COMPREHENSIVE COORDINATION CHEMISTRY
The Synthesis, Reactions, Properties & Applications of Coordination Compounds
Editor-in-Chief: Sir Geoffrey Wilkinson, FRS Executive Editors: Robert D. Gillard & Jon A. McCleverty
Pergamon An imprint of Elsevier Science
COMPREXENSIVE COORDINATION CHEMISTRY IN 7 VOLUMES
PERGAMON MAJOR REFERENCE WORKS
Comprehensive Inorganic Chemistry (1973) comprehensive Organic Chemistry (1979) Comprehensive Organometallic Chemistry (1982) Comprehensive Hererocyclic Chemistry (1984) International Encyclopedia of Education ( 1985 ) comprehensive Insect Physiology, Biochemistry & Pharmacology (1 985) Comprehensive Biotechnology (1985) Physics in Medicine & Biology Encyclopedia (1986) Encyclopedia of Materials Science & Engineering (1986) World Encyclopedia of Peace (1986) Systems & Control Encyclopedia (1987) Comprehensive Coordination Chemistry (1987) Comprehensive Polymer Science (1988) Comprehensive Medicinal Chemistry (1989)
COMPREHENSIVE COORDINATION CHEMISTRY The Synthesis, Reactions, Properties & Applications of Coordination Compounds Volume 4 Middle Transition Elements
EDITOR-IN-CHIEF
SIR GEOFFREY WILKINSON, FRS imperial College of Science & Technology, University of London, UK EXECUTrVE EDITORS
ROBERT D. GILLARD University College, Cardiff. UK
JON A. McCLEVERTY University of Birmingham, UK
PERGAMON PRESS
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First edition 1987 Library of Congress Cataloging-in-PublicationData Comprehensive coordination chemistry. Includes bibliographies Contents: v. 1. Theory and background - v. 2. Ligands - v. 3. Main group and early transition elements - [etc.] 1. Coordination compounds. . I. Wilkinson, Geoffrey, Sir, 192111. Gillard, Robert D. Ill. McCleverty, Jan A. QD474.C65 I987 541.2’242 86-12319 British Library Cataloguing in Publication Data Comprehensive coordination chemistry: the synthesis, reactions, properties and applications of coordination compounds. 1. Coordination compounds. I. Wilkinson, Geoffrey, 1921. 11. Gillard, Robert D. 111. McCleverty, Jon A. 541.2’242 QD474 ISBN 0-08-035947-7 (~01.4) ISBN 0-08-026232-5(set)
Printed in Great Britain by A. Wheaton & Co. Ltd., Exeter
Contents vii
Preface Contributors to Volume 4
ix
Contents of All Volumes
xi
41
Manganese B. CHISWELL and E. D. MCKENZIE,University of Queensland, St Lucia, Queensland, Australia, and L. F . LINDOY,James Cook University, Townsville, Queensland, Australia
1
42
Technetium A. DAYISON, Massachusetts Institute of Technology, Cambridge, M A , U S A , and C. J. L. LOCK,McMaster University, Hamildon, Ontario, Canada
123
43
Rhenium K. A. COUNER and R. A. WALTON,Purdue University, West Lafayette, IN, USA
125 See below
44.1 Iron(I1) and Lower States 44.2 Iron(II1) and Higher States S. M. NELSON,The Queen’s University of Belfast, UK
217
45
Ruthenium M. SCHRODER and T. A. STEPHENSON, University of Edinburgh, UK
277
46
Osmium W. P. GRIFFITH, Imperial College of Soieacc and Technology, London, U K
519
47
Cobalt D. BUCKINGHAM and C. R. CLARK,University of Otago, Dunedin, New Zealund
635
48
Rhodium P. S. SHERIDAN, Colgate University, Hamilton, N Y , USA, and F . JARDINE, North East London Polytechnic, UK
90 1
49
Iridium N. SERPONE and M. A. JAMIESON, Concordia University, Montreal, Quebec, Canada
1097
44. I
Iron(11) and Lower States P. N. HAWKER, Johnson Matthey Catalytic Systems Division, Royston, U K , and M . V. TWIGG,IC1 Chemicals and Polymers Group, Billingham, UK
1179
Subject Index
1289
Formula Index
1309
V
Preface Since the appearance of water on the Earth, aqua complex ions of metals must have existed. The subsequent appearance of life depended on, and may even have resulted from, interaction of metal ions with organic molecules. Attempts to use consciously and to understand the metal-binding properties of what are now recognized as electron-donating molecules or anions (ligands) date from the development of analytical procedures for metals by Berzelius and his contempones. Typically, by 1897, Ostwald could point out, in his ‘Scientific Foundations of Analytical Chemistry’, the high stability of cyanomercurate(1I) species and that ‘notwithstanding the extremely poisonous character of its constituents, it exerts no appreciable poison effect’. By the late 19th century there were numerous examples of the complexing of metal ions, and the synthesis of the great variety of metal complexes that could be isolated and crystallized was being rapidly developed by chemists such as S . M. Jerrgensen in Copenhagen. Attempts to understand the ‘residual affinity’ of metal ions for other molecules and anions culminated in the theories of Alfred Werner, although it is salutary to remember that his views were by no means universally accepted until the mid 1920s. The progress in studies of metal complex chemistry was rapid, perhaps partly because of the utility and economic importance of metal chemistry, but also because of the intrinsic interest of many of the compounds and the intellectual challenge of the structural problems to be solved. If we define a coordination compound as the product of association of a Bronsted base with a Lewis acid, then there is an infinite variety of complexing systems. In this treatise we have made an arbitrary distinction between coordination compounds and organometallic compounds that have metal-carbon bonds. This division roughly corresponds to the distinction -which most but not all chemists would acknowledge as a real one -between the cobalt(II1) ions [Co(NH3),I3+ and [ C O ( $ - C ~ H ~ ) ~Any ] + .species where the number of metal-carbon bonds is at least half the coordination number of the metal is deemed to be ‘organometallic’ and is outside the scope of our coverage; such compounds have been treated in detail in the companion work, ‘Comprehensive Organometallic Chemistry’. It is a measure of the arbitrariness and overlap between the two areas that several chapters in the present work are by authors who also contributed to the organometallic volumes. We have attempted to give a contemporary overview of the whole field which we hope will provide not only a convenient source of information but also ideas for further advances on the solid research base that has come from so much dedicated effort in laboratories all over the world. The first volume describes general aspects of the field from history, through nomenclature, to a discussion of the current position of mechanistic and related studies. The binding of ligands according to donor atoms is then considered (Volume 2) and the coordination chemistry of the elements is treated (Volumes 3, 4 and 5 ) in the common order based on the Periodic Table. The sequence of treatment of complexes of particular ligands for each metal follows the order given in the discussion of parent ligands. Volume 6 considers the applications and importance of coordination chemistry in several areas (from industrial catalysis to photography, from geochemistry to medicine). Volume 7 contains cumulative indexes which will render the mass of information in these volumes even more accessible to users. The chapters have been written by industrial and academic research workers from many countries, all actively engaged in the relevant areas, and we are exceedingly grateful for the arduous efforts that have made this treatise possible. They have our most sincere thanks and appreciation. We wish to pay tribute to the memories of Professor Martin Nelson and Dr Tony Stephenson who died after completion of their manuscripts, and we wish to convey our deepest sympathies to their families. We are grateful to their collaborators for finalizing their contributions for publication. Because of ill health and other factors beyond the editors’ control, the manuscripts for the chapters on Phosphorus Ligands and Technetium were not available in time for publication. However, it is anticipated that the material for these chapters will appear in the journal Polyhedron in due course as Polyhedron Reports. We should also like to acknowledge the way in which the staff at the publisher, particularly
...
Vlll
Preface
Dr Colin Drayton and his dedicated editorial team, have supported the editors and authors in our endeavour to produce a work which correctly portrays the relevance and achievements of modern coordination chemistry. We hope that users of these volumes will find them as full of novel information and as great a stimulus to new work as we believe them to be.
JON A. MCCLEVERTY Birmingham
ROBERT D. GILLARD Cardifl GEOFFREY WILKINSON London
Contributors to Volume 4 Professor D. Buckingham Chemistry Department, University of Otago, Box 56, Dunedin, New Zealand Dr B. Chiswell Department of Chemistry, University of Queensland, St Lucia, Queensland 4067, Australia Dr C. R. Clark Chemistry Department, University of Otago, Box 56, Dunedin, New Zealand Dr K. A. Conner Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA Professor A. Davison Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 021 39, USA Dr W. P. Griffith Department of Chemistry, Imperial College of Science & Technology, South Kensington, London SW7 2AY, UK Dr P. N. Hawker Johnson Matthey Catalytic Systems Division, Royston, Herts SG8 5HE, UK Dr M. A. Jamieson Department of Chemistry, Concordia University, Sir George Williams Campus, 1455 de Maisonneuve Boulevard West, Montreal, Quebec H3G lM8, Canada
Dr F. Jardine Department of Chemistry, North East London Polytechnic, Romford Road, London E15 4LZ, UK Dr L. F. Lindoy Department of Chemistry and Biochemistry, James Cook University, Townsville, Queensland 48 1 1. Australia Professor C. J . L. Lock Institute for Materials Research, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 3M1, Canada Dr E. D. McKenzie Department of Chemistry, University of Queensland, St Lucia, Queensland 4067, Australia Dr S. M. Nelson (Deceased) Department of Chemistry, Queen's University, Belfast BT9 5AG, UK Dr M. Schroder Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK C.O.C. &A*
iX
X
Contrihlttors to Volume 4
Professor N. Serpone Department of Chemistry, Concordia University, Sir George Williams Campus, 1455 de Maisonneuve Boulevard West, Montreal, Quebec H3G 1M8, Canada Professor P. S . Sheridan Department of Chemistry, Colgate University, Hamilton, NY 13346, USA Dr T. A. Stephenson (Deceased) Department of Chemistry, University of Edinburgh, West Mains Road, PO Box 1, Billingham EH9 355, UK Dr M. V. Twigg Research and Technology Department, IC1 Chemicals and Polymers Group, PO Box 1, Billingham, Cleveland TS23 1LB, UK Professor R. A. Wilton Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA
Contents of All Volumes Volume 1 Theory & Background
General Historical Survey to 1930 Development of Coordination Chemistry Since 1930 2 Coordination Numbers and Geometries Nomenclature of Coordination Compounds 3 4 Cages and Clusters Isomerism in Coordination Chemistry 5 6 Ligand Field Theory Reaction Mechanisms 7.1 Substitution Reactions 7.2 Electron Transfer Reactions 7.3 Photochemical Processes 7.4 Reactions of Coordinated Ligands 7.5 Reactions in the Solid State Complexes in Aqueous and Non-aqueous Media 8.1 Electrochemistry and Coordination chemistry 8.2 Electrochemical Properties in Aqueous Solutions 8.3 Electrochemical Properties in Non-aqueous Solutions Quantitative Aspects of Solvent ElTects 9 Applications in Analysis 10 Subject Index Formula Index 1.1
1.2
Volume 2 Ligands 11 12.1 12.2
13.1 13.2
13.3 13.4 13.5
13.6 13.7 13.8
14
15.1 15.2
15.3 15.4 15.5
Mercury as a Ligand Group IV Ligands Cyanides and Fulminates Silicon, Germanium, Tin and Lead Nitrogen Ligands Ammonia and Amines Heterocyclic Nitrogen-donor Ligands Miscellaneous Nitrogen-containing Ligands Amido and Imido Metal Complexes Sulfurdiimine, Triazenido, Azabutadiene and Triatomic Hetero Anion Ligands Polypyrazolylborates and Related Ligands Nitriles Oximes, Guanidines and Related Species Phosphorus, Arsenic, Antimony and Bismuth Ligands Oxygen Ligands Water, Hydroxide and Oxide Dioxygen, Superoxide and Peroxide Alkoxides and Aryloxides Diketones and Related Ligands Oxyanions xi
Contents of All Volumes
xii
15.6 Carboxylates, Squarates and Related Species 15.7 Hydroxy Acids 15.8 Sulfoxides, Amides, Amine Oxides and Related Ligands 15.9 Hydroxamates, Cupferron and Related Ligands Surfuv Ligands 16.1 Sulfides 16.2 Thioethers 16.3 Metallothio Anions 16,4 Dithiocarbamates and Related Ligands 16.5 Dithiolenes and Related Species 16.6 Other Sulfur-containing Ligands Selenium and Tellurium Ligands 17 Halogens as Ligands 18 Hydrogen and Hydrides as Ligands 19 Mixed Donor Atom Ligands 20.1 Schiff Bases as Acyclic Polydentate Ligands 20.2 Amino Acids, Peptides and Proteins 20.3 Complexones 20.4 Bidentate Ligands Multidentute Macrocyclic Ligands 21.1 Porphyrins, Hydroporphyrins, Azaporphyrins, Phthalocyanines, Corroles, Corrins and Related Macrocycles 21.2 Other Polyaza Macrocycles 21.3 Multidentate Macrocyclic and Macropolycyclic Ligands Ligands of Biological Importance 22 Subject Index Formula Index
Volume 3 Main Group & Early Transition Elements 23 24 25.1 25.2 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Alkali Metals and Group IIA Metals Boron Aluminum and Gallium Indium and Thallium Silicon, Germanium, Tin and Lead Phosphorus Arsenic, Antimony and Bismuth Sulfur, Selenium, Tellurium and Polonium Halogenium Species and Noble Gases Titanium Zirconium and Hafnium Vanadium Niobium and Tantalum Chromium M o1y bdenum Tungsten Isopolyanions and Heteropolyanions Scandium, Yttrium and the Lanthanides The Actinides Subject Index Formula Index
Volume 4 Middle Transition Elements 41
Manganese
Contents of A I1 Volumes 42 43 44.1 44.2 45 46 47 48 49
Technetium Rhenium Iron Iron(I1) and Lower States Iron(II1) and Higher States Ruthenium Osmium Cobalt Rhodium Iridium Subject Index Formula Index
Volume 5
Late Transition Elements
Nickel 50 Palladium 51 Platinum 52 Copper 53 Silver 54 Gold 55 56.1 Zinc and Cadmium 56.2 Mercury Subject Index Formula Index
Volume 6 Applications
57 58 59 60
61.1 61.2 61.3 61.4 61.5 62.1 62.2 63 64 65 66
Electrochemical Applications Dyes and Pigments Photographic Applications Compounds Exhibiting Unusual Electrical Properties Uses in Synthesis and Catalysis Stoichiometric Reactions of Coordinated Ligands Catalytic Activation of Small Molecules Metal Complexes in Oxidation Lewis Acid Catalysis and the Reactions of Coordinated Ligands Decomposition of Water into its Elements Biological and Medical Aspects Coordination Compounds in Biology Uses in Therapy Application to Extractive Metallurgy Geochemical and Prebiotic Systems Applications in the Nuclear Fuel Cycle and Radiopharmacy Other Uses of Coordination Compounds Subject Index Formula Index
Volume 7 Indexes
67
Index of Review Articles and Specialist Texts Cumulative Subject Index Cumulative Formula Index
...
Xlll
Manganese BARRY CHISWELL and E. DONALD McKENZlE University o f Queensland, Australia and LEONARD F. L1NDOY James Cook University of North Queensland, Australia 41.1 INTRODUCTION 41.2 MANGANESE(1) AND LOWER OXIDATION STATES
41.2.1 41.2.2 41.2.3 41.2.4
Introduction ManRanese( - I) Manganese(0) Manganese(I1
2 6 6 1 7. 8
MANGANESE(I1) 41.3.1 Introduction 41.3.2 Carbon Donor Ligands, Including Cyanide 41.3.2.1 Cyanides 41.3.2.2 Alkyls and aryls 41.3.2.3 Fulminates and other carbon ligands 41.3.3 Nitrogen Ligands 41.3.3.1 Unidentate ligands 41.3.3.2 Bidentare ligands 41.3.3.3 Terdenrate ligamds 41.3.3.4 Quadridentate ligands 4 I .3 3.5 Quimquedentote ligamdr 41.3.3.6 Sexudenrare and septadentate ligands 41.3.3.7 Uses of munganese nitrogen compounds 41.3.4 Phosphorus, Arsenic, Antimony and Bismuth Ligands 41.3.5 Oxygen Ligands 41.3.5.1 Unidentate ligands 41.3.5.2 Oxyanions 41.3.5.3 Bidentate ligands 41.3.5.4 Muitidentare ligands 41.3.6 Surfur Ligands 41.3.6.1 Surfides nnd thiolates 41.3.6.2 Dithiolemes 41.3.6.3 Thiocarbonyls 41.3.6.4 1.1-Dithiolates 41.3.7 Halides 41.3.7.1 Crystalline halides with octahedrul coordination 41.3.7.2 Tetrahedral [MnX,/’- and [ M n X , L ] - species 41.3.7.3 Halo compounds of Mn” in the fluid phases 41.3.7.4 Fluor0 anions of the nonmetals as ligands 41.3.8 Hydrogen and Hydride Ligands 41.3.8.1 Hydrogen 41.3.8.2 Hydride ligands 41.3.9 Mixed Donor Atom Ligands 41.3.9.1 'Ambiguous' N-0 and bidentate N--0 ligands 41.3.9.2 Mixed N-0 donor atom terdentate ligands 41.3.9.3 Mixed N-0 donor atom quadridentate ligands 41 3.9.4 Mixed N--0 donor atom quinquedentate ligands 41.3.9.5 Mixed N-0 donor atom sexa- and ocfadentaceligands 41.3.9.6 Mixed N-S donor atom ligands 41.3.9.7 Mixed 0-S donor atom ligands 41.3.10 Macrocyclic Ligands 41.3.10.1 Porphyrin ligands
41.3
1
9 9 11 11 12
14 15 15
23 26 21 29 29 30 31 34
35 41 47
52 53 53 54 54
54 55 56 59 60 60 60 60 61 61 61 65 66 69 69 70 71 12 72
Mangunme
2 41.3.10.2 Pk ihalocyanint. ligands 41.3.10.3 Other nitrogen donor macrocycles 41.3.10.4 Oxygen donor macrocycles 41.3.10.5 Mixed donor macrocycles
MANGANESE(III) 41.4.1 Carbon Ligands 41.4.2 Nitrogen and Phosphorus Ligands 41.4.3 Oxygen Ligands 41.4.3.1 Waier, hydroxide and oxide 41.4.3.2 Oxyanions 41.4.3.3 Carboxylates 41.4.3.4 Alcohol and phenol ligands 41.4.3.5 Acctylacelonate and related ligands 41.4.3.6 Other oxyzen ligands 41.4.4 Su&r Ligan& 41.4.5 Halides 41.4.6 Hydrogen and Hydride Ligands 41.4.7 Mixed Donor Atom Ligands 41.4.7.1 Mixed N-0 donor atom bidentate ligands 41.4.7.2 Mixed N-0 donor atom terdentate ligands 41.4.7.3 Mixed N-0 donor atom quadridentate ligan& 41.4.7.4 Mixed N-0 donor atom quinque- and sexadenfate ligands 41.4.7.5 Mixed 0-S and N--,O--.S donor atom ligand 41.4.8 Macrocyclic Ligands 41.4.8.1 Porphyrin ligands 41.4.8.2 Phthuhcyyanine ligands 41.4.8.3 Other nitrogen donor ligands
41.4
41.5 MANGANESWV)
41.5.1 Carbon Ligands 41.5.2 Nitrogen and Phosphorus Ligands 41.5.3 Oxygen Ligands 41.5.3.1 Water. hydroxide and oxide 41S.3.2 Peroxo 41.5.3.3 Oxyanions 41S.3.4 Di- and polyhydroxy ligands, including caiechol 41.5.3.5 Other ligands 41.5,4 Sulfur Ligands 41 S.5 Halides 41.5.6 Mixed Donor Atom Ligandy 41.5.7 Mactocylic Ligands 11.5.7.1 Porphyrin ligands 41.5.7.2 Phthalocyanine ligands 41.6
MANGANESE(V), (VI) A N D (VII)
41.6.1 Teerraoxomanganates 41.6.2 Other Oxo and Uxohalo species 41.6.3 Porph-vrin Ligands 41.7
REFERENCES
75 76 80 80 82 83 84 85 85 87 88
89 89 90 90 91 93 93 93 93 94 96 96 97
97 99 100
102 102 103 104 104 105 105 106 106 106 107 108 108 108 109 109
109 110 110 111
41.1 INTRODUCTION
In preparing a review of the coordination chemistry complementary to that by Treichel' in the companion volumes on organometallic chemistry, we have been fortunate in the way in which manganese chemistry is so readily divided between the two. Probably as much as for any other metal, the organometallic chemistry of manganese, Le. compounds with Mn-C bonds, is the chemistry of the lower oxidation states (I, 0 and - I); and the chemistry of manganese(I1) and the higher oxidation states (I11 to VII) is that of classical coordination chemistry with the donor atoms nitrogen, oxygen and the halogens. Compounds with other Group V and Group VI donors have been prepared, but their known chemistry is not yet extensive. There are, of course, overlaps between the two areas, and the elegant work by Wilkinson and co-workers on the manganese(II), (111) and (rv)alkyls immediately comes to mind (see Section 41.3.2.2); similarly, many of the classical ligands of coordination chemistry, such as heterocyclic nitrogen ligands, and the phosphines and arsines, are widely used in organometallic chemistry. But this division into lower oxidation state/organometallic and higher oxidation state/coordination chemistry is as neat a division as one could hope for in chemistry.
Manganese
3
Manganese has been a metal much neglected by the coordination chemist. Interest has recently been aroused, however, by the recognition of its biological role, especially in photosynthesis.' This neglect is reflected in the treatment manganese has received, for example, in the Chemical Society's 'Annual Reports', and it is only within the last decade that significant volumes on the chemistry of manganese have appeared within the Gmelin ~ y s t e mWe . ~ often searched reviews on the chemistry of particular ligand systems in vain, or for just one or two references to manganese. In this regard it is pleasing to see the new series of annual reviews of the particular metals in Coordination Chemistry Reviews, which give adequate coverage to manganese.G Available reviews are, of course, of diverse quality: they range from good comprehensive, critical reviews to simple compilations of published material, some of which do not even have the virtue of being comprehensive. But, since none of the recent reviews covers large areas, we note them in the sections for the various ligands. Reasons for the neglect of manganese coordination chemistry undoubtedly relate to the dominance of the Mn" state, its half-filled d s configuration, and all that follows from this, especially the lack of any reliable guide to the structure of the coordination polyhedron from anything other than X-ray methods. The great majority of Mn" compounds are high spin, and, in the 6S ground state, d-d transitions are forbidden by both the LaPorte and the spin state selection rules. Thus the d-d electronic spectra are of very low intensity and are of little use in determining metal stereochemistry; the compounds are colourless or very pale coloured. Thus in the period when transition metal coordination chemists believed in the d-d spectra as a reliable guide to structure, manganese could elicit but little interest. Manganese(1T) compounds are quite labile; the metal shows distinct class (a) c h a r a ~ t e rand ; ~ its 'ionic radius' (defined by the M-H,O distanoes in Table 1) is large compared with the other first row transition metals. These lead to distinct parallels with magnesium(I1) rather than the latter, although there are also significant parallels with octahedral high spin nickel(I1). Many, but by no means all, manganese(I1) compounds also are sensitive to oxidation by oxygen and their preparation is often complicated by the marked tendency for aqueous manganese solutions, under ambient conditions, to deposit the ubiquitous, highly insoluble manganese dioxide. Both add complications to manganese(I1) chemistry and have undoubtedly made it less attractive to some workers. This sensitivity to oxygen and especially the products of the reactions are a major reason for the increasing interest in manganese chemistry. The last decade has seen significant developments: both manganese(II1) and manganese(1V) have been properly characterized in a number of systems, and it appears that the + 4 oxidation state is more accessible to manganese than, for example, iron. However, a great deal more needs to be done, and done properly, before we can pretend to have a reasonably balanced view of the relative accessibilities and stabilities of Mn"' and Mn'". Much has been, and is being, published on the reaction of manganese(I1) species with 02,the possibility of the existence of Mn-0, species and their structure and properties, and the nature of the final oxidized products. But there is very little light yet shed in this area. The higher oxidation states V, VI and VI1 are all known, and are largely represented in the chemistry of the oxo ligand manganates. The highest oxidation state VI1 corresponds to the total number of 3d and 4s electrons-a feature of the earlier transition metals Sc to Cr. Manganese is, except for a few unstable oxoferrates, the last of this series, and although potassium permanganate is one of the best known manganese compounds, and is widely used, yet the chemistry of manganese and oxo ligands is by no means as extensive as that of chromium and vanadium. In other aspects, especially in the dominance of the + 2 oxidation state, manganese shows parallels with the later transition metals. It is the first transition metal for which + 2 is the common oxidation state. Table 1 MI'-OH, Metal:
M-OH,
(A)
a
b
Bond Distances in the Hexaaquo Cations
-w?
Mn
Fe
CO
NI
CU
Zn
Cd
2.09 2.10 2.05
2.19 2.20 2.15
2.17 2.14 2.10
2.11 2.09 2.08
2.09 2.08 2.04
2.22 2.10 1.96
2.13 2.12 2.08
2.30 2.30 2.24
2.20
2.12
2.08
2.04
2.43 1.94
2.08
2.29
the Tutton salis there are three independent pairs of lrtrns Iigands: H. Montgomery, R. V. Chastdin and E. C. Lingafelter, Acra Crysrallogr.. 1966. ZO, 711; C. S. C.Phillips and R. J. P. Williams, 'Inorganic Chemistry', Oxford, 1966, vol. 2, p. 249. bAqueoussolutions (X-ray data): H . Ohtaki, Rev. Inorg. Churn., 1982, 4, 103.
4
Manganese
Thus, in its broad features, manganese chemistry reflects its position in the middle of the transition series. As any scientific classification creates problems of definition, so our present classification based on oxidation state leads to difficulty in the placing of material. This is particularly, but not exclusively, true of nitro.&. It is common to classify M-NO moieties into compounds of: (a) the monoanion NO-, characbond; and (b) the terized by lower energy NO stretching frequencies and a bent (- 120") M-N-0 nitronium cation NOf, characterized by higher energy NO stretching frequencies and a short, linear M-N-0 bond. Thus, for example, the compound [Mn(NO),PPh,], with three linear M-NO bonds, becomes a compound of Mn-"' and this we believe to be a chemical nonsense or at best a useless formalism. Further, we note also that a recent X-ray structural determination* of a compound of type (b) above shows significant bending in the M-N-0 bond, and the authors suggest that such bonding may be more widespread in type (b) compounds. Accordingly, we adopt the convention, in agreement with IUPAC nomenclature recommendations, that NO be regarded as a neutral ligand, and deal with the above trinitrosyl as a compound of manganese(0). This also means, for example, that the anions [Mn(CN),NO]"- (n = 2,3) become here compounds of manganese(II1) and (111, although they are often classified elsewhere under manganese(I1) and (I), respectively. The nature of the data: Since we have attempted to be critical in our assessment of the published data, it is as well that we note the bases used for much of this critical assessment. Mixing any ligand ( L ) , especially ligands containing one (or many) nitrogen or oxygen donor atom(s), with MnX, (X = a monoanion) in a suitable solvcnt system will lead to the separation of crystalline species (or a number of them) MnX,L,; deprotonation of the ligand, if possible, may give further compounds. Such information is, of itself, often of little interest, even though the products be 'new' compounds. Yet this is often the only established, meaningful fact in what may be quite long papers. Sadly, many papers are published in which not even such a simple basic fact has been established. Despite published (in 1978) analytical figures, we find it hard to believe that a black residue, obtained by heating manganese(I1) with an oxygen donor ligand in an aqueous alkaline solution in open vessels, is a manganese(I1) compound; we think it more likely to have been MnO,, perhaps admixed with some carbonaceous material. Powders, even of the expected colour, are suspect unless X-ray diffraction shows them to be a discrete crystalline species, or recrysallization can be achieved to give a product with say an IR spectrum identical with the original. Ideally all solids should generally be characterized by their X-ray diffraction patterns and TR spectra. But having characterized a crystalline product, what then? What are the authors trying to achieve beyond preparing a new compound? If it is to show what compounds can form, then many papers fail to explore a proper range of reaction conditions; if it is to show how the ligand is bound to the metal, then there is currently but one reliable physical method for this job -X-ray diffraction. Sometimes this may simply mean demonstrating isomorphism with the compound of another metal of known structure (especially if supplemented with IR data). But practice shows that such isomorphism often does not happen, and then full X-ray analyses are the only reliable answer. It may be fun-perhaps quite absorbing fun- to play with the so-called 'sporting methods', such as vibrational spectra, electronic spectra and magnetism. But, especially in manganese chemistry where electronic spectra are of but little use for indicating structure, we have taken little note of structural assignments based on these 'sporting methods', although we may have noticed the work for other reasons, such as an intrinsic interest in the reported compounds. Whilst electronic spectra do not have the structural use they have (with discretion) with nickel(II), it is at least possible to make some useful deduction from the colour of the products. We can, for example, conclude with some confidence that a black substance described as a manganese(I1) compound with oxygen donors was not; that a white substance described as Mn'" or Mn" was not; and that a green compound, having the same intense colour as some related Mn"' species, was probably not manganese(II), despite the apparent stoichiometry (Section 41.3.3.2). Yet each of these has appeared in the recent literature. It is arguable that the study of magnetic moments of coordination compounds has been as much a hindrance to the elucidation of stereochemistry as it has been of use. The earlier confidence that it would be possible to correlate 'stereochemistry of an atom with its effective magnetic moment" has not been confirmed, although some impressive work has come out of magnetic studies. What is not arguable is that the chemistry of manganese would be no less further advanced if no paper had been wasted on arguments about structure of solid polymeric materials from magnetic moments.
Manganese
5
Manganese with its great variety of oxidation states, each with varying numbers of unpaired 3d electrons, is magnetically interesting but not simple. The theoretical 'spin only' magnetic moments for oxidation states* + 2, + 3 and + 4 are in Table 2, and for comparison some experimentally determined moments of some representative compounds are in Table 3. Monomeric species, with no exchange interaction between adjacent metal atoms, typically exhibit moments close to the 'spin only' values. Most known compounds of manganese(I1) and (111) are high spin, with the cyano compounds best known among the few low spin species. But these moments give no information about stereochemistry of the metal atom, and their purpose can be only to give information about the spin state and perhaps to help to confirm oxidation state. In practice, measured moments range between 0 and 6.5 BM, and those differing significantly from 'spin only' values often are called 'anomalous', in the sense of a deviation from a norm. Such nomenclature, however, is probably in all cases fatuous- the deviation from the norm often itself being the norm. Table 2 Theoretical Magnetic Moments (in BM) for Mn", Mn"' and Mn'" Compounds (Based on 'Spin Only' Formula) Spin State Oxidation State
spin Free
Intermediate Spin
Mn" Mn"' Mn"
5.92 4.90 3.87
3.87 -
Spin Paired
I
Oh Field
Td Field
1.73 2.83 337*
1.73 0 1.73
* In an Oh field spin pairing will not occur. Table 3 Experimental Room Temperature Magnetic Moments (in BM) of some Mn", Mn"' and Mn" Compounds (a) Magnetically Dilute Species" Oxidation State
Mn" Mn" Mn" Mn"' Mn"' Mn'" Mn'"
Compound
[Mn(py)61Br2 byW2[MnC1,1 K,[Mn(CN),I IWacac)31 K3P M C N) J K~i ~ n ~ ~ i K~~M~(cN),I
Spin State
Stereochemistry
pel7
Spin free Spin free Spin paired Spin fre.e Spin paired Spin free Spin free
Oh Td Oh Oh Oh Oh Oh
6.00 5.95 I .80 4.95 3.18 3.90 3.94
(b) Magnetically Interactive Species
Oxidation State
Mn"
Compound
Stereochemistry
[Mn(salen)]
Five-coord. Dimeric?
1
Mn"
5.24
Ref b
Square planar
4.34
Trimeric
3.98
d
2.07
e
Octahedra? 0-bridged [Mn(salen)(OAc)] IMnO(pc)l
1
&7
? ?
4.68 3.7s
r g
Ligand abbreviarions: acac = acetylacetone; OAc = acetate; pc = phthalcqanine; py = pyridine; salen = MN-ethylenebis(salicylaldimine) References: B. N. Figgis and J. Lewis in 'Modern Coordination Chemistry', ed. J. Lewis and R. C. Wilkins, Interscience, New York, 1960, p. 4 0 6 7 . bC. J. Boreham and B. Chiswell, Inorg. Chim. Acfa, 1977, 24, 77. 'C. G. Barraclough, R. L. Martin and S . Mitra, J. Chem. Phys., 1970, 53, 1638. dA. R. E. Bdikie, M. B. Humthouse, D. B. New and P. Thornton, J. Chem. Soc., Chem. Cornrnun.. 1978,62. e A. Yamamoto, L.K.Phillips and M. Calvin, Inorg. Chem., 1968, 7, 847. ' A . Earnshaw, E. A. King and L. F. Larkworthy, J . Chem. Soc./A), 1968, 1048. A. B. P. Lever, J. Chem SOC.,1965, 1821. a
* States lower than Mn" are usually stabilized by strong field ligands giving rise to spin-paired systems; whereas the magnetic properties of MnV,MnV1and Mn"" have not been widely studied.
6
Manganese
Where magnetic moments are lower than the ‘spin only’ values, and full experimental evidence of antiferromagnetism is obtained, the results are often interpreted in terms of structures for which the best basis appears to be nothing more than the author’s previous experience. The only proper procedure is to interpret the magnetic data after the structure is known and not vice versa. The use of magnetic data as primary evidence for any structure is precluded by the variety of structural types- oligomeric and dimeric- which are known to give antiferromagnetism. In addition, manganese compounds exhibiting antiferromagnetism are now known to have metal atoms in different oxidation states and there is the further possibility of different spin states. Yet another complication of which not sufficient heed is often taken is the presence of the ubiquitous, insoluble and paramagnetic MnO,. It must be suspected as a by-product of the preparation of almost any compound of oxidation state greater than 1, and so many products that are reported as dark powders, or even grey powders, probably possess appreciable amounts of MnO,, thus invalidating the reported magnetic data, and possibly even the reported stoichiometry. Although nuclear magnetic resonance studies have been of great use in elucidating the structures in solutions of the compounds of many of the metals, the paramagnetic nature of many manganese compounds, and in particular high spin Mn”, has limited the applicability of this technique. Nevertheless, the very difficulty associated with obtaining Mn” NMR spectra has been turned to advantage by using Mn” as a magnetic relaxation probe in the study of biomechanisms and biomacromolecules.‘O Although a drawback to NMR studies, the five unpaired electrons of high spin Mn“ compounds give rise to excellent electron spin reJononce spectra. An early review” noted the effect upon ESR spectra of changing ligands about the Mn” ion while retaining high symmetry, whereas a recent review summarizes the work that has been done upon compounds with a low-symmetry environment about the metal.12 ESR studies have been applied in a most interesting manner to ligand speciation studies of manganese in aqueous environmental ~amp1es.I~ Although also paramagnetic, high spin d 4 Mn”’ compounds appear to have short spin-lattice relaxation times and generally do not give good ESR spectra. Little work has been undertaken on the ESR of the higher oxidation states of manganese, although Mn’” compounds are known to yield spectra.” There is another basic lesson which many authors publishing in this area of chemistry have failed to heed; when labile compounds are dissolved in any solvent, information about the properties of the solution (e.g. a conductance measurement) gives no direct information about the solid compound. For some metal compounds, it may be possible to show (e.g. from electronic spectra) that the coordination sphere remains unchanged from the crystalline solid to the solution, but this appears not to be possible for manganese. For these sorts of reasons we have found it necessary to neglect a significant portion of the published work, especially on ligand systems that offer the metal a choice of donor atoms. The story of the coordination chemistry of manganese is very incomplete; it is only at an early stage of development compared with most of the other transition metals; and there is an obvious need for X-ray structural techniques to be used much more routinely in those studies of manganese chemistry that rely on the isolation and characterization of solid compounds for their purpose. Finally a genera1 point -we do not usually distinguish between ‘binary compounds’, ‘salts’ and ‘complexes’. All give interesting examples of metal coordination polyhedra and the terms are arbitrary distinctions, derived from the history of chemistry as our understanding has developed. They have little meaning now beyond their use as jargon words by inorganic chemists. Whilst we (meaning coordination chemists) all understand what they mean and how to use them, they impose a barrier to communication with nonspecialists and inhibit the learning process. Unfortunately, also, they are tendentious words, leading almost inevitably to the belief that the ‘simple’ chemistry of a metal ion can be understood by a study of only the binary compounds and salts, and that a study of the ‘complexes’ is a higher-order, more esoteric branch of knowledge. For these reasons, we have attempted to show in much of the following how the coordination chemistry of one metal can be described without the use of the tendentious word ‘complex’.
+
41.2 41.2.1
MANGANESE(1) AND LOWER OXIDATION STATES Introduction
This area of coordination chemistry gives the greatest overlap with the area generally classified as organometallic chemistry and reviewed by Treichel in the companion volume. We attempt in Table 4 to give an overview of the range of oxidation states covered by the organometallic chemistry of manganese, and thus the bulk of the lower oxidation state chemistry
Manganese
7
Table 4 The Oxidation States of the Organometallic Chemistry of Manganese
Sce also thc borane and carborane-manganese carbonyl compounds: R. N. Grimes, in 'Comprehensive Organomclallic Chemistry', d. G. Wilkinuon, F. G. A. Stone and E. W. A M , Pergamon, Oxford, 1982, vol. 1, p. 459.
of manganese. It is important to note, however, that in this area the oxidation state formalism is probably less clear cut and less useful than elsewhere. Apart from the organometallic chemistry, there is but little manganese chemistry left for us to review and most of this is of the cyanides and nitrosyls, most of which could have reasonably been embraced within organometallic. Previous assignments of lower metal oxidation states to species such as [Mn(phthalocyanine)]- and [Mn(bipy),]* now are known to be incorrect and these species are treated under manganese(I1). Furthermore, present indications are that [Mn"N,] macrocyclic systems, in general, cannot be reduced to Mn' species, unlike those of the later transition metals, especially cobalt, nickel and copper. Although reduced species can and have been obtained, these, when properly studied, turn out to be not Mn' or Mn' species, but rather Mn" species of radical anion ligands (see Section 41.3.10). As discussed previously, nitrosyls are classified according to oxidation state on the convention that NO is a neutral ligand. 41.2.2 Manganese( -I) The only compound to be noted here is the recently de~cribed'~cyano nitrosyl K,[Mn(CN),(NO),]. This green compound has been made by the reduction of the dimeric species K,[Mn,(CN),(NO),] with potassium in liquid ammonia. The IR spectrum suggests a tetrahedral structure similar to the isoelectronic [Fe(CN),
41.2.3 Manganese(0) The interpretation" of the observed paramagnetism of some yellow products of the reduction of [Mn(CN),I4- with K in liquid NH, as evidence for Mno species is by no means unequivocal. The only other Mno species which come within our sphere of interest are nitrosyls, primarily derived from the carbonyl nitrosyls of Table 5. Of these, the two Mno species have already been reviewed by Treichel,', as have the various series of currently known compounds of which these are respectively, the parents: series such as [MnL(NO)J, [Mn(CO),L(NO)] and [Mn(CO),L,NO] etc. attempt^'^ to prepare monocyano derivatives of [Mn(CO),NO] and [Mn(CO)(NO),] by treating with KCN in liquid NH3gave instead; for the former K,[Mn(CN),(CO),] and K3[Mn(CN),(CO)J, and for the latter K,[Mn,(CN),(NO),]. Evidence for a dimeric structure K4[Mn2(CN),(NO)J has been isolated in cis and trans f0~rns.I~ with Mn-Mn bonds derives from the diamagnetism of the compounds, the IR spectra and cis-trans conversions. Yellow crystals of the cis are obtained from the reaction in liquid Table 5 The Primary Carbonyl Nitrosyls of Manganese ~
Compound
Isoelectronic Series
Ref-
8
Munganese
NH3{vNo= 1664 and 1737cm-' (solid), and 1680, 1685 and 1731, 1734.5cm-' (ethanol solution)} and these can be converted to bluish needles of the trans by heating in ethanoi in a sealed tube at 100°C for several days {vN0 = 1709, 175Ocm-'). More recently Behrens and co-workerd6 were able to prepare the series of monocyano nitrosyl carbonyls by reactions such as equation (1). The Mno compounds obtained in this work are listed in Table 6. Na[Mn(CN)(CO),(NO)] dissolves readily in polar solvents such as water, ethanol, acetone or THF, but the solutions, even in the absence of light and air, are unstable. Crystalline salts, which are air stable, were obtained by adding such a solution to an excess of the appropriate cation. The related isonitrile species were prepared by alkylation of the cyano anions with R,O' . [Mn(CO)(NO)J iNaN(SiMe&
2Na[Mn(CN)(NOXI + OISiMeA
(1)
The compound K,[Mn(CN),NO] has recently been prepared" as a dark green, highly pyrophoric solid by the reduction of K,[Mn(CN),NO] with K in liquid NH,. (vEo = 1370cm-', cf. vNo for K,[Mn(CN)5NO] = 1730cm-'}.
41.2.4
Manganese(1)
The white hexacyanomanganese(1) cation was first recognized by Manchot and Ga11,I8although a white crystalline solid, previously reported by Grube and B r a ~ s e , ' was ~ , undoubtedly the potassium salt. Manchot and Gall then reported2' both the potassium and sodium salts, obtained by reduction of the appropriate Mn" species in aqueous alkaline solution with AI powder, but a variety of other preparative methods now are available.21,22 Both Na,iMn(CN),] and K,[Mn(CN),] are robust in dry air, but are 50011 oxidized in moist air, the more soluble Na+ salt decomposing at the faster rate. They are also decomposed in a dry CO, atmosphere. Aqueous solutions are yellow and are strongly reducing: the potential for the reaction Mn' --+ Mn" + e- in aqueous 1.5 M NaOH is - 1.06V at 25 "C. The Mn' species are reoxidized by water at an appreciable rate, so that all studies of [Mn(CN)& in aqueous solutions (containing excess CN-) refer to solutions also containing Mn" species. It has been shown p~larographically*~ that the process [Mn(CN),]'- e [Mn(CN),I4- is reversible, and there is no evidence for disproportionation to Mn" and Mn'. Decomposition of the yellow solutions in aqueous CN- has been followed by monitoringz3the fading of the intense single broad band at 365nm. The data were interpreted as evidence for [Mn(CN)50H]5- and [Mn,(CN),oO]lo- in these aqueous solutions, and the involvement of these species in the oxidation of Mn' to Mn". Closely related to the hexacyano species are the various isonitrile cations [Mn(CNR),]+ and [Mn(CNAr),]+ , but these are covered in Treichel's review.' Two manganese(1) cyanocarbonyls are known. cis-K, [Mn(CN),(CO),] has been preparedI4from Mn,(CO),, or [Mn(CO),NO] with KCN in liquid NH3. The cis configuration was assigned to the colourless crystalline compound on the basis of the IR spectrum. fac-K,[Mn(CN),(CO),] is also a product of the reaction of KCN in liquid NH, on [Mn(CO),NO], but is best prepared from [Mn(CO),Cl] and an ethanol solution of KCN at 120°C under pressure. Again the isomeric structure is assigned14from the IR spectra. A range of dinitrosyl compounds of manganese(1) is known and most of them have been included in Treichel's review.' An interesting recent addition to the list is the compound (l),prepared*, from Table 6 Monocyano Nitrosyls" Compound
a PPN =
bis(rripheny1phosphine)immonium.
Physical form
vNo(cm-') (KBrdisc)
Red-brown, amorphous solid Red needles Red needles Red,microcrystalline Red oil Grey-green, amorphous solid Green oil Green oil
1665 1687 1675 1720 1720 1773, 1660 1795, 1692 1790, 1682
Manganese
9
[MnBr(NO),{P(OMe),),], AgPF6 and Na+L- in THF. The dark red crystals had vNo = 1709, 1643 cm-.’ (cyclohexane).
Another class of compounds which formally comes within the guidelines for this review is the series of dicarbonyl cations [Mn(CO),L,]+, [Mn(CO),L,(L-L)]+ and [Mn(CO),(L-L),]+ -both cis and trans isomers. But, since these have been reviewed by Treichel,‘ as have the monocarbonyl compounds, we simply note that several recent papers add significantly to the list of known
corn pound^.^^^' An interesting new area of manganese(1) chemistry is opened up by the recent report**of the first transition metal compound of the tetrahydroalane(II1) anion: [Mn(AlH,)(dmpe),] {where dmpe = 1,2-bis(dimethylphosphino)ethane}. This yellow, diamagnetic compound was prepared from [MnB~~(dmpe)~] and excess LiAlH, in toluene. It appears to be monomeric in solution, with a bidentate AlH; moiety, but an X-ray structure determination of the solid shows that two alane moieties bridge to form the dimer (2).
41.3 MANGANESE(1I) 41.3.1 Introduction This is the dominant oxidation state of manganese. It is the stable oxidation state of hexaaquomanganese, and of almost all of the manganese ‘salts’ that one finds in reagent bottles in the laboratory. Although many, but not all, manganese(I1) species can be oxidized readily, and the chemistry of manganese(II1) and manganese(1V) is receiving increasing attention, these oxidized species find use as oxidizing agents. On the other hand, reduction to manganese@ is unusual, except for carbon ligands, and is apparently impossible for species with exclusively 0 , N or halogen donors, as evidenced by the scarcity of material in Section 41.2. Manganese is the first transition metal for which + 2 is the ‘common’ oxidation state-a feature of the remaining first row transition metals from iron to zinc. By comparison with its two neighbours to left and right-chromium and iron-manganese(I1) is more stable than expected. This is commonly ascribed to the stable half-filled d electron shell of the high spin compounds. The oxidation state diagrams in Figure 1 give a perspective on the relative stability of manganese(I1) for OH,, OH- and 02-ligands. In many of its features, the chemistry of manganese(I1) spans that of the alkaline earth metals {especiallymagnesium(I1)) and that of the later first row transition metals. There are also parallels with cadmium(I1) rather than zinc(I1). These are all related to the more significantly ionic bonding for manganese and to the size of the metal ‘ion’. Manganese(I1) is quite distinctly a class ( a } metal.’ This is manifest, for example, in the preference for 0 donor rather than N donor ligands, so that amino acid compounds become simply those of substituted carboxylates, and there is currently little evidence for the N bonding of peptides that leads to the interesting chemistry of the peptide compounds of the later transition metals. Of P and As ligands but little is known, although recent work suggests the possibility that this may become
10
Manganese
+4
t2
I
t
AGO
1
-2
I
I
I
I
0
I
I1
I
m
I
N
V
VI
VI1
Oxidation State Figure 1 Oxidation state diagrams for manganese
one of the more important areas of Mn" chemistry (see Section 41.3.4). Of S ligands, as well, there is yet little known (Section 41.3.6), although this does give an impression of difficulty in obtaining pure compounds because of lability and low stability. The state of manganese-sulfur chemistry is summed up in the statement that one recent paper adds enormously to this area. The marked lability of manganese@) species is undoubtedly one of the reasons for the neglect of manganese chemistry. This lability, often coupled with low stability of species (see IrvingWilliams Series)29ensures that there is little correlation between solutions and solids, and adds an uncertainty to this area of chemistry that is largely absent from the chemistry of inert compounds. The spin state of most manganese(I1) compounds is high spin d 5with a 6S ground state- the few known exceptions are noted in the text (see also Table 7). The coordination geometries are quite variable; and, in keeping with the lack of LFSE for the high spin d configuration, ligand steric and electronic effects appear to play a major role in defining these geometries. We have already noted (Table 1) that manganese(I1) is larger (whether defined by ionic or covalent radii, or more simply by comparative metal-ligand distances, as in Table 1) than other Table 7 Coordination Polyhedra and Spin States for Manganese(I1) Coordination Number
Donor Atoms
( a ) High Spin Polyhedra (per= 5 . 8 4 . 2 BM; S = 3) 8a
I" 6 (Octahedral) 6 (Trigonal Pnsmalic) 5* 4 (Tetrahedral)
4 (Planar) 3 (Planar) 2 (Linear) ( b ) Intermediate Spin Polyhedra (peB 4 BM; S = I) 4 (Planar) ( c ) Low Spin Polyhedra (peB= 1.8-2.2 BM; S = 1) 6 (Octahedral)
-
'' We do not distinguish the various ideal polyhedra. Constraintswithin the molecules usually ensure that some 'intermediate' shape is adopted. bThe tris(ethylnilrosolato)mwganese(ll) anion (P,Gouzerh, Y. Jeannin, C. Rocchiccioli-Dellcheff and F. Valentini, J . Coord. Chcm., 1979, 6, 221) has been described as a low spin species; but the details given are unique in manganese chemistry, and need further confirmation.
Manganese
11
first row (+ 2) transition metals, and, in some ways, more akin to what are regarded as ionic metals, such as magnesium, calcium and cadmium. There are also indications that Mn" is more plastic than the later transition metals. (This view derives from presently available X-ray structural data, although more comparative data are needed for confirmation.) By more plastic we mean that metal-ligand bond distances appear to be quite variable, and change more in response to ligand steric effects. We associate this 'plasticity' with the greater ionic component in the bonds. Coordination number six appears, from the limited available data, the most common (Table 7). Lower coordination numbers are known but appear to be fairly rare: three is represented by some silylamides (Section 41.3.3.1) and in a dehydrated zeolite (Section 41.3.5.2.i); four-planar is known only for macrocyclic [N4] systems and as [MnO,] in a few unusual crystalline compounds; and four-tetrahedral is well characterized but still apparently less common than for Zn, Co and possibly even Ni. Coordination number five is also known, but few examples are yet characterized. On the other hand, higher coordination numbers seven and eight are less unusual here -hardly surprising in view of the larger size of Mn". Examples include seven coordinate [Mn(edta)H,O]- (Section 41.3.9), [Mn(py,tren)]'+ (Section 41.3.3.6) and an [MnO,] species (Section 41.3.5.3.iii), and eight coordinate [Mn(0,)J2+ species (Section 41.3.5.4). As noted in Section 41.1, the electronic spectra of Mn" compounds have not been particularly useful in the elucidation of structure. Although there is some justification for the claim3' that tetrahedral compounds are yellow to yellow-green in colour and have molar extinction coefficients of up to 100 times more intense than those for the pale pink or colourless octahedral compounds, yet these distinctions are not, in may cases, clear cut enough to lead to unambiguous stereochemical assignments. It is true that tetrahedral3' [MnCl4I2-,[MnBr,]'- and [MnI,I2- all have pale yellow or yellowand i s ~ c y a n a t o[MnX,]*~~ species. It also appears to be green colours, as do the i~othiocyanato~~ true that these anionic species have d-d solution spectra (molecular structures appear to be unchanged in the solvents used, including CH3CN)with molar extinction coefficients of the order of 1 to 15-values which are indeed some 100 times more intense than those for the peaks in the spectrum34 of the pale pink octahedral [Mn(H20)6]2+r Nevertheless, it does not follow that all pale yellow or green compounds of Mn" are tetrahedral; the octahedral [Mn(acac),(H,O),] is yellow,35and the compounds K,[Mn(NCS),] 3H,O and Cs,[Mn(NCS),] are described as red36and yell~wish-green,~~ respectively. Furthermore [Mn(salen)] (salen = dianion of N , N'-bis(salicy1aldehydo)ethylenediamine), whch is yellow to orange (depending on crystal size),38is more likely to be planar than tetrahedral, although a five-coordinate dimer structure39is most likely for the solid; and in acetonitrile solution its d-d spectrum has molar extinction coefficients of between one and two. Thus, whilst there may be some use for electronic spectra in confirming structure of Mn" compounds,40it is not likely to be great. Many more spectra of known species will need to be accumulated for assessment, and a major difficulty with these labile systems will be to ensure that we know structure in solution first before we start trying to assign spectra.
-
41.3.2 Carbon Donor Ligands, Including Cyanides
41.3.2.1 Cyanides Preparation of a pure binary cyanide Mn(CN),, which would of course involve bridging cyano groups in a polymeric structure, has not yet been achieved, nor has the preparation of any hydrates, but the crystalline solids Mn(CN),-L (L = DMSO, DMF and DMA) have been described re~ently.~' However, many mixed ligand species containing cyanide have been described, and the anionic hexacyano species [Mn(CN),I4 has been known42since at least 1869. It is a Iow spin species (Table 3); has been isolated in a range of salts M:IMn(CN),] and M!'[Mn(CN),] and their hydrates; yet it is not very robust in aqueous solution. As well as being subject to oxidation in the air, its aqueous solutions soon lose CN-; unless such aqueous solutions contain an excess CN- concentration of at least 1.5 M, green compounds of empirical formulae M'Mn(CN), or M"Mn,(CN), are precipitated. The half-life of exchange of labelled CN- with [Mn(CN),I4- at pH 11.8 is only approximately five minutes.43 Recent work on aqueous cyanide solutions of K,[Mn(CN),J -3H,O indicate that [Mn(CN),(H,O),-,,](" 2, with n = 1-5 and the binuclear species [Mn(CN),-CN-Mn(CN),I7 are present. On the addition of NaOH, the hydroxo species [Mn(CN),0HJ4-is most likely formed, as are dimeric species with oxo or hydroxo ligands.* K,[Mn(CN),J. 3H,O is usually a blue-violet material, although the more soluble Na,[Mn-
12
Manganese
(CN),]. 12Hz0 has the expected yellow colour; both dissolve in aqueous cyanide to give yellow solutions. They are made4' by treating manganese(I1) acetate or carbonate with an excess of KCN or NaCN under rigorous exclusion of oxygen. The blue colour of the potassium salt suggests the presence of some Mn"' or Mn'" in the solid and an interoxidation-state electron-transfer band. An X-ray structural analysis4, of Na,[Mn(CN),]. 12H,O shows the expected octahedral MnC, anion, with Mn-C = 1.95(3) A. The magnetic moment of the potassium salt at 300 K is 2.18 BM, corresponding to the presence of one unpaired electron with some orbital ~ o n t r i b u t i o n . ~ ~ The acid species H,[Mn(CN),], or perhaps more properly [Mn(CNH),(CN),], has been prepared48 from Pb,[Mn(CN),] and H2S. The dark green insoluble products such as 'KMn(CN),' obtained from aqueous solutions of K,[Mn(CN),] (4.v.) are apparently more properly formulated as, e.g. K,Mn"[Mn"(CN),]. The X-ray powder photograph can be indexed on a simple cubic cell of a = 8.89A; the IR spectrum shows a single 'bridging' vCN of 2062cm-'; and the magnetic moment of 4.53 BM all are consistent with a structure related to Prussian blue (see iron cyanides), c ~ n s i s t i n gessentially ~~ of equal numbers of low spin [Mn"C,f and high spin [Mn"N,] octahedra. Another product formulateds0 as K, szMn(CN)3.72has been made by fusing K,[MII(CN)~]and KCN at 650 "C. Available data suggest that it is merely an impure form of 'K Mn(CN),'. Interestingly, the action of NOCl on a suspension of green 'K Mn(CN),' in sulfolane give?' a tan product of approximate composition &,Mn(CN), . It appears to be identical with the product made by mixing" aqueous solutions of manganese(I1) acetate and K,[Mn(CN),]. Available physicochemical data on both products-vcN "N 2150cm-', pefl= 4.2 BM per Mn, and a cubic unit cell of a = 10.63A-suggest again a Prussian blue type structure KMn'LIMn'l'(CN),]. It is intriguing to note that the colours of green 'KMn(CN),' and tan 'K,,Mn(CN),' are the reverse of what one might expect from parallels in the iron Prussian blue system; 'KFe(CN),' or K,Fe"[Fe"(CN),] is colourless, whereas 'K, 5Fe(CN),' or KFe"[Fe"'(CN),] is deep blue because of an interoxidation-state electron-transfer band. Clearly, further detailed study of these manganese systems is required, and the final explanation will probably need to take account of the availability of the Mn" state -note especially the tendency, under some conditions, for [Mn"'(CN),I3- to disproportionate to Mn" and MnIV. Species such as Mn1'[M(CN),].8H,0 (M = Fe, Ru, Os) and Mn\'[M(CN),] .nH,O (M = Cr, Co) have been reviewed by Ludi and G ~ d e 1 X-Ray . ~ ~ structural analysesdpschow that the former have fac-[MnN,O,] octahedra from bridging cyanides and both terminal and bridging aquo groups, whereas the latter have54[MnN402]octahedra. A brown product, formulateds5as K, Mn(CN),, has been prepared by reducing K3[Mn(CN),] in hydrogen at 420 "C.The magnetic moment is 4.4 EM, and there appear to be bridging and terminal cyanides; but the exact formulation, including metal oxidation state, is not established from available data. [Mn(CN)jN0]3- is best prepared% as the purple, crystalline, diamagnetic potassium salt K,[Mn(CN),NO] -2H,O by adding hydroxylamine to an aqueous solution of a manganese(I1) 'salt' and an excess of KCN. Yields from the reactionj7 of NO with aqueous solutions of [Mn(CN),I4are usually poor. Treatment of the potassium salt with a non-oxidizing acid gives" carmine-red H,[Mn(CN),NO]. An X-ray structural analysiss9 of the potassium salt shows an almost linear Mn-NO with Mn-N = 1.66 A and N-0 = 1.21 A.The vibrational spectrum57gives uN0 = 1706cm-' ; and the electronic spectrum shows three bands at 18 500, 28 600 and 45900cm-' with molar extinction coefficients of 24, 114 and 16000. Photolysis (UV) of anaerobic aqueous solutions leads" to [Mn(CN),I4- and NO, whereas, in the presence of oxygen, [Mn(CN),(N0,)l3 is formed.,' Br, or HNO, oxidizes the nitrosyl anion to [Mn(CN),N0I2-, and the standard reduction potential'6 for this reversible process is 0.6 V. Methylation6* with methyl fluorosulfonate gives [Mn(CNMe),NO](SO,F), .
41.3.2.2
Alkyis and aryls
Although normally classified within organometallic chemistry, the recently defined alkyls are a significant part of manganese(I1) chemistry and we include them here. The compounds described in four recent papers from Wilkinson's g r o ~ p ~ "are ~ ~summarized "~ in Scheme 1. Comments on the various types of product follow the order of this scheme:
Manganese
tetrahedral
\
13
tetrahedral
LiR
[Mn2R41
or or
J
[Mn4Rsl [MnR21,
I
(E)
\
[MnRdPR‘d2 1 tetrahedral
Three-coordinate or tetrahedral
N donors
(P- P)
IMnRdNhI (C) tetrahedral
(F) [Mn(P-P)Rzl tetrahedral
[Mn(dmpe),Br,]
+
MgMe,
(Et20)
(GI +
[Mn(P-P),R21 octahedral
Scheme 1
( A ) Only three binary alkyls MnR, have been isolated in pure form,63and are described as yellow to red crystals. X-Ray structural determinations of all three are known. They involved bridging alkyls and range from a dimer with pseudo three-coordinate Mn” (3; R = CH,CMe,Ph); through a linear tetramer with two terminal three-coordinate metals and two central tetrahedra (R = CH,CMe,); to a long-chain polymer with mostly tetrahedral metals (4; R = CH,SiMe,). The low magnetic moments indicate antiferromagnetism.
(4)
( B ) Tetraalkylmanganate(I1) specied3 are light sensitive. Colours range from yellow-orange to red. The known manganate with a bulky alkyl Li,fMn(CH,SiMe,),] is orange-brown and behaves as a simple high spin tetrahedral [MnR,]’- species, but the chemical and physical properties of the Only when Lit is known tetramethylmanganates(I1) are complicated by the small Li+ solvated, as in [Li(TMEDA),][Mn(Me),] {TMEDA = 1,2-bis(dirnethylamino)ethane), do the solids appear to contain the simple high spin, tetrahedral [MnR,]’- species. (C) Solids isolated from the reaction with nitrogen donor ligands were [MnR,(N-N)] (where N-N is a chelating diamine: TMEDA or N,N’-dimethylpiperazine)and [MnR2py,] (py = pyridine). All but one were described63as colourless solids and all apparently are high spin tetrahedral manganese(I1) species. A related oxygen donor compound [Mn(CH,Ph),(dioxane),] also has been
(0) Addition of unidentate phosphines to the alkyls gives a series of orange coloured dimers [Mn,R,(PMe,),]. The X-ray structural determinations of three of themMshow that they are dimers with essentially tetrahedral [MnC,P] polyhedra, although there is also an interesting close contact between one hydrogen atom on each of the bridging methylene groups and the manganese atom. The solid compounds are antiferromagnetic. A related aryl compound Mn(Ph),P(cyclo-C,H, , ) 3 has been described,6’ but its structure is unknown. ( E )When an excess of phosphine is added to solutions of the dimers, paler coloured bisphosphine monomers are formed65(reaction 2). These tetrahedral monomers are high spin and are
Manganese
14
thermally unstable. Indeed most of the solids isolated were found to lose phosphine readily, reverting to the dimers. However, [Mn(CH,CMe,Ph),(PMe,),] is indefinitely stable at room temperature in the absence of air, and the X-ray structure of the colourless crystals shows6’the expected (distorted) tetrahedral geometry.
+ 2P
Mn,R.,P,
2[MnR,P2]
(2)
( F ) With chelating phosphines, on the other hand, more robust [MnC,P,] tetrahedra are obtained. Four solids [MnR,(dmpe)] were described6’ (dmpe = 1,2-bis(dimethylphosphine)ethane, and R = CHIPh, CH,CMe,, CH,CMe,Ph and CH,SiMe,} as yellow or colourless solids, containing high spin, tetrahedral monomers, which are thermally stable but air sensitive. (G) Some of these alkyls can add two molecules of chelating phosphines to give red, low spin ( S = +), octahedral [MnC2P4]species. The first described65was the product of the chelating dialkyl o-xylyl-[Mn(o-C6H4(CH,),}(dmpe),]-in which the M-C bonds (2.1 1 A) are shorter than in the tetrahedral species (2.15 A); the other is the dimethyl” species [Mn(CH,),(dmpe),]. The only other well-characterized alkyl or aryl compounds of manganese(I1) appear to be two interesting isomeric species6*(5) and (6). The aryl compound (5) has been isolated in a complex form MnL, -2LiI.2THF ( L = C6H4CH,NMe,) as pale-yellow air-sensitive crystals (peff= 5.7 EM) of unknown molecular structure, and as the yellow-green [MnL,] (peff= 5.5 BM) which is apparently a ‘tetrahedral’ monomer. On the other hand the bis-benzyl compound (6) parallels the alkyl-bridged dimers found for the above phosphines, and gives the only known example of a five-coordinate manganese(I1) alkyl; its crystal structure shows a dimeric alkyl-bridged molecule (7) in which one metal atom is five-coordinate and the other four-coordinate. The expected antiferromagnetism is reflected in a room temperature magnetic moment of 2.6 BM.
Me N
’
41.3.2.3
I
‘Md?
Fulminates and other carbon ligands
Several mixed ligand fulminates have been described. These include a poorly characterized ammine compound which was obtained6’ by reacting Hg(CNO), with manganese amalgam in anhydrous methanol, which had been saturated with NH,; and there are two reasonably robust yellow compounds Mn(CNO),(N-N), (N-N = 2,2’-bipyridyl and 1,lO-phenanthroline).” Various formulations have been suggested for these latter, but there appears to be no good evidence to suggest that they are not simply octahedral cis-[Mn(CNO),(N-N),] species. On heating, they do not explode, but deflagrate vigorously. It dissolves in liquid NH, in the An interesting acetylide is the rose-coloured [Mn(C2H)2(NH3)4].7’ presence of KC,H, apparently forming K,[Mn(C,H),]. Also possibly involving Mn-C bonds is the compound72 of the tricyanomethanide ion Mn{C(CN)3},((Me,N),PO),. This colourless compound is not isomorphous with the octahedral Ni” species, but also probably has an octahedral polymeric structure with bridging anions.
Manganese 41.3.3
41.3.3.1
(i)
15
Nitrogen Ligands
Unidentate ZiganB
Ammonia and amido
Manganese(I1) are distinctly less stable and robust than those of the later transition metals. In aqueous solutions, buffered with NH:, the species [Mn(NH3),(0H,),-,]2+, for n = 1 4 , have been identified and the stability constants have been measured; for example, in 0.4M NH,CI at 25 “C, log K , = 0.90, log K2 = 0.67, log K3 = - 0.40, and log K4 = 0.30. Apparently, only at high NH, concentrations, ( > 2 M) is the hexaammine present in any significant concentration. The known solid ammines are summarized in Table 8. Preparations with maximum n were generally made either by reacting solid MnX, with gaseous NH,, or reacting MnX, or a hydrate with liquid NH,. All lost NH, at rates depending on X, and the storage conditions. The species for n less than maximum value were characterized generally as steps in the thermal decomposition of the higher species. Composition does not necessarily, of course, reflect coordinate number at the metal. This is especially so for the species for which n > 6; there is no evidence for [MnN,] or [MnN,] polyhedra except where speciaI ligand steric effects promote higher coordinate numbers. However, most of the species MnX, -6NH, are white powders, and probably contain the cation [Mn(NH,),]’+ (as do those for n > 7), and this is proved by X-ray powder diffraction for X = C1, Br, I, Clod, SO,F and BF,, which have cubic crystal structures belonging to the CaF, type: These compounds are sensitive to air, especially to moist air, generally turning brown, owing to oxidation and displacement of at least some of the NH3. Sensitivity is anion dependent, with, for example, decreasing sensitivity from F- to 1- amongst the halides. The species MnX, .4NH, are surprisingly unusual: MnCl, -4NH, exists only under supercritical conditions, the thiocyanate needs further confirmation and the [SiF,I2- species appears to be almost an exception; and in none of these cases are we certain of an [MnX,N,] structure. The diammines, on the other hand are well characterized and almost certainly have a linear polymeric structure as in (8) for (L = NH,). The monoammines also are undoubtedly representatives of a wetl-characterized polymeric structural type, like the monopyridine compounds (4.v.). L
L
L
L
L
L
Table 8 The Composition of Known Manganese(I1)
X-
ammine^'^ MnX2(NH,), Comments
n NH,a
F
CI Br I
b 12
11
8
10 10
c10, BF4 CN NCS SO3F )SO, tCr04
@iF,
7
6 6 6 6 6 6
4
2 2 2 2 2 2
1 1
C
d
4
6 6 6
4
12
e
’ A few unusual species. listed in Gn~elin.’~ but for which we think the evidence unsatisfactory, are omitted. bAt least two papers report the reaction of MnF, NH,, but products appear ill-defined and soon reven to MnF,. Under normal conditions, the species for which n = 6 , 2 and 1 only arc observed. The other s+es (including n = 4) exist only under
+
supercritical condilina5. dThc rpecies Mn(CN)2+2NHJSeemb suspect; colour was described as yellow-green and pea = 6.3 BM. In air it turns brown, losing HCN. Ref. 74.
Manganese
16
Amido species of Mn" are known. The binary compound Mn(NH,), is obtained75as an impure light yellow product from KNH, and Mn(SCN), in liquid NH,, but is stable only in the absence of air and water and below room temperature. The compounds M,[Mn(NH,),] (M = Na, K, Rb, Cs), however, are relatively robust in the absence of water. The yellow crystalline compounds were ~ b t a i n e d 'by ~ dissolving Mn in a liquid NH,solution of the alkali metal. An X-ray analysis of the K + salt confirms7' the expected tetrahedral [MnN,] anion, with Mn-N = 2.12(4) A.
( i i ) Aliphatic and aromatic amines and substituted amides
Available data7' on the products from the reaction of MnX, species with amines are both limited and often of poor quality. For the primary amines, data refer to the reaction of solid MnX, with gaseous RNH2;but for most of the other amines, products were obtained from ethanol solutions. In the latter case, initial products often contained ethanol, MnX, -A, (EtOH), (A = amine), but this was lost either on storage over CaCl, or on heating. It is not known whether the ethanol is bonded to the Mn or simply solvent of crystallization. The poor quality of much of the data is evident both from the dark colours of many of the products, and from the lack of any meaningful structural data; of course, all pure species MnX,-A, should be white or perhaps a very pale colour. Most, but not quite all, of the high spin species MnX,.A, are unstable to oxidation in the air and especially in moist air. Observed stoichiometries for the compounds with the primary amines (Table 9) are for integral. values of n in (MnX, -A,) of 6, 4,2 and 1 (the two non-integral values, assigned to nitrates, need further confirmation). For n = 6, known compounds are confmed to methylamine and ethylamine; these are probably [MnA,IX, species. Tetrakis species are better defined here than for NH, and probably have [MnX,AJ monomeric structures; but the most common stoichiometry is MnX,A,. These latter species probably have the linear polymer structure of (8). Very little is known of compounds with the more sterically-demanding secondary amines. Two white aziridine (C,H,N) species MnCI,A, and MnI,A, have been described, but all other products (with piperidine, dialkylamines and substituted anilines) are described as 1 : 1 species MnX, .A. (Unfortunately, all of the latter were described as brown or other dark colours and are therefore suspect.) Tertiary amine compounds also are generally poorly defined. One attempt to obtain compounds of R3N (R = Me, Et) from ethanol gave products which were not properly characterized, but which were formulated as containing R,N and OR- ligands. However, a white unstable solid (MnC1, .Et,N) was obtained, which probably has a similar layer structure to (MnCl, mpy),,. Trioctylarnine can be used to extract Mn" from aqueous acid solution into organic solvents, but the nature of the species involved is unknown. The only available X-ray structural data refer to a compound of hexamethylenetetraamine {(CH2)&]. A crystalline bis species has been shown7' to have an all rrans-[MnCl,N,O,] structure with long Mn--N bonds of 2.40A. The related cationic ligand (9) formsso the bis species [MnX,(LH),]X (X = C1, Br); whereas ligand (10) does not bond to Mn; although Co, Ni and Cu form the species [MnX,(LH)] (X = C1, Br, I), manganese forms" instead (LH),[MnX,]. Amido species of the secondary amine NH(SiMe,)* have been studied in some depth. Table 9 The Composition of Known Manganese(I1) Amine Compounds MnX,(Am)- (Am = RNH, or ArNH,) n
X
Me
RNH, Et
B2
C,H,
2-MeC6Hd
2
2 2 4, 2
2, 1 2
2
2
3
2
c1
6
4
Br I NCS NO,
6 6
4
+CrO, :Crl 0,
6
is04
(6Ib, 4
ArNH, 3-MeC6H4 4, 2
bObtained only at low temperature.
2 2 4, 2
6
4
" 3-CIC6H,, 4-CIC,H4, 3-BrC6H,, 4-BrC,H4, 3,4-diMeC,H3,
4-MeC6H4
and a- and p-naphthylamines.
P
Other?
2 2 2 2
17
Mangnnese
I
Me
(9)
(10)
Mn[N(SiMe,),j, was first made by Burger and Wannagat" from MnI, and NaN(SiMe,), in THF, but Bradley and co-workerss3have now shown that a reaction of MnCl, and the Li salt in THF gives pink [Mn{N(SiMe3),},(THF)1, which is quite robust; it can be distilled without losing THF and is a paramagnetic (pem= 5.91 BM) monomer.84At 120 "C,in a stream of argon, the THF is lost and recrystallization of the residue from pentane gives pink platess3of the dimer (11). An X-ray structure of this dimer shows Mn-N = 1.99414 (terminal) and 2.1848, (bridging), with a metal-metal distance of 2.841 A. The magnetic moment of 3.34 BM (at 296 K) shows the expected antiferromagnetism.
Me3Si
\
Me,Si /N-Mn
,SiMe, /"\ \ SiMe, \N /h Me3Si SiMe, (1 1)
More recently Horvath et al." have improved the synthesis of [Mn(NR,),(THF)] and have shown that another series of yellow tetrahedral compounds [Mn(NR,),L,] (where L = THF, py or But-CN) can be formed. All are thermally robust, but show exteme oxygen sensitivity. There is also an intriguing benzene species of unknown structure ~n{N(SiMe,),},(C,H,)z], and a series of mixed ligand species MnXY (where X =C1, NO3 and Bu; and Y = N(SiMe,), or OR). Attempts to prepare compounds of NEt; gave instead compounds of a reaction product of the amide (Section 41.3.3.2).
(iii) Hydrazine and hydroxylamine Three types of compound of hydrazine and MnX, species have been characterized? MnX,(N,H,), for n = 1, 2 and 3. Within these dasses, they are of varying robustness-some are stable in air, others not. The white n = 1 compounds are known only for MnF, and MnCzO,, and they are the stable species at ambient conditions for both. They can be prepared either by direct reaction or by loss of N2H4 from the bis species. The structure is unknown, but is undoubtedly polymeric and probably consists of both bridging hydrazine and anions. A similar compound has been described for 1,Zdimethylhydrazine, and with 4-nitrophenylhydrazine. The bis compounds MnX2(N,H4),are the most common and have been characterized for X = F, CI, Br7NCS, HCOO and OAc, and X, =r oxalate and picramate. Crystalline hydrates of these also have been defined, as have hydrates for X = I and NO3,but whether they have significantly different molecular structures from the anhydrous species is unknown. A full X-ray structural analysis of MnCl,(N,H,), characterizes the bis species as iinear polymers based on double N2H4 bridges (12) (Mn-N = 2.27 8, and Mn-Cl= 2.57 A). Bis compounds have also been described with phenylhydrazine and MnX, (for X = C1, Br, I). Structures could be polymeric based either on bridging hydrazine as in (12), or on bridging halide as in (8).
'I'
'I'
18
Manganese Table 10 The Known Manganese(I1) Pyridine Compounds MnX,.npy
n
Br
J J
d d d d
; i
d
d
V'
6 4
3 2 1 other
I
CI
( n = $1
(n = 5)
X NCZ a
NO3
d d
.
d
CI04/BF4
Other db
d
/
>
d
fl
(n = $)
(n = 8)
" Z = 0,S or Se. b X = ;SnBr6.
' X = CF,CO, , CCI,CO;, SbCI,, tHgCI,, +Cr,O,, Reo,. 'At least ten others have been described. X = Me3CCO;, $C204.
A tetrakis compound of 4-nitrophenylhydrazine and MnC1, may be an example of a monomeric [MnCI,N,] chromophore. A few compounds of hydroxylamine and MnXz have been described,P7but it is uncertain that any of them contain Mn-N bonds (see Section 41.3.9).
( i v ) Pyridines and quinolines
Almost all of the known compounds'' appear to be high spin six-coordinate species; they are generally colourless and sensitive to air, turning brown with loss of a nitrogen ligand. The solid pyridine compounds are summarized in Table 10. UnIike the amines, tetrakis compounds are quite common, and hexakis species are less usual. This is probably related to the steric requirements of the a-hydrogens of pyridine. The room-temperature-stable combination of MnX, and py is usually a bis species. In aqueous solution, potentiometric measurements have been interpreted in terms of only four species [Mnpy,(H,0)(,_,,]2+(n = 1 to 4), with log K, determined as 1.86, 1.59, 0.9 and 0.6, respectively. Perhaps the n = 5 and 6 species are formed at higher concentrations of pyridine, but, even in the solid state, there is yet no certainty that the compounds MnX,-6py actually contain [Mnpy6I2'. It is virtually certain that the compound Mn(C10,), * Spy is not eight-coordinate. The tetrakis compounds, which are quite common here, probably all (because of the steric requirements of py) have a trans-[MnX,N,] octahedral structure. X-Ray proof of this, however, is available only for [Mn(NCS),py,] and MnSbC1, -4py, and is obtained by comparison of the powder patterns with the octahedral Ni species. Bis compounds are usually the final product of the other species when left at room temperature. They appear generally to have the polymeric structure (8) and show some antiferromagnetism, except where the bridging ligands, such as [Ni(CN),J2- and [C,O,]'-, cause sufficient separation of metal centres. A full X-ray analysis of MnCl,py, shows the polymeric structure with Mn-CI = 2.56, 2S7A and Mn-N = 2.24A. Moaopjridine compounds result from thermal degradation of the bis species. They appear to have the polymeric structure (13) (MnC1,py is isomorphous with NiBrZpy).Further thermal degradation ,sometimes leads to a well-characterized phase { Mn,X, *2py},,.
PY
PY
Manganese
19
Pentakispyrddine compounds are known only in Mn(C10,), * 5py*3H20which is of unknown structure, and there is an isoquinoline compound MnCl, * 5L. Tris compounds have been proposed only in [PyzH][MnBr,py,] (which could be instead [MnBr,py,] SpyHBr) and [R,NI[MnBrCl,py,] (R = Me, Et) and a 4-picoline analogue, but even their formulation as discrete species needs X-ray confirmation. Substituted pyridines (b or y) generally give compounds of the same type as pyridine, although no hexakis compounds have yet been described. We especially notice, however, the compound Mn(NCS),(3-Br-py),, which is ‘freely soluble in the common organic solvents’, unlike the other bis species. A tetrahedral [MnN,] has been proposed, although small oligomers, such as a fivecoordinate dimer, are also possible. 2-Substituted pyridines, including quinoline, have more stringent steric requirements, and generally form only 1:2 and 1: 1 species. Those so far described appear to be polymers, with structures such as (8) and (13),yet it is here especially that we might expect to encounter tetrahedral [MnX,L,] species or five-coordinate dimers as in the biquinolyl compounds (Section 41.3.3.2). A series of compounds formulated as [RH],IMnCl,py,] are more likely to be six-coordinate, with free pyridine in the crystal lattice, if indeed they are discrete compounds and not mixtures.
(v)
Pyrazoles, imidazoles. triazoles, etc.
The compounds of these ligands,” especially those of imidazole, have been more effectively studied than those of the more classical ligands, z.e. pyridine and ammonia. The biological role of imidazole moieties as donors has given impetus to these studies in the 1970s, and the compounds are generally more robust in the air and at ambient temperature, although some are still unstable, especially with respect to oxidation in moist air. Structural data are more complete; more full X-ray determinations are available, and nearly maximum use has been made of X-ray powder data, especially by Reedijk and co-workers? for comparison with other species of known structure. Table 11 gives the stoichiometries of the compounds MnX, -nL. These were generally obtained as crystalline species from anhydrous ethanol, although a few of the species of lower n values were made by thermal decomposition. All are high spin, with some polymers showing antiferromagnetism, and most are, as expected, colourless; a few are pale pink, and there are some yellows, fawns and browns, but the detail is unimportant. A common feature seems to be bonding through the unsaturated nitrogen (-N*), and not through the NH nitrogen.
The compounds for n = 6 appear generally to contain [MnNs12+octahedra; proof comes from comparative X-ray data in some cases and in others reasonable evidence is available from ESR data. The one observed compound of higher rz, that is Mn(ClO,), .7(5-Me-pyrazole), is isomorphous with an Ni species, known to be [NiL,](ClO,),.L, and thus belongs to this class. Where there are ring substituents adjacent to the bonding N, steric crowding destabilizes the [MnN,JZ+octahedra and species of lower n are obtained. In Table 11 there are two possible exceptions: Mn(NCS),-6(3,5diMe-pyrazole) and Mn(N03), 6(2-Me-imidazole).The former appears from available data to have the structure [Mn(NCS),L4]-2L, and there is only doubtful IR evidence that all the imidazoles are bonded in the latter. Note that in this latter case, both C104 and BF, form only 1:4 species. There are many n = 4 species here, most of which appear to have a trans octahedral [MnX,L,] structure. Indications of this come from one available X-ray structure” ([MnBr2(5-Me-pyrazole),], which has trans bromines at 2.73 A and Mn--N of 2.25 A}, from the ESR data and perhaps also from some of the IR data. Two crystalline forms of Mn(NCS),(S-Me-pyr),, both of which are isomorphous with Ni species, have been interpreted as examples of cis and trans isomers, but the data do not absolutely require the acceptance of the view that one is a cis isomer. The n = 3 species are of some interest in providing one of the few known examples of fivecoordinate Mn”.Fourteen of these species have been described (Table 11); some have been assigned (undefined) polymeric structures; but structure is known certainly only for [MnC12(2-Me-imid),]. This is a five-coordinate with ‘intermediate’ structure and bond distances Mn-C1 of 2.392 and 2.525A, and Mn-N of 2.19, 2.20 and 2.25A.
-
C.O.C. b B
20
Manganese Table 11 Known Compounds MnX,.nL for the Pyrazoles, Imidazoles and Triazoles
L Pyrazole" 5-Me 3,5-diMe Imidazole" I-Me 2-Me 4-Me 1,2-diMe I-Et 1-Pr I-BU I-Vinyl
Br
I
nfar X = NCS
F
CI
4 4'
2
4,2b 4' 2
4c 4' 6
6, 4, 2, 1 6,3, 2, 1 3b, 2, 1 4, 3 2' 4', 3 6,2 4 4, 3
6, 4, 2 6, 4
6, 4, 2 4, 3
t
e
Benzimidazole" 1-Vinyl 2-Me 2-BZ
6 2'
6, 3 4' 4 4, 2 4' 4, 3 2
6 6 6 6
2 2 2
2 2
2
a Substituent positions
CIO,
BF,
4d 4' 3, 2 6 6, 3 6 6 3 6, 3 6', 3 6' 6
6
6
2
2
1,2,4-Triazole" 4-Me
NO,
4h
7f.9
6' 6'
$0,
6
4 6
6 6 6 6. 4
2 2 Ib
3b are given in diagrams (14) to (17).
I,Full X-ray structural analyses available- see text. Isomorphous with Pe, Co, Cu and Zn analogues. Isomorphous with Cu analogue. e
Tetrameric [Mn.,Fq] species formed-see
text.
'Isomorphous with Ni analogue (and usually others) thus identifying Oh or D4h symmetry.
BThe structure is [MnL,](CIO,)*L. h Explosive above 200°C. ' See text for X-ray powder data.
Within this class are a series of compounds MnF(BF4)-3L(where L = 5-Me-pyrazoleYl-Et- and l-Pr-imidazole). They resulted from attempts to prepare Mn(BF4), compounds, and all appear to have the tetrameric 'cubane' structureg3of (18). L L
I
I
L
The n = 2 compounds appear to be generally antiferromagnetic polymers of structural type (8). One full X-ra study is available% on MnCl,(pyrazole),, which gives Mn-Cl = 2.593A and Mn-N = 2.201 There are, however, some interesting exceptions with the sterically hindered ligands. For 3,5-dimethylpyrazole, the F, C1 and NCS species are antiferromagnetic (especially the F), but the Br species which is not isomorphous with the C1, is isomorphous with the Ni, Co and Zn species, and there are suggestions that it has a tetrahedral monomeric structure. Similarly, for MnCl2(1,2-di-Me-imid),, the magnetic moment is invariant between 100 and 300 K, and the compound also appears to be a tetrahedral monomer, whilst the alcohol solubility of the Me- and Bz-substituted benzimidazoles may point to tetrahedral monomers or five-coordinate dimers. But few n = 1 compounds have been described. They probably have a polymeric chain structure like (13). There is also a series of tetrahedra [%Nl[MnX,L] (where X = C1, Br and L = 1-Me, l-Et, 1-Pr, l-vinyl and 4-Me-imidazole).
1.
1 Manganese
21
The few known triazole (17) compounds are of different type. MnSO, forms a mono species under conditions where Ni forms a tris, and this has a monomeric structureg5[Mn(SO,)(H,O)L]. On the other hand Mn(NCS), forms a polymeric Mn(NCS)?-2L species, in which the polymerization occurs through the triazole ligand (N-2 and N-4) bridging trans Mn(NCS), units.% The other known triazole compound is Mn2(NCS),(4-Me-triazole), which has97the dimeric structure (19)
Another unusual species is the compound of Mn" and the imidazoyl anion [Mn,(imidH)6- 2 imid], made from manganese carbonyl. The colourless crystals are air sensitive, and have98a polymeric structure with alternating [MnN,] octrahedra and [MnN,] tetrahedra, bridged by the imidazole ligands. Each octahedral Mn is surrounded by four tetrahedral Mn's and each tetrahedron is surrounded by two other tetrahedra and two octahedra.
(vi)
Other neutral nitrogen ligands
Very little work appears to have been done on manganese nitrile compounds, although several authors have described* [Mn(RCN),]'+ and [Mn(ArCN),]'+ species. Nitrosyl compounds of manganese(II), in addition to the cyano nitrosyls (Section 41.3.2.2) and porphyrin species (Section 41.3.10. I), are represented by the compounds [Mn(L)NO](ClO,), (L = 20) and [Mn(L)NO] (L' = 21). The former was prepared by simply treating the compound with the pentaamine ligand with NO, and an X-ray structure analysisIa) shows an octahedral [MnN,]'+ species with Mn-N(0) of 1.58A and the other Mn-N distances ranging between 1.91 and 2.12 A. The Mn" compounds of the salicylaldiminate ligands (21) reacted reversibly"' in solution with NO, many lost NO when attempts were made to isolate them, and even though crystalline, purple-red, diamagnetic solids were obtained, they were thermally unstable, reverting to yellow [Mn"(L')] and NO. The solutions, however, reacted with O2to give [Mn"'(L)(NO,)] species. Interestingly, the compounds of the sexadentate ligands (22) (n = 2, 3) also react'" with NO to give red nitrosyl compounds in solution (CHCI3) and these then react with O2 to give green [Mn"'(L)(NO,)], which may be seven-coordinate species.
X &o-
22
Manganese
Thiazyltrifluoride (NSF,) is an interesting new ligand which forms colourless [Mn(AsF, j2(NSF,j4]. The X-ray analysis'03shows Mn-N bonds of 2.19 A and trans F atoms (from AsF,) at 2.19 A.
(vii)
Unidentate anions (including thiocyanate and azide)
The nitrite anion appears to bond to Mn" through its 0 atoms exclusively, and is treated in Section 41.3.5. Thiocyanato, but not cyanato and selenocyanato species have been studied in some depth.lw All three apparently prefer N-bonding to Mn", although there is at least one example of Mn-NCS -Mn bridgmg; polymeric Mn(NCS),(EtOH), has'" the layer structure represented in (23). In golden-yellow HgMn(SCN),, the bridging is Mn-NCS-Hg like the Fe, Co and Zn analogues.
I
There is also some evidence for Mn-N-Mn pyridines, but this needs X-ray confirmation.
bridging in some compounds of the substituted
Thc binary thiocyanate has been knownlM as the hydrate Mn(NCSj,*3H20 since 1842, and Mn(NCS), can be obtained by heating this to 170 "C,or by heating the ethanol compound (23)to 110 "C.The selenocyanate has only been obtained as a THF or dioxane species, decomposing when attempts are made to remove these other ligands. All three (NCO, NCS, NCSe) form a range of anionic species, and the known solids are summarized in Table 12. Some studies of solutions are available, but much remains to be learnt of the detailed structures adopted by Mn" in these solutions. Both tetrahedral and octahedral species seem well characterized in the solid state, but it would be reassuring to have some X-ray confirmaTable 12 Anionic Species with Cyanate, Thiocyanate and Selenocyanate Anion
[Mn(NCO),]'[Mn(NCS),]'[Mn(NCS),(AA)]'-' Mn(NCS):-' [Mn(NCS),Y[Mn(NCSe),12[Mn(NCSe),14-
Number of Salts"
Confirmed Structuresb
First Reported
1 10
1964 1905
2 2 8 1 3
1981 I905 I900 I965 i 965
Ref: 33 LO6 107 106 36, 37 108 108
This refers to the numbers of solid compounds described with appropriate stoichiometry. 'The test adopted here is a demonstrated isomorphism with other metal compounds of known structure- through X-ray powder photography. These may be five-coordinate, but appear not to have been studied since the original work. K,[Mn(NCS)J4HI0 is an isotype of the rhombohedral K4[Ni(NCS)6]-4A,0 and K3[Cr(NCS),]-4H,O, and the quinolinium salt has been shown to be isomorphous with the Fe and Ni species. AA = bipy or phen.
a
Mangunese
23
tion of the tetrahedron. There are also two solids which may be examples of five-coordinate [Mn(NCS)J3- species. The recently reported yellow anions [Mn(NCS),(AA)I2- (where AA = bipy or phen) were prepared readily from aqueous ethanol. Related to these probably are the compoundsio9of the hexamethylenetetraamine monocation, (C6H,3N4)2 Mn(NCS),. The manganese compound is isomorphous with the Fe, Co and Ni analogues and the structure may well be an [Mn(NCS),(N),] octahedron (cf. Section 41.3.3.1.2). Both the binary azide Mn(N3)2, which, of course, detonates readily, and the tetrakis anion [Mn(N3),12- are known."' The latter has been isolated as the NEt: salt from various non-aqueous solvents and the n-cetyltrimethylammonium salt precipitates from aqueous solutions of the former. Products obtained have been described as light grey; the evidence for a monomeric tetrahedron is so far only IR; the compounds are light sensitive, turning brown. They are insensitive to shock, but do detonate in a flame. Azido compounds with other ligands also have been studied. As with thiocyanate, so there is some evidence that azido can bridge through the one terminal N atom in at least some substituted pyridine compounds, a form of bridging that is well characterized"' in the Mn" tricarbonyl anion (24).
41.3.3.2 Bidentate ligands With higher multidentates, but beginning with the bidentates, Mn" compounds with nitrogen ligands become rather more robust. The 1,Zdiaminoethane species [MnenJ2+ is still quite sensitive to air; aqueous solutions are oxidized and the solids [MnenJX, turn brown at varying rates; but [Mnbipy,]'+ and [Mnphen3l2+are not at all easily oxidized. Stability constants in water are generally quite low: log K,'s for Mn(en):' are 2.8, 2.1 and 0.9, respectively (with log p3 rising to 10.1 in DMSO); and for phen they are 4.0, 3.5 and 3,O. Table 13 lists the solid compounds MnX, *nL,all of which are high spin. There are also significant reports of compounds with diamines, such as m and p-phenylenediamines, which are incapable of chelating. These compounds,li2 thus, can be treated simply as variations on the unidentate theme.
NH, (25)
Except perhaps for the special case of the n = 4 species, the values of n seem invariably to define the number of bidentate ligands bonded to Mn:tris compounds are octahedral cations (one salt, the ethylxanthate, has been shown123to be isomorphous with the octahedral Fe, Co and Ni salts); bis
24
Manganese Table 13 The Numbers of Known Compounds MnX,.nL for the Bidentate Nitrogen Ligands
Ligand
NH,CH,CH,NH, NH,(CH*),NH, NH, (o-C, HJNH, NH, (2-CbH4-2-Cs H, )NH, NH,NHCONHNH, 2-pyCH2NH: 2-pyCH2NHCH; 6-Me-2-pyCH2NH! 2-pyNHNH2 8-quinNH2 (25) (26) (27) (28) (29) (30) (31) biPY phen (32) (33) (34) 2,9-Me2-phen (35) (36)and (37) (38)b (39) R = H (39) R = Me
n=l
6
1
Number of CornpoundP n=3
n=4
n=2
1 1 3 4 1 4 2
6 1 1 3 I
1
2
2 1 2
1 4 5
1 1
1
1 1 1
8 8
8 8 9 14 1
8 7
5
1
1 1
3 5 4 1
1 I
Ref. 112, 113 113 112, 113 112 114 115, I16 116 116 I13 113, 117 118 119 120 121 122 122 123 123 124 125 126 113 127 128 129 129
aThe totals here include solvates and species known lo be, for example, [Mn(N-N),(H,Oh]X,. bThese products were described as brown and are suspect.
compounds are ~ c t a h e d r a l ' ~ [MnX,(N--N),] ~,'~' or [Mn(N-N),(H,0),]2+, etc. species; and, whilst the mono compounds in the solid state appear generally to be (MnX,(N-N)), polymers with bridging anions,132they dissolve to give [Mn(N-N) (solvent),12+ cations. Several anions [Mn&(N -N)]*- also are k n ~ w n , "but ~ have not been significantly explored. The bis-chelate compounds of bipyridyl and phenanthroline a pear to be invariably cis ~somers130~131~133 as a result of the steric effect of their a-hydrogens;' although, with the larger Mn-N distances, trans isomers may be available here.]" Full X-ray structural analyses of [Mn(NCX),bipy,] (X ;= 0, S) are with the most reliable data giving13' Mn-N(bipy) of 2.17-2.31 8, and Mn-N(CS) = 2.145 A. The ~ t r u c t u r eof ' ~ the ~ closely related cis-[Mn(NO,),LJ (where L = 32)shows a significant feature of these Mn compounds: it is eight-coordinate, with both nitrates bidentate (Mn-0 = 2.30-2.47A (average = 2.42) and Mn-N = 2.29-2.33 A). Such eight-coordinate structures may be a common feature of the compounds with the oxyanions, including carboxylates. With substituents in the a-positions, the steric requirements of bipy and phen become more demanding, and ligands (33) and (34)give five-coordinate species.'25~126The pyridyl-quinolyl ligand (33) gives a monomeric [MnBr,(H,O)(N-N)], whilst the biquinolyl ligand (34)gives a dimeric [(N-N)ClMn-Cl,-MnCl(N-N)], no other compounds of these ligands appear to have been reported. With the bis-chelate cornpounds'13 of 2,9-dimethyl- 1,lo-phenanthroline (35), it will be interesting to see if the larger Mn" will allow octahedral [MnX,(N-N)J species, or whether they will be five-coordinate [MnX(N--N),]X.
P
25
Manganese Table 14 Compounds with Bipyridyl Anions
Compound
Coiour
[MnbiPY3 1 [Mnbipy,] * 2THF Li[Mnbipy,].THF Na,[Mnbipy,] * 3THF * Sdioxane
Black-brown needles Brown needles Deep red crystals Black-violet crystals
Magnetic moment(BM) 293 K 80 K 4.09 4.1 3.6 5.99
3.81 3.8
The n = 4 compounds (Table 13) have been k n ~ w n " ~for , ' ~phenanthroline ~ since 1933. Analogous Sr and Ba species, but which are not isomorphous with the Mn species, now are knownI3' to be [Mphen2(H20)J2+-2phen-nH20species, and Drew et have reported another example of a non-bonded phenanthroline. Attempts to obtain suitable crystals of Mn phen,X, species for X-ray analysis have not been succe~sful,'~~ so the question of structure of the Mn species is still open. Eight-coordination is, however, established in the l,8-naphthyridine (39 for R = H) compound [Mn(N-N),](ClO,),, which is isomorphous129 with the Fe, Co, Ni, Cu and Pd species; it also occurs again in the [Mn(NO,),(N-N),] species; but, for the dimethyl ligand (39 for R = Me), steric effects reduce the coordination number to six.
-
The [Mnbipy,lzs cation undergoes at one-electron oxidation at 1.4V (v saturated calomel), and a number of reduction ~ t e p s . ' * ~ * ' ~ ~ Chemical reductions'23in THF solutions have produced several crystalline reduced species (Table 14). The best available explanation of these species defines them as Mn" compounds of radical anion ligands, either bipy- (for the first three) or bipy2- for the Na salt. The magnetism of the compounds containing bipy- is an interesting theoretical problem. A range of other anionic bidentate ligand compounds also has been described (Table 15), including several [MII(N-N)~]' tetrahedra, but these are generally very sensitive to air and the studies so far have been of limited scope. We have real reservations about the dipyrromethane compound: there is little in the report, except stoichiometry, to distinguish it from the tris-Mn"' species.
R
R
--.
Manganese
26
Table 15 Compounds with Anionic Bidentate Ligands Ligand
k i n compound
(40)
(26) (41) (42) R (43)
= H,
Me
(44) (45)
Mn(L-H),(H,O)* Mn(L-H), L Mn(L-H), ? [MnL,I [MnL,I WL,I MnL, 2H, 0
Colour
Yellow Dark green White
Probable structure
'
[Mfi%I Mn"'-&Mn"' ? Tetrahedral WnN,] Tetrahedral [MnN,]
Dark violet
Rej 138 I18 139 I40 141 142 I43
(44)
41.3.3.3
Terdentate ligands
( i ) Linear ligands
The study of the Mn" compounds of the terdentate ligands has generally occurred as an addendum to the study of the compounds of the other metals, the one significant exception being a recent paper by Chiswell and Litster.14 bis(2Stability constants in aqueous solution are available for dien (bi~(2-aminoethyl)amine),'~~ pyridyln~ethyl)arnine'~~ and terpy (2,2'2"-terpy1idyl).'~~The determined log K,'s and log K2's are, respectively: 4.0 and 2.8; 3.5 and 2.6; and 4.4(K2not measured for terpy). Both 2:l and 1:l solid MnX,.nL species have been well characterized. The 2:l species are unexceptionable bis-ligand [MnN6]X, octahedra: those with dien are white, and those with the unsaturated heterocyclic ligands related to terpyIa are generally yellow. Electrochemical studies of the bis-terpy species, and analogues with related ligands,I4' have revealed a series of reduced species, which undoubtedly relate to the Mn"-radical-anion species observed with bipyridyl (previous section). Monoanions of the ligands (46 for X = NH), formed by loss of the hydrazone proton, produce neutral molecules [Mn(N,),]': four of these species have been described, and despite an earlier report to the contrary,I4' they are all high spin.'44
In the 1: 1 compounds MnLX,, five-coordinate monomeric [MnX,N,] structures appear to be dominant. Such structures were first seen concurrently in the compounds of pentamethyl-dien (X = C1, Br),'49terpy (X = Cl)'sOand paphy (46 for X = NH, R, = R, = R, = H)."' No full X-ray studies of Mn compounds are available, but powder patterns relate them to known Co and Zn species. Octahedral dimers [Mn2X4L,], and possibly polymers, probably exist also, as postulated by Madden and Nelson's2 for the chloro compound of bis(2-pyridylethyl)amine, but none are yet proven, The most complete study of the [MnX,L] species14 produced 58 compounds with X = C1, Br, 1 or NCS and L being one of 20 variations on the theme (46) or related ligands using quinolyl, benzthiazyl or oxime moieties. They had colours ranging from yellow, through orange, to brown; were all high spin; and X-ray powder patterns of fourteen of them (X C1 or NCS) showed ten to be isomorphous with the almost certainly five-coordinate Co" analogues, but not with the octahedral Ni" dimers. . I
27
Manganese (ii)
Tripod ligands
But few compounds of this class have been described. A high spin compound {MnL2](C104)2 has been described'" for L = (47, X = C-OH) and a compound [MnCl,L] has been described'54for L = (47, X = N). The latter was thought to be tetrahedral, but seems more likely to be a fivecoordinate monomer. The other ligands studied have been the monoanions (48), where R = H or Me, X = GaMe,15' BH or B-pyraz~yl.'~' These ligands form colourless, high spin [MnN,]' octahedral species.
41.3.3.4
(i)
Quadridentate ligands
Linear ligunds
-
The classical linear tetraamine, triethylenetetraamine (trien), gives a 1:1 species in aqueous with a log K of only 5. Such aqueous solutions are very oxygen sensitive. The solid compounds prepared from this ligand and the other Iinear quadridentates are in Table 16. The most common type MnX, *L,which are yellow, orange or light green in colour, probably are largely octahedral [MnX,N4] species. The sterically hindered 3,22,3-tet species'57were isomorphous with their Fe" analogues, but not with those of the later transition metals, for which five-coordinate [MX(3,22,3-tet)]Xstructures have been demonstrated: perhaps the larger Mn" and Fe" atoms allow [MnX2(3,22,3-tet)]octahedral coordination. Also pertinent here is the compound of ligand (56):ring closure with formaldehyde to give macrocyclic species (Section 41.3.10) was achieved""' for the other later transition metals but not for the (larger) Mn". The cream or yellow [Mn,L,]X, species are of a type well characterized for such linear quadridentates: they probably have a structure in which each ligand acts as a bridging group between two metals, being bidentate to each, and thus giving two connected [MnN,] octahedra. Table 16 Compounds Reported with the Linear Quadridentate Nitrogen Ligands Compomd- defined by X Mnz L, 1x4
Ligand
MnX,. L
trien 3,22,3-tet" (49h B = (CH,), (49),B = (CH2)7b*C (49),B = CH(CH,)CH, (51), B = (CH,), (SI), B = (CH,), (51), B = o-C6H4 (521, B = (CH212 (521, B = (CH213 (53), R' = R2 = R3 = H (53), R' = R2 = Me, R' = H (53), R' = R' = H, R' = Me (53), R' + R2 = (C,H,), R3= H (53), R' = R3= H, RZ= But (53), R' = R2 = R3= Me
Br, C10, C1, Br c1, ClO, C1, Br, I, NO,, NCS, ClO, Cl, c10, Cl, ClO,
(55) (56), R = H (56), R = Me
c10,
r
Br, BPh,
Br
Clod, BPh, ClO,, BPh,
CIO, CIO, c10,
C1, Br, NCS
ClO,, BPh, c10,
c1
ClO,, BPh,
c1 C1, Br, NCS ClO,
c1
C1, NCS
Ref: 156 157
ClO, BPh, ClO,
ClO, ClO,
c1 c1
Mn X,L2
a 0 4
158 158, 159 158 158 158 158 158 158 158 144, 158 I58 158 144 144 160 161 161
* 3,22,3-tcl = bis-N..V'-(3-aminopropyl)piperazine. This ligand also forms a compound Mn2L(N,), which is probably of structural type (SO), but with further polymerization on the azide anion. A compound MnLClPF, also has been descnbed. C O C +B*
Manganese
28
A n n NH,
NY N ,H
Mn
Mn
/ \X
/ \X .,
(49)
,NH,
X
X
(50) Me
Ri-42
HN-N
Me
Ri4@
HN-N
N-NH
N-NH
N-NH R-N
I NH,
N- R
I
hH*
But of more interest are the species MnL,X,, which are possibly eight-coordinate. They tend to form with the more rigid, planar ligands (but CJ trien), and they are all ligands which show cumulative ring strain (C-~train),'~~ especially when bound to the larger Mn". Thus, they are eminently suitable for two of them to fit comfortably, mutually perpendicular, around a manganese(II) atom. As with the tetrakis-bidentates (Section 41.3.3.2) X-ray studies await the collection of suitable crystals. Ligands (53) to (57) are capable of losing two protons, to give [Mn(quad-2H)I0molecular species, and those of ligands (53) to (55) have been studied.'m,'" Attempts to prepare them by reaction (3), generally gave mostly MnO,; but thirteen compounds of the dianions of Iigands (53), (54) and (55) have been prepared by irradiating solutions of the ligand and Mn,(CO),, in THF under N,. They are generally deep violet to red in coIour, the colours arising from ligand based charge transfer transitions, and generally appear to be monomers, rather than dimers with two N-C-N bridges as demonstrated for the Ni" ana10gues.I~~ A few examples of the dimers were found,164although, with the larger Mn,"they might have been expected to be more common. Perhaps the compound of ligand (55),lWwith magnetic moment between 4.8 and 5.4 BM, is a pure dimer of structural type related to the Ni dimer.16'
y
HN-NH
N'
N' I I NH2 NH2
29
Manganese Mn(quad)X,
+
20R-
4
[Mn(quad-2H)]
+
2ROH
(3)
The monomeric Mn" compounds derived from ligands (53) and (54) appear closely related to [Mn(phthalocyanine)] (Section 41.3.10.2) in their magnetic properties, and some, with temperature independent magnetic moments of near 4.0 BM, appear to be examRles of pure S = 3 species. The reactions of these neutral species with O2in pyridine have been studied in depth (see Section 41.3.9.3 for a discussion of the oxygenation of Mn" compounds of multidentate ligands).
(ii)
Tripod ligands
Me6tren{tris(2-dimethylaminoethyl)amine} forms166five-coordinate species IMnX(Me,tren)]X, which were among the first five-coordinate Mn" species identified. More recent work has shown that the unsubstituted tren molecule also forms five-coordinate Mn" specie^:'^' the solid compounds [MnX(tren)]BPh, were isolated for X = N,, C1, Br, CN, NCS and NCO; and the X-ray structures of the last two have shown a basically [MnXNJY structure, although specific H bonding holds the cations in pairs in the solid state. Mn-N(H2) distances were near 2.2& Mn--N(apical), nearer 2.4A and Mn-N(CX) were near 2.1 A. Related ligands (58, R = H, Me)'46and (59)168 also have been studied. For the ligand (58, R = H), the data in aqueous solution have been interpreted as evidence for an WnN4(H,0)J2+ sevencoordinate s t r ~ c t u r e . ' ~ ~
41.3.3.5 Quinquedentate ligands
The linear aliphatic ligand (NH,CH,CH,NHCH,CH,),NH forms169a 1: 1 species in water with log K , of 7.s7.5. Irreversible oxidation occurs in the air, but attempts to isolate Mn"' species or solid Mn" species were unsuccessful. Except for several reports of single compounds, with no meaningful structural information, data on compounds of linear quinqudentates are restricted to a report from Coleman and Taylor.'70 They prepared nine compounds of the ligands (60) MnX,L-nROH (X = C1, Br, T or NCS; R = H, Me or Pr). All were high spin, with colours ranging from yellow to orange; the compounds were soluble in water, methanol, acetone and DMF; and all were said to show no observable reactivity with O2or NO, neither in the solid state nor in solution (DMF).
(61)
Early attempts'7' to prepare an Mn" compound of the ligand (61) (see Section 41.3.3.6), gave instead white crystals of a compound MnC1,. L, where L is a partially hydrolysed form (61), having lost one pyridylaldimine residue.
41.3.3.6 Sexadentale and septadentate ligands (i)
Linear ligands
The only report"' of h e a r N, ligands gave three compounds [MnLIX, (X = NCS or I) of the ligands (62) (for R = 2 or 3). The bright yellow crystals of the high spin Mn" compounds were stable in air for at least several weeks and do not react with 0, or NO, neither in the solid nor in solution.
30
Mangunese
(ii)
Bifurcated ligands
Data are available on the ligands (63) (for n = 2 or 3) and (64). Ligand (63 for n = 2) forms a 1 : 1 species in water with log ICl of 9.3 and the solution data have been interpreted16' as evidence for a seven-coordinate polyhedron [Mn(N,)H,O]*+, comparable with that proven for [Mnedta(H,O)]'(Section 41.3.9.5). Given the steric constraints of this amine ligand, it may be possible to obtain [MnLXIX species here.
By contrast, the ligand (63 for n = 3 ) has a log Kl of only 5.3 and it readily forms binuclear species Mn,(N,):l in water. however, has a log K of 10.27 and the solid compound The pyridyl ligand (a), [MnL](CtO&. H,O has been isolated as yellowish needles.'46Available data were interpreted as indicating an [MnN,] octahedral structure, with the water molecule not bonded to the metal.
(iii)
Tripod ligands
The ligand Et-C(CH,NHCH,CH,NH,), has a log Kl of 8.2 in water: no solid derivatives have been r e ~ 0 r t e d . l ~ ~ But the more interesting tripod ligands (65) to (67)are those which induce trigonal prismatic six-coordination and seven-coordination on Mn". Ligands (65) and (66)present their donor atoms at positions for trigonal prismatic rather than octahedral coordination, and, whereas the Mn" d S high spin metal atom, in its yellow compounds, is largely content to accept the ligand's geometry, the later transition metals induce ligand distortions to approach octahedral geometry.172
The closely related ligand (67)is potentially septadentate, and although full details are not published, it seems that the Mn" compound is indeed seven-coordinate, with an Mn-N (apical) distance of2.79
41.3.3.7
Uses of manganese nitrogen compounds
The following lists some of the more significant uses of the compounds noted in Section 41.3.3.
Manganese
31
Manganese(I1) nitrogen species, and especially Mntrien;:, show strong catalytic activity towards the decomposition of W,O, (only iron compounds seem more effective), and are used to accelerate the photopolymerization of vinyl monomers, in the presence of cc1,.'56 Some of the tetrakis compounds of substituted pyridines [MnX,L,], especially where X = NCS, form clathrates with hydrocarbons, such as xylenes, cumenes, mesitylene, etc.; and specific compounds have been used to separate isomers of these hydrocarbons." Various nitrogen compounds have found use in corrosion inhibition, as fire-proofing agents for epoxy resins and flame retardants for polyurethane foams. For these and other uses, the reader is tells of the use of Mn" chelates referred to the indices of Chemical Abstmcts. One recent of phenanthroline and bipyridyl as contraceptive and antivenereal agents. The antibacterial and antifungal properties of these compounds have long been known.17'
41.3.4
Phosphorus, Arsenic, Antimony and Bismuth Ligands
Although many tertiary phosphine, and to a lesser extent arsine, compounds of Mn' have been known for many years, the corresponding complexes of Mn" are relatively rare. The preference of Mn" for oxygen, nitrogen and halogen donors has meant that few compounds with the heavier Group V donor atoms have been isolated. However, recent work demonstrates that, given anhydrous conditions, simple Mn" salts will interact with tertiary phosphines to give compounds with a Mn"-P bond. Although the compounds [Mn(PPh3),jX2(X = halogen), [MnX2(EPh3)2](X = halogen; E = P or As),176and [MnX,(diars),] (X = halogen; diars = o-phenylenebi~dimethylarsine)"~were reported some years ago, work since then has indicated that in these original preparations, the actual ligands were phosphine or arsine oxides, i.e. that the Group V donor had oxidized in solution by reaction with ~ x y g e n . ' ' ~ . ' ~ . ' ~ The lack of early success in obtaining tertiary phosphine and arsine compounds by direct interaction of the relevant ligands with Mn" halides, led to the use of preparative methods incorporating oxidation of suitable Mn' carbonyl compounds. Thus [MnBr,(CO),(diars)] was obtained by bromine oxidation of [MnBr(CO),diar~],'~' while [Mn(C0)2(dppe)2]2+(dppe = 1 ,Zdiphenylphosphinoethane) was prepared by oxidation of [Mn(CO),(dppe),]+ with nitric acid.", Such Mn" carbonyl compounds were shown by infrared studies to possess trans-carbonyl structures and this fact was used to explain why the Mn' compounds [Mn(CO),TP]+ and [Mn(CO),QP]+ {TP = bis(odiphenylphosphinopheny1)phosphine;QP = tris(o-diphenylphosphinophenyl)phosphine},in which the carbonyls must be in the cis configuration, could not be oxidized to give stable Mn" comp o u n d ~ ." ~ A further example of the preparation of stable Mn" phosphine and arsine compounds is shown by the oxidation with NOPF, in benzene of trans-[MnBr(CO),L,] (L = PMe,Ph or AsMe,Ph) and cis-[MnBr(CO),L,] (L2 = dppe or L = AsMe,Ph) to yield fa~-[MnBr(C0),L,]~.'~~ Nevertheless, the complexity of the interaction of dppe with Mn,(CO)lois a warning to those who postulate invariant ligand integrity in reactions. Although this reaction was originally claimed to yield three products, viz. [Mn(CO),(dppe),][Mn(CO),], Mn(CO),dppe and Mn(CO)(dppe),, an X-ray crystal study has shown that the last of these three compounds should be formulated as a Mn' compound with a carbonyl-insertion reaction occurring between one of the phenyl rings on one of the phosphorus atoms and the manganese atom to yield a terdentate ligand (68).185 PI11
$
Ph,
The early work on bidentate phosphine compounds of manganese was reviewed in 1972.Is6More recent preparations of the compounds [MnX,(diars),l, and preparations of similar compounds with the ligand o-phenylene(dimethy1arsine) (dirnethylstibine), have indicated that if care is taken in
32
Manganese Table 17 Bond Lengths for [MnCl,(diphos),l"+ Compounds Bond Lengths
Compound (MnC&(diphos)
Mn", n = 0 Mn"', n = 1 Mn", n = 2
/"
+
Mn-CI
2.502 2.239 2.195
(A) Mn-P
2.625 2.345 2.428
excluding water from the reactions, these compounds, and not the arsine oxides, can be prepared;'" however, compounds of the two tertiary stibines, Me,Sb(CH,),SbMe, and C6H4(SbMe2-o),could not be obtained under similar conditions. Other workersI8' have reported that the tertiary diphosphine, o-phenylenebis(dimethy1phosphine) (diphos) reacts with anhydrous manganese(1I) halides to yield yellow octahedral compounds of type [MnX,(diphos),] (X = C1 or Br), in which the halogens have a trans configuration. Unlike the corresponding diarsine compounds, these diphosphine compounds are readily oxidized to octahedral orange Mn'" and then orange-brown Mn" species by the reactions: [MnX,(diphos),] + Ph,CPF6 + [MnX,(diphos),]+ ; [MnCl,(diphos),]+ + HNO, + [MnCl,(dipho~),]~+.The X-ray structures of all three chloro compounds have been reported'88 and are of particular interest as this work allows a rare direct comparison of changes in metal-ligand bond lengths with changes in meta1 oxidation state (Table 17). The reduction in Mn-Cl bond length with increase in positive charge on the metal is clearly observable, as is the decrease in Mn-P bond length in going from Mn" to Mn"'. The unexpected increase in Mn--P bond length when Mn"' is oxidized to Mn" is found to be due to the closer chlorines in the latter compound 'pushing away' the phosphine methyl groups. Trans-bromo structures are also found in the similar ditertiary phosphine bidentate complex the Mn-P bond length in this [MnBr,(dmpe),] (dmpe = 1,2-bi~(dimethylphosphino)ethane);~~ compound (2.655A) being comparable with that found in [MnCl,(diphos),] (2.625A). The reaction of [MnBr,(dmpe)j with LiAlH, yields a Mn' compound of stoichiometry Mn(AIH,)(dmpe), (Section 41.2.4). The reactions of anhydrous manganese(I1) halides with unidentate tertiary phosphines, R3P,have been reinvestigated, and it has been shown that polymeric compounds, of formulation MnX,(PR,) (X = halogen or thiocyanate), can be isolated from tetrahydrofuran solutions.'89These compounds, isolated for a large range of phosphines, but excluding triphenylphosphine,lW undergo reversible oxygenation in both the solid state and in solution yielding deep red, blue, purple or green compounds of type MnX,(PR,)O,. A typical example is MnBr,(PBu;) for which a dioxygen binding isotherm similar to that shown by myoglobin is claimed.'89There appears to be no dioxygen bond breakage during these oxygenation reactions."' Equilibrium data for the oxygen absorption have been deduced, and it is claimed that, for example, [MnI,(PBu;)] will reversibly absorb and desorb completely one mole of dioxygen per mole of complex for more than 400 times, if the reaction is carried out in solution at - 20 "C. In the solid state only the thiocyanato compounds of formulation [Mn(NCS),(PR,)J (where R = alkyl) reversibly bind ~xygen."~ It has been shown'93that the X-ray structure of the compound Mn12(PMe2Ph)is polymeric with alternating octahedral and tetrahedral Mn" entities (69). However, full details of the structure are as yet unknown. Binding of dioxygen in the solid state must be, one assumes, into the lattice structure of the crystalline compound. Thus crystal structures are most important in ascertaining how the gas molecules fit into the lattice.
These oxygenation reactions have been hailed as being of great importance4 but the findings have been challenged by other workers,'" who had not been able to isolate other than Mn" tetrahydrofuran complexes from supposedly identical reaction procedure^.'^^
Mangunese
33
claim^'^^^'^^ with no details supplied yet, that many of the compounds of type [MnX,(PR,)] reversibly bind not only dioxygen, but carbon monoxide, carbon dioxide, nitric oxide, sulfur dioxide and ethylene as well as dihydrogen, would appear to indicate that these polymeric complexes are a most versatile form of molecular sieve if the reactions proceed in the solid state. What may be the nature of binding of such a diverse group of molecules to the metal compounds is difficult to conceive. A further unresolved problem of the work lies in the results which indicate that reversible oxygenation occurs for many compounds both in solution as well as in the solid state. It is obvious that the mechanisms for oxygenation must be entirely different in these two situations even if in both cases each molecule of the compound can bind one dioxygen molecule. The stabilization of Mn"-alkyl and Mn'l-aryl bonds by use of tertiary phosphines has been shown by the ability of [Ph,MnP(cyclohexyl),] to undergo various insertion reactions, e.g. CO insertion to give [(PhCOO)2MnP(cyclohexyl),j.6' Further work" of great interest has shown that Mn" dialkyls yield orange or yellow dimeric compounds of general type [Mn,R,(PR,),] (R = CH,SiMe3, CH,CMe, and CH,Ph) (70) when interacted with a number of unidentate tertiary phosphines. The X-ray structures of three compounds of this type have been shown to possess two asymmetrically bridging alkyl groups, with an additional terminal alkyl and one phosphine per manganese atom. In the presence of excess tertiary phosphine in light petroleum, the orange solutions of these dimeric compounds fade to a pale yellow colour. ESR spectra showed that monomers form due to a cleavage reaction (4).
[Mn,(CH,CMe,Ph),(PMe,),]
f
2PMe, e 2[Mn(CH,CMe,Phh(PMe,),l
(4)
The X-ray crystal structure of this colourless monomer shows that it has a severely distorted tetrahedral geometry; the compound is high spin with a magnetic moment of 5.9 BM. Monomeric compounds of this type readily lose phosphine to re-form the dimer, but they can be stabilized by use of the bidentate phosphine, 1,2-bis(dimethyIphosphino)ethane (dmpe) to replace the two unidentate phosphine moieties. On the other hand, if the two unidentate alkyl moieties are replaced by the bidentate ligand o-xylylene (o-(CH,),C,H,), a low spin octahedral compound Mn[o(CH,),C,H,](dmpe), (71) can be prepared. The metal-ligand bonds in this low spin compound are all noticeably shorter than those in the tetrahedra1 high spin monomer (Table 18).
Table 18 Mn-Ligand Bond Lengths (A) for High Spin [Mn(CH,CMe,Ph),(PMe,),] (A) and Low Spin [Mn(o-(CH,},C,H,)(dmpe),](B) ~~
Mn-L Bond
Mn-C Mn-P
Compound A
Compound B
2.149
2.110 and 2.104 2.230 (Mn-P,)*
2.633
2.297 (Mn-P,) *see (71).
34
Manganese
in the area of Mn" alkyls has led to the disproportionation of Mn"' to yield Mn" Other and Mn" species in the reaction (5). iMn(acac),]
+ dmpe + MeLi
+
[MnM~,(dmpe)~] IMnMe,(dmpe)]
(5)
Phosphide compounds of manganese(I1) have also been isolated. The tetra(dipheny1phosphide)manganate(I1) ion has been prepared by the reaction sequence (6),lg8while in liquid ammonia at -75°C '[(Mn(NCS),(NH,),]' is claimed to react with KPPh, to yield either colourless Mn(PH2),-3NH, or Kz[Mn(PHz),].2NH3.'99 MnX, -tKPPh2.2dioxane
KMn(PPh,),
K2[Mn(PPh2),] + Mn(PPh,),
(6)
(X = halide)
The Mn" to phosphorus or arsenic bond can also be stabilized by use of hybrid multidentate ligands containing one or more nitrogen donors. Two early examples of compounds in which both nitrogen and phosphorus were bound to the Mn" are: (i) [Mn(N-P)JBr2, (N-P = 72):Oo (ii) the apparently five-coordinate [Mn(N,-P)Cl]Cl, (N,-P = 73j.20'
The linear terdentate hybrid ligands (74) with the donor set N-N-As, also yield compounds of the types [MnX,(N-N-As)] (X = C1, Br or I), [Mn(N-N-As),](ClO,), and [Mn(NCS),(N-N-As)], in which all three donor atoms appear to be bonded?02Similarly, all four donors of the quadridentate N-N-N-As ligand (75) appear to be coordinated in the compound [Mn(N-N-N-As)], as in which both an imine proton and an arsenic methyl group are lost upon reaction of the ligand with Mn,(C0),,.'60
R = Me or Et
(74)
The simple bidentate N-As ligand (76) yields the cornpounds [MnBr,(N-As)] (N-As),](ClO,), in which both donors are bound.203
41.3.5
and [Mn-
Oxygen Ligands
Although the lowest limiting oxidation state for all oxygen ligands, Mn" is also, on balance, the most stable. This comment is certainly true for neutral, and especially acid aqueous solutions, and, although alkaline solutions are oxidized by O,, this is at least partially because of very low solubilities of the higher oxides, especially MnOz. Despite the importance of this area of Mn'l chemistry, we still have much to learn about basic structural preferences: the variety of polyhedra adopted; their relative importance, distortions of
Manganese
35
them and the energy costs of such distortions. Available data, however, do suggest that nondirectional, ionic bonding predominates in the Mn-L bond, even though covalency is not negligible. A consequence of the predominantly ionic bonding is that effects on the ligands of Mn-L bonding, and therefore the energy profiles of the ligand vibrations, are not very different from the effects of H-bonding and 'non-bonded' interactions in crystalline solids. Thus attempts to define stereochemistries by vibrational methods are unreliable, a view that is strongly reinforced by the variety and complexity observed, for example, in the carboxylate compounds (Section 41.3.5.2.ii). We make no further apology for giving prominence in the following to the X-ray structural data.
41.3.5.1
Unidentate ligands
( i ) Water, hydroxide and oxide
There is little doubt that the basic species formed when almost any compound MnX, is dissolved in is !J4n(H,0),]2f. It is seen in various crystalline solids, and the solution data give no real hint that we need to suspect the other coordination polyhedra, either higher or lower, are present in the aqueous solutions. So, [Mn(H,0)6]2+ it is-mainly on the basism5of the optical spectra and correlation with the crystalline solids. In this respect, we note that XAFS studies on aqueous solutions derive an Mn-OH, distance of 2.18& in good agreement with the observed distances in solids (Table 1). The rate of exchange of bonded water with bulk solvent gives evidenoe that the process is associative, that is, it occurs through a seven-coordinate intern~ediate.~'~ The two reported determinations give (averaged) kinetic parameters of; k , = 2.15 x IO'S-'; AH* = 7.8 kcal mole-'; AS* = 1.2caImale-'K-'; and AV* = -5.4mlmoIe-'. It is the negative BY' which implies the associative mechanism. In the competition between a longer Mn-OH, bond favouring dissociation, and the greater space, made available by the longer bonds, favouring assaciation, it appears that the latter wins. We can only notice the result and correlate it with the greater tendency shown by Mn" to form seven-coordinate polyhedra. All aqueous solutions of MnX, are, of course, further complicated by the formation of [MnX,(H,0),,](2-"'' species, and a perusal of the reported stability constants for thesezo6indicates that they are formed to much the same extent as for the other first row transition metals. They are, however, less evident because of the lack of colour in the solutions. Such species also are encountered in X-ray structural analyses of Mn" compounds. The species with n > 2 are formed in water in significant concentrations only in the presence of an excess of anion. The 'hydrolysis' of Mn: parallels that of its nearest neighbours CrzCand Fez+;at pH as low as 3.5, proton transfer to solvent, forming [Mn(OH)(H,O),]* occurs, and the limiting value at low pH for K,,has been measured (25 "C) as 5 x 10-5mole1-'. At higher pH, polymerization (apparently dimerization) occurs,Zo7leading eventually to the precipitation of solid, polymeric Mn(OH), . Mn(OH), is a well-characterized, stoichiometric, crystalline compound with the hexagonal Mg(OH), structure. Although preparations of it require careful exclusion of 02,the pure, isolated solid has good stability in the air; it occurs naturally (pyrochroite); and finds use as an industrial catalyst. It is weakly amphoteric, and can be recrystallized from hot, concentrated alkali solutions. This, of course, implies the formation of hydroxo and/or oxomanganates(II), and, indeed, solid, yellow Na,[Mn(OH),] has been desscribed?" as have Ba, Mn(OH), and Sr,Mn(OH),. Little is known of the 'basic salts':209Mn(0H)Cl has been decribed in two crystalline forms, which are said to have layer structures related to Mg(OH)?; and the compounds Mn2(0H),X (X = C1, Br, I) also are known, with the chloro compound again in two different crystalline forms, one of which occurs in the mineral kempite, isotypic with &Cu,(OH),Cl. Crystalline phases containing both OHand oxyanion ligands are covered in Section 41.3.5.2. Manganese(I1) oxide exists in only one modification under normal conditions, and this has the simple rock salt cubic structure.210,2"In addition, Mn" occurs in some of the other binary oxides, and in a wide range of mixed oxides (Table 19). Most of these have been studied mainly for their magnetic and electrical properties, but we are here interested in any light they can shed on the structural preferences of Mn". Table 19 summarizes available data, referring largely to species for which three-dimensional refinements give bond length data, and confirm structures implied by symmetry and unit cell data. The area of interest shades imperceptibly into the compounds of the oxyanions, which we have separated into Tables 22-25. The data show that Mn", unlike Mnrvand some of the other metals, is not particularly fussy about
36
Manganese
C
i
Manganese
37
its neighbours -neither their number, nor their arrangement. Certainly, [MnO,] octahedra are predominant, deriving from the occupation of the octahedral holes of close-packed oxygens. But often these octahedra are quite distorted, with Mn-0 distances ranging from 1.99 to 2.50 A and individual angular distortions of at least 15". And four-planar, four-tetrahedral, five-, six- trigonal prismatic, seven- and eight- (both cubic and dodecahedral) coordinate polyhedra are observed; and most of these now have parallels in the more discrete coordination polyhedra upon which coordination chemists have largely concentrated to date.
(ii) Monohydric alcohols, alkoxides and phenoxides Solutions of MnX, in the alcohols give [Mn(ROH),]2+ and these cations have been isolated as crystalline solids for R = Me, Et. In more concentrated solutions, the species [MnX,(ROH),J(2")' have been detected by EPR and 'HNMR spectra. The alcohol ligands are easily replaced by water. A number of other crystalline solids of type MnX,-nROH also have been isolated for primary alcohols, methyl to pentyl, (but not a secondary or tertiary alcohol) and n = 1 to 4, but not necessarily an integer. Many appear to be variations on MnX, polymeric systems, with ROH donors, as are the only two of known structure: Mn2C1,*3EtOHconsists of polymeric chains of alternating cis-[MnC&(EtOH),] and [MnCl,(EtOH)] ~ c t a h e d r a ; whilst ~ ' ~ Mn,Cl,, 14EtOH has three different Mn octahedra making up chains2I4of trans-[MnCl,(EtOH),], cis-[MnCl,(EtOH),] and mer-[MnCl,(EtOH)J (Mn-0 distances range from 2.16 to 2.21 A in the two structures), Mn" alkoxides and phenoxides have been studied only quite recently. Druce''' reported two Mn(OR), species in 1937, and the pale pink, insoluble Mn(OMe), was described2I6in 1966, but the only reasonably complete study is that reported2I7by Horvath et al. in 1979. They prepared a total of eighteen solid alkoxides and six phenoxides by reaction (7). These ranged from amorphous polymers and gelatinous fibres to crystalline species. The pale coloured polymers were assigned octahedral [MnO,] structures, but some of the more crystalline species were more reactive, soluble in common organic solvents, and were thought to be monomers or 'loosely associated'. Wilkinson and co-workers, however, have shown'" that Mn(OBu'), is trimeric in freezing benzene, and drew parallels with the three- and four-coordinate oligomeric alkyls (Section 41.3.2.2). It seems likely that In a few, if any, 'of these alkoxides or phenoxides will turn out to be two-coordinate number of cases, Horvath et al. observed compounds Mn(OR),-nL (where L = py or THF) and characterized two yellow phenoxides [Mn(OR),(THF),]. All of these alkoxides were extremely air-sensitive, especially when wet, and, with traces of 02,all the pale coloured products darken to brown or black materials. Several are pyrophoric, and the ethoxide reacts explosively with excess 02.The solid 2,4-di-t-butylphenoxide reacts with controlled amounts of 0, to give a green intermediate, which may be a compound of dioxygen, and recalls the interesting phosphine work of McAuliffe and co-workers (Section 41.3.4). Further reaction with O2g m s a black material, from which the purple quinone (77) can be isolated in 76% yield?I7 Mn{A1(OPr'),)2 is a light brown viscous liquid, which can be distilled in vacuo and is monomeric in boiling benzene?" Within the phenoxide class, there is a compound with a four-coplanar [MnO,] polyhedron: in [Mn{Pt(NH,),(L-),),]C1,~ 10H20{where L- is the anion of l-Me-thymine (78)) the thymines are N-bonded to Pt and each Pt moiety is bidentate to a central Mnl*,so that the latter is bonded to four phenoxides in a planar array.220
-
Mn{N(SiMe,),},
+ ROH + Mn(OR), $- 2HN(SiMe3)*
(7)
Manganese
38
(iii)
Ethers
It is clear, from the behaviour of various MnX, species in diethyl ether,"' that the ether molecules bind to the metal, although no significant crystalline solid compounds have been isolated and characterized, at least partially because of the volatility of the ether. Two other commonly used solvent ethers (THF and dioxane), both of which have less significant steric problems, form somewhat more robust species. For the solid compounds MnX, -2THF (X = C1, Br, I, NCS) the data~21.222 at least for the halides, are consistent with an expected chain polymeric structure of type (8). The same proposed trans[MnX,O,] polyhedron is observedzz3in the crystal structure of [Mn{Ti"'(Cp),Cl,),(THF),Iin , whch the trans THF oxygens are at 2.227(3) A. The hexakis species [Mn(THF),][SbCl,], seems to be the only known compound with an all-ether fMnO6I2' polyhedron,224A previously described perchlorate"' is probably [Mn(THF),(H,0)z](C104)z. Although capable of bidentate function to one metal, steric and conformational preferences generally seem to dictate that dioxane22' acts either as a unidentate hgand or that it bridges two different metals, as in (79). The compound MnC1, -dioxane, for which (79) is the proposed structure, is not affected by six hours in vacuum at 100 "C!Other known MnX, - ndioxane species include those defined by n = 1, for X = C104, NCS and NCSe, and n = 2 for X = Br and I, and some mixed aqu-ther compounds.22'
0
I \
6
I \
(iv) CrarbonyI ligands
The most commonly described class is that of the octahedral compounds [MnL,]X,, which are summarized in Table 20. All appear to be simple octahedral cationic species, although only the pyrone and 4-butyllactam compounds have been shown to be isomorphous with a reliably octahedral Ni" analogue. Robustness vanes from the aldehydes, which rapidly turn brown, even if kept below 0 "C, to the lactam, which is 'stable to air and light, and moderately stable to heat'; but all are rapidly attacked by water. In parallel with some of the nitrogen ligand compounds, a few of these carbonyl ligands give crystalline species MnX, -nL, where FI z 6 . These include X = NCS, n = 8 and X = Br or I, n = 10 for urea;22sand X = SbCl,, n = 7 for 4-chlorobenzaldehyde.226
39
Manganese Table 20 Carbonyl Ligand Compounds [MnLJX, Ligund Class
Aldehydes Ketones Esters Amides Lactams Ureas
Specific Ligands
Acet-, propion- and benz-aldehyde 2-C1,4-C1 and 4-Me-benzaldehyde Acetone, chloroacetone, methylethylketone and acetophenone 2,6-Dimethyl-4-pyrone Methyl formate Ethyl acetate Dimethylformamide, acrylamide 6-Capro and 4-butyl-lactam Urca, t-butyl-, dimethyl- and diethyl-urea
XFeCI,, InC1, SbCI, FeCI,, InC1, ClO,, BF, FeCI,, InC1, SbCI, ClO, ClO, Br, I, ClO,. NO,
Ref.
a 226 a b C
C
d, 231 e, f
225, g. h
W. L. Driessen and W. L. Groeneveld, Recl. Trav. Chim. Pays-Bas, 1969, 88, 977; 1971, 90, 87 and 258. b A . De Jager, J. J . De Vrieze and J. Reedijk, Inorg. Chim. Acta, 1976, ZO, 59. W. L. Driessen, W. L. Groeneveld and F. W. van der Way, Red. Trav. Chim.Pays-Bas, 1970,89, 353. W. Schneider, Helv. Chim. Acla, 1963, 46, 1842. e S . K. Madan and H. H. Denk, J. Inurg. Nucl. Chem., 1965,27, 1049. ‘ S . K. Madan and I. A. Sturr. J . Inorg. Nucl. Chem., 1967, 29, 1669. Kircheiss, U. Pretsch and U. Borth, 2. Chem., 1980, 20, 349. hB. C. Stonestreet, W. E . Bull and R. J. Williams, J . Inorg. Nucl. Chem., 1966, 28, 1895.
Compounds MnX, -4Lare described only for amides and ureas: with the former being isomorphous with the Ni” dimethylacetamide (X = NCS, C104)2279228 analogue and octahedral; the lactams (80) (rn = 5 , 7 and 1 1; X = NCS)229-similar compounds are formed by Ni, Cu and Zn, but Co and Cd form [ML,][M(NCS),]; formamide (X = Cl),230and acrylamide (X = NO,);231 urea (X = NCS, C1, Br)225,232-an unrefined X-ray structure2,, proves the trans-octahedral structure for X = NCS; dimethyl- and diethyl-ureas (X = NCS).22s
[MnBr,(dimethylurea),] has a fiveOnly three MnX, -3Lcompounds have been coordinate structure234with Mn-0 at 2.04 and 2.18 A; and [MnCl,(propionamide),] and [MnC1,(4butyllactam),] probably have a similar structure. A more exhaustive study of these ligand systems will probably produce more such five-coordinate species. Well-defined crystalline compounds MnX, -nL for n = 2, 1 and < 1 also are d e s ~ r i b e d . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Most are apparently polymers of types (8) and (13), although tetrahedral [MnX,L,] species may eventually be identified. A different polymer, with cis unidentate ethylacetates, at 2.17 and 2.24 A, has been identifiedz3’in [Mn(PO,Cl,),(ethyla~etate)~].
(v)
Ligands with NO, PO,AsO, SO and SeO groups
With possible minor exceptions in the case of RNO,, all of these ligands provide a single oxygen donor atom and all form similar compounds. The numbers of known compounds for the different ligand types are summarized in Table 21, both as an indication of areas of concentration of effort and an entry to the literature. Unfortunately, this contains very little basic structural data (Table 21)-only two full X-ray analyses and a few correlations of powder data. After early problems in preparing some of these species, the use of special solvent systems, including ethyrlorthoformate as a dehydrating agent, and the SbCl; anion, has allowed significant extension of the class. The NO species are covered in a recent Gmelin v~lurne”~ and the PO species were reviewed in 1971.240 The compounds MnX,-nL, for n = 6 , are found quite commonly in the pyridine-N-oxides and sulfoxides, but oniy one PO group compound is known. This is generally ascribed to steric (bulk
40
Manganese Table 21 The Numbers of Known Compounds MnX,-nL for the N, P, As, S and Se Oxide Ligands
Ligand Clam
6
5
NumbeP of Compounds for n = 4 3 2 (2) 1 1
2 3 15 1
I 3(1)
6
Ref
0.5
2
239 239 239, c
d, e
6
240, 243, 244, 246, f g h, i
1 12
j k, 1
I
3t2)
I
6 2 1 1
2
243, f 1
m-r 3, t U
The numbers in parentheses refer to hydrates. This includes quinoline, etc., species. A. N. Speca, L. S. Gelfand, F. J. Iaconianni, L. L. Pytlewski, C. M. Mikulski and N. M. Karayannis, Inorg. Chim. Acta, 1979, 33, 195. W. L. Driessen, L. M. Van Geldrop and W. L. Groeneveld, R e d . Trav. Chim. Pays-Bas, 1970, 89, 1271. e C. Dragulescu, E. Petrovici and I. Lupu, Monatsch. Chem., 1974,105, 1176. G. A. Rodley, D. M. L. Goodgame and F. A. Cotton. J . Chem. Soc., 1965, 1499. C. M. Mikulski, J. Unruh, L. L. Pytlewski and N. M. Karayannis, Transition Met. Chem., 1979. 4, 98. h N. M. Karayannis, C . M. Mikulski, L. S. Gelfand and L. L. Pytlewski, J . Inorg. Nucl. Chem., 1978,40, 1513. 1 N. M. Karayannis, C. M. Mikulski, M. J. Strocko,L.L.Pytlcwski and M. M. Labes, Inorg. Chim. Acra, 1974, 8, 91. M. W. G. De Bolster and W. L. Groeneveld, Red. T ~ w . Chim. Pays-Bas. 1971, 90, 1153. M. W.G. De Bolster and W. L. Groeneveld, R e d . Trav. Chim. Pays-Bas, 1971, 90,477. M. W. G.De Bolster, I. K. Kortram and W. L. Groeneveld, J. Inorg. Nucl. Chem., 1973, 35, 1843. mG.V. Tsintsadze, R u n . J. Inorg. Chem. (Engl. Trans[.), 1971, 16, 614. " A. H.M. Flew and W. L. Groeneveld, Red. Trav. Chim. Pays-Bas, 1972, 91, 317. ' W. D. Hamson, J. B. Gill and D. C. Goodall, J . Chem. Soc., Dolton Trans., 1979, 847. W. F. Currier and J. H. Weber, Inorg. Chem., 1967, 6, 1539. P. W. N. M. Van Leeuwen and W. L. Groeneveld, Reel. Trav. Chim. Pays-Bas, 1967,86,721. F. A. Cotton and R. Francis, J . Am. Chem. SOC.,1960,82,2986. ' C. D. Desjardins and I. Passmore, J . Fluorine Chem., 1975, 6, 379. P. W. Dean, J . Fluorine Chem., 1975, 5, 499. R. Paetzold and P. Vordank, Z . Anorg. Allg. Chem., 1966, 347, 294. a
'
'
'
'
or B-strain) effects and the argument probably is valid. But arguments, that 2,6-dimethylpyridineoxide forms only a tetrakis compound for steric reasons, are confounded by the hexakis compound formed by the 2,4,6-trimethyl analogue. These hexakis compounds generally are pale yellow, are assumed to be [MnL,]X, octahedra, and such a structure is proven for ligand (Sl)."' They tend not to be very robust; the p y 0 species generally are sensitive to air, light and heat, apparently more so than the compounds for n < 6 .
n
Whether steric bulk or crystal-packing effects are more important, many of the ligands, and especially the PO and As0 species, do form pentakis and tetrakis compounds several of which are hydrates. The n = 5 compounds may be five-coordinate or may contain an anion in the sixth coordination site. The former is known" for [Mn(Me,A~0),](C10,)~. It absorbs H20in the air, forming octahedral [M~(M~,AsO)~H,O](C~O,)~. In the p1 = 4 series, there are k e d points in the known s t r u c t ~ r e sof~ ,the ~ ~square ~ pyramidal cations [MnI(Ph,PO),]+ and [MnClO,(Ph,MeAsO),]+, and there has been a tendency to relate most of the other compounds to these. However, other compounds of the group may turn out to be tetrahedral [MnO,]X, species*, and some, especially the hydrates, may be octahedral. Some of the correlations with Co and Ni structures, assigning tetrahedra, were made before the variety of five-coordinate structures was recognized, and must be considered doubtful.
Manganese
41
The n = 3 species, on simplest interpretation, may be five-coordinate monomers [MnX,L,] or ionic [MnL,][MnX4]species. There has been a tendency to believe more in the latter than the former. There are also some hydrates, such as Mn(CI04),-DMS0.4H,0, for which it is impossible to make even a reasonable guess about structures. A wider variety of structural assignments is available for the n = 2 species: possible structures include tetrahedral monomers, five-coordinate anion-bridged dimers; other oligomers; and polymers, such as (8). Many tetrahedral monomers have been postulated, but none have yet been confirmed. The only fixed point in the sea of data is the structure of [MnCl(NCS)(Ph,PO),] which is a five-coordinate with chlorine bridges. The compounds for n = 1 or 0.5 undoubtedly have polymeric (layer) structures, of which (13) is an example, There is also an example of a 'cubane' tetramer like (18) in this class, MnF(BF4).(2,6diMepyO) * MeOH, which resulted239from the reaction of 2,6-dimethylpyridine N-oxide with Mn(BF4)2.
(vi ) Dioxygen Reversibly formed Mn" compounds with O2are persistent in the 1iteratu1-e.~~ However, whilst it is clear that they do have a limited existence, no firm detail of the bonding in any of them is available through X-ray analyses. Attempts to obtain crystalline species have been thwarted by reactions of the [MnO,] moiety, giving other Mn"' and/or Mn" species. Equally strong claims have been made for an [Mn'V-0,2-]~, formulation in porphyrins, and an [Mn"'-02-]qIJ superoxo formulation in phthalocyanines. Probably both can exist for the more electrovalently versatile manganese; but eventual hard proof, through X-ray work would seem at this stage to require some elegant chemical and physical experimentation, or perhaps just further exploration. (?Ve think that the reversible combination of 0, with the solid phosphine compounds (Section 41.3.4) is likely to have its final explanation given more in terms of clathrate formation, especially in view of the wide range of other gases reported to bind reversibly.) It is somewhat surprising that Mn compounds with 0,moieties appear to have such limited existence; 'peroxo' compounds of the higher oxidation states are an important feature of the earlier transition metals (and we shall notice a few, poorly characterized, examples for Mn in Section 41.5.3); and the metals which follow manganese -Fe and Co -have an extensive and important chemistry of reversible combination with the oxygen molecule. It is perhaps pertinent to recall that some Mn" species are very efficient catalysts for the decomposition of hydrogen peroxide (Section 41.3.3.7).
41.3.5.2
Oxyanwns
In this section we deal with the oxyanions of the non-metals, which usually have well-defined polyhedra, in contrast to the mixed oxides of Section 41.3.5.1(i). Such distinction is, of course, not always sharp.
( i ) Borates, carbonates and silicates
A variety of crystalline borates of Mn" has been characterized, and structural data are available for many%*(Table 22). These show that the dominant Mn polyhedron is the octahedron, although two exceptions have been identified, derived from distortion of tetrahedra: in LiMnBO,, the Mn atoms are displaced towards a fifth oxygen and are at the centres of [MnO,] polyhedra; and in MnB40,, the MnO, tetrahedra have four other oxygens along normals to the faces, making an effectively [MnO,] polyhedron. The octahedron also is adopted in the known carbonatesz4' (Table 22). MnCO,, although thermodynamically unstable in O2 or air, decomposes only slowly, and the pink, almost white, compound occurs naturally as the mineral rhodochrosite. Only one crystalline form is known which has the calcite structure; and both CaMn(CO,), and BaMn(CO,)p have the dolomite structure. The carbonate mineral sidorenkite (Na,MnCO,PO,) has a bidentate carbonate group, which is somewhat unusual in inorganic structures. However, there is distinctly more complexity in the range of known compounds, and in the observed coordination polyhedra, in the crystalline silicates.250There are only two basic silicates: Mn,Si04, which occurs naturally (tephroite); and MnSiO, the metasilicate, which also occurs
42
Manganese Table 22 The Coordination Polyhedra of Crystalline Mn" Borates and Carbonates
Compound
Coordination PoIyhedra with (av.j Bond Lengths (A)
MnB,O,
[MnO,,] n = &8 (2.442-3.188)
MnB,04 MnB, 0, Mn,(BO, jz
wo,i (i) [MnO,] (2.184) (ii) [MnO,] (2.214)
Comments
Distorted tetrahedra with four other oxygens on normal to faces. Isomorphous with Fe and Mg. Isomorphous with known Co. Jimboite.
Isomorphous with Fe and Co.
MnB,0,,*8H20 MnB,0,.9H20 MnB,O,- 3H,O LiMnBO, Mn,(OHhtB(OH)d'O4 Pinakiolites CaMnB, 0 , - H 2 0 MnCO, CaMn(C0, ), Na, MnCO, PO,
[MnO,] 2.15-2.22 [MnO,l (i) [MnO,] 2.135 (ii) [MnO,) 2.156 (i) [MnO,] 2.204 (ii) [MnO,] 2.203 [M~O,I (it(iii) [MnO,] 2.2 1 [MnO,] 2.196 [MnO,l [MnO,] 2.092-2.350
Sussexite-chains of edge-linked octahedra, H.c.p. oxygens; Mn in tetrahedral holes, but displaoed towards fifth oxygen. Very distorted, edge-sharing a bidentaie BO, unit; Mn-0 up to 2.41 A (Seamanite). A family of Mn"/Mn"' borate minerals. Roweite. Calcite structure. Dolomite structure. Sidorenkite- bidentate CO,.
Table 23 Structural Data for Crystalline Mn" Silicates Compound
Phase. Mineral
I (Rhodonite)
MnSiO,
11 (F'yroxmanganite)
Mn, SiO, Mn"/Mn'" mineral
Mn"/Mn"' mineral Mn,(OH, F ) , W , ) , Mn, (OH), (SiOJ, Na,Mn,Si,O,
IV Garnet-type, tetragonal p-form Tephroite Ungbanite
Braunite Allegheneyite Leucophoenicite
Na, Mn(Si0, jz Na, (Mn, Na),MnSi,O,, (Li, Na)Mn4Si,0,,0H
Narnbulite
K, Mn, Si,, 0,. H20 CaMnSi,Oq (ca, Mn)& 4 CaMn, OH(SiJ 0,,OH). H, 0 Ca2Mn,(OH),Si,,0,,.5H,0 CaMn,(BeSiO,), Zn, Mn(OH), SiO, K,Zn,Mn,(SiO,), Si207 Na, InOH{SiO,(OH)}, PbMn(Si,O,), 13Mn0 Sb, 0,-2A1,0, 2Si0, Mn,(BeSiO,), S
Johannsenite Bustamite Inesite Hodgkinsonite
Catoptrite Kelvite
M n Pobhedrn und Bond Lengths (av. in
A)
(ij(iii) [MnO,] 2.219; 2.242; 2.223 (iv) [MnO,] 6 x -2.2; 1 x 2.778 (v) [MnO,] 3 x 2.743; 4 x 2.139 (iHiv) [MnO,] 2.2 (v) WnO,] 3 x 2.743; 4 x 2.141 (vi) [Mn07] 6 x 2.2; 1 x 2.796 (vii) [MnO,] 3 x 2.696; 4 x 2.144 (i) [MnO,] dodecahedral (ii) [MnO,] [Mn061
[~no,i
(i) [MnO,] 2 x 2.25; 4 x 2.34 (ii) [MnOJ TBP 2 x 2.23; 1.92; 1.96; 2.07 (iii) [MnOB]distorted rhombohedron 2 x 2.22; 6 x 2.46 (i) [MnO,] 2.33 [MnO,]( x 3 ) 2.221; 2.199; 2.215 [MnO,](x 4) 2.203; 2.258; 2.217; 2.206 (ij [MnO,] 2.0CL2.10 (ii) [MnOJ 2.03-2.42 [MnO,] ( x 2) 2.10; 2.33 [(Mn,Na)O,] ( x 3) 2.39; 2.27; 2.40b WnQ] 1.962.24 [MnO,] ( X 3) 2.233; 2.242; 2.188 [MnO,] 3 x 2.752; 4 x 2.145 [MnO,] (these polyhedra share an edge) [MnO,l [MnO,] 2.17 [MnO,] ( x 2) 2.245; 2.203 [MnO,] ( x 3) 2.204; 2.199; 2.205 [MnO,l( x 4) [MnO,] 2.1 13-2.460; 2.133-2.513 [MnO,] 2.222 [MnO,] ( x 2) SP 1.91-2.22 [MnO,] TP ( x 2) 2.15-2.53 [MnO,] 2.26 [MnO,] ( x 3) 2.22; 2.24; 2.19 [MnO,S] 2.082
~~
Unless otherwise stated [MnO,] polyhedra are octahedral, albeit often somewhat distorted; and [MnO,] polyhedra are tetrahedral. The more 'flexible' [MnO,] polyhedra are not defined, and the [MnO,] polyhedra only when they are very close to the extremes of lngonal bipyramid (TBP)or square pyramid (SP). TP = trigonal prism. Quoted bond lengths are usually averdges, bul hyphenu, in same cams, indicate the range. Mn" and Na' share these sites, so the (averaged) dibtdnm okwerved experimentally probably overestimate the size of the hole in which the Mn" is located.
43
Manganese
naturally in several forms (Table 23). Both were described by Gorgeu in 1883. The former is known as a single phase, and is the end member of the olivine group of silicates. Its structure is simply based on hexagonal close packing of oxygens, with Mn" in half the octahedral holes, and Si'" in one-eighth of the tetrahedral holes. By contrast, the metasilicate has been characterised in five distinct phases, some of which are known only in minerals and appear to require the presence of small amounts of other metals for stability of the phase. Available structural details for these silicates, and for the wide range of other known species containing Mn" and silicate, are summarized in Table 23. Coordination numbers four (tetrahedral), five (both square pyramid and trigonal bipyramid and 'in-between' structures), six (mainly octahedral and distorted, but including one trigonal prism), seven (of various shapes) and eight (dodecahedra and distorted rhombohedra) all are defined; and there is evidence for three-coplanar in a zeolite: the hydrated form is five-coordinate (3 x 2.288, and 2 x 2.05A) but, on dehydration of this Mn-exchanged zeolite, the solid contains planar three-coordinate Mn" with Mn-0 = 2.1 1
(ii)
Monocarboxylates
(Monocarboxylates with other, different, functional groups, such as salicylate and picolinate, are discussed in other sections.) This is one of the few areas of Mn" chemistry for which a significant amount of structural data is available. Yet, the only pattern to emerge is one of variety of structure, mainly polymeric, and the absence of any hard evidence for a dimeric tetra-bridged species of the copper(I1) acetate structure. There is a distinct impression, when first following through the literature, of encountering with almost each new structure some unexpected, novel feature, and the data certainly underline the sheer futility of attempting to define structure of crystalline solids by other than X-ray methods. Two groups have been most prolific; Kennard's have been looking mainly at the structures of herbicide carboxylates, and Ciunik and G l o ~ i a kthe ~ ~amino ~ acid compounds, generally MnX,(AA),.nH,O. These latter all are bonded in zwitterion form, so that they are generally examples of substituted Mn" carboxylate hydrates. The rest of the literature is accessible through the latest publications of these g r o ~ p sand ~ ~G~m. e~l ~i ~~~ . ~ ~ ~ The majority of the Mn" carboxylates and their hydrates are robust in the air, but some, for example Mn(ButCOO)I, react readily forming red-brown trinuclear Mn"' species. It is with the same carboxylate, that so-far-unsubstantiated claims have been made for dinuclear species,2sssuch as [Mn,(B~'C00)~py,~* 2Bu'COOH. Both formate and acetate have given compounds M2Mn(RCOO),*nH10 (M = Na, K , NH4).25a AIthough structures are yet unknown, it is tempting to draw parallels with the tetrakis-nitrito and nitrato-manganate" species (Section 41.3.5.2.iii). Table 24 contains data on approximately half the known structures, and a similar story emerges from the amino acid Monomers are known and there is the first example of a transition metal conipound [M(RCOO)(W20),]RC00;but most structures involve carboxylate bridging of three basic types (82)-(84}, although one structure has cis-unidentate carboxylates and chooses to polymerize on a single bridging water molecule. The Mn polyhedron in the tetrahydrate for R = (89) is the only one to depart from the pattern of [MnO,] octahedra. It has trans-H,O molecules at relatively normal distances of 2.28 and 2.31(2) A,and in the plane normal to these are three Table 24 Representative Structures of Carboxylate Compounds Mn(RC0;)2*nH20252
R
n
Mn Polyhedra
H
2
Me
4
Et
2 2 2
mfnO,(H,O),I [MnOsl c-[MnO4(H20)2l [MnO,l t-WnO4(H20)21 t-[Mn04(H20)21 t-[MnO4(H20 1 2 1 f-[MnO2(H20)4l [MnO(H,O),I+ ~-[MnOdH,O),I c-[MnO,(H20)41 c-[Mn0,(H20)41 W n O 2 (H20 ) 4 l
(87) (88) (8911 I1 I11 (90)
(91) (92)
4"
5 4 4 5 4
'Full formula is [Mn(RCO,), (H2O),]*2RCO2H
Bridgo Type
Comments (Mn--0
Bond Lengths in
A)
(82)
Polymer (linear) (2.14-2.22)
(821, (84)
Polymer (chain) (2.15-2.26) (2.16-2.23) Polymer (chain) (2.125-2.369) Polymer (two-dimensional) (2.152.22)
(W, (W, (85) (82) (82)
(W (86) (85)
Monomer An unusual [MnX(H20),]X species Dimer (see text) (2.28-2.46) Polymer (chain) (2.11-2.29) Polymer (linear) Monomer (2.19-2.22)
Manganese
44
carboxylate oxygens (at 2.35-2.46(2) A and O-Mn-0 angles of 80 and SS'), an ether 0 (at 2.96 A) and a ring C1 (at 3.07 A). It is pertinent to add that, in at least one case, the phenoxy ether oxygen bonds more closely to Cu" but not at all to Mn".
Tr
Mn
Mn
(iii) Nitrites and nitrates
Known details of the nitrites are largely confined to three papers.u637 Anhydrous Mn(NOJ, is stable below 40°C,and the pale yellow compounds M,[Mn(NO,),j, including the Cs salt and the yellow Cs,Mn(NO,), require storage under mother liquor at 0 "C.The tetrakisnitrito species appears to have an [MnO,] polyhedron with all bidentate nitrites and is closely paralleled by other larger M" species, i.c. those of Ca, Sr, Ba, Zn, Cd and Hg. A few mixed ligand compounds, with various neutral ligands, have keen described but none give acceptable evidence for an N-bonded nitro. Anhydrous manganese(I1) nitrate is readily obtainable as pale red crystals, and like the hydrates, is hygroscopic.'" Hydrated Mn(NO,),, apparently largely as the tetrahydrate, is used commercially to colour porcelain and as an intermediate in the preparation of specific forms of MnO,. There are four well-characterized hydrates Mn(NO3),*nH20n = 6, 4, 2 and 1. The former appears to be another [Mn(H,O),]X, species, but X-ray structural analyses of the others give examples of the modes of bonding of nitrate. Mn(NO,), .4H20, isostructura1259with Zn and Ni, is a monomeric cis-[Mn(NO,),(H,O),] octahedral species, with unidentate nitrates (Mn-0 = 2.13-2.19 8);the dihydrate is isostmctural with Zn?@ and has trans-[MnO,(H,O),] octahedra (Mn-0 = 2.05-2.14 A), with nitrates bridging as in (82); and the monohydrate has both [MnO,] and [MnO,] polyhedra:26'the former are eis-[MnO,(H,O),] octahedra (Mn-0 = 2.20 A) and the latter are approximately dodecahedral, with four bidentate nitrates (Mn-0 = 2.31-2.37 A). The same eight-coordinate polyhedra are found in the compounds M2[Mn(N03),] (M = Na, R,
45
Manganese
Ph,MeAs and Ph4A~)262 -the crystal structure of the latter showing two crystallographically distinct nitrates, one of which has Mn-0 distances of 2.25 and 2.29A and the other 2.29 and 2.40A. Nitrate groups also are found in various mixed ligand species with the other neutral ligands, and one of the few species to have been explored crystallographically is [Mn(N0,)2(N-N),],124 which also has bidentate nitrato groups (Section 41.3.3.2).
(iv) Oxyanions of P and As
The oxyanions of phosphorus and arsenic take up the major portion of a recent large volume of Gmelin (C9),which includes more than 100 compounds, hydrates and phases containing phosphate To pursue the detail of the latter is an exercise in phosphorus chemistry, so we concentrate on a survey of the available data on the Mn" coordination polyhedra. Mn" phosphates find significant use in industry, including MnHPO, - 3 H 2 0 for the preparation of luminescent materials, and Mn(H2P04),-2.5H,O in fertilizers and especially for manganese phosphate coatings on metals to improve wear resistance. The pyrophosphate Mn,P,O, is a useful calibrant for magnetic measurements. There appears to be but one referencei79to a phosphinite Mn(Ph,PO),, although phosphinates are receiving attention: the compounds Mn(R2P02)2(R = Bun, But, pentyl and octyl) are polymeric264 and probably octahedral; and structural data are available on five compounds Mn(R2P02)2*L, showing close parallels with the carboxylate^,^^^,^^^ including zwitterion species.'& PO, bridging like (82) and (84) occurs in these. Only one phosphonate, the rather insoluble MnHPO, *H,O, appears to have been described, and there are six known palyphosphonates, ranging up to Mn,H,,P702, -4H,O. A fluomphosphate Mn(P0,F) .nH,O and several chlorophosphates Mn(Cl,PO,), * n Lalso are c h a r a c t e r i ~ e d . ~ ~ ~ * ~ ~ ~ The remainder of the P oxyanion compounds, hydrates and phases are phosphates of various types, including pyro- and meta-phosphates, and polymers. Of the > 100 described species, including such mixed ligand species as Mn2ClP0, and Mn,(OH),(PO,),, X-ray anaIyses are available on twenty-two and all but one { F-Mn3(P0,)2}have [MnO,] octahedra with Mn-0 generally averaging between 2.18 and 2.24A. This predominance of the octahedron probably derives in significant part from the general compatibility of Mn in octahedral holes and P in tetrahedral holes of close-packed oxygen. This 'compatibility', however, is seldom perfect for most of the polyhedra are distorted: in one casex8 bond length variations of 2.08-2.59w occur in the one octahedron and angular distortions of 10" are seen. Such distortion reaches a peak among known structures in F-Mn,(P0,)2, the nine independent atoms of which all have five nearest neighbours (range = 2.03-2.36A; average = 2.13-2.19A) and distinctly longer bonds to a sixth oxygen, that may be as close as 2.39A in the more regular octahedra to > 3.0A in the least.269 Table 25 Mn" Arsenate Species with Non-octahedral Coordination Polyhedra Compound Mineral
Mn, (OH)AsO., Sarkinite Mn, (OH)AsO, Eveite Mn"Mn: Sr,As,O; Mn"Mn: Ba, As,O, (Mn, Mg, Cu)dOH, C1)(As03), Magnussonite (cubic)
Mn,,[As,(OH),O,,I(As,O,,),CO, Armagite Mn, [AsSi, O,,(OH)] Tiragalloite Mn,oSbzAs2Si, 0% Parwelite
Coordination Polyhedra
[MnO,l ( x 4) [MnO,] Octahedron ( x [ M n o J ( x 4) [MnO,] Octahedron ( x [MnO,] Square plane [MnO,] Square plane [MnO,] Cube [MnO,] Octahedron [MnO,] Trigonal prism [MnO,] Square plane [MnO,] Octahedron ( x [MnO,] Trigonal prism [MnO,] Octahedron ( x [MnO5+2 1 [MnO,] Cube [MnO,] Octahedron [MnOJ [MnO,] Tetrahedron
'Related Sb and Bi compounds have the same structure.
4)
4)
4) 3)
Bond Lengths (A) Average
2.129 2.212 2.12 2.19 2.078 2.124 2.42 2.19 2.24 2.10 2.217 2.237 2.222 2.198 ( x 5), 2.625, 2.974 2.42 2.21 2.09 and 2.12 2.07
Range
2.065-2.223(6) 2.149-22.344
2.099-2.366 2.102-2.366 2.17-2.24
46
Manganese
The arsenate polyhedra, on the other hand, show distinctly more variability; of the > 60 species described:' many of which are minerals, structural analyses are available for one-third and, of these, one-third appear to have non-octahedral Mn polyhedra (Table 25). (The proviso results from a reportz6' that the earlier analysis, describing arsenoclasite as containing [MnO,], [MnO,] and [MnO,] polyhedra, was in error, and that there are only [MnO,] polyhedra -albeit distorted.) One other interesting feature occurs in mixed Mn, Zn species: where there is a choice of octahedral and tetrahedral holes, Mn occupies the former and Zn the latter. Undoubtedly the size difference is the major, if not only, cause.
(v) Oxyunions of S , Se and Te
Robust manganese(I1) sulfznates and sulfonates undoubtedly can exist, but there is very little detail on them in the literature. Lindner and co-workersZ7'claimed evidence for both 0 , O - and S-bonded forms of PhSO;, but the latter seems very unlikely. The binary sulfite MnSO, and five distinct hydrates have been defined,272-274 and the structures of the two different forms of the trihydrate are available. They show .fat-[MnO,(H,O),] and cis[MnO,(H,O),] octahedra. Various compounds M,Mn(SO,),, KzMn2(S0,), and Na2Mq(S0& were described by Berklund in 1879 and Gorgeu in 1883.272They are unlikely to represent discrete [Mn(S0,),12p anions. The suIfate MnSO, and its hydrates (7, 5, 4, 2 and 1) represent the stable oxidation state under ambient conditions. Compounds are colourless in finely-divided form and pale pink or rose coloured when crystalline. All appear to be examples of high spin octahedral Md'. Aqueous solutions have been much studied, to define the physical parameters, because of their importance in Mn production from its ores. Stability constants for 1:1, 1:2 and 1:3 species have been defined but detail (structurally) of the labile Mn polyhedra is unclear. Examples of unidentate and bridging sulfato are seen in the known crystal structures, but not bidentate or terdentate to the same metal atom. (However, bidentate function may occur in some of the mixed-ligand compounds described earlier.) In all, some 80 compounds, phases and hydrates are listed in a 1976 volume of GmeZin (C6), including a few thiosulfates and polythionates. Of these, structural data are available only on ea. IO%, but they refer to at least one third of the types, and all show [MnO,] octahedra, often containing water molecules as well as oxygens of the sulfates. The octahedra [MnO,(H,O),] are described for x = 6, 5, (c- and t-) 4 and 2, and the mineral mooreite (Mg,Zn4Mn,(S0,),(OH),,-8HZO}275 has [Mn(OH),(H,O),] octahedra (and, incidentally, [Zn(OH),] tetrahedra as well}. but they are unlikely to represent Species like KzMn(S04)Zand KzMnz(S0,)3 are discrete 'complex' Mn anions. The 'complexity' of the solids will be defined primarily by close packing of the sulfates, with charge neutralization achieved by MnZt and other cations in appropriate 'holes'. Of the 20 known selenite and selenate species,272structures are now available on more than one-third, and all show [MnO,] 0ctahedra.2~~ Manganese(I1) selenite (MnSeO,) has the perovskite (CaTiO,) structure, with each [MnO,] octahedron provided by three SeO, roups (Mn-0 = 2.235 A); the monohydrate has [MnO,(HzO)] octahedra (Mn-0 = 2.14-2.31 ); and the dihydrate has cis-[MnO,(H,O),] octahedra (2.142.29 A). This system also gives an example of an unexpected redox reaction: hot solutions of Mn" and H2Se0,, and various other reaction routes, give orange-red crystals of Mn(SeO,)?, which is apparently another Mdvcompound, like MnO,, stabilized by its insolubility. There is an interesting structural parallel with the insoluble, white iodate Mn(TO,),. MnSe,O,, MnSeO, and their hydrates also are known, as well as species like Na, Mn(SeO,), . Fewer tellurites and tellurates are known:272MnTeO, is isostructural with the Se analogue, and tellurates are represented by species like Mn,TeO, -no MnTeO, species appear to have been described.
1
( v i ) Oxyanions of the halogens
Very few binary compounds of this class have been described, but C10; is widely used as a counter ion for the preparation of many of the cationic species of the other neutral ligands, despite the instability that gives some risk of explosion! For many years, it was regarded as the standard for 'non-coordination' -especially in aqueous solution -and both the chlorate and perchlorate crystallized from aqueous solution as [Mn(H20),]Xz. However, the anhydrous Mn(C10,)2 can be
Munganese
47
Table 26 Isopoly- and Heteropolyanion Compounds of Manganese Compoumd Type and Structural Data
Gmelin ref.
Isopolyvanadates Tetravanadomanganates(1V) 1 l-Vanadomanganate(1V 13-Vanadomanganate(IV) 12-Niobomanganate(IV); [MnO,] (1.87A) 6-Molybdomanganate(Ii) 9-Molyhdomanganate(lV); [MnO,] (1.90 A)" 1 2-Molybdornanganate(IV) Mn" 1I-molybdogermanate Mn" 12-molybdosilicate Triheteropoly-rnolybdomanganosilicatesof Mn" and Mn"' Mn" 1I-molybdophosphate Mn" 12-molybdophosphate; isomorphous Mg, Ca Triheteropoly-molybdomanganophosphates of Mn" and Mn"' 1 1-Tungstornanganate(IlII) 5-Tungstomanganate(IV) 6-Tungstomanganate(IV); [MnO,] (1.94 A)b 12-Tungstomanganate(IV) Heteropoly species IMMnW,, 0,J- (M = Zn, Ga, Ge) of Mn" and Mn"' Mn" 12-tungstosilicales Triheteropoly-tungstomanganosdicatesof Mn" and Mn"' Tungstomanganophosphates (PMnW,, and P1MWI7)of Mn" and Mn"' Tungstomanganoarscnates of Mn" and Mn"'
C3, 162 C3, 164 C3, 165 C3, 166 C3, 173, 174 C3,209 c3,210 C3,211 c3,212 C8, 331 C8, 331 C9,208 C9, 209 C9, 209 C3, 223 C3. 224 C3, 225 C3,225 C8, 332 CR, 332 c9,210 C9, 363
"R.Allmann and H. d'Amour, Z. Kristallogr., 1975, 141, 342; T. J . R. Weakley, J . Less-Common Metalr, 1977, 54.289 bV. S . Sergienko, V. N. MaEanov, M. A. Porai-KoSic and E. A. Torknkova. Koord. Khim., 1979, 5,936.
prepared,277and both the di-and tetra-hydrates have been defined. Beginning with Mn(ClO,),, other anhydrous metal perchlorates,277and some copper(I1) and nickel(I1) compounds,278perchlorato coordination has been increasingly recognized in the literature, and one of the first to be defined crystallographically was the arsine oxide specie^"^ [MnClO,(Ph, AsO),]ClO,. Other perchlorato species have been noticed in the various sections on neutral ligands. Two anhydrous iodates also are known279and both were first reported by Rammelsberg in the 19th century. The insoluble Mn(IO,), precipitates from aqueous mixture of Mn" and TO,. It is relatively robust in the short-term, but on long keeping releases 12. The periodate Mn,(IO,),, however, is stable only below 15°C.
(vii) Zsopoly- and heteropolyanions of V , Nb, Mo and W
Anions of this type containing Mn (11, I11 and IV) are listed in Table 26. Most of these have been isolated as crystalline solids. There is a tendency for stability of the higher oxidation states (I11 and IV), but a number of Mn" species are defined,"o~28' the most robust of which appear to be the triheteroporyanion tungstomanganophosphates [PMnW, 019(OH2)]5-and [P,MnW,,O,, (OH,)]'-. Bath are readily oxidized to Mn"'. The Mn atoms are high spin and magnetically dilute; and the anians are yellow-brown because of a metal-ligand charge-transfer band in the near UV. A cation exchange resin, in the H+ form, completety removes Mn".
,
41.3.5.3
Bidentafe ligands
( i ) Diethers, diols and diphenols
The ether compounds have been little studied: only the 1,Zdimethoxyethane species MnLX, (X = CI, Br), [MnL2(H,0),] (CIO,), and [MnL,][SbCl,], appear to have been described,'" and all may contain ether chelates. Dioxane, on the other hand, acts as a bridging group in quite robust polymers (Section 41.3.5.1.iii). Diol compounds are known, both of the neutral aliphatic ligands and in their deprotonated (alkoxide) fonn,2'2but generally have been studied for their reaction with O2and other oxidizing
Manganese
48
agents, to form Mn"' and Mn'" species. Ethane-1,2-diol species [MnX,L,] (X = Cl, Br, NO,, +SO,) are characterized by an X-ray analysis of the chloro compound, showing an ~ n C l , O , ]octahedron. The alkoxide Mn(L-2H) was studied for its catalytic activity, but not characterized structurally. Various poiyols, such as mannitol, inositol, etc., appear also to act as diols, forming similar chelate compounds.282These have been studied mainly for their reaction with 0, to give Mn"' and Mn" species, and little is known of the structures they adopt. Catechol, with its more readily ionized protons (and other o-diphenoh), in aqueous alkaline solution forms soluble, colourless anionic species. Stability constants for the 1: 1 and 2:l species have been measured (log K,= 7.5, log K2= 5.7) and the 3:l species forms at pH > 11. In 0.3 M NaOH, the latter is readily oxidized by ferricyanide to Mn"'. Some crystalline species have been described, but need further characterization and most seem to be very air Again the interest in these compounds has been directed mainly towards their oxidation to Mn"' and Mn'" species.
(ii)
Diketones and related ligands
Significant data are available only on the x- and fl-diketones and structurally related ligands.2s3 A single tris-chelate cationic species of dimethylphthalate appears to be the sole representative of the y-diketone type. Neutral a-diketones are represented only in two crystalline quinone species MnBr, L of unknown structure, but there is a litle more detail available on the anions of hydroxyquinones (93b(97),the related tropolone (98) and the dianions of the cyclic carbonyls (99) for n = 2, 3 and 4.The latter give polymeric hydrates based on [MnO,] octahedra.284The hydroxyquinonate anions (93)-(97) have all given crystalline species Mn(0, O),, one of which (97)has been shown to be tetrarne1ic.2~~ T h s compound was prepared from Mn,(CO),, and the hydroxyquinone in toluene; reaction in pyridine gave [Mn(O,O),py2]-2py. The anion (94) has also been reported to give a solid MnClf0,O) -4EtOH.
-
(93) R (94)
=
H
(95)
R = Me
0-
0-
0-
0-
The tropolone compound Mn(O,O),, where ( 0 , O ) = (98), was prepared from the ligand and Mn(OAc), in aqueous methanol, and, after drying, was sublimed. Undoubtedly, it is polymeric, but the details are unknown. Acetylacetonate is usually considered the reference ligand for the p-diketonates and it has been comprehensively studied. Neutral ligand compounds are known: acacH acts as a unidentate ligand A related ligand on an (MnBr,), linear polymer, like (S), and it chelates in [Mn(a~acH),]~nCl,],.~*~ diamidomalonate has been proven to chelate in [Mn(NO,),LJ, which has bidentate amide and unidentate But most of the chemistry of acetyiacetone and related ligands concerns their behaviour as monoanions, bonding essentially in the keto-enol form (100). The types of ligands of this general
(103)
(104)
R = H R = Me
(105)
class, for which Mn" compounds have been described, are summarized in Table 27, together with the structural types of the Mn compounds. The binary compounds Mn(O,O), are probably mostly, if not all, octahedra1 polymers3' like the known Ni" and Co" species, but the detail is unknown beyond a report that Mn(acac), is trimeric in benzene. However, for most of the other structural forms, X-ray analyses are available of proof of type: [Mn(acac),(H,O),], [Mn(dbm),(THF),]-2THF (dbm = (100) for R = R = Ph; R = H},288[Mn(acac),phen], [Mn,(acac),(allylNH,),] and NH,[Mn(O,O),] ((0,O) = (100) for R = CF,; R' = H; R" = f~ryl}.*'~All show Mn octahedra and there is no compelling evidence in any of the literature for alternative polyhedra in related compounds. Most are pale-yellow crystalline products and are robust in the air. Mn(acac), and its hydrate are sensitive to oxidation, as are many of the others, when wet (that is in equilibrium with a solution) but are reasonably robust when dried. Yet other compounds are distinctly less 0, sensitive. The dihydrate [Mn(aca~>,(H~O)~] readily loses water, and the anhydrous species can be sublimed at low pressures.290Mn(acac), has been used as a catalyst for a variety of chemicaI reactions, including polymerizations.
(iii ) Dicarboxy lates and hydroxycarboxylates The dicarboxylates have been separated from the compounds of Section 41.3.5.2(ii) because of the possibility of the formation of chelate rings (106). Such chelation, however, is by no means universal in these compounds. The 1:1, 1:2 and 1:3 species, which have been shown to exist in aqueous
I Manganese
50
Table 28 Mn" Oxalato Compounds" Structural Detail
Ref.
trans-[MnO,(H,O),], type (107) bridging
b
K trms-[MnO,(H,O),j, (106) and (1W) [MnO,(H,O)], (107) and (108)
c
Compound
d
Gmelin (ref. 3), 1980, D2,85. R.Deyneux, C. Berro and A. Pdneloux, Bull. SOC.Chim. Fr., 1973,25. H.Siems and J. Lohn. 2. Anorg. Allg. Chrm., 1972, 393, 97. dH.Schulz, Acta Crysrallogr., Sect. B, 1974, 30,1318. a
solutions and which, for oxalate, have log K's of approximately 4.5,2 and 1.5 respectively, probably all have chelated ligands.293Certainly the 1:2 and 1:3 species can be oxidized to the chelated Mn"' species. However, most of the solid compounds (see Table 28 for the oxalate species, which have been most studied) are polymers, with the dicarboxylates currently known to bridge in the forms (107)-(110).
Malonate and the dianions of the other aliphatic dicarboxylic acids H 4 C(CH,),CO,H (n = 1-8) have been isolated as crystalline hydrates, and some of these have been dehydrated.293X-Ray structural analyses have been reported for the malonate dihydrate," which has trans H, Os and bridging malonates of type (110), and for the succinate tetrahydrate and adipate dihydrate:29' the succinate reputedly has a trans-[MnO,(H, 0)4] polyhedron with bridging &carboxylate, presumably as in (109). The monoanion of maleic acid (111) is only unidentate in crystalline [Mn(H-maleate),(H,0)4];296 but the equivalent compound of phthalic acid is a bis-chelated octahedral [Mn(O,O)z(H,O),] species.297Clearly other interactions in the solids, as well as the bonding interactions with Mn", define structure in the carboxylate compounds. All of these species are relatively robust in the air, even in solution; but, especially when the pH is raised, they can be readily oxidized to Mn"'.
Manganese
51
Another interesting structural variation has recently been reported298as occurring in some mixed metal compounds of dithiooxalate, Le. MMn(S,C,O,), -7.5 H 2 0 (for M = Ni and Cu). These have the linear polymer structure (112), with seven-coordinate Mn".
- ~ 'the ~ The hydroxymrboxylates are known in a range of crystalline binary Mn" h y d r a t e ~ , ' ~ ~but only ternary compounds are a 5NO,-salicylate, which is probably Na,[MnL,(H,O),], and the Na and K tartrates M,MnC,H,O, ~ 6 H ~ 0Both . ' ~ the aliphatic and aromatic (only salicylates) hydroxycarboxylates form 1:1 and 1:2 species in aqueous solution, which are of relatively low stability, and much of the interest to date has been in their oxidations to Mn"' and MnIV- such oxidations being readily achieved in alkaline solutions, especially as the number of hydroxy groups on the ligand is increased. Table 29 contains the known structural All are octahedral [MnO,] species and a common feature is the chelate ring (113). Several of the compounds are cis-[Mn(O,O),(H,O),] monomers, but the more complex acids {(113) for R containing OH andlor CO,H groups} form acts as a terdentate ligand to one Mn and polymers. For example, citrate in Mn,(C6H507)2-9HZ0 bridges to two others: there are [Mn(H,0),]2+ and [MnO,(N,O)] polyhedra.
R
Io>.
'0
(iv)
H
Q
Other ligands
A range of bidentate 0 donor ligands, based on nitrogen oxides, have been reacted with Mn". Although several compounds of ligands (114) and (115) in their monoanion form have been described,304they are not fully characterized as Mn" species, and the reported dark colours suggest the oxidation may have occurred during the preparations. Compounds of the other potentially anionic bidentates (116H119) are somewhat better characterized, although (116) appears to have been detected so far in only neutral unidentate function.305One compound of ligand (117) has been shown to be a trans-[Mn(O,O),(H,O),] octahedral species,3o6and the ligands (118) and (119) form rather insoluble Mn(L-H), species, which are probably polymeric with [MnO,] o ~ t a h e d r a . ~ ~ ' , ~ ~ ~ The orange tris-chelated compounds of 2,2'-bipyridyl-N,N'-dioxide (120) are very sensitive to light and to oxygen: the most robust, the [PtC14j2'-salt, has been described as being stable in the light for one hour. Like the [Mn(terpyO3),I2+cations, they can be oxidized in CH,CN to Mn"' and to Mn" (Section 41.3.5.4). Also described, have been [Mn(bipyO,),(NCS),] and Mn(bipyO,)CI,, which appear to be somewhat more robust.309 Table 29 The Mn" a-Hydroxycarboxylates of Known Structure Compound
Mn(glycollate), -2H, 0 Mn(L-lactate),. 2H,O Mn(malate).3H,O Mn,(citratc),+9H, 0 Mn(~-gluconate)~-2H~ 0 Mn(H-salicylate),. 2H,O
C.O.C. 4--c
Coordination Polyhedra
cis-wn(O,O), (H,O),] monomers cis-lJfn(O,O), (H,O),] monomers cis-[Mn(O,O)O, (H,O),] polymer [Mn(H2O),l2+and [Mn(0,0,0)02(Hz0)] polymer cis-[Mn(O,O)O, (H,O),] polymer [MnOJ
Re$ 30 1 30 1 302 300 300 303
52
Manganese
(119)
A few tris-bidentate di-sulfoxides also are known,310and a bidentate phosphate ester3' and amide3". Two compounds of sulfoacetic acid ( H 03SCH,CO,H) were described by Hahn and Wolf in 1925, but there appears to have been no detailed study since on this ligand, or any other alkylsulfonic acid with Mn".
41.3.5.4
Midtidentate ligands
For the cyclic, or crown, ethers, see Section 41.3.10.4. There appears to have been no systematic study of multidentate all-oxygen ligand compounds of Mn. Two well-defined neutral terdentates terpyridyl-N,N',N"-trioxideand the phosphoramide (121) both form MnX, L and [MnLJX, species. For terpyridyltrioxide, the bright orange crystalline compounds MnCl,(terpyO,). 3H, 0 and [Mn(terpy03),](C10,), -2H, 0 are light sensitive, but the latter forms stable solutions in CH3CN. Oxidation to both Mn"' and Mn'" has been achieved electrochemically in reversible one-electron steps at half-wave potentials E,,, = 1.06 and 1.77V (versus SCE).314 The phosphorarnide (121) also forms 1:l and 1:2 crystalline NMe2
h i o a k terdentates are certainly represented in a crystalline citrate (Section 41.3.5.3(iii)), and probably in some compounds of glycerol {CH,(OH)CH(OH)CH,OH} and higher polyols which have been studied mainly for their oxidations to Mn"' and Mn" species?I6 Quadridentale ligands appear to be represented solely by the ligands (122) and (123), which form eight-coordinate bis-chelate [MnL,]'+ cationic specie^.^" The related [Mn(N,),]'+ compounds (Section 41.3.3.qi)) have not yet been structurally characterized. And there is a recent example of a sexadentate tris(j3-diketone) ligand (124). Perhaps not surprisingly the trianion of this ligand gives a very robust Mn"' neutral moIecule, which can be reduced to Mn"; and this latter species also is kinetically stable to ligand exchange.318
Manganese
53
Table 30 The Coordination Polyhedra of the Binary, Ternary and Quaternary Mn" Sulfides
Tetrahedra
j?-MnS (Zinc blende) y-MnS (Wurtzite) Cs, Mn, S, BaMnS, Ba,MnS, BaMnS, Sn A12MnS4(I)(ZnlnqS4tYp.1 AI2MnS, (TI) (Cubic spinel) Ga2.,7Mn,.75S4 MnYb,S, Cu,MnS,Sn MnCr, S, MnIn, S,
Bond Lengths
2.41-2.54 2.38-2.41 2.39-2.42 2.38-2.51 2.36 2.39
(A)"
Octahedra
I-MnS MnS, Ga,MnS,(MgGa,S, type) Mn,,~a,,f,,~f&l MnM,AlS, MnM3GaS, Mn, SnS, Mn, GeS, Mno.,,NbS, La,MnSi, S,, MnAg,S,Sb, MnIn,S, (35% Inverse spinel) MnV2S4 (Fe,S4 type)
Bond Lengths
(A)
2.612 2.59 2.38-2.86 2.62 2.6CL2.62 2.52-2.15 2.64-2.61 2.60-2.63
length data usually refer to the observed range. Where single values appear, these are averaged values. For phases without bond length data, inclusion in the table relies on X-ray powder data showing isomorphism with known species.
a Bond
41.3.6 Sulfur Ligands Manganese sulfur chemistry has been much neglected. Most detail is known of the simple solids, Le. the bi-, ter- and quaternary sulfides, which so far have been generally neglected by the coordina-
tion chemist; something is understood of the 1,l-dithiolates, such as the dithiocarbamates, and of the 1,Zdithiolates or dithiolenes; but only now are results being reported of a systematic study of the thiol compounds. All Mn" compounds are high spin d 5 species, with moments significantly below 5.9 BM being observed only where structures allow antiferromagnetism.
41.3.6.1
Su@des and thiolates
Structural data are available (Table 30) for a range of binary, ternary and quaternary sulfides of manganese, almost invariably Mn", and these set the scene for the structures to be expected in the compounds with the more discrete polyhedra.319Indeed, the structural pattern is established in the simple binary compound MnS. Whereas, the stable modification of this (a-MnS) is green and has the cubic rock salt structure with [MnS,] octahedra, the well-known flesh-coloured precipitates of the qualitative analysis system are metastable p- and y-modifications, which have [MnS,] tetrahedra with respectively the zinc blende or diamond (cubic) and wurtzite (hexagonal) structures. And so, in the rest of the known solids, there are almost equal numbers of four-coordinate tetrahedra and six-coordinate octahedra with no other polyhedra having been detected. Although this same structural pattern is found in the discrete polyhedra formed by the thiols, there is a marked difference in redox properties; although there are no binary sulfides of higher oxidation state and very few ternary sulfides, yet many of the thiol species are very easily oxidized to stable Mn"' species. Hence, anaerobic conditions are usually required for the preparation of the Mn" thiol species.320 The only monothiol studied, thiophenolate (PhS- ), forms pink tetrahedral [Mn(SPh),]'-, isolated as [NRJ+ and [PPh4]+ and a red-orange, tetrameric anion [Mn4(SPh),,I2-, which has the adamantane-like structure (125)320(which is, of course, a small fragment of the zinc blende or diamond structure of p-MnS). Each of these species has parallels in the other transition metals. The anion [Mn(SPh),12- was first prepared32' from the manganese compound of dithiosquarate (dts (126)) and [Mn(SPh),dtslZp also was isolated. It probably represents another example of a tetrahedral [MnS,] species with a bidentate dithiol. The original described the pink solids K,Mn(dts), -2H,O, [Ph,P],Mn(dts), and [Ph,P],Mn(dts), -2EtOH. Perhaps they also are examples of bis-bidentate dithiolates, but the structural detail is unknown. However, a dithiolate chelate compound has certainly been identified for 1,2-dithi0ethane~~O (-S(CH2), S- = edt): yellow-brown [Me,NI,[Mn(edt),]*CH,CN and orange-yellow [Ph,P],[Mn(edt),] have been characterized; and an X-ray analysis of the former proves the bisbidentate tetrahedral structure, with Mn-S = 2.43 A (average) and an interchelate angle of 91.5". This anion is particularly sensitive to oxidation by Q, giving green MnII' S-Mn-S species (Section 41.4.4).
Manganese
54
1 O O x s S--
A related chelate compound-the first to be reported323,and the most stable towards air (pink crystals were prepared by slow evaporation of solvent in the air) -is [Mn{SP(Ph),NP(Ph), S},]. This also is tetrahedral, with Mn-S distances of 2.426-2.457 A and interchelate S-Mn-S angles of 106 and 112". 41.3.6.2
Dithiolenes
Manganese(I1) is but poorly represented in the chemistry of the d i t h i ~ l e n e s . ~ "Only . ~ ~ ~one red-brown paramagnetic compound, [PPh,], Ih.ln(mnt),] {mnt = (CN),C,S;-}, appears to have been described. In addition, several unstable nitrosyls, such as WEtJ, [Mn(mnt)*NO], are known.326 They are described as being extremely unstable in air, decomposing rapidly in solution, but more slowly in the solid state.
41.3.6.3
Thiocarbonyls
This section appears to be represented so far by but one ligand and that by but one fully-characterized compound: trans-[MnCl, tu,] (where tu = thiourea) is isomorphous with the Ni" ana10gue.I~~ Dithio-P-diketonates have not yet been described for Mn. Under conditions which give w(SacSac),] species for M = Co, Ni, Cu, the only Mn"products {and Fe") obtained3,' were the dithiolylium salts of [MnCl,]'-.
41.3.6.4
IJ-Dithiolates
Data are available on the dithiocarbamates f(127) for X = R,N-abbreviated as R,dtc}, xanthates {(127) for X = RO-abbreviated as R ~ a n t h ] , ~ ~dithiophosphinates '?~'~ (R,PS; for R = Me, Ph and CF,)331and dithiocacodylate (M~,AsS;),~~ but mostly refer to the CS, ligands.
Generally, preparation of the Mn" species requires anaerobic conditions -Mn(Et,dtc), is pyrophoric, although some of the other compounds are reasonably robust. On mixing aqueous solutions of MnCl, and the Na or K salt of the ligand, yellow MnL, compounds precipitate. The dithiocarbamates are rather insoluble, but the ethylxanthate has significant water solubility (0.1 gl-' at 25°C). One structural d e t e r m i n a t i ~ nthat , ~ ~ of ~ Mn(Et,dtc),, shows a linear polymer with [MnS,] octahedra (Mn-S = 2.51-2.78A), no chelation but bridging of type (84). A recent study has explored the
Manganese
55
antiferromagnetism of this compound.334A related compound (1,2-bis-dithiocarbamato-ethane)is the fungicide 'maneb'. Through an electrochemical study of the Mn"' compounds, the anionic species [Mn(R,dtc),]have been ~haracterized,~~' but not isolated. Reversible reduction to Mn", as in equation (8), is followed by dissociation (9) to MnL, and L-; and reoxidation to Mn"' then is irreversible. Such tris aniono species, which appear to contain three chelating ligands, are apparently more stable in the xanthate series and [MEt,][Mn(Etxanth),] has been isolated as well as [Mn(Etxanth),phen] and two forms of [Mn(Etxanth), bipy]. This difference has been ascribed to the insolubility of the bis-dithiocarbamates. However, the electrochemical shows that the dissociation (9) occurs in solution, and there is an alternative explanation: the small chelate ring for the RCS; ligands confers significant instability on the larger Mn-S polyhedra of Mn", but this instability is reduced and the 'bite' of the chelate ring increased as Mn-S distances are reduced in Mn"' and Mn". --c
[MII~(R,NCS~)~]-[MnU'(R2NCS2),]O [Mn(R,NCS,),]-
+[Mn1V(R2NCS,),]C
+
Mn(R2NCS2)2 R2NCS;
(8) (9)
The enhanced stability of the tris-chelate species in the higher oxidation states is thus explained.
41.3.7
Halides
The artificiality of attempting to separate the compounds of the metals into 'salts' and 'complexes' becomes perhaps most apparent with the halides. As with the other transition metals, Mn" forms, for example, at least the crystalline chlorides MnCl,, M'MnCl,, M'Mn,Cl,, M'Mn4C1,, M:MnCl,, M\MnCl, and MiMnCl, and other ternary and quaternary phases. As well, many of these, when crystallized from water, form hydrates; and, equally, there are 'solvates' formed from the other solvents including traditional 'ligands' like pyridine. In almost all of the series of binary and ternary compounds known, there are examples of the dominant structural theme in the crystalline halide compounds- the [MnClJ octahedron. These octahedra are derived, according to the point of view, from Mn2+ (and the other M + ) fitting into appropriate 'holes' in close-packed halide lattices, or from face-, edge- or corner-sharing of interconnected octahedra, forming polymers, In the hydrates and the other solvates, other octahedra, containing molecules of solvent, occur. Tn the above list, there are also examples of discrete [MnCl,]'- tetrahedra, especially in the MlMnCl, and M:MnCI, series, and especially when Mi is large. There are no known examples of tetrahedral fluoro species, but they are quite significant in the bromides and iodides. In describing such solids, it is perhaps not illogical to talk of Mn" 'complexes'. However, the problem of definition of a 'complex' is well illustrated by the ternary hydrate (NH4),MnC14 2H,O. Its structure is derived (Figure 2) from that of NH,Cl, simply by replacing two adjacent NH: ions by a trans-Mn(H,O), unit; and a continuous series of solid solutions are known in the range 6NH4C1*MnClZ-2H,O to 2NH4C1-MnC12.2H20.Which is the complex? Surely the ternary hydrate is at least as much a complex of NH4C1as it is a complex of manganese(I1). Any attempt to separate, for example, MnC1, from ternary phases like KMnCl,, by classifying one as a 'salt' and the other as a 'complex', is not only illogical but also denies their close relationship. Accordingly we have tried a novel (to the coordination chemist) classification: taking firstly the solid compounds which contain linked octahedra; treating the tetrahedral Wn&]' species as a separate problem; and then addressing the problem of the polyhedra formed in the fluid phases in another separate section. On current knowledge, these are the only coordination polyhedra to be considered for Mn" halo compounds: four-planar is unknown, and five-coordinate species have been detected only in a few examples of [MnX2L,]and [Mn,(p-X),X,L,], which have been discussed in the sections on the appropriate unidentate ligands. The basic data are available in the Gmelin volumes C4 and C5, with earlier reviews in Colton and and by Kernmitt;*" and more recent data are in the annual and Canterford's from Structure Reports.
-
Manganese
56
41.3.7.1
Crystalline halides with octakeriral coordination
(i) Fluorides All known crystalline Mn" fluoro compounds have octahedral coordination for the metal. A feature of these fluorides is the lack of crystalline hydrates: only MnF, -4H,O (isostructural with Zn) appears to have been characterized. The pink binary compound MnF,, known since the time of Berzelius, is reasonably stable chemically and barely soluble in most common solvents. It is the most thoroughly studied of any manganese fluoro compound, being of interest for both its spectroscopic and magnetic properties: it was the compound in which the exciton and exciton-magnon structure of electronic transitions in antiferromagnetic insulators was first discovered and identified. The cubic perovskite RbMnF, also has been much studied because its magnetic behaviour is very close to that of the ideal isotropic Heissenberg antiferromagnet. MnF,, in its normal crystalline form (at least four others are known), has a tetragonally distorted rutile (TiO,) structure, with Mn-F (average) = 2.11 A. Similar bond lengths are found in the known structures of the ternary and quaternary phases (Table 3 l), many of which also can exist in different structural forms at different temperature and pressure. In only one known case, that of Ba, [MnF,] at high pressure, is there evidence for a discrete [MnF,I4- octahedron. (However, it may well be possible to define conditions for the separation from solution of compounds [MLJ4+ [MnFJ4-, using suitably sized inert cations.) The normal form of Ba,MnF, is best represented as BaZnFz,(MnF4),,with chains of corner sharing [MnF,] octahedra-a common feature of the M,MnF, species. Probably one of the major driving forces for the formation of such 'polymers', rather than discrete [MnF,I4- anions, is the electroneutrality concept, which leads one to expect that highly charged ions will be unstable.
(ii)
Chlorides
The structural variation observed here is greater than for the other halides; there is almost as much variety in the crystalline hydrates as in the (often hygroscopic) anhydrous compounds; and undoubtedly further variants will be found as the systems are further explored. MnCl, is certainly the stable manganese chloride, as MnCl, is unstable above - 40°C, and claims for MnCl, have never been substantiated. Recent report^^^,'^' describe reactive forms of the anhydrous material useful for preparing various compounds of MnCI, (and the other halides). However, in the normal form, crystalline MnCI, has the hexagonal layer-type structure defined first for CdCl,. The pink solid is hygroscopic, very soluble in water and forms hydrates MnCl,.nH,O (for n = 1, 2, 4 and 6). It also, of course, forms compounds MnCl,-nL, often called complexes, with an enormous range of other ligands (L): those for unidentate ligands and n = 1 have structures like (13); the great majority of the n = 2 compounds appear to be chain polymers like (8); and MnC1, .4H,O, like many other of the n = 4 species, is an octahedral monomer cis-[MnCl,(H,O),]. The hexahydrate, which is almost certainly [Mn(H,O),]Cl,, is stable only below - 2°C. For some Table 31 Mangdnese(I1) Ternary and Quaternary Fluorides with WnF,] Octahedra Comment
M' MnF, MgMn,F, and Mg2MnF, M'Mn,F, M: MniF,
Mi MnF, MI"M nF, Ba,MnF, LiMn[GaF,] Na,MnM'"F,
Na, IC,Rb, Cs, NH,, TI-usually cubic perovskites (Mn-F = 2.11-2.13w) Both have MnF, structure Na only-no structural detail M = K, Rb- tetragonal M, = NaCd or NaCa-cubic M = K, Rb, Cs- tetragonal (Mn-F = 2.09-2.1 1 A) M, = Ba-orthorhombic, Mn-F = 2.037-2.151 A (av. 2.098A) M = AI, Ga, Cr Normally Ba,F,(MnF,X, but high pressure form has the K,[GeF61 structure Cation ordered version of Na,[SiF,] (Mn-F = 2.121A) M = Fe, V (Mn-F = 2.08 A)
57
Manganese
Figure 2 The relationship of MnCl,-2NH4C1-2H,0 to the structure of NH,CI: (a) two adjacent cubic cells of NH,CI; (b) replacement of 2NH: by Mn(H,O):+
of the other unidentate ligands, we have also noticed compounds MnCI, *nLfor n < 1 These have structures shading from the double chains of (13) into the fully layer structures of MnC1,. Mixing MnC1, and MC1 (M = alkali metal cation, NH:, etc.) in a melt of appropriate composition gives on cooling the ternary compounds MMnCl,. Most of these pink or red solids are very hygroscopic and their preparation from water usually gives hydrates of them. At different stoich+
iometries, compounds of the series M,MnCl., and sometimes also M,MnCl, and M,MnCl, can be Table 32 Structural Variety in the Ternary Mn" Chlorides and Their Hydrates Phase
NqMnC1, (NH4)6MnC1, *2H,O MMn,Cl, CsMn,CI, Na, Mn, C1, MMnCl, (a)
(4 (4
(d )
M,MnCl, (a)
(4
Struc?ural Dota
Examples
ReJ
C .C .~.CI-, with Mn and Na in ordered octahedral sites See Figure 2
-
Tetragonal, with C.C.P. C1- -MnCl,] octahedra share five edges and one corner Both C.C.P. and hexagonal stacking of C1-, Mn in $ of octahedral sites Hexagonal (P6,lm) infinite chains of face-sharing octahedra [MnCk] of (a) Hexagonal (P6,/mmc)-vaiiation Rhombohedral (R3)-Ilmenite (FeTiO,) structuretrigonal distortion of h.c.p. C1Rhombohedral ( R h )Mn,CI,, trimers of face-shared octahedra, interconnected in spirals by cornersharing (128) Tetragonal (P,mm)-distorted perovskite, see ( f ) Cubic-perovskite [MnCl,] -formed by threedimensional comer-sharing Monoclinic (P2, /c) -infinite chains of face-sharing octahedra, like (uf Edge-sharing linear polymer [MnCI,O] Orthorhombic (Pcca). Corner sharing cis-[MnCl,O,] Orthorhombic (Cmcu).Comer sharing cis-[MnCI40,] Monoclinic. Comer sharing cis-[MnCl,O,] Triclinic. Dimers, as in (129) trans[MnC14Oz1 Hexagonal. Face-sharing, infinite chains of [MnCl,] and discrete [MnC1,I2- tetrahedra Tetragonal (14/mm). Double perovskite layers, with each [MnCl,] sharing five comers Cubic inverse spinel [MnCl,] Tetragonal (14/mm). Perovskite layers of K,NiF4 type, each [MnCI,] sharing four corners in plane Orthorhombic (Pbam). Edge-sharing chains of [~nc1,1 Complex of phases based on K,NiF, perovskite layer structure -variations depend on RNH, groups Orthorhombic (Pnma) [MnCI,]'- tetrahedra Cubic P2,3 [MnCl4I2- tetrahedra Triclinic [MnCl,]'- tetraheha Tetragonal (14/mmm) trum-(MnC140,] (Figure 2) Triclinic. Linear chains of trms-[MnCI,O,] edge-sharing M,[MnCl,]-MCl, with tetrahedra Octahedral
Na, PYH
NMe,, NEt, Rb, Cs", MeNH, Na CS'
K NH,, Cs"' Me2W2 pyH, quinH a-Rb, CS Me,NH, Mew, fl-Rb, CS Pr'NH, Me,NH
337
K. Rb
340
Li Rb, CZ-CS
341
338 338 339
Na RNH,
342
m e , , B-Cs [Ph,MeAs] NEb, PYH NH,, K Rb, Cs
Cs,dienH,
343 344
58
Manganese
prepared, as well as hydrates of them, but especially of the former. In addition, a variety of more complex species, including the NH4CL complex of solid solutions (Figure 2), have been identified structurally (Table 32). The use of larger quaternary ammonium, phosphonium and arsonium cations to precipitate appropriate ternary compounds from organic solvents, like the alcohols and acetone, has further extended the range of known types.
In the great bulk of the known from MnCl, to M4 MnCI,, and including M,MnC14, the dominant coordination polyhedron for Mn is the octahedron: [MnCl,] units are formed by face-, corner- or edge-sharing of bridging chlorides in the anhydrous species (Table 32). Many of the compounds listed also undergo phase changes at different temperature and pressure, but the manganese retains octahedral coordination. The only example of a discrete hexachloromanganese(I1) anion appears to be in K,[MnCl,], although others may be obtainable with appropriately sized 4+ cations, based, for example, on Pt"'. The few examples of M,MnCl, are examples of the only other structural theme, that of the tetrahedral [MnCl,]'-. Despite apparent stoichiometry, few of the known M, MnC1, solid compounds contain this tetrahedral anion: although they give tetrahedra in melts,346most of the solids have layer type structures related to K,NiF,, containing corner-sharing [MnCl,] octahedra. Only one example of mixed coordination polyhedra for Mn"-chloro species is known, the recently described (Me3NH),Mn,Cl,, which contains both [MnCl,] in chains of face-sharing octahedra and discrete [MnCl,]*- tetrahedra. As expected, the tetrahedra are most common in the compounds of the larger cations (Cs for the alkali metals). Measured bond lengths in the MnC1, octahedra range from 2.46 to 2.63 A, but they are generally near 2.53-2.55 8, for bridging Cl and 2.50 A for terminal
c1.
In the various 'hydrates', the water molecules usually are in the Mn coordination sphere. They make up the octahedra without the embarassment of high negative charges, and thus there are more examples here of monomers or small oligomers, like the dimers of (129). Other neutral ligands can replace the water, but very little work seems to have been attempted in this area for Mn. One recent report347postulated both [MnCl4L,I2- and [MnC14L,]2- species for N donor ligands, but, whilst the former are quite acceptable, the latter are most unlikely to occur and cannot be accepted without full X-ray evidence. Much of the interest in these compounds has been in their physical properties such as spectra, which have been reviewed recently in the Gmelin volumes,"* and magnetism. Of particular interest for magnetic studies have been the compounds, such as [NMe,]MnCl,, which have one-dimensional magnetic coupling in the chains of face-sharing MnCl, octahedra.
( i i i ) Bromides and iodides
The bromo, and especially the iodo cornpounds of Mn" are somewhat less robust than the chlorides, so that the variety of observed compounds and the structural patterns are less (Tables 33 and 34). Where compounds have been studied (and the structural data are less complete), there are close parallels between the chloro and bromo species, with one significant variation: there is a noticeable tendency for the bromides to form [MnX,]*- tetrahedra more readily. Thus, Rb,[MnBr,] is tetrahedral, whereas the chloro cornpound (Table 32) is known only as an octahedral polymer; and there is no octahedral polymer form of Cs,MnBr, as there is for the chloro analogue. Similarly, in the various onium salts, discrete [MnBr,12- tetrahedra are more common. One intriguing compound in
59
Manganese Table 33 Structural Detail for the Mn” Bromo Compounds
Compound
Coordinaiion Polyhedra
MnBr, MnBr,.2H, 0 MnBr,-2H, 0 MMnBr, (K, TI) (Rb, Cs, NMe,) MMnBr3*2H,0(Cs) (Me,NW M,MnBr, (Rb, RNH,) (Cs, PYH,NMe,) (Ph,MeAs) Cs,MnRr, K,,MnBr,
Comment
CdI, layer structure Monomer Chains-edge sharing, as in (8) Double chains-edge sharing (NH4CdC1, type) Face-sharing chains’ of CsNiC1, type Corner-sharing chains, as in CsNiC13-2H,0 p2, Im Tetragonal, KZNiFIlayer structure350 Pnma. Discrctc tetrahedra Cubic. Discrete tetrahedra Isomorphous with chloride
this series is [Et,OH]MnBr,, which is described as a light yellow solid. Perhaps it is an example of an [Mn,Br,] dimer of tetrahedra. K4[MnBr,] is ax3 example of a discrete octahedron, and there is a recent report3” of the preparation of [Me,NH,], MnBr,. Both MnBr, and MnI, have the CdI, layer structure, rather than the CdCl, variant of the chloride, but all form similar ‘hydrates’ of similar properties, with the tetrahydrates being the normally stable form. The compounds with the other neutral ligands appear generally to parallel the chloro species; but a recent reportfg3of the structure of MnI, .PMe,Ph raises an interesting structural variant: [Mn14P,) octahedra alternate with Mnl, tetrahedra, as in (69). Ternary iodo compounds are formed only with large cations: the onium salts appear to be invariably examples of [MnI,]’- tetrahedra, although both CsMnI, and TlMnI, give examples of [MnI,] polymers (Table 34).
41.3.7.2
Tetrahedral (MnX,]’- and [MnX, L ] - species
Tetrahedral [MnX,]’- species were identified by Nyholm and in the first unequivocal demonstration of tetrahedra in transition metal compounds. More recent structural studies” 7,a2R.343.345,349.35n have confirmed the existence and range of such tetrahedra some of which are listed in Tables 32-34. They are known for CI, Br and 1, but not for F. Cotton and Goodgame3’first studied the electronic spectra, details of which have been summari~ed.”~ There are also descriptions in the 1iteratu1-e~~~ of mixed ligand species [MnBr,C1,IZ-, [MnCl,I,]*and [MnBr,IJZ , but the data do not appear to prove the existence of such definite species rather than mixtures. The conditions under which tetrahedra form are becoming clearer, although some of the identifications rely on spectra and a doubt must remain. (How different will be the spectra of [MnC1,I3-, if and when identified, from that of [MnCI,]*-?) The structural data (Tables 32-34) show that, of the binary and ternary compounds, few of the chloro species, more of the bromo species, and most of the iodo species have [MnX$ tetrahedra in the solids. This progression needs no further explanation than size of halide (see especially the Cs and Rb species M,Mn&). In melts, however, M2MnCl, compounds give tetrahedra.”s,,346,349 Even in aqueous solution, and in other solvents (Section 41.3.7.31, in the presence of an excess of X-,such tetrahedra have been detected, and it is assumed that [MnX, solvent]- tetrahedra also are present. Such latter species are yet poorly Table 34 Structural Detail for the Mn” Iodo Compounds
Compound
Coordination Polyhedra
MnI, MMnI, (Cs)
(TI) M,MnI, (various) MnI,.PhMe, P
C.O.C. 4--c‘*
[Mnl,] and [MnI,P,]
Comment CdI, layer structure Hexagonal, related to CsNiCI,, chains of face-sharing octahedra’ Edge-sharing double chains, like bromide No X-ray data, but all known compounds appear to contain discrete tetrahedra Octahedra and tetrahedra alternate in the chains’’j
60
Mungunese
characterized for Mn": among the few crystalline species are pR,][MnX,L] (R = alkyl; X = C1, Br; and L = a unidentate imidazole N ligand).39In another pertinent example, whereas Co, Ni and Cu form compounds [MnX,L] {X = C1, Br, I; and L = (lo)}, Mn forms L,[MnX,] species.''
41.3.7.3
Halo compounds of Mn" in the fluid phases
In the gas phase, all MnX, compounds are linear symmetrical molecules with zero dipole m ~ m e n t .In~ addition, ~ . ~ ~ ~dimers [Mn,(p-X,)X,] have been detected for at least the chloride and Parallels with the alkyl (3) and amido (11) species are evident. Liquid (melt) phases of the ternary compounds, or of MnX, in MX melts3&give only octahedra for the fluorides, but tetrahedra for Cl, Br and I. In water, all MnX, species dissolve to give [Mn(H,0),]2', although at higher concentrations [MnX,(H,0),-,]-(n-2) species For F-, Kl is only of the order of five to ten. With excess halide, such species assume greater importance, and there is reasonable evidence for [MnX,H,O]and [MnXJ- for X = C1, Br. However, without the spin-allowed transitions found in the electronic spectra of the other transition metals, it is less easy to detect the various halo species for Mn". In non-aqueous solutions, such as in MeNO, and CH,CN, the tetrahedral anions [MnXJ(X = C1, Br, I) appear to maintain their integrity sufficiently, despite their lability, to allow meaningful measurement of some of the physical properties of these tetrahedra.
41.3.7.4
Fhoro anions of the nonmetals as ligands
Fluoro anions of the nonmetals, such as BF;, PF; and SiFi- are usually used, instead of the explosive ClO; , especially in non-aqueous solvents, when a counterion is required that does not compete too effectively for a place in the metal coordination sphere. However, since the acceptance of the reality of C10; ~oordination,2~'~~~' it has become to be recognized that these fluoro anions also can bond: BF; is as good a ligand as ClO;, although more subject to side reactions and therefore leading to unexpected products like the tetrameric species (18); PF; is less good than C10; , and was originally by contrast, to demonstrate perchlorate coordination; but direct bonding of AsF; and SbF; have now been demonstrated, both in the binary compounds MnX,335 and in the compound [M~(ASF,),(NSF,)~].'~~
41.3.8
Hydrogen and Hydride Ligands
This is but a fringe area of Mn" chemistry, although some detail of each class is known.
41.3.8.1
Hydrogen
The classification 'Mn hydrides' is persistent in the literature. The earlier view that H, 'only dissolved' in metallic Mn, and formed no well-defined hydrides, was probably too simplistic, although Mn-H species are unlikely to be of great interest to the preparative chemist. MnH, has been trapped in Ar ox N matrices at 4 K, and spectroscopic suggest a bent molecule with H-Mn-H of 117 & 30"; other authors357think that the results of some mathematical calcurations in favour of a linear molecule should take precedence. MnHI.1.5THF has been described as resulting from the reaction of MnI, with AlEt, in THF, whereas related reactions usually produce binary MnAl derivatives. in 1973. But, most of the The role of Mn" hydrides in organometallic chemistry was recent papers on Mn hydrides refer to the effect on the magnetism of intermetallic phases when H, is absorbed. In a few cases, stoichiometry has been determined -examples are Mn,ErH,, Mn,ErH4,6,360M ~ , S C H , , ~and ~ I Mn,ZrH,.362A neutron diffraction analysis of the latter shows 12 hydrogen (or deuterium) nearest neighbours for Mn, at 1 . 7 4 1 -77A. Although no pattern is yet emergent, it seems not improbable that the oxidation state formalism will have but little use in this area of chemistry.
I Munganese
61
41.3.8.2 Hydride iigands
Both Mn(BH,), and Mn(AlH,)* have ‘been described,363but little is known of them and their reactions. Noth364also described the species Li,,Mn(BH,),X, *rnEt,O (n = 1,2,3 and X = C1, I) and LiMn,BH,I, as yellow or orange oils, obtained by reacting ether solutions of LiBHLand MnX, in various ratios and then evaporating to dryness. The Mn” compounds of the boranes and carboranes were covered in the companion volume365 and they include at least one example of a low spin Mn” ~ a r b o r a n e . ~ ~ ~
41.3.9 Mixed Donor Atom Ligands Macrocyclic systems are discussed in Section 41.3.10.5.
41.3.9.1
‘Ambiguous’ N - 0
and bidentate N - 0 ligands
This is an area of chemistry in which most that has been attempted has been at a very superficial level. It is very easy in particular to synthesize Schiff base N-0 donor set hybrid ligands and to make Mn” compounds from them. It is not easy, unless one uses X-ray techniques, to unambiguously assign which ligand donor atoms are bound. Furthermore, the marked tendency of oxygen donors to form bridges, (such behaviour is very common for carboxylic acid residues, see Section 41.3.5.2.ii)often complicates the ligand binding pattern immensely. Thus in [Mn(pro)(H,0),J2+ {pro = D,L-proline (130)) one might simplistically expect a monomeric octahedral ion with both N and 0 donors of the proline coordinated. In fact X-ray analysis indicates that the N atoms do not bind to the Mn’l, but that the carboxylic acid groups bind both 0 atoms to adjacent Mn atoms to give a continuous polymer and an MnO, ~hromophore.2’~
This section is constructed to deal firstly with the group of ligands which possess potential N and 0 donors which cannot both bind to the one Mn atom. The coverage will then extend, with increasing ligand denticity, to deal with ligands that can potentially bind more than one type of donor atom to the one Mn atom. The major analytical technique that has been used to assign ligand binding atoms has been IR spectroscopy. Large numbers of workers have based their claims for or against the binding of specific donors on the change (or lack of change) of such ‘parameters’ as amine stretching frequencies, pyridine breathing modes or carbonyl stretching frequencies, upon binding of the free ligand to the metal. Thus 2-benzoylpyridine (131) in [MnCl,L,(H,O),] is postulated on I R data to be bound only by the pyridine nitrogen with the carbonyl group, whose TR stretching frequencies are virtually unchanged upon complexation, unbound.367On the other hand, MnCl,(tp) (tp = 2pyridone, 132) appears to be isomorphous with the corresponding Cu” compound, which has been shown to possess a four-coordinate dichloro-bridged structure with unidentate oxygen bonding of tp. Again, in this case, the IR carbonyl stretching frequencies are unchanged upon c ~ r n p l e x a t i o n . ~ ~ ~
To demonstrate the difficulty of using IR evidence, Table 35 lists the CO stretching vibrations for cytosine (cy = (133)) and some cytosine compounds.369It is known that the structure of CuCl,(cy),
I 62
Manganese Table 35 IR Carbonyl Stretching Vibrations for Cytosine and Some Cytosine Metal Compounds
C=O stretching frequencies (cm-')
Compotcnd
1703, 1662, 1615 "1676,1658, 1601 1676, 1656, 1621 1676, 1656, 1621, 1593 1683, 1660, 1630
is effectively planar with a CuCl,N, chromophore, however the TR evidence would indicate that the carbonyl groups are bound in all cases. Some ligands, which possess both potential N and 0 donors, but whose structure forbids chelation of both donors, are listed in Table 36 along with proposed bonding evidence. Although structures are in most cases unproven and may well be polymeric, many workers appear to favour the 0-bonded system rather than the N-bonded. Such bonding is proposed for probably the simplest of these 'ambiguous' ligands, viz. hydro~ylamine;3~~ however the somewhat similar ligand, cyanoacetic acid is claimed to form N and 0-bonded polymers with Mn'1.380The claim3" that in MnCl,(biuret), the ligand (148) binds only 0 atoms to the metal would appear to support the contention that Mn" prefers 0 donors. However, as this is the preferred method of binding of biuret to other transition metal ions, when the ligand is not acting as an anionic group, such support is questionable.
HO H (133)
(134)
(135)
(136) X = C@2H, Y = H (137) X = H, Y = C 0 2 H (138) X = H, Y = CH@
(139) X = H, Y = C(O)NEt,
(144) R = 1I (145) K = Me
Possibly the clearest indication of the preference of Mn" for 0 rather than N donors is seen in the compounds formed by the amino acids. The early work of Berezina and P o z i g ~ n indicated ~~' that MnX,(aa),, MnSO,(gly), and Mn(NO,),(gly), (where X = C1 or Br; aa A either glycine (gly) or a-alanine (a-ara); and n = 2 , 4 or 6 ) could be prepared. IR evidence was used to show that the amino acids coordinated as zwitterions through only the 0 donor, except in the two cases, MnCl,(cc-ala),, and Mn(NO,),(gly),, in which both the N and the 0 were bonded. A series of Polish papedN3has shown that there is no X-ray evidence that, while the amino acid residue does not lose a proton, the N atom bonds. There are two types of amino acid compounds formed: (a) octahedral chain polymers in which the carboxylic acid group acts as a bridge between two adjacent Mn" atoms, e.g.
Manganese
63
Table 36 Some ‘Ambiguous’ Ligands and Their Proposed Bonding to Mn”
Ligand and Structure m-Proline (130) Cytosine (133) &-Caprolactam(134) p-Quinonedioxime (135) Isonicotinic acid (136) Nicotinic acid (137) Nicotinic aldehyde (138) N,N-Diethylnicotine amide (139) 2-Pyrrolidinone (140) Cyanuric acid (141) Tetrahydra- 1,4-thiaza-3-one (142) Thiazolidine-2-thione (143) Uracil (144) Thymine (145) Flavoquinone (146) 4-Amino-3,5,6-trichloropicolinate (147)
-
Bonding and Evidence
Ref
0-bonded polymer; X-ray N-bonded; IR 0-bonded; IR 0-bonded; IR 0- and N-bonded polymer; IR 0- and N-bonded polymer; IR N-bonded; IR 0-heterocyclic N-bonded bridging Mn atoms; X-ray 0-bonded; IR 0-bonded; IR 0-bonded; IR 0-bonded; IR 0-bonded; IR 0-bonded; IR 0-bonded; NMR 0- and N-bonded polymer; X-ray
253 369 229 370 371 372 372 373 236 374 375 375 376 376 377
[MnC1,(gly)(H20),1, and [Mn(gly),(H,O),],Br,, and (b) monomeric octahedral compounds in which the amino acids bind only through one of the 0 atoms, e.g. [MnCl,(gly),(H,O),]. Stability constant measurements have been undertaken to show that in basic solution mixed amino acid ligands with two amino acid moieties present, Le. NH2-C(R)-C=N-C(R’)COOH), yield complexes with log K values of the order of 6 to 7;384such compounds appear to bind the amine nitrogen.38s Berezina et a1.386claim, using IR evidence, that the anionic glycine residues in Mn(gly), * 2 H 2 0 chelate as N-0 bidentates. If this is the case, glycine is here paralleling what is believed to be the behaviour of the bidentate salicylaldimines (149), and the behaviour of the interesting low spin [Mn“L,]- ion {HL = ethylnitrosolic acid (150)}, which has been shown by X-ray work to be octahedral with binding of the N of the nitroso group and the oxime 0 atom. However, there are always exceptions to any rule; in the compound [MnL2(H20)J {HL = (151)}, X-ray structural analysis388 has shown the Mn atom to be bonded to the nitrogen atoms of two trans ligands. Me
I
1 N8
OH
( 1 49)
The bis-bidentate salicylaldiminato compounds (149) of Mn” have not been studied systematica11y.173,389 Since Tsumaki3” reported the red-gold benzyl compound (149) (R = CH,C,H, and X = H) in 1938, a number of yellow or orange solid compounds of empirical formula MnL,(H,O), (see Table 37). Many of these for various values of R and X in (149), have k e n prepared39.391-393 , ~ ~one ~ ~of~the ~ few certain pieces of compounds appear to be polymeric392or at least d i r n e r i ~but structural data is that reported on the orange compound (149) (R = Me, X = H), which has been shown to be isomorphous with its Zn a n a l o g ~ eThis . ~ ~Mn compound thus has the dimeric structure (152), with two five-coordinate Mn atoms. The compound does not exhibit antiferromagnetic although other compounds of type (149) with different values for R and X appear to have low magnetic moments and their structures have been postulated as dimeric on the basis of temperature dependent magnetic studies.”
Manganese
64
Table 37 Mn" Compounds of Formulation (149)
X
R
Pho-Me-Phm-Me-Php-Me-PhPh-CHlp N H , SOZ-Ph-
H H H H
Ph-
5 4 0 3H 5-SO3H
Ref.
H H
PNHlSOl-PhPhp-NHzS02-PhMeMeo-Me-Ph-
3-COlH 3-CO2H H Fused Ph ring Fused Ph ring
394 394 394 394 390 391 391 391 392 392 292 39 39
Undoubtedly, a whole series of Mn" compounds of type (149) are readily obtainable, and should be of comparable air stability to the analogous Co" species. One attempt to prepare the compound (149) for R = Pr'and X = H, by mixing Mn(NO,), in water with salicylaldehyde and excess amine in ethanol, gave yellow crystals of formulation [MnL,] .Pr'NH,NO,. Identical reactions for Co, Ni, Cu and Zn nitrates yielded only the corresponding compounds of type (149).396The stability constants of some of the bis-bidentate salicylaldiminato compounds of Mn" have been found to be of the order expected in the Irving-Williams Undoubtedly much more work is required in the area of synthesis and structural analysis before a clear picture of this system can be obtained and the relevance of magnetic data properly assessed. The possible bidentate ligands with a chelating N-0 donor set constitutes a virtually infinite set and a large number of this set has been thrust upon an unsuspecting Mn" atom, often with very few conclusive results. Thus the Mn" and Mn"' compounds of 8-hydroxyquinoline (153) and its . ~ ~Mn" ' both deprotonated MnL, and substituted derivatives attract some 16 pages in G m e l i ~ ~For protonated, e.g. [MnCl,(HL),], ligand compounds are described, but there are no X-ray structures. It appears likely that both the N and the 0 atoms are bound in the MnL, compound, as the structure of the Mn"' compound, [MnL,], has been published.398 A number of N-0 bidentate ligands developed from a-substituted pyridines (154) have been reacted with MnCl, to yield MnCl,L, (see Table 38). It is claimed that these compounds bond via the pyridine N atom and the 0 atom. Other workersM3have looked at similar ligands such as (155) and claimed, from IR studies, that both the N and the 0 atoms are bound in compounds of types of MnC1,L and [MnL,(H,O),],.
( 1 53)
( I 54)
(155) R = NH2 or Me
A development of the ligands derived from a-substituted pyridines is the potentially quadridentate ligand (156). It is suggested that the anhydrous cream-white [MnCl,L] compound is polymeric with each N-0 pair of the ligand binding to a different Mn" atom.* This tendency to polymerization is ably demonstrated in the X-ray structure (157) of the compound [Mn,ClL3(H,0)3]C13*L.5 (L = isonicotinyl h y d r a ~ i d e )and ~ ~ ,in the possible structureM6of di(bis[N-phenylaminomethyl]phosphinato)manganese(II) (158).
Munpnese
!
65
i
\
"")
=O
41.3.9.2 Mixed N-0
donor atom terdentate ligands
Mn" compounds of several types of terdentate Schiff base ligands are known.'73Of interest for derived from condensation of supposed biological reasons are ligands of donor set 0-N-0 a-amino acids with sali~ylaldehyde,~~ pyruvic acid:'* pyridova1"O' and substituted salicylaldehydes.4" Although X-ray analysis4' indicates that the compound [Mnl' (pyridoxal-valine),] is octahedral, the structures of most of these compounds have not been fully elucidated. Claims of monomeric, dimeric and polymeric molecules are based mainly upon analytical results and upon room temperature magnetic susceptibility measurements. Nothing of special biological interest has come from this work. or S-N-0 donor set Schiff base The Mn" compounds, derived from the terdentate 0-N-0 ~ ' ~ u n s u b s t i t ~ t e d ~o-hydroxyan'~~~'~ ligands obtained from the condensation of both s ~ b s t i t u t e dand iline, anthranilic acid4" or o-amin~benzenethiol~'~ with various salicylaldehyde residues, have the
'
Table 38 MnCl,L, Compounds from Ligands of Type (154)
R
Siruciure Proposed
Ref.
Octahedral Mn" with CI,N,O, chromophore from X-ray structure
399
-cHz--F--NH20
Bidentate N-0 IR spectra
ligands from
400
-C
Bidentate N-0 spectra
ligands from IR
121
-f-NH-NF -Me Me 0
Bidentate N-0 ligands from IR spectra; tetrahedral?
40 1
-S-Pll
Bidentate N-0 ligands from IR spectra; polymeric?
402
-f-NH--NHI 0
=N-OH
I
Ph
0
66
Manganese
stoichiometry MnL (where H, L = the ligand). Temperature dependent magnetic data4I2support the polymeric nature of these compounds but their ready ~ x i d a t i o n ~suggests ' ~ * ~ ~that ~ such results may be due to the presence of higher oxidation state manganese. The preference of Mn" for oxygen donor atoms is evidenced by the fact that with (159) Mn" coordinates by a N-N-0 donor set to yield the distorted trigonal bipyramidal compound [MnCl,L].415Comparison of the X-ray structure of this compound with the corresponding Cu" compound shows that the latter compound uses an N-N-N donor set with both pyridine nitrogens c o ~ r d i n a t i n g .Nevertheless, ~'~ this preference is not always exhibited. In [MnL,](C104)2, tris(2-pyridyI)methanol (47; X = COH) is claimed to possess an N-N-N donor set rather than How the ligand would coordinate upon removal of the methanol the possible N-0-N donor set appears to be present in the white airproton is of interest; certainly the N-O-N sensitive compound [MnL,] obtained when the sodium salt of (160) is reacted with MnC1,.24 Nevertheless, one must be impressed with the predilection of Mn" for oxygen donors when one finds that the structure of [MnL(H,O),] (where L = iminodiacetic acid) is octahedral with an MnO, chr~mophore.~'~
41.3.9.3
Mixed N - 0
donor atom qua&i&ntate ligands
The most comprehensively studied Mn" Schiff base compounds are those of the quadridentate hybrid O-N-N-0 donor set ligands of structure (161); the ligand salenH, ((161) where Y = H and X = (CH,),} being the object of a number of different ~ t u d i e s . ~ *The ~ , ~orange-yellow, '* crystalline, air-stable compound [Mn(salen)(en)] was isolated in 1933 and shown to rapidly oxidize in ~ o l u t i o n -The ~ ' ~compound [Mn(salen)]has been prepared by the four main methods used to prepare metal compounds of ligands derived from the interaction of salicylaldehydes and primary amines:" these are: (i) Preparation and isolation of the ligand and subsequent reaction with the metal salt (often the acetate); (ii) Preparation and isolation of the bis(salicylaldehydato)metaI(II) compound and subsequent reaction with the amine; (iii) Reaction of the stoichiometric amounts of salicylaldehyde, amine and metal salt; (iv) Interaction of a metal carbonyl with the prepared ligand in non-aqueous solvent under a nitrogen atmosphere (for cases in which the metal compound may be air sensitive)?" The magnetic properties of [Mn(salen)] have been the subject of a number of ~ t ~ d ialleof ~ , ~ ~ ~ ~ ~ ~ which have indicated the presence of antiferromagnetic interactions. As [Mn(salen)] has an X-ray powder pattern similar to [Cu(salen)],, a dimeric compound with mutual sharing of one of the its magnetism has been analysed in terms of such a structure. oxygen atoms of each
The reaction of [Mn(salen)] with oxygen has been the subject of much study since the early claim that the product was [Mn"'(~alen)(OH)].~'~ The topic has been re~iewed,~" but the nature of the product is still uncertain and the field badly requires definitive work, not least because the oxidation of Mn" species is rapidly assuming great biological importance. Some results of oxygenation
67
Manganese Table 39 Oxygenation Reactions of [Mn(salen)]
Solvent and Temperawe of Oxygenation
Postulated Compounds Obtained
[Mn"'(salen)O, ,?In [Mn"'(salen)],O- H 2 0 [Mn"' (salen)], 0, [Mn'v(salen)o], [Mn(salen)(OH)] [Mn(salen)Oq MnOz
(i) Pyridine, 20°C (ii) DMF, 25°C (iii) DMSO, 25°C (iv) Pyridine, 25°C (v) Pyridine, - 15°C (vi) Pyridine, 115°C
1.92 1.97 1.96 1.99 2.01
+
39 39 425 425 426 426
experiments of [Mn(salen)] are summarized in Table 39. Such reactions are not reversible. Oxygenation of Tth" compounds of quadridentate N ligands has been noted in Section 41.3.3.4(i). It is also claimed that nitric oxide reacts in DMSO with [Mn(salcn)] to yield [Mn11'(salen)O],0,.427 This is surprising, as an ethanolic solution of manganese(I1) acetate and salenH, is claimed to react with NO to yield [Mn"'(~alen)(OAc)].~~ On the other hand, it has been claimed that [Mn"'(salen)CO]ClO,, a dimer with bridging carbonyls, can be isolated from the interaction of [Mn(salen)(H20)2]with oxygen in 50j50 MeOH/EtOH in the presence of Cloy and NaOH."' Much work has been published on the Mn" compounds of (161) with X = (CH,), and Y = various substituents; a summary is provided in Table 40a. The remainder of Table 40 demonstrates the large number of compounds of general structure MnL (L = 161) that have been prepared; structures of such components are not known, excepting that in which X = meso-2,3-butane, Y = H, which has been to be isomorphous with the corresponding Co" compound whose structure is square The oxygenation reactions of many of these MnL compounds have been studied. MnL*H,O (X = (CH,),; Y = H) in refluxing pyridine reacts irreversibly with 0, to yield a dimeric Mn"' Table 40 Mn" Compounds Prepared from Quadridentate Ligands of Structure (161) ~
X
Ligand
Y
H H H H H 3-COl H 5-CI 5-Br 540, 5-SO3 3-Me0 3-NO2 Fused-Ph H 3-C02H 5-C1
5-Br Fused-Ph
H
H H H H 5-C1 H H H 5-CI H 5-C1 H 5-CI
H
Mn" Compound Formulation
MnL MnL MnL MnL MnL MnL MnL. EtOH MnL. EtOH MnL MnL MnL. H,O MnL+H,O MnL.2H2 0
& (BM af R.T.) 5.25
Ref.
5.89 5.96 4*74 4.74 5.92 5.96 5.37
426 39 422 38 392 392 39 39 39 1 429 425 39 39 1
5.29 5.47 5.86 5.68 5.38
432 392 39 39 39 I
MnL MnL MnL MnL
5.92 5.97
426 426 426 426
MnL*H,O MnL- EtOH MnL MnL MnL MnL MnL MnL
5.93 5.99 5.74 5.63 5.64 5.63 5.76
MnL
5.63 5.76
MnL MnL MnL MnL MnL
MnL MnL
5.27 5.29
5.24 5.28 5.88
5.83 5.98
5.88
5.7 1
430 39 43 1 43 1 43 1 43 1 43 1 43 1 43 I 43 1 43 1
68
Manganese
c0mpound,4~~ which can also be obtained by heating the Mn" compound in the solid state in air at 80"C;4jo however in benzene at room temperature the oxygenation reaction yields [MnL(0)-&H,].4'0 All the compounds of Table 40(c) react with 0, in (i) boiling pyridine to yield [MnLOH] + MnO,; (ii) pyridine at - 15°Cto yield [MnLOH]. These apparently Mn"' compounds do not reversibly lose The preparation and oxygenation of a series of MnL compounds with L = (161) and X of varying aliphatic chain lengths from (CH,), up to (CH2),0has been reported4" (see Table 40(d)}. The compounds with X = (CH,), to (CH2)10react with 0, in DMSO to yield insoluble red-brown powdery materials of formulation [Mnt(O)], excepting where X = (CH,), in which case [MnL(0),j2]is obtained. The magnetic moments of the compounds [MnL(O)] fall in the range 3.2 to 3.7 BM, and it is claimed that Mn" p-dioxo-bridging dimers are present. Oxygenation is not reversible. As noted above definitive (and hard) work is required in this area. A 'double-sided' version of (161) has been produced in the ligand (162), which has shown to yield a Mn" compound of formulation M I I , L . H , O ~ this ~ ~ compound being similar to the Mn2L-3H,O compound obtained from (163).
0,p HO
OH
(163)
CHzCHZOH / NTCH~CO~H CHZCO2H
CHzCHzOH N-/ CH2CHZNEtz \
CHzCHZNEtz
CHZCHZOH
OH
I30
'
I Manganese
69
Quadridentatc hybrid ligands of the tripod variety appear to favour polymerization, although. such behaviour is not shown by the sexadentate edta. The ligand (164) loses two protons to yield the compound [MnL(H,O)];nH,O (165),435while it is suggested that the ligand (166) dimerizes through the oxygen atoms in [MnzL,]Zf.436 However, the somewhat similar ligand (167) has been shown by X-ray analysis of the compound [MnL(NCS),] to form a monomer with a distorted octahedral Mn" at0m.4~'
41.3.9.4
Mixed N - 0
donor atom quinquedentute ligands
Structure (161) can be expanded to yield quinquedentates by the inclusion of a donor atom in the X chain to yield for example X = (CH,),-Z-(CH,), (where n = 2 and 2 = NH or S; n = 3 and 2 = 0).Such ligands react with manganese(I1) acetate to yield yellow [MnL-nW,O] (where n = 0.5 or 1),438 whose structures may well be polymeric as in the [CuL] compound (L = 161 in which X = (CH,),-NH-(CH,), and Y = H).439These Mn" compounds react with dioxygen irreversibly to yield probably Mn"' compounds. Various mechanisms of the interaction are fully discussed in a re~iew.4'~ A most interesting quinquedentate is ligand (168) (R = 2'-pyridyl), which has been shown to yield the compounds [Mn(LH,)Cl,]. 5H,O and [MnL(H,0)2]*7Hz0.In both compounds X-ray structural work proves that the ligand binds through two N and two 0 atoms of the ligand with the two pyridine residues at either end of the ligand chain non-bonding; the ligand forming a pentagonal plane about the metal with the two chlorine atoms in the former compound, and two water molecules in the latter, occupying positions six and seven of the coordination sphere above and below the ligand plane.440A similar seven-coordinate structure has been found in (168) in which R = NH,."' The preference of Mn" for 0 donors (although it is very doubtful if both terminal pyridines could bind in a planar situation due to steric interaction), and the lack of preference of Mn" for any particular stereochemical shape, are both clearly shown in these results. It is of interest that other workers believe that for the similar ligand (169), in the neutral compound [MnL(H,O),] the ligand binds via two N and two 0 atoms, with the octahedral coordination being completed by the two water molecules.442
41.3.9.5
.
Mixed N--0 donor atom sexa- and ocdadentate ligands
Long-chain sexa- and octadentates are very often derived from Schiff base condensations. Some recent examples are shown in ligands (170)443and (171);444in both cases it is believed that the six and eight potential donor atoms respectively are bound in the Mn" compounds.
Po
Me
O=(
Me
The edta compound of Mn" has played on important role in expanding the coordination chemist's demonstrated that the large vision of the occurrence of higher coordination numbers. Hoard et size of the Mn" ion pushed back the sexadentate ligand and allowed a water molecule to coordinate, thereby producing a seven-coordinate ion, [Mn(edta)H,O)]-. On the other hand, the tetrarnethyl-
Manganese
70
substituted ‘edta’ appears to be octahedral and possess at least some non-bonded oxygen donors in [MnCl,L].446Bi- and tri-nuclear edta compounds of Mn” have been prepared. In the compound Mn,edta*9H2Oa seven-coordinate bridging structure is p r o p ~ s e d , ~ whereas ’ in Mn,(edta),- 10H,O each of the edta residues retains a proton and adjacent seven-coordinate [MnedtaH,O] units are bridged by an [Mn(H,O),] unit bound to carboxyl oxygens of the edta residues.448
Mixed N-S donor atom ligands
41.3.9.6
As might be expected by reference to Section 41.3.6 on sulfur compounds of Mn”, little work of real consequence has been done in this area. A reviewMgof the early 1970’s details the manganese compounds of sulfur-containing amino acids, and quotes evidence that cysteine (172; R = SH) and penicillamine bind via both the N and S donor atoms at higher pH values; however, analysis of the papers quoted yielded little evidence for S binding. On the other hand, there is sound evidence that when the thiol group is methylated, as in S-methylcysteine and methionine (172; R = SMe and CH,SMe respectively), the S does not coordinate, either in solution450or in the solid state.45’
Work up to 1974 on N-S chelating agents is comprehensively covered by Ali and Li~ingstone;~~’ although one has to delve deeply into t h s review to find the few references to Mn. Table 41 lists some of the bidentate N-S ligands that are claimed to bind both donor atoms to Mn”. Evidence for such binding is usually based upon doubtful TR interpretations coupled with compound stoichiometries; even the X-ray isomorphism of the compound of ligand (174) to a similar Co” compound is somewhat doubtful.
(173)
Long-chain terdentate ligands of donor sequence N-N-S (178), (179) and (180) have been shown to yield compounds of formulation MnLX, (X = halogen); such compounds have been claimed to be six-coordinate five-coordinate monomericm and tetrahedral46’respectively, with both N and S bonding. However, there is little evidence to justify any of these claims. When the field is expanded to cover ligands of even higher denticity, the proof of binding of both while Nand S is indeed difficult. (181) is claimed to possess an N4Schromophore in [Mn(L)I]C104,462 (182) is proposed as a quinquedentate in [MnI’L], but not to form a compound with Mn11’.463 Table 41 Some Mn” Compounds in which Bidentate N-S Ligand (173) (174) (R = H) (174) (R = Me) (175) (1761
(177)
Ligands are Claimed to Bind both Donor Atoms
N-S Binding
Compound Stoichiometry MnL, Mn(HL),fl,
Mn(HL),CI, (and Br,)
-
MnL, 2HL Mn(HLkC1, MnL,Cl, (and Br2)
~
Note: Except for (177), HL = protonated and L
= deprotonated
form of the ligand.
Evidence
Ref
IR X-Ray isomorphism IR and ESR IR IR IR
453 454 455 456 457 458
71
Manganese
41.3.9.7 Mixed 0-S
donor atom ligands
There is sound evidence that bidentate 0-S ligands can bind both donors to Mn". Ligand (183) has been shown to yield the compound [Mn"(L),(py),],464in which proton loss and both 0 and S binding appear certain. Ligands such as (184) may well bind both 0 and S to Mni',465although it appears doubtful if (185) does.466
R
There is a small literature on multidentate 0 - S ligands possessing thioether sulfur atoms and carboxylic acid or hydroxy groups. Table 42 lists some of these compounds, together with their proposed stereochemistries and structures. In all compounds listed, binding of both all the sulfur atoms and all the carboxylic acid or hydroxy groups is proposed.
Table 42 Some Mn" Compounds of Multidentate 0-S Ligand
(186) (R = CH,OH) (186) (R = CO,H) (187) (187) (188) (n = 2) (188) (n = 4) (189) a
Compound Stoichiometry
Ligands in Which Both 0 and S Donors are Bound Stereochemistry
Ref.
Mn(L)CI, Mn(L-2H).H,O Mn(L-2H).2H2O M'Mn(L-3H). n H , O
Five-coordinate Oh polymer Oh polymer
Mn(L-2H)
Oh polymer Oh polymer Oh dimer
467 468 469 469 470 47 I 472
Mn(L-2H)+2H20 Mn2(L-4H)*12H,O
MI = Li,Na or K and n = 3+ 2 or 2 respectively.
72
Manganese
41.3.10 Macrocyclic Ligands
41.3.10.1 Porphyrin ligands Since the first report of a manganese porphyrin in 1904,473there has been a great deal of research involving such compounds largely because of their unusual properties and their possible relevance For example, it is believed that a to biological systems such as green-plant manganese porphyrin may function as the oxidant for the natural photosynthetic oxidation of water to molecular oxygen. This latter aspect has provided a motivation for a number of redox and photochemical The manganese porphyrins were reviewed in detail by Boucher in 1972.492Porphyrin complexes in the + 2 to + 5 oxidation states all exist although the + 3 state is most stable. Mn" porphyrins may be obtained by direct reduction of the corresponding Mn"' porphyrin; for example, the tetraphenyl porphyrin (190), [Mn(TPP)], may be obtained by reduction of [Mn"' Cl(TPP)] with Cr(acac), in toluene" to yield the purple product [Mn(TPP)]* 2(t0luene)?'~
A number of other reductants have also been used to reduce Mn"' porphyrins in solution.a2 With dithionitePgOthe reaction is stoichiometric for a one-electron reduction above pH 8 but below this pH the [Mn"(TPP)] product undergoes demetallation. Related reduction can be induced by irradiation of a Mn"' porphyrin with visible light although, in the absence of an added electron donor, the quantum yields are Mn"' porphyrins may also be electrochemically reduced in a single electron step to yield the corresponding Mn" porphyrin in both aqueous and non-aqueous media."85The potentials at which these reductions occur are sensitive to the nature of the solvent, counter ion present and pK, of any bound ligands, as well as to the basicity of the porphyrin ring involved.4B24B4 Similar factors also cause variation in the visible spectra of the respective compounds!82 In contrast to fMn(phtha1ocyanine)l (see Iater) and [Fe(TPP)] which adopt intermediate-spin configurations, [Mn(TPP)] appears to be fully high-spin.@' Treatment of this compound with a heterocyclic base such as 1-methylimidazole(l-MeImH), 2-methylimidazole (2-MeImH) or pyridine (py) leads only to 1:l adducts being obtained; there appears to be no tendency for six-coordinate species to form even when the bases are present in large excess.a2-486All the solid adducts have magnetic properties which correspond to high-spin d5 configurations (with magnetic moments of 6.2-6.6BM). X-Ray analyses of [Mn(TPP)] and the THF solvate of [Mn(TPP)(l-MeImH)] have been perforThe X-ray data for the former compound suggestsqs7that the Mn" ion, with its med.@"487~488 relatively large radius, is positioned out of the N,-plane of the macrocycle. This result is in The five-coordinate [Mn(TPP (l-MeTmH)] also accordance with earlier theoretical predi~tions.4~~ has the Mn" displaced from the porphyrin plane; in this case, the Mn" lies 0.56 above the plane towards the I-methylimidazole ligand. Calculations of the charge and spin distribution in the related Mn" porphyrin containing a coordinated water in the axial position indicate that the Mn" is strongly bound and that about 30% of the unpaired spin population ( S = 512) is delocalized away from the Mn" on to the attached ligands.," The Mn"(TPP) complexes of imidazole (ImH) and the imidazolate ion (Im) have also been isolated; both are high spin and five-co~rdinate.~~' The increased field strength of imidazolate over imidazole is insufficient to cause spin pairing (which was proposed to be a prerequisite for six-coordination to occur). In part, because the Mn" ion is not held in the donor plane of the porphyrin, demetallation of such complexes in acid becomes facile as shown in one study (where loss of metal was found to be
B
Manganese
73
lo6 times faster than far the analogous Mn"' species).492Other studies involving the kinetics of exchange of zinc or cobalt for Mn" in a porphyrin compound in ammoniacal medium have been reported.493 Mn"-substituted haemoglobin undergoes irreversible oxidation to the corresponding Mn"' compound on exposure to oxygen494and the five-coordinate imidazole adducts mentioned previously also undergo similar oxidation in solution. For mn(TPP)], oxidation is very rapid although it is apparent that coordination of an axial base decreases the ease of oxidation. This effect is illustrated by the behaviour of the five-coordinate Mn" species incorporating the 'picket fence' porphyrin495in the presence of a slight excess of the axial ligand. For this system the rate of oxidation to the Mn'" species is very slow (hours). At low temperatures (- 78 to - 9 0 T ) a number of Mn" porphyrins have been observed to react reversibly with oxygen in solvents such as toluene or toluene/ TH F.483,484,496-498 Examples of such behaviour are illustrated by equations (10) and (1 I).
A comparison of the affinities of the compounds of type [M(TPP)py] (M = Mn, Fe, Co) for molecular oxygen (under one atmosphere pressure) in toluene at - 79°C gives Co" < Fe" c Mn". This order also corresponds to the ease -of oxidation of the base-free metalloporphyrins from MI1 MI11 497,499 Optical and ESR measurements were initially used to study the reversible coordination of dioxygen by the above Mn"(TPP) compound. The oxygenated product differs from the corresponding Cor' and Fe" porphyrin adducts since it is of intermediate spin and contains three unpaired electrons ( S = 3/2) rather than being low spin (as are the Fe" and Co" analogues). The ESR results suggested a bonding scheme in which the three unpaired electrons are localized on the manganese thus corresponding to a 'Mn'"-peroxo' valency formalism. However subsequent IR studies suggest that the negative charge on the O2 in [Mn(TPP)O,] lies between those of the superoxo (0;) and ~ ' , ~ ~ that ~ O2does not add to Wn(TPP)py] but rather, peroxo (O:-) ions.499The s t ~ d i e s ~indicate adduct formation results in loss of the pyridine Iigand to yield five-coordinate [Mn(TPP)O,] as the product. The equilibrium constants for replacement of a metal-bound pyridine moiety by 0, in [Mn(TPP)Py] and in the related compounds of a series of p-substituted tetraphenylporphyrins at - 78°C are all less than unity.496Thus a high concentration of the coordinating base will not favour oxygen-adduct formation; this provides a rationale for the reported inability of Mn" haemoglobin (with its high 'local' concentration of imidazole in the region of the metal ion) to reversibly bind molecular oxygen." The visible, ESR and IR studies mentioned previously are in accordance with the symmetric sideways coordination of dioxygen in [Mn(TPP)O,] and such a coordination geometry is also consistent with the results of MO calculations.s00The mode of bonding of the dioxygen thus appears to differ from that in the corresponding Fe" and Col' porphyrin adducts which contain the oxygen bound in an end-on fashion. Recent IR studies of the dioxygen adduct of octaethylporphyrinatomanganese(II), [Mn(OEP)O,], prepared by matrix condensation techniques, indicate that the v ( 0 , ) stretch for this compound, as well as for the corresponding TPP and phthalocyanine complexes, all fall in the range 922-983 cm-' .50' Deuterium NMR spectroscopy has been used to define the electronic structure of compounds generated by reaction of the superoxide ion with a Mn" porphyrin.502The position of a deuteratedpyrrole signal together with a solution magnetic moment of 5.0 BM were interpreted as indicating the presence of a superoxomanganese(I1) porphyrin system with spin coupling between the superoxide ligand and the manganese ion. This system has been proposed to be an excellent model for what has been postulated to be (formal) superoxide coupling to low-spin iron(II1) in the oxyhaemoglobin molecule. Although the superoxide ion is not a usual substrate for haemoproteins, it is significant as an important resonance configuration for oxygenated haemoglobin and cytochrome P-450. In contrast to its behaviour towards 02, manganese-substituted haemoglobin binds NO reversibly at room temperature. Moreover, the Mn" + NO system is isoelectronic with Fe" + CO and has Examples of nitrosyl been argued to be a close model for the binding of CO to haemog10bin.~~ adduct formation by simple Mn porphyrins have also been reported.504 A number of dimetallic 'face to face' dimeric porphyrins containing Mn" have been ~ynthesized.~'' For example, urea-linked TPP dimers (191) have been obtained containing imidazolate ligands ~
74
Manganese
bridging between Mn"/MnI1 and Mn"/Co" centres. The study of such dimeric species is timely because of the growing number of bi- and tetra-nuclear metalloproteins (haemocyanin, copper oxidases, haemerythrin, superoxide dismutase and cytochrome oxidase) which have been found to exhibit interaction betwen heteronuclear metal sites. In particular, the imidazolate-bridged compound has been proposed to be a spin model for cytochrome oxidase even though it contains Mn"/Co" rather than Fe"'/Cu" as occurs in the natural system. However, in each system there is an S = 5/2 metalloporphyrin interacting with an S = 1/2 centre. From the measured small value of - J ( = 5 cm- ' 1 for the antiferromagnetic coupling in the synthetic dimer it was concluded unlikely that histidine acts as a bridging ligand to Cu, at the haem a3 site of cytochrome oxidase.
\
NH
A related series of mixed-metal 'face to face' porphyrin dimers (192) has been studied by Collman et a1.% A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 0, to H,O via H,Oz. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co"/Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemically." However for most of the mixed-metal dimers, including a Co"/Mn" species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced.
Mmganese 41.3.10.2
75
Phtha&ocyanineligands
Much of the early interest in manganese porphyrins had its origins in prior ~ ~ r kinvolving , ~ ~ ~ ' ~ ~ ~ manganese phthalocyanine 193; [MnPc]). Phthalocyanines (including those of manganese) were reviewed by Lever in 1962.51Since then, the general metal-ion chemistry of Pc and its derivatives has been discussed in numerous books and articles; two more recent reviews are by Boucher5" and Bere~in.~'~
b
Self condensation of phthalonitrile or o-cyanabenzamide in the presence of manganese metal gives a product which, on sublimation, yields pure W ~ P C ] .Alternatively ~'~ this product may be obtained by the reaction of phthalonitrile with manganese acetate. Although similarities exist, the chemistry of manganese phthalocyanine species show many differences to that of the corresponding
porphyrin^.^,^'^ [MnPc] (/?-polymorphic form) is unusual in that it provides an example of an intermediate spin state with S = 312 for the d5 configuration. This compound also provides a rare example of a ferromagnetic molecular crystal (J = + 7.6 cm-' ). [MnPc] has been the subject of both X-ray513,514 and neutron diffraction5I5studies. The coordination environment of the Mn ion is square planar and the overall molecule is isostructural with a range of other divalent phthalocyanines. Stacking of the planar [MnPcf molecules occurs in the lattice such that two pyrrole nitrogen atoms from each molecule lie exactly above or below the manganese belon ing to adjacent moIecules. The distance between manganese and the axial nitrogen atoms is 3.4 . The Mn" atoms are aligned in linear arrays parallel to the b axis of the molecule such that [MnPc] may be considered to contain magnetic linear chains along the b axis of the crystal. The magnetic properties of [MnPc] are consistent with the presence of ferromagnetic intrachain exchange together with weak antiferromagnetic interchain interaction; the Curie temperature is 8.6 K.5'"5's The X-ray and neutron diffraction data mentioned previously have been used in conjunction with the technique of polarized neutron diffraction (at 4.2K) to deduce spin-density distributions in [ M ~ P C ] . ~In' ~further - ~ ' ~ investigations5I4it proved possible to determine individual 3d and 4s orbital populations on the manganese ion together with an estimate of 24% for the d-orbital electron density delocalized in the macrocyclic ring. From these studies it appears that the charge on the manganese is approximately 1 ; this charge appears to be achieved primarily by the loss of a 3d electron rather than a 4s electron. Largely because of the possible involvement of manganese in the photosynthetic production of oxygen, the energetics of manganese phthalocyanine redox processes have been of considerable have used a number of electrochemical and other techniques to effect and interest. Lever et characterize the oxidation of Mn"(Pc) to Mn"' (Pc) species, ligand oxidation of Mn"'(Pc) and two successive one-electron reductions of Mn"(Pc). The electrochemical studies were carried out using pyridine, dimethylsulfoxide or dimethylacetamide as solvent and perchlorate, chloride or bromide as the supporting electrolyte anion. Heterogeneous rate constants were obtained for several of these couples in the presence of perchlorate anion. Each of the couples showed near ideal behaviour with the exception of the chloride and bromide systems at higher scan rates. No evidence for the formation of Mn' species was obtained. [MnPc] reacts with a number of monodentate bases to yield low spin ( S = 4) adducts of type [MnPcLJ (L = pyridine, imidazole, triethylamine In the solid state [MnPc(py),] loses its two pyridines to yield [MnPc] on heating the adduct in vacuo at 100"C.5*3 Despite earlier reports to the contrary [MnPc] does not react with oxygen in pyridine provided
a
+
e
t
~
.
)
.
~
~
~
7
~
~
~
76
Manganese
the pyridine is rigorously dried and purified. If the latter condition is not met then reaction does occur with the end-product of the oxidation being a dinuclear Mn"' species of type Nevertheless, reaction does occur in pure N,N-dimethylacetamide to yield a solution of the oxygen adduct. Reversal of the oxygenation occurs over many days on degassing the solution. The deoxygenation occurs more rapidly upon exposure to bright white light or upon addition of an electron donor to the solution in an evacuated flask. For the first reaction type the released molecular oxygen was detected using mass spectrometry. In contrast, when the adduct is reconverted to [MnPc] with electron donors such as imidazole, triethylamine, catechol etc., no oxygen apears to be released. In this case the adduct seems to be mimicing oxidase activity of the (see later). Further, addition of certain electron donors, such as type proposed by Uchida et ~21.~'~ imidazole, to the adduct in the presence of oxygen results in formation of a dinuclear species of type [PCM~~'~--O-M~"'PC];the latter may be reconverted to the oxygen adduct by reaction with oxygen in N,N-dimethylacetamide. The oxygen adduct has been isolated and has S = From the results of physical measurement^,^^^,^^^,^^' the adduct was proposed to contain a bound superoxide and to be of type [Mn"'PcO,]. The various reactions involving this compound are summarized in Scheme 2 (modified from reference 522 using, where appropriate, imidazole as the electron donor).
+.
[Mnl'Pclm]
Imidazole
IMn"Pc1
4
Dezassing with N,
o f imidazole
Lniidazolc condilionr)
(anerohic
Vacuum or nitrogen degassing of dimethylacetamide solution (aided by visible light)
0,in dimethylacetamide (pure)
Imidazole in presence oro,
[IJcMn''i-O
-
-Mn"'Pc]
c
Scheme 2
[MnPcjhas been demonstrated to show high catalytic activity for the oxidation of 3-methylindole The major product of (equation 12) and thus mimics the action of tryptophan-2,3-dio~ygenase.~~~ the catalyzed oxygenation is 2-formamidoacetophenone, with 2-aminoacetophenone and the 2-isonitrile derivative as minor products. The model studies indicate that the substrate specificity for the oxygenation of indole derivatives is different to that of singlet oxygen or radical autoxidation. In other studies, the [MnPc] catalyzed autoxidation of phenols has been in~estigated',~together with other catalytic redox reactions involving a number of different substrate types.5" 0
41.3.10.3 Other nitrogen donor macrocycles In contrast to the large number of studies concerned with porphyrins and phthalocyanines, other ~ ~ " ~this ~ macrocyclic ligand compounds of Mn" have received somewhat less a t t e n t i ~ n . " ~ ,In~ part, appears to reflect the fact that many compounds of this type tend to be of only moderate thermodynamic stability and in some instances are air and/or moisture sensitive. In addition, Mn" has been shown to be ineffective as a template metal for certain cyclization reactions which occur in the presence of other ion^.^^^,^^^*^^^,^^^ This effect may be a consequence of the relatively large size of the Mn" ion not enabling it to fit in the cavity of the target macrocycle. Alternatively, the often low affinity of Mn" for the macrocycle precursors may contribute to the absence of a template effect for particular systems. The meso form of the saturated tetradentate macrocycle (194) has been used to synthesize the
77
Manganese
colourless compounds [MnX,(194)] (X = CF,SO,, CHjC02).533The synthetic procedure used to obtain these species is more demanding than often required for other first row transition metal complexes since these compounds are both oxygen and water sensitive. Further, the macrocycle appears to exhibit only low affinity for Mn". The compounds were obtained by reaction of (194) with Mn(S03CF3),-CH,CN or Mn(CH,CO,), in hot degassed acetonitrile under anhydrous conditions. Both compounds are high spin and the solid state ESR spectra indicate that the manganese ion is experiencing a relatively small tetragonal distortion in each case. Using a template procedure involving condensation of 2,6-diacetylpyridine and the appropriate triamine in the presence of Md', compounds of type [Mn(NCS),(195)] and [MnCI(195)]PF6have534 been isolated. The synthesis of [MnC1(195)]PF6is illustrated by equation (1 3). Both compounds are high spin with magnetic moments of 5.94 BM; they appear to contain either five or six coordinated Mn". It is of interest that attempts to chemically oxidize these compounds to Mn"' species using NOPF, or H,Oz in acetonitrile in the presence of excess Cl- ion were unsuccessful even though the Mn" compounds of the fully-saturated (14-membered) macrocycle (194) readily oxidize under similar conditions. This difference in redox behaviour is undoubetedly related to the presence in (195) of a conjugated fragment containing three nitrogen donors. However it is inappropriate to speculate further about this since larger ring (N5-donor) macrocycles incorporating this moiety do undergo oxidation to Mn"'species.
r"" HNl NH HN
MeuMe Me
MnClz
+ Me &Me
+
(-
Mn" compounds of the rigid dianionic macrocycle (1%) of type [Mn(l96)(amine)] (where amine = triethylamine or tri-n-propylamine) have been synthesized directly by reaction of the preformed macrocyle with Mn(S0,CF3), in acetonitrile followed by addition of the amine.535These high spin compounds were assigned square pyramidal structures with the metal ion displaced from the plane of the macrocycle towards the axial ligand. Such a structure has been confirmed for [Mn(196)NEt3]by X-ray diffraction.536This compound has the Mn" ion 0.73A out of the N,macrocyclic plane. The hole size in (196) is smaller than in the porphyrins; the five-coordinate geometry appears to be a direct consequence of the Mn" ion being too large to fit completely in the macrocyclic cavity. The coordinated macrocycle adopts a saddle shape reflecting steric interactions between the methyl groups and the aromatic rings. A detailed resonance Raman study of these Mn" species has been ~ n d e r t a k e n . ~Relative ~ ' * ~ ~ ~to the spectrum of the free ligand, the vibrations observed to undergo large shifts on incorporation of the metal ion were those associated with the 'porphyrin-Iike' six-membered chelate rings.
N
N
Mangunese
78
Descriptions of the synthesis of the tetraaza macrocycle (197) by a non-template procedure from o-phthalonitrile and 2,6-diaminopyridine as well as the synthesis of its Mn” compound have been published.s39The manganese species is very insoluble; it is high spin with a magnetic moment of 5.80 BM and obeys the Curie law between 85 and 300 K. Manganese compounds of a number of N,-donor macrocycles have been investigated. For Alexander et ~ 2 . ’syn~ ~ example, following earlier work involving iron complexes of (198),540754’ thesized a range of high spin ( S = 512) complexes of type [MnX2(198)]*nH,0(X = C1, ClO,, rz = 0, 1 or 6 ) . For example, orange [MnCl,(198)] -6H,O was obtained by condensation of 2,6-diacetylpyridine with triethylenetetramine in methanol in the presence of manganese chloride. This product is isomorphous with the corresponding magnesium compound which was postulated to contain a [Mg(198)(H,0),]2+ moiety.543The single-crystal ESR spectrum of the Mn” compound exhibits an . ~ ~ and the other compounds in the series unusual angular dependence of the ESR t r a n s i t i ~ n sThis were assigned seven-coordinate structures. For [Mn(C10,),(198)], infrared evidence suggests that the perchlorate groups are coordinated in the solid. The macrocycle in [MnCl, (198)] H,O can be reduced using a nickel-aluminium alloy under basic condition^.^' Addition of cold concentrated nitric acid then leads to isolation of the reduced macrocycle (201) as its tetrahydrogennitrate salt.
-
(198) n = m = 2 (199) m = 2 , n = 3
(200) rn
=
3, n = 2
showed that Mn” acts as Using a related in situ procedure to that just described, Drew et a template for the synthesis of compounds of all three macrocycles (198)-(200) containing 1 5 , 16and 17-membered great rings respectively. The products are of type M,X,L.xH,O and MnXL(CIO,)*xH,O (X = C1, NCS or BPh,; x = 0,0.5, 2 or 6). As before, all the compounds are high spin with S = 5/2 ground states. The compounds of the 15- and 16-membered macrocycles have been assigned pentagonal bipyramid structures in which the macrocycle coordinates in the equatorial plane with the negatively charged monoanions or water molecules occupying the axial positions. These species are all mononuclear with the exception of those of type [{Mn(NCS)L},][ClO,], which were assigned NCS-bridged, polymeric structures in the solid state. The latter complexes exhibit Curie-Weiss behaviour with Weiss constants of - 13 K in each case; this behaviour has been interpreted in terms of weak antiferromagnetic exchange. The 17-membered ring appears to yield metal compounds which have a somewhat different structure to those of (198) and (199). This was confirmed for [Mn(NCS),(200)] by an X-ray diffraction study which shows that, while the coordination sphere is best considered as a distorted pentagonal bipyramid with the thiocyanate groups in axial sites, the macrocycle is distorted from planarity. The pyridine nitrogen is 0.92A out of the plane containing the other four ring nitrogens and the metal ion. The remaining compounds containing the 17-membered ring, [MnC1(200)][C104]and [MnC1(200)]C14.5H,O, appear to be six-coordinate and have been proposed to have pentagonal pyramidal structures with the vacant (axial) site being partially shielded by the folded macrocycle. In separate work, Dabrowiak er al.’” also prepared manganese compounds of the 16- and 17-membered rings; these were used as precursors for the corresponding Mn”’ compounds (see later). A crystal structure of the 1:l adduct of 1,lO-phenanthroline and the seven coordinate species, [Mn(l!39)(H,0),](C10,)2, indicates that the phenanthroline molecule is not coordinated to the manganese but is sandwiched between [Mn(199)(H2O),I*+ units in a lamellar structure+136 The phenanthroline nitrogens are hydrogen bound to water molecules in adjacent complex ions; the structure also appears to be held together by 71 donorln acceptor and dispersion forces.
Mangunese
79
Compounds of type [MX,(202)] (X = CIO,, NO3) have also been rep~rted.”~ Like the closely related dimethylated species, these compounds have been assigned distorted pentagonal bipyramidal structures and for the compound with X = ClO,, an X-ray structure determination confirms this geometry. Mn” complexes of type [MnClL]’ (where L = (203) with R = Me or CH,CH,CH,OH; R‘ = Me and R = Me; R = H) and [MnX,L].nH,O (where L = (203) with R = Me, R = H or R = Me, R = Me) have been synthesized by Lewis and co-workers using a template For example, to prepare the complex of (203; R = H, R’= Me),5492,6-diformylpyridine and 2,9-di(lmethy1hydrazino)- 1,lO-phenanthroline monohydrochloride were condensed in a methanol/water mixture containing manganese chloride. X-Ray structure determinations of each of the species [MnClL](BF,) (L = (203) with R = CH, or CH,CH20H),549~550 indicate that they have similar coordination geometries: the Mn” ion is six-coordinate (pentagonal pyramidal) and is bound by five macrocyclic nitrogens in a plane and by an axial chloro group. The metal ion is displaced from the N,-plane towards the chloro ligand in each compound. For the metal compound containing the peripheral hydroxyethyl substituents, these groups are not coordinated. They lie on the same side of the macrocycle as the axial chloro ligand. The compounds of type [MnX,L]-nH,O have been assigned pentagonal bipyramidal coordination geometries. The electrochemistry of a number of such six-coordinate compounds [MnXL]+ and seven-coordinate compounds [MX,L] (with L = (203), R , R = Me and X = halide, water, triphenylphosphine oxide, imidazole, 1-methylimidazole or pyridine) has been investigated.’” The redox behaviour of these compounds was of interest because it was considered that the potentially z-acceptor macrocycle (203; R = R‘ = Me) may promote the formation of Mno or Mn’ s p i e s or may yield a metal-stabilized ligand radical with the manganese remaining in its divalent state. For a number of macrocyclic ligand systems, it has been demonstrated that the redox behaviour can be quite dependent on axial ligation; it was also of interest to study whether this was the case for the present systems.
(202)
(203)
Cyclic voltammetry on the above compounds in acetonitrile gave a reversible one-electron reduction wave near - 1.4V (versus Ag/AgNO,) in each case. Thus the reduction potential showed little variation with change of axial ligand. This suggests that the redox behaviour is confined to the coordinated macrocycle since, as just mentioned, axial ligand type would be expected to markedly influence redox behaviour directly involving the metal centre. Quantitative reduction using controlled potential electrolysis led to isolation of the corresponding one-electron reduction products. These were shown by ESR studies to be Mn” ligand-radical species; it was postulated that the additional electron in these species resides in the diimine pyridine portion of the ligand. No evidence for metal-ion reduction was obtained even when n-acceptor axial ligands such as CO or phosphines were added to the solution in the electrochemical cell. Few stability constants for manganese compounds of cyclic ligands have been reported. A log K value of 15.0 ( I = 1.0 M, KNO,; 25°C) for the 1 :l compound of the dicyclic ligand (204) and Mn” has been rep~rted.’~’ The log value for the corresponding compound of the related non-cyclic ligand (205) is 9.3.553The large difference in these values illustrates well the operation of the ‘macrocyclic e f f e c P in the bicyclic system.
80 41.3.10.4
Manganese Oxygen donor macrocycles
Mn" compounds incorporating crown ether macrocycles have been reported. The coordination chemistry of crown ethers has been mainly concerned with the compounds of non-transition ions.55' This is due, in part, to the generally weaker affinity of such ligands for transition metals. X-Ray studies indicate that a number of transition metal compounds containing crowns (which were isolated from aqueous solution) have the crowns H-bound to coordinated water molecules rather than being bound directly to the metal ion. Such an arrangement occurs in [Mn(H,0),](CI0,),-L,5'6 L (where L = (206); 18-~rown-6).'~~ In [Mn(NO,)(H,O),](NO,)~ L s H , O ' ~and ~ ~n,Clz(H20)8]C12contrast, it has proved possible to isolate yellow-orange [MnL,](Br,), (L= (207); 12-crown-4) in which the macrocycles bind directly to the Mn" ion; the synthesis was performed in methanol under anhydrous conditions.s59The X-ray analysis of this compound indicates that the cation consists of two 12-crown-4 macrocycles bound in 'sandwich' fashion to the Mn" ion. The Mn" is located on a crystallographic mirror plane whch requires that the 0,-donor set of each crown ligand be coplanar. The overall coordination geometry is square antiprismatic. It is evident from the mean length (2.31 A) of the Mn-0 bonds that the ether ligands are weakly bound. Other Mn" compounds of polyethers have been reported. For example, a mono-crown species of composition [MnL(MeNO,)](SbCl,), (where L = 206) has been isolated after addition of 18-crown6 in CH,Cl, to [Mn(MeNO,),](SbC&), in MeN0,.508 The product rapidly decomposes in the presence of water. From infrared studies it was concluded that the MeN0, molecule is bound to the Mn" and that at least one of the ether oxygens is not coordinated. Compounds of composition (MnBr2),L*3/2acetone*H,0 (L = (20s) benzo-15-crown-5) and (MnX,),L*SH,O (L = 206 X = C1, Br) have also been isolated from non-aqueous reaction Comparative infrared studies have also been used to investigate the nature of these species; however, it has not been possible to assign their structures with certainty.
n
co "7 Q e, .s
n O)
O W 0
L/
41.3.10.5
Q-.)
e, oJ W
Mixed donor macrocycles
Studies in which 2,6-diacetylpyridine and 3,6-dioxaoctane-l,8-diamine were condensed in the presence of a stoichiometric amount of Mn" led to isolation of orange [Mn(NCS),(209)].562As found for the Mn" compounds of the related N,-ligand system, an X-ray study indicates that this high-spin species is seven-coordinate with the macrocycle occupying the pentagonal plane and the axial positions being filled by isothiocyanate groups. The five donor atoms of the macrocycle are almost coplanar although the structure is slightly distorted from true pentagonal bipyramidal geometry. The interaction in 95% methanol (1 = 0.1, Et,NC104) of Mn" with a series of 02N3-macrocycles of type (210) has been in~estigated.~'~ 4n all cases compounds with a 1:l metal to ligand stoichiome-
(209)
Manganese
81
try were observed. As expected from the position of Mn" in the Irving-Williams stability order, these compounds are all less stable (with log K values in the range 3.54.0) than the corresponding complexes of Co", Ni", CUI' or Zn". Pentagonal bipyramidal compounds of type WnX,L] (where L = (211) with Y = 0 or S; X = ClO,, NO3) have been synthesized547using in situ procedures. In other studies it was observed that reaction of MnCl, with the appropriate ligand precursors in ethanol yielded initially a bright orange-yellow product of composition MnC12L-2H,0 (L = (211) with Y = 0.564 If this product was removed from the reaction solution, needles of the free macrocycle (212) crystallized. In this product ethanol has added across each of the imine bonds of (211) with Y = 0. Additions of this type have been documented in other imine-containing complexes,s65a driving force for these reactions may be the relief of strain in the highly conjugated diimine precursor. If (212) is returned to the reaction filtrate and the solution warmed then red-brown [MnCI, (212)] crystallizes. In a further experiment the condensation was performed in the absence of the Mn" and (212) was still isolated indicating that its formation is not controlled by a metal-template reaction.
Reaction of 5-methylisophthalaldehyde with 1,3-diaminopropane in the presence of manganese(I1) chloride and manganese(I1) acetate in methanol yields a dimetallic complex of type in which the dianionic form of the macrocycle acts as a novel Mn, C1, L 2H,O (where LH, = 213)566 binucleating species. The proposed structure is one in which each metal ion is surrounded by an O,N,-donor set in an approximately planar arrangement with a chloro group occupying an axial position on each Mn"; the coordination geometry of each Mn" is approximatery square pyramidal. A stereochemistry of this type has been confirmed for the analogous dicopper(I1) species by X-ray diffra~tion.'~'The study of metal-metal interactions in the dimetallic compounds of (213) is simplified by the fact that both metal ions are in identical environments. However, in contrast to Mn,C1, L * 2 H 2 0 (where the compounds of this ligand with some other metal ions,566,s68,569 LH, = 213) exhibits little evidence of antiferromagnetic interaction between adjacent Mn" ions - the compound shows Curie-Weiss behaviour with a Weiss constant of - 7°.566 In other studies, the anhydrous compound Mn,C1, L has been demonstrated to contain a slight ferromagnetic exchange interaction with J = 0.2 ~ m - ' . ' ~ ~
-
+
Magnetic and electrochemical studies involving mixed metal compounds of deprotonated (213) have also been r e p ~ r t e d . ~These ~ ~ , ~compounds ~' include Cu" /Mn" and Cu'/Mn" species. For the [Cu"CU"L]~+compound mentioned previously, two sequential reductions corresponding to conversion of this species to [Cu"Cu'L]+ and then to [Cu'Cu'L] were observed at -0.93 and - 1.32V, respectively, relative to Fe/Fe+ in dimethylformamide. In contrast, for [ C U " M ~ " L ] ~the + couple. It is Cu"/Cu' wave occurs at - 1.OV; a wave at + 0.150V corresponds to the Mn"'/Mn''
82
Manganese
perhaps surprising that substitution of Fe", Co", Ni" or Zn" for the Mn" in the above heteronuclear compound does not alter (within experimental error) the value of the Cu"/Cu' couple. This difference between the redox behaviour of the dicopper(T1) species and [CU"M"L]~+(M = Mn, Fe, Co, Ni or Zn) has been postulated to reflect the fact that only the [Cu'Cu"L]+ species can exist to any extent as a resonance-stabilized delocalized ion. The Cu' in a series of [Cu' M" L]+ compounds (including [Cu' Mn''L]+ ) has been demonstrated5'* to have an axial affinity for some or all of the bases: carbon monoxide, ethylene, tris(umethoxyphenyllphosphine and 4-ethylpyridine. In all cases the binding constants for adduct formation are low; for example, for [Cu'Mn''L]+ and CO or 4-ethylpyridine the values are 0.9M-' and 22.5 M-' , respectively. In contrast to the constant Cu''/CuI reduction potentials mentioned above for the [CurrM'rL]2+ species, the binding constants for the attachment of axial ligands to the CUI in the respective [Cu'M''L]" compounds show a slight dependence on the nature of MI'. Thus the binding studies indicate that communication occurs between adjacent metal ions. While the nature of these effects are as yet little understood, studies in this general area show potential for providing a fuller understanding of a number of metal-containing biological systems in which communication between adjacent metal ions has been demonstrated to occur.
41.4 MANGANESE(II1)
Oxidation of Mn" to Mn"' has long been known to occur, but details of the, sometimes complex, products are only now emerging. Whereas, in many systems, simple one-electron oxidation gives well-defined Mn"' species, yet in others the products may be mixtures of oxidation states, and there are examples of direct oxidation to Mn" without any detectable Mn"' intermediate (Section 41.5). Accordingly, many compounds which are called Mn"' in the literature, especially the various products of the 'oxygenation' of Mn" salicylaldiminates (Section 41.3.9.3), must be assigned to the 'not proven' category at this stage. There are few cationic compounds with neutral ligands: the known chemistry largely refers to compounds of the anionic ligands and generally the more electronegative donors 0 and F. However, there is an extensive chemistry with the macrocyclic [N4]ligands; [Mn"'S,] species are well characterized with the dithiolates, which can satisfy both charge and coordination shell requirements; and there is a recent report of [MnI, (PR3)J (Section 41.4.2) which opens the possibility of new vistas. Compounds of the oxygen ligands have found good use as oxidizing agents in hydrocarbon chemistry, as has MnF, as a fluorinating agent. Figure 1 indicates, and the observations for the various ligand systems (4.v.) add further detail, that Mn"' is often subject to disproportionation (14), a reaction which, in aqueous solutions, is often driven by precipitation of MnO, (equation 15). 2Mn"'
e
Mn"
+
Mn"
(14)
Table 43 Coordination Polyhedra" and Spin States for Manganese(II1)
Coordination Number
Donor Atoms
f a ) High Spin Polyhedrn (peA= 4.8-5.3 BM) 7 6 (Octahedral)
5
4 (Tetrahedral) (Planar) ( b ) Low Spin Polyhedra 6 (Octahedral) a
beR= 3.1-3.2BM)
[O,jd [c,i,
w6irI ~ , N I [ P ~ ~ ~ , I
In most cases, the polyhedra are included only if X-ray proof of structure i s available.
'Macrocyclic ligands.
Spin state not yet reported. This refers LO the basic chloride in Table 46.
I 83
Manganese
Mn"' is formally d 4 , and therefore the high spin configuration in an octahedral field is one of those, like the d g of Cu", which the Jahn-Teller theorem predicts should distort. Such distortions are indeed observed in the great majority of, but not all, known structures; and they often serve to identify the Mn"' polyhedra in mixed oxidation state compounds. There are few examples of other spin states (Table 43): [Mn(CN)6]3-is an example of a low spin ( S = 1) compound, and there are other examples in the macrocyclic compounds (Section 41.4.8). Magnetic measurements, although useful for confirmation of spin state, have little diagnostic value, and ESR signals are usually not observed, reputedly because of fast spin relaxation. Although again of little proven diagnostic value, except within limited classes like the solid halides,348the electronic spectra of these d 4 systems have spin allowed d-d transitions; and accordingly the compounds have more colour than those of Mn". Except where specific ligand steric effects add kinetic stability, the compounds of Mn"', including the low spin [Mn(CN>,]3-, are quite labile.
41.4.1
Carbon Ligands
The literature of Mn"' and Mn" cyano c o m p o ~ n d s " , ~is~somewhat * ~ ~ ~ confused by claims for non-existent species, which have since been disproved. Hence, we list in Table 44 only the known species for which acceptable proof has been published. K,[Mn(CN),], which was one of the few examples of low spin Mnl", was first prepared in 1837. Standard methods of preparation have been described; but, in some hands at least, these may lead to contamination of products with [Mn2O(CN),,I6-.Hence, much of the early work on the physical properties of [MII(CN)~]~is in doubt. There is but one form of the compound K,[Mn(CN), -red, monoclinic crystals-in which measured Mn-C distances are 1.94, 1.99 and 2.00( x 2) fk, giving the same average value of 1.98 8, as for the cubic Cs,Li[Mn(CN),]. Magnetic measurements (peE= 3.50 BM at room temperature) indicate a low spin &'f configuration. The undiluted K+ salt showed no ESR signal between 12 and 290K. Exchange of CN- with [Mn(CN),13- in water is very fast, and there are indications" of an 'associative' mechanism involving [Mn(CN),H,0l3-. Aqueous perchloric acid solutions often precipitate Mn(CN), .nH,0,574which is another example of disproportionation of Mn"', being formulated as Mn" [Mn" (CN),]*nH,O (Section 41.5.1). A major contaminant of many of the earlier preparations of K,[Mn(CN),]~" and often described as a different form of this, was a golden brown dimeric oxo-bridged species [(O5Mn-OMn(CN)J-. It is also formed when KMnO, is reacted with KCN, and an X-ray analysis of the crystalline compound K,[Mn,O(CN),,, CN showed a linear Mn-0-Mn bridge, with Mn0 = 1.72 A and Mn-CN = 1.97-2.06 . It is almost diamagnetic because of antiferromagnetism. Various metal cations form precipitates with [Mn(CN),I3- in neutral or acid solution. One of them, Fe, [Mn(CN),l2 undergoes an interesting reaction: on heating, rearrangement occurs to [Fe(CN),I4-. With our formalism, accepting NO as a neutral ligand, [Mn(CN),NO]'- becomes an Mn'" compound, although it is not very comfortable here, especially when placed against [Mn(CN),(NO,)l3-, which it forms when irradiated (UV) in the presence of 0,. The nitrosyl is yellow, has a magnetic moment of 1.73 BM and a vNo of 1885cm-'. Attempts to prepare them Mn"' alkyls appear to be particularly prone to disprop~rtionation.'~~ from [Mn(acac),] and Li alkyls in the presence of phosphines give transient colour changes at low temperatures which appear to be the Mn"' species, but only Mn" and Mn" alkyls (4.v.) have been isolated from the solutions. An interesting which has not yet been confirmed, describes
i
Table 44 The Mn" and Mn'"Cyano Compounds Compound
Oxidation State
I11 II/IV I11 ?
I11 IV
C.O.C.+D
Structure
Colour
[Mn(CN),]'- octahedral Mn'1[Mn'V(CN)6].nH,0 [(CN),MnOMn(CN)J6[Mn(CN),NO]'- octahedral
Red Green Golden-brown Yellow
[Mn(CN),]'- octahedral
Yellow
1 Munganese
84
Mn"' monoalkyls [MnR(N,O,)H,O] {where (N202)is a Schiff base ligand (161)}, which is analogous to the well-characterized Co"' species. Six compounds were described, all with magnetic moments near 5.0 BM.
41.4.2
Nitrogen and Phosphorus Ligands
The known chemistry of Mn"' with nitrogen ligands, other than the macrocycles (Section 41.4.8), is still quite limited {but see also the mixed N, 0 ligands, Section 41.4.7). Perhaps the tendency for Mn"' to disproportionate, in many ligand systems, is responsible for the failure to obtain [Mn{N(SiMel),),], when these species have been obtained84for all other metals Sc"' to Fe"'. from the reactions of MnCl, with uni-, bi- and Most of the data for this section come78+210~s72 ter-dentate ligands (Table 45). The green monopyridine compound (214) is not at all robust, but reacts further to give a more stable, brown tris compound [MnCl,py,] (215). Compounds of the latter class generally are sensitive to moisture (although the NEt, compound is distinctly less so), but the py compound is stable up to 100°C under N,. No physical measurements were reported, but the related terpy compound (219) is a typical, high spin, octahedral species. The air stable red compounds (215) formed with the bidentate ligands, appear to be dimers, with magnetic moments reduced by antiferromagnetism; but the aquo species (217) are almost certainly [MnCl, (H,O)(N,)] octahedra. A structural analysis of the related, but more versatile, nitrato species [Mn(NO,),bipy] shows a seven-coordinate polyhedron with two bidentate nitrates and one bipy nitrogen almost co-planar (Mn-0 = 2.1 1-2.40; Mn-N = 2.09 A), and axial N (2.00 A) and unidentate NO3(Mn-0 = 1.90A). ESR spectra have been observed for the compounds (215), but they are very broad single lines (6000e). Table 45 Solid MnX,-L, Compounds for Nitrogen Ligands Ligand and Cornmenis
Class (214) MnX,N (215) MnX,N, (216) (217) [MnX,N,(H,O)l (218) [MnCl,N, j
(219) (220) [(Mn"'O2-Mn'")(N2),]X, (221) [M4N,),I(CIO,),
(X = c1-) (X = is@-) (X = a - ) (X =NO;) (x = F-) (X = Cl-)
py, quin: green crystals from cold ether py: chocolate-brown from hot py bipy, phen: red or red-brown bipy, phen: red crystals phen: orange-brown bipy, phen: red-brown N = NH,,RNH, (R = Me, Et, Pr,Bu): brown R,NH (R = Me, Et): brown R,N (R = Et, Bz): Et, brown; Bz, green py, quin: brown N3= terpy: brown N2 = bipy, phen (X = ClO,, PF,, fS,Oi-): green Nz = en: green
Pef
(BM)
3.9
5.0 4.9 5.0
4.9 2.1-2.8
4.9
Various reactions of excess phenanthroline or bipyridyl with Mn" or Mn"' species in the air, or with KMnO, solutions, produce the compounds (220), which have the dimeric, dioxo-bridged, Mn1"/Mnw structure (222). These green compounds, isolated with various X-, are stable in moist air at room temperature and in the daylight for several days, but decompose somewhat faster in solution. The Mn"' and the Mn'" octahedra are well defined in the X-ray structural analysis of [MnzO2bipy4](CIO.,),-3H20,with the Mn"' having the longer bond lengths and the expected tetragonal Magnetic moments are low because of antiferromagnetism and ESR spectra showing hyperfine coupling to both Mn"' and Mn'" have been obtained.
Manganese
85
The green dimeric 3 + species can be oxidized to the 4+ Mn'" species (Section 41.5.2), but, although there is good electrochemical evidence (in CH,CN solution) for the quasi-reversible step (a) in reaction (16), the Mn"' dimer (2+) is unstable. Equally protonation of the dioxo-bridge atoms leads to increased instability; green solutions of the Mn"'/Mn'" anion turn red on the addition of H+; the process is immediately reversible; but reversibility decreases with time as the dimer decomposes to Mn".
[(N,N)2Mn"102Mn11'(N,N)2]2+ [(N,N)2Mn"'0,Mn"(N,N)2]3+ [(N,N)2MniV0,Mn'V(N,N),]4+ (16) Dihydroxy-bridged species with other N ligands have been postulated for some biguanide compounds, but the details here need to be confirmed by X-ray structural studies. However, both dihydroxy- and dimethoxy-bridged dimers are known for Mn"' compounds of salicylaldiminate ligands (Section 41.4.7). The only reference to a tris-bidentate species is the compound (221), which is reported578to be a green crystalline compound with peE= 4.85 BM and g = 2.08. Many other [MnN,] compounds should therefore be accessible. One interesting recent example is the Mn"' compound of the ligand (223). This molecule lies on a crystallographic three-fold axis; is nearly octahedral [MIIN,], but with a significant twist towards the eclipsed, trigonal prism, with Mn-N (pyrrole) 2.054 and (azomethine) 2.148(2) A; and is the first example of Mn"' or any d 4 system, with a crossover from the high spin S = 2 state to the low spin S = 1 state at 4&50K.579
The low spin configuration was earlier found in the deep-coloured, tris chelated amino-troponiminates [Mn(L,)J (for L, = 43). Black [Mn(L,),], prepared from Mn,(CO),, and LNH, had a magnetic moment of 3.12 BM; whereas the brown-black [Mn(acac),(L,)] and black [Mn(acac)(L,),] both were high spin species with per = 4.9 BM (in CHC13).14' An X-ray structural analysis of the latter showed a tetragonal distortion of the [MnN,O,] octahedron.580 Phosphorus ligand compounds of Mn"', except for alkyl and hydride species (Section 41.4.6), are confined to a report of [MnCl,(diphos),]Cl (see Section 41.3.4 and Table 17); and now the unexpected, dark-green, phosphine-iodo compound"' [Mnl, (PMe,),], which has a trigonal bipyramidal five-coordinate structure, with axial phosphines (at Mn-P = 2.43-2.44 A). Undoubtedly the latter compound is but the 'tip of the iceberg', and we can expect to see a considerable further development of this area of Mn"' chemistry; although it is pertinent to recall that the phosphine alkyls of Mn'" disproportionate. 41.4.3
Oxygen Ligands
Many compounds, with a range of ligands, have been isolated as crystalline solids. They are almost invariably good oxidizing agents; and, as such, are used in organic (hydrocarbon) chemistry, especially the acetate, but also now the sulfate and diphosphonate.-
41.4.3.1
Water, hydroxide and oxide
[Mn(H,O),I3+ is perfectly well characterized in the red crystalline alum CsMn(SO,),* 12H,O, but we have significant doubts about the current orthodoxy that it also is a significant component of green aqueous acid Mn"' in water was well in 1968, and nothing published since appears to add significantly to the basic data. Interpretations were given in terms of Mni:, with hydrolysis to MnOH: ; which translates, not unreasonably to [Mn(H20)6]3' and [Mn(OH)(H,0),]2+. The aqueous solutions are described as green and rather unstable. All data referring to [Mn(H,O)J+ were obtained by extrapolating to some ideal, either of time or concentration. But, even so, the extinction coefficient obtained for the visible spectrum of this species is ten times that obtained for it in
Manganese
86
CsMn(SO,),* 12H20.Thus it seems not improbable that the solution data, even in concentrated acid solutions may refer instead to [Mn(OH)(H,0),]2+ or the dimer [(H,0)4Mn(OH)1Mn(H20),]4+; with the second species required by the analyses of the various kinetic datass3being a further 'hydrolysed' (proton-transfer) species, Complications which militate against the observance of pure [Mn(H, 0),13+ in aqueous solution are its oxidizing properties, the usual electroneutrality-driven 'hydrolysis' (or, more exactly, proton transfer to solvent) and the disproportionation reactions (14) and (15). The instability of the cation is shown in the behaviour of the alums MMn(SO,),. 12H,O (M = K, Rb, Cs, NH,): all four were isolated by Christensen in 1901, as red crystalline compounds, but only the Cs compound is stable at normal room temperatures and even this one begins to blacken and decompose at 33°C. No other solids containing the cation appear to have been described. The aqueous solutions of Mn"' are stabilized by the presence of Mn" and H C . They become cloudy on standing, and at a rate which is accelerated by decreasing [H+] or [Mn"], or by increasing [Mn"']. The cloudiness results, of course, from the formation of oxo/hydroxo polymers; but the detail of these and the kinetics of their formation are not understood, except by extrapolation from the behaviour of the other metal compounds. At least for the solid state, we are on firmer ground, and the structural data2" for most of the Mn"' oxo and hydroxo compounds are in Table 46. Mn(OH), has not been detected, instead MnO(OH), in two major crystalline forms, is the species first formed when Mn(OH), is oxidized in contact with alkaline solutions, Several other crystalline solids are known to contain OH- (Table 46). However, we can only speculate that the green crystalline Na, Ba and Sr hydroxomanganates(III), described by Scholder and Kyri584in 1952, may contain the [Mn(OH),J3- anion, although the authors postulated [Mn(OH)J2- and [Mn(OH),I4- as well. The coordination polyhedra are generally [MnO,] octahedra, with the sorts of distortion (Table 46) required by, but not certainly attributable to, the Jahn-Teller theorem. Such distortion is as often rhombic (2 + 2 + 2) as tetragonal (4 + 2). Bond distances average between 2.0 and 2.1 A, and Table 46 Structural Data for the Mn"' Oxo and Hydroxo Compounds Compounds
Bond Distances (AT
Polyhedra
1.963 ( x 2); 1.996 ( x 2); 2.050 ( x 2) (2.00) 1.960 ( x 2); 1.998 ( x 2); 2.047 ( x 2) (2.00) 1.88-1.92; 1.962.01; 2. W 2 . 3 1 (2.05) 1.896 ( x 2); 1.968 ( x 2); 2.178, 2.340 (2.04) 1.868, 1.878; 1.965, 1.981; 2.199, 2.333 (2.04) 1.92 ( x 2); 1.92 ( x 2); 2.40 ( x 2) (2.08) 1.91 ( x 2); 1.95 ( x 2); 2.41 ( x 2) (2.09) 1.89 ( x 2); 1.96 ( x 2); 2.29 ( x 2) (2.05) 1.953 ( x 2); 1.941 ( x 2); 2.068 (S.P.) 1.85I.95 (tetrahedral dimer) (2.05 and 2.07) 1 . s 1 . 9 6 ( x 4); 2.29, 2.46 1.93 ( x 2); 1.93 ( x 2); 2.22 ( x 2) (2.03) 1.88 ( x 2); 1.88 ( x 2); 2.24 ( x 2) (2.00) 1.905 ( x 2); 1.959 ( x 2); 2.187 ( x 2) (2.02) Trigonal bipyramidal 1.911 ( x 2); 1.926 ( x 2); 2.020 (S.P.) 1.91-2.10 ( x 4); 2.94 (S.P.) 1,916 ( x 2); 1.976 ( x 2); 2.147 ( x 2) (2.01) 1.903, 1.923; 1.945, 1.964; 2.196, 2.217 (2.04) 1.94 ( x 2); 1.97 ( x 2); 2.03, 2.27 (2.02) 1.95 ( x 2); 1.99 ( x 2); 2.17 (S.P.) 1.90 ( x 2); 1.91 ( x 2); 2.24 ( x 2) (2.02) 1.90 ( x 2); 1.91 ( x 2); 2.20 ( x 2) (2.00) (2.02) 1.87 ( x 2); 1.91 ( x 2); (Cl) 2.83, 2.84 1.84 ( x 2); 1.92 ( x 2); (Cl) 2.84 ( x 2) 2.01 ( x 4); 2.03 ( x 2)
MnO(OH)b (cc)
~~
~
~~~
~ ~ ~ ~ ~ _ _ _ _ _ _ _ _ _ ~ ~ ~ _ _ _ _ ~ ~ ~ ~ ~ ~~~
~~~
~~
Bond distances for the octahedra are listed in tram pairs to feature the distortions. The figure in parentheses is the average. 'The shorter distances refer to 02-. ' Ref. 585. 'Ref. 586. Ref. 587. Ref. 588. *The mineral bermanite-ref. 263. a
Manganese
87
thus are intermediate between those for Mn" and Mn'". These distances, and the distortions, often serve to identify the + 3 oxidation state in the solids. Similar octahedra are observed in silicate mineral^,^" such as braunite and henritermierite, and borate minerals,"* such as the pinakiolites and gaudefroyite. One apparent exception to the rule of distortion is the [Mn"'06] octahedron in Mn,O,,Cl,, which has bond lengths of 2.01 and 2.03& but the [MnCl,O,] octahedra, with Mn-C1 of 2.84& are so distorted as to closely approach a planar [MnO,] geometry. Five-coordinate and four-coordinate (tetrahedral) structures also are known. The five-mordinate polyhedra usual1 are square pyramidal with a distinctly longer axial bond; and, in one case, this is so long (2.94 ) as again to suggest planar [MnO,] coordination. The tetrahedron is known so far only in the dimeric oxoanion [MnzO6I6-,which was made by heating MnO and K,O together at 610°C for ten days.586
K
41.4.3.2
Oxyunions
Nitrato compounds appear to be confined2" to the thermally unstable Mn(NO,),, NO,[Mn(NO,),] and [Mn(NO,),L] (where L = bipy or phen). The latter are dichroic (redlgreen), and an X-ray analysis of the bip shows a (5 + 2) seven-coordinate structure, with axial (unidentate) nitrate (1.90 ) and N (1.99&, and equatorial N (2.09A) and two bidentate nitrates (Mn-0 = 2.11, 2.40 and 2.20, 2.24A). Phosphato compounds,590however, are more significant: stable crystalline solids have been known since 1844 (Table 47); aqueous solutions of diphosphatomanganese(I1) species have been used analytically; and NH4MnP207is a violet pigment ('manganese violet'). Even so, reported studies on the solids are quite limited-in many cases nothing has been reported since the original preparations, more than 60 years ago. X-Ray structural analyses of the red-violet compounds Mnki,P,06.2H,0S90and Mn(P0,)359'both show the expected, distorted [MnO,] octahedra. The phosphate system is dominated by the insoluble green-grey MnPO,. H,O. However, violet solutions, which appear to contain mainly [Mn(PO,),I3-, can be obtained by dissolving Mn"' acetate in concentrated H,PO,. Phosphonato compounds have been studied in Ac,O, and evidence reported for the 1:1, 2:3, 1:2 and 1:3 species with Mn"'; but only the 1:3 species could be detected in MeOH. But the most intensively studied system, the most stable and that used as an oxidometric reagent, has been that of diphosphate (P,O;-) and Mn"' in aqueous solutions. The violet solutions, prepared x stored unchanged by oxidizing Mn" withKMnO, or HN03, are most robust at pH 4 6 and can l for at least one month. There is no agreement in the literature about the nature of the molecular anions present but at, and below, pH 4, [Mn(H2P,0,),l3- appears to predominate; and, above pH species probably becomes more significant. 4, HP20;The suIfate Mn,(SO,), can be prepared from Mn,O, in hot, concentrated H,SO, as dark-green (H,O),]Mnneedles, which are thermally stable but very hygrosc~pic.'~~ From 70% H,SO,, red [Hf (SO,), can be obtained and the compounds MMn(SO,), (M = NH,, K, Rb) have been described. (Unstable, hydrated 'alum' forms were noted in the previous section.) Sulfato compounds are formed in solution, are somewhat more robust than the aquoloxo species, but details are still not clear. The blue coiour described for Al,Mn,(SO,), by Etard in 1878/79 (and there are other compounds with other MI1' sulfates)592suggest that it is worthy of further study.
K
Table 47 Crystalline Phosphato and Arsenato Compounds of Mn"'
Compound MnH, P, 0,. 2H, 0 MMn(HPO,), * nH, 0 MnPO,.H,O Mn(POd3 NaMn(P0, ), MndP, O,),*nH,O MMnPz07 MMnP, 0,.nH,O MnAsO,*H,O Mn(H, AsO,), 3H,O Mn~lMn"'(OH),AsO, +
Comments Red-violet-[MnO,(H,O),] octahedrasw (M = Li, Na, K, NH,) Red crystals-water
Grey-green, insoluble Red-violet-[MnO,] octahedra%' From Mn,03, HPO, and NaPO, at 1OOO"C Violet (M = K, NH,) Red or violet (M = Na, K, NH,) Red or violet Brown-violet. X-Ray pattern related to PO, analogue Dark violet The mineral FLinkite -[MnO,] octahedra
soluble
88
Manganese
Selindtes, on the other hand, have attracted recent interest: compounds MMn(SeO,),-nH,O (M = H, NH,, K, Rb, Cs) have been described593and X-ray analyses5" of Mn2(Se0,),.3H,0 and MnH(SeO,)(Se, 0,) reveal distorted [MnO,] octahedra. Iodates of Mn"' are perhaps represented in the yellow to brown-yellow solids M,Mn(IO,),. There is also a range of isopoly- and heteropoly-anion compounds which we have summarized in Table 26.
41.4.3.3
Carboxylates
There are two basic classes here: (a) the compounds of the monocarboxylates, such as acetate, which turn out to have a rather complex chemistry; and (b) the bidentate carboxylates, such as oxalate, malonate and salicylate which are straightforward bidentate anionic chelates. Contrary to the assumptions in most of the literature,254it is now becoming clear that the monocarboxylates of Mn"' are 'basic' species, that is, they are compounds with oxo, as well as carboxylato, ligands. For acetate, neither the cinnamon brown 'dihydrate' first reported by Christensen in 1901, nor the 'anhydrous' species made from Mn(N0,)2-6H,0 and acetic anhydride by Spath in 1912, are examples of simple binary acetates. Rather, they are variants on a trinuclear structural theme [Mn30(00CR)6L3]+, with the planar structure (2241, in which each Mn atom is bridged to its neighbours by two carboxylate bridges. This is proven for the product from acetic anhyd~ide,~" but is yet only an assumption for the 'dihydrate', for which the story may be more complex: similar reaction conditions to those used for its preparation, but with 50% more KMnO,, produce a red-black mixed Mn"' /Mn'" described below. Known variants of the structural theme (224) are: [Mn,O(OAc),HOAc] -the so-called anhydrous acetateSg5-in which L is an oxygen from an acetate which links the trimers into linear in which L is an oxygen from polymers, or, in one case, HOAc; K,(Mn"(H20)2[Mn~"O(HC0,)9]2), a formate which links the Mn" atoms to the trimer; and [Mn,O(OAc),L,] C10, for L = py or B-pi~~~~--the py compound is isomorphous with the Cr, Fe, Ru, Rh and Ir analogues. A minor variation on this theme is seen in the neutral compounds [Mn,O(OAc),L,] where L = pyS9'or 3Cl-py.600In these, one of the Mn atoms is Mn", which is distinguished experimentally in the X-ray analysis of the 3C1-py compound,600but not in that of the py compound.s99 A further variation on this theme is seen in a mixed Mn"'/Mn" acetate, prepareds96as red-black crystals as for 'Mn(OAc),-2H20', but using more KMnO,. The compound is a dodecanuclear 2 H O A c - 4 H 2 0 ,in which the central Mn,?OI2unit has the strucspecies [Mn1201z(OAc),6(H,0),]ture (225). There is a central cubane-like (Mn,O,) cluster of Mn" atoms, each linked via further triply-bridging oxides to two Mn"' atoms. The Mn atoms are then further linked by single or double acetate bridges of type (82) and water molecules complete the octahedra for half the Mn"' atoms. Bond lengths and distortions of the Mn"' polyhedra here, and in the other acetate compounds, serve to distinguish the different oxidation states. Mn
L
Mn-o\
I
Manganese
89
Undoubtedly further variations will be identified as these carboxylates are further explored. Oxalate forms red [MnOx,13- and green (cis) or yellow trans-[MnOx,(H,O)J, which have been Usually, preparations of the bis species give the cis isomer, but it is the trans known since 1936.293+572 isomer which predominates in aqueous solution. The compounds are all rather unstable and are particularly sensitive to light. However, the malonates, known since 1922, are rather more robust and especially towards light. X-Ray structural analyses are available on trism' and trans bis602 species, with all showing 2 + 2 + 2 distortions of the Mn"' octahedra. Hydroxycarboxylate compounds of Mn"' are known for salicylates and various aliphatic acids.299 Some crystalline solids have been isolated, including bis-chelate species of the salicylates M[Mn(Xsal),L2] (L = H,O, py); but the only tris-chelate compound isolated is the black Cs, [Mn{(CF,),C(O)COO),]. Various red or green crystalline solids also are known with tartrate ligands and they are reasonably robust, but no structural data are available. Much of the known detail on the hydroxycarboxylate ligands refers to the redox properties of aqueous alkaline solutions; and especially to electrochemical studies on the oxidation of the Mn" species of glyceric, tartaric, gluconic, glucaric and citric acids to Mn"' and Mn'V.299,603 For each ligand, two Mn"' species were detected, and they appear to be a monomeric tris-chelate species and a dimeric oxo/hydroxo-bridged bis-chelate species.
41.4.3.4 Alcohol and phenol ligands
Orange-red Mn"' compounds of the polyhydroxy ligands, including the polyhydroxycarboxylates noted in the previous section, are formed either by bubbling air through the Mn" compounds in aqueous alkaline solution or by dissolving 'manganese(II1) acetate' in the liquid polyhydroxy alcoho1.212,m3,604 Such solutions are relatively robust, but in acid solutions the Mn"' compounds appear only as intermediates in the oxidation of the alcohol to the ketone. Stability (robustness) of the Mn"' compounds increases with the number of alcohol goups, so that, for example, the sorbitol compoundsm4are more robust than those of glycerol. Catechol and other diphenols have been known since 1926 to form Mn"' species, and they have recently been receiving closer a t t e n t i ~ n . In ~ ~aqueous ~ . ~ ~ alkaline solution, tris-chelate species apparently are formed. However, these systems have the added complication of ligand oxidation to the semiquinone, which forms more (thermodynamically) stable species than the diphenol (see also Section 41.5.3.4): there is evidence for a tris-semiquinone Mn"' molecule, which is a four electron oxidizing agent according to equation ( 17).m3 Another semiquinone compound of Mn"' has been described.ws [Mn(salen)(DBSQ)] (DSSQ = 3,s-di-tert-butyl-o-benzosemiquinonate) [Mn'['(semiquinone),]
+
4e- S [Mn"(diolate), J4-
(17)
41.4.3.5 Acetylacetonate and related ligands
A range of Mn"' compounds of acetyla~etonate~~~ is well established; they are used for preparing other Mn"' species; and have been studied as catalysts, especially for vinyl polymerizations. Brown [Mn(acac),], somewhat less robust than its Cr, Fe and Co analogues, has been prepared by a variety of routes, including the action of KMnO, on the ligand, reaction of the metal with the ligand in the presence of O,, various oxidations of Mn" species or reaction of the ligand with other Mn"' compounds. Details of a convenient preparation, from KMnO, and acacH, have appeared recently.m6 The molecular species can be recrystallized from a variety of hydrocarbon and halocarbon solvents, and it has been identified in three crystalline forms, of which X-ray structural analyses are . ~ ~ a misleading report in 1964, the [MnO,] octahedra are available on the 8- and y - f o r m ~Despite distorted: there is a 'compressed' (2 + 4) distortion for the 8-form (Mn-0 = 1.93-2.02 A) and an 'elongated' (4 + 2) distortion for the y-form {Mn-0 = 1.94( x 4) and 2.1 I( x 2)). for many of the usual range of Similar green or brown tris-chelated compounds are available P-diketonates, the most stable compound being that formed with the sexadentate ligand (124).,18 Solutions of [Mn(acac),] react with HX to give brown Mn(acac),X species (X = Cl, Br, I), and other compounds of chis type (X = N,, NCS, RC00)m8have been made by direct reactions. These compounds also are used for the preparation of other Mn"' compounds, especially of the por-
90
Manganese
phyrins. X-Ray data on the azido and thiocyanato compounds show planar Mn(acac), units (Mn-0 = 1.91 A) linked into linear polymers by bridging of the anion (for azide, MnN = 2.245& and for thiocyanate, Mn-N = 2.189 and Mn-S = 2.880A). In water, the cationic species [Mn(a~ac),(H,O),]~form and several salts of this have been isolated, while with pyridine and other neutral unidentate ligands [Mn(acac),XL] have been Other mixed ligand systems studied include the troponeiminates (43) and 8-hydroxyquinoline: in both cases [MnL,L'] and [MnLL;] have been characterized. [Mn(acac)(salen)] represents a nonplanar form of the salen ligand, related to the well-characterized Co"' In the tris-tropolonato compound (L = 98), the two independent molecules show either a 4 + 2 or a 2 + 2 f 2 distortion of the [MnO,] octahedra.'"
41.4.3.6
Other oxygen ligands
Several ether compounds of MnCl, have been described,"' but most are thermally unstable well below normal temperatures. Of neutral carbonyl ligands, only purple [Mn(urea),](ClO,), appears to have been described. All Mn-0 bond lengths are constrained to equality by the space group, but an analysis of the 'temperature factors' showed that there was scope for distortion of the octahedron and this was suggested to be dynamic.612 Related red [MnL,]X, compounds with pyridine N-oxides have been but no structural data are yet available. These unidentate pyridine-N-oxides also red or green compounds [MnCI, L,], which are probably octahedral, and green compounds MnCl,L,, which may well prove to be five-coordinate monomers. Blue MnCl,(Ph,PO), and MnCl,(Ph,AsO), also are known; and there is a bromo species [MnBr,(quinO),]. Most of these tris- and bis-ligand compounds of MnX, are quite robust in dry air at normal temperatures, but are instantly decomposed by water and slowly decompose in moist air. The tris-ligand cation of bipy-N-oxide (l2O),6I3and the bis-ligand cation of terpy-N-oxide314can be prepared by electrochemical oxidation of the Mn" species in CH,CN, and the robust dark-red crystalline perchlorates have been isolated.
41.4.4
Sulfur Ligands
No binary Mn"' surfide is known and this oxidation state is known3I9only in several ternary sulfide phase: perhaps in cubic spinel species, such as ZnMn,S,, and probably (magnetic evidence) in several Nb and Ta ternary phases, formulated as M n , ,NbS,, MII,,~~T~S,and M%,,NbS,. Bond lengths for the first two are reported as 2.52 and 2 . 4 9 i . On the other hand, oxidation of the thiol compounds to Mn"' occurs so readily that Mn" compounds have to be prepared anaerobically. For simple thiols, only one species has been isolated and characterized to date. The dianion of 1,Zdithioethane (edt) in methanol solution with Mn" forms [Mn(edt)J2- and reacts rapidly with 0, to give green solutions, from which dark green crystalline [Et,NJ,Nn,(edt),] and the [Ph4P] salt were obtained.32oX-Ray analysis of the former showed the dimeric structure (226) with asymmetric bridges Mn-S = 2.346 and 2.632(2)A. The other three S atoms have shorter distances (2.316 2 . 3 2 3 A), and the overall five-coordinate structure is reasonably close to the square pyramidal extreme with four short bonds and one long (axial) bond. The magnetic moment (3.96 BM) suggests some antiferromagnetic coupling.
Manganese
91
Table 48 Bond Length Data for the Mn"'Dithiocarbamate Compounds [Mn(R,dtc),]
4
Mn-S
Et2
O(CH2CH2)2
O(CH,CH,),
2.373(9) 2.383(8) 2.385(3) 2.385(3) 2.344(1) 2.365(1)
bond lengths
2.428(7) 2.43l(7) 2.447(3) 2.486(3) 2.433(1) 2.483(1)
Ref.
'
2.54l(7) 2.557(7) 2.529(3) 2.534(3) 2.527(1) 2.584(1)
a
b C
P. C. Hedly and A. H. While, J. Chem. SOC.,Dalton Trans., 1972, 1883. bCH,CI, solvate: R. J. Butcher and E. Sinn, J. Chem. SOC.,D o h n Trans., 1975, 2517. ' CHCl, solvate: R.J. Butcher and E. Sinn, J . Am. Chem. Soc., 1976, 98, 5159. a
In solutions in donor solvents (DMF, DMSO and MeCN), dissociation equilibria occur, with the formation of presumably [Mn(edt),(solvent),]- monomers. Magnetic moments in these solutions (5.0-5.1 BM) are close to the spin-only value for high spin d4,and the electronic spectra have been ~haracterized.~~' Dithiocarbamates of Mn"' have long been known because of the ease of oxidation from Mn": Marcel Delepine first described some of the dark violet [Mn(R,dtc),] compounds in 1907. They r e s ~ l t from ~ ~ ~the, reaction, ~ ~ ~ *in~open ~ ~vessels, of aqueous Mn" with the Na or K salt of the ligand; are reasonably robust under ambient conditions, but sensitive to light and moisture; and they have magnetic moments near 5 BM. The Se analogues also are known and have similar proper tie^.^'^ Three X-ray analyses structurally characterize the compounds as tris-bidentate with fourmembered chelate rings (Table 48); and show that the [MnS,] octahedra are distorted, not only trigonally by the chelate rings, but also by the (2 + 2 f 2) variation of the 'tetragonal' distortion observed in most Mn"' octahedra. An electrochemical study has shown reversible reduction to Mn" and oxidation to Mn'" species; and a IH NMR study shows that isomerization, of the compounds of unsymmetrical ligands such as MePhdtc, is slow on the NMR time scale at - 60°C and occurs via a trigonal twist mechanism rather than a dissociative one. Bis-dithiocarbamato compounds, such as Mn(R,dtc),Cl, appear to be available, but not studied; and there is a recent report6I8of a nitrosyl [Mn(R,dtc),Cl(NO)].
41.4.5
Halides
The coordination chemistry of Mn"' with the halides210,336.572 is distinctly more limited than that of Mn"; the former are good halogenating agents for hydrocarbon compounds and the only binary compound able to exist at normal temperatures is MnF,. Black solids, stable only below - 40°C probably represent MnCl,, but the bromo and iodo compounds have never been observed. There is more of a tendency here for the stoichiometry of the ternary compounds and their hydrates to reflect the coordination polyhedra. For example, there are a number of examples of M, [MnF,] compounds, and those of formulae MMnF4-2H,0 generally appear to be of structural type M[MnF4(H20)2],with trans octahedra. However, the series M,MnF,, M, MnF,-H,O, MMnF, and MMnF,.H,O all are examples in which octahedra are formed by bridging offluorides; although discrete [MnC1,l2- five-coordinate polyhedra exist in the series M,MnCl, and there are some unstable examples of [MnCI,]'-. All of these coordination polyhedra are quite labile, and the structures of the Mn'" polyhedra in any solution depend only on the nature of the solvent and not the starting material. Stability constants K , to K,, for aqueous fluoro species in perchlorate media, have recently been reported.619 MnF, is a red-purple crystalline solid which is readily soluble in water. It is antiferromagnetic only at low temperature. Also known are a mixed oxidation state fluoride, Mn,F,, and a hydrate which is said to be isostructural with NiSnC16-6H,0. Perhaps the latter represents an example of disproportionation, and may be [Mn"(H,O),][MnWF6]. MnF, is structurally related to the other fluorides TiF, to COF,, but with the significant 2 2 2 distortion expected for the high spin d 4 configuration (Table 49). The M,WnF,] series of ternary compounds generally have tetragonally distorted fluoroperovskite structures, in which there are two sites for M, and hence mixed cation species such as K,Na[MnF,] are readily prepared. Large tervalent cations, however, like [M(NH,)J+ (M = Cr, Co, Rh) give cubic crystals which contain discrete [MnFJ- octahedra with experimentally equal Mn-F bonds, although the data can be interpreted in terms of a dynamic (or random) disorder.
+ +
C.O.C. b D *
Manganese
92
Table 49 Bond Length Data for [Mn'"FF,]Octahedra Bond Lengths (A)a
Compound 1.79
MnF, K, NaMnF, [Cr(NH3 [NH,I,MnF, K,MnF,-H,O Rb,MnF,.H?O Cs, MnF, .H,O RbMnF, -H,O
1.91
2.09 2.06
1.86b 1.92p 1 .84b
1.821 1.835 1.835 1.83
2.09 2.072 2.089 2.127 2.157
1.842 1.860 1.868 1.85
(1.93) (1.93) (1.92) (1.92) (1.91) (1.93)d (1.94)d (1.94)c
'These are generally given in rrun,y pairs, with averaged vnlues in parentheses. Data are from Cmelin (C4) unless otherwise stated. 'Four equal bonds. e All bonds equal in the cubic space group. dRef. 621. e Ref. 622.
The violet M, MnF, compounds and their monohydrates, for which a new, convenient method of preparation recently has been given,',' all have chains of corner-sharing [MnF,] octahedra with trans bridging fluorides6" (Table 49). The water molecules of the hydrates are accommodated between the chains. CsMnF, has a layer structure with corner-sharing [MnF,] octahedra, and there are at least two different hydrates here: RbMnF,*H,O has chains of alternating [MnF,] and trans-[MnF,(H,O),] octahedra, whereas both Cs and NMe,~nF,(H,O),] have discrete trans-diaquo octahedra.6z2 There is even less variety in the known chemistry of the chloro compounds because of their lower redox stability.623Although MnCl, is stable only below - 40°C apparently decomposing through a disproportionation reaction,6" the oxidation state is stabilized with other Iigands. Thus MnCl, is more robust in donor solvents, undoubtedly as species like [MnCl,L,], and compounds of this type are well established for both N and 0 donors (Sections 41.4.2 and 41.4.3.6). There is also a series of pyridine-N-oxide compounds (Section 41.4.3.6) MnCl,L, which may well be five-coordinate. Certainly five-coordination is well established in the series of green compounds M, MnC1, , ~ ~ ~ an X-ray analysis of the bipyridylium (M = alkyl ammonium, 2M = bipyH,, ~ h e n H , ) through salt.625Several purple or red-brown monohydrates (M = K, Rb and NH,) also are known.624There is an interesting recent report4 that Mn"' in a mixture of acetic acid and acetyl chloride, also containing MOAc, gives crystalline M,[MnCl,] (M = Na, R,NH,). The brown hexachloro anion also is well established, but not very robust. It is isolated from solution6",626with large tervalent cations, such as [Coen,I3+,[CopnJ3' and [Rhpn,I3+.The solids are instantly decomposed by water or on keeping at room temperature. Of brumo compounds, there are yet only one or two compounds [MnBr,L,] (Section 41.4.3.6), so the advent of a well-proven iodo compound [MnI,(PR,),]58' was unexpected. It would be most surprising if the latter were to remain the sole representative of Mn"' iodo compounds: we can surely expect interesting further developments here. Table 50 Mn"' Compounds"of Bidentate N - 0
Magneric Moment
Compound
R
Stoich iomezry
MnL,OAc MnL, MnL,OAc MnL,Xd MnL, MnL, MnL, MnL, MnL,OAc MnL,OAc MnL,OHe MnL, a All
Alkylb c
PP, Pr' Pr" o-Me-C6 H, C6H5
p-cl-c6H, C6H5
m-N0,-C6 H, p-NOz-C6H, -CH,-c6& -CH,-CSHS
compounds,excepting e which is dark brown, are d&kd
See footnote a above.
X
(BM at R.T.)
Ref.
H H H H H 5-Br 5-Br H H H H H
4.%5.1
628 629 629 629 630 630 630 63 1 63 1 63 1 419 632
4.8-5.0 4.9-5.0 4.9-5.0 4.97 5.03 4.84 5.06 4.95 4.98 -
as being green to dark gnen in colour.
bCH,,C,H,, n-ClH7, n-C,H,, n-C,H,, or n-C6H,3. R = H, n-C,H,, cyclohexyl, -CH,-C,H,, C,H, or p-Br-C,H,. d~ = CI or Br. e
Ligands of Type (227)
93
Manganese 41.4.6
Hydrogen and Hydride Ligands
There is very little information available: MnH, has possibly been observed3s6in Ar and N, matrices at 4 K; MnH,(dmpe), appears to result from the reaction of H, with MnH(C,H,)(dmpe), under p r e s s ~ r e and ; ~ ~there ~ ~ ~is~ a report of the preparation of Mn(BH,h, but no confirmatory
41.4.7
Mixed Donor Atom Ligands
The chemistry in this section is almost exclusively that of Schiff base ligands; this reflects the predominance of the aldehyde/carbonyl condensation reaction with primary amines in the preparation of mixed donor atom ligands.
41.4.7.1
Mixed N - 0
donor atom bidentate ligands
Salicylaldehyde has been condensed with many primary amines to yield bidentate N-0 ligands. Such ligands react with Mn"' acetate dihydrate to give compounds of stoichiometries Mn(L), or Mn(L),(X) (where HL = (227); X = C1, Br, OH or OAc); examples are listed in Table 50.
Reliable structural information on most of these compounds is not available, although there is mass spectral evidence that the compounds listed against reference 629 are polymeric when X = OAc, but monomeric when X = CI or Br. There has been an X-ray structural determination undertaken on the compound Mn(L), for reference 632; bond lengths are listed in Table 51 (compound 1) and the structure has been shown to be a trans array of the three ligands around a pseudotetragonally distorted Mn atom. Such distortion, which is predicted by the Jahn-Teller theorem, has also been found in the structures of the Mn(L), compounds derived from non-Schiff base N-0 bidentate ligands, such as the octahedral tris(8-hydroxyquinolato)manganese(III)~CH30H (or 0.5C,Hi3OH) and the octahedral tris(cr-picolinato)manganese(III) compounds (see compounds 2 and 3 respectively in Table 5 1).
41.4.7.2
Mixed N - 0
domr atom terdentate ligands
Once again the Schiff base ligand type predominates. The most common stoichiometry of compounds formed by the interaction of such ligands with Mn"' acetate, is that of Mn(L)(OAc) (L = dianionic Schiff base terdentate), obtained at higher pH values. Such compounds have been obtained from ligands (228) (n = 2 and X = H, 5-Br, 5-NO,, 3-OMe or 3,5-diBr;635n = 3 and X = H),636(229) (R = Me and R = 2- or 4-pyridy1;6" R = H, Me, Et or Pr and R = Ph638),and (230) (R = H, 5-C1 or S-BI-~~'). X-Ray structural analysis636of the compound Mn(L)(OAc) (where L = (228); X = H and n = 3) shows that it is a dimer with octahedral Mn"' atoms (232); bond Table 51 Bond Lengths for [Mn(N-O),]
Compounds of Mn"'
Bond Lengths Axial Donors
N-0 Ligand (227) R = Bz; X = H (153) (154) R = CO,H
Mn-N 2.251, 2.271 2.231, 2.265 2.217, 2.254
Mn-N
(A) Equatorial Donors Mn--0
2.056
1.882, 1.896, 1.917 1.912, 1.913, 1.920
2.059
1.891, 1.908, 1.942
2.082
Ref. 632 633 634
94
Manganese
lengths are, for the terdentate, Mn-0: 1.849 to 1.951A and Mn-N 2.006& and for the axial acetates, Mn-0: 2.2084 to 2.251 A. In the presence of anions other than acetate, compounds of similar stoichiometry, Mn(L)(X) (where X = C1 or Br;638OH639and OMew) have been prepared. The compound in which X = OMe was obtained by hydrolysis of the potential quadridentate ligand (231) in an alkaline solution of MnCl,, to yield (233) in which the terdentate ligand is (229) (where R = H and R' = o-NH,C6H4). Structure (233)indicates the marked tendency for these ligands to form polymers with Mn"' in the solid stale. Nevertheless, the monomeric Mn"' anionic compound [Mn(L),]'- (where L = N-salicylideneglythe crystal structurew' has cinato ligand) has been shown to exist in ~n1'(H,O),][Mn"'(L),].2H,0; a distorted octahedral Mn"' with two planar ligands with bond lengths: Mn-0 2.236 and 2.046, Mn-N, 2.065A for one ligand, and Mn-0, 1.934 and 1.861, Mn-N, 1.981 A for the other ligand. The compounds [Mn(L),](NCS) (where L = (234); n = 1 or 2) are also claimeda2 not to be polymeric.
(229)
I
A
/"---------O \ /=N
,N
41.4.7.3 Mixed N - 0
donor atom quadridentate ligands
Once again Schiff base ligands predominate, and such work up to 1980 has been extensively Many of the compounds claimed to be Mn"' species have been reviewed by Connors et
Manganese Table 52 Mn'" Compounds" of Formula Mn(L)(Z) in which L
95 E
(161) and Z = Monoanionic Ligand
L = (161) Y
X
2
(BM at r.t.1
Ref
H H H
OAc OH OH
4.68 4.90
38 38 419
H H
Meb Phb c1
H H H H H 5-Br H 4-Me 4-Bd 4-OMe 4-Br H H o-C, H4 0-CgH4 o-CGH~ o-C6H4 o-C6H4 ~-NO,-CJ-C,H, CH2--CH(OH)--CH, CH2-CH(OH)-CH, CH2--CH(OH)-CH, CHz-CH(CH3) CH,-CH(CH, ) CH,-CH(CH,)
H 4-Me 4-OH 5-Br
5-c1
H
H H H 4-Bd H H
-
4.98
4.85 4.96 4.98
646 646
C1
4.92 4.84 4.86 4.30
C1
4.74
c1
4.54
641 647 647 629 629 649 653 653 653 653 653
4.98
646
5.16 4.92 4.81 5.03
648 648 648 648 648 648 650 650 650 65 1 652 652
Br 1 Br 1 OAc
c1 C1
Meb
Phb OAc
OAc OH OAc OH OH C1 Br I C1 C1 NO,
4.75 4.80
5.40
5.06 4.80 4.84 4.76 5.00 5.02 4.94
compounds are brown in colour. 'These compounds analyse as monohydrates; they may well be akoholates. a All
obtained by oxygenation reactions with Mn" compounds of ligands of type (161). This is an area of research which has generated much information, but little definitive result. The literature has been extensively reviewed by Coleman and Tay10r."~ Products of such oxygenation reactions are nearly always brown powders, and usually analyse as Polymeric structures are Mn"'(L)(+X) or Mn"(L)(X) (where L = (161) and X = 0 or OH).425@3,426 suspected in most cases, and indeed black crystals of the compound Mn(sa1tn)OH -4py (where saltn = (161) with X = (CH,), and Y = H), obtained by the oxygenation of the corresponding Mn" compound in pyridine solution, have been treated to X-ray structural analy~is,"~and the structure shown to be dimeric with two five-coordinate Mn'" atoms, each bridged by two hydroxy groups; the pyridine moieties are not bound directly to the metal. Many compounds of formulation Mn(L)(Z) (where L = (161) and Z = a monoanion) have been prepared (see Table 52). Structural evidence upon such compounds is not abundant, but [Mn(salen)to possess approximately planar Mn(sa1en) units linked in a polymeric (OAc)] has been chain by singly bridging acetate ions. The very similar compound, in which the benzene rings of salen are replaced by naphthalene rings, also possesses the same polymeric Other similar types of Mn"' compounds, in which Z is a non-charged ligand, e.g. Wn(L)H,O] (see entry in Table 52 against reference 651), have been isolated, while compounds of type [Mn(salen)(bident)] (where bident = acac or salicylaldehyde monoanions) and [Mn(salen)(bident)]ClO, (where bident = en or bipy), which were first reported4" in 1933, have been studied more recently.654 ' studied the Schiff base compounds of formulation [Mn(L)(X)] (where Boucher et ~ 1 . 6 ~have L = (235) and X = monoanionic ligand); X-ray structural analysis of the compound in which X = C1 shows that it has a distorted square pyramidal structure with a non-planar ligand arrangement .656
96
Manganese Me\
,CH,-cH2
/C=N
CY2
41.4.7.4
Mixed N - 0
Me
N=C
/ \
/CH2
,c=o Me
\
o=c
\
Me
donor atom quinque- and sexadentate ligands
The Schiff base quinquedentate ligands (161), in which X = (CH,)3-NH-(CH,)3 and Y = 5NO, or 3-Me0, form Mn"' compounds of the type Mn(L)(OH) when the corresponding Mn" compound is oxygenated (see Section 41.3.9.4), whereas in similar compounds, in which Y is not an electron-withdrawing group, the ligand undergoes reaction when the Mn" compound is oxygenated.657 An X-ray structural analysis of the compound [Mn(L)(NCS)] (where L = (161), X = (CH2)3NH-(CH,), and Y is a fused C6H, ring) shows an octahedral Mn"' atom, with the NCS group and the centra1 N atom of the quinquedentate ligand occupying the apical p~sitions.~" Coleman et have described the preparation and cyclic voltammetry of a series of compounds (n = 2 or 3 and tn = 2, 3 or 4), [Mn(L)(NCS)] (where L = (161); X -. (CH,),-NH-(CH,), (CH,),-S-(CH,), and (CH2)3-O-(CH,)3; Y = H, 3-N02, 5-N02and 5-OMe). Similar compounds, in which X = (CH,),-P(Ph)-(CH,),, have been de~cribed!'~ In a series of interesting papers,443it has been shown that the Mn" compounds from both quinque(n = 2 or 3, and and sexadentate ligands based on (161), in which X = (CH,),-NH-(CH,), m = 2 or 3) and (CH,),-NH-(CH,),-NH-(CH,),, (n = 2 or 3, m = 2 or 3 and p = 2 or 3) respectively, will react with oxygen and nitric oxide. However, replacement of the salicylaldehyde group in (161) by a 2-pyridyl residue renders the Mn" compounds inactive to such reactions. The interesting potential septadentaee tripod ligand, obtained by the condensation of 5chlorosalicylaldehyde with tris(2-aminoethane)amine, has been shown to yield an almost undistorted octahedral Mnrrlcompound, MnN303,with the apical N atom of the ligand unboundvW Early work on the dark red compound of Mn"' with edta suggested that its structure was K{Mn(edta)H,O]* 1.5H20?' However, X-ray work shows the compound to possess an octahedral structure with a sexadentate edta residue. Axial bond length distortion is observed, with two tram 0 atoms being 2.050 and 2.022 8, from the Mn atom, while the two equatorial Mn-0 bonds are A very similar structural 1.907 and I .889 A and the two Mn-0 bonds are 2.21 1 and 2.237 result was found in the case where edta was replaced by trans- 1,2-diaminocyclohexane tetraacetic acid in K[Mn(L)]*H,0.663 Mechanisms of reduction by hydrazine, hydroxylamine and substituted benzene- 1,2-diols of Mn"' compounds containing edta-type moieties have been studied.664
41.4.7.5
Mixed 0 - S and N-0--S
donor atom ligands
A few interesting Mn"' compounds of 0-S bidentates have been prepared. The interaction of Mn"' acetate with (183) (R = Ph) leads to ligand oxidation, but when R = But the tris-ligand Mn"' compound was A similar compound, [Mn(L),] (HL = 236), has been shown to possess an octahedral stereochemistry with marked tetragonal distortion; the bond lengths for the MnO,S, chromophore are: Mn-C& = 1.958 and 1.941; Mn-O,,, = 2.132; Mn--S(,, = 2.324 and 2.354; Mn-S(,, = 2.584A.
Manganese
97
The sexadentate ligand (161) (where X = (CH,),-S-(CH,),;IS-(CH,), and Y = H) has been to bind all three pairs of N, 0 and S atoms to yield a Mn compound; no Mn" compound could be obtained (see Section 41.3.9.6).
41.4.8 Macrocyclic Ligands 41.4.8.1
Porphyrin ligands
Mn"' porphyrins have been extensively studied4'' with compounds containing the metal in this ',~~ (such oxidation state being especially stable. Mn'"derivatives of partially r e d ~ c e d ~porphyrins as the chlorins) have also been synthesized. A number of physical studies have been performed on Mn"' porphyrins in an attempt to understand their eiectronic structure. The visible spectra of such compounds have been of particular interest. For most metal porphyrins the visible spectrum is insensitive to the nature of the coordinated metal and this has been interpreted as indicating little interaction between the metal and the porphyrin .n-orbitals in such compounds. However this is not the case for Mn"' porphyrins which exhibit metal-dependent charge transfer absorptions. This dependence appears to reflect significant .n orbital-Mn'" interaction. Resonance Raman and linear dichroism spectral studies are also consistent with this conclu~ion.~."~ X-Ray structures of several Mn"' porphyrin compounds have been determined. Five-coordinate square pyramidal structures occur for [MnCl(TPP)],"8 [MnN, (TPP)f@and [MIICN(TPP)]~~' while trans pseudooctahedral geometries are found in [MnN, (TPP)(CH, OH)],""."" and For all these compounds the average inplane Mn-N bond lengths are near [Mncl(TPP)(~y)l.~'~ constant at 2.02 f 0.1 A. The manganese is displaced from the N,-donor plane in each case with the displacement tending to be greatest for the five-coordinate species (0.23-0.27 A). As expected from the smaller size of Mn"' the displacements in the latter compounds are less than that found in the related (high-spin) five-coordinate Mn" compound, [Mn(TPP)(l-methylimidazole)], In this case the Mn" is positioned 0.56 A above the donor plane.483.488 Using spectrophotometric measurements the binding constant for formation of the latter 1:1 adduct with l-methylimidazole was found to be 24.7M-' at 21"C.673 [Mn(TPP)(CH,OH),]' as its perchlorate salt exists as two near identical independent ions in the solid state. The Mn"' is tetragonally coordinated with long Mn-0 bonds in the axial positions.673 The metal lies in the N,-donor plane in each case. The structure of the Mn"' compound [Mn(TPP)imidazolate]. consists of a linear polymer with imidazolate bridges between adjacent [Mn"'(TPP)JS m0ieties.4~'A curious feature of the structure is a short-shortjlong-long alternation of the Mn-N(1m) bond distances along the chain. This has been suggested to reflect alternating (predominantly) low- and high-spin Mn"' atoms in the chain. Before the synthesis of this compound all previously reported six-coordinated Mn"'porphyrins were high ~pin.4~' The unusual magnetic properties of the polymeric complex appear to reflect the enhanced ligand field strength of the deprotonated imidazole moiety. In accordance with this, the bis(imidazo1ate) compound [Bu,Nl[Mn(Im),(TPP)] is completely low spin. Kinetic and equilibrium studies of the interaction of imidazole with a Mn"' porphyrin in aqueous solution have been reported.675Imidazole bonds to the Mn"' porphyrin in a step-wise fashion to yield initially an imidazole-bridged dimeric species followed by a monomeric bis(imidazo1e)-Mn"' porphyrin adduct. Comparative variable-temperature magnetic susceptibility studies have been undertaken for [MnCI(TPP)] and ~nC1(TPP)(py)]676*677 and accurate estimates (- 2.3 and - 3.0 cm-' respectively) of the zero-field splitting of the ground state of Mn'" in these compounds obtained. As expected the magnetic moments of both compounds at room temperature are close to the spin-only value for s = 2. The reduction of Mn"' porphyrins [to Mn" species] has been treated in Section 41.3.10.1. Mn"' porphyrins may be electrochemically oxidized in non-aqueous media to yield Mn"' cation radicals and dications as the probable products.485The redox potentials depend on the nature of the solvent, counter ion present, pK, of any axially-bound ligands as welk as on the basicity of the porphyrin ring involved. Water-soluble Mn"' porphyrins are oxidized by a range of oxidants in aqueous alkaline solution to the corresponding Mn'" porphyrins which appear to exist as p-oxo dimers;&' using hypochlorite as oxidant, a second oxidation step also occurs. In this case the product has been postulated to be a Mn" oxo porphyrin. The Mn" and MnVporphyrins show only limited stability in water and revert to the stable Mn"' porphyrin upon standing in the dark for several
98
Manganese
hours. Although in alkaline (or neutral) solution the initial oxidation product is a Mn'" porphyrin, in acid the first electron may be removed from the n system of the porphyrin ring. In other studies, reaction of [MnX(TPP)] (X = N; or OCN-) with iodosylbenzene in hydrocarbon or halocarbon solvents yielded products formulated as p o x 0 dimers, [Mn'VX(TPP)],0.6'8 [Mn"NN,(TPP)],O has been characterized by X-ray diffraction (see Section 41.5.7.1). Infrared evidence the formulation of the dimers as being metal-centred oxidized products rather than containing porphyrin cation radicals; the possibility of ligand-centred versus metalcentred oxidation in higher-valent metallo porphyrins has been discussed by several authors."M82 Meso-a,cr,cc,a-tetrakis(o-nicotinamidophenyl)porphyrin(237)is able to bind two metals in square planar environments which are orientated in planes coaxial with each other.683A number of studies using this ligand were instigated in attempts to obtain models for cytochrome c oxidase. However the coordination chemistry of this system is less straightforward than originally appeared, especially When an inert metal such as when involving substitution-labile metals such as Fe", Fe"' or Ru" is reacted with (237), coordination of the four pyridyl groups occurs to the ruthenium; ruthenium did not insert into the prophyrin under the conditions used. Once coordinated, the Ru" appears to lock the nicotinic 'pickets' into place and hence inhibit the ligand isomerization reported to occur in the presence of more labile metal ions.684From this starting product, a mixed Ru"/Mn"' product has been prepared. From cyclic voltammetric and spectrophotometric data, it was concluded that the proximity (- 5A) of the two metal centres does not result in a major electronic perturbation of either metal in this case.
(237)
One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. In model studies by Groves et ~ 1 . , 6 ~an ' iron porphyrin was shown to catalyse the insertion of oxygen from iodosylbenzeneinto such hydrocarbons as cyclohexane; the enzyme can also cause the transfer of oxygen from iodosylbenzene.6s6It has been suggested6" that the model reaction may proceed by oxygen atom transfer to yield an oxo-metal compound which then removes an aliphatic hydrogen atom followed by capture of the resulting carbon radical. Manganese porphyrin complexes, in the presence of a variety of reduced oxygen sources, have been demonstrated to act as a catalyst for the oxidation of alkanes, alkenes, alcohols, ethers, amines, phosphines and sulfides.687492 Equation (18) gives a typical reaction sequence.692
X = C1, Br, I, N3
Manganese
99
In the majority of these reactions, higher valent (Mn" or MnV)compounds have been implicated in the respective reactions (vide supra). As mentioned previously, two p o x 0 Mn" porphyrin species have been characterized from the reaction of iodosylbenzene with Mn"' compounds of the type [MnX(TPP)].678,690 These latter compounds are capable of oxidizing alkanes in good yield at room temperature, apparently via the intermediacy of alkyl free Within this general context, the formation of an acylperoxymanganese(II1) species and its decomposition to an oxomanganese(V) species which undergoes oxygen atom transfer to alkenes has been in~estigated.~" Anchoring [MnOAc(TPP)] to a rigid polymer support considerably enhances the rate of cyclohexene epoxidation using sodium hypochlorite as the oxygen source.*95This enhancement may be the result of site isolation restricting the formation of (less reactive) p-oxo dimers during the formation of the higher-valent manganese species. In the natural system, cytochrome P-450 catalyses reductive dioxygen activation to give the reactive iron oxo compound which is involved in the oxidation of the organic substrate. Such using a NaBH,/Mn"'(TPP)/ reductive dioxygen activation has been modelled by Tabushi et 0, redox system. In the presence of the reductant, [Mn"'(TPP)]+ is converted to [Mn"(TPP)] which activates dioxygen to yield the strongly oxidizing species. 1n.a related reaction [MnCl(TPP)] has been reported to catalyse the specific oxidation of certain alkenes to ketones by molecular oxygen in the presence of [NBU,][BH,];~*~ the product ketones were then further reduced to alcohols under the conditions used. Related reactions involving NaBH, have been shown to convert ethylene to ethanol, propylene to acetone and i-propyl alcohol, and cholesIn these systems, molecular oxygen plus a reducing agent can terol to 3/?,5a-dihydro~ycholestane.~~~ be considered to be a substitute for the single oxygen donors such as iodosylbenzene. Colloidal platinum/H, wil1 also effect the [Mn"'(TPP)]+ to [Mn"(TPP)] reduction."' The relative reactivities of alkenes using the H2/[Mn1''(TPP)]+/O, system have been shown to closely resemble those for the [Mn"'(TPP)]'' /C6H,IO system. A related biphasic system involving ascorbate as the reducing species has been r e p ~ r t e d . ~ ~ In an extension of the above studies involving oxygen atom transfer, Breslow and Gellman701have carried out a related amidation involving tosylamidation of cyclohexane to yield N-cyclohexyltoluene-p-sulfonamide in the presence of [MnCl(TPP)]; equation (19). ~
1
.
+
~
~
~
3
~
~
'
other products
There appears to be considerable promise for other related metal-porphyrin catalysed nitrogen insertions for obtaining amides and amines. In this context, Groves and Takahashi702have reported the activation and transfer of nitrogen from a nitridomanganese(V) porphyrin complex, the reaction investigated being the aza analogue of epoxidation.
41.4.8.2 Phthalocyanine ligands A number of Mn"' compounds of type [MnXPc] (where X is a singly charged monodentate ligand) have been prepared from [M~Pc].~*~,'" For example, reaction of [MnPc] with SOC1, or SOBr, yields [MnClPc] and WnBrPc], re~pectively.~"All these compounds exhibit high-spin d4 magnetic moments of about 4.9 BM. Reaction of fMnPc] with oxygen in pyridine which has not been rigorously purified yields the pox0 dimer, IpyP~Mn~"-O-Mn~~~Pcpy] .522-524 This purple compound has been studied by X-ray diffraction.'& The structure contains two near flat and parallel MnPc systems joined by an oxygen atom which is midway between the manganese atoms. Each manganese has a pyridine bound opposite to the oxygen atom. As discussed in Section 4.3.10.2, the dinuclear species reacts with oxygen in dimethylacetamide quantitatively to form the adduct [MnPcO,] which is formulated as containing Mn"' and a superoxo ion.522The dinuclear species is unstable in dimethylacetamide (or dimethyiformamide) in vacuo to yield a mixture of mononuclear M" and M"' compounds which in white light are cleanly photoreduced to [M~Pc].'"~ The electrochemistry of the dinuclear compound has also been investigated; this compound undergoes a one-electron oxidation (+ 0.35 V vs. W E ) in pyridine to yield a species which is presumably mixed valent. It also undergoes a four-electron reduction (at -0.9V) to yield two
100
Manganese
molecules of a species formulated as Mn"(Pc3-). An investigation of the reduction process indicates that an ECE mechanism takes place in this latter case. The presence of a Mn-0-Mn moiety in the dimeric species appears to shift the redox behaviour of the metal centres to potentials that are 700 mV more negative. As such, a Mn" state appears accessible for this system. The electrochemical generation of Mn"' phthalocyanine from [MnPc] under different conditions has already been discussed in Section 41.3.10.2. Related photochemically induced redox transitions have also been reported.'05 A study of the electrochemistry of mononuclear [Mn"'OHPc] has also been performed.7"6This high-spin hydroxy species is readily prepared by treatment of the binuclear compound with water; the Mn-0-Mn bridge is quite susceptible to cleavage by nucleophiles. The voltametry of the hydroxy species is typical of [Mn"'XPc] compounds with X being strongly bound. The cyclic voltammogram of the hydroxy compound in pyridine gives potentials for the Mn"'/Mn" couple and for two successive one-electron reductions which are, as expected, virtually the same as those obtained for MnI'Pc species under related conditions.521 An artificial 'haemoglobin' has been synthesized in which Mn"' tetrasulfonated-phthalocyanine (TsPc) was used to replace the haem gr~up.~''On the basis of spectroscopic data, the product has been formulated as [Mn'"OPc] *globin. Electrophoresis and molecular weight determinations indicate that the compound is dimeric. The compound does not reversibly bind oxygen and cannot act as a physiological carrier of oxygen. However, [MnOPc]*globin does have an affinity for additional ligands such as CN-, CO, NO and imidazole. In In the solid, the dioxygen adduct of [Mn"(TsPc)] is best formulated as [M~"'(TSPC)O;].~~~ solution, the ESR spectrum as a function of pH suggests that intramolecular electron transfer may occur between [Mn" (TsPc)O,] and [Mn"'(TsPc)O; 1. The superoxide is only stable within the approximate pH range 11.5 to 13.5. Rcaction of [Mn(TsPc)O,] with hydrazine hydrate has been reported to produce [M~(TSPC)].'~~
41.4.8.3
Other nitrogen donor ligands
Largely because a manganese porphyrin containing the metal in a high oxidation state appears to be involved in the photosynthetic process,'" a number of studies involving nitrogen donor synthetic macrocycles have been carried out in order to examine the oxidation states accessible to this ion as a function of macrocycle structure. Mn"' compounds of such macrocyclic ligands have been generated both chemically and electrochemically, and for a number of systems of this type, Mn"' appears to be the preferred oxidation state. Mn'" species of cyclam (238) of type [MnX,(238)]+ (X = Cl, Br, NCS or N,) have been prepared.''' All compounds are high spin and appear stable indefinitely in the solid state. From spectral evidence the compounds were assigned trans octahedral structures. The species [MnC1,(238}1* was obtained by reaction of (238) with manganese(I1) chloride in methanol; oxidation was then effected by bubbling air through the solution for six hours. The product precipitated as its chloride salt on addition of concentrated hydrochloric acid to the reaction solution. The other compounds in the series were obtained from [MnC12(238)]C1by exchange reactions involving the chloro ligands. In other studies, Mn"' compounds of cyclam (238) and the related 15-membered ring (239) have been r e p ~ r t e d . "The ~ compounds are of types [MnXz(238)]Y(X = Cl, Y = BF,; X = Br, Y = Br,, PF6) and [MnBr,(239)]Y (Y = Br,, PFs) and were synthesized by reaction of anhydrous Mn" chloride or bromide with the appropriate macrocycle in refluxing methanol (under nitrogen) following by oxidation to the Mn"' products using chlorine or bromine. The BF; and PF; salts were obtained by metathesis reactions involving the initial product from each preparation. Once again, the compounds have been assigned six-coordinate, high spin (d4)structures in which the macrocycles coordinate to the Mn"' in a planar fashion.
Manganese
101
Mn"' species of the mesa form of the fully saturated N,-macrocycle (194) may be prepared by oxidation, using a range of oxidants (such as chlorine, bromine, oxygen, nitrosonium salts and hydrogen peroxide), of the corresponding Mn" species (see Section 41.3.10.3)? However the most efficient route involves oxidation of the product formed from Mn" acetate and the macrocycle with NOBF, or NOPF, followed by addition of HX to give products of type [MnX,(194)]Y2 (where X = C1, Br; Y = BF,, PF;). These Mn"' species display magnetic moments and electronic spectra which are consistent with high spin, tetragonally distorted d4systems. They are more stable to pH variation than are the Mn" analogues. These Mn"' compounds are more readily reduced than a number of Mnrlrporphyrins. The racemic Corresponding complexes of the racemic form of (194) have also been ~btained."~ macrocycle was initially resolved as its nickel species and, after removal of the nickel, the opticallyactive macrocycle was used to prepare Mnl" compounds by a similar procedure to that given above for compounds of the meso ligand. Two high spin Mn"' compounds of the pyridine-containing 15-membered macrocycle (198) of type [MnX2(193)]PF6(X = C1, Br) have been isolated after oxidation of the corresponding Mn" complexes in acetonitrile with NOPF, .534 These compounds have been assigned pentagonal bipyramidal coordination geometries in which the macrocycle coordinates in a planar manner; reports of such a geometry for Mn"' are rare. Both compounds are high spin with magnetic moments of 4.94BM. The analogous 16- and 17-membered N, ligands appear not to stabilize Mn"' and it has been suggested that these ligands may fold to yield octahedral Mn" species in which the nitrogens of the macrocycle occupy one axial and four equatorial positions. The remaining axial position is filled by a halide ion. It is perhaps significant in this context that attempts to oxidize the octahedral Mn**species of the N4 macrocycle (195) to the corresponding Mn"' compounds were unsuccessful. High spin Mn"'products of the dianionic N4macrocycle (196) of type [MnX(196)] where X is Cl, Br, SCN and N, have been prepared.s35The chloro and bromo derivatives crystallize with a molecule of acetonitrile while the thiocyanate and azide derivatives were obtained unsolvated. In the former compounds, the acetonitrile may be weakly coordinated to the manganese ion. An X-ray structure determination of wnNCS( 196)] indicates that it has a square pyramidal coordination geometry with the NCS- group N-bonded and coordinated in the axial position. As in other structures of this ring system, the macrocycle has a pronounced saddle shape. The Mn"' ion is displaced 0.356 A from the N,-donor plane.'14 The overall structure is similar to that of the Mn" species, [Mn(196)NEt3], discussed previously.536 The azide compound exhibits a slightly reduced magnetic moment (peE= 4.78 BM);535this suggests the presence of antiferromagnetic coupled Mn"' ions (via an N; bridge) in this compound. Except for the azide derivative, the various Mn"' compounds gave similar electrochemical beha~iour.~ Each ' ~ compound showed two one-electron oxidation steps, the first being reversible and the second somewhat irreversible. Evidence suggests that these oxidations essentially involve the coordinated macrocycle. Each compound exhibits a one-electron reduction wave which has been assigned to the conversion of MnI" to Mn". It is evident that, relative to the saturated macrocycle (194),533 the dianionic ligand (196) stabilizes Mn"' over Mn". No evidence for the generation of Mn" species was obtained during these studies. All the above Mn"' compounds of (196) are stable as solids under dry nitrogen although they slowly decompose in air.s35They also decompose in aqueous base but in acid undergo reversible protonation. The protonation site is probably the alkenic carbon of the 8-diketone fragment; equation (20). Ni" complexes of related macrocyclic ligands have been reported to undergo similar protonation reaction^.^'^'^^^
In situ condensation of 2,6-diaminopyridine and acetylacetone has been proposed to yield compounds of type WXL] (where LH2 is the N,-cyclic ligand formed by condensation of two molecules of each of the above precursors:" M = Cr, Mn, Fe or Co; and X is a range of mono anions). The structure of the Mn"' species (and, indeed of all of the compounds) was proposed to be square pyramidal; the pyridine nitrogens of the organic ligand were assumed not to bind to the Mn"'. However non-coordination of these donors would appear to induce considerable strain into the macrocyclic system and hence, in the absence of full X-ray structural data, the assigned structure should be considered tentative.
Manganese
102
Reaction of tris(2,4-pentanedionato)manganese(III) with either of the macrobicyclic ligands of type (240) in acetonitrile leads to isolation of compounds of type [MnCIL](PF,), in each case.'I9 These compounds are five-coordinate and high spin. Both display slow but efficientcatalase activity; both react smoothly with hydrogen peroxide in acetonitrile but the rate of oxygen evolution is much slower than for an effective catalase model compound based on Fe"' studied previo~sly.~~" The manganese species gave no evidence that they exhibit peroxidase activity since, in the presence of phenolic substrates, no oxygenation activity was dete~ted."~
Electrochemical reduction of the Mn"' species leads to isolation of the corresponding high spin Mn" compounds. The latter react irreversibly with dioxygen to regenerate the deprotonated forms of the respective Mn"' compounds.
41.5
MANGANESE(1V)
Manganese(1V) chemistry is dominated by the insoluble MnO,, which forms so readily in aqueous solutions; and indeed the great bulk of Mn" chemistry involves compounds in which the metal's coordination partners are 0-donor ligands. As well, in recent years, the range of known coordination partners for Mnlv has been significantly extended: we now know of reasonably robust tetraalkyls [MnC,P,], there are [Mn" s,], [Mn'VCl,] and [Mn'" Fs] compounds and some compounds with nitrogen ligands including porphyrins. Except in the few cases where there is antiferromagnetic coupling between adjacent metal atoms, the compounds are high spin d 3 ( S = 2) species and the great majority of them are octahedral. Some at least of the observable Mn13 chemistry derives from the disproportionation of the Mn"' species, rather than from direct oxidation of the latter. Many of the Mn" species are redox unstable and have significant existence because they react slowly. The ubiquitous MnO, is one example; and there is another in the insoluble Mn(SeO,),, which precipitates from a solution of Mn" and H,SeO, in water. One of the more interesting developments in Mn chemistry has been the recent demonstration of a thermal equilibrium between oxidation states I1 and 1V in an [MnO,N,] system, details of which may well offer the key to an understanding of the role of Mn in 0, production in the photosynthetic system. Undoubtedly the continuing search for an understanding of this system will produce further advances in our understanding of Mn chemistry. 41.5.1
Carbon Ligands
Cyano compounds of Mn" are properly characterized only for the hexacyano anion [Mn(CN),I2-: it has been isolated, as the yellow Kf salt,51from the oxidation of the Mn"' anion in DMF with NOCl. It is immediately decomposed by deaerated water, with the separation of MnO,. Magnetic measurements (peR= 3.94 BM) show it to be a high spin d 3 species. Perhaps surprising, in view of the reactivity of this anion, it has now been that Mn(CN),*nH,O, which precipitates from aqueous acid solutions of [MII(CN),]~-, has a cubic, Prussian blue type structure and should be formulated as Mnl'wn'V(CN),]-izH,O. Impure samples of analogous compounds, in which Zn" and Cd" replace the Mn",also have been obtained.
Manganese
103
'K,Mn(CN),' does not exist. It is one of those chimeras that seem to persist through acquiring textbook status before being debunked: products, which were described as such, were mixtures containing Mn'['-O-Mn"' species. The relativeIy robust, tetrahedral, green, volatile MnR, (R = norbornyl) was reported in 1972 from the reaction of LiR and MnCl, in petroleum,'*' and the oxidation mechanism leading to the Mn" species in still apparently unexplained.63Related, but less thermally robust green compounds of 2-methyl-2-phenylpropyl and trimethylsilylmethyl were described by Wilkinson and c o - ~ o r k e r s ~ ~ as arising when traces of 0, were admitted to a solution of the Mn" alkyl, and were identified particularly by their ESR spectra as S = 3 species. More recently octahedral [MnMe,dmpe] has been isolated from a reaction that involves disproportionation of the Mn"' specie^."^ The reaction of [Mn(acac),] with dmpe and MeLi in diethylether at -78OC gives a clear yellow solution, from which yellow crystals of the Mn'" compound (and then the orange [MnMe,(dmpe),]} were obtained. In another similar reaction [MnMe,(PMe,),] also was obtained. Such reactions would appear to offer a route to the exploration of an extensive chemistry of Mn" alkyls. Yellow IMnMe,dmpe] is a high spin compound oL,8 = 3.87 BM in solution) and an X-ray analysis of its stru~ture''~shows Mn-P = 2.50 8, and Mn-C = 2.07 8, (trans to P) and 2.12 A (trans to C).
41.5.2 Nitrogen and Phosphorus Ligands Knowledge of nitrogen ligands, apart from the macrocyclic, and especially the porphyrin ligands, (Section 41.5.7) I s confined to just a few compounds, which also have other ligands in their coordination sphere.572 Black [MnCl,bipy] (with peR= 3.82 BM) is one of the compounds formed in the reactions of KMnO, and bipy with concentrated aqueous HCI. It loses C1, at room temperature. Still reactive, but nonetheless well characterized are the p-dioxo dimers noticed in Section 41.4.2. The reaction of bipy or phen and KMnO, in concentrated HC1 produces green p-dioxo species (222), which contain both Mn"' and Mn". The latter has, of course, the shorter bond lengths (Mn-0 = 1.784 and Mn-N = 2.02-2.08& compared to 1.85 and 2.13-2.23 for Mn"'). Oxidation to the red, fully Mn" dimer, according to equation (16), occurs in CH,CN at E,, = 1.33V (vs. SCE), for both bipy and phen; and the phenanthroline species has been isolated as [Mn,O,(phen),](C104)4.H20.Its magnetic moment is 1.86BM at room temperature, reducing to 1.36BM at 94K. ESR data confirm the equivalency of the two Mn atoms and the absence of any significant electron pairing between them in solution (for bipy, g = 2.05 and A = 83 G). Again, as for Mn"' (Section 41.4.2), there are some biguanide which have been described as Mn" p-dioxo species of similar type to the above; but there is a need for further characterization, especially, if possible, by X-ray techniques. An even more interesting vista is opened up by a recent report of a tetranuclear [MniV0,] compound with the terdentate ligand (241).723The reaction of aqueous solutions of the amine and MnCl, in water with 0, produces a black solution, and the addition of NaBr, and adjustment to pH 8, gives needle-like crystals of [Mn,O,L,]Br, 50H0.56H,O.
An X-ray analysis shows the Mn,06, adamantane-like nucleus (cf. the S analogue in Section 41.3.6.1) with each Mn atom also bonding to the terdentate amine ligand, giving [MnO,N,j octahedra {average bond lengths: Mn-0 = 1.79p\ and Mn-N = 2.08(1)& and O-Mn-0 = 128.7(8)").The magnetic moment of 3.96 BM per Mn suggests very little, if any, antiferromagnetism. It would be surprising if this is not the first of many such cluster compounds to be detected, and they may form a class like the basically trinuclear carboxylates (Section 41.4.3.3).
Manganese
104
Phosphine ligand compounds of Mn" are confined to the phosphine alkyls of Section 41.5.1, and [MnC1,(diphos),]C12 of Table 17 (Section 41.3.4). There are no authenicated arsine compounds.
41.5.3
Oxygen Ligands
41S.3.1
Water, hydroxide and oxide
There is no evidence for the significant existence of an Mn" aquo species in water, nor for the binary hydroxide Mn(OH),, nor indeed for a definite oxide hydrate: the aqueous chemistry of Mn" is dominated by the insoluble Mn0,.2".472 Yet the [Mn(OH),]'- octahedron is found in the yellow-green to orange mineral j o ~ r a v s k i t e ' ~ ~ Ca3Mn(OH)6.CO3.SO4. 12H20,in which the Mn-OH distance has been measured at 1.87A; and in d e s p ~ j o l s i t eCa,Mn(S04),*(OH),-3H,0 ~~~ (Mn-0 = 1.92 A). MnO, exists in an almost bewildering array of different structural types.'*2'0,s72*726 Most of them are non-stoichiometric, and contain more or less H 2 0 , OH- and/or other metal atoms. It has been suggested that such phases, in the range of MnO, (n = 1.7-2.0), whether synthetic or natural, should be designated by the German language name ' B r a u n ~ t e i n ' . Of ~ ~ ~the known phases, only the fl-modification (pyrolusite) corresponds in oxidation state and chemical purity to 'MnO,'. The different modifications find widespread use in quite diverse areas of industry: from the Leclanchi dry cells; through [0] sources in welding rods, safety matches, explosives and fireworks; as additives in glasses and cements; as ion-exchangers in water; to a wide range of applications as a catalyst, especially in oxidizing reactions. Known forms of MnOz and the great bulk of other binary and ternary oxides2" all contain octahedrally coordinated Mn", with Mn-0 distances averaging near 1.9A (Table 53), paralleling the situation in all known compounds of the chelating oxygen ligands. However, there is at least one example of an [MnO4I4-tetrahedron: Ba,MnO, has the Cs3CoClS structures7' and thus contains discrete tetrahedra. Such tetraoxomanganate tetrahedra dominate the chemistry of the higher oxidation states V to VII, and have been observed in the dimeric Mn"' compound I(6Wn,O6] (Section 41.4.3.1). It is not unlikely that, as the solid state structural chemistry of MnN is further elucidated, more of these tetrahedra will be found and they may be significant in aqueous alkaline solutions. There are probably other examples in the solids reported by Scholder and co-workers. Bridging oxide ligands are beginning to be detected in various oligomeric/cluster compounds: [Mn30] (Section 41.4.3.3), [Mn,O,] and [Mn,O,] (Section 41.5.2) units are now known.
Table 53 Some of the Binary and Ternary Oxides of Mn" Compound
MnO, @-form) Ramsdelli te Mn,O, MilMn,O, Li,MnO, u
B
BaMnO, NiMnO, Mg,MnO4 M,MnO,
Bond Lengths (A) Av. Range
Comments Tetragonal rutile type
[MnO,]
(Mn: Mny 0,) M = CU,CO,Cd NaCl structure type NaFeO, superstructure of NaCl CsNiE, structure Ilmenite structure Cubic spinel (M = Ca, Sr, Ba) Tetragonal K,NiF, layer structure
wno61 [MnO,l [MnOJ [MnO,] [Mal [MnOJ
wno6i
(1.88) (1.89) 1.861.92 1.85-1.92 1.81-2.00 1.88-1.9 I
[MnO,]
CaMn, 0, Ca,Mn,O, Ca4Mn3010 ZnMn, 0,* 3H, 0 Mg, MnO, Sr,CrMnO,
Sr,Ti, 0,type Isostructural with CQTi,O,, Chalcophanite
IMnO,l wno6i
Disordered perovskite
iMn061 iMno61
CaCu, Mn,O,, Ba, MnO,
Perovskite-Iike structure mo,i Isotype of Cs,CoCI, -tetrahedral [Mn0,I4-
(1.93) (1.94) 1.85-1.96 (1.91) (1.92)
105
Mmganese 41.5.3.2
Peroxo
There are arguments in the literature that the products of reversible oxygenation of Mn" porphyrins should be regarded as Mn'v-peroxo ( q 2 )species (Section 41.3.5.1.vi); and, although yet unproven, this does seem to fit with the emerging pattern of Mn chemistry. Unlike its neighbour Cr, which also forms high oxidation state tetraoxo species, Mn does not form robust compounds containing peroxo ligands. The reaction of alkali tetraoxomanganate(V1) or Mn" species with H 2 0 2in cooled, concentrated alkaline solutions gives slightly soluble red-brown to brown crystalline products, which are probably peroxomanganate(1V) species. ' ~ ~ the The product was dependent on concentration of alkali, and Scholder and K ~ l b claimed preparations of pure MzHZIMn(O,),O] (M = Na, K), and of mixtures containing the compounds M,H[Mn(O,),] (M = Na, K), K3H[Mn(02),0]and K,[Mn(O,),]. Undoubtedly, they were peroxo compounds, but confirmation of their structures must await X-ray data. They are unlikely to find much use: when dry they detonate above 0°C. K,H,[Mn(O,),O] gives a red-brown aqueous solution, from which O2 is slowly evolved, with precipitation of MnO,.
41.5.3.3
Oxyanions
In these compounds the [MnO,] octahedron is clearly becoming unstable: the known compounds are not generally robust, but there are several interesting exceptions. There are yet no indications of any coordination polyhedron other than the octahedron. Oxalate forms the only known carboxylato species- the green K,[Mn,O,(Ox),] -nH,O and apparently another orange coloured compound of unknown type.'72,728 The green dimer can be kept for ca. one week at - 692,in the dark; but useful physical measurements have been made and its decomposition Mn"' phosphato species are formed729in solution only at very high [H3P04],by the reaction of KMnO,. Such brown-red solutions, on standing or upon dilution, deposit the insoluble Mn"' species MnPO,.H,O. Black plates, formulated as ~H,],Mn(PO,),-H,O, and the flesh-pink coloured solid obtained from concentrated aqueous H, AsO, and formulated as Mn(H,AsO,),, may be examples of Mn" species; but further characterization is needed. With the group VI oxyanions, sulfates and tellurates are probably represented in some of the reported data, but the major interest is in the insoluble selenite Mn(SeO,),. MII(SO,)~may be representeds9' in the unstable black solid that can be prepared from MnSO, and KMnO, in 55% H2SO4.It dissolves in cold 4&70% H,S04, forming a brown solution, which probably contains. Mn" sulfato species. And some tellurates have been described as Na,H,Mn(TeO,), 3H,O and K,H, Mn(TeO,), * 5H,0, but not firmly characterized. Thus the ready preparation of Mn(SeO,), stands in contrast: it can be made7j0as orange-red crystals in high yields by reacting aqueous Mn" and H,SeO, on the water bath; MnO, and SeO, in boiling water, or in a sealed flask at 250-300°C; Se and MnO, in aqueous H,SO,; or KMnO, and SeO, in water. The reaction (21) is reversible. Clearly, Mn(SeO,), represents a significant energy minimum for the Mn/Se/O system, and it is interesting to note that Mn"(IO,), is an energy minimum in the Mn/I/O system.
-
MnSeO,
+ SeOl
2M"C
Mn(SeO,),
(21)
Iodato compounds of Mn" are known, having been first described by Berg in 1899; but do not appear to have been obtained free from Mn"' materials.279Amber or brown solutions can be obtained by dissolving MnO, in iodic acid and adding some periodate. Such solutions are robust in sealed quartz tubes, but in glass deposit red K2Mn(I0,),. A recent731study of the solid products has detected a range of solid solutions from K2Mn(10J6 to K,H2Mn(103)9;and salts with other cations are known. Periodate oxidizes Mn to [MnO,]-, but, with the proper choice of conditions, Mn" compounds (unstable now to oxidation rather than reduction) can be isolated. Red Na7H.,Mn(IO6),. 17H,O and K7H,(I0,),*8H20 have been described, and an X-ray analysis of the former showed a tris-bidentate [MnLV06] species with Mn-0 = 1.90A.The red, insoluble compounds NaMnIO, and KMnIO, are i s o s t r u ~ t u r a lwith ~ ~ ~NaNiIO,. Another octahedral oxyanion [Mn"O,] species has been c h a r a c t e r i ~ d 'in ~ ~one of the products
I06
Manganese
-
of evaporation of an aqueous solution of HMnO,: violet (H,O),[Mn(MnO,),] 5H20is unstable above -4°C. And there is a range of heteropoly- and isopoly-anion compounds of Mn" with V, Nb, Ta, Mo and W (Table 26), most of which appear to be [MnO,] species.
41.5.3.4
Di-and polyhydroxy ligands, including catechol
A range of polyhydroxy and polyhydroxy-carboxylate manganese compounds has been shown to be oxidizable to Mn", and one of them, the tris-chelate of the dianion of sorbitol, has been isolated734as a red-brown hygroscopic solid (Nh.le,)2[Mn(C6H,,0,), 6H,O. Such compounds have been known3I6since Schottlander reported some yellow glycerate species Na, [MnL,] and Sr[MnL,] in 1870; but they have only recently been studied in some Although X-ray data are not available, the sorbitolate compound appears to be another example, like the periodate (Section 41.5.3.3) of a tris-chelated Mn'" anion, using two adjacent hydroxy groups of the sorbitol. More detail is available on the chelated MnTVcompounds of the anion of one of the catechols -3,5-di-ter-butylcatecholate(242). The Mn compounds of this dianion, in the presence of an excess of the ligand, can be oxidized to the Mn"' (Section 41.4.3.4) and Mn" species.282X-Ray analyses of the tris-chelated Mn" anion [MnL,]'- give Mn-0 bond length^^^^.',^ between 1.89 and 1.92 A. There are conflicting reports about the possibility of this anion supporting reversible binding of 0,.282,737 The final explanation of the observed phen~mena~~'.'~' will probably involve ligand oxidation to the semiquinone form (243).
The latter is almost certainly invohed in an interesting thermal equilibrium (22), which has been observed in both pyridine and toluene solutions. When [Mny L,] (L = 243) (Section 41.3.5.3.G) was treated with py, purple crystals [Mn"L,py,] -2py (L = 242) were obtained. An X-ray analy~is'~' proved the Mn'"/catecholate formulation (Mn-0 = 1.85412) and Mn-N = 2.018(3)A}, and a spectrophotometric study of the equilibrium (22) showed that the Mn" form prevails at low temperature in the solutions. There seems to be a real possibility that some such redox changes may offer an explanation of the role of Mn in biological photosynthetic water oxidation.
41.5.3.5
Other ligands
Electrochemical s t ~ d i e s " ~ have ~ ~ shown '~ that the [Mn(bipyO,), 12+ and [Mn(terpy0,)JZf cations can be oxidized to the Mn" species in CH3CN.The spectra of the Mn" cations were measured, but they are rather unstable and were not isolated. There have also been some reports in the literature of Mn'" compounds of acetylacetonate, but they have not yet been fully substantiated.
41.5.4
Sulfur Ligands
No binary or ternary phases containing Mn" sulfides appear to have been described; but well characterized, although reactive and somewhat difficult to handle, compounds are known with both 1,l- and 1 ,Zdithiolates. As noticed in equation (8), the tris-dialkyldithiocarbamato-manganese(II1) compounds [Mn(R,dtc),] are readily oxidized to the monocations329~330 (oxidation potentials for the I6 compounds studied varied between 0.25 and 0.53 V). Such oxidations have been achieved chemically by
I Manganese
107
adding BF3 to CH2C12solutions of [Mn(R,dtc),] in the air, whence oiidized [Mn(R,dtc),]BF, species were obtained; and, alternatively, by mixing benzene solutions of the Mn”’ compound with acetone solutions of [Mn(H,O),]X, (X = C104, BF,), whence either the C10; or BF; salts were obtained. (The oxidizing agent in each case appears to have been O,.) The dark purple compounds [Mn(R,dtc),]X have magnetic moments typical of high spin d 3 systems (3.7W.25 BM), and give EPR signals with isotropic g values and small hyperfine interaction. Confirmation of the [Mn”S6] structure of these compounds is given by an X-ray analysis of the piperidine compound [Mn{S2CN(CH2)5),~CIO,-CHCl,: the tris bidentate species has Mn-S distances between 2.325 and 2.353 A, averaging at 2.33 A. Tris-dithiolenemanganesecompounds are k n o w r ~ ~only ” , ~in ~~ the Mn” oxidation state, just as the bis compounds are only known as Mn” (Section 41.3.6.2). Green crystals of M2[Mn(mnt),] (M = PPh: and AsPh:) were prepared from MnCOAc),, and black crystals of ~R,],[Mn(S,C,C1,),], giving magenta solutions, were prepared either from Mn“ in the air, or from KMnO,. All have magnetic moments near 4 BM which is typical of high spin d 3 systems. Electrochemical studies have shown that they can be reduced in two one-electron steps and oxidized in two one-electron steps, but none of the reduced or oxidized species have been isolated. Identity of X-ray powder patterns with the Fe analogue suggests that [AsPh,], [Mn(mnt),] has the same octahedral structure defined for the Fe compound. A recent report73ydescribes several mixed ligand compounds [Mn(R,dtc),X,] (X = C1, Br).
41.5.5
Halides
The only binary halide known is the fluoride MnF,; and, although ternary compounds are well established for both F- and C1-, the latter are particularly unstable at normal temperatures and all are, to varying degrees, sensitive to MnF,, known since 1928, has now been obtained in single crystal^,^^ which have a trigonal unit cell, but the structure has not yet been reported. The ultramarine blue solid is very hygroscopic, loses F, slowly at normal temperatures and hydrolyses instantly in contact with water. Three classes of ternary fluorides are known (Table 54). Red 0;[Mn,F,]- is the only representative of this class;741but all of the alkali metal species MMnF, are known. The former is polymeric, with double chains of linked [MnF,] octahedra, and, although structural analyses are not available, it seems certain that the brick-red MMnF, spp. also contain chains of [MnF,] octahedra. Magnetic moments, except for the 0: species, are near 3.8BM. They were first prepared by Hoppe and co-workers by fluorinating MMnF, at 45&510”C in a flow system, but details have now been given7” of a wet method using 48% HF. This method also gave an interesting pyridine compound formulated as MnF,*py*H,O. which is described as quite robust at normal temperatures, although having a faint odour of pyridine. Undoubtedly a wide range of N-donor Compounds of similar type will be obtainable. The third class of ternary fluorides is the simplest one- the compounds M,[MnF,]. All are yellow or orange-yellow with magnetic moments near 3.8BM, as expected for octahedral Mn”. They crystallize in various forms: the alkali metal salts, being known in two hexagonal forms (one having the Na,[SiF,] structure), a trigonal and a cubic form (the latter having the K2[F’tCl,l structure). The salts M[MnF,], for M = Mg, Ca, Cd, Hg have the Li[SbF,] structure; and, for M = Sr and Ba, the Ba[GeF,] structure. Mn-F bond lengths for six different measurements range between 1.72 and 1.75 A, and average at 1.735A.These yellow, hygroscopic compounds are prepared by the action of F, on a mixture of MF and MnF, or by reduction of [MnOJ in HF. Recently, (NF,),[MnF,] Table 54 The Ternary Halide Compounds of Manganese(1V)
Compound Type 0: Mn, F; MMnF,
Comments Ruby-red needles or plates = 5.63 BM [MnF,] octahedra Iinked in double chains M = 0:. Red Li, Na, K, Rb, Cs. Brick-red [MnF,] octahedra, but details unknown M = Li-Cs, NO, NF,; 2M = Mg-Ca Yellow or orangeyellow [MnF6I2- octahedra M = K, Rb, Cs, NH,,N k . Dark red, unstable solids. K2mCI,] structure Black crystals
Ref 74 1 740 742 572, 743 623 512
108
Manganese
has been as 'an outstanding solid oxidizer, with good thermal stability and a high active fluorine content'. It is, however, shock sensitive, especially in the presence of fuels, and reacts violently with water. MnC1, is not known in crystalline form, although The chloro analogues are much less [MnCLbipy] has been prepared (Section 41 -5.2) and characterized. Like the hexachloromanganate(1v) species, however, it loses C1, at normal temperatures. The very dark red, unstable solid K,[MnCl,j was first prepared by the reduction of Ca(MnO,), with 40% HCl, followed by the addition of KC1; but it has also been made from 'Mn"' acetate' and potassium acetate in a large excess of acetyl chloride. Other salts of Rb, Cs, NH4 and NMe, have been made, and all have face-centred cubic lattices of the K,PtC16 type. The Mn-CI distance in the potassium salt is 2.28 A.
41.5.6 Mixed Donor Atom Ligands Compounds of Mn" with the various multidentate ligands with different donor atoms have been postulated, especially as products of the 'oxygenation' of the Mn" salicylaldiminato ligands (Section 41.3.9.3). But there is no unequivocal proof of this oxidation state in these compounds. It would be surprising if Mn" species do not form, even if they turn out to be oxidatively unstable. But we currently know nothing of consequence about the detail.
41.5.7 Macrocyclic Ligands 41.5.7.1 Porphyrin ligands Higher-valent manganese porphyrins have been of interest since they have been shown to be important intermediates in the activation of hydrocarbons under mild conditions (Section 41.4.8). Their use as sensitizers for the photooxidation of water to oxygen has also provided a motivationpB although the involvement of a manganese porphyrin in the natural system has not been fully documented. MIv porphyrins may be generated by treatment of the corresponding Mn"' porphyrin with a strong oxidant, such as hypochlorite (Section 41.4.8.1). The local environment affects the ease of such oxidation and also the nature of the oxidation products.4p0In alkaline solution, such oxidation is facile and yields both Mn" and MnVporphyrins. The oxidation is similar, but less efficient, when it occurs in membranes or positively charged micelles. However, in both negatively charged micelles and microemulsions the oxidation is very inefficient: under these conditions no MnVwas observed. In contrast to their redox behaviour, such changes in environment have little effect upon the spectroscopic and chemical properties of Mn"' porphyrin species. Radiochemical (y-irradiation) one-electron oxidation of [MnCl(TPP)]has also been demonstrated in tetrachloroethane at 77 K.744ESR and optical spectroscopy indicate that oxidation occurs at the metal centre to yield a Mn" compound. The Mn" porphyrins tend to be of limited stability in water at neutral pH but are stable under strongly alkaline condition^?^^*^*^ In acid, rapid decomposition to Mn" species occurs via a complex redox mechanism.479The reduction is catalyzed by redox catalysts, such as RuO, in acid, to yield O2in low yield as one product. This latter observation documents (in part) a route for the catalytic oxidation of water. Hill and coworker^"^ have reported a range of MnJVporphyrin species, including the monomeric species [Mn(OCH,),(TPP)],746[M~(P~I(OAC)O)~(TPP)]~~' and [MnX, (TPP)] (X = N,, NC0)74' as well as the two types of dimeric species mentioned previously in Section 41.4.8.1, [(MnX(TPP)),0]678(X = N, and OCN) and [(MnX(OIPh)(TPP)),0].6w [Mn(OCH,),(TPP)] is obtained by oxidation of [Mn(OAc)(TPP)]in basic methanol using sodium hypochlorite or iodosylb e n ~ n e . 7X-Ray ~ ~ diffraction indicates that this molecule is pseudooctahedral with the methoxy groups coordinated above and below the N,-donor plane of the macrocycle. The compound has a high spin d 3 Mn" ground state with a magnetic moment of 3.9 BM. X-Ray studies of a number of the other monomeric species indicate related pseudooctahedral coordination geometries for these compounds. The X-ray structure of wnN,(TPP)],O indicates an oxygen-bridged dimeric structure in which each manganese ion achieves a six-coordinate geometry.678The ground state of this compound is best described as containing two antiferromagnetically coupled Mn'" ( d 3 )ions.
Manganese 41.5.7.2
109
Phthalocyanine ligands
As noted in Section 41.4.8.2, there are suggestions in the literature of Mn'" phthalocyanine species, such as [MnOPc]; but they appear much less accessible than for the porphyrins; and have yet to be fully characterized.
41.6
MANGANESE(V), (VI) AND (VII)
The coordination chemistry of these higher oxidation states is dominated by, but not completely confined to, the tetraoxomanganates.
41.6.1 Tetraoxomanganates The tetraoxomanganate ion can be isolated, although it appears to be but of minor importance, for Mn" (in Ba,MnO,, Table 53), Mn"' (in &[Mn,O,], Table 46) and even for Mn" (in Na,,(MnO,),O, Table 19); and it seems not unlikely that further examples will be found, especially for Mn". At the other end of the oxidation scale, purple tetraoxomanganate(VI1) is, of course, the permanganate that one learns about so soon after the first introduction to chemistry; green MnOi-, which is stable in alkaline solutions, has been 'known' since Glauber described a green melt obtained by heating pyrolusite (MnO,) with saltpetre (KNO,) in 1659; but MnOi-, although noticed in the older literature as hypomanganate, was not recognized as such until the work of Lux in 1946. Thus the tetrahedral [MnO,y- unit is remarkable in being the only coordination polyhedron that has been detected throughout the whole range of oxidation states Mn" to Mn'". It has not proven possible (at least not yet) to study the full range of these redox changes in any solution, but at least for the last three of the series (MnV to MnV") there is considerable detail available on their relative stabilities."' Tetraoxomanganate(V), first characterized by Lux, has been isolated in a range of solid compounds. Relatively pure salts M,[MnO,], for M = Li, Na, K, Rb and Cs, have been des ~ r i b e d , ' ~ * ,as ~ ~well , ' ~ ~as Ba,[MnO,],, Ba,(MnO,),OH and halo-apatite species such as Sr and Ba,(MnO,),Cl. Although a complete X-ray analysis is not available, there is no doubt about the and [PO4],existence of the [Mn0,j3- tetrahedron, through comparative powder data with species; and mixed crystals Li, (PO,/MnO,) and Sr5(PO4/MnO4),C1 have been studied. Preparations rely on the reduction of [MnOJ in concentrated aqueous alkali, or oxidation of Mn" or MnO, in an alkaline melt. The blue solids are very moisture sensitive; magnetic moments are near 2.8 BM, as expected for a d 2system; and details of their spectra have been studied, especially in Li,PO, and Sr,(PO,),CI matrices, The study involving the latter reports'" a 'dynamic Jahn-Teller effect' in an excited state of [Mn0,l3-. The relative stabilities of MnVand Mn" in nitrite, nitrate, hydroxide, oxide and peroxide melts have recently been studied in some detail.',' In NaN0, and NaNO,/Na,O, melts, [Mn0,I3- is favoured over [Mn04]*-, whereas in KNOz or KN03 melts it is the MnV' species which is favo~red.~'' In aqueous alkaline solutions of blue Mnv, the concentration of OH- must be at least 8 M to prevent disproportionation to [MnOJ- and MnO,. Green tetraoxomanganate(VI), however, is robust in aqueous alkaline solutions only slightly above 1 M in [OH-]: lower [OH-] leads to rapid disproportionation to MnOz and MnO; (Figure 1). This anion has been isolated in a number of crystalline solids, especially the alkali metal salts M,[MnO,] (M = Li 4 Cs), and an X-ray analysis of the K salt has proven the tetrahedral [Mn0,I2unit, with Mn-0 distances between 1.647 and 1.669 (average = 1.659 A). The green manganate(V1) ion is a d I system, and measured magnetic moments are near 1.8 BM. Rapid exchange reactions between MnOi and MnOZ- have been studied, and cation effects were interpreted in terms of an outer-sphere bridged activated complex. Permanganate [MnOJ is relatively robust for a strong oxidizing agent. The potassium salt is widely used industrially for bleaching; as an antiseptic, preservative and astringent; as an oxidizer in water and air treatment; and for oxidizing hydrocarbon compounds: world production in the early 1970s was estimated at near 40000 tonnes. It is as an oxidizing agent, rather than for its coordination chemistry, that it is best known chemically, being the source of a five electron oxidation in acid solution (reduced to Mn") or three electron oxidation in alkaline solution, where reduction stops at Mn'" because of precipitation of MnO,. Aqueous solutions are unstable, as any
110
Manganese
student of its analytical applications will know; and indeed MnO; is a good example of a kinetically stabilized compound. A range of salts of the anion has been described and it is known as a ligand -one notable example being (H,O),~Mnlv(MnO,),]~nH,O, which is one of the products of dehydration of HMnO, in water. An X-ray analysis of the potassium and calcium salts has given Mn-0 at 1.63 A-somewhat shorter, as expected, than in the manganate(V1) species. One area of MnOi oxidations that has not been greatly explored, and which may lead to new Mn compounds in lower oxidation states, is reaction in non-aqueous solvents. A recent description752 of the salt of the [(Ph,P),N]+ cation, which is rather soluble in a number of dipolar aprotic solvents, gives a t least one starting material for such work; and the use of crown ethers and cryptates, to 'solubilize' MMnO, species, gives another.
41.6.2 Other Oxo and Oxohalo Species The compounds for which reasonable evidence is available of their existence are listed in Table 55. Other than the tetraoxomanganates, the compounds are rather unstable, are also very strong
oxidizing agents and are liable to detonation, especially in contact with a fuel.
41.6.3
Porphyrin Ligands
Reactions of hypochlorite with Mn]"porphyrins (Section 41.4.8.1) under alkaline conditions appear to produce, as well as the MnIV species (Section 41.5.7.1), some MnV species, such as [Mn(N,)ClO]; but these have a very limited existence.687 However, quite robust crystalline Mn' nitrido compounds are produced when these oxidations are carried out in the presence of NH,.757 These red compounds contain a Mn-N triple bond, are diamagnetic, five-coordinate and show considerable chemical stability. Thus while the isoelectronic oxochromium(1V) or oxomolybdenum(IV) porphyrins behave, respectively, as weak oxidants and reductants, the corresponding manganeseonitrido species exhibits enhanced redox stability. {MnN(TPP)]and related porphyrin species were also independently prepared by the reaction of iodosylbenzene with [Mn"'X(porphyrin)] (X = C1, Br, OAc) in dichloromethane ixt the presence of excess a m m ~ n i a . " ~ Table 55 Compounds of Manganesew), (VI) and (VII) Compound
HMnO, MnO, F
MnOC1, MnO, C1, MnO, Cl MnO, -SO,CI(7)
MnV porphyrins
Cnmnts
Blue anion-characterized in crystalline compounds, in concentrated alkali solutions and in melts. Green anion-stable in aqueous solutions with [OH-] > 1M; crystalline salts. Purple anion -reasonably robust in water and in crystalline salts. Very unstable red/geen liquid. Liable to detonation but IR and UV spectra recently obtained on matrix-isolated samples.753 Permanganic acid or hydroxotrioxomanganate(VI1). Also as a dihydrate. Both crystalline:54 strong oxidizing agents, with parallels to HCIO,. Highly volatile (Wohla 1828).572Microwave data give Td with Mn-0 = 1.586(5) and Mn-F = 1.724(5)A. Dark green crystals at -78°C. Very moisture sensitive and decomposes explosively. Green liquid from KMnO, reduced in HS0,C1.'55 Decomposes to MnC1, above 0°C and to [MnO,]'in aqueous alkali. Least stable of oxochlorides-decomposing at - 30°C. Brown solutions in CCl,?55 Green in CCl, solution. Detonates in air above 0°C. Slowly hydrolysed to MnO; and Cl- in water.755 This species may exist in solution, but decomposes, during isolation attempts, to MnO*S0,Cl.756 See Section 41.6.3
Muttganese
111
X-ray structural ana lyse^'^^,'^^ show square pyramidal structures with the metal 0.4 A out of the [N4]donor atom plane and short, axial M n e N distances of 1.51 A. One of the structural analyses refers to a porphodimethene r n a c r o ~ y c l ewhich , ~ ~ ~ survived (12% yield) the reductive methylation of the analogous nitrido(octaethy1-porphyrin)manganese(V) species, illustrating the redox stability of such formally MnV compounds.
41.7
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___
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585. 586. 587. 588. 589.
~
Manganese 653. 654. 655. 656. 657. 658. 659. 660. 661. 662. 663. 664. 665. 666. 667. 668. 669. 670. 671. 672. 673. 674. 675. 676. 677. 678. 679. 680. 681. 682. 683. 684. 685. 686. 687. 688. 689. 690. 691. 692. 693. 694. 695. 696. 697. 698. 699. 700. 701. 702: 703. 704. 705. 706. 707. 708. 709. 710. 711. 712. 713. 714. 715. 716. 717.
718. 719. 720.
121
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122 721. 722. 723. 724. 725. 726. 727. 728. 729. 730. 731. 732. 733. 734. 735. 736. 737. 738. 739. 740. 741. 742. 743. 744. 745. 746. 747. 748. 749. 750. 751. 752. 753. 754. 755. 756. 757. 758. 759.
Manganese B. K. Bower and H. G. Tennent, J . Am. Chem. Soc.. 1972, 94,2512. J. Bera and D. Sen, Indian J . Chem.. Sect. A , 1976, 14, 880. K. Wieghardt, U. Bossek and W. Gebert, Angew. Chem., Inr. Ed. Engl., 1983, 22, 328. M. M. Granger and J. Protas, Acra Crystallogr., Sect. B, 1969, 25, 1943. G. Gaudefroy, M.-M. Granger, F. Permingeat and J. Protas, Bull. SOC.Fr. Mineral. Cristallogr., 1968, 91, 43. Gmelin (ref. 3), 1973, C1, 126. R. Scholder and A. Kolb, Z. Anorg. Ailg. C k m . , 1949, 260, 31, 231. T Uehiro, I. Taminaga and Y. Yoshino, Buff.Chem. SOC.Jpn., 1975,48, 2809. Gmelin {ref. 3), 1983, C9, 159, 190 and 345. Gmelin (ref. 3), 1976; C6, 299. V. L. Pavlov and A. V. Melezhik, Run. J. Inorg. Chem. (Engl. Transl.), 1975, 20, 378 and 532. I. I).Brown, Can. J. Chem.. 1969,47,3779. B . Krebs and K.-D. Hasse, Angew. Chem., Int. Ed. Engl.. 1974, 13, 603. D. T. Richens and D. T Sawyer, J. Am. Chem. SOC.,1979, 101, 3681. J. A. R. Hartman, B. M. Foxman and S. R. Cooper, J. Chem. SOL Chem. Cotnmun., 1982, 583. D.-€3.Chin, D. T. Sawyer, W. P. Schaefer and C. J. Simmons, Inorg. Chem.. 1983, 22, 752. S. R. Cooper and J. A. R. Hartman, Inorg. Chem., 1982, 21,4315; D.-H. Chin and D. T. Sawyer, ibid.. 4317. M. W. Lynch, D. N. Hendrickson, B. J. Fitzgerald and C. G. Pierpont, J. Am. Chem. SOC.,1984, 106, 2041. B. Pradhan and D. V. R. Rao, J. Indian Chem. SOC.,1981,58,733. R.Hoppe, Isr. J. Chem., 1978, 17, 48. B. G. Mueller, J. Fluorine Chem., 1981, 17,409; Chem. Abstr., 1981, 95, 34 507. M. K. Chaudhuri, J. C. Das and H. S. Dasgupta, J. Inorg. Nucl. Chem., 1981, 43,85. K. 0.Christe, W. W. Wilson and R. D. Wilson, Inorg. Chem., 1980, 19, 3254. S. Konishi, M. Hoshino and M. Imamura, J . Phys. Chem., 1982, 86, 4537. J. A. Smegd and C. L. Hill, J. Am. Chem. Soc., 1983, 105, 2920 and references therein. M. J. Camenzind, F. J. Hollander and C. L. Hill, ZnorR. Chem., 19x2, 21,4301. M. J. Camenzind, F. J. Hollander and C . L. Hill, Inurg. Chem., 1983, 22, 3776. G. Meyer and R. Hoppe, Z. Anorg. Allg. C k m . , 1876, 424, 249. Gmelin (ref. 3), 1975, C2, 2, 111 and 256. P. Day, R. Borromei and L. Oleari, Chem. Phys. Lett., 1981, 77, 214. R. B. Temple and G,W. Thickett, A u t . J. Chem., 1973, 26, 2051. A. Martinsen and J. Songstad, Acra Chem. Scand., Ser. A , 1977, 31, 645. W. Levason, J. S. Ogden and J. W. Turff, J. Chem. Soc., Dalton Trans., 1983, 2699. N. A. Frigerio, J. Am. Chem. Soc., 1969,91,6200. T. S. Briggs, J. Inorg. Nucl. Chem., 1968, 30, 2866. 2.A. Siddiqi, Lutfullah, S. A. A. Zaidi and K.S . Siddiqi, Bull. SOC.Chim. Fr., Part I , 1980, 228. J. W. Buchler, C. Dreher and K. L. Lay, Z. Narurforsch., Teil B, 1982, 37, 1155. C. L. Hill and F. J. Hollander, J. Am. Chem. SOC.,1982, 104,7318. J. W. Buchler, C. Dreher, K. L. Lay, Y.J. A. Lee and W. R. Scheidt, Znorg. Chem., 1983, 22, 888.
42 Technetium ALAN DAVISON Massachusetts institute of Technology, Cambridge, MA, USA and COLIN J. L. LOCK McMaster University, Hamilton, Ontario, Canada Because of ill health and other factors beyond the editors’ control, the manuscript for this chapter was not available in time for publication. However, it is anticipated that the material will appear in the journal Polyhedron in due course as a Polyhedron Report. ,
C.O.C. &E*
123
43 Rhenium K. A. CONNER and RICHARD A. WALTON Purdue University, West Lafa yette, lN, USA 126 128
43.1 INTRODUCTION 43.2 RHENIUM@) COMPLEXES
RHENIUM(I) COMPLEXES 43.3.1 Cyanide, Alkyl Isocyanide and Aryl Isocyanide Complexes 43.3.2 Mixed Dinitrogen-Phosphine Ligand Complexes 43.3.2.1 Mononuclear derivatives 43.3.2.2 Mixed metal dinuclear and irinuclear derivatives 43.3.3 Other Phosphine Complexes 43.3.4 Phosphite Complexes
128
RHENIUM(I1) COMPLEXES 43.4.1 Mononuclear Complexes 43.4.1.1 Cyanide, d k y l isocyanides and aryl isocyanides 43.4.1.2 Phosphines and arsines 43.4.2 Dinuclear Complexes Containing the Re:+ and R d + Cores 43.4.2.1 Monodentaie phosphines 43.4.2.2 Bidentaie phosphines and arsines 43.4.2.3 Sulfiur ligand 43.4.2.4 Hydides 43.4.3 Trinuclear Complexes
135
RHENIUM(II1) COMPLEXES 43.5.1 Mononuclear Complexes 43.5.1.1 Cyanide, alkyi isocyanides and aryl isocyanides 43.5.1.2 Amines. N-heterocycles and amido ligands 43.5.1.3 Dinitrogen, diazenido and triazenido ligan& 43.5.1.4 Cyanare and thiocyanate 43.5.1.5 Nitriles and amidinates 43.5.1.6 Phosphines, phosphites, arsines and stibines 43.5.1.7 Oxygen and suljiur donors 43.5.1.8 Mononuclear and dinuclear hydrides 43.5.1.9 Mixed donor atom ligands 43.5.2 Dinuclear Complexes Containing the R$,+ Core 43.5.2.1 Halides 43.5.2.2 Thiocyanate and selenocyanate 43.5.2.3 N-Heterocycles and amidine ligands 43.S.2.4 Phosphines and arsines 43.5.2.5 Sulfates and alkyl and aryl carboxylates 43.5.2.6 Sulfur ligands 43.5.2.7 Mixed donor atom ligands 43.5.3 Trinuclear Complexes Containing the R d + Core 43.5.3.1 Cyanide, alkyl isocyanides and aryl isocyanides 43.5.3.2 Nitrogen ligands 43.5.3.3 Phosphines and arsines 43.5.3.4 Oxygen ligands 43.5.3.5 Sulfur and selenium ligands 43.5.3.6 Halides 43.5.4 Hexanuclear Clusters of Rhenium(III)
143 143 143 144 144 145 146 147 149 150 153 154 154 156 156
RHENIUMOV) COMPLEXES 43.6.1 Mononuclear Complexes Cyanides nnd alkyl isocyanides 43.6.1.I 43.6.1.2 Ami?~oniuand N-heterocycles 43.6.1.3 Diazenido m d imido ligands
165
43.3
43.4
43.5
43.6
128 129 129 132 134 135 135 135 136 136 136 139 142 142 142
157 158 160
160 160 161 161 162 162 163 163 164 165 165
166 166
125
126
Rhenium
43.6.1.4 C‘jJanateand thiocyanate 43.6.1.5 Nitriles and amidates 43.6.1.6 Phosphines, arsines and stibines 43.6.1.7 Oxygen and surfur ligands 43.6.1.8 Thr mixed oxide-halide anions [Re, (p-O)X,,]”43.6.1.9 Complex halides, [ReX6l2-, [ R e X , ( 0 H ) l 2 - , [ReCI,L]- and related species 43.6.1.IO Hydrides 43.6.1.11 Mixed donor atom ligands 43.6.2 Dinuclear Complexes Containing a Rhenium-Rhenium Bond and Oxide and/or Halide Bridges
166 167 168 170 171 172 174. 175 176
43.7 RHENIUMW) COMPLEXES 43.7. I Mononuclear Complexes Containing the [ReOj3+ Moiety 43.7.1.1 Cyanides and aryl isocyunides 43.7.1.2 Amines and N-heterocycles 43.7.1.3 Thiocyanate and other nitrogen donors 43.7.1.4 Phosphines,phosphites, arsines and stibines 43.7.1.5 Oxygen and surfur ligands 43.7.1.6 Halides 43.7.1.7 Mixed donor atom ligands and macrocycles 43.7.2 Mononuclear Complexes Containing !he [Reo,]’ Moiety 43.7.2.1 Cyanide and aryl isocyanides 43.7.2.2 Amines and N-heterocycles 43.7.2.3 Phosphines 43.7.2.4 Other ligands 43.7.3 Complexes Containing the p-Oxo-bridged /Re,O,]‘+ Moiety 43.7.3.1 Cyanide 43.7.3.2 Ethylenediamine und N-heterocycles 43.7.3.3 Diulkyldithiocarhamates 43.7.3.4 Other ligands 43.7.4 Niirides and Imides (Nitrenes) 43.7.5 Other Complexes 43.7.5.1 Oxygen and surfur ligan& 43.7.5.2 Halides 43.7.8.3 Hydrides 43.7.8.4 Miscellaneous ligands
177 177 177 177 179 179
43.8 RHENIUM(V1) COMPLEXES 43.8.1 Nitrides, Imides (Nitrenes) and Amides 43.8.2 Halides and Oxide Halides 43.8.3 Oxoanions and Alkoxides 43.8.4 Suqur and Mixed Sulfur-Nitrogen Ligands
194 144
43.9 RHENIUM(V1I) COMPLEXES 43.9.1 Nitrides. Imides (Nitrenes) and Amide3 43.9.2 Oxo Species 43.9.2-1 Perrhenates and related species 43.9.2.2 Other oxygen ligands 43.9.2.3 Amines and N-heterocycles 43.9.2.4 Halides 43.9.2.5 Thdoperrhenates 43.9.3 .Thiolate Ligands 43.9.4 Halides 43.9.5 Hydrides
196
43.10 NITROSYL COMPLEXES A N D OTHERS WHERE A N AMBIGUITY M OXIDATION STATE ASSIGNMENT MAY EXIST 43.1 1 MISCELLANEOUS COMPLEXES 43.12 REFERENCES
43.1
1x1 1s3 1S4 1S5
185 1S6
186 1S7
187 187 187 188 188 188 192 192 192 192
193
195 196 196 196 197 197 198 199 199 200 20 1 20 1 20 1
202 204 204
-
INTRODUCTION
The coordination chemistry of rhenium, the last of the stable elements to be discovered (in 1925), has in the last 25 years undergone a tremendous growth. This element now occupies an important place in contemporary inorganic chemistry. Entry into its chemistry is usually afforded by access to the commercially available (but relatively expensive) perrhenate salts NH,ReO, and KReO,, although rhenium metal is sometimes used as the starting material. In surveying the coordination chemistry of rhenium we have been greatly assisted by the availability of the superb review entitled ‘Recent Advances in the Chemistry of Rhenium’ that was written by George Rouschias and published in 1974.’ This comprehensive review covers all aspects of
Rhsnium
127
rhenium chemistry and can usefully be consulted and used in conjunction with the present review. Other useful reference sources are an early review by Fergusson2on the coordination chemistry of rhenium dating from 1966, and two monographs on rhenium chemistry published around this same time.3.4Additionally, we have made use of the more recent editions of Annual Reports (Section A ) and the Specialist Periodical Reports, published by the Royal Society of Chemistry, in surveying material for use in this review.* Rhenium occupies a very important place in the development of the chemistry of complexes containing metal-metal double, triple and quadruple bonds. This chemistry involves coordination complexes that contain pairs or trinuclear clusters of rhenium atoms and has been surveyed in a recent monograph entitled 'Multiple Bonds Between Metal Atoms'.' This source, which covers the literature up to early 1981, has also been invaluable in preparing the present review and can be consulted for a more complete and detailed coverage of these classes of complexes. Since the organometallic chemistry of rhenium is surveyed in the companion series Comprehensive Organometallic Chemistry$ every effort has been made to minimize overlap with these topics, although an occasional reference is made in the present review to certain alkyl, aryl, carbonyl and isocyanide complexes that may be covered more fully in the excellent review by Boag and Kaesz.6 Salts of the perrhenate anion (including perrhenic acid) are not only the single most important group of starting materials for the synthesis of other rhenium compounds but are also of importance in catalysis and other commercial processes. Although they are formally coordination complexes of ReV"and they appear quite extensively in the chemical literature (e.g. see Chemical Abstracts), much of the interest in them is outside the confines of the topic of coordination chemistry as it is commonly accepted. Accordingly, an effort has been made here to keep the coverage of their chemistry to a minimum. to Reo will be the subject of this review. Of these, ReV',Re' Oxidation states ranging from and Reo are rare, while Re" is encountered mainly in triply bonded dirhenium complexes, Le. those possessing the Re:+ core. Examples of mononuclear complexes that possess these oxidation states, together with their stereochemical properties, are presented in Table 1, while some properties of coordination complexes containing multiply bonded pairs or trinuclear clusters of rhenium atoms are given in Table 2. Several systematic studies have been carried out that correlate Re 4f binding energies (as measured by the ESCA technique) of complexes ranging from those of Re"" to Re' with metal oxidation These results can be useful in helping to identify complexes of unknown structure. Table 1
Oxidation States and Typical Stereochemistries of Mononuclear Coordination Complexes of Rhenium"
Oxidolion slate
Coordination number
Re', d 6
6
Re", d S Re"', d 4
6 7
Re", d
Geometry Octahedron Trigonal bipyramid Octahedron Pentagonal bipyramid Octahedron Trigonal bipyramid Octahedron ?
ReV,d 2 ReV',d
'
ReV1',d o
5
4
Octahedron Square pyramid Square antiprism? Pentagonal bipyramid? Octahedron Trigonal prism Square pyramid Tetrahedron Tricapped trigonal prism ? ? Octahedron Trigonal prism Square pyramid Tetrahedron
Examples ReCl(N,)(PMe,Ph),, K, Re(CN), ReCKdppe), tReC1(N2)(PMeJW4l+ K4Re(CN),- 2H, 0 mer-ReC1, (PMe,Ph),, [Re(acac),Cl,]Re(SAr), (NCMe)(PPh,) ReCl, (acac), , K2ReCl, ReHdPR,), [ReFJ, ReOCl,(PEt,Ph),, [ReO,(en),]+ [ReOBrJ , [ReO(SPh),][ReF,]'[RcF71 [ReN(NCS),]' , R&Cla(H20) Re(%C2Ph2l3 [ReNCIJ [Reo,]*[ReH$ [RW[ReOF,]Reo, Cl(bipy), [Reo, Cl,j2[Re(NHSC,H4M+ Re(NPPh,)(SPh), [Reo,]-
'There are no well-documented mononucledr non-organometallic complexeb of Reo known as of this time.
* Several important papers (references 574-639) dealing with the coordinationchemistry of rhenium were published after the submission of the original manuscript. The chapter has been updated by inclusion of paragraphs discussing these recent developments at appropriate points throughout the text.
Rhenium
128
Table 2 Some Properties of Multiply Bonded Dirhenium and Trirhenium Complexes Possessing the Re,L, and Re,L,, Units Oxidation state
Metal core
Electronic configurationa
Metal-metal bond order
Example
Re"
Given for the dirhenium cores only. For a description ofthe Re--Re bonding in Rei' and Re!+ see B. E. Bursten, F. A. Cotton, J. C. Green, E. A. Seddon and G. 0.Stanley, J . Am. Chew. SOL,1980, 102, 955 and references therein.
In the systematic treatment of the coordination chemistry of rhenium, we shall start with the lowest oxidation state and proceed, in order, to the next highest. Compounds will be considered, within each oxidation state, in order of increasing nuclearity (i.e. mono-, di- and tri-nuciear); we define nuclearity in terms of the number of metal atoms held together by metal-metal bonds. Some complexes where an ambiguity in assigning the metal oxidation state may exist (e.g. nitrosyls) are discussed in a separate section. In the case of hydrides we have arbitrarily given the hydride ligand a charge of - 1 (ie. H - ) and assign the metal oxidation state accordingly; for example, Re,H,(PMe,Ph), is treated as a hydride complex of rhenium(II1).
43.2 RflENIUM(0) COMPLEXES
This is, as expected, the rarest oxidation state as far as non-organometallic complexes of rhenium are concerned. The dirhenium phosphite complex Re, [P(OMe),],, has been in low yield by several methods, viz. the potassiun-potassium iodide reduction of ReOC1, (py), or an R e C k pyridine complex, followed by treatment with trimethyl phosphite. This yellow complex displays a "P{'H] NMR spectrum that appears as a first order AB, pattern, and is isoelectronic with Re,(CO),,. It reacts with H,upon photolysis in THF solution to produce hydridorhenium(IT1) and other ~ p e c i e s .The ' ~ related triphenyl phosphite derivative Re, [p(OPh),l,, has been described14as a product of the reaction between ReH, (PPh,), and P(OPh),. A report describing the isolation of K,Re(CN), could not be substantiated in a later investigation of this system."
43.3 RHENIUM@)COMPLEXES The most important complexes of rhenium(1) are those containing the dinitrogen ligand, in which the rhenium atom is also complexed by phosphine ligands.
43.3.1
Cyanide, Alkyl Isocyanide and Aryl Isocyanide Complexes
Through a careful reexamination of several literature reports, the existence of salts of the rhenium(1) [Re(CN)& anion has k e n ~onfirmed.'~ It can be prepared by the reduction (using BH; or K/Hg as reducing agents) of mixtures of [ReCl,]'- and CN-.'%'' The potassium salt is isomorphous with its manganese and technetium analogues.'6 Other methods, including the reaction of Re,C1, with aqueous KCN solution and the reaction of KZReC1,with KCN under N,(g), are also believed to give K,Re(CN),." The pentacyanonitrosylrhenate(3 -1 ion, [Re(CN),(N0)I3-, can be prepared by the reaction of hydroxylamine hydrochloride, KReO, and KCN in alkaline ~olution.'~ This preparative procedure had previously (and presumably incorrectly) been described as affording K, [Re(CN),(NO)] -4H,0.Ls The diamagnetic, red crystalline trihydrate K3[Re(CN3(NO)] 3H20is formally a derivative of rhenium(I), with coordination from NO+. Its vibrational spectrum (Raman and IR), with v(N0) at 1650cm-l, has been assigned in terms of C,, symmetry."
-
-
Rheaium
129
While non-carbonyl-containing isocyanide complexes of rhenium(1) are discussed elsewhere: some recent developments are worthy of note here. The multiply bonded dirhenium(II1) complexes Re,(O,CMe),Cl, and (Bu!N), Re2C1, are reductively cleaved by alkyl and aryl isocyanides under reflux conditions to give salts of the homoleptic [Re(CNR),]' (R = CMe, or C&l) and [Re(CNAr),]' (Ar = phenyl, p-tolyl, 2,6-dimethylphenyl or 2,4,6-trimethylphenyl) cations.Ig-" Similar forcing reaction conditions can also be used in the formation of the same cations by reaction of the isocyanides with K,ReX, (X = Br or I) and Re,& (X = C1, Br or I).'' Milder conditions (reaction in acetone at room temperature) lead to the conversion of K,ReI, to Re(CNAr),I (Ar = phenyl or 2,ddimethylphenyl), complexes that react reversibly with I, to give the rhenium(1) derivatives Re(CNAr),I, that contain coordinated triiodide.2, While [Re(CNR),]+ is inert to substitution by monodentate tertiary phosphines, routes to species of the type [Re(CNR),(PR,),]+ are provided by (a) the reductive deavage of the triply bonded complexes Re,C14(PR,),(PR, = PEt, or PPr;) by RNC ligands or (b) the reductive elimination of H2 from the polyhydride complexes ReH,(PR,), (PR, = PPh, or PEtPh,) or ReH,(PPh,),(NH,C,H, , ) . 1 9 These rhenium(1) species exhibit electrochemically reversible one-electron oxidations (E,,!2= f 0.6 to -t0.8 V vs. SCE €or the RNC comand can be oxidized by the plexes and + 1.05 to + 1.20 V vs. SCE for the ArNC cornple~es),'~~~' halogens to give [Re(CNR)6X]2+and [Re(CNAr),Xj2+ (X = C1 or Br)."s2' The reduction of ReCl,(THF), by Na/Hg in the presence of t-butyl isocyanide is a high yield route .B' )The to R ~ C ~ ( C N B U ~ trimethylphosphine derivatives of this, ReCl(CNBu'),(PMe,), and ReCl(CNBu'), (PMe,),, are prepared by the reaction of ButNC with ReCl(PMe,), (see Section 43.3.3). Both have been shown by X-ray crystallography to have octahedral structures and are protonated and [ReC1(CNBul),(CNHBut)by fluoroboric acid to gtve [ReC1(CNBut)(CNHBu')(PMe,),]BF, (PMe3),]BF4,respectively, in which the CNHBu' ligands are believed to be trans to the Re-Cl bonds. The X-ray crystal structure of trans-ReCl(CNBu')(dppe), (dppe = Ph,PCH, CH,PPh,) as its tetrahydrofuran solvate has been and the Re-CrN-C unit found to be close to linear. The preparation of cis- and trans-[ReCl(CNMe){@-CF,C6H4),PCHzCH,P(C6H4-pCF3),),] has also been They are prepared by reacting ReCl{@CF,C6H4)2PCH2CH2P(C6H4-p-CF3)2}2 with MeNC in THF at room temperature and reflux, respectively. The complexes trans-[ReL,(dppe),JBF4 (where L = CO or RNC, with R = Me, But, p-MeOC6H4, p-MeC,H,, p-C1C6H4 or 2,6-C&C&) have been prepared from the reaction of trans-ReCl(N,)(dppe), with L in THF in the presence of T1BF4.576
43.3.2 Mixed DinitrogewPhosphine Ligand Complexes 43.3.2.1
Mononuclear &r.ivatives
The chelated benzoyldiazenido complex of rhenium(V), ReCl,(N, COPh)(PPh,),, is the key compound for the preparation of a series of rhenium(I)4initrogen complexes."*2~226 It reacts with monodentate and bidentate phosphine donors to form yellow complexes of the types ReC1(N2)(PR,),, (PR, = PMe,Ph, PMePh,, Me,NPF, or PHPh,) and ReCl(N2)[Ph2P(CH2),PPh2],. Of these, the complexes ReCl(N,)(PMe,Ph), (1) and ReCl(N,)(dppe), (2; dppe = 1,2bis(dipheny1phosphino)ethane) are by far the most important, the structure of (1) having been established by X-ray crystallography which demonstrated the presence of a terminally bound dinitrogen ligand.*' Analogous derivatives with arsine ligands such as arphos (1 -diphenylphosphino2-diphenylarsinoethane) and the mixed phosphinephosphite species ReCl(N,)L, (PPh,), (L = PF,, PH,Ph or P(OCH,), OMe) and the tetrakisphosphito derivatives ReCl(N,)[P(OMe)J, and ReCl(N,)(dmope), (dmope = (MeO),PCH,CH,P(OMe),) have also been prepared by this same general procedure, or variations thereof (Scheme l).24,25J8*29
N
N
N
N
ill
(1) P = PMe,Ph
Ill
.(2) P - P
= dppe
130
Rhenium ReCI(N,)(PMe,Phj,
f
PMe,Ph MeoH reflux
ReCIz(NzCOPh)(PPh3)z
PMe,Ph
I
*
ReC1,(N,COPh)(PMezPh)3
Scheme 1 Routes to some dinitrogen complexes of rhenium(1)
If the benzoylazo complex ReCl,(N, COPh)(PPh,), is first converted into the related organodiazenido derivatives ReCl,(N,COPh)(PMe,Ph),, ReCl,(N, COPh)[P(OMe),], or ReC1,(N,COPh)(PPh,)[P(OEt),],,25,29~30 then the latter Re"' species can be used as precursors to the mixed phosphinephosphite complex ReCl(N,)(PMe, Ph),[P(OMe,),], and the methyl isocyanide complexes mer-[ReCI(N,)(CNMe){P(OMe), >,] (3) and ReCl(N,)(CNMc)(PPh,)[P(OEt,)],(see Scheme 1). The structure of (3) shows it to be analogous to that of (l).30 A listing of a selection of the more important dinitrogen complexes of rhenium(1) together with several of their of properties is given in Table 3. Other characterization studies have included I5N NMR, X-ray photoelectron (ESCA) and low frequency vibrational spectroscopic r n e a s u r e m e n t ~ . ~ ~ * ~ ~ - ~ ~
c1
Ill
N
(3) P'
= P(OMe)3; C = CNMe
-
An alternative route to a dinitrogen complex of rhenium(1) involves the Na/Hg reduction of THF solutions of the phenylimido complexes of Rev, Re(NPh)Cl, (PPh,), or Re(NPh)Cl,(PMe,),, under a nitrogen atmosphere in the presence of excess PMe, .34 This gives Re(NHPh)(N,)(PMe,), (4), which has been shown by X-ray crystallography to have a structure containing a cis disposition of N, and NHPh ligands. The Na/Hg reduction of ReCl,(THF), in THF solutions of PEt2Phunder Table 3 Some Properties of Representative Dinitrogen Complexes of Rhenium(1) Complex
R a N , )(PMe, Ph), (1) ReC1(NJ(dppe), (2) ReC1(N,)lP(OMe), 14
ReCI(N,)(PMe,Ph),[P(OMe),],
ReCI(N,)(CNMe)[P(OMe),], (3) Re(NHPh)(W(PMe, (4) ReH(N,)(PEt, Ph), ReH(N2 )(dPPe)* ReCl(N2 ) O ( P M e P h ) , R d S J N E t , )IN,)PMe* Ph), Re(N,)(PMe,Ph), PMe,(C,H,)I
Color
v( N N ) a ( c m - ' )
Yellow Yellow White Yellow Yellow Yellow Yellow Yellow Orange Red
1925 (CHCI,) 1980 (CHCI,) 2103 (CH,Cl,) 2000 (C,H,) 2030 (N.S.)d 2000 (Nujol) 1945 (Nujol) 2006 (Nujol) 1905 (Nujol) 1945 (Nujol) 2000 pujol)
Medium given in parentheses; N.S. = not specified. Bond distances in A. e Disorder problem precluded a precise determination of this distance. 4 C E N ) of MeNC at 2140cm-'. a
w
Srmciural derail9 r(Re-N)
=
1.97(2), r(N-N)
r(Re-N) r(Re-N) r(Re-N)
= I .98(1), r(N-N)
r(Re-N)
= 1.960( lo), r(N-N)
1.955(13), r(N-N) = 2.055(5), r(N-N) =
=
1.06(3)'
= 1.04(2) = 1.10(2) =
1.018(8)
=
I . 1 17( 13)
25, 27
25 29 25 30 34 35 36 25 26 38
Rhenium
131
P
an N,(g) atmosphere gives a separable mixture of ReH(N,)(PEt,Ph), and ReCl(N,)(PEt,Ph),.35 The identity of the hydride derivative (for which 4Re-H) = 1945cm- ') has been confirmed crystallographically (Table 3). The hydrido species ReH(N2)(dppe), results from the reaction of (Et,N), ReH, with dppe in propan-2-01 under an N, a t m ~ s p h e r e An . ~ ~aiternative preparation of this complex involves the T h s reaction can be reaction of N, with photogenerated coordinatively unsaturated ReH(d~pe)?.~? reverskd through the photo induced loss of N,. Reaction of this complex with MeX (X = Br or I) gives ReX(N,)(dppe),, while displacement of the N2ligand occurs upon its reaction with C2H, and C0.36 The properties of the hydride and phenylimido complexes resemble those of the other derivatives (Table 3). An interesting and unusual route to a dinitrogen complex of rhenium(I) occurs upon reacting photogenerated ReH, (PMe, Ph), and 1-butylethykne in benzene under N,. This yields the arene complex (C6H6)Re(PMe,Ph)(CH,CH, CMe,) together with the ortho-metallated dinitrogen containing product fac-Re(N,)(PMe,Ph),(PMe,C,H,)(Table 3).38 Although the coordinated N, ligand in the Re' derivatives has not yet been protonated or alkylated," these complexes exhibit, nonetheless, an interesting reaction chemistry. The reactions of ReCl(N,)(PMe,Ph), with pyridine ligands and bidentate monoanionic sulfur ligands proceed as shown in Scheme 2,24-26to give species in which the Re-N, unit is retained and which possess properties similar to those for the complexes listed in Table 3. Treatment of the dithiophosphinato with MeNC in THF under argon and tungsten filament light complex Re(q2-S2PPh2)(N2)(PMe,Ph), (5) as yellow crystals (Scheme 2) gave mer-[Re(q' -S,PPh,)(N,)(CNMe)(PMe,Ph), . ~ ~crystal structure shows Re-N and N-N bond [v(CN) = 2100cm-', v(N2) = 1 9 8 0 ~ m - ' ] Its lengths [1.83(1) and 1.13( 1) A, respectively] somewhat shorter and longer, respectively, than those of (3), a result which is in accord with differences in the values of the v(N,) modes E2030cm-I for (3) versus 1980cm-' for (S)].
/
[ ReCI(N,)(PMc,Ph),
ReC1,(N,COR)(PMe2 Ph), ( R = M y h )
J'
c1, CHCl,
RCOCl reflux C4H8
ReC1(N2)(PMezPW4 Na[S,CNR1]
py, benzene
reflux
I I
:::",""
I I
Re(S2PPh2)(Nz)(PMeZPh)3
THF
MeNC THF
[KeH1(S2CNR,)X(PMezPh), I X (X = CI or Br) Scheme 2
R~C~(N~)(PY)(PM~,PW~ K[S,PPh, 1
Re(SzCNRz)(Nz)(PMe,Ph), HX
+
hv
Re(~'-S2PPhZ)(N2)(CNMe)(PMelPh),
Some reactions of dinitrogen complexes of rheniurntl)
111
N
(5) P = PMe,Ph; C = CNMe
132
Rhenium
Cationic complexes may be obtained via abstraction of C1- from ReCI(N,)(dppe), by I B F , (equation 1) in the presence of CH, CN or PhCN.” An interesting by-product in the reaction with benzonitrile is the non-dinitrogen containing product [Re(NCPh),(dppe),]BF, which was isolated in low yield as orange plates. Hydrido complexes of Re“’ result upon protonation of ReH(N,)(d~pe),,~and Re(S2CNR)(N,)(PMe,Ph),26 with acids such as HBF,, HCl and HBr in non-aqueous media (Scheme 2). When Re(NHPh)(N,)(PMe,), is treated with HBF,/THF, the BF; salt of [Re(N,)(PMe,),]’ can be isolated.,, This cation has also been isolated as its chloride salt upon The N, ligand in reacting ReCl(PMe,), with N, in methanol (v(N,) = 2030cm-’ (N~jol)).~’ Re(NHPh)(N,)(PMe,), is lost upon reaction of this complex with IJbenzene and CO,/toluene; the complexes [Re(NHPh)I(PMe,),]I and Re(NHPh)(q2-COz)(PMe,), are formed, The lability of the N2 ligand towards alkyl and aryl isocyanides has been demonstrated by equation (2) which occurs readily with both alkyl and aryl isocyanides.40The resulting complexes are ‘electron rich’, the alkyl isocyanide derivatives ReCl(CNR)(dppe), (R = Me or But) undergoing electrophilic attack upon treatment with HBF, to form the carbyne complexes trans[ReCl(CNHR)(dppe)2]BF,.41 Other noteworthy features of the series of complexes ReCl(CNR)(dppe), and ReCl(CNAr)(dppe),, are the low value of v(CN), the presence of a ‘bent’ M-CNR coordination mode, and their ease of oxidation (as measured by cyclic voltammetry) to the corresponding rhenium(I1) monocations.40 ReCI(N,)(dppe),
+
TIBF,
irans-ReCl(N,)(dppe),
+
RCN
RNC
+ TIC1 trans-ReCl(CNR)(dppe), + N,
[Re(N,)(NCR)(dppe),]BF,
--+
(1)
(2)
Many of the dinitrogen complexes of Re’ have been found to undergo a reversible one-electron oxidation to the corresponding 17-electron Re’*cations (E1,?between +0.05 and -0.26V vs. SCE in CH,Cl,/MeOH solution with (Bu! N)BF, as supporting e l e c t r ~ l y t e ) . Such ~ ’ ~ ~an ~ oxidation can be accomplished chemically using C1, /CHCl, (Scheme 2) and other oxidants to give paramagnetic purple or green salts of [ReCl(N,)(PMe,Ph),]+ or [ReCl(N,)(dppe)J+ . Other important reactions that lead to the oxidation of the metal center involve the formation of rhenium(II1) acyl- and aroyl-diazenido complexes [ReCl, (N,COR)(PMe,Ph), (R = Me or Ph) by the treatment of ReClOn the basis of ” N NMR (N,>(py)(PM%Ph), or ReCl(N,)(PMe,Ph), with RCOCl (Scheme 2).24342 spectroscopy,@ the diazenido ligands in these complexes are suggested to be ‘singly-bent’, i.e. M---N=N The formation of ReCl,(N,COR)(PMe, Ph), by this procedure is especially - ‘Re noteworthy since it represents essentially the reverse of the original preparation of the N, complexes starting with ReCl,(N,COPh)(PMe,Ph), . Attempts to acylate or aroyiate ReCl(N,)(dppe), have not yet been ~uccessful.~~ A wider range or complexes of the type ReCl(N,)(R,PCH,CH,PR,), have now been prepared by the reaction of rhenium(\? complex ReCl,(N,COPh)(PPh,), with the appropriate diphosphine The relationship ligand (R = Et, C6Hl,,Ph, p-MeOC,H,, p-MeC,H,, p-ClC,H, or p-CF3C6H4).575 between E;; (determined by cyclic voltammetry) and V(N2)for these complexes and others of this ‘type has been examined.”’ The I5N and P NMR spectra of rrans-ReC1(N2)(R,PCH2CH2PR2), (R = Ph or m-MeC,H,) and trans-ReCl(N,)(PMe,Ph), are part of an extensive study of the NMR spectra of complexes that contain terminal dinitrogen ligand^.'^' The treatment of transReCl(N,)(dppe), with P h C r C C H , in refluxing benzene or THF gives the q2-phenylallenecomplex ReCl(rf2-H,C - C = C H P ~ ) ( ~ ~ P ~ ) , , ~while ’ * terminal alkynes HCECR (R = Et, Ph or C0,Et) react to give the vinylidene complexes ~rcans-ReCI(C=CHR)(dppe),.~’~
43.3.2.2 Mixed metal dnuclear and trinuclear derivatives The complex ReCl(N,)(PMe,Ph), (1) shows a remarkably strong tendency to react with other transition metal centers via its terminaliy bound dinitrogen ligand to give dinitrogen-bridged di- and tri-nuclear complexes.24A listing of these complexes, together with the preparative routes employed and certain of their properties, is given in Table 4. Two of these complexes, viz. (6)and (7) in Table 4,have been fully characterized structurally by X-ray crystallography, and shown to possess N-N bonds longer than those in the mononuclear complexes containing terminally bound dinitrogen (Table 3). Solution studies that have monitored the formation of adducts between ReCl(N2)(PMqPh), and other Lewis bases (including several other Re‘--N, complexes) and the Lewis acids AIR, (R = Me, Ph or Cl), have demonstrated that this particular Re’ complex ranks high in
Rhenium
133
134
i p ,,,,\\\"'
C]-
Re -N=N
FPI
-Mo
c1.
rI
Rhenium ,$\,&I -0Me
@ ,\, "
4 3-Re pV
c1
r*,, &k
I -N=N--Mo P f ---N=N-k C l V I Pfl 1 I c1 P P
P
( 6 ) P = PMe,Ph
,, \ '\ '
-C1
( 7 ) P = PMe,Ph
ReCl(N2)(PMe2Ph), + ReOCl,X(PPh,),
e
ReOCI2X(PPh,)[(NZ)ReCI(PMezPh),]+ PPh, (3)
X = CIorOMe
the order of base strength; 1:1 adducts of AlMe, with ReCl(N,)(PMe,Ph),, ReCl(N,)(dppe), and ReCl(N,)(py)(PMe,Ph), have been isolated.49Apparently, the greater the transfer of electron density from the metal into the E* dinitrogen orbitals (i.e. the lower v(N2) is) the more basic the terminal nitrogen atom is. In the case of the interaction of ReCl(N,)(PMe,Ph), with ReOCl,(PPh,), and ReOCl, (OMe)(PPh,),,36an equilibrium (equation 3) is established in CHC1, with Kegvalues of 0.28 f 0.03 and 0.004 k 0.002, respectively, at 27°C. Following the establishment of the structure of (7), a detailed study of the bonding in this molecule has been conducted through a combined electronic, IR, Raman and resonance Raman spectral investigation." The Raman-active v(N,) mode is found close to 1800cm-', while v(Re -N) is a t 680 cm-' . The bonding in this complex can approximately be described in terms of Re =N=N=MO.~I
-
Dinitrogen-bridged complexes of trans-ReCl(N,)(PMe,Ph), with NbC1, and TaCl, (1: 1 adducts) and ZrCI, (2:l adduct like TiC14) have been The "N NMR spectra of these and other complexes of this type as well as dinitrogen complexes of rhenium(1) with the main group acceptor AlMe, have been st~died.'~'Fenske-Hall MO calculations have been carried out on the molecule [{MoCL (OH)) GU-W ){ ReC1(PH3l4 >I.%' 43.3.3 Other Phosphine Complexes The sodium amalgam reduction under N, of ReCl,(THF), in THF containing an excess of PMe, affords the yellow chloride ReCl(PMe,), and the white hydride ReH(PMe,),.35 Both complexes are quite air stable and fairly reactive. The hydride can be hydrogenated to the trihydride ReH,(PMe,),, whereas the chloride ReCl(PMe,), can be methylated (using LiMe) to ReMe(PMe,),. ReCl(PMe,), reacts directly with N, (in methanol) and CO (in diethyl ether) to form [Re(N,)(PMe,),]Cl (see Section 43.3.2.1) and ReCl(CO),(PMe,),, re~pectively.~~ Although the Na/Hg reduction of Re(NPh)Cl,(PMe,), in THF under N,(g) in the presence of PMe, leads to Re(NHPh)(N,)(PMe,), (see Section 43.3.2. l), a prolonged reaction time gives this complex as a minor product and the $-dimethylphosphinomethyl derivative Re(q2CH,PMe,)(PMe,), as the major one. Irradiation of ReH,(dppe), with 366nm light leads to the reductive elimination of H, and the formation of HRe(dppe), or a solvated derivative thereof.37This photoproduct reacts with, among other things, N2 and CO,, to form the complexes ReH(N,)(dppe), (see Section 43.3.2.1) and the formate Re(O,CH)(dppe),. The ReH(dppe), intermediate can also be trapped by CO, C2H, and CZH2,and can undergo reversible intramolecular insertion into the C-H bonds of the phenyl groups of the dppe ligand^.^' Related chemistry can also be developed from ReCl(N,)(dppe), (and to a certain extent also from ReCI(N,)(PMe,Ph),) which upon irradiation in methanol generates ReCl(dppe), (S)." Under these conditions, the reactive photogenerated ReCl(dppe), reacts with the solvent to yield ReCl(CO)(dppe), and ReClH,(dppe), . The X-ray crystal structure of (8) has recently been reported5, and found to be that of a trigonal bipyramid.
(8) P-P
= dppe
The complex [ReCl(p-CF,C6H,),PCH,CH,P(C6H,-pCF,)2} 2] is obtained in moderate yield from the reaction of ReCl(N,)(PMe,Ph), with the diphosphine in refluxing toluene.s75
Rhenium
135
43.3.4 Phosphite Complexes Using the same synthetic procedures that produce the Reo derivative Re,[P(OMe),],, (Section 43.2),I2,l3the monohydrido complex HRe[P(OMe),], is often formed in low yield and is also generated, along with other products, upon photolyzing Re,[P(OMe),],, in the presence of H2. It has been characterized by 'H NMR spectroscopy (C7DS,6 - 9.5 as a quintet of doublets with J(P -H),,,, = 12 Hz and J(P-H),, = 24 Hz) and mass spectroscopy, but has otherwise been little studied. When solutions of ReCl, in THF are reacted with trimethyl phosphite for one day, the resulting solution stripped to dryness, treated with fresh P(OMe),, and then reduced with K/Hg, the complex [(MeO),Pj,Re-0-P(OMe), can be isolated in low yield.', It is forrnally a derivative of Re' and may be formed by loss of a CH, radical from the putative 19-electron intermediate Re[P(OMe),],. It reacts with Me1 to give [Re(f(OMe), )J+ I-. An additional hydridorhenium(1) phosphite complex that has been isolated, albeit in low yield, from the K/Hg reduction of higher valent rhenium species in the presence of P(OMe), ligand, is HRe[P(OMe),], [(MeO),P-0P(OMe),] (9) containing the novel tetramethyl diphosphite ligand. This stereochemically rigid molecule ('H NMR spectrum in C,Ds&ws the Re-H resonance as a multiplet at 6 - 8.8) is believed to contain a four-membered MPOP ring.
( 9 ) P' = P(OMe),
The triphenyl phosphite complex ReCl[P(OPh),], is obtained as white crystals in high yield by the reaction of P(OPh), with ReCl,(N,COPh)(PPh,), in benzene.,' This reaction clearly proceeds in a fashion different from that found with P(OMe),, vzz. the formation of ReCl(N,)[P(OMe),],.
43.4 RHENIUM(I1) COMPLEXES The most extensive group of complexes that are known to have this oxidation state are the mixed halide-phosphine derivatives of the triply bonded Re:+ core, a moiety that possesses the electronrich 02n462S*2electronic configuration (Table 2).5
43.4.1 Mononudear Complexes
43.4.1.1 Cyanide,, alkyl isocyanides and aryl isocyanides Early reports of cyano complexes of rhenium(I1) of the types [Re(CN),I4- and [Re(CN),I3- could not be confirmed in a reinvestigation of the reactions that were said to lead to these product^.'^ The only species definitively identified was the [Re(CN),I4- anion. Accordingly, the claims4 that 'A&[Re(CN),]' can be methylated to give [Re(CNMe),]'+ cannot now be supported. Indeed, solutions containing the authentic [Re(CNR)J2', [Re(CNR),(PR,)JZt and [Re(CNAr),]'+ cations and do not appear to have properties (R = alkyl, Ar = aryl) can be generated electro~hemically,'~~~' that are in accord with those reported54 for '[Re(CNMe),I2+'. The rhenium(1) complexes ReCl(CNR)(dppe), and ReCl(CNAr)(dppe), also undergo a reversible electrochemical one-electron transfer to generate the corresponding monocations, but these latter species have not otherwise been characterized.MHowever, a series of quite stabIe, paramagnetic, mixed-ligand complexes of the type [Re(CNR),(NCMe)L(PPh,),](BF,), (R = Pr' or But, L = MeCN or py) have been prepared, and the crystal structure of [Re(CNBu'), (NCMe),(PFh,),](BF,), (10) has shown that it possesses the all-trans ge~rnetry.~' Through an investigation of the reactions between RNC (R = Pr' or But) and a range of monohydridorhenium(II1) species [ReH(NCMe), L(PPh,),](BF,), (L = MeCN, py, C,H,,NH, or Bu'NH,), it was demonstrated that the sequence of reactions which involve the formation of (10) and other complexes of this type is as shown in Scheme 3 . Note that these complexes are easily converted into their 18-electron rhenium(1) congeners, and are also formed
Rhenium
136
p
3
PPh3 (10) C= C N R ; N=NCMe [ReH(NCMe)3L(PPh3)2 I '*
RNC
[ReH(NCMe)2(CNR)L(PPh3)2] 2+ + MeCN
CNR [Re(CNR)dPPhd21+ + MeCN
+L
RNC
[Re(CNR),(NCMe)L(PPhs)a]* + H+
+
MeCN
I
Zn acetone
[ Re(CNR)2 (NCMe)L(PPh& ] 2+ (10)
Scheme 3
from them upon treatment with acid and other mild oxidant^.^' Some properties of (10) and other mononuclear rhenium(I1) species are given in Table 5 . Recent attemptsz2to repeat the preparations of compounds that have been formulated6 as the rhenium(I1) derivatives Re, I, (CNCy), and Re, I,(CNCy), (Cy = cyclohexyl) were unsuccessful. The reactions of K2ReI, and Re3I, with cyclohexyl isocyanide and other isocyanides give well-defined complexes of Re' and Re"' only. Also, the existence of trans-ReBr,(CNCy), has not yet been confirmed, despite attempts to do 5 0 . ' ~
43.4.1.2 Phosphhes and arsines Such complexes occur exclusively as derivatives of the rhenium(I1) halides. The neutral, presumably octahedral, complexes ReXJdiars), and ReX,(qas) (X = Cl, Br or I; diars = o-phenylene(bisdimethylarsine); qas = tris(o-diphenylardnopheny1)arsine) are formed by reduction of the corresponding cationic rhenium(II1) species.2The species ReX,(tas) (X = C1, Br or I; tas = bis(o-diphenylarsinopheny1)phenylarsine) are prepared similarly but may be five-coordinate.' The complex ReCI,(PMe,Ph), and some of its properties have been described (including its ESCA spectrum and electrochemical properties)," but details of its preparation have not. The paramagnetic complexes trans-[ReCl(N,)(dppe),]X (X= Cl- , Br; , I;, ClO; , BF;, PF;, FeClT or CuC1; ) and trans-[ReC1(N2)(PMe2Ph),]FeCl, are prepared by oxidation of the rhenium(1) analogues using oxidizing transition metal salts such as AgNO,, FeCl, or CuCl, in ethanol or, alternatively, a stoichiometric amount of halogen in chloroformn2'These complexes, which exist in green or purple forms, can be oxidized further with concomitant loss of Nz.Typical properties are given in Table 5 , where it can be seen that these complexes resemble other isoelectronic rheniurn(I1) complexes, including the carbonyl derivatives tmns-ReC1,(CO),(PR,)2."s6
43.4.2 Dinuclear Complexes Containing the Rei+ and Re:+ Cores 43.4.2.1
Monodentate phosphines
The most important complexes that possess these cores are those of stoichiometry Re2&(PR3), (X = C1, Br or I; PR3 = PMe,, PEt,, PPi;, PBu;, PMe,Ph or PEtlPh).5*56 They are best prepared by the reaction between monodentate tertiary phosphines and the salts (BGN),RezX, (X = C1, Br or These reactions proceed in a stepwise fashion, first to give the unreduced substitution product Re,X,(PR,),, and then Re,X,(PR,), (ll),followed by RezCl,(PR,), (12). While most reactions with phosphines have been found to terminate at the stage Re2C14(PR3),,in the case of PPh, the reaction does not proceed beyond Re,Cl,(PPh,),. Thls may be due in part to the insolubility of the resulting
Rhenium
37
138
Rhenium
p
c1
(11) P
= PR3
P
c1
(12) P = PR3
dirhenium(II1) product in the reaction medium, although steric and/or electronic factors also dictate the course of this reaction.56The diphenylalkylphosphines PRPh,, where R = Me or Et, give the 'mixed-valent' species Re2X,(PRPh,),.57The extent of reduction (it.to Rei+ or Re:+) is apparently dependent to a large extent upon the basicity of the phosphine. Thus, the addition of NaBH, to a reaction mixture containing (Bu: Nj,RezCl, and PEtPh, leads to further reduction to Re,Cl,(PEtPh,)4.62 While other methods have been used to form Re,X,(PR,), and Re2X,(PR3),, (for example, the reductive cleavage by PR, of the Re,& clusters (X = C1, Br or I) under reflux conditionss6), these procedures are less convenient than the ones which utilize (Bu,"N),Re,X,. Those reactions between (Bu:N),Re,X, (X = C1 or Br) and phosphines that lead directly to the dirhenium(I1) complexes Re,X,(PR,), are difficult to control so that they stop at the intermediate stage of reduction corresponding to RezX5(PR,),. However, quite simple methods exist for reoxidizing Re,X4(PR3), back to Re,X, (PR3)3.56 The X-ray crystal structures of two complexes of the type Re2C1,(PR,j4 (12) (PR, = PEt3 or PMe,Ph), together with spectroscopic and magnetic studies on the paramagnetic Re,X,(PR,), (11) compound, have demonstrated that these two groups of complexes possess non-ligand bndged M2L8 eclipsed rotational geometries. Relativistic Xu-SW c a l c u l a t i o n ~on ~ ~the ~ ~hypothetical molecule Re,Cl,(PW,), and its monocation have shown that the two electrons in excess of those necessary to ensure a quadruple bond (a'7r4d2 configuration) reside in the Re-Re 6* orbital. The spectroscopic properties, redox behavior and general reaction chemistry of complexes of types (11) and (12) all serve to support this picture of the electronic structure., Thus, they can be oxidized (both chemically and electrochemically) back to complexes of Re"' containing the Re!+ core. Refluxing CCl,, oxidizes Re,Cl,(PEt,), to [Et,PCl], Re, Cl,, while with the bromide analogue a mixture of Re, C14Br,(PEt3)2and [Et3PC1],RezC1,Br4 is p r o d ~ c e d. ~ Note ' that the salt (Bu:N)Re,Cl,, which like other [Re,&]- -containing species can reacts with the phosphines PPh,, be formed by halogen (Cl, or Br,) oxidation of (BU:N),R~,X~,~ PEtPh, and PEt, to give Re,CI,(PPh,),, Re,Cl,(PEtPh,), and Re,Cl,(PEt,),, respectively." The preceding redox results are summarized in Scheme 4. CCl,
J
[ R e z X 9 ] - A IRezXs12(oZ7r4)
I
(ffZn462)
PRPh,
Re2X5(PR& (oZn4626 * I )
PRPh,
t
&ReZX4(PR3)4 (rJ2n4626.2)
PRa
t
Scheme 4 Chemical redox behavior of the RGC core
Although oxygen-free solutions of Re,&(PR,), are quite stable, the presence of small amounts of oxygen can lead to the formation of species containing the Re:' In the case of the reaction between Re,Cl,(PR,), and O,,it is believed that the [Re,C&(PR,j,]+ cation is the dirhenium species that is formed initially, and that it is then converted into Re,Cl,(PR,), by scavenging chloride ion. This conclusion is supported by cyclic voltametric st~dies',~'on solutions of Re,X,(PR,), (X = C1, Br or I) in 0.2M (Bu:N)PF6-CH2C1, which reveal that these complexes possess two one-electron oxidations that approach electrochemical reversibility. The first oxidation occurs at quite negative potentials (e.g. El:, = -0.3OV vs. SCE for Re,Cl,(PMe,Ph),), thereby explaining the ease of oxidation to the monocation. In the presence of chloride ion, a series of coupled electrochemical-chemical reactions are possible for the chloro complexes as shown in Scheme 5 . The difference between pathways A and B is the potential used for the oxidation of Re,CI,(PR,), ( d . ~ . -t 1.OV in A and +OSV in B). It is also important to realize that either potential is sufficient to oxidize the first chemical product Re,CI,(PR,), to its monocation [Re,Cl,(PR,),]+.
I Rhenium
139
Note that both Re,CI,(PR,), and Re,Cl,(PR3)2 have their own well-developed redox c h e m i ~ t r y . ~ ~ The former exhibits a one-electron oxidation in the neighborhood of +0.40V YS. SCE and a one-electron reduction at 'v - 0.7 V vs. SCE. For the complexes Re,C&(PR,),, a well-defined one-electron reduction to [Re,Cl,(PR,),]- occurs very close to - 0.05 V vs. SCE. In all instances, the species which may be generated electrochemically (viz. [Re2C1,(PR3),J+, [Re,C1,(PR,),I2+, [Re,Cl,(PR,),]+, [Re,Cl,(PR,),]- or [Re,Cl,(PR,),]-) can be prepared chemically using NO'PF; as the oxidant or cobaltocene as the reductant. By this mean the dirhenium cations can be isolated as their PF; saIts and the anions as their [($-C,H,),Co]* salts.6M' These studies have included the isolation of the three complexes Re,Cl,(PMe,Ph),, [Re,Cl,(PMe,Ph),]PF, and [Re,Cl,(PMe,Ph),](PF,),, and their X-ray structural characterization.66This has provided a series of three complexes possessing M-M bonds of order 4,3.5and 3 with identical sets of monodentate ligands. The Re-Re 'bond lengths change from 2.241(1) to 2.218(1) to 2.215(2)A as the bond order increases from 3 to 3.5 to 4. All three complexes have the same basic eclipsed structure that characterizes all compounds o f the type Re,X,(PR,), (12). The expected shortening of the bond occurs on going from Re, Cl,(PMe,Ph), to [Re,Cl,(PMe,Ph),] +,but there is no change between [Re,Cl,(PMe,Ph),]+ and [Re,C1,(PMe,Ph),]2+. Any decrease expected on the basis o f an increase in Re-Re bond order (from 3.5 to 4) is nullified by a second effect which apparently results in a lessening o f the CT and/or z bonding. The latter change results from orbital contraction effects which accompany the increase in charge (Le. from Re:+ to Re$+).
Scheme 5 Coupled electrochemical-chemical processes for Re;+ species
The use of cobaltocene has also been applied to the reduction of the dirhenium(II1) carboxylates Re,(O,CR),Cl,, where R = Pr, CMe, or Ph, to give the paramagnetc salts [($C,H,),Co][Re,(0,CR),C12] which are derivatives of the Re:+ ~ o r e . These ~' same anions have also been generated electrochemically.68 In addition to the rich redox chemistry that characterizes multiply bonded dirhenium complexes containing Re-Re bond orders of 4, 3.5 and 3, there are instances where facile Re-Re bond cleavage is encountered. Perhaps the best examples are those which involve reactions with z-acceptor ligands such as CO and RNC. Under such circumstances, the species [Re,CI,(PR,),]"+, where n = 0, 1 or 2, react to form mononuclear complexes such as ReCI,(CO),(PR,),, ReCl(CO),(PR,), and [Re(CNR), (PR,),]+,the actual product isolated depending upon the starting material and choice of reaction ~ o n d i t i o n s . ' This ~ , ~ ~is a reaction course that is encountered with many multiple bonded species containing M-M triple and quadruple bonds.'
43.4.2.2
Bidentare phosphines and arsines
Starting from (Bu;N),Re,X, (X = C1, Br or I) or Re,X4(PR3),, the dirhenium(I1) complexes Re,X,(LL), can be prepared6'," with the ligands LL = 1,2-bis(diphenylphosphino)ethane (dppe) or 1-diphenylphosphino-2-diphenylarsinoethane(arphos). The method that involves displacement of the PR, ligands from Re,Cl,(PR,), is the preferred synthetic procedure." The spectroscopic properties of RezX,(LL), are in accord with these molecules possessing a structure in which the ligands
Rhenium
140
LL bridge !he two metal centers. ,Because of the conformational demands of the resulting sixmembered Re-Re-P-C-C-P and Re-Re-P-C-CAs rings, the rotational conformation about the Re-Re bond is expected to be staggered, a result which has been confirmed7' by an X-ray crystal structure determination on the compound Re,Cl,(dppe), (13). The mean torsional (rotational) angle is 39", reflecting the absence of any inherent electronic barrier to rotation in a compound possessing the 027r4482d*2 configuration. An example of a triply bonded Re,X,(LL), complex has been encountered in which the two ligands LL each chelate to a single metal atom and the conformation is eclipsed; this occurs for LL = Ph,P(CH,),PPh (14)." This complex is prepared in modest yield upon refluxing an acetonitrile solution containing (BuiW2Re,Cl, and the bidentate phosphine ligand.
(13) L-L
= dppeorarphos
(14) P-P
= Ph2P(CHz)3PPh2
Like the complexes Re,X,(PR,),, the staggered species Re,X,(LL),, where LL = dppe or arphos and X = C1, Br or I, exhibit two accessible one-electron oxidation^.'^ These occur at N 0.25 V and E -t 1.OV vs. SCE for solutions of these complexes in 0.2 M (BuI:N)PF,-CH,Cl,. The first process has been accomplished chemically using NO" PF; as the oxidant to give paramagnetic, ESR-active [Re2C14(LL)21PF6* The chemistry of the dirhenium(1I) complexes changes somewhat when the dppm ligand (Ph,PCH,PPh,) is used. Although this bidentate phosphine can chelate, it shows a propensity for bridging two metal atoms. The complex Re,Cl,(dppm), can be prepared by displacing the PPG ligands in RqCl,(PPg),, although the use of Re,Cl,(PEt,), gives the mixed phosphine complex Re,C14(PEt,),(dppm).70 Related procedures have been used to prepare Re,Cl,(dppa), and Re,Cl,(PMePh,),(dppa) (dppa = bis(diphenylph~sphino)amine).'~ The bis-dppm complex can also be prepared, somewhat more conveniently, from the reaction between dppm and Re, Cl,(PBu;), in methanol, whereas the 'mixed-valent' species Re, C15(dppm), results when diethyl ether is used as the reaction solvent7, While the structure of Re,Cb(dppm), (15) has not yet been determined crystallographically, there is little doubt that it contains two bridging dppm ligands in a trans disposition to each other. It exhibits, as expected, two one-electron oxidations (E,,, = + 0 . 2 7 V and +0.80V vs. SCE).74When the electrolysis of dichloromethane solutions of Re,CL,(dppm), containing an excess of C1- is carried out at + 0.60 V, the paramagnetic complex RqC15(dppm), is first formed, and this is in turn converted to Re~cl6(dppm),,a complex that contains a Re-Re double bond (see Section 43.5.2.4). This conversion of Re,Cl,(dppm), to higher oxidation state dirhenium species is represented in Scheme 6.
+
A
(is) P
Bond order
3
P = dppm
3.5
2
Scheme 6 Coupled electrochemical-chemicalreactions involving Re, CI,(dppm),
141
Rhenium
In the reactions between 2-(diphenylphosphino)pyridine (Ph,Ppy) and the dirhenium(II1) complexes (Bu,"N),Re,Cl, and Re,C&(PR,), (R = Et or Bu"), reduction takes place and several dark The most noteworthy reaction red-purple dirhenium(I1) complexes can be isolated (Scheme 7).75*76 is that between (Bu,"N), Re,C18 (or Re,CI, (PR3)z) and Ph,Ppy in methanol which first affords Re,C14(PhzPpy), (16); this complex in turn eliminates HC1 in the presence of base to give the ortho-metallated complex RezC1,(Ph2Ppy),[Ph(C6H4)Ppy](17). This complex has been subjected to an X-ray structure determination. Another product from this same reaction is the dirhenium(I1) salt [Re,Cl,(Ph,Ppy),]Cl,, which contains four bridging Ph2Ppy ligands. It can be metathesized with KPF, to give the salt [Re,Cl,(Ph,Ppy),](PF,), (IS), whose structure has been determned by X-ray crystallography. When acetone is used as the reaction solvent, Re,Cl,(PR,), reacts with Ph,Ppy to give RezC1,(Ph,Ppy),(PR,) (19) which can be converted into (17) upon reaction with one or two equivalents of Ph,Fpy.76
i,MeOH,reflux,M:L = 1 : Q S ; ii,MeOH,reflux,M:L = 1:>6; iii, MeOH,reflux, M:L = 1 : 4 3 ; iv, MeOH, reflux, M:L = 1:>4; v, acetone, reflux, M:L = 1 :6; vi, MeOH, reflux, M:L = 1.2. Nore: M represents the stoichiometry of the metal complex, L that
of the Ph&y liqand
Scheme 7 Dirhenium(I1) complexes of Ph,Ppy
I + $ .,I Re-Re
2*
n
n N P
N
P
n
N
P
/--
C1-
c1 (16) N-P
-Cl I
c1
= PPhZPpy; P' = PR,
(17)
(18)
X-Ray crystal structures have now been carried out on the triply bonded complexes Re2C1,(dpprn), (dppm = PhzPCH2PPh2)and a-Re,CI,(Me,PCH,CH,PMe,),.'82Studies have been made of the reactions between Re,Cl,(dppm), and both CO and nitrile ligands; the metal-metal bonded complexes Rez(p-Cl)(p-dpprn), C1, (CO),583 Re2(p-Cl)(p-CO)gl-dppm),C1, (CO)583 and [Re,@-dppm),Cl,(NCR),]X (X = C1- or PF; ; R = Me, Et, Ph or 4-PhC,&)584have been isolated and characterized. The Re-Re bond is also retained in the reactions of Re,Cl,(dppm), with isocyanide ligands from which 1:l and 1:2 complexes have been isolated, viz. Re,Cl,(dppm),(CNR) As a by-product of the reaction (R = Me, But or xylyl) and [R~,C~,(~~~II~),(CNBU')~]PF~.~~~ between Re, CI,(dppm), and two equivalents of Bu'NC, the novel paramagnetic p-iminyl dirhenium(I1,III) complex [Re,@-Cl)(p-C=NHBu' )(p-dppm), C12(CNBut),]PF6has been isolated.586 The chemical oxidation of [Re,C1, (dppm), (NCR),]PF6 to [Re,Cl,(dppm),(NCR),](PF,), can be accomplished using NOPF,,587and this same reagent converts Re,Cl,(dppm),(CNR) to its monocation.585The complexes [Re2X3(LL),L']"+ (n = 1 or 2; X = C1 or Br; LL = Ph2PCHzCH2PPh,or Ph,PCH,CH,AsPh,; L = nitrile or isocyanide ligand) have also been prepared and characterized.588
142 43.4.2.3
Rhenium Sarfur iigands
There are two compounds which fall into this category, viz. Re,Cl,(DTH), and [ReBr,(DTH)j, (DTH = 2,5-dithiahexane). Both are prepared by reaction of the appropriate (Bu:N), Re2XEsalt with DTH in refluxing acetonitrile., The structure of the bromide complex is unknown, although n is probably 2. In the case of Re,Cl,(DTH), (20), this paramagnetic air-stable complex can be isolated as beautiful red-black dichroic crystals and its X-ray crystal structure shows that it possesses a staggered rotational conformation in which the two rhenium atoms are in quite dissimilar environment^.^^ Formally, at least, it can be considered as the 'zwitterion' Cl(DTH),Re'lRe'l'C1,.
(20) S-S
43.4.2.4
= DTH
ffy&i&s
The reaction of two equivalents of the phosphite ligand P(OCH,),CEt (abbreviated P') with the dirhenium(1V) hydride Re, H,(PMe, Ph), in benzene produces cleanly the complex Re, H,(PMezPh),P; (21), the unusual unsymmetric structure of which has been established by an X-ray crystal structure d e t e r m i na t i ~ n.The ~ ~ two metal atoms have different coordination numbers and different formal oxidation states. Protonation of this complex with excess HBF,-Et,O in benzene at room temperature gives the dirhenium(II1) hydride [Re,H,(PMe,Ph),P;]BF,. Consistent with the fact that (21) has no symmetry, four Re-H resonances are seen in the ' H NMR spectra at - 48°C; by + 57°C these have coalesced to a single broad resonance implying that hydride migration to all four environments occurs.79
(21) P
PMcZPh; P' = P(OCH,),CEt
43.4.3 Trinnclear Complexes The trinuclear rhenium(II1) cluster halides Re,X, (X = C1 or Br) react with many Lewis bases to form adducts of the type Re,X,L,, where n = 2 or 3., However, in the case of reactions with pyridine and related tertiary amines, reduction to the dark green-black trinuclear rhenium(I1) complexes [Re,X, L,], occurs quite r e a d i l ~ . ' ~The . ~ ~three-electron reduction of the Re;'+ cluster unit to give Re!+ is in accord with current ideas" concerning the electronic structures of these species. These reactions proceed via the unreduced adducts Re3X9L3,but whether reduction does or does not occur depends upon the nature of X and L. The more basic amines (pK, values > 4.9), i.e. pyridine, 3- and 4-methylpyridine, isoquinoline, 2-methylquinoline and benzimidazole, reduce Re,CI, to [Re,Cl,L,],, while the relatively weak bases pyrazine, 2,6-dimethylpyrazine and 3-chloropyridine (pK, values < 3) give the dark red-purple unreduced adducts Re>Cl,L,. Tn the case of 2-methylpyridine, 2-vinylpyridine, 2,6-lutidine and quinoline, materials of 'intermediate' composition, approximating to [Re,Cl,L,],, are produced. These phases correspond to a two-electron reduction of the Re:+ core.5 A similar situation to that described above pertains to Re,Br,, but in this instance the more readily reducible bromide cluster is also reduced (to [Re,Br&,],) by the weak base 3-chlor0pyridine.~~ Based upon a consideration of the electronic absorption and ESCA spectral properties of the complexes [Re,X, LJn, and their conversion to the rhenium(II1) salts (amineH),Re3Xll,it has been suggested' that they possess a halogen-bridged 'polymer of trimers' structure. A variation in the nature of the reaction products that are formed by the amine reductions of Re3C1, is encountered in the case of acridine. The dark green salt (AcrH),Re,Cl, is produced rather
Rhenium
143
than the adduct [Re, CL,(AC~),],,~~ a noteworthy result since this complex contains the only known chloroanion of rhenium(I1). The reactions of Re,X, (X = C1 or Br) with 2,2'-bipyridyl, 1,lO-phenanthroline and 2,2',2"-terpyridyl in acetone produce dark purple insoluble solids whose properties are in accord with the stoichiometry (amineH), Re,Cl,(amine),, in which the formal oxidation state of the rhenium is + (3 - x). The degree of reduction depends upon the reaction conditions that are used.5 43.5 RHENIUM(1II) COMPLEXES
Most complexes of this oxidation state are derivatives of the rhenium(TI1) halides. They are classified according to the other ligands (if any) that are present. For example, ReCl,(PR,), will be found under 'phosphine Complexes' rather than 'halide complexes'. Mononuclear complexes of rhenium(TT1) are mostly of the types [ReX,L.,]+ and ReX,L,, but rarely [ReX,L,]-. Additionally, two very important classes of complexes exist that are derivatives of the quadruply bonded dinuclear [Re,XSl2- and doubly bonded trinuclear [Re3X,,l3- anions (X = C1, Br or I),'
43.5.1 43.5.1.1
Mononuclear Complexes Cyanide, 0 k y l isocyanides and lwyl ismyanides
The best characterized cyanide complex of rhenium(II1) is K4Re(CN)7-2H,O. It can be prepared as pale-yellow crystals through a variety of reactions between RCN and &Rex, (X = C1 or I), Re,Cl, or KReO, in both aqueous and non-aqueous media.15 The mixed salt NaK,Re(CN),.3H20 can be formed when NaCN is used in place of KCN. The [Re(CN),I4- anion is now known to be the product, often admixed with other well-defined higher oxidation state cyano rhenium species, in reactions that have previously been described as giving [Re(CN),],-, [Re(CN),I3- , [Re(CN)6I3-, [Re(CN),]- and [Re(CN),I3-. Indeed, there remains to be definitive structural proof that any of these latter anions have been isolated in a pure state, although there is no good reason to believe that such species cannot be prepared. For example, the X-ray powder pattern of the complex purported to be 'K, Re(CN),-H,0'83 resembles closely that of K, Re(CN),. 2H20.The X-ray crystal structure of K4Re(CN),- 2H,O showss4that this complex contains a pentagonal bipyramidal anion, = 2.077(3)A and r(Re-C,) = 2.095(5)& and is isostructural with with r(Re-C,) K,V(CN),- 2Hz0. This structural result confirms the conclusions based upon vibrational spectroscopic studies'59R5 that this geometry holds for [Re(CN),I4- in aqueous solution, as well as solid K,Re(CN),*2H20. The diamagnetism of this potassium salt is in accord with the presence of a low-spin d4 configuration. The following characterized isocyanide complexes of rhenium(I1I) are described elsewhere: ReCl,(CNMe)(PPh,),, ReCl,(CNMe)(dppe), ReCl,(CNMe),(PPh,), [ReCl,(CNMe),(PPh,)]+, A recent attempt to isolate ReBr, (CN-p-tol),, through the ReBr,(CNMe), and ReBr,(CN-p-t~l),.~ reaction of K, ReBr, with p-tolNC in ethanol,, gave a material with the appropriate microanalytical data, color and IR spectroscopic properties, but it is by no means certain that this is the correct structural formulation.22The reaction between K,Rel, and Bu'NC in ethanol at room temperature gives golden-brown crystals of [Re(CNBu'),12]13." The related seven-coordinate chloride and bromide derivatives, [Re(CNBu'),X,]PF, (X = C1 or Br), are formed by the non-reductive cleavage of quadruply bonded (Bu,"N)zRezX,upon their reaction with Bu'NC in rnethan01.l~The mixed isocyanidehalide-phosphine complexes of rhenium(III), [Re(CNBu'),(PEtPh,)Cl2]PF, and [Re(CNBu'), (dppe)Cl,]PF,, are likewise prepared by isocyanide cleavage of Re2C1,(PEtPh2), or dimeric Re, Cl,(dppe), (the latter containing Re@-Cl), Re bridges but no Re-Re bond)." These complexes show IR active ~(CIN) modes (at 2200 cm-') that are in accord with them being derivatives of Re"'. The halogen (C12,Br, or 12)oxidation of [Re(CNBut),]PF6and [Re(CNPh),]PF, in chlorocarbon or acetone solvents in the presence of KPF,, gives the salts [Re(CNBU'),X](PF,), (X = C1, Br or I) and [Re(CNPh),X](PF,), (X = C1 or Br).'932'Cyclic voltametric studies have demonstrated that these complexes are quite easily reduced back to the parent rhenium(1) cations, the ease of reduction being I > Br > C1 and PhNC > Bu'NC.'~,~' When the reactions between various aryl isocyanides ArNC and (B$N), Re2C1, are carrried out in methanol at room temperature, then rhenium(II1)-containing complexes of the types [Re(CNAr),]+ [ReCI,(CNAr),]- and ReC1, (CNAr), can be isolated.2' These compounds display welldefined electrochemistry (cyclic voltammetry) in 0.2 M (Bu:N)PF,-CH,C12, the mononuclear Re"'
-
144
-
Rhenium
species displaying the redox processes
Re'
+Re"' +e-e-
+e-
-e-
Re"
Characterization of the salts [Re(CNAr),]+ [ReCl,(CNAr),]- has shown that freshly prepared samples contain the trans anion, but that CH2C1, solutions slowly isomerize to the corresponding cis isomer. The H NMR spectra of these same salts reveal substantial Knight shifts associated with the paramagnetic d [ReCl,(CNAr),]- anions." The properties of [ReCI,(CNAr),]- resemble those for the nitrile analogues [ReCl,(NCR),]- described in Section 43.5.1.5.
43.5.1.2 Amines, N-heterocycles and mi& ligands
There are few simple complexes that contain two or more of these ligands bound to a single Re"' center. There are, on the other hand, a variety of mixed ligand complexes where one of these ligands is present.' Red-brown crystalline ReCl,(py), is prepared by the reaction between ReCl, (benzil)(PPh,) and pyridine, while with ReCl,(NCMe)(PPh,), as the starting material, orange-brown ReCI, (py),(PPh,) is produced.B6A similar reaction between ReC1, (NCMe)(PPh, )z and 2,T-bipyridyl in mixed dichloromethane-benzene solvent gives green crystalline ReCl, (bipy)(PPh,).86 The dark green compIex ReBr,(NH,-p-tol), is apparently the product from the reaction of g-toluidine with KzReBr6.22 The rhenium(I) complex Re(NHPh)(N,)(PMe?),, which is prepared by the Na/Hg reduction of Re(NPh)Cl, (PMe,), under an N,(g) atm~sphere,~, reacts with I2 to give [Re(NHPh)I(PMe,),JI. 35 This constitutes a quite rare example of an amido complex of Re"' (see also Section 43.5.1.8). Deprotonation of the rhenium(V) alkylimido (Le. nitrene) complexes trans-ReC1, (NMe)(PFWh,), and trans-ReC1, (NMe)(PMe,Ph),, using pyridine and PMe,Ph/NEt, mixtures, respectively, gves the methyleneamido complexes of rhenium(III), ReCl,(N=CH,)(py)(PRPh,), (R = Me, Et or Ph) and ReClz(N=CH,)(PMe,Ph),.' New methods for obtaining 2,2'-bipyridyl complexes of the typesfac-Re(bipy)(PR,)Cl, and trans, cis-[Re(bipy)(PR,), Clz]+ have been described.''' The electrochemical and spectroscopic properties of these complexes have been measured.
43.5.1.3 Dinitrogen, diazenido and triazenido ligands
Dinitrogen complexes of rhenium(II1) are rare while those of rhenium(1) are comparatively common (Section 43.3.2.1). The mixed hydriddinitrogen complex [ReH,(N,)(dppe),]BF, is mentioned in Section 43.5.1.8. The reactions by which the rhenium(I1I) acyl- and aroyl-diazenido complexes ReCl,(N,COR)(PMe,Ph), (R = Me or Ph) can be formed by treatment of the rhenium(1) complexes ReCl(N,)(PMe,Ph), and ReCl(N,)(py)(PMe,Ph), with RCOCl have been described in Section 43.3.2.1 (see Scheme 2). These complexes display a v(C0) mode at 162Ocm-' and v (") at 1505cn-'.,* The complex ReCl,(N,COPh)(PMe,Ph), (22) has also been prepared by reacting ReCl,(N,COPh)(PPh,), (23)with excess PMe,Ph. This is one example of a more general type of reaction where (23)reacts with various ligands (MeCN, py, phosphites and other ph~sphines)~~.'' to give the products that are shown in Scheme 8. Further reaction with some of these same nucleophiles can give dinitrogen complexes of Re' (see Section 43.3.2.1). An X-ray structure determination on complex (22) supports its formulation as a derivative of On the other hand, the green crystalline triphenylphosphine complex (23), which is prepared by the reaction of ReOCI, (PPh,), with monobenzoylhydrazine and dibenzoylhydrazine in benzene-ethanol in the presence of hydrochloric acid, can be described formally as a derivative of Re" or Re"'. The same However, is true for other complexes of this type that have been prepared by a similar pr~cedure.'~
-
C'
(22) P
=
PMe,Ph
(23) P
=
PPhS
145
Rhenium ReClz (N2CbPh)(PPh,), IL
(e.g.
L = MeCN or py)
(e.g.
L = P(OMe),)
(ex. L
= PMe2Ph)
Scheme 8
while v(C0) and V(") modes have not been assigned in the IR spectra of these complexes, the ESCA spectrum of (23)has been interpreted in terms of its formulation as a derivative of Re"'.*' In this event, the conversion of (23)to (22) constitutes a non-redox ligand substitution process. When mer-ReX,(PMe,Ph), (X = C1, Br or I) is heated with phenylhydrazine in ethanol, a mixture of the phenyldiazenido complexes ReX,(N,Ph)(PMe,Ph), (24) and ReCl,(N,Ph)(NH,)(PMe,Ph), (25) is produced.88)wThe X-ray crystal structure of (U), when X = C1, has confirmed its identity @(Re-N) = 1.77(2)A; r(N-N) = 1.23(2)A):' and (25) is believed to have a very similar structure. The NH, ligand of (25) is easily displaced by CO, PMe,Ph or PEt,Ph, but not by MeCN, MeNC, py or PPh,.88*90 Reversible protonation of (25) can be achieved using HCI or HBr to give the salts [ReCl, (NNHPh)(NH,)(PMe,Ph), JX. Structural studies on the bromide salt of (X)have shown that protonation occurs at the second nitrogen c1
CI P
I
I *,*-N=N # Pfl I NH3
C1-Re-N=N *\NP p@
1
C1 --Re 'Ph
P
(24) P = PMe,Ph
'Ph
[
CIH ;p .
( 2 5 ) P = PMe,Ph
R&-N=N
:q+
(26) P = PMe,Ph
The compound ReOCl,(PPh,), reacts, in the presence of PPh,, with the lithium salts Li(RN,R) (R = Ph, p-MeC,H,, p-FC,H, or p-ClC,H,) in boiling THF to yield the mononuclear, paramagnetic Re"' complexes ReClz(RN,R)(PPh,),. The reaction is believed to proceed as shown in equation (4).92The X-ray crystal structure of the derivative where R = p-MeC,H, (27) shows the presence of chelating triazenido ligands. These complexes display the distinctive Knight-shifted H NMR spectra that characterize paramagnetic mononuclear Rerr1species.92
'
ReOCl,(PPh,),
+
PPh,
+
Li(RN==N=NR)
+
ReCl,(R,N,)(PPh,),
+ Ph3P0 + LiCl (4)
(27) P = PPhS; R = p-tol
The syntheses and X-ray crystal structures of the diazenido complexes Re(SCH,CH,SCH, CH, CH, SCH, CH, S)(PPh,)(N, COPh)5wand (Et, NH)[Re, (N2Ph)2(SPh),]s9' have been published. The complex ReCl(N,Ph),(PPh,), is formed by the reaction of ReOQ,(PPh,), with excess phenylhydrazine in methan01,~"or with Me, SiN,Ph in di~hloromethane.5~~ The bromo analogue can be made in an analogous fashion.s92They react with excess HX in methanol to give the hydrazido(2 -) ligand complex Rex, (NNHPh)(N,)(PPh,),.592
43.5.1.4
Cyanate and thiocyanate
The isocyanate complex ReCl(MeNCO)(dppe), (a), which has been formulated as a complex of Re"', is prepared by a sequence of reactions that first involves the formation of the orange methyl
I46
Rhenium
isocyanide complex ReCI,(CNMe)(PPh,),, from the reaction of a mixture of PPh, and ReOCl, (PPh,), with MeNC, followed by its conversion, via a ligand-exchange reaction, to paramag= 1.91 BM), and subsequent hydrolysis-deprotonation. netic ReC1, (CNMe)(dppe)
Electrochemical studies have shown that in non-aqueous media (MeCN or CH,C12), the [Re(NCS),]’- anion possesses a very accessible and quite reversible one-electron reduction (Eli2 between - 0.1 and - 0.3 V depending upon solvent, et^.).'^.'^ This result has led to the design of a procedure for the reduction of (Bug N), Re(NCS), to pale yellow crystalline (Bu:N), Re(NCS), using resulting complex is extremely air sensitive in both the solid state N,H,-H,O as the red~ctant.~‘The and in solution, reverting to the stable [Re(NCS)$ anion. In contrast to the instability of [Re(NCS),],-, a variety of quite stable mixed ligand complexes are k n o ~ n . 9 ~ The 9 ~reactions ~ of the quadruply bonded dirhenium(Il1) salt (BuiN), Re,(NCS), with monodentate and bidentate tertiary phosphines (PPr;, PEt2Ph,PPh,, dppm and dppe) produce the green complexes (BuiN), Re,(NCS), L, which contain dimeric thiocyanate-bridged anions (29) with magnetically dilute Re’*’centers.97In the case of the dppm and dppe complexes, these phosphine ligands are believed to be bound in a monodentate fashion. These two complexes are unstable in acetonitrile solutions due to rapid intramolecular attack of the unbound ‘dangling’ tertiary phosphine groups upon the thiocyanate bridges. This leads to the formation of the yellow monomeric anions [Re(NCS),(dppm)]- and [Re(NCS),(dppe)J- (30). The orange-brown mononuclear complex (Bu:N)Re(NCS),(PEt,), is formed by the reaction of (Bu,“N),Re,(NCS), with PEt,, via the green intermediate (Bui N),Re,(NCS), (PEt,)2!7
N
c
C S
S
(30) P--P
= dppmordppe
Upon reacting (Buf;N),Rez(NCS),(PEt2Ph), (29;P = PEt,Ph) with the bidentates dppe, bipy and phen, the paramagnetic mononuclear complexes Re(NCS), (PEt,Ph)(bidentate) are f ~ r m e d . These ~’ complexes display three electrochemically reversible one-electron waves in their cyclic voltammograms (recorded in 0.2 M (Bu~N)PF6-CH,C1,) attributable to the processes [Re]+
---e-
[Re?
-e-
+e -
[Re].
-e-
+e-
[Re]’
([Re] represents the complex).94The X-ray crystal structure of the dppe derivative has confirmed that these compounds possess distorted octahedral structures.”
43.5.1.5
Nitriles and amidinates
The most important nitrile compound of Re”‘ is ReCl,(NCMe)(PPh,),, a complex which finds much use as a starting material for the synthesis of other Re“’ complexes and some derivatives of Re”.’ This complex and others of its kind are discussed in Section 43.5.1.6. Other complexes that contain a single nitrile ligand, e.g. [ReOBr,(NCMe)]- and [ReBr,(NO)(NCMe)]-, are most conveniently considered in other sections, i.e. ‘Rhenium(V) Complexes’ (Section 43.7) and ‘Nitrosyls’ (Section 43.10), respectively, for the two examples cited here. The reduction of ReCl,(NCMe), by NMe, in ethanol or by Cd/Hg in acetonitrile generates the paramagnetic [ReCl,(NCMe),]- anion whch can be isolated as its Cs+, Cd2+or Me,NH+ salts.”
I Rhenium
147
These compounds, like ReCI,(NCMe)(PPh,),, do not display v ( C r N ) modes in their TR spectra (see Section 43.5.1.6). The cis stereochemistry of [ReCl,(NCMe),]- has been confirmed by X-ray crystallography.' Reoxidation of the orange anion to cis-ReCI,(NCMe), can be accomplished using cold nitric acid, iodine or various oxidizing metal salts. The potential of the [ C ~ ~ - R ~ C ~ , ( N C M ~ ) , ] ~ , ' couple is -0.33V YS. SCE, as measured by cyclic voltammetry on 0.2M (Bu:N)PF6&H,C1, solutions.65The salt [Me,NH]ReCI,(NCMe), reacts with APh, (A = Pi As or Sb) to give ReC1,(NCMe)(APh,),.98 A novel route to the [ReCI,(NCMe),]- anion involves the photochemical cleavage of the [Re,C1,I2- anion. Ultraviolet irradiation of acetonitrile solutions of (Bu:N),Re, C1, affords both tan-coloured (Bu,"N)ReCI,(NCMe), and orange ReCl,(NCMe),." A most unusual reaction takes place between ReOC1, (PPh,), and malononitrile in benzene to give a crystalline red product of stoichiometry ReCI,(C6W4N4)(PPh3),.The yield of this product is increased in the presence of added PPh,. It has been proposedgRthat this complex contains the 4-(dicyanomethylene)azetid-2-imine ligand (NC),C=CNHC(NH)CH,, formed by intramolecular nucleophilic addition of the enamine amino group to the non-conjugated nitrile group of the malononitrile dimer (NC),C=C(NH,)CH,CN (abbreviated MND). When this complex is heated with pyridine, free MND is formed, while an orange crystalline complex, analyzing as 'Re(OMe)(MND)(NH,),Cl', is produced when a methano1 solution is treated with gaseous ammonia.98This compound is suspected of being the unusual amidine derivative (31); it can be isolated as its C1- or BPh; salt. +
43.5.1.6 Phosphines, phosphites, arsines and stibines The most extensive and important group of complexes are those with phosphine ligands. They are usually encountered as derivatives of mononuclear rhenium(II1) halides. Compounds of the type Rex, L3,where L represents a trialkyl-, dialkylphenyl- or alkyldiphenyl-phosphine, or -arsine, constitute an extensive class of mononuclear rhenium@) complexes; examples are ReCl, (PEt,),, ReBr, (PMePh,), and ReCl, (AsEt,Ph), .'.Iw These complexes, which range in color from yellow to dark brown, are paramagnetic, exhibit Knight-shifted ' H NMR spectra and invariably possess the mer octahedral configuration. The X-ray crystal structure of mer-ReCl,(PMe,Ph), has been determined."' They are most conveniently prepared by reacting ReOC1, (PPh,), or ReC1, (NCMe)(PPh,), with the appropriate phosphine in boiling the reaction of rrans-ReC1,(PR3), with excess PR, in boiling ethanol' or, in the case of mer-ReX,(PMe,Ph), (X = C! or Br), by heating KReO, in concentrated hydrohalic acid with PMe,Ph in ethan01.I'~By reacting ReOCI,(dppe) with PEt,Ph in boiling benzene, the mixed phosphine complex ReCl,(dppe)(PEt, Ph) can be produced.'" In the reactions involving the reduction of higher valent oxo-rhenium species, the phosphine ligand serves as the reducing agent by facilitating oxygen transfer from the metal through formation of the appropriate phosphine oxide. Spectroscopic studies (e.g. electronic absorption, IR and NMR) have been carried out on many of these When ReOCl,(PPh,), is reacted with an excess of PPh, in refluxing acetonitrile, reduction to Re"' again occurs, but in this instance the product is ReCl,(NCMe)(PPh,), (32).s6This same complex has also been prepared as a relatively minor product in the reaction between B-ReCl, and PPh, in acetonitrile.'04The mechanism of its formation from ReOCl,(PPh,), has been looked at in some This complex is a very important starting material for synthesis of many rhenium(II1) species,'.86including some of the ReCl, (PR,), complexes described above. Several complexes of the type ReX,(NCR)(PPh,), have been prepared using other nitrile solvents (R = Pr", Bu', n-C,H,, or Ph) and ReOX,(PPh,), (X = C1 or Br) as the starting material.86 The reactions of ReX,(NCMe)(PPh,), with the phosphines PEtPh,, PPr"Ph, and PBu"Ph, result in displacement of the PPh, ligands and the formation of ReBr,(NCMe)(PEtPh,), and ReX,(NCMe)(PRPh,), (X = CI or Br;
148
Rhenium
R = Pf or Bun) rather than ReX,(PRPh,),.'OO The AsPh, and SbPh, analogues ReC1,(NCMe)(APh,), (A = As or Sb) are prepared by the reaction of We,NH]ReCl,(NCMej, with AsPh, or SbPh3.98 P
P ( 3 2 ) P = PPh3
Although complexes of the type ReX3(NCR)(PPh,), do not exhibit an identifiable V(C=N) mode in the IR mull spectra, the X-ray crystal structure of ReCl,(NCMej(PPh,), (32) confirms the presence of normal N-bound acetonitrile, with Re-Cl distances of 2.35 A (trans to Cl) and 2.40 A (trans to MeCN).'05 These complexes exhibit the expected paramagnetism for d' Re"' complexes.86 The complex ReC1, (NCMe)(PPh,), and its analogues are readily oxidized by halocarbons to give ReIv complexes. When solutions of ReCl,(NCMe)(PPh,), are allowed to undergo oxidation in air, the pale green crystalline complex Re0Cl3(OPPh,)(PPh,) is formed; the latter complex in turn reacts with PPh, to give ReOCl,(PPh,), and Ph3P0.86 Studies of the electrochemical behavior of mer-ReCI,(PMe,Ph), in 0.2 M (BuiN)PF,-CH,Cl, show that it possesses both a reversible one-electron oxidation (E,,2= +0.67V vs. SCE) and a one-electron r e d ~ c t i o n .A~ ~later investigation of this system showed that the electrochemical oxidation of ReCl,(PMe,Ph), in acetonitrile at -k 0.9 V with NaClO, as supporting electrolyte could be used to prepare the violet colored rhenium(1V) complex [ReCl, (PMe,Ph),]ClO,. In the presence of (Et,N)Cl, this electrolysis produced trans-ReCl,(PMe,Ph),.'" When ReCl,(PMe,Ph), is reduced in acetonitrile, it is believed that the resulting monoanion reacts rapidly with solvent to produce ReCl,(PMe,Ph),(NCMe), although a pure sample of the latter compound has not been isolated. However, its Re"' analogue, viz. [ReCl,(PMe,Ph),(NCMe)]ClO,, is apparently formed upon reacting mer-ReC1, (PMe, Ph), with AgClO, in acetonitri1e.lo6 Complexes of the type trans-ReCb(PR,), (PR, = PEt,, P P e , PBu;, PMePh, or PPh,) exhibit a very accessible one-electron reduction in the potential range - 0.1 to - 0.5 V vs. SCE (in 0.2 M (Bu~N)PF,-CH,C1,).6s Solutions containing the yellow Re'" monoanions have been generated but pure salts have not yet been isolated.IM A variety of well-characterized paramagnetic salts of the cationic species [ReX,(diars),]+ and [ReX,(dppe),]+ (X = C1, Br or I) have been known for many years.2 They are believed to possess a tram configuration. Their preparations have involved the reaction of KReO, with dppe in ethanolic hydrochloric acid, or ReOX,(dppe) with dppe in boiling EtOH, and the reaction of HReO, with &ars in hot EtOH in the presence of hypophosphorous acid and the appropriate HX.'J,'07These two general synthetic methods (or variations thereof) are probably the best ones, although these complexes have been prepared by several other less direct routes.' More recent developments have included the synthesis of [ReC1,(Ph,P(CH,),PPh2),]C1and [ReCl,(Ph,PCH =CHPPh2),]C1 by the usual method starting from KRe0,,Im and the preparation of [ReCl,(dppa),]X (X = C1 or PF,; dppa = bis(dipheny1phosphino)amine) from the cleavage of (BuiN),Re, Cl,." Salts containing the diamagnetic [ReCl(dppe),12+ dication can apparently be produced by oxidizing ReCl(N,)(dppe), beyond the stage [ReCl(N,)(dppe),]+, using AgNO, or AgC10, as oxidants.25Another class of well-defined complexes that contain bidentate phosphines and arsines are the halide-bridged species Rez@-X)2X,(LL), (X = Cl or Br; LL = dppe or arphos). These magnetically dilute compounds are the major products formed when (Bu:N),Re,X, (X = C1or Br) is reacted with dppe or arphos in acetonitrile at room temperature?o*'wAn earlier report' describing the existence of diamagnetic, 'five-coordinate' ReCl,(diars) is worth reinvestigating in view of these more recent findings. The X-ray crystal structure of the magenta colored acetonitrile solvate Re2(p-C1),Cl,(dppe),.2MeCN(33) has shown that the Re-Re distance is very long (- 3.81 A).'o9 When complex (33) is refluxed in acetonitrile, the orange-brown monomer ReC1, (NCMe)(dppe) is formed through cleavage of the Re-C1-Re bridging bonds." When this reaction is carried out in the presence of an excess of dppe, a yellow insoluble complex of stoichiometry [ReCl,(dppe),,], is produced.m It is unclear at present whether it is a dppe-bridged polymer, or ionic BeCl2(dppe)zl[ReC1, (dppe)l. Complexes with polydentate phosphine and arsine ligands are known but in most instances their structures are in doubt and they remain inadequately characterized? The reaction between ReC1,(NCMe)(PPh,), and (Ph,PCH,),CMe (abbreviated P,) gives paramagnetic (pC8 = 1.8 BM), green-
Rhenium
c1
C1
Cl
CI
(33) P-P
149
= dppe
yellow crystalline fac-ReC1, (P3).'O0A complex of this same stoichiometry had previously been prepared by the reduction of ReOCl,(P,) in aqueous acetone using sodium dithionite."' This buff colored complex could be converted to the bromide by reaction with LiBr. Both the chloride and bromide are paramagnetic kfl 1.7 BM) and exhibit sharp ' H NMR spectra. However, the two different samples of ReCl, (P,) have quite different melting points ( 154-6°C"0versus 225-227"C'"(') thereby implying that they are not one and the same. Other less well-defined species are mentioned briefly in Section 43.5.3.3 (see Table 7). There are a few reports which describe phosphite complexes of rhenium(II1) that bear a close analogy to the phosphine derivatives ReX,(PR,),. Thus, the sythesis of fuc-ReCl,~(OEt),], has been accomplished through the reaction of ReOCl,(PPh,), with an excess of P(OEt),. This structural formulation is based upon IR and NMR spectra characterization."' A more extensive study has led to the isolation of paramagnetic mer-ReX,[P(OR),], (X = C1, Br or I; P(OR), = P(OEt), or P(OCH,), CMe)."' The triaryl phosphite complexes ReI, [P(OPh),],, ReI, [P(O-p-tol),], and ReX,(PPh,),[P(OPh),] are also known,'.' and the suggestion has been made that ReI, [P(OPh),], exists in the mer config~~ration."~ Phosphido complexes are not at present an important facet of rhenium(lI1) chemistry. The reaction between ReOC1, (PPh,), and PHPh, in hot benzene gives ReC1, (PFh2)(PHPh2),, but, surprisingly, this complex is said to be diamagnetic.' Preliminary details of the isolation of the diamagnetic homoleptic dicyclohexylphosphide complex [Li(DME)]Re(PCy,), have been reported.II4 The [ R e ( P c ~ , ) ~ lanion may be four-coordinate but in good donor solvents (e.g. THF or DME) it is believed to equilibrate with a purple paramagnetic adduct. The complex trans-[ReCl, (Me, PCH, CH,PMe,),]PF, has been synthesized and its X-ray crystal structure determined.593The related Re-labeled complex has been made from ['*4Re0,]- .593
-
43.5.1.7
Oxygen and suljiir donors
b-Diketone complexes of rhenium(II1) are the most extensive and best characterized of those that contain oxygen and sulfur ligands. The tis-acetylacetonate (pentane-2,4-dionate, acac) of rhenium(III), Re(acac),, was first reported in 1960 as being a product of the reaction of Reo, with a~etylacetone."~Twelve years later more convenient synthetic procedures were reported that involved the reaction between Re(acac),Cl, or ReCl,(acac)(PPh,), and Na(acac)."6 Other synthetic methods involve the Zn or Zn/Hg reduction of Re(a~ac),Cl,.'~' The complex Re(hfac), been prepared by reacting (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) has ReCl,(hfac)(PPh,), with Na(hfac).Il6 Dark red crystalline Re(acac), and Re(hfac), are volatile and = 1.7-1.8 BM at 290 K) and exhibit exhibit well-defined mass spectra.'I6 They are paramagnetic (pea temperature independent paramagnetism. A consideration of the X-ray powder data for these two complexes indicates that they possess the typical trisw-diketonate)metal structure. Re(acac), is extremely susceptible to oxidation thereby making its handling quite difficult.'I6 A rhenium(I1I) complex of salicylaldehyde, ReCl, (salH)(PPh3),, is also known (Section 43.5.1.9). When the rhenium(V') complexes ReOX,(OEt)(PPh,), (X = C1, Br or I) are reacted with acacH in hot benzene, the species ReOXz(acac)(PPh,)2 are formed first, and then react further to give paramagnetic, orange-red ReX,(acac)(PPh,),."* In the related reactions between ReOCl,(OEt)(PEt,Ph), and acacH, and of ReOCl,(OEt)(PPh,), with hfacH and tfacH (tfac = 1,1, l-trifluoropentane-2,4-dionate), the complexes ReCl,(acac)(PEt,Ph),, ReCl,(hfac)(PPh,), and ReCl,(tfac)(PPh,),, respectively, can be produced."' When ReOCl,(OEt)(PPh,), is refluxed with acacH in the absence of the benzene solvent, it appears that further substitution occurs to give ReCl(acac),(PPh,) (after 16 hours), and that this is followed by oxidation to the Re'" complex Re(acac),Cl, as the reflux time is increased (to 50 hours)."' An X-ray structure determination on a crystal of ReCl,(acac),(PPh,), has shown"' that it is the isomer that possesses a trans arrangement of phosphine ligands and cis disposition of Re-C1 bonds."' The one-electron reduction of cis- or trans-Re(acac),CI, to [truns-Re(acac),Cl,]- can be accom-
Rhenium
150
plished through the reaction of these Re'" complexes with Tl(acac) in ether, Na/MeCN or K/ MeCN. The bromide complex K[Re(acac), Br,] has also been prepared using the potassium reduction method. Cation exchange between K[Re(acac),Cl,] and (Ph,As)Cl in aqueous solution affords blue-green crystals of (P~As)~Re(acac),C1,],"7the X-ray crystal structure of which has shown that the anion possesses a tram octahedral structure.!20 The major difference between the structure of the anion and trans-Re(acac),Cl, is the lengthening of the Re-C1 bond lengths of the former.I2' When ReCl,(NCMe)(PPh,), is heated with b e n d (PhCOCOPh) in benzene, dark purple ReCl,(PhCOCOPh)(PPh,) is formed along with truns-ReC&(PPh,),. The analogous reaction with 9,lO-phenanthraquinone (phenquin) gives the blue complex ReCI, Cphenq~in)(PPh,).'~The benzil complex is paramagnetic (pC8 = 1.85 BM) and reacts with pyridine to give ReCl,(py), and benzil, an important reaction since this is the only route to the tris-pyridine complex (see Section 43.5.1.2). Coordination complexes of rhenium(II1) that contain sulfur ligands are relatively uncommon, although a couple of derivatives with the diethyldicarbamato ligand resemble the related acac complexes. The reaction of ReC1, (NCMe)(PPh,), with NaS,CNEt,- 3H,O in acetone leads to the formation of red-brown Re(S,CNEt),. The related reaction with NaSzCNPh,.2H,0 gave dark purple crystalline Re(S2CNPh,), (PPh,) in which one of the diphenyldithiocarbamate ligands is believed to be monodentate.I2' The complex ReCl,(S,CNEt,)(PPh,), is best prepared by the reaction of ReOC1, (S2CNEt2)(PPh3)Z with PPh, by prolonged refluxing in ethanol.12' The sevencoordinate carbonyl-containing rhenium(II1) complex Re(S,CNEt,),CO is produced by the reaction of ReCl(CO), with (Et,NCS)* and has been structurally characterized by X-ray crystallography.6 Mixed hydriddithiocarbamate complexes of Re"' of the types ReH(S2CNR2)X(PMe2Ph),and ReH(S,CNR,),(PMe,Ph), are also known (see Section 43.5.1.8). Thiourea reacts with (BuiN),Re,X, (X = C1 or Br) with the resulting cleavage of the Re-Re quadruple bond and the formation of insoluble complexes formulated as Rex, (tu),.' Similar products are formed upon reacting thiourea with Reo; in 2 M hydrochloric acid, but all these products remain incompletely characterized.' The reactions'of K2ReX6(X = C1, Br or I) with cysteine (cystH,) have been investigated and preliminary details reported describing the isolation of the crystalline, non-ionic, diamagnetic compounds of formula Re(cystH),X (X = C1, Br or I).',, A well-characterized set of thiol derivatives are the five-coordinate complexes Re(SAr),(NCMe)(PPh,) (Ar = Ph, p-tol, C6F, or C6Cl,) that are formed by the reaction of [ReO(SAr),]- with excess PPh, in refluxing MeCN. The X-ray crystal structure of the derivative where Ar = Ph has shown that the geometry is trigonal bipyrarnidaLzo4
"'
43.5.1.8
Mononuclear and dinuclenr hydrides
Hydride complexes of rhenium, including those of Re"', were the subject of an excellent review by Kaesz and Saillant that covered the literature up to 1971.'23Included in this, and also in the slightly later review by Rouschias,' was coverage of the important complexes ReH,(PPh,),, ReH,(dppe), and ReH,(PPh,),(dppe), together with ReH,X(dppe), (X = C1, Br or I) which is formed by the halogenation of ReH, (dppe), , plus the acac-containing complexes ReH,(acac)(PPh,), and ReHX(acac)(PPh,), (X = C1, Br or I). The complexes ReH,(dppe), and ReW, (E'Ph3)2 (dppe) are prepared by reacting ReH,(PPh,), with d ~ p e , ' ,while ~ ReH3(PPh,), is formed upon treating ReO(OEt)I,(PPh,), with NaBH, in the presence of an excess of PPh,.',' The synthesis of the analogous PMe,Ph derivative ReH,(PMe,Ph), has been accomplished by the LiAlJi&reduction of a THF solution of mer-ReCl,(PMe,Ph), and PMe,Ph."$ An attempt to prepare the tri-p-tolylphosphine analogue of ReH, (PPh,), by a related procedure gave a red product that was formulated as [ReH,(P-p-t~l,),],.'~~ This compound, which purportedly has a PPh, analogue of this same stoi~hiometry,'~~ is most likely Re,H,(P-p-tol,),, or some closely allied dirhenium polyhydride, and therefore possesses a formal oxidation state higher than Re"'. The white PMe, complex ReH,(PMe,), can be prepared by treating ReH(PMe,), or Re(?*-CH2PMe,)(PMe,), with H,.,' The spectroscopic properties of ReH3(PR,), (PR, = PPh,, PMe,Ph or PMe,) and ReH,(dppe), (see Table 6) show that in solution they are fluxional, as evidenced by a high field quintet for the Re-H resonance in the ' H NMR spectra. The temperature dependence of the ' H NMR spectra of ReH,(dppe),, ReH, (dpae),, ReH, (PPh,),(dppe) and ReH, (PPh,),(dpae) (dpae = Ph,AsCH,CH,AsPh,) have been studied down to -90°C or For ReH3(dppe), and ReH,(dpae),, - 50"C, the decoupled spectrum appearing two of the three hydride ligands were equivalent at as an AB2 pattern with Jw,-H, w 9.5 Hz. The ' H NMR spectrum of ReH,(PPh3),(dppe) consists of a triplet of triplets, as expected for three equivalent protons coupled to two non-equivalent pairs of phosphorus atoms.'36While the solid-state structures of the complexes with monodentate phos-
-
151
Rhenium
f
4
Rhenium
152
phines, ReH, (PR,),, have not been determined by X-ray crystallography, those of ReH,(dppe), and ReH,(PPh,),(dppe) have and, in both instances, the geometry has been described as a distorted pentagonal bipyramid.128 The reaction chemistries of ReH,(PR,), (PR, = PPh, or PMe,Ph) and ReH,(dppe), have to date been explored more thoroughly than those of any other hydrido complexes of rhenium(II1). While ReH,(PPh,), reacts with an excess of HX (X = C1 or Br) in benzene to give the rhenium(1V) salts (Ph,PH)ReX, (PPh3),Iz9ReH, (dppe), and ReH, (PMe,), can be protonated to give the tetrahydrido cationic c ~ r n p l e x e s ,as ~~ shown ~ ' ~ ~in equation (5) for ReH, (PMe,),. Upon treatment of ReH,(PPh,), with P(OPh),, substitution of up to three of the PPh, ligands can occur to give ReH,(PPh3), [P(OPh)& and ReH,(PPh3)[P(OPh),],.IM Few other mixed hydride-phosphite complexes of rhenium(II1) are known at present. A mixture of [ReH,p(OMe),],]+ and ReH,[P(OMe),], (02CCF,) is believed to be produced upon protonating Re, [P(OMe)J,, with trifluoroacetic acid, while ReH,[P(OMe),], has been characterized by IH NMR spectroscopy (Table 6) as one of the products from photolyzing H,-Re, [P(OMe),],, mixtures in THF.13 ReH,(PMe,),
+
HBF,
[ReH,(PMe,),]BF,
(5)
A very important reactivity difference between ReH,(dppe), and ReH,(PR,), lies in their photochemical properties. While the initial photoproduct of the 366 nm irradiation of ReH,(dppe), is the highly reactive species ReH(dppe), (see Section 43.3.3), in the case of ReH,(PMe,Ph), phosphine loss rather than the elimination of H2occurs."' Under an H, atmosphere the 16-electron species ReH,(PMe,Ph), smoothly converts to ReH,(PMe,Ph), via the intermediacy of ReH,(PMe,Ph), (equation 6). The putative 14-electron intermediates ReH,(PAr,), (Ar = aryl) are other examples of coordinatively and electronically unsaturated hydrido species. They are believed to be formed in reactions of ReH,(PAr,), that result in the dehydrogenation of alkanes.13' ReH,(PMe,Ph),
PMe2Ph
+
ReH,(PMe,Ph),
PMe2Ph
+ ReH,(PMe,Ph),
(6)
Protonation of ReH(N,)(dppe), with HBF, in benzene gives the cream colored salt [ReH,(N,)(dppe),]BF,. When HCl is used in place of HBF, then ReHCl,(dppe), is formed, presumably via the ~ ~a similar fashion, treatment of the rhenium(1) complex intermediacy of [ReH, (N2( d ~ p e ) , j C l .In ReCl(PMe,), with one equivalent of aq. 48% HBF, produces the yellow monohydride [ReHCl(PMe,),]BF, (Table 6).,, Other dinitrogen complexes of rhenium(1) provide a route, via protonation, to hydrido-rhenium(II1) species. The complexes mer-Re(S,CNR,)(N,)(PMe,Ph),(R = Me or Et) react with an excess of HX (X = C1 or Br) in THF to give [ReHz(S2CNRZ)X(PMe,Ph),]X, which on treatment with NEt, or pyridine give ReH(S,CNR,)X(PMe,Ph),. The latter species are converted to ReH(S,CNR,),(PMe,Ph), when reacted with Na(S,CNR,) in acetone solution.26 Spectroscopic properties of representative complexes of these types are given in Table 6. When slurries in acetonitrile of the heptahydride complex ReH,(PPh,), and the pentahydrides ReH,(PPh,),L (L = py or CsH,,NH,) are treated with HBF,, then H, is evolved and quantitive yields of the yellow colored salts [ReH(NCMe)3(PPh3)2L](BF4)2 (L = MeCN, py or C6H,,NH,) are obtained.13' This constitutes an alternative and quite simple strategy for the preparation of monohydridwrhenium(II1) complexes. These complexes exhibit a reversible one-electron oxidation at 1.OV vs. SCE, as measured by the cyclic voltammetric technique. Bulk electrolysis of CH,Cl, solutions of [ReH(NCMe,),(PPh,)2L]2+ affords purple [ReH(NCMe),(PPh,),LI3+, a rare example of a mononuclear hydriderhenium(1V) species.13* A more complete account is now avaifable'" describing the protonation reactions of the complexes ReH,(PPh,),, ReH,(PPh,),L (L = py, C,H,,NH, or Bu'NH,) and ReH,I(PPh,), with HBF,*Et,O in acetonitrile or propionitrile (for a preliminary report see ref. 132). This procedure affords the seven-coordinate monohydrido complexes [ReH(NCR),(PPh,), L](BF4)2in good yield. The very air-sensitive complex ReH, (NHPh)(PMe,), can be obtained by the Na/Hg reduction of Re(NPh)Cl,(PMe,), in the presence of excess PMe, under an H, atmosphere or by the oxidative addition of hydrogen to Re(NHPh)(N,)(PMe,),. This reaction (equation 7) is reversible at room temperature. The ' H NMR spectrum of this complex has been interpreted in terms of the presence of two isomers (Table 6).,, When the reduction of Re(NPh)Cl,(PMe,), is carried out under argon in the presence of excess PMe, then the yellow crystalline monohydride ReH(NHPh)($ CH, PMe,)(PMe,), is believed to be the reaction product.34
-
Re(NHPh)(N,)(PMe,),
+ H, e
ReH2(NHPh)(PMed4
+ N2
(7)
I Rhenium
153
The tetrahydriddirhenium(I1) complex Re, H.,(PMe,Ph),P; (21) (P' = P(OCH,),CEt) (see Section 43.4.2.4) can be protonated using HBF, * Et,O in benzene solution to give the dirhenium(II1) complex [Re,Hs(PMe,Ph)4P;]BF4.78An X-ray structure determination (carried out at - 160"C) shows it to have the symmetric structure (Me,PhP),(P)HRe(p-H), ReH(P)(PMe,Ph), with an Re -Re distance (2.605(2) A) very similar to that in Re,H,(PMe,Ph),P; (2.597(1) A).'* At - 60°C this complex exhibits (in CD,C12) three hydride resonances in its ' H NMR spectrum, two of which are due to inequivalent bridging hydride environments (ratio 2: l), and one "P ABX pattern consistent with the solid-state symmetry. By 16°C the 31 P{'H) spectrum appears as an AzXpattern, and the two types of bridging hydride ' H NMR resonances coalesce but remain distinct from the terminal hydride triplet. The latter triplet pattern arises from coupling of each of the equivalent terminal hydride ligands to the two 'local' PMe,Ph ligands." The characteristics of the 'H NMR spectrum of [Re,H,(PMe,Ph),P;]+ at + 16°C are also seen in the related spectrum of [Re2H5(PPh,),(CNBu'),]PF, (34)over the temperature range 35 to - 70°C. This complex is prepared and an X-ray through the reaction of [Re,H,(PPh,),]PF, with Bu'NC in di~hlorornethane,~~~~'~~ crystal structure determination has shown that it possesses a structure very similar to that of the The maroon mixed phosphine-phosphite derivative with an Re-Re distance of 2.6O4(1) complex (34)can be oxidized by (NO)PF, in acetone to afford the paramagnetic. ESR-active, dark In contrast to the lack of reaction between blue compound [Re2H, (PPh3)4(CNBu'),](PF,)2.132,134 (34)and Bu'NC, its oxidized congener reacts rapidly to give [Re(CNBu'),(PPh,),]PF, and gaseous H,, along with significant quantities of (34)that are formed by back reduction of the dication without concomitant isocyanide coordination,
+
+
P
1'
(34) P = PPh3; C = CNBU'
Photolysis of benzene or isooctane solutions of ReH,(PR,), (PR3 = PMe2Ph, PMePh, or PPh,) results in the photoextrusion of a PR3 ligand rather than H,.'25.'3SThis leads (as shown in Scheme 9) to the formation of several products, including Re,H,(PR,), in the case of PR, = PMe,Ph or PMePh, (but not PPh,). A further product obtained in the case of the photolysis of ReH,(PPh,), is the trihydride ReH3(PPh3)4.'25 The structural characterization of Re,H,(PMe,Ph), has included an X-ray structure determination, but not all the metal-bound hydrogens have been lo~ated.'~' The metal-metal separation is 2.538(4) 8. ' H NMR spectral evidence is consistent with the structure (Me,PhP),HRe@-H),ReH,(PMe,Ph),, so that this complex could be viewed as a mixed oxidation state specie~.'~' ReH,(PR&
hv
-ReHS(PR&
+ fR3
Scheme 9 Photochemistry of ReH,(PR,), (ref. 135)
43.5.1.9 Mixed donor atom ligands An important recent development has been the isolation of several mixed N-0 ligand complexes of Re"'. The complex ReC1, (NCMe)(PPh,), reacts in boiling salicylaldehyde (salH) to produce green ReC1, (salW)(PPh,), in low yield."' In this paramagnetic complex the salH ligand is believed to be bound through its carbonyl oxygen, at least on the basis of its IR spectral properties. The Schiff base r h e n i u m 0 complexes ReOX,L(PPh,) yX = C1 or Br; L = N-methylsalicylideneiminate,Nphenylsalicylideneiminate, +N,N'-ethylenebis(salicylideneiminate)] react with an excess of PMe,Ph to give the red rhenium(II1) complexes ReX,L(PMe,Ph), via the intermediacy of ReOX,L-
Rhenium
154
(PMe2Ph).206These paramagnetic complexes exhibit well-defined Knight-shifted H NMR spectra . and are believed to have a trans disposition of PMe,Ph ligands and a cis arrangement of Re-X bonds. The complex ReCl, (Ph-sal)(PMe,Ph), is said to isomerize when heated under argon in toluene to give the isomer with the phosphine and halide ligands mutually cis to one another.207The Schiff base complex ReC1, (Ph-sal)(PMe, Ph), reported previpusly,206has now been structurally characterized by X-ray ~ r y s t a l l og r a p h yThe . ~ ~complex ~ ReCl,(Me-sal)(PPh,), has been prepared by the reaction of trms-ReCl,(PPh,), with the lithium salt of the ligand in refluxing benzene.M7 The pyrazolylborate complex ReCl,(FTh,)[HB(pz),] can be prepared in high yield by the reaction of ReOCl, [HB(pz),] with PPh, in hot toluene.572
43.5.2 Dinuclear Complexes Containing the Reif Core
Section 43.4.2 dealt with the chemistry of coordination complexes that are derivatives of the triply bonded Re:' core, possessing the c2n46'8*' electronic configuration. The removal of two electrons from this configuration gives rise to complexes that contain the quadruply bonded Re';+ core. The literature covering the period between the discovery of this class of complexes in 1964 and the latter part of 1981 is surveyed in the text 'Multiple Bonds Between Metal Atoms' by F. A. Cotton and R. A. Walton that was published in 19X2.5 This source (Chapters 2 and 8 in particular) should be consulted for an exhaustive survey of this field. Accordingly, we s h a l in the present section restrict our coverage to a relatively brief consideration of the literature for the period 1964-81 and a more thorough treatment of the work that has been published since that time. Since salts of the octahalodirhenate(II1) anion serve as the most convenient starting point for the synthesis of almost all dirheniurn(II1) complexes, they will be considered first. The quadruply bonded dirhenium(II1) acetate complex Re,CI4(O,CCH,),*2H,O reacts with PPh, in refluxing ROH (R = Me, Et or Pr") to give the red crystalline complexes Re,Cl,(OR),(PPh,), which have proved to be the Re'"Re" mixed valent species (RO), C1, ReReC1,(PPh,),.596 The one-electron oxidation and one-electron reduction of the complex Re,(p-C1),(p-dppm),C1,74 can be accomplished using NOPF, and cobaltocene as the oxidant and reductant, respectively, to give paramagnetic ions that possess Re-Re bond orders of 1.5.5g7*597
43.5.2.1
Halides
The tetra-n-butylammonium salt (BGN), Re2C1,, because of its relative ease of preparation and favourable solubility properties in many organic solvents, has been the most commonly used starting material in dirhenium(II1) chemistry. This blue crystalline material was for many years most conveniently prepared by the hypophosphorous acid reduction of [Reo,]- in aqueous hydrochloric acid.I3' However, because of the relatively low yield in which this salt was prepared (40% or less), another more convenient method has been sought and, very recently, d i ~ c o v e r e d . ' ~The ' * ~ ~reaction ~ between (Bu,N)ReO, and refluxing benzoyl chloride affords a solution which upon the addition of an HCl(g) saturated solution of (Bu:N)Br dissolved in ethanol gives (Bu~N),Re,Cl, in 90% yield. Both [ReCl,]'- and Re2(02CPh),C14have been isolated as intermediates in this rea~ti0n.I~' An alternative starting material for the synthesis of (BuflN),Re,Cl, with PhCOCl is ReOCl,(PPh,),, which is itself prepared from perrhenate. This reaction proceeds via red rrans-ReCl,(PPh,), which can be Cation exchange reactions can be used to obtain other salts of this anion,' while halide exchange reactions can be used to prepare the related fluoride, bromide and iodide anions as shown in Scheme 10.5~14n~'4' An alternative route to (Bu~Nj,Re,X,(X = C1, Br or I) involves the treatment of solutions of the benzoate complex Re,(O,CPh),Cl, with HX(g) in the presence of (Buf;N)X (equation 8).61A variety of other less convenient procedures exist for the preparation of salts of the [Re,X8]2p anions (X = C1 or Br) that contain the cesium cation or various organic cation^.^ Of these, the conversion of the trinuclear cluster Re,Cl, to [Re2C1,I2- by reaction with molten Et2NH: C1- is of special note since it proceeds in quite high yield.'P2 However, it suffers from one big disadvantage, in that it requires a ready source of Re,C&, a compound that is itself best prepared from [Re, Cl8I2- via the quadruply bonded acetate complex Re2(0,CMe),C12. Another recent noteworthy studyIM showed that the dirhenium octahydride complex Re,H,(PPh,), (see Section 43.6.1.lo), reacts with allyl chloride or bromide to give (Ph3PC3H5),Re2X,in 70-80% yield (equation 9). When Re,H:,(PPh,), is treated with HCl(g) in MeOH the mixed salt (Ph, PH),Re, C18-5(Ph,PH)ReCl,(PPh,) is formed.
Rhenium
/
48% HEW,reflux
(Bu:N), Re,Cl,
MeOH
\
b
(Bu;N),Re,Br,
Scheme 10
Rc,(02CPh),C1,
+
8HX
+
4(Bu,"N)'
ROH
(BdN)?Rc2X8f 4PhzC0,H
+
4Ht
(X = CI, Br or I) Re2H,(PPh,),
+
+
excess C3H,X 3 (Ph3PC3H5)2Re2Xs C3H6
+ H2
(X = C1 or Br)
With the exception of Cs,Re,Cl,-H,O and Cs2Re,Br,, other salts possessing alkali metal cations (K+ or Rb+) and the NH,+ cation have generalIy been prepared (admixed with the corresponding salts of [ReCl,]2-) via the high pressure reduction of [Reo,]- by molecular hydrogen in concentrated hydrochloric acid.5 These compounds, while being structurally similar to (Bu: N)zRe,X, and possessing the same electronic structure, have such low solubilities in non-aqueous organic solvents that their chemical reactivities have not been explored in any great detail. Several X-ray crystal structures have been determined for salts containing the [Re2C1,I2- and [Re2Br,IZ-anions, all of which were published before 19S2.5,145 The only recent structure determinaWhile the crystal structures of tion is that of [(DMA),H], Re,Br, (DMA = dirnethyla~etamide).'~~ the fluoride and iodide salts (Bu,"N),Re,X, have not yet been determined, there can be no doubt that they possess the typical [Re,X,]'- (35) structure, which is characterized by an eclipsed rotational conformation and a very short Re-Re distance (2.22-2.25 A).5 Following the isolation of the complex Re,& (CO), by the I, oxidation of Re,J,(CO),(THF), in heptane, an X-ray crystal structure determination showed that this tetrarhenium complex could be viewed formairy as a complex between two RefCO): fragments and [Re, I,]'-, the association occurring viu bent Re-I-Re bridges that involve six of the iodine atoms of [Re,TJ2-. However, as a consequence of this structure, the rotational conformation within the central RezT, unit is staggered rather than eclipsed and, as a result, the Re-Re bond length increases to 2.279(1)A.I4' In accord with the bonding scheme for such a species: as the conformation shifts from eclipsed to staggered, the 6 bonding diminishes (Le. d,-d:y overlap) and the metal-metal bond order decreases from four to three.
(35)
The electronic structures of the [Re2X,l2- anions have been the subject of much interest. Molecular orbital calculations have established the correctness of the bonding model that pictures the Re -Re bond as being one of order four with the 027c4d2ground state electronic c o n f i g u r a t i ~ n . This ~"~~ has further been supported by electronic absorption spectral studies, including the definitive assignment of the 6 + 6' t r a r ~ s i t i o n , ~and ~ ' ~vibrational ~ ~ ' ~ ~ spectral measurements (including resonance Raman spectra).'49The latter have led to the assignment of the vfRe-Re) frequencies both in the ground and excited An appreciation of the bonding in these molecules has led to studies of the emission spectra of [Re2C1,I2- and [Re, BrJ2- including an investigation of the electron transfer chemistry of the luminescent excited state of the octachlorodirhenate(II1) anion, Le. {[Re,ClJ2- 1*. These experiments have revealed that ([RezC1,I2-}*functions as a strong oxidant as well as a moderately good reductant in non-aqueous media."' In particular, the reaction of the 66* singlet {[Re,Cl,]*-}* with
,'
C O T 4-F*
Rhenium
156
electron acceptors such as TCNE provides a facile route to the powerful inorganic oxidant [Re2C1,]-.150In this context it should be noted that the [Re,C1,]2- anion has a quite well-defined electrochemistry. A one-electron reduction occurs close to - 0.85 V vs. SCE in acetonitrile, but approaches electrochemical reversibility only at high sweep rates.5 This reduction is chemically irreversible. Cyclic voltammetric measurements have shown2's'50that the [Re,C1,I2- /[Re,Cl,]couple occurs at + 1.24 V vs. SCE in MeCN and at + 1.20 V vs. SCE in CH, Cl,. The electrochemical properties of (But N), Re,Br, have not been as extensively studied, although acetonitrile solutions of this complex have been shown by cyclic voltammetry to possess irreversible reductions at - 0.70 V and - 1.32V vs. SCE.' Chemical oxidations and reductions of the [Re2XJ2- anions and their derivatives which give rise to dirhenium complexes possessing formal charges either greater or less than + 6 5 are discussed in those sections that survey the appropriate oxidation states, Le. [Re,Cl,]- is discussed with other Re'" compounds while the complexes Re,Cl,(PR,), (and Re,Cl,(PR,),, etc.) are included with Re" species. Few methods have yet been developed for preparing mixed octahalo anions, although the reaction of (ButN),Re,Cl, with calculated quantities of 48% hydrobromic acid has been used to prepare [Re,C1,Br,]2-, [Re,C1,Br51z- and [RezC1,Br,]2- .Is1 Also, the CC14 oxidation of Re,Br,(PEt,), produces the salt (Et,PCl),Re,Cl,Br,, along with a quantity of Re,C1,Br,(PEt,),s7 (see Section 43.4.2.1). The analogous reaction of Re,Cl,(PEt,), with CCl, gives (Et,PCl),Re,C1,,s7 an illustration of the relative ease with which complexes possessing the R 4 + and Re:' cores can be oxidized back to Re;'.'
43.5.2.2
Thiocyanate and selenocyanate
The reactions of (Bu;N),Re,Cl, with NaSCN and KSeCN in non-aqueous media give dark red (Bu4nN), Re,(NCS), and purple (BuYN), Re,(NCSe),, re~pectively.~ The spectroscopic characterizations of these two complexes accord with them possessing the type of structure depicted by (35)with N-bound NCS and NCSe ligands.' In the case of [Re,(NCS),]'-, cation exchange reactions can be used to give other salts. Polarographic and cyclic voltammetric measurements, in MeCN and CH,C1, respectively, have shown that (But N), Re,(NCS), possesses two accessible one-electron reductions close to - 0.1 V and - 0.75 V vs. SCE.s,94 In the reaction between (ButN),Re,Cl, and NaSCN, acidified MeOH is the solvent which is used in the preparation of the aforementioned (But N), Re, (NCS),, whereas the use of acetone produces red-brown (Bu:N),Re(NCS), and dark green (Bu;N), Re,(NCS),, (36), both of which are oxidation The X-ray crystal structure of paramagnetic (36)shows that products of (But N), Re, (NCS),.1sz*1s3 this symmetric dirhenium complex possesses an edge-shared bioctahedral structure with an Re-Re bond (2.613(1)A). The structural details imply that the oxidation state description is (+ 3.5, 3.5) rather than (+ 4, + 3). Note that in the reaction of (ButN),Re,(NCS), with phosphines, the Re -Re bond is cleaved to a similar bioctahedral structure [Re, (pNCS),(NCS),(PR,),]2- (see Section 43.5.1.4) which, however, does not possess an Re-Re bond, Le. the latter complexes are magnetically dilute (+ 3, + 3) species.
+
43.5.2.3 N-Heterocycles and amidine ligands
A poorly characterized, insoluble, gray-black 2,2'-bipyridine complex formulated as [ReC13(bipy).3/2H,Ojn (the value of n is unknown) has been obtained from the reaction of (Bu:N),Re,Cl, with bipy in n-butanol. It is paramagnetic & = 1.36 BM), but its spectral properties give few clues as to its structure.lo8 (PhC(NPh),H), N,N'-diThe reactions of (ButN),Re, CI, with A',"-diphenylbenzamidine
Rhenium
157
methylbenzamidine (PhC(NMe),H) and N,N'-diphenylacetamidine (MeC(NPh),H) have been found to produce the amidinato-bridged species Re,[(PhN),CPh],Cl,, Re,[(PhN),CPh],Cl,.THF, Re,[(PhN),CMe], Cl, and Re,[(MeN),CPh],Cl,.CCI,, all of which have been characterized by X-ray crystallography. The first three complexes have a trans disposition of amidinate ligands as shown in the case of Re,[(PhN),CPh],Cl, (37), whereas Re,[(MeN),CPh],Cl,*CCl, has the closely related tetracarboxylato-type structure (Le. Re,(O,CR),Cl,, see Section 43.5.2.5) with two weakly bound axial chloride ligands. The THF solvate is of special interest since the THF molecule occupies one of the empty coordination sites colinear with the Re-Re bond and the rotational conformation is twisted 6" from the perfect eclipsed one found in the ~ a r e n tThe . ~ Re-Re distances in these complexes fall in the narrow range 2.18 to 2.21 A . ' 9 7 ' 5 5
43.5.2.4
Phosphines and arsines
The reaction of the [Re,C1,]2- and [Re,Br, 1'- anions with monodentate tertiary phosphines (e.g. PEt,, PPG, PBu;, PEt,Ph, PEtPh, and PPh,) in methanol gives the green substitution products Re,X,(PR,),. The X-ray crystal structure of Re,Cl,(PEt,), (38) has confirmed the close structural relationship of these complexes to the parent anion^.^^^'^^"^^ The related reaction of [Re2X,IZ-with AsPh, gives Re,X,(AsPh,),. The phosphine complexes have been found to undergo one-electron reductions to give the monoanions [RezX,(PR3),]- possessing Re-Re bonds of order 3.5 (see Section 43.4.2.1). The oxidation of the triply bonded dirhenium(I1) complex Re,Cl,(PMe,Ph), using (NO)PF, has been shown to give the salts [Re,CL(PMe,Ph),]PF, and [Re,Cl,(PMe,Ph),](PF,), (Section 43.4.2. 1).6" The latter diamagnetic quadruply bonded dicationic complex is isoelectronic with the neutral derivatives R Q X ~ ( P R ~ )ItZreacts . with C1- to give Re,Cl,(PMe,Ph), and can be cleaved by CO to give the mononuclear rhenium(1) carbonyl complex ReCI(CO), (PMe2Ph)Z.65g69 P
CI
(38) P = PEt,
While the reactions of [Re2Xs12- (X = C1, Br or I) with dppe or arphos give either the triply bonded dirhenium(II) complexes Re,X4(LL)2(LL = dppe or arphos) (see Section 43.4.2.2) or the dimeric halogen-bridged magnetically dilute rhenium(II1) complexes Re,(pX),X,(LL), (see Section 43.5.1.6), the complex Re,Cl,(dppm), (39) can be prepared by the reaction of (Bu~N),Re,Cl, with bis(dipheny1phosphino)methane in acetonitrile or d i c h l o r ~ r n e t h a n e .It~ ~can * ~also ~ be generated by the electrochemical oxidation of Re,Cl,(dppm), or Re,Cl,(dppm), in the presence of chloride ion (see Section 43.4.2.2). The X-ray crystal structure of purple crystalline (39) confirms the structure proposed previously on the basis of ESCA Furthermore, the structural details, including the Re-Re bond distance of 2.616(1)&74 are in accord with the structure that had been predicted on the basis of this compound possessing a rhenium-rhenium double bond (a22628*2configuration)."' The properties of (39) are very similar to those of the complexes RezC1,(Ph,Ppy),76 and Re, Cl, (dppa), (dppa = bi~(dipheny1phosphino)amine)'~ that have been prepared by the reactions of Re2C1,(PBu;), and (Bu;N),RezCls with Ph2Ppy and dppa, respectively. Accordingly, these three complexes are believed to have similar structures and to be members of the class of complexes that contain Re-Re double bonds. Other such derivatives are the alkoxide complexes Re,Cl,(OR)(dppm), (40) (R = Me, Et, Pr" or Bun) that are formed preferentially over
Rhenium
158 Re,Cl,(dppm), when (Bu:N),Re,Cl, solvent .7d
is reacted with dppm in the appropriate refluxing ROH
h
(39) P
P = dppin
(40)
Electrochemical studies on solutions of (39) and (40) in 0.2 M (Bu,N)PF,-CH,Cl, show that these complexes exhibit a well-defined and rich redox behavior. In the case of (39) four couples are observed in the potential range 1.8 to - 1.4V vs. SCE; two correspond to one-electron oxidations and two to one-electron reduction^.'^
+
43.5.2.5
Suua fes and aIkyl and aryl carboxylates
The sulfato-bridged complex Na, Re,(SO,),- 8H, 0 has been prepared by the reaction of (Bu;N),Re,Cl, with sulfate anion and may be reconverted to the chloro anion by reaction with refluxing hydrochloric acid in the presence of (BuiN)CI. The structure shows the presence of weakly coor= 2.28 A).159 dinated axial water molecules (r(Re-Re) = 2.214(1) A, r(Re--0),2, 0-0
f41) 0-0
= O&R; X = C1. Br. or I
Dirhenium(TI1) carboxylates of the type Re,(O,CR),-,X,+, ( x = 0, 1 or 2) are known for a variety of carboxylate ligands and for X = C1, Br or I. They constitute derivatives of [Re2X,I2- that are formed by the stepwise replacement of up to three pairs of syn halide ligands by bridging carboxylate ligands. They possess electronic structures that are closely related to those of the [Re,X, 1'- anions. Their syntheses, structures and chemical reactivities have been fairly extensively reviewed for the period up to late 1981.5"54An example is shown in (41). The conversion of the [Re,X,]'- anions (X = C1, Br or I) into the orange-brown alkyl carboxylate complexes of the type Re, (O,CR),X, is most conveniently accomplished by reaction with refluxing mixtures of the appropriate acid and anhydride (if a ~ a i l a b l e ) . ~In ' +the ' ~ ~reactions between [Re,X,]'and XCH,CO,H (X = C1 or Br) both carboxylate coordination and halide exchange can occur, e.g. [Re,C1,J2- + CH,BrCO,H -,Re,(0,CCH,Br),Br,.160 Rather than starting with salts of the [Re2Br,J2- and [Re,I,]'- anions, it is actually more convenient to react the chloro complexes Re,(O,CR),Cl, with liquid HBr or HI (equation In the presence of an excess of HX and the appropriate (Bu:N)X salt, this latter reaction may be used to regenerate ( B u ~ N ) , R ~ , X ,The .~~ corresponding aryl carboxylate complexes Re, (0,CAr),X, are best prepared from the acetates through exchange reactions (equation 11). While several other procedures have been used to prepare Re,(O,CR),X, complexes (X = C1 or Br) (e.g. using Re,Cl, or tran~-ReOX,(PPh,),~),none are as convenient or as generally applicable as the methods that employ [RezX8]'-. Re2(02CR),C1,
+
2HX
-
Re2(02CR),X,
+
2HC1
t 10)
(X = Br or I)
+ 4ArC02H
Re2(02CCH3),X2
Re2(02CAr),X,
+
4CH,COzH
(1 1)
159
Rhenium
X-Ray structural studies on several complexes of the type Re,(O,CR),X, (41) (R = alkyl or aryl; X = C1 or Br) have established the now familiar multiply bonded carboxylate-bridged structure containing a very short Re-Re distance (- 2.24 A).53145 This structural unit is also present in [Re,(02CR)4](Re0,)2,'61a 'mixed-valent' complex that contains weakly coordinated axial perrhenate anions and which is formed, along with Re2(O,CR),C1, and [Re,(O,CR),Cl,](ReO,) (vide ~ perrhenate-containing infra), from the reaction of Re, C1, with refluxing carboxylic a ~ i d s ; "the species arise when oxygen is present. The existence and stability of [Re,(O, CR),](ReO,), (the derivative with R = Pr" has been characterized by X-ray crystallography)I6' point out the lability of the axial halide ligands of Re,(O,CR),X,, a result that accords with the formation of Re,(O,CR),(NCS), and [Re,(O2CR),(H,0),]SO, upon the reaction of Re,(O,CR),Cl, (R = Pr") with AgSCN and Ag,S04, re~pectively.~ Electrochemical measurements of Re,(O,CR),X, (R = an alkyl or aryl group and X = C1, Br or I) have revealed that CH,C1, solutions of these complexes are fairly easily reduced to the corresponding monoanions [Re, (0,CR),X,]-. The voltammetric half-wave potentials fall in the range - 0.13 to - 0.42 V w. SCE, and show a dependence upon the nature of the halogen (becoming more negative in the order I < Br < C1) and upon the Taft o* parameter for R.@The reduced anions can be prepared chemically using cobaltocene as the reducing agent to give [(q5-C5H5),Co][Re,(0,CR),X,].67 ESR spectral measurements and other characterization studies show that these species possess the $7r44626*1 electronic configuration and, therefore, an Re-Re bond order of 3.5. An especially important set of reactions of Re,(O,CMe),Cl, occurs with gaseous HX (X = C1, Br or I). When run at temperatures between 300 and 350"C, these reactions afford the trinuclear rhenium(II1) halides Re,X, in close to quantitative yield. This is the one procedure that can conveniently be used to prepare all three halides.]" Complexes of stoichometry Re,(O,CR),X, and Re2(02CR),X3represent intermediate degrees of substitution between [Re,X,]2- and ReZ(O,CR),X,. Of these two sets of derivatives, the former are the more commonly encountered, the dark blue hydrated acetates Re,(0,CMe),X4*2H,0 (X = C1 or Br) being prepared either from [Re,X,I2- or by the high pressure hydrogen reduction of KReO, or K,ReX6 in a mixture of HX and acetic a ~ i d . ' , ~These ~ ~ , 'hydrated ~ species react with ligands (L) such as py, DMF, DMSO, dimethylacetamide and Ph,PO to form other adducts of the type Re,(0,CMe),X,L,.5~'65In those instances where the X-ray crystal structures of such molecules have been dete~mined,~,'~' there is a cis arrangement of bridging acetate groups (42). This is also the case for the formate complex (NH4)2[Re, (O,CH), CI,] and its diphenylformamide (DPF) derivative Re,(O, CH),CI,(DPF), where a cisoid arrangement of carboxylate groups is present.' A close similarity in the spectral properties (IR and electronic absorption) of this group of acetate complexesI6' supports the notion that they are structurally very similar. Several mixed halide derivatives of the type Re,(O,CMe),Cl,-,Br,L, (L = DMF, DMSO or dimethylacetamide) were obtained as by-products during the autoclave synthesis of eis-Re,(0,CMe)2 Br4Lz.Apparently, the presence of chloride in these products arose from impurities adsorbed on the wall of the autoclave from preceding experiments that involved an HCI-containing reaction mixture.'66
x' (42) 0-0
I
x =
x'
I
x
02CR; X
=
CI, Br, or I
While thermal studies show that the axial ligands L of Re,(O,CMe),X,L, can be lost upon heating, it is clear that this can be a complex process, although in the case of L = H,O, this gives These same anhydrous compounds rise to the anhydrous form of the acetates Re, (0,CMe)2X4.164*167 have also been formed through the thermal decomposition of Re, (0,CMe),X, and the decomposition of the pivalate complex Re,(O,CCMe,).,Cl, in vacuo gives green crystalline Re, (0,CCMe,),Cl, or pink Re, (0,CCMe,),Cl, (vide infra) depending upon the temperature.I6' X-Ray structural studies on anhydrous Re,(O,CMe),X, (X = C1 or Br), Re,(O,CCMe,),CL, and Re,(O,Cthereby Ph),I, have shown that a trans arrangement of carboxylate groups is present (43),5*168,'69 demonstrating that a loss of axial ligands is accompanied by a cis -,trans isomerization. A few examples of the 3:3 carboxy1ate:halide complexes Rez(02CR)3X3have been obtained. One such example is Re,(O,CCMe,),Cl, which is formed during the thermal decomposition of
160
Rhenium
Re2(O,CCMe3),C1, in vacuo at 240°C.168Violet Re2(0,CMe)3Cl, has been prepared by dissolving Re,(O,CMe),Cl,, in cold glacial acetic acid, while the analogous bromide is formed upon treating a chloroform solution of Re2(0,CMe),Br, with the stoichiometric quantity of acetic acid.','" T h e structure of Re,(O,CCMe,),Cl, represents a situation intermediate between that of (41) and (43), with [Re,(02CCMe,),C1,]+ units linked by bridging C1- ligands into chains. A similar structure exists for Re,(O,CH),Cl,, while in the case of [Rez(0,CR),CI,]Re04, a complex formed upon refluxing Re, C1, with carboxylic acids in the presence of o ~ y g e n , ' ~perrhenate ~ , ~ ' ~ substitutes for an axial halide as shown by an X-ray crystal structure for the derivative where R = Pf (44). 0 0
(44) 0-0
=
OzCR
A variety of dirhenium(II1) carboxylates of the types Re,(O,CMe), Br,, truns-Re,(O,CMe),X, (X = C1 or Br), cis-Re,(O,CMe),X,L, (L = py, DMSO, etc.), Re,(O,CH),Cl,L, (L = DMF or DMSO) and (NH,),[Re2(0,CH),C1,] have been studied by 35C1or Br NQR spectroscopy and the results compared with data for salts of the [Re2C1,]'- and [Re,Br,j2- anion^.'^' Also of interest are some screening studies that were carried out on the use of [Re2(0,CEt),]S04 and several derivatives of the type Re,(02CR),X4*2H,0 as antiturnour agents.'72 Mixed alkyl-carboxylato complexes of dirhenium(II1) have been prepared starting from either Re,(O,CMe),Cl, or [Re,Me,l2-. The chemistry of these multiply bonded complexes (e.g. Re,(O,CMe),& and Re,(O,CMe),Me,) is discussed elsewhere (see refs. 5 and 6).
'*
43.5.2.6
S@ir ligands
The complexes Re,X,(tmtu), and Re2X,(DTH), (X = C1 or Br; tmtu = tetramethylthiourea; DTH = 2,5-dithiahexane) are formed upon reacting (Bu;N),Re,X, with these ligands under mild reaction conditions. These complexes are believed, on the basis of spectroscopic data, to possess structures akin to that of Re,Cl,(PEt,), (38),although in the case of the DTH derivatives this means that this potentially bidentate ligand may be bonding in a monodentate fashion.'
43.5.2.7
Mixed donor atom ligands
Only two such complexes will concern us here. One of them, the doubly bonded dirhenium(I1) complex Re, Cl,(Ph,Ppy), which contains the mixed P-N bidentate 2-(dipheny1phosphino)pyridine ligand, has been described in Section 43.5.2.4. The other complex is Re2(hp),C1, (Hhp = 2hydroxypyridine), which is prepared by the reaction of (BuiN), Re,Cl, with the molten ligand and has a structure ver similar to that of Re,(O,CR),X, (41), but with an even shorter Re-Re bond distance (2.206(2) versus 2.24
1
43.5.3
Trinuclear Complexes Containing the Re;+ Core
The trinuclear rhenium(II1) halides Re,X, (X = C1, Br or I), and the mixed halides Re,X,-,Y, double bonds", and are the precursors to an extensive class of coordination complexes that are formed by the addition of up to three ligands to the available (e.g. Re,Cl,Br,), contain Re-Re
I Rhenium
161
coordination sites on these cluster halides, i.e. Re,X,L,, where n = 1, 2 or 3 (e).’ Many such complexes exist for X = C1 or Br,’ but for Re,I,, only complexes with L = alkyl or aryl isocyanide ligand are known. Note that the reaction of Re,I, with ligands such as tertiary amines or tertiary phosphines leads to the decomposition of the cl~ster.’~’ The synthetic and structural chemistry of Re3Xghas been reviewed and this source’ can be usefully consulted for coverage of the literature up to late 1981. Note that in addition to the ability of Re,X, to form coordination complexes involving minimum perturbation of the cluster structure, other reactions of note are the reductive cleavage of Re,& (X = C1, Br or I) by isocyanide ligands to give [Re(CNR),]+ (see Section 43.3. 1)22and by phosphine ligands to give Re,&(PR,), and ReZX,(PR3), (see Section 43.4.2.1).56Also, heterocyclic tertiary amines (L) can reduce Re,X, (X = C1 or Br) to give trinuclear clusters of the type [Re,X,L,], (Section 43.4.3). The coordination chemistry that has been developed for trirhenium(II1) cluster species containing Re-C (alkyl) bonds (Le. -CH, and -CH,SiMe,) is discussed elsewhere (see refs. 5 and 6).
43.5.3.1
Cyanide, aiiiyl isocyanides and aryi isucyanides
The only cyanide-containing complexes known are (Et,N), [Re, Cl,(CN),] and (Et4N),[Re,C1,(CN),] which can be isolated by the reaction between (Et4N)CN and Re,Cl,. In these reactions, no more than six of the Re-Cl bonds of the parent Re3C1, cluster can be replaced by cyanide.”, The Re-Cl bonds that remain intact involve the Re-C1-Re bridging units of (45).
(45) X = CI or Br
While the earlier literature on isocyanide ligands has been reviewed recently: subsequent studies have demonstrated the generality of the reactions that occur at room temperature between Re,X, (X = C1, Br or I) and alkyl or aryl isocyanides that lead to Re,X,(CNR), (R = Me, CHMe,, CMe,, C,H,, or p-toly1).22The spectroscopic properties of these complexes show that the Re,& unit is intact. The electrochemical properties of solutions of the chloride and bromide complexes in 0.2 M (Bu,“N)PF,-CH2Cl,show a well-behaved one-electron oxidation close to f 1.6V YS. SCE and an irreversible reduction at -0.15V.22 Note that when the reactions between ResX, and RNC are carried out in refluxing acetone and alcohol solvents, reductive cleavage of these clusters occurs to give [Re(CNR),]+ (Section 43.3.1).
-
43.5.3.2 Nitrogen ligands The reactions between Re3X, (X = C1 or Br) and the more basic heterocyclic tertiary amines (L) that result in the reduction of the cluster and the formation of [Re,X,L,], (Section 43.4.3) are known to proceed via the intermediacy of Re,XgL,, since purple Re,Cl,py, can first be isolated by careful control of the reaction conditions.” The less basic amines such as pyrazine, 2,&dimethylpyrazine and 3-chloropyridine give unreduced Re,X,L,. Similar 1:3 adducts have also been prepared by the reactions of Re3C1, with acetonitrile and ben~onitrile.~ that ammonolysis occurs, and that the A careful study of the Re,Cl,-NH, (1) system has purple material that remains upon evaporation of the resulting reaction solution is a mixture of Re,Cl,(NH,), (NH3)3and NH,Cl. This work has permitted a meaningful reassessment of earlier studies dealing with the reaction of Re,X, with ammonia, and with primary and secondary amines, and their reinterpretation in terms of solvolysis of the Re-X bonds.5 Azide and thiocyanate react with Re,Cl, in a similar fashion to that described for cyanide (Section 43.5.3.1) to give salts of the [Re,C1,X3l3-, [Re3C1,X,I3-, [Re,C1,X,l3- and [Re,C1,X,l2- anions (X = N, or NCS).’76*’78 There is also evidence that [Re,(NCS),,I3- can be formed from Re3X, (X = CI or Br) under forcing reaction conditions, further reflecting the fact that the order of Re-C1 bond lability is Cl, % Cl,.’78 ‘
I62 43.5.3.3
Rhenium Phosphiiies and arsines
The preparation and structural characterization of Re, CI, (PEt,Ph), was an important early advance in the development of the chemistry of the halides Re,X,.' An X-ray crystal structure determination established that it possessed structure (45) (L = PEt,Ph) with short Re-Re bonds ~ ~ mon(2.49 A) but long Re-P distances (2.70 A), the latter arising from steric ~ r 0 w d i n g . lOther odentate phosphines (PMePh,, PEtPh, and PPh,) and AsPh, from crystalline purple-red tris adducts with Re,Cl, and Re3Br, that bear a close structural relationship to Re,Cl,(PEt,Ph),.' When the reactions between phosphines PR,, PR,Ph or PRPh, (R = alkyl) and Re,X, (X = C1, Br or I) are carried out in refluxing acetone or alcohol solvents, the lower valent dirhenium complexes Re,X,(PR,),, Re,X,(PR,Ph), and Re,X,(PRPh,), are p r o d ~ c e d . ~ ~ The ~ ~coordination ~~"' of several phosphole ligands to Re,Cl, has also been investigated. Ethanol solutions of 3-methyl-1-phenylphosphole (mPP) and 3,4-dimethyl-1-phenylphosphole(dPP) react immediately to form diamagnetic Re,CI,L, (L = mPP and dPP). With bulky phospholes like 1,2,5-triphenylphosphole(TPP), complex formation requires reflux conditions and gives the less stable solvated complexes Re, C1, L,-CH,C1,.180 In contrast to the well-defined adducts with monodentate phosphines and arsines, the reactions of Re,C19 and Re,Br, with polydentate ligands give products that with few exceptions are not well characterized. An exception is Re,Cl,(dppe),., which is formed upon mixing acetone or THF solutions of Re, C1, and dppe. Spectroscopic evidence favors it being an authentic Re, C1, derivative, containing intermolecular dppe bridges."' There is also evidence that small quantities of mononuclear [ReCI, (dppe),]Cl are formed in this reaction."' The corresponding reaction between Re,Cl, and o-phenylenebisdimethylarsine (diars) in acidified (HC1) ethanol gives red insoluble Re, Ch(diars), together with small quantities of [ReCl,(d~ar~),]Cl.~~~ In the case of tri-, tetra- and hexadentate and phosphine and arsine ligands, the true nature of the resulting products is poorly ~ n d e r s t o o d . ~A * ' summary *~ of the reactions that have been carried out together with the proposed stoichiometries of the products is given in Table 7.
43.5.3.4
Oxygtk ligands
Most complexes containing oxygen donors are of the type Re,X,L, (X= Cl or Br).' These have been prepared with Ph,PO, Ph,AsO, DMF, HMPA and sulfoxide ligands and they possess properties that dearly demonstrate their close structural relationship to Re,Cl,(PEt,Ph), ( 4 9 ' The ligand 1,2,5-triphenylphospholeoxide reacts with Re, C1, in refluxing dichloromethane to give the solvate With the bidentate ligand acetylacetone (acacH), both in-plane coordination Re, Cl,L,.CH,Cl,. and solvolysis of one of the out-of-plane terminal Re-X bonds per rhenium atom occurs to give R e ,C l ,( a c a ~ ) , . The ~ ~ ~ rather novel cluster complex Re,Br,(AsO,),(DMSO), is apparently formed upon reacting an acetone solution of Re,Br, with silver arsenate, followed by extraction of the resulting solid into dimethyl s ~ l f o x i d e .The ~ ' ~ structure, on the basis of spectroscopic measurements, has been suggested to involve the capping of the faces of the Re, triangle by two tridentate arsenate ligands. This is a further illustration of the lability of the terminal Re-X bonds in the Re,& clusters. A recent and quite original approach to the synthesis of Re,& cluster complexes involves the cocondensation of rhenium atoms, generated from a positive hearth electron-gun furnace, and reactive halocarbons such as 1,Zdibromo- (or dichloro-) ethane. Extraction of the reaction matrix at room temperature with THF gives Re,X,(THF), (X = C1 or Br) in yields of ~ 9 0 % . This '~~ strategy will likely see further developments in the future. Table 7 Products from the Reactions of Re,C1, with Tri-, Tetra- and Hem-dentate Phosphine and Arsine Ligands
Ligand (abbreviation)
Product (s)
(Ph,PCH,),CMe P3) (PhZPCH, CH,)zPPh (Pf-Pf-Pf) (P~,AsCH,CH,)~PP (Asf-Pf-Asf) ~ (Ph, PCH, CH, ) J (P(PD3) Ph,P(CH,),P(P~j(CHz)zP(Ph)(CHz)zPPh, (Pf-Pf-Pf-Pf)
Re3Cl, (P3) Re3C1,[(Pf)312 Re, Cl, (Asf-Pf-Asf), Re,Cl,p(Pf),],, {ReCl,[P(Pf),]]," {ReCMPf),l},"
(Ph2PCH,CH2)2PCH,CH2P(CH,CH2PPh,j2 ReC12p,(Pf),1'" P2(P041 Product i s bclieved
10
be a mononuclear rhenium(II1) derivative.
Comments
Red crystals; THF as solvent Reaction solvent dependent Brown solid; MeCN as solvent Reaction solvent dependent Reaction solvent dependent 2-Methoxyethanolas solvent
Ref:
5 5 183 5
5 5
I
Rhenium
163
The nitrate-containing salt Cs,Re,Cl,(NO,), has been prepared by the addition of CsNO, to a solution of Re,C1, in ice-cold nitric acid. This reaction is accompanied by the oxidation of some of the rhenium and the formation of Cs, ReCl,.'8' There also exist several stable derivatives of Re,X, that contain coordinated water molecules. This situation arises with certain complex halides of Re,X, (Section 43.5.3.6) and in the case of the diethyl sulfide complex Re3C1,(Et2S),(H20) (Section 43.5.3.5). The aforementioned sections should be consulted for further reference to these complexes.
43.5.3.5
Suljur and selenium ligands
Few such complexes are known and once again they are of the type Re,X,L, (45). The complexes with l,Cthioxane, Re3Cl,(C4HBOSj,,and diethyl sulfide, Re,Cl,(Et,S),(H,O), have been described, the former possessing S-bound 1, 4- t hi o ~ a ne . ' ~In ' ~ acetone '*~ solution Re,Cl, reacts with 2,5-dithiahexane (DTH) to give polymeric Re, Cl,(DTH),.,, possessing, it is believed, bridging DTH ligands that are in the trans rotational conformation."' Under more forcing reaction conditions, the polymer structure breaks down and up to three monodentate DTH ligands may coordinate to the Re, C1, cluster. Both the chloride and bromide clusters Re,X, react with sodium diethyldithiocarbamate (Nadtc) to form Re,X,(dtc),, species that are very similar to the corresponding acac derivative Re,Cl,(acac), (Section 43.5.3.4). The remaining terminal Re-Cl bonds of the chloride complex Re,Cl,(dtc), can be replaced upon reaction with silver thiocyanate to yield Re,Cl,(NCS),(dtc),. ''* The only derivative of Re,X, that contains a selenium ligand is one of stoichiometry Re,Cl,L,-CH,Cl, that is formed by the reaction of Re,Cl, with 1,2,5-triphenylphospholeselenide in refluxing dichloromethane.'86
43.5.3.6
Halides
Salts containing the complex halides [Re3XgtJ- (X = C1 or Br; n = 1,2 or 3) constitute the most important group of coordination complexes of the trinuclear halides of rhenium(II1). The key example is Cs, Re, Cl,,, because its structural characterization by X-ray crystallography first demonstrated the trinuclear structure of Re,X, and its derivatives (see structure 451, and its identification played a decisive role in opening up the field of multiple metal-metal bond ~hemistry.~ The cesium salt is prepared by mixing Re,Cl, with CsCl in concentrated hydrochloric acid.I8' This simple synthetic procedure can be effectively used to prepare nearly all such complex halides of Re3C1, and Re, Br,.5 Salts have been prepared with both alkali metal cations and various organic cations (e.g. pyH+ and P ~ , A s + )The . ~ success relies in large part upon the quite remarkable stability of such solutions to oxidation to mononuclear rheniumw) species. The [Re,Cl,J- anion and species like it bear a close structural relationship to Re,Cl,; each of the intermolecular Re-C1 bridges of Re3Cl, are cleaved and replaced by a terminally bound chloride ligand.'89,'90The Re-Re distance in Cs,Re,Cl,, is 2.477(3)A. The specific salt of an [Re3X9+J- anion that can be isolated depends primarily upon the choice of cation and of X, although in a few instances crystals have been grown under conditions sufficiently different that different anions can be stabilized by the same cation, e.g. Cs, Re, Br,, and Crystal structure determinations on the salts (Ph4As),Re3Cl11 (H,O), Cs,Re,Br,, Cs,Re,Br,, and Cs,Re,Br,, have confirmed their identity, the first complex containing a coordinated water m~lecule.~ These studies have further demonstrated that in cases where one of the rhenium atoms is deficient (i.e. CsZRe,Br,,),the structure distorts in such a way that the two Re-Re bond lengths that are associated with the deficient Re atom are shorter (2.43 A) than the remaining Re-Re bond (2.49 A).'91 An important radiochemical exchange experiment using H36Clhas shown that for [Re3ClI2l3-, the order of substitutional lability is C1, (in-plane) > Cl,(out-of-plane) & Cl,, a conclusion that is further supported by chloride-pseudohalide exchange reactions using N;, CN- and NCS- (see Sections 43.5.3.1 and 43.5.3.2). In addition to these lability studies, there are other reactions that have revealed that the Re,X, clusters are susceptible to oxidative disruption under certain conditions. Thus it has been found that while the quinolinium salt (quinH),Re,Br,, can be isolated upon treating a saturated solution of Re,Br, in 48% hydrobromic acid with an excess of quinoline, the salt (quinH),Re,Br,, can also be isolated from this reaction medium.'92 Use of pyridine or Et4N+ in place of quinoline gives the pyH+ or E t N + salts of [Re,Br,,12-^. An X-ray crystal structure determination of (quinHj,Re,Br,, shows that it should be formulated as (quinHj,[ReBr,].53178
164
Rhenium
[Re, Br,(H20),].193The closely related green-brown complex ( B U ' P C ~ ~ ) ~ [ R ~ C ~ , has ] [ Rbeen ~~C~~] prepared by a different route, namely, the reaction between ReCl,, PC1, and Bu'Cl in CS,. Its structural formulation is based upon IR, electronic absorption and mass spectral studies.'94 When solutions of Re,Br, in hydrobromic acid are subject to air oxidation, salts of [ReBr,]'- and ~ ' a similar vein, the addition [ReOBr,]- can be isolated upon addition of the appropriate ~ a t i 0 n . l In of a hydrobromic acid solution of Re,Cl, to an aqueous solution of CsBr can lead to crystals of CS, Rec&, CS,ReBr,, CS~Re,C1,Brl, (believed to be CS,ReBr6'CS,Re,Cl,Br,) and CsRe,Cl,Br,(H,O), (46) provided one is patient in the work-up procedure.'" The X-ray crystal structure of complex (46) has revealed that the [Re,C1,I6+ core is intact and that water molecules occupy two of the three in-plane terminal coordination sites.'96These oxidations of the Re3C1, and Re3Br, units in hydrohalic acids presumably arise, at Ieast in part, from oxygen. However, another important reaction that can lead to Re" is the well-documented disproportionation of Re,X9 (equation 12) in the solid state at elevated temperatures or in molten halide media>1,'97 Br
Br
I
Br
BrlB ! Br
43.5.4
I
Hexmuclear Clusters of Rhenium(I1l)
The ternary sulfide phases M',[Re,S,]S,,,(S,),I, (MI = N a or K) have been prepared by the reaction of Re, KReO, or Res, with a large excess of Na,CO, or KzC03and sulfur (alternatively X-Ray crystal structure detera stream of H,S can be used) at a temperature of 750-800"C.198~'94 minations have confirmed the presence of the [Re&] core in these Re"'comp1exes (Re-Re distance 2.59-2.63 A) and the presence of intra- and inter-cluster sulfide bridges and bridging disulfide units. These red crystalline complexes have been found to transform to black crystals upon standing in air or various solutions with little loss of crystal integrity but with a change from insulating to semiconducting behavior.I9' The preparation of similar ternary sulfide phases using barium or strontium carbonates in place of sodium and potassium has led to the isolation of BazRe6SIIand Sr,Re,S,, when the preparations are carried out at z 1400°C in a stream of H,S.'(@These rhenium(1II) complexes are, like their aLkali metal analogues, diamagnetic and have a structure containing the {[Re6S8]S6/2}4- framework, thereby differing from Mi [Re,Cl,]S,,, (S2)zlTin the absence of bridging disulfide units. One of the stimuli for studying these complexes is their close relationship to the well-known Chevrel phases MMo,Y, (Y = S , Se or Te). Accordingly, a noteworthy recent development is the discovery of octahedral rhenium-selenide clusters. The phase Re6Se,C1, has been prepared by the high temperature reaction ('I 1300 K) of ReC1, with mixtures of Re and Se."' An X-ray crystal structure has shown the presence of the [Re,Se,] core possessing Re-Re distances of 2.63-2.67 A. The presence of two terminal Re-Cl bonds (trans to one another) inhibits interlayer linkages.20' The semiconductor-like rhenium telluride phase Re,Te, has been characterized by X-ray crystallography and shown to possess the usual octahedral Re, core (r(Re-Re) = 2.67-2.72 A) within the structure {[Re,Te,]Te,}Te, in which the [Re,Te,] clusters are linked via butterfly-like [Te(Te -Te-Te},] units.202 Mixed-metal clusters that contain rhenium, e.g. the phase Mol ,Re, Se,, can also be prepared by direct reaction of the elements. They contain the octahedral (Mo,Re), cluster.203 Preliminary X-ray structural data have been reported598for Re6~,-Se),(~,-Cl),Cl,. The phases Re,X,X',, (X= Se or Te; X = C1 or Br) react with various ligands L in DMSO to give materials of stoichiometry [Re,X,X',*L *DMSO], (L = Et3N, Me,NCH,CH,NMe, or b i ~ y ) . The ~ , ~ clusters M:[Re,S,]S,,,(S,),12 (MI, = Rb, or K,Rb,) have been synthesized by the reaction of alkali metal carbonates with Re in a stream of H,S at 800°C. The analogous selenides M:Re,Se,, have also been obtaindm
165
Rhenium 43.6 RHENTUM(1V) COMPLEXES
The most numerous complexes o€ this oxidation state are halide species of the types ReX,L,, [ReX,L]- and [ReX,]'-.l Also, in contrast to the lower oxidation states, Re" begins to display the bridges) that becomes so tendency for oxo-rhenium bonding (in this instance in Re-0-Re dominant in the chemistry of the higher oxidation states ReV,ReV'and Re"". At the same time, the propensity of rhenium to form multiple metal-metal bonds that was apparent in the chemistry of Re" and Re"' begins to diminish in the case of Re". One final general point concerns the fact that the Re" oxidation state, like that of Re", is apparently present in many of the complexes that contain multidentate mixed donor ligands which have been used for the spectrophotometric determination of rhenium. These complexes, which are usually generated by the SnC1, reduction of Re"' in hydrochloric acid, are for the most part of ill-defined structure and often of uncertain oxidation state, although the Re:ligand stoichiometry is usually known. Such poorly characterized systems, recent examples of which include rhenium complexes of N-(4-methoxyphenyl)-a-thiopicolinamide and various thiohydrazones,2°8'209 will not be discussed further.
43.6.1
43.6.1 .I
Mononuclear Complexes Cyanides and alkyl isocyanides
Salts of the [Re(CN),+,](n+z)- anions are unknown, and a report that K.,[ReO,(CN),] can be prepared by the reaction of K,ReCI, with an excess of aqueous KCN at room temperature ave, The in other hands, a mixture of K3[Re02(CN),], K,[Re(CN),] and K5[Re,(CN),(OH),]*2H,0.' reactions of KzReCl, with KSCN/KCN or KSeCN/KCN melts at 250 and 320°C respectively give the diamagnetic, red-brown, cubane-like clusters [Re,(p3-S),(CN),,]4- and ~e,(p,-Se),(CN),,]4-. When KOCN is used in place of KSCN then Reo, is f ~ r m e d . ' ~The - ~ 'sulfide ~ and selenide species have been isolated as their K' , Cs', (Ph,P)+ and (Ph,As)+ salts, and the X-ray crystal structures of (Ph4P),[Re,L,(CN),,].3H2O (L = S or Se) (47) determined.210These results contradict an earlier report that [Re(CN),j- can be prepared using this procedure. The I R and Raman spectra of the crystalline salts and their solutions accord quite nicely with the crystal structure data.''
B
The reaction between NaReO, and H2S in an excess of aqueous NaCN solution yields salts of the diamagnetic [Re,@,-S),(CN),I4- ian.I5 The blue-green Cs+, (Ph,P)+ and (Ph4As)+salts have been isolated and the X-ray crystal structure of (Ph4P)4[Re,(p2-S)2(CN)8]-6HI0 (48)shows it to be an edge-shared bioctahedron with Re-S-Re bridges."' The Re-Re distance of 2.60 A is substantially shorter than the comparable distance(s) (2.755 A) present in tetranuclear (47), but the extent of Re-Re bonding in these complexes remains unknown. i-
14-
N C
Nc
"I/,//,/
1
NC C
-
N
N
C
I
I Rhenium
I66
The methyl isocyanide complex (Bu:N)[ReCI,(CNMe)] is prepared in rather low yield in the reaction between (Bu:N),Re,Cl, and MeNC in ethanol.'12 It is a member of a relatively extensive class of complexes of the type [ReX,L]- (X = halide) (Section 43.6.1.9).
43.6.1.2
Ammonia and N-heterocycles
Complexes of Re" with ammonia and aliphatic amines are not well defined and the chemistry has been littte explored. ReC1, reacts with NH,Cl at 195°C to give a material formulated as [ReQ (NH,)] which at higher temperature (25OOC and above) decomposes first to [ReCl, (NH,)] and then to ReNCl and Re2NC1.2" The pyridine complex cis-ReC&(py), has been obtained by dissolving b-ReCl, in dry pyridine, a reaction which also gives (pyH),Re,Cl,.'" It has also been obtained, as has its bromide analogue, by the more traditional method of pyrolyzing (~yH),Rex,,'.~a procedure that has been used for the synthesis of other compounds of the type ReC14L,. The complexes ReX,(bipy) and ReX,(phen) (L = C1 or Br) can be prepared by the pyrolysis of (LH),ReX,(L = bipy or phen), the reaction of ReC1, or K2ReBr, with molten bipy, or the reaction of ReBr(CO),(bipy) with refluxing dry Br,.',214 A variation of the last of these procedures to include the reaction between ReCl(CO),(bipy) and Br, gives the mixed halide derivative ReClBr, (bipy)?', The brown or red py, bipy and phen complexes have electronic absorption spectra and magnetic properties that accord with their being magnetically dilute tk3 species, and low frequency IR spectra that show v(Re-X) patterns consistent with cis octahedral geometries. This contrasts with the properties of a black complex purported to be Re(OH)CI,(phen) that was prepared by the reaction of an aqueous solution of K,ReCl, with an ethanol solution of phen.*" A magnetic moment of 2.1 BM is not consistent with its formulation as a normal magnetically dilute Re" complex. However, the presence of a low-spin configuration is a possibility. Previous reports of the isolation of Rel,(py), from the interaction of ReI, with pyridine in acetone2could not be confirmed in a later reinvestigation of this system.'I6 Also, since ReI,(bipy) and ReI,(phen) are not formed in the analogous reactions involving 2,2-bipyridyl or 1,l0-phenanthroline,2I6 a report describing the preparation of ReI,(phen) by such a procedure' is at this time still open to question. However, the treatment of [ReO,(py),]I - H 2 0with freshly distilled HI has been reported to give ReI,(py),. The = 3.59 BM) supports it being a Re" derivative.25s magnetic moment of this complex GCR The mixed phosphine-pyridine complex ReCl,(py)(PPh,) is formed as orange crystals by the with boiling pyridine for ten minutes.'" Salts of the [ReCl,(py)]- and reaction of rr~ns-ReCl,(PPh,)~ pyrazine-bridged [(ReC1,),pyzl2- anions are also known and are discussed in Section 43.6.1.9.
43.6.1.3
Diazenido and imido ligands
While rhenium(II1) complexes containing diazenido ligands are quite numerous and well characterized (Section 43.5.1.3), those of rhenium(1V) are rare. The benzoyldiazenido complex ReCI,(N,COPh)(PPh,), (23)reacts with C1, in CC1, in two steps. The brown, crystalline, air stable Re" complex ReCl,(N=NCOPh)(PPh,), is formed first, and this on further treatment with C1, is converted to trans-ReCl,(PPh,), according to equations (13) and (14).*' This Re" complex exhibits a v(N=N) mode at 1580cm-' in its IR spectrum and has a magnetic moment of 2.85 BM. ReCl,(N,COPh)(PPh,)2
+ +C12
I--*
ReCl,(N,COPh)(PPh,),
ReCl,(N,COPh)(PPh,), f Clz -+ ReCl,(PPh,),
f
PhCOCl
+
(13)
N2
(14)
The one example of an imido complex of rhenium(1v is Re(NPh)CI,(PMe,), which has been prepared by the reduction of Re(NPh)Cl,(PMe,), in THF using one equivalent of Na/Hg in the presence of an excess of trimethylphosphine. This air sensitive, dark red crystalline complex is paramagnetic with a magnetic moment of 1.66 BM at room temperature, and is further characterized by a broad ESR signal centered at g = 1.74 for a frozen benzene solution.34These observations have been interpreted in terms of a low-spin Re" species with a ligand field of low symmetry.
43.6.1.4 Cyanate and thiocyanate
The brown hexacyanato complex (Ph,As),[Re(CNO),] has been reported to be the product of reaction KzReClswith KOCN in molten dimethylsulfone followed by dissolution of the residue in
Rhenium
167
~ " magnetic moment (4.1 BM) is water and its rapid precipitation by the addition of ( P ~ , A s ) C ~ .Its significantly higher than that found for other species of the type [Rex,]'- (e.g. X = thiocyanate) and is not readily explained. The interpretation of the IR spectrum of this salt, based upon a consideration of the v(CN), 4CO) and v(NC0) modes,21s in terms of 0-bound cyanate ligands should be treated with caution in the absence of definitive X-ray structure data. Also, the ease with which [Re,(p-S3)4(CN)12]4-is formed in a K,ReCl,/KSCN/KCN melt system by sulfide abstraction from the KSCN (Section 43.6.1. l), raises the question as to whether an analogous reaction course (i.e. 0 abstraction) might occur to some extent in a KOCN/dimethylsulfone melt system. Indeed, the reaction of a KCNO/KCN melt with K,ReCl, is known to give Reo,. The well-characterized and stable hexakis(isothiocyanato)rhenate(lV) anion is a key species (dong with [ReO(NCS),]'-) in the spectrophotometric determination of rhenium in solutions generated by the reduction of [Reo,]- with thiocyanate and tin(T1) ion in hydrochloric a~id."~~"" The dark red-brown salts Cs2Re(NCS), and (BU!N),Re(NCS), are best prepared by the reaction of K,ReC1, with molten KSCN, followed by cation exchange in aqueous CsC1, and of (Bu,"N),Re,Cl, with NaSCN in acetone, re~pectively.'~~*'~' The addition of thallous acetate to an aqueous solution of Cs,Re(NCS),,22' and the mixing of a methanol solution of (Ph,As)Cl with a THF solution containing [Re(NCS),]2-,152have been used to prepare TI, Re(NCS), and (Ph&s),Re(NCS),, respectively. The Sn]' reduction of [Reo,]- and SCN- in hydrochloric acid produces red solutions from which the salts ( P ~ , A SRe(NCS), )~ and (Ph,As), ReO(NCS), have both been These results throw into doubt the recent notion that [ReO(SCN),I2- is present in such The [Re(NCS)J- anion has been thoroughly characterized spectroscopically,1'2~220and magnetic moment determinations on the Cs+,T1+, (By"N)+ and (Ph4As)+salts show them to be well-behaved octahedral Re" species; the room temperature magnetic moments are 3.66,3.36, 3.50 and 3.38 BM, respectively.'s2,22' Electrochemical measurements have shown that the [Re(NCS),]'- anion exhibits an irreversible one-electron oxidation above + 1 .O V vs. SCE (+ 1.23V in MeCN and -I- 1.03 V in CH,Cl,) and a reversible reduction at -0.2V vs. SCE (-0.11 V in MeCN and -0.28V in CH,C1,).94,95The one-electron reduction is both electrochemically and chemically reversible as demonstrated by the isolation of ( B u ~ NRe(NCS), )~ upon using N,H,*H,O as reducing agent (Section 43.5.1.4).', A report' describing the isolation of the complex ReO(SCN),(py), through the addition of pyridine to the solution generated by the SnC1, reduction of KReO, has not been confirmed. Such a complex would be most unusual for it would contain the hitherto unknown terminal [Re=O]'+ moiety.
-
43.6.1.5
Nitriks and amidates
Like other higher valent halides of the transition metals, ReCl, is reduced by acetonitrile to give the rhenium(1V) complex ReC14(NCMe),.98Other nitrile complexes ReCl,(NCR), (R = Pr" or Ph) can be prepared by this same route or by ligand exchange from ReCl,(NCMe),. Later work has shown2,, that this synthetic procedure for ReCl,(NCMe), gives predominantly the crystalline green cis isomer together with a small amount of the brown-green trans form. A detailed comparison has been made of the spectroscopic properties (IR, electronic absorption and ESCA) of cis- and truns-ReCl,(NCMe),. 224 This complex can also be prepared by the dissolution of /3-ReCl, in boiling acetonitrile.'" The oxidation of acetonitrile solutions offuc-Re(CO),X(NCMe), (X = C1 or Br) with Cl, or Br, gives pale green cis-ReCl,(NCMe), and brown cis-ReBr,(NCM:),, respectively in yields exceeding 50% .225 This method provides a convenient alternative and, furthermore, is applicable to the bromide complex. The acetonitrile complex [Et,N)[ReCl,(NCMe)] is also known (Section 43.6.1.9). The reactivity of cis-ReCl,(NCR), has been studied in considerable detail.98 Reduction to [ReCl,(NCMe),]- takes place quite readily (Section 43.5.1.5), while displacement of the MeCN ligands can be accomplished using APh, (A = P, As or Sb) (Section 43.6.1.6). The complexes ReCl,(NCR), react rapidly in the cold with primary aromatic amines (ArNH,) to give the stable, orange amidato complexes ReCl,[NH=C(R)(NHAr)J, (Ar = p-tol, m-C,H4F and p-C6H,(CO2Et) for R = Me; Ar = p-C,H,(OMe) for R = Ph). These behave as typical Re'" species (e.g. pefl= 3.5 BM) and are believed to possess a trans octahedral structure (49). The corresponding reactions of ReCl,(NCMe), with methanol and ethanol give the green crystalline complexes ReCl,[NH=C(Me)OR], (M = Me or Et) (50); in the case of the methanol reaction, Cs,[Re(OMe)Cl,] (Section 43.6.1.9) can be isolated from the reaction filtrate upon addition of CsCl.
Rhenium
168
(SO)
(49)
43.6.1.6
R
= Meor Et
Phosphines, arsines and stibimes
The complexes rrans-ReC1, L, that are formed with monodentate phosphine, arsine and stibine ligands constitute a very stable and easily prepared class of compounds. This is especially true in the case of zruns-ReC14(PPh,), which can be obtained by what seems to be a myriad of procedures. This is illustrated in Table 8 wherein are listed a selection of the more important methods that are known to produce complexes of the type trans-ReX,(AR,), (X = Cl, Br or I; A = P, As or Sb). Note that in the reaction between cis-ReC&(NCMe),and PPh,, a short reaction time followed by a rapid There are numerous other examples quenching of the reaction gives C~S-R~C~~(NCM~)(PP~,).'~~ where such complexes, especially the dark red-scarlet phosphine derivatives, are formed as reaction by-products; e.g. the formation of trans-ReCI,(PR,), in the carbonylation of Re,C&(PR,), (PR, = PPrf or PMe2Ph).69.234 Several mixed halide complexes of the types ReX3Y(PPh3), and ReX,Yz(PPh,)z are known, and can be prepared either by the reactions of hydrido-rhenium species with ~ ~ 2 2 7 . 2 3or 5 the halocarbon oxidation of ReX,(NCMe)(PPh,), (Scheme 11).86Salts of the [ReX,(PR,)J- anions are also known (Section 43.6.1.9). The vibrational spectrum (a single IR active v(Re-X) mode),' magnetic properties (pea = 3 . 4 3 . 7 BM),'" and an X-ray crystal structure determination on trans-ReCI,(PMe,Ph),,'" all Table 8 Preparative Methods for truns-ReX,(AR,)2 (A = P, As or Sb)
X C1, Br
AR,
Synthetic procedure ReOX,(PPh,), + boiling RC0,H + HCl ReX,(NCMe)(PPh,), -thalocarbons ReCI, PPh, in dry Me,CO ReOCl,(PPh,), boiling PhCOCl ReH,(acac)(PPh,), or ReHX(acac)(PPh,), HX in benzene ReI, + PPh, in dry Me,CO ReH,(PPh,), HX ReH,(PPh,), chlorocarbons (CCl,, CHCl,, CH, Cl, Me, CCI and C, H, C1) Pyrolysis of [LH],ReX,
PPh,
+
I
c1
PPh, PPh, PPh,
C1, Br
PELPh,-,
CI CI, Br
PR,Ph
C1, Br
c1 CI
a
+ +
ReCl,L, CCl, or Cl, ReX,L, X, ReH,(PEt,Ph), + HCl
+
ReCI, (NCMe)(PPh3)2
cBr*
HCI
ReH4I(PPh,),
benzene
-
ReC1, Br(PPh3 )2
c ReC1, I(PPh3 )z
H, and hydrocarbons as by -produ'cts n = 1, 2 or 3 [La+ = [ph3-,EtnPH]+ R = Me, Et, P f or Bun
No evidence for chlorohydride intermediates A = P, As or Sb A = As or Sb
ReCl.,(NCMe), + APh, ReC1, (NCMe)(APh,), CCl,
I
Re$ 217 86 226 138 227
+
+
PEt,Ph APhj
+
+
PPh,, PEt,Ph APh,
Comments
Rex3Y (PPh3)z
Scheme 11 Mixed halide complexes of the types ReX,Y(PPh,), and ReX,Y,(PPh3)2
216 127 144 228-23 1 102 228 232
98, 104 98
169
Rhenium
serve to confirm the magnetically dilute trans-octahedral structure of this class of complexes. Electrochemical measurements on acetonitrile and &chloromethane solutions of several trunsReCl,(PR,), complexes show that a reversible one-electron reduction to the monoanion [trunsReCl,(PR,),]- is very accessible (E1,,in the range - 0.1 to - 0.5 V vs. SCE).6s,106 A suggestion some years ago that the reaction of [Reo,]- with PEt,Ph and hydrochloric acid in boiling ethanol yields the well-characterized cis and truns isomers of ReOCl,(PEt,Ph), together with has apparently been confirmed by an the violet rhenium(1V) complex tr~ns-Re(OH)Cl~(PEt,Ph),,'~~ X-ray structure determination of the latter complex.236The observed Re-0 bond length of 1.795(4) A and the observation of an IR-active NO-H) band at 2920 cm-' support this content i ~ n . *Some ' ~ degree of double bond character in the Re-O(H) bond may explain the low magnetic moment (1.92 BM),'" since this could impart a low-spin configuration (one unpaired electron) to this system. Complexes of the type ReX,(LL) containing bidentate phosphine and arsine ligands have been prepared by two general methods. The halocarbon oxidation of the rhenium(II1) complexes Re, (pX),X,(dppe), (X = C1 or Br) (Section 43.5.1.6) leads to the halocarbon solvates of cisReX,(dppe)."' The X-ray crystal structure of cis-ReC1, (dppe).3/4 CCl, has confirmed the nature of these products.237Orange-red crystalline cis-Re&(dppe) can also be prepared upon heating Re,(p-X)z)6(dppe)2 with acetic acid-acetic anhydride mixtures but, depending upon the reaction conditions, Contamination by Re, (0,CMe), X, or ReOCl, (dppeO,) can ensue."' Orange cisReCl,(dppe) has also been prepared by the C1, oxidation of ReH,(dppe)(PPh,), in benzene solution.',, A second general approach involves the reaction of cis-ReC1, (NCMe), with bistdiphenylphosphino)methane (dppm), bis( 1,2-diphenylphosphino)ethane(dppe) and 1-diphenylphosphino-2-diphenylarsinoethane ( a r p h ~ s ) but, ~ ~ ' surprisingly, the reaction products are very dependent upon the reaction conditions. These results233are summarized in Scheme 12, together with details of the route to cis-ReBr,(dppm), via the Br, oxidation of ReBr(CO),(dppm).*14 Complexes of the types cisReCl,(LL), trans-ReCl, (LL), or polymeric truns-ReCl,(LL) with LL in a t r m , bridging conformation, can be isolated depending upon the nature of LL, the reaction stoichiometry and the reaction conditions (solvent and temperature).',, These complexes have been thoroughly characterized on the basis of their far IR,electronic absorption and ESCA spectra, and also their magnetic properties.214.233 cyclic voltammetric measurements on CH, Cl, solutions of cis-ReCl,(LL) reveal the existence of a reversible one-electron reduction at - 0.05 V vs. SCE.65The differences in behavior observed with the dppm, dppe and arphos ligands can be rationalized in terms of truns-ReCl,(LL),
-
i n
c-ReBr(CO)3(dpprn)
ma:CHCI,
-ReBr4(dppm) red
C ~ S
crs-ReCI4(dppm) orange
dppm (1:l): MeCN, CH,CI,
or Ms,CO (reflux) dppm (1:Z): Me,CO (reflux)
Ir
dppe (1:l); MsCN (rdlux) or CH,Cl,
cis-ReC14(NCMe)2
.
c
( 0 "C)
i t
rrans-ReC14(dppm)z purple ci~-ReC14(dppe)orange
dppe (l:l);CH%Cl,(r.t.)
rrans-[ReC14(dppe)l in red
f arphos (1 :l):Me,CO (r.t.)
>
arphos (1:l): MsCN (re&)
Larpho8 (13); Ms,CO {reflux)
t r u n s -[ReC14(arphos)]
red
cis-R&14(arph0~) orange iii
t
p'
rrans-R&14(arphos)z purple
i, refluxing CHzCIz; ii, excess dppm, refluxing acetone; ii, refluxing CHClS or DMF; iv, excess arphos, refluxing acetone; v, cia- ReCL+(NCMe),, refluxing acetone.
Scheme 12
170
Rhenium
being the thermodynamically favored product for LL = dppm or arphos but not dppe. In the case of cis-ReCl,(dpprn), the strained chelate ring is unstable in the presence of excess dppm. Although a more stable five-membered ring is present in cis-ReCl,(arphos), the formation of a second Re -P bond provides the driving force for its conversion to trans-ReCh(arphos),. On the other hand, when CH2Cl,, CHCl, or DMF solutions of trans-ReCl,(LL), are kept at room temperature for many hours or heated for short periods, the orange complexes cis-ReCl,(LL) can be regenerated, provided that excess ligand is not present. The course of these reactions is clearly favored by an increase in entropy upon loss of one of the bidentate ligands. While attempts to prepare the complex ReCl,(dpae) (dpae = 1,2-bis(diphenylarsino)ethane) were unsuccessfu1,233complexes with other bidentate arsines have been isolated. ReX,(diars) {X C1 or Br; diars = o-phenylenebis(dimethylarsine)} has been prepared by the halogen oxidation of ReX(CO),(diars),’ while the complexes ReCl,(dtcb) and ReCl,(dhcp) (dtcb = 1,2-bis(dimethylarsino)3,3,4,4-tetrafluorocyclobut-l-ene and dhcp = 1,2-bis(dimethylarsino)-3,3,4,4,5,5-hexafluorocyclopentene) are formed as green solids from the reaction of ReC1, with dtcb and dhcp in carbon t e t r a c h l ~ r i d e ? ~The ” ~ dtcb ~ ~ complex is also isolable, admixed with yellow [ReC12(dtcb),]C10,, from the reaction of dtcb with solutions generated by the H3P02reduction of the KReO,/HCl/EtOH/ H 2 0system.23sThe room temperature magnetic moments of ReCl,(dtcb) and ReCl,(dhcp) are 2.97 and 2-55 BM,respectively, and are lower than normally found (- 3.4BM) for Re” c o m p l e ~ e s . ~ ~ Nonetheless, these are authentic mononuclear Re” species containing cis chelating arsine ligands as demonstrated by an X-ray crystal structure determination of R e C l , ( d t ~ b ) . ~ ~ ~
Oxygen and su&izr ligands
43.6.1.7
The chemistry of Re” with oxygen ligands is not especially well developed. The simple aqua anions [ReCl,(H,O)]- are known (Section 43.6.1.9),as are the hydroxo species [ReX,(OH)]’- and the methoxide [ReC1,(OMe)l2- (Section 43.6.1.9).Rhenium(IV) is also believed to be incorporated and [P2W,7061]10-with [ReCl,]’- under into certain heteropolyacids upon treating [SiW,, 039]8nitrogen. The [Rex, (OH)],- anions exist in equilibrium with the p-oxo-bridged dirhenium(1V) species [Re,OX,J- (Section 43.6.13).The latter constitute examples of a class of complex that possess a linear Re’v-O-RelV unit, and which may contain additional oxygen or sulfur ligands. These Complexes include Re,OX,(SEt,), and RezOX6(EtSCH2CHISEt)2 (X = C1 or Br),240 K2[Re20L,CI,] (HL = a dipeptide or tripeptide ligand such as glycylalanine, glycylleucine or glyclglycylglycine),”’ K2H2[Re20L,(SO,),] (HL = picolinic acid, nicotinic acid or isonicotinic as well as an acid), K2[Re, OL,(SO,),] (L = picolinamide, nicotinamide or isonic~tinamide),~~~ assortment of pox0 species containing OH- ligands, e.g. [Re,O(OH), (LL),],- (LL = oxalate, citrate, tartrate or gallate)’ and K2[Re,OL,(OH),(SO,),] (HL = picolinic acid or isonicotinic a~id),”~ and Re’” complexes that purportedly contain the [Re20,] moiety, e.g. H, [Re, 0,L4] (HL = penicillamine).,@ The identities of these complexes require confirmation since they are based mostly upon conductance, spectroscopic and magnetic data. In no instance has an X-ray crystal structure been carried in order to support the formulations of any of these systems. The complex trans-Re(OPh),(PMe,), (51) is one of the two well-characterized alkoxides of Re”. It is prepared in low yield by the reaction of ReOCl, with NaOPh in diethyl ether/THF followed by the addition of PMe,.,,’ The red crystalline complex (51) has been shown by X-ray crystallography to possess the trans structure that usually characterizes Rex, L, type complexes. The other complex is Cs,[ReCl,(OMe)] that is discussed in Section 43.6.1.9.
’
P
P (51) P = PMe,
An earlier report that the reaction of ReOCl,(OEt)(PPh,), with hot acetylacetonate produces dimeric Re,Cl,(acac),, via the intermediacy of ReCl(acac),PPh, (Section 43.5.1.7), was later shown to be incorrect; orange Re, Cl,(acac), is in reality t r a n ~ - R e C l ~ ( a c a cThis ) ~ + conclusion ~ has been confirmed by an X-ray crystal structure determination.247Alternative methods have been developed
$ 1 Rhenium
171
to prepare trans-ReC1, (acac),, viz. refluxing ReOCI,(acac)(PPh,) in acacH under NZ,or refluxing solutions of ReCl,(acac)(PPh,), {or ReC1, (acac)(PPh,)(OPPh,)) in acacH in air overnight. By work-up of the reaction filtrates, small amounts of red-brown cis-ReCl,(acac), can be isolated.246 The reactions between ReOX,(OMe)(PPh,), o( = C1, Br or I) and the P-diketones acacH or dbmH (dbmH = dibenzoylmethane) have been used to prepare ReX,(acac), (X = Br or I) and cisReCl,(dbm),. The acac complexes display characteristic mass spectra, exhibiting an abundant parent ion peak, and they have room temperature magnetic moments of 3.1-3.3 BM. Their IR spectra, dipole moments (in benzene) and X-ray powder patterns support the notion that the bromide complex consists primarily of the cis isomer (with a small amount of trans), while the iodide is the trans isomer.’46 In an extraction-photometric method for the determination of rhenium, a sulfito complex [(Me,CHCH,CH,),NH,],ReO(SO,)Cl,is supposedly formed when Relv is extracted from 3.2 M hydrochloric acid containing SO: ~- with diisoamylaminein ethyl acetate or CC4.248This formulation should be treated with some scepticism until it can be confirmed. Complexes of Re‘” with organic acids have been prepared by the reaction of K,ReCl, with the appropriate acid in a 1:3 or 1:2 molar ratio.249These products were claimed to contain the [Re(C20,),I2- or [ReC1,L,l2- (LH, = phthalic, glutaric or aspartic acid) anions but have not been characterized in much detail, although upon hydrolysis they apparently give Reo,. There is a report describing the isolation of the violet 1:l nitromethane complex ReCl,(MeNO,) following the dissoiution of ReC1, in MeNO,,’% but this complex remains otherwise uncharacterized and its existence unsubstantiated. By a similar procedure, namely reaction with ReCl,, the solvents THF, dioxane and 1,Coxathiane reduce this halide to give trans-ReCl,L, (L = THF, dioxane or oxathiane), the complex trans-ReCI,(C,H,OS), containing S-bound thi~xane.’~’ The related bromide complex trans-ReBr,(C,H,OS), can be prepared by reacting K,ReBr, with 1,4-0xathiane in HBr.2s’Upon heating trans-ReCl,(PPh,), in air to 200°C,the corresponding triphenylphosphine oxide complex is obtained. This shows a characteristic v(P=O) mode (at 1 l5Ocm-’) in its IR spectrum.229This behavior contrasts to some extent with the related thermal transformations of trans-ReCl,(PEtPh,), and trans-ReCl,(PEt, Ph), which give ReCl,[OP(OEt)Ph,], (via ReThe course of these Cl,(OPEtPh,),) and ReCl,[OPEt,-,(OEt),Ph], (n = 0, 1 or 2), respectively?’0s231 reactions can be followed by IR spectroscopy through monitoring of the v(P=O) and v(P-0 -Et) vibrations. The salts (Et,N)[ReCl,L] (L = DMSO and DMF) are known, as is the thiourea (tu) derivative (Et,N)[ReCI,(tu)] (see Section 43.6.1.9). Thioether (Et,S, EtSCH,CH,SEt and 1,Coxathiane) and thiol (penicillamine) sulfur ligand complexes of Re’’ have already been mentioned in this section, and constitute, along with some dithiocarbamate derivatives, the majority of the complexes of Re’” that are known to contain Re-S bonds. The reaction of ReCl(CO), with tetraethylthiuram disulfide Both brown [Re(S,CNEt,),][ReCL,(S,CNEt,)] gives several products of which two contain and red crystalline ReC1, (SZCNEt2)2 can be isolated from a benzene solution of the reactants. These complexes have room temperature magnetic moments of 3.7 and 2.9 BM, respectively. IR spectral measurements on ReCl,(S,CNEt,), show two v(Re-C1) modes (305s and 278 mcm-’) in accord with a cis octahedral stereochemistry. This complex can also be prepared through the reaction of trans-ReCl,(PPh,), with this same thiuram disulfide in benzene, while the related dibenzyl dithiocarbamate analogue ReCl,[S,CN(CH,Ph),j, is obtained by the interaction of ReCl(CO), with tetrabenzylthiuram disuifide.252The mixed sulfide-cyanide and selenide-cyanide species [Re,(p, X),(CN),,]4p (X = S or Se) and [Re,(p-S)2(CN)8]4- are described in Section 43.6.1.1.
-
43.6.1.8
The mixed oxide-halide anions [Re2 ( p - 0 ) X , J -
Salts of these anions (n = 4 or 3) constitute the best characterized of the Re” complexes that contain a linear Re-0-Re unit.’,’ Prepared by the reduction of KRe0,,253the nearly diamagnetic red-brown salt K4(Re,0C1,,] was shown by X-ray c stallography to possess a structure with a linear Re-0-Re unit (Re-0 distances = 1.865(2) ) which joins to two [ReOCt,] octahedra.,,, The question as to the nature of the blood-red product that is formed upon treating [Re20C1,,]4in dilute hydrochloric acid with H 20 , or other oxidants’,2s3has apparently been answered by an X-ray crystal structure determination on a product, Cs,Re,OCl,,, isolated from one such solution.’” Formally, this mixed-valent complex is derived by a one-electron oxidation of [Re,OCl,,l4but in view of its symmetrical structure with r(Re-0) = 1.832(3)a, it must be considered an Re (+3.5, +3.5) spe~ies.2’~ There is no explanation for the earlier reported magnetic moment of These 3.57 BM for this complex, the implication being that this value is probably in error.253,255
x
172
Rhenium
complexes contain the type of linear Re-0-Re unit that is believed to be present in the 'parent' oxide-chloride [ReOC12],.256Furthermore, these results provide the structural base from which to consider the nature of other derivatives that supposedly contain the Re1"-0-Re1" unit (Section 43.6.1.7). A report describing the preparation of oxyfluoro species purported to be M,Re,OF,, (M = K or Cs) has been published.60'They have been characterized on the basis of spectroscopic, magnetic and conductivity measurements.
43.6.1.9
Complex halides [ R c X , f - , [ReX,(OH)]'-, [ReCI,L ] - and related species
Stable and easily prepared hexahalorhenate(1V) species [Rex,]'- are known for X = F, CI, Br and I. The preferred method for making [ReX6I2-(X = Cl, Br or I) is by the reduction of [Reo,]in concentrated HX in the presence of a reducing agent such as I-, Sn" or hypophosphorous acid (which gives yields greater than go%).' [ReF6I2-can be prepared by reducing ReF, with alkali metal iodides in liquid SO2or by reacting molten KHF, with either a KReO,/KI mixture or KzReBr6.257 The remarkable stability of the [ReX,I2- anions is demonstrated by their formation as byproducts in numerous reactions. For example, treatment of [ReO2(py),]Br.2H2O with hot concentrated HBr produces ReOBr,(py), along with (pyH), Reaction of (Bu;N),Re2C1, with CO(g) in MeCN gives ReCl(CO), as the major product, but [Re(NCMe),(CO),],[ReC1,] is also formed.z9 When ReH,(PPh,), is reacted with allyl halide (C,H,X; X = C1, Br or I) in an ether solvent the salt (Ph,PC3H5)zReX,is formed in quite good yield.'" The addition of PCl, and BUT1 to a solution of rhenium(V) chloride in carbon disulfide, results in a product containing both RqC1, and [ReC1,I2-, (Bu'PC~,)~[R~,CI,] [ReClJ.'9* K2ReCl, is also formed as an 'intermediate' in the preparation of (Bu,"N),Re,Cl, in the method involving the reaction of PhCOCl with KReO,. K2ReC16does not react further due to its insolubility in the reaction solvent (PhCOCl), and reduces the yield of the desired product, however, use of (Bu;N)ReO, in place of KReO, allows the production of the [Re2C1,I2- to proceed with yields up to 95%.'38A final example is from a spectroscopic study done on the products of the H2 reduction of KReO, in HC1 solution. These products were determined to be [ReC16j2-and [Re2C1,I2-, both of which could be precipitated from the solution by addition of NH,C1.2m The mixed ligand species of the type [ReX,,X,-,]2- (n = 1-5; X = C1; X = Br or I) can be prepared by heating a mixture of K,ReX, and K2ReX', in a quartz tube at high temperature, and then separating the products by ion column chromatography. Apparently the cis and trans isomers for n = 2, 3 and 4 of the mixed Re-1-Cl complexes cannot be separated.261-2M Mixed Re-Br -C1 complexes have also been obtained by pouring concentrated solutions of K,ReC& and K, ReBr, dropwise into The mixed ligand complexes have been studied by electronic absorption261p262,264 and far IR spectroscopy,263as well as by X-ray d i f f r a ~ t i o n . ~ ~ ' , ' ~ ~ The structure of [ReX,I2- is expected from simple ligand field considerations to be a regular octahedron. Crystals of &ReF, have been studiedz6, by X-ray diffraction and found to be of the K'GeF, type. The Re-F distance is 1.953 A, and the [ReF,]'- octahedron is slightly compressed. K2ReCI, and K2ReBr, have regular octahedral structures of the K2PtC16type; all C1 and Br atoms are crystallographically equivalent. K2Re&,, however, is less symmetrical and has three different iodine environments.' Structural studies done on [ReX,I2- salts with cations other than K include recent neutron diffraction measurements on (NH,)2ReCl,.267 The spectroscopic and magnetic properties of [ReX,I2- anions have been thoroughly studied; the reviews by Rouschias' and Fergusson' should be consulted for a detailed discussion of these studies through 1972. Subsequent work in this area has expanded or refined previous measurements, and in many cases has been confirmatory or complimentary. Sharp-line absorbance, luminescence and Raman spectra have been measured for [ReF,]'- in pure and host crystal environments.268[ReC1,]2and [ReBr6]'- have been studied by high resolution IR2,' and sharp-line luminescence spectrosand the far IR region of M',ReCl, (MI = K, Rb, Cs, T1, NH,, etc.) has been examined a t very high pressures (up to 20 kbar).27' Accurate values of the Raman-active fundamentals v, (alg), v2 (e,) and v, ( t t ) of [ReCl,]'- have been reported along with calculations of the parallel and perpendicular bond polarizability deri~atives.2~~ These results have been discussed in terms of the pre-resonance Raman effect. The resonance Raman spectrum of (Et,N), ReI, has been measured at 80K; the ulg vibrational mode was found to be coupled to the electronic transition Ts (4A,g) -,Ts (2 T ~ ~ ) . ~ ~ ~ The hexahalorhenate(1V) anions have been the subjects of extensive electronic absorption spectra studies. Their spectra have been recorded in aqueous and non-aqueous solvents, in halide melts, in the solid state and doped into other crystal hosts (e.g. K2PtC1, or Cs,ZrCl,). The energy level
Rhenium
173
diagram for Re” in an octahedral crystal field has been assigned and crystal field parameters have been measured.’ More recently, the low temperature single crystal absorption spectra of [Rex,]’(X = F, C1 or Br) in a variety of Sn host crystals with trigonal symmetry have been recorded using polarized light.z74,275 Ligand field calculations were used to rationalize the type and degree of trigonal field splitting. The electronic transitions of [ReF,]’- in single crystals of Cs,ReF, and doped into Cs,GeF, have been measured at liquid He ternperat~res.’~~ Crystal field parameters and spin-orbit coupling information have been derived and the vibrational structure has been interpreted on the basis of K # 0 lattice effects and a small distortion in the pure Cs,ReF, crystal. K,ReCl, single crystals have also been studied at liquid He temperature^.'^^ The magnetic circular dichroism and absorption spectra of [ReCl,]’- and [ReBr,]’- in cubic hosts at liquid He temperatures have been measured and most of the bands have been assigned to allowed or parity forbidden charge transfer transition~.’~~ Earlier work had favored a d-d interpretation of these bands. Another study has analyzed the charge transfer (ligand-to-metal) spectra of a series of octahedral transition metal complexes including [Rex,]’- (X = C1, Br or I) using a simple crystal field Most of the recent investigations of the magnetic properties of [Rex,]’- have involved ammonium or substituted ammonium salts. K,ReCl, and K2ReBr, are ordered antiferromagnetics at 4.2 K.’ Antiferromagnetic behavior is also found for the (MeNH,)+ and (Me,NH,)+ salts of Additionally, the synthesis and properties of (NH&ReCI, have been [Rex,]’- (X = C1 or Br).280,28’ the subjects of a recent study,2” as have those of (RH),ReCl, (R = cyclohexylamine, PhNEt,, PhNMe, or 2,5-1~tidine).’~~ There have been several polarographic studies done on [ReCl,]’- and [ReBr,]’- in which a one-electron reduction has been The cathodic one-electron reduction of [Rex,]’is complicated by a secondary process involving the reduction of (X = C1, Br or I) to give e- + ReX,(OH)3- H + ) which is formed in solution by the [ReX,(H,O)]- jviz. [ReX,(H20)]- ihydrolysis of [Re&]’- .z86J87 Cyclic voltammetric measurements on acetonitrile solutions of (Bu:N),ReX, (X = C1 or Br) have shown that in addition to the one-electron reduction (El,’= - 1.18 and - 0.90V vs. SCE for X = C1 and Br, respectively), a reversible one-electron oxidation is also observed.95 In general, the [Rex,]’- anions are quite inert to substitution. Aquation of [ReC1,]2- is especially slow, but is catalyzed by Hg”, Tl’”, Cd” and For [ReBr,]’-, aquation is somewhat easier, [ReF,]’- is stable to hydrolysis and can be recrystallized from aqueous but is also catalyzed by Tl‘”. sol~tions.’~~ The hydrolysis and the kinetics of metal-ion-catalyzed aquation have been investigated for both [ReClJ- and [ReBr,]’- .2887289 The aqua anion [ReC1,(HzO)]-, which can undergo hydrolysis to [ReCl, (OH)]’-, has been structurally characterized by X-ray crystallography in the bis(dithiobisformamidinium) salt [(NH,),CSSC(NH,)2]2[ReCl,(H,O)]Cl,. 2H10.290 T h s green crystalline substance was obtained as one of several products in a study of the [ReO,]-/thiourea/HCI system. The Re-0 distance in the pseudooctahedral anion is quite short (2.076(10) A)?w An alternative means of preparing [ReCl,(H20)]- is through the reaction of the salt [(P~N)ReCI,],, which contains the polymeric chlorine~ ‘ of [(ReCl,)-], can be prepared by bridged [(ReC1,)-], anion, with a moist a t m ~ s p h e r e . ~Salts reacting ReCl, with an equimolar amount of (R4N)C1(R = Et or Pr”) in boiling CH,Cl, for several days. If ReCl, is reacted with Et4NC1in a 1:3 mole ratio, then (Et4N)2ReC1,is formed in almost quantitative yield. The air-stable mustard-colored salts [(R,N)ReCl,], react with many donors (MeCN, DMF, py, DMSO, PPh,, tu or H,O) to form the salts (R,N)ReCl,(L). Reaction with the bridging ligand pyrazine (pyz) gives [(&N)ReC15]2pyz.291This is the most general synthetic route for salts of [ReC15L]”,although other methods are known, such as the treatment of ReH,(PPh,), with HX to give (Ph,PH)[ReX,(PPh,)] (X = C1 or Br),’29and the reaction of (pyH),ReX, with pyridine at 190°Cwhich gives (pyH)[ReX5(py)](X = Cl or Br).292These Complexes possess spectroscopic (far TR and electronic absorption) and magnetic properties that are typical of magnetically dilute Re” specie^.'^' Salts of the [ReX5(OH)l2-anions (X = C1, Br or I), which are related to [ReX,(H,O)]- through the hydrolysis reaction given in equation (1 51, have been isolated in a few instances‘,’ and appear to have properties in accord with the expected pseudooctahedral structure. These anions are in equilibrium with the y o x o dirhenium(1V) species [RezOX,,]4- (Section 43.6.1.8). The cream-yellow colored methoxide derivative Cs,[ReCl, (OMe)] has been prepared from the reaction of cisReCl,(NCMe), with hot MeOH saturated with gaseous HCl.98
+
[ReX,(HzO)]-
e
[ReX,(OH)]2-
-t
H+
(15)
The complexes [ReCl, (PhC, Ph)], and ReCI, (FhC, Fh) have been prepared by the reaction of ReC1, with diphenylacetylene, and the former converted into the salt Ph,P[ReC15(PhCzPh)].602
Rhenium
I74
Some interesting radical cation salts of dibenzotetrathiafulvalene (DBTTF) and tetramethyltetrathiafulvalene (TMTTF) with inorganic anions, including [ReC1,]2-, have been prepared which possess metallic conductivity. These compounds, having the general formula (DBTTF),(MCl,), (M = Sn, Pt, Re or Te), have identical electrical properties, with their metal-insulator transition occurring in the range 120-1 50 K.293,294
43.6.1.10
Hydrides
Mononuclear hydride compelxes of Re" are very rare. The only class of such complexes that has been isolated in the solid state is ReHX,(acac)(PPh,), (X = Cl, Br or I).'', The iodide complex is apparently prepared by the reaction of ReH,(acac)(PPh,), with I,, whereas the chloride and bromide are formed upon dissolving this same complex in CHX,. Although the chloride and iodide were shown by magnetic susceptibility measurements to be paramagnetic, their moments are quite different. All three complexes display weak v(Re-H) modes in their IR The electrochemical oxidation of the yellow monohydride of rhenium(II1) [ReH(NCMe), (PPh3)2(py)](BF4)2 generates solutions that contain the violet colored tricationic species [ReH(NCMe), (PPh,),(py)]3+. Similar behavior characterizes the systems [ReH(NCMe)3(PPh,), (NH,C6HlI)](BF4)> and [ReH(NCMe),(PPh,),](BF,),, but in none of these cases have salts of the rhenium(1V) trications been isolated. 13' The most thoroughly studied hydrido complexes of rhenium(1V) are those of the type Re,@ H)4H4(PR3)4(PR, = PPh,, PEtPhj, PMePh,, PEtzPh or PMe,Ph) which can be prepared by thermolysis of ReH, (PR3)2r232 the photolysis of ReH,(PR,), or ReH, (PR,)3'2s3135 and/or the reaction of mixtures of phosphine and (Bu;N),Re,Cl, with NaBH, in methanol or ethanol.62,'" The triphenylarsine analogue Re,H,(AsPh,), has been prepared in low yield (12%) by reacting ReOCI,(AsPh,), with LiAlH, in THF at OcC.134 The chemistry of these complexes, which had originally been reported as 'agnohydrides' because of the uncertainty as to the number of hydride ligands present,?,' was not explored in any detail until their structure was established through a neutron diffraction analysis of Re2H,(PEt, Ph), (52).295The short Re-Re distance (2.538(4) A) may indicate some degree of multiple Re-Re bonding in these complexes but this is by no means certain. The terminal H,P, units and the bridging H, unit are in a mutually staggered arrangement.'" In the high field ' H NMR spectra of these complexes, the resonance due to the hydride ligands is seen as a symmetric quintet in the region 6 - 5.0 to - 8.1, with J(P-H) 8-9.5 Hz.125,232,295,296 For the arsine complex Re,H,(AsPh,),, the ' H NMR spectrum shows a hydride singlet at 6 - 15.66.'~~ These observations are in keeping with t h s class of molecules being fluxional at room temperature; details of variable temperature studies have not yet been reported, with the exception of a limited ' H NMR spectral study of Re,H,(PEt,Ph), which did not yield a frozen-out spectrum at low ternperat~res.'~' These structural studies have been followed by a consideration of the bonding and the conformational preferences in molecules of the type represented by (52).29'Also, it has been shown that the dinuclear core of Re,H,(PPh,), is preserved upon the reactions of this complex with allyl chloride and bromide which give a mixture of (Ph,PC,H,),Re,X, (X = C1 or Br), H, and propene. However, this dimetai unit is disrupted when reacted with allyl iodide since, in this instance, the mononuclear rhenium(N) complex (Ph, PC3H,),ReIs is formed.14 Electrochemical measurements by the cyclic voltammetric technique have shown that solutions of Re2H8(PEt,Ph,-,), (n = 0, 1 or 2) and Re,H,(AsPh,), in 0.2 M (Bu:N)PF,-CH2Cl, possess two one-electron oxidations, the first of which ( E l / ,= -0.16 to -0.40 vs. SCE) is reversible. Solutions of the paramagnetic ESR-active cations [Re2H,L,]+ (L = PEt,,Ph,-,, or AsPh,) can be generated by controlled potential electrolysis and, in the case of the PPh, derivative, the salt [Re,H,(PPh,),]PF, can be prepared using Ph3CfPF; (or C,Hj+PF6)as the oxidant in CH2Ci2.'"-296Interestingly, when a nitrile solvent RCN (R = Me, Et or Ph) is used in place of CH,Cl, in this reaction, hydride abstraction rather than oxidation takes place to give {Re,H, (PPh,),(NGR)]PF,. In these complexes the RCN ligands are easily substituted by ButNC to give [Re,H,(PPh,),(CNBu')JPF,,and this complex like its nitrile precursors can be oxidized reversibly in two one-electron steps. The most accessible one-electron oxidation (El!*= -0.03 to +0.19V vs. SCE) can be accomplished chemically using NOPF6.134 The paramagnetic, monocationic species [Re,H, (PPh3)J+ displays a tremendous enhancement in reactivity compared to its neutral parent Re2H,(PPh,),, as does [R~,H,(PP~,),(CNBU')]~+ compared to [RezH7(PPh3),(CNBut)]+.This aspect of the chemistry of these complexes is shown in Scheme 13, wherein the redox chemistry and substitution chemistry of the dirhenium polyhydrides towards Bu' NC are summarized.lM Another means of activating the complexes Re2& (PR,), (PR, = PMePh, or PMe,Ph) involves their interaction with [CU(NCMe),]PF, according to equation
-
Rhenium
175
(16).307These novel hexanuclear species display an unprecedented planar rhomboidal metal array consisting of two intact Re, units and two Cu+ cations. These clusters can be considered as ‘raft-like’ aggregates and therefore represent a geometric analogue of a metal surface.M7
(52) P
=
PE12Ph
RCN (R = Me, Et, Ph), ii, Ph3C+PF6-, CH2C12, 0 O C ; iii. NO+PF6-, acetone; iv, Zn,CH2C12; v, BdNC, CH2C12, room temperature; Vi, BdNC, CH2C12, 0 ‘C. P = PPh3 1, Ph&%-,
Scheme 13 Redox and subsitution chemistry of Re,H,(PPh,),
43.6.1.11 Mixed h n o r atom ligands Certain of these complexes have been described in Section 43.6.1 -7, viz. those with peptide ligands, pyridinecarboxylic acids and derivatives, and penicillamine. The most extensive studies have been carried out on Schiff base complexes in which the Schiff base ligands are either tetradentate and dianionic, or bidentate and monoanionic. The complexes of the former type, trans-ReCl,(N,O,), where N,O, represents the dianionic Schiff base ligands derived from sal,enH, = N,N’-ethylenebis(salicylideneimine), sal,propH, = N,N‘-trimethylenebis(salicylideneimine), sal, phenH, = N,N’-o-phenylenebis(salicy1ideneimine) and acac,enH, = N,N’-ethylenebis(acetylacetimine), are formed by refluxing rrans-ReCl,(PPh,), with one equivalent of the Schiff base and two equivalents of NEt, in dry toluene under nitrogen.298These complexes are paramagnetic and show a single v(Re-Cl) mode in the IR spectra in accord with the trans geometry (53). In the absence of NEt,, the preceding reactions of ReCl,(PPh,), with sal,enH, and sal,propH,, when carried out in dry THF, give ReCl,(sal,enH,) and ReCl,(sal,propH,). The complexes are believed to contain the neutral Schiff base ligand bound only through its N-donor atoms.298
I
c1
Rhenium
176
The complex truns-ReCl,(PPh,), is also the starting material for the synthesis of several complexes derived from the bidentate Schiff bases Me-salH = N-methylsalicylideneimine and PhIn benzene or toluene solvents the Schiff base (LH) or its salH = N-phenyl~alicylideneimine.~~ lithium salt reacts to give ReCl,(LH)(PPh,), ReCl,(LH),, ReCl,(L)(PPh,) or ReCl,(L),. The related PMe, Ph derivatives ReCl, (L)(PMe,Ph) can be obtained by refluxing cis-ReCl,(L)(PMe,Ph), in CCl, or by heating it in toluene under a stream of CO for a long time. The detailed stereochemistries of these complexes have not been established, although ReCl,(L), probably has a trans arrangement of Re-Cl bonds on the basis of a single v(Re-C1) mode in the IR spectra.207The analogous 8-hydroxyquinoline (oxineH) complexes ReC1, (oxine)(PPh,) and ReCl,(oxine), were prepared by Also, paramagnetic complexes procedures similar to those used for their Schiff base analog~es.2'~ of the type Re(OH)Cl(L), (LH = 2- or 8-hydroxyquinoline) have been described but they remain inadequately characterized. They are said to be formed when the appropriate (LH,), ReCl, salts are heated with excess of LH in acetic acid?15 There is a report that describes the isolation of several complexes of Re" (and of Rev) with thiosemicarbazide and its derivatives. These are obtained by the reactions of [ReCl,]'-, and the so-called a-ReC!,, with thiosemicarbazide and the thiosemicarbazones of acetone, quinolinealdehyde and 5-chlorosalicylaldehyde.~9g While the thiosemicarbazide complex is paramagnetic, in accord with its being an Re'" species, the structural formulation proposed for this complex (and for others of its kind) is based primarily upon IR spectroscopic data and should therefore be regarded as highly speculative. Neutral complexes ReX,(L),, where X = C1 or Br, and LH = 8-quinolinol or 8-quinolinethiol, are formed in the dehydrohalogenation ofthe corresponding (LH?),Rex, salts.603Several complexes of rhenium(1V) with pyridinecarboxylic acids have also been described.6MThese include ReCl,(CO,2-py), , ReCl, (3-pyC0, H), and ReCl, (4-pyC0, H),.
43.6.2
Dinuclear Complexes Containing a Rhenium-Rhenium Bond and Oxide and/or Halide Bridges
The nonahalodirhenate(1V) anions [Re,X,]- (X = C1 or Br) (54) possess Re-Re triple bonds (a2$ configuration) and can be prepared by halogen oxidation of (BuiN), Re,X,.300 The salts (Bui N)Re, Cl, (dark green) and (Bui N)Re,Br, (dark red) have k e n isolated and while they are quite stable in the solid state their solutions are easily reduced to produce either (Bu:N),Re,X, (an Re (+ 3.5, f 3.5) derivative) or ( B U , " N ) , R % XThe ~ . ~paramagnetic, ~ violet [Re,C1,J2- anion has also been prepared by the reactions of j-ReCl, with (PhAs)Cl in MeOH-canc. HC1 and with PPh, in anhydrous a c e t ~ n e . ~These - ' ~ ~ reactions lead to the salts (Ph, AS)^ Re,Cl, and (DOTP),Re, Cl,, respectively, where DOTP represents the 1 ,1-dimethyl-3-oxobutyltriphenylphosphoniumcation which is formed by the acid-catalyzed condensation of acetone followed by a Michael addition of triphenylphosphine. Various salts of [RezC1,]2- (with (DOTP)' , (Ph,PH)+, [(Ph,PO),Hj+, (Ph,As)+ and (Et,N)+) have also been prepared starting from ReCI,, but in several of these instances the yields are rather 10w.s*226'301 An X-ray crystal structure determination has been carried out on (54) for X = C1. The ESCA spectrum of the complex (Bu;N)Re,Cl, shows that the bridging C1 atoms have higher C12p binding energies (by 1.3 eV) than those in the terminal Re-C1 bonds.302The complexes (Bu,"N)Re,Cl, and (Bu: N),Re, C1, display identical reactivities towards tertiary phosphines, reactions with PPh,, PEtPh, and PEt, giving Re,Cl,(PPh,),, Re,Cl,(PEtPh,), and Re2CI,(PEt3),, respectively." ) ~ = C1 or Br) are reacted Amongst the products that can be isolatsd when trans-ReOX, ( P P I I ~(X with carboxylic acids in boiling toluene are dark green Re20X5(02CR)(PPh3)z (X ;= Cl or Br) and purple Re,0C13(0,CR),(PPh,)2.2'7 X-Ray crystal structure studies on Re20C1,(02CEt)(PPh3)2(55) and Re, OCl3(O2CEt),(PPh,), (56) have shown that these complexes possess short Re-Re bonds (2.51-2.52A) although it is unclear what these values reflect in terms of actual Re-Re bond order^.^'^,^^ Complexes of the types (55) and (56) represent intermediate stages of reduction (Re(+ 4) and Re(+ 3 3 , respectively) in the conversion of ReOCI,(PPh,), to Re,(O,CR),Cl,, the final product of these reactions. There is one example of a structurally characterized p-dioxo complex of rhenium(1V). Dissolution of Reo, in an oxalic acid-potassium oxalate mixture (Re:H,C,O,:K,C,O, = 1:3:1) gave, amongst . ~ ~ of these products were other things, three groups of crystals (dark-red, red and g r e e n - ~ d )Yields not reported but an X-ray structure determination on the green-red material showed it to be K,[Re,0,(C20,),].3H20, containing an Re@-O),Re unit and an Re-Re distance (2.36211) A) which is shorter than that in (55) and (56) and presumably reflects a considerable degree of Re
-
I Rhenium
177 R 0
0\ , / O
R (54) X = CI or Br
(55) P = PPh,
(56)P = PPh,
-Re multiple bond character. This result raises the important question as to whether some of the unit (Section 43.6.1.7) rhenium(1V) complexes that are believed to contain a linear Re-0-Re might not in reality have structures related to this oxalate derivative. The suggestion that the Re'" complex with 5,6-diphenyl-2,3-dihydro(asym)triazine-3-thione(HL), that has been used for the spectrophotometric determination of rhenium, might be Re2(p-O),L,, containing bidentate (N and S) coordination from L, is not without precedent. This complex is prepared by the usual SnCl, reduction of [Reo,]- in HCl in the presence of HL.,'' Other oxide ligand complexes that can be mentioned here are the ternary rare earth rhenates. These include a few phases that contain dirhenium(1V) centers, of which La.,Re,O,, and La,Re,O,, are of special note since the eclipsed [Re'",O,] units that are present can be considered to contain Re-Re bonds of order three.'-3o8The phase NH,ReO,~,F,*H,O has been shown to possess a structure that contains isolated [Re,O,,F,] units. This material is paramagnetic and is an ins~lator.~~
43.7 RHENIWMW) COMPLEXES
The most prevalent complexes of this oxidation state are those which contain the [Reo]'+, [Reo,]+ and [Re,0J4+ moieties, a reflection of the increasing stability of the rhenium-oxygen bond as the oxidation state increases. The tendency of Re" to form multiple bonds (as in [Re=0I3+ and [O=Re=O]+) is also seen in its behavior toward nitrogen, viz. the stabilization of the [Re-NI2+ (nitrido) and [Re=NRI3+ (nitrene) moieties. With few exceptions, complexes of Rev have been found to be essentially diamagnetic, i.e. to contain a spin-paired d2 configuration. Because of the dominance of the [Reo]", [Reo,]' and [Re20,]4" units in the chemistry of Rev, such complexes will be considered first in the chemistry of this oxidation state.
43.7.1 Mononuclear Complexes Containing the [Re0I3+Moiety 43.7.1.1
Cyanides and aryl isocyanides
The species [ReO(OH)(CN)4]2-is formed upon protonation of K3Re0,(CN4) and then eliminates water to give [Re,O,(CN), J4While other methods have been described for generating [ReO(OH)(CN),]2L, cyano derivatives containing the [Reo],' unit are rarely encountered. However, another example is the monoanionic species [ReO(H,O)(CN),F]-, which is believed to be formed upon the treatment of [ReO,(CN)J- with hot, 40% HF.," The reaction ofptolyl isocyanide (p-tolNC) with ReO(OEt)12(PPh,), is reported to give [ReO(OEt)I(PPh,),(CN-p-tol)]Ior ReO(OEt)I, (PPh,)(CN-p-tol) depending upon whether hot ethanol or acetone is used as the reaction solvent ,312
43.7.1.2 Amines and N-heterocycles
There are several reports in the older literature that describe the isolation of ReOCl,(py),, ReOC13(en) and ReOX3(bipy) (X = Cl or Br), often via the protonation of the corresponding dioxo-rhenium(V) species."' In more recent years interest in these particular molecules, none of which has been characterized crystallographically, has continued. The thermal decomposition of (NH4),ReOCl5is said to lead to (NH,)[ReOCI,(NH,)] and ReOC13(NH3)2,whereas the reaction of
178
Rhenium
(NH,),ReOCl, with various nitrogen donors is described as giving ReOCI,(L), (L = py, Et,NH, 1/2bipy or 1/2phen), (NH,)[ReOCl,(Et,NH)] and ReOCl, (DMF).313 Green ReOCl, (py), is formed upon reacting ReOCl, or ReOC1, (POCI,) with ~yridine.~’, ReOCl, (bipy) has been prepared by the reaction of bipy with ReOC1,,3’4 and as a brown solid through a similar reaction involving The former is ReOCl,(PPh,)(OPPh,),86 while green and violet isomers have also been de~cribed.~” isolated both by the H,PO, reduction of a mixture of HReO, and bipy in WCl/EtOH, and upon heating (bipyH),ReCl, in 6 M hydrochloric acid in a current of air; dark green ReOBr,(bipy) is prepared by a similar procedure. The violet form is produced, along with [ReO(OH)(bipy)J2+,when the latter reaction is carried out in more dilute hydrochloric acid (0.4 M). The violet form converts to green ReOCl,(bipy) when heated in 6 M HCl and has a much higher magnetic moment (2.1 vs. 0.4 BM).315This high magnetic moment raises the question as to whether Re’” is present, perhaps as an impurity. The protonation of the ethylenediamine complex [Reo, (en),]’, using the hydrohalic acids, has been described as a means of forming complexes that have been formulated ReO(OH)X,(en) These (green), [ReO(OH)(en),]X, (purple) (X = C1, Br or I) and [Re(OH),Cl,(en)]Cl Structural formulations are based largely upon the spectroscopic and magnetic properties of the products. In no instance has the result of an X-ray structure determination been reported, although a brief mention was made (in 1978) of the forthcoming publication of the structural details of [ReO(OH)(en),](ClO,),, for which the Re=O and Re=OH distances are 1.72 and 1.83& respect i ~ e l y . ~Crystallographic ” measurements may be necessary in order to distinguish the ReO(0H)X,(en) formulation from the alternative one of Re,O,X,(en),. These reactions are also complicated by the fact that in boiling concentrated HX, the complexes Cs, ReOCl,, (enH,)ReBr, and ReI,(en) are also f~rmed.’’~ While the protonation of [ReO,(py),]+ in cold moderately concentrated HBr and HI is said to give ReO(OH)X,(py), {X = Br (green) or I (light brown)), complexes which are analogous to ReO(OH)X,(en), more concentrated hot acid produces ReOBr, (py), and (pyH), ReEr, and ReI,(py),, respectively.258 The complex [ReO,(py),]Cl is the final product in the reaction of ReOCl,(PPh,), (or ReC1,) with wet pyridine. Its formation has been postulated to occur via the intermediacy of the three complexes shown in Scheme 14.3’8While the hydroxy species has not been structurally characterized, its ethoxide analogue ReO(OEt)Cl,(py), has been shown by X-ray crystallography to have a structure (57) with the E t 0 ligand trans to the terminal Re-0 bond.319Of special note is the shortness of the Re-0 bonds (1.896(6) and 1.684(7)& an observation that has led to the suggestion that the Re -0 bond orders lie somewhere between two and three for Re-O,, and between one and two for Re-OEt.3’9 The reaction of ReOCl,(py), with ethanol has been found to be a route to this same ethoxide
Scheme 14
0 II ,\,\’
P Y W
‘i“
PY
c1
Rhenium 43.7.3.3
179
Thiocyanate and other nitrogen donors
The most important thiocyanate complex of rhenium(V) is the oxopentakis(isothiocyanate) species [ReO(NCS),]'-. It has been implicated as one of the species that is present (along with [Re(NCS),I2- ) when [Reo,]- is reduced with tin(I1) ion in acidic thiocyanate solutions.220Several well-characterized salts (Et,N+, Bu,"N+,Ph,P+ and Ph,As+) have been prepared by ligand exchange reactions involving salts of the [ReOBr,(NCMe)]- , [ReOC1,I2- or [ReOCl,]- anions.220.320 The salt (Ph,As), ReO(NCS), has also been prepared by the tin(I1) chloride reduction of NaReO, in an acidic NH,NCS solution.220It has been suggested that the samples of the salts M1,ReO(NCS)5 (M' = Cs+, T1+ or Me,N+), that can be prepared by the reaction of Cs,ReOCI, with KSCN in hot water,,,' are contaminated with [Re(NCS),]' salts and possibly with other complexes as The [ReO(NCS),I2- salts display characteristic TR and electronic absorption spectral properties, e.g. a v(Re0) mode close to 960 cm-' in their TR spectra, but variations in the magnetic properties reported for the salt (Ph,As)' ReO(NCS), need to be ~ l a r i f i e d .Other ~ ~ ~ ,thiocyanate ~~~ complexes that have been reported include those formulated as being of the types ReO(SCN),(PR,)2 (PR, = PPh, or PEt,Ph), ReOCl(NCS), (PPh,), and ReO(OH)(NCS)?(PPh,),, some of which remain inadequately characterized especially with regard to the mode of thiocyanate binding.',' Also, a brown solid analyzing as (Et,N),ReOCl,(NCS), has been isolated from the reaction mixture that is obtained upon reacting KSCN with a compound purported to be ReC13(OEt)2.322 The acetonitrile-containing anion [ReOBr,(NCMe)]- is described in Section 43.7.1.6. The only known dialkylarnido compound is red, diamagnetic, crystalline ReOm(SiMe,),], (v(Re=O) at 978cm-'), It is a unique four-coordinate compound of rhenium(V), that can be prepared by the interaction of ReOCl, with LiNMe,.24s
43.7.1.4 Phosphines, phosphites, arsines and stibines Monodentate tertiary phosphines and arsines form a very large number of six-coordinate complexes of the types ReOX,(L),, ReOX,(L)(L)' and ReO(OR)X,(L),, where X = halide or NCS; L = a tertiary phosphine, arsine or stibine ligand; L = DMSO, Ph,PO or a phosphite ligand; and R = alkyl group.'., Of these, ReOCl,(PPh,), is by far the most important compound since it is commonly used as a starting material for the synthesis of many other rhenium complexes. The preparations of ReOX,(L), and ReO(OR)X,L, can be quite varied but usually utilize solutions of [Reo,]- in alcohol in the presence of an excess of the appropriate hydrohalic acid.')2 Alternative procedures include reactions of [ReOX,I2- (X = C1 or Br) with phosphines,323phosphine exchange reactions using ReOX3(PPh3), (X = C1 or Br)323,the thermal decomposition of (Ph,PH),ReOCl, and thiocyanate or which gives ReOCI,(PPh,), via the intermediacy of (Ph,PH)[ReOCl,(PPh,)]~24 Complexes of the type ReOX,(L), have been halide exchange reactions of ReOX, (PR,),.107.325 prepared for X = C1, Br, T or NCS, and phosphine and arsine ligands (L) such as PPh,, PEt,Ph, PEt,, AsPh,, AsMe,Ph and the stibine Ph, Sb.'.*Themixed halide complex ReOCl2I(PPh3),has also been described; its preparation involves the reaction of [ReOI,(PPh,),]+ with HC1.' In the case of the complexes ReO(OR)X, (L)', both alkyl and aryl alkoxy derivatives have been prepared.' Further details of these and other synthetic procedures are described by Rouchias.' However, note should be taken of the remarkable ease with which rhenium halides partake in 'oxygen abstraction' reactions to afford oxorhenium(V) species; the formation of ReOCl, (PPh,), in the reaction between PPh, and ReC1, or /I-ReCl, in acetone is an illustration of this tenden~y.'~~,''~ Assignment of the structures of the ReOX, L, complexes [trans (one form) or cis (two f o h s ) ] has until recently rested largely upon spectroscopic data (IR, NMR and electronic absorption) and ' ~ special ~ ~ ~ ~ ~note ~ ~ is ~ ~the ~ ~very ~ ~ characteristic v(Re0) mode that dipole moment r n e a s ~ r e r n e n t s . Of occurs as a sharp intense band in the vicinity of 9 7 0 ~ r n - ' . ' ~ Thus, ~ ~ " ~ReOCI,(PEt,Ph), has been shown to exist in cis and trans forms (blue and green re~pectively);"~ a third 'isomeric' violet form has been shown to be in actuality the rhenium(1V) complex trans-Re(OH)C1,(PEt,Ph),?36The X-ray crystal structure of trans-ReOC1, (PEt,Ph), (58) has confirmed this structural assignment, as is also the case for cis-ReOC1, (PEt,Ph), (59), although in this particular instance the structure determination is not yet ~ o m p l e t e . ~ ' Some ' , ~ ~ ~structural details of these complexes and related species are given in Table 9. The structural characterization of the alkoxide derivatives trans-ReO(OEt)CI,(PPh,), and transReO(OR)T,(PPh,), (R = Me or Et) show that these complexes resemble closely the structure of the pyridine complex mns-ReO(OEt)Cl,(py), (57) (Section 43.7.1.2), with a m n s O=Re-OR unit
c.0 c. u .
180
Rhenium
Rhenium
181
c1
P
(58) P = PEtZPh
(59) P = PEt2Ph
and a trans arrangement of Re-P bond^.^^^"^ This is, in turn, similar to (58) but with OR replacing the trans chloride ligand. There is some evidence that complexes of the type ReO(OR)X, (PPh,), isomerize quite rapidly in certain organic solvents."' Cationic species of the type [ReOX,(PPh,),]ReO, (X = Cl, Br or I) have been described as being produced when ReO,I(PPh,), is reacted with oxygen or ReO(OEt)Xz(PPh3), is treated with perrhenic acid in acetone, but their identity requires confirmation. Several well-characterized mixed ligand complexes of the type ReOX,(L)(L) are also known, where L represents a phosphine ligand and L' is DMSO, DMF, Ph,PO, PhEt,PO or one of several phosphite ligands. The mixed phosphine -phosphine oxide complex ReOC1, (PPh,)(OPPh,) is formed when a current of air is passed through a suspension of ReCl,(NCMe)(PPh,), in boiling benzene,86and probably has a structure similar to that of ReOX,(PEt,Ph)(OPEt,Ph) (X = C1 or Br) (Table 9) in which the phosphine oxide ligand is trans to the terminal Re=O bond.327The DMSO analogue ReOX,(PPh,)(DMSO) (X = C1 or Br) is formed by the interaction of DMSO with ReOX,(PPh,), or ReO(OEt)Cl,(PPh,), and concentrated hydrohalic acid.',' The DMF-containing complex ReOCl, (PPh,)(DMF) has also been prepared and possesses a structure where the 0-bound DMF ligand is trans to the terminal Re =O When trans-ReOX,(PPh,), is treated with P(OMe), and phosphites of the type P[(OCH,),CR] (R = Me or Et), the mixed phosphinephosphite complexes ReOX,(PPh,)(phosphite) are formed."' In the case of the P(OMe), complex ReOCI, (PPh,)[P(OMe),], two isomers can be isolated (blue and green); these are best distinguished on the basis of differences in their low frequency IR spectra (v(Re-C1) modes). Another type of mixed ligand complex is that which contains an anionic ligand, as exemplified by the acetylacetonate derivative ReOCl,(acac)(PPh,). This green complex, as well as its bromide and iodide analogues, can be prepared by the reaction of ReOX, (OEt)(PPh,), with acacH using very short reaction times."' ReOCl,(acac)(PPh,) can also be prepared by the interaction of ReOCl,(PPh,)(OPPh,) with acacH.86The related PEt, Ph complex ReOCl,(acac)(PEt,Ph) has also been prepared."' These diamagnetic complexes are characterized by a v(Re=O) band at 980 cm-' in their IR spectra, and a structure in which one of the oxygen atoms of the chelating acac ligand is trans to the terminal Re=O unit (Table 9)-332 Several complexes of [Re0l3' that contain bidentate and tridentate phosphine/arsine ligands have been prepared and characterized. Some of these, uiz. ReOC1, (dppe), ReOC1, (EtzPCHzCH,PEtz), ReOCl, (diars) and ReOX, (tas) (X = C1 or Br; tas = bis(o-diphenylarsinopheny1)phenylarsine) are described in the older literature which has been summarized by Fergusson? ReOC1, (dppe) and ReOC1,(Et,PCH,CH,PEt2) can be prepared by the reaction of the phosphine with NaReO, in a boiling alcohol-conc. HC1 solution.107More recent preparations of ReOX,(dppe) (X = C1, Br or I) have involved the reactions of [ReO,(dppe),]X with HX in ethanol (X = C1 or Br) and of ReO,I(PPh,)(dppe) with H1.330,333 The X-ray crysta1 structures of ReOX,(dppe) (X = C1 or Br) have been determined (Table 9).327The pale green dppm complex ReOCl,(dppm) has been obtained by the reaction of cis-ReCl.,(NCMe), with dppm in hot acetone for a period of 48 hours. This complex apparently forms by oxygen abstraction from the acetookwzwne The reaction between KReO,, (Ph2PCH,)3CMe(abbreviated P3) and H,PO, in ethanol-conc. HCI gives blue ReOCI,(P,) and upon prolonged reflux (24 hours) a small quantity of a green isomer. Although both complexes possess a v(Re=O) mode at N 985 cm-I, it bas been suggested that the green isomer is a seven-coordinate complex containing tridentate P, while the blue form is six-coordinate with bidentate coordination from the P, ligand."' The reaction of either the green or blue form with ethanol gives ReO(OEt)Cl, (P,), in which one of the phosphine groups is uncoordinated."'
-
43.7.1.5
Oxygen and suvur ligands
Mixed phosphineoxygen donor complexes of [Re0l3+that contain DMSO, DMF, acac, Ph3P0 and PhEt,PO are described in Section 43.7.1.4. Complexes of the type ReOX,(OPR,), (X = C1 or
182
Rhenium
Br; R 3 P 0 = Ph,PO, Ph,EtPO, Ph,HPO, Ph2P(0)CH2CH2P(O)Ph,or Ph,P(O)CH=CHP(O)Ph,) can be prepared by reaction of the ligand with solutions of K,ReOX, (X = C1 or Br) in acidified (HCl or HBr) acetone.323Alternatively, the thermal oxidation of the phosphine derivatives ReOX,(PR,), in air at temperatures up to 200°C can give these same phosphine oxide s p e ~ i e s . ~ ~ ~ , ~ ~ ~ The reaction between ReOCl,(AsPh,), and dioxane or 1,4-oxathiane leads to displacement of AsPh, by the latter ligands; the oxathiane complex presumably contains S-bound ligands.,,' In fact, a range of suifur donors are capable of stabilizing the ReVoxidation state containing the [Re0I3+ moiety. 1,2-Bis(ethylthio)ethane (EtSCH, CH, SEt) reacts with K,ReOCl, in hydrochloric acid to give a mixture of diamagnetic blue needles and green plates (v(Re=O) at 980 and 970cm-', respectively) that are believed to be ReOC1,(EtSCH2CH, SEt) and ReO(OH)Cl,(EtSCH2CH2SEt), respectively.334Several complexes of Rev with thiourea (tu) and N,N'-ethylenethiourea (entu) have been reported, but the structural formulations are based largely upon microanalytical data and IR spectroscopic and conductiometric measurements, e.g. ReOCl, (L),*nH, 0, [ReOC1(L),]C1-2H20, K[ReOC14(L)]-2H,0 (L = tu or entu). The formation of these complexes has been investigated by spectrophotometric and potentiometric methods through monitoring the reaction between While none of the complexes isolated in these studies [ReOCl,]*- and tu or entu in 6 M HC1.336,337 have been subjected to X-ray crystal structure analyses, closely related species have been prepared by reacting KReO,, with tu in concentrated hydrochloric acid. The structures of two of the resulting complexes, one obtained as blue crystals, the other being green, have been deterrr~ined.~~'.~~~ These complexes, [ReOCl,(tu),(H,O)]Cl and ReOCl,(tu)(H,O), both possess octahedral structures in which the water molecule is bound trans to the oxo 0 atom. The terminal Re-0 distances are 1.654(10) and 1 -713( 19) A, respectively, in the cationic and neutral complex, while the corresponding Re-OH, distances are 2.231(10) and 2.291(20) A. These results are of additional significance in view of the application of the ReV"-HC1-Sn"-thiourea system in the analytical chemistry of rhenium. However, while ReVand Re" thiourea complexes are said to be formed depending on the SnCl, concentration, attempts at the structural characterization of any tin-containing species have so far been f r ~ s t r a t e d . ~ ~ ' . ~ ~ ' Synthetic procedures for complexes of oxorhenium(V) containing thiol ligands are now well established and several of these derivatives are well characterized structurally. The reaction between (Ph,As)ReOBr, and PhSH in acetonitrile in the presence of the base NEt, has proved to be a convenient means of preparing (Ph,As)ReO(SPh), (60). The crystal structure of this complex as its acetonitrile solvate has shown that the anion possesses a square pyramidal geometry with an Re -0 distance of 1.686(9) An alternative route to this anion and others of the type [ReO(SAr),](Ar = Ph, p-tol, C,F, or C,Cl,) involves the reaction of ReOCl3(PPh,), with excess aryl thiolate anion in refluxing methanol.204These species, which have been isolated as their Ph4P+salts, show an intense v(Re-0) band in the range 99&950cm-' in their IR spectra.2" The selenol andogue (Ph, As)ReO(SePh), has also been prepared and ~haracterized.~~'
( 6 0 ) S = SPh
The analogous ethabedithiol derivative (Ph,As)ReO(SCH,CH,S), can be prepared by one of two procedures, namely, the reaction of NaReO, with ethanedithiol and NaBH, in THF, or the reaction of (Bu;N)ReOBr, with a solution of the ethanedithiolate in THF.j4, Similar procedures have been developed for the synthesis of A,[ReO(dtox),] (A = BuZN or Ph,MeAs; dtox = lY2-dithiooxalate), (Ph,As)ReO(mnt),(mnt = maleonitriledithiolate) and (BuiN)ReO(tdt), (tdt = 3,4-dimercaptotoluene) using the appropriate [ReOBrJ salt as the synthetic starting material.,,, The mnt derivative has also been prepared by the reaction of [ReC&I2- with Na,mnt and I,.344These orange-brown crystalline compounds exhibit the usual v(R-0) modes in the range 990-950 cm-', and possess low magnetic moments which are field strength dependent.,,, In all instances, they most likely possess a mononuclear square pyramidal geometry. This has been confirmed in the case of the Cs' salt of [ReO(dtox),]- by an X-ray crystal structure determinati~n.~~, A rhenium(V) complex containing dithiooxamide (dto) of stoichiometry ReOCl,(dto), is formed upon reacting dto with a solution containing [ReOC1,]2 in acetic a~id-HCl.,,~ The voltammetric behavior of [ReO(L-L),]- (L-L = SCH2CH2S,tma, dtox, rnnt or tdt) has been ~
Rh eniwn
183
examined at Hg and Pt electrodes. A reduction occurs at potentials more negative than - 0.9 V (vs. SCE) but oxidatively there appears to be little tendency to form ReV' c~mplexes."~ An especially interesting complex is the one derived from mercaptothioacetic acid (HSCH,COSH, tmaH), viz. (BuI;N)ReO(tma),. This has been prepared by the reaction of (BuiN)ReOBr, with commercial samples of thioglycolic acid in MeOH following adjustment of the pH to 7.5 with 10 M NaOH. The formation of [ReO(tma),]- reflects the presence of sizeable concentrations of mercaptothioacetic acid in the thioglycolic acid.,,, These observations are important since several studies have been made of the complexation of Re" (and Re"") by thioglycolic acid in acidic media, although the resulting complexes are poorly defined.347*348 The dialkyl dithiocarbamate complexes ReOC12(Etzdtc)(PPh,) and ReOX(R,dtc), (X = C1 or Br; R = Me, Et or +(CH2)5)are best prepared by the reaction of ReOX,(PPh,), in dry acetone with the appropriate tetraalkylthiuram disulfide, or, in the case of the Et,dtc derivatives, by treatment with thallium diethyldithiocarbamate.121The methoxide-containing complex ReO(OMe)(Et,dtc), is formed by the reaction of Re,O,(Et,dtc), with dry methanol. Like the other R,dtc complexes it displays a characteristic v(Re=O) mode (94Ocm-I) in its IR spectrum.349 Salts of the [ReO(SAr),]- anions (Ar = mesityl, 2,6-diisopropylphenyl or 2,4,6-triisopropylphenyl) have been isolated as their Et,PH+ and Ph,P+ salts.605 Attempts to recrystallize Re(SC6H3(pr' 123) (MeCN)(PPh312 from CHI C12 -EtOH gave the salt [Ph,PSC,H,(Pr'),]* [ReO(SC,H,(Pf),),]-. The X-ray crystal structure of this complex and (Ph,P)[ReO(SC,H, Me3),] have been determined.m5
43.7.1.6 Halides The oxo species that will be considered here are those of the types [Re0X,Iz-, [ReOX,] - and [ReOX,L]-, where L represents a monodentate donor and X = C1, Br and/or Salts containing the pseudooctahedral [ReOX,I2- anions (X= C1 or Br) are often easily prepared but can be contaminated with paramagnetic [ReC1,I2- and diamagnetic [Reo,]- due to the disproportionation of ReVto ReV" and Re" that can occur in aqueous media (equation 17).1,3'0The [Re0C1,l2- anion may be obtained by reducing [Reo,]- with iodide in concentrated hydrochloric acid, or by dissolving ReOCl, or ReCl, in concentrated hydrochloric acid and adding a large The bromide analogue [ReOBr5l2- is probably best prepared as its Cs' salt by the dissolution of ReC1, in 48% hydrobromic acid followed by the addition of CsBr, but can also be prepared from the ~ ~ ~exchange * ~ ~ can ~ also be used to prepare other reduction of ReOBr, in hydrobromic a ~ i d .Cation sa]tsv3Z4,3S4 Only a very brief mention has been made of the [ReOT,]2- anion and it is not well characterized.' 3Re"
+
2Re"'
+
ReV"
(17)
Magnetic studies on pure samples of MiReOX, salts have shown that these complexes are essentially diamagnetic, so that claims3,, of significant residual paramagnetism can most likely be attributable to [ReX,I2- con tarn in ant^.^^' Potassium, rubidium and cesium salts of [ReOX,I2(X = C1 or Br) are seemingly isostructural with those of other closely related transition metal species of the type [MOX,]*- (M = Cr, Mo, W, Tc).355,356 A careful study has been made of the electronic absorption spectral properties and vibrational spectra (IR and Raman) of Cs,ReOX, (X = C1 or Br), the latter dealing with the assignment of the Re=O and Re-X stretching and deformation modes.351,3~S Studies of the equilibria existing in solutions of [Re0C1,l2- in hydrochloric acid reveal that the Re-Cl bond trans to Re=O is labile and that substitution by water can occur to give [ReOCl,(HZ0)]-,357*358 which is a derivative of the [ReOCIJ- anion. The best procedure for the preparation of salts of [ReOXJ- (X = C1, Br or I) involves the reduction of [Reo,]- with zinc in concentrated sulfuric acid with HX present.,,' Reactions of [Reox,]- with monodentate ligands can be used to The triphenylphosphine prepare the six-coordinate anions [ReOX,(L)]- (L = H 2 0 or MeCN).3593360 derivative (Ph3PH)[ReOCI,(PPh,)] can be obtained by the treatment of ReOCl,(PPh,), with hydrogen chloride and through the thermolysis of (Ph,PH), ReOC1,,324,331 while the treatment of (NH,),ReOCl, with Et2NH, MeCN and EtOH is said to give complexes of stoichiometry (NH,)[ReOCl, (L)].313X-Ray structural data have confirmed the octahedral structures of several [ReOX,(L)]- salts in which L is weakly bonded trans to the Re=O bond. Such data were first ~)]~ (Et4N)[ReOBr4(H20)],3M) ~' and later supplemented by reported for ( P ~ . , A s ) [ R ~ O B ~ , ( N C Mand crystallographic data for (P~,AS)[R~OC~,(H~O)]~~~~~~~ and the salt [(NH,),CSSC(NH,),][ReOCl,-
184
Rhenium
H20)]C1-H20.343 All revealed a very short Re-0 distance (1.63-1.73 A) that accord with its multiple bond character. The crystals of [(NH,),CSSC(NH,),]~ReOCI,(H,O)]Cl~H,O were obtained (together with [ReOCl,(tu),(H,O)]Cl and ReOCl,(tu)(H,O)- see Section 43.7.1.5) in a complicated reaction involving the reduction by thiourea (tu) of KReO, in concentrated HCl. The dithiobisformamidinium cation is an oxidation product of the thiourea.363 The complex (Ph,P),ReOCl, which can be obtained as its bis-CH2C1, solvate from the reaction of ReCl, with Ph,PCl in the presence of water has been characterized by X-ray crystallography.606
43.7.1.7
Mixed donor atom ligands a d macrocycles
Complexes of [Reo],' containing Schiff bases constitute the most extensive group of complexes derived from mixed donor atom ligands. Salicylaldehyde (salH), which serves as a precursor to most of the Schiff bases, also forms several well-defined complexes. The compound ReOC1, (sal)(PPh,), can be prepared by reacting salH (or its lithium salt) with ReOCI,(PPh,),, or with a mixture of KReO,, PPh, and excess hydrochloric acid in boiling EtOH?05 Treatment of this complex with PMe2Ph leads to the formation of ReOCI,(CHOsal)(PMe,Ph), through the opening of the chelate ring and the conversion of the monoanionic bidentate sal ligand to a monodentate donor, bound through its phenolic oxygen (in which form it is abbreviated CHOsal). The reaction of salH with (Ph,As)ReOCl, in refluxing ethanol gives (Ph,As)[ReOCl, (sal)]. These three diamagnetic complexes each possess a v(Re=O) mode in the range 975-960 cm-', and probably have structures in which the phenolic oxygen in bound trans to the terminal Re=O unitPo5The salt (Ph,P)[ReOCl,(sal)] has also been reported and is synthesized by heating neat salicyaldehyde with (Ph,P), ReCl, or a solution in ethanol with (Ph,P)ReOC14.3M The bidentate monoanionic Schiff bases N-methylsalicylideneiminate(Me-sal) and N-phenylsalicylideneiminate (Ph-sal) react with ReOX,(PPh,), or ReOX,(OEt)(PPh,), (x = Cl or Br) to afford the six-coordinate species ReOX,(L)(PPh,) or ReOX(L), (L = Me-sal or Ph-sal), depending upon the reaction s t ~ i c h i o m e t r y .A~ ~similar ~ procedure can be used to obtain the analogous complexes derived from 8-hydroxyquinoline (oxineH), viz. ReOX, (oxine)(PPh3) and ReOX ( ~ x i n e ) , , ~while ~ ' the reaction of (Ph,As)ReOCl, with a two-fold stoichiometric amount of Schiff base in hot ethanol can be used to prepare ReOCl(L), (L = Me-sal, Ph-sal or ~xine).~" The reaction of ReOX,(L)(PPh,) with PMe,Ph has proven to be a simple means of preparing ReOX,(L)(PMe2Ph).2MAn X-ray crystal structure determination on ReOCl(Me-sal), (61) has shown that the phenolic oxygen of one of the Me-sal ligands is, as expected, trans to the short multiply bonded Re =O unit (1.680(4) A).367 In the case of ReOX,(L)(PPh,), some systems have been isolated as cis and trans isomers, and the pale green 'intermediate' ReOCl, (Me-salH)(PPh,) has been prepared by the reaction of ReOCI, (PPh,), with Me-salH in benzene at room ternperat~re.~~' 0 'fl,,,,,,,
N
II
4
%
,\,,\\\I"
O)
n
(61) N 0 = Me-sal
When a stoichiometric quantity of the Schiff base Me-salH or Ph-salH, or the bidentate oxineH, is reacted with (Ph,As)ReOCl, in anhydrous THF, the salts (PhAs)[ReOCl,(LH)] are produced, whereas thede same reactions when carried out in boiling ethanol give ( P ~ , A s ) [ R ~ O C ~ , ( L ) ] . ~ ~ Studies involving the tetradentate, dianionic Schiff base ligands derived from sal,enH, = N , N'ethylenebis(salicylideneimine), sal, propH, = N,N'-trimethylenebis(salicylidenamine), sal,phenH, = N,N'-o-phenylenebis(salicy1ideneimine) and acac,enH, = N,N'-ethylenebis(acety1acetimine) have shown that analogous procedures to those described above can be used to prepare the Schiff-base-bridged species [ReOX,(PPh,)],(sal,en) (X = C1 or Br), [ReOCl,(PPh,)],(acac,enH,), ([ReOCI,],(~al,en))~- and { [ReOCl& (sal,enH,)'-, as well as the mononuclear complexes ReOCl(L) ( L = sal,en, sal,prop, sa1,phen or The latter mononuclear species have been proposed to contain a structure with the Re-Cl bond tram to Re=O?'* although such a stereochemistry is not found in the case of the X-ray crystal structure of green crystalline [ReOCI,(PPh,)],(sal,en) (62).3h9In this complex (r(Re=O) = 1.68(1) A and v(Re=O) = 969cm-') a phenolic oxygen is trans to the terminal Re=O bond. The mustard-yellow complex ReOCl,(a-
Rhenium
185
’
cac,enH,) can be isolated upon reacting ReOCl,(PPh,), with acac,enH,in refluxing THF. H NMR spectral data for this complex have been interpreted in terms of a bidentate chelating N-bound acac,enH, ligand in the enolized form.29s
C1
-Re-N.-
I‘
.
I‘
P
0
P
0
Several complexes derived from the tridentate Schiff base N-(2-hydroxyphenyI)salicylideneimine (HOPh-salH) have been synthesized and the X-ray crystal structures of trans-ReOCl(L)(OPh-sal) (L = H,O or MeOH) and cis-ReOCl(PMe,Ph)(OPh-sal)determined.m7 The structures of the previously prepared Schiff base complexes trans-ReOBr, (Ph-sal)(PPh,),@ and cis- and trunsRe0Cl,(Me-~al)(PPh,)~ have been reported. Rhenium(V) complexes of vinyl amides RCOCH=C(R’)NH, of the type ReOCl, [C(O- )= CHC(R)=NHJ(PPh,) have been prepared by reacting the amides with ReOC13(PPh3), in anhydrous benzene or THF. These complexes, for which R = CH,CH,CO,H when R = Ph or C6H1,, or R = Me or CH2CH2COzMewhen R = Ph, probably possess the usual octahedral structure with When mixtures of ketones and ReOCI,(PPh,), are the ligand oxygen donor trans to treated with acyl- or aryl-hydrazine hydrohalides in refluxing benzene, derivatives of the ketone hydrazone are formed. The reaction of ReOBr, (PPh,), with benzoylhydrazine hydrobromide and acetone gives an analogous bromide complex.87An X-ray crystal structure on the derivative [acetone benzoylhydrazonido(I ) N , O]dichlorooxo(triphenylphosphine)rhenium(V) shows that these species have the structure depicted in (63).371
I
R’ (63) P = PPhj
An assortment of other mixed donor ligand complexes of oxo-rheniumv) are also known but these are not particularly well characterized and, generally speaking, their structures are uncertain. These data include the complexation of ReVby such ligands as 2-aminoben~enethiol:~’thiosemic a r b a z i d e ~ l-methyl-2-mercaptoimidazole374 ,~~~ and 2-rnercaptobenzothia~ole.~~~ Recently, the pyrazolylborate complex ReOCl, [HB(pz),] has been prepared.572 Macrocylic complexes of rhenium are rare. The complex [ReO(OPh)L], where H,L = octaethylporphine, has been prepared starting from the oxide Re, 0,in refluxing phenol. Its spectral properties have been described, including the assignment of v(Re=O) at 946cm’-’.37s
43.7.2 43.7.2.1
Mononuclear Complexes Containing the peO,)+ Moiety Cyanide and aryl isocyanides
One of the most stable and best characterized cyano complexes of rhenium is the [ReO,(CN),I3anion. It is prepared quite conveniently by the reduction of [Reo,]- using hydrazine hydrate in the presence of CN- .2 The structure of the potassium salt IS,ReO,(CN), has been determined by both X-ray376and neutron diffraction377and shown to contain a truns dioxo arrangement of Re=O bonds, with r(Re-0) N 1.78 A. The protonation of [ReO2(CN)J3- leads first to [ReO(OH)(CN)4]2-, which then eliminates water to give [Re,03(CN), 1‘- .3’03378 Rates of oxygen exchange between rran~-[ReO,(CN),]~-and water have been determined in both acidic and basic
186
Rhenium
media. In acid, a mechanism involving exchange of an oxo ligand that is facilitated by protonation is believed to occur. On the other hand, in basic media exchange occurs only after replacement of an equatorial cyano ligand by a solvent water molecule or hydroxide ion.379 An MO treatment of the bonding in diamagnetic [ReO,(CN),I3- has been used to interpret the electronic absorption spectrum of this species,380while a detailed analysis of the vibrational spectra of K,ReO,(CN),, K,Re0,(I3CN), and K3Re'80'60(CN), is available. 381 The V ( ' ~ O = R ~ = ' ~ O) modes are a t 775 (IR) and 879 (Raman) cm-'. The electronic absorption and emission spectra of [Re0,(CN)4]3- (and the ethylenediamine and pyridine complexes [ReO,(en),]+ and [ReO,(py),] -see Section 43.7.2.2) have been A p-tolyl isocyanide complex of stoichiometry [ReO,(PPh,), (CNR),JI has been reported312but remains incompletely characterized. Other isocyanide-containing derivatives are unknown. +
43.7.2.2 Amines and N-heterocycles Diamagnetic species of the types [ReO,L,]+ (L = py, picolines, NH, or primary amines) and [ReO,(LL),]+ (LL = ethylenediamine (en) and other bidentate primary amines, or bipy) constitute an extensive series of complexes whose preparations and reactivities, including the protonation of the dioxorhenium unit, are thoroughly reported in earlier reviews.',' These species are usually encountered in salts with the following anions: C1-, Br-, I-, CIO; and Reo;. The most extensively studied systems are the yellow-orange species [ReO,(py),]+ and [ReO,(en),]+, both of which display characteristic v(O=Re=O) vibrations at 820 cm- ' in their IR spectra.326 Their syntheses can be accomplished using starting materials like K, ReC1, and KRe0,,3n2,38'but they can also be formed easily from compounds such as ,/3-ReCl, and Re,O,Cl,(py), (and many others) when oxygen and water are p r e ~ e n t . ~ ' ~ ' ~ " The X-ray crystal structure of trans-[ReO, (en),]Cl has been the subject of three separate studies, The results of X-ray structural the most recent determination giving r(Re-0) = 1.765(7)A.317 studies are also available for trans-[Reo, (en),]N0,385and truns-[Re0 (py),]Cl .2H,0,3'7for which the Re=O bond distances are reported as 1.764(9) A and 1.764(13) respectively. Several complexes are also known where more than one type of ligand (including an amine or N-heterocycle) is present in the [Reo,]' coordination sphere; examples include [ReO,(py), (PPh,)]+ and [Reo2(py)2(PPh,),]+ (see Section 43.7.2.3), and some mixed aqua-fluoride-pyridine derivatives, e.g. [ReO,py,]F, [ReO,(py),(H,O)w and [ReO,(py),(H,O),]F, that have been described as being formed starting from [Re0,(py),]F.386 The electronic absorption spectra of various dioxorhenium(V) trmzs-{ReQ,L,]+ species have been investigated (L = py, 4-Mepy or 4-Bu'py), and these species have been shown to exhibit luminescence in the solid state and in aprotic solvents but not in water."' In addition to spectroscopic studies on [ReO,(en),]+ and [ReO,(py),]+ ,610 the reduction of the en complex and its 1,3-diaminopropane analogue with the aquated electron has been found to generate transient d3 Re" complexes.6" Re,O, is reduced by PEt, in the presence of pyridine, 4-methylpyridine or 4-phenylpyridine, to give [ReO,L,]ReO, in good yield. The X-ray crystal structure of trans-[ReO2(4-Mepy),]ReO, has been The incorporation of rhenium into zeolites via [ReO,(en),]C1 has been found to lead to lower catalytic activity (in the hydrogenation of benzene) than is the case for catalysts obtained by impregnating zeolite NaY with NH, Reo, solution.388 N
A,
43.7.2.3
Phosphines
The six-coordinate complexes ReO,I(PEt,Ph), and ReO,I(dppe)(PPh,) are formed when fivecoordinate ReO,I(PPh,), is reacted with PEt,Ph or one equivalent of d ~ p e . ~ In , ' a similar fashion, the reaction of ReO,I(PPh,), with an excess of py can be used to prepare [ReO,(py),(PPh,)jX (X = I or ClO,), which in turn reacts with PPh, to give [Re0,(py),(PPh,),]+.330 The treatment of ReO,I(PPh,), with an excess of dppe gives [ReO,(dppe),]+ , which can also be generated directly from Reo; by reaction with HX in is itself most easily The complex used to prepare the preceding complexes, viz. ReO,I(FFh,), (a), obtained as violet crystals by stirring a suspension of ReO12(OR)pPh3)2(R = Me or Et) in wet a c e t ~ n e . " ~This . ~ ~reaction ~ is believed to occur as shown in equation (18), and an X-ray crystal structure determination has shown that (64) possesses a geometry somewhat intermediate between that of a trigonal bipyramid and a square pyramid, although it resembles the former more closely. angle of This structure is characterized by a trans disposition of phosphine ligands, an O-Re-0
187
Rhenium
139" and Re=O distances of 1.742(1l)A.329This is the only structure known to date in which the iwo oxide ligands in an [Reo,]' moiety are not trans to one another. P
P
(64) P
ReOL(OR)(PPh3),
+
H,O
=
PPh,
acetone
ReO,I(PPh,),
+
ROH
+
HI
(18)
The complexes Reo, I(PPh,), and ReOI, (AsPh,), are useful synthetic starting materials for the preparation of the 2-butyne complex R ~ O I ( M ~ C = E C M ~Dirhenium ) , . ~ ' ~ heptaoxide Re,O, reacts while when the reduction is carried out using PEt, with PMe, to give tran~-[ReO~(PMe,),]ReO,,6'~ in the presence of pyridine, 4-methylpyridine or 4-phenylpyridine, the mixed valent salts [Reo, LJReo, are 43.7.2.4 Other ligands
Complexes in which both fluoride and water have been incorporated into the inner coordination sphere are described in Section 43.7.2.2. A DMSO derivative ReO,I(DMSO), has been reported to exist but remains inadequately c h a r a c t e r i ~ d ,as ~ ~are ' the thiocyanate-containing species formulated as [ReO,(CNS)J-, for which evidence of solid compounds is doubtful,, and [ReO,(NCS),(PPh,),]-, the latter having been isolated as its Na', NH: and Ph,As+ salts through the reaction between ReO(OH)(NCS),(PPh,), and OH- in the presence of an excess of SCN- .389 The existence of the thiourea-containing cation [Reo, (tu)J+ has been described in the literature before 1963, but its identity remains questionable.,
43.7.3 Complexes Containing the p-Oxo-bridged [Re, O,I4+ Moiety 43.7.3.1 Cyanide
The acidification of solutions containing [ReO,(CN)J3- leads to [Re, 0,(CN),T-, a species which has been isolated as its [Pt(NH,),],Re,O,(CN), salt and structurally identdied by X-ray crystall~graphy.~'~-~" The linear O=Re-0-Re=O unit is characterized by Re-0 distances of 1.698(7) and 1.915(1)A. A detailed analysis has been made of the vibrational spectrum of [Re,O, (CN),I4- and the v(Re=O) and v(Re-0-Re) modes assigned, viz. v(Re=O) (Raman), 1017cm-', v(Re=O) (IR), 1008 cm-', v(Re-0-Re) (IR), 720cm-' and v(Re-0-Re) (Raman), 205 ern-' The rates of oxygen exchange of the anion have been determined, the mechanism by which the exchange of terminal oxo ligands occurs being facilitated by association with H + .379 The diamagnetic, mixed alkali metal salt K,Na[Re,0,(CN),J*2H20 has also been prepared and spectroscopically characterized by IR and electronic absorption spectroscopy. Its IR-active v(Re=O) and v(Re-0-Re) modes have been assigned to bands at 995 and 725 cm-', res p e ~ t i v e l yAn ,~ ~M ~ O analysis of the electronic structure of [Re,O3(CN),l4- is a~ailable.~"
.,''
43.7.3.2 Ethylenediamine and N-heterocycles
The neutral ethylenediamine complex Re,O,Cl,(en), is formed upon allowing a solution of [ReO(OH)(en),]'+ in 2 M HC1 to stand.,,' The green crystalline material that results has a structure . ~ ~ related ~ very closely related to that of [Re,03(CN),I4- as established by X-ray c r y s t a l l ~ g r a p h yThe pyridine and 2,2'-bipyridyl complexes have been prepared by reacting ReCl, or ReCl, with py or bipy in wet acetone, by the reaction of ReOCl, with bipy in absolute ethanol, and by the reactions of ReOX,(PPh,), or ReO(OEt)Cl, (PPh,), with py in wet b e n ~ e n e . ~ ' ~The .~'~ reaction "~ scheme by which ReOCI,(PPh,), is converted into Re2O,C1,(py), is shown in Scheme 14. The pattern of Re -0 stretching modes of the py complex (65) is similar to that of the analogous CN- and en complexes. However, this complex differs from the CN- and en derivatives in havings its two sets of equatorial ligands in a staggered arrangement one to the other (in this molecule the twist angle
Rhenium
188
is 1 13°).3’3The Re-0 1.903(16)
distances are as follows: Re=O,, 1.715(16), 1.764(16); Re-Ob,
1.943(16),
CI 0 =Re -0
-R e = O
(65) N = py
43.7.3.3 Dialkyldithiocarbamates The dialkyldithiocarbamate complexes Rez03(R2dtc),(R = Me, Et or Ph) are best prepared by reacting ReOCl,(PPh,), or Re,O,Cl,(py), with Na+R,dtc- in acetone. ‘21*349 The best characterized complex is the diethyldithiocarbamate derivative Re203(Et,dtc),, whose X-ray crystal structure has been d e t e r m i ~ ~ e d. ’The ~ ~ . ~complex ~’ has a structure which closely resembles that of (65), but with an angle of twist of 40” away from the fully eclipsed conformation. Its vibrational spectrum exhibits the following characteristic Re-0 stretching modes: v(Re=O) (Raman), 961 cm-’, v(Re=O) (IR), 955cm-’, PEt,Ph (+ 0.29V) > py (+0.17 V) > C,H, I NH, (+0.16 V) > CSH,,NH (+0.1 1V). Oxidation is easiest with the complex where the build-up of electron density at the metal is greatest.296Interestingly, the reaction of the mild oxidant [Cu(NCMe),]PF, with ReH,(PR,), (PR, = PMePh, or PMe,Ph) in THF does not result in a redox reaction but rather in the formation of the novel complex [(R3P),H,Re(p-H),Cu(pH), ReH,(PR,),]PF, according to the stoichiometry given in equation (22). The structural formulation is based upon an X-ray structure determination together with H NMR spectral measurement~.~" A redox reaction occurs between ReH,(PPh,),L (L = C6HIINH2or C5HloNH)and allyl chloride leading to expulsion of H2 and the formation of (Ph,PC3Hs),ReCI, and some propene.'"
'
2ReH5(PR,),
+ [Cu(NCMe),]PF,
-*
[(ReeH5(PR3),),Cu]P% -t 4MeCN
(22)
(PR, = PMePh, or PMe,Ph)
Photolysis of benzene or isooctane solutions of ReH,(PR,), (PR, = PPh,, PMePh, or PMe,Ph) results in the formation of the reactive 16-electron species ReH,(PR, j2 as a result of the photoextrusion of a PR, ligand. Several hydridc-phosphine products can be produced following the formation of ReH,(PR,), as shown in Scheme 9.1257r35 These include ReH,(PR,), which is formed if the photolysis is carried out under an H, atmosphere.Iz5The photochemistry of ReH,(PMe,Ph), has also been investigated by laser and conventional flash photolysis.433While the species ReH,(PMe,Ph), is observed in saturated hydrocarbon solvents, photolysis in benzene gives (q2C,H,)ReH,(PMe,Ph),. Also, dimerization to give Re,H8(PMe2Ph), (see Scheme 9 and refs. 125 and 135) proceeds by the thermal reaction of a phototransient with the ground state saturated species ReH, (PMe,Ph), . The phototransient species ReH,(F'R,), exhibits some interesting organometallic chemistry as demonstrated by its reaction with saturated and unsaturated organics, the former in the presence of a hydrogen acceptor.3s
43.7.5.4
Miscellaneous ligands
The hexacyanate anion [Re(OCN),]- has been claimed to be the species that is formed upon reacting ReCl, with KCNO in dimethyl sulfone:'8 while the ammonolysis of ReC1, in liquid ammonia gives ReCl, (NH, j2.2NH, .352 Halogens oxidize the [Rex, (diars),]+ cations to eight-coordinate [ReX,(diars),]+ (X = C1 or Br).2
Rhenium
194
Rhenium pentachloride reacts with diisopropylcarbodiimide in CCr, to form the monomeric six-coordinate amidino complex ReC14[PriNC(Cl)NPr'].623
43.8 43.8.1
RHENIUM(V1) COMPOUNDS Nitrides, Imides (Nitrenes) and Amides
The reaction between anhydrous liquid ammonia and ReOCl, at - 33°C affords brown insoluble [ReO(NH, ),I2 together with an ammonia-soluble species formulated (tentatively) as mode at NH,[ReO(NH,j,Cl]. The IR spectrum of [ReO(NH,),], displays a v(Re-0-Re) chain sy~tern.4~~ 830cm ', thereby supporting the presence of a polymeric [-0-Re-0-Re-] The nitrido complexes A[ReNCl,] (A = Me,N+ or Ph,As') are formed upon reacting ReNC1, with ACI in P0C13.435The P b A s f salt has also been formed by reacting Ph,AsCl with the chlorothionitrene complex Re(NSCl)CI,(POCl,) in CH, C1, s01ution.4~~ The analogous bromide Ph,As[ReNBr,] is formed by the thermal decomposition in vacuo at 210°C of dark violet crystalline PkAs[Re(NBBr,)Br,] which is itself prepared by reacting Ph,As[ReNCl,] with excess BBr,.,." The salts of the [ReNX,]- anions possess IR-active v(ReN) modes between 1085 and 1100cm-', as does the salt P~,AS[R~(NBB~,)B~,].~~~,~~' The X-ray crystal structures of the salts Ph, As[ReNX,] (X = C1 or Br) have been determined and show that the [Rem,]- anions possess a distorted square pyramidal geometry (69) with very short Re-N distances of 1.628, and the Re atom located above the plane of the four X The ESR spectrum of [ReNClJ doped into the Ph,As[ReNCl,] host has been reported, and the spectrum of [ReNCl,]- in frozen MeCN solution is indicative of the presence of [ReNCl,(NCMe)]- .438
(69) X = Clor Br
The reaction of ReNCl, with py, bipy and PPh, gives ReNCl,(py),, ReNCl,(bipy) and ReNCl,(PPh,),, respectively, whereas when Ph,As[ReNC&] is treated with py or PPh,, Ph,As[ReNCI,(L)] (L = py or PPh3) is The admixing of KNCS and Ph,As[ReNCl,] in refluxing methanol in the presence of an excess of Ph,AsNCS gives the salt (Ph,As),[ReN(NCS),], which possesses the expected octahedral structure with a short Re-N (nitrido) distance of 1.66 A."" Other nitrid+rhenium(V) complexes include [ReNCl, (POCl,)], 2POCl,, which forms eight membered Re,N, rings with alternating R e E N . * -Re units (r(Re=N) = 1.68(2)A) and is prepared upon treating ReNC1, with excess POCl,.441Another example is a compound formulated as (Ph,As),[Re,FN, C17],which is proposed to be a p-fluoro compIex with terminal nitrido ligands.*, It is formally a derivative of ReV'and is prepared by mixing ReNCl, and Ph,AsF in acetonitrile. Its structure has not yet been determined by X-ray crystallography, but magnetic susceptibility meas= 1.66 BM urements (over the range 4.8-280 K) indicate that the complex is magnetically dilute (per per Re atom), behavior that is similar to that found for Ph,P[ReNCI,] (peR = 1.67 BM per Re atom).&' The p-tolylimido complex Re(N-p-tol)Cl,(Ph,PO) is believed to be formed as one of the products in the oxygenation of [Re(N-p-tol)Cl,(PPh,)], in CCl, or benzene.420Its magnetic moment u ,( = 1.61 BM) supports this formulation!20 The N-chloroalkylated derivatives Re(NR)Cl,(POCl,) (R = CCl, or CzCls)are formed upon reacting ReCl,, dissolved in POCl,, with cyanogen chloride or CCl,CN in the presence of Cl,. An X-ray crystal structure determination has shown that for the complex Re(NC,Cl,)Cl,(POCl,), the Re-N distance is 1.69(1) 8, and the Re-N-C angle approaches linearity ( 169")?, When cyanogen is used in place of cyanogen chloride and CCl, CN in the above reactions, the bis-nitrido complex (Cl, PO)Cl, M(NCCl,CC1,N)MC1,(POC1,) is formedaWThe treatment of Re(NR)Cl,(POCl,) (R = CCl, or C,Cl,) with Ph,AsCl gives the salts Ph, As[Re(NR)Cl,] .445 A novel reaction occurs when ReC1, reacts in POCI, solution with (NSCl), to give the formally rheniurn(V1) chlorothionitrene complex Re(NSC1)C1,(POCI3), together with the rhenium(VI1) derivative Re(NSCl),C1,(POCl,).436 The chlorothionitrene ligand is related to the arylthionitrene moiety as found in Re(NSAr)Cl(Et,dtc), (Ar = 2,4-(N0&C6H3) (Section 43.7.4).
-
195
Rhenium
43.8.2 Halides and Oxide Halides The only complex halides of Re"' that do not contain a terminal Re=O moiety are the fluororhenate(V1) anions [Ref,]- and [ReF,]*- which are prepared by the reaction of ReF, with alkali metal fluorides. The action of ReF, on M',ReF, is also a route to M'ReF, (MI = K, Rb or CS)."~ While [ReF,I2- has a square antiprismatic geometry, the vibrational spectra of [ReF,]- salts is in accord with pentagonal bipyramidal structure (D5* symmetry). The magnetic moments of M',ReF, salts are apparently higher (1.61.7 BM) than those reported for M'ReF, (0.64.7 BM).,,, The controlled hydrolysis of M'ReF, (M' = Rb or Cs) in wet acetonitrile is a suitable method for preparing the light green oxidefluorides M' ReOF, .446 The [ReOFJ anion has also been generated by the partial hydrolysis of ReF, in aqueous HE and characterized by ESR spectroscopy at 77 K.447 The vibrational spectrum of M' ReOF, (MI = Rb or Cs) has been assigned in terms of C,, symmetry for the anion.448The parent oxidefluoride ReOF, forms a pale blue 1: 1 adduct with SbF5which has a dimeric structure in which two Re atoms and two Sb atoms are linked through fluorine bridges into distorted eight-membered rings.444 The oxidechloride ReOCl, reacts with stoichiometric amounts of water, MeCN and POCI,, to ~ . ~complexes ~~ are form the adducts ReOCl,(L), in which L is trans to the Re=O g r o ~ p . " " ~These paramagnetic z 1.7 BM) and possess v(Re=O) modes close to 1020m-i.3'4.434 The ESR spectra at low temperature of the MeCN and POCI, adducts have been recorded and analyzed in considerable and comparable spectral data are also available for anhydrous toluene and CCI, solutions of ReOCl, in the presence of Ph,PO, urea, MECN, acetone, Et,O, DMG and dioxane.452The X-ray crystal structure of the bright red, volatile hydrate ReOCI,(H,O) confirms that the strongly bonded oxo group (Re-0 = 1.63(2)A) is trans to a weakly bonded H 2 0 ligand (Re-0 = 2.27(2)A)."MA gas-phase electron diffraction study has been made of ReOC14.h25 The reaction between ReOX, (X = Cl or Br) and A + X - (A = Et,N, Ph4P or Ph,As) in anhydrous CHCl, or CHIC&is a convenient method for preparing A[ReOX,j.434.453 The action of gaseous HC1 upon suspensions of [Reo,]- in various aqueous and non-aqueous media has also been used to prepare the salts (Ph,As)ReOCl, and (Ph, P)ReOCl, ,362,4y The [ReOCIJ- anion is also generated in the [ReOCl,f2--H, SO4-HCl and [ReOCI,]2--H,P, 0,-HC1 systems where it has been characterized by ESR and electronic absorption s p e c t r o s c ~ p yIn . ~the ~ ~pyrophosphoric ~~~~ acid system, the existence of [ReOC1,(H3P,0,)]- has also been proposed. A study of the electronic absorption spectrum of the {ReOBr,]2--H, SO,-HBr system indicates that ReOBr, and [ReOBrJ are formed,'", a result that has led to the development of a good synthetic procedure for ReOBr, through the reaction of NH,ReO, with an excess of NaBr in concentrated sulfuric acid.'** In the case of the analogous [ReOF,I2--H,SO4-HF system, it is believed that [ReOF,] is generated.457 ' * ~the ~~*~~~ The results of various studies on the ESR spectrum of [ReOCl,]- are a ~ a i l a b l e , 4 ~and magnetic properties and IR and electronic absorption spectra of the paramagnetic salts A[ReOCl,] (A = Et4N, P b P or Ph,As) have been r e p o ~ t e d . 4 ~The ~ " ~partial ~ hydrolysis of the [ReOCl,]- ion gives mauve [Re,03Cls]*-which can be isolated as its Et,N+ and Ph,As+ salts.453These complexes, which display v(Re=O) and v(Re-O-Re) modes at 970 and 740cm-', respectively, are essentially diamagnetic, but other than the presence of an O=Re-0-Re=O moiety, it remains unclear as to which of several possible isomers has been isolated.453The use of a different method to generate the [Re,0,C1,]2- anion was reported subsequently, namely, the reaction of [Reo,]- with acetyl chloride in acetic acid-HCl.,@' However, the salts that were isolated (tetraphenylphosphonium and 2,3,5-triphenyltetrazolium)did not have the same properties as those that had been prepared via [ReOC15]-. For example, they are described as possessing a magnetic moment of 1.5BM and their @e-0-Re) mode is purportedly close to 9 l O ~ m - ' . ~ An interesting 'derivative' of [Re, 03C1J2- is the complex oxidehalide Re, 0, a,(Reo, Cl), (70) which is formed in the reaction between Reo, and ReCl,, and by the UV irradiation of a mixture of ReOCl,, ReOC&(H,O) and ReOlC1.461The X-ray crystal structure of (70) shows that each of the two perrhenyl chloride fragments are weakly bound to the Re20,C1, unit via an oxygen atom. The unit (Re short Re=O terminal bonds (Re-0) = 1.69(2)w) are cis to the bridging Re-0-Re -0 = 1.847(1)A).&' ~
-
C1-
0 C' II #' Re -0
Reo, C1 O c1 -Re
18"" -Cl
N
196
Rhenium
43.8.3 Oxoanions and Alkoxides There exist quite a large number of ordered perovskites A,BRe06 (A = Ca, Sr or Ba; B = Ca, Sr, Ba, Mg, Mn, Fe, Co etc.) and other mixed oxides that contain ReV1,'but these are not considered to be coordination complexes and, accordingly, they will not be considered here. The interesting violet colored salt (Me,N), Reo,, which contains the paramagnetic [Reo,]'- anion, can be prepared by the controlled potential one-electron reduction of [Reo,]- in acetonitrile ( - 2.30 V vs. SCE).462,463 In water, the first electron transfer is believed to be followed by the disproportionation of Re" to Re"' and Re", thereby leading to an overall three-electron process at the electr0de.4~~ In weakly acidic solutions, oxalate and citrate increase the ease of reduction of the [Reo,]- anion through the reversible formation of 1:1 complexes.w A study has been made of the reaction of [Reo,]- with the equated electron in neutral aqueous solution to give [Re0,I2- .624 The salt (Me,N), Reo, crystallizes in an antifluorite-like cubic face-centered lattice. The magnetic and spectroscopic properties of this complex have been rep~rted.,~' The simplest alkoxide of ReV' is Re(OMe), which can be prepared by reacting ReF, with Si(OMe),. It is volatile in high vacuum at room temperature and displays the parent ion in its mass s p e c t r ~ r n . The 4 ~ ~ alkoxide ReO(OBu'), is formed when ReOCl, is treated with LiOBu' in diethylether. This paramagnetic blue crystalline solid is thermally unstable above O"C, but has been characterized by IR spectroscopy (v(Re=O) at 992 cm-').245The analogous reaction of LiOPr' with ReOCI, can lead to the isolation of green crystalline {Li[ReO(OPr'),]. LiCl(THF)},. This ESRactive complex possesses a v(Re=O) mode at 987cm-' in its IR spectrum and a complicated dimeric structure in the crystalline state in which octahedral [ReO(OPr'),]- anions are present and some of the alkoxide groups are involved in forming bridges to the Li+ ions in the lattice."' When ReOCl, is dissolved in methanol in the presence of a tertiary amine, orange air-sensitive crystals of the complex (MeO),ORe(p-O)(p-OMe), ReO(OMe), (71) are formed. This diamagnetic complex has Re-0 stretching frequencies at 982 and 968 cm-l assignable to u(Re=O) and at 756cm-' to v(Re -O-Re)."* Its X-ray crystal structure shows the presence of a short Re-Re contact (2.559 A) and Re=O and Re-0-Re distances of 1.70 and 1.91 A, respecti~e1y.~~~
-
Me
-
0
Me
Me
43.8.4 Sulfur and Mixed Sulfur-Nitrogen Ligands The trigonal prismatic complexes Re(S2C2Ph,), and Re(tdt),, where S,C,Ph, and tdt are the cis-l,2-diphenylethene-l,2-dithiolatoand toluene-3,4-dithiolato ligands, respectively, can be prepared from the reactions of the appropriate ligands with ReCI,.' The X-ray crystal structure of Re(S,C,Ph,), reveals it to be a near perfect trigonal prism.' Detailed studies have been made ofthe ESR and electronic spectra and redox properties of these ~ o m p l e x e s . Similar * ~ ~ ~ ~complexes ~~~ to c m be prepared using 2-amithose obtained with these 1,Zdithiolate ligands (1,2-dithiolene~)~~ nobenzenethiol and 4-chloro-2-aminobenzenethiol. The blue-green complexes Re(NHSC,H4), and Re(NHSC,H, Cl), (abbreviated Re(abt), and Re(abtCl),, respectively) were prepared by dissolving the ReV" complexes Re(NSC6H4)(NHSC6H&and Re(NSC6H,Cl)(NHSC,H,C1), in aqueous ace t ~ n eBoth . ~ ~complexes are paramagnetic (peg N 1.46 BM) and are found to undergo reversible one-electron oxidation and reduction processes. Thus, this series of complexes resembles the corresponding tri(dithio1ene) set in attaining formal oxidation states of V, VI and VII. These complexes are believed to have a trigonal prismatic geometry:'' An ESR study of trigonal prismatic Re(abt), (where abtH, = 2-aminobenzenethiol) has been carried out to study the effects of concentration and solvent composition on the extent of molecular aggregation.626
43.9
RHENlUM(VI1) COMPLEXES
43.9.1 Nitrides, Imides (Nitrenes) and Amides The only simple nitrido complex is the salt K,Re03N, which is formed in the reaction between Re,O, and KNH, in liquid ammonia.' The reaction of PPh, with ReNCl, gives the phosphinimato
Rhenium
197
complex Re(NPPh3)C&(PPh3),"39 which is analogous to the compound Re(NPPh, )(SPh),. This latter molecule can be isolated, in low yield, following air oxidation of the solution that is generated by the interaction of ReNCl,(PPh,), with excess phenylthiolate anion in hot acetonitrile.*" The role of oxygen in this reaction may involve the oxidation of a [ReN(SPh),]'- intermediate to the ReVtK nitride ReN(SPh),. The X-ray crystal structure of square pyramidal Re(NPPh,)(SPh), shows that the Re-N-P angle is 163" and that the Re-N and N-P distances of 1.743(7) and 1.634(9) A are in accord with considerable Re-N and P-N multiple bonding. The formation of these phosphinimato complexes of ReV1'presumably reflects the considerable electrophilic character of the Re""-nitride moiety. The tris(tert-butylimido) complex (Me3SiO)Re(NBu'), can be prepared by the reaction of trimethylsilylperrhenate with tert-butyl(trimethylsily1)amine in hexane (equation 23).469When a deficiency of amine is used in this reaction, the yellow crystalline complex (72) is formed.470It can by Me, SiOReO,. The addition be viewed as the product of the silylation of [(M~,S~O)R~(NBU')~O], of four equivalents of HC1 to Me,SiORe(NBu'), in CH,C1, produces Re(NBu'),Cl, in high yield. This monomeric complex is believed to possess a trigonal bipyramidal geometry with the two imido ligands in equatorial p0sitions.4~' The analogous bromide complex is best prepared by reacting Re(NBu'),Cl, with Me, SiBr in toluene, and various alkyl, alkylidene and alkylidyne complexes can be generated by addition of the appropriate alkylating
-
MeSSiOReO,
+
3Bu'NH(SiMe,)
+
M~,S~OR~(NBU +' )3Me, ~ SiOH
(23)
I
SiMe3
While the treatment of Reo, C1 with diisopropylamine gives the volatile, moisture sensitive dialkylamine compound (Pr'),NReO,, the related reaction with (Me, Si),NLi in diethylether produces the red, air-stable, crystalline complex (Me, SiO), Re[N(SiMe,),](NSiMe,),, which is believed to be five-coordinate with trimethylsilyl-amide, -imide and -oxide groups.47'Another imido complex is the bis-chlorothionitrene derivative P~As[ReCl,(NSCl),] which is formed upon reacting Re(NSC1)2C1,(POCl,) with Ph,AsCl in CH,C12.436An X-ray crystal structure determination reveals that the two NSCl ligands have a cis arrangement with nearly linear Re=N=S ~ n i t s . 4 ~ ~ Other nitrene-containing compounds are the pair of molecules ReF, (NCI) and ReF,(NF) which are formed upon reacting ReF, with Me,SiN, at low temperature in the presence of ClF,. The resulting complex reaction mixture also gives rise to small and variable quantities of ReNF4.473 X-Ray crystal structure determinations on ReF,(NX) (X = C1 or F) show that the Re-N bond is short (- 1.73A), a result which accords with the high frequency of the IR active v(Re-N) mode (1205 cm-').473The nitride-chloride ReNCI, has been prepared by the reaction of ReCl, with NCl,, and it has been structurally characterized by X-ray crystallography; it possesses a structure similar to that of WOCl,.4" The direct fluorination of ReNC1, between 80°C and 130°C gives the moisture-sensitive material ReNF,. ReF, (NCl), an N-chloronitrene in which the ReNF, and ReF,(NCl) molecules are linked by a linear asymmetric fluorine bridge. The black crystalline salt (Ph,PCl)[ReCl,(N, S,)] is obtained by the reaction of Re(NSCl), CI, (POCI,) with PPh,. An X-ray crystal structure determination reveals the presence of an ReN,S, five-membered ring.628 43.9.2 43.9.2.1
Oxo Specie
Perrhenutes and related species
The aqueous chemistry of rhenium(V1I) is dominated by the great stability of the [Reo,]- ion.' Dirhenium heptoxide (Re,07) is a yellow volatile crystalline compound which dissolves in water to
198
Rhenium
form a colorless, strongly acidic solution of perrhenic acid. The anhydrous acid can be obtained as pale yellow hygroscopic needles, the crystal structure of which shows the presence of 0,Re-0 -ReO,(H,O), molecules possessing a linear Re-0-Re bridge?75In more dilute solutions the acid is completely dissociated to the [Reo,]- anion.' A study has been made of the oxygen exchange between [Reo,]- and water under acid and while molecular ReO,(OH) has not been isolated, several esters of perrhenic acid have been prepared. These include R, MOReO, (M = Si, Ge or Sn; R = Me, Et or Ph), which can be prepared by the reaction of AgReO, with R,MCl, and BuiSiOReO,, which is formed by treating Re,O, with t r i - t - b ~ t y l s i l a n o lThe . ~ ~alkoxides ~ ~ ~ ~ ROReO, (R = Me or But) are formed through the interaction of Re0,Cl or Me,SiOReO, with the silyl ethers Me,SiOR in hydrocarbon solvents!72 An X-ray crystal structure on Me, SiOReO, has confirmed its monomeric tetrahedral structure,4R0while the properties of MeOReO, suggest that it has a polymeric structure with methoxide bridges.472NQR studies (Re-185 and Re-187) have been carried out on Me,MORe0,.481A variety of other derivatives of the type Re0,X exist, including the cases where X = halide, NO, or N(Pr'),.1.472 These compounds react with various ligands to form coordination complexes that are described in Sections 43.9.2.3 and 43.9.2.4. Perrhenic acid can be extracted into non-aqueous organic solvents by aliphatic amines, tributyl phosphate, phosphine oxides, arsine oxides and amine N - o ~ i d e s . ' Most ~ ~ ~metal ~ - ~ ~cations form very stable perrhenates. This is especially true of the main group elements, the first transition series and the rare earths.',308 The X-ray crystal structure of some of the simplest salts have been determined, e.g. NH,ReO, and KRe0,.',484,485 Rhodium(II1) perrhenate Rh(ReO,),-nH,O has also been des~ r i b e d , and ~ ' ~ is apparently the first of the platinum metal derivatives to be isolated. While the [Reo,]- ion is strictly tetrahedral in solution, lower symmetries may be encountered in crystalline salts as monitored by IR and Raman spectroscopy.' The electronic structure of the [Reo,]- ion has been studied experimentally and theoretically,' including a study of the valence level electron structure by X-ray photoelectron spectro~copy.~~' There is a quite extensive chemistry associated with the so-called mesoperrhenates and orthoperrhenates, which are derivatives of the [ReO,I3- and [Reo$ anions, and various mixed metal oxides. These phases have been surveyed in some detail in the review by Rouschias.' More recent developments have included studies on rhenium-apatites containing the square pyramidal [Reo,],; hexagonal perovskites with cation vacancies that contain [ReO,]'-,"" and the double oxides M,Re,O,, (M = Sr or Ba) and MSRe,O,, (M = Ca or Sr).49' The [Reo,]- ion is known to be a weak 1igand.l Recent examples of this are encountered in the case of [Cu(HL)(OReO,)], where H, L = 2,2'-( 1,3-propanediyldiirninobis(2-methyl-3-butanone)dioxine), which has been structurally characterized by X-ray cry~tallography,4~~ the cation [Co(NH,), (OReO,)]'+ ,494 and the structurally characterized species [Re,(O,CR),](ReO,), and [Re,(O, CR),ClJReO, (see Section 43.5.2.5). The perrhenate anion is present in the organic superconductor materials (TMTSF), Reo,, where (BEDT-TTF), (Reo,),, where BEDTTMTSF = tetramethyltetraselenafulvalene, and and is also an important precursor to Re-Pt TTF = bis(ethylenedithiolo)tetrafulvalene~95~496 reforming catalysts and metathesis catalysts. The latter role has led to studies of the thermal d e c o m p o ~ i t i o nthe ~ ~laser ~ ~ ~Raman ~~ spectra499and the X-ray photoelectron s p e ~ t r a ~ ~of~ * ~ ~ , ~ " catalytically important perrhenates, especially NH4Reo, and AI(Reo,), .
43.9.2.2
Other oxygen ligands
In addition to the structurally characterized species Re,O,(H,O), (Section 43.9.2. l), the complex Re, O,(H,O), (dioxane), can be isolated from solutions of Re,O, in H,Wioxane.l In the presence of smaH amounts of water, the compound Re2O,(OH),*3dioxane has been obtained."' The crystals contain dimeric centrosymmetric [0,Re(p-OW), Reo,] units with the octahedral coordination sphere around each rhenium being completed by bridging 1,4-dioxane molecules (one 0 donor per Re), the latter linking the dimeric units into endless chains. The remaining dioxane molecules are hydrogen bonded to the p O H groups.5o1 Evidence has been presented that in 1-IOM H,SO, rhenium(VI1) forms 1:l and 1:2 complexes, the latter being formulated as [Reo, (S0,)J The complexation of [Reo,]- in concentrated alkali with polyhydroxy1 compounds (e.g. D-gluconolactone and D-mannitol) is said to occur,* while edta (H4L), when treated with an aqueous Re"" solution, precipitates a complex formulated as Ba[Re03(HL)]*Ba,L*4H,0 upon the addition of BaC03.505 The addition of B(OTeF,), to ReF, produces the complex Re0,(OTeF,),,'06 while ReCI, reacts
Rhenium
199
with Cl,O, in the presence of POCl,, to give dimeric [ReO,(O,PCl,)(POCl,)], in which bridging 02PC12groups are believed to be p r e ~ e n t . ~ ’ Several carboxylate complexes of the type RC02Re0,(L) {R = Me, CF3,CMe, or Ph; L = THF, py or tetramethylethylenediamine(tmed)) have been prepared and c h a r a c t e r i ~ d . 4The ~ ~ treatment of Re,O, with the acid anhydrides in THF allows the isolation of colorless crystalline RC0,ReO,(THF) (R = Me, CF, or CMe,). The benzoate complex can be prepared by carboxylate exchange in diethyl ether. The complexes with py and tmed are formed by ligand exchange involving the displacement of the THF ligand of RCO,ReO,(THF). All these complexes are believed, on the basis of TR and NMR spectroscopic studies, to be six-coordinate. In the case of the tmed complexes (73), monodentate RCO, groups are pre~ent.4~’ The same is apparently true in the case of an analogous complex that contains the chelating 2-hydroxypyridine ligand, MeCO,ReO, (Hhp), and which is prepared by reacting MeCO, ReO,(THF) with Hhp in ether or chlorocarbon solvents.472
0
Complexes that contain dimethylformamide, viz. ReO,X(DMF), (X = F or Cl) and Et,N[ReO, C1, (DMF)], and other oxygen donors are described in Section 43.9.2.4.
43.9.2.3 Amines and N-heterocycles
The complexes RCO,ReO,(L), where L = py, tmed or Hhp, have been described in Section 43.9.2.2. The analogous alkoxide derivatives ROReO,(tmed) (R = Me or But) have also been prepared and so have the stable, crystalline diisopropylamido species (Pr’),NReO, (py) and [(Pt),NRe0,],tmed~72Reference to the complexes ReO,Cl(L), (L = py, 1/2bipy or 1/2phen) is found in Section 43.9.2.4 along with other halide containing species. The thermal decomposition of [ReO,(py),]X*nH,O (X = Br or 1) at temperates of 135°C (X = I) or 155°C (X = Br) gives a pale brown solid formulated as Re,O,(py),. This product is described as having v(Re-0) bands at 970, 910 and S&Ocm-’. The material obtained by the pyrolysis of [ReO,py,]Cl. 2H20 is, however, violet and its structural relationship to the pale brown material described above is uncertain at present.258 The X-ray crystal structure of the complex Re,07py,6*2has been reported. It is composed of two inequivalent Re atoms, linked by a single p - 0 bridge, which exhibit coordination numbers of five and six.629
43.9.2.4 Halides A variety of neutral adducts of the halides Re0,F and Re0,Cl are known. Both form 1:2 complexes with DMF, the fluoride being formed by the reaction of Re,O, with HF in DMF,S08while the solvolysis of [Re0,Cl,]2- in DMF first gives Et,N[ReO, Cl,(DMF)], followed by the formation of Re0,C1(DMF),.S09 Other complexes of the type ReO,Cl(L), (L = DMSO, MeCONMe,, (Me,N),CO and (Me,N),PO) have also been prepared but are rather unstable.’ Complexes of Reo, C1 with 8-hydroxyquinoline (oxineH) have been prepared and have been formulated as ReO,Cl(oxineH), and ReO,Cl(oxineH)*noxineH, in which oxineH is believed to be bound in a monodentate and bidentate fashion, respectively. In DMF solution the complex ReO,(oxine)(DMF) is The py and bipy complexes ReO,Cl(py), and ReO,Cl(bipy) can be obtained quite easily from the direct reaction of Re0,Cl with the corresponding ligand in CCl, An alternative method that has been used for the bipy complex, which is also applicable to its phen analogue, Re0,Cl(phen), involves the reaction of HReO, with bipy (or phen) in the presence of concentrated
200
Rhenium
hydrochloric acid.g The IR spectral properties of the py, bipy and phen complex (the v(Re-0) are in accord with the complexes possessing similar structures. The X-ray crystal structure derivative Reo, Cl(bipy) (74) has shown that it has a &stereochemistry, with Re-0 distances of 1.708(9)A (trans to N) and 1.718(15)A (trans to C1).51'ReO,Cl(phen) is believed to be isostructural with this. However, when attempts were made to grow crystals by dissolving it in concentrated hydrochloric acid and allowing the solution to stand in a desiccator over P205,a resulting yellow crystalline complex was identified as (phenH,)[ReO, Cl, (H20)]C1by X-ray crystallography.5'2The structure solution is not of high precision because of a disorder problem, but the anion clearly possesses a fac octahedral 0
(74) N--N
= bipy
Complex oxide fluorides of Re"", which can be regarded as derivatives of ReOF,, Re02F3and ReO,F, are all known. The only salts that contain [ReOF,]- are NO[ReOF,] and NO,[ReOF,] which are produced by the reaction of NOF and N 0 2 F with ReOF5?57The reaction between ReO,F, and MF (M = Na, K, Rb or Cs) in a staled quartz tube at 200°C has been used to prepare MRe02F,.513On the other hand, two procedures can be used to prepare salts of the [ReO,FJanion. The action of IF, upon a mixture of KReO, and K F gives K,ReO,F,, a white solid that is hydrolyzed by moist air,446while (Me,N),ReO, F, is formed by reacting ReO,F(DMF), with Me,NF in The vibrational spectra of KRe02F4and K,ReO,F, have been studied and system have led to assigned assuming C,, symmetry for each."8 Studies of the [ReO,]--HF-H,O suggestions that the [ReO,FI2-, [Reo, (0H)F2l2-and [ReO,(OH)(HF,),]- ions exist under certain conditions, but none have been i~olated.~" While the isolation of CsReO,CL,. CsCl has been reported and its thermal stability in~estigated,~'~ the best known oxide chloride anion of R e ' " is [ReO,C1,I2-. Various salts have been prepared by the reaction of Re0,Cl with the chlorides ACI (A = Me,N+, Et,N+, pyH+, Me,NH+ and Mepy+) in inert soIvents,5" and the important cesium salt Cs2Re0,C1, by the dissolution of [ReOJ in concentrated hydrochloric acid saturated with hydrogen chloride.'I6 The X-ray crystal structure of Cs,ReO,Cl, has confirmed thefac octahedral geometry of the anion (similar to that in 74), with an This geometry is the same as that deduced on the basis average Re-0 bond length of 1.70(2) A.517 of IR spectral mea~urernents.~'~ A somewhat different mode of coordination is encountered in the complexes Reo, C1-AlCl,, Reo, C1-NbC1, and Re0,Cl. SbCl,, which are believed (from IR spectral evidence) to contain Re =0-M (M = Al, Nb or Sb) bridges, and thereby do not involve any increase in the coordination number about rheni~m.~'**''~ The fluoride ion donor properties of ReOF, have been demonstrated by the preparation of [ReOF,]AsF, and [Re,02Fg]Sb2F,, which have been characterized by vibrational spectroscopy and mass spectrometry. The X-ray crystal structure of [Re2O2Fg]Sb,FIconsists of discrete monofluorine-bridged dimeric anions and cations.6MThe I9FNMR spectra of solutions obtained by dissolving Re,O, in 60% HF in ethanol have been interpreted in terms of the presence of Re03F, [ReO,FJ, [ReO,(OEt)F,]- and ReO(OEt)F4.631 The five-coordinate anion [Reo, Cl,]'- has been isolated as its Ph,P+ salt from the reaction of Re0,Cl with Ph4PCl in di~hlorornethane.~~~
43.9.2.5
Thioperrhenates
When H,S is passed into an aqueous solution of [Reo,]-, the electronic absorption spectrum changes, signaling the formation of the species [MO,_,S,]- (n = l-4).',520Of the four possible species only [Reo, SI- and [Res,] - have been completely characterized structurally in the crystalline state.' The electronic absorption spectra of all four species have been reported,520and a detailed study has been made of the resonance Raman spectrum of [Res,]- as part of a more general investigation of the vibrational spectra of some of these specie^.'^"^ In this case, particularly informative spectra were obtained for (Ph,P)ReS, where the overtones of the v, mode were seen out
20 1
Rhenium
to I O V , . The ~ ~ ~ laser-excited emission spectrum of this same salt reveals a mixture of relaxed and partially relaxed luminescence thought to originate from the vibronic levels of the excited state.'" Further studies of the resonance Raman spectra of (Ph,P)ReS, have been reported.633
43.9.3 Thiolate Ligands Oxidation of the Re" complexes Re(NHSC6H,), and Re(NHSC,H,CI), (abbreviated Re(abt), and Re(abtCI),, respectively) in THF in the presence of an excess of p-toluenesulfonic acid can be This gives the diamagnetic salts [Re(abaccomplished using 0, and H,02, re~pectively.~'~ t)J* (C7H7S03)- and [Re(abtCl),]+ (C,H,SOJ. These complexes resemble the corresponding tris(dithio1ene) derivatives Re(S,C, R2)3 in attaining a formal oxidation number of VI1 upon undergoing a one-electron ~ x i d a t i o n When . ~ ~ KReO, is reacted with o-aminobenzenethiol, the brown complex Re(NSC6H,)(NHSC6H,),, i.e. Re(abt-H)(abt),, containing one doubly deprotonated and two singly deprotonated coordinated amines, is formed. The analogous complex These two complexes derived from 4-chloro-2-aminobenzenethol is made by a related can be converted to the Re"' species Re(abt), and Re(abtCl), (see Section 43.8.4) upon prolonged treatment with acetone!"
43.9.4 Halides Rhenium heptafluoride reacts with nitryl and nitrosyl fluorides to form NO,ReF, and NOReF,.257 The Raman spectra of the solids and 19FNMR spectra of their HF solutions have been interpreted in terms of Du symmetry for the [ReFJ anion. The cationic species [ReF,]' can be formed by heating ReF, with SbF, by which means [ReF6]+[SbF6]-and [ReF,]' [SbZFII]-can be f ~ r m e d . ~ " More extensive studies of the ReF,-SbF, system (see ref. 257) have led to the isolation of [ReF6]+[Sb, F,, 1- and [ReF,]+ [Sb, F16]- which have been characterized by vibrational spectroscopy.630
43.9.5
Hydrides
Hydrido complexes of the types [ReH,]'-, [ReH,(L)]- and ReH,(L), (L = a phosphine or arsine ligand) constitute one of the more important groups of polyhydride complex of the transition elernents.'23,429 Of these, [ReH,]'- is historically of most importance since it was the first polyhydride complex to be prepared. It can be isolated as its sodium and potassium salts which can be prepared The reaction of [ReH,]*- in 2-propanol with by the reduction of [ReO,]- using Na or K meta1.523~524 the phosphines PPh,, PBu; or PEt,, and with AsPh, gives the substitution products [ReH,(L)]-,43' while the reactions of ReCl,(L),, ReOCl,(L), or ReO(OR)Cl, L, with LiA1H4 provide convenient routes to ReH,(L), (L = P-p-tol,, PPh,, PEtPh,, PEt,Ph, AsEt,Ph or 1/2dppe).'26,232 A neutron diffraction study of K,ReH, has shown that the anion possesses a tricapped trigonal prismatic geometry with an average Re-H distance of 1.68(5) The H NMR spectrum of [ReH,I2- in aqueous solution reveals a single proton signal, and detailed studies have been made of the vibrational spectrum of this anion, including measurements on single crystals of K2ReH9.'v526 On the other hand, structural data on [ReH,(L)]- and ReH,(L), are incomplete. The monoanions are characterized by $Re-H) modes at 1980, 1930 and 1850cm-' in their IR spectra, and a doublet for the Re-H resonance is found at S x -8 (J(P-H) x 18Hz) in the NMR s ectra denoting dynamic i~ornerization.4~' SCF-Xu-SW calculations have been carried out on [ReH,] and [ReH8(PH,)]- assuming a close structural relationship between these two species, with the PH, ligand in an equatorial position. Fluxionality is also found for solutions of ReH,(L), as studied by ' H NMR spectroscopy; a triplet is observed in the region between 6 - 4 and 6 - 6 for phosphine derivatives (J(P-H) : 13-20Hz) and a singlet at 6 - 6.0 for R ~ H , ( A s E ~ , P ~ ) , .The ' ~ * $Re-H) *~~ modes of these complexes have been located between 1980 and 1870cm-' in their IR X-Ray crystal structure determinations have located the R,P-Re-PR, unit of ReH,(PPh,),, ReH,(PMe,Ph), and ReH,[P(Pr'),Ph],, the P-Re-P angles being in the range 139 tis 147". However, whether this is in accord with the expected distorted tricapped trigonal prismatic geometry is ~ n c l e a r . ~ ~ # ~ * ~ The deuterides ReD, (PPh,),, ReD, (PEt,Ph),, and ReD,(dppe) are easily prepared and undergo acid-catalyzed H-D exchange with ethanol but the mechanism of this exchange is
-
-
F-
202
Rhenium
Exchange also occurs between ReH,(PEt,Ph), and deuterobenzene at 1OO"C, and ReD,(PPh,), is slowly converted to the hydride by hydrogen gas under ambient conditions.232ReH,(PPh,), also serves as a key starting material for some important organic transformation^.'^' The interaction of LiAlH, with Re(NPhjC1, (PMe,), followed by methanol hydrolysis at low temperatures gives ReH,(NHPh)(PMe,),. The ' H NMR spectrum of this complex in the hydride region shows a triplet at S- 5.73 with J(€-H) = 21 Hz, while the 3'P{'H) spectrum possesses a singlet at S - 30.9. Accordingly, this complex, like other nine-coordinate rhenium hydrido species, is fluxional.s30
43.10
NITROSYL COMPLEXES AND OTHERS WHERE AN AMBIGUITY IN OXIDATION STATE ASSIGNMENT MAY EXIST
While this section deals primarily with nitrosyl complexes of rhenium, there are other systems where ambiguity in assigning oxidation states can arise. These include rhenium complexes derived from the 2-aminobenzenethiolligands, viz. [Re(NHC,H,X),]+,a.- (X = H or Cl), and the dithiolates [Re(S,C, R2)3]f70*-,species which are described in the appropriate sections dealing with Rev, Re" and Rev" oxidation states (see Sections 43.7.5.1, 43.8.4 and 43.9.3). Other examples are the tris(2,2'-bipyridine) and tris( 1,IO-phenanthroline) complexes Re(bipy), and Re(phen),, which can be prepared by the Na/Hg reduction of ReC&(THF), in THF in the presence of bipy or phen. These air-sensitive, purple solids have magnetic moments of 4.1 and 4.BM, re~pectively.~~' In the case of the nitrosyl derivatives, a quite extensive chemistry has been developed. For convenience, we consider these complexes as a single group to avoid any confusion in the assigning of formal oxidation states. [Re(NO)X,]'- salts are known for X = F, Cl, Br and I. General methods for preparation of the F,s32C P 3 and BJ?'~ derivatives include the addition of NO gas to hydrated Reoz in either aqueous or ethanolic HX solutions and the reaction between so-called Re(NO)(OH), and aqueous HX. The iodide salt has been made by the latter method: Cs2C03or RbzC03dissolved in aqueous HI is added to a solution of Re(NO)(OH), in warm aqueous HI to produce black crystalline M:{Re(NO)I,] (M' = Cs or Rb).535 Neutral Re(NO)X, (L-L) and cationic [Re(NO)X(L-L),]PtCl, (L-L = bipy or phen) can be prepared from [Re(NO)X,]'- (X = F, C1, Br or I).532-535 Reactions of (Et,N),[Re(NO)X,] with a variety of polar solvents produce (Et,N)[Re(NO)X,L] (L = MeOH, EtOH, MeCN, DMSO, erc.) which upon heating yield (Et,N)[Re(NO)X,].536Crystal structures of (Et4N)[Re(NO)C14(py)]537 and (Et4N)[Re(NO)Br4L]538 (L = EtOH or MeCN) have been performed showing the molecules to have a distorted octahedral geometry with the solvent molecules situated trans to linear Re-NO units. The reaction of (Et,N),[Re(NO)X,] with N-donor ligands in EtOH or MeCN also gives salts with the formulation (Et,N)[Re(NO)X,L], where L = pyridine, aniline or PhCH,NH,, while refluxing [Re(NO)X,]'- in diglyme produces the trisubstituted complexes Re(NO)X, L,, where L = pyridine or 4-pi~oline.~~' The reactions of H2[Re(NO)X5](X = C1 or Br) have been studied and include the addition of L = bipy or phen to concentrated HX solutions of H,[Re(NO)X,] to give LH,[Re(NO)X,],540which gives Re(NO)X,L on heating, and the reaction of H,[Re(NO)X,] with pyHX in HX solution which yields (pyH),[Re(NO)X,].54' Heating the latter complex to 250°C in a COz atmosphere produces (pyH)[Re(NO)X,(py)]; heating to 400°C under CO, gives [Re( N O K (PY)21. Another reaction producing [Re(NO)X,]'- (X = C1 or Br) as a variety of R4N' salts (R = Me, Et, Bu or Phj is that of ReO(OEt)(PPh,),X with NO gas in refluxing EtOH in the presence of RqNX.542Re(NO)X3(PPh3),is also produced in this reaction. The electronic absorption spectra and far IR spectra of tertiary phosphine complexes of the types Re(NO)X,(PR, jz and Re(NOjX,(PR,), have been compared,325and an IR spectral study made of [Re('4NO)Xs]2-and [Re(15NO)X5]2- Several polarographic studies5%% have been carried out on the [Re(NOjX,I2- anions and their related substitution products. One study5,, compared [Re(NO)X,]'- with [Rex,]'-, finding the two species electrochemically similar and implying that Re is 'quadrivalent' in the nitrosyl complex. X-Ray photoelectron spectroscopic (XPS) evidence, in conjunction with electrochemical and dipole moment measurements," also indicates that NO is present in a neutral or negatively charged state in a variety of Re-NO complexes with formal oxidation states ranging from + 1 to + 4 even though the Re-NO interaction is essentially linear, and thus can be considered formally as a three-electron donor, NO'. Whether the nitric oxide is present as NO+ with Re", or as NO- with Re'", or as some intermediate situation has not been determined. In another study focusing on the charge distribution in these complexes, it was concluded, on the basis of electrochemical measurements, that the electron transfer was from the Re, with NO making little contribution to the redox
Rhenium
203
A more recent XPS study of [Re(NO)X5I2-and some related complexes reports evidence for a linear to bent transformation in the bonding mode of coordinated NO when the samples are exposed to X-rays.548 ReH,(NO)(PPh,), has been prepared by refluxing [Re(NO)Xsl2- (X = CI or Br) and PPh, in ethanol followed by slow addition of NaBH,. This hydride complex reacts with X, to give Re(NO)X,(PPh,), (X = C1, Br or I) and either ReH,(NO)(PPh,), or [Re(NO)X,]’- reacts with CO gas to give Re(NO)(CO),(PPh,),.’49 The crystal structure of ReH2(NO)(PPh,),’50shows it to have distorted octahedral geometry with two PPh, ligands in axial positions and the other PPh,, the two hydrides, and the NO in equatorial positions. The NO is linked linearly as NO+. In the reaction of hydrated Reo, with gaseous NO in the absence of HX, the complex Re(NO)(OH),-H,O is said to be formed.533Both this reaction and that of Reo2and NO in the presence of HX have [ReO,J- as a by-product. Re(NO)(OH),*H,O and the related Re(NO)Cl,-xH,O have been the subjects of a number of investigations with regard to their synthesis, interconversion and hydrolysis reaction^.^^'-'^^ The acid H[Re(NO)(C,O,)(OH), (H, O)]is purported to be formed from the reaction of Re(NO)(OH),*H,O with oxalic acid in aqueous medium, and the K + , NH: and Pb2+salts of the acid have been de~cribed.”~ The anion of this acid reacts with organic diimines to produce the mixed ligand complexes, Re(NO)L(C,O,)X (L = bipy or phen; X = OH or Cl). Re(NO)(OH),*H,O does not react directly with acetic acid, but when refluxed in a mixture of acetic acid and acetic anhydride, a product proposed to be [Re,(NO),(O,CCH,),O(OH),] has been reported.”’ Most rhenium nitrosyl complexes have been prepared using NO, NOX or HNO, as the nitrosylating agent. Recently, the reductive nitrosylation of [Reo,] with NH,OH-HCl in the presence of NCS or Ny in an alkaline medium has been ac~omplished.’’~ The complexes formed in this way include [Re(NO)X,(H,O)] - (X = NCS or N3) and [Re(NO)X,(L)(H,O)] (L = bipy or phen), containing the [ReN0Izf moiety and existing in several isomeric forms. Another method of introducing a nitrosyl group is by the use of N-nitrosoamides. An ethanol PPh, and NaBH, gives ReHsolution of ReCl,(PPh,),, N-methyl-N-nitrosotoluene-p-sulfonamide, (NO),(PPh,),. Addtion of the same N-nitrosoamide to ReH(CO),(PPh,), in refluxing benzene yields Re(NO)(CO),(PPh,),.”7 The reactions of these complexes with X, (X = C1, Br or I) and RX (X = halide; R = H, Me, MeCO and PhCO) have been investigated. A phase that is said to possess the stoichiometry Re(NO)Cl, has been prepared by passing NO gas through a CCl, solution of ReCl,. Using benzene instead of CC1, yields Re(NO)Cl,-C,H,.”8 So-called Re(NO)Cl, has been reacted with a variety of donor ligands to produce materials analyzing as Re(NO)Cl,(NCMe), Re(NO)Cl,(py),, Re(NO)Cl(Ph, PO)(NCMe) and Re(NO)CI,hen),.^^^ The X-ray crystal structure of Re(NO)Cl, (NCMe) shows the molecules to have an octahedral geometry with a cis arrangement of the NO and MeCN ligands.559 A complex which was first reported to be [Re(NO)C12(PPh3)2],5M made by passing NO gas through a suspension of ~ReC12(N2COPh)(PPh,)2] in benzene-methanol (1: l), has been reformulated as [Re(NO)C1,(OMe)(PPh,),].561 Reactions of this complex with L (L = CO, NH,, SO,, MeCN, PhCN, py, MeNHNH,, NH,CH,CH,OMe or TCNE) result in the substituted products [Re(NO)Cl, (PPh,), L], while reactions with tertiary phosphines tend to give products in which the PPh, ligands are displaced, such as [Re(NO)Cl,(PR,),]. The reaction with PMe,Ph yields
[Re(NO)C12(PMe,Ph)2(PPh,)].’60 [Re(NO)Cl,(OMe)(PPh,),] also reacts with NaBH, and PPh, to give a high yield of ReH,(NO)(PPh,),, which in turn is a useful starting material for a variety of rhenium nitrosyl comple~es.’~’ ReH, (NO)(PPh,), reacts with HCl in ethanol to form Re(NO)Cl,(PPh,),, while the addition of the latter complex to a CH,CI, solution saturated with CO gas results in Re(NO)Cl,(CO)(PPh,),. Re(NO)Cl, (PPh,), also reacts with p-tolyl isocyanide in refluxing benzene to yield Re(NO)Cl,(CN-p-tol),(PPh,). The fluoro cation [Re(NO)F(CO)(PPh,),]+ can be prepared by the reaction of ReH,(NO)(PPh,), with HBF, or HPF, in the presence of CO. This cation reacts with X- = H-, OCH; or F- to give the neutral species Re(NO)XF(PPh,),. The crystal structure of the perchlorate salt of this fluoro cation has been determined; the PPh, ligands are meridonal and the fluoride is trans to the linear Re-NO group. A thiolatc+nitrosyl complex [Re,(NO),(SPh),]- has been made in low yield by reacting [Re(NO)Cl,(PPh,),] (later reformulated as [Re(NO)Cl,(OMe)(PPh,),j)560~56’ with thiophenol in refluxing methanol in the presence of triethylamine. The crystal structure performed on the [Ph,P]+ salt of the p-tolylthiol derivative shows the complex to consist of two pseudooctahedral rhenium units linked by three bridging thiolato ligands. The Re-Re distance of 2.783(1)A indicates significant metal-metal intera~tion.’,~ Thionitrosyl complexes of rhenium have been prepared either by the addition of S,C1, or SCl, to nitride-containing rhenium species or by introduction of NS via NS+ SbF;, Thus,ReNCl,(PRPh,), (R = Ph or Pr”) and S,CI, give Re(NS)Cl,(PRPh,),; ReNX,(PR,), (PR,= PMe,Ph, PEt,Ph or
204
Rhenium
PMePh,; X = C1 or Br) and half an equivalent of S2C12yield Re(NS)ClX(PR,),; an excess of either SzClzor SCl, added to ReNCl,(PR,), gives Re(NS)Cl, (PR3)2;and [ReNCI(dppe),]Cl with S2Cl, forms [Re(NS)Cl(dppe),]C1.563The reaction between Re(CO), Br and NS’ SbF; leads to formation of [Re(CO),NSI2*,a cation which can also be prepared by elimination of F- from [Re(CO),(NSF)]+ using AsF,.’”,’~~ There are very few monomeric rhenium complexes with two nitrosyl ligands. The earliest examples also contained PPh,, such as Re(NO),X(PPh,), (X = C1, Br or 1)542,566 and Re(NO),X,(PPh,), (X = C1 or Br).,% More recently, Re(NO),X, (X = C1 or Br) and several related dinitrosyl complexes have been prepared. Re(NO),C1, is associated by C1 bridges and can be obtained in quantitative yield from the reaction between ReCl, and CC13N02+56’ The bromo derivative cannot be made directly from ReBr,, but addition of excess BBr, to Ph, P[Re(NO),Cl,] jields Ph,P[Re(N0)2Br,] which can then be reacted with AlBr, in CH2Br,to produce Re(NO)zBr,.5* The NO ligands are in a cis arrangement; Re(NO),Br, is dimeric with Br bridges. Re(NO),Cl, adds an MeCN molecule when dissolved in this solvent giving Re(NO),Cl, (NCMe). The Addition of PbAsCl to the MeCN product in CH,Cl, produces the salt P~,As[R~(NO),C~,].~’ X-ray crystal structures of the MeCN adduct and the Ph,As+ salt have been solved; the NO groups are cis to one another in both cases. The reaction of Ph,As[Re(NO),Cl,] in CH,Cl, in the presence of MeCN and H20 gives Ph,As[Re(NO)CI, (OC(NH,)CH,)].569 Other reactions of Re(NO),CI, include the addition of PPh, in CH2C1, to form the phospheneiminato complex [Re(NO)Cl, (NPPh3)(0PPh,)]570and the addition of excess SbPh, in CH2C1, to produce [Re(NO),C12(SbPh3),].571 Further examples of rhenium nitrosyls have been reported. The reactions of Re(NO)C12(OCH3)(PPh3)2 with thiophenols which contain methyl or isopropyl groups in their ortho positions gives trigonal bipyramidal complexes of the type ReNO(SR),. The X-ray crystal structure of the complex where R = 2,6-diisopropylphenyl has been detem1ined.6~~7~~~ Thiophenols without ortho substituents give dinuclear [Re,(NO),(SR),]- which contain a triple thiolate bridge:34*635The reductive nitrosylation of [ReOJ using an excess of CN-, OH- and NH,OH*HCl has been used to prepare K3[Re(NO)(CN),] 2H,0.636A similar procedure using NCS- in place of CN- followed by acidification and treatment with NO;, is said to provide a route to the dinitrosyls A[Re(NO),(NCS),] (A = Me,N, Et4N, Ph,P or Ph,As) and Re(NO),(NCS),(LL) (LL = bipy or hen).^^^ Another interesting example of a thionitrosyl complex has been described. ~~As],[Re(NS)(NSCl)Cl,]has been synthesized from the reaction between S,N,, ReNC1, and ~ ~ analogous reaction of S,N, Ph,AsCI in CH,CI,. Its X-ray crystal structure has been s 0 1 v e d . ~The with ReC1, gives [ P ~ , A S ] R ~ ( N S ) C ~ , , ~ ~ *
-
43.11 MISCELLANEOUS COMPLEXES
A series of polyoxotungstate anions which contain high valent rhenium, e.g. [SiW,, Re20,0]2-’3-’4- ha ve been synthesized in the form of BuiN+ ~alts.6~’ The anions contain rhenium in oxidation states VII, VI and V. Cyclic voltammograms, ESR and electronic absorption spectra have been used to characterize these complexes.
43.12
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Rhenium 571. 572. 573. 574. 575. 576. 577. 578. 579. 580. 581. 582.
213
V. D. Fenske, V. N. Mronga and K. Dehnicke, Z. Anorg. Allg. Chem., 1983,498, 131. M. J. Abrams, A. Davison and A. G. Jones, inorg. Chim. Acm, 1984,82, 125. U.Muller, Acta Crystallogr., Sect. C , 1984,40,571. M. A. A. F. De,C. T. Carrondo, A. M. T. S. Domingos and G. A. Jeffrey, J. Orgunornet. Chem., 1985,289, 377. J. Chatt, W.Huswn, G. J. Leigh, H. M. AIi, C. J. Pickett and D. A. Rankin, J. Chem. SOC.,Dalfon Trans., 1985, 1131. A. J. L. Pombeiro, Inorg. Chim. Acta, 1985,103,95. S. Donovan-Mtunzi, R. L. Richards and J. Mason, 1. Chem. Soc., Dalton Trans., 1984,469. D. L. Hughes, A. J. L. Pombeiro, C. J. Pickett and R. L. Richards, J. Chem. Soc., Chem. Commun., 1984,992. A. J. L. Pombeiro, 3. C. Jeffery, C. J. Pickett and R. L. Richards, J. Organornef. Chem.. 1984,277, C7. S. Donovan-Mtunzi, R. L. Richards and J. Mason, J. Chem. Soc.. Dalton Trans., 1984,2429. C. B. Powell and M. B. Hall, Inorg. Chem, 1984,23,4619. T.J. Barder, F. A. Cotton, K. R. Dunbar, G. L. Powell, W. Schwotzer and R. A. Walton, Inarg. Chem., 1985,24,
2550. 583. F. A. Cotton, L. M .Daniels, K. R. Dunbar, L. R. Falvello, S. M. Tetrick and R,A. Walton,d. Am. Chem. Soc., 1985, 107, 3524. 584. T.J. Barder, F. A. Cotton, L. R. Falvello and R. A. Walton, Inorg. Chem.. 1985,24, 1258. 585. L. B. Anderson, T. J. Barder and R. A. Walton, Inorg. Chem., 1985,24, 1421. 586. T. J. Barder, D. Powell and R. A. Walton, J. Chem. SOC.,Chem. Commun., 1985,550. 587. K. R.Dunbar, D. Powell and R. A. Walton, Znorg. Chem., 1985,24,2842. 588. L. B. Anderson, S. M. Tetrick and R. A. Walton, J. Chem.Soc., Dalton Trans., 1985,55. 589. J. V. Casper, B. P. Sullivan and T. J. Meyer, Inorg. Chem., 1984,23, 2104. 590. T. Nicholson, N. Shaikh and J. Zubieta, Znorg. C h h . Acta, 1985,99,L45. 591. T. Nicholson and J. Zubieta, Znorg. Chim. Acta, 1985,100, L35. 592. J. R. Dilworth, S. A. Harrison, D. R. M. Walton and E. Schweda, Inorg. Chem., 1985,24, 2594. 593. J.-L. Vanderheyden, M. J. Hegg and E. Deutsch, Znorg. Chem., 1985,24, 1666. 594. J. D. Allison, G. A. 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44.1 Iron(ll) and Lower States PELHAM N. HAWKER Catalytic Systems Division, Johnson Matthe y PL C, Ro yston, and
UK
MARTYN V. TWlGG IC1 Chemicals and Polymers Group, Billingham, UK Because of factors beyond the editors’ control, the submission of the manuscript for this chapter was delayed. In order to minimize any delay in publishing ‘ComprehensiveCoordination Chemistry’ as a whole, the coverage of iron(I1) and lower states appears at the end of this volume, commencing on page 1179.
215
44.2 Iron(ll1) and Higher States S.MARTIN NELSON Queen Is University, Belfast, Northern Ireland 44.2.1 INTRODUCTION 44.2.2 CARBON LIGANDS 44.2.2.1 Cyanide Complexes
217 220 220
44.2.3 NITROGEN LIGANDS 44.2.3.1 Monoden taie Ligands 44.2.3.2 Bidentate Ligands 44.2.3.3 Ter- and Poly-dentate Ligands
222 222 222 225
44.2.4 PHOSPHORUS AND ARSENIC LIGANDS 44.2.5 OXYGEN-CONTAINING LIGANDS 44.2.5.I Complexes of Neutral Monodentute Ligand 44.2.5.2 Complexes of Carboxylates and Related Anionic Ligands 44.2.5.3 Complexes of Diketones and Related Ligands 44.2.5.4 Complexes of Phenols, Catechols, Quinones and Related Systems 44.2.5.5 Complexes of Hydroxamic Acid, Thiohydroxamic Acid and Derivatives
226 226 226 228 229 230 233
44.2.6 SULFUR LIGANDS 44.2.6.1 Surfid0 and Thiolato Complexes 44.2.6.1.1 Mononuclear 'FeS, ' systems 44.2.6.1.2 Dimclear ' F e S ' systems 44.2.6.1.3 Trinuclear 'Fe-S' systems 44.2.6.1.4 Tetranuclear 'Fe-S' systems 44.2.6.1.5 Hexmuclear %e-S' systems 44.2.6.1.6 'MeFe-S' complexes 44.2.6.2 complexes of Dithiocarbamates and Related Ligands
234 234 235 235 237 238 24 1 24 1 242
44.2.7 HALIDE COMPLEXES 44.2.8 COMPLEXES OF MIXED DONOR ATOM LIGANDS 44.2.8.1 Complexes of Sckiz Base Ligands 44.2.8.1.1 Spin-crossover iron(III)-Schif base complexes 44.2.8.2 Complexes of Arninocarboxylates, Amino Acidr and Related Ligands 44.2.8.2.I Hemerythrin and model compounds
241 249 249 252 253 253
44.2.9 MACROCYCLIC LIGANDS 44.2.9.1 Synthetic' Macrocyclic Ligands 44.2.9.2 Porphyrins and Reiated Ligands 44.2.9.2.1 The heme proteins 44.2.9.2.2 Electron transport heme proteins 44.2.9.2.3 Cytochrome PG0,peroxidase and catalase enzymes
255 255 259 262 263 264
44.2.10 IRON(KV), IRON(V) AND IRON(V1) 44.2.1 1 REFERENCES
266 261
44.2.1
INTRODUCTION
The common oxidation states of iron are + 2 and + 3. The relative stability of the two oxidation states in acid aqueous solution is defined by the standard electrode potential of + 0.77 V for the Fe3+/Fez+ couple.laThis potential is such that the hydrated Fe" cation is thermodynamically unstable with respect to atmospheric oxidation (equation 1). The oxidation is even more favourable in basic solution (equation 2). It is apparent, therefore, that the chemistry of iron, including its 217
218
Iron
importance in biology, is closely associated with the ready interconversion of these two oxidation states and with the dependence of the redox potential on the ligand environment.
+ +02+ 2H+ Fe(OH),(s) + OH2Fe2+
-+
* We3+
+
H20
&Fe203.3H20+ e-
Ea
=
0.46V
(1)
(2)
Ea = -0.56V
The small size (Goldschmidt radius 0.53A)and relatively high charge of the Fe3+ ion are responsible for its strong tendency to hydrolyze in aqueous solution. Thus, the hexaaqua ion is a Bronsted acid of moderate strength (pK, = 3.05) existing in the undissociated form only below pH 2 (equation 3). At higher pH further proton dissociations lead to the formation of hydroxoand oxo-bridged di- and poly-nuclear species such as (1) and eventually to precipitation of hydrated iron(lI1) oxide.'' The high charge density of Fe3+is also responsible for its strong affinity for 'hard' or 'class a' donors such as oxygen and F-. It is for this reason that Fe3+,unlike Fez', forms few stable complexes with ligands such as ammonia or simple amines which are also good Bronsted bases in aqueous solution.
-
[Fe(H20)6]3+ e [Fe(H20),(OH)]2+
+ H+
(3)
The Fe"' ion has a d Selectron configuration. While it is high spin (S = $) in most of its complexes there is the possibility of the stabilization of low-spin (S = +) ground states in strong octahedral fields such as generated by CN- and many bi- or poly-dentate ligands containing unsaturated nitrogen (Figures 1 and 2). In addition, intermediate spin (S = 2) states may be produced in fields of lower symmetry. Magnetic susceptibility measurements and ESR and Mossbauer spectra provide convenient means for distinguishing between different possible spin states in most cases. For the common case of high-spin octahedral iron(II1) which has an orbitally non-degenerate ' A , ground state the magnetic moment is given by the high-spin-only value of 5.92BM which should be temperature independent. Where deviations from this value occur either an antiferromagnetic or, less commonIy, a ferromagnetic interaction between adjacent paramagnetic centres of a thermally controlled equilibrium between different spin states is usually indicated. Spin-doublet and spin-quartet ground states are associated with magnetic moments, at ambient temperature, close to 4.0 BM and 2.2 BM, respectively, corresponding to the presence of one and three unpaired electrons (Figure 1). Because of the orbital singlet nature of high-spin Fe"' there are no excited states of the same spin multiplicity and all ' a d ' transitions are therefore spin forbidden as well as Laporte forbidden. The free-ion ground state 6 S becomes 6 A ,in a cubic field while the first excited state 4Gsplits into two T states, T, and T,, and into a degenerate pair, 4A, and 4E(Figure 2). The first four ligand field
(01 s = 2 2
S=$
( b l S =2 z
$. 3 2
Figure 1 The occupation of the 3d orbitals in weak and strong field (a) octahedral and (b) square pyramidal Fe"' complexes
Iron(I11) and Higher States
219
801
3
A(B, Figore 2 A semiquantitative energy level diagram Fanabe-Sugano diagram) for the d 5 configuration'
transitions for the high-spin d5 ion are therefore those indicated in equation (4). Since these sextet-quartet transitions are both spin and parity forbidden, intensities seldom exceed 0.01 dm3mol-' cm-' ; in tetrahedral symmetry a measure of d-p mixing occurs and somewhat higher intensities are usually observed. In strong octahedral fields the 'T2 component of the 2 I free-ion excited state becomes sufficiently lowered in energy to become the ground state (Figure 2) corresponding to the spin-paired ti, d orbital electron occupation shown in Figure 1,
-
As a result of the spin- and parity-forbidden nature of the ligand field transitions of the high-spin d 5 ion many simple salts and complexes of iron(II1) have little or no colour. The hexaaqua ion, for
example, has a very pale violet colour while [FeF,I3- is virtually colourless. Where strong colour in high-spin iron(II1) compounds does occur this can usually be assigned to ligand-to-metal charge transfer absorption (or to internal transitions of the coordinated ligand) as in the yellow and red-brown [FeClJ and [FeBrJ complexes. The analytical applications of the blood red colour of Fe3+/NCS- complexes are well known. The commonest coordination number of Fe"' is six but a range of other coordination numbers -three, four, five, seven and eight -are also well established. Examples of all of these will be given in the following pages along with discussion of the structural and electronic factors which determine them. The treatment of the coordination chemistry of iron(III), and higher states, adopted in this review is, as elsewhere in this volume, by means of ligand type. We begin with Main Group IV ligands and follow with ligands of Groups V, VI, VII, mixed donor atom ligands and macrocyclic ligands. Naturally occurring ligands are not considered separately as in other chapters but, instead, are treated together with synthetic ligands of the same donor atom class. Some overlap between sections is inevitable. Iron(II1) complexes are considered first followed by complexes of higher oxidation
Iron
220
states. Because of the comparable stabilities of iron(I1) and iron(1II) no discussion of the coordination chemistry of iron(II1) can ignore that of iron(I1). However, it is hoped that the unavoidable reference to iron(II), the subject of the preceding chapter, will be illuminating rather than merely repetitive. In preparing this review emphasis has naturally been placed on recent rather than early work while at the same time hopefully giving due place to the latter. The coverage is far from comprehensive since limitations of space dictated ruthless selection of material. Such selection is necessarily subjective and, in this chapter, probably reflects as much on the author’s preferences and prejudices as on his good judgement. Valuable early accounts of various aspects of iron(III), (IV) and (VI) chemistry may be found in refs. 3-9. For more recent general treatments see refs.
44.2.2
CARBON LIGANDS
Organometallic compounds of iron have been comprehensively covered in the companion work, ‘Comprehensive Organometallic Chemistry’.16 The present review is therefore restricted to Fe’” complexes of cyanide.
44.2.2.1
Cyanide Complexes
A comprehensive monograph” on cyano complexes of the transition metals, including Fer”, is available. Cyanides and fulminates as ligands are also the subject of Chapter 12 of Volume I of this series.” The ambidentate nature of the CN- ion has also been reviewed.” The short C-N distance of 1.16 A in CN- is in accordance with the existence of a CEN triple bond. The highest occupied 0 orbital is localized on the carbon atom (Figure 3) and this dictates that the carbon end of the ion is more basic than the nitrogen end?’ It is thus expected, and observed, that when acting as a monodentate ligand CN- coordinates viu the carbon atom, with a linear, or near linear, M-C-N bond system. CN- can, of course, act as a bridging ligand between neighbouring metal ions, using both carbon and nitrogen atoms. The most important cyano complex of Fe”’ is the red hexacyanoferrate(II1) anion, [Fe(CN)J3-. This is usually prepared, as the potassium salt, by the oxidation of hexacyanoferrate(II), [Fe(CN),I4-, with chlorine. [Fe(CN),j3- has a magnetic moment of 2.25 BM at 300 K in accordance with the presence of one unpaired electron as required for a ’ T , ground state (Figure 1) with appreciable orbital contribution?’ IO
0
x
5
I
VI
-10
-20
Figure 3 Molecular orbitals of the cyanide ion. Orbital energies calculated by a semiempirical SCF-MO procedure”
Iron (III) and Higher States
22 1
K3[Fe(CN),] is dimorphic, existing in monoclinic and orthorhombic forms, depending on how the ordered layers are stacked in the crystal lattice. The com lex anion has near octahedral symmetry with Fe-C and C-N distances close to 1.95 and 1.14 , respectively.22Interestingly, the Fe-C bond in [Fe(CN)J- is longer than in [Fe(CN),I4- (1.92 A) despite the smaller sue of the Fe3+ion. This is attributed to the n-accepting capacity of the CN- ion, the n bonding being more pronounced in the lower valence state ~ o m p l e x ' ~However, ~'~. the Fe"' complex is nonetheless thermodynamically more stable (by about lo7) than the Fe" complex. This apparent anomaly has been attributed to a very negative entropy of hydration of the highly charged [Fe(CN),I4- anion. The same considerations account, of course, for the less positive standard electrode potential ( E " = +0.36V) for [Fe(CN),]3-/[Fe(CN),]4- than observed for the hydrated ions ( E " = + 0.77V).'7[Fe(CN)J3 is therefore a moderately effective oxidizing agent. In contrast to [Fe(CN),I4- the corresponding Fer'' complex is poisonous and is generally more reactive in regard to ligand substitution. Several substitution products of the type [Fe"'(CN),L]"-- are known where L = e.g. H,Q, NH,, N;, SCN-, heterocyclic Like the parent [Fe(CN),j3- ion, all have magnetic properties and Mossbauer spectra27characteristic of low-spin Fe"'. In the IR spectra the v(CN) stretching vibrations occur in the range 2090-21 50 cm-'. Further details may be found in Sharpe's monographi7and references therein. In the pentacyano complexes a new band in the electronic spectra has been assigned as L + Fe charge transfer in origin with the aid of magnetic circular dichroism measurements." The bridging capacity of the CN- ion has been mentioned, this arising from the presence of a non-bonded electron pair on the nitrogen as well as the carbon atom. Many polymeric transition metal cyanides are known, the structures of which may involve chains, sheets or three-dimensional networks. For the case of iron the best known are the Prussian blues and the Turnbull's b l u e ~ . * ~ * ' ~ , ' ~ The former are obtained by treating a solution of Fe"' with [Fe(CN),I4- and the latter by treating a solution of Fe" with [Fe(CN),I3-. IR,,'magnetic3' and M O ~ s b a u edata r ~ ~ have shown that the two substances are essentially the same although variations in composition accompany variations in preparative conditions (Figure 4). These materials are now formulated as iron(II1) hexacyanoferrate(I1) complexes regardless of the combination of starting materials, the iron(II1) component being high-spin. The intense colour is attributed to electron transfer between Fe" and Fe"'. Significantly, K,Fe[Fe(CN),], which contains only Fe", is white. A single crystal X-ray study of Fe,[Fe(CN)6],*xH,0 ( x = 14-16) has revealed3, that the average distances are Fe"-C = 1 . 9 2 k C-N = 1.13 A and Fe"'-N = 2.03A. A neutron diffraction study has also been carried The complexes Fe[Fe(CN),X]-xH,O (X = H,O, NH, or CO) have been prepared.35These also contain high-spin Fe" -N and low-spin Fe"-C centres. The structure of the [PPh4I4 salt of the binuclear complex anion [Fe,(CN),,(NH,)I4- formed by reaction of [Fe(CN),13,' with [Fe(CN),(NH,)]'- has been described.36The polynuclear complex formed by reaction of [Fe(CN)J- with [Cu(dien)]*+ contains both linear and non-linear Fe--C-N-Cu h~kages.~' The bifunctional capability of the CN- ion also means that, as well as acting as a bridge between adjacent metal centres, the nitrogen end may act as a base towards Hf and various Lewis acids. The acid H,Fe(CN), is somewhat unstable and difficult to obtain pure; it is said to be a strong acid in aqueous solution. Not surprisingly, when charge effects are considered, the coordinated CN- group has poorer basic character in Fe"' complexes than in corresponding Fe" complexes.38A number of adducts with Lewis acids such as BF, have been reported." An interesting series of dinuclear iron-cyano complexes [(NC), Fe-L-Fe(CN),]4-,S-36in which the metal centres are bridged via bifunctional heterocyclic amines such as pyrazine, 4,4'-bipy-
B:
Figure 4 The structure of Prussian blue and related compounds. If none of the cube centre sites are occupied, the structure is that of ferric ferricyanide (both black and white Fe positions occupied by Fe"'); if every second cube centre site (marked with a dotted circle) is occupied by K + , the structure is that of soluble Prussian blue (black = Fe", white = Fe"'); if all the centre sites are occupied by K+ (crosses as well as dotted circles) the structure is that of dipotassium ferrous ferrocyanide (black and white sites = Fe")
222
Iron
ridine and trans- 1,2-bis(4-pyridyl)ethylene have been described.36Mixed valence species [Fe"-L -FeJ'1]5p are generated as intermediates in the two-electron redox process (equation 5). However, conproportionation constants for the [II.II] [III.III] e 2[II.III] equilibria corresponded fairly closely to a statistical distribution of the three species, rendering unlikely the isolation of the pure mixed valence complex. An intervalence transfer transition for solutions of the mixed valence species was observed in the near IR at 8000 cm-', the energy depending only slightly on the nature of the bridging ligand. The intensity of the intervalence band indicated only a small degree (- 1%) of electron delocalization. These systems therefore appear to belong to Class I1 of the Robin-Day classificationMfor which there is only a weak coupling between the metal centres.
+
-
[Fe11'-L-Fe1i']4-
+ 3-
e
[Fe'l-L-Fe"]n-
(5)
Other cyano complexes of Fe"', containing only one or two cyano groups, will be considered in later sections which focus principally on the coordination chemistry of the associated ligands.
44.2.3
NITROGEN LIGANDS
The affinity of iron(II1) for nitrogen is relatively low except where stability is enhanced by chelation, particularly with multidentate ligands containing anionic oxygen, or in macrocyclic ligands, or where stability is enhanced via ligand field stabilization effects accompanying spin pairing.
44.2.3.1 Mondentate Ligands
Addition of ammonia or simple amines to aqueous solutions merely precipitates the hydrous nH,0, though under anhydrous conditions the poorly stable hexaammines oxide, Fez0,. [Fe(NH,),]X, (X = e.g. C1, Br) are said to be formed.41These of course, are readily decomposed by water. [Fe(NH,),(NO)]Cl, has been formulated42as a complex of Fe"' with NO- rather than of Fe' with N O + . A number of Fe'" compounds containing pyridine have been reported. Some of these4, have salt structures containing the pyridinium cation, [pyH]+, however, together with e.g. [FeClJ- . Some genuine complexes do appear to exist, however. Reaction of FeCl, with anhydrous pyridine gives a red complex formulated as,fac-Fepy,Cl,-py on the basis of IR and ESR spectra.44An analogous thiocyanate complex, Fepy,(NCS),, has also been prepared from cthanolic solution.45 Use of pyrazine or pyrimidine in place of pyridine afforded complexes believed to be dimeric or polymeric containing pyrazine or pyrimidine bridges.43The green complex [FeL&l,] (L = 4-cyanopyridine), prepared by reaction of anhydrous FeCl, with 4-cyanopyridine in trichloromethane, has a trigonal bipyramidal structure in which the three chlorine atoms occupy the equatorial positions and the = 5.56 BM at 296K, pyridine nitrogen atoms the axial positions.46The magnetic properties (peE 4.77 BM at 9 K) suggest a measure of intermolecular antiferromagnetic interaction. The Fe"' atom in the complex salt [ P ~ A s ~ ~ [ F ~ ( Nalso , ) , ] has ~ ' a trigonal bipyramidal geometry with the equatorial Fe-N distances slightly shorter than the axial distances.48 The complex is high-spin with the magnetic moment (6.0BM) close to the spin-only value. In contrast to the five-coordination found for the azido complex the cyanato and thocyanato complexes, [PkAsJ[Fe(NCO),] and [Me4N3[Fe(NCS),], isolated from non-aqueous media, are, respectively, tetrahedral and ~ c t a h e d r a l . ~IR ~ . ' ~and electronic spectral data indicate that the bonding atom of the pseudoharide ion is nitrogen in both Among the relatively few examples of three-coordinate transition metal compounds is the Fe'" The structure has been determined.s2The complex of bis(trimethylsilyl)amine, [Fe{N(SiMe,)2}3].51 'FeN,' and 'FeNSi,' units are planar, the latter making an angle of 49" with each other. The magnetic moment is 5.91 BM at 298K and the susceptibility obeys the Curie-Weiss law with 8 = - 10". A very large quadrupole splitting (5.12mms-I) was observed in the Mossbauer spectr~m.~,
44.2.3.2
Bidentate Ligands
[Fe(en),]Cl, has been prepared by the slow addition of 1,Zdiaminoethane (en) to anhydrous FeCI, in absolute ethanol. The complex is low-spin having a magnetic moment of 2.45BM at room
Iron ( M ) and Higher States
223
temperature, falling to 2.17 BM at 80 K. The data are in accord with those predicted assuming a spin-orbit coupling constant of -400cm-', a delocalization constant of 1.0 and little or no axial distortion. Mossbauer data also indicate low distortion, the quadrupole splitting being much smaller (1.09 mm s-' at 290 K) than observed for [Fe(phen)J3' (1.67 mm s-') or [Fe(terpy),l3+ (3.09mms-')." The absence of low energy charge transfer absorption allowed a determination of and tentative assignment of the ligand field spectrum in the solid state and evaluation of the ligand field parameters Dq, B and C as 1950, 500 and 2000cm-', respectively; the value of B corresponds to 50% reduction from the free ion value. Corresponding values of Dq and B for [Fe(CN),I3-have been reported as 3500 and 720 cm-', re~pectively.'~ Oxidation of the tris complexes of Fe" with the a-diimine ligands such as 2,2'-bipyridyl (bipy) and 1,lO-phenanthroline hen)^^ affords the corresponding Fe"' complexes [Fe(bipy)$+ and [Fe(phen)J3+. These blue complexes have low-spin ( S -- +) ground states, as conclusively shown by m a g n e t i ~ , ~M "~ o s~~~b~a ~ u e rand ~ ~ ESRm measurements. The crystal structure of [Fe(phen),][ClO,],.H,O has been determined.&The Fe"' ion is octahedrally coordinated by three symmetrically bidentate phen ligands, the Fe-N distance being 1.973(8)A. The ESR spectrum determined at 85 K showed the principal g values to be along the molecular pseudo C, axis (8, = 1.459) and in plane normal to it (g2= 2.615, g, = 2.727). The ESR measurements together with the temperature dependent magnetic susceptibility data were interpreted to yield a splitting of the orbital degeneracy of the * TZgground term such that the 'AIg orbital singlet lies lowest, some SOOcm-' below ,Eg. Unlike the CN- ion, replacement of H,O in the coordination sphere of the hexaaqua Fez+or Fe3+ by bipy or phen raises the standard redox potential (from 0.77 V to 1.12 V in the case of phen). This stabilization of the lower oxidation state is attributed to extensive back-coordination of the t; electrons into the vacant p: orbitals of the a-diimine ligands. The intense red colour of the Feis complexes is likewise due to this metal-to-ligand charge transfer. The reversibility of the redox reaction and the easily detectable colour change make these complexes very useful redox indicators.6',62 The dicyano complexes [Fe(bipy),(CN),]X and [Fe(phen),(CN),]X (X = C104, NO,) are also low-spin with magnetic propertied, and Mossbauer spectra5' (Table 1) comparable with those of the tris complexes. Little or no t2gelectron delocalization was apparent in the magnetic results. The stereochemistry of these mixed ligand complexes is presumably cis.
-
N
Table 1 Mossbauer Parameters for Some IronOlI) Complexes with Nitrogen Donor Ligands
Complex
T (W
Isomer shqt (6)" (mm s-I)
Quadrupole splitting (AEQ) (mm s- ') 1.09 1.69 1.go 1.62 1.71 1.63 3.09 3.43
a
Relative to iron foil.
Treatment of Fer''salts in aqueous media with phen gives not the blue tris complex [Fe(phen),13+ but brown dinuclear complexes of composition Fe,O(phen),X,-nH,O (X = ClO;, NO;, C1-, +SO4'--)." These binuclear complexes have temperature dependent paramagnetic susceptibilities corresponding to ca. 1.9BM per Fe atom at room temperature and decreasing with decrease in ~ of antiferromagnetic superexchange interactemperature. The results are best i n t e r ~ r e t e din~terms tion*' between high-spin (S = +) paramagnetic centres linked via an oxo bridge as in structure (2). Analysis of the magnetic data gave values of the exchange coupling constant J of ca. - 100cm-'. In support of the presence of a p-oxo bridge was the observation of a strong band in the IR at stretching vibration, this being a commonly observed 840-860 cm- ' assigned to the Fe-0-Fe feature of such compounds. Mossbauer isomer shifts confirm the assignment of S = % spin states to the interacting Fe'" ions.5g [CKphen), Fe-O-Fc(phenj2CI1
*+
(2)
A number of studies of the complex of empirical formula Fe(phen)C1, have been made. First prepared by Simon et it has been formulated as [Fe(phen),Cl,][FeCl,] on the basis of magnetic,
Iron
224
spectrophotometric, conductimetric and IR data and of the isolation of a perchlorate salt [Fe(phen)2C12]C10,.6s7 On the other hand two modifications prepared by Fitzsimmons and coworkers" in glacial acetic acid and in aqueous HCl have been assigned a chloride-bridged structure since neither form displayed the Mossbauer resonance expected for the [FeCl,]- anion. More recently, a single crystal X-ray structure determination6*on the product of reaction of FeC1, with phen in acetone has revealed the [Fe(phen),Cl,][FeCl,] structure. The six-coordinate cation has the expected cis configuration. The complex is high-spin, the magnetism conforming fairly closely to the Curie law. The high-spin nature of this complex shows that, as with Fe", coordination of only two phen molecules does not affect spin pairing. Not unexpectedly, the Fe--N distances are significantly longer than in the low-spin complex [Fe(phen)3][C104]3.w The red complexes [Fe(phen),ox].5H20 and [Fe(phen),rnal]-7H2O (ox = oxalate, mal = malonate), as well as related complexes containing 4,7-dimethyl- 1, IO-phenanthroline and 2,2'-bipyridyl as ligands, originally formulated69as containing Fe" in the rare spin-triplet ( S = 1) ground state, have been ree~amined.~'It was found that when the complexes were prepared under the rigorous exclusion of oxygen the products were violet in colour and showed different magnetic properties, typical of Fe" complexes exhibiting a T2 ' A , spin equilibrium (the oxalate) or having a 'T2 ground state (the malonate). The originally prepared red species are now believed to be [Fe"(diimine),], [Fe"'(dianion),] (dianion)+.nH,O by analogy with the properties of complexes such as [Fe" (diirnine),],[Fe"'(dianion),],.nH,O obtained directly by reacting solutions of [Fe" (diimine)#+ and [Fe"'(dianion),13-. These mixed valence salts contain low-spin ( S = 0) cations and high-spin ( S = $) anions thus displaying average magnetic moments per Fe atom of close to 4.0 BM, i.e. in the range expected for spin-triplet Fe" systems. A reinvestigation7' of the complex first thought to be an 0, adduct of [Fe(bt)2(NCS)2](bt = 2,2'-bi-2-thiazoline) with a spin-triplet ground state7* Spin-quartet ground states has similarly led to its reformulation as [Fe"(bt)3]2[Fe11'(NCS),](C10,). ( S = 'f) have been proposed73 for some Fe"' complexes of biguanides on the evidence of their = ca. 4.0 BM) and Mossbauer spectra. The structures of the complexes are magnetic properties unknown, however. Whereas the complexes [Fe(bipy)J3+ and [Fe(phen),13+are low spin the tris ligand complexes of other or-diimine ligands with Fe"' may not be. Thus, [Fe(bi),][ClO,], [bi = 2,2'-bi-2-imidazoline (311 is high spin ( S = 2) while [Fe(bhp),][ClO,], [bhp = 2,2'-bi-1,2,3,4-tetrahydropyrimidine(4)] has a T' ground state but with a thermally accessible 6A, excited state in the 8&300 K temperature range.74 The corresponding Fe" complexes of (3) and (4) also show thermally controlled spin transitions in this temperature range; (4) is therefore an example of a ligand which generates spin-cross-over conditions in both iron(I1) and iron(ll1) cornplexe~.~~ The orange tris ligand complexes of (3) and (4) with Fe"' can be reversibly deprotonated using NaOMe, but not weaker bases such as pyridine, to afford the dark green complexes [Fe(bi)2(biH)]2' (5) and [Fe(bhp)z(bhpH)]2f(6) in which one of the three ligands is monodeprotonated. The latter complex (6)has ,ueE= 1.87 BM at 93 K rising to 3.70 BM at 353 K indicative of a T 2 e 6 A ,spin equilibrium in this complex also, a conclusion supported by Mossbauer spectra. In contrast, the complex [Fe(bi),(biH)][ClO,], (5) is fully in the high-spin form &E = 5.91 BM) at 293 K though there are indications of the beginnings of a transition to 'T2below 100K.74
(5)
( 6 ) Only monodeprotonated ligand shown
The green Fe"' complex [Fe(bi), (biH)I2+(5) containing one monodeprotonated ligand can also be prepared by aerial oxidation of the red Fe" complex [Fe(bi),l2+ (7) in dry acetonitrile solution. The stoichiometry of the oxidation (equation 6) was established by measurement of the O2 consumed (pressure change at constant volume) and of the water produced (GLC) and the kinetics of
Iron(III) and Higher States
225
the reaction were monitored spectrophotometrically.75 The reaction occurs in two stages, each is low. An unusual essentially first order in complex and independent of [O,] except where [O,] feature of the reaction is that while all the 0, is consumed in the initial (fast) reaction (tIlzca.6 m at 30°C in MeCN) only half of the Fe"' product is formed during this stage. It follows that the second reaction (t)!, ca. 60 m) involves the (slow) decay of some oxygen-containing intermediate to '~ no the remainder of the product. The mechanism outkined in Scheme 1 has been p r ~ p o s e d . While direct evidence for the intermediacy of the iron(1V) species (11) was found the scheme is consistent with the dissociative kinetics observed during the initial rapid uptake and associated formation of half of the Fe"' product (5) [steps (ib(v)], followed by the formation of the remainder to (5) via the slow step (vi). It is considered that the ligand-to-metal proton transfer does not occur until after the two-electron transfer step (iii) and that the initially formed adduct (9) may be stabilized via a hydrogen bonding interaction involving the coordinated O2 and the acidic hydrogen of the ligand. 2-(2'-Pyridyl)imidazole (pyim) forms low-spin tris complexes both of the neutral ligand and of the monodeprotonated anionic ligand.76
U
\
Hb- Fe"'
-\
)(-)N[ N
N
H 'H (12) + 2
Y
( 5 ) + 2Hz0
(5)
Scheme 1 r.d. = rate determining step. Only one of three ligands i s shown
4Fe"(bi)31z+ + O2
--t
41Fe"'(bi)2(biH)]Z+ + 2H,O
(6)
44.2.3.3 Ter- and Poly-dentate Ligands Rather few well-defined complexes of Fe"' with ter- and poly-dentate nitrogen ligands are known other than those of macrocyclic ligands, synthetic and naturally occurring, which are considered in later sections. Direct addition of 2,2',2"-terpyridyl to Fe"' salts leads to reduction and formation of the [Fe"(terpy),]*+ cation. However, oxidation of the Fe" complex gives Fe"' complexes the nature of which depends on the particular anion present. For the case of X = halide or NCS the product have the formula Fe(terpy)X, and may have a high-spin five-coordinate (distorted square pyramidal) ~tructure.'~For X = C104 the low-spin [Fe(terpy),][ClO,], is obtained while for X = NO, the product appears to have a p-oxo binuclear structure with strong antiferromagnetic coupling7* similar to the situation in analogous complexes of bipy and phen. In di-Zpyridylamine (13) and tri-2-pyridylamine (14) the central aliphatic nitrogen atom is unfavourably positioned for participation in a chelating coordination mode and usually only bidentate (13) or tridentate (14) behaviour is observed. Three classes of complex with Fe"' have been identified.79One class comprises salt structures such as [dipyamH],[Cl, Fe-0-FeCl,] containing protonated ligand cations and binuclear y-oxo anions. These complexes show the expected V(NH) vibration at 860-890 cm-'. The crystal and vibration in the IR along with a strong v(Fe-0-Fe) molecular structure of the related complex [pyHj, [CI, Fe-0-FeCl,] -py.cuntaining the pyridinium cation has been determined.*' Tripyam also forms a series of apparently mononuclear complexes [Fe(tripyam)X,] (X= halide) while for dipyam a third class of complex, e.g. Fq,Cl4O(dipyam),-5H,O of uncertain structure has been obtained,m The tetradentate ligands N , N'-bis(2-pyridylmethyl)ethylenediamine (pmen) and 2,2':6,2":6",2'"tetrapyridyl (tetpy) form y-oxo binuclear Fer'' complexes in which the pairs of S = ions are
+
226
Iron
(14)
antiferromagnetically coupled with J = - 89 cm-' for [Fe,(pmen),(H,O),O][SO,],*H,Oand - 83 cm-' for [Fe2(tetpy),(H,O),0][SO4],. 5H,0.82383 The former complex is assigned a cis configuration while the latter complex of the less flexible tetpy ligand has the trans configuration. Both complexes largely retain the binuclear structure in aqueous solution but differ in stability as pH is increased. This stability difference is attributed to the different stereochemistry of the two quadridentate ligands, cis-a for pmen and trans for tetpy. The corresponding mononuclear complexes of pmen, viz. high-spin [Fe(pmen)C12]C1 and low-spin [Fe(pmen)(OH,)(OH)][ClO,],, have been preparedaS4An interesting spin-state change (5' = to X = 5) accompanies a structural rearrangement from cis-a (15) to cis-,!? (16) geometry when the conjugate dihydroxo base of both salts is formed.
+
(15) cis-a
(16) cis-0
Although the Schiff base { (pyrol), tren) (17)formed by condensation of pyrrole-2-carboxaldehyde with 2,2',2"-tris(ethylamino)amine is potentially heptadentate the neutral low-spin complex [Fe{(pyrol), tren)] has a six-coordinate (trigonally distorted octahedral) structure, the distance between the metal atom and the central tertiary aliphatic nitrogen being greater than the van der Waals contact distance."
44.2.4
PHOSPHORUS AND ARSENIC LIGANDS
Unlike the lower oxidation states of iron the affinity of Fe'" for phosphorus, arsenic and antimony donor atoms is relatively low. Simple adducts Fe(PPh3)C1, and Fe(AsPh,)Cl,, presumably of distorted tetrahedral structure, have been prepared by reaction of Fe,(CO),, with PPh3 or AsPh, in CHC138487 while direct reaction of Fe"' salts with the tertiary phosphines and arsines in diethyl ether afforded bis complexes of the type Fe(PPh,),X, (X = Cl, NCS, NO,). These were non-electrolytes in nitrobenzene and high-spin with magnetic moments close to 5.9 BM but in the absence of further information the structures must remain uncertain.88-gg Complexes with bidentate ligands are better defined.97Common reaction products of FeCl, and, to a lesser extent FeBr,, with bidentate P- and As-containing bidentate ligands, are salt structures of the type trans-[Fe(E-E')Cl,][FeCl,] containing low-spin cations (E,E = P or Many of these complexes have been studied by Mossbauer s p e ~ t r o s c o p yas~well ~ ~ as ~ ~by~ more conventional techniques. The complex anion [FeCl,]- may be replaced by e.g. halide, CIO;, BF; and PF, in many c a ~ e s .Examples ~ ~ ' ~ ~ of complexes containing tri- and tetra-dentate ligands are the neutral complex [Fe(TAS)(NCS),] (TAS = methylbi~(3-propanedirnethylarsine)arsine)'~'and the complex salt [Fe(TTA)Cl,][FeCl,] (TTA = tris(3-dimethylarsinopropyl)arsine)'02 in both of which the complexes Fe"' ions are low-spin. Consideration of the [Fe'"] oxidation products of diarsine Fe"' complexes is deferred.
44.2.5 44.2.5.1
OXYGEN-CONTAINING LIGANDS Complexes of Neutral Monodentate Ligands
Most simple hydrated Fe"' salts of oxyacids such as nitrate and perchlorate are presumed to contain the hexaaqua ion [Fe(H,0)J3+ though few crystallographic studies are a ~ a i l a b l e . ' ~The ~,'~
Iron(1iI) and Higher States
227
structure of Fe(NO,), -9H,O comprises two crystallographically distinct [Fe(H20)J3+octahedra, each possessing crystaIlographic (I)symmetry, connected by a complex hydrogen-bonded network involving the nitrate anions and lattice water molecules.103The mean Fe"'-O distance (1.986(7) A) is 0.14A shorter than the FeII-0 distance in [Fe(H,0)6]Zf and is, in fact, the shortest Fe1''-O distance yet determined for coordinated water molecules. The introduction of other ligands into the coordination sphere results in a lengthening of the Fe"'-0 distances of the remaining water molecules. Salts of other anions including sulfate and chloride usually contain one or more coordinated Thus, for example, FeC13.6H,0 contains the trans-{Fe(H,O),Cl,]+ cation'" while p H 4 l 2FeC1,-H,O contains [Fe(H,0)5Clj2+.'06The strong yellow colour of these salts contrasts with the pale violet colour of this hexaaqua cation and is, of course, due to chloride-to-Fe"' charge transfer absorption. ESR spectra of FeCl3-6H2Oin A1C1,*6H2Oare consistent with the presence of trans-[Fe(H20),C1,]'C ions.44 FeCl,-2.5 H 2 0 , however, is made up of a cis[Fe(H,O),CJ,]+ cation, a [FeCl,] anion and a water of crystalli~ation.'~~ X-Ray diffraction studies of concentrated aqueous solutions of FeCl,, FeC1,.6H,O and Fe,(SO,), have revealed the presence of a range of species, mononuclear and polynuclear, depending on concentration and pH.'08,'w [FeCl,] may be extracted into ether from hydrochloric acid solutions. Iron(II1) chloride, and other simple salts, react with a range of organic ligands such as alcohols, ethers, aldehydes, ketones, amides, sulfoxides, amine and phosphine oxides, efc. The structures of many of these complexes are unknown; some appear to be simple addition compounds while others have salt structures. The alcoholates FeCl,.2ROH (R = Me, Et, efc.) have been isolated from solutions of FeC1, in benzene containing the appropriate alcohol.109When base is present the corresponding alkoxides may be obtained.'" These have trimeric [Fe,(OR),] structures containing a triangular core of three antiferromagnetically coupled Fe"' ions linked by oxo bridges."' In contrast, reaction of FeC1, with Li[OCH(CMe,),] gave the binuclear complex (f8).1'2
Ethers generally have poor coordinating capacity for transition metals except where the ether function forms part of a multidentate chelating ligand. They appear to interact best with metal ions having few d electrons, and complexes of cyclic ethers appear to be more stable than those of open-chain ethers. A four-coordinate, presumably distorted tetrahedral, high-spin complex [FeCl, (THF)] (THF = tetrahydrofuran) has been prepared by direct reaction under anhydrous conditions.'I3 Molecular weight and other measurements indicate some concentration-dependent association in benzene and 1,Zdichloroethane. A distorted octahedral structure has been assigned to the perchlorate salt of the cationic complex [Fe(THF),(H20),l3+,however.'" Although amides and sulfoxides have a potential ambidentate character, complexes with first row transition metal ions appear, invariably, to be oxygen bonded. Many six-coordinate Fe"' complexes [FeLJX, (L = e.g. DMF, DMA, urea, HMPA, DMSO; X = C10; ,NO;, CI- )have been prepared and their physical properties (IR, UV/vis, ESR and magnetic) reported in many cases."' Complexes of other stoichiometries such as FeCl, * 2DMSO and FeCl, -4DMSO have also been reported,'l6the latter having the familiar salt structure [Fe(DMSO),Cl,][FeCl,]. Analogous complexes of the cyclic amides ethyleneurea and propyleneurea may have the same structure."' Treatment of metallic iron with DMSO and CCl, at above 100°Cis said to give a mixture of cis- and trans-[Fe(DMSO),Cl,].X (X = C1- or FeCl;)."* A number of hexakis(pyridine N-oxide) complexes [FeL,]X,.nH,O (L = pyridine N-oxide or ring-substituted derivative, X = ClOi, NO;) have been prepared.'"I2' As expected for an octahedral field of 0-donor ligands the complexes are all high-spin kR N 6.0 BM). The v(Fe-0) stretching vibration has been observed in the 3 2 U O O cm-' range. ESR measurements suggest considerable distortion from octahedral symmetry in at least some of these complexes.'2oUse of Fe"' chloride or bromide led to complexes of the type [FeL,Cl,][FeC1,]120~'21 or [FeL3C13]120 as found also for complexes with other 0-donor ligands.
228
Iron
In contrast to the complexes of pyridine N-oxides, it appears that not more than four phosphine N-oxides can coordinate to one Fe"' ion, presumably for steric reasons. With Ph,PO and Ph3As0 and analogous ligands, complexes of stoichiometry FeL,(ClO,), and FeL,X, (X = C1, Br, NCS, NO3) are ESR and vibrational spectra imply that the perchlorate complexes contain planar [FeL, J 3 + ions, the arsine oxide exercising the larger ligand field, that the halide complexes have the salt structure trans-[FeL,X][FeX,], and that the thiocyanate and nitrate are five-coordinate r n o n o r n e r ~ .Perchlorate '~~ ion coordination may be involved in some complexes of n-alkylphosphine oxides reflecting lower steric effects than in corresponding aromatic phosphine oxides.IZ4Bidentate bitertiary phosphine oxides (L-L) in which the phosphorus atoms are linked viu methylene, ethylene, tetramethylene and hexamethylene groups react with Fe"' chloride and nitrate in organic -nH,O, [Fe(L-L)(ONO,),], [Fesolvents to yield complexes of the types [Fe(L-L),Cl,][FeCl,] (L-LL)(ONO,),][NO, J or [Fe2(L-L)3(0N02)4][N03]z-2HZ0. Analytical, spectroscopic (IR, UV/ vis, ESR, Mossbauer), magnetic and conductance measurements have been used to assign probable structures, these being five- or six-coordinate depending on the nature of the bis-ph~sphine.'~~
44.2.5.2
Complexes of Carboxylates and Related Anionic Ligands
An important aspect of the chemistry of iron(III), briefly referred to in the introduction, is the tendency of Fe"' to undergo hydrolytic polymerization in aqueous media, leading to di- and poly-nuclear complexes in which the metallic centres are linked by oxygen bridges.Iz6Perhaps the simplest of these species is the dihydroxo-bridged dimer di-p-hydroxo-octaaquadiiron(II1)[(H,O),Fe(OH),Fe(H,0),]4+ postulated to be the dominant species present at Iow pH on the basis of spectral, magnetic and other physical proper tie^.'^' More recently, confirmation of the nature of this dimer has been obtained from EXAFS studies.'**The results show that each Fe"' is octahedrally coordinated to two bridging OH- ligands and to four H,O molecules. The planar ring (19) comprises Fe-O(H)-Fe and 0-Fe-0 angles of 101" and 79", the Fe-(OH), Fe-(OH,) and Fe..-Fe distances being 1.89, 2.03 and 2.91 A, respectively. These parameters compare well with those found for the same structural unit occurring in di-p-hydroxo-bis[2,6-pyridinedicarboxylatoaquairon(III)] and derivatives.lZ9All of these doubly bridged di-Fe"' systems show magnetic moments below the spin-only value of 5.92BM expected for high-spin Fe"' which further decrease with decrease in temperature. Generally, the susceptibility vs. temperature data can be fitted to the spin-spin interaction model based on the exchange Hamiltonian H = - 2J$, $, with 3, = Sz= 5, for which the variation of molar susceptibility with temperature is given by equation (7), where x = exp(J/kn, J = coupling constant and Na is the temperature-independent-paramagnetism term.'*'" For the doubly bridged dimers the antiferromagnetic coupling is usually relatively weak (J -= - 20 cm-') whereas for the single bridged p-oxo dimers (vide infru) J is commonly of the order of -90 to - l00cm-I. H
0 F r < >Fr 0
H
55
+
30x"
+
14x"
+
5 ~ ?+~ 2'
1
+
(7)
Fe"' forms a number of basic carboxylates of general formula [Fe,O(O,CR),L,]X where R = Me, CMe,, amino acid residue, erc., L = e.g. H,O, MeOH, py, DMF and X = X-Ray diffraction studies have shown that these complexes are trinuclear (see structure 20) containing an These O-centred triangular cluster of Fe"' atoms, the F e 3 0 core being planar'33 or nearly complexes also show weak antiferromagnetic interactions. A variety of models have been considered.'" The most appropriate appears to be that, consistent with the known structures, based on an equilateral triangular arrangement of Fe"' atoms with a small degree of intertrimer antiferromagnetic exchange in addition to the (larger, i.e. J x - 3Ocm-I) intramolecular interaction. The mixed valence complexes [Fe"Fe~"O(O,CMe),L,] (L = H 2 0 or p ~ ) ' ~ ' -have ' ~ * been studied by Miissbauer, IR and optical spectroscopy and magnetic susceptibility methods.'39aVariable-temperature magnetic susceptibility data for the aqua complex are interpreted in terms of a Heisenberg-Dirac-Van Vkck S, = 2, SI= S, = 5 spin-exchangemodel with J,2 = & = - 50.0cm-'and J13 = - 14.5crn-'.
I Iron (III) and Higher
States
229
An intervalence electron-transfer band, not present in the 'Fe:" ' analogue, was observed in the room temperature electronic spectrum at 13 800 cm-'. Mossbauer spectra indicated distinct Fe" and Fe"' sites at 17 K while at 300 K a single absorption was observed. The thermal barrier to electron transfer in the trimer was estimated as about 470 cm- . Triiron clusters of this type, in the presence of zinc powder, acetic acid, aqueous pyridine and oxygen, are reportedla to effect the oxidation of saturated hydrocarbons. The exchange interactions in the series of complexes [Fe~"M"0(0, CMe),py,]*py (M = Mg, Mn, Co, Ni or Zn) which, M = Ni excepted, are isomorphous with the mixed valence complex referred to above have been measured.139b
'
( 20)
Tetranuclear and pentanuclear Fe'" clusters can also be p ~ e p a r e d . ~ ~The ? ' ~Fe, ' complex is antiferromagnetic while the Fe, complex is ferromagnetic with a very asymmetric Mossbauer spectrum.' 34 The complex K,[Fe,O(SO,),(H,O),], like the corresponding acetate, also contains a planar F e 3 0 core, with, in this case, each pair of Fe atoms additionally bridged by two S q - groups.'" The magnetic behaviour likewise conforms to that predicted by the triangular cluster model (H = - 2JS1.S2+ S,-S, + S,*S,) with J = -26.Ocm-', g = 2.00 and Nu = 0. The generally similar behaviour to that of the structurally related carboxylates indicates that magnetic exchange originates predominantly from interaction of different Fe'" ions via their mutual overlap with appropriate filled valence orbitals of the cemral oxygen atom; superexchange via the bridging sulfate ions is considered to be small or negligible.'43 In addition to the polynuclear carboxylate complexes considered above, a number of mononuclear complexes are also known. Among these are the tris(oxa1ato) complexes. K,[Fe(ox),] 3H20 is readily prepared by H,O, oxidation of the Fell oxalate in the presence of oxalic acid and excess oxalate ion.'" The green crystals are photosensitive, the oxalate ligand being oxidized to C 0 2with concomitant reduction of the metal to Fe". The structure has been determined;'45the Fe06coordination sphere has approximate octahedral symmetry with some trigonal distortion. As expected for a magnetically dilute mononuclear Fe"' complex the magnetic moment at 300°C (5.96 BM) is close to the spin-only value.'& The tetranitratoferrate anion [Fe(NO,),]-, like the corresponding anionic complexes of Mn'f , Co" and Zn'l, contains eight-coordinate metal ion involving four almost symmetrical bidentate nitrate groups.147The anion has essentially dodecahedral (DW) symmetry.
44.2.5.3
Complexes of Diketones and Related Ligands
Fe"' readily forms a range of stable, highly coloured complexes with the anions of j-diketones, such as acetylacetone (acacH).I4' The best known of these is the neutral tris complex [Fe(acac),] for which a convenient new high yield synthesis has recently been de~cribed.'~' A single crystal X-ray structure determinationI5* has confirmed the expected near-octahedral arrangement of the six oxygen donor atoms. Considerable distortion from octahedral symmetry is found in the corresponding tris complex of the tropolone anion (21), largely because of the smaller bite of the chelating group.'" These complexes are, of course, high-spin; some magnetic anisotropy of the 6AIgground A simple one-electron MO state has been detected and interpreted in terms of zero-field ~p1itting.I~~ scheme has been used to treat the W / v i s spectra of [Fe(acac),] and derivatives containing Me, CF, and Ph substituent^.'^^ The three main absorption bands a t 30 00&37 000 cm-',24 00&29 000 cm-'
Iron
230
and 20 000-23 000 cm- ' were assigned to n: + n*, tb + x* and II + e, transitions, respectively. [Fe(acac),] has been reported to have catalytic activity for stereoselective P-epoxidation of cholesterol and analogues with H,0,.'53 The complex [Fe(acac), Cl] is mononuclear and five-coordinate. The Fe"' atom is located slightly above (0.51 A) the plane of the four acac oxygen atoms (Fe-O,1.95 A) with the C1 atom occupying the apical position (Fe-Cl, 2.22 A) of an overall square pyramid.'54 Magnetic properties and Mossbauer spectra have been mea~ured.'~' A series of dimeric Fe"' complexes [L2Fe(OR)I2(where L = acac or the enolate of dipivaloylmethane, R = Me, Et or Pr') have been prepared and characterized." These contain the dialkoxybridged structures (22) as judged by molecular weight, IR and magnetic susceptibility measurements. Values of J in H = 2J3, * $ (3,= S, = +) were determined to be ca. - 1Ocm- '. A similar structural framework occurs in the dinuclear complex formed from Fe"' and the dianion of 3,4-dihydroxy-3-cyclobutene,better known as squaric acid (23).'56In this complex the bridging groups are hydroxide rather than alkoxide groups and the superexchange coupling in somewhat smaler (J = - 7.0 cm-'1.
&44.2.5.4
Complexes of Phenols, Catechols, Quinones and Related Systems
Much of the recent interest in iron complexes of hydroxybenzenes derives from the importance of such compounds in controlled iron transport within bacteria. Since, at physiological pH, iron is largely present as insoluble hydrated iron(II1) oxide (K, 2 x it is not readily available. Nature has therefore developed mechanisms for the controlled solubilization, transport and release l ~ ~ mechanism utilizes chelating agents called sideroof the iron as needed by the 0 r g a n i ~ m . This phores.'" Nearly all the siderophores employ negatively charged oxygen-donor chelating functional groups. The two prinicpal classes are the catecholates and the hydroxamates. (Hydroxamate complexes are considered in the following section.) Fe"'-phenolate groups also occur in catechol dio~ygenases.'~~ Despite the large amount of recent work devoted to iron(III+atecholate derivatives rather less is known of the coordination chemistry of Fe"' with monophenols. The colour reaction of Fe"' with the phenolic group has been employed analytically for a long time but only very recently has a complex of monodentate phenoxide been structurally defined. 160 The red-orange compounds [Et4NI[Fe(OC,,H,,)4] (24) and [Ph4P][Fe(OC6H2C1,),] (25) of 2,3,5,&tetramethylphenolate and 2,4,6trichlorophenolate were obtained from reactions of FeCl, with the appropriate lithium phenolate in the presence of the quaternary ammonium (or phosphonium) cation in anhydrous ethanol. Both complexes have somewhat distorted FeO, coordination geometry, the distortion being most marked in (25) where the 0-Fe-0 bond angfes range from 99.3" to 122.9".The Fe-0 distances in both complexes are, at 1.847 A in (24) and 1.866 in (25), significantly shorter than the average Fe-0 distance (2.02 A) observed in the FeO, core of [Fe(catecholate),13- complexes. Both (24) and (25) undergo a reversible one-electron reduction at - 1.32 and - 0.45 V, respectively (vs. SCE in MeCN), reflecting, in the case of the tetramethylphenolato complex particularly, strong stabilization of the + 3 oxidation state. Electronic spectra are dominated by three intense charge transfer transitions, the two low energy bands allowing a determination of the tetrahedral ligand-field splitting parameter, 4, of 8000cm-' (24) and 7400cm-' (25), Le. about twice as large as for [FeClJ. Catechol is a very weak acid and at low pH is a poor ligand forming only a transient complex which undergoes a redox reaction to give Fe" and o-quinone as products. At higher pH, however, very stable iron(II1) tris(catecho1ate) complexes are formed which are indefinitely stable in the absence of air.I6' The structure of K,iFe(cat),]* 1.5 H 2 0has been determined by single crystal X-ray diffraction,'6' in order to probe the coordination occurring in the Fe"' complex of the microbial iron 1-serine) transport compound enterobactin (26) (the cyclic triester of 2,3-dihydroxy-N-benzoylwhich is itself a tricatech~l.'"~'~~ The complex anion has approximate D3 symmetry, the primary distortion being a bending of the catecho1 rings away from planarity with the 0-M-0 planes. The
-
23 1
Iron (ZII) and Higher States
average Fe-0 bond lengths are 2.01 5 A,somewhat longer than the Cr-0 bonds (1.986 A) in the isostructural Cr"' complex. The high-spin Fe"'-enterobactin complex is exceptionally stable thermodynamically having the largest formation constant (ca. los2) known for any Fer" complex.'64 A number of synthetic analogues for enterobactin which contain 1,2-dihydroxyphenyl chelating groups have been investigated.162,165 One of these, hexadentate 1,3,5-tris(2,3-dihydroxybenzoylaminomethyl)benzene (27) has been shown'65to form a 1:l [FeLI3- complex with a formation constant of 1045,8.
c'=o
I
HN
O=C
H$H X
An important biological requirement of iron-enterobactin systems is the release of iron from the highly stable [Fe(ent)I3- complex once inside the cell. The redox potential for [Fe(ent),13- (estimated as ca. - 750mV at pH 7) is probably too negative to allow reduction to Fe" by known biological reductants at physiological pH.16*At lower pH the redox potential may shift towards positive values, however. In one mechanism for the release of iron from enterobactin at low pH it is p r ~ p o s e d ' ~ ~ , ' ~ ~ that an internal redox reaction occurs between Fe3+and catechol to generate a bIue FeI'semiquinone species.'*7However, this mechanism in aqueous solution has been challenged by Raymond and co - ~ o r k e r s ' who, ~ ' using Mossbauer and ESR techniques, found no evidence for reduction to Fe" in the pH range 2-10, though some indication of a quasireversible redox reduction at low pH in methanol was apparent. An earlier mechanism"' for iron release involves cleavage of the central triester ring of enterobactin by a specific esterase which would raise the redox potential. However, it has since been shown that catechoylamides which lack the triester ring retain the very negative redox couples. An alternative mechanism involves three protonation steps with a concomitant change from a 'catecholate' (28) to a 'salicylate' (29) mode of bonding which has the effect of dramatically raising the redox potential and bringing it within the range of physiological reductants. 164~170 In an attempt to model the specific iron-binding site of the human milk protein, lactoferrin, believed to involve two or three tyrosyl residues per iron atom, a range of Fe"' phenolate complexes have been prepared.'72These exemplify the donor sets FeO, to FeO,N,O; where 0 represents a phenolate oxygen, N an aliphatic or aromatic nitrogen and 0' a carboxylate or methanol oxygen. Electronic and ESR spectra have been reported along with the results of an X-ray structure determination of one member of the series, [FeL,(MeOH),]NO,*MeOH where LH is 2-(5methylpyrazol-3-y1)phenol(30). The high-spin six-coordinate complex has an all-trans configuration, the Fe-0 (phenolate) bond being 1.888 A.
232
Iron
The non-heme iron enzymes catechol 1,Zdioxygenase and protocatechuate 3,4-dioxygenase catalyze the dioxygenation of catechols to cis,cis-muconic acids (31).'59Spectroscopic studies suggest the active site to contain a mononuclear high-spin Fe'" centre coordinated to at least two tyrosines, this being responsible for the LMCT absorption resulting in the red colour of the enzymes. Substrate binding alters the colour but does not replace the bound t y r ~ s i n e . ' ~Kinetic ~ , ' ~ ~ and resonance Raman (RR) studies17' further suggest that the cathechol substrate may be bound in a monodentate fashion. This coordination mode has recently been confirmed in the model compound [Fe(saloph)(catH)] (saloph = N,N*-(1,2-phenylene)bis(salicylidinimato); (32).'76In this mononuI clear high-spin complex the metal ion has a square pyramidal geometry in which it is raised 0.55 ! above the mean basal plane and with a rather short (1.828A) apical Fe-0 (cat) bond. The analogous complex [Fe(salen)(catH)] has also been prepared. This can be deprotonated with strong base to give the anionic complex [Fe(salen)(cat)]- having markedly different properties'" and in which the catecholate is chelated in a six-coordinate structure in which the tetradentate salen ligand has rearranged from an equatoria1 to a cis+ configuration in order to accommodate the bidentate catecholate dianion. An interesting observation is that the monodentate catecholate complex reacts readily with 0, to form the chelated semiquinone while the chelated catecholate is unreactive.17* While these results may imply a monodentate catecholate interaction between substrate and enzyme it should be noted that other workers found that chelated 3,5-di-tert-butylcatecholin [Fe(NTA)(cat)] (NTA = nitrilotriacetate) does undergo the catechol ring ~1eavage.l'~ Fe"' Complexes of semiquinones may also be prepared by reaction of Fe"-Schiff base complexes The product complexes [Fe"'(salen)(SQ)] have Mossbauer such as [Fe(salen)] with o-quinones.180-181 spectra characteristic of high-spin Fer" and magnetic moments near 4.86 BM at room temperature and are therefore formulated as high-spin Fe"' complexes in which the unpaired electron of the coordinated o-semiquinone is antiferromagnetically coupled to the Fe"' ion (- J > 600 cm-') to give complexes with S = 2 ground states.I8' The crystal structure of the complex in which SQ = phenanthrenesemiquinone (phenSQ) has been determined.'78 This complex has a similar distorted octahedral geometry to that of [Fe(salen)(cat)].- The complex [Fe(phenSQ)J -phenQ (phenQ = the quinone form of phenSQ) reacts with bipy or phen to give complexes of formulation [Fe(phenSQ)(phenCAT)(bidentate nitrogen base)], which, on the basis of magnetic properties and Mossbauer spectra, are considered to be high-spin Fe"' complexes.1s2The mixed valence character of the complexes is indicated by the observation of an intervalence transfer band at 1100nm in the solid state and in solvents of low dielectric constant. The non-appearance of the intervalence band in high dielectric constant solvents may be due to an intramolecular electron transfer to a [Fe" (phenSQ)(nitrogen base)] species. Reaction of [FeI'salen] with p-quinones gives, in contrast, the binuclear complexes [Fe,(~alen)~Q].'*~ It was shown by a variety of properties (magnetic measurements and IR, ESR and Mossbauer spectra) that the compounds consist of high-spin Fe"' ions bridged by the dianion Q (structure 33) of the hydroquinone which transmits weak (- J c 6 cm-I) antiferromagnetic interaction.Is3 This structure has been confirmed, for the case of Q = the dianion of hydroquinone, by a single crystal structure determination.'"
I
0-
0-
I 233
Zron(II1) and Higher States
The electrochemistry of 8-quinolinol and 2-methyl-8-quinolinol complexes of iron in DMSO has been studied with a view to probing the redox chemistry of iron proteins containing histidine and tyrosine functional groups. Four reversible Fe"/Fe"' redox couples occur between 0.25 V and -0.3OV (vs. SCE) as a function of solution acidity.Ia4The methyl-substituted ligand also forms a p-oxo bridged binuclear complex comprising high-spin five-coordinate Fer" with J = - 80 cm-' .Iss
+
44.2.5.5
Complexes of Hydroxamic Acid, Thiohydroxamic Acid and Derivatives
The second main class of Fe"'-specific sequestering agents (siderophores) responsible for the acquisition and assimilation of iron employ hydroxamate ligating groups (34)+'57,158,'86 Two important subclasses are the ferrioxamines and the ferrichromes (Figure 5). Examples are ferrioxamine B, a linear trihydroxamic acid and ferrichromt A, a cyclic hexapeptide carrying three hydroxamatecontaining side chains.
Figure 5 The structure of ferrichrome
-
The hydroxamic acids (35) are rather weak acids (p& 9) which in acid solution' form deeply coloured 1:l complexes with Fe"', a reaction which has long had analytical application for the hydroxamate functional group. Complexes of hydroxamic acids with a range of metal ions, including Fell', have been reviewed.'87Mossbauer spectra are typical of high-spin Fe"' complexes.'88In neutral solution a high-spin, octahedral tris(hydroxamato)iron(III) complex (36)is formed, the hydroxamate anion acting as a bidentate ligand. Such complexes are very stable with formation constants CB,) approaching lo3'. In sharp contrast, the stabilities of corresponding Fe" complexes are much lower ('J2 for the bis complex with acetohydroxamic acid = 3 x 10') so that reduction of the Fe"' provides a mechanism for the release of the iron in the natural situation. The neutral tris complexes of Fe"' (36)can be further deprotonated in strongly basic media to generate the trianionic hydroximato complex [FeL313-(37). O=CR HO-NHI
.[. ] '=CR
+ M3+
(35) Hydroxamic acid
\o
/MI I
- M I
-
(36) Hydroxamato complex (37) Hydroximato complex
234
Iron
The asymmetry of the hydroxamate ligand leads to the possibility of the isolation of geometrical (and optical) isomers in the tris complexes. However, for high-spin Fe"', expected to be kinetically labile, all structures so far determined have the cis configuration. Thus, for example, the most stable as have also ferricrystalline form of tris(benzohydroxamato)iron(III) has the cis and ferric N,N',N"-tria~etylfusarinine.~~' However, for the inert Cr"' complexes, on chrome A,1w9191 the other hand, both the cis and trans geometrical isomers have been separated and characterized in solution;'93in addition, the trans isomer of tris(benzohydroxamato)chromium(III) has been structurally defined by X-ray diffraction.194More recently, the structures of both cis and trans isomers of the tris(benzohydroximato)chromate(III) anions have been determined.19' Interestingly, the cis and trans isomers were obtained by deprotonation of the neutral hydroxamato complexes of opposite geometry. I n view of the kinetic lability of high-spin Fe"' it might be expected that the metal ion undergoes ready exchange in hydroxamate complexes. However, as shown by Raymond and co-workers, this is not always the case. For example, tris(hydroxamato)iron(III) complexes have been resolved into their component optical isomers and have been found to be very stable in various non-aqueous solvents.'% Even slower substitution kinetics may be expected for natural siderophores comprising a single hexadentate ligand. Experiments on the kinetics of iron exchange and iron removal from complexes of ferrioxamine and ferrichrome A using s5 Fe and W / v i s spectrophotometric techniques have in fact shown that exchange is extremely slow; half-lives for exchange are > 200 h under certain conditions. 19' The stabilities of Fe" complexes of hydroxamates are much lower than those of Fe"' (J2 for the bis Fe" complex of acetohydroxamic acid = 3 x lo8).It is thought, therefore, that the mechanism of release of iron from hydroxamate siderophores may occur via reduction of Fe"' to the much more weakly bound Fe" state,15' as also proposed for the tris(catecholat0) siderophores. Cyclic voltammetry has shown that the Fe"' hydroxamate complexes undergo reversible or quasi-reversible one-electron reductions under suitable conditions. The observed formal potential EEis pH dependent, decreasing from -0.388mV (vs. SCE) at pH 7 by 60mV per pH unit, for the case of tris(acetohydroxamato)iron(III), consistent with progressive deprotonations of the coordinated ligand. Thus, deprotonation stabilizes the Fe"' YS. the Fe" Recently, a potentially new class of siderochrome has been isolated from bacteria such as Pseudomonas~uorescenscontaining thiohydroxamic acids which form very stable Fe"' complexes.m Both the parent acids and their chelates show antibiotic properties and may be involved in bacterial iron transport. The thiohydroxamate chelates, like their hydroxamate analogues, display high-spin S = 4 properties, though some evidence for a high-spin low-spin equilibrium in one case has been reported.'" The structure of tris(ZV-methylthioformohydroxarnato)iron(III) has been determined.202 The crystals examined contained a A-cis arrangement of ligands though the related complex, tris(benzothiohydroxamato)iron(III), could be resolved into the A and A isomers.203Electrochemical s t ~ d i e shave ' ~ ~shown ~ ~ ~ that replacing the carbonyl oxygen (in hydroxamates) by sulfur (as in thiohydroxamates) increases the reduction potential which would make the biological release of iron by reduction easier. The bis(thiohydr0xamate) complex [FeL,Cl] (L = N-methylbenzothiohydroxamate anion) has a five-coordinate (pseudo square pyramidal) structure comprising two trans bidentate ' S O donor groups and a C1 atom.205,206 It has been shown205(by magnetic, Mossbauer and ESR properties) to be high-spin indicating that the overall ligand field is smaller than that occurring in intermediatespin dithiocarbamate systems CS4C1' donor set).
44.2.6
44.2.6.1
SULFUR LIGANDS Sulfido and Thiolato Complexes
Few areas of coordination chemistry can have undergone such rapid advances in the last decade as the chemistry of iron-sulfur cluster compounds. The stimulus for the rapid growth in this field has been the goal of modelling the active sites in the non-heme iron proteins involved in photosynthesis, mitochondrial respiration and nitrogen fixation. Following the recognition of the occurrence of the 'FeS,' coordination unit in mono-, di- and tetra-nuclear proteins which mediate rapid and reversible electron transfer reactions, inorganic chemists have accepted the challenge to prepare synthetic analogues having the same distinctive physical and chemical properties. A remarkable degree of success has been achieved. Many excellent reviews2'' are available and these should l x consulted as sources to the detail contained in primary publications. There are at least five structural types of iron-sulfide-thiolate complex. The best known are
Iron ( I l I ) and Higher States
235
mononuclear tetrahedral [Fe(SR),]*--'- binuclear [Fe, S2(SR)4]3-,2-containing the planar [Fe2(p2S),] core, and tetranuclear [Fe,S4(SR),]3-,2- containing the cubane [Fe4(p3-S)J core (Figure 6). Recently, two other classes of cluster containing three and six iron atoms have been d i s c o ~ e r e d . ~ ~ * * ~ ~ ~
(40)
Figure 6 Structural units occurring in iron-thiolate complexes
44.2.6.1.1 Mononuclear 'FeS, ' systems
This structural unit occurs in the one-iron proteins, rubredoxins (Rd), obtained from bacteria, in which the active site is [Fe(S-Cys),], where S-Cys is a cysteinyl residue of the protein chain [structure (38)of Figure 61. These proteins have molecular weights typically about 6000. The variety. of physical investigations (magnetic susceptibiiity,2" ESR,," Mossbauer,212optical a b s ~ r p t i o n , * ~ ~ , ~ ' ~ MCD2I4)have demonstrated that the two redox states, Rd,, and Rered,coupled by one-electron transfer at E o - 0.4 to - OAV, contain Fe"' and Fe", respectively.215The X-ray structure216of Clostridium pusfeurianum Rd,, at 1.2 8, resolution has indicated a distorted tetrahedral 'FeS,' active angles from 104" to 114'. site; the Fe-S distances range from 2.24 to 2.33 A and the S-Fe-S Rather few well-characterized synthetic compounds containing the 'FeS,' coordmation unit are known;*" most contain Fe" and are not oxidizible to Fe"', often because of the tendency of the Fe"' complexes to undergo autoredox reactions to Fe" and RSSR. A notable exception is the complex l?IEt4][Fe(S2-o-xyI),] (S,-o-xyl = o-xylyl-a&-dithiolate) prepared by reaction of the dithiol with [FeC1,I2- followed by aerobic Comparison of the structures of the Fe"' and Fe" complexes has shown that in both cases the metal atom is in a slightly distorted tetrahedral environment. There is some lengthening (- 0.09 A) of the mean Fe-S bond distance on reduction. Both complexes are high-spin ( S = 3,S = 2). In the 77 K Mossbauer spectra the isomer shifts (S, cf. natural iron) and quadrupole splittings (AE,) are 0.13 and 0.57mms-' (Fe"') and 0.61 and 3.28 mm s-I (Fe"). Cyclic voltammetric measurements in DMF confirmed that the two complexes are related by a quasi-reversible one-electron process at 1.O V, a somewhat more negative potential than would lx expected for the proteins in the same medium. More recently three other mononuclear [Fe(SR),]- complexes containing monodentate thiolate have been structurally defined. In these SR = SPh-,'I9 SEtP2l9and 2,3,5,6-tetramethylbenzenethiolate.220Sterically hindered thiolate ligands appear to stabilize the [Fe(SR),] coordination unit and thus provide a means for the preparation by ligand exchange of the previously elusive (non-hindered) monodentate tetrathiolato iron(II1) complexes.
-
N
44.2.6.1.2 Dinuclear ?;e-S' systems
A wide variety of synthetic binuclear iron complexes (39) bridged by sulfide, disulfide and/or thiolate groups are known. They have proven to be good models for the two-iron ferredoxin proteins, as demonstrated by comparisons of structure and properties. Early attempts to isolate two-iron complexes by direct reaction of a monothiol with FeCl,, NaSH and NaOMe afforded only four-iron products suggesting that a particularly high stability is associated with the tetrameric
236
Iron
-
cubane structure (40) relative to other possible Fe-S reaction products.221It was argued that use of a chelating dithiol which is unable to span the 7 A distance between thiolate coordination sites in the tetramer might lead to the desired two-iron product. This strategy proved successful for the case where RSH is o-xylydithiol (41) (equation 8), a crystalline product (42) being isolated by the use of a quaternary cation. It was further discovered that related complexes of non-chelating aryl thiols could be readily prepared by ligand exchange. The structures of the o-xylyl dimer (42) and of the corresponding complex containing o-tolylthiol ligands have been determined.22'Both are centrosymmetric sulfide-bridged dimers containing a planar Fe2S, core with each Fe"' atom in an approximately tetrahedral environment. The short Fe...Fe separation suggests some direct metalmetal interaction. Subsequently, improved syntheses for the two-iron complexes were developed.2u These involved reaction of FeCl,, eiemental sulfur and thiolate in the presence of tetraalkylammonium ions in MeOH solution at ambient temperature; in the absence of WR,]"the final reaction products were the four-iron tetranuclear clusters. Analogous selenium compounds were obtained by the use of elemental selenium in place of sulfur.
2FeCI,
+4
EsH SH
+
2NaSH
+
2NaOMe
-+
A remarkable similarity in properties between the synthetic two-iron complexes and the oxidized forms of the two-iron ferrodoxins has been observed and subsequently placed on a firm structural s . ~ ~ ~ spectra of basis by the X-ray structure determination of Fd,, of Spiruha p l ~ ~ t e n s tOptical [FezS2(S,-o-xyl),12-(42) and Fd,, are closely similar.224Both the synthetic and natural systems have magnetic susceptibilities well below those expected for isolated Fe"' ions and which approach zero at low temperature, indicative of strong intramolecular antiferromagnetism. The data lead to coupling constants J = - 148cm-I for (42)"' which compares with J = - 143 to - 185cm-* for various Fd,,.226 Mossbauer parameters are also in good agreement. The isomer shifts for spinach Fd,, is 0.22mms-'227while for the synthetic analogues values of0.17 mm s-' have been observed.225 The smaller quadrupole splitting (- 0.35 mm s-I) in the synthetic systems225compared to spinach Fd,, ( 0 . 6 5 r n m ~ - ' )may ~ ~ ~reflect a lower site symmetry in the latter. Electrochemical investigation2" of the synthetic dimers in DMF has revealed two quasi-reversible one-electron reduction waves at - 1.25V and - 1.49 V (w. SCE), for the o-xylyl system, attributable to the three-member electron transfer series given in equation (9). Since two-iron Fd,, proteins undergo a single one-electron reduction (to give Fd,) it is concluded that Fdd contains the total oxidative level (formally Fe" 4-Fe"') as the synthetic trianion. No protein oxidation level corresponding to the tetraanion (Fe", Fe") has yet been detected. The antiferromagnetic coupling present in Fd,, proteins in retained on reduction to Fdred(J x - 1OOcm-') leading to a spin doublet ground state.226Spectroscopic studies226(ESR, NMR, optical, Mossbauer) have revealed the occurrence of two (spectroscopically) distinct metal sites having the properties of high-spin tetrahedral Fe"' and Fe", thus characterizing Fdredas a class I1 mixed valence species, Le. one having a localized FeI'I, Fe" oxidation state configuration. Similar properties were found for the one-electron reduction product (generated in solution) of [FeS,(S,o-xyl),J2- (42) leading to the conclusion that the localized mixed-valence behaviour is not a consequence of the protein structure but is an intrinsic property of any [Fe2S,(SR),l3- species and, indeed, of any species containing the [Fe2S2]core.229 [Fe2S2(SR),14-
[Fe2S2(SR)4]3-
[Fe2S2(SR)4]2-
[Fe", Fe"]
[Fe", Fe"']
[Fe"', Fe"']
(9)
The exchange of o-xylyl dithiolate by aryl thiolate has already been noted. Such exchange (equation 10) is fairly general for one-iron and four-iron systems as weIl as two-iron and is the basis of the core extrusion method for the identification of active sites in iron-sulfur proteins. This type of reaction can be extended to the replacement of coordinated thiolate by other nucleophiles. For example, reaction of the complexes [Fe,S,(SR),]2L (and also the four-iron clusters) with benzoyl chloride afford [Fe2S2C14]2-(or [Fe,S4C1,]2-).230Comparison of the crystal structure of [NEt.,],[Fe,S, C t ] with those of the previousiy investigated 0-xylyl dithiolate and p-tolyl thiolate complexes demonstrates that the replacement occurs virtually without structural change and reinforces the view that the Fe,S, core in two-iron systems (and the Fe,S, core in four-iron systems) is an integral
Iron(Ii1) and Higher States
237
substructural unit which can be transferred intact from one ligand environment to another, including those in proteins. Both the two-iron and four-iron dimers and tetramers are also reactive with respect to substitution of core S by Se as demonstrated by ' H NMR spectroscopy in MeCN solution.23' [Fe2S,(S,-o-xyl),]2- + 4RSH * [Fe,S,(SR),]Z-
44.2.6.1.3
+ 20-xylSH
(10)
Trinudear 'Fe-S' systems
Three-iron clusters have been detected only very recently. An Fe3(p-S), unit from Azotobacter vinelandii ferrodoxin I has been identified crystallographically.zo8Tlus protein also contains a [4Fe-3S] cluster. The [3Fe-3S] unit has a near planar cyclic structure (43). Each tetrahedral Fe atom has two terminal ligands of which five are cysteinyl side chains while the sixth appears to be an oxygen atom from H,O or OH-. The Fe.,.Fe distances, unlike those in two-iron and four-iron clusters, are Iong at 4.1 A. On the other hand, EXAFS measurements on two other proteins, both containing isolated ferrodoxin I1 from Desulfovibrio gigs?'* and beef heart three-centre Fe cores, have indicated Fe..Fe distances of 2.7A. This closer metal..-metal separation is incompatible with a planar [3Fe-3S] ring. Moreover, it has been shown that aconitase contains four labile S atoms, not three.233The situation is further confounded by the resonance Raman results of Spiro and collaboratorsu4on ferredoxins from A. vinelandii. The observed spectra are not compatible with the planar [3Fe3S] structure obtained in the crystallographic study. Instead, they strongly indicate a [3Fe4S] structure of type (44) or (45) derived from the [4Fe-4S] it must cubane core. Since there is no reason to question the validity of the crystallographic resultsZoS be concluded that the two groups of investigators examined different chemical species. Since the [3Fe-4S] structure appears to occur in all other three-iron proteins so far examined it has been speculated234that one S atom was lost during the isolation procedure for the planar [3Fe-3S] specimen, with accompanying structural rearrangement.
-
s
Structural diversity in trinuclear iron sulfide complexes has been extended by the isolation of a number of synthetic systems having a pyramidal IFe,(p3-S)] central unit (46)"' or a linear arrangement [FeS,FeS,FeJ (47).236The former structure (46) occurs in Fe" and Co" complexes of stoichiometry [M,S(S,-o-xyl)J- and comprises an apical pa-Satom bonded to three M atoms arranged in a nearly equilateral triangle.235Each M atom is coordinated to one terminal and one bridging sulfur atom of a dithiolate ligand, affording distorted tetrahedral MI'S4 units in structures that closely approach overall C, symmetry. The reduced, somewhat temperature dependent, magnetic moments (2.28 EM per metal ion at 301 K for M = Fe) and other properties are consistent with the occurrence of magnetic coupling. The Fe" complex shows a quasi-reversible oxidation (at - 0.59 V vs. SCE).235
Iron
238
The complexes [Fe,S4(SR),l3- (R = Et or Ph), isolated as the [NEt,]+ salts, were prepared by reaction of the strongly reducing [Fe"(SEt),]'- complex with 1.4 equivalents of sulfur in acetone. The corresponding benzenethiolate was then obtained by ligand exchange. Indeed, the [Fe(SEt),12compounds, affording complex was found to be a very versatile precursor to a range of Fe-S-SR direct entry to bi-, tri-, tetra- and hexa-nuclear clusters, as outlined in Scheme 2 which also indicates conversion pathways between [3Fe-4S] and higher order clusters.236 [FeCI,]
I
'-
4NaSEt. MeCN
[Fe(SEt),]
'-
I
ZHSH MeCN
Scheme 2 Preparation routes to two-, three-, four- and six-iron sulfur-thiolate clusters
The anion of [NEt4]3[Fe,S,(SPh)4]consists of two tetrahedral FeS, units linked by a central Fe atom also in tetrahedral sites; the two outer Fe atoms are each coordinated by two thiolate groups (structure 47). The Fe-..Fe separation is 2.714& i.e. similar to that deduced from EXAFS experiments for D.gigas ferredoxin II. Despite this, the structures of the three-iron cores in the natural and synthetic systems are considered to be quite different as judged by their different ESR spectra (g x 2.01 in the oxidized form of the protein, g x 4.2 in [Fe,S,(SEt),13-) and other properties.
44.2.6.1.4
Tetranuclear
'Fe-S' systems
The four-iron clusters are the most thoroughly studied group of iron-sulfur cluster comp o u n d ~ .In~ nature ~ . ~ ~they ~ occur in the four-iron and eight-iron ferredoxins (Fd) and the so-called high-potential (HP) proteins and many synthetic analogues have been studied. Their structures and properties are naturally rather more complex than those of the systems considered above. The physiologcal function of the ferredoxins is to transfer electrons and a major challenge of the inorganic chemist has been to develop simpler synthetic systems which reproduce, as far as possible, the distinctive electrochemical and other properties of the active sites of the proteins. As in the case of the one-iron, two-iron and three-iron sulfur compounds already considered, our present understanding of the chemistry of the ferredoxins owes much to the elegant synthetic analogue approach of Holm and co-workers whose publications should be consulted for fuller detail. The ferredoxin and high-potential proteins contain one or two [Fe,S,(S-Cys),] clusters (48) each of which acts as a one-electron transfer agent. The same structural unit occurs in many synthetic systems in which a thiolate ion takes the place of the cysteinyl residue of the proteins. The close similarity in the structures of the [Fe,S,(SR),] units between proteins and synthetic analogues has been demonstrated by X-ray structure determinations23K2244 on members of both classes, as can be seen from an inspection of the structural parameters collected in Table 2. Clearly, the synthetic compounds can be regarded as accurate structural models for the proteins. The basic geometry of this common structural unit is that of a tetrahedron of four iron atoms interpenetrating a second (larger) tetrahedron of four S atoms. Each S2- ion (designated S*) is bonded to three Fe atoms and each Fe atom bears a terminal SR group, cysteinyl residues in the case of the proteins. In all cases' the central 'cube' is compressed towards DZdsymmetry. An important feature of results of HP,, and HP,, is that the structural parameters appear very insensitive to the overall oxidation level. Moreover, there is no evidence for localized Fe", Fe"' states in any of these mixed valence structures. This is in accord with the findings of spectroscopic investigation^'^' capable of detecting localized Fe'l, Fe"' states with lifetimes of I O L 4 to 10-'6seconds. Physical properties have shown that the Fe-S clusters in [Fe,S,(SR),]'" are isoelectronic with those occurring in the Fd,, and HPredproteins thus
I 239
Iron (111)and Higher States
establishing that these forms of the proteins contain, formally, two Fe"' and two Fe" ions. Since the magnetic moments of the dianions are well below the values for high- and low-spin Fe"' and high-spin Fe", and since they are further reduced on cooling, it follows that the metal centres are antiferromagnetically coupled giving a resultant singlet ground ~tate.2,~
Table 2 Average Dimensions (A or ") in Some [Fe,S,YSR),] Units"
&-S* Fd,, from P. uerogenrs HP,,, from Chromatitm HPOxfrom Chromatiurn [Fe, S?(SPh),l2[Fc,S;(SCH,Ph),l2[Fe, S,YSCH,CH,C0,),]6[Fe, SKSCH, CH,OH),]*-
2.29 2.33 2.26 2.286 2.286 2.281 2.281
Fe--S
2.18 2.22 2.22 2.251 2,263 2.250 2.256
Fe--.Fe
S-Fe--S*
S*-Fc-S*
Fc-S*-Fe
2.85 2.82 2.11 2.146 2.736
116 115
101 103
114 114.4 115.1
104 104.1 104.3
14.6 13.1 13.8 13.5
2.155 2.729
114.6 114.2
103.9 104.3
14.1 13.5
I1
aData from ref. 207 and 238-244 in which original sources may be found
Electrochemical studies',' have shown that [Fe, S,(SR),]'-'complexes undergo both one-electron reductions and one-electron oxidations (equation 11). Property comparisons with the proteins have allowed the equivalence, in overall oxidation state level, between [Fe,S,(SR),]3- and Fe,, and between [Fe,S,(SR),]- and HP,,, to be established. The 'super-reduced' species HPsPredand the 'super-oxidized' species do not appear to be active in the natural protein metabolism. However, the [Fe,S4(SR),l3- complex (S = +) has been isolated and shown to have similar optical ESR and Mossbauer spectra to those of Fd,,,, pr0teins.2~' In contrast to the [4Fe-4S] core of the Fd, analogue, [Fe4S,(SPh),l3-, which is distorted from cubic to elongated DZdsymmetry, the core of the [Fe4S,(SCH,Ph),l3- anion consists six short and six long Fe-S bonds in C,, ~yrnmetry.'~'This kind of distortion appears to be exceptional, however, the usual structural effect accompanying [Fe,S,(SR,)]'- to [Fe4S4(SR),l3- being compressed DU --t elongated D2&
HPs-red
HPEd
H P O X
A point of difference between the natural and synthetic Fe-S compounds lies in the potential values at which oxidation/reduction occurs, these generally being more negative for the analogues (E,,' - 1.05 to - 0.75 V). Discrepancies originally ascribed to effects due to the influence of the polypeptide structure can now be attributed, in part, to differences in solvent medium.247However, the polypeptide must still have an attenuating effect on redox potentials, possibly by protecting the active site from solvent. A ' H NMR studyzmof the redox equilibria of [Fe4&(SR),]2-*3- (X = S, R = CH,Ph or p-C,H,Me; X = Se, R = CH,Ph) showed rates ca. lo4 greater than for Fd or HP proteins, the differences being attributed to the steric influence of protein structure. Preparative methods and reaction sequences leading to [FezS,(SR),]'- complexes, and pathways for interconversion to related clusters have been described and d i s ~ u s s e d . ~ ~Th * ~e ~series ' * * ~of ~ complexes [F~,S,(SC,H,-O-X),]~- (X = NH,, OMe, OH or SMe) have been prepared by ligand substitution from [F~,S,(SBU'),]~-and the structure of the complex in which X = OH has been N
Iron
240
solved by X-ray crystallography. This cluster contains three conventional tetrahedral FeS, sites and one distorted trigonal bipyramidal FeS,O site achieved by chelation of one o-SC6H,0H ligand. The Fe-S bond to the chelated ligand is only 0.035A longer than the other three though the Fe-O(H) distance (2.3 18 A) reflects a weaker, secondary interaction. Mossbauer spectra displayed separate quadrupole-split doublets for the case of X = OH and SMe but only a single doublet for X = NH2 suggestive of no secondary bonding in this case.253An interesting feature of the Mossbauer results is the high isomer shifts (6> 0.5 mm s- at 4.2 K relative to natural Fe) very close to the values found for the P clusters of nitrogenase.*% Several other tetranuclear iron-sulfur cluster compounds of general formula [Fe,S,&J’”*”~, in which X is a non-thiolate terminal ligand, have been studied in some detail. These include systems in which X = Cl, q5-C5H5,NO and dithiolene. The structure of the dianion [Fe,S,C1,]2- has been determined; it exhibits the same cubane-like stereochemistry, distorted from Tdto D2d,found in the corresponding thiolate tetramer dianion, showing that the core structure is rather insensitive to the nature of the terminal ligands.2MSimilar considerations apply to the case of complexes containing or X = [S2C2(CF3)JZ5’ terminal ligands. Variations in core dimensions occur, the X = q5-C5H52559256 however, according to the overall charge of the core. Thus, the geometry of the parent neutral diamagnetic [Fe, S,(q-C, H5),] complex distorts from tetragonal DU to orthorhombic D2 on oxidation (via Ag’, I2 or Br2) to the monocation, while, on further oxidation to the dicationZs8there is a further distortion of the cube towards a flattened iron tetrahedral structure with four short and two long Fe...Fe distances; this contrasts with the four long and two short bonds in the neutral compounds?56Nonetheless, the basic structural features of the cote framework remain intact as is of four electrochemically reversible one-electron waves in nicely i l l ~ s t r a t e d ~by~ *the * ~occurrence ~~ the cyclic voltammogram (Figure 7) of [Fe, S,(q5-CSH5),l”in MeCN corresponding to the couples 3+/2+ through to O / - 1 (equation 12).
‘
Volts vs SSCE
Figure 7 Cyclic voltammogram of [Fe,(tl5-C,H,),(~,-S),IZ+in acetonitrile at a platinum bead electrode with a scan rate of I vs-‘
t3
1.41 V e
4-2
0.88V e
4-1
0.33V e 0
= -0.33v
-1
The nitrosyl complex [Fe,S,(NO),] reveals two reversible O/- 1 and - 1/- 2 couples in CH2C12 solution. A structural comparison of the monoanion with the neutral parent again reveals core deformation (T, 4&) on reduction involving changes in Fe-q-Fedistance within the core. The architectural variations have been rationalized in terms of a cluster bonding model in which the unpaired electron in the monoanion occupies a t , level of largely antibonding character.260Analoin which two of the bridging sulfide ions of the core gous complexes [Fe4S2(p3-NCMe3)2(NO)4]ob-’ have been replaced by the electronically equivalent triply bridging N-tert-butyl anion, have been described.”’ A different core structure is found in the black Roussin salt [AsPh,][Fe,S,(NO),]. Here, the anion comprises a trigonal pyramid of four Fe atoms with triply bridging S2- ions and Fe-Fe bonds linking the apical Fe(N0) fragment to each of the three basal Fe(N0)2
Iron (III) and Higher States 44.2.6.1.5
241
Hexarruciear 'Fe-S' systems
The most recent additions to the family of polynuclear iron-sulfur cluster compounds are those having a hexanuclear six-iron core. Two types have been isolated, exemplified by [Fe,S,(SR),I4- (49) (R = Et, But or SCHzPh)'63~256~2w and [Fe,S,(PEt,),]'+ (50).265 The complexes (49) have a delocalized (4Fe"', 2Fe") mixed valence formulation while paramagnetic (50) contains all six metals, formally, in the Fe"' oxidation state. Complexes (49) were prepared by methods outline in Scheme 2 while (50) was prepared by reaction of Fe(BF,),.6H20 and PEt, (molar ratio ca. 1:3) with H,S in 0,-free acetone-ethanol at room temperature, followed by a period of air exposure, and, finally, precipitation as the [BPhJ salt.264The two complexes have quite different structures. The dication (50) is built up of an octahedron of iron atoms with the sulfur atoms triply bridging all octahedral faces; each square pyramidal Fe atom is additionally coordinated by a PEt, molecule.z65The core structure in (49) on the other hand contains three different types of bridging sulfur atom, six ~$3, two p3-S and one p4-S. The structure can be regarded as derived from two incomplete [Fe,S,] units, each lacking one Fe atom, linked by the common p4-S atom.2363264
i (49) The structure of the hexamclear [Fe,S,(SR),]'- cluster
44.2.6.1.6
(50) Each of the eight sulfur a t o m triply bridges an octahedral
'Mo-FeS' complexes
The isolation by Shah and from nitrogenase enzyme of an iron-molybdenum cofactor (FeMo-co) containing Fe, acid labile S and Mo in the approximate ratio (68)Fe:(4-6)S:Mo, believed to be the catalytic site (in a reduced oxidation level) for the reduction of dinitrogen to ammonia, has greatly stimulated attempts to prepare and characterize synthetic compounds containing Mo and Fe together in the same cluster. This goal was given added significance by the recognition from X-ray absorption spectra (XAS) and EXAFS studies267of several Fe-Mo proteins that the coordination unit might contain structures such as (51) or (SZ),i.e. types of cluster very similar to those already well established for Fe-S compounds, and for which synthetic procedures via 'spontaneous self-assembly' were available. Application of these synthetic methods, involving the use of ~ o S 4 I 2 -known , to act as a as a soluble source of Mo led to the isolation of a series of heteropolynuclear products via reactions such as given in equations (13) and (14).269Structure determination^^^^^^^' of crystalline salts of anions (53) and (54) have confirmed the occurrence in both of the cubane-like MoFe,S, unit, one of the proposed structural models for FeMo-co. The two cubane MoFe3S, subclusters are linked by different types of bridging unit to afford the so-called 'double cubane' cluster. Other smaller, Fe-Mo-S complexes such as (55) and (56) have also been prepared.'72.273The structural similarity of (52) to the other proposed model for FeMo-co is obvious. The tungsten analogues of many of these. complexes have also been prepared.'" A significant observation is the occurrence of a spin-quartet (S = 3) type of ESR spectrum in some of the synthetic clusters273as also observed in FeMo-~o.'~'For a detailed description of the and structures, physical properties and reactions of these complexes the review article by the references therein, should be consulted. 2MoS:-
+
6FeC13
+
17RS-
----*
+
+
18C1-
(13)
+
+ 21C1-
(14)
[ M O * F ~ ~ S ~ ( S R ) ~4RSSR ]~(53)
2MoS,'-
+
7FeC13
+ 20RS-
-+
[MO~F~,S,(SR),,]~- 4RSSR (54)
More recently, the preparation of the trianion fM~Fe,S,(SEt),(cat),]~-(57) (cat = catecholate), which contains a single cubane type [MoFe,S,] core, by a cleavage of the Fe"'-bridged double cubane [Mo, Fe,S,(SEt),,13- (54) has been described. The EXAFS spectrum of (57) is in qualitative
Iron
242 -S
/ Fe
S-
S
/
.
’
S-
Fe
\ (52)
SR
1
3
R
R
‘SR
RS
(53)
r
RS
\
,,FxS, S-Fc
-
44.2.6.2
,S\
R
/Fe-s
\R/
S R
4-
/ SR
,sxkeySR
Ie-s
S-Mo-S-Fc--S-Mo-S
4\d/ RS
R
,S,
7 3-,
\R/
S
K
\Y / S-Fe
\SR
RS
-
Complexes of Dithiocarbamates and Related Ligands
There are a number of negatively charged ligands capable of coordinating via two sulfur atoms (or one sulfur and one oxygen atom) to a metal ion with formation of four-, five- or six-membered chelate rings (see structures 58 to 68).Other modes of coordination, Le. unidentate or bridging, are also known for the same group of ligands. Several reviews are available.279-z86
( 5 8 ) Dithiocarbamate
(59) Xanthate
(60)Thioxanthate (61) Dithiocarboxylate
(62) Dithiocarbonate
243
Iron ( I U ) and Higher States S
S +NR,
+S S (63) Trithtocarbamate
0 (64) Dithiophosphinate
(67)DithiOhte
(65) Monoth,ocarbamate
( 6 6 ) Monothiocarbonate
(68) Dithio4-diketonate
The iron(II1) dithiocarbamates are the most extensively studied, largely because of their interesting magnetic properties. Cambi and co-workersZs7first showed that tris(dialky1dithiocarbamato)iron(II1) displayed spin cross-over phenomena between high-spin and low-spin states. As indicated earlier (Figure 1) Fe'" in a weak octahedral field has a 6AIg(t&ei)ground term and gives rise, in the absence of magnetic interactions with other paramagnetic centres, to magnetic moments close to the spin-only value of 5.92 BM (S= 3). In a strong field, on the other hand, the term 'Ta ( t i g )lies lowest and in this case room temperature moments are commonly close to about 2.0 BM corresponding to the presence of one unpaired electron in the tzRorbital set, Le. S = +. The difference between the observed moment and the spin-only value of 1.73 BM arises from small contributions from the non-zero value of the orbital contribution of the angular momentum in the & configuration. In practice, deviations from standard high- or low-spin behaviour in magnetically dilute systems can arise whenever the ligand field is comparable in strength with the mean electron-pairing energy appropriate to the Fe"' d 5 configuration. In cases where the energy of the hgh-spin state lies only slightly (from ca. 50 to ca. 400cm-') above that of the low-spin state there will be a thermally controlled population of the two states which will be manifested by a temperature dependent magnetic moment between the limiting values of 5.9 BM at high temperatures and 2.0 BM at low temperatures. Temperature dependent magnetic susceptibility measurements of several tris-N,N-dialkyldithiocarbamate complexes have been successfully interpreted in these term^?^*-*^' However, only in the case of solutions can a simple Boltzmann distribution between high- and low-spin states be e~pected.~" This is because of complicating secondary solid state effects such as vibrations in crystal packing, solvation and, in certain cases, the existence of the complex in more than one crystal modification leading to phase changes on temperature variation. Mossbauer effect studiesz9' show only one, averaged, temperature dependent quadrupole-split doublet. The failure to observe separate signals due to the two spin states has been attributed to high-spin =$ low-spin cross-over periods shorter than the Mossbauer timescale (1.5 x 10- 's). The same applies to NMR shifts. Also, Hall and Hendri~kson~'~ demonstrated, from ESR measurements, that the spin-flipping rate is of the order of 10" s-'. It should be noted, however, that separate signals for the two spin isomers could be detected in the Mossbauer spectra of some tris(monothi0-/?-diketonato)iron(III) complexes, indicating slower exchange in the case of this unsymmetrical chelating ligand?g' It has been observed that the position of the 6A,e 2 T 2equilibrium in dithiocarbamate complexes [Fe(S,CNR,),] is quite sensitive to the nature of the NR2 substituent.290As is apparent from the collected data in Table 3 the room temperature magnetic moments shift in favour of the * T2state as the NR2 groups change from the cyclic pyrrolidyl group of N,N-di-n-alkyl, to N-alkyl, N-aryl, to N,N-di-sec-alkyl, It has been suggested that this trend may have a steric origin since the RNR angle is expected to increase in the same order leading in turn, to an increased contribution of resonance form (70), compared to (69, this having the stronger ligand field. However, other workers296have argued that limiting structure (70) will have the lower field and that its importance will increase with increase in electron releasing ability of the NR, group. In agreement with this argument an increase in peEwith increase in pK, of the secondary amine was observed, except for those amino groups having a secondary carbon atom. The exceptional position of these systems in the correlation was explained in terms of non-coplanarity of the CSz and NR, planes. It was further observed290that xanthates [Fe(S, COR)3] are almost completely in the low-spin form while dithiophosphinates [Fe(S,CPR,),] (R = OMe, OEt, OPr", O p t ) are almost completely high-spin in the 9&300 K temperature range. The magnetic properties of diselenocarbamato complexes are very similar to those of corresponding thio compounds but with the position of the spin equilibrium shifted towards the low-spin state in most examples.297The thioxanthates, in contrast to xanthates, also appear to be examples of spin equilbrium systems.298 C.O.C. L l
244
Iron
Table 3 Magnetic Moments in Solid and Solution Phases at 23°C of the Complexes [Fe(S,CNR,),]
NR, = pyrrolidyl NR, = piperidyl NR, = morpholyl R=Me Et Pr" Bun isoamyl n-hexyl allyl R,, R, Me, Ph Et, Ph Pr",Ph Am', Ph diknzyl di-wc-alkyl PI' B ui cyclohexyl aB =
5.83 4.01 5.12 4.17 4.24 4.48 5.32 4.30 3.52 4.40 2.99 4.70 4.68 3.36 4.02 2.62 3.02 2.75
5-81 (B) 4.16 (C) -
4.20 (C) 4.41 (B, C) 4.42 (B) 4.34 (B) 4.57 (B) 4.42 {B) 4.34 (B) 3.33 (C) 3.63 (C) 3.55 (B) 3.43 (B) 3.60 (B) 2.32 (B) 2.88 (B, C ) 2.63 (C)
Benzene, C = Chloroform.
Since it can be predicted that depopulation of the (high-spin) sextet level in favour of the (low-spin) doublet level should be accompanied by a contraction of the [FeS,] core in spin transition systems, it is to be expected that the position of the spin equilibrium K will depend on pressure. This has been experimentally verified for several tris(dithiocarbamato)iron(III) complexes in chloroform solution at pressures up to 5000 atmospheres at room temperature. On the assumption that the molecular volume contraction AV arises entirely as a result of the spin change, equation (15) can be applied to the calculation of the volume change. Values of A Y of the order of 5 cm3mol-' were obtained for systems with a wide variety of N substituents.289~2w The values correspond to a contraction of the Fe-S bond length of about 0.1 A, during the ' A -+ 'T transition. AV = - R T6 In( K~ ) T
and Crystallographic confirmation of this effect has been obtained, first by Hoskins and subsequently by Healy and White.3" In the complex [Fe(S,CNBu~),] which is largely in the high-spin form at room temperature (peff= 5.32 BM) the mean Fe-S bond length is 2.42A whereas = 2.72 BM at 296 K), the in the xanthate complex [Fe(S,COEt),], largely in the low-spin form kff mean Fe-S distance is 2.32A. The same difference of 0.1 A in mean Fe-S distance was observed between the high-spin (pcR = 5.9 BM at 300 K) tris(N-pyrrolidyldithiocarbamate) and the mainly = 3.0 BM at 300 K) N-methyl-N-phenyl derivative. In addition, there are certain low-spin alterations in the overall symmetry of the coordination polyhedron as a consequence of the spin change. Thus, the high-spin dithiocarbamate with Bun as substituent has a structure intermediate between trigonal prismatic and octahedral, whereas in the low-spin xanthate the,structure is closer to octahedral.299 In [Fe(S,CNEt,),] the Fe-S distance changes from 2.357 A at 297 K ken = 4.3 BM) to 2.306A at 79 K ('p= 2.2 BM) but in this case crystal packing is also different at these two temperature^.^" The contraction in Fe-S bond length accompanying the 6 A -+, T spin change is also reflected in a change in the Fe-S stretching frequencies. With the aid of 54Feand 57Feisotopic substitution the assignment of v(Fe-S) vibrations in the regions 21&240cm-] and 300-340cm-' for high- and low-spin forms, respectively, has been made.302 T, spin transition in dithiocarbamatoiron(II1) complexes is sensitive not only to The 6 A , temperature, pressure and chemical modification but also to the number and nature of solvent
Iron(I1I) and Higher States
245
molecules included in the lattice s t r u c t ~ r e .With ~ ~ ~solvents - ~ ~ ~ such as benzene and nitrobenzene the equilibrium is shifted towards a low-spin ground state while in solvents such as H,O, CHC13and CH,Cl, capable of hydrogen bonding the equilibrium is shifted towards the high-spin state. Although the symmetry of six-coordinate tris(dithiocarbamato)iron(III) complexes is that of a trigonally distorted octahedron rather than pure octahedral, the distartion would seem to be too ) to become the ground state. However, small for a component, 4A2to E, of the split T, ( 2 % ~ ~level it was speculated303that this might occur in the case of the CH,C1, solvate of tris(morphoiny1dithiocarbamate)iron(III) whose magnetic moment appeared not to fall below values (- 4 BM) appropriate to a spin quartet ground state even at very low temperatures. However, a subsequent detailed Mossbauer examination of this system by the same and by others?" has shown conclusively that only sextet and double states are involved. Treatment of the tris(dialkyldithiocarbamato)iron(lII)complexes in benzene with a controlled amount of concentrated hydrohalic acid affords the black bis(1igand) complexes [FeX(S,CNR,),] (X = Cl, Br or I).306For X = Cl the complexes may also be prepared by irradiation of the tris(ligand) complex in a halogenated solvent. This free radical reaction is believed to proceed via excited-state tabilization of one ligand followed by attack of solvent.307Analogous complexes of pseudohalide ions (X = NCO-, NCS- or NCSe-) have been obtained from reaction of the parent tris complex with the appropriate Ag+ salt.380Representative complexes of this class have been shown by X-ray diffraction methods to have square pyramidal structures (71) in which the sulfur 2.228-2.30 A) with the halide atoms of the two bidentate ligands comprise the basal plane (Fe-S 2.26-2.28 8L).309,310In the cases examined the metal atom sits ion in the apical position (Fe-Cl 0.6 A out of the mean S, plane in the direction of the apical halide ion.
-
-
N
i (71)
While for the trisldithiocarbamate) complexes the symmetry is not sufficiently distorted from cubic to generate a spin-quartet ground state ( S = ;), this is no longer true for the square pyramidal bis(comp1exes) and these exhibit nearly temperature independent magnetic moments close to 3.9 BM corresponding to three unpaired electrons (Figure I). ESR and magnetic anisotropy data are consistent with a ' A 2 ground ~ t a t e . ~Large ~ ? ~ "quadrupole splittings ( A E = 2.7 m m s-I) are consistent with the large electric field gradient at the iron nucleus expected for the C,, point ~ymrnetry.~" Fe"' dithiocarbamates [Fe(dtc),] undergo reversible or nearly reversible one-electron oxidations and one-electron reductions in polar organic solvents to generate Fe" and Fe" species, respectively (equation 16). The bright yellow air-sensitive Fe" complexes appear to be high spin while the Few systems (vide infra) have low-spin d 4 configurations.282b [Fe'" (dtc),]
+
e
[Feu'(dtc)J
e [Fe"(dtc),1-
(16)
1,1 -Dicyanoethylene-2,2-dithiolate(72),which like the dithiocarbamates forms four-membered chelate rings, gives tris complexes with Fe"' which are high spin (S = i)with, presumably, six-coordinate structure^.^'^ The geometrical isomer 1,2-dicyanoethylene- 1,Zdithiolate (73;R = CN) and related dithiolenes give five-membered chelate rings and form Fe"' complexes with a variety of The bis complexes structures and properties. The [PPh4]+salt of [Fe{SzC2(CN),},J3-is low [Fe{S,C,(CN),},]~ are dimeric in the solid state and constitute an electron transfer series with n = 0, - 1 and - 2. The structure of [NBU,],[F~(S,C~(CN)~)~]~ has been determined. Each Fe atom has a square pyramidal geometry in which the basal plane contains four sulfur atoms (of two bidentate dithiolene anions) the apical position being occupied by a bridging sulfur atom of a second monomer unit. The magnetic properties are consistent with antiferromagnetically coupled S = The dimeric bis(dithio1ene) complexes are cleaved by various monodentate bases to afford complexes of the type [Fe(S,C,R,),B]-. Those complexes in which B is a 'soft' ligand such as PPh,, AsPh, RNC or CN- have doublet ground states, while those where B is e.g. amine, phosphine oxide, halide or N;, exhibit quartet ground
+
246
Iron
The low-spin complex [Fe@-to1CS,)? (p-to1 CS,)] contains two four-membered chelate rings and one five-membered chelate ring (74) involving the metalklisulfide linkage. Fe-S distances in the five-membered chelate ring are slightly smaller than in the four-membered ring, while the S-.-S'bite' distance is distinctly larger, as in dithiolene complexes."6 The dithiooxalate ion (75)binds to metal ions via the sulfur atoms, and the tris complexes with Fe"' may be low- or high-spin depending on the charge distribution within the ligand. Thus when = 2.30 BM at 300 K) complex ~h,As],[Fe(dto),] -3MeN0, (76) was treated with the low-spin (pCR the cations [M(PPh,),]+ M = CUI or Ag' (reaction 17), the crystalline high-spin complexes (77) were obtained (pert.2 5.9 BM).317Although the dithiooxalates show some electrochemical activity they are not subject to the same extensive electron delocalization which is a prominent feature of the dithiolenes; they are thus closer in properties to the thioxanthates in this respect.
(75)
Me
(74)
Six-membered chelate rings are found in complexes of dithioacetylacetonate (Sacsac). [Fe(Sacac),] may be obtained in crystalline form by reaction of Fe" salts with acetylacetone and H,S in acidified alcohol. The 'FeS,' unit is a slightly distorted octahedron, two of three ligands being related by a two-fold symmetry axis with the third inequivalent ligand subtending a larger S-Fe -S angle. The mean Fe-S distance is 2.25A in good agreement with values found in other low-spin dithio chelates, and, as before, the metalkhelate rings are essentially planar, The low-spin = 2.00 BM at 293 K, nature of this complex inferred from magnetic susceptibility measurements (pcK 1.75 BM at 100 K) is confirmed by the large values of the quadrupole spitting (1.96 mm s- I at 4.2 K) in the Mossbauer spectrum. A rhombic character is apparent from the observation of three g values in the ESR spectrum of the powder (gl = 2.14, g2 = 2.09, g, = 2.01).3'8 The spin-pairing effect of sulfur as a donor atom is nicely illustrated by a comparison of the magnetic properties of the series of complexes [Fe(acac),], [Fe(Sacac)J and [Fe(SacSac),]. Thus, while the first has a high-spin ground state and the last is essentially fully low-spin, the asymmetrical '0,s' ligand Sacac forms both high- and low-spin complexes (78,79)depending on the nature of the ligand substituents. Thus, for R = R' = Ph the room temperature moment is 5.50BM, while for R = CF,, R' = Ph, it is 2.31 BM. A trigonal antiprismatic rather than trigonal prismatic arrangement of thefuc-O,S, donor set is seen in the values of the twist angles &52" in 78,60"in 79) of the two parallel triangles defined by the three oxygen and the three sulfur donor There is a marked difference, for systems of comparable spin state, between the Fe-S bond lengths of four-membered chelate rings and those of six-membered chelate rings. The values of
-
247
Iron (111) and Higher States
-
2.41 and 2.32 A found for high- and low-spin dithiocarbamates and xanthates are significantly larger than the values of 2.36 and 2.25 A observed for (78) and (79)."9 Complexes of the monothiocarbamate anion (R2mtc) promise to have a coordination chemistry as rich as that of the dithiocarbamates and are the subject of a recent review.2s4The tris bidentate arrangement of ligands found in the six-coordinate dithiocarbamates is also found in the monothiocarbamates. In the complex [Fe(mtcMe,),J the twist angle # between the triangle formed by the three sulfur atoms and the parallel triangle formed by the three oxygen atoms was found to be 33.2", Le. intermediate between that (# = Oo) for a trigonal prismatic arrangement and that (# = 60") for an trigonal antiprismatic (pseudooctahedral) arrangement, similar to the situation found in most analogous dithiocarbamato iron(II1) complexes.3mThe two complexes (80) and (81) are also 6Ae T spin-equilibrium systems between 6 and 300 K although the former can also be obtained in a crystalline modification that remains high-spin down to 10K. Mossbauer spectra at low temperatures indicate that the 6 A + 2 T interconversion rates are > 107s-'. A single ESR signal at g = 4.3 probably arises from rhombically distorted high-spin Fe"' isomers.32'
-
Fe
[$) \
R'
3
(78) K = R' = Ph (79) R = CF3, R ' = Ph
(80) K = Me ( S I ) R = Et
44.2.7 HALIDE COMPLEXES In keeping with the 'hard' or 'class a' nature of the Fe3+ion the most stable complexes are formed with the small non-polarizable F- ion.322Consecutive formation constants for some fluoro and chloro complexes in aqueous solution may be found in ref. 322. Bromo complexes are even less stable than chloro complexes while with the easily oxidizable I- ion no stable simple iodides have been prepared although a very few complexes containing Fe"'-I bonds are known in combination with other ligands. Consistent with the stability order F > C1' > Br- is the occurrence of higher coordination number FeI'' complexes with F than with Br-. Thus, while F- readily forms hexacoordinate [FeFJ- complex ions and no tetrafluoro complex ions, C1- forms both [FeC1,I3and [FeCl,] while Br apparently does not form a stable hexabromo complex [FeBr,]". The anhydrous binary halides FeX, (X = F, C1 or Br) have near regular octahedral coordination as reflected in the absence of resolved splitting in the Mossbauer spectra (isomer shifts 0.49-0.55 mm s- I ) . The hexahydrate, FeC1,-6H20, however, has a trans octahedral arrangement [Fe(H2O),Cl,]Cl-2H,O and exhibits pronounced quadrupole ~ p l i t t i n g . ~ The ' . ~ ~ dihydrate ~ and heptahydrate of pentafluorodiiron, Fe2F,-2H,0 and Fe2F,-7H,O are Fe", Fe"' mixed valence systems. The structure of the red dihydrate has been determined.324It consists of a three-dimensional network with distinct Fe" and Fe"' sites. The formally Fe"' ions occupy vertex-sharing 'FeF,' octahedra while the Fe" sites in 'Fe(H20)2F4'share the vertex positions of the fluoride equatorial plane. The Fe"'-F and Fe"-F distances are 1.941 and 2.060& respectively. The structure is ferromagnetically ordered below 48 K but above this temperature distinct quadrupole-split absorptions for the two sites are apparent showing that this is a class IT4' mixed-valence The yellow heptahydrate, on the other hand, is formulated as [Fe"(H,O),][Fe"'F,(H,O)J and therefore as a class I mixed-valence system. A number of tetra-, penta- and hexa-fluoride anions are known, the last mentioned being the most imp~rtant.~" The tetra- and penta-fluorides have reduced magnetic moments and are probably polymers with fluoride bridges;327the hexafluorides containing the discrete [FeF613-have magnetic moments close to 5.90 BM at room temperature.328The complex K2[FeF5(H20)]also consists of discrete six-coordinate anions with one F- ion replaced by a water molecule, although the anions are linked into chains by strong hydrogen bonds (O...F, 2.54 A).329 A large number of complexes containing the tetrahedral [FeCl, 1- anion have been prepared; some have already been mentioned. Several single crystal X-ray structure determinations are available. ],~~~ and [Fe(NCAmong the most accurate are those of [ A S P ~ J F ~ C ~ , [PC1,]~eC1,]31' Me),][FeCI,], .332 Fe-C1 distances (2.182-2.1 87 A) agree well among the different structures alangle from the tetrahedral value in some cases. though there are small departures in CI-Fe-C1
Iron
248
The complex of stoichiometry (NMeH,), FeBr, should be formulated as ~MeH,],[FeBr,]Br since it contains the tetrahedral [FeBrJ complex anion; mean Fe-Br distances are 2.32A while the Br-Fe-3r angles vary up to 4" from the true tetrahedral value.333It has also been that the complex of formula (pyH),Fe,Cl, contains only the [FeCl,]- ion and not the triply chloridebridged dinuclear [Fe,C1,I3- anion as first supposed; the complex should thus be formulated as the mixed salt @yH]2[FeCl,],. [pyH]Cl. The complete series of complexes ~Et,][FeX,-,Y,]- (X = C1, Y = Br, n = 0-4)has been synthesized and the far IR and Mossbauer spectra reported. The vibrational spectra are consistent with the lowering of the molecular symmetry from Tdto C,, and from Tdto C,, (Table 4). The reduction in symmetry is not reflected in the Mossbauer spectra, however, since a narrow unbroadened single peak was observed in all the spectra.335 Table 4 IR and Mossbauer dataa for [Fe"'X~4-,IYn]anions Isomer Shifi Anion
v(Fe-Xj
(cm-I)
d(X-Fe-X)
(an-')
~
fFeCL1[FeCI,Br][FeCI,Br,](FeCIBrJ [FeBr4]-
388 391, 350, 296 385, 348,296, 266 384, 295, 270 294
137 135, 117 126, 225, 74 120, 73 98
ut 77K(mms-') ~
0.282 0.285 0.321 0.333 0.350
Data from ref. 335. Relative to natural iron.
The hexachloroferrate(II1) anion [FeC1,I3- may be stabilized in the solid state by the use of large [Co(pn)3]3' (pn = 1,3-diaminopropane) and [NMeH,]'. Magnetic cations such as [CO(NH~)~],+, moments lie in the accepted range for high-spin (S = 2) octahedral configurations as do the Mossbauer spectra also which consist of a single absorption at ca. + 0.5 mm s-' (vs.natural and optical spectra336have been assigned. The vibrational Slow evaporation of aqueous solutions of FeC1,.6H,O and e.g. NH,C1 or KC1 affords salts of the six-coordinate [FeC1,(H,0)]2- anion. Single crystal Raman studies have been reported for this ion.338Several other complexes, in which one or more of the chlorides of [FeCl,]- or [FeCl6l3-have been replaced by other ligands, have been described and structurally characterized. The p-oxo dinuclear complex [PyH], [Cl, Fe-0-FeCI,] 'py (82) contains distorted tetrahedral 'FeOCl,' units bridged by the oxygen atom via short Fe-0 bonds (1.755&, the Fe-0-Fe angle being 155.6". The C1-Fe-C1 and Cl-Fe-0 angles are in the range 108-111" and the Fe-C1 distances (2.208-2.219 A) are somewhat lengthened with respect to those in [FeClJ- . As expected for the local C,, symmetry about each Fe"' atom two TR active Fe-Cl stretching vibrations, e and a l , are observed, at 360cm--' and 31 1 cm-'. Strong absorption at 8 6 0 m - ' is assigned to the Fe-0-Fe antisymmetric stretch. Strong antiferromagnetic coupling occurs between the S = 3 spins. The coupling constant (J = - 92 cm-' ) is not appreciably different from values reported for other binuclear p-oxo complexes in which the Fer'' ions are in a square pyramidal or octahedral environment. This is the first example of a binuclear Fe"' complex containing, apart from the bridge, only monodentate ligands, and the first example of a p-oxo Fe"' complex having tetrahedral coordinati~n.~~'
(82)
The complex [Fe(CCN-py), Cl,] (4-CN-py = 4-cyanopyridine) is a trigonal bipyramid with the three C1 atoms occupying the equatorial sites and the pyridine ring nitrogens the axial positions.M This is one of a very few structurally defined trigonal pyramidal Fe"' complexes. An earlier example, [Fe(N3)5]2-,has already been mentioned;,' the complex FeCl,(diox) (diox = dioxane) may have a similar ge~rnetry.'~' The molecular structure of the dimeric species [Fe,C16] has been investigated by vapour phase electron diffraction. The two tetrahedral Fe"' atoms are bridged by two C1 atoms in a puckered FezC1,ring. As for related dimeric molecules such as [Al,Cl,] the terminal Fe-C1 bonds (2.127A) are significantly stronger than the bridging Fe-C1 bonds (2.326 A).M2
Iron(1II) and Higher States 44.2.8
44.2.8.1
249
COMPLEXES OF MIXED DONOR ATOM LIGANDS Complexes of Schiff Base Ligands
In this section complexes of acyclic Schiff base ligands only are considered, macrocyclic Schiff base complexes are included in a later section. Condensation reactions (equation 1 8) between carbonyl compounds and primary amines have provided one of the most important and widely studied classes of chelating ligand. A particularly important carbonyl precursor for the synthesis of Schiff base ligands is salicyaldehyde first used by Pfeiffer and Tsumaki."4' A wide variety of ligand types may be obtained via the Schiff base condensation reaction which vary in denticity, flexibility, nature of donor atoms (in addition to nitrogen and oxygen) and in electronic properties. Some of the more important classes of ligand derived from salicyaldehyde are shown, in the anionic (deprotonated) form, in Figure 8. Within each class subtle alterations in coordinating characteristics can be achieved by variation in the nature of the R substituents and in the nature and position of the phenyl ring substituents. A wide diversity of coordination geometries and magnetic behaviour has been found.
c"a 0
NR
(86) Figure 8 Some important classes of Schiff base ligands derived from salicylaldehyde
The bidentate N-substituted salicyaldimines (83) form complexes of stoichiometry [Fe(salNR),X] (X = C1 or Br) which have high-spin ( S = 2) monomeric five-coordinate ~ t r u c t u r e s An . ~ ~X-ray investigation of the R = n-propyl, X = C1 derivative indicated a stereochemistry intermediate between trigonal bipyramidal and square ~ y r a m i d a l . ~The ~ ' derivative with R = 2-phenylethyl, X = C1 apparently exists in two crystalline modifications, one having a monomeric trigonal bipyramidal the other having a distorted structure intermediate between a trigonal bipyramid and a square pyramid which allows a loose association between adjacent monomer This weak association Ieading to a phase change to a six-coordinate modifkation is suggested to be responsible for the reduction in magnetic moment, from the normal high-spin value, at temperatures below 140K. The complex [Fe(saIpa)Cl,],, where salpa is the tridentate dianionic ligand of type (87) formed from the reaction of salicyaldehyde with 3-aminopropanol, also contains five-coordinate Fe"' but, this time, in a genuinely dimeric structure (88). The iron atoms of the dimer are bridged by the
250
Iron
alkoxide oxygens to form a four-membered ring with Fe-0 distances of 1.983 and 1.9348L and angles of 75.9" and 104.1O at iron and oxygen, respectively. Considering the geometry as (distorted) square pyramidal the basal plane is made up of the donors of the tridentate ligand and the bridging alkoxide oxygen with the chlorine atom in the apical position. The magnetic behaviour accords with the occurrence of antiferromagnetic superexchange interactionmediated by the dialkoxy bridge, per having the reduced value of 4.52 BM at room temperature and falling further to 2.37 BM at liquid nitrogen temperature. Reaction of [Fe(salpa)Cl], with Na,O, afforded the red crystalline complex '[Fe,(salpa)(salpaH),] -toluene in which salpaH is the monomeric form of the tridentate ligand.34sIn this dinuclear complex each Fe"' atom is six-coordinate and linked to the other via bridging propoxide groups in, once again, an almost planar four-membered FezO, ring. Despite having nearly identical distorted octahedral geometries (with trans nitrogen atoms) the iron atoms are nevertheless nonequivalent. One iron atom is coordinated by two doubly deprotonated tridentate salpa ligands, the two propoxide groups which are in cis positions providing the bridging atoms for the dimeric unit. The second iron atom is coordinated by the two monodeprotonated salpaH ligands each acting in a bidentate fashion with uncoordinated OH groups. As in the dimeric five-coordinate complex described, the Fe"' centres are antiferromagnetically coupled with the coupling constant being in the range - 5 to - locm-'. CI
c1
I
(88)
(89)
(90)
Condensation of two equivalents of salicyaldehyde with a diprimary amine, commonly 1,2diaminoethane (en), provides a convenient route to tetradentate Schiff base ligands of type (85). For the case where the amine is en the product ligand is abbreviated as salen. Different modifications of the complex of formula Fe(sa1en)Cl may be obtained according to the method of preparation or isolation, one monomeric and one dimeric.349The monomeric form (89), obtained by recrystallization of the dimeric form from nitromethane and containing a molecule of solvent in the lattice, has a square pyramidal structure with the metal atom sitting 0.46 8L above the best plane defined by the 'N402'donor set of the tetradentate ligand, in the direction of the apical C1 atom.350In the dimer (W), isolated from acetone solution, two molecules of monomer each share one oxygen atom from each salen ligand to give an overall octahedral coordination for each metal ion.35' The bridging Fe-0 bond, trans to the Fe-Cl bond, in the dimer is 0.24A longer than the mean of the Fe-0 bonds in the N,O, planes and may reflect the tendency for the dimer to dissociate into monomers in chloroform solution. The monomeric and dimeric Fe(sa1en)X (X = C1 or Br) complexes, and various ring substituted derivatives are readily distinguished on the basis of their magnetic properties.349The monomers have magnetic moments at room temperature close to the spin-only value of 5.92 BM and obey the Curie-Weiss law with small Weiss constant. The dimers, on the other hand, have subnormal room temperature moments which are further reduced on decrease in temperature. In these cases the magnetic data have been successfully fitted to the theoretical equation describing antiferromagnetic superexchange between pairs of interacting S = 5 spins, yielding coupling constants J of between - 6.5 and - 8.0 cm-'in different cases. IR and Mossbauer spectra also provide useful criteria for the recognition of monomeric and dimeric forms of Fe(sa1en)X complexes. Those compounds known or suspected to be dimeric displayed up to three bands between 800 and 900 cm- ' ,while in compounds believed to be mononuclear a single band was observed in this region. Similarly Mossbauer isomer shifts and quadrupole splittings both tended to be smaller for the five-coordinate (mononuclear) complexes than for the six-coordinate (dinuclear) complexes.352 The preparation and properties of some unusual organometallic Fe'" complexes of salen have been described by Floriani and C a l d e ~ a z z oIt. ~was ~ ~ found that the iron(I1) complex (91) can be reduced to the corresponding monoanion (92) by sodium in THF. The extremely air-sensitive N
,'
25 1
Iron (IiI) and Higher States
monoanion, believed to be a high-spin Fe' d7 species, behaves as a strong nucleophile and reacts at - 60" with benzyl chloride to give (93) (see Scheme 3), a rare example of a compound containing an Fe"'-carbon (T bond. This complex has a high-spin ( S = $) configuration as evidenced by the magnetic moment of 5.87 BM at room temperature and presumably has a mononuclear square pyramidal structure similar to that of the mononuclear form of Fe(sa1en)Cl. The complex (93) is both thermally unstable and 0, sensitive (Scheme 3) leading to reduction to Fe(sa1en) and oxidation to the p o x 0 complex (94), respectively, along with dibenzyl. The Fe-C bond is also cleaved by Lewis bases such as pyridine and cyclohexyl isocyanide and by reaction with benzyl chloride and I2 (Scheme 3).
+ IFr1"(salen)C1l + PhCH,CH,Ph Scheme 3
Preparation and reactions of organometallic iron(II1) complexes
(94)
A second series of dinuclear Fe"'-salen complexes [Fe(salen)O,] (94) has been extensively investigated354-357 following Pfeiffer and Tsumaki's first report.343These complexes have five-coordinate structures, in contrast to the six-coordinate [Fe(salen)Cl], dimers, in which the two approximately square 'FeN,O,' moieties are linked by a single oxygen bridge. In two solvate modifications the Fe -0-Fe bridge angle has been shown354*355 to be 139" and 142". A much larger antiferromagnetic coupling occurs in the singly-bridged p o x 0 compounds than in the doubly oxygen-bridged [Fe(salen)C1], dimers already discussed, J having values in the range of - 87 to - 100cm-' for various members of the former class. Various possible reasons for the different exchange interactions can be considered. Firstly the Fe-0 distances (1.97-1.98 A) are appreciably shorter in the p-oxo complex than those (1.92-2.18 A) in [Fe(salen)Cl],. This would suggest a measure of multiple bonding in the former, possibly allowing transmission of exchange coupling via n as well as 0 pathways. There is also a large difference in Fe-0-Fe angle in the two systems. Since a ferromagnetic interaction is predicted when the bridge angle is close to 90" it may be that the relatively small angles ( 95") in this net antiferromagnetism in [Fe(salen)Cl], is a result of the small Fe-0-Fe case. The products of reaction of [Fe, (~alen)~O] with trichloroacetic, trifluoroacetic, salicylic and
-
I
252
Iron
picric acids are dimers of composition [Fe(salen>X],,where X is the conjugate base of the acid HX, with magnetic properties very similar to those of [Fe(salen)Cl],. A monomeric compound is formed on reaction with picolinic acid, however. The reduced form of salen, salen-H, in which the two imino groups have been hydrogenated, also forms a dimeric complex [Fe(salen-H)Ck]-2H,O believed3'* to have a similar phenoxy-bridged six-coordinate structure to that of the parent Schiff base complex [Fe(salen)Cl],. However, this ligand gives a di-p-hydroxo dimer [Fe(salen-H)(OH)],. 2H20.2py rather than a p o x 0 dimer and, as expected for a 'Fe0,Fe' bridging unit in which the bridge angles are relatively small (102.8" in this case), the superexchange coupling (J = - 10.4crnL') is relatively weak.359
44.2.8.1 .I
Spin-crossover iron(lli)-Schifl base complexes
Several examples of high-spin ( S = 5)S low-spin ( S = +) systems have already been encountered in the section dealing with dithiocarbarnato and related ligands. Iron(II1) complexes of the tridentate and hexadentate Schiff base ligands of types (84) and (86) constitute new families providing many examples of spin c r o ~ s o v e rThe . ~ bis(tridentate) ~~~ and mono(hexadentate) ligand complexes both contain the metal ion in a distorted octahedral geometry, FeN,O, core, in which the two terminal oxygen atoms occupy cis positions and the remaining four nitrogens (two cis amines and two trans imines) complete the coordination sphere structures (95) and (96). These complexes may be high-spin or low-spin depending on the nature and position of substituents, counterions, lattice solvent and even geometrical isomerism. In addition, the position of the spin equilbrium and the nature of the transition is significantly influenced by pressure and often by grinding366of the solid. Structures of several complexes of this class, in both spin states, are now a ~ a i l a b l e . ~It~ 'is- ~of~ ~ particular interest to examine the effects of the spin change on bond lengths and other structural parameters. It has been observed, for example, that the average Fe-ligand bond length is longer by about 0.12 A in the high-spin complex hr = 5.75 BM at room temperature) [Fe(X-salmeen),]PF, (X = 5-OMe, salmeen = N-methylethylenediaminesalicylaldiminato anion, i.e. R = Me in structure 84) than in the predominantly low-spin complexes where X = 3-OMe (peK = 3.20BM) or X = 5NOz = 2.58 BM). However, the bond length difference is not uniform, the Fe-N bonds varying much more (- 0. I6 A) than the Fe-0 bonds ( 0.02 A).363Closely similar effects are also observed for complexes of the hexadentate ligands (86),361 The ,T $6A.spinequilibria are dependent on the nature of ring substituent X and, for solids, on the nature of the counterion. NO, as substituent favours the low-spin form while OMe as substituent favours the high-spin form relative to the parent unsubstituted complex. In solution the position of the spin equilibrium is solvent dependent. The effect of solvent is interpreted largely as arising from a specific hydrogen-bonding interaction of the solvent with the N-H groups of the trien backbone.360The [Fe(X-sal), trim]+ complexes exhbit a reversible one-electron electrochemical reduction. From the temperature dependence of the half-wave potentials for the reduction it has been possible to determine the electron-transfer entropy change (AS,,) and to compare with this the entropy change (AS,) associated with the spin equilibrium. It was found that while ASscis almost constant for different systems, ASet shows a considerable dependence on the high- and low-spin isomer populations, becoming larger as the low-spin fraction increase^.^^ The question as to whether spin crossover occurs before (equation 19) or after (equation 20) electron transfer was approached by measurement of heterogeneous electron transfer rates. Results obtained strongly point to an overall electron transfer mechanism that involves spin conversion of Fe"', prior to electron transfer, Le. equation (19) and not equation (20).365
-
I Zron(ZIZ) and Higher States
253
Both ultrasonic and laser Raman temperature jump techniques have been applied to the measurement of the spin state interconversions (equation 21) in s o l ~ t i o n . " ~On - ~the ~ ~basis of measurements on a range of complexes it was concluded that intersystem crossing rates are faster in solution than in the solid state, as judged from Mossbauer spectra, and faster for the bis complexes of the tridentate ligands than for those of the hexadentate ligands. Data pertaining to some differently substituted [Few-salmeen),]+ complexes are in Table 5.367 'T
4 e k-I
(21)
6A
Table 5 Equilibrium Constants (&I, Relaxation Times (T) and Intersystem Crossing Rate Constants (kl, k-, in Equation 21) For Some [Fe(X-salmeen)] Complexes in Acetone/Methanol Solution" +
~~
H 3-OMe 4-OMe SOMe SNO, 5-NOZ
1.31 (20) 2.79 (25) 1.67 (19) 0.72 (- 7) 0.34 (21) 0.18 (22)
9 8 1I 9 8 7
7.1 x 9.2 x 5.7 x 4.6 x 3.2 x 2.2 x
10' io7 107 107 107 io7
5.4 x 3.3 x 3.4 x 6.4 x 9.3 x 1.2 x
io7
lo7 107
IO7 IO1 108
"Data from ref. 367. 'Kcq = [hsJ/ps]at temperature T in "C in parenthesis.
A limited number of Fe"' complexes with sulfur-containing Schiff base ligands derived from thiosalicylaldehyde have been prepared. Among these is the complex [{Fe(tsalen)),O] spy obtained from reactions of the Fe" complex Fe(tsa1en) with O2(tsalen is the sulfur analogue of the tetradentate ligand salen). Like the salen complex the sulfur analogue also contains a p-oxo linkage and a square pyramidaI arrangement for each Fe(tsa1en)O unit. The Fe-0-Fe bond angle is 159", Le. somewhat larger than in corresponding salen complexes but within the range of bond angles found in Fe'''-O-Fe"' complexes generally. A point of difference between [{Fe(salen)},O] and [(Fe(tsalen)},O] is that while the metal ions in the 'oxygen' ligand complex are in the S = state and strongly coupled, in the 'sulfur' complex the Fe"' ions are S = and weakly coupled. Low-spin Fe"' also occurs in the bis complexes of the tridentate ligand (tsaleten), the sulfur analogues of ligand type
+
+
(MI 368-370 f
44.2.8.2
Complexes of Aminocarboxylates, Amino Acids and Related Ligands
The palydentate anions of the polyamine-polycarboxylicacids are well known to form stable high coordination number complexes with a wide range of metal ions. Edta is, of course;the best known. In the complexes Li[Fe(dta)(H,O)] 2H,O and Rb[Fe(edta)(H,O)] H,O the hexadentate edta tetraanion and one H,O molecule provide the seven donor atoms for a pentagonal bipyramidal coordination shell for each Fe"' ion.371In [Fe(edtaH)(H, O)] the coordination number of the metal ion is six since one carboxylic acid group of the ligand is protonated and u n ~ o o r d i n a t e d .The ~~~ complex Ca[Fe(DCTA)(H, 0)12 contains seven-coordinate Fe"' (DCTA = trans- 1,2-diaminocyclohe~ane-N,N'-tetraacetate).~~~ The coordination numbers of the central Fe"' atom in the chelates of diethylenetriaminepentaacetic acid H,DTPA and several of its salts are variable, six in the complex of the parent acid, seven in the mono [NH,]' salt and eight in the didkali metal The dimeric complex [Fe,(edta),Ol4- is formed by base hydrolysis of the mononuclear comp l e x e ~The . ~ ~X-ray ~ structure of the related complex [enH,][Fe,(Hedta),O] -6H,O has been determined (Hedta = N-hydroxyethylenediaminetriacetate). Each Hedta ligand is pentadentate and the bridge.376As found for other two Fe(Hedta) moieties are linked by a near linear (165") Fe-0-Fe p-oxo bridged di-Fe"' complexes already mentioned, these complexes are also antiferromagnetically coupled with J z - 95 cm-' . The complexes also exhibit a series of ligand field bands of enhanced intensity in the visible region and a number of higher energy absorptions which have been assigned to simultaneous pair eIectronic excitations arising from coupled LF tran~itions.3~~
-
44.2.8.2.1
Hemerythrin and model compounds
An important group of naturally occurring amino acids complexes of iron are the hemerythrins -the oxygen-carrying proteins in a variety of marine worms. Most hemerythrins have molecular
254
Iron
weights of about 108000 and consist of eight subunits each of which contains a binuclear iron active site. As a result of the application of modern physical methods such as EXAFS, resonance Raman and Mossbauer spectroscopy, as well as crystallographic studies, together with conventional chemical and biochemical studies, a very useful picture of the nature of the active site in hemerythrin has now emerged.37' The oxidation states present in the different forms of hemerythrin are indicated in Figure 9. The colourless deoxy form reversibly binds 0, in the ratio of 2Fe:0, to give the burgundy coloured oxy form. In addition, an oxidized met form containing only Fe"' but no 0,may be obtained as well as a mixed valence semi-met form [Fe"', Fe"]. Assignment of oxidation states originally based on chemical reactions378has been amply confirmed by s p e c t r o s c o p i measurements. ~ ~ ~ ~ ~ ~ ~ Moreover, the presence of a p-oxo bridge between the metal centres in the oxy and met forms of the protein became apparent from magnetic studies and from Mossabuer,"' optical and vibrational spectra. The exchange coupling constants J of - 134 cm-' for met hemerythrin and of - 77 cm I for oxyhemerythrin383are not too different from the range of values of -90 to - 130cm-' found in synthetic compounds containing p-oxo linkages. These structural conclusions have been confirmed by various crystallographic s t ~ d i e s ~ ~of~ the . ~ *met ' and azidomet forms, the latest of these being carried out at 2.0A resolution. In the met form one of the two iron atoms is octahedrally coordinated while the second iron atom is in a highly distorted trigonal bipyramidal environment. In addition to the p-oxo bridge the metal centres are also linked by the carboxylate groups of glutamate and aspartate groups. Remaining coordination sites are taken by nitrogen atoms from histidine residues of the amino acid side chains (Figure lo). Addition of azide converts the five-coordinate centre in met hemerythrin to octahedral with accompanying minor changes in bond lengths and angles and conformational changes in the protein. Although there is some disagreement between X-ray and EXAFS studies in relation to bond angles and distances, the main geometrical features of the oxy and met forms of the protein appear well established. [Fr"', Fc"' ] O2 oxy
semi-mct
Figure 9 Interconversion of different forms of hemerythrin His 73
Figure 10 The coordination of the two Fe"' ions in met hemerythrin
A (p-oxo)bis(p-carboxylato)diiron(TII) core in a model for met hemerythrin has been prepared by spontaneous 'self-assembly' in aqueous solutions of iron(II1) perchlorate, sodium acetate and sodium tri- l-pyra~olylborate.~~~ The resulting binuclear complex was shown to be very similar in structure and properties to that of met hemerythrin. Thus, the two Fe"' atoms are linked via B p o x 0
Iron(IIi) and Higher States
255
bridge (Fe-0-Fe = 123.5') and two p-acetato bridges, the remaining coordination sites being filled by the tridentate pyrazolylborate ligands. Antiferromagnetic behaviour with J = - 122cm-' compared well with that (J = - 134cm-I) for metaquo hemerythrin.386 EXAFS spectra3*'of the deoxy[Fe", Fe"] form of hemerythrin suggest the absence of the pox0 bridge, in agreement with the lack of antiferromagnetic coupling. Analysis of the data suggests that on conversion of oxy- to deoxy-hemerythrin (equation 22) the bound 0, is replaced by OH- and the short bond to the bridging oxygen atom is broken, leaving it bound to one iron. Thus, one iron becomes five-coordinate while the other remains six-coordinate. . [Fe"'--O-Fell'-O,] + H,O e [FeII-OH, Fe"-OH] -t 0, (22) Of central interest in the chemistry of hemerythrin is the mode of interaction of 0,with the binuclear Fe"' active site. Several structural models (97-101 in Figure 11) have been considered, all incorporating a peroxido diiron(II1) formalism, as required by the resonance Raman identification isotopic The use of v ( 0 - 0 ) at 844cm-', and its displacement to 796 m-' on '*02 of unsymmetrically labelled dioxygen,1601s0,has been of service in discriminating between the symmetrically bound 0, adducts (97), (98) and (101) and the asymmetric models (9) and (100). For the former a single symmetric 0-0 stretch would be expected whereas v ( 0 - 0 ) should be a doublet if either of the unsymmetrical structures (99) or (100) is the correct one. The RR results In fact, structure (100) in obtained strongly favour the unsymmetrical structure (99) or which the peroxo group is terminally bound to one Fe"' centre, taking the place of the azide ligand in metazido hemerythrin, is the preferred structural candidate on present knowledge.
P
0
I
(99)
(101)
(100)
Figure 11 Idealized structural representations for the Fe,.O, active site in hemerythrin
44.2.9 MACROCYCLIC LIGANDS Complexes of large ring compounds containing a set of suitably positioned donor atomsmacrocyclic ligands -have engaged the attention of chemists for a long time but particularly over the last two decades. A major stimulus for the study of macrocyclic compounds has been the desire to mimic and probe the function of certain metal ions in natural systems where they are known to be ligated to macrocyclic molecules. These include, of course, the well-known porphyrin and corrin rings containing a planar array of four nitrogen atoms. Iron(II1) complexes of porphyrins will be considered in a later section. We first consider some iron(II1) complexes of synthetic macrocylic ligands.
44.2.9.1
Synthetic Macrocyclic Ligands
Nearly all iron complexes of synthetic macrocyclic ligands contain nitrogen either as the only ligand atom or as the major donor present. Moreover, most macrocyclic ligands are tetradentate usually presenting a roughly planar 'N4' donor set to the centrally complexed metal ion. The comprehensive review by Melson3'' of the coordination chemistry of macrocyclic compounds should be consulted for work published up until 1978. Iron(II1) complexes of tetradentate 'N,' macrocyclic ligands may be five-coordinate (square pyramidal) or six-coordinate (approximately octahedral), and high-spin (S = ;), intermediate spin
256
Iron
( S = 5) or low-spin ( S = +),depending on the nature of the macrocyclic ligand, i.e. its size and degree of unsaturation, and on the number of the axial ligands. Examples of iron(II1) complexes displaying all three ground states within the same family of macrocycles are found among complexes of the 14-, 15- and 16-membered ligands (102), (103) and (104).39'The five-coordinate complexes [Fe"'N,X] have the intermediate ( S = 2) spin ground state when the macrocycle is 14- or 15-membered and where X = halide or carboxylate, whereas analogous compounds of the 16-membered macrocycle (104) are high spin ( S = 5). For the case of X = thiolate, all three spin states were found: viz. [14]-SPh, S = 3;[15,16]-SPh, S = +;[16]-SCH2Ph, S = 5. The results demonstrate the effects of ring size and axial ligand on spin state (Table 6). The spin state of the six-coordinate complexes [Fe(cyclam)X,]+ [cyclam = 1,4,8,1l-tetraazacyclotetradecane (lOS)] is governed by the geometrical configuration of the 14-membered macrocycle, being high-spin for the cis complexes and low-spin for the trans complexes. However, rrums-[Fe(cyclam)Br,][C10,] with pefl = 3.90BM at 295 K may exist in a high-spin+ low-spin eq~ilibrium.~'~
n
(102) R = R' = (CH,), (103) R = (CH,),, R = (CH,), (104) R = R' = (CH,),
( 105)
Magnetic and Mossbauer Dataa for Fe"' Complexes [FeN,SR] of Tetradentate Macrocyclic Ligands (102)4104)
Table 6
Isomerb Shift Complex
Spin State
Kfr(BM)
d(IMIl-1)
Quadrupole Splitting dE, (mm s-I) 3.60 1.93 0.71
a
From ref 391
Relative to iron metd
Iron has been found to be very effective in promoting the oxidative dehydrogenation of secondary amine groups bound to the metal as part of a macrocyclic ligand generating new macrocyclic complexes having higher degrees of u n ~ a t u r a t i o n One . ~ ~ example ~ ~ ~ ~ is the preparation of, successively, the triimine complex (108) and the tetraimine complex (109) from the diimine complex (106) (see Scheme 4). Atmospheric oxygen readily performs the oxidation under the influence of Fe'", while stronger oxidizing agents are generally required in the case of nickel or copper complexes. Although reactant and product complexes contain the metal ion in the + 2 oxidation state there seems little doubt that a first step in the oxidative dehydrogenation is an oxidation of Fe" to Fe"', this being followed by an electron transfer, with accompanying deprotonation, from the coordinated secondary amine group to Fell'. Oxidation of the radical so formed then generates the new imine centre (Scheme 5).393 The intermediate iron(II1) complex [Fe([14]4,11-dieneN,)(MeCN),][ClO,], (107) has been isolated. This, and other derivatives containing coordinated Clor NCS- in place of MeCN, are low-spin (pcR = 2.15-2.30 BM).3M MeCN
Me
Me
Mr MrCN
MeCN
MeCN
(106)
(107)
( 1 OS)
Scheme 4
( 1 09)
Proposed pathway for the generation of triimine and tetraimine complexes vin oxidative dehydrogenation
Iroio(III) and Higher States
257
Scheme 5 Proposed radical mechanism for the iron-promotcd oxidative dehydrogenation of a coordinated secondary amine
An interesting Fell'-mediated macrocyclic ligand oxygenution has been observed.'% When the Fe"' complex (110; X = MeCO,) in DMF was heated for 6 h at 80°C black needles were slowly deposited. X-Ray analysis revealed that the product is the p-oxo dimer (111) in which the -N-CH2-CH2-Nbridge of the reactant macrocyclic has been oxidized to the oxamido ligand. It was suggested that this oxidation also proceeds via a radical mechanism in which the initial step is elimination of acetic acid and generation of an Fe" radical species. Ligand oxygenation (112 + 113) of Fe" complexes of bis(p-diimine) macrocycles by molecular oxygen has also been observed. Again, an Fe"'-mediated radical mechanism has been suggested for this oxidati~n."~
(112)
(1 13)
Many of the synthetic macrocyclic ligands have been synthesized by metal ion template methods. One of the best examples is provided by the self-condensation reactions of a-aminobenzaldehyde."o~R98 In the absence of metal ions, o-aminobenzaldehyde gives a variety of polycyclic structures. In the presence of certain metal ions complexes of the tridentate macrocycle (114) and/or of the tetradentate macrocycle (TAAB) (I 15) may be obtained. Among those in the latter category are the dinuclear Fe"' complexes [Fe2(TAAB),0]X,-4H,0 (X = C10, or NO,) (116) and the mononuclear complexes [Fe(TAAB)F]X2-2H,0. Magnetic and other properties have established p-oxo structures for the binuclear complexes, with the expected strong antiferromagnetic coupling (J = - 65 to - 111 cm-I). The p-oxo bridge is cleaved, with difficulty, by acid fluoride to generate the low-spin mononuclear species. An interesting reaction of the p-oxo dimers is that with methoxide ion giving complexes (117) of the new dianionic ligand formed by nucleophilic addition of the OMe- to two imine bonds.39R An early demonstration of the template effect in the synthesis of macrocyclic ligands was the metal-ion-controlled cyclic condensation of 2,6-diacetylpyridine with 3,3'-diaminodipropylamine to give complexes of the 1Cmembered W4' ring (lHQ usually abbreviated (Me,pyo[ 14ltriene N4).39y Effective template ions include Ni", Co" and Cu" . Well-characterized iron complexes have not been
Iron
258
--
4Me0
0
4H'
OMe
OMe
OM c
OMe
(117)
(1 16)
obtained. However, the two C=N bonds of (118)may be hydrogenated to afford the macrocycle (119), Mezpyo[14]ene N4,which can exist in meso and racemic forms. Fe"' derivatives of the meso ligand have been prepared. These have the formula [Fe(Me,pyo[14]eneN4)Xz]Y(X = C1 or Br; Y = C10, or BF,) and, on the evidence of the magnetic properties and Mossbauer spectra, are assigned low-spin six-coordinate structures. They show excellent catalytic activity for the decomposition of H 2 0 2in aqueous acid solution.400
(118)
(119)
Extension of the metal ion template method to condensations between 2,6-diacetylpyridine with other linear polyamines has proven successful in many cases. Thus, for example, Fe"' salts are effective temilatks for the synihesis of the pentadentate macrocycles (120),(121)and (123)denoted (222-N5),(232-N5)and (222-N34), respectively, but not for the (323-N5)macrocycle (122).a'403The template action of the metal ion was evidenced by the fact that in its absence the products of reaction were oils or gums, indicative of an oligomeric or polymeric constitution. Such materials showed IR in bands at 1700cm-' and/or 3300cm-' indicating the presence of unreacted keto and/or primary amine groups. In contrast, in the presence of an equimolar quantity of Fe"' salt, crystalline high-spin complexes of stoichiometry [Fe(mac)X,]Y (X = e.g. CI, Br, I or NCS; Y = C104, NO,, BF,, BPh, or PF,) were obtained from acid solutions. In basic media antiferromagnetically coupled
-
N
I
Iron(II1) and Higher States
259
dinuclear complexes containing p-oxo bridges are obtained. In both series of complexes, mononuclear or binuclear, the pentadentate macrocyclic ligand maintains an approximately planar array of donor atoms around the central metal ion. Two monodentate ligands above and below this pentagonal plane complete the pentagonal bipyramidal geometry of the complexes. Single crystal [Fe(232-N5)(NCS),][C10,]402 and structure determinations of [Fe(222-N,)(NCS),][C104]402@""''' [Fe,(222-N,),(C10,),0'J[C10,],~H,0404 have been carried out. Cyclic voltammetry has shown that the mononuclear Fe"' complexes undergo two one-electron processes in acetonitrile solution. The more positive (reversible) process can be ascribed to the Fe"'/Fe" couple, and the more negative reduction (reversible at fast scan speeds) corresponds to the formation of a ligand radical anion, Le. addition of an electron to the lowest antibonding orbital of the conjugated pyridyl-diimino system. The Fe"'/Fe" couple occurs in the range i-0.3 to -0.4V vs. SCE depending somewhat on the nature of the axial ligands as well as on the particular macrocyclic ligand. The values of the couples suggested that the Fe" complexes should have reasonable stability to oxidation. In agreement with this prediction crystalline Fe" complexes of all three macrocycles could be prepared by dithionite reducti0n.4'~Structural studies and various physical properties have established the retention of the basic pentagonal bipyramidal structure on reduction. Despite the differences in size of Fe3+ and Fez+ there is little change in the average Fe-N(macrocyc1e) bond length on change of oxidation state. What does occur, however, is a movement of the metal closer to the unsaturated (trimethine) segment of the macrocycle on reduction.406This is attributed to a measure of metal-to-macrocycle back coordination in the lower oxidation state. The apparent instability of the Fe"' complex of the 17-membered 'N5' macrocycle (122) is presumably a consequence of a mismatch in size between metal ion and macrocycle cavity in this case. Mossbauer spectra of several of the high-spin seven-coordinate complexes [Fe(222-N,)X,]C104 have been measured over a range of temperatures. The temperature dependent spectra were interpreted in terms of paramagnetic relaxation effects; a correlation between the electric field gradient and the zero-field splitting was demonstrated and shown to change sign as the field strength of the axial ligand X was varied?" The complexes are photochemically a ~ t i v e ~ ' ~ to. ~a 'degree ~ dependent on the nature of the axial ligands, leading to reduction of the metal centre and oxidation of the axially coordinated ligand.409 Reaction of the Fe" complexes of 222-N5 and 232-N5 with O2 leads to formation of the p-oxo dimers. It is conceivabIe that an intermediate in the formation of the p-oxo dimer is a p-peroxo-diiron(II1) species as proposed for reactions of other Fe" complexes with 0,. Evidence for the stabilization of the p-peroxo species in aqueous solution has been adduced by Kimura et ~1.4'' for the case of the Fe" complex of the related saturated macrocycle (124). This yellow Fe" complex absorbs O,, non-reversibly, in aqueous solution in the ratio of one 0, per two Fe, forming a red purple solution stable for up to 3 h at ambient temperature?''
(120) m = n = 2 (121) m = 2, n = 3 (122) m = 3, n = 2
44.2.9.2
( 1 23)
(1 24)
Porphyrins and Related Ligands
Iron porphyrins serve as the prosthetic group )r the biologically important LJSS of proteins known as the hemoproteins. These include the oxygen transport and storage proteins, hemoglobin and myoglobin, the electron carrier cytochromes, as well as various oxidase and oxygenase enzymes. As a result, the hemoproteins and synthetic iron porphyrin complexes have received much attention. A vast literature has now accumulated, particularly over the past few years, and it is therefore quite impossible to do other than indicate the principal classes of iron(II1) porphyrin complex known to occur, together with a brief description of the structures and properties of some representative examples. Many comprehensive reviews are available.4"4'6
Iron
260
The parent ligand porphine has the planar conjugated tetrapyrrole ring structure (125) containing a 16-membered inner larger ring. In the naturally occurring metalloporphyrins all eight pyrrole carbon atoms are completely substituted. Commonly used synthetic porphyrins are octaethylporphyrin (H,OEP) (126) and meso-tetraphenylporphyrin (H,TPP) (127).
(125)
(127)
(126)
In the metal complexes the two pyrrole hydrogens are usually replaced by the metal ion so that porphyrin is coordinating as the dianion. Because of the conjugated nature of the dianionic porphyrin ring the hole size is fairly well fixed (some variability via ring puckering is permitted) at about 2.01 & 0.08 A. Thus, while some metal ions may sit in the hole in the same plane as the ring, others, being too large, may be forced to sit out-of-plane. Usually, though not always, the metal ion will be bound to all four nitrogen donors ofthe porphyrin ring. The metal ion can then acquire fiveor six-coordination by binding one or two monodentate ligands in axial positions. Indeed, higher coordination numbers are found in several cases but for iron porphyrin complexes coordination numbers are restricted to four, five and six, and to five and six only for the case of Fe'" complexes. The properties of a particular iron porphyrin are controlled in several different ways-by variation in the nature and position of porphyrin ring substituents, the number and nature of axial ligands, and the spin state and oxidation state of the metal ion. Not least, the protein itself in the natura1 systems plays a major role in the tailoring of the properties of the prosthetic group for the particular biological function it is to perform. Most six-coordinate Fe"'-porphinato complexes, [Fe(por)L,]' have low-spin ( S = +) configurationS.413.417420In such cases, the equatorial Fe-N distances are close to 1.998, and Fe-N(axia1) distances usually less than 2.0 A. Displacements of the Fe atom out of the porphyrin '&' plane are usually small. However, a number of Complexes in which the axially ligating group is relatively weak field display high-spin ( S = $) S low-spin ( S = +) spin equilibria at accessible temperature^^^'^^^ or are actually fully high-spin as in the case of [Fe(TPP)F,]- and [Fe(TPP)(EtOH)?]".423*424 The distinctive differences in magnetic properties and in Mossbauer and ESR spectra of the two classes are clearly reflected in the structural parameters as may be seen from an inspection of the data in Table 7. Thus, for the low-spin complexes the average Fell'-N(porphyrin) distance in the low-spin complexes (vacant dx2-y2 orbital) is < 2.00 A whereas in the high-spin complexes (occupied dx2-,2 orbital) this bond length is an excess of 2.00A. Table 7 Structural Data for Some Six-coordinate [Fe"'(porphyrin)L,] Complexes Porphyrin
Spin state
L
Average Fe- N (por) (A)
PP-IX
1-Meim
1 2
1.990
TPP
im
f
1.989
TPP
PY
+
1.990
N;
CN-
1
TPP TPP TPP OEP
THTa PMSb 4-Meim 3-Clpy
1
TPP TPP
EtOH F-
TPP
2
2.000
2
1.985
1 2
t
4 (98 K)'
t H +(293 K)d 5 2
i
1.982 1.998 1.994 2.014 2.03 2.064
Fe-L
(A)
1.966 1.988 1.957 I .991 2.085 1.926 1.975 2.357 2.341 1.928, 1.958 2.031 2.194 2.14 1.966
Ref.
417
413
41 9
420 420 428 42 1 423 424
aTeetrabydrothiophene.hPhentamethylenesulfde. Predominantly low-spin at 98K. About 55% in high-spin form at 293 K.
Five-coordinate (square pyramidal) Fe"' containing a single axial ligand are generally high-spin ( S = $) with the metal atom displaced from the porphyrin 'N4' plane in the direction of the apical Structural data for some representative examples are in Table 8.
26 1
Iron(I1I) and Higher States Table 8 Structural Data for Some Five-coordinate [Fe"' (porphyrin)X] Complexes
Porphyrin
PP- IX-DME TPP PP-IX-DME TPP" TPPb 'In [Fe,(TPP),O].
X
Spin-State
OMe
t
Average Fe-Nipor)
SC,H,-p-NO, 0 N
Fe-X(&
2.073 2.049 2.064 2.087 1.991
4
ci
(A)
-2
5 2
I5
1.842 2.192 2.324 1.763 1.661
,,
Displacement of Fe out of 'N4 'plane
Ref
0.49 0.38 0.448 0.50 0.32
425 426 421 439 440
[Fe,(TFP),N].
However, more recently, a number of intermediate spin ( S = ): iron(II1) complexes have beenprepared.42M34Stabilization of this spin state can be expected, in either square pyramidal or tetragonal symmetry, whenever the axial ligand field is very weak compared to the equatorial ligand field, such that dx2-,+ lies well above dz2in energy and remains unoccupied. Examples of S = 2 complexes are [Fe(OEP)(C104)]429[Fe(OEP)(EtOH)~][C104],429~434[Fe(TPP)(Cl0~)]43o and [Fe(TPP)(H,0),][C104]430in which the axial ligands are the weakly bonding C10, ion or EtOH and H,O molecules. SeveraI of the complexes of thic class have effective magnetic moments at room temperature in the range 4.5-5.5 BM, significantly larger than expected for an S = 2 state. Since Mossbauer spectra and other properties exclude the existence of a thermal equilibrium between magnetically distinct spin states, it has been postulated435that there is a quantum admixture of S = 3 and S = 3 states. Quantum mechanical admixing, presumed to occur via spin-orbit coupling when the energy separation between the pure states is small, may be recognized also on the basis of unusual g = 4ESR signals as well as by a drop in magnetic moment at low temperatures. A common product of the autoxidation of iron(T1) porphyrins is the p o x 0 dimer436*439 (equation 23), a reaction of importance in relation to 0, transport and utilization in nature. These p-oxo dimers contain two high-spin ( S = $) Fe"' ions which are antiferromagnetically coupled through the oxo bridge,437.438 as noted above for several other classes of p-oxo Fe"' complex. An alternative route to the p-oxo dimers is reaction of mononuclear [Fe(por)X] (X = e.g. C1) with OH-. The geometry439 of the coordination sphere in the five-coordinate p-oxo dimers approximates very closely to that found in the uncoupled five-coordinate monomers (see Table 8). In [Fe,(TPP),Oj the bridge is near = 174.5'. It is interesting to compare the p-oxo complex [Fe,(TPP),0]439 linear with Fe-0-Fe with the p-nitrido complex [Fe,(TPP),NJ.440Structural parameters, specifically the short FeN(por) distance (1.99 and the relatively small out-of-plane displacement (0.32 A) of the Fe"' atom from the porphyrin ring are suggestive of a low-spin ( S = 3) ground state (compare with data in Table 8). Similar conclusions were reached on the basis of X-ray photoelectron studies.@' On the other hand Mossbauer spectraM2appear to favour a high-spin description with a formal oxidation state of Fe'", Fe'" or rapidly exchanging Fe"', Fe", while a resonance Raman studya3 likewise favours the high-spin Fe"' formulation.
w)
4Fe"
+
O2
+
2Fe"1-O-Fe111
(23)
The mechanism of the autoxidation of Fe" porphyrins has been studied in detail by visible and H NMR spectroscopy.444This is an important reaction and its understanding, and circumvention, have formed the basis of the strategies for the design of synthetic iron(1I) complexes capable of the reversible binding of 02.As mentioned above the usual ultimate product of Fe" porphyrin autoxidation is the p-oxo dimer. La Mar and BaIch and co-workersM have shown that addition of 0, to toluene solutions of Fe"P (128) (P = a porphyrin dianion) leads to the formation of an intermediate (130) which may be detected at - 80°C by visible and ' H NMR spectra. This intermediate has been formulated as a peroxo-bridged diiron(II1) species on the basis of its antiferromagnetism in solution and the stoichiometry of its formation. Upon warming, this p-peroxo complex decomposes quantitatively to the g o x o species (132) together with 02.This reaction has been shown to be first order in (130). The overall mechanism for the autoxidation is summarized in equations (24)-(30). The formation of the Fe'" species (131) is consistent with other information on this oxidation state of iron (vide infra). Evidence for a different kind of Fe"' porphyrin-peroxo complex in solution has been presented. This mononuclear species formulated as [Fe"'(por)(O,)]- (por = TPP or OEP) appears to be produced via a redox reaction between Fe" porphyrin and 0;. Visible, IR, resonance Raman and ESR spectra support a formulation of the complexes in which the peroxide is bidentately coordinated to Fe"' in the 'sideways-on' (Griffiths) fashion (structure 133).445 The low temperature spectroscopic observation of the Fe'" ferry1 complex (134) [P = TPP, B = py, pip or N-Me-im] formed via reaction (31) has been followed by the demonstration by the same group of its role in oxygen atom transfer reactions. Thus, it was foundM6that PFe" catalyzes 1
262
Iron
(133)
the oxidation of triphenylphosphine in toluene to triphenylphosphine oxide. It is consideredu6 that PFe"0 is the active oxidant and that the catalytic cycle in Scheme 6 applies. Fe"' species such as [FePClJ,[FePB,]+ and [PFe-0-FeP] were inactive, though the p-oxo dimer of phthalocyanatoiron(II1) PcFe-0-FePc] can apparently perform the same oxidation in the presence of pyridine under mild condition^.^' PFe"'-O-O-Fe'L'P (130)
3Ph3PO
+ 2B
-+
2BPFe'"
(3 1)
(134)
'Ph3P
Scheme 6 Proposed catalytic cyde for the oxidation of triphenylphospine via Fe" porphyrin complexes
44.2.9.2.1
The heme proteins
In the heme proteins the prosthetic group consists of iron coordinated to a substituted porphyrin. T h e versatility of the heme proteins is apparent in the range of different functions they perform, mediate or catalyze. These functions are associated with, for example, dioxygen transport (hemog-
Iroa(Irij and Higher States
263
lobin), dioxygen storage (myoglobin), electron transfer (cytochromes), catalysis of oxygenation reactions such as hydroxylation (cytochrome F4%),and of oxidation involving hydrogen peroxide (peroxidases and catalases). The adaptation of the heme protein to a particular function is controlled by a variety of structural and other factors, often in intriguing and subtle ways. These include alteration of the substitution pattern of the porphyrin ring (affecting e.g. the redox potential of the metal centre), the nature of the axial ligation at the iron centre (affecting spin state, reactivity and redox potential), and the conformational/structural/interactiverelationships of the prosthetic group with the amino acid chain of the protein. In all of these metalloproteins the redox activity of the coordinated metal ion is central to the biological function, all involving changes in oxidation state during an operational cycle. Usually the cycling in oxidation state is between i2 and 3 but higher oxidation states may also be involved. The dioxygen-carrying proteins, hemoglobin and myoglobin, as well as the many synthetic model systems that have been developed recently,48 will not be considered here in this chapter since their chemistry is mainly that of iron in the 2 oxidation state. The present chapter is therefore restricted to a brief description of the structure and reactivity of some cytochrome and peroxidase proteins.
+
+
44.2.9.2.2
Electron transport heme proteins
The cytochromes are the electron carrier heme proteins occurring in the mitochondrial respiratory chain.449There are five cytochromes linking coenzymes Q (ubiquinone) and O2 in this electron transport chain (Scheme 7). Cytochromes are also involved in energy transfer in photosynthesis. The iron atom in cytochromes cycles between the Fe" and Fe"' states, Le. they are one-electron carriers, in contrast to CoQ and the NADH flavins they act upon which are two-electron carriers. Thus, one molecule of reduced CoQ transfer its two high potential electrons to two molecules of cytochrome b, the next member of the electron transport chain. CoQ Scheme 7
-, cyt
b
-, cyt
c, -, cyt c
+
cyt(a
+ a,)
-+
0,
The electron transport chain linking coenzyme Q and 0,
The five cytochromes between CoQ and 0, (Scheme 7) increase incrementally in redox potential, spanning an overall potential of about one volt. The various cytochromes differ in structure and properties. In cyt b, cyt cI and cyt c the prosthetic group is heme itself, iron-protoporphyrin-IX, as in hemoglobin and myoglobin. Cytochrome c is structurally well defined in both the Fe" and Fe'" It consists of a polypeptide chain of 104 amino acid residues and a covalently attached heme group. One axial site of the iron atom is occupied by the nitrogen from a histidine residue and the other axial site by a sulfur atom from a methionine residue of the protein chain (Figure 12). Thus, unlike myoglobin and hemoglobin, cytochrome c functions without change in axial ligation. Indeed, as reinforced by studies on model compounds of low-spin Fe" and Fe"' porphyrins containing axial thioether ligands, there is little alteration in Fe-S bond length as change of oxidation state of the metal?"
,
Met 80 /
t
His 18
Figure 12 The heme group in cytochrome c showing the axial ligation to methionine and histidine residues of the protein chain
An interesting feature of cytochrome c, which is present in all organisms that contain a mitochondrial respiratory chain, is that it has remained virtually unchanged since it evolved more than 1.5 x lo9 years ago. Reduced cytochrome c passes its electron on to the next member of the respiratory chain, cytochrome (a f a,). These two cytochromes, together known as cytochrome c oxidase, comprise two iron-porphyrin prosthetic groups and two copper centres, Electrons are transferred to cytochrome a and then to a3which contains Cu which cycles between the -t- 1 and + 2 oxidation states. The net function performed by this terminal member of the electron transport chain is the four-elec-
Iron
264
tron reduction of O2to H 2 0 with release of energy which is stored in the ADP-ATP cycle, and the reoxidation of reduced cyt c to the oxidized form (equation 32).452"53Various spectroscopic (ESR,4% MCD455) and magnetic studies4" have shown that the enzyme contains one isolated heme unit (cytochrome a) which is low-spin in both the oxidized and reduced forms and one isolated copper centre which is ESR active. The second heme unit (cytochrome a3),believed to be the O2binding site, is associated with the second copper atom which is EPR undetectable. Variable temperature magnetic susceptibility measurements have shown that the iron atom of the fully oxidized (resting) form of the enzyme is in the S = spin state and strongly antiferromagnetically coupled to the EPR The data indicate a coupling constant - J of non-detectable copper atom in the S = $ spin 2 200 cm-', leading to a resultant S = 2 ground state, for the spin-coupled [Fe-B-Cu] dimer. One axial position of the coordination sphere of the iron atom in cyt a, is believed to be an imidazole while the other is occupied by a bridging ligand B responsible for the superexchange interaction with the Cu" ion (structure 135). The nature of the bridging ligand B remains uncertain. Several candidates have been proposed on the basis of the properties of various model binuclear complexes containing coupled Fe"'-Cu" pairs. These include the imidazolate ion (from a histidine residue of the an oxo (or hydroxo) bridge (derived from O2i t ~ e l f ) ~and ' ~ ,a~thiolate group (from a cystein residue)?
+
4cyt c'l
44.2.9.2.3
+
O2
+
4H+
+
4cyt c"'
+
2H20
(32)
Cytochrome Pds0,peroxidase and catalase enzymes
Cytochrome P450,named after the wavelength (in nm) of the absorption maximum of the carbon monoxide complex of the reduced form, is not an electron transport protein but actually a monoxygenase enzyme which catalyzes the incorporation of one atom of 0,into an organic substrate while the second atom is reduced to H,O. A common activity is the catalytic hydroxylation of substrates (equation 33) which include hydrocarbons and steroids. Other oxidations mediated by P450enzymes are epoxidation, oxidative group transfer involving involving a I ,Zcarbon shift with concomitant incorporation of oxygen as a carbonyl group at the C-1 position, and heteroatom The active site of P450comprises an iron protoporphyrin IX moiety in a hydrophobic cleft in the surface of the apoprotein. The metal ion is always five- or six-coordinate. One axial ligand is believed to be the thiolate anion from a cysteine residue of the polypeptide chain. The second axial position is the site for O2 binding. The mechanism of hydroxylation of substrates by cyt P450was first proposed to be an oxenoid process in which an oxene species inserted into a carbon-hydrogen bond in a manner analogous to carbenes. Subsequently, Groves and ~ o - w o r k e r sdemonstrated ~ ~ , ~ ~ ~ that alkane hydroxylation proceeded by a stepwise process involving the formation of a carbon-centred free radical via hydrogen abstraction by a perferryl (Fev=O) intermediate (equation 34). A significant discovery was that exogenous oxygen donors such as iodosylbenzene and alkyt hydroperoxides are effective oxygen sources for the enzyme in the absence of dioxygen and a reducing agent. This suggested that dioxygen is bound and reduced to metal-bound peroxide at the active site. The epoxidation of alkenes by iodosylbenzene (equation 35) was also found to be catalyzed by synthetic iron porphyrins. The mechanism466for the epoxidation using either O2 or PhIO is . summarized in Scheme 8. Peroxidase and catalase enzymes catalyze oxidations by hydrogen peroxide and the disproportionation of hydrogen peroxide into dioxygen and water (equations 36 and 37). The two classes of RH
+
[FeV=O]
O2 RH
-+
+
+ 2e- + [Fe(OH) + R.] 2H+
ROH
---*
+
H20
Fe"' -k ROH
(33) (34)
Iron(IIIj and Higher States
265
y:-
,120\
2HC
m P 1 s o
Scheme 8 A catalytic cycle for oxygen activation and transfer by cytochrome P4* (S = substrate, e.g. alkene; X iodosylbenzene)w
= e.g.
enzyme have strong similarities, not only in function but also in structure and mechanism of a ~ t i o n . These ~ ~ ~ heme , ~ ~ ~proteins have attracted much attention in recent years because, like cytochrome P450,they appear to involve novel high oxidation state species of iron as intermediates in the redox cycles and because the oxo-iron species, in synthetic compounds, may have practical value as catalysts in the functionalization of organic molecules. The best characterized enzyme of this class is horseradish peroxidase (HRP). This contains protoporphyrin IX and in the resting state the iron is in the + 3 oxidation state, probably coordinated to an imidazole nitrogen atom (from a histidine residue) in one axial position with, possibly, an exchangeable aquo group in the sixth p o ~ i t i o n .There ~ ~ ~ .is~evidence ~~ for both high- and low-spin forms of the Fe'" ion. Upon reaction with H 2 0 2or organic peroxides the iron(II1) prosthetic group undergoes a two-electron oxidation to generate a green compound HRP-I which then may undergo a one-electron reduction to give the red species HRP-11. The collective evidence of a variety of spectroscopic and other studies strongly favours a low-spin (S = 1) Fe'" formulation for both HRP-I and HRP-II, with the porphyrin ring Similar considerations forming a z-cation radical in the case of HRP-I (equations 38 and 39).47w73 probably apply to the catalase family of en~yrnes.4'~
Hz0, + SH, HI02
[PFe"']
peroxidase
-t HZ02
+
[.PFe'"=O]
€I202
+
cs&c
+
R
2H20
+
S
(36)
2H2O
+
0 2
(37)
[-PFe'v=O]
+
H,O
(38)
.R
(39)
[HRP-I]
[PFe'V=O]
+
[HRP-111
Oxidation products of Fe"' porphyrin complexes may not always involve Fe", however. Thus, magnetic, Mossbauer and structural studies on [Fe(TPP)C1][SbC16]allow assignment as an Fe"' ( S = $) complex of a radical porphyrin cation.475 Recently synthesized iron porphyrin<arbene complexes (136) may be considered as carbon analogues of the oxo-iron(1V) porphyrin (HRP-I1 and CAT-11) species.476The one-electron oxidation of [Fe(TPP)(C=CR,)] (R = aryl) by CuCl, affords the stable complex [Fe(TPP)(C=CR,)] C1 (TPP = mess-tetra-p-tolyl porphyrin and R = p-ClC6H4)which has a visible spectrum very similar to that of CAT-I. X-Ray analysis of this complex has revealed a structure (137)in which the vinylidene moiety has inserted between the iron atom and one pyrrole ring of the porphyrin and it is suggested that this may constitute a model, Le. structure (138), for the active site of CAT-I as an alternative to the Fe'" cation radical
266 44.2.10
Iron
LRON(IV), IRON(V) AND IRON(V1)
Usually, high oxidation states of metals are stabilized most effectively when in combination with the most electronegative atoms, fluorine and oxygen. Rather surprisingly, no Fe" or higher oxidation state compounds with fluorine are known. Indeed rather few stable or well-defined higher oxidation state iron compounds are known. Despite this, there can be no doubt about the importance of such compounds. We have already encountered the Fe" and FeV states in reactive intermediates in the reactions of several metalloproteins and of several synthetic porphyrin complexes. The S P and Ba2+salts of the ferrates(IV), Sr,FeO, and Ba,FeO,, may be prepared by oxidation of a mixture of hydrated iron(IT1) oxide with the alkaline earth metal oxide or hydroxide with oxygen at XOG9OO"C (equation 40).47sThese black compounds do not contain discrete [Fe0,I4 ions and are really mixed metal oxides. They are readily decomposed by mineral acids to Fe"' and 02. Vibrational spectra of some of these systems have been recorded."' Sr,[Fe(OH)&
+ Sr(OH), + to, -,
2Sr[FeO,]
+
7H20
(40)
The oxidation of the iron(I1I) complexes [Fe(DAS),X,][FeX,] (DAS = o-phenylenebisdimethylarsine, X = C1, Br) with concentrated nitric acid followed by addition of the appropriate anion led to the isolation of black [Fe(DAS),X,]Y, (Y = BF;, ClO;, The magnetic properties indicate two unpaired electrons consistent with a Iow-spin tig configuration in a tetragonal (trans halide) structure. Mossbauer quadrupole splittings are large (- 3.2mm S K ' 1 and temperature independent.," The most stable iron(1V) complexes are those containing dithiocarbamate and related ligands. Reaction of tris(dithiocarbamato)iron(lII) complexes [Fe1"(S2CNR,),] (R = Me, Et, Pr', cyclohexyl) with BF, in the presence of air yields the complexes [Fe'v(S,CNRZ)3]BF,.Salts containing other counterions such as PF, and C10; may be obtained by variations in the preparative method^.^'^,^^ I, may be used as oxidant in certain cases.483 These intensely coloured complexes have magnetic moments at room temperature in the range 3.2-3.4 BM, measured in chloroform solution. These values are consistent with low-spin ( S = 1) ground states when allowance is made for some distortion from cubic symmetry. Mossbauer isomer shifts in the range 0.14.2 mm s-' relative to iron metal are lower than those characteristic of Fe"' compounds consistent with the removal of one d electron from Fe"' ( d 5 )to give Fe" (d4).Quadrupole splittings are in the range 2.c2.5 mm s-' , i.e. somewhat smaller than observed for the tetragonal [Fe(DAS)X2l2+complexes mentioned above, ~ ~ ~structure . ~ ~ ~ of but still much larger than the splitting usually observed for Fe"' c o r n p l e ~ e s . The the cation in [Fe(S,CNR,),]ClO, [R, = (CH,),] is very similar to that of the corresponding Fe"' complex. There is, however, a significant shortening (0.11 A) in the average Fe-S distance as expected for a depopulation of the antibonding eg orbital, as well as for the higher formal charge on the metal.48sAs found for many Fe'" complexes of 'sulfur' ligands, the 'FeS,' core in the Fe" complex conforms neither to the ideal octahedral nor trigonal prismatic arrangements, the angle of twist between the triangular faces being 38". Despite the ambiguity regarding the assignment of formal oxidation states to metal and ligand in many dithiolate complexes, the structure and properties of the benzyltriphenylphosphonium salt of tris( 1,1-dicarboethoxyethylene-2,2-dithiolato)ferrate [BzPh,P],[Fe(DED),] (DED = structure 139) are considered486to be best described in terms of an Fe" complex. The structure is close in important details to that of the dithiocarbamate complex described ab0ve.4~' Mixed valence Fe"', Fe" oxo-bridged dimers [Fe,L,O]X (L = TPP or salen; X = PF,, BF,, CIO, or I,) have been prepared by oxidation of the pox0 diiron(IT1) complexes.4877488 Antiferromagnetic exchange interaction was indicated by the variable temperature magnetic susceptibility data which could be fitted to the theoretical equations for an isotropic exchange ( H = - 25 SI- S , ) in an SI = $, S, = 2 dimer. The exchange parameter J was found to fall in the range - 7.5 to - 17.6 cm-' for [Fe,(salen),O]X and in the range - 82.5 to - 119cm-' for [Fe,(TPP),O]X. Mossbauer spectra of both series of complexes, recorded at 4.2 and 77 K, consisted of a single quadrupole-split doublet, showing that the rate of electron transfer between the metal centres is faster than IO7 s-'. The ESR spectra of the salen complexes were reported to consist of a single isotropic signal at g 1: 2.0, this being somewhat temperature dependent in line width. This behaviour is consistent with a thermal intervalence electron transfer rate of the order of 10'Os I . For the [Fe,(TPP),O]X complexes either a high-spin Fe"' ESR signal or no ESR signal was observed suggesting that thermal electron transfer [Fe,(TPP),O] can also be oxidized electrochemically to rates are less than 10" s-' in these [Fe,(TPP),O]'+ which may be formulated either as [Fe'V-O-Fe'V] or [Fe1V-O-Fe111(por+)]."89 An organometallic compound containing iron formally in the -k 4 oxidation state is the purple
-
267
Iron (JII) and Higher States
tetralkyl [Fe(not),] (140) (nor = I-norbornyl) formed by reaction of FeCl,*OEt with I-norbornyllithium. The stability of this complex and of those of other transition metal ions is probably associated with the bulky nature of the alkyl ligand which serves to protect the metal from attack by reagents.490 orit
O=C
\
/
= :;‘
O=C’,
S-
s-
OEt
( I 39)
The + 5 oxidation state of iron is very rare and has been firmly established only for the ferrate(V) oxyanion [Fe0,I3-. The K + salt has been prepared by heating K,FeO, with KOH in 0, at 70&800°C or from KO, and Fe,O, in 0, at similar t e r n p e r a t u r e ~ . An ~ ~ ”impure ~ ~ ~ form of Na3Fe0, has been obtained on heating Na,FeO, in O2 at high pressure. X-Ray investigations indicate a tetrahedral structure for the [Fe0,13- anion. Iron(V) has been suggested to occur as intermediates in the reactions of various biological systems, e.g. in HRP-I. However, as noted above, this species is probably best regarded as an Fe’” complex of the porphyrin radical cation. The chemistry of iron(V1) is confined to that of the ferrate(VT) [FeOJ- ion. KJFeO,] may be prepared by the oxidation of hydrated Fe20, in concentrated aqueous alkali with KClO or KBrO Electrolysis of concentrated alkali solutions with an iron anode has also been (equation 41)49’.493. Pure materials appear to be limited to employed as a method for the preparation of [Fe0,]’-.494,495 the salts with alkali metal ions and Ba2+.The purple salts contain discrete tetrahedral [FeO,]*anions and the alkali metal salts have the orthorhombic B-K, SO, The magnetic moments are in the range 2.8-3.2 BM consistent with the presence of two unpaired e l e ~ t r o n s . ~ ~ ~ , ~ ~ ~ Electronic,498ESR4Y9and Mossbauers””spectra have been reported. The ferrate(V1) ion is moderately stable in concentrated alkali solution but decomposes rapidly in neutral or acidic solutions according to equation (42). The strong oxidizing power of [FeO4I2is reflected in the redox couple of 2.0 V for the half reaction (equation 43). The ferrate(V1) ion is a stronger oxidizing agcnt than pcrmanganate and has found application in the oxidation of alcohols and other organic compounds.s0’-503
-
F e 2 0 ? - 3 H 2 0t 4 0 H -
2[Fe0,I2[FcO,]’-
+
3C10---+
+ 10H’ + 8Ht +
+
3e-
2[Fe0,I2-
2Fe3+
+
+
5H20
e Fe3+ +
3CI-
+
+
$02
4H,O
5H,O
(41) (42)
(43)
44.2.11 REFERENCES 1. (a) G . Charlot, A. Collumeau and M . J. C. Marchon, -Oxidation-Reduction Potentials of lnorganic Substances in Aqueous Solution’, Butterworths, London, 1971; (b) L. G . Sillen, Q.Rev., Chem. Soc.. 1959, 13, 146. 2. Y. Tdnabe and S. Sugano, J . Phys. SOC.Jpn.. 1954+9, 753. 3. Gmelin, ‘Handbuch der Anorganischen Chemie’ Eisen Teil B, Leifcrung 1 5, System Nummcr 59, Berlin, 192%1932. 4. N . V. Sidgwick, ‘The Chemical Elements and their Compounds’, Oxford, 1950, vol. 2, p. 1316. 5. H. Remy, ‘Treatise on Inorganic Chemistry’, Elsevier, Amsterdam, 1956, vol. 2, p. 249. 6. E. de B. Barnett and C. L. Wilson, ‘Inorganic Chemistry’, 2nd edn., Longmans, London, 1957, p. 220. I . A. F. Wells, ’Structural Inorganic Chemistry’, 3rd edn., Oxford, 1962. 8. C. S. G. Phillips and R. J. P. Williams, ‘Inorganic Chemistry’, Oxford, 1966, vols. 1 and 2. 9. P. J. Durrant and B. Durrant, ‘Introduction to Advanced Inorganic Chemistry’, 2nd edn., Longmans, London, 1970, p. 958. 10. D. Nicholls, in ‘Comprehensive Inorganic Chemistry’, ed. J. C. Bailar, H. J. Emeleus, R. S. Nyholm and A. F. TrotmanDickenson, Pergamon, Oxford, 1973, vol. 3, p. 967. 11. S. A. Cotton, Coord. Chern. Rev., 1972, 8, 185. 12. J. E. Huheey, ‘Inorganic Chemistry’, Harper and Row, New York, 1972. 13. K. F. Purcell and J. C. Kotz, ‘Inorganic Chemistry‘, Saunders, Philadelphia, 1977. 14. F. A. Cotton and G. Wilkinson, ‘Advanced Inorganic Chemistry’, 4th edn., Wiley-Interscience, New York, 1980. 15. A. G.Sharp, ‘Inorganic Chemistry’, Longmans. London, 1981. 16. ‘Comprehensive Organometallic Chemistry’, ed. G. Wilkinsun, F. G . A. Stone and E. W.Abel, Pergamon, Oxford, 1982.
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17. 18. 19. 20. 21. 22. 23.
’
Iron
Iron(II1) and Higher States 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104.
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107. 108. 109. 110. 111. 112. 113. 114. 115.
116. 117. 118. 119. 120. 121, 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134.
135.
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IronIIII) and Higher States 434. 435. 436. 437. 438. 439. 440. 441. 442. 443. 444.
445. 446. 447. 448. 449. 450. 451. 452. 453. 454. 455. 456. 457. 458. 459. 460. 461. 462. 463. 464 465. 466. 467. 468. 469 470 471. 472. 473. 474. 475. 416. 477. 478. 479. .
”
480. 481. 482. 483. 484. 485. 486 487, 488. 489.
215
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45 Rutheniurn MARTIN SCHRODER and T. ANTHONY STEPHENSON University of Edinburgh, UK 45.1 INTRODUCTION 45.2 MERCURY-CONTAINING LIGANDS 45.3 GROUP IV LIGANDS 45.3.1 Cyanides and Fulminates 45.3.1.1 Ruthenium ( I ) cyanides 45.3.1.2 Ruthenium ( I I ) cyanides 45.3.1.3 Ruihenim(III) cyanides 45.3.1.4 Ruthenium(IV}, ( V ) and (VU) cyano complexes 45.3.2 Silicon-containing Ligands 45.3.2.1 Carbonyl complexes 45.3.2.2 Carbonyl complexes containing group IV, V or VI donor ligands 45.3.2.3 Tertiary phosphine and arsine complexes 45.3.3 Germanium-containingLigands 45.3.4 Tin-containing Ligands 45.3.4.1 Rutheniurn(II) haIide complexes 45.3.4.2 Ruthenium ( I I ) carbonyl and carbonyl halide complexes 45.3.4.3 Ruthenium(II) complexes containing group IV3 V or VI donor ligands 45.3.5 Lead-containing Ligands
279 280 28 1 28 1 28 1 28 1 283 284 284 284 285 287 287 287 287 288 288 289
45.4 NITROGEN LIGANDS 45.4.1 Ammines 45.4.1.1 Mononuclear complexes [ R u ( N H 3 } , Y f 45.4.1.2 Mononuclear complexes [RuL, (NH,),-,Pt 45.4.1.3 Bmucfeur complexes 45.4.1.4 Tri- and poly-nuclear complexes 45.4.2 Amines 45.4.2.1 Mononuclear complexes [RuL,]" 45.4.2.2 Monuntrcleeor complexes [ R U L , ] ~ ' ~ ~ ' 45.4.2.3 Mononuclear complexes with nitrogen donor ligands 45.4.2.4 Mononuclear complexes with oxygen donor ligands 45.4.2.5 Mononuclear complexes with halide ligands 45.4.2.6 Binuclear complexes 45.4.3 Heterocycles 45.4.3.1 Mononuclear complexes 45.4.3.2 Binuclear and polynuclear complexes 45.4.4 Nitrosyls 45.4.4.1 Nitrogen donor complexes 45.4.4.2 Oxygen donor complexes 45.4.4.3 Suifiu donor complexes 45.4.4.4 Halide complexes 45.4.5 Hydrazines, Diimines and Related Ligan& 45.4.6 Nilrides 45.4.7 Thiocyanates 45.4 8 Nitriles
289 289 289 29 1 31 1 320 321 32 1 32 1 322 323 324 325 325 325 357 361 361 362 362 362 363 364 365 365
45.5 PHOSPHORUS, ARSENIC AND ANTIMONY LIGANDS 45.5.1 Mononuclear Ruthenium (0) Complexes 45.5.1.1 RuL,. RuL,,L;-,, RuL,,Li-, complexes ( L = P donor: L' = P donor, MeCN, alkene) 45.5.1.2 Nitrosyl complexes 45.5.1.3 Carbonyl and/or isocyanide complexes 45.5.2 Bi-, Tri- and Tetra-nuclear Ruthenium(O] Complexes 45.5.2.1 Nitrosyl complexes 45.5.2.2 Carbonyl complexes 45.5.3 Ruthenium ( I ) Complexes 45.5.3.I Mononuclear complexes 45.5.3.2 Bi- and ietra-nuclear complexes
277
366 366 366 367 370 372 372 373 374 374 374
278
Ruthenium
45.8.4 Mononuclear Ruthenium(II) Complexes 45.5.4.i fRuL,I2+, [RuXL,]+ (n = 4.5), [RuX(L-L), J ' , fRuL'/2+ carions ( X = halide, alkyl etc.) 45.5.4.2 [RuXYL,] complexes (n = .2.3,4; X , Y = halide) 45.5.4.3 fRuXY(L-L),] complexes (n = 2.3; X , Y = halide) 45.5.4.4 Polydentate P, As or Sb donor ligand halide complexes 45.5.4.5 Carbon donor ligand complexes 45.5.4.6 Nitrogen donor ligand complexes 45.5.4.7 Oxygen donor ligand complexes 45.5.4.8 Sulfur donor ligand complexes 45.5.4.9 Mixed donor atom (P-N, P-0, P-Sj ligands 45.5.5 Bi-. Tri-, Tetra- and Poly-nuclear Ruthemium(l1) Complexes 45.jS.1 Triple halide-bridged complexes (and related species) 45.5.5.2 Double halide-bridged complexes (and related species) 4.7 S.5.3 Binuclear complexes w ith poly den la te phasph ine bridges 45.5 .S.4 Miscellaneous 455.6 Mixed Vulence Rurheniuml~1)iRvthnium( [ [ I ) Complexes 45.5.7 Mizorruclear Ruthenium(IIIj Complexes 45.5.7.1 Monodenlate P , As and Sb donor ligand complexes 4.7.5.7.2 Bi- and poly-dentate P and As donor ligund complexes 45.5.8 Bi- and Tri-nuclear Ruthenium {UIj Complexes 45.5.9 Ruthenium(IV) and Higher Oxidation State Complexes 45.5.10 Useful Books and Reviews
376 376 377 379 380
380 39 I 399 404
408 410 410 41 3 414 415 415 416 416
417 418 419 420
45.6 OXYGEN LIGANDS 45.6.1 Aqua Complexes 45.6.2 Hydroxy Complexes 4S.h.3 Oxo Complexes 45.6.4 Ketoenolate Complexes 45.6.5 Carboxylate Complexes 45.6.5.1 Monucarboxylales 45.6.5.2 Dicarboxylates 45.6.6 MisceIluneous
420 420 42 I 421 423 425 425 429 429
45.7 SULFUR LIGANDS 45.7.1 Sulfido, Thiolato and Thiazole Complexes 45.7.2 Thioerhers 45.7.3 Metallothioanions as Ligands 45.7.4 Dithiocarbarnato and Related Complexes 45.7.5 Dithio-phosphinato and -phosphate Complexes 45.7.6 Dithiolene, Dithiocarhoxylute and Related Complexes 4.5.7.7 Dithio- nnd Monothio-j-diketonate Comp1exe.v 45.7.8 Thiourea Complexes 4.5.7.9 Suffoxide Complexes
430 430 432 433 433 436
45.8 SELENIUM AND TELLURIUM LIGANDS 45.9 HALOGEN LIGANDS 45.9.1 Ruthenium(1) Halides 44.9.2 Ruihenium(I) Carbonyl Hulides 45.9.3 Ruthenium(II) Halides 45.9.4 Ruthenium(II) Aquahalide Complexes 45.9.5 Ruthenium ( I I ) Carbonyl Halides 45.9.8.1 Carbonyljuorides 45.9.5.2 Neutral carbonyl chlorides, bromides and iodides 45.9.7.3 Anionic carbonyl halides 45.9.6 Rulhenium ( H I ) Halides 45.9.6.1 Pure neutral halides 45.9.6.2 Pure anionic halides 459.6.3 Aqua- and hydrmxo-hdide complexes 45.9.6.4 Carbony 1 halides 45.9.7 Ruthenium(IV) Halides 45.9.7.1 Pure neutral halides 4S.9.7.2 Pure anionic halides and related species 45.9.7.3 Oxo-, hydroxo- and aqua-halide complexes 48.9.8 Ruthenium( V ) Halides 45.9.9 Ruthenium( V I ) Halides 48.9.10 Ruthenium( VIII) Halides
439 440 440 440 441 442 442 442 442 443 443 443 444 445 446 446 446 447 447 448 449 450
45.10 HYDROGEN AND HYDRIDE LIGANDS 45.I0.I Ruthenium(0) Complexes 45.10.2 Ruthenium(1) Complexes 4i,10.3 Mononuclear RutheniumTU) Complexes 45.10.3.I Dihydrido complexes 45.10.32 Monohydrido complexes
450 4 50 450
436 436 437 437
450
450 452
Ruthenium 45.10.3.3 Anionic complexes 45.10.4 Bi-, Tri-. Tetra- and Poly-nuclear Rutheniumf I I ) Complexes 45.10.5 Higher Oxidaiion State Hydrido Complexes 45.10.6 References to Catalytic Reactions of Ruthenium Hydrido and Related Complexes 45.1 1 MIXED DONOR ATOM LIGANDS
45.11.1 N - 0 Ligan& 45.11.1.1 &hi# base complexes 45.11.1.2 Oxime complexes 45.11.1.3 Amino acid, 8-hydroxyquinoline. dipicoiinic acid and related complexes 45.11.1.4 6-Meth~.'l-2-p~rid~olate and acetamide complexes 45.1 1.1.5 Edia and related complexes 1.5.11.1.6 Triorene I-oxide complexes 45.11.2 N-S Ligmdp 45.11.3 N - . CLigan& 4.5.11.4 S-0 Ligan& 45.12 MACROCYCLIC COMPLEXES
45.12.1 45.12.2 45.12.3 45.12.4 45.12.5 45.13
Porphyrins Phthalocyanines Tetraaza Macrocjcles Triaza Macrocycles Tetrathia Macrocycles
REFERENCES
279 460 46 1
462 463 463 463
463 464
465 466 466 461 467 468 468 469
469
414 415 411 411
418
45.1 INTRODUCTION
Ruthenium was discovered by Karl Karlovitch Klaus in 1844 in Tartu, Estonia (Ruthenia is latin p.p.m. The for Russia). It is a rare metal with a natural abundance in the earth's crust of ca. main sources are laurite, the native alloys osmiridium and iridiosmium and other platinum metal concentrates. Ruthenium has an atomic number of 44, electronic configuration [Kr]4d75s' and an atomic weight of 101.07. It possesses seven stable isotopes 9 6 R(~I = 0, 5.5%), 98Ru( I = 0, 1.87%), 99Ru (Z = 4, 12.72%), IooRu( I = 0, 12.62%), '"Ru (I= 9, 17.07%), Io2RU ( I = 0, 31.61%), Io4Ru( I = 0, 18.58%), and exists in four allotropes, the stable a-phase being grey-white in colour. The metal is insoluble in all acids including aqua regia and is resistant to oxygen up to 600T whereupon it forms RuO,. The metal dissolves in fused KOH to give K2[Ru0,]. Ruthenium exhibits a wide range of oxidation states from VI11 to - 11, the most common being I11 and I1 for 'classical-type' ligands. The least common oxidation states are VII, V, I and - I. The higher oxidation states are stabilized by small n donor ligands such as F-, O'-, N3- (e.g. [RuVIF,], ); conversely n acceptor ligands such as CO, PR, stabilize the lower [RuV"'04], [RuV1NC14]oxidation states (e.g. [Ru-"(C0),I2-, [Ruo(CO),], [Ruo(CO),(PPh,),]). Ligands which are good cr donors but show no substantial n acceptor or donor properties (e.g. H,O, NH,) are usually associated with Ru"' and Ru". Table 1 summarizes the oxidation states and coordination numbers for complexes of ruthenium. Ruthenium(O), d': The chemistry is primarily one of mononuclear, polynuclear carbonyls and tertiary phosphine complexes; a range of Ruo nitrosyls are known. Ruthenium(I), d': Very few monomeric complexes are known with dimerization and polymerization reactions viu Ru-Ru metal bond formation being prevalent. Ruthenium(IIj, d': A wide range of Ru" Complexes incorporating carbonyl, phosphine, ammine and heterocyclic ligands are known. Virtually all Ru" complexes are octahedral and diamagnetic with a tk6 configuration (unless steric constraints are present). Catalytic processes utilizing Ru" phosphine complexes, kinetic studies on substitution reactions of [Ru(NH,), LIZ+ and the photophysics and photochemistry of [Ru(bipy),]'+ and related systems have been areas of major advance in recent years. Ruthenium (IIZ), d : Ru"' is often associated with 'classical-type' ligands, e.g. ammine, water, halides. They are octahedral low spin t,; species with one unpaired electron and generally substitutionally inert. The electronic structure of polynuclear carboxylates and mixed valence R U ~ ' " ~ ~ complexes of the type [Ru(NH,),],L5+ has been the subject of much interest particularly with respect to the degree of unpaired electron delocalization within these molecules. Ruthenium(IV), d 4 :These complexes are often octahedral or distorted octahedral with a t z i low
Ruthenium
280
Table 1
Oxidation state
Coordination number
Complex
RU-'I Ruo
Ru' Ru"
Ru"'
6
RuN
spin configuration giving = 2.7-2.9 BM which decreases with decreasing temperature. 0x0- and nitrido-bridged polynuclear complexes of Ru" are of particular interest, while more recently organometallic and hydrido Ru" species have been synthesized. Ruthenium(V), ( V I ) , (VII), (VIII) ( d 3 ,d 2 ,d ' , d " ) :These complexes are restricted to halo, oxo and nitrido species such as [RuNCl,]-, [RuNCl,]'- and [Ru~,]*-''-'~. Oxo complexes of ruthenium have been used as oxidation catalysts. This article reviews the coordination chemistry of ruthenium up to mid-1984 with particular emphasis on work published since 1967. The organometallic chemistry of ruthenium has been reviewed' and is not covered herein. Previous reviews of the coordination chemistry of ruthenium have been published by Griffith,' S e d d o P and Gmelin,6with a more general review by Cotton and Wilkinson.7 Since ruthenium complexes often show multi-electron redox behaviour, the ordering of the contents within the review has been taken according to the ligands bound to the metal, with subdivisions where applicable, for metal oxidation states. The main reason for this approach is to facilitate data retrieval and cross referencing.
45.2 MERCURY-CONTAINING LIGANDS Very few papers have been published on compounds containing direct ruthenium-mercury bonds and of these, several are strictly outwith the scope of this review since the other ligands present are either carbocyclic rings or other carbon-bonded groups, e.g. [(Ru(q-C,H,),),Hg, Br,],' [Ru, (CO),(H~B~)(C,BU')],~ and [RUC~~(CNE~),(H~C~,)].'~ The first examples of compounds containing Ru-Hg bonds were [Ru(HgX)(CO),(PPh,),]HgX, (1) (X = C1, Br, I) prepared by the reaction of mercury(I1) halides with [Ru(CO),(PP~,),].~'Similar reactions, however, with [RuI(CO),(PPh,),]I and HgI, (1:l molar ratio) give [RuT(CO),(PPh,),] HgI, and [(RuI(CO),(PPh,),),]Hg14(2:1 molar ratio)." Oxidation of [Ru(CO),(q4-C,H,)] with HgX, affords [Ru(X)(HgX)(CO),], (X = C1, Br or SCN), in which the halogen bridge can be cleaved with pyridine to give [RuX(HgX)(CO),pyj (X = C1, Br).I3 Reaction of [Ru(CO),(PPh,),] or ~u(CO),(PPh,),] with Hg(CF,), has been shown by X-ray The average C-F distance in the Ru-bound analysis to give [Ru(HgCF,)(CF,)(CO), (PPh,),] 4I.)@ CF3 group is 0.1 A longer than in the Hg-bound CF, group and the Ru-Hg bond length is 2.628( 1)A.The compound reacts with aqueous HClO, to convert the CF, group to a CO group with retention of the Ru-Hg linkage, viz. [Ru(HgCF,)(CO), (PPh,),]ClO,. Treatment of RuCl, in acetone with HgCl, followed by EPh, is claimed to give [RuCl,(HgCl,)(EPh,),] (E = P, As, Sb) (vRuHg 178-1 72 ~ r n - ' ) . ' ~Interaction of cis-[RuClMe(PMe,), J with 1% sodium amalgam over a period of three days at ambient temperature yields the trimetallic complex (3) in which the methyl groups are orientated cu. 90" from one another.'6a Reaction of [RU,(NO)(CO),~]with HgCl, gives [Ru, (NO)(CO),o]2Hg.'6bFinally, the Hg2+-catalysed aquation of the [ R U C ~ ( N H ~ ) cation ~ ] ~ ' has been studied in both aqueous solution and in a variety of mixed water-organic solvents." Kinetic
Ruthenium
28 1
data in aqueous solution are consistent with preequilibrium formation of a chloro-bridged intermediate [(NH,), Ru-C1-Hgl4+ followed by dissociation to release [HgCl]+ and the rapid addition of water to give [ R u ( N H ~ ) ~ ( O H ~ ) ] ~ + . PPh3
PPha
I
OC “tttt/,,
PMe3 HgCF3
PMe3
I
Me #e*‘ Me3P -Ru Me3PH PMe,
PMe3
1
PPhj (2)
(1)
Me
(31
GROUP IV LIGANDS
45.3 45.3.1
Cyanides and Fulminates
45.3.1.1 Ruthenium(I) cyanides
Gamma-ray irradiation at 77 K and X-ray irradiation at room temperature of single crystals of the diamagnetic &[Ru(CN),] 3 H 2 0 produce paramagnetic low spin Rut species.18At room temperature, the ESR spectra were interpreted in terms of the species [Ru(CN),@IC)]’- whereas at liquid nitrogen temperature [Ru(CN),(NC)J5- is observed. However, parallel studies by Symons and Wilkinson on K,[Ru(cN),] in various host lattices suggest that y irradiation produces such species as [Ru(CN),(CN)H20I4-, [Ru(CN),(CN),]’ ’-, [Ru(CN),(CN)X]’- and [Ru(CN),XJ(X = C1, Br), where C N indicates a cyanide ligand tilted off the z axis.”
-
45.3.1.2 Ruthenium(Z1) cyanides
(9
Ru(CNl2 and the [Ru(CN),14- ion
Ru(CN), was prepared in 1920 by addition of KCN to the blue ‘ruthenium dichloride’ solution.*’ No structural information is available on the material. K,[Ru(CN)~]was first made in 1896 either by the action of KCN on KZ[RuO4]or by boiling a solution of RuCl, with an excess of KCN until the solution loses its colour.2’ As described elsewhere,2various other metal salts of this anion have been prepared together with the anhydrous free acid H,[Ru(CN),] and in several earlier studies, the electronic, IR and Raman spectra” have been recorded and assignments proposed. The IR spectra of solid Y,[Ru(CN)~](Y = H, D) have been measured and interpreted as showing unsymmetrical (N-Y. . .N) bonds.23The 99RuNMR spectrum of &[RU(CN)~]has been recorded.24 The X-ray structure of Mn,[Ru(CN),]-XH,O confirms that the ruthenium ion is coordinated by six cyanide ions in a slightly distorted octahedron (average Ru-C and C-N distances are 2.028(6) and 1.160(8)A respectively) and all the CN groups are bonded through nitrogen to manganese.25 A detailed investigation of the surface-enhanced Raman scattering from adsorbed K,[Ru(CN)~] clearly demonstrates that the molecule interacts with the metal surface via a CN bridge. The strength of this interaction is dependent upon the metal surface (Ag or Cu) and the applied potentiai. Furthermore, the spectra from both metals show vibrational modes which are only IR active in bulk phase, indicating a symmetry lowering on adsorptionsz6Mossbauer parameters for K,[Ru(CN),] and other ruthenium-cyano compounds such as &[Ru(CN),NO2].2H,O and K2[Ru(CN),NO] - 2 H z 0have been reported and disc~ssed.~’ A method for the preparation of thin films of Fe,[RU(CN)& (‘ruthenium purple’) involving electrochemical reduction of K3[Ru(CN),] in a solution of Fe,(SO,), has been developed.28This ‘ruthenium purple’ modified electrode is claimed to be one of the best catalysts for evolution of oxygen and chlorine. Electrochemical studies on polyammonium macrocyclic complexes of [Ru(CN),I4- indicate a 1: 1 stoichiometry with a monoelectronic, reversible, oxidation for these complexes; this illustrates the control of redox potential of anions by complexation with appropriate receptor molecules.29The kinetics of oxidation of [Ru(CN),J4- by WnOJ in HC10, have been investigated by stopped-flow techniques. It is found that [Ru(CN),I4- is quantitatively oxidized to [Ru(CN),I3 in accordance with equation (1) and that two protonated intermediates [RuH(CN),I3and [RuH,(CN),]’- are involved in the oxidation process.30
282
Ruthenium MnO;
+
5Ru"
+
8H+
+
Mn2+
+
5Ru"'
+
4H20
(1)
Finally, reaction of [Ru(CN),I4- with alkylating agents such as dimethyl sulfate3' or triethyloxonium tetrafluoroborate in acetone32gives high yields of the isocyanide cations [Ru(CNMe),]" and [Ru(C=N-C(Me),CH2(CO)Me),j2' respectively. Treatment of the former with simple .3' amines leads to various carbene complexes such as [Ru(CNM~),(C(NHM~),),]~+
(ii) Monomeric Ru" cyanides containing N donor ligands
K2[Ru(CN),(NO)], the only fully established nitrosyl cyanide ruthenium complex, is prepared by reaction of &[Ru(CN)~] with HNO, ?3 Its Mossbauer spectrum,27bwhich shows a more positive isomer shift than K4[Ru(CN)J, is ascribed to the greater n acceptor ability of NO+ compared to CN- . For K4[Ru(CN)5(N02)],prepared by reaction of K2[Ru(CN)5N0]with OH-,34aa decrease in isomer shift27bis attributed to the poor x acceptor ability o€ NO;. The X-ray structure of Na,[Ru(CN),NO]-2H20, prepared from 'RuCI,*xH,O' and NaCN by oxidation with HNO, has been reported. Some reactions of this anion with various nucleophiles are discussed.34hInterestingly, a recent paper claims that the final product of the [Ru(CN),]~-(HNO, reaction is [Ru(CN),(NO)(H,O),I.~~ Reaction of [Ru(NH,),OH,]~+ and Na[BH,CN] under argon leads to the formation of [Ru(BH,CN)(NH,),]+ which on exposure to air gives the ruthenium(II1) dication [Ru(BH,CN)(NH,),I2+. Dissolution of the former in 3 M HCI produces the hydrogen cyanide complex [Ru(NCH)(NH,),]+ coordinated at the nitrile nitr~gen.~' More recent studies by Isied and TaubeJ7 have shown that this HCN complex can be prepared by direct reaction of HCN with [Ru(NH,),OH,]'+ and that its instability is due to intramolecular rearrangement of N- to C-bound ligand, followed by labilization of the trans ammonia group and subsequent polymerization to [RuCN(NH,),],"+ . Similarly reaction of [RuCN(NH,), OH,]2+ with equimolar amounts of NaCN gave [RuCN(NH,),]+ . Reaction of either the latter or [Ru(NCH)(NH,),]'+ with isonicotinamide (isn) gave trans-[Ru(CN)(NH,),(isn)] + . The synthesis of [Ru(CN),~Z)~(pz = pyrazine) has been described (equations 2 and 3); the N-methylated derivative is similarly prepared. The effective pKa of [Ru(CN),pzH]'- (0.4 5 0.1) has been measured by spectrophotometric titration and its value contrasted with that of [Fe(CN)spzH]2 (0.065 & 0.06) and [ R u( NH , ) , ( ~ z H ) ](2.85 ~ ~ & 0. 1)?8 The electronic spectra3gaand voltammetric behaviour3" of a series of [Ru(CN), LJ3--anions (L = various aromatic nitrogen heterocycles) have been reported and fully discussed. [Ru(CN),I4-
+
Br2
H?O
[Ru(CN)~OHJ- + BrCN
+
Br-
(2)
A substantial amount of work has been published on c i s - [ R ~ ( c N(bipy), )~ J and the corresponding [Ru(CN),(phen),]. Much of this is concerned with their interesting photochemical and photophysical properties and these are fully discussed, together with those of the well-known and exhaustively studied [Ru(bipy),]*+ in Sections 45.4.3.1.viii.a and 45.4.3.1 .vi.b respectively. The compound cis-[Ru(CN),(bipy),] 3H,O was first prepared from NaCN, 2,2'-bipyridyl and KZIRuCls(OH1)lfollowed by reduction with bisulfite ion. Tt is protonated on the cyanide nitrogen atoms by perchloric acid in acetic acidwand its IR spectrum has been recorded and discussed.'" An alternative synthesis from reaction of [Ru(C,O,)(bipy),] .4H,O with NaCN is available.42 (iii) Monomeric Ru" cyanides containing ofher ligands
A few cyano phosphine and arsine complexes of ruthenium(I1) have been reported. These include tram-[RuH(CN)(dmpe),] (dmpe = Me,P(CH,),PMe,) obtained from the corresponding chloro complex and aqueous KCN@ or more recently by treatment of [RuH(Znp)(dmpe),] (2-np = 2naphthyl) with HCN. The latter gives a 8 5 % cis, 15% trans isomer mixture ('H NMR evidence).44 The complex trans-[Ru(CN),(eda), J (eda = ethylene-1,2-bis(diphenylarsine))45and quadridentate tertiary phosphine and arsine complexes such as (Ru(CN),[E(o-C6H4EPh,),]}(E = P, As) are
283
Rurhenium
prepared by reaction of the corresponding chloro compounds with NaCN!6 Reaction of [Ru(y2CS2Me)(C0)2(PPh3)2]C104 with CN- gives [RuCN($ -CS,Me)(C0)2(PPh,)2] which then loses CO to generate [RU(CN)(~*-CS,M~)CO(PP~,)J.~~ Two carbonyl complexes K, [Ru(CN), I, (CO),] (from ruthenium carbonyl iodide and KCN)48and [RuC~(CN)(NH,)(CO)(PP~,)~] (4) (from treatment of [RuCl, (CCl,)(CO)(PPh,),] with ammonia) are known. Reaction of the latter with CO gives [RuCI(CN)(CO),(PPh,),] (5).49 PPh3
PPh3
(iv) Binuclear Ru" cyanides
Most of these complexes are formed via reaction of [Ru(CN),I4- with other transition metal compounds and involve bridging cyano groups. For example, DMF solutions of cis-[Ru(CN),(bipy),] and either K[PtC13(C2H4)]or cis- or trans-[PtCl,LL'] (L = C2H4, Me,S, L' = 4-methylpyridine) react to give 1: 1 and 1:2 adducts. Spectroscopic studies indicate that adduct formation results ] can be in appreciably enhanced emission lifetimes. The compound (RU(CN)~(~~~~),[P~CI,(C,H,)], isolated but no structural information is available as yet." Similarly aqueous solutions of [Ru(CN)J4 and bis(histidinato)cobalt(III) develop an orange-red colour when exposed to visible light and the formation of the single cyano-bridged complex anion [(~-hist)~Co(NC)Ru(CN)~l~is propo~ed.~' The closely related species [(en),NH, Co(NC)Ru(CN),]- has been synthesizeds2"and many other references to cyano-bridged Ru"/metal complexes are to be found in references 52b and 53 with the latter describing the synthesis, photophysical properties and excited state redox behaviour of [CN(bipy), RuCNPtdien]'+ and [dienPtNC[Ru(bipy),]CNPtdienI4+. Polymeric cations made up of Ru(bipy), units linked by cyanide bridges can be prepared by reacting [Ru(CN), (bipy),] with [R~(C,O,)(bipy),].~~ The mechanism of formation of these species probably involves initial formation of an outer sphere complex and then reorganization to form a cyano-bridged compound. For example, mixing of [RuC1(NH3)J2' and [Ru(CN),I4- solutions produces a new absorption maximum at 510nm (e = 20) assigned to an outer sphere intervalence transfer transition from Ru" to Ru"'. Irradiation of this band leads t o the formation of [(NH,),Ru(NC)Ru(CN),]- as a stable photoproduct whereas intervalence excitation of the starting ion pair yields its redox isomer [Ru"Cl(NH,),]+ / [Ru"'(CN),]~- which can diffuse apart.55 Similarly, saturated DMSO solutions of [Co(NH,),I3+ / [Ru(CN),I4- exhibit an absorption band at 360 nm ( E = 580) assigned to the intervalence transfer from a reducing Ru" to an oxidizing Co"' centre,56and in a very thorough study, Curtis and Meyer have examined outer sphere charge transfer (OSCT) transitions for a series of mixed metal ion pairs [M(CN),, Ru(NH,),L]- (M = Fe, Ru, Os; L = pyridine or substituted pyridine). From a detailed analysis of these OSCT bands, a number of proposals regarding the 'structure' and the extent of electronic coupling in these species are Related studies on the binuclear [(NH,),RuNCRu(bipy),CN]" and trinuclear [(NH,), RuNCRu(bipy), CNRu(NH,)J6 have been Finally, the action of H2S04on K4[Ru(CN)J is reported to give the formally mixed valence [Ruz(CN),OH,] as a blue powder, which reacts with ammonia to produce NH4[Ru,(CN),(NH,),].5Y However this is old work and more evidence is required to substantiate these claims. +
45.3.1.3
Ruthenium(III) cyanides
Treatment of a solution of K4[Ru(CN),] with Cl? results in several colour changes and warming with H, SO, then precipitates a green-black product of composition Ru(CN),* %I, 0; with concentrated ammonia this gives Ru(CN),.2NH3.H, 0 which is also insoluble.59Much more recently, because coordinated methylamine ligands are more susceptible to oxidation than the free base, it has been found that exposure of [Ru(NH, Me),]'+ cations to oxygen under ambient conditions produces Ru(CN), 3H20.h0 Earlier studies reveal that a yellow solution is obtained by reaction of [Ru(CN)J4' with H 2 0 2 ,
-
C O C LJ'
Ruthenium
284
Ce" or bisrnuthate in acid conditions, or with ozone in neutral. This solution presumably contains the [RU(CN),]~-ion (cf. more recent work involving [MnO,]- oxidation -equation (l), Section 45.3.1.2.i). The potential of the [Ru(CN),I3- /[RU(CN)6I4- couple is 0.86 & 0.05 V vs. SCE.61 Hydroxyl radicals produced by pulse radiolysis also converts [Ru(CN),I4- into [Ru(CN),I3- ,62 Studies on the decomposition of aqueous solutions of [Ru(CN),I3- reveal that reactions such as aquation (to [Ru(CN),OH,I2- 1, dimerization and ligand oxidation to form [Ru(CN),(CN0)I4- are in~olved.~, Finally, Gillard et al. report that the [Ru(CN),(bipy),]+ cation (obtained by oxidation of [Ru(CN),(bipy),] with C1, or cerium(1V) nitrate) reacts with water to give the [Ru(bipy),(H,0),I3+ cation. Reduction of this solution in the presence of SCN- gives [R~(SCN),(bipy),].4H~0.~
+
45.3.1.4
Rubhenium(IV), ( V ) and (VU) cyano complexes
Treatment of K, [Ru,NCl,(OH,),] with KCN gives the very stable species K,[Ru2N(CN),,]. 3H20.On the basis of IR and Raman studies, an eclipsed (D4*) structure with a linear RuNRu unit is proposed, although of course, these indirect methods cannot distinguish between this and the staggered (DM)configuration. Cyano complexes of Ru" are unusual and it is suggested that in this compound the strong .n donor effects of the nitrido ligand force the ruthenium to behave as a Ru"' or even Ru" centre. Reaction of RuO, with ice cold solutions of NaCN in the presence of caesium ion did not yield the expected Cs,[Ru(CN),(O),] anion (cf: Cs,[RuCl,(O),], Section 45.9.9).Instead a green paramagnetic salt formulated as Cs, [Ru"(CN),(CNO),(O)] is formed. The reaction of K, [RuO,] with NaCN in the presence of Cs+ give a yellow paramagnetic material Cs,[Ru,(CN),(O),] (containing formally Ru'"?).~~
45.3.2 45.3.2.1
Silicon-containing Ligands
Carbonyl complexes
Many of the complexes with silicon-containing ligands and carbonyl groups are outwith the scape of this review in that they are octahedral with four CO groups per ruthenium. Nevertheless, since these compounds are often the precursors for species with three or less CO groups per ruthenium, a brief outline of their syntheses and characterization is given in this section.
-
[RU~CO),~I
co
. .
CO
HMZY,
[Ru(CO),I
[RuH(MZY,)(COhI
5
co
co
MZY,
co
co
MZY,
(7)
(8) Scheme 1
As discussed in detail elsewhere,' most of these silicon-containing tetracarbonyl complexes are made via either photochemical or thermal reaction of [RU~(CO)~,] with a silicon hydride (Scheme 1; M = Si). This yields an intermediate [RuH(SiZY,)(CO),] (6) which may decompose to [Ru(SiZY,)(CO),], (7) by loss of H, or may react with more HSiZY, to give cis- and/or trans-[Ru(SiZY,),Cl;66367 Z = Me, Y = Cl;66,68369 Z = C1, Y = Me6' (not all combina(CO),] (8) [Z = Y = Me, Et, tions of products formed)]. The latter can also be formed by direct reaction of [Ru(SiZY,)(CO),], with HSiZY,. The stereochemically non-rigid behaviour of (8) (M = Si, Ge, Sn) has been invesRelated tetracarbonyl complexes with chelating disilanyl tigated by "C-('H} NMR spectro~copy.~~ ligands have been formed directly from [Ru,(CO),,] and an extensive range of mixed Group IVB ligand complexes have been made from [Ru(SiMe,)(CO),]-. (See reference 1, p. 909 for details.)
OC
”..,I
-
,**\\\” SiZCI,
I
oc%/*,# **#
HRU\ oc C1,ZSi
Q,, 4%
c18o y o y ; o . *e,\\
~y
Tcok
H
co
oc
“QQ/,
R’R2Si /
sizclz
-_
C‘O
co
r/To\
1 /R u jbSiRzR‘ co
co b
(11)
T)@**co i
(12)
Reaction of [Ru(SiMe,)(CO),], with bromine or iodine gives truns-[RuX(SiMe,)(CO),] and pyrolysis of these materials produces the halide-bridged species [RuX(SiMe,)(CO),], as an isomeric mixture.74However, as shown in Scheme 2, treatment of cis-[RuH(SiRCI,)(CO),} with CBr, results in the formation of [RuBr(SiRCl,)(CO),], (single isomeric form) together with trans-[RuBr(SiRCl,)(CO),]. Similar products are obtained when the Ru-Ru bond in [Ru(SiRCl,)(CO),], is cleaved with Br2.6SReaction of [Os(CO),] with ~rms-[RuBr(SiCl,)(CO),] at 0°C gives [(CO),OsRuBr(SiC1, j(CO),] which on standing in solution isomerized to [Br(C0)40~R~(SiC13)(CO)4].75
co
co
co
CO
CO
&/
ir I
A
CO
CO
Scheme 2
45.3.2.2
.
Carbonyl rampiexes containing group iV, V or VI donor ligands
In cis-[Ru(SiCl,),(CO),] only the CO groups trans to silicon exchange with ‘3C0.’6 Substitution by PPh, to giver ~ner-[Ru(SiCl,),(C0),PPh,]has a similar rate and a wide range of other mer-
286
Ruthenium
[Ru(S~CI~)~(C ) ~= PF,, P(OPh),, AsPh,, SbPh,, CNC(Me), tltc.) are readily ~repared.6~ L]O(L Similar results are obtained with cis-[Ru(SiMeCl,),(CO),], and likewise with [RuH(SiRCl,)(CO),], P donor ligands displace the CO group trans to -SiRCI, (equation 4).6s Synthesis of the related [RuX(SiZY,)(CO),L] (L = P(OMe),, PPh, etc., X = C1, Br, I, ZY, = Me,; CI,; Z = Me, Y = Cl) are readily achieved by cleavage of [RuX(SiZY,)(CO),], with L.68,74 Further substitution in mer-[Ru(SiCl,), (CO),L] could only be accomplished when L had a small cone angle. Thus, the second CO group is not replaced by PPh, even at 80°C whereas P(OMe), reacts L,] (L = PF3, at room temperature to give [RU(S~CI~)~(CO),PP~, {P(OMej, }I and [Ru(S~C~,)~(CO), . ~ ~ studies on CO substitution reactions PPhMe, etc.) are also formed under mild c o n d i t i o n ~Kinetic of [RU(S~C~,)~(CO),L] complexes indicate a dissociative mechanism and a cis effect of L which is almost entirely steric in origin.77The dicarbonyl complex [Ru(SiMe,),(CO), (PPh,),] can however by synthesized by reaction of [Ru(SiMe,)(CO),], (7)with PPh3(equation 5) or by oxidative addition of HSiMe, to [Ru(CO),(PPh, j2] under UV irradiation.66 Reaction of [RuH,(CO)z(PEt,),] with SiCkMe, also gives [RU(S~M~~)~(CO),(PE~,),].~~" A study of the photochemistry of [RuH(SiEt,)(CO,)PPh,] indicates that both facile loss of CO and reductive elimination of Et,SiH can
co OC % ',,,/,t
I ,"*'
SIRCI,
+
PR3
OC+H
-
co [Ru(SiMe,)(CO),l,
co OC,,% , ' ,,t
I *"' SiRCI, I H'
RU9
RIP(
+ co
(4)
co
+ PPh,
+
+
Ru(SiMe,),(CO)z(PPh3)z Ru(CO),(PPhdz
(5)
Substitution of the equatorial groups in cis-[Ru(SiCl,),(CO), J by a variety of bidentate ligands (L-Lj gives [Ru(SiCl,), (CO), (L-L)] via monodentate intermediate [Ru(SiCl,),(COj, (L-L)] complexes (L-L = diene, MeS(CH,),SMe, Ph,P(CH,),PPh,, o-C,H,[AsMe,], etc.). With PhzE(CH2)2(L-L)).79 EPh, (E = P, As), it is also possible to isolate the binuclear complexes {[RU(S~C~,),(CO)~]~ The trinuclear cluster anions [Ru,(SiR,R')(CO),(PR,),]- are prepared from Na[Ru,H(CO),,,(SiR,R),] and isolated as the m(PPh,),]+ salts. Structure (13) is proposed on the basis of NMR data.80 Finally, reaction of [Ru,(CO),,] with Ph PCH,SiMe,H affords the symmetrically bridged diruthenium compound (14) (Ru-Ru 3.056(1) Xj. Treatment of (14) with CF,CO,H gives the mononuclear phosphinomethyl-dimethylsilanol complex [Ru(PPh,CH, SiMe,OH)(CO), (CO,CF, j2Et,0] (15) in which the Et,O molecule is hydrogen-bonded to the silanol-0 with 0...O 2.64 A.
r
co R'R,Si, "I,,,,
L
CO
I
1-
co ,&*'
co
Rurhenium
45.3.2.3
287
Tertiary phosphine and arsine complexes
Silyl-ruthenium(rV) complexes [RuH,(SiR,)L,] (R, = various combinations of H, F, C1, Me, Ph, OMe, OEt; L = PPh,, AsPh,, P(C,H,Me-p),; n = 2, 3) are formed by reaction of an excess of HSiR, with [RuH,L,], [RuHClL,], [RuCl,L,] or [RuCl,(AsPh,),]. Complexes of the type [RuH,X(SiR,)L,] (X = C1, I) are obtained by reaction of the chlororuthenium(l1) complexes with HSiC1, or of [RuH,(Si(OEt),) (PPh,),] with CDC1, or I,. These dihydrides are unstable in solution in the absence of an excess of silane and revert to [RuHX(PPh,),]. In some cases, [Ru(SiR,),(PPh,),] are formed.82,83.84 These silyl-ruthenium complexes are very reactive. For example, incomplete €3-D exchange occurs between [RuH,(SiR,)L,] and D, at room temperature. Treatment with excess WC1 gives the free silane and [RuCl,(PPh,),], whereas with CO, [RuH,CO(PPhl),] is formed. Exchange of silyl groups occurs on reaction of [RuH,(SiR,)(PPh3),] (R, = (OEt),, C1,Me) with HSiF, whereas iodine gives [RuH,I(Si(OEt),)(PPh,),] and H,. Finally, extensive investigations of the ability of [RuCl, (PPh,),] to cataIyse the hydrosilation of a range of organic substrates have been made. (See references 1 and 85 for full details.) For details of other silyl-ruthenium compounds containing polyalkenes, azulenes and arenes, see reference 1, pp. 915-919.
45.3.3 Germanium-containing Ligands Although in comparison to Si much less work has been published on the reactions of germyl hydrides with [Ru~(CO),~], reaction with HGeMe, gives compounds [Ru(GeMe,)(CO),lz (7), [Ru(GeMe,),(CO),] (8) and [Ru(p-GeMe,)(GeMe,)(CO),], (cf. 11) (Scheme 1, M = Ge). In addition, pyrolysis of [Ru(GeMe,),(CO),] (8) a t 140°C gave [Ru@-GeMe,)(CO),], (16) and a small amount of [Ru,@-GeMe,), (CO),] (cf. 12).86A number of tetracdrbonyl complexes containing different Group IVB ligands can be made from the [Ru(GeMe,)(CO),]- anion which is obtained by reduction of [Ru(GeMe,)(CO),], with Na/Hg (see reference 1, p. 912 for details). X-Ray structural analysis of the cis and trans isomers of [Ru(GeCl,),(CO),] reveals a short Ru-Ge distance (2.48 A) indicative of the high E-bonding ability of the GeCl; anion.87 The compound [Ru(GeMe,), (CO), (PPh3)J (isomeric mixture) is formed from [Ru(GeMe,), (CO),] and PPh3.86 No germyl -ruthenium(IV) compounds of type [RuH,(GeR,)L,] have been reported although in one instance hydrogennylation of 1-hexene catalysed by [ R ~ c l , ( P P h , ) ~isl observed (equation 6)." Treatment of RuCl, with GeCl,Et, followed by EPh, is reported to give [RuCI,(GeEt,Cl)(EPh,),] (E = P, As, Sb). Other germyl-ruthenium compounds containing hydrocarbon moieties are discussed in reference 1, p. 915.
45.3.4 Tin-containing Ligands 45.3.4.1
Rutheniurn(11) halide complexes
It has long been known that interaction of tin(I1) chloride with solutions of platinum metal salts gives coloured species which have been utilized analytically but until recently, the nature of the
288
Ruthenium
species present in solution was uncertain. Reactions of SnCl, with hydrated RuCl, or RuO, in dilute hydrochloric acid give yellow or orange salts containing the [RuC~(S~CI,),]~anion,89previously described as [ R uC ~ , ( S ~ C ~ , ).90 , ] ~The - structure has been confirmed by X-ray analysis and the first example of "9Sn-99Ru coupling has been observed in the Il9Sn FT NMR spectrum of this anion?' The "'Sn Mossbauer spectrum of '[RuCl,(SnCl,),]-' is reportedg2Reaction of [RuCI(S~C~,),]~with L (L = thiourea or its substituted derivatives) is claimed to give Me,N[Ru(SnCl,),L,], [Ru(SnCl,), L4]and [ R ~ L , ] ( s nc l , ) , . ~ ~ Another route to the [RuCI(S~C~,),]~anion is from reactions between SnC1, and [RuCl,NO]'in HC1 solution; intermediates such as orange [RuCI, (SnC1,),N0l2 - and brownish-black [Ru, C1, (SnCl,), (N0),I4- were ~haracterized.9~ The [RuC1,(SnCl3).,I4- ion is reported in a Japanese patent" and further work has substantiated this claim.96 Reaction of 'RuCl3.xH2O' with SnF, in aqueous H F followed by-addition of M F (M = Na, K or NH,) gives M4[RU(SnF,),] which on treatment with HX in the presence of [Me,NJ+ gives [Me,N],[RuX(SnF3),] (X = C1, Br).9' The {RU(SnF3)6]4- anion is also formed by treatment of H,[RuCI(S~(OH)~),]with SnCl, in HF.98 The Il9Sn NMR spectrum of the corresponding in situ by reaction of 'RuC1,-xH20' with a ten-fold excess of SnC1,-2H,O in [ R u ( S~ C ~ , ) , ](made ~3 M HCl) has been p u b l i ~ h e d . ~ ~ , ~ ~
45.3.4.2 Ruthenium(II) carbonyl and carbonyl halide complexes As for silicon and germanium, reaction of [Ru,(CO),,] with HSnR, gives good yields of [RuISnR,),(CO),]; for R = Me, a small amount of [Ru(p-SnMe,)(SnMe,)(CO),], (cf. 11) is also produced;1aDthe structure of the latter has been Confirmed by X-ray analysis.'" Other ways of forming the [Ru(SnR,),(CO),] complexes are by treatment of [RuH,(CO),] with HSnMe, or SnPh,Cl, reaction of [Ru(MMe,),(CO),] (M = Si,Ge)66,86with HSnMe,, or by reaction of Me, SnCH,NMe, with [Ru,(CO),,].102Reaction of the [Ru(MMe,)(CO),]- anions (M = Si,Ge) with SnR,C1 gives the mixed group IVB ligand complex [Ru(MMe,)(SnR,)(CO),j and treatment of these with SnRiCl gives [Ru(SnR;)(SnR,)(CO)J. With SnCl, Me,, the anion [Ru(SiMe,)(CO),]- yields [R~(pSnMe,)(C0),1,.~~ The X-ray structure of [Ru(S~M~,)(CO)~], shows the linear Sn-Ru-Ru -Sn unit and eclipsed configuration of the CO groups (cf. 7; Scheme I).lo3 The reaction of SnC1, with [RU,(CO)~~] (xylene, 135°C) gives IRu2Cl3(SnCl,)(CO),] (17); a trinuclear derivative [Ru3(C0),2SnC1,jis formed at room temperature and is thought to contain a linear Ru, sequence. On heating this may form [RuCl(SnCI,)(CO),] which can dimerize with loss of SnCl, to form (17). Under CO (70atm) the product is trans-[Ru(SnCI,),(CO),] which does not isomerize to the cis form in s ~ l u t i o n . ' ~ * ' ~ ~
(17)
The 'RuCOX (X = Cl or Br) sohtions react with SnX, to form lemon-yellow [RuCl,and SnCl, in HCl solu(SnX,),(C0),]2-; the chloro complex is also formed from [RUCI,(CO)~~, tion.106.107.f~8 The related [RuCl(SnMe,)(CO),] is prepared by cleavage of one of the Ru-Sn bonds in [Ru(SnMe,),(CO),], using I, at - 5"C.74
45.3.4.3
Ruthenium(I1) complexes eonfaininggroup ZV, V or VI hmr ligands
Treatment of the 'RuCOCl' solutions with PPh, and SnCl, in acetone gives lemon-yellow crystals but shown by X-ray analysis to be initially formulated as [Ru,C1,(SnCl,)(CO),(PPh,),(Me,CO),]'06 the monomer (18). This is converted to the bridged derivative (19) by warming in benzene; both complexes readily eliminate SnCl,, the latter giving [ R U ~ C ~ , ( C O ) ~ ( P P(see ~ , )Section ~ ] ~ ~ 45.5.5.1).
PPh, (18)
289
Ruthenium
White truns-[Ru(SnCl,), (CNEt),] and C~S-[RUCI(S~C~,)(CNE~)~ (EPh,),] are formed by insertion of SnCl, into [RuCl,(CNEt),] or cis-[RuCl,(CNEt),(EPh,),] (E = P, As, Sb) respectively.''~"' , ) , ] treat'~ Similarly reaction of RuC1, with SnCl, followed by EPh, gives [ R U C ~ ( S ~ C ~ , ) ( E P ~and ment of trans-[RuHX(PHPh,),] or [RuCl,(DMSO),] with SnCl, gives trans[RuH(SnCl,)(PHPh,),]'" and [Ru(SnC1,),(DMSO),]"Z respectively. Reaction of the [RuCl,(SnX,),(C0),]2- anions with pyridine (X = C1, Br) gives [Ru(SnX,),(CO),(py),]; with SEt,, the cationic complex [Ru(SnCl,)(CO),(SEt,),]+ is formed.''* Finally, reaction of ~RuI(CO),(PPh,),]I with SnI, (1: 1 mole ratio) gives [RuI(CO)~(PP~,),]S~I,. The same compound is formed by oxidation of [Ru(CO),(PPh,),] with SnI,; in C,H, under reflux the latter reaction produces cis-[RuI,(CO),(PPh,),] and SnT,."
Lead-containing Ligands
45.3.5
The only reference to ruthenium-lead complexes is the preparation of cis-[Ru(PbMe,), (CO),] from reaction of [Ru(C0),I2- with PbClMe, (reference 1, p. 912).
45.4
NITROGEN LIGANDS
45.4.1 Ammines 45.4.1.1
Mononuclear complexes [Ru(NH3)J4
Rutheniuna(l1): [RU(NH,),]~+(20) is a diamagnetic orange complex which can be prepared by reaction of RuC1, with NH,OH in the presence of zinc dust (equations 7 and 8),"3-'15 by treatment by refluxof RuCl,, [RuCl, H2012-,[RuCI(NH,),]~+or [RuC1,I2- with hydrazine and NH, C1,'16,117 ing [Ru(NH,),N,]~+ with concentrated NH, s o l ~ t i o n "or ~ by reduction of [Ru(NH3),I3+ (see below). The single crystal X-ray structures of [RU(NH3),]X2(X = I-, Cl-) (20) have been reported118,119 and show octahedral geometry around ruthenium(I1) with Ru-N = 2.144 A for the Isalt."' The UV visible spectrum of (20) shows an absorption at 267nm,'16while its Mossbauer spectrum has also been reported.I2' The IR spectrum of [Ru(NH3),l2+has been a n a l y ~ e d , " ~ , ' ~ ' - ' ~ ~ and on the basis of deuteration shifts using highly concentrated nujol mulls the band predominantly due to the T I , (Ru-N) stretching vibration, vRRuN, has been reassigned to the absorption at 430~rn-I.'~~ 2RuC13
+
Zn
+
16NH3
-
~ [ R U ( N H ~ )+~ ][Zn(NH3)4]C12 C~~
(7)
(20)
[RU(NH,)~]C~> + [Zn(NH,),]CI,
+ 4HC1
+
[Ru(NH,),][znC&]
+ 4NH,Cl
(8)
120)
[Ru(NH,),I2+ is a good reducing agent (E" = -0.214V vs. SHE for [ R u @ H ~ ) ~ ] ~couple),'24 +'~+ and has been studied in relation to outer-sphere electron transfer reactions."s"2"'26 The self exchange between [Ru(NH,),I2+ and [Ru(NH3)J3+ (equation 9) has a rate constant k = 3.2 x 103M-'s-' in CF,S0,H,127 with A H # = 43.1 l.OkJmol-' and A S # = - 11 f 3 J K-' mol-*.12' Reductive quenching'" of excited state [Ru(bipy),]'+* by [Ru(NH,),I2+ via an outer-sphere mechanism has been reported129(see Section 45.4.3.1). Oxidation of [Ru(NH,),]'+ by [Ru(NH3),pyl3+occurs with a rate constant, k = 7.2 x lo5M-' s-'.I3OElectron transfer reactions between [Ru(NH, I6l2+ and a series of cobalt(II1) complexes have been s t ~ d i e d . ' ~ ' The - ' ~ ~one electron reduction of [CO(L)(OW,),]~+ (L = tetra-4-N-rnethylpyridylporphine) (equation 10) occurs = 8.9 x lo5) higher than that observed for the at a rate ( k k Z= 1.2 x 105M-'s-' at 15.5T; reduction of [CO(NH,),(OH,)]~+and [CO(NH,),]~+ This is consistent with the values for the Co"'/Cs" couples, with the rate of reaction decreasing with increasing P H . ' The ~ ~ reduction of .1353136
Ruthenium
290
[CoX(NH,),I2+ (X = NCS-, SCN , N;, F-, C1-, Br , I-, OAc-, RCO;)124,'26,131,113,13&141 [CoX(NH,),I3+ (X = H20,14, nitri1e,l4-'glycine, alanine144)and [Co(en),(L-L)]"+ (L-L = thiolate, thioether, alkoxy, ~arboxylate)'~' by [Ru(NH,),I2+ has been studied kinetically, the specific rates for reduction of the nitrile complexes being increased due to electron withdrawal by the C=.N For the glycine and alanine complexes, no pH dependence was observed,14 the electron transfer being thought not to occur via coordination of carboxylate to rutheni~m.'",'~'Kinetic studies have also been carried out on the reduction of [Fe(L)(OH,),]'+ (L = tetra-4-N-methylpyridyl porphine),14, [Fe(edta)]- ,I4' cytochrome aa3,la cytochrome c(III),I4' platin~m(IV)''~and copper(II)/ (TTT)'24*15' complexes and perchlorate salts.'24The reduction of Fe'" is found to be faster than that of [Fe0Hl2+.I2' The importance of non-adidbaticity in the above outer-space mechanisms has been discussed. 129~152 [Ru(NH3)6]2+ 4- [Ru(NH,),I3+
-
+
[Ru(NH~)~]*+ [Co"' (L)(OH,)2j5C
[Ru(NH,)J3+
+ [Ru(NH3)6I2'
[Ru(NH3)J3+
+
[CO"(L)(OH~),]~+
(9) (10)
Under protic conditions [Ru(NH,),I2+ is oxidized by 0, to give [Ru(NH3),I3+and H,O, (equaThe tion l l),Is3 the reduction of 0, being lo4 faster than the corresponding reduction of H202.153 reduction of H,O, to H,O by [Ru(NH,),j*+ shows first order dependence on [Ru"] and saturation dependence on [H,0,].154 A mechanism involving addition of H,O, to a distorted, activated six-coordinate complex *[Ru(NH,),]'+ to give a seven-coordinate intermediate (21) (equation 12) has been po~tulated."~ The photooxidation of [Ru(NH,),]'+ at a ferrocene derivatized photoanode The aquation of [RU(NH,),]~+is found to be [H J catalysed with rate constant has been rep~rted.'~' k = 1.24 x 10-'M-'s-' at 25°C,'56 an interaction of H'+ and Ru" n-d electron density being proposed to labilize the Ru-N bond.'"' The hydroformylation of ethylene is catalysed by [Ru(NH3),l2+ on zeolites; the active species is thought to be [Ru(OH)(NH,),I2+
--_4 0 2 1 dt
- k[[Ru"(NH3),]2'][0,]
[Ru(NH3)6I2'
e
*[RU(N&)6I2'
k
1.26
=
H202
X
lo2
M-lS-'
at 25"C, /i = 1
-
(1 1)
W ( N H 3 )612+
[ R u ( N H , ) ~ ( H ~ O ~ ) ] ~ +2H + (21)
2[RU(NH,)6]3f
+ 2H2O
(12)
Rutheniurn(1II): [Ru(NH,)J3+ (22) is colourless and is prepared usually by the oxidation of [Ru(NH7)J2+.I1' Oxidizing agents that have been used include hydrazine hydrochloride,'I6 Na,[PtCl,],"' N ~ [ A u C ~ ~acidified ] , " ~ NO; salts,ls8Br, in the presence of NaBr'I3 and Br, ~ s t p o u r . ' ~ ~ Heating of [RUCI(NH,)~]C~~ with hydrazine hydrochloride,"" or treatment of [Ru(NH,)~(OH~)]'+ with hydrazoic acid',' also gives [Ru(NH,),j3+. The latter reaction is proposed to occur via a nitrene intermediate (23) (equations 13-15).'6L [Ru(NH3)6]3+shows an absorption band at 276 nm,116.'62 while X -+ Ru charge transfer bands for [Ru(NH,),]X3 (X = C1-, Br-, I-) have also been assigned;'" for X = I- and Br-, two maxima are observed due to 2P3:2-2Pl:2 doublet splitting of the for the complexes [Ru(NH,),]X, (X = C1-, halide ion.156The Ru-N stretching vibration v,,-, Br-, I-, BF;) occurs in the IR in the region 4 8 5 5 4 2 4 ~ m - ' . ~The ' ~ single crystal X-ray structures of [Ru(NH,),]X, (X = BF;,Ii8 C1-Il9) confirm an octahedral geometry for ruthenium(II1) with Ru-N = 2.104 A for the BF; salt."' Magnetic susceptibility measurements in the range 8&300 K show [Ru(NH,),]X, to have a low spin d 5configuration with pc,,= 1.90 BM (87 K), 2.17 BM (292 K) for the BrAqueous solutions of [Ru(NH,),I3 turn yellow on addition of NaOH with a new absorption at 400nm due to the formation of [RU(NH,)(NH,),]~+(24) (equation 16)'" [Ru(NH,)(NH,),]~' is believed to play an important role in the base catalysed proton exchange reactions of [Ru(NH,),I3+ in H20.1MWhen [Ru(NH,),I3+ is dissolved in HPOi- /NaOH buffer (pH 12), however, no yellow solution is obtained; in addition absorptions for [Ru(NH,)(NH,),I2+ were found to show marked dependence on anion^.'^' Subsequently the optical spectra of basic solutions of [Ru(NH,),I3+ were analysed assuming the presence of a hydroxide ion pair and [Ru(NH,)(NH,)J2+, the formation constants of which were 5.4 M-' and 1.8 M - ' respectively.166 The aquation of [Ru(NH,),I3+ proceeds via two parallel reaction pathways: (a) pH independent, (b) base catalysed, pK, of [RU(NH3)6I3+= 13.1.16, [Ru(NH,)~]~+ ,like [Ru(NH,),]'+, participates in outer-sphere electron transfer reactions, a range of such reactions involving the reduction of [Ru(NH3),I3+ having been reported. [Ru(NH,)~]~+ quenches the excited state [Ru(bipy),I2+* and has been suggested as a possible oxidant in related [V(H,O),]*+ redox systems.'67 The reduction of [Ru(NH3)J3+ by C?+,'*' ~(bipy)3]2c,'6s (AHf =2.5kJmol-I, A S z = - 4 2 J K - ' m o l ^ ' at 298K'69 and U"' ( A H z = l.OkJmol-',
A S # = - 39 J K ' mol -1)170q171 has been studied. No acid dependence was observed in the oxidation of Np'" by [Ru(NH,),]~+,'~* while the second order reduction by Ti"' was found to be inversely proportional to [Hf], suggesting that Ti(OH)'+ was the active reductant.173[Ru(NH3),l3+forms a range of mixed valent salts with [Ru(CN),I4-, [Fe(CN),I4- 5g~i74and [Fe(CN),LI3- (L = CO, DMSO, pz, py, imid)'" which show outer-sphere intervalence charge transfer bands in the visible-near IR region. A plot of optical electron transfer energy versus difference in redox potential of the complexes increases Iinearly; these results have been d i s c u ~ s e din ~ ~terms ~ ' ~ ~of Hush t h e ~ r y . ' ~ ~ . ' ~ ~ Photolysis of [Ru(NK3),l3+ at 185 nm leads to substantial photoreduction in the presence of 2-propano1, methanol or ethanol which act as radical scavengers of the excited state [Ru(NH3),I3+*.177{ R u ( N H ~ ) ~has ] ~ +been r e d u d by and by organic radicals via outer-sphere mechanisms, the order of reactivity being CH,OB < CH,cHOH < (CH3)2cOH< CH3COHCO; < CH3COC(O- )CH3;178*'79 in general Rc(OH)CO,H was found to be less reactive than Rc(OH)CO, .IKoThe low yield UV photoaquation of [Ru(NH3)J3+ has been reported.IM'The adsorption-desorption of [Ru(NH3),I3+on pyrolytic graphite electrodes coated with Nafion 125 and on clay-modified electrodes has been described.lS2ls3 Rutheniurn(IV) and ( V ) : Oxidation of [Ru(NH3)J3+by 6 H gives [Ru(NH,)~]~+ which rapidly decays to [RU(NH,),]~+and [Ru(NH,),I3+ with a rate constant k = 4.5 x 109M-1s-'.'78
45.4.1.2
Mononuclear complexes [RuL, (NH,),-,]"+
Ruthenium ammine-phosphine and -selenium complexes are discussed in Sections 45.5.4.6 and 45.8 respectively.
(i)
Carbon donor ligand complexes
(a) Cyanides and related ligands Cyano-ammine complexes are also discussed in Section 45.3.1.2.ii [Ru(NH,), OH2]'+ reacts with isocyanide L in ethanol to give [RU(L)(NH~)~]'+, and on further reaction, the bis adduct trans[Ru(L), (NH3),]'+ (L = benzyl isocyanide, cyclohexyl i~ocyanide).'~~" Aquation of trans[Ru(CNC, H,),(NH3)4]2+yields ~~~~S-[RU(CNC-,H,)(NH~)~(OH~)]~~ .I8& Measurements on the rate of aquation of [RU(CNC,H,)(NH,)~]~+ to trans-[Ru(CNC7H7)(NH,),(OH,)1"+ indicate that isocyanide labilizes the frons NH, by a factor of forty compared with the aquation of [Ru(NH3),I2+ is also enhanced by isocyanide, Likewise the rate of aquation of trans-[R~(CNC,H,)(NH,),(isn)]~+ values for the above a low affjnityof ruthenium(I1) for isn being observed in these systems."& complexes have been measured, the electrolysis of [Ru(CNC7H7)(NH,)J2+ at 0.79V vs. NHE yielding the ruthenium(II1) species [Ru(CNC, H7)(NH3)J3+ The solid state thermal reactions of [Ru(NH,), LIZ+(L = MeNC, CO) have been analysed and compared with the reactivity of analogous complexes of N,, NCMe and NO.184b (b) Carbonyls and alkenes Carbonyl and alkene complexes of ruthenium have been previously reviewed.' The syntheses of [Ru(CO)(NH,),]'+ are summarized in Scheme 3. The dependence of vcE0 upon counterion and upon solvent bas been discussed,'8* The Mossbauer spectrum of [Ru(CO)(NH,),]'+ has been r e p ~ r t e d . ~ ' "The ' ~ ~replacement of H,O by isn in trans-[R~(L)(NH,)~(OH,)32fhas been assessed, the lability increasing in the order L = CO = N2< isn < py < imid (N bound) N NH,< OH- < CN< SO:- < imid (C bound)*189 For 71 acid ligands good correlation between specific rate and E,,, values for the ruthenium(II)/(III) couple was observed.'8g Treatment of [RuCI(NH,),]~+ with Ag+ followed by reduction by zinc amalgam in aqueous
[RuX*CO(NH3)31
cb-1K u C I ~ ( N H :I)CI )~
rmi5-
[ Ru(I*~CS)CO(NH~)~ 1' c
i , Zn/Hg, H', CO;"' ii, CO, H 2 S 0 4 . 48 h;I8' iii, NIIJ ;Is9 iv, CO, HCO'H, 70 "C, 3 h;"' v, NaNCS;'59 vi, Zn/Hg, H f , CO;'j9 vii. H', H2O;Is9 viii, NH3;'59 ix, HX:'Sg x, CO, HC1, 80 "C, 16 h;Is6 xi. H2. 8O"C, 5 h;lS6 xii, NH,OH, 25 "C, 48 h;lS6 xiii. CO, NHzNHz;187a xiv, (a) CO, EtOH, (b) NHzNHz;187a xv, CO?, Z I I / H ~ ; " ~ ~ Scheme 3
solution and addition of L yields [Ru(L)(NH,),]~+(L = fumaric acid, ethylene, isobutene, 1,4-cyclohexadiene, acetylene, phenylaoetylene, 3-hexyne).IgnThe complex of 3-hexyne has also been prepared by addition to [Ru(NH,),(acetone)12+.I9' The complexes show d -,n* charge transfer bands in the range 200-300nm while the C=C and C s C stretching vibrations v,,, vczc are lowered in energy by 100-200cm-' on complexation. The single crystal X-ray structure of the fumaric acid adduct (25) shows the alkene to be carbon v2 bound to the metal centre, C=C = 1.413 A, Ru-C = 2.172 A, Ru-N = 2.143-2.154 A, with the axial NH, ligands bending away from the equatorially bound alkene.'" The stabilization of ruthenium(I1) in these systems (E1,*= 0.54V vs. NHE for [Ru"(3-he~yne)(NH,),]~+)is ascribed to back-bonding from metal to alkene.'"
(25)
Carbon bonded imidazole derivatives are discussed in Section 45.4.1.2.ii.b.
(ii) Nitrogen donor ligand compIexes ( a ) Amines
Treatment of [RuCl(NH,),]'+ with Ag(O,CCF,), followed by zinc amalgam reduction and addition of amine yields [Ru(L)(IJH,)J2+ (L = cyclohexylamine, benzylamine, methyla~nine).'~~ Oxidation of these complexes with Br, produces the corresponding ruthenium(TT1) species [ R u ( L ) ( N H ~ ) ~ ] ~Subsequent +.~~' oxidation of the amine ligand can readily occur to give imine and nitrile products, explaining the relatively few complexes of this type that have been isolated (see Section 45.4.2). (6) Monodentate heterocycles RutheniumfII): A range of complexes of the type [RU(L)(NH,),]~+(e.g. L = p y r i d a ~ i n e ,p' ~y~r a ~ i n e , ' ~ ~ .pyrimidine,lg3 '~~,'~' triazine,'932-cyanopyridine, 3-cyanopyridine, 4-cyanopyridine,I9*2-, 3- and 4-RC(O)-substituted pyridines (R = CHMe,, CH,Et, CH2Pr',19'OMe, NH,, 1,2-bi~(4-pyridyl)ethylene,~~~ hypOH,194 H,Ig3 PhI9'), N-rnethylpyra~ine,'~~ 4,4'-bi~yridine,'~~ oxanthine,2mguanine,"' xanthine,2M4-dimethylamin0pyridine'~~) have been prepared by reaction of [RuCI(NH,),]Cl, with a Ag+ salt (AgO,CCF, is often used) followed by zinc amalgam reduction
Ruthenium
293
and addition of ligand L. Alternatively L may be added directly to [Ru(NH,),(OH,)]~+. Reduction of [RuL(NH,)J3+ chemically by H,S203 or electr~chernically~~ has been used to generate [RuL(NH3),I2+ (L = pyridine, 4-aminopyridine). The single crystal X-ray structure of [Ru(NH3),pzI2+ (26) shows octahedral ruthenium(I1) with Ru-N(NH,) = 2.15-2.17 A, and a shortened Ru-N(pz) = 2.006A due to dn(Ru) + *nlpz) back-bonding.,: The plane of the pyrazine ring intersects the equatorial plane containing N-1, N-2, N-3 and N-4 at 45" thus minimizing steric effects.
(26)
The electronic spectra of these complexes show solvent dependent Ru"
4
L charge transfer
band^'^^,'^^ which have been used to assess the degree of metal + ligand back-bonding.,06 The
Ru"/Ru"' redox couples for the complexes have been correlated with the energies of the Ru" + L charge transfer bands,lg3 e.g. for L = py, A = 407nm ( E = 7700 M-' an-'),Eii2= + 0.38 V vs. SSCE.'" Mossbauer studies of [Ru(NH,), LI2l (L= pyridine, pyrazine) have been reported.12' The degree of back-donation from the ruthenium(I1) centre has been probed by 'Kand 13CNMR for the complexes IRU(L)(NH~)~]"(L = pyridine, 4-methyl pyridine, 4-benzoyl pyridine, pyrazine, The shifts in the I3C NMR spectra for the 3- and 4-positions were found to methyl pyra~ine).~"~'~ be consistent with back-bonding to the ligand from the metal In addition, the CFN stretching vibration vCCN for 4-cyanopyridine shifts to low energy on coordination to the [Ru(NH,),]'+ moiety.'% [Ru(NH3),LI2+(27)(L = 4aminopyridine) shows a Ru + L charge-transfer band at 360 nm. At Iow pH this absorption decreases in intensity due to protonation of the free amine (equation 171," suggesting that the lone pair on the uncoordinated NH, influences the M -+ L charge transfer process. On protonation of (27), LH+ becomes a better n acceptor shifting to longer wavelength?" On coordination of pyrazine, 4,4'-bipyridine and 1,2-bis(4'-pyridyl)ethylene to [Ru(NH,),I2+, the basicity of the free N site is increased (pK, for complexes = 2.6,4.4, 5.0 respecti~ely).'~~ This effect has been discussed in relation to stronger metal + ligand back-donation in the protonated complexes'93and in terms of the degree of delocalization of the protonated species.19' The enthalpy of protonation of [ R U ( N H , ) , ~ ~is ] ~found + to be - 20.5 kJmol-'.208This work has been further extended by the synthesis of complexes [Ru(NH,),LHI3+ incorporating potentially bidentate ligands (L = e.g., 1,1'-bis(4-pyridyl)amine).*oy The rate of reduction of pyrazine N-oxide was found not to be increased on coordination to the [Ru(NH3)J2+moiety in spite of the N-0 stretching vibration vNp0 decreasing from 1313cm-l in the free oxide to 1258 cm-' in the The ~ynthesis,'~~ electronic, IR and resonance Raman spectra of [Ru(L)(NH3),I3+ (L = l-methyI-4,4-bipyridine) have been published?'l The pseudoexchange rate for the [Ru(NH,),PY]~+'~+ couple has been determined as k = 4.7 x 1O'M-l s-' at 25°C.212The reduction of cytochrome c,213,214 r n y o g l ~ b i n , ~Co"' ' ~ ~ and '~ CuIl/I1I complexes by [Ru(L)(NH3),I2+(L = histidine, pyridine, pyrazine, 4,4'-bipyridine) has been of 0, by [Ru(NH3),(isn)12+,and cis- and trms-[Ru(NH,),(isn)r e p ~ r t e d . ' ~ ' , ~The ' ~ , ~reduction '~ (OH,)]" occurs via outer-sphere formation of O2 with rate constants k = 1 .OS x lo-', 1.38 x IO-' and 3.03 x 1W2M-'s-' respectively (equation 1 8).220Ruthenium(II1) and decreasing acidity were found to inhibit the reduction due to competition between the reaction of 0; and Ru"' and protonation of 0; followed by reaction of Ru" (equations 19-22)."' [Ru(NH,), ( i ~ n ) ] ~is+oxidized by HO, with a rate constant k = 9.07 x 106M-'s-' via an outer-sphere mechanism that shows anomalous behaviour with regard to Marcus The mechanism and kinetics of the reduction of 0, in the presence of C1- has been discussed.22' Reduction of H 2 0 2by [RuL(NH3),I2+(L = 1methylimidazole) is first order in [Ru"].'"
Ruthenium
294
-d[Ru"] dt
=
0; + H'
+
HOz
Ru"
+
Ru" H+
+
2k[Ru"][02]
e H02 Ru"'
+
H02
-+
+ Ru"'
(20)
HOT
(21)
+
I221
HIOz
[ R u ( N H , ) ~ ~ aquates ~ ] ~ + at a slower rate than [Ru(NH,),]*+ , while electron withdrawing groups on coordinated pyridine slow the aquation still f ~ r t h e r . 'The ~ ~ kinetics , ~ ~ ~ of aquation of protonated (27) have been studied to give a rate law (equation 23) with rate constant kl = 4.4 x 10 ' s The photoactivation of the relatively non-labile [Ru(NH,),py]*+ has been reported. Photosubstitution of [Ru(NH,),py]*+ is found to be wavelength dependent and often leads to the formation of several p r o d ~ c t s (Scheme * ~ ~ ~ ~4)~(4 ~ the photochemistry of Ru(bipy):+ which shows photoluminescence and is relatively photosubstitution inert,225.2z6 see Section 45.4.3.1.vi). The quantum yield for the aquation of [Ru(NH3),py12+on irradiation at the Ru + L charge transfer band was found to be pH dependent suggesting competitive acid-catalysed and acid independent reaction pathways.*" A proposed mechanism for photoaquation of [ R U ( N H , ) , ~ ~is]given ~ ~ in Scheme 5.'" Studies on charge transfer excited species [RuL(NH,),I2+* showed evidence for ligand field excited states, and indicated that the metal + ligand charge transfer excited state (MLCT) was in fact substitutionally is therefore the excited state responsible for photosubstitution in [€t~(NH~),py]~+ believed to be ligand field (LF) in character.***Thus, if the LF state is of lower energy relative to the MLCT state, photolability and substitution would be expected to readily occur, whereas for systems where the MLCT state is of lowest energy, photostability would be prevalent.228The role of solvent in the relative ordering of these levels has been intimated.228Flash photolysis experiments on a series of complexes [RuL(NH,),J2+(L = substituted pyridines, aromatic heterocycles) show the formation of long-lived transients for those complexes that are photosubstitution inert.*>' These intermediates have been proposed to involve a pyridine ligand n-bonded to Ru" with an uncoordinated N which can be reversibly protonated with aquation of the complex.229UV irradiation of [Ru(NH,),py]'+ in aqueous NaCl solution yields initially [Ru(NH, (OH,)]*+ which is oxidized to [RuCI(NH,),f2+ The photolysis of the pyridyl complex (28) at 313 nm leads to a novel carboncarbon bond cleavage reaction on the ligand site (equation 24).lw
>,
-d
[Ru'L] - ~,K,[H+][Ru"L] dt
-
1
+ K,[H+l
(23)
295
Ruthenium
The rate of aquation of [R~(NH,),(irnid)]~+ was found to be 500 times faster than for the pyridyl derivative. A series of substituted imidazole adducts was therefore prepared and studied by 'H NMR. This clearly showed an intramolecular arrangement of N-bound to C-bound imidazole The single crystal X-ray structure of (29) confirms2j2the formation of a Ru-C (Scheme 6).23'-234 bond (2.218A), with CO trans to imidazole (R'= R = Me). In general, the N-bound imidazole complexes are unstable below pH 2 due to facile aquation via highly labilizing*@C-bound interm e d i a t e ~The . ~ ~electrochemistry ~ and reactions with H,O, and 0, of N- and C-bound imidazole complexes have been reported.235C-bound xanthine derivatives have also been p r e ~ a r e d . ~The '~.~~~ binding of [Ru(NH3)J2+ to double helical and single stranded DNA has bean reported to occur at N-7 of g ~ a n i n e . ~ [RU(NH,),]~+ ~'*'~~ has been anchored to graphite electrodes using polyvinylpyridine and p~lyacrylonitriie.~~~ The rates of substitution of H,O in cis- and trans-[RuL(NH,),(OHz)]z+ have been measured, and the effect of L on the lability of H,O assessedlS9(see Section 45.4.1.2.i.b). Equilibrium constants for equation (25) have been measured239for a range of ligands L (Table 4).
12+
Z+
iij lii
r
i. CO;
12+
ii.
U Z CI-; r iii, Zn/Ilg, HzO Scheme 6
Complexes of the type trans-[R~L,(NH,),]~~ can be prepared by treatment of [RuCl,(NH,),]+ with Ag(O,CCF,), reduction by zinc amalgam and addition of L (L = pyridine, pyrazine, 3- and 4-RCO pyridine; R = OMe, NH,, OH).194.2M Reaction of cis- or trans-[R~Cl,(NH,),]~with pyHCl and zinc amalgam gives cis- or trans-[Ru(NH, )4py2]2+,241 while reduction of [Ru(NH3),py2'J3+by H2S also yields [ R U ( N H ~ ) , ~ ~ , A ] ~ series ~ . ~ ' ~of mixed ligand complexes of type trans[RuLL'(NH,),I2+ (L = pyridine, L' = pyrazine, pyrazine H + , 4-acyl pyridine; L = pyrazine, L' = pyrazine, pyrazine H+;z40.z4z L- pyridine, 3-chloropyridine, 3,5-dichloropyridine, L = 4cyano-N-methyl pyridinium, N-methyl-4,4'-bipyridine'96)have been prepared by treatment of trans-[Ru(SO,)L(NH,),]' with zinc amalgam and L'. The basicity of these complexes was found to decrease in the order [ R U ( N H ~ ) ~ ~>Ztruns-[Ru(NH,),(py)(pz)]*+ ]~+ > truns[Ru(NH3),(pz)J2' > pz > truns-[Ru(pzH)(NH3),pzl3+> [Ru(NH3),pzI3+.2w The visible spectrum of some of the mixed ligand complexes shows two metal + ligand charge transfer absorption bands,',' these being found to be highly solvent dependent.196The single crystal X-ray structure of cis-[Ru(NH,),(isn),]'+ shows a net decrease in the Ru"-N(isn) distance relative to Rd"-N(isn) in ~is-[Ru(NH?),(isn),]~+?~~ this is attributed to metal -+ isn d + 3t* back-bonding in the ruthenium(I1) Reaction of [ R U ( N H ~ ) , N ~with ] ~ + py yields [Ru(NH,),py,I2+ and [ R U ( N H ~ ) ~ ~ ~ ~ ] ~ +
296
Ruthenium
while photolysis of trans-[RuLL'(NH3),I2+ in the visible region leads to photoaquation of N H ~ . ~ ~ . ~ ~ ~ and [ R u ( L ) , ( N H ~ ) ~ ] ~have + / ~ +been The redox couples for a series of complexes [RUL(NH,),]~+'~+ measured and correlated with the electronic properties of L.246 RutheniumfZII): [RuL(NH3),l3+ and [Ru(L),(NH,),]~+ (L = pyridine,'94 N-methylpyrazine+ 3- and 4-RCO pyridine; R = OMe, NH,, OH'") have been prepared by Ag' or Ce'" oxidation of the corresponding ruthenium(I1) complexes. The preparation of [RuL(NH3),l3+by air oxidation of [RuL(NH,),]*+ (L = imidazole, 1-methylimidazole, 4-methylimidazole, 3,5-dimethylimidazole, benzimidazole) has also been a c h i e ~ e d , ~with ~ ' . the ~ ~ formation ~ of [R~Cl(NH,),(imid)]~+ as a side product.23'Reaction of cis-[Ru(NH,),(OH,),]'+ with isn in HCl yields [RuCI(NH,),~S~]~* :22 while cis- or trans-[RuCl(NH,),SO,]+ with L in aqueous solution gives cis- or trrans-[R~(SO,)(L)(NH~)~3' (L = pyridine, isonicotinamide,222 3-chloropyridine, 3,5-di~hloropyridine,~~~ substituted imi d a z o l ~ ~ ~which ' , ~ ~reacts ~ ) with HX to form [RuX(L)(NH,)J2+ (X = C1-, Br-, T-).222.239 The E1,2 values for the Run'/'' couples in these complexes are influenced by X rather than L, while the energy of the Ru"' + L charge transfer is also markedly dependent on X.='The electronic and vibrational spectra of [RuL(NH,),]Cl, (L = 4-dimethylaminopyridine) have been analysed;211For L = 4aminopyridine protonation of the complex leads to loss of the L + M charge transfer band at 505 nm indicating lone pair involvement in the CT process.2@' The single crystal X-ray structure of [Ru(NH,),pzI3+ prepared by PbO, oxidation of [Ru(NH3),pz]'+ ,Iy7 shows a longer Ru"'-N(pz) distance (2.076 A) and slightly shorter Ru"'-N(NH,) bonds (2.10-2.13 A) as compared with the ruthenium(I1) analogue which has bond lengths of 2.006 A and 2.15-2. I7 8, for those linkage^.^" Similar lengthening of the Ru"'-N(isn) + basic than the distances is aho observed for ~is-[Ru(NH,),(isn)~]'+.243 [ R U ( N H ~ ) ~ P Zis] ~less for [RuL(NH3)J3+(L = Cruthenium(I1) analogue,240a similar decrease in pK, being bound, N-bound imidazole, pK, for coordinated imidazole = 11.OO, 7.06 respectively) compared with free imidazole (pK, = 14.2). The synthesis of the protonated complex [Ru(LH)(NH,),I4+ (L = 1 ,l'-bis(4-pyridyl)amine) by C1, oxidation of [Ru(LH)(NH,),]~+in HC1 has been a~hieved."~ The interaction of ruthenium(II1) with biologically active ligands has been studied in complexes [RuL(NH,),f3+ (L = g ~ a n i n e , ~xanthine ~ ' . ~ ~ ~derivatives?02 hypoxanthine,200cytidine, adenosine histidine of ribonuclease A*''). These products show low energy L + M t~bercidin?~' charge transfer bands, the cytidine and adenosine adducts dissociating or isomerizing on reduction to r u t h e n i ~ r n ( I I ) .The ~ ~ ~single crystal X-ray structures of [RuL(NH,),]Cl, (L = hypoxanthine, 7-methylhypoxanthine) have been determined.257Hypoxanthine is bound through N-7 (Ru-N-7 = 2.087 A) (M), while for 7-methylhypoxanthine, N-3 to N-9 linkage isomerization can O C C U ~ ~ ~ * ~ ~ ' the crystal structure of (31) showing Ru"' bound to N-9 (Ru-N-9 = 2.094A).251The X-ray structure of [Ru"'L(NH,),]~~(L = 1-methylcytosine ) (32)shows a short Ru-N(cytosine) bond (1.983 A), with hydrogen bonding between a ring N atom of cytosine and two NH, ligands.2S2 +
3+
KH3 H3N%*,
I **'NH3
RU"
H,N@]
bNH '
Y
0
The outer-sphere reduction of [ R U ( N H , ) , ~ ~ ] ~with ' [Fe(CN),I4- and [RU(NH,),]~+ (k = 4.3 x lo6, 7.2 x lo5M-'s-' respectively) has been studied.',' [RuL(NH,),I3+ (L = pyridine, substituted pyridine) forms ion pairs with [M"(CN),14- (M = Fe, Ru, Os) which show in concentrated solutions outer-sphere charge transfer bands in the near IR (A = 915nm for [Ru(NH,),py],[Fe(CN),],), interactions between w(CN)6]4- and an ammine face or with the pyridine ligand being p o s t ~ l a t e d . ~The ~ ~ reduction ' ~ ~ . ~ ~ of ~ [ R u ( N H , ) , ~ s ~ ]by ~ +Cr"' has been noted to be 2 x lo4 faster than Cr" reduction of C O ' " ; ' ~the ~ formation of a binuclear Ru"'-Cr'' intermediate with Cr" bound to the amide or carbonyl of isonicotinamide was proposedLw(see Section 45.4.1.3). Kinetic 1 and [Co(edta)l2-*'' studies on the reduction of [ R U ( N H , ) ~ P ~ ]by ~ + Cr'1,253V" ,253 T""254 ( k = 3.4 x lop3, 1.2 x lo5, 4.2 x lo3, 32M-Is-' respectively), [Ru(NH3),pzI3+ and cis[Ru(NH,j,(i~n),]~+by Ti"' ( k = 1.5 x lo5, 1 x 106M-'s-' re~pectively)~'~ and [RuL(NH3)*I3+ (L = pz, 4,4'-bipy) by [Co(edta)]'- ( k = 77 M-' S K I for 4,4'-bipy c ~ m p l e x ) ~have ' ~ . ~been ~ ~ reported
297
Ruthenium
and the mechanisms discussed, The oxidation of horseheart ferrocytochrome and related species by [Ru(NH,),LI3+ at low pH has been studied.2i~2'7~2s6~257 0; reduces [Ru(NH,)&~]~+ with a rate constant k = 2.18 x lo8M-'s-'.,,' The efficient hydrolysis ofp-nitrophenylacetate in the presence of [Ru(imid-)(NH,),]2+,258 and the reduction of [RuL(NH,),I3+ (L = py, isn) and [Ru(NH3),(isn),I3+ by Cu' have been reported.259Outer-sphere ligand to metal charge transfer absorption bands have been observed in the electronic spectra of [Ru(NH3),pyl3+with halide ions.260 Ruthenium (IV): [Ru(NH,),pyI3+ disproportionates at high pH to give [Ru(NH3),pyI2+and [RU(NH,),PY]~+ .26i Formation of Ru" is second order in [Ru"'], with [ R u ( N H , ) , ~ ~ ] being ~+ reformed from [Ru(NH,),pyl4+ at pH 8-8.5?6' ( e ) Bidentate heterocycles Ruthenium (11): (See also Section 45.4.3.1.ix). Reaction of [Ru(NH,),(OH,),]~+, [Ru(NH7)5(OH2)]2+or [Ru(NH,),(acetone)12+ with L-L gives complexes [Ru(L-L)(NH,),]*+, while [Ru(L-L), (NH,),]" may be prepared by treatment of [Ru(L-L),Cl,], or preferably [Ru(LL)2(OH2)2]2+ with NH, (L-L = 2,2'-bipyridine, 1,lO-phenanthroline, 2,2'-bipyrimidine, or related heterocycles.196.20332-267 The ruthenium(I1) complexes show intense metal + ligand charge transfer bands (not present in ruthenium(II1) products)267and a related one electron Ru"!'" redox couple. ~~~,~~~~ The electrochemistry of[Ru(bipy),(NH,),I2+ ( E , / 2= + 1.27V vs. SCE) has been r e p ~ r t e d ,and a spatially isolated redox orbital model proposed for this and related systems.268b Aerial oxidation of [RU(L-L)(NH,),]~+ (L-L = 2-aminomethylpyridine) yields the corresponding 2-pyridinalimine ' H NMR of [Ru(L-L), (NH3),I2+(L-L = 4,4-dimethyl-2,2'-bipyridine)indicates that the .~~~ rates have been measured'27for complex is in the sterically preferred cis c on f ~ gu r a t i o nExchange [Ru(bipy)(NH,),]3"'2+ (k = 7.7 x 10' in CF,SO,H, 2.2 x 106M-' s e i in HClO,, AH* = 16.7 kJmol-', A S # = - 100JK-Imol-'), and for [R~(bipy),(NH,)~]~+'~+ (k = 8.4 x 107M-'s-' in HC104). Reduction of 0, by [Ru(phen)(NH3),I2+ occurs with a rate constant k = 7.73 x 10 - 3 M-'s 1:20 while exchange reactions with [Co(0H2)J3+have also been reported.270 Ruthenium (IlI): {RU(L-L),(NH,),]~+ and [Ru(L-L)(NH,),]" may be prepared by chemical or electrochemical oxidation of ruthenium(I1) analogues and are found to be strong oxidizing agents. Reduction of [Ru(terpy)(bipy)(NH3)I3+,[Ru(bipy),(NH,),I3+, [Ru(terpy)(NH,),I3+ and [Ru(bipy) (NH3),I3+ with [Fe(OH,),I2+ has been reported and related to Marcus t h e ~ r y . ~Oxidation " of [(NH,),Ru],bipym4+ at 1.02V vs. NHE leads to binuclear cleavage and the formation of [Ru(bipym)(NH,),I3+ and [Ru(NH,),(OH,),]~+ The reduction of [Ru(bipy)(NH,),I3+ with Cu' has been reported.272 (d) Nitrosy Is Transition metal nitrosyl complexes have been the subject of several RutheniudZI): [Ru(NO)(NH,),I3+ was originally synthesized by treatment of [Ru(NH3),l3+with NO, in HC1.i58*278 It can also be prepared by aerial oxidation of [Ru(NH,),I3+ at pH 13 with (equaNaOH279(equation 26), by reaction of [Ru(NH3)J3+with NO under acidic conditions28w2E2 tion 27) or by direct substitution of H,O in [Ru(NH,),(OH,)]~+ by NO.158 The N-0 stretching ' single crystal X-ray structure28s285 vibration Q~ for [Ru(NO)(NH3),l3+occurs at 1908C T I - ' ; ~ ~ the of (33) shows Ru-N(N0) = 1.770, N-0 = 1.172, Ru-N(NH,) = 2.017-2.133A with a near linear nitrosyl ligand.**,Thus the binding of NO in [Ru(NO)(NH,),I3+ should be regarded as [Ru" -NO+] rather than [Ru"'-NO], consistent with the diamagnetism of the complex. Analysis of the electronic absorption spectra of related complexes rules out a magnetically coupled [Ru"'-NO] Mossbauer studies of [RuN0(NH3)J3+have been reported.27c,'"
+
0
ltt
N
+
Ruthenium
298
Whereas the aquation of [Ru(NH,)J3+ is very slow (several days), the reaction of [Ru(NH,),]X, with NO is relatively fast with rate constants k = 2.0 x IO-' for X = Cl-, I .9 x IO-' M-'s-' for X = Br- with first order dependence on B O ] and [ ( R U ( N H , ) , ) ~ ~ ] The . ' ~ ~ dependence of the Reaction of [RUB~(NH,),]~+ with NO gives [Ru(Nkinetics on pH has been 0)(NH3),l3+and [RuB~(NO)(NH,),]~'.158 The 'H NMR of [Ru(NO)(NH,),]~+shows increased H" exchange at the trans NH, group relative to [Ru(NH,),CO]*+. Schemes 7 and 8 summarize the reactivity of ruthenium ammine-nitrosyl complexes. The single crystal X-ray structures of [RuCl(NO)(NH,), J[Ru(NO,), (OH) (NO)] and [Ru(OH)(NO)(NH,),][Os(NO,),(OH)NO] *0.5H20 have been reported.296b
liii
i, 0.5 M[-OH] IV.
ii, [Ru(NO)(NH,),] 3';2ay iii. 5.OMI-OH1
:88
A;ass
Y. H';"'
;28u
vi, Ag+, NO, H20;2q0 vii, HX;2q"
Scheme 7
frQFIS-[Ru(OAc)(NO)(NH3), ] 2+
RuCI(NH3), 1 2+
\
tmns-[Ru(N,)(NO)(NH,),I2',
viii
, / '
'+
~ ~ Q H A - [ R u C ~ ( N O ) ( N H ~ xii ),~
xi"/
m [Ru(OH)(NO)(NH~)~I"
zn8.289.291 ii. NH 3 r,292 111, ... -OH, PhCHO catalyst;293 iv, NH3;2" ~,-OH;ZW
(a) CI, (b) NH40H reflux (c) O°C;1'4 vi, (a) Cl, (b) N&OH (c) HCl;114.286.29s N;. 75 "C, H 2 0 , H+;286 ix, NaOAc. HOAC. 9 0 ° C ; z s 6 ~ 2 9 s x, KNCO, 90°C;286 xi, (a) NH:, (b) CI2. NH3 ( c ) A I 80 "C fdl HC1;"4.286 xii, NH40H;1'4.2*6xiii, 6aOH. xiv, HCI04;296
V,
vii. Br-:286 viii,
Scheme 8
For arans-[Ru"X(NO)(NHS),l"+, the N-0 stretching vibration vNp0 has been found to be a good indicator of trans effects of X; yN decreases in the order NH, > NCO- > N; > - 0 A c > CI- > Br-.268 'H NMR studies of cis- and trans-[Ru(NO)(NH3)Q(OH2)]3+ in concentrated H,S04 show the complexes to be stable for approximately fifteen minute^."^ IR298and X-ray structural analyses283of trans-[Ru(OH)(N0)(NH3),]C1, have been reported and indicate binding of NO+ to Ru" (< RuNO = 173.8'). For [Ru(NO)(NH,),I3', E" = -0.12V vs. SCE at 23°C.299Radiolysis of [Ru(NO)(NH,),j3+ yields a one-electron reduction product [RU@O)(NH,),]~+(34)which rapidly adds organic radicals (equations 28 and 29).299Reduction of [Ru(NO)(NH,),l3+ in the presence of a range of alcohols, amines and carboxylic acids RH yields products [Ru(N(0)R)(NH,),j2' .3009301 [Ru(NO)(NH,), J2+ is proposed as an intermediate in the redox catalysed aquation of [Ru(NO)(NH,),I3+ to trans[Ru(OH)(NO)(NH,),]~' which occurs with rate constants k = 3-1 x lo9, 5.5 x 108M-'s-' for eo2 and CH, t O H respectively.302The rates of reaction are independent of [{Ru(NO)(NH,), I?'] but depend linearly upon [{ R u( ~ ~ O ) ( N H, ) , }] ~produced + during the radical reaction. [Ru(hO)(NH3),I2+is scavenged by 0, with a rate constant k = 7.6 x 106M-'s-'.299 Nitrosyl ammine intermediates have been detected when [Ru(NHJ6I3+ is treated with O2 in
(34)
zeolites at 450 K303while reaction of [Ru(NO)(NH3),I3+with [Cr(OH2)6]2+affords a dimeric species
[(NH,)4(OH,)Ru-NH-Cr(OH,),]5+.282 ( e ) Dinilrogen The activation of N, by transition metal complexes has been Ruthenium(1) : Pulse radiolysis of [Ru(NH,),NJ2+ yields [Ru'(NH,),N,]+ with a rate constant k = 4.2 x 109M-'s-'. The product dismutes to [Ru(NH,),NJ2+ and [RU(NH,),N,]~ (2k = 2.7 x lo9M-' s - I ) with subsequent deposition of Ru Ruthenium(II): The synthesis of [Ru(NH,)~N,]~+ was first prepared in 1965 by refluxing RuC1, in aqueous hydra~ine.~" Since then a range of methods have been developed for the synthesis of this complex. Scheme 9 summarizes the main routes to [RU(NH,),N,]~+.
\/
[ RUC16 1
'+
[(L R u=(diazirine) N H ~L)]~
'-
//
IRuCl,(diene) I,
RuC1, bi
xx,,,
[RuCI,(ON,)J'-
v vi
xxii
IRu(NH,)(OII,)]~'&
[Ru(NHs),Nzl*'' "N N = 2110cm-I-
[Ru(NH,)c,12'
+
- [Ru(NH3)6I3+
v,i viii
[1ZuC~l(NlI,),J2'
y/ \\
P \ IRu(Nd(h'H,)j 1(Nd2
xvii xviii xix
IKu("J)(NH3)s 13+
[ R W H , ) 5 l2NY
[Ru(NH3),(OHz)12+
'+
[Ru(NH~)~N~(OHZ)I
t
xii
[Ru(NH3)sNzO12'
cis-[ RuCI,(NH,),
1'
i , NH,NH,:16a244~310 ii, Zn, NH40H;3"~3'2 iii, NH2NH2;Is7 iv, NH3;3I3 v , NOI-OH;280,314vi, NH2NHZ;'16vii, Zn/Hg, CF3S03H,N2;315viii, Zn, N20;316 ix, concentrated HC1;'" x, (a) Ag+ (b) NaN,?" xi, decomposition A;3L7 xji, NaN,, H c ; l S 9 xiii. NH3;Isy xiv, Cr2f;3'4-3'8 x v , Nz.311),320X V ~ CI?, , 0 0 ~ 1 1 4
xvii. RNH,, pH xviii, NI€2NH2;292)"' X ~ X , - O H ;xx, ' ~ NaN3,'NH3. ~ H+;I6"'* xxiii, 0 2 or H202, Ht:323 x x i , NH,NH,, 1 11;244 xxii, NH2N1i2; xxiv, NI12NI12, 12 l1;2"" Scheme 9
High yields of [Ru(NH3),N2I3+can be obtained from the reaction of [Ru(NH,),]'+ with NO at pH 8.45,2803314 the yield dropping with lowering of pH and falling to zero at pH 6.280The proposed mechanism for the formation of [RU(NH,),N,]~+ from [Ru(NH3)J3+ is illustrated in Scheme 1 Labelling experiments confirm that N, formation in the complex arises from combination of NO and coordinated NH, (equation 30),314 whereas in the reduction of [Ru(NH,),N,Ol2+ by Cr", N, arises solely from coordinated N20 (equation 31).3'4+3'8The mechanism of formation of [Ru(NH3),N,I2+ from [RU(NH3)6I3' and N2H4involves oxidation of coordinated N,H4 to N, (equation 32).'16The kinetics of formation of [RU(NH,),N,]~' from [RU(NH,),OH,]~+and N, has 104 324.325 = 3.3 been reported (equation 33); k, = 7.3 x lop2,k-, = 2.03 x 10--6s-1, 0.15892807287
/I.
[Ru(NO)(NH3), 13+
IRu(NH3)613'
[ Ru(NII3)5(NHz)I2'
-OH
\LlRu(NH3),(NH2)N01*'
NO
\
F ( N H 31 5 Nz I
*
H2 [Ru(NH3)5"01
*+
-
1
NHR N4N
,#'
H%#J
Ru
I
'NH~ NH3
[Ru(NH3)5N212+
[Ru(NH,),]'+
(36)
I = 276nm [WNH3)5N2Pf
[Ru(OH)(NfI3)5I*+
(37)
1 = 29011m Reaction of diazirine L with [Ru(NH,),OH,]~+yields [ R u ( N H ~ ) ~ L (36) ] ~ + which is oxidized in air or by H z 0 2to [RU(NH,)~N,]~+ and c y c l ~ h e x a n o n eThe .~~~ single crystal X-ray structure of (36) indicates strong d +pn* (N=N) back-bonding with Ru-N(N,) = 1.911 A, Ru-N(trans NH,) = 2.147A, N=N = 1.28A, < RuNN = 141.6°.323
Ruthenium
30 1
r
12r
L
J (36)
Ruthenium(1llj: Reaction of [Ru(NH,),NJ2+ with 6 H yields [Ru"'(NH~)~N,]~+ which rapidly aquates to [ R U ( O H ) ( N H , ) , ] ~ +For . ~ ~[Ru(NH,),N,]~+ ~ the specific rate of aquation k = OS-'.^'' (f) Nitrous oxide Ruthenium(i1): Reaction of [Ru(NH,),(OH,)]~+with oxygen free N,O at 3 M O atm yields the adduct [Ru(NH,),N,O]~+ (equation 38).3'4*343*344 A t lower pressures, [RU(NH~),N,]~+ and {[Ru(NH3),],N,)4+ are formed. [Ru(NH,),N,0l2+ may also be prepared by reaction of [Ru(NO)(NH,),I3+ with NH,OH under basic c ~ n d i t i o n s . ~Scheme ' ' ~ ~ ~ 1~1~gives ~ ~ the proposed mechanism for this reaction, with the N of NO remaining bound to the Ru ~ e n t r e . , ' ~[RU(NH,),N,O]~+ .~~~ can be isolated as a by-product in the reaction of [Ru(NO)(NH,),]~+with N,H4.322X-Ray powder results and the electronic and vibrational spectra of [Ru(NH3),N,0j2+ are consistent with linear bound N'O, R u N ~ N O , with , ~ ~characteristic IR bands at 1160cm-' and 2 2 5 0 ~ r n - ' . ~ ' ~
+
[ R u ( N H ~ ) ~ ( O H ~ ) ] ~ "N"NO +
[ R U ( N H ~ ) ~ ~ $ ~NH2"H+ I~'
+
[ R u ( N H ~ ) ~ ~ ~ N ' ~ N+O ]H,O *+
R [RU(NH~)~;-NH~OH]~+ -Ht
(38)
0
II
[Ru(NH3),$-NHOH12+
0
I
[ Ru(NB3),-~=NOH]+
[ Ru(NH,), !NO]
'+
Scheme 11
The reduction of complexed N,O by Cr" and V" is increased by facts of 10' and lo7 respectively over reduction of frw N,0.346-'47The kinetics of these reactions have been d i s ~ u s s e d . ~ ~ ~ , ~ ~ ~ (8) Cyanates and thiocyanates The complexes [Ru(L)(NH,),I2' (L = - NCO, NCS, -NCSe) can be prepared by reaction of [RUCI(NH,),]~+ with L in refluxing aqueous solution, or by treatment of [RuC1(NH3),l2+with Agf followed by reduction with zinc amalgam and addition of L.34*,349 With excess NCS, further reaction leads to the formation of a range of aminethiocyanate compIexes including [Ru(NCS),(NH,),] + 350-352 and the proposed [Ru(NCS),(SCN),(NH,),]- ?52,353 A low energy charge transfer band is observed for [Ru(NCO)(NH,),]~+and is assigned to e(nL) + tz(44dRu)transition.354 On the basis of the energy of d-d transitions in [Ru(L)(NH,),I2+, a spectrochemical series of L (NH, > -NCO > - 0 A c > -NCS > -NCSe) was proposed.349 Reaction of the dimer [(NH,), RuSSRU(NH,),]~+with CN- gives S-bound [Ru(SCN)(NH,),]+ which rearranges to the N-bound [Ru(NCS)(NH,),]~+.~~~~~~~ An alternative preparation of [RU(SCN)(NH,),]~+has been reported.3S2The hydrolysis of [Ru(NCO)(NH,),]*+ is acid catalysed with the proposed formation of [Ru(NH,C02H)(NH3),]'+ as a reactive i n t e r ~ n e d i a t e . ~ ~ The reduction of [Ru(NCS)(NH,),]*' by Ti'*' and Ti(Hedta) shows second order kinetics with rate constants k = 840 and 2.8 x 104M-l s-'"'' inner-sphere electron transfer via -NCS bridges being proposed for both systems.357 (h) Nitriles Ruthenium (11); Treatment of [RU(NH,),(OH~)]~+or [Ru(NH,), (acetone)]'+ with L or [RuC1(NH,)J2+ with zinc amalgam in the presence of L yields [RuL(NH3),]'+ (L = acetonitrile, a c r y l ~ n i t r i l e hydrogen ,~~ ~ y a n i d e , ~ethyl ~ ' ~ ' cyanoben~onitrile,'~'substituted ben~onitrile,'~~,~~'~~~~ formate,361dicyanamide, malononitrile, substituted malononitrile, tri~yanomethanide,~~' 4-cyano- 1methylpyridini~m"~).Reaction of a hundred-fold excess of RCHO (R = Ph, Me) with [Ru(NH~)~],+ under alkaline conditions yields [RU(NH,),NCR]~+.363-365 The likely mechanism of this reaction is given in Scheme 12. An alternative route to nitrile comprexes is by reaction of [ R U ( N H ~ ) , O H ~ ]with ~ + aldoximes, e.g. RMeC=NOH, to afford [Ru(NH,), (NCMe)]" and
302
Ruthenium
A wide range of alkoximes have been surveyed for this reactivity, the reaction proceeding ROH.366-367 via deaquation and elimination of ROH with no net redox change occurring at the metal centre.s368
I Scheme 12
Complexation of RCN to ruthenium(I1) centres leads to shifts of the C=N stretching vibration vcEN to lower frequencies as compared with vC--Nfor the free ligand.369This is due to strong d x
+ 7t*
back-bonding from Ru" to the nitrile ligand,358~3" and is consistent with the remarkable unreactivity ~ ~ ~below). ~ ~ ~ ' The of these d' complexes compared with the corresponding d 5 Ru"' ~ p e c i e s (see moiety has been studied by 'H NMR and confirms binding of acrylonitrile to the [RU(NH~),]~+ terminal N binding rather than IT bonding through C=N or C=C.360 'H NMR has also been used to probe the degree of back-bonding from Ru" to coordinated nitrile, the observed shielding of geminal protons for coordinated acrylonitrile and 1-methylacrylonitrile, and of the 3- and 4-protons of benzonitrile being consistent with an extensive back-bonding The electronic and resonance Raman spectra of [RuL(NH3)J3+ (L = 4-cyano-1 -methylpyridinium+)2" and Mijssbauer studies on [RUL(NH,),]~+(L = benzonitrile, acetonitrile)'20 have been published. The photolysis of [Ru(NH3),(NCMe)12+ in aqueous conditions in the presence of C1- gives [ R U C I ( N H ~ ) ~and ] ~ +[Ru(NH,),(OH,)]~+.~*~,~~~ Ruthenium (m) : Ruthenium(II1) nitrile complexes [RuL(NH,),I3+ may be synthesized by reaction of L with [ R U ( N H ~ ) , O H ~ ] ~oxidation + , ' ~ ~ of [Ru(L)(NH3)J2+with Ce'" or Ag',3s9by direct treatment of [RuCl(NH,),]*+ with L under r e f l ~ x ~ ~or~ by , ~ "aerial oxidation of coordinated The TR spectrum of these complexes shows the C=N stretching vibration v , - , ~ to occur at higher frequencies than in the free ligand,358.360 indicating net .n back-donation from f l (L) 3 dn (Ru"'). This is also reflected in the reactivity of nitrile ligands coordinated to Ru'". The specific rates of base hydrolysis of [Ru(NH,),NCRI3+ are 2.2 x lo2 (R = Me) and 2 x lo3M-' s-' (R = Ph) which is IO6times more reactive than the corresponding Ru" complex, and 10' times more reactive than the free ligand.370,371,373 General base catalysis is observed in these reactions suggesting -OH attack on coordinated nitrile as an initial step leading to amido complexes ]~+ [Ru(NHC(O)R)(NH3),l2+.370,371 On coordination of tert-butylmalononitrile to [ R U ( N H ~ ) ~the pK, of the ligand decreases from 13.1 to 3.85,362the enhanced acidity of tert-butylmalononitrile on coordination to Ru"' being attributed to the relative net 7c acceptor ability of the metal centre; cf. the increase in basicity and decrease in basicity of pyrazine on coordination to [Ru(NH,),]'+ and [Ru(NH,)J3+respectively.240The acid hydrolysis of [Ru(NH,),NCRI3+ has been reported to occur with specific rates k = 1.2 x lop5(R = Me), 3.4 x 10-5M-1s-I (R = Ph).373 (i) imines, umides and related ligan& Imine complexes have been involved as intermediates in the condensation of aldehydes with [Ru(NO)(NH3),I3 under basic conditions to form nitrile c o m p l e x e ~ ;in~ certain ~ . ~ ~ ~cases the imine species have been isolated (equation 39)?@ Reaction of [RuC1(NH3),l2+with Ag(O,CCF,) followed by addition of ligands L (L = quinone diimine derivatives of l,Cdiaminobenzene, l-amino-4dimethylaminobenzene, 1,4-diarninonaphthalene,4-aminophenol) gives complexes [RuL(NH,),12+, e.g. (37).374These products show intense metal to ligand charge transfer bands near 550nm ( E = 3 x 104M-'cm-'), the basicity of the diimine ligand being increased on coordination to ruthenium(I1). A reversible couple between Ru"(diimine) and Ru" (diamine) is ob~erved.~" Reaction of [Ru(NH,),I3+ with diketones such as biacetyl yields under basic conditions the corresponding bidentate diimine complexes (38).375,376 The kinetics of the reaction have been elucidated and +
303
Ruthenium
shown to proceed with [-OH] dependence via the intermediate [Ru(NH,)(NH,),]*+ (24) (Scheme 13). The diimine complex (38) shows a reversible oxidation couple at + 0.56 V vs. SCE, controlled potential electrolysis of chemical oxidation with Br, yielding the ruthenium(II1) a n a l ~ g u e . ~ ~ ' , ~ ~ ~ 0
13+
d [ Ru" --
1 -
/IIRLI["I[biacctyl] [-OH1
d/
Scheme 13
The base hydrolysis of Ru"' nitrile complexes to amido products has been reported (equation 40).370 Reaction of [Ru"(NH,),(NCCO,E~)]~' (39) with A@ yields [Ru"'(NHC(O)C0,Et)(NH3)5]2+(40)which in mild alkali gives the oxamic acid derivative (41) (equation 41).361The hydrolysis of [Ru(NCO)(NH,)J2+ gives [Ru(NH,CO,H)(NH,),]~+.348 Amide derivatives of [ R u ( N H ~ ) ~may ] ~ - also be prepared directly from the reaction of [Ru(NH,),OH,I2+ with RCONHz (R = Me, Ph),37'while sulfamide complexes have been synthesized by reaction of SO:- on azide complexes377or by attack of S20:- on coordinated NH, (Scheme 14).378 RU
-
NZCR
i
Rt1-N
H
- M-NYo I1 2
Y K
R
[ R u ( N H ~ ) ~ ( N C C O ~ E ~ ) ] ~ + [Ru(NH,),(NHC(O)CO,Et)]*+
(39) [Ru(NH3),(OH2)I3+
[Ru(NH,)~(N~)I~+
i , ( a ) LiOH (b) LiN,. H+;378 ii, SO:;
+
[Ru(NH,),(NHC(O)CO,H)]~+ (41)
-
(40)
(441)
[RU(NH~)~(NH)I~+
iii, Na2S20,, 0 2 26 , h, Scheme 14
(40)
IY. HBr3'*
304
Ruthenium
(iii) Oxygen donor ligand complexes (a) Aqua and hydroxy ligands Ruthenium (11): [RU(NH,),OH,]~+can be most efficiently prepared by zinc amalgam reduction of M ore recently, an alternative route avoiding Zn2+and [RuCl(NH,),]Cl, in aqueous C1- ion has been developed based on the aquation of electrochemically reduced [Ru(O, SCF,)(NH,),]2+ Alternative routes include the photolysis379and acid-catalysed hydrolysis3*’of [Ru(NH,)J2+ and the reduction of [Ru(NH,),(OH,)]~+.’03 The lability of the aqua ligand in these systems makes [Ru(NH,), OH,]’+ an excellent starting material for the synthesis of substituted pentaammine complexes, and for the study of their kinetics of formation. Tables 2 and 3 give data on the substitution reactions of [Ru(NH,),0H,12+ with a range of ligands ~ + 3 8 383 1 The kinetics of substitution on [Ru(NH3),OH,l2+ are first order in [[Ru(NH,),(OH,)]’+] and [L] with rate constant in the range k = 0.048 -,0.32 M-’ s-’ at 25°C for L = pyridine, 3- and Ccyanopyridine, acetonitrile, benzonitrile, perfluor~benzonitrile.~”,~~’ Electron-donating groups on L were found to enhance the rate of substitution, while electron-withdrawing groups decreased the rate. An S,l mechanism was suggested, with pyridines reacting relatively slowly due to steric The enthalpies for substitution of [Ru(NH,),OH2I2+by acetonitrile, imidazole, pyridine, thiodiethanol, [Ru(NH,),~z]~+, isonicotinamide, pyrazine, N-methylpyrazinium+, dimethyl sulfoxide and CO have been found to be - 38.4, - 38.9, - 53.1, - 57.3, - 57.7, - 64.0, - 70.3, - 75.3, - 80.3 and - 160.3kJ mol-’ respectively.208 Table 2 Kinetic Data for Substitution Reactions L A [Ru(NH3),LI2‘ f H 2 0 [Ru(NH,),(OH,)]’+
+
5.5 x 10-2 30.2 x 9.3 x 5.6 x 3.1 x 10.5 x 10-2 3.6 10-3 20 x 10-2 2.7 x lo-‘ 3 x 10-3
NH, MeCN PY PZ PZH + isn isnH+ imid imidH+ 3-Methylpyridine 2,h-Dimethylpyridine Isoniazid Cyanoace tate Pyrazine N-oxide
SO:- > imid(N bound) > isn > pz > Mepz+.387For S C l - and N-bound imid, bond making was proposed to be substantial in the activated complex, whereas for isn, pz and Mepz+, noticeable outer-sphere complex formation was indicated.38’ Reaction of trans-[Ru(SO,)(WH3),isn]+ with CF, SO,H and zinc amalgam gives tr~ns-[Ru(NH~),isn(OW,)]~+ The labilizing effect of L in tran~-[RuL(NH,),(0H~)]~+ was probed by kinetic measurements and the labilizing
305
Ruthenium
effect of L was found to increase in the order L = N2 iCO < isn < pz < imid(N bound) < NH, < -OH < P(OEt), < -CN < SO:- 46 x 10'
'
Electron transfer between [Ru(NH,),(OH,)]~+ and [RUC~(NH,),]~"[k = 1.0 x lo3M A s-"') yields [RU(NH,)~OH~]'and [ R U C I ( N H , ) ~ ] ' .The ~ ~ ~rate of electron transfer is reported to be independent of [€€+I, and decreases in the order 1- > Br- >Cl, the reaction proceeding via an outer-sphere type me~hanism."~Subsequent aquation of [RuCI(NH3),]+ occurs to give [Ru(N]~+ by H3),OH2I2" (k = 4.7 s-') thus rendering the overall aquation of [ R u C ~ ( N H ~ ) ~catalysed reducing agent.125.'78 The kinetics of reduction of [CoX(NH,),I2+ (X = F-, C1-, Br-, I-, -NCS, -SCN, N;) and [Co(ox)(NH,),]- by [RU(NH,),OH,]~+have been reported and discussed in terms of inner- and outer-sphere electron transfer mechanism^.'^^'^^^,^^^ [Ru(NH, (OH,)],' with V02+ yields green binuclear complexes [(NH,), Ru"OV" (L),,I4+(L = Hz0,380edta derivative^^^') which show M L charge transfer bands near 600 nm. The ESR spectra and electron transfer processes in these systems have been reported.39' Reduction of H202and cytochrome c by [RU(NH~),OH~]~' while [ R u ( N H ~OH2I2+ )~ has been bound to polyvinyl pyridine coated has been electrodes.A93 Rutheniurn(I1I):[ R U ( N H ~ ) , O H ~has ] ~ +been prepared by reaction of [RuCl(NH,),]CI, with dilute with H,0,2'* by NH, and acidification to pH 3,394by reaction of [Ru(O,SCF,)(NH,),)](O,SCF,), protonation of [Ru(OH)(NH3),l2+with 3 M CF,S0,H362or by treatment of [Ru(NH,)~]C~, with HCl.395[ R U ( O H ) ( N H , ) , ] ~ +can ~ ~be ~ *isolated ~ ~ ~ from the reduction of [RuC1(NH,),]C12 with zinc amalgam followed by air oxidation of the product, from the reaction of [RuCl(NH,),]CI, with NH3,290,397 and from the aquation of [Ru(NH,),l3' under basic conditions.164The latter reaction proceeds via two independent pathways: (a) pH independent, and (b) base-catalysed aquation via [Ru(NH,)(NH,),]'+ intermediate;166for [Ru(NH,),OH,]~+ pK, = 4.2.235,3x Substitution reactions of [RU(NH,),OH~]~+ have been reported (Table 3).383Thermolysis of [Ru(NH,),OH,]X, in the solid state takes place via a two step process to give initially [RuX(NH,),0H2]2+ in a rate-determining step, with subsequent formation of [RuX(NH,),I2+ (X = C1-, Br-, I-, NO;);399"the activation energies and entropies for these reactions have been assesSed.399a.399b In solution, anation of [Ru(NH,), OH2)],* shows second order kinetics.124*399c*4m The photolysis of [Ru(NH,),(OH,)]~+ has been reported.'77 Electron transfer reactions of [RU(NH,),OH~]~+ with Sm11,"O',402 Yb",401,402 Eu" 401402 Fe"' [FeII10H]2f,128 UIII 170 125,402 V11'%02 and [Ti"'(OH)I2+ have been studied. [Ru(NH3),I3+bn zeolites can be converted to a complex of the type [Ru(NH,),(OH),,(CO),]"+ which is an active catalyst for the water gas shift reacti0n.4'~ [Ru(NH,),(OH,),]~+ is prepared via addition of CF, S0,H to [Ru(ox)(NH,),]+;~"the affinity of trans-[R~"'L(NH,),0H,]~+ (L = C-bound imidazole) for imidazole, py, isn, C1-, Br-, I- is very low (K,,,,, = 1.2 for isn) except for NCS- (Kam = 3.3 x l0,M-l); a large kinetic labilization by C-bound imidazole is implied.235 Electrochemical evidence for the interconversion of [Ru'" (O)(NH,),]'+, [Ru"' (OH)(NH,),I2+ and [Ru"'(NH,), (OH,)],+ has been obtained using activated glassy carbon electrodes.4wb --f
9
7
306
Ruthenium
[Ru(O),(NH,),]'+ and related ruthenium(V1) complexes incorporating the trans dioxo moiety have been reported.65 (b).Carboxylates Ruthenium(ZZ) : The polarographic reduction of [Ru"'L(NH,),]'+ (L = formate, acetate, propionate, butyrate, glycolate, glycinate) shows a reversible one electron transfer to give Ru" at a DME in aqueous s o l ~ t i o n .E~ ~v*alues ~ ~ for this couple were found to become more positive with increasing [H+ The reversible uptake of CO, by [RU(NH,)~(OH,)]~+ has been described.187b Ruthenium(III): Reaction of [Ru(OH)(NH,),]Cl, with acetic anhydride followed by acid hydroly.397 Oxalic acid with [ R U( OH ) ( NH , ) , ] ~ or +~ sis and treatment with Ag+ gives [Ru(OAC)(NH,),]~+ [RuCI(NH,),]~+~"" affords cis-[Ru(ox)(NH,),]+ . Carboxylato complexes can be prepared generaly by treatment of [RUC~(NH,),]~+ with Ag(0,CCF3) followed by addition of carboxylate buffer in DMF.402Charge transfer bands for the complexes [RU(O,CR)(NH,),]~+are observed near 300 nm, and are related to the Ellzvalues for reduction of the complexes.4o5 Aquation of [RU(O,CR)(NH,),]~+proceeds viu two pathways: (a) an acid-catalysed S, 1 process, and (b) an acid independent SN2process for R = CH,F, CH,Cl, CH,Br, CH,I.,07 For R = CHCl,, CCl, the aquation reaction is not catalysed by H + due to the high acidity of the coordinated Data for the aquation of non-halo carboxylate complexes are given in Table 5; the mechanisms of the reactions have been discussed!1M12 Anation of [Ru(O,CCH,C~)(NH,),]~+is reported to proceed via predominantly an S, 1 mechanism.w Table 5 Kinetic Data for Aquation of [RU(O,CR)(NH,),]~+~'~ for which Aquation Rate Law is Rate = [complex](kH,o+ k H [ H + ] ) -_
.
Formate Acetate Propionate Isobutyrate Glycolate Glycinate
.
5.37 x 10-5 11.8 x 10-5 14 x 10-5 13.7 x 10-5 5.1 x 1 0 - ~ 6.75 x lo-'
3.3 2.3 2.5
10-3
10-3 10-3
1.7 x 10-3 0.75 x 10-3 3.1 x 10-3
The reduction of cis-[Ru(ox)(NH,),]+ , [Ru(oxH)(NH,),]~+ and [Ru(OAC)(NH,),]~+ by [Ru(NJ4,)J2+ and [Ru(NH,),OH,]~+has been studied;4I3the rates of reduction are acid dependent and increase with increasing [H+] with outer-sphere mechanisms being proposed.413Inner-sphere reductions of [Ru(O~CR)(NH,)~]~+ have been reported with C I - ' ' ~and Ti"' .414,415. Ac.id dependent second order kinetics were observed for Til1' reactions which occur via electron transfer in a deprotonated binuclear intermediate, the rate constant for reduction being identified as the rate constant for substitution on For the salicylato complex, no acid dependence was 0bserved;4~' " cross-bridging between Ru"' and Ti'" through the n-system of carboxylate was p r ~ p o s e d . ~Reaction of fRu(NH,),I3+, [RUC~(NH,),]~+ or cis-[RuCl,(NH,),]+ with LH, (LH, = catechol, 2,3-dihydroxynaphthalene) yields cis-[RuL(NH,),]+ , while reaction with LH, (LH, = 3,4-dihydroxybenzoic acid, 3-(3,4-dihydroxyphenyl)propanoicacid) yields [RuL(NH,),];~'~ the properties of these complexes have been (c) Sulfoxides and related ligands Ruthenium(ZI): Refluxing [Ru(NH,),N,]~+ in aqueous DMSO yields [RU(NH,),DMSO]~+ (42):'' ' H NMR and IR spectral data for [Ru(NH,), (S(0)(Me),}f2+,[Ru(ND,), (S(O)(Me),}I2+, [Ru(ND,), { S(O)(CD,), )I2+ and [Ru(NH,),{S(O)(CD,),}]~+ indicate that the DMSO ligand is Sb ~ n d e d . ~This " is confirmed by the single crystal X-ray structure of [Ru(NH,), DMS0I2+(42) which shows Ru-S = 2.188 and S - 0 = 1.527A with lengthening of Ru-N distance trans to S to 2.209 A range of SO,, HSO; and SO:- complexes of [Ru(NHJ5I2+ and [Ru(NH3),I2'"have been prepared.2,419'4" Reaction of [RuCl(NH,), SO,]' with NH, yields [Ru(SO,)(NH,),] which on acidifthe Mossbauer of which has been reported.'" Treatment of ication gives [Ru(NH,)~S02]2+,419-4zo [RUCI(NH,),]~+with HSO; /SO2yields trans-[Ru(HSO,), (NH,),] which can be converted to trans[RuX(NH,),SO,]+ with HX (X = CI-, Br-).'96341942'The single crystal X-ray structure of trans-[RuCl(NH,),SO,]Cl (43)shows4*' the SO2 ligand to be S bonded with Ru-S = 2.072& S-0 = 1.462, 1.39481 Ru-Cl = 2.41581 with a planar RuSO, moiety. Truns-[RuL(NH,),SO,j2+ may be prepared by reaction of tran~-[RuCl{NH,),S0,]~with L (L = pz, isn, imid).239,422 Photolysis of trans-[RuCl(NH,),SO,]' leads to linkage isomerism of the SO, ligand, from S-bonded SO, (vso = 1255, 1 110cm-I) to q2-bonded SO, (vm = 1165, 940crn-') (equations 42 and 43).423
Ruthenium
307
[Ru(NH,),(OH,)(SO,)]'+ may be prepared by reaction of cis-[RuCl,(NH,),~Cl with zinc amalgam, H S K followed by a ~ i d i f i c a t i o n . ~In ~ ~solution, ,~~' the SO, complex is in equilibrium with 5 ) : " The rates of substitution of pyrazine into the HSO, and SO:- derivatives (equations 44 and 4 SO, (44),HSO, (45) and SO:- (46) complexes have been measured; rate constants k = 0.033, 0.2, 13.5 M - ' s at 25°C respectively.422The rate of substitution for c~.T-[Ru(SOJ(NH~)~OH~] is independent of [pz], the rate-determining step being release of NH, trans to
tr~ns-[Ru(NH,)~(0H~)S0~]~ ' + H20 -(W trans-[Ru(HS03 )(NH, )4 (OH2)]
+
e Lrans-[Ru(HSO,)(NH,),{OHz)]+-t
H+
(44)
(45) pK, = 2.15
e
trans-[Ru(SO3)(NH, )4 (OH,)]
(45)
(46) pK, = 5.05
+
H
+
(45)
1
Ruthenium(ZI1): Oxidation of [Ru(NH,), {S(0)(Me),)]2+(42) to the Ru"' complex leads to S --* 0 isomerization at a specific rate k = 7.0 x IO-' SKI;reduction back to Ru" reverses the isomerization (k = 30 s- I ) reforming (42).424Aquation is associated with both reactions. The 3 +/2 + couples are 1 .O V and 0.01 V vs. SCE for the S- and 0-bonded complexes re~pectively.~'~ The electrochemistry oxidation at 0.5 V vs. SCE leads to the formation of the of [Ru(NH,),S0,I2+ has been Ru"' complex with hydrolysis of coordinated SO2at a rate constant k = 2,4 x 10 s- ' >25 Treatment ,"26,427 of [RuCl(NH,),]Cl, with CF,SO,H leads to a high yield of labile [Ru"'(OS02 CF3)(NH3)J2+ which readily undergoes substitution reactions to [RuL(NH,),I3+.208,426*427 Reaction of [Ru(Nwhile truns-[Ru(SO,)(L)(NH,),]+ can be H3),0H2I3+with S,O:- yields [RU(S~O,)(NH,),]+~~~ prepared by reaction of trans-[RuC1(NH3),SO,]' with L (L = pyridine, 3-chloropyridine, 3,5-dichloropyridine) in aqueous s01ution.I~~ Oxidation of [RuCl(NH,),SO,]+ with H,O, in the presence of Na2C03and py gives [RU(SO,)(NH,),~~]+.~'~
+
(iv) Sulfur donor ligand complexes
(See also Section 45.7). Reaction of [RU(NW,),OH~]~+ with an excess of L gives products MeS(CH2),SMe,4" H2S,43' HS(CH2)4SH,432 [RuL(NH,),]'+ (L = 2,6-dithiaspir0[3.3]heptane,"~~~~~ Me2S,433S(C,H,),S,434S(C3H6)2S434). For [Ru(NH,),(SH,)]~+ and [Ru(NH,),(SH,)j3+, pK, = 4.0 and - 10 respectively, while for [Ru(NH,),(SHEt)]'+ and [Ru(NH,),(SHEt#+ pK, = 9 and - 7 = 1.5 x lo3M-1,"31 re~pectively.4~' For the formation of [Ru(NH,),(SW,)]~+ (equation 46), thioethers having generally a greater affinity for Ru" than for Ru"'.~~' Reduction of [RuL(NH,),I3+ (L = S(CH2)4S) with zinc amalgam gives [RuL(NH,),]*+, which on further reduction yields [RU(NH,)~HS(CH~),SH]~+;~~~ reoxidation to IRuL(NH3),l3+has been shown to occur with 0, or Ce'" (Scheme 15)."2 The aquation of [ R u ( N H ~ ) ~ X ( M ~ )to , ] ~[Ru(NH,),(OH, + )X(Me),]'+ occurs (X = S), 1.0 x lo-, (X = Se), 1.2 x lo-, (X = Te) reflecting an with rate constants k = 4.2 x increasing labilization order of (Me),S < (Me),& < (Me),Te.4'3
i, HS(CH2hSH; ii. O 2 or Cclv; iii, [O] ; iv, Zn/Hg;
V,
(e'"
Scheme 15
Oxidation of [Ru"(LH)(NH,),]~+ with O2 or Ce'" gives [RU"'L(NH,),]~+ (L = SCH,Me, S(CH,),SH, S(CH,),SH, SPh, S(Me)2)>3*3433 while treatment of [RU(NH,),OH,]~+with [(Me),S]PF, yields [Ru"(NH,),S(Me),13+ .435 The Ru'" complexes show strong ligand -,metal charge transfer 2 = 453 nm ( E = 3 x lo2M-' cm-').434,435 The lability and reacbands e.g. for [Ru(NH,),S(M~),]~~ tivity of thioethers bound to Ru" and Ru"' have been fully discussed in relation to the electronic and redox properties of these s y ~ t e m ~ . ~ ~ ~ , ~ % , ~ ~ ~ ~ ~ ~
(v) Halide complexes Rutkenium(1l) : [RuCl(NH,),]' can be prepared by reduction of [ R U C ~ ( N H ~ )with ~ ] ~Ti"' + 173 or ~,173 1 photolysis 1 of [Ru(NH,),OH,]*+ in the presence of C1-379 or by radiolysis of [RuC1(NH,),12+,I7' For [RuCI(NH,),]~+,cis- and tran.~-[RuCl,(NH,),]~, for Ru"/Ru"' couple = - 0.282V, - 0.328 V and - 0.412 V vs. SCE r e s p e c t i ~ e l y[RuCl(NH,),]+ . ~ ~ ~ ~ ~ ~ ~ aquates rapidly to give [Ru(NH,),OH,]~+ together with [RUCI(NH,),]~+(equation 47).37934259437 The oxidation of [RuCl(NH,),]+ by CoL;+ (L = en, cyclohexanediamine) follows first order dependence on [Co"'] and and [Ru"] via an outer-sphere electron transfer with rate constant k = 3.8 x 6.2 x 10-3M-1s-1 respecti~ely?~~ [RuX(NH,),]+ H 2 0 [Ru(NHMOWr+ + X(47) ~
+
X X
= C1-, =
Br-,
lyequ,, = 0.7M,
KqUd=
A H + = 50kJrnol-', A$
0.92M, AH' = 59kJmol-', A S i
= =
-67JK-'mol-',
k
-42JK-'mol-',
k
= =
6.3s-'
5.4s-'
Ruthenium(III): [ R U X ( N H ~ ) ~can ] X ~be prepared by treatment of RuC1, or [RuC~,(OH,)]~-with hydrazine followed by HC1,"7*325-439 by refluxing [Ru(NH,)J3+ in ~ 1 1 3 , 1 ' 4 , ' 9 3 , 3 9 4 , 4 2 ' , ~or 2 by solid state thermal substitution of [Ru(NH,),jX, (X = C1-, Br-, I-)."3.444 The single crystal X-ray structure of [RuC1(NH3),]C1, (47) shows Ru-N = 2.07-2.1 1 A, Ru-C1 = 2.34 A:39 Reaction of cis-[Ru(ox)(NH,),]+ or [RuX(NH,),,SO,]+ with refluxing HX (X = C1-, Br- , or thermolysis of [RuCI(NH,),]C~,~ yields [RuX,(NH,),]+ . RuC1, (NH,), has been prepared by action of O2on bIue solution of [RU(NH,),]~+ in HC1,QQ2,448 by reaction of HCl with cis-[RuC12(NH,),]+ and . ~ single crystal X-ray structure of fuc-[RuCl, (NH,),] by thermolysis of [ R u C ~ , ( N H , ) , ] C ~The shows Ru--N = 2.105-2.467& Ru-Cl= 2.323-2.529 A+"@
Halo-ammine complexes show halide -+ Ru"' charge transfer bands450which have been characterized by circular dichroism s t ~ d i e s . Calculations 3~~ predict that the Ru-N bond strength increases in the order cis-[RuX2(NH,),]+ < [RUX(NH,),]~+< [Ru(NH,),OH,]'+ < [Ru(NH,),],+, with trans to X weaker than when cis to X.451The magnetic and ESR properties of Ru-NH, [RuCl(NH,),]Cl, have been r e p ~ r t e d . ' ~ ' * ~ ' ~ ~ ~ ~ ~ by C1- and Br- proceeds via first order kinetics;388with Anation of trans-[Ru(NH,), (i~n)(0H,>]~+ I- however, more complex kinetics are observed with the rate law being given by equation (4X).388 The ka term can be accounted for by equations (49) to (51), with the kb term suggesting a possible
309
Ruthenium
electron transfer from I- to Ru"' not involving free 12.3ssThe role of halide in the reduction of 0, by trans-[R~(NH,),(isn)(OH~)]~+ has been discussed. Trans-[RuI(NH,),(isn)]T, can also be prepared by treatment of trans-[Ru(SO,)(NH,), (isn)]' with NaI.388
+ 31[Ru"(OH,)] + I2[Ru1'(I)] + I;
2[Ru"'(OH2)]
+
2[RuI1(OH2)] + I;
+
[Rul'(I)]
+
+
HzO
2[Ru"'(I)] f 31-
(49) (50)
(51)
The ligand exchange reactions of [RuX(NH,),12+ (X = CI-, Br-, I-) (equation 52) have been studied and are proposed to take place via a two step process with the rate-determining step being the formation of [RU(NH,)~OH,]~+ (equations 53 and 54).4,, The mechanisms for Br- exchange for [RuBr,(NH,),]+ in aqueous and mixed solvent systems have been The aquation of [RuX(NH3),l2+(X = C1-, Br-, I - ) has been reported to occur via an associative SN2mechanism,456,457 with base hydrolysis being substantially faster than the corresponding acid h y d r o l y s i ~ . ~ ~ ~ , ~ ~ ~ Scheme 16 illustrates the proposed SN2cb and SN2 processes for the base hydrolysis of = 0.03-0.2M,4s9a~4s9b although an S, Icb mechanism [RuX(NH,),l2+ (X = C1-, Br:, I - ) for [-OH] has been attributed to the reaction at lower -OH concentrations of 3.78 x 10-4-2.65 x 10-3M.398 The aquation of [RUCI(NH,),]~+is catalysed by Hg2+17and [Ru(NH,),(OH,)]*+,'*~the former Acid hydrolysis of cisreaction occurring via the formation of a chloro-bridged inte~mediate.'~ [RuC12(NH3),]+ takes place with retention of config~ration.4~~ The UV photoaquation of [RUCI(NH,),]~+has been reported in very low yield,"' while photoaquation of cis-[RuX,(NH,),]+ (X = Br-, I-) gives C~~-[RUX(NH,),(OH,)]*+.~~~' No photoaquation is observed for X = Cl-;46' [RuCl(OH)(NH,),J+ has been prepared by treatment of RuC1, with NH, in aqueous
F o r X = C I - . [-OH] = 0 _ 0 3 - 0 _ 2 M ; k I - 9 . 2 9 ~lo"', k - ] = 2.65 x k , = 6.78 x I O - ' , k 3 = 1.13 x 10-'M-'s-'
Scheme 16
The solid state thermal deamination-anation of [RuX(NH,),]Y, (s) to [RuXY(NW,),]Y (s) (X = C1-, Br-; Y = Cl-, Br-, I - , NO;) has been investigated and the mechanism of the reactions di~cussed.4~~ Thermal degradation of [RuX(NH,),]X, occurs via the reduction of Ru"' by halide or with the initial formation of [RuX,(NH,),] (X = C1-, Br-). The thermal by stability of [RuX(NH,),l2+ was found to decrease in the order C1- > Br- > I-.443 The reduction of [RuX(NH3),l2+by Ti"' shows second order kinetics, with the rate of reaction being inversely proportional to [H'] indicating that TiOH2+ and not Ti3+ is the reductant; rate The reduction of constant k = 12, 56.0, 112.0M-'s-l for X = C1-, Br-, I- re~pectively.'~~.~~' [RuCI(NH,),]~+,CLY-[RUC~,(NH,),]+ and C~~-[RUC~(NH,),(OH,)]~+ by Cr'$ follows pseudo first order kinetics at higher [Cr"] with rate constant k = 460, 117 and 154 s-' re~pectively.~~~'~~''~~~ For trans-[RuCl,(NH,),]+ second order kinetics is observed.468Reductions with Cr" occur via an
I 310
Ruthenium
inner-sphere mechanism with the formation of binuclear chloro-bridged intermediates [Ru"'-Cl-Cr"] followed by electron transfer to give [Ru"-Cl--Cr"'] and dissociation to Ru" and Cr"' Reduction of [RUX(NH,),]~' (X = C1-, Br-, I - ) by U"' probably proceeds via an inner-sphere mechanism,"' whereas for reduction by V" an outer-sphere mechanism predomiby contrast, reduction of [Ru(NH,),OH,]~+ by U"' is thought to occur via an outersphere process."' The reaction of [RuX(NH3),I2+with organic radicals has been reported,'" while I- /I2 coupling experiments using electrodes chemically modified with [RuCl(NH,),]Cl, exhibited electr~catalysis.~~ An intervalence charge transfer band at 1 = 510nm has been observed on mixing [RUCI(NH3)5]*' with [Ru(CN)6I4-.s5'470
(vi) Amino acids, esters and mixed donor complexes
Addition of L to [Ru(NH,),OH,]~+under basic conditions yields the complexes [RuL(NH, )J2+ (L = ethyl glycine,383methyl sarcosine,383glycinamide,4" N-ethyl gly~inamide?~, glycylglycine,4" gly~ylglycinamide,4'~ethyl glycylglycir~e~~~). Oxidation of the glycine complex [RU*'(NH,),(NH,CH,CO,H)]~+leads to N --t 0 isomerization to give [Ru"'(NH~),(O,CCH~NH~)]~+ (Scheme 17).471 For the ethyl glycine complex [Ru"(NH,),(NH,CH,CO,Et)I3+, N 40 rearrangement occurs within the Ru"' oxidation state to yield [Ru"'(NH,),OC(OE~)CH,NH,]~+ under acidic conditions.473[H'] inhibits reversion to the Nbonded complex, with the hydrolysis of the ethyl glycinate ligand being found to be much faster for this system than for the [Co(NH,),I3.+ anal0gue.4~~Under acidic conditions (pH = 3) [RuL(NH3),l3' (L = glycinamide, N-ethyl glycinamide, glycylglycine, glycylglycinamide, ethyl glycylglycine) reacts to form [RuL(NH3)J3+in which the ligand L is N,O bonded.472At higher acidities k"[H+] aquation to [Ru(NH3)'0H2l3+is a competing reaction, with the rate law being k = k where k corresponds to chelate formation and k" to a q u a t i ~ n . ~ "Treatment of cis[Ru(NH,),(OH,),]~+ at pH 1 with L gives [Ru(L)(NH3),l3+ (L = glycylglycine, glycinamide, N'ethyl glycinamide) with the ligand L being N,N' bonded.404"Rearrangement to the N,O-bound isomer is slow, but can be achieved more readily by reduction to the Ru" species in acidic solution and reoxidi~ing.~~" It is interesting to note here that the N-bonded complex [RuL(NH,),l3+ (L = acetamide) does not rearrange to the 0-bonded isomer. However, for L = formamide, reduction does lead to N 40 isomerization to form the 0-bonded Ru" species which on reoxidation, The above linkage decays with a specific rate k = 1 s-' to the N-bonded starting isomerization reactions and relative stabilities of the products have been fully d i s c u ~ s e d . ~ . ~ ~ ' ~ ~ ~ Radiolabelled Ru"'-.ammine complexes of amino acids have been reported."'
+
f
[(H3N),RU"(NH,CH,C0,11)l
z+
i, (a) Zn/Hg. (b) sodium glycinate; ii, glycine, 3 h; iii, [O]
Scheme 17
Reaction of [RU(NH,),(OH,)~]~+ with L (L = 2-pyridine ~ a r b o x a l d e h y d e2-pyridine ,~~~ alcoh 0 1 ~ ~ yields ~ " ) the complexes [ R U ' ~ L ( N H ~ ) ~ These ] ~ + .products can be interconverted by oxidation of the pyridine alcohol derivative (48)to the acetyl pyridine complex (49) (Scheme 18).476aThe complexes [Ru(NH,),L]+ (n = 43;LH = pyrazinecarboxylic acid) have been generated in which L can be mono- or bi-dentate.47hb Metalloflavin complexes of type [RuL(NH,),I2+ have been synthesized by reaction of [Ru(NH,),0Hz]2+ and C~~-[RU(NH,),(OH,),]~+!~+ with flavin ligand, L.477479 The single crystal X-ray
Ruthenium
311
b
f '
+
(48)
Scheme 18
structure of the 10-methylisoalloxazine complex (50) has been reported and confirms a bidentate N,O chelate with Ru-0 = 2.088& Ru-N(flavin) = 1.980A, Ru--N(NH,) = 2.12A, the short Ru-N(flavin) distance being consistent with back-bonding from Ru".~",~'~ (50) shows two, one electron r e d ~ c t i o n s . ~More ~ * . ~ ~recently, ~ resonance Raman studies on flavin derivatives of [Ru(NH,),I2+ have been p~blished.~" Condensation of [Ru(NH,)J3+ with oxalic acid is reported to yield (51).375
r I+
45.4.1.3
Binuclear complexes
Ruthenium ammine-phosphine and -selenium complexes are discussed in Sections 45.5 and 45.8 respectively.
(2')
Nitrogen donor bridged complexes ( a ) Amines
Refluxing RuCl, for two hours in ethanol followed by treatment with NH,at 90°C for two to three hours yields a black product [(NH,),Ru(NH,), Ru(NH3),]C1,.4H,0 (52):" The single crystal confirms the presence of two bridging NH, ligands with RuX-ray structure of , Ru-N(NH,) = 2.130-2.146F1.~~~ N(NH,) = 2.011, r
14+
312
Ruthenium
( b ) Heterocycles and nitriles Several reviews on binuclear pentaammine ruthenium complexes have been Since the initial report by Creutz and Taube in 1969 on the synthesis of the mixed valence dimer [RU(NH,)~]~PZ'+ (53);" a wide range of binuclear ruthenium ammine complexes have k e n prepared afid characterized. The basis for this work was to study the extent of metal-metal interaction across the bridging ligand, and in particular to assess the charge distribution in the mixed valence complexes [Ru(NH,),],L'+. These complexes can be described as either (a) charge localized systems with no interaction between Ru"/Ru"' centres (Class I), (b) having only partial metal-metal Hush theory coupling (Class 11) or (c) fully delocalized with Ru"~/Ru"! centres (Class 111).'7'~4g0 that a moderately coupled Class I1 mixed valence system will show an intervaience charge transfer band for the light-induced transition (equation 55). The analysis of intervalence More bands, particularly their solvent dependence, has been the subject of much discussion.4843491 recentIy, the vibronic coupling modei4g24whas been applied to the analysis of intervalence charge transfer bands and good agreement has been observed for localized systems while discrepancies have occurred for supposed delocalized ones.495A full discussion of the theoretical basis for these models is outside the scope of this review, and is summarized in Creutz's review.484The measurement of comproportionation constant K, for equation (56) has also been used to assess the stabilization due to delocalization of [Ru(NH,),]~L'+ relative to [Ru(NH,),], L4+ and [Ru(NH,),],L6+ .491 The properties of intervalence charge transfer bands and evaluation of 4 have been commonly used parameters for the assignment of Class I1 or Class 111 behaviour to these types of mixed valence complexes. I-
(53)
[Ru", Run'Isi -% [Ru"', Ru"]'+ [2,q4+
+
[3,316'
Kc
e
(55)
2[2,3]"
(56)
Reaction of [Ru(NH,), OH,I2+ (generally prepared by treatment of [RuCI(NH,),]~+with Ag(0,CCF, ) at SOT, removal of precipitated AgC1, acidification with NaHCO, in aqueous solution
to pH 2-3 and reduction by zinc amalgam) with half an equivalent of L yields the binuclear complex ~yrimidine,''~~ 4,4-bipyridine,'95~2w~497 1,2-bis(4[Ru(NH,),],L4+ (L = for example, pyraZine,L9s,197 1,2-bis(4-pyridyI)pyridyl)ethyler~e,''~l,l'-bi~(4-pyridyl)methane,*~~1,l '-bi~(Cpyridyl)amine,~~~ cyanacetylenqZM3,3'-dimethyl-4,4'-l~ipyridine?~dicyanoben~enes:~~d~cyanonaphthalenes,~~~ ~ g e n ~ ~Dithionite '). has been used as a counterion for the isolation of the complexes in the solid ~tate.4~' The [2,2] species [Ru(NH,)~]L~+ can be oxidized to the [2,3j5+and [3,316+ions electrochem. ~ ~ ~the ~ ~ Creutz-Taube ~ complex [Ru(NH,),],pz5+ may be prepared by ically or c h e r n i ~ a l l y Thus, oxidation of the [2,214+ion with Ag+, while further oxidation with Ce4+ yields the [3,316+ion (equation 57).197 [Ru(NH3)s
4 g+
12 PZ4 +
wi4+ I, = 547nm
(Ru" + L)
no IVCT
[Ru(NH3)SI2PZ5 + [2,3i5+ L = 565nm (Ru" -+ L) 1, = 1562nm (IVCT)
ce4+
[ R U ( N H ~ ) ~ ~ ~ P Z(57) ~+ [3,316+
no RuIl + L no IVCT
The single crystal X-ray structure of ~ R u ( N H, ) , ] ~ ~ z(53) ' + has been this shows Ru-N(pz) = 2,002 A, Ru-N(NH3 trans to pz) = 2.135 A, Ru-N(NH, cis to pz) = 2.1 10A.502a A full crystallographic study of the series [(H3N)5Ru(pz)Ru(NH,),Y+ (n = 4,5,6) has been published.502bThe [2,3]" complex shows an intervalence charge transfer band in the near IR at 1562nm ( E = 5 x lo3M-I ~ m - ' ) , ' ~in~ addition to a Ru" + ligand charge transfer band at 565 nm ( E = 2.1 x 104M-' cm-' )197,489 while the [2,214+complex shows an analogous charge transfer band at 547 nm ( E = 3 x lo4M-' cm-')."89Neither the [2,214+nor [3,3j6+ions show an intervalence charge transfer band at 1562 nm; the [3,3j6+ion shows no absorbance at 565 nrn.lg7Although the observat i o ~ ~of ' an ~ intervalence ~ + ~ ~ ~ band for the mixed valent species [Ru(NH,),],pz'+ was consistent with Hush theory,176the solvent dependence of the absorption suggested that a more delocalized formalism for the complex should be invoked.486In addition, the cornproportionation constant & of 4 x IO6 indicated a relative stabilization of the [2,3]'+ ion.197The question of whether the CreutzTaube ion should be regarded as a Class I1 or Class I11 system is still the topic of much debate.
313
Ruthenium
Highly contradictory reports on the degree of delocalization in the complex have been published, with many techniques being used in an attempt to distinguish between the two possible classifications. Conflicting ~R,'97~486~489~502~s04-506 photoelectron spectroscopy,s05~507"" 99 Ru Mossbauer,s029s'2 powder ESR (8-50 QSo6 single crystal ESR,502a~s's,516 magresonance R a m a ~ ~ , " -'H ~ ' ~NMR,s06,s14 netic s~sceptibility,~'~ e l e c t r ~ c h e m i c a l and ' ~ ~ , ~ ~ ~ studies have been published. The contradictory results obtained for the [2,3]'+ complex may be partially reconciled however by regarding the rate of intramolecular electron transfer k, in (53) as being rapid (equation 58). In principle, experiments with a faster resolution time than k, could detect Class I1 behaviour, while techniques with a slower resolution time would suggest Class 111 characteristics for (53). A value for k, of loi3s-' > k, > 10" s has been p r o p o ~ e d , 4 ~suggesting ~.~~' a substantial degree of delocalization in the molecule. The Creutz-Taube complex appears to be at the frontier of Class II/Class I11 behaviour, and as such, a description of delocalization within the molecule in terms of k, would seem most appropriate.484
The complex [Ru(NH,),],4,4-bipy4+ (4,4'-bipy = 4,4'-bipyridine) shows one, two electron oxida[Ru(NH,),], 4,4-bipys+ t i ~ n , "the ~ [3,316+ion being formed on addition of Br, or C1, (Scheme 19).497 can not be prepared directly from the [2,214+ion, but is generated by addition of Ru" to the [3,316+ i0n?y7,500 The mixed valence [2,3]'+ ion shows an intervalence charge transfer band at 1030nm (6 = 880 M-' cn--'),jm and has a comproportionation constant K, = 20 near to a statistical suggesting no special stability for this mixed valence complex. Solvent dependence of the intervalence charge transfer absorption agrees well with the Hush m ~ d e l , ~ ' * -and ' ' ~ the rate of intramolecuIar electron transfer for [Ru(NH3),],4,4-bipy5+ has been estimated as k, < 108s-' 484 indicating weakly coupled Class I1 behaviour. [(H3N)5Ru(OH2)12+
4*4-b'py
*
[ Ru(NH3)F J ,4.4'-b1py4+
Br, or CI,
* [Ru(NH,), ] 24.4'-bipyh+
'
Ku(NH,),OH,
]I*
D
[ Ru(NH~)Tl?4,4'-blpys+
Scheme 19
A good example of a Class TI1 binuclear complex is the cyanogen adduct [Ru(NH,)~],NCCN'+; K, is estimated as > with an intervalence charge transfer band at 1429nm ( c = 4.1 x 10'M-l ~ m - ' ) The . ~ ~IR spectrum of the [2,3]'+ ion shows vCN at 2210cm-', intermediate between the values of 1960cm-' and 2330cm-' for the [2,214+and [3,316+ species respectively. Thus for [Ru(NH,),],NCCNSf, the rate of electron transfer k, between metal centres has been estimated as k,> l O I 3 s-1!84,486,498 A theoretical molecular orbital analysis of the mixed valence complexes [Ru(NH,),],LS+ has proposed5" that for L = pz, the electron is in an orbital coupled over both metal centres. For L = non-conjugated dinitrile or 4,4'-bipyridine, non-equivalent metal centres were suggested, while for L = cyanogen, a highly coupled, symmetric ground state was found."' Table 6 lists data for binuclear mixed valence pentaammine ruthenium complexes. The bridging of dinitrile ligands in binuclear pentaammine ruthenium complexes can be accompanied by deprotonation of the nitrile. Thus, on oxidation of [Ru(NH,),],L4+ (L = malononitrile, rert-butylmalononitrile) to the mixed valence [2,3j5+species, deprotonation of the bridging ligand occurs to yield [Ru(NH,),],(L-)~+ (equation 59).362f'z8~526~sz7 The enormous enhancement of pK, for and [Ru"'-L the nitrile ligands on binding to Ru"' can be illustrated in the [Ru"-L--Ru'"]~+ -Ru"'I6+ complexes where pK, = - 0.7 and - 4.8 for L = malonitrile, pK, = 1.6, - 3.0 for L = tert-butylmalononitrile respectively.362Similar p% enhancement has been observed for mononuclear systems.362 [(H,N), Ru"(L)Ru" (NH3 )sf+
-
[(H3 N)5 Ru" (L-
)Ru"' (NH3)514+
(59)
[2,L- ,314 +
[Ru(NH,),], bipym4+ (bipym = 2,2'-bipyrimidine) has been prepared by treatment of [Ru(NH,),OHJ3+ with bipym in the presence of zinc amalgam.262The complex shows two oxidations at + 0.830V and + 1.02 V vs. NHE, the second oxidation leading to cleavage of the bimetallic unit. Photolysis of the [Ru(NH,),], bipym4* causes only slight decomposition of the complex.262
314
Ruthenium
Table 6 Cornproportionation Constants (K,)and Intervalence Charge Transfer (WCT) Bands for Mixed Valence Binuclear Complexes [Ru(NH,),],L'+ 4R1
K,
L
nN
N
w
ZVCT(nm)
e(M-' cm-')
5 x lo3
Re$
4 x IO6
1562
197,498, 503, 521
3 x Id
1399
41
1030
880
209, 486, 495, 497, 500
9.8
890
165
209, 486, 495
6.7
810
30
209,486,495
26
920
1010
209, 495, 519
14
960
760
209, 486, 495
14
920
640
209.495
N-N
bJ
20
< 10
H
N
O
N
H
10'0
496, 503
522
486, 495, 522
855
70
1090
450
486
830
60
523
950
6
1710
900
486. 524
525
Ruthenium
315
Table 6 (Continued)
loLo
HN&H
1770
800
525
496. 503
M . .e.
NCQCN
23
862
507
QCN
11
752
8
33
1055
98
1000
600
503
Nr QCN
496, 503
CN
1020
1O8
4.5 x
IO4
495
1.4 x Id
315,484, 503
935
1100
503
847
23
503
NC
f-J -
1429 925
4.1 x
Id
495, 498, 503 526
180
NC
B=XBz
NC
CN
G" &
10
990
1.6 x
ld
362
975
150
527
78 1
135
496. 503
cN
7
CN
503
8.5
CN NC
10
769
20
503
10
769
140
503
CN
acN CK
NC
1,3-Dicyanoadamantane p-Pseudo-p,p'-dicyanoparacyclophane
C.U.C 4-K*
503
9
10 4
752 800
12 10
503 503
Ruthenium
316
Table 6 (Continued)
L
NC
x
CN
NC-CN
Tricyanomethanide Dicyanamide
NC*CN NC
A
Tricyanomethanide Dicyanamide a For
CN
784
2.8
lo4=
362
694"
1.7 x lo4"
362
562" 400"
4.5 x 1038 2 x 1038
362 362
1.6 x
362, 428, 526, 527
> IO'Ob
1 170b
>
1280b 2.3 x 103b 2 x 102b
1550b
1lOOb
X
> 104b
362, 527
6.2 x 2.8 x IO-''
362 362, 428
[(H,N),RU"'(L-)RU"'(NH,),]~~.bFor [(H3 N), RU"(L-)RU'"(NH~)~]~'
Reaction of trans-[RuCl(NH,),SO,]Cl with NaHCO,, pz and then H 2 0 , yields the binuclear complex [(SO,)(H,N),RU"~~~RU~~~(NH,)~SO~]~+ which can be converted to [H~~(H,N),RU~'~ZRU~'(NN,),~~H]~+ by treatment with zinc amalgam, pyrazine and acid.242T h s latter species is a useful intermediate in the synthesis of tri- and tetra-nuclear complexes. A range of complexes of type trans-[L(NH,),Ru(imid)Ru(NH,),L]"* ( L = SO,, SO:-, H,O; h i d = imidazole derivatives; L = SO:-, NH,) have been synthesized by reaction of [Ru(SO,)(NH,),OH,] and the corresponding mononuclear fragment.247,528a These complexes have been studied electrochemically and intervalence charge transfer bands observed for mixed valence Irradiation of the [RuC1(NH3),I2+/[Ru(CN),I4- ion pair leads to the formation of [(H,N), Ru"'NCRu"(CN),]- .55 The binuclear species [(H,N)5Ru(pz)Ru(CN),]-!' have been prepared and their electronic properties compared with related symmetric and asymmetric complexes.528b Reaction of [Ru(NH,),pzI2" with [R~"~(Hedta)0H,] produces a trapped valence complex [(H3N)5R~1'pzdimer (55) which Ru"'(edta)]+ (54). Photolysis of (54) leads to an activated [Ru"'--pz~~--R~~~~] decays back to (54).529
0 w
( H 3 N ) s R u ' L ' c ~-
Ru"' (&a)
~ - - t
1 J
Binuclear complexes [(H3N)5Ru(L)RuX(bipy),13+ (X = C1-, L = pz, 4,4'-bipy, 1,2-bis(4pyridyl)ethylene, 1,2-bis(4-pyridyl)ethane; X = NO;, L = pz) have been synthesized by reaction of [Ru(NH,),OH,]~+with [RuX(L)(bipy),]+.530The [2,314+and [3,3]'+ complexes have been generated, and the intervalence charge transfer bands for the mixed valence species a n a l y ~ e d . ~ ~For '"~~ X = C1-, L = pz, 4,4'-bipy, 1,2-bis(4-pyridyl)ethylene, absorptions were observed at 960nm ( E = 530), 695 nm ( E > 300) and 680nm ( E > 300 M-' cm-') re~pectively.~~' No such band was The metal-metal coupling in [(H, N), Ru"pzobserved for the 1,Zbis(Cpyridyl)ethane analog~e.~,' Ru"'Cl(bipy),14+has been estimated to be three times lower than for the Creutz-Taube i ~ n .The ~ ~ ~ , ~ ~ ~ preparation of [(H3N)5RupzRu(NCCH3)(bipy),15+has been reported.5M
Ruthenium
317
The interactions of the [Ru(NH,),I2+ moiety and its derivatives with other metal centres in ligand-bridged heterobimetallic complexes of the type [(NH3)5Ru-L--MI have been investigated. M = Ni", L = Systems that have been studied include M = Cr"',,L = 4-a~nidopyridine;'~~q~~~ M = CUI', L = M = CUI, L = 4-vinyl pyridine;539*540a M = C0 [I1 7 540b L = 3-02 CPY,%' 402Cpy,54'3-02CCH2py,541 4-02CCH2py,54'~ - N C P ~ , ' ~ ~4,4'-bipy,522,923,3'-bip~,~~, 1,l'-bis(4p~ridyl)methane,5~~ 1,2-bis(4-~yridyl)ethane,~~~ 1,2-bi~(4-pyridyl)ethylene,~~ l,l'-bis(4-pyridyl)1,4-dicyanobicyclo[2.2.2]ocmethanol,5233,3'-dimethyl-4,4'-bipyridine,sz2bi~-(4-pyridyl)sulfide,~~~ tane;543M = Rh"', L = 4-NCpy,s42,sapz?42 4,4-bipy;s42M = Fel', L = PZ,'~~ Cp derivative^."^"^^ Ni", Cu" and Zn" bind to [Ru(NH3),pzl2+(equation 60) with association quotients K = 17, 32 and Photolysis of the Ru"-pz--Cu" complex leads to bleaching of the charge 3.1 M-' re~pective1y.~'~ transfer band with the formation of a R~~~'---pz--Cu~ dimer.537Binding of Cu' to [RuL(NH,),]'+ (L = 4-vinyl pyridine) via the alkene (KeqUll = 8.4 x lo3)has been reported, with subsequent electron ~ complex [(NC),Fe"pztransfer from CUI to Ru"' at a rate constant k = 9 . 7 M - ' ~ - ' . ' ~The RU"(NH~)~]'-, prepared by reaction of [ R U ( N H ~ ) ~ ~with Z ] ~[Fe(CN),OH,I3-, + shows two, one electron oxidations at 0.45 V and 0.72 V vs. NHE for oxidation of Ru" and Fe" re~pectively.~~' The neutral species has been formulated as a trapped valence Class I1 system,.with an intervalence charge transfer band at 1330 nm.545The first order specific rate constants for electron transfer from Ru" to Co"' in [(H3N),Co"'L Ru"(NH,),OH,]"+ (L = substituted pyridines, 4,4'-bipyridyl derivaMore recently, a range of Co-Ru dimers tives) are in the range k = 1 x low4to 44 x 10-3s-1.5223548 with polypeptide bridges have been prepared, and electron transfer reactions within the binuclear system inve~tigated."'~~'Ferrocene-ruthenium heterobinuclear complexes (56) and (57) have been synthesized and compared with polynuclear ferrocene derivative^.^^^,^^ The Fe", Ru" dimers show one electron oxidations at 0.44 V and 0.85 V for (56) and 0.44 V and -t 0.66V vs. SCE for (57) with intervalence charge transfer bands in the near IR being observed for the Fe", Ru"' species.546
+
+
+
+
[(H,N),Rupz]*+
+
e
M"
+
[ ( H ~ N ) ~ R-Nu
nN\s/
M"]'+
1lC
Fe
(56)
(e) Dinitrogen [(Ru(NH,),),N,]~+ can be synthesized by treatment of [RU(NH,),OH,]~+with N2,313,338 by reaction of [Ru(NH,), OH2I2+with [Ru(NH,)~N?]~+ ,3243338 by reduction of reaction solutions of N 2 0 and [RuCI(NH,),]~+with zinc amalgam,316by reaction of [Ru(NO)(NH,),]~+with [Ru(NH,),I3+ under basic conditions,321 or by decomposition of [Ru(N,)(NH,),I2+ under acidic condition^.^" The ]~+ 61) follows first formation of the dimer from the reaction of N, with [ R U ( N H, ) ~ OH ~(equation with k , = 7.3 x 10-2M-'s-', order kinetics in [NJ, with second order kinetics Kequil = 3.3 x lV, AH" = -42.3kJmol'-], AS' = - 5 4 J K - ' r n 0 1 - ' . ' ~ ~ * ~ Fo ~r~ the reaction of = 7.3 x IO3, AH' = - 46.9 kJmol-I, [Ru(NH,),OH,]~+with [ R u ( N H ~ ) ~ N ~(equation ]'+ 62), Kequi, ASo = - 84 J K - ' mol-' .324 [(Ru(NH,),),N2l4+ shows no strong IR band for the stretching vibration vN=N suggesting a linear symmetric structure for the dimer.,,' The Raman spectrum of [(Ru(NH,),),N,]~+ shows vN-N at 2100 cm-I, and v15N--15N at 2030 cm-I A linear structure with an eclipsed conformation is confirmedss5by the sinae crystal X-ray structure of (58) with Ru = 178.3", N--N = 1.124A (N-N(N,) = 1.928A7 Ru.* .Ru = 4.979A, ]+with the formation of small amounts of [ R ~ ( e n ) , ] * + . ~ ~ ' ~
8,
c1
( 6 5 ) [RuCl,(nbd)(pip), I ( 6 6 ) IRuClz(nbd)(anil)21
NHTR
H
(67) [RuClZ(cod)(hex),1
(68)
Rutheniwn(II1): Reaction of [Ru(ox),I3- with amine L followed by HX yields cis-[RuXzL,] * (L = en, 1,Zdiaminopropane, +trien);45'.6'8,620 [RuX,(en)]- has been isolated from these reactions.620 The absolute configuration of (-)-cis-[RuCl,(en),]+ has been deterir1ined,6~'and can be converted " may be to the trans isomer by heating for 1 min at 60°C with ethylene g l ~ c o l . ~Trans-[RuX,L]+ alternatively prepared by treatment of [RuCI,(OH,)]~- with L (L = (en),, (1,2-diaminopropane),, (1 ,2-diamino-N, N'-dimethylethane), , 1,9-diamino-3,7-diazanonane, l,lO-diamino-4,7-diazaOxidation of [RuC1(enH)(en),l2+ with [Ru(en)J3+ decane, 1,l l-diamin0-4,8-diazaundecane).~~~"~~ gives [ R ~ C l ( e n H ) ( e n ) , ] ~ while + ~ ~ ~ solvation effects in cis-[RuBr,(en),]+ , trans-[RuX,(en),]+ (X = C1-, Br-, I-, NCS-) and trans-[RuX,L]+ (X = C1-, Br-; L = 1,9-diamin0-3,7-diazanonane) have been probed by ESR The acid and base hydrolysis of cis- and truns-[RuCI,L]+ (L = (NH,),, (en),, (1,Zdiaminopropane),, trien, 1,9-diamino-3,7-diazanonane) have been s t ~ d i e d ; ~ ~the~ role . ~ of ~ increased ' . ~ ~ ~ . ~ ~ ~ ~ ~ ' ~ ~related ~ " ~ ~ 'to the Eli,values chelation in the observed rates of hydrolysis has been a s s e ~ s e d ~ ~ and
Ruthenium
325
for the Ru"'/" redox c o ~ p l e s . ~ Trans-[RuCI, ~~,"~ (en)J+ hydrolyses more slowly than the cis isomer, the aquation of cis-[RuCI,(en),]" being Hg*+ catalysed (equation 71).&20 An S, 1 mechanism via a square pyramidal intermediate has been propo~ed,6".~~"~~',~' hydrolysis occurring with retention of config~ration.~~'.~~.~~~.~~' Base hydrolysis occurs via a conjugate base mechanism, with a second order rate law (equation 72):" The ligand field (LF) and ligand-metal charge transfer (LMCT) excited state photochemical aquation of trans-[RuX,(en),]X has been For X = I-, LF excitation led to aquation with greater than 85% stereochemical change, whereas for LMCT excitation retention of configuration was observed. For X = C1-, Br-, the LF and LMCT bands overlap and stereochemical mixing was found in the products.64' The reduction of trans-[RuXL]+ (X = 2 W , 2Br-, 21-, BrC12-; L = (en),, 1,9-diamino-3,7Reduction by Cr" follows a rate expression diazanonane) by Cr" and V" has been reported.b247M5 (equation 73) consistent with an inner-sphere mechanism involving bridging chloro specie^;^-^^ an outer-sphere mechanism has been suggested for reductions with cis-[RuCl,(en),l'
4
Hg"
-
~is-[RuCl(en),(OH,)]~+
- d[complex]
dt
Thermolysis of (LH),[RuCl,] (L = Et2NH).646
45.4.2.6
=
( , i s - [ Ru(en)2(OH,),1
'*
I
k[-OH][complex]
(L = Et,NH)
has
been proposed
to yield
[RuCl,L,]
Binuclear complexes
Treatment of [RuL,(N,)(OH,)]~+ with [RUL,(OH,),]~+(L = en, itrien) yields the dimer [RuL,(OH,)]2N:+.320On reduction with zinc amalgam cis-[RuBr,(trien)] reacts with N2 in the presence of KX to give [(trien)XRu"(N,)Ru"X(trien)]X, (X = Br- , I-).," [(en),C1Ru(OH)R~Cl(en),]~+is reported to be the product of photolysis of ~is-[RuCl(en),OH,]~+ .621 The nitrido-bridged complex [ R ~ , N ( e n ) , j ' + is ~ ~discussed in Section 45.4.6.
45.4.3
Heterocycles
45.4.3.1 Mononuclear complexes (i) Monodentate heterocyclic complexes [RuL,]'+ "' Reaction of [RU(OH,),]~+with excess of L (L = pyridine, pyrazine, pyridazine) yields [RuL,]'+ Analogous products have also been prepared by extended reflux of pyridine with [RuH~ ) ~ ]2-~ + (HOMe),(PPh,)J+ or [RuH(OH,)~(PP~,),]+ or by reaction of [ R U ( C O ~ ) ( N ~ Hwith methylpyridine.650 The single crystal X-ray structure of [Rupy,]*+ (69) shows Ru-N = 2.10-2.14A with octahedral Ru". (69) shows a metal to ligand charge transfer band at 341 nm, with a larger Ru-N force constant compared with data for the Ru--NH, bond.649For the [RUpy6]3+/2+ couple E" = + 1.29V vs. NHEM9
,@'
2+
326
Rut hen ium
(ii) Monodentate heterocyclic complexes with carbon donor ligands Treatment of [RuCI,(CO),]~with py in THF yields [RuCl,(CO), py]65'which reacts further to give [ R U C ~ , ( C O ) , ~ ~Reaction ~ ] . ~ ~of' ~[RuX,(CO),] ~~ (X = Br-, I-) with L at room temperature leads to the formation of [RuX,(CO),L] (L = py, 3-methylpyridine, 3.4-dimethylpyridine).6s4The complexes [RuCl,(CO)L,] (L = py, 3-methylpyridine) have been isolated from the reactions of [RuC12(CO)diene], (diene = 1,5-cod, nbd) with L;6>jfor L = quinoline, the complex [RuCl,(CO),L,] was obtained.'" Replacement of C1- in [RuCl,(CO),py,] has been achieved by treatment with Ag+ to yield [Ru(OAc),(CO),py,] in the presence of -OAc, or [Ru(CO),(NCMe),py,I2+ in acetonitrile.6s6 The decarbonylation of [RuCI,(CO),L,] with bidentate ligands has been reported.657Decarbonyhtion of [RuX,(PPh,),(CO),] (X = C1-, Br -1 with E t 3 N 0 in pyridine gives [RuX,(PPh,),(CO)py] (equation 74) with inversion of stere~chernistry.~~~
PPh3
Reaction of [RuCl,(CO)nbd]- with py yields [RuCl,(CO)py,]- and [R~Cl,(CO)(nbd)py]~~~ while refluxing [RU(CO~)(N,H,),]~+ with excess of L affords [Ru(cod)L,]'+ (L = py, 4-methylpyridine);650 the catalytic reduction of nitroalkenes and N-oxides by CO and H 2 0 has been achieved using [Ru(cod)py,]'+ .660 The complex [Ru(NCSj, (CO)zpy,] has been prepared by reaction of [Ru(NCS),(CO),], with pyridine.h6'Amine exchange reactions of [RuClH(cod)L,] (L = amine) with pyridine, 4-methylpyridine and 4-dimethylaminopyridine have been studied; the single crystal X-ray structure of [RuCIH(cod)py,] (70) showshh2 Ru-N = 2.167, 2.1 53 A, Ru-Cl = 2.584 A, RU--H = 1.60W.
(70)
The pK, of [Ru(CN),(pzH)]'- is 0.4 compared with 2.85 for [Ru(~zH)(NH,),]~+ .38 Rutheniumcyano complexes are discussed in Section 45.3.1
(iii) Monodentare heterocyclic complexes with nitrogen donor ligands Treatment of [Ru(NO,),py,] with HX yields [RuX(N0)py,l2+ (X = C1-, Br-); with X = ClO;, [Ru(OH)(NO)py,12+is obtained.663 The single crystal X-ray structure of trans-[R~Cl(NO)py,]~+ (71) shows a linear RuNO unit with < R uN O = 174.8", Ru-NO = 1.760& N-0 = 1.132& Ru -CI = 2.314A shortened due to the trans effect of nitrosyl, Ru-N(py) = 2.1 11 The nitrosyl in [RuX(NO)py,]'+ behaves as an electrophile, reaction with -OH leading to the reversible formation of [ R u X ( N O , ) ~ ~ ,The ] . ~ ~C10; ~ salt of (71) reacts with N; to give a mixture of [RuCl(N,)py,j and [Ru(Cl)py,(OH,)]+, while the PF; salt of (71) yields [RuCl(N,)py,]+ which rapidly loses N, in s ~ l u t i o n The . ~ ~ one electron electrochemical reduction of (71) generates [ R U ( N O ) ~ ~ , ( O H ~; ) ] ~ + further reduction either electrochemically or with zinc amalgam produces [RuCl(NH,)py,]" !63 [RuCl(N(O)SO,)py,] can be prepared by treatment of (71) with SO:-,665while reaction of [RuI,(NO)] with py is proposed to give a dimeric species of stoichiometry [ R U I , ( N O ) ~ ~ , ] , . ~ ~ r
2+
Ruthenium
327
Thermolysis o f (LH), [RuCI, (NO)] (L = benzimidazole, imidazole, 5-nitrobenzimidazole, benzotriazole, py) leads to the initial formation of [RuC14(NO)L]- with subsequent reaction to afford [RuC1,(NO)LJM6 When solutions of RuC1, with NaNCS in pyridine are refluxed for three days, a product formulated as [Ru(NCS),py,] can be isolated.667Nitrosyl and thionitrosyl complexes containing heterocycles with arsine or phosphine ligands have been
(iv) Monodentate heterocyclic complexes with oxygen donor ligands
(L = N-methylpyrazinium+), [RuL,(OH2),j2' (L = py, pz) and The complexes [RuL(OH,)~]~+ ] ~ Reaction + of [R~py,(0H,)~]'+can be isolated from the reaction of L with [ R u ( O H ~ ) ~.648 K,[Ru(ox),] with py under reflux for 1 h followed by reduction with zinc amalgam yields y , ] ruthenium(V1) .~~ [R~(ox)py,].~@' Treatment of [RuCl,py,] with NO; produces [ R ~ ( N 0 ~ ) ~ p The complex [RuO,C1,pyz] with a trans-dioxo O=Ru=O moiety can be prepared by reaction of RuO, with py in the presence of HCl.65A product of stoichiometry RuO,py, has been isolated from the reaction of RuO, with pyridine; the exact nature of this complex is unknown but is likely to involve Ru"' and/or R U " . ~ ~Trans-[Ru" ~ ' ~ (O)Cl(py),]+ can be prepared by oxidation of trans-[RuCl( N o ) ( ~ y ) , ] ~the + ; single cr stal X-ray structure of the Ru"-oxo product shows an unusually long Ru-0 distance of 1.852 .674b
x
(v)
Monodentate heterocyclic Complexes with halide ligands
Treatment of [RuC1612 with L under reflux conditions yields [RuCl,L,] (L = py, 2-, 3- and
4-rnethylpyridir1e.~~~'~~~ [Ru(ox)py,] reacts with HCI to give cis-[RuCl,py,l. Recrystallization of cis-[RuCl,py,] from pyridine leads to the isolation of the more stable trans isomer.669Extended reflux of [RuCl,(PPh,),] or [RuCl,(AsPh,),], with L yields [RuCl,L,] (L = quinoline, 2-methylpyridine, 2-aminopyridine) together with mixed phosphine- and arsine-amine complexes.677The IR spectra of complexes of type [RuCl,L,] have been r e p ~ r t e d . ~ ~ ~ , ~ ~ ~ The complexes [RuCl,L,] can be prepared by reaction of [RuCl,L,] with HC1 (L = py, 2-, 3- and 4-meth~lpyridine),6'~ by reflux of RuC1, with L (L = imidazole, substituted imidazole^)^'^ or by [RuCl,py,] and fusion of RuX, (X = C1-, Br-, I-) with molten L (L = thia~olidine-2-thione).~'~ [RuCl,pyL,] (L = 2-methylbenzonitrile) show a reversible RU'"'" couple near 0.OV vs. SCE, and a reversible Ru""" couple."" The differences between fac and mer isomers towards redox reactions have been discussed.a0 Treatment of [RuCI.,(NCR),] with pyridine gives [R~Cl,py,]-.~'~ The IR and magnetic properties of complexes [RuC13L3]have been reported.675Halo Complexes of 3-methyl rhodanine, adenosine and xanthosine have been prepared by reflux of RuC1, with ligand in alcoholic solver~ts.~~~@~ Thermolysis of (LH), [RuCl,] yields [RuCl, L,] (L = imidazole, 5-nitrobenzimidazole, benzimidazole, pyridine. benzotriazole, Et,NH);646.6B4 for L = pyridine thermolysis at higher temperatures gives [ R ~ C l , p y , ] ~ . ~ ~ ~
(vi) Bidentate heterocylic complexes [RuL,J"
(a) (RuL.*I+ [Ru(bipy),]' can be generated by reductive quenchng of excited state [Ru(bipy),]'+* (see below) or by controlled potential electrolysis of [Ru(bipy), 1,' at - 1.42V.6'5,"6 [Ru(bipy),]'+;'''- have been generated electrochemically and their absorption spectra are consistent with the formation of charge localized ruthenium(I1)-bipy radical s p e ~ i e s . ~Comparison * ~ . ~ ~ ~ of the electronic spectra for [Ru (bipy),12+"+'*"- shows the progressive growth of a band near 340 nm which is observed also for free (bipy') radical anion, and the concomitant collapse of the (bipy') z-+rc* transition near 290 nm.687440 Bands assigned to intervalence charge transfer transition between coordinated (bipy') and (bipy-) occur at 4500cm-' ( E = 210, 345 M-' cm-') for [Ru(bipy),]'+;* re~pectively;~'~ no such absorptions are observed for [Ru(bipy),j2+'[-.691 [Ru(bipy),]+ shows a metal to ligand charge transfer band at 495-5 10 nm.685,688,689,692-695 The ESR spectra of [Ru(bipy),]'+'' indicate interligand electron hopping with line broadening of the isotropic signal near g = 2,6y6,697 no such line broadening being observed for [Ru(bipy),]' .h97 The charge localized assignment for [Ru(bipy),]+" is further confirmed by resonance Raman data6'* which show bands assigned to both (bipy') and (bipy;) in
Rut hen ium
328
the complexes. The redox orbital localization in [Ru"(bipy')(bipy),]+ has been compared to the electron distribution in charge transfer excited state [Ru(bipy),]'+ * 687,688s6919697-700 (see below). Spectrwlectrochemical studies on reduced complexes of [RuL,]'+ (L = 3,3'-biq~inoline,~''5,5'-bis(etho~ycarbonyl)-2,2'-bipyridine,6~~ 4,4-bis(ethoxycarbonyl)-2,2'-bipyridine,687~702 4,4-dimethyl-2,2'b i ~ y r i d i n e , ~4,4'-bis(diethyl~arbamyl)-2,2'-bipyridine~~~) '~ have been published. Voltammetry of [RuLJ'+ (L = bipy, 4,4-bis(ethoxycarbonyl)-2,2-bipyridine) at low temperatures indicates spatially isolated, localized orbitals for these ~ ~ m p l e ~ e ~ . ~ ~ ~ - ~ ~ ~ ~ [Ru(bipy),]+ is oxidized to [Ru(bipy),]'+ by O,, [Co(bipy),13+and [NiCN4I2-at pH 11 with rate constants k = 7.4 x lo9,1.6 x lo9 and 4.1 x IO7M-' s-' r e s p e c t i ~ e l y [Ru(bipy),]O . ~ ~ ~ ~ ~ ~ ~obtained ~~~~ by controlled potential electrolysis of [Ru(bipy),]*+ at - 1.65 V generates H, on reaction with H20;M6no evolution of H, is observed on treatment of [Ru(bipy),J+ with H,0.686
tw
rRuL3i2+
Over the past fifteen years a vast literature has built up around trisldiimino) complexes of Ru", particularly [Ru(bipy),J2+.This has been based upon the photochemical behaviour of [Ru(bipy),]'+ and the nature ofits excited state [Ru(bipy),]'+* in relation to the use of such complexes as potential solar energy converters. [Ru(bipy),12+is stable up to 300°C irrespective of the nature of the anion, the decomposition product being metallic Ru or R u O , . ~ 'Early ~ ~ preparations of [Ru(bipy),12+and [Ru(phen),I2+ were achieved by fusion of RuC1, with molten ligand at 250°C706,707 or by refluxing The prepara[RuCl, (OH,)I2- with ligand in the presence of tartrate708'709 or hypophosphite.707~710~711 tion of [Ru(bipy),]'+ by reduction of RuCI, with H, over Pt black followed by treatment with bipy By addition of optically active tartrate salt, A-[R~(bipy)~]~' has in MeOH has been rec~rnmended."~ been isolated'oa~70y~7~3 and ion association measurements for [ R ~ ( p h e n ) ~ ] ~ and + ~ ' the ~ , ~optical '~ activity of [Ru(bipy),]'+ and [R~(phen),]"~'~ have been studied. A-[Ru(phen),]'+ has been used in The intercalation clay column chromatography for the optical resolution of metal complexes.7179718 of [R~Cphen)~]~' into DNA has been NMR studies of bipy complexes of Ru" have been p ~ b l i s h e d . ~Table ~ ~ . ~ 8~ 'summarizes selected data on the synthesis, electronic spectra and redox properties of complexes of type [RuL,]*+. A comprehensive account of the photochemistry, photophysics and theoretical models derived from absorption and luminescent data for [Ru(bipy),12+ is beyond the scope of this review. Several excellent reviews of this area have appeared the accounts by Seddon and S e d d ~ n , ,and ~ K a l y a n a s ~ n d a r a r n ~~~ in the 1iteratUre,3-5,487b,727,731,767-784 being particularly detailed. The electronic absorption spectrum of [Ru(bipy),]'+ shows bands at 185nm, 208 nm and 285 nm transitions at 323 nrn, 345 nm, 238 nm, 250 nm.'27q73'The assigned to bipy n -,n* transitions, and intense absorption at 454nm ( c = 14600M-'cm-')724 is assigned to a metal to ligand charge 790 this latter transition shows a long tail absorption above 500 nm which has been t~dnsfer;'~.'~~ The luminescence of [Ru(bipy),]" assigned to a spin-forbidden metal to ligand charge tran~fer?"~'~' was first reported by Paris and Brandt in 1959 as an emission near 600nm and was assigned as charge transfer in nature.79' Later this was reassigned to a singlet &d t r a n ~ i t i o n ~ "and . ~ ~then ~ to a spin-forbidden triplet-singlet charge transfer from (tZg)'(e)' to (t2g)6(equation 75).790+794*795 The orbital nature of the MLCT absorption and emission spectra of [Ru(bipy),I2+, and hence the electronic structure of the excited state [Ru(bipy),I2+*, has been the subject of much controversy. [Ru(bipy),12+ --. (t2d6
[Ru(bipy), 1,'
*
(t29)s(n*)'
--T h "'
[Ru(biPYhI2+
(75)
t t2,)6
To rationalize the luminescence characteristics of [Ru(bipy),j*+* Crosby and co-workers have an electron-ion parent (EIP) coupling m ~ d e l ~ "to~ * ~ ~ developed in a series of papers727~736an~793~802407 describe the orbital symmetries of the excited states. In this model, the electronic structure of [Ru(bipy),]'+ * could not be simply described as singlet or triplet; since the luminescence is charge transfer <* in character, spin-orbit coupling would be expected to occur thus mixing the emitting states and thus the emission was described as occurring from a 'manifold' of spin-orbit states ,CT-+ A,,,790~803-805~s08 with three emitting levels of A , , E and A , symmetries. [Ru(bipy),I2+* was therefore described as a d Ru"' centre with one electron delocalized over three bipy ligand^'^^.^^' and the temperature dependence and polarization of the luminescence More recently, however, the EIP model has been modified and alternative models that generate the requisite manifold of emitting levels have gained favour. Photoselection data for the emission of fRu(bipy),12+* were found to be inconsistent with the EIP mode1789~s09~s'o with a far greater degree ~ ' ~ the single crystal X-ra of polarization than would be expected for an ion of 4 ~ y m m e t r y . * "Since the complex to have D3 symmetry with short Ru-N = 2.056 structure of [Ru(bipy),]*+
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Ruthenium
distances (consistent with strong Ru" -,bipy back-donation), more recent proposals have led to the thesis that the excited state in [Ru(bipy)J2+*is of lower symmetry than D,; e.g. with a localized bipy radical such as [Ru"' (bipy ')(bipy), ]'+ . Direct experimental evidence for localized charge distribuand resonance Raman spectral studtion in [Ru(bipy),]'+* has come from absorption699~700~815-817 ies81%823on the excited state species. The absorption spectrum of [Ru(bipy),]'+ * shows a band near 360 nm assignable to coordinated (bipy:) radical anion,699~7M),8'ss'7 while frequency shifts between [Ru(bipy),]'+ and [Ru(bipy),12+* have been found to be analogous to those observed in the IR spectra for (bipy') and (bipyT).8L&822 This has led to the conclusion that on the vibrational time scale, the electron is localized on only one bipy ligand, rather than delocalized over all three. Voltammetric data s ~ g g e s t ' ~ a localized ~ - ~ ~ ~ formalism ~ for the redox orbitals of [Ru(bipy),]'+ and this is confirmed by spectroscopic studies on [Ru(bipy),]+. Polarized single crystal emission,a24 absorption data,720*825*826 and resonance Raman s t u d i e ~ ~on ' ~[Ru(bipy),]'+ ,~~~ have been published. Analysis of the charge transfer absorption spectrum of [Ru(bipy),I2+ by CD786,8'5,826.829,830 and by molecular orbital X, and Huckel calculation^^^^^^^^^^^^ ha s led to a model7" which involves only weak coupling (- 600cm-I) between bipy ligands. This is consistent with spectral studies for [Ru(bipy),]'+ * and [Ru(bipy),]+ . More recently an electronic structural models33based on the absorption spectrum of [Ru(bipy),I2+ has indicated that the luminescent excited state can be regarded as a triplet with less than 10% singlet character. The exact nature of the excited state species [Ru(bipy),]'+* is still the subject of much discussion and is likely to remain so in the near future. [Ru(bipy),]'+* can not only be generated by photolysis of [R~(bipy)~]'+, but also via electron transfer reactions in the dark to give chemiluminescent activity. This has been achieved by the reduction of [Ru(bipy),I3+ with -OH,834N,H,, NaBH,, dta,s347835 e,q:36 H2S04,837 oxalates3' or [Ru(bipy),]"729 (Scheme 22). The chemiluminescent spectrum obtained from the reduction of [Ru(bipy)J3+ was found to be identical to that of the photoluminescent spectrum of [Ru(bipy)J2+ .836 Chemiluminescence has been extended to electrogenerated chemiluminescence (ECL) in which reduced and oxidized forms of [R~(bipy)~]*+ are cogenerated by rapid cycling between E,, of the Ru"'"' couple and Eredof the first reduction potential followed by annihilation of [RuElectron transfer during ECL experiments (bipy),l3+ with [Ru(bipy),]+ (equation 76).725,729,838,839 occurs via an outer-sphere mechanism conforming to Marcus t h e ~ r y . ' ~ , The ' ~ ' efficiency obtained from ECL and pulse radiolysisg36experiments is generally higher than that for photolumiand more recently ECL studies have been carried out a t electrodes modified by polymer surface film^.^^,^^^ The photoinduced disproportionation of [Ru(bipy),]'+ on photolysis in a glass matrix to yield [Ru-(bipy),]+ and [Ru(bipy),13+has been reported (equation 77 and 7 Q W [ R ~ ( b i p y ) ~ I-t~hv' '
t
[Ru(bipyh I'
Scheme 22
Quenching of the luminescence of [R~(bipy)~]'+* by a quencher Q can occur via energy transfer to give [Ru(bipy),]'+, by reductive quenching to [Ru(bipy),]+ or by oxidative quenching to yield [Ru(bipy)J3+ (equations 79-8 1).849The parallels between quenching mechanisms and photodiode behaviour at a molecular level have been made.*" The relative ,Eliz values for [Ru(bipy),l3'!'+''+ and Q'-'*" play an important role as to which mechanism is observed.849The oxidation potential of malung it a stronger reducing agent by [Ru(bipy)J2+*has been estimated at - 0.84V vs. NHE686"5' ' [R~(bipy)~j'+* is more reactive towards both approximately 2.1 V compared with [Ru(bipy),]'+ .% consistent with its reduction and oxidation than the ground state m ~ l e c u f e(Scheme ~ ~ ~ 23)767*775 +
Rurheniurn
333
formalism as [R~"'(bipy;)(bipy)~]~+. A range of inorganic and organic reagents have been used to ~ ~ , * ~ ~quenching of [Ru(bipy),]'+ * and quench the excited state fRu(bipy),I2+* s p e ~ i e s . ~Oxidative related excited state complexes has been reported with S,Oi- ,695.8083839,85k856 0,,769,857-863 [ C O X ( N H , ) , ] ~ +(X = Cl-, Br-, 1-),85128" [ C O L ( N H , ) , ] ~ + , ~[~C~O, ' (~O X ) , ] ~ - , ~ ~ ~ , ~ ~ ~ [CO(NH,),]~+,'~ [ C o ( a ~ a c ) ~ ]Co"' , ' ~ ~macrocyclic complexes,870~871 [CoCl(Hedta)]- ,8" [Co(phen),I3+,863binuclear Co"' species,872,873 [Fe(CN),]3- 863,874 [Fe(ox)3]3- 864,867 Fe3+ 129,728,863.875-880 [cr(ox),]3- ,867368 ~1111,878 [os(bipy),]3+ ,863 [Ru(NH, I6]i+863,880 H g11,88l-8h bipy der~vatives86'.879.880.88k890 and nitrobenzene.884,891 Reductive quenching has been studied with [Fe(CN), LI3- ,849 [Mo(CN),I4- 1874,892 Eu11,694,695*892,893 dtc- ,899 methoxyben[Os(CN),]4- ,695,873,874 amines,884,894897 do amine,'^' adrenaline,898~-dopa,8~' ~ e n e saromatic , ~ ~ ~ ethers,894hydroquinonesg4and water,894while energy transfer mechanism has been Quenching reactions with Fe",'29*878 RuI1 851 R u111,907 observed for Cr"',728*868,s74,W(rPo4and 0,'". Os"Yn7Os'", Os", 0 ~ ~ ' ~ ' ~ CuII 908,909 Ag' 910.911 EulIi ,129,695 [PtC14]2-,s68[Ni(CN)4]2-,874 9 anb N, N'-dirnethylar~iline,~~~*~'~,~'~ have also been reported. Quenching of [Ru(bipy),]'+ * does not occur with [Co(CN),I3-, [Pt(CN),I2- or [Pd(CN),]'- !74 Energy transfer quenching is thought to be favoured when absorption and emission spectra of the reacting species o ~ e r l a p . ~ ~ ~ ~ ~ ~ 9
7
Reductive quenching
[Ru(bipy),]'+*
+Q +Q
Oxidative quenching
[Ru(bipy),]'+*
+Q
Energy transfer quenching [Ru(bipy),]*+*
t
+ Q* + Q'
+
[Ru(bipy),lzt
+
[Ru(bipy),]+
+
[Ru(bipy),13+ + Q-
0.84 e V
1.26 eV
Scheme 23
The use of electron transfer quenching of [Ru(bipy),]'+ * (equations SO and 8 1) for the conversion of photolysis energy to generate radical species has been developed. Bipyridinium salts such as N ,N'-dimethyl-4,4-bipyridinium2+(MV2+) and N-methyl-4,4'-bipyridinium have been used successfully as electron acceptors.865Flash photolysis studies indicate that the formation of free radicals occurs via one electron transfer from [Ru(bipy),]*+*,865 while the quenching of [Ru(bipy),]'+ * by nitrobenzene has been described in terms of encounter complex and ion pair formation (Scheme 24).89'Quenching by binuclear superoxy Co"' species has been formulated as being due to 90% electron transfer and 10% energy transfer.873The relaxation processes of excited state polypyridyl complexes916and electron transfer kinetics in these systems768have been discussed. The oxidation of and MV" (equations Ph,N to Ph, N+ has been achieved using photosensitized [Ru(bi~y)~]*+ +
82-84).9'7
Scheme 24
Ruthenium
334
With the abiiity of photolytically sensitizing [Ru(bipy),12+to [Ru(bipy),]'+ * and transferring an electron to a quencher Q to generate a strong oxidant [Ru(bipy),I3+and reductant Q- simultaneously, much attention has been focused on the possible utilization of [Ru(bipy),],+ as a photocataiyst for the photochemical splitting of water to H, and 0,. Although [Ru(bipy),12+*does not cleave water itself, a series of papers have appeared in the literature describing the decomposition of H,O to H2 and/or 0, and related redox processes using [Ru(bipy),I2+ or a derivative as a photosensitizer in conjunction with a range of radical carriers and homogeneous and heterogeneous cata l y s t ~ . * " . * ~ ~Scheme . ~ ' " ~ ~ ~25 illustrates a typical cycle utilizing the oxidative quenching of [Ru(bipy),]2+*.9'8The photochemical splitting of H,O by tris(diimin0) Ru" complexes is discussed in Chapter 62.5. [Ru(bipy),]*+ mediates the photoreduction of alkenes with 1-benzyl-1,CdihydronicotinamideY5'and the photooxidation of phenol^.'^' More recently, electron transfer reactions of [Ru(bipy)J'+ derivatives (particularly those incorporating vinyl substituents) have been studied in with at ion exchange surfaces,97yin polymeric films at electrode surfaces,'83.846.952-977 zirconium phosphate lattices,897and zeolites980in order to investigate the formation of photocurrents; surfactant bipy derivatives have also been studied in micellar solutions.854~s88~963~964~98'-987a Work has been directed also to the development of new photosensitizers of type [RuL,]'+ .987b The cation [Ru(bpz),]'' (bpz = 2,2'-bipyrazine) is a superior photosensitizer to [Ru(bipy),12+and shows emission at 603 nm with a half-life of the excited state I,,, = 1.04 ps,7so-752compared with a t,,, = 6.0p s for [Ru(bipy),]'+ *.711,753 Whereas {Ru(bipy),]'+ * is oxidatively quenched by triethanolamine to [Ru(bipy)J3+, [Ru(bpz),I2+* is reductively quenched to [Ru(bpz),]+ which undergoes electron transfer with MV2+ to yield MV+.7soThis is consistent with the redox properties of [Ru(bpz),12+ which is oxidized and reduced at potentials 0,s V more positive than [R~(bipy)~]*+ .752 Protonation equilibria for [Ru(bpz),]'+ * have been measured9**and ECL has been generated by reaction of [Ru(bpz),]+ with [Ru(bpz),I3+.752 The single crystal X-ray structure of [RuL,I2+(L = 4,Tdimethyl2,3,8,9-dibenzo-5,6-dihydrophenanthroline)has been reported.'" Photochemical studies on [RnL,I2+(L = 2,2'-bithiazoline) indicate that aromatized cyclic ligands are not essential for chargetransfer emission,gwwhile [RuL,] * (LH = 2,2'-dipyridyl amine) has been reported to show spatially isolated single ring emi~sion."~The luminescence of complexes containing nitro and phosphonium bipy ligands has been ~tudied.9~'" More recently, covalently linked photosensitizer (bipy) and electron acceptor (MV2+)complexes have been
-
~
Scheme 25
ir+
The photochemical stability of [Ru(bipy),12+towards ligand substitution reactions has been the mainstay of its use as a p h o t o s e n ~ i t i z e r More .~~~~ recently ~~ however photosubstitution reactions have been r e p ~ r t e d . ~The ~ ~EIP , ~model ~ ~ , has ~ ~been ~ modified by Houten and watt^‘'^^,^^^ to take into account high temperature substitution reactions by introducing a higher set of non-luminescent Substitution photochemistry of {Ru(bipy),l2+ by Br- is quenched by ferrocene three times as strongly as luminescence suggesting that the photoreactive LF state is not in thermal equilibrium with the luminescent excited state.999Photolysis of [Ru(bipy),12+with X- yields [RuX,(bipy)J (X- = Cl-,'Oo0 Br-,IW' I- ,lo' -NCS,IW3-NCO,lm -NCSe,Iw NO; ,IW2 N; ,loo, malonate,lW2- CNIM2).Solvolysis may also occur to give the complexes [Ru(bipy),(NCMe),l2' ,757,1Ws [R~Cl(bipy)~S] (S = MeCN, MeN02)'006and [RuX(bipy),DMF]+ (X = Br-,'" -NCSIW3);the role of chlorinated solvents in these photoreactions has been discussed,'DW Association of H+ in ground
Rudhenium
335
state and excited state [Ru(bipy),I2+ has been s ~ g g e s t e d ~ ~leading ~ * ' " ~ to decomposition via loss of bipy ligand^.^^^'^^^,'^ Monodentate bipy intermediates have been proposed in these reactions767,994,1010.1Ul1 which is consistent with data for the photoracemization of [Ru(bipy),I2+ via rotation of five-coordinate intermediate species (72) (equation 85).'OI2 The photoanation of [Ru(bpz),12+ in MeCN by C1- yields cis-[RuCl(bpz),(NCMe)]+ and cis-[RuCl, ( b p ~ ) , ] . ~ ~ '
-
2+
N f \ hv
There have been several reports of covalent hydration and pseudo-base formation in the reactions of [RuL3I2+(L = bipy,64*709 phen, 5-nitro- 1,10-phenanthroline,743~'o'3-'022 2,2'-b ipy~imidine"~~) with a range of nucleophiles. These reactions are proposed to involve nucleophilic attack on the coordinated heterocyclic ligand to give in the case of CN- attack on phen and 5-nitro-1,IO-phenanthroline complexes, 2-, 6- or 9-cyano derivative^.'^'^ The proposals for pseudo-base formation have come under severe criti~ism'~"''~~ although the yields of these products in solution are likely to be low. 1023,1n~n,1031 For [RuL,]'+ (L = 2,2'-bipyrimidine) ring cleavage is reported to occur in the presence of -OH to yield [Ru(OH)(bipym),(pym)]+ .Ioz3 The acidity of the 3,3'-protons in [Ru(bipy),I2+ has been noted although covalent hydration was not observed d i r e ~ t l y . ' ~ ~ ~ , ' ~ ~ ~ ( c ) [RuL3I3+ Complexes of type [RuL3I3+can be prepared by chemical or electrochemical oxidation of [RuL3I2+(Table 8) or as a product in the oxidative quenching of excited state [RuL3I2+* (see above). C12722and Celv'oN has been reported. [Ru The oxidation of tRu(bipy),]'+ by Pb0,s37T111',1033 with no MLCT band near 454 nm as (bipy),13+shows a band near 680 nm ( E = 407 M-' cm-')257,723 observed for [Ru(bipy),12+.724 The ESR spectrum of [Ru(bipy),I3+ is consistent with a Ru"' d 5 configuration.6w [RuLJ3+ (L = bipy, phen) have been reported not to racemize or dissociate in water, but react to give covalently hydrated speciesa The electrochemical formation and spectroscopy of [Ru(bipy),13+in AlCl,/N-( 1-buty1)pyridinium chloride melts have been described.'035 [R~(bipy)~],+ undergoes outer-sphere electron transfer with reducing agents such as alkyl radicyclohexanone,'038 - OH,1039 [Ru(bipy),I2+,152,1040 Co" '040 toll' Io44 c a l ~ , " ~BH; ~ ,Iu3' [Fe(CN),I4[Mo(CN),I2SbC13kwand Hg1?22For [Cr(bipy),I3+* ,lW5[Fe(oH2)6]2',271"0461048 the oxidation of [Fe(OH,),]'+ by [Ru(phen),I3+ and [Ru(bipy),13+,A H # = - 5.65, - 1.26 kJmol-I; A S z = - 151, - 138 JK-'mol-' re~pectively.'"~The reaction of [Ru(phen),],+ with [Ru(bipy),12+ proceeds with a rate constant k = 1.2 x 109M-'s-' and AHf = 32.2kJmol-', ASf = -27.6JK-'mol-','W6 The oxidation of H 2 0 to O2 has been achieved using Co" in the presence of [Ru(bipy),13 .IO4' The electrochemistry of [RuL,I2+ (L = bipy, phen) has been extended by the measurement of the [RuL3I4+l3+ redox couples in liquid SO, at -t 2.78 V and + 2.60 V vs. SCE respectively.7233726 +
(vi9 Bidentate heterocyclic complexes f RuL2L ' r + , [RuLL'L]"+
A range of mixed ligand complexes [RuL,L']*+ have been prepared and their spectroscopic, luminescent and redox properties studied (Table 9). The absorption and CD spectra of [Ru(phen),(bipy)12+and [R~(bipy)~(phen)]~+ have been discussed.'0s2[Ru(bipy), LI2+[L = 2-(aminomethy1)pyridine, 2-( l-aminoethy1)pyridine (73)] undergoes oxidative dehydrogenation on oxidation at + 1.16V vs. SCE (Scheme 26).1068,1074 The rate-determining step of reaction is deprotonation at the a carbon atom with concomitant two electron transfer from ligand to metal.'074The single crystal X-ray structure of (73) Ru-N(py) = 2.062A, Ru-N(NH,) = 2.121 A, Ru--N(bipy) = 2.064, 2.046, 2.044, 2.057 A. The single crystal X-ray structure of the related complex [Ru(bipy),(NH) C6H4I2+(74) has been reported and shows short RU--N(NH)~C~H~ distances of 2.038, 1.996k suggesting strong back-bonding from Ru" to the diimino ligand.""
Ruthenium
336
3t
H -+
bipy), Ru'"
Scheme 26
The luminescent properties of the excited state species [RuL,L']'+ * have been investigated with the aim of developing more efficient photosensitizers than [Ru(bipy),I2+.733 Mixed ligand Ru" complexes incorporating surfactant long chain ester derivatives of bipy have been synthesized, for and their luminescence investigated in micellar example (75) and (76),888~1055~1"h1-1n63~107~'083 These surfactant complexes are generally found to be inactive for the solutions. 106'-1063~1079~108' photochemical cleavage of ~ a t e r , ~ hydrolysis ~ ~ ~ of ' the ~ ~ ester~ moieties ~ ' ~and~ destruction ~ ' ~ ~of ~ ~ ~ ~ ~ assemblies being a particular problem with these systems.9s6Irradiation of [R~(bipy)~L]'+ (L = 2,2'bipyridine-4,4-dicarboxylic acid) in the presence of edta, MVz+ and Pt catalyst produces H, at one tenth of the rate as observed for [Ru(bipy),I2+ This is ascribed to the electron withdrawing effect of the carboxyl or ester functions to bimolecular quenching of the excited state by water;log6H+ transfer in the excited state species has been studied and the luminescence measured as a function of pH.Io8'The deprotonated complex was found to be a stronger base in the excited state than in the ground state,Io8' while for complexes of 4,7-dihydroxy- 1,lO-phenanthroline, an increase in acidity has been observed in the excited state.'053The linking of complexes [RuL,L'I2+ to polystyrene and chemically modified eIecsupports,Ioss and the preparations of vinyl trodes10W.1091 have been reported.
r
2+
L
(74)
(75) R =
w
The lifetimes of excited state [l(u(bipy),Ll2+* vary according to L; for example, the half-life for s compared [RuL,12+*(L = 2,2'-bibenzimidazole) t I T2= l.OOps, for [Ru(bipy),L]'+* f,,, = 3 . 0 0 ~as with t,,?= 4.0 jis for [Ru(bipy),]*+*.711.'5f This has led to the study of excited state species for mixed ligand systems particularly with regard to localized and delocalized formalisms for [RuL2L'I2+*. Using the EIP model, Crosby has proposed ligand-ligand coupling in the excited and ground state of [Ru(bipy),L]'+ (L = 4,4'-dimethyL2,2'-bipyridine, 4,4-diphenyl-2,2'-bipyridine,5,ddimethyl1,10-phenanthroline, 4,7-dimethyl- 1,IO-phenanthroline, 4,7-diphenyI- 1,lO-phenanthroline):M
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Ruthenium
345
[Ru(bipy),phen)]'' and [Ru(bipy)(phen),I2+.w7'0s0~1092~~0y~ More recently, the absorption spectra of [Ru(bipy),(phen),_,]'+ have been interpreted as superpositions of the tris chelate [Ru(bipy),]'+ and [Ru(phen),]'+ end points,Io5'suggesting localized, weakly coupled excited states. For the complexes 2,2'-biquin0line,'~~~~~~~~~~ 3,3'-biq~inoline,~~'~ [Ru(bipy), LI2+(L = 2-(2'-pyridyl)quin0line,~~~~~~~~'~~~ 2,2'-bipyridine-4,4'-dicarboxylic acid,''" 2,2'-bipyridine-4,4-diethyl carboxylate,'0574,4-dinitro2,2'-bipyridineIw4), [Ru(bipy)L,l2+ (L = 3,3'-biquino1ine)log4 and [Ru(phen), LIZ+ (L = 2-(2'pyridyl)q~inoline,'~"t ~ i p y , "272'-biquinoline,''N ~~ dmch'Og4)no clear dual emission has been observed, emission occurring from the lowest energy charge transfer excited state localized on the ligand which is easiest to r e d u ~ e . ~ ~ ~ . 'A~ d" egree . ' ' ~ ~ of energy transfer from phen to 2-(2-pyridyl)quinoline (L) has been suggested however for [Ru(phen),L,-.]*+ The electrochemistry of [RuL, LIZ+ (L = 2,2'-bipyrimidine, L' = bipy, 4,4'-dimethyl-2,2'-bipyridine, 5,5'-dimethyl-Z,2'-bipyridine) indicates that for the primary reduction product, the electron is localized on a single 2,2'-bipyrimidine ligand.734Kinetic and emission data for a range of mixed ligand complexes [Ru(bipy), xLx]2+ (L = 2,2'-bipyrazine, 2,2'-bipyrimidine), [RUL,-,L',]~+ (L = 2,2'-bipyrimidine, L' = 2,2'-bipyrazine) and [RuLL'L'l'' (L = bipy, L' = 2,2'-bipyrimidine, L = 2,2'-bipyrazine) have been reported.1098a For synthetic and electrochemical work in this area see references 1098a-d. Spectro-electrochemical studies on [R~(bipy),L]'+~~~'(L = 2,2'-biquinoline, phen, dmch) have been carried out and the results related to a localized redox orbital m ~ d e l . ~ " [Ru(bipy), L]" (L = 4,4-dichloro-2,2'-bipyridine) is susceptible to attack by nucleophiles at the 4,4' positions of the substituted bipy, reactivity being higher for the coordinated than for the free ligand.IoYY (viii) Biden fate kederocyclic complexes with carbon donor ligands A wide range of complexes of type [ R U L ~ X Y ] ~(L ~ ~=~bidentate +~' heterocycle) have been prepared and their chemical and photochemical properties inve~tigated.~~~~~~~'~,~~'~''~ (a) Cyanides (see Section 45.3.1) Cis-[Ru(CN), (bipy),] can be isolated by treatment of [Ru(ox)(bipy),] with NaCN in aqueous MeOH and chromatography on silica or alumina,40~41~s5~266 by photolysis of [R~(bipy),](CN),'("'~ or by reaction of RuCl, with L (L = bipy, phen or heterocyclic derivatives) followed by NaCN to yield [ R U ( C N ) , ( L ) , ] . ~ ~ ~The ~ ~ ' interaction ~*~ of [Ru(CN),(bipy),] with Ag+,I1*' [Pt(dien)]'+ (dien = diethylenetriamine),"02 [PtC13(C2H4)]-and cis- and trans-[PtCX,(L)(L)] (L = CzH,, Me,S; L' = 4methylpyridine)50."03yields .CN bridged 1:1 and 1 :2 adducts and the luminescence properties of these complexes have been described, The excited state chemistry'lWand electron transfer quenching Cu2+,'Io5 Fe3+7:y paraquata7' and reactions of [Ru(CN)?(L),]* with pyridinium salts,"" 02r857 aqueous a ~ i d " ' ~ ~ have " ' ~ been reported. The electrochemistry of [Ru(CN),(bipy),] has been reported.268b Hydrolysis of [Ru"'(CN),(bipy),]+ yields [Ru(bipy), (OH,),]3+ .64 (b) Carbonyis and related ligands Reaction of the red solution obtained from refluxing RuX, with CO in EtOH with L yields [RuX,(CO),L] (X = C1- , Br- ; L = bipy, phen,48~"S~652,"08"1" d i p h ~ s ~ " ). , ~Treatment '~ of [RuX,(CO),L] with H X gives [RuX;(CO),L] (X' = CF,CO;, MeCO;, CF,SO;) from which [RU(CO),(NCM~),L]~+ can be generated."08 The electrochemistry of [RuCl,(CO), L] (L = bipy, phen) has been reported.625The decarbonylation of [RuX,(CO),L] (77) (L = bipy, X = CI-, CF,CO;; L = phen, X = C1-, Br-) with Et,NO in pyridine yields [RuX,(CO)L(py)] (78) with while treatment of [RuCI,(CO),L], [RuCI,retention of stereochemistry (equation 86):'' (CO)py(phen)]or [Ru(CO),(phen),I2+with Me,NO in 2-methoxyethanol in the presence of L gives [RuL(L'),]'+ (L = phen, bipy, L' = bipy, phen, 3,4,7,8-tetramethyl-l, 1O-phenanthr~line).~~~ [RuCl, (C,H,)COj- reacts with L to produce [RuCl,(CO)L], and [RuCl,(CO)L]- (L = bipy, hen).^^' Treatment of [Ru(O,SCF,),(CO),phen] with L (L = bipy, phen, 3,4,7,8-tetramethyl-l ,lophenanthroline,t108 4,4-dimethyl-2,2'-bipyridine,1"2" 4,4-dii~opropyl-2,2'-bipyridine'''~") yields the complexes [Ru(CO),(phen)L]'+; likewise reaction of [Ru(CO),(NCM~),L]~+with L gives cis[Ru(CO),L,]2+."0s~"1?a [Ru(CO),(bipy),](PF,), catalyses the reduction of CO, to formic acid and CO under alkaline conditions, and to CO only under acidic conditions."'2b The formation of [R~(C0),py,L1~+ (L = bipy, phen) has been achieved by treatment of [ R u ( C O) , ( NC M ~ ) , ~ ~ , ] ~ + with L.656Reaction of [RuCl,(CO),L] with L'H (L'H = acetylacetone, benzoylacetone, dibenzoylmethane, 2-hydroxyacetophenone, 2-hydroxbenzophenone, 3-hydroxy-4-methoxy-benzophenone, 8-hydroxyquinoline, salicylaldehyde, L = phen, bipy) yields [Ru(CO), LL;].1"3 Scheme 27 illustrates the synthesis of some heterocyclic complexes of ruthenium carbonyls.
Ruthenium
346
Et,NO
PY
i, L (L = p h e n , hipy);""8~1"2~"14 ii, bipy, H,O, EtOH. A;"14 iii, hv, S ( S = McOII. HZO,McCN);"'~ iv, HOAc or Ag0,CR;6Sh v. C'F,SO,H (I = phen);hS6 vi, Ag02CCF3;656 vii, L' (L' = phen, bipy, 3,4,7,8-tctramcthyl-I 10-phcnanthroline)6s6~'10s~ "I2 ~
Scheme 27
Reaction of RuCI, with two equivalents of bipy in DMF yields [RuCI,(bipy),] together with Cis-[RuCl(CO)L,]+ (L = bipy, phen) can also cis-[RuCl(CO)(bipy),]+ (79) in 30-50% yield."15-1'17 or by reaction of cis-[RuCl(bipy),OH,]+ with Phbe prepared by carbonylation of CECH in water at 100°C.11'9Extended carbonylation of [RuCl,(bipy),]'in the presence of Ag+ yields [Ru(CO),(bipy),12+.1116,1118The single crystal X-ray structure of cis-[R~~Cl(CO)(bipy)~]* (79) shows a six-coordinate Ru" centre with Ru-C = 1.861,Ru-CI = 2.396,Ru-N = 2.097, 2.066, 2.177, 2,104 A."" (79) catalyses the photochemical water-gas shift r e a ~ t i o n . " ~ ~ [RuCI"'~~ (CO)(bipy),J+ reacts with N donor ligands L' under reflux to give cis-[Ru(CO)(bipy), LIZ+, whereas The relevance of this reactivity to the the photochemical reaction yields cis-[RuCl(bipy), L']+ .'I5 possible synthesis of [RuCl(bipy), (q'-bipy)]+ has been discu~sed."'~Cis-[RuH(CO)L,]+ can be prepared by treatment of [RuCI(CO)L,]+ (L = bipy, 4,4'-dimethyl-Z,2'-bipyridine) with BH;, and reacts with aqueous acid to give [R~(C0)(bipy),OH,]~+and H2 (equation 87).II2' Reaction of [Ru(CO,)(bipy),] with HC0,H for 1 h affords [Ru(O,CH)(bipy),CO]+ which loses C 0 2in refluxing 2-methoxyethanol to yield cis-[RuH(bipy),CO]+ . I i 2 ' The role of [RuH(CO)(bipy),]+ as a possible intermediate in the photochemical generation of H, from H 2 0 by [Ru(bipy),12+ has been intimated,"2'
Ruthenium [WH)ICO)(bipy),lC f H,O
----t
347
IRu(CO)(bipy),(OH,>f*
+ H,
(87)
Reaction of ~is-[Ru(bipy),(OH,),]~+with PhC-CH yields cis-[Ru(CO)(q' -CH2Ph)(bipy),],"lg while treatment of czs-[Ru(bipy),(acetone),]2+ with nbd gives [Ru(C,H,)(bipy),]'+ (A = 421 nm, E = 6780M-'cm-', E,!2= + 1.70V vs. SSCE, in MeCN.I9' Trans-[RuCl,(C,H,)(bipy)] (A = 390 nm, E = 1460M-'cm-', El,, = 1.04V vs. SSCE in MeCN) has been prepared by reaction of [RuCl,(C,H,)], with bipy in acetone."' The single crystal X-ray structure of trans-[R~(bipy),(PPh,),]~+ 1122~1123and the synthesis of related bis(phosphine) cornplexe~'*"~ have been reported. Reaction of [RuCl, J (L = bipy,"", 2-(arylaz0)pyridine'"') with phosphine, arsine and stibine ligands (L') affords the complexes can be isolated."" (See Section 45.5). [RuCl(L),L]+ while with bidentate ligands L [R~(bipy)~L']'+
+
(ix) Bidentate heterocyclic complexes with nitrogen donor ligands
( a ) Tertiary amines A range of complexes of type cis-[RuL,L'Jz+ and [RuLL'&]'' ( L =bidentate heterocycle; L' = tertiary amine) has been synthesized and their electronic spectra a n a l y ~ e d . The ~ ~ ~synthesis ~'~~ of [Ru" L2LJ+ (L = bipy, phen, L' = py,264-266 ~ 2 ,pyrazole,"26 ' ~ ~ ~ cis- and arans-4-stilbazolee~127~'128 -NCS649266 substituted t r i a ~ o l e s , "MeCN,266.'s7."26 ~~ N; 3- and 4-nicotinic acid,'n583- and 4-amin0pyridine,'~' 4,4-bipyridine;8sn,'09' L = 4,4'-dimethyl-2,2'-bipyridine, L' = py1122*"23) can be typically achieved by refluxing [RuCl, L2] with L',"'1,"267'1'7*113' treatment of [RuCI, L, J with Ag+ followed by L'"z66."32or by reaction of L' with [ R u L ~ ( O E I ~ ) ~ Th]e ~absolute + . ~ ~configuration ~~~~~~~ of cis-[Ru(phen),py,]'+ has been assigned via absorption and CD s p e ~ t r a ? ~ Reaction ~ - ' ' ~ ~ of trans[RUL,(OH,),]~' with L' (L = bipy, phen, L' = py, MeCN, pyrazole) yields the complexes trans3 single crystal X-ray structure of trans-[Ru@hen),py,I2+(80) shows [RuL,L,]~+.61z*'w5~1'2'*'' 3 ' ~ 1 1 3The Ru-N(py) = 2.097, Ru-N(phen) = 2.096, 2.100 A"22,"33with unfavourable cc-H interactions being proposed between heterocyclic rings;'005structural analysis of trans-[RuL,py,]'+ (L = 4,4'dimethyl-2,2'-bipyridine) confirms distorted bipyridyl ligand^.^'^^"^^ Photolysis of trans-[Ru(phen),py,12+ yields the cis isomer in good yield.'133Photolysis of [Ru(bipy),]'+ or [R~(bpz),]~+ yields substituted products via loss of one bipy or bpz ligand.7s',757,1002,'W4~1"5 The electrochemistry of cis- and trans-[R~(bipy),py,]~~ has been reported26Sa,268b,69' ; cis- and trans-[Ru(bipy), pyJ+ have been electrogenerated and show intervalence charge transfer bands at 4350 cm( E = 121 M -'cm-') and 4090cm-l (E = lOOM-'cm-') re~pectively.~~' Deprotonation reactions of [Ru(bipy),L,I2+ (L = pyrazole) have been studied,'Iz6while photochemical isomerization reactions of [R~(bipy),L,]~+ (L = truns- and cis-stilbazole) and [Ru(bipy),LX]+ have been reported to occur The formation in butyronitrile via CT excited state to give ligand radical anions (equation 88)."2751'28 of pseudo-base adducts between nucleophiles and [Ru(bipy),py,l2+ has been rep~rted,"~'but later refuted by other workers as a simple substitution reaction with pyridine loss.'o" Reaction of [RuCl,(bipy)] with py in refluxing EtOH for 24 h gives [Ru(bipy)py, ]2+;612*1131 further reaction with L (L = en. 2-methvlaminopyridine. 2-ethylaminopyridine, phen, bipy) yields [Ru(bipy)(L)py, 1' + .61i ,2663113n
'
C.O.C. e l :
348
Ruthenium
[RuCl(bipy),py]+ can be prepared by treatment of [RuCl,(bipy),] with py followed by NaCl;1136 substitution reactions of ~is-[Ru(phen)~py,]~+ with a range of anions proceed with retention of config~ration."~~ Photolysis of [Ru(bipy),py,12+ and [Ru(bipy),(NCMe),l2+ in CH,Cl, with X yields [RuX(bipy),py]+, [RuX(bipy),(NCMe)]+ and [RuX,(bipy),] (X = ClO;, NO;, -NCS, Br-, C1-, NO;) via a dissociative mechani~rn."~~ A range of vinyl and carboxypyridyl derivatives of [Ru(bipy),]'+ have been prepared and electron transfer reactions and the photochemical activity of these complexes as polymer films at electrode surfaces s~U~~e~~745,861,953,1058,1138-l14B Some chemical transformations involving polyvinyl pyridine (PVP) are illustrated in Scheme 28."38~'142-'~u~1149 The charge transfer luminescence spectra of cis- and tram-[Ru(bipy), pyJ2+ ,1096,1I5O cis-[Ru(bipy), LJ4+ (L = 2-methyi-4,4'-bipyridinium+),'I5' cis- and tram-[R~(bipy)(phen)py,]~+,[Ru(bipy)py,(L)]'+ (L = en, (NH312, 2-methylaminopyridine, 2ethylaminopyridine) and [RuCl(bipy)py,] have been measured.lW6
I" i, solvent S; ii, pz; iii: PVP, A: iv, C1-; v, PVP, A; v i , c'1-. A : vii. hu, solvent S; viii. hv. solvent S, ix, PVP, A;
X,
PVP
Scheme 28
For electron transfer reaction between [Ru(bipy),py,13+ and [Fe(OH2),fZ+, AH' = + 1.26kJmol-' and AS' = -1305K-'m01-'.'~~ ( b ) Nitrosyls and related ligands Reaction of [RuX,L,] (X = Cl- , Br-; L = bipy, phen) with NO; yields [Ru(N02),L,]; acidification with HCI forms [RuC1(NO)LJ2+while with HPF, [Ru(NO2)(NO)L2]*+ is pr~duced."~""" The thermal decomposition of (phenH), [RuCL,(NO)] gives [RuCl(NO)phen]*+ while reaction of [RuCl, (NO)],619,"55,"56 or [RUCI~(NO)]~-"'~ with L yields [RuCl,(NO)L]. Treatment of trans[ R ~ ( b i p y ) ~ ( O H ~ )with , ] ~ +NaNO, followed by acidification gives ~ran.~-[Ru(NO)(OH)(bipy)~]~+ .Iws Scheme 29 summarizes work on the interconversion of NO+, NO; and NO; c o m p l e x e ~ . "The ~~ reduction of [Ru(N0)(bipy),Lj3+ (L = NH,, py, MeCN) and [RuX(NO)(bipy),12+ (X = N;, C1-, NO; ) electrochemically or by I- yields [Ru(NO)(bipy),LI2+and [RuX(NO)(bipy),]+ respective~y~l159,1165,1166 There was found to be a linear correlation between vNo and El,, for the reversible one electron reduction of these complexes. The site of reduction is essentially n* (NO) in character and thus can be regarded as reduction of coordinated NO+ to NO with a lowering in frequency of the N-0 stretching vibration vNo from 1940 and 1970 cm-' for [RuCl(NO)(bipy),]'+ and [RumO)(bipy),NCMeI3+to 1650 and 1610cm-' for [RuCl(NO)(bipy),l+ and [Ru(NO)(bipy),NCMe]'+ respecti~ely."~~~'"~~''~~ [RuCl,(bipy),] reacts with [Ru(NO)(bipy),NCMeI3+ (rate constant k '> IO6 M-' s - ' ) to give [RuCl,(bipy),]+ and [Ru(NO)(bipy),NCMe]'+ ."s9*1'66 Th e oxidation of Cis-LRuCl[RuCl(NO)(bipy),]+ at chemically modified electrodes has been reported. (NOJbipy),] can be oxidized to give a transient Ru"' complex [RuCl(NO,)(bipy),]+ which decomposes via 0 transfer to yield [RuC1(NO)(bipy),12+ (81) and tRuCl(NO,)(bipy),]+ (82) (equation 89).'167*''68 This observation has led to the use of [RuCl(NO,)(bipy),] and [Ru(NO,)(bipy),py]+ for the electrocatalytic oxidation of PPh, under alkaline conditions whereby the NO; complex can be regenerated from the NO+ species by reaction with base (OH or lutidine) (Scheme 30)."67This electrocatalytic cycle has been extended by immobilization of [Ru(NO,)(isn)(bipy),]+ on alkyl amine silanized modified PtO electrodes, thus enabling the catalytic oxidation of PPh, at a modified electrode surface. I169~1 I7O A series of electrophilic reactions of coordinated nitrosyl group has been reported. [RuCI(NO)(bipy),]*+ reacts with aryl amines ArNH, to give [RuCl(N,Ar)(bipy)]*+which shows a degree of diazonium ion character with vNZN in the range 19SO-2100~rn-'.''~~~~'~~ Cleavage of these complexes occurs with KI in aqueous solution at 60°C to yield [RuI,(bipy),], N, and iodoaryl
Ruthenium
349
[ RuCl(bipy),S]+ \
i, hv, Me1:N;'15' ii. N;; S (S = H2O, McCN. MeOH. a c e l o 1 1 c ) ; " ~ " ~ ~ ~ ~ " - ~ ~ ~ ~ iii, ~ ~ ; : l l B l iv. - ~ ~ ; I l 5 4 . 1 1 v, 6 3II', SnCI, vr BF3;'lh3 vi, N;;II5' vii. HPF6;"" viii, Cerv;1'53i x . ( a ) KN3 (b) py(I\;H,, MeCN, H20);'i534i159,1161 X , H+:11s1'15'1 ~ + 12c-, xi, + 6e-: + 5H+;llb4 xii. L' (L' = MeCN. NH3, PPh3, P Z ) ; " ~ xiii, xiv MeCN, HPF,;"" xv, (a) NO; (b) HC1;"" xvi, xvii, NO, ' I s 2
-
2[R~Cl(NO,)(bipy)~]
- 3e-
+ l'i!H+;1164
Scheme 29
[RuC1(NO)(bipy),I2+ + [RuC1(NO3)(bipy),]+ @1)
(82)
Scheme 30
I
produ~ts."'~[RuCl(p-N2C6H,Me)(bipy),]*+ shows an irreversible one electron reduction to give a coordinated N, Ar radical product which decomposes in MeCN to [R~Cl(bipy),NCMe]+.~'~~ Reacwith C6H,NRR yields [ R U X ( N ( O ) C , H , N R R ) ( ~ ~ ~ (X ~ ) ~=] +Cl-, tion of [R~X(No)(bipy)~]~' R = R' = Me, R = H, R' = Me; X = NO;, R = R' = Me, R = H, R = Me).'173"N labelling The same experiments indicate that the N of nitrosyl remains bound to the metal ~ e n t r e . " ' ~ complexes can be prepared by reaction of [RuCl(bipy),(acetone)]+ with 4-N(O)C,H4NMe, and show dn-+ A (nitrosoarene) charge transfer bands near 600 nm in addition tu bands assigned to d n -,7f+ (bipy) transition^."^^ [RUC~(N(O)C,H,NRR'}(~~~~)~]+ shows a Ru"I"' couple and a one electron reduction on the nitrosoarene ligand."" Further reaction with a second mole of arylamine can occur to yield [Ru(N(O)C,H,NMe, ),(bipy),12+ (83 in equation 90).'173,'174 Addition of arylhydrazones RCH=NNHR (R = Me, Ph, p-MeC,H,, R = Ph, p-MeC,H,) to [RuC1(NO)(bipy),12+ gives the products [Ru(bipy),L]+ (84) in MeOH/MeO-."76 (84) can also be synthesized by direct reaction of NaL with [RuCl,(bipy)J. Protonation raises the RU"'~''couple from 0.8 V for (84) to 1.O V vs. SCE for [Ru(LH)(bipy),12+ . ' I T 6 Treatment of [RuC1(NO)(bipy),I2+ with phenol yields [RuCl(N(O)C,H,OH)(bipy), 1' .Ii7, Reactions of [R~Cl(NO)(bipy)~]~+ and [R~(NO)(bipy),py]~+ with RO- (R = Me, Et, P?) yield the products [RuCl{N(O)OR)(bipy),]+ and [R~{N(O)OR}(bipy),py]~+re~pectively;''~~
+
+
1
,
R
(84)
dxRu" -+ f i (N(0)OR) back-bonding is significant in these species, the N(0)OR ligand being a better 72 acceptor than NO, py or MeCN."77 The photo induced addition of -OH to [RuCI(NO)(bipy),]'+ gives the metastable product [RuCl(NO,H)(bipy),]' which regenerates the starting material in the dark."78 SO:- reversibly adds to [RuX(NO)(bipy),]+ (X = C1-, Br-) to give [RuX(N(O)SO,}(~~~~),].~~~ The single crystal X-ray structure of the chloro complex shows a distorted octahedral environment around Ru" with Ru-N(N(O)SO,) = 1.90 A and a long N-S bond Length of 1.82 Addition of Hacac to [RuCl(NO)(bipy),]'+ gives N-bound MeCOC(R0)COMe."" ( c ) Other nitrogen donors Thermal substitution reactions of [RuClz(bipy),] with diazadienes (dad) yield [Ru(bipy)?dad]'+ species."" The N-bound thiocyanate complex [R~(NCS)~(bipy),l~~' can be prepared by treatment of RuCl, with bipy in refluxing EtOH for 48 h followed by NaNCS for 24 h,667,'18' or by photolysis of [R~(bipy),l(NCS),.~~~,~~~~ The latter method also yields {Ru(NCS)(bipy),DMF]+ as a side product.'003[Ru(N,),(bipy),] and [Ru(N,)(bipy),L]+ (L = py, MeCN) can be electrochemically o r chemically oxidized with Br, or Ce" to the corresponding Ru"' species which undergo thermal and photo induced decomposition in MeCN via electron transfer from N; ta Ru"' (equation 91)."*' Treatment of [Ru(nbd)(NCMe),I2' with L (L = bipy, phen, 5-methyl-1,IO-phenanthroline) yields [Ru(L)(NCMe),12+.'Is3 Oxidation of [ R U ( ~ ~ ~ ~ ) , ( N H , C H , Rby) ~eight ] ~ + electrons leads to the formation of the dinitrile complex [Ru(bipy),(NCR),I2+ .llX4 [Ru(N, ),(biPY),l
(x)
[Ru(N,)(bipy),(NCMc)l +
D
2
191)
Bidentate heterocyclic complexes with oxygen donor ligands
Ruthenium (ZI) : Acidification of [Ru(CO,)L,] (L = phen, bipy. bpz) in aqueous solution yields cis-[Ru(bipy),(OH,),]2f 1119,1133.1185-1 187 which on photolysis gives the trans isomer via a five-coor[RuCI, LJ (L = 2-(pheny1azo)pyridine. 2-(3-tolylazo)pyridine)underdinate intermediate.1005,"33."s7 goes stereoretentive displacement of C1- in the presence of Ag+ to yield [RuL,(OH,),]~+which can be deprotonated to [Ru(OH)L, (OH2)]+ and ~ R u ( O H ) , L , ] . " ~The ~ , ~diaqua ~ ~ ~ complexes are useful starting materials for the synthesis of products of type [Ru(bipy),XZ]"+ (X, Z = e.g. py, pz, MeCN, NO;, NO+, halides, PR,).6"~'oos~1'33,"61,i 1 8 i , 1 1 8 s , 1 1 8Scheme 8 3 1 shows some substitution chemistry of the [Ru(bipy),I2+ moiety. The second order rate constants for substitution on [R~(bipy),(OH,),]~+ , " ~catalytic ' photooxidation of water using decrease in the order N3 > NO2 > SCN > p ~ . " ~ ~The trans-[Ru(bipy),(OH,),] on hectorite has been reported, the active complex being thought to be [Ru(bipy),(OH,)],04+ '9I. Treatment of [RuX(NO)(bipy),]*+with KN, in solvent S yields [RuX(bipy),S]+ (X = C1-, NO,; S = acetone, methanol, H,0)."61,"62Likewise [R~(bipy),py(OH,)]~+ can be prepared by reaction of fR~(NO)(bipy),py]~+ with KN, in water or by treatment of [RuCl(bipy),py]+ with Ag+ in water."36 while [RuCl, Cis-[RuCl,(bipy),] in refluxing DMSO yields cis-[RuCl(bipy),DMSO]+ ,'I9' (DMSO),] with 8-aminoquinoline (L) gives [RUCI,(L)(DMSO)~].'~~~ The single crystal X-ray structure of [Ru(CO,)(bipy),] (85), prepared by reaction of [RuC12(bipy),] with NaHC03,"h"."93shows bidentate C0:- with Ru-0 = 2.087, Ru-N(cis to 0) = 2.042, Ru-N (trans to 0) = 2.013 Treatment of [RuCJ,L,] (L = bipy and phen derivatives) with carboxylates (L') gives products of type [RuL'LJ+ (L' = 2,5-pyrazine dicarboxylate,5h2oxalate,264.369,"95 aCaC268a.269). Reaction of
[Ru(CO,)(bipy),] with hydroxamates (H,L) yields the complexes [Ru"(HL)(bipy),]+ and [Rd'(L)(bipy),], which show Ru"~"' and Rullr'lvcouples in MeCN. A series of partially characterized complexes involving hydroquinone have been reported.' 19'
0
(85)
Ruthenium (III): [R~(bipy),(OH,),]~+has been proposed as an intermediate in the reaction of RuCl, with bipy in aqueous conditions at pH 2."" In the presenp: of ClOi-, [R~(bipy),(OH,),]~+ is oxidized to [Ru(OH)(bipy),OH,]~+. The single crystal X-ray structure of trans-[Ru(OH)(bipy),OH,]'+ (86) shows Ru-N = 2.090, 2.099 and Ru-0 = 2.007 8, with intermolecular hydrogen bridges between coordinated OH, and OH Oxidation of [Ru(bipy),py(OH,)I2+ at + 0.8 V vs. SCE yields [Ru111(OH)(bipy)~py~2j,11~~ while oxidation by NO; gives in acidic via a Ru" intermediate (Scheme 32).'19* In the presence of conditions [R~"'(bipy),py(OH~)]~+ NH,SO;, the HNO, produced is reduced to N2 and on electrochemical regeneration of the Ru" starting material, the system becomes catalytic for the reduction of NO; to N2.'I9'The oxidation of H,O, by [R~(OH)(bipy)~py]~+ has been studied."99 [Ru(bipy),py(OH,)I3+ has a pK, = 0.85 compared to its Ru" analogue which has a pK, of 10.79.1136 The formation of [R~(OH)(bipy),py]~+ from [R~(bipy),py(OH,)]~+ and [Ru(O)(bipy),py]*+is first order in [Ru"] and [RU'"].'~" Oxidation of hydroxamate complexes [Ru"(LH)(bipy),]+ and [Ru(L)(bipy),] with Ce'" yields the corresponding Ru"' species. 2t
Scheme 32
Ruthenium
352
Rutlaenim(IV): [Ru(bipy)zpy(OH,)j2+can be oxidized by Ce" or by controlled potential elec( vRu-o at 792 cm-') which oxidizes PPh, to OPPh, (Scheme trolysis to [R~(O)(bipy)~py]~+ 33).'136,'201-'204 "0labelling shows that 0 transfer from Ru=O to PPh, occurs via first order kinetics has been used electrocatalytically with rate constant k = 1.75 x lo5M-'s-' .1202 [R~(O)(bipy),py]~+ for the oxidation of alkenes, alcohols, aldehydes, aromatics'204and chloride to chlorine.'205The H-D kinetic isotope effects for the oxidation of H 2 0 , by Ru"' and Ru" have been measured as 16 and occurs via a two electron 22 respectively.' 199 The oxidation of 2-propanol by [R~(O)(bipy)~py]~+ hydride transfer from the a-carbon to Ru=O; no transfer of coordinated 0 was observed, while a slower oxidation of substrate by Ru'" via a one electron transfer was detected.I2O3Electrode supported oxidation catalysts based on Ru" have been achieved by deposition of films containing [Ru(bipy),(OHJ2+ bound to poIy-4-~inylpyridine.l'~~ [Ru(0)L2pylZC (L = 2-(phenylazo)pyridine), prepared by Ce4+ oxidation of [RuL,py(OH,)]'+ (F= 1.20V), catalyses the oxidation of H,O to 0,.'206
+
t 0.53
[R~(bipy),py(OH,)]~+
V
[Ru(bipy),py(OHjI
'+
PPh,
+ O 42
[RLI(~I~Y)~P~(OII
")
[R~i(bipy)~py(OPPh~)l 2+
'+
* [Ru(bipy),py(MeCN
j1 2+
+ OPPh3
Scheme 33
Ruthenium( V j , ( V I ) , (VII), (VJII): Electrochemical studies on ci~-[Ru(bipy),(OH,),]~+show a series of one electron couples to Rul'Ti'viv~vT (Scheme 34),'207,'208 the Ru"' species being thought to be the active complex in the oxidation of C1-.'2"5Reaction of RuO, with bipy and phen yield products of stoichiometry Ru0,bipy and Ru,0,phen2 which have been assigned as Ru""' and Ru"" complexes respectively;1209 from their spectroscopic and chemical properties however they appear to be lower valent species. A series of partially characterized products have been synthesized by reaction A genuine Ru" complex [Ru(O), C1, (bipy)] has of these complexes with bipy in MeOH.674"~'195~1210 been prepared from the reaction of RuO, with HCI and bipy.65,12" cis-[ R~(bipy)~(OH,),] 2+
-
[ Ru(bipyj,(OH)(OH,)]
2+
'0.76 "+ [ R ~ ( b i p y ) ~ ( O ) ( O H , ) l ~ ' ~
Scheme 34
(xi) Bidentate heterocyclic complexes with su1fur donor ligands
The synthesis and catalytic behaviour of [Ru(S2MoS,)(bipy),] (87) have been
(xii) Bidentate heterocyclic complexes with halide ligands Ruthenium(1I): A range of complexes of type [RuCl,L,] have been prepared by a variety of routes, 264-266.738 Cis-[RuCl,(bipy),] can be prepared by reaction of RuCl, with bipy in refluxing DMF for 8 h in the presence of LiC1,1"5J'" by substitution of py in ci~-[Ru(bipy),py,]~'by Cl- ,'I3' or by activation of RuCI, in refluxing H20/EtOH, followed by evaporation and dissolution in aqueous HC1 and addition of bipy in H,0/acetone.'212bTrans-[RuCl,(bipy),] has been synthesized by
c
Ruthenium
353
addition of LiCl to trans-[R~(bipy),(OH,),]~~ or by photolysis of [Ru(CO,)(bipy),] in HCl.Ioo5 Refluxing solutions of [Ru(bipy)py,l2+ with bipy in EtOH followed by addition of HCl yield mixtures of cis- and rrans-[R~Cl,(bipy),].l'~~The cis isomer can be prepared by repeated treatment of the cisltrans mixture with py to give [Ru(bipy),py,12+ followed by conversion to the dichloro complex with HCl."" Other routes to [RuCl, (bipy),] include reaction of [RuCl, (Hdma)] (dma = dimethylacetamide) with b i ~ y , " ' ~IRUC~,(OH)]~-with bipy and X- (x- = C1-, Br-, I-),369*708,1214 photolysis of [R~(bipy),py,]X,"~' or [Ru(bipy),]Cl, in acetone,'006heating [RuCl(bipy),(NCMe)]Cl in CH,C1,,'006vacuum pyrolysis of [Ru(bipy),]Cl, at 290°C for 2 h369or treatment of [Ru(ox)(bipy),] with HCl.369The electronic spectra of [RuX,(bipy),] (X = C1-, Br-, I - ) have Thermolysis of (phenH)[R~Cl,phen]~~',"~~ or reaction of [Ru(phen),py2j2+ been reported.267,738*785.1214 and 2,2'-biq~inoline'~~~ derivatives of with X- 'I3' yields [RuX,(phen),] while 2,2'-bipyrazine''85,'2iS [RuCl, L,] have also been reported. Treatment of (Hpq)[RuC14(pq)]with HC1, Hg and KC1 yields Refluxing RuCl, with 2-(pheny1azo)pyridine (L) in [RuCl, (pq),] (pq = 2-(2'-pyridyl)q~inoline).'~~ EtOH gives blue j-[RuCl,L,] (88).I2l6Reaction of L with [RuCl,(DMSO),] in acetone for 17 h1216 or refluxing RuC13with L in MeOH for 8 h'046leads to the formation of green y-[RuCl,L,] (89) which can be converted to the blue ct isomer [RuCl,LJ (90) by reflux in xylene for 3 h.I2l6Substitution reactions of [RuCl, L2] have been reported.611*'066
[RuCl,(bipy),] undergoes solvolysis in H,0,1'81DMS0'19' and MeCN.'O06[RuCl(bipy),OH,]+ has been partially resolved by ( +)-tartrate.l2I7 Reduction of [RuCl,(bipy)]- by Ti2+ yields [RuCl,(bipy)]'- which catalyses the hydrogenation of maleic acid to succinnic acid via a [RuC13(H)bipy12intermediate.I2" The reductive electrochemistry of [RuCl,(bipy),] and related [Ru(bipy),]'+ species has been interpreted in terms of a spatially isolated redox orbital model with R* electron density localized on individual bipy ligands.268a Ruthenium(III1: For [RuCl,(bipy),], El,, (Ru"'"') = 0.35V vs. SCE, with electron withdrawing [R~CI,(bipy)~]+ can be prepared by oxidation of [RuCl,groups on bipy increasing El,2.268a,738 (bipy),] ,369.675.1 Ig7 by Cl- substitution on [RuCl(p-N,C, H, Me)(bipy)2]2+1'4s or by CI, oxidation of [Ru(ox)(bipy),] in the presence of C1- .369 [RuCl,(bipy),]' undergoes rapid solvolysis in H 2 0to yield [R~Cl(bipy),OH,]~* ; both [RuCl,(bipy),l+ and [RuCl(bipy),0H2]2+have been resolved with (+)tartrate.I197.1217.1219 [RuCl,(bipy)OH,] can be prepared by reduction of [RuCl,(bipy)] in H 2 0 and EtOH26sor by refluxing RuCI, with aqueous HCl and bipy for several day^.^^^,^^^,^^^^ The complex is often used for Refluxing [RUC~,OH]~the preparation of tris chelate species [Ru(bipy)L,I2+ (Table with L in the presence of C1-,264*265 or treatment of RuC13with H2in MeOH followed by aerial reflux of the resulting green solution with L and KCl has been reported to give [RuCl.,L]- (L = bipy, phen)."" The magnetic properties of [RuCI,bipy]- have been studied.'63 Ruthenium f I V ) :[RuCl,L] has been reported to be the product of the oxidation of LH[RuCl,L] with C1, or Ce'" in HN0,,2647,265 while thermolysis of (phenH,)[RuC1,] yields [R~Cl,phen].~'~ The magnetic properties of these complexes have been reported.12"
+
9).6129712,74531131
(xiii) Bidentate heterocyclic complexes with mixed donor ligands
Reaction of [Ru(azb)Cl(CO),], (azb = 2-(pheny1azo)phenyl) with bipy gives [Ru(azb)(CO),bipy]+ [Ru(azb)Cl,(CO),]- (91) with the azb ligand binding through N and C.I2" Treatment of [RuCl,(bipy),] with L- yields [RuL(bipy),]+ (92) which shows redox activity at = 0.39, - 1.67 and - 2.00 V vs. SCE.'070*107' Complexes (93) have been prepared by reaction of L with [Ru(CO,)(bipy),] or [R~(bipy),(EtOH),]~+;~~~~ oxidation of (93) to [Ru"'L(bipy),]*+ can be achieved chemically or electrochemically, the energy of the L -,metal charge transfer band in the Ru"' complexes correlating with the values for the Rull'lli redox The complexes [Ru-
+
' I
354
Ruthenium
( b i p ~ ) ~ L(L ] += azophenols, azonaphthols) have been reported.1224 Reaction of [RuCl,(bipy),] with As for the complexes (93), isonitrosoketonates yields complexes of type [RuL(bipy),]+ (94).'225*'22b (94) show correlation between E,,, for the RdI'I'I couple and the energy of L -+ metal charge transfer bands in the Ru"' species; protonation of (94) raises the Ru"~'" redox An alternative route to (94) is by the reaction of cis-[R~Cl(NO)(bipy),]~+ with propiophenone in MeOH/Me0-.'227
L
L
c
Q (93)
(94j
Chiral complexes of amino acids and related ligands of type A and A [RuL(bipy),]+ (L = (-)serine,'=' (- )-tryptophane,I2*'5-hydro~ytryptophane,'~~' phenylala~ine,'~~' ( - )-alanine'23'.1232) and [RuL(bipy),] (L = ( -)-aspartate,1233( -)-glutamate'234a) have been reported. Inversion of the A and A isomers has been studied and equilibrium constants for the isomerization reactions deter228.1 22Y,1233,1240a . More recently, chiral complexes A, A-fRu(L),L']+ (L = bipy, phen; L'H = Sthreonine, S-allothreonine) have been prepared and studied cry~tallographically.'~~~~ Reaction of [RuCl, (bipy),] with 8-mercaptoquinoline (L) under basic conditions yields the N,S complex [RuL(bipy)J+; alkylation of the coordinated ligand at S has been achieved with MeI.'235
(xiv) Tridentate heterocyclic complexes /RuL2Y+
[R~(terpy)~]'+ can be prepared by fusion of RuCl, with the ligand at 250-260°C,1236 reduction of RuC1, with H2 over Pt black followed by reaction with terpy in Me0Hso7 or by reaction of [RUC~,(OH~)]~with terpy in the presence of H3P02.728 The synthesis, electronic spectra and redox properties of related bis chelate complexes [RuL2I2+are summarized in Table 10. The electrochemis~ ~ ~ ~voltammetry * ' ~ ~ ' ~ 'shows ~ ~ ~ a one electron oxidation try of [Ru(terpy)J2+ has been s t ~ d i e d ; ~ ~ ~ cyclic at + 1.264V, vs. SSCE in DMF.697ESR studies on the reduced products indicate that the first two reductions are ligand based with the redox orbitals being located on single chelate rings.'2d2The charge transfer luminescence of [RuL,]'+ (L = terpy, 4'-phenyl-2,2',6',2"-terpyridine, 4,4',4"-triphenyl-2,2',6',2-terpyridine) has been interpreted on the basis of a delocalized excited ~ t a t e , ' "but ~ more recently, also interpreted using a charge-trapped (see Section 45.4.3.1.vi). The half-life of [Ru(terpy),12+* is estimated as approximately 1.2 ns and undergoes electron transfer quenching with [Fe(OH,),I3+ to give [Ru(terpy),13+ and [Fe(OH2),I2' The reduction of [Ru(terpy),13+ by [Fe(oH2),I2+ is first order in [Ru"'] and [Fe"] with AH' = - 11.72kJmol-', A S # = - 172 J K-' The luminescence of [RuL,I2+ (L = 2.6-di(2'-quinolyl)pyridine) has been studied and assigned as charge transfer in nature.'238[RuL2I2+( L = 2,4,6-tri(2'-pyridyl)- 1,3,5triazine) is reported to undergo pseudo-base formation by attack of -OH on the triazine More recently, the complex [Ru(L),I2+ (L = 5,5"-diphenyl-2,2',2-terpyridine) has been synthesi~ed.'"~
Ruthenium
Pi
I
I
I
-
d
t I
I
-+
d N
I
355
356
Ruthenium
( x v ) Tridentate heterocyclic complexes with carbon donor ligands
Reaction of RuCl, with terpy in refluxing DMF gives trans-[RuCl (CO)terpy]1’16.’248 the single crystal X-ray structure of which shows Ru-N = 2.070, 2.013, 2.084i, Ru-Cl = 2.400, 2.403 A, Ru-C = 1.865 A.1248 The single crystal X-ray structure of cis-[RuCl,(CO)terpy], prepared by decarbonylation of [RuCl,(CO) terpy] with Me,NO in CH2C1,, shows Ru-N = 2.080, 1.969, 2.081 A, Ru-C1 = 2.420,2.438 Ru-C = 1.886A.124B [RuX,(CO),], (X = C1-, Br-) reacts with terpy to give [RuX2(CO),terpy]; the single crystal X-ray structure of two forms of [RuBr,(CO),terpy] shows bidentate terpy Iigand which can be protonated to yield [RuX,(CO),(Hterpy)]’ .Iz4’ [RuCl,terpy] reacts with L (L = PPh,, PTol,) to give trans-[RuCl,(terpy)L] which can be converted to the cis isomer in refluxing I,2-dichloroethane.124’The cis isomer can also be isolated from the reaction of [RuC12(PPh3)Jwith terpy; carbonylation of cis- or trans-[RuCl,(terpy)(PPh,)] gives cis- or trans-[RuCl, (CO)(terpy)]. Cis-[Ru(terpy)(OH,), PPhJ2+ with PhC=CH yields [Ru(qlCH,Ph)(CO)(terpy)(PPh,)]+ and PhMe.”” The preparation of irons-[RuCl(terpy)(PPh,),]’ has been reported.’249 Reaction of [Ru,(CO),,] with KHB(pz), yields [RuX(CO)~L](X = C1-, Br-, I-; L = tris (pyra~olyl)borate).’~~~ Treatment of [RuCl,(C,H,)], with B(pz); (tetrakis( 1-pyrazoly1)borate) in MeCN gives [Ru(B(pz),)(C,H,)]+, the single crystal X-ray structure of which shows octahedral Ru” with Ru-N = 2.105, Ru-C = 2.20A.1251
k,
(mi)
Tridenlute heterocycles with nitrogen dunor ligands
Reaction of [R~(terpy)(bipy}(OH,)]~’or [Ru(terpy)(bipy)Cl]’ with L (L = NH3,’IX4 py,IZs24-vinylpyridine,1252 (wmethylbenzyl)amine,254 i~opropylarnine,~~” 4-chlorostilb a z ~ l e , ’ 1,2-bi~(4’-pyridyl)ethylene,’~~ ’~~ b e n ~ o n i t r i l e yields ~ ~ ~ ) the complexes [R~(terpy)(bipy)L]~+. The 4,4‘-dimethyl-2,2’-pyridyl species have been p~epared.’’~Oxidation of [Ru(terpy)(bipy)(NH, CHMe,)]” and related amine complexes by four electrons gives the Ru’” product [Ru(NCMe,)(terpy)(bipy)13+ via a two electron oxidation intermediate [Ru(terpy)(bipy)(NH=CMe2)]’+ ?59,1253 The single crystal X-ray structure of [Ru(NCMe,) terpy)(bipy)13+ shows a linear Ru-N-C moiety with a short Ru-N(NCMe,) bond of 1.831 suggesting multiple bonding between Ru-N.”~ The oxidation of coordinated NH, in [Ru(terpy)(bipy)NH,I2+ to NO; in [Ru(NO,)(terpy)(bipy)]’+ has been achieved by controlled potential electrolysis at + 0.8 V vs. SCE in water.1’64.’184 The reaction occurs via [Ru(NO)(terpy)(bipy)]’+ and [Ru(NO,)(terpy)(bIpy)] intermediates. The reverse process, reduction of NO; to NH,, has also been observed (Scheme 3 5). I 84 Comparisons between this redox system and its osmium analogue have been made in relation to E,,, values and the differences between Ru--NO and Os-NO mixing.’25sQuenching of the excited state [Ru(terpy)(bipy)NH,]*+* using Fe3’ (aq) and paraquat ha been r e p ~ r t e d . ~ ”
8,
+
‘
RuCI,
7
*
terpy A [RuCI3(terpy)l A [RuCl(terpy)(b~py)]+
11
Ru(NH,)( terpy)(bipy) I
11 11
11
I1 IRu(NHO)(terpy)(bip~)I’* i , EtOH, A, 3 h;’”
ii, bipy, EtOH, H 2 0 ,A, 3 h;259 iii. NO;, HzO, A, 3 h:”’ v, -6e; +H,O,-5H+1164.11S4
’+
[Ru(N)(terpy)(bipy)l’+
iv. HPF,, MeOH;259
Scheme 35
Reaction of [RuCl,(terpy)] with L yields the mer isomers of IRu(terpy)L,] (L = 4+vinylpyridine, trans-stilbazole, 4-chlorostilbazole, 1,2-bis(4-pyndyl)ethylene); [RuCl(terpy)(stilbazole),l’ was also
Ruthenium
357
isolated from the reaction.'254 Likewise, treatment of the tripyrazolylmethanc complex [Rucl, {HC(pz),)] with 4-vinylpyridine gives [RuCl, { HC(pz), ) ( ~ p y ) ~ ] ~ ' [RuCl,(NO)terpy]+ is the product from the reaction of [RuCl,(NO)] with t e r ~ y . "[RuCI~~ (NO)(tetrapy)] has been reported.'Is7
(xvii) Tridentate heterocyclic complexes with oxygen and sulfur donor ligands [Ru(terpy)(bipy)OH2]2+ can be prepared by treatment of [RuCl(terpy)(bipy)]+ with h g + in water,265.1068,1252 or by aquation of [Ru(03SCF?)(terpy)(bipy)]+ A range of substitution products of [Ru(terpy)(bipy)OH2l2+has been prepared.1158Oxidation to [R~"'(OH)(terpy)(bipy)]~' and [Ru" (0)(terpy)(bipy)l2 has Seen acliievzd electrochemically in solution and on surface film (equation 92)~ll86,I2~3.l204,I257a.lZ57bThis system has been used for the catalytic oxidation of 2-propanol, p-toluic acid and xyIenes.1186~'203~12~~1258 The kinetics and mechanism of the oxidation of aromatic hydrocarbons1258 and 2-propan01'~~~ by [Ru(0)(terpy)(bipy)12+have been d i s ~ u s s e d . ' ~A~slower -'~~~ reaction of [Ru"' (OH)(terpy)(bipy)]" occurs via a one electron outer-sphere tran~fer.'"~ +
[Ru(terpy)(bipy)(OH,)I2'
049V
[Ru(terpr)(bipyPHl2+
0.62 V
[Ru(terp~)(bi~r)(O)l~~ (92)
Treatment of [RuCl(terpy)(bipy)]+ with CF, SO3H under N, gives [Ru(O, SCF,)(terpy)(bipy)]+ , which when treated with dry air at 110°C for 5 h yields the Ru"' analogue [Ru(O,SCF,)(terpy)(bipy)j2'. 1256 Aquation of [Ru(O,SCF,)(terpy)(bipy)12+ occurs readily to give [R~(terpy)(bipy)OH,]~+ ,1203~1204~1256 The luminescence and redox chemistry of [Ru(terpy)(bipy)LI2+ (L = OH,, phenothiazine, N-methylphenothiazine, thianthrene) have been reported.'259
(xviii) Tridentate heterocyclic complexes wifh halide ligands [RuCl, (terpy)] can be prepared by reaction of RuCI, with terpy in refluxing EtOH for 3 h.259*1249,1252 The corresponding complex of 1,3-bis(4-methyl-2-pyridylimino)isoindoline(L) [RuCl, L] can be prepared by the same method1240,1260 and has been used for the catalytic oxidation of alcohols.1260 The preparation of [RuCl(terpy)(bipy)]+ has been achieved by reaction of [RuCl,(bipy)] or [RuCl, bipyl- with terpy,264,265,1181 by treatment of [RuCl, terpy] with bipy259,'252 and by decarbonylaA range tion of [RuCl,(CU), (bipy),] with Me,NO in the presence of terpy in 2-metho~yethanol.~~~ of substitution products of [RuCl(terpy)(bipy)]+1068~1181 and related complexes have been reported.2M,26*
45.4.3.2
Binuclear and polynuclear complexes
(i) Nitrogen donor bridged complexes This area has been reviewed.484A range of binuclear [2,2], [2,3] and [3,3] complexes of type [RuCl(bipy),],L"+, [Rupy(bipy),], LE+and [Ru(bipy),]L"+ and related systems have been prepared (Table 11). Dimerization reactions have been carried out using a general strategy involving labile solvent coordinated mononuclear substrates. For example, [RuCl(N0)(bipy),l2+can be activated (96) to by N; in acetone to give [RuCl(bipy),(dcetone)]+ (95) which readily adds [RuCl(bi Y)~L]+ produce the binuclear complexes [RuCl(bipy),], L2' (97) (L = pz,"25~'L49~1261.126111265 4 4'-b'IPYI,Z-bi~(4'-pyridyl)ethane,'~~~ pyrimidine1266*1265) idine,l 149,1265 1,Z-bi~(4'-pyridyl)ethylenel~~~,~~~~ can be prepared by an analogous route.'269 The purifica(Scheme 36). [Ru(bipy),py], (4,4'-bi~y)~+ tion of these binuclear complexes has been Oxidation of the [2,2] complexes to the [3,3] species can be carried out electrochemically or chemically, while the mixed valence [2,3] species may be generated directly from the [2,2] species, or by reaction of [2,2] with [3,3] in 1:l molar ratios.1132,1149.1263.1265.1266 The degree of delocalization in the mixed valence [2,3] complexes has been assessed. Magnetic properties of the complexes IRuCl(bipy),], L5+ (L = pz, pyrimidine, 4,4'-bipyridine, 1,2-bis{4'-pyridyl)ethylene, 1,2-bis(4-ppridyl)ethane) suggest very little interaction between the metal centres;"" this, together with electrochemical, K, and intervalence charge transfer absorption data (Table 1 l), indicates trapped valence Class I1 behaviour for these compleXes.484,530,1 261,1263.1265-1267 Electron transfer in these systems has been likened to the outer-sphere + .1261,1263.1265,1267 N o dramatic difelectron transfer between [RuCl(bipy),py]*+ and [R~Cl(bipy)~py] Y
Ruthenium
358
3 0
P
2
3
z
3
b N
0
z
5
QI
d
v/
++++++++++
mrnrnmmmmmmv,
+
m
+++
mv,m
I l l
+
v,
I
+
VI
I
3
d c" x
a 3
x
a
3
Ruthenium
J
J
IRuCl(bipy)2acetonel+ + [RuCl(bipy),LI+
(95)
359
-
[(bipy)2C1RuLRuCl(bipy)2I
'+
(97)
(96)
Scheme 36
ference was observed in metal-metal interactions in [RuCl(phen),], L3+ compared with [RuCI(bipy),], L3+.I' 32 A range of tetradentate ligands have been used to bridge [Rt~(bipy)~]'+moieties (Table 1 ~)~746,762,1054,1064.l067.1072,1073,1~40,i2~,i273Reaction of [RuCI, (bipy),] with L under refluxing conditions gives [Ru(bipy),], L4+ (L = 2,2'-bipyrimidinej,746~762.'0"4 while for L = 2,3-dipyridylquinoxaline, [RuCl,(bipy),] was treated with Ag+ in acetone followed by addition of L.'OMBinuclear complexes incorporating pyrazole and benzimidazole anion bridging ligands have been developed (Scheme 37).1072,ii26 Oxidation of the pyrazole [2,212+species (98) to the [3,213+product leads to the formation of a very strong Ru"'-pyrazole bond with concomitant bridge cleavage."" [Ru(bipy),], L4+ (L = 2,3-bis(2'-pyridyl)pyrazine) has been prepared from [RuCl,(bipy),]; the [3,215+ion decomposes in solution. The synthesis and electrochemistry of bi-, tri- and tetra-nuclear species ~~ incorporating a 2,2',3,3'-tetra-2-pyridyl-6,6'-biquinoxaline bridge have been d e s ~ r i b e d , "while binuclear complexes of isoindoline derivatives and the binding of Cu and Pd salts to related The characteristics of intervalence transfer in monomeric complexes have also been mixed metal Ru/Os dimers have been fully discussed.i273bHetero-binuclear species based on [Ru(CN),(bipy)J are discussed in Section 45.4.3.1 .viii. [RuCl,(bipy), I
I
/i
IRu(bipy),(pyrazH), 1,'
-
*
[Ru(bipy),(acetone),j2+
[{(bipy),Ru~(pYraz),l*+
IRu(bipy),(p);raz), 1
(98)
i, Ag', acetone; ii, p y n z H ; iii, KOH Scheme W S
A series of t r i n u ~ l e a r , tetran~clear~~"' ~ ~ ~ ~ ~ ' ~ ~'41~ and higher p o l y n ~ c l e a r "products ~~ have been prepared. Treatment of [RuL3I2"(L = 2,2'-bipyrimidine) with three equivalents of [RuCl,(bipyj,] in refluxing water yields the tetranuclear species [Ru(bipy),(bipym)],Ru*+ .746 The [2,212+species [RuCl(bipy),],pz2' (99) reacts with N; in acetone to give the dimer [(bipyj2C1RuLRu(bipy),(acetone)]'+ (100); reaction with half an equivalent of pz, or one equivalent of [RuCl(bipy),pz]+ leads to the formation of the tetramer [2,2,2,216+ (101) and trimer [2,2,214+(102) respectively (Scheme 38)."49 The electrochemistry of (101) and (102) has been investigated (equa~,'~~~ tions 93 and 94) and metal-metal interactions in the mixed valence products d i s c ~ s s e d . " ~The trinuclear complex [N(CH, CH,OphenRu(bipy), J3J6+ has been prepared.
j/
\;1
[ RuCl(bipy), I z p z 4 + L [~bipyjZCIRupzRu(biDy),(acetone) j 3+
(99)
[ (bipy),C'IKupzRu(bipy ),pzRu(bipy), pz RuCl(bipy),
(100)
1'+ [ (hipy),CIRupzRu(bipy ),pzRuCl( bipy), I 4+
i, KN3. acetone; ii, Yzpz: iii, [RuCl(bipy),pzl+ Scheme
-
[2,2,214+ 5 [3,2,3]"
[2,2,2,2jhf
[3,2,2,318+
[3,3,317' [3,3,2,3]"
(93)
[3,3,3,3]"'+
(94)
360 (ii]
Ruthenium oxygen donor complexes
Reaction of [RuCl(L), acetone]+ (95), prepared from [RuCl(NO)(L),]'+ with N; in acetone, with 0, for 72h yields the oxo-bridged dimer [RuC1(L),],O2+ (103) ( L = bipy, phen).Iz7,Treatment of [RuCl,(bipy),] with Ag+ for 8 h in refluxing water gives [Ru(bipy),(OH,)],04+ (104) which can be readily converted to the nitro derivative [Ru(N0,)(bipy),],O2+ (105) on reaction with NaNO, The redox properties of [RuCl(bipy),],Oz+ have been investigated and the (Scheme 39).'207,'2'2b,'274 oxidized [3,413+ (106), [4,414+ and reduced [2,3]'+ ions generated.Iz7' PES studies on the mixed valence [3,413 ion indicate strong coupling between equivalent Ru atoms.'274The single crystal X-ray structure of [Ru(NO,)(bipy)J,O2+ (105) shows < RuORu = 157.2" with Ru-0 = 1.876, 1.890 A, with a Ru to Ru distance of 3.692 A.1275 The oxo-bridged dimers [RU(L)~OH~],O~* (L = bipy, phen) catalytically oxidize H 2 0 to 041207 and 2C1- to C121205~'207 in the presence of excess of Ce'". The mechanism of H 2 0oxidation 1s believed to occur via the four electron oxidation species [5,514+(equation 95).'207The use of these binuclear complexes on chemically modified electrodes has been reported, lZo5 and their involvement in the photooxidation of HzO at the surface of hectorite mentioned.''w Reaction of [RuCl,(bipy),] with L2- (L2- = pyrazine-2,5-dicarboxylate) gives [Ru(LH)(bipy),]+ and a green binuclear species [Ru(bipy),], Lz+.562 No intervalence charge transfer absorption was observed for the mixed valence [2,313+ ion in contrast to the complexes [Ru(NH,),],L3+ and (107) shows a [(NH,), R u ( L ) R ~ ( b i p y ) , ] ~ The + . ~ ~single ~ crystal X-ray structure of [Rupy,],(ox centrosymmetric planar oxalate bridging Ru" ions with Ru-0 = 2.096, 2.133 +
I+
[R~Cl(bipy)~(~cetone)l~
TRuCl(bipy)zI2O2'A [ RuCl(bipy), I2O3+ ( 1 03) (106)
(95)
t
[RuCl,(bipy), J A IRu(bipy)~(H;rO)l2 0 4 + ( 104) I.
02. 72 h:
ii.
Clz,
iii.
IRu(NOz)(bipy), I2O2' (105)
AgNO,, HzO, A, 8 h , n. NahO,
Scheme 39'*74 2c
(105)
[(bipy),RuV-O-RuV 0 'I
II
0
( b i ~ y ) ~+] ~1H20 +
-
[(bipy),Ri~"'-O-Ru'"(bipy)~
I
OH,
I
14' t 0,
(95)
0112
Reaction of RuO, with phen in CCl, yields a supposed oxy-bridged Ru"" dimer [RuZOt(phen),.1209(see Section 45.4.3.1.x).
(iii)
Suijur donor complexes
The binuclear complex (108) has been synthesized and its use as a catalyst for the electrochemical reduction of acetylene in MeOH/DMF discussed.'212a
Ruthenium
36 1 2+
t 108)
(iv) Halide complexes Reaction of [RuCI,(AsPh,),], with bipy is reported to yield a dimer of stoichiometry [Ru2C14of {Ru(CO,)(bipy),] with cis-[RuCl,(bipy),] under acidic conditions gives ( b i ~ y ) , ] .Reaction ~~~ [RuCl(bipy),],'+; the complex can be oxidized to a [3,213+ion which decomposes in MeCN to [RuCl,(bipy),]+ and [R~(bipy),(NCMe),]~+."~~ The weakness of the Cl- bridge in the mixed valence compound is assigned to the high strength of the Ru"'-Cl bond leading to bridge cleavage.' 126~1160
45.4.4
Nitrosyls
Ruthenium shows a wide nitrosyl chemistry and several reviews of the area have been published,273-277 ISN NMR studies on nitrosyl complexes show that N is deshielded by up to 450 p.p.m. in bent as compared with linear NO ligands.'277PES studies of core binding energies for nitrosyl complexes have been reported and comparisons between coordinated NO+ and +N2R assessed.1278,i279WR u Mossbauer spectra of [RuX,(NO)y+ (X = -CN, NH3, -NCS, C1-, Br-) have been measured at 4.2K, and the chemical isomer shift found to decrease as ligand field strength of X decrease^;^'^+^^*^ the changes in quadrupolar splitting have been discussed in terms of 0 donor and acceptor abilities of coordinated ligands.'28"
45.4.4.1 Nitrogen donor complexes
Nitrosyl ammine, amine, heterocyclic and phosphine complexes are discussed in Sections 45.4.1, 45.4.2, 45.4.3 and 45.5 respectively. A range of nitrosyl complexes incorporating NO; and NO; ligands have been reported. Treatment of RuC1, with NaNO, yields [Ru(OH)(NO,),(NO)]~(109), 1281-1283 the single crystal X-ray structure of which shows a linear Ru-N-O(nitrosy1) with -OH and NO+ mutually trans; Ru-N(N0,) = 2.079 A, Ru-N(N0) = 1.748A.1284 The IR spectrum of [Ru(OH)(NO,),(NO)]~- has been assigned by ',NO and "NO labelling experiments.2Yx Reaction of ruthenium(I1) nitrosyls with HN03 gives a wide range of products of general type e.g. [Ru(NO,),(NO,),(NO),]"+ ;the separation of some of the products has been [Ru(NO,),(NO)(OH,),I, CRu(NO,)(NO,)(NO)(OH,),I+,[Ru(NO,)(NO,),(NS))(?H,),I:In the presence of tributyl phosphate (tbp), [Ru(NO,), (NO)(OH,),] gives an equilibnum mixture of [Ru(NO,), (NO)(tbp),], which can be extracted into dodecane.1286
(109)
362
Ruthenium
Treatment of [RuCl, (NO)]'- with KNCS yields [Ru(NCS), (NO)]'the Mossbauer spectrum of which has been rep~rted.'~'The reaction between [Ru(CN),I4- and HNO, yields [Ru(CN), NO],- 128!.i283.1293 (see Section 45.3.1).
45.4.4.2
oxygen donor complexes
[Ru(NO)(OH,),]~+can be prepared by reaction of [Ru(OH,)~]~+ with N0,2s2or by protonation with HNO, of [Ru(OH),NO], formed by alkaline hydrolysis of [RuCl, Reaction of [Ru(N0)(OH2)J3+with [Cr(OH2)J2+yields the dimeric product [(H20)5Ru-NH-Cr(OH,),]S+, the rate of formation of which is first order in [Cr"] and [RU''].'~' [RuCl, (NO)(OH,),] reacts with diketones, e.g. acacH, to give the halo-bridged dimer [RuC12(acac)(NO)], .619 At pH 3 cis-[R~Cl(acac)~(NO)] is formed while a tetramer [Ru(acac),(NO)], has also been propo~ed;'~''at'pH 6 [RuCl, (NO)(OH,),] with Hacac yields [Ru(acac),(hia)] (110) which can also be prepared directly from cis-[RuCl(acac),(NO) .lZy7 The single crystal X-ray structure of the dimer [Ru(acac),(NO)], (111) shows Ru-0 = 2.031 , Ru-N = 1.918 A with a planar R u ( p NO),Ru unit.'298The structure of [RuL,(NO,)(N0)I2- (L = 2,4,5,6-(lH,3H)pyrimidinetetrone-5oximate) has been reported, chelation of L being through 0 of oximate and the 6-carbonyl moiety Reaction of [RuC13(NO)(OH,),] with glycinate gives [Ru(OH),(gwith cis NO and ly)(NO)J- .'0 Oxidation of [RuBr,(SEt,),(NO)] produces [RuBr3(NO)(OSEt2)],(1121, the X-ray structure of which shows a Br--bridged dimer with sulfoxide trans to NO; Ru-N = 1.71 A, Ru-0
A
= 2.058
y I , 'i"\N,i"\;o
&o os**, I
9
\-A'
(111)
(110)
+o **to
*%I
3 r,
B
Ru-
V
I
*,,*"Br% ,,,,t
0
N
I
,**OB'
B ' rfTB ' ,. 0
N
'._-
( x ~ c ) , R L I?/'=O0
45.4.4.3
,****,N,"t,,,,t
gEt2
SEt2
0
(112)
Suvw donor complexes
Reaction of [RuCl,(NO)] with thioethers (L) in EtOH gives complexes [RuCl,(NO)L,] (L = E t , , SEtPh, SPr, S(CH,Ph),, SPrPh, SMefh, SeEt,, SePhEt) with L being mutually trans;6'99'"2*1303 the synthesis of [RuBr,(NO)(SEt,),] has been rep~rted.'~" The 'Hand I3CNMR and IR spectra of these complexes have been a n a l y ~ e d . ~The ~ ' ~complexes , ~ ~ ~ ~ [RuX,(NO)L,] (X = C1-, Br-; L = PFh,, AsPh,) react with elemental sulfur to give the sulfide dimers [RuX,(NO)L,J,S which can be oxidized with perbenzoic acid to the sulfoxy-bridged species [RuX2(NO)L2J,S0.'3" Reaction of [RuCl, (NO)] with dithioacetylacetonate (sacsac-) in water yields trans-[Ru(sacs a ~ ) , C l ( N 0 ) ] . 'Polarography ~~ of this product shows two reversible reductions at - 0.27 V and - 1.43 V vs. Ag/AgCl, compared with [Ru(sacsac),] which shows only one redox couple at 0.04V More recently dithiocarbamate derivatives w. Ag/AgCl corresponding to the RuI1'!'I cis- and trans-[RuX(S,CNRR'),(NO)] (X = C1-, Br-, I - , Fe-, NO;, -OH, -NCO, N;, SCN-; R = R' = Me, Et; R = Me, R' Et; R = Me, R' = Ph) have been prepared from [RuCl,(NO)] and Na' (-SzCNRR') in MeOH followed by acidification and treatment with X-.'308 Cis-[Ru(S,CNMe,)' (15N02)(NO)]undergoes exchange between NO and NOz ligands.1308Cis-[Ru(S, CNRR),(NO)] can also be isolated from the above rea~tions'~''or prepared directly from [Ru(S,CNRR'),] with NO.'309The ' H NMR spectrum of the complex with R = R = Me differentiates between unidentate and bidentate dtc Iigand~;'~" this is confirmed by the single crystal X-ray structure of [Ru(S,CNEt,),(NO)] (113) which shows two bidentate dtc ligands with Ru-S = 2.415 A and one monodentate dtc with Ru-S = 2.398A cis to NO. Ru-N = 1.72A.'3'0
+
3
45.4.4.4
HaIide complexes
Reaction of RuCI, with NO in EtOH yields [RuCl,(N0)]1292.'300~1302 while [RuX,(NO)], (X = C1-, In aqueous solution, Br-, I - ) can be prepared by treatment of RuO, with HX and HN03.131"1312
I Ruthenium
363
Et2" (113)
these complexes may be regarded as [RuX,(NO)(OH,),]~~'~ and are highly useful starting materials for the synthesis of ruthenium nitrosyls. The kinetics of hydrolysis of [RuCI,(NO)(OH,),] show displacement of three C1- ligands at decreasing rates with no loss of NO+, pK, and pK, for [RuCl, (NO)(OH,)J being 4.9 and 7.5 respectively (Scheme 40).l3I3 The interaction of [RuCl,(NO)(OH,), with glycine and alanine has been reported.I3l4
reaction of RuCl, with HCl and NO for 48 h Anation of [RuX,(NO)] with halide,'3'1.'312.'3'S followed by NH,Cli3" or treatment of RuQ, with NOCl in EtOH2sz3'316 yield [RuC1,(N0)I2-. [RuCl,(NO)l2- with X- gives [RUX,(NO)]~-(X = Br-, The single crystal X-ray structure of [RuCI,(NO)]'- (114) shows octahedral Ru" with Ru-Cl(equatoria1) = 2.373-2.379, RuCl(trans to NO) = 2.357, Ru-N = 1.738, N-0 = 1.131A, -= RuNO = 176.7",'3'7a with [RUF,(NO)]~-being found to be i s o s t r u ~ t u r a l . ' The ~ ' ~ ~IR spectrum of [RUC~,(NO)]~has been 1876, a ~ s i g n e d ? ~ ~with ~ ' ' v,4No ~ ~ ~=' ~ ~ 1850, 1844cm-' for X = C1-, Br-, I- re~pectiveiy.'~''The 19F NMR spectrum'32s and Mossbauer dataz7 for [RUX,(WO)]~- have been reported. The rate of hydrolysis of [RuC15(NO)]2- is more rapid under basic conditions although hydrolysis of [RuC14(OH)(N0)]2- is pH independent at pH < 2.5 and pH = 7,5-9.5.'315,1326 The crystal structure of [RuC14(OH)(N0)]2-(Ru-N = 2.04 A) has been reported.'327
[RuCl, (NO)]'- shows a reversible one electron oxidation, the electrogenerated product being assigned on the basis of IR, ESR and magnetic data to low spin [RU"'C~,(NO)]-.'~~~~'~~~~ Reaction of RuCl, with S3N3C13gives a brown product '[RuCl,(NS)J' which, on treatment with Ph,PCl in aqueous solution, yields PPh,[RuCl,(NS)(OH,)], the single crystal X-ray structure of which shows octahedral geometry with H 2 0 trans to NS and ,I and [(dppe)PtRu,(CO),], see reference 1, p. 927.
45.5.3 Ruthenium(1) Complexes
45.5.3.1 Mononuclear complexes There are very few monomeric Ru' complexes containing P or As donor ligands; dimerization with formation of a metal-metal bond is preferred in most instances. Brief references are made to [RuCl,(NO)(PPh,),] obtained as a minor side-product in the preparation of [ R U C I , N O ( P P ~ , ) , ~ ' ~ ~ ~ and also by reaction of [RuCl,NO(PPh,)], with PPh,.'329This product shows an ESR spectrum indicative of a paramagnetic Ru'(d7) complex. However, reaction of [RuI,(NO)], with AsMePh, gives a compound of empirical formula '[RuI,NO(AsMePh,),]' which is essentially diamagnetic and is therefore tentatively formulated as the metal-metal bonded dimer [RUI,(NO)(ASM~P~,),],.~~~~ Electroreduction of [RuCl(dppp),]PF, leads to the Ru' species [RuCl(dppp),] and then the Ruo anion [RuCl(dppp),]- although onky the ortho-metallated product [RuH(C,H,PPh(CH,),PPh,)(dppp)] was i~olated;''~" [Ru(NO)(dppp),]+ also undergoes two reversible reductions."" A series of compounds such as [RuCl?(NO)(AsMePh,),l, fRuCl,NO(DMSO)L,], (L = AsPh,, SbPh,, AsMePh,, AsMe,Ph) and [RuCl,NO(L-L),] (L-L = dppm, dpae) have been reported. A These have been formulated as Ru" species but have vNo bands characteristic of NO+ (!).I5" further critical examination of these complexes is very desirable.
45.5.3.2
Bi- and tetra-nuclear complexes
In contrast to Ru', Ru"' and especially Ru", relatively few Ru' complexes containing P or As donor Iigands are reported. Reaction between [Ru,(CO)~,] and Q R , in C,H, gives [Ru(pER2)(CO),], (139) (E = P, As; R = Me, C,F,).1512-'514 Reduction of (139; ERz = PMe,) produces the Ruo diamagnetic dianion; a paramagnetic radical anion (formally Ru,',' is formed on mixing (139) with its dianion and analysis of the NMR spectra of the latter and the ESR spectrum of the radical anion indicates that the general geometry, including the Ru-Ru bond, is preserved in all three complexes. Iodine cleaves the Ru-Ru bond of (139; ER1 = PMe,) to give the Ru" dimer [RUI(~-PM~~)(CO)~]~.'~'~ The complex (139) (ER, = PPh,) has been synthesized, together with various trinuclear hydrido complexes, by irradiation ( > 300 nm) of [Ru,(C0)9(PPh2H)3].1516 The mixed P- and S-bridged complex [RuZ(p-PPhZ)(p-SPh)(C0),](140) is obtained from [ R U ~ ( C O ) ~J and Ph2PSPh by cleavage of the P-S bond.'s17
375
Ruthenium
Closely related binuclear Ru' complexes [Ru(p-Cl)(CO),(PBu',R)], (141) are generated by reaction of the 'Ru(CO)CI' solution with the bulky phosphines PBut2R (R = But, Ph or p - T 0 1 ) . " ' ~ ~ ~ * ~ ~ With smaller substituents, [RuCl,(CO),(PR,),] are formed (see Section 45.5.4.54. Compounds (141) undergo facile PR, exchange to yield [Ru,(p-Cl),(CO),(PR,)(PR',)] and metathetical reactions with heavier halides give the bromo and iodo analogues; [Ru(p-I)(CQ),(PPh,)], is also formed by reaction of [Ru,~-CI),(p-Fe(CO),)(CO),(PPh,),] (Section 45.5.5) with KIl5Igawhereas interaction of [Ru,(CH,),(PMe,),] (Section 45.5.8) with water gives [Ru2(p-H)(p-OH)(PMe3),](142).Is2' [Ag(OCOMe>]and (141) produce [Ru(p-OCOMe)(CO),(PBu', R)I2(143), and other routes to these carboxylate compounds are by reaction between [RU~(CO)~,] and RCOIH in the presence of P B u ' , , ' ~treatment ~~ of [ R u ~ ( C O ) ~ ( P Pwith ~ ~ )MeC0,H's23 ~] or reaction of [Ru(OCOR)(CO),], with F ) ~ , R = H, Me, Et-not all combinaequimolar amounts of L (L = PBu,, PPh,, P ( ~ I - C ~ H ~AsPh,; t i o n ~ ) . The ' ~ ~ ~corresponding [Ru(p-OMe)(CO),(PBu',)], is readily obtained from reaction of [Ru(CO),(PBu',),] with MeOH.'522",152"
(142)
(141)
(143)
Reaction of [Ru(OCOMe)(CO),], with PBu, (2:1 mole ratio of Ru:P) at 150°C in C6H6has (144) together with a minor amount of (143). yielded the tetrameric [Ru,(OCOM~),(CO)~(PBU,),] Compound (144) can also be prepared by reacting fRu(OCOMe)(CO),], with (143) in C6H, at 150°C. Treatment of the tetramer (144) with the stoichiometric amounts of PBu, then gives (143). Similarly [Ru, (CO),,] and glutaric acid in boiling toluene give [Ru, (OCO(CH,), OCO)(CO),], which on treatment with PBu, (1:l mole ratio of Ru:P) yields ~Ru,(OCO(CH,),OCO),(CO),(PBu,),] (145). X-Ray analyses of (144) and (145) reveal that the structures consist ofcouples of dimers linked by carboxylate The corresponding [Ru4(OCO(CH,),0CO),(PBu,),l (n = 0-2) are also known. ' 527 Another Ru'/PR, complex is [RUCI(PM~,)~], obtained in low yield from the reaction of [Ru,(OCOMe),Cl] and [Mg(MeOC,H,),] in the presence of excess PMe3'528and formulated as the dimer (146) on the basis of its diamagnetism, 31PNMR and far IR spectra.'529The binuclear Ru' nitrosyls [Ru12NO(AsMePh,),], and [RuCI,NO(PPh,)], are covered in Section 45.5.3.1.
(144)
376
Ruthenium
For completion, although carbocyclic ring complexes are not the province of this review, [Ru,(CO)~PR? (q-CSHs),], made by treatment of [Ru(CO),(q-C,H,)], with PR,, are other formally Ru’ binuclear complexes.’s30
45.5.4 Mononuclear Ruthenium(1I) Complexes 45.5.4.1 [RuL,I2+, [RuXL,Jf (n = 4,5), [RuX(L-L),]+, [RuL’]’+ cations ( X = hulide, alkyl etc.)
To date the only routes to the fully substituted cations [RuL6]’+ are by reaction in MeOH of [R~X,(nbd)l,’’”,”~~ (X = CI, Br) or [ R u C ~ , ( P P ~ , ) , ]with ” ~ ~ an excess of P(OMe), or P(OMe),Ph. For L = P(OMe),, some [RuCl{P(OMe),),]” is also isolated from the mother liquor. If less P(OMe),Ph is used, the binuclear cations [RuZXJ(P(OMe),Ph),]+are formed, (see Section 45.5.5.1), and these are the only products isolated on reaction of [RuX,(nbd)], with bulkier PR, groups such Reaction of [Ru{P(OMe),},] or [RuMeas P(OEt),, P(OEt),Ph, P(OR)Ph, (R = Me, Et).1531,1532 {P(O)(OMe),) {P(OMe),},] with Me1 gives [RUM~{P(OM~),},]I,’~~~ and one of the products from the reaction of trans-[RuCl,(PMe,),] with the lithiated ylide Li[MqP(CH,),] is [RuCl(PMe,),(CH,PMe, )IC1 (147).1380~’534
1
c1
1 ,,,,,,, I bPMr3
M e 3 P“,,/,*
\”
Me,P*
R1 1 P Me 3 !c L CH2PMe3
(147)
Treatment with Ph,C[BF,] of the reaction mixture from [Ru,(OCOMe),Cl], [Hg(MeOC,H,),] and PMe, gives [RuCl(PMe,),]BF, (31P-{’H}NMR spectrum is a sharp singlet), formulated as a fluxional five-coordinate monomer. A compound of empirical formula [RuCl(PMe,),]Cl is also obtained from trans-[RuCl,(PMe,),] left in C6H6 for several weeks. However its 31P-{’H)NMR spectrum consists of an A X attern at ambient temperature which suggests that it is the binuclear dication [ R u C ~ ( P M ~ , ) , ] ~The ~ I ~analogous .~~~ cations [RuCI(P(OMe)Ph,),]+ and [RuCI(P(OEt)Ph2)4]22+ can be synthesized by reaction of [RuCl,(PPh,),] or [RuCl,(P(OR)PhJ,] with an excess of P(OR)Ph, in EtOH. These undergo further rearrangement to form [Ru1CI3(P(OMe)Ph,),]+ and [Ru,Cl,(P(OEt)Ph,),]+ respectively (see Section 45.5.5. 1).1533The deeply coloured pentacoordinate cations [RUx(L-L),](PF,) (L--L = dppp, 2-(diphenylphosphinowith ethanolic sohethy1)pyridine; X = C1, Br) are formed by treatment of trans-[RuX,(L-L),] tions of NH,[PF6], and these take up small neutral ligands to give trans-[RuXL(L-L),]PF, (L‘ = CO, MeCN).153sFive-coordinate cations are not obtained by this route with the less bulky dppm or dppe ligands,1s3sbut they have been prepared for the unsymmetrical diphosphine ligands Ph,P(CH,), PMe, and Ph,P(CH,),PMePh. Non-fluxional trigonal bipyramidal structures (148) are proposed on the basis of 31PNMR spectral studies.’536aHowever, there is evidence for square pyramidal geometry (when L = PMe,Ph).’53hbPreliminary studies have also indicated that one of the products from reaction of cis-[RuCl,(dppm),] and Na[Mn(CO),] in THF is the very air-sensitive [RuCl(dppm),THF]Mn(CO), .Im The related [Ru(PhC=C)(PMe,Ph),]PF, is known.’s37 /-fPMe,
Ph 2 %,/, Ph,P/
I I
Ru --I
\ PMe,
A number of Ru” cations containing polydentate P or As donor ligands have been reported. For example, prolonged reflux of [RuCl, (PPh,),] in C6H6 with the hexatertiary phosphine P ,P,P’,P’-tetrakis(2-diphenylphosphinoethyl)-a,a’-diphospha-p-xylene (TDDX) gives as the main product the binuclear [Ru2C1,(PPh,),(TDDX)] (see Section 45.5.5.3), but treatment of the filtrate
Ruhnium
377
with NH4[PF,] precipitates some [Ru(TDDX)](PF,), in which pentadentate coordination of the ligand is tentatively The corresponding ligand As,As,As‘,As’-tetrakis(2-diphenylarsinoethyl)a,d-diphospha-p-xylene(TDADX) gives the chocolate brown [RuCl(TDADX)]Cl under the same conditions. In this case TDADX acts as a tetradentate ligand which may be coordinated to a single Ru” atom either through one P and three As or through all four As atoms. Carbonylation The five-coordinate cation [RuCl(tet-1)]C1 (tet-1 yields [RuCI(CO)(TDADX)]C~.’~~~ = Ph2P(CH,)2PPh(CH2)2P(Ph)(CH,)2PPh2) is also obtained by reaction of [RuCI2(PPh,),] with tet-1 in dry C6H6,and again carbonylation affords [RuCl(CO)(tet-l)]Cl. In contrast, direct reaction of [RuCl,(DMSO),] with either tet-1 or tet-2 (P[(CH,),PPh2],} in C,H,/MeOH gives the S-bonded DMSO cations [R~Cl(DMS0)tet]Cl.l”~ 45.5.4.2
(i)
[RuXYL,] complexes ( n = 2,3,4; X,Y = halide)
L = monodentate P donor ligand
The products of reaction of tertiary phosphines with ‘RuCl, -xH,O’ are very dependent upon the EtOH1542 nature of PR, and the reaction conditions. Thus with an excess of PPh, in MeOH,’06,’541 or isopropanol’”’ at reflux for several hours, red-brown [RuCl, (PPh,),] is formed whereas shaking the reaction mixture at room temperature yields [RuCI,(PP~,),]”’~(shown by 31P-{1H}NMR to be best formulated as [RuCl,(PPh,),. PPh,] with one PPh3 group trapped in the lattice). Prolonged reaction at room temperature in MeOH using a 1:2 molar ratio of Ru to PPh, gives green [RuCl,(PPh,), MeOH]106(see Section 45.5.7.1), whereas these reactants in isobutanol or cyclohexanol yield black, polymeric [RuC12(PPh3 (which can also be made by heating [RuCI,(PPh,),l under reflux in methyl ethyl ketone,’545 or by reduction of [RuCI,(PPh,),MeOH] with hydrogen).’546,’547 Solvents such as 2-methoxyethanol and benzyl alcohol take part in the reaction producing [RuHCl(CO)(PPh,),] (see Section 45.10.3.2.iii)’”’ and [RuCI, (CO)(PPh,),] (Section 45.5.4.5.i)Is4’respectively. Other routes to [RuCl, (PPh,),] include the treatment of [RuH,(N,)(PPh,),] or [RuH,(PPh,),] with HClis9 or reaction of C S [ R U ( ~ ] - C ~ H , ) C(or ~ ~the ] ” ~reduced ‘ruthenium blue’ solution)676with PPh,. An X-ray structural analysis confirms the five-coordinate structure (149); a distorted square pyramidal structure with one of the phenyl ortho hydrogens blocking the sixth coordination position.’55’Osmometric molecular weight studies in acetone on [RuCl, (PPh,),] (n = 3,4)Io6suggest extensive dissociation in solution and spectrophotometric measurements in deoxygenated C6H6 supported these conclusions (equations 96 and 97).lS5’However, variable temperature 31PNMR studies indicate that the dissociation product [RuCl,(PPh,),],, is a five-coordinate dimer (150) (n = 2) rather than a fom-coordinate monomer (n = 1) and equation (98) rather than (97) is a better representation of the species present in non-polar ~ e d i a . ’ ~ ~ ’Note > ’ ~ “that the different ,‘P NMR spectrum of ‘RuCl,(PPh,),’ reported later by Vrieze et is in fact a spectrum of [RuCl,(PPh,)z(2-MeCOC5H,N)f.’554 The 31PNMR spectra also reveal the fluxional nature of all the ruthenium complexes shown in equation (98) and quantitative kinetic and thermodynamic data are available.
’”’
PPh,
[RuC12(PPh,),]
e [RuC12(PPh,)2] +
2[R~Cl2(PPh3)3]
e [RuC12(PPh3)2]2 +
PPh,
(97) (98)
In more polar media, conductometric,lo6 spectrophot~metric’~~~ and ’IF NMR spectroscopic reveal the presence of an alternative dissociation pathway for [RuCl,(PPh,),], namely loss of C1- to produce Ru” cations (equations 99 and 100) although unpublished work indicates that the triply-bridged chloride dimer [Ru,CL,(DMA)(PPh,),] is present.lSs5 Synthesis of pure [RuBr,(PPh,),] by reaction of methanolic ‘RuCl,*xH,O’/LiBr solutions with PPh, has also been
378
Ruthenium
claimed,Io6but subsequent "P NMR studies revealed that a mixture of [RuCl,Br,_ ,(PPh,), J (x = &2) is formed.1s43However, pure [RuBr,(PPh,),] can be synthesized by reaction of [Ph,(PhCH,)P], [Ru,Br,] with PPh, in MeOH. This has very similar properties to [RuCl,(PPh,),].1556 [RuCl,(PPh,),] [RuCl(PPh,),]CI
e [RuCI(PPh;);]' + e
[RuCl(PPh,),]CI
+
C1-
PPh,
(99)
(100)
A number of other complexes of type [RuCl,L,] (L = P donor ligand, n = 2,3 or 4)can be made by either &rect reaction of 'RuCl,*xH,O' or the reduced 'ruthenium blue' solution with L. These P(p-MeOC,H,), (n = 3),'559 PMe,(CH,Ph) (n = 4),1560 include L = P(p-Tol), (n = 3,4),1557,1558 PMe,(l-np) (n = 3),'561 P(OMe), (n = 4, PPh,(C,H,SO,H) (n = 2),ls6, PHPh, ( n = 4, tr~ns),'~",'~~~ PHEt (n = 4),15" various substituted phospholes (n = 3,4),156631567 and 3,3',4,4-te~ ~ ~trans isomer of [RuCl,(PHMe,),] is also traMe- 1,l '-diphosphaferrocene (n = 4, t r ~ l a s ) . 'The prepared from 'RuC1,-xH,O and Me,PPMe, in H,O or EtOH, the hydrogen being abstracted from For the whereas treatment of [Ru,CI, (PMe,Ph),]Cl with PHPh, gives the cis isomer.1565 the preparation of [RuCl,(P(OEt),),j, 'RuCI, -xH,O' and P(OEt), are reacted in the presence of (124) with HX (X = C1, Br) leads to Na[BH,j.'570 Treatment of [RuH(q2-Me,PCH,)(PMe,),] cis-[RuX,(PMe,),] whereas with CH,X, (X = C1, Br, I) or MeI, a mixture of [RuX,(PMe,),] and [RuX(~~-M~,PCH,)(PM~,),] is produced.1391b However, most P donor ligands react with 'RuCI,~xH,O' in aqueous EtOH or 2-methoxyethanol lo give the triple chloride-bridged complexes [RuZCljL,]CI, (see Section 45.5.5. l), which do not generdly react with an excess of L to give [RuC12L4].A better route to the latter is therefore via reaction of L with [RuCl,(PPh,),,,,], usually in non-polar media to minimize formation of ionic species. This method affords both [RuCl,L,] (L = PEtPhZ,l5&P(OR)Ph, (R = Me, Et),'533PRPh, (R = Me, MeCH,CH(Ph)CH,),L571PMe(Ph)(PhCH,),'572 etc.) and [RuCl, L,] (L = P(OPh),1573 t r ~ n s ' ~P(OMe)2Ph,'533 ), P(OMe)3,'533PEt3,lm PMePh,,1400PMe,,1529,1574 PRPh, PMe,Ph (cis;" etc.). For (R = Me, MeCH,CH(Ph)CH,)1571and various 1-substituted 3,4-dimethylph0spholes'~~~ n = 4, both cis and trans isomers are found and these undergo thermal and photochemical isomerization in solution (see references 1533 and 1575 for details). Reaction of [RuH(C,H,PPh,)(PMe,),] with HCl gives [RuC~,(PM~,),].'~~'' Many of these monomeric complexes also undergo facile rearrangement in solution to form various bi- and sometimes tri-nuclear species as discussed in Section 45.5.5.1. A number of mixed P donor ligand complexes [RuX,L,L',] are also known. Most of the compounds in this category involve a combination of PPh, and fluorophosphines. For example, treatment of c ~ ~ - [ R u H , ( P F , ) , ~ P ~with , ) ~ ]gaseous HX gave cis-[RuX,(PF,),(PPh,),] (X = CI, Br)1576 whose structure (151) was verified by X-ray analysis for X = C1.L577 Other routes to (151) include reaction of [RuCl,(PPh,),] or [RuHCl(CO)(DMA)(PPh,),] with two moles of PF,1578and (C,,H,, = 2,7-dimethylocta-2,6-diene-1,8-diyl) with PF, and then of [RuC~~(C,~H,,)(PF,NM~,)] PPh, Compound (151) is also formed via binuclear, triple chloride-bridged intermediates (Section 45.5.5.1), on treatment of [RuCl,(PPh,),] with equimolar amounts of PF,.'580Reaction of [RuCl,(PPh,),] with allyl difluorophosphite gives [RuCl, (PF, CH,CH=CH,),(PPh3),],'5s1 whereas the sterically bulky phosphole 1-Me3CDMP (152) gives the stereochemically rigid, pentacoordinate [RuCl,(PPh,)(Me, CDMP),] (cf. other 1-substituted 3,4-dimethylphospholes (RDMP) which afford [RuCL(RDMP),] (R = Me, Bu, Ph, PhCH,)].'575It has been shown that trans-[RuCl,(PMe,),] reacts with P(OMe), to give trans, mer-[RuC1,(PMe3),P(OMe),] and all trans-[RuCl,(PMe,), {P(OMe),},] whereas trans-[RuCl, {P(OMe),}] and PMe, gives trans,sis,cis-[RuCl,{P(OMe)3},].'39'c
,
(151)
( 152)
These monomeric [RuCl,L,] complexes (particularly L = PPh,) are very useful precursors for the synthesis of a wide range of Ru" compounds of type [RuCl,Y,-,L,] (x = 1-3; Y = C, N, 0 or S donor ligands). Complete displacement of the chloride groups with other anionic ligands is also
Ruthenium
379
readily achieved to give the six-coordinate complexes [Ru(X-X),L,] (X-X = - S2CNRZ,-SOCPh, -OCOR, -acac, eft.). For detailed examples see reference 85 and Sections 45.5.4.5 to 45.5.4.9. References to the widespread catalytic properties of [RuCl,(PPh,),] are given in Section 45.10.6, together with those for the closely related [RuHCl(PPh,),] and other catalytically active species.
(ii) L = monodentate As donor ligand In contrast to the large amount of work on [RuCl,(PR,),] compounds, relatively little has been published on the corresponding tertiary arsine complexes. Attempts to synthesize [RuCl,(AsR,),] the via 'RuC1,-xH, 0' and AsR, in MeOH gave [Ru"'Cl,(AsR,),MeOH] (R = Ph,Ia6 bromo derivatives are also available. The above reaction in EtOH gives a mixture of [RuCl3(AsPh,),] and [RuCI,(ASP~,),E~OH]'~~' and not [RuCl,(AsPh, j3] as previously claimed.'583 However extended reflux (17h) of 'RuCI,-xH,O' and AsMe,Ph in MeOH gives cis- and trans[RuCl,(AsMe,Phj,] whereas shorter reflux times (1 hj in 2-methoxyethanol yield [RuCl,(AsMe,Ph),].1584Similar extended reflux in EtOH produces [RuCl,L,] [L = AsRPh, (R = Me, Et, Pr, BU);'~'~ AsMe,(PhCH2)'5m].Another route to these compounds is via ligand exchange in non-polar Treatsolvents with either [RuCI, (PPh,),] or [RuCl, (AsPh,), MeOH] in the presence of Zn/Hg.1586 ment of the reduced 'ruthenium blue' solution with AsMe,Ph and AsMePh, in alcohol is reported to give the five-coordinate [RuCl,(AsR,),] cornple~es.'~'' Earlier claims1s87 that reaction of 'RuCl,.xH,O' and an excess of AsPh, in boiling 2-butanol gives the brown solid [RuCl,(AsPh,),], are incorrect. On the basis of magnetic, ESR and electrochemical evidence, this has been reformulated as the mixed valence [Ru,CI,(AsPh,),] (see Section 45.5.6).1588 It is likely that [Ru,Cf,(AsPh,),] and not [RuCI,(AsPh,),], is also formed on treatment of [RuCI,(AsPh,),MeOHf with AsPh, in The insoluble mixed ligand polymer but this may also be a mixed valence species. However, it [RuCl,(AsPh,)(PPh,)], is appears that genuine ~RuCl,(AsPh,),], can be prepared by treatment of [RuCl, (AsPh,),MeOH] with 1 atm H, in DMA at 25"C.1546J547
(iii) L
= monodentate
Sb donor ligand
The only complexes of this type are with SbPh,. Reaction of 'RuC1,-xH,O' and SbPh, in MeOH the same compound is at ambient temperature gives the very insoluble deep red [RuCI,(S~P~,)~],,; A more soluble pink form (presumably obtained under reflux in MeOH or 2-methoxyeChan01.'~~ monomeric) has been prepared by reaction of alcoholic solutions of the reduced 'ruthenium blue' solution with SbPh3.I5"An earlier report on the preparation of [RuCl, (SbPh,),] from K,[RuCl,] and SbPh, is availabte'5xqand the corresponding [RuBr2(SbPh3),]is known.I5"
45.5.4.3
(RuXY(L-L),] complexes (n = 2,3; X,Y = halide)
Earlier work by Chatt et al. on reactions of 'RuC1,-xH,O' with various diphosphines R,P(CH,),PR, (R = Me, Et or Ph) and Nyholm et al. with o-C,H,(AsMe,),(diars) led to the preparation of cis- and trans-[RuCl, (R2P(CH2),PR2)2]437'591 and trans-[RuCl,(diar~),]'~~~ respectively. Replacement of chloride with Br-, I-, -OCOMe, SCN- was achieved. More recently, a wider range of the truw compounds [RuCI,(L-L),] have been obtained by treatment of either [RuCl,(PPh,),], [RuCI,(py),], K,[RuCl,(OH,)] or 'RuCl,-xH,O' with L-L in alcoholic solution 2-(diphenylpho~phinoethyl)pyridine,'~~~~ (L-L = dppm,'s93-t595dppe, I571.1594.1595 dppp,"3SL~1s36a~'5Y4 Ph,As(CH=CH)AsPh, (eda),45+'593diars, "" 0-C, H, (PMePh),, ' 5 9 7 o-C,H, (AsM~P~),,"'~ Ph,P(CH,),PMe2,'536a Ph,P(CH,),PMePh,'536a etc.), or with excess dppm, eda and Ph,As(CH,),AsPh,, by displacement of all the CO from 'RuCOC1' solutions.'593 Some trans[RuCl,(dmpe),] is also formed by treating [RuH(2-np)(dmpe),l with CDC1, Metathesis of trans[RuCl,(eda),] with NaX (X = NCS, I, N,, CN) yields frans-[RuX,(eda),] but with NaBr, [RuClBr(eda),] is formed.45 The complexes cis- and trans-[RuCl,(dppmj,] undergo isomerization reactions induced by light, heat or oxidation to Ru"'. The conversion rrans -,cis occurs thermalIy while cis -, trans occurs photochemically apparently by irradiation of the lowest lying d-d transiti~n.'~''In water/alcohol mixtures photolysis of cis- 01:trans-[RuCl,(dmpe),] gives trans-[R~CI(OH,)(dmpej,]~, while trans[RuCl(DMSO)(dmpe),]' is always observed in DMSO. These results are consistent with the C 0 . C 4-M'
380
Ruthenium
formation of a five-coordinate excited-state fragment as the primary photochemical process and the thermodynamic preference for [RuCl(dmpe):]* to rearrange to the isomer with C1- in the apical position A detailed investigation of the stereochemistry of cis- and trans-[RuCl,(L-L),] (L-L = oC,H,(EMePh),; E = As, P) has been undertaken. The optically active, racemic, meso, syn, and anti forms of the trans compound were isolated for both ligands; each of these subsequently isomerized to the corresponding cis complex by reaction with AlEt, and the various diastereoisomers of the latter separated and characterized. Whereas the trans isomers are relatively inert, the cis complexes readily undergo stereospecific halogen substitution by I- and C0.’597A minor product obtained from the reaction of ‘RuCl,.H,O’ with ko-C6H4(PMePh),in the presence of formaldehyde is trans-[RuCl, {( )-o-C,H,(PMePh), )( +)-o-C,H,(PMePh)P(O)MePh] containing P , P and 0,P‘ bidentate ligands respectively.’m Reaction of [Ru(OH,),](tos), with dppe gives [Ru(dppc), (tos),] (tos = p-toluenesulfonate).Imb Reaction of [RuCl,(PPh,),] with MeC(CH, PPh,), (triphos) affords the white compound IRuC1,(triphos),] shown by ” P NMR spectroscopy to have structures (153) with non-coordinated P atoms,Isw whereas Ph,AsCH,AsPh, (dpam) forms [RuCl, (dpam),] which contains one bi- and‘two mono-dentate dpam ligands.’593The reinvestigation of the reaction of [RuCl, (PPh,),] with equimolar amounts of Ph,P(CH2),PPhz ( n = 1-4) has been reported. Only for n = 4 is the complex [RuCl,(PPh,)(chelate)] isolatable. ” P NMR evidence is also presented for binuclear species such as [Ru?Cl,(L-L), (PPh3)2]1m1 (see Section 45.5.5).
+
(153)
Various [RuCl,(L-L),] have been prepared starting from either [RuC12(DMSO)4]1S’1 or reduced solutions of [Ru,OC~,,]~-and X-ray structures of cis- and tru~-[RuCl,(dppm),] are reported.lm” Reaction of [RuCl,(PPh,),] with Ph,P(CH,),PPhCH,CH,CpH (LH) (n = 3,4) has been shown to give neutral [RuLCl] complexes containing a coordinated Cp group.’602b
45.5.4.4
Polydentate P, As or Sb donor ligand halide complexes
Although reaction of MeC(CH,PPh,), (triphos) with [RuCl,(PPh,),] gives compound (153) where triphos acts as a bidentate I i g a ~ ~ dthe , ’analogous ~~ reaction with PhP{(CH,),PPh2 gives the In contrast, reaction of this tritertiarz five-coordinate monomer [RLIC~,P~P{(CH,),PP~,},].’~ phosphine with ‘RuC1, * xH, 0 in boiling EtOH gives a binuclear chloride-bridged complex (see Section 45.5.5.2). Likewise, reaction of ‘RuCl,*xH,O’ with PhP{(CH,),PPh,}, (ttp) and PhP((CH,),PCy,), (Cyttp) affords [RuCl,(ttp)], and [RuCl,(Cyttp)] respectively. The former is polymeric and the latter monomeric, presumably due to steric effects of the larger cyclohexyl groups.’486Incomplete displacement of PPh, from [RuCl, (PPh,),] occurs on treatment with P[(CH,),PPh,], (tet-2) to produce [RuC12(PPh,)(tet-2)], in which tet-2 acts as a terdentate ligand.’540 Interestingly, the same reaction with Ph,P(CH,),PPh(CH,),PPh, (tet- 1) gives the five-coordinate cation { R ~ C l ( t e t - l ) ] C l although , ’ ~ ~ ~ the product from reaction of tet-1 with [RuCl,py,] is formulated elsewhere as [RuCl,(tet-l)], and [RuCl,N{(CH,),PPh,},] is also rep~rted.”~’ A series of six-coordinate complexes [RuCl, (ligand)] containing the quadridentate ligands (oPh,L*C6H4),L, (L = P or As; L‘ = P, As or Sb but not all combinations), have been prepared by reaction o f ‘RuCl,~xH,O’ or [RuCl,(PPh,),] with the appropriate ligand. Metathesis with NaX gives the corresponding [RuX,(ligand)] (X = Br, I, NCS, CN but not ail combinations).46An X-ray analysis o f [RuBr, {As(o-C,H,AsPh,),}] confirms the distorted octahedral Reaction of (L) gives ~rans-[RuCl,L].’~~~~ [RuC12py,] with Ph,P(CH,),P(Ph)(CH,),P(Ph)(CH,),PPh,
45.5.4.5
(0
Carbon donor ligand complexes
Carbonyl halides
A very large number of compounds of general formula [RuX,(CO),L,-,] (X = Cl, Br, I; L = PR,, AsR,, SbPh,; x = 1-3) have been synthesized in the last 25 years. Since this is the subject
38 1
Ruthenium
of detailed review^,'"^^^^'^^^'^^^ only a brief account is presented here. For x = 3, the best route is via reaction of the cluster compounds [Ru, (CO),L,] with halogens or hydrocarbon solvents'5m(e.g. L = PPh,, PMe,(CH,Ph), AsMe,(CH,Ph); x = Cl, Br, I).'560,'"hFor x = 2 and 1, several methods are available and these lead to the various geometrical isomers (154-157; x = 2) and (158, 159; x = 1) which can also interconvert under thermal or photochemical stimulation (see references 1607-1610). For example, reaction of [RuI,(CO),],, with L gives [RuI,(CO)~L,] (154) whereas [RuI,(CO),(p-MeC6H4NH,),] and L under mild conditions gives products of stereochemistry (155) which isomerize thermally to (154). These iodo compounds are converted into the corresponding chloro or bromo compounds by treatment with the appropriate halogen, usually with preservation of the original The dicarbonyl species can also be obtained by direct reaction of [RuX,(CO),], (X = C1, Br), [RuC~,(CO)~],,[RuC~,(CO),THF]'~'~ or [ R U C ~ , ( C O ) ~ ] *with - ' ~ ' ~L or by treatment of carbonylated, alcoholic solutions of RuCl, with L.106*'s"4,'614" As detailed in Table 1, reference 1, p. 693, the latter route also leads to [RuC12COL,] in some instances. Bromo and iodo analogues of [RuCl,COL,] can be obtained by metathesis, the ease of replacement of Cl dependent upon the trans ligand; C1 trans to PR, is substituted more rapidly than C1 trans to Cl or CO which allows the isolation of some intermediate [RuCIY(CO)(PR,),] complexes (X = Br, 1, SCN).I6l4"The same intermediates can be obtained by treatment of [RuHCl(CO)(PR,),] with HX (X = Br, I), together with some [RuX, (CO)(PR,),] (158).'614b
( 154)
( 155)
( 1 56)
(157)
(158)
(159)
An alternative method is by carbonylation of complexes of Ru" or Rd" containing P, As or Sb donor ligands, e.g. [RuCl,(PPh,),] reacts with CO to give [RuCl,(CO),(PPh,),] (the isomer formed depending on condition^),'^^^'^^ whereas [RuCl,(P(OMe)Ph,),] produces [RuCl,(isomer (CO)(P(OMe)Ph,),] (isomer 158),'615and [RuCl,(PMe,),] forms [RuCl,CO(F'Me,),] 159).'39'cMore examples are given in Table 2, reference 1, p.695. Solvent0 complexes [RuCl,(CO)(PR,),(solvent)] (solvent = DMA, DMF, DMSO, MeOH) have also been isolated from these reactions. 155%1607,l616 In some instances, decarbonylation of the solvent provides the source of CO, e.g. reaction of 'RuCl,*xH,O' and PPh3 in benzyl alcohol gives [RuC12(CO)(PPh3),];'"' [Ru,C13(PEt2Ph),JCl and reaction of decarbonylates saturated aldehydes RCHO to give [RUCI,CO(PE~,P~),]'~'' 'RuCl, -xH,O', Pfh,, KOH, 2-methoxyethanol and aqueous HCHO, followed by CCl, produces [RUC~,(CO)~(PP~~)~];~~'~~ a similar recipe yields [RuX,CO(AsRPh,),] (R = Me, Et, Pr).1618 Other routes to [RuCl,COL,] are by reaction of [RuCl,(PPh,),] with C0,1619and cocondensation of Ru with (COCI),, followed by addition of PMe3.1620 Reaction of [Ru(CO), (PPh,),] with (NSCl), gives [RuCl,(CO),(PPh,),], "" and X-ray structures of some of these mono- and di-carbonyl complexes have been p~blished.'~~~,'~~~,'~~~ Detailed studies of the thermally and photochemically induced interconversions of the various isomers of [RuX,(CO),L,] reveal that the key step is dissociation of CO to form five-coordinate intermediates.'60*,161D A similar study 'on [RuCl,CO(PMe,Ph),L] (L = P(OMe),, P(OMe),Ph) suggests that the isomerization mechanisms invohe phosphorus ligand dissociation.'m An electrochemical study of [RuCI,(CO)(PMe,Ph),] reveals that the compound undergoes a reversible oneelectron oxidation.625 Treatment of carbonylated, 2-methoxyethanol solutions of RuCl, with bulkier phosphines such as PCy, gives the five-coordinate [RUCI,CO(PC~,),~'~~~"~'~~~~ shown by X-ray analysis to have the distorted square pyramidal structure (160);'624a cyclohexyl hydrogen atom blocks the sixth coordination position (Ru.-.H = 2.31 A). Reaction of (160) with L' (CO, fumaronitrile, CH,=CHCN) gives [RuCl,(CO)L(PCy,),].'623"~'625 Even bulkier phosphines such as PBu',Ph and PBu',(p-Tol) react with the 'RuCOCl' solution to give dimeric ruthenium(1) complexes [Ru(~-CI)(CO)~PR,], (see Section 45.5.3.2). These probably form because the PBut2Ph groups are too bulky to permit six coordination. As discussed elsewhere, related compounds containing SnCI; , (e.g. [RuCl(SnC13)CO(Me2CO)(PPh,),]),'og SiMe, {e.g.[RUC~(S~M~,)(CO),PP~,]),~~~~~ and SiCl, (e.g. [Ru(SiCl,),(C0)zPPh3(P(OMe),)])67(Section 45.3.2) are available. There are relatively few examples of cationic and anionic carbonyl halides containing monodentate P, As or Sb donor ligands. Reaction of [Ru(CO),(PPh,),] with I2 or %I4 gives
382
Ruthenium CO
(160)
[RuI(CO),(PPh,),]Z (Z = I,'626 SnT,"); on heating, these form [RuT,(CO),(PPh,),]. Similar cations [RuX(COj, (PR3),]+ are obtained by reaction of phosphine-substituted carbonyl halides with Lewis acids (AICl,, AIBrl, FeCl,) in benzene under CO (X = C1, Br; R = Cy, Et, Ph).'627In some systems, Lewis acid adducts can be isolated. Thus treatment of [RuCl,(CO),(PMe,),] with FeC1, in CH,Cl, at 25°C gives [RuCI(Cl + FeCl,)(CO),(PMe,),]. Addition of more FeCl, affords [Ru(CI + FeCl,),(CO)z(PMe7)2].162s The related cations [RuCl(CO),(CS)(L-L)]BPh, are known (see Section 45.5.4.5.ii).14'9aDissolving [RuCl,CO(P(OMe)Ph,),] in MeOH and adding Na[BPh,] gives the five-coordinate cation [RuCl(CO)(P(OMe)Ph,),]+; with [RuCl,CO(P(OEt)Ph,),], the binuclear (cf. [RuCI(P(OMe)Ph,),]+ and [RuCL [RuCl(CO)(P(OEt)Ph,),], (BPh,), is (P(OEt)Ph2)4]22' (Section 45.5.4.1).1533The chiral octahedral cations [RuR(CO),L*(PMe,),]+ (L* = PhPMeR' etc.) have been prepared and ,JPpbetween the diastereotopic PMe, groups determined. '629b Reaction of [Ru(CO),(PPh,),] or [Ru12(CO),(PPh,),] with Me1 gives MePh,P [ R U I ,( C O ) , P P ~ , ] . 'Closely ~ ~ ~ related carbonyl anions [RuCI3COL2]- (L = AsPh,, SbPh,) are prepared by treatment of Ph3(PhCH,)P[RuCl, CO(nbd)] with stoichiometric amounts of L; some [RuCl,COL(nbd)j (Section 45.5.4.5.vi) is also formed. With excess SbPh,, [RuCl,CO(SbPh,),] (158) is the main product.6s9Treatment of [RuCl,CO(MeOH)(PPh,),] with [AsPh,]Cl*HCl in acetone gives ASP~,{R~C~,CO(PP~,),].'~'~ Relatively little work has been published on carbonyl halide complexes of polydentate phosphines and arsines. Thus reaction of [RuX,(CO),], (X = C1, Br, I) with dppe, eda and Ph,E(CH,),EPh, (E = P, As) gives [RuX,(CO),(L-L)] (161; X = Cl),1631 whereas treatment of the ethanolic 'RuCOC1' solution with dppm or eda gives isomer (162). With dpam, [RuCI,(CO),(dpam),] (163) with unidentate dpam coordination is formed.'593Detailed studies on the analogous dicarbonyl complexes cis- and trans-[RuCl,(CO),(L-L)] (L-L = o-C,H,(EMePh),; E = As, P) have appeared. The trans isomers were prepared from [RuCl, (CO),], and L-L in boiling EtOH or 2-methoxyethanol whereas treatment of [RuCl,(CO),j, with L-L in EtOH at room temperature affords the less stable cis isomers. For L-L = o-C,H,(PMePh),, the tricarbonyl cations [RuCl(CO),L-L]Cl can be prepared and the dicarbonyls undergo reversible decarbonylation to give the chloro-bridged dimers [RuCI,(CO)(L-L)],. Reaction of these dimers or the netural dicarbonyls with DMSO gives exclusively cis-[RuCl,(CO)(DMSO)(L-L)] (with S-bonded DMS0).1632*1633 Carbonylation of [RuC1,(ttp)], gives cis- and trans-[RuCI,(CO)ttp] whereas only trans-[RuCl,(CO)Cyttp] is formed with [ R ~ C l , ( C yt t p ) ] . The ' ~ ~ ~cations [RuX(CO),(Cyttp)]X' ( X X = HC1, HBr, HBF,, Cl,, Br?) are prepared by oxidative addition of halogens and acids to [Ru(CO),(Cyttp)] whereas treatment of [RuCl,(tpp)], with CO and Tl[BF,] gives [RU(CO),(~~~)](BF,),.'~~~ Halogenation of [Ru(CO),triphos] (triphos = MeC(CH,PPh,),) gives initially the salts [RuX(CO),triphos]X (X = C1, Br) which lose CO reversibly to give [RuX,(CO)(triphos)]. The neutral chloro complex contains a labile Ru-P bond since it also reacts reversibly with SO, to give [RuC12CO(S0,)(triphos)](164j.Ia5 Treatment of these and related [RuX(CO),(triphos)]X cations with a variety of neutral ligands L (L = Bu'NC, P(OMe),, PMe,Ph etc.) in the presence of trimethylamine N-oxide yields chiral complexes of the type [RuX(CO)L(triphos)]X'. The compound [RuMe(CO)(CNBu')(triphos)]PF,has been resolved, and the absolute configuration of the ($)-enantiomer has been determined.'6M L
Ruthenium
383
Reactions of [RuCl,(L-L),] (L-L = dppm or dppe) with CO and silver salts of non-coordinating anions produce ERu(CO),(L-L),]~+ (both cis and Trans isomers). A series of compounds of general formula [Ru(CCl)),(dppm),AgYIX, may also be isolated (Y= 02PF2,C1, ClO,, BF(OH),; X = BF,, AsF,, ClO,), which are proposed to contain a dpprn ligand bridging Ru and Ag, the bond between which is reversibly cleaved by MeNO, on the NMR time scale.1635 Treatment of the dicarbonyl cations [Ru(COj, (L-L),]'+ with Na(HB(OEt),) produces the formyl species [Ru(CHO)(CO)(LL),]+ as confirmed by the X-ray analysis of trans-IRu(CDO)(CO)(dppe),]SbF,.CH,C12.'636Studies have revealed that trans-[Ru(CHO)(CO)(dppe),fSbF, decomposes in CH,CL, to give cis[RuH(CO)(dppe),]SbF, which subsequently isomerizes to the trans isomer. However, cis[Ru(CHO)(CO)(dppm),]SbF, gives [RuH(CO), (dppm),]SbF, with a monodentate dppm ligand but chelation occurs on photolysis in CH,Cl, to give trans-[RuCI(CO)(dppm),]SbF, via trans[RuH(CO)(dppm),]SbF, It has been shown that ~rans-[Ru(CHO)(CO)(dp),jSbF,[dp = 1,2bis(dipheny1phosphine)benzenel decomposes to brans-[RuH(CO)(dp),]SbF, via a free radical mecha n i ~ m .Monocarbonyl '~~~~ cations of type [RuCl(CO)(L-L),] are also prepared by direct reaction of trans-[RuCl,(dppm),] with C0'593or by reaction of the five-coordinate cations [RUCI(GL),]~ (L-L = dppp, 2-diphenylphosphinoethylpyridine)with C0.1535 Hydridocarbonyls and carbonyl complexes containing C, N, 0 or S donor ligands are covered in later sections of this review. +
(ii) CS and CSe complexes
Reaction of [RuCl,(CS)(CO),] (made from [RU~(CO)~,] and CSCI,) with PPh, gives [RuCl,(CO),CS(PPh,)] (165). With bidentate ligands L-L (L-L = Ph,E(CH,),EPh2; E = P, As) a mixture of species is obtained and only [RuCl(CO),CS(L-L)]BPh, could be isolated in pure (Section 45.5.5.2) with CO gives form.'629d Bridge cleavage of [RuC~,(CS)(PP~,),]~ [RuCI, CO(CS)(PPh,),], the isomer formed depending on the reaction conditions,'638whereas with MCl/HCl, M[RuCl,(CS)(PPh,),] (M = Ph,As, Et,N, Ph,(PhCH,)P) is obtained.'639Treatment of the double chloride-bridged dimer with DMF gives [RuCl,(CO)DMF(PPh,),] which the aqua on recrystallization from MeOH/CH, C1, produces [RuCl, (CS)MeOH(PPh,),] (166);1616 analogue [RuCl, (OH,)(CS)(PPh,),] is formed by reaction of [RuCl,(PPh,),] with CS, in xylene.'640 Reaction of [RuCl,(PPh,),] with neat CS, gives the binuclear complexes [RuCI,(CS)(PPh,),], and [RU,C~,(CS)(PP~,),]'~~~~'~~ (see Section 45.5.5.1) and the monomeric [RuCl,CO(CS)(PPh,)] (?) is claimed .Ia2 The alkoxyphenylphosphine complexes [RuCl,(CS)L,] [L = P(OR)Ph2 (Ti = Me, Et), P(OMe),Ph] are prepared by reaction of either [Ru,Cl,(CS)(PPh,),] or [RuClzCS(PPh,),], with an excess of L in C,H,. Addition of Na[BPh,] to aicoholic solutions of these P(OR)Ph, complexes then gives [RuCl(CS)(P(OMe)Ph,),]BPh,and [RuCl(CS)(P(OEt)Ph),],(BPh,), . No cationic complexes are generated under these conditions for [RuClz(CS)(P(OMe),Ph),1.'515 The Ruo complex (Ru(CO),(PPh,),] reacts readily with CY, (Y = S, Se) to form [Ru(CY,)(CO)2(PPh,),].'470 These compounds are readily methylated with Me1 to give [Ru(CY,Me)(CO),(PPh,),]+ . Displacement of CO with X- affords neutral [RuX(CY, Me)(CO)(PPh,),] and treatment of these dithio- or diseleno-methyl ester complexes with HX produces the mixed thio- or seleno-carbonyl complexes [RuX,(CO)(CY)(PPh,),] (X = C1, Br).Ia3 These compounds are also formed on treatment of [RuCl,(CCl,)(CO)(PPh,),] with H,Y.49An X-ray structure determination shows [RuCl,CO(CSe)(PPh,),] to have structure (167). The long Ru-Cl bond (2.477A) trans to CSe compared to 2.428 A (Ru-Cl trans to CO) demonstrates the strong trans influence of the CSe l i g a ~ ~ d .Chromatography '~~~,'~ of [RuI,(CO)(CSe)(PPh,),] on alumina affords [RuI(OH)(CO)(CSe)(PPh,),] (with OH trans to CSe).'@' A report on the preparation of hydride complexes such as [RuHCl(CS)(PPh,),], and [RuH(q2-OCOR)CS(PPh,),] (R = H, Me) has appeared (see Section 45.10.3.2.iii). The corresponding [RuC1(q2-OCOHjCS(PPh,),] is also synthesized from [RuCl, (OH,)(CS)(PPh,),] and Na[OCOH].164sSome CNR complexes containing CS and CSe are described in the next section.
Ruthenium
384
(iii) CNR, CNHR and related carbene complexes Reaction of [RuX,(AsRPh,),] (X = Br, C1; R = Et, Me, Ph) with p-tolyl isocyanide in acetone A more detailed study on [RuCl,L,] or alcohol gives [RuX~(ASRP~,),(CN-~-TO~),].'~~ (L = PMe,Ph, PPr",Ph, PBu",Ph) shows that the ruthenium(TI1) cations [RuCl,(CNR)L,]+ are first formed and these undergo reduction to various isomers (168-170) of [RuCl,(CNR), L,].'647Other routes to [RuX,(CNR),(ER,),] involve direct reaction of [RuCl,(PPh,),], [RuX, (AsPh,),MeOH] or [RuX,(SbPh,),] (X = Cl, n = 3; X = Br, n = 4) with CNR;'4'','590-'648 the I3CNMR spectrum of [RuCl, (CNPh),(PPh,),] (170) is r e ~ 0 r t e d . These l ~ ~ isomers undergo facile interconversion in solution on thermal or photochemical excitation, and the mechanism of isomerization is proposed to occur via the photoelimination of PPh,.'s90*'650 The analogous [Ru(OCOMe),(CNMe),(PPh,),jare prepared from the action of CNMe on [Ru(OCOMe)z(PPh,),]'6s' and treatment of cis-[RuCl,(CNEt),(EPh,),] (169) with SnC1, in 2-methoxyethanol gives cis-[RuCl(SnCl,)(CNEt),(EPh,),] (E = P, AS, Sb).'" Direct reaction of [RuX,CO(AsRPh,),] (X = C1, Br; R = Me, Et, Pr) with p-tolyl isocyanide gives [R~X,(CO)(CN-~-TO~)(ASRP~,),]'~~~ and the X-ray structures of [RuCl,(CO)(CN-pClPh)(PPh,),]'653 (171) and [RuCl(Ph)CO(CNCMe,)(PMe,Ph),](172)'654have been reported. Treatment of the five-coordinate [RuCl(R)(CO)(PPh,),] with CNR affords [RuCl(R')(CO>(CNR)(PPh,),](R = R = p-Tol). Heating this in boiling CH,Cl, gives an orange material, shown by X-ray analysis to be the iminoacyl complex (173). Protonation of (173) with HC1 gives the aminocarbene complex [RuCI, [C(NHR)R](CO)(PPh3),].'655 Reaction of 0, with the zerovalent [Ru(CO),(CN-p-Tol)(PPh, ),I (Section 45.5.1.3) gives the carbonate (174).'480 The product of reaction between [RuCl,(PPh,),] and CS, in boiling xylene reacts with CNR to give the yellow trans isomer of [RuCl,(CNR)(CS)(PPh,),]. Isomerization to a colourless cis isomer occurs in toluene under reflux and this reacts with Ag[C104] to give [RuCI(OCIO,)(CNR)(CS)(PPh,),] which in turn gives [RuCl(CNR)(CO)(CS)(PPh,),]ClO, on carbonyiation. This cation reacts with SeH- to give (175) which on methylation with Me1 affords [Ru(qZ-Se=CSMe)(CNR)(CO)(PPh3),]+ (176). Treatment of (176) with aqueous HCl in toluene/ EtOH solution under reflux liberates MeSH and produces [RuCl,(CNR)(CSe)(PPh,),] in high yield.'640*'645 The dicarbonyl complexes [RuCl, (CNR)(CO),PPh,] can be made by reaction of [RuCl, (CO),(CS)PPh,] with NH,R, this reaction goes via a labile amino(mercapto)carbene complex which eliminates H2 Other, isocyanide complexes containing hydride ligands are covered in Section 45.10.
I
PPh3 (173)
se
PPI13
(174)
(175) La = (CO)(CIVR)(PPhA),
( 176)
Roper and c o - w ~ r k e r s have ' ~ ~ ~reported the synthesis of a number of formimidoyl (CNHR) complexes. For example reaction of [RuHI(CN-p-Tol)(CO)(PPh,),] with acetate ion promotes hydrogen migration from ruthenium to isocyanide to give the formimidoyl complex (177) (L-L = OCOMe). Treatment with -S?CNEt, gives (177) (L-L = SZCNEt2).The nitrogen atom of the CNHR group is attacked by electrophiles to generate a range of secondary carbene complexes such as (178) (with HC104)and (179) (with HCl) (Scheme 45). With dimethylsulfate, methylation of (177) (L-L = OCOMe) leads to the cationic N-p-tolylmethylaminocarbenecomplex (180). The bidentate acetato group is then removed by addition of LiCl to give the neutral carbene (181). Other examples of P donor ligand compounds containing these C-bonded ligands are to be found in the above references.
Ruthenium
385
!-
J
r
+
L
1'
L = PPh3 I R = p-Tol i, -0COMe; ii, HI; iii. -S2CNEt2 ; iv. HC104; v, HCI; vi, MeZSO,; vii, LiCl
scheme 45
Reaction of [RuCl,(PPh,),] with electron-rich alkenes (RNCH,CH,NRC=), (LR,,R = Me, Et, CH2Ph) affords tru~zs-[RuCl,(L~)~] which for R = Me react with PF, and P(OMe), to give trans[RUC~(L~')~(PF,)]C~ and tr~ns-[RuCl(L~'), {P(OMe)3},]C1respectively. More complicated reactions occur with aryl-substituted electron-rich alkenes. Thus, reaction of LRz( R = Ph, p-To1 or p-MeOC,H,) with [RuCl,(PPh,),] in refluxing xylene gives the cyclometallated product (182). Further reactions undergone by this stereochemicaiiy rigid five-coordinate complex include dis-R' placement of both Pfh, groups by other PR, to give [RuCl(L )(PR3),] or of one PPh, group by LE', to give [RuC1(LR)(LE')PPh,]. The latter compound is also formed by treatment of [RuCl,(LR)(NO)PPh,] (made from [RuCI,NO(PPh,),] and LR,) with LEt2.Small ligands L' add to -R' -R (182) and [RuCI(L )(PR3)J to give the six-coordinate [RuCl(L )(PR,),L'] ( L = CO, PF,, P(OMe),, N,, MeCN).'4M-'657.'6s8a The compound [ R U C ~ , ( L ~ ' is ) ~ a] good hydrosilylation catalyst.'658b
- -
-
[ RuCI(LR')(PPh&l
(182) X = H, Me OF OMe; L = PPh,
386
Ruthenium
(iv) Alkyl, aryl and acyl complexes Most of the early work in this areal6" was concerned with reactions between cis- and trans[RuX,(L-L),] (X = halide; L-L = dmpe, dppe or dppm) and various alkylating and arylating reagents. Thus, cis-[RuCl,(L-L),] and LiR (R = Me, Ph) give the corresponding dimethyl and diphenyl complexes, cis-[RuR,(L-L),] respectively. Cis-[RuEt,(dmpe),] has been prepared from trans-ERuCl, (dmpe),] and MgEt, whereas with trans-[Ru(BCOMe),(dmpe),], the trans diethyl isomer is formed.'6aUse of the less reactive AIR3compounds affords only the monoalkyl derivatives [RuClR(L-L),] (alkyl = Me, Et, PF) from both the cis and trans dichloro complexes; the monoaryl analogues (R = Ph, p-MeC,H,) are also available. Further reaction of cis-[RuClMe(L-L),] with LiMe gives cis-[RuMe,(L-L),] whereas treatment with Br- , I- or SCN- produces [RuXR(L-L),]. The latter are also formed by reaction of [RuHR(L-L),] (see Section 45.10.3.2.i) with hydrogen halides. The compound cis-[Ru(2-np)(Cl)(dmpe),] is also the major product together with CCI. 20% [RuCl, (dmpe),] on treatment of cis-[Ru(2-np)H(dmpe),] with CDC1,,44and [RuClMe(diars),] is obtained from [RuCl,(diars),] and LiMe.'659Detailed NMR studies are reported on many of these alkyl and aryl c o m p l e ~ e s . ~ ~ ~ ~ ~ ' ~ ~ ~ The corresponding dialkyl and diary1 complexes cis-[RuR, (PMe,),] (R = Me,'661Ph'52s)are best prepared by treatment of [Ru, (OCOMe),Cl] with MgR, and PMe, in THF. The dimethyl derivative has also been synthesized by reaction of trans-[RuCl, (PMe,),] and LiMe in ether.1380a,1534 Studied6 have shown that treatment of this di-Me compound with one equivaient of HC1 at - 78°C gives cis-ERuCl(Me)(PMe,),] whereas with MeCO,H, cis-[Ru(q' -OCOMe)Me(PMe,),] is formed; more HCl produces [RuCl(PMe,),],CI,. The mono-Me complex also reacts with 1% Na/Hg to give cis-[RuMe(PMe,),],Hg (see Section 45.2). The five-coordinate PPh, complex [RuCl(Me)(PPh,),] results from the reaction of [RuCl,(PPh,),] with AlMe, in C,H,.'662 Related mono-Me complexes generated by heating [Ru{P(OMe), >,I in n-hexane in include cis-[RuMe{P(O)(OMe)~}{P(OMe),},] a sealed tube and [RuMe{P(OMe),),]I, obtained by reaction of Me1 with either of these isomers.'38o The ylide complex [RuCl(CH,PMe,)(PMe,),]Cl (Section 45.5.4.1) results from reaction of trans[RuCl,(PMe,),] with Li[Me,P(CH,),].'534 Trifluoroiodomethane oxidatively adds to rrans-[Ru(CO), {P(OCH,)3CEt},] in C6H6 to give [RuI(CF,)(CO),L,]~~~~ whereas the photochemical reaction of chlorotrifluoroethylene with trans[Ru(CO),(PMe,Ph),] affords an isomeric mixture of [RuCl(q'-CF=CF,)(CO),(PMe,Ph),j.'664" Various compounds containing a Ru-CF, bond, e.g. [Ru(CF,)(HgCF,)(CO),(PPh,),] (Section 45.2) and [RuCI(CF,)(CO),(PPh,),], are also reported and conversions to difluorocarbene species d i s c ~ s s e d . ' ~Reaction of [RuCl,(CF,)CO(PPh,),] with HOCHzCMe, gives [RuCl,(CFOCH2CMe3)CO(PPh3)2].'"h A preliminary communication on reactions of [RuC12(CCl,)CO(PPh,),] is a ~ a i l a b l e . ~ ~ In an interesting insertion reaction, [RuHCl(CO)(PPh,),] and RCH=CHR' dimethyl fumarate, methyl sorbate, methyl acrylate, N,N'-dimethylacrylamide or CH2=CH-2-py) give I
I
[RuCl(CO)(RCH-CH, R')(PPh,),] in which the alkyl residue is bidentate, coordinated through the a-alkyl bond and also via R'. With CH,=CHCN, however, the halide-bridged [RuCl{CHMe(CN)}CO(PPh,),], with a more normal monodentate alkyl group is formed.lffisa,b Alkyl acrylates insert into [RuHClCO(PPh,),] under mild conditions to give Ru-C g-bonded compounds [RuCl-
(CH,CH2C0,R)CO(PPh3)2].166sc Reaction of trans-[RuCl,(CO),(PMe,Ph),] with HgR, (R = Me, Ph) provides a convenient route to [RuCl(R)(CO),(PMe,Ph),] since even when HgR2is added in large excess, there is no replacement of the remaining chloride ligand.16" The methyl derivative [RuCl(Me)(CO),(PMe,),] is also obtained by photolysis of [RuCl,(CO), (PMe,),] and dimethyltitano~ene.'~~~ Treatment however of (R = Me, Ph, 3- and either the cis or trans isomer with LiR leads to [RUR,(CO)~(PM~,P~),] 4-MeC6H,, 4-MeOC6H,, 4-CIC6H,, 4-FC6H,), and the mixed [RuRR'(CO),(PMe,Ph),] are readily synthesized from [RuCl(R)(CO),(PMe,Ph),] and LiR.'668Spectroscopic (IR, NMR) studies reveal that all these compounds have configuration (183) irrespective of the stereochemistry of the starting materia1.'666,1668 The proposed mechanism of formation of the monoalkyl or aryl complexes involves loss of CO and formation of intermediate Hg adducts. However, for reactions with LiR (Scheme 46) it is suggested that the mechanism involves nucleophilic attack by R - on the carbon atom of a CO ligand to yield the acyl intermediate (184). In support of this proposal, reaction of [RuMe,(CO),(PMe,Ph), J with CO gives first [RuMe(COMe)(CO),(PMe,Ph),] and then [Ru(COMe),(CO), (PMe,Ph),]. Similarly, [RuClMe(CO),(PMe,Ph),] and [RuMePh(CO),(PMe,Ph),] afford [RuCl(COMe)(CO),(PMe,Ph),] and [RuPh(COMe)(CO),(PMe,Ph),] respectively on carbonylation and with L' (L' = PR,, P(OR),, AsMe,Ph), [RuX(COMe)(CO)(PMe,Ph),L') (X = C1, Ph) are formed. 1668~1669 However no evidence is found for benzoyl complexes when [RuX(Ph)(CO),(P-
Rut heniurn
387
MezPh),] (X = C1, Br, I, Me, COMe, Ph) are treated with CO at atmospheric pressure or with PMe,Ph. This is in contrast to studies on related complexes [Ru(CpH,Me-4)X(CO),(PPh,),] (X = C1, Br, I) (for preparation, see Scheme 47) which are in equilibrium in solution with acyl species [RUX(COC~H,M~-~)(CO)(PP~,)~].~~'~ The low value of vco (1550 cm- ') suggests the acyl group is dihapto rather than monohapto and this is confirmed by crystal structure analyses of [RuI(?~-COC,H,M~-~)(CO)(PP~,),] and of the related [RuI(q2-COMe)(CO)(PPh3),] (185; X = I; R = Me) which is prepared from [Ru(CO),(PPh,),] and MeHgI in benzene.I6" Interestingly, it has been suggested that the corresponding [RuCl(COEt)(CO)(PPh,),] contains an q' -acyl group and that it does not isomerize to [RuCl(Et)(CO),(PPh,),] in Complexes of formula [RuCI(COEt)CO(PPh,)2Sotvent] (Solvent = C6H6,CH2ClI, THF) can be obtained by reaction of PPh, with [RuCI(COEt)(CO),], (prepared by treating [RuCl(CO),(q-C,H,)] with C2H, under pressre)."^^ Reaction of [RuHCl(PPh,),] and [RuHCl(CO)(PPh,),] with aldehydes has also been claimed to give [RuC1(COR)(CO)(PPh,),] (R = Me, Et).1674However, it has been pointed that the IR and NMR spectra of these compounds are identical with those of [RuCl(OCOR)(CO)(PPh,),], and since it is known that [RuCl,(PPh,),] and PhCHO give [RuCl(OCOPh)CO(PPh,),]'675a(see Section 45.5.4.7.vi), the acyl formulation seems unlikely here. The binuclear complex [Ru,(CO),PPh, (p-PPh2)(p-O=CPh)] containing bridging phosphido and acyl groups is obtained by pyrolysis of [Ru, (CO),(PPh3),].16'sbThe double acyl-bridged compound [Ru,(p-O= CEt),(CO),] undergoes facile substitution of a CO by PPh, et^.^^^^^ PMe2Ph
x%,l
,/"
oc+co PMe,Ph (183) X = C1, Me, aryl
PMe,Ph
PMe,Ph
PMe,Ph
PMe,Ph
PMe,Ph
PMe,Ph Scheme 46
Scheme 47
It has been revealed1676that the diaryl complexes [RuRR(CO),(PMe,Ph),] will react with Me,CNC to give [RuR'(COR)(CO)(CNCMe,)(PMe,Ph),] shown by X-ray analysis (for R = R = Ph) to have structure (186).16" Kinetic studies indicate that the rate-determining step is combination of the aryl and CO ligands followed by rapid attack of the isonitrile trans to the acyl ligand (Scheme 48). In unsymmetrical diaryl compounds, the aryl ligand bearing the more electronreleasing substituent becomes incorporated in the acyl ligand. Similar complexes [RuR(COMe)(CO)(CNCMe,)(PMe,Ph),]are formed from [RuMeR(CO),(PMe,Ph),] (R = Ph, COMe, Me) and Me, CNC. In this instance [RuMe(COMe)CO(CNCMe,)(PMe,Ph),](186) reacts with more Me,CNC to form (187); in the absence of added Me,CNC, complex (186) (R = R = Me)
Ruthenium
388
slowly disproportionates into [RuMe2(CO),(PMe,Ph),] and (187).1678a Treatment of [RuH(CO),], with PPh,, followed by elution from a silica gel column with EtOH gives [Ru(OH)(OCEt)(CO),PPh3].'67sb PMe2Ph R R'**,** ,,*e\*
PMezPh
I
L
R'""%
I
/R
PMe,Ph \*,*e'R'
1
Me-****,, Me,CNCt
7
\O
oc+coPMeaPh
* C V iUkC/R
o c WPMezPh r-C
PMezPh \O (186)
Scheme 48
The complexes [RuRR(CO), (PMe,Ph),] also decompose intramolecularly in CHCl, at 298 K to yield ketones RR'CO. A proposed mechanism (Scheme 49) involves reductive elimination by C-C bond formation. For R = R' = p-Tol, the unusual product (188) is isolated and it is suggested that this arises from oxidative addition of the ketone in the proposed Ruo intermediate to form (189) and subsequent reaction with CHC1, replaces hydride by chloride.'679"There is a report of the crystal structure of (188) and further examples of the synthesis of ortho-metallated ketone complexes.1679b
[ RuRR'(CO),(PMe,Ph),
'[ Ku(CO)(PMe,Ph),
[ Ru(CO)(OCRR')(PMe,Ph),
'
(for = ''
/ I Ps
1 $
1 ' t RR'CO
[ Ru(C0) { C,H3MeC(0)C6H,Me
} H(PMe,Ph), I
CHCI,
( 189)
PPhMe, Me
Scheme 49
( I 88)
Alkyl, aryl and acyl nitrosyl phosphines and hydridophosphine complexes of Ru" are covered in Sections 45.5.4.6.iii and 45.10 respectively.
(v)
Cyclometallated complexes
Cyclometallation is a reaction in which a chelate metal alkyl or aryl is generated by cleavage of a C-H bond in a coordinated ligand, a new metal-carbon r~ bond being formed at the point of cleavage (e.g. compound (182) in Section 45.5.4.5.iii). Since most of the work on these types of complex is contained in another review (reference 1, p.734), only a brief survey is included here. Reaction of [Ru, (OCOMe),Cl] with [Mg(Me, SiCH2)J and PMe, in THF gives the metallacycle (190 X = Si). On reaction with [Mg(CH2BuT)2J the neopentyl compound (190; X = C) is whereas [Mg(2-MeOC6H,),] produces (191).'528Compound (190; X = C) is also obtained by reaction of cis-[RuClMe(PMe,),] or [Ru,Cl, (PMe,),]BF, with [MgCl(CH, Bu')]; reaction of cis-
389
Ruthenium
[RuCI(Me)(PMe,),], truns-[RuCl,(PMe,),] or [RuCI(PMe,),],Cl, with [MgCl(CH,Ph)] gives (192).16Treatment of rrans-[RuCl, (PMe,),] with the lithiated ylide Li[Me, P(CH,),] gives complex (193) containing both a three- and four-membered metallacycle together with the cation [RuCl(PMe3),(CH,PMe,)]C1 (147) (Section 45.5.4. 1).1380,'534 Another example of a three-membered obtained by reaction of [RuH(q2-h4e,PCH,)(PMe,),] metallacycle is [RuI(~~-M~,PCH,)(PM~~),] with MeI; some trans-[RuI, (PMe,),] is also formed.'391The metallacycle [RuCl(CH,PMe,)(PMe,),] and spirocycle [Ru(CH,PMe,), (PMe,),] have been prepared by reaction of cis[Ru(OCOMe)Cl(PMe,),] and cis-[Ru(OCOMe), (PMe,),] respectively with Li~(SiMe,),].'680 Reaction of [RuHCl(CO)(PPh,),] with alkenes such as dimethyl fumarate and 2-vinylpyridine give the metallacycles (194) and (195) respectively,'665and with 2-formylpyridines, insertion into the Ru-H bond gives cis-[RuCl(OCH, C,E-T4N)(CO)(PPh,),].'68' The related complex (196) is prepared by reaction of [RuCl(OCOMe)(CO)(PPh, j2]with acetophenone at elevated temperatures'682[cf. compound (188) in Section 45.5.4.5.ivl.
(190)
'1
(192)
(191)
(193)
PPh,
H C0,Me
***t*
+ ,o.
PPh,
(194)
OMe
PPI13 H~
oc%,@o,l /'c
.FT+-J PPh, (195)
PPh3
I
C'****,
)O
*cq*; (196)
A number of complexes containing one or more ortho-metallated tertiary phosphite or phosphine ligands are known. For example, reaction of [RuHCl(PPh,),] and P(OPh), in C,H, at room temperature gives hydrogen and the ortho-bonded complex (197); on treatment with H2 (or Dz), [RuX((2,6-X,C,H3O),P],] (X = H, D) is readily formed,'683while CO in boiling benzene affords [RuCl((C,H,O)P(OPh),)CO~{P(OPh)3)2].1684 Reaction of [RuHCI(CO)(PPh,),] with P(OPh), in boiling xylene leads to displacement of PPh, to give low yields of [RuCl(CO)P(OPh,),(P-C)] (P-C = C,H,OP(OPh),) whereas with [RuH,(PPh,),], the dimetallated product (198) is ob(as does reaction tained.'685Sodium amalgam reduction of [RuC12(P(OPh),),] also gives (198),1665" of (197) with sodium phenoxide)'686band the analogous dimetallated phosphite species cis-[Ru(P -C),(c~d)]'~~'and C~~-[RU(P-C),(CO),]'~~~ are reported. Most of the cyclometallated tertiary phosphine complexes also contain Ru-€3 Ponds (see Section 45.10.3.23). However, the yellow cyclometallated PPh, complex [RuCl(C,H,PPh,)(PPh,)], has been identified as the product of stoichiometric hydrogenation of aJkenes such as maleic acid with [RuHCl(PPh,), 3. It reacts with CO in solution to give ERUC~(~,H,PP~,)(CO)(PP~,)].'~~~ Reaction of [RuHCl(PPh,),j with an excess of Li, Zn or Mg methyls gives some [RuMe(C,H,PPh,)(PPh, )Z]Et20 together with various hydridomethyl complexes (Section 45, IO. 3.2.i).1690The chloride analogue [RuCl(C,H,PPh, )(PPh,),] (199) is prepared by reaction of [RuCl,(PPh,),] with [Zr(qC5H& and thermal decomposition of [RuH(C,H,PPh,)L(PPh,),j (L = MeCN, py) gives a mixture of products including the bis-ortho-metallated complex [RU(C,H,PP~,),L(PP~,)].'~
Ruthenium
390
Finally, as detailed in reference I, p.739 various ortho-metallated complexes have been generated from the ligand o-CH,=CHC,H,PPh, (SP); e.g. reaction of [RuCI,(CO),(SP),] with boiling 2methoxyethanol gives, as the major product [Ru(o-CHMeC,H,PPh,)Cl(CO)(SP)] (200). Other more complex products from this reaction are discussed in reference 1489. See Section 45.5.4.6.v for an example of a cyclometallated formazan derivative.
(vi)
Alkene complexes
Reaction of all-trans[RuCl, (CO), (PMe, Ph),] with C,H, in CHCl, gives [RuCI,(CO)(PMe,Ph),CzH4],shown by X-ray analysis to have structure (201). The 'H and I3CNMR data are consistent with retention of this structure in solution, and show free rotation of the C2H, group about the The alkenyl complexes [RuCl(CO),(EMe,Ph), (CCO,R)= Ru-C,H, axis down to - 40°C.L692 C(CO,R)Cl}] (202) (E = P, As; R = Me, Et) have been synthesized by reaction of trans-[RuC1,(CO), (EMe,Ph),] with the alkynes R 0 2CC-CCO,R. The proposed mechanism of formation of (202) involves initial formation of the alkyne complexes [RuCl, (CO)(EMe,Ph),(RO, CCCCO, R)] which then undergo intramolecular nucleophilic attack by a chloride ligand. Further reaction of (202; R = Me) with EMe,Ph gives [RuCl(CO)(EMe,Ph),(C(CO,Me)= C(C0,Me)C1]1,'693Treatment of the dimer [RuCl(q' -C,H,)(CO),($-CH2=CHCN)]2 with PPh, Other monoalkene complexes include gives [RUC~(~'-C,H,)(CO)~(PP~~)(~~-CH,=CHCN)].'~~~ [RuCl,CO(PCy,),L'] (L' = fumaronitrile, CH,=CHCN), [(RUCI,CO(PC~,)~ ),TCNE] (with a bridging TCNE ligand)'625and [RuCl(NO)(PPh,),alkene] (see Section 45.5.1.2). (E = P, As) gives [RuX,(L-L),] Treatment of 'RuC1,-xH,O' with o-CH,-CHC,H,EPhz (X = C1, Br). The compound [RuCl,(SP),] (E = P) exists in three isomeric forms all with both vinyl groups coordinated, two with trans chlorides and hence cis and trans phosphines, the most stable isomer having structure (203).The bromo phosphine and the arsine complexes exist only in form (203). Treatment of [RuCl,(SP),] in refluxing 2-methoxyethanol with CO results in displacement of the vinyl groups to form [RuCl,(CO),(SP),] whereas direct reaction of 'RuCl,*xH,O', SP and CO in 2-methoxyethanol yields [RuCi,(CO),SP] (204), which decomposes to [RuCl, (CO)SP], on further heating. On reaction of [RuCI,(CO),]~ and SP, [RuCl,(CO),SP] (205) is initially produced.1695The X-ray structural analysis of [RuCl,CO(BDPK)J (BDPH = 1,6-bis(diphenylphosphino)-trans-3-hexene), in which the BDPH moiety is bound via the alkenic group and two P atoms occupying mutually trans positions, has been reported.'696 Reaction of [RuCl2(PPh3),]( n = 3,4) or [RuX,(EPh,),MeOH] with norbornadiene gives the very insoluble [RuX,(EPh,),(nbd)] (E = P, As; X = Cl, Br),'06*655,'697 shown by X-ray analysis (for E = P) to have structure (206).1698a A more soluble analogue [RuX,(PMe,Ph), (nbd)] is prepared from Ph, (PhCH,)P[RuX,CO(nbd)] and PMe2Ph.With [RuCl,CO(nbd)]- and PMePh, (1:2 molar ratio), a mixture of [RuCl,(PMePh,), (nbd)] and [RuCl,CO(PMePh,),] is formed. Reaction of the nbd anion with EPh, in 1:2 molar ratio gives [RuCl,CO(EPh,)nbd] and some [RuCl,CO(EPh,),]- anion (E = AS, Sb).659
To
H C-CH2 Phw+,:
Clv
T
,,, ,,,,,,*co RLI" kPMe2Ph
I
CI
PhMe,P%,*"4 Ru'\,,,\,\KO
c1q
\PMe,Ph
c
/B
R02C
CCICOzR
(201)
(203)
C'
co
PPh3
(204)
(205)
( 206 1
The insoluble cycloheptatriene complex [RuCl,(chpt)], undergoes bridge cleavage reactions to give [RuCl,L(chpt)] (L = AsPh,, PBu,, PPh,, P(OPh),). These Lewis base derivatives react with thallium P-diketonates to give the cyclohexadienyl complexes [Ru(dike)L(C,H,-dike)] resulting from attack of diketone on the coordinated hydrocarbon as well as on the Ru" hydride alkene complexes are covered in Section 45.10, but other Ru" P, As or Sb donor
Ru then izkm
39 1
ligand complexes containing delocalized n-electron ligands such as $-allyl, q5-cyclopentadienyl, $-cyclohexadienyl, q5-carborane and $-arene are outside the scope of this review (see references 1 , 3 and 1698b for further information). Binuclear compounds containing tertiary phosphines and ethene are discussed in Sections 45.5.5.1 and 45.5.5.2.
45.5.4.6
Nitrogen donor ligand complexes
(i) Ammine, amine, hydrazine and carhamoyI complexes Surprisingly few ammine or amine complexes containing P or As donor ligands are known. The (R = Me, Et, Bu, P i ) can be prepared by treatment of cations trans-[R~(NH,),(P(0R)~}~]~+ [Ru(NH3),0Hz]”+( n = 2,3) or tran~-[Ru(NH,)~(0H,)(S0,)]~” cations with P(OR), in acetone. In aqueous CF3SO3H, hydrolysis to trans-[Ru(NH,), (OH,){P(OEt), }] occurs and reaction of this with P(OR), gives trans-[Ru(NH,),P(OR), {P(OEtj, >I (R = Me, Bu, Pr‘). Displacement of water by The binuclear complex p-TEPP-transpyrazine affords trans-[Ru(NH,),P(OEt),p~]~+ [Ru(NH,),P(OEt),],(PF,), has been prepared. This readily hydrolyses to form trans[Ru(NH,),(OH, )(P(OEt),)]’+ Reaction of trans-[RuCl(N,)(diars),] with dry HCl gas, followed by addition of BPh; , gives first trans-[RuCl(N,)(diars), JBPh, and then trans[R~Cl(NH~)(diars)~]BPh,.”~~ The neutral [RuX,(NH,R)(PMe,Ph),] (207) (X = CI, Br; R = H, Me, Bu) arise from reaction of mer-[RuX,(PMe,Ph),] with NH2R in anhydrous ethanol. Hydrazines aho yield [RuCl,(NH,NHR)(PMe,Ph),] (R = H, Ph) and the binuclear [RuC12(NzH,)(PMe,Fh),], but secondary and tertiary amines give only [RuX,(EtOH)(PMe,Ph),]. However, when small amounts of water are present, dealkylation of secondary amines occurs to produce the corresponding [RuX,(NH,R)(PMe,Ph),]. ”02 Similar monosubstituted compounds are obtained by reaction of [RuCl,(PPh,),], with aniline, o-chloroaniline and o-t~luidine.’~’~ However NH,, most hydrazines and amines react with [RuCl,(PPh,),], or [RuCl, (PPh,),] to form the disubstituted [RuCl,L,(PPh,),] (L = NH,R; R = H, PhCH,, p-MeOC,H,, Me(CH,),, (PhCH,)HNNH,, NHEt,, NEt,; L, = en, 1,2-(H,N),C,H,Me-p, et^.);^^^,^^^^^^^^ [RUC~,(NH,CH,P~),(PP~,)~] catalyses the oxidation of NH,CH,Ph to PhCN.1703 The AsPh, analogues [RuCl,L,(AsPh,),] (L = NHEt,, NEI,, N?H4, NH,, MeNH,, etc.) are also k n o ~ n . ~ Tr ~ eatment ~ . ’ ~ ~ of [Ru(N,H, j,cod](BPhq), with various PR, in acetone gives the hydrazone complexes truns[Ru(NH2NCMe,),L,](BPh,), (L = P(OMe),, P(OEt),, P(OMe),Ph, P(OCH,), CR; R = Me and Et) whereas [Ru(NH*NHMe),(cod)lIPF,), and P(OMe),Ph produces [Ru(NH,NHMe),(P(OMe),Ph),](PF,),.6M*’705 Reaction of [RuH,(PPh,),] and o-phenylenediamine gives the diimine complex [Ru{0-C,H, (NH), )(PPh,), 1. ”06 .169y91700a
x I,
A
(207 )
Other ammine, amine and hydrazine complexes include [RuCl, L(CO)(PPh,),] (L = N H p T o l , [RuCl(CN)NH,(CO)(PPh,),] NH,-p-OMeC,H,, PhHNNH,) from [RuC12(PPh3)?],L and from reaction of [RuCl,(CCl,)CO(PPh,),] with NH3;’ [Ru(SOCPh),L,(PR,),] (L = NH,, NH,Et; L, = HNC(Me)CH,CMe,NH,, en) (see Section 45.5.4.8.~),~~’~~~~’* and [Ru(dttd)(N,H,)PPh,] (see Section 45.5.4.8.iii).I7O9 The carbamoyl complexes [RuCl(CO), (CONHp-Tol)(PPh,),] are generated by displacement of NH, R from [RuCI(CO), (NH, R ) , ( C O N H ~ - T O ~ ) ]whereas ~ ~ ~ ” [Ru(CO),(CONHR)(PPh,),]BF,, (prepared from [Ru(CO),(PPh,),(RNCO)] and HBF,), gives [RuCl(CO)(CONHR)(PPh,),], (R = COPh) on addition of LiC1.1472
(io
Mono-, bi- and tri-den fate N-heterocyclic complexes
Treatment of [RuC12(PPh3)J2or [RuX,(PPh,),] (X = Cl, Br) with pyridine leads to stepwise replacement of PPh, groups giving [RuCl,Ipy),(PPh,),], [RuX, Cpy),PPh,] and trans-
-
c1 , , ,% ,",
1
PPh3
-co
CI+
I
%,,/#,*****X"*#/#,
*,+*co
PPhj
I
& ,a.*
Nljxz
(.Wpx+.
PY
PPI13
PPh3
.
Reaction of mer-[RuCl, (PMe,Ph),] and an excess of bipy in MeOH followed by recrystallization from acetone-light petroleum gives [RuCl(bipy)(PMe,Ph),]Cl*2 H 2 0 (210) and [RuCl(bipy)(PMe,Ph),(OCMe,)]Cl, whereas in CH,C12 [RuCl,(bipy)(PMe, Ph),] (211) and a small amount of bipyH[RuCl,(bipy)(PMe,Ph)] are formed. With phen in CH,Cl,, [RuCl,(phen) (PMe,Ph),] is produced but in MeOH followed by recrystallization from CH, C1,-light petroleum, the main product is [RuCl(phen)(PMe,Ph), (CH, C12)]Cl, together with small amounts of phenH[RuCl,(phen)PMe,Ph] -H,O. Treatment of mer-[RuCl,(PMe,Ph),] with 3,4,7,8-tetramethyl-l ,lophenanthroline (Me,phen) in MeOH gives [RuCl(Me,phen)(PMe,Ph),]Cl. In contrast, with 2,9-dimethyl-1,lO-phenanthroline(Me,phen) only [Ru,Cl,(Me,phen),]Cl, and [RU,Cl,(Plkle,Ph),]C1 are isolated.' 'l Other bipy and phen compounds include [Ru(SC,F,)bipy(PPh,),],C1, (from reaction of (209); N-N = bipy, X = C1) with [Pb(SC6F,)2])'714 and the two isomers of [Ru(SOCPh),(N-N)(PMe, Ph),] (see Section 45.5.4.8.v)."'* The reaction of [Ru(terpy)Cl,] with an excess of L (L = PPh,, P(p-Tol),) in the presence of Et,N yields violet trans-{RuCl, L(terpy)] (terpy = 2,2',2"-terpyridyl). This complex can be converted into cis-[RuCl,L(terpy)] (212) by boiling in 1 ,2dchloroethane and the cis isomer can also be prepared by treatment of [RuCl,(PPh,),] with terpy in boiling C,H,. The chloride in the cis isomer is significantly more labile than that in the trans isomer and addition of PPh, gives the trans-[RuCl(PPh3)?(terpy)]' cation. With CO, the cis and trans isomers give cis- and trans-[RuCl,CO(terpy)] respectively.'249Reaction of cis-[Ru(0H,),(PPh,)(terpy)J2+ with PhCECH gives the alkyl complex [Ru(q-CH, Ph)CO(PPh,)(terpy)]" .''I9
393
Ruthenium
(iii) Nitrosyl, lhioni6rosy1, azido and dinitrogen complexes
The ruthenium(T1) nitrosyl complexes [RuCl,(NO)L,] (L = PR,, AsR,, SbR3)were first prepared by the reaction of [RuCl,NO(OH,),] with L in ethanol; treatment with NaBr or LiI gave the corresponding [RuX,NOL,] (X = Br, I).'23,1715,'716 Alternative routes to [RuCl,NO(PPh,),] are shown in Scheme 50. Compounds of this type are also generated by the method shown in equations (101)(104). A reinvestigation of the original synthetic route by Townsend and C ~ s k r a n ' ~revealed ~' that for L = PMe2Ph, AsMe,Ph, the kinetically favoured isomer (213) could be isolated which readily converts on heating to the thermodynamically favoured isomer (214). For other L, only isomer (214) is observed. X-Ray analyses of [RuCl,(NO)L,] (L = PPh3,'718 PMePh217") confirm this observation and reveal a linear RuNO group; multinuclear NMR studies are also Studies have indicated that irradiation (at 365 nm) of both (213) and (214) (for L = PMe,Ph) induces isornerization to (215); reconversion to (214) requires thermal energy. For L = PPh3, the photochemical reaction is reversible at ambient temperature and in the presence of SbPh,, [RuCl,NO(PPh,)(SbPh,)] is
\ I- /
'RuC13 .xH,O'
[ RuHCI(CO)(PPh,),l
[ RuCI3(WO)(PPh3),14
[ R U C I , ( P P ~ ~ ) ~ ]viii
[ RuCINO(PPh,),l
iv
[RuH4(PPh3),1
"'
i. N 0 ; I ii, P P h , / N - m c t h y l - . ~ - n i t r o s o - ~ ~ - ~ ~ l u e n e s u l f o ~ i a'm i d ciii, ; ~TsCl ~ ~ ~;"I4,~O iv. h'OC1:'408 v. C12;'438vi, N0,/LE1;''18 vii, NOC1;17'9 viij, NO/CHC1i4"
Scheme 50
[Ru(CO),(NO)(PPh,),]+ [RuC12(PPh3)3]
+
C12
+ NO
+
acetone oI benzene
[RuC12(PPh3)J + [CoNO(DMG),] [Ru(NO),(PPh,),]
+
PhCH,Br
-+
[RuCI~NO(F'P~,)~] + ci~-[RuCl~(CO)2(PPh3)2]~~' (101) +
+ [Ru(NO), (PPhj)21'411 (102) [RuCI~NO(PP~,)~] + [R~Cl(N0)02(PPh3)2]'~(103)
[RuCt,NO(PPh3)2]
tolr[ R u B ~ , N O ( P P ~ , )I-~ ] [RuB~,(NCP~)~(PP~,),]'~'~ (104)
NO
CI
CI
c1
Cl
c1
Reaction of [ R u C~ , N O ( E P ~ ,(E ) ~ ]= As, Sb) with various P(OR), and dppe gives [RuCl,(NO)L,] [L = P(OR), (R = Me, Et, Bu), Wppe] and kinetic studies indicate that ligand dissociation to generate a five-coordinate [RuCl,NO(EPh,)] intermediate is an important step."" Other complexes [RuCl,NO(L-L)] (L-L = dppm, cis-Ph,AsCH=CHAsPh,, Ph,PCH,OCH,PPh,) are formed from [RuCl,NO] but again dpam is anomalous forming [RuCl,
-
394
Ruthenium
N O ( d p a n ~ ) , ] . ’Some ~ ~ ~ studies on anchoring of [RuCl, NO(SbPh,),] to polymer-bound phosphinic ligands are rep~rted,”’~ and the mixed ligand complex [RuCl,NO(AsPh,)OAsPh,] is known.’726 in dry Treatment of ‘RuCI, -xH,O’ and P(OPh), with N-methyl-N-nitroso-p-toluenesulfonamide tert-butanol gives green [RuCl, NO{P(OPh), I,]. However, the same reaction in EtOH affords the unusual yellow dimer (216), shown by X-ray analysis to contain unsymmetrical RuPOHOP moieties.I7,’ Other binuclear nitrosyl complexes are the unusual [RuX,NO(EPh,)],Y (X = C1, Br; E = P, AS; Y = s, s0).1304 As detailed in Scheme 43 (Section 45.5.1.2), [RuCI(NO)(PPh,),] undergoes oxidative addition reactions with various halogens, alkyl, aryl and acyl halides to give the corresponding Rut*NO c o r n p l e ~ e s . ’ Similar ~ ~ ’ ~ ~studies ~ ~ ~ are reported for [ R U C ~ ( N O ) ( A S P ~With ~ ) ~ ][RuCl(NO)(P.~~~~ MePh,),] and I,, trans halide addition is followed by facile isomerization in solution to give (217).‘”* Mixed halide derivatives are also prepared by treatment of [RuX(CO)(NO)(PPh,),] with halogens Y, giving [RuXY2NO(PPh,),] (with trans PPh, The corresponding sulfato complex [RuCI(SO,)(NO)(PPh,),] is obtained by reaction of [RuCI(NO)(PPh,),j or [RuH(NO)(PPh,),] with whereas [Ru(NO),(PPh,),] or [RuH(NO)(PPh,),] and R,CO,H afford [Ru(OCORF),NO(PPh,),] (R = CF,, C2Fg)’4’7~1’28a,’728b and [RuCl(NO)(PPh,),] gives [RuCl(NO)(C,X,O,)(PPh,),] (X = CI, Br) on treatment with quinones.’”’ In reactions of [RuCl,NO] with L in ethanol, the cations [RuCl2(NO)L3]+(L = PMqPh, AsMe,Ph) can be isolated from the filtrate as either BPh,- or C1- (for L = PMe,Ph) salts. NMR evidence suggests configuration (218).’720The cations truns-[RuCl(NO)(diar~),]~+ can be similarly prepared from [RuCl,(NO)] and d i a r ~ . ’ ~As~ ’shown in Scheme 51, this cation exhibits a rich chemistry producing azido, dinitrogen, ammine and nitro cornplexes.‘70’~’730 Reaction of cis-[RuCl,and similarly, trans(diars),] with NaN, in boiling 2-methoxyethanol gives cis-[Ru(N,),~diar~),]”~~ [R~(N,)~(eda),jis formed from tran~-[RuCl,(eda),]~’and [RU(N~)~(N(CH,CH~PP~,),)] from [RuCl, (N(CH, CH,PPh,),}].’s95 Cis-[Ru(N,),(PMe,),] has been synthesized from cis[RuMe, (PMe,),] and Me, SiN,.Ii3’ Addition of N, to the electron rich carbene complex (182)gives [RuCl(LR‘)(PR3), N,] (see Section 45.5.4.5.iii).
-
t-[RuCl(N,)(diars),I’
-
t-[RuC1(N3)(diars)z]
ji
iii
r-[RuCI(NO)(diars)z I Clz
J
t-[RuC1(N02)(diars)l 1
t-[R~I(NO)(diars)~l I2
[SbF6], or [NO] [PFh] or HCl; !NO] IPFs I ; iv, NaOH; v, hu/O,; vi, excess Lil
i, NaN3 crrN,H4; ii, [NO,] iii,
Scheme 51
Finally, reaction of various ruthenium(I1) and (111) compounds with [NSCl], in THF gives thionitrosyl complexes. Examples include [RuCI, (NS)(EPh,),] (partial X-ray analysis has been reported for E = P),I7,’ [RuBr,Cl(NS)(EPh,),] (E = P, As) and [RuCI,(NS)(AsPh,)(EPh,)](E = P, Sb).“’
(iv) Diuzenido ( N N R ) and diazene (RN-NR) complexes Reaction of [RuCI,(PPh,),] with various aryl diazonium tetrafluoroborates in the presence of LiCl gives the diazenido complexes [RuCl,(NNAr)(PPh,),] (Ar = p-Tol, p-MeOC,H,, p-ClC6H4,
Ruthenium
395
C1%,"*,
r;,,\*c1
-+
OC.*, '04
Ru
ClwI
\N, PPhj
N
,Ar
1
H I ,,,,,,+N=N-Ar
RU, ,
\CI
OCwI
L
-
( v ) Triazenido ( R N N N R J ,formamidinato ( R N c ( H )N R ) , formazan (RNNC(R)NNR), formamido ( R N c ( H j O ) , thioformamido ( R N c ( H ) S ) and ureylene (RNCONRzp) complexes
Bidentate 1,3-diaryltriazenido complexes can be synthesized by reaction of 1,3-diaryltriazenes, (ArNNNHAr), with either Ruo or Ru" hydride complexes. For example, [RuHXCO(PPh,),] (X = H, Cl) and [Ru(CO),(PPh,),] give [RuH(ArNNNAr)(CO)(PPh,),] in boiling C,H, or EtOH, whereas [RuHCl(CO)(PPh,),] affords [RuC1(ArN"Ar)(CO)(PPh3),] in C6H6 at room temperature. An X-ray analysis on [RuH(p-TolN,Tol-p)(CO)(PPh,),] confirms configuration (221).1740,'741 Alternative routes to triazene complexes involve displacement of halide ion from halo-ruthenium complexes with triazenido anions (e.g. equations 105 and 106j.'742,'743 Diarylcarbodiimides (ArN=C=NR) insert into Ru-H bonds to give the corresponding formamidinato (ArN-CH---NAr) complexes (equation 107). Configuration (222)assigned on the basis of NMR evidence has been confirmed for X = H by X-ray analysis.'744Compound (222; X = H)is also prepared from [RuH,(PPh,),] a n ~ I p - T o ~ N C 0 .In ' ~boiling ~' C6Hs,Pr'NCNPr' inserts into the Ru-H bond of [RuHCl(CO)(PPh,),] but concomitant dehydrogenation of a Pr' group gives [RuCl(CH,=C(Me)NCHNCHMe, )CO(PPh,),] (223)as confirmed by X-ray analysis.'746 PPh3
f'
396
Ruthenium [RuHCl(PPh,),] -t Lip,Ph,]
+
--+ [RuH(PhN,Ph)(PPh,),]
+ 2C1-
[RuCl,(PPh3),] -t Li[N3Phz] -+ [Ru(PhN,Ph),(PPh,),] (orHN,Ph, + Et,N) PPh,
[RuHX(CO)(PPh,),] + TolNCNTol (X = H, C1, Bv, OCOCF,)
+
(105) ( 106)
T"'
1 #\A?/,,,
X%y.l, Ru
&I
LiCl
\N-W
.>''CH
(107)
1
pph3 To1
PPh3 Me
cI 1% CH2
(223)
(224)
Reaction of [RuH,(CO)(EPh,),] (E = P, As) or [RuHCl(CO)(PPh,),] with 1,5-diphenylforin boiling 2-methoxyethanol or DMF affords the deep green mazans (PhN==NC(R)-NNHPh) (224) (as cyclometallated fonnazan derivatives [Ri{(0-&H,)N=Ik(R)=NNPh}CO(EPh,),], established by X-ray analysis for R = Ph, E = P). Isomeric hydrido intermediates of form (REiuH(PhN=NCH=NNPh)CO(PPh,),] have been isolated as pink (with cis-PPh,) or purple (with trans-PPh, ) complexes and convert to the green cyclometallated compound on further heating."47 Alkyl and aryl isothiocyanates, RNCS, undergo insertion reactions into Ru-H bonds to give thioformamido, (RN-===CH=S), complexes (equations 108 and 109)1748 whereas aryl isocyanates, RNCO, and [RuHX(CO)(PPh,),] (X = H, C1, Br) afford the analogous formamido complexes (225; Y = 0).Under more forcing conditions, reaction of the dihydride with p-TolNCO results in cleavage of a molecule of isocyanate and formation of the ureylene complex [Ru(TolNCONTol)(CO),(PPh,),] (226).1745 Compound (226; R = p-TolSO,) is also obtained by reaction of [Ru(C0),(PP$),] with toluene-p-sulfonyl isocyanate at room temperature in C6H,.1472 [RuHX(CO)(PPh,),] + RNCS (X = H, C1, Br, OCOCF3) [RuH*(PPh3)4] + RNCS
PPh,
K
(225) Y = 0,S
+
[RuX(RNCHS)(CO)(PPh,),] (R = Me, Et, Ph, p-Tol)
(108)
[Ru(RNCHS),(PPh,),]
(109)
PPh3
1226) R = p-TOl, p-TOlSO,
(vi> Nirriles and N bonded thiocyanates and selenocyanates Reaction of [RuCl,(PPh,),] and various alkyl or aryl nitriles RCN gives [RuCl,(RCN),(PPh,),] (R = Me, Pr", Pr', Ph, PhCH2, CH,=CH etc.); in boiling acetone the cis isomer (277) is obtained whereas toluene yields the truns isomer (228). Recrystallization of (228) from acetone gives (227). With dinitriles, [RuCl,L,(PPh,),] is formed (L = NC(CH,),CN (n = 14), p h t h a l ~ n i t r i l e ) .Re' ~~~ crystallization of the bis(MeCN) complex from C6H, affords the binuciear [RuCI,(MeCN)(PPh,),],.1404 The corresponding [RuCI,(RCN),(AsPh,),] (R = Me, Pr, CH,=CH)
Rut hen ium
397
are k n o ~ n , h ~ ~and , ' ~atso ~ ' the ethyl(dipheny1phosphino)acetate analogue (equation 1 An alternative route to (228) (L' = various PR,) is via bridge cleavage of [RuCl,(PhCN),], while treatment of Et,NIRuCl,(RCN),] with PMePh, yields [RuCl, (RCN)z(PMePh,),].681Treatment of the polymeric [Ru(NCS), (PPh,),],,, (made from [RuCl,(PPh,),] and SCN- ), with MeCN gives [R"NCS)2(MeCN),(PPh,),],'7'' and the closely related [Ru(NCE),(CO),(PPh,),] (E = S , Se) are prepared from [Ru(CO), (PPh,),] and (ECN)z.66',1626 L
RCN~I
L
I
I
RCNf,,//,,
c1'%,,, ,,&" NCR Ru
Ru'
bci
RCNVI ~ C
[RuCI,(o-MeC,H,CN),]
+
PPh2CH2C(0)OEt
I
L
L
-
[RuC1,(o-MeC,H,CN),(PPh,CH2C(O)OEt),](1 10) (228)
The cations fac-[Ru(RCN), L3]'+ are generated either by treatment of [RuZCl,L6]CI (L = PMe,Ph, PMePh,) with Ag[BF,] and RCN (R = Me, Et, Pr, Ph, MeOC(0)CH,-),'7s2 by protonation of [Ru,(OHj,(PMe,Ph),]+ with HPF, in MeCN'753or by reaction of [Ru(MeCN),(cod)],+ with P(OMe)?Ph or P(OMe), in nitr~methanc.'~~' Thefac isomer isomerizes to the mer form on recry~tallization.'~~~ In MeCN, [Ru(MeCN),codj2' and L give [Ru(M~CN),L,]~' (L = PPh,, PMePh?, PMe,Ph, P(OMe),Ph and P(OMe),). The monocations [RuCl(MeCN),L,]+ (L = PPh,, PMePh,, PMe,Ph) can be synthesized from [RuCI(MeCNj, (cod)]PF, and L in refluxing MeCN,'370 and treatment of [RuX(L-L),]PF, (L-L = dppe, Ph,P(CH,),NC,H,) with MeCN gives the sixcoordinate trans-[RuX(MeCN)(L-L)z]PF,.L535 Treatment of [Ru(OCOMe)(OH,)(PPh,),]BF,with MeCN gives [Ru(OCOMe)(MeCN)(PPh,),]+ which reacts with aqueous HBF, to produce the [Ru(OH,)(M~CN),(PP~,),]~+ di~ati0n.I~'~ Nitrile complexes containing Ru-H bonds are covered in Section 45.10.
(vi9 Oxime and Schir base complexes In the absence of alkali, hydroxyoximes, a-dioximes and a-carbonyloximes displace PPh, from [RuC12(PPh,),] and form complexes of type (229),(230)and (231)respectively. Small simple oximes give the six-coordinate bis(oxime) compounds (232) whereas with more bulky substituents the pentacoordinate [RuCl, (N(OH)C(Me)(Bu'))(PPh,),] is formed. In MeOH, reductive decarboxylation of solvent occurs with formation of [RuHCl(CO)(LH)(PPh,),] -MeOH (L-H = cyclohexanone oxime, acetophenone oxime and n-butylphenylketone oxime). In the presence of base, chloride displacement occurs and both a-dioximes and a-carbonyloximes give trans-[Ru(oxime), (PPh,),]. Similar products are formed with pyrinaldoxime but with salicyaldoxime, stepwise replacement of chloride produces first (233)and then (2M).1755 Treatment of [RuCl,(PPh,),] with the sodium salts of Schiff bases also leads to chloride displacement. For example, N-benzylsalicylideneimine gives (235) whereas N,N'-ethylenebis(pentane-2,4dione-monoimine) yields (236).'756,17s7a91757b The air sensitive compound (237) has been reported.'75" Redox reactions of the compound [RuC,H,(M=CHC,H,O-o),(PPh,),] with 12, FeC1, and O2have been Reaction of [RuCI, (PPh,),] with the sodium salt of N,N'-bis(salicyla1dehyd0)ethylenediamine (salenH,) gives [Ru(salen)(PPh,),] whereas with the free ligand in methanol, the ruthenium(II1) complex [RuCl(salen)PPh,] is formed.1756,1758c H PhC/
I
yH
PPh3
I
, , , # 'CI
Ru P h C \ NIW I \Cl
OH PPh3
MeC/N" ,,,,,t I
MeC\N( I
PPhB **\**a
I
k"
OH PPh3
PPh3
I ,***"'
MeC /0"084~*,
I I
OH I
PPh3 N%#,,/, ,#\\\*C]
I
Me,C/, N 1 qWC,
MeC\+,, OH
Me&
PPh3
OH
pph3
398
Ruthenium PPh3
(viii) MiscelZaneous
Reaction of [RuCl,(CO)(PPh,),] and S,N2-2A1Cl, in CH,Cl, gives [RuCI(S~N~)(CO),(PP~,)~]AIC14, (238). the first example in which the S2N2group acts as a monodentate ligand through
Aminoacid complexes [RuClL(PPh, 12] (LH = glycine, L-serine, L-hydroxyproline and Lallohydroxyproline) can be prepared by reaction of [RuCI2(PPh,),] with the appropriate amino acid in the presence of Na[HCO,]. Glycerin is oxidatively dehydrogenated in the presence of [RuClL(PPh,),] and N-methylmorpholine-N-oxide.'760 On heating [RuH, (PPh,),] with 2-hydroxypyridine and 8-hydroxyquinoline, [RuX, (PPh,),] (X = 2-OC5H4N,8-OC9H6N) is formed whereas 2-mercaptopyridine, 2-aminothiophenol and 2-aminophenol form [Ru(chelate),(PPh,),] even at room temperat~re.'~'~ Reaction with N-sulfinylaniline (NSA) gives [RuC12(EPh,),(NSA),] (E = P, As) via N-c~ordination.'~~' Reaction of [Ru(CO), (PPh,),] with 2-nitrophenol in boiling C,H, gives the unusual Ru" complex (239)in which on nitrophenol ligand has undergone reduction to a nitrosophenolate anion and another coordinates via only the phenolate
399
Ruthenium 45.5.4.7
Oxygen donor ligand complexes
Water, alcohol, acetone, dimethylformawide, dimethylacetamide and tetrahydrofuran complexes (1)
A number of 0 donor solvent0 complexes containing P donor ligands have been reported. Treatment of [RuCI,(CS)(PPh,),], with DMF gives [RuCl,(CS)(PPh,),DMFj which on recrystal[RUC~,(CS)M~OH(PP~,),].'~'~ Similarly lization from MeOH/CH,Cl, produces DMF), forms [RuCl,(CO)DMF(PFh,),], (from [RuCl,(PPh,),] and CO in and not the five-coordinate [RuC12(CO)(PPh,),] as originally [RuCl, (CO)MeOH(PPh,),]659,16'6 suggested.'@"The aqua analogues are obtained as shown in equations (111) and (112), and carbonylation of [RuCl,(PPh,),] in DMA gives [RUC~,(CO)DMA(PP~,)~].'~~ 'RuCI,*xH,O'
+
HCOzH
-
[RuCI,(PPh,),]
+
'RuQ,(OHz)(CO)' CSI
PPh
2 [RuC~~(OH,)(CO)(PP~~)~]'~~~ (1 11)
[RuClz(OH2)CS(PPh,)2]1640
(1 12)
On heating [RuCl,(PPh3),] in acetone, red [RuC12Me,CO(PPh,),], precipitates whilst with higher ketones, black {RuCl,(PPh,),], is pr~duced."~'Secondary and tertiary amines react with mer[RuX,(PMe,Ph),] in anhydrous EtOH to give [RuX,(EtOH)(PMe,Ph),] (X = CI, Br)1702and treatment of [RuH,(PPh,),] with SO, in wet toluene gives the aqua intermediate [RuSO,(OH,)(S0,)(PPh,),].'7MComplexes of formula [RuC1(COEt)CO(PPh,),Solventj(Solvent = C6H6,THF, CH,C12) arise from reaction of PPh, with [RUCI(COE~)(CO),],.'~~~ Direct reaction of [Ru(OH,),](tos), with PPh3 gives [Ru(~os),(OH~),(PP$)~] (tos = p-toluenesulfonate).'mb ),]C10, has been synthesized by reaction of cisThe aqua cation [RuCl(OH,)(CNR)(CS)(PPh, [RuCl,(CNR)(CS)(PPh,),] with Ag[C104] in EtOH/CH2C1,1M5and protonation of [RuH(OCOMe)(PPh,),] with aqueous HBF, in acetone gives [Ru(OCOMe)(OH,)(PPh,),]BF, as the end product. Intermediate hydrido cations such as [RuH(Solvent)(PPh,),]+ are produced in this reaction (see Section 45.10.3.2.~).'~'~ Other solvated hydrido complexes include [RuH(OH,)(CO),(PPh,),-.]BF, (n = 1,2),1765 [RuH(MeOH)(PMe,Ph),]PF,, [RuH(EtOH)(dppe),]PF6,1766 [RuH(OH,)(CO),(PPh,),]+ , [RuH(OH,),(MeOH)(PPh,),]+ 1754 (Section 45.10.3.2.v) and various hydroxo species containing H,O, THF, acetone and tert-butanol (Section 45.5.4.7.ii). The reactive intermediate [Ru(Me, CO),(PMe,Ph)J2+ is generated by treatment of [Ru,(OH),(PMe,Ph),]' with HPF, in a ~ e t 0 n e . I ' ~[RUCI(S~C~,)(M~,CO)CO(PP~,),]'~ ~ and [RuCl(dpprn),THF]Mn(CO),'506are discussed in Sections 45.2 and 45.5.4.1 respectively.
(ii) Hydroxo c o ~ p L x w Reaction of [RuCl, (PPh,),] with NaOH in THF containing 10% water gives is converted to [RuC~(OH)(OH,)(PP~,)~]~ but on prolonged heating this [RuH(OH)(PPh,),(THF)],. As shown in Scheme 52,1767these and other hydride-hydroxo dimers, together with their monomeric analogues [RuH(OH)(PPh,),Solvent] (Solvent = H,O, THF), are obtained by treatment of [RuHCl(PPh,)J with aqueous carbonate-free NaOH or KOH in, THF or acetone. Another route to [RuH(OH)(OH,)(PPh,),] is by reaction of [RuH(C6H4PPh,)Et,O(PPh,),] (Section 45.10.3.2.ii) with water in THF.Ia2 Further complications arise because of subsequent transformation of some of these products into tetranuclear complexes containing terminal carbonyls and bridging OH, H and PPh, ligands, e.g. [RU~H,(OH)~(P-
Phz)z(CO)z(Me,CO),(PPh,)6].'767
i, NaOH or KOH in THF/H,O; 25 "C; ii, NaOH or KOH in Me,CO/> 10% H,O; iii, KOH,Me,CO/< 1% H,O; A; iv, KOH, Me,CO/< 1% H 2 0 ;25 'C; v, KOBu' in Bu'OH Scheme 52
400
Ruthenium
Other binuclear hydroxo species are [Ru(OH),(PMe,),], (240)'52'(equation 1 13), [ R u ~ ( O H ) ~ L ~ ] + (241) (L = PMe,Ph, PMePh,, P(OMe)Ph,, PMe,) and [Ru,(OH)~(PP~,)~(BF,)~] (242). The PMe, cation is best prepared by reaction of [Ru,(CH~)~(PM~,),] with [Ph,C]BF, in THF'768whereas treatment of [RuH(NH,NMe,), (cod)]+ with PR, in acetone/MeOH followed by addition of water provides a high yield route to the others.'753Compound (242) arises from reaction of [RuH(OCOMe)(PPh,),] with HBF, in MeOH. The low conductivity in MeNO, suggests anion coordination or i~n-pairing.'~~' The related hydroxo-bridged complex (243) is produced by reaction of [RuHCl(Pr"CHCHNPt)(PPh,),] with water in t01uene.I~~~
Trimetallic hydroxo-bridged complexes such as [Ru, @-OH), (p-Fe(CO),}(CO),L,] (L = PPh,, PMe,, AsPh,) and [RuZ(p-OH)(p-H){p-Fe(CO),)(CO),(PPh3),] are covered in Section 45.5.5.
(iii) Ketoenolate (dike) and quinone complexes
Reaction of [RuH,(PPh,),] with acetylacetone in boiling 2-methoxyethanol in the presence of Et,N gives orange crystals of [Ru(acac),(PPh,),] shown by 'H NMR spectroscopy to have the cis configuration (244). Similar compounds are obtained using trifluoroacetylacetone and hexafluoroa~etylacetone."~' The rate of inversion between the optical isomers of (244) is slow at ambient temperature and hence the enantiomers can be res01ved.l~~'[R~(acac)~(PPh, ),I can also be prepared from [RuCI,(PPh,),], acacH and Et,N in boiling C,H, and on recrystallization from MeOH and C6H, gives orange and green forms respectively. These products have identical NMR and electronic spectral properties and it is suggested that they may differ in the solid either in the orientation of the phenyl groups or because of the existence of a trans isomer.'545In the absence of base, compounds such as [RuCl(acac)(PPh,),],, which are formulated as chloride-bridged dimers are isolated; the corresponding AsPh, compounds are also known.'704 Treatment of [RuHCl(CO)(PPh,),] with acacH in boiling 2-methoxyethanol gives [RuCl(acac)(CO)(PPh,),] (245) whereas in the presence of Et3N, the hydrido complex [RuH(acac)CO(PPh,),] (245) is produced. The latter also arises from direct reaction of [RuH?(CO)(PPh,),], [RuH(NO,)COCPPh,),] or [Ru(CO),(PPh,),] with ~ c ~ c H , ' or ~ ~[Ru(OCOCF,),(CO)(PPh,),l ','~~~ with acacHj KOH.I7" Complexes of similar stoichiometry result from reaction of [RuHX(CO)(PPh,),] (X = H, C1) with acacH in C,H, The fluorinated P-diketonates fR~(dike)~(PPh,),]and [RuX(dikete)CO(PPh,),] (X = H, C1) are also k n o ~ n . ' ~ ~Interestingly, ','~~~ reaction of [RuH,(CO)(PPh,),] with [Pd(facfac),] in boiling C6H, gives a mixture of [RuH(facfac)(CO)(PPh,),] and cis-[Ru(facfac),CO(PPh,)] which can be separated by c h r ~ m a t o g r a p h y . ' ~Treatment ~~ of [Ru(NO,),CO(PP~,)~](Section 45.5.4.7.v) with acetylacetone gives [Ru(NO,)(acac)CO(PPh,),] containing a monodentate NO3- ligand,'773and likewise [Ru(OCOCF,),(CO)(PPh,),]gives [Ru(OCOCF,)(dike>(CO)(PPh,),] (dike = acac, f a ~ f a c ) . ' ~ ~ , Tetrachloro- and tetrabromo- 1,2-benzoquinone react with [Ru(CO),(PPh,),] to give the orange complexes (246). The same compounds are obtained using tetrachloropyrocatechol and base. Cyclic voltammetry indicates that compounds (246)undergo reversible one-electron transfer reactions and these oxidation products can be chemically generated.'777aTreatment of [RuCI(NO)(PPh,),] with various quinones yields [RUCI(NO)(C,&O~)(PP~~)~]~~~~ whereas [RuCl, (PPh,),] affords the ruthenium(1V) complexes [RuC1,(C6X402)(PPh3),](X = C1, Br).1777" The catecholate-bridged dimer
40 1
Ruthenium
[Ru(CO),(PPh, )(y-0-0, C6C14)],undergoes bridge cleavage with Group V donors and halide ions to give [RU(CO)~(PP~,)L(O-O,C,C~,)] (L = PPh,, AsPh, etc.) and [RuX(C0)2(PPh3)(o-02C,C1,)](X = C1, Br, I) respectively. Chemical oxidation of these monomers gives semiquinone comp l e ~ e s . ~Direct " ~ ~ reaction of [RuC~,(CO),(PP~,),-~] (n = 0-3) with o-semiquinolates of Na, T1 or et^.).'^^^^ Cu gives [RuClL(CO),(PPh,),-,] (HL = 3,6-di-ters-butyl-l,Zsemiquinone
(245)
(244)
X
= H,C1
(246) X = C1, Br
(iv) Dioxygen complexes
Apart from the formally Ruo compounds [RuX(NO)O, (PPh,),] and [Ru(CO)(CNR)0,(PPh,),],iQ80 the only claims for Ru"0, species are concerned with 0, absorption by [RuCI, (PPh,),] and 'KuCl,(AsPh,),'. However, the green material obtained by exposure of solutions of [RuCI,(PPh,),] to air is attributed to formation of unspecified triphenylphosphine oxide ~ o m p l e x e s ' ~ "and ~ ' ~the ~ ~compound '~~ [RuCI, (AsPh,),O,] has been reformulated as the binuclear p-peroxo complex [(AsPh,), C1, RuO, RuC1, (AsPh, ),]C12.Iy7' Note however that earlier workersiH7 suggested that Ru"' arsine oxide complexes may be present here.
(v) Nitrato, sulfato, carbonato and perchloraro complexes Reaction of [RuHCI(CO)(PMe,Ph),] with aqueous HNO, gives [RuCl(NO,)(CO)(PMe,Ph),] containing a monodentate NO3 ligand.i614c Similarly, treatment of [ R u H , C O ( F P ~ ~with ) ~ ] HNO, gives [Ru(NO,),CO(FPh,),] (247)which exhibits facile intramolecular exchange of its uni- and bi-dentate NO3- groups. A preliminary communication reports the preparation of [Ru(NO,),(PMe,Ph),] from reaction of [Ru,X,(PMe,Ph),"J+ (X = OH, H) with HNO,; [RuH(PMe,Ph),]+ and HNO, give [Ru(NO,)(PMe,Ph),]+ The dicarbonyl complex [Ru(NO,),(CO),(PPh,),] (with unidentate NO3- ligands) is prepared either by reaction of (247)with CO or treatment of [Ru(CO),(PPh,),] with HNO,. Reaction of (247)with EtOH or propanol gives the hydrido complex [RuH(NO,)(CO)(PPh,),] (248) and with PMePh,, [Ru(NO,),CO(PMePh,),] is formed. Facile cleavage of one or more Ru-0 bonds in (247) or (248) with ligands such as acacH (Section 45.5.4.7.iii), -0COMe (Section 45.5.4.7.vi) and dithioacid anions (Section 4554.85) can also occur. Compound (247) is a good catalyst for dehydrogenating primary and secondary alcohols. 1773,1779 Treatment of [Ru(CO),(PR,),] (PR, = PPh,, PPh,Ip-Tol)] with SO2in C,H, gives the air sensitive compounds [Ru(CO),(PR,),(SO,)] (see (127) in Section 45.5.1.3) which on exposure to air affords [Ru(SO,)(CO),(PR,),].'"~ Other routes to this compound are shown in equation (1 14).'474,1477 An X-ray structure analysis confirms the presence of bidentate sulfate.i7s0The related complex [RuCl' ~ ~unusual ~ reaction occurs bet(SO,)(NO)(PPh,),] (Section 45.5.4.6.iii) is well c h a r a c t e r i ~ e d .An ween [RuH,(PPh,),] and SO, in toluene. The first product is the yellow aqua complex [RuS04(OH,)(S0,)(PPh,),] which slowly rearranges in solution to the binuclear toluene solvate (249). X-Ray analysis reveals that this contains unidentate SO, and bridging SO:- groups.i761 ~
0
/
Ruthenium
402
[Ru(CO),(PPh,),]
iu=,, ,o
P
CI
0
In common with all these P-0 ligand complexes which contain long Ru-0 bonds, X-ray structural analysis of [RuCl(Me)(cod)P(ZMeOC,H,),] * CH2C12,(prepared from [RuCI(Me)(cod)MeCN], and P(ZMeOC,H,),), reveals a distorted octahedral configuration with a 2-methoxyphenyl oxygen atom occupying the sixth coordination site [Ru-0 = 2.58 A).18”An analogous The complex trans-[RuCl, {( +)-ostructure is found for [RuC~(M~)(CO~){PP~,(~-M~OC~H,)}].’~~~ C6H4(PMePh),} ( )-o-C,H,(PMePh)(O)MePh)] containing P , P and 0 , P bidentate ligands respectively is covered in Section 45.5.4.3.
(iii) P-S ligand complexes
The only reference found to complexes of this type is a paper by Sanger and Day on reactions of Ph, P(CH, )z SPh (PS) with [RuCl,(CO),], which gives P-bonded [RuCl,(CO), PSI and then [RuCl, (CO),(PS),]. However, when an ethanolic solution of ‘RuCl, .xH,O and excess Ph,P(CH,),SPh are refluxed for a prolonged period, an insoluble pink solid [RuCl,(PS),], which
410
Ruthenium
probably contains bidentate Ph,P(CH,)2 SPh is precipitated. Reaction of 'RuCl,~xH,O' with PhtP(CH2)2SPhunder an atmosphere of CO gives first [RuCl,(CO)(PS),] and then some [RuCl,(CO),(PS),]. lgZ9 The complexes [RuC$ (4-R2PDBT),] (R = Ph, p-Tol; DBT = dibenzothiophene) were prepared in high yield from [RuCl,(PPh,),] and 4-R2PDBT. X-Ray diffraction studies reveal that for R = p-Tol, an all cis geometry is adopted.Ia3' A range of P-C ligand complexes are known and these are fully discussed under the headings cyclometallated (Sections 45.5.4.5.v and 45.10.3.2.iii) and alkene Complexes (Section 45.5.4.5.vi). Macrocycle complexes containing P, As or Sb donor ligands are covered in Section 45.12.
45.5.5 45.5.5.1
Bi-, Tri-, Tetra- and Poly-nuclear Ruthenium(I1) Complexes Triple halide-bridged cornpiexes (and related species)
Compounds of the type [L,RuCl,RuL,]Y (L = various alkyl and aryl substituted PR,; Y = CI, C104, SCN, BPh,) were first prepared in 1961 by reaction of 'RuCl,*xH,O' with L in aqueous MeOH or EtOH.'59'aA wider range of these cations (L = P(OR)Ph,, P(OR),Ph, P(OR), (R = Me, Et), AsRPh,, AsR,Ph etc.) are now available, usually from reaction of 'RuCl,.xH,O', [RuCl,(nbd)],, [RuCl,(PPh,),], Ru-arenes or the reduced 'ruthenium blue' solution with L in polar media~29b~676~1531-153z~'533.1544.1550~'56'~1585~16'4b The confacial bioctahedral geometry (280) has been verified by X-ray structural analyses for L = PEt,Ph (Y = mer-[RuCl3(PEt2Ph),]-),'"' PMe,Ph (Y = PF,)'832and PMe, (Y = BF,).'& The related cations [L3RuX3RuL3]+ (X = OH-, L = PMe,Ph, PMePh,, P(OMe)Ph,; X = F, I, SH, SR; L = PMe,Ph, PMePh,) are synthesized by treatment of [RuH(NH,NMe,),(cod)]+ with L in acetone/MeOH followed by addition of HX.'7s3The [Ru,(OH),(PMe,),]+ cation is obtained from [Ru,(CH,), (PMe,),] and [Ph,C]BF, in THF1768 and crystal structures for this and the PMe,Ph analogue are published. The [Ru2H3L6]+cations [L = PMe,Ph, PMe,) are discussed in Section 45.10.4. Treatment of these hydroxo complexes with HX in MeNO, provides a high yield route to the pure [Ru,X, L6]+cations (L = PMe,Ph; X = C1, Br, I).'556The iodo and bromo derivatives can but 31P also be prepared from [Ru,Cl,L6]+ and NaI or 'RuC1,-xH,O', LiBr and L respectively'614b NMR evidence indicates that these are a mixture of [Ru,Cl,X,-,L,]' cations.1556 Pyrolysis of [ R u , C I ~ ( P E ~ ~ P in ~ )methyl ~ ] C ~ acetate or n-propyl propionate gives [Ru,Cl,(PEt,Ph),][RuCI, (PEt,Ph),] and/or the neutral binuclear complexes [(PEt2Ph),RuCl, RuCI(PEt,Ph),] (281) (X-ray analyses a~ailable).'~''~'~~~*~~~~ An alternative route to the latter is by reaction of [RuC12(PPh3)J with L in non-polar solvents such as hexane or petroleum ether. Monomeric [RuCl,L,,,) are first produced (see Section 45.5.4.2.i) and in solution these rearrange to [Ru,CGL,] and/or [Ru,Cl,L,]Cl depending on the nature of L and of the solvent (Scheme 56). High yields of [Ru2CI4Lj](L = PCIPh,, PEtPh,, PEt,Ph) can be prepared by this intermolecular coupling meth~ d . ' For ' ~ L = P(OR),Ph, P(OR)Ph, no neutral [Ru,Cl,L,] are observed and this is attributed to the high nucleophilicity of the Ru-P bonds. Instead, various ionic species such as [RuZCl3L6]+, [RuCl{P(OMe)Ph,},]+, [RuCl{P(OEt)Ph,},]~+ and [Ru,Cl,(P(OEt)Ph,},]+ (282) are isolated (Scheme 57).1533 Reaction of [Ru, C1, (PEtPh,),] with equimolar amounts of AgCl gives the fluxional heterotrimetallic complex [Ru,Cl,(PEtPh,),AgPEtPh,] in which a Ag' ion is linked to one terminal and two bridging-chloride ligands.1832a The double chloride-bridged [RuCl(PMe,),],Cl, can be synthesized by leaving rrans-[RuCl,(PMe,),] in C6H6 for several weeks.'529 Pyrolysis of [RqCl, {P(OMe)Ph,},]Cl produces the unusual compound (283) in which all the Ru-P bonds are
Ruthenium
\
41 1
-solvent
L
I.
r
l+
L
Scheme 57
retained but some cleavage and partial hydrolysis of O-Me bonds has occurred'834(cf. (258) in Section 45.5.4.8.ii). The neutral, binuclear complex [(PPh,), (CS)RuCI, RuCI(PPh,),] (284) (X-ray analysis available)'835together with some [RuCl,(CS)(PPh, ),], and [RuCI, (S,CPPh,)(PPh,),] are obtained by reaction of [RuCl, (PPh,),] with CS,.'639,164' Possible reaction pathways are shown in Scheme 58 and these suggestions are supported by the high yield synthesis of [Ru,Cl,Y(PR,),] (Y = CO, CS; PR, = PPh,, PCp-Tol),) from [RuCl,(PR,),] and [RuCl,Y(PR,),DMF] (equation 122).1558,'6'6 Extension of this intermolecular coupling method to prepare [(PPh,),(CO)RuCl, RuCI{P(p-Tol), )2]1558 and [(PPh,),CORuCl, OSC~(PP~,)~]'*'~" is possible. Furthermore, heating [RuCI,Y(PPh,), MeOH] in CH,Cl, gives an isomeric mixture of [PPh, Cl(Y)RuCl, Ru(Y)(PPh,),] and reaction of these with PPh, and Na[BPh,] then affords the cations [(PPh3)2YRuC13RuY(PPh,),]BPh4 (285) (Y = CO, CS).I6I6Other routes to [Ru,CI,(CO)~(PP~~)~] are by reaction of [RuCI,CO(nbd)]- with an excess of PPh, in CH,C1,,1616treatment of [RuHX(CO)(PPh,),] (X = H,IsmC1"") with HC1 in c&6 or elimination of SnCl, from [Ru,C1,(SnC13)(CO),(PPh,),] (19 and Section 45.3.4.3).lWSome NMR evidence for compounds of formulation '[Ru, C14(L-L),(PPh,),]' (L-L = dppe, dppp) has been published'm' and unpublished suggests that the anionic [Ru,Cl,(PPh,),]- is formed by addition of CI- to [Ru,Cl,(DMA)(PPh,),]. A high yield method of incorporating ligands such as N,, alkenes, alkynes etc. into the terminal positions of [L,-,C~,RUC~,RUC~~ L,-,] by displacement of terminal chloride using Tl[BF,] has been p ~ b 1 i s h e d . l ' ~ ~ ~ [ R u C ~ ~ Y ( P DMF] R~)~
+
[RuC12(PR,),]
acetone 7 [(PR,jlYRuC1, RuCI(PR,),I + PRJ
iDMF
(122)
An isomeric mixture of the analogous PF, complex [Ru2Cl4(PF,),(PPh,),j can be generated by reaction of [RuH,(PF,)(PPh,),] with HC1. Other methods of preparation of this compound are shown in Scheme 59. Methods (iii) and (iv) also afford [(PPh,)Cl(PF,NMe,)RuCl, Ru(PF,NMe,)(PPh,),] whereas reaction of cis-[RuCl,(PF,NMe,),(PPh,),] with equimolar amounts of
Scheme 58
L (285) Y = c o , cs
[RuCI,(PPh,),] gives the isomers (286) and (287). The mono-PF, complex [Ru,Cl,(PF,)(PPh,),] is obtained from reaction of [RuCl,(PPh,),] with PF, in 2:l molar ratio and the mixed PF,/CO complex [Ru,C~~(CO)PF,(PP~,)~] from [Ru,C14CO(PPh,),] and PF, (1:l mole ratios).'5m The heterobimetallic complexes (288) can be synthesized by intermolscular coupling of equimolar amounts of [RuCl,(PPh,),] and mer-[RhCl,L,] (L = PMe,Ph, PEt,Ph, PBu,, PBu,Ph, PPh3).'837
...
fRuH2(PF3)(PPh3j31
I(PPh3)CI(PF3)RuC13Ru(PF3)(PPh3j21 A [ RuCI,(PPh,),]
cis-[R U C ~ ~ ( P F ~ ) , { P 1P ~ ~ ) ,
[ R u C ~ ~ ( P F ~ ) ( PDMF] P~,)~
i, HCI gas; ii, PF3; iii, PF3 (1: 1 mole ratio); iv, A; v, [RuCI,(PPh,),]
( 1 : 1 mole ratio)
Scheme 59
Another example of a neutral triple halide-bridged complex of this type is [(PPh,)? ClRuCl, RUN,(PPh,),] formed by treating [RuCl,(PPh,),] in THF, in a reverse osmosis cell with nitrogen under pressure. This compound reacts with N2BIoHgSMeZ to give [(PPh3)2CIRuC13Ru(N, B,,H,SMe2)(PPh,),].1838 The interesting diazadiene complexes (289) and (290) have been prepared by reaction of [RuCI,(PPh,),] with diazadiene in 1:2 and 2: 1 molar ratios respectively. The unusual hydroxo-bridged complex [(PPh,)(Pr"CHCHNPr')RuCl(OH),RuH(PPh,),](243) (Sec-
Ruthenium
413
tion 45.547%) is produced by the reaction of [RuHCl(Pr'NCHCHNPr')(PPh,),]with water in t~luene.'"~A similar reaction between [RuCl,(PPh,),] and a 1.5-fold molar excess of the electronrich alkene (MehCH,CH,NMeC=), (LMe),affords [Ru, C14(LMe),(PPh3),].1657a
Some examples of electrogenerated triple halide-bridged ruthenium(I1) anions are given in Section 45.5.6. Finally, a series of related trimetallic halide- and hydroxo-bridged complexes have been reported. Thus reaction of [Ru(arene)Cl, L] with [Fe,(CO),] gives the novel complex [Ru,(p-Cl), (p-Fe(CO),)- ' (CO),L,] (291) which undergoes further reaction with aqueous Na,CO, in acetone to give (292) and in isopropanol to give (293) and (292) (Scheme 60).I5lqaA report on the preparation of [Ru2(pCl), (p-Fe(CO), )(CO),Ph,P(CH,),PPh,] is available.'4gg
L = PPh,, PMe,, AsPh,, Ph,P(C=CBu') i, Na2C03/acetone; ii, Na2C03/isopropanol
Scheme 60
45.5.5.2
Double halide-bridged complexes (and related species)
Reaction of [Ru(CO),PPh,] with halogens in boiling acetone or with chlorinated solvents at room temperature gives the dimeric species [RuX,(CO),PPh,], (X = C1, Br, The same compounds are formed by heating [RuX,(CO),PPh,] in C6H6.'*' Related complexes [RU&(CO)~L]~ (X = Cl, Br; L = PBu',Ph, PBu',@-To]), PBu:) are obtained by treatment of the ruthenium(1) dimers [RuC~(CO),L]~ with haiogens.1519-1520b or (for X = I) by reaction of [RuI,(CO),j, with PBd3for short [RuCl,(CO), L,], (L = PMe,, PEt,) are also known.1840All these periods in 2-metho~yethanol;'~~~ compounds have terminal CO groups and bridging and terminal halides. Reaction of 'RuC1,~xH20 with ethylene- 1,2-bis(diphenylarsine) (eda) in boiling 2-methoxyethanol gives [R~Cl,CO(eda)l,.~~ Analogous compounds [RuCl, CO(L),], with PR, and AsR, ligands are obtained by treatment of [RuCl,CO(nbd)]- with PPh,,659heating [RuCl,CO(AsPh,),] in or by leaving all trans[RuCl,(CO),(PMe,Ph),] to stand in acetone for several days.Im8The thiocarbonyl complexes P ( ~ - T O ~ ) ,are ' ~ ~also ~ ) available via [RuCl,L,] and CS2. [RuCl,(CS)(L),], (L = PPh,,1638,'639,1611 Reaction of 'RuCl, -xH,O', PMe, and CO gave [RuCI,(CO)~(PM~,)], which catalysed the alcoholysis of Et,SiH.'84'b CO and CS double halide-bridged species include the cations Other [RuCIY(P(OE~)P~~)~]~(BP~~), (Y = CO, CS) obtained by dissolution of [ R U C I ~ Y ( P ( O E ~ ) Pin~ ~ ) ~ ] EtOH containing Na[BPh,],'615and the novel anionic complex (294) from reaction of 'RuC1,mx-
414
Ruthenium
H 2 0 , NaI, PBu', with CO in refluxing 2-metho~yethanol.'~~~ As discussed elsewhere (Scheme 57, Section 45.5.5.1) the [ R U C ~ { P ( O E ~ ) P ~ cation ~ ) , ] : ~can be prepared from [RuC12(PPh3),]or [RuCl,(P(OEt)Ph,),] treated with P(OEt)Ph, in ErOH'S35and [RuCI(PMe,),]:+ is also known.L529 Under UV irradiation, reaction of [RuCl, (PPh,),] and [ZrCp,MeCl] affords the interesting zirconiumruthenium complex (295)1691and direct reaction of 'RuCl, -xH,O' tritertiary phosphine PhP((CH,),PPh, }, (L) in boiling EtOH gives the binuclear chloride-bridged [RuCl, L]2.1842 Treat) , ] 3,3',4,4'-tetraMe- 1,l'-diphosphaferrocene (DPF) gives the unusual ment of [ R u C ~ ~ ( P P ~ ,with binuclear complex [RuCl, (DPF)PPh,], whose structure (296) was confirmed by X-ray analysis.lm Reduction of [Ru,Cl,(PEt,Ph),] with Na[BH,] in the presence of C,H, or PhC=CH(L) leads to the double halide-bridged complexes [(PEt, Ph)3C1Ru(p-C1)2R u L ' , C ~ ( P E ~ ~ P ~ ) ] . ' ~ ' ~ ~
PPh3
C1
(296)
The following double halide-bridged compounds are discussed elsewhere: [RuCl,(PPh,),], (Section 45.5.4.2.i), [RuCl,(AsPh,),], (?) (Section 45.5.4.2.ii), [RuCI,(CO)L-L], (L-L = oC,H,/EMePh),; E = P, As) (Section 45.5.4.5.i), [RuCl,CO(SP)], (Section 45.5.4.5.vi), [RuCl(CO)(CONHR)(PPh,),], (Section 45.5.4.6.i), [RUC~(N-N){PP~~)~]~CI, (N-N = bipy, phen) (Section 45.5.4.6.ii), [RuCI{CHMe(CN)) CO(PPh,),], (Section 45.5.4.5.iv), [RuCl, NO({P(OEt)2O},H)], (Section 45.5.4.6.iii), [RuCl2(NNAr)(PPh3),]:+, [Ru,C~~(M~CN),(NNA~),(PP~,),]~+ (Section 45.5.4.6.iv), [RuCl,(MeCN)(PPh,),], (Section 45.5.4.6.vi), [RuCl2(PPh3),Me2CO], (Section 45.5.4.74, [RuCl(acac)(EPh,),], (Section 45.5.4.7.iii), [RuCI,(SPPh,)PPh,], and [RuCl,(SPPh,)CO], (Section 45.5.4.8.vii).
45.5.5.3 Binuclear complexes with polydentate phosphine bridges
The chiral ditertiary phosphine, diop, displaces all three PPh, groups from [RuCl, (PPh,),] to give which is a good catalyst for asymmetric the green binuclear complex [Ru2C1,(diop),] (297),1571,'843 ~ e' analogous ~~~'~~~ transfer hydrogenation of prochira1 unsaturated acids or esters by a l ~ o h o l s . ' ~ ~ 'Th compound [RuzC14(dppb),]is obtained either by reaction of dppb with [RuCl,(PPh,),] or K,[RuC1s(OH,)]'S'S or by decomposition of [ R ~ H C l ( d p p b ) , ] . 'Reaction ~~~ of {RuCl,(cod)], with 2,2'-bis(diphenylphosphine)- 1,l '-binaphthyl (BINAP) gives the chiral [Ru, C&(BINAP),NEt,] which is an T ' P
Ruthenium
415
excellent catalyst for asymmetric hydrogenation of alkenes and some cyclic a n h y ~ 3 r i d e s .Prolonl~~~~ ged reaction of [RuCl,(PPh,),] in hot C6H6 with the hexadentate ligand TDDX gives the bridged complex (298),15,'and the related complexes [RuC~,(CO),]~L(L = Ph,P(CH,),P(Ph)(CH,),P(Ph)(CH,),PPh, (tet-1) and P[(CH,),PPh,], (tet-2) are also known.Im The cation [(bipy),CIRu(pdppm)RuCl(bipy),I2+ can be prepared by treatment of [RuCl, (bipy),] with dppm in EtOH.1267
45.5.5.4 MisceUaneous
Other bi-, tri-, tetra- or poly-nuclear Ru" complexes mentioned in earlier sections are [(RuCl,CO(PCy3)2),TCNE] (Section 45.5.4.5.vi), [RuCI,(N,H,)(PM~~P~)~], (Section 45.5.4.6.i), [Ru(NCS),(PPh3jJn (Section 45.5.4.6.ii), various bi- and tetra-nuclear hydrido and halo/hydroxo compounds (Section 45.5.4.74, [RuX2NO(EPh,),],Y (Y = S, SO) (Section 45.5.4.8. I), [Ru(SC6F, )(bipy)(PPh, )2 Jz, [RuI(SH)(CO>(CNR)PP~,I, , [RuC1(2-HSC,H,N)(2-SCjH,N)PPh,l,, [Ru,CI,S,(PPh,),] (Section 45.5.4.8.i), [RuSO,(SO,)(PPh,),], (Section 45.5.4.7.~)~[Ru, C1,(SO, j3(PPh,),] (Section 45.5.4.8.vi) and [RuPdCl, (CO)(PPh, py),] (Section 45.5.4.9 .i).
45.5.6
Mixed Valence Ruthenium(II)/Ruthenium(III) Complexes
The first formally mixed valence Ru,"'"' complexes, namely [Ru2Cl5L4](L = PBu",, P(pent)",) were synthesized in 1967 by the prolonged interaction of excess L on 'RuCl,*xH,O' in EtOH'847and the 'symmetric' structure (299) was later confirmed by X-ray analysis for L = P B U " , . ~More ~~~ recently, the corresponding red-brown As@-Tol), and AsPh, complexes have been prepared by reaction of 'RuCI,.xH,O' with excess AsR, in EtOH and butan-2-01 respectiveIy.'588The latter was previously reported as the Ru" polymer [RuCl, ASP^,)^],.'^^^ However, reaction of 'RuCl, -xH, 0' with As@-Tol), in butan-2-01 or with As(p-C6H,C1), in MeOH gives the yellow-green isomer (300). Evidence for these formulations comes from detailed ESR, magnetic and electrochemical studies and is supported by the single crystal X-ray structure of [(PEt,Ph)Cl, RuCl, Ru(PEt, Ph),] (300), synthesized by aerial oxidation of [(PEt,Ph),C1RuC1,Ru(PEt2Ph),] (Section 45.5.5.1) in HCl/ MeNO, solution.i58*The related complexes [(PR,),YRuCI,RuCl,PR,] (Y = CO, CS; R = Ph, p-Tol) are produced by aerial oxidation of [(PR,)2YRuCl, RuCl(PR,),] (Section 45.5.5.1) and are believed to have structure (301).'556~1h39~'83611.1*49 T h e mixed valence complexes [Ru, C1, (mpp1)4]"67 (mppl = 3-methyl-1-phenylphosphole)and [Ru, C1,(dpae)2]"88are also known.
Other mixed valence triple halide-bridged Ru,"-"' complexes can be generated electrochemically from stable Ru2II*I1 or RU~*II,~I~ compounds and characterized by in situ spectroscopic measurements. Examples include [ R U ~ C ~ ~ ( P E ~[Ru,C&(PEt,Ph),]+, ~P~)~]~' [Ru2CI,Y(PPh,),]+ (Y = CO, CS), and [Ru,CI,{As(p-Tol),},]-~. An examination of the intervalence charge transfer bands in the optical spectra of these complexes reveals that the degree of metal-metal interaction decreases as the molecular asymmetry increases. Bulk electrogeneration, followed by in sidu characterization, o€ Ru,"~" anions such as [(PEt,Ph)ClzRuCl, Ru(PEt,Ph),]- and [As(p-Tol),Cl, RuCl, RuCl(As(pTO^),},]^- is also possible.'ss6~'8'0 cation, which is a valence-trapped The mixed valence [(bipy), ClR~(p-dppm)RuCl(bipy)~]~+ species with an intervalence charge transfer band at 8020 cm-', is synthesized by stoichiometric oxidation of [(bipy),ClR~(p-dppm)RuCl(bipy),]~~with [Fe(bipy),I3+ Compound (302)has been prepared and in the semioxidized Ru"'. -.* Ru" complex derived from (302), some interaction between Ru atoms is proposed although no intervalence charge transfer band is dete~ted.'~''" The synthesis and reactivity of complexes such as [Ru,O(OCOMe),(PPh,),] in which the ruthenium has a mean oxidation state of 2f are covered in Section 45.6.5.
+
416
45.5.7 45.5.7.1
Ruthenium
Mononuclear Ruthenium(1II) Complexes Monodentate P, A s and Sb donor ligand complexes
An extensive series of paramagnetic Ru“’ complexes [RuCl,LJ (L = PR, or AsR,) can be synthesized from ‘RuC1,-xW20’ concentrated HCl and L in EtOH, using much shorter reaction The bromo compounds are times than those required to give [Ru2C13L6]C1or also available and IR and ESR studies suggest meridional structure^.'^^^^'^^^^'^^^ The phosphole ~~ complex [RuCl,(dmppl),] (dmppl = 3,4-dimethyl- 1-phenylphospholefis also r e p 0 r t e 4 ’ ~However, for L = AsPh,Io6and A s ( p - T ~ l ) , ~reaction ~ , ’ ~ ~with ~ ‘RuCl3*xH,0in boiling MeOH gives the green methanolates [RuCl, (AsR,),MeOH] (303) and the PPh, analogue is obtained in low yield by shaking MeOH solutions of PPh, with ‘RuCl,.xH,O’ (2:l molar ratio).’% The bromo compounds are known and the preparation of [RuCl, (AsPh,),] (two isomers?) and [RuCl, ( A s P ~ , ) , O A S P ~ , ] ~ ~ ~ ~ (previously formulated as [RuCl, ( A S P ~ , ) ( O AS P ~ , ) , ]is ) ’ also ~ ~ ~ published. Note however that the compound reported as [ R uC ~ , ( A S P ~, ) , O, ]has ’ ~ ~been ~ reformulated as the binuclear p-peroxo complex i(AsPh,), Cl, RuO, R u C ~ , ( A S P ~ , ) , ] Calthough ~ , ’ ~ ~ ~ earlier workers1547 suggested that all the physicochemical properties could be ascribed to Ru”’ arsine oxide complexes. Further work to resolve these differences is now required. As shown in Scheme 61, these [RuX,(EPh,),MeOH] complexes are excellent precursors for the synthesis of a wide range of low spin Ru”’ compounds containing 0, N or S donor ligands.106.1697.18S5,1856 The same types of compound can be prepared from [RuX,(As(pT o ~ ) , } , M ~ O H ] ,[RuCl,(AsPh,),] ~ ~ ~ * ’ ~ ~ ~ or ‘[RuCl,(AsPh,),0AsPh,]’.‘726~’749 Thus the labile MeOH group can be replaced by a variety of donor ligands L to give [RuX,(EPh,),L] (304). For L = pyridine, the monopyridine complex could be isolated only by adding a large excess of light petroleum to quench the reaction immediately after addition of pyridine.’8s5The related mer,trans-[RuCl, (NH,)(P(OMe),},] is obtained as a by-product from reaction of ‘RuC1, *xH,O’(containing nitrogen impurity) with P(OMe), in refluxing MeOH.Is6’ Ligands such as pyridine, Me,S, isocyanides and the bidentate bipy and phen also displace one of the EPh, groups from (303) to give [RuX,(EPh,)LJ (305). The mixed [RuCl,(PPh,)py(CNR)] is prepared by treatment of the reactive [RuCl, (PPh,),py] with CNR.‘”’ Spectroscopic and dipole moment measurements support the proposed structures. Treatment with halide ion in the presence of a large cation gives the Ru”’ anions (306). For E = P, the same compound is produced by reaction of [RuCl,(PPh,),] with MCl/HCl (M = AsPh:, Me4N+)or Ph,As[Ru(N)Cl,] with PPh, in cold acetone.’857Other anions can be prepared either by direct exchange with [RuCl,(EPh,),]- (e.g. L = P(OPh),, PEt,, PMqPh) or, as their phosphonium salts, by reaction of the ‘RuCOCl’ solution with excess L (L = PEt,, PEt,Ph, PMe2Ph).1749*1858 The trans structure is established by ESR and IR ~ p e c t r o s c o p yand ~ ~their ~ ~ ~magnetic ~ ~ ~ ~ circular ~~~~~ dichroism and optical spectra have been analysed.1860 Under more forcing conditions, reduction of (303) to R d’ complexes generally occurs (see Section 45.5.4 for examples). However, rather surprisingly, mer-[RuCl,L,] (L = PPh,, PMePh,, PMe,Ph, PEtPh,) have been synthesized by reaction of [RuCl,(AsPh,),MeOH] with an excess of L in boiling C6H61B6 An electrochemical survey of many of these compounds has been reported. Most of the neutral compounds show a reversible one-electron reduction and an irreversible one-electron oxidation whereas the anions (306) exhibit irreversible reduction and quasi-reversible oxidation behaviour.62s Reaction of (303) or [RuC13(EPh,),]with P-diketones leads to partial replacement of halide and
Ruthenium
417
(X = C1, Br; E = P, As - not all possible combinations) i, L = Me2C0, THF,MeNO,, DMF,DMSO, RCHO, NH3, MeNH,, Et2& CS, py, RCN (R = Me, Pr, Ph, PhCH2,CH2 = CH) etc.; ii: L = % bipy, ?hphen, Me$, py, 2-MeC5H,N, pyrazine, 4,4'-bipy, 1,4-dithian, CNR;iii, X = CI-;Br-; iv, various P-diketones Scheme 61
formation of [RuX, (dike)(EPh,),] (307);'861,'862 [RuCl,(acac)(dppe)] is also known.1863 The bromotropolonate complexes [RuCl,L(EPh,),] (L = bromotropolonate, E = P, As) are given by reaction of L with [RuCI,(EP~,),],'~~ whereas [RuCl, (PPh,),] and N-phenylcarbamoylpyrrole-2-thiocarboxamide (PTH) afford S-bonded [RuCl, (PTH)2(PPh3)].'8'9Treatment of IRuX,(PPh,),] with RCOzH in aerated solvents affords [RuX,(OCOR)(PPh,),] (R = Me, Et, Ph, p-To1 etc.) whose structure (308) is established by X-ray analysis (for X = Cl; R = Ph).Is6' This compound is also prepared by reaction of [RuCl,(PPh,),] with dibenzoyl peroxide.'866 A route to [RuCl,(OCOR)(AsPh,),] (R = Me, Et) is published although it was suggested these are dimeric with chloride bridges and unidentate - OCOR ligands."" PPh3
A number of cationic Ru"' compounds containing P or As donor ligands are known. Examples include [RuC12(CWR)(PR3),]' (from reaction of mer-[RuCl,(PR,),] and CNR, see Section 45.5.4.5.iii),IH7[RuCl, bipy(PPh3),]BPh4(from [RuCI, (PPh,),(MeNO,)], bipy and Na[BPh,] heated in MeOH) and [RuCI,(PhCN)(bipy)(PPR,)]Cl-H,O(from [RuCl, (bipy)PPh,] in MeOH treated with an excess of PhCN)."I3 Studies have revealed that oxidation of [RuCi2(PMe,),] with AgPFs gives [RuCI, (PMe, )43PF6.'391c
45.5.7.2
Bi- andpoiy-dentate P and A s donor ligand complexes
The ruthenium(II1) cations cis- or trans-[RuCl, (L-L),]C104 (L-L = dmpe, depe) are readily prepared by treatment of the corresponding cis- or trans-[RuCl,(L-L),] with HClO,; their ESR spectra are r e p ~ r t e d . ~ ~Oxidation ~ ~ " - ' ~ of ~ ~trans-[RuCl,(dppm),] with NO[BF,] affords trans[RuCl, (dppm),]BF, although this must be isolated rapidly to avoid conversion to nitrosyl comor electrochemical oxidation of cis-[RuCl, (dppm),] also gives transp lex e ~ .' ' ~Chemical ~
418
Ruthenium
[RuCl, (dppm),]+ Oxidation of [RuCl, (diars),] with Cl, in the presence of X- produces trans[RuX,(diars),]X (X= C1, Br, I)'592 and other Ru"' cations generated by oxidation of their neutral (X = C1, Br)'777aand [RuCl, {PPh,(2Ru" precursors include [Ru(1,2-02C,X,)(CO),(PPh3)2]+ MeOC,H,)},]+ Direct reaction of 'RuCl, .xH,O' with the neopentyldi(tertiary phosphine), (Me,CCH,),P(CH,),P(CH,CMe,),, gives [RuCI,(L-L),]+ whereas with Ph,P(CH,),SPh (SP), [RuCl, SP] is first p r o d u ~ e d . ' ' ~ ~ Few Ru"' complexes containing polydentate P or As donor ligands are known. Thus, reaction of 'RuCl,.xH,O with the hexatertiary phosphine [Ph,PCH,CH,], P(CH,), P[CH, CH,PPh,], (L) in boiling EtOH gives the brown Ru"' cation [RuCl,L]+ isolatable as its PF; salt (,netf = 1.7 BM); tetradentate coordination of the ligand is proposed.ls6*In contrast, reaction of the closely related TDDX ligand (see Section 45.5.4.1) with 'RuC1,-xH,O' gives the binuclear Ru"' cation [Ru,CI4(TDDX)](PF& (cf, [Ru,Cl, (PPh, ), TDDX] (298), Section 45.5.5.3).'53RChlorine oxidation of [RuCl, {N(CH,CH2PPh,), }] affords the corresponding Ru"' cation.'595
45.5.8
Bi- and Tri-nuclear Ruthenium(II1) Complexes
Heating the methanolates [RuX, (AsR,), MeOH] (303) (X = C1, Br; R = Ph, p-Tol) in non-coordinating solvents such as C6H, or CH2C12provides a high yield route to the paramagnetic triple halide-bridged complexes [(AsR,)X, RuX, RuX(AsR,),] (309). These undergo stepwise, one-electron reductions to form the [Ru,X,(AsR,),]"- anions (n = 1,2).'ss0 Under milder conditions, loss of MeOH from (303) in the above solvents gives brown compounds of empirical formula [RuX,(AsR,),]. Monomeric trigonal bipyramidal structures were initially proposed for these speciesl 582.1697.1859 but on the basis of electrochemical evidence (two reversible one-electron reductions), they were reformulated as the double halide-bridged [(AsR,),X, R u X ~ R U X ~ ( A S R ~However ) ~ ] . ~ ' ~a comparison of electrochemical, magnetic, ESR and optical spectral properties strongly indicates that the latter are also the triple halide-bridged [Ru, X, (AsR,),] (309) containing one molecule of clathrated AsR,'~~'(cf, [RuCl,(PPh,),] PPh,).'543,1544 Related binuclear complexes [Ru(NCS),(EPh,),], (E = As, Sb) can be prepared by reaction of [RuCl,(EPh3),] with NH4[NCS].17043'71t These are formulated as double bridged species but they might be [Ru,(NCS),(EPh,),]EPh,. There are two brief references to [RuX, (PPh3)Jn complexes; [RuI,(PPh,),], from reaction of [RuH,(PPh,),] with I,1870and [RuCl,(PR3),], (R = Bu, Pr, Pent) from 'RuCl,.xH,O' and PR, in EtOH.'847The latter is formulated as a double chloride-bridged species but more definitive evidence is now required. The interesting complex [RuCl, (mppl)] (mppl = 3-methyl- 1 -phenylphosphole)is also known.1567 Electrogeneration and in situ spectroscopic characterization of triply halide-bridged binuclear Ru,"'~"' cations such as [Ru, CI,(PEt, Ph)4j+, [Ru2C13(PEt, Ph),I3+ and [ R U ~ C I ~ ( P E ~ ~have P~)~ been ] ~ ' accomplished;'*" oxygenation of [RuCl,(AsPh,),] is claimed to give the binuclear p-peroxo complex [(AsPh,), Cl, RuO, RuC1,(AsPh,),]Cl, rather than [RuCl,(AsPh,),(OAsPh,)] (see Section 45.5.7. l).'77x Reaction of [Ru,(OCOMe),Cl] with Li[C,NH,NH] in THF followed by addition of PMe, Ph gives dark red crystals of [Ru, (cgNH4NH),(PMe,Ph),], shown by X-ray analysis to have structure (310), in which the anionic ligand exhibits three modes of coordination. This is the first edge-sharing
Ph PhC-
0
Ruthenium
419
biooctahedral molecule having a bond between two d 5metal atoms and also the first in which the (p2,q') bridging atoms are imido nitrogen
Treatment of [Ru,(OCOMe),Cl] with excess molten benzamide (PhCONH,) affords [Ru2(PhCONH),C1] which reacts with PPh, in DMSO/MeOH to generate the novel complex [Ru, (Ph),(PhCONH),@-NC(Ph)OPPh,),] (311). This consists of two e,quivalent Ru"' atoms surrounded by a distorted octahedron of ligand atoms. The Ph, POC(Ph)N- ligand coordinates through its P and N atoms to form a five-membered chelate ring and the nitrogen atom also serves as a bridge to the other Ru atom. This chelating ligand is (at least formally) the product of attack of a PhCONH- oxygen atom on Ph,P with concomitant shift of a phenyl group to Ru and loss of H.187lb
The reaction between [Ru,O(OCOMe),(OH,),]OCOMe and MgMe, in the presence of PMe3 yields the diamagnetic tri-p-methylene complex (312). The short Ru"' -Ru"' distance of 2.650( 1) A probably indicates the presence of a metal-metal bond and hence accounts for the observed diamagnetism. Compound (312) reacts with one equivalent of HBF, or [Ph,qIBF, to give [(Me,P),Ru(p-CH,), (p-CH,)Ru(PMe,),]BF, (313) and with an excess of HBF, or CF,S03H to eliminate methane and produce [(Me, P), Ru(p-CH,),Ru(PMe,),](BF,), (314). The above protonation can be readily reversed by addition of alkyllithi~ms.'~~'"'~*~~~~ The remarkable trinuclear cation (315) is isolated by protonation with HBF, of the red oil which is also obtained from the [Ru, O(OCOMe),(OH,),]OCOMe, MgMe,, PMe, reaction mixture. This formally contains two Ru"' and a central, tetrahedrally coordinated Ru" ion.'52',176s~1872
45.5.9 Ruthenium(1V) and Higher Oxidation State Complexes There are few RuIv complexes containing soft P, As or Sb donor ligands. Treatment of Bu",N[RuNCl,] with EPh, in boiling acetone gives the diamagnetic [RuV'C1,N(EPh,),l (E = Sb, As). Further reaction of the AsPh, compound with various PR, in cold acetone produces the novel phosphineimidato complexes [RuCl,(NPR,)(PR,),] (PR, = PPh,, PEtPh,, PEt,Ph, PMePh,, PEt,); with PMe,Ph, the intermediate [RuCl,(NPMe,Ph)(AsPh,),] is is01ated.I~~~ Magnetic moments typical of Ru" & ca. 2.8BM at room temperature) are observed and the structure (316) is confirmed by X-ray analysis of the [RuCl,(NPEt,Ph)(PEt,Ph),] complex.'873
(316)
Other Ru" complexes discussed elsewhere include [RuC~~(C,X,O,)(PP~,),~ (X = C1, Br) (Section [RuH,(PPh,),],, 45.5.4.7.iii), [RuH,(SiR,)(PPh,),] (Section 45.3.2.3), [RuH,(PPh,),], [RuH,(PR,),]+ (Section 45.10.5), [RuF,(PF,)J, and RuF,.AsF, (Section 45.9.7.2).
Ruthenium
420
Higher oxidation state complexes are confined to [RuO,PF,] and [RuO,(PF,),] (from RuO, and
PF,). Similar reactions with PCI, or PBr, reduce the tetroxide to RuO,PX, (X = C1, Br).’874 Preliminary communications on the hexahydrides [RuH,(PR,)J (R = Ph, Cy) have recently appeared (see Section 45.10.5).
45.5.10
Useful Books and Reviews
For more detailed information about P, As, and Sb donor ligand complexes, the following books and reviews are recommended. ‘Transition Metal Complexes of Phosphorus, Arsenic and Antimony Ligands’ edited by C. A. McAuliffe, McMillan, 1973; ‘Phosphine, Arsine and Stibine Complexes of the Transition Elements’ by C. A. McAuliffe and W. Levason, Elsevier, 1979; ‘Comprehensive Organometallic Chemistry’ edited by Sir Ge&rey Wilkinson, F. G . A. Stone and E. W. Abel, Pergamon Press, 1982, Volume 4, Sections 32.1 to 32.9, ‘The Chemical and Catalytic Reactions of Dichlorotris(triphenylphosphine)ruthenium(II) and its Major Derivatives’ by F. H. Jardine in Progress in Inorganic Chemistry, 1984, volume 31, pp. 265-370, ‘The Chemistry of Ruthenium’ by E. A. Seddon and K. R. Seddon, Elsevier, 1984, chapters 7-14, and volume 7 (Comprehensive Review Index) of this series.
45.6
OXYGEN LIGANDS
Ruthenium Complexes of oxygen donor ligands incorporating other donor ligands are discussed in the corresponding section for the other ligand. Ternary and related compounds are outside the scope of this review.
45.6.1
Aqua Complexes
The chemistry of ruthenium aqua complexes has been compared to the corresponding ammine Ruthenium(II): [Ru(OH,),]~+ can be prepared by reduction of RuO, with metallic Pb6483’876 or or by electr~chemical’~~~ or chemical reduction of [RUC~,(OH,)]~using Pt/H2.28231879PurificaSn1S77 tion of the pink product as its ditosylate salt by ion exchange column chromatography has been d e ~ c r i b e d . ~The ~ ~ ~single , ” ~ ~crystal X-ray structure of [Ru(OH,),](tos), (317) shows an average EXAFS,’”’ IR, Raman’”’ and electronic6” spectral data for Ru-0 distance of 2.122 [Ru(OH,),]~+have been reported, Dq and B being evaluated as 2100cm-’ and 423 cm-’ respectivel ~ . ’ ”The ~ force constant for the symmetric Ru-OH, stretching vibration has been calculated as 1.91 mdyn k’ For the redox couple [Ru(OH,),]~+!~+, Et = + 0.205 V vs. NHE.M8,’s82 The rate of self exchange between [Ru(OH,),]~”” (equation 123) has been determined as -5OM-’s-’ by 170,99Ru NMR,’883IR force constantls8’ and kinetic (Marcus Theory) [Ru(OH,),]~+ is readily oxidized to [Ru(OH2),J3+by 02:82 ClO; ,1885 [RuL(NH,),]~+(L = py, isn),18@[Ru(bipy),13+, [0s(bipy)J3+, [Co(phen),13+and [Fe(OH2),J3+.Igs4 The oxidation of [Ru(OH2)J2+by C10; in the presence of halides follows kinetics given by equation (124), where k, = 3.2 x lo-, M-’s-’, K = 1.2M-’, the rate being limited by slow substitution on Ru”.’885,1886 The same rate-determining step has been invoked in the [Ru(OH,),]’+ catalysed substitution reactions of [Ru(OH,),I3+ by halide ion X- with second order rate constant k = 8.5 x lo-’, 10.2 x lop3, 9.8M-’s-’ for X = C1-, Br-, I- respectively.”” The rate of water exchange on [Ru(OH,),]’+ has been measured as 1.44 x 10-2s-1at 24.5”C by ”0NMR studies.1ss3 [RU(OH,)6]2+ is a good starting material for the synthesis of substituted aqua complexes, e.g. [Ru(NO)(OH~),]~+ [Ru(~~),(OH,),-,]~+ (x = 4,2,0);648reaction of [Ru(OH,),l2+ with N, at
.’”’
42 1
Ruthenium
5 atm for 72 h yields the N,-bridged dimer [ R U ( O H ) , ] , N ~ +Addition . ~ ~ ~ ~ of N2to [RU(OH2)6]" to give [Ru(N2)(0H2)J2+was found to proceed approximately 35 times more slower than the addition of N, to [ R U ( N H , ) , O H , ] ~ + . ' ~ ~ ~
[Rll(OH,),j]2+-I- [RU(OH&]'+
&
[RU(OH,),j]"
4-
[Rh(OH2)6]*'
(123)
Rutheniurn(II1): [Ru(OH,),I3+ is readily prepared by oxidation of [Ru(OH,),]'+ .1876*1887The single The crystal X-ray structure of [Ru(OH,),](tos), shows an average Ru-0 distance of 2.029 A.1876 with temperature independent magnetic moment of [Ru(OH),I3+has been measured as 1.92 BMlUE8 powder and single crystal ESR spectra in CsGa(S04),.12H20 giving g,,= 1.494, g, = 2.517, A,, = 2.2 x cm-' and A , = 1 x lop4cm-*.lsSs IR and Raman'881spectra of [ R u ( O H ~ ) ~have ]~+ been reported, the force constant for the symmetrical Ru-OH2 stretching vibration being estimated as 2.98 mdyn k';"" molecular orbital calculations,'s90EXAFSISs0and NMR'8*9data have also been published. Absorption spectra for [Ru(OH,),]~+ give values for Dq and B of 30OOcm-' and 600 cm-' r e s p e c t i ~ e l y . ' ~Fo ~ ' r' ~the ~~~ formation of [Ru(OH)(OH,),]~+,pk = 2.4 at 20°C.1891The reduction of [Ru(OH,),]~+by [Ru(NH3),]*+, [RuCI(OH,),]~+and [V(OH,),]'+ has been reported.Iss4The preparation of [Ru(OH2),I4+in solution has been claimed.1892
45.6.2
Hydroxy Complexes
The chemistry of hydroxy and high valence aqua complexes of ruthenium is generally very ill-defined; for a full discussion see reference 3. The most important hydroxy complex is the red/brown Ru'" product formulated as [ R U ~ ( O H ) ~ ,] prepared ~+ by oxidation of Ru"' nitrosyls and nitrates with HN03,1893 by electrolytic1894 or chemica11895-'898 reduction of RuO, or in potentiometric studies of Ru" in methylsulfonic and nitric acid a t pH 1.5-4.1899 [Ru,(OH),,]~+ shows multi-redox behaviour, the generation of intermediates formulated as [ R u ~ ( O H ) , ~ ][RU,(OH),]~+, ~+, [Ru,(OH),]'+ and [RU~(OH),]~+ being available.1s93~1895~1s97~1s99~1900 The direct formation of [Ru4(0H),l8+by electrolysis of [ R u ~ ( O H ) ~at ~ ]a~Pt+ electrode has been reported, the Ru"' complex decomposing to a binuclear species of type [Ru,(OH),I4+ via first and zero dependence in [Ru"'] and [H'] respecti~ely.'~~' Ruthenium hydroxy species are active hydrogenation
45.6.3
Oxo Complexes
5
Oxo (M=O) complexes of ruthenium,Iw3associated high valence chemistry'9o4and the properties of ruthenium(1I) and (111) oxidess5 have been reviewed. Ruthenium ( I V ) : RuO, can be prepared by high temperature oxidation of Ru or RUC1,,1907-191 0 by dehydration of hydrated Ru0,191'*i912 or reduction of RuO,, [Ru04]- or [RuO,]'by NaBH,.'913,1914 The compound has a rutile structure in crystalline form with octahedral Ru with no direct Ru-0 = 1.984 A for four connectivities and Ru-0 = 1.942 A for the other PES,192o M o s s b a ~ eand ~ magnetic ~ ~ ~ ~suscep~ ~ ~ ' ~ ~ ~ Ru-Ru bonding occurs within the tibility1908s'923 data for RuOz have been published with = 1.10 x lop6and 1.76 x 10L6EMUg-'at 1.3 and 1033 K respectively. In recent years Ru02 and related oxides have found uses in catalytic (see Section 45.4.3.l.vi) and the systems, particularly in the generation of O2 from H2019w1929 water-gas shift reaction.1930 The chemistry of ruthenates MRuO, has been reviewed (see reference 3, pp. 116135). Ruthenium( V ): Several reports on the preparation of hydrated ruthenium(V) oxide have appeared. 1910,1913-1936 These products are not well characterized and require further detailed analysis. Cluster species of type [h4016]'- have been proposed to exist in N ~ , R u O , . " ~ ~ Ruthenium( V I ) :[RuO,]'- is stable in aqueous aIkaline conditions and can be generated by fusion of RuCl, or RuO, with KNO, and KOH,'938-1942 reduction of RuO, with aqueous KOH (equations 125 and 126),'943" oxidation of RuO, by Na,01943b or NaBi0,1944*'945 and by the oxidation of RuCl, with alkaline IS2S20812'1.1946-1947 or hypochIorite.lw Problems with overoxidation to RuV"and Ru""' and h y p ~ c h l o r i t e . ' ~[RuO,]'~ " ~ ~ ~ ~can be isolated most conexist with reactions using Na,01943b
422
Ruthenium
veniently in the solid state as its relatively insoluble barium salt, the single crystal X-ray structure of which shows the complex to be Ba[Ru(O),(OH),] (318) with trigonal bipyramidal Ru"' and trans hydroxides, Ru=O = 1.755 A, Ru-OH = 2.02 A.195@1952 The IR and Raman spectra of the solid are consistent with this s t r ~ c t u r e , " ~ although ~ , ' ~ ~ ~solution ~ ' ~ ~ ~studies suggest that the major species is [Ru0,I2-, deuteration experiments indicating the absence of hydroxy species.1953-1956 [RuO,]'shows absorptions near 480 and 380nm1949,1957-1962 and an isotropic ESR signal at g = 2.0, with a magnetic moment ,ne== 2.75 BM.IN3The Mossbauer spectrum of [RuO,j'- has been reported.""
K,[RuO,] and Na,[Ru04] are stable in aqueous alkali giving a bright orange solution, disproportionation to Ru"" and Ru" occurring in acidic and neutral condition^.'^^^.^^^^ The electrochemistry of [RuO,]*- has been r e ~ 0 r t e d . I [RuO,]'-'~~ acts as a two electron oxidant for the stoichiometric and catalytic oxidation of alcohols, aldehydes and amines to carboxylic acids and ketones. 121I , 19441947,1966-1974 The reagent is less reactive towards carbon-carbon double bonds than RuO,, [RuO,]- or OsO, and has been used for the oxidation of unsaturated alcohol^.^^"*^^^^ The use of Ba[Ru(O),(OH),], [Ru(O),CI,]- and [Ru(O),Cl,(bipy)] for the oxidation of alcohols to aldehydes and ketones has been described.lZ1' Ruthenium(V1) complexes of periodate"' and sulfate1953~'975*1976 have been reported. RuO, can be and has been proposed to be an intermediate in generated in the vapour phase at 1 200"C1~5*'977-1980 the oxidation of substrates by Ru04.'966~'9s'~1982 Rutkenium(V11): [Ru04]- may be most readily prepared by reaction of [RuO,]'- with C12 or NaOC1.1948,1957,'960 Alternative preparations include fusion of RuC13 with KNO, and KOH1983or reduction of RuO, with KOH at 0°C (equation 125).196071969 K[RuO,] is less soluble than K,[RuO,] forming very dark green almost black crystals. The single crystal X-ray structure of [RuO,]- shows a distorted tetrahedral Ru centre with Ru-0 = 1.79A and (ORuO = 106°'9R4 (cf. with Ba[Ru(0)2(OH)3]). IR and Raman spectra for [RuOJ have been reported.'954-195691984 The electronic spectrum of [Ru04]- shows absorptions near 390 and 320 nm; the assignment of these transitions and their relationship to absorption bands observed for RuO, and [Ru04]'- have been the subject The Mossbauer spectrum of [RuOJ has been reported."@ of much debate.'948,1949,1957-1961,1985,19g6 [Ru04]- is reduced under alkaline conditions to [RuO,]*- (equation 126) via a rate law given in equation (127); the proposed mechanism of reaction is given in equations (12 8 H 132).193531987 Electron exchange between [Ru04]- and [Ru0,I2- occurs with second order kinetics, rate constant k > 3 x 1 e M - l s-' at 0°C; with [MnO4I2- the specific rate constant for electron transfer k = 5.7 x t02M-1s-1 at 20°C with AH" = 17kJmol-', A S = -46JR-'moI-', A H # = 29KJrnol-I, A S # = - 4 6 J K - ' r n 0 1 - ~ . ~ ~ ~ ~ " Rate [RuO,][RuO,(OH),]~-
+
+
=
k[[Ru041-12[OH-13
2[OH-]
[RuO,]-
e
e
[RuO4(OH)2l3-
[RUO,(OH)~]~-+ [Ru0412-
(127)
(128) (129)
Ruthenium
423
[RuO,]- oxidizes alcohols and aldehydes to ketones and carboxylic acids, and readily cleaves The use of Bu,N[RuO,], prepared by the fusion of RuCl, carbon-carbon double with KNO, and KOH and subsequent reaction with Ci, and addition of Bu,NOH, for the oxidation of alcohols to aldehydes and ketones in acetone or CH,Cl, has been described.'98sb Ruthenium( VIIIJ:The chemistry of RuO, has been reviewed (reference 3, pp.45-55). RuO, is a toxic, highly volatile yellow product (m.p. 25.4"C, b.p. 40°C) which tends to explode in the solid state. It can be prepared by oxidation of RuCl,, RuOz or [RuCl6l2- with Ce'V,1989319w ~10, ,1994 cr 0 2- ,1995 B ~ O1957 - or BrO; ,1946,'997 oxidation of [RUO,]~~~10,,1943a,1991 p~~0~1,1992,1993 2 7 with Cl,1998or K[Mn0,],'999or by hydrolysis of RuOF,, RuF, or RuF, followed by oxidation with F2.2000 The most convenient method of preparation is by oxidation of RuO, by aqueous NaIO, at 0°C followed by extraction of RuO, into CCl,; RuO, can be stored as a yellow solution in CCl, over NaTO,. Electron diffraction studies on RuO, in the gaseous phase show the Ru-0 distance to be 1.705 A, with no deviation from tetrahedral geometry being observed.2w2The photoelectron spectrum of RuO, has been reassigned to give an orbital ionization sequence 2t, < 2a, < l e < 3t2 < lt1?w3,2004 previously studied samples being contaminated with C022w5or N2.2w6Theoretical calculations based on S C F - X B ,extended ~~ Hiicke12wsand Hartree-Fock-Slater Discrete Variation MO'9582i986 methods have been used to predict the energy level sequence and have inverted the energies of the ~ " ~study le and 2a, levels. The electronic absorption spectrum of RuO, has been r e p ~ r t e d , ~a CD M o ~ s b a u e r ,and '~~ showing that the absorption near 390 nm is due to a 1 t , -+ 2e IR and Raman1954."l>2019 spectra for RuO, in the gaseous, liquid and solid states have been published, RuO, retaining its Tdsymmetry in CCl,. 9 9 R ~lolRu , and " 0 NMR studies on RuO, show the coupling constant J17 wRu = 23.4Hz.24,2020 The redox chemistry oPRu0, has been investigated, Scheme 62 illustrating the potentials for the Electrochemical reduction of RuO, in the interconversion of the oxy anions of ruthenium.1949~2021*zoa2 presence of a series of anions yields partially characterized nitrato and hydroxy complexes of Ru" and Ru11'.2023 Reaction of RuO, with two electron donors L (L = NH,, PF,) yields adducts RUO,-L.'~'~ In the case of L = PF,, an apparent binuclear complex [Ru0,],PF3 was formed at - 100oc.~874 1943a~198171991,2001
RuO, is an extremely powerful oxidizing agent that has found important uses in the stoichiometric and catalytic'wl"O" oxidation of organic substrate^.'^^^^^^^' The reactivity of RuO, towards organic substrates has been reviewed,3-1982,2025 and compared with corresponding reactivity of O S O , .The ~~~~ oxidations of alcOho~s,1957,1966,1981,1997,2027-2030 aldehydes,1977 alkenes,1993,1W7,2031,2032 alkynes,2033.2034 ethers, 1997,2035-2038 lactones,2039 amines,2@W041 aides,1997 aromatics,2024,2o35,204'-?050s ~ l f i d e s , 2oxalic ~~~ acid,2051 and Se1V'989 by RuO, have been reported, with reaction occurring via two successive, two electron transfers.146h The oxidation of 2-propanol in 1 4 . 5 M HClO, solution occurs via a pathway where hydride abstraction is rate determining, while at high pH, the rate-determining step is carbonium ion f o r r n a t i ~ n . ~ ~ ~ ' - ~ ~ ~ ~
45.6.4
Ketoenolate Complexes
A range of ketoenolate complexes [Ru"'L,] (HL = RC(O)CH,C(O)R', R = R' = Me,163,2052 R = CF,, R' = ph,X55,2061 But 2061 ph,2052,2057,2058 4-~0,ph,2058,2059 ~~t.2052.2055,2060 CF3?2052-2054 , Me;2054~2056~2057~206'~2042 R = Me, R' = Ph;,06*HL = RC(O)CHR'C(O)R", R = R" = Ph, Me, CF,, But, R' = Ph)2052 have been prepared by reaction of blue ruthenium(II1) chloride with the corresponding diketone under basic conditions or by ligand exchange with [Ru(acac),]. The tris chelate complexes of (+)-3-acetylcarnph0?~~~ and (+)-3-trifluoroacetylcamphot064 [RuL,], for example, have been prepared by ligand exchange reaction with [Ru(acac),] in ethyl benzoate at 160°C;the diastereomers have been separated by tlc on silica and assigned by IH NMR studies?063For tris complexes of 9
424
Ruthenium
asymmetric ligands (L = A-B), mer and fuc isomers of [Ru(A-B), J can be isolated.2054~2057~206z The formation of Io3Rulabelled [Ru(acac),] has been described."" [Ru(acac),] has been resolved by chromatography on montmorillonite containing A-[Ni(phen),I2+:065,2066 by complex formation with (2R, 3R)-( -)-dibenzoyl tartaric acidM67and partially resolved by chromatography on D-( +)lactone.2068 The single crystal X-ray structure of [Ru(acac),] (319) shows octahedral Ru"' with average The magnetic properties of [Ru(acac),] are consistent with low spin d5 Ru-0 distance of 2.0 A.2069 Ru"' with = 1.91 and 1.75 BM at 298 K and 85.2 K while the ESR spectrum of [Ru(acac),] in [Al(acac),] shows a three line signal with g, = 1.28, g, = 1.74, g, = 2.82207'suggesting a dynamic Jahn-Teller effect in the complex.2o72IR2073and ~ ~ ~ 2 0 6 2 , 2 0 7 4 , 2 0spectra 75 for [Ru (acach] have been reported. [Ru(acac),] shows absorptions at 476nm ( E = 1.68 x lo3), 324 (E = 1.7 x lo3) and 265nm ( E = 2.7 x 104M-I cm-') in r n e t h a n 0 1 ~ ~ with ~ - " ~D~q being estimated as 1540 cm-' .2076 Kinetic studies on ligand exchange in [Ru(acac),] in acetyl acetone have been carried out; the first order rate constant k = 9.5 x lO-'s-' at 158°C with exchange occurring without decomposition. The rate-determining step was proposed to be formation of complex involving unidentate acac.2077,2078 Mass spectral data for [RuL,] (HL = RC(O)CH,C(O)R, R = R = Me, CF,; R = Me, R = CF,) suggest relatively little tendency for internal redox reactions in these complexes.2056 Several studies on the electrochemistry of tris ketoenolate complexes have been published.20~2,20~~,2062,2064,~79 [Ru(acac),] shows a one electron redox couple to [Ru(acac),]- at E: = - 0.72 V and - 0.51 V vs. SCE in MeCN and H 2 0 respectively.2079 The effect of substitution on acac on the RUI''/~'redox couples for the corresponding complexes has been assessed and related to the redox couples [ R u ( N H ~ ) ~ ] ~[+R/ ~~(+e,n ) , ] ~ +and / ~ +[ R U ( C N ) ~ ] ~.2055 -/~A - linear relationship was observed between El values for the couples and the sum of the Hammett constants for the fun& tionalities on the coordinated k e t o e n ~ l a t e Association .~~~ between [RuLJ and alkali metal ions has been noted;2052the rate of aquation of [Ru(acac),]- was found to be relatively slow, with a rate constant k < 1 x 102M-'s-1.207sH[Ru(acac),] is observed to be a relatively strong acid with pK, < 3.5.2078 The complex [RuL,] (LH = (+)-3-acetylcamphor) can be reduced by treatment with Na/Hg for 40 h, or oxidized by controlled potential electrolysis to the corresponding Ru" and Ru'" species respectively with retention of configuration.2064The Ru"' diastereomers are found to isomerize at 170"C.m Electron transfer and reduction of derivatives of [Ru(acac),] by Ti"' have been investigated,2080,m2 the electron transfer in the ketoenolato bridged Ru-L-Ti binuclear intermediate occurring with a rate constant k = 21 s-' .m8'[Ru(acac),] has been found to be an active catalyst in carbonylation and homologation reactions.208"088 [Ru(acac),(CO),] has been synthesized by reduction of [Ru(acac),] in THF at 160°C with H,and CO at high pressure,2089 in benzene with CO at 150"C,2090 by treatment of [Ru(acac),(NCMe),] with CO a t 11O"C2O9'or reaction of Ru,(CO),, with Hacac at 150°C in THF.'08' [Ru(acac),(cod)] (320) can be prepared by treatment of RuC1, with cod and oxalic acid in refluxing ethanol for 2 h, followed A similar route yields by treatment with acetic acid, Hacac and K2C03.2053*209232093 [R~(acac)~(nbd)].'~'~ [R~(acac)Cl,(OH,)~] has been synthesized by reaction of RuC1, with acac in aqueous HCl.'s63 A highly unusual pentanuclear RuFe, complex linked by ketoenolate and Cpfunctions has been reported; the complex is capable of undergoing an eight electron change, four at the Ru atom and one each at the Fe
$ P o
pf '\--
\*
v
5 6 0'
%*,JIftO
fl k o
o$ O
(319)
9
(320)
The complexes [RuL,] (LH = tropolone, a-isopropenyltropolone) have been prepared by reaction of RuC13with LH in the presence of NaHCO,; the NMR spectra of these products show them Ketoenolate complexes incorporating phosphine, to have predominantly the mer configuration.2095a ammine and amine ligands are discussed in Sections 45.5, 45.4.1 and 45.4.2 respectively.
Ruthenium
425
45.6.5 Carboxylate Complexes 45.6.5.1
Monocarboxylates
Treatment of [RU~(CO)~,] with carboxylic acids at reflux gives, as the major product, the insoluble For R = Me, this is also formed by polymeric species [Ru(OCOR)(CO),], (R = Me, Et, CgH19).1524 carbonylation (1 atm) of 'RuC1,-xH,O and Na[OCOMe] in MeCO,H/acetone at 80°C?096Interesshown by tingly, reaction of [Ru,(CO),,] with 4-fluorobenzoic acid (HL) gives [Ru~L~(CO)~OH,], X-ray analysis to have bridging carboxylate groups and a terminal aqua ligand.2095b The reaction of [RU~(CO)~,] with C,H4 and H,O under pressure also gives a propionato complex containing the [ R U ( C O ) ~ ( ~ - O ~ C E ~ ) ] ~ Reaction of [Rub-OCOMe)(CO),], with CO (50 atm; XOT) in hexane"24 or carbonylation (1 atm) of 'RuCI,*xH20 and Na[OCOMe] in aqueous MeC0,H at 800 ~ 2 0 9 6affords [RU~(OCOM~),(CO)~J of probable structure (321). Both [Ru(OCOR)(CO),], and [Ru, (OCOMe)2(C0),] react with a wide range of ligands to yield the complexes [Ru(p-0COMe)(CO),L], (L = MeCN, AsPh,, py, piperidine and various PR3) (143; Section 45.5.3.2).1s'9~'524.2097~*098 Tetrameric [Ru4(OCOMe),(CO),(PBu3),] (144; Section 45.5.3.2) can also be prepared from [RU(OCOM~)(CO),],.~~~~,~~~~ The complexes [Ru(OCOMe)(CO),], and [Ru,(p-0COMe)(CO),(pip)], have been shown to be active catalysts for the selective homogeneous carbonylation of a variety of cyclic secondary amines;z09s.2099 [Ru(OCOMe)(CO),], also catalyses the isomerization of alkenes."O" (R = CF,, CCl,, The binuclear Ru" carboxylates [Ru~(OCOR)~(~-OCOR)~(~-OH~)(CO~)~] CH2C1)have been synthesized via protonation of [Ru(p3-C,H,R),(cod)] with haloacetic acids. Some reactions of these complexes (R = CF,) are shown in Scheme 63.*"' Me
/\ Me 0
p
**,*o/c-I>\o Ru,,\\''
-co
OC-Ru" OIC'
co
oc'(
co
i, CO( 1 a h ) , THF/SO O C ; ii, L2 = dppm or dppe (4 equiv), Et,O, 25 O C ; iii,-PMe,Ph (4 equiv), THF/25 "C; iv, L = PMe,Ph, PMePh,, PPh3 (6 equiv), Et20, 25 OC
Scheme 63
-
Reaction of 'RuCI, xH,O' with acetic acid/acetic anhydride mixtures gives the brown solid [Ru,(OCOMe),Clj (322) and a green solution (see below)?1o2Higher yields of (322) are obtained if A range of complexes of type the reaction is performed in the presence of an excess of LiC1.1769 [Ru~(OCOR)~X] (R = Me; X = C1, Br, I, SCN, NO3, OCOMe; R = Ph, H; X = Cl, Br; R = Et, Pr", CH,C1; X = C1) can be prepared either from 'RuC1,~xH20' or by carboxylate or anion X-R ay structural analyses on the butyrate:'" propionate:106 exchange reactions on (322).2102-2'04 (X = Cl) and formate2109(X = Br) reveal [Ru,(OCOR),]+ units linked into infinite acetate210G2'08 chains by axially bridging halide groups (Ru..+.Rn 2.28 A). Variable temperature magnetic z 4 BM per molecule at 300 K (Curie-Weiss susceptibility studies on all these compounds show ,uem constant l3 = 20 K for R = Pr"), which indicates the presence of three unpaired electrons (quartet ground state).2102-21"*2"o This information together with extensive spectroscopic evidence (ESR,Z"o e~ec~ron~c,2102.2105,2108.2111-21 14 re~onance-Rarnan~~~'-~''~ IR, Raman,2"1*21'5 and SCF-Xu-SW calculations'''' suggest the ground state electronic configuration c ~ ~ p ~ 66*'. ~ 7 rThe * ~ ESR spectrum2'"
426
Ruthenium
indicates that the three unpaired electrons are fully delocalized. Cyclic voltammetric studies (vs. SCE) on [Ru,(OCOPr"),Cl] in CH,C12 reveal two reduction waves, one of which disappears on addition of an excess of C1- i0n."'~3~''~ The proposed reduction process is shown in Scheme 64 and the electronic spectra of the reduced species have been pub1ished."l6 Rate constants for the reduction of [Ru,(OCOMe),]+ by Ti3+ are available.2080 [Ru, (OCOMe),(THF),] has been successfully isolated by reaction of [Ru, (OCOMe),Cl] with one equivalent of Me, SiCH,MgCl and its structure confirmed by X-ray analysis. Magnetic measurements w 3.1 BM per molecule at 305 K) suggest a r~~71~6'6*' 71*3 electronic onf figuration.^"^
(322)
.
11
EK = 0.0ov
I1
te
[ Ruz(OCOPr "),
+e-
1
E5 =
-.0.34
V
[Ru,(OCOPr"),CI, 1 n -
decomposition products
decomposition products Scheme 64
Hydrolysis of (322) affords [Ru,(OCOMe),(OH,),]+ whereas treatment with CsCl gives Cs[Ru,(OCOMe),Cl,]; these have the discrete dimeric structure (323)?'06With pyridine, (322)gives [Ru(OCOMe),py,] and [Ru(OCOMe), py2],with uni- and bi-dentate OCOMe coordination respecThe corresponding [Ru(Otively,'Io2 whereas ButNC produces trans-[Ru(OCOMe),(Bu'NC),].2"s COH),py,] (n = 2,4) can be prepared from reaction of [ R u ~ O ( O ~ C H ) ~ ( O H ~ ) ~with ] O ~ pyCH ridine.,Il9 Related complexes include Na[Ru(OCOH), (OH,)] (from 'RuCl,.xH,O' and NaEOand [Ru(OCOMe)(azb)(CO),] (from [RuCl(azb)(CO),], and Ag[OCOMe]).'222Some other reactions of [Ru,(OCOMe),Cl] are shown in Scheme 65 and further discussed in the appropriate section. Calorimetric measurements of the enthalpies of adduct formation to [Ru,(OCOPr"),Cl] by various Lewis bases are now available.2121c Reaction of (322)with HBF, in MeOH, followed by addition of PPh, gives a solution which contains a very active alkene and alkyne hydrogenation Other monocarboxylate complexes containing P or As donor ligands are discussed in Sections 45.5.4.7.vi and 45.10. [ Ru,R6 J
\ ti /
+ [ Ru,(OCOMez)R4]
-
[ R U , ( O C O M ~ ) , ( P P ~1 ~ ) ~ vi
[Ru,(CF,CONH),Cl
J
[R u ~ ( P ~ C O K H ) ~ C ~ I
[Ruz(OCOMe),C1l &[ Ruz(N4CI2 H22)21 BF,
i: CF,CONH,/A:"6 ii, molten PhCONH2;'B7'b iii, Cz2H22N;4-,NalBF,] EtOH;2120 iv, Li(C5NH4NH); PMC,P~;'~''" v, 12 M HC1, [Et4N]C1;Z1al vi, PPh3/MeOH;1769 vii, RMgUl (R = Me3SiCHz, Me3CCH2)22'as221b
Scheme 65
Ruthenium
427
The main component of the green solution, formed by reaction of ‘RuCl,.xH,O’ with acetic acid/acetic anhydride, was believed to contain a binuclear ruthenium acetato but an X-ray structure determination on its PPh, adduct, [Ru,O(OCOMe),(PPh,),] (324), (mean oxidation Later, more state + 2 9 , suggested it should be reformulated as an oxotriruthenium species.2123 detailed investigations2124 of the reaction of ‘RuCl,~xH,O with Na[OCOMe], MeC0,H in EtOH showed the product to be the dark green [Ru,0(OCOMe),(OH2),]OCOMe (325) Oreff at 298 K is 1.77 BM/molecule). In aqueous solution the hydrolytic equilibrium shown in equation (133) is established with the cation favoured at pH < 2 and the anion at pH > 9. Compound (325) is an active catalyst for the conversion of unsaturated carbinols to saturated ketones,212’and some oxidative transfer dehydrogenation Detailed vibrational spectral studies on (325)and related species are Me ,r\O 0
Ph3P\
I
/
\ O
/
Me
Q
,PPh3
/\[/[\r MeC
1
‘ 0
McC-NI
‘RL1yo/CMe \O PPh3
(324)
[Ru,O(OCOMe),(OH,),]+
-H+
G
- H+
[Ru,O(OCOMe),OH(OH,),]
[Ru,O(OCOMe),(OH),(OH,)]-
(325)
(133)
Reduction of (325) with H,in aqueous media gives the light green, diamagnetic intermediate [Ru, O(OCOMe), (OH,),] which undergoes a further two electron reduction to yield yellow [Ru,(OCOMe),(OH,),] (uCrat 298 K is 0.4 BM/molecule). The latter is very oxygen sensitive and rapidly reinserts oxygen to regenerate [Ru, O(OCOMe),(OH,),]. Reaction of (325)with pyridine affordsthe corresponding blue [Ru3O(OC0Me),py,]+ cation which is readily reduced by H, to [Ru,O(OCOMe),py,J and then [ R U , ( O C O M ~ ) ~ ~ ~ , ] . ~ ~ ~ ~ The diamagnetic [Ru30(OCOMe),(PPh,),] (324), formed by reaction between PPh, and [ R u , O ( O C O M ~ ) , ( ~ H ~ ) (~n]= ” +l,O), is not reduced by H, at ambient temperature although it does form an extremely air-sensitive, reduced yellow solution (formulated as ‘Ru(OCOMe),PPh, ’) under more forcing conditions. Reoxidation of this does not regenerate (324) but instead the purple, diamagnetic [Ru,O(OCOMe),(PPh,),] (326). Some other routes to (326) are shown in Scheme 66.1769.2124 Related complexes are known for other carboxylates (OCOH,2”9OCOEt,2’24 OCOPr,2‘24 OCOCH,C13-n (n = 0-2)2128etc.). I
Me
‘RuC13 .xH,O’
.-
I
[ R U ( O C O M ~ ) ~ (1 P P ~ ~ ~
MeC
I*
co/\
P
I
*.O*+%/Ru
Ru9a“* ‘*E.,C M e
\owlpow 0 PPh3
[ RuCh (PPh313 I &‘Ru(PPh3),’
A
Me
i, PPh3, Wa[OCOMe] , MeOH;. ii, 02/MeOH; iii, Ag’; iv, 02/Na[OCOMel
Scheme 66
428
Ruthenium
If [Ru,O(OCOMe),(OH,),]OCOMeis treated with H, at 80°C in DMF, the monohydride [Ru,(H)O(OCOMe), (DMF),](OCOMe) is first formed which undergoes a subsequent intrarnolecular reduction to produce [Ru, O(OCOMe),(DMF),][OCOMe]. Further reduction with H, yields dimeric ruthenium(1) species such as [Ru,(OCOMe),(HO,CMe)MeOH]together with [Ru(OCOMe)(CO)L], (L = PPh,, SbPh,, py etc.), isolated by addition of L to the DMF solution. The final products are a mixture of [Ru(OCOMe>(CO),L],and [RuH(CO),],.'~~~ Further studies indicate that [Ru,(H)O(OCOMe), (DMF),f(OCOMe) catalyses the hydrogenation and isomerization of alkenes. Interestingly, only one ruthenium centre is involved in the catalytic process.2130 Carbon monoxide reacts slowly with [Ru30(OCOMe),(MeOH),]+ in MeOH but the reduced forms, [Ru, O(OCOMe),(MeOH),] and [Ru, (OCOMe),(MeOH),] rapidly give purple, paramagnetic [Ru,O(OCOMe),(CO)(MeoH),] (327a); with pyridine (327a) gives [Ru, O(OCOMe), (CO)py,] (32%) which is also prepared from [Ru,O(OCOMe),py,]+ and CO. Reaction of (327a) with PPh, yields turquoise [Ru20(OCOMe),CO(PPh,)] (328)also formed from [Ru30(OCOMe),(PPh,),] and C0.1651Note, however that other workers suggest that [Ru3O(OCOMe),(CO)py2]has a symmetric structure of type (324).'13' Treatment of [Ru,O(OCOMe),(MeOH),]+ with dppm or dppe gives the binuclear cations (329) whereas dppm and [Ru,O(OCOMe),(MeOH),] affords two isomers of [Ru,(OCOMe),(dppm),]; some [Ru(OCOMe),(dppm),1 is also produced. With dppe and [Ru,O(OCOMe),(MeOH),], the product analyses for [Ru60,(OCOMe),,(dppe),] and this is tentatively formulated as two [Ru,O(OCOMe),] units linked by three bridging dppe
Me (328) CO
(327) (a) L = MeOH; (b) L
= py
Reactions of [Ru, (OCOMe),(MeOH),] with MeNC, NO, SO, are fully discussed in reference 1651. For example MeNC gives [Ru,O(OCOMe),(NCMe),MeOH] and NO followed by PPh,
affords [Ru, O(OCOMe),(NO)(PPh,),]. Electrochemical studies on [Ru,0(OCOMe),py2L]+ (L = pyrazine, etc.) (see Scheme 67 for synthesis) reveal the existence of a series of redox states [Ru,O(OCOMe),py,L]" (n = 3 to - 2). The electronic properties have been interpreted via a MO scheme which assumes strong Ru-Ru interaction via the central oxygen atom and also direct metal-metal b ~ n d i n g . ~ ' Rates ~ ' , ~ 'of~ electron ~ self-exchange between [Ru,O(OCOMe),(py),]+ and [Ru,O(OCOMe),(py),] have been determined by ' H NMR line broadening The pyrazine-bridged dimer (330) is prepared as shown in Scheme 67. Detailed cyclic voltammetric studies on this and related N-heterocyclic ligand bridged dimers indicate a weak clustercluster interaction across the bridge at anodic potentials but significant interactions at cathadic potentials as the electron content of the clusters increases. Electronic spectral measurements support this ~ o n c l u s i o n . ~ ' ~ ~ ~ ~ ' ~ ~ The ligand bridged trimeric cluster (331) (PF, salt) has been prepared as shown in Scheme 68. This complex shows spectacular electron-transfer behaviour with a total of ten one- or two-electron waves between + 2.2 and - 1.7 V (in MeCN vs. SSCE). Again, these electrochemical studies suggest that when the total electron content is low, interelectronic cluster interactions are weak but as the
+
429
Ruthenium
electron content increases, electronic coupling between the cluster sites is enhanced. In fact at negative potentials, a series of closely spaced, one electron waves suggests the beginning of band-like behaviour in this discrete molecular [ RLI,O(OCOM~),COPY~ I
.
[Ru~O(OCOM~),CO~~~I+
(327b) [Ru,O(OCOMe),py,MeOHl*
:u,O(OCOMe),(CO)(MeOH),
I
+ CO
pz
*
[ R u 3 0 (O C O M e )6 (~ y )2 ~ z l +
+~[RU~O(OCOM~)~(~~)~.PZ]+ [ { py2Ru,0(OCOMe),pz}
:u30(0COMe)6CO(pz),
/
I + 2[ R u , O ( O C O M ~ ) ~ ~ ~ ~ M ~ O H ~ +
+2MeOH
{ Ru30(OCOMe),CO}
]'+
(331)
Scheme 68
Reaction of [NH4],[RuC1,] with 10% HCOIH gives [NH,],[RuC14(OCOH)(OH,)] whereas with is Na[OCOH], [NH,],~RuCl,(OCOH)]~2H20
45.6.5.2
DicarboxyZates
The ruthenium(II1) trisoxalato anion [Ru(C, 0 , ) ~ ~was - first prepared by Charonnat as its potassium salt by reaction between K, [RuCl, (OH,)] arid K, [C20,].2137 Later workers2138 found that this method led to KCI contamination and prepared K , [ R u ( C ~ O , ) ~ ] * ~ Hby~ Oreaction of K,[Ru,OBr,,] with H, [C,O,] and HCHO, followed by neutralization with K[HCO,]. Treatment of RuO,.xH,O with an excess of ammonium hydrogen oxalate gives [NH4I3[Ru(C204),]*1.5H20.588 The magnetic proper tie^,^^^^^^^^ e l e c t r o n i ~and ~ ~ESR ~ ~ ~spectram ~ ~ ~ and electrochemical behav i o ~ r " ~of' this anion are fully documented. The [Ru(C,O,)(OH,),]+ ion has been reported to exist in aqueous solution and the [Ru(C,O,),(OH,),]- anion has been isolated as its sodium salt by treatment of [Ru,(OCOMe),Cl] with aqueous Na, [C, O,] under an oxygen atmosphere. This anion undergoes a facile one-electron reduction by Ti"' via an inner-sphere mechanism.2143 Other [Ru(CZO4),L2]-anions (L = N donor ligands) are discussed in Section 45.4. Reaction of K3[Ru(C204),]with [enH,][C204]gives the cis-[Ru(C,O,)(en),]+ cation.620 Other oxalato complexes include various nitrosyl species such as K2[RuCl, (C, 04)NO], K2[RuClfrom (Ct04)2NO],K[Ru(C,O,),NO(py)], [RuCl(C, 04)N0(py)2],2'"the black Kz[R~(Cz04)3]2145 oxidation of K, [Ru(C,O,),] with H z 0 2and Csz[Ru0,(C,0,),],65 obtained by treatment of RuO, with ice cold solutions of Cs,[C,O,] and oxalic acid. The latter contains a trans RuO, unit. The mixed valence anion [Ru, (OCOMe), (C204),(OH2),]- has been synthesized by reaction of [Ru2(0via an interCOMe),Cl] with oxalic acid. Reduction with Ti"' gives [RuZ(OCOMe)2(CZ04)2]2mediate containing two ruthenium atoms and one titanium atom."46 Aqueous solution studies between Ru"' or Ru" with citric acid indicate that 1:3 and 1:2 complexes respectively are f0rrned.2'~'
45.6.6
Miscellaneous
Ruthenium complexes incorporating only nitrate, sulfate2148or related oxy-anion ligands are generally ill-defined.' The chemistry of heavy metal nitrites2150and d i ~ x y g e n ~ 'com~'"
Ruthenium
430
plexes has been reviewed. The fluorosulfate complexes [Ru(SO,F),j, [Ru(SO,F),]- , [Ru(SO, F)i]and [Ru(SO, F)6]2- have been reported.2151b Reaction of [RuBr2(CO),] with L (L = pyridine N-oxide, OPPh,) in CHC1, yields the complexes [RuBr2(C0),L).2152The products [RuCl, L3] (L = 2,dlutidine N - ~ x i d e , ~phosphole ”~ N-oxide der i v a t i v e ~ 8-hydroxyquinoline-N-oxide2155~z~56) ,~~~~ and 0 bonded [RuCl, L2] (L = thiomorpholin-3one)679have been prepared. 45.7 SULFUR LIGANDS 45.7.1
Sulfido, Thiolato and Thiazole Complexes
No simple sulfides of Ru in formal oxidation states less than four are apparently known; thus ’~ work2 reveals that the Ru” ‘Rugs8’ has been shown to be a mixture of RuS, and R U . ~ ’Earlier complex RuS, can be made from the elements at high temperatures and that it has the pyrite structures.2158More recently, poorly crystallized samples of diamagnetic RuS, were prepared by reaction of the anhydrous [NH,],[RuCl,] with H2S at 180°C for 4h. These amorphous products were annealed under various conditions and the resulting degree of crystallinity was determined.2159,2160 The high temperature enthalpy and decomposition pressure of RuS, (to form Ru(s) and S2(g))2161and IR data and normal coordinate calculations2162are available. Synthesis of members of the Col-,Ru,S2 (0 6 x < 1) and Rh,-,Ru,S2 (0.5 < x < 1) systems has been accomplished by the sulfurization of mixtures of [NH,],[RuCl,] with either [Co(NH,),Cl]Cl, or [NH,]3[RhCI,] and variable temperature magnetic susceptibility measurements The polysulfides RuS, and RuS, can also be prepared by reaction of K2[RuC16]and H,S; RuS6 is oxidized by air to a pyrosulfite [Ru(S,O,),] and a ‘thiometallate’, possibly [Ru(SH),I3-, can be obtained from polysulfides and [RuCI,]*- However, this is old work and further studies in this area are desirable. Reaction of RuO, with SCI, in CCl, gives the brown, moisture sensitive [RuCl,S],. A binuclear structure with a RuS2Ru bridge is proposed.2164Another sulfido-bridged complex is [Ru,H,(p3~ ~ ] Na,[SO,] and alcoholic KOH.,16’ A higher yield S)(CO),] (332) from reaction of [ R U ~ ( C O )with with sulfur under a CO/H, atmosphere in refluxing route to (332) is by reaction of [RU~(CO)~,] Compound (332)is also generated by addition octane; some [Ru, S2(CO),] (333) is also of water to the trihydrido cation [Ru,H,@-S)(CO),]+ .2167,2L68Variable temperature I3CNMR studies on (332)reveal both H and CO scrambling processes.2169 With [Ruj(CO)12]and cyclohexene sulfide, a yellow solid of composition [RuS(CO),(C,H,,)], has been isolated.”65
.’
(333)
S u ~ d complexes o containing P or As donor ligands, e.g. [RuX2NO(EPh,)l2S(X = C1, Br; E = P, As), [Ru,Cl,S,(PPh,),], [Ru(CO),(PPh,)S], etc., are discussed in Section 45.5.4.8.i. Reaction of NEt,[RuCl,(MeCN),] with 4 equivalents of the Li salt of either 2,3,5,6-tetramethylbenzenethiol or 2,4,6-triisopropylbenzenethiol(LH) and 0.5 equivalents of the corresponding sulfide in MeOHJMeCN solutions give high yields of [RuL,(MeCN)], shown by X-ray analyses to have trigonal bipyramidal structures with axial MeCN groups. Electrochemical studies reveal that both [RuL,MeCN]+ and [RuL,MeCN]- can be generated.2170a Reaction of [RuL,MeCN] with CO at A series of room temperature gives [RuL,CO], a rare example of a Ru” carbonyl complex.2170b six-coordinate octaethylporphyrin complexes [Ru(SR)(OEP)(CO)] can be synthesized by treatment of [Ru(OEP)CO] with SR- (R = CMe,, Et, CH2CH2C0,Me).21’1 Reaction of the reduced ‘ruthenium blue’ solution with thiols such as 2-aminobenzenethiol and 4-toluenethiol (LH) gives highly insoluble compounds of stoichiometry [Ru(L),], which are prob-
431
Ruthenium
ably polymeric with sulfur bridges. Attempts to cleave these bridgesiwith PPh, and p-toluidine were unsuccessful. The benzothiazole-2-thiol (LH) complex of stoichiometry [RuLJ is weakly paramagnetic, dimeric in CHCI, and structure (334) is proposed. Reaction of the reduced 'ruthenium blue' solution with pyridine-2-thiol gives the highly insoluble and polymeric [RuCl,(pySH),], which reacts with PPh, to yield [Ru(pyS), (PPh,),].676A Ru"' complex of 3,4,5-pyridazinetrithiolhas also been rep~rted."~'" Also reaction of [Ru(acac),] with HSPh has been shown to give polymeric [R u (S P ~ ) ,] ,.~ ' ~ * ~
(334)
Treatment of the 'RuCOCl' solution with thiophenol produces the light brown [Ru(SPh),(CO),],, formulated as a trimer on the basis of an osmometric molecular weight determination. Similar reactions with dithiols gave yellow complexes of uncertain c o m p o ~ i t i o nbut ' ~ ~ with 2-aminothiophenol, the orange brown [Ru(SC,H4NH2j2(C0),] was isolated.''* Reaction of [Ru3(C0),,] with thiols (including thiophenol) affords the binuclear [Ru(SR),(CO),], and the polymeric [Ru(SR),(CO),], (R = Me, Et, Bun, Ph); the formation of [Ru(SPh),] is also mentioned.2173Similar products are formed with organic disulfides. For example, Ph,S, and [RU~(CO)~,] produces [RuSPh(CO),],, [RuSPh(CO),],, [Ru(SPh),(CO),], and two forms of [Ru(SPh),(CO),],; Et2Sz gives [RuSEt(CO),], and [Ru(SE~),(CO),],~~~~ whereas Me's, generates [Ru(SMe), (CO),],, (n = 242) which on bromination gives [RUB~(SM~),(CO),],?~~~ The mixed P- and S-bridged complex [Ru,(pPPh,)(p-SPh)(CO),] [(140); Section 45.5.3.21 is obtained from [Ru,(CO),,] and Fh,FSfh.'517Under milder conditions, [Ru, (CO),,] and RSH afford the trinuclear hydrido complexes [Ru,H(pSR)(COj,,] (335) (R = Et, Br")2176which for R = Et undergo protonation in 98% H,SO, to generate the [Ru,H,(p-SEt)(CO),,]+ ati ion.^^^^*^^^^ Treatment of [Ru(azbjCl(CO),], (azb = 2-phenylazophenyl)with sodium benzenedithiolate gives [Ru(azb)(SPh)(CO),],, a compound containing bridging SPh groups whereas sodium 2-aminobenzenethiolate (abt) yields the monomeric [Ru(azbj(abt)(CO),] with a chelating abt ligand.'222Reacin MeOH gives a mixture of products but tion of mercaptobenzothiazole (mbtH) with [RU~(CO)~,] addition of pyridine and recrystallization affords [Ru(mbt), (py),(CO),], (with cis CO, cis py and mutually trans mbt ligands coordinated in the thiolate form via the exocyclic sulfur atom), and [Ru(mbt)py(CO),], (336), probably via initial formation of [RuZ(mbt),(CO),]. In the latter, the Und er mild conditions (toluene, 5OoC/1h), Ru-Ru bond is bridged by the thiazole ligand?177,2178 [Ru3(CO),,] and mbtH generate [Ru, H(C7H,NS,)(CO&] (337)in which the heterocyclic ligand is attached to the cluster via a bridging thiolato group and a third ruthenium by nitrogen. Under the same mild conditions, mercaptoethanoic acid and [Ru, (CO),,] afford [Ru,H~-SCH2COzH)(CO)l,] with a structure similar to that of other [RU~H(II-SR)(CO)~,] (335) complexes?'79 Reaction of [Ru, (CO),,] with thiobenzophenones produces the novel ortho-metallated complex (338) in high yield.2180
/pe#
PQ ( w 4 R u\ I@*** sR OC%,+ I Ru-PY 1 ,/o (CO),
RUM
(cob
(335)
s/c\N
w- 1
wco
CPs \ /
(336)
as>+, N
I
&I\
4F0)3 /
R4COh
(337)
432
Ruthenium
(338) R = OMe, Me
Other examples of carbonyl clusters containing sulfido or thiolato ligands include the hydrocar[RU,(~~-S)(CO)~(~~-$-C~H~)]~~~~ and also bon complexes [Ru,(~-SBU~)(CO)~(~~-~~-C,H~)], [RU6H2(SEt)2(CO)ls]and [RU~HC(SE~)~(CO)~~].~'~~ Cations such as [Ru(SR)(NH,),]'+, [(NH,), RuS, Ru(NH3)J4+, are discussed in Sections 45.4.1.2.i~and 45.4.1.3.iii respectively and thiolato complexes containing P donor ligands in Section 45.5.4.8.i. References to the preparation of other ruthenium-thiol and -thione complexes are given in reference 3. Reaction of [Ru(edta)(OH,)] with RSH gives [Ru(edta)SRI2- (see Section 45.1 1.1.5). Finally, although outside the scope of this review, various &H, and q--C,H6 complexes containing thiolato groups are known. Examples include [Ru(SR)(CO),(q-C, H5)], [Ru(p-SR)(CO)(q[Ru,(~-SR)~(CO)~(?&H,),] (reference 1, p.781) and [Ru,(p-SEt)3(qC5H5)]2, C6H6)2]BPh4.MeOH.2L83
45.7.2
Thioethers
The Ru"' monomers [RuCI,(SRR),] (R = Ph; R' = Me, Et, Pr", Bun; R = R' = Me, Et) are generally prepared by heating under reflux an ethanolic solution of 'RuCl,.xH,O' with an excess of RRS.2'8G2'86 The corresponding bromides are prepared by shaking the chlorides with LiBr in acetone. With Me,S and Et,S, the red [RuCl,(SR,),] was contaminated with black [RUCI,(SR,),]~ which can be separated by fractional crystallization. Under milder conditions of reaction, small amounts of yellow [RuCl,(SMe,),] are also The low pefl of the binuclear species (ca. 1.O 3M/Ru at 293 K) suggests some Ru-Ru interaction probably via chloro bridges. Initially, [RuCl,(SEt2)J was assigned afac configuration on the basis of its IR spectrum and the similarity of its X-ray powder pattern to fac-[IrCl, (SEt2)3].2184,2187 However, further ' H NMR studies on the latter indicate it has a mer configuration which suggests that all the [RuX3L3]have a mer structure;2'8stheir ESR spectra are consistent with this conclusion.'852.2'85 Interestingly, reaction of 'RuC1, -xH,O' with PhPr'S in boiling MeOH gives [RuCI,(SPhPt),MeOH] (cf. [RuC13(EPh,)2MeOH](E = P, As); Section 45.5.7.1). Treatment of this with MeCN gives first [RuCl, (SPhPr'),MeCN] and then [RuCl, (MeCN),]. MeCN. Facile SR, displacement is a general feature of the [RuCl,(SPhR),] complexes (see equation 134).2185 [RuCl,(SPhR),]
+
L
3 [RuCl,L,]*0.SC6H,
(L
=
PhNH,, py)
(134)
Other dialkyl sulfide complexes include the unusual cation [Ru(SnCI,)(CO),(SEt,)J+,obtained by treating [RuCI,(S~CI,),(CO),]~-with three moles of Et,S,'O8 [Ru(MeCN), (SEt2)(cod)l2' (from [Ru(MeCN),cod]*+ and EtzS),l"o various [RUCI,NO(SRR')~](see Section 45.4.4.3),123,130',t30L1303 [RuCl,(PPh,)(SMe,),] (see Section 45.5.7. I), [RuCl,CO(SMe,),]- (from reaction of [RuC1,CO(nbd)j- and Me,S),659 and ammine complexes such as [RU(NH,),SM~,]~', [Ru(NH,),(OH2)SMe212+, [RU(NH~)~LJ'+(L = MeS(CH,),CH(NH2)C02Me), and [(NH,),RuLRu(NH,),]"+ (L = l,Cdithiane, 1,5-dithiocane; n = 4 3 ) etc. (see Sections 45.4.1.2.i~ and 45.4.1.3.iii). The reaction of 'RuC1,-xH,O' with various disulfides in boiling EtOH affords [RuCl,(RS(CH,),SR),,,], (R = Et, Pr", Ph). For R = Et, reaction with pyridine gives [RuCl,(EtS(CH,I2SEt)py] suggesting that one sulfide molecule is bridging and the other chelating in the parent reports the synthesis of other examples of [RuCl, L,,,], m~lecule."~'Parallel work by Rice et a1.2189 [L = l,Cdithiane, RS(CH,),SR (R = Me, Et)] and also [RuCI,(RS(CH,),SR)], (R = Me, Ph; n = probably 2) and [RuCl,(PhS(CH,),SPh),], (n = probably 2). In boiling 2-methoxyethanol, reduction occurs to give [RuX2(RS(CH2),SR),] (X = C1, Br) (see reference 2190 for variable temperature 'HNMR studies on [RuX,(MeS(CH,),SMe),]) and for
Ruthenium
433
R = Et, chemical oxidation with HClO, or Br, then generates the Ru"' cations [RuX,(EtS(CH,),SEt),]+ . For R = Ph, carbonylation in boiling 2-methoxyethanol gives [RuX,(CO),(PhS(CHz),SPh)(with cis-CO and trans X groups).21SsCompounds of this type are also formed as shown in equation (135). Similarly, [RuX,(CO),(SR,),] (various isomers) and in some Interestingly, reaction of ethane-1 ,Zdithiol instances [RuX,(CO)(SR,),] can be prepared.'61'~2'9*~2'92 with [RuI,(CO),], gives the unstable [RuI,(CO),(HS(CH,),SH),] in which the sulfur ligands may be monodentate.I6"
[RUI,(CO)~],+ RS(CH2)zSR
+
[RuI,(CO),(RS(CH,),SR)]
(R
=
Et, Ph)'61',219' (135)
The Ru"' anion [RuCl,(MeS(CH,),SMe)j- is obtained as its AsPh: and NMe; salts from 'RuCl,-xH,O, 2J-dithiahexane and [AsPhJHCI, in acetone or [NMe,]CI in absolute Treatment of 'RuCI,-xH,O' with the trisulfide 1,1,1-tris(ethylthiomethyI)ethane in boiling 2methoxyethanol gives [RuCl, {MeC(CH, SEt),}] which on further heating with DMF affords [RuCl,(CO)(MeC(CH,SEt),}]. This is a rare example of a solvent decarbonylation reaction with a sulfide complex. Carbonylation in 2-methoxyethanol also yields [RuCl,(CO), {MeC(CH,SEt), >I formulated as a seven-coordinate Ru" compound.218s The corresponding [RuCl, \S(CH,CHzCHzSMe),}] is obtained by reaction of 'RuCl,~xH,O' with the trithioether in EtOH.' 94
45.7.3
Metallothioanions as Ligands
The complexes [Ru(CoL,),]X, *nHzO(339) (X = C1, I, BPh,) (per 1.8 BM at 303 K) are prepared by reaction of RuC1, with a large excess of CoL, (L = NH,CH,CH,S-), followed by addition of NaI or NaCBPh,]. A study of the ESR spectra of this and the corresponding Fe"' complex suggests severe distortion from octahedral geometry. Hyperfine structure from the two equivalent cobalt nuclei indicates that the unpaired electron is delocalized over all three metal ions.219s
(339)
Treatment of [RuCl, (bipy),] .2H, 0 with equimolar amounts of [NHJ2[MoS4]in aqueous EtOH for 18 h at room temperature produces the bimetallic complex (87). Reaction of (87) with more [RuCI,(bipy),]*H,O then affords the trimetallic cation (lot?),isolated as its PF; salt. Electrochemical reduction of these species at - 1.7 V in the presence of acetylene produces some ethylene and ethane"" (see Sections 45.4.3.1.xi and 45.4.3.2.iii respectively).
45.7.4 Dithiocarbamato and Related Complexes Several reviews which cover the chemistry of ruthenium dialkyldithiocarbamates have appeared in the last fifteen year^."^"^'^^ Cambi and Malatesta=" and later M a l a t e ~ t areported '~~ the low spin, monomeric [Ru(S,CNR,),] (R = Me, Et, Bun) which they prepared by reacting K,[RuCl,] with aqueous Na[S,CNR,]. More recently, compounds of this type have been prepared by the reaction A crystal structure determination for of Na[S,CNR,] with solutions of 'RuCl,~xH20'.2201~2z0z R = Et shows the RuS, core to have approximate D, symmetry and a twist towards trigonal prismatic geometry?20392204 Essentially the same RuS, core geometry is found for [Ru(S,CN(CH,),O),] 2.5CHC1, with the hydrogen atom of each CHC1, hydrogen-bonded to the sulfur atoms of various ligands.z202The temperature-dependent ' H NMR spectra of various [Ru(S, CNRR'),] reveal both intramolecular metal-centred inversion and ligand-centred geometric isomerization which are attributed to a trigonal twist mechanism and SZC-N bond rotation respectively. The distortion of the RuS, core towards trigonal prismatic coordination may be the reason for ease of metal-centred inversion whch involves a trigonal twist pathway.2201Mass spectralzzosand ESR data604on several [Ru(S,CNR,), J are available. Several studies2262210 have been published on the redox behaviour of [Ru(S2CNR,),]. A one electron reversible reduction to [Ru(S,CNR,),]- is observed but the one electron oxidation is
434
Ruthenium
quasi-reversible and the product [RU(S~CNR,)~]+ is unstable towards further reaction or decomposition. For example, in MeCN, the electrochemical oxidation product of [Ru(S,CNEt,),] is the "" had indicated that [Ru(S,Cdiamagnetic [Ru(S,CNEt,), MeCN] cation."" Earlier studies'Zo6 NEt,),]' was synthetically accessible and aerial oxidation in the presence of BF, was reported to yield [Ru(S, CNEt2)3]BF4.22n7 However, X-ray analysis revealed the product to be the binuclear Ru,Itl,II' cation [ R U , ( S ~ C N E ~ , ) ~ ](340) B F ~(B-isomer). Electrochemical studies on (340) in MeCN showed a four member series (equation 136) and the one electron reduction product [RU~".I~~(~&CNEt,),] was isolated.22"* Independent studies involving chromatographic separation of the reaction products of 'RuCl,.xH , O arid Na[S,CNPr;] led to the isolation of some [Ru2(S,CNPf2),]X (X = C1-, Ru,CIs")), shown by X-ray analysis to have structure (341) (a-isomer)."" The a-[Ru,(S,CNMe,),]+ cation is converted to the /I-isomer by heating in CHCl, and the interconversion can be monitored by 'H NMR spectroscopy. Reduction of the a-isomers is more facile than the fl-isomers but the p[Ru,(S,CNR,),]- rapidly isomerizes to give the a-[Ru,(S,CNR,)J anion which is the reverse of the situation for the cations.220g A detailed reexamination22'0of the preparation of compounds (340) and (341) reveals that chemical oxidation of [Ru(S,CNR,),] using BF, gas in the absence of oxygen yields the a-isomer; in the presence of oxygen, only the fl-isomer is formed. The 'H NMR spectra of a- and /l-[Ru,(S2CNMe2),]+ have been assigned by variable temperature and ligand-exchange studies and an isomerization reaction mechanism proposed. +
(340)
(341)
The r- and /I-isomers of [Ruz(S,CNRZ),]f can be reduced to the paramagnetic a- and 8[RU,(S,CNR~)~] respectively by reaction with Na[BH,] in ethanol. For R = Et, the green a-isomer converts in toluene to the purple fl-isomer with a first-order rate constant of 8 x 10- s-' at 30°C. Photolysis in CHCI, yields the oxidized cations with chloride counterions and retained a and /3 stereochemistry.2210 Seven-coordinate, diamagnetic Ru" complexes [RuCl(S,CNR,),] (R = Me, Et) are obtained by photolysis of [Ru(S,CNR,),] in CHCl, or CH,Clz or by reaction with gaseous HCl in C6H,. X-Ray anaiysis reveals a distorted pentagonal bipyrimidal structure (342) although the compound is non-rigid in CD,Cl, solution even at - 95"C.2212More detailed photochemical studies on [Ru(S,CNR,),] at /z = 265-366 nm in various chlorinated solvents show that in addition to (342), some a-IRu,(S,CNR,),]Cl is formed; a reaction pathway involving the formation of an excited Ru" species [Ru(S,CNR,), (S: CNR,)]* is proposed to explain these results.2213 Interestingly, photolysis at 366nm in the presence of a large excess of benzophenone gives the unusual product [RuCl(S,CNMe,),(q2-SCNMe,)] (343) in greater than 90% yield. Variable temperature 'HNMR studies on this non-rigid, pentagonal bipyramidal molecule are reported2*14In donor-type solvents S such further reaction with PPh, gives as MeCN and DMSO, compound (342)forms [Ru(S,CNE~~)~S]CI; [ R U ( S ~ C N E ~ , ) ~ P Pand ~ , ] Call~ these seven-coordinate Ru" complexes are stereochemically nonis undissociated and exhibits a one electron rigid. In propylene carbonate, /RU(S,CNE~~)~CI] oxidation and a two electron reduction step to form [Ru(S,CNEt,),]Addition of iodine to [Ru(S,CNRR'),] in CHC1, affords the diamagnetic Ru'" complexes [Ru(S,CNRR),I,] (344) (R = R = Me, Et, Ph, PhCH,; R = Me; R = Ph). X-Ray analysis (for R = R = Me) shows the molecuke to be seven-coordinate with one iodide ion coordinated in an axial position while an I, molecule strongly solvates the iodide atom. Again all the R groups are Addition of Ag[BFJ in magnetically equivalent on the 'H NMR timescale even at - 90"C.221n,2215 acetone to these [Ru(S,CNR,)~X] compounds gives ultimately [Ru, (S2CNR2)5]BF4and with Na[S,CNR,], [Ru(S,CNR,),] is formed. Both these reactions illustrate the lability of the RuLX bond,2210,2z12,2215 Interestingly, [Ru(S2CNEt,),N0] is six-coordinate with one of the - SZCNEt2groups monodentate. This is not too surprising since the complex contains a Ru" d 6 ion which has a marked preference for an octahedral tlg6 c ~ f i f i g u r a t i o nHydrogen-1 .~~~~ and "C NMR studies of [Ru(S,C-
Ruthenium
435
\,/I'
I' \
(343) (344)
NR,),NO] (R = Me, Et) (made by passing NO through CHC1, solutions of [Ru(S,CNR,),]), show hindered rotation about the C=N bond in the two bidentate sulfur ligands but free rotation at ambient temperature in the monodentate ligand; there is no exchange between mono- and bi-dentate ligands at room t e m p e r a t ~ r e . ' ~ ~ ~ - ~ ~ ' ~ The complexes tuans-[RuCl(S,CNR,),NO] (R, = Me, Et, MePh, MeEt) are prepared by reaction of [RuCl,NO] with two equivalents of Na[S,CNR,]. A series of cis-[RuX(S,CNR,),NO] (X = C1, Br, I, NO,, SCN, N,, NCO, F) can be synthesized by treatment of the trans chloro compound with either HX or AgX. Heating some of these cis isomers to 16(r210°C results in quantitative conversion to the trans isomer. Several cationic solvates trans-[Ru(S,CNR,),NO(S)]+ (S = H,O, MeOH) were also isolated and 'H and I3C NMR studies on all these compounds indicate stereochemical rigidity at ambient temperat~re.'~~' The binuclear [Ru,N(S,CNEt2),]C1 has been prepared by reaction of K, [RuzNCI,(OH,),] with Na[S,CNEt,]. It is a non-electrolyte in acetone solution and may be polymeric in the solid state with - - -Ru-C1-Ru-N-Ru-C1- - chains.65 The yellow crystalline 'Ru(S,CNR,),CO' (R = Me, Et) and [Ru(S,CNR,),(CO),] (R = Me, PhCH,) were prepared by reaction of ethanolic 'RuCOCl' solutions with Na[S,CNR,] or tetrasubstituted thiuramdi~ulfide.'~~ The latter (R = Me) is also prepared by treating [RU~(CO)~,] with tetramethylthiuramdisulfide or by the prolonged reaction of [RuCI,(CO)~(PPh,),] with Na[S2CNMe,]."98 The related thioxanthate complex [Ru(S,CSMe),(CO),] is formed by addition of CS, to THF solutions of Na,[Ru(CO),], followed by reaction with methyl Prior ageing of the 'RuCOCI' solution and treatment with tetrabenzylthiuramdisulfide gives [Ru S,CN(CH,= 1.8 EM)."' However, X-ray analyses by Raston and reveal that Ph)2)2(CO),]C1krn 'Ru(S,CNEt,),CO has the dimeric structure (345) in the solid state. Compound (345) is also formed, together with some cis-[Ru(S,CNEt,),(CO),] by reaction of P-[Ru~(S,CNE~,)~] with CO Slow electrochemical oxidation of (345) affords the mixed valence Ru,"~"' under UV irradiation.2210 cation, [Ru,(S,CNEt,),(CO),]+ (346), isolated in high yield as its BF; salt. The most important feature of this structure is that the CO groups are now both coordinated to the same Ru atom, which indicates that the oxidation reaction has resulted in an unusual CO rearrangement. The Ru-Ru Another interesting Ru" trinudistance of 3.6 14(1) 8, indicates no significant Ru-Ru bonding.2219 clear complex (347) was isolated in minute quantities by prolonged reaction of methanolic solutions of 'RuCOCl' with the ester MeSC(S)NEt2.2220
co
(346)
(345)
(347)
Treatment of (345) (R, Me,, Et, or MePh) with bis(perfluoromethy1)-1,2-dithietene in boiling THF gives violet, diamagnetic crystals of [Ru(S~CNR,),(S,C~(CF~)~)] which were characterized by 'H NMR and IR spectroscopy.2221Some [Ru(S,CNMe,),diene] (diene = nbd, cod) have been prepared by reaction of [RuX,diene], with Wa[S,CNMe2].2222For the preparation of pentamethylenedithiocarbamate and related ligands, see references 2223-2225. E
C.O.C. a
436
Ruthenium
A range of complexes containing dithiocarbamate or dithiocarbonate groups and P donor ligands are described in Section 45.5.4.8.ii.
45.7.5
Dithio-phosphinato and -phosphate Complexes
The violet complex [Ru(S,PPh,),] can be prepared by the reaction of [RuCl, (AsPh,),MeOH] (or 'RuCl,.xH,O) with Na[S,PPh,] in acetone.'697Its NMR'697and ESR spectram have been reported. The black compound [Ru(S,PEt,),] hf= 1.93 BM) is mentioned but no preparative details are = 1.7 BM), (from 'RuCl,*xH,O' and given.2226The analogous [Ru{S2P(4-ClC6H,),},] NalS, PR,] in water)2227 and [Ru{S,P(OEt), >J(from K,[RuCl,(OH,)] and NH4[SZP(OEt)2]),2228 are also known. The reaction between [Ru,(CO),,] and Ph2PS,H gives C~~-[RU(S~PP~~)~(CO),]."~* Various Ru" complexes containing P or As donor ligands as well as [Ru(S,PMe,),(diene)] (diene = cod, nbd)lsooare described in Section 45.5.4.8.ii.
45.7.6
Dithiolene, Dithiocarboxylate and Related Complexes
Apart from early reports of the preparation of [Ru(S,C,Ph,),] (originally formulated as [ R u ( S ,C ,P ~ , ) , ] ) , ~and ~ ~ ~the synthesisz29 and ESR spectrum of the [Ru(rnnt),l3- anion,b04the chemistry of ruthenium dithiolenes is relatively unexplored. Other examples of dithiolenes and related complexes include (AsPh,), [Ru{S,C=C(CN),},] (from [RuCl,]'- and [S,C=C(CN),]2- in [RuL,] (L = S2CCH2Ph,2231 SzCPh,2232 S2CC6H,-4Me,2232 2-hydroxybenzenedithiocarb0xylate,2~~~ 8-quinolinedithiocarboxylate),2234(Ph, As), [Ru(dto),] {dto = dithiooxalate)m and various Ru" and Ru"' quinazoline-2,4(1H,SH)-dithione (QH) complexes such as [RuCl, (Q)(QH)py,], [RuCI, (Q)(QH)(OH,),], [ R U ( Q ) ~ ( ~ Y ) The , ] ? ~ESR ~ ~ spectra of several of these compounds are analysed in detai1.m,2230Ruthenium(II1) complexes with the following ligands have also been reported piperazinedithiocarboxylate, piperazinebi~(dithiocarboxylate),2~~~1,2,4-triazoline-5thione derivatives.2237 As discussed elsewhere (Section 45.5.4.8.iii), reaction of [Ru,(CO),,] with S,C,(CF,), in boiling n-heptane gives an impure orange coloured carbonyl adduct which reacts with various P and As donor ligands to give clean products. In boiling n-decane, [Ru3(CO),,]and S2C2(CF3),produces an impure green material, believed to be [Ru{S,C,(CF,),},], which also reacts with P and As donor ligands.'sosReaction of the reduced 'ruthenium blue' solution with toluene-3,4-dithiol (L'H,) gives dark brown, polymeric [Ru,L,] (,uCR = 0.6 EM)."' with Reaction of [RuCl,(CO),THF] with Li(Sc,H,SMe-o] yields C~T-[RU(M~SC,H,S)~(CO)~]; Li[l,2-S,C,H4], ~ ~ ~ - [ R U ( C , H , S ~ ) ( CisOobtained ) ~ ] ~ ~ and isolated as its NMe: salt. Treatment of the latter with 1,2-C,H4Br, gves cis-[Ru(dttd)(CO),] (H,dttd = 2,3,8,9-dibenzo-1,4,7,10-tetrathiadecane).I7O9Further reactions of these compounds with PR, ligands are discussed in Section 45.5.4.8.iii. Dithiocarboxylate complexes containing P donor ligands are covered in Section 45.5.4.8 .iv.
45.7.7
Dithio- and Monothio-/&diketonateComplexes
There is a review on transition metal dithio-b-diketonate complexes2238 but very little work has been published on those of ruthenium. Treatment of [RuCl3NO(OHJ] and acetylacetone with H2S in a saturated HCl/EtOH solution affords the red-brown [RuCl(sacsac),NO] for which a truns structure (348)was proposed on the basis of 'H NMR spectroscopy. Polarographic studies in acetone/Et,N[ClO,] reveal two reversible one electron reductions at - 0.27V and - l.44V,1305 whereas [Ru(sacsac),], (made as above starting from 'RuC1, *xH,O), undergoes a one electron reduction at + 0.04 V (relative to Ag/AgCl). The magnitude and reversible nature of these reduction potentials suggest that the isolation of sacsac compounds in unusually low formal oxidation states should be possible.1306~'307~2055 The ESR spectrum of [Ru(sacsac),] is consistent with low spin Ru"' ( t Z g 5 )with a large low symmetry d i ~ t o r t i o n ~ "and , ~ ~its ~ ~pees of 1.72BM at 290K supports this conclusion. Detailed electronic spectra (35 OO(L10 000 cm-' ) are reported and some tentative assignments proposed.u39 The related U-ethylthioacetothioacetate ruthenium(II1) complex [Ru(EtOsacsac),] (349) is prepared by direct reaction of 'RuCl,*xH,O', EtOsacsacH and Na[OCOMe] in EtOH. The dark green solid is thermally unstable above 60°C.2240
43 7
Ruthenium
A series of papers224'-2244 have appeared on the preparation and physicochemical properties of the monothio-&diketonates [Ru(RC(S)=CHCOCF,),] (R = Ph, p-MeC,H,, m-MeC,H,, 2-thienyl, b-naphthyl, Pr', rn-ClC,H,, m-BrC,H,) and [Ru(RC(S)XC(O)Ph),] (X = CH, R = Ph, NHPh, NBu,; X = N, R = piperidino, NBu,).'~~'" Information given includes mass spectra (molecular ion peak observed), dipole moments (which suggest a facial octahedral configuration (350) in which all S atoms are mutually cis), and magnetic data over the temperature range 83-293 K. The compounds are all low spin Ru"' with magnetic moments in the range 1.68-1.84 BM (at 293 K). They obey the Curie-Weiss Law with large negative values for the Weiss constant. Brown, diamagnetic [Ru(PhC(S)CHC(O)Ph)], was reported as the product from the reaction between K2[RUCl,] and PhC(S)CHC(O)Ph but it was very poorly ~haracterized."~'~
(348)
(349)
(350)
Some monothio-carboxylate, -carbonate and -carbamate complexes have also been synthesized but these all contain P donor ligands (see Section 45.5.4.8.v).
45.7.8
Thiourea Complexes
The reaction of thiourea with Ru"' ions is of analytical importance and much work has been done towards developing a reliable method of analysis.2z45 Although earlier studies2246 suggested that only two thiourea complexes were present in HC10, solution, namely [Ru(tu)]'+ and [Ru(tu),], Youmans has shown that reaction of aqueous HCl solutions of 'RuC1,.xH20' with an excess of thiourea at 100°C gives [Ru(tu),I3+ isolated as its wgI4l2- salt. Magnetic measurements (per= 2.0 BM at 300 K) support the Ru"' formulation and IR studies indicate Ru-S b~nding."~' More recently:'& [RuCl(tu),]Cl, has been isolated as have a number of complexes containing substituted thioureas such as diphenylthiourea, allylthiourea, naphthylthiourea eic. For other (mainly Russian) references to studies of the interaction of thiourea and substituted thioureas with ruthenium(III), see reference 3, p. 212. For ammine complexes containing SOz,e.g. [RU(NH,),(SO,)(OH,)]~' [RuCl(NH,),S02]C1 , etc., see Section 45.4.1.2.iii.c.
I
45.7.9 Sulfoxide Complexes These are covered in this section because most ruthenium sulfoxide complexes contain predominantly S-bonded groups. Two reviews on transition metal sulfoxides have been p ~ b l i s h e d . ~ ~ ~ ~ , ~ ~ ~ ~ Yellow [RuX,(DMSO),] (X = C1, Br) were first prepared by the reduction of 'RuCl,-xH,O' (or Later studies revealed that the same compounds are formed RuBr,) in DMSO at 80°C with H2.2251 in quantitative yield by heating the halides for a few minutes with DMSO under reflux.", The bromo compound is also formed by prolonged shaking of [RuBr, (AsPh,),MeOH] with The O ) ~obtained ] in analogous fashion. deuterated products [ R u X ~ ( ~ ~ - D M S are The IR,'12,U51 1H"2,225' and 13C2253 NMR spectra of yellow [RuCl2(DMSO)J suggest the presence of both S- and 0-bonded DMSO groups and X-ray analysis22s4 confirms structure (351) with a facial arrangement of S-bonded ligands. A less stable, red isomer of [RuCI,(DMSO)~]has also been described and it was concluded on the basis of IR evidence that it should be formulated as [Ru(OSMe2),(SOMeZ),-,Clz] (n > 2).'Ig6 For [RuBr,(DMSO),] however, spectroscopic evidence indicates exclusively S-bonded 697.2252 and an X-ray confirms structure (352), with a trans arrangement. IR measurements suggest that [RuI, (DMSO),] has a similar structure.2252 This difference in structure between the chloro and bromo complexes helps to rationalize the fact that the latter is a much more active sulfide oxidation ~ a t a l y s tCo .~ mpounds ~ ~ ~ ~of~tetramethylene ~~ sulfoxide of type [RuX~(TMSO)~] and the thermal decomposition of (X = C1, Br, I) with only S-bonded groups are known,2258*225y [RuCl,(DMSO),] has been studied by tga and dta techniques.2186 Hydrolysis of [RuC12(DMS0),] can be monitored by 'H NMR spectroscopy and various aqua complexes
43 8
Ruthenium
Some reactions of [RuC~,(DMSO)~] in which either the chloride or some or all of the DMSO groups are replaced are shown in Scheme 69.”’ Similar studies have been reported using trans[RuB~,(DMSO),].~~~”” Note that in the absence of DMSO, adenine and cytosine are claimed to react with [RuCI,(DMSO),] to give the binuclear species [Ru, Cl,(ade), (DMSO),] and [Ru2Cl,(cyt), (DMSO)z(MeOH)4]respectively.226’ A paper has revealed that reaction of [RuCl,(DMSO),j with various anionic bidentate ligands such as o-HO, CC6H,NHCOMe (AABH) etc. provides a convenient route to [Ru(AAB), (DMSO),]. With uninegative terdentate ligands, (L), complexes of formula [Ru(L),] are formed.2262
[ R ~ [ a d e ) ~ ( D M S 0 C12 )~1
[ RuCI,(L-L)(DMSO)~]
i, NO; ii, CO; iii, L-L = 2,2‘-bipy, 1,lO-phen or 2-aminopyridine; iv, PPh,; v, 2-mercaptobenzothiazole (mbtH); vi, Na[S2CNEt2]; vii, SnC1,; viii, py; ix, adenine/DMSO 2260 Scheme 69
In the presence of Ag+ in D,O, [RuCl,(DMSO),] gives ~~C-~RU(DMSO),(OD,),]~+ and in DMSO, this provides a route to the [Ru(DMSO)J2+ cation, with counterion C10,-”2 or BF,- .2263 Equation (1 37) shows another route to this cation. The diene/DMSO cation is made by reaction of [Ru(N2H,),~odl(BPh,)~with DMSO in acetone. IR,65032263 ‘H6’0~22”.2’63 and 13C2253 NMR studies on this hexakis cation suggest both S- and 0-bonded DMSO groups which is confirmed by crystallographic evidence (structure 353).2263 Hydrolysis of (353) leads to stepwise displacement of Reaction of 0-bonded DMSO to form [Ru(DMSO),-, (OD,),I2+ (n = 1-3) (353) (BF,- salt) with adenine in DMSO gives [Ru(ade),(DMSO),](BF,),.u60” Treatment of [RuCl,(DMSO),] with stoichiometric amounts of Ag[BF,] in DMSO generates [RuCl(OSMe2)?(SOMe2),]+.2252 Reduction of ‘RuCl,.xH,O’ with H, at 80°C in the presence of a stoichiometric amount of DMSO in DMA solution affords yellow NMe,H, [RuCl,(DMSO)J, the cation being a reduction product of the solvent. The deuterated analogue is readily prepared and X-ray analysis shows the fac S-bonded structure (354).2264 In water loss of chloride produces some [RuCI,(DMSO), (OH,)] and treatment with Ag+ /D,O forms [RU(DMSO)~(OD,)~]~+ .2252,2263 With adenine in DMSO, [RuC1, (ade)(DMSO),] is produced.”% [Ru(DMSO),cod](BPh,),
(353)
[RU(DMSO)~](BP~,)~~~~
(354)
( 137)
439
Ruthenium
Complexes (351), (353) and (354) will catalyse the hydrogenation (DMA, 60"C, latm) of acrylamide to 1- a m i n ~ p r o p a n e ~but ' ~ ~are , ~ inactive ~ ~ ~ for the hydrogenation of alkenes.2263Compounds (351)and (354)catalyse the hydrogenation of methyl vinyl ketone to methyl ethyl ketone and the reduction of unsaturated carboxylic acids."6s Neutral, monomeric ruthenium(I1) complexes containing chiral sulfoxide ligands have been prepared by sulfoxide exchange reactions with [RuCl, (DMSO),], and these show some catalytic activity for the hydrogenation of prochiral unsaturated carboxylic acid substrates.22" With bulkier sulfoxide ligands (L), trimeric complexes [RuCI, L,], are formed and racemic methyl phenyl sulfoxide (mpso) affords polymeric [RuCl,(mps0),]~.2~~~*'~~~ Other monomeric Ru" complexes containing DMSO ligands include S-bonded [Ru(NH,),(DMSO)]'+ (from [RU(NH,),N,]~+and DMS0),244A418 [RuCl,NO(DMSO),] (from 'RuCl,.xH,O', DMSO and NO),''* [RuClX(DMSO)(q-C,H,)] (X = C1, H).2250References to the preparation of DMSO compounds containing P or As donor ligands are given in Section 45.5.4.8.vi. The binuclear O-bonded [ R U B ~ , N O ( O S E ~(355) ~ ) ] ~is formed by photochemically-induced aerial oxidation of [RuBr, NO(Et,S),] in CHC13,13U' whereas heating moist toluene solutions of [RuCl, (DMSO),] affords [Ru~C~,(DMSO),].''~~ Detailed IR and NMR ('H and I3C) studies establish the triple chloride-bridged structure (356)containing exclusively S-bonded DMSO groups. In contrast to [RuCl,(DMSO),], (356)undergoes a reversible one electron oxidation to generate in situ the mixed valence cation [Ru2Cl4(DMS0),]+.2253 NO
OSEt,
&Et2
NO
0 II
0
Relatively little evidence exists for monomeric Ru"' sulfoxide complexes. Thus [RuCl, (DMSO),] was prepared by the reaction between DMSO and the reduced 'ruthenium blue' solutions676but later reports'I2 indicate that it is unrepeatable. However, direct reaction of 'RuX,-xH, 0' with di-n-propyl or di-n-butyl sulfoxide (L) is claimed to give [RuX,L,] (X = C1, Br) and IR data suggest the presence Other Ru"' sulfoxides include [Ru(DMSO),]X, (X = C1, ClO,), of both 0-and S-bonded ligands.2258 [Ru(DMSO),CI]Cl2and MIRUCI,(DMSO)~](M = Bu,N+, Na'). The first two contain both 0and S-bonded DMSO but the latter only S - b ~ n d e d . " ~ ~ , "Reaction ~~ of [RuBr, (AsPh,),MeOH] with DMSO/H,O (1: 1 v/v) gives O-bonded [RuBr,(AsPh,),DMSO]; in neat DM SO, [RuBr, (DMSO),] is formed.1697
45.8 SELENIUM AND TELLURIUM LIGANDS In contrast to sulfur (Section 45.7), relatively little has been published on ruthenium complexes containing selenium and tellurium ligands. Ruse, and RuTe, are normally prepared from the elements at high temperature or by heating RuCI, with Se or Te.' These have the pyrite s t r u c t ~ r e . ~Other ~ ~ * measurements *~~~~ on these compounds include TR spectra, force constant and normal coordinate calculations?'62a lzsTeMossbauer spectrum2269 and the determination of the high temperature enthalpy and decomposition pressure of R U S ~ , .A* low ~ ~ ~temperature modification of RuTe, of the marcasite type has been prepared. At Rh, -xRu,Se, (x < 0.45) is 620°C, this transforms monotropically into the pyrite type known and exhibits s u p e r c on d ~ c t i vi t y . ~ ~ ~ ~ Reaction of [Ru,(CO),,] with selenium or tellurium under a CO/H, atmosphere in refluxing octane gives [Ru,H,(~,-E)(CO)~](E = Se, Te) (see (332) Section 45.7.1) and [Ru,Se,(CO),] (see (333),Section 45.7.1).2'66Low yields of (332)are also obtained from reaction of [RU,(CO)~,]with The interesting binuclear complex [(NHJ5 Ruse, Ru(NH,),]Cl, has EO, and alcoholic KOH.2165 recently been synthesized.2273 The yellow binuclear compounds [Ru(EPh)(CO),], (E = Se, Te) are minor products from reactions between [Ru3(C0),,] and Ph,E,. These react further with Ph,E, to give two forms of orange, polymeric [Ru(EPh),(CO),], (n = 6-7 and 12-14) as major products. The seleno compound [Ru(SePh)(CO),], may exist as two isomers, but only one, probably with the anti conformation was isolated.2274Irradiation of [RU(CO),(~&H~)]~ with Ph,Se2 gives [Ru(SePh)(CO),(r]-C,H,)] and
440
Ruthenium
[Ru(p-SePh)(CO)(q-C,H,)], (reference 1, p. 781). [RuL4(MeCN)] and [RuL4(C0)](LH = 2,3,5,6(CH,),C,HSeH or 2,4,6-(Pr'),C,H,SeH) have been prepared.2170b A number of seleno- and telluro- ether complexes are known. For example, treatment of [RuI,(CO),],, with various selenides or tellurides affords [RuI,(CO), L2] and/or [RuI,(CO)L,] [L = Et2Se, Ph,Se, Bu2Te, Ph,Te]. Isomeric mixtures are frequently formed and separated by fractional crystallization. The chloro and bromo analogues can be prepared by treatment of the iodo complex with C1, or Br, re~pectively.'~'',~'~' Similarly, Ph,PSe gives IRuI,(CO),(SePPh,),] which decomposes in DMF to produce [Ru,I,(CO)(SePPh,),DMF]. [RuI,(CO),PhSe(CH,),SePh]is formed unexpectedly from [RuI,(CO),],, Ph,Se,, EtOH and C,H, at 150°C. The ethano bridge is presumably generated by catalytic dehydration of EtOH.I6" On the basis of IH NMR and molecular weight measurements, the complex of empirical formula [RuCl,(Pr'Se(CH,),SePr')], is proposed to have the dimeric structure (357j.227s
(357)
Complexes of type [RuX,NOL,] (L = Et,Se, EtPhSe) are readily prepared13" and exhibit facile inversion at selenium.'303Ammine cations such as [Ru(NH,),EMeJZt, [Ru(NH,),(OH,)(EMq)]'+ (E = Se, Te),433[ R u ( S ~ P ~ ) ( N H , ) , ] *are + ~also ~ ~ known (cf. SR2 analogues in Section 45.4.1.2.i~). The only diselenocarbamates of ruthenium mentioned in the literature are [Ru(Se,CNEt,),] (from Et,NCSe,H and N ~ , [ R u C ~ , ] ) ,and ~ * ~[ ~ R u ( S ~ , C N M ~ , ) , N O ] .Selenourea ~~'~ is reported to form a blue-green complex with Ru'" solutions in 6 M HC12277 and interactions between selenourea and Ru"' have been investigated.2278 Other seIenium-containing complexes include the benzeneseleninato compounds [Ru(RC, H, SeO,),] and [Ru(RC,H,SeO,),X], (R = H; p - and m-Cl, Br; p-Me; X = C1, Br), whose IR spectra indicate 0,O'-seleninato coordination. The tris-complexes are octahedral and the bis are polymeric, octahedral with bridging halides.2279.2280 Reaction of [Ru(CO), (PPh,),] with (SeCN), affords
[RU(NCS~),(CO),(PP~,),].~~~' Selenocarbonyl complexes and related species are discussed in Sections 45.5.4.5.ii and 45.5.4.5.iii.
45.9
HALOGEN LIGANDS
This section covers the chemistry of pure halides, aqua-, hydroxo-, oxo- and carbonyl halides. For nitrido and nitrosyl halides see Sections 45.4.6 and 45.4.4.4 respectively. Unlike most other sections' of this article, much of the published work on ruthenium halides is pre- 1970 and excellent, detailed have appeared in the literature. Therefore in order to conserve space and avoid undue repetition, only a relatively brief account containing earlier key references and more recent data is presented here.
45.9.1
Ruthenium(1) Halides
Although reactions of hypophosphorous acid with aqueous solutions of RuX, (X = C1, Br, I) are claimed to give RuX in further work in this area is now required. More recently, Ru' chloro compounds of possible formula H[Ru,Cl,(DMA),] have been isolated from reaction of 'RuCl,.xH,O' with H, in DMA; these species are effective catalysts for hydrogenation of alkenes and unsaturated carboxylic acids.22s3
45.9.2
Ruthenium(1) Carbonyl Halides
Brief reports on the preparation and properties of colourless [RuBr(CO)],, (from RuBr, and CO at 350atm and 180"C),2284and orange [RuI(CO),],, (from RuCI, or [RuI,(CO),], with CO or [Ru(CO,)] and I,),''1o have appeared in the literature. A reinvestigation of these compounds would be of considerable interest.
Ruthenium 45.9.3
44 l
RutheniumflI) Halides
Although there are a number of reports of the preparation of RuCI, in the early literature, it appears that this species has never been prepared in a pure state. In fact the reported method of preparation is similar to that established for P-RuCl, (see Section 45.9.6.1)22s5and incomplete chlorination with consequent contamination of RuCl, by ruthenium metal was probably responsible for the erroneous claims. However, residues with Ru:C1 ratios of ca. 1:2 have been isolated from the reduced 'ruthenium blue' solution (see below); a black solid with a Ru:Br ratio of ca. 1:2 is obtained from the corresponding violet-blue RU" bromide No RuX, (X = F, I) are known. Chemical or eiectrochemical reduction of aqueous or alcoholic solutions of 'RuCl, *xH,O', K,[RuCl,] or K4[Ru20CI,,] produce the well-known reduced 'ruthenium blue' solutions which are both useful synthetic precursors for many rutheniurn(1T) and (ITT) and active catalysts for alkene hydrogenation and isomerization.2283 However in spite of 180 years of effort the composition of these blue solutions still remains uncertain. For an excellent and detailed account of this fascinating but extremely complicated topic, see reference 3, p. 342. In brief, a survey of the experimental data clearly reveals that there are several species present in solution, in an equilibrium whose position depends on the pH and chloride ion concentration of the solution. Earlier workers suggested that an important component of these solutions was [RUC~,]~and various solids purporting to contain this anion were isolated by addition of cations such as Cs' , pyH+, enH,*+ etc. Later workersUgshave proposed a trimeric structure (358) for this anion. Rose and Wilkinson,2288by treatment of the methanolic blue solutions with various cations, have trapped out stable solids of composition M[Ru,ClIz] (M = Fe(bipy)32+, Ru(bipy),'+, [C6H4(CH,PPh,),lz+ and proposed structure (359). The ESR spectra of these materials are essentially identical to those of the blue solutions in various solvents, so it is reasonable to assume that the ion present in the solids is also present in the blue solutions. However, the ESR spectrum and = 1.3 BM per five ruthenium atoms) are not consistent with what magnetic properties of (359) (peE might be expected to be a diamagnetic cluster. Furthermore, the blue solutions obtained by electrolytic reduction of K2[RuC15(OH2)]in 0.01 M HC1 and which can be separated into the binuclear blue components [Ru2C1,I2+,[Ru, C14]+,[Ru,Cl,], (and unidentified anionic species), by It has been suggested that ion exchange techniques at 0°C have the same ESR spectra as (359).2289 these RuZII.'' mixed valence complexes are isostructural with the ammines [(NH,), Ru(pC1)3R~(NH3)3]2+ (see Section 45.4.1.3.iv), e.g. [(H,0),RuC1,Ru(H,0),]2+ etc. Clearly more work is now required to resolve these differences of structural formulation. Upon saturation with HC1 gas, the blue methanolic solutions turn green and addition of a cation gives green solids of empirical formula MIRuCI,] (M = pyH+, Et4NC)for which the tetrameric structure (360) has been proposed.22ss Interestingly, reaction between 'RuCl,*xH,O' and *2CHC13,which conNa[S,CNPr;] gives as one of the minor products, [RU~(S~CNP~;),]~[RU,C~,] tains the only crystallographically characterized chloro complex (361) of RU".~*''Clearly this dimeric formulation must be considered as a possibility for Rose and Wilkinson's green complexes M,[RuC~,],.~*~ The claim of brown trimeric anions [Ru, ClgI3- (from reaction of aqueous solutions of 'RuCl,.xH, 0' with H,02)2290 has been withdrawn; they are now reformulated as the binuclear Ru'" anions [Ru,OC&(OH,)]~- (reference 5, p. 97).
Ruthenium
442
Dissolution of [Ru,(OCOMe),C1] in 12 M HCl also gives a deep blue solution and addition of [Et4NlC1and solvent evaporation leads to the slow precipitation of the deep green, mixed valence [H, O,],[Et4N],[Ru, Cl12](362) which has been characterized It is believed that this complex is derived from the components of the ruthenium blue solution by trace amounts of aerial oxidation. The ground state electronic structure and electronic spectra of (362) have been calculated by the SCF-Xa-SW method.2291 Violet-blue solutions are obtained from chemical reduction of RuBr, in ethanolic or HBr/H,O media.2287*u92 These give similar electronidzy2and ESR spectra2288 to their chloride analogues. Blue solutions are also generated by reduction of green ethanolic solutions of R U I ~ . ~ ~ "
45.9.4
Rutbenium(I1) Aquahalide Complexes
Polarographic half-wave potentiah for the reduction of various monomeric aquachlororuthenium(II1) complexes [RUC"(OH,),_,]'3-"'' (0 d n d 3) have been reported. The various Ru" products then undergo rapid aquation to give [RU(H,0)6]2+(see Section 45.6.1). A cyclic voltammetric study on [RuCl(OH,),]+ reveals that at high sweep rates, only the oxidation wave due to [RuCl(OH,),]+ is observed; at lower rates, a wave due to [Ru(H,O),]~+ grows in. The [RuCl,(OH,),]- anion aquates too rapidly to be studied by cyclic voltammetry but the rates of aquation of cis- and rruns-[RuCl,(OH,),] were calculated as 0.13 and 0.14 s- ' respectively.2293
459.5 45.9.5.1
Rutheniurn(I1) Carbonyl Halides Carbonyl fluorides
Reaction of [RuF,(CO),*RuF,], (see Section 45.9.8) with CO (100 atm/20OoC) gives pale yellow [RuF,(CO),], of structure (363). Compound (363) is also formed as shown in equation (138). The mass spectrum of (363) has been analysed in terms of an initial fragmentation to yield [Ru,F,(CO),]+ and [Ru,F,(CO),]+. It reacts with HCl to give [RuCl,(CO),], and with an excess of XeF, to formfac-[RuF,(C0)3].2294,2295
co
45.9.5.2
Neutral carbonyl chlorides, bromides and iodides
For detailed references, see the excellent accounts of this area in references 1 and 3. The first report of ruthenium(I1) carbonyl halides was in 1924 when polymeric [RUX,(CO),]~were prepared by passing CO over RuX, (X = C1, Br, I) at ea. 270°C.2296Other routes to these compounds include reaction of [Ru,(CO),,] and halogens in inert solvents above 140"C,237evaporation of aqueous HX solutions containing [ R uX , ( C O ) , ] * - , ' ~ ' ~ , thermal ~~~ decomposition of [RuX,(CO),] (or [Ru2X,(CO),]) at 200°C in vacuo,2297or carbonylation of [ R ~ C l ~ ( c h t ) ] ,IR . ' ~data ~ ~ show two vco bands, indicating a cis-(CO)2 arrangement in a 'kinked' structure (364).2299 Other neutral Ru" carbonyl halides are of the type cis-[RuX2(C0),], [RuX,(CO),], and [Ru,X,(CO)~~].The best route to cis-[RuI,(CO),] is shown in equation (139).2300Treatment of
Ru rhenium
I
443
[RuBr,(CO),], with CO (8&90 atm/20-l2O0C) gives pure cis-[RuBr,(CO),]; the chloride reacts For other preparations see similarly but rapidly reforms [RuCl, (CO),], in the absence of C0.6s1 reference 1, p. 672. X-Ray (for X = I)?301IR, "C NMR and mass spectral data are a ~ a i l a b l e . * ~ ~ * * ~ ~ ~ [Ru,(C0),21
+
I2
COiC6H6 IOO'C:.IOatmj13h'
cis-[RuI2(CO),I (goo/,)
(139)
Carbonylation of 'RuC1,-xH, 0' in methanol (6OCC/10atm/24 h) and subsequent evaporation of with CHX, in the presence solvents is the best route to [RuC~,(CO)~]~.~~".' Treatment of [RU,(CO),~] of ethanol also affords [RuX,(CO),], (X = CI, Br)653and oxidation of [Ru(CO),(g4-C,Hs)] with I, gives [RUT,(CO),],.~~"~ Other routes include thermal decomposition of ci.~-[RuX,(C0),]~~~ and carbonylation of [RuCI,(nbd)], or [RuCl,(C, Hh)J2in a I c o h o l ~ . ' Detailed ~~" mass spectral data have been recorded.2302 The structure (365) of [RuX,(CO),], (X = CI, Br)'"' is consistent with the IR data obtained in pure CHCI,. Discrepancies between authors in the earlier IR solution spectral results in CHCI, are ascribed to the presence of ethanol stabilizer producing solvated monomers such as L~] [RuCI,(CO),(E~OH)].~~~ Reactions with Lewis bases to give [RuX,(CO), L] and [ R U X ~ ( C O ) ~are covered in other sections. Reaction with SbCI, and HCI in CH,Cl, gives E R U C ~ ~ ( C O ) , ] ~ S ~ C ~ , . ~ ~ ~ ~ At room temperature [RuX,(CO),] rapidly trimerize to form [Ru,X,(CO)~~] which are proposed to have structure (366). The chloride is most conveniently prepared by direct chlorination of [RU,(CO)~,]in refluxing CHC1,. In solution, they readily lose CO to give [RuX2(C0)3]2.22y7s2307
45.9.5.3
Anionic carboayl halides
Reaction of 'RuCl,.xH,O' with HC1/HC02H (1:l mixtures), under reflux produces a series of striking colour changes (deep red to green to orange to yellow), over a period of 30h.22y8,230* Treatment of the solution at the appropriate time with CsCl led to isolation of red Cs,[RuCl,CO], green Cs,[truns-RuC~,CO(OH,)](confirmed by X-ray analysis)?309orange Cs,[cis-RuCl,(CO),] and yellow Cs[fbc-RuCl,(CO),] salts respectively. Evaporation of the final solution gave [RuCI,(CO),],. Less convenient routes to these anions include carbonylation and subsequent reduction by H2 of 'RuCl, *xH,O'/H2OiHCl solutions, carbonylation of reduced 'ruthenium blue' solutions and addiand [RuCI,(CO)~]tion of chloride to [RuCI,(CO),], and [RuCl,(CO),],, (giving [RUCI,(CO)~]~respe~tively).'~'~,~~~~ Analogous bromo and iodo anions are formed by reaction of 'RuX,-xH,O' with HX/ HCO, ~ 2 2 9 8 , 2 3 0 8or, for Cs,[RuX,(CO),], by treatment of Cs,[RuC~(CO),]with HX. The red 'RuCOC1' solution exchanges with iodide to give a red-brown solution from which dark brown [Et4N],[ R U ~ I ~ ( C O(with ) ~ ] ch Ru(CO), groups) is obtained by addition of [Et4NlC1.23"For schemes and further details see reference 1, p. 670. For reactions with Lewis bases see Sections 45.4.3.15 and 45.5.4.5.i.
45.9.6
Ruthenium(1III) Halides
45.9.6.1 Pure neutral halides Ruthenium(II1) fluoride can be prepared by the reduction of rutheniumw) fluoride with either iodine or sulfur at elevated temperature^^^" or in an impure form by reduction of [RuF5], with ruthenium It is a dark brown powder shown by X-ray powder diffraction studies to consist Neutron diffraction of RuF, corner shared octahedra, (with Ru----Ru separations of 3.37 A).2314 studies reveal no evidence of magnetic ordering even at 4.2 After considerable confusion and effort (see reference 3, p. 156 for a comprehensive review of the earlier literature), it was shown by Fletcher and co-workersZ31hthat two modifications of water COC
+-o*
'
Ruthenium
444
insoluble RuC1, (namely a and 0) could be prepared in a pure state. The dark brown B-form is best obtained by heating ruthenium sponge at 330-340°C in a 1:3 mixture of CO and C12and the black a-form by heating the B-form in a current of chlorine to 700°C. The /? + a transition occurs at 4 5 0 T The a-form is isostructural with violet CrC1, and a-TiC1, and has a strongly and is irrever~ible.~ defect-ordered layer structure with Ru----Ru = 3.46 8. The 8-form has a distorted octahedral environment of chloride ions, with a Ru----Ru distance of 2.83 A. Detailed magnetic measurements reveal that P-RuCl, is antiferromagnetic k l o w 600 K and shows extremely low values of xm, whereas a-RuCI, has much higher magnetic susceptibilities, e.g. pcf = 2.14 BM at room temperture, and only becomes antiferromagnetic below 13 K.13’7 The optical properties of the latter have been studied in IR, visible and UV regions by reflectance Information on other physicochemical properties is to be found in reference 3. Water soluble hydrated or commercial RuCl, (Le. ‘RuCl,.xH,O’) is usually prepared by the evaporation of a solution of [RuO,] in hydrochloric acid. However, it is important to note that it is a heterogeneous, ill-defined mixture of variable oxidation-state, oxochloro and hydroxochloro monomeric and polymeric ruthenium complexes. Nevertheless, it is by far the most common starting material for the preparation of ruthenium complexes. Note that evaporation of aqueous solutions of ‘RuC1,-xH20 (pH ca. 1.5) several times on a waterbath removes most of the HC1 contaminant and this purified ‘RuCl, - xH 20 ’is much more reactive towards, for example, various organic ligands such as 1,3-~yclohexadienes.~~~~ Commercial ‘RuCl,.xH,O’ is widely used as a catalyst (see references 3 and 2320 for details of earlier studies). Examples include the oxidation of amines and the production of glycol esters from synthesis and the generation of 1,3-diol derivatives from reaction of dienes and alkenes with aldehydes and carboxylic acids.2323 Pure samples of water insoluble, black-brown RuBr, can be prepared by prolonged reaction between ruthenium and bromine at 45&500”C and 20atm.2324X-Ray analysis reveals that the structure is based upon chains of (Ru2Br,)Brt groups with the metal in approximately octahedral coordination with a Ru----Ru separation of 2.73 It exhibits only a very weak temperatureindependent paramagnetism, indicative of strong metal-metal interactions.2326A water soluble version of ‘RuBr,-xH20 prepared from [RuO,] (or ruthenium(I1I) hydroxide) and HBr is also
RuI, can be synthesized either by the metathetical reaction between ‘RuC1,*xH20’and KI in aqueous solution, by the action of hydriodic acid on [ R u O , ] , ~ *or ~ ,by ~ ~thermal ~ decomposition of [RuIWH, ),]I, or C ~ ~ - [ R U I , ( N H , ) , ] IThe .~,~ black ~ ~ powder is sparingly soluble in water with a similar structure to RUB^,.^"' For other properties see reference 3, p. 161.
45.9.6.2
Pure anionic halides
K3[RUF6]can be prepared by fusing RuX, (X = Cl, I) with K[HF2] in the absence of oxygen. It is a brownish-grey powder, isomorphous with K,[RhF,], with pfi = 1.25 BM at room temperature. Its electronic and IR spectra are Virtually all the methods of preparing [RuCl6I3- are a variation on a method developed by Charonnat which involves recrystallizing salts of [RuC~,(OH,)]~-from 12 M hydrochloric A convenient synthesis of K,[RuCl,] is to add the correct molar ratio of KC1 to the methanolic ‘ruthenium blue’ solution, heat under reflux in air and then recrystallize the product from 12M HC1.1220 Reduction of [RuC1,I2- by H22332 and by (generated by pulse radiolysis) has been studied. Extensive physicochemical studies on this anion with a variety of cations include X-ra analysis (for [A1(H,O)J3+2331 and IPhNH,]” cations2334),electronic and IR s p e ~ t r a ~ ~ ’ ” ~ , ’ ” ’ , ~ ~ ~ ~ ~ ~ ~ magnetic circular magnetic measurements (ca. 2 BM at 300 K),2338 ESR,Z3,’W H M e m and SCF-Xu-SW calculations on the ground state electronic structure2291 and measurement of the kinetics of isotope exchange for the reaction shown in equation (140).2M’ [RuCI,]’-
+
“C1-
e [RuCI,~~CI]~+ C1-
w 9
[RuC&]’- in aqueous HCI solution will activate H, homogeneously and hence catalyse its reduction of Ru’” and Fe’’’.2342 It will also catalyse the isotopic exchange between H 2 0 and D,2343 and the hydration of alkynes, although the active species in the latter process are probably the aquachloro species [RUCI,(OH,)]~- and [RuCl,(OH,),]- .I6’, The preparation and properties of these and related aquahalide Ru”’ complexes are discussed in the next section. There are many reports in the early literature of Complexes of the general formula A4[RuCi7]
Ruiheniurn
445
which are probably the crystal aggregates A3[RuC16]AC1.' Thus, [N(CH2CHzNH3)3]H[RuC1,]2H,O has spectroscopic properties identical to K,[RUCl,] and is formulated as [N(CH,CH,NH,),][RUCI,]HCI.~H,O~~~~ and [PhNH,], [RuCl,] is isomorphous with its bromide analogue, whose crystal structure reveals the formulation [PhNH,], [ R u B ~ , ] B ~Compounds , . ~ ~ ~ ~ of formula A,[RuCl,] are also known but attempts to repeat many of these preparations have resulted only in the isolation of (see Section 45.9.7.3). Nevertheless dehydration of K,[RuCl,(OH,)] or heating K2[RuCl,] in HC1 at 600°C gives K,[RuCl,]. Magnetic susceptibility measure= 1.64 BM at 350 K; 1 * 14 BM at 80 K) are significantly lower than for ments on this material kf the [RuC1,I3- ions (ca. 2BM at 300K),2338which might suggest it should be formulated as a [Ru,C~,,]~-ion?344Thermal stability studies on this anion and other chlororuthenates such as a-RuCl, and K 3[RuCI,] have been reported.2345 on the preparation of the [Ru,Cl,y- anion has been verified. Cs3[Ru2C1,] Earlier Russian can be prepared by solid state reaction at 700°C between stoichiometric proportions of CsCl and = 0.51 BM per RuCI,.'~~'Its X-ray structure (Ru-Ru = 2.725(3)A), magnetic susceptibility bef Ru at 300K),a347redox properties (see equation 141 for [NBu,]+ salt in CH,Cl,; E+ values vs. Ag/AgCl reference electrode)2348 and electronic s p e ~ t r ahave ~ ~been ~ , ~ reported ~ ~ ~ and SCF-Xa-SW calculations performed which establish a 'a';e'4e"4' ground state electronic structure.2291 The analogous K,[Ru2Brs] is readily prepared by reaction of 'RuCl,-xH,O' with a concentrated HBr/ethanol mixture followed by addition of KBr. The magnetic, redox and electronic spectral properties and Ru-Ru internuclear distance (2.86A) of this anion are very similar to those of [Ru,CI,]~- .2335,2348
35e-
33e-
34e-
32e-
Salts of the [RuBr6I3- anion (with counterions such as [PhNH3]+2334and [N(CH,CH,NH,)3]1+ 2338) have been prepared by analogous routes to those of the chloride derivatives and fully characterized by structural and spectroscopic technique^.^ Compounds of formulation A,[RuBr,] (probabiy A,[RuBr,]ABr) and A, [RuBr,] (probably A,[Ru,OBr,,]) are also reported.3
45.9.6.3
Aqua- and hyduoxo-halide complexes
Ion exchange techniques have been used to separate the species present in aqueous solutions of ruthenium trichloride. The species present are [RuCI,(OH,),-,](l-")' (0 < n < 6) and their electronic spectra are available.'ss7*2349-23S' A number of these aquachloro complexes have been isolated and characterized by X-ray analysis. Thus, red M,[RuCl,(OH,)] can be prepared by ethanolic reduction of [RuO,] in HC1 in the presence or by reaction between of MC1 (M = K, Rb, Cs, NH,),2352by reduction of K,[Ru(C,O,)~] and HCl.2353 The bromo derivatives M,[RuBr,(OH,)] are made by exactly analogous The X-ray structure of the potassium2355and caesium chloro salts2356 confirm the octahedral structure with Ru-C1 = 2.31 1 (trans to HzO), 2.353 A (cis to H,O). Detailed magnetic (p&ca. 2 BM at 300 Kj2357 and IR have been reported. Oxidation in HCl/HClO, solutions give dimeric oxo-bridged Ru'" species2359 whereas in 2 M HCI, [RuC1,I2- is formed r e ~ e r s i b l y . ~ ~ ~ ~ ~ ~ ~ ~ The red cis-[RuC1,(OH,),]- anion is prepared (as the Hf salt) by evaporation of a solution of [RuO,], HC1 and or, as its NH,+, Rb+ or Cs+ salts, by reduction of ammonium ruthenate with SnCl, and HC1 in the presence of the appropriate cation.2363 The latter type of route also gives M [ R U B ~, ( O H , ) , ] . Interconversion ~~~ between H[cis-RuCI,(OH,),] and the green trans isomer is possible (equation 142).2362 Solutions of trans-[R~C1,(0H,),]~are also obtained by aerial oxidation of the reduced methanolic blue sohtions and addition of [AsPh4]CI then precipitates The X-ray structure of the red Asgreen AsPh,[trans-RuCl,(OH,),] (v(Ru-C1) 336cm-1).2288 Ph,[cis-RuCl,(OH,),] complex has been reported with Ru-C1 = 2.325 (trans to H,O), 2.355 8,(cis to c1).2365 A(Et0H)
cis-H[RuC1,(OHz),l
trans-H[RuCl, (OH,),
(142)
Orange mer- and fa~-[RuCl,(OH~)~l can be separated by elution at low temperature of aged ruthenium(II1) chloride2350 (or K, [RuCl, solutions through successive cation and anion exchange columns, which removes any charged species. Studies on these solution species include
446
Ruthenium
dipole TR2367 and electronic2350 spectral measurements. The kinetics of interconversion of the two isomers have been determined and it is found that the mer isomer is more labile than the fuc isomers towards aquation, producing cis-[RuCI,(OH,),]+ .2351 Both cis- and tr~ns-[RuCl,(OH,)~]+ cations can be successfully separated from aged solutions of 'RuCl,-xH,O' in 0.3 M HC1 by elution from a cation exchange column.2349 The trans isomer, which is also made by heating blue hydrochloric acid solutions of Ru" under N,, has an ESR spectrum consistent with axial symmetry.22saFor the interconversion cis trans, K = 1.235' The pentaaqua cation [RuC~(OH,),]~+ can be isolated in solution using cation exchange techniques.'887It electronic spectrum'887and rate of aquation (ti > 1 year) have been measured (cf. for [RuC1J3 , aquation to [RUCI,(OH,)]~- takes only a few seconds). Thus, cationic and neutral aquachlororuthenium(TT1) species are relatively substitutionally inert, compared to anionic specie~.~~~' As mentioned earlier (Section 45.9.6.2) aqueous solutions of 'RuCI,*xH,O' catalyse the hydration of alkynes. Detailed mechanistic studies indicate that the catalytic activity is dependent upon the combined concentrations of [RuC~,(OH,)~]-and [RuCl, (OH,)]'The binuclear mixed valence chloro species [Ru,C~,,+~](~-")+ (0 d n d 2) are discussed in Sections 45.9.3. Finally, there are some earlier reports of ill-defined hydroxo- and oxo-halide species such as [Ru, Cl,(OH),], [Ru,OCl,], [RuC1,(OH)(HIO)] and [en,H,],[RuCl,OH] etc. (see reference 2, p. 135).
45.9.6.4 Carbonyl halides
Reaction between [ R u ~ ( C O ) and ~ ~ ]an excess of XeF, in either CFCl,CF,Cl or anhydrous H F at = 1.92 EM), solid [RuF,(CO),]. It is room temperature affords the air sensitive, paramagnetic, (pen also formed by reaction of [RuF,(CO),], with an excess of XeF,. The ESR spectrum in H F solution at 77 K confirms the monomeric, facial structure (367).2295
As discussed in Section 45.9.5.3, reaction of 'RuX,-xH,O' with IX/HCO,H (1:l mixtures) under reflux produces initial deep red solutions containing the [RuX5COIZ-anions (X = C1, Br) which can For X = C1, the same anion is formed by be precipitated (for X = Cl) as its caesium sa1t.2298,2308 carbonylation of aqueous HC1 solutions of 'RUC~,.~H,O','~~,~~'~,~~'~ or treatment of [RuCl,CO(nbd)], with [Ph,(PhCH,)P]Cl/HCl in X-Ray structures of the M2[RuC15CO]salt (M = CS+,'~'~ Ph3(PhCH2)P+236832369) are available. Evaporation of these red solutions yields an impure form of 'RuX,CO' (possibly contaminated with RuX,). No Ru"' iodocarbonyl complex has been isolated owing to its facile reduction but some IR evidence for its existence in solution is reported.229X
45.9.7 Ruthenium(1V) Halides 45.9.7.1
Pure neutral halides
Yellow crystals of RuF, can be synthesized by reaction of [RuF,], with iodine and iodine pentafluoride. It has a room temperature magnetic moment of 3.04BM which drops to 2.51 BM at Reaction with water gives Ru01.2ms2370 88 Ruthenium(1V) chloride has been detected in the vapour phase on heating RuCl, and C1, between 400_X00"C.2371 It is the principal vapour phase species up to 853"C2372 and it is reported that solid RuCl, can be isolated by cooling the above vapour to liquid air temperatures. Above - 3WC, it Note however that Fletcher and C O - W O ~ suggest ~ ~ ~ Sthat ~ the ~ ~ decomposes to RuCl, and C1,.2373,2374 above observations are compatible with oxochlorides such as {RuzOC16]and are not, per se, evidence for the existence of RuCl,. All attempts to prepare RuX, (X = Br, I) have given only the trihalides (see Section 45.9.6.1).
~
Ruthenium 45.9.7.2
447
Pure anwnic halides and related species
K,[RuF,] can be prepared by the action of water on K [ R u F , ] ~and ~ ~has ~ a trigonal structure.2376 An alternative preparation, which gives a cubic structure, is to fuse K,[Ru(NO,),OH(NO)] (or RuC1,) with K[HF,] at 250°C.2376,2377 Other alkali metal salts (Na+, Rb+, Cs') are and also rhombohedral Ba[RuF,] (from RuCl,, BaCl,, F, at 350°C)2379 and [N01,[RuF6] (by prolonged reaction between [NO][RuF,] and an excess of NOF at 150°C).2380 Magnetic IR2382and of solution of K2[RuF,] in water electronic spectral s t ~ d i e have s ~ been ~ ~published. ~ ~ ~ ~The ~ enthalpy ~ at 298 K has been measured and the single ion hydration enthalpy of gaseous [RuF,l2- deterReaction of [RuF,] with PF, in anhydrous hydrogen fluoride gave polymeric [RuF,(PF,)],. The low magnetic susceptibility values (per= 1.81 BM per Ru at 293K) probably arise from strong magnetic exchange in the solid state. Treatment of RuF, with AsF, gives the adduct RuF,-AsF, formulated on the basis of IR evidence as (RuF,),"+ (AsF,),"-.~~*~ Earlier methods of preparation of K,[RuCI,] include the oxidation of K2[RuC15(OH,)] with Cl,, reaction of' K, [RuO,] with cold dilute HCl, recrystallization of K4[Ru,OCl,,] from concentrated HC1 and the reduction of [RuO,] with concentrated HC1 in the presence of KCL3 Unfortunately most of these methods produce some K4[RuzOCIlo]contaminant and, therefore, more elaborate preparations to obtain purer samples have been devised.2360,2386 For example, reduction of K,[Ru,OCl,,] to K2[RuC15(OHz)]with ethanol and then oxidation by C1, to K, [RuCI,]. Other alkali metal and organic cation salts are available. For example, 'RuCl,-xH,O' and [Et4NlC1in thionyl chloride gives [Et4N],[ R u C ~ , ] . ' ~ ~ ~ The potassium salt forms black crystals which have a cubic lattice and a Ru-C1 distance of 2.29(4) h ~ . Extensive ~ ~ ~magnetic ~ ~ measurements ~ ~ ~ 9 (pea ~ cu. 2.8 BM at room temperature), IR, Raman and electronic spectral studies have been published on the [RuClJ- anion (see reference 3, p. 94 and references 2336b and 2389b for details), although much of the earlier work is suspect because of contamination by diamagnetic K4[R~20C1,0]. The results are consistent with a model in which both the spin-orbit coupling constants and the interelectronic coupling factors are close to their free-ion values, although the possibility of contributions from higher excited states cannot be ruled Similar results are obtained for the [RuBr,]'- anion, prepared as its potassium salt, either by oxidation of concentrated aqueous solutions of K,[RuBr,(OH,)] with 8r22391 or by passing Br, through solutions of K,[RuCl,], K, [RuCl,], K,[RuC1,(014,)], K,[Ru,OCl,,] or RuC1, in hydrobromic The NQR spectrum of K,[RuCI,] shows a single resonance frequency over the temperature range 77-298 K2386but on standing its aqueous solutions are slowly reduced and polarographic studies show this involves irreversible reduction to [RuCl5(0Hz)J2-probably via [RuCI6l3-.23w The latter is formed directly by reaction of [RuC161ZPwith H2.2332q2392 H *2333 or Br-2393and the kinetics of this and process have been measured. Heating K,[RuCl,] in vacuo gives K3[RuCl6],a-RuCI, and treatment of Et,N[RuCl,(RCN)] with DMF affords the ERuCl,(DMF)]- anion.681 The binuclear mixed valence anions [ R u ~ ' " , ' ~ X(X~ = ] ~C1, ~ Br) have been electrogenerated from [Ru,X,I3-. Variable temperature magnetic measurements on analyte solutions of these 33 valence electron anions (per= 4.40 BM per molecule at 273 K for X = Br), indicate three unpaired electrons per molecule suggesting a spin quartet electronic ground state. Electrogeneration of the [RU,'~~'~X,]anions is possible but compound instability makes quantitative magnetic measurements Observation of the intervalence charge transfer transition in the near IR spectrum of [Ru,CL,]~- [but not [Ru,CI,]"- (n = 3,1)] supports a spin-trapped description of the bonding.'"' No hexaiodoruthenates(1V) have been prepared
45.9.7.3 Oxo-, hydroxo- and aqua-halide complexes K4[Ru,OC1,,], originally formulated as KZIRuCls(OH)],can be made by the reaction of [RuO,], K2[Ru04] or Ru0,-xH,O with potassium chloride dissolved in 5 M HCl. The rubidium and caesium salts are made by similar methods.2395 Its formulation as a binuclear complex followed the observation that the complex was diamagnetic, and structure (368) with a linear Ru-0-Ru skeleton was subsequently confirmed by X-ray A molecular orbital description of the bonding in (368) has been given, which accounts for both the diamagnetism and the short Ru-0 bond lengths.2396Extensive electronic, resonance Raman and IR spectral measurements have been reported for this anion and various detailed assignments p r o p o ~ e d . ~Polarographic ~ ~ ~ , ~ ~ ' ~ reduction .2399 of (368) leads to irreversible formation of [RUC~~(OH,)]~-
Ruthenium
448
Black crystals of K4[Ru20BrlO] can be prepared from [RuO,], HBr and KBr and measurements of its IR, electronic and resonance Raman spectra suggest it is isostructural with [Ru,OC~,,]~-.~ Hydrolysis of [RuFJ- in neutral or acid solution is reported to form the corresponding [Ru,OF,J- anion. This will react with Cl- to give [Ru,OCI,,]~- which polymerizes to [RuC12(OH)2],?400Reaction of 'RuC1,-xH,O' with Cl, is claimed to give [R~~OC16] and [Ru,OC~,]~'*~ (see Section 45.9.7.1). Other studies on the Raman spectra of aqueous solutions of K4{Ru,0C1,,] or K,[RuQ,] indicate the presence of a range of aqua- or hydroxo-chloride species. These include [Ru,OCI,(OH,)]~(isolated by Crisp and Seddon'), [ R U ~ OC ~ , ( O H, ) ~and ] ~ -[RuC&(OH),l2- anions.2401 In aqueous methanol solutions containing Ru" chlorides, [Ru2C1,(MeOH)(OH,)(OH),] is claimed to be present.2402The preparation and properties of [Ru,OC~,(NCR)~] are discussed in references 1373 and 1374. A number of publications have appeared on the effect of adding halide ions to solutions of Ru" in HCI04. Although there is now general agreement as to the colour and electronic spectra of the various species formed, there has as yet been no hard identification of any of the chromophores. A range o f formulations have been suggested which include monomers such as [RuC~,(OH),]~-, [RuOC1412-, [RuC~,(OH),(OH,)~],binuclear [ R u ~ O C ~ ~ ( O H , ) ][Ru,OCI,(OH,),]~-, ~-, [Ru202C1,(OH,),]2- and a variety of polynuclear species. For detailed references to this complicated area see reference 3, p. 108. Finally, treatment of aged 'RuCl,.xH,O' in 12M HC1 with [Et,NlCl in the presence of a few drops of Hg produces a complex described as '[Et,~,[Ru,Cl,(OH),]'. This may contain a Ru-0-Ru linkage.,"
45.9.8
Ruthenium(V) Halides
The only ruthenium(V) halides reported are those containing fluoride. Green ruthenium(V) by reduction of [RuF,] with fluoride can be prepared by direct fluorination of the metal at 300°C,2403 B203,24u4 BiF,, SbF,, Ru or H22385or by reaction of the metal and BrF,. The latter method first generates [BrF,] [RuF,] which decomposes at 120°C in vacuo to give [RuFJ, and BrF,.2375The tetrameric structure (369) has been confirmed by X-ray analysis:405 but its mass indicates that the vapour consists of an equilibrium between monomer, dimer, trimer and tetramer units. Magnetic measurements reveal the presence of significant interactions between adjacent metal Mos~baue?'~~ and IRZmspectral measurements are also available. Some of the reactions of [RuF,], are discussed below but further details are to be found in reference 3, p. 79. The hexafluororuthenate(V) anion [RuFJ is readily prepared with a variety of cations such as alkali metals, alkaline earths, Ag+, T1+, PeF]', [Xe,F,]+, [O,]', [NO]+, [SF,]+, [BrF,]+. Some of the synthetic routes are shown in Scheme 70. Measurements on this anion include various X-ray structural analyses, magnetic data (ca. 3.60BM at room temperature), IR, Raman and diffuse reflectance ~ p e c t r aInterestingly, .~ XeF[RuF,] can be oxidized by F2at 350°C to give [XeF,][RuF,]. X-Ray structures of both species have been determined and reveal significant interaction between the [XeF,]+ cation and [RuFJ anion, producing substantial distortion (particularly for n = I), of the latter from regular octahedral g e ~ m e t r y . ~All ~ ' ' the [RuF,]- metal salts are stable in dry air but in water dissolve to produce [RuF,I2-, O2and RuO,. This is believed to be due to two competing reactions, a reduction and a disproportionation (equations 143 and 144).2375 Reaction of XeF, and [RuF,], in 1:2 molar ratio yields [ X e ~ [ R u 2 F , , ] ~ 4 1 The 1 ~ 2 same 4 1 2 binuclear anion is produced upon prolonged storage at room temperature of O,[RUF,];~~'~ [KrF][Ru,F,, ] is also known.242" The mixed Ru"-Ru" complex [RuF, (CO),.RuF,], can be prepared by reaction of [RuF,], with CO in a flow system at 200°C or by heating [Ru,(CO),,] with a large excess of XeF, at 100°C in CFClzCF,C1. A tetrameric structure (370)has been proposed on the basis of IR and magnetic
I 449
/ K
or xi
RuF6
C S [~RuOC141
i, ACl, BrF, (A = Li, Na);2410 ii, ACI, F,(A = K, Rb, C S ) ; ~ ~iii,' ~TlF, SeF4 (A = Tl);23'5 iv, XeF,, BrF, (A = XeF+ or XezFi depending on mole ratio of [RuF5I4to X F F ? ) ~ or ~ ~XeF,(fuse) ' (A = XeF+);2412V, SF4, 108 "C (A = SF;);2413 vi, BrF,, AX (AX = KBr, CsBr,AgBr) or M[BrOJJ2(M = Ca,Sr, Ba);2375,2379vii, F,/Oz, A, 300°C (A = O:);2414 viii, KF, anhydrous HF (A = K);23s5 ix, F,, 15OoC (A = Cs); x, HzO(A = H30+);2415xi, NO or NOF (A = NO+);2416 xii, F,,200-300°C (A = Na, K, Rb, C S ) ' ~ ' ~ Scheme 70
measurements. Further reaction of (370) with CO (100atm/200"C) gives the pale yellow [RuF,(CO),], (363 and Section 45.9.5.1).2294~22g5 4[RuF6]-
+
6H20
4RuV F
45.9.9
-
+
41RuF6]'-
+ 4 H 3 0 + + O2
+
RuVU1 3RdV
(143) (144
F
Ruthenium(VI) Halides
Dark brown [RuF,] can be made by direct fluorination of the metal at elevated temperatures, the product being distilled rapidly out of the reaction zone or condensed at low ternperat~re.~,~' [RuF,], which is much less volatile is also formed during the reaction. X-Ray powder patterns show the molecule to be isostxuctural with other platinum metal hexafluoride^^^^',^"^ and, together with the IR spectrum,2423 suggest an octahedral structure. The solid is volatile at room temperature decomposing rapidly a t 200°C to give [RuF,], and Fa. It is also highly reactive and will attack Pyrex glass at room temperature.'''' A recent paper describes the redox reactions of RuF, to produce species such as [RuF,(PF,)I, (with PF,) RuF,.AsF, (with AsF,), RuF,-ClF, (with ClF,) and RuF, (using various reagents -see Section 4 ~ . 9 . 8 ) . ~ ~ ' ~ The compound originally formulated as K , [ R u F , ] ~ ~has ' ~ now been shown to be a mixture of K[RuF,] and KwF2].23's Ruthenium(V1) oxide tetrafluoride, [RuOF,], is reported to have been prepared by the action of a mixture of BrF, and Br, on ruthenium. The enthalpy and entropy of vaporization, X-ray powder = 2.9 1 BM at room temperature) have been measured.2406 pattern and magnetic properties (petf However more recently other workers also claim to have prepared [RuOFJ (by the fluorination of Ru02 at 40&500"C), b'ut it shows different properties to the earlier product. Evidence for its formulation is based on mass spectroscopy (which shows the molecular ion [RuOF,]' ), elemental analysis and I R spectrum (v(Ru-0) = 1040cm-'). The solid is unstable, even at room temperature, decomposing to [RuF,] and 02.2000 A report on the physical and chemical properties of RuF, and RuOF, is available.2424
450
Ruthenium
The diamagnetic, tetrachlorodioxoruthenate(V1) salts AJRuO,Cl,] (A = Cs, Rb) can be prepared by treating [RuO,] with an excess of AC1 in dilute HC1. Likewise, CsBr/HBr affords CS,[RUO~B~,]. Their IR spectra suggest they contain a trans RuO, unit.65Cs,[RuO,Cl,] reacts with F, at 150°C to give Cs[RuF,], with boiling 10 M HC1 to give Cs,[RuCl,], and with water to produce [RuO,] and R U O ~ . ~ ~ ”
45.9.10 Ruthenium(VII1) Halides Several claims for the formation of [RuF,] by fluorination of Ru or RuO,, have been published. It i s a highly reactive yellow solid which exists only at low temperatures (or in the presence of an excess of fluorine), and is far more volatile than [ R u F , ] , . ~ ~ ~
45.10 HYDROGEN AND HYDRIDE LIGANDS
45.10.1 Ruthenium(0) Complexes Apart from hydridoruthenium carbonyl cluster compounds (see Sections 45.10.3.3 and 45.10.4) the only Ruo hydride complexes are the deep brown [RuH(NO)(PR,),] (R3 = Ph,, PhzMe, Ph,Pt, Ph,Cy) which are fully discussed in Section 45.5.1.2.
45.10.2 Ruthenium(1) Complexes The only hydrido complex containing a formally Ru’ centre is the diamagnetic, air-sensitive [Ru,H(OH)(PMe,),], obtained by heating [ R U ~ ( ~ - C H , ) ~ ( P at M ~60°C ~ ) ~in ] light petroleum or T H F with an excess of water. On the basis of 31P,I3C and ‘H NMR spectra, structure (142) (see Section 45.5.3.2) is pr~posed.’~’’
45.10.3 Mononuclear Ruthenium(1T) Complexes 45.10.3.1
Dihydrido complexes
(i) [RuH,L,L~-,] ( L = L’ = P donor ligand, x = 4,3,2), [RuH2(L-L),]
and related specks
The yellow compound [RuH,(PPh,),] was first prepared in 24% yield by the reduction of RuC1, or [Ru(acac),] with AIEt, in the presence of PPh,.is70Some higher yield routes to this complex are shown in Scheme 71. An X-ray analysis shows a distorted octahedral configuration with essentially cis hydride ligands,,,,, and in solution facile dissociation occurs to give free PPh, and [RuH, (PPh,),].’s38,1870 A compound of the latter formula can be isolated by successive addition of ‘RuCl, .xH,O’ and Na[BH,] to hot ethanolic PPh, (three moles/mole As discussed elsewhere (Section 45.10.3.1 i ) , these [RuH,(PPh,),] complexes exhibit a range of interesting reactions.
i, NaLBH41, PPh3, H2, MeOHIC,H6;1408 ii, Na[BH41, PPhg,,MeOH/C6Hg;140a2427-2429 ifi, Li[NMe21(2 e q ~ i v . )iv, ; ~PPh3, ~ ~ ~EtOH/A;2431 v, PPh,, H1-THF;x432 vi, Na[BH4], PPh3, EtOH/A;’*2427 vii, PPhj;’54~x2R2433 viii, pPh,;24251419.2433 ix, PPh3;1s1’,2433x, PPh3/hu;a4” xi?KOH/Bu*OH 1767 Scheme 71
Ruthenium
45 1
Similar routes to those shown in Scheme 71 can be used to prepare other [RuH2L,j complexes. For example, method (v) works well for L = PMePh,, PMe,Ph and P(OMC),,'~~'and method (vi) is excellent for the preparation of various tertiary phosphine and phosphite c o m p l e x e ~ . ~The ~~~.~~~' or with KOH reaction of [Ru,Cl,(PMePh,),]Cl with hydrazine, PMePh, and H, under pressurez436 in isopropanol'752affords [RuH,(PMePh,),] which is also produced by treatment of [RuH(PMePh,),(MeOH)]PF, with H,.1766[RuH,(PMe,),] can be prepared by reduction of either [Ru2(0COMe),Cl] or ~Ru,O(OCOMe),(H,O),]OCOMe with Na/Hg in THF under H2 (3 atm) in the presence of PMe,,I5" or from cis-[RuClMe(PMe,),] and LiAlH, or NaOMe,'6a whereas [RuH,(PHPh,),] is prepared by reaction of cis- or traw[RuCl,(PHPh,),] with an excess of LiOMe in boiling MeOH."' The majority of these [RuH, L,] complexes adopt the cis configuration in the solid state, although and crystals of Irans-[RuCl, cis-trans mixtures are sometimes observed in (P(OEt),Phj,] can be Some of these compounds (but not for PMe,) are stereochemically non-rigid in solution, undergoing intramolecular ligand exchange which can be monitored by ' H and 31PNMR s p e c t r o ~ c o p y .The ~ ~ tertiary ~ ~ , ~ ~phosphines ~~ (except for PMe,), unlike the phosphite complexes, readily lose a PR, group on heating and hence provide a ready synthesis of [RuH,YL,J compounds (e.g. L = PMePh,, Y = CO, PhCN;14352436 see Section 45.10.3.1.ii for more examples). The PF, complex cis-[RuH,(PF,),] is obtained in high yield from the reaction of PF, (400 atm) and H, (120 atm) with anhydrous RuC1, and Cu powder at 250°C for 20 h. Reaction with Et,N generates the monoanion [RuH(PF, ),I-, which is stereochemically non-rigid,13@and with K/Hg, K2[Ru(PF3),] is formed.'3R1hThe mixed fluorophosphine/PPh, complexes [RuH,L(PPh,),], [RuH,L,(PPh,),] (L = PF,, PF,(NMe, j} and [RuW,(PF,)(PF,NMe,)(PPh,),] are also reported.2w Reaction of [RuH,(PPh,),] with dppm gives (RuH2(dppm)(PPh3),].1399 Reduction of trans-[RuHCl(L-L),] (see Section 45.10.3.2.i) with an excess of LiAlH, gives [RuH,(L-L),] (L-t = depe, o-CsH4(PEt2),) which were initially formulated as trans isomers.'591c However, the 'H NMR spectrum for the depe compound indicates a cis configuration2437 and this is supported by the X-ray analysis of cis-[RuH,(dppe)2]:432 made by method (v), Scheme 71. Other routes to these types of compound are by displacement of 2-naphthyl by Hz (or D,) from cis[R~H(2-np)(drnpe),],'~~* treatment of [RuH(L-L),]' (L-L = dppe, dppp, dppb) with H,, deprotonation of [RuH,(dppp),]PF6 with Et3N,1766 or reaction of [Ru(cod)(cot)] with dppe or dppm under H,.13987'399 Finally rare examples of polydentate P donor ligand complexes containing two hydrido groups are cis-[RuH, { P(OMe),}(ttp)] and trans-[RuH,(PPh3)(ttp)] (ttp = PhP(CHzCH,CH,PPh,),), prepared by reaction of [RuH(BH,)(ttp)] (Section 45.10.3.2.i) with the appropriate P donor ligand and Et,N in THF.'"6*244'
(ii) [RuH2 YSL,-,] ( L = P donor ligand, Y = C,N or 0 donor ligand; x = 1,2,3,4)
Reaction of [RuHCl(PPh,),] with AlEt, in ether in a N, a t m o ~ p h e r e , ' ~ ~reverse ~ , ~ ~ ~osmosis ' - ' ~ ~ of [RuH,(PPh,),] under N2 pressurels3*or treatment of [RuH(OCOH)(PPh,)J with N,'81'32433 affords the dinitrogen compound [RuH,(N,)(PPh,),] as an air-sensitive white solid (vNN = 2147 cm-I). The P(p-Tol), analogue is obtained by borohydride reduction of [RuCl, (PCp-Tol), under N,.1599,2428 The N, group is readily displaced by various Lewis bases to give [RuH,Y(PPh,)J (Y = PPh,, CO, PhCN, NH,, N,B,,H,SMe,) and by H, to form [ R U H , ( P P ~ , ) , ](see ~ ~ ~Section ~ 45.10.5);treatment of the latter with N, regenerates [RuH,N,(PPh,),] and with CO, and RCN, [RuH,CO(PPh,),] and [RuH,(RCN)(PPh,),] (R = Me, Ph) respectively are formed.14"* Interestingly, IR and NMR ev.~~' idence indicates that N2 has a much greater affinity than 2-pentene for [ R u H , ( P P ~ , ) ~ ] X-Ray 3C6H6has a linear RuNNR linkage with cis analysis reveals that [RuH,(N,B,,H,SM~)(PP~,)~] Some other routes to the white [ R u H , C O ( P P ~ ~(371) ) ~ ] are shown hydrides, trans PPh, in Scheme 72. Similarly [RuH, CO(PMePh,),] is obtained by reaction of [RuH,(PMePh,),] with co,243532436 treatment of [RuH($-BH4)(PMePh2),] with CO in the presence of Et,N or heating [Ru2C1,(PMePh,),1C1 with ethanolic KOH."" Reaction of the 'RuCOCl' solution with PEtPh, and a large excess of NatBH,] or Na[BH, CN] affords [RuH,CO(PEtPh,),].*"' Other compounds of type [RuH,YL,] are [RuH,(CO)(PPh,)(L-L)] (L-L = Ph,P(CH2),PPh2 (n = I d ) , Ph, P(CH,), AsPh,, 1,2-(Ph, P),C, H,),'786 [RuH, CO(ttp)]:441 [RuH,(PPh,), (q'~ C5H,o)]?"6 [RuH, (PPh,),THF] (from [RuCl,(PPh,),] and Na/Hg or MgiHg in THF),IW and [RuH,(RNCHCHNR)(PPh,),,] (n = 2,3; R = Ph, CHMe,, CMe,, 4-MeO-Ph) which are believed to contain monodentate diazadiene ligands in some i n s t a n c e ~ . ~ ~ ' The dicarbonyl complexes [RuH,(CO),(PPh,),] (372)can be synthesized by treatment of [Ru-
-
[ RLIHC{(PP~,)~]
, 'RuC13 xH20'
i, EtOH/A;2431 ii, Co;lSll.Z433 iii, HCH0;1767,2427 W . , PPh,, HCHO/KOH(A);'4'0a v , N ~ O M C / M ~ O Hvi,; 'Hz, ~ ~ Et,N, ~ ROH;2a5 vii, EtOH;2445 viii, -0Et 1467 Scheme 72
H
PPha
CO
(371)
(CO),(PPh,),] with H2 under pressure'452or [Ru(CO),(PPh,),] with H2 (1 atm).'469Other methods involve the reaction of the thermally unstable [RuH,(CO),] with L (either PEt, or PPh,) and treatment of [RuCl,(CO),L,] with LiA1H4.78"The latter method can also be used to prepare [RuH, (CO), (dppe)] and reaction of the 'RuCOCl' solution with 5-phenyl-SH-dibenzophosphole (DBP) and Na[BH,] yields [RuW, (CO), (DBP),].243'Evidence for the existence of a hydrido-tricarbony1 complex has been presented; at high pressures [RuH,(COh PPh,] is in reversible equilibrium with {Ru(CO),(PPh,] and H,.,'Other [RuH,Y,L,] complexes are [RuH,(bipy)(PPh,),] (from [RuH,(PPh,),] and bipy), [RuH, (C,H4(CN)2}(PPh,)3],1706 [RuH,(CNBu'),(PMe,),] (from [RuMe,(PMe,),] and CNBu' in THF),'444 and [RuH,(MeCN)(PPh3),].'* Finally, information on the preparation, Raman, IR, 'H, I3C NMR spectra and reactions of [RuH,(CO),] is given in references 78a, 2448 and 2450 and references to dihydridoruthenium carbonyl cluster compounds are cited in Sections 45.10.3.3and 45.10.4.
45.10.3.2
Monohydrido complexes
(9
[&HXL,,], [RuHX(L-L),] ( n = 3,4; L = P,As donor ligand; X = halide, alkyl, aryl, BH,, OCOR) and related species
Treatment of [RuCl, (PPh,),] in C6H6/EtOHwith H, for several days gives violet-black crystals of [RuHCl(PPh,),]. This compound is also formed if the reaction is carried out under H, pressure (100 atm; 12h) or in pure C,H, in the presence of an organic base.2445-24s' Other routes to this reactive material are shown in Scheme 73. In method (viii), use of HX or Me1 gives the corresponding and [RuH,(PPh,),] with EtBr and I, respectively also affords [RuHX(PPh,),] (X = Br, 1)'437"''-2433 these complexes.'*70A single crystal X-ray analysis of [RuHCl(PPh,),] 'C6H6 reveals a highly distorted trigonal bipyramid with the two axial PPh, groups being bent away from the perpendicular Another compound of empirical formula position to accommodate the equatorial PPh, 'RuHCI(PPh3),' results from treatment of [RuH,(PPh,),] in C,H, with 1,4-Cl2Si,Ph, in the presence of excess Et3N. However, this is a red dimeric compound with different physical and spectroscopic properties from purple [RuHCl(PPh,),] C6H6.2455Other monomeric [RuHClL,] complexes [L = ASP^,,^^^' AsMePh,,"" PPh,(C,H,S03H),'563*2456 P(2-py),,ISz1dmpp1;'566.'S67 L, = triphos, tet-1 1539.1540) are available. Several compounds of type [RuHXL,] are known. For example, trans-[RuHX(PHPh,),] (X = C1, Br, I, SCN) are made by reaction of cis-[RuH,(PHPh,),] with NaX. The chloro compound is also prepared as shown in equation (145); it reacts with SnCl, to give trans-[RuH(SnCl,)(PHPh,),]."' Low yield routes to cis-[RuHCl(PMe,),] are shown in equation (1 46), but high yield routes have been reported, namely by dropwise addition of one equivalent of HCl to an etherial solution of
-
i, Na[BH41, C6H6/H,0 (A);2445 ii, R3SiH (R = Me, Et):' iii, Li[NMe2] (1: I mole ~ a t i o ) / T H F iv, ; ~ Et3SiH, ~~~ PPh3, Et3N in C6Hb;83v, RCHzOH;24s224s3vi, CHC13;2430vii, CDC13;84viii, HCl;181'.2433 ix, hv (366 nm)1456 Scheme 73
cis-[RuH,(PMe,),] at - 78°C or by reaction of cis-[RuClMe(PMe,),] with one equivalent of either Li[BH4] or LiA1H(OMe),.'6a Bidentate P and As donor ligand monohydrido complexes rrans-[RuHCl(L-L),] were first prepared by reduction of cis-[RuCl,(L-L),] with an excess of Li[AlH,] in THF (L-L = diars, dmpe, C H , ) , (n = 2-4) and depe, dppm, 0-C, H,(PEt,),).'59' More recently, ~ F ~ ~ - [ R U H C ~ ( P ~ , P (PPh,),] [RuHCl(diop),] were synthesized by exchange of the appropriate L-L with [RuHCl(PPh,),]. Reaction of [RuHCl(PPh,),] with Ph,As(CH,),AsPh, gives [RuHCl(Ph, As(CH,), AsPh,),] but Ph,As(CH,)AsPh, yields [RuHC1(PPh,),(Ph,As(CH,)AsPh,)].1846 Equation (147) shows another route to this type of complex. Although both 'H and 31PNMR studies on [RuHCl(diop),] show inequivalent P atoms characteristic of a cis configuration, X-ray analysis reveals a distorted octahedron with H trans to C1.2457This compound is also an active alkene hydrogenation c a t a l y ~ Treatment t ~ ~ ~ ~ of [Ru(dppe), (styrene)] with P(QPh), (or P(OMe), at higher temperatures) gives [RuH{P(O)(OR),}(dppe),] (R = Me, Ph).'392 cis-
or trans-[RuCl,(PHPh ) 1
LioMe
*
tr~ns-[RuHCl(PHPh,)~]
(145)
C~~-[RUHCI(PM~,).,J'~~~
( 146)
cis-[RuCI, (PHPh,),] tr~ns-[RuC1~(PMe,),]Nq'Hg T d
4 MgEtl
[Ru~(OCOM~)~C~
Er$N, H2
(PPh3)31 + L-L &me&a)ehanof [RuHC1(L-L)21 (L-L = dppp, Ph,P(CH,},PMePh, Ph2P{CH2}3PMe,)'536 LRuC12
cis-IRuH(2-C,oH,)(dmpe),l T=
IRu(C,,H,)(dmpe)*I
(147)
(148)
The corresponding hydrido/alkyl (and aryl) complexes cis-[RuHR(L-L),] (L-L = dppe, dppm, dmpe; R = Me, Et, Ph) are readily prepared from cis-[RuClR(L-L),] and Li[AIH,]'659whereas treatment of cis- or trans-[RuCl, (dmpe),] with arene radical anions affords cis-[RuH(q'aryl)(dmpe),] (aryl = phenyl, 2-naphthyl, anthryl, phenanthryl).IJg9In solution, these compounds are in tautomeric equilibria with significant concentrations of Ruo complexes (e.g. equation 148) although X-ray analysis for aryl = 2-naphthyl confirms the presence of the six-coordinate Ru" species (373) in the solid state.2459 Some reactions of (373) with various substrates to produce other Note that the compound of empirical formula hydrido complexes are shown in Scheme 74.40324ma ['Ru(dmpe),'] obtained by pyrolysis of [RuH(2-np)(dmpe),] (reaction (iv); Scheme 74)is a binuclear Ru" hydrido complex, resulting from intermolecular oxidative addition of methyl groups to ruthenium. Reaction of cis-[RuClMe(PMe,),] with excess NaOMe in THF affords cis-[RuHMe(PMe,),] which is also obtained by methylation of cis-[RuHCl(PMe,),] with MeMgCl.'&Higher homologues such as cis-[RuHR(PMe,),] (R = Et, Prn) can be prepared from truns-[RuC1,(PMe3),] and excess MgR, in THF.'393The reaction of [RuHCl(PPh,),] with LiMe in diethyl ether also affords alkyl derivatives but the product is dependent on the Li:Ru ratio used. Thus, with a small excess of LiMe,
',''
Xuthenium
454
i, HX (X = CN, CH,CN, CHxOMe, ChHaCN); ii, C6D6, 60 cC;iii, Hz (D2);iv, A Scheme 74
yellow [RuHMe(PPh,),(Et,O),] contaminated with some LiCl is formed; a large excess of LiMe gives [RuHMe(PPh,),(LiMeOEt2)21. These complexes are extremely reactive e.g. dissolution in THF and warming gives methane and MuH(C~H,PP~,)(PP~~)(THF),].'~*~ The preparation of hydrido complexes by means of [BH4]- may involve Ru-BH, intermediates and in some cases these can be isolated. Thus reaction of 'RuC13.xH20', [RuCl,(PPh,),] or the 'reduced ruthenium blue' with Na[BH,] in the presence of a large excess of PPh, gves [ R u H ( B H , ) ( P P ~ ~ ) ~Treatment ] ? ~ ~ ~ of [RuCl,(ttp)], with excess Na[BH,] in boiling THF gives [RuH($-BH,)ttp] which at ambient temperature shows discrete 'HNMR signals for the Ru-H, each of the two bridging protons and the two terminal BH's. Variable temperature studies indicate scrambling of the BH,- protons at two different temperatures which is interpreted as a two-step process. The latter compound in the presence of Et,N catalyses the hydrogenation of I-octene to octane at rates comparable to that with [RhC1(PPh,)3].2441The active hydrogenation catalyst [RuH(q2-BH,)(PMePh,),] is synthesized from fuc-[Ru(MeCN),(PMePh,),](BF,), and Na[BH4]'752 and [RuH(q2-BH,)(PMe3),]is also known (equation 149). X-Ray analysis of the latter confirms the mer octahedral configuration (374).15"The analogous rner-[RuH(q2-BH,)(PMe2Ph),]has been prepared by treatment of mer-[RuCl,(PMe,Ph),] with NaBH, in EtOH at 25 oC.2460b &-[RuClMe(PMe,),]
> 2 equiv. Li[BH4 I H
\ trans-[RuCl, @'Me3
I
1
PMe 3 (149)
Ru'
Me3pT!!
excess Li[BH4]
I kH 3
H (374)
A suspension of [RuCl,(PPh,)J in boiling MeOH containing the sodium salt of a carboxylic acid is reduced by H2 or sodium hypophosphite to give the yellow [RuH(OCOR)(PPh,),] (R = Me, shown by X-ray analysis (R = Me)2a62 to have the distorted CH,Cl, CF,, Et, Pr, Pr', Ph octahedral structure (375)which is retained in A more rapid preparation of these active e t ~ . ) , ' ~ ' ' 3 ~ '
homogeneous catalysts for hydrogenation of 1-alkenes is by treatment of [RuH,(PPh,),] with the appropriate RCO,H in boiling 2-methoxyethanol or by rapid successive addition of 'RuC1, *xH,O', RC0,H and KOH to a solution of PPh, in boiling EtOH;17*'the latter method can be used to give [RuH(OCOM~)(P~,PP~SO,H-~),].~~~~ Carbon dioxide reacts readily with [RuH,(PPh,),], [RuH,(PPh,),] or [RuH,(N,)(PPh,),] in aromatic solvents to give the formato complex [RuH(OCOH)(PPh,),]'437~'g'1~2433 whereas [RuH2(PPh3),] and C 0 2 in ROH produce [RuH(O,COR)(PP~,),]'~~'and in NHMeJEtOH, the carbamato complex [RuH(O,CNMe,)(PPh,),] is formed.'792 PPh,
(375)
Reaction of [RuH, (PPh,),] and dicarboxylic or a-hydroxy acids in boiling isopropanol yields [RuH(OCOCH,OCOH)(PPh,),], [RuH(OCOH(OH)R)(PPh,),] and the dimeric complexes [{ RuH(PPh,), }, (OCO(CH,), OCO)] (n = 2 4 ) t 4 " with five-membered cyclic dicarboxylic anhydrides such as phthalic anhydride, C--0 bond cleavage occurs to give [RuH(o-OCOC,H,CHO)(PPh,),].2465 Displacement of PPh, groups from jRuH(OCOR)(PPh,),] is also possible. Thus [RuH(OCOMe)(PPh,),L,-,] (n = (r3; L = PIOMe),, P(OCH,),CMe, PF,NMe,, ButNC, g p p e -not all combinations) are reported1386 and !j3uH(0COMe)(Ph2P(CH,),PPh,),] (n = 2,3) is ment i ~ n e d . ' ~ The ~ , complex [RuH(OCOCF,)(PF,),(PPh,),] can be prepared by reaction of [RuH,(PF,),(PP~,)~]with CF,C0,H.'576
(ii)
Cyclometalluted complexes
Cyclometallation is defined in Section 45.5.4.5.v where some examples of complexes which do not contain Ru-H bonds are given. The majority of orsho-metallated ruthenium tertiary phosphine complexes are also monohydrides. For example reaction of [RuHCI(PPh,),] with LiR (R = Me, ( S = Et,O, Me, SiCHz) in ethers at ambient temperature produces [RUW(C,H,PP~~)(PP~,)~S] THF) which are formed by loss of CH4 or SiMe, from [ R U H R ( P P ~ ~ ) ~ By S , ]use . of an excess of LiMe, ZnMe, or MgMe, in Et20, yellow crystalline compounds containing Li, Mg or Zn as well as ortho-metallated PPh, ligands are obtained e.g. (376).1690 Treatment of [RuH(C~H,PP~,)(PP~~)~E~~O], [RuH2(PPh3),]or [RuH,(PPh,),] with C2H4 gives a white solid of empirical formula [Ru(PPh,),C,H,] which converts to a pale orange isomer on heating. Initially these were formulated as Ruocomplexes2466 but further studies reveal that they are the Ru" ortho-metallated species (377) and (378) respectively. An analogous reaction with cod gives (379) and a mechanism of formation involving Ru" hydrido intermediates is proposed. Reduction of [RuCI,(PPh,),] (n = 3 or 4) with Na/Hg or Mg/Hg in MeCN/THF gives [RuH(C,H,PPh,)(MeCN)(PPh,),]whereas reduction in THF/py affords a mixture of two isomers of the corresponding ITiuH(C,H,PPh,)Cpy)(PPh,),]; in the presence of 2,2'-bipy, the dark red [RuH(C,H,PPh,)(bipy)(PPh,)] is produced.Iw 1
L
-
PPh3
Me
I
""., *,& HHYbC PPh3
(376) L = PPh,
(377)
')
Hkh,/P) Ik c
I
Ph3P
Ph3Pf
PPh,
(378)
(379)
Ruthenium
456
Electrochemical reduction of [RuCl(dppp),]PF, affords unstable anionic complexes which rapidly Direct synthesis of this cominternally metallate to give [RuH(C,H4P(Ph)(CH,)3PPh,)(dppp)].'509" plex is achieved by reaction of [Ru(styrene),(PPh,),] with dppp and [RuH(C,H,P(Ph)(CH,),PPh,)(dppe)] (two isomers) is formed by treating [Ru(styrene)(dppe),] with toluene. Hot toluene gives (380) whereas at room temperature isomer (381) is isolated. On standing for three days at room temperature, the dimetallated complex (382) can be is01ated.I~~' Other cyciometallated tertiary phosphine complexes include K[RuH, (C6H4PPh2)(PPh3),]CIoH,-Et,O (formed by reaction of [RuHC1(PPh3),] with potassium naphthalide in THF at - 80"C),x67[RuH(C,H,PPh,)(CO)(PPh,),] (from thermal decarboxylation of [Ru(o-Tol)(OCOH)CO(PPh,), [RuH(q2-Me,PCH,)(PMe,),] (see Section 4551 .l) and 'Ru(dmpe),' (see reaction (iv), Scheme 74 in Section 45.10.3.2.i).Reaction of trans-[RuCl,(PMe,),] with Na/Hg in the presence of PPh, gives fuc-[RuH(C,H4PPh,)(PMe,),] which reacts with Me1 and CS, to give mer-[RuI(C,H4PPh,)(PMe3),] and fuc-[Ru(SCHS)(C,H,PPh,)(PMe,),] respe~tively.'~~'~ Several ortho-metallated tertiary phosphite ruthenium hydrides have been characterized. For examand likewise reaction of ple treatment of [Ru(CO), {P(OMe),},f with P(OPh), produces (383)1380 [RuH(g2-BH,)ttp] with P(OPh), yields the ortho metallated complex (384) even at room temperature.2441In boiling C,&, [RuH,(CO)(PPh,),] and P(OPh), affords [ljuH{(C6H40)P(OPh), CO(PPh,)P(OPh), j whereas more forcing conditions also produce some [RuH{(C,H,O)P-
>-
(OPh)2)CO(P(OPh)3}2].1685 The compound [RuH2(PPh3)J will cleave the vinyl CH bonds of neat alkylmethacrylates to give the five-membered metallacyclic complex (385), confirmed by an X-ray analysis of the n-butyl derivative?&'
c%%I /p\ *NP
%+I TP\ ,P.*'P '
P
H+P
Ru
C
I
B
H (380)
[
C""..n /
&*a'
*pp
C
d
LP'
(381)
(382)
OR
I
(iii) [RuHXYxL,-,] ( L = P or As donor ligand, X = halide, OCUR, NU,, dike, BF.,, dithioucid; Y = CO andlor CNR; x = 1,2) Many hydridc-carbonyl complexes of ruthenium are obtained by decarbonylation of oxygen containing solvents (commonly alcohols). Thus Ru"' halides react with EPh, in high boiling point alcohols such as 2-methoxyethanol or ethylene glycol to give [RuHX(CO)(EPh,),] (E = P, As; X = C1, 3r).2469Later papers reporting the preparation of these complexes include HCHO in the recipe.1410b,1841& Ruthenium tertiary phosphine halide compounds also react with alcohols, particularly in the presence of base to give stable hydrido-carbonyl complexes (equation 150).'s84~'614b The strong trans Iabifzing effect of the hydride ligand in [RuHCl(CO)(PR,),] can then be used to effect substitution with various P and As donor ligands (L) to form [RuHCl(CO)(PR,),L] (often in equilibrium with starting material). 1581.1685*2470*2471 The corresponding [RuHCl(CS)(PPh,), J is prepared by reaction of [RuCl(OCOB)CS(PPh,),] (Section 45.5.4.7.vi) with PPh,.IM5 [RuzC13PR3)61C1
KOH/EtOH
A
'
[RuJWCO)(PR,),I
(1501
Decarbonylation of 2-methoxyethanol provides the CO ligands for formation of the five-coorPBut,R (R = Me, Et)I7"]. These dinate bulky phosphine complexes [RuHCl(CO)L,] (L = PCY~,"'~ compounds readily take up CO to give [RuHCl(CO),L,] (L = P C Y ~ , P' ~B' u~' , R ~ ' ~with ~ ) drum L and
Ruthenium
457
cis CO ligands. Treatment of cis-[RuC12(CO)~(PButPr"z)2] with 2-methoxyethanol/KOH yields [R~HCI(CO),(PBU~P~",),]'~'~ and [RuHCI(PPh,),] and CO affords [RuHCI(CO),(PPh3),].'449 The latter is also produced from [RuCI,(PPh,),] in DMA treated with a H 2 / C 0 mixture.'607 The five-coordinate [RuHCl(CO)(PBu',Et),] also reacts with MeNC to givf: [RuHCl(CO)(CNMe)(PBu', Et),]'712 and other neutral isocyanide monohydrido complexes include Pyridine and SO, add to [RuH[RuHX(CO)(CNR)(PPh,),] (X = C1, Br, I).1480~1652,'656a~'7'7 Cl(CO)(PCy,),] to give [ R U H C ~ ( C O ) ~ ~ ( P C ~ ,and ) , ] ~[RuHC1(CO)(S02)(FCy3)2]'813 ~" respectively. With acetylene or phenylacetylene (L), [RuHCl(CO)L(PCy,),] is formed.1623b Reaction of a six-fold excess of PPh, with 'Ru(C0)Cl' in EtOH in the presence of Na[BH,] affords a mixture of isomers of [RuH(BH,)(CO)(PPh,),] which both contain a monodentate BH4 ligand. The bulky PCy, generates [RuH(BH,)(CO)(PCy,),] and with DBP and Na[BH, CN], [RuH(BH,CN)(CO),(DBP),] is formed.z43' Other carbonyl complexes of type [RuHX(CO),L,] (x = 1,2) include [RuH(OCOR)(CO),(PPh,),] (from [RuH,(CO)(PPh,),] and RC02H),172sa,'774 [RuH(NO,)(CO),(PPh,),] (Section 45.5.4.7.v), [RuH(dike)CO(PPh,),] (Section 45.5.4.7.iii) and [RuH(S-S)CO(PPh,),] (SS = -SZCNR2,-&COR) (Section 45.5.4.8.ii).
(iv) Other neutral monohydrido complexes containing C, N , 0 or S donor ligands
Norbornadiene reacts with [RuHCl(PPh,),] in C6H6 to give [ R ~ H C l ( P P h , ) ~ ( n b d )shown2473 j,~~~ to exist in solution as a single isomer. The analogous complexes [RuHCL,(cod)] (L = piperidine, C6HIiNH2,NHMe,) are formed by reaction of [RuC1,(cod)], with amines at high temperature; X-ray analysis for L = piperidine establishes structure (386).628Reaction of the unusual dimer [(RuHX(cod)},(NH,NMe,)1 (X = C1, Br) with Lewis bases in boiling acetone or MeOH generates a range of [RuHX(cod)L,] (L = PMePh,, SbPh,, AsPh,, py). These are stable in the solid state but decompose in solution; spectroscopic (NMR, IR) data suggest structure (386)?474
Cl
r
Propene oxidatively adds to [ R U H ( C ~ H ~ P P ~ ~ ) M ~ C N(or ( P to P ~the ~ ) ~isomeric ] [Ru(qwhich then reacts with an MeCN)(PPh,),]MeCN to give [RuH(q-C, H,)(MeCN)(PPh,),]1404,2475 whereas excess of isobutene to give the 2-methylallyl complex [RuH(q3-C,H,)(MeCN)(PPh,),], A similar hydrido-q3-l-phenylallyl treatment with PHPh, affords [RUH(~-C,H,)(PM~P~,),].~~~~ complex [RuH(q3-C, H,Ph)(MeCN)(PPh,),] is isoiated in high yield from reaction of [Ru(qMeCN)(PPh,),]MeCN with allylbenzene in MeCN.Im5A series of q-1-3and q-1-5 cyclohexadienyl Ru" hydrides such as [RuH(PPh,),(q-CsH7)] can be obtained by reaction of [Ru(PPh,),(styrene),] with cyclohexane,2476 whereas l-hexene gives a mixture of syn- and unti-[RuH(PPh3),(styrene)(l-3-qC6Hl Other C donor ligand complexes include [RuH(PPh,),(q-C,H,)], [RuHCl(PPh,)(qC,Me,)], and a range of carborane complexes (reference 1, p. 806). The diamagnetic red complex [RUH(N(S~M~,)~}(PP~,),] can be isolated from reaction of [RuHCl(PPh,),j with Liw(SiMe,),]. It is a rare example of a d 6 14-electron species and is proposed to have a distorted tetrahedral ge~metry."~' With diazadienes, [RuHCI(PPh,),] yields [RuHCl(dad)(PPh,),] which disproportionates in polar media to give [RuH,(dad)(PPh,),], [RuH,(CO)(PPh,),] and [Ru2C1,(dad),(PPh,),]C1.1770 Treatment of [RuH,(PPh,),] with cyclic imides such as (+)-camphorimide, phthalimide, succinimide gives [RuH(imido)(PPh,),] in which the imide acts as a bidentate ligand coordinating via its N and 0 atoms.2478The potentially bidentate ligands 2-hydroxypyridine and 8-hydroxyquinoline react with [RuH, (PPh,),] at room temperature to give 8-OC9H6N) and 2-aminopyridine yields [RuHX(PPh,),] (X = 2-OC5H,N, [RuH(HNC,H,N)(PPh,),]. With pyrocatechol and resorcinol, the yellow crystalline complexes [RuH(OC,H,OH)(PPh,),]-n[(OH),C,H,] ( n = 2 and 0 respectively) are isolated and 'HNMR studies indicate that the phenolic rings are ~r-bonded.'~'~
458
Ruthenium
Tn contrast, [RuHCl(PPh,),] and 2,2'-bipyridyl give a poorly soluble red-brown material which may be [RuHCl(bipy)(PPh,),], although it could not be purified?4qsThere is also a brief mention in the literature of the reaction of [RuH(PhN,Ph)(PPh,),] (see Section 45.5.4.6.v) with N, in a reverse osmosis cell to give [RuH(N,)(P~N,P~)(PP~,),].~~~~ As noted elsewhere, various monohydrido complexes containing diazenido, diazene (Section 45.5.4.6.iv), triazenido, formazan and thioformamido (Section 45.5.4.6.v) ligands are available. The yellow compound [RuH(CH,NO,)(PPh,),], obtained from the reaction of MeNO, with is thought to contain bidentate 0 bonded [RuH, (PPh,),] or [RuH(C6H4PPh,)(PPh,),Et,0] CH,NO,- as in (387).1402 Reaction of [RuCl,(PMe,), {P(OMe),),] with Na/Hg in benzene gives the aryl hydrido complex cis-[RuH(C,H,)(PMe,), { P(OMe),),].'391c
H
A range of Ru hydrido/hydroxo Complexes are known, e.g. [RuH(OH)(PPh,),solvent] (solvent = H 2 0 , THF) (see Section 45.5.4.7.ii for details) and the related [RuH(SR)(PPh,),] (R = H, Me, Ph, PhCH2)have been prepared (see Section 45.5.4.8.i). The sulfoxide complexes [RuHCl(DMSO),(tri[RuHCl(DMSO),(AsPh,),], [RuHCi(DMSO)(AsMePh,),] and phos)], [RuHCl(DMSO)(tet-l)],"4" [ R u H C I ( D M S O ) ( A S M ~ , P ~ )are , ~ 'also ~ ~ ~reported.
(VI
Cationic monohydride complexes
Triphenylmethyl hexafluorophosphate abstracts one hydride ion from [RuH,(PPh,),] to give of a five-coordinate sixteen electron cation in preference to the [ R u H ( P P ~ , ) , ] P F , .The ~ ~ ~formation ~ eighteen electron species [RuH(PPh,),]' is attributed to the steric bulk of the PPh, ligands since smaller ligands such as P(OR),Ph, P(OR), (R = Me, Et), PHPh?,PMe,Ph, PMe, do form [RuHL,]' cations. Some methods of synthesizing the latter are shown in Scheme 75. In method (iv), treatment and the mixed with only four equivalents of PMezPh in MeOH gives [RuH(M~OH)(PM~,P~),]+'~~~ ~ ) ~ and HBF,.'576 Interescations [RuH(PF,), (PPh,),]BF4 are generated from [ R U H ~ ( P F(PPh3)J tingly, [Ru(CNBu'),] deprotonates nido-2,3-Me,-2,3-C,B,H6 to give the related [RuH(CNBu'),]+ cation stabilized by a carborane anion.24R2
i, L = P(OR),Ph (R = Me, Et) or P H P I I , ; ~ii,~ P(OMe), ~~ or P(OMe),Ph, or PHPh,; PFg;111.1531.1532 iii, PHPh,/MeOH; PFg;1565 iv, excess L (P(0Et)B or P(OMe),Ph or PMe,Ph), MeOH/A;'766~2480v, PMe,; PF;,1399'vi, P(OMeI3 or P(OMe),Ph 2474. 2481
Scheme 75
The [RuH(PMe,Ph),]+ cation has a distorted octahedral structure2480 which shows evidence of the strain caused by non-bonded repulsions between the substituents on phosphorus and is consequently very reactive. For example, reaction with CO, in ROH gives the monoalkylcarbonate esters [Ru(O,COR)(PMe,Ph),]+ (253; Section 45.5.4.7.vii) and with CS,, [Ru(S2CH)(PMe,Ph),]+ is formed (Section 45.5.4.8.i~). The red solution of [RuH(PPh,),]PF, turns yellow on standing and a pale yellow solid of empirical formula [RuH(PPh,),]PF, can be isolated. Spectroscopic data are consistent with struc-
Ruthenium
459
ture (388) in which one of the PPh, ligands in q-bonded to Ru via one of the phenyl rings,'754J479 and this has been confirmed by X-ray analysis. Compound (388) is also formed in good yield by reaction of [RuH,(PPh,),], [RuH,(PPh,),] or [RuH(OCOMe)(PPh,),] with HBF,/MeOH in the presence of excess PPh3.'7s4
In the absence of an excess of PPh,, the initial red species on protonation of [RuH(OCOMe)(PPh,),] with HBF, is [RuH(solvent)(PPh,),]+ (solvent = MeOH, acetone) which in MeOH forms the yellow [RuH(H,O),(MeOH)(PPh,),]+ cation. This cation is also obtained by treatment of [Ru(OCOMe)(H,O)(PPh,),]BF, with aqueous HBF, under hydrogen. Treatment of [RuH(OCOMe)(PPh,),] in MeOH or acetone containing MeCN with aqueous HBF, gives the [RuH(MeCN),(PPh, j3]+ cation.17s4Cations of this type are also prepared as shown in Scheme 76 and assigned an octahedral structure with RCN trans to hydride.2479
i, MeCN/MeOH, A; ii, Ph,C[BF,I,
CH2C1,, MeCN; iii, RCN, MeOH, KPF,
(R = Me, Pr, Pr', Ph, p-Tol, MeOC,H4, ClC6H4) Scheme 76
Reaction of [RuH(NH,NMe,)cod]PF, (see later) with the ditertiary phosphines dppp and dppb gives the five-coordinate [RuH(L-L),]PF, whereas dppe in EtOH yields the six-coordinate [R~H(Et0H)(dppe),jPF,.'~~~ Protonation of [RuH,(dppm),] with HBF, in aqueous MeOH gives trans-[R~H(OH,)(dpprn),]BF,.~~~~ The corresponding [RuH(Me,P(CH,),PPh,),]BPh, has been synthesized from trans-[RuHCl(Me, P(CH,),PPh,),] stirred with ethanolic solutions of Na[BPh,]. It is not fluxional at room temperature and a trigonal bipyramidal structure is More recent work suggests these compounds are square pyramidal in structure.1536b Several routes to the monocarbonyl hydrido cations [RuH(CO)(L-L),]+ and [RuH(CO)L,]+ are available (Scheme 77). Method (iii) with CNBu' in place of CO gives trans-[RuH(CNBu')(depe),]+ ?484 Other monocarbonyl cations include [RuH(S, CPCy,)(CO)(PCy, j2]+ (from CS, cis-[RuH(CO)(bipy),]+ (prepared by heating cis-[Ru($ -0COH)and [RUHC~(CO)(PC~,),]),'*~~ of cis-[RuCI(CO)(bipy),]+ with NaBH4)1121C CO(bipy)2]+in 2 - r n e t h o ~ y e t h a n o l "or ~ ~by~ ~treatment ~ and [RuH(H,O)(CO)(PPh,),]BF, (from treatment of [RuH,(CO)(PPh, j3] with aqueous HBF,). Further reaction of the latter with CO gives [RuH(H,O)(CO),(PPh,),]BF,*EtOH(389) whose X-ray analysis reveals an extensive hydrogen-bonding network between the BF4- ion, the coordinated water molecule and the EtOH of crystal1i~ation.l~~~ Compound (389) is also prepared by carbonylation of [RuH(acetone)(PPh,),]BF4. 1754 Treatment of [RuHCI(CO)(PPh,),] with Ag[ClO,] (390) and since the MeCN groups are of different in MeCN gives [RUH(CO)(M~CN),(PP~,)~]CIO, lability (trans to H and CO), a range of complexes are accessible by stepwise replacement. For example, treatment with CO, followed by PPh, gives [RuH(CO),(PPh,),]+ with trans CO groups.'467 The tricarbonyl hydrido cation [RuH(CO),(PPh,),]+ is obtained by treatment of [Ru(CO), (PPh,),] with NO[BF,] or NO[PF6]2486a and a preparative route to the cations [RuH(N,)(DMSO),(AsPh, )JC1 and [RuH(N,)(DMSO)(AsMePh,),]Clhas appeared."" A series of monohydrido cations containing the tridentate ligands ttp and Cyttp have been reported. For example, reaction of [RuH(u2-BH,)(ttp)] with HBF, and the appropriate ligand gives [RuHL,(ttp)]BF, (L = P(OMe),, MeCN, CO). For L = P(OMe),, three different isomers (Le. trans, cis- syn and cis- arzrz] are obtained depending on such factors as solvent used, reaction conditions
460
Ruthenium
~ ~Me,P(CH2),PPh2, ~ Li[C1041, EtOH;1536 ii, CO (L-L = dppe or ~ i p p p ) ; " ~iii,~ C O / a ~ e t o n e ; ' iv, PHPh2;'s65 vi, CO, PF6;2485vii, CH,CI, 1637
i, CO;"" V,
Scheme 77
and
sequence
of
addition
of
reagents.
Some mixed
ligand
complexes
such
as
[RuH(MeCN)(PF,)(ttp)]BF,and [RuH(CO)(P(OMe),)(ttp)]BF4are also reported. Oxidative addition of acids to [Ru(CO),(Cyttp)] affords the [RuH(CO),(Cyttp)]+ (CO),triphos]Cl is formed from [Ru(CO), triphos] and acetyl
whereas [RuH+
Treatment of a suspension of [RuCl,(cod)], in MeOH with N,N-dimethylhydrazine gives [RuH(NH,NMe),cod]+ isolated as its BPh,- or PF6- salts6" and shown by X-ray analysis2487to have structure (391) with coordination via the less basic but more sterically favoured NH, nitrogen atoms. The NH,NMe, groups are readily replaced by other N and P donor ligands to give [RuHL, (cod)]+ (L = NZH4, MeNHNH,, py, 4-Mepy, P(OMe),, P(OCH,),CMe, PPh,OMe, PMe,Ph, PMePh2).65032488 Reaction of the dimer [{RuHX(cod)),NH,NMe,] (X = C1, Br) with 4-methylpyridine in EtOH or with AgNO, in MeCN also produces [RuHL,cod]' (L = 4-Mepy, MeCN) respectively,2474 and the coordinated cod in [RuH(PMe,Ph), (cod)]PF, is readily replaced by 1,3-dienes to give [RuH(PMe,Ph),fdiene)]PF, (diene = buta-1,3-diene, he~a-1,3-diene)?~~'
45.10.3.3 Anionic complexes There are relatively few examples of anionic ruthenium hydrido complexes. Thus, several papers have discussed the preparation of the [RuH6I4-ion from Ru, H, and various lanthanide dihydrides at 8OOOC.~490,2491The 99RuMossbauer spectra at 4.2 K for various M,[RuH,] (M = Ca, Sr, Eu,Y) are consistent with diamagnetic Ru" centres.2492 An unusual anionic dihydrido ruthenium complex is formed by reduction of [RuHC1(PPh3),], with potassium naphthalide at low temperature in THF, followed by recrystallization from toluene containing a small amount of diglyme. The physicochemical evidence is consistent with its formulation as the binuclear [{Kdiglyme] {RuH,(PPh,)(PPh,)} -]?. Note that reduFtion of [RuHCl(PPh,),] with potassium naphthalide gives the ortho-metallated complex K[RuH,(C6H,PPh,)+
(PPh3)2]C,oH8-Et,0.2467~2493 A number of new anionic ruthenium hydrides have been synthesized by Halpern et al. These includefuc-[RuH, (PPh,),]- ,isolated as its K+ salt by reaction of K[RuH, (C6H4PPhz)(PPh3)2] with H,(1 a m ) in THF at 25"C, and K[RuH(PPh,),(anthracene)]. Treatment of the latter with H2 gives K[RuH,(PPh3)J whose spectral data are consistent with a fluxional pentagonal bipyramidal structure. This pentahydride complex exhibits a rich chemistry, reacting with 1-hexene to produce the coordinatively unsaturated [RuH,(PPh,),]- anion and with cyclohexadiene to give [RuH(PPh,),cyclohexadiene]- .Anthracene regenerates [RuH(PPh,),anthracene]- whereas ethylene affords the ortho-metallated [Ru(C, H,PPh,)(PPh3)(C2ET,)]-. Several of these complexes are good catalysts for the hydrogenation of anthracene to 1,2,3,4-tetrahydroanthra~ene.'~~~
1 Ruthenium
46 1
Other anionic ruthenium hydrides include the stereochemically non-rigid [RuH(PF,),]- (from reaction of cis-[RuH, (PF,),] with Et3N),'3wthe protonated intermediates [RuH(CN),I3- and [RuH,(CN),]*- (see Section 45.3.1.24 and an extensive range of ti-, tetra- and hexa-nuclear ruthenium hydride carbonyls, such as [Ru,H(CO),, I-, [Ru,H(CO),~]-, [RU~H,(CO),~]and [RU,H(CO),,]- (see reference 1, pp. 889-906).
45.10.4 Bi-, Tri-, Tetra- and Poly-nuclear Ruthenium(I1) Complexes The unusual hydrido-bridged complex [(RuHX(cod)),NH,NMe,] (X = C1, Br) is prepared by addition of either LiCl or LiBr to a boiling acetone/MeOH solution of [RuH(NH,NMe,),(cod)]Y Cy = PF, or BPh,).2474An X-ray analysis24g5 (for X = C1) shows that its structure (392)consists of two Ru atoms linked by three different bridging ligands. Some reactions of this compound involving bridge cleavage and substitution are discussed in earlier sections. The X-ray structure of [{RuH(pz)cod),pzH] has shown structure (393) with a semi-bridging hydride ligand.2496
Me2 H2
(393)
(392)
Treatment of IRy(p-CH,),(PMe,),] with €3, (3 atm) in light petroleum affords the red crystalline diamagnetic [Ru,H,(PMe,),] shown by X-ray analysis to have structure (394). This double bridged species is quantitatively converted to [RuZH3 (PMe,),]BF, (395) by reaction with aqueous HBF4 in THF. It can also be obtained by reaction of H2 (3atm) with either [RuZ(p-CH2),(pCH,)(PMe,),]BF, or [Ru&-CH~)~(PM~,),][BF,],in MeOH. The Ru-Ru distance in (395) is 2.540( 1) A.1521A complex analysing for [RU,H3(PMe,Ph),]PF, is produced by refluxing [Ru(OCOH)(PMe,Ph),]PF, in MeOH in air.'753 The related hydroxo-bridged complex (243 and Section 45.5.4.7.ii) is produced by [PPh, (Pr'NCHCHNPr')Ru(p-OH),(p-Cl)RuH(PPh,),] with water in toluene.'770The novel compound the reaction of [RuHCl(Pr'NCHCHNPr')(PPh,),] In [Ru2H4N2 (PPh, >,I with three bridging hydrides and a multiple metal-metal bond is known.za97a the ruthenium catalysed conversion of methanol to methyl formate, [RuCI2(PPh3),]is converted to the dinuclear cation [(PPhJ)2(CO)Ru(p-H)(p-C1)2Ru(CO)(PPh3)2]+, isolated as its PF,- salt.2497c
+Me3
H
(394)
r
1
L
1 (395)
As discussed elsewhere (Section 45.10.3.2.i) reaction of [RuH,(PPh,)J with 1,4-dichlorooctaphenyltetrasilane in the presence of Et,N gives the dimeric [RuHCl(PPh,),],, tentatively formulated on the basis of 'H, 31PNMR and IR spectroscopy as a bis-p-chloro complex with terminal hydrido ligands.245sReaction of H, with [RuX,(EPh,),MeOH] in toluene or DMA in the presence of bases such as 'proton sponge' is reported to yield five-coordinate halide-bridged dimers [RuHX(EPh,),], (X = Cl, Br; E = P, As). Some evidence was also found for the existence of [Ru"'HB ~ , ( A s P ~ ~ ) However, , ] . ' ~ ~ ~ unpublished indicates that the above reactions also produce the Ru"' dimers [RuH, X(ER,),], with possible structure (396) and these are in equilibrium with [RuHX(ER,),],. Reduction of anhydrous RuCl, with sodium sand in neat Me,P gives [(RuH(P-
Me,),}(p-MqPCH,),].2497d In a communication, reaction of the Ruo complex [Ru(cod)(cot)] with two equivalents of dppm in the presence of H2 gives a compound of empirical formula [RuH,(dppm),]. On the basis of 31P and 'H NMR evidence, this was tentatively formulated as a trinuclear complex,'398but further studies indicate that a mixture of monomeric cis- and trans-[RuH,(dppm),] is formed.'399A similar with reaction with dppe gives [R~H,(dppe),],'~~~ also formed by reaction of [R~(tos),(dppe),]'~" NaAlH,. Pyrolysis of [RuH(2-np)(dmpe),] gives a compound of empirical formula '[Ru(dmpe),]' which is shown by structural analysis to be the dimeric Ru" hydride (123; Section 45.5.1.1).
462
Ruthenium
Treatment of [RuH,(PPh,),] with an excess of hydroquinone in THF gives ;o highly insoluble compound tentatively formulated as the binuclear complex [ R u , H , ( O , C ~ H ~ ) ~ ( P P ~(with ,)~] a bridging hydroquinone ligand)I7O6whereas [RuH, (dppm),] and [RhCl(CO),], affords the unusual [RuRhH,Cl(p-CO)CO(dppm),] (397).'507a Reaction of [RuH, (dppm),] with [RhCl(cod)], produces [RuRhH,Cl(cod)(dppm),] (containing one bridging C1, H and dppm Treatment with Na[BH,] gives the coordinatively unsaturated [RuRhH, (dppm),]?497'Reactions with CO, P(OMe)3 and LiMe are also reported. Reaction of [MoRu(CO),(dppm),] with H, gives the dihydrogenbridged complex [MoRuH, (C0)5(dppm),].1507b
(397)
Examples of trinuclear hydrido complexes containing ruthenium include [Ru, (p-H)(p-OH){p(293 and Section 45.5.5. l), [Ru,(p-H),(p-P, q2-CH2= Fe(CO),](CO),(PPh,),] CHC&PPh,)(CO),] (from pyrolysis of [Ru,(CO),,SP] at 80°C; SP = U - C H , = C H C ~ H , P P ~ , ) , ~ ~ ~ ~ [Ru, (p-H), (CO),(SiZCl,),] (9 and Section 45.3.2.1) [RuRh,H(CO),, 1, [RuCo,H(CO),o(PPh,),]24~b and a series of phosphido-bridged clusters derived from photolysis of [Ru, (CO),(PHPh,),] or reaction of [RU,(CO)~~]with PHPh, e.g. [Ru, (p-H)2(p-PPh,)2(CO),], [ R u ~ ( ~ - H ) ~ ( ~ PPh,),(CO),(PHPh,)] etc.I5l6Many other hydrido carbonyl clusters containing three to six ruthenium atoms are known but these are outside the scope of this review (see reference 1, p. 889 for further details and reference 2499 for the X-ray structural analysis of the chiral cluster [Ru,H4(co)9 (PMez Ph)[P(OC,H,Me-p),f~(OCHz 1 3 CEt]] . 45.10.5
Higher Oxidation State Hydrido Complexes
In the presence of proton sponge, the complexes [RuC12(PPh3),],, [RuHCl(PPh,), (nbdjj or [RuX,YL,], (X = Y = C1, Br, or X = C1, Br; Y = 0,CR; L = PPh,, P(p-Tol), or AsPh,), react with H, in DMA or toluene to give the compounds [RuH,XL2Iz (X = C1, Br). The Ru"' product [RuH, CL(P(p-Tol), l2l2has been characterized by analytical, molecular weight, ' H and 31PNMR methods and chemical reactions and the data suggest the asymmetric structure (396). A reinvestigation of the reaction between [RuCI,(PPh,),] and H, in DMA to form [RuHCl(PPh,),] also indicates that in the presence of excess C1-, a long lived intermediate, tentatively formulated as [Ru,H,Cl,(PPh,),], is formed.'555In earlier work, reaction of [RuBr,(AsPh,),] with H, in DMA at 25°C was claimed to give [ R u H B ~ , ( A s P ~ , ) , ~ . ' ~ ~ ~ The tetrahydrido complex [RuH,(PPh,),] is readily obtained as very air sensitive white crystals by reaction of [RuCl,(PPh,),~ with NaBH, in C,H,/MeOH; in the presence of excess PPh,, [RuH,(PPh3),] is produced and this can be converted to the tetrahydride by passing a stream of H, through a C,H, solution of the compound.'408~'549~2427,2428 The tetrahydride is also generated from [RuH, (N2)(PPh3)3]and H,, a conversion which is readily reversed on exposure to nitrogen. Other Solid [RuH,(PPh3),] reacts with various other subroutes are shown in equation (15 i).'s'1q2433,2442 strates (CO, NO, SO2, NOCI, RCN) with displacement of H, to give compounds containing these ligands.'w8Thus it might be effectively considered as a Ru" complex containing neutral dihydrogen. The corresponding [RuH,(DBP),] (DBP = 5-phenyl-5H-dibenzophosphole)can be synthesized from [RuCl,(DBP),] and NaBH,2431and it has been shown that [RuH,(PR,),] (R = Pr', Cy, NEt,) can be prepared by treatment of [Ru(cod)(cot)] with three equivalents of PR, under H,. These complexes in deuterated aromatic solvents exhibit H-D exchange between the solvent and the coordinated phosphine^.^'^ [RuH, (P@,-FC,H,),}] is also known and has been shown to dehydrogenate cyclooctane at 150°C in the presence of a hydrogen acceptor.2501
The cationic Ru" monohydrides [RuH(MeOH)(PMe,Ph), ]PF6, [RuH(EtOH)(dppe),]PF, and [RuH(dppp),]PF, oxidatively add H, under ambient conditions to form the Ru'" trihydrido cations [RuH,(PMe,Ph),]PF, and [RuH3(GL),]PF, (L-L = dppe, dppp), a process which can be reversed
Ruthenium
463
on heating. Addition of HPF, to [RuH,(PMePh,),] and [RuH,(dppb),] also gave very unstable trihydrido cations which revert to the dihydride on recry~tallization.'~~~ The corresponding [RuH,(PPh,),]+ has been obtained, together with the dimer [RuHCl(PPh,),],, by treatment of [RuH,(PPh,),] with 1,4-CIzSi,PhBin the presence of Et3N.2502A related I6 electron species [RuH,(PPh,),]' is tentatively claimed'754but no NMR data were reported because of rapid H2 evolution in solution. The novel compound [Ru,H,N, (PPh,),] can be prepared by recrystallization under nitrogen of the hydrogenation product of [Ru(PPh,),(styrene),]. An X-ray analysis reveals structure (398) with four bridging hydrides and a short Ru-Ru distance (2.556(3) A), consistent with a Ru-Ru double bond (although it is possible that this compound might be [Ru,H,N,(PP~,),]).~~'~ 'H and "P NMR spectra indicate that the crude hydrogenation product is a mixture of [RuH,(PPh,),] and [Ru,H,(PPh,),].2s03 Similar diamagnetic binuclear complexes [Ru,H,L,] (L = PPr',, PBu',), are produced by reaction of [Ru(cod)(cot)] with L a t 40°C (although further work suggests these should be reformulated as [Ru2H6L4]~ p e c i e s ) , whereas ~ ~ ' ~ with PCy,, is isolated.'398The latter loses H2 thermally, photochemically or in vucuo to give [RuH,(PC~,)~] [Ru,H,(pCy,),]. Protonation of [RUH,(PCY~)~] with HBF,/H,O leads to [Ru(OH,),(PCy,)]BF, whereas reaction with ethylene produces the metallated complex [RuH(C,H,,P(C6H,I)2}(PCy3)(C, H,),].2497a
The silyl-rutheniurn(1V) complexes [RuH,(SiR,)L,,] (n = 2,3; L = PPh,, AsPh,, P(p-Tol),) and [RuH,X(SiR,)L,] (x = CI, I) are discussed in Section 45.3.2.3.
45.10.6 References to Catalytic Reactions of Ruthenium Hydrido and Related Complexes
A number of the compounds discussed in this section e.g. [RuHCl(PPh,),], [RuH(OCOR)(PPh,),], [RuH,(PP~,)~], [RuHCl(CO)(PPh,),], etc. and some from other sections e . ~ [Ru(CO),(PPh,),], . [RuCl,(PPh,),], [RuCl,(EPh,),MeOH] (Section 45.5)[RuCl,(DMSO),] (Section 45.7.9), 'RuCl, xH,O' (Section 45.9) are excellent homogeneous catalysts (or catalyst precursors) for such reactions as hydrogenation, isomerization, hydro formylation, polymerization, oxidation etc. of various organic substrates. Further references to these catalytic reactions are to be found in Chapters 61.1-61.5 and 67 of the present work and also in the following books and reviews. 'Comprehensive Organometallic Chemistry' edited by Sir Geoffrey Wilkinson, F. G. A. Stone and E. W. Abel, Volume 4 Section 32.9, Volume 8 and Volume 9 (Table 9 of Review Index), Pergamon Press, 1982. 'The Chemical and Catalytic Reactions of Dichlorotris(tripheny1phosphine)ruthenium(1I) and its Major Derivatives' by F. H. Jardine in 'Progress in Inorganic Chemistry', 1984, Volume 31, pp. 265-370. 'The Chemistry of Ruthenium' by E. A. Seddon and K. R. Seddon, Elsevier, 1984. 45.11 MIXED DONOR ATOM LIGANDS
45.11.1 N-0 Ligands 45.11.1.1
Schifl base complexes
Treatment of [Ru,(CO),,] with N,N'-bis(salicyla1dehydo)ethylenediamine (salenH,) in either DMF2OS9or toluenezm gives the dimeric yellow complex [Ru(salen)CO],; this compound is also formed by carbonylation of tr~ns-[Ru(salen)(PPh,),]'~~~ (see Section 45.5.4.6.vii). Reaction with pyridine forms [Ru(salen)COpy] which undergoes facile oxidation with [PhN,]BF,, [pNO,C,H,N,]BF, or @t30]BF4to generate [R~(salen)COpy]BF,.~~@" Interaction of [RuCl,(CO),], or [RuCl,(CO),], with Na,[salen] yields cis-[Ru(salen)(CO),] and with [RuCl,(nbd)],, [Ru(salen)(nbd)] is Nitrosation of [Ru(salen)(PPh,),] in THF at W C affords trans[Ru(ONO)(salen)NO] (399) with an 0-bonded nitrito Reaction of [Ru(acac),] with N-benzylethylsalicylideneimine (LH) in ethyl benzoate gives the chiral tris complex [RuL,] which can be separated by TLC into its enantiomers.2s06
Ruthenium
464
0
\
N
I
0
(399)
For Schiff base complexes containing tertiary phosphines, see Section 45.5.4.6.vii.
45.1 I .I. 2 Oxime complexes ( i ) IsonitPosoketones
Reaction of 'RuX, *xH,O' with a-benziloxime [PhC(=O)C(=NOH)Ph] (HB) gives products formulated on the basis of IR, ESR, magnetic and electrochemical data as trans-[Ru"'X,(HB)B] (400) (X = C1, 3r).134772507 A wider range of isonitrosoketone (hydroxyiminoketone) complexes of type (400) are now available, (R = R = Me; R = Ph, R = H, Me; R' = Me, R = Ph), and these undergo both chemical and electrochemical reduction to generate the blue [RuX,(HB)B]- anions. Addition of Et,N deprotonates (400) to g v e [RuX,B,]- and reprotonation occurs on treatment with HC104?508 Similar reactions occur with arylazooximes [RC(=NOH)N=NAr] to give and some ruthenium isonitrosoketonates containing 2,2[RuX,(HL)L] (see Section 45.4.5),1347*1349 bipyridyl ligands are described in Section 45.4.3.1 .xiii. O"CMe
I
0N'
7 RL p%/,,,,,
* p ; c / R'
c
I\ R/ckN(
@*
Ru
I
;I
ky>'\~
I X O---H----0 (400)
//C\,,Me
/p%*, I ,*,*' d Me
Hc 4-CF3TPP(equation 153), [Ru(OEP)(CO),] (436) being stable to loss of CO under vacuum over several h o ~ r s . 2 ' ~ ~ The rates of phenyl ring rotation and ring current effects in ruthenium complexes of TPP Exchange of 4-z-butylpyridine in derivatives have been probed by NMR spectroscopy.258~2s82~2596 [Ru(porph)(CO)(4-Butpy)](porph = 4-XTPP; X = C1,Me, OMe, Et,N, CF3) occurs via a dissociative mechanism with AG* = 77.4-82.8kJmol-l, AH* = 82.&92.5kJrnol-' and AS' = 2142 J K-' mol-' ?583 Multiple resonance studies on [Ru(porph)(CO)(imid>]show that imidazole tautomerism (equation 154) occurs via a dissociative, intermolecular exchange (AG' = 76.5 kJ mol-'),2s96rather than intramolecularly as suggested p r e v i o ~ s l y ~Inter~ ' ~ *and ~~~~ 7
,258292583
Ruthenium
470
Rzo
(429) R = mesityl (430) HzTPrnP R = Pr"
R'
R' -
RZ
H CF,
H
Me Pr
H H €I H H H H H Me nicotinamide
c1 Br '
F OMe Et2N
so; H
H
Formulae H2TPP HZ4-CF3TPP
H
H24-MeTPP H24-Pr'TPP Hz4-CIITPP H24-BrTPP H24-FTPP H ,4-MeOTPP Hz 4-E t ZNTPP H ZCSOBTPP H22-MeTPP Hz2-nicotinamide TPP
R02C
COzR
(431) R = H (432) R = M e (433) R = ClsH37
& co
K
t
HN
(434) H,Etio I
[Ru(OEP)CO]
+
CO
(435)
e
[Ru(OEP)(CO),]
1153)
(436)
intra-molecular exchange reactions have been observed for 43- and 3,6-dimethylpyridazine adducts, intramolecular site exchange via a seven-coordinate ruthenium(I1) intermediate (437) (equation 155) occurring at 20-85 times faster than intermolecular p r o c e s ~ e s . ~"P ' ~ NMR data for adducts of [Ru(OEP)CO] with bidentate phosphine ligands have been tentatively attributed to the formation of seven-coordinate adducts (438) (equation 156).2579A molecular orbital scheme for [Ru(porph) (CO)L] has been The complexes [Ru(porph)(CO)L] generally show two, one electron oxidations and one, one electron reduction. For [Ru(OEP)(CO)(THF)] and [Ru(EtioT)(CO)(THF)], 'Et = + 0.61 and 0.64V vs. SSCE respectivelyz592 while for [Ru(4-XTPP)CO] (X = H, Me, MeO, F, Cl, Br), 'Eox= 0.74-0.86, 2Eox= 1.18-1.27 and Eed = - 1.24 to 1.35 V YS. SSCE in DMS0.2576,2s93 On the basis of electronic, IR and ESR data, the first oxidation has been assigned as ligand-based to give the n-cation radical [Ru" (porpht )(CO)L]+~576~2592,2593~2597,2598Interestingly the osmium analogue [Os"(OEP)(CO)py] is oxidized at the metal centre at +0.4V vs. SSCE to afford [Os"'(OEP)-
+
I Ruthenium
47 1
co
co
I
co
uo
I
c'o
I
a I
',
*N-N
fJ
Me
Me
I
(155)
0 '
Me Me
PSP
I
I
co
(CO)py] '.2592 The effect of variation of porphyrin ligand?''' solvation and axial l i g a t i ~ n " ~ ~on *~'~~*~~ the porphyrin ring oxidation has been assessed. The 'H NMR spectra of n-cation radical complexes incorporating OEP and meso-porphyrin IX dimethyl ester have been reported and related to data on horseradish peroxidase and myoglobin derivatives.2s48The second oxidation has been assigned to a metal-based redox couple to yield [R~"'(porph*)(CO)L]~+, with the reduction process being sited on the The stabilization of ruthenium(1) in the reduced complexes has also been proposed.2m' For [Ru(4-Et2NTPP)(C0)(4-Butpy)] three oxidation couples at E* = + 0.50, 0.73 and 1.06 V vs. SCE in CH2C12are observed.25wIntramolecular electron transfer reactions between porphyrin and metal centres induced by changes in axial ligation have been studied and are summarized in Scheme 78.2568,2592~2602~263 For example, addition of Br- to purple ?Azl, ground state) [Ru(OEP')CO]+ yields green ('A,, ground state) [Ru(OEP')Br(CO)] which can be converted to [Ru"'(OEP)(PBu" 3)2]+ on addition of P B u " ~ ? ~Addition ~' of CO to [Ru"'(OEP)(AsPh,),]+ yields [Ru" (OEP')(CO)(AsPh,)]+ r e ve r ~ i b l y . ' ~ ~
+
+
11
--O
[ Ru"'(0EP)py
,I+
11 11
-e-
[ Ru(0EPf)COpy ]+
ll
-e
[Ru(OEPf)CO]+
-e
Scheme 7%
11
-e-
5 [Ru"'(OEP)(PR,),l+
472
Ruthenium
-
Flash photolysis of [Ru(OEP)CO] has been investigated.260qThe lifetime of the excited state species [Ru(porph)CO]* (porph = TPP derivatives) has been measured as 3 0 p s with emission occurring near 730 nm.2576 Oxidative and reductive quenching of [Ru(TPP)CO]* and related species has been reported, E* for the [Ru(TPP)CO]*’+ redox couple being assessed as - 0.57 V vs. SSCE.2576 [Ru(TPP)CO] photocatalyses the generation of H2 from H,O in aqueous micellar solutions in the presence of h y d r ~ g e n a s e , ~ and ~ ’ catalyses the oxidation of sulfides to sulfoxides under O2at 100 psi (7 x IOskgm-2) and 100°C.137s [Ru(4-SO3TPP)C0I4- has been reported to catalyse the water-gasshift reaction.2569 Reaction of two equivalents of [Ru(TPP)CO] with one equivalent of 4,4‘-bipy produces the binuclear species [Ru(TPP)CO],* 4 , 4 ’ - b i p ~ . ’ ~ ~ Photolysis of [Ru(porph)(CO)(HOEt)] at 300 nm in the presence of L yields [Ru(porph)L,] (L py, porph = Tpp,25” OEp,2571J5’2Jm~268 Etio I,26o6meso-porphyrin IX dioctadecyl ester?591 L = MeCN, porph = OEP, TPP;2M)4 L = DMSO, porph = OEp6O8).The single crystal X-ray structure of trans-[Ru(OEP)py,] (439) shows octahedral ruthenium(1I) in the plane of the porphyrin ligand with Ru-N(py) = 2.1 A?6o8The resonance Raman spectrum of [Ru(OEP)py,] has been Oxidation of [Ru(porph)py,] (porph = TPP, OEP) at 0.08 V vs. SSCE yields [RulI1 [Ru(TPP)py,]+ has been detected in redox quenching reactions of excited state (porph)py,]+ ;2592,2593 [Ru(TPP)py,]* with [Ru(NH&~+.lBS Photolysis of [Ru(OEP)(CO)py] in MeCN yields [Ru(OEP)(NCMe)py] (440) which can be converted to [Ru(OEP)Brpy] (441) on oxidation with Br,.2611Reaction of [Ru(OEP)Brpy] with AgPF, in MeCN affords [Ru(OEP)(NCMe)py]+.26’1 Scheme 79 summarizes the synthetic routes to ruthenium porphyrins complexes.257372607s2611
+
PR3
0
e
&
I
II
co
It
0
Br
NCMe
I
B ’ 2
*&
(443)
I
I
PY (441)
H C-CH2
I
H~C-CHZ \ /
’T
(416)
THF
OEt
0
I C \ / C W H
OEt
(445)
H
HOEt
\
N,CHCO,Et, Ph
t
(446)
(447)
CY
N,CHMe
HOEt (444)
CI
H
I
C II
(23 I I
0
4
HCI
CH,CI,
e23 I
Me
\ /
co
0
I
I
(23 I
I
c1
OH
(449)
(448)
co
Scheme 79
Refluxing solutions of [Ru(porph)(CO)(HOEt)] with phosphines L yields the complexes iP(4 [Ru(porph)L,] (porph = OEP, TPP; L = PPh3, +PPh,, PBu”,, P(4-MeOC&),, M ~ O 6CH4 )3 r 2564,2567,2568 PEt,, PMe,Ph, P(OMe)3,260’bis(dipheny1phosphino)methane (dpm), bis(diphenylphosphino)ethane, bis(diphenylphosphino)butane, bis((dipheny1phosphino)ethyl)ethyl-
Ruthenium
473
mine;2579,261 2 porph = 4-CF3TPP, L = P(OMe),2580).The single crystal X-ray structure of trans[Ru(OEP)(PPh,),] (442) shows six-coordinate ruthenium(II), Ru-P = 2.438, Ru-N = 2.057, 2.044A with the central metal atom in the plane of the macrocyclic ring!567 An X-ray structural analysis of [Ru(TPP)(dpm),] shows monodentate dpm ligands with Ru-P = 2.398, RuThe dissociation of phosphine ligand L in the complexes [Ru(porph)L2] has N = 2.038,2.045 been as~essed,~~~*~'@' equilibrium constant K = 1.2 x lo-' M for dissociation of [Ru(OEP)(PPh,),] in toluene at room temperature (equation 157).26'3[Ru(TPP)(PPh,),] is an efficient catalyst for the decarbonylation of aldehydes.2614 IRu(OEP)(PPh,),]
[Ru(OEP)(PPh,)]
+
PPh,
(157)
The electrochemistry of bis-phosphine ruthenium porphyrin complexes has been reported 0xidation of [Ru(porph)L,] (porph = OEP, TPP; L = PPh,, PBu",) affords (Scheme 78).2568Jm1-2603 the ruthenium(IT1) species [Ru(porph)L,]+ which converts to [Ru(porph)BrL] in the presence of Br .2568*2602*2611Likewise direct oxidation of [Ru(porph)(CO)L] or [Ru(porph)L,] with Br,, or HBr The complexes [Ru(OEP)ClL] have been prepared by an in air gives analogous procedure using C12, or HCl in air.26" The single crystal X-ray structure of [Ru(OEP)BrPPh,] (443)shows ruthenium(II1) out of the porphyrin plane by 0.049 A towards the PPh, ligand, with Ru-Br = 2.552 A, Ru--P = 2.415 A, Ru-N = 2.025-2.047 A?6i'Reduction of [Ru(OEP)BrPPh,] by zinc amalgam yields [ R u ( O E P ) P P ~ , ] .[Ru(OEP)BrPPh,] ~~'~ in the presence of iodosyl benzene catalyses the oxidation of alkenes, cyclohexane and PPh3.26i5The ruthenium(1V) species [Ru(OEP)(O)j and [Ru(OEPf )(O)Br] have been tentatively proposed as intermediates in the Oxidation of PPh, to OPPh3.26'5The formation of ruthenium(1) species in the reduction of [Ru(TPP)(PR,),] has been Treatment of [Ru(OEP)py,] with KCN over a prolonged period affords [Ru"'(OEP)(CN),]which can be converted to [Ru(OEP)(CN)py] on reaction with py.2574Reduction of [Ru(OEP)(CN)py] with sodium amalgam in the presence of CSCl, yields [Ru(OEP)(CS)py]; the species [Ru"(OEP)(CN)L]- (L = py, CO) have been identified in sol~tion?~"Isocyanide adducts [Ru(porph)(CNR),] (porph = OEP, TPP, R = Bz;26'6.26'7 porph = 4-CF3TPP, R = Bu'2580)can be isolated from reactions of [Ru(porph)CO] with RNC; for [Ru(TPP)(CNBz),], yNC = 2148 Addition of L (L = py, 1 -methylimidazole, 4-terr-butylpyridine) gives [Ru(porph)(CNB~)L].~~'~*~~'~ Kinetic measurements on axial ligand exchange in these compounds indicate that their lability increases in the order Ru(Pc) < Ru(porph) 4 Fe(Pc) < Fe(porph) (Pc = phthalocyanine).261632617 Reaction of [Ru(porph)CO] (porph = meso-porphyrin IX dimethyl ester) with NO in benzene yields [Ru(porph)(NO),], vN0 = 1786 and 1838 cm-'.258992590 [Ru(OEP)(CO)py] with NO in MeOH affords [RU(OEP)(OM~)NO].*~~~ The aryldiazonium adducts [Ru(porph)(N,Ar)solvent]+ (porph = TPP, Ar = Ph, 4-MeOPh, solvent = acetone; porph = TPP, Ar = 4-N02Ph, solvent = EtOH; porph = OEP, Ar = 4-N02Ph, solvent = EtOH) have been prepared by reaction of [Ru(porph)(CO)(HOEt)] with ArNz+ in refluxing CH,C12.2618 The stretching vibration vN=N occurs near 1800 cm- for these complexes.2618 The binding of 0, to ruthenium(I1) porphyrins has been investigated; the formation of adducts [Ru(porph)(O,)py] (L = rneso-porphyrin IX dioctadecyl ester)2m and [Ru(OEP)(O,)(NCM~)]~~~' has been reported. [Ru(OEP)PPh,] and [Ru(OEP)(PPh,),] react with 0, in toluene to give OPPh,, RuO, and H20EP.2613 The reaction is thought to occur via initial binding of 0,to [Ru(OEP)PPh,] An outer-sphere and subsequent formation of [Ru(OEP)OI-&O as a proposed electron transfer between [Ru(OEP)(PPh,),] and O2 to give superoxide and Ru"' has been (444),prepared by reduction of [Ru(TPP)OR],O (445) with NaBH, n ~ t e d . ~ ~[Ru(TPP)(HOEt),] ".~~'~ in THF and addition of EtOH, is oxidized by 0, to [Ru(TPP)(OEt)]20.2573 {Ru(TPP)(OEt)(HOEt)] (446)can be isolated from the reaction (Scheme 79) and its single crystal X-ray structure shows a centrosymmetric Ru"' with Ru-0 = 2.019A.2573Oxidation of [Ru(porph)], (447) with O2 in the presence of trace H20,2607 or oxidation of [Ru(porph)COj (porph = OEP, TPP) with tert-butylhyd r o p e r ~ x i d egives ~ ~ ~the ~ binuclear Ru'" complex [Ru(porph)OW],O (448).An X-ray structural analysis of [Ru(OEP)OH],O shows a linear RuORu backbone with Ru-O(bridge) = 1.847 A, Ru-OH = 2.195A, Ru-N = 2.067A with a staggered conformation of porphyrin rings.2620A range of related binuclear Ru'" species [Ru(porph)X],O (porph = OEP, TPP, T P r n PX = C1- ,Br- , - O h , CF,CO,-, HS04-, -OMe, -OEt, 4--OC6H4Me, 2--OC6H40H) have been prepared.2573,2607.2619,2621 The single crystal X-ray structure of [Ru(OEP)Cl],O (449) shows a linear RuORu moiety with Ru-0 = 1.793, Ru-C1 = 2.320, Ru-N = 2.038 A,2621awhile an X-ray structural analysis of [Ru(TPP)OR],O (OR = 4-OC,H4Me) shows < RuORu = 177.8", Ru-0 (bridge) = 1.789 and Ru-OR = 1.964A.2573 Cyclic voltammetry of the binuclear Ru'" complexes
Ruthenium
474
[Ru(OEP)X],O (X = C1-, Br-, -OH) shows three, two electron redox couples in CH,CI, assigned to Ru" Ru" /Ru'" Ru"' and RulllRull'/Ru'lRu"processes and oxidation of the porphyrin More recently, oxidation of [Ru(porph)CO] (porph = 429) with MCPBA or iodosyl benzene has yielded the trans-dioxo ruthenium(V1) species [Ru(porph)O,] which reacts with P(OMe), to give two equivalents of Me,PO and [Ru(porph)(P(OMe),),]. Interestingly, oxidation of the tolyl complex (419) yielded the Ru'" p o x 0 dimer, suggesting that steric hindrance of (429) is important for the generation of the mononuclear Ru" oxo Vacuum pyrolysis (ZlO'C, l o p 5torr) of red [Ru(porph)py,J (439) yields a green dimeric product [Ru(porph)], (porph = TPP, OEP, 4-MeTPP).2577~260"260s The single crystal X-ray structure of [Ru(OEP)12(447)shows Ru-Ru = 2.408 A, Ru-N = 2.050 8, with the Ru" ion 0.30 A out of the plane of the porphyrin ligands which are twisted by 23.8" with respect to one a n ~ t h e r . ~ ~ " . ~ ~ ' Paramagnetic shifts in the 'H NMR spectra of these complexes have been analysed and related to a qualitative molecular orbital diagram suggesting a formal Ru-Ru bond order of two with an electronic configuration a:ge;fb:,~~e*,2.2577.2622 The binuclear complexes bind CO in the presence of py to yield [Ru(porph)(CO)py]. The binding of first row metal ions Mn"', Fe", Cot', Ni", Cui' and Zn" to [Ru(porph)CO] (porph = rneso-tetra(2-nicotinamidophenyl)porphyrin, 428) has been investigated by electronic spectra and cyclic v ~ l t a m m e t r yReaction . ~ ~ ~ ~ of R U~ ( C O)with ~ , (450) in refluxing THF for 3 h leads to metal insertion to yield (451), together with the complex (452), formed by oxidative addition of The single crystal X-ray structure ruthenium into a pyrrole carbon-nitrogen bond (Scheme 80).2627d of (452) shows cis carbonyl ligands on six-coordinate Ru" with direct bonding of ruthenium to a carbon atom of the macrocyclic ligand (Ru-C = 2.086A) and a non-bonding pyrrole nitrogen atom?623"Very recently, the synthesis of alkylidene and alkene complexes of ruthenium porphyrins reduction of (447) with K in THF yields a violet-black precipitate of the has been ruthenium porphyrin dianion K2[Ru'lporph)] (Scheme 79).'621b RCR Ph
II
c t
Scheme 80
45.12.2
Phthalocyanines
The coordination chemistry of the phthalocyanine dianion Pc2- (H,Pc = 453) has been reviewed+2562.262&2626Early work on the complexation of ruthenium by phthalocyanines reported the reaction of 2-cyanobenzamide or 1,2-dicyanobenzenewith RuC13to give '[Ru(Pc)]', '[Ru(Pc)Clj' or '[Ru(C!PC)]'.'"~2631 More recently treatment of RuCl, with 2-cyanobenzamide or 1,2-dicyanobenzene at 250°C for 4 h is reported to yield [Ru"(Pc)] together with [Ru"'(Pc)Cl] as a minor product and less than 5% yield of [ R U ( P C ) C O ] .Alternatively ~~~~ [Ru(Pc)] can be prepared by reaction of RuC1, with 2-cyanobenzamide in naphthalene at 290°C for 1 h,2633 while the direct synthesis of [Ru(Pc)CO] has been achieved by treatment of Ru,(CO),, or RuCl, under CO with 1,2-dicyanobenzene or H,Pc at elevated tempeatures (greater than 200°C).2632-2634 Extraction of [Ru(Pc)] and 4-methyl pyridine:633 [Ru(Pc)CO] with ligands L affords the adducts [Ru(Pc)L,] (L = py,2632.2633 4-rert-b~tylpyridine,2~~~ aniline,2627*z632 2-meth~laniline,,~'~ PBu"!, P(OBU"),:~~~ and 4-methyl pyridine, 4-tert-butyl p y r ~ d i n e ' ~respectively. ~~) The species [Ru(Pc)(CO)L](L = py,2632*2633 [Ru(Pc)(DMSO),] is likely to incorporate S-bonded sulfoxide and has been found to be a useful precursor to a series of bis-adduct complexes [Ru(Pc)L,] (L = MeCN, py, imid, DMF) formed by photolysis of the DMSO adduct in the presence of L.2635Carbonylation of [Ru(Pc)L,] affords [ R U ( P C ) ( C O ) L ] . ~Refluxing ~ ~ ~ - ~ ~solutions ~~ of [Ru(Pc)] in DMF gives [Ru(Pc)CO(DMF)] which can be converted to [Ru(Pc)L2] on photolysis in L.2635The polymeric pyrazine-bridged species [Ru(Pc)pz], has been rep0rted.2~~~ Scheme 8 1 illustrates the synthetic chemistry of ruthenium
phthalocyanine^.^^'^,^^^^
Rurheniurn
475
Q (453) HZPC
I
Me-,mid
[ Ru(Pc)(CNBz)(Me-imid)]
I
DMF
[ Ru(Pc)(CO)(DMF)
1
",k
[ Ru(Pc)(DMF)~I
I
[Ru(Pc)(DMSO), 1
// I
'E:d
[ Ru(Pc)(imid),]
7 [Ku(Pc)(NCMe), 1 hv
\
[ Ru(Pc)(CO)(NCMe)l
Scheme 81
For [Ru(Pc)COpy] v , - _ ~ occurs at 1965 cm-I compared with 1939cm-' for [Ru(TPP)COpy] indicating that back-bonding from Ru" to CO is greater in the latter Pc complexes are found to be less labile than the corresponding porphyrin species, ligand substitution occurring via a dissociative mechanism.z617~z634 The complexes [Ru(Pc)(CO)L] and [Ru(Pc)L,] (L = py, 4-methylpyridine, 4-tert-butylpyridine, imid, MeCN, DMF, DMSO) each show a reversible one electron oxidation in CH,Clz; E: = + 0.91 V and + 0.77 vs. SCE for [Ru(Pc)COpy] and [Ru(Pc)py,] respectively.2635 Quantitative oxidation gives the R radical cation products [Ru(Pc')(CO)L] and [Ru(Pct)L2] which have been ~ ~redox ~ ~ activity of ruthenium phthalocyanicharacterized by electronic and ESR s p e ~ t r o s c o p ythe nes have been compared to the corresponding porphyrin analogues.2635 The photochemistry of [ R ~ ( P c ) p y , ]and ~~~ the ~ quenching of excited state species [Ru(Pc)L,]* (L = DMSO) and [Ru(Pc)(CO)L]* (L = py, DMF)2638have been investigated. [Ru(Pc){(CD,),S0}2] has been used as a 'H NMR shift reagent.2639Protonation of [Ru(Pc)] at uncoordinated N atoms of the macrocyclic ring has been studied~628~2640~261 '[RuPc]' has been found to catalyse the decomposition of H202,2629 the oxidation of propene by 022642 and the carbonylation of nitro compounds to isocyanates.2643 The exact nature of the complex '[Ru(Pc)CI]' remains unknown. Formulations of a ruthenium(II1) complex with axial C1- , [ R u ( P c ) C ~ ] , ~~ or a~ 'ruthenium(I1) species with chlorinated Pc, [R U (C I P C ) ] have ,~ ~ been proposed, although 'H, "C NMR and electronic spectral data appear to The complexes [Ru(ClPc)L,j (L = PPh,, py, show no evidence for chlorination at the Pc ring.5.2634 methyl imidazole, P(OBu"),, P(Bu"),) have been The synthesis of [Ru{Pc-(SO,),}]~(H,Pc-SO, H = phthalocyanine tetrasulfonic acid) has been mentioned.5
45.12.3 Tetraaza Macrocycles A range of low spin, octahedral ruthenium(II1) complexes incorporating tetraaza macrocyclic ligands have been synthesized. Reaction of [RuCl, (OH,)]'- with L in alcoholic solvent under reflux affords trans-[RuCl,(L)j+ (L = 454463).632a.632b,634,2645-2649 Oxidative dehydrogenation of the coordinated macrocyclic ligand can occur2646 and has been used to prepare trans-[RuCl,(L)]+ (L = 460 and 461) by aerial oxidation of trans-[RuC1,(L)]+ (L = 458 and 459) respectiveIy?&18The single
476
Ruthenium
crystal X-ray structure of trans-[RuCl,(L)j+ (L = 454) shows distorted octahedral Ru"' with Ru-Cl = 2.343, Ru-N = 2.083A.'" ESR spectra of trans-[RuCl,(L)]+ (L = 454) have been reported.635Reaction of [RuCI,(OH,)]~- with cyclam (454) in MeOH over three days yields a mixture of cis- and truns-[RuCl,(L)]+ which can be separated by ion exchange and gel filtration and characterized by UV-visible s p e ~ t r o s c o p y ? ~The ~ " single crystal X-ray structure of the cis isomer shows Ru-CI = 2.371 A with Ru-N = 2.104-2.i17A.2615bReduction of these products by zinc amalgam yields the corresponding Ru" species (Et = -0.23V, -0.19V vs. NHE for cis- and truns-[RuCI, (L)]+ respectively) which react with isn to afford cis- and tr~ns-[Ru(L)(isn),]~+ .2645a Cis-[RuX,(L)]+ may also be prepared by reaction of [Ru(ox),]~- with L (L = 454, 455) in the presence of HX (X = C1-, Br-).633More recently, the complexes cis-[Ru(L)(L'),I2+ (L = 454, L' = py2, bipy, phen) have been synthesized.2645b Substitution reactions of tranls-[RuCl,(L)]+ to give trans-[RuX,(L)j+ (X = for example, Br- ,I-, NCS-, NOz-, OAc ) by direct reaction with X- or via reduction to Ru" have been des~ r i b e d . ~ ~ " . ~The " ~ kinetics , ~ ~ ~ ' of ~ ~ligand substitution in [RuX,(L)]+ and [RuX,(L)] (X = Ci -, Br-) have been measured and suggest a dissociative mechanism for Ru"' complexes with the rate The acid and base hydrolysis of halide release decreasing in the order L = 459 > 458 > 454.638,26s0 of complexes of cyclam (454) have been studied and related to data for tetraammine, bis-en and related open chain species, an S, 1 cb mechanism being operative for base h y d r o l y ~ i s . ~ ~ ~ , ~ ~ " ~ ~ ' ~ ~ ~ The electrochemistry of the ruthenium macrocycles cis- and trans-[RuX,(L)l"+;("-"+ (L = 454462, X = C1- ,Br- ,I-, NCS- ,OH,, NO,-) have been investigated; the Ru""~'couple was found to vary over the range - 0.80 to + 0.83 V vs. Ag/Ag+ and was related to the electronic and steric effects of L and X.633*639*2" Kinetic data for the reduction of 6runs-[RuCl2(L)]+(L = 454,455, 458, 459) by Cr'l and V" suggest that electron transfer occurs via inner- and outer-sphere mechanisms r e s p e c t i ~ e l y . ~ @ The ~ variation in redox properties of [Ru(CN),]~--'~-in the presence of macrocyclic polyamine receptors has been i n ~ e s t i g a t e d . ~ ~
(458) C-meso
(459) C-rac
(460) C-meso (461) C - ~ U C
(463)
The absorption spectra of trans-[RuX,(L)]+ (L = 454456,458,459, X = CI- ,Br.-, I - ) show two ligand to metal charge transfer bands. Irradiation at the lower energy CT band leads to stereoretentive aquation of X-; the effects of L on the photoreactivity of these complexes have been discussed and related to their open chain analogues.&22 Reduction of [RuCl,(L)]+ (L = 463) at -0.20V vs. SCE in aqueous 4-toluene sulfonic acid affords [Ru(L)(OH,),]~+which can be oxidized to [Ru" (0)(L)(OH,)]2+ with H, O2or electrochemically at 0.6-0.7V vs. SCE. This latter species shows a Ru=O stretching vibration vR,,=* at 855 cm-' .2649a More recently, this work has been further developed, the single crystal X-ray structures of ~rans-[Ru'~ (0)(L)(NCMe)12+,2629b ~runs-[Ru'~ (0)(CI)(L)]+2619c and trans(o),(L)]~2M9d being reported. The chemical and electrochemical interconversion of these The template products has been achieved, and the oxidative activity of Ru" analogues assessed.2M9c synthesis of [Ru(L)(OH,)] (CIO,), (L = 464) has been reported although the exact nature of this product is Reaction of [Ru,(OAc),Cl] with LH, (LH, = 465) in ethanol yields the +
Ruthenium
477
mixed valence Ru" Ru"' binuclear cofacial s cies [RuL],+ befi = 1.Si5 BM) the single crystal X-ray structure of which shows Ru-Ru = 2. 26 7 r w i t h a formal Ru-Ru bond order of 2.5?12' Reduc= 2.88 BM) with a tion of [RuL),+ with NaBH, in ethanol affords the Ru"Ru" dimer [RuL], (peR Ru-Ru bond distance of 2.379 A and formal bond order of two.212'Reaction with CO and py yields [Ru(L)(CO] and [Ru(L)py, ] respectively .2120
45.12.4
Triaza Macrocycles
A range of complexes incorporating the triaza macrocycle, 1,4,7-triazacyclononane (446), have been synthe~ized?~"~ these include [Ru"Cl(L)(DMSO),]+, [Ru"(L),I2+, [Ru"'X3(L)] (X= C1-, (C1-OH),~-MeCO,)l3''*', Br- ) and [Rul"(ox)(I)(L)] and the binuclear species, [Ru'"~(L)~ [ R U " ~ ~ ( L ) ~ ( ~ - O Hand )~C [ R~U~,]( ~L + ) ~ ( ~ - C I ) ~The ] ~ +single . crystal X-ray structure of [Ru"',(L),(p OH),(p-MeC0)2]I,-H,0 shows a short Ru-Ru bond distance of 2.572 A, indicating the presence of a single Ru-Ru bond for the diamagnetic The electrochemistry of the binuclear complexes was
45.12.5
Tetratbia Macrocycles
Reaction of RuCl, or {RuCl,(OH,)]'- with L (L = 467469) in 2-(2-ethoxy)ethanol for two days gives cis-[RuCl, (L)] + .632a,639.2652 The complexes incorporating (467) were originally formulated as trans isomers on the basis of IR data,"', but were later reformulated as cis the single crystal X-ray of cis-[RuCI,(L)] (L = 467) shows Ru-Cl = 2.471, Ru-S = 2.262, 2.333A.2653A range of substituted derivatives [RuX,(L)]+ (X = C1 ., Br- , I-, NCS-, NOz-",N3-) have been prepared and their electrochemistry in~estigated.6~~7~~~' More recently the reaction of RuCl, with (467) in ethanol for 2h has been reported to afford a brown productfuc-[RuCl,(L)] in which the macrocycle is though to be tricoordinate; treatment with LiBr in acetone yieldsfa~-[RuBr,(L)].'~~
ACKNOWLEDGEMENT We are very grateful to Professors M. C. Baird, P. Braunstein, M. I. Bruce, B. Chaudret, D. J. Cole-Hamilton, .I. P. Collman, E. C. Constable, F. A. Cotton, P. H. Dixneuf, V. L. Goedken, M. L. H. Green, G. A. Heath, B, R. James, A, Ludi, P. M. Maitlis, R. J. Mawby, D. W. Meek, T. J. Meyer, J. H. Nelson, R. K. Porneroy, T. B. Rauchfuss, E. A. Seddon, K. R.Seddon, N. Sutin, H. Taube, L. M. Venanzi and G. Wilkinson for reprints and/or preprints of their work.
Ruthenium
47 8 45.13
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1209. 1210. 1211. 1212.
Ruthenium 1275. 1276. 1277. 1278. 1279. 1280. 1281. 1282. 1283. 1284. 1285. 1286. 1287. 1288. 1289. 1290. 1291. 1292. 1293. 1294. 1295. 1296. 1297. 1298. 1299. 1300. 1301. 1302. 1303. 1304. 1305. 1306. 1307. 1308. 1309. 1310. 1311. 1312. 1313. 1314. 1315. 1316. 1317.
.
1318. 1319. 1320. 1321. 1322. 1323. 1324. 1325. 1326, 1327. 1328. 1329. 1330. 1331. 1332. 1333. 1334. 1335. 1336. 1337. 1338. 1339. 1340.
497
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~
Ruthenium 1406. 1407. 1408. 1409. 1410. 1411. 1412. 1413. 1414. 1415. 1416. 1417. 1418. 1419. 1420. 1421. 1422. 1423. 1424. 1425. 1426. 1427. 1428. 1429. 1430. 1431. 1432. 1433. 1434. 1435. 1436. 1437. 1438. 1439. 1440. 1441. 1442. 1443. 1444. 1445. 1446. 1447. 1448. 1449. 1450. 1451. 1452. 1453. 1454. 1455.
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499
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Ruthenium 2551. 2552. 2553. 2554. 2555. 2556. 2557. 2558. 2559. 2560. 2561. 2562. 2563. 2564. 2565. 2566. 2567. 2568. 2569. 2570. 257 1. 2572. 2573. 2574. 2575. 2576. 2577. 2578. 2579. 2580. 2581. 2582. 2583. 2584. 2585. 2586. 2587. 2588. 2589. 2590. 259 1. 2592. 2593. 2594. 2595. 2596. 2597. 2598. 2599. 2600 260 1 2602 2603 2604 2605 2606 2607 2608 2609 2610 261 1 2612 2613 2614. 2615. 2616.
517
N. K. Roy and C. R. Saha, J. Inorg. Nucl. Chem., 1980,42, 37. K. Hiraki, Y . Obayashi and Y. Oki, Bull. Chem. SOC.Jpn., 1979, 52, 1372. R. L. Bennett, M. I. Bruce, B. L. Goodall, M. 2.Iqbal and F. G. A. Stone, J. Chem. Soc.. Dabon Truns., 1972,1787. M. Nonoyama and S. Kajita, Transition Met. Chem., 1981, 6, 163. M. I. Bruce, M. Z. Iqbal and F. G. A. Stone, J. Organomel. Chem.. 1971, 31,275. R. L. Bennett, M. I. Bruce and I. Matsuda, Aust. J. Chem., 1975,28, 1265. M. I. Bruce, B. L. Goodall and F. G. A. Stone, J. Organomet. Chem., 1973, 60, 343. J. M. Patrick, A. H.White, M. J. Bruce, M. I. Beatson, D. St. C. Black, G. B. Deacon and N. C. Thomas, J. Chem. SOC.,Dalton Trans., 1983, 2121. D. Dolphin, B. R. James and P. D. Smith, Coord. Chem. Rev., 1981, 39, 31. L. J. Boucher, ‘Coordination Chemistry of Macrocyclic Compounds’, ed. G. A. Melson, Plenum, New York, 1979, pp. 517-536. K. M. Smith (ed.),‘Porphyrins and Metalloporphyrins’, Elsevier, Amsterdam, 1975. (a) D. Dolphin red.), ‘The Porphyrins’, Volumes I-VII, Academic, New York; (b) B. D. Berezin, ‘The Coordination Compounds of Porphyrins and Phthalocyanines’, Wiley, London, 1981, E. B. Fleischer, R. Thorp and D. Venerable, Chem. Commun., 1969,475. B. C. Chow and I. A. Cohen, Bioinorg. Chem., 1971, 1, 57. D. Cullen, E. Meyer, T. S. Srivastava and M . Tsutsui, J. Chem. SOC., Chem. Commun., 1972, 584. J. J. Bonnet, S. S. Eaton, G. R. Eaton, R. H. Holm and J. A. Ibers, J . Am. Chem. Sue., 1973, 95, 2141. S. Ariel, D. Dolphin, G. Domazetis, B. R. James, T. W. Leung, S. J. Rettig, J. Trotter and G. M. Williams, Can. J. Chem., 1984, 62, 755. M. Barley, J. Y. Becker, G. Domazetis, D. Dolphin and B. R. James, Can. J . Chem., 1983, 61, 2389. M. Pawlik, M. F. Hog and R. E. Shepherd, J. Chem. SOC.,Chem. Commun., 1983, 1467. M. Tsutsui, D. Ostfeld and L. M. Hoffman, J. Am. Chem. Soc., 1971,93, 1820. A. Corsini, H. Mehdi and A. Chan, Can. J. Chem., 1980, 58, 527. A. Antipas, J. W. Buchler, M. Gouterman and P.D. Smith, J. Am. Chem. SOC.,1978, 100, 3015. J. P. CoHman, C. E. Barnes, P. J. Brothers, T. 3. Collins, T.Ozawa, J. C. Gallucci and J. A. Ibers, J. Am. Chem. Soc.. 1984, 106, 5151 and references therein. P. D. Smith, D. Dolphin and B. R. James, J. Organomet. Chem., 1981, 208, 239. G. R. Eaton and S. S. Eaton, J. Am. Chem. Sac., 1975, 97, 235. D. P. RiIlema, J. K. Nagle, L. F. Barringer and T. J. Meyer, J. Am. Chem. SOC..1981, 103, 56. J. P. Collman, C . E. Barnes, P. N. Swepston and J. A. Ibers, J. Am. Chem. SOC.,1984, 106, 3500. R. G. Little and J. A. Ibers, J. Am. Chem. SOC..1973, 95, 8583. G. Domazetis, 3 . R. James and D, Dolphin, Inorg. Chim. Acta, 1981, 54, L47. S. S. Eaton and G. R. Eaton, Inorg. Chem., 1976, 15, 134. S. S. Eaton and G. R. Eaton, J. Chem. SOC.,Chem. Commun., 1974, 576; J. Am. Chem. SOC.,1975,97,3660. S . S. Eaton and G. R. Eaton, J. Am. Chem. Soc., 1977, 99, 6594. S. S. Eaton and G. R. Eaton, Inorg. Chem., 1977, 16, 72. S. S. Eaton, G. R. Eaton and R. H. Holm, J. Urganomet. Chem., 1971, 32, C52; 1972, 39, 179. C. J. Malerich, Inorg. Chim. Acta, 1982, 58, 123. C. M. Elliot and R. R. Krebs, J . Am. Chem. Sou., 1982, 104,4301. T. S. Srivastava, Biochim. Biophys. Acta, 1977, 491, 599. M. Tsutsui, D. Ostfeld, J. M. Francis and L. M. Hoffman, J . Coord. Chem., 1971, 1, 115. T. S. Srivastava? L. Hoffman and M. Tsutsui, J. Am. Chem. Sou., 1972,94, 1385. K. G. Srinivasamurthy, N, M. N. Gowda and G. K. N. Reddy, J. Inorg. Nucl. Chem., 1977,39, 1977. F. R. Hopf and D. G . Whitten, J . Am. Chem. Soc., 1976,98,7422. G. M. Brown, F. R. Hopf, T. J. Meyer and D. G. Whitten, J. Am. Chem. SOC., 1975,W, 5385. G. M. Brown, F.R. Hopf, J. A. Ferguson, T. J. Meyer and D. G . Whitten, J . Am. Chem. SOC.,1973,955939. T. S. Srivastava, Inorg. Chim. Acta, 1981, 55, 161; 7.S . Srivastava, Indian J. Chem., Secl. A , 1979, 18, 284. D. R. Paulson, A. W. Addison, D. Dolphin and B. R. James, J. Eiol. Chem., 1979, 254,7002. J. W. Faller and J. W. Sibert, J . Urganomet. Chem., 1971, 31, C5; J. W. Faller, C. C. Chen and C. J. Malerich, J. Inorg. Eiochem., 1979, 11, 151. T. Malinksi, D. Chang, L. A. Bottomley and K. M .Kadish, Inorg. Chem., 1982, 21,4248. I. Morishima, Y. Shiro and T. Takamuki, J. Am. Chem. SOC.,1983,105,6168. K. M. Kadish and D. Chang, Inorg. Chem., 1982, 21,3614. K. M. Kadish, D. J. Leggett and D. Chang, Inorg. Chem., 1982, 21, 3618. T. Boschi, G. Bontempelli and G. A. Mazzocchin, Inorg. Chim. Acta, 1979, 37, 155. M.Barley, J. Y. Becker, G. Domazetis, D. Dolphin and B. R. James, J. Chem. SOC.,Chem. Commun., 1981, 982. M. H. Barley, D. Dolphin and 8. R. James, J. Chem. SOC.,Chem. Commun., 1984, 1499. M. Barley, D. Dolphin, B. R. James, C. Kirmaier and D. Holton, J. Am. Chem. Soc., I984, 106,3937. I. Okura and N. Kim-Thuan, J. Ghem. SOC.,Chem. Commun., 1980, 84. G. W. Sovocool, F. R. Hopf and D. G. Whitten, J, Am. Chem. SOC.,1972,94,4350. J. P. Collman, C. E. Barnes, T. J. Collins, P. J. Brothers, J. Gallucci and J. A. Ibers, J. Am. Chem. SOC.,1981, 103, 7030. F. R. Hopf, T. P. OBrien, W. R. Scheidt and D. G. Whitten, J . Am. Chem. SOC.,1975,97,277. N. Farrell, D. Dolphin and B. R. James, J. Am. Chem. SOC.,1978, 100, 324. G. A. Schick and D. F. Bocian, J . Am. Chem. Soc.. 1984,106, 1682. B. R. James, D. Dolphin, T. W. Leung, F. W. B. Einstein and A. C. Willis, Can. J . Chem.. 1984, 62, 1238. R. G. Ball, G. Domazetis, D. Dolphin, B. R. James and J. Trotter, Znorg. Chem., 1981, 20, 1556. B. R. James, S. R. Mikkelsen, T. W. Leung, G. M. Williams and R. Wong, Inorg. Chim. Acta. 1984, 85, 209. G. Domazetis, B. Tarpey, D. Dolphin and B. R. James, J. Chem. Sou., Chem. Cornmun.. 1980, 939. T. Leung, B. R. James and D. Dolphin, Inorg. Chim. Acto, 1983, 79, 180. C. E. Holloway, D. V. Stynes and C. P. J. Vuik, J . Chem. Soc,, Dalton Trans., 1982, 95.
518 2617. 2618. 2619. 2620. 2621. 2622. 2623.
2624. 2625. 2626. 2627. 2628. 2629. 2630. 2631. 2632. 2633. 2634. 2635. 2636. 2637. 2638. 2639. 2640. 2641. 2642. 2643. 2644. 2645. 2646. 2647. 2648. 2649.
2650. 2651. 2652. 2653.
Ruthenium F. Pomposo, I?. Carruthers and D. V. Stynes, Inorg. Chem., 1982, 21, 4245. N. FarrclI and A. A. Neves, Inorg. Chim. Acta, 1981, 54, L53. H. Sugimoto, T. Higashi, M. Mori, M.Nagano, Z. Yoshida and H. Ogoshi, Bull. Chem. Soc. Jpn., 1982, 55, 822. H.Masuda, T. Taga, K. Osaki, H. Sugimoto, M. Mori and H. Ogoshi, J. Am. Chem. Soc., 1981, 103, 2199. (a) H. Masuda, T. Taga, K. Osaki, H. Sugimoto, M. Mori and H. Ogoshi, Bull. Chem. Soc. Jpn., 1982,55,3887;I. T. Groves and R. Quinn, Inorg. Chem.. 1984, 23, 3844. J. P. Collman, C. E. Barnes and L. K. Woo, Proc. Natl. Acad. Sei. USA, 1983,80,7684. (a) Y.-W. Chan, F. E. Wood, M. W. Renner, H. Hope and A. L. Balch, J . Am. Chem. Soc., 1984, 106,3381; Y.-W. Chan, M. R. Renner and A. L. Balch, Organometallics, 1983,2, 1888; (b) J. P. Collman, P. J. Brothers, L. McEIweeWhite, E. Rose and L. J. Wright, J. Am. Chem. Soc., 1985, 107,4570; (c) B. R. James, A. W. Addison, M. Cairns, D. Dolphin, N. P. Farrell, D.R. Paulson and S . Walker, Fundam. Res. Homogeneous Cutal., 1979, 3, 751. L. J. Boucher, ‘Coordination Chemistry of Macrocyclic Compounds’, ed. G . A. Melson, Plenum, 1979, pp. 461-516. K. Kasuga and M. Tsutsui, Coord. Chem. Rev., 1980, 32, 67. F. H. Moser and A. L. Thomas, ‘The Phthalocyanines’, C.R.C., Boca Raton, FL, 1983, volumes I and 11. : P. C. Krueger and M. F. Kenncy, J. Inorg. Nucl. Chem., 1963, 25, 303. 13. D. Berezin and G. K. Sennikova, Dokl. Akad. Nauk SSSR, 19M, 159, 117; 1962, 146, 604. A. N. Shlyapova and B. D. Berezin, In. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnoi., 1970, 13, 1218. I. M. Keen and B. W. Malerbi, J. Inorg. Nucl. Chem., 1965, 27, 1311. I. M. Keen, Platinum Met. Rev., 1964, 8, 143, S. Omiya, M. Tsutsui, E. F. Meyer, 1. Bernal and D. L. Cullen, Inorg. Chem., 1980, 19, 134. N. P. Farrell, A. J. Murray, J. R. Thornback, D. H. Dolphin and B. R. James, Inorg. Chim. Acta, 1978, 28, L144. M. M. Doe r and D. A. Sweigert, Inorg. Chem., 1981, 20, 1683. D. Dolphin, B. R. James, A. J. Murray and J. R. Thornback, Can. J . Chem., 1980, 58, 1125. M. Hanack, A. Datz, W. Kobel, J. Koch, I. Metz, M. Mezger, 0. Scheider and H. J. Schulze, J. Phys., Colloq. (Orsay, Fr.), 1983, C3, 633. D. R. Prasad and G . Ferraudi, Znorg. Chem., 1982, 21, 4241. D. R. Prasad and G . Ferraudi, Inorg. Chem., 1983, 22, 1672. C. K. Choy, J. R. Mooney and M. E. Kenny, J. Magn. Reson., 1979,35, I. 6 . D. Berezin, Zh. Obshch. Khim., 1973, 43, 2738. B. D. Berezin, Izv. Vyssh. Uchebn Zaved., Khim. Khim. Tekhnol.. 1968,11, 537. I. I. Ioffe and G . A. Zapevalov, Latv. PSR, Zinat. Vestis, Kim. Ser., 1977, 430 (Chem. Abstr., 1911, 87, i52 579). M. Hiraoka and M. Ito, Jpn. Pat. 74 14 731 (Chem. Abstr., 1974, 81, 120 195). L. J. Boucher and P. Rivera, Inorg. Chem., 1980, 19, 1816. (a) S. S. Isied, Znorg. Chem., 1980, 19,91I; 1980, 19, 3552; (b) C.-M. Che, S.-S. Kwong, C.-K. Poon, T.-F. Lau and T. C. W. Mak, Inorg. Chem., 1985, 24- 1359. C.-K. Poon and C.-M. Che, Inorg. Chem., 1981, 20,1640. D. D. Walker and H. Taube, Inorg. Chem., 1981, 20, 2828. C.-K. Poon and C.-M. Che, J . Chem. SOC.,Dalton Trans., 1981, 1019; C.-M. Che, S . 4 . Kwong and C.-K. Poon, Inorg. Chem., 1985, 24, 1601. (a) C.-M. Che, T.-W. Tang and C.-K. Poon, J. Chem. Soc., Chem. Commun., 1984,641; (b) C.-M. Che, K.-Y. Wong and T. C . W. Mak, J. Chem. Soc.. Chem. Cornmun., 1985, 546; (c) C.-M. Che, K.-Y. Wong and T. C. W. Mak, J , Chem. Sou., Chem. Commun., 1985, 988; (d) T.C. W. Mak, C.-M. Che and K.-Y. Wong, J. Chem. SOC.,Chew. Comrnun., 1985,986; C.-M. Che, K.-Y. Wong and C.-K. Poon, Inorg. Chern.,1985, 24, 1797. C.-K. Poon and T.-C. Lau, Inorg. Chem., 1983, 22, 1664. (a) C.-K. Poon and C.-M. Che, J. Chem. Soc., Chem. Commun., 1979, 861; (b) K. Wieghardt, W. Hemnann, M. Koeppen, I. Jibril and G. Huttner, 2. Naturforsch., Teil E, 1984, 39, 1335. C.-K. Poon and C.-M. Che, J. Chem. SOC..Dalton Trans., 1981, 495. Dalton Trans., 1982, 1465. T.-F. Lai and C.-K. Poon, J. Chem. SOC.,
46 Osmium WILLIAM P . GRlFFlTH Imperial College of Science and Technology, London, UK 46.1 INTRODUCTION 46.1.1 History 46.1.2 Review on Osmium 46.1.3 General Features of the Coordination Chemistry of Osmium 46.1.3.1 Oxidation slates 46.1.3.2 Coordination numbers 46.1.3.3 The ligands in the complexes 46.1.3.4 Special mpects of osmium coordination chemistry
522 522 522 522 522 523 524 524
46.2 MERCURY AS A LlCiAND 46.2.1 OsmiuM-Mercury Complexes
525 525
46.3 GROUP N COMPLEXES 46.3.1 Cyanide Complexes 46.3.1.1 OJmiumfIj 46.3.1.2 Osmium(Ir) 46.3.1.3 Osmium(II1) 46.3.1.4 Osmium(IV) 46.3.1.5 Osmium(VI) 46.3.2 Fulminaro Complex 46.3.3 Osmium-Tin Complexes 46.3.3.1 Complexes containing SnX; and halo ligan& 46.3.3.2 Cumplexes coniaining SnCl; and other ligmdr
525 525 525 525 526 526 526 526 526 521 527
46.4 NITROGEN LIGANDS 46.4.1 Ammine Complexes 46.4. I . 1 Unsubstiturcd ammines, [Os( N H , laI '' 46.4.1.2 Substitured ammines 46.4.2 Complexes with Monodentate Amines 46.4.3 Complexes with Bidenrate Amines 46.4.3.1 Complexes with ethylenediamine 46.4.3.2 Complexes with deprotonated ethylenediamine 46.4.3.3 A complex of 2,3-butanediimine 46.4.4 Complexes with Nitrogen Heterocycles 46.4.4.1 Comple.Tes of pyridine and of substituted pyridines 46.4.4.2 Comple.res with chelating pyridines 44.4.4.3 Piperidine ('C6H,, N ) complexes 46.4.4.4 Bifunctional heterocyclic donors 46.4.4.5 Tetrafwctional ligands 46.4.4.6 Polypyridyi complexes of osmium 46.4.5 Nitrosyl Complexes 46.4.5.1 [OsjNO)'f ond /Os( NO)]' complexes 46.4.5.2 [Os(.VO) f ',[ O S ( N O ) ]and ~ [0s(NO),I8 complexes 46.4.5.3 ~ O S ( N O ) [] O~s,( N O ) ] ' " and [ O S ( N O ~ ~complexes ]'~ 46.4.6 Complex of HNO 46.4.7 Diazenido Complexes 46.4.8 Thionitrosyl ( N S ) Complexes 46.4.8.1 OsmiumfIIj 46.4.8.2 O s m i u m ~ I ~ ~ j 46.4.9 Complex of NSCI 46.4.10 Dinitrogen Complexes 46.4.10.1 Mononuclear complexes 46.4.10.2 Binuclear dinitroRen-bridged complexes 46.4. I 1 Hydrazine Cotnplexes
527
519
528 528 529 530 530 530 53 1 533 533 533 534 535 535 537 537 544 545 549 551 55 1 55 I 552 552 553 553 554 554 556 556
520
Osmium
46.4.12 Hydroxylamine Complex 46.4.13 Amino (NH,) and Organoamido (NR,j Complexes 46.4.13.1 Amino complexes 46.4.13.2 Alkanolaminato and diaminato complexes 46.4.14 Imido (=NH) and Organoimido {=NR) Complexes 46.4.14.1 Complexes with =NH 46.4.14.2 Complexes with =NR 46.4.15 Nitrido Complexes 46.4.I5.I Mononuclear complexes 46.4.15.2 Polynuclear complexes 46.4.15.3 Polynuclear complexes of mixed oxidation state 46.4.16 Triazenido Complexes 46.4.17 Azido Complexes 46.4.18 Complexes of Pseudohalide Ligands 46.4.1 8.1 Thiocyanato complexes 46.4.19 Nitrile Complexes 46.4.19.1 Osmium( I I ) 46.4.19.2 Osmium(III) 46.4.20 Miscellaneous Nitrogen Donor Complexes 46.4.20.1 Eiguanide complexes 46.4.20.2 Eenrotriazole complexes 46.4.20.3 Phosphineiminato (NPR,) and phosphineaminato (NHPR,) complexes 46.4.20.4 Other nitrogen donor complexes
46.5 PHOSPHORUS, ARSENIC AND ANTIMONY LIGANDS 46.5.1 Teriiary Phosphine Complexes 46.5.1.1 Phosphine hydrido complexes 46.5.1.2 Halo phosphine complexes 46.5.2 Tertiary Phosphile Complexes 46.5.2.I O.rmium( 0 ) 46.5.2.2 Osmium( I I ) 46.5.2.3 Osmium(III) 46.5.2.4 Osmium(IV) 465.3 Phosphorus Trihalide Complexes 46.5.3.1 Osmium(- 11) 46.5.3.2 Osmium ( 0 ) 46.5.3.3 Osmium(11) 46.5.3.4 Osmium(III) 46.5.3.5 Higher oxidation states 46.5.4 Tertiary Arsine Complexes 46.5.4.1 Hydrido arsines 46.5.4.2 Halo ursine complexes 463.5 Tertiary Stibine Complexes 4 6 S A Bidenlate Phosphine and Arsine Complexe.T 46.5.7 Complexes with Polydentate Phosphines and Arsines
46.6 OXYGEN-CONTAINING LIGANDS 46.6.1 Aqua and Hydroxo Complexes 46.6.2 Osmium Oxo Complexes 46.6.2.I Mononuclear complexes 46 6 2 . 2 Binuclear oxo-bridged complexes 46.6.3 Dioxygen, Superoxo and Peroxo Species 46.6.4 Alkoxo Complexes 46.6.5 B-Diketonate Complexes 46.6.5.1 Osmium( I I ) 46.4.5.2 Osmiurn(III) 46.6.5.3 Osrniurn(IV) 46.6.6 TropaIanato Complexes 46.6.7 Catecholato Complexes 46.6.7,I Tris catecholato complexes Os (cat) and Os (Rcat) 46.6.7.2 Bis caiecholato complexes 46.6.7.3 Polymeric catecholato complexes 46.6.7.4 Complexes with naturally occurring catechols 46.6.8 Ketone Complex 46.6.9 Oxoanions as Ligands 46.6.9.1 Nitrato and nitro complexes 46.6.9.2 Phosphato and sulfato complexes 46.6.9.3 SulJto and hydrosulfito complexes 46.6.9.4 Tr$ato complex 46.6.9.5 Carboxylato complexes 46.6.10 Dimethyl Sulfoxide Complex 46.6.11 Complexes with Carbohydrates I
557 557 557 557 557 558' 558 560 560
563 565 565 565 565 565 566 567 567 567 567 568 568 569 569 570 570 572 575 575 575 575 575 575 575
516 576 576 576 576 576 576 577 578 579
,
579 579 579 580 593 596 596 596 597 597 597 597 597 597 597 598 598 598 598 598 599 599 600 600 602 602
Osmium 46.7 SULFUR-CONTAINING LIGANDS 46.7.1 Hydrosuuido and Disulfur Ligands 46.7.2 Surfida Phosphine Complexes 46.7.3 Thioether Complexes 46.7.3.1 Dialkyl suJfides 46.7.3.2 Chelating &des 46.7.4 Dithiocarbamaio fS,CNR; ), Trithiocarbamato (S,CNR; ), Dithiocarbonaio ( S , C O R - ) and Phosphinodithionate (S, PR; ) Complexes 46.7.4.1 Dithiocarbamates 46.7.4.2 Trithiocarbamates 46.7.4.3 Alkyl dirhiocarbonato complexes 46.7.4.4 Alkyl- and aryl-phosphinodithionates 46.7.S Dithiolene Complexes 46.7.6 Sul/ur Dioxidp Complexes 46.7.7 Di- and Mono-ihio-fi-diketonates 46.7.7.1 Dithioacetylacetonate (4-thio$ent-jl-ene-Z-thione, S,C, H; , sacsac) 46.7.7.2 Monothio-8-diketonale complex 46.7.8 Monothione Complexes 46.7.8.1 Imida~olidine-2-thiones 46.7.8.2 Thiazolidine-2-thione 46.7.9 Thiolato Complexes 46.7.9.1 Osmiumflllj 46.7.9.2 Osmium(IV) 46.7.10 Dithiocarboxyate m d Thiocarboxylate Complexes 46.7.11 Thiourea Complexes 47.7.1 I .I Osmium fll) 46.7. I I .2 Osmium (IIIJ 46.7.11.3 Osmium(V1j 46.7.12 Miscellaneous S,S Donor Ligands
52 1 603 603 603 603 603 603 604 604 604 605 605 606 606 606 606 606 607 607 607 607 607 607 607 608 608 608 608 608
46.8 SELENIUM- AND TELLURIUM-CONTAINING LIGANDS 46.8.1 Seienido Complexes 46.8.2 Selenoether Complexes 46.8.3 Diethylselenocarbamato Complex 46.8.4 Imidazolidine-2-selones 46.8.5 Tellurium-containingComplexes
609 609 609 609 609 609
46.9 HALOGENS AS LIGANDS 46.9.1 Fluoride,?and Fiunrn Complexes 46.9.1.1 Osmium f WJ 46.9.1.2 Osmium( V ) 46.9.1.3 Osmium(VIj 46.9.1.4 Osmiurn(VI1) 46.9.1.5 Osmium( YIIIj 46.9.2 Chloro, Bromo and lodo Complexes 46.9.2.1 Osmium(llI) 46.9.2.2 Osmium ( I V ) 46.9.2.3 Osmium(Vj
609 609 609 61 1 611 612 612 613 613 613 615
46.10 HYDROGEN AND HYDRIDES AS LIGANDS 46.10.1 Tetrahydroborate Complex 46.10.2 Borane Complex
615 616 616
46.1 1 MIXED DONOR ATOM LIGANDS 46.11.1 Bidentate A-0 Complexes 46.11.1.1 Benzamido ligand 46.1 1 .l.2 Hydroxyhenzunzido IiRands 46.1 I .2 Complexes of Miscellaneous Sulfur-Nitrogen Ligan& 46.1 1.2.1 Sulfarnaro complexes 46.11.2.2 2-Thioamidopyridine complexes 46.1 I .2.3 Miscellaneous sulfur-nitrogen complexes 46.1I .3 Complexes of Miscellaneous Sulfur-Oxygen Ligands
616 616 616 616 616 616 617 617 617
46.12 MULTIDENTATE MACROCYCLIC LIGANDS 46.12,I Porphryin Complexes 46.12. I . 1 Osmium(1V) 46.12.1.2 Osmium f V I ) 46.12.1.3 Dimeric complexes 46.12.1.4 Complexes amiih other prophyrins 46.12.2 Phthaiocyonine Complexes 46.12.2.1 Osmium(1lJ 46.12.2.2 Osmium(II1)
617 617 618 618 618 618 618 618 619
Osmium
522 46.12.2.3 Osmium(IV) 46.12.2.4 Osmium( V I ) 46.13
REFERENCES
619 619 619
46.1 INTRODUCTION 46.1.1
History
Since osmium was first isolated as the tetroxide, one of its most important and celebrated coordination complexes, a brief account of the history of its coordination chemistry is not out of place. The tetroxide, and subsequently the metal, was first isolated in 1803 by Smithson Tennant‘ (1761-1815) by distillation with nitric acid of the black material derived after aqua regia treatment of platinum metal concentrates. Of the tetroxide Tennant wrote: ‘As the smell is one of its most distinguishing characters, I should on that account incline to call the metal Osmium’ (from m p q , a smell). Tennant narrowly escaped being overtaken by Fourcroy and Vauquelin2q3 in the discovery of osmium. Early work on the coordination chemistry of the element was carried out by Berzelius; but the most extensive of these early studies were those of Claw (17961864),‘ who in I844 had discovered ruthenium.
46.1.2
Reviews on Osmium
The classic works of Gmelin6*’give the most extensive coverage of the coordination chemistry of the element, although the volumes on its organometallic chemistry have not yet appeared. The latest edition' covers the period 1939 to 1980 and the present author wrote the sections concerned with the coordination chemistry. There are other less comprehensive but useful sources of information. Pascal covers the literature up to 1958’ while Mellor’s treatise is still useful for the older work (up to 1936).’ The author has written a book on the four rarer platinum metals (osmium, ruthenium, iridium, rhodium) covering the literature to 1967 and a short early review on osmium (1965);” Livingstone has written a book on the six platinum metals (1975).12There are recent reviews on the high oxidation state chemistry of osmium,13“and on standard potentials for osmium and ruthenium.‘3hOther reviews in the osmium chemistry of specific ligands are mentioned at appropriate points in the text. The organometallic chemistry of the element is of course covered up to 1982 in Comprehensive Orgunometullic Chemistry (vol. 4, p. 967).
46.1.3
General Features of the Coordination Chemistry of Osmium
It is hoped that these will become apparent as the text proceeds, so the following comments are very general.
46.1.3.1
Oxidation states
The three elements osmium, ruthenium and rhenium are distinguished from all others by the variety of the oxidation states which they display in their Complexes, and of these three osmium is probably the most versatile in this respect. Examples of the main classes of complexes found for each oxidation state are listed in Table 1 . It will be seen that all the states from VI11 (do) to - I1 ( d ” ) are represented. It is this ability of the element to adopt different oxidation states at the behest of the surrounding ligands which gives osmium chemistry its special flavour and attractions. The principal reason for its versatility in this respect is surely its position within the block of transition metals in the Periodic Table. Its membership of the third row means that its outer ( 5 4 orbitals are relatively exposed so that their electronic occupancy is susceptible to the nature of the coordinating ligands, and its central position within that third row means that attainment of the d oconfiguration typical of elements to the left, and of the d’’ typical of elements to the right, are both possible for osmium. The element can thus be used as a convenient backdrop or exemplar of acceptor properties when discussing the donor properties of ligands. Thus, strong IT donors such as F-,0’-and N3-
523
Osmium
favour high oxidation states (Os""',Os"), strong n acceptors such as CO and NO4 are associated with Os" or Os" while NO+ attains, in a formal sense at least, the Os-" state. 'Conventional' ligands such as halide, ammonia and ethylenediamine tend to favour the I11 or IV states, while ligands with moderate n-acceptor capabilities (CN-, bipy, phen) will tend to the Os" ( d 6 )state. Table 1 Oxidation States in Coordination Complexes of Osmium Typical stabilizing ligands
Oxidation state
d"
Comments
VI11
do
Fairly rare but important
F-,
VI1
d' d2 d3
Rare
F-, 02F - , O'-,N3-
d4
Relatively common Rare Common
111
d5
Common
I1
d6
Common
VI
V IV
=
F-
Doubtful Fairly rare
d9 d''
Very rare Fairly rare
NO NO', CO, PF,
- I1
alkyl or aryl; X
= CI,Mr,
OSF,, rosa.,1 , OSOF, OsF,, tvans-[Os02X41", [OsNX,].. [OsFsl.-, [OsF,], [OsX,I'-, IOs(NH3),X,l2+, W U ~ - O S ( L RX4~ } ~ [OsX,l3-, IO4NH3),I3', [Os en,]", Os(LR,),X, [OS(CN),]~-,(Os(bipy)3]2+, [Os(phen),]'+ [Os(NO)X,]'- ~rans-Os(LR,),X, Os(NO)(PPh,hCI, Os(CO)s, Os,(cO)n, Os{P(OR),}s, OsW,), Os(NO)depe, Os(NO),(PPh, 12, [WCO)412[OsPF, b l2 -
dS
-I
L = P, As or Sb; R
OsO.,, [OsO,F,]-, [OsO,N]-
F-, C1-, E l - , I-, O H " , PR,, OH-. LR, C1-, Br-, I-, NH,, acac, en, LR, CN-, bipy, phen, NO+, krpY, LR3 ? NO+ CO, P(oR),, PF3
d7
I 0
d-, N3-
Examples"
I
I
I
I.
Since the oxidation state concept is so important for osmium we shall make extensive use of it as a means of classification of material within ligand groups, covering the lowest oxidation states first.
46.1.3.2
Coordination numbers
Osmium, and indeed all six platinum group metals, display rather little enterprise and versatility with respect to coordination numbers, although of course the ability to change coordination number lies at the heart of many catalytic reactions of platinum group complexes. The great majority of osmium complexes are octahedral. In Table 2 we give examples of the rarer coordination numbers. There are few examples of eight or seven coordination (nine coordination has not yet been observed for the element although its possibility for osmium has recently been briefly disc~ssed'~). There are examples of square-based pyramidal geometry, mainly for the higher oxidation states, and trigonal bipyramidal for the lower. Tetrahedral coordination is rare, though found both for osmium(VII1) Table 2 Unusual Coordination Numbers for Osmium CompIexes ~~
-
Coordination numher
Examples
Page in text
8
[Os(en - H),]12, [Os(en - H),en]I, Os(PMe, Ph), H, ? OsF, Os(edta)(H,O) OsF, Os(PMe,Ph),H,, Os(PEt,Ph),H, [Os(PR, ) 4 H 3 1 f Os(PPh3)3HC1, OsO(02 R h 0% 0.4(0, R)z IOsNX,]-, OsOC1, OSPPfi,),X, OS(CO), [Os(PF3)4 HIOs(P(OMe),I, oso, , [OSO,]-[OsO,N]Os(NOh(PPh,L
53 1 57 1
Unknown
612 602
Unknown Monocapped octahedron Pentagonal bipyramidal Distorted pentagonal bipyramidal Unknown Square-based pyramidal Square-based pyramidal Squawbased pyramidal Trigonal bipyramidal Trigonal bipyramidal Trigonal bipyramidal Tetrahedral Tetrahedral Distorted tetrahedral
I
7
5
4
9
612 57 1 570, 571 584 563, 584 573 516 515 588 562 55 1
Shape
Unknown
Osmium
524
(in OsO, and [OsO,Nl-) and for os m h n ( - IT) in Os(NO),(PPh,),. Square planar coordination might be expected for osmium(0) ( d 8 )but does not seem to have been observed for it.
46.1.3.3
The ligands in the complexes
(i) Group VII donors All four halide ions from octahedral complexes with the metal. Fluorides or fluoro complexes are known for osmium(VT1) to osmium(1V) inclusive, with rather tenuous evidence for OsF,; OsF, is a rare example of seven coordination for the metal. The other halides prefer the IV and I11 states, though osmium(V) chloride and chloro complexes have recently been made and also [OsBr,]-. Although [RuF,I3- is well known there is no osmium analogue, in keeping with the general tendency of third-row elements to display higher oxidation states than their second-row partners.
(ii)
Group VI
The oxo (O*-) ligand is dominant in the coordination chemistry of osmium, participating in the VIII to IV oxidation states inclusive. The tetroxide OsO, is the single most important compound of osmium; OsO, and the recently discovered [OsO,]- ion are tetrahedral. The trans-[O=Osv'=O] 'osmyl' moiety displays an extensive chemistry, comparable with that of the 'uranyl' O=U=O unit, and there is an extensive and unique cyclic oxo-ester chemistry (p. 584). There is surprisingly little information on hydroxo, aqua, sulfato, nitrato or phosphato complexes, but much recent work has been carried out on carboxylato species, and clearly much work remains to be done on the 0-donor chemistry of the element. There are a reasonable number of sulfur-donor complexes but few with selenium or tellurium.
(iii)
>
Group Y
There are now a substantial number of nitrido complexes of osmium(V1) and osmium(1V) as well as the 'osmiamate' ion [OsV"'O,N]-. In addition to the ammine and ethylenediamine complexes, much recent work has been carried out on the bipy, phen and terpy complexes, often in connection with research into the photodissociation of water, The nitrosyl chemistry of the element, though seemingly not as extensive as that of ruthenium, has received much attention, and there has been considerable work on the phosphine, arsine and stibine complexes.
(iv) Group IY The wide area of osmium cluster carbonyl chemistry lies beyond the scope of this chapter (see refs. 15 and 16), though a few carbonyl complexes are covered, and of course there is full coverage of the cyanide chemistry of osmium.
46.1.3.4 Special aspects of osmium coordination chemistry We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of cis-hydroxylation, hydroxyamination or amination of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 5711, osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research.
Osmium
525
46.2 MERCURY AS G LIGAND 46.2.1
Osmium-Mercury Complexes
These are reported as 1:l or 2:l adducts with osmium phosphine or arsine complexes with mercury(I1) chloride, but presumably involve Os-Hg-C1 units. They are the black Os(PMePh,), C1, -HgCl,, the white Os(dppm),C1,-HgC12 and the brown Os(SbPh,),Cl.HgCl,, the grey Os(PPh3),C1-2HgC1, and Os(dppe),CI, 2HgC12,the red Os(dmpm),Cl- 2HgC1, and the black Os(AsPh,), C1.2HgC1,. All are made from (NH.&[oSC&] and the phosphine, arsine and stibine ligands in water-methanol with HgCl, .I7 Reaction of Os(PEtPh2),H, and Os(PEtPh2),H2 with HgCl, gives unidentified species containing Hg:Os ratios of up to 9: 1; evidence for the existence of Os(HgCl)(PR,),H(CO) and Os(HgCl),(PR,),(CO) (PR, = PMe,Ph, PEt,Ph) was presented.'*
46.3 GROUP 1V COMPLEXES The chemistry of osmium is dominated here by the cyanide ligand; there is one fdminato complex, and a number of species containing Os-Sn bonds.
46.3.1 Cyanide Complexes The cyanide ligand is a good cr donor and a fairly effective .n a c c e p t ~ r , ' ~so, ~it* is not surprising that the most stable osmium cyano complex is the d 6 species [Os(CN),I4-, while [os(CN)6]3'- (d') is fairly strongly oxidizing. In truns-[Os02(CN),jZ- (p. 582) and trans-[OsN(CN),H,O]~(p. 562) it is likely that the high oxidation state is sustained by the x-donor oxo ligands (the high vCN IR frequencies, 21 S 1 cm-' in [OSO,(CN),]~-~'and 21 5 5 cm-I in [OSN(CN),(H,O)],~~ suggest that there is relatively little TC bonding in these Os-C linkages). Cyano complexes of osmium, amongst other transition metals, have been reviewed." There is a brief section on osmium cyano complexes in ref. 16, p. 976.
46.3.1.1
Osmium( I )
A species believed to be [Os(CN),I4- can be generated by electron bombardment of [Os(CN),I4in a KCl matrix at 77 KSz3It was detected by EPR; if genuine it is one of the very few examples of monovalent osmium.
46.3.1.2
Osm'um(Il)
The potassium, barium, nickel(I1) and copper(I1) salts have long been known," the normal method of preparation being to treat truns-K2~Os0,(OH),]with excess aqueous potassium cyanide followed by fusion with potassium ~yanide.2~3" A free acid, H,[Os(CN),], can be made by addition of concentrated hydrochloric acid to K, [OS(CN),].~'It is soluble in alcohol but can be precipitated as an etherate by diethyl ether. A study of its IR spectrum and that of its deuteriate suggests that it contains asymmetric -Os-C-N...H-N-C-Oshydrogen bond^.^^-^' There have been a number of studies on the vibrational spectra of [oS(cN)6]'-, mostly on the potassium ~alt.~'-~'. The latest data are given in Table 3 (with comparative results for [Fe(CN),I4and [Ru(CN),I4- ); the Tlgand T,, modes, which are inactive in the Raman and IR, were measured by IETS (inelastic eiectron-tunelling spectroscopy).30The increase in vMPc (e.g. v2, v,) from Fe to Os is consistent with an increase in M-C II bonding in the same sequence,3' though it is interesting to note that this is apparently not borne out by v , . Electronic spectra of [OS(CN),]~-have been measured3, and a~signed;,~.~, the lODq value for the ion exceeds 34000m-', much greater than those for [Fe(CN),I4- and [Ru(CN),I4-." Likewise, I3CNMR chemical shifts for [0s(CN),l4compared with [Fe(CN),I4- and [Ru(CN),I4- are consistent with greater (a + x ) metal-carbon overlap in the former.3s There are also ESCA,36'830sMOssbauer3'" and I4NNMR data2' for [Os(CN),I4-, and spin-lattice relaxation times for the water molecules in K,[OS(CN)~].3 H 2 0 have been determined.37b have been measured. Reaction rates between [Os(CN),I4- and [MnO,],- ,38 and also with A mixed-valence complex of the Prussian blue type, KFe1'1[OS1'(CN)61,has been rep~rted.~'
Osmium
526
Table 3 Raman and Infrared data (cm-I) on [M(CN)&I4-(M = Fe, Ru,
oSp [Fr ( CN j , 1'-
[Ru ( C N ) ,14-
[Or(GK), 1'-
2090 385 2055 409 354
2108 425 2068 425 344 2048
2109 458 206 1 449 362 2038
2044 586 415
5 55
(390)
-
-
-
506
465
476
( 105) 474
a
550
371 -
-
49 1
513
Raman spectrum, aqueous solution. IETS spectroscopy. 'IR of K,[M(CN),] in KBr disc.
A few substituted osmium(I1) cyano complexes are known (for [OS(NO)(CN),]~-see p. 546). Reaction of (NH,),[OsCl,] and 2,2'-bipyridyl followed by treatment with NaCN gives Os(CN),(bipy),-2H,O in which the CN groups may be cis. Protonation is said to occur in perchloric acid to give Os(CN),(bipy),.2HCI04 .* 46.3 .I .3 Osmium(1Il)
The salt (Buf;N),[Os(CN),] is made by an ion-exchange procedure from [Os(CN),I4- in air. The magnetic moment is 2.12 BM a t 296 K. The electronic spectrum was measured and assigned; 10 Dq is 34 950 for [Fe(CN),],-, and 38 000 m-' for [OS(CN)6I3-.34 The [oS(cN),]3- /[Os(CN),]*couple of +0.634V compares with + 0.356V for the [Fe(CN),]3-/[Fe(CN),]4- couple and with + 0.860V for fRu(CN),I3- /[Ru(CN),p- A higher value might have been expected for osmium, may arise but apparently the palarographic value of + 0.99 V, reported in the earlier literat~re,~' from reduction of cyanohydroxo complexes rather than from [0s(CN),l3The redox potential for the [Os(CN),(bipy),]' /Os(CN),(bipy), complex is 0.778 V.40 The (by electrolytic reduction of an OsO,/CN- mixture) and [Os(Cspecies [OS(CN),(OH)~]~N)4(OH)z]3-(reduction of trans-[OsOz(CN),]*-) have been .1993334L
46.3.1.4
+
Osmium(IV)
Although [Os(CN),]'- species have been claimed, it is clear that they are in fact [os(cN)6]4p salts."
46.3.1.5
Osmium( VI)
For trans-[OsO,(CN),I2- see p. 582; for rruns-[OsN(CN),(H,O)]-, see p. 562. There are also reports of [OSO,(OH),(CN),]~-
46.3.2
Fulminato Complex
The only reported example of an osmium fulminato complex is (Ph,As),[OsO,(CNO),], made by reaction of OsO, and potassium fulminate in a sealed tube followed by precipitation with tetraphenylarsonium chloride. The I R spectrum has v(C=N) at 2297 and 2181 cm-' and vas (OsO,) at 824 cm-
46.3.3 Osmium-Tin Complexes Osmium forms relatively few non-organometallic metal-metal-bonded complexes (for species containing Os-Hg bonds see above, p. 525). In common with the other platinum group elements, however, it does form species of the type [Os(SnX,),XI2-, and these have received some recent study.
Osmium
527
46.3.3.1 Complexes containing SnX; and halo ligan& The known examples are listed in Table 4. The general method of preparation is the treatment with SnX, in the presence of HX (for X = F, C1, Br), while the hydroxo species is made ~ , ~substituted ~ complexes with phosphines, arsines and by hydrolysis of the chloro c ~ m p l e x .The stibines are similarly made in the presence of the ligand.”
O f K,[OSC16]
Table 4 Complexes Containing Osmium - Tin Bonds Cornplen”
Colour
Spectroscopic and other siudies
White Pale yellow Bright red
NMR53b IR, M6ssbauerds IR:SAb UV/vis?6 MO~sbauer?~ NMR” IR,4’“6 UV/vis,& MO~sbauer~~
Red Red Red
MOs~bauer~~ IR, MOssbauer4’ IR, MOssba~er,~ IR, MOs~bauer~~ IR, Raman”
a L = thiourea.
The best studied of these species is the chloro complex, initially t h o ~ g h t , to ~ ,be ~ ~a tetramethylammonium salt since (Me,N)Cl was used as the cation; subsequent X-ray data, however, showed it to so the same may be true be an ammonium salt or a mixed ammonium-tetramethylammonium for other anionic complexes listed in the table. The structure shows the presence of two crystallographically independent but very similar anions in which five pyramidal SnCl; groups and a chloro ligand are at the apices of a slightly distorted octahedron. The Os-C1 distance is 2.42(2) A; the mean Os-Sn distance cis to it is 2.580(5) I$ and the trans Os-Sn distance is 2.530(5)A. The Os-C1 bond length is some 0.08 A longer than in other osmium(I1) species so there appears to be a trans influence by the SnCl; ligand, manifested also in the shorter Os-Sn distance trans to the chloro ligand.48A force constant analysis was carried out on the molecule .wThe A very high ‘I7Sn-”’Sn coupling constant of 18 600 6 Hz was found for [0~(SnCl,),Cl]~complex catalyzes alkene isomerization, as does [Os(SnBr,), BrI4:7349
44.3.3.2
Complexes containing SnCl; and other ligunds
Species of the type Os(SnCl,), L2 (L = methyl-, ethyl-, phenyl-, ethylphenyl- and diphenylthioureas and also thioacetamide) have been made from [0~(SnCl,),Cl]~-and L, and the thiourea complexes Os[SnCl,),L,], [Os(SnCl,), LJ, OS(S~CI,),L,]~-and [OsL,](SnCl,),Cl can also be obtained.” The phosphonium salt (Ph, P),[OS(S~C~,)C~,]~is obtained from [Os(SnC15)ClJ4- and Ph,P or Ph,P0.S3a The reaction of (NH,), [0sCl6] and tin(I1) chloride in concentrated hydrochloric acid and phosphines, arsines or stibines gives Os(SnCl,)(PR,), C1, (PR,= PMePh,, PPcPh, PPh,); Os(SnC1,) (LPh,),Cl (L = As, Sb); Os(SnCl,)(L-L),C1 (L-L = dppm, dppe). IR spectra were measured and also, in the cases of Os(SnCl,)(PPh,),Cl,, Os(SnCl,)(dppe),Cl and Os(SnCl3)(SbPh3),C1, Raman spectra.” The II9SnNMR spectra of (PPH,)4[Os(SnC13),j and of [0~Cl(SnCl,),]~have recently been meas ~ r e d , and ’ ~ ~[ o ~ ( snF , ) , ] ~is- also known.,’ For the crystal structure of (PPh,),[Os(NO)CI,(SnCI,),]see p. 549 below.
46.4
NITROGEN LIGANDS
Nitrogen is, after oxygen, the most frequently encountered donor atom in the coordination chemistry of osmium. There is a large and growing body of work on the ammine, pyridine, ethylenediamine and porphyrin complexes, but the most popular and rapidly growing field of study at the present time is that of the ‘polypyridyls’, the 2,2’-bipyridyl, 1,lO-phenanthroline and 2,2’,2,6‘terpyridyl complexes of the metal.
Osmium
528
46.4.1
Ammine Complexes
Ammonia forms a number of complexes with osmium (see Table S ) , though the only fully established unsubstituted osmium ammine which has been isolated is the hexaammine [Os(NH,),]'+ ; there is, however, electrochemical evidence for the existence both of [Os(NH,),]'+ and of [OS(NH,),]~+.Amongst the substituted ammines which have been isolated and characterized are several of osmium(II), all with supporting (or stabilizing) n-acceptor ligands, e.g. [Os(NH3)5C0]2fI 1Os(NH3)5(NO)l3+and [Os(NH3)s(N,)I2+. TaMe S Representative Ammine Complexes Complex
Colour
White
pK,
Spectroscopic and other studies"
15
0
Yellow Yellow Bright yellow
4.0
4.9 6.5
White Lavender X-ray" a
M = magnetic studies; CV = cyclic voltammetry. Bromo and iodo analogues also known.
There is also a wide range of osmium(II1) species, e.g. [Os(NH,),XI2+ (X = C1, Br, I, OH), cisand truns-[Os(NH,),X,]'+ and mer-Os(NH,),X, (X = C1, Br, I), and in fairly recent work the isolation of the corresponding osmium(1V) ammines has been demonstrated. In the presence of a strongly n-donating ligand such as N3-, [ O S ! ' ~ N ( N H ~ ) ~ X(X ~ ]=~ + C1, Br, I, NCS, N,) and the ill-characterized [OS'~N(NH,),(H,O)~+are formed, while two oxo ligands suffice to render trans[OsV'Ol(NH,),]2+ a stable species. In this section we consider unsubstituted complexes first, then aqua, hydroxo, halo, carbonyl and phosphine ammines. For nitrido ammines see pp. 562, 563, nitrosylammines p. 546,and dinitrogen ammines p. 554. Other substituted ammines are dealt with under the section concerned with the substituting ligand.
46.4.1.1
Unsubstituted ammines, [Os(NH, ),$+
Although both the brown [Os(NH,),] and the yellow [Os(NH,),]Br have been claimed as products of the reaction of [Os(NH,),]Br, with potassium in liquid ammonia,54the products were not well characterized and could be hydrido ammines or mixtures. There is evidence from the polarographic reduction of [Os(NN3),l3+for the existenoe of [Os(NH,),]*+ but it appears to be labile to substitution and has not been isolated:55in this respect osmium differs significantly from ruthenium, for [Ru(NH,),]*+ is isolable and is in fact a useful synthetic precursor. The tetravalent complex [Os(NH,),I4+ has also been detected electrochemically, by cyclic voltammetric oxidation of [Os(NH3),j3+;" in acidic solution; however, the first product of one-electron oxidation of [Os(NH3)J3' is probably [OS(NH,),(NH,)]~+.55 Salts of [Os(NH3),I3+ are made from [OsC&]'- and ammonia with zinc dustS6 or from (NH& [OsBr,] and ammonia under pres~ure,~' but a recently devised high-yield procedure involves reaction of [Os(NH,), (NO)],+ with perchloric acid and amalgamated zinc." The magnetic moment of [Os(NH,),]Br, has been measured from 77 to 300KS8Both [Os(NH3),][OsBr,]59and [Os(NH,),]Br[OsBr,] H2Omare known; presumably both contain [Os(NH,),13+ though this is not immediately obvious.
Osmium 46.4.1.2
529
Substituted ammines
(i) Aqua, hydroxo and trijlato ammines The aqua complex [OS(NH,),(H,O)]~+is best made from [OS(NH,),(N,)]~+with HClO, and ~ er iu m ( I V )a; ~reversible ~ polarographic reduction, presumably to [OS(NH,),(H,O)]~+,is observed at pH 4.6' The hydroxopentaammine [OS(NH,),(OH)]~+is made by addition of base to the aqua complex, and this too exhibits a one-electron reversible polarographic reduction to the corresponding osmium(I1) species.61The aqua complex of osmium(II1) is a useful precursor for [OS(NH,),X]~+ species (X = C1, Br, I; see below). The 'triflato' complex [Os(NH,), (OS01CF3)]' , made from [OS(NH,),(N,)]~+and triflic acid with bromine, is a very useful precursor for the preparation of mononuclear [Os(NH,),L]"+ species (L = pyridine, pyraine, pyrimidine, etc., see p. 534) and for binuclear pyrazine-bridged complexes (p. 526).62For trans-~O~O,(NH,),]~+, see p. 582. +
(ii) Halo ammines For convenience we group chloro, bromo and iodo complexes together; no fluoroammines have yet been reported. (a) Osmium(I1). Evidence from cyclic voltammetry suggests that [Os(NH,),XI*+ (X = C1, I) can be reduced to the divalent state, presumably to [Os(NH,),X]+, but that these are labile to sub~titution.~, are made from [Os(NH,),(H,0)j3' and HX (b) Osmium(Z1l). The pentaammines [0s(NH3),XJ2+ (X = C1,55*64Br, P5).Both cis and trans isomers are known of [Os(NH3),X2]+;the cis species are made from ci~-[0s(N,),(NH,),]~+and HX,S5 and the trans complexes are prepared from trans[OSO,(NH,),]~+and HX with SnCl, (X = C1, Br), while for the iodo complex iron wire replaces SnCl, as the reducing agent.,, There is a brief report of the trihalo species mer-Os(NH,),X, (X = C1, Br, I).,, The monoammine complex K2[Os(NH3)C1,], made by reduction of K[OsO,Njwith SnC1, in HC166or with vanadium(I1) or europium(II),S5was once thought to be an amide, K,[OS(NH,)C~,].~~ Spectral data on osmium(II1) haloammines are available (see Table 5); simplified MO theory has been applied to the charge-transfer spectra of [OS(NH,),X]~+and [OS(NH,),X,]+.~* ( c ) Osmiuna(1V). No pentaammine halide species seem to be known oFosmium(N), but cis- and trans-[Os(NH,),X,]'+ and mer-[Os(NH,),X,]+ are made by oxidation of the corresponding osmium(II1) complexes with iron(TI1) in the appropriate acid.55 Osmium(1V) ammines have unusually low pPC, values (see Table 5 ) and so persist only in strongly acidic media; the complexes are more acid by an order of five to six than their platinum(1V) counterparts but only slightly more acidic than the corresponding iridiumlrv) complexes. This has been correlated with the decreasing tzgelectron occupancy in the series Pt'V-Ir'v-Os'V and attributed to charge transfer from ligands to the empty E orbitals on osmium(1V). The cis isomers are more acidic than the trans, and this can be correlated with differences between the osmium(II1) and osmium(1V) complexes for the isomers.55 Nitric oxide reacts rapidly with osmium(1V) di- and tri-haloammines giving, via a 'diazotization' reaction, osmiwn(I1T) dinitrogen ammine Complexes (see p. 555).@ At higher, pH, di- and trihaloammines are unstable and undergo disproportionation reactions to osmium(II1) haloammines and trans-[OsO, (NH,),]'+ .M For [Os(NH,)X,][Os(NH,),X] (X = Cl, Br), see p. 557.
(iii) Carbonyl and phosphine ammines Reaction of [Os(NH,),C1I2+ with formic acid and zinc gives [ O S ( N H , ) ~ ( C O ) ]and ~ ~ ,this ~ ~ can be oxidized to [Os(NH,),(CO)l3+ with [IrCi6l2-; oxidation of this with cerium(N) leads to [Os(N,)(NH,), (C0),l4+ in which there is a p-dinitrogen bridge linking two cis tetraammine carbonyl osmium(I1) moieties (see p. 556).70 The reaction of N,H,. H,O and PPh, with trans-K,[OsO,(OH),] yields Os(PPh,),Cl,(NH,), and the X-ray crystal structure reveals the structure shown in Figure 1. In this, a = 2.136(9), b = 2.41 1(2), c = 2.362(1) and the trans chloro distance d = 2.360(3)A. There is some distortion, the Cl(cis)-Os-Cl(cis) angle being 169.37(1)0?
530
Osmium
Cl Figure 1 Structure of Os(PPh,),Cl,(NH,)7'
Reaction of Os(PMezPh),X, (X= C1, Br) with Os(PMe, Ph)3X2(N H3) .72
ammonia
in
dry
ethanol gives
(iv) Miscellaneous ammines
For tran~-[OsO,(NH,),]~+, see p. 582, and for [OSN(NH,),(H,O)]~+,see p. 562.
(v) Binuciear ammine complexes These constitute an important recent feature of osmium chemistry, but for convenience (and chemical sense) they are considered under the sections dealing with bridging ligands. For such complexes with bridging dinitrogen, see p. 556, with bridging pyrazine, see p. 536, and bridging nitride, see p. 563. The ill-defined [Os,(NH,),Br,]Br, is formed when [Os(H,),]Br, is heated to 305°C;the iodo complex [Os,(NH,),I,]I, is formed when [Os(NH,),]I, is heated to 234 "C.'5
46.4.2
Complexes with Monodentate Amines
We consider here the remarkably small number of complexes of osmium with primary and aromatic amines RNH, ; all these involve phosphines as coligands. For complexes of pyridine and its analogues see p. 533; complexes of polydentate amines are considered in the sections which follow this one (p. 534). Reaction of Os(PMe,Ph),X, with RNHz (X = Cl, R = Me, Et, P f , Pr', Bu"; X = Br, R = Me, Bu") gives Os(PMe,Ph),X,(RNH,)." The secondury amines R2NH give, with Os(PMe,Ph),CI,, Os(PMe,Ph),Cl,(RNH,), which contains the coordinated primary amine (R = Me, Et, Pr", P t , Bun, "hexyl, cyclohexyl, piperidine)."
46.4.3
Complexes with Bidentate Amines
46.4.3.1 Complexes with ethylenediamine As ligands for osmium, ammonia and ethylenediamine (en; 1,2-diaminoethane) would be expected to have broadly similar properties, and this is on the whole true -there are stable osmium(I1I) complexes such as [0s(en),l3', trans-[Os(en),X,]+ (X = C1, Br) and [Os(en)Cl,] ,and more recently the tetravalent Os(en)X, (X = C1, Br, I) has been made. There is, however, an interesting chemistry of the 1,2-ethanediaminato-(l -) and 4 2 -) ligands with osmium in the higher IV, V and VI states, and another unusual feature is the existence of the hydride [0~H,(en),]~*. Table 6 Representative Ethylenediamine Complexes
Complex
Colour
[os(en)3]13 [Os(en),Cl,]Cl. H 2 0 b [Os(en),H,I[ZnCl41 Os(en)Br," [Os(en - H)en,]Rr,
Yellow
Pink
Spectroscopic md other studief
IR,19 M," ESR,*' IR, Raman, UVivis, M7' IR, 'H NMR, M" IR, UVivis'' X-R~Y?~
a M = ma netic studier; CV = cyclic voltammetry. bBrornn cumpkx also kn0wn.7'~ Chloro and iodo complexes also known?'
Osmium
531
We deal first with ethylenediamine complexes, then with 1,2-ethanediaminato-( 1 -) and -(2 -) ligands (the so-called (en - H) and (en - 2H)), then related mono- and di-imines, and finally with substituted ethylenediamine complexes (see Table 6 ) .
(i)
Osmium(I1j
There is a brief mention in the literature of [Os(en),12+,made by a one-electron reduction of [Os(en),]'+. See also Scheme 1, p. 532.
(ii )
Osmium IIlI}
The tris chelate [Os(en),13+is most easily made by reduction of [Os(en - H)(en)J3+ with ethanol, conc. HBr, Na,S,0,76 or NaHSO, .77 The iodide is paramagnetic with a magnetic moment close to 1.6 BM at room t e m p e r a t ~ r eThe . ~ ~ two trans-[Os(en),X,]X (X = C1, Br) complexes can be made from [0~(en),H,]~+ and HX. The trans configuration for the chloro complex is suggested by the angle close Raman and IR spectra, while that for the bromo species is indicated by a Br-Os-Br to 180 from a partly resolved X-ray analysis of the compound. Both complexes are paramagnetic in solution (peR= 1.91 and 1.95 BM at 323 K)." The salts Cs[Os(en)X,] (X = C1, Br) were obtained from [Os(en)(en - H)2]Br, and HX (a similar reaction with HI gave '[Os(en),I,] I.,'). The bromo species with HCl gave Cs[Os(en)Cl,Br,-,] (n = 1-3)." A rate of proton exchange with [0s(en),l3+ has been given.sn
(iii ) Osmium ( I V )
Although [Os(en),],+ has not been isolated, it seems possible that an oxidation wave for [0s(en),l3+ near + 2V may be due to this species; a rate of proton exchange with [Os(en),14+ has been given." It is clearly unstable with respect to [Os(en)(en - H),I2+ and [Os(en,)(en - H)I3+(p. 532).
There are however several substituted ethylenediamine complexes, the most unusual being the hydride [Os(en),H,]'+ which can be obtained as a [ZnClJ- salt (by reaction of [OsO,(en),]Cl, with zinc in HCl) or as a chloride. The salts are diamagnetic or almost so (per 0.22 BM at 298 K and 0.10 BM at 183 K for the [ZnC1,I2- salt). Although this is formally an osmium(1V) complex there seems to be no abnormality in the 'H NMR shift (d,, = 15.4 p.p.m.); the Os-H stretch lies at 21 50 cm ' in the IR." The species Os(en)X, (X = C1, Br) can be prepared from [Os(en - H),(en)] Br, and HX; a similar reaction with HI leads to '[Os(en),I,] I,', but Os(en)I, can be obtained from either Os(en)Cl, or Os(en)Br, with HI.79 There is also an oxalato complex Os(en)ox,, obtained from [Os(en - H),(en)]Br, and oxalic acid.79
(iv) Osmium( VI)
The osmyl complex trans-[O~O,(en),]~+ is discussed on p. 583 (see also Table 21). The only other species in this category is the deep red crystalline '[Os(en),I,],', obtained by treatment of [Os(en)(en - H),]Br2 with HI.79Although analytical data fit the formulation reasonably well it seems unlikely that this really is an eight-coordinate osmium(V1) species; an alternative possibility is [Os(en)(en - H),](13),.
46.4.3.2
Complexes with deprotonated ethylenediamine
These species, in which the 1,2-ethanediaminato-(l-) and -(2 -) ligands, (HN(CH2),NH2)- and (HN(CH,),NH)2-, are denoted by (en - H) and (en - 2H) respectively are relatively common for the higher oxidation states of osmium; indeed the normal preparation of the 'simple' [0s(en),l3+ cation is by reduction of [Os(en - H),enI2+. They are in effect .rr-donating amido Iigands and so it is perhaps not surprising that osmium(1V) and osmium(V1) complexes should be formed with them. Precise characterization of some of them is lacking, however, particularly those of osmium(III), but there is a recent X-ray crystal structure of [Os'"(en - H),en]Br,.76 C.O.C. &R
532
Osmium
There is also evidence from NMR of the existence of mono- and di-imine complexes containing the coordinated HN(CH,),NH, and HN(CH2)2NHligands; these we refer to below as im and diim respectively.
(i )
Osmium (IZ)
The monoimine complex [O~(im)(en>~]~+ is formed at pH 5-7 by the spontaneous oxidative dehydrogenation of [Os(en - H>(en)J3'; formation of other iminato complexes is given in Scheme 1.76 The complexes were detected by electronic and 'Hand I3CNMR spectros~opy.~~ [Os(en-H)2en] *+
.
H+
' [Os(en-H)en, 1 3*
Scheme 1 Reactions of deprotonated ethylenediamine c ~ rn p l e x e s ' ~
(E) Osmium(II1) Reaction of [Os(en),]13 with three, two or one moles of potassium amide yields respectively the brown Os(en - H)3, the yellow-brown [Os(en - H),en]I and the pink [Os(en - H)(en)& the magnetic moment of the latter is 1.58BM at room temperature.82Formulation as iminato complexes would also be possible; structural data on these species would be of interest.
(iii) Osmium (IV)
The reaction of (NH,),[OsBr,] and ethylenediamineat 60 "Cyields the pink [Os(en - H)2en]Br,,'7 and this formulation of 1955 as an ethanediaminato(1 -) complex has recently been c o n h e d by a single crystal X-ray study. The two deprotonated ends of the ligands are cis to each other, and the Os--N bond length to these ends (c in Figure 2, is 1.90 A,indicative of some multiple bonding, since the Os-NH, bond lengths a and b are respectively 2. I9 and 2.11 In Scheme 1 the reactions of this and other en - H complexes are depicted; the p& for the [Os(en - H),enI2+-[Os(en - H)en,j3+ equilibrium is about 6.16 The pyrophoric species [Os(en - H),]I has also been claimed, made by treatment of [Os(en - H),en]I, with potassium amide in liquid ammonia.'*
L
J
Figure 2 Structure of [Os(en - H)ze~]2+7a
533
Osmium (iv) Osmium ( V ) and Osmium ( VZ)
A black microcrystalline material formuiated as [Os(en - H),en]I, .4H20 is obtained from = 1.78 BM).83A similar [Os(en - H),en]I, and ethylenediamineat 100OC," and is paramagnetic beR preparative method in the presence of air gives [Os(en - H),]I,. 3H20,77a,green-brown diamagnetic material thought to contain os r ni ~ m ( V I )As . ~ ~formulated both these species presumably contain eight-coordinate osmium, a very rare coordination number for the element, and clearly they merit further investigation. 46.4.3.3 A complex of 2,3-butanediimine
Biacetyl reacts with [Os(NH,),]I, in alkali, and the diammine-2,3-butanediiminespecies (Figure 3) is formed as brown-black crystals. The 'H NMR spectrum is consistent with the trans str~cture.'~
Figure 3 Proposed structure of the 2,3-butanediiminecomplex*'
46.4.4
Complexes with Nitrogen Heterocycles
46.4.4.1 Complexes of pyridine and of substituted pyridines (i)
Complexes with pyridine
Osmium forms a number of complexes with pyridine (Table 7), mostly of the I1 oxidation state although there are exampIes of osmium(II1) and osmium(1V) pyridine complexes. The important osmium(V1) species Os,O,py,, a binuclear pp'-oxo 'osmyl' species, is considered on p. 595. There is also some important recent chemistry of osmium nicotinic and isonicotinic acid complexes (the 3- and 4-pyridinecarboxylic acids respectively). Table 7 Complexes of Pyridine and of Substituted Pyridines Complex
Cofour
Spectroscopic and other studieP ~~
C ~ s - o S ( p Y ) ,(CO),C1,
"ms-os(py),(co), C1I OS(PY)3pPh3)c& [Os(py)(NH,),IC1,.2H,O [Os(isonic)(NH,),]Cl,b OS@Y),W h , Kl3 mr-Os(PY), c1, [05(PY)(cO)c~,Imer-Os(3-Mepic),13 [oS@Y)c1,1-
Red Orange
Orange
~~
IR, Raman, UV/visW IR, Raman, UV/vis, MSS9 ESCA, C v 3 6~~ IR, ram an:'^^^ U V / v i ~ , CVb2.909' uv/vis,wJ" M C D , ' ~ Cv90 ESCA, CV, M93 I R ,M ~ ~, ~ OESRIOZ I IR, Raman, UV/visg5 IR, MIo'
WV/V~S~~*~'
aM
= magnetic studies; CV = cyclic voltammetry. bisonic = isonicotinamide. Also bromo and iodo complexes.
(a) Osmium(ZI). The reaction of [0sXJ2- and pyridine gives Os(py),X, (X = C1, Br);86for the The carbonyl species Os(py), (CO)X, and iodo complex a frunsconfiguration has been p~stulated.'~ Os(py),(CO)XI (X= C1, Br) are obtained from [Os(CO),I,]- with pyridine followed by metathesis -the chloro ligands occupy trans position.88The dicarbonyls cis-Os(py),(CO),X, (X = C1, Br, I) are made from pyridine and c~s-[OS(CO)~X,]~and their configurations have been determined by vibrational spectroscopy; on heating in vacuo the thermodynamically more stable trans isomers are formed.89 ,6239039' Cyclic voltammetry of [ o ~ ( p y ) ( N H ~ ) demonstrated ~]~+ the existence of [0~(py)(NH,),]~" and the surface-enhanced Raman spectra (SERS) of both species were measured ~ e c e n t l y , 9with '~~~
Osmium
534
normal, pyridine-d, and ammine-d, substitution. The metal-nitrogen, ammine and internal pyridine modes are enhanced; IR spectra were also recorded." Reaction of pyridine with OsOzCl,(PPh,), yields Os(py),(PPh,)Cl,, which can be reversibly oxidized to the osmium(II1) and osmium(1V) derivatives; electrochemical reduction of Os(py),(PPh,)Cl, gives [Os"(py),(PPh,)Cl,]- ?3 ( B ) Osmium (III). The iodo complex mer-Os(py),I, can be made from [os16]2- and pyridine, and from this the corresponding Os(py),X, species (X = C1, Br) can be obtained with X,;" fieOs(py),CI, is made from [OsCl,]'- and pyridine.86 The mixed methyl pyrazine (R) complexes Os(py),R,-,X, (n = 3, 1; X = C1, Br, 1) and Os(py),RX,I (X = CI, Br, I) can be made from [OsI,]' , pyridine and X- .y4 The carbonyl halo species trans-[Os(py)(CO)X4] (X = C1, Br, I) can be made from anhydrous pyridine with [OS(CO),X,]~-or [Os(CO)X,j-." Replacement of the dinitrogen ligand in [OS(N,)(NH,),]~+by pyridine under aerobic conditionsPo or of the weakly bound triflate ligand in [Os(OSO,CF,)(NH,),]'+ by pyridine6' yields [ 0 ~ ( p y ) ( N H ~ ) , ]cyclic ~ + ; voltammetric studies show reversible reduction to [0~(py)(NH,),]~+ 62.90,9' and the surface-enhanced Raman s p e c t r ~ m ~(SERS) ' ~ ~ ~ of the complex (both normal and deuteriated) shows Os-N, ammine and internal pyridine vibrational modes. Infrared spectra were also mea~ured.~' Reaction of trans-Os(PPh,),Ci, with pyridine yields Os(py), (PPh,)Cl,, which can be oxidized or reduced by one-electron reactions, while electrochemical oxidation of Os(py), (PPh,)Cl, gives [OS(PYI, (PPh, W*l+.93 (c) Oxrniunz(1V). Salts of [Os(py)X,]- (X = C1, Br, I) can be obtained from [OSX,]~ and pyridine, and the mixed species [Os(py)Cl,I]- has also been isolated.06 The chloro and bromo complexes can be made photochemically in the same way.y7 Electrochemical oxidation of Os(py)?(PPh,)CI, gives [Os(py),(PPh3)Cl,]+, and oxidation of [Os(py),(PPh3)Cl$ gives [Os(py)?(PPh3)Clzlz+ .93
(ii) Complexes of substituted pyridines (a) Picoline (methylpyridine) complexes. The 3- and 4-methylpyridine complexes Os(pic), l3have been made from [OSI,]~-and the ligand.94 (b) Isonicohic mid (pyridine-4-carboxylic acid) and isonicotinamide complexes. The dinitrogen can be replaced by isonicotinic acid to give [0~(nic)(NN,),]~+ ,4° and ligand in [OS(N~)(NH,)~]~' reaction of this ligand or of isonicotinamide with [Os(N,), (NH, ),I2+ yields [OSL(N,)(NH,),]~+.The latter isonicotinic acid complex [OsL(N 2)(NH3)4]2-' can be oxidized anaerobically in the presence of HC1 to give cis-[OsLCI(NH,),]C12 H20, while the isonicotinamide complex [OsL'(N,)(NH,),]+ reacts with NaNO, and HC1 to give a triammine, [OsL'(N,), (NH,)JCl, :Evidence was also obtained for the existence of [Os(nicotinic ~ C ~ ~ ) ( N , ) ( N H , ).98 ,]~+
-
46.4.4.2
Complexes with chelating pyridines
A number of unusual complexes in which pyridine is substituted in the 2 or ortho position with 0- or N-donor ligands are known.
(i)
2-Hydroxypyridine (hp) complexes
Reaction of the ligand with OsCl,'o"'04or with O S ~ ( O C O M ~ ) ,gives C ~ ~Os,hp4C1,S,, '~~ where S is a solvent mokecule. The X-ray crystal structure of Os2hp,C12*OEt2 shows this to have a 'lantern' type of structure similar to that found for Os,(OCOR),CI, (R = Me, Et, Pr"; see Figure 34). In the complex the OsN,O, moieties have a trans arrangement and the Os-Os distance is 2.344(2) A,'', while in Os,hp4C1,-2MeCN it is 2.357(1)&'04 The magnetic moment of 0s2hp4C1,is 1,44BM at 300 K,'" larger than that for the carboxylates Os,(OCOR),Cl, (p. 601), possibly due to a greater population of states derived from configurations such as aLn462n*2 or arn4626*n*.'06 Reduction of Os,hp4C1, with (n-C5H5),Cogives [(n-C5H5)2C~][O~Zhp4C12], which contains the Os:' core. This olive-green material has a magnetic moment of 2.68 BM at 308 K and of 2.63 BM at 173K in dichloromethane solution (unlike the corresponding carboxylates for which the moment drops substantially with temperature). The ESR spectrum of the complex was also measured.Iw The ligand 2-aminomethylpyridine(Rpy) functions as a chelating ligand in its own right or as the monodeprotonated 2-iminomethylpyridine (RpyH) species. This reaction of Rpy with [OsBr,12-
Osmium
535
gives the red osmium(1V) complex [Os(Rpy)(RpyH),]Br,, with a magnetic moment of 1.2BM at room temperature, together with the green [Os(Rpy),(RpyH)](ClO,), (p& = 1.1 BM) isolated from perchloric acid solution of the reactants. This can be reduced with HSO; to the yellow-green [Os(Rpy),](ClO.,), , an osmium(II1) complex (peR= 1.8 BM)."' Isomeric complexes of 2-(pheny1azo)pyridine (pap) and 2-(rn-tolylazo)pyridine (tap) (Figure 4a) have been prepared from the ligand and (NH4)2[OsX,]. These have stoichiometries corresponding to OsL,X2 (X = C1, Br), but exist in various forms such that Os(pap),Cl, and Os(tap),Cl, have structures (b) and (c) shown in Figure 4.IwaA recent X-ray crystal structure study shows that Os(tap),Cl, has the structure shown in Figure 4(b); mean Os-C1, 2.394; mean Os-N, 2.06; mean OS-N,, 1.974 AIDYb
(a) tap, R = H; pap, R = Me
X
X
(b)
(C)
Figure 4 Isomers of 2-(arylazo)pyridine complexes'@'"
46.4.4.3
Piperidine (C,H,, N ) complexes
For convenience these are placed with pyridine. Only two complexes with osmium seem to have been reported. The ligand reacts with OsL,Cl, (L = PMe,Ph,72As(p-MeC,H,),"') to give OsL,Cl, (piperidine).
46.4.4.4
Bifunctional heterocyclic donors
In this section we consider the osmium complexes of the three potentially bifunctional ligands pyrazine (pyz), pyrimidine (pym) and pyridazine (pyd) (Table 8).* All form terminal complexes with
Pyrazinc (PYZ)
Pyrimidine (wm)
Pyridazine (pyd)
osmium, but only pyrazine forms binuclear species, the others not doing so presumably for steric reasons. Of particular interest is [ O S ~ ( ~ ~ Z ) ( N H the ~ ) ~osmium ~ ] ~ + , analogue of the celebrated 'Creutz-Taube' complex [ R u ~ ( ~ ~ z ) ( N H . l l '~ ) , ~ ] ~ ~ Table 8 Pyrazine Complexes of Osmium Complex
Coiolrr
Spectroscopic and other studies'
Blue-green Orange
IR, UV/visq4 iR, UV/visg8 W/vis,62CV,90"'IR, resonance Raman9I Uv/viss8 UV/vis, cVl4 UVjvis, Cv"4 UV/vis, c V i l 4 UV/vis, cV'l4 uv~vis,lo0.114 MCD IW cvll3.ll4 UV/vis, cV'l4 UVjvis, MCD,IWCV"'
Orange Violet Red
UV/vis, CV1" X-Rayl14 'CV
= cyclic
voltammetry. bChloro and bromo complexes also known. 'L
*For bipyridyl, phenanthroline and terpyridyl see p. 537
el
seq
= pyz,
pyzc, bprn, pyz, py7s. bprn.
Osmium
536 (i)
Mononuclear complexes
(a) Osmium(ZZ). All three ligands form species of the type [OSL(NH,),]~+,these being obtained by e l e c t r o ~ h e m i c a lor ~ ~chemicalw ~~~ (zinc) reduction of the corresponding osmium(II1) species. In basic solution these osmium(I1) complexes are relatively stable in the absence of oxidizing agents, but are less so in acid: the values of p& for deprotonation of [OS"(LH)(NH,),]~+are 7.4, 2.1 and 3.7 for pyrazine, pyrimidine and pyridazine respectively, compared with pK, values of 0.6, 1.3 and 2.33 for the ligands." Recently the surface-enhanced Raman spectra (SERS)of [Os(pyz)(NH,),]*+ and of [0~(pyz)(ND,),]~+ have been recorded." The tetraammines c~s-[OSL(N,)(NH,),~*+ (L = pyrazine or 3,5-dimethylpyrazine) are made from cis-[Os(N,),(NH3)4]2' and the ligand? complexes of the monoprotonated species eis[Os(LH)(N,)(NH,),]+ (L = N-methylpyrazinium, pyrazinium and 3,5-dimethylpyrazinium) are also known.98 Recently the complex [Os(pyzc)bipy,]+ ( p y x = 2-pyrazinecarboxylic acid) has been made by reaction of Os(bipy), C1, with the (b) Osmium(ZZI). The tris complex OsL,X, (L = pyrazine, methylpyrazine and pyrimidine, X = C1,3r, I) can be made from [OSI,]~-and the ligands followed by metathesis where appropriate; mer configurations were assigned to Os(pyz), C1, , Os(pyz), I, and OsL,X, (L = N-methylpyrazine, X = Cl, Br, I).% The mixed species OsL(py),Br,I,-, (n = 1, 2) are made from mer-OsL(py),I, (L = N-methylpyrazine) and b r ~ m i n e . " ~ Salts of [OsL(NH,),13+ (L = pyrazine, pyrimidine, pyridazine) can be made by displacement of nitrogen from [OS(N,)(NH~),]~+'~ or from the triflato complex [OS(OSO,CF~)(NH~),]~+ and the ligands.62Cyclic voltammetry showed reduction to the osmium(I1) species,62.90and the surfaceand of their ammineenhanced Raman spectra (SERS) of [Os(pyz)(HNH,),I3+, [0s(py~)(NH,),]~+ d5 derivatives were measured, as were the IR spectra." Reaction of cis-[Os(py~)(N~)(NH~)~]~" with HX yields ci~-[Os(pyz)X(NH,)~]~+ (X = C1,Br).98
(ii) Binuclear pyrazine-bridged complexes A number of these have been prepared using the synthetic procedures shown in Scheme 2. Of particular interest is [0s,(py~)(NH,),,]~+which has a radicalIy different electronic absorption spectrum from either [ 0 ~ ' ~ ~ ( p y z ) ( N H ~or) ,[~0]~~ ~+ ~ ~ ~ ( p y z ) ( Nand H ~is) believed ~ ~ ] ~ +to, be a class 111delocalized mixed-valence dimer, as is the dinitrogen analogue [Os,(N,)(NH,)lo]5+ (see p. 556). A similar situation probably obtains for [ 0 ~ ~ ( p y z ) C l ( N H ~ ) ~ ] ~ +
// HCI. 0,
Scheme 2 Preparation of binuclear osmium ammines*l 4
Osmium
537
The pyrazine-bridged complex [OsR~(pyz)bipy,Cl,]~' has been reported and its electronic spectrum and cyclic voltammetry have been measured; two one-electron oxidations are observed.'" In recent work the X-ray crystal structure O f [Os2(pyZ)(NH3),,]C16.2H,O has been determined; the Os -N (pyrazine) distance is 2.101(7) Reaction of [Os(pyzc)bipy,]+ (pyzc = 2-pyrazinecarboxylic acid) with cis-Ru(bipy), Cl, gives the bridged complex [OsRu(pyzc)bipy412+; electronic absorption spectra and cyclic voltammograms were measured. Two one-electron oxidations were observed; the osmium(1I) site is oxidized first and then the ruthenium site."'
46.4.4.5
Tetrufmctional ligands
The ligand 2,2-bipyrimidine (bpm) has been used to form terminal and bridgmg complexes with osmium. Reaction of cis-Os(bipy),Cl, and the ligand yields [Os(bpm)bipy,]'+, and with cis-
Ru(bipy), C1, this gives the bipyrimidine-bridged complex [OsRu(bpm)bipy4l4+; the electronic absorption spectra and cyclic voltammetric properties were measured. Two one-electron oxidations were observed, the first probably occurring at the Os" site and the second at Ru"."'
46.4.4.6
Polypyridyl complexes of osmium
It has now become fashionable to call the bipyridyl, phenanthroline and terpyridyl ligands, and substituted forms of these, the 'polypyridyls' (see Table 9). In section 46.4.4.6.i. we consider the first two together, while terpyridyl complexes are dealt with in Section 46.4.4.6.iii. Table 9 Properties of Bipyridyl, Phenanthroline and Terpyridyl Complexes Complex
Spectroscopic and other propertie?
Colour
Green
Brown Purple Green-brown Violet Red Brown Green a CV
(i)
= cyclic voltammetry; M = magnetic studies.
Bromo and iodo complexes also known.
Complexes of 2,2'-bipyridyland 1,IO-phenmthroline
In accordance with accepted practice we abbreviate these as bipy and phen, and the substituted ligands as Rbipy and Rphen (thus 5-Clbipy is 5-chloro-2,2'-bipyridyl and 4,7-Ph2phen is 4,7-diphenyl-1,lO-phenanthroline).The preparative routes to bipy and phen complexes are m general very similar, as are the chemistries of the two sets of complexes, so it is convenient to deal with them together. We consider first the tris species [Os(LL),j2+and [Os(LL),I3+and, in the following section, complexes involving LL and other ligands (LL = bipy, Rbipy, Rphen). Since, like terpyridyl (p. 542), these ligands are good R acceptors by virtue of their considerable ring conjugation, it is the osmium(I1) (d6)state which is the commonest and generally the most stable; thus [0s(LL)J3+ species are good one-electron oxidants. Nevertheless there are also examples of osmium(IV) and even a few of osmium(V) and osmium(V1) (the 'osmyl' osmium(VI) complexes are considered on p. 581). The chemistry of these complexes has been extensively researched in the past two decades: the role of [Os(LL),J3+ and [0s(LL),l2+ in outer-sphere electron transfer processes has been much studied,
Osmium
538
and two early papers in 1964 uncovered a large area of chemistry of bipy and phen complexes with other ligands. Recently, however, some of the considerable current interest in polypyridyl complexes of ruthenium as photosensitizers in the photodissociation of water has stimulated similar work on the corresponding complexes of osmium; as is indicated below it is the mixed complexes of osmium rather than [Os(LL),]"+ which have excited the most interest. There is also much work likely to be done on electroactive polymeric species containing these ligands as electrode coatings. (a) The tris complexes [Os(bipy) I"+,[Os (phen),/n , [Os (Rbipy)*]n+and [Os( Rphen), I"+ Osmium(IZ). Salts of the green [Os(bipy),12+ ion can be made from OsO, and bipyridyl in the presence of hydrazine hydrate,"' but the commonest procedure is to treat K,[OsCl,] or [Os(bipy),Cl,]Cl with bipyridyl at 260 0C;116 a similar reaction of (NH4),[OsBr6] with phen at 270 "C gives the dark brown [Os(phen)J2+ Recent convenient preparations of both species involve refluxing solutions of the ligand with O S ~ ( O A C ) ~orCwith ~ , ~ 0sCl3'". ~~ Resolution of salts of both ions has been effected with the antimonyl tartrate anion.I17 The substituted complexes [Os(Rbipy),12' and [Os(Rphen),12+ are normally made from [OsX612-(X = C1, Br) and the ligand (e.g. 6-Mebipy,' l9 4,4'-Me,bipy,120,'215,5'-Me,bipy,'2' 6,6'-Me2bipy,"', 2-Mephen and 2,9-Me2phen;' 4,7-Me2phen and sulfonated brief mention has been made of the complexes with 5-Me~hen,',~ 4,7-Me,phen).'13 and the ligand The [Os(phen),12+cation is said to be formed in aqueous solution from [OSC~,]~Recently EPR data on [Os(bipy),]- and at pH 2.5-5.0; in strong HC1, Os(phen),Cl, is [O~(bipy)~]+ have been reported.'24b An X-ray crystal structure determination of [Os(phen),)((ClO,),- H,O shows that the Os-N distances in the cation lie between 2.06617) and 2.08217) A. Spectroscopic properties: the complexes as photosensitizers. The emphasis of these volumes is on the chemistry of the coordination complexes and so we can provide only a summary and brief guide to recent work on the photophysics and photochemistry of the polypyridyl complexes. Although most bipy and phen complexes of osmium, like their ruthenium analogues, are highly luminescent, it is only comparatively recently that the appropriate spectra have been measured and attempts made to harness their properties for such important applications as the photodissociation of water. The lifetimes of low-lying metal-to-ligand charge transfer (MLCT) excited states of [Os(LL),j2+ complexes are in general much shorter than those of their ruthenium(I1) counterpart^.'^^^'^^ This probably arises from the greater spin-orbit coupling in osmium than in ruthenium complexes leading to enhanced radiative and non-radiative decay rates for triplet-singlet transitions which have the effect of shortening the lifetimes of excited states.Iz7The electronic absorption and emission spectra and in most cases quantum yields and lifetimes of excited states have been measured for [os(bipY>,]2+,119.123.12&137,145 [O~(bipy-d,),]~+ 1 3 4 ~ 1 3 6[Os(phen), 12 + , I IY,123,127-131.13~137,~39.145, and the following fOs(Rbipy)12+and [Os(Rphen),]" species with Rbipy: 6-Mebipy,l194,4'-Me bi 1 2 6 ~ 1 3 4 ~ 1 3 7 6,6'-Me, bipy,"' 4,4-Ph2bip~i; with Rphen: 5-Clphen and 5 - M e ~ h e n , ' ~5~,6-M * ' ~e,phen, ~ py'126,134.137 5 - N O , ~ h e n , 2,9-Me2phen, '~~ 4,7-Ph,phen;'26v'37also for [Os(phen),(Ph,phen)12+ and [Os(phen),SO,Phphen]'+ .Iz3 In most cases aqueous media were used but in some micellar solutions of sodium lauryl sulfate were e m p l ~ y e d . ' ~The ~ , ' positions ~~ of MLCT absorption bands for [Os(bipy),j2+are solvent-dependent, this dependence being interpreted on the basis of dielectric continuum theory.Im The lifetime of excited [0s(bipy-d,l2- is approximately three times that for [Os(bipy),I2+,an effect ascribed to radiationless decay theory. 36 A parametric model, including the effects of spin-orbit coupling, has been developed for the MLCT excited state of [Os(bipy),]*+ Quenching of excited states of [Os(bipy),12+ and [Os(5.6-Me2phen),]*+ by copper(I1) cornp l e ~ e s , 'by ~ ~iron aqua c ~ r n p l e xe s , 'by ~ ~ [ C ~ ( p h e n ) ~ ] [Ru(NH3)J3', ~+, [Fe(CN),]3-;'4' of [Os(phen),12+ by HgCl, ,I3' by iron aqua complexes'42and by cobalt(li1) speciesIu has been studied. Such quenching by iron aqua species was also studied for [O~(S,S'-Me,bipy),]~+and for [Os(5Clphen),]2+'42and by 0, for [ O ~ ( b i p y } , ] ~ + , '[Os ~ ' , ' phen,I2+ ~~ and [Os phen,]" (LL = Ph,phen, S02Phphen)'". Circuiar dichroism spectra for [Os(bipy),I2+ and [Os(phen),I2+ have been reported. 1463'47 Resonance Raman spectra of [Os(bipy),12" have also recently been measured.'& For redox and electron-transfer properties of [Os(bipy),12+ and [0s(phen),l2+ see p. 539. Osmium ( I l l ) . Oxidation of [O~(bipy),]~+ with chlorine yields the violet [Os(bipy),13+ ion,"5*149 while the red IOs(phen),l3+ is similarly prepared by oxidation of [Os@hen),12+.1499150 Resolution of [Os(phen,)]-'+ has been achieved by oxidation of the corresponding [0s(phen),l2+ species. Spectroscopic properties. The electronic absorption spectra of [Os(bipy),13+136~'45~1467151and of [0s(phen>J3+ ha ve been measured, as have their circular dichroism and also the spectra of [0s(4,4-Me2bipy),I3+. I 5 ' The lifetimes of the charge transfer (MLCT) excited states of +
1457146
Osmium
539
[ O ~ ( b i p y ) , ]and ~ ~ [Os(phen),]’+ are some lo5 to lo6 times shorter than those of the MLCT states for the corresponding [Os(bipy),p+ and [O~(phen)~]’+ species (see also p. 538); it is thought that the excited states of the trivalent complexes are so short-lived that their lower energy ligand field states are too high to be populated. The lifetime for [Os(bipy-$),I3+ is roughly twice that for for the [Os(bipy),13+136 (see also p. 538). A brief mention has been made of the use of [O~(bipy),]~+ non-catalytic photooxidation of water in the presence of cobalt(I1) hydroxo complexes.152The rate of quenching of luminescence of [Ru(bipy),I2+ by [Os(bipy,)I3+is close to the diffusion-controlled limit. 153 The ‘ H NMR spectra of [Os(bipy),]-” and of { O ~ ( p h e n ) ~have ] ~ + been measured and contact, ESR were also measured.149The pseudo-contact and Fermi-contact shifts s t ~ d i e d ; “ ~ .the ’~~ , ’ ~spectra ~ 13C resonance spectra of [Os(bipy),12+, [0~(4,4-Cl~bipy),]~+, [Os(4,4-(OEt), bipy)J’+ , [Os(4OMebipy),]’+ and \Os{4-NMezbipy),I2+ have been recorded.IS3The resonance Raman spectrum of [Os(bipy),12 has been measured.l18 Redox and electron transfer properties of [Os(LL),I3+.There have been a number of determinaValues of E,, of + 0.858 V for [Os(bipy),13+/ tions of the [0s(LL),l3+/[OS(LL),]~+ [Os(bipy),12+ in 0.01 M H2S04and 0.855V in 0.1 M HCl have been given; for [Os(phen)J3+/ [Os(phen)]’+ it is + 0.859 V in 0.1 M HCl.‘57a Polarographic studies show, in addition to oxidation waves for [0s(bipy),l2+, several reduction and [Oswaves; 130,158 this is also the case for [0~(4,4‘-Me,bipy),]~+, [Os(5,5’-Me, bipy),]’+ (phen),12+.Recent cyclic voltammetry and coulombetry studies on [O~(bipy),]~+ and [Os(phen),12+ in liquid SO, show successive one-electron oxidations to [0s(LL),l3’ and [0s(LL),l4+,‘I8 There is a ” ~ ~ potential for [Os(phen)J+ in aqueous and in nonsmall but real difference in the O S ~ I redox aqueous sodium lauryl sulfate micellar Correlations have been made between the oxidation potentials and charge-transfer transition frequencies in complexes [M(LL),]’+ (Me = Fe, Ru, Os; LL = bipy, 4,4’-Me,bipy, 5,5’-Me,bipy).’” These high potentials make [Os(LL),],+ systems efficient one-electron oxidants, and most of these oxidations proceed via an outer-sphere process with second-order rate constants often close to the diffusion-controlled limits. Amongst those reactions which have been studied are [Os(bipy),I3+with [Mo(CN),I4-, [Mn(NO)(CN)J- ;IM)[Fe(cN),l4-, ~(CN),]4--;161 [Ru(bipy), 1’ ,[1rCl6l2-,I6’ Fe(phen),(CN),;I6l [RU(H,O),]~+;I6, [0s(LL),l3+ with [0s(LL),l2+, (LL = bipy, hen,'^^ 4,4-bipylw); [Os(bipy),12+with [Mo(CN),I3- ,I6’ [Fe(bipy),l3+;16’ [Os(phen),]’+ with [Fe(bipy),]’+ , [Fe(phen),13+ and [Fe(4,4‘-Me,phen),]2+;166 [0s(LL),l2+ with [Fe(LL),I3’ (LL = 4,4-Me,phen, 4,7-Me2phen, 5,6-Me2phen, 3,5,6,8-Me,phen).I6, Detailed kinetics of some of these and other processes have been studied, e.g. oxidation by [0s(bipy),l3+ of J - , SCN-,I6’OH-;’*’ reaction of [0s(LL),l3+ with OH- to give 0, (LL = bipy, phen, 5-Mcphen, 4,4-Me,bip~),I~~ oxidation of ascorbic acid by [OS(LL),]~+(LL = bipy, 4,4‘Me, bipy);’” and the photoinduced electron transfer reactions of acidopentaamminecobalt(II1) complexes with [Os(bipy),]’+ .I7” The outer-sphere oxidation of ascorbic acid by [Os(bipy)J2+ incorporated in a coating on a graphite electrode has recently been repprtsd.liO The thermodynamics of formation of outer-sphere complexes in solution, including those of osmium, have been reviewed.”, Osmium(IVJ. Evidence has been obtained from electrochemical oxidation of [O~(bipy),]~+ and [Os(phen),12+in liquid SO, for the existence of [0s(phen)J4+. Magnetic susceptibility measurements show that [O~(bipy),]~+ so generated is diamagnetic, and the I H NMR spectra and electrochemistry suggest that the oxidation is metal-centred for the bipy complex at least (whereas oxidation for the corresponding iron and ruthenium complexes is likely to be ligand-centred). Resonance Raman spectra of [Os(bipy),14+ are more complex than those of [Os(bipy)J2+ or [Os(bipy),13+ suggesting that simple D, symmetry is not maintained: a Jahn-Teller splitting leading to inequivalent bond orders amongst the bipy rings was suggested as one explanation for this.”’ (b) Substituted complexes [Os(bipy),X, /n+, [Os(phen),X,,P+, [Us/Rbipy),Xy]”+, [Os(Rphen) XyIn+ Monomeric species. There are many of these, and for convenience and brevity we have illustrated the preparation of most of them schematically (see Schemes 3 and 4). The two main precursors are osmium(II1) species: [Os(LL),X,]- (LL = bipy, phen; X = C1, Br, I), made by treatment of K, [OsX,] with the ligands LL in dimethylf~rmarnide’~~”~~ (reduction with Na, S20, gives OS(LL),X,~’~);and [Os(LL)C14]-, made by heating the bipyridinium or phenanthrolinium salts (LLH),[OsCI,] to give Os(LL)CI, which is then reduced with H,P02 to [Os(LL)ClJ .86 Recently a number of complexes of the form c~~T-[OS(LL),X,]~+ (e.g. LL = bipy, phen; X = MeCN, $(PhlPCH,PPh,), +(Ph,PCH=CHPPh2), PMe,Ph) have been made by reaction of +
+
C.O.C. +R*
Osmium
540
LL = coordinated bipy (B) or phen (P); X = Cl, Br, I. All from ref. 173 unless otherwise indicated
Scheme 3 Reactions of Os(LL)ZXz
cis-Os(LL),C1, with X;127 similar procedures are used to make complexes of the type OSl~(~L),XX'. 127.131.138 Some of these have been used as photosensitizers (see below). Surface-enhanced Raman spectra (SERS) of [Os(NH,), bipy13+ and its reduced form [Os(NH3)5bipyI2+ have been measured." Elecironic spectra and redox properties. The electronic absorption spectra of a number of osmium(I1) bipy and phen complexes containing other ligands have been studied, viz. [Os(bipy),LLI2+ (LL = phen, en, ox).12oOs(bipy),X, (X = Cl,120,172 Br, 1,'" CNIz7),[O~(bipy),py,]~+, [Os(bipy), a ~ a c ] + [Os(bipy),X(py)]+ ,~~~ (X = CI, Br, I),173[O~(bipy)en~]~+ ,I2' [O~(bipy)en,]*+,'~' Os(phen)20x.120 Circular dichroism spectra of [Os(bipy),phenI2+ [Os(bipy)py, XI+ (X = C1, Br, and of IOs(bipy)phen212+have also been measured.175 For electronic spectra of species in this category used as photosensitizers see below. For redox properties of [O~(bipy)~acac]+ see p. 541. Polarographic data have been obtained for oxidation and reduction of Os(bipy),X, (X = C1, Br) and [Os(bipy),Lz]'+ (L = py, CN, +en).'5sFor electrochemical oxidation of ~is-[Os(bipy),(H,O),]~+see p. OO0."6 In [Os(phen)2(Ph,PCH2PPh,)]2', [Os(phen),(cis-Ph,PCH=CH PPh,)12+, [Os(phen),(CO)Cl]+ and [O~(bipy),(CNMe),]~+ there are small but real differences in the Os"'" couples in aqueous as compared with aqueous micellar sodium lauryl sulfate media.'38 Spectroscopic properties: the complexes as photosensitizers. In general, substituted osmium polypyridyl complexes are more reactive electron transfer sensitizers than their ruthenium(I1) counterparts and, more significantly, have longer MLCT excited state lifetimes than the corresponding [OS(LL),]~+species. The replacement of one of the polypyridyl ligands to enhance the MLCT state lifetime can be accomplished by choosing the ligand X (in 0s"LLX4 or 0s"LL2X2)or X and X' (in Os" LL X X ) in such a way that its n-acceptor capacity controls the Iifetime.Iz7Species which have been studied in this way include [Os(phen)py412+,[Os(bipy)diars, J 2 + , [Os(phen),pyJ2+, cis[Os(LL), (CO)CI]2 ;I I cis-[Os(LLIz(CO)CI]Z ;I I cis-[Os(LL),O\ICMe),] 2 + ;127,131,141 cis[0~(bipy)~(NCMe)(PMePh~)]~+;'~' rrans-[O~(phen),(PMe,Ph),]~';'~~,~~~O~(bipy),(CN),;~'~ trans[Os(bipy), R2]" (R = PMePh,, PPh,); ~is-[Os(bipy),(PMePh,),]~+, [Os(bipy),diars]'+ ;I3' [Os(LL),dppm]2+;'27,13'[O~(LL),dppe]~+;'~''~~~' [Os(phen),dppI2+, [O~Cphen),dape]~+,~~~ [OsL, (cisPh, PCH=CHPPh,), ]I+ .l27,13O,lY, '41 trans-[Os(bipy),(rrans-Ph,PCH=CHPPh,)]*+ [ O ~ ( LL) , d a pe ] ~ +[ ,O ' ~S~( ~ ~ ~ ~ ) , ( P ~ ~ P C , H ,and , P P[O~(bipy),(Me~SO)~].~~~*~~~ ~~)]~+ The properties of a number of these photosensitizers have been studied in aqueous and sodium lauryl sulfate micellar media,13' their electronic and emission spectra, electrochemistry and quenching of their luminescence by HgC1, in both media.I3' In some cases solvent dependence of MLCT excited state lifetimes has been observed, e.g. in [Os(bipy),py,j*+ , [O~(bipy)~(NCMe),]~+ and [0~(bipy)~(Ph,PC~H,PPh~]~+. For these, dielectric continuum theory has been invoked to explain such solvent dependence, and it is necessary to assume that the excited electron in the excited state is localized on one ligand rather than delocalized over all three." There is a preliminary report of an excited state photoelectrochemical cell based on the reductive quenching of the MLCT excited state of [Os(bipy)diars,12+ by W(p-C6H,Br), in acidic, aerated acetonitrile: it produces H 2 0 2and bromine with high quantum effi~iency.~" +
+
~
[OsBen,] 2 +
[OsBacac,]+
+
OsBacaKI,
LL = coordinated bipy (B) or phen (P). "Ref. 173. bRef. 86 Scheme 4 Reactions of [Os(LL)Cl,]-
Osmium (IZI) . The osmium(II1) complexes K[Os(LL)CI,] (LL = bipy, phen), made by reduction of Os(LL)Cl, with H,P02 and KCl, are useful precursors for other osmium complexes (Scheme 4). In general, osmium(II1) complexes in this category are made by oxidation of the corresponding osmium(I1) species with chlorine or with cerium(1V) (see Schemes 3 and 4). Magnetic susceptibilities of [Os(bipy),CI,](ClO,) were measured from 81 to 303 K (pemis 1.61 BM at 81 K and 1.87BM at 303K); for (Os(phen),Cl,]ClO, peEis 1.61 BM at 85K and 1.73BM at 300.3 K.'" The complex [0s(NH,),(4,4'-bipy)l3+ has been mentioned, and its resonance Raman and IR spectra recorded." Electronic spectra and redox properties. Electronic spectra have been measured for the following osmium(II1) complexes: [Os(bipy),phenI3+ , [O~(bipy),acac]~+ ,I5' [Os(bipy),(py)XI2+ (X = C1, Br, I),171[Os(bipy)Cl,]-, [Os(bipy)acac,]+ ,15' [Os(bipy),X,]+ (X = C1, Br, I)'5'.172[Os(phen),X,]+ (X = Br, I)151.172 and [O~(bipy)(py),X]~+ (X = C1, Br, I).86 Circular dichroism spectra were also measured for [Os(bipy),phenI3+and for [Os(bipy)phen,13 .I7, The redox potential for [Os(bipy),acacI2+/[Os(bipy),acac]+ is 0.1539 V as compared with 0.8836 V for [0s(bipy),l3+/[0s(bipy)J2+ , showing graphically that osmium(IT1) is stablized relative to osmiurn(1) is stabilized relative to osmium(I1) by replacement of one bipyridyl ligand.151For (LL = py, polarographic oxidation of Os(bipy),Cl,, Os(php),X, (X = Cl, Br), [O~(bipy),LL,]~+ see be10w.I~~ There are $n) and [Os(bipy),(CN),] see p. 540, and of ~is-[Os(bipy),(OH)(H,O)]~+, also electrochemical (redox) data for [OSL,(L-L),]~+ (L-L = Ph2P(CH,),PPh,, Ph,PCH= CHPPh,, dape) and [Os(biby),X,I2+ (X = MeCN, Osmium(IV), (VI and (VI). Heating (LLH),[OsCl,] to 300°C gives Os(LL)Cl4 (LL = bipy, phen).86 Cyclic voltammetry and differential pulse polarography of cis-[Os(bipy), (H,0)J2+ at pH 1.4 show oxidation to osmium(II1) (E: -t 0.16V), followed by a two-electron step to osmium(V) (E4+ 0.61 V) and the one-electron step to Os(V1) (E30.81V); in the pH range 2.0-5.5 the Os""-Osv oxidation was resolved into two reversible one-electron waves showing an intermediacy of OS'". From pH dependence studies on the complexes the Os'" complex is likely to be cis-[Os(bipy),(OH)and the Osv ~is-[Oslbipy),O(OH)]~+, while the (H,0)I2+,the Osw probably ~is-[Os(bipy)~O(H,O)]~+ (see also p. 584). As with the analogous ruthenium comOs(V1) species may be ~is-[OsO~bipy]~+ plexes the redox system occurs within a remarkably narrow potential window, with AE+for M v 1 ~ to MlII/11 176,178 Dimeric species. Reaction of cis-Os(bipy),Cl, and Ph, PCH2PPh2(dppm) gives [Cl(bipy), Os"(dppm)O~"(bipy),Cl]~+which, on oxidation with bromine, yields [Cl(bipy),Os"' (dppm)Os"'(bipy),C1I4+; mixture of these two dimers gives the weakly coupled mixed-valence species [Cl(bipy)20s1'(dppm)Os111(bipy),C1]3+.The intervalence transitions were measured.179 A pp'hydroxo species O ~ ~ ( O H ) ~ b i p ycan ~ C l be ~ made by prolonged action of hot water on K[O~(bipy)Cl,].~~ The near IR and magnetic circular dichroism spectra of [(NH,),Os(4,4'-bipy)Os(NH3),16$have +
+
+
542
Osmium
been recorded; exchange coupling is likely to occur in this species.’00The mixed metal species [0sRu(4,4’-bipy)C1,(bipy),l2+ has been prepared from [0s(4,4’-bipy)Cl(bipy;\,l2+ and cisRu(bipy),Cl, ; cyclic voltammetric and eiectronic spectral data were recorded.IL2 For the p and pp’-imido complexes [Os,(NH)(bipy),14+ and Os,(NH),(bipy), see p. 558. Polymeric species. A field of rapidly growing interest is the reductive electrochemical polymerization of vinyl-containing ruthenium and osmium polypyridyl complexes. For osmium, the starting materials are cis-[Os(bipy), L2](PFs), (L = 4-vinylpyridine (n = 2). N-(4-pyridyl)cinnamamide (n = I)), made from the ligand and cis-Os(bipy),(CO,), and ctr-~Os(bipy),L’Cl](PF,)( L = 4vinylpyridine and bis(4-pyridyl)ethylene), made from cis-Os(bipyj,Cl, and the ligands. These species can be made to form polymeric fibres (homopolymer, copolymer or spatially segregated bilayer) and electrode coatings, and are electroactive because they contain a redox centre in each repeat unit.’” The redox conduction of the film prepared from electrochemical polymerization of cis-[Os(bipy), (N(4-pyridyl)~innarnarnide),]~+ has been measured by steady-state voltammetry and is close to the E O of the polymer 0s1”/Os” couple, involving the intermediacy of a mixed-valence state of the polymer. 181a-18’c Coated platinum electrodes using poly-[Os(bipy),(4-vinylpyridine),]3+ have been used to study the kinetically unfavourable oxidations of Os(bipy),Cl, and [0s(Rphen)J2+ (Rphen = 5-Clphen, 5-N02phen and 3,4,7,X-Me,phen).Is2
(ii) Complexes of 2,2’-biquinoline and 2- (2’-pyridyl) quinoline
These ligands are clearly related to 2,Z’-bipyridyl. They react with K, [OsCl,] in refluxing glycerol to give, after addition of sodium perchlorate, black crystals of [Os(biq),](ClO,), H,O and [O~(pq)~](ClO,), * H, 0. The absorption and luminescence spectra were measured; the difference between the energies of the x and n* bands was less than that found for [Os(bipyj,]’+, as expected from the additional a-electron delocalization in these ligands.1Y0
-
(iii) Terpyridyl (terpy) complexes As a tridentate conjugated ligand this would be expected, like 2,T-bipyridyl and 1,lO-phenanthroline, to stabilize osmium(I1) and this is indeed the case, with most of the reported complexes containing divalent osmium. The unsubstituted bis complex [0s(terpy),l2+is remarkably stabIe. It seems likely that terpyridyl complexes of osmium(I1) are good candidates for further investigation as photosensitizers, having so far received less attention than the corresponding bipyridyl or phenanthroline species. Osmium(I1).The unsubstituted complex [Os(terpy),]Cl, was made in 1937 by fusion of the ligand with a mixture of K2[OsCI6]and osmium metal, and is singularly unreactive towards either acids or alkalis.’” The green chloride gives a red solution. For its redox and spectroscopic properties see below. A number of substituted terpyridyl complexes have been reported, most of them made by Dwyer and co-workers in 1964.19, Preparative routes are given in Scheme 5. Spectroscopic properties: the complexes as photosensitizers. Spectroscopic (absorption and emission spectra),’28138~187redox and photochemical properties (MLCT lifetimes, quenching of the Os” MLCT states by HgC1,) have been studied for [Os(terpy),12+ in aqueous and in aqueous sodium lauryl sulfate micellar solutions;I3*the solvent accessibility of the complexes correlates with the hydrophilicity of the ligands (other osmium(I1) bipy and phen complexes were included in the study).139Absolute luminescence, quantum yields and lifetimes of [Os(terpy),]I, in an alcohol glass at 77 K have been determined.lZ9 The complexes [Os(terpy)LCl]+., [Os(terpy)LRI2+ (L = cisPh,PCH=CHPPh,, dppm; R = py, MeCN, CO), [Os(terpy)dpprnJ2+ and [Os(terpy)(dppm)CI]+ are the first complexes observed to exhibit strong MLCT-based luminescence in fluid solutions at room temperature^;]^^ the absorption and emission spectra of [Os(terpy)(dppm)Cl]+ and of
Osmium
543
[OsTLC11+'
-
OsTC13/ p y
[OsTpy,]
,+
[OSTLX]~~' X = py, CO, NCMe
[OsTpy313+b
'I'
[OsTpy,Cl] *+
[OsTpy,CIl+ / ~
2
1
0
5 s
~
c1 z [os-r,13+d
IOSTJ~+C
\
[OsTB(H,O)] *+ e
bipyH', heat
& [OsTF%(NCMe)]3+
[OsTB(NCMe)l 2*
\ /AN
OSBCL,,I
T,X -
-
[OSTEK~]+~
X-
t
[OSTBX]'~
1
[ OSTBX]
'+
'Ref. 193. bRef. 192. 'Ref, 191. dRef. 189. eRef. 195. 'Ref. 86. T = terpyridyl; B = bipyridyl; L = dppm,cisPh,PCH=CHPPh2
Scheme 5 Preparation of terpyridyl complexes
[0s(terpy)(ci~-Ph,PCH=CHPPh,)(NCMe)]~+ at room temperature have been measured.'93 The 'H resonance spectrum of [Os(terpy),12+has been recorded.18' Redox properties. For oxidation of [Os(terpy)J2+ see below. Treatment of [Os(terpy)(bipy)Cl]+ with trifluoromethanesulfonic acid gives [W?O-O~(terpy)bipy]~+,and it has been shown by electrochemical and spectrophotometric studies (equation 1) to be a member of the series (at pH 7).lg4
= [H20--Os(terpy)CbiPY)13+ -e-
[H,0--Os(terpY)(biPY)12+
C-
=
-2H+,
-e-
[o=os(tcrpY)(biPY)12+
(1)
The hydroxo species [HO-Os(terpy)bipy12+ can also be made by oxidation of [H,O-OS(terpy)bipy]*+.lW Redox properties of [Os(terpy)LCl]+, [Os(terpy)LR]'+ (L = cis-Ph,PCH= CHPPh,, R = py, CO, MeCN), [Os(terpy)dppm Cl]+ and of [Os(terpy)dppmj2+have been measured.19, At pH 13, electrochemical reduction of [OsO,(terpy)(OH)]+ yields [Os(terpy)O,(OH)], at pH 7, [Os(terpy)(OH),]. and [O~(terpy)(H~O)~]~+ at pH 1." Osmium(I1I). The unsubstituted [Os(terpy)J3+ ion can be obtained as a sparingly soluble green perchlorate by oxidation of [Os(terpy),12 with C12.Iz9 A number of substituted osmium(II1) terpyridyl complexes are known (see Scheme 5). Redox properties. In general [M(terpy),13+ ions are less stable than the corresponding [M(bipy)J3+ species (M = Os, Ru, Fe); in 0.1 M H Z S 0 4the [M(terpy)2]3+/[M(terpy),]2+potentials are +0.951 (Os), + 1.28 (Ru) and + 1.08 V (Fe).IS9Although the colour change from green ([O~(terpy),]~+) to red [Os(terpy),12+ is in principle favourable for redox titration purposes the instability of the tripositive complex renders it unsuitable for the purpose.1s9The complex [HO -0s(terpy)bipyl2+ is obtained by electrochemical reduction of [0=Os(terpy)bipyl2+ or oxidation of [HO-Os(terpy)bipy]+ or [H,O-O~(terpy)bipy]~+,lg5and electrochemical reduction of [OsO,(terpy)(OH)]+ gives [Os(terpy)(H,0),l3+ at pH 1, and Os(terpy)(OH), at pH 7." Osmium('IV), ( V ) and (' VI). The existence of [O=OS'~(terpy)bipy]*+ has already been mentioned (see above).lg5 Irreversible electrochemical oxidation of this occurs from pH 1 to 12 at f 1.05V, and the initial product is thought to be [O=OsV(terpy)bipy13+ which then loses a bipyridyl ligand and undergoes a further one-electron oxidation to [OsO,(terpy)(OH)]+. At pH 13.5 a
Osmium
544
two-electron, one-proton reduction wave is observed (equation 2), while at lower pH the intermediate is [HO-O~"'(terpy)bipy]~+-Iy5 2e-,H+
[O=Os(terpy)(bipy)P+
[HO-Os"(terpy)(bipy)]+
-
(2)
The osmium(V) species OsO,(terpy)(OH) has also been detected electrochemically at pH 13, from reduction of [OsO,(terpy)(OH)] ; the latter is made from trans-K, [OsO,(OH),] and t e r p ~ . ' ~ ~ Binuclear complexes. For [Os,O(terpy),(bipy),]4+ and [O~~O(terpy),(bipy),]~+, see p. 594.19' +
(iv)
Complexes of ligands related to terpyridyl
(a) Bis(2,6-di(2'-quinolyl)pyridine) complexes. This ligand (dqp) is closely related to terpyrjdyl. It reacts with K,[OsCl,] in refluxing ethylene glycol to give, after addition of sodium perchlorate, the dark purple [Os(dqp),](ClO,), , The absorption and luminescence spectra were recorded; the luminescence spectrum is similar to that of IOs(terpy)J2+, the benzo substitution in the dpq complex producing a slight red shift in some of the bands.196
(b) (-)-Tub xrarine complexes. Reaction of Os(bipy)(Cl, with (-)-tubocurarine (L) gave the dichloro-p-[I ,4-~yclohexylbis(2,2',2"binuclear complex [Cl(bipy)Os(LL)Os(LL)O~(bipy)C1]~+, terpyridyI)-6-methy1)-1,4-diol]bis(2,2'-py~dyl)diosmium(II) dichloride. The two coordinated chloro ligands can be replaced by pyridine on refluxing this with the complex in aqueous solution. The former complex has potent curariform biological activity'" and there are possible applications as stains for electron microscopy for the complexe~.'~'
Hoo::*-te
terpy-CH,
L
46.4.5
Nitrosyl Complexes
As with nitrido and oxo complexes we shall consider osmium nitrosyl (NO) complexes as a group, irrespective of the nature of the other ligands present. Just as N3- and 0,- are strong n donors and tend to dominate the overall bonding in their complexes, so too the nitrosyl ligand-certainly in its NO+ guise-is a very effective 7c acceptor and is the dominant ligand in complexes containing it. The normal classification of material by oxidation state is inappropriate for nitrosyl complexes because the oxidation state concept is very much a formalism for them. Instead we shall use the generally accepted [M(NO)J'+ classification in which x is the number of coordinated NO groups and n the number of metal d electrons, the latter being calculated on the basis that NO+ is the coordinated moiety. As will be apparent, osmium complexes within each such category do in fact show considerable similarities of structure and reactivity, and also with their ruthenium analogues. Osmium is unusual in forming an [M(NO)]' type of complex. Since a far greater number of nitrosyl complexes have been isolated for ruthenium than for any other metal it might be expected that osmium should rival ruthenium in this respect. There seems no reason why this should not be so, and the present disparity in numbers of known osmium and ruthenium nitrosyls simply suggests that much less work has been carried out on the osmium systems. There are several recent and comprehensive reviews on metal nitrosyl complexes which give good coverage to the osmium complexes;'9s a review on ruthenium nitrosyl complexes is particulady
I
545
Osmium
useful for discussion of bonding aspects and is reievant to the analogous osmium n i t r o ~ y l s . The '~~ reader is also referred to the appropriate section in 'Comprehensive Organometallic Chernistry'.zoo
46.4.5.1
(i)
[ O s ( N 0 ) l 5 and [ 0 s ( N O ) l 6 complexes
[WNO)P
The recently reported [Os(NO)Cl,]- anion, isolated as NO+ and PPh,Me+ salts from the reaction of Os,Cl,, with nitric oxide, is a rare example of a 17-electron nitrosyl system.*"
(ii) [ O s ( N O ) f This is the most numerous class and encompasses the widest diversity of coligands. Most of the known species are octahedral and in all the Os-N-0 linkage is likely to be essentially linear with the nitrosyl moiety bound as NO+, giving the osmium a formal oxidation state of 11. An interesting area, well investigated for [Ru(N0)l6 species but less studied for the osmium analogues, is that of the attack of the NO group in some of the species by nucleophiles.'99As with the corresponding ruthenium complexes the N-0 stretching frequency (vNo) lies between 1800 and 1900cm-' for most of the complexes which behave in this way. Table 10 Representative Osmium Nitrosyl Complexes Complex
Colour
"NO ( E d )
Spectroscopic and other properties
Golden-yellow
1823
IRm'
Red Red
Yellow
1824 1866.1854 1822 1891, 1870 1808 1785, 1764 1807 1873 1832
IR, EL,m' NMR,'03 X-rayz3' Raman,Z08.212 1R208312.250 IR, X-ray2" Raman, IR?" I4NNMRZ5' I R , , ~X-rayu6 ~ IR,2I4 '5NNMR2" IR?" X-rayU6"3B IR,Z'~ X-ray236 Raman,m IR,ms X-ray:37 14NNMRZS2
Red Orange Brown Orange Yellow Orange Yellow Yellow Yellow Red Deep red
1905 1825 1850 1852 1899 I820 1820 1861, 1837 1872 1805 1770
IR,222223 ESCA,36 M o s ~ b a u e r ' ~ ~ IR,
Yellow
1845
1~2'3
Brown Green Orange Orange
1635 1795 1750 t 700
IR, 'H NMRZj9
Orange
1842, 1632
Brown
1417
IR. CV24'
Deep red Red
1665, 1616 1778, 1500
IR, ESCA, X-ray:448 I5NNMRZs3 p , Z 3 Z ~ , 1 ~ j ~ , 2 3 2 . 2 5I H 5 ~ ~ ~ 2
Yellow Yellow Yellow
1r226 IRZY IR, X-ray2" IR, 19FNMR," X-rayz24*229 IR, X-ray" IR, 'HNMRm5 IR, 'HNMRm5 IR, X-raJ3' IR, UV/vis,232MSU2
1~227
IR, X-ray'" IR, X-raJ4'
5
Bromo and iodo complexes also known. bChloro, bromo and iodo m lexes also known. 'Bromo and iodo complexes known, also with arsine and stibine ligands. Methoxo and fluoro complexes also known!32
5
546
Osmium
In Table 10 we list representative complexes of the [0s(NO)l6 type; for conciseness we normally give only the potassium salts of anionic species and chloride salts of cationic complexes; for neutral species involving halide ligands we concentrate on the chloro examples. The coverage is representative but not comprehensive.
(iii) Nitrosyl halides All four [OS(NO)X,]~-species are now known. The fluoro complex is made from [OsX,]’- with excess KHF2,’OZ the reaction yielding [Os(NOjF,X,-,]2- (X = CI, Br);’”3 stereochemistries of the products were deduced from I9FNMR data.lo3For X-ray data see p. 548. The other nitrosylpentahalogeno complexes have been established longer. They are prepared from salts of trans[Os(NOj(OH)(N0,),]2- together with an excess of HX (X = C1, Br, I);204another useful method is Other salts for which relatively to react salts of [oSx6]’- with nitrite ion and HX (X = Cl, Br, Rb,[Osrecent preparations have been given include (NH4),[Os(NO)Xs] (X = C1, Br, 1),2°6,207 Br,2053208 IZo5 1(NO)X,]’*’ and Cs,[Os(NO)X,] (X = Cl~05~207~”8 More recently the preparation of (NO),~Os(NO)C1,]from 0s2C1,, and CC1,NO has been reported; when the complex is heated to 170°C the polymeric chloro-bridged species [Os(NO)CI,], is f0rmed.2~~ With PPh, Me+, (NO),[Os(NO)Cl,] gives (PPh, M ~ ) [ O S ( NO ) C ~ , ]the ; ~ ~X-ray ~ crystal structure of this is described below on p. 548. Studies on the I9FNMR of [Os(NO)F,I2- in aqueous solution provide evidence for the existence in such solutions of [Os(NO)F,(H,O)]- , cis- and trans-Os(NO)F,(H,O), , [Os(NOjF,(H,O),]+ and [Os(NO)F(H,0),]z+.203The hydrolytic behaviour of [Os(NO)X,I2- (X= C1, Br, I) indicates that NO+ is exerting a trans effect in these complexes; the stability decreases from X = C1 to X = I, and the product is initially trans-[Os(NO)(H,O)X,] - which then deprotonates to trans[OS(NO)(OH)&]-.~~~ Irradiation of [OS(NO)X,]~- (X = C!, Br) with mercury light gives [Os(H,O)X5I2-.’I1 There are comprehensive Raman and IR spectroscopic data on [OS(NO)X,]~-salts (X = C1, Br, I) .%212
(iv) Nitrosyl ammine and nitro complexes
The chloride, bromide and iodide of [OS(NO)(NH~)~]~+ are made from the corresponding [Os(NH,)J3+ salts and nitric oxide,s6 and X-ray data are available for waras[Os(NOj(OHj(NH,),]Br,, Os(NO)(OH)(NH,)Br, and [Os(NO)(H20)(NH,),Cl]Br, (see p. 548). There does appear to be a trans influence of the NO group in that the protons in the trans ammine group in [Os(NOj(NH, ),I3+ are acidic; treatment with base yields rrwzs-[Os(NOj(NH,)(NH3),]’+, and another product of the reaction is trans-[Os(NO)(OH)(NH,)4]2+. This will react with HX .’I4 (X = CI, Br, I) to give tran~-[Os(N0)X(NH,),]~+ Other substituted ammine complexes such as cis-Os(NO)X, (NH,), ,[Os(NO)X(NH,)3(Hz)I2+ (X = C1, Br, rrans-Os(NO)Cl,(NH,),~15 OS(NO)X,(NH,)~(OH)(X = C1, Br,2L6,’17 NOZY ’16 Bu, OAC”’), cis-Os(NO)Br,(NH,),C1217 and [Os(NO)(OH),(NH, j,(H,0)]+215have been prepared. Salts of ‘[OS(NO,),]~-’ have long been reported in the literature’” with sodium, potassium, ammonium, magnesium, calcium, strontium and barium cations, but the anion in these salts is now known to be tr~ns-[Os(NOj(OH)(N0,),]~.’I9 The potassium salt, made from [OSC~,]~and excess KNO2,2I8is a useful starting material for other nitrosyl complexes since the NO; groups are easily displaced. The X-ray crystal structure of the anion is discussed below (p. 548). The anion exhibits a complex hydrolytic behaviour, alledgedly in accordance with the trans effect of the nitrosyl ligand.’”
(v j Nitrosyl azido and cyano complexes The complex (Ph,P),[Os(NO)(N,),] is made from OsO,, hydroxylamine and sodium azide. It has an unusually low vNp0 frequency (1715 m-' j. With o-phenanthroline, Os(NO)(N,), phen is produced. The red nitrosylpentacyano complex K,[Os(NO)(CN),] *2H,O can be made from transK,[OSO~(OH)~], KCN and nitric acid?= The salt, which is unusual in that it is barely soluble in water, is like the nitroprusside ion in that it gives a coloured complex with sulfide ion.’” With nitric acid it gives a number of species such as trans-[Os(NO)(H,O)(CN),]- and Os(NO)(CN), 2H20.223 For nitrosylcyano complexes with thiourea see p. 608.
547
Osmium ( v i ) Nitrosyl complexes with monoteriary phosphines, arsines and stibines
The hydrido complexes Os(NO)H(PPh,),(OCOR), (R = CF,, C,F,) have been prepared by reaction of R C 0 2 H with Os(NO)H(PPh,), ,224 and a similar reaction will yield [Os(NO)H,(COCF,)](H(OCOCF,)2);224 Os(NO)z(PPh,), and RC0,H also give [Os(NO)H(PPh,),(OCOR),]+ .225 The neutral hydride is thought to have a structure in which the hydride is trans to the nitrosyl group and the monodentate carboxylato ligands are trans to each other.224 in Reaction of [0sX6l2- (X = C1, Br) with MNTS (N-methyl-N-nitrosotoluene-p-sulfonamide) the presence of PR, (R = Bun, Ph, p-MeC6H4,p-ClC,H4, X = C1, R = Ph, X = Br) yields Os(NO)X,(PR,),;226other methods involve reaction of ClNO or BrNO with OsC1, in the presence of PPh3227 or reaction of [OsX,]' - with nitric oxide and LPh, (L = As, Sb), this latter reaction giving Os(NO)X,(AsPh,),, and Os(NO)X,(SbPh,), (X = C1, Br, The structure for all these species is likely to be that of Figure 12, with NO replacing NS.226,227
(vii)
With chelating arsines
Reaction of Cs,[Os(NO)X,] and o-phenylenebis(dimethy1arsine) (diars) gave trans[Os(NO)(diars)zX]2+(X = C1, Br, I); cis-[Os(N0)(diars),Cl]Cl2is made from Os(N,)(diars),Cl and HCl.205The systems provide a good illustration of attack by nucleophiles upon a coordinated nitrosyl group which has some electrophilic properties by virtue of the residual positive charge on its nitrogen atom (see Scheme 6).205
/ Os(NO,)diars,Cl
~
-
.:.I
\
rrans-lOs(NO)dian,Cl]*+
0,
Os(N(0)NNHPhl diars,Cl
Os(N,)diars,Cl
'
[ Os(N2)diars,C11
Scheme 6 Reactions of nitrosyl diarsine complexes205
( v i i i ) Miscellaneous [Os(NO)/" species
Reaction of Os(NO),(PPh,), with trifluoroacetic acid gives Os(NO)(ON= C(O)CF,)(OCOCF,)(PPh,) (Scheme 7), in which both coordinated trifluoroacetate and 0,O-trifluoroacetato-hydroxamate ligands are present (see p. 548 for X-ray s t r u c t ~ r e ) ? ~There ~ . ' ~ ~are also two metal-metal-bonded species for which X-ray crystal structures have been determined (p. 548):
Scheme 7 Reactions of Os(NO)z(PPh3)2
548
Osmium
Os(NO)Cl,(HgCl)(PPh,),, made by the action of HgC1, on Os(NO)(CO)Cl(PPh,)2,230and (Ph4P)2[Os(NO)CI,(SnCl, ),I, obtained from [Os(NO)Cl,], and PPh,(SnC1,).23' Octaethylporphyrin (OEP) affords three osmium nitrosyl complexes: Os(NO)(OMe)(OEP), made from Os(CO)py(OEP), methanol and NO:,* with H F this yields OS(NO)F(OEP)~~' and with HClO, is it gives Os(NO)(OC103)(OEP).255 The nitrosyl carbonyl complex [Os(NO)(CO)Cl,(PPh3)2]BF, made by reaction of Os(NO)(CO)Cl(PPh,), with HBF, .223 Two complexes have recently been reported with polypyridyl ligands: [Os(NO)(bipy), CI](PF,), , obtained from Os(bipy),Cl, and nitric oxide with PF; ,and [Os(NO)(terpy)(bipy)](PF,), , obtained from [Os(terpy)(bipy)Cl]+ ,NO; and HPF,. The acid-base equilibrium (equation 3) was compared with that for the ruthenium analogue, and potentials for the [O~(NO)(bipy),Cl]~+'~+ and [Os(NO)(terpy)(bipy)13' I 2 + couples were compared with those for the ruthenium analogues. The differences were attributed to greater Os-NO electronic mixing and greater spin-orbit coupling at the osmium atorn.ls3
+ 20HlOs(NO)(terp~)(bipy)]~+
e
[Os(NO,)(terpy)~bipy)]++ H 2 0
(3)
Reaction of OsO, with hydroxylamine hydrochloride and thiocyanate ion gives [Os(NO)(NCS),]'-; with 2,2'-bipyridyl or 1,lO-phenanthroline (LL), Os(NO)(NCS), (LL) complexes are formed.234
( i x ) X-Ray structural studies on [ O s ( N O J f complexes
Until fairly recently there were relatively few structural studies on osmium nitrosyls, but of late the field has received a fair amount of attention. As expected"' the Os-N-0 moiety is essentially linear in all these complexes, and most of them have octahedral coordination. There is evidence for a slight lengthening of the bond trans to the nitrosyl (there are more examples of this in the more extensively studied structural chemistry of [Ru(N0)I6 complexes'g8);it has been noted that such trans bond shortening is a feature of octahedral [M(N0)l6 complexes where the N-0 stretching frequency exceeds 1800cm-1,198 as it does in all these compounds. Structurally the simplest complexes to be studied are those of NaK[Os(NO)F,] . H 2 0 (I) and Cs2[Os(NO)F5].H,O (11), For these the Os-N and N-0 distances are 1.707(9) and 1.18(1) (I) and 1.675(16) and 1.21(2)A (11); the mean Os-F (cis) distances are 1.991(8) and 1.986(9)A in I) and (11) respectively, while the corresponding Os-F (trans) distances are 1.966(5) and 1.947(5) The Os-N-0 angles are essentially linear in (I) and (II).235This slight shortening of trans ligands as against cis has been noted and discussed for [Ru(N0)l6complexes of the form [Ru(NO)XJ- . I q 8 In [PPh, Me][Os(NO)Cl,] there is no tram ligand, the structure being square-based pyramidal with an 177(2) "); the four equatorial axial nitrosyl ligand (Os-N 1.89(2), N-0 0.90(3) A, Os-N-0 angle of ligands are at a mean distance of 2.36(1)A from the osmium, with a mean N-Os-C1 93.8 "'.O The N-0 distance of 0.90(3) A is described as 'unnaturally shorty2" and presumably would have a more normal value if low-temperature data were obtained. It seems likely that the five-coordinate structure is a consequence of the presence of the large organic cation rather than a marked trans effect of the nitrosyl ligand; often 1:1 complexes rather than 2: 1 cation:anion species are preferentially formed when the cation is bulky as is the case here (a similar instance is found for [OsNXJ species (X = C1, Br, I); see p. 560). Two other structures of highly symmetrical species are those for trans-[Os(NO)(OH)(NH,),]Br, (III)z36and the anion in trans,trans-[Os(NO)(OH)(N02),][Ru(NO)(OH)(NH,],] qH,O (IV).237Here the Os-N and N-0 distances are respectively 1.74(2) and 1.19(3) (111) and 1.762 and 1.15A (11);
a ~
Figure 5 Structures of [0s(NO)lb complexes containing meid-metal bonds230,23
Osmium
549
the Os-N-0 angles are 176(1)' (I) and 175.7(8)" (IV). The trans Os-0 (OH) distances are 2.01(2) (111) and 1.943A (IV), while the cis Os-N distances have mean values of 2.14 and 2.09A for (111) and (IV) respectively. For the trans-[R~(N0)(0H)(NH~)~]~+ cation in (IV) the Ru-N and angle at 178.4O, the tram Ru-0 (OH) N-0 distances are 1.739 and 1.16 A with the Ru-N-0 distance 1.956A and the mean cis Ru-N (NH,) distance 2.1 15A.237 In (PPh,),[Os(NO)Cl, (SnCl,),] (V) the trans Os-C1 distance (e) is siightly but significantly shorter than that for the cis (d) (Figure 5a) (0 = 1.734(2), b = 1.166(21), mean c = 2.648(1), mean d = 2.380( l), e = 2.364(4) A; Os-N-0 angle 180 0).231 Another species containing a metal-metal bond is Os(NO)(HgCl)C12(PPh3)z(VI) (Figure 5b); this is an X-ray structure determination of rather lower accuracy (a = 1.79(4), b = 1.03(6), c = 2.42(2), mean d = 2.39(2), e = 2.577(6), f = 2.39(3), g = 2.37(2) A, Os-N-0 180 Br,236,238(VII) (Figure 6a) and Os(NO)(H,O)(NH,), C1236"38 (VIII) (Figure In OS(NO)(OH)(NH,)~ 6b) the bond distances are: a = 1.75(1), b = 1.16(2}, mean c = 2.533, mean d = 2.1 1 e = 1.98(l)A (for VII) u = 1.64(3), h = L21(4), c = 2.343(15), mean d = 2.34, e = 2.13(3).236 r
1
Figure 6 Structures of nitrosyl ammine complexes
In the unusual structure of Os(NO)(ON=C(b)CF3)(OCOCF,)(PPh,), (VII) (Figure 7) the metal -ligand bond lengths between cis and trans ligands are the most marked for any [0s(NO)l6 complexes, though the nature of the ligands is admittedly different. The coordinating atom trans to the nitrosyl group is from an 0,O-triffuoroacetohydroxamate ligand. In this a = 1.726(7), b = 1.209(9), c = 2.115(6), d = 2.063(6), e = 2.406(2), f = 2.375(2) and g = 1.984(6)A224(see also ref. 229). there is disorder between the carbonyl and nitrosyl ligands; the In [Os(NO)(CO)Cl,(PPh,),]BF4 phosphine groups are trans to each
46.4.5.2 J O s ( N 0 )J 7 , [Os(NO)J8 and [Os(N0),J8 complexes
(i)
[OsCNO)]' species
It has recently been shown that OsCl,(PPh,), reacts with QNCl and ONBr to give Os(NO)XCl(PPh,), (X = Cl, Br); EPR data were rep~rted."~
( i i ) ) [Os(NO)]' species
There are a few of these, all with phosphines or arsines and with six- or five-coordination. The vNo frequency in general for these lies lower than for those in [0s(NO)l6 species, in the range 1650-1 800 cm- ' .
Osmium
550
The hydrides Os(NO)H(PR,), (PR, = PMePh,, PPh,) are made from Os(NO)CI,(PR,), with ethanolic KOH; the PMePh, complex may be fluxional in solution.239 The complex Os(NO)Cl(PPh,), is made from Os(NO)(CO)Cl(PPh,), in CH,CI, and air (a procedure giving Os(NO)Cl(CO, )(PPh,),) followed by treatment with PPh, .240 It reacts with diazwith the halogenated diazoalomethane to give the carbene complex OS(NO)(CH,)C~(PP~,),;~~,~~~ kane IC(N,)CO,Et an aldoxime dinitrogen complex Os(N,)Cl, (N(OH)CHCO,Et}(PPh,), is formed (see p. 556).24' The complexes Os(NO)(NO,)(PPh,), and Os(NO)Cl(AsPh,), , made from OsC1, and, in the first instance, N 2 0 3and PPh, and in the second ClNO and ASP^,,^' have unusually high vNo frequencies for [OSNO]~ species (Table 10). and dppeZ4'will undergo The complex [Os(NO)(dppej,]+, made from [Os(NO)(CO),(PPh,),]BPh, electrochemical reduction in two reversible one-electron steps to give [Os(N0)I9 and [OS(NO~]'~ species (equation 4j.243 [Os(NO)dppezlz+
@-'I
Colour
Os(NO)dppe,
-1.33V
E+(vs. SCE) VNO
e
[Os(NO)dppe,]- 1.95V
1673
1417
1420
Red
Brown
Red
(4)
There are also a number of nitrosvl carbonvl comdexes in this cateaorv. a retxesentative selection of which are listed in Table 10; general types are [OS~NO)(CO)PR,)~]~, [ijs(NO)(CO),(PR,),J+ and OS(NO)(CO)X(PR,),.~~~ In the latter category the chloro complex Os(NO)(CO)CI(PPh,), is of particular interest. It is made by reaction of Os(CO)ClH(PPh,), with N-methyl-N-nitroso-p-toluenesulfonamide, and reacts with HC1 to give Os(HNO)Cl,(CO)(PMe3j,. This latter complex was shown by X-ray crystal structure analysis to contain an N-bonded HNO so presumably electrophilic attack on NO has occurred, possibly via a 20-electron intermediate such as [Os(NO)(CO)Cl,(PPh,),]- . Another curious feature of Os(NO)(CO)Cl(PPh,), is that it exhibits three vN-o frequencies (at 1769, 1629 and I560cmP', shifting to 1742, 1598 and 1534cm-' on "N substitution). It has been surmised that the solid may contain more than one form of the species in dynamic equilibrium.233Another carbonyl complex of interest is Os(NO)H(COj(PPh, ),, made from Os(NO)(CO)Cl(PPh,), and NaBPh4;244 its X-ray crystal structure has been obtained (see p. 551, Figure 9). (iii) X-Ray studies on [Os(NO)/8 species Two studies have been reported, both on carbonyl nitrosyl complexes, but in both cases disorder made from prevented the NO and CO groups being distinguished. In [Os(NO)(CO),(PPh,),]C104, Os(NOjH(CO)(PPh,), and HCIO,, the trigonal bipyramidal structure has apical phosphine ligands,24 and this is also the case245in [Os(NO)(CO)(CNR)(PPh,j2]C10,245(R = p-tolyl). In both, however, the Os-N-0 unit is essentially linear.
(iv)
[Os(NO), 7 species
Few of these are known but there is an X-ray study on one, [Os(NO),(OH)(PPh,),]PF,, made by reaction of oxygen with O S ( N O ) , ( P P ~ in ~ ) ~the presence of HPF,. The crystal structure shows a distorted square-based pyramidal structure in which there is a bent apical nitrosyl group (Os--N -0 = 133.6(1)") and a linear basal nitrosyl ligand (Os-N-0 = 177.6(12) "). The bond distances are: a = 1.86(1), b = 1.17(2), mean c = 2.434, d = 1.935(8). e = 1 . 6 3 ( l ) , f = 1.24(2jA (Figure 8)." The two stretching frequencies of 1842 and 1632cm-', recorded for the BF, salt,247presumably correspond to those of the linear and bent moieties.
Figure 8 Structure of [Os(NO),(OH)(PPh,),]PF,246
Osmium
551
There is a brief report of [Os(NO),(OCOR)(PPh,],OCOR (R = CF3, CzF5);225 reaction of Os(NO), (PPh,), with strong non-coordinating acids gives [Os(NO),H(PPh& 1' but the species was not isolated.247
46.4.5.3
[Os (NO)J9,[OS(NO)]'~and [Os (NO),]'o complexes
j O s ( N 0 ) P species
(i)
The only example would appear to be Os(NO)dppe, , made by electrochemical reduction of [Os(NO)dppe2]+(see below). It is dark;brown and very air-sensitive, giving an ESR spectrum which seems to suggest that the unpaired electron is largely localized on the NO ligand. The very low vN0 frequency of 1417 cm-' suggests that it may contain a bent Os-N-0
(ii) [Os(NO)]" species
Again the only example seems to be [Os(NO)dppe,]- (see below);243the low v ~ frequency - ~ of 1420cm-' would be consistent with the presence of a bent Os-N-0 ligand, equivalent to a coordinated NO- group (a linear Os-N-0 group would give an electron count of 20).
(Eii)
[Os ( N Oj 2
species
A very interesting complex is Os(N0)2(PPh,)2*$C,H,, made from reaction of Os(NO)H(PPh,), with N-methyl-N-nitroso-p-toluenesulfonamide; it crystallizes as a hemisolvate from benzene.248 The = 139.1(3)", P-OsX-ray crystal structure shows it to be tetrahedral (N-Os--N P = 103.51(6) ") with two essentially linear Os-N-0 moieties (Os-N = 1.771(6) and 1.776(7)& N-0 = 1.195(8) and 1.211(7)& Os-N-0 = 178.7(7)' and 174.1(6)0)?48Although the Os-N -0 fragments are linear the v ~ frequencies - ~ are low, as is to be expected in view of competition for E-electron density on the osmium atom. ESCA studies suggest that the effective oxidation state of the osmium lies between I1 and O.,,' The hydrido complex [Os(NO),H(PPh,),]+ is formed by the action of strong acids on the complex, and the reaction of CO under pressure with this gives Os(CO),(PPh3), with CO, and N, as by-products."' Anothcr interesting complex in this category is Os(NO),(OEP) (QEP = octaethylporphyrin), made from nitric oxide and O S ( C O ) ~ ~ ( O E Pone ) ~vNo ~ ~frequency , ~ ~ ~ is very low (1500 cm-') so that one Os-N-0 group could be substantially more bent than the other.
46.4.6
Complex of HNO
As mentioned above (p. 550), reaction of Os(NO)Cl(CO)(PPh,), with HC1 gives Os(NHO)Cl,(CO)(PPh,),, a rare example of a complex containing a coordinated HNO group. The X-ray crystal structure (Figure 9) has been determined. In this a = 1.915(6), b = 1.193(7), c = 0.94(11), d = 2.440(2), mean e = 2.434(2),f= 1.899(8),g = 2.429(2) A; ONH = 99(7) '. The vNo frequency is at 1410cm-', shifting to 1393cm-' in I5N sub~titution.'~~
QI
,5,*\fPh3 Clos-co P h 3 Y
I#
c1 Figure 9 Structure of Os(HNO)(CO)(PPh,),233
46.4.7
R
Diazenido Complexes
Reaction of Os(PPh3),X,, LiX and aryldiazonium salts (RN,)BF, (X = CI, R = Ph;256X = Br, = Ph,zs6P-C,H,M~~~'") gives OS(N,R)(PP~,)~X,, with the likely structure shown in Figure
Osmium
552
ESCA data on [Os(N2-p-MeC,H,)(PPh,),JBr, and on Os(Nz-p-C,HqF)(PPh3)zBr3 suggest that, in the latter case at least, the spectra could distinguish between the two nitrogen
Cl Figure 10 Likely structure of Os(N2R)(PPh,),X:56
46.4.8
Thioaitrosyl (NS) Complexes
Despite the relatively large number and diversity of osmium nitrosyl complexes (p. 544) rather few thionitrosyls of the metal are known (see Table 1l), and this is clearly a field of potential interest for further study. It should in particular be possible to obtain complexes containing a bent Os-N -S moiety. It is interesting to note that two examples of bis-thionitrosyls, a very rare class of complex, are known with osmium. The formal oxidation state of the metal in all its known thionitrosyls is I1 with the exception of (Ph,As)[Os(NS)Cl,]; again by analogy with the nitrosyl chemistry, osmium(0) thionitrosyls might be expected to exist. Table I1 Thionitrosyl and NSU Complexes
Complex
Mp.
Colour
Os(NS), C1, (P%As)[Os(NS), c1'cl4 1 Os(NS)Cl,(PMe,Ph), os(Ns)c13 pph3 h Os(NS)Cl, (AsPh,), Os(NS)CI, bipy O s ( N W 4 PY2 (Ph4P)*[WNS)(NCS),) (Ph,A5)[~(NS)CI,J
I" C)
Brown
Black Green Green Green Green Green-yellow Green-brown Black irons-(Ph,P)[Os(NS)CI,(H20)] Green
145- 147 15G152
"Y
Speciroscopic and other proper8ie.v
1388, 1326, 1308 1298, 1215 1285 1294:6' 1290'60 1282,2621290" 1282 1284 1270 1340 1285
IR26' IR, X-ray26' IR, 'HNMRz62 11~ 2 6~2 . 2X-ray2a 6:3 ~ ~ IRZb2 IR262 I~ 2 6 5 1R2"' IR, X-ray'"
Preparative routes involve sulfur abstraction by osmium(V1) nitrido complexes from SfC12or SCN- ,though the mechanisms of these reactions are not clear; reaction of halides with (NSC1)3has also been used. The subject of thionitrosyl coordination chemistry has recently been reviewed.258The NS ligand has been variously described as being a better 0 donor and n accept^^^*^^^^ or a better c donor but a worse ?E acceptor than NO.26o
46.4.8.1
(i)
Osmium( I I )
Bis thionitrosyls
Two such species are known for osmium. Reaction of Os,Cl,, with (NSCl), yields Os(NSCl),Cl, (see p. 553 below) which, with (PbAs)Cl, gives (Ph,As)[Os(NS),Cl~Cl,]. The X-ray crystal structure of this (Figure 11) shows the thionitrosyl ligands to be cis and deviating only slightly from
linearity (the OsNS angle is 169.5 "). A loosely bound chloro Ligand is attached to the sulfur end of an NS group ( S a *C12.28 A); in the crystal it is statistically &longs to both sulfur atoms with an occupation probability of 50%.
-
c1
C/
Figure 11 Structure of the anion in (Ph,As)[Os(NS),Cl.C1,]"'
Osrnium
553
The Os-N distance of 1.83 A is comparable with those found in nitrosyl complexes and is clearly indicative of a measure of back-bonding, but there is no apparent trans shortening or lengthening effect on the op osite chloro ligands.26’In the structure, a = 2.36, mean b = 1.835, mean c = 1.46, mean d = 2.27 .’“ Reaction of this material with GaCl, gave the neutral bis thionitr,osyI, Os(NS),Cl,. The IR spectrum suggests a cis configuration for the complex; although three bands appear in the N-S stretching region (1388, 1326 and 1308cm-’) the latter two are thought to arise from Fermi resonance. The Os-N stretches appear at 385 and 369cm-’, though the latter could be an Os -C1 mode.26’
s:
( i i ) Mono thianitrosyls
The first thionitrosyl complexes of osmium, Os(NS)X,L, (X = C1, Br, L = AsPh,, PMe,Ph, )bipy), were made by Chatt by reacting the nitrido species OsNX,L, with S,CI,; the pyridine complex Os(NS)Cl,py, was made from [OsNChI-, pyridine and SzC1,.262The mechanism of the reaction is not at all clear. In the corresponding reaction of SZClzwith rhenium(\? nitrido complexes to give thionitrosyls it was postulated that nucleophilic attack of S,C1, at the sulfur atom by the nitrido ligand might occur, but this is less likely for osmium since osmium(VI) nitrido complexes exhibit pronounced electrophilicity at the N3- ligand (p. 561). The ’ HN M R spectra of Os(NS)Cl,(PMe,Ph), was measured and a structure proposed akin to that of Figure 12, with trans virtually coupled phosphorus ligands; the complexes are diamagnetic and probably contain linear Os-N-S Reaction of ’OsCI,’ with (NSCl), and L (L = PPh3, AsPh,) yields Os(NS)Cl, L, ;263an improved method for preparation of Os(NS)Cl,(PPh,), involves the use of OsCl,(PPh,), and (NSCl),. The unit to be the structure is X-ray crystal structure (Figure 12) shows the Os-N-S analogous to those for Os(NO)Cl,(PPh,), and OsNCI,(PPh,),, with a = 1.779(9), b = 1.503(10), mean c = 2.459(2), mean d = 2.387(3) and e = 2.399(3)A. S blli
N
Figure 12 Structure of Os(NS)Cl,(PPh,),
A number of other monothionitrosyls of osmium have recently been reported. Reaction of Os,Cll0 and (NSCl), in the presence of POCl, yields Os(NS)Cl,(POCl), from which (Ph,As)[Os(NS)Cl,] can be made,20’and the latter can also be obtained, as the PPh; salt, from ‘Os(NS)CI,’ (a polymer, made from OsCl, , (NSCI), and PPhZ ’@).Recrystallization of this from water-methanol gives trans-(PPh,)[Os(NS)CI,(H, O)],the structure having been elucidated by X-ray angle of 174.9(3)”; Osdiffraction (Os-N = 1.731(4), N-S = 1.514(5)A with an Os-N-S 0 = 2.165(4), mean Os-C1 = 2.356& mean N-Os-C1 = 94.8 ’).’@
46.4.8.2
Osmium(I1I)
Another example of sulfur abstraction by an osmium nitrido complex occurs in the reaction of [OsNCl,]- with (PPh,)NCS to give (Ph4P)2[O~(NS)(NCS)5], in which the thiocyanate groups are thought to be bonded to the metal via nitr0gen.2~’For reactions of the complex see Scheme 10, p. 563.
46.4.9
Complex of NSCl
The precursor species for [Os(NS),Cl -C14]- and Os(NS),Cl, is Os(NSCl),Cl,, a black material obtained by reaction of (NSCl), with Os2C1,,. It is thought to have a cis configuration with bent N --S-C1 ligands bound to osmium via the nitrogen atoms. The N-S stretching vibration appears at 1335
554
Osmium
46.4.10
Dinitrogen (N,) Complexes
The most stable osmium dinitrogen complexes occur with the metal In the divalent state, as would be expected for this n-acidic ligand, but a substantial number of osmium(II1) dinitrogen complexes are also known, though these tend to lose N2readily and are kinetically much more labile than their osmium(1I) counterparts (see Table 12). A few bis(dinitrogen) species are known with a cis arrangement of N, ligands; such a configuration is best adapted to n-acidic ligands, there being less competition for the metal tb orbitals than for the trans arrangement. An interesting feature of osmium dinitrogen chemistry is the formation of bridged species containing the Os-N=N--Os and Os--N-N-Ru units with apparently mixed oxidation states in some cases. Dinitrogen complexes of osmium are often synthetically useful because the Nz group can easily be displaced, especially by acids. Table 12 Dinitrogen Complexes Complex
Cnlour
vN (em-')
YeIIow White
2022 2179 2028 2003 2167, 2104 2180, 2125 2212
Yellow Yellow Yellow
2205 White
Brown Yellow Green Green White Blue
2080 2092 1979 2059 2085 2080 2015 1999 2125
Specfroseopic and other propertied
IR, UVjvis98 IR, CV" UV/vis, Cv64 TR, UV/vis, CVM UV/vis@ lR,'X-rayz" IR, X-ray"' IR,273.274 1 H NMR 273 15NNMR288 IR, 'HNMR,274"NNMRZg8 IR, 1HNMR,27'I5NNMRz8' IR. ' H NMR'" IR; 'H N M R ~ D S IR, Raman, U V / V ~CV:Soa*280b S ~ ~ ~ MCDZ8Ob UVlVis. CVZN Raman; Uv,vis,2'' ESCA282,287 IRI Raman, CV'" X-Ray282b
'Bromo and iodo complexes and t r n m isomers also known. b L = isonicotinamide. 'Also PEG, PMePh2, PEt2Ph, PEtPhz, PPr",Ph, AsMezPh, AsE.r,Ph. dCV = cyclic vultammetry.
46.4.10.1 Mononuclear complexes (i)
Complexes with ammines and amines
(a) Osmiurn(II). Reaction of salts of [OsCl,]'- with hydrazine hydrate gives [Os(N,)(NH,),)'+ ;266 chloride, bromide, iodide, perchlorate, tetrafluoroborate and tetraphenylborate salts have been made. The X-ray crystal structure (Figure 13) shows the trans ammine group to be slightIy but significantly Ionger than the cis ammines (a = 1.842(13), b = 1.12(2), mean c = 2.144, d = 2.151(15) A) while the Os-N (N,) distance is clearly much shorter than that expected for a pure 0 bond.'" Photolytic studies on the ion have been reported,z68and y irradiation of the complex in .269 It is possible methanol generates the OH radical which reacts to give [OS(N,)(NH~)(NH,),]~+ that a trans effect may be operative such that the trans ammine protons become more acidic.
NH 3 Figure 13 Structure of [Os(N,)(NH,),]CI,"
Osmium
555
Reaction of [Os(NH,),C0I2+ with HCl and KNO, yields c~s-[OS(N,)(NH,),(CO)]~+; trans[Os(N,)(NH,), I]’ is made from [Os(N,)(NH,),]*+ and chromium(I1) with HCl and KNO,. A slightly different route was used to make cis- and trans-Os(N,)(NH,),X, (X = C1, Br, I), by treating cis- or trans-[Os(NH,),X,]+ in HCl with KNO, and NO.@ Complexes of the form cis[Os(N2)(NH3),LIZ+( L = isonicotinic acid, isonicotinamide, pyrazine and substituted pyrazines) are considered on p. 535. There are also bis(dinitrogen) complexes of osmium(I1). The first to be made was cis[Os(N,),(NH,),]’+ by a ‘diazotization’ procedure using [OS(N,)(NH,),]~’ and nitrite in acid soluti~ n . , ~Vibrational ’ spectroscopic data using both ’H and 15Nsubstitution were obtained for the which are useful precursors for other osmium complexes involving pyrazine (see p. 536). A similar preparative procedure involving [ O S ( N ~ ) ( N H ~ ) ~gives L ] ~ +cis-[Os(N,),(NH,), L]’+-(L = isoni~otinarnidc).~’ (h) Osmium(II1). Oxidation of [Os(N,)(NH,), J 2 + with cerium(1V) gives [Os(N,)(NH,),13+ ,61 and the existence of the ion has also been shown electrochemically.272Reaction of cis- or trans[OS(NH~)~X,]+ with HCI and NO; gives cis- and trans-[Os(N,)(NH,),X,]+ respectively (X = Cl, Br, I), and a similar method yields cis- and trans-[Os(N,)(NH,),X,]+ from mer- and facOs(NH,),X, (X = C1, Br, I); mer-Os(N,)(NH,),I, was also prepared.@ The kinetic stability of these osmium(II1) dinitrogen species, while less than that of the corresponding osmium(I1) complexes, is far greater than for ruthenium(II1) and shows that osmium(II1) is capable of effective metal-to-ligand donation. Comparisons have been drawn, using electrochemical and pK, data, between the n-acceptor properties of CO and N, in [OsL(NH,),I3+ (L = CO, N,) lies near 16,55that of [Os(N,)(NH,)J3+ is about 6.6 and that species;@thus, the pK,o€[OS(NH,),]~+ of [OS(NH,)~CO]~+ about 2.5. In basic solution osmium(TI1) dinitrogen complexes disproportionate to an osmium(I1) dinitrogen species and o~rnium(VVI).~~ (c) Osmium ( I V ) . The possibility of the existence of [OS(N,)(NH,),]~’ has been rnenti~ned.~~’
(ii) Complexes with phosphines and arsines (a) Osmium(1I). These are summarized in Table 12. Most of them take the form mer(Os(N,)(LR,),X, and are made by the reaction of Os(LR,),X, in THF with zinc amalgam and N, under pressure.273The dinitrogen ligand has weakly basic properties; thus, mer-Os(N,)(PEt,Ph), Cl, forms a weak complex Os(”AlMe,)(PEt,Ph), CI, with the etherate of trimethylalurnin~m.~~~ IR data (from comparison of intensities of the N=N and C=O stretching frequencies in Os(N,)(PEt,Ph),Cl, and Os(CO)(PEt,Ph),C12 suggest that N, is a weaker d donor and weaker 7c acceptor than CO in these complexes.275 There are also hydrido dinitrogen complexes. Reaction of OsH,(PEtPh,), with toluene-p-sulfonyl azide yields Os(N2)H,(PEtPh2),,276 while monohydrido complexes Os(N2)(PR3),HX (PR, = PMe,Ph, X = C1, Br; PR, = PEt,Ph, X = C1) are made from Os(N,)(PR,),X, and sodium borohydride. The structure is thought to involve a mer arrangement of phosphine groups with the hydride cis to the coordinated dinitr~gen.”~ A dinitrogen complex with the chelating phosphine depe, [Os(N,)H(depe),]BPh,, is made from trans-[OsHCl(depe),]+ , nitrogen and sodium tetraphTwo dinitrogen phosphine enylb~ron.~ With ~ ’ HCl, Os(N,)(diars), C1 gives [O~(N,)(diars),Cl]Cl.~~~ complexes of osmium have been studied by X-ray methods. Reaction of mer-Os(N,)(PMe,Ph),Cl, with Pb(SC,F,), gave mer-Os(N,)(PMe,Ph),Cl(SC,F,) which has the structure shown in Figure 14 in which a = 1.909, b = 1.112, mean c = 2.37, d = 2.507, e = 2.31 and f = 2.42/j.279
Figure 14 Structure of Os(N,)(PMe,Ph), C1(SPh)279
An unusual dinitrogen complex generated from the halogenated diazoalkane IC(N,)CO, Et and Os(NO)Cl(PPh,), in the presence of HCl yieids Os(N,)[N(OH)CHCO,Et](PPh,),Cl,;an X-ray crystal study shows the structure to be that shown in Figure 15 in which a = 1.927(4),h = 1.063(5), c = 2.026(4), d = 2.412(1), mean e = 2.407 a n d f = 2.431(1),kZ4’
Osmium
556
Figure 15 Structure of Os(N,){N(OH)CHC02Et)(PPh,),C1,24'
46.4.10.2
Binuclear dinitrogen-bridged complexes
A number of these have been isolated, including species with mixed oxidation states and heteronuclear species with ruthenium. In their general chemistry these remarkable complexes show clear analogies with bridging pyrazine complexes (p. 536):
The best characterized species is [OS,(N,)(NH,),,]~+ made as a bromide or as a tosylate from reaction of [Os(H,0)(NH3),13+ with [OS(N,)(NH,),]~+in the presence of zinc amalgam. The stability of the mixed valence system (formally Os1"-")has been ascribed to electronic delocalization effects and an MO scheme proposed,'* but it is not clear why it appears to be more stable than its reduction product, [OS(N,)(NH,),~]~+. Magnetic circular dichroism spectra and cyclic voltammetry of the ion have been measured.2s0bA number of other mixed homonuclear species are known, viz. [OS,(N,)(NW~),(H?~)~~+, [ O S , ( N ~ ) ( N H ~ ) ~ C ~[I O ~ +S,~ ( N ~ ) ( N H ~ ) ~ C and ~~]~+ [Os,(N,)(NH,),(N,)]'+. The latter has one terminal and one bridging N, ligand, and is made by reaction of [OS(N,)(NH,),]~+with cis-[Os(N,), (NH,),I2+ ?" There is some evidence from ESCA data that the ion [OS,(N~)(NH,),C~,]~+, as the trichloride, is indeed a delocalized Class I11 mixed valence species.282aAnother binuclear dinitrogen-bridged complex is [OS~(N,)(NH,),(CO),]C~~, made by oxidation of [OS(NH,),CO]~+with persulfate, cerium(IV), permanganate or electrolytically. A possible route for formation of the complex by oxidation of [Os(NH3),C0l3+to a nitrido species of higher oxidation state, possible osmium(V) with coupling of nitrido moieties to give the product, was suggested.70The isolation of the blue [OS,(N,)(NH,),,]~+ by oxidation of the pentapositive species with chlorine has recently been reported.282b Heteronuclear dinitrogen-bridged complexes are also known. Reaction of cis-[Ru(NH,),(H20)J2+ with [OS(N,)(NH,),]~+ yields [(NH,), O~(N,)RU(NH,),(H,O)]~+,and this can be oxidized to [(NW,),O~(N,)RU(NH,)~(H,O)]~+.~~~ There is also a very brief mention of [OsRU(N,)(NH,),~~+ (n = 4, 5).2&1
46.4.11
Hydrazine Complexes
A few of these are known, all with osmium(I1) and with phosphine or arsine coligands. Reaction of Os(PMe,Ph),CI, with N,H, or N,H,Ph in ethanol yields Os(PMe,Ph),Cl,(N,&) ~ C=~ Et, , Bun) as starting materials and Os(PMe2Ph),Cl,(NH,NHPh); the use of O S ( P R ~ P ~ ) (R and N2H4under reflux gives apparently binuclear species, Os, (PR2Ph)4C1,(N, H4),, with the proposed structure shown in Figure 16."
Figure 16 Proposed structure of Os, [PR,Ph),Cl.,72
The only reported example of an arsine complex is Os(AsCp-MeC,H,),},Cl,(N,H~), made from @(As@-MeC,H,), C1, and N,H, .ll'
557
Osmium
46.4.12 Hydroxylamine Complex It seems strange that no hydroxylamine complexes of osmium are established. There is a brief report of a deprotonated complex Os(NO)(NHOH)Cl,(PPh,),, made from Os(NO),(PPh,), and HC1. The vNo band is at 1860 cm-I .247
46.4.13 Amino (-NH,)
and Organoamido (-NR,)
Complexes
Relatively few of these are known, though it might be anticipated that more osmium complexes with these ligands should exist.
46.4.13.1 Amino complexes
The only established example is ~~u~~-[OS(NO)(NH,)(NH,),]~+ (see p. 546); the species [Os(N,)(NH,)(NH,),]" has also been claimed (see p. 554). Further investigation of '[Os(NH,)X,][Os(NH,), XI' (X = Br, I), made by heating [Os(NH,),jX, :5 is clearly needed; 'K2[0s(NH2)C1,]' is known to be K,[Os(NH,)Cl,] (see p. 529). There is also a need to investigate Cs,[Os,(NH,),X,j and Cs,[Os,(NH,),X,,j (X = C1, Br, I), made from HX and [Os(NH,)#kBr,]?89 The amido ammine [OS(NH~)(NH,)~]~+ may be the first product of the one-electron oxidation of [Os(NH,),13+ in acidic media."
46.4.13.2
Alkanolaminato and diaminato comgi~?xes
Reaction of OsO, (NBu'), with dimethyl or diethyl fumarate gives the osmium(V1) 'esters' OsO,(NBuf(CHCO,R),NBut) (R = Me, Et); the complex from diethyl fumarate is monomeric in benzene and the structure given in Figure 17(a) was proposed.m A related complex OsO, (NBu'(CHCN), NBu') has recently been made from OsO, (NBu'X and fumar~nitrile?~' Reaction of OsO(NBu'), and dimethyl fumarate gives OsO(NBut)(NBut(CHC02Me)zNBu'),'~' presumably having the structure shown in Figure 17(b).
(a)
(b)
Figure 17 Proposed structures of diaminato complexes'"
These monomeric and presumably tetrahedral complexes are intermediates in the diamination of alkenes (see p. 559). It is curious that, whereas OsO, and Os03(NR) react with alkenes R to give square-based pyramidal dimers Os204(02 R), (p.593) and [OsO,(OR(NR))], (p. 559) respectively, the reaction products of OsO,(NR), (and presumably of OsO(NR),) with alkenes are monomeric:29o there is no obvious reason for this. It is significant that Os-N rather than Os-0 bonds are formed in these reactions.240 the Alkanolaminato complexes [OsO,(OR"R)], are made from OsO, (NR) and alkenes R;291,300 X-ray crystal structure of [OsO,(OMeCH,NBu')j, shows this to have structure (I), Scheme 8.2917291a
.
46.4.14 Imido (=NH) and Organoimido (=NR) Complexes This is an interesting area of osmium chemistry which stands much in need of development. Apart from the two incompletely characterized NH comprexes, most of the organoimido chemistry is that of osmium(VII1); species such as OsO,(NR) will undergo oxyamination reactions with alkenic double bonds, and such reactions may be made catalytic. However, because of the high reactivity of osmium(VIII), only a small range of tertiary imido complexes have been prepared (R = But, Am', I-adamantyl, Oct').
558
Osmium
There is a good review on organoimide chemi~try,'~'and organoimides are also mentioned in the course of a review on nitrido complexes?93
46.4.14.1
Complexes with =NH
Two binuclear complexes containing 2,2'-bipyridyl (bipy) have been reported in which the imido made from ligand appears to function in a bridging role. These are [Os2(NH)Cl,(bipy)4](C104),, [Os(bipy),(NH,)Cl]+ and cerium(IV), and Os,(NH,)(bipy),, made from Os(bipy), Cl, and ammoThe IR spectrum of the first complex has been The oxidation states seem too low t~ia.''~ for these to be true imido complexes and further investigation would certainly be of interest.
46.4.14.2 Complexes with =NR ( i ) Osmium( V ) and osmium( VI)
It now appears that 'Os(NC,H4X)CI3(PPh3),' (X = H, p-MeO, P-C~),~' made from 'OsOCl,(PPh,),' (now known to be a mixture containing OsO,C1,(PPh,),; see p. 584) are nitrile complexes OS(NCC6H4X)Cl3(PPh~),(see p. 566).296 may perhaps be regarded as an imido complex The complex (PPh4)[Os(NC(CC1,)NCCl(CC1,)C1,] of osmiurnfV1); it is made by reaction of Os, Cl,, with trichloroacetonitrile, and X-ray studies show it to have the structure shown in Figure 18. In this, a = 1.97(1), h = 1.34(1), mean c = 2.32, d = 2.279(3), Os-N-C = 142.1{8) '. The Os-N distance ( 0 ) is slightly shorter than that expected angle is 142.1(8)".297 for a single bond; the Os-N-C
E
c1-os
Q1 *,***C' -c1
c1vd l
Cl Figure 18 Structure of (PbP)[Os{ NC(CC1, )NCC I (CCl,))Cl, 1''
(ii )
Osmium (VIIZ)
The established organoimido complexes all contain osmium(VII1) and are listed in Table 13. Table 13 Organoimido Complexes Mp.
cj
Complex
Colour
("
OsO,(NBu') OsO, (NAmt) OsO,(NAd)" OsO,(NOct') [OsO,(NOct')j, DABCOb OsO,(NBu'),
Yellow
100 57-59
Yellow Yellow Red Yellow
OsO, (NAm' ), OsO,(NAdX OsO, (NAm' )(NAd) OsO(NBu'),
Yellow Yellow Yellow
2W211 72-74
OsO(NAd), (NBu')
Orange
aAd = 1-adamantyl
'OEN
(cm-I)
vosu
17&177
1184 1210 1215
925, 912 930,915 925,915
51.5
1224
928, 910
1210 1200
875, 850 888, 878
114-116 (dec)
1175 1180 1190, 1100
880,865 885, 875 838
146 ( d x )
1160, 1100
838
11%120 (d)
Spectroscopic and other properties
IR;% U V / V ~ S :I3C ~ ~NMR2'233M IR, 'HNMR," IR, 'HNMR,301X-raym IR,291UV/vis290 Raman, IR, X-ray3M IR, 'HNMR, MS:w X-ray,'02 "C NMR3@ IR, I H N M R ~ W IR, 'HNMR"' IR, 'H NMR, MS:" I3CNMR3M IR, 'H NMR"'
DABCO = 1,4-diazabicyclo[22.21octane Not specifically assigned as such in reference.
(a) Monoimzdo species, OsO,(NR). The first to be made were Os03(NBut) and Os03(NOctt), obtained from OsO, and the tertiary amine in Subsequent preparations of OsO,(NBu') used ~ e n t a n e ~ ~or + ~water2w *' as solvent. Other species made by similar routes are O S O , ( N A ~ ' ) ~ ~ ~ ' and the I-adamantyl complex OSO,(N-~-A~).~~,~~~
559
Osml'Um
The X-ray crystal structure of the latter complex has been determined. The coordination about the osmium is tetrahedral; the Os=N distance of 1.697(4)A is slightly shorter than the mean Os =O distance of 1.714(4)A and not much longer than the Os=N distance in terminal nitrido complexes below. The N-C distance is 1.448(6)A and the Os-N-C angle 171.4(4)0.302 The complexes will effect oxyamination reaction with alkenes in a stereospecific reaction (Scheme 8).290After reductive cleavage of the intermediate alkanolaminato complex (I) (see below) vicinal amino alcohols (11) are formed. The reaction is unusual in that the new C-N bond is always formed at the least substituted terminal alkenic carbon atom, and there is a clear preference for the imido complex to use its NR group for coordination to the osmium despite the steric restraints of R.2yy,300 However, the least sterically hindered part of the alkene moiety is attached to. the nitrogen atom.290~291-300 The yields of amino alcohols in the reaction can be improved by addition of tertiary alkyl bulkhead amines (see below).
(1)
(11)
Scheme 8 Oxyamination by O S O ~ ( N R )3 0~3 ~ ~ ,
The oxyamination reactions can be rendered cataIytic by using c h 1 0 r a r n i n e - T ~or~ ~ N-chloro-N~~~ argentocarbamatesnM6 (b) The adducts OsO, (NR).L and [OsO,( N A ) I, L'. Just as OsO, forms weak adducts with nitrogen donors (see p. 592), OsO,(NR) (R = I-Ad, Oct')2y' will form 1 : 1 species OsO,(NBut)*L (L = quinuclidine, 3-quinuclidinone, 3-quinuclidinyl acetate307)and OsO,(NR). L (R = 1Ad, Oct'; L = quinuclidine).2y'There are also 2:l species [OsO,(NBu')],*L with L' = hexamethylenetetramine (HMT) or 1,4-diazabicycl0[2.2.2]octane (DABC0)307and the same nitrogen donors form similar complexes with OsO,(NR) (R = Am', Oct').2y' The X-ray crystal structure of [OsO,(NOct')], .DABCO has recently been determined; the DABCO ligand bridges two OsO,(NOct)' moieties (Os-N = 2.45(1) A), a little longer than the Os-N distance in (OsO,),.HMT due perhaps to a slightly greater trans weakening effect of the imido group over the oxo ligand). The osmium atoms have trigonal bipyramidal coordination, the apical positions being occupied by one DABCO nitrogen atom at the imido nitrogen (Os-N = 1.73(1) A) while the equatorial positions are occupied by the oxo ligands (mean Os=O = 1.71I$).30s ( c ) Bl's imido species, OsOz(NR),. Reaction of OsO, and N-tert-butyltriphenyIphosphineimine or tert-butyltrimethylsilylamine30z gives OsOz(NBu'), ,290 and the 1 -adamantylimido complex OsO,(NAd), was similarly made; a mixed species OsO,(NAmt)(NAd) was prepared from Os0,(NAm') and N-l-adamantyltriphenylphosphineimine.2Yo The X-ray crystal structure of OsO,(NBu'), again shows the expected tetrahedral structure with Os=O at 1.744(6) A; however, one of the imido groups is essentially linear (Os-N = 1.710(8) A, Os-N-C = 178.9(9) ") and the other substantially bent (Os-N = 1.719(8)& 0s-NC = 155.1(8)A.301The presence of one linear and one bent imido group in the complex has been partly rationalized on electron-counting procedures and MO consideration^,^^^^^' although both IR and 'H NMR data indicate that both the imido groups are equivalent in solution?'' Reaction of OsO, (NBu'), with alkenes followed by reductive cleavage gives vicinal diamines (IV; Scheme 9), and the intermediate 'ester' (I) (see p. 557) was isolated using dimethyl or diethyl fumarate as the reacting alkene. Again, the preference for Os-N rather than OS-0 bond formation is noteworthy." R
Scheme 9 Diamination b y OSO~(NR),~'~
Efforts to obtain adducts of OsO,(NR), with nitrogen donors were ~ n s u c c e s s f u l ~ this ~ ' is not surprising since, if the osmium were to be trigonal bipyramidal as in OsO,-L (p. 592) and [OsO,(NR)];L (p. 592), there would be considerable steric interaction between L and a necessarily equatorial NR group.
560
Osmium
( d ) Trisimido species, OsO(NR), . Reaction of OsO, , OsO, (NBu*1 or OsOz(NBu‘ )2 with N-rerfbutyl-tri-n-butylphosphineimine gives OsO(N,Bu’)),, and the mixed complex OsO(NAd),(NBut) can be made from OsO,(NAd), and N-rert-butyl-tri-n-butylph~sphineimine.~~~ With alkenes, OsO(NBu‘), effects vicinal diamination; the intermediate ester (111) (see Scheme 9) was isolated from the reaction with dimethy fumarate.290 ( e ) Tetrakisimido species, Os(NR), . Species of the type Os(NBu‘), (NS0,Ar) have been briefly mentioned (Ar = 2,4,6-trimethylphenyl, 2,4,6-triisopropylphenyl,p-t~lyl).*’~
46.4.15
Nitrido Complexes
In this section we consider together all the osmium complexes involving the nitrido (N7.1 ligand since it appears that this is the dominant factor in the bonding of these complexes (for the same reason all oxo (02-) and nitrosyl (NO) complexes are grouped together). Osmium forms a reasonably diverse number of nitrido complexes, particularly in the VI oxidation state (see Table 14); in this it resembles rhenium (as in many other respects). The terminal nitrido ligand is believed to be the strongest (or certainly one of the strongest) R donor^^^^,^" and will prefer, like the isoelectronic oxo (02-) ligand, to coordinate to metals in oxidation states of low d electron occupancy (e.g. d o , OS""^, ReV”; or d 2 , Os”’, Re”). Such confiwrations are readily accessible to rhenium and osmium, placed as they are in the centre of the third row of the transition element block. When in a bridging role the nitrido (and oxo) ligands prefer a d 4 electron configuration and this of course is again readily accessible for osmium, for which the Tv state is one of the commonest. Table 14 Nitrido Complexes VOaO
Compfex
Colour
K2[OsNCI,] K2[OsNBr5] trans-K[OsNCI,(H, O)] truns-K[OsNBr, (H20)] Ph,As[OsNCI,] Ph, As[OsNBr,] Ph,As[OsNI, J OsNCl,(PPh, Me),
Purple Purple
Kt050,Nl K,[Os,NCla (H,O), I” Os, N(S2CNM% 15. c,7 [%N(~3kCW4
Yellow
1084 1085 1109
Pink Pink Deep red Brown Brown Black Yellow
Spectroscopic and other parameters
[em-’)
1123 1119 I107 1072, 1063 1021 I137 LO52 1104
IR,Z2,3’a,325 R a m a ~ ~Mossba~er,”~ ,’~~ IR, Ra~nan,~’ Uv/vi~,’~’ X-ray323 IR?” UV/vis,3” X-ray326 IR, R a m a ~ ~ UY/vis,33’ ,~” 3’ X-ray329,330 IR, R a r n a ~ ~UVjvis~’*~”’ ,’~~ X-ray”’ IR,’65 U V / V ~ SX-ray3j2 ,~~’ IR, ’H NMR315 Raman,325.3M.358IR325.241,356,359 “v,+iS 342.3-362 Raman, IR:u UV/visw IR, UV/vis, ‘H NMR, CV?” X-rayJ4, Raman, IR344U V / V ~ S ~ “ . ’ ~ ~
cv,363 ~ - ~ ~ 3 3 > 3 3 8
aBromo analogue also known. Bromo and i d 0 analogues also known.
All nitrido complexes of osmium so far reported are diamagnetic, with the exception of [OS,N,(NH,),(H~O),]~+(see p. 565). The subject of transition metal nitrido complexes has been
46.4.15.1 Mononuclear complexes ( i ) Osmium(fV) and osmium( V )
No mononuclear osmium(1V) nitrido complexes have been established, but p-nitrido species containing a linear Os-N-Os unit are known, and there are trinuciear species of osmium(1V) and of mixed oxidation state containing Os-N-Os-N-Os units. The existence of both types is predictable from bonding considerations. They are dealt with on p. 5M. An osmium(V) nitrido complex [OSN(CO)(NH,),]~+may be involved in the formation of [Os,(N2)(NH,),(C0)z]4+by oxidation of [Os(NH,), (CO)]” (ii ) Osmium ( VI)
This, having a d 2 configuration, is particularly suitable for octahedral mononitrido complexes such as [ O S N C ~ , ] to ~ ~which , the Ballhausen-Gray bonding model for vanadyl species can be applied.312Taking the nitrido ligand to lie on the z axis the Os=N triple bond is formed from a 0
Osmium
561
bond and two 7c bonds, the latter being formed by overlap of filled 2p, and 2p, orbitals on N3- with empty 5dxzand 5 4 : orbitals on the osmium, the remaining d electrons pairing up in the Sd,, orbital. It is curious that, whereas no [Osv’OX,]”- species are known, trans dioxo [OsV’O,XJ- complexes are numerous, the reverse being the case for the nitrido ligand: numerous octahedral osmium(V1) mononitrido complexes exist but no bis nitrido species. It may be that N3-is too powerful a .n d ~ n o r ~ for ’ ~ a, ~metal ’ ~ atom to sustain the presence of two such ligands, and indeed, as shown below, in some osmium(V1) nitrido species the ligand has electrophilic proper tie^^^^^^'^^'^ while in the less electronegative but isoelectronic rhenium(V) counterparts the nitrido ligand sometimes has weak nucleophilic proper tie^.^'^ (a) Nitrido halogeno complexes. Reaction of salts of the ‘osmiamate’ ion [Os03N]- with HC1 gives [OsNC1,l2-; the free acid and the potassium, rubidium, caesium and ammonium salts were made bv Werner.j” and more recent urenarations of the Dotassiumm and cesium3’’ salts were described. Similarly, [OsO,N]- with HB; gfves [0sNl3r,l2-; the rubidium and cesium salts have been is~ la t e d .~ ’ ’ ,~ ’ ~ The X-ray crystal structure of K,[OsNCl,] shows the anion to be severely distorted with an apparently large tram weakening effect produced by the nitrido ligand320(an earlier X-ray study on ‘KZ was later shown to concern traneK[O~NCl,(H,0)]~~~ and there is also another early, low-accuracy study of K2[OSNC~,]’*~)>. In K2[OsNCl,] (Figure 19a) = 1.614(13), mean b = 2.361(4) and c = 2.605(4)A with an NOsCl (cis)angle of 96.5(5) ’. The abnormal length of the trans Os-C1 bond was attributed to non-bonding steric interactions leading to N*..Cl (cis) and Cl...Cl (cis-cis) distances of ca. 3.06 and 3.33 A respectively.320
Figure 19 Octahedral halo nitrido complexes
Vibrational spectroscopic data on [OsNX,]’- (X = C1, Br) have been given.325 It is clear that the nitrido ligand exerts a trans weakening influence, however. The trans halide atoms in [OsNX,]’- are easily replaced by there are X-ray crystal structural studies on trans-K[OsNC1,(H20)] (Figure 19b)323and on trans-K[OsNBr,(H,O)] (Figure 1 9 ~ ) . ~In’ ~(b), a = 1.74(7), mean b = 2.34(2) and c = 2.50(3)&323and in (c) a = 1.67(5), mean b = 2.486, and c = 2.42(3)A. The N-Os-C1 (cis) angle is 90.8 and the N-Os-& angle 101 0.326 Earlier determinations on the chloro complex (then thought to be Kz[OsNC1,]323,324) and on the bromo complex are of Iower accuracy. The Os-0 distances are much longer than in other osmium(V1) species. Thus, Os-0 (OH) in trans-K2(Os02(OH),]is 2.03 A327;clearly a trans weakening effect is operative. The axial water molecules are easily removed (leading in the case of the bromo complex to a polymer, K,[OSNB~,],).~’~ The larger the cation in trans-M’[OsNX,(H,O)] the more labile is the water molecule; salts of [OsNCl,(H,O)]- are known for M = K, Cs or Me,N, but for M = Et,N or larger organic cations the five-coordinate M’[OsNXJ is formed. Presumably lattice energy forces are responsible for these differences.328 The square-based pyramidal halo complexes [OsNX,]- can be made from [OsNX5l2- (X = C1, Br) with ( P ~ , G S ) X ,and ~ ’ ~the deep red, beautifully crystalline iodo complex Ph,As[OsNI,] is made ~ ~ are X-ray crystal structure analyses of all three from [OsO,N]- and HI with ( P ~ , A s ) + ?There [OsNX,]- species (Figure 20 and Table 15). They all have a distorted square-based pyramidal structure in which the four halo ligands lie we11 below the osmium atom, to such an extent that the X..*Xdistances approach the van der Waals interaction distances. It has been suggested that steric N...X repulsion is not the main cause of the distortion since the N..-X and X.*.X distances are virtually identical despite the differences in size between N and X. Rather it is likely to be an N
Figure 20 Structure of [OsNX.,]‘
Osmium
562
Table IS Bond Parameters for [OsNXJ
(4
os-x (4
NOsX
N...X
Complex
(a)
(bj
(”)
(A)
(4
Ref
[Ph, As][OsNCl.,] [ph, As][OsNBr,] [Ph, As][OsN14]
1.604(10) 1.583(15) 1.626(17)
2.3lO(2) 2.457(1) 2.662(1)
104.53(5) 104.29 103.73(2)
3.125(7) 3.238( I) 3.433(1)
3.162(4) 3.364 3.657
329, 330 33 1 332
OS-N
cis-X...cis-X
electronic repulsion effect: the 2p: and 2p2, ~telectrons on the nitrido ligand are drawn towards the electronegative osmium centre with concomitant repulsive interactions with the lone pairs on the halo Iigand~+32~.330.332~ Electronic spectral data on [OsNCl,]- and [OsNBr,] are consistent with such distortions. Vibrational spectroscopic data on [OsNX,]- (X = C1, Br)32’.33’and [OsNh J-z6s have been obtained with ”N substitution data in the case of the chloro and bromo complexes.32s Chemical reactions of [OsNX,]- in acetone solution are summarised in Scheme 10. The elmtrophilicity of the nitrido ligand to phosphines has already been mentioned but it does not extend to arsine, stibine and simple nitrogen donors. Some of the latter, however, do form weak 1: 1 complexes with monodentate ligands L and are presumably coordinated in positions trans to the nitrido ligand. Bifunctional ligands L give 2: 1 complexes [(OSNCI,),L’]~-.26s The [OsNCl,]- ion will abstract sulfur from t h i ~ c ya na t e ~to~give ’ [OS(NS)(NCS),]~-;another unusual feature is the relative lack of reactivity of [OsNT,]- which, with its formal oxidation state of VI, is the highest oxidation state sustained by any transition metal iodo complex. It does not react with phosphines, arsines or aromatic amines, but with SbPh, it gives O S N I , ( S ~ P ~ ,in) ,which ~ ~ ~ there must be considerable steric strain and an overall symmetry similar to that of a lump of coal. In recently reported work it has been shown that [OsNCI,]- can be alkylated to [OsNR,Cl,-,]with MgR,, XMgR or AIR, (n = 2 or 4, R = CH,CMe, or CH,CH,Ph; n = 4, R = Me).332b ( h ) Nitride complexes with other ligands. The existence of complexes with a water molecule trans to the nitride is quite common: thus, reaction of K[OsO,NI with HCN or oxalic acid gives trans-K[OsN(CN), (H, O)] and trans-K[OsN(ox), (H, O)], while trans-K[OsNF, (OH), (H,O)] and trans-K~OsN(ox)(OH),(H20)]are also known.*’ In some osmium(V1) nitrido complexes the N3- ligand is subject to nucleophilic attack by phosphines PR, to give phosphineiminato complexes Os(NPR,)X, (PR, )2;313-315 similar complexes The formation of these is illustrated in Scheme 10 below, see also pare formed by ph~sphinites.’~~ 568. The complexes OsNL,X, (L = PR,,AsR,, SbPh,, tbipy, iphen, py; X = C1, Br) can be made as shown in Scheme 10 and are diamagnetic. IR spectra and, in the case of OsN(PMe,Ph),Cl,, ‘HNMR, suggest a trans structure similar to that shown in Figure 12, N replacing NS3” The abstraction reaction of sulfur from S, by OsNL,Cl, (L = PMe,Ph, AsPh,, tbipy, iphen, py)262is noteworthy, as is a similar abstraction of sulfur from SCN- by [OsNCl,]- to give [OS(NS)(NCS),]~-.~~’ The porphyrin complexes trans-OsN(0EP)X (OEP = octaethylporphyrin; X = F, OC103, OMe) are made from OsO,(OEP) and hydrazine hydrate with HF, HClO, or MeOH.255The existence of trans-[O~N(NH,),(H,0)]~+ has been suggested though not fully confirmed.” Recently, the preparation of (Bu~N)[OsN(OSiMe,),]has been accomplished by reacting [OsNCl,]- with sodium siloxide; the product can be alkylated without cleavage of the Os-N bond.322a ( c ) Osmium( VIII). With a few minor exceptions the only established osmium(VII1) nitrido species are the ‘osmiamates’, salts of the tetrahedral [OsO,N]- ion. The osmiamate in is the source of most other osmium nitrido complexes. The free acid is said to be OsN(OH)O, in the solid stateZ2.333-334 and the BunqN+,Ph,P+, P b A s + , lithium, rhodium, potassium, rubidium, cesium, ammonium, silver, thallium, mercury, zinc, barium and lead salts are also known. The preparation of the potassium salt of OsO, in concentrated KOH solution with NH,OH is a simple method for obtaining this useful starting A number of single crystal X-ray studies have been published, on the p o t a s s i ~ m , ~ ~ ~ , ~ ~ ~ and cesium”8salts. In the potassium and rubidium salts there appears to be a statistical distribution of nitrido and oxo ligands so that Os--N and Os-0 bond lengths seem to be equal (e.g. 1.72A in K [ O S O , N ~ However, .~~~ in Cs[OsO,Nl, the Os-N and Os-0 distances are 1.676(15) and and an X-ray radial distribution analysis gave the Os-0 and Os-N 1.739(8)A re~pectively,~~’ distances in K[OsO,N] as 1.78(3) and 1.63(3) A re~pectively.’~~ Vibrational spectra of normal and isotopically substituted [OsO,N]- have been measured (see Table 16).325.740.341,758-360
[Os(NS)(NCS),]
'-
trans-[O~rPPh3)2C14]- 4
Xvl
Os(NHPPh3)C14(PPh3)
i, C1: PEt,, PMe,Ph, PMePh,, PEt,Ph,? P(OEt)Ph2,b Br: PEt,Ph, PEtPh,? ii, Cl,, PEt3, PYe,Ph, PMePhz, PEt,Ph, PEtPhz: Br: %bipy;B v: PPh,;"
iii, PPh3 (Cl, Br):
iv, CZ: Yzbipy? '/zphen,h
vi, PPh3,MeOH? vii, NCO-; viii, PPh,;b ix, CZ: AsEt,, AsPh3,
SbPh3,B Si-: AsPh,, SbPh,,' . I : ShPh,?
x, S,CI,, PMe,Ph, AsPh3$ xi, CI,: AsPh3;b xii, SCN-b
xiii, H*? xiv, p y p xv, S2C12,AsPh,;b xvi, HCI.'
' Ref. 3 15 Ref. 265 Rcf. 262. Scheme 10 Reactions of [OsNX4]Table 16 Vibrational Spectra of [OsO,EJ1-
(4)
(E)
vOsN
v2 voso
60,
420
VS 800sN
"6 'Os0
[OsO, N]-
1021
897
309
[oso,l5~].
1029 998
K[Os"O, NI"
309 309
87 1 872 87 I 826
372 374 368 363
309 302 302
1024
898 896 844
81
a IR
v3
"4
282
Ref: 340 325 325 34 1
data for the solid; others are Raman spectra in aqueous solution.
Electronic spectral data have also been measured for the ion.342The only other osmium(VII1) nitrido complexes claimed in the literature are of the type [OOsN(Rbig),]OH- S04.nH20(Rbig = biguanide, methylbig~anide).~~,
46.4.15.2
Polynuclear complexes
( i ) Binuclear species ( a ) Osmium(1V). The known dimers are of the form [OS~NX,(H,O),]~-(X = C1, Br) and (Y = C1, Br, I, NCS, N,, N03);344 there is also a dithiocarbamato complex [Os,N(NH,),Y,]" O S ~ N ( S ~ C N M ~ ,) , . C , H ,The , ? ~ ~first complexes of this type were [ O S ~ N ( N H ~ ) ~ X (X~= ] XC1, ~ Br), made from (NH,),[OsX,] and ammonia under pressure;34G348 the routes in Scheme 11 were developed later.344The chloro species [ O S ~ N C ~ ~ ( J3H ~can O ) also ~ be made in low yield by refluxing an aqueous solution of K,[Os(NH,SO,)Cl,], and there is some evidence for the existence of K,[Os,NC1,,].349However, although there is a brief mention in a review of an X-ray crystal structure of ' C S ~ [ O S , N C ~ , ,this ~ ' ~species '~ is nowhere else reported in the literature, and it seems likely that , ~ Jp. 593). the structure referred to is really that of C S , [ O ~ , O C ~(see The dimethyldithiocarbamato complex was originally made from OsCl, .nH,O and Na(S2CNMe,),345and both it and the diethyldithiocaxbamato analogue were later prepared from The X-ray crystal structure of the 1: 1 heptane Os(S,CNR,),, Na(S2CNR2)and Kz[OsNC15],350 C.O.C. 4-
s
564
Osmium
X = Cl-, Bi;Y = Cl-, Br-, I-; NCS-, N3-, NCSScheme 1 1 Preparation of nitrido-bridged binuclear complexes 3 4 4
adduct of Os,N(S,CNMe,), shows this to contain a slightly bent, symmetric nitrido bridge accompanied by a bridging dithiocarbamato ligand, the octahedron being completed by the other four chelating dithiocarbamato ligands. The Os-N distance is 1.76(3) A and the OsNOs angle 164.6(18)*; the mean Os-S distance is 2.39(2) Structure and bonding. All these osmium(1V) dimers are diamagnetic and Raman and IR spectra of [ O S ~ N X ~ ( H ~ O )and , ] ~ -of [ O S ~ N ( N H ~ ) ~ Yshow , ] ~ + no coincidences of fundamental skeletal vibrational modes, indicating a centrosymmetric structure.M Although, with the exception of Os,N(S,CNMe,),.C, HI,, there are no crystal structure studies on the osmium complexes, that of K, [Ru,NCl,(H,O),] shows that this contains a centrosymmetric anion with a linear, symmetric Ru -N-Ru bridge. The Ru-N distance is 1.720& indicating considerable multiple bonding in the linkage, and the chloro ligands are bent back from the nitrido group (N-Ru-Cl angle = 94.3 ”). The long Ru-0 (OH,) distance of 2.175 A is also suggestive of a trans weakening influence by the nitrido ligand.35’The considerable similarities in the vibrational spectra of the ruthenium and osmium complexes3” makt it very likely that the structures will be similar in both groups. A similar structure has also been found recently from X-ray and vibrational spectroscopic studies on K,[Ru,N(CN),,].~~~ The bonding in these species is likely to be similar to that proposed for [OS,OC~,,]~(p. 594),p,-d, interactions for 2p orbitals on N3- and 5d,,, 5dYzorbitals on osmium giving rise to the observed diamagneti~m.~’~ were recorded; isotopic Raman and IR spectra of [Os,NX, (HzO)2]3- and [OSN(NH~)~Y,]~+ substitution (’H, lSN)was used to aid assignments for [OsZN(NH3),Br,]Br,.3”
(ii)
Trinuclear species
The known chemistry of these species can all be systematized on the basis of the skeletal structure shown in Figure 21, though there are as yet no single crystal X-ray studies on complexes believed to contain it. r
Figure 21 T h e likely basic structure of [Os,N,X,Y,ZJ+
1 n+
species3”
Reaction of (NH,),[OsCl,] or of [OS(NH,)~CI]CI,with ammonia gives the diamagnetic purple species [Os3N,(NH3),(H,0),]C1,, ‘osmium violet’, for which the structure above in Figure 21 (with X = NH,,Y = Z = H,O) was proposed; with aqueous cyanide, [Os,Nz(CN)l,(H20),]4- is formed (X = Z = CN-, Y = H,0).352,3”Resonance Raman and IR spectra were measured, and from these The bonding in the data it appears that the Os3Nzunit has considerable multiple bond ~haracter.~” complex can be rationalized by using the molecular orbital scheme for ruthenium red, [Ru30,(NH,),,]6C,355 and its ethylenediamine analogue [Ru, 02(NH3),oenz]6+ .356 The complex ‘0s3N7O9Hz,’, made from OsO, and ammonia,344is possibly Os3N3(OH),(NH3),, with a structure related to that of Figure 21 but having an extra terminal nitrido ligand at the central atom (Y site).344
565
Osmium 46.4.15.3
Polynuclear complexes of mixed ox&lion state
Electrochemical oxidation of Os,N(S,CNR,), (R = Me, Et) showed that a reversible one-electron oxidation occurs to [Os,N(S, CNR,),]+ , an osmium(1WV) species and a further irreversible oxidation, possibly to an osmium(V) dimer.3s0 Trinuclear species apparently containing mixed oxidation states are also known. Reaction of (NH4),[OsC16] or [OS(NH,),C~]~+with air and ammonia gives, in addition to [Os3N2(NH3)8(H20)6]6+,'osmium brown' [Os3N,(NH3),(H,0),]C1,; the latter is parammagnetic (pefi= 0.87 BM per trimer unit). IR, resonance Raman and ESCA data are consistent with its formulation as in Figure 21 with X = NH3 and Y = Z = HzO. Treatment of 'Claus's salt', obtained from OsO, and aqueous ammonia, with cyanide ion gives [OS~N,(CN),(H,0),]4- (X = CN-, Y = Z = H20),354 while HX gives [Os,N,X,, (NH3)3]3-(X = CI or Br, Y = C1 or Br with one H 2 0 ligand, Z = NH,).3" Again, the bonding may be rationalized by the use of MO diagrams for trimeric ruthenium oxo-bridged s p e ~ i e s . ~ ~ ~ ' ~ ~
46.4.16 Triazenido Complexes The trihydro species OsH, (RNNNR)(PPh,), have been made from OsCpPh,), H, and the triaZeneS (R = Ph, p-MeC,jH,, p-ClC6Hq); the complex OsH, (p-MeC6H,)N"(p-MeOC6H4) was similarly prepared. These complexes are likely to be seven-coordinate and involve osmium(1V); I H NMR spectra suggest that they are stereochemically non-rigid. Their remarkable stability (the diphenyl complex is unaffected by boiling with PPh3 in toluene) may arise from n-delocalization from the triazene to empty metal d orbitals.% The osmium(1I) species Os(RNNNR),(PPh,)z (R = Ph, p-MeC,H,, p-MeOC,H,) are made from [OSC~,]~and the triazene with PPh, and KOH. On the basis of IR and 'HNMR data the following structure shown in Figure 22 was proposed. PPh3
1 1
,*e*
R>-Os-NR N k - ~ q ~
R N.yN .Y
PPh3 Figure 22 Likely structure of O S(RN N N R),(PP~&~~
46.4.17 Azido Complexes The only unsubstituted azido complex is (Ph,As)[Os(N,),], made from NaN, and (Ph,As)Cl with [OSC~,]~-. It is an explosive yellow-orange complex.365 The p-nitrido complex [OS,N(NH,),(N~)~](N,),is a brown-yellow material obtained from [OS,N(NH,),C~,]~~ and KN3 under reflux (vNN = 2077 and 2040cm-').344 The phosphine complexes Os(N,)(PR3),C12 (PR, = PMe2Ph, PEtzPh, PPf,Ph, PBu",Ph) are made from NaN, and mer-Os(PR,), C1, ; these purple complexes have vNN near 2060 cm-' and probably have the structure shown in Figure 12, N3 replacing NS.366A bis azido complex, cisOs(N,),(PMe,Ph),, is a white material made from Os(PMe,Ph),Cl, and NaN,; vNN is at 2050 cm-' .366 The diarsine complex Os(N,)(diars), C1 is known, prepared from trans[O~(NO)diars,Cl]~+ and N,H,-HCI; with HC1 it gives the yellow [O~(N~)(diars)~Cl]C1.~~~
46.4.18 Complexes of Pseudohalide Ligands We have already considered azido complexes above; the only other pseudohalide to form complexes with the element is thiocyanate.
46.4.18.1
Thimyanato complexes
Apart from a rather uncertain report of [OS(SCN),]~-the only unsubstituted thiocyanato complex to be fully established is [Os(NCS),]'-. However, it appears that for the ambidentate NCS-
566
Osmium
ligand osmiurn(II1) lies on the hard-soft boundary so that N or S bonding occurs almost in a random fashion; all the isomers of [OS(NCS),(SCN),-,]3- are known except for [OS(SCN),]~(n = 0). This is probably the first system in which linkage isomerism and geometrical isomerism are so intertwined in an otherwise unsubstituted [ML,Y- system.
(i)
Osmiurn(1I)
The only unsubstituted species claimed are R, [Os(NCS),] (R = diantipyrylmethane, C,,H,N,02H+, and diantipyrylpropylmethane, C2GH,,N40,H+).367 There are several substituted osmium(I1) thiocyanato complexes, however. There is no indication from spectral data whether the ligand is N or S bonded in (Ph,P),[Os(NO)(NCS),] (see p. 545);368in the thionitrosyl complex (Ph,As),[Os(h~S)(NCS),](see p. 542) IR data suggest that there may be Os-N rather than Os -S bonding of the thiocyanate ligand. In Os(diars), (SCN), , made from Os(diars), C1, and SCN- , there are no spectral data to indicate the nature of binding of SCN-,369 but in Os(Ph,AsCH,AsPh,),(SCN), , made from (NH,),[OsBr,], SCN- and the arsine, the IR spectrum is suggestive of S- rather than N-bonded thi~cyanate.~~' The complex Os(SCN),(bipy), is made from KSCN and Os(bipy),CI, ."'
(ii
Osmium (III)
Reaction of K,[OsCl,] and RSCN with precipitation of the products by (Bun4N)+yields (Bun4N)3[O~(NCS)6]; IR,37'electronic absorption3'* and Mossbauer spectra were measured,184and the magnetic susceptibility was measured from 90 to 295 K (per = 1.78 BM at 295 K).373On the basis of the IR spectrum in particular no clear conclusion could be drawn as to whether the thiocyanate ligand was N or S bonded. Subsequent investigation showed that a mixture of isomers (Bu"~N)~[~s(NC~),(~CN)~-,] is formed in the reaction,374and these may be separated by highvoltage electrophoresis or ion e ~ c h a n g e . ~ ' ~ . ~ ~ ~ Of the ten possible isomers of [ o ~ ( N c ~ ) , ( s c N ) , ~ , ]nine ~ - have now been separated by ion at high (refluxing) temperatures the K2[OsCl,]-KSCN reaction gives cis and fac complexes, while at lower temperatures (60"C) the trans and mer species are formed.376In general, longer boiling periods produce a higher degree of N-bonded substitution while shorter reaction times at lower temperatures favour the formation of S-bonded isomers.375The only missing species is the fully N-bonded isomer, [0s(NCS),l3--.The complexes were characterized by their Raman and ~"~~ of isomer assignment was IR spectra as well as electronic absorption s p e ~ t r a . ~ 'Confirmation given by EXAFS measurements on the various salts: the Os-N-C-S bonds are essentially linear while the Os-S-C-N bonds have an Os-S-C angle of approximately 110 '. The Os--N bond length decreases with increasing n in [OS(NCS),(SCN),-,]~- (2.21 8,for n = 1,2.13 for n = 6 ) with the Os-S distance remaining relatively constant at 2.50 A.377
(iii) Osmium ( I V ) Four osmium(1V) complexes have been reported. The salts [OS,N(NH,),(NCS)~]Y,(Y = c1, NCS) are made from [OS,N(NH,),C~,~~+ and NCS-. The IR and Raman spectra suggest that it contains N-bonded t h i ~ c y a n a t e consistent ,~~ with the probable 'harder' nature of the OsiV2N3' moiety (see also p. 563). It has recently been reported, however, that osmium and ruthenium can be separated via their [M(NCS),]'- complexes using polyurethane foam.378 Recently, the isomeric pair [OS(NCS)C~,]~-and [Os(SCN)Cl,]*- have been isolated by ionexchange separation of the reaction product of [OSC~,I]~with (SCN),. IR, Raman and electronic spectra were measured.379
46.4.19
Nitrile Complexes
Surprisingly few osmium nitrile complexes are reported in the literature.
Osmium
567
46.4.1 9.1 Osrnkni( I ] )
The reaction of Os(PMe,Ph),H, and HBF, in acetonitrile gives [OS(NCM~),(PM~,P~),](BF,),,~*~ and there is a brief mention of Os(NCMe)H,fPR,),, made (but not isolated in the pure state) from Os(PR,),H, (PR, = PMePh,, PMe,Ph, PEtPh,).276 see under amides, p. 558. For (PPh,)[Os(NCMe)(CCl,)NCCl(CCl,)CI,],
46.4.19.2
Osmiurn(iII)
The complexes Os(NCAr)Cl,(PPh,), (Ar = Ph, p-tolyl) can be made from OsCl,(PPh,),, PPh, and ArNC; with MeCN, Os(NCMe)Cl,(PPh,), is formed. These species are likely, on the basis of TR data, to have the structure shown in Figure 23(a). Reaction of Os0,C1,(PPh3), and of OsCl,(PPh,), respectively with p-toluenenitrile gives cis- and trans-Os(NCR), (PPh, jzC1, (b and c; Figure 23) re~pectively.2'~ R C N
R C N
I 1
Ph,P-Os-PPh, C l H
,*@$1
Ph3PC
C1
(4
1 1
R
C
N
c1 ,,,&
Os-PPh, f
I
,."" N C R
Ph3P-00s-PPh3
' a
I
YR
CI
(b)
tC1
Figure 23 Structures of nitrile phosphine complexes296
= 1.87 BM) The acetonitrile complex Os(NCMe)Cl, (PPh,), was found to be paramagnetic with a broad room-temperature ESR spectrum (g = 2.16). The cyclic voltammogam showed a one-electron reversible reduction, and there is also a one-electron ~xidation.~, The salt (Et,N)[Os(NCMe)Br,], obtained from [OsBr,12- and PbO, in acetonitrile, has recently been r e p ~ r t e d .~ " A recent X-ray crystal structure determination on mer-Os(NCMe)Br, (PPh3), shows the Os-N distance to be 2.024(10) A with the bromo ligand trans to it at 2.499(1) A; the mean Os-P distance is 2.419 A and the trans bromo ligands are at 2.490A. The complex was made by treating (Bu",NjZ[Os,Br,,] in acetonitrile with triphenylpho~phine.~~'
46.4.20
Miscellaneous Nitrogen Donor Complexes
46.4.20 Biguanide complexes
There seem to be no unsubstituted complexes of biguanide with osmium, though a few tris species of substituted biguanides have been reported.
( i ) Osmium(I1)
The tris complexes [Os(Rbipy),]Br, have been made from (NH4), [OsBr,] and Rbig .HCl (Rbig = N' ,N' -dimethylbiguanide, N' -morpholinebiguanide, N' pchlorophenyl- NS -isopropylbiguanide (paludrine)). Electronic spectra were recorded, and the paludrine complex resolved via the antimonyl tartrate.383
Osmium
568 ( i i ) Osmium(II1)
Paludrine hydrochloride and (NH4)2 [oSc&]give [Os(paludrine),]Cl, , or [O~(paludrine),]~+ can be oxidized with hydrogen peroxide. The magnetic moment of the chloride at room temperature is 2.45 BM. The [Os(paludrine),X,]X complexes (X = C1, Br) have also been prepared by reaction of (NH,), [OsX,] with the ligand, and the magnetic moments at room temperature are respectively 1.60 and 1.68 BM. Electronic spectra were recorded.383
(iii)
Osmium( VZ)
Reaction of (NH,),[OsCI6] and biguanides with alkaline H 2 0 , and OsO, with biguanide yields OsO,(big),; with H2S0, this gives ~OsO,(bigH),]SO,~H,Oand HC1 yields [OsO,(bigH),]C1. The latter will react with 2,2'-bipyridyl (bipy) to give [OsO,(big)(bipy)]+ . Piperazine biguanide (L') reacts with tr~ns-[OsO,(OH),]~-to give [OsO, L;I2+ .384
46.4.20.2
Benzotriazole complexes
Benzotriazole, C,H,NHN, (btz), forms a few complexes with osmium; fuller characterization would certainly be desirable.
(i)
Osmium(l1I)
Osmium tetroxide in alkali with the ligand in ethanol yields Os(btz),(OH),; Os(btz),X, (X = C1, ~ , HX.,'' Salts of [ O~ O, ( b t z ) , ]are ~ ~ said to be formed from Br) can be made from ' [ O s O , b t ~ , ] ~and the ligand and OsO, in alkali; red sodium, potassium, silver, barium, lead and calcium salts were isolated. "j
46.4.20.3
Phosphineiminato (NPR, ) and phosphineaminato (NHPR,) complexes
A number of osmium(1V) phosphineiminato complexes are known which also contain phosphine or arsine coiigands; they are prepared by nucleophilic attack of phosphines on the nitrido ligand in osmium(V1) species. There is also one example of a phosphiteiminato complex, one of a phosphinearninato species, both formally involving osmium(IV), and two examples of osmium(1) aroylphosphineiminato complexes.
(i) Phosphineiminato complexes, Os(NPR3)X , (LR,), The preparation and principal properties of these species have been summarised in Scheme IO. They are made by reaction of alkyl aryl phosphines PR, (PEt,, PMe,Ph, PMePh,, PEt2Ph,PEtPh,), with OsN(AsPh,),X, (X = C1, Br) while the triphenylphosphineiminato complexes OS(NPP~,)X,(PP~,)~ (X = CI, Br) are best made directly from [OsNXJ and PPh,. There is one example of a phosphineiminato complex containing an arsine group, Os(NPPhMe,)Cl,(AsPh,), ,m and there is a phosphiteaminato species Os{NP(OEt)Ph, 1Cl, {P(OEt)Ph,)2.265 The complexes are paramagnetic with moments close to 1.8 BM at room temperature. The 'H NMR and IR spectra of Os(NPMe,Ph)CI,(PMe,Ph), suggest the trans configuration (I; Figure 24)*', and this is indeed the type of structure found for the analogous ruthenium complex Ru(NPEt,Ph)Cl, (PEt2Ph)2for which X-ray data have been obtained.3s6These complexes can be oxidized by chlorine to the nitrido species OsN(PR,),CI,, and the triphenylphosphineiminato species can be protonated successively to O S ( N H P P ~ , ) C ~ , ( P P ~This , ) . ~bright ~ ~ red material is made by reaction of [OsNC14]- with PPh, and HCl, or by addition of one mole of HCl to Os(NPPh3)C13(PPh,),.On the basis of IR data and on the ease of preparation of the complex from species of structure (a), structure (b) was proposed (Figure 24). The complex is readily deprotonated to Os(NPPh,)Cl,(PPh,),, but its own further protonation by a mole of HC1 leads to formation of (Ph3PNHz)C1and salts of [Os(PPh,)Cl,]- .294
Osmium
Yh3
569 H
N
\ N/PPh3
Cl
Cl
(a)
Ib)
Figore 24 Structures of phosphineimtnato and phosphineaminato c o r n p l e x e ~ ~
( i i ) Aroylphosphineiminato complexes, Os(NPFh3C(O)Ar)Cl, (PPh, i2
The N-toluolyl and N-benzoyl complexes have been made from Os02C12(PPh3),,the phosphineimine and PPh,, and the toluolyl complex was also obtained from Os(PPh,), Cl,, toluoylphosphineimine and PPh, . A structure with the imine functioning as an N,O donor has been proposed (Figure 25).296
PPhl Figure 25 Structure of Os(NPPh,COAr)Cl,(PPh,),”6
46.4.20.4
Other nitrogen donor complexes
Two complexes have been reported with 8-amino-7-hydroxy-4-methylcoumarin, NO, C,, Hs. Reaction of OsO, with this ligand in aqueous alkali and ethanol yields [Os(NO,C,,,Hs),(OH),]~ 3H20, a paramagnetic species (peK= 1.22 BM at room temperature, suggesting the presence of osmium(1V)). A similar reaction but in acidic aqueous ethanol yields the violet diamagnetic [Os02(N03C,oH8)2]C12;IR and electronic spectra were m e a ~ u r e d , Reaction ~ ~ ~ ~ ~ ~of*OsCl, with 4-p-dimethylaminoanilino-3-penten-2-one gives grey crystals of OsL,. 2H,O; the electronic spectrum of this allegedly osmium(1V) complex was measured, but its diamagnetism3” is unexpected -perhaps it is some type of ‘osmyl’ complex. A complex formed between Na,[OsC&] and strychnine sulfate was formulated as For Os(N2C2,H,,02)3390but later reformulated as a strychninium salt (strychnineH), [OsC16].39”392 other complexes of osmium with alkaloids see p. 587.
46.5
PHOSPHORUS, ARSENIC AND ANTIMONY LIGANDS
The chemistry of these species with osmium is quite extensive, particularly in the case of the phosphines. In order to keep the sections to a manageable length we concentrate here on the hydrido and halo phosphines, arsines and stibines; when other ligands are involved the material is normally given under that ligand, but cross-references are provided. There are also separate sections on phosphorus trihalide complexes and on tertiary phosphites, while the relatively small sections on chelating phosphines and arsines are considered together. Also, in order to reduce the complexity of the treatment, much use has been made of reaction schemes, and it is in these rather than in the main text that some carbonyl complexes with these ligands are considered. For a fuller treatment of carbonyl phosphines, arsines and stibines the section in ‘ComprehensiveOrganometallic Chemistry’ is recommended,’(‘ as are the reviews of MCA ~ l i f f e . ~ ~ ~ , ~ ~ ~ General chemistry. The commonest oxidation states in the phosphines, arsines and stibines of osmium are I1 and 111, but there are a number of osmium(1V) species and, with oxo, nitrido and hydrido ligands, osmium(V1); with carbonyls there are examples of osmium(0) and with nitrosyls (pp. 549-55 1) aIso of the 0 and - I1 states. Although the stereochemistry is octahedral in most cases
570
Osmium
there are examples of eight- and seven-coordination (the latter notably in the case of OsL,H,) and five-coordination; steric factors undoubtedly play a role particularIy in the latter case. Osmium is not as catalytically active in its phospine, arsine and stibine chemistry as is, for example, ruthenium or rhodium, and most of the examples of catalytic reactions are with the carbonyls which are not covered in this chapter. The element nevertheless exhibits a rich chemistry with these ligands.
46.5.1 Tertiary Phosphine Complexes 46.5.1.1
Phosphine hydrido complexes
A wide variety of these are known, and the principal preparative routes to most of them are indicated in Scheme 12. The examples amongst these of seven- and even eight-coordination -very unusual stereochemistries for osmium- is noteworthy, as is the fact that, in a formal sense at least, some of them contain osmium(1V) or osmium(VI), very high oxidation states for phosphinecontaining species. [Os(PEt,Ph),H,]+
IOscI61
---...I)
Os(PPh,),HX
vi
OsL,H,
/ \ /
vii
mer-OsL,CI,
Os(PMe,Ph),H,KO)
OSL3(CO)cI,
OsL3H2CI2
c
p
ci.s-OsL4H
truns-Os( PHP~I,)~CI,
X = 01, Br, I ; R = Me, Et
Scheme I 2 Some typical preparations and reactions of osmium hyhrido phosphine conlplexes
The subjects of h y d r i d ~and ~ ~r ~n u l t i h y d r i d ~complexes ~~~ have recently been reviewed.
( i ) Monohydrido species
These span the formal oxidation states of TT to TV. The carboxylato species Os(PPh,),H(OCOR) (R = Me, Et,CF,, CZFS)are dealt with on p. 600; Os(PPh,),HX (X = C1, Br, I) have been briefly reported as products of the reaction of Os(PPh,),H, with HX, and the chloro complex is also made from Os(PPh,),Cl, and H,. With CO, Os(PPh,),HCl(C0)2 is formed and pyridine gives Os(PPh,), HCI(py), . The chloro complex Os(PPh,), HCl catalyzes the homogeneous hydrogenation of terminal alkenes, and in some cases alkene isomerization occurs.397 The earlier reported3’*Os(PBu”,Ph),HCl, (n = 2 and 3) are now known to be mixtures,399but the formally tetravalent Os(PR,),HCi, (PR, = PMePh,, PEt,Ph)277are well established. For hydrido carboxylato phosphine complexes see p. 601.
( i i ) Dihydrido species
There are two main classes: cis-Os(PR3),H, and OS(PR~)~H,CI, (Scheme 12 and Table 17). The cis configuration of the former is established, in the case of Os(PMePh,),H, at least, by phosphorus the hgh-field ‘NNMR spectra of Os(PMePh,),H,Cl, and of decoupling NMR
571
OSWliUm
Os(PEt,Ph), HZC12suggest that the hydrido ligands are in equivalent environments with respect to the phosphine ligands, a situation possibly caused by coupling effects.*" The complex cisOs(PEtPh,),H, catalytically isomerizes and hydrogenates 1 -octene, a process much more rapid in the presence than in the absence of hydrogen.276 Table 17 Hydrido Phosphine and Arsine Complexes' Complex Os(PMePh,), HCI, cis-Os(PMe, Ph), H, Os(PMePh,),H,CI, [Os(PEt,Ph),H,]BPh, Os(PEt,Ph), H, Os(PMe, Ph), H6 Os(AsEtPh,),H, cis-Os(AsEtPh, ), Hz
Colour
M.p. I" C )
Pink 104-108
Grey White White Yellow oil
110-118 (d) 199-203 19-80 91-99 158-1643
"OsH
6,H
(cm-')
Icrn-']
2216, 2004, 1900 1950, 1920 2171, 202Y
830
I
2033, 1984, 1864 2028, 1980, 1869 2041, 1986, 1813 1992, 1975
842 826 810 835
ZOSH
(p.p.m.)
Ref:
19.42q 20.35 19.55q 21.09 19.52q 18.60t 19.66 23.14
211 216 217 276 404 404 216 216
Recently the complex cis-Os(PMe,),H, has been obtained by reaction of trans-Os(PMe,),Cl, and NaC,,H,; with PF; this gives [Os(PMe,),H,]+ (see below).400Photolysis of Os(J?Me,Ph),H, gives a reactive intermediate Os(PMe,Ph),H,. This reacts with more phosphine to give cisOs(PMe,Ph),H,, and a dimeric product Os,(p-H),(PMe,Ph),H, also results from the reaction (p. 572).400
( i i i ) Tri- and terra-hydrido species
A number have been isolated: [Os(PEt,Ph),H,]BPh,, made from cis-Os(PEt,Ph),H, and HC1 followed by addition of NaBPh,,276 and [Os(PMe,),H,]PF,, from cis-Os(PMe,),H, and NH,(PF,).@la It seems from ' H N M R data on the latter species that is fluxional at room temperat~re;~"" an alarming observation on [Os(PEtzPh),H,]BPh, (and its deuteriate) was than no IR band could be found to assign to the Os-H or Os-'H stretches, though a band a t 842cm-* was assigned to an Os-H deformation mode.276 There is evidence for the existence of [Os(PEt,Ph), {PhzP(CH,),PPh,)H,]+ .276 A recent X-ray crystal structure of K[OsH, (PMe,Ph),] shows this to contain weak dimers with an H-K interaction; the main Os-H distance is 1.67 A.401b Tetrahydrido complexes of the form Os(PR, I3H4 are the most important phosphine hydrides of osmium. Preparations are shown in Scheme 12 and typical data in Table 17, and some of the reactions are also given in Scheme 14. There is a full neutron diffraction study on Os(PMe,Ph),H, which shows this to have a distorted pentagonal bipyrimidal structure with two apical and one equatorial phosphine ligand (mean Os-P apical distance 2.312A with a P-Os-P angle of 166.1(1) '; the bending occurs presumably as a result of the position of the equatorial phosphine ligand (Os-P = 2.347(3)A). The hydride ligands are all in the equatorial plane (mean OsH = 1.663 A).396.402 There are preliminary X-ray data on Os(PEt,Ph),H, which indicate a similar structure (mean axial Os-P distance 2.296& and an equatorial Os-P distance of 2.339Am3). 'H NMR data on the tetrahydrido complex indicate equivalence of the phosphine ligands, an effect which probably arises from fluxionality in ~ o l u f i o n . "IR ~ ~spectra ~~ show three or four Os--H stretches in the 180&2100cm-' region (though in some cases five stretches are observed in solution)!04 Both ' H N M R and IR data are available for a number of OsL,H, complexes (L = PBu",, PMe,Ph, PEt,Ph) and also for Os(PMe,Ph),LH, ( L = P(OEt)3, P(OMe)2Ph, PEt,Ph, PPh,, AsMe,Ph); these latter species are made from mer-Os(PMe,Ph),L'Cl, and BH; in methanoLm These OsL, H, complexes have an extensive chemistry, representative examples of which are illustrated in scheme 12. In the presence of hydrogen, Os(PEtPh,),H, catalyzes the hydrogenation of oct-l-ene.2'6 Photolysis of Os(PMe,Ph),H, gives a reactive intermediate Os(PMe,Ph),H, (see above).
(iv) Penta- and hexa-hydrido complexes
Although salts of [OS(PR,)~H,]+do not seem to have been isolated there is evidence from 'H NMR spectra that [Os(PMe2Ph),H,]+ is present in solutions of cis-Os(PMe,Ph),H, in trifluoro-
Osmium
572
acetic acid.401Such an [OsL3H,]+ species would be a member of the series WL,H,, ReL,H, and. IrL, H, .404 The hexahydrido species Os(PMe,Ph),H, is made by treatment of Ph,As{Os(PMe,Ph),Cl,] with LiAIH,; it was detected by 'H NMR."4 The species OsH,(PPhPri), and OsH,(PPhPr;),HgCl, have recently been reported.
(v) Polynuclear hydrido complexes Photolysis of Os(PMe2Ph),H4in benzene or THF yields, amongst other products, red crystals of Os,(PMe Ph),H4. The X-ray crystal structure (Fig. 26) shows it to have a short Os-Os bond of 2.818(1)1, formally a double bond if the electron-count of 18 is to be achieved; the Os,H, bridge appears to be asymmetric with long and short distances of 2.18(10) and 1.59(10)A respectively, while the Os-H terminal bonds are at 1.61(9) A. The Os(PMe,Fh), unit about each metal atom has afac geometry; the mean Os-P distance tram to the Os,H, plane is 2.259 A and that trans to the Reaction of this complex with HBF, terminal hydrido ligand is substantially longer at 2.340(3) A.40" leads to [Os,(PMe,Ph),H,]BF,, which has a tricapped trigonal prismatic type of structure (akin to that found in K,[ReH,]) with three bridging hydrido ligands (mean Os-H distance = 1.75(11)A, Os-Os = 2.558(1)A) and the mean Os-P distance is 2.291(4)A. Formally there is an Os-Os triple bond here.m Finally, there is 31PNMRevidence for the existence of Os,(PMe,Ph),H, in solutions of Os,(PMe,Ph),H,, in which three hydrido ligands bridge two osmium atoms; one metal atom has three phosphine ligands with afac arrangment and the other similarly bears two phosphine ligands and a terminal hydride ligand.m PMe,Ph
PhMe2P
H
Figure 26 Structure of Osz(PMe2Ph)6H4m
46.5.1.2
Halo phosphine complexes
A very substantial number of these are known (examples are given in Table 18); as with the halo-arsines (p. 576) and -stibines (p. 577) the metal oxidation states represented are 11,111 and IV with a preponderance of the trivalent state; the early-reported 'Os(PPh,),X' (X = C1, Br) complexes are now known to be hydrides, Os(PPh,), HX(C0).408 Table 18 Physical Data for Halo Phosphine, Arsine and Stabine Complexes Complex
Colour
M.p. (0 C )
tram-Os(PMqPh), Cl, trans-Os(PPh, ), Br2a mer-Os(PMe, Ph), C1, ~ u c ~ s ( P B uPh),CI, , trans-Ph, As[Os(PEt, Ph), Cl,] OsPPb3 )2INH, IC13 rner-Os(AsEY,), C1, fac-Os(AsMe, Ph), Ck, mer-Os(SbPh, l3C1, fac-Os(SbPh,), Br, trans-Os(PMe, Ph),CI4 rrans-Os(AsPr",)z C1, trans-Os(SbPh,),CI4
Yellow Grey Red Maroon Yellow Orange Orange Red-brown Green Black Brown Green-brown Black
252-258 202-205 (d) 200-204 16 I- I62 198-202 223-228 159-160 158-1 86 238-240 (d) 263-265 (d) 20&207 155-156 280-282 (d)
a
Spectroscopic and other properties
~~
~~~
IR, 'H NMR, ESR370 IR,'H NMR'70 IR,'HNMR?" M 411s423 X-raya3 IR,4I3UV/V~S?'~ 'HNMR,428M4" IR"'
AsPh, and SbPh, analogues also known?m
( i ) Osmium(II)
Examples both of cis- and trans-Os(PR,),Cl, and of the five-coordinate Os(PPh,),X, (X = C1, Br) are known. Preparations of these are outlined in Scheme 13; in addition, Os(PMe, Ph), {P(OMe),Ph}Cl, can be obtained from P(OMe),Ph and O S ( P M ~ , P ~ ) , ( N , ) and C~~
573
Osmium
truns-Os(PMe,),Cl, from Os(PPh,),Cl, and PMe, (a preliminary X-ray study of the complex was rep~rted).~" The cis form of OsCl,(PMe,), is also known.401
,
[Os(PMe3).,C12]+
\
C~S-OSL,X,~ /
, Os(PPh,),X,
i, HX,L (Cl: BuyPh, Pr:PI;, Br: PPr:); 4 1 1 ii, CIz, L = PMe,Ph, CC14, L = PMe,Ph, PEt,Ph; 4 0 4 fi, CY: PMePh,, PPCPh, PPh3; 3 7 0 iv, BH,, PMe,Ph, PEt,Ph; 4 0 4 v, HX, L (CZ: PEt3, PPr';, PBu;, PMe,Ph, PEtZPh, PPr;Ph, PBu%h, 4 1 1 PPh,(l-naphthyl), 4 1 4 Br: PPG, PMe,Ph, PEt,Ph, PBu:Ph); vi, N2H4 (c1, PBu2Ph); ' 1 3 vii, HCl, PBu;, PMe,Ph, PEtPh2; 4 0 4 viii, HX, PPh3 (CI, Br); 370 ix. Zn, PEt,, PMe,Ph, PEtPh2; 2 5 4 x, PMePh,, PEt,Ph, PEtPh,; 4 2 0 xi, Zn, HCI, PMe2Ph; 2 7 6 xii, Zn, PMe,Ph; 2'6 xiii, HCl; 276 XiV, PHPh2; 4 0 7 xv? PPh, (Cl,
397, 410
Br , ' I ) ;
xvi, AgPF6; 4 0 1 xvii, PMe,
(c1).4 1 6
Scheme 13 Preparative routes for osmium halo phosphine complexes
IR spectra were used to indicate configurations of cis- and rr~ns-Os(PHPh,),C1~~~ and of rru n ~ - O s ( PPh ~ ) ~ B while r ~ ? ' ~ ,'PNMR spectra of Os(PPh,),X, (X = CI, Br) suggested a squarebased pyramidal structure for these in which, however, intramolecular rearangement renders the two phosphorus environments e qu i ~ a l e n t . ~ 'In ~ " a recent paper electroreduction of merOsCl,(PMe,Ph), to mer-[OsCI, (PMe,Ph),]- was described; this releases C1- to give the five-coordinate trans-OsCI,(PMe,Ph), .410b
( i i ) Osmium (III)
mer- and fac-OsL, X,. These are important starting materials for other osmium phosphine complexes, particularly the mer isomers (see p. 574); general methods of preparation are given in Scheme 13. Other starting materials which have sometimes been used for mer complexes are K,[OsNCl,], trans-K,[OsO,(OH),] and rran~-OsO,Cl,(PPh,),1I! In some cases mer-OsL,Cl, can be converted to mer-OsL, Br, with LiBr in refluxing e t h a n ~ l . ~ " *The ~ " species Os(PMe,Ph),LCl, (L = P(OMe),Ph, P(OEt),, PEt,Ph, PPh,) are made from trans-Os(PMe,Ph),Cl, and L in hot benzene.w These trivalent complexes are useful starting materials for other osmium phospine complexes, and some of the reactions are indicated in Schemes 13 and 14. In not all of the cases is it clear whether the mer orfac isomer was used and so we have omitted these prefixes, although it is likely in most cases that the mer was in fact the starting isomer. In order to simplify the schemes, general reactions only are indicated. There is one X-ray crystal structure of a mer isomer, mer-Os(PMe,Ph),Cl, (Figure 27). Bond lengths are a = f = 2.347(6), b = c = 2.408(6), d = 2.350(5) and e = 2.439(6) There are IR data on many mer and fac complexes (mer-OsL,X,; X = C1, L = PPr"3r4'3 PBun3,@' PMe,Ph,w PEt,Ph,w P B u " , P ~ ;X ~ '= ~ Br, L = PPrn,;4'3fac-0sL,C1,; X = C1, L = PBu",, PMe,Ph, PEt,Ph,404 P B u " ~ P ~ ,PPh,i3'O ~'~ X = Br, L = PPh,370), and electronic (nzer-Os(PPr",),C1,,"'4 mermer- and fac-Os(PBu,"Ph), C1, mer-O~(PBu",Ph),Br,~'~) and ESR spectra Os(PPr,Ph), CI, (mer-Os(PMe,Ph),Cl,, rner-Os(PEt,Ph),C1,,'02~413 mer- andfa~-Os(PBu",Ph),Cl~'~). Magnetic moments for these complexes lie in the range 1.9-2.1 BM at room There are ESR data on mer-OsCl,(PR,), (PR, = PMe,Ph, PBu",Ph, AsMe,Ph) and on mer-Os(PBun,Ph),Br3.'oz
,:'
S,P/
cis-OsL, (S2PRz), 4 1 2
k
R
2
c ~ ~ Q s L ~ ( & C N RI ~ ) ~
see p. 571
L = phosphine and arsine; L' = phosphine only Scheme 14 Some re.actions of OsL,X, and OsLgH4
Figure 27 Structure of mer-O~(PMe,Ph),C1,~~~
OsL,X,. Only one example of this type is reported; Os(PPh3),C1,, made by refluxing facOs(PPh,), C1, in t - b ~ t a n o l . ~ ' ~ trans-[OsL,X,/-. Apart from the preparative methods given in Scheme 13 transPh4As(Os(PEt,Ph),C14] and trans-I3un4N[Os(PEt2Ph),Br4]are made from the phosphine and the corresponding [OsNX,][Os(PMe, j,Cl,]PF,. This is made from AgPF6 and trans-Os(PMe,),Cl,, and reacts with CO to give trans-Os(PMe,), Clz(CO).401 [OsZL6C13/+. These are made as indicated in Scheme 13, and are likely to have a p3-chloro structure?" trans-OsL,X,. Apart from the preparative routes outlined in Scheme 13, trans-Os(PPh,),C14 can be made from Ph,As[OsNCl,] with PPh, or from Os(PPh,),Cl,(NPPh,) and HCl.3i5 A single crystal X-ray structure shows Os(PMe,Ph),C14 to have a trans configuration, with Os -P = 2.448(3) A and Os-CI = 2.319(3)A.403 A trans configuration in solution is also suggested by TR and 'HNMR studies on species with L = PMe,Ph, PEt,Ph, PPf2Ph and PBU""~;~'~.~'' this is also the conclusion from Raman data on Os(PMePh,)2C14 and O S ( P P ~ ~ ) ~and C ~ TR , , ~data ~ ~ on Os(PMePh,),CI,, Os(PPr",Ph),CI,, O S ( P P ~ , ) , C ~and , ~ ~Os(PPr",),Br, ~ ,413 There are electronic spectra for Os(PPr",),X, (X = C1, Br)4'4 and ESCA data on OS(PE~~)~CI,, Os(PMe,Ph),C1,42' and Os(FPh,),C1,;93 magnetic moments are normally between 1.5 and 1.6BM at room temperature for these complexes.4" [OsLX,]-. The sole example appears to be Ph,As[Os(PPh,)Cl,], a red complex made from Os(PPh,)Cl,(NHPPh,).Me,CO and Ph4As(H502)C1,.315 MisceElaneous phosphine complexes. For phosphine complexes with other ligands see as follows: acetonato, p. 598; alkoxo, p. 596; ammine, p. 529; azido, p. 565; carboxylato, p. 601; diazenido, p. 601; B-diketonate, p. 597; dinitrogen, p. 555; dithiocarbamate and dithiocarbonate, p. 604; hydrazine, p. 556; complexes with metal-metal bonds, p. 525; nitrile, p. 567; nitrido, p. 562; nitrosyl, p. 547; oxo, p. 584; phosphineiminato, p- 568; phosphinodithionato, p. 605; pyridine and piperidine, p. 533; sulfido, p. 603; triazenido, p. 565; trithiocarbonate, p. 605.
Osmium 46.5.2
575
Tertiary Phosphite Complexes
It is a singular circumstance that the known chemistry of the tertiary phosphite complexes of osmium differs quite significantly from that of the tertiary phosphines, arsines and stibines. The closest analogue to P(OR), in osmium coordination chemistry would seem to be PF,, but even here the similarities are not marked. The oxidation states found are 0, 11, 111 and IV (there are no established zerovalent unsubstituted osmium phosphine complexes), and the phosphites form unsubstituted species of the type OsL, and [0sLJ2+ which have no counterparts in phosphine chemistry, The reason for these differences must be associated in part at least with the different cone angles and basicities of P(OR), ligands as against PR, . Further similarities and differences between the chemistries of osmium phosphines, phosphites and phosphorus trihalide complexes would obviously constitute a worthwhile study.
46.5.2.1
Osmium(0)
Reaction of Os{P(OMe),),Cl, and P(OMe), with sodium amalgam in THF yields the white Os(P(OMe),}, . Study of the ,*P['H]NMR spectra showed simultaneous slow exchange of axial and equatorial ligands consistent with a Berry mechanism.429 46.5.2.2
Osmium(f1)
Amongst the osrnium(l1) phosphito complexes known are the octahedral [OS(P(OR)~},]~+ ,made from Os(PPh,), Br, and P(OEt), . The tetraphenylboronate is white; the trimethyl phosphite analogue could not be isolated.430Other osmium(I1) species include [Os{P(OEt),),X]* (X = Cl, Br), made from Os(PPh,),X, and P(OEt)3,430Os(P(OR),},Br, (R = Ph, p-ClC,H,, p-MeC,H,), also made from Os(PPh,),Br, and P(OR), and Os(P(OPh),),Cl,, made from OsNCI,(AsPh,), and P(OPh), .265 For Os(P(OMe),},(octaethylporphyrin) see p. 618.
46.5.2.3
Osmium(IH)
For Os(PMe,Ph),(P(OR),Ph)Cl, (R
46.5.2.4
= Me,
Et) see p. 573.
Osmium(IVj
Reaction of Na,[OsCl,] Os(P(OMe),],Cl, :29
and P(OMe), in the presence of amalgamated zinc gives
46.5.3 Phosphorus Tribalide Complexes The PF, ligand is similar to CO in many aspects of its coordination chemistry, and this is well exemplified by the small area of its osmium chemistry so far explored. The oxidation stages - 11, 0 and IT are represented, and there is an 'adduct' ofosmium(VII1). Little is known about complexes of PCl, with osmium.
46.5.3.1
Osmium( - iI)
The salt K,[OS(PF~)~] constitutes the most unequivocal instance in which osmium attains this rare oxidation state (it may always be argued for Os(NO),(PPh,), , which has been crystallographically characterized, that - I1 is only a formal oxidation state). The salt is made from Os(PF,),H, and potassium amalgam in ether. It is colourless, and the 'H NMR and IR spectra are consistent with a symmetrical, presumably tetrahedral, structure."'
576 46.5.3.2
Osmium Usmium(0)
Osmium trichloride and Cu metal react with PF, at 250 "C under 500 atm to give the colourless Os(PF3),, for which a trigonal bipyramidal structure was inferred.433
46.5.3.3
Osmium(II)
The dihydride Os(PF3),H2is a colourless liquid (m.p. - 72"C), prepared from anhydrous OsC1, and PF, at 400atm with H, at 120atm at 250°C. Both 31Pand ' H NM R spectra suggest a cis structure.432With triethylamine [Os(PF,),H]- is thought to be formed; the species was not isolated, but the 'HNMR spectrum of a solution containing it was suggestive of a trigonal bipyramidal structure with the hydrido ligand occupying an axial positiont3' However, later ' H and ''F NMR data suggest that it is fluxional in solution.434 A number of tertiary phosphine complexes of osmium(I1) substituted by PF, have been reported. Thus, reaction of Os(PPh,), H, and PF, yields cis-Os(PF,)(PPh,), H, and cis-Os(PF,)(PPh,), H, 53: and the former reacts with HC1 to give c~s-OS(PF~),(PP~,),HC~."~ This in turn with HCI under pressure gives cis-0~(PF~),(PPh,),Cl,,4~~ and reaction of PF, with mer-Os(PMe,Ph),Cl, with zinc will give both the cis and trans forms of both Os(PF,),(PMe,Ph),C1, and Os(PF,)(PMe,Ph),Cl, :37
46.4.3.4
Osmium(III)
For Os(PMe2Ph),{P(OR),Ph)CI, (R = Me, Et) see p. 572. Reaction of osmium metal with PCl, at 600°C gives a brown, volatile solid OsCl,-PCl,, the magnetic susceptibility of which was measured from 77 to 373 K; at the fatter temperature peffis 1.94 BM.4,* Presumably this material has a polymeric structure so as to give it octahedral coordination.
46.5.3.5
Higher oxidation states
At - 100 "C OsO, will react with PF, to give (OsO,),.PF,; at 60 "C OsO,-PF, is the product. With E l , , OsO, gives a black material, O S O ~ - P C I , ~ ~ ~
46.5.4
Tertiary Arsine Complexes
A substantial number of these have been reported for osmium, particularly with AsPh,. Predictably the chemistry is quite similar to that for the analogous phosphine species.
46.5.4.1
Hydrido arsines
This is an area which has been much less explored in osmium chemistry than that of the hydrido phosphines. Some physical data are given in Table 17. No monohydrido arsines seem to be known; reaction of Os(AsR,),H, and AsR, in toluene gives the dihydrido species cis-Os(AsR,),H, (AsR, = AsEtzPh, AsEtPh,), and the ' HN M R spectra of these suggest a cis configuration as is the case for the phosphine analogues.276No trihydrido complexes have been reported, but examples of Os(AsR,),H, are known, made by reaction of mer-Os(AsR,),Cl, with sodium borohydride (AsR, = AsMe,Ph,404 A s E ~ , P ~ , * ' ~ ,AsEtPh, ~" ,276 ASP^,^'^). Carbonylation of Os(AsEtPh,), H4 with CO yields Os(AsEtPh,), (CO)H, .276
46.5.4.2
Halo arsine complexes
The general chemistry here, though not as extensively investigated as yet, follows the general pattern of the corresponding halo phosphine complexes. (see Table 18). As with these and with the halo stibines, oxidation states of 11, IT1 and IV are represented. Reaction of [0sXJ2- (X = C1, Br) with H3P04, HX and AsMe,Ph or AsMePh, gves Os(AsR,),X,; although the configurations were not established in this relatively early workM it
Osmium
577
seems likely that they may be trans. The IR spectra of Os(AsPh,),Br,, made by refluxing (NH,),[OsCl,] and HBr with AsPh, in 2-methoxyethanol, suggest that this has a trans config~ration.~" There has been a more extensive study of osmium(II1) complexes of the form Os(AsR,),X,, and these are summarized in Scheme 15 (see also Table 18). OsL2(Nz)CI,
oso,
i, HCI (AsMe,Ph, A S E ~ ~ P ~ ii,) ;Zn, ~ ' Nz ~ (AsMe,Ph, A ~ E t , p h ) ; ' ~iii, ~ HI (AsMe,Ph, A ~ M e p h ? ) ; ~ ~ ' iv, HX l l ~ E t ~ p h ~ vi, ' ~ )NzH4 ; ASP^;);^^' AsMe,Ph411 AsEtPh, 2 7 6 ) ; v, CC1, (ASP$, A ~ M e , P h , ~ A vii, CI, Br: A S M ~ ~ PAsMePhz,4223440 ~ , ~ ~ ' AsRPhz (R = Et, Pr, Bu);,*~ Br, ASP^^.^^' viii, HC1 (ASPS,, A s E t ~ p h , ~AsPh33'o)); '~ IX, AsPh3 (C1,370 Br4*'): x , H 2 P 0 2 , HX (X =Cl, Br) (AsMe2Ph, A ~ M e p h , ) . ~ ~ '
Scheme 15 Preparative routes to haloarsine complexes
46.5.5
Tertiary Stibine Complexes
As is the case with most elements the stibine chemistry has been a largely neglected area and this is certainly so for osmium: very few osmium stibine complexes have been isolated. All of the species which have been isolated are with SbPh3, though oxidation states span the range TI to IV. Preparative routes and typical properties are summarized in Scheme 16 (see also Table 18). The claimed trans configuration for Os(SbPh,),X, (X = C1, Br) and for Os(SbPh,),Cl, is based at l , , the present on the simplicity of the IR spectra and, in the case of the black 0 ~ ( S b P h ~ ) ~ Con 'H NMR spectrum also."0 There is confusion however about the structure of Os(SbPh,),Cl,. On the basis of ESR, 'H NMR and IR data a mer configuration has been proposed3'' and this would indeed seem to be the likely structure; an earlier IR study however was interpreted in favour of afuc structure.44' The magnetic moment over a temperature range has been reported (100 to 300 K; peR= 1.62 BM at 100 K rising to 1.73BM at 300 K)442(or 1.92 BM at 293 K).441The bromo complex Os(SbPh,), Br,, made from (NH),[OsBr,] and SbPh, in 2-methoxyethanol, is purple, melting at the same preparative route using methanol as the solvent gives a black 215 OC4" or at 185-188 0C;443
[OSX,I
2-
i, L (X = C1, Br);370 ii, L (X = C1,370,441Br,370 v, NO;22s3441 vi, C1, CC14;370 vii, C 0 . 4 4 *
iii, L, BH; (X = C1);441 iv, C1, RNC;447
Scheme 16 Representative preparations and properties of triphenylstibine complexes
Osmium
578
form melting at 263-265 0C.370 On the basis of IR and ' H NMR spectra afac configuration for the black form was suggested;370a mer configuration for the purple form has been assigned on the basis 1 of its IR spectra.443Clearly there is room for some further work in this area. For nirrido stibines see p. 562, and for nitrosyl stibines see p. 547.
46.5.6
Bidentate Phosphine and Arsine Complexes
For convenience we consider these together. As with tertiary phosphines, arsines and stibines the oxidation states featured in the chemistry of these bidentate ligands are TI, I11 and IV, with the former being predominant. General preparative routes to these are summarized in Scheme 17. The abbreviations used below, in Scheme 17 and in Table 19 are the conventional ones, namely: dmpe: bis( 1,2-dimethylphosphino)ethane,Me,P(CH,),PMe, depe: bis( 1,2-diethylphosphino)ethane, Et, P(CH,),PEt, dppm: bis( 1,2-diphenylphosphino)methane,Ph, PCH, PPh, dppe: bis(l,2-diphenylphosphino)ethane, Ph,P(CH,),PPh, dpp: bis( 1,2-diphenylphosphino)propane,Ph,P(CH,),PPh, pdmp: o-phenylenebis(dimethylphosphine), o-C,H,(PMe,), pdep: o-phenylenebis(diethylphosphine), O-C6Hq(PEt,)2 diars: o-phenylenebis(dimethylarsine), o-C6H,(AsMe,), dapm: 1,2-bis(diphenylarsino)rnethane, Ph, AsCH, AsPh, dape: 1,2-bis(diphenylarsino)ethane,Ph,As(CH,), AsPh,
i, BHi, L= P(OMeI3, P(OPh),, PhCh:278 ii, LiAIH,, d ~ p e , , , ~ de~m:~,'' iii. LiAlH,. , ~ ~ de~e?'~ iv, &Me,, dppm, d ~ p e ; ~ v, " X-. Li.41H4:Cl': dmpe, d ~ p e , d~ ~, p~ r n ; ~I - ~: 'd e ~ e ; SCN-: vi, dppm, dmpe, depe, dppe, ~ d e p ; ~ *vii. ' Na/Hg, d e ~ m ; , ~ ,viii, CI: AgBF,, dppe. dpp;445 H z 0 2 , p d m ~ ; ~ C&, " d i a r ~ ; Br: , ~ ~Br,, diars:I.SC!V: HCIO,, I- or SCN-, d i a r ~ ; ~ix.~ 'C1: dmpe, dppm, depe, d ~ p e , p~ d~r *n ~ , d~ ~~ p' , , ~d' i a r ~ Br: : ~ d~ i~a r ~ ? ~I, SCN: [-.with X = Br. d i a ~ s X, ~= Br, ~ ~ SCN-;370 x, HNO,: CI ( p d m ~ , d~i~a r' ~ ) ; , , ~Br, I: ( d i a r ~ ) ;xi,~ ~I -~( d ~ p e ) ; ~ " xii, BF; (CI, Br) dpp;445 xiii, CI, dpp;445 xiv, dapm. 3 7
Scheme 17 General preparations for chelating phosphine and arsine complexes Table 19 Chelating Phosphine and Arsine Complexes
Complex
Colour
M.p.
cis-Os(dppm), C1, cis-Os(dppe),C1, !rans-Os(dmpe), C1, trans-Os(dppe), C1, IOS(dPP)2 ClIBPh, trans-[Os@dmp),Cl,]ClO,
Yellow Yellow Yellow Yellow Brown Purple
290-29 1 298-300 (d) 293-296 260
'CY
= cyclic
voltammetry; D = dipole moment
(0
C)
Spectroscopic and other properties" X-rayw
W20 UV/V~S'~~ UV(vis4" IR, UV(vis, 3'PNMR,MSCV452 UV/vis, EPR,CV4"
Osmium
579
The X-ray crystal structure of Os(dppm),Cl,~C€iCl shows this to be cis, with mean bond lengths of 2.452(2) (Os-Cl), 2.297 (P cis to Cl) and 2.339( 1) (P trans to Cl), and a CI-Os-Cl angle of 83.08(7) o.444 The cis configuration of Os(depe),Cl, and of Os(dppe),Cl, in solution is demonstrated The trans configurations of by the dipole moments of 9.3 and 8.3D re~pectively.~~’ ) been [0~(dpp),Cl,]BF,~~ and of the arsine Complexes OsL,Cl, (L = d i a r ~ , ~ d~a,~~e ~, ~’’have inferred from the relative simplicity of their far IR spectra. The complexes Os(SCN),(dapm), and OsI,(dapm), have been reported; in the former the thiocyanate ligand is thought to be S-b~nded.,~’ Magnetic susceptibility measurements on truns-[OsL,Cl]BF,~~CH,Cl,(L = dppe, dpp) give surprisingly high magnetic moments at room temperature (pcE= 1.98 and 2.07 BM respe~tively):~~ while moments of [O~(diars)~X,] are more normal (1.85 (Cl), 1.88 (Br), 1.93 (1) BM).369For the osmium(1V) species [Os(diars),X,](ClO,), they are, not unexpectedly, lower: 1.25 (Cl), 1.22 (Br) and 1.16 (I) BM.369Another osmium(1V) complex is trum-Os(dapm)Cl,*EtOH, made from the ligand and [osc16]2- in ethanol.370 The IR and ”PNMR spectra of [Os(dpp),Cl]+ suggest that this has a trigonal bpyramidal structure in solution with a chloro ligand in an axial p o ~ i t i o n . ~On ’ electrochemical reduction, it gives Os(dpp),Cl (a rare case of osmium(1)) and, on further reduction, the osmium(0) species [Os(dpp), C1]- ?52 For complexes of bidentate phosphines with metal-metal bonds see pp. 525, 527; for dinitrogen complexes see p. 555.
1
46.5.7 Complexes with Polydentate Phosphines and Arsines The ‘triphos’ ligand (bis(2-diphenyl(phosphinoethyl)phenylphosphine, [Ph2P(CH2)J2PPh) reacts with [osc&]z2 to give the yellow Os(triphos)CI, 1455 while reaction of bis(a-diphenylphosphinopheny1)phenylphosphine (QP) with [OS(NO)(OH)(NO,),]~- and HC1 gave the pale yellow Os(QP)C12.456 One osmium complex of the phosphorus-arsenic chelate As-Pf-As (bis(2-diphenylarsinoethyl)phenylphosphine, (Ph, AsCH, CH,X PPh) has been made from OsO, and the ligand in HCl; it is the orange Os(As-Pf-As)Cl, .457 There is one complex with the tetradentate arsine qas (tris(o-diphenyIarsinophenyl)arsine, C,,H,,As,): Os(qas)Cl,, made from K, [Os(NO)(OH)(NO,),] and the ligand with HCl.4j6
46.6 OXYGEN-CONTAINING LIGANDS 46.6.1 Aqua and Hydroxo Complexes No unsubstituted aqua complexes of osmium seem to have been reported, though we have already referred to a number of species in which H,O is a ligand. Species such as [OS(H,O),]~+and [OS(H,O),]~+would be expected to exist. There has also been little work on osmium hydroxo complexes apart from the now well-characterized ci.~-[0sO,(OH>~]~(p. 592), tram-[OsO,(OH),]’- (p. 58 1) and the two p-hydroxo species structurally characterized, M[Os,(OH)O,] (M = Rb, Cs) (p. 596). The [Os(OH),]2- species should certainly exist but does not seem to have been mentioned in the literature; even [OS(OH),]~-might be expected to be stable in the absence of oxygen. Other hydroxo (and aqua) complexes are considered in sections dealing with the other ligands present.
46.6.2 Osmium Oxo Complexes The oxo complexes of osmium dominate the chemistry of the element, the duster carbonyls notwithstanding, and the tetroxide is the single most important compound of the element. In the area of osmium oxo chemistry there are three main topics of interest: the tetroxide itself, the ‘osmyl’ complexes which contain the trans O=Osv’=O unit, and the cyclic ‘oxo ester’ species which contain the Osw moiety. This is perhaps the most appropriate point at which to comment briefly on the reasons for the stability of these three systems. Apart from ruthenium, osmium is the only element to form a neutral octavalent tetroxide, and certainly OsO, is chemically much more stable than RuO,. It is a relatively mild oxidizing agent and
580
Osmium
undoubtedly owes its stability largely to the ability of osmium, by virtue of its central position in the third row of the transition elements, to sustain the very high VI11 oxidation state in the presence of n-donor ligands (0,- alone, 0'- with N3-, 0 2 -with F-). The much greater oxidizing power of RuO, must surely derive from the fact that ruthenium is a second-row element and thus barely able to sustain the VI11 state, while FeO, is unknown. Whereas OsO, and the oxo esters considered below possess certain features unique to osmium, analogues of the 'osmyl' complexes are found for other elements capable of attaining a d 2configuration (Ru"', Re", Tc", W'", Mo'")), although it is true that the diversity of the coligands X in trans-[Os"O,X,~-is rivalled only by that in the uranyl system. The reason for adoption of a trans geometry is electronic (see p. 579); cis dioxo species cis-[MO,X,Y- are found for the d o state (Rev", Mo", W"', V") but are curiously missing from osmium(VII1) and ruthenium(VII1) chemistry. The cyclic oxo esters are very much a feature of osmium(V1) chemistry but analogous rings are rare with other elements: they have been detected for rutheniurn(V1) and manganese(V), but only for chromiurn(V) has any substantial oxo ester chemistry been uncovered. The osmium species are formed by two main routes: (a) by reaction of OsO, with alkenes, dienes or alkynes, in which case two electrons are transferred from the multiple bond to osmium(VII1) which is thus reduced to ) ~ ]= H, Me) with elimination osmium(vI), or (b) by reaction of cis-glycols with t r a n ~ - [ O s O , ( O , R(R of water or methanol (see p. 582). Both routes are facile for osmium and the d 2 osmium(V1) state is particularly stable for oxo species; the redox and kinetic properties of OsO, are such as to favour route (a) (whereas for RuO,, for example, C=C cleavage would occur, and [Reo,]- does not possess sufficient oxidizing power for the reaction to occur at all). Route (b) is facile because of the stability of the osmyl system in [Os0,(OH),j2- and [0~0,(OMe),]~-whereas analogues of these species are so far not known for other elements. Nevertheless, oxo ester rings for other elements should surely be commoner than they seem to be; the most fruitful approach to obtain them might be exploitation of a route analogous to (b) with, for example, trans dioxo species of rutheniurnfVT) or of rhenium(V). The arrangement of this large section on osmium oxo complexes follows the pattern of other sections in this chapter. We consider mononuclear complexes first, by increasing oxidation state and, within each oxidation state, we consider those complexes with the largest number of oxo ligands first. The same procedure is then followed for binuclear species. For convenience, however, mononuclear and polynuclear oxo esters are considered together.
46.6.2.1
Mononuclear complexes
( i ) Osmium(IV) and ( V ) complexes ( a ) Osmium(IV). For [O=0s(terpy)(bipy)l2' see p. 543.
( b ) Osmium(V). The salt Li,[OsO,] has been reported, made by reaction of Li,[OsO,] and Li,O with osmium metal. On the basis of X-ray studies of this hexagonal lattice the Os=O distance was estimated as 2.07 A, suggesting that this is a lattice compound rather than a true complex.459There is a brief report of L~,[OSO,]~~* but again there is no evidence that [OSO,]~-tetrahedra are involved.
(ii) Osmium ( V I ) complexes As mentioned above the chemistry here is dominated by that of the osmyl species (see p. 581) and by oxo ester complexes (p. 584). ( a ) Hexa-, penta- and tetra-oxo species. A substantial number of these have been reported, but in no single case is it certain that they contain discrete mononuclear species. Those reported include [OSO,]~-, [0sO5l4- and [OsO,]'- ions. Examples are Li,[OsO,], Na,[OsO,], Ba, [OsO,], Ba, [OSO,]~"and Ba[OsO,] *nH,0;46' the IR spectrum of BaJOsO,] has been measured.462 For many years the 'osmates' M'2[Os0,]~2H20 (M = Na, K)463and Ba[OsO,]*H,O were regarded as being tetrahedral osmium(V1) species, but it is now known that the sodium and potassium (OH),]'- ion;327p629465 the barium salt could be Ba[OsO, (OH),] salts at least contain the trans-~OsO, or, possibly, contains the [OSO~(OH)~]'anion (see below). ( b ) Trioxo species. Few of these are known for osmium(V1). The species [OSO,(OH),]~-is said ~ . is ~ ~possible ~ that 'Ba[OsO,]-nH,O', made by to be present in solutions of [ O S O , ( O H ) , ] ~ It could be Ba[OsO, (OH),] since 'Ba[Ru04]-2H,0' has reaction of Ba(NO,), with IS,[OsO, been shown by X-ray studies to be Ba[RuO,(OH),] (in which there is a trigonal bipyramidal anion with three oxo ligands in equatorial p~sitions).~~'
Osmium
58 1
A number of trioxo species are claimed in the earlier literature. Thus, ‘OsO,(py),’ has long been regarded as such but is now known to be os,06(py))4‘68’469 (though in aqueous solution there is an equilibrium between the monomer and dime^'+''^^'^). Likewise the claimed ‘oxy-oxmyl’ salts ’ ~ probably [os20&]4- (see pp. 593 and 595). [OSO,X,]~-(X= Cl, Br, NO;, $ 0 ~ ) are (c) Dioxo (‘osmyi’)species. As mentioned above (p. 579) this is one of the most important classes of osmium complexes, rivalling in number and diversity the oxo esters to be discussed in the next section. The trivial name ‘osmyl’ denotes the trans Os=Osvl=O grouping and, like ‘uranyl’ for O=Uv*=O, has passed into common parlance and will be used here. We have already mentioned that the easily attained d 2 configuration for osmium favours the formation of these species (just as d o , for other elements but apparently not for osmium, favours the formation of cis dioxo specie^^',^^^). The realization in 19604wthat the Long-kn~wn“~and supposedly tetrahedral ‘K2[Os04] 2H20’ was diamagnetic and its consequent reformulation as trans-K,[OsO, (OH),]4u (subsequently confirmed by X-ray s t ~ d i e s ~ ? led ~ , , to ~ ~a) rationalization of the osmyl complexes. Almost all osmyl complexes are octahedral (the few exceptions are treated on p. 582) and the great majority have linear or almost linear O=Os=O systems (Table 20); most are mononuclear but a few are binuclear with a pp’-dioxo bridge.
-
Table 20 X-Ray Structural Data on ‘Osmyl’ Complexes Complex
(4 /oso,x,/n+
trans-K2[OsO,(OH),] trans-K, [OsO,CI,j rrans-[OsO,en,](HSO,),
(b) os02 ( 0 , R l P h OS02(0, C , H6 N?O?)PY> OsO,(O,C,H,N, O,)py, - H 2 0 3py OsO, (0,C , ,HI, N, 0,)py2 oso,(0,c,,HI2 1PY,’C,H, (c) Miscellaneous Os0,(OH),phen2(X = 0, Y = N) OsO, (NH,CN, CO, ), K[OsO,(OAc),]*2AcOH
-
doso ( A )
OOsO (>)
do,
(A)
XOsX (“)
doSy( A )
YOsY
(“I
ReJ
1.77 1.750(2) I .74(1)
180 180 180
2.03 2.379(5) 2.11(1)
90 90 80.2
1.85 1.744
162 164.0
1.90 1.967
85 84.4(4)
2.16 2.169
94 89.8
515 498
1.78(4) I .72
164(3) 169
1.95(5) 1.95
90(3) 86
2.18(6) 2.11
91(3) 91
516 517
1.742(4)
166.2(2)
1.983(4)
91.0(2)
2.130(5)
78.9(2)
469
1.731(3) 1.711
180.0
2.038(3) 2.026
78.9(1)b
2.1 14(3) 2. I 48b
59.2b
495 483
125.2(3)
-
I
-
-
327,465 487 493
a Chelating OOsN
angle. Chelating acetato ligand. For Os,06py, and K,[O~O,(NO,),] see Table 25 and p. 595; for O ~ 0 , ( 0 2 C 6 H 1 0 ) 2 ( N ~ Hsx , 3 Table ~ 25 and pp. 587; for Os,04(0,C,Hl,)py,~4H,0 see p. 586 and Table 21.
They are all diamagnetic, and this can be attributed to the large tetragonal compression of the octahedron by the trans O=Os bonds causing the two d electrons to pair up in the dxy orbital (assuming the oxo ligands to lie on the z axis).464 Another approach is to envisage the formation of two three-centre, four-electron MOs involving 2 p ~ - d x z o - 2 pand ~ 2p,Z-dV,O-2p,2orbitals. We classify here mononuclear osmyl complexes truns-[0sO2XJ- by the nature of the ligand X; binuclear osmyl complexes are considered on p. 595. Osmyl complexes with 0-donor ligands. Preeminent amongst these are the ‘osmates’, sometimes called ‘osmiates’ in the early literature, and still often incorrectly formulated as salts of ‘[0sOJ2- ’. The sodium, potassium and barium salts were reported in 1844,474but it is only recently that the rubidium and cesium salts were prepared.475The purple potassium salt is the best known and is a useful starting material for the preparation of other osmyl or osmium complexes. It is easily made from OsO, in excess KOH with added ethanol functioning both as reducing and precipitating and can also be made from osmium metal by fusion with KOH and KN03.477The sodium salt, though more soluble, is less easy to characterize and has a variable degree of hydration The X-ray crystal depending on the amount of ethanol added to the Os0,-NaOH mixture!” (Table 20) structure of the potassium salt clearly shows it to have the trans structure (1)327*465 postulated earlier on the basis of its diamagnetism and electronic spectra.464The acid dissociation constants of ‘H2[OsOz(OH),]’have been The possibility that ’Ba[Os04]*nH20’(and hence B ~ [ O S O , ( O H ) ~ ] * ~ H could ~ O )be Ba[OsO,(OH),] has been mentioned above (p. 580).
Osmium
582
A closely related species is the olive-green K,[OsO,(OMe),], made from KOH and OsO, in methano1.48M82 Its use in electron microscopy of biological tissue has been proposed.4B2From it and ~ ~recrystallization ~ from acetic acid this acetic acid can be prepared the blue K [ O ~ O , ( O A C ) , ]on gives K[OsO,(OAc),]-2AcOH in which the osmyl group is emphatically not linear (Figure 28). 0
I
/c\
0
QI
c
0
0-c-0
0-O b SL, O,,,\\"' /
o q 0
'C'
0
I
0
Figure 28 Structure of K[OSO,(OA~),].~ACOH"~
The bent osmyl unit (OOsO angle = 125.2(3) ") is trans to a chelating acetato ligand, the octahedral coordination being completed by two monodentate acetato ligands. (mean a = 2.026, mean h = 1.711, mean c = 2.148.A).4x3 The oxalato complex [OsO,ox2]'- has long been known as the sodium, potassium, ammonium, rubidium, cesium, magnesium and silver salts; the normal method of preparation is to treat OsO, with oxalic acid and the appropriate cation.204Recent electronic spectral studies on the tetrabutyland [O~O,(rnalonate),]~-show vibrational fine s t r u ~ t u r eThese .~~ ammonium salt of [OSO,OX,]~are clearly true osmyl species, but the ion '[Os0,0x,]2-'204 may be better formulated as [OS,O~OQ]~-, rather than as the previously suggested [Os02(OH),ox]'- ." The salts M',[OsO,(salicylate),] are also reported (M = Na, K, NH,, Rb, C S ) . ~ ' ~ The diolato complexes K,[OsO,(O,R),] containing oxo ester rings are dealt with on p. 585 (see also Table 21). Formally related to these are the catecholato complexes [OsO,(Rcat),]*-, but they differ from them in that the metal-catecholato ring is likely to be planar (see p. 597). These are made for catechol or substituted catechols (Rcat = 4-methyl-, 4-fur-butyl-4-nitro-cholato) and K, [OsO,(OH),]. They react with pyridine to give OsO, (Rcat)py, .486 Table 21 Vibrational and Other Data for Osmyl Complexes
~'p$~oso2) v,!OsO,) Complex
a CV =
cyclic voltammetry
Colour
(cm-I)
Green Blue Yellow Yellow Red Red Red Red Yellow Yellow
825 845 860 855 803 824 837 843 828
Brown Brown Brown Olive-green Red Tan Brown Tan Brown Brown
824 826 898 825 830 842 840 s47 830 850
S(Os0,)
(cm-')
(em-')
852
304
910 890 859 864 904 899 865 917
870
300 361 366 330 290 308 292 270
Spectroscopic and other propertie8 . ESCA?, Raman,&' lR:',462,5'8a UV/VIS;"" X-ray327.465 IR,a W!V~S~' IR?- X-ray4" IR, Ra~nan,'~ IR, RamanG4 Raman, IR*' Raman, IR,&' I3CNMRSiRh Raman, IR:h2,51AX-ray,'87 U V / V ~ S ' ~ " ~ Raman, I R , uv/vis5I4 ~ ~ ~ ~ ~ ~ ~ Raman,462 IR462.518' RP6x-ray493 X-ray46
292
IFP
310
]R"9
310
1 ~ ~ ' ~ IR, UV/vis, 'HNMRSW Raman,&' IR,21*461 CVszO Raman, IRm7 IR, ESCA9, IR, ESCA513 Raman, IR"' X-rayszz Raman, IR, X-ray469
886 858
280
879 895
290
583
Osmium
Halo osmyl complexes, ( O S O , X , ] ~ . The potassium, ammonium and cesium salts of M',[OsO2CI4]and potassium and ammonium salts of M',[OsO,Br,] are known;204no fluoro or iodo osmyl complexes seem to have been reported, although tr~ns-[OsO~F,]~would be expected to be stable. Reaction of traw-K, [OsO,(OH),] with HCl gives trans-K,[O~O,Cl,]~~~ and a convenient preparais to treat K4[O~Z06(N02)4] with HC1 or HBr.'" tion for salts of [OSO,C~,]~-or [OSO,B~,]~The X-ray crystal structure of K,[OsO,Cl,J shows it to have a trans structure for the anion (see Table 20).487 The so-called 'oxyosmyl' complex 'K,[OsO, ClJ has been made from K , O S , O~ ( N O~ )and ~] a deficiency of HC1.'04 Although the pyridinium analogue488of this has been reformulated as (pyH),[OsO,(OH), C12],461 a more attractive formulation is perhaps K,[OS,O~C~~], analogous to its nitro precursor. The species [OsO,CI,(H,O)]- is thought to exist in equilibrium with [OsO, C14]2-(equation 5).466
e [OsO,Cl,(H,O)]- -+
[ O S O ~ C I ~ ]-t ~ -H,O
C1-
(5)
The electrochemical reduction of [OSO,C~,]~to osmium(1V) and osmium(II1) chloroaqua complexes has also recently been Osmyl complexes with nitrogen donor 1igands.The ammine [OsO, (NH3),]CI, was first made by Frkmy in 1844," and is still sometimes called ' F r h y ' s salt'. It is most easily made by reaction of trans-K, [OsO,(OH),] with ammonium chloride4%and is pale yellow and only sparingly soluble in water. On heating it decomposes to hydroxoammine complexe~.~~' The ethylenediamine complex trans-[OsO,enJC1, is made from trans-K, [OsO, (OH),] and ethylenediamine d i h y d r ~ c h l o r i deThe . ~ ~ ~X-ray crystal structure of the bisulfate salt (Table 20) confirms the trans geometry (the O=Os=O grouping, though linear, is not precisely perpendicular to the OsN, plane). The rate of exchange of H,I80 with t r ~ n s - [ O s ' ~ O ~ e nis~slow ] ~ + but increases with increasing PH."~ It reacts with [Fe(H20)6]2+to give a deep blue species, possibIy [(H,O),Fe-0 - 0 ~ O e n , ] ~ + ,for which preliminary X-ray data were given.494 Direct reaction of OsO, with a series of amino acids in aqueous solution gives trans-OsO,(an), (an = glycinato, DL-glychato, DL-valinato, DL-lcucinato, DL-isoleucinato and DL-phenylalaninato), and the X-ray crystal structure of trans-OsO,(NH, CH,CO,), has been elucidated (Table 20).495 There are also X-ray data (Table 20) on Os0,(OH),phen-5H,0,"69 made by reaction of transK,[OsO,(OH),] with 1,lo-phenanthroline hen).^% The species exists in another monomeric form in which the hydrogen bonding of water molecules is slightly different; there is also a dimer, O~,O,phen,.~~' Equilibria for this, and for OsO,(OH),TMEN Os,O,(TMEN), (TMEN = N,N, N',N'-tetramethylethylenediamine), have been studied496and there arc probably equilibria such as OsO,(OH), bipy OsO, bipy, and OsO, (OH),py, z+ Os,O, py, .471 The oxo esters OsO,(O,R)py, are considered on p. 584 but since they are osmyl complexes we list X-ray data for four of these in Table 20; there arc also IR data for them.4w-4Y7 It is worthwhile at this point to note that the O=Os=O moiety is linear only in complexes of the form trans-[OsO,X,l'-, where D , symmetry exists by virtue of the equivalence of the four X ligands. In species of the type OsO,X,Y, (e.g. such as discussed above) it deviates substantially from linearity, by some 10 to 18 O, in the complexes which have been studied by X-ray methods. In all cases the distortion is away from the relatively short Os-0 (ester or hydroxo) bonds and towards the longer Os-N bonds. This effect has plausibly been explained on the basis that the .n electrons in the O=Os=O system have a powerful repulsive effect towards electrons in the Os-0 and Os -N moieties; these are minimized by bending of the O=Os=O system, and are also manifest in a relatively large OOsO angle (ca. 85 ") and relatively large NOsN angle (ca. 94 ") (though these are of course affected by ligand bite consideration^).^^^ There are obvious parallels here with the distortions observed in OsO(O,R), (p. 584),4993s00 Os,O,(O,R), (p. 584)50' and [OsNX,]- (p. 561);329-332 similar effects are operative in the structures. of Os,O,py, (Table 25)469and K4[Osq0,(NO,),] (Table 25).'02 The nitro complex cK2[O~03(N02)2] 3H,0"" is not, as previously suggested, K,[OsOz(OH),fp. 595).'02 For (N0,),]2'*503but has been shown by X-ray studies to be K,[OS,O~(NO~)~] [OSO,(NO,),]~- and [OsO,(NO,), see p. 598. An olive-green octaethylporphyrin (OEP) complex OsO,(OEP) can be made by oxidation of Os(CO)Cl(OEP) with H, 0, .M4 Osmyl complexes with miscellaneous donors.The cyano complex K;[Os02(CN),] is made from the action of aqueous KCN solution on OsO, There are a number of osmyl complexes with sulfur donors. Reaction of OsO, with sodium sulfite in alkali gives Na2[Os0,(S0,),];S"6 Raman and IR spectra suggest that the sulfite is S rather than 0 bonded.507There are complexes with thiourea, e.g.
+
+
.'"'
Osmium
584
[OsO,(SC(NH,), ),]SO4 and dithiocarbamato species have been Osmyl complexes with thiosemicarbazides are made from the ligand and Os04.5’0~51’ The species originally thought to be OsOC13(PPh3), and O S O B ~ ~ ( P P ~ ,are ),~~ now ~ known to be mixtures containing O S O ~ C I , ( P P ~and , ) ~O~S~O~ ,~B~~~~~( P P ~they , ) ~are ; ~ made ~ ~ in a pure state by reaction of OsO, in HCl or HBr w t h ethanol, and a number of Os02C1,(PR,), species (PR, = PMePh,, PEtPh,, PEt,Ph) have been so made.’13 The ‘osmyl’ thioether complexes OsO,X,(SR,), (SR2 = SMe,, MeS(CH,),SMe, oC,H,A(SMe,), o-C,H,(PPh,)(SMe); X = C1, Br) have recently been reported, made by reaction of the ligand with OsO, in concentrated HX. The IR, electronic and ‘H NMR spectra were recorded; the seleno complexes MeSe(CH,), SeMe were also One example of a cis dioxo complex has been reported, cis-[OsO,bipyl](C104)z,made by addition of Ce” to a solution containing ci~-[Osbipy,(H,O),]~+.The ‘HNMR and IR spectra (v,,(OsO2)= 878, vs(OsO,)= 858 cn-’)support this very unusual structure for an osmium(V1) dioxo An X-ray crystal structure would clearly be of great interest. For osmyl complexes with phthalocyanines see p. 619. ( d ) Monooxo species. The simplest of these are the oxotetrafluoride and oxotetrachloride, but the large body of oxo esters come into this category also and are separately considered below. Oxohalides (see Table 22). The oxotetrafluoride is a blue-green material, made by reaction of OsF, and OsO, at 175°C,’23 and is diamagnetic. The oxotetrachloride OsOCl, is a dark-red diamagnetic liquid, very sensitive to moisture, made by reaction of an 8: 1 C1,:0, mixture on osmium metal at 400 0C,524 from OsO, and thionyl chloride under or, in a new preparation, from OsO, and BCl, .525b The IR spectrum of the gas suggested a square-based pyramidal structure for the compound?26It is isoelectronic with [OsNCl,]- and so, like the latter, might be expected to undergo nucleophdic attack by phosphines at the axial ligand, but this does not appear to be the case.2hsThe compound reacts with cesium fluoride in HC1 to give the green ‘Cs, [OS~OC~,]’,~*~ an apparently heptacoordinate material which should be further investigated. The so-called ‘Os, OCl,’, also made from osmium with a chloride: oxygen mixture,527is probably OSOC~,.~*~” Table 22 Monooxo Complexes Complex
OsOF, OSOCI,
&OF, *0(02C?Hd? 090(02C2Me4)2
-
Coiour Blue-green Deep red Green Black Black
vos=o
1028 963 992 978
Spectroscopic and other properties
Rarna~?’~ 1~,525b.526 ~ v / ~ i ~ 5 2 5 b
Raman, IR,534M,5” X-ray5” IR,49: ~ - ~ ~ 4 % 5 2 9
IR,4g7X-raysoo
For Os,04(02C,Me4), see p. 000.
Osmium( Vi) oxo esters. A large body of osmium(V1) complexes containing the cyclic system OsOCCO is known, generally made by reaction of OsO, with an alkene R.It is convenient to deal with these complexes as a group; we abbreviate the above ring as Os(O,R), the hypothetical 02R2‘diolato’ unit being derived from an alkene R (or diene or alkyne). Although ring complexes of this type are known for a few other metals it is only with osmium that such wide diversity and chemical stability are achieved. The early work on the oxidation of alkenes and alkynes by OsO, (see p. 590) was little concerned with the osmium-containing intermediates. It was Criegee who showed, in 1936’” and in 1942,480 that alkenes R would react with OsO, to give isolable ‘esters’, formulated as OsO,(O,R) cmonoesters’) and, in some cases, OsO(O,R), (‘diesters’); on hydrolysis these gave cis diols in high yield. He also showed in 1942 that the reaction of OsO, with alkenes was greatly accelerated by the presence of nitrogenous bases L (L = pyridine, isoquinoline) and that this reaction gave OsO2(O2R)L2 which, on hydrolysis, gave the cis glycol. The possible mechanistic pathways for ester formation from alkenes are briefly considered on p. 590. In this section we consider first the esters formed without nitrogenous bases (i.e. monoesters, diesters and salts of [OsO,(O,R),]’-) and then the even larger body of information on esters containing nitrogenous bases. The latter section is sub-divided according to the Os: L ratio. Ester complexes formed in the absence of nitrogenous bases. Criegee’s ‘monoesters’ were formed by direct reaction of OsO, with the alkene R (e.g. tetramethylethylene, cyclopentene, indene, ,camphene,cholesterol) and were easiIy i ~ o l a t e d . ~ He ~ * ~formulated ** them as presumably tetrahedral species (Figure 29a) (Le. as OsO,(O,R)). Subsequent work showed them to be dimers, Os,O,(O,R), (b)r497and the complex obtained from tetramethylethylene, O S , O . , ( O ~ C ~ Mwas ~ ~ )shown ~, by X-ray
Osmium
585
diffraction to have an anti structure (Figure 29b) the osmium having square-based pyramidal coordination. The terminal oxo ligand adopts the apical positions (Os=O = 1.675(7) A) with the basal plane being formed by two diolato oxo ligands (mean Os-0 = 1.870A) and the two bridging oxo ligands (Os-0 = 1.922& OsOOs (bridging) angle = 103.9(4)”). The bridging Os202ring is planar. The mean O(terminal)OsO(based) angle is 111.1 c,501,529 a distortion comparable to that found in [Osv”X4j- species (see p. 561) and probably arising from the same causes. A study of the ‘H NMR spectra of the complex showed that, in solution, there is a solvent-dependent mixture of the syn and anti isomers, the former being relatively more stable in solvents of high dielectric constant .536
(b)
(a)
(C)
(d)
Figure 29 Strctures proposed for osmium ester c ~ r n p l e x e s ~ ~ ’ ~ ~ ~
It has been shown that the hydrolysis of the monoester of 3-cyclohexenecarboxylic acid in H, I8O gave 3,4-dihydroxycyclohexanecarboxylicacid with no I8Oincorporation, which implies that cleavage occurs at the Os-0 bonds.530 ~ , normaily prepared by reaction of a cis diol R(OH), The ‘diesters’ of osmium(VI), O S O ( O ~ R )are with trans-K, [OsO,(OMe),] or KIOsOz(OAc),], the resulting K,[OsO,(O,R),] (vi& infra) then being acidified.528Tn a few cases diesters can be obtained directly from an alkene R with 0 ~ 0 ,or~ ~ ‘ by prolonged reaction of a diol R(OH), with OSO,!~’ A single crystal X-ray of the species derived from ethylene glycol, OsO(0,C2HJ2, shows this to have a square-based pyramidal structure (Figure 29c).449.s32 The coordination about the osmium is very similar to that found for the monoesters, the p,p’-dioxo bridge being replaced by a second oxo ester ring. The Os=O distance is 1.670(12) A and the mean O(terminal)OsO(ester) angle is 110.1 ’; the mean Os-0 (ester) distance is 1.885A and mean C-0 distance 1.452 Ver similar parameters have been found for Os0(02C,Me4), (Os=0 = 1.62, Os-0 (ester) = 1.84. There are a few examples of salts of the type K,[OsO,(O,R),], made, as mentioned above, by reaction of diols R(OH), with trans-K, [OsO,(OMe),] or K[OsO,(OAc),]. No structural data are available but the IR and Raman spectra of K,[OSO,(O,C,H,),]~~~ are fully consistent with an osmyl structure (Figure 296; see Table 21). Oxo ester complexes with nitrogenous hases ( L ) . Although most of these species contain the osmyl unit there are so many of them that it is appropriate to deal with them in a separate reaction. Most of the complexes have an 0 s : L ratio of 1:2 (the majority taking the form OsO,(O,R)py,) and so we consider such species first and include those complexes of the type OsO, (0,R)(L-L), where L -L is a bidentate N donor (usually 2,2’-bipyridyl). Within this first section we consider first the and then species derived from alkenes R or diols R(OH),, viz. OsO,(O,R)L, or OsO,(O,R)L-L, species derived from dienes, trienes and alkynes. We then deal with the much smaller body of complexes where the 0s:L ratio is 1:1. ( I ) Species containing OsL,or Os(L-L) up ti^ Complexes OsO, (U,R) L, and OsO,(0,R ) L--L formed from alkenes R or cis diols R(OH),. Criegee showed that the reaction of alkenes R with OsO, was greatly accelerated by the addition of N donors such as pyridine or isoquinoline (L); the products, of stoichiometry OsOz(O2R)L2,were easy to isolate and crystallize, and on hydrolysis they would give the pure cis diol R(OH),.480 Another route to OsO,(O,R)L, is the reactiona0 of a cis diol R(OH), with ‘OsO,py,’ (now known to be 0 ~ , 0 , p y ,though ~ ~ ~ in solution the active species may well be OsO,py, or 0~0,(OH),py,)!~~ On p. 590 we discuss briefly the mechanism of the Os0,-alkene-L reaction. The reaction in which cis diols R(OH), react with ‘OsO,py,’ is thought to proceed via nucleophilic attack by the diol on osmium pyridine species, This is consistent with kinetic data and also with “0-labelling experiments on thymine glycols which demonstrated that there was no C-0 bond cleavage.53o The reaction between ‘OsO,py,’, and K,[OsO, (OMe),] or K[OsO,(OAc),] and R(OH), normally occurs only with acyclic 1,Zdiols and cyclic cis 1,2-diols; trans 1,2-diols do riot react with the exception of tram- 1,2-cyclohexanediol and tram- 1,2-~ycloheptanediol.~ Behrman showed that trans-thyminediol, trans-thymidinediol and trans-l,3-dirnethylthyminedioldid react, albeit slowly, f4.4993532
K
Osmium
586
with ‘OsO,py,’ as did the cis analogues;470presumably transcis isomerization had occurred. There are other kinetic data on the reaction of diols with oxoosmium pyridine,537*538 substituted pyridine,4’O 2,2/_bipyridyl,47~.S37,538,5~ 1,1O-phenanthr~line’~’ and TMEN539,541 complexes. Studies have also been made of ‘transesterification’ reactions with L = ~ y r i d i n e , ~ $~ b~ i-~’ ~y ~r i d y l , i~~~h~e -n ~a n~t~h r o l i n e ~ ~ ~ and $TMEN.’39,54’ These OsO,(O,R)L, species are of course of the osmyl type; for convenience we give structural and vibrational data in the osmyl section (Table 21, pp. 581 and 582). There are four X-ray structures available (Table 20). In the complexes derived from thymine, 0 ~ 0 ~ ( 0 , C , H , N , O , ) p y , , ~ ’~ ~erstwhile H~O-~ 5,6~double ~ , ~bond ~~ and from l-methylthymine, O S O ~ ( O , C , H , N ~ O ~ ) ~it~is,the of the ligand which is spanned by the osmium, leading to a saturated pyrimidine ring system. The ester ring in the thymine complex has a twisted half-chair onf formation^^^^^'' while that from 1-methylthymine is of a half-~hair:,~~ this difference may be due to disorder problems, however:% In the adenosine complex OsO,(O,C,,H,,N,O,)py, it is the cis 2’,3’-diol o f the adenosine molecule which is so spanned, the nucleoside in the complex adopting a syn conformation rather than the anti arrangement in the free nucleoside.516The complex OsO,(O,C,, H,,)py, * C7H8 is derived from 9-rnethylben~anthracene.”~All of these complexes have O=Os=O units deviating substantially from linearity; this is discussed in the context of the osmyl complexes on p. 583. The IR spectra of a number of OsO2(O2R)L,complexes (L = pyridine, isoquinoline) have been In general (see meaSUred468,497,519and, in some cases, “0 labelling used to help in a~signments.4~~ Table 21) v,,(OsO,) lies near 83 0 m - ’ and ~ ( O S Onear , ) 300cm-’, as in other osmyl complexes. From dienes and trienes. Reaction of Os04and an excess of diene R in the presence of excess or i~oquinoline~~’ gives OsO2(O2R)L2,while a similar reaction but with a 2: 1 mole ratio ofOs0,:diene gives OS,O,(O,R)L,.~~~ It is likely that. Os02(0,R)L2 species have the structure illustrated for gives cyclooctene-5,6cycloocta-l,5-diene in Figure 30(a), since hydrolysis of OsO, (O2CSHl2)py2 diol. Raman and IR data are also consistent with this f ~ r m u l a t i o n . ~ ~ ’
(a)
(b)
Figure 30 Likely structures of (a) OsO,(O,C,H,,)L, and (b) OS,O,(O,R)L,~~’
In the case of Osz0,(0,C,Hl,)py,-4H,0 a crystal X-ray study shows that, as expected, a binuclear osmyl complex is formed with a bridging tetrolato ring (Figure 31).522 The conformation of the central ring is essentially that of a skewed chair-boat form. The osmyl units (Os=O = 1.72A) are substantially bent (OOsO angle = 163.5(7)”) from the ester group (Os -0 (ester) = 1.95 .$) towards the nitrogen atoms (Os-N = 2.19 as in other osmyl complexes (p. 583).
Figure 31 Structure of Os,O,(O,C,H,,)py,-4H,O (reproduced with permission from B. A. Cartwright, W. P. Grifith, M. Schroder and A. C. Skapski, Inorg. Chim. Acfa, 53, L129, copyright 1981 Elsevier Sequoia SA)”’
The formation of 1:1 and 2: 1 Os0,:furan oxo esters in the presence of pyridine and of 2,2’-bipyridyl has been studied kinetically using H NMR.543 There are a few complexes with trienes. Reaction of methyl linolenate and OsO, in excess pyridine gives Os30,(0,C,,H,,)py,~2H,0 in which it seems that each of the three double bonds is spanned ~ ) ~ from ~ ~ , ergosterol, OsO, and pyridine, it seems by an OsO,py, moiety;544in O S O , ( O , C ~ ~ Hmade that only one of the double bonds is so spanned since hydrolysis yields 3,5,6-ergo~tadientriol.~~
’
587
Osmium
From alkynes. Reaction of OsO, and p y ~ i d i n e ~ ’5.2~1 or ’ ~ is~quinoline~~~*”’ yields Osz0,(0,R)L4. On the basis of IR497 and Raman, IR and ‘HNMR5” spectroscopy a tetrolato structure was proposed with the inevitable osmyl unit (Figure 30b)?97,521 (2) Species containing 0s.L units. ( a ) From alkenes. The propensity of OsO, to form 2 : 1 or 1: 1 adducts with nitrogeneous bases (p. 592) led to a study of reactions of such species with unsaturated organic substrates. The L (L = pyridine, isoquinoline, phthaIazine, quinuclidine) react with alkenes R to complexes 0~0,give [OsO, (0,R)L], .s193545 The dimeric formulation derives from an X-ray crystallographic study carried out on Os,04(02C,H,,),(NC7Hl,),, the species obtained from cyclohexene with the OsO,. quinuclidine adduct. The structure is unusual, showing a weak Os,O, bridge formed by one oxo ligand per metal atom of a distinctly bent osmyl moiety ( a = 1.73, mean b = 1.94, c = 1.78, d = 2.22A; Figure 32a).*46The OOsO osmyl angle is 154”; the terminal oxo bond length of 1.73 A is slightly shorter than that involved in the asymmetric brid e (1.78 A), the two bridge distances being 1.78 and 2.22A. The Os-0 (ester) distances (1.90,1.97 ) and 0 (ester)OsO(ester) angles are comparable with those for other oxo ester complexes. In solution the complex is monomeric and is thought to have a trigonal bipyramidal structure with the oxo ligand in the equatorial plane (Figure 32b).
x
Figure 32 Structure of Os0,(02R)L: (a) in the solid state and (b) in
Addition of excess ligand L to OsO,(O,R)L solution gives OSO,(O,R)L,;~~~ reaction of 2: 1 adducts such as C,H,,N,*20s04 formed from hexamethylenetetramine with alkenes R gives a mixture (equation 6).’19
+
~ C ~ H ~ > N ~ ( O S O4R . + ) ~-+
+
~[C~H,,N,.OSO,(O,R)]
O~204(02R)2
(6)
It is a consequence of the unusual structure of these complexes in both the solid (dimer) and solution (monomer) states that the IR and Raman spectra show v(Os=O) frequencies to be higher than in osmyl c o r n p l e ~ e s . ~In~particular ~ ~ ~ ~ ’ the IR band assigned to v,,(OsO,) lies near 890cm-’ in all these oxo esters containing 0 s . L units rather than the 830cm-’ absorption typical of those containing OsL, or Os(L) units; such behaviour is more typicalM’of cis rather than of trans dioxo complexes. Finally we may mention the reaction products of the alkaloids brucine (N,O,C,, Hz6) and strychnine (N,O,C,, H,) with OsO,. Initially red 1: 1 adducts are formed when these are reacted with OsO, (p. 593) but then green osmium(V1) species OsO,(O,R) are formed. Since brucine and strychnine (R) contain alkene linkages it is likely that an internal oxo ester has been formed in which an OsO,-N unit spans the double bond, with the N donor atoms within the molecule being coordinated to the osmium. Models show that such a structure, which would be analogous to that found for Os0,(0,C,H,,)(NC7H,,) (see above), is feasible. Indeed, OsO, with brucine will react with cyclohexene to give [OsO,(02C6Hlo)(brucine)],.”53547a Complexes formed from dienes and alkynes. The quinuclidine adduct OsO, . L reacts with dienes R to give Os,O,(O,R)L,; this is likely to have a structure analogous to that found for (p. 586) but with trigonal bipyramidal coordination about the osmium. With 0s2O4(O4C8Hl2)py4 cyclooctadiene in excess, OsO,-L gives OsO,(O,C,H,,)L for which vibrational IH NMR spectra suggest that only one double bond is spanned by an Os0,L moiety.545 Just as OsO, in excess pyridine reacts with alkynes R to give Os204(04R)py,,OsO,.L gives Os,04(04R)L2;again, there is likely to be trigonal bipyramidal coordination about the osmium.s45
588 (iii)
Osmium Osmium ( VZI)
( a ) Hexa- and pelzta-oxo species. A few of these have been prepared, e.g. M1,[Os06](M = Li, Na),460*547b B ~ , [ O S O , ] ,(the ~ ~ ~IR spectrum of this was measured)*' and M',[OsO,] (M = Na, K).460 On the basis of IR data it was tentatively suggested that K,[OsO,] might in fact be K6[OS20],].462 Magnetic susceptibility measurements were made on Li,[OsO,] from 39 to 251 K.547b ( b ) Tetraoxo species. Recently the salt (Ph,As)[OsO,] has been reported, made by reaction of (Ph,As)I with OsO, in CH,Cl, solution. The grey-green complex has a magnetic moment of 1.37 BM at room temperature and the susceptibility follows the Curie-Weiss law from 4.2 to 140K; (e = 2.6 K). The low moment has been attributed to spin-orbit coupling effects. Cyclic voltammetric couple (E, = 0.1 V vs. studies in CHzClzshow a reversible one-electron process for the 0s~,/0s0,~ standard calomel electrode). The TR spectrum shows bands at 834 and 852cm-' assigned to v1 ( A , ) and vj (F2),and the deformations ti2 and v4 are at 240 cm-' .546 The ion functions as an oxidant for activated alcohols, converting primary alcholos to aldehydes.549 (c) Monooxo species. The oxopentafluoride OsOF, is well characterised and is a discrete molecular species. It is made by reaction of osmium with a 5: 1 F,: 0, mixture at 300 "C under 5 a t ~ n . ' ~ ~ The emerald green crystals melt a t 59.2 and boil at 100.5 "C with a transition point at 32.5 "C.The X-ray crystal structure shows it to have the expected C,, structure (Os=O = 1.74, Os-F (cis) = 1.78(3), Os-F (trans) = 1.72(3) A).,,' It is interesting that the trans fluoro ligand seems to be slightly closer to the metal than the cis. Thermodynamic parameters were collected from vapour pressure data; magnetic susceptibility measurements from 77 to 295 K showed the solid to obey the CurkWeiss law with 0 = 6 "Cat pcff 1.47 BM at 298 The compound OsO,F, is probably polymeric (see p. 594).533
(iv)
Osmium( VZII)
Obviously the chemistry here is dominated by that of the tetroxide. We briefly consider the hexaand penta-oxo species, then OsO,, followed by species of the type OsO,*L and OsO,-L,. The few ,tri-, di- and mono-oxo complexes of osmium(VII1) conclude the section. (see Table 23). Table 23 Osmium(VII1) Oxo Complexes Complex
POI0
see Table 24 Os0,-NH, OS04.PY OsO,.NC,H,, cis-Li,[OsO, (OH),] (OH),] cis-Na, [OsO, cis-Ba[OsO,(OH),] cis-Cs2[Os04F,] Ph4P[Os0,C1]-CHzCIz
917, 885 928, 915, 907, 885 925, 910, 900 92M50 845740 88M20 950-900 910, 895, 885
Spectroscopic und orher properks
X-rays5' Raman, IR, UV/vis (see p. OOO), Mossbauer,'& NMR,583.584 ESCA,SS2 electron diffractions6' IR" Raman, IR4'* Raman, IR?" X-ray634 IR,628X-ray624 1~,628,629
IR, Raman,'',6'9 X-ray6,' Raman, IR462,63' X-ray632
For binuclear species see Table 25.
( a ) Hexa- and penta-oxo species Acid dissociation constants of 'H,OsO,, H,, OsO,+", H2OsO,' and 'H,OsO, H,O' have been published on the basis of measurements of the variation with pH of the electronic spectra of OsO, in aqueous solution.479 The X-ray crystal structure of the salt Sr[OsO,(H,O)] shows this to have a distorted octahedral (C,) structure with Os=O distances varying from 1.72 to 1.93A and an Os-0 (H,O) distance of 2.12A.'" However, it has recently been suggested that this is incorrect and that the salt is Sr[OsO,(OH),(H,O)](OH),.H,O(see p. 592). ( b ) Tetraoxo species Osmium tetroxide, OsO,. This is undoubtedly the most important compound or complex of osmium, and it was as the tetroxide that osmium was first separated and, as explained on p. 522, gave its name to the element. It is industrially important both in the fine organic chemicals industry (for the cis hydroxylation of alkenes) and in biological chemistry and medicine for the 'fixation' or
-
589
Osmium
preservation of biological tissue. Its use for the latter purpose can be dated to 184955'and for catalytic hydroxylation to 1913.ss2It is also an important precursor for other osmium complexes (p590). ~ 'there ~ is a good recent review on the The author has written a short general review on 0 ~ 0 ,and chemistry of its reactions with alkenes, dienes and alkynes.554In this section we describe its preparation, then its physical properties and chemical reactions. Preparation. The oxidation of the metal or of any osmium salts will, if vigorous enough, produce the tetroxide; this is very volatile and can be distilled off or extracted from aqueous solution into ~~~ an inert solvent such as CC1,. Treatment of the metal at 300°C with oxygen gives O S O , , and powdered osmium will also react with aqueous hypochlorite to give it.497Various methods of treating osmium residues to give OsO, have been described: oxidation with chromic acid and a nitric-sulfuric acid mixture followed by steam distillati~n;'~~ oxidation by permanganate in acid s o l ~ t i o nor~ conversion ~~ of residues to a thiourea complex followed by oxidation with HzOzgives 0 ~ 0which, , ~ ~ in both ~ of these latter methods is then extracted into CCI,. Properties. Osmium tetroxide forms pale yellow crystals with a very characteristic odour (a possible description is that of a mixture of ozone and damp hay). It has a considerable vapour pressure at room temperature and so must be kept in stoppered container or (preferably) in sealed ampoules. The vapour is toxic (TLV 2.5 p.p.m.).sS8aThe long-term occupational exposure limit (OEL) is 0.002 mgm-3.558b It is The compound melts at 40.46 & 0.1 "C and there is a transition point at 72.54 0.4 0C.559 slightly soluble in water to give a very pale yellow solution but is very soluble in organic solvents, polar or non-polar (see below). Thermodynamic data of the compound have been reviewed.s60 In the solid and gaseous states it is tetrahedral, with X-ray studies on the solid indicating a very slight distortion (two Os=O distances of 1.68417) and two of 1.710(7)& with OOsO angles of 106.7(4) and 1 10.7(3)").56'It seems reasonable to ascribe these very slight distortions to solid-state effects (in the gas phase the Os-0 distance is 1.711(3)As62).Raman data (see below) suggest that the tetrahedral symmetry is retained in the pure OsO, in solution. The solubility of the tetroxide in water at 25 "C is 72.4 g dm-3 .'@Although earlier studies on the partition coefficient of OsO, from its aqueous solution with CCl, seemed to indicate tht some [OsO,], polymers were present, this is now thought unlikely;M5certainly the Raman spectrum of the aqueous solution is very similar to that of the solid.563There is, however, evidence of aggregation in propane or NF3 solutions at 90 K.566In aqueous solution OsO, behaves as a very weak dibasic Extraction constants for OsO, in a variety of organic solvents have recently been Polarographic and electrochemical studies. Earlier polarographic data for OsO, have been reMost data concern alkaline aqueous solutions; the reductions Os'"' + Os", Osv1+ Os" and sometimes Os" + have been observed, and in oxygen-free alkali the Os" -+ Os" and Os" + Os"' steps have E+ values of - 0.41 and - 1.08V (vs. SCE).56s Spectroscopic data. This attractive molecule has long been a favourite for spectroscopic studies and only the most comprehensive or recent data are given here. The four fundamental vibrational frequencies have often been measured for the molecule; since all four are Raman active but only two (the F2) are IR active we concentrate on the Raman data (Table 24). Table 24 Vibrational Spectra of OsO, (cm-') (A,,'
Gas, 373K Pure liquid Aqueous solution
vz (E)
964
333.1 338
962
333
965.2
v3
(FJ
960.I 953 952
v4 (F2)
Ref
322.7 334 333
569 563 563
IR spectra of gaseous OsiSO,show v3 and v, to lie at 91 1.6 and 3 2 1 . 0 ~ m - ' ;there ~ ~ ~are recent IR data on 1920s04.57' Raman values for v , and v2 are, for solutions in CCh, 964.5 and 335.2cm-' for OsI6O, and 909.7 and 316.5cm-' for OS'*O,.~" There have been many force contact treatments (see for example ref. 573) and determinations of Coriolis coupling constant^."^ The use of transitions within the OsO, molecule as high-precision frequency standards using CO, lasers has been described in a number of publication^.^^'*'^^ There have been detailed studies on the electronic absorption spectrum of the gas, and the magnetic circular dichroism spectrum has also been recorded and a ~ s i g n e d ; " ~such * ~ ~spectra ~ * ~ ~were ~ also obtained for the aqueous solution.5s' The electronic spectrum of the solid in an argon matrix at 20 K was measured and assigned.580An energy Ievel diagram was constructed on the basis of the
590
Osmium
gas phase There are also studies on the photoelectronic spectra of gaseous 0 ~ 0 , and ’~~ the ‘890sMossbauer spectrum was ~ e c 0 r d e d . lUsing ~ ~ molten OsO, a very weak Ig70sNMRresonance was Chemical reactions of OsO, . We concentrate on the important oxidation reactions. (a) Oxidation of alkenes. The subject has been recently and comprehensively reviewedss4so we give only an outline here. The first observation that OsO, reacted with alkenes was apparently made in 1894;ss’by 1913 Hofmann had established that OsO, could be used catalytically for the cis hydroxylation of alkenes with C10,- as cooxidant.ss2However, it was Criegee who, in three classic papers~80~481~528 laid the foundations of our knowledge of this important reaction. He showed that OsO, would react with alkenes R to give ‘OsO,(O,R)’ and in some cases OsO(0, R)2.The reaction was greatly accelerated Hydroloysis of any of these gave by addition of bases such as pyridine to give OsOz(0,R)py,.48’~528 the cis glycol R(OH),. It is now known that ‘OsO,(O,R)’ is in fact dimeric, Os,O,(O,R), (see p. 5 85).m’3s2y The reaction of alkenes with OsO, is commonly thought to involve a concerted six-n-electron cyclization leading directly to a five-membered ring (Figure 29a), accounting for the exclusively cis addition to alkenes. However, S h a r p l e ~ s has j ~ ~ suggested that the direct nucleophilic attack of carbon on oxygen that this entails is unlikely and that, moreover, the intermediate (Figure 29a) involves an uncomfortable angular strain on the ester ring. The proposed mechanism involves initial formation of an asymmetric oxametallacycle intermediate via initial nucleophilic attack of the alkene at the metal rather than at oxygen.586This then rearranges and could dimerize to give (b) in The scheme also accounts for the observed4” rate increase in the OsO,-.alkene reaction Figure 29.586 consequent on addition of pyridine: attack by the latter would induce a reductive insertion of the Os-C bond of the oxametallacycle into an oxo group giving an ester which then reacts with more pyridine giving Os0,(0,R)L,.s86 Unequivocal direct evidence for this appealing mechanism is so far l a ~ k i n though ~ ~ ~intermediates ~ * ~ [OsO,.LI2, albeit in dimeric form, have been i ~ o l a t e d , ’ and ~~,~ there is a report of the existence (but not the isolation) of a complex of OsO, with adamantyliden e a d a m a ~ ~ t a neKinetics .’~ of the reaction of OsO, with alkenes in the absence of nitrogenous bases have been as have those of the Os0,-alkene reaction in the presence of amm~nia,’~’ pyridine,537-592,593 substituted pyridine^,'^' 2,2‘-bipyridy1s37~s38~s92~594 and TMEN.594The Os0,-alkene reaction is also substantially accelerated by meth~lamine,’~’ cyanide’” and thi~cyanate.’~~ Although OsO, is thought of as a specific oxidant for cis alkenes there are a few examples of hydroxylation of trans alkenes, e.g. cb- and trans-cyclodecene can be hydroxylated to the respective cis and trans diols.597The OsO,-alkene-pyridine reaction can be made catalytic by the use of suitable secondary oxidants such as H 2 0 2 ,C10,-, Bu‘OOH, N-morpholine N-oxide, IO4-, O2and C10- .554 (b) Reaction with dienes and trienes. Criegee noted that ergosterol could be oxidized to 3,5,6-ergostadientriol by OsO, and pyridine and he isolated the intermediate ester?’ there are scattered reports in the literature of the cis hydroxylation of dienes and trienes, e.g. norbornadiene to exo-cis-norborThe nane-2,3-di01~~~ and of cis,trans- 1,5,9-~yclododecatrieneto 8-cyclododecane-trans-1,2-di01.’~~ kinetics of reaction of Os0,-pyridine and Os0,-bipyridyl to furans have been studied.s43For a discussion of the oxo esters formed from OsO, and dienes, see p. 586. (c) Reaction with alkyne^."^ The first to react OsO, with alkynes was Phillips in 1894,58sand an early analytical method for osmium was based on the reaction of OsO, with acetylene.6wAgain it was Criegee who in 1942 noted that acetylene reacted with Os0,-pyridine to give an ‘ester’.4goLater work showed that alkynes R (acetylene, phenylacetylene, methylphenylacetylene, diphenylacetylene) with Os0,-pyridine gave Os,O,(O,R)py,; on hydrolysis the esters of the latter three alkenes gave benzil, benzoic acid and 1-phenylpropane-1,2-dione respectively. The reactions can be rendered catalytic by use of an OsO,-KClO, reagent.521The oxo esters derived from alkynes are considered above (p. 587). (d) Reaction with glycols and catechols. It has been claimed on the basis of electronic spectra that OsO, reacts with ethylene glycol, pinacol or trimethylethyleneglycol (R) to give Os02(OH),(0,R);60’ kinetics of the oxidation of ethyleneglycol to formaldehyde by OsO, have been reported.m2Certainly ) ~ , in the presence of the prolonged reaction of OsO, with ethylene glycol gives O S O ( O , C ~ H ~and With catechol or substituted catechols (Rcat), species pyridine OsO, (OC2H,(OH)),py2 is f0rmed.4~~ of the form Os(Rcat),, [OsO(Rcat),], and tran~-[OsO,(Rcat),]~-can be isolated (see p. 597).486 (e) Reaction with amino acids, peptides andproteins. The direct reaction of OsO, with amino acids can be difficult owing to the facile oxidation of the NH, but recently a number of osmyl complexes OsO,(an), have been isolated from amino acids and OsO, (see p. 583), and an X-ray crystal structure of OsO,(glycinate), has confirmed the osmyl formulation. Earlier studies, perhaps carried out under less rigorous conditions, indicated that amino acid oxidation occurred when
Osmium
59 1
0s045u-592q6” or Os0,-pyridines44~603 was used. Blocking of the reactive NW2group and esterification of the carboxylate group give complexes of the form Os,O,L, and the kinetics of reaction of OsO, and pyridine with 1-methy-a-N-acetyl-m-tryptophan were studied; attack is across the double band , ~ ~ some cleavage may of tryptophan.592Peptides seem relatively unreactive towards 0 ~ 0though occur.6o3 The reaction of OsO, with proteins is a complex subject needing much further study. The evidence so far suggests that OsO, can cross-link and that the tryptophanyl residue in particular may be i m p l i ~ a t e dIn . ~biological ~ tissue, where OsO, reacts with membranous material containing both unsaturated lipids and proteins, there is evidence for lipid-protein cross-linking (see below).544m4 (f) Reaction wirh nucleosides, nucleotides and nucleic acids. The literature up to 1977 has been r e v i e ~ e d .Of ~ ~the ~ nitrogenous ,~~~ bases in nucleic acids the ones most likely to react with OsO, are the pyrimidines, and indeed there has been much work on its reaction with thymine and its d e r i v a t i v e ~ , 4 ~adenine ~ ~ ~ ~ ~derivatives,537 ,~’ cytosine and 3-methylyto~irte.’~~ A sequence-specific osmium reagent for thymine-cytosine pairs has been devised.607Nucleosides on their own react slowly ~ ~2,2’-bipyridy1537,S38 .~” the reaction is with OsO, but in the presence of donors such as p y ~ i d i n e ~ or much faster, giving osmyl species Os0,(02R)py, or OsO,(O,R)bipy. Reactions of this type are observed with thymidine,m cytidine,538adenosines3’ or substituted adeno~ine.’~’ We have already discussed the crystal structure of the adenosine complex Os02(0,C,oHl,Ns0,)pyz(Table 20 and p. 586).’16 In the presence of pyridine, nucleotides react with OsO,; kinetic data were obtained for the reaction of desoxythymidine-5’-monophosphate(dTMF),608and other nucleotide-0s0, reactions were also i n v e ~ t i g a t e d . ”Polyuridylic ~~~ acid (poly U) is easily oxidized by OsO, with ammonia,591 and direct visualization of individual osmium atoms in polyuridylic acid treated with Os0,-bipyridyl has been achieved (shown in the paper in a remarkable series of photograph^).^^ In in vitro studies there is little observable reaction between OsO, and native deoxyribonucleic acid (DNA), but it does react with denatured DNA, in which the thymine bases have probably been hydroxylated to 5.6-dihydroxythymine via an osmium(V1) (g) Osmium tetroxide and biological tissue. The oldest use of OsO, is for the ‘fixation’ or preservation of biological tissue and dates back to 1849,5s’although in 1804 Tennant’ had noted that it stained the hands; there is a considerable literature on the subject.611Osmium tetroxide is unique in having a dual role: applied as a dilute aqueous solution to tissue followed by dehydration and mounting procedures it both preserves the biological material and stains certain parts, particularly membranes, with an osmium-containing material. These stained parts are visible in optical microscopy as black or grey areas but also, by virtue of the electron-scattering power of osmium, delineate those areas in electron micrographs. It has been demonstrated that the uptake of OsO, by membranous material in particular is related to the content of unsaturated phosph0lipids,6’~ and this with other evidence has led to the suggestion that cross-linking of double bonds in such lipids could 0ccur497,6 13.6 14 by ‘monoester’ or ‘diester’ formation (p, 584). An earlier suggestion that simple ~ ’is~also possible that proteins and hydrogen bonding of OsO, was involved“” has been d i ~ p r o v e d .It unsaturated lipids are cross-linked in membranes by OsO, to give osmyl species;”604 the presence of osmium(V1) as well as osmium(1V) and osmium(I1) in tissue has been demonstrated by ESCA data,616though such data have to be treated with some caution.617It is also clear that OsO, reacts with catechols (’phenolics’) in plant tissue to produce a very deep stain, probably arising from [ O s O ( ~ a t ) , ] , , 4and ~ ~ ~will ~ ~ ~almost certainly stain alkaloid^.^^^'^^^ Miscellaneous sectiuns of OsO, There is a recent review on the role of OsO, in the catalysis of redox reaction^,^'^ and the Os0,xatalyzed decomposition of HzOz is thought to proceed via peroxo With C032- or SO,’- in aqueous solution OsO, gives [OSO,(OH),]~-, but with a ) ~ ][0s(SO,),l8~are formed.621 mixture of S 0 3 z - and C032- it is said that [ O S O ~ ( S O ~and
( v ) Tetraoxo osmium( VZII) complexes with other ligands
( a ) Oxo hydruxo and oxo aqua species. It has long been known that OsO, will react with alkali metal hydroxides to give deep red solutions from which unstable solids can be isolated (called ‘perosomates’, ‘perperosmates’ or ‘osmenates’ in the older literature). Early workers made M’2[O~04(OH),](M= K,622,623 Rb,623 Cs622.623 ) and B~[OSO,(OH),]~~~; the ammonium salt was also claimed622(though it might be thought that (NH,)[OsO,NI would be the product of the reaction of OsO, and NH,OH). By using a deficiency of M‘OH with OsO,, M’[OsO,(OH)] (M = Rb, C S ~ ’and ~) Cs[Os2(OH)0,]623were claimed. Recent X-ray work has shown the existence of c i s - [ O ~ 0 , ( O H ) ~ ] ~ - , [Os,(OH)O,]- and of {Os05(H20)]-also.
Osmium
592-
The X-ray crystal structures of cis-Liz[Os04(OH)z],624 c ~ ~ ~ N ~ , [ O ~ O , ( Oand H ) , ]cis~~~ M”(OsO,(OH),] (M = Ca, S f Z 6arid Ba626*62’) have been given, as have improved procedures for the Rb, Cs629.630). The anions have cis conisolation of cis-MI, [OsO,(OH),] (M = Li,628Na, f i g u r a t i o n ~as , ~predicted ~~ by earlier Raman In Li, [OsO,(OH),] the Os-0 (OH) distance is 2.15A with mean Os=O 1.75A;62.’in the sodium salt there are two Os-0 (OH) distances of 2.10 and 2.178, with the Os=O bond lengths varying from 1.74 to 1.82 in the barium salt there is disorder between one 0-0 (OH) and Os=O bond, while the other Os-0 (OH) is 2.15A and the mean Os=O = 1.73 It has been suggested that the species formulated as Sr[OsO,(H,O)] and for which a crystal structure has been reported (p. 588) is in fact Sr[Os03(OH),](OH)-2H,0 or Sr[Os03(OH),(H,0)](OH),~H20.629 For M,[Os,O,(OH)] (M = Rb, Cs) see p. 596. The [OSO,(OH),]~-ion has been implicated in the oxidation of ethylene glycol by OsO, in alkali, and a mechanism proposed for this reaction involving the [OsO,(OH),(OCH2)20H]3-ion.602 ( b ) Oxo halo complexes. The yellow Cs,[OsO,F,] has been made from excess CsF and 0 ~ 0 and also the rubidium salt;622its Raman and IR spectra suggested a cis structure for the ani0n.4~~”’ More recently, both it and Rb2[Os04Fz]have been made from RbF and O S O , . ~A~ ’salt of approximate composition Cs[OsO,F] has been obtained from OsO, and CsF; its Raman and IR spectra suggest that it may contain pentacoordinate osmium.631 The isolation and X-ray crystal structure of (Ph,)[OsO,Cl] * CH, CIzhave been reported. Reaction of OsO, in CH2C1, with (PPh&l gives the orange-red crystals. The anion is trigonal bipyramidal with the chloro ligand axial; the mean Os-0 distance is 1.72A and the Os-C1 distance is very long at 2.76(2) ( c ) ‘Adducts’ of OsO, with N donors. Although ‘ O S O , . ~ ~ is , ’now ~ ~ ~known to be o s ~ o & y 4 ~ w there are loose 1 : 1 and 2:1 adducts of OsO, with N-donor ligands. These are unusual in that they appear to have very weak, long Os-N bands such that the symmetry of the OsO, fragment is essentially undistorted and retains the reactivity of OsO, (see Figure 33 for structures of OsO,.NC, H,, and [0sO4],.N4 C, H,,634). 0
n
0
0
(a)
I b )
Figure 33 Structures of (a) OsO,*NC,H,, and (b) (OSO,)~*N.,C,H,~ (reproduced with permission from W. P. Griffith, A. C. Skapski, K. A. Woode and M. J. Wright, Jnorg. Chim. Acta, 31, U13,copyright 1978 Elscvier Sequoia SA)”‘
They are made by reaction of OsO, in aqueous solution or emulsion of the amine L to give Os04-L (L = p y ~ i d i n e , ~ ~NB, , ~ ~,66*333 ~ , ”pyridazine, ~ quinuclidine (NC, H,, ), phthalazine and isoL (L’ = hexamethylenetetramine q u i n ~ l i n e , ”hexamethylenetetramine ~ mandelate635)or Os2O8* (N, C6HI2),1,4-diazabicyclo[2.2.2]octane, 5-methylpyrimidineand pyrazine).’lg With the exception of the pyridine, pyrazine and 5-methylpyrimidineadducts these produce little or no vapour pressure of OsO, at room temperature and so can be used as ‘safe’ forms of OsO,; indeed, [OSO,]~.N,C,H,, is now commercially used in histochemistry as a replacement for O S O , . ~ ~ ~ The X-ray crystal structures of the quinuclidine and hexamethylenetetramine adducts show very long Os-N bonds (2.37 and 2.42A respectively) while the symmetry of the OsO, unit is little disturbed: the Os-0 bond lengths are between 1A97 and 1.722A,and the O(axial)OsO(equatorial) angle of 100.6 is close to the tetrahedral angle of 109.47O (Figure 33).634
~
~
~
Osmium
593
The reluctance of the remaining two nitrogen atoms in hexamethylenetetramine to coordinate may be due to unfavourable angles at these atoms brought about by coordination of the other two donor ~entres.''~ It has been noted that a wide variety of alkaloids L containing one or more N-donor atoms will react with OsO, to give red solutions which almost certainly contain OsO,.L adducts (L = strychnine, brucine, quinine, sparteine, cinchonine, atropine, yohimbine, cocaine, emetine).545%547 In the cases of strychnine and brucine an internal reaction subsequently occurs to give green osmium(1V) complexes OsO,(O,L) (see p. 587). There is also evidence for the formation of 1 :1 OsO,. L adducts with blocked amino acids (e.g. z-N-benzoyl-L-histidine, a-N-benzoyl-DLmethionine) and imidazoles.s44 NC,H,, with alkene^,"^-^^'-"^ dienes545and alkyne^^' are considered on p. The reactions of 0~0,. 590 and those of [Os0,j2*N,C,H,, with alkenes54son p. 587. 6.2.1.4.3. Trioxo osmium( VIII) complexes The salts M',[OsO, F,] are made by reaction of OsO, and KBr, CsBr or AgIO, in a 1:1 mole ratio with BrF, ,533 or from M F (M = K, Rb, Cs) and Os0,F2.631 The IR and Raman spectra of the solid cesium salt suggest a fac structure$62perhaps indicating that 0,- is a stronger n: donor than is Ffor osmium(VTI1) (there are also more recent vibrational data6,,). The postulated dimer [OsO,F,], is considered on p, 596; the monomeric matrix-isolated OsO,F, has been shown by Raman and IR spectra to have a trigonal bipyramidal (D3J structure.636 Two possible trioxo intermediates have been mentioned: [OsO,(OH), {O(CH2)20H}]Z-, from reaction of ethylene glycol with OsO, in base,602and OsO,(O,R), thought to be formed when monoesters Os,O,(O,R), are oxidized with HzOz.s2a 6.2.1.4.4. Dioxo and monooxo osmium( VIII) complexes. Reaction of glycols R(OH)* with OsO, is said to give Os0,(OH),(02R) as an intermediate.@' There is no unequivocal evidence for the existence of these, though in view of the remarkable ring the existence of osmium(VII1) analogues seems feasible; a reductive stability of the OswOO,R route from alkenes is not of course possible. Two possible candidates, OsO,(O,R) and Os0,(OH),(O,R), are mentioned above and on p. 590. The acid dissociation constant of 'OsO,(OH),' has been given.479
46.6.2.2
Binuclear oxo-bridged complexes.
A number of these are known (see Table 25), particularly with osmium(IV) and osmium(V1). It is clear that the bridging oxo ligands are functioning as n: donors, particularly in the p o x 0 osmium(1V) dimers. Table 25 Binuclear Oxo-bridged Complexes Complex
v, (Os, 0 )
cS4[oS~oc1m1 852 (Me4N)4[O%OC&Br,] 846 (Et4N)4[OszOBriol 846 Os, O(dppm)2C1,+2CHC1, K2 [OS10(q4-~hbd-Et)2(OPPh,)z OS,O(OAC)~CI, (PPh,), Et, 0 os2 O(OEP)z (OH), 2'' 04(02c2 98 I Rb[Os2(OH)O81 960-800 CS[OS, (OH)O8I 960-885 [ 0 ~ 0 4 1 zN4 * C, Hn 930, 917, 909
v, ( 0 3 2 0 )
Spectroscopic and other properties
225 223 237
Raman,MO*M'.65' IR,462.M0 U V / V ~ S ,X-raya' @~ Raman, IRa2 Raman,&' lRmm UV/vis, CY, X-rayM7 X-raya8 CV, ESCA, X-ray"' UV/vis, CV, 'HNMR650 IR;" 'H NMR,536X-rap'.529 I R , ~ x-raf5' '~ 117:~~X-rayag Raman, IR?" X-ray6%
-
VJOSO2)
OS2O6PY4 %[0s206(N02)41 0s204(02C6H10)2(NC~H~3)Z Os,O(OEP),(OMe), For Os2O4(0,C2Me4~ see p. 000.
833 84 1 903
V,l~SO,) 87 5 884 898
V(O~*O,)
640,448
Raman, IR,468.wX-raY Raman, IR,w X-rayso2 Raman, IR,%'X-rayM X-rayM
594 (i)
Osmium Osmium(II) and osmium(II1)
The complex [Os'", O(bipy), (terpy>,](ClO,), has been claimed (made from reaction of [OsCl(bipy)(terpy)]Cl with HClO,); reduction with Na,S,O, is said to give [Os,O(bipy),(terpy)2](C10,),, while oxidation with ceric ammonium nitrate gives [O~~O(bipy)~(terpy)~](ClO~)~.'~~ These are the only 0s"'''' p-oxo complexes so far reported, but iron forms Fe"'-O--Fe"' and Fe"' -N-Fe'" moieties.
(id)
Osmiuw(W)
( u ) Halo complexes. The most comprehensively studied example is [Os,OCl (isoelectronic with [Os,NCI,,]'-, see p. 563). A number of salts containing [OS(OW)C~,]~(e.g.M12[Os(OH)C15]; M = Na, K, Rb, Cs and some
organic were reported in the earlier literature, but these almost certainly should be reformulated as M',[Os,OCl,,]. There is evidence for the existence in solution of the [0s(OH)Clj]'ion, however (see p. 615). The potassium, cesium and ammonium salts can be made by reaction of OsO, in concentrated HCl with OsO, in acid, in the presence of FeSO, functioning as the reducing agent.63w The solids are diamagnetic639(this was one of the features which suggested that they were not in fact salts of [Os(OH)C1,IZ-,for which paramagnetism would be expected). The single crystal X-ray structure of Cs, [Os,UCI,,] shows the anion to have a p-oxo bridged structure with the equatorial chloro ligands angle at 180 c; the in the eclipsed position. The Os-0 distance is 1.778(15) A with the Os-0-0s cic Os-C1 distance is 2.370 A and the frans Os-C1 distance is only slightly longer at 2.433 A.M1Tt is instructive to compare the Oslv-O-Os'v grouping with the OS'~--N-OS'~moiety (see p. 615); clearly the nitrido ligand has a more powerful trans weakening effecl, for the [Os,NC1,0]5-ion has only a tenuous existence, the trans ligands being easily replaced by water to give [Os,NCI, (HzO),I3-.334.349 The structural parameters for [OszOC1io]4-can be compared with those found for K,[Ru20Cllo]and K3[Ru,NCl,(Hz0)2j;the Ru-0 distance is 1.808, with a 0-Ru-Cl angle of 94 ',@* while the Ru-N distance is 1.720(4)A with an N-Ru-CI angle of 94.7(2)0.3s' Raman studies have been carried out on the potassium,640cesiumw and ammonium and on the [ O S ~ O C ~ion ~ ~in] ~ acid - solution;@3resonance Raman enhancement was observed for the v I (symetric Os-0-0s stretch) mode.640a3IR data have also been reported.462.640 A molecular orbital scheme for the bonding in [OS,OC~,,]~can be derived from that proposed by Dunitz and OrgelW for the analogous [ R u ~ O C ~s p~e~~ ]i e~s . 6 ~Such ~ , ~ a' scheme has been used to interpret the electronic .&13 and vibrational spectra of [OS~OCI,,]~ A few other pox0 halo complexes have been reported but incompletely characterized, viz. salts of' [OS,OC~,B~,]~,*',[OS;OI,,]~ and [ O S ~ OI , C ~8I,'. ] ~ -The species [OS~OC~,(H,O),]~is thought ~ ] ~ -[OS~~~C~,(H,O),]~in acid solution.489 to be a product of the reaction of t r ~ n s - [ O s ~ ~ O, C 1with There is a recent report of the X-ray crystal structure of trans-Os,O(OEP),]OMe),, a deep violet material made by reaction of Os(OEP)(CO)(OMe)and 2,3-dimethylindole in dichloromethane. The Os-0-0s skeleton is essentially linear (Os-0-0s = 178", Os-0 = 1.808(3)A); the octaethylporphyrin (OEP) ligands are twisted at a dihedral an le of 23 ' but are planar (mean OsN = 2.033(78)A); the Os-0 (OMe) distance is 1.997(29) .646 (bj Other p o x o osmium(W) complexes. Reaction of OsC1, in methanol with dppm (Ph, PCH, PPh,) followed by recrystallization of the product from ethanol yields Os,O(p-dppm), C1,.2MeCI, in which a linear Os-0-0s unit (Os-0 = 1.792(1)?1) is spanned by two bridgmg chelating phosphine ligands, the chloro ligands occupying equatorial (cis Os-C1 = 2.387(3) A>and axial (trans Os-C1 = 2.307(3) A) positions. The Os-P distance is 2.443(4) A. As in Cs,[0s2OC1,,] the configuration is eclipsed; unlike the interaction in the latter complex, though, the chloro ligands trans to the oxo ligand are slightly closer to the metal than are the cis. The OOsP angle is 85.51(8) ', brought in presumably by the strained bridge, while the OOsCl (cis) angle is 87.57(8) '. The complex could be electrochemically oxidized (presumably to a mixed Os'"''' species) and reduced in two one-electron steps.@' Reaction of the tetradentate N,N.O.O ligand 1,2-bis(3,5-dichloro-2-hydroxybenzamido)ethane (abbreviated as H,chbaEt; see p. 616) with trans-K,[OsO,(OH),] in ethanol yields the osmyl complex K,[Os0,(q4-chbaEt)], which can then be reduced by PPh, to the poxo-bridged species K,[Os2O(q4-chbaEt)(OPPPh,),)64" As in Cs,[Os,OCl,,] the equatorial ligands are eclipsed, but the Os-0-0s bridge is not quite linear [OsOOs angle = 175.5 ', Os-0 (bridged) = 1.795 A>. The trans Os--0 (PPh,) distance is 1.971 8, so there is no obvious trans lengthening effect.&*,
R
595
Osmium
Another p-oxo complex containing a bent bridge is Os2O(OAc),CI,(PPh,),, made from transOs02C1,(PPh,), and acetic anhydride (other complexes, such as Os,O(OCOR),X,(PR;),, R = Me, Et; X = C1, Br; PR; = PPh3 or PEt,Ph, were made in similar fashion). In Os,0(OAc),C14(PPh,),.Et,O the Os-0-0s moiety (OsOOs angle = 140.2(4)", Os-0 (bridge) = 1.830(10)1$) is spanned by two bridging acetato ligands (Os-0 = 2.083(9)), and it is presumably these which cause the very substantial bending of the OsOOs unit and consequent lengthening, in comparison with the other systems, of the Os-0 bridge distance. The complex is diamagnetic, and the Os...Os distance is 3.440(2)A. The equatorial planes, as viewed down the Os...Os axis, are not eclipsed due to the strain imposed by the bridging acetato ligands. Both this and the other Os,O(OCOR)X, (PR;), complexes exhibit a reversible one-electron reduction, and the purple paramagnetic osmium(IIT)/(IV) complexes M [ O S ~ O ( O A C ) ~ C ~ ~were ( P P isolated ~ ~ ) ~ ] (M = Na, Ph,As); pcKwas 1.3 BM for the sodium salt and 1.7 BM for the [Ph,As]+ salt.w9 A number of p-oxo osmium(1V) porphyrin complexes with octaethylporphyrin (OEP) have been obtained by reaction of Os"(OEP)(CO)(MeOH) with air and 2,3-dirnethylindole in dichloromethane. The product is the dark purple Os,O(OEP),(OMe),; the methoxo group is labile with respect to substitution and both Os,O(OEP),(OH), and Os,O(OEP),(OMe)(OH) can be made. The complexes are thought to contain a linear bridge with the methoxo or hydroxo ligands trans to the oxo bridge. Cyclic voltammetric studies show that Os,O(OEP),(OMe), can be reduced to an OS'" -0s"' and then to an OS'~'-OS'~'dimer.650 The dimeric species Os,0C16, made from OsO, and HC1, has been isolated; IR and mass spectral data were obtained.''' The so-called 'OsBr,' may be O S ~ O B ~ ~ . ~ ~ ~ ~
(iii) Osmium(V)
I
The so-called Os,OCI,, made from osmium metal and a 15:l chlorine-xygen 700 0C,s27is thought from its IR spectrum to be OsOC1, (p. 584).525
mixture at
(iv) Osmium (VJj
A number of these are known. Most fall into two categories: one, of the type Os206L4(A) contains trans dioxo osmyl moieties, and the other 0s,04L4(B) contains one terminal oxo ligand per osmium, as in the 'monoesters' already treated on p. 584. In both cases the two metal atoms are linked by a p,p'-dioxo bridge in which the Os-0 distance is typically ca. 1.95 A. There is clearly system found in osmium(IV) far less 7t bonding in this bridging system than in the linear Os-0-0s species; this may be a result of the fact that two of the three orbitals suitable for such bonding are used for n bonding to the terminal oxo ligand in these osmium(V1) systems, whereas in the osmium(1V) species two such orbitals are available for n bonding in the Os-0-0s bridge. The compound Os206py4, prepared by the reaction of OsO, with pyridine in the presence of a little ethanol as reducing is formulated in the older literature as O~O,py,4*~ (the complex ' 0 ~ 0 , * p y , ' ,made ~ ~ ~ from the slow reaction of OsO, with pyridine, is almost certainly Os2?6py,470). The single crystal X-ray shows it to be a dimer, as previously proposed on the basis of vibrational of type spectroscopic The osmium is octahedral with trans osmyl linkages (Os=O = 1.73 A, the osmyl OOsO angle being bent away from the Os,O, bridge at 163.5(8)"); the Os-N (py) distance is 2.20(2)1$, and the mean Os-0 (bridge) distance is 1.94 A.469 There is evidence4" that, in solution, the dimer is in equilibrium with monomer (equation 7) and there may be such equilibria between monomer and dimer forms with Os,O,L, (L = 3-picoline, 3-~hloropyridine),~~' 2,2'-bip~ridyl,"~~ imidazole,544 1,10-phenanthroline4% and N , N , N , N tetramethylethylenediamine (TMEN)496).Species of the Os206L4 type have also been found with imidazoles and x-N-benzoyl-L-histidine isobutyl ester.544
-
Another example of the type (A) structure is K , [ O S ~ O ~ ( N O 6H20, ~ ) ~ ] a deep brown crystalline material obtained by prolonged reaction at room temperature of KNO, with OsO, in aqueous solution. It was long formulated as 'K,[OSO~(NO,),]-~H,O'~" and is a useful precursor for other osmium(V1) and osmium(1V) complexes since it releases NO, with acids. A single crystal X-ray study showed it to be of type (A) with the osmyl Os=O distance 1.79(l)A and the osmyl OOsO C.O.C. 4 - T
596
Osmium
angie 164.1 bent awa from the Os,O, bridge; the Os-N distance is 2.17(2)A and the Os-0 bridge distance is 1.99 with an OsOOs bridge angle of 1020.502 The species formulated478as ‘Na4[02(0s0,(0H),},]~nH20,a product of reaction of OsO, and NaOH in water-ethanol, could presumably be reformulated as Na,[Os20,(OH),] .nH20. An X-ray crystal structure of a type (B) complex, Os20,(0,C2Me4), ,is discussed on p. 584. Other binuclear ‘oxo ester’ complexes which have been studied by X-ray methods are the ‘tetrolato’ species O S , O , ( O ~ C , H , ~ (see ) ~ ~p.~586 ,~~ and ~ Table 21) and the species Os204(02C6H,o),(NC,H,,)2,546 (p. 587 and Table 25). It is known too that some complexes obtained from OsO, and nitrogenous bases are binuclear (p. 592). O,
(v)
8:
Osmium( VZZ)
The yellow-green OsO,F,, made by reaction of a 1: 1 mixture of OsO, with OsF, at 750 “C, is probably a tetramer or an infinite chain structure with single fluoro bridges.’”
(vi)
Osmium (VZII)
The binuclear species M’[Os,(OH)O,] (M = Rb, Cs) are prepared by reaction of a deficiency of M’OH with 0s04.629*630 The X-ray crystal structures of the rubidium65’and cesium629salts show the osmium atoms to have a trigonal bipyramidal configuration with the bridging hydroxo ligand in the apical position. The Os=O distances vary from 1.66 to 1.75A in the rubidium salt, with the Os -0 (OH) distance 2.16A and the OsOOs angle 153°.65’In the cesium salt Cs[Os,(OH)O,] the structure is similar (Os=O axial = 1.71(3), mean 0=0 equatorial = 1.67 A) linked by a p-hydroxo ligand constituting the remaining axial positions (Os-0 = 2.22(2) A>in an essentially symmetrical but bent bridge (OOsO angle 133(1)0)~*9 is made by reaction The compound OsO, F2(indistinguishable by X-ray methods from 0 s 0 2F3)533 of osmium metal with a 2: 1 oxygen-fluorine mixture5’, or from OsO, and BrF,.”, There is much doubt about its structure, but the compound made at normal temperatures exists in at least three different crystalline forms and polymeric structures seem more
46.6.3 Dioxygen, Superoxo and Peroxo Species Surprisingly, no non-organometallic complexes containing these ligands appear to have been isolated,6% though peroxo species in particular might well be involved as intermediates in some The complex Os(O,)(CO),(PPh,), reactions (e.g. in the Os0,-catalyzed decomposition of H, OZQo). is made by slow reaction of O2with Os(CO)z(PPh,),,655while Os(O,)(CO)HX(PCy,), (X = CI, Br) complexes are made from O2and Os(CO)HX(PCy,), . With SOzthe chloro species gives a sulfato complex.656It is likely that an u2-peroxo (or ‘dioxygen’) complex is implicated in ligand oxidations at organic centres, e.g. oxidation of Os(CO)C1(NO)(PPh3), to Os(C0,)C1(NO)(PPh3), by o~ygen.‘~’
46.6.4 Akoxo Complexes A few alkoxo complexes are known with phosphine coligands, and the methoxo species bransK2[0sO2(OMe),] is quite a useful synthetic reagent (see p. 584). Reaction of (Bun,N)[OsNC1,] with PPh, in methanol gives Os(PPh3)2C1,(OMe),,3’5 and Os(OMe,Ph),Cl, and Me,N in ethanol yields the yellow 0~(PMe~Ph),C1,(0Et).~~ The ethoxo species fac-Os(PMe,Ph),(S, CNMe,)(OEt) is made from mer-Os(PMe,Ph), (SzCNMe2)C1 and K(S,COEt) in ethan01.~”
46.6.5 8-Diketonate Complexes
As might be expected for chelating 0 , O donor ligands it is the intermediate trivalent state of osmium which is stabilized by P-diketonates (as is the case for catechols, p. 597, and tropolonato complexes, p. 597). In the presence of macceptor chelates such as bipy, phen and terpy the I1 state is favoured, while with halides the IV state is preferred. The apparent absence of an osmyl complex, e.g. OsO,acac,, is surprising.
I Osmium 46.6.5.1
597
Osmium( I l l
Although it has not been isolated, the product of polarographic reduction of Os(acac), is probably [Os(acac),]- .6s8 Reaction of Os(PPh,), H4and P-diketonates L gives Os(PPh,), L, (L =,XH(CO), made from SO2 in b e n~ e ne . ~ " and OS(PC~,)~XH(CO)
46.7.7
Di- and Mono-thio-j?-diketonates
As with much of osmium sulfur chemistry the data are scattered and incomplete; the best characterized complex is the dithioaetylacetonate complex Os(sacsac), .
46.7.7.1 Dithioacetylacetonate (4-thio$ent-3-ene-2-thione, S, C,H,-
,sacsac)
The reaction between OsO, and acetylacetone with H2S gives dark brown glossy plates of Os(sacsac),. IR and electronic spectra were recorded for the complex, as well as the ESR spectrum.'" The magnetic susceptibility from 110 to 290K shows that pfff is 1.65BM at the latter temperature."' Polarographic reduction of M(sacsac), (M = Ru, Os) shows reversible reduction at - 0.01 and - 0.151 V (vs. Ag/AgCl e l e c t r ~ d e ) , ~while ~ ~ ~for ~ "Os(acac), the potential is - 1.244V,hSs i.e. Os(sacsac), is more easily reduced than Os(acac),, consistent with the 'softer' nature of sacsac as against acac, but less easily reduced than Ru(sacsac),, again as to be expected since ruthenium is a second row element. A dark green complex, Os(S,CsH,), , is made from I-ethoxy- 1-thiolobut-I-ene-3-thione and [OsC1,]2- .7'2
46.7.7.2 Monothio-/I-diketonate complex Monothiobznzoylmethane reacts with [OSCI,]~-in aqueous ethanol to give the black complex Os(SOC,jHj2)3, which has a magnetic moment of 1.66 BM at room temperature. The same prepara-
Osmium
607
tion but with sodium acetate yields the black Os(SOC,,H,,),, with a magnetic moment of 1.89 BM.713
46.7.8 Monothione Complexes For the imidazolidine-2-thiones, which have received the most extensive study in this class of complexes, the monosubstituted imidazolidines stabilize osmium(II1) while the disubstituted ones stabilize osmium(1V) and o~mium(VI).~'~
46.7.8.1 Imidazolidine-2-thiones
The reaction of (I) or (11) with OsO, in aqueous ethanol gives [OsL, J 3 + (L = (I), R = H, Me, (L = 11, R = Me, Et); IR and electronic spectra were measEt)7143715 and trans-[OsL,L,]2+716 Ured714.715,717 and the kinetics of the reactions f o H o ~ e d . ' ~ * ~ ~ ~ ~
The X-ray crystal structure of truns-[OsL4C1,](C10,), (L = 11, R = Et, Le. the N,N'-diethylimidazolidine-2-thione) confirms the trans configuration (Os-C1 = 2.351 A, mean Os-S = 2.386 A). The reaction is curious, for the chloro ligands were apparently derived from HCIO, since the reaction was of OsO,, the ligand and aqueous HCIO,. The osmyl complex trans-[OsO,L,](CIO,), can also be isolated from the reaction
46.7.8.2
Thiazotidine-2-thwne
This ligand (L) will react with OsX, (X = CI, Br) to give OsL3X,; IR and electronic spectra of these were
46.7.9 Thiolato Complexes 46.7.9.1
Osmium(III)
The complex Os(SC,,H,,),(CNMe) is made from OsCl,, the lithium salt of 2,3,5,6-tetramethylbenzenethiolate and 2,3,5,6-tetramethylphenylsulfide; it is green-yellow. The X-ray crystal structure of the ruthenium salt shows it to be trigona1 bipyramidal, with the acetonitrile in the axial position. The osmium complex is isomorphous; it would seem therefore to be the only example so far reported of a trigonal bipyramidal osmium(1V) complex. In an attempt to make a complex of lower coordination number 2,4,6-triisopropylbenzenethiolate(SCl5H23)was used but the complex was still pentacoordinate, i.e. Os(SlSH,,)4(MeCN).719
46.7.9.2
Osmium(IV)
The species Na[Os(SNC,H,,)(OH),] and Os(SNC,H,,),(OH), have been claimed, OsO, with dimethylaminoethanethiol giving the former7" whiIe diisopropylaminoethanethiolwith 'osmic acid' gives the latter, which supposedly contains osmium(IV) (per is 1.2 BM at room temperature).
46.7.10 Dithiocarboxylate and Thiocarboxylate Complexes There is some work on dithioformato complexes, but clearly thiocarboxylate osmium chemistry is an area in need of further study. Complexes of the dithioformate ligand S2CH- have been made from Os(PPh,),H, and CS2. By analogy with the ruthenium analogue (for which an X-ray crystal structure has been obtained) a
Osmium
608
structure involving bidentate S,CH- ligands cis to each other was proposed for Os(PPh,),(S, CH), . Reaction of CS, with Os(PR,),HX(CO) (PR, = PMe,Ph, PMePh,, PPh,; X = Cl, Br) gives two forms of Os(PR, j2(S2CH)X(CO);in one the sulfur atoms are thought to be trans to the PR, ligands, and in the other the halide and PR, ligands are trans to the sulfur atoms.424 A number of complexes of p-chlorodithiobenzoate (S, C,H, C1- ; L) have been claimed, involving There is also Os(SO,Cl6H,),*2H,0, a complex osmium(II1) and osmium(1V); OsL, was i~olated.~” of 5,5’-thiodisalicylic acid, made from the ligand and ‘osmic acid’, and which has a magnetic moment of 1.35 BM at room temperat~re.~”
46.7.1 1 Thiourea Complexes
The principal oxidation states involved in the rather few thiourea (thio) complexes which have been studied are 11, I11 and (as the osmyl complex) VI. In no case is the mode of bonding properly established. In view of the uncertainty surrounding some of these species we consider both unsubstituted and substituted complexes together, using the apparent oxidation state for classification purposes.
46.7.11 .I
Osmium(l1)
Reduction of [Os(thio),13” with [Cr(H,O)JZf gives [Os(thio)6]z+ The reaction of thiourea with [OS(WO)CI,]~-is said to give a brown, seven-coordinate ‘associaThe X-ray photoelectron spectrum of OsL,*Cl, (L = dition complex’, K,[Os(NO)(CN),* thio].723 phenylthiourea) has been m e a s ~ r e d . ~”
46.7.11.2
Osmium(1II)
It appears that the main reaction product of OsO, with thiourea is [Os(thio),13+, together with formamidine disulfide, via initial formation of [OsO, thio,)’+ .508,725 The earlier-reported species [O~(thio),](OH)C1,~~~ is likely to have contained the osmium(II1) species. Salts of [Os(thio), J3+ with [Cr(NH,),(SCN),]- and [Cr(SCN),I3- have been isolated using OsO,, thiourea and the complex anion in HC1.727,728 [Os(thio),](SnCI,),Cl has been reported;52ESCA data on [Os(thio>,]Cl, have been given. The Os0,-thiourea reaction has recently been investigated k i n e t i ~ a l l y . ’ ~ ~ ~ ~ ~ ~ Reaction of [OS(NO)(CN),]~-in hydroxide (which presumably generates [Os(NO,)(CN),]‘-) and The X-ray photoelectron spectrum of OsL,Cl, has been thiourea gives K,[Os(thi~)(CNj,].~~~~~’~ measured (L = diphenylthi~ureaj.”~
46.7.11.3
Osmium( VI)
The osmyl-complex [OsO,thio,]SO, (brown, made from OsO, and the ligand in H2S04)M8 and a dark green osmyl complex, made from N-cl-pyridyl-N’-benzoylthioureaand trans-K, [OsOz(OH),], have been reported.731 For ethylenethiourea complexes see imidazoline-2-thione, p. 607.
46.7.12
Miscellaneous S,S Donor Ligands
The diisopropyldithiophosphate complex, made by reaction of the ammonium salt of the ligand with [osc&]z-,has a dimeric structure with two bidentate (Pr‘0)2PS,- groups per metal atom and two bridging Pr‘P-S-S ligands, according to a preliminary X-ray Complexes of the form Os(mdtp), and Os(ptp), (mdtp = bis(2-methyl-5-chlorophenyl)dithiophosphinate and ptp is the corresponding p-tolyl derivative) were made from [osc16]2- and the sodium salt of the ligand; the ESR data for these species were very briefly described. The complex (Os(ptp),(PPh,),]Cl and [Os(ptp),phen]Cl were also prepared.’33
Osmium
609
46.8 SELENWM- AND TELLURIUM-CONTAINING LIGANDS There are relatively few of these, and it is obviously an area worthy of further investigation, as is that of osmium-tellurium chemistry.
46.8.1 Selenido Complexes Reaction of Os(PPh,),(CO), with Se, yieids 0~(q~-Se,)(PPh~),(C0)~, the crystal structure of which shows a distorted octahedral structure. The side-bonded SeTligand (Os-% = 2.5403(8) and 2.5526(7)A) with Se-Se = 2.321(1)A) is trans to two carbonyl ligands (mean Os-C = 1.874A); the two phosphine ligands are trans to each other {mean Os-P = 2.406A).W3
46.8.2 Selenoether Complexes Reaction of MeSe(CH,),SeMe and also of Me,Se with HX and OsO, gives OsO,X,{MeSe(CH,),SeMe) (X = C1, Br); IR, electronic and 'HNMR spectra were measured.514
46.8.3 Diethylselenocarbamato Complex The species Os(Se,CNEt,), is obtained from [OsBr,]'- and (NH,)(Se, CNEt,). The complex has a magnetic moment of 1.63 BM at room ternperat~re.'~~
46.8.4 Imidazolidine-2-selones Reaction of these with OsO, in acid solution gives [0sL6l3+(L = SeN,C,H,R; R = H, Me, Et); the kinetics of the reactions were studied as well as the IR spectra of the complexes.735
46.8.5 Tellurium-containing Complexes Attempts to prepare OsO,X,(TeMe,) (X = CI, Br) from OsO,, HX and Me,Te
46.9 HALOGENS AS LIGANDS The chemistry of osmium fluorides and fluoro complexes differs sufficiently from that of the chloro, bromo and iodo species to warrant separate treatment here. The early review of Canterford and Colton is still useful for the general halide chemistry of the element.736
46.9.1 Fluorides and fluor0 complexes Oxidation states IV to VI11 inclusive are represented in the fluorine chemistry of osmium, as compared with 111 to VI for ruthenium and I1 and IT1 for iron. Thus the fluorides of the ironruthenium-osmium triad well exemplify the greater tendency of second and third row elements to higher oxidation states with n-donor ligands. The oxofluorides of osmium, amongst others, have recently been review@731we have already dealt with them on p. 584.
46.9.1 .I
Osmium( I V )
The tetrafluoride OsF, is not a discrete molecular species and so is not considered in detail here: it is a yellow material, prepared by reduction of [OsFJ, with hydrogen over platinum gauze under UV irradiation at 50 0C.738
Osmium
410
A number of salts of [OsF,I2- are known: the free acid7,' (in aqueous solution only), the hydra~inium,~~' tridodecylamm~nium,~~~ p o t a s ~ i u m , ~ ~ ~ . ~ ~ ~ and c e ~ i u r n ' ~ ~ J ~ ~ salts have been isolated. The general method of preparation is to treat the corresponding [OsF,]salt with aqueous alkali;743a convenient way of making K,[OsF,] is to fuse K2[OSX,] (X = C1, Br or I) with KHF, at 400 0C.744They are white solids giving stable aqueous solutions. Lattice parameters have been measured for the sodium, potassium and cesium salts; the sodium salt exists in a metastable orthorhombic form and a stable hexagonal form,742while K2[OsFs]and Cs[OsF,] have hexagonal structures related to that of K,[GeF,].739The IR stretching frequency ( v 3 , Flu)of K,[OsF,] is 548 cm-I .745 Electronic spectra have been measured'& and assigned for solid Kz[OsF6]and Cs2[OsF6];for the [OsF,J2-ion 10Dq is given as 26000cm Studies on the magnetic susceptibility of K2[OsF6]and Cs,[OsF,] from 89 to 291 K have been carried out and show that the molar susceptibility (xM) is essentially independent of temperature over this range (see Table 27).748The essential invariance of lM with temperature has been d i s c ~ s s e d . ' ~It~would , ~ ~ ~be of considerable interest to study, as has been done for K,[OSX,] (X = Cl, Br, I), the effect on the magnetic susceptibility of M',[OSF,] salts of diluting them in isomorphous diamagnetic host lattices such as M',PtF,]. Table 27 Magnetic Data on [OsX,]*- Complexes'48
Complex
Temperature range (KI
x, over same range (10-6 cgsu)
peg at 300 K (EM)
88.9%291.0 89.g278.5
71 3-71 3 957-937
1.31 1S O
%300 91.0-246.5
94 1-94 1 1257-1162
1.51 1.67
87.4-295.3 98.G295.4
617403 1392-1285
1.21 1.76
88.4-296.2 90.7-294.4
877-789 1191-1 132
1.38 1.65
Studies on the I9FNMR spectra of solid M,[OsF,] (M = K, Rb, Cs) suggest that the anions have distorted structures.750
(i)
Mixed~7uorohaloosmates(IVj, [OsF, X, .n / 2 -
Many such complexes have been prepared. The fluorochloro series [OSF,,C~,-,]~-has been made from [OSC~,]~and BrF,; both cis and trans forms of [OsF,C1,12- were isolted as was trans[OsF, Cl4I2-.751 Recently the reaction of BrF, and [OSC~,]~has been shown to give all five members of [OSF,CI,-,,]~- with the cis isomers for n = 4 and 2 and thefac isomer of [OsF,Cl,]-; reaction of C1with [OsF, Cl]'- and with ~is-[OsF,Cl,]~-gives tr~ns-[OsF,Cl,]~-,tran~-[OsF,Cl,]~-and mer[OsF,Cl,l2-. The isomers were separated by ion exchange and their vibrational spectra in earlier work vibrational spectra and unit cell dimensions were listed for some isomers as the potassium salts.7s1 Most members of the series [OSF,B~,-,]~-and [OSF,,I~-,,]~have been isolated from' the reaction The salt trans-Cs[OsF,ClBr], made by of [Os&]'- (X = Br, I) and tridodecylammonium has been reaction of K,[OsF,Cl] with HBr followed by ionophoresis and addition of cesium studied by single crystal X-ray analysis. The mean Os-F distance is 1.94A, with Os-C1 = 2.43(1) and Os-Br = 2.49(1)k7"
( i i ) Miscellaneous osmium(IV) fluoro complexes
Reaction of OsF, with PF3 a t - 196 "C gives a yellow compound, amorphous to X-rays, giving analyses corresponding to OsF,-2PF3. On the basis of IR data the formulation trans-OsF, (PF, ), was suggested. The essentially diamagnetic nature of the material (aR = 0.3 BM at 300 K) is curious, but possibly it arises from the tetragonal distortion of the complex if it is indeed an osmium(1V) complex containing coordinated PF, ligands.755
Osmium 46.9.1.2
61 I
Osmium( V )
This is an unusual oxidation state for osmium. We include osmium pentafluoride in our coverage here since it is a discrete molecular species. Osmium pentafluoride, [OsF,],, is made by the reduction of OsF, with iodine and IF, at 55 "C7,, or from the action of silicon on O S F , . ~ It ~ 'is a blue-green solid melting at 70 "Cto a viscous green liquid which becomes bluer as the temperature rises, boiling at 233 "C to a colourless ~ a p o u r . ~ ' ~ The X-ray crystal structure of the solid shows it to be tetrameric in the solid state with a structure closely akin to that of [RuF5I4,the metal atoms lying at the corners of a rhombus and linked by cis fluoro bridges (Os-F (bridge = 2.03 A, mean Os-F(bridge)-Os angle = 137 "1. The mean Os -F terminal distance is 1.84 (Figure 36).
R
Figure 36 Structure of [OsF,], (reproduced with permission from J. H. Canterford and R. Colton, 'Halides of the Second and Third Row Transition Metals', p. 20, copyright 1968 Wiley)
The substantially bent Os-F(bridge)-Os angle is in agreement with the E-bonding hypothesis put forward by Canterford and C~lton.'~'In pentafluorides [MF5], containing cis fluoro bridges, the angle degree of bridge fluoro-to-metal n: bonding will be at a maximum if the M-F(bridge)-M is 180" and M has a d o configuration (e.g. M = Nb, Ta). As the d orbital electron occupancy increases with concomitant lower bridge F to M n: bonding the angle is observed to decrease, from d' species (Mo, W), d 2 (Tc, Re), to d 3 (Ru, Os) and beyond; thus, in [OsF,],, it is likely that such .n bonding is of little irnp~rtance.~'~ Magnetic interactions in [OsF,], have been studied by susceptibility and neutron diffraction measurements; it is magnetically ordered at TN= 6 K.759 The magnetic moment varies from 1.73 BM at 101.5Kto 2.06BM at 295.4K.756
( i ) Salts of [OsF,]-
pota~sium,'~' rubidium,76' A number of salts of this ion are known; those of lithium,760 cesium,743~ i l v e r , ' ~NO+ ~ , ~ ,762 ~ * XeFf and Xe2F,i763and BrF,+.'" The alkali metal salts are made N awith ~ ~ ~BrF, ) (K742,Rb,761C S ~ , ~ ) . by reaction of a 1: 1 OsBr,, MBr mixture with f l ~ o r i n e ( L i , ~ ~ O or These are colourless, decomposing in moist air, and in alkaline solution giving [OsF,I2-. These salts Xe salts763). of non-metallic cations are made from OsF, or OsF, (NO+,762 Unit cell parameters have been measured for the alkali metal salts, which have rhombohedral structure^.'^*'^^ A single crystal X-ray of K[OsF,] shows this to have a slightly distorted octahedral structure with a mean Os-F distance of 1.82 A.765Raman spectra of (NO)[OsF,] and Xe,F,[OsF,] have been measured76' and are listed in Tabie 28. From the electronic reflectance spectrum of Cs[OsF,] a value of 36000cm-' was obtained for 10Dq.747 Magnetic susceptibility data for K[OsF,] and Cs[OsF,] show that the magnetic moment is essentially invariant from 1 15 to 295 K, giving peff 3.34 and 3.23 BM for the potassium and cesium salts re~pectively.'~~
46.9.1.3
(i)
Osmium( VI)
Osmium hexajuoride
OsF, is made by the direct fluorination of the meta1;767-769 as noted on p. 612 it now seems quite clear767,769 that the 'OsF,' of the early literature'" is in fact OsF,. It is a bright yellow solid, melting at 33.9 "Cand boiling at 47.5 "C with a triple point at 33.4 "C (at 474.5 mrnHg);768there is a phase transition at 1.4' C n l In water it disproportionates to OsO, and [OSF,]~-.
612
Osmium
Electron diffraction data on the vapour at - 39 "C show the molecule to be octahedral in the gas phase with an Os-F distance of 1.83I(&) A;it is interesting to note that the Os-F distance for this d 2species is very close to the value of I .833(3) E\ observed in WF, (do)and 1.830(8) A in IrF, (d3).772 It has been plausibly suggested that this constancy of distance is a consequence of n donation from the Auoro ligands to the metal decreasing from W to Ir because of increased d electron occupancy, this effect being counterbalanced by the shrinkage of the M"' size from left to right brought about by shielding effects.773 In the solid state OsF, is cubic at room temperatures, undergoing a transition at 1.4 "C to a form of lower symmetry (probably orth~rhombic).'~~ Thermodynamic data have been calculated for the compound on the basis of vapour pressure data.768 There has been much work on the vibrational spectroscopy of the molecule; the best data for the vibrational frequencies are given in Table 28,774based on the IR spectrum of the gas and the Raman spectra of the and Electronic Raman spectra of a solution of OsF, in MoF, have recently been measured.778The unusual breadth and low intensity of the v2(F)and low intensities of combinations involving v2 s ~ g g e s t ' ~ ~that ' ' ~a dynamic Jahn-TeIler effect is operative for this i2g' molecule. There have been many force constant treatments for the and determinations of Coriolis coupling constant^.^^^^^^^ Electronic spectra of the gas782and its solutions in M O F , ' have ~~ been measured and assignments proposed. Magnetic susceptibiIity data were measured from 117 to 297 K and show that the Curie-Weiss law is obeyed (6 = 66 ",peE= 1.47 BM at 267.8 K).769 Theoretical interpretions of these data have been given.783,784 Table 28 Vibrational Spectra for [MXJ- Complexes Complex
R R, IR R, IR R R, IR R, IR R, IR R R, IR R, 1R R, IR
690 73 1 310 346 354 375 189 21 1 218 140 155 ~~~~
"Solid bGas 'Solid and solul~on.dSolulion.
214 269 302 180' 169 162 113" 120
720 294'
268 185'
325 325 200'
178 168 116'
221 140' I70
122
205 165 171 I83 94' 100
111'
91
80
763 717 79 1 818 820 833
79 1 818 828, 829 79 I 82 1
~
[Co en3][OsX,].
The "FNMR resonance has been measured from 115 to 245 K at different field strengths of solid OsF, and spin-lattice relaxation times determined.785The electron affinity of OsF, has been compared with those of other metal hexafluoride^.^^^^^^^^^^^ The Mossbauer spectrum has been recorded.Is4 The apparently heptacoordinate (NO)[OsF,] has been made from FNO and OsF,; it has a cubic structure.762
46.9.2.4
Osmium( VII)
The heptafluoride OsF, is made by fluorination of the metal at 600 "C under a fluorine pressure structure. of 400 atm. The IR spectra suggest that the complex has a pentagonal bipyramidal (D5,,) The magnetic susceptibility was measured from 90 to 195 K and, as with [OsF,]*- salts, the moment is essentially invariant with temperature over this range (pes = 1.OXBM).788
46.9.1.5
Osmium( VZZZ)
The octafluoride OsF, was claimed in 1913 to be the product of reaction of Os with fluorine770but It is said that some OsF, , however, may be produced by reaction clearly OsF, had been made.767,769 of osmium with fluorine at very high pressure^.'^'
Osmium
613
For oxo fluorides and oxo fluoro complexes see p. 584, 588 and 596.
46.9.2
Chloro, Bromo and Iodo Complexes
There is only one binary halide, Os2C1,0,which can be classed as a 'complex' in the sense that it has a discrete molecular structure, and we consider it on p. 615 below. In this section we consider unsubstituted halo complexes [OsX,Y-, mixed species [oSxflY6-,~-(X and Y both halides) and aqua and hydroxo halides. Other halo species substituted with other ligands are considered under the section dealing with the substituting ligand (e-g. oxo halo species on p. 584, nitrido halo complexes on p. 563, halo ammines on p. 529). Although there is a small chemistry of [OsXJ- and an even smaller chemistry of [OsCl,]-, it is the IV oxidation state that predominates in the chloro, bromo and iodo chemistry of osmium, whereas for ruthenium it is probably true to say that the chemistry of [RuX,I3- is more extensive, or at least has been more extensively investigated, that that of [RuXJ2-. Much work has been done on the mixed halo species [OSX,Y,-,]~-. There is a short review on osmium halo complexes in solution.789For carbonyl halides of osmium, see p. 973 of ref. 16.
46.9.2.1
Osmium( I l l )
There are very early reports of the potassium and ammonium salts of [OSC~,]~-, made from [Os03Nl and HCl or for OsO, with NH,Cl and H,S,5 but the compounds were not fully characin acid solution with silver powder gives solutions thought to contain terized. Reduction of [OSX,,]~(X = Cl, Br, I),790and recently the salts [Co(en),][OsX,] have been isolated from such The potassium salt K3[OsBr,] has been isolated by electrolytic reduction of [ O S B ~ , ] ~ - ; ~ ~ ~ the [OSBr,]2-/[OSBr6]3- couple is + 0.31 V in 4 M HBr (vs. NHE).7y3 Vibrational spectroscopic data for [OsX6J3-(X = C1, Br, I) are given in Table 28,'" and there are ESR data on apparently uncharacterized [OsClJ- specie^.^^,'^^ Dimeric [Os,X,]'- (X = C1, Br) complexes have recently been made.
46.9.2.2
Osmiuna(1V)
Salts of [OsX,]*-, especially the chloride as Na,[osC16]-nH,0 (conveniently soluble in water or polar organic solvents), as Kz[osc16] or (N&),[OsCi6], are good precursors for preparations of other osmium complexes; indeed, Na,[OsCI,] nH,O rivals OsO, as a useful starting material.
-
( i ) Unsubstitured [Os&]'-
species
For [OSC~,]~the free acid796and salts with a variety of organic cations are k n o ~ n , ~and ~ ~the -'~ sodium,'" pota~sium,'~~~~" rubidium,803cesiumw and a m m o n i ~ m ' ~salts ~ ~ 'together ~~ with a number of heavy metal salts (e.g. silver and thallium(I)s06).The best method of preparation is from OsO, and concentrated HCl in the presence of a reducing agent such as ascorbic acid,p8 iron(I1) chlorideBoS and the appropriate cation. The [OsBr,]'- ion is obtainable as the free acid in solutions"" and salts of organic cation^^'^^^^^^^''^ as well as sodium,* potassium,B".'oS cesi~rn"~~'"and amr n o n i ~ m , "while ~ ~ [osI6]'- is similarly represented by the acid in solution (and also, apparently, as ~ ~ organic ), cations'" and as the p o t a ~ s i u r n ~ ' "and ~'~ a solid isomorphous with (NH,), [ O S I ~ ] ~with ammonium"4 salts. For [OsBr,12- and [OsI,]'- the acids themselves will reduce Os04,8099814 or [oSc16]*- can be refluxed with There is a single crystal X-ray structure of (Ph4P)2[OsCl,] which shows the mean Os-C1 distance to be 2.332 &'I5 and unit cell data obtained for a number of cubic salts of [OsC&]*-and [OsBr,]'- .'I6 In a recent paper correlations between Os-C1 distances and Os-C1 stretching frequencies have * been made, and similar data presented for rhenium and iridium chloro specie^.^" In particular there have been a number of studies on the Raman spectra, both non-resonance and resonance, of [osc16]2p818-822 as well as electronic823and electronic resonance Raman spectra$24similar studies have been carried out with [OsBr,]2- 818,819.823-826.828.824 and [Os16]2-803,821,823,827 (for the latter, magnetic, In Table 27 we also electronic and Raman spectra and optical activity were also
614
Osmium
include data on the 1R spectra of [osc&]2-,820~828~82y[OSBr6]2-820 and [OSI6]'-- ,791 There have been a number of studies on the electronic absorption spectra of [OsXJ- (X = C1, Br,790~798~806~81'~819~E24~83[Cg12 1811~823~824~831); vibrational intervals were located in the luminescence spectra of solid K, [OSCl,] doped into K,[FtC16].832 There have also been NQR807,s34 and ESCA427studies on K2[Os&] (X = C1, Br) and Mossbauer work on K,[OSC&].1s4An MO description of the bonding in [OsC1,]2- has been given.83s ( a ) Magnetic susceptibility data. The magnetic properties O f K2[Os&] and cS,[o&,] (X = F,Cl. Br, I) were first systematically studied by Earnshaw et al., who found relatively little variation in the atomic susceptibility (1,)over a wide temperature range (Table 28).7a It was assumed from these data that the relative constancy of X, with temperature arose from high spin-orbit coupling; where significant variation of xM did occur with temperature it was suggested that a small amount of paramagnetic impurity (e.g. [OsXJ- ) was perhaps re~ponsible.7~~ Later work showed that the susceptibilities of K,[OsCI,] and K,[OsBr6] diluted in the isomorphous diamagnetic host lattice K2[PtC&]or K2[PtBr6]were much higher than for the pure salts, and a 'superexchange' mechanism via d,-d, metal ligand orbitals for the non-dilute systems was proposed to account for these observations.83M38 Some of these measurements were repeateds08*839 and susceptibility data presented for (NH,),[OsX,], (Bu",N),[OSC~~] and (Me4N),[OsBr6],and intermediate coupling schemes p r o p o ~ e d . ~ ~Theoreticai ~ ~ ~ " O calculations on the basis of a cubic field successfully fitted experimental results for the susceptibility and electronic absorption spectra of K, [OsCl,j.84' ( b j Ofherdata for [ 0 s X , l 2 - . Kinetics of the reaction shown in equation (1 1) were measured and compared with those for [IrCI,]'- and [IrC1,I3 ;','there are also mechanistic studies on the reactions of [OsCI,]*- with [Fe(CN),I4- ,843 bromidexMand iodide.',' There are recent thermogravimetric and differenbal thermal analysis data for ML2[OsXh] (M = Rb, Cs; X = Br, I)."' [OsC16]*- + HzO
[OsCl,(H,O)]-
+
C1-
(1 1)
The species OsC1,.2MC14 (M = S,847Se, Tea,') have been reported; their IR spectra suggest that they contain [osc16]z- ions and so they have been formulated as (MCl,), [ O S C ~ ~ ] . ~ ~ '
d complexes, [OSX,YbPn]*( i i ) ~ h e halo
Species of this type containing the fluor0 ligand have already been considered (p. 610). ( a ) [OsCI,Br,-,]2-. All the geometrical isomers in this system, made from HBr and [OsCI,]'-, chromatographyx50~u*1 and HPLC8s2have been used, infrared have been identified; ionophore~is,"~ spectra measured of the various complexes831and their stability constants determined."' Kinetics of exchange for the eighteen possible ligand exchange reactions were investigated by r a d i ~ r n e t r y . " ~ ~ ~ ~ ( b ) [OsClnI,~,]zPand [ O S B ~ , I , - , ] ~ -Members . of the former series were made from HI and [OSC~,]~-, and all the isomers were separated chromat~graphically,s~~ whilst ionophoretic methods were used on the HBr-[OsI,]'- system to give all the isomers. IR spectra of the members of the Cs, [OsBr,I,-,] series have been as have electronic spectra.8s6bHydrolysis of [osci,I6-,]'- is catalyzed by rner~ury(lI).~"
(iii) Dimeric osmium(IV) species
The dimeric salt (Bun4N)JOs2Clto]can be made by heating (Bun,N)[OsCI,] for a short time to 240 "C. The X-ray crystal structure analysis shows a chloro-bridged dimer structure (OsCl(bridge) = 2.41 1(2), mean Os-C1 trans to bridge = 2.306, mean Os-Cl cis to bridge = 2.301 A). The Os...Os distance is 3.626(1) A. The magnetic susceptibility was measured from 94 to 336 K; it obeys the Curie-Weiss law with 8 = - 6 and pcff= 1.36 BM (per metal atom).8S8Recently the bromo analogue has been made by refluxing (Bun4,N),[OsBr,] in trifluoroacetic acid. The structure is very similar to that of [Os,Cl, 12- ; the bridging Os-Br distances are 2.544(4) A, the mean terminal Os-Br distance is 2.454i, and the Os---Osdistance is 3.788(3)A. Although there is clearly no direct Os-Os interaction in either of these complexes the magnetic moment peffis remarkably low, 0.26 BM at 296 K3**
I Osmium 46.9.2.3
61 5
Osmium( V )
( i ) Salts of [OsX, j - ( X = CI, Br)
The tetraphenylarsonium salt is made from Os2Cl,, and ( p h , A~ ) C l ' and ~ ~ tetraethylammonium salt from (Et,N)[OsX,(CO),] (X = Br, I) and C1, at 120"C;833oxidation of [osc16]2- by PbO, gives [oScl,]- 860,86' and SCI, [oSc&] can be got from OsO, and SC1, .861 The redox potential [oScl,]-/ [osc16]2- iS 0.8 v (VS. Hg/Hg,S04).833 The X-ray crystal structures of ( B U " ~ ) [ O S Cand I ~ ] (PPh4)[OsC1,]8'5 ~~~ show these to be octahedral, with mean Os-C1 distances of 2.303 and 2.284 A respectively. The difference is surprising given that only the cation has changed (see also ref. 861), but both are shorter than the Os-C1 distance of 2.332A in ( P~ ,F ) ,[ O S C I ~ ]as , ~ expected '~ in view of their higher oxidation states. The magnetic susceptibility of (Bu",N)[OsCl,] over the range 94 to 336 K obeys the Curie-Weiss law (0 = 18 K, pC8= 3.03 BM)'" The Raman, resonance Raman and IR spectra of [osci6]- were recorded (see Table 28).", Attempts to isolate pure salts of [OsBr,]- were not successful, but the ion was generated in solution to form [OsBr,12- and solid PbO,, and electronic spectra were recorded. The [OsBr,]-/ [OsBr,]*- potential is + 1.20 V.860
( i i ) Dimeric osmium pentachloride, Os, CI,,
This is made by reaction of OsF, with BC1, at - 196 OCS6'or from OsO, and SC12.859 The X-ray crystal struture shows it to be a chloro-bridged dimer with a bridging Os-C1 distance of 2.42& and a mean terminal Os--Cl distance of 2.24 A. The Os-**Os distance is 3.63 A. The magnetic susceptibility over the range 80 to 293 K obeys the Curie-Weiss law (0 = 230, peR= 2.54 BM per Os atom). IR, electronic and ESR spectra were also recorded."'
(iii) Substituted halo complexes ( a ) Aqua and hydroxo species. Recent work on the electrochemical oxidation of tran~-[OsO,Cl,]~gives evidence for the existence of labile [Os"'Cln(H, 0)6-n](3-*)-species; at high C1- concentrations [OsCl,(H,O),]- reacts with trans-[OsO,C1,I2- to give OsCl,(H,O), .489 Although the main product of aquation of [0sX,l2- appears to be [OsX,(H,O)]- (X = C1, there is evidence for the existence of [OSC~,(OH)]~,R63 cis-[OsCl,(OH)(H,O] , fuc[OsC1,(OH),(H20)]- and mer-OsCl,(OH)(H,O), as well as oxo-bridged dimers in the hydrolysis [OSCI,(H~O)~-~](~-")-~~~~*~~ products of [OsCl,]'Evidence for the existence of OsC14(H20)2~89,865 and [ O S I , ( H , O ) ] - ~ has ~ ~ ~been * ~ ~presented. Studies of the equilibria in equation (I 2) have been made (X = Cl, B r ) * 4 2 7 * 6 3
[OsX,(H,O)]-
+
OH-
+
~OSX,(OH)]~-
H,O
(12)
There are electronic spectral data on [OsC1,(HzO)lThe species OsCl,(H,O) is probably formed by oxidation of [OsC1,(H20)]- by C12-; electronic spectra have been re~orded.'~' The salts of 'iOsX,(OH)]*-' (X = C1, Br) reported in the earlier literature637are almost certainly those of the binuclear symmetrically bridged p-oxo complexes [OS,OX,,]~- (see p. 594), but [OsCl, (OH)]*- and possibly [OSC~,(OH),]~-are probably present in aqueous solutions of [os,oc1,0]4-.639 ( b ) Aqua and hydroxo complexes of mixed halo species. The equilibria between cis[OsCl, 12(H20)]- and cis-[OsCl,I,(OH)~- have been studied.*,, For the mixed halo species see under the appropriate substituting ligand (e.g. oxo halo complexes, p. 594; nitrido halo species, p. 563). .8429863
46.10 HYDROGEN AND HYDRIDES AS LIGANDS
Hydrido complexes have been dealt with already under the appropriate coligands (e.g. hydrido nitrosyls, p. 545; hydrido phosphines, pp. 570, 572; hydrido arsines, p. 576).
Osmium
616
In tetrahydroborate and borane complexes the effective ligand is hydrogen so we deal with these small sections here.
Tetrahydroborate Complex
46.10.1
The tetrahydroborato complex Os(BH,)H3(P(cyc10-C,H,),}, has been made by reaction of OsH,{P(cyclo-CSH,),O}, and the THF adduct of BH3. The X-ray crystal structure shows it to be distorted pentagonal bipyramidal: the axial ligands are the phosphines (Os-P = 2.345(2) A, the BH,- is bound by two bridging hydro ep atoms (Os-H = 1.90, B-H (bridge) = 1.31, OsB = 2.30(1), E--H (terminal) = 1.09(7) ), and the equatorial plane is completed by three terminal hydrido ligands (Os-H = 1.57A). At 90 "C there is rapid exchange of the bridging H atoms with the hydrido ligands bound to osmium.868
R
Borane Complex
46.10.2
The cluso-type 1l-vertex metalloborane (PMe,Ph),OsB,,H,(OEt), will react with MCl,(PPh,), (M = Os, Ru) to give the mixed-valence complexes (PPh3)2C1MCIOs(PMe2Ph)B,,H,(OEt),. The Os or Ru atoms are bound exopolyhedrally to the initial metalloborane by two two-electron M -H,-B links and an Os-Cl-M four-electron bridge.'@
46.11
MIXED DONOR ATOM LIGANDS
46.11.1 46.11 .I .I
Bidentate N-0
Complexes
Benzamido ligand
The benzamidato complex Os,Cl,(NHCOPh), is made from Os,(OAc),C1, and molten benzamide, and OszBr,(NHCOPh), from the chloro species and Br-. The X-ray crystal structures show these to have 'lantern' structures with four bridging benzamido ligands and the halide ligands axial. Each has two closely related molecules in the unit cell with slightly different structures. For the chloro complex the mean distances are Os-Os = 2.367, Os-C1 = 2.491, Os-0 = 1.99 and Os-N = 2.02t4; for the bromo s ecies the mean values are Os--0s = 2.384,Os-Br = 2.610, Os-0 = 2.02 and Os-N = 1.93
1!707s71
46.11J . 2
Hydroxybenzamido Iigands
An interesting piece of recent research concerns the osmium complexes of 172-bis(3,5-dichloro-2hydroxybenzamide)ethane (H,chba-Et) and of 172-bis(3,5-dichloro-2-hydroxybenzamido)-4, 5-dichlorobenzene (H,chba-dcb) which function as tetradentate tetraanions. They are difficult to oxidize and their amide functional groups tend to stabilize higher oxidation states of metals. The intention is to use complexes of these with osmium as specific oxidants; by controlling the ligands on the metal, control of oxidative properties may be achieved. Reaction of the ligand with transK2[Os0,(OH),] gives tvans-K,[OsO,(q4-chba-Et) (characterized analytically, by 'H NMR and by the observation of vas(OsO,) at 820cm-' in the IR shifting to 788cm--' on "0 substitution). The structures of the complexes Os(g'-chba-Et)py,, made from pyridine and trans-& [OsO2(q4-chbaEt)], and Os(q4-chba-dcb) bipy, made from Os(q4-chba-dcb(PPh3),and bipyridyl, were established crystallographically.87*For K,[OsO(q4-chba-Et),(OPPh,),] see p. 594. For aminocoumarin and aminopentenone complexes, see p. 569;for complexes with amino acids, see p. 583, and with alkaloids, p. 587.
46.11.2
46.11.2.1
Complexes of Miscellaneous Sulfur-Nitrogen Ligands Sulfamato complexes
The salt K,[Os(NH,SO,)Cl,] was made as yellow plates from reaction of [0sC1J2-, SnCl, and sulfamic acid. The magnetic moment is 2.21 BM at room temperatures, and on the basis of the IR
7 Osmium
617
spectrum it seems likely that the sulfamato ligand is N-bonded to the metal. Another species, K,[OsCl,] -NH, SO,, not fully characterized, was also obtained. Refluxing of [Os(NH, S0,)C1,]2- in aqueous solution gave a very low yield of K,[Os,NCl,(H,O),] and another nitrido complex, probably K,[Os,NCI,o].349
46.11.2.2
2- Thioamidopyridine complexes
These, [Os(N,SC,H,),]Br, (per= 1.71 BM at room temperat~re),'~~ and the diamagnetic 6methyl-2-thioamidopyridinecomplex Os(SN,C7H,)zCl,,874 are made from [OsX6]'- and the ligands. Complexes of thiazolidine-Zthione (S2NC,Hs)with OsX, (X = C1, Br) and the ligands, gives the complexes OsL,X,. IR and electronic spectra were measured, and the magnetic moments of the thiornorpholin-3-one complex are 1.9 (X = C1) and 1.8 (X = Br) BM respecti~ely.~'~
46.11.2.3 Miscellaneous sulfur-nitrogen compkxes
These, for the most part, lack effective characterization. The following have also been reported, made from the ligand and OsO,, sometimes with HCl: (from 2-mercaptobenzimidazole);87~ Os(C,H,N,S), and OS(C~H,N,S).(C,H,N,SH)C~~-~H,O OS(C~H,NSO)~ (from 2-mercaptobenzoxazole);876( ~ ~ H) , [ OS O( S ~ N, C(from , ) ~ ] 2,5-dimercapto1,3,4-thiadiazole and pyridine);877Os(SN20,C,ZH,o)(OH)2and Os02(SN,0,C,,H,o)C12(from 3,3'diamin0-4,4'-dihydroxyphenylsulfone);~~~ ~OsO,(SON,C,H,,),]Cl, (from 4benzylamidothiosemicarbazide);"' Os(SNO), C3H& (from cysteine);544Os(SNO,C,H,),*L and Os(SNO,C,H,),.L (from 1-methylirnidazole (L) and cysteine or a-N-acetylonethylcysteine re~pectively);~"Os@N306C10H15)2 (from glutathione)?'' OsO, (SNOC7H6)2(from thiosalicylamide)~7g
46.11.3 Complexes of Miscellaneous Sulfur-Oxygen Ligands
There are relatively few of these; as with many of the sulfur-nitrogen donor complexes, little more than the stoichiometries is known. Thiomorpholin-3-one gives complexes of the type OsE3X,with OsX, (X = CI, Br); their magnetic Osmium tetroxide reacts with moments are respectively 1.8 and 2.0 BM at room ternperat~res.~'~ z-mercaptopropionic acid in NaOH to give Na, [Os(OH), (OOCSCHMe),]880,and 'thiovanol' (3mercapto- 1,2-propanediol) gives allegedly tetra- and penta-meric species [OsL,], [OsL,], with OsO, .'I Some osmium-(111) and -(IV) complexes with 8-mercaptoquinolines have been reported. 2,7-dirnethy1-8-mercaptoq~inoline~~~ with The ligands include 2,6-dimethyl-8-mercaptoquinoline,88' osmium(III), 6-methyl-8-mercaptoquinoline884with osmium(IV), 5-fluoro- and 5-iodo-8with osmium-(111) or -(VI). Two mercapt~quinolines~~~ and 5-alkylthio-8-mercaptoquinoline886 species which have been more fully characterized with this type of ligand are Os(NSC,H,), , made from K, [OsBr,] and 8-mercaptoq~inoline,~~~ and Os(NSC,H, Cl), , prepared using 5-chloro-8mercaptoquinoline
.'*'
46.12 MULTIDENTATE MACROCYCLIC LIGANDS 46.12.1 Porphyrin Complexes The porphyrin ligands impose an N, plane on their complexes and, by virtue of the extensive aromaticity of the ligand, are effective 7r acceptors. Thus it is that most of the osmium complexes are octahedral and of the form trans-Os"~porphyrin)L,or -Os"(porphyrin)LL'. Most of the work on osmium complexes has been carried out by Buchler and his collaborators using the octaethylporphyrin (OEP- ) ligand. There are two reviews on the s ~ b j e c t . ~ * ~ * ~ ~ We shall deal with complexes of OEP first, and then with those of other porphyrins.
(i)
Octaethylporphyrin (OEP) complexes
(a) Osmium(I1).Most of the known complexes involve osmium(I1). The two commonest starting materials are Os(OEP)(CO)py, made from HZ(OEP)and OsC1, under CO in pyridine,891from OsO,,
Osmium
618
H,(OEP), CO and pyridine,892or from OS,(CO),~,pyridine and the porphyrin893or the dinitrogen complex Os(OEP)(N,)(THF), prepared from Os(OEP)02and N2H, .894 Many complexes of the form Os(OEP)L, have now been isolated (e.g. py, OMe, P(OMe),, NO (p.551), THF, 0 (pp. 582, 594 and 595), MeCN, imidazole, 1-methyiimidazole, NH,, NMe,") and most are made from Os(OEP)(CO)Cl or Os (OEP)py, (the latter is prepared from the former by treatment with ~yridine).'~' electronic2323s95 as well as mass spectral datas2on many of There are 'H NMR, electrochemical,sgO these complexes. A rich resonance Raman spectrum of Os(OEP)py, has been measured; many of the porphyrin ligand fundamentals are resonance-enhanced.'96 Iterative extended Huckel calculations have been carried out on a number of these complexes and used to interpret absorption and emission spectra of Os(OEP)py,, Os(OEP)(CO)py, Os(OEP)(N,)(THF) and Os(OEP)(P(OMe), j2.x95 ( 6 ) Osrniurn(II1). Although no osmiurn(II1) porphyrin complexes seem to have been isolated, electrochemical data indicate the existence of [Os(OEP)L,]+, made by electrooxidation of Os(OEP)L, (L = py, y-picoline, piperidine, imidazole, 1 -methylimidazole, NH,, NMe,, P(OMe),, MeCN). In some cases correlations between Et for this oxidation reaction and electronic spectral absorptions could be d r a ~ n . ' ~ ' Similar . ~ ~ ~ electrochemical data for Os(0EP)LL' oxidized to [Os(OEP)LL']+ shows Et values intermediate between those for [Os(OEP)L,]+ and [Os(OEP)L',]+; analogies have been drawn between this and the cytochrome system.897
46.12.1.1
Osmium(IV)
Few examples are known. One which has been isolated is Os(OEP)Br,, made from Os(OEP)(N,)(THF) and CBr4,"' and Os(OEP)(OMe), obtained from Os(OEP)O, and SnCI, in methanol.R94In a recent report, reaction of Os(OEP)(CO)Cl with CHC1, or CCl, under photolysis at 365 nm gave Os(OEP)Cl,. Quantum yields were measured.899
46.12.1.2 Osmium( VI) For the osmyl species Os02(OEP) see p. 582 (an improved procedure for its preparation has recently been described"').
46.12.J.3
Dimeric complexes
The dimer [Os(OEP)], is made by heating Os(OEP)py,, and a mixed dimer OsRu(OEP), is prepared in similar fashion by thermolysis of a mixture of Os(OEP)py,, and Ru(OEP)py2. It was suggested that M=M bonds are present in these specie^.'^'
46.12.1.4
Complexes with other porphyrins
Reaction of K2[OsC16]900or O S C ~ ,with ~ ~ ' mesob-toly1)porphyrin (H,TTP) with CO in the presence of pyridine yields Os(TTP)(CO)py which with pyridine gives Os(TTP)py, .900 On heating this gives a dimer [Os(TTP)], which presumably, like its OEP analogue, contains an Os=Os bond.891 A number of other complexes of the form Os(porphyrin)(CO)py can be obtained from K,[OsCl,], the porphyrin and CO in pyridine, e.g. mesoporphyrin-IX dimethyl ester, mesoporphyrin-XI dianion (I) and mesotetraphenylporphyrin;m the complex with (I) can also be made from Os,(CO),,,pyridine and the p0rphyrin.8~~
46.12.2
Phthalocyanine Complexes
Rather surprisingly the phthalocyanine chemistry of osmium is far less developed than that of the porphyrins. Here we use Pc for [C32H16Ns]2I
46.12.2.1
Osmium( I t )
The polymeric [OsPc], has been made by reaction of OsO, with pyromellitic dianhydride or with
Osmium
619
phthalic anhydride in the presence of ammonium m~lybdate.’~’ An ‘osmium phthalocyanine’ is said to be formed by reaction of (NH,)z[OsC1,] and phthalonitrile.9°2
46.12.2.2
Osmium(Ill)
The complex OsPcClC,H,(CN), is made from (NH,), [OsCl,] and excess 1,Zdicyanobenzene at 360 “C, and is dark blue. It has a magnetic moment of 1.1 BM at room t e m p e r a t ~ r e . ~ ~
46.12.2.3
Osmium(ZY)
The polymeric [OsPc-SO,],, has been made by reaction of OsO, and excess molten phthalonitrile followed by extraction with H,SO,. It is dark blue and d i a m a g n e t i ~it; ~has ~ however been suggested The ion [Os(PcH)(HSO,),]+ may be that it could be an osmyl complex, Os0,Pc~C,H4(CN),.”3~’n5 and OsPcX, has been very briefly mentioned present in solutions of ‘[OsPc~SO,],’ in H2S04,906,907 (X = C1, Br, I).9o8
46.12.2.4
Osmium( VI)
Reaction of OsO, and 1,2 dicyanobenzene at 180 “C gives the dark blue OsO,*Pc{C,H,(CN),} = 1.4BM at 20 uC);903 however, all other osmyl complexes reported (p. which is paramagnetic (peR. 581) are diamagnetic, so this could well be an osmium(1V) or even an osmium(ZI1) species. Another osmyl species claimed is OsO,-Pc*3PhNH,, made from OSO~*PC*{C,H,(CN)~} and aniline; no magnetic properties were reported for this.’03
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"
Osmium 361. 362. 363. 364. 365. 366. 367. 368. 369. 370. 371. 372. 373. 374. 375. 376. 377. 378. 379. 380. 381. 382. 383. 384. 385. 386. 387. 388. 389. 390. 391. 392. 393. 394. 395. 396. 397. 398. 399. 400. 401. 402. 403. 404. 405. 406. 407. 408. 409. 410. 411. 412. 413. 414. 415. 416. 417. 418. 419. 420. 421. 422. 423. 424. 425. 426.
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Osmium 758. 759. 760. 761. 762. 763. 764. 765. 766. 767. 768. 769. 770. 771. 772. 773. 774. 775. 776. 777. 778. 779. 780. 781. 782. 783. 784. 785. 786. 787. 788. 789. 790. 791. 792. 793. 794. 795.
,'
796. 797. 798. 799. 800. 801. 802. 803. 804. 805. 806. 807. 808. 809. 810. 811. 812. 813. 814. 815. 816. 817. 818. 819. 820. 821. 822. 823. 824. 825. 826. 827. 828. 829.
63 I
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632 830. 831. 832. 833. 834. 835. 836. 837. 838. 839. 840. 841. 842. 843. 844. 845. 846. 847. 848. 849. 850. 851. 852. 853. 854. 855. 856. 857.
858. 859. 860. 861. 862. 863. 864. 865. 866. 867. 868. 869. 870. 871. 872. 873. 874. 875. 876. 877. 878. 879. 880. 881. 882. 883. 884. 885. 886. 887. 888. 889. 890. 891. 892. 893. 894. 845.
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Osmium 896. 897. 898. 899. 900. 901. 902. 903. 904. 905. 906. 907. 908.
633
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47 Cobalt DAVID A. BUCKINGHAM and CHARLES R. CLARK University of Otago, Dunedin, New Zealand
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47.1 INTRODUCTION 47.1.1 General Scope 47.1.2 Synthesis Reference Works 47.1.3 Structural Reference Works 47.1.4 Properties and Reactivity Reference Works
636 636 637 637 637
47.2 CARBON LIGANDS 47.2.1 Carbon Dioxide 47.2.2 Carbon Disulfide 47.2.3 Cyanide 47.2.3.1 Cohalt(-Ij und cobalt(0) 47.2.3.2 CobaiffIj 47.2.3.3 cobwrri 47.2.3.4 Cobalt! I l I )
637 637 646 646 646 647 648 652
47.3 SILICON LIGANDS 47.3.1 Silyl Complexes
655 655
47.4 NITROGEN LIGANDS 47.4.I Nifrosyl 47.4.1.1 Synthesis 47.4.1.2 Bonding and structure 47.4.1.3 Reactivity 47.4.2 Nitro ( N O , ) and :Vitrito ( O N O ) Ligandy 47.4.3 Nitrikr 47.4.3.I Cobult(3Il) 47.4.4 Cyanme, Thiocyanaic and Sslenocyanare 47.4.4.1 CobalilIIl) 47.43 Urms. Amides and Peptides 47.4.S.l CobuItfIIlj 47.4.6 Pyridine, Pyrazine. Triazine and Purine 47.4.6.1 Cobalt{lIj 47.4.6.2 Cobalt(IIIj 47.4.7 Phenanthroline, Bipyridyl and Terpyridvl 47.4.7.1 Cobalri-il, cobait(0) and cobalt(I) 47.4.7.2 Coball(1I) 47.4.7.3 Cobait(3II) 47.4.8 Imidazole, Pyrazole. Triazole and Tetrazole 47.4.8.1 Cobali(II) 47.4.8.2 Cohali(I1i)
657 657 657 663 664 666 674 674 679 679 682 682 683 683 687 690 69 1 691 692 693 693 697
47.5 PHOSPHORUS LIGANDS 47.5.1 Phasphorw. 47.5.2 Phosphine.r 47.5.2.1 Hydrides 47.5.2.2 Diniirogen 47.5.2.3 IVitrosyI 47.5.2.4 Monodentale phosphines 47.5.2.5 Bidentate phosphines 47.5.2.6 Tri- and mulii-dentate phosphines 47.5.2.7 Phosphites, phosphonites and phosphinites 47.5.2.8 Phosphates, cobalt(III)
699 699 70 1 704 709 71 1 718 728 738 746 750
47.6 ARSENIC LIGANDS 47.6.1 Cohalt(-ij, CoBalt(0) and Cobalt(1) 47.6.2 Cobalr(11)
767 769 769
63 5
636
Cobalt
47.6.3 Cobalt(lI1) 47.6.4 Arsenatm
773 774
47.7 OXYGEN LIGANDS 47.7.1 Superoxo and Peroxo 47.7.1.1 qI-Superoxo 4 7.7.1.2 q ':ql:Superoxo 47.7.1.3 q2-Peroxo 47.7.1.4 ql:q'-Peroxo 47.7.2 Carboxylates 47.7.2.1 Synthesis, cobalt (IiI) 4 7.7.2.2 Monocarboxylates 47.7.2.3 Dicarboxylates, cobalt(1Ir) 47.7.2.4 Hydroxycarboxylates, cobaltfill) 47.7.2.5 Tridentates and quadridentates. cobalt(III) 47.7.2.6 Quinquedentates and sexidenrates, cobali (III) 47.7.3 Carbonate 47.7.3.i Cobalt(II) 47.7.3.2 Cobalt(II1) 47.7.4 Oxyanions of Cobalt(III) 47.7.4.1 Sulfate, selenate and tellurate 47.7.4.2 Thiosulfate 47.7.4.3 Sulfile and selenite 47.7.4.4 Chromate, molybdate and tungstale 47.7.4.5 Iodate, perrhenate, chlorite and perchlorate 47.7.4.6 Polyoxyanions of Nb', Mo", and I" 47.7.5 Ethers. Cobalt(II)
775 775 776 78 1 784 785 . 190 790 79 1 500 502 803
47.8 SULFUR LIGANDS 47.8.1 Sulfides and Disulfides 47.8.2 Thioiates, Sulfenates and Sulfinates 47.8.2.1 Low oxidation states of cobalt 47.8.2.2 Cobalt(l1I) 47.8.3 Thioethers 4 7.8.3.1 Cobalt [I I ) 47.8.3.2 Thioethers, sulfoxides and muones, cobalt(li1) 47.8.4 Disulfides ( R S S R ) , Cobalt(III) 47.8.5 Trans Effect of Coordinated Sulfur 47.8.6 Thiosemicarbazones. Cobalt(II) 47.8.7 Thiosemicarbazides, Cobalt ( I I I ) 47.8.8 Thionitro and Thionitrosyl 47.8.9 Surarnaies, SulJonamides and Isothiocyunates, Cobalt(lI1) 47.8.10 I ,I-Dithioacids and 1.1-Dithiolates 47.8.IO. 1 I .1- Rithioacid complexes 47.8.10.2 1,l-Dithiolate complexes 47.8.10.3 Thiophosphinates and selemophosphinates 47.8.11 1 ,I-Dithiolates 4 7.8.11.I Ti-is(1.bdithiola tes) 4 7.8.1I .2 Bis (dithiolates) 47.8.12 1,S-Dithiolenes
829
47.9 SELENlUM LIGANDS 47.9.1 Selenaie and Selenite 47.9.2 Selenolute. Selenate and Seleninate 47.9.2.I Cobalt( I I ) 47.9.2.2 Cobalt(III)
880 880 88 1 88 1 88 I
47 10 REFERENCES
882
wi
47.1 47.1.1
n08 81 1 81 1 811 817 817 817
820 825 825
826 828 829 833 833 835 849 849 849 855 856 857
857 858 858 860
860 868 869
871 876 876 880
INTRODUCTION General Scope
The coordination chemistry of cobalt can be summed up in three words: synthesis, structure and reactivity, with synthesis being the most important because it comes first. This chapter concentrates on synthetic aspects and its aim is to provide the reader with starting points to the preparation of cobalt complexes in addition to surveying their chemistry. The chapter deals with complexes formed with groups IV (C, Si), V (N, P, As) and VI (0,S, Se) donor atoms with the following exceptions. Organocarbon chemistry including organocobalt(II1) model systems for vitamin B,, have been considered by Kemmitt and Russell in the companion series
‘Comprehensive Organometallic Chemistry’;’ only CN-, C 0 2 and CS2 complexes are considered here. The extensive chemistry of cobalt with ammonia and saturated amines is dealt with by House, N-containing heterocyclics by Reedijk, and N macrocycles (porphyrins, corrins, phthalocyanines), polyaza macrocycles, and all-encompassing ligands (cryptands, crown ethers and sepulchrates) by Dolphin, Curtis, and Lehn and Mertes respectively in companion chapters in the present series. Similarly, the mixed O,N donor ligands including Schiff bases are dealt with by Randaccio, Calligaris and Bresciani Pahor.
47.1.2 Synthesis Reference Works Table 1 lists useful general texts which may be consulted on preparative methods and recipes for cobalt complexes. Table 2 gives references and preparative procedures for some especially useful Co”’ starting materials and complexes since these are critically important to most investigations; Table 3 gives a more detailed listing of preparations which are to be found in publications which can be relied on for their accuracy and reproducibility. Some comment is appropriate: preparations given in ‘Inorganic Synthesis’ and Brauer’s ‘Handbook of Preparative Inorganic Chemistry’ can be relied on, but the comprehensive information given in the 1974 edition of Gmelin’s ‘Handbook of Inorganic Chernsitry’ is non-critical in this respect.
47.1.3 Structural Reference Works
A structural index of cobalt complexes is inappropriate since this is readily available from standard data bases.’ Specific articles and reviews dealing with structures or structurally related aspects are given in Table 4. Structures, where appropriate to the present chapter, are given in the text or in tables but no attempt has been made to be comprehensive. 47.1.4 Properties and Reactivity Reference Works Table 5 gives references to recent general accounts or reviews of aspects of cobalt chemistry not specifically dealt with in this chapter. Other chapters in the present series also contain much detailed information.
47.2 CARBON LIGANDS 47.2.1 Carbon Dioxide Whereas the highly basic Ir‘, Rh’ and Nio complexes which are active in CO, fixation are monofunctional systems, bifunctional [Co(salen)M] complexes (M = K, Na) which bind C 0 2 Table 1 F’nmary Reference Works: Preparations of Cobalt Complexes or Preparative Methods Authorleditor
A. Werner G. Brauer Various (series) Gmelin W. G. Palmer
G. G. Schlessinger W. L. Jolly M. Shibata
Title, reference
‘Neuere Anschauungen auf dem Gebiete der anorganischen Chemie’, 4th edn., Freidrich Vieweg und Sohn, Brunswick, 1920 ‘Handbook of Preparative Inorganic Chemistry’, 2nd edn., Academic, New York, vol. 2, 1965 ‘Inorganic Synthesis’, McGraw-Hill, New York, vols. 1 (1939)-17 (1977); Wiley-Interscience, New York, vol. 18 (1978)‘Handbuch der Anorganischen Chemie’, SpringerVerlag, Berlin; Kobalt, vol. 58, 8th edn.; new supprement series vols. 5-6, 1973 ‘Experimental Inorganic Chemistry’, Cambridge, New York, 1954 ‘Inorganic Laboratory Preparations’, Chemical Publishing Co., New York, 1962 ‘Preparative Inorganic Reactions’, Interscience, New York. vol. 1. 1964 ‘Topics in Current Chemistry’ series, Springer-Verlag, 1983,vol. I10
Comments
Earliest account of ‘classical’ Co”’ chemistry Preparations of several cobalt salts and complexes, cJ Table 3 Extensive source of primary preparalions; checked, CJ Table 3 (complete vols. 1-22) Comprehensive listing of all reported preparations; not checked for reliability A few basic preparations; good early account A few useful preparations, methods Preparation methods; a few recipes Co”’ preparations; some recipes
638
Cobult Table 2 Particularly Useful Cobalt Complexes for Synthetic Work Complex Cobalril) ICoH(N,)(PPh, ),I [CoH!P(OPh), 1 4 1
Cobalt(III) K[Co(acac),(CN),] [Co(S,COEt)3I Na, [Co(NO,),I [cO(co, )(NH, ),]NO, [Co(C03)(phen),]C1.5H,0 [CO(DMGH),X@Y)l cis-[Co(NO,), (en),]NO,, resolution Cl trrms-[Co(CI), ( p ~ ) ~ ] -6HZ0 [CO(OSO~CF,)(NH~),I(CF, SO,), [ W C O , )(NH,),IC104 tmns-[Co(C1)2(en),]Cl*H20 cts-[Co(OSO,CF,~:(en),](CF,SO,) [Co(DMG),Br(Me, S)] (-)-Na[Co(C,O,),en], resolution fac-[Co(OSO, CF,); dien]
25 26
Cobalt{IIj [Co(IKMe),](BF,), [Co(amc), 1 (Et, .y), [Co{ s2c2 (CN), } * 1
Cobulr{III) Na, [Co(CO,),]. 3H,O CoO(0A) [Co(H)-(PPh3)3] K, [CoiCN), I] Na,[Co(CN),(SO, ),I 3H,O (+)-Ag[Co(edta)], resolution [Co(KH,), (H20)31C13 1.
Complex
Ref.
1
2 3
4 5 6
I 8 21 23
Ref.
9 10 24 11 12 13 14 15 16, 23 17 18 16, 23
19 20 23
B. J. Hathaway, D. G. Holah and A. E. Underhill, J . Chem. Soc., 1962, 2434. M. Shibata, Top. Curr. Chem.,
1983, 110, 15. 2. I 6.Ellem and R. 0. Ragsdalc, lnorg. S.ynrh., 1968, 11, 84. 3. J Bray, J. Locke, J. A . McCleverty and D. Coucouvanis, Inorg. Synlh.. 1972, 13, 189. 4. H. F. Holtzclaw, Inorg. S p l h . . 1966, 8, 202; H. F. Bduer and W. C . Drinkard, J . A m . Chem. Soc.. 1960, 82, 5031 5. G . Brduer, 'Handhook nf Pmparalive Inorganic Chemistry', 2nd edn.: Academic, New York, 1965, vnl. 2, p. 1520. 6. A. Sacco and M. Rossi. Inorg. Synth., 1970, 12, 19. 7. .A W. Adamson, A. Chiang and E. Zinato. J. Am. Chcm. Soc., 1969, 91, 5467. 8. L. Cambi and E. Paglia, Gazz. Chim. Ira/., 1970. 88, 691. 9. L J. Boucher, C. G. Coe and D. R. Herrington, Inorg. Chim. Acta, 1974, 11, 123. IO. F. Galsbol and C. E. Schaffer. Inorg. Synth., 1967, 10, 45. I I . R.J. Angelici, 'Synthesis and Techniques in Inorganic Chemistry', W. B. Saunders, Philadelphia, 1969; Inorg. Synth., 1960, 6, 173. 12. A. V. Ablov, Russ. J. Inorg. Chem. (Engl. Trans/.),1961,6, 157; A. V. Ablov and D. M. Palade, R u n . J. Inorg. C'hern. (Engl. Transl.), i961, 6, 306. 13. Ci. N. Schrduzer, Inorg. Synlh.. 1968, 11, 61. 14. F. P. Dwyer and F. L. Garvan, Inory. Synlh., 1960, 6, 195. 1 5 . J . Glerup, C. E. Schatfer and I . Springhurg, Acta C'hem. Scand., Str. A , 1978. 32, 673. 16. N. E. Dixon, W.G. Jackson, G. A. Lawrance and A. M. Sargeson, Inorg. Synth., 19x3, 22, 104. 17. S. Ficner, D. A . Palmer, T. P. Dasgupta and G. M. Harris, Inarg. Synih., 1977, 17, 152. 18. .I. Springborg and C. E. Schaffer, I R O ~Synllr., ~. 1973, 14, 70. 19. J . Bulkowski, A. Cutler, D. Dolphin and R. B. Silverman, Inorg. Synth.. 1980, UI, 128. 20. J H. Worrell, Inory. Synrh., 1972, 13, 195. 21. C . W. Maricondi and B. E. Douglas. Inorg. Chem., 1972, 11, 688; W. T. Jordan and L. R. Froebe, Inorg. Synth., 1978, 18, 100. 22. R. B. Hogel and L. F. Druding, Inorg. Chem., 1970, 9, 1496. 23. 1\1. E. Dixon, W. G. Jackson? M. J. Lancaster, G. A. Lawrance and A. M. Sargeson, Inorg. Chem., 1981,20, 470. 24. R-ef. 5 , p. 1541. 25. A. Sacco and M. Rossi, Inorg. Synth., 1970, 12, 18. 26. J J. Levison and S. D. Robinson, Inor%. Synth., 1972, 13, 107.
reversibly haie also been described (equation l).3.4TI appears that the alkali metal ion acts as a Lewis acid in a concerted acid-base attack on C 0 2 , with the basic Co' centre as the other partner. The resultant diamagnetic red or red-brown adducts liberate 1 mol of CO, on treatment with acids and have been characterized by IR s p e c t r o ~ c o p yand ~ ~X-ray ~ crystallography.4 The solid complexes are. however, unstable in the absence of a CO, atmosphere. The structure of [Co(Przsaleni(CO,)(K)(THF)], shows that carbon dioxide is C bonded to the cobalt centre with each oxygen bonded to two different K f ions. The Co-C distance of 199pm resembles that found for many alkyl derivatives of [Co(salen)] and the coordinated CO, is bent (0-C-0 angle 135 ' j and has C-0 distances of 122 and 124 pm. The overall geometry about cobalt is square pyramidal (CoN,O,C chromophore). Increased stability of the C0, adducts may result from the binding of bases such as pyridine in the vacant sixth position.' [Co(salen)MJ
+
CO,
e [Co(salen)(CO,)M]
(3 1
Coball
s
w
639
Cobalt
640 PI I-
f
0
2 0;
wrc.
8
2
.^ W
. I
P4
5
3
I3
W"
W 00
f
Cobalt
64 1
642
Cobalt
643
j .-
L,
VI
3
0
I
'
644
Cobalt
Cobalt
& I
.-c *
8
J 0
e c
.-
645
Cobalt
646 47.2.2
Carbon Disulfide
The tris (tertiary phosphine) ligand MeC(dppm), forms both mono- and di-nuclear cobalt-carbon disulfide complexe~.~ The monomer [Co{MeC(dppm),}(CS,)] is formed by the direct reaction of [Co,(CO),] with MeC(dppm), and CS,, or by treating the binuclear Co' complex [{MeC(dppm),]Co},(p-CS,)](BPh,),with sodium naphthalenide or with NaBH, in the presence of excess CS,. The monomer is an air-stable red solid (p = 1.91 BM) with the Co atom coordinated to the three phosphorus atoms of MeC(dppm), and to CS, by a a bond to C and a 71 bond to one C=S group. The S-C-S angle (133.8") and the average length of the C-S bonds (166pm) approach those found for uncoordinated CS2 in the 3A, excited state (135.8", 164pm).' The coordinated CS, group is electron rich at sulfur and is activated towards attack by electrophiles. Thus treatment of [Co{MeC(dppm),}(CS,)] with [Co(H,O),]" in the presence of MeC(dppm), gives the diamagnetic dimer [{[MeC(dppm),]C~},@-cS,)]~+, and the uncoordinated S atom displaces coordinated THF from [Ct(CO),(THF)] and [Mn(CO),(THF)Cp] to form [{MeC(dppm),}Co(p-CS2)Cr(CO),] and [{MeC(dppm),}Co(p-CS,)Mn(CO),Cp]respectively. Structural data for these binuclear complexes are given in Table 6. The linkage arrangement of the CS,-bridged dimers is shown by (1) and (2). In both [([MeC(dpprn),]C~),(CS,)]~+ and [{MeC(dppm),}Co(CS,)Cr(CO),jthe 7c C=S bond is retained and the bond lengths and angles in the Co(CS,) units differ little from those found in [{MeC(dppm),)Co(CS,)]. Apparently 7[: bonding of CS, to the Co atom in (1) does not greatly influence the a-bonding capabilities of the sulfur atoms since the bond lengths of the two Co-S c bonds are nearly identical (- 230pm).
(1)
(2)
With molecular oxygen [{MeC(dppm), )Co(CS,)] in THF solution forms the yellow-green dithocarbonate complex [Co{MeC(dppm), }(S,CO)]. Related reactions using elemental sulfur or selenium give the corresponding trithiocarbonato and selenodithiocarbonato complexes re~pectively.~
47.2.3
Cyanide
The chemistry of cobalt cyanide complexes has been extensively reviewed by Sharpe.8
47.2.3.1
Cobalt( -Z)
and cobaZt(0)
The cyano complexes of cobalt( - I) are restricted to nitrosyl and carbonylnitrosyl mixed ligand complexes. all of which obey the 18-electron rule. Dark-violet K3[Co(CN),(N0)] results from the action of KCN on [Co(CO>,(NO)] in liquid ammonia and the intermediate compounds K[Co(CN)(CO), (NO)] and K2[Co(CN),(CO)(NO)] may be obtained when the reaction is carried out using stoichiometric ratios of the reactants? The conversion of [Co(NO),Br,] to brown Na[Co(CN),(NO),] by treatment with ethanolic NaCN has been described," and similar preparations using [Co(NO),BrL] reactants (L = PPh,, AsPh,, SPPh,. SePPh,) give the corresponding Na, [Co(CN)(NO), L] complexes. The reduction of K, [Co(CN),] by potassium in liquid ammonia d o r d s the air-sensitive brownviolet complex K,[CO,(CN),],~' which is the only cyano complex of Coo so far described. On the Table 6 Structures: CobaltCarbon Disulfide Complexesa Complex
[Co(CS,)IMeC(dPPm), }I
[{[MeC(dppm)3]Co},(CS2)](BPh,), [WC(dppm), JCo(CS,)Cr(CO),l
Comments q2 linkage; Co-S(I)
= 220.6pm, Co-C = ISSpm; planar CoCS, fragment Bridging CS?; Co(1)-CS(2) bound q2; Co(2) bound via u bond to each S atom; Co(I)-S( 1) = 229 pm, Co(1)-C = 172 pm, C0(2)-S = 227, 231 pm; planar Co(CS,)Co fragment Bridging CS,, Co-CS(2) bound q2; Cr octahedral; Cr-S(2) = 244pm, Co-S(2) = 218 pm, Co-C = 187 pm
'C. Bianchini, C. Mealli, A. Meli, A. Orlandini and L. Sacconi. Inorg. Chem., 1980, 19, 2968.
Cobalt
647
basis of IR and Raman spectra bridging CN- groups appear to be absent and the centrosymmetric structure of DU symmetry (3) has been assigned." CN CN
CN
I \ J-CN NC -CO -CO 4% I CN CN CN
47.2.3.2
-la-
Cobalt(Z)
The pentacyanocobaltate(1) ion has been postulated as an intermediate in the electrochemical and chemical reductions of [CoH(CN),I3- as well as in the electrochemical reduction of [Co(CN),I3-. Pulse radiolysis of solutions of [Co(CN),I3- produces [Co(CN),I4-, observable as a transient intermediate which reacts with water to yield [CoH(CN),I3-. The kinetics of these processes has been studied.', The isolation of K, [Co(CN), J as a yellow air-sensitive solid from the chemical reduction of K, [Co(CN),] has been reported but the product has not been fully chara~terized.'~ Carbonylation of solutions of K,[COH(CN)~]yields [Co(CN),(C0),I2-, however attempts to isolate pure salts of this species have been unsuccessful owing to its tendency to decompose and On treatment of the product solutions with phosphines moderately stable di~proportionate.'~ five-coordinate Co' complexes of the type [Co(CN)(CO),(PR,),], [Co(CN),(CO)(PR,),]- and [Co(CN),(CO),(PR,)]- are obtained (Scheme I).'' An alternative route to [Co(CN)(CO),(PR,),] complexes (PR, = PEt,Ph, PEtPh,, PPh,) is provide by direct carbonylation of [Co(CN),(PR,),]. In C,H,Cl, solution addition of CO gives [Co(CN),(CO)(PR,),] initially, which is then reduced by CO to the cobalt(1) product.17 [Co(CN),(CO),I
[Co(CN),(CO)(PR,)I-
PR3
* [Co(CN)z(CO)(PR,)zI- + CO
Scheme I
Oxidation of [Co(CN),(CO)(PEt,),]- (4) by [Fe(CN),13- proceeds quantitatively in accordance with Scheme 2 to yield the cyanide-bridged iron(II)-cobalt(III) dimer I(CN), Fe(CN)Co(CN),(PEt,),(OH,)I3- (5) and CO:-. [CO(CN),(CO)(PE~,)~I+ 4[Fe(CN),l
3-
+ 40H-
-
(4)
Scheme 2
Detailed studies of this reaction agree with the mechanism in Scheme 3." The intermediate (6) is observable as a transient species (A,,, 500nm, E = 5 x lo2), which may decay by pH-dependent pathways to the final product (5), or to the intermediate cyano-bridged iron(II)-cobalt(I) dimer (7). The existence of (7) is inferred from trapping experiments with acrylonitrile (yielding stable [Co(CN),(PEt3)(CH,=CHCN)]- ), or allyl alcohol (yielding the n-allyl complex [Co(CN),(PEt,),(q-C3H5)]. The observation that (7)is also trapped by CO provides a basis for the catalytic oxidation of CO to CO, (CO:-) via the cycle (4) -+ (6)-+ (7) (4). -+
(CN)2(CO)(PEt,),l(4)
IFe(CN),I
'-
[ (NC),Fe"(CN)Co"'
(6)
1
(CN12(PEt3 12(C02H) I 4- Cot/--H*
[(NC),Fe1tlCA1)Co'(CN),(PEt3)21 5 (7) Scheme 3
-'O
H+,a,
= ~(NC)SFe"(CN)Co"'(CN)z(PEt3)2(0H2)l3 (5)
648
Cobalt
Hydrazine reduction of [Co(CN),(dpe),] affords the air-sensitive five-coordinate complex [Co(CN)(dpe),].” This process involves dsplacement of coordinated cyanide ion accompanied by chelation of the dangling dpe ligand. One diphosphine ligand in the product is readily displaced by CO giving five-coordinate [Co(CN)(CO),(dpe)J.
47.2.3.3 Cobalt(II) Light-brown precipitates of cobait(1I) cyanide containing varying amounts of bound water are obtained from addition of stoichiometric quantities of cyanide ion to aqueous solutions of cobalt(1I) salts. On heating to 250 “C the hydrates lose water to give blue [CO(CN),],.’~This form of cobalt cyanide possesses the ability to form inclusion compounds with a wide variety of small orgnaic molecules, and X-ray powder patterns indicate an open structure based on a face-centred cube with a = 1 0 1 2 ~ m . ~The ’ structure was originally suggested to be related to that of Prussian blue, KFe[Fe(CN),], with Co2+ replacing R’ and Fe2+ and [Co(CN),I4- replacing [Fe(CN),I4-. However, this structure would need to contain 12 formula units in the unit cell, which is incompatible ). similarities exist between the structure of with the observed crystal density (1.87 g ~ m - ~Although [Co(CN),], and those of both Prussian blue and Co3[Co(CN),], the nature of the differences remains uncertain?*” The olive-green [Co(CN),I3- ion is the predominant cobalt(I1) cyanide species in deoxygenated aqueous solutions of cobalt(I1) salts to which excess cyanide ion has been added. The pentacyanocobaltate(I1) ion is paramagnetic and undergoes rapid exchange of bound CN- .” UV-visible and ESR studies have been interpreted in terms of a purely five-coordinate structure (Ch but the anion may be hexacoordinate with a weakly bound water molecule in the sixth position and with C,, symmetry maintained.24Studies of the ion in solution are complicated by its tendency to dimerize to diamagnetic violet [ C O ~ ( C N ) , ~the ] ~ -structure , of which has been ~haracterized.2~ The dimer possesses D4dsymmetry with a Co-Co bond separation of 279.8 pm and with the two sets of equatorial cyanide ligands in staggered conformation (8).
(8)
A yellow form of the pentacyanocobaltate(I1) ion has been observed in DMF solution. Various monomeric (NR,),[Co(CN),] complexes have been crystallized” and a structural study on one shows the anion to possess a truly five-coordinate square-pyramidal geometry.27Electron irradiation of solid K, [Co(CN),] gives a presumed pentacyanocobaltate(I1) ion,2sthe visible spectrum of which is virtually identical with that of [Co(CN),]&. The structural parallels between the green and yellow forms of [Co(CN),]’- and isoelectronic [CO(CNR),]~’ ions indicate that all the green forms observed both in the solid state and in solution are weakly coordinated in the sixth axial position. The existence of a tetracyanocobaltate(I1) ion in aqueous solution has been inferred from studies at high dilution and a brown polymeric complex of composition K2[Co(CN)s] has been isolated from solutions of [Co(SCN),] and KCN in ammonia.” Recently, the monomeric air-sensitive ’ ~ complex is complex [(PPh,),N],[Co(CN),] * DMF has been crystallized from DMF s ~ l u t i o n , The essentially square planar but does feature a very weak axial interaction to a neighbouring DMF molecule (Co-0 separation 264pm). The [Co(CN),I2- anion appears to be the sole example of a square-planar low-spin ( p = 2.15 BM) cobalt(I1) complex containing solely unidentate ligands. Of the various cobalt(I1) cyanide complexes the reactions of [Co(CN),I3- have been the most widely studied. Oxidative addition is the characteristic reaction of the low-spin ion, and the driving force for reaction derives from the increase in stability achieved on passing from the 17-valence-electron configuration of the reactant to the closed-shell, 18-valence-electron configuration of the cobalt(II1) product. These reactions have been reviewed.,’ Two-electron oxidizing agents with [Co(CN),I3- either give bridged [(NC),Co(X)Co(CN)J- species (X = 0, p-benz~quinone,~~ C, F, ,33 CZH2,34 SOz,3s S I I C ~ , ~or ~ ) undergo reductive cleavage with formation of monomeric ICN,38 H20,39H,O, ,38 alkyl halide,39 cobalt(I1I) products (equation 2, X-Y = H,,36 hal~gen,~’ NHzOH3*).Except for the reaction of H, (and possibly H,O) these processes occur through a
I Cobalt
649
free-radical stepwise mechanism (Scheme 4):'~~~For alkyl halides, the rate-determining step is halogen-atom abstraction and the alkyl radicals so produced may combine with [Co(CN)J- giving organocobalt(II1) products or may disproportionate to alkanes or alkenes. a,w-Dihalides are dehalogenated.
-
+
~ [ C O ( C N ) ~ ] ~ - X-Y [Co(CN),l
3-
[Co(CN),] 3 -
+ X-Y
+ Y.
[Co(CN),X]'-
-
+ [c0(cN)5y]3-
[Co(CN),X] 3-
+Y
(2)
*
[Co(CN),YI 3-
Scheme 4
Hydrogen absorption by aqueous solutions of [Co(CN),]'- was first reported by Iguchi in 1942.'"' H,uptake corresponds to 0.5 mol per mol Co and is accompanied by formation of the colourless hydrido complex [CoH(CN),]'-?' The same product results from the reduction of [Co(CN)J3electrolytically or by means of NaBH,. Hydrogenation is reversible with X = IOs dm3mol-' at 25 "C (equation 3). The forward reaction follows third-order kinetics with rate = k[Co(CN):-]'[H,]. This is consistent with a mechanism involving rate-determining reaction of H, with [CO,(CN),,]~- since the latter exists in rapid equilibrium with [Co(CN5I3-?' The further observation that both [CoH(CN),13- and [CoD(CN),I3- are produced when the hydrogenation is carried out in D,O media has been claimed to support a mechanism involving heterolytic fission of H, ?2 However, homolytic fission with subsequent Co-H/D, 0 exchange would lead to the same result and this issue is 2[C0(CN)5l3-
+
Hz
(3)
2ICoH(CN)5l3-
Several low-spin cobalt(1I) cyanide complexes containing tertiary or secondary phosphine ligands, and corresponding to the general formula [Co(CN),P,] have been prepared by halide-cyanide exchange (CN--form anion resin) from tetrahedral [CoX,P,] complexes (X = C1, Br) and an excess of phosphme (Table 7).44345 With ditertiary phosphines the reaction gives either diphosphinebridged binuclear complexes [(P,P)(CN),Co(P,P)Co(CN),(P,P)](P,P = dppp, dppb) or mononuclear complexes [Co(CN),(P,P),] (P,P = dppe) in which one of the diphosphine ligands is monodentate. In all cases the reactions yield orange or red complexes which are five coordinate with a C2P, donor set. In solution (C2H4C12,benzene) they are frequently unstable and are decomposed by a dissociative process to yellow or green products presumed to be four c o o r d i ~ a t e . ~ ~ In many respects the reactions of the monomeric mixed phosphine-cyanide complexes parallel those of [CO(CN),]~-.Thus, [Co(CN),(dppe),] undergoes oxidative reactions with SOz, SnCl,, and alkyl and hydrogen halides, and forms a peroxo dimer on exposure to dioxygen. This latter species is unstable however and in methanol reacts with solvent giving CH,O and [C~(CN),(dppe),]+.~ In C2H4C1, and in the presence of dioxygen the dimer oxidizes solvent to CO, and this process is accompanied by formation of the zwitterionic complex [(~~~~),(NC)CO(CN)COC~,].~~ a
Table 7 Five-coordinate Cobalt(I1)-Phosphine Cyanide Complexes Complex
Red or red-brown, PR, = PMe,, PMe,Ph,
[Co(CN)dPR,),I [Co(CN), (PEt, W,l [Co(CN),(PEtPh,), 1 [Co(CN),IPhz W H J , PPhZJ1.51
~ ~ ~ ~ ~ N [Co(CN),(PHRZ13 1
ReL
Comments
)
I
~
~
,
PMePh,, EPR Dark red, decomposes at 11&113"C, p = 2.OBM, unstable in solution Red, decomposes at 108-1 I O T , p = 2.OBM Pink-red, n = 3, 4; I.( = 2.3-2.4 BM,possibly dimeric ~ Red-brown, ~ ~ ~ decomposes z ~ , at ~ 251 ~ "C, ~ p ~= 2.2 z BM l ~ ~ Red or orange red, R = cyclohexyl, Ph; p = 1.9-2.0 BM
1. R. S. Drago, J. R.Stahlbush, D.J. Kitko and J. B r m , J. Am. Chem. Soc., 1980, 102, 1884. 2. P. Rigo, M.Bressan and A. Turco, Inorg. Chem., 1968,7, 1460. 3. P. Rig0 and M.Bressan, Inorg. Nucl. Chem. Let!.. 1973, 9, 527. 4. P. Rig0 and A. Turco, Coord. Chem. Rev., 1974, 13, 133.
1 2
2
2 ~
~3 , 4
~
650
Cobalt
Cobalt
652 4 7.2.3.4
Cobalt Cobalt (ZIZ)
The preparation and properties of a number of cobalt(III)-cyano complexes are given in Table 8. Many reactions involving formation of cobalt(II1)-CN bonds are catalyzed by cobalt(I1) species. Adamson3' first proposed that catalysis of cyanide substitution in [CoX(NH3),I2+ions (X = 3r-, I - ) to form [Co(CN),XI3- involved the participation of the substitutionally labile [Co(CN),I3(Scheme 5 ) . Subsequent mechanistic studies have been interpreted in terms of inner-sphere electron transfer between [Co(CN),I3- and [CoX(NH3),]*+through bridged [(NC), CoXCo(NH,),] intermediates (X = C1-, N; , NCS-, OH-). For a number of other pentaarnminecobalt(II1) complexes, including those where X = PO:-, RCO;, COi-, SO:- and NH,, cyanide substitution, while still catalyzed by Co", follows a different stoichiometry (equation 4). These processes are believed to proceed via outer-sphere electron transfer between [Co(CN),I4- (presumed to exist in equilibriuin with fCo(CN),I3-) and the [CoX(NH,), 3' r e a ~ t a n t .These ~ . ~ ~ latter reactions have no real preparative value however and the {Co(CN),I3- ion is usually prepared as its potassium salt by aerial oxidation of hot solutions of cobalt(I1) salts containing excess KCN. The product is obtained as a pale-yellow crystalline solid, soluble in water but insoluble in common organic solvents. Its parent acid, H,[CO(CN),] .0.5H20,is readily obtained by chromatography on acidic-form cation-exchange resin and may be used to prepare various other M"+ or quaternary ammonium salts.49 ~
[Co(CN)S] 3-
+ [CoX(NH,), 1
[Co!CN),XI
2+
+ -
[Co(NH,),12*+ 5CN-
3-
+ ICo(NH,)Sl2+
[Co(CN), I 3 - t 5NH,
Scheme 5
[ C O X ( N H ~ ) ~ ~6+cW-
[CO(CN)~]'- + x"-
+
5NH,
(4)
Strong acids, bases and oxidants leave [CO(CN),]~-unaffected and the only substitutions of preparative significance are photosohation or photoanation. Reduction gives either Coo- or Co"cyano complexes depending on conditions .' The spectroscopy and photochemistry of the [Co(CN),I3- ion have been studied extensively and have been reviewed.,' Bands at 3 12,260 and 202 nm in the electronic spectrum of aqueous solutions are assigned to the ' A ! , ITlg,'A!,-, ' T , and CT transitions respectively,5' and similar spectroscopic features are exhibited by solid-state ~arnples.'~ Recent studies have identified a weak absorption band at 390 nm as being associated with the singlet to tripiet 'A,, ' T I gt r a n ~ i t i o n . ' ~ + ~ ~ and lran~-Na,[Co(CN),(S0,),]~~) Crystalline K3[Co(CN)6](and also c~~-K,N~,[CO(CN)~(SO,),] exhibits phosphorescencefrom the 3T,,excited state of the molecule at low temperature^,^, and laser irradiation of [CO(CN),]~- in a glass matrix at 94K gives rise to a transient spectrum (tl!z = 1.5 x lo-, s) in the 350 to 500 nm region assigned to absorption by this excited-state species.s4 In aqueous solution [Co(CN),I3- undergoes efficient CN- photoaquation (equation 5 ) with a wavelength-independent quantum yield ( 4 = 0.3) irrespective of whether irradiation occurs in the 2 5 4 3 6 5 nm region,,' corresponding to spin-allowed LF absorptions, or in the 405-436 nm region,'* corresponding primarily to direct triplet excitation. The aquapentacyanocobaltate(II1)ion is effectively the terminal product in these processes since it is practically photoinert with respect to further substitution of cyanide. A value of A f l of 1.3 cm3mol-' has been reported for the reaction." --$
-
--f
+
[Co(CN),I3-
+
H20
3 [CO(CN),(OH,)]~-+ CN-
(5)
Photolysis of aqueous solutions of [Co(CN),X]"+ (X = CI-, Br-, I-, N; , SCN-, H 2 0 , NH,) and [Co(CN),(SO,)V- (Y = SO:-, OH-, H 2 0 ) in the LF region involves replacement by H,O ofthe X and SO:- ligands respectively. Unlike the photoreaction of [CO(CN),]~-,photoaquation efficiencies depend on ionic strength and these reactions appear to occur through dissociative mechanisms involving five-coordinate intermediates. In contrast, photoaquation of [Co(CN),13- is interpreted in terms of a photoactivated interchange mechanism between inner-sphere CN- and an outer-sphere water molecule.50This photoactivated interchange has been shownw to be the primary step in the photoanation of [Co(CN),I3- by nucleophiles (Y = I-, N;) and photoproduction of [Co(CN),Yj3occurs exclusively from the immediate product [Co(CN),(H,0)I2- (Scheme 6). Irradiation of cobalt(III)-cyano complexes in the CT region results in photoredox processes in parallel with substitutions. The former are comparatively inefficient, presumably because of effective conversion mechanisms from CT to LF states. However, UV irradiation of rrans-[C0(CN),(SO,)(H,0)]~-,6' [Co(CN),(NO,)l3- 50 and [Co(CN), (0H)l3- 62 produces Co'l species, and oxidized acidoligand products have been detected during photolysis of [CO(CN),X]~-anions (X = N;, I-, SCN-)!3764
I 653
Cobalt [CO(CN)~] 3- -k [Co(CN),(OH,)I
''
H20
'-+ Y-
'-+ CN"
ICO(CN)~(OH,)]
-
[Co(CN),Y] 3-
+ H20
Scheme6
Early kinetic measurements on the thermaI anation of [Co(CN),(H,0)l2- by anionic nucleophiles (Y = N; , SCN- ) indicated that substitution proceeded with a less-than-first-order dependence on [y1 and approached a limiting rate at high concentration^.^^ This, together with the substantial agreement between the limiting rates for the thiocyanate and azide systems and the apparent water-exchange rate on [CO(CN),(H,O)]~-,indicated that substitution occurred via a limiting dissociative mechanism involving five-coordinate [Co(CN),]*- as a reaction intermediate (Scheme 7). (Co(CN),(OH,)I
[Co(CN),] 2 -
'--+
+ Y-
[Co(CN)SI
-
'-+ HzO
ICo(CN),Y] 3 -
Scheme 7
Similar kinetic behaviour is found for substitution of H,O by CN- in trans-[Co(CN),(S0,)(H,0)]4-.66However, a reactive intermediate [Co(CN),(SO3)I3-species generated by loss of H 2 0 from the coordination sphere need not, in this case, be five coordinate since the possibility exists for rearrangement to a configuration where SO:-. is bidentate (Section 47.7.4.3). The reactions of [CO(CN),(H,O)]~-with N; ",'* and SCN-68 have been reinvestigated and important differences found which indicate that the evidence in favour of limiting kinetics is not as clear as originally supposed. In particular, deviations from the simple second-order rate law for anation, rate = k[Co(CN), (H,0)2-]v-], appear to be small and the question of reaction mechanism remains somewhat open at present. The effect of solvent on reaction between [Co(CN),(H20)]'- and Ny6' and the small positive activation volumes for anation by Br-, I- and SCN-70 are indicative of a dissociative mechanism, but the existence of a long-lived five-coordinate intermediate has not been definitively established. The thermal reaction between truns-[Co(CN),(S0,),l5- and 13CN- in aqueous solution results in stereospecific incorporation of label to form aran~-[Co('~CN)(CN)~(S0,)1~-, whereas when the reaction is carried out photochemically, labelled cyanide is randomly in~orporated.'~Photodissociative loss of SO:- appears to lead to an electronically or vibrationally excited intermediate which has a chemistry quite distinct from the intermediate produced in the thermal reaction. Whatever its exact nature, the photoexcited intermediate has a short configurational lifetime and this causes random introduction of I3CN-. Structural data for cobalt(III)-cyano complexes are given in Table 9. Numerous studies have been carried out on hexacyanobaltates(II1) and on Prussian blue analogues of the type M, [Co(CN),],. 12H20.Various structural models have been proposed for the latter complexes and a discussion of these and the physical changes which occur on dehydration has been given by Bea11.72In Co"' cyanide complexes the Co-C and C-N bond distances (typically 189 2 and 115 k 2 pm respectively) show little variation irrespective of whether the ligand is terminal or bridging. Better sensitivity to the coordination mode is shown by the stretch frequencies, vm, which generally occur at a wavenumber 60-80cm-' higher for bridging cyanide ligands than for terminal. In a series of related cyanoaminecobalt(II1) complexes the respective frequency ranges of 2190 k 10cm- ' and 2130 f IOcm-' are characteristic of these two modes. Structures have been reported for both the [(Tic), Co(CN)Co(NH, and [(NC),Co(NC)Co(NH,),] linkage isomers as well as for [Co(CN)(DMGH),(H,O)] and its isostructural linkage isomer [Co(NC)(DMGH),(H20)]. The electronic spectra of the binuclear complexes do not correspond to those expected for an average environment of CN- and NH, ligands on a single Co"' centre but rather to the sum of the spectra of the two cobalt(II1) moieties. Similar spectral relationships are found for the thiocyanate-bridged dimers [(NC), Co(SCN)Co(NH,),] and [(NC),CO(NCS)CO(NH,),].~~ The isocyano complex [Co(NC)(DMGH),(H,O)] appears to be the only Co"' complex containing a monodentate isocyano ligand, and is prepared from the reaction of [Co(DMGH),(NH,),] with [Ag(CN),]- in aqueous solution." The similar reaction with CN- in the absence of silver ion yields the corresponding C-bonded isomer. [Co(NC)(DMGH),(H,O)] is hydrolytically stable but is converted to its linkage isomer on heating to 250 "C.
0-
cc
F
c
l l
Cobalt
655
The [CoH(CN),I3- ion results from treatment of solutions of [Co(CN),]’- with H2 (Section 47.2.3.3). The hydrido complex is somewhat unstable in solution and salts are obtained with difficulty, however Cs2Na[CoH(CN),] has been isolated as a pure colourless solid which displays vCN = 21 13cm- I and vCoH= 1840 cm- ’. Freshly prepared solutions of the ion have = 305 nm and a ‘H NMR signal 17.6p.p.m. upfield from the H,O resonance.8 The [CoH(CN),I3- ion has been used extensively as a catalyst in various homogeneous hydrogenations and reviews of the catalytic applications are a ~ a i l a b l e . Under ~ ~ ~ ~neutral ~ ~ ’ ~conditions the classes of compounds reduced include conjugated dienes, styrenes, cr,fi-unsaturated acids (as either salts or esters) and aldehydes, 1,2-diketones and epoxides. Many of these reactions involve a-complex formation and under favourable conditions stable organopentacyanobaltate(II1) products may be isolated.m Simple alkenes and alkynes are not reduced but terminal alkynes incorporating an OH function are either selectively hydrocyanated to saturatsd secondary nitriles or reduced to alkenes (Scheme 8).77C, allylic alcohols are catalytically hydrogenolyzed to alkenes with the product distributions being highly dependent on reaction condition^.^' Similar selectivities have been noted in the catalytic hydrogenation of conjugated dienes to rnonoalkene~.~~ RC=CH
-I -I [CoH(CN),I’-
[RC=CHzI
RC=CHz
Co(CN)s
I
[CoH(CN),
1’-
]EoH(CN)~ ]
’-
RCH-Me
RCH=CH2
I
CN Scheme 8
Carbonylation of [CoH(CN),]’- in strongly alkaline media results in the formation of [Co(CN),(CO),]*- and the intermediacy of the short-lived tetracyanocobaltate(1) ion [Co(CN),I3 in this process has been inferred from mechanistic studies (Scheme 9).15 Under very similar conditions [CoH(CN),I3- catalyzes the cyanation of vinyl halides to form ci,j?-unsaturated nitriles with retention of configuration (equation 6). Oxidative addition of vinyl halide to the intermediate {Co(CN),]’- ion is stereoselective and in the (T complex so produced reductive coupling of the vinyl and cyano ligands gives the stereoretentive organic product and regenerates [CO(CN),]”.~~
1 4-
[Co(CN),
[ Co(CN), ] 3 -
+ 2CO
-
[Co(CN),]
+ H20
3- f
[Co(CN),(CO)J
CN-
’-t CN-
Scheme 9
>c=c,
/
CN
47.3 47.3.1
SILICON LIGANDS Silyl Complexes
A few trialkylsilyl (SIR;) complexes of cobalt are known and a recent account of the reactions of transition-metal trialkylsilanes is a~ailable.~’ The cobalt complexes are often prepared via oxidative addition of HSiR, to a low-valent carbonyl or dinitrogen complex (equation 7) or by elimination of H, from a cobalt hydride (equation 8). They are usually air sensitive, and unstable with respect to displacement of HSiR, on heating or on treatment with N,, H, or CO, sometimes in a reversible manner. Deuteration (Scheme 10) provides a useful catalytic route to DSiR, systems through the intermediacy of the 16-electron [CoD(PPh,),] complex, and 0-silylation to give %(OR), species (R = Me, Et) is possible at ambient temperatures via [CoH,{Si(OEt),](PPh,),] in the appropriate alcohol.s0
656
Cob& [CoH(N,)(PPh,),] [CoH(CO),(PR;),J
+ HSiR3 +
HSiK,
e
-
657 [Co(H),(SiR,)(PPh,),] f Nz
(7)
[CO(S~R,)(CO),(P'R,),-~] + H2
(8)
Scheme 10
Various substitutions are possible (equation 9)8' and [CoH(CO),] is known to catalyze the addition of HSiMe, to C,H, at room temperature to give high yields of SiMe,Et.**Treatment of [Co(SiMe,)(CO),] with ROH or NH,R rapidly results in SiMe,OR or SiMe,(NR,) species and [CoH(C0)I4, while GeF, and AsMe,Cl generate SiMe,F and SiMe,C1.8'
47.4
NITROGEN LIGANDS
47.4.1
47.4.1.1
Nitrosyl Synthesis
Phosphine complexes containing the NO ligand are considered in Section 47.5.2.3. Most remaining cobalt nitrosyls are simply prepared from the corresponding isolated or in situ Co" complex and NO,, (Table 1 1). General articles dealing with synthetic methods are available.*3-86 There appears to be no known synthesis via oxygen abstraction from coordinated ONO-, NO; or NO;, or by oxidation of coordinated NH, or NH,OH. Dimeric [Co(NO),X], (X = C1-, Br-) is a potential starting material for bis(nitrosy1) complexes, but few examples of its use outside the phosphine complexes (Table 34) exist. Most cobalt nitrosyls are diamagnetic with an 18-electron count if NO is considered a three-electron donor in the four- and five-coordinate systems and a one-electron donor in six-coordination; paramagnetic [Co(NO)(N(dppe), }](BPh,) (17-electron, tetrahedral) and [Co(NO)Cl(das),](ClO,), (1 7-electron, probably octahedral) are exceptions (Table 1 1). The chemistry of the reaction of NO with ammoniacial solutions of Cof& has had a long and interesting history. As early as 1903, Sand and GensslerS7demonstrated the existence of two isomeric forms of [Co(NO)(NH,),]X,, a black series (X = C1-, NO;, IO;) and a red series (X = Br-, I-, NO;, SO: - ); the former liberated NOe, rather readily in aqueous solution but the latter was far more stable, especially in dilute acid. Based on empirical evidence, Werner and Karrer" considered the black series to be monomeric, designated (9), and the red series to be dimeric with a hyponitro structure (10). Since that time the reported paramagnetism of the black series has been corrected (both are diamagnetic) but most investigators (and there have been a large number) considered Werner to be wrong and assigned the same or similar structure in the opposite sense. However Werner and Karrer were indeed correct (or nearly so) as the known structures (11) and (12) testify (Table 12).
( 1 1) black
(12) red
658
Cobalt
659
6 m
2P 000
I
2
Cobalt
66 1
0
c. I
P z \
x =O
/
s
I
s'1
662
Cobalt
Preparaticn of the black monomer is achieved at 0 "C after short reaction times; the red dimer i s formed at ambient temperatures using longer reaction times (Table 11). Also the red dimer can
be prepared from the black monomer and isomer and an intermediate hyponitrito monomer [CO(N,O,)(NH,),]~+appears to be involved:' Miki and Ishimorigosuggest that by using ammoniacal solutions which have been pre-exposed to the atmosphere a second hyponitrito isomer, assigned strscture (13), can be formed. The isomer is less stable in solution giving rise to [CO(NO,)(NK,),]~+ and Co(OH), in boiling water and its thermal decomposition appears to differ from the other^.^^^^' However structural clarification is necessary. (NH,),co~+-N-0-0-N
-co(Pr'H,):'
(13)
Treatment of black [Co(NO)(NH,),]Y,(Y = CI-, NO;) with KCN at 0°C gives yellow K3[C~(NO)(CN)5]-H,0,which may weit have a dimeric hyponitrito structure, although direct structural evidence is l a ~ k i n g . ~IR ' - ~data ~ support a cis or skewed hyponitrito bridge and earlier claims that the same compound could be prepared from red [CO(NO)(NH,),],(SO~)~.~H,O have been d i s c o ~ n t e d Further .~~ treatment with CN- leads to N20,, (equation 10). Several mixed carbonyl-nitrosyls having the 1 8-electron count have been prepared from [Co(NO)(CO),] or [Co(NO), Brl2 (Table 1 1). All are four coordinate and probably tetrahedral, although the low vNo for K,[CO(NO)(CN)~](1485 cm-') may imply square-planar coordination with a bent nitrosyl. Rates of substitution in poorly coordinating solvents (THF, xylene; equations 11 and 12) foilow the rate expression k,, = k, + k,[L], with k,and k2being attributed to dissociative paths involving the sovlent and ligand respe~tively.~~ For L-L chelates formation of the monodentate species is rate determining." [Co(NO)(CN),];-
+
[Co(NO)(CO),] [Co(NO)(CO),]
+
2CN-
+
+L
L-L
2H' --*
-+
+
2[C0(CN),l3- -t H2O
[Co(NO)(CO),L]
+
[Co(NO)(CO)(L-L)]
+ N20
(10)
CO,gi
(1 1)
+ 2CO,,
(W
Oxidative addition of NONOz was suggested many years ago by Rossi and S a ~ c for o ~reaction ~ (13) but "N experiments are necessary to confirm this. However Lippard and coworkers97have demonstrated NO+ addition in the reaction of dimeric [Co(SNNS)], (SNNS = - SCH,CH,N(Me)CH,CH,N(Me)CH,CH,S-)with NOPF,. Here NO+ adds across the @-SR),-bridging dimer to form the triply bridged diamagnetic Co"' complex (14; Table 12). [Co(NO)(PPh,),] -t 7 N 0
+
[Co(NO),(NO,)(PPh,)]
+
ZOPPh,
+
N,O
+
0.5N2
(13)
hl e
(14)
Structures of two other p,-bridged nitrosyls [{Co(Cp)},(p-NO),], [{Co(Cp)},(p-NO,CO)] are known (Table 12) and the preparation of [(Co(C,R,)},(p-NO),~+ (R = H, Me; n = 0, 1, 2) and [(Co(C5Me,)), (p-NO,CO)y' (n = 0, 1) from [Co(CO),(C,R,)] and NOPF6 in dichloromethane, and their electrochemistry, have recently been discussed by Connelly, Payne and Geiger?8 Reversibility in the coordination of NO to Co" is not as widespread as it is with 0,, and this is supported by theory.99The Co-NO bond is stronger than the Co-0, bond, particularly in the absence of a sixth ligand, and with spin-pairing complete there is no driving force to form a Co--NO-Co dimer similar to that found with Co-0,-Co complexes. Reversible coordination is however fclund in a few cases: in [C~(NO)Cl(dipy),]+'~ and in some ill-defined [Co(NO)(amino acid)(base)] species.I0' Nitrosyl transfer from [Co(NO)(DMGH),] to various electron-deficient Fe, Co, Ru and Rh complexes (e.g. to [ C O C I , ( P P ~ , ) ~to ] , 'iron ~ ~ hemoproteins (e.g.hemoglobin),'03and may result From low concentrations of NO generated in to O2 leading to [CO(NO~)(DMGH),]'~ equilibria such as (14) and (15) rather than from the direct addition of coordinated NO.
[Co(NO)(DMGI-I)Z] [Co(NO)(DMGH),] 47.4.1.2
+
CohaIi
663
e
(14)
B
[Co(DMGH),] -t NO
e
ICo(DMGH),B]
+ NO
(15)
Bonding and structure
A recent review on structural aspects is available,'05 and accounts of bonding have been given by Enemark and FelthamIo6and Mingos.lo7Three distinguishable coordination modes occur in transition metal nitrosyl: terminal linear (15) or (16), terminal bent (17), and bridging (18). These formally correspond to the coordination of NO and NO", NO-- and NO- respectively. Terminal Co-ON coordination has not been found since N is the more basic end of the N O dipole, but it may be possible to form this isomer via reactions of Co=O or Co-0,, or by appropriate cleavage of dimeric Co-N(0)NO-Co systems (see below). Some unusual bridging structures involving 0 and N coordination are known (Section 47.5.2.3). 0
II " I
0
II
N
I
//O :N
I
co
CO'
co
(15)
(16)
117)
F
; /N\
co
co
(18)
Low-oxidation-state cobalt complexes are invariably four or five coordinate with linear or nearly linear NO bonding. Tn these (15) is considered as NO coordinated to Co" uiu a three-electron bond and (16) as NO+ coordination to d 8 Co'; the latter description is usually preferred although an overall molecular orbital is involved and individual oxidation states have no reality, and perhaps little usefulness. A general (MNO}" picture has been developed by Mingoslo7and by Enemark and Feltham"' (n is the sum of electrons associated with the metal d orbitals and the n*(NO) orbitals of NO groups), and a more specific {MNO)' scheme by Hoffmann and coworkers.'" Larkworthy and coworkers have recently correlated "NO and "Co NMR shifts in five- and six-coordinate cobalt-nitrosyl complexes."* Shielding parameters decrease with decreasing MNO angle consistent with the argument that (as a general rule) bent nitrosyl is a weaker ligand than linear nitrosyl. All known [Co(NO)L,] systems have close to tetrahedral symmetry (C,), with linear or nearly linear NO coordination (Table 12) consistent with a closed shell {CoNO)" onf figuration.'^^^'^^^''' A square-planar structure should contain bent NO1'*Io' but no example has yet been found. Fenske and coworkers have used extended MO calculations to predict acceptable ionization energies for [CO(NO)(CO),],"~and emphasize the relationship between isoelectronic CO and NO+ by relating the electronic structure of [Co(NO)(CO),] to symmetry lowering in Ni(CO), and Fe(CO), . Evans and Zink use photochemical results to predict a bent NO in the excited state of [CO(NO)(CO),]."~ Many four-coordinate systems contain two NO ligands with [Co(NO), Cl], being tetrahedral and dimeric (C1 bridges), while [Co(NO),I], is tetrahedra1 with infinite --I-Co-Ichains and no Co-Co bond (Table 12). Canadell and Eisenstein discuss factors influencing this choice of structure.II4 For tetrahedral monomeric [M(NO), L2] systems, Martin and Taylor have correlated M-N-0 bending with the ON-M-NO angle; bending away of the 0 atoms accompanies larger NCoN angles while smaller angles are found when the nitrosyls point toward each other."' This is in agreement with theory,Io6and all [Co(NO),L,] structures are either close to the cross-over point (NCoN = 131 "3 or the two nitrosyls point towards each other. However Kaduk and Ibers'I6 point out that although available bonding schemes allow rationalization of known structural results, they are not yet able to predict confidently the ground state geometry or more detailed stereochemistry. The 'suck it and see' experimental approach is still of value. [Co(NO){N(dppe), ]](BPb) has the { C O N O )configuration ~ and has a slightly distorted tetrahedral structure (Table 12) not unlike that found in high-spin [CoCl(N(dppe),]](PF,) (Table 40). The one unpaired electron (p = 1.98 BM) lies predominantly in a metal-based orbital,lo8 which is consistent with the considerable bending in the CoNO grouping (165 "). For { CoNO)* systems the following conclusions arise."' (i) for 6- and x-donating ligands a range of geometries is possible from a strongly bent square pyramid (21; C,>to a less bent trigonal bipyramid (19; C2v).A bent nitrosyl will tend to move its Co-N coordination axis in the direction of increased TC overlap; in the square pyramid the bent structure (C,) is favoured over the linear one (20; Ch).
664
Cobali
(ii) For n-acceptor ligands the trigonal bipyramid is favoured over the square pyramid with NO being equatorial (C;,), and if a nitrosyl bends it will do so in the axial plane rather than in the equatorial plane. (iii) Linear coordination is expected for axial coordination in a trigonal bipyramid and for basal coordination in the square pyramid. Geometrical deductions have also been made for mixed L systems.'DsObserved structures (Table 12) agree with these conclusions in most instances; thus [Co(NO)(das),](ClO,), is trigonal bipyramidal with linear NO coordination, [Co(NO)(Me,dtc),], [Co(NO)(Pridtc),] and [Co(NO)(salen)] are square pyramidal or tetragonal pyramidal with a bent NO, but [Co(NO)Cl,(PPh,Me),] adopts an intermediate bent geometry with a stereochemistry of bending at variance with prediction. The second isomer found in the [Co(NO)Cl,(PPh, Me),] system"' has not been structurally defined but is thought to be a rotational isomer of the C, manifold rather than one of the more limiting geometries.'O' N o paramagnetic {CoNO}' systems have been found, but a dramatic change in stereochemistry accompanies the conversion of five-coordinate [Co(NO)(das),](C10,), (22), to six-coordinate [Co(NO)(NCS)(das),](ClO,) (23).'18The increased energy of the dzzorbital shifts the electron pair to the N atom, increasing the tendency to bend with NO- coordination to Co"'.
(22) v N o = 1852cm-' CoNO = 179"
C S
(23) v N o = 1567 crn-' CoNO = 135"
Bent structures (17) and (18) are favoured for six-coordinate Co'" systems (CoNO, 12Ck-130O; vNo z 1 6 0 0 ~ 1 ~and ' ) are clearly distinguished from the others. Although often synthesized from d7 Cot' complexes (or Co:& L) and NO,,,, internal redox and coordination of NO- is clearly indicated with the electron pair on N available for subsequent attack on electron-deficient species (e.g. 0, or NO, see below). In the (MNO}*formalism the bending arises from the higher energy of the dz2metal-based orbital compared to 7c*(NO).'O7 Noel1 and M o r o k ~ r n a "discuss ~ this in some detail for [CO(NO)(NH,),]~+and predict the observed eclipsing of an adjacent ammonia ligand (Table 12). The large structural trans effect in octahedral {MNO)' complexes is well recognized.'" The rrans Co-NH, bond in [Co(NO)(NH,),]*+ is some 24pm longer than the cis Co-NH, bonds, and the large majority of bent Co"' systems have no trans ligand at all (Table 12). Bridged p N 0 structures are found in a few instances (Table 12) and are in agreement with NOcoordination and the absence of Co-Co bonding.
+
47.4.1.3
Rewtivity
General accounts of the reactivity of coordinated nitrosyb are a ~ a i l a b I e . ' ~ ~ ~ A ' ~ 'metaI - ' " ion or metal complex is unable to generate the reactivity of ionic NO+ towards nucleophiles (i.e. OH-, RS-,RHN-) or of NO- towards electrophiles (Le. H + , PhCH,X), but it can present NO in a less reactive form under conditions inappropriate to the free ion. Thus linear coordination with vNo > 1886cm-' may promote nucleophilic reactions at N, and bent coordination with the higher electron density on N (vNo < 1700cm-') may promote electrophilic attack by H ' , 02,NO and PhCH,Br.'2s No cobalt nitrosyl is known to undergo nucleophilic attack on the nitrosyl group. Normally Iinear coordination is associated with four or five coordination and the nucleophile adds to the unsaturated metal instead. However a number of electrophilic reactions are known (Table 13). A recent discussion of these is r e ~ om m e n d e d . ' ~ ~
Cobalt
666
Protonation is found with [Co(NO)Br(das),]', but it remains uncertain whether at N or 0. Electrophilic attack by 0, occurs with most uncharged tetragonal pyramidal five-coordinate systems (Table 13), but prior association with a base ligand such as pyridine appears necessary. The mechanism proposed by Clarkson and Basolo (Scheme 11) is probably general,lZ6although neither intermediate has been identified. The function of B appears to be to make the N atom more nucleophilic by promoting the back-bonding component. However it has not been shown that coordinated NO, derives directly from coordinated NO, that exchange or reaction with uncoordinated NO is absent, and that only one of the 0 atoms in the product derives from 0,. Likewise the reaction with an excess of NO remains uncertain in detail although the major product is the on [Co(N0)(8-quin),] an same, Le. the coordinated nitro group. In a recent 15Nstudy by I'~liki'*~ 80% yield of NO[Co(N02)(N0,)(8-quin),] was obtained (equation 16); liberated N,O and coordinated NO; derived exclusively from NO,, , while coordinated NO; derived exclusively from coordinated NO. This supports a role for N202(g) and eliminates homolytic N-0 fission in an intermediate such as (24). [Co(NO)L41 + B
K
4 CotPr'O)(B)L,
0. cs,;w,
+
/O BCoL4N '0-0
BCuL4N/"
\
+
Co(NO)(B)L,
P\
A BCOL~N
0-0
0-0
rate =
"\NL,CoR /
faSl2LCo(NO,)(B)L,]
kK 1BI [ 0 2 1 t W N O ) L I 1 +K[Bl
--
Scheme 11
[C0('~N0)(8-quin)~] + 815N0
1SNO[Co(14N02)('SN0,)(8-quin)2] + 3"N20 N--0
co-y,/
(16)
\,,N-Co
Further studies are required to support and extend these mechanistic aspects. Likewise ''N tracer studies are needed to determine the source of coordinated NO; in [Co(NO,)(DMGH),py] produced in the aerial oxidation of [Co(NO)(DMGH),] in the presence of pyridine,lo4and to determine the source of N in N, and N,O produced in the very unusual reduction of NO by NH, (or aliphatic amines) as catalyzed by cis- and truns[Co(NO,),(en),]X (X = NO;, NO;)."* Bergmann and coworkers have used [Co(Cp)(p-N0)IZto investigate NO insertion into metalcarbon bonds (Scheme 12; crystal structure, R = Et),'29and to study addition of coordinated NO to alkenes (Scheme 13).I3O7l3'The product dinitrosoalkene complex (25) can be reduced by LiAtH4 to the primary diamine and can undergo both thermal and photochemical alkene exchange. The catalytic reduction of nitric oxide by amines to N,O and N2,g)has been found to occur in the presence of cis-[Co(NO,),(en),](NO,) and other Co@ amines both in aqueous solution and in ' Y-type ~ e o I i t e s . ~ ~Co" * , ~ions ~ * incorporated into Y-type zeolites are also effective catalysts in the presence of NH, and '[CO(NO),(NH,)]~+'has been proposed as a steady-state intermediate.'33 Likewise the catalytic reaction between NO and CO on the surface of C0,04 generating N, and CO, has considerable interest in the control of nitrogen oxide emission^.'^^ A general account of the use of cobalt nitrosyls in pollution control has been given recently by Pandey.13' 47.4.2
Nitro (NO,) and Nitrito (ONO)
[Co(NO,)(NH,),]X, and [Co(OWO)(NH,),]X, represent the first examples of linkage isomerism in coordination chem~itry.'~"'~' The former nitro isomer is yellow, the latter nitrito is orange, and
667
Cobalt
fCp)Co
/R
PPh,
(CP)CO / N R
~
'PPh3
\NO
Scheme 12
0
/ UAIH,
f Q
0 Scheme 13
they are easily ~ r e p a r e d . ' Nitro ~' complexes are usually formed via nitrito intermediates and heating a Co-OH';' complex with NaNO, at 6&80 "C in 0.01 M H + for 20 minutes is often sufficient to isomerize the Latter. For systems where water exchange is rapid, direct displacement of coordinated H 2 0 by NO; may occur. Some useful preparations are collected in Table 14. Na,[Co(NO,),] is particularly useful in synthesis since the first four or five nitro groups can usually be displaced successively by other ligands, particularly chelating amines. Co-ON0 compiexes are formed by addition of Co-OH to N, 03(aq). Pearson, Henry, Bergmann and Baso10'~~ first described the rate law, rate = k[CoOH2][HNO,] [NO;], and van Eldik and coworkers have recently confirmed the second-order dependence on [NO; IT over a wide pH range.14' Murmann and Taube demonstrated retention of the Co-0 bond by "0 tracer methods (equation 17).142Other Co-OH"+ systems behave similarly and the reaction is relatively fast at room temperature and maximizes at a pH between 3 and 5. Specifically labelled complexes ( e g . CO--'~ONO,Co-ON" 0)have recently been CO-OH
iNt03
-
~ c 0 - 0t
Co-O---H I
0-h-Nb2
I
I
HNOz
(17)
dN
Structures of several Co-NO;+ and Co-ON@+ complexes are known (Table 15, for examples). The Co-NO, group results in no major structural trans influence although Juranic and coworkers'@report a 5 pm shortening in the Co-NO, bond when the trans ligand is carboxylate. The Co-NO, bond is usually very robust and nitro complexes are well known for their stability in aqueous solution. A kinetic trans influence of the NO, group has not been clearly demonstrated. Hydrolysis is very slow in water but does occur (cis,tvans-[Co(en),(NO,>,lf3 x 10 's-', 25°C);'45 heating in strong acids or in NaOH solution results in more rapid hydrolysis. Photolysis of the solid C.O.C. 4-v*
~
Cobalt
648
3
+*
w
vr
F'
N m
VI
3
Cobalt
670
I
Cobdt
67 1
can result in complete isomeri~ation'~~ and this may prove a useful method for the complete removal of the nitro group. Acid hydrolysis of [CO(NO,)(NH,),]*~follows the rate law kobs= k(anti1og H o ) (Ho= Hammett acidity constant) and is very slow at ambient temperature^.'^^ JoIly and coworkers showed many years ago that the 0 atom in the [CO(NH,),OH,]~+product derives from coordinated NO; and suggested Co-ON02+ as a possible intermediate (equation 18).j4 The [Co(NO,),en]ion hydrolyzes more rapidly with k, = 2.8 x lOP4s-' and k , = 2.2 x lO-*rnol-' dm' s-' at 25 "C, kobs= k, + k,[H+]; this reaction is reversible in weakly acidic solution.'49 Murmann and Rahmoeller in a recent elegant study demonstrated the independence of the rate of the NO; exchange reaction of (26)on [NO;], solvent and added electrolytes by using "N- and '80-labelled NO; ;Is0 (26) is especially labile, and the aqua-nitro complex is known to anate rapidly. The mechanism given by equation (19) probably holds in a general sense.
0 CO-N
I
/o 4 slow CO-0-N
+
fastC0-0HZ
NO;
"NO; fsst
* CO--N15 No
' 0
' 0
(19)
The Co-ON0 bond is less robust and can be cleaved by mild acid (the reverse o f its formation) ~. ' ~ ' it can be cleaved by the action of but this appears to occur with 0 ~ c r a m b l i n g ; ' ~ alternatively, HN, . Acid hydrolysis is markedly catalyzed by C1-, Br-, 1- and SCN- with major terms in the rate k,[H+] B-].'52 Thus the half-life for aquation of [Co(ONO)(NH,),]*+in law being kobs= k , m * ] i), (0.9 mol drnp3)is ea. 1 h at 25 "C,but with the same HCIO, containHCIO, (0.1 mol ~ l m - ~LiClO, ing LiCl(0.9 r n ~ l d r n - ~it )is ca. 1.5 min. A transition state (27;equation 20) is proposed."* Such C1catalysis could possibly be used in conjunction with the photolytic isomerization of Co-NO:+ to cause rapid hydrolysis of stable nitro species. The HN, -promoted reaction has been extensively studied by See1 and coworker^.^^^^'^^ The rate law takes the form kobs= k, f k2[HN3],where kl is the spontaneous isomerization rate (Table 16; k, = 5.53 x and 2 x 10-2dm3mol-' s-', 25 "C for [Co(ONO)(NH,),]'+ and ~is-[Co(ONO),(en),]+,'~ respectively). In this case N; attack results in gaseous N, and N,O products (equation 21). The lower AHt for the assisted reaction (74 vs. 93 kJmol-I) means that isomerization can be prevented at low temperatures ( - 5 "C) by using reasonable Ny concentrations.
I
Co-0NO2+
CO-ONO"
+ H+ + X--
+ H+ +
N;
-L
H
Co-0---
CO-0-N
1'
,t
r
N=,"
'O
\N---N=N
-Co-OH2+
-Co--OH2+
f
NOX
+ N2 +N,O
(20)
(21)
Present interest lies in the Co-ON0 to Co-NO, interconversion. This is spontaneous in the forward direction (Co"' favours N and 0 coordination) and light catalyzed in the reverse. The solid-state reaction has been studied in some detail by Grenthe and N ~ r d i n . ' ~These ~ , ' ~authors compared the structures of trans-[Co(NCS)(ONO)(en),]X and trans-[Co(NCS)(NO,)(en),]X (X = I-, ClO;) and propose an intramolecular rotation in the Co-ON0 plane for the spon-
672
f
m
a... m +I
"m
+
P m
m m e-
12 I l
+&a
3
+ 3
Cobait
673
taneous reaction (equation 22).15' An intermolecular process would involve at least a 400 pm movement of N in the crystal. Powder photography (X = ClO;, 293 K) shows a gradual transformation indicating a homogeneous change (Le. separate phases do not appear to coexist in the sample). Photochemical irradiation reverses the process (80% complete) with full retention of crystallinity and of structure. The authors conclude that minimal atomic and molecular movement occurs and that both processes are topochemical. A somewhat more complex solid-state reaction change is occurs with [Co(ONO)(NH,),]Cl, .Is6 A fast spontaneous Co-ON0 -+ Co-NO, followed by a slower and more substantial structura1 change to thermodynamicallystable crystalline [Co(N0,)(NH,),]C12 (monoclinic, CJc, ). The initial isomerization retains the original space group (orthorhombic, P,,nb) but ionic and intermolecular contacts apparently prevent a simple rotation in the Co-ON0 plane in this case, and a more complex structural movement is necessary. The reverse photochemical reaction of the stable [Co(NO,)(NH,),]Cl, crystal gives a new crystalline modification of [Co(ONO)(NH,),]Cl,, which easily reverts to the original on standing. No ionic C1is incorporated in these processes, and crystal dimensions again imply intramolecular reactions. Johnson and Pashman claim to have observed the 'transition state' complex involving bidentate species (A) in a low temperature photolytic study.'"
The solution chemistry of the Co-ON0 to Co-NO, isomerization has now been widely studied, mostly with the pentaammine system. Basolo and coworkers initially showed the rate to be independent of added NO; (Table 16) and the intramolecular nature was confirmed by I80-labelling studies.142Murmann earlier had shown that optically active ( + ) s s 9 -cis-[Co(NO,)(ONO)(en),]+ isomerized with full retention of c ~ nf i gu r a t i o nand ' ~~this has been confirmed for the OH- -catalyzed reaction in a recent The rate for the pentaammine complex has been recently measured in 16 solvents and the 100-fold variation (THF slowest, H 2 0 fastest) has been interpreted in terms of various solvent parameters; mild catalysis by F-, AcO- and NEt, is found in Me,S0.'59H$+, Cd2+ and Ag+ catalyze the reaction in water (Table 16) and in acetone (Zn2+ also), and the lack of competitive coordination by H,O, AcO- and NO; for the induced path in water again point to a concerted intramolecular process.'59 The rate is also OH- catalyzed (Table 16), kobs= k + koHIOH-], and the large AVI for koH (27 1.4cm3mol-') points to the normal &lcb mechanism with a large AT (22cm3mol-') for ammine depr~tonation.'~~ A similar situation has been found with cis- and trans-[Co(ONO),(en),]+ (Table 16).l6OThe reaction has also been studied in NH,,,, ,where kobs= k[NH4C10,]-', indicating isomerization via [CO(NH,)(ONO)(NH~)~](C~O~) (Table 16) and experiments using the specifically labelled trans-I5NH, complex show retention of geometrical configuration and the absence of competition for entry by NH, .I6' Oxygen scrambling has been demonstrated by Sargeson and coworkers using specifically labelled Co-I7ONO and Co-ONI7O pentaammine ~ornp1exes.l~~ Acid cleavage results in CO-'~OH:+ from both reactants and this confirms the earlier 50% scrambling result obtained using "0-labelled co m p l e x e ~ .'The ~ ~ spontaneous reaction gives a k ( 0 -,O)/k(O+ N) ratio of 1.2 indicating a slightly faster rate for the 0 to 0 interconversion. This is difficult to reconcile with the intramolecular rotation mechanism (equation 22) proposed for the solid-state reaction and suggests a more complex twist mechanism or momentary complete bond cleavage and re-entry of 0 and N. The solution photochemistry of Co-NO, leads to both redox decomposition (Coil ) and isomerization to Co-ON0 ([CO(NO,)(NH,),]~+,'~~ cis-[Co(NO,),(en),]+ .),61 The ratio of products is wavelength independent, indicating efficient mixing of reactive LMCT and LF singlet excited states.'" A semiquantitative model has been described by E n d i ~ 0 t t . IThe ~ ~ isomerization pathway is facilitated by increasing solvent viscosity suggesting that the redox p r o m s may come at a later stage.166A possible mechanism is given by Scheme 14. Scandola and colleagues report acetone
I
674
Cobalt
sensitization of both kredox and k,,,, by an energy-transfer process and have discussed the relative energetics in some detdi1.167 No attempt has been made to see if the excited-state species compete for is not formed. In aerated 'spectator' anions (e.g. ion-paired NO;) but apparently Co-OHiadduct, while in the absence of 0, another acetonitrile O2 acts as a scavenger forming a Co"'-O; Co"' complex is formed.I6' There has been a report that photolysis of cis-[Co(OC,O,)(NO,)(en),] leads to cu. 10% chelated [Co(O,C,O,)(en),]+ as well as Cot& '69 but whether this results from direct entry of the free carboxylate group in the excited state or from subsequent chelation in the aqua complex is not yet known. Co-NO2
OW^^ ^ /
A '[Co-N021 *
'[2C~11.N02]
singlet CT state
3[4C~".N02] high,spin Ilrndox
co"'
7-O
-0
CO:~,
+ NO2
Scheme 14
Unlike Co-NO complexes (Section 47.4.1.3), the reactivity of coordinated nitro and nitrito ligands toward electrophiles is not well explored. However Tovrog and his colleagues at Allied Chemical Corporation have recently shown that (28) catalytically oxidizes PPh3 in a bimolecular transfer of 0 from coordinated NO, (equation 23),'" and that (211) and the similar porphyrin complex [Co(TPP)(py)(NO,)] both catalyze the oxidation of primary and secondary alcohols to aldehydes and ketones in the presence of the Lewis acids BF,-H,O or LiPF,; non-radical pathways are pr~posed.'~' Internal redox in [Co(NO,),(bipy),]X also occurs on heating, with oxygen transfer, to give the Co" complex [Co(NO,)X(bipy),] and NO (X = C1-, Br-, I-, C10; , C10;).172
~
/o \ $=NGN=$
47.4.3
\ 4
+
PPh3 ~
-
~Co(NO)(saloph)(~y)I ~ ~ + OPPh3
o
123)
0 2
Nitriles
47.4.3.1 Cobalt (III)
Cobalt(II1) nitrile complexes are easily prepared via displacement of a poorly coordinating ligand such as (MeO),PO, (EtO),PO or CF,SO, by RCN. Thus trcatment of [CO(N,),(NH,),-,](~with NOX (X = ClO;, BF;, OS0,CF;) in (EtO),PO, followed by addition of the nitrile and ((MeO), PO)6-T](C104)3and warming, or dissoluwarming, or addition of the nitrile to [C~(amine),~ (x = 5,4,3) in the appropriate nitrile and warming, tion of [Co(amine),(OSO,CF,),-,](CF, leads to excellent yields (Table 17). Jordan, Sargeson and Taube were the first to use such methods,'73and subsequent reports by Balahura,Iy4Sargeson and coworkers'75and Kupferschmidt and Jordan"' are recommended for more recent preparations. Most nitrile complexes are stable in neutral to acidic aqueous solution, and have yellow-orange colours characteristic of amine complexes. The reactivity of coordinated nitriles towards nucleophiles, and electrophiles when deprotonated, has been of major interest. Addition of OH- to the nitrile carbon occurs readily in alkaline solution, kobs= koHIOH-], and the rate is increased some 106-107 times by coordination (equation 24).'76.*77 koHhas values of 1-50m01-~dm3s - ' for R = [e.g. 3.4 (Me); 35 (CH=CH )] and varies over a wider range fox substituted benzonitriles following a Hammett relati~nship.'~'For Z-cyanobenzonitrile addition of one equivalent of OH - is followed by intramolecular amidine formation on treatment with further alkali (29), or by rearrangement in acid and hydrolysis to the diamide (311, or cyclization to the alternative amidine isomer (32;Scheme 17)."' The alkaline hydrolysis of the
~
Cobali I-
r-r*
h N
67 5
676
Cobalt
coordinated 2-cyanobenzamide intermediate (30) is very fast (koH = 5.8 x 106mol-’dm3s-l and assistance by the adjacent amide substituent is l i k e l ~ . ” Attack ~. by deprotonated coordinated amine centres on nitrile groups of coordinated ligands occurs under mild alkaline conditions to form bidentate amidines (33; equation 25).” Attack by other nucleophiles such as C0:- and CN- are known,ig1,1a2 with the latter being followed by successive additions of coordinated ammonia and CN- with the tridentate bis(amidine)aminomethylmalonate complex (34) resulting (Scheme 18).‘** (NH3),Co3+-N_CR
+ OH-
(NHj)SCo3+-NC
+
OH-
-
(NH3),Co2+-NH
-
(24)
Hi[fast)
(NH3),Co3+-NC
Q
o=c
CN
C N
\
NH2 (30)
(NH,),Co3’-NH=C NH
OH-
NH
Scheme 17
H,N’
H2N’
H (NH3),Co3+-NeCMe
+ 2CN-
lN7:
(NH3),Co, ’+
N H2 CN
H
NH2
(34) Scheme 18
Methylene groups adjacent to coordinated nitrile carbon are acidic (e.g. for R = CH2CN, pK, = 5.7) and rapidly undergo base-catalyzed proton exchange (for R = Me this is faster than hydrolysis), and addition of electrophiles other than H’ to the resulting carbanion (35; Scheme 19).
677
Cobalt
In the absence of an added electrophile the anions undergo spontaneous electron transfer to the metal with release of Cot;, and the nitrile radical, which dimerizes. Alternatively, the coordinated radical anion induces polymerization of methacrylate, styrene, acrylonitde and methylacrylonitrile monomers under mild non-aqueous conditions.'8' Reduction of coordinated nitrile to the parent amine can be accomplished with NaBH, in slightly ( N H ~ ) S C O - N C C H ~ C O+~ E OH~
e (NH3)5Co-NCCHC02Et + H 2 0 (35)
(NH3),Co3+-NCbC02Et
(NH3),Co3+-NCdC02Et
I
I
I
Br
I
IOH
OBr
II I (NH3)sCoZC-NHCCC02Et I
NH3 + Coil
I
Me Ho-
OMe
II I
NCCHCOZEt
I
NCCHC02Et
I
(NH3)5C02+-NHCCC02Et Me
Br
Scheme 19
alkaline aqueous solution (equation 26);lS3in acid solution, reduction results in the very rapid formation of Cot& and Coo. Adjacent double bonds are activated towards species possessing an acidic H atom. Thus coordinated acrylonitrile adds to acetylacetone, nitromethane and PO:- to form saturated nitrile species (Scheme 2O).'@
H,C(COMe),
(NH3)5 Co3+-NCCH=CH2 Po:-
1
t
,COMe (NH3)sC~3+-NC(CH2)2CH, COMe
\-
\ MeNO,
(NH,),Co3'-NC(CH,)20PO:-
0 It
(NH,)SCo3'-NC(CH,)3N02
,COMe
(NHj)5C02+-NHC(CH2)2CH
,
COMe Scheme 20
Coordinated cyanamide does not add OH- to form the N-bound urea complex since it deprotonates (pK, x 5.2) to give the unreactive [Co(NCNH)(NH,),I2+ ion, but in acid it readily loses NH: to form N-coordinated cyanate (equation 27).lS3The analogous dimethylcyanamide complex cannot deprotonate and adds OH- to form urea (Scheme 21),18' but this does not proceed further to coordinated carbamic acid and NHMe, in a process analogous to that suggested for the function of Ni2+ in the metalloenzyme jack bean urease (which catalyzes the conversion of urea to these nor does it lose NHMe, to form the cyanato complex as suggested by Balahura and Jordan for the similar urea derivatives (equation 28).Ig3 Alternatively, in acid solution the protonated urea rapidly isomerizes to the 0-coordinated complex (tljzz OS, 25 "C; Scheme 21).Is5 This isomer undergoes OH--catalyzed hydrolysis at the metal rather than at acyl carbon, so the function of the metalloenzyme has not been duplicated in this system. A similar property is found for 0-bound urea (equation 29).'87 (NH3)5C03f-NCNH2
+
(NH3)&03+-NCNMe2
H30+
+ OH-
-
(NH3),Co2+--NCO
-
+
0
1I
(NH3),Co2+-NHCNMe2
scheme 21
NH: n+
+
e OH-
H+
(27)
Cobalt
678
+
(WH,)5Co'+--OH2
R NH,
(NH3)3C~2'-NH R
-+
+ H,O +
H+
--L
(NH,),Co*+-NCO
+-
RH (28)
R = NH,,OEt, NHCH,, NHPh
(NH,),co~+-o=c
+ OH-
/ m 2
'NR,
4
(NH3)5CoZ*-OH
+
Pathways for reduction of Co"'-NCR complexes by Mz+ reductants (C3+ < Eu*+ < V2+ < [Ru(NH,),]'+ in a rate sensej are a continuing source of interest.'8&190 Most coordinated nitriles (36; equation 30) reduce via an outer-sphere pathway (Le, without specific prior attachment of the reductant), but an inner-sphere route becomes available when R = CH,CO,H, CH,CO; in an appropriately substituted aryl species (37j, especially with C(en),] has also been demonstrated (Scheme 58a) and a phosphorane intermediate analogous
Scheme 58
COT b y
Scheme 58a
I Cobalt
756
U
757 Cobalt 00
n
Cobalt
758
.~ rn r-
P,
-
E
Z
I
v
L
7 59
Cobalt
Cobalt
760
to (163) is suggested.'*I6The advantages of metal-coordinated hydroxide over alkoxide in facilitating such cyclizations and hydrolyses are discussed in this
+
(NH,),Co-OP(OMe):+
0HJ.H-
+
(NH3)5Co-OH:C /OH2+
+
PO(OMe),
(1 19)
Ligands closely related to orthophosphate esters include acetylphosphate, acetylphenylphosphate and fluorophosphate. Alkaline hydrolysis of [Co(OP(O),OCOMe}(NH,),]+ occurs exclusively at the carbonyl centre, and the acceleration provided by cobalt(II1) four atoms removed is minimal (10 times, equation 120).546This is a good comparative example of phosphoryl versus acyl hydrolysis, since an excellent leaving group at P is provided (AcO..) and P is two atoms closer to the activating metal. Likewise the coordinated-hydroxide-promoted hydrolysis of acetylphenylphosphate occurs exclusively at C (equation 121), but the rate enhancement over solvolytic OH- is in this case significant (lo2for a 10 mM Co"' solution).546Hydrolysis at P does not occur even with 0-bound monofluorophosphate, with ca. 50% hydrolysis occurring at Co"' and the remainder involving intramolecular attack of a coordinated amide ion (equation 122).547The latter path presumably involves a four-membered chelated amidate intermediate and an enhancement of ca. 10'' over hydrolysis of FPOi- is calculated for this part. Attack by phosphate species such as is found in ADP and ATP synthetases is more difficult. Apparently heating [Co(0P(0,H)F}(NH3),](C10,), with a solution of NaH,PO, leads to some pyrophosphate complex [Co(OP20,H)(NH3),] but the published details are not very
R
(NH,),Co+-0-P-0-C-Me I 0
R
f
OH-
+
(NH,),Ca-OPO,
k = 0.53mol-ldm3s-', 25"C, Z
(NH3)5C~-OHZ++
0
0
'I
I'
0-P-0-C-Me
=
+ MeCOO
~
(120)
1.OM
k = 0.029 mol-' dm' s-'
0 0
(NH3)&02+-@CMe +
R
(NH,),CO+-O-P--F
I
0
+ OH-
+
(NH,),CoZ'-OH
,OH + F+ FPOi- + ( N H ~ ) ~ C O , NH,PO,
(122)
k = 3.3 x iO-3mol-ldm3s-l
(iii) Pyrophosphates, triphosphates, ADP and ATP Preparations are listed in Table 47. Main interest lies in determining how metal ions catalyze the hydrolysis of ADP and ATP. ATP plays a crucial role in the energy metabolism of all living cells and divalent metal ions (M$+, Mn2+and Calf) play an important role in these phosphoryl transfer processes.549Divalent metal ions such as Mgz+, Ca2*, Zn2', Cu2+and Mn2+ provide only modest in vitro ~ a t a l y s i and s ~ ~stronger, ~ ~ ~ ~ more specific coordination to phosphate units appears to be required by the enzyme. Cot" (and Cr"') complexes of ADP and ATP have been shown to mimic many of the biological functions of the Mg'+ enzyme, and since the cobalt(II1)-phosphate coordination remains intact, the specificity of alternative coordination sites, and the stereochemical requirements at phosphorus, have been elucidated in some cases.ss5Often the Co"'-enzyme species is biologically active and several enzymic functions of ATP have been examined in this manner. HP,O:- coordinates to Co"' either as a monodentate ligand (terminal 0-) or as a bidentate chelate (six-membered -O,O-). The complexes are easily made in aqueous solution (equation 123),556or by heating sparingly soluble [Co(N),-,(H,O),-,](HP,O,) (N = NH,, en, tn) under a good vacuum at 100 0C,557 Chromatographic separation on Dowex or Sephadex anion-cation resins leads to pure species (Table 47). Crystal structures of [CO{OP(O)~OPO~H}(NH~),] * H,O, [Co(02P20,(OH)}(NH,),]~2H20 and A,h-[Co{0,P20,(OH)}(en),] 2 H 2 0 are available (Table 46) with the six-membered chelates showing no apparent strain about the Co or P centres. The alternative four-membered ring isomer has not been identified and is unlikely to occur in an equilibrium sense because of chelate ring strain. This lack of ring strain in the six-membered chelate stabilizes the system towards hydrolysis at
-
-
I 76 1
Cobalt
...
,
.
0
0
Cobalt
762
P, and for (N)4= (NH,),, (en),, tren and (tn), (equation 124), hydrolysis is unimportant over the wide range pH 3-14. At pH values above 11 hydrolysis at Co occurs with the ultimate release of unaffected (no 0 exchange) P, 0;-. The monodentate intermediate appears to have some stability since it has been observed, but not isolated. For (N), = (NH,), (equation 124), reduction to Co" becomes an increasing problem at high pH. 0
Using 31PNMR in a semi-quantitative sense Hubner and MilburnSSshave shown that with the = (tn), system multiple coordination to the HP,O:- unit is possible when excess Co"' labile reagent is present. Then hydrolysis at P (probably stoichiometric rather than catalytic) ensues with a maximum rate at pH x 7. This suggests [Co(OH)(H,0)(tn)J2+ is the reactive ingredient. At pH 5 ([Co]= 0.3, [H2P,0:--] = 0.1 r n ~ l d r n -a~half-time ) for H,PO; production of about 1 h compared to 10 years for the uncoordinated ligand demonstrates the accelerating effect of the additional Co"' unit(s). Although no reactive species have been identified, Scheme 59 gives some of the possibilities. Crystal structures are available for the diphosphonate (PCP) and diphosphoamidate (PNP) analogues (164) and (165) (Table 46) and these have been used as probes for H transfer, Ca2+ binding, and cleavage patterns in phosphoryl transfer. Thus the potato enzyme apyrase catalyzes the hydrolysis of [Co(PXP)(NH,),] species (X = 0, N) to give [Co(OPO,),(NH3)4]with the possible in the case of X = N.573 intermediacy of [CO(OPO,)(O~O,NH,)(NH,)~]
or
Scheme 59
763
Cobalt
ii ,OH
0-P
+/ (NH,),Co\
\
CR’R’
0-P,
/
II
0
OH
(164) R‘, R 2 = H, Me, OH, But, NH,, C1, NMe,
H,P,O:, can act as a monodentate (terminal 0 - ) ,bidentate (p,y, six membered; a,y, eight membered) and tridentate (a,p,y, six membered) ligand system (crystal structures Table 46). The P,y six-membered chelate is particularly interesting since it is the only structure to have an asymmetric P centre. Although prepared from [Co(NH,),(OH,),] and H,P30:, as a racemic mixture, the crystal structure was carried out on the isomer isolated from [Co(ATP)(NH,),] by periodate cleavage, and has the absolute ( S ) chirality. All structures (Table 46) indicate little or no strain at either Co or P, although sharply folded units are often involved. As with chelated HP& little or no accelerated hydrolysis at P occurs in the absence of additional metal activation and this appears to be independent of ring size (equation 125).559,’60.561 This is consistent with the absence of distortion at the tetrahedral P centres. Some accelerated hydrolysis at alkaline pH values results from the slower hydrolysis for the parent P,Of, compared to the protonated species rather than from a significant increase in kobsfor the chelate. Table 48 gives some hydrolysis data for [CO{O,P,O,)(NH,),]~- and the p and y isomers of [CO(OP,O,)(NH,),]~-. The presence of additional Co’” reagent significantly accelerates the reaction, especially with species having two available cis coordination sites.562*563 The pH variation in kobs has been interpreted as intramolecular hydrolysis by coordinated hydroxide in the bifunctionahed system (167; Scheme 60) and the derived rate k , = 0.01 s-’ represents an acceleration of 3 x lo4 compared to the initial chelate (166) and 5 x lo5 compared to P30:;, so considerable activation has been achieved. [Co(OH), (cyclen)]+ also initiates hydrolysis in monodentate p- and y-{Co(OF,O,)(NH,),]*- systems564(formation of PO:- + [Co(OP,O,)(NH,),] is 50fold faster for the fl isomer) but the effect is considerably smaller and does not appear to show the same pH variation. None of the reactive intermediates have been isolated in these studies, or indeed clearly identified, so that considerable uncertainty remains as to details. However distortion about the P centre undergoing hydrolysis seems a necessary requirement, and it is likely that this can only be achieved in four-membered chelates.
-
~
0-P 0- P03H-
0
0-P (NH,),CJ
II ,o\0
/
o-i\O0 Scheme MI
+ (cyclen)Co,+/
0
,‘ 0 -
0
. .
Cobalt
764
Table 48 Accelerated Hydrolysis at Phosphorus in fl,y-[Co(NIf,), 0,P,08]2- I PH
A ctiva ring specie? .
1
10‘ keds-
Relative rate
:
.
~
P30:; (free ligand) None None [C0(teta)(H,O)(0H)]~ [Co(Me6dien)(H,0)(0H)]2f [CO(NH~),OHZ~’~ [c~(NH,),(H,~),I~+ [Co(cyclen)(H,O)(OH)]Z+ +
6.0 6.0 9.0 6.0 6.0 6.0 6.0 6.0 7.0 8.0 8.5 10.12
‘1
~~
0.16 0.34
13
7 160
-
7 12 26
-
-
700
1.5 6.2 33 50 54
66 -
I
1.
P. R. Norman and R. D. Cornelius, J . Am. Chem. SOC., 1982, 104,2356.
2. 3. 4.
[Co(NH3),O2P,O8]*- = 25mM; [activating complex1 = 20mM. At pH 6 k, = 2.3 x s-’ (40°C). For /?-[CO(NH,),OP~O,]~- /cobs < lO@s-’ and for ~-[CO(NH,),OP,O,]~/cobs < 10-8s-’ (i.e. little or no acceleration over free ligand). T.P. Hammy, P. F. Gilletti, R D. Cornelius and M. Sundaralingam, J . Am. Chem. Soc., 1984, 106, 28 12.
Several complexes of the [Co(HATP)(NH,),] (n = 2, 3, 4) and [Co(ADP)(NH,),] (n = 4, 5 ) variety have been prepared and isolated (as solids or as aqueous solutions; Table 47). Spectral data (‘H and 3’PNMR, UV-visible) have been reported.ss6 Likewise the diastereoisomeric ADP(aS) coordinates with a preference for (0,s) binding (equation 126).565N o crystal structures are available, but the configuration of the inactive form of [Co(HATP)(NH,),], isolated from the yeasthexokinase-mediated synthesis of glucose 6-phosphate ADP, has been characterized following periodate cleavage and a crystal structure on the resulting [Co(OzP,O, H2)(NH3),]566is available (Table 46; Scheme 61). The (S) configuration at P requires the ( R ) diastereoisomer of B,y-[Co(HATP)(NH,),] to be the active ingredient in the enzymic This strongly implies similar @,y coordination and an ( R )configuration for MgATP. Likewise, phosphoribosyl-pyrophosphate (PRPP) synthetase specifically utilizes the PJ-( -):.-(S)-[Co(ATP)(NH,),] diastereoisomer in the phosphorylation of ribose 5-pho~phate’~~ but in this case the phosphorylated ribose product containing the metal has the opposite CD from the starting material implying that inversion at P has occurred (Scheme 62). The (R)-and (S)-cr,p-[Co(ADP)(NH,),] diastereoisomers have been separated on cy~loheptaarnylose~~~ and assignments of configuration follow the above ATP complexes. S
*-o3p-0-p
II /QAd
‘0(R)-ADP(aS)
+ [Co(OH&(NH3hI3+
-
0
0
0-P
I1,o-
II ,o-
,0 +(NH3)4Co,P-p\0
+/ \ (NH~~CO, s -P
/
‘OAd 0
O-P,,
11
P,y-[Co(ATP)(NH,), J
I S
Ad
+ glucose + hexokinase
0-P
II ‘ 0 0 1 P03Ad
I
hexokinase
(PH7)
0, r-(+)~~-(R)-[Co(ATP)(NH,),]
+ glucose Scheme 61
(126)
Cobalt 0
2-0,p
I1 ,o-
0
,
+p-9o P, ~+):P~-(NH,)~CO \
765
\
O--CHZO
+
H
/
0- p,, I O
0,
OH
HO
PKPP synthetase
OH
POjAd
11 ,o-
0- P
Scheme 62
As found with H2P20:- and H,P,O:; hydrolysis of ATP and ADP can be accelerated by [CO(OH)(OH,)(N),]~+s p e ~ i e s ~ ~and ~ , ' ~by ' the hydrolysis products of [Co(Cl), (dien)].572The (N)4 = (tn), complex shows no acceleration for a 1:1 ratio, but the 2:l metakATP mixture (0.14.01 m01drn."~ATP) shows lo5enhancement at pH 7. Scheme 63 summarizes these findings. At high pH hydrolysis of the Co"' complex predominates. [Co(cyclen)(H,0),j3+ also catalyzes the hydrolysis of [Co(NH,),(ATP)]- at pH 8-9, and (168) has been claimed as the reactive species.s81 In all these studies however no intermediates have been positively identified, and the immediate products, coordinated or otherwise, also lack definition.
-
K
a 0.002
, -
[ C C ~ A T P ) ( ~+~ [) C ~ IO ( O H ) ( O H ~ ) ( ~*+~ ) ~ I
M-'
{c~o(tn)~}~(ATP)
I
khvd = 6 X l O - ' S - '
ADP + AMP + HP0:-
T
khYd = 4
f
x
H2P203~ O - ~ S - ~
Scheme 63
-o\p/o'0
I 0-P"O
'OAd
(168)
Stable Co'" ADP and ATP complexes have been used as competitive inhibitors in a number of enzymic studies and some progress has been made at unravelling the requirements of the active sites. Steady-state kinetic studies show /3,y-[Co(ATP)(NH,),] to compete with MnATP for (Na+ K+), Mgz+ and Ca2+ATPases derived from kidney medulla.574K,values for fl,y-[Co(ATP)(NW,),] are similar to the K , values for MnATP for both the m a + + K') and M d + enzymes, and "P NMR shows that the Co'" complex acts as a substrate for the Mg2+ and Ca2+ systems. Likewise,
+
Cobalt
766
( -)505-[Co(ATP)(NH3)4]and ( -)532-[Co(ADP)(NH3)3 (H,O)] compete with MgADP for the active
site of creatine kinase although they are not as effective in this role as similar Cr"' species.575 Werber and coworkers have used an uncertain Co(phen)ATP complex to probe the active ATPase site of the energy-releasing myosin e n ~ y m e . ~ ' A ~ ,more ~ ' ~ recent publication suggests that the initial inhibitor may be a Co"(phen), species and that the site of inactivation is not the true active site of the enzyme.578The paramagnetic effects of enzyme-bound Mn2+on the 'H and 31PT , relaxation rates of P,y-[Co(ATP)(NH,),] have been used to describe the active site of CAMP-dependent bovine-heart kina~e.~'~.~'' P.y-[Co(ATP)(NH,),] competes with MgATP (and MnATP) for the nucleotide binding site in the enzyme and the TI data suggest that Mn2+ (attached to an adjacent inhibitory site) coordinates to either all three r$,y or the cr,y phosphates of ,&p[Co(ATP)(NH,),]; the latter possibility is favoured.s79A decision on the two possible orientations of the adenosine ring about the glycosidic bond has been made possible by nuclear Overhauser studies, with the antiglycosidic structure shown by (169) being favoured.'"
(169) Co(ATP)(NH,),-bound protein kinase
( i v ) Phosphonates, phosphoamidates, phosphites and their esters
Busch and Pennington have prepared the [CO(OP(O,H)CH(M~)NH,}(NH,),]~~ ion containing 0-bound 1-aminoethylphosphoric acid, but it remains to be isolated and fully characterized.s82The terminal amine group has a pK, of 3.2 and the deprotonated complex acts as a bridge for intersphere electron tramfer with Cr:& .582 A series of diphosphonate chelates of type (164) have been prepared Crystal structures on the parent (R',R2 = H) bis(ethy1enedi(Table 47) and fully ~haracterized.~'~ amine) and tetraammine complexes are available (Table 46). Complex (164) binds Ca2+strongly ( K varies from 4 x lo3to 2 x 106moldrnp3),particularly when R = OH, and this implies a similar role for radioactive '9mTc-bound pyrophosphonates and pyrophosphates, which are used as skeletal imaging reagents."' has been prepared by treathg N-bound phosphoamidate in [CO(NH~PO,H)(H~O)(NH~)~]CI~ 0-bound [Co{OP(O,H)PhNO, )(NH,),]'+ with 3 mol dm-, NaOH.54'It is easily removed from the metal by treating with acid, but further characterization seems desirable; 0,O-chelated amidates such as (165) have been prepared and a crystal structure is available (Table 46). Several [CO(OH)(OH,)(N),]~+ systems catalyze the hydrolysis of p-nitrophenylmethylphosphonate (PMP) and its ethyl ester (EPMP) [(N), = tme > tn > trien % bipy], possibly via the sequence shown by Scheme 64 with rate-determining coordination of the ester.584This involves an acceleration of > IO4 at pH 7.6 (1.5mM complex) but none of the intermediates have been characterized. Recycling of the [CO(OH)(OH,)(N),]~+reagent appears to be involved.
MeP03H-
+ (N)&o*+/OH -----OH, Scheme 64
I _
Cobalt
*
.
767
Klaui has recently described the first clear example of a 5T2(high spin) ' A , (diamagnetic) spin equilibrium in an octahedral Co"' complex using the phosphito complex [Co(OPR2),Cp] (R = OR, Et) as a tridentate ligand.585,586 Complex (170) has been isolated (C10, and PF; salts) following electrochemical oxidation of the neutral Co" complex. The six phosphite oxygens provide a ligand field about the central metal intermediate between H,O (low spin) and F- (CoFi- ,high spin) with the complexes being green and paramagnetic at elevated temperatures (p = 4.1 BM, R = Et, 393 K) and yellow and diamagnetic below 200 K. The behaviour is reversible, occurs both in the solid and in solution, and is R de~endent."~"P NMR studies show little solvent influence on the spin ~ *is~conequilibrium, with AH' = 24kJmol-', A S 3 = 70 Jrnol-'K-' in four different ~ o l v e n t s . It cluded that inner-sphere electronic effects are responsible for the equilibrium position.
@
+
I
L
Phosphine oxides, phosphoramides and phosphinates
These are largely restricted to Co" and Co"'. The former are invariably tetrahedral and the latter octahedral, sometimes with ligand-induced distortion. An early paper by Goodgame and Cotton discusses some spectroscopic properties5*' and MCD spectra have been investigated by Kat0 and ~ 49 lists some structural A k i r n ~ t o . ~A' ~review covering recent chemistry is a ~ a i l a b l e . 'Table information. irninophosphoranes Co" complexes containing phosphoramides (0 = P(NR2),)"' and phosphinates (-0,PR' R2)593 are known, as are distorted octahedral complexes (RN = PPh3)592 and (173).596 containing the neutral chelating ligands (171),'% (172)595
47.6
ARSENIC LIGANDS
The softer and larger As atom results in weaker coordination to all first-row transition elements. As(OR), and AsX, (X = halogen) coordination to cobalt has not been well characterized. Arsines (AsR,) form longer and weaker bonds with the 0-donor power decreasing in [he order NR,
Cobalt
768 m
r .
d
-
9
. i.
r c
2 E
t L
.*
3
...
o
.^ m
Cobalt
1
769
(Co-L z 200 pm), PR, (225), AsR, (233). Few low-oxidation-state complexes have been reported and no crystal structures. The alkyl-substituted arsines of Co" are usually easily air-oxidized to C O " ~ complexes, but this is difficult with phenyl- and aryl-arsines. Further oxidation to Co'" does not appear to be possible.
47.6.1
Cobalt(-I), Cobalt(0) and Cobalt(I)
AsPh, displaces alkene from [Co(C8H,,)(C,B,,)] under H, to form [HCoH(AsPh,),], and under suitable conditions [CoH(N,)(AsPh,),] and [Co(H),(AsPh,), J can be obtained.59s Several Coo complexes containing CO and NO ligands in addition to AsR, are known. [CO(NO}(L),,~(ASR,)~,,] and [Co(NO), L(AsR,)] are tetrahedral or close to tetrahedral with linear or nearly linear bonding of NO. Some preparations are given in Table 50 and structures in Table 51. Several nominally Coocomplexes containing chelated arsines are known and a recent study of the coordination of (174) to [Co, (C, {CF,),)(CO),] resulting in structure (175)599 typifies organocobalt work in this area.
Me
0
( 174) ttas
Diamagnetic [Co(MeC(dpam),}(CO),](BP~)containing the tridentate arsine tris(dipheny1arsinomethy1)ethane is formed on treating the paramagnetic Co" dimer (176) with CO, and a similar five-coordinate complex [Co{As(dpap), }(CO)](BPh,) containing ligand (177) has been prepared from [Co,(CO),] (Table 50). Structures of these monomeric systems are not known.
47.6.2
Cobalt(II)
Unlike their phosphine analogues monotertiary arsines do not readily combine with simple Co" halides. Anhydrous COX,with AsEt, in vacuo forms unstable [Co(X),(AsEt,),] species,600and [Co(I), (AsPh,),] has been isolated from nitromethane solution.m' Bidentate arsines form more stable complexes and some preparations are gwen in Table 50. The electronic spectrum of red-brown [Co(l),(dpav)] (178) is inconsistent with a tetrahedral geometry and it may be planar ( p = 2.9 BM), but [Co(X),(dpae)] (179) (X = C1-, Br-, I - ) is almost certainly tetrahedral.," The asymmetric phenyl analogue forms [Co(mpae),](CoX,) as the solid and a planar structure is possible for the cation (p = 2.1 BM for the C10; salt).,'' Bis(dipheny1arsino)methane forms green [Co,(dpam),I,] of unknown structure and low stability. Four-coordinate planar structures for complexes [CO(AS-AS),](C~O,)~have not been positively identified with the poorer aryl or vinyl As donors (see below) but dark green [Co(dmav),](CoCl,) is probably planar (p = 2.6 BM for c a t i ~ n ) . ~Attempts , to prepare five-coordinate [CoX(diars),]+ complexes (diars = 180) have not been successful for X = C1-, Br-, I-, but for X = NCS- excess ligand results in [Co(NCS) (As-As),]+ (As-As = dpav, dpae), and using a 1:l ratio of ligand to metal the non-electrolyte [Co(NCS),(diars),] has been isolated which may be planar ( p = 2.6 BM).@'Irradiation of a mixture of cis and tram bis(dimethy1arsine)ethylene with COX,results in [CoX(dmav),]X, which is five-coordinate; a 1:l electrolyte in solution (X = C1-,Br-, I-), its magnetic moment in the solid state
770
. ..
I
Cobalt
77 1
772
Cobalt Table SI Structures: Some Cobalt-Arsine Complexes Complex
Structural data"
Comments
cobaitfrr)
[Co(OC10,)(OAsMePh2),]C10,
CaO(As) 202; OCoO 85-95
trans-[Co(OClO,), (das),]
COAS229-230; COO(C10,) 28C330 COCO237.7; COAS232-237; COH 157-184;
Cobalt (III) trans-[Co(Cl), (das),]ClO, trans-[Co(Cl), (das),]CI [Co(No)(das)*l(Clo,),
CoAs 234; CoCl 222; AsCoAs
88 CoAs 2334; CoCl226; AsCoAs 88.6" COAS233-236; ASCOAS85 CoNO 178"; CON 168 CoN(CS) 212; CoAs 234235; ASCOAS86"; NCOAS85-92 CoAs 231, 232; CoC1228.6; CON 190(av): As0 176 Co"0 208; COP 230; CoAs 236; CON 187-191; As0 166 @
trans-[Co(NO)NCS(das),]NCS trans-[CoCI{As(OH)MePh}(DMGH),] * H,O
Co[Co{As(O)MePH}(DMGH)(DMG)], A-( +)54h-~i ~p- [ C ~(R,R)-tetars}]ClO, 02) {
COO 186.7; COAS229-232; 00 142.4; OCoO 44.9"
Dist. tet. pyr.; four equiv. arsine oxide groups Planar; two crystalline forms; with v. long COO bonds in axial sites p,-bridged hydride dimer
i
2 6
Oct.
3
Oct.; isostructural with Ni"' complex Trig. bipyr.; NO in eq. plane
4
Oct.; rrans NO, NCS
5
Oct.; probably MePhAs(0H) coord. Oct.; trans CoAs bond with As(O-) bonded to central co" ion Dist. oct.; side-on bonding of 0;
7
5
8
9
a Bond
I. 2.
3. 4. 5. 6. 7. 8. 9.
lengths in pm. P. Pauling, G. B. Robertson and G. A. Rodley. Nature (London), 1965, 207, 73. F. W. B. Einstein and G. A. Rodley, J . Inorg. .Vud Chem., 1967, 29, 347. P. J. Pauling, D. W. Porter and G. B. Robertson, J . Chem. SOC.( A ) , 1970, 2728. P. K. Bemstein, G. A. Rodley, R. Marsh and H. B. Gray, Inorg. Chem., 1972, 11, 3040. J . H. Enemark, R. D. Feltham, J. Tiker-Nappier and K. F. Bizot, Inorg. Chem., 1975, 14, 624. P. Dapporto, S. Midollini and L. Sacconi, Inorg. Chem., 1975, 14, 1643. L. M. Mihichuk, M. J. Mombourquette, F. W. B. Einstein and A. C. Willis, Inorg. Chim. Acta. 1982, 63, 189. W. R. Cullen, D. Dolphin, F. W. B. Einstein, L. M. Mihichuk and A. C. Willis, J . Am. C k m . Soc., 1979, 101, 6898. D. B. Crump, R. F. Stepaniak and N. C. Payne, Can. 6.Chem., 1977,55,438.
(p = 1.962.16 BM) is consistent with penta~oordination.~~~ [CoX,(das),] (das = 181) is likely to be
trans octahedral in the solid state although magnetic data are in the range found for five-coordinate CO''.'" The structure of [Co(OClO,),(das),] shows long bonds in the axial positions of a tetragonally distorted octahedraon (Table 51). In ionizing solvents such as nitromethane a planar geometry for [Co(das>,l2+is possible, although Bosnich, Jackson and Lo suggest dimerization in acetonitrile.606 The related ligand (182) forms green paramagnetic [Co(nas),](CoC~)and yellow diamagnetic [Co(nas),](ClO,),, and dimerization is possible for the latter and octahedral coordination for the f ~ r m e r . ~Reduction ' of [Co(H,O),(das),](ClO,), with primary alcohols leads to the dimer (183), which is essentially diamagnetic (p = 0.7 BM).606
(178) dpav
(179) dpae
(180) diars
(181) das (182) nas
The branched ligand (184) forms unstable [Co{MeAs(dmap), )J2+ salts with possibly the apical As remaining uncoordinated,"* but no Co" complexes have been reported with the softer ligand (185) (the apical phosphine analogue PhP(dmap), forms squarepyramidal [CoCl, {PhP(dmap),31,
Cobalt
773
Section 47.5.2.6.iii). Reduction of Co(BF,), with NaBH, in the presence of (1%) forms the mauve p,-hydrido-bridged dimer (176)375(structure Table 5 l), which reacts with HCI or CO generating 3 mol of H, and Co2+or [Co{MeC(dpam),)(CO),](BPh,) respectively.
47.6.3
Cobalt(I1I)
Few monotertiary arsine complexes are known for Co"'. Displacement of trans sulfito groups from Na, [Co(CN), (SO,),] gives Na[Co(CN), (AsPh,),], which presumably also has the trans config ~ ra t i o n .~Some ' [CoY(DMGH),(AsR,)] cobaloximes are also known (R = Me, Ph; Y = C1-, SnPh; )610 as are some a-bonded R'R2As- and R'R*(O)As complexes (see below). Bidentate diarsines are more common and Nyholm many years ago reported the preparation of several trans-[CoX,(das),]+ complexes by air oxidation of the corresponding Co" systems (X = C1-, Br-, I-, NCS-).6" He also isolated [Co(das),](ClO,), . Subsequently, the cis-[CoX,(das),]+ isomer was prepared, initially following treatment of tram-[CoI, (das),]I with Ag+,612 and later via [C0(CO,)(das),]Cl0,.~~~ In this later work Bosnich and coworkers resolved cis-[Co(Cl),(das),]+ using (+)-arsenyltatrate and showed optical retention in various substitutions with H,O, CO:-, NO;, NO; and C,O:-. Racemization in dry MeOH occurs with some 80% conversion to the trans isomer.613 Some preparationsare given in Table 50 and crystal structures in Table 51. The green trans complexes are usually tetragonally distorted in the solid state but the trans configuration is thermodynamically favoured in solution. Peloso and Dolcetti have followed rates of substitution (Cland Jackson, McLaren exchange, NCS- ) and isomerization for cis- and ~runs-[Co(Cl),(das),]+,~'~ Some unusual self-induced catalytic and Bosnich report an interpretation of their CD ~pectra.~" substitution and racemization processes have also been reported and solutions should be protected from the light.613The cis-diaqua, hydroxyaqua, and dihyroxy ions do not racemize rapidly although water and hydroxide exchange occurs within hours at 25 0C.613Green, presumably trans, [Co(X), (dmav),]X complexes have also been prepared, as has [Co(drnav),](BF,), (Table 50). Bubbling NO through methanol solutions of trans-[Co(X), (das),]X results in trans[CoX(NO)(das),](CIO,) and trigonal-bipyramjdal [Co(NO)(das),](C10,) (Table 50), and Brant and Feltham have recently correlated their photoelectron spectra with those of other Co and Fe nitrosyls. A linear correlation between vNO and the N Is binding energy was demonstrated as well as insensitivity of the As 3d binding energy to the other ligands.616 A number of multidentate arsine complexes have been prepared by air oxidation of the corresponding Co" complexes (Table 50). Perhaps the most interesting work in this area is that of Bosnich and his colleagues using the quadridentates (187), (188) and (189); all contain two central chiral As atoms. Air oxidation of CoC1, in acidified MeOH in the presence of (187) gives five [Co(Cl),(tetars)]Cl isomers, three containing the racemic (RR,SS) ligand and two the meso (RS) form.6" Resolution of a-[Co(Cl), (RR,SS-tetars)]+ followed by reduction using KCN gives the optically-active ( S S ) ligand, which is optically stable and stereospecific in its subsequent coordination to Co"'. However an interesting and unexplained halide-induced racemization process was found for the complexes in acetonitrile. Metathesis leads to a wide range of complexes (X = COi-, H,O, Br-, NO;, NCS-, CN-, N;) with retention at the chiral As centres but with changes in stereochemistry between isomers (NO), (191) and (192).6'8The meso ligand prefers the trans structure (192) but the racemic ligand exists in all three forms. A rationalization of their CD spectra has also been rep0rted.6'~Reduction of a-[Co(Cl>,(RR,SS-tetars)]Clwith NaBH, in MeOH-H20 mixcontures produces a yellow solution which absorbs O2forming /3-[Co(0,)(RR,SS-tetars)](C1O4) taining side-on-bonded dioxygen (204; Table 5 1). A similar property holds for trans-[Co(Cl), (R,Stetars)]Cl and trans-[Co(Cl),(das),]Cl. If the solutions are kept acidic hydrido complexes trans[COH(CI)(A~--AS)~](C~O,) are formed (193; Scheme 65). Use of excess NaBH, at pH 8 leads to almost white [Co(H),(das),](ClO,), which slowly absorbs O2 in the solid forming pink [Co(O,)(das),](CIO,) (194).620Likewise treatment with Me1 results in (195). The trans-labilizing effect of hydride is seen in the very facile exchange of aqua ligand in [ C O H ( H , O ) ( ~ ~ ~ ) ,A] ~ + . ~ ~ ~ similar study using fars (188) and qars (189) has given the meso and optically active ligands and
-
774
.
interconversions similar to the above are possible.62'The (RR,SS) qars ligand forms only the c1 stereochemistry (190) due to the rigid ligand backbone but fars gives a, p- and trans complexes. Reduction of trans-[Co(Br), (RR,SS-fars)]Br with NaBH, in basic solution and subsequent 0, absorption forms [Co(RR,SS-fars)O,j+, while in acidic solution trans-[CoH(Br)(RR,SS-fan)]+ is formed. A stable dihydride [Co(H),(RR,SS-qars)](C104) was also isolated.621
r .
. .'
Cobalt
Ph
Ph
I 1 M~,As(CH,)~A~(CH,),A~(CII~)~A~M~, (187) (K,S)tetars ( w m o form) (RR,SS)tetan (racemic f o r i ) (188) fars
x
I X
AsMe, (189) qars
I+
(190)
(191)
P
(192) trans
Few mechanistic studies on ligand replacement have been reported. Nanda and Tobe many years ago studied the displacement of C1- by Y = Br-, NCS- in [Co(Cl),{MeAs(dmap)}], and the independence of rate on the nature and concentration of Y - suggested rate-limiting dissociation. Also it appears that [CoX(solv)(As-As), ]'+ species rapidly undergo anation by X- (halide, pseudohalide). The rate is very much faster than for the corresponding amine complexes. Some interesting 0-bonded R,(0H)As complexes containing As"' result from treating [Co(DMGH),] with Ph,AsCI or MePhAsCl in methanol, and a structure on [CoCl{As(OH)MePhJ(DMGH),]-H,O (Table 5 1) establishes a Co"'-As bond.623Treatment of Co' cobaloximes with R'R'AsX (X = C1, I) leads to R'R2As- coordination. Thus addition of [Co(DMGH),py]- to AsMe,CI and [Co(DMGH),(Bu'py)f- to AsMe21in methanol gives a product which approximates to [Co(DMGH),(AsMe,)-MeOH. This is thought to be polymeric with AsMe, bridges?" Using [Co(DMGH),(PBu;)]- as the nucleophile R1R2AsC1forms green solutions (R', R2 = Me, Ph) thought to be (196),which air oxidize to (197; Scheme 66). A crystal structure (Table 51) shows two [Co(DMGH)(PBuy)(As(O)MePh)] units joined to a central Co" ion.624Complex (196) also reacts with Mer, or [Co(DMGH),(py)Me], to form AsMe, (Scheme 66), and this is seen as a possible model for the aerobic biological methylation of arsenic.
47.6.4 Arsenates Comparisons with phosphate (and other oxyanions) can be made with reference to Section 47.5.2.8.(i). Substitution at As" is much faster than that at Pv due to its more polarizable nature and readiness to expand its covalency from four to five or six. Thus 0 exchange in As0,H~3-")-is considerably faster than in P analogues and this is paralleled by their ability to react with CoOH;' species with displacement at As rather than at the metal. The reaction given by equation (127) is reasonably fast s-I in 0.1 mol dmm3arsenate, pH 6)597and when assumptions in the forward direction (k,= 5 x are made concerning ion pairing the rate for displacement at As within the ion pair is somewhat faster (ca. 10 times) than that for the unassisted 0 exchange in HASO:- (kcx;Table 74). Proton
Cobalt ,H....
0 -
‘
775
‘0
1
- Co[Co{As(O)MePh}(DMGH)(PBu,)I ( 197)
+
AsMeZI- + [Co(Me>!DMGH),(PBu,)l Scheme 66
transfers in a transition state of type (253) have been proposed with the lower reactivity of H,AsO; being attributed to the smaller ion-pairing constant.597The reverse hydrolysis reaction (khyd;Table 74) is fastest with [CO(OASO,H~)(NH~)~]~+ IPK, x 3.3), and an acid-catalyzed route via [CO(OA~O,H,)(NH,),]~~ (pK, x - 0.5) is also availabre. The more basic complexes are relatively stable in aqueous solution. Comparison of k,,, and k,, (Table 74) shows that the COO-AsO,H, bond is more inert than the HO-AsO,H, bond. [(NH?)~COOH,I 3+
+ [HASO,] 2-
kf
,
‘hyd
,[(NH3)5CoOAs0,HIf -t H20
(127)
The chelate form of arsenate (equation 128) has not been clearly identified. It is more likely than phosphate to dissociate to the monodentate form in aqueous solution.
47.7
OXYGEN LIGANDS
47.7.1 Superoxo and Peroxo
Cobalt-dioxygen chemistry has its origins with FremyQ’ who, in 1852, reported that the exposure of ammoniacal solutions of cobalt(I1) salts to air resulted in the formation of brown ‘oxo-cobaltiates’. However, it was not until 1898 that these cobalt-dioxygen complexes were correctly formulated by Werner and Mylius as containing the [(NH,),CO(O,)CO(NH,),~~ cation.626In the late 1930s Tsumaki6” observed that solid samples of the cobalt(I1)-Schiff-base complex [Co(salen)] possessed the capacity to bind dioxygen reversibly. This discovery prompted a widespread and continuing interest in the oxygen-carrying ability of cobalt chelates. Much of the early work in this area concerned solid state cobalt(I1kSchiff-base systems, in particular the square-planar complexes derived from [Co(salen)J (198) and the trigonal-bipyramidal complexes derived from [Co(salpr)] (199). Both types of compound can be prepared in active and inactive forms, with dioxygen-carrying ability being dependent on purification procedures and the methods of preparation employed. Calvin and his coworkers prepared, characterized and tested for dioxygen-carrying properties a total of 57 Co-Schiff-base complexes6’*and many more were examined by Dieh1.629Compounds of type (198) bind 1 mol of O,/mol of Co while those of type (199) bind 0.5rnol O,/mol of Co. Oxygenation can be reversed either by heating or by evacuation, however after many oxygenationdeoxygenation cycles dioxygen-carrying ability decreases. In the solid state [Co(3-Fsalen)] deterioand this deterioration is linked to rates to -60% of its original activity after 1500 irreversible oxidation or to strain imposed on the crystal leading to inactive modifications. In non-aqueous solution oxygen uptake by both the type (198) and (199) chelates was complicated by problems of irreversibility and the failure of the complexes to appreciably oxygenate under oxygen A significant development came with pressures sufficient to cause oxygenation in the solid e ~ ~the ~ cobaIt(I1kamino-acid complex the discovery of Burk, Hearon, Caroline and S ~ h a d that
m
Cobalt
776
[Colhis),] combined reversibly with dioxygen in aqueous solution. This was an important result, for [Co(his),] was the first synthetic compound to undergo reversible oxygenation in aqueous solution and it was evident that oxygenation was a general rather than an isolated phenomenon. This early chemistry of cobalt-dioxygen complexes has been r e v i e ~ e d . ~ ~ * - ~ ~ ~ ~ ~
It is now recognized that cobalt dioxygen complexes may be classified according to their molecular geometries and electronic distributions into four related categories. These structural classes were originally proposed by V a ~ k in a his ~ ~general ~ review on dioxygen-metal complexes and allow the cobalt systems to be viewed as either superoxo (types Ia and Ib) or peroxo complexes (types IIa and IIb) of cobalt(II1). This formalism is supported by the collective experimental evidence for cobalt-dioxygen adducts, which indicates that binding of O2to cobalt involves considerable electron transfer from the metal to the coordinated dioxygen moiety. Unlike the situation with chromiumand iron-O, adducts the degree of electron transfer, and consequently the nature of the bonding in the cobalt-dioxygen complexes, is relatively clear.
(Id
(Ib)
(114
(Ilb)
One of the best measures of the degree of charge transfer to dioxygen is the stretching frequency vo2, and selected data for cobalt4ioxygen complexes are given in Table 52. Several early assign-
and ments of vo2 are known to be incorrect but the increasing use of isotopic substitution ('*0,/''04) Raman and resonance-Raman techniques has now eliminated many of the problems and ambiguities associated with the measurements. The frequency ranges 1075-1 195cm-' and 790932 cm-' are widely recognized as diagnostic for superoxo and peroxo complexes respectivelp36and virtually all stable cobalt-dioxygen adducts are found to have stretch frequencies lying in one or the other of these ranges. The unstable complexes formed between cobalt(I1) porphyrins and 0, doped into Ar matrices have recently been ~ h o w nto~have ~ ~voz, values ~ ~ ~(- 1280 cm-') significantly greater than the upper limit of the superoxo range. Single-crystal X-ray structures have now been determined for a large number of cobalt-dioxygen complexes and comparisons of the observed 0-0 bond lengths with those of superoxide anion or peroxide dianion (0-0 separation = 128pm, KO,; 149 pm, Na,O,) also form a reliable guide to classification. Recent reviews on the chemistry of metaldioxygen complexes with particular relevance to cobalt binuclear superoxo and peroxo systems include a number dealing with general complexes,M2.6$3 reversible complex catalytic oxidation64Y,6M and e l e c t t o n i ~ and ~ ~ ' EPR652spectral properties.
47.7.1. I
1/' Superoxo
The 1: 1 dioxygen adducts of cobalt(I1) complexes containing tetradentate ligands have been widely studied. In these systems dioxygen binding requires the presence of a Lewis base in the coordination site trans to the dioxygen moiety, which gives rise to six-coordinate adducts. Complexes of this type include those with Schiff bases, porphyrinato and various other macrocyclic ligands. Other ligand types are also suitable however. Certain pentadentate ligands provide the additional donor atom to eliminate the need for an external Lewis base, and a stable six-coordinate five-coordinate cobalt-dioxygen adduct containing solely unidentate ligands has k e n
I Cobalt
777
Table 52 Stretching Frequencies in Cobalt-Dioxygen Complexes
Superoxo complexes
[CoWO,)l [CoMb(O, )I [Co(OEP)(Oz)I [Co(TPP)(Oz11 [Co(TTP)(I -Meim)(O,)] [Co(Cap)( 1-Meim)(O,)] [Co(TpivPP)(1 -Meim)(O,)] [Co(TPP)(PY)(O*)l tCo(salen)(o,)l [Co(saWb~)(O~ll [Co(acacen)(B)(Oz11
1122, 1153 1122, I153 1275 1278 1142 1176 1150 1144 1011 888 1123-1195
[{NH,)sCoJ,(O,)Ifl,* H,O Ks[{(NC),CoJ,(O,II* H,O
1122 1099
[{(NH,),CoJ2(NH,)(0,)1CI,-3H,0 I. 2. 3. 4. 5. 6. 7.
8. 9.
IO. 1I.
1066, 1095 1066, 1095 I202 I209 1071 1084
1077 1084 943 833 -
*
1075
-
-
Res. Raman, H,O Res. Raman, W,O Res. Raman, Ar matrix Res. Raman, Ar matrix IR, CH,CI, IR, CH2C12 IR, nujol Res. Raman, CH,C1, Res. Raman, cryst. Res. Raman, cryst. IR, nujol, B = py, 4-Mepy, C N H ~ P Y~ ,- C N P Y Raman, cryst. Raman, cryst. Raman, cryst.
3 3 4 5 6 6 I 8 9 10 11
2 2 2
J. Halpern, B. t.Goodall, G.P. Khare, H.S. Lim and J. U. Pluih, J . Am. Chem. Soc.. 1975, 97, 2301. T. Shibahara and M. Mori, Bull. Chem. SOC.Jpn., 1978, 51, 1374. M. Tsubaki and N. T. Yu, Proc. Nutl. Acnd. Sci. USA. 1981.78. 3581. M. W.Urban, K. Nakamoto and J. Kincaid, lnorg. Chim. Acta. 1932, 61, 77. M. Kozuka and K. Nakamoto, J . Am. Chem. Sac., 1981, 103, 2162. R. D. Jones, J. R. Budge, P. E. Ellis, Jr., J. E. Linard, D. A. Summerville and F. Basolo, J . Orgunomet. Chem.. 1979, 181, 151. J. P. Collman, J. I. B r a w n , T. R. Halbert and K. Suslick, Proc. Nutl. Acad. Sci. USA, 1976,73, 3333. K. Bajdor and K. Nakamoto, J . Am. Chem. Soc., 1983, 105,678. M. Suzuki, T. Ishiguro, M. Kozuka and K. Nakamoto, borg. Chem., 1981, 20, 1993. K. Nakamoto, M. Suzuki, T. Ishiguro, M. Kozuka, Y. Nishida and S. Kida, Inorg. Chem., 1980, 19, 2822. A. L. Crumbliss and F. Basolo, J . Am. Chem. Soc., 1970,92, 55.
cobalt(I1)-trisphosphine complexes bind dioxygen but dissociate a phosphine ligand in the process to form five-coordinate terminally bound dioxygen complexes.456 In many systems there is a marked tendency for p-peroxo dimer formation (Scheme 67) but the 1:1 adducts may be stabilized by the use of polar solvents and by bulky groups in the ligand which hinder the dimerization step. They may also be stabilized kinetically by use of low temperatures, however bases with the ability to hydrogen bond will accelerate dimeri~ation~'~ and CT basicity is also important. Detailed discussions of the factors influencing stability are (BML4)Co
+0
KO,
2
, A (B)(L,)Co(Oz)
Scheme 67
Table 53 gives a list of a number of cobalt(l1l)-superoxo complexes which have been isolated as crystalline solids. There is a marked preponderance of complexes of the type [Co(SB)(B)(O,)] (SB = Schiff base). The base adducts of simple Co" porphyrins have low affinities for dioxygen at room temperature and consequently their 1:l adducts with 0, are not isolable. In contrast, exposure of a solid sample of Collman's picket fence porphyrin system [Co(TpivPP)(N-Meim)] to 1 atm of The pivalamido pickets in this dioxygen for 24 h produces [Co(TpivPP)(N-Meim)(O,)] compound exercise control of solvation about the coordinated dioxygen moiety and the stability is comparable with that of COM~O,,~'' where the globin protein environment performs the same function. Virtually all water-soluble cobalt dioxygen complexes are binuclear but (NR,),[Co(CN),(O,)J (R = Et, Pr") complexes may be obtained following the reaction of 0,with [CO(CN),]~-in DMF
= Cobalt
778
Table 53 Crystalline Monomeric Cobalt(II1)-Superoxo
B
Complex *
.:
DMF ..
pr
4-Mepy 4-CNpy 4-W,PY
PY
[Co(3-Me0salen)(B)(O2)] [Co(3-MeOsaltmen)(B)(O,)] [Co(3-Fsaltmtm)(B)(02)] [Co(3-Bu‘salen)(B)(02)] [co(saltm~)(B)(o,)l [Co(N-Mesalpr)(O,)][Co(N-Mesalpr)] [Co(J-en)(B)(OdI [Co(J-pn)(B)(Oz11 [Co(2-Xacacen)(3)(0,)]
H2 0 I-Meim PY Btim
PY
Orange
Py
9v
4.
5. 6. 7. 8. 9.
IO. 11. 12. 13. 14. 15.
16.
..
.
pem= 2.22 BM
Ref. 3 3 3 3 3
peR= 1.89 BM pER= 1.54 BM peE= 1.71 BM peff= 1.49 BM pea= 1.65 BM
4 S 12 7 -
BM = 52Snm (CH,Cl,) I, = 525nm (CH2C1,) X = C1, &- = 370nm (CH,Cl,) X = SUCCNH, I,,, = 355, 525nm (CH,CI,) X = SUCCNH, I,,, = 360, 52Snm (CH,CI,) p c m =3.26
-
-
I, = 560 nm (CH2C1,) R = Et, Pr” I,, = 330 nm (EtOH/MeOH)
1-Mcim
-
i , , = , 4 12, 549 nnl (toluene); peerr = 2.54 BM
-
..
-
Comments
Black-purple Red-brown Dark brown
~
.
Black Red-brown Carmine Red-black Brown Red-brown Red Burgundy
PY
PY
Py. PPh,
I. 2. 3.
Colour
Dark red Black Black Purple Dark red Orange
-
Complexes
6 8 2, 9 2 1 1 1
10 11
13 14 IS, 16
.
K. Kasuga, T. Nagahara, A. Tsuge, K. Sogabe and Y . Yamamoto, Bull. Chern. SOC.Jpn., 1983,56,95. K. Kubokura, H. Okawa and S. Kieda, Bull. Chem. SOC.Jpn., 1978, 51, 2036. A. L. Crumbliss and F. Basolo, J . Am. Chem. Soc.. 1970,92, 55. C. Floriani and F. Calderazzo, J . Chem. SOC( A ) . 1969, 946. B. T. Huie, R. M. Leyden and W. P. Schaefer, Inorg. Chem., 1979, 18, 125. R. S. Gall and W. P. Schaefer, Inorg. Chem.. 1976, 15, 2758. W. P. Schaefer, B. T. Huie, M. G. Kurilla and S . E. Ealick, Inorg. Chem., 1980, 19, 340. R. H. Nisuinder and L. T. Taylor, Inorg. Chim. Acta, 1976, 18, L7. K. Nakamoto, Y. Nonaka, T. Ishiguro, M. W. Urban, M. Suzuki, M. Kozuka, Y. Nishida and S. Kida,J. Am. Chem. SOC.,1982.104, 3386. J. D. Landeis and G. A. Rodley, Synrh. Reuai. h r g . Metal-Org. Chem., 1972, 2, 65. W. Kanda. H. Okawd and S. Kida, J . Chem. Snr.. Chcm. Commun., 1983, 973. A. Avdeef and W. P. Schaefer, J . Am. Uhem. Suc., 1976, 98,5153. D. A. White, A. J . Solodar and M. M. Baizer. Inorg. Chem., 1972, 11, 2160. G . P. Klare, E. Lec-Ruff and A. 8. P. Lever, Cm. J. Chem., 1976,54, 3424. S . R. Rckens. A. E. Martell, G . McLendon, A. B. P. Lever and H . B. Gray, Inorx. Chem.. 1978, 17, 21W. J. P. Collman, J. I. Brauman, K. M. Doxsee, T. R. Halbert, S. E. Hayes and K. S. Suslick, J . Am. Chem. SOC.,1978, 100. 2761. J. P. Collman, R. R. Gagne, J. Kouba and H. Ljusberg-Wahren, J . Am. Chem. Soc.. 1974, %, 6800.
and oxygenation of ethanolic solutions of cis-[Co(Cl), (s-Me,en),] .gives cis-[CoCl(sMe2en)2(02)]+isolated as its PF; or Cloy salt.656That the monomeric rather than the dimeric complexes are obtained appears to be due to the influence of solvent in the first case and to steric hindrance in the ligand in the second. The five-coordinate Co’I-Schiff-base complexes [Co(salpr)] and [Co(N-Mesalpr)] differ only by substitution of H for Me at the axial nitrogen atom, yet on oxygenation in non-polar solvents the former gives a stable p-peroxo dimer, [{C~(salpr)},(O,)],~~~ while the latter gives the unusual superoxo complex [Co(N-Mesalpr)(O,)] * [Co(N-Mesalpr)].658 Clearly, the balance between formation of the mononuclear and binuclear complexes is delicate.
779
Cobalt
.
Structural studies on the monomeric dioxygen complexes have often been hampered by solid-state disorder or excessively high thermal motion of the coordinated dioxygen moiety. Despite these problems a number of structures have been reported (Table 54) and with one exception the 0-0 bond distances all lie within the relatively narrow range 124 to 135pm. Both the 0-0 bond lengths and the Co-0-0 bond angles (range 117 O to 153 ”) are consistent with the formulation of these 1:l adducts as cobalt(II1)-superoxo complexes. The variation in the Co-0-0 bond angles has been attributed to packing forces.@’ Magnetic susceptibility measurements on [Co(L,)(B)(O,)] complexes correspond to the presence of one unpaired electron and EPR spectra exhibit a well-defined eight-line 59C0hyperfine spectrum with a peak separation of 12 G. Studies using 170,-labelled [Co(bzacen)(py)(Oz)]show the two oxygen atoms to be non-equivalent in the solid state and in frozen solution,659but equivalent in fluid The equivalence may be due to a rapid equilibrium between two bent conformations: Co-(OAOB) f Co-(OBOA). These experiments also showed the electron to be localized essentially on dioxygen. The commonly held view that there is a formal transfer of an electron from the cobalt metal centre to a dioxygen .n* orbital in the cobalt-dioxygen adducts (Co”’-O; ) has been challenged by Tovrog, Kitko and Drago.662On the basis of EPR experiments they suggest that on combination of a Co” complex with dioxygen only partial transfer occurs, with the degree of transfer being dependent on the ligand environment about the metal. For a series of cobalt(I1) complexes containing a range of different ligand types they estimate between 0.1 and 0.8 electrons transferred to the coordinated dioxygen fragment. The proposed alternative bonding interaction involves the spin pairing of unpaired electron in an antibonding orbital of O2with an unpaired electron in a dz2 orbital on cobalt(II).ffi3That the unpaired electron in the adducts resides in an oxygen n* orbital is not in dispute. Smith and F i l b r o ~ hslve ~ ~ ’ discussed the merits of this proposal, but have concluded from a recent analysis of the EPR spectra of several cobalt-dioxygen complexes that electron transfer cannot be estimated reliably at present.6” The results of EPR studies on a wide range of cobalt-dioxygen complexes, including CoHbO,, CoMbO, and the dioxygen adduct of vitamin B12r,have been summarized in recent r e v i e ~ s . ~ ~ ~ ~ ~ Several cobalt-dioxygen adducts have been studied theoretically. These investigations include studies on [Co(Cl),(B)(O,)] systems (B = none, NH,, py, im) by BocaMSand the construction of Walsh diagrams by Teo and Li666for the interaction of 0, with the fragments [Co(CN)J3- and [Co(PH,),]+. Calculations on [Co(acacen)(B)(O,)] (B = none, H,O, CN-, CO, im) by Veillard and
-
Table 54 Structures: Mononuclear Cobalt(IJ1kSuperoxo Complexes
Compound [Co(bzacen)(PY)(O,)l
126
186
126
[wacacen)(PY)(O,)l [Co(salpeen)(O,)]-MeCN
-
195
106
190
134
[Co(3-Bu‘salen)(py)(02)]
135
187
116
[Co(3-MeOsaltmen)(HzO)(O,)] dme
125
188
117
[Co(saltmen)(Bzim)(Oz)] [Et“ [Co(CN), ( 0 2 )I ’ 5H*O
128 124
189 190
120 153
[Co(3-Bu‘saltmen)(Bzim)(O,)]
127
187
117
[Co(N-Mesalpr)(OJ
I35
I88
133
130
I88
I17
.[Co(3-Fsaltmen)(N-Meirn)(02)] 2ac 1. 2. 3. 4. 5. 6. 7.
8. 9.
10.
High thermal motion of 0, 0, disordered, - 40 “C 0, irresolvably disordered C,H4 bridge disordered 0,two-fold disordered -
Oz two-fold
disordered At -152°C and room temp. 0, two-fold disordered At -171°C
G. A. Rodley and W. T. Robinson, Nature {Londonj. 1972,235,438. M . Calligaris, Inorg. Xucl. Chem. Lett., 1973, 9, 419. G. B. Jameson, W . T. Robinson and G. A. Rodley, J. Am. Chem. Soc.. Dalton Trans., 1978, 191. B. T. Huie, R. M. Leyden and W. P. Schaefer, Inorg. Chem., 1979, 18, 125. A. Avdeef and W. P. Schaefer, J. Am. Chem. Soc.. 1976, 98, 5153. R. S. Gall and W. P. Schaefer, Inorg. Chem., 1976, 15, 2759. L. D. Brown and K. N. Raymond, Inorg. Chem., 1975, 14,2595. W. P. Schaefer, E. T. Huie? M. G. Kurilla and S. E. Ealick, Inorg. Chem., 1980, 19, 340. R. S. Gall, J. F. Rcdgers, W. P, Schaefer and G, C. Christoph. J . Am. Chem. Soc., 1976,98, 5135. R. Cini and P. Orioli, J. C h e w Soc.. Chem. Commun., 1981, 196.
I
2 3 8 4 6 7
9 IO 5
780
Cobalt
c o ~ o r k e r s ~ ~ ~ the s h obent w ql geometry to be more stable than the q2 by 19G-340kJmol-' depending on B, and also preferred over the linear v' arrangement. In agreement with the related calculations of Fantucci and Valenti on [C~(acacen)(NH,)(O,)]~~~ the unpaired electron is found to be localized essentially in a xforbital of dioxygen. Veillard's calculations667also show a relationship between the a-donor ability of the axial ligand B, the ease of oxidation of cobalt(1I) to cobalt(III), and the ease of oxygenation. Qualitative support for this result comes from the observation by Carter, Rillema and BasoIo6@ that for a number of complexes of the type [Co(SB)(B)] (SB = Schiff base) a linear correlation exists between log k2 (&, = [Co(SB)(B)(O,)]/[Co(SB)(B)lpO,) and the oxidation potential cobalt(I1) to cobalt(Il1). The correlation holds both for variation in B within the [Co(bzacen)(B)] series and for variation in the Schiff base within the [Co(SB)(py)] series, thus giving a measure of the effects of axiai and in-plane ligands on the stability of the [Co(SB)(B)(O,)] adducts. Baso10~~'suggested the correlations exist because the oxidation potentials provide a measure of the charge density on the cobalt centre. The greater the charge density on Co the greater will be the electron transfer from cobalt to dioxygen, and consequently the more stable the dioxygen complexes. The correlation between KO,and the base strength of axial phosphine ligands in [Co(porphyrin)(PR,)] complexes6m also relates to the amount of electron density at the cobalt metal centre. An analogous correlation relating electronic effects arising from substitution in the in-plane ligand to the dioxygen-binding ability of [Co(porphyrin)(py)] complexes has been described by Walker, Beroiz and Kadi~h.6~' The reduction of [Co(CN),(0,jl3- by MoVspecies in aqueous solution produces the bimetallomer (201), which is slowly decomposed in organic solvents to the q2-Mo(0,) species (202).672Tracer studies using "0, show however that the peroxo bridge in (202) does not derive from [Co(CN), (0,)I3-,673
(201)
(202)
Kumar and E n d i ~ o t t have ~ ~ , examined the one-electron oxidation-reduction of [Co([14]aneN4)(H20)(02)]*+in aqueous solution. Reduction competes successfully with dimerization in this system and even mildly reducing metal ions such as Fez' react via an inner-sphere process. Presumably this pathway is preferred owing to the large free-energy changes associated with formation of the p-peroxo adducts. With Fe2+the adduct is observable as a transient CoOOFe species, which decays to Fe3+and unspecified cobalt products. The coordination of 0, to Co" centres not only enhances the basicity of the dioxygen fragment but also increases its tendency to undergo free-radical reactions. The radical nature of q' cobaltdioxygen complexes is ROW well established and is particularly demonstrated through their participation in hydrogen-abstraction reactions. The autooxidation of phenols to the corresponding p-quinones and diphenoquinones catalyzed by cobalt(I1) complexes provides the best example. N i ~ h i n a g a ~has ' ~ demonstrated that the dioxygen complex [Co(salpr)(O, j] activates 2,4,6-tri-bbutylphenol via hydrogen abstraction by following the simultaneous decrease in the Co-0, EPR signal and appearance of the phenoxy radical signal. The product of the reaction of [Co(salpr)Jwith the radical has since been isolated and found to have the structure (203).649Similar H-atom abstractions are observed for [Co(saIen)(py)O,)] and the dioxygen adduct of vitamin BLkwith Z ,Cdihydroxybenzene, and with N,N"-tetramethyl-p-phenylene.676 Drago and coworkers have also examined the oxidation of phenols catalyzed by cobalt(T1)-Schiff base c ~ r n p l e x e sand ~ ~ report ~ - ~ ~ a~ high selectivity toward p-quinone production over solution analogues when the complex is bound to a polymer A further illustration of the radical character of the q' Co-0, compIexes has been given by the reaction of [Co(lV-Mesalpr)(O2)]with the spin trap 5,5-dimethyl-1-pyrroline spin a d d u ~ t . ~ " N-oxide @MPO) to form a stable Co-0-0-DMPO
A I I
1 Cobalt
,
~
'
78 1
The role of cobalt-dioxygen complexes in autooxidations other than phenol oxidation is less certain, and ostensibly similar reactions appear to follow radically different pathways. Thus, in the oxidation of thiols to disulfide catalyzed by Co" species catalysis by the phthalocyanine complex [Co"(TSPc)l4- apparently proceeds via a Co' intermediate and without participation of Co-0, whereas catalysis by [Co"(TPP)] appears to involve initial formation of an ql cobalt-dioxygen complex from which 0; is displaced by thiolate.681Several reviews giving extensive coverage to oxidations catalyzed by cobalt(I1) complexes are available.649,650~682.683 There have been a number of reports describing the formation of q1 cobaltdioxygen complexes by reactions not involving oxygenation of cobalt(I1) chelates. Ligand substitution at the axial positions of the cobalt(lI1) complex aquacobalamin (vitamin B,,,) is sufficiently rapid to allow coordination of electrochemically generated 0; ion in DMF solution.684The product displays an EPR spectrum identical with that of the dioxygen adduct of the Co" complex vitamin B12r. Condensation of [Co(CO),], produced from the thermal dissociation of [Co,(CO),], and 0, at 77 K produces [CO(CO)~(O,)]~~' which has the EPR characteristics of an ql cobalt-dioxygen adduct. The same product is reported from the cocondensation of Co atoms with CO/O2mixtures at 10-12 K.686 Simic and Hoffman687have found that 0; adds to both [C0(4,11-dieneN~)(H,O),]~+ and [Co( 1,3,8,lO-tetraeneN4)(H,O),12f in aqueous solution to produce transient species displaying intense charge transfer bands. The spectra do not correspond to Co' or Co"' complexes, excluding 0; as a simple electron transfer agent. It is concluded that 0; adds to the Co" centre at a labile axial site. 47.7.1.2
#:#
Supcroxa
The only p-superoxo complexes to have been characterized are those which contain cobalt. They are readily prepared by treatment of the corresponding p-peroxo complexes with strong oxidants. Thompson and Wilmarth688observed that MnO,, HOC1, Br,, BrO; and NO; are all effective oxidants toward [(en),Co(p-NH, ,02)Co(en),13+ in acidic solution whereas Fe3+, H , 0 2 , Agf and Cr,O:- are not. In other systems Cl,, PbO,, persulfate and Ce" in nitric acid solution have also effected oxidation.642A number of well-characterized p-superoxo cobalt(II1) complexes are listed in Table 55. There are no reports of p-superoxo complexes containing Schiff base ligands and all that are known contain either terminal amine or cyano groups. The p-superoxo cobait(II1) complexes are usually green light-sensitive compounds and their measured magnetic susceptibilities (p % 1.6 EM) correspond to the presence of one unpaired electron. originally formulated the complexes as containing a peroxide group briding Co"' and Co'" centres, however the equivalence of the two metal atoms has been demonstrated by EPR experiments. Room temperature EPR spectra of both the single-bridged p-supecoxo6w~691 and p - a r n i d o - ~ - ~ ~ p e rions ~ x oare ~ ~well-defined ~ ~ ~ ~ ~ isotropic spectra consisting of 15 lines with A,,, 10 G. This is the expected spectrum for two magnetically equivalent cobalt nuclei with the unpaired electron residing primarily on the dioxygen bridge. A theoretical study of [(NH,), CO(O,)CO(NH,),]~+using SCCC methods6" is in agreement with this result. Several structural studies on the p-superoxocobalt(II1) complexes have been undertaken (Table 56) and the observed 0-0 bond lengths (range 124 to 135pm) also support the structural formalism Co"'(0:- )Co"'. Recently, Raman spectroscopy has allowed the determination of 0-0 stretching frequencies for a number of single-bridged superoxo c o m p l e ~ e s .As ~ ~is ~the - ~case ~ ~ for the corresponding p-peroxo species, this vibration is IR inactive, presumably because the molecular symmetry is such that dipole changes due to this vibration are small. A general discussion of the Raman and IR spectra of psuperoxocobalt(II1) complexes has been givenm and values of voz appear in Table 52. The purple salt [(NH,),Co(0,)(NH3),] [(NC), Co(O,)Co(CN),] has been prepared@'and is stable in the solid state, whereas [(NH3),Co(0,)Co(NH,),](C10,), decomposes with loss of oxygen within 2 to 3 d at room temperature.696In acidic solution [(NH3)5Co(02)C~(NH3)S]5+ and the doubly bridged [(NH,),Co(p-NH, ,O,)CO(NH,),]~+ion decompose by pH independent paths to yield Co"' and Co" species in equal amounts. These processes appear to be simple aquations at cobalt(III), following which the leaving group is further decomposed. Valentine and Valentine698found that irradiation of the LMCT bands of [(NH,),Co(O,)Co(NH3),15+and [(NH3),Co(p-NH2,02)Co(NH,),]4+ gives efficient decomposition into 1:1:1 mixtures of [Co(NH, )5(H20)]3+,Co:& and O2independent of the temperature, acidity or ionic strength of the solutions. Attempts to trap electronically-excited intermediates gave negative results and the quantum yields fall sharply if the ammine ligands are replaced by chelating amines such as t e t r e x ~ . ~ ~ ~ N
782 r-
"
s
. C
K
+ z"
+ a
s
0
s
c
++++
t
+ "F: E i
Cobalt
784
Table 56 Structures: Dimeric Cobalt(II1)-Superoxo Complexes Compound
0-0 hJm) 132 131 129 129, 144
1. 2. 3. 4. 5. 6. 7.
co-0
co-0-0
Ref.
(Pm)
("1
190 189 191 192, 194
117 118 118 121, 122
1 3 4 2
121 120 I19
5 6 7
R. E. Marsh and W. P. Schaefer, Acta Cry.%dIogr., Sect. B, 1968, 24, 246. F. R. Fronczek, W. P. Schaefer and R . E. Marsh, Inorg. Chem., 1975, 14, 611. W. P. Schaefer and R. E. Marsh, Arlo Crystdlogr., 1966, 21, 735. V. M. Miskowski, B. D. Santarsiero, W. P . Schaefer, G. E . Ansok and H. 8. Gray, Inorg Chem., 1984, 23, 172 G. G. Christoph, R. E. Marsh and W. P. Schaefer, Inorg. Chem., 1969, 8, 291. U. Thewalt and G. Struckmeier, Z . Anorg. Allg. Chem., 1976, 419, 163. U. Thewalt and R. E. Marsh, Inorg. Chem.. 1972, 11, 351.
Photolysis of [(NC),CO(O,)CO(CN),]~-gives [CQ(CN),(OH,)]'-, 0, and H,0,698 with quantum yields being dependent on the wavelength of e x c i t a t i ~ nSuperoxide .~~ ion appears to be generated in the primary photolysis step. ~ Kinetic data have been reported for reduction of pL-superoxocomplexes by Fez+,'" M O " , ' ~C0'1703 and Ru" complexes,'04 and Vz+, CI?' and These processes involve outer-sphere electron transfer and in some cases703,706 the Marcus theory has been appiied to the rate constants obtained. Electron transfer quenching of the excited state of [Ru(bipy),]'+ by various p-superoxo cobah(II1) complexes leads to production of [Ru(bipy)J3+ and the corresponding p-peroxo species.706
47.7.1.3
# Peroxo
Few examples of this type are known and all appear to be formed irreversibly. Oxygenation of the planar Co' complex [Co(vpp),](BF,) either in the solid state or in solution gives [Co(vpp)(O,)](BF,), which shares with [Co(b~acen)(py)(O,)]~~ the distinction of being the first monomeric cobalt4ioxygen complex to be structurally ~haracterized.~~' BosnichYo8 found that the similar reaction of 0, with the Co' species produced by BH; reduction of cis-cl-[Co(Cl),(R,R:S,Stetars)]+ gave red cis-/3-[Co(R,R:S,S-tetars)(O2)]+for which a structural study7(@ has also confirmed the q2-bondedmode of the dioxygen moiety (204). This complex is also produced exclusively when an equilibrium mixture of the &-a and cis-p isomers of [Co(R,R:S,S-tetar~)(H,0),]~+ in water is treated with Hz02.This specificity appears to arise through Co"-catalyzed isomeric equilibration of the products since the H,O, reaction is far faster than the cis-u- to cis-S-diaqua isomerization, The corresponding reaction with ~is-[Co(diars),(H,O),]~+ gives cis-[Co(diars),(O,)]+, which is also ) neutral methanol solution is exposed tc formed quantitatively when cis-[Co(H),(diars),](ClO,in dioxygen. B o s n i ~ hreports ~ ~ ~ an alternative method of preparing q2-dioxygen adducts via the base-induced cleavage of Co-Co bonds in dimers of the type trans,trans-[(X)(As,)Co-Co (As4)(X)J4+ (X = H,0 or MeCN; As, = (diars),, R,S-tetars, R,R-tetars). The yields are always less than 50% and the reactions involve heterolytic cleavage of the Co-Co bonds to produce Co"' compIexes and the O2sensitive Co' species (Scheme 68).
Cobalt J
785
Under certain conditions two one-electron-donor Co" centres can interact with dioxygen as if they were a single two-electron donor. This occurs in the formation of [Co,(CN),(PMe,Ph),(O,)] (205) from the reaction of dioxygen with [ C O ( C N ) ~ ( P M ~ ~ In P ~this ) ~ case ] . ~ ~the ~ binuclear complex arises from a cyanide-bridged inner-sphere electron-transfer reaction between [Co(CN), (PMe,Ph),] and the immediate 0,-substituted product [Co(CN),(PMe,Ph),(O,)]. The formal oxidation states of the final product thus correspond to Co"'NCCo"'(O:-). All the $-dioxygen adducts are kinetically robust, which is surprising in view of the small bite angles (OCoO % 45 ") reported for these species (Table 57).
PR,
47.7.1.4
l/:q' Peroxo
The reaction of cobalt(I1) complexes with dioxygen in aqueous solution normally produces diamagnetic p-peroxocobalt(lI1) complexes, and isotopic labelling shows the peroxo bridge to be derived entirely from the 0, used in the greparati~n.~''Formation of the dimer is preceded by formation of a monomeric superoxo species (Scheme 69), which may be stabilized by steric hindrance in the ligand. Generally the kinetic andlor energy balance favours the dimer. The ligands include simple monodentates (H,O, NH, , CN-, NO; ,halide), aminocarboxylates, polyamines and various macrocycles.
Scheme 69
Under basic conditions acidic ligands (H,O, NH,) cis to the peroxo group can be deprotonated to form dibridged p-(OH,O,) or p-(NH,,O,) species. The effect of the second bridge is to stabilize the peroxo dimer toward dissociation. However, while the amido bridge is stable in acidic solution the p-hydroxo bridge is readily cleaved. Ethylenediamine7" and tren'" dibridged p-peroxo species have also been reported. The one-electron oxidation of the usually brown p-peroxo cobalt(1II) complexes with strong oxidants such as Ce", Cl, or S 2 0 i - affords green paramagnetic p-superoxo complexes. Table 5 5 gives details of the preparation of water-soluble p-peroxo and p-superoxo complexes where these have been isolated as crystalline solids. Ligand exchange on many of the p-peroxo Complexes is rapid and the facile substitution of monodentate ligands frequently provides an alternative to direct oxygenation as a synthetic route, e.g. equation (128). [(NH3)5C002Co(NH3)5]4+ + 2dien
+
[(NH,),(dien)C~(O,)Co(dien)(NH,)~]~+ + 6NH3 (128)
It has recently been shown that the rates of substitution of NH3 ligands in [{(en),'',and dioxygen exchange in [{(tren)(MeNH,)C0),(0~)]~+ 714 are identical with (NH,)CO)~(O,)]~+ the pH-independent rates of decomposition of these complexes to cobalt(1I) species and dioxygen. Similarly, the rates of formation of p-hydroxo bridges, following loss of amine ligand, in [{(tren)(NH,)Co) (0,)]4+,715 [{(en)*(NH,)Co], (0,)j4+ 'I3 and [{(tren)(MeNW,)Co), (O,)]" 4I' are found to be identical with the rates of decomposition of the complexes to cobalt(I1) and 0,. Thus the fast ligand-exchange rates observed for the p-peroxo complexes arise from the lability of the cobalt(I1) species produced via Scheme 69.
Cobdl
786
Not all singly bridged p-peroxo complexes decompose as indicated by the reverse pathways of Scheme 69. I1 has long been known that in acidic solution [{(NQCO],(O~)]~is cleaved quantitatively to ICo(CN), (H20)I2- and hydrogen peroxide,” and a number of other p-peroxo complexes behave similarly. Also, in acidic media tr~ns-[{($CN)([14]aneN~}Co),(0,)]~~ is quantitatively converted to trans-[Co(SCN),([ 14]aneN4)]+in the presence of SCN- in a process which produces H,O, .’I6 Solutions of [{(H20)([14]aneN,)C0),(0J]~+ possess both oxidizing and reducing properties’” and it is probable that H,O, formation in this reaction is due to an outer-sphere electron transfer between the oxidizing Co”’-(0,) and reducing Co” species produced in the first step of peroxo-dimer decomposition. Concentrated solutions of strong acids protonate the peroxo bridge to produce red p-hydro,0,)Co(N4)j3+ ions [(N,) = (NH,), , (en)?; peroxo Complexes. Protonation of [(N4)Co(pL-NHZ Scheme 701 initially gives the orange intermediate (206), which slowly isomerizes to the red isomer (207),710,718 Crystallographic studies on (207) for (N4) = confirm the end-on coordination mode for the hydroperoxo group and give the 0-0 bond distance as I42 pin. The three bonds to the bridging oxygen atom are not coplanar. N H ~ (WXo’ ‘,Co(N), \0-0
13+
, --H’ ‘IN),Co’
N H ~ ‘Co(N), \0-o+ I
14+
NH
e (K)+Co’ ‘0+
14+
>Co(NI,
I
Scheme 70
Zchnder and Fallab720 have demonstrated peroxo bridge formation via anation of czs[C0(en)~(H?0),]~+ with H20,. Reaction at pH 4.8 initially gives the hydroperoxo complex cis[CO(OOH)(~~>,(H,O)]~+, which at higher pH reacts with [Co(OH)(en),(H,0)]2+ to yield racemic [(en>,~C~(p-OH,O,)Co(en),]~-. Crystal structures of the dibridged species are available for the meso”’ and racemic722forms. The question regarding isomer distributions in p-peroxo complexes containing multidentate ligands has been addressed by Fallab and coworkers. In the formation of [(trien)Co(p-OH,O,)Co(trien)P+ eight chemically distinct isomers are possible of which three are observed.723Methylation at the secondary nitrogens of the trien ligand destablizes isomers with the p configuration and oxygenation of [Co(Me2trien)(H,0),j2+ in alkaline solution gives the dibridged complex (208)7’3 with an X,CI configuration. Three achiral isomers of [(tren>Co(~i-OH,O,)Co(tren)]~+ are possible. however only (209) is formed by oxygenation of [C~(trem)(OH,)]”.”~
The electronic spectra of p-peroxo cobalt(II1) complexes are dominated by LMCT transitions from filled n*(Oi-) orbitals to empty Co”’ du* orbitals, giving rise to intense broad bands in the near UV Ligand field bands often appear as poorly defined shoulders on the rising UV absorption but are resolved for certain [ 14]aneN4cornplexe~.~’~ The monobridged p-peroxo dimers often exhibit two band maxima at 320 and 380 nm, whereas the dibridged complexes b-(0H,02)0 p-(NH2,OJ display one intense band at 360 nm with a shoulder at higher energies. These features arise from splitting of the z*(O:- 1 level and are related to the planarity of the Co(0,)Co unit!” Electronic716and sgCONMR726spectral studies indicate that the p-peroxo group possesses norma? spectrochemical properties for an oxygen donor ligand bound to Co“’. Selected structural data for p-peroxo cobalt(TT1) complexes are included in Table 57. Orbital bonds in N,Co(O,)CoN, species show that trans bent bonds diagrams constructed for Co--0-0 and this is what is observed for the monobridged complexes. Dibridging forces are the most the p-peroxo group to adopt a skewed cis geometry except in cases (e.g. [(tren)Co(p-tren,O,)Co-
-
Cobalt
787
Table SI Structures: Monomeric and Dimeric Cobalt(I1ItPeroxo Complexes Compound '
.
Monomeric [CO(VPP), (O,)I(BF,) [CO, (Me, PhP), (CN),(O,)I *0.5C,H,
cis-/?-[Co(R,R-tetars)(O,)](ClO,)
~~~~~
I. 2. 3. 4. 5. 6. 7. 8. Y. IO. 1 I.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
22. 23. 24.
~~~~~~
0-0 (Pm)
co-0
142 144 142
189 190 186
I47 147 146 I49 IS3 149 147 145 131 138 145 134 146 143 I46 149 142 145 147 143
co--0--0
~~~
~
Ref.
("1
bm) -
44.6 (0-CO-0) 44.9 (0-co-0)
1 2 3
188 189 189 190 189 192 188 199 193, 200 191 193 191
113 111 115 110 110
4 5 6 7 8
186 189 187 188 192 174, 184 186 188
112 107 1IO 116 111, 119 116, 1 IO i 10
112
9
111 119 118 120 119 120
10
111
11 21 22 23 24
13 14 15 16 17 18 19 20
~
N. W. Terry, 111, E. L. Amma and L. Vaska, J. Am. Chem. Soc., 1972,94,653. J. Halpern, B. L. Goodall, G. P. Khare, H. S. Lim and J. J. Pluth, J . Am. Chem. Soc., 1975, 97, 2301 D. B. Crump, R. F. Stepaniak and N. C. Payne, Can. J. Chem., 1977,55,438. W. P. Schaefer, Inorg. Chem.. 1968, 7, 725. U. Thewalt, Z . Anorg. Allg. Chem., 1982, 485, 112. J. H. Timmons, A. Clearfield, A. E. Martell and R. H. Niswander. Inorg. Chem., 1979, 18, 1042. J. R. Fritch, ci. G. Christoph and W. P. Schaefer, h o r g . Chem., 1973, 12, 2170. T. Shibahara. S. Koda and M. Mori, Bull. Chcm. SOC.Jpn., 1973, 46,2070. M. Zehnder and U. Thewalt, Z . Anarg. Allg. Chem., 1980, 461, 5 3 . F. R. Fronczek, W .P.Schaefer and R. E. Marsh, Acta Cry..rrailogr.,Sect. R, 1974, 30, 117. F. R. Fronaek and W. P. Schaefer, Inorg. Chim. Acta. 1974, 9, 143. S. Fallab, M. Zehnder and 11. Thewalt, Helv.Chim. A c ~ u ,1980, 63, 1491. M. Zehnder, U. Thewdt and S. Fallab, Helv. Chim. Acta, 1976, 59, 2290. H. Macke, M. Zehnder and U. Thewalt, Helv. Chim. Acfa. 1979, 62, 1804. U. Thewalt, Z . Anorg. Allg. Chem., 1972, 393, 1. M. Zehnder, U. Thewalt and S. Fallab, Helv. Chim. Acta, 1979, 62, 2099. U. Thewalt and R. Marsh, J. Am. Chem. Soc., 1967,89, 6364. F. Bigoli, M. Lanfranchi, E. Leporati and M. A. Pellinghelli, Crysr. Struct. Commun., 1981, 10,1445. U. Thewalt and G. Struckmeier, Z. Anorg. Allg. Chem.,1976, 419, 163. T. Shibahara, M. More, K. Matsumoto and S. Ooi, Bull. Chem. Sac. Jpn., 1981, 54, 433. B. C. Wang and W. P. Schaefer, Science, 1969. 166, 1404. A. Avdeef-and W. P. Schaefer, h o r g . Chem., 1976, 15, 1432. L. A. Lindblorn, W. P. Schaefer and R. E. Marsh, Actu Cr,wtal/ogr.,Sect. E, 1971, 27, 1461. M. Callignris, G . Nardin, L . Randaccio and A. Ripamonti, J. Cham. SOC.(4), 1970, 1069.
(tren)14+) where the additional bridging ligand possesses sufficient conformational flexibility to allow the trans structure. The 0-0 bond lengths (mean 147pm) are comparable with that found * *the solid-state structures hydrogen-bonding effects, for ionic peroxide in Na, O2 (149 ~ m ) . ~In packing forces and the nature of the counterion play a role in determining the planarity of the co-0-0-co Unit.w3729 Rate studies on the oxygenations of cobalt(I1) complexes in aqueous solution are numerous. According to Scheme 69, and assuming a steady state in L,Co(02), production of the p-peroxo complex follows equation (129), which for k2[CoL,] % k , reduces to equation (130) and k , is obtained directly. At ambient concentrations of 0, this is generally the case, implying that for most Co" complexes formation of the monomeric L,CoO, species is rate determining under these conditions. Values of the rate constant kl (Table 58) mainly lie in the relatively narrow range lo3 to 105dm3mol-ts-' at 25 "C irrespective of the nature of the ligands on the Co" reactant. This
I
788
Cobalt
constancy appears to arise, at least in part, from a mechanism dominated by water exchange; however the oxygenation rates are slower than the estimated rates of water exchange at Co" centres by a factor of lo3, and the data indicate that there is a strict orientation requirement of the 0, moiety in the substitution process. It is of some interest to find that the low-spin [C0([14]aneN,)(OH,),]~+and high-spin [Co([l 5]aneN4)(OHi),]2+ions combine with 0, at comparable rates.730Since reaction of the latter complex requires a change in spin multiplicity, the intrinsic barrier to adduct formation appears not to be sensitive to the spin state of the Co" reactant.
-
A recent kinetic study has monitored oxygenation of the primary Co" photoproduct produced following flash photolysis of cis-[Co(NO,),(en,]+ in air-saturated acetonitrile solution.73' Hyde and S y k e ~have ~ ~ carried ~ out a detailed study of the Cr" reduction of [(en)2Co(pNH, ,o,)C~(en),]~+. The reaction proceeds in five identifiable stages; the first involving inner-sphere attack by C?+ on the peroxo bridge to give a Cr11'-{0,)2--Co11' species. The stepwise reduction is complicated, with isomerization and decomposition being involved in the intermediate stages before final Cr2+reduction of the Co"' centre. The Fez+ reduction of [(N4)Co(p-OH,0,)Co(N,)]3+ ions in acidic solution is slower than hydrolysis of the OH bridge, and the previously reported Fe2+ reduction of the doubly bridged species7j3has been shown to be in error.734 Fallab'35has shown that reaction of I- with [(en),Co{p-OH,O,)Co(en),J3+ gives [C0(en),(0H,)]~ and I, in acidic solution. Hydrolysis of the OH bridging group is followed by I- attack on the mono-bridged species. The latter process is related to that occurring when I- reacts with [{N4)Co(pNH, ,O2)Co(N4)l3+ions to produce [{N,)Co{p-NH, ,OH)Co(N4)l4+species,736and to the C1-- or Br--catalyzed disproportionation of the p-amido-p-peroxo complexes to produce the correspondTable 58 Rate Constants for Oxygenation of Co" Complexes in Water at 25 "C Complex
[Co(tren)(H,O)j2+ [Co([141aneN,)(H,0)z12+ [co([~~I~~w(H~),P [Co(NH,),(H,0)1 [Co(en),(H,O),P+ [Co(dien),]* [co(trien)(n,O),]*+ [Co(OH)(trien)(H,O)]+ [CO(Lhis)] [Co(gly- d Y )I2[co(cY~),l[Co(sdtma)(H,O),]+ [Co(udtma)(H,O),]+ [Co(sedda)(H,O),I [Co(uedda)(H,O),] [Co(terp~)(~he~)(H,0~1*+ [CO(terPY)(biPY)(~1~)1~~ +
+
+
I. 2. 3.
4. 5. 6. 7. 8. 9. 10. 11.
k, (M-ls-')
Ref:
2.2x 102 5 x IC+
2
3.8 x io3 2.5 x 104
4.1 x 105 1.2 x 10' 2.5 x 104 2.8 x l@ 3.5 x 103 1.0 x 104 2.0 x io5 3 x 103 1.4 x 104 2s 22 8.5 x 105 1.0 x lo6
1 2 3 4 4 5 5
6 7 8 9 9 10 10 11
11
H. Macke, Help. Chim. Acta, 1981, 64, 1579. C. L. Won& J. A. Switzer, K.P. Balakrishnan and J. F. Endicott, J . Am. Chem. Suc., 1980, $02, 5511. J. Simplicio and R. G . Wilkins, J . Am. Chem. Soc.. 1969, 91, 1325. F. Miller, J. Simplicio and R. G. Wilkins, J . Am. Chem. Soc.. 1969, 91, 1962. F. Miller and R. G. Wilkins, J . Am. Chem. SOC.,1970, 92. 2687. K. L. Wallers and R. G. Wilkins, Inorg. Chem., 1974, 13, 752. G. McLendon and A. E. Martell, Inorg. Chem., 1975, 14, 1423. R. G. Wilkins, A&. CAem. Ser., 1971, 100, 111. G. McLendon, D. T. MacMillan, M. Hariharan and A. E. Martell, Inorg. Chem.. 1975, 14, 2322. G. McLendon, R. J. Motekaitis and A. E. Martell, Inorg. Chem., 1975, 14, 1993. D.Huchital and A. E. Martell, Inorg. Chem., 1974, 13, 2966.
789
'
'
ing p-amido-pa-hydroxoand p-amido-p-superoxo species.737In those systems it is likely that the reactive form of the complex is the hydroperoxo species, which undergoes initial attack by halide ion at the non-bridging oxygen atom (Scheme 71). Sulfite and nitrite ions with [(en)2Co(pNH2,0,)C0(en),]~+ give respectively [(en),Co(p-NH, ,SO,)Co(en),]'+ and [(en),Co(pNH,,NO,)CO(~~),]~+.~~~
NH2 -I3+
co/ 'co \ I
co'
> -
HOCI/HODI
\
NH2 -I4' 'co
I
0-0
0-0
Scheme 7 1
Reduction potentials for p-superoxolp-peroxo-cobalt(II1)couples have recently been obtained by cyclic v o l t a m ~ n e t r y . Decomposition ~~~*~~ or dissociation of one or the other of the components of the couple, which frustrated measurements by other techniques, may be overcome by the use of fast scan rates. Protonation of the p-peroxo group stabilizes the complex and Ee values are pH dependent below pH 3. A similar stabilization on protonation of the peroxo bridge has been noted in the CrZf-,Vz+-and Eu2+-promoted redu~tion''~of the [(NH3)5Co(02)Co(NH3)5]4' ion. The protonated species has K, 10dm3mol-1at 25 T 7 0 5 Dimeric cobalt porphyrins linked at the porphyrin periphery to provide a cofacial configuration, and capable of forming p-bridged oxygen adducts, have been Chang7,* demonstrated that peroxo-bridge formation occurred on exposure of a dicobalt porphyrin to dioxygen and that the p-peroxo complex was readily oxidized to the corresponding p-superoxo species. More recently, Collman's has investigated electroreducing properties of a number of dicobalt porphyrins bound to graphite electrodes in acidic aqueous solution. Electroreduction of 0,to H,O, or H,O is observed in all cases with the mode of catalytic activity being dependent on the degree of separation of the two Co centres. One possible mechanism for the four-electron reduction of dioxygen to water in this system is shown in (Scheme 72), but it should be emphasized that bridging dioxygen species have not been directly observed in this process.
-
+ 2H,O
0-OH
Scheme 72
790
Cobalt
47.7.2
Carboxylates
Synthesis, cobelt(ZZZ)
47.7.2.1
Until recently the heating of Co-OH;+ species with RCO,H or RC0,Na in neutral aqueous solution, first used by Gibbs and Genth,745. was the recognized method of preparation. G o ~ l d , ’ ~ ~ ’ ~ ~ following earlier work by Sebra and Ta~be,’~ prepared numerous [CO(OCOR)(NH,),]~+ ions by this method (equation 131). Some improvement is possible using [Co(DMF)(NH,),](ClO,), or [Co(DMSO)(NH,),](C10,), in DMF or DMSO, by using [Co(OH,)(NH,),](ClO,), in ethylene The Ag+-induced glycol, and by using [Co(OCO,)(NH,),](NO,) in d i g l ~ m eor~ ~ ~ removal of C1- from [CoCl(NH,),]Cl, or cis-[Co(Cl),(en),](ClO,) in MeOH at room temp e r a t ~ r e ’ ~ ~is*’another ~: possibility but is not recommended.
-
[C~(OH,),_,(arnine),]~++ RCO;
[C~(OCOR),_,(arnine),]‘~-”’+f (6 - x)H20
(131)
The new method, rediscovered by Jackman, Scott and P~rtrnan,~’* originates with Werner who prepared [Co(OCOR)(NH,),](NO,), (R = Me, Et) using a heterogeneous mixture of solid [Co(OH)(NH,),](NO,), and the appropriate anhydride (equation 132).753 Jackman and coworkers use non-aqueous conditions and have extended the reaction to include a large number of acid anhydrides, with the hydroxo complex being generated in situ by adding the sterically hindered base N ( M ~ ) , B z . ’The ~ ~ rate law for reaction (132) in aqueous solution takes the form kobs= kK,[Co],/ (K, -t [H+]), which is consistent with Co-OH2+ being the reactive species (K, = acidity constant for Co--OH:+). The reaction is normally quite fast (k = 3.1 mol-’ dm3s-’ for R = Et, 25 “C I = 1.0) and “0tracer experiments confirm that the oxygen atom in the coordinated carboxylate derives from the hydroxo ligand.755The difficulty with using aqueous solutions in a preparative Sense is the ready hydrolysis of many anhydrides. In non-aqueous solvents (DMF, DMSO, (MeO),PO) the anhydride can often be generated in situ from the parent acid (equation 133), but in DMF it is important to generate the Co-OH species before adding RCO,H (i.e. add the sterically hindered base before adding RC0,H) otherwise Co-DMF3+ is formed in rather large amounts (equation 134).756Such side reactions can also be fast and occur via nucleophilic attack of Co-OH at the unsaturated C centre.7s7This new preparative method has general utility since any acyl compound with a good leaving group can be used (equation 135). This probably includes dicarboxylic acid species (monodentate or bidentate) but such preparations (equation 136) have not been examined in detail. However chelated acetylacetonate (210) has been prepared in this manner.758The r a n t preparations of cis- and trans-[Co(NO,)(OC,O,)(en),] uses displacement of the aqua Jigand by
c2 0 2 4
169
-
0 0 [(WJ5Co-0Hl2+ RC02H
+
+
PhN=C=NPh
II I1
RCOCR +
- -
0
tl
I CR
(NHd5Co2+-m
PhN=CNHPh
RCOIH
+
RCO,H
PhNHCNHPh
I1
I
+ (RC0)20
(132) (133)
0
OCR
II
0 PhN=CNHPh H I OCR
+
Me,NCHO
I1
-+
Me,N=CHOCR
II
I(NH,),CoOH,13t
0
l o
(NH3)5C03f-@CHN(Me), Co3+-@H,
+
RCX
\I
+ 2B
+ Co2+--@CR
[I
0 X = C1-, PhO-, p-N02C6H40-,RCO;;
+
B
=
+
PhNHCNHPh
RC02H
II
0
+ Hf
+ X- +
2BH’
(1341 (1356
0 NEt,, or similar non-nucleophilic base
(136j
Cobalt
79 1
47.7.2.2 Monocarboxylates
(i) Formato, cobalt(I)
, .*
The reaction of CO, with [CoH(N,)(PPh,),] or [Co(H),(PPh,),] in benzene containing excess In the absence of excess ligand phosphine leads to the green formato complex [Co(O, CH)(PPh3)3].+"'9 only brown [Co(CO)(PPh,),] and a cobalt(t1) formate species, which converts to Co(HCOO),.2H20 on exposure to moist air, are formed. Attempts to insert C 0 2into the Co-H bonds of [CoH(dppe),] and [CoH(dppv),f lead to poorly defined products which release CO, on treatment with H2S04 and which do not contain coordinated f ~ r m a t e . ~ "By contrast, [CoH(N,)(RR;P),] and [Co(H),(RR;P),] (R = Et, R = Ph; R = Ph, R' = Et) both give formate complexes on treatment with CO, . These are less stable than the corresponding PPh, complexes but few experimental details are available. [CoH(PMe,Ph),] does not react with C 0 2in the solid state but in benzene solution it takes up C 0 2 reversibty to give [Co(OzCH)(PMe2Ph),].'@This has antisymmetric and symmetric vibrations of the CoOCHO group at 1610 and 1360cm-I. A general scheme for the insertion of CO, into Co-H bonds involving initial formation of cobalt-carbon dioxide adducts has been proposed (Scheme 73).7"
Scheme 73
(ii) Formato, cobalt (IIIj
[Co(O,CH)(NH,),](CIO,), containing 0-bonded formate has been prepared via [Co(OH,)(NH,),](ClO,), (pink, 69.6),761but it can undopbtedly be made uia [Co(OH)(NH,),]*+ and Me0,CH (equation 139, or by using [Co(DMSO)(NH,),](C104), and Na0,CH. The carbonyl 0 undergoes H+-catalyzed exchange, k = 6.2 x mol-' dm3s - I , ~ ~ ' but the complex is quite stable in aqueous solution (Table 59). The cis-[Co(O,CH)(H,O)(en),I2+ ion has been reported in aqueous solution (A,,, 300, 500 nm).761 Major interest lies in the oxidation of Co"'-OC(0)H species to CO,, its use in electron transfer, and its photochemistry. Candlin and Halpern showed that oxidation by MnO; (1.0O . l m ~ l d r n - ~ H -O"C)followedtheratelawk,,,= +, k[MnO;](k = 2.5 x 10-2mol-'dm3s-') but with the [CO(NH,),(OH,)]~+product showing an additional [MnO; ] dependence over the less abundant Co& product (ratio % 3.5 in 10 ~ 2 m 0 1 d m ~ 3 M n 0 ; ) ~This 7 6 3was interpreted in terms of Scheme 74 with the primary act bein H atom abstraction to give the formate radical anion (211). Presumbly the k2 path leads to CO'~--Q=C=O or Co"'--CO,, which rapidly aquates. More recently Brezniak and Hoffman generated (but did not directly observe) the same radical inter" mediate (211) using pulse radiolysis methods with H atom abstraction by HO*and H O . ~Only Cot$) was observed in this study (k 2 x 109mol-1dm3s-1)and using an estimate of k, > 106s-' for the electron-tunnelling process (equation 137) they estimate k2 of Scheme 74 as > 3 x 108mol-' dm3s-'. Clearly the COjradical coordinated to Co"' is a powerful reducing agent. and a Photolysis at 254nm also leads to Coi& and C 0 2 (also H, by a secondary long-lived intermediate, assigned as the C-bonded formato isomer (212; Scheme 7 9 , has been ~ h a r a c t e r i z e dThis . ~ ~ ~species is attributed to a pathway involving ligand excitation (Scheme 75) and has a pK, = 2.6. It is much more stable than the radical anion, existing for at ieast one minute in acidic solution but more rapidly decaying a t pH 5 ( 50 s - ' ) . ~ ~ ~
-
N
792
Cobalt Table 59 Rate Data for Aquation and Base Hydrolysis of Some Carboxylate Cobalt(II1) Complexes ~
Ligand
Complex
HCO; MeCO; MeCO, H (MeO)C,H,CO; (MeO)C,H4C02H c,0: ~
HC, 0; HO,CCH,CO; M2+0,CCO;
HC, 0;
c,o:-
HO,C(CH,), CO; HO,C(CHOH),CO; HO, CC, H, COT HOC, H, CO;
c20:-
HC, 0; HC20; i- H+ HOC,H,CO; (also derivatives) - OC6H4CO~ (also derivatives)
[CoX(NH,)(trien)r* trans-[CoX(OH)(en), ] cis-[CoX(OH)(en), ] [C0(~2C202~(en),l'
5.8 x
13 -
3.0 x 40°C 1.2 10-4, 40°C 1.3 x 10-5 2.7 x 10-4 1.1 x 40°C
(K,0.65) -
6.3 x 2.9 x 40°C (ZnZt), 60°C 1.5 x 2.0 x (Ni2+),60°C 1.5 x IO-' (Co2+),60°C 1.0 x 10-4 (c + ),60cc 2.2 x IO-', 25°C 1.5 x IO-', 25°C 1.0 10-7,40~ 5.0 x 40°C 1.0 x 1 0 - 7 , 4 0 ~ 4.6 x 70°C 4 1.0 x IO-,, 60°C 4.9 x IO-,, 60°C 8x 60°C (mol-' dm3s-' ) 1.66 x 60°C 1.39 x 10 -4, 30.4"C
(K, 0.14) 2.45 x (kOH) 6.1 x (kbH) (K, 116)
1 1 2 -
7
-
2.75 x IO-', 30°C
7 1 1 1 12
9 9 9 10
2.95 x IO-, (koH), 30°C 3.56 x IO-' (koH), 30°C 6.5 x 1 0 - ~(koH) 6.3 x IO-' (koH) 6.2 x 10-5 (voH)
Chelated C,b,-
1 1, 5
-
1 . 1 3 ' ~IO-', 30.4"C
c,o:-OC6H4CO; c,o:c,o:-
1 1 1
11
8 8 3 4
6
25 'C. D. Smith, M. F. Amira, P. D. Abdullah and C. B. Monk, J . Chem. SOC.,Dalton Tram., 1983, 337. 2. P.B. Abdullah and C. B. Monk, J. Chem. Soc.. Dalton Trans., 1983, 1175; A. C . Dash and R. K . Nanda, Inorg. Chem., 1974,13,655. 3. G. M. Miskelly, C. R. Clark, J. Simpson and D. A. Buckingham, h o r g . Chem., 1983, 22, 3237.
a At
1.
4. 5. 6.
7. 8. 9. IO. II. 12. 13.
G. M. Miskelly and D. A. Buckingham, Comments Inorg. Chcm., in press. N . S. Angerman and R. B. Jordan, Inorg. Chem., 1967, 6, 1376. D. A. Buckingham, C. R. Clark and G. Hiskelly, J. Am. Chem. SOP.,1986, 108, 5202. C . Andrade and H. Taube, Inorg. Chem.. 1966,5, 1087. A. C. Dash and M. S. Dash, J . Inorg. Nard. Chem., 1981, 43, 2873. A. C. Dasb and R. K. Nanda, J . Inorg. .Vuc1. Chem., 1974, 36, 2071. A. C . Dash and B. Mohanty, J. Inorg. Nucl. Chem.. 1977,39, 1179. A. C. Dasb and R. K . Nanda, J . Inorg. Nucl. Chem., 1978,40, 309. A. C. Dash and R. K. Nanda, Inorg. Chem., 1973, 12, 2024. W. E. Jones, R. B. Jordan and T. W. Swaddle, Inorg. Chem., 1969, 8, 2504.
0
(NH3)&f;-
II
OCH
+ MnO,
(NH3),C?-O&3
+ HMnO;
cf;+ co2+ 5NH3 Scheme 74
(iii)
Alkyf and aryl monocarboxylates, cobalt(III)
Both monomeric (213),chelated (214) and bridged dimeric (215) and (216) complexes are known. The latter were first prepared by WerneP' and in the last decade have received some attention in relation to remote attack and outer-sphere electron transfer (see below). The monomeric complexes
Coholt
793
I
I Scheme 75
’
(213) are very abundant and simple to prepare (Section 47.7.2.1). Formation via anation of Co-OH;+ by RC0,H or RCO; is slow and occurs via displacement of coordinated water (Scheme 76). Table 43 contains some recent kinetic data and extensive compilations are to be found e l ~ e w h e r e . ~ Saturation ~ ~ . ’ ~ ~ kinetics are often encountered, kobs = k, K,,[RCO; ]/(1 Kip[RCO; I), with rate-determining substitution from within the ion pair (the alternative possibility of non-reactivity of the ion pair would be counter producti~e).’~’ A dissociative mechanism is implied since kl bears some resemblance to the water-exchange rate and has never been known to exceed it, but whether a totally dissociative process is involved within the ion pair with the five-coordinate intermediate existing long enough for unfavourably positioned ions to reorient themselves is not known.
+
(NH,),Co-0
12+
\
C-R
//
0
(213)
0 (N)4co; ‘C’-@ I Y-(CHR),,
72+
/x, COP-"^)^ I o + pI
(NH,),Co
C
R
(214)
(215)
A
(NH-,)~CO
X, Y = NH2, OH
Co(NH313
1 1 o++)
CR
(216)
Scheme 76
The reverse hydrolysis in acidic to neutral solution often follows the rate law kohs= ko + k , [H’ ] and the reaction is again very slow; usually such studies are carried out at elevated temperatures. Table 59 contains some relevant data and extensive compilations are a ~ a i l a b l e . ’k~, ~is’ ~regarded ~~ as solvolysis of the protonated substrate (Scheme 77) and represents the reverse of the anation by RC0,H. Acid-catalyzed 0 exchange into the coordinated acyl function is generally slower than However, Jackman and hydrolysis except for electronegative R (H, CF, , C02H)where it is coworkers have found intramolecular scrambling between the 0 atoms in [Co{OC(O)Me](NH,),]Y, both in solid and in solution.757In the solid it is about one-third complete in 90 d at 23 “C, and is two-thirds complete after 15 min at SO “C in aqueous solution. It is not known whether this ~ ~ ~it may we11 resemble Co-SCN/Co-NCS isomerization process is H f - or O H - - ~ a t a l y z e dand (Section 47.4.4.1). 0 scrambling has not been observed in chelate complexes of type (214), probably because rotation about the acyl function is now more difficult. Alkaline hydrolysis is also slow, but is faster than acid hydrolysis; it follows the rate law kobs= koHIOH-] in most complexes (Table 59). An SJcb mechanism is generally accepted with aquation of the amine-deprotonated conjugate and cleavage of the Co-0 bond (equation 138).’72.’73For some electronegative R (CF,,CCl,, CHCl,, COT) k0bs = kOHIOH-]-t&[OH-]*with OH- attack at the acyl function and both Co-0 and COO-C bond cleavage, the latter via the deprotonated addition intermediate (217; Scheme
Cobalt
794
78).7767'hOH-"catalyzed exchange of carbonyl oxygen is generally slower than alkaline hydrolysis for most alkyl groups with monodentate systems,777but for the above electronegative species it is comparable?743775 HO
0
Co-OH?+RCO;
*
ko H,O
II C$-O-CR+H+.
-
d
,
CF-€)*R
1
kH
>Co-OHY
+ RC02H
Scheme 77
*11
0
1 I
(NH3)5Co-OCR
(NH&Co-OH
+ OH-
+ RCeO-*
eOH-
, A (NH3)sCo-OCR
(NH,),Co-OCR
H20
dl
I
R
(NH3)4( NH,)Co-OCR
\"'
0-
I , z (NH~)~CO-O-CR OH-
(NH3),CZ-OH
J
*-I
(217)
+ RCe;
Scheme 78
The bridged complexes (215) and (216) are generally prepared from p-(OH) species (Table 60).78' Thus the rate law for the formation of [{CO(NH~)~)~(~-NH~,O,CM~)]~* in the [H'J range 0.1-1.5 takes the form kobS= (k,4- k,[H+])[MeCO,H] with kl= 1.4 x 10 mol - I dm3s-', k, = 6.5 x 10-6m01 'dm6s-' at 25 3C.768Initial anation at one Co"' centre is considered to be rate determining with rapid subsequent bridging. Protonation of the bridging p-(OH) also plays a part. The reverse hydrolysis is also H+-catalyzed and is somewhat lower.'^ Recently the novel Coi" Cr"' complex (218) has been reported767and obvious extensions are possible. Carboxylate-Co"' systems have been extensively used to study electron transfer. Taube and Gould pioneered this work and have remained major contributor^.^^^^^^^ A recent review by Pennington is recommended,769and articles by Haim (bridged activated complexes), Meyer (excited state), Sutin (theory), Endicott (structural and photochemical probes), Ford, Wink and DiBenedetto (photochemical), Espenson (free radical) and Creutz (mixed valence complexes) in a volume reductant dedicated to Taube7" provides up-to-date background. Inner-sphere paths using a probably proceed via an intermediate such as (219),while if R contains a coordination centre then radical intermediates such as (220) have been observed.778The same or similar reduced-ligand With bridged intermediates have also been directly generated via pulse-radiolysis carboxylates (216), an outer-sphere mechanism is more likely since both oxygens are already coordinated but if R contains an available coordination site then bridged intermediates are again observed and are probably the reactive entity.360Other reductants, such as the photoexcited [Ru(bipy),*I2+ species, are thought to act by both outer-sphere and ligand-reduced inner-sphere mechani~rns."~
Cobalt
795
2 +a $
cv
2 I .^ E QI vl
I
Y Vl vl
o:, 1
A
+
h N
4
a 3
C.O.C. b 2 '
796
N
N
n-
N
n
.
..
Cobalt
798
e9-z
vl
Iz:
I
c
.c
m
d
s
c
Cobalt
800 47.7.2.3 Dicwboxylates, cobalt(ZIZ)
As well as regular monodentate complexes (213), R = (CH,),,CO,H, two types of chelate have been prepared (221) and (222), and one other bidentate (223). Some preparations are given in Table 60 and some structures in Table 61. The extended dimeric complexes (223) were first reported by D&04 and his method has been recently improved and extended to include a number of symrnetrical but asymmetric or heterometal complexes are possible. Sykes and coworkers have prepared p-bridged chelated oxalato complexes of type (226) but also including (224) and (225) (X, Y = NH,, OH).806Wieghardt has made similar complexes with other dicarboxylic acidsw7and Nishide and Saito some related oxalato and carbonato complexes containing 2-aminopropan01.~~ Complex (226) aquates slowly to the ring-opened species [(NH,),CoOCOCOOCo(NH,),(H,0)14+ in 1.O mol dmP3HClO,, and this is representative of others. X
/ \
X
lh+
14+
o++o C
Simple bidentate chelates (221) have been extensively studied. Their formation via anation of cis-Co(OH,)~+systems is slow (Table 60),but saturation rates consistent with the intervention of ion pairs (Scheme 76) are observed, especially with C20ip.783Both monodentate and bidentate coordination are possible. With [Co(OH,)(N),I3+ and oxalate [CO(OC,O,H)(N),]~+is formed [(Nj5= (NH3)5,784 (en)2(NH3)].’s5 With cis-[C0(0H;),(N),]~+ the chelate [CO(O~C,(CH,),}(NH,)~]+is the final outcome but intermediate monodentate carboxylates [Co{OCO(CH,), CO, )(N),(OH,)]+ become increasingly stable as the subsequent chelation step With H,C,?, slows down. This occurs with increasing ring size (n > 1) and steric req~irement.~~’ and HC,O; biphasic kinetics are not observed however (ci~-[Co(H~O),(en),]~+,~~~*~’ CIS[CO(OH,),(NH,),]~+,~~~ pH < 1) and [Co(O,C,O,)(N),]+ appears to be the only observed product (equation 139). In 2.0 mol dmP3Hf marked catalysis by NO; has been but this is not apparent with [CO(OH,)(N),]~+.The effect is not understood and marked H+ catalysis is also inv0lved.7~’Possible decomposition products such as HNO,, N z 0 3or H,NO: may be formed under the acid conditions and elevated temperatures (50-70 “C) or the water-exchange rate (k,,,Kx; Scheme 76) may be increased by the presence of nitrate species. A biphasic reaction is however found C~~-[CO(H,O),(NH,),]~+)?~~ For with C,@- in the pH range 3-7 ([CO(OH)(H,O)(~~),]~+,~~ [Co(OH)(H2O)(en),l2+water exchange has been increased significantly over that for [Co(H, O),(en),I3+ and the resulting increased anation rate for both cis and trans reactants leads to cis- and trans-[Co(0C,O3)(OH,)(en),]+ intermediates (Scheme 79).793These intermediates have been recovered and studied independently with the trans isomer isomerizing to cis (kis,, = 8 x 1W6s-’) before chelating (kch= 4.6 x IOp5 spl An “0tracer study has shown that chelation in the cis monodentate (227) proceeds partly (- 3&40%) via attack by coordinated water, while cis[Co(OC,O,)(OH)(en),] reacts entirely by this path (equation IN).’% Under acidic conditions (pH < 2) the cyclization of cis-[C0(0C,O,H)(en),]~+ has increased three-fold over that at pH 5 and
I Coball
80 1
the reaction is also believed to occur exclusively via intramolecular oxygen exchange at the carboxyl centre rather than at the metal (equation 141).796
+ H2C204/HCzOi
kl
[Co(OH2),0413+
ICo(OH2)2(N)413'.H2C204/HC,0,- TLF
+
[CO(O~C~O~)(N)~]'2H2O ctr,iruns-[Co(OH)(OH2)(en)~l "+
Kip
'
+ 2H'
(1 39)
cis,Irulzs-[Co(OH)(OHz)(en),] z'.C,O~-
Scheme 79
Both monodentate and chelated oxalato complexes undergo H+- and OH--catalyzed exchange this is faster (H+) or comparable (OH-) with hydrolyof 0 atoms. For [CO(OC,O,/H)(NH,),]+~~+ sis of the complex, while for [Co(O,C,O,)(en),]' the two exocyclic 0 atoms exchange much faster than chelate ring opening. Some 0-exchange data are given in (Table 62). Hydrolysis of chelated oxalate and malonate is very slow and both systems can be considered inert at ambient temperatures. There is no report of H + - or M"+-catalyzed ring opening; there is no easy point of attachment of the catalyst in such systems. Acid hydrolysis of monodentate complexes at elevated temperatures (Table 59) often follows the rate law kobs= k, + k , w + ] with k2 correspondThe acid-catalyzed pathway becomes less ing to hydrolysis of [CO(OC(O)(CH,),CO,H}(N),]~+. Metal ions (Fe3+,A13+,Co2+,Ni2+,ZnZ+,Mn2+)catalyze the important with increasing n.797,79s ~ - ~and ~ Harris reaction in the same manner by coordinating to the free carboxylate f ~ n c t i o n .Dash have studied this initial rapid binding of Fe3+ species to [CO(OC~O,)(NH,),J+.~ Alkaline hydrolysis is somewhat faster and generally follows the rate law k,, = k,,[OH-] + k;IH[OH-l2, although sometimes koHis not observed (Table 59). For Dash and [Co(OC,O,)(NH,),]+, koH corresponds to Co-0 and kbH to C-0 bond Table 62 "0-Exchange Data for Oxalato Complexes Complex
Exchangeable 0"
Rate, rate law
kobs= kH[H+]; k , = 2.3 x lo-' mol-' dm3s-I. 25 "C kobs= koHIOH-]; koH = 2.9 x mol-' dm3s-', 25 "C a Exchangeable
1. 2.
3. 4.
0 indicated by asterisk.
C. Andrade, R. B. Jordan and H.Taube, Inorg. Chem., 1970,9, 711. C. Andrade and H. Taube,J. Am. Chem. SOC.,1964,86, 1328. N. S. Angerman and R. B. Jordan, Inorg. Chem., 1967,6,1376. G. Miskelly, C. R. Clark and D. A. Buckingham, J. Am. Chem. SOL, 1986, 108, 5202.
1
4
Cobalt
802
coworkers have carried out a number of kinetic studies.*" For chelated [Co(O,C,O,)(en),]+, Andrade and Taube showed that close to three 0 atoms had exchanged on complete alkaline hydrolysi~"~ and recent studies by Buckingham and coworkers have demonstrated that two of these exchange rapidly in the chelate before hydrolysis, and that both the koHand koH paths for chelate cleavage occur with substantial (60% and 100% respectively) C-0 bond fission (Scheme 80). The subsequent hydrolysis of the monodentate cis hydroxo product occurs entirely via Co-0 bond cleavage.796Similar mechanisms have been suggested for the alkaline hydrolysis of other chelated dicarboxylate complexes.802
Scheme 80
The photochemistry of Co"' oxalate complexes was studied and reviewed at an early stage.8o4 Thermally unstable green K, [Co(O, C,O,)J and KICo(O,C,Ol),en] undergo photoredox decay both in solution and in the solid with the C,Oi radical being a major product (Scheme 81). An intermediate bidentate oxalate radical coordinated to high-spin Co" (228) has been Endicott and coworkers have shown by studying the photolysis of monodentate [Co(OC,O,)(NH,),]+ and chelated [Co(O,C,O,)(en),]+ that an additional pathway exists (Scheme 82) whereby excitation results in heterocyclic C-C cleavage and the formation of C 0 2and a C-bonded formato complex of Co"' (229);8'2this latter species spontaneously decays via internal electron transfer to Co:& and *CO,H or CO; radicals. More stable Co" products are formed in the photolysis of [Co(O,C,O,)(N-N),]X (N-N = phen, bipy), with [Co(O,C,O,)(N-N),] and [Co(X),(N-N),] Some MO calculations on these latter systems have been described.8" being
[ c o ( o * c ~ 0 2 )I 3j- iczo:
-
[co(o,c,o,),lz- + cz0:- + 2c02
Scheme 81
'' * [ C O ( C O ~ H ) ( H ~ O ) ('+ ~ ~+)COZ ~I
[C0(0~C~O~)(en)~l+
(229)
'low
* Cot.& + COzH + 2en
pK, = 2.6
Scheme 82
47.7.2.4 Hycivoxycarboxylates: cobak(ZII) (i)
Salicylate
The formation and hydrolysis of monodentate salicylate complexes (230)parallels other carboxylates (Tables 60 and 59, respectively). Metal-ion-accelerated hydrolysis (Fe3+ spe~ies,"~
Cobalt
-
803
AI”+”’) is also found, and a pathway via the phenolate anion (pK, 11.2) exists for both aquation and alkaline Chelated salicylates (231)are known [(N)4 = (NH&, (en),, P-(trien)] and [Co(sal)(en),]+ has been resolved into enantiomeric forms (Table 60). These chelates are very stable in aqueous solution and hydrolysis is very difficult. 0-exchange studies do not appear to have been carried out. Garbett and Gillard have noted an interesting oxidation-decarboxylation of ( - ) 5 8 9 [Co(sal)(en),]+ in concentrated HNO, in which the five-membered oxalate chelate (232)is formed Nitration of the chelate precedes ring oxidation and in -90% yield together with picric Yamamoto and coworkers have carried out an extensive study on the green nitro product first noted by Morgan and Smith.”* Green [C0(5-N0~sal)(N),]~+ is paramagnetic ( p = 1.61.9 BM) and appears to be formed during the nitration since similar treatment of authentic orange [Co(5NOzsal)(NH,),]+ does not appear to lead to the paramagnetic species.819Y amamoto has investigated a number of such systems [(N)*= (NH3)4,(en),, (trien), (phen),] and has shown that the related nitroso species [Co(S-NOsal)(NH,),]+ and its brown Paramagnetic oxidation product [C0(5-N0sal)(NH,),]~+are not formed as intermediates in the nitration p r o c e s ~ . ’Photoelectron ’~~~~ spectroscopy indicates substantial ligand as opposed to metal oxidation.g2o”’
(ii)
Tartrate, malate and citrate
Some preparations using these hydroxy acids are given in Table 60 and some structures are listed in Table 61. An early report*= of the stereospecific reaction between (R)(+)-tartaric or (R)(+)-malic acid and [Co(CO,)Cphen),]Cl at ambient temperature has been refuted and both A- and A[Co{(R)L}(phen),]+ ions (233)are f~rmed!*~,~*~ The distribution ratio A(R)/A(R)= 0.38 for the @)-tartrate complex. Using [Co(Cl),(phen),]Cl at 60 “C and Na( +)-tartrate the ratio is 0.70, but thermodynamic distributions remain unknown. A(R) and A(R) diastereoisomers of both acids have recently been isolated and characterized by CD and 360 MHz ‘H NMB.s25The reaction of mesotartaric acid results in four diastereoisomeric possibilities (depending on which asymmetric carbon is in the chelate ring) and all four have been separated (A(S)R:A(S)R:A(R)S:A(R)S = 26:24:27:23).”‘From these results it appears likely that the reported stereospecific reaction of [Co(CO,)(en),]Cl with (S)(-)-tartaric acid resulting in only A[C~((S)tart)(en),]Cl**~ is in error. The reaction of sodium citrate with [Co(Cl),(trien)]Cl also leads to a preference for the five-membered chelate involving the hydroxyl group (234)as has recently been shown by a crystal structure of fi-[Co(cit)(trien)]. 5H20.827
(233) R = OH [(,H)-tart] R = H [(R)-mal]
47.7.2.5
Tridentates and quadridentates, cobalt(1ZZ)
Variations in ligand type and geometrical configuration provide an almost endless array of possibilities, and far too many to consider here. Bernauer”’ considers such possibilities and some recent preparations are given in Table 63, and crystal structures in Table 61. Main interest centres on correlating alterations in stereochemistry with CD and ‘ H NMR spectra, and [Co(edda)XY]“+ systems figure prominently in this regard. An early review is availableaZgarid a selected studys3’may be consulted. Na[Co(edda)(CO,)] serves as a good starting material for many
h
I I I
v I 3
D
j-
1 3
e , Q Q
0
> 0 0 > 0 >
0 >
888 8 222 2 (r
e,
0
S
2
805
Cobalt
r
:
c4
k
N.
N
+I m
x
4
I
3 11
807
= 808
Cobalt
c~mplexes.'~'The potential tri- and quadri-dentate hydroxy acids (malic, tartaric and citric) show bidentate chelation and are considered in Section 47.7.2.4.(ii).
47.7.2.6
Quinquedentates and sexidentates, cobalt(ZZZ)
Brintzinger, Thiele and Muller appear to have been the first to prepare a sexidentate polycarboxylate cobalt(II1) complex when they reported Na[Co(edta)].4H20.832The sexidentate nature of the ligand was confirmed by a crystal structure of NH,[Co(edta)] -2H,0.833Busch and Bailar first reported the resolution of [Co(edta)]- into its enantiomeric formssMand its absolute configuration (235) was assigned by comparisons with K( -I-)~~~-[CO{ (R)pdta]] (236)(the (&methyl substituent prefers the equatorial disposition)83sand by its reaction with ethylenediamine, which leads to a preponderance of A-( -t)589-[Co(en)3]3+8xSuch stereospecific effects have been discussed by Sarg e ~ o n . Subsequently '~~ other methods of preparation and resolution have become available and these are collected in Table 64. A crystal structure of ( -)s46-K[Co(edta)] *2H,O has confirmed its enantiomeric configuration as (235).838Koine and Tanigaki report the isolation, resolution and 400 MHz 'H NMR spectrum of ( -)546-[Co((R)pdta}]- containing the axial @)-methyl orientation Similar stereospecific effects are found and confirm that it is present to only 0.1YOat eq~ilibrium.'~~ oxidation of Co" with [Co{(R,R)-cdta}]- containing trans- 1,2-cyclohexanediaminetetraacetate;83s.83' with H 2 0 2in the presence of excess ligand leads to a 1:2 Co"':cdta complex of unknown structurenm Lengthening the N-(CH,),--N chain to n = 4 gives [Co(bdta)]- (Table 64) but no suitable oxidizing agent has been found for n = 6.84' Proton assignments in the 'HNMR spectrum of [Co(edta)]- have recently been confirmed by Overhauser enhancement studies.862
(236) (+)s~n-[Co((R)-~dta)l -
(235) (+),,,-ICo(edta)l-
An early general account of the properties and some reactions of [Co(Y)JL(Y4- = edta, pdta, cdta) has been given by Garvan,@*and B e r r ~ a u e rhas ~ ~ discussed ~ more recent stereospecific e f f a t s of related sexidentates. 'H NMR assignments and proton exchange data are availablcW Some reversible E" values and stability constants have been reported by Ogino and OginoM5and are given in Table 65. A good parallel exists between the stability of the Co" and Co"' complexes. The crystal structures of NH,[C0(edta)l*2H,O~~~ and (-)546K[Co(tdta)] *2H20838 imply that equatorially placed acetate arms are more strained than axial ones and can be displaced to form quinquedentate complexes (237) containing a free carboxylic acid [CoX(HY)- or carboxylate [CoX(Y)12- residue (equation 142). Preparations exist for X = Ci-, Br-, NO;, H,O, OH- (Table 64) and extension is obviously possible. SchwarzenbachM6first reported the preparation of the quinquedentate complexes and the uncertainty in the geometrical purity of [CoBr(Hedta)]- has now been traced to the use of CoCl,.6H20 as starting mate~ial.~'Some have been resolved into optical isomers in their own right (Table 64) but reaction (142) occurs largely (75%; Y = edta; X = Cl-), Table 65 Redox and Stability Constant Data for Some Co"/Co"' Polycarboxylate Complexes'
E"
Couple
log Kcdr Y
log KcJrl Y
16.03 15.39 15.58 17.40 16.9 18.73 14.8
40.92 41.70 37.6 42.4 42.4 43.9 31.5
(mv)b ~~
~~
--
[Co(edta)]-''[Co(tdta)]-''[Co(bdta)]+[C~@dta)]-~[Co(rn-bdta)]-''[Co(cdta)]-'? [Co(Heedtra)(H20)p'a Data taken from
0.374 0.295 0.547' 0.367 0.339 0.356 0.507
H. Ogino and K. Ogino, Inorg. Chem., 1983,22, 2208. b v ~hydrogen . electrode; 0.1 mol dmU3NaCIO,. I.Orn~ldm-~ NaClO.,.
Cobalt
809
or completely (Y = pdta, cdta) with retention of absolute configuration. The exclusive disengageand by a structure ment of one equatorial acetate arm is suported by ‘H and I3C NMR dataa48.a49 However the reactions of ( +)Y6-[Co(edta)]- and (-)%on K[Co(NO,)(Hedtra)]- 1.5H20(238).850 [CoX(edta)12- (X = CI-, Br-) of the same absolute configuration with ethylenediamine to give an excess of A( -)58g-[Co(en),]3+,x5’and the similar reactions of ( -)546-[C~((S)pdta}]-with ethylenediamine to give A( +)5as-[Co(en),]3+,and with (22,s)-propylenediamine to give A( +)ss9-[Co{(S)pn}3]3+7852 suggest that a rather specific complete unravelling occurs.861
fi,~o*,**\-o FI& N\
)+
HX
I)
0
0I /
(+)5464CO(Y)I -
,COzH
f,~o,*\\\\\J N Z k N \
(142)
CH2
(237) (-)w~-[CO(HY)XI-
Including two six-membered chelates as in (239) appears to position these in the equatorial plane?’, and a similar result is found in the crystal structure of the ethylenediaminedisuccinic acid (edds) complex (240).8” Removal of these less-strained equatorial chelates to form quinquedentate complexes (equation 142) appears to be difficult. Substitution of methyl groups on two of the acetate arms seems to force these into axial positions.855The free acetate arms of [CoX(edta)F- (X = H,O, NH,, NO;) can coordinate to other metal ions and (241) has been prepared.
(239) A-[Co(eddda)l -
(240) A-[Co{(S,S)-edds}]
(241)
Higginson and coworkersR5h have investigated the rates and equilibria of ring closure and have shown that protonation of the acetate arm (pK, = 3 . l ) results in considerable stabilization of the quinquedentate chelate (> 50% in 1.Omol dmP3Hf). A related equilibrium is found with the N-hydroxyethyl deri~ative.~~’ The faster chelation rate for the anion (k:) compared to the protonated carboxylic acid (kr; Scheme 83) was considered to demonstrate anchimeric assistance in removing the coordinated water molecule. However the corresponding rates for [Co(H,O)(cdta)]which is contrary to the above and [Co(H,O)(Hcdta)] are 7 x 10-4s-’ and 1.6 x 10-2s-17x57 interpretation; a change in mechanism is possible. Some relevant data are given in Table 66. Spontaneous halide removal from [CoX(HY)]- and [CoX(Y)12- (X = C1-, Br-) in aqueous solution is slower than chelation of the aqua complex so that a decision as to whether the immediate product is [Co(H20)(Y)]- or [Co(Y)]- cannot be made. However studies by Wilkins and coworkers using quinquedentates which do not contain a free carboxylate arm and hence must form [Co(H,O)(Y)] suggest that the sexidentate [Co(Y)]- is the immediate product in the aquation with loss of X- being assisted by the carboxylate arm in some fashion (Table 66).858Many M2+ and some M3+ ions catalyze the removal of CI-, and for H$+ and Ag+ the induced reactions lead directly to
Cobalt
810
the coordinated carboxylate with full retention of configuration @.e.[Co(H,O)(Y)]- is not formed). Higginson and coworkers relate the relative abilities of M2+ (M’+) to bind the carboxylate and coordinated halide in [CoX(Y)12- with assistance from both being considered important (Scheme 84).859*s60 With Hg2+and several M3+ions (In3+,Sc3+,La3+)limiting rates of reaction were observed with KHgbeing independent of protonation of the carboxylate residue but with k,, being considerably faster for the anion. This suggests strong binding to chloride in an equilibrium sense but with assistance in its subsequent removal by carboxylate. [Co(edta)]-
rCo(11,O)Cedta) I -
[Co(H20)(Hedtn)]
25 “C, I = 0.1 mol dm-3
Scheme 83
Scheme 84 Table 66 Rates of Ring Closure and Loss of Halide for Some Quinquedentate Polycarboxylate Cobalt(lI1) Complexes Complex
[Co(Hedta)H,O] [Co(edta)H,O][Co(edta)OH]*[Co(Hcdta)H, 01 ICo(cdta)H,O](Co(Hpdta)H,01 [Co(pdta)H,O][Co@dta)OH]*[Co(Heedtra)H,O] [Co(Heedt ra)OH] [Co(H edta)CI][Co(edta)C1]2[Co(edta)Br]*[Co(Hcdta)Cl] [Co(~dta)Cl]~ [Co(Hpdta)CI][Co(Hpdta)Br][Co(Heedtra)Cl][Co(Heedtra)Br][Co(Meedtra)CI][Co(Meedtra)Br]1. 2. 3. 4. 5.
k
Cammenis
Ref.
pK, = 3.1; reversible reaction, K, = 0.78mol-’dm3 pK, = 8.1; poorly reversible, K, = 830
I I 1 2 2 3 3 3
(3-I)
2.3 x 10 - 4 1.9 10-3 4.3 x 1 0 - ~ 1.6 x IO-’ 7.0 x LO-4 2.7 x 10V4 2.0 x IO-’ 2.5 x 1.9 x 10-5 3.3 x 3x 3 x 2 x 10-5 1.5 x 10-4 7 10-5
-
pK, x 1; unusually fast ~
-
pK, = 7.94; very reversible, K , = 5.8 x lo-’ Poorly reversible, K, = 35 Very slow No apparent assistance pK, = 2.8
1.2 x 1x
5x 2.8 x lo-’ 2.1 x
-
1 1 2 2 2 3
-
3
-
4x
4 4
Loss of c1Loss of BrLoss of c1Loss of Br-
R. Dyke and W. C. E. Higginson, J. Chem. SOC.,1960, 1998. M. H. Evans, B. Grossman and R. G. Wilkins, Inorg. Chim. Acta, 1975, 14, 59. K. Swaminathan and D.H. Busch, Inorg. Chem., 1962, I , 256. S. P.Tanner and W. C. E. Higginson, J. Chem. Soc. (4), 1966, 537. M .L. Morris and D. H.Busch, J. Phys. Chem... 1959, 63,340.
5 5
2 2
Cohalt
’
’
81 1
Rates and mechanisms for the reduction of [Co(edta)]-, [Co(H,O)(Hedta)], [CoX(edta)12-, [Co(edta)(NH,),]- and [Co(NH,), (Co(edta)(H,Oj]]’+ by Cr2+,Fez+, Fe(CN):-, Ru(NH&+ and Ti3+ have been investigated by many research workers and an article by P e n n i n g t ~ nis~ ~ ~ recommended for background reading. Recent studies by Ogino and C O W O ~ ~ ~and ~ Sby ~ ~ Marec and O r h a n ~ v i c ’are ~ ~also relevant. The photochemistry of [Co(edtaj]- and [CoX(Hedta)j- has been investigated by Natarajan and Endicott.8a Besides a small amount of photoaquation the limiting yields of Cof,‘ and CO, differ (+co = 0.025, 0.14) but for the quinquedentate complex +co is nearly independent of X (X = C1-, Br-, NO;) over the excitation range 300 nm > 1 > 229 nm. This suggests a rapid thermal equilibration between singlet excited states with rate-determining competitive formation of products and unreactive reactant states of lower energy.865
47.7.3 Carbonate 47.7.3.1
CobaltrII)
Few well-defined Co”+arbonate complexes are known. The two structures given in Table 68 are probably representative of chelated and monodentate carbonate. K,[Co(C03), (H20)J crystallizes as rose-coloured plates from warm solutions of Co” salts in the presence of K, C 0 3870 and contains trans monodentate carbonate. Bidentate carbonate in [Co(CO,)(H,O), (imH),] appears to exert a measurable structural trans influence on the trans water molecules (COO,216.6pm vs. 209.4 in a similar effect is found in Co’5arbonato complexes. Violet crystals of C O ( H , O ) ~ + and )~~~ [Co(CO,)(H,O), (NHpy,)] have been isolated on reacting Na, [Co(CO,), ] with 2,T-bipyridylarnine and presumably contain chelated carbonate ( I R p = 5.23 BM).”’ 47.7.3.2
Cobalt(1II)
Cobalt(III)-carbonato complexes provide a very useful starting point for the preparation of other cobalt(II1) complexes so that high yield synthetic methods are of great practical importance. Table 67 lists preparations for the major structural types (241)-(244) and variations for substituted or related ligand systems are based on these. Recent reviews covering the wider aspects of transition metal carbonates are available.872474
II
0
~
,
~
~
~
*
~
~
~
Cobalt
812
Table 67 Some Recent Preparations o f Carbonato Cobalt(iI1) Complexes Complex Monodentate [CO(OCO,)(NH,)~]NO,-1.5HZO
cis-[Co(CO,)(NH3)(en),]C1O4 traans-[Co(CO, )(NH,)(en),]C104 a,/l-[Co(CO,)(tetren)]ClO,* 3H,O trans-[Co(CO, )Cl(en),]-H20
Ref.
Properties
Preparation
+ +
Co(NO,), i(NH4),C03 NKOH, air, r.t., 24h cis-[C0(0H)(NH,)(en),]~+COz trans-~Co(OH)(NH,)(en),]2fCO, a,/l-[Co(0H)(tetren)lZ++ COz trans-[Co(OH)Cl(en),]'+ + CO,
+
Red;
94; 8237 13800
33
74
34 34 34 35
Red Red Red Blue;
Bridging monoden ta te
[(NH,),CoOC(D)OCo(NH,),I(SO4),. 4H2O Bridging-p-bidm rate 3 c 0 3 }Co(NH31 3 Is04 * 5H20 [(S-pra), Cob-OH,CO,)Co(S-pra),] [pH3)3
[(S-prah Co(p-CO,), Cop-pra),] Chelated Na,[Co(CO,),]
*
3H,O
cir-K[Co(CO,), (NH,),]
+
+
CoS04*7R,0 (NH,),CO, NH,OH; 0°C 0,, 24h Co(C0,):- + S-praH; heat, 60 "C, 2h; chrom. As above
Red; v. soluble; em, 138; E,= 152
1
Red; crystal structure
2
Hygroscopic; uncertain
3
As above
3
+
Co(NO,), NaHCO, + H,O,; OcC, lh Resolved using ( +),,-[Co(en),I3' Ico(co,)t13- + (NH,)zCO,; resolved
Olive-green; E ~ , , 155; em 166 Optically v. unstable Blue-violet; 657s174, E , ~ , 258; racemizes with I,,, 3 min Purple; 165; 232 [Co(C0,),l3py; 100 h, 25 "C Blue-black; 154; [Co(CO,),]'(enH),CO,; 4 0 ° C 164 2 h; chrom. Violet-red; 220 [CO(CO,),]~- KNO,; 0 ° C 3 4 h 92; E~ 108 [Co(CO,),]'2KCN; r.t. L; EtOH, r.t., [Co(CO,),]'E562 152 (bipy, phen); Y = bipy resolved using (-)58praoemizes with t,,, 7 h in aq. soln. AE = 5.93 [Co(MO,),(en)zI(OAc), Y = phm with ( -)s89-[Co(C20q)(en),]OAc (bipy), 7.72 (phen) ~ ~ , Purple; E ~ 100 Co(C0,):- -tKNO,; O T , I h; Orange HOAc, 3 4 h ICo(C0, )z (CN)213- Hz CzO, 8535 80; E435 118 Co(C0,):glyH; 60 "C, 3 h Red-violet Co(CO,):.- NH,OH; heat Red; eSzO104 CoCl, 2en f CO, +,H,O,; Red; 133; 124 &35 "C, resolved using [ ~ t1250' ] ~ ~ (X = I) NdCo(Cz 0 4 1 2 (en)] Na,Co(CO,), H,edda Violet; 114 (sym); €533 234 (asym) Co(C0,):2Y; 2&35 "C 250 Red; em, 118, (bipy); em9 136, 200 @hen); crystal structure ICO(CO,)C~(~Y)~I + Hg(py),(C104)2 Red; 170; 189 py; 50 "C, 45 min K [ C O ( C O ~ ) ~ ( ~NH40H ~)] -t Red; 125, E , ~ , 148 NH,CI; 50°C; chrom; resolved. cis-[Co(CI),(NH,),(en)]Cl + Ag,CO,; Red; cSI295; 103 dark Red KlCo(CO,),(~H,),I + biPY + HClO,; pK 5.5, O T ; r.t., chrorn., resolved with Na( +) [Co(edta)] CoCl, H,O, KHCO, -t- glyH; 6 M 5 " C , 3h CoCl, tren 3HC1+ NaHCO, Red; t j M130; E,% 110 PbO, ; 25-70 "C AgClO4 a-(Co(Cl),(trien)]CI + NaHCO, ; Red; 124; resolved, evap. r.t. + NaClO, 1230" [CojCI),(trien)]Cl+ Li,CO,; heat Red E ~ 180; , resolved, M * 9 770 [Co(CI),(cyclen)]C1 Li,C03; 80 ('C, Red; 280 6h
+ + + + +
+
+
+ +
+
+
-
+
cis-K[Co(CO,)(gly),] H,O [Co(C03)(tren)]C104*H,O
sy~-[Co(C03>(trien)J(C10,)
asym-[Co(CO,)(trien)](C10,) [co(co,)~cyclen)]cl~ 2H,O
+
+ +
+
+
+
+
-
4 5
6
7 8 9, 10 11 12
15 10
11 13 14 16 17
18
19
20 37 21
37 22 23 24 24 25
Cobalt Tabk 67 '
F .
.
.
[Co(CO,)(cyclam)lCl [CO(co3)([15,161aneW1(C1O4) [Co(CO, ); (trand 14]diene)](C104)
[Co(C0,)(3,2,3-tet)](C1O4) [CO(CO, )(tetb)l(ClOt)
813
(continued)
+
Na,[Co(CO,),] cyclam 3.5HC1 [Co(X),(l15,16]aneN4)]X (X =C1-, Br-) Li,CO, Na,[Co(CO,),] L; heat trans-[Co(Cl), (3,2,3-tet)]C104 Na,CO,; heat, 30min Na,[Co(CO,),] + tetb; heat, pH 8
+
+
+
[Co(CO,)(diars),]Y
trans-[Co(Cl),diars]CI + Li,CO,; MeOH, H,O; ISmin, heat
[Co(CO,)(tetars)](ClO,)
[Co(Cl),(tetarsllC1 Na, CO, ; MeOH, H,O, 80°C aans-[Co(Cl),(qars)]Cl Li,CO,;
asym-[Co(CO,){(R,S)-qars)](ClO,)
asrm-[Co(CO,){(R,~-fars)l 1.
2. 3. 4. 5. 6.
I. 8. 9. 10.
11. 12. 13. 14. 15. 16. 17. 18.
19.
20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.
+
heat, 20min trans-[Co(Br),{fars)]Br lOmin, heat
+ + Na,CO,:
Red;
204;
Red;esB 138 (15);
140 em
26 21
146 (16)
Red;em 121; 135 Red; l27; egm 125
36, 38
28
Purple; cSjq 200 (a); cSS, 184 (8) Orange, resolved using NaAsO(tart), 880 * Red; .vym and osym isomers; both resolved Red
29
Red
32
30 31 32
K. Koshy and T. P. Dasgupta, Inorg. Chim. Acta, 1982, 61, 207; M. Abedini, Inorg. Chem., 1976, 15, 2945. M. R. Churchil, R. A. Lashewycz, K. Koshy and T. P. Dasgupta, Inorg. Chem., 1981, 20, 376. T. Nishida and K. Saito, Bull. Chem. SOC.Jpn., 1977, 50, 2618. H. F. Bauer and W. C. Drinkad, J . Am. Chem. Soc., 1960,82,5031; Inorg. Synth., 1966,8, 202. R. D. Gillard, P. R.Mitchell and M. G. Price, J. Chem. Soc., Dalton Trans., 1972, 1211. M. Mori, M. Shibata, E. Kyuno and T. Adachi, Bull. Chem. SOC.Jpn., 1956,29,883; S. Muramoto and M. Shibata, Chem. Lett., 1977, 1499. G. Davies and Y.-W. Hung, Irtorg. Chem., 1976, 15, 704. N. S. Rowan, C. B. Storm and J, 8. Hunt, Inorg. Chem., 1978, 17, 2853. V. A. Golowys, L. A. Kokl and C. K. SokoI, Russ. J. Inorg. Chem. (Engl. Transl.), 1965, 10, 829. H. lchikawa and M. Shibata, Bull. Chem. SOC. Jpn., 1969, 42, 2873; 1970, 43, 3789. S. Fujinami and M. Shibata, Chem. Lett., 1972, 219. Y. Ida, K. Imai and M. Shibata, Bull. Chem. SOC.Jpn., 1978, 51, 2741. K. Kawasaki, J. Yoshii and M. Shibata, Bull. Chem. SOC.Jpn.. 1970,43, 3819; M. Shibata, Top. Curr. Chem., 1983, 110, 36. M. Mori, M. Shibata, E.Kyuno and K. Hoshiyama, Bull. Chem. SOC.Jpn., 1958,31, 291; G. Schlessinger, Inorg. Synth., 1960, 6, 173. Y. Ida, K. Kobayashi and M. Shibata, Chem. Lett., 1974, 1299. J. Springborg and C. E. Schaffer, Inorg. Synth., 1973,14, 64. F. P. Dwyer, A. M. Sargeson and I. K. Reid, J . Am. Chem. SOC., 1963,85, 1215. L. J. Halloran, A. L. Oillie and J. I. Legg, Inorg. Synth., 1978,18,104; P. J. Garnett and D. W. Watts, Inorg. Chim. Acta, 1974,8,293. K. Kashiwabara, K. Igi and B. E. Douglas, Bull. Chem. SOC.Jpn.. 1976,49, 1573; E. C. Niederhoffer, A. E. Martell, P. Rudolf and A. Clearfield, Inorg. Chem., 1982, 21, 3734. J. Springborg and C. E. Schaffer, Acto. Chem. Scand., 1973,27, 3312. R. J. Robbins and G. M. Harris, J. Am. Chem. Soc., 1970,92, 5104. M. Shibata, H. Nishikawa and Y. Nishida, Inorg. Chem., 1968.7, 9. T. P. Dasgupta and G. M. Harris, J . Am. Chem. Sac.. 1975,97, 1733. A. M. Sargeson and G. H. Searle, Inorg. Chem., 1967, 6, 787. J. P. Collman and P. W. Schneider, Inorg. Chem.. 1966, 5, 1380. C. K. Poon and M. L. Tobe, J. Chem. SOC.( A ) , 1968, 1549. Y. Hung, L. Y. Martin, S. C. Jackels, A. M. Tait and D. H.Bush, J. Am. Chem. Soc., 1977,99, 4029. M. D. Alexander and H. G. Hamilton, Inorg. Chem., 1969, 8, 2131. N. A. P. Kane-Maguire, J. F. Endicott and D. P. Rillema, inorg. Chim. Acta, 1972, 6, 443. B. K. W. Baylis and 1. C . Bailar, Inorg. Chem., 1970, 9, 641; B. Bosnich, W. G. Jackson and J. W. Mchren, Inorg. Chem.. 1974, 13, 1133. B. Bosnich, W. G. Jackson and S. B. Wild, Inorg. Chem,. 1974, 13, i121. B. Bosnich, S. T. D. Lo and E. A. Sullivan, Inorg. Chem., 1975, 14, 2305. F. Bssolo and R. K.Murmann, Inorg. Synth., 1953, 4, 171. S. Ficner, D. A. Palmer, T. P. Dasgupta and G. M. Harris, Inorg. Synth., 1977, 17, 152. T. Inoue and G. M. Harris, Inorg. Chem., 1980, 19, 1091. N. Sadasivan and J. F. Endicott, f. Am. Chem. Soc., 1964, W, 5468; N. Sadasivan, 1. A. Kernohan and J. F. Endicott, Inorg. Chem.. 1967, 6, 770. K. Tsuji, S. Fujinami and M. Shibata, Bull. Chem. Soc. Jpn., 1981, 54, 1531. R. W. Hay and G . A. Lawrence, J. Chem. Soc.. Dalton Trans.. 1975, 1467.
The usefulness of cobalt(III)-carbonato complexes derives from their acid lability, with COz and the Co"'-aqua compIex being rapidly generated in most cases (equations 143,144 and 145), with full retention of geometric and optical configuration. The coordinated water molecule can then be replaced by the ligand of choice. In non-aqueous solvents the cleavage pattern (equations 143, 144 and 145) would predict an easy route to [Co(~olv)(H,0)(N),]~+ systems, and if finely divided [Co(CO,)(N),]X (N4= (NH,),, (en&) is suspended in dry acetone and concentrated HX added (X = C1-, Br-, CF3SO;), [CoX(OH,)(N),]X, results. However dissolution in concentrated HX invariably results in largely [Co(X),(N),]X. Na, [Co(CO,)3],s'6 or the [Co(CO,),]'- ion prepared in situ, has been extensively used for prepar-
814
Cobalt
ing [Co(CO,),(AB)I"- complexes, and from these, mixed [Co(CO,)(AB)(A'B')] systems, especially where AB, A'B' are bidentate ligands such as oxalate, amino acids, phen, bipy and related chelating diamines. Shibata and colleagues have been largely responsible for developing this area and a recent account of their work is a ~ a i l a bl e . 8The ~ ~ green [Co(C03),13- ion has been resolved into its optical forms but racemizes rapidly,878and it is only moderately stable between pH 8 and 10 in the presence of 0.1-1.0moldm-3HC0;;879 it is best prepared and used immediately. Many of the [Co(CO,)(AB)J+ and [Co(CO,)(macrocycle)]+ systems can be prepared directly from CoC1, or Co(OAc), and the ligand and NaHCO, , others from [Co(X), (AB)$ or [Co(X),(macrocycle)]X, where X = C1-, Br-, using Ag,C03, and still others by oxidation of the Co" complexes in the presence of C02(Table 67). For the macrocycles cyclam,880@'[15 or 16]aneN,,8s2trans[14]dieneNtR3 and tet b,&p4[Co(CO,)(macrocycle)]' is particularly valuable since bidentate carbonate requires adjacent cis coordination sites and this forces a strained stereospecific geometry on the macrocycle, e.g. (245). This forced configuration is retained on removing the carbonate moiety in acid solution. This property is valuable since it can present a Co"' centre to potential ligands in an alternative configuration and it might be useful in the investigation of biologically active sites.
(245) Ico(CO,)~trans(S,R)[ 141dieneN4}l'
Monodentate systems [Co(OCO),(N),y+ are easily formed by saturating an aqueous solution of the aqua complex with CO, at pH x 6. Under such conditions maximum concentrations of dissolved CO, occur and many [Co(OH,)(N),Y+ complexes have pKa values in this range. It is the complexed hydroxo conjugate which is the reactive species (equation 146)875and if the pKa is a little higher (i.e. the complex is somewhat less acidic) then the reaction is a little slower under any pH condition. The reaction of HCO; and C0:- (pH 8-10) is very slow by comparison and can generally be disregarded for those complexes which have slow water exchange rates. But contributions by such species have been found.886The rate law takes these factors into consideration and some rate constants corresponding to equation (146) are given in Table 69. A recognizable Bronsted relationship exists between logk and the pK, of the Co-OH;" complex,ss5and the p value of -0.15 suggests control by the properties of CO,,,, rather than by the nucleophilic ability of coordinated hydroxide. A AV study gives a value of - 10.1 & 0.6cm3mol-' for the pentaammine system and this has been interpreted in terms of significant bond formation in the transition state.887 [(N),Co-OHH12+
+
COXaq, -S t ( N ) J o - O
L C02]+
+
H+
(146)
Four basic structural types have been found: monodentate (241), bridging monodentate (244), bridging p-bidentate (243) and the four-membered chelate (242). Some structures are given in Table 68. Koshy and Dasgupta have recently reinvestigated the preparation of (2.44) [(N)5= (NH3),lsssbut its very rapid hydrolysis to [Co(OCO,H)(NH,),]+ and [CO(OH,)(NH,),]~+still leaves some doubt as to its structure. Churchill and colleagues have described the structure of p-bridged [((NH3)3C~]Z \p-(OH),C03}]S04- 5 H 2 0 together with preliminary data on its acid hydrolysis and reconstitution. 89 The cleavage patterns for the two structural types (243) and (244) have not been described in any detail. Monodentate complexes (241) are well characterized (Table 67) but only one structure, that of [Co(OC02)(NH3),]Br.H,O, is available (Table 68). Considerable unsaturation into the coordinated 0 atom is suggested by the similar C-0 bond lengths. These complexes undergo rapid acidcatalyzed decarboxylation (reverse of equation 146) and some rate data are given in Table 69. A c y l 4 bond cleavage has been established for [ C O ( O C O ~ H ) ( NH , ) , ] ~and + , ~ the ~ rapidity of the reactions makes this certain in a general sense. Bidentate (242) shows some 10pm longer bond lengths for C-0 in the chelate ring with 68-70 O anglds at the metal (Table 68), which indicates considerable strain. Acid catalysis follows the rate law kobs= k, + k,[H+] with k, and k , values of 1-8 x s-' and 0.1-1 mol-' dm3s-' respectively for tetraammine systems and 10- s-' and 10-4-10L5mol-' dm3s-' for the aromatic amines py, phen, b i ~ y . ~Note " the very much slower k , values for the complexes containing aromatic amines. lSO tracer experiments have established
-
Cobalt
815
Table 68 Crystal Structures of Carbonato Complexes COqJkX
Structural datd
Comments
ReJ
CO"
trans-KdCo{(CO),)2(H20)41
P2,/n; COO207; OC 128-129; (H,O)CoO 87.8 "; OCO 119-120" A2Ja;COO 214; OC 130; OCoO 61.2"
Pna2,; COO 193; OC 131-120; OCO 117, 123, 118" Pcmn; COO 190.5; OC 113.6, 123.7; OCoO 70.5' P2,/c; COO 189; OC 132, 133, 122; OCoO 69.3 O C2/c;COO 191; OC 130.6(2), 123; OCoO 68.2' COO 192; OC 132(2), 122; OCoO 68.8 P2,/c; COO 188; OC 133, 128, 120; OCoO 69.4 C2jc; COO 189; OC 131, 133, 121; OCoO, 69.3' C2/c; COO 188.7; OC 131, 124; * 5H,O [CO(CO~)(~~PY)~INO, OCoO 69.9 * Na[Co(CO, )(pren)]. 3H,O P2,; COO 191; COO 131, 132, 123; OCDO 69.1 P2,2,2,;COO 191; OC 126, K[CO(CO,){(R)-V~~}]* 2HlO 133, 129 P2,2,2,;COO 192; OC 131, (+)589 -[Co(CO,)(cyclen))CIOO,~ H,O 132, 122; OCoO 68.4" P2,2,2,; COO 192; OC 131, ( +)589-[Co(C0,)(3,8-Me,tnen)]ClO, 130, 124; OCoO 68.6" PT;Coo 192; OC 129, 129, [Co(C03)(terpy)(OWI -4H20 126: OCoO 68.0 P2,jc; COO 192; OC 131(2), [(NH,),Co(p-OH,NH,)Co(CO,),lI,.H,O 124; OCoO 68.5' [(NH,),Co{p-(OH),,CO3)Co(NH,),]SO,~5H2O PI;COO 190; OC 129- 130, 126 [Co(CO,)(ms-bn),]l.H,O P2,2,2,;COO 191-192; CON 194-197; OCoO 48.8'
.,
Co" complex, trans monodentate OCOi-
1
cis(OH,), ; trans(imH), bidentate
18
Monodentate
2
Bidentate
3
Bidentate
4
Bidentate
5
Bidentate
6
Bidentate
7
Bidentate
8
Bidentate
9
Bidentate
10
A( +), cis(N), cis@) configuration Bidentate
11
Bidentate
13
Bidentate
14
Bidentate
15
p-bridging p-bridging
16 17
O
a Bond
1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 1I . 12. 13.
14.
IS. 16. 17. 18.
'
12
lengths in pm. R. L. Harlow and S. H. Simonsen, Acra Crystallogr.. Sect. B, 1976, 32, 466. H. C. Freeman and G. Robinson, J . Chem. Soc., 1965,3194. G. A. Barclay and B. F. Hoskins, J. Chem. Soc., 1962, 586; M. R. Snow, Aust. J . Chem., 1972, 25, 1307. K. Kaas and A. M. Sorenson, Acta Crystallogr.,Secr. E, 1973, 39, 113. F. Bigoli, M. Lanfranchi, E. Leporati and M. A. Pellingelli, Crysf.Sfruct. Commun., 1980,9, 1261; P. C. Healy, C. H. L. Kenndard, G. Smith and A. H. White, Crysr. Srruct. Commun.. 1981, 10, 883. R. J. Geue and M. R. Snow, J . Chem. Soc. ( A ) , 1971, 2981. B. C. Guild, T. Hayden and T. F. Brennan, Crysr. Srmct. Commun., 1980,9, 371. H. Hennig, J. Sieler, R. Benedix, J. Kaiser, L. Sjoelin and 0. Lindquist, 2. Anorg. A&. Chem.. 1980, 464, 151; ref. 9. E. C. Niederhoffer, A. E. Martell, P. Rudolf and A. Clearfield, Imrg. Chem., 1982, 21, 3734. T. C. Woon, M. F. Mackay and M. J. OConnor, Acto Ciytallogr., Sect. E. 1980, 36, 2033. M .G. Price and D. R. Russell, J. Chem. Soc, Dalton Trans., 1981, 1067. J. H. Loehlin and E. B. Fleischer, Acra Crystallogr.,Sect. 8, 1976, 32, 3063. K. Toriurni and Y . Saito, Acra Crystallogr., Sect. B, 1975, 31, 1247. E. S. Kucharski, B. W. Skelton and A. H. White, Aust. J . C k m . , 1978, 31, 47. M. R. Churchill, G. M. Harris, R. S . Lobanova and R. N . Shchelolzov, Zh. Neorg. Khim., 1981, 26, 718. M. R. Churchill, R. A. Lashewycz, K. Koshy and T. P. Dasgupta, Inorg. Chem., 1981, 20, 376. M. F. Gargallo, J. D. Mather, E. N . Duesler and R. D. Tapacott, Inorg. Chem., 1983, 22, 2888. E. Baraniak, H. C. Freeman, I. M. James and C. E. Nockolds, J. Chem. Soc. ( A ) , 1970, 2558.
bond cleavage for k, in [Co(CO,)(NH,),]+(Scheme 85)892followed by faster O-C02H hydrolysis of the monodentate (Table 69). in strongly acidic solutions kl can exceed k, for the monodentate with a limiting rate (KHin Scheme 85) being approached. In this case the monodentate The reverse ring closure for [Co(OCO,)(OH,)(N),]' is fast is observed as an (k0 = 0.024.1 s-', Scheme 85). For [Co(CO,),(en)]- and [Co(CO,),(py)J- loss of the first carbonate chelate is very fast and rate data are only available for the intermediate [Co(CO,)(H20)2(N)2]+species [(N)z= (p~)~~']. Co-0
Cobalt
816
Table 69 Rates of CO,,,, Uptake and Release for Some Monondentate Cobalt(lI1) Carbonato Complexes"
PK!,,
Complex
[Co(OCOzH)(NH3)SI'+
k (mol-'dm's-l)
PK
cis-[Co(OCO,H)(OH2)(en)2]2+
6.3 5.3 (PKJ 7.9 W 2 ) 6.I
trans-[Co(OCO,H)(0H,)(en)J2+
-
220 f 40 A V i - 10.1cm3mol-' 166f 15 44+2 170 10 225 f I5 -
cis-[Co(OC0,H)(NH,)(en)2]2+ rruns-[co(KO, H)(N H3)(en),1'+
-
-
-
-
trans-[Co(OC02H)Cl(en),]'
-
-
4.9 8.0 29 7.2 1.6
57 4 196 4 24 37 & 0.1 70 f0.2 338 32
-
-
6.9 -
-
-
-
-
-
-
6.6
cr,p-S-[Co(OCO,H)(tetren)]"
[Co(OCO, H)(OH, )(tren)]'+
c~.~-[Co(OCO,H)(OH,)(cyclam)]+ trans-ICo(OC0, H)(OH,)(cyclam)]+ trans-[Co(OCOzH)(CN)(NH,)d]+ [Co(OC02H)(OH,)(nta)]sym-(Co(OC0, H)(OH,)(edda)] asp-[Co(OCO,H)(OH,)(edda)]
+
+
+
6.1, 6.4
k,
@-I)
Re$
6.4 5.9
1.10, 1.25 AVr +6.8cm3mol-' 0.28 0.03 1.5, 1.2
1, 2 13 4 5, 6
5.8 5.6 6.7 6.3 6.5
0.37 2.1 f 0.1 0.60 f 0.02 0.66 f 0.02 I .02 f 0.05
3 3
-
-
+
7
7 8 9 9
-
0.38
0.01
IO
51
11
55 2.3
12 12
+
aFor reaction (R)5CoOH"+ COl(aq) (R),CoOCO,H"+, where pK(,,) relates to (R),CoOd$+" and pK to (R),CoOC02H"+ I. E. Chaffee, T. P. Dasgupta and G. M. Harris, J . Am. Chem. Soc., 1973,95,4169. 2. T P. Dasgupki and G. M. Harris, J. Am. Chtm. Soc., 1968, 90, 6360; D . A. Palmer and G. M. Hams, Inorg. Chem., 1974, 13,965. 3. G. M. Harris and K. E. Hyde, Inorg. Chum.. 1978, 17, 1892. 4. T. P. Dasgupta and G. M. Harris, Irwrg. Chem., 1978, 17, 3304. 5. T. P. Dasgupia and G. M. Harris, J . Am. Chem. Soc., 1971, 93,91; 1975,97, 1733. 6. T. P. Dasgupta and G. M. Harris, Inorg. Chem.. 1978, 17, 3123. 7. S. A. Ficner, Ph.D. Thesis, State University of New York (Buffalo), New York, 1980. 8. T. Inoue and G. M. Harris, Inorg. Chem., 1980, 19, 1091. 9. T P.Dasgupta and G. M. Harris, J . Am. Chem. Sac., 1977, 99, 2490. 10. C. R Krishnamoorthy, D. A. Palmer, R.van Eldik and G. M. Harris, Inorg. Chirn. Acta, 1979, 35, L361. 11. T.P. Dasgupta and G. M. Harris, Inorg. Chem., 1974, 13, 1275. 12. R. van Eldik, T. P. Dasgupta and G. M. Harris, Inorg. Chem., 1975, 14, 2573. 13. U. Spitzer, R. tan Eldik and H. Kelm, Inorg. Chern., 1982,21, 2821.
+ co2 Scheme 85
Under alkaline conditions pathways first order in [OH-] become available for both ring opening and subsequent hydrolysis of the monodentate carbonate intermediate (equation 147). Both reactions are quite slow (koH = 3.6 x l o p 3mol-' dm3s-I, kbH = 8.2 x 10-6mol-' dm3s-' for [Co(CO,)(tren)]+).R93 Both reactions involve Co--0 cleavage for (N)4= (en)2 and probably occur via S,kb cleavage in the deprotonated amine base conjugate.894Endicott and coworkers have discussed this reaction in some and Dasgupta and Sadler report a recent example using [Co(CO,)(~yclam)]+.~*~ The reverse cyclization is also very slow under alkaline conditions (k--OH= 6.5 x lO-'s-' for (N)., = (en)2) and the ring-opened form (246) is stabilized for [OH-] > 0.01 rnoldmw3.Carbonate exchange in monodentate (241) and chelate (242) systems therefore nearly always occurs via hydrolysis and Co"'-hydroxo attack on solvated COz. Dobbins and Harris have discussed this for chelate systems899and Francis and Jordan for [Co(OCO,)(NH3)S]+.900 However, displacement of carbonate in [CO(CO,),]~-by pyridine, diamines and SO:is thought to occur both directly as well s via [CO(CO~),(H,O)(OCO~H)]~specie^?^^*^^*
il.
817
Cobalt 47.7.4
Oxyanions of Cobalt(II1)
47.7.4.1 Sugate, s e h a t e and tellurate
The group oxidation state anions SO:-, Se0:- and TeO,(OH):- show an increasing tendency to polymerize with SO:- being relatively inert to oxygen exchange and dimerization, while TeO,(OH):- rather easily dimerizes to Te,O,(OH)i- ? The racemic chelate [Co{O,Te(OH),}(en),]' also rapidly equilibrates to the dimer [Co(en), {O,Te(OH),O,}(en), Co12+in aqueous solution and tetrameric [{ Co(en),),(O4TeO,TeO,)~(en),Co),lC1, has also been i s ~ i a t e d .This ~ ' increasing tendency to raise the valence of the non-metal by substitution with other oxy species increases or Co"'--OH attack. Some preparations are listed in Table 70. the possibility of Co"'-OH, SO:- coordinates in three structural modifications (247), (248)and (249) with the chelate (247) being unstable towards pH-independent ring opening [(N), = (en),, k = 7.3 x 10-5s-'].906The monodentate (248)is quite stable but undergoes slow aquation, kobS= k , f k2[Hf], and base hydrolysis (Table 44). Ring opening and hydrolysis almost certainly occur without S - 0 bond cleavage.90' Formation of monodentate sdfato complexes [Co(OSO,)(NH,),]+ and [Co(OSO,)(H,O)(en),]+ is also very slow (Table 43) and assumes a rate independent of [SO:-] above 0.02 mol dmP3indicative of strong ion pairing.908*w9 Bridged species (249) are easily formed by bubbling SO, through a solution of the p-superoxo dimer [(N),Co(p-(0, ,NH,))Co(N),I4+ [(N), = (NH,),, (en)J with oxidation at S and reduction at 0 occurring rapidly.'" A corresponding p-selenato complex can be formed in a similar manner using SeO, but the reaction is lower.^" Selenate analogous of (247) and (248) appear to be unknown. As mentioned above the preparation of the Te"' chelate [Co(O,Te(OH), )(en),]CI - H 2 0corresponding to (247)has been reported and it has been resolved into its optical enantiomers?'' H
47.7.4.2
Thiosu@te
Some interesting chemistry is now available on the mono sulfur analogue of sulfate, the thiosulfate ion. Both S- and 0-bound isomers of [Co(S,O,)(NH,),]+ are known, and a structure on the former (Table 72) confirms it, Le. (250), as the stable form. Preparations are given in Table 70. The 0-bound isomer has been observed as a minor product (ca. 3%) in the alkaline hydrolysis of [CO(OSO,R)(NH,),]~' (R = Me, CF,) in 1.0 M Na,S,0,.947 It rather rapidIy isomerized intramolecularly in solution to the S-bound isomer ( t l i 2x 27min). The 0,s chelate (251) has been prepared by Deutsch and coworkers but is unstable towards hydrolysis to monodentate trans[CO(SSO,)(OH,)(~~),]+?~~ Such hydrolysis is some four times faster than that for the sulfato chelate [Co(O,SO,)(en),]', which suggests a small trans influence for the coordinated sulfur (cf. Section 47.8.5). Oxidation of cis-[Co(SSO,),(en),]- with an excess of 1, leads to trans-[Co(S(S)SO,}(OH,)(en),] containing the stabilized disulfane monosulfonate anion (252), possibly via an unstable tetrathionate intermediate (equation 148). The product was isolated as trans[Co{S(S)SO,)Cl(en),] and shows trans labilizing effects.',' Stoichiometric oxidation of trans[Co(SSO,),(en),]- using I; gives truns-[Co(SSO,)(H,O)(en),]+, and anation of this ion with NCS-, NO;, NCO-, C1- and C20:- is fast, consistent with the kinetic trans labilizing order SO:- > RSO; > SS0:- (cf. Section 47.8.5)?50,951 Aquation of the resulting trans-[Co(SSO,)(Y)(en),] is also fast. +
rn Cobalt 818
2
f
Cobalt
d i
d
819
Cobalt
820
-
47.7.4.3 SuGte and selenite
The lower oxidation state group IV anions SO:- and SeOi- rapidly undergo substitution and oxygen exchange and associative interchange processes involving expansion of covalency are attractive routes to their metal complexes?'*Coordination is often much faster than the unassisted water exchange at the metal, and proton transfer and bond making in associated transition states of the type (253) and (254) are important for Co"'. H---O/ \ (N),co-o------M(O~
fr---(j \
H
(253)
0-
\ y(O), 'H---d 1
(N)SCo -O;----
H
(254)
Tetrahedral SO:- is known to coordinate to Co"' in three ways (255), (256) and (257), although structures are only confirmed for (255; Table 72). Preparations to some 0-bonded complexes are given in Table 70 and some S-bonded systems in Table 71. Since SO:- can be removed by treatment with hot concentrated acid, S-bonded complexes (255) can sometimes be useful in synthesis but reduction to Co& is often a competing They are reasonably stable in alkali. Baldwin many years ago prepared orange-brown [Co(O,SO)(en),]X (X = C1-, ClO; ,NO; ) and on the basis of molecular weight and IR data represented them as cheIates (257).'14 More recently some phenanthroline and bipyridyl complexes of similar constitution have been prepared (Table 7 1). The chelate structure has not been confirmed but Baldwin's observations suggest that it is quite robust.
The 0-bonded isomer (256) has been examined recently by Harris and his colleague^^'^^^'^ and can be easily prepared by the addition of the aqua complex to NazS,O, or NaHSO, solutions at pH 5-6 (Table 71). These species are much less stable, undergoing reduction to Co" or hydrolyzing in acid solution (pH < 5), and isomerizing to the S-bound isomer in alkali. Addition of to Co"'-OH is fast at pH % 5 where the concentration of the two reactants is maximized. Compared to the reaction with C02(aq) reaction (149) is some 10' times faster. Some kinetic data are collected in Table 73. The resulting red-brown products (Table 70) are typical of other 0-bound The reverse group VI anions, and a recent I7O experiment verifies the source of the 0 hydrolysis is subject to H + catalysis as required by equation (149; Table 74). For the pentaarnmine system reduction to Co" also occurs at pH < 5 but the 0-bound isomer is stable in alkaline solution and apparently isomerizes to [Co(SO,)(NH,)]+;this latter aspect has not been described in much
-
Cobalt
C
82 1
Cobalt
822
Table 72 Structures of Some Cobalt(1ll) Sulfito ( S ) Complexes ~~
~
Structural data; trans effecp
Complex
COS 221.8; SO 148; CON (trans) 205.5; CON(cis) 196.6;Ar 8.9 COS 218.1; COO(trans) 203.7; CON trans-[C~(SO,)€i,O(en)~](CIO~)* H20 (cis) 194.2 COS 220.3; CON (trans) 197.4, CON trm.r-[Co(SO,)(W S ) ( e n ) , ] (cis) 196.2 COS 222.3; CON (trans) 200.8, CON [rcms-[Co(SO,}~Lm)(en)2]C10,2H20 (cis) I96,3; Ar 4.5 CI.T-NH,[CO(SO )*(NHT)d]* 3HlO COS 222; CON (trans) 199.3, 202.3; CON (cis) 197.0;Ar 5.3, 2.3 COS 220.6, 223.3: CON (trans) 201.5, crs-Na,[Co(SO, (en),](CIO,) .H, 0 200.3; CON (cis) 196.4, 195.9; Ar 4.4, 5.1 COS 226.7; SO 148.9 (av); CON (trans) 195.8 (av) COS 220.5; SO 148; CoCl 237.7; CON (trans) 196 COS222.7; SO 147.5; CON (rram) 207.0; CON (cis) 196; Ar 1 1 COS 223; SO 147; CON (trun.5) 202.3; CON (cis) 196.5; Ar 5.8; CoSC 112.90" COS 228.7; SS 204.8; SO 146; CON (trans) 198.6; CON (cis) 196.7;AF 1.9;
~~
Kinetic trans lability (log k,,); comments
Re$
- 1.98
la, 7
2.4
21,b
[CoSO,(NH, ) 5 ];(SO,) (also Cl.H,O salt)
0.8
3, 2b
- 1.5
4, 2b
~
~
Sa, b
0.82
6
V. long Co-S
bonds
8
V. long Co-CI
bond
9
V. long Co-NH, -
bond
10 7 11
COSS 110" a Data
in pm.
I.
R. C. Elder and M. Trkula, J . Am. Chem. SOC.,1974, %, 2635.
2.
(a)
3. 4. 5.
6. 7.
8. 10.
I I.
E. N. Mzslen, C. L. Raston, A. H. White and J. K. Yandell, J . Chem. Soc., Dalron Trans., 1975, 327. (b) J. K. Yandell and L. A. Tomlins, Avst. J . Chem., 1978, 31, 561. S. Baggie a i d L. N. Becka, Acta Crystallogr., Sect. E, 1969, 25, 946. C. L. Raston, A. H. White and J. K. Yandell, Ausr. J . Chem., 1978, 31, 993. (a) C. L . Raston, A. H. White and J. K. Yandell, A u t . J . Chem., 1978, 31, 999. (b) A. C. Rutenherg and J. S. Drury, lirorg. Chem., 1963, 2, 219. C. C. Raston, A. H. White and J. Yandell, A w l . J. Chem., 1979, 32, 291. R. C . Eldzt., M. J. Heeg, M. D. Payne. M. Trkulaand E. Deutsch, Inorg. Chem., 1978, 17, 431. G. D. Fallen. C. L. Raston, A. H. Whitt and 3. K. Yandell, Ausr. J . Chem.. 1980, 33, 665. C. L. Raston, A. H.White and 3. K. Yandell, Aust. 3. Chem.. 1980, 33, 419. C . L. Raston, A. H. White and J. K. Yandeli, A w r . J . Chem., 1980, 33, 1123. R.J. Rcstil 1. G . Fcrguson and R. J . Balahura. Znorg. Chem., 1977, 16, 167.
detail."' The ~~,fl-[Co(OSO,)(tetren)]+ ion is more stable toward reduction and isomerizes to a,/J-[Co(SO,)(tetren)]+ at a pH-independent rate (pH 6 7 . 5 , 2.7 x 10-4s-', 25 "C). This is some lo3times faster than the related isomerization of 0-chelated sulfinate in [C~(cystO,)(en),]~+ (Section 47.8.2.2.iii). The resulting S-bound isomer undergoes slow hydrolysis in alkali, k,, = 8.15 x lO-'mol-' dm3s-'?16 [Co(OSO,)(OH,)(tren)]+ reduces to Co" in the pH range 2.9-5.4 (koFl2 1.0 x IO-'s-') but adds a second SO:- ligand in the pH range 7.2-8.9 to form [Co(SO,),(tren)]- .918 Unlike [CO(NO,)(NH?),]~ and S-bound [Co(cysO,)(en),]+, [Co(S03)(NH3)J+shows no signs of photoisomerization.'" +
(NW3),Co2+-eH t- SO+q) /
kf khyd
f H'
(NH,)>Cd+-@ \
(149)
so;
The direct reaction of Co"'-OH2 or Co"'-OH species with SO:-, as distinct from the above reaction with SO,,,,,, is not well documented but appears to be moderately fast. Stranks and coworkers9" many years ago found that tr~ns-[Co(OH)(OH,)(en),]~+ reacts with SO:- to form tr~ns-[Co(SO,)~(en),]-within one second at pH 7.5, which is faster than expected for substitution at the metal centre, and Spitzer and van Eldik'" have recently discussed the relatively fast reaction of [CO(OH)(NH,),]~+with SO:- at pH9-10 to form trans-[Co(SO,),(NH,),]- (Table 71). It is very likely that these reactions occur by the addition of coordinated water or hydroxide to the sulfur atom of SO: with subsequent isomerization of 0-bound sulfite to the favoured S-bound isomer (equation 150). Crimson-red selenito complexes are formed within milliseconds by the addition of CoOH;+ species to Na,SeO, at pH values between 3 and 6.92'-923 Bidentate chelates of type (247) do nof
I 823 Table 73 Rate Data for Addition of COOH;+/C~O€€(“-’~+ to SO, and CO,, and the H+-catalyzed Hydrolysis of COOSO$+’)+ and C O O C O ~ - ~ ) + Complex
k (mol-’ dm’s-’ 1
Reacrant
4.7 x IOE 2.2 x 10’ 1.0 x lob 2.8 io2 9 x 106 5.3 x lo? 2,4 x io9 44 3.3 x los
Ico(oH)(NH3),12+
cis-[C0(0H)(OH,)(en),]~~ ICo(H, 0),(tren)]*+ [Co(OH)(OH2)(tren)12+ [C~(OH),(tren)]~+ [Co(OH)(OH,)(tren)]* ?r,b-[C~(OH)(tetren)]~+ +
I.
2. 3. 4. 5. 6. 7. 8. 9.
IO.
k (s-’)
Complex
1.0 x 103 1.10 cis-[C0(0S0,H)(OH~)(en)~]~+6.0 x 103 cis-[C0(0C0~H)(OH,)(en),]~+ 0.45 [Co(OS02H)(OR2)(tren)]’+ 3.8 x 103
Ref:
-
[Co(OS0,H)(NH3),IZ+ [cO(oCo, H)(NH3
-
-
[Co(OCO,H)(OH,)(tren)l2 [Co(OSO,H)(tetren)]”
+
1.19 1 . 1 x 103
I I I 8, 9 10
R. van Eldik and G. M. Harris, Inorg. Chem., 1980, 19, 980. E. Chaffee, T. P. Dasgupta and G. M. Harris, J . Am. Chem. SOC.,1973,95,4169. D. A. Palmer and G. M. Harris, Inorg. Chem., 1974, 13,965. T. P. Dasgupta and G. M. Harris, Inorg. Chem., 1984, 23,4399 (probable error in quoted hydrolysis rate constants). J. M. D e Jovine, W. K. Wan and G. M. Harris, unpublished data; cf. ref. 7. G. M. Harris, and K. E. Hyde, Inorg. Chem.,1978, 17, 1892. A. A. El-Awady and G. M. Harris, Inorg. Chem., 1981, 20, 1660. T. P. Dasgupta and G. M. Harris, J . Am. Chem. Soc., 1975, 97, 1733. T. P. Dasgupta and G. M. Hams, Inorg. Chem., 1978, 17, 3123. A. C. Dash, A. A. El-Awady and G. M. Harris, Inorg. Chem., 1981, 20, 3160.
(NjSCd-OHz
+
*
SO:-
(N),Cozt-O
[“SI
\
so;
(N),CO+-SO,
(150)
coexist with the monodentate (248) at equilibrium and equilibrium constants for formation of the monodentate are such that the monodentate is stabilized in solutions above pH 3. Rate data have been interpreted in terms of addition of CoOH:+ and CoOH2+ to HSeO; (Table 74) and for the aqua complexes rate saturation can be attributed to ion pairing.92’The fast rate for CoOH:+ is attributed to efficient proton transfer to selenite oxygen in the associative transition stare (253).The reaction of Se0:- is some 15-24 times slower if OH- is considered the leaving group on Se. Dimeric selenite species become important contributors to the reaction at pH > 7.923 It is well known that S-bonded SO:- complexes (255) labilize monodentate ligands wans to them in octahedral coordination. Thus Siebert and Wittke showed that [Co(SO,)(NH,),]+ could be prepared and purified only in solutions containing an excess of NH,(,!, otherwise trans[Co(SO,)(OH,)(NH,),]+ was isolated.924This trans labilizing effect is also discussed in Section 47.8.5.It is found in the reactions of [Co(SO,)(NW,),] ,425,926.930 trans-[C~(§O,)(H~O)(en),]+,’~~ ~~U~~-[C~(SO,)X(DMGH),]~~+,~~~~~~~~~” and with trans-[C0(S0,)(CN),(OH,)1~.669932 The large body of evidence suggests that SO:- promotes an S, l(lim) displacement mechanism with complete dissociation of the rrans group prior to interaction with the incoming ligand, especially with ~~~~~-[CO(SO,)(CN)~(OH,)]’(Scheme 86). The alternative dissociative interchange process with saturation of the rate being attributed to saturation within a preformed ion pair (Scheme 87) is possible for cationic systems with the rate variation being ascribed largely to variation in K,p.927 However the S, l(1im) mechanism seems certain for [CO(SO,)(CN),(OH,)]~-where ion pairing with Y”- should not cloud the issue and where k, is independent of the concentrations and identity of Y’- over a wide range of entering groups (SO:-, CN-, NCS-, NH,, py; k, x 2.5 0 . 5 ~ ~A’ H; f 92.4 kJrnol-’, ASt 73 J mol-’ KL1).932 Wilmarth’s contributions to this area have now been suitably ackn~wledged.~” Perhaps the [CO(SO,)(CN),]~-intermediate is stabilized as the chelatld octahedral ion (258). Such a species would allow the solvent cage to reach or approach the equilibrated state, but its subsequent preference for other ligand donors over H 2 0 , e.g. 140 (NH,), 340 (py), remains a mystery. Studies such as these establish the dissociative influence of coordinated SO:-- , whereas Farrell and Murray report the the similar influence of coordinated CN- is now open to q~estion.~’ possibility of 0 entry in the reaction of [Co(SO,)(OH,)(en),]+ with SO$- at pH7.1 with the [Co(OSO,)(SO,)(en),]- product rearranging subsequently to [ C O ( S O , > , ( ~ ~ ) , ] - . ~ ~ ~
+
[co(so,)(CN),l
3-
4- Y -
ka
Scheme 86
[Co(S0,)(CN),Y14-
824
h
PI
2
h
Cobalt
825
Scheme 87
47.7.4.4 Chromate, molybdate and tungstate
A relatively rapid red-orange colour change accompanies the addition of Na,Cr04 to [Co(NH,),(OH2)l3+in acid solution and this can be attributed to equilibrium (151). Equilibrium studies indicate the existence of a very stable ion pair [CO(NH,),(OH,)]~' .Crb,- in addition to the inner-sphere complex (256) (K, = kf/khyd= 3.07 x 10') and limited kinetic data indicate a rate first order in both Crvl and [H+] (Table 74).935Hydrolysis of [Co(OCrO,)(NH,),]+ is reasonably rapid (il,2< 7min, 5 "C).Early reports on the preparations of chelate complexes [Co(O,MO,)(NH,),]X and monodentate [Co(OMO,)(NH,),]X need to be reexamined. It is likely that the fast rate of oxygen exchange at M (Cr,936 W3')makes the chelates (247) unstable with respect to the monodentate [Co(OMO,)(H,O)(NH,),]+ and these complexes need to be isolated and carefully characterized. (NH3)5C03f-OH,
+
HCrO;
(NH3)sCof-OCr03
+
H30+
(151)
'hyd
(256)
Anation of [CO(OH)(NH,),]~+by MOO:- in the pH range 7.1-8.0 is in the stopped-flow range indicating rapid oxygen displacement at MO''?~' Rate data have been interpreted in terms of [CO(OH,)(NH,),]~' and [CO(OH)(NH,),]~+reacting with HMoO; (Table 74) even though the pK, of the latter is 3.53. Both reactions are faster than water exchange (k,) although slower than substitution by anionic species at HMoO; . Reaction (152) is reversible ( K = 475 mol-' drn-,) with the acid-catalyzed path being the reverse of HMoOi- addition. It is clear that exchange of Col"coordinated molybdate is reasonably fast. (NH3)5C03+-oH2 f MoOf
kf e kh,d
(NH,),Co'-OMoO,
+
H20
(152)
Subsititution by [Co(OH),,(OH2)0,1 (en),1+S2+ into W0:- in the pH range 8.0-9.0 is in the stopped-flow range indicating a very fast attack by Co-OH or Co-OH, at Wv'.941Rate data given in Table 7 4 are for substitution into HWO; even though its pK, = 3.5. The alternative of [C0(H,0),(en),]~+ and [CO(OH)(H,O)(~~),]~+ reacting with W0:- gives forward rates (6.60 x lo2 and 2.8 x 104mol-'dm s-I), which suggest exchange within the ion pair in excess of 66 and 560 s-'. These values are very much faster than the unassisted water exchange rate ( 0 . 4 4 ~ ~and ' ) an associative interchange process (see 253) is again likely.
47.7.4.5
Iodate, pervlrenate, chlorite and perchlorate
0 exchange into IO;, Reo; and C10; is fast and their cobalt(II1) complexes are formed by the addition of Co-OH, to the group VI1 centre. The immediate red-purple colour change which accompanies the addition of NaIO, to [Co(OH,),(en),13' must be followed by relaxation metho d ~ ! ~Both ~ the forward and reverse reactions (equation 153) are acid catalyzed ([H'] = 0.03-0.005mol drn-,) and have been interpreted in terms of addition of coordinated water to H I0 3 (Table 74). Using a value of 0.3 mol-' dm3for the association constant, the formation rate within the associated reactant (253) is 5 x 104s-', which is almost 10 times faster than unassisted water exchange into HIO, . Clearly IO; exchange on Co"' is very fast and it has not been possible
Cobalt
826
to isolate solid iodate complexes.943Early reports in the literature of isolated cobalt(II1) complexes containing IO;, BrO; and C10, are probably erroneous. (en),(H20)Co3+-OH2
+
HI03
kf
khyd
(en)2(H20)Co2'-OI0,
+
H30+
1153)
Addition of perrhenate to CoOH;+ or CoOH("-l)+complexes has not been studied in detail but heating [Co(OH,)(NH,),](ReO,), results in displacement of coordinated water (Table 70). Both [H ] and [OH- 1 paths for 0-Re bond cleavage in [Co(OReO,)(NH,),]'+ exist (Table 74) and these processes are strongly catalyzed by acetic acid and by acetate and biphthalate ions.944 The chlorito complex [Co(OClO)(NH,)JNO,), has been prepared by C102oxidation of Co" in the presence of ammonia (Table 70); it is stable for considerable periods when protected from light.Y45 The reverse reaction is acid catalyzed. Attempts to prepare a chlorato complex by oxidation were unsuccessful. Reduction using Fe2+,HSO; , or VOz+ results in substantial retention of the Co-0 bond (equation 154), but oxidation with Co;;, results in some 66% Co-0 bond cleavage.945 +
[(NH3)5C00-C1012'
+
S0,H-
+ 2Hf -,
[(NH,),CoOHJ3+
+
HSO;
+
C1-
(154)
Few cobalt complexes exist containing coordinated tetrahedral perchlorate, and those which do show weak coordination. Table 75 gives some structural data. Early reports on the isolation of [Co(OCIO,)(NH,),](ClO,), are erroneous but Sargeson and coworkers have prepared it as pink in concentrated HClO, Coordinated perchlorate crystals by nitrosation of [CON~(NH,),](C~O,)~ is rapidly displaced by water in aqueous solution (tljZz 7 s) and cobalt in any oxidation state prefers almost any other donor atom over perchlorate oxygen. 47.7.4.6 Polyoxyunions of Nbv,Mu", Wv'and Iv" Some interesting chemical and structural patterns are beginning to emerge in the coordination of cobalt with oxyanions of these elements. Together with Vv, TaV and CrV1the simple oxyanions readily condense to form structural units based on M 0 6 octahedra by sharing an edge or a trigonal COO,or COO,units fit into these polyhedra as integral parts and the high group valence face.9s2-953 often stabilizes Co"' octahedra. The structural unit is often large but is robust and many have been known for a long time. Some preparations are given in TabIe 76 and structures in Table 77. Several Co"' complexes containing the Nb,O;; unit have been prepared (Table 76). Nb,O;& (259) consists of six NbO, octahedra condensed by edge sharing and one of the triangular faces coordinates as a tridentate to forrnfur-[C~(Nb,O,,)L,]~ complexes. K, [Co(Nb,O,,)(CO,)(NH,)] has been prepared from K[Co(CO,),(NH,),] and has been used to coordinate (9-aspartate, glycinate, (8-valinate and (S)-alaninate, with the last two being resolved into enantiomeric When Table 75 Structures of Cobalt Complexes Containing Coordinated C10; Complex
Structural datd
[Co(OC10,)(CNPh),](C10,)~0.5(CH,Cl)2 P2,C; CoC 195, 184 (av); CoO(C10,) 259 [C0(0C10,)(0AsMePh~)~](ClO~) P4jn; CoO(AsR,) 202; OCOO85-95", COOCl 130";CoO(C10,) 210 rruns-~Co(OCIO,),{S(Me)CH,CH,S(Me)),]P2,C; COS229; COO 234; SCoS 88. I O ; SCoO 88 P2/C; CON 197 (av); rrans-[Co(OClO,)(NO)(en),](C10,) CoO(C10,) 236; NCoN 93", 85' P2/C; COAS229-230; trans-[Co(OClO,),(diars),] COO28C330 a Bond
I. 2. 3. 4. 5.
Comments
Re$
Sq. pyr.; green form, v. long Co-0 bond into basal site Dist. sq. pyr.; disordered oxygens in coord. C10;
1 2
Dist. oct.; rrans C10,; meso (RS) dithioether Dist. oct.; long Co-0 bond
3
Planar with v. long bonds to axial CIO;; oxygens not located
5
lengths in pm. F. A. Jumak, D. G. Greig and K. L. Raymond, Inorg. Chum., 1975,14,2585. P. Pauling, G. B. Robertson and G. A. Rodley, Xahture (London), 1965,207, 73. F. A. Cotton and D. L. Weaver, J . Am. Chem. Soc., 1965, 87, 4189. P. I,. Johnson, J. H. Enemark, R. D. Feltham and K. B. Swedo, Inorg. Chem., 1976, 15, 2989. F. W. B. Einstein and 0.A. Rodley, J. Inorg. Nucl. Chem.. 1967,29,347.
4
Cobalt
827
Table 76 Preparations: Cobalt(II1) Complexes Containing Polyoxyantions of Nbv, Mo"', Wv' and 1'' Complex
+
K7[CO(Nb6019)(CO3)INHJ)I Na, [Co(Nb,O,,)(dien)]
H3
lC04I3 o12(oH)121'
3H2
H?tCo41701s(NH1)61'9.5H20
Na4[Co41,0,,(OH,),{(5')-al.8H,O a)] (NH,),[COMO,O,~(OH),]. 12H,O (NH416 iC02 Mo10034 (OH),]7 H 2 0
K4H[CoW,, O,]. 20H,O 1.
2. 3. 4. 5. 6. 7.
K[Co(CO,),(NH, h] K,HNb,O,,13H,O + KOH; pH 10.6 mer-[Co(Cl),dien] Na,HNb,O,,15,O NaOH; conc. heat, MeOH, cool trans-[Co(Cl),(en),]CI+ Na, HNb,O,,- 15H,O; boiling H,O, N a O ~ CoC12*6H,0+ NaIO, -t- NaOH; NaOCl; Dowex 50 ( H t ) H7[Co,l,0,H,,]*8H,0 -I- 98% en; 55 "C,3 h H3 OMHI 21. 8H20 + NH3(l); 30 h, r.t. H,[ C O~I ,O~~H,, ]-~+ H ,(5')-alanine; O H,O, 65T, 1Oh Co& Mo,O& Br, [COMO,O,,(OH)~]~- C; H,O, heat
+
H3[Co411018 (enhl. 5 b O
'
Properties
Prepamtion
+
+
+
+
[COW,,O,,]~-; 2 €4H2S04, K,S,O,, boil
Re$
Green; useful starting material Purple; css0 63 Green-blue; em 64;
1
1 51
2
Green;very stable
I
Red-brown; cSS5400
3
Red-brown;
3
350
Green; 370 resolved on Sephadex Green; em 20 Green; zm5 90 resolved using (+ )sss-[Co(en)313+ Yellow
3 4 5
6
Y. Hosokawa, I. Hidaka and Y. Shimura, Bull. Chem. SOC.Jpn., 1975, 48, 3175. C. M. Flynn and G. D. Stucky, Inorg. Chem., 1969,8, 178. T. Ama, J. Hidaka and Y. Shimura, Bull. Chem. Soc. Jpn., 1973, 46,2145. R. D. Hall, J . Am. Chem. Sor., 1907,29, 692; Y. Shimura, H. It0 and R. Tsuchidd, Nippon Kogukar Zmshi, 1954,75, 560. H. T. Evans and J. S. Showell, J. Am. Chem. Sac., 1969,91,6881; T . Ama, J. Hidaka and Y. Shimura, Bull Chem. SOC.Jpn., 1970, 43, 2654. L. C. W. Baker and T. P. McCutcheon, J . Am. Chem. SOC.,1956,78,4503. C. J. Nyman and R. A. Plane, J. Am. Chem. Soc., 1961,83, 2617.
Table 77 Structures: Cobalt Complexes of Polyoxyanions Complex (NH4)6[Co,Mol,03,(OH),~I. 7 H 2 0
(NH,),,[Co2(NH,)(H,0),As,W,,0,,,]-nW,0 K,[CoW,, O,,] *20H,0
Li(H*0)2H[Co,I,O,,(OH),*1.3H,O a Bond
1.
2.
3. 4.
Structural datd Dimeric COO,joined by two p(0)bridges; COO 189, 196195; bidentate O,MoO, units Four A S W ~ Ofour ~ ~ WO,; , two Co" in external sites Dist. COO,tetrahedron surrounded by WO, octahedra; COO 188 Co3+ at centre of planar hexagon with alternate Co3*and 1''- at vertices; COO, octahedra
Ref 1
2 4
3
lengths in pm.
H. T. Evans and J. S. Showell, 1.Am. Chem. SOC.,1969,91, 6881. F. Robert, M. Legrie, G. Herve, A. Teze and Y. Jeannin, Inorg. Chem., 1980, 19, 1746. L. Lebicda, M. CiecharowicL-Rutkowska, L. C. W. Baker and J. Grochowski, Acta Crysfallogr.,Sect. B, 1980, 36, 2530. L. C. W. Baker, in 'Advances in the Chemistry of the Coordination Compounds', ed. S . Kirschner, Maemillan, New York, 1961, p. 608.
'
,
.
solutions of Co?;,:,are mixed with the stable Mo70:: ion in the presence of an oxidizing agent such as Br, the central Mo"' atom is replaced by Co"' and the remarkably stable, emerald-green [ C O ( M O , O ~ ~ ( O H ) ion ~ ) ] ~results (Table 76). Its structure is presumably the same as that of the analogous Cr"' species955 (260) with four-membered chelates surrounding a Cot" centre in a symmetric configuration. Treating (260) with activated charcoaI or Raney nickel forms the racemic dimeric unit (261)956and this has been resolved into its enantiomeric forms using (+)sss-[Co(en),]3f.957 Heteropoly complexes based on 10;- units have been surveyedg5* and the structure of [Co,13O2,HI2j3-(262; Table 77) shows a central COO,octahedron surrounded by alternating COO, and IO, octahedra. The three terminal Co"' centres contain cis-aqua ligands, which can be readily replaced by NH, ethylenediamine or amino acids (Table 76). The [ C O ~ I ~(OH,), O , ~ ((S)ala}I4- ion has been separated into its diastereoisomeric forms by chromatography on Sephade~."~ The [CoW,,O,]'- ion has the classic 'Keggin' structure with a central tetrahedrally situated Co"' atom surrounded by four W , 0 1 2clusters making up an almost regular cubo-octahedron (263).953 This unit is structurally very stable, does not undergo rapid 0 exchange at pH 1-2,960and can be used as an outer-sphere electron transfer oxidant in both inorganicg6' and organic965reactions. C.O.C. 4-AA'
rn Cobalt
828
(259) Nb60y9
Recent oxidations to be studied include those of p-methoxytoluene,962gl~tathione'~~ and cysteine and related mercapto acids.'@
47.7.5
Ethers, Cobalt(I1)
Well-defined crystalline complexes possessing cobalt(I1)ether oxygen bonds are comparatively rare. There are no authenticated examples of monodentate ether complexes. Light blue or lavender adducts of cobalt(T1) halides with 1 ,Cdioxane are formulated as containing six-coordinate cobalt(I1) and bridging dioxane ligands966but have not been fully characterized. Only one example of bidentate ether oxygen coordination has been reported. Addition of SOCI, to a solution of Co(CF3COO)2in 2-dimethoxyethane (dme) gives the red trimeric complex [CO,(C~)(SO,)(CF~COO)~ (dme),] in which dme acts as a chelating group and the remaining ligands bridge octahedral metal centres.967 Octahedral coordination is tils0 found in the dimeric complex [Co(Cl)(diglyme)],(SbCl,)2 (diglyme = triethyleneglycol dimethyl ether), where the chlorides act as bridging ligands and the coordination sphere of each metal is completed by binding of one molecule of the tetradentate polyether.gh8 Lindoy and coworker^^^^^^^^ have reported cobalt(I1) complexes of various 14- to 17-membered macrocycles with O,N, donor sets. For ligands of type (264) high spin [CoX,(macrocycle)] complexes (X = Cl-, Br", NCS- ) are octahedral for 15- and 1&membered ring systems,.but tetrahedral and without Co-0 (ether) bonds for the 17-membered ligand. The complexes [CoX,(macrocycle)] of the ligand (265; R = R' = H; n = 2; m = 3) have been isolated as orange and purple isomers for X = NCS-. IR studies indicate thiocyanate to be N bound in both species and a crystal structure on the purple isomer confirms a six-coordinate geometry in which the quadridentate is folded about the Co atom giving a b-cis configuration (266).
Cobalt
829
Table 78 Structural Data for Cobalt(I1)-Ether Complexes
Complex
Ligand type
[CO,gl-C02bI(SbCI,)r [Co3013-C1)(~3-SO,)01-CF3COO)3 L31 [CO(CO, LI
0, tetradentate 0, bidentate N,O, macrocyclic
[coL][cocl,j
N,O, macrobicyclic
[Co(NO,),LI
0,macrocyclic
[Co(H, 01, LI(NO31,
0, macrocyclic
[CoClL](PF, ) [Co(NCS), Ll
NOP, tripodal N20,macrocyclic
1. 2. 3. 4. 5.
6. 7.
Comments
Ref.
Oct.; COO211.5 to 213.6 pm Oct.; COO214 pm Trig. bipyr.; N,OC1, donor set; COO 231.9 pm Pent. bipyr.; N,O, donor set; COO 209.6 to 221.9 pm Seven-coordinate; COO (ether) 216 to 221 pm; NO; mono and bidentate Pent. bipyr.; COO (ether) 213.1 to 234.7 pm;H,O ligands axial Trig. bipyr. Dist. oct.; COO217-233 pm
1 2 3 4
5
5 6 7
A. J. Kinneging, W. J. Vermin and S. Gorter, Acta. Crystdogr., Sect E , 1982, 38, 1824. J. Estienne and R Werss, J . Cfiem. SOC.,Cfiem. Commun.. 1972, 862. G. R. Newkome, D. P. Kohli and F. Fronczek, J . Chem. Soc., Cfiem. Commun., 1980, 9. F. Mathieu and R. Werss, J . Cfiem. Soc., Cfiem. Comnaur., 1973, 816. E. M Holt, N. W. Alcock, R. R. Hendrixson, G. D. Malpass, Jr.. R. G. Ghiradellr and R. A. Palmer, Acta CrystaNogr., Sect. E, 1981, 37, 1080. P. Dapporto, G. Fallani and L. Sacconi, J . Coord Chem.. 1971, 1, 269. L. G. Armslrong, L. F. Lmdoy, M. MCPArtlm, G. M. Mockler and P. A. Tasker, Inorg. Chem., 1977, 16, 1665.
There are now several structural studies on cobalt(I1) crown ether and cryptand complexes (Table 78), which show the coordination mode to be markedly sensitive to the macrocycle cavity diameter. Both 12-crown-4 and 15-crown-5 (cavity diameters 120-1 50 pm and 170-220 pm respectively) can include Co2+ions to form structures in which every ether oxygen atom is bound to the In contrast Co-0 (ether) bonding is destabilized in the larger 18-crown-6 and dicyclohexyl-18-crown6 polyethers. In the blue complexes obtained from the reaction of these compounds with CoC1, the role of the cyclic polyether is to solvate discrete [Co(H,O),]” cations and [CoClJ’- anions and there are no direct Co-0 (ether) bond^.^",^^^ A similar effect is seen in the Co” complex of a 27-membered-ring macrocycle where the pentacoordinate structure (267) features only one long Co-0 (ether) bond.974
47.8 SULFUR LIGANDS
47.8.1 Sulfides and Disulfides Monomeric cobalt complexes containing SH- or S2- as ligands are comparatively rare since there is a tendency toward formation of binary sulfides. Di Vaira, Midollini and S a ~ c o n i ’showed ~~ that bonds tri(tertiary phosphines), when used as coiigands, are capable of stabilizing Co-S and were able to prepare the five-coordinate complexes [Co(SH){P(dppe), }](BPh,) and [Co(SH){N(dppe),}](BPh,). These species are low spin with trigonal-bipyrarnidal geometries, and with the sulfur atom in an axial position. Treatment of [Co(SH)(W(dppe), ]j(BPh4) with NaOEt produces the diamagnetic complex [(N(dppe), )Co(p-S)Co{N(dppe),)l which features a linear Co-S-Co bridge and tetrahedral sp3 coordination about each Co atom. Diamagnetism in this complex appears to arise through superexchange coupling of two d’ metal cent~es.”~
Cobalt
8 30
Binuclear ions containing only disulfide as a bridging ligand are also rare but oxidation of [CO(CN),]~-by elemental sulfur leads to [(NC),Co(S,)Co(CN), J6-, isolated as its potassium salt, and the same species is formed on treatment of [CO(CN),(OH)]~-or ~(NC),CO(O,)CO(CN),]~with has also been reported.977A recent studyws H, S. The selenium analogue [(NC),CO(S~,)CO(CN),]~suggests that the bridging disulfide ligand in [(NC),Co(S,)Co(CN),j6- is slowly oxidized on exposure to air to thiosulfite (SSO:-) but the preparation appears to suffer from problems of poor reproducibility. A crystal structure of K,[(CN),Co(SSO,)Co(CN),] characterizes the product as containing a p-(thiosulfito-S,S') unit indicating that oxidation of the sulfide bridge does not involve Co-S rupture. Both sulfide and disulfide are more commonly encountered as bridging ligands in cobalt clusters. There are few systematic syntheses. For example, [Co,(CO),] reacts at room temperature with CS, either neat or in the presence of hydrocarbon solvents to form [Co, (S)(CO),], [Co,(C)(S),(CO),,j and at least three other sulfur-containing derivatives in addition to sulfur-free species such as [CO,(C)(CO),].'~Similarly, [Co,(CO),] with elemental sulfur in hydrocarbon solvent gives [Co,(S)(CO),1, [Co,(S), (CO),,,],[Co, (S), (CO),] and [Co,(S), (CO) 14]:ao with the product distribution dependent on the reaction conditions. Details of severa1 cluster preparations are given in Table 79 and structural data in Table 80. Single crystal EPR studies of paramagnetic [Co,(X)(CO),] species (268; X = S, Se) diluted in the diamagnetic [FeCO,(X)(CO),] analoguesB' demonstrate that the unpaired electron in (268) resides in an antibonding orbital. The antibonding nature of the HOMO is also demonstrated by the lengthening af the M-M bonds in (268) relative to those in [FeCo,(X)(CO),] species, which contain one electron less.ys2The effect of electron occupancy of antibonding orbitals on Co-Co bond Table 79 Preprations and Properties of Cobalt-Sulfide Clusters
Compound
Preparation
[Co,(CO),] + PhSH f CO, 0 "C [Fe,S(CO),] [Co(CO),], hexane
+
(Bu"),S + ~(vs-cP)co(co~,l (Bu"), s + ~(v5-CP~CO(CO),1 [W-CP), c03S A + 1, [Co,(CO),] S, heptane, 30 "C [(qs-Cp)Co(CO),] + S, hexane [($-CP)~CO~ S6]t- PPh,, toluene, reflux [(~s-Cp)4C04S,] AgPF,, CH,CI,/MeOH [Co,(CO),] +- S, hexane [CO,(CO)~]+ CS,, hexane, r.t. [Co,(CO),] + EtSH, hexane CoC1, PEt, + H,S, acetone/EtOH
+
+
+
+
[Co608(PEt3),] sodium naphthalenide C 02+ Na,(S,-o-xyl) Li,S
+
[Co,S, (SPh),]j[co6s8(co)61'3% 1. 2. 3. 4. 5. 6. 7. 8.
9.
IO. 11.
12. 13. 14. 15. 16.
+ [($-CP)CO(CO)(PP~~)] + RNCS [Co4(SPh),,12- + HS-, acetone [Co,S6(SPh),Ji- + sodium naphthalenide [Co,(CO),J -b S + CO, petroleum ether, 40°C
Comments
Black; air sensitive Dark violet; air stable; m.p. 107-112°C Black; pd = 2.83 BM Black; diamagnetic Black; peA= 1.95 Red-brown Black; air stable; diamagnetic Red-black peg= 1.73 BM Black; air sensitive Dark green; air stable Dark brown; air stable; k a =2.15 BM Deep brown; diamagnetic Black; air sensitive; pG = 1.93-2.09 BM Black; air stable; R = Me, Ph, C ~ H I , Black; air sensitive; uC"= 1.56 BM Black; air sensitive; pcO= 1.35 BM Air stable
C. H.Wei and L. F. Dahl, Inorg, Chem,. 19C7,6- 1229. S . A. Khattab, L. Marko, G . Bor and B. Marko, J. Organomet. Chem.. 1964, 1, 373. S.Otsuka, A. Nakamura and T. Yoshida, Liebigs Ann. Chem.. 1968, 719, 54. P.D. Frisch and L. F. Dahl, J. Am. Chem. Soc., 1972,94, 5082. L. Marko, G . Bor and G. Almassy, Chem. Ber.. 1961, 94, 847. V. A. Uchtrnan and L. F. Dahl, J . Am. Chem. SOC.,1969,91, 3756. G. L. Simon and L. F. Dahl, J . Am. Chem. Soc., 1973,95, 2164. L. Marko, G. Bor, E. Klumpp, B. Marko and G. Almasy, Chem. Ber., 1963, %, 95% G. Bor, G. Gervasio, R. Rossetti and P. L.Stanghellini, J . Chem. Soc., Chem. Commun., 1978, 841. E. Klumpp, L. Marko and G. Bor, Chem. Ber., 1964,97, 926. F. Cecconi, C. A. Ghilardi and S. Midollini, Inorg. Cbim. Acta, 1981, 64,L47. G. Christou, K S. Hagen, J. K. Bashkin and R. H. Holm, Inorg. Chem., 1985, 24, 1010. K. S . Kagen. G. Christou and R. H. Holm, horg. Chem., 1983, 22, 309. F. Cecconi, C. A . Ghilardi, S. Midollini and A. Orlandini, Inorg. Chim. Acta, 1983.76, L183. J. Fortune, A. R. Manning and F. S. Stevens, J . Chem. Sac.. Chem. Commun., 1983, 1071. G. Gervasio, R. Rossetti and P. L. Stanghellini, Inorg. Chim. Acta, 1984, 83, L9.
Ref: 1
2 3 3 4
5 6 7
7 8 9 10 11
14 13
15 12 12
16
Cobalt
83 1
Table 80 Structures: Cobalt-Sulfide Clusters Complex
K O 3 S(CO),l
[FeCo, S(CO),l
Core structure
JA
co co
JL
co
-co
Comment.t
Ref.
CoCo = 265 pm; COS= 213 pm
1
MM = 255pm; MS = 216 pm
2
Cubane-like, CoCo = 324-334 pm; COS = 230 pm (n = 0); CoCo = 3 17, 331 pm; COS= 221 pm (n = 1)
3
Rectangular bipyramid; CoCo = 248 and 260 pm; apical S atom 137pm above the Co4 plane
4
Two Co3 (Ir-S) units linked by disulfide bridge; CoCo = 246-254 pm; SS = 204pm
6
SS = 203,204pm; COS= 21S223pm
7
co
co -
[Co,S(SEt),(CO),, J
[CO, s(s,-o-xyl),]*-
LL
co-c o [Co, S6(SPh),]‘-is-
Concentric Co, cube and S, octahedron
(c0)6i [c06 S8(PEt3)l”+
Concentric S, cube and Co, octahedron Concentric S, cube and Co, octahedron
tC06 s8
8
Co,(p-SEt)(y-CO), unit linked by three (p-SEt) bridges to the Co atoms of a Co,(Ir-S) fragment Distorted tetrahedral COS, units; CoCo = 28 1 pm; COS= 232 p@), 224 pm(t) Distorted tetrahedral COS, units; CoCo = 266pm (4-1, 267pm (5-) CoCo = 288 pm; COS = 228 pm CoCo = 279 pm; COS = 223 pm (n = 1); CoCo = 28 1pm; COS= 223 pm (n = 0)
9
10 11 12
Cobalt
8832
Table 80 (continued) Complex
Comments
Core structure
R = C6HII ; COCO= 249-251 pm;
Re$
13
COS= 21 1 pm; CoC = 188-206 pm N
I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13.
C. H. Wei and L. F. Dahl, Inorg. Chern.. 1967, 6 , 1229. D. L . Stevenson,C. H. Wei and L. F. Dahl, J . Am. Chem. Soc., 1971, 93, 6027. G. L. Simon and L. F. Dahl, J . Am. Chem. Soe., 1973.95, 2164. C. H. Wei and L. F. Dahl, Crysf. Sfrucf.Commun., 1975, 4, 583. P. D. FriSCh and L. F. Dahl, J . Am. Chem. SOL,1972, 94, 5082. D. L. Stevenson, V. R. Magnusson and L. F.Dahl, J. Am. Chem. Soc., 1967,89, 3727. V. A. Uchtman and L. F. Dahl, J. Am. Chem. Soc., 1969,91, 3756. C. H. Wk and L. F. Dahl, J . Am. Chem. Soc., 1968,90, 3977. K. S. Hagen, G. Christou and R. H. Holm, Inorg. Chem., 1983, 22, 309. G. Christou, K. S. Wagen, J. K. Bashkin and R. H. Holm, Inorg. Chem.. 1985, 24, 1010. G. Gervasio, R. Rossetti and P. L. StangheUi, Imrg. C h h . Acla. 1984, 83, L9. F. Cecconi, G. A. Ghilardi and S. Midollini, Inorg. Chim. Acfa, 1981,64, L47; F. &coni, C. A. Ghilardi, S. Midollini and A. Orlandini, borg. Chim. Acra, 1983, 76, L183. J . Fortune, A. R.Manning and F. S. Stevens, J. Chem. Soc., Chcm. Commun., 1983, 1071.
lengths is also seen in the family of clusters [Co,(S)(X)(Cp)l"+ (269; X = CO, n = 0;X = S, n = 1; X = S , n = O).983Approximate MO calculations using Fenske-Hall procedures have been carried out on these ~ystems.9~~
(26s)
(269)
VahrenkampgBSfirst synthesized the chirai tetrahedrane type clusters (270), which have been resolved for M = Mo via the diastereoisomeric phosphane (271),986and a directed synthesis9" has -S-bridged tetranuclear CoFeMoW cluster (272). achieved the construction of the pLj
(270) M = Mo,W, Cr
A chemistry of cobalt-sulfide-thioiate molecular clusters comparable with that of iron systems with HS- in acetone affords the has also begun to emerge. Treatment of [CO,(~-SP~)~(SP~),]'octanuciear cluster [C08(pd-S)6(SPh),]4- isolated as its PriN+ salt?88In MeCN solution the Complex is red-purple with intense sulfur-core charge transfer bands which obscure the Co" d-d transitions. This behaviour contrasts with that of both mono- and poly-nuclear cobalt"-thiolate complexes, which all display LMCT bands below 440nm and have well-developed v 2 and v3 features. The [Co, ( ' J , - S ~ ] ] ~ +core sustains reversible one-electron oxidation and reduction ( E l j z= - 0.54, - 1.18V, MeCN) and chemical reduction with sodium acenaphthylenide in THF gives [Co8(p4-
Cobalt
.
833
S,)(SPh)6]'-, which has also been structurally characterized (Table 80). A similar Co, S6 core unit is found in the synthetic mineral pentlandite, Cogs, ."a9 In systems containing cobalt(I1) salts, sulfide and thiolate substitution of the bidentate S2-o-xy12ligand (S2-0-xyl' - = o-xylene-a,a'-dithiolate) for monofunctional thiolate directs cluster assembly away from the C0,S6 core and toward the Co3(p3-S)unit. A high yield preparation (50%) of the ~ , ~ the ~' complex (Et4N),[Co,(p3-S)(S2-o-xyl),] has been reported990and structural s t u d i e ~ ~show pyramidal Co, b3-S) unit to be unsupported by additional metal-sulfur bonding. The solution properties of the complex ('H NMR, UV-visible spectra and magnetic moments) are consistent with magnetically coupled tetrahedral COS, chromophores. Structural studies on [Co,(p3-S)*(CO),] 3S,992and [Co6(p, -S)8(PEt,),](BPh,)993show the cluster components to consist of Co, octahedra encapsulated in S8 cubes. This geometry is the inverse of cores, where Co, cubes are circumscribed by S, octahedra. that found for [co,(~~-s)6]4+'7+
-
47.8.2 Thiolates, Sulfenates and Sulfinates A useful review of the stereodynamic possibilities for inversion at chiral and prochiral sulfur (and Se, Te) in metal complexes is available.994
47.8.2.1 Low oxidation states of cobalt Structural data for Co-SR complexes are given in Table 81; selected data for Co-S-Co systems are included in Table 80. Klumpp, Marko and Borgaodescribed the synthesis of presumed [Co,(SEt),(CO), J and [Co,(SEt),(CO),] from the reaction of [Co,(CO),]with EtSH in hexane. However subsequent X-ray have shown the products to be [Co, (SEt),(CO),] (273) and structures by Wei and Dah1995,996 Table 81 Structures: Cobalt(I1jThiolate Complexes
Complex
Comments
Monomeric, tetrahedral CoS,N; COS = 228 pm, CON = 204 pm Monomeric, distorted tetrahedral COS,C1, ; COS = 232 pm, CoCl = 227 pm Monomeric, tetrahedral COS,; COS = 230pm Dimeric, Co2(pS), core; CoCo = 279 pm, COS,, = 230pm Monomeric, distorted tetrahedral COS, ; COS= 232-234pm Dimeric, Co, (p-S), core; CoCo = 305 pm (wti), 302 pm (syn); COS, = 235-238pm, COS,= 226229 pm Co,@-S), core; CoCo = 387 pm, COS, = 232 pm, COS,= 226 pm Co,&-SEt),(p-CO) core; CoCo = 255pm, COS = 220-223 pm Co,(p-SEt), core; each pair of Co atoms joined by two briding SEt groups; COS = 224229 pm; minimum CoCo = 250 pmA Co,(p-SEt),(ji-CO),(CO),fragment connected by two (rc,-SEt) bridges to a Co, @-SEt)&-CO) O.lmoldm-'. Comparative data are given in Table 88. A similar situation holds for S,Oi-. Coordinated thiolate is about as nucleophilic as NCS- and the reduced nucleophilicity of coordinated sulfenate is probably why it is stabilized relative to organic sulfenates. The ligand field strength of sulfur increases with formal oxidation
841
Cobalt
?
3
f
3
N -
842
Cobalt Table 84 UV-Visible Spectral Data for Sulfur-Bonded Co'" Complexes [nm (e)]
r
Thiolato (RS-) [C0(CYS)3l3- (preen) [Co(cyst),l (green) [Co(cysH),]-2H,O (red)
444 (568)
582 (269) 570 (568) 510 (24700)
H, fCo(O,CCH,CH, S),] (red) [Co(c~s)(en),l+ [C~(cyst)(en),]~+ [Co(NH2C,H4s)(en),12+
600 sh (37)
600 sh (44) 580 sh (80)
[Cottga)(cnM+ [Co{O2CCH(Me)S)(en),]+ [C0{02CC(Me),s)w,l+
510 (22600) 483 (126) 482 (142) 470 sh (170)
279 (1640)
434 (1240) 417 (MOO)
400 (240), 350 sh (300)
270 (9800) 225 (8300) 270 (8200) 283 (I 1 700) 282 (13 800) 279 (19 400) 260 (14 600) 282 (I 1 700) 280 (12600) 282 (10900)
371 (5800) 365 (6700) 375 (6000) 360 (6000) 357 (6000)
287 (3750) 287 (3750) 285 (4400) 285 (3000) 287 (3500)
432 (220) 430 (250) 430 (190) 448 (435)
288 (14 200) 285 (13400) 287 (1 1 900) 294 (IO 200)
415 (6400)
518 (152) 515 (149) 514 (153) 480 (600) 470 (500) 475 sh (800) 493 (350) 480 (390)
487 (177)
282 (8380) 257 (4140) 280 (7300) 270 (7210)
499 (168) Disulfde (RSSRJ [Co(cystSMe) (en),],
489 (149)
+
501 (161)
340 ( 1640)
343 (1710), 278 (7550) 319 (1340) 285 (7680)
Table 85 Structural Data for Sulfur Chelates Sirucfurd data (pm)
Ref.
COS224.3; trans CON 200.5; Ar 4.3 COS222.6; trans CON 200.1; Ar 4.1 COS 223.4; trans CON 202.4; Ar 4.9 COS225.2; trans CON 201.1; Ar 3.9 COS 224.6; trans CON201.1; Ar 4.4 COS225.9; CON 197.4; SCoN 12.5" COS 222.2, 222.9; CON 192.2, 198.3; COO 197.1, 194.2 COS 225.5; trans CON200.7; Ar 4.6
1 1 3 3 4
Complex Thiokate [Co(tga)(en),]CI. H,O [Co(cyst)(en),J(SCN), A-[Co{(R)-cysI(en)*I(CIO4) A-[Co{(R)cys)(en),](CIO,) .H,O [Co(NH,C, H,S)(en)21C1(C104) [Co(Me, pymt),] acetone
-
K[C~{(S)-P~~){(R)-P~~)I.~H,O [Co{(R)-pen)(dien)]Cl.H,O
11 13 14
Thioether
[Co{[(R)-c~sMeI,en)l(C10~)
COS226.7; Ar 0 COS221.4; Ar 2.0 Cambridge Crystallographic Data Centre COS227.6; CON195198; Ar 0 COS227.1; COO 190.6; CON 194.6; XCOY 83.8-98.9" COS 225.7, 226.4; trans (0)isomer
Disulfide [Co(cystSEt)(en),](C104),
COS 227.2; SS 203.3; Ar 1
9
[Co{S[SC(Me),CO2H]C(Me),CO,)(en),](C10,),~2H,O COS 226.0; SS 205.2; Ar 0 ICo(en)? (cYSh(c104)4- 6HzO
SS 203.2
10 15
[Co(~ystMe)(en)~] [Fe(CN),] * 4H,O [Co(tgaCH,Ph)(en),](SCN),
A(S)-[Cpt(Rj-c?lsMe)(en),I(NCS), [Co([cyst(CHCON)Et]COCH,)(en),(C10,),
-
[Co{(R)-cysMe),](CIO,) H, 0
M e l d bridged rhiolates { NS)-ICo(en), (tgallzAg)(CQ), A,h-[Co(en), (cyst)],[Cu(MeCN),](C10, I6 -2H, 0 [iCO(CY5t), I: P6(C0)12 ( S 04P 4
COS 224.7; Ar 1.4 AgS 237 COS 221.3; CuS 240.6, 234.2; SCuS 97" COCO285.7; Co,S 223.8; Co,S 226.2; CON 199.6; SCo,N 88.3 '; Co,SCo, 78.8"
5
5 6 7 12 16
8 17 18
Cobalt
843
Table 85 (continued) I.
2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13. 14. 15.
16. 17. 18.
R. C. Elder, L. R. Florian, R . E. Lake and A. M. Yacynych, Inorg. Chem., 1973, 12, 2690. W. G. Jackson, A. M. Sargeson and P. 0. Whimp, J. Chem. Soc.. Chem. Commun., 1976,934. H. C. Freeman, C . J. Moore, W. G. Jackson and A. M. Sargeson, Inorg. Chem., 1978, 17, 3513. M. H. Dickman, R. J. Doedens and E. Deutsch, fnorg. Chem., 1980,19, 945. . R. C. Elder, G. J . Kennard. M. D. Payne and E. Deutsch, fnorg. Chem., 1978, 17, 1296. G. J. Gainsford, W. G. Jackson and A. M. Sargeson, J. Chem. Soc.. Chem. Commun., 1979, 802. L. Roecker, M. H. Dickman, D. L. Nosco, R. J. Doedens and E. Deutsch, fnorg. Chem., 1983,22,2022. M. J. Heeg, R. C. Elder and E. Deutsch, Inorg. Chem., 1980, 19, 554. D. L. Nosco, R. C. Elder and E. Deutsch, Inorg. Chem.. 1980, 19, 2545. L. D. Lydan, R. C. Elder and E. Deutsch, Inorg. Chrm.. 1982, 21, 3186. B.A. Cartwright, P. 0 . Langguth, Jr. and A. C. Skapski, Acra Crjsdlugr., Sect. B, 1979, 35, 63. P. de Meester and D. J. Hodgsw, J . Chem. Sac., Dalton Trans.. 1976, 618, H. M. Hclis, P. de Maester and D. J. Hodgson, J. Am. Chew So(en),l+compared to the slower oxidation of the equatorial lone pair in A(S)-[Co{(R)cysO}(en),l+ supports this idea.IM1 However the structure of A(S)-[Co(cystO)(en),](SCN)2 (287; Table 87) clearly shows that oxidation in this complex has occurred at the equatorial site. The specificity in this latter system has not been detailed. Equatorial oxidation is also found with [Co(cyst),] (Section 47.8.2.2.ii), so some flexibility is possible. Theg- and r-[Co(cystO>(tren)l2'ions having S'" as the only centre of asymmetry have been resolved into their (R)and ( S ) enantiomers (288) and (289).lWsThis is the first reported resolution of enantiomeric sulfenate complexes in which coordinated sulfur is the only source of asymmetry.
Cobalt
844
\ o r - c a
mm
m
I
I
Cobalt
845
Table 88 Rate Data (at 25 “C) for Oxidation of Coordinated Thiolate by H,O, (see text)
Complex r
. .
k,(mol-’ dm3s-’)
ReJ
0.24 0.36 9.9 ( I = 0.513, 19.9”) 0.85 3.2 0.14 0.34 x 2.5 2.2 0.52
1 2 4 4 4
I.
D.L. Herting, C. P. Sloan, A. W. Cabrd and J. H. Krueger, horg. Chem.. 1978, 17, 1649.
2.
1. K. Adzamli, K. Libson, J. D. Lydon, R. C. Elder and E. Deutsch, Aorg. Chem., 1979,18,
3.
303. 0.Vollarova and J. Benko, J. G e m . SOC.,Dalton Trans., 1983, 2359. I. K. Adzamli and E. Deutsch, Inorg. Chem., 1980, 19, 1366.
4.
A(S)-[Co{(R)-HcysO}(en)2 I *’
(288) t(~~[Co(cystO)(tren)1 2*
A(S)-[Co(cystO)(en), 1
’+
(289) p (S)-[Co(cystO)(tren)I
‘’
Mutarotation at coordinated sulfenate S is slow, which is analogous to the situation with organic thiols. Introduction of the S=O group imparts a high resistance to inversion. Racemization in p-(S)-[Co(~ystO)(tren)]~+ occurs at the rate 9.92 x iO-’s-’ (30 “C; pH independent) and the t-(S) isomer is some 80 times slower.’045Other diastereoisomeric sulfenate systems have also been shown to be optically very stable. Protonation to form Co-S(0H)R (pK, x 0) does not alter this, but addition of alkali does with mutarotation of A(S)-[Co{(R)cysO}(en)2j+ being rapid at both S and Co in 0.1 mol dmP3OH- (but not at C). This may occur via a labile Co”--SO(R) inte~mediate.”~~ Mutarotation at S is also reported to be catalyzed by light,’04’possibly via an O-bound sulfenate intermediate (equation 162). Adamson and coworkers report that photodecomposition to Co& is the final outcome.’W9For sulfinate complexes donor S to 0 isomerization O C C U ~ S ’ ~ and ~ ~ ’ red ’~~ O been isolated.1029This process (equation 163; hv O-bound (N,O)[Co(cystO,)(en),](ClO,),~H,has 350-450nm) is thermally reversible (z,,~ x 600h, r.t.) but which 0 atom is involved, axial or equatorial, is not known. The O-bound form is now asymmetric, and some specificity might be expected.
Cobalt
846
Root and Deutsch discuss other oxidations of coordinated thi~late’~’’and Scheme 94 outlines some of these. Nucleophilicity towards Me1 is similar to that of uncoordinated RSH or RSR. Some crystal structures of the oxidized products are given in Table 87. The reactivity of sulfenic acids is masked by coordination to the metal and these complexes are reasonbly stable in neutral or weakly acidic solutions, but strong acid gives mostly Cot;, . Unlike organic sulfoxides (which preferentially 0-alkylate and then slowly rearrange to the sulfoxonium ion R3SO+)only S-methylation is observed when [Co(cy~tO)(en),]~+ is treated with Me1 (it may however initially occur at O).Iw This product then isomerizes to the 0-bound isomer (equation 164). Both processes occur with full retention of configuration at S and Co and the six-membered ring is apparently more stable than the five-membered chelate. Treatment of [Co(tgaO)(en),]+ with Me1 however gives rise to the thioether [Co(tgaMe)(en),12+ rather than the sulfoxide possibly via disproportionation of the intermediate U-methyl ester to the thiolate and formaldehyde (equation 165), the process being Other oxidants may react to give oxidation at the activated by the acidic a-carbon centre.1043J044 with N-chlorosucadjacent C atom rather than at coordinated S. Thus treatment of [C~(tga)(en)~]+ ~inimide’’’~ or acetic anhydridelD5*in DMF or DMSO leads to the monothiooxalate chelate (290; equation 166).
\l
0 0
(en)zco(
1
NHa
12+
/R
(en),Co I’;+’
”’s3
/SR
(en),Co\ \
NHZ
NHz
Cobalt
847
(iv) Tridentate, quudridentate and sexidentate chelates
-
The (S,O,N) tridentate ligands (R)-cysteine, (R)-methionine and (R)-penicillamine coordinate stereospecifically but often give several geometric isomers of [Co{(R)L),]-.+. Table 89 lists some preparations and Table 85 some structures. Ail three isomers (291), (292) and (293) of [Co{(R)cys),]- have been isolated and their visible and CD and 'H NMR spectra c ~ r n p a r e d . ' ~ * ' ~ ~ Isomerization to a common equilibrium slowly occurs in aqueous solution. Treatment of trans(l\r)[Co{(S)pen),] with Me,S04 in aqueous solution results in all three geometric thioether complexes [C~((S)penMe),]Br,~~' whereas similar treatment of the three [Co{(S)pen)(dien)]+ isomers appears to proceed with geometric integrity.IWs'H NMR also suggests that methylation at the asymmetric S atom is stereospecific, The isolation of the tridentate [Co{(R)cys),j- has not been reported (cf. Section 47.8.2.2.ii for [Co{(R)cys},]'-). Some mixed-ligand complexes are included in Table 89. The quadridentate ligand HS(CH,),N(Me)CH,CH,N(Me)(CH,),SH forms two [Co(eddt)(en)]+ isomers (294) and (299, and a crystal structure establishes stereospecific coordination of the sexidentate thioether ligand N,N'-ethylenebis{(R)-cysteine-Me) (296; Table 89). ~
(1))
Metal binding to coordinated thiolate
Soft metals such as Ag", Cut and MeHg+ coordinate to Co"' SR"+ complexes and uncoordinated thioethers with about equal ease. Thus it seems that Co"' and R + have similar effects on the binding power of RS-,reducing it by about IO" (Table 90; equation 167). This property has implications for electron transfer since ligation of coordinated thiol to a second metal centre is probably the first step in inner-sphere electron transfer. Data given in Table 90 demonstrate that Co"'-SR"+ retains some Lewis basicity towards soft metals. These studies have utilized [CO(R-S)(~~),]++~+ systems (R-S = cyst, tga)Io5' but the metals Fe111,105','052 Ru1111052 and C0111*0s3 also form very stable [ C o ( c y ~ t ) ~ ] ~cations M ~ + via bonding to the trigonal faces of two Co(cyst), octahedra,"" and Zn", CU"and Cd" form { [Co(cyst),],M}MBr, species which are thought to involve p-Br bridges.'053 Resonance Raman studies show two strong absorptions which have been assigned to the somewhat weakened stretching and bending modes of the Co(cyst), moiety,"" and oxidation by H,O, appears to be modified by the presence of the second The prochirai S centre generates two diastereoisomeric possibilities in a CoS(R)M product. Structure (297;Table 85) shows the axial lone pair to be more nucleophilic generating A(S),A(S) and A(R),A(R) dimers. The Ag+ ion lies on a two-fold symmetry axis and the dimer unit is chiral with two units existing in the crystal related by a centre of inversion. Both lone pairs are involved in the binding of Cut (structure 298;Table 85). This red-brown dimer has been suggested as a model for ESR-silent type 3 copper in multicopper
Cobalt
848
Table 89 Preparations: Some Tri-, Quadri- and Sexi-dentate Thiolate and Thioether Chelates of Cobalt(II1) Preparation
Ligand
Complex
NH,CH(CO; )C(Me), S-
CoC1,-6H2O, H20, pH 7, 6 h. Brown [truns (N)] and green [trans (O)] isomers isolated Co(NO,h, HzO, PH 7 [Co(dien)Cl,] AgNO,; H,O, N,; chrom; three brown isomers CoC1,.6H2O, H,O, 70"C,charcoal; PbO,, chrom.; three isomers CoCl2.6H,O, H,O, pH 8; charcoal, PbO,, 65 "C; chrom. CoCIz-6H,O, H,O, p H 8; charcoal, PbO,, 65°C; chrom. CoCl,.6H,O, H,O, 70 "C, charcoal; Pb02 20 min; chrom; three isomers K[Co{(S)-penj,], H,O Me,SO, chrom.; three isomers [Co(NH,),]Cl,; H,O; pH 2.5, charcoal, 40"C, 16 min, chrom.; MeSO,, chrom. [Co(dien)((R)-pen}]CI Me,SO,, H,O, chrom. [Co(NH,),j€13 OH-, 90°C; chrom., two isomers, resolution of cis-(S) [Co(en),]Cl, -k OH-; 2h, r.t. chrom., two isomers CoCI,~6H,O PbO,, 70°C 20min; chrom.
+
NH,CY(CO;)CH,CH,SCH,
NH2CH(C0; )CH, SMe
NH,CH(CO;)C(Me),SMe
[- S(CH,), N(MWH21,
[CH,NCH(CO; )CH,SMe], I. 2. 3. 4. 5.
6. 7. 8.
+
K. Okamoto, W. Wakayama, H. Einaga, S. Yamada and J. Hidaka, Bull. Chem. SOC.Jpn.. 1983, 56, 165. H. M. Helis, P. de Meester and D. J. Hodgson, J. Am. Chem. SOC..1977, 99, 3309. K. Okamoto, K. Wakayama, H. Einaga, M. Ohmasa and J. Hidaka, Bull. Chem. Soc. Jpn., 1982,55, 3473. J. Hidaka, S. Yamada and Y. Shimura, Chem. Leu., 1974, 1487. K. Wakayama, K. Okamoto, H. Einaga and J. Hidaka, Bull. Chem. SOC.Jpn., 1983, 56, 1995. T. Isago, K. Igi and J. Hidaka, Bull. Chem. SOC.Jpn., 1979, 52, 407. K. Yamanari, N. Takeshita, T. Komorita and Y. Shimura, Chem. Lett., 1981, 861. K. Okamoto, T. Isago, M. Ohmasa and J. Hidaka, Barll. Chem. Sor. Jpn::, 1982,55, 1077.
Me
c
N
Me
c
N
(298) AA-{[Co,&yst)(en)2I [Cu(NCMe), I 2}6' Table 90 Equiiibriurn Constants for Ag+ and MeHg+ Binding to Thiolate (RS-)and Thioether (RSR) Sulfur13
Species
K (dm3mol-')
HOCH,CH,SAg (HOCH,CH,),SAg+ [CO(CYstAg)(en), 13+ [Co(tgaAg)(d, 1, (HOCH,CH,),SAg+ S(CH, CH,OH), [Co(en), (tga)I2+Ag+[@ga)(en)2C0l2+ HOCH,CH, SHgMe [Co(cystHgMe)(en),13+ [co(tgaHg~e)(~hlz+ [Co(en),(cy~t)]~+HgMe+[(cyst)(en), Co]'+
1.6 x 1013 3.4 x 103
+
'
2 3
+
+
(NH, CH,),CHS-
1
4.0 x 104 4.8 x 104 1.9 x lo2
1.3 x 103 1.3 x 10l6
4,6 x io4 3.3 x 105 5.3 x 102
In water at 25 "C 'M. J. Heeg. R. C. Elder and E. Deutsch, Inorg. Chem., 1979, 18, 2036.
5
7 7
8
I Cobalt 41.8.3
849
Thioethers
The coordination chemistry of thio-, seleno- and telluro-ethers in transition metal complexes has recently been reviewed in detail.Iog4
47.8.3.1
Co&alt(ZI)
Co" is generally regarded as forming only weak bonds to thioether sulfur. This is indicated by the absence of complexes containing monodentate thioether and by only a few reports dealing with bidentate thioether complexes. The latter are of the [Co(X),{RS(CH,),SR},] type (R = Me, Et, Pr; X = C1-, Br , I -,NCS-)'062.'063 and they display spectral and magnetic properties consistent with a trans octahedral geometry. This has been verified for [Co(ClO,), { MeS(CH,),SMe),], which incidently was the first complex containing coordinated perchlorate to be structurally characterized (Table 75).IW Its magnetic properties resemble Co" in a square-planar environment due to the weak coordination of ClO, and have been interpreted in terms of a doublet-quartet equilibrium.'o63 Various mixed-donor bidentate systems have been investigated. N-S bidentates generally form high-spin octahedral [Co(X), (N-S),] complexes but increasing ligand bulk can result in tetrahedral [Co(X),(N-S)]. Ligands investigated include 2-(methy1thio)aniline (mta),lM5 2-[(methylthio)methyl]aniline (mtma),'066 2-((methy1thio)methyl)pyridine ( m t m ~ ) 'and ~ ~ trans-2-(ethylthio)cyclopentylamine (etcp).'068Tris complexes containing these ligands are rare but [Co(mtmp),](ClO,), has been reported.'067 The structure of trans-[Co(Cl), {HO(CH2)2S(CH2)20H}2] shows the potentially tridentate thioether acting as a bidentate with normal Co-0 bonds (207pm) but long Co-S bonds (251 ~ r n ) . ' ~Structural ~' studies have also been carried out on high-spin Co" complexes of alkylligan~s~1070.1071 With PhZP(2-MeSPh) and thioacetate (RSCH,COO- ) and [CoX(PSSP)]+ have Ph,P(CH,), S(CH,), S(CH,), PPh, low-spin five-coordinate [CoX(P-S),]' been characterized by magnetic and spectral studies.6°4,'072 Similar complexes exist with the related As-P and Se-P ligands. The NSN tridentate Me,N(CH,), S(CH,), S(CH,)2NMe, (Me,daes) forms high-spin ( p = 4.44.5 BM) five-coordinate [Co(X),(Me,daes)] complexes (X = C1-, Br-, I - ) and visible spectra indicate trigonal bipyramidal structures in the solid^.'^' This geometry also occurs with [CoBr(Bu'-NS,)]PF, (Bd-NS, = N(CH,CH,SBu'),). All three sulfur atoms lie in the equatorial plane with the Co atom displaced 35 pm toward the apical bromine (Co-S, 238 pm)."" Cobalt(I1) complexes of polythioethers include those of 1,2-bis-(u-methylthiophenylthio)ethane,"" and the macrocyclic ligands 1,4,7-trithiacyclononane ([9]a11eS,)'~'~ and 1 ,4,7,10713,16-hexathiacyclooctadecane([l 8]aneS,).'077The structure of [Co(ll 8]aneS6)](picrate), shows the cobalt atom in a distorted octahedral environment with each three-sulfur segment coordinated facially to give the meso geometry (299).Facial coordination of the trithia ligands is also found in the tetragonally distorted COS, complex [c0([9]aneS,),](BF~)~(300),'076but here Co-S,, > Co-Sa,, whereas the reverse occurs with [Co([ 1S]aneS,)](picrate), The low spin ( p = 1.8 BM) of the latter complex is unusual since thioether donors are normally regarded as relatively weak-field ligands. However this is the only Co(SR)i+ system for which magnetic and ESR data have been reported. "
47.8.3.2
Thioethers, sulfoxides and sulfones, co&aIt(ZZI)
Thioether complexes are mostly of the bidentate (301) or multidentate (302)type. Monodentate and in such cases they provide useful systems (303) are rare but are known (equation 167a),1078 starting materials since the thioether ligand is easily displaced. CoWO,),
+ ZDMGH, + NaBr + Me,S
02
E,OH,hcat
~ ~ ~ ~ S - [ C O B ~ ( M ~ , S ) ( D M(167a) GH)~] brown
Cobalt
8 50
R-.
(302)
(301)
(303)
Coordinated thioether S is often asymmetric. In (302) the asymmetry is frozen by the configuration of the attached chelate rings but in (301) it is not and inversion is fast. Conventional resolutjon via diastereoisomeric salts has never been substantiated for a purely enantiomeric Co-SR'Ra"+ system devoid of other chirality. "C and 'H studies on diastereoisomeric systems'079however show that inversion rates are normally of the order 0.1-10 sP1, which makes coordinated thioethers decidedly more configurationally labile than organic sulfonium salts. The effects of asymmetry in (302) are readily seen in diastereoisomeric systems where the lone pair on S is often stereospecifically orientated. It usually directs stereochemical change and ligand replacement processes.
(i)
Bidentates
Kolhari and Busch were the first to describe the alkylation of Co"'-SR when they treated (N,S)[Co(cys)(en),]I with MeT in aqueous rnethan01.'~~'However they incorrectly described the product as (N,0)[Co(cysMe)(en),]T2 containing carboxylate rather than thioether coordination. The (N,S) and (N,O) chelates have subsequently been clearly distinguished (equations 168 and 169), and the methylation reaction gives the (N,S) isomer with retention of the Co-S bond.'OR0Deutsch and coworkers in a comprehensive investigation of alkylating reagents describe the versatility of this reaction.'08'~L0s2 Some of this information is collected in Table 91. Primary and secondary alkyl groups, activated double bonds, carbocations, and strained groups have been added. Alkylation of [Co(cyst>(en),]'+ with methyl vinyl ketone is first order in both reactants and in [Hf], consistent with the mechanism given by equation ( l7O).'Os2 Where crystal structures are available (Table 85) they show alkylation occurs stereospecifically at the axial lone pair with the R group directed between adjacent ethylenediamine chelates,* e.g. (304). Subsequent mutarotation does not usually occur.
A-(N,S)[CoC(R)-cys}(en),IC10, ~is-[Co(DMSO),(en)~](C10,),
+
+-
McI DMSO. ~-(N,S)[Co~(R)-cysMe}(en),l(C1O4), brown
(R)-HcysMe
1168)
A,A-(N,O)[Co((R)-cysMe)(en),](ClO,),(169) orange
Bidentate thioether complexes are reasonably stable in neutral and acidic aqueous solutions but readily hydrolyze in alkali, and the process appears to be reversible. Oxidation to the sulfoxide is difficult but once thioether S is dissociated from the metal becomes relatively easy (Scheme 95). Table 91 Preparations:Thioether Complexes [Co(en), (cystR)](ClO,),
R
Me. CH2R1(where R1is almost anything) (CH2),C0,H, (CH,),CONH,, (CH,),COMe (CH,),CHO, (CH,),CN, CHCHCO,H, CH(COZH)CH,CO, H, CHCONHCOCH,, CHCON(Me)COCH,, CHCON (CH,Me)COCH,, CHCON(Ph)COCH, Me, Et, But
Reagent. conditions
Ref
Rx (X = c1,Br, I); DMF; r.t.
1
or alkene, r.L, 6 M HClO, 0.5 2 h, chrom.
2
ROH, MeSO,H, r.t., &1 week
2
+ o
CH(Me)CH,CO, H
1. 2.
R. C. Elder, G. J. Kennard, M. D. Payne and E. Deutsch, Inorg. Chrm.. 1978, 17, 1296. L. Roecker, M. H. Dickman, D. L. Nosco, R. J. Doedens and E. Deulsch. Inorg. Chem.. 1983, 22, 2022
*The IUPAC designation (S)or ( R ) at sulfur depends on the nature of the substituent. When R = H, Me, Et, the A-[Co(NH,-SR)(en),]'+ complex takes the (R) configuration but for heavier substituents (e.g. 0,S) the configuration is IS). For the sake of consistency with the sulfenate and disulfide complexes we will designate the substituent as taking precedence in all cases, Le. (S) for all substituents.
85 I
Cobalt
.-
. . ..
(304) A(s)-lC~(H,r\i-SR)(en),l'' ,~
Subsequent coordination now ocurs via the 0 atom, probably with displacement of coordinated water. This chemistry resembles that of organic sulfonium salts (R, S+) and thioethers.
Me
Scheme 95
Inversion at Co"'-SR1R2 is fast (0.1-10 s-I) but diastereoisomeric systems have been detected by IH and I3C NMR spectroscopy. Thus (305)shows clearly resolved signals of the tren ligand arising from the chiral t h i ~ e t h e r . Also ' ~ ~ ~ the related t-(N,S)[Co((R)cys)(tren)l3 chelate shows immediate 'H-NMR spectral changes on deprotonation of the dangling equatorial carboxylate arising from rapid (< 5 5) mutarotation of the (R)S[R)C, (S)S(R)Cdiastereoisomeric mixture. Thus inversion appears to be 105-107times more rapid than for uncoordinated sulfonium salts, a result in keeping with other M-SR'R' +
Photolysis in the ligand field region leads to photoaquation (equation 171), while irradiation in the charge-transfer region leads to some reduction to CO~&."'''~ The thioether S atom in [Co (RSCH,CH,NH,)(en),13' (R = alkyl, aryl) has been shown to act as a lead-in centre for innersphere reduction of the metal by Cr&, with more bulky R slowing down the rate and carboxylate substituents increasing the rate by chelating to the reductant.'097
c.0c. 4-68
Cobalt
852
(ii)
Multidentates
Preparations and structures are given in Tables 92 and 93 respectively. Bridging thioether (302) prefers the bent configuration with adjacent chelate rings in different planes. Thus [Co(daes),13+ adopts only the u-fac configuration (306), whereas the analogous all N system prefers the mer geometry (307). Similarly, [Co(X),(eee)y+ is s-cis (308) for X = C1-, Br-, NO;, NCS-, C,O:-, bipy, as is [Co(Cl),(tet)]+, while [Co(X),(ete)l"+ prefers the u-cis geometry (309) but does adopt rrans (310) for X = Ci- (60% u-cis, 40% trans at equilibrium).loWThe corresponding all-N complexes [Co(Cl),(trien)]+, [Co(CI),(3,2,3-tet)lC and [Co(C1),(2,3,2-tet)lC adopt s-cis, u-cis and trans,trans, and trans configurations respectively. This stabilization of the bent geometry at S is not well understood but may be connected with its larger size. The reader is referred to a recent article by Abel, Bhargava and Orrell on such aspects.994In (308) the lone pair on S is stereospecifically orientated and only A(R,R) and A(S,S)configurations obtained, but for (309) and (310) the planar S may be ( R ) or ( S ) . This can direct subsequent stereochemical ~ h a n g e . ' Thus ~ ~ ~inversion ' ~ ~ ~ at S must occur for the stereochemical change (309) R,S to (308) R,R, and (310) R,R gives stereospecific (309)-A(R,R)and (308)-A(R,R)products if the asymmetry at S is maintained. Little is known about such processes compared to the well-documented chemistry for coordinated N. Worrell and colleagues have synthesized several thioether chelates in order to study the effects of the long Co-S bonds The cations (311) and (312) show on inner-sphere electron transfer reactions via trans ligands.'0*9~'090 increased rates of reduction by Fe$, compared to their all N analogue^.'^^'^^^ One recent studyim6 serves to highlight the ease of inversion at thioether S and suggests that the restriction of geometrical configuration resultant upon retention at S is not as certain here as it is in N multidentates. The ready interconversion of the optically active brown (313) and green (314) isomers of [Co{l,lObis(salicylideneamino)-4,7-dithiadecane)]+ (equation 172), can be accomplished via a trigonal twist process with simultaneous inversion at both chiral S centres. The alternative possibility of retention at S would require a complex series of edge displacements and predicts the opposite absolute configuration about the metal. Rapid (0.1-10 s-I) inversion at coordinated S allows for flexible rearrangement in such multidentates. The structure of (313; Table 93) contains the very unusual planar thioether ligand.
n = i n = 2 (eee) n = i n = 3 (ete) n = 3, m = 2 (tet)
(310) trans green, X = C1-
(311)
[CoN,(NSNSN)]
'+
(312) ICoN3(eee)] 2+
Cyclic 14- and 15-membered tetrathioether ligands (315) and (316) also form [CoX,(S,)]+ complexes with (315) preferring the cis geometry and (316) being trans with planar S donors (Table 92). cis-[CoCl,(ttp)]+ undergoes normal metathetical reactions to form cis-[CoX,(ttp)]+ species but trans-[CoCI,(ttx)]+ decomposes readily in hot water to give Co;: .lWs
853
._ ... z z x z
+ 4+ + +
4
E
e
00 i hl
0" 0:
z
ii
+
I I
+
h N
Y Q. 3
Cobalt
854
Table 93 Structures: Multidentate Thioether-CobaMJII) Complexes
Sac-[Co(Cl),(daes)] (- )ss~-as~~far.-[Co(daes),]C1, .2H,O
asym,cis-[Co(C!)(NO,)(ete)]Cl A- trans(S,O)-[Co{ (5')-aehc}gly](ClO, )
COS 221.6; CoC1227-8; CON 193.3, 195.9; SCOCl 175-6 '; CSC 102-7" COS 224.4, 224.8; CON 197-9; SCOS 19.2 "C: CSC 102.2' COS 2222; CON 196-9; CoCl 227; CoN(0,) 209 COS222.3
[Co(ONSSNO)jI (two isomers)
COS224-5; COO 189-190; CON 1946
[Cu(~azacapten)j(S,O,),*4H,O
COS 217-224; CON 194-198 COS 222.6; CON 200.9 COS 2234; CoC1226.8; CON 196.8, 197.8
+
( )j,o-[Co(azacapten)](ZnC14)Cl
[CoCl(eeee)jCI(ClO,)
Cummenis
Structural data"
Complex
Ref.
Bent configuration at S; no trans Ar Bent configuration at S; no trans Ar C1 trans to S
(R) configuration for asymmetric S atom Brown isomer (NSSN planar); green (two forms NSSN k n t , planar) Sexidentate, encapsulated Optically active complex Racemate; all-bent Configuration
a Bond
1. 2.
3. 4. 5.
6. 7.
lengths in prn. G. Baumann, L. G. Marzilli, C. L. N i x and B. Rubin, Inorg. Chim. Acta, 1983, 77, L35. A. Hammershoi, E. Larsen and S. Larsen, A c f a Chem. Scund.. Ser. A, 1978, 32, 501. J. Murray-Rust and P. Murray-Rust, Acta Crystollogr., Sect. B, 1973, 29, 2606. K. Okamoto, M. Suzuki and J. Hidaka, Chem. Lerz., 1983, 401. A. M. Sargeson, A. H. White and A. C Willis, J . Chem. Soc., Dalton Trans.. 1976, 1080. L. R.Gahdn, T. W. Humbley, A. M. Sargeson and M.R. Snow, Inorg. Chem.. 1982,21, 2659. J. D. Korp. I. Bernal and J. H. Worrell, Polyhedron, 1983, 2, 323.
(313)
(+)~,-(S,S)-[CO(ONSSNO)~ +
brown
n
(; ;)
= ttp
U Steric or canformational restraints imposed by substituents on the ligand can impose stereosebctivity or can enforce some stereospecificity. Thus (R)-epe stereospecifically forms (317) with the methyl group equatorial rather than axial,loe7and (S,S)-pep also prefers (65%) the A configuration (318) with the methyl groups adjacent to X.'088In both systems the configuration about Co and S
I Cobalt
855
is maintained in various displacements of X. Gahan and O'Connor have used the optically-active binaphthyl fragment (319)to stereospecifically form the cobalt sexidentate (320)in which the two S donors have the ( S ) c ~ n f i g u r a t i o n . ' ~ ~ ~
0""
CH=NB(CHz)ZS
p-)
"0
S(CHz)ZNH=CH
@
,*\@
S"%,,,
c-0
I
SW] k o L N /
(319)
(320) A-(+)546-[Co{(S.S)-nme~~ I+
Sargeson and coworkers have synthesized a number of thioether cryptate sexidentates including (321)Iw2and (322)lw3condensing the appropriate trithiol with aziridine. The trigonally positioned amine groups in their Co"' complexes (Table 92) have been capped by condensation with HCHO
and an appropriate nucleopile to form the totally encapsulated complexes (323; Schexe 96). These have been resolved into their optical enantiomers which can be reduced to the chiral Co" chelates and reoxidized by 0,with full retention of chirality. Indeed it is very difficult to remove the metal from within the cage under any condition. 1 3 4
+
(321) ten
(322) atch
HCHO NH, or MeNO,
Co(0Ac) 2 _
o,,c
Rz = 0 2 N
)I+) and CS,-bridged binuclear (e.g. [{ MeC(dppm), 1Co(p-CS,)Cr(CO),]) complexes respectively. (ii) Di- and tri-thiocarbonates, cobalt(1II) Olive-green [CO(S,CS),]~-has been prepared and isolated (Table 98) and a brown-green complex precipitates from ammoniacal CoSO, when treated with Na,CS3."62 IR data for the latter have been interpreted in terms of structure (343),'16,and [CO(S,CS),]~-(344)probably contains the bidentate S, donor set although it is less stable than the unstable all-oxygen analogue [Co(CO,),]'- (Section 47.7.3.2). of composition [Co,(C,S,)(NH,),]
S II S'CYi
(344)
(343)
(iii) Alkenedithiolates and dithiocarbimates The preparation and properties of known cobalt(I1) and cobalt(II1) complexes are given in Table 98. The dithiocyclopentadienyl anion is a resonance hybrid and the low spin ( p = 2.55 BM) and ESR of (345) suggest a distorted planar structure. The 1,l analogue of maleonitrileditholate (346) has reduced electron delocalization and McCleverty and his colleagues have investigated the electrochemistry of the mixed [Co{S, C, (CN),}, (S, C=X)]'- system.'Ibl Only one-electron oxidation is possible in CH2CI, at Pt (equation 193), and chemical oxidation with I, has resulted in the isolation of (Ph,P),[Co(S,C,(CN),)(S,C=X)] (X = CHNO,, C(CN)CO,Et). Likewise diamagnetic (347) can be reversibly oxidized (Et,2= 0.14 V).'I6' The dithiocarbazic acid anion [Co(S,C=NNH,),I3has been discussed in Section 47.8.10.1. (i) and the corresponding nitrile [CO(S~C=NCN),]~has been isolated (Table 98).
(345)
(347)
47.8.10.3 Thiophospfiates and selenophosphinates These ligands can be easily made (equations 194 and 199, but obviously extreme care is necessary with the Se c o r n p ~ u n d . ~ ' ~ ~ ~ ~ ' ~ ~ PZSs
+ 4EtOH
PPh2Cl
+
S,
--
N2,heat
-
1
2(EtO),PS,H
Ph2PSCl
+
HZS
Ph,PS;Na+
Cobalt
870 (i)
Cobalt(I1)
The synthesis of [Co(S,PR,),] (R = alkyl, aryl, OEt, CF,) involves addition of the sodium or ammonium salt of the ligand to solutions of Co" halides, or the reaction of Co metal with the acid [Co{S2P(OEt),>,I has also been made by photoreduction ligand ( R = F, CF,; Table 99).1166,1168*1169 of the Co"' complex (equation 196)."" Although many of these tetrahedral species are quite soluble in organic solvents, this is not the case for R = Me, Ph, and the former complex at least is polymeric in the solid state."69In contrast the structure of [Co(S,PEt,),], consists of dimeric units (348)."69 Benzene sohtions of this complex appear to be associated into larger polymeric units with retention of tetrahedral geometry about the metal. Attempts to prepare [Co(S,PMe,),] in aqueous solution lead to [Co(S,PMe,)(SOPMe,)] but with other dialkyldithiophosphinatesthis substitution of 0 for S is not stoichiometric. Monomeric [Co(S,PR,),] is stabilized towards oxidation at the metal by the formation of trigonal-bipyramidal five-coordinate (349)and octahedral adducts with unidentate amines and phosphines (Table 99).1'70,117' Dimeric structures such as (350) are also k n ~ w n . " ' ~ 2[Co{S,P(OEt),}3]
* [Co{S,P(OEt)*},] + (EtO),P-S-S-P(OEt),
/I
196)
II
S
S
I
'
S\,/S Et0 (348)
\OE.t
(350)
(349)
Table 99 Preparations: Some 1, I-Dithio- and Diseleno-phosphinate Complexes of Cobalt Cvmplrx
Preparalion
[COIS,P(OEt)l),l [Co(Se, PPh2 121 [Co(S2PPh,), (quinoline)]
[CotS,P(OR),J(L)l (R = Me, Et; L = PPh,, benzothiazole, benzim, pip, pyz)
Cobolr(III) [CO(S?PPh?)31 [CoPe,P(OEt),
131
[CoIS,P(OE%),l 1.
2. 3. 4.
5. 6. 7. 8.
9.
Ref.
CO + HSI PF, (- 80 "C)
Green; volatile; m.p. 44-44.5 "C: u = 6.1 BM
I
+ HS,P(F)Et COX, + HS,PMe,
Gr&; decomposes on storage Green; air stable; p = 4.38 BM Blue; p = 5.49 BM
2
Green; p = 4.15 Turquoise; trig. bipyr. structure (COS,, 263, COS, 232, CON 203 pm); p = 4.69BM Mostly five-coordinate; trig. bipyr. structure (R = Me, L = PPh,)
4 5
Brown; diamagnetic, 600; &dm 93000 Red;trig. dist. oct.
7
Olive-green; diamagnetic
9
Co [CoIS2P(Me),l,l
Properties
+
CoC1,-6H20 NaS, P(OEt), CoCl NaSe,PPh, [Co(S,PPh,),] quinoline; H,O-rexal .CHCI,
+
CoCI,.6H,O HlO
+
+ L; benzene,
CoC1*6H,O -t L; benzene
+ +
NaS,PPh,; Na,Co(NO,), EtOH CoCI,.6H,O L; EtOH, 60 "C, N2 CoCl2*6H2O+ L; EtOH
I
F. N.Tebbe and E. L. Muetterties, Inorg. Chem.. 1970, 9, 629. H. W .Roesky, Angcw. Chem., Int. Ed. Engl.. 1968, I, 815. R. G. Cavell, E. D. Day, W. Byers and P. M. Watkins, Inorg. Chem., 1972, 11, 1759. A. Muller, P. Christophliemk and V. V. Krishna Rao, Chern. Ber., 1971, 104. 1905. A. C. Villa, C. Guastini, P. Porta and A. A. G. Tomlinson, J. Chem. Soc.. Dalton T*uns., 1978, 956. M. Shimoi, A. Ouchi, T.Uehiro, Y. Yoshino, S. %to and Y. Saito, Bull. Chem. Soc. Jpn., 1982, 55, 2371. A. Muller, V. \I. Krishna Rao and G. Klinkiek, Chem. Ber., 1971, 104, 1892. A. A. G.Tomlineon, J. Chem. Soc. ( A ) , 1971, 1409. C. K.Jorgensen, Acta Chem. Scnnd., 1962, 16, 2017.
3 3
6
8
871
Cobalt
Cobalt(T1) complexes of the mixed thiophosphinic and selenophosphinic ligands R, PXY(X = S, Se; Y = S,Se, 0)were first investigated by Kuchen and coworkers,1'6"''h7 and Calligaris and his group describe the structure of polymeric [Co(SOPPh,),In as chains of alternate single- and triply-bridging Ph, PSO units coordinated to distorted tetrahedral Co Their preparation is similar to the dithiophosphinate complexes (Table 99) and polymerization in solution appears to be e ~ t e n s i v e . " ~ They ~,"~~ are susceptible to hydrolysis and oxidation but can normally be stored for extended periods in sealed tubes. and [Co{Se(0)The preparation and spectral properties of [Co(Se,PPh,),] have been PEt, >,I has been de~cribed."~~ Both are rather unstable.
( i i ) Cobalt(lII1
Dark green [Co{S2P(OR),},] can be prepared from the ligand and C O F , " ~or~by oxidation of [Co{S,P(OR)2},] with H,O, (Table 99).11'6The analogous Se complex forms red crystals having a distorted octahedral stere~chemistry."~~ Indigo-coloured (351; Scheme 102) reacts with diphenylpho~phine'~'~."~ and diphenylarsine"'' chelates in acetone or CH,Cl, in two stages with the second stage involving transfer of a methyl group to the released (MeO), PSZ- nu~leophi1e.l'~~
Scheme 102
47.8.11
1,f-Dithiolates
Well-documented articles and reviews are a ~ a i l a b l e . " ~ ~These " ~ ~ "ligands ~ ~ form five-membered chelates with most metals. Early interests lay in the difficulty in assigning a formal oxidation state to the metal (352k(354),the discovery of the rare trigonal-prismatic structure for several neutral tris complexes (but not for Co) in some facile one- and two-electron transfer reactions, and in some unusual five- and six-coordinate adducts, including some with bonding to saturated carbon. No monodentate 1,Zdithiolate system is known and the trans configuration (355) has not been described. No attempted oxidation of trans dithiol-metal complexes has been described and it might be expected that (355) would be quite stable with rotation about the C-C bond occurring only in one tautomer.
(352) dithiolate
(353) dithiene
(354) dithiolene
Cobalt forms diamagnetic [CO(S,C,R,)~]~-, a series of monomeric square planar [ C O ( S ~ C ~ R ~ ) ~ I ' ions ( n = 1-3) and square-pyramidal dimeric [Co(S,C,R,),];- species (n = 0-2). All are highly coloured, indicating extensive electron delocalization, and all are reasonably labile both in a geometric sense and towards the displacement of one or more dithiolate ligands by other chelating groups. The square planar [Co(S2CzR,),]- ion adds both monodentate and bidentate ligands forming five- and six-coordinate complexes. Their lability, wide range of oxidation states, and the rather rare square-planar [Co(S,C,R2),3- ion distinguishes these complexes.
Cobalt
872
3
N
rn
W
d
z W
3
x
P 0
B
c C
I
I
I
N 3
A * v
0
Cobalt N
0-
2
3
fc
m
w
w
m
^
873
'
I
. Cobalt
-
d.
m
N
U
c
9
b,
N
2
I
I
2
c
N
L
Ba -
n
I Cobalt m
m
9
-
I
m p' N
N
d rn N
2 N
00
PI 3
L'
v
LL
u
875
Cobalt
876 47.8.Xl .I
Trisfl,2-dithiolates)
Representative preparations are given in Table 100 and structural information in Table 101. Diamagnetic [Co(S,C2R,),I3- can be prepared from Na, [Co(CO,),] or [CoBr(NH, >J2+ by heating with Na,S,C,R, in water or alcohol or by adding excess ligand to the more stable square-planar monoanion (356; equation 198). [Co(S,C,R2),l3- tends to dissociate to [Co(S,C,R,),]- in the absence of excess ligand and to [Co(S2C2R,),]'- in sunlight. It is probably octahedral, although no structure is available; (Ph,As), [Co{S2C2(CN),}J is isomorphous with the Cr"' complex, and the analogous FeIV complex is octahedral. In general the more reduced or negatively charge [M(S2C2R2),ln- systems appear to be octahedral while the oxidized or neutral systems (e.g. of Re"', V"', Mo" ) have the uncommon trigonal-prismatic structure (357; equation 1991, possibIy because the dithiolene structure would prefer to be trans, e.g. (355). [V{S,C2(CN)2],]2- adopts an intermediate distorted geometry. Attempts to resolve [Co(S,C,R,),] - into its enantiomers have failed and the suggestion is that the trigonal-prismatic geometry (357) provides a low-energy pathway to intramolecular racemization (equation 199). Spectrally, however, these complexes resemble [Co(thiooxalato),]'-, which has been resolved. The reduction potential, E,,, x - 0.6 V, suggests considerable reductive stability in neutral solution but the reduced [Co(S, C, R2)3]4-ion rapidly dissociates to [Co(S, C, R2)2]2-and free ligand. Oxidation is unknown although likely. Compared to the more common 0- and N-donor octahedral chelates [CO(S,C,R,),]~- is rather labiie.
(356)
47.8.11.2
green-brown
Bis(dithio1ates)
The [Co(S,C,R,),]- ion can be prepared by the stoichiometric addition of Na,S,C,R, to CoCI,*6H20in EtOH or MeOH, or by the careful oxidation (e.g. I,) of [CO(S,C,R,),]~- (Table 100). This ion is chemically more robust than [Co(S2C2R2),j3-,although dimerization does occur and is usually rapid and reversible (equation 200). The equilibrium position depends on substituents with electron-withdrawing groups (CF,, Ph, CN) stabilizing the dimer, and R = H, p-MeC6H,, giving monomeric products. Thus [Co(p-MeC,H,dithiolate),] is monomeric and square planar both as a solid and in non-coordinating solvents, while the tetrachlorobenzene derivative [Co(S,C,Cl,),]~- is diamagnetic and dimeric with a square-pyramidal geometry in the solid. In THF solution the latter dissociates to give the full spin-triplet ( S = 1) ground state monomer (equation 201). Structures of both the square-planar monomer and square-pyramidal dimer (Table 101) show similar bond lengths to Co in the bridges and in the chelate rings with no Co-Co interaction. With [Co{S,C,(CF,),}], Co is 37 pm out of the basal plane of the square pyramid. The monomer unit has two unpaired electrons, the dimer is diamagnetic. Reversible oxidation (R = CF,, Ph) is possible has been isolated (equation 202), and the intermediate product (Et,N)[Co{S,C,(CF,),},], u (, = 1.91 BM, Table 100). The fully oxidized [Co(S2C2R2)J2systems (R = CF,, Ph) are diamagnetic and adopt a square-pyramidal geometry (358), presumably because this allows the pairing up of spins in the semi-oxidized ligands, Thus both the electronic nature of the substituent and the oxidation state of the complex as a whole appear to control the structure and magnetic behaviour. The tendency of [Co(S2C2R,),1- to dimerize is paralleled by its ability to add additional ligands, those systems with electron-withdrawing substituents (R = CN, CF,,halogenated phenyl) forming
Cobah
877
Dz c1
s\ co/s
CI& 2
c1
C1
s/ \s
-I-
c1
stable five- and six-coordinate systems but the others doing so only with difficulty (Table 102). Ligand addition is both fast and reversible. Addition to the square-planar monomer (359; equation 203) is rate determining (L = PPh,, AsPh,, py, p h e r ~ ) . ' ' ~ With ~,"~L ~ = phen the process is two stage with chelation and stereochemical rearrangement following formation of the five-coordinate square pyramid. Subsequent reaction of [Co(S2C,R2),L]- with chelating ligands such as phen, mnt2-, bipy appears to occur via both associative ,and dissociative routes (Scheme 103). Green or tan five-coordinate (BU,N)[CO{S~C~(CN)~}~L] (L = pip, morph) and six-coordinate (Bu4N)[Co{S,C2(CN),),L,] (L = py, imH, NH,, BuNH,, PhNH2) are representative of these systems."" Crystal structures of two L = phen complexes (Table 101) show typical octahedral stereochemistries and these six-coordinate systems are spin paired and diamagnetic. The five-coordinate [Co(S,C,(CN),},L]- ion appears to be on the borderline between spin-triplet (S= 1) and ~~"~~ diamagnetic ground states although there seems to be some uncertainty on this p ~ i n t . " ~The monodentate six-coordinate systems may adopt cis (preferred) and/or trans geometries. 1Co(S*CzRZ),l:-
fast
+ L slow
2~c~(szc*Rz)~I(359)
2[Co(SzC*RZ12Ll-
(203)
Seheme 103
Ligand addition to the fully oxidized diamagnetic dimer [Co(S,C,R,),], (R = CF,, Ph) cleaves the bridge forming spin-triplet [Co(S,C,RZ)2L] monomers of presumed square-pyramidal geometry (equation 204). These may be considered as Co" dithiolates or as Co'" complexes containing the
878
Cobalt
5
c
d
e
Cobalt
879
oxidized ligand (more likely). Ligand exchange studies using mixtures of the reduced dimers [Co(S,C, R2)& have shown'190that the monomer-dimer equilibrium is rapidly set up, that ligand exchange at the metal is slow (several hours) and probably occurs intramolecularly within the mixed dimer, and that dithiolate exchange does not occur in the five-coordinate system formed on adding L (Scheme 104). L
Scheme 104
The fully reduced monomeric anion [CO(S~C,R,),]~-can be prepared from CoC1,.6H,O or Co(OAc),*6HZOand Na,S,C,R, in alcohol providing air is excluded (Table 100). Solutions are orange to red, the solids are paramagnetic with one unpaired electron and the structures are square planar. The complexes do not appear to dimerize and show a marked reluctance to add additional ligands except when the latter is a good z acceptor (e.g. (Ph,P),[Co(S,C,(CN),(NO)}] has been has been reported to be strongly isolated). The photochemical oxidation of [CO{S~C,(CN),}~]~wavelength dependent."'' The electronic structure of square planar [Co(S,C,R,),p- formed part of the considerable debate in the 1960s as to the extent of electron delocalization in the 1,2-dithiolene complexes, i.e. is [CO{S,C,(CN),),]~- d7 Cot' with two dianionic dithiolate ligands (360)or is it spin-paired Co' with an odd-electron dithiene radical anion (361),"92*'193 The general consensus now seems to be that (360)is more correct with the odd electron being largely metal based but with some ligand character."" The intense colour is consistent with extensive electron delocalization. Likewise MO calculations on the oxidized [Co(S2CZR2),]-ion conclude that the two unpaired electrons are also largely metal based"94making these species examples of very rare square-planar paramagnetic d6 Co"' (362)(cf. discussion above). -72-
71-
1-
Electrochemical oxidation of [Co(S, C2(CN),},]'- in THF produces initially monomeric [Co(S,C,(CN), I,]-, which quickly dimerizes to [Co(S,C,(CN,},]~- (Scheme 105). In DMSO this subsequent dimerization is only ~1ight.l~" Further reduction to [Co{S, C2(CN)2},]3-is possible and weak acids are reduced to HIby this ion. A metal-based protonation mechanism (Scheme 105) has been proposed.'lW
880
Cobalt
Scheme 105
Attempts to alkylate on sulfur in [ C O { S ~ C ~ ( C N ) , } ~have ] ~ ~been ~ ’ unsuccessful. Reaction of Me1 in THF with the former leads to decomposition while the latter appears to be u n r e a ~ t i v e . ” ~ ~ ~
47.8.12 l,%Dithiolenes The syntheses and general properties of 1,3-dithiolene transition metal complexes have been discussed in a recent Martin and Stewart1Ig9first described the preparation of the deep-violet complex [Co(SacSac),] (363;R = Me) from the reaction of CoCl, with acetylacetone (acacH) and H,S. A similar synthesiP” where CF,acac is substituted for acac yields [Co(CF,SacSac),] (363; R = CF,). The preparation of [Co(PhSacSacj,] via BH, reduction of the dithiolium salt (364) in the presence of CoCI, has aIso been reported.I2’’
The magnetic properties of [Co(SacSac),] ( p = 2.35 BM, 290 K; 2.30 BM,112 K)”98are consistent with the known square-planar geometry in which the Co atom is precisely coplanar with the four S donor atoms.’202Low-spin five-coordinate base adducts [Co(SacSac), B] (B = py, piperazine, ~ ~ ’ ~oxidative ” addition of amines (phen, BiPh,, SbPh,, AsPh,, PPh, j have been r e p ~ r t e d , ’ * ~ and bipy) gives octahedral Co”’ complexes [Co(SacSac),(N-N)]+ .Im5 With nitric oxide [Co(SacSac),] yields two nitrosyls; dark brown, weakly paramagnetic [Co(SacSac),(NO)] and red-brown, diamagnetic [Co(SacSac)(NO),]. The mononitrosyl is unstable in solution and disproportionates to the dinitrosyl and [Co(Sa~Sac),].’~~~ A crystal structure of tetrahedral [Co(SacSac)(NO),] is available.”‘ Dark-brown [Co(SacSac),] is prepared by aerial oxidation of solutions of [Co(SacSac>J in the presence of acetylacetone and H, S.Izo7The initial product is [Co(SacSac), (Sacac)] which converts to [Co(SacSac),] under forcing conditions.
47.9 SELENIUM LIGANDS
47.9.1 Selenate and Selenite These have been discussed together with oxyanions (Section 47.7.4), specifically Se0:- (Section 47.7.4.1) and Se0:- (Section 47.7.4.3). An early report on the preparation of (365)”OS requires further confirmation.
88 1
Cobalt
47.9.2 Selenolate, Selenenate and Seleninate 47.9.2.1 Cobait(tZ) Pink-violet seleninato complexes [Co(O, SePhX),(H, O),] (X = H, m-,p-C1, m-,p-Br, p-Me, p-N02) can be obtained by the addition of cobalt(I1) salts to aqueous solutions of substituted benzeneseleninato anions;120g IR studies indicate that the arylseleninato groups are (0,O) chelated to the metal. Reaction with N,N-bidentate ligands (phen, bipy) gives octahedral orange-yellow [Co(O,SeArX),(N,N),] species in which the RSeO; groups are bonded via a single oxygen donor.'*''
4 7.9.2.2 cobalt (m) Co(II1)-Se chemistry is in its infancy. Only a few compounds are known and those which are have been prepared via the oxidative Bennett approach"" using the diselenide and Co" (equation 205, Table 103). Deutsch and coworkers were the first to make selenolate complexes.'21'Generally they resemble their S counterparts, with H,O, oxidation to the orange-yellow selenenato (-Se(0)R) and seleninato (-Se(O>,R) complexes being even easier, and methylation with Me,SO, in water giving selenoether Co-Se(Me)R complexes (Table 103).1212 The structural rrms effect of Co-SeR is somewhat greater than Co-SR (Ar = 6.3 vs. 4.1 pm) and all Co-Se complexes show intense charge-transfer spectra tailing into the visible.1211~1212The 0 atom in Co-Se(0)R is more basic than the corresponding oxygen atom in Co-S(0)R (pK, % 4 vs. % This is in keeping with a longer Co-Se bond compared to Co-S ( 236 pm vs. 224 pm).
-
+
2C0(C10~)~ 5en 4 (H3N + CH2CH,Seh SO:-
N2,
I
2[Co(cystSe)(en),1(CIO,),
t , H20
(205)
One important difference from S is in the isomerization from Co-Se(0,)R bonding to Co--OSe(O)R on standing an aqueous solution of [Co(cystSeO,)(en),](NO,), for 1 d in a refrigerat~r."'~ The structure of the resulting spontaneously resolved A(S)-( -)$-; [Co{cystSeO(O))(en),](NO,), complex (366)gives the ( S ) configuration on Se with the exo 0 atom over an ethylenediamine ring.'*I4The longer Co-Se bond makes dissociation from the metal easier and this fact will probably limit the coordination chemistry of Se with Co. The lower energy of the ligand field absorption compared to S analogues ( 10nm shifts in the 49&500 nm absorption) is in keeping with this view. The corresponding sulfinate complex requires strong light for isomerization from S to 0 coordination (cf. Section 47.8.2.2.iii). or on methylation to The Se atom becomes asymmetric on oxidation to Co-Se(0)R C e S e ( M e ) R . Comparisons of CD spectra suggest that Se in A-[Co{~ystSe(O)}(en),]~+takes on the
-
Table 103 Selenium Complexes Complex
Preparations, properties structure?
Re$
(H,N+CH,CH,Se),SO~-,N,, r l . , H , O CoSe 237.8, tram CON 203.4, Ar 6.3; e4% 170, eB7 18 500 (HO,CCH,Se),, N,, r.t., H,; CoSe 235.5, trans CON 200.6, Ar 4 . 0 e520 150, ezDS17000 Ref. 1; resolved, K,(Sb-+-tart), 1 mol equiv. H,O,, r.t., 15min
1
3. 4. 5.
2 2 2
0-bound seleninate; spontaneous resolution; SeO 145.6, 147.6
3
MeI, DMSO
4 4 4
MeI, DMSO
4
-
1. 2.
2
Excess 5% H,O,,HCIO,, 30min Me,SO,, overnight, A-25 Sephadex
-
a Crystal
1. 5
structures. Bond lengths in pm. C. A. Stein, P. E. Ellis, R. C . Elder and E. Deutsch, Inorg. Chew., 1976, IS, 1618. T.Konno, K. Okamoto and J. Hidaka, Chem. Lett.. 1982, 535. K. Okamoto, T. Konno, M. Nomoto, H. Einaga and J. Hidakn, Chem. Lett., 1982, 1941. K. Nakajima, M. Kojima and J. Fujita, Chem. Leu., 1982, 925. R. C . Elder, K. Burkett and E. Deutsch, Acta Cryslaflogr., Secr. 8, 1979, 35, 164.
Cobalt
882
equatorial ( R ) configuration (367) but this remains ‘ H and ”C spectra suggest the opposite A(S) configuration* for the selenoether, and a stereospecific product.12” The long Co-Se bond should allow greater flexibility in this respect. Protonation at the 0 atom in A(R)-[Co(cysSe(O))(en),]’+ is easy (pK, z 4) and appears to result in mutarotation at Se possibly via ready dissociation from the metal. Unlike thioether sulfur where racemization or mutarotation is reasonably fast (0.1-10 s-’) the two optical Se isomers, (S)- and (R)-t-[Co{cystSe(Me)}(tren)12+(368) have been separated on SP-Sephade~.’~’~ The mutarotation rate, 2x s-’, demonstrates that inversion at Se is much slower than at S. The similar p complex could not be resolved chromatographically and it appears that the analogous p-(R)-[C~(cystSe(O)}(tren)]~+ complex (369) racemizes some 50 times faster than its t congener. This may result from the greater lability of the Co-Se bond, or from differing rates of inversion at the metal. Photolysis of Co-SeR complexes leads to photoreduction at a11 wavelengths.
-
Me
(368) r(S)-[ Co(cystSeMej(tren)l”
47.10 I.
REFERENCES R. D. W. Kemmitt and D. R. Russell, in ’Comprehensive Organometallic Chemistry’, ed. G. Wilkinson, F. G. A.
Stone and E. W. Abel, Pergamon, Oxford, 1982, vol. 5. 2. The following databases for crystal structures are available: (1) ‘Cambridge Crystallographic Database’, Olga Kennard, University Chemical Laboratory, Lensfield Road, Cambridge, CB2 IEW, UK (all crystals containing organic carbon); (2) ‘Inorganic Crystal Structure Database’, G. Bergerhoff, Anorganisch-Chemisches Institut der Universitat Bonn, and I. D. Brown, Institute for Materials Research, McMaster University, Hamilton, OnPario, Canada, LSS 4M1 (complements (1) above); (3) ‘The Metal Data File’, L. D. Calvert, Chemistry Division, National Research Council of Canada, Ottavia, Canada, KIA OR9 (intcrmetallic compounds); (4) ‘Protein Data Bank‘, T. F. Koetzle, Chemistry Department, Brookhaven National Laboratory, Upton, NY, 1 1973, USA (includes metalloproteins). 3. C. Floriani and G . Fachinetti, J. Chem. Soc.. Chem. Cornrnun.. 1974, 615. 4. G. Fachinetti, C. Floriani and P. F. Zanazzi, J . Am. Chern. Soc., 1978, 100, 7405. 5. C. Bianchini, C. Mealli, A. Meli, A. Orlandini and L. Sacconi, inorg. Chem., 1980, 19, 2968. 6. B. Kleman, Can. J . Phys., 1963, 41, 2034. 7. C. Bianchini and A. Meli, J . Chem. Soc., Chem. Commun., 1983, 156. 8. A. G. Sharpe, ‘The Chemistry of Cyano Complexes of the Transition Metals’, Academic, London, 1976. 9. H. Behrens, E. Lindner and H. Schindler, Chern. Ber., 1966, 99, 2399. 10. W. Hieber and H. Fuhrling, 2. Anorg. Allg. Chem., 1970, 373, 48. 1 I . W. Hieber and C . Bartenstein, 2. Anorg. Allg. Chem., 1954, 276, 12. 12. W. P. Griffith and A. J. Wickham, J . Chern. Soc. ( A ) , 1969, 834. 13. G . D. Venerable, I1 and J. Halpern, J. Am. Chem. Soc.. 1971, 93, 2176.
*See footnote on p. 650 concerning absolute assignments.
Cobalt 14. 15. 16. 17. 18. 19. 20.. 21. 22. 23. 24. 25.
.
’
26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82.
883
G. W. Watt and R. J. Thompson, J. Inorg. Nucl. Chem., 1959, 9, 311. G. Guastalla, J. Halpern and M. Pribanic, J. Am. Chem. Sac., 1972,94, 1575. J. Bercaw, G. Guastalla and J. Halpern, Chem. Commun., 1971, 1594. M. Bressan, B. Corain, P. Rig0 and A. Turco, Inorg. Chem., 1970, 9, 1733. J. E. Bercaw, L. Y. Goh and J. Halpern, J. Am. Chem. Sac., 1972,94, 6534. P. Rig0 and M. Bressan, Inorg. Nucl. Chem. Lett., 1973, 9 , 527. D. M. S. Mosha and D. Nicholls, Inorg. Chim. Acia, 1980, 38, 127. A. Weiss and W. Rothenstein, Angew. Chem., 1963, 2, 396. A. W. Adamson, J. Am. Chem. Soc., 1951,13, 5710. J. J. Alexander and H. B. Gray, J. Am. Chem. Soc., 1967, 89, 3356; D. Guenzburger, A. 0. Caride and E. Zuleta, Chem. Phys. Leu., 1972, 14, 239. J. M. Pratt and R.J. P. Williams, J. Chem. SOC.( A ) , 1967, 1291. G. L. Simon, A. W. Adamson and L. F. Dahl, J . Am. Chem. Soc., 1972,94,7654; L. D. Brown, K. N. Raymond and S. Z. Goldberg, J . Am. Chem. Soc., 1972, 94, 7664. D. A. White, A. J. 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Chem. Soc., 1972,94, 1563. 1145. R. A. Winograd, D. L. Lewis and S. J. Lippard, Inorg. Chem., 1975, 14, 2601. 1146. C. L.Raston, A. H. White and G. Winter, Ausr. J. Chem., 1978, 31, 291. 1147. C. Bianchini, A. Meli and A. Orlandini, Inurg. Chem., 1982, 21, 4161, 4166, 1148. C. Furlani and M. L. Luciani, Inorg. Chum., 1968. 7, 1586. 1149. G. Cauquis and A. Deronzier, J. Inorg. Nucl. Chem., 1980, 42, 1447. 1150. S. R. Hansen and S. E. Harnung, Acta Chem. Scmd., Ser. A , 1979, 33, 41. 1151. L. R. Gahan and M. J. OConnor, Aust. J . Chem., 1979, 32, 1653. 1152. M. Nakashima and S. Kida, Bull. Chem. Soc. Jpn., 1982, 55, 809. 1153. H. T. V. Hoa and R. J. Magee, J. Inorg. Nucl. Chem., 1979, 41, 351. 1154. P. Deplano and E. G. Trogu, Inorg. Chim. Acta, 1982, 61, 261, 265. 1155. P. Deplano and E. F. Trogu, Inorg. Chim. Acta, 1983, 68, 153. 1156. T.-C. Woon and M. J. O’Connor, Aust. J. Chem., 1981,34, 2039. 1157. R. Heber, R. Kirmse and E. Hoyer, Z. Anorg. Allg. Chem., 1972, 393, 159. 1158. R. P. Burns, F. P. McCullough and C. A. McAuliffe, Adv. Inorg. Chem. Radiochem.. 1980, 23, 248. 1159. W. A. Deskin, J. Am. Chem. Soc.,, 1958, 80, 5680. 1160. J. P. Fackler and D. Coucouvanis, J. Am. Chem. Suc., 1966,88, 3913. 1161. C. Bianchini, C. Mealli, A. Meli and G. Scapacci, J . Chcm. SOC.,Dalivn Trans., 1982, 799. 1162. 0. F. W i d e and K. A. Hoffmann, Z. AnorR. AIIg. Chem., 1896, 11, 379. 1163. A. Muller and B. Krebs, 2. Anurg. Allg. Chem., 1966, 345, 165. 1164. J. A. McCleverly, D. G. Orchard and K. Smith, J. Chem. SOC.( A ) , 1971, 707. 1165. D. Coucouvaniq Prog. Inorg. Chem., 1979. 26, 437. 1166. W. Kuchen and A. Judat, Chem. Bur., 1967, 100, 991. 1167. W. Kuchen and H. Hertel. Angew. Chem., Inr. Ed. Engl.. 1969,8, 89. 1168. F. N. Tebbe and E. L. Muetterties, Inorg. Chem.. 1970, 9, 629. 1169. R. G. Cavell, E. D. Day, W. Byers and P. M. Watkins, Inorg. Chem., 1972, 11, 1759. 1170. A. C. Villa, C. Guastini, P. Porta and A. A. G. Tomlinson, J . Chem. Soc., Dalton Trans., 1978, 956. 1171. M. Shimoi, A. Ouchi, T. Uehiro, Y. Yoshino, S. Sat0 and Y. Saito, Bull. Chem. SOC.Jpn.. 1982, 55, 2371. 1172. G. A. Williams and R. Robson, Aust. J , Chem., 1981,34,65. 1173. M. Calligaris, A. Ciana, S. Meriani, G. Nardin, L. Randaccio and A. Ripamonti, J. Chem. Soc. ( A ) , 1970, 3386. 1174. A. Muller, P. Christophlicmk and V. V. Krishna Rao, Chew. Ber., 1971, 104, 1905. 1175. D. E. Goldbcrg, W. C. Fernelius and M. Shamma$Inurg. Synth., 1960, 6, 142. 1176. C. K. Jorgensen, Acra Chem. 8:und.. 1962, 16, 2017. 1177. A. A. G. Tomlinson, J. Chum. Soc. ( A ) , 1971, 1409. 1178. A. A. G. Todinson, L. Mattogno, V. Di Castro and C. Furlani, Inorg. Chim. Acta, 1978, 26, L l l . 1179. L. L. Costanzo, I. Fragala, S. Giuffrida and G. Condorelli, Inorg. Chim. Acta, 1978, 28, 19. 1180. E. Hoyer, W. Dietzsch and W. Schroth, Z. Chem., 1971,11,41. 1181. R. P. Burns and C. A. McAuliffe, Adv. Inorg. Chem. Radiochem., 1979, 22, 323. 1182. J. A. McCleverty, Prog. Inorg. Chem., 1968, 10, 49. 1183. H. B. Gray, in ‘Transition Metal Chemistry’, ed. R. L.Carlin, Edward Arnold, London, 1965, vol. 1, p. 239. 1184. H. B. Gray, R. Eisenberg and E. I. Stiefel, Adv. Chem. Ser.. 1967, 62, 641. 1185. E. L. Muetterties, J. Am. Chem. Soc., 1968, 90,5097. 1186. I. G. Dance, Inorg. Chem., 1973, 12, 2381. 1187. A. Vlcek and A. A. Vlcek, J. Electroanal. Chem., 1981, 125, 481. 1188. I. G. Dance and T. R. Miller, Inorg. Chem., 1974, 13, 525. 1189. C. H. Langford, E. Billig, S. I. Shupack and H. B. Gray, J. Am. Chem. Soc., 196486,2958. 1190. A. L. Balch; Inorg. Chem.. 1971, 10, 388. 1191. D. M. Dooley and B. M. Patterson, Inorg. Chum., 1982, 21,4330. 1192. A. H. Maki, N. Edelstein, A. Davison and R. H. Holm, J. Am. Chem. Soc., 1964,%, 4580. 1193. R. H. Holm, A. L. Balch, A. Davison, A. H. Maki and T. E. Berry, J. Am. Chem. Soc., 1967,89,2866. 1194. P. J. Van der Put and A. A. Schilperoord, Inorg. Chem.. 1974, 13, 2476. 1195. A. Vlcek and A. A. Vlcek, Inorg. Chim. Acta, 1982, 64,L273. 1196. A. Vlcek, Inorg. Chim. Acta, 1980, 43, 35. 1197. A. Vlcek and A. A. Vlcek, Inorg. Chim. Acta, 1980, 41, 123 1198. T. N. Lockyer and R. L. Martin, Prog. Inorg. Chem., 1980, 27, 223. 1199. R. L. Martin and I. M. Stewart, Nature (London), 1966, 210, 522. 1200. A. Ouchi, M. Nakatani and Y. Takahashi, Bull. Chem. SOC.Jpn., 1968,41,2044. 1201. T. Takahashi, M. Nakatani and A. Ouchi, Bull. Chem- Soc. Jpn.. 1969, 42, 274. 1202. R. Beckett and B. F. Hoskins, J . Chem. Soc., Dalron Trans., 1974, 622. 1203. K. M. Erck and 3. B. Wayland, Inorg. Chem., 1972,11, 1141. 1204. J. F. White and M. F. Farona, Inorg. Chem., 1971, 10, 1080.
900 1u15.
1206. 1207. 1208. 1209. 1210. 1211. 1212. 1213. 1214. 1215. 1216. 1217.
Cobalt R. K. Y.Ho and R. L. Martin, Ausr. J. Chem., 1973, 26, 2299. A. R. Hendrickson, R. K. Y. Ho and R. L. Martin, Inorg. Chem., 1974,13, 1279. G. A. Heath and R. L. Martin, A u t . J. Chem., 1971, 24, 2061. J. Meyer, G. Dirska and F. Clemens, Z . Anorg. Allg. Chem., 1934, 139, 333. C. F’reti, G. Tosi, D. De Filippo and G. Verani, J. Inorg. Nucl. Chem., 1974, 36, 2203. C. Preti and G. Tosi, Inorg. Chem.. 1977, 16, 2805. C. A. Stein, P. E. Ellis, R. C. Elder and E, Deutsch, Znorg. Chem., 1976, 15, 1618. T. Konno, K. Okamoto and J. Bidaka, Chem. Lett.. 1982, 535. T. Konno, K. Okamoto, H. Einaga and J. Hidaka, Chem. Lett., 1983,969. K.Okamoto, T. Konno, M. Nomoto, H. Einaga and J. Hidaka, Chem. Lett.. 1982, 1941. K. Nakajima, M. Kojima and J. Fujita, Chem. Lett., 1982, 925. D. R. Jones, L. F. Lindoy and A. M. Sargeson, J. Am. Chem. Soc., 1983, 105, 7327.
D. A. Buckingham and C. R. Clark, unpublished results.
_,.
I .
48 Rhodium FRED H. JARDINE North East London Polytechnic, London, UK
I
and PETER S. SHERIDAN Colgate University, Hamilton, NY, USA 902 904 905 905 905 906
48.1 INTRODUCTION 48.2 RHODIUM( - I) COMPLEXES 48.3 RHODIUM(0) COMPLEXES 48.3.1 [Rh, (PF,),f 48.3.2 [Rh( PPh,), ] Complexes 48.3.3 [Rh(diphos),]
906 906 906 906 908 908 912 914 918 92 1 92 I 924 924 925 925 926 929
48.4 RHODIUMQ) COMPLEXES , 48.4.1 Anionic Complexes 48.4.2 [RhXL,] Complexes 48.4.2.1 Three-coordinatecomplexes 48.4.2.2 Complexes containing bidentate anions 48.4.2.3 Bridged complexes 48.4.2.4 [RhX(LL)] complexes 48.4.3 [RhXL, f Complexes 48.4.3.1 [RhXL,L'] complexes 48.4.4 [RhXL,] Complexes 48.4.4.1 [RhHL,] complexes 48.4.4.2 Other [RhXL4j complexes 48.4.4.3 [RhH(LL),] complexes 48.4.5 [ R h L J X Complexes 48.4.5.1 Complexes containing monodentate ligan& 48.4.5.2 Complexes containing bidentate ligands 48.4.6 [ R h L S ] X Complexes 48.5 RHODIUMQI) COMPLEXES 48.5.1 Monomeric Complexes 48.5.1.1 Tertiary phosphine complexes 48.5.1.2 Tertiary arsine complexes 48.5.1.3 Tertiary stibine complexes 48.5.2 Dimeric Complexes 48.5.2.1 Synthesis and structure 48.5.2.2 Bonding and electronic structure in Rh" dimers 48.5.2.3 Reacrivily of Rh" dimers 48.5.2.4 Miscellruwous polynuclear complexes of R.4"
930 930 932 933 933 93 3 934 944 947 951
48.6 RHODIUM(II1) COMPLEXES 48.6.1 Group IV Ligands 48.6.1.1 Cyano complexes 48.6.1.2 Miscellaneous group I?' complexes 48.6.2 Nitrogen Ligan& 48.6.2.I Monodentate amines 48.6.2.2 Bidentate aliphatic amines 48.6.2.3 Photochemistry of monodentate and bidentate' amine complexes 48.6.2.4 Polydentate aliphatic amine complexes 48.6.2.5 N-heterocycle complexes 48.6.2.6 Halonitrile complexes 48.6.2.7 Porphyrin and phthalocyanine complexes 48.6.2.8 Miwellaneow nitrogen ligand complexes 48.6.3 Complexes of Heavier Group V Ligands
952 952 952 953 953 953 965 980 989 995 1005 1006
90 I
1012 1015
I Rhodium
902
48.6.3.1 Anionic complexes 48.6.3.2 [ R h X , L ] complexes 48.6.3.3 /RhX3L,] complexes 48.6.3.4 [RhH,XL,] and [RhHX,L,] complexes 48.6.3.5 [RhX3L3]complexes 48.6.3.6 [RhX,L,]X complexes 48.6.3.7 [RhX, (LL),]X' complexes 48.6.3.8 Complexes containing polydentate ligands 48.6.4 Mi.xed A-0 and N-S Ligand Complexes 48.6.4.1 N - 0 ligand complexes 48.6.4.2 N-S bidentate complexes 48.6.5 Oxygen Ligands 48,63,1 The hexaaqua ion 48.6.5.2 The tris(oxu1ato) ion 48.6.5.3 Acetylacetonuto complexes 48.6.5.4 Dioxygen complexes 48 6.5.5 Miscellaneous oxygen ligands 48.6.6 S+r and Heavier Group VI Ligands 48.6.6.I Monodentate surfur ligands 48.6.6.2 SuQiur chelates 48.6.6.3 Miscellaneous group VI ligands 48.6.7 Hexahalo and Aquahalo Complexes
i015 1016 1017 1026 1028 1032 1035 1040 1044 1044 1048 1049 1049 1049 1050
1052 1053 1054 1054 1054 1056 1057
48.7 RHODIUM(1V) COMPLEXES 48.7.I Anionic Complexes 48.7.2 Neutral Complexes 48.7.3 Miscellaneous Rh" Compkxes
1061 1061 1062 1062
RHODIUM(V) COMPLEXES 48.8.I Anionic Complexes 48.8.1.1 [RhF,]48.8.1.2 0.ryanions of RhV 48.8.2 RhF,
1063
48.8
48.9 RHODIUM(V1) COMPLEXES 48.10 MISCELLANEOUS RHODIUM COMPLEXES 48.10.1 Complexes of Mixed Oxidation State 48.10.2 Nitrosyl Complexes
48.10.2.1 [ R h ( N O ) L 3 ]complexes 48.10.2.2 [Rh ( N O )X,L,] complexes 48.10.2.3 Cationic complexes 48.10.2.4 Thionitrosyl complexes 48.10.3 Bimetallic Complexes 48.11
REFERENCES
1063 1063 1063 1064 1064 1065 1065 1065 1066 1068 1072
1074
1074 1077
48.1 INTRODUCTION
Rhodium, as befits an element of the platinum metal sextet, occurs mainly as a minor constituent of platinum group metal ores. Thus, it occurs as a component of the ore deposits at Sudbury, Ontario, in the Merensky reef at Rustenberg in the Republic of South Africa, and in the platinum metals' ore deposits in the Ural range of the USSR. The major production centers are the Urals and South Africa, since the Sudbury ores in particular have a very low rhodium content. Minor deposits of the metal ores which contain some rhodium also occur in British Columbia, the United States of America, Columbia, Spain, Borneo and Australia. Nevertheless, rhodium is an exceedingly rare element with an abundance in the earth's crust of some lo-'%. The element was first isolated by Wollaston in 1804 who obtained it from crude platinum. The crude platinum was of Spanish origin and Wollaston complained of the mercury added by the Spaniards in an attempt to remove the gold from the material.' Within a decade the first rhodium complex, [RhC1(NH3),]C1,, had been prepared by Vauquelin? Later this complex was studied by CIaus whose name is often associated with the salt.3The naming of a complex after a later worker in the field has since become something of a tradition in rhodium chemistry. The stereochemistry of rhodium(II1) complexes was established by Werner who prepared the cation [Rh(en),13+ and resolved the racemate via its (+)-3-nitrocamphorate or (+)-tartrate salts.' However, progress in rhodium complex chemistry was slow, and it was not until the early 1940s that Dwyer and Nyholrn',' prepared further examples of rhodium complexes. These included the
Rhodium
903
tertiary arsine complexes, which were later to exert such an important influence on the direction rhodium chemistry would take. Again rhodium chemistry languished for two decades. In the 1960s four disparate lines of research suddently burgeoned, making the chemistry of the element one of the most active research areas of the past two decades. The kinetic and thermodynamic studies on the octahedral rhodium(II1) complexes’ suddenly gained momentum. This field was later to give rise to the important discovery of the photosensitivity of rhodium complexes.’ A totally new field of rhodium chemistry was opened up by the discovery of the remarkable ~.~~ important complex has sticatalytic properties of [RhCl(PPh,),] by Wilkinson’s ~ c h o o l .This mulated a rapid expansion in the chemistry of the lower oxidation state complexes. Much of this interest has, inevitably, spilled over into the organometalic chemistry of the element and is thus outside the scope of this chapter. The facile changes of oxidation state exhibited by rhodium complex catalysts pointed the way to the employment of rhodium complexes in the photochemical decomposition of water.” The interest in the lower valence states of rhodium also led to the synthesis of the rhodium(I1) carboxylates,” which, although shown to be diamagnetic, excited little interest until the end of that momentous decade when an X-ray crystal structure revealed the source of the diamagnetism to be their rhodium-rhodium bonds.’, However, the rapid pace of rhodium chemistry has not been fully reflected in the review literature. The two extant English language reference works have been well overtaken by There is an invaluable compendium of the early work by Griffith,I6and a new edition of Gmelin’s handbook has been published very recently.” However, apart from these last two books and the ever increasing space devoted to the chemistry of the element in undergraduate texts there has been no systematic attempt to review the chemistry of this element apart from its organometallic compounds, which form the subject of the companion series.” To some extent this situation has been rectified by a number of reviews of more limited range. Among these are the review by Robinson on the rhodium(I1) carboxylates,’’ and the reviews on the chemistry of [RhCl(PPh,),]20and [RhH(CO)(PPh,),].21 The tertiary phosphine, arsine and stibine complexes of the element have also been covered in two reviews of these ligands’ complexes with the transition elements.22 Apart from catalytic reactions in organic and industrial chemistry there are few practical applications of rhodium complexes. Recently some interest has been shown in the use of rhodium(I1) carboxylates in chemotherapy. Generally it appears that the rhodiurn(T1) Complexes are less effective although they also appear to induce less unwelcome side effects than than those of plalin~rn(J1),2~ do the platinum(T1) complexes. They may have some promise as radiation sensitizers, although this is more probably due to the high atomic number of rhodium than any specific pharmaceutical properties. As might be expected, the lipophilicity increases with increasing chain length of the carboxylato ligands’ alkyl group. This increases the body burden of rhodium, which in turn increases both the toxicity and antitumor activity of the rhodium complex. However, it has been stated that too great an increase in the chain length (i.e. beyond the pentanoate) reduces the therapeutic proper tie^.^^ It seems that optimal properties are shown by the b~tyrate.’~-*~ The mode of action of the carboxylates is, as yet, uncertain. It is believed that acetate is mainly decomposed in the liver. The labelled acetate produces 14C02,but only 5% of the rhodium is eliminated in the urine in the first 24 hours after admini~tration.~~ The acetate, propionatez8and butyrate29inhibit DNA and RNA syntheses. This has been demonstrated for the butyrate by in vivo studies on Erlich tumor cells in mice. It is possible that rhodium(I1) acetate owes its antineoplastic activity to its inhibition of the enzymatic deamination of arabinosylcytosine. The latter is an antineoplastic agent in its own right, and it has been suggested that its joint administration with rhodium(I1) acetate might prove efficaci~us.~’ The complexes of rhodium will primarily be considered in order of increasing oxidation state. However, complexes in which the oxidation state of the metal is uncertain (e.g. nitrosyls) or dinuclear complexes of mixed oxidation state will be dealt with last. The complexes wili be assigned to sections according to the number of neutral ligands they contain. Polydentate ligands will, for the purposes of assignation, be considered to be equivalent to the number of donor atoms. However, polydentate ligands’ complexes will normally be considered after those of similar monodentate ligands. For example, discussion of the properties of [Rh(NH,),]’+ will precede that of the reactions of [Rh(en)?I3+.Mononuclear complexes will precede bridged complexes. Within these classes the donor atom will determine the order to citation and
Rhodium
904
within each of these sub-classes the anionic ligand will then determine the order. Polyatomic anions will be cited in the order appropriate to their central atom. The final determinant of precedence will be increasing relative molar mass, which can generally be equated with complexity of the ligand set. For example mer-[RhCl,(PMe,),] will be discussed before mer-[RhCl,(PPr,),], which in turn will be followed by mer-[RhCl,(PPr\),]. To this end, invoking the complexity criterion, PR, complexes will, for example, precede PR'R, complexes. All complexes will be included at the first opportunity. Thus, [RhC1,(H,O)j2- would be considered to be the complex of an oxygen ligand rather than of five chloro ligands. This order will be adhered to except where its slavish implementation would lead to extensive cross referencing and unnecessary duplication of material. The bald order will be supplemented by sundry reaction schemes since, historically, many areas of rhodium chemistry have been developed by preparing derivatives of some dozen or so key complexes, and it would be unwise to discard this aspect of the subject.
48.2
RHODIUM(-I) COMPLEXES
With one exception the only non-carbonyl complexes in this oxidation state are those of the [Rh(PF,),]- ion. Complexes of this ligand were reviewed by Nixon in 1970a3' The salts of this ion, which is presumably tetrahedral like the isoelectronic anion [Co(CO)J, can be obtained by neutralizing [RhH(PF,),] with bases (equation l).3* [RhH(PF3)41
+ PY
4
(PYH)[R~(PF,)~I
(1)
The potassium salt can be prepared by reducing [Rh,(PF,),] with potassium amalgam (equation 2),32-35 or by allowing the latter reagent to react with the hydridorhodium(1) complex (equation
-
3).32,35
[Rh(PF,hI [RhH(PF,),]
+
+
2K/Hg
2K/Hg
2K[Rh(PF,)41
2K[Rh(PF,)d]
+
12)
+ HZ + Hg
(3)
The dimeric chloro complex [RhCl(PF,),], can be similarly reduced.36 Large cations form isolable salts of the [Rh(PF,),]- anion (equations 4 and 5).32 [Rh(PF3141
+
wWF3)'il
--
[Fe(~hen>~l~'
+
I~O(CP>,l+
[ F d ~ h d1[Rh(PF3 , 1412
(4)
[Co~CP>,l[WPF3)41
(5)
The reactions of the [Rh(PF,),]- anion are showing in Scheme 1. The most noteworthy reactions include the formation of metal-metal bonds with tin33,36and gold." Other [LnM-Rh(PF3),] complexes can be prepared by different routes (see Section 48.10.3). The anion is oxidized by SnCb, TlCl, or {RhCl(PF3)2],.33
i, Ph,SnC1;3bii, [P~,PAuCI];'~ iii, TICll or S ~ I C ~ ,iv, ; ~ [RhCI(PF,),], ~ Scheme 1 Reactions of [Rh(PF,),J-
+
PF3;33v, 50% H,S0434
I
Rhodium
905
The only other non-carbonyl rhodium( - I) complex that has been isolated is the closely related [Rh(PF,NMe,),]- . The potassium salt can be obtained by reducing [RhCl(PF,NMe,),] with potassium amalgam.3'
48.3 RHODIUM(0) COMPLEXES
48.3.1 [Rhz(PF,),] This complex can be prepared by allowing PF, to react with anhydrous rhodium trichloride and copper powder under severe conditions (equation RhCl,
+ 6Cu +
8PF3
*:;:
[Rh2(PF3)J2 -t 6CuCl
(6)
A more convenient preparation is to oxidize K[Rh(fF,),] with [RhCl(PF,),], (Scheme 1, Section 48.2 above).33The diamagnetic, orange-red crystals of the product melt at 92.5OC.3' The dimeric formulation (1) is consistent with its diamagnetism, and with the "F and 31PNMR spectra recorded Poor crystal quality prevents the solid state structure being at temperatures below - 15°C.33,38
dete~mined.~'
[C13SiRhU'F3)41
[Ph,GeRh(PF,hI
\
/
1
iv
/
\
[RhH(PF3h1 + [Ph3SiRh(PF3),1
[RhH(PF,hl
viii
48.3.2 [Rh(PPh,),,] Complexes Pale green [Rh(PPh,),], can be prepared by the polarographic reduction of [RhCI(PPh,),] in acetonitrile. The product is believed to be dimeric since it is diamagnetic. Attempts to demonstrate the presence of an Rh-H bond in the material failed.3y*40 The complex can also be prepared by electrochemically reducing solutions of RhC13*3H20 in the presence of triphenylphosphine.4' There is a patent claiming that magnesium amalgam reduced [RhCl(PPh,),] to [Rh(PPh3)2]2.4' However, since the high field 'HNMR spectra of these complexes have not been obtained, there
906
Rhodium
must be some doubt as to their authenticity. They may be hydridorhodium(1) complexes or even o-metallated rhodium(1) complexes. 48.3.3 [Rh(diphos), 1 The ionic complex [Rh(diphos)JCl can be reduced electrochemically in a variety of solvents to form the reactive species [Rh(diphos),]. The usual fate of this species is to abstract a hydrogen atom from the solvent and form [RhH(diphos),]? There is one instance where attack on the electrolyte, [Bu4NJ[C1O4],has been 0bserved.4~Another school disputes this mechanism of [RhH(diphos),] formation, and rightly questions the validity of deuterium labelling experiments which use alkaline mixtures of H 2 0 and D20.&
48.4 RHODIUM(1) COMPLEXES 48.4.1 Anionic Complexes Rhodium does not readily form anionic complexes in its lower oxidation states, probably because n-acid ligands are usually required to disperse the electron density from the metal in neutral or even cationic complexes. It is not surprising, therefore, that the few examples of rhodium(1) anionic complexes contain powerful n-acid ligands. The [RhCI(SnCl,),]:- anion can be obtained by allowing [SnCl,]- -to reduce rhodium trichloride (equation 7). The tetramethylammonium salt has been isolated. 2RhCI,
+
G[SnCl,]--
3M HCI
[RhCl(SnCl,)J-
+
2ISnClJ
(7)
Tetramethylammonium chloride cleaves the dimeric complex [RhC1(PF,)2]2 and forms [Me,~[RhCl,(PF3j21.36 48.4.2 [RhXI,]Complexes There are three principal types of complex which share this stoichiometry. The first consists of authentic three-coordinate complexes containing three monodentate ligands. The second type comprises complexes of mononegative bidentate anions. Tn the last type rhodium achieves four coordination by forming bridged species of general formula [RhXL,],. Complexes where one of the neutral ligands is untypically bidentate are discussed in Section 48.4.2.4.
48.4.2.1
Three-coordinate complexes
These complexes (see Table 1j are formed by very bulky, electron-donating tertiary phosphine ligands since even relatively large ligands of this type from RhXL, complexes (e.g. [RhCl(PF'h,),]j. Electron-withdrawing ligands give rise to halo-bridged complexes, for example [RhC1(P(C6F,),},],. There has been considerable interest in the synthesis and properties of such three-coordinate Complexes since they are isolable analogs of the important catalytic intermediate [RhCI(PPh312]. Bis(tricyclohexy1phosphine) complexes are probably the most accessible and hence widely invesTable 1 Physical Properties of [RhXL,] Complexes Complex
a With
decomposition.
Color
M.p. ("C)
Ref
Rhodium
907
tigated three-coordinate complexes. Numerous PCy, complexes can be prepared from [RhF(C, HI,),], (equation 8).46*47 r
The bromo and iodo complexes can also be prepared from [RhC1(C8H,,)2]2.47 The NMR spectrum shows [RhCl(PCy,),] to be in equilibrium with [RhCl(PCy3),]2.46 The bis(tricyclohexy1phosphine) complexes are 14-electron species and are highly reactive. They are easily oxidized by air,4sand even decompose when stored in an inert atmosphere. However, they will readily coordinate an additional small ligand. Many of these small ligands bind through carbon and are, therefore, outside the scope of this review. Both d i n i t r ~ g e n ~and ~ , , ~sulfur dioxide form adducts (equations 9 and
The Auoro complex does not react with dinitrogen, but the degree of reactivity increases with the atomic number of the halo ligand. The sulfur dioxide complex (2) has been shown LO contain an S-bound sulfur dioxide ligand.5’
T
Cy3P-Rh-pcY3
1
(2)
The RhXL, complexes can also be prepared by expelling one neutral ligand from RhXL, complexes. Tricoordinate or weakly solvated square planar complexes are known to exist in so l~ ti o n ,’ ~ but ~ ’ ~ unsolvated examples have seldom been isolated. The phenolate complex [Rh(OPh)(PPh,)2] can be prepared by recrystallizing the tris(tripheny1phosphne) complex from toluene, but it may be dimeric with anion bridges.” The best examples of this method of preparation are those of [RhHLJ complexes. These three-coordinate hydrido complexes are obtained due to the high trans effect of the hydrido ligand labilizing the third neutral ligand in the four-coordinate precursor. Several {RhH(PR,),] complexes can be obtained from the dinitrogen complexes [RhH(N2)(PR,)3].55.56 The corresponding PCy, complex can be obtained by decomposing trans[RhHN,(PCy,),] at 60°C (equation 1l).”
At lower temperatures dinitrogen coordinates to [RhH(PCy,),] forming the four-coordinate mononuclear complex [RhH(N,)(PCy,),].46,47.49355556 Less commonly, three-coordinate complexes can be obtained containing a large anion. The yellow three-coordinate complex [Rh(N=C(CF,),}(PPh,),] can be prepared by allowing [Me, Sn (N=C(CF,), ]] to react with [RhC1(PPh,),].s7 Poorer yields are obtained when [LiN=C(CF,),] is used. Presumably the anion is bound solely through nitrogen as it is in [Rh (N=C(CF,), } (C(NMeCH,), )(PPh,),] whose structure has been determined by X-ray crystal10graphy.’~ The green bis(trimethylsily1)amido complex can be prepared by allowing [RhCl(PPh,),] to react with LiN(SiMe,), in THF.59It will be noted that neither of these anions can undergo 8-hydride abstraction reactions that destroy simpler amido complexes.
908
Rhodium
48.4.2.2
Complexes containing bidentare anions
The pentan-2,4-dionato (acac) complexes may be prepared either by cleaving dinuclear penran2,4-dionato complexes with tertiary phosphines,60.61or by displacing the halo ligands in bridged, triphenyl phosphite or PF2(NEt2)Complexes (equations 1 2-16).61,62
+ 4PR3 -,
[Rh(acac)(C,H4),l2
2[Rh(acac)(PR3),1
-
+ 4C2H4
(12)
R3 = Ph,,Ph,Me@' [Rh(acac)(C8H14)z]z R,
+ 4PR3 =
2[Rh(acac)(PRdZ1+
4G H14
(13)
PF,NEt,, PF(NMe,)261
[Rh(a~ac)(CO)~]+ 2PR3
[Rh(acac)(PR,)J i- 2CO
+
(14)
R3 = PF2NEt2,P(NMe2)361 [RhX{P(OPh)3)2]2
+
2acacH
,
2[Rh(a~ac){P(OPh),}~]
(15)
2[Rh(acac)(FF2NEt2),]
(16)
X = C1, Br6' [RhCl(PF,NEt,)J,
+ 2acacH
+
Closely similar is the preparation of the N-methylsalicylaldiminato complexes (equation 17).63 [RhCl(C,H,),], f 2Tl(SalNMe)
0 + 4PR3 d Et
R,
=
2[Rh(SalNMe)(PR3)2J
+ ZTlCl + 4CzH4
(17)
Ph,, Ph,Me6,
Orange crystals of the trifluoroacetato complex [Rh(0,CCF3){P(OMe)3>,I can be obtained if the trimeric hydrido complex [RhH(P(OMe),},], is allowed to react with CF,C02H in diethyl ether." Generally, however, this class of complex is the preserve of the disulfur anion. If [RhCl(PPh,),] is allowed to react with ammonium dialkyldithiophosphinates or diaryldithiophosphates in benzene, the products are [Rh(S, PCy,)(PPh3),] and [Rh(S,P(OPh), )(PPh,),] respectively (equation 18).6s [RhCl(PPh,),]
+
NH4S2PR2
,
R
=
[Rh(S,PR,)(PPh,)J
+
PPh3
+
NbCI
(18)
cy,OPP
Other di- or mono-thioanions (3HS) form complexes.66 The secondary phosphinethiol MeCH(SH)CH, PHPh also displaces the chloro and one triphenylphosphine ligand from [ R h c l ( P P l ~ ~when ) ~ ] it is dissolved in aqueous ethanolic ammonia.67The structures of these complexes have been inferred from their 'H and 31P NMR spectra.
48.1.2.3
Bridged complexes
These complexes can be prepared from phosphorus ligands that are themselves substituted by electron-withdrawing groups. It is often beneficial, however, to employ a low ratio of phosphorus ligand to rhodium since in several instances excess ligand cleaves the halo bridges and forms four-coordinate monomeric complexes. Table 2 lists those complexes of this structure that have been isolated. Many of the complexes can be prepared by displacing ethylene from Cramer's compound (equation 19). However, it is more difficult to displace all the cycloocta-1,5-diene from [RhCl(cod)], (equation 20); unless excess ligand is used an intermediate cyclooctadiene complex is formed."
Rhodium
909
Table 2 Physical Properties of (RhXL,], Complexes
X C1 SCN
L
Color
-
Salmon Beige
PPhl
c1 c1
Red-brown
CI CJ C1
Orange-brown Dark red Green Dark red Dark red
c1 Br C1 Br
I
CI Br
Dark red Purple-black
C1 H
Yellow Green
Br SCN PF2
Yellow Yellow Yellow -
-
c1 c1
Br
I Q
c1 C1
I
c1
c1
c1 a With
PF; PF, PF3 PF2CC13 PF2NMe, PF,NEt2 PF,NEt, PF, (Ofr) SPPh, PFS PhZN,
24043a 153 151-53
Oil
Rd
Dark red orange
Yellow Yellow
76 64 62, 77 62, 77 78 32 33, 34, 79, 80 79 79
80 61, 81 82 61 76 83, 84 85
112-13 103-04
Oil
Orange Brown -
+
14
150-75 140 70 6142 6243 118-19
Yellow
68 69 70 71 71 72
73, 74 74 74 75 75 74
216-18 175-85 180-85 300 300 219-21 190-93 205 123"
-
Red
Re$
M.p. ("C)
300
-
decomposition. b R= (MeSiO),MeSiCH,CH,,
The enthalpies of these exothermic reactions have been determined for chloro, bromo and iodo species.86 Other complexes that have been obtained from the cyclooctadiene complex include [RhC1(PF3)2]2,36 [RhCl(P(OMe),},], and [RhCI{PF, (OPr)}2]2.76 It is even more difficult to prepare [RhClL,], complexes from [RhCl(CO),], (equation 21). Only the stronger .n-acid ligands are capable of totally displacing the carbonyl ligands of the starting material. The triphenyl phosphite7' and PF, complexes79can be made in this way. [RhCl(CO),]2
+ 4L
-*
[RhClb],
+
4CO
(21)
L = PF3,79P(OPh),, P(p-OCsH,Me),, P(p-oc6H,,c1)3'8
The mixed Iigand complex [Rh2C1,(PF,)2(PhN,Ph),] was prepared by equilibrating the two dimeric complexes (equation 22).*' [RhCl(PF3)2]2
+
[RhCl(PhN2Ph),],
* 2[RhCl(PF3)(PhN2Ph)],
(22)
The complexes can also be prepared from other monomeric rhodium(1) complexes. Both the triphenylphosphine and the tri(p-toly1)phosphine complexes can be prepared by refluxing solutions of the appropriate [RhCl(PAr,),] complex in an inert atmosphere (equation 23).6','' Alternatively one ligand of an [RhXL,] complex can be oxidized (equation 24).87,88 2[RhC1(PAr3),]
[RhCl(PAr,),],
Ar = Ph,6Bp-CsH4Me7"
+ 2PAr3
(23)
Rhodium
910 2[RhCl(CO)(PPh,),]
+
Bu'OOH
--*
[RhCI(PPh,),],
+
2Ph3P0
(24)
Finally several complexes can be prepared by utilizing the ligand to reduce hydrated rhodium trichloride in homogeneous solution (equation 25).71,7f75,83,84 2RhC13*3H,0 + 6L
[RhCIL2I2 -i- 2LO
(25)
P(CgF5)3:3"4 PPhz(CsF~h'~ PPh;!(CgC15)75
L = (9),
(10)
(9)
The kinetics of the formation of [RhCl(PPh,),], from [RhC1(PPh3),] have been s t ~ d i e d . ~The '.~~ dimeric complex has the planar structure (11); this is similar to that found for [RhCl(cod)],, but differs from the folded structure adopted by [RhC1(CO),]2.93Details of the far IR spectra, which indicate similar halo-bridged structures are collected in Table 3. Of all the complexes detailed above, those containing fluorine-substituted ligands have been the most studied. Their 31PNMR spectra are summarized in Table 4. The dimeric complexes can undergo bridge cleavage, neutral ligand exchange, or anionic ligand exchange when allowed to react with suitable reagents. Again the reactions of [RhCl(PF,),], have been most widely studied. These reactions are summarized in Scheme 3. Triphenyl phosphite complexes undergo several of the reactions of the PF3complexes and, unlike the latter, are cleaved by treatment with excess ligand (equation 24). In the presence of a noncomplexing anion the chloro ligand is displaced.62 Excess triphenyl phosphite will convert [RhC1{P(C6F,)3)J2 to the triphenyl phosphite complex (equation 27). Triphenylphosphine reacts similarly.73374
(11) Table 3 Rh-CI
Stretch Frequencies in (RhClL,], Complexes
'
Vm-a (cm- )
Complex
Frequency
4F
Ref.
Solvent
(MHz)
(p.p.m.)
J (P-F) (Hz)
J (Rh-F) (Hz)
ReJ
CHC12 CHCI, C6H6 CHCI, CHCI,
56.47 56.47 56.47 56.47 56.47 56.47 94.1 94.1 94.1 56.47
+ 16.0 + 17.7 + 15.2 + 13.9 + 15.6 + 17.2 + 17.1 + 15.9 + 14.2 + 20.5
1340 1311' 1 3Wb 1316b a
31.6 31.5 31.5 31 29.8
89 89 89 89 89 33 79 79 79 33
C6H6 C6H6 C6H6
C6H6
C6H6
i, +PF,, -l2O'Cs9 ii, -PF,, 27°C:' iii, C,H,, benzo[c]cinnoline, 80"C;90iv, PPh,:k7'.w.m v, PPh3;89vi, AsPh,;SP vii, diph0s;8~viii, P(OPh),, i-CsH,2;89ix, CO;7nx, excess PF,;" xi, [ B u , N ~ C xii, ~ ; ~C1,CPFZ;"" ~ xiii, Tl(acac);3L xiv, K/Hg;36 xv, [RhC1(PhN2Ph),]z:5 xvi, PPh,, 25"C;4]xvii, excess PPh3;n9xviii, AsPh3w Scheme 3 Reaction of [RhCI(PF,),],
[RhX(P(OPh),},],
+
2P(OPh),
-
----f
2[RhX{P(OPh),],]
(26)
X = C1, Br6'
+
[RhCl(P(C6F5)3}2]2 6L
L
=
2[RhC1L3]
+ 4P(C6F5h
(27)
P(OPh)3,73PPh374
The chloro ligands of [RhC1{P(OPh),}2]2can also be displaced by pentan-2,4-dione."~~~ The dialkylamidodifluorophosphine complexes are also cleaved when allowed to react with excess ligand. However, these reactions are easily reversed. The yellow dimethylamidophosphine complex is also cleaved by triphenylphosphine and reduced by potassium amalgam.37 The triphenylphosphine dimer is not very soluble, and this together with its ready reaction with oxygen has deterred investigations of its chemical properties. Whilst liquid sulfur dioxide cleaves the molecule and forms [RhC1(S02)(PPh,),],96 solutions of SO, in ethanol/chloroform yield the cenClosely trosymmetric dimer (12),97whose structure has been determined by X-ray cry~tallography.~' similar to this complex is the cation (13) which can be prepared by passing sulfur dioxide into a methanolic solution of [RhCl(cod)],, NaClO, and PPh,. The orange perchlorate salt has had its structure determined by X-ray crystall~graphy.~~
The allyl complex [Rh(q3-C,H,)(cod)] reacts with trialkyl phosphites to form [RhHL'], complexes, The dimeric triisopropyl phosphite complex has already been mentioned. Trimethyl and triethyl phosphite, however, form trimeric products.My'OO The structure of the trimethyl phosphite complex (14) has been determined by X-ray Finally there is a report of an ill-characterized benzothiazole complex [RhC1(C7H,NS)JZ.The benzothiazole ligands may be N-bound.Io3
Rhodium
912
48.4.2.4 [ R h X t L L ) ] complexes
There are a few analogs of the bridged complexes described above which contain bidentate neutral ligands in place of the four monodentate neutral ligands. They may be prepared by careful displacement of cycloocta- 1,5-diene ligands from [RhX(cod)12 (equation 29j.L04.105 Other complexes have been prepared by removing the single cyclooctadiene ligand from [RhCl(LL)(codj], complexes by hydrogenation (equation 30).'06 [Rh(PPh,)(cod)],
+ 2LL
+
[Rh(PPh,)LL],
+ CSH12
(28)
LL = Ph2PCH2PPh2,diphos, Ph2P(CH2)3PPh2,Ph2P(CH2)2A~Ph2lW [RhCl(cod)12
+
(15)
[RhLL(cod)]Cl
+
3 [RhCl(15)], + H2
+
[RhCILLIl
+
2C8H,,
(29)
(30)
CSH14
LL = diphos, (16), diopLo6
When the diarsine ligand (17) is allowed to react with rhodium trichloride in refluxing DMF, the bridged complex [RhCl(17)I2is formed (equation 31).'07 RhC13*3H20$- (17)
>
[RhCl(17)12
(3 1)
(17)
The complexes prepared by hydrogenation of their cyclooctadiene precursors may be solvated ionic monomers. This type of complex is formed if the anion is non-coordinating. For example, complexes of the ligand (18) can be obtained containing two methanol ligands (equation 32). However, the unsolvated complex [Rh,(18),][C104], is the minor product in the reaction. It seem quite likely that this complex contains an $-phenyl group, as has been demonstrated €or the corresponding diphos complex.loa,lm 10[Rh(18)(nbd)]C104
+
20H2
8[Rh(18)(MeOH),]C104
+
[Rh(18)],[C104],
+
10C7HI2(32)
Recently, hydrogenation of the q3-allylcomplex [Rh(C2H4P2(OR)4}(q3-C4H7)J (R = Me, Et, Pr') has yielded [RhH{C,H,P,(OR),)], complexes (R = Me, Et, n = 4;R = Pr', n = 2) (equation 33)."' I R ~ { ( R O ) ~ P ( C) H 2W ~ R ) ,ICs' -GHT 11
H, 3atm
C,Hg
+
tRhH{ (RO),P(CH2),P(OR),}l,
(33)
If the anion is bidentate then bidentate neutral ligands form [RhX(LLj] complexes. Strangely, only one example of such a complex is known. The orange 5-methyl-8-hydroxyquinolinatocomplex
Rhodium
913
(18)
can be prepared if its rhodium(1) dicarboslyl complex is allowed to react with the unsaturated ditertiary phosphine (19) (equation 34).
+ 2co
(34)
[RhXL,] Complexes
48.4.3
This is the most populous class of rhodium(1) complex. Interest in this type of complex has All comundoubtedly been inspired by the important catalytic applications of [RhCl(PPh,)3j.'o*20 plexes of this stoichiometry are 16-electron compounds and are formally coordinately unsaturated. However, for steric reasons they do not readily add a fifth ligand and pentacoordinate rhodium(1) complexes are rare. Many of the complexes undergo important oxidative addition reactions. The rhodium(II1) products from these reactions are described in Section 48.6. Isolable complexes of this stoichiometry are listed in Table 5. Table 5 Physical Properties of [RhXL,] Complexes
L
X ~
c1
c1 c1 H
c1
c1
H c1 H CN SnCI, SnBr, NCO
N3 OH OPh OCN 0,CMe 0,CEt 0,CPr 4 C C 5 HI I 0, CCH, C1 0 2 CPh PO,CC6N4 NO2 p-O,CC,H,CI 0,CCF, 0,CCF, CI 02CCF5
Color ~
PHPh, PHCY, PMe, PEt, PEt, PPrl PPr; PPr; PPh, PPh, PPh, PPh,
Yellow-brown Yellow Red
PPh,
Yellow -
PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh3 PPh,
M.P. Pc)
ReJ
~~
Red Red-brown Brown Pale yellow Brown Orange Orange Dark red Bright red -
Red-brown Orange Orange Orange Orange Orange Orange Orange Orange Orange
173-75 193 205-10
oil
185-95 120-30 45
3&35 -
tom2 128-29 -
100-03 207-08 -
-
I
233-35 136-38 236-37
112 113 114-116 117 114 114 117 114 118 69, 119 120-122 122 123 124 125 54, 126 69, 123 127, 128 127 121 121 121 121, 129 127 127 130 121 121, 130
Rhodium
914
Table 5 (Continued)
X 02cc6F5
F
c1
c1 Br
I
c1
L PPh, PPh, PPh, PPh, PPh, PPh,
C1
n
C1 c1
c1 c1
c1 c1
c1 c1 c1
c1 c1
c1
PPh;CI6H;, PPh, (m-C,H, SO3Na) PPh, Rb
c1 C1 c1 1
SCN
c1
SCN c1 I CI CI CI
Map.("C)
ReJ
15941b 130" 158" 130" 116" 178-83 14w3 179-80 154-55 -
130 47 131 68, 131, 132 68, 131, 132 68, 131 70 71 133 71 134 135 134 1I4 136 118 137 137 138 72 139 139 139 62, 73, 77, 78. 89 62, 77 78 78
I
170-75 185-95 230-50 Oil 82 82 1O M 6 1 70-80
183-86 135-36 92-94 135-39
78
78 78 78 61 81 37, 61, 81,
85
Cl Br
AsPh, AsPh,Et AsPh,Et
CI
SbPh,
CI
SPPh,
a With
Orange Orange Red Orange Red-brown Beige Brown Orange-red Orange-red Orange Orange Orange Brown-red Orange Yellow Orange Orange Orange" Dark red Orange Red Orange Yellow Yellow Yellow Yellow Yellow Yellow Yellow Orange Yellow Yellow
Br
c1
Color
Brown Orange Orange- -red Purple -
150-55" 197-200 t 92-96 170
-
140, 141 142 142 140, 141, 143
84
decomposition. b R = (MeSiO),McSiCH,CH,. 'Ethanol hemisolvate.
The neutral ligands usually possess some z-bonding capacity and it is rare, even in complexes containing two different neutral ligands, for a purely a-bonding neutral ligand to coordinate to rhodium. The anionic ligand can be virtually any uninegative ion. Anions of low coordinating power, however. do not form complexes of this stoichiometry and structure. There is an ionic perchlorate complex [Rh(PPh3)3]C104in which the cation is 'T' shaped.14 It has been calculated that a cation of this structure lies near an energy rninjrn~rn.'~' Two ionic PMe, complexes are also The complexes can be prepared by a wide variety of routes. The important complex [RhCI(PPh& has probably received the most attention, and preparations of this complex are shown in Scheme
4. The main preparative routes to other complexes of this class usually involve the displacement of less firmly bound ligands from other rhodium(1) complexes (equations 35 and 36). This method has proved popular in catalytic studies since it enables the catalyst to be prepared in situ and obviates the necessity of isolating the catalytic complex. A limited range of tertiary phosphine complexes can be prepared by reducing hydrated rhodium trichloride with the ligand itself (equation 37). Triarylphosphines containing electron-withdrawing substituents do not form monomeric complexes. Tris(perfluorophenyl)phosphine, for example, forms a dimeric complex (equation 25), and even heating the latter compound with neat ligand in a Carius tube for 24 hours does not form a mononuclear complex.74Under other conditions certain other tertiary phosphines can form a wide range of products which embraces rhodium(II1), rhodium(II), and hydrido or carbonyl complexes.
915
Rhodium RhC13.3H20
~ ~ Q > zIRhCl(CO)(PPhA s1
[RhCl(cod)lZ
ii, PPh,, N,, MeOH;'".'" iii, PPh3;139 iv, PPh,, EtOH, reRux;l4' v. PPh,, C,H,, r e f l ~ x ; ~ ~ , ~ ~ i, 6PPh,, N,, EtOH. reflux,6s.132 vi, excess PPh3;'49vii, PPh,, PhCH,Cl, reflux;lSO viii, [NiCL(PPh ) 1. C,H,/Me,CO, 50°C;"' xi, HgCl,, PPh,, N,, C,H,, reflux;lS2x, PPh,, C6H6;6B3$, PPh,, C,H,@ Scheme 4 Preparations of [RhCl(PPh,),]
L
=
PHPh2.'12P(p-C,H4Me),,70PPhzMe,'I8PPhzEt,'35PPh2(CH,)2SiC1,,PPh2(CH2)8SiC13,
PPh,(CH,)2SiMe(OMe)2,'2 PPh(C4H,N0)2,PP~I(C~H~,,N)~,'~~ (9), (IO),'' PF,NMe,,37 PF2NEt2FAsPh3,1mSbPh3'40.'43 IRhCI(CBH14)2]2 L
=
+
6L
-
2[RhCIL,]
+ 4CBH14
(36)
P(p-C6H,0Me),, P(p-C,H4C1)3,134 PPh(C4HBNO),,PPh(C,Hl,N),,'39 PF(NMe,)261 RhC1,.3H20
+
excess L
-+
[RhClL,]
(37)
L = P(p-C,H,F),,'35 A~Etph,,'~' SPPh,B4 Many other triphenylphosphine complexes of this stoichiometry can be prepared by anion metathesis of [RhCl(PPh,),]. Some of these reactions are illustrated in Scheme 5 . R h IN (SiMe3)2HPPh3)Z
I
Rh{Ii=C(CF,),} (PPh3)z 1
t
[RhOCN(PPh,), 1
[RhH(PPhJh I
I
IRhNa (PPha 131
[ W&CNMc2)(PPh3)2
1
i, LiN(SiMe,),, THF;59 ii, [Ph,As][NCO], EtOH;'" iii, ph,As][NCO], MeCN;"' iv, Dowex resin (CN form), CH2C1,/EtOH;M"'y v; [Et4NJN,;IZ4 vi, NaS,CNMe,;& vii, NdBH,, EtOH;'53 viii, NaBH,, PPhl, EtOH;IS3 ix, TlClO,, Me,CO;'* x, LiN=C(CF3)2;57 xi, PPh,;I5' xii, PPh3Is3 C.0.C. L D D
Scheme 5 Anion substitution reactions of [RhCl(PPh,),]
Rhodium
916
Table 6 IR Spactra of Carbaxylatorhadium(1) Complexes Complex
ycooB
VCW
(cm-’)
(cm-’)
AV (cm-I)
Ref.
r .
1373,’ 1379 1383 1389 1355 1405 1418
225 218 235 239 255
127 127 121 121 121 121 121, 130 121 121, 130 I27 127
-
-
-
1350 1371
250 209
aAsymmetric. Symmetric.
The hydrido complex RhH(PPh,), is also a good starting point for preparing other complexes. Upon treatment with acids it eliminates hydrogen and forms an [RhX(PPh,),] complex (equation 38). LRhH(PPh,),] =
+ HX
[RhX(PPh,),j
+
+
(38)
H,
3, 4; x = Oph,54+126 0 CMe ’27.128*154 O,CEt, p-O2CC6H4NO2,~ - O Z C C ~ H ~ C ~ ~ ” 2
7
The carboxylato complexes (Table 6; equation 39) can also be prepared from the uncharacterized dimeric rhodium(I1) species obtained by treating rhodium(I1) acetate with HBF.,. 12’ {Rh;+}
+
LiOzCR
+ PPh,
+
[Rh(O,CR)(PPh,)J
(39)
R = Me, Et, Pr, CSHL1, Ph, CH,CI, CCIF,, CF,, C2F5
If rhodium(II1) acetate were known, it would be possible to believe the patent claim that [Rh(O,CMe)(PPh,),] can be prepared from it.’” Chlorotris(triphenylphosphine)rhodium(I) is also the precursor of numerous chloro complexes which may be obtained by exchanging PPh, for other ligands. The main use of this reaction, however, is to prepare [RhXL, L ] complexes (see Section 48.4.3.1 below). The triphenylphosphine substitution reactions of [RhCl(PPh,),] are shown in Scheme 6. The hydrido complex, [RhH(PPh,),], is obtained in a number of anion exchange reactions o f [RhCI(PPh,),] due to hydride abstraction from the anions (equation 40). [RhCl(PPh,),] MX
=
+ MX
+
[RhH(PPh,),]
(40)
NaBH4,’53Alp?, ,54*118LiNHPr‘,ImLiNMe,, LiNPr:,I6’ KOH/EtOHI5’
The trialkyl or triaryl phosphite complexes are more accessible than those of tertiary phosphines. The phosphite ligands complex strongly to rhodium, and easily displace alkene,I6l and even carbonyl7’ ligands from [RhC1(C0)J2 (equations 41 and 42), a displacement that cannot be achieved by tertiary phosphines. indeed, even tertiary phosphine ligands themselves can be displaced by the phosphite ligands (equation 43).16’ The phosphite complexes can also be prepared by cleaving [RhX{P(OR), },I, complexes with additional ligand (equation 26). [RhCl(CaH,I)]2
+
6L
+
2[RhClL3]
+
2CgH12
(41)
L = P(OMe)3,76P(OPh),6227’ [RhCI(CO)J,
+ 6P(OR)7
2[RhCI(P(OR),),]
-+
+ 4CO
(42)
R = Ph, p-c6 H4 Me, p-c6H4 [RhCl(CO)(ZPh,),]
+
~ X W S SP(p-OC,H,Cl),
+
[RhCI(P(p-OC~H,CI)3},]
+
2ZPh7
+
CO (43)
Z = P, AS, Sb16*
By contrast tertiary arsine and stibine ligands do not displace alkadiene ligands’63and are only ligands from other rhodium(1) complexes. able to displace weakly bound alkene’40.143 or Few of the [RhXL,] complexes have had their physical properties investigated in great detail. As
xi
cis-[RhCl~Ph3)(PFzNEt2)7I
i, DMSO, 100"C;1s6ii, benzo[c]cinnoline,C6H,;w iii, PPh, R (R = C,H,,, C,,H,,,I3' m-C6H,S0,Na);"* iv, P(Zpy),, C,H,, CCIJ,Bo NMe,, NEt282);vii, (+)-PhCHMeNH,;1s9 viii, SO,,,,, 25"C;I5' v, P,, CH,Cl,, -78'C;158 vi, PF,X (X = F,80,89 C,Hl,;96 ix, SPPh,? x, PMe,, CaH,4;116 xi, PF,NEt280
Scheme 6 Triphenylphosphine substitution reactions of [RhCI(PPh,),]
in the case of their chemical properties nearly ail interest has been focussed on the commercially and catalytically important [RhCl(PPh,),]. A few of the complexes have had their structures determined by X-ray crystallography. Both the red164,165and orange165 isomers of [RhCl(PPh,),] have been investigated, as has [Rh(0,CPh)(PPh,),j.'29~166 The latter compound was probably investigated because of its unusual preparation (equation 44).lz9 [RhPh(PPh,),]
+
COZ
+
[Rh(O,CPh)(PPh,),]
(44)
The structures of the tris(tripheny1phosphine) complexes are somewhat distorted from a true square planar arrangement of ligands. However, the structure of [RhCl(PMe,),] (20) also deviates significantlyfrom square planar geometry and this is unlikely to arise from interligand As might be expected, the large PPh3 ligands of [RhH(PPh,),] encroach severely upon the small hydrido ligand which has not been accurately located.'6'
PMe3 (20)
The structure of [RhBr(PPh,),] has been investigated by the relatively untried technique of X-ray absorption. It was concluded that its RhBrP, core was similar to the RhCIP, cores of the chloro complexes.I6' The deviations of the cores from orthogonality are also indicated by the ,'P NMR study of solid [RhCl(PPh,),]. The spectrum is complicated due to the many side bands caused by the 31Pchemical shift anisotropy, but is sufficiently well resolved to detect the inequivalence of the two trans PPh, 1iga11ds.I~~
Rhodium
918
The solution chemistry of [RhCl(PPh,),] has been dominated by the catalytically engendered interest in the dissociation of triphenylphosphine from the complex. The degree of dissociation has proved difficult to determine. Early results were erroneous due to aerial oxidation of the soluti0ns,6~.’~’ and anaerobic isopeistic methods failed to provide an answer due to the slow precipitation of [RhCI(PPh,)2]2.170However, more recent experiments in which oxygen has been rigorously excluded have shown the degree of dissociation in unadulterated solutions to be ~ m a l l . ~ ” -The ’~~ exchange of PPh, with P(p-C6H, Me), has been shown to proceed by a dissociative mechani~rn.”~ Data from the 31PNMR spectra of some [RhXL,] complexes are collected in Table 7. The Io3Rh NMR spectrum of RhC1(PPh3)3 has also been obtained. In addition to the coupling constants recorded in Table 7 the additional coupIing constant 3J[P(1kRh-P(2)] was evaluated as - 38 H Z . ” ~ The NMR data have been used in Hiickel molecular orbital calculations for some [RhCI(PR,),] species.I7’ The X-ray emission spectrum of [RhCl(PPh,),] has been recorded.”’ However, X-ray photoelectron spectroscopy is claimed to show that the complex is 40% contaminated with a dioxygen rhodium(II1) specie^."^ This seems highIy unlikely since two independent X-ray crystal structure determinations have been made. The thermal decomposition of [RhCI(PPh,),] has been investigated twice. The first investigation revealed that the complex reacted with oxygen between 100 and 165”C,but decomposed in a helium atmosphere at 210°C.1’0The second proposed that biaryls were formed when [RhCl(PPh,),] and its congeners were heated above 140”C.’81
48.4.3.1
[RRXL, L’] complexes
The [RhXL,L’] complexes exist in two isomeric forms. The trans complexes are listed in Table 8, and the few cis complexes in Table 9. The trans complexes predominate since L is usually smaller than L and the trans disposition of the L ligands is of lower energy than the cis arrangement. The complexes can be prepared by a few general routes. The first of these involves cleaving [RhXL2I2complexes with L or the addition of L’ to the three coordinate, monomeric complexes [RhXL2] (equation 46). In another method (equation 47), one neutral ligand is displaced from [RhXL,] complexes by L (see Scheme 6 for displacement of triphenylphosphine ligands from [RhCl(PPh,),]). 1
+
[RhXL,], X
=
Cl, L = PF,, L
=
2L’ + Z[RhXLlL]
PPh,,3AL
=
PF,NEt,, L‘
=
(45)
RPh3;82X = C1, Br, I,
L = PPh,, L = SO,(,,%
[Rhx(PC~,)zl + Nz
+
[RhX(PCy,)21NdI
(46)
X = C1, Br, I& [RhXL,] X = C1, L
=
+
L‘
--*
[RhXLzL]
+L
(47:
P(rn-CsH4Me),, P(p-C6H,Me),, AsPh,, L‘ = P.,;’56X = Br, L = PPh3, L’ = PF,;*9X
=
Br, I, L = PPh,, L’ = DMSO’56
Table 7 “ P NMR Data for some [RhX(PR,),] Complexes Complex
W1l
JP(2)’
(p.p.m.)
(p.p.m.) -
-41.5 -48.0 - 46.8 - 43.2 - 46.2 - 42.7 - 45.0
J[Rh-P(I)] (H4 131d 145d - 142 - 141 - 139 143d 140d 140d
J[Rh-P(Z) J
J[P(I)-P(Z)]
(H4
(H4
Isod I 72d - 189 - 192 - 194 189d 187d 18Sd
2Sd - 38 - 37 - 36 3Xd 39 4od
‘R = (CH,),SiCI,. bR’ = (CH,),SiCl,. ‘P(2) denotes the phosphorus atom rrms to the anionic ligands. dSign not determined.
Ref
919
Rhodium Table 8 Physical Properties of [MXL,L] Complexes
L'
L
1
~
PPr; PPr; PPr; PBu; PPh, PPh, PPh, PPh, PPh, PPh, PPh, PPh3 PPh, PPh, PPh, PPh, PPh, PCY, PCY, PCY3 PCY, PCY3 P(m-C,H,Me), P(P-C6H,M43 PPhBu: AsPh, AsPh, a With
N,
N,
so2 N, N,
X
Color
M P ('C)
H CI CI H CI
Yellow
-
c1
Yellow
C1
Yellow
194"
-
-
c1
c1
so, SO2
so2
DMSO DMSO DMSO N* N2
-
so,
-
-
Yellow Yellow Yellow
-
H
-
p4
c1
N2
H
-
-
-
-
-
-
110-15 132-34
-
-
-
c1 c1
p4 PF,
I
-
c1 c1
Pd
-
Br I CI Br I CI Br I
Nz N?
-
171-73 16368
C1 Br
PF, PF, PF?(CCI, ) PF,NMe, PF,NEt,
0"
-
Yellow Yellow Yellow
c1
p4
I
I
ReJ 55
182, 183 51 55 184 158 34, 89 89 80 37 82 96 96 96 156 156 156 55 46 46 46 51 158 158
-
55
104-06
158 89
I
decomposition.
Table 9 Physical Properties of cis-[RhCIL, L ] Complexes L
L'
Color
M.p. ("C)
ReJ
PPh, PF, PF,NMe, PF,NEt,
C,,H,N, C,,H,N, PPh, PPh,
Yellow-orange Yellow
113-15" 140 45"
Lemon yellow
123--24"
90 90 37 82
a With
-
-
decomposition.
Less commonly, treatment of the dimeric complexes with excess ligand both cleaves the dimers and displaces one of the original neutral ligands (equation 48). Some dinitrogen complexes can be prepared in a single reaction from hydrated rhodium trichloride (equation 49). Other dinitrogen complexes can be prepared by displacing cyclooctene from [RhCl(C,H,,),], with PPr, under nitrogen (equation 50).lS3The most ingeneous preparation of a dinitrogen complex is the low temperature reaction of Irrans-RhCl(CO)(PPh,),] with butanoyl azide (equation 51).IE4 [RhClL;],
+
4L
+
2[RhClL2L']
R,
=
[RhCl(C8H14)2]2+ 4PPr', ~rans-[RhCl(CO)(PPh,),]
2L'
(48)
[RhH(PR,),(N,)I
(49)
ASP^,^^
L = PF,, L = PPh,,
RhCl3-3H20 + excess PR3
+
N W & NZ
THF
B u ~ , 'P~~ B u ; ~ " ' ~ ~
3
+
2[RhCl(Nz)(PPr'3),] + 4GH,,
PrCON,
+
[RhCl(N,)(PPh,),]
(50) (51)
Tetrakis(trimethylphosphine)rhodium(I) chloride loses two trimethylphosphine ligands when allowed to react with PPh, at elevated temperatures (equation 52). The ionic compound also undergoes a more complex reaction with benzene and sodium. In this a trimethylphosphine ligand is converted to a PMe,Ph ligand (equation 53).'% The geometry adopted by bath these complexes
Rhodium
920
is unknown. When [RhCI(PPh,),j is allowed to react in refluxing benzene with I-aryltetrazoline-5thiones (equation 54), two PPh, ligands are replaced by two thione molecules. These are believed to be S-bonded, but again information on the geometry around rhodium is 1a~king.I~~ Similar remarks apply to the geometry of [RhCI(PPh,),(MeCN)]'88 and [RhCl(PPh3),SnCH(SiMe,),].'89 [Rh(PMe,),]Cl
+ PPh3
[Rh(PMe,),]Cl [RhCI(PPh,),]
C7H8:THF
+
Na
+ ArN, NHCS
* trans-[RhCl(PMe,), (PPh,)]
+
2PMe,
[RhCl(PMe,),(PMe,Ph)] [RhCl(PPh,)(ArN,NHCS),]
(52)
(53)
+
2PPh,
(54)
By a similar reaction to that shown in equation (50), the unusual complex [RhCl(PPrj),(pMeC,H4,NSO)] can be prepared. In the solid state the ligand is S-bound but isomerization to a p-bonded species occurs in solution. The latter compound is hydrolyzed by water to a sulfur dioxide complex (equation '9I.)%
Other complexes whose crystal structures have been determined include the tetraphosphorus complex (Zl),"' and the dinitrogen complex (22).IB2The latter complex was originally believed to contain a sideways-on dinitrogen ligand;Is3however, once corrections were made for disorder and anisotropy of the dinitrogen ligand, the more usual end-on coordination was proposed.'82 c1
(21)
(22)
The hydrido dinitrogen compkxes, whilst failing to fulfill the modern alchemists' dream of a catalyst for the low temperature low pressure ammonia synthesis, have been thoroughly investigated and some of their spectroscopic properties are listed in Table 10. Bidentate ligands very seldom form related complexes of the type {RhXL(LL)], possibly because one of the bidentate ligand's donor atoms must be trans to a tertiary phosphine, a factor that also accounts for the small number of [cis-RhXL,L'] complexes known. A rare example of a complex containing a bidentate ligand is provided by the orange complex (23),which can be obtained from [RhCl(PPhl), (Scheme 6 ) .157 There are also a few dimeric complexes containing a bridging bidentate ligand. Particularly noteworthy are the binuclear dinitrogen complexes [(RhH(PR,), N2] (R = Pr';' Cys5."). FinaHy it should be noted that there are a few ionic [RhLJX complexes besides [Rh(PPh,),](CIO,) Table 10 'H NMR and IR Data for [RhHL,] Complexes
a
Hexane solution, all other Ill spectra from Nujol mulls.
92 1
Rhodium
mentioned above.'44Surprisingly these contain the smallest tertiary phosphine, PMe,, rather than very large ligands whose bulk might be expected to favor three-coordinate cations. Even more surprisingly the complexes have been obtained directly from [Rh(PMe,),]CI rather than [RhCI(PMe,),] (equation 56)."6,'46 [Rh(PMe,),]Cl
+
KX
-,
[Rh(PMe,),]X
+
PMe,
+
KCI
(56)
X = BPh+ PF6"67'46
48.4.4
48.4.4.1
[RhXL,) Complexes
(RhHL, ] complexes
There are three limiting forms of structure for hydrido complexes of this stoichiometry: trigonal bipyramidal, square pyramidal and monocapped tetrahedra. The structure of a given complex is difficult to classify since, in addition to the hydrido ligand being difficult to locate by X-ray crystallography, many of the complexes suffer radiolytic damage during data collection. This damage reduces the quantity and accuracy of the data available. However, examination of the structure of the rhodium ligand core gives a clue to the overall structure. Similar difficulties occur in attempts to study solution structures by NMR spectrometry. Here ligand exchange is the problem. A further difficulty in IH NMR spectrometry is the multiple splitting of the hydride resonance by both Io3Rhand ,'P nuclei in the molecule. The ultimate example is found in [RhH(PF,),] where every nucleus in the molecule has a non-integral spin. It has been calculated that the hydrido resonance should be split into 130 peaks!"' In the solid state [RhH(PPh,),]lg3and the isomorphous [RhH(PPh3)3(AsPh,)]"* have been shown to have monocapped tetrahedral structures, and [RhHIPPh,), (PF,)]'" is a trigonal bipyramidal species. It has been stated that the solution structures, at low temperatures, of [RhH(PF,),] and [RhH{P(OEt)3}4]r195 and [RhH{P(OCH2),CPr),]'% are all monocapped tetrahedra. However, these three complexes dearly exhibit fluxional behavior in solution and pass through intermediate This stereochemical structures which can be represented as distorted trigonal bipyramid~.'~~,'~' non-rigidity may account for their J(Rh-H) values of 9-9.5 Hz,"' which may be compared with the coupling constant of the square pyramidal complex [RhH(PPh,Me),] of 7 Hz' Is (see below). Nevertheless, it has been found that the 'H NMR spectrum of RhH(PPh3), (J(Rh-H) = 0 Hz) is similar (see Table 11) to that of the known trigonal bipyramidal species [RhH(CO)(PPh,),] (J(Rh-H) = 0.8 Hz). The 'H NMR spectrum of [RhH(PPh,Me),] shows only the cis P-H coupling of 18 Hz, without any trans P-H coupling being observed. Additionally, J(Rh-P) is 7 Hz, which implies that [RhH(PPh,Me),] is square pyramidal in solution.'I8 The triaryl phosphite complexes Table 11 Complex
IR and 'H NMR Spectra of [RhHL4] ComplexPs TRI-
Ref
x
19.9 20.6 22.1 20.6 -
21.12 22.89 21.06
5.8" ca.0.v 7.v 9.5' F ~a.8~
8.82'
1992b 2140d 2070 2180, 2125 . 2140
I97 118,204 1 I8 133 213 204 I95 195 198 I96
1431my'. Ref. 118. vRh--,, 1548 ern-', 'JRc,+H 18.OHz. JP-H57.4 Hz,JF-" 16.6Hz. rJp_H 35Hz. *JP-"21.5Hz. hJp-Hca. 45Hz.'J+,, 44.12H2.
I
Rhodium
922
[R~W{P(OAJ)~}~] have similar coupling constants and are probably also square pyramidal in solution. One of the more important complexes in this class is [RhH(PPh,),]. It can be prepared from a variety of starting materials (Scheme 7). The four PPh, ligands form a tetrahedron around rhodium. The hydrido ligand could not be located by X-ray crystal10graphy.l~~ The complex is very reactive; the hydrido ligand can be replaced by other anionic ligands (Scheme 8) and the lability of the triphenylphosphine ligands allows other complexes of this stoichiometry to be prepared. The most interesting product from these reactions is the dirhodium carbonate complex (24). This is the product when [RhH(PPh,),] is allowed to react with carbon dioxide in toluene at 25°C.205Originally the product was thought to be a C 0 2 complex,2o6but X-ray crystallography showed it to be (24)."O' The additional oxygen atom may arise from traces of moisture or oxygen in the gas stream. 31PNMR studies on benzene solutions of (24) imply that the dimeric structure is retained in solution.
The action of PF3and hydrogen at high temperatures and pressures upon mixtures of anhydrous rhodium trichloride and copper forms (equation 57) the colorless liquid [RhH(PF,),] (b.p. 8 9 T / 725 torr)?2335~208 The liquid decomposes slowly at 20°C to form [Rh(PF2)(PF3)J2,but on heating to 140'C it decomposes to rhodium metal.32The hydrido complex can also be formed by the action of strong acids on the potassium salt K[Rh(PF,),] (equation RhCl,/Cu
+ PF3 + Hz
K[Rh(PF,),I
+
H2S04
200 am
,o0c
+
+
[RhH(PF,),I
[RhH(PF3)41
(57) (581
The complex behaves as a strong acid (Scheme 9). The deuterio complex can be prepared by treating the hydrido complex with D2S04.32 The exchange of hydrogen is much easier than in the case of [RhH(PPh,),] which only exchanges very slowly with deuterium gas (equation 59).209 RhC13 -3H2 0
[ Rh(02CMe)(PPh3)3 1
i, NaBH,, PPh,, EtOH;1yn,19Y ii, Et,Al, PPh3;zw iii, EtOH/KOH, reflux;20'.'" iv, NaBH,, PPh3;IS3 v, N2K,PPh,, C6Hb/EtOH,H,,reflux;"' vi, NaOPr', PPh,, Pr'OH;i25 vii, PPh3;118~iss viii, H,, PPh,, C,Hb;203 ix, H,, PPh,2w Scheme 7 Preparations of [RhH(PPh,),]
[Rh(O,CR)(PPh3)3 1
[ RhH(PPh3 ),(ROH)
1
i, CO,, C,H,, 25"C?05 ii, diphos$OOiii, ROH;"' iv, RC02H;'27,'28,'30~154 v, - PPh,1'a~'53 Scheme 8 Rcactions of [RhH(PPh,),]
i, slow, 25"C3' ii, K/Hg, Et20, 20°C;" iii, [CO(CP),]+;~~ iv, [Fe(~hen),)~+:~ v, D,S0,32 Scheme 9 Reactions of (RhH(PF,),]
[RhH(PPh,),]
f
D2
3
[R~D(PP~I,)~] f HD
(59)
The next most important complexes of this type are trialkyl or triaryl phosphite complexes. These are best prepared by allowing the ligands to react with [RhH(CO)(PPh,),] (equations 60 and 61). [RhH(CO)(PPh,),] X,
=
[RhCl{PPh(OEt),)4]
+ 4PX3
+
[RhH(PX,)4]
+
3PPh3
+ CO
(60)
Et3,197 Ph3?'O Ar3r198 PrC(CH20)3'96
+ PPh(0Et)z
NaBH4, Ar EtOH
[RhH{PPh(OEt),)4]
(61)
Little is known of their chemical properties. If the electrochemical reduction of [RhH(P(OPh),},] is essayed, loss of a phosphite ligand occurs before reduction to [RhH{P(OPh),}J takes place. The anionic complex is unstable and o-metallates, eliminating hydrogen. " COC GDD'
924
Rhodium
Apart from [RhH(PPh,Me),], which can be prepared from [RhCl(PPh, Me),] by treating it with hydrazine (equation 63),II8 and [RhH(PMe,),], which is prepared by reducing the rhodium(TI1) complex [RhH,(PMe,),]Cl with sodium amalgam (equation 62),Ig6the remaining example of this type of complex is the dibenzophosphole complex [RhH(10),] (equation 64). The last complex dissociates readily in solution (equation 65).52,’33,212 [RhH2PMe3141C1
NaiHs
THF,25-c)
[RhH(PMe3)4I
N2H4. HI, 80°C
[RhCI(PPh,Me)3] iP%Me IUICI,-3H20 i(30)
[RhH(lO),I
5
’
(62)
!RM(PPbMehl
(43)
[RhH(lO),]
(64)
+ (10)
(65)
[RhH(10),]
The one complex of this stoichiometry not to have a set of four phosphorus ligands is [RhH(PPh,),(AsPh,)]. This can be prepared by allowing AsPh, to react with [RhH(PPh,),] (equation 66). It has been shown to be isomorphous with [RhH(PPh,),]. Phosphorus and arsenic randomly occupy the coordination The corresponding tris(tripheny1arsine) complex can be prepared from [RhPh(cod)(PPh,)] (equation 67).204 [RhH(PPh,),]
+ Ad%,
[RhH(PPh3)3(AsPh,)]
-+
[RhPh(cod)(PPh,)] f AsPh, -I-Hz
48.4.4.2
-*
[RhH(AsPh3)3(PPh,)]
(66)
(47)
Other [RhXL,] complexes
Very few complexes of this type are known. Some [RhX(PF,),] complexes can be prepared from [RhX(PF,),], but these readily lose PF, on warming to room temperature?’ The complexes containing PF2NR, ligands are slightly more stable. The complexes [RhCl(PF,NMe,),], [RhCl(PF,NEt,),], and [RhI(PF,NEt,),] have been isolated. Their decomposition temperatures have been reported to be 7 9 , 2 8 , and 58°C respectively. The difluorodimethylaminophosphine complex is yellow and the others orange. They have been prepared by addition of excess ligand to various dimeric rhodium(1) complexes (equations 68-70).61
+ excess PF,NMe2 [RhC1(PF,NEt,),l2 + excess PF,NEt, [RhI(cod)], + PF,NEt, -+ [RhCI(CO),],
[RhC1(PF2NMe2),]
(68)
5 [RhCI(PF,NEt,),]
(69)
[RhI(PF,NEt,),]
(70)
The yeliow diethyl phenylphosphonite complex is also known. It can be prepared directly from hydrated rhodium trichloride (equation 71).”’ RhC13*3H20+ NaBA,
48.4.4.3
+ excess PPh(OEt),
[RhCI{PPh(OEt),},]
(71)
I R h H ( L L ) , / complexes
Despite the isolation of numerous [RhHL,] complexes only two of the corresponding complexes containing bidentate ligands have been prepared. The first complex [RhH(diphos),] may be prepared by the displacement of neutral ligands from [RhH(PPh,),] (equation 72),200or by allowing the chloride salt to react with [BHJ (equation 73). This reaction is reversed when the hydrido complex is dissolved in carbon tetrachloride. The deuterio complex can be prepared similariy from Li[AlD,]. The hydrido complex eliminates hydrogen when allowed to react with HC104 (equation 74).2‘4
[RhH(PPh,),]
+
2diphos
+
[RhH(diphos),]
+
4PPh3
172)
925
Rhodium NaBHI, N1,EtOH
[Rh(dipho~)~]Cl
[RhH(diphos)*]
(73)
w 1 4
[RhH(diphos)*] + HC104
+
[Rh(diphos),]CIO,
+
H2
(74)
The hydrido complex is formed as an intermediate in the electrochemical reduction of [Rh(dip h o ~ ) ~ ] C but l ? ~ is itself oxidized in the system (equation 75).215 [Rh(diphos)&I
[Rh(diphos),]'
MeCN
[RhH(diphos),]
3 [Rh(diphos),]+
[RhH(diphos),]+ (75)
The light orange hydrido complex melts with decomposition at 280°C. Since the hydrido ligand is coordinated to rhodium ( vRh-" 1902cm- I , &h-D 1465 cm-' ), the complex is coordinatively saturated and exhibits poor catalytic properties. The second complex of this type has been prepared directly from hydrated rhodium trichloride (equation 76).*16Later work has shown that the addition of formaldehyde is superfluous.217An alternative preparation uses [BH,]- as the reducing agent (equation 77)?'* RhC13.3H,0
+
(+)-diop
RhC13*3H20+ (+)-diop
yzc
KaBH,
[RhH{(+)-diop},] [RhH((+)-diop}*]
(77)
The yellow complex decomposes at 210°C. Upon recrystallization from dichloromethane the monosolvate is formed which melts at 185°C. In soIution the complex exhibits fluxional behavior. The high field ' H NMR spectrum shows the hydride resonance to be composed of a quintet of doublets centered at T z 28.4. In benzene solution [XI: = - 3.40.218 In the solid state %h-" = 2 0 4 0 ~ m - ' . ~X-Ray '~ crystallography shows the complex to have the trigonal bipyramid structure (25).219
48.4.5
48.4.5.1
[RhLJX Complexes Complexes containing monodentate ligands
There are comparatively few complexes of this type, which, given the populous nature of the related [Rh(LL),]X dass (see Section 48.4.5.2 below), seems a little strange. However, the small bite of the bidentate ligands causes fewer steric problems than the accommodation of four large, monodentate ligands around the central rhodium atom. As a consequence, isoiable complexes of this stoichiometry usually contain small or medium sized neutral ligands (see Table 12). The complexes can be prepared from other ionic rhodium(1) complexes (equation 78)22'or, in the presence of non-coordinating anions, they can be prepared by allowing excess neutral ligand to react with other rhodiurn(1) complexes (equation 79).220.222
In general, the complexes are stable and unreactive. Their simple 31PNMR spectra are unchanged upon addition of free ligand, showing the inert nature of the complexes. Their main reactions are
Rhodium
926
Table 12 Physical Properties of [RhL4]X Complexes
X
L
Ref
M.p. (“C)
Color
~
Yellow Yellow Yellow Yellow Yellow Yellow Yellow Lemon yellow Brown Red-orange Red-brown Red-brown Orange Yellow Yellow Yellow Yellow Yellow
PHEt, PHEtPh PHPh, PHPh, PHPh, PHPhCy PHCY, PMe,
PBu, PPh, Me PPh2MG PMe,Ph PMe,Ph PPh,(OMe) PPh(OMe), PPh(OMe), P(OPh)3 P(OW3 a With
.,
113
147= 156“
113 220 220 220
2 1CL20”
18GW 150‘
113
196” 193”
113 116, 146 76
-
221,222 222 221,222 222 222 221, 222 222
179-80 -
124-30 -.
175-76b 177 177
-
76 76
decomposition. bCH,C12 solvate. ‘Explodes.
oxidative addition. The complexes prepared by way of equation (78) oxidatively add molecular hydrogen, in contrast to the complexes of secondary phosphines, which are unreactive even at high hydrogen pressures.220 Tetrakis(trimethylphosphine)rhodium(T) is more reactive, but tends to lose a PMe, ligand readily (see equations 52, 53 and 56).
48.4.5.2
Complexes containing bidentate iigands
A very large number of complexes of this stoichiometry have been prepared. Isolable complexes are listed in Table 13. Like the analogs containing monodentate ligands, the cations are somewhat inert and many of their reactions involve anion exchange. The most accessible complex appears to be [Rh(diphos),]Cl which can be obtained from a variety of starting materials (Scheme IO). The ligand will reduce rhodium(lI1) chloride or displace other ligands from many rhodium(1) complexes. Unlike tertiary monophosphines it will totally displace carbon monoxide from [RhCl(CO),], (equation SO). It does, however, require excess ligand to bring about the r e a c t i ~ n . ~Less ’ ~ , diphos * ~ ~ forms an unstable carbonyl complex,233which slowly disproportionates (equation 8 [RhCI(C0)2]2 + 2diphos
-
4[RhCl(CO)(diphos)]
+
2[RhCI(CO)(diphos)]
2[Rh(diphos),]Cl
+
2CO
(80)
+ [RhCI(co)2]2
(81)
The other complexes are prepared similarly by utilizing the power of the bidentate ligands to displace (equations 82 and 83) monodentate ligands from rhodium’s coordination sphere.
+ 4LL
[RhCl(cod)], LL
=I
--*
2[Rh(LL),]CI
+
2~0d
182)
PhMePCH,CH,PPhMe, cis-(Ph2P)CH=CH(PPh,), diars, cis-CPh,As)CH=CH(AsPhl)226 [RhCl(CaHi4)2]2
+ 4LL
+
2[Rh(LL),]Cl
+ 4CBH14
(83)
LL = (Ph2P)2CH2,(Ph2P)CH2CH2CH2(PPh2), (Ph,P)(CH,),(PPh,), ( + ) - d i ~ p * , ~
Other ionic complexes which have a non-complexing anion can be prepared from chloro complexes and an inorganic salt of the anion (equations 84 and 85). [RhCl(cod)12 + 4LL
+
2MX
-
2[Rh(LL),]X
+ 2 ~ 0 d+ 2MC1
(84)
MX = NaBPh,, LL = (Ph,P),NCHICO,Me, (Ph2P)1NCHPiC02Me,236 Ph2PCH2CH2SEt,Z” PhTFCHpCH2SPh242pN3; MX = NH4PF6, LL MX = AgPF,, LL
=
=
(B~’CH~)*PCHLCH~P(CH~BU~)*~~~;
(q5-C,H,AsPh,),Fe, PhMePCH2CH2PPhMe,cis-Ph,AsCH=CHAsPh,, d i a r ~ ~ ~ ’
Rhodium
927
Table 13 Physical Properties of [Rh(LL),]X Complexes
LL
X
M.p. ("C)
Color Orange Yellow
(Ph, P)2 CH, (Ph, P)Z CH2 (Ph,P), CHMe cis-{(Ph2P)CH) cis-{(Ph,P)CH), &-{(Ph,P)CH)z cis-{(PhZP)CH), m e 2 W H 2 32 {(PhMeP)CH21 2 {(PhMeP)CH2)2 {(PhMeP)CH, 12 {(PhMeP)CH, 12 diphos diphos diphos 23 1 diphos diphos diphos diphos diphos diphos diphos {(ButCH, P)C% I I (Ph,P)L(CH2)3 (Ph2Ph(CH*11 (Ph2P)2(CH2 14 (Ph,P),(CH? 1 4 (Ph,P),NCH, C0,Me (Ph,P),NCHMeCO,hle (Ph, P),NCHFY CO, Me {(Ph, PICK I2 0 ( - )-diop (+)-diop (+)-diop cis-{(Ph, As)CH), cis-{(Ph, As)CW), cis-{(Ph, As)CH)? cis-{(Ph, As)CH], cis-{(Ph2As)CH] {(PhMeAs)CH,), { (Ph,As)CH,}2 {(Ph,As)CH, 1 2 {(Ph,As)CH, diars diars diars diars (26)
Ref:
124d 97d, -
-
223,224 224 223 225, 226, 221 226 226 225, 226 227 228, 229 226 226 226, 230 65, 214 223,224 2670
-
Yellow
-
-
-
-
Yellow
225d
-
-
Yellow
24CL50' -
-
Yellow Pale yellow Pale orange Pale yellow
-
246 273-76 264-70
-
26Sd -
-
Yellow Yellow Yellow Yellow Yellow Pale orange Yellow Red Yellow Yellow Yellow Yellow Yellow Yellow Pale orange Yellow
235 224 224 224 224 236 236 236 225 231 224 224 226 226 238 226, 230 226,239 240
logd 206-09 204 20344 225d -
174-7Td 79d -
-
Orange Yellow
23gd I
~
Ph,P(CH2), SMe Ph,P(CH,), SEt Ph,P(CH,)2 SEt Ph, P(CH2)rSPh Ph,P(CH&SPh
65 65 141, 214 214
215 282 221-27 180' I 62-Md 1f~9-73~
-
(27)
232 232 233, 234
~
-
.I_
Yellow Yellow Yellow -
-
65
-
65
-
-
-
-
-
65
-
-
-
Golden brown Yellow Yellow Yellow Yellow Yellow
-
226 226 226 226 107 24 I 242 242 242 242, 243 242
-
-
-
'CH,CI, solvate. bMeOH solvate. Monohydrate. With decomposition.
[RhCl(C,H,4),]2
+ 4LL +
2MX
+
2[Rh(LL),]X
+
4CsH14
+
2MC1
(85)
MX = NaBF,, LL = (PhzP),CH2, (Ph,P),CHMe, diphos, PhzPCH2CH2CH2PPh2223
+ 4( -)-diop + 2NaBPh + 2[Rh{( -)-diop],]BPh + 2NaCI [RhCI(CO),], + 4cis-Ph2AsCH=CHAsPh, + 2NaBF, + ~[R~(~~~-P~,ASCH=CHA +~4CO P ~ ~+) ~2NaCI ]BF~ [Rh(Ph2P(CH2)2SR),]BF4 [IZh(cod)(acac)] + Ph3CBF4 + 2Ph2P(CH2)2SR [RhCl(nbd)],
-+
R = Me, Et, Ph242
(86) (87)
(88)
928
Rhodium [RhCl(PPh3)j ]
RhCI,
[Rh(LL),]CI
IRhCW'FA
+
MX
I )
[Rh(LL)JX
+
12
MCI
(89)
MX = AgBF,, LL = (Ph2P)2CH,, Ph2PCH2CH,CH2PPh2,Ph2P(CH2),PPh2,(+)-di~p?~, MX = NaBPh,, NaBF,, LiPF,, LL
=
PhMePCH,CH,PPhMe, cis-(Ph,P)CH=CH(PPh,),
cis-(Ph, As)CH=CH(AsPh,),
Although (But CH2)2P(CH,),P(CH,B~')2forms a typical [Rh(LL),]+ complex when allowed to react with [RhCl(cod)], and NH,PF6 (equation X4),234the closely related ditertiary phosphine Bu: P(CH2),PBu: forms complexes which contain hydrido ligands or the diphosphine bound additionally to rhodium through Apart from anion exchange reactions the complexes are inert. One of the few reagents to bring about a change in the coordination sphere is sulfur dioxide which coordinates to rhodium without displacement of a bidentate ligand. However, the product is not well characterized and the crystals contain clathrated sulfur dioxide.2s0 If [Rh(diphos),]Cl is allowed to react with an anion exchange resin in the cyanide form, the corresponding cyanide complex (vCPN 2089 cm- ') is formed. Upon dissolution of the product in benzene a diphos ligand is lost and [Rh,(CN),(diphos),] is formed."' The dinuclear product probably contains a bridging diphos ligand (cf. [Cu,(N,),(diph~s),]'~'). These reactions and the anion exchange reactions of [Rh(diphos),]Cl are shown in Scheme loa. By way of contrast, the unsaturated ditertiary phosphine complex [Rh{cis-(Ph,P)CH= CH(PPh,)],]ClO, forms a five-coordinate cyano complex ( vCPN 2070 cm-') when allowed to react with an anion exchange resin.252 The structure of [Rh(diphos),]CiO, has been determined. X-Ray crystallography showed the RhP, core to be planar, but the aliphatic carbon atoms of the ligands do not lie in this plane.2s3 The electronic spectra of several of the complexes have been studied and attempts have been made to correlate the absorption bands with the energy levels of proposed molecular orbital &agrams .254.255 An Rh-P coupling constant of 133 Hz has been observed in the "P NMR spectrum of [Rh(diphos),] + ?56 Many complexes of this type (e.g. [Rh{(HOCH2)CH(PPh2)CH2PPh,}2][BF4]257) have been prepared in situ and used as homogeneous catalytic hydrogenation catalysts, but have never been fully characterized.
Rhodium
929
[ Rh(diphos), I CN
[Rh(diphos),] [C1041
[Rh(diphos), IC1
[ Rh(diphos),
[Rh(diphos), 1 [PFs 1
I [BPh41
i, NaBH,, N,, EtOH or LiAlH,, N,, THF?I4 ii, NaClO,, EtOH;l4 iii, K[PF,], MeOH;233iv, NaBPb, EtOH?l4 v, Dowex resin (cyanide form), CH,C12;"' vi, CbH6;'" vii, + HCIO,, - H,'l4 Scheme 10(n) Reactions of [Rh(diphos),]CI
48.4.6
[RhL,]X Complexes
In this type of complex rhodium(1) achieves its maximum coordination number with neutral ligands. As might be expected, very few complexes of this type are known. The isolable examples are unstable and easily lose neutral ligands. The complexes so far isolated contain five trialkyl phosphite ligands. These ligands have high %-aciditybut are poor a-donors. All six [Rh(P(OR),},]X complexes are white. They ionize in acetone solution. Their physical properties are shown in Table 14.
The salts have been prepared by adding excess trialkyl phosphite to [RhCl(cod)12in the presence of an inorganic salt of a non-complexing anion (equation 90). [RhCl(cod)],
+
excess P(OR)3 i2MX
+
2[Rh{P(OR),},]X
+
2cod
+
2MC1
(90)
R = Me, Et, Bu', Bu", MX = NaBPh,; R = Me, MX = NH,PF6; P(OR), = P(OCH2)3CMe,
MX = NaBPh,,Z22
The 'H NMR spectra of the complexes show the ligands to be labile. There is the further possibility that the cation is fluxional, having trigonal bipyramid and square pyramid configurations as limiting forms. The ligands can be displaced by alkenes, and the trimethyl and triethyl phosphite complexes lose three ligands from the cation to yield $-tetraphenylborato complexes. In the case of the P(OMe), complex the reaction can be reversed by adding excess P(OMe), (equations 91a and 91b).222 Table 14 Physical Properties of [RhL,]X Complexes Complex
M.p. ("C)
Conductance(S)
Ref.
Rhodium
930
48.5
RHODIUM(I1) COMPLEXES
Despite the vast number of cobalt(1T) complexes known, the dipositive oxidation state is rare for rhodium. The bulk of the rhodium(l1) complexes known are dimeric, the classic examples being the diamagnetic carboxylato complexes that adopt the classic lantern structure (26). However, even the aqua complex is dimeric so the bridging carboxylato ligands are not essential for the formation of the rhodium-rhodium bond.
Rhodium(I1) complexes with tertiary arsines were erroneously reported over 40 years ago. These complexes were, with the advent of NMR spectrometry, later proved to be hydridorhodium(II1) complexes. Nevertheless, the only stable isolable monomeric rhodjum(T1) complexes are those containing tertiary phosphine and other similar group VB ligands. A further difference between rhodium(I1) and cobalt(I1) complexes becomes apparent from a study of the monomeric complexes. Whereas virtually all cobalt(I1) complexes are high spin d 7 species, with the exception of [Co(CN),I3- and related species, the rhodium(I1) complexes are a11 low spin complexes. Thus, because of the lack of spin reorientation required in forming a low spin rhodium(II1) complex, they are excellent reducing agents. The stability of the rhodium-rhodium bond in the dimers prevents their facile oxidation. Because of the great differences in behavior between the few monomeric complexes and the dimeric complexes, this section i s divided principally into two parts.
48.5.1 Monomeric Complexes Few monomeric rhodium(I1) complexes, other than those stabilized by group VB ligands, have anything but a transient existence. Whilst the former can be prepared by reduction of rhodium(II1) salts with the group VB ligand, the latter are generated by photolytic or radiolytic methods. The short lifetimes of these species have occasionally been extended by trapping them within a crystal lattice. It has been demonstrated that RhBr,-doped AgBr crystals give rise to Rh2+ centers upon exposure to light. These rhodiurn(l1) centers become more stable with decreasing temperatures and However, it was at 35 K it was shown that the low spin Rh" center was in a Ddh believed that the D4,,symmetry observed for photolytically generated Rh2+centers in LiH or LID It is reasonable to assume less than ideal geometry for lattices was an artifact of site a t2,'e; ion since Jahn-Teller effects will be prominent for such an electronic configuration and a static Jahn-Teller effect has been observed for Rh2+ in a CaO matrix at 4.2K.260 Gamma irradiation of Rh3+-doped NH4C1 crystals again gives rise to Rh2+ centers of Da symmetry. In this system further reduction to Rho was also observed.261Pulse radiolysis has proved to be a powerful tool for the investigation of the behavior of rhodium(I1) species in aqueous solution. Various reducing agents in the system can bring about the reduction of rhodium(EI1) complexes besides the ubiquitous solvated electron.262 Both [RhCl(NH,),]+ generated from [RhC1(NH,),I2+, and [Rh(bipy),]'" generated from [Rh(bipy)J3+, have been studied. The former (Scheme 11) shows that Jahn-Teller effects persist in the solution chemistry of rhodium(II).263The principal reactions of the latter (Scheme 12) involve disproportionation to Rh' and Rh"'.z6kZ" The complexes described in this section contain either (i) tertiary phosphines or stibines in
93 1
Rhodium
t [ Rh(NH,),(OH~)I 2+
+ NH3
Scheme 11 Generation and reactions of [RhCl(NH,),]+
LRh(biw),
I
1 '+
slow
[Rh(bi~y)~]~+
[ Rh(bipy),
I 2+
-
[Rh(bipyhl?
Scheme 12 Generation and reactions of [Rh(bipy)3]2'
conjunction with halo ligands, or (ii) bidenlate tertiary monophosphine ligands. They are clearly different in nature from the [Rh,(02CR)4(ZX3)l] (Z = P, As, Sb) complexes, which are best regarded as derivatives of carboxylatorhodium(I1) complexes (Table 14a). Table 14a Physical Properties of Rhodium(1I) Complexes Containing Tertiary Phosphines and Stibines Complex .
~
~
Color
M.p. ("C)
Ref
~~~~~
Red-brown [RhCl,WY,),l Green [RhBrzPCY,121 [cis-RhCl, {P(o-C,H,Mej, },] Purple [trm-RhCl, {P(o-C, H, Me), }2] Blue-green [trms-RhCl, (PBu; Me),] Purple [traW-RhCl, ( P B u~ Et),] Green Green (tram-RhCl, (PBuiPr),] Red [RhCI, {PBu: (C=CPh)},] Green [trans-RhCl, (PBui R),Y Dark blue [frm-Rh(PB$ (o-C~H,O)},] [RhCl, {Bu;P(CH,),CO, Et}] Blue [ ~ C ~ Z I B U ; P ( C H Z ) S C O Z E ~ } I Green [fiCl, (diphos j] Gray-green [RhCl, {Sb(o-C, H, Me),}] Red-brown Red-brown [RhBr, { Sb(o-C, H4Me), }]
'R = -CH2(2-Me-5-OMeC6HS).
bWith decomposition.
170-200 165-75 175-95 140-45
20147b 350 14-7 90-96 -
261 267 265 265 268,269 268,269 268, 269 270 272 273 275 275 277 278 278
Rhodium 48.5.1 .I
Tertiary phosphine complexes
Many large tertiary phosphines fail to reduce hydrated rhodium trichloride and form rhodium(1) complexes. Under mild conditions they usually reduce the salt to paramagnetic rhodium(1I) complexes. This was first discovered when P(o-C,H,Me), was employed, as in equation (92).265 RhC13.3H,0
+
P(o-C,H,Me),
RhClz{P(o-C,H,Me),),
(92)
The blue-green complex is paramagnetic (p = 2.3 BM) and has a single band due to vRh ,-, in its far I R spectrum. Accordingly it has been assigned the trans configuration. If the preparative reaction is carried out at O T , or if CH,Cl, solutions of the blue-green complex are evaporated in vacuo, purple crystals of a less stable and less well characterized isomer are obtained. The purple isomer is also paramagnetic (see Table 14b for magnetic and TR spectral data). If these preparations are carried out at high temperatures, or if either of the above complexes is heated, o-metallation occurs. No reduction to rhodium(1) complexes was observed It has been claimed that the attempted borohydride reduction of the dichloro complex in ethanol forms the unstable diamagnetic rhodium(l1) complex [R~H(BH,)(P(O-C,H,M~),}].~'~ The large steric requirements of the tricyclohexylphosphine ligand are presumably responsible for the isolation of [RhX,(PCy,),] (X = C1, Br) complexes under mild conditions. Both the red-brown chloro complex and the green bromo complex are paramagnetic.'6' The ligands PBuiR (R = Me, Et. Pr) also yield trans-rhodium(l1) complexes when aIlowed to react with ethanolic solutions of RhCl3.3HIO. If the reaction is carried out above 25°C o-metallation does not occur, instead [RhHCl, ( P h i R),] complexes are f ~ r m e d . ' ~ *The * ~ ' ~closely related alkynylphosphine ligand PBul,(C=CPh) also forms a hydridorhodium(l IT) complex when allowed to react with hydrated rhodium trichloride. Upon dissolution in chloroform, however, the red rhodium(II1) hydrido complex decomposes and forms the orange binuclear rhodium(I1) complex (27)."O 2653266
C1\Rh/cl\Rh/ (PhC=C)Bu:P'
PBu:(C-CPh)
\C1/
\C1
(27)
Tri(neopenty1)phosphine and phenyldi(neopenty1)phosphine also form complexes of this stoichiometry. However, these complexes are apparently diamagnetic. In their ' H NMR spectra the resonances of the l-butyl groups are well defined. Accordingly they have been formulated as mixed Rh'/Rh"' complexes. They may adopt either structure (28) or (29).'" C1
(29) (28) Table 14b Magnetic Properties and Far-IR Spectra of Rhodium(1I) Complexes of Tertiary Phosphines and Stibines
[RhCI,(PCY,),l IRhBr,(PCYd,l [cis-RhC1, {P(o-C, H,Me),},] [trans-RhC1,{P(o-C,H,Me), [trans-RhC1, (PBuS Me),] [frans-RhCl,(PBu:Et),] [trans-RhCl,(PBu;Pr)J [RhCl, {PBu: (C_CPh)J2] [RhCl,(PBu\ R),F [RhCl, { Bu:P(CH,),CO,Et)] [RhCl, {Bu\P(CH,),CO, Et}] [RhCl, (diphos)] W C l , ISb(o-C,H,M~),lI
' R = -CH,(Z-Me-5-OMeC,H,).
2.24 2.24 2.0 2.3 1.15 P P P P P P P D
354, 343 288, 270 -
35 1 352, 348 334, 360 -
29 5 349. 342 352
358 262 340
P, paramagnetic; D, diamagnetic.
267 267 265 265 268, 269 268, 269 268, 269 270 272 275 275 277 278
-
Rhodium
.
933
Emerald green crystals of truns-[RhCl,(PR,),] can be obtained from the tertiary phosphine and hydrated rhodium trichloride at room temperature. The NMR spectrum is degraded and multiple peaks from v , ~ - ~in, the far IR spectrum suggest that the product is a trans rhodium(I1) complex.272 The closely related ligand, PBu:(o-C,H,OMe), also forms a paramagnetic rhodium(I1) complex. However, in the reaction the ligand is demethylated, and the product has a totally different structure to the halo complexes so far described in this secti0n.2~~ A similar reaction takes place between RhCl, .3H20 and the keto phosphine PBu:(CH,COPh), except that a rhodium(II1) complex is first formed containing the phosphaenolate. In the presence of sodium methoxide this rhodium(II1) complex is reduced to the paramagnetic rhodium(I1) species (30).274 The carbethoxy tertiary phosphine BukP(CH,),CO,Et (n = 1) forms an octahedral, cationic rhodiurn(JT1) complex (311, in which the carbonyl group coordinates to rhodium. Homologous ligands (n = 2, 3) from four-coordinate rhodium(1I) complexes in which the Ligands are only phosphorus bound."'
(30)
(31)
Although hydrated rhodium(I1) propionate merely has its axial aqua ligands replaced when it is allowed to react with PPh,, both the formate and acetate form orange mononuclear complexes under mild condition^.'^^ If diphos is allowed to react with the q3-2-methallylcomplex [RhCl, (q3-C,H,Me)] in the presence of hydrogen the gray-green complex [RhCl, (diphos)] is
48.5.1.2
Tertiary arsine complexes
There are no tertiary arsine complexes of rhodium(I1). Early reports of such complexes are erroneous.'Y6 The complexes have been shown to be hydridorhodium(TI1) species.
48.5.1.3
Tertiary stibine complexes
Rhodium(II1) halides form [RhX, (Sb(o-C, H, Me), )] complexes when allowed to react with tri(o-toly1)stibine. Despite their stoichiometry and diamagnetism there is no evidence for their being either o-metallated or hydridorhodium(II1) complexes. It is believed they have the planar dimeric structure (32).Intramolecular interactions in such dimers could account for their diamagnetism and the non-equivalence of the methyl protons in their 'H NMR spectrum.278
48.5.2
Dimeric Complexes
In 1960, Chernayev and co-workers reported that refluxing [RhC1,I3- in aqueous formic acid leads to a stable, dark green compound.279Despite its initial formulation as a Rh' formate, a formulation quickly retracted280when an X-ray study of the acetate analog showed it to be dimeric,28'this dark green material is generally acknowledged to be the first well-characterized complex containing the Rh;' moiety. Since then, interest in the structures, the metal-metal and metal-ligand bonding, and the reactions (e.g. substitutions, electrochemistry, chemotherapy, catalytic properties) of these complex has caused an explosion of research activity. These complexes
Rhodium
934
contain what is now generally recognized as a Rh-Rh single bond, and most, but not all, contain four ligands which span the Rh-Rh bond. They will be considered according to the type of bridging ligands, followed by brief discussions of the bonding and reactions of these complexes. Numerous excellent reviews have appeared recently and while overlap is unavoidable, the discussion here concentrates on work presented since their appearance.282-2s6
48.5.2.1
Synthesis and structure
( i ) Oxo-bridged dimers
The prototypical Rh" dimer is [Rh,(O,CMe),] and is readily prepared by refluxing RhCl, - 3 H 2 0 and (Na+/H+)O,CMe in ethanol, a mildly reducing solvent (equation 93).2R7,2XR Recrystallization of the dark green precipitate from methanol gives the dimethanol adduct, [Rh,(O,CMe),(MeOH),], which forms the parent [Rh,(02CMe),] when heated to 100°C for 30min. Yields as high as 85% (based on initial RhCl,) have been rep~rted."~ RhCI,*3H20 + (Na+jH+)(O,CMe-)
EtOH
[Rhz(O,CMe),]
(93)
The dimeric Rh" acetates, readily soluble in a wide variety of solvents, are air-stable Lewis acids which readily form 1:l and 1:2 adducts with many Lewis bases. They generally display two visible absorption bands at -600 and -450nm. The 450nm band is reasonably unaffected by adduct formation, while the position and intensity of the low energy band is extremely sensitive to the number and nature of the axially bonded ligands; two other characteristic transitions in the UV region near 250 and 220 nm are also axial-ligand dependent.2'o,29'(Spectral assignments are discussed in Section 48.5.2.2.)The thermal stability of [Rh,(O,CMe),L,] is dependent on the nature of the adduct L, and on the nature of the carboxylate bridge^.,^,-'^^ Much of our current understanding of [Rh,(02CMe), L2] complexes comes from numerous single crystal X-ray structures. Table 15 lists, in order of increasing Rh-Rh bond length, the Rh-Rh and average Rh-L bond lengths for the tetraacetate complexes whose X-ray structures have been reported. The Rh-Rh bond is short and remarkably invariant within this series, with less than 0.08 A difference between the longest and shortest examples. The structure of the dihydrate (33)297 is representative of the structure of all of these tetracarboxylato species. The [Rh,(O,CMe),] core symmetry, with the Lewis base adducts H,O in (33) coordinated to the sites has approximately D4,, trans to the Rh-Rh bond. There is little evidence of strain, as the average bond angles (e.g. Table 15 Bond Lengths (A) for [Rh,(O,CMe),L,J
L
MeOH MeCN
Rh-Rh
PF, MeNC VPh), PPh, (NO)(NO,)
2.377 2.384(1) 2.3855(5) 2.393(1) 2.395(1) 2.3959(6) 2.397(1) 2.3963(2) 2.4020(3) 2.4020(7) 2.4024(7) 2.404(1) 2.406( 1) 2.408 2.412(6) 2.4121(4) 2.413(1) 2.4193(3) 2.4215(6) 2.4333(4) 2.4434(6) 2.4505(2) 2.4537(4)
P(OM4,
2.4556(3)
H2O
4-CNpy
Caffeine" CI Ci PY PhCH, SH NHm, PhSH AAMP (Me), SO 7-Azaindole Theophylline" PhN(H)C(OMe)S Tetrahydrofuran
co
" S e e (36). hCited (ref. 93a) in ref. 282.
Rk--L
2.286 2.254(7) 2.31 O(3) 2.243(4) 2.315(9) 2.585(9) 2.601(1) 2.227(3) 2.55l(2) 2.301(5)
2.548(1) 2.292(9) 2.45l(2) 2.243 2.23(3) 2.556(2) 2.517(1) 2.091(3) 2.340(2) 2.158(5) 2.412( 1) 2.4771(5) 1.933(4) 2.010(4) 2.437(1)
commerns -
-
One MeCN in lattice -
As [C(NH,),]+ salt As Li+ salt
-
-
One-dimensional polymer S atom wordinahon -
S atom coordination One toluene in lattice I
L-NO L--N02 (N coordinated) -
Re$
295 296 297 282 298 299 300 301 302 303 299 304 305 b 300 301 305 303,306 306 299 307 307 303 306
Rhodium
935
Rh-Rh-O,,, 88.07"; Rh-0-C, 119.5O0; Rh-Rh-O,,, 176.03") suggest a comfortable fit of the acetates around the Rh:+ moiety.297The softness of the Rh" center is apparent, as both DMSO and PhN(H)C(OMe)S are sulfur bonded, and the nitrite ion is bonded through the nitrogen atom (Table 15). The binding to the axial sites is relatively weak, and long Rh-L bond lengths are a general feature of these tetracarboxylate dimers; for example, in [Rh,(02CMe)4(H,0)2], the bonds from the acetate bridges. Rh-OH, axial bonds are 0.27 A longer than the Rh-O,,
The readily prepared [Rh,(02CMe),] is the starting point for the synthesis of many other oxo-bridged Rh" dimers. While the mechanisms of these reactions are not known, a carboxylate exchange must occur, as heating [Rh2(O2CMe)Jin an excess of the desired carboxylate, usually in the protonated form, generally leads to virtually quantitative conversion to the substituted carboxylate (equation 94). Substituted carboxylate dimers have also been prepared by refluxing ethanolic solutions of RhC1,*3H20with the desired carboxylate, analogous to equation (93). As shown in Table 16, X-ray studies have shown that there is still little variety in the Rh-Rh bond length, and excluding the phosphate-bridged species, the difference between the shortest and longest Rh-Rh bond in this series is only 0.031 A. [Rh,(O,CMe),]
+
4H0,CR
+
[Rh2(O2CR),]
+
4H02CMe
(94)
While X-ray structures have not been reported, numerous [Rh, (Q)4LJ complexes have been Recent examples include complexes where Q is reported with a variety o f bridging ligands.282,283,286 the conjugate base of glutaric acid [-02C(CH2)3 COOH], a-propionic acid:" or mandelic acid (with L = H,O, NH,, N,H,, thiourea, DMSO, PhNH2);3'9the CD spectrum of the tetramandelate [Rh,(L-( +)-O,CC(OH)HPh),(H,O),] has been reported.320Reaction of RhCl, 3H,O with ethanolic solutions of sodium cinnamate (trans-PhCH=CHCOO-- Na+) leads to [Rh,(cinnamate),], which forms 1:2 adducts with DMF, MeCN and pyridine when dissolved in these solvents.32' Cotton recently reported the single crystal X-ray structures of two [Rh,(O,CR), L,] complexes, where R is a bulky hydrocarbon (R = 1-adamantane or 2-(PhC6H,).308The structures show little
-
Table 16 Bond Lengths Coniplex
(A) for some Rh:'
Oxo-bridged Dimers
Rh-Rh
Rh-L
2.371 (2) 2.371(1) 2.38 2.386(3) 2.387(I)
2.296(9) 2.29S(2) 2.45 2.33(1) 2,324(6)
2.396(1) 2.403(1) 2.404(1)
2.233(3) 2.275(6) 2.362(4)
2.4 17(1) 2.407( 1) 2.385(2)
2.413(3) 2.449( 1) 2.30(1) 2.30( 1) 2.344( 5) 2.601(3) 2.237(2) 2.29(1)
2.378(1) 2.380(2) 2.402(1) 2.483 I )
Comments
Ref.
-
308 305 309 310, 311 312
-
Only one H,O adduct -
Zigzag chains with DDA bridges -
Linear chains with PHZ bridges S atom coordination Rh-O(H)Et Rh-OK, -
Phosphate bridges
308 312 312 312 313 314 315 315 316 317
~
936
Rhodium
evidence of steric crowding, as the bulky groups art: far from each other at the tips of the bridges. When R = CPh,, a mixed bridged species [Rh,(02CMe)2{0,CCPh,)?(MeCN),] forms (see Section 48.5.2.1 .iii>. The salicylate ion (sal = o-02CC6H40H-) forms, via a bridge-exchange with [Rh,(O,CMe),], complexes of the form [Rh,(sal),L,] (L = NH,, py, aniline, en, urea, MeC(S)NH, and NZH,).~" Reaction of salicylate with RhC13. 3H20 in aqueous EtOH, followed by recrystallization of the solid product from absolute ethanol, leads to the interesting diadduct species [Rh,(sal),(C,H,OH)(H,O)], which has been characterized by a structural Bonding to the Rh, core is through the carboxyl oxygens, the phenyl rings are coplanar with the Rh-Rh bond, and three of the salicylate hydroxyl groups face toward one end of the dimer. This 3,l geometry is presumed to be sterically determined, as the ethanol is bonded to the Rh at the end of the complex with the three hydroxyl groups, while the water is coordinated at the more open end; there is evidence of hydrogen bonding between the phenyl hydroxides and the bridging carboxyl oxygens as in (34).3'2
(34)
Steric crowding is more evident when bulky ligands occupy the axial sites. Cotton has presented3I2 the single crystal X-ray structures of [Rh(O,CEt),(L),] (35; L = acridine (ACR), 7-azadine (AZA), phenazine (PHZ), or 2,3,5,6-tetramethyI-p-phenylenediamine (DDA); n = 1 or 2). For L = ACR or AZA (monobasic ligands), n = 2 and the normal coordination geometry occurs, consisting of discrete dimeric units with an ACR or an AZA bonded to each end of the Rh, core. In contrast, the dibasic ligands (L = PHZ or DDA) serve as bridges between Rh, units, and complexes of the stoichiometry [Rh(prop),L] are formed. For L = PHZ, almost linear chains occur (the Rh-Rh-N angle is ca. 174"), while the approximately tetrahedral amine of the DDA results in zigzag chains of -[Rh-Rh]-DDA-[Rh-Rh]-DDA-[Rh-Rh]-.3'z
ACR = acridine
AZA = 7-azaindole
PHZ = phenazine DDA = 2,3.5.6-tetrarnethylp-p henylenediamine
(35)
Interest in the antitumor properties of [Rh,(O,CMe),] has led to studies of adduct formation with bases of biological importance. Cyclophosphamide (36a) does not generally coordinate with metal systems, but adducts of the form [Rh2(0,CR),L,] (R = Me, Et, Pr; L = cyclophosphamide) have been reported323although the nature of the phosphamide-Rh bond is not clear. Single crystal X-ray structures of [Rh,(O,CMe),L,] (L = theophylline, caffeine; 36b) show they have the normal Rh" dimeric structure.298The surprising feature of the theophylline complex is that bonding to the theophylline is through N(9), rather than the N(7) generally observed for other theophylline-metal complexes. It is suggested that the repulsion between the carbonyl oxygen at C(6) and the bridging carboxyls of [Rh,(O,CMe),] hinders axial coordination at the N(7) site. Coordination through N(9) is also observed for the caffeine complex, where the methyl group prevents coordination at N(7).298 and C(8) coordinaCoordination at N(9) and at C(8) are observed in Cu'I-caffeine complexes,3243325 tion is observed in the [Ru"'(caffeine)(NH,), Cl,]' ion.326
(36a) cyclophosphamide (36b) theophylline, R = H caffeine, R = Me
(36c) AAMP
(36d) adenine
(36e) thiamine
Rhodium
.
931
Aoki has shown that AAMP (4-amino-5-aminomethyl-2-methylpyrimidine; 36c) reacts with [Rh,(O,CMe),] to form the 1:l adduct [Rh,(O,CMe),(AAMP)].3.5 H 2 0 , a one-dimensional polymer reminiscent of the DDA c o m p l e x e ~Coordination .~~ to the Rh2 units is through the N( 1) and the amino group from N(5), and does not involve the basic N(3) site. The bonding is end to end, with two amine nitrogens [from N(5)] bonded at each end of the same Rh, unit, and two pyrimidine N(l) atoms bonded at either end of the next unit; this can be represented as: --N')-[Rh-Rh] Lack of binding by N(3) is attributed -(N' -N5)-[Rh-Rh]-(N5-N')-[Rh-Rh]-(N'--. to steric crowding induced by the carboxylate oxygens, and suggests why tRh,(O,CMe),] does not react with polycytidlic acid bases, where the N(3) ring nitrogen is flanked by bulky carbonyl and amino groups; it also led Aoki to suggest that [Rh2(02CMe),]could develop into a useful probe for estimating the adenine (36d) containing regions of nuclei acids, as it should bind adenine much more strongly than the other nucleic acid bases. Aoki also reports the preparation of [Rh,(O,CMe),(thiamine)(H,O)]Y (36e;Y = NO;, PF; , EFT, ClO; ); the electronic spectrum of this cation suggests the thiamine is N bonded to the Rh, with N(1) favored over N(3) for the same steric reason.3w Use of fluorinated carboxylates as bridges between Rh" centers began with a report from Drago's group of the ESR spectrum of the 1:l adduct of [Rh,(O,CCF,),] and 2,2,6,6-tetramethylpiperidine N-oxide (Tempol; 37a).3'7Not surprisingly, the electron-withdrawing bridges enhance the Lewis acidity of the Rh centers. Numerous perfluorinated carboxylate dimers have been structurally characterized (Table 17), and while the variation in Rh-Rh bond lengths is a bit longer than that observed for the unfluorinated carboxylates, the total range is still small (0.086A). The increased acidity of the Rh centers is evident, as such weak bases as dimethyl sulfone (the first example of a structurally confined sulfone complex)328and nitroxide radicals (37b)330,332 form structurally characterized adducts. Unless the axial ligands are similar, it is inappropriate to compare the structures of fluorinated and unfluorinated carboxylate-bridged complexes, but for the H 2 0 and alcohol adducts, the complexes with the electron-withdrawing bridges have marginally longer Rh-Rh, and shorter Rh-0, bonds. R
A
ANA
But
\
I
N
/
But
I
0-
0.
(37a) Tempo, R = H Tempo1,R = OH
(37b) t-butyl nitroxide
The electron-withdrawing fluorides also seem to harden the Rh centers. For the soft phosphine and phosphate adducts, fluorinated bridges have the opposite effect on the Rh-L, bond lengths, as the Rh-P bonds are longer with the fluorinated bridges than with the electron-donating hydrocarbon substituents. More dramatic evidence of the effect of the electronegative bridges comes from studies of the potentially ambidentate DMSO adducts. Numerous DMSO adducts of unfluorinated tetracarboxylates have been r e p ~ r t e d ~ ~ and ~ ~the' electronic ~ ~ ~ ~and, IR ~ ~ ~ ~ ~ ' ~ ~ ~ ~ ~ * spectra of these orange complexes suggest sulfur coordination. In contrast, the DMSO adduct of [Rh,(O,CCF,),] is blue. Structural studies of [Rh,(O,CR),(DMSO),] (I2 = Me, Et, CF,) have confirmed the ambidentate nature of DMSO, as it is S-bonded in the unfluorinated c o r n p l e ~ e s ~ ~ ~ ~ ~ ' ~ An interesting 'structural isotope effect' is observed in the but is 0-bonded when R = CF3.313 Table 17 Bond Lengths
L
MezSO, EtOH TempoIb (CD,),SO H2O Tempob Me,SO Tempoh"
P(OPh), PPh,
(A) for
Perfluorinated Complexes of the Form IRh(O, CRhLzJ"
Rh-Rh
Rh-L
2.400(1) 2.403(6) 2.405(1) 2.408(1) 2.409(1) 2.417(1) 2.420(1) 2.424(1) 2.470(1) 2.486(I )
2.287(3) 2.27(1) 2.240(3) 2.248(3) 2.243(2) 2.220(2) 2.240(3) 2.238(5) 2.422(2) 2.494(1)
Unless otherwise stated, R
Comments
Re$
Oxygen-coordinated sulfone 4-OH coordination of Tempol 0 coordination of &-DMSO Two r-butyl nitroxides in lattice Oxy coordination of Tempo 0 coordination of DMSO Oxy coordination of Tempo
328 3 29 330 33 1 330 332 313, 331 332
= CF,. bSee (37) for
-
333 333
-
radical structures. R
= C,F,.
938
Rhodium
structures of [Rhz(OzCCF3)4(L)Z] (L = Me$O or (CD,),SO), as these complexes crystallize in different space groups, and the deuterated adduct has a surprisingly shorter Rh-Rh bond (0.012A). Such large packing modifications upon substitution of D for H are unusual, and may reflect the effect of the smaller size of the CD, Other sulfur-bonding adducts of the Rh:+ core have been reported for thiols,338t h i o e t h e r ~ , ~ ~ . ~ ~ ~ alkyl ~ u l f i d e ~ , thiourea 2~~.~~~*~~~ and benz~thiadiazole.~~’ Reaction of NCX- (X = S , Se) with [Rh,(O,CMe),] is reported to lead to the anionic 1:l adduct [Rh,(0,CMe),(NCX)]-.M3 A structure has not been reported, but based on analytical data, one-dimensional chains with bridging NCX- anions were p r o p o ~ e d . ~ ~ The nitroxyl radical adducts (Table 17) are dramatic evidence of the increased acidity of Rh centers when encased by the fluorinated bridges. Hendrickson did variable temperature magnetic studies on [Rh,(O,CR),(Tempo),] (R = CF3, Pr, Ph) and [Rh2(02CCF3)4(Tempo1)2].332 For the Tempo biradicals, an ‘astonishingly large antiferromagnetic exchange interaction’ between the biradical centers was observed. At 342K, the magnetic moment is 1.06BM, but by 80K the complexes are essentialy diamagnetic; the fact that the biradical centers are ca. 6.8 A apart makes this exchange all the more unusual. (The J values are 2-3 orders of magnitude greater than those observed for the tetrafluorocarboxylate dimer of molybdenum.) The significance of this biradical coupling to our understanding of the bonding in Rh” dimers is discussed in Section 48.5.2.2. Dirhodiumtetracarboxylates with N-donor adducts have been reported for numerous monodentate and bidentate bases, varying in complexity from ammonia through nucleic acids and adenine nucleosides and nucleotides; Table 18 Iists references for studies on such N-donating adducts. The structures of a variety of these complexes have been determined (vide supra), and while the range of Rh-Rh bond distances is greatest for N-donating bases, the range is still only 0 . 0 6 7 ~ . 2 s 2 Straightforward correlations between the Rh-Rh bond length and a specific property of the axial ligands have not yet appeared. Carbon-bonded adducts include cyanide ions’5h and is~cyanides.’~~ The structure of [Rh,(O,CMe),(MeNC),] is reported to confirm the formation of an axial Rh-C bond.299Interest in the CO adduct has been sparked by the question of whether (r or ‘/t bonding is invoived in the Rh-La, bond. A structural study shows that in [Rh,(02CMe),(CO)zJ the CO is linearly bonded through the carbon atom, and that the C-0 bond is 0.03 A longer than in free carbon monoxide.352This is difficult to understand, as several groups have reported that a small (between 39 and 60cm-’) decrease in the v, stretching frequency occurs upon coordination to the Rh;? moiety.335,352’58.359 The different experimental conditions used in the spectroscopic studies may account for the different C--0 stretching frequencies, but it is difficult to rationalize the apparent contradiction in the IR and structural information. Bridging by carbonate, sulfate and dihydrogenphosphate (02CR,for R = 02-, SO:- or P(0H); ) have also been observed in dirhodium(I1) complexes. The first report of a carbonate-bridged species came from Chernyaev’s and Wilson and Taube later prepared [Rh(CO3),I4- by the reaction of aqueous carbonate with the unbridged dimer [Rh,(H,0),,]4+ (abbreviated as Rh&).Z89 The similarity in the electrochemical properties of the carbonate and the acetate complexes led Taube to propose a carbonate-bridged dimeric structure, which was confirmed by structural studies on the diaqua and dichloro adducts (Table 16).3’5Bicarbonate-bridged complexes (R = OH-) have been r e p ~ r t e d , ~ ’but , ~ ~have ’ not been confirmed by structural studies. Taube also noted that the mobility of Rh:,&) on a cation-exchange column was markedly increased when sulfuric acid was used Table 18 Studies Dealing with N-coordinated Adducts of [Rh2(02CR),] Type of compound
R&h.
NH, or N2H4
290, 291, 336 340. 341
Aliphatic/cyclic amines
290,291, 303, 335, 336, 345
Pyridine
281, 290, 291, 301, 334-336, 340, 344-347
Other heterocyclic amines
291, 308, 312, 335, 341, 345, 347, 351, 438
Type of compound
Refs.
Aromatic amines
312, 351, 340
Nitriles
291, 299, 308, 334, 335
Nitric oxide Peptides Guanidine Imidazole Nitro Caffeine Amino acids Adenosine derivatives
291, 352 438 344, 353 354, 438 344 298 354, 355 350
Rhodium
.
939
as the eluant;364elution of Rh& with (NH,),SO, led to the isolation of the presumably sulfato-bridged (NH,),[Rh2(S04)4]-4.5H20. The diaqua adduct [Rh,(S04)4(H20)2]4-, isolated as the Cs+ salt, can also be prepared by heating [Rh,(O,CR),] (R = H or Me) in sulfuric A variety of H,PO; -bridged species have been reported,3'9,361.365 and structural studies on [Rh,(H,PO,),(H,O),] confirm the standard dioxo-bridged Rh core, but with an unusually long Rh-Rh (ii) Asymmetrically bridged dimers The dimers discussed so far have been bridged by four identical oxo ligands, with D4hsymmetry of the primary coordination sphere. Following work with Cr and W, a new class of Rh" dimers with substituted 2-oxypyridine bridges (38) has been developed. These oxypyridines are prototypes for the bridging ligands with two different coordinating sites, considered in this section. This ligand asymmetry introduces the possibility of geometric isomerization in the tetrabridged dimer; as shown in (39), the four possible isomers can be labelled the 4,O (C,"),3,l (C,), 2,2-1rans (&), and 2,2-cis (C,) isomers. Examples of all four of these forms have now been characterized.
chp = 6-chloro-2-oxypyridine, X = C1 fhp = 6-tluoro-2-oxypyridine,X = F
4,o
3,1
2,2-truns
2,2-cis
(39)
Synthesis of oxypyridine-bridged dimers can be achieved by the direct reaction of RhC13-3H,0 with the appropriate ligand in a mildly reducing solvent, as in equation (93), or by bridge exchange (equation 94), with the ubiquitous [Rh,(O,CMe),] as the common starting material. Data from structural studies on a variety of asymmetrically-bridged dimers are presented in Table 19. Initial studies of [Rh,(Q),] (Q = mhp or chp) showed that as with the Cr adducts steric crowding of the pyridine methyl groups hinders coordination at the axial sites, and the complex adopts the 2,2-trans g e ~ m e t r y .The ~ ~ ~1:1, ~adducts ~~ of imidazole, acetonitrile and Hmhp with [Rh,(mhp),], and the imidazole adduct of [Rh,(chp),] have been structurally characterized, and all adopt the 3,l geometry.368*369 With three methyl groups around one end of the molecule, the Lewis base is able to bond at the less-crowded end. Synthesis of [Rh,(mhp),(Hmhp)], by reaction of [Rh,(O,CMe),(MeOH),] with Na(Hmhp), also generates a tetrameric complex, [Rh, (mhp),],-2CH2C1,. This tetramer consists of two [Rh,(mhp),] units, each in a 3,1 geometry. with the open ends joined by two Rh-0 bonds between the Rh (at the open end of each dimer), and an oxygen from one of the bridging oxy pyridine^.^^ This structure is represented in (40).The 12.65MHz Io3RhNMR spectrum (in CD,Cl,/CH,Cl, at -300K) had a single resonance at 5745p.p.m. for [Rh,(mhp),], while the tetramer had a pair of doublets (split by 35 Hz) centered at 7644 and 4322p.p.m., consistent with the presence of two sets of inequivalent Rh Table 19 Bond Lengths (A) of Some Rh" Dimers with Four Identical, Asymmetric Bridges Rh-Rh
Rh-L
-
[Rh,(OSCM44 (MaS)OH),I [Rh,(ChP),l [Rh, (chp), (imidazole)] 3H,O [Rh,(fhP)me,SW
2.359(1) 2.365(1) 2.369(1) 2.372(1) 2.383(1) 2.384(1) 2.550(3) 2.379(1) 2.385(1) 2.410(1)
[Rh*(TFA),(PY),la
2.472
2.29
Complex
[Rh,(mhp),l [Rh,(mhp),l iRh2 (mhP)412*2CH2 c12 [Rh, (mhp), (M&N)I
+w
[Rh,(mhp),(Hmhp)J.O.5C,H, [Rh,(mhp),(imidazole)]-0.5MeCN
-
-
2.236(3) 2.152(7) 2.195(4) 2.144(4) 2.521(5) 2.129(9) 2.332(3)
aTFA = conjugates base of trifiuoroacetamide = CF,C(O)NH
Commenis
ReJ
2,2-transgeometry 2,2-zrans geometry mhp bridges 2 Rh, groups 3,l geometry 3,l geometry 3,l geomety 2,2-trans geometry 3,l geomety 4-0 geometry, S-bonded DMSO 2,2-cis geometry
366, 367 368 369 368 369 368 370 368 368 371 369
940
Rhodium
The complexity of the tetramer’s structure convincingly demonstrates the power of single crystal X-ray studies as a primary analytical tool. Perhaps an even more dramatic recommendation comes from the three imidazole adducts listed in Table 19, as imidazole was not a reagent used in their Hmhp (or Hchp) syntheses. The dimers were all prepared from a solid melt of [Rh,(O,CMe),] under argon, and the resulting solids were recrystallized from various solvents. The imidazole was identified by assigning atomic scattering parameters for either N or C to the different atoms in the ring; the only configuration with reasonable thermal ellipsoids was that of imidazole. The authors do not speculate (in print) about the source of the imidazole, and invite rational suggestions as to its The 4,O geometry is observed for [Rh,(fhp),(DMSO)] (38; X = F).3”As observed with the Cr, Mo and W analog^,^" the four fluorine atoms form a square around one end of the Rh, core, with a short F-F distance of 2.925 A. The DMSO is S-bonded, with an unusually short Rh-S bond (2.332 vs. 2.449 8, for [Rh,(O,C),(DMSO),]). Bear and co-workers have recently presented several reports on the chemistry of [Rh,(tfaa),], The tfaa bridge can where tfaa is CF3C(0)NH- , the conjugate base of triflu~roacetarnide.~~~~~’~~~~~ be considered to be a trifluoroacetate with a coordinated oxygen replaced by an isoelectric WH group, and represents the first N-0 bidentate bridges of Rh, not based on the oxypyridine system. Similar to the carboxylate-bridged dimers in gross structure and adduct formation the tfaa dimers display some distinctive properties. Reaction of [Rh,(mhp),] with Htfaa leads to a mixture of products, separable into four bands by HPLC. The approximate distribution of Rh in the four bands as 4%, 94%, 2% and < 1%; NMR analysis of the first 4% band suggests the 3,l isomer of [Rh,(tfaa),]. Structural studies of the bis(pyridine) adduct of the second band showed it had the 2,2-cis geometry with a relatively long Rh-Rh bond.372(This structural study was marred by an unidentified ‘mysterious’ diatomic molecule, which occupied 15% of the axial sites.) An anaiogous bridge-exchange of [Rh,(O,CMe),] with N-phenylacetamide (MeCONHPh), also leads to four HPLC separable products. NMR analysis suggests that the four products correspond to the four geometric isomers (39) of [Rh,(PhNOCMe),J, with the predominant product being the 3,l isomer.376 Much of the work with [Rh,(tfaa),] and its adducts centers on its interesting electrochemical properties, which are discussed in Section 48.5.2.3.(ii). Bridge exchanges between [Rh,(O,CMe),] and 2,4-dithiouracil (dt~)~’’ or thiobenzoic acid ( t b ~ ) ~ ” generate the diamagnetic complexes [Rh,(Q),(L),] (L = H,O for Q = ttu; L = py, thiourea, DMSO or DMF when Q = tbz). IR analysis indicates that a dtu nitrogen is deprotonated upon coordination, and that the dtu exhibits N and S bonding to the Rh, core, although a specific geometry is not suggested. Reaction of RhC1,- 3H,O with 5,S-thiodisalicylic acid in acid (pH 1) solution is reported to give an ill-characterized, paramagnetic (p = 2.60 BM) brown complex of stoichiometry [Rh(C,,H, o6s)*4H20].379
+
-
(iii) Mixed-bridge dimers
Recently, Rh”dimers with more than one type of bridging ligand in the same complex have been characterized; these will be considered along with a few complexes with less than four bridging
Rhodium
‘
.
94 1
ligands. Reaction of the Rh’ dimer [Rh,CI, (p-CO)(DPM),] (DPM = bis(dipheny1arsino)methane = Ph,AsCH,AsPh,) with HA (A- = C1-, SO,Ph-, M e - , BFi ) does not cause the expected oxidative addition, but leads to [Rh, Cl,(p-H)(p-CO)(DPM), (A)].’” The single crystal X-ray structure of the HC1 adduct, [Rh,Cl,(p-H)(p-CO)(DPM),] shows it to have two mutually trans bridging DPM ligands; the mutually trans H and the CO also serve as bridges, and the IR bands ( vco between 1748 and 1818 cm-’ and vfi-H between 1230 and 1267 cm-’) suggest that all of these complexes have structures similar to the trichloro complex. The Rh-Rh distance (2.7464 A) is Iong, but still within the bonding range. This complex is unique among Rh” dimers in several ways, as it has three different bridging ligands, and has three axial ligands (CI-), two at one end and the third at the other end of the dimer.3s0Oxidative addition across the Rh‘-Rh’ core (a transannular oxidative addition) does occur when I2 reacts with [Rh~(CO),CI,(DPM),], generating the (DPM)-bridged Rh” dimer, [Rh(CO),Cl, I, (DFM),].38’ Similar reactions have led to the diamagnetic dimers, [Rh,(CNR),(X,)(DPM),] (X = I, Br),3B2 and [Rhz(CNR),(Ph,AsCH,AsPh,)T,], and
[Rh(CNR)4(DPM),(SCF3),].383 Mixed-bridge complexes based on a bridging carboxylate frame have been obtained from the products of the bridge exchange between [Rh,(O,CMe),] and Five different complexes apparently crystallized from this single brew (vide supra), and two of them were [Rh,(O,CMe), (mhp),(im)] and [Rh,(O,CMe),(mhp),(im)] * 2CH,CI,. Both complexes exhibit mutually trans acetates and oxypyridines, with both pyridine-methyl groups pointing toward the same end of the dimer; the imidazole is coordinated to the other, more open, end. As with the [Rh,(mhp),(im)] (vide supra) the source of the imidazole is unknown.368All five members of the series [Rh,(O,CMe), (NH(O)CMe),-,] ( n = 0,1,2,3,4) have been prepared via bridge exchange between [Rh,(O,CMe),] and MeC(0)NH- .384The five complexes were separated by HPLC, but the exact geometric structure of the substituted species was not determined. In their studies of [Rh2(O2CR),] with bulky R groups, Cotton and Thompson noted that the bridge exchange between [Rh,(O,CMe),] and Ph3CCO; generates [Rh2(02CMe),(0,CCPh,),(MeCN),].308 Other bulky substituents (R = 2-PhC6H, or 1-adamantane) replaced all four acetate bridges, and it is not clear why the reaction with triphenylcarboxylate stops after the replacement of only two acetates. (Kinetic studies of acetate replacement by trifluoroacetate in [Rh,(O,CMe),] indicate that the reaction slows markedly after replacement of two acetate bridges?’’ so a kinetic factor may be involved.) Steric crowding cannot be important, as [Rh,(O,CMe),(O,CCPh,),(MeCN),j has the cis configuration, and there is adequate room for two additional triphenylcarboxylate bridges. Cotton points to a stacking of the phenyl groups from adjacent bridges as a possible contribution to the stability of the cis geometry.”‘ A new series of mixed-bridge dimers, recently characterized by Ford and his group, are based on Upon mixing the crescent-shaped bpnp with [Rhz(02CMe),] a 1,8-naphthyridine frame (41).38”388 (in acidic methanol under argon in the presence of NH,PF,), substitution of one acetate bridge occurs, and the ‘croissant complex’ [Rh2(0,CMe),(bpnp)]PF, can be isolated. A single-crystal X-ray study (Table 20) reveals that the bpnp acts as a bridge, through the 1,s-naphthyridine nitrogens, allowing the pendant pyridines to bond to the axial sites at either end of the Rh, core. Other complexes in this series include [Rh,(O,CMe),(L)]+ (L = dinp or pynp), [Rh,(np),]C&, [Rh, (0,CMe),(pynp),]*+ ,387 and [Rh, ( ~ y mCl,](PF,), )~ .384 NMR analysis of [Rh, (0,CMe),(pynp),]’+ indicates that the acetates and the pynp bridges have a mutually cis ge~rnetry.”~ A single-crystal X-ray study of [Rhz(pynp),C12](PF6)2revealed its unusual geometry, represented in
PYnP
dinp
942
Rhodium Table 20 Bond Lengths (A) of some Miscellaneous Rh" Dimers Compkx
Rh-Rh
Rh-L
Cornmenis
2.388(2) 2.388(1) 2.388(2)
2.17(1) 2.133(7) 2.19(1)
368 368 308
2.405(2) 2.7464(7)
2.196(16) 2.358(2) 2.393(2) 2.462(2)
Trans configuration Trans configuration Cis configuration, with a toluene of crystallization Axial nitrogens from bpnp bridge Trans phosphines, with three terminal chlorides
2.5668(7) 2.576 2.6 18(5)
2. I75(15) 2.506 2.485(9)
Axial nitrogens from pymp bridges
3 88 385 390
2.625(2) 2.785(2) 2.936(2)
-
2.837(1)
2.447(1)
Long R h n bond
3 94
2.317
2.22
Oxidation decreases both Rh-Rh and Rh-OH? distances
395, 3%
-
Eclipsed DMG groups
Ref
386, 387 380
39 I 392 393
2.735( 1) 2.438(4)
C,,H,,N, = 7,16-dihydr~6,8,15,17-tetramethyldibenzo[h,1~[1,4,8,1 l]tetraazacyclotetradecinate (a macrocyclic dianion); im = imidazole; mhp = 2-owy4-methylpyridinz; DPM = bis(rtipheny1ph~sphino)mcthane; bpnp = 2,7-bis(2-pyridyl)-l ,haphthyridine (41b); pynp = 2-(2pyridyl) 1,8-naphlhyridine (41c).
a
(42), as it has only two bridging ligands. The pyridines pendant to the pynp bridges act as the axial donors, while the other pynp acts as a non-bridging bidentate ligand; the two chlorides bond in equatorial positions to the other Rh atom.3s8The two naphthyridine-bridged complexes characterized to date have longer Rh-Rh bonds than observed for the purely acetate-bridged complexes, but the length is still within the range of a Rh-Rh single bond. Two other complexes with only two bridging ligands have been characterized by X-ray studies. CH),(phen),]Cl, is a 1:2 electrolyte in aqueous solution, suggesting that the chlorides are [Rh2(02 not axially bonded, or that they are readily labilized in aqueous solution.3s9This bis(bridge) complex has a Rh-Rh bond (2.576 A) which is CQ. 0.2A longer than that observed for the tetracarboxylatebridged dimers, but is comparable to the 2.61 8 A observed for the other bis(bridge) complex, [Rh?(0,CMe), (DMG), (PPh, )21-390 While its structure has not been determined, the first well-characterized bis(bridge) complex was characterized by Ugo and co-workers, who noted that the reaction of [Rh,(O,CMe),(H,O),] with acac, trifluoroacetylacetone, or hfacac (Q) gives complexes of the form [Rh,(O, CMe),(Q),] (H20),. Adducts with py, 2-methylpyridine and 2-chloropyridine were characterized, and analysis of electronic and IR spectra, and the 'H and P NMR led Ugo to propose bridging acetates, with the acacs each chelated to a single Mixed a~etate/salicylate,~~~ acetate/pho~phate,~~ and formate/ bipyW complexes have also been reported, but the nature of the bridging has not been determined.
(iv) Dimers without bridging ligands
Several claims could be supported for the first preparation of a Rh" dimer with only the Rh-Rh bond to hold the dimer together. In 1961 Martin and co-workers reported that sodium amalgam reduction of [Rh(bipy),Cl,]X (X = NO;, ClO;) leads to red-violet solids with the stoichiometry [Rh(bipy),C1(X)],-4H2O (X = NO;, ClO,-)." They are diamagnetic, and were presumed to be chloro-bridged dimers; the preponderance of Rh"-Rh" bonds discovered since 1961, and the lack of halogen-bridged dimers, suggests that this work should be reinvestigated. In 1967, Shchepinov and co-workers used BH; to reduce [Rh(DMG),Cl,]+ in the presence of PPh,, generating [Rh,(DMG),(PPh,),], a diamagnetic, air-stable solid.402 Shchepinov's suggestion that [Rh,(DMG),(PPh,),] contained a Rh-Rh bond was confirmed in a structural study, which found a long (2.936 A) Rh-Rh bond, the longest Rh"-Rh" bond yet determined.393The nearly planar bidentate DMG ligands are staggered on the two Rh atoms, and are bent towards each other and away from the axial PPh, ligands; steric repulsions between the DMG ligands are thought to be responsible for the long Rh-Rh bond.3Y3Keller and Seibold prepared [Rh,(DMG),] by reacting
I
Rhodium I
943
HDMG with [Rh2(OzCMe),], an interesting bridge + no bridge c o n v e r ~ i o n .Complexes ~~ with DMG derivatives, and the py and THF adducts, were prepared by this same An X-ray photoelectron study of several DMG-containing Rh" and Rh"' complexes has been r e p ~ r t e d . ~ ~ Soon after the preparation of [Rh,(DMG),(PPh,),], Maspero and Taube reported that Crf& reduces [Rh(H,0),C1J2+ to give a species with cation exchange behavior which implied a charge greater than 3 + (equation 95).362This complex was formulated as the [Rh,(H,0)l,]4+ dimer [Amax at 630 (~39.8),412 (59.1) and 250nm (l0600)]. Attempts to isolate a solid with a variety of anions (PF;, BF;, BPhy , BIoH:i, B,,C1:; and [Fe(CN),I4-) failed, and halide-catalyzed disproportionation to Rh' and Rh"' also interfered. A solid salt of this important cation has not yet been reported. The complex is reported to be 'slightly paramagnetic', which led Taube to suggest that the dimers are in equilibrium with the paramagnetic Ziolkowski later suggested that the paramagnetism may result if the HOMOS are two degenerate .n* orbitals populated by two unpaired electrons;'' and a study of the magnetic susceptibility of a variety of tetraacetate-bridged dimers also showed that a small temperature independent paramagnetism (pcEnear 0.5 BM) was a common feature of these Rh" dimers.407 2[Rh(H20)5C1]2+
+
[CI-(H~O)~]~' -,
+
[ C T ( H ~ O ) ~ C ~ ] ~[Rh,(H20),,]4+ +
(95)
Reduction of [Rh(OEP)Cl] (OEP = octaethylporphyrin) with H2(g)leads to the Rh"' hydride [Rh(OEP)H], which, when heated in benzene or oxidized with O,, forms the brown diamagnetic [Rh,(OEP),] (equation 96).4°8*409 The OEP ligand is a rigidly planar cyclic dianion, so ligand bridging is impossible. The dimer undergoes a variety of reactions with substituted hydrocarbons, leading to complexes with Rh"'-C bonds (Section 48.5.2.3.G).
2[Rh(OEP)CI]
+
H,
+
2[Rh(OEP)(H)]
{-;
$02
[RhIOEP),I
f
HzO
[Rh*(OEP),I
+
H,
(96)
A. CbHh
During studies on a series of Rh' isonitriles, Olrnstead and Balch found that a series of dimeric Rh"-isonitrile complexes could be generated by the reaction of appropriate Rh' and Rh"' complexes (equation 97):'' This reaction was found to be quite general (for X = I, R = Me, Bun, Ph, C6HlI; and R = C6Ht,for X = Cl), but not universal, as no reaction is observed if R = But and X = Br or I. A halogen-bridged dimer would require LittIe atomic motion, but IR analysis supports a Rh-Rh bond, with axial ha1ides:'O Support for this proposed structure came when it was shown that analogous compounds can be prepared by the transannular oxidative addition of monomeric [Rh(p-MeC,H,NC),]PF,,192 or dimeric [Rh2(bridge)4]2+(bridge = 1,3-diiso~yanopropane).~" Irradiation of [Rhz(bridge),l2+in conc. HCl causes H, formation and the generation of a similar dimer [Rh,(bridge),Cl,]'+ .394 A single-crystal X-ray structure confirmed the traditional Rh-Rh bonded structure, with short non-bonded interactions between eclipsed C-N groups on adjacent Rh atoms contributing to the length of the Rh-Rh bond (2.837 A).394Another (presumably bridged dimer) has been prepared by the two-electron electrochemical oxidation of [Rh, (dimen),]" (dimen = 1,8diisocyanomenthane). In the presence of axially coordinated C1- , oxidation occurs a t low potentials, and in the presence of PF; (in MeCN or CH,Cl,) a yellow salt, identified as [Rh,(dimen),CI,]fPF,),, has been isolated.412 [Rh(RNC),]+
+
[Rh(RNC),X,]+
e
[Rh2(RNC)8X2]z+
(97)
While studying Schiff base complexes of &I1' (see Section 48.6.4.1 for a discussion of structures), West and co-workers reported that if RhC13.3H20 or [Rhpy,Cl,]Cl (in pyridine) is mixed with methanol solutions of N,N'-ethylenebis(salicy1ideneiminate) (H, salen) and a reducing agent (Zn This complex remains dust), a diamagnetic, pale yellow solid, formulated as [Rh(salen)py], form~.~'' ill characterized, and while it displays an uncharacteristic chemical inertness, West suggested that Reaction of equimolar it may be an unbridged Rh" dimer, analogous to the bis(DMG) ~omplexes.4'~ mixtures of [Rh(C0),Clz],, salenH, and a base (Et3N or NaHCO,) in refluxing methanol leads to a weakly paramagnetic (peE= 0.90 BM) material formulated as Rh(salen):l4 The corresponding Schiff bases with 1,Z-propylene (salpn) and o-phenylene (saloph) chains between the two salicylideneiminato groups also form 1 :1 complexes with Rh, but both [Rh(salpn)], and [Rh(saloph)], are diamagnetic, suggesting that they are undissociated dimers, while [Rh(salen)], is in equilibrium with its monomeric form. All three react with 0, in air to give paramagnetic oxygen adducts (e.g. [Rh(salen)O,]), formulated as hyperoxo Rh'" ~pecies.4'~ The 0, oxidation of [Rh(CO),CI,], in dimethylacetamide (Me,NC(O)Me = DMA) in the
944
Rhodium
presence of LiCl and Ph,As+ leads to a solid identified as (P~,As),[R~~C~,(DMA),].~~~ The solid has a low magnetic moment ( p = 0.95 BM), and is a 2: 1 electrolyte in DMA, although in CHCI, its molecular weight is ca. 23% lower than expected. Further oxidation leads to [Rh,C1,I3-, a characterized anion in which two pyramidally distorted RhC16octahedra share an octahedral face (Section 48.6.7). This led to the proposal that the two Rh atoms in [Rh*C&(DMA),]'- are bridged by a pair of chloride atoms (two [RhCl,(DMA)] square pyramids sharing a basal edge)!" If true, this is a unique Rh" dimer; an alternate structure with a Rh-Rh bond is not difficult to imagine. 48.5.2.2
Bonding and electronic structure in Rh" dimers
Two related features of Rh;+ dimers of obvious interest are the mcta1-metal bond, and the range of colors displayed by various adducts. The colors range from blue to green for oxygen donors, red to orange for nitrogen and sulfur donors, and red to yellow for phosphorus donors. The spectra generally consist of two relatively weak ( E 200-300) transitions centered near 600 nm (Band I) and 450nm (Band 11). The position of Band I is very dependent on the number and nature of axially bound ligands, while Band I1 is virtually invariant to the presence or absence of numerous ligands. Two intense bands (Band I11 near 250 nm and Band IV near 220 nm with E > 10000) occur in the UV. These two bands undergo a ligand-dependent red shift as the axial ligands become more tightly bonded. Structural studies have shown that the Rh-Rh bond length varies by more than 0.5& from 2.371 to 2.936A (see Table 17 and 21). To understand the spectra and structural features of these complexes clearly requires an understanding of their electronic structure. Few areas in rhodium chemistry have received such heated discussion, and while a consensus seems to have emerged on the Rh-Rh bond order, questions remain to be resolved. The original report (in 1962) on the structure of [Rh,(02CMe)4(H20)2]281 suggested a Rh-Rh single bond; the Rh-Rh bond length (2.45 A) and the observed diamagnetism could be rationalized with such a formulation. In 1970, Martin used extended Hiickel calculations to arrive at an 7c*4,290 supporting the single bond electronic configuration for the Rh, core (14e-) of 7c4, a', S2, hypothesis. The axial-ligand dependent absorption (Band I, 600 nm) was assigned as a n* + o* (Rh:+) transition. Band 11, relatively unaffected by axial ligands, was assigned as a 7c + (6, a*) or 7c -,(6*, a*) transition, again within the Rh, core. Just before Martin's extended Hiickel analysis, Cotton and co-workers suggested that the manifold of occupied frontier orbitals included two non-bonding orbitals which were essentially the 5s and 5pz orbitals of the Rh atoms (the Rh-Rh axis generally defines the z-axis). This led to a 14ecore configuration of 0 2 ,7c4, S2, ai, a':, P 2 ,with two empty x* and an empty a* orbitals, and a Rh-Rh triple bond.393,416,417 This configuration also accounted for the diamagnetism, and was soon supported by a reevaluation of the Rh-Rh bond length in IRh,(02CMe),(H,0),]. The new value, 2.3855 A,297was ea. 0.065 A shorter than the earlier reported and more than 0.5 shorter than the only other Rh-Rh bond length known at the time (2.936 8, in [Rh,(DMG),(PPh,) An estimate of the Rh" covalent radius (1.4G1.45 set a 'rather firm lower limit of 2.80 for the Rh"-Rh" single bond.297,393 Any bond which is 15% shorter than the minimum single bond length must be considered a serious candidate for a multiple bond. The triple bond formulation did not receive theoretical support, however, as in 1978 Norman and Kobdri presented their SCF-Y-SW calculations on [Rh,(O,CH),] and [Rh2(02CH)4(H20)2]~19 Their calculations indicate that the non-bonding 5s and 5p, Rh-centered orbitals, important in Cotton's analysis, are at high energy and remain unoccupied. Their 14-electron core is a,, n4,S2,7 ~ * ~ , 6*2 with only the o* unoccupied; this represents a single, 0 bond, as predicted by Martin."' The electronic transitions were assigned as: Band I, n* -+ a* (within the Rhz core); Band 11, E* -+ o* (Rh-0,); Band 111, o + c* (Rh,); and Band IV, a(L) + a*,in reasonable agreement with Martin's earlier assignment.290Norman notes that the electronic spectra for the unbridged Rh&, and the bridged [Rh,(C0,),I4- and [Rh(S0,),]4- ions are very similar to the spectra of the carboxylate-bridged dimers, and suggests analogous electronic configurations for the i0ns.4'~ When water molecules are brought in along the Rh-Rh axis, Norman's analysis indicates that there is very little 7c interaction with the metal, and that the 7c and 6 MOs of the metal-metal bond are virtually unperturbed by the presence of the H 2 0 .As for a bonding, within ulgsymmetry axial binding by water stabilizes both the Rh-OH, o-bonding and antibonding orbitals so that both are fully occupied, and virtually no net bonding results. In a2"symmetry, the Rh-Rh bond destabilizes the Rh-Rh 6 % orbital, lessening its interaction with the a orbitals on the water. This nicely accounts for the observations that the metal-axial-ligand bonds are significantly weaker than normal in these Rh:+ dimers, and that the stronger the metal-metal bond (that is the higher in energy the Rh-Rh
i'
945
Rhodium
c* orbital) the weaker the metal-axial-ligand bond. Thus, there is a mutual interaction, as the presence of axial ligands weakens the Rh-Rh bond, while the Rh-Rh bond weakens the Rh-axial-ligand bond.4' This analysis also accounts for the observed changes in the electronic spectra. Axial ligands strongly influence the energy of the Rh-Rh cr* orbital, so the energy of Band I [n* + a* (Rh,)] would be expected to be dependent on the number and nature of the axial ligands. Band I1 (- 450 nm) was assigned to the n* -+ a* (Rh-O,), which is virtually unaffected by the presence or absence of axial ligands. The axial-ligand dependency and the greater intensity of Bands I11 and IV are consistent with their assignment as a -+ cr* transitions. These spectral assignments were supported by a single crystal polarized absorption s t ~ d y . ~ " In 1981, Nakalsuji et al. presented the results of an ab initio SCF study on [Rh,(O,CH),], various adducts, and the corresponding cation^.^" WhiIe their proposed electronic configuration was not exactly the same as that suggested by Norman and Kolari, they did agree that a Rh-Rh single bond of a symmetry exists in these dimers. Their analysis showed just how sensitive the electronic configuration is to the nature of the axial ligands, for when there are no axial ligands, or when L = H,O or NH,, they put the electronic configuration of the 14electron core (for [Rh2(0,CH)4(L),] at n4, 62, n*4,8*,, 0'); when L = PH,, the ordering of the two lowest levels in the Rh, core reverses with the net result still being a single cr bond. This analysis was extended to the monocation radical dimer,,', and the calculations are consistent with ionization of an electron from the HOMO u orbital, leading to a cation with the unpaired electron in an orbital of t~ symmetry. This assignment is supported by ESR ~ t u d i e s . ~ ~ ~ , ~ " was also explored The differences caused by axial coordination by P, vs. bonding by either N or 0, in an SCF-X'SW molecular orbital calculation by Bursten and Cotton."' These studies are of special interest, as Drago has presented several studies (vide infra) which imply the importance of n back bonding in phosphine adduct formation. Neither the X"calculations of Cotton425nor the ab initio calculations of N a k a t s ~ j i ~ *supports ' , ~ ~ ~ the existence of any significant amount of K bonding, as the only effective Rh-P bonding is through the Rh dz2orbital, and is thus of c symmetry. The ESR spectra on [Rh,(O,CR),(PX,),]+ cations also imply the dominance of Rh-P u bonds. Gray and co-workers have recently presented a single-crystal polarized spectral analysis for [Rh,(O,CMe),(H,O),] and for Li,[Rh2(02CMe),C1,]*8H,O, and propose new assignments for the visible a b ~ o r p t i o n s Both . ~ ~ complexes showed similar spectra, with Band I split into two electricdipole-allowed transitions near 650 (z-polarized) and 600 nm (x,y-polarized). The 650 band was assigned to a 6*(Rh2)-+ u*(Rh-0) band, while the band at ca. 600nm was assigned as n*(Rh,) + a*(Rh-0). Weak but structured transitions (c z 10-20) between 800 and 650 nm are assigned to the spin-forbidden components of these 6*, n* + u*(Rh-0) transitions. The assignment of Band I as a Rh, + Rh-0 transition, rather than the previously assigned Rhz core t r a n s i t i ~ n , ~ "is~based ~ ' ~ ~ on ~ ~the ~ x,y-polarization of the major 600nm band and the 300cm-' vibrational spacing, which corresponds to a Rh-0, not a Rh-Rh, stretch. The reasons for the sensitivity of Band I to axial ligands is not as straightforward as in the previous assignment, and is proposed to be due to axial-ligand-induced variations in the energy of the occupied K* orbital. Band I1 is assigned to n(Rh-0) -+ a*(Rh-0). Isotropic (solution) spectra were then used to assign the two UV bands to the a(Rh,) -+ o*(Rh,) and cr(Rh-L) -+ t ~ * ( R h , ) . ~ ~ Use of vibronic spacing for electronic band assignment depends on knowledge of stretching frequencies, and estimates of the Rh-Rh stretching frequency in Rh;' carboxylates have varied: 155cm-' for [Rh2(O,CMe),lQ6, 170 cm-' for [Rh,(0,CMe)4(MeOH)z]427and between 288 and 351 cm-' for a variety of carboxylates, with 320 m-' assigned to the Rh-Rh stretch in the diaquatetraacetate dimer.428In light of work with Ru, tetracarboxylate dimers (with significantly shorter metal-metal and the value of 184cm-' for the Rh-Rh stretch in [Rh,(mhp),],430 Gray questions the 288-351 cm-' range proposed by Oldham428and suggests that a range of 150-170 cm-' is more re~onable.~''" Gray then makes the interesting observation that this is the same region as the Rh-Rh stretch in [Rh2(bridge)4X2]2+, which has a Rh-Rh bond 0.4A longer than the tetracarboxylate complexes. This represents an unusual, but explicable, breakdown of Badger's rule, which correlates a given bond length with the cube root of its force c0nstant.4~'(It is ironic that in the same manuscript Gray invokes Badger's rule to dispute the 288-351 cm- ' assignments of Rh-Rh stretching frequen~ies.~"") Electronic transitions in [Rh,(O, CMe),L,], and in its cation radical, have been qualitatively interpreted in terms of a molecular orbital model which assumes that metal-ligand interactions are larger than intermetallic interaction^."^^ The intense near UV bands (Bands 111and IV) are assigned to intermetallic -+ n* transitions; the bonding model qualitatively accounts for the observed red shift as ligand a-donating ability increases. This is in agreement with the assignments of Gray et
-
Rhodium
946
Table 21 Proposed Models for the Rh-Rh Method used I
Bond in Rh” Tetracarboxylate Dimers
Rh-Rh bond order
Molecular orbitals ordering
.
Dubicki, Martin (240) Cotton et ul. (416, 417) Norman, Kolari (419) Nakatsuji el ai. (421)
Extended Hiickel
1
Qualitative MO and empirical arguments SCF-X”-SW
3
&Za462U?;a’:6*2
1
dn4
Ab initio-SCF
1
If L = H,O, n4 6 2 n + 4 p 2 ,I
.
n4a2626*2n*4
NH,, or no L
If L = PH, oZ , 4 n * 4 p
Nakatsuji et ul. (423)
#2
For cation radical, if L = H,O, NH,
-
Ab initi+HF
52n47c*46*2 u’
For cation radical, if no L singly occupied 6*
and Norman and K0lari.4~~ For the cation radical, however, the corresponding transition does not to suggest that the odd electron in the show the same ligand dependency, leading cation radical is not accommodated in the Rh-Rh 0 bond, directly contradicting the conclusions reached by N a k a t s ~ j i ? ” , ~ ~ ~ To summarize the current position on the Rh-Rh bond, the ‘triple-bond windmill’36*seems to have stopped spinning, and workers in the area seem to have reached a consensus, and refer to a Rh-Rh single bond. Agreement ends at that point however, as generally accepted electronic configurations and spectral assignments have not yet emerged, and will doubtless remain a contested and stimulating subject for some time. Table 21 summarizes the conclusions reached by the major theoretical studies on Rh”-Rh’’ systems. The nature of the bond between Rh and an axially coordinated ligand (Lax)has also been debated in recent years. There is no argument that axial Rh-L bonds are significantly weaker and longer than bonds to the equatorial bridges, and that the ligands at axial sites are more readily r e p l a d than the bridging ligands. The strong trans effect (and influence) of the Rh-Rh bond is often invoked to account for these observations.”’ Debate has centered around whether the Rh-LaX bond involves CT or n bonding. The opening salvo came from Drag0 and coworkers, who studied the thermodynamics of the formation of the 1:l and 2:1 adducts of eight different bases with [Rh(buty),] (buty = C,H,CO;> in benzene solution (equation 98).’>’They found K , was approximately three orders of magnitude Despite the differences larger than K,;if uncoupled, statistical arguments would have Kl = 4K2-359 between K , and K,, AH, w AH,, implying a large entropy effect. Drago suggested that [Rh,(buty),] forms 2: 1 adducts with benzene when dissolved in that solvent, but that coordination of an added base to one end causes the release of both benzenes (equation 99); the large entropy increase of this step contributes to the first equilibrium constant. Also noting that in benzene solution both acetonitrile and carbon monoxide form adducts with a decrese in C-N and C-0 stretching frequencies respectively, Drago suggested that extensive .n back-bonding must be involved in the Rh-L, bonding. The simplified MO diagram (Figure 1) shows that .n bonding lowers the n* level, and increases the energy of the .n* + CT*transition. The presence of n back-bonding was supported by an analysis of the reduction potentials of the oxidation products [Rh,(buty),L,]+ (n = 0, 1, 2)359 and by a similar thermodynamic study in CH, Cl,, a non-coordinating solvent.433 [Rh,(bUtY),l
Kl
[Rhz(buty),LI
Kl
=y [Rh,rO~tY)‘iLl
‘6”6
[Rh2(buty)41
e [Rh2@UtY)4(C6H6)21 4,
[Rh2(buty)4Ll
+
(98) 2C6H6
(99)
Bursten and Cotton then presented the results of an X“-SW molecular orbital calculation on [Rh,(O,CH),(PH,),] in which they found no evidence of Rh -+ P back-bonding, and commented Other that the ‘evidence on the occurrence of .n-bonding is neither extensive nor ~traightforward’.~~ theoretical studies have implied that Rh-L, bonding is predominantly The X-ray photoelectron spectra of [Rh,(O,CMe),] and its 1:l adduct with 4-cyanopyridine show that the adduct displays lower binding energies (Rh 3d5,?and Rh 3p,,;) than the parent complex. If extensive back-bonding to the 4-cyanopyridine existed, a decrease in electron density of the Rh, core (and an increase in binding energies) would be expected; this was interpreted to point to the dominance o f
I Rhodium
u only
947
U + T
Figure 1 Simplified MO diagram for adducts of [Rh@uty)J with bases in benzene solution
'
cr effects over any back-bondingP4 (Interestingly, the two Rh" centers were indistinguishable in the 1 :1 adduct with Ccyanopyridine.) Drago has presented additional thermodynamic arguments and a comparison between Rh$+and Mo:+ systems to support the position that for some ligands, the admittedly weak Rh-L, bond can have a significant n: ~ o m p o n e n tA. ~recent ~ ~ ~ESR ~ study of a variety of 1: 1 and 2:I adducts of the oxidized [Rh,(buty),L]+ ion led to the suggestion that the ordering of the filled molecular orbitals depends on the nature of the axial ligands, and that for the 2:l phosphine adducts the HOMO is singly degenerate (presumably a cr* orbital), while for the 1:l phosphine adducts, or for complexes with weak donors, the HOMO is the doubly degenerate n:* The debate over the importance of n bonding of axially coordinated ligands has not yet been resolved, but the case for the existence of some 'IIinteraction seems to be growing. For example, Bear and coworkers determined the equilibrium constants for the 1:l and 2:l adducts of [Rh2(02CMe),] with a variety of di- and tri-peptides, substituted pyridines and imidazole, and found that the stability of the L-Rh bond does not correlate with the base strength of the entering ligand; this led Bear to suggest that more than cr bonding is involved, and that n: bonding must be ~rucia1.4~' Ironically, a recent study of the thermodynamics of the formation of 1:l adducts between [Rh,(02CCF3)4]and I 1 different alkenes found that the K,, values (between 6.1 for styrene and 578 for 2-methylpropane) do not correlate with association constants between these alkenes and more traditional 'IC donors such as Rh', PdI', Pt" and Ag'.439Activation by the electron-withdrawing fluoroacetates is necessary, as no reaction is observed between alkenes and [Rh,(O,CMe),], suggesting that back-donation from the Rh-Rh system may not have a dominant iduence on Rh-La, stability. The case for n bonding is supported by the large antiferromagnetic exchange interaction between radical centers in [Rh(O,CR),(Tempo),] (R = CF3,Pr,C6F5) which was rationalized in terms of a strong 7~ interaction between the Rh and the axial radicals.332(The Mo-Mo n* orbitals would be empty in the Mo analog, making such a 7t interaction impossible, consistent with the negligible antiferromagnetic exchange observed in ~MO(O~CCF,),(T~~~O),].~~~
48.5.2.3
Reactivity of RH' dimers
The reactivity of Rh" dimers cannot be divided cleanly into substitution and redox processes, as the presence of the very stable + 1 and + 3 oxidation states means that processes which might be expected to cause substitution can lead to redox processes.
( i ) Substitutions at Rh" dimers
The most obvious reaction of Rh" dimers is the formation of adducts by ligand substitution in the axial sites. An outline of the various bases which have been reported to coordinate to the Rhz core is given in Section 48.5.2.1. Numerous studies concerning the kinetics and thermodynamics of axial substitutions have been p r e ~ e n t e d . ~ ~ In ' , ~general, * ~ ~ *adduct ~ ~ formation occurs in two kinetically separable steps at rates which suggest that the Rh-Rh bond has a strong trans-labilizing influence. The stability of the adducts is dependent on the nature of the bridging ligands, and Bear and co-workers have shown that for the reactions of [Rh,Q,] (Q = acetate, propionate and methoxyacetate) with a variety of ligands, both K and k values follow the order pro~ " ~order was rationalized in terms of a desolvation effect of pionate > acetate > M e - a ~ e t a t e . 4 ~This the bridging groups, and parallels the order observed in antitumor studies. The lability of the axial
948
. .
I
.
:
..
Rhodium
site led Bear to use [Rh,(benzoate),] suspended in squalene as the stationary stage in GLC columns, which were used to separate alkene mixtures and various ethers, ketones and esters.444A multitemperature study of the 'H, I3C, 3'P and Io3RhNMR of the trimethyl phosphite (TMP) adducts of [Rh,(O,CMe),] was able to detect the 1:l and 2:l adducts, and found rapid TMP exchange at 298 KM5 Few kinetic studies on bridge substitutions have been presented, but studies of the reaction of CF,COOH with [Rh,(O,CMe),] showed that the relative rates of bridge exchange is 80:40:2:1.& The geometry of the unisolated [Rh,(O, CMe), (O,CCF,),] was not determined, but it is interesting to note that after substitution of the first two fluorinated bridges, the rate of further exchange slows drama tically. There is no characteristic reaction in acidic solution, as the reaction depends on the nature of the conjugate base of the acid used. Bridge exchange is observed in carboxylic, thiocarboxylic, sulfuric and orthophosphoric acids, while arenesulfinic acids (RS0,H) cause oxidation to RhrRSO,), species.447Hydrochloric and hydrobromic acids cause disproportionation to Rh metal and RhX3.448.449 The potential bridge, dipicolinic acid, reacts with [Rh,(O, CMe),], generating a structurally uncharacterized material identified as Rh(dpc) 3H20.450An early report2*' indicating that reaction of [Rh,(O,CMe),] with perchloric, trifluoromethylsulfonic or fluoroboric acids leads to Rh;&, was later questioned by Taube4" and Pinna~aia,~,who showed that the products are [Rh,(O,CMe),]+ and [Rh2(02CMe),j2'. In acetonitrile solutions of CF,SO,H or HBF,-Et,O (both strong, non-complexing acids), [Rh, (0,CPr),] loses butyric acid and generates the cationic [Rh, (0, CPr),l*+ dimer, although acetonitriles are presumably coordinated. The product gives no ESR signal, implying negligible amounts of monomeric radicals, but attempts to precipitate the dication with sulfide ion leads to black, intractable solids.453 In the presence of oxidizing agents the bridged dimers show surprising stability as [Rh,(O,CMe),] does not react with 02, but does react with ozone to form the trimeric [Rh,@0)(0,CMe),(H20),]+?54 A structural study of the perchlorate salt shows a central oxygen atom surrounded by a triangle of Rh and is discussed in Section 48.5.2.4. Oxidation to [Rh,(O,CMe),]+ has been reported with Cl,, PbO, and Ce4+,while basic peroxide and persulfate oxidations lead to Rh"' specie^.‘"^ In the presence of H,, the unbridged Rh$,, is reduced to Rh metal, and the strongly reducing environment of H, over a THF solution containing PMe, and Na(Hg) causes the decomposition of [R~I,(O,CM~),]!~~ Low pressure CO leads to the formation of a labile CO a d d ~ ~ but t , under ~ ~ ~ ~ ~ ~ ~ ' ~ extreme conditions (up to 300 atm and 120°C) reduction to Rh,(CO),, occurs.358 In a fluoroboric acid/rnethanol solution, PPh, reduces [Rh2(02CMe),] to [Rh(PPh,), (BF4)]?57 while oxidation occurs in the presence of PMe,, yielding fac-[Rh(PMe,), (H)(O, CMe)2]."58In the presence of P(Me),, Grignards react with [Rh,(O,CMe),] to generate monomeric Rh"' alkyls and Rh' CJ aryls.457 Little is known of the reactivity of the Rhz,, ion, although Ziolkowski and Taube point out that it is probably labile, as it can be prepared by the Cr2+reduction of [Rh(NH3),C1I2+.No ammonia is retained in the primary coordination sphere of the resulting Rh" dimer.459The dimer reacts with dihalogens (X,) to give [Rh(H,O),XI2+ (X = C1, Br, 1),362*460 and with 0, to give [Rh(H,O),]'+ with a green transient, identified as the peroxo-bridged dimer [(H20)5Rh-02-Rh(H20)s]4+ .45g e
( i i ) Electrochemistry of Rh" dimers
T h e lack of Rh" complexes generated from the reduction of Rh"' species was noted by Taube in 1 9 6 P 2and in his recent review Felthouse notes that electrochemical studies of other metal-metal bonded systems have been 'a fertile area for study.. . although relatively few studies have been reported on dirhodium(I1) compounds.'282That is rapidly changing, as Rhi' electrochemistry has become a vigorously pursued research area. Wilson and Taube first reported that while Rh:, is not oxidizable by cyclic voltammetry, the tetraacetato-, tetracarbonato- and tetrasulfato-bridged dimers undergo reversible one-electron oxidations at 1.225V, 1.022V (inp-toluenesulfonic acid solution), and I .112V (0.2M H2S04)versus NHE respectively.289Oxidation of [Rh,(02CMe),] at a bright Pt electrode in 1.0M HClO, leads to [Rhz(OzCMe),]+,and the precipitation of uncharacterized solids IRh,(O,CMe),] and [Rh2(0,CMe),Clj.46' ESR and IR spectra of [Rh,(O,CMe),]+ showed (within the time of the experiments) suggesting a [Rh2.5]2formulation that the two Rh nuclei had identical electronic rather than [Rh"-Rh"']. This was supported by the X-ray photoelectron spectroscopy, which
I
,
1 showed a single binding energy for the Rh 3d5!, orbitals (310.1 eV) in [Rh,(O,CMe),]+, implying identical Rh atoms within the time scale of the XPS experiment.434 Bear and coworkers have shown that a reversible, one-electron oxidation is a general phenomenon for numerous [Rh,(02CR)4]complexes (R = C(Me),, C,H,, Pr, Et, Me, PhCH,, MeOCH,, PhOCH,, MeCHCI), but not when R = CF,.462The oxidation potential is dependent on the nature of R, as dimers with electron-donating bridges (R = C(Me),, C,H,) are oxidized near 0.90 V (versus SCE in DMF solution) but ea. 1.28 V is needed to oxidize the complex with the electronegative R = MeCHCl bridge; oxidation was not observed (up to 1.7 V} for R = CF,.462None of these complexes showed a second oxidation wave (up to 1.7 V), but all were irreversibly reduced (between - 1.04 and - 1.68V); further electron absorption was observed, leading to uncharacterized reduction products. Again, the reduction potentials are dependent on the nature of the bridge, with complexes containing electron-withdrawing bridges the easiest to reduce. Bear showed4'* that the half-wave potentials (both oxidation and reduction) could k linearly correlated to the polar substitution parameters of Taft.&j Bridges which effectively withdraw electron density from the Rh, core stabilize the higher oxidation state, making reduction of [Rh,(O,CR),] easier and oxidation more difficult, while the electron-donating bridges stabilize the lower oxidation state, and the opposite trend in oxidation potentials is observed.462A more recent study of the one-electron oxidation of [Rh, (Q}J (Q = pivolate, butyrate, acetate, benzoate, methoxyacetate and chloroacetate) also found a LFER between E: and the Taft polar substituent parameters. Similar relationships were also observed for several 1:l and 2:l axially substituted Bear,462Bottomleyw and drag^^^' have noted that the oxidation potential of [Rh2(O2CR),Lz]is also sensitive to the nature of the axial ligands (L), and that axial coordination reduces the oxidation potential of the dimer. Increases in the CT donor strength (as mesured by pKBH+)of the axially bonded ligand should destabilize the dz2molecular orbital, leading to the observed cathodic shift in oxidation potential. Adduct stabilization of the cation was supported by estimates of the equilibrium constants for the reactions shown in equations (98), for L = py, and (100) as K , and K2 (8.20 and 4.38) were three orders of magnitude less than for the analogous reactions involving the cation (K: z 11, and K: x 8).4@The electrochemical reactions observed for [Rh,Q,] can be summarized in equation (101). K+
K+
[fi2(butY)41c IRhzQ.+]+
[Rh,(butYhLl+
(100)
[Rh2(bUtY)'IL21+
= [Rh2Q4] = [Rh2Q4]- * products? i e-
le-
ne-
(101)
Cannon and co-workers have used stopped-Bow techniques to study the kinetics and thermodynamics of axial substitutions at [Rh2(O1CMe),]+.485 Their results are summarized in Table 22. The monoadduct [Rh2(O2CMe),Br]was detected by rapid scanning stopped-flow spectrophotometry, with A,,a,, between 520 and 530, and between 790 and 800 nm. Oxidation of [Rh, (O,CMe),] by C1, (reaction 5 in Table 22) is independent of [ H + ]and [Cl-1, indicating that Cl, (not HOC1 or OCl-) is the actual oxidizing agent.%' Rates of reduction of [Rh2(02CMe),jf with cationic reductants (to hinder the inner-sphere path) vary between 10.7 M-l s-' with Fe2+,to 6.0 x 104M-'s-' for Ce4+.466 Marcus analysis of these electron transfer rates allowed Cannon to estimate the rate of self-exchange between [Rh,(O,CMe),] and [Rh,(OzCMe),]+ (equation 102) at k = 16 M-' s - ' . , ~ ~ [Rhz(OzCMe)41+ + [RhdO~CMehl
[Rh2(02CMe),l -t [Rh,(O,CMe),l+
(102)
Bear, Kadish and co-workers have led a recent burst of activity centered on the electrochemical behavior of Rh" dimers bridged by trifluoroacetamide ttfaa HN(O)CCF,). First prepared via bridge exchange375with [Rh,(O, CMe),], [Rh,(tfaa),] undergoes one-electron oxidation between +0.91 V and + 1.08V (vs. SCE) in a variety of non-aqueous ~ o l v e n t s . ~ ' ~The ' ~ ~oxidations ' are reversible in MeNO,, RCN (R = Ph, Me, Pr), acetone, THF and DMF, but are not reversible in Table 22 Kinetic and Thermodynamic Parameters (From ref. 465)
k, (M-ls-')
Reaction
+ +
[Rh,(O,CMe),]+ CI[Rh,(O,CMe),Cl] [Rh,(O,CMe),]+ f Br[Rh2(02CMe),Br] [Rh,(O,CMe),Br] Br[Rh, (0,CMe),Br,][Rh,(O,CMe),BrJ e [Rh,(O,CMe),I+ Br; [Rh,(O,CMe),l + CIl & [Rh,(O,CMe),]+ + C1,
e
+
4.7(f1.6) x 10' 16.4(+0.5) x lo3
-
6.0 x 10, 120
LWL) 60 & 14 18 k 3 -
K 80
890 150
-
Rhodium
950
py or MeCNlpy solutions; in py, the &om of [Rh,(tfaa),(py)] is ca. 250, implying that the irreversibility is due to a rapid reaction of the oxidized 1:l py adduct, although 'the exact nature of the irreversibility has not been identified'.467Unlike the acetate-bridged dimers, the tfaa dimers are not reduced at potentials as low as - 1.9 V. Electrochemical oxidation of [Rh2{NH(0)CCF,},] contrasts with the behavior of the triff uoroacetate analog, which could not be oxidized electrochemically. The generalization -electron-withdrawing bridges destabilize the cationic species, making the oxidation potential more positive, while electron-donating bridges make oxidation less difficult -applies here, as the effect of the electron-donating nitrogen offsets the electron-withdrawing effects of the CF, group. The net effect is that the oxidation behavior of [Rh,(tfaa),] compares to that of [Rh2(02CEt)4].467 This generalization was further explored in a study of the electrochemistry of the five dimers [Rh,(O,CMe),(HN(O)CMe),_ ,] (H = 0, 1, 2, 3, 4)+3"4.M8 Prepared by bridge exchange with [Rh,(O,CMe),] and separated by HPLC, the oxidation potentiaIs vary linearly with the number of amidate ligands. In acetonitrile solution the oxidation potential of [Rh2(0,CMe),] is + 1.19V (vs. SCE), and is reduced by 0.25 f 0.04 V for each amidate ligand which replaces an acetate; oxidation of [Rh, {NH(O)CMe),] occurs at 0.15 V.'s4 All of the oxidations are reversible (or 'quasi-reversible'), and a second oxidation wave was observed for [Rh2(O,CMe){NH(O)CMe),] and [Rh, {NH(O)CMe},] at 1.65 and 1.41 V respectively. Study of the mixed acetate-amidate complexes (where n = 0 and 1) in the presence of various axial ligands and in different solvent systems supports the existence of a n: interaction between the Rh, core and potential n acceptors such as DMSO and PPh,. The sensitivity of the orbital structure to the nature of the bridging ligands is emphasized by the observation that the nature of the HOMO changes as n goes from 0 ([Rh,(O,CMe),]) to 1 ([Rh, (0,CMe), {NH(O)CMe) The effects of N-substituted phenyl groups has also been explored in electrochemical studies of [Rhz{ P I I N ( O ) C M ~ ) , ] . ~3ridge ' ~ , ~ ~ ~exchange from [Rh,(02CMe),] followed by HPLC separatior! leads to a green isomer, identified by NMR analysis as the 3:l isomer (see Section 48.5.2.l.ii), and a blue isomer, a 1:l water adduct of unknown geometry. Both complexes undergo two reversible one-electron oxidations. The oxidations (+ 0.55 and + 1.65V (vs. SCE) for the blue isomer; 0.53 and + 1.63V for the green form) are separable in CH,Cl,, MeCN and PrCN, and the two-electron oxidation product is stable within the time of the cyclic voltammetry experiment, although a slow reaction to Rh"' products occurs. This suggests the interesting possibility that with very electrondonating bridges, stable Rh;" species might be prepared.46g
+
(iii) Miscelluneous reactions of R F dimers One obvious reaction of Rh" dimers which has been little explored is the cleavage of the Rh-Rh bond. Taube suggested that the slight paramagnetism of Rh& may be due to the d i m e r e 2(monomer) eq~ilibrium,~~' but the small paramagnetism beff near 0.5 BM) of tetraacetatod!rhodium(T1) c o m p l e ~ e swhere , ~ ~ Rh-Rh bond cleavage is unlikely, led Taube to suggest2" that the paramagnetism of Rh;& is due to some other phenomenon. Espenson has shown that flash GI continuous photolysis (visible irradiation) of [Rh,(DMG),(PPh,),] homolytically cleaves the Rh-Rh bond, generating the [Rh(DMG),PPh,] radical monomer?'* This monomer may play ar important role in FeCl, oxidation of [Rh,(DMG),(PPh,),] (equation 103) as the reaction is firs; order in [Rhi+] and zero order in [Fe3+j (AH* = 85.6 f 2.6kJmol-' AS* = 19.2 k 7.7 J K- I mol- I ) , implying that the reaction proceeds via rate-determining cleavagi of the Rh-Rh bond, followed by rapid oxidation of the resulting radical monomers!m For thl reaction (dimer) 2(monomer), Espenson estimated that K 1.5 x lo-", with A H o z 86 kJrnoland ASQz 100 J K-' mol-', Ignoring solvent effects, this A H c value is an estimate of the Rh-FX bond dissociation energy when the Rh-Rh bond distance is 2.97 A; in Rh metal, the Rh-Rh bone is 2.68 A long, with a bond energy of 93 kJ IRh, (DMG),(PPh,),]
+
2FeC1,
---*
2[Rh(DMG),(l'Ph3)Cl]
+ 2FeC1,
(1C3
There are scattered reports of the catalytic activity of Rh" dimers, although this field is dominater by the thoroughly studied catalytic activity of Rh' monomers. Rempel and coworkers investigate1 the [Rh2(02CMe),]-catalyzed hydrogenation of terminal alkenes, substituted ethylenes and ethylefi in DMF s0lution,4'~and found that only one of the Rh atoms is involved in the reaction, which 1 proposed to proceed via a hydride complex. Hydrogenation activity has been reported :"c: [Rh, (phen),(O,CMe),Cl, and Rh" (and Pd") carboxylates efficiently catalyze the cyclopropand
Rhodim
95 1
tion of alkenes by dia~oesters.~'~ A more thorough discussion of the catalytic activity of Rh" dimers is in the companion series." The unbridged [Rh, (OEP),] undergoes a variety of reactions with substituted hydrocarbons in which the Rh-Rh bond is cleaved, and organometallic Rh"' complexes are formed. Several such reactions are represented in Scheme 13.409These reactions are intimately tied to reactions of Rh"' OEP complexes, discussed in Section 48.6.2.7.
[(OEYhRhzl
T Etl, PY
[(OEP)Rh"'-I(py)l
PhCH,llC
[(OEP)Rhl'L-Br]
CH,=CHCH*R
~(O~P)Rh"'-CII~CH=CllK] (R = Ph. CN,Pr")
CH,=CHOEt
[ (OEP ) R h"'-CH,CHO
HCZCR
+
[(OEP)Rh"'-CH2Phl
1
[(OEP)Rh'"-CH=CR-Rh(OEP)]
(R = H. Ph)
Scheme 13
Interest in the chemistry of tetracarboxylate Rh" dimers has doubtless been spurred by the observation that [Rh,(O,CMe),] has shown antitumor activity against L1210 leukemia and Ehrlich ascites tumors in m i ~ e . The ~ rate ~ ~of *formation ~ ~ ~ and ~ ~ the stability of 1:l adducts in [Rh,(Q),] follows the order Q = propionate > Q = acetate > Q = methyl acetate, paralleling their antitumor a ~ t i v i t y . ~Preliminary '~' screening of the cyclophosphamide adducts of the tetrapropionate and tetrabutyrate dimers indicate that the complexes are inactive.323
48.5.2.4 Miscellaneous polynuclear complexes of R P Coordination complexes with more than two Rh" are relatively rare. Numerous Rh-bridged (bridge = 1,3-diisocyanopropane) polynuclear species, and other organometallic clusters, involving Cp, CO (often as bridges), and phosphine ligands have been characterized. These have been efficiently reviewed by Fe1thouse2" and are outside the scope of this report. Haines has reported the structure of a unique dimer containing Rh", [Rh2(CO)(L),C1,]-2CHC1, (L = (PhO),PN(Et)P(OPh)2- a neutral symmetric diphosphorus bridge),dP which nicely evades classification. One of the Rh atoms is in a square planar environment, coordinated to two mutually cis phosphite bridges, a CO and the other Rh atom; it has bond lengths and a coordination geometry characteristic of Rho. The other Rh is five coordinate, being bonded to two mutually cis phosphite bridges, two cis chlorines, and the other Rh atom. Its geometry and bond distance are characteristic of Rh". The Rho-Rh" bond distance of 2.661 A is well within the range observed for the more traditional Rh" dimers. An interesting compound, prepared by the ozonization of [Rh,(O,CMe),], and formulated as [Rh,(0)(02CMe),(H,0)3]C104 was reported by Wilkinson and coworkers?" On the basis of electronic, TR and NMR spectra, they proposed a Rh30core; such M,O cores were then known for cr,480 ~ ~ , 4 ~8~ 14 8 0 a . 4 8and 2 R u . The ~ ~same ~ trimer was also prepared by reacting RhC1,-3H2O with AgO,CMe, and was shown to be isomorphous with the Cr analog.484A single crystal X-ray structure confirmed the presence of the planar Rh,O core with the Rh atoms in an almost equilateral triangle around the central 0 atom (43).455While formally a Rh"' trimer, assuming a central 0 2 -it, is instructive to compare this structure to the numerous structures of RhI' dimers discussed in Section 48.5.2.1. Each Rh pair in the trimer is bridged by two acetate groups, but the average Rh-Rh ! distance (3.33 A) precludes the possibility of strong Rh-Rh interaction. The average distance between the Rh and the 0 atom of the bridging acetates (2.020 A) is similar (even a bit shorter) than that observed in [Rh,(O,CMe),(H,O),] (2.039AZ9')despite a Rh-Rh distance which is almost 1 A longer in the trimer. This is dramatic evidence that the acetate is a most accommodating bridge,303 and that the C-0-Rh bond is flexible, expanding from ca. 119.5" in the dimer to cu. 132" in the trimer. Such clear evidence of flexibility confounds the problem of explaining why the Rh-Rh bond is so consistently short in the tetracarboxylate dimers, as the carboxylate bridges do not appear to be all that constraining. The average Rh-OH, distance in the trimer (ca. 2.13) is significantly
Rhodium
952
shorter (ea. 0.18A) than the Rh-O,, distance in [Rh,(O,CMe),(H,O),], or in any of these axially bonded dimers. In the trimer, the Rh-OH, bond is not tram to a Rh-Rh bond, while the characteristically long Rh-0 bonds in the dimers are directly trans to a Rh-Rh bond, dearly illustrating the often-cited trans influence of the Rh-Rh bond.
48.6 RHODIUM(II1) COMPLEXES
48.6.1 Group IV Ligands 48.6.1.1
Cyano complexes
The hexacyanorhodate(II1) anion was first prepared by the fusion of [Rh(NH,),Cl]Cl, with RCN,485and Schmidtke prepared it by reacting RhCI,.3H20 with a five-fold excess of CN-; aqueous KCN workup of the resulting ‘Rh(CN),.3H2O’ leads to K, [Rh(CN),].486Wilkinson and co-workers reported that the reaction of an aqueous solution of CN- with [Rh(CO),Cl], leads to [Rh(CN),(H)(H,0)]2- ,”” but Jewsbury later identified the product as [Rh(CN), (H)I3- An intermediate in this reaction was said to be polymeric [Rh(C0)2CN],,487while Jewsbury later spoke of [Rh(CO),(CN)Cl]-, trans-[Rh(CO),(CN),]- and [Rh(CO)(CN),I2- as intermediates in the same reaction.4ssA stopped-flow kinetic study of the reaction of CN- with [Rh(CO),Cl], indicated that successive CN- substitutions lead to the tetracyanorhodate(1) anion, [Rh(CN),],-, which then undergoes oxidative addition with HCN to give [Rh(CN),(H)]3-.489In the presence of Mer, pentacyanoal kylrhodium complexes are The [Rh(CN),(H)I3- anion forms adducts with 0,, NO, and C,F,, although the nature of the oxygen adduct has been d i s p ~ t e d . “ ’Reaction ~ ~ ~ of trans-[Rh(py), Cl,]Cl with excess KCN in boiling water gives a yellow solid, identified as the first mixed cyano complex of Rh”’, [Rhpy,(CN),C1]’-2H,O. Its insolubility in a variety of solvents and its IR spectrum suggest that it is a cyano-bridged dimer.4g1 Gray and co-workers showed that photolysis (254nm) of [M(CN),],- (M = Rh, Ir) in acidic aqueous solution leads directly to [Rh(CN), (H20)]2-,and even extended photolysis leads to the loss of only one CN- !92 Thermal anation of the resulting aquapentacyanorhodate(II1) anion provides a convenient synthetic path to [Rh(CN),X]”- (X = C1-, Br-, I-, OH- and NCMe). The electronic spectra of the resulting complexes are given in Table 23; the spectrum of the [Rh(CN),I3- ion has Table 23 Electronic Spectra of some Cyano Complexes of Rh“’(from Ref. 492)
K, [Rh(CN),NCMej [Rh(CN),H, 01’[Rh(CN),OHIK3 [ ~ ( C N ) , C l J K3 [ W C N ) , Brl
K, tRh(CN), I1
a Also
260sh (170) 225 (555) 245 (550) 265 (605) 258 (500) 277 (425) 278 (402) 237sh (1700) 205 (24000) 313 (1500) 258 (4200) 220 (42 000)
assignedw to [Rh(CN),(H,0)lz- contamination.
W l g r
‘AI.
+
lA-=
18
‘TI, IE;i
+
’ A , , -t ‘EB ‘A,, --t ‘ E a ‘A,, --t ‘ E a ‘ A , , -t ’Ea
T-LMCT U-LMCX ‘ A , ,+ ‘E” E-LMCT 0-LMCT
ST&)’
.. Rhodium
953
been the source of some c o n t r ~ v e r s y . ~ ~S~hrnidtke~'~ ~~~~~""'~ originally assigned the 225 nm band to the spin-allowed 'Al, + IT,, transition, and the weak shoulder at 260nm to the spin-forbidden ' A , , + (3Tlg, 3T2g) transition. Gray and co-workers agreed4" with the 225nm band assignment, but suggested that the ill-defined shoulder near 260 nm, as well as the 325 nm (initially thought to be the ' A , , --* IT band494),is due to contamination by the [Rh(CN),(H,O)l2- i0p.4'~
. . . . ..
r.. : ;I
. ._
48.6.1.2
MiscelIaneous group I V complexes
Acyl chlorides oxidatively add to [RhCl(L),] (L = PMe,Ph) to generate [RhCi,(COR)L,] = Me or Et)!% In the absence of oxygen, reaction of [RhCl,(COR)L,] with NH,PF, gives a square pyramidal Rh" acyl salt [RhCl(COR)L,][PF,] with an apical acetyl g r o ~ p . In 4 ~the ~ presence of oxygen, however, the same conditions lead to a dirhodium, triply-chloro-bridgd acetyl Rh"' cation [Rh2C1,(COR),L4]PF6.497 A single-crystal X-ray structure of this dimer shows both rhodiums to be octahedrally coordinated by three shared chlorides, two phosphines and the carbon atom of the acetyl group. The trans influence of the acyl group is apparent, as the Rh-C1 bond distance for the C1 trans to the acyl group is significantly longer (about 0.2A) than those trans to the phosphines. The long (3.328 A) Rh-Rh distance suggests there is no metal-metal bond. A mechanism for the formation of the dimer, involving the dimerization of an unisolated [RhCl(L),(Me)(CO)]+ ion, is proposed.497 The SnC1; moiety acts as a monoanionic ligand toward Rh"', forming complexes with Rh-Sn bonds. Following the method of Kimura et ~21.14'~ Cs,[RhCl,(SnCl,),] and (Me4N), [RhX,(SnX,),-,] (X = C1, Br; n = 1, 2, 3) have been prepared and characterized by conductivities, far IR spectra (Rh-Sn stretches found near 210 cm-' ), Ii9SnMossbauer spectroscopy and by their reactions with water and iron(II1) ion.499All of the complexes are diamagnetic, and SnX; shows a strong trans-labilizing effect. Mossbauer spectra show a significant decrease in the density of the 5s electrons at the Sn" atoms, suggesting CT donation to the Rh center, while the quadrupole splitting suggests the SnX; ligand is also a n acceptor.4w The three anions [RhC1,-,(SnC13),]3- (m = 1, 2 or 3), have been studied by "'Sn NMR spectroscopy. The II7Sn satellite peak intensity is directly related to the number of coordinated tin atoms, and these satellites also reveal that fast intramolecular scrambling of the ligand occurs.5ooReaction of these Rh"' complexes with a six-fold excess of Sn" leads to a Rh' complex with five Sn" ligands. The reaction of DMF with HC1 solutions of RhCl,*3H20 in the presence of excess SnC1, leads to a Rh"' complex with one hydrido ligand, and four equivalent (by 'I9Sn NMR) SnC1,DMF ligands. Occupancy of the sixth coordination site is ~ncertain.~"An X-ray analysis of blue-violet crystals of the composition [RhL,][Rh(SnC1,),(SnCI,)][SnC16].4H,0(L = NH, or 0.5 en) has been presented;'"' such salts have been used as hydrogenation or alcohol dehydrogenation catalysts. An X-ray analysis of Cs4[RhSn6F,,(H20),] has shown that the structure consists of Cs' ions, H,O molecules and [Rh{SnF,(H,0),],[Sn4F,,}]- ani0ns.5'~The Rh atom is Coordinated to the six Sn atoms, with the two mutually cis SnF,(H,O), groups, and a tetradentate Sn4F,, group held together by fluoride bridges between the 4 Sn atoms.
(R
...
,
48.6.2 Nitrogen Ligands 48.6.2.1
Monodentate amines
( i ) Pentaammine complexes Complexes of the form [Rh(NH,),X]"+ have been studied and restudied for many years, and the work continues. Firmly bonded to the Rh"', the five ammines are thermally quite inert, leaving the sixth site for ligand substitution studies. Numerous thermodynamic and kinetic studies of ligand substitutions have been reported and the reactivity at the sixth site is generally, but not always, between Co and Ir. Ligands at the sixth site are often susceptible to photoinduced substitution, but not to redox processes, making [Rh(NH,),XJ"+ complexes popular candidates for photochemical studies. (a) Synthesis and characterization. The starting material for this family of complexes is almost always the yellow [Rh(NH,), Cl]Cl,, initially prepared by Vauquelin, and Claw3 but most modern . ~ ~former ~ references are to the methods of Johnson and B a s o1 0 ~or Wilkinson and c o - w ~ r k e r sThe
954
Rhodium
is adapted from the method of Lebedin~ki,~'~ and involves the direct reaction of RhC13*3H20with (NH4),C03/NH4C1in aqueous solution (equation 104). This leads to chloride salts of the chloropentaammine and dichlorotetraammine cations, but these are easily separated as [Rh(NH3),C1]C12is insoluble in cold water, while the dichlorotetraammine is quite soluble.504The latter synthesis takes advantage of the ability of reducing agents to catalyze substitutions at Rh"' centers (Section 48.6.2.5.iii), as an aqueous ethanolic solution of RhCl, -3H,O reacts rapidly with aqueous NH, to generate, in 80% yields, pure [Rh(NH3),C1]C1,.S0'
+
RhC13*3H20
(NH4)2C03/NbC1
%
+
[Rh(NH3)5Cl]C12
(104)
[Rh(NH3),C12]CI
Generation of other pentaammines [Rh(NH,),X]"+ generally involves simply heating an aqueous solution containing [Rh(NH3)5C1]2+and an excess of X. Direct substitution of the chloride, without significant ammine labilization, is the rule, although a claim has been made that heating solid [Rh(NH,), Cl]Cl, to 296°C causes ammine labilization, cleanly yielding [Rh(NH,)4C1,]C1.507This was later questioned, as significant decomposition, yielding Rh metal and N, gas, was also observed under these harsh condition^.^^' The chloride is readily removed by boiling a solution of [Rh(NH,),Cl]CI, with Ag+ (usually as the nitrate or perchlorate salt), generating [Rh(NH,), (H20)I3+and the easily removed AgCl; the aquapentaamminerhodium(II1) cation can then be isolated, if desired, as the perchlorate salt.50gThe coordinated water is weakly acidic (cited pK, values range from 6.14s'0to 6.24 in 0.47 M NaC104,S'L and the free energy, enthalpy and entropy of neutralization are similar to 6.93 in 1.0 M NaC104512), to those for the Co"' ana10g."~ Replacement of the coordinated water proceeds more readily than removal of the chloride (the mechanisms of such direct anations are discussed below). The coordinated oxygen atom is also a reaction site in itself, and some of the apparent anations involve oxygen attack, and do not involve cleavage of the Rh-0 bond (e.g. reaction with C 0 2 to yield ( 7 0 2 - Sl4,SlS , SO, to yield NO;'I7 and acetate'"). These reactions are also discussed below. A variety of ligands have occupied the sixth site of the pentaamminerhodium(II1) moiety. The electronic absorption spectra for many of the resulting complexes are recorded in Table 24. Some of the more unusual and/or useful ligands merit mention here. Reduction of [Rh(NH,),Cl]'+ with BH, 536 has been reported to yield the monohydrido complex [Rh(NH3),HI2+,although later reports say that reduction with Zn in ammonia is a more convenient preparative route.sos7541 The hydride ligand has a strong trans-labilizing effect, so the hydridopentaamminerhodium(II1) cation is a convenient starting material for the synthesis of a variety of complexes, as both H- and NH, labiiization can be effected.s42 Table 24 Electronic Spectra of Some [Rh(NH,),XY+ Complexes
Nitrogen Ligands (neutral ligands) NH,
NH,CI
NH, C(0)OH Imidazole NCMe NCCH=CH, NCC(Me)=CH, NCPh NC(o-C,H,F) NC(m-C,H,F) NCCH, Ph NCCH=CHPh NCCH2(2-MeC,H.,)
306 (135) 307 (135), 256 (105) 305 (131), 256 (94) 395 (225), 258sh (304), 21Osh (2416) 299 (150), 250 (171) 315 (152), 259 (112) 235 (12 900) 308 (157), 256 (143) 314 (IN), 259 (122) 305, 250 304 (146). 250sh (153) 301 (158), 253 (126) 300 (380), 210 (8300) 301 (190), 213 (8300) 301sh (324), 282sh (1400), 275 (1700), 268sh (1660), 245 (16 OW), 234 (20 400) 305sh (316), 288sh (2450), 281 (2570), 243 (14500), 235 (18200) 300sh (275), 288 (1700), 265 (813), 237 (8400) 299 (221), 266 (294), 262 (368), 256 (422), 251 (389), 247 (350) 300 (183). 267 (208), 263 (276), 257 (341), 251 (311), 247 (271) 310 (310), 290 (2100), 284 (2200), 247 (1300), 237 (16000)
519 520 52 1 522 523 52 1 524 525 520
526 527 528 528 528 528
528 528 529 529 529
Rhodium
NCCH, (4-MeOC6H4) NCCH=CH(4-OHC, H4) 4-Mepy 3-Clpy Pyridine Anionic Ligan& NO;
NCONCSN; Imidazolato NHC(0)NH; NHC(O)CH, NHC(0)Ph-
L-3
N
N-Fe(CN),
w
Oxygen Ligand (neutral ligands) H2O
HC(0)O MeC(0)OMeCH, C(0)ONH,C(O)OHC,O; OS0,CF;
co:so:
Miscellaneous Anionic Lirands H-
c1Br-
ICN-
C.O.C. L E E .
955
300 (249), 279 (1200), 273 (1400), 226 (9300) 307 (143), 280 (460),274 (520), 223 (2500) 302 (170), 256 (2500) 302 (170), 272 (2930) 302 (160), 259 (2700)
529 529 529 529 529
296 (330) 295, 245 323 (276), 269 (160) 318 (474) 251 (6569) 251 (5200) 251 (6760) 310,252 307 (166), 252.5 (216) 318 (158), 259 (153) 313 (164): 259 (153) 240 (7700)
519 526 520 531 522 532 533 526 527 521 525 524
481 (5400)
534
400 (= 4500)
534
571 (8200)
534
312 (105), 261 (89) 312 (log), 261 (91) 314 (112), 261 (101) 314 (112) 315 (1051, 262 (95) 315 (106), 263 (95) 316, 267 3 16 (I 05), 264 (94) 314 (107), 262 (92.7) 330 (163), 266 (I 14) 324, 258
519 531 535 514 532 512 536 537 51 I 52 1 526
319 (130): 277 (119) 319 (132) 321 (124) 319, 277 322, 266 322, 259 322 (156), 265 (125) 324 (145), 260 (100) 322 (159) 333 (103), 267 (84) 325 (178) 258 (2100)
512 514 519 526 526 526 538 52 1 537 539 514 516
310 (120) 312 (108), 261 (91) 347 (101), 276 (106) 349 (loo), 274 (103) 344 (104), 274 (113) 359 (122) 424sh (29), 360 (125) 388 (230) 419 (219), 381sh (230), 276 (3690) 287 (146), 246 (139)
536 531 511 519 53 1 519 531 519 53 1 540
Rhodium
956
The conjugate base of trifluoromethanesulfonic acid, OSOl CF;, nicknamed ‘Triflate’, is becoming a popular ligand as it is an excellent leaving group. If [Rh(NH3),Cl]C12is heated (9&110°C) in neat trifluoromethanesulfonic acid, the chloride-free solid, identified as [Rh(NH3r5(OS0,CF,)] (OSO,CF,),, is generated. The ‘H NMR spectrum shows two peaks in a 4:l ratio (3.16p.p.m. vs. 3-(trimethylsi1yl)propanesulfonate for the cis protons, and 2,90 p.p.m. for the trans protons), and the electronic spectrum (Table 24) suggests that the triflate ligand exerts a weak ligand field.539Its uses as a leaving group were demonstrated by Taube and c o - w o r k e r ~ ,who ~ ~ ~ reacted [Rh(NH,),(0SO,CF3)]*+ with [Os(NH,),(pyrazine)13+, and isolated the pyrazine-bridged [(NH,), Rh(pyrazine)Os(NH,),]C16. TriAate’s versatility was shown by Jackman and cowith acetic anhydride in the workers,”~4,~~~ who reacted a DMF solution of [Rh(NH,),(OSO, CF3)J2+ presence of N,N-dimethylbenzylamine (DMB). The resulting acetyiation, forming acetatopentaarnrninerhodium(III), is thought to proceed in two steps, with the basic DMB initially reacting with the triflate ligand, to generate the hydroxo complex, which then rapidly reacts with the acetic anhydride to form the acetatopentaamminerhodium(II1) cation. The hexaammine can be prepared by heating [Rh(NH,),Cl]Cl, dissolved in concentrated aqueous ammonia in a sealed tube at 100°C €or several day^.^'^,'^^,'^' Detaiied IR studies of the deuterated hexaammine,% and of [Rh(NH3),I3+as the GaFi-, ScFi-549and MnFi-550salts provide evidence of significant hydrogen bonding between the ammine hydrogens and the hexafluorometalate anions. Emission studies of crystalline powders of [Rh(NH,),][Rh(CN),], and the deuterated analog, suggest that the luminescent ’TIgexcited state exhibits Jahn-Teller distortions.551An alternative route to the hexaammine involves the acid hydrolysis of [Rh(NH,),(NCO)]” ;”O the details of this reaction and the synthesis of a series of organonitrilepentaammineRh(II1) complexes are discussed below Photolysis of [Rh(NH3)5(N3)]2+ can be used to generate [Rh(NH,),(NH,C1)]3+, or the M 2 0 H a n a l ~ g : ~ the ’ ~ .proposed ~~~ mechanisms are discussed in Section 48.6.2.3. One of the more unusual pentaamminerhodium(II1) complexes was reported by Wieghatdt,’” who reacted the monodentate oxalatopentaamminerhodium(II1) cation with [Co,@0H),(NH3),I3+ to generate the [Co,RhI5+ trimer, isolated as the bromide salt (44).IR spectra in the C-0 stretching region for several oxalatopentaamminerhodium(II1) complexes, and the deuterated analogs, are also recorded.552
.’”
15+
(44)
The ‘HNMR spectra of a variety of [Rh(NH,),XY’ ions dissolved in concentrated H,S04 show only one ammine proton peak, at - 3.82, -4.13, - 3.78 and - 3.65p.p.m. (vs. external TMS) for X = NH,, HSO; , C1- and Br- respectively.547The Ir analogs also have only one ammine resonance, but the Co species display two peaks in the expected 4:l ratio. Neither intraligand nor proton exchange with the solvent could be used to explain the missing peaks; the magnitude of the ‘diamagnetic’ and ‘paramagnetic’ shielding in the heavier metals was cited to account for the single peak in the Rh and Ir cases. This is not a universal phenomenon, however, as other [Rh(NH,),X]”+ complexes do show two ammine peaks in the expected 4:l ratio.539 ( b )Reactions of [Rh(NH3),7X]”’.The aquation and anation of [Rh(NH3)5X]”+in acidic aqueous solution (equation 105), was first studied by Lamb5,, who used conductance changes to monitor the ‘interconversion of acido and aqua rhodium complexes’, for the chloro and bromopentaamminerhodium(II1) cations. He found that both aquation and anation are first-order in [Rh”‘], and that the hydrolysis is accelerated by the presence of OH-. The forward form of equation (105) (aquation) OH,,557-559 SO:- ,560 NO; 561 and N; .562 Br- , I- , has since been studied for X = Cl-,5543556 Representative results for the aquation of various pentaammine salts are given in Table 25. For aquations of halides or nitrate ions, rate expressions which are first-order in [Rh] and zero-order in [H+] are found. P0eS3lgives a detailed discussion of the thermodynamic, kinetic and intrinsic-kinetic trans effects, and compares the aquation and anation of [Rh(NH3)5X]2+ (X = Cl, Br and I) with the [Rh(en),XY]+ system. When X = SO:-, a two-term rate expression was observed 5319556
[Rh(NH,),X]”+
+
H20
H+
[Rh(NH,)5H20]3+-k X‘3”””)-
(I O 3
Rhodium Table
25 Kinetic
Parameters
T X
c1-
50 60 77
50
Br-
60 50 60 25 25
IH2O
so:-
25
NO; ONO-(NO;) MeCO;
25 -
53.8
Me,CCO;
68.6
CF,CO; O=C(NH,),
64 25
+
1.13 x 8.8 x 2.27 x 1.00 x 5.4 x 2.4 x 10-7 2.51 x 1.07 x 1.07 x 1WS
-
-
k
Acidic
for the Aquation of [Rh(NH,)5X]"+ in [Rh(NH3)5X]"i H 2 0 [Rh(NH&H20I3+ f X
k (s-')
("C)
957
1.6 x k , = 1.1 x 1.23 x lo-' k, = 2 . 0 x k,,=2.0 x kH =3.3 x k , = 3.2 x k , = 3.3 x =
4.0 x
AH* (kJmol-I) 101 1.3 94.4 102 103 f 0.8 105 109.6 f 0.8 114 100 1.3 101 103 & 1.3 89.5 119 97.5 65.7 (68.6) 96 96 105
+
-
Aqueous
Solution
+
AS * (JK-'mol-')
-46.4 f 3 -41.4 & 2.5 -32.6 3
Comments p = 0.2M -
AH'
=
- 14.6; A S o = - 113
p = 0.2M -
p=02M
-
-
12.5
-
- 2.7
-
-
Ref.
f3 . 3 f 4.6 - 55.6 3% - 12.5
+
AV* = -4.1 0.4cm3mol-' Rate = k + k,[H+]
+
Aqueous H, SO, solutions k,, = k,*O + k, [H I kobs = k,,o + M H + I kHvery small Oxygen-bonded urea
-
- 17 -
531 556 554 53 1 516 531 556 557 558 563 560 561 564 518
+
- 25 - 25 -
518 518 521
+
(rate = k , k 2 F + ]implying ), significant acid catalysis as well as the normal, first-order aquati01-1.~~' The reverse of equation (105), the anation of aquapentaamminerhodium(III), has also been extensively studied. After Lamb's initial observations,s53the first detailed study was reported by Po& and co-workers (for X = C l ~Br, , I- and NCS- 31:) but was quickly followed by anation studies for X = C1-,531,555,565 Br- and ,565 propionate,538 NO; and MeCOO- .566 The results are summarized in Table 26. Interpretation of the kinetic results is complicated by unavoidable interference from ion-pairing, and while some differences exist, most systems showed: (a) AS* values near zero; (b) rates which are less than, or comparable to, the rates of water exchange; and (c) rates which depend on the ionic strength and the nature of the nucleophile. The general consensus of these workers is that anation of pentaamminerhodium(II1) complexes is associative in nature, and while not purely associative, the anations, and by implication the aquations, are probably more associative than the analogous reactions of [CO(NH,),(H,O)]~+; the I, designation is the most commonly used description. In the oxalate system, vanEldik points to the very negative values for A S * (Table 26), the similarity in rates of anation by HzC204,HC,O; and CzO:-, and their relative unreactivity
Sa-
+
Table 26 Kinetic Parameters for the Anation [Rh(NH,)5(H20)]3+ X"-
X
c1-
T ("C) 50 65
k (M-I s - I ) 2.01 4.2
-
Br1-
NCS-
so:-
N; MeCO; EtCO, H EtCO; H2C204
HC20;
c,o:co2
IO-^ 10-3 -
50 65 50 50 65 65 60 60 60 60 60 60 25
1.44 x 10-4
25
1.8 x IO*
7.9 x 10-3 1.15 x
1.19 x 10-4 1.7 LO-3 5.97 x lo-' 1.83 x 1.75 x 3.67 x 10-4 1.5 x 10-4
1.4 x 10-4 1.2 x 10-4 4.9 f 1.2 x 102
AH* (kJ mol-') 107 f 2.1 -
106 f 1.3
-
102 i 0.8 99.6 96.2 107 & 2.1 89.1 92.9 35 f 2.1 56.1 f 2.9 54.4 & 2.1 71 f 4.2
AS* (J K-lmol-' ) +13+4 -
+8.4 k 4.2 I
- 4.2 - 13 f 4.2 - 20.0 f 0.1 f 4 . 8 5 f 1.3 -71 + 4 -33 5 4 -213 & 4.2 - 142 f 8.4 - 138 & 4.2 +50 f 12.6
+ H,O
+ [R~I(NH,),X](~-"'-
Comments
Ref.
p = 0.2M 4.0 M NaCIO, AV* = 3.0 0.7m3mol-l p = 0.2 M 4.0 M NaC10, p = 0.2M p = 0.2M 4.0 M NaC10, k in s-' p = 1.OM p = 1.OM; k in s-' p = 1.0M; k in s-' No Rh-0 cleavage; 7 < pH < 9.9 AV* = -4.7 & 0.8cm3mol-' No Rh-0 bond cleavage
531 565 561 531 565 531 531 565 566 566 538 538 537 537 537 514
+
535 516
I 958
Rhodium Table 27 The Partial Molar Volume (p), the Volume of Activation (dV*) and the Partial Molar Volume of the Transition State (AP") during H 2 0 Exchange at [M(NH,),H,U]3+ (in cm3mol-') (from ref. 568)M
P
AV*
AP*
co
70.9 77.2 71.3 81.3
1.2 -4.1 - 3.2
72.1 73. I 74. I
- 5.8
75.5
Rh Ir Cr
when compared to anation by halides, sulfate, azide, etc., to argue that a different mechanism (he suggests I d ) is operating during oxalate a n a t i ~ n . ~ ' ~ A powerful technique for exploring the mechanisms of such ligand replacements is the determination of the volume of activation, AV*, by monitoring the effect of pressure on the reaction rate. Swaddle first reported the A V* for the water exchange of aquapentaamminerhodium(II1) had a negative value (- 4.1 & 0.4cm3mol-' ),563 comparable to values determined for aqua exchange at aquapentaamminechromium(II1) (- 5.8 cm3mol-') or hexaaquachromium(II1) (- 9.3 cm3mol-'); such anations of Cr"' centers are generally thought to react via an associative path. The contrast to the more dissociative [CO(NH,),(H,O)]~'was also sharp (AV* = + 1.2 cm3mol-'), further supporting the idea that substitutions at Rh"'centers are more associative than those at Co"' (Table 27). The effects of pressure on the kinetics of the anation of aquapentaamminerhodium(II1) by C1were aiso studied by vanEldik et al.567They assumed an ion-pairing mechanism, represented in equation (106), but could not determine K , under the high ionic strength conditions used. They concluded that the value of K , was 'immeasurably small', and discounted the importance of ion-pairing during the anation of aquapentaamminerhodium(II1) under high ionic strength conditions. (vanEldik commented on the difficulty in distinguishing between I, and I d mechanisms on the basis of kinetic interpretations. At low ionic strengths, one can determine the value of K,, but will not have enough nucleophile present to determine k; but with high ionic strengths, k, but not K,,can be determined.) A positive AV* (+ 3.0 0.7cm3mol-') was found, similar to the values found for Co'" anations, but much more positive than the NCS- anation of aquapentaamminechromium(II1) (- 4.9 cm3mol-'). For the rhodium system, vanEldik disagreed with Swaddle's earlier conclusion,563and spoke of a 'dissociatively activated transition state'.'" [LsRhH20l3++ C1-
e ([L5RhH20]*Cl-}2C& KY
[L,RhClI2+
+ HZO
(106)
Swaddle has determined the partial molar volumes568and reexamined the effects of pressure on the water exchange of [M(NH,)j(H,0)]3+ (M = Co, Rh, Ir and Cr).56Y The partial molar volume of the transition state, (AV*) was defined to be the difference between the volume of activation and the showed that partial molar volume of the ground state (A V* = A I/* - A P).While earlier the volume of activation (AV*) did vary as the central metal changed, the partial molar volume of the transition state (AT*) is quite insensitive to the nature of the central metal (Table 27). Swaddle concludes that 'contrary to intuition. . . the transition states of a series of aqua exchange reactions must resemble one another surprisingly closely, regardless of detailed mechanism'.569 Pavelich has taken a different approach to kinetic studies of aquapentaamminerhodium(II1) anations, and argues that the common practice of doing kinetic studies at high ionic strength is invalid, as ion pairing with the 'inert' electrolytes (typically perchlorates) confuses the interpretation of the results. He has presented a study of Cl- anation of aquapentaamminerhodium(II1) at low ionic strength, with the only anion added to the solution being the anating ligand. Ion-pairing was treated by potentiometric and spectral data, and was combined with kinetic data on the formation of [Rh(NH,),C1I2+. A complex modeling analysis (to fit idealized A , I and D thechanisms) was used and Pavelich concluded that the reaction could most effectively be modeled assuming an interchange ( I ) path.'" The activation parameters cited in Table 26 are based on that assumption. It is fair to conclude that the mechanism of substitutions in acidic solution of [Rh(NH,),X]"+ has not yet been settled. The recent trend has been to question the earlier assignment of an I, process, and to just refer to it as an interchange 1, with less credence given to experimentally verifiable differences between Za and Idmechanisms. The Hg'+ ion induces aquation of [Rh(NH,),Xl2+ (X = C1, Br and I) and numerous lunetic studies have been p r e ~ e n t e d . ' ~The ~ - ~mechanisms ~~ represented in equation (107) appear to occur;
Rhodium
959
the rate of aquapenhammine formation is proportional to [Hg2+]for X = C1 and Br, but approaches a constant value for X = The [Rh(NH,),1-Hg]4+ intermediate has been detected (A,,,,,, at 370nm), and the K,, of this intermediate at 11.4"C is 225 16."' At that temperature, the Hg2+-induced aquation is about seven orders of magnitude faster than the spontaneous aquation, and the rate constant for the dissociation of [Rh(NH3),I-Hgl4+ is 1.26 x 10" exp(- 66500 -t 5000)lRT (at 25"C, k = 2.75 x ~ O - ' S - ' ) . ~The ~ ~ enthalpy of activation for the Hg'+-induced aquation of the bromopentaammine complex is 32 kJmol-' lower than for the spontaneous a q ~ a t i o n . ' The ~ ~ AV* is negligible (- 1.0 f 0.4cm3mol-') for the Hg2+-induced aquation of [M(NH,),X]*+ (M = Co, Rh, Ir; X = C1, Br, I), suggesting that the rate-determining A t; e: configuration for step is not purely dissociative, but better described as an inter~hange.,~~.,'~ the Rh"' center has been suggested for the [Rh(NH3)5X-Hg]4+ intermediate?' A full kinetic study of the Hg'' -induced aquation of trans-[Rh(NH,),CI,]+ has also been pre~ented;~" the rate of release of the first chloride is significantly greater than for the second chloride. [L5RhX]'+
+
HgZf
K
e [L,Rh-X-Hg]4+
[L5RhI3+ + HgX'
2
(107)
[L, RhH20I3'
Lamb5,, first noted that OH- ion accelerates the hydrolysis of [Rh(NH,), Cl]'+, but the increase is not as dramatic as observed for Co"' complexes. Kinetic studies have revealed a rate law of the form: kobs= k, + k,,[OH-] for the hydrolysis of [Rh(NH3)5X]"+for X = C1-, Br-, I-,578 N0cs6' and N F .Isokinetic ~ ~ ~ plots (E, plotted as a function of log A ) are linear for halopentaammine complexes, but the data from the base hydrolysis of [Rh(NH,),N,]'+ arc well off the line, for both Rh"' and Go"' pentaarnmines, suggesting the azido complexes have some unspecified 'distinguishing feature' in their mechanisms.533For carboxylato complexes (X = RCO, ), a threeterm equation is needed to account for the kinetic results: kobL = k2[OH-] + k3[OH-I2. The thirdorder path (second order in [OH-]) was attributed to a path involving OH- attack of the coordinated acetate.579For most systems, the base-catalyzed reaction is presumed to proceed via the well-known dissociative conjugate-base mechanism, S, 1CB. Palmer reported that the pressure dependence of the rates of base hydrolysis of [Rh(NH,),X]'+ (X = C1-, Br-, I-, NO;) are consistent with an Id model for X = C1 and Br, although he questions the utility of trying to draw fine distinctions between I, and I d steps, For X = I- the volume data suggested some ion-pairing between the I~ and the coordinatively unsaturated conjugate base, [Rh(NH3),(NH,)l2+, while for X = NO; an I mechanism was favored.580Table 28 summarizes some of the kinetic parameters determined for the base hydrolysis of pentaammine complexes of Rh"'. The kinetics of substitutions at pentaamminerhodium(I1i) in ammonia solutions have been studied by Bait and J e l ~ r n a . ~ ~For ' - ~ ' the ~ two-step reaction represented in equation (1OS), they invoke an associative conjugate-base mechanism (SN2CB) as the reaction appears more associative than the Co analog.5s2A major study of the ammoniation of [Rh(NH,),X]'+ (for X = Br- or NO;) revealed a two-term rate expression, kobs= k, + kb[NH4+]-', with AS* values for the rate-determining step which are ca. 420 J K-' mol-' less than for the Co analogs, again suggesting a mechanism more associative than for For [Rh(NH,),(NOJZ+ , the hydrolysis follows the pattern observed for the Co"' analogs, with NH, being released in acidic solution, and NO; being released in base. This was interpreted in terms of the kinetic tram effect of the nitrite ion.5" Balt concludes that the acid reaction (ammine loss) 'proceeds via the same mechanism in the Co"' and Rh"*complexes (Le. dissociative interchange), whereas aquation with loss of NO; is probably more associative for Rh"' 581 [RhwH3)4(0H)21C
68-8O'C
NH1(aq)
'
[Rh(NH,),0H]2+
-
[Rh(NH3)6]3+
(108)
Several of the apparent aquations and anations at the pentaamminerhodium(II1) moiety have been shown to be reactions of the coordinated ligands, and do not involve cleavage of the rhodiumligand bond. The aquation of [Rh(NH,), (RC00)I2+(RCOO = Ac- ,Bu' COO- or CF, COO-) has a two-term kinetic expression, characteristic of a reaction proceeding via an uncatalyzed and a catalyzed path, kob5= k , + k, [H+].,'' Indirect evidence suggests that in the acid-catalyzed reaction (but not the uncatalyzed path), carbon-oxygen cleavage leads to the final aquapentaammineby rhodium(II1) pr~duct.~''More detailed kinetic studies of the uptake of C0,5'4 and aquapentaamminerhodium(II1) have been presented. The reactive species in both cases is the hydroxopentaammine complex, and as shown in Table 29, the rates of decarboxylation of /M(NH,),(CO,)J+ are very rapid, and virtually invariant for M = Co, Rh or Ir.5'4This mechanism is supported by volume of activation studies,535 which also suggest that both C-0 bond formation
Rhodium
960
Table 28 Kinetic Parameters for Base Hydrolysis of [Rh(NH,),X]"+ ~-
- -~
k (M-ls-' )
T
X .I
c1-
rC)
40
4.93 4.18 5.77 3.98
I
I
40 50 -
78
1.65 x 5.25 x 1.61 x 1.8 x 1.3 x 4.76 x
-
-
40 40 -
Br-
I-
NO;
so:NT NO; OHCF,CO;
cc1,co;
40
40
25 25
60 25 25
CHF,CO;
25
CH, FCO;
25
HCO;
45
MeCO;
55.7
NC-H NCC,H, O=C(NH,),
25 25 25
AS* (JK-'mol-')
AH' (kJmol-I)
x 10-4
115 119 124 128
x 10-3
x 10-4 x 10-3
+66 25
+ + 93 + 33 + 114 + 42 -
-
-
134 137
10-4
10-3
~
lo-' lo-' 10-3 10-4
.,
** *
~
118 7 109 5 42.8 t 121 9
116*2 115 & 1
*
102 2 144.8 f 1.3 168 4
No reaction k, = 1.0 x 75.3 k, = 2.9 x lo-' 31.4 k, = 3.0 x 110.5 k2 = 1.5 x lo-' 35.1 k , = 5 x lo-' 126 k, = 6.5 x 43.9 k, = 4.1 x 10-4 133 79.9 k, = 1.6 x k, = 1.3 x I39 k, = 2.1 x lo-' 107 k, = 3.3 x 133 k, too small to be detected 1.0 & 0.1 4.7 6.0 x lo-' -
-
- 54 - 151 58 - 163 92 - 134 109 -71 + 117 f8 92
+ +
+
-
-
Comments
p=O.lM Calculated to p = 0 AV* = 19.3cm3mol-' p = 0.1 M Calculated to p = 0 AV* = 20.2cm3mol-' p=OO.lM Calculated to p = 0 A V * = 20.4cm3mol-' AV* = 22.3cm3mol-I
ReJ 556 578 580 556
578 580 556 578 550 580
56 1 560
Aqueous ammonia solution r,!2 > 150h Rate = k,[OH-] k,[OH-l2 k i n M-'s-'
+
533 581 557 579 579
k in M-'s-' As K, of acid decreases, k, and k, decrease
579
-
579
k in M-'s-' -
5 79
-
579
Yields [(NH,), RhNHC(O)MeI3+ Yields [(NH,), RhNHC(O)Ph]'+ Oxygen-bonded urea
525 524 52 1
-
uptake, and C-0 bond cleavage during decarboxylation are approximately 50% complete in the transition state. This suggests a transition state represented by (45).535
0
The classic example of an apparent aquation/anation which does not involve cleavage of a metal-ligand bond involves the [M(NH,),NO,]*+ ion, First reported (for M = Rh"', Ir"' and Pt") by Basolo and J o h n ~ o n , ~the " rate of formation of [Rh(NH,),N0,]2' from [Rh(NH3)5H10]3+ Such kinetic behavior is followed the rate expression: rate = k[[Rh(NH,),H,0J2+][NO;][HN0,]. reminiscent of the nitrosation of amines, and suggests a mechanism involving NO+ attack at the coordinated hydroxyl oxygen (equation 109-1 1 l ) , followed by intramolecular linkage isomerization of the resulting nitrito complex (equation 112) to the stable N-bonded nitropentaammineTable 29 Parameters for the Decarboxylation of [M(NH3)sC03]+(25"C, p = 0.5 M) (from ref. 514)
[M(NH,),CO,]*
+
K,
H+ e [M(NH,),(CO3H)l2+ ~ ( N H 3 ) s O H ] 2 + CO,
+
M
k(s-1)
K,(MI
AH,* (kJ mol-')
Co Rh Ir
1.10 i 0.05 1.13 i 0.06 1.45 & 0.07
2.0 f 0.1 1.1 +O,l 1.6 f 0.1
70.3 f 0.8 71.1 & 2.1 79.5 f 2.1
AS:(J
k
e
K-'mol-') -8+4 -4L4 f 2 5 k4
Rhodium
' .
96 1
rhodi~m(III).~'~ Aquation in acid solution of both the nitro and nitritopentaamminerhodium(II1) complexes are consistent with this me~hanism.~" The nitro complex is hydrolyzed by the loss of NO+ ion from the transition state, and the entering water is not involved in the transition state. Aquation of the nitrito complex is less straightforward, and reacts via a mechanism involving both mono- and di-protonated species. Results with Rh parallel the results for other pentaammine complexes (Cr, Ir and Co), confirming that these reactions do not involve cleavage of the metal -ligand bond.S64 [(NH,)sRh-H20]3+
[(NH3)sRhOH]2+ + H +
1
(109)
2+
[(NH3)5Rh--OH]"
+
Nz03
[(NH,)IRh-y/W
-+
y+-ol -
-+
[(NH,)5Rh-ONO]2+
+ HN02
NO;
[Rh(NH3)5-ONO]Z+
[Rh(NH3)5-N02]2'
(1 12)
The spontaneous isomerization from the nitrito (-ONO) to the nitro (-NO,) pentaamminerhodium(II1) complex has been well studied, and compared to the isomerizations of the analogous Co, Ir and Cr complexes. Isomerization was shown to occur in both the solid state and, about an order of magnitude more rapidly, in aqueous The volume of activation (A V * )in aqueous solution is comparable for [M(NH,)50NO]2+ (M = Co, Ir and Rh, AV* = - 6.7, - 5.9 and - 7.4 -t 4 cm3mol-' respectively), which is consistent with the anticipated intramolecular isomerizat i ~ n . ' *The ~ linkage isomerization is catalyzed by OH-,585and by Hg2+.5K6 In alkaline solution, the linkage isomerization of nitritopentaamminerhodium(II1) obeys the rate expression: rate = k, + koH[OH-], and at 25"C, k, = 9.6 0.2 X 10-4S-', koH = 7.6 0.2 X 10-3M-iS-', AS: = - 18 & 9JK-'mol-' and AH: = 85 k 2.5kJmol-l, AHZH = 96 k 2kJmol-', AS:, = + 38 k 6 J K - ' mol-'.58s For both the spontaneous and base-catalyzed paths, the rates of isomerization for the three pentaammines follow the order Rh > Co > Ir; for the base-catalyzed path the reaction rates are of the order 278:217:1, while for the spontaneous path (ks), the relative rate constants are 32:2.3:1. The greater reactivity of the Rh complex, attributed to a lower AH* for both the spontaneous and base-catalyzed paths, contrasts sharply with the order generally observed for the aquation of d 6 pentaammines. For example, for M = Co, Rh and Ir, the aquation of [M(NH3)5Br]2+in acidic solution occurs with relative rate constants of 4: 1 :0.001.37 The ordering is not always Co > Rh B Ir, however, as the rates of water exchange in aquapentaammine complexes in the Rh triad have relative rates of 0.53:1:0.0059,559and the relative rates of acid aquation of the [M(NH,),(CO,)]' ions are l:l:0.02.518 Linkage isomerization has also been observed in the [Rh(NH,), (urea)I3+ ion."' The direct substitution of urea into [Rh(NH,), (OS02CF3)]2+in sulfolane solution Ieads to [Rh(NH3), -3H,O, with both N- and 0-bonded ureas. Recrystallization leads to the pure (~rea)]~(S~O,), 0-bonded isomer. The pK, of the N- and 0-bonded isomers are 13.19 and 13.67, respectively, compared to a pK, of about 14 for free urea. The ' H NMR suggests that the N-bonded form is in the imidol, not the amide form (46). In alkaline solution, the 0 form isomerizes to give the deprotonated N-bonded ureapentaammine complex and [Rh(NH,),01-€]2+.In acidic solution, both the N- and 0-bonded forms lead to [Rh(NH&H20l3+and [Rh(NH3)6]3+,with the latter complex coming from a [Rh(NH,),NC0I2* intermediate (vide infra). The isomerizations and the hydrolyses of both the N- and 0-bonded urea complexes showed spontaneous and base-catalyzed paths. As with the Co"' analog, there was no evidence for the 'substantial production of [Rh(NH,), (OOCNH,)I2+'; formamide is the product of urease-catalyzed hydrolysis of urea, and apparently the Rh"' does not polarize the C-0 bond to the extent of the Ni2+ in the urease. Formation of the hexaammine does represent a 30 000 fold increase in the rate of urea hydrolysis, and is presumed to proceed through a Rh-NCO inter~nediate.'~' H
I
[(NH,),Rh---N:
OH
0
1
II
C --NH2]3t
[(NH3)sRh-NH2-C-NH2]3t amide form
imidol form (46)
962
Rhodium
This last reaction, the hydrolysis of coordinated nitriles, has been extensively studied by Ford and co-workers; acidic hydrolysis of [Rh(NH,),NC0I2+ has been used as an alternate route to the hexaammine (zquation 113).52nThe reaction was shown to go through a carbamic acid intermediate, which was isolated and characterized. The kinetic study of the hexaammine formation was complicated by the presence of these two consecutive reactions, and by two different reaction paths. The mechanism proposed for the hydrolysis of the coordinated cyanate is shown in Scheme 14; at low acid concentrations (0.005 to 0.025 M) the rate determining step is H 2 0 attack of the protonated species leading to a rate expression which is first-order in [H+]: rate = k,[Rh][H+]. At higher acid concentrations (0.2 to 1.O M), decomposition of the carbamic acid complex is rate determining, and a more complex rate law is observed: rate = k,[Rh] + k-,[Rh][H+lP'.At 2 5 T , k, = 0.62M-'s-] (almost four times as reactive as the Co analog, and an order of magnitude faster than the Ru s-', k P l = 9.6 x IOp4 M s-.' ."' analog), ko = 2.1 x [Rh(NH3),(NCO)IZL+ H,O+
[(NH3),RhNCOl
2+
H,O*
-+
[Rh(NH,),13+
+
CO,
(113)
0
II
[(NH&Rh(NH,COH)]
[Rh(NH3)61 3 t
3+
OH* Scheme 14
In basic solution, however, the hydrolysis of benzonitrilepentdamminerhodium(II1)ion (equation 114) is two orders of magnitude slower than the Ru"' analog, yet still represents a major rate enhancement (about six orders of magnitude) over the rate of hydrolysis of the free ligand. For the free ligand and the benzonitrilepentaammine complexes of I ~ " ,Rh"', Co"' and Ru"', the relative . ~ ~reaction ~ ~ ~ ~ ~is rate constants are 1,2.9 x lo5,6.5 x lo6,2.5 x lo6 and 2.8 x 10' r e s p e c t i ~ e l yThe first-order in both [Rh complex] and [OH-]. The intermediate amino complex can be protonated, but only a! low pH, as its pK, is 11.8.jZ4 H O [(NH,), Rh-N_CPhl3+
+
I OH
-+
II
[(NH3)5Rh--N-cPhlZ+ amido form
(1 14)
A kinetic study of the nitrous-acid-induced aquation of [Rh(NH,),N,I2+ indicated that the The results suggest the reaction cannot be interpreted in terms of a single reaction mechani~rn.~~' mechanism proposed for the Co analog. As shown in equations (1 1 5 j ( 119), a series of nitrous acid preequilibria is followed by attack at the coordinated azide with (Rh(NH,)sN3, NO}3f proposed as a likely intermediate. The [Rh(NH3),N,I2+ ion is a factor of four less reactive than the Co analog:26 and at least an order of magnitude less reactive than HN3.589
H+
+
HNO,
e
NO+
+
H20
(1 15)
Rhodium
963
The electrochemical reduction of [Rh(NH3)50H}2C has been studied in buffered ammoniacal solutions, and leads to a hydroperoxo species (equation 120).590Using a DME, E! = - l.lOV (vs. saturated calomel), but below pH 9.4, the pH dependence of the reduction potential led to the conclusion that a rapid proton uptake (presumably to give the aquapentaarnmine ion) occurs before electron transfer could occur. Gross electrolysis (- 1.19V, Hg cathode) leads to [Rh(NH,),Hl2+, although a Rh' intermediate is thought to be involved. Curtis et ai. measured the reduction potentials of a large number of [Rh(NH,),X]"+ complexes in 0.1 M NaClO, solution and irreversible reductions occurred between - 0.86 and - 1.45 V."' Despite several references to apparent correlations between reduction potentials and numerous other physical properties of coordination complexes, Curtis could find no correlation between the reduction potentials and absorption spectra, 'H NMR, or rates of aquation or base hydrolysis. The reduction potentials for the complexes studied are given in Table 30. Generally, the Rh'" complexes are ca. 0.8 & 0.1 V more difficult to reduce than their Co analogs, and about 0.7V easier to reduce than the corresponding Ir complexes. Early reports of some correlation between reduction potential and the wavelength of maximum d-d a b ~ o r p t i o n ' ~were ~ , ' ~ assumed ~ to be based on limited data as no such correlation could be found in this huge study involving dozens of pentaammine complexes of CoI", Rh"' and Ir11'.591When carefully considered, the lack of such correlation is really not surprising. and is carefully rationalized by Curtis et al. CRh~NH3),OHl2++
0 2
'
NH3e,NHNH:
[Rh(NH,),(OH)(O,H)l+
(120)
(ii) Other monodentate amine complexes The ubiquitous WH3ligand also forms a variety of tetraammine complexes, [Rh(NW,),XYJ"', but the chemistry of these complexes is closely related to that of the bis(ethy1enediamine) analogs and relevant references will be included there. The acid dissociation constants for [Rh(NH,),(H,O)X]"+ (X = NH,, H,O, OH-, C1-, or Br- ;Table 3 1) have been determined from potentiometric titrations; the acidity of the coordinated H 2 0can be rationalized in terms of electrostatic and o 2nd R bonding at 120°C for 15 hours leaus to 'thermal effects.'I2 Heating solid cis-[Rh(NH3),(OH)(H20)]S,0, deaquation' (equation 121) and the formation (after recrystallization from NH,Br solution) of the bis(hydrox0) bridged salt, [(NH,), Rh(OH)2Rh(NH,)4]Br4,595which is isomorphous with the well known Co"' and Cr"' analogs. The electronic spectra of these complexes, and other monodentate amine complexes not covered in the pentaammine section (Section 48.6.2.l.i), are given in Table 32.
cis-[Rh(NH,),(OH)(H,O)]S,O, [(NH,), Rh(OH)2Rh(NH,),]Br., + 2Hz0 ( I 2 1) Monodentate amines other than NH, also form complexes with Rh"'. Edwards and co-workers prepared and characterized 12 complexes of the form [Rh(Az),X,], [Rh(Az),X,]+, [Rh(Az),X]*+, [Rh(Az)J3+, [Rh(NH,),AzI3+ and [Rh(Az), (H,0),(OH)]2+, where Az = aziridine (azacyclopropane), NH(C,H,), and X = halide.S96Prepared by the direct reaction of RhC1,-3H,O with aziridine, these complexes are similar to the well-studied ammine complexes; the aziridine acts as a typical monodentate ligand, halide aquation occurs in aqueous solution, and Br- substitution of coordinated Cl- was noted. Reaction of [Rh(Az),Cl,]+ with I- causes reduction to [Rh(Az),I], and HCl does not open the aziridine ring, suggesting that coordination to the Rh"' stabilizes the three-membered aziridine ring, presumably by saturating the N atom, and hindering the standard Table 30 Reduction Potentials of some [lZh(NH,),X]"+ Complexes (vs. SCE) (from ref. 591)
Nitrogen ligands NH, NH, C(O)NH, Imidazole NCMe Imidazolate NCONO; NH,C(O)NHMeC(0)NH-
- 1.12 - 1.24 - 1.10 -0.95 - 1.17 - 1.14 - 1.33 - 1.45
- 1.35
Oxygen ligands H2O OHHC(O)NMe, O=C(NH, )* HCO; NH, co;
- 0.96 - 1.02 - 0.94 - 0.86 - 1.06 - 1.09
Hulide ligand c o-
-0.96
Rhodium
964
Table 31 pK Values for Some RhlL1Amine Complexes
Complex c~s-[R~(NH,),(H,O),]~ trans-[Rh(NH,),(H, O),I3+ +
C~S-[R~(NH,),(C~)(H,O)]~+ trans-[Rh(NH, )4 (C1)(H,0)l2 cis-[Rh(?W,),(Br)(H, O)]” truns-[Rh(NH, ),(Br)(H2 0)12 +
+
C~~-[R~(NH,),(CN)(H,O)]~+ rrans-[Rh(NH,), (CN)(H,O)]” [Rh(NH,Me), H,O]’+ [WNW5H,0I3+
[Rh(NH3)5(NH2COMe)]3+ [Rh(NH,),(HN, COPh)]’+ [Rh(NH,),(imidazo1e)l3+
P 4
PK2
6.40 4.92 7.84 6.75 7.89 6.87 6.8 7.8
8.32 8.26
6.10
-
6.63 6.27 6.40 6.53 6.93 6.24 3.3 2.2 9.97
-
25T, 25T, 25T, 25T, 25T, 25T,
-
-
-
Conditions
Ref.
1.OM NaClO, I.OM NaClO, I .O M NaClO, 1.0 M NaClO, 1.0 M NaC10, 1.OM NaC10,
512 512 512 5 12 512 512
-
22”C, 0.047 M NaCIO, 9.4”C, p P 0.2 M 1 5 T , p = 0.06M 1 5 T , fi = 0.12M 22‘C, 0.047 M NaClO, 25‘C, 1.OM NaClO, 3 5 T , )I x 0.2 M 1.OM C10,1.0 M ClO; p = 0.1
-
-
511 53 1 594 594 511 512 53 1 525 524 527
Table 32 Electronic Spectra of some Monodentate Amine Complexes of Rh”’ ..
Compound [Rh(NH,Me),CI]Zt [Rh(NH, Me),H20I3+ [RhAz,C131 [RhAz,Cl,]Cl [RhAz,Cl,]Br [RhAz,Cl,]I [RhAz, CI]Cl, [RhAz, Br,l [RhAz, Br,]Br [RbAz, 411 [Rh(NH,), AzICl, cis-[Rh(NH,),(H,O),]’cis-[Rh(NH,),(OH),]’ [(NH3)4Rh(OH), fi(NH3)414+ trans-[Rh(NH3),(HzO)C1l2’ trans-[Rh(NH,),(H, O)CN12+ trans-[Rh(NH,), (OH)CN]+ trans-[Rh(NH,), (CI)CN]+ cis-[Rh(NH,), (H2O)CN]” cis-[Rh(NH,), (OH)CN]+ cis-[Rh(NH3)4(Ci)Cr\rl+
..
a,, (nm) (4 357 (122), 284 (144) 321 (164), 269 (150) 394 (180), 326 (lIS), 223 (9800) 417 (250), 340 (250), 287 (410), 223 (9700) 415 (170), 336 (170), 284 (400), 225 (3900) 417 (400), 336 (400), 288 (616), 228 (16 100) 410 (230), 330 (340), 289 (460), 221 (6300) 418, 283, 223 290, 225 488, 358, 281, 221 324 (370), 219 (940) 326 (107), 268 (91) 335 (122), 281 (118) 331.5 (523) 392 (54.3), 286 (124.6) 296 (160), 253 (138) 295 (184), 260 (192) 302 (171), 259 (142) 291 (119), 251 (133) 305 (142), 266sh (191) 366 (- 109), 262 (- 166)
Ref-
511 594 596 596 596 596 596 596 596 596 596 595 595 595 577 540 540 540 540 540 540
ring-opening reactions. The electronic spectra (Table 32) are dificult to interpret, as no information on the geometric configuration of these complexes is given; an unusual feature is suggested, as a band identified as the spin-forbidden ‘ A,p -+ 3T2gtransition, previously observed for [RhCI,I3-, [Rh(C,0,),I3- and [Rh(NH,),Br]2+,597 is also observed for [Rh(Az),Cl,] and [Rh(Az),I,]I (488 nm). All the aziridine complexes showed strong intraligand transitions (21 7-227 nm), and IR peaks (850-890cm-’), confirming that the aziridine ring is intact. The molar absorption constants for several of these complexes were reported to increase upon dilution, but no explanation for this phenomenon was suggested.596 A series of monomeric and chloro-bridged oligomeric (or perhaps polymeric) allylamine complexes [RhCl,L,] (L = H,NCH,CH=CH,; n = 1, 2, 3) have been characterized (for n = 1 a coordinated HzOis also present) and have been shown to undergo a variety of inter- and intra-rnolecular reactions.s98As shown in Scheme 15, the reactions are thought to begin by a breakdown of the chloro bridges, followed by coordination of the unsaturated C=C moiety of the allylamine, H
Rhodium
965
transfer, and rapid collapse to the final products. (The organic products were identified by GLC, but the exact nature of the Rh"' product was not indicated.) Coordination to the Rh"' has no effect on the stretching frequency of the C=C double bond of the allylamine, showing that they are N-bonded. Analogous complexes with the saturated n-propylamine were prepared: [RhCl, L,] (n = 1, 2, 3) and [RhL5Cl]C1, (L' = H,NCH,CH,Me); while similar to the allyl complexes, the saturated ligands do not undergo the reactions represented in Scheme 15.'% r
-.
7
LntramoIecular
Intermolecular
Amino
'LoRh(NH3)'
+ CH,=CHMe
MeCHzCH
+ 0
II
H,C=CHCH
Scheme 15
Swaddle has studied the H + - and OH--induced aquation of [Rh(NH2R),C1lZ+(R = H or Me), and found, contrary to expectations, that [Rh(NH,Me),(H,0)]3+ is more acidic (pK, = 6.10) than the pentaammine complex (Table 3 l), suggesting that methylamine is a poorer electron-donor than ammonia.'" Methyl substitution of the amine only slightly retards the acid hydrolysis of [Rh(NH2R),C1]'+; the difference is in the AS* term (-50.2JK-'mol-' for R = Me vs. - 44 J K-' mol-' for the pentaammine) not the AH* (101.9 kJmol-' for R = Me vs. 102 kJmol-' for the pentaammine). In addition, the change in AS* upon methyl substitution is opposite to the change observed for the analogous Co"' system, further supporting the assignment of an Z, mechanism for the aquation of Co"' amines in acidic aqueous solution, and a more associative (I,) path for the Rh"' analogs. In contrast, methyl substitution accelerates the OH- induced aquation of [Rh(NH2R),C1l2+by a factor of 29, consistent with a dissociative rate-determing step which is accelerated by steric crowding. The S, 1CB (or DcB) mechanism is suggestds''
48.6.2.2
Bidentate albhatic amines
( i ) [Rhlen),I'+
Werner first reported that the [Rh(en),]Cl, salt results when hydrated ethylenediamine reacts with Na,[RhCI,]," and Jaeger showed that it couid be formed when 50% en/H,O reacts with Wilkinson and co-workersYm1 and Gasbo1,602 have taken advantage of the catalytRhC1,- 3H,0.599*b00 ic properties of ethanol to prepare [Rh(en)>I3+by reacting RhCl, - 3H,O with ethylenediamine in aqueous ethanol, and Watt has used the direct reaction of 98% ethylenediamine with solid RhC1,*3H,O with some SUCC~SS."~ The electronic spectrum has been reported by a variety of workers,m',60z,m and is consistent with a RhN, chromophore (Section 48.6.2.2.ii; Table 35). Werner resolved it as the 3-nitro-( +)-camphor and (+)-tartrate salt: and the tartrate resolution has been repeated.m*@6The ( -)-[Rh(en)J3+ ion is isostructural with ( +)-[Co(en),I3+ and (+)[Cr(en),I3+ and has the A configuration.610 Several researchers have used 'H NMR to analyze configurational interactions of the ethylenediamine bidentates in [Rh(en),I3+.41'6's Hawkins noted6'' that the non-equivalence of protons in such tris(ethy1enediamine) complexes is not necessarily due to conformational differences, for even if
966
Rhodium Table 33 Population of A-[Rh(en),]" Conformers (from ref. 61 5)
I' ,
..
Ring conjiguration
Nme
111 116,161,611 166, 616,661 666
A B C D
Mole fractions 0.15M 0.3 M PO:A-[Rh(en),]'+ A-[Rh(en), 1'
8 92 0 0
45
55 0 0
rapid configurational changes occur between (T and 2. forms, the four C--H groups in a given ethylenediamine ring are still inequivalent, leading to an AA'BB' 'H NMR pattern. The ' H NMR spectra of [Rh(en),]X, (X = C1-, I- or CiOi) have virtually identical chemical shifts and band widths (for both the methylene and amine bands), and the spectra are virtually unchanged between 4 and 8 2 T , indicating rapid conformational averaging. As observed with [ C ~ ( e n ) , ] ~ " , ~ion '~,~'~ pairing to multiply-charged oxyanions is directional, and conformational ordering effects can be observed for D,O solutions of [Rh(en)J3+ in the presence of phosphate, selenate and selenite These changes were interpreted to imply directional association favoring the (Zel)ring configuration, an assignment supported by the similarity of the spectra of [Co(en)J3+ and [Rh(en),I3' in the A thorough conformational analysis of hydrogen-bonding solvent, trifluoroacetic [Rh(en),]Cl, (D,O solutions with a 220 MHz 'H NMR) disputed this assignment, h~wever,~'' as the expected AA'BB' pattern was found to be slightly perturbed by the stronger coupling to the low field proton; spectra were interpreted in terms of a conformationally labile ethylenediamine system which gave an AA'BB'X spectrum. Analysis of the molecular conformation population is represented in Table 33. Theoretical considerations and analogy to [Co(en),13+ion suggest that configurations A and B would have approximately equal populations, yet the B configuration (Zls) dominates, and is presumed to be the lowest energy configuration for A-[Rh(en),13+.In the presence of phosphate ions, directional association favors the A configuration ('ZIF), and at 0.3 M PO:- the A and B configurations are almost equally p o p ~ l a t e d . ~ ~ ' Petersen and co-workers used 13C NMR to study configurational changes in [Rh(en),13+ and [Rh(en),XY]"+ (vide infra), and observing only one 13Cresonance for [Rh(en),I3+ (46.35 p.p.m. vs. TMS), also concluded that ring configurational changes are fast on the NMR time scale.Mn Little chemistry of [Rh(en),I3+ has been presented, as it is thermally inert, although Watt and co-workers have reported on the products formed upon deprotonation of [Rh(en),I3+ by KNH,.603,6'M'sDeprotonation occurs as represented in equation (122), and all three products (x = 1, 2 or 3) have been characterized by IR analysis.603Reaction of [Rh(en),I3+ with five equivalents of KNH, (25"C, 5 days) reportedly leads to dark green crystals of a paramagnetic GCR = 1.5 & 0.1 BM) solid identified as K[Rh(en - 2H)(en - H),].6'6Reaction of this material with water regenerates [Rh(en)J3+, and quantitative reduction to Rh metal and the IR spectrum show that K[Rh(en - 2H)(en - H),] is a Rh"' complex without a Rh-H linkage. Watt concludes that the 'origin of paramagnetism remains obscure'.616Deprotonation of [Rh(dien),] with amide ion has also been shown to lead to the doubly deprotonated [Rh(dien - H)JI, a diamagnetic H,O-sensitive solid:" Further deprotonation leads to a pyrophoric solid (presumed to be [Rh(dien - H)(dien - 2H)]), and an ill-characterized species assumed to be the hexadeprotonated [Rh(dien - 3H)2]3-.617 [Rh(en),]T,
+
xKNH2
-+
[Rh(en - H)x(en),-x]T~s-xl+ xNH, x = 1,2or3
+
xK1
(1 22)
The deprotonated tris(ethy1enediamine) complexes are, not unexpectedly, nucleophilic. The pale yellow doubly deprotonated complex [Rh(en - H),(en)]I acts as a nucleophile toward MeI, MeBr, SOCl2,SO2CI2,and triply deprotonated [Rh(en - H),] is electrophilically attacked by Me1 (Scheme 16).6'8The methylated species are air and H,O stable, but while air stable, the BF, and SO,Cl, adducts hydrolyze rapidly. The low S-0 stretching frequencies observed led to the suggested bonding configurations shown in Scheme 16, with the sulfur atoms bonded to two amine nitrogens.6'8
(ii)
(Rk(en),XYp+complexes
Basolo and Johnson opened the now extensive field of bis(ethy1enediamine)Rh"' chemistry with of complexes of the form [Rh(A),ClXy+ (&= (NH3)4,(en),, (meso-bn),, (+-bm),,
9 T P ~ W rt1dy619
Rhodium
[Rh(en)(en - H),]I
[Rh(cn){(en
-
967
-
H),SO}]Cl, I
2MeI
[Rh(en)(enMe),] I,
[Rh(en){(en - H),SO, f]Cl,I
(C,C,C',C'-tetramethylen),, tren or trien). The general synthetic route requires reaction of RhCI, 3H,O with a stoichiometric amount of the hydrochloride salt of the appropriate amine, followed by the gradual addition of a stoichiometric amount of NaOH to the refluxing solution. As part of this study, cis and trans isomers of [Rh(en),X,]+ (X = CI, Br, I, NO, or N3) as well as [Rh(en),ClY]"+ (Y = SCN , NO; or NH,) were prepared and characterized by UV/visible and IR spectroscopy, with geometric assignments made on the basis of optical resolution (for the cis form), absorption spectra, and base-catalyzed hydrolysis of the cis c o m p l e x e ~ . Reaction ~'~ of trans[Rh(en),X,]+ with BH; was shown to lead to the monohydride (with retention of the trans geometry)$" and the large trans effect of the H- ion led to trans-[Rh(en),Hz]+. Vibrational TR and Raman spectra for a variety of [Rh(en)2XY]"+complexes showed that the Rh--N stretch occurs at 555-575 cm-', with N-Rh-N deformations at about 250 cm-' and the Rh-X stretch (X = C1, Br or I) between 150 and 3 5 0 ~ m - ' . ~ *Conductivities ' (in DMSO solution) of cis- and trans[Rh(en),CI,]Cl, as well as some cis dichlorobis(substituted diamine) complexes are similar to those for the Cr"'and Co"' analogs, and are characteristic of 1: 1 salts.622The [Rh(en),(H,0),]3+ ions are weak diprotic acids, with the trans isomer an order of magnitude more acidic than the cis form (Table 34). Direct synthesis of the trans form of [Rh(en),XY]"+is generally easier than that of the cis isomer, but the cis configuration can be ensured by bonding a bidentate to the two non-ethylenediamine coordination sites. The [Rh(en),ox]NO, salt was prepared by refluxing an aqueous solution of cis-[Rh(en),CI,]NO, with Na,ox for 2.5 h.629In the presence of trace amounts of BH4 (at pH 4.7), a refluxing solution of either cis- or ~r~ns-[Rh(en)~Cl~]+ reacts with ox2- within seconds to form [Rh(en),ox]+ .630.631 This BH, catalyzed substitution (with trans to cis isomerization) was shown to be a general route to [Rh(en),AA]"+ complexes (for AA = en, ox2-, gly-, ala-, maIonate2-).63M3s Reaction of the [Rh(en),ox]+ with various anions in strongly acidic solution was then developed as The electronic spectra of a rich a convenient route to cis-[Rh(en),X,]+ ions (X = C1, Br, variety of [Rh(en)2Xv+ ions are recorded in Table 35, and depending upon the position of the charge-transfer bands, generally consist of two spin-allowed d-d transitions (descended from ' A , + 'T,(O,,)and the higher energy ' A , -+ 'T, transitions). The spin-forbidden ' A ,+ 3Tltransition has been observed in ~rans-[Rih(en)~Cl~]ClO~ at - 1 8 9 T as a very weak shoulder at 469nm.644 Resolution of cis-[Rh(en),XY]"" has been effected by numerous workers; Basolo isolated ( -)535 [Rh(en),CI,]+ as the bromocamphor-7c-sulfonate (BCS) Gillard separated ( +)589-[Rh(en)2(CZ04)]+ion (initially as the (+)-[Co(EDTA)]- salt), and found it to be isomorphous with
-
Table 34 Acid Dissociation Values for [Rh(en),(H,O),]'+
trans-[Rh(en),(H,O),]'+
cis-[Rh(en)z (H20)2]'+
4.43 4.73 4.33
7.81 7.64 7.72
4.47
7.91 7.98 8.08 8.24
5.82 6.09 6.34
-
20, 26.4 2s 25.0 25 25 25.0
0.20M NaNO, 1.OM NaCIOO, 0.5 M NaCIO, 1.OM NaCIO, 0.01 M NaClO, 0.5 M NaClO, 1.OM NaC104
623 624 625 626 627 627 627
Rhodium
968
Table 35 Electronic Spectra for [Rh(en),XYj"+
rrapIs- [Rh(en), C1, 1+
NO; NO;
c1-
c10;
NO;
cis-[Rh(en),Cl,]'
c10; c1-
trans-[Rh(en), Br,]+
NO;
Cl0,-
ris-[Rh(en)?BrJ+
Br-
trans-[Rh(en), 12]+
NO; I-
c10,-
I-
cis-[Rh(en), Ill+ trans-[Rh(en),ClBr]
cloy
+
trans-[Rh(en), BrI]'
c10; c10,-
tranr-[Rh(en), CII]'
cloy
trans-[Rh(enjzClrJ+ rram-[Rh(en), CIN, I+ rruns-[Rh(en), BrN, ]+ trans-[Rh(en)21N,]+ trans-[Rh(en), (Cl)(OH,)]2+
c10; C1Q; c10;
t runs-[Rh(en), (CI)(OH)]
c10;
+
cloy
cloy c10,-
406 (83), 286 (134) 406 (75), 286 (130) 406 (79.4), 312sh (79), 289 (123), 242sh (1585), 206 (40OOO) 406 (84), 286 (130), 240 (1350), 206 (38 900) 407 (75), 286 (125) 407 (82.5), 287 (126.4) 410 (82), 289 (121), 240sh (1700), 297 (39 600) 407 (82.5), 287 (120) 352 (147), 295 (189) 352 (155), 295 (180) 352 (203), 295 (205) 394sh (loo), 352 (191). 295 (186), 238(sh) (1000) 350 (195), 296 (202) 352 (194), 295 (189) 429 (115), 327sh (91), 278 (2570), 234 (37 000) 425 (loo), 276 (1800) 425 (100) 425 (120), 276 (3000), 231 (- 31 000) 431 (1 15), 277 (2900), 234 (37 800) 429 (120), 276 (3000) 425 (120), 276 (3000) 435sh (79.4), 368 (240). 278sh (1000) 362 (210), 276sh (900) 363 (260), 280sh (900) 366 (255), 28Osh (980) 367 (258) 465 (263), 340 (24 500), 270 (49 OOO), 223 (30000) 462 (260), 341 (lOOOO), 269 (30000), 222 (20000) 467 (271), 342 (l6000), ca. 270 462 (322), 339 (2178), 269 (45 450), 220sh (- 16 820) 462 (260), 340 (143001, 269 (31 OOO), 222 (20000) 444sh (447), 375 (953, 290 (4677), 225 (44 700) 375 (1200) 486sh (- 18), 419 (94), 270 (1590), 222 (31 700) 486sh (- 10),.413 (95), 262 (- 1600) 455 (260), 311 (7500), 253 (41 500) 488 (162), 453 (172), 310 (4540), 253 (25 700), 226 (17 600) 440 (154), 300 (4350), 242 (38 500) 490 (260), 440 (160), 301 (4300), 241 (38300) 486 (243), 440 (151), 298 (3870), 242 (38 100) 377 (560), 263 (9600) 388 (580), 272 (15 100) 408,292 383 (46), 283 (143) 386 ( 5 3 , 282 (147) 350-380 (26); 280 (73) 384 (56), 283 (143) 363 (123), 285 (172) 360 (52), 280 (76) 370 (94), 293 (134) 367 (94), 285 (129) 372 (102), 289 (111) 325,285 470 (32), 405 ( 5 3 , 280 (520j, 235 (5260) 465 (26), 395 (48) 465sh (31), 396 (58), 252sh (603)
-
-
-
-
607 619 633 62 1 634 624 635 625 607 619 607 633 636 631 633 619 620 634 635 637 634 633 619 636 63 1 637 633 619 636,653 621 638 633 635 621 634 638 621 638 639 62 1 638 638 638 62 1 607 640 640 62 1 640 635 641 642 607 62 I
640 641
Rhodium
969
Table 35 (Continued)
cis-[Rh(en),(3r)(H2O)]*'
Cl0,NO;
trans-[Rh(en),(Br)(OH)]+
c10;
trans-[Rh(en), (T)(OH2)]'+
c10;
c10;
trans-[Rh(en), (I)(OH)]'
frans-[Rh(en),(NH,),1, cis-[Rh(en), (NH,),j3+ trans-[Rh(en), (NH3)(OH,)]3+ trans-[Rh(en), (NH3)C1]?+
c10;
cis-[Rh(en),(NH,)C1]2+
NO,
fruns-[Rh(en)l (NH3)Br]"
NO;
f runs-[Rh(en)2(N H,)I]2c
1-
+
cis-[Rh(en),(en - H)C1I3+ Irans-[Rh(en), (Cl)(OCO,)] trans-[Rh(en), (Br)(OCO,)] trans-[Rh(en),(I)(OCO,)] trans-[Rh(enX(OCO, )(OH2)]+
trans-[Rh(en),(OCOz)(OH)j
ClO, ClO;, NO; NO;
CI-
-
-
\
Cl0,-
trans-[Rh(en),(OCO,),]cis-[Rh(en), (OH, )(C€O,)]+ cis-[Rh(en),(OCO,)]+ ~rans-[Rh(en),(OH,)~]~'
-
truns-[Rh(en),(OH)(OH,)J2+
pH = 6
c10; c10, pH=2
trans-[Rh(en), (OH), 1' p H = 11
cis-[Rh(en), (OHz)2]3i
ao;
cis-[Rh(cn), (OH,)(OH)]'+
c10;
cis-[Rh(en), (OH),]'
Cl0,-
trans-[Rh(en), (N0,)J cis-[Rh(en), (NO,)2]' trans-[Rh(en), (NCS)Cl]+ trans-[Rh(en), (N, )2]+ trans-[Rh(en), NO,Cl]+ trans-[Rh(en), H,]+ trans-[Rh(en), (H)Clj+ truns-[Rh(en), (H)Br]+ truns-[Rh(en)2(H)I] trans-[Rh(en)2(H)(OH)]'
NO; NO; NCS-
+
358 (133) 357 (175) 370 (138), 275sh (1066) 370 (103), 290sh (- 200) 376 (105), 290sh (148) 373 (109), 295sh (146) 476 (150), -410, 332sh (-760), 300sh (- 1240), 270sh (- 2030) 475 (95), 300 (1000) 480 (263), 298 (1417) 445 (149), 391sh (- 120), 338 (1195), 270 (6072) 450-400 (71), 270 (2150) 440sh (175), 397 (196) 300 (182), 252 (166) 302 (204), 254 (158) 302 (120), 259 (122) 342 (80), 275 (113) 342 (95), 275 (120) 345 (105), 275 (132) 342 (150), 276 (195) 344 (151), 276 (182) 355 (117), 286sh (158), 244 (912) 357 (116) 391 (347), 333sh (- 479, 276 (4170), 227 (49000) 345 (140), 274 (239) 376 (1 18) 384 (i4ij 456 (232). 415 (216) 345 (ioij' 346 (121) 347 (146) 332 (224) 327 (312) 342 (66), 272 (125) 342 (64), 262 (148) 351 (61.6), 274 (130) 350 (60), 274 (130) 351 (58.4), 274 (130) 337 (88), 270 (158) 343 (89), 282 (154) 344 (87), 284 (149) 338 (136), 290 (136) 336 (94), 289 (154) 346 (98), 297 (130) 344 (99), 296 (134) 344 (97), 295 (132) 318 (174), 271 (148) 314 (184) 318 (175), 271 (137) 323 (199), 283sh (155) 327 323 (203), 280sh (161) 329 (I 84), 280 (1 79) 330 (189) 329 (179), 278.5 (171) 330sh (590), 255sh (2400) 290 (660), 245sh (3500) 363 (340) 375 (780), 282 (12000) 310sh (310), 255-265sh (860) 340 (250), 290 (500) 365 (84) 371 (160) 385 (220) 295 (323), 257 (272)
c1-
NO; NO; NO; NOT NO; NO;
,
631 637 62 1 640 635 641 62 1
640 641 621 640 641 633 633 62 1 607 619 633 619 633 633 637 633 632 641 641 641 625 625 625 627 627 62 1 623 624 625 626 623 625 626 62 1 623 635 625 626 627 636 626 627 636 626 627 636 626 619 619 619 619 619 620 620 620 620 675
Rhodium
970
Table 35 (Continued)
[Rh(eni2(bidentatel]"+ complexes NO; 301 (238), 255 (191) 301 (243), 255 (194) 306 (251), 257 (246) 300 (250) 350sh, 296 (244), 253 (195) cloy 325 (260) 325 (270) 325.5 (253) 316 (250), 268 (286) 316 (330), 270 (335) 316 (298), 268 (350) 316 (286), 265 (363) 316 (335), 266 (350) 316 (320), 266 (355) 317 (285), 265 (430) 345 (300), 270 (900) 352 (700), 276 (1800) 450sh (150), 348 (500), 288 (900)
[Rh(en)?C, O,]'
~
~
610 607 606 608
609 659 630 63 1 643 643 643 643 643 643 643 618 618 618 618 628 628
( -),,y-[Co(en),(ox)]( +)[Co(EDTA)], implying the L (now called the A) configuration for ( + ) 5 8 9 [Rh(en>,ox]+.636 The ( +)589-[Rh(en),(N02),]+ ion was also separated by the same technique.
Spontaneous resolution can be observed if a solution containing (+)-[Rh(en),ox]+ is added to a saturated solution of racemic [Rh(en), oxIC1, as ( +)ss9 -[Rh(en), ox]C1 precipitates; a solution of ( -)589-[Co(en),ox]+ can also be used to initiate the precipitation of ( +)ss9-[Rh(en),o~]C1.636 BaiIaP3 also generated ( -)589-[Rh(en),C1,]+ as the BCS salt, and showed that ammoniations (in liquid and aqueous ammonia) also occur with retention of configuration. Such substitution studies (summarized in Scheme 17), and a comparison of the electronic and CD spectra with their Co analogs has allowed the assignment of absolute configuration for a variety of cis-[Rh(en),XY]"+ ions (Table 36).
I-
(+)D -[ Rh(en),
(ox),]
+
-
HCLO.
-'
(+)D
A Configuration
I
,iConfiguration
1
Converted to or from the of the dichloro cation
Converted to or from the enantiomer of the dichloro cation
(-)" enantiomer
Scheme 17
97 1
Rhodium Table 36 Electronic and Optical Spectra of cis-[Rh(en),XYj+ Complexes (Less Soluble Diastereomer)
Compound
Electronic (nm) (4
.2m,,
ORD" [I.;
300 (250)
(+I
306 (251)
-79
257 (246) [Rh(en),Cl,]'
-
-
394 (100) 352 (191) 295 (186) 350 (195) 296 (202)
(+)
-
-
435 (79) 368 (240) 278sh (1.0 x 103) -
(+)
363 (260) 280(sh) (900) 302 (204)
-
254 (158) 344 (151)
-
276 (1 82) 325 (270)
(+I
290sh (660) 245sh (3500) 314 (184) 327 330 (189) [Rh(en), dYII2 [Rh(en), (Sala)]I, [Rh(en), (S-val)]I, [Rh(en), (S-leu)]l, [Rh(en),(S-ser)]12 [Rh(en), {(2S,3R)-thr)II, [Rh(en), (S-met)]I,
(+)
(+I (+I (+I
276 (189) 316 (250) 268 (286) 316 (330) 270 (286) 316 (298) 268 (350) 3 16 (286) 26.5 (363) 316 (335) 266 (350) 316 (320) 266 (335) 317 (285)
aRotation, at D line of sodium, or the sign of the rotation proposed by IUPAC.M5
+ 34
+37 + 36 -t35
+ 37 f
36
+ 27 at
Absolute configurution
Re$
A
608
A
605
A
633
A
636
A
633
A
636
A
633
A
633
- 2.88
A
636
1.25 - 0.8
A
636
A A A
636 636 636
A
643
A
643
A
643
A
643
h
643
A
643
A
643
Circular dichroism a (AE),,
'
320 288 320 287 258 446 394 345 304 400 307 467 406 350 303 480 416 353 310 325 293 266 362 29 1 263 348 260 300
-2.01 0.09 + 2.0 -0.1 + 0.8 -0.10 - 0.52 +0.33 + 0.72 + 0.54 - 0.82 -0.15 - 0.62 0.62 + 0.20 -0.17 + 0.28 - 0.47 -0.15 0.48 - 0.06 0.20
314 320 410 335 272 335 260 335 265 335 265 335 265 335 265 335 265 342 293 260
- 0.63 - 0.99 + 0.03 - 0.38 - 0.29 -2.31 - 0.38 -2.31 - 0.59 - 2.34 - 0.61 - 2.35 - 0.62 - 2.36 -0.51 - 2.37 - 0.49 - 1.64 -0.16 - 0.09
-
+
+
+ + 40.51 + 0.77 + 0.2 ~
listed w d v e h g t h . bAbsolute configurations based on the helical model
Amino acid complexes of Rh"' are relatively rare,M6 but some [Rh(enjZ(AB)T+ complexes (AB = glycino, alanino, (-)-leucine, (-)-methionine, (-)-valine, (-)-phenylalanine, (-)-tyrosine) have been prepared by reacting equimolar mixtures of cis or tram-[Rh(en),Cl,]+, OH- and the appropriate amino acid in an ethanoljwater (1/3) ~olution."~ In the absence of the reducing ethanol, no reaction is observed; the preparation of the gly and ala complexes using BH; catalysis has been reported:30 Resolution of such cations, and a comparison of their CD spectra with ( -)589-[Rh(en)3]3+(with the A configuration) implies that all the ( -)ssg-[Rh(en)2AB]+ ions also . ~ ~electronic and CD spectra of the methionine complexes suggest that have the A c o n f i g ~ a t i o nThe the methionine is bonded through the N and 0 atoms.M3 While electronic, IR and 'H NMR spectra have been used to assign isomeric form for [Rh(en)2XY]"4 ions, '?C NMR has recently been used to unambiguously assign their geomeThe 13Cchemical shifts for several such compounds are given in Table 37. The cis species show more complex spectra than the t r m s isomers, and the 13Csignal in truns-[Rh(en),XY]"+ is
Rhodium
972
Table 37 I3C Chemical Shifts of some [Rh(en),XY]"+ Complexes (from ref. 607)
"
46.36 45.68
[~h(en)~i~+ trans-[Rh(en), C1,]+ trans-[Rh(en),(H2O)C1]" trm-[Rh(en), (NH3)C112+
,
45.75 45.64
cis-[Rh(en),Cl,]+ czs-[Rh(en),(H,O)Cl]Z+ cis-[Rh(en),(NH,)Cl]?+ cis-[Rh(en),(enH)ClI3+
,
47.25 46.15 47.78 47.07 45.98 45.20 47.07 46.27 45.81 (double intensity) 47.15 46.56 45.99 45.94 43.30 49.59 (broad)
relatively independent of the nature of the non-amine ligands, while it is quite sensitive to the nature of X and Y in cis-[Rh(en),XY]"+ ions?'' Few X-ray structures of these well-characterized ethylenediamine complexes have been reported, but an interesting geometry is displayed by the HgCI, adduct of cis-[Rh(en),Cl, JC1. It has a layered structure with each Hg atom in an environment of six C1 atoms, two each from the adjacent ~is-IRh(en)~Cl~]' moieties.619 Numerous studies on the substitutions at various [Rh(en)2XY]"f complexes (equation 123) have been reported. Basolo and c o - w ~ r k e rpresented s ~ ~ ~ a kinetic study of a large variety of [Rh(N),Cl,]+ complexes (cis and trans complexes for N, = (en),, (tetrameen),, (C204),; trans complexes for N, = (NH,),,(m-bn),, (f-bn),; and cis-[Rh(trien)Cl,]+, and {Rh(NH,),CI]*+). As with the Co"' analogs, the nature and concentration of the incoming nucleophile has little effect on the rate of C1release; for example, the rate constant for C1- release from trans-[Rh(en),Cl,]+ is between 4.0 and 5.2 ( x l v s-') for anation by OH-, NO;, I-, 36Cl-,NH, and thi0urea.5~~ Unlike Co'", however, the Rh"' complexes display rates of chloride release which are insensitive to the total charge on the complex, not dramatically catalyzed by hydroxide ions, and which appear to be completely stereoretentive. Basolo suggested that the mechanism for these substitutions is probably more associative than the dissociative route proposed for the Co'" analogs, and that a seven-coordinate Rh"I intermediate may be involved. truns-[Rh(en),W'
+
Z-
+
truns-[Rh(en),XZ~'
+ Y-
(123)
Z = C1, Br, I; X,Y = halide, H 2 0 , OH-, or NH3
Poe and co-workers have extensively studied the thermodynamics, kinetics and the trans effect in the reactivity of such c ~ m p l e x e ~ . ~ ' ~The - ~thermodynamics ~ ~ ~ ~ ~ ~ ,of~the ~ successive ~ , ~ , ~ anations ~ , ~ ~ ~ of [Rh(en),X(H,O)Y+ by halogens suggests that the Rh center is a marginally soft acid, and that its softness is increased by the coordination of a soft base (such as I-) in a position trans to the reaction site; a halide ligand weakens the metal-ligand bond trans to itself, and an I- causes a more The dramatic weakening than a Br-, which has more effect than a coordinated C1-.634,638640 spectrophotometrically obtained thermodynamic parameters are summarized in Table 38. The kinetic parameters for reactions in acidic solution are consistent with the thermodynamic results, and they suggest the trans-labilizing series: I- > Br- > C1- x NH, > H,O (Table I
Table 38 Thermodynamic Parameters for Halogen Substitutions (Anations and Aquations) of trans-[Rh(en)zXY]+ Starting complex
[Rh(en)2Cl,]+ [Rh(en),Cl,]* [Rh(en),ClBr]+ [Rh(en),ClBr]+ [Rh(en)&lBr]+ [Rh(en)2ClI]+ [Rh(en),Cll]+ [Rh(en),Br,J+ [Rh(en),BrI]+ [Rh(en),Cl,]+ [Rh(en),ClBr]+ [Rh(en),ClBr]+ [Rh(en),ClI]+ (Rh(en),BrT]+ [Rh(en),IJ+ [Rh(en),(OH)I]+ a At
85%.
Reactant
+ + + ++ + + + + + + + + + ++
BrIBr-~ II-
I
'
BrIIH,O H,O H,O H,O H20 H,O H,O
Rh product
-e
z=Z
A
& A
e e
A
E
e e z= e e
'Indirectly determined' parameter.
K
AH" (kJ mol-')
Anations (at UO'C)
[Rh(en),ClBr]+ I .9" [Rh(en),ClII+ 7.0 [Rh(en),Br,]+ 0.8" [FLh(en)tCII]+ [Rh(en)$rI]+ [Rh(en),I,]+ 2.2 [Rh(en),BrIj+ [Rh(en),BrI]+ 7.0 1.9 [Rh(en),I,]+ Aguations (K values at 50°C) 0.0020 [Rh(en)rCl(H,0)]2+ 0.0006 [Rh(enhCl(H2O)l2+ [Rh(en)2Br(H20jc+ 0.0006 [Rh(en),lW,O)] 0.0OIO IRh(en)21(H20)12+ 0.0006 [Rh(er1)~1(H~O)]~+ 0.0002 [Rh(en),(OH)H20]2' 0.002
-0 - 3.8
+ 2.5
7.9 -0 f 2.5 - 0.4 k 3.3b - 10.9 & 3.3b -26 k 1.7 - 11.7 f 2.1b - 10.9 f. 1.7 - 14.2 k 1.3 -2.5 f 1.7
+ 13.4 f. 1.7 + 5.0 k 2.9 +6.7 f. 2.1 + 18.8 f 1.3 + 29.7 f 2.1 + 17.1 f.0.8
Re$ 634 638 634 638 638 638 638 638 638
640 640 640
640 640 640
639
Rhodium
973
Table 39 Kinetic Parameters for the Anations trans-[Rh(en),(H,0)L]2* Trans ligand ( L )
Anating figand (X-)
C1
c1
C1 Br Br
Br
I I I
C1
C1
Br Br
I
NHJ
c1
H2O
Br
OH
I
+
X-
--t
trans-[Rh(en),(L)X]+ f H,O
R"
AH*
(M-'s-')
(kJmol-')
7.2 6.2 6.8 x 5.3 x 8.5 x 7.4 x 5.8 x 3.4 x 7.3 x 1.5
106 f 1.3 102 f 1.3 92.0 f 2.1 93.3 f 1.3 81.6 f 1.7 77.8 4 1.3 75.3 & 1.3 105.2 0.8 99.6 f 0.6 133.8 f 0.9
10-4 10-4
IW3 10-3
IO-' lo-' IO-' 10-4
10-3 10-4
A S (J K-' mol-')
Ref.
21.8 f 4.2 8.4 4.6 2.1 6.3 1.7 & 4.2 2.5 & 5.0 - 5.9 k 4.6 1.26 4.6 13.8 f 2.6 21.8 f 2.0 95.6 2.6
640 640 640 640 640 640 640 623 623 623
*
'At 50°C
poe reminds us that to realistically compare the kinetic trans effect of different ligands, one cannot just consider kinetic parameters, but can only compare reactions in which the thermodynamic driving forces are the same; comparisons of AH* (the kinetic term) with the thermodynamic AH' can be used to derive a kinetic trans-effect series.651 Table 40 demonstrates what little effect the incoming nucleophile has on the reaction rate, as well as the large role played by the trans ligand. (The rate of halide anations5"was subsequently taken to be identical to that of direct aquation,635,639,"653 although specific data on aquations are not presented.) When the anating ligand has a high kinetic trans effect, such as I-, no evidence of an intermediate aquahalo complex is observed, and the anations proceed directly, with retention of isosbestic points, to the diiodo c0mplex.6'~The similarity of the anation activation parameters to those for the analogous reactions of the pentaammine complexes supports the associative Substitutions at trans-w(en), (H, 0),13+ appear unique. However, as the large entropy of activation suggests, a different mechanism (presumably a more dissociative one) operates for this complex.640 [Chloride anation of [Rh(H20),I3+ is also thought to involve a dissociative mechanism (vide infra).6M]For the anation of trans-[Rh(en),XI]+ (X = C1 or Br), an interesting catalyzed reaction was noted,640but not pursued. BasolosS5noted that reactions of Rh"' amine complexes were not dramatically accelerated by hydroxide ion, but did show that substitutions in base do follow the standard kobr= k, 4- k,[OH-] format, with k , representing the first-order aquation observed in acidic solution, and k, representing the second-order base-catalyzed path. Poe has studied the kinetics of the base hydrolysis of a variety of trans-[Rh(en),Xv+ complexes (Table 41). Studies on the base hydrolyses of trans[Rh(en),(OH)X]+ (X = C1, Br, I) showed that the coordinated hydroxide has an intrinsic kinetic trans effect comparable to that of C1- but that its position in a thermodynamic trans effect series is much higher.635For trans-[Rh(en),X,]+ (X= CI, Br), virtually complete trans + cis isomerization occurs upon hydrolysis in base, and ca. 50% isomerization is observed when X = I.653No such 39).623,610,650,651
Table 40 Kinetic Parameters for the Substitution [Rh(en),XL)"+
Cl CI Br Br Br Br I I I I I OH
C1
OH
fI,o
Br
I C1
I
Br Br Br
CN
c1 c1
Br Br I C1
c1 Br Br I 1
F
I Xb
+Y
+
1.39 x 3.34 5.19 x 5.45 7.25 x 3.94 x 8.16 8.84 x 4.36 x 5.24 x 1.2 x
+
[Rh(en)2XY]m+
lo-* 10-7 10-4
IO-* 10-4 10-4 10-4
10-4 10-4 10-4 1.2 10-3 7.3 x 10-4
L
104 f 2.5 115.6 0.5 104 & 0.63 108 f 0.84 106 0.1 97.0 f 2.0 87.2 f 1.1 91.3 f 1.1 96.7 f 1.3 96.1 f 1.3 107 f 3.1 117 & 0.67 99.6 _+ 0.63
'At 50°C. bData are an average for reactions when entering ligand is either C1- or Br-
- 35.6 f 7.9 - 12.1 f 4.6 24.7 f 2.1 12.6 & 2.5 18.0 f 6.3 - 49.0 Ifi 6.3 - 35 f 3.3 -22 f 3.8 - 10.9 f 3.3 - 11.3 f 1.3 -+IO f 12.6 22.3 f 2.0 + 21.8 2.0
+
65 I 65 1 65 1 65 1 651 65 I 651 65 1 65 1 651 65 1 639 639
-
Rhodium
974
Table 41 Kinetic Parameters for the Base Hydrolysis of truns-[Rh(en)2XYl''*
4 Starting complex
(s-')
AH: (kJmol-")
AS: (JK-'mol-')
109 & 2.6 103 & 1 107.9 It 1.4 105 & 1 102 & 0.5 105 1 I05 t- 0.63
-21.5 f 7.7 -38&4 - 13.4 f 4 -21 f 4 -5.6 f 1.6 +4+4 1.9 -2.8 8.0 & 2.4
+
109 & 0.79
4 (Mi's-')
AH: (kJmol-')
AS: (JK-'mol-')
3.85 x 1.68 x 1.81 x lo-'
146.7 5.9 140f7 130 & 3
87.9 f 17.3
-
1.87 x IO-' 1.50 10-5
-
108 I05
+
-
84.8 & 7.7 73.9 9.0 -
2.9 5.4
- 2 f 8.8 - 13 f 16
Ref.
65 3 650 653 650 653 650 635 635
"At S0"C.
isomerization is observed upon base hydrolysis of the halohydroxy complexes.63sIt has been ~ u g g e s t e dthat ~ ~an ~~ acidic ~ ~ ~N-€3 group must be coordinated trans to the reaction site in order for k, (the OH- dependent term) to be significant, but tr~ns-[Rh(en),Cl(py)]~+ has a significant [OH-] dependent term, as do the trans-[Rh(en),XY]"+ ions (Table 41). Pavelich6" has presented a thorough study of the chloride anation of tr~ns-[Rh(en),(H,O),]~+ is much (equation 124), which implies that the conjugate base form (trms-[Rh(en),(OH)(H,0)]2+) more reactive than had previously been thought. Between [H'] values of 0 . 0 2 M to 0.2M, the observed rate is strongly, and linearly, proportional to [H+]-'. Presumably due to the strong trans-labilizing influence of the coordinated hydroxide, the aquahydroxo complex is calculated to be at least three orders of magnitude more reactive than the diaqua ion. Pavelich calculated that at a pH of 1.7 ([Hf]-' = 50M-')), less than 1 % of the complex is deprotonated, but that 95% of the anation still proceeds through this reactive aquahydroxo complex. Addition of the second chloride is 20 times faster still, presumably due to the trans influence of the coordinated chloride, so only the dichloro product is observed. The rate of chloride anation is not linearly dependent on [Cl-1. This non-linearity was also observed by Poe for bromide anations of the same complex623and is presumed to be due to ion-pairing to both the diaqua (3 +) and aquahydroxo (2+) cations. Analysis of this deviation from linearity in the C1- system could not be fitted to an interchange stoichiometric mechanism rate law, but could be fitted to a dissociative stoichiometric mechanism rate law. Interestingly, the data were similar to Poe's, but Pavelich reaches the opposite conclusion, and suggests that trans-[Rh(en),(H2O>,13$,and its conjugate base, react via a dissociative mechanism.6z4 trans-[Rh(en),(HzO)z]3C + 2CIH+
]I
+H+
1 rruns-[Rh(en),C12]+ i2H20
tr~ns-[Rh(en),(H,O)(OH)]~+ -t 2C1-
(12)
/
Pavelich also points out that there may be valid chemical reasons for the variety of different mechanisms which have been postulated for seemingly straightforward substitutions at Rh"' centers. Associative interchanges have been proposed for the pentaammine cations (Section 48.6.2.1 .i). Both associative and dissociative interchanges have been proposed for the [Rh(en),XY]"+ series, and dissociative interchanges, as well as purely dissociative paths, have been proposed for the hexaqua and hexachloro species (Sections 48.6.5.1 and 48.6.7). Such variation in mechanism suggests that the inert ligands have a profound effect on the substitution mechanism of a given rhodium complex. Pavelich speculates that when Rh"' is compared to Co"', the increased polarizability of the Rh center (or perhaps its softness) makes it more susceptible to electronic changes induced by the innocent ligands, so that while a wide range of Co"' or Cr"' complexes may display similar substitution mechanisms, differing mechanisms can be expected for the analogous Rh"' or Ir"' complexes.624 Following from their work on the reactions of pentaammine complexes in liquid ammonia's'-583, Balt and Jelsma studied the kinetics of the ammoniation of some dihalobis(ethy1enediamine)rhodium(II1) cornplexe~.~'~ While trans-[Rh(en),Cl,]+ proved 'too stable toward ammoniation to be studied', the cis isomer and trans-[Rh(en),I,]+ were both sufficiently reactive. Both cations react in two distinct steps to form [Rh(en)2(NH3)2]3'; substitution of the dichloro species goes with retention of the cis configuration, while the geometric configuration of the [Rh(en)*(NH3)J3' formed from the diiodo ion was not determined. In excess NH4C104,both displayed the rate expression observed in the pentaammine studies: kobs= k, + kbwH4+]-I.For these complexes, k,, the rate constant for the 'spontaneous' reaction, is ca. 0 for both reactions, and k b = k,&,, where k, is the
Rhodium
975
constant for the rate-determining step, and GBis the equilibrium constant for the acid-base preequilibrium. (When no NH,C104 is added, an unusual rate expression is obtained: kobs= k , (KCB)*[Rh]-f.)As observed for the pentaammine complexes, the A S * for base hydrolysis is lower for Rh'" than for the Co"' analogs, consistent with a more associative conjugate-base mechanism for the Rh complexes.656 There are few examples of acid-catalyzed substitutions of [Rh(en),Xv+ complexes; attempts to detect H + -catalyzed aquation of trans-[Rh(en), (NO,),]+ were unsuccessful.657Hancock and coworkers found acid-catalyzed substitutions in a kinetic study of the dinuclear diol, A,A-[(en),Rh(OH), Rh(en)J4+.628 The synthesis, 'thermal deaquation' and structure of this ion are analogous to [(NH,), Rh(OH), Rh(NH,), ]4+,595 and the A,A configuration of the ethylenediamine complex is inferred from X-ray powder patterns, which show it to be isomorphous with the well-characterized Co"' and Cr"' ana10gs.~~~ In aqueous solution, a ring opening/closing equilibrium is established, as representmi in Scheme 18, with rate and equilibrium constant values given in Table 42. The Rh"' system is similar to the analogous Cr"' system, except that Rh"' displays acid catalysis (as does the Co complex). The similarity in AH* and AS* for the k - , and k - , paths suggests that similar mechanisms are operating in the two reactions, and since the nucleophiles are so different (OH- and OH,, respectively), it is argued that a dissociative mechanism seems likely. The two pK, values of the diaquamonool differ by seven orders of magnitude (pK: = 2.37 and pK: = 9.13), too large a difference to be rationalized on the basis of charge considerations (note pK, values in Tables 31 and 34), and Hancock suggests that the aquahydroxomonool is stabilized by intramolecular hydrogen bonding, as represented in (47).628A base-catalyzed path was also observed, but it only becomes significant at pH values above 9.5.
I1
-H* +H+,Kk
Scheme 18 Table 42 Parameters for the Equilibration of [(en), Rh(OH)?Rh(en),l'+ (ref. 628)" Kineric parameters Spontaneous path k , = 4.6(1) x io-4s-1 AH+ 86(3)kJmol-' AS:= - 1 9 ( 8 ) J K - 1 ~ ~ l - ' Acid path k,/Ka3= 0.134(3)M-'~-' AH* - W = 0.134(3)M-' S-' AS* - A 9 = - 68(7) J K-I mol-] Base path (0.1 M NaOH) k, = 2.10(2) x 1 0 - 3 - 1 Thermodynamic parameters pKI = 2.372(1) AHo 28(4)kJmol-' pK: 38i 9.128(7) K , = 11.2(5) AH" = - 14(3)kJmol-' 25T, #
=
1.OM (Na,H)CIO,, see Scheme 18 for labeled reactions.
k-, AH!' AS:,
= 4.1(1) x IO-'S-~ = lOlkJmol-' = 9(9)JK-'mol-'
k - , = 5.1 x 10-5s-1 AHZ, = 100kJmol-' AS:, = 9(9)J K-' mol-'
AS" = 49(13)JK-lmol-l -
ASo = - 28(&)J K-' mol-'
= 976
Rhodium 14+
H I
(47)
Acid-catalyzed aquations were also observed in a major kinetic study of [Rh(en),(ox)]+ .659 The overall reaction studied is given in equation (125), which belies the complexity of the system. All four of the oxalate oxygen atoms exchange with the solvent, but at two kinetically distinct rates. The reactions involved are summarized in Scheme 19, which assumes that there is no direct exchange between an ‘inner’ oxygen (coordinated to the Rh), and the solvent; an ‘inner’ oxygen must become an ‘outer’ oxygen before exchange can occur. There are three possible acid-catalyzed paths for outer oxygen/solvent exchange: two which involve a ring opening step (A + D 4A or A + B D -+ A, and A + C + A), and one which does not require ring opening (A + B --$ A). A two-term rate law expression, k = k2[H+]+ k3[HfI2is needed to describe the reaction shown in equation (125). -+
[Rh(en),(ox)]+
+ 2 H3 0 + e
~is-[Rh(en)~(H~O}~]~’ + Hzox
(125)
kc: \
Scheme 19
Comparison of the kinetic parameters for the reactions in Scheme 19 to those for the analogous reactions of the anionic [Rh(ox)J3- (Section 48.6.5.2) is instructive.w2 The kinetic parameters (Table 43) for the exchange of the outer oxygens are similar for the two systems, implying that the net charge on the complex has little influence on the kinetics of that step. For the tris(oxa1ato) complex, the inner/outer oxygen exchange is always slower than the exchange of the outer oxygens with the solvent; in contrast, the rates of inner/outer interchange and outer/solvent exchange are similar for [Rh(en),(ox)]+, and reach a crossover point at about 60°C. The faster inner/outer exchange in the latter system is presumably due to the more favourable AS* values (Table 43). In both systems, acid hydrolysis of the oxalato ligand is much slower than the inner/outer exchange at any temperature. For the tris(oxa1ato) anion aquation proceeds to completion, while with
977 Table 43 Kinetic Parameters for Oxygen-exchange Processes at 25°C Process
[Rh(en),(ox)]+
Outer-oxygen-solvent exchange Inner/outer oxygen exchange Acid hydrolysis (k = k,[H+] k3[H+I2)
71.5
3.5 x 10-5
Outer-oxygen-solvent exchange Inner/outer oxygen exchange Acid hydrolysis (k = k,[H+] + k3[H+l2)
k 1.3
AS* (JK-'mol-l)
Ref.
-90.4
4.2
659
k 17
659
-21 zk 25 -97.9 & 8
659
- 83.7 It 25
660
109 & 4.2
1.0 x 10-5
+
[Rh(0x)7l3-
AH* (kJmol-')
k (M-'s-I)
Complex
k2 = 1.08 x k, = 3.0 x
75.3
(M-'s-') 9.2 x 10-5
70.7 f 8.4
1.83 x
91.9
+24
101 f 8.4
k2 = 2.5 x IO-* k, = 1.49 x
10
8.4
-26
107 1 8 . 4 108 f 8.4
2.5
660
-33 & 25 - 12 2 25
660
(M-2 S - 1)
[Rh(en),(ox)'J+ equilibrium is' established after about 7&75% of the oxalate is aquated, presumably due to the coulombic attraction between ox2- (or Hox- ) and the cationic ethylenediamine comple~.''~ A thorough analysis of the complex kinetics for this system is inappropriate here, and despite the lack of direct evidence for the intermediates in Scheme 19, the predicted acid dependence was observed. (Interestingly, aquation of the analogous [Co(en), (ox)]' ion is not acid A relative ordering of the reaction rates gives: (a) k,, 9 kBA> k,, > kAC> kDDE z k&; and (b) kcA % kcD k x k D k . The chemistry of aqueous solutions of CO, in the presence of aquated transition metal complexes has been extensively studied by a group of researchers led by Harris, Palmer and vanEldik, and they have presented extensive kinetic studies on the carboxylation and decarboxylation of a variety of bis(ethy1enediamine)carbonato complexes of Rh"i.625.627,a1 The decarboxylation of [Rh(en), (CO,)]' (bidentate carbonate), occurs according to the two-term rate expression kobs= k, + k , [H+]. The kinetic parameters for this reaction are given in Table 44, and the reaction is presumed to occur via the route represented in equations (126)-(128). The rapid decarboxylation of the intermediate ion. cis-[Rh(en),(CO,H)(H,O)]+ was independently studied, as was the rate of CO, uptake by cis[Rh(en),(OH)(H,0)]2+ and ~is-[Rh(en),(H,O),]~+; the relevant parameters are also given in Table 44. The AH* values for the carboxylation and the decarboxylation of the monodentate carbonate complexes are atypically low for substitutions at Rh"' centers, suggesting that these reactions do not involve the cleavage of a Rh-0 bond; carboxylation must involve electrophilic attack of the C 0 2 at the oxygen atom of the coordinated hydroxo group.627 The markedly slower rate, and the higher cis-[Rh(en),CO,]+
+
H20
kobs
cis-[Rh(en),(H20)(OC02)]+
cis-[Rh(en),(H,0)(OC0,H)]2"
=
+
(k, + hIH+I)
H+
==2
~is-[Rh(en),(H,0)(OC0,H)]~+
e cis-[Rl~(en),(H,O)(OH)]~+ A2
-
+
CO,
-2
cis-[Rh(en),(OH),]+
(126)
cis-[Rh(en),(H20)(OCO2)]+
co2, k3
ow e Hf
cu-[Rh(en),(OH)(OCO,)]
TaMe 44 Kinetic Parameters for the Decarboxylation of cis-[Rh(en), (OC02)Xp+ (from ref. 627) Process
k (2YCja
AH* (kJmol-I)
AS* (J K-' mol-')
Ring opening (kobs)
k,, = 9.33 x 1 0 - ~ S - l k , = 1.00 x lo-' M-'s-' k, = 7.2 x IO-' M-' S-I
95.4 6.3 105 f 15 81.2 1.3
-21 & 21 8 i:46 24.3 k 8.3
66.5 f 2.1
16.3 & 6
Decarboxylation of protonated form CO, uptake by aquahydroxo ion CO, uptake by dihydroxo ion
k - , = 69 & I8 M-I
S-'
k, = 215 & 2 9 W ' s - '
'Rate constant labels given in equations (126)-(128).
61.1 & 0.8
4.6
2.9
(127) (128)
= Rhodium
978
AH* values for the decarboxylation of the bidentate carbonate, imply that the first, rate-determining step in this reaction must involve cleavage of a Rh-0 bond as the carbonate ring is opened, and rapid decarboxylation of the monodentate complex can then occur with only C-0 bond cleavage. and trans-[Rh(en),(OCO,),]The trans carbonato complexes, trans-[Rh(en),(OCO,)(H,O)]+ " were also characterized, and the kinetics of their formation and decarboxylation s t ~ d i e d . ~Scheme 20 represents the complex reaction pattern observed. In the absence of added carbonate, decarboxylation of the monocarbonato complex has a rate expression of the form kobs= k , [H+I/ ([Hf] + K 3 ) (Scheme 20). Decarboxylation of the bis(carbonat0) complex is unusual in that the researchers could find no evidence of an intermediate monocarbonato complex, and concluded that both carbonate ions are lost simultaneously (presumably as 2C0,). To support this supposition, they noted that at pH 8.82, CO, uptake by trcms-[Rh(en),(OH),]+ is proportional to [CO,]'. The trans geometry was maintained throughout these reactions, and the general pattern observed for the cis cornpIexe~,6~~ i.e. rapid carboxyiation of the Rh-OH- or decarboxylation of the Rh-OC0,H moieties, also held for the trans complexes.621 This work was extended to include the kinetics o f the formation and decarboxylations of the neutral [Rh(en),(X)(OCO,)] complexes (X = C1, Br, I), so the effect of changing the ligand trans to the reactive site could be monitored.@' These complexes displayed kinetic rate expressions similar to the trans aquacarbonato ions,625and a simpler reaction sequence, represented in equations (129) and (130), was observed. The pK, of trans-[Rh(en),(X)(H,O)]'+ increases in the order C1 < Br < I, which can be considered as the order of increasing effectiveness for weakening the Rh-0 bond, and hence strengthening the 0-H bond; as expected, the first-order rate constant for decarboxylation decreases in the order Cl > Br > I. Summarizing the results given in Table 45,the ability of various ligands to labilize a trans bicarbonate decreases in the order: H,O > OC0,H- > OCOZ- > C1- > Br- z NH, > I- (the listing for NH, is from the cis series of complexes-Table 44). A graph of log k , vs. pK, for a variety of M-CO, complexes shows a strong correlation between the acidity of the parent species and the rate of decarboxylation. The rate of CO, uptake by trans-[Rh(en),(X)(H,O)~+ increases in the order X = C1 < Br < I, which is reflected in a steady increase in AH* and A S (Table 46). When compared with the rates of CO, uptake by a variety of aquated metal complexes (including complexes of Cu", Zn", CrI'', and Ir'") a clear correlation exists between the pK, of the coordinated water and the rate of CO, uptake. bis carboxylates
mono carboxylates
R(W02);
OH-
H+ LJECARBOXY LATION
-
CARBOXYLATION
Scheme 20
trans-[Rh(en),(X)(OH)]+ + Ht
(129)
S trans-[Rh(en),(X)(OCO,EI)]+
(130)
tr~ns-[Kh(en),(X)(H~O)]~* COZ
+
rrans-[Rh(en),(X)(OH)]'
trm-IRh(en),(X)(OCO,)]
+
H+
Rhodium
979
Kinetic Parameters for the Dmrboxylations of trans-[Rh(en),(X)(OCO,€I)~'
Table 45
trans-[Rh(en),(X)(OH)r+ 4- CO,
trans-[Rh(en),(X)(OCO, H)r+Trans ligand (X)
k (25°C)
C1 Br I
1.26 f 0.04 1.12 0.03 0.55 f 0.04 2.92 0.08 1.3 & 0.1 2.26 f 0.05
HzfJ CO, CO,H
AH*
AS*
(kJ mol- I )
QK-'mol-')
PK3
Ref.
- 0.4 f 6.7 3.3 -9.6 f 1.2 - 82.0 1.7
6.36 f 0.01 6.42 f 0.05 6.58 f 0.01
641 641 641 625 625 625
72.8 f 2.1 76.1 0.8 72.0 & 1.2 45.6 1 0 . 4
+ 10.9
+ - 8.8 + 3.3
-
68.2
-
-
0.8
-
I
-
T a b 46 Kinetic Parameters for the Carboxylation of rruns-[Rh(en)2(X)(OH)r' rrans-[Rh(en)2(X)(OH)r+
Br I H20 CO, OH
rruns-[Rh(en),(X)(OCO,H)j"+
AH*
k (25°C) (M-ls-')
Trans iigand (X) C1
+ CO,
(kJmol-')
260 f 12 395 f 33 422 f 43 81+3 330 f 80 1.4 (+ 0.2) x (M-, s - I )
52.3 f 5.4 63.2 i 4.6 73.6 3.3 54.0 & 3.8 64.4 2.9 68.2 3.8
+
AS* (JK-'mol-') -22
I8
+ 16 f 16 + 5 0 + 11
-23 f 13
+ 19 f 10 +69 f 13
Ref. 641 641 64 1 625 625
625
Thus the rate of uptake does not depend on the nature of the metal, but depends on the nucleophilicity of the bound hydroxide ion, which correlates well with the strength of the 0-H bond in the conjugate acid. Isokinetic plots (AH* vs. AS*) show parallel lines for CO, uptake and decarboxylation, confirming that these complexes share a common mechanism which does not involve cleavage of a metal-ligand
(iii) Other aI@atic bidentate complexes The synthesis and reactivities of substituted aliphatic bidentate complexes are similar to the reactions of analogous ethylenediaminecomplexes. Basolo and co-workers p r e p a 1 - 4 ~and ' ~ studied the rates of chloride aquationss5of a variety of trans-[Rh(NN),Cl,]+ complexes (NN = meso-bn, where bn = 2,3-diaminobutane; (f)-bn, and tetrameen-both cis and trans isomers); the rate of chloride release is not dramatically dependent on the nature of the aliphatic amine ligand. Hancock used the BH; -catalyzed path to generate [Rh(tn),(ox)]+ (tn = 1,3-diaminopropane), and then showed that substitutions at both trans- and cis-[Rh(tn),X,]+ occur with retention of geometry (Scheme 21).664A kinetic study of the bromide anation and base hydrolysis of trans[Rh(tn)2Cl,]+ showed that the alkaline reaction displays the familiar kobs= k, kZIOH-] rate expres~ion;''~ the large entropy of activation for the OH- dependent step (AS" = 109 k 30JK-'mol-' vs. 71 f 5JK-'mol-' for the ethylenediamine analog) was noted, and an &1CB path was suggested.6" When compared to the bromide anation of trans[Rh(cn),Cl,]", the effect of the six-membered ring on the reaction rate is negligible.665The cis and trans-[Rh(tn),(H,O),j3f ions have been prepared by Ag+ -induced release of the C1- ligands,"' and these complex ions have electronic spectra and pK, values (Table 47) comparable to their ethylenediamine analogs.
+
RhCI3.3H20
+
tran~-[Rh(tn)~Cl,]+ % trans-[Rh(tn),Br,]+ ox2-
I
BH;
ci~-[Rh(tn)~Cl~]+ E cis-[Rh(tn),ox]+ &heme 21
C . 0 C . &FF
% cis-[Rh(tn)2Br,]'
980
Rhodium Table 47 Electronic Spectra and pK, Values for 1,3-Diaminopropane (tn) Complexes of Rh"'
trans-[Rh(tn), (H2O),l3 tr~m-[Rh(tn),( H 2 O)(OH)]2+ trans-[Rh(tn), (0H)J czs-[Rh(tn), (HzO),]3+ cis-[Rh(tn), (€€20)(OH)]2+ czs-[rUl(tnh (OH),]+ tr~n.~-[Rh(tn), (H,OjCI]Z a s - [ Rh(tn)2(HzO)CIl2+ rrans{Rh(tn)z(OH)Cl]+ cis-[Rh(t n), (OH)Cl]+ trans-[Rh(tn),(H, 0)Brl2+ crs-[Rh(tn), (H, O)Br]'+ trans-[Rh(tn),(OH)Br]+ cis-[Rh(tn), (OH)Br]+ trans-[Rh(tn), a * ] + ~is-[Rh(tn)~Cl,]+ trans-[Rh(tn)?BrJ+ cis-[Rh(tn), Br,]" +
+
+
357 (60), 274 (171) 348 (88), 283 (179) 351 (97), 294 (150) 325 (123), 270 (147) 325 (134), 270 (135) 330 (135), 279 (130) 395 (53, 282 (164) 342 (116), 281 (131) 374 (96), 288 (1 5 1) 343 (121), 279 (106) 410 (59), 475 (36) 358 (128) 385 (112), 284 (170) 354 (130) 418 (80), 315sh ( - 85), 289 (136) -387sh (-89), 353 (123), 293 (125) 441 (119), -342Sh(-58), 281 (3170) -405sh (- 125j, 368 (158), 278sh (1 140) 329 (172)
-
-
4.39 8.20 6.15 8.20
-
6.39 7.53 6.46 7.51 -
0.9 M NaCIO,, 0.1 M HClO, 1.O M NaClO, 0.96 M NaClO,, 0.04 M NaOH 0.9 M NaClO,, 0.1 M HC10, 1.OM NaClO, 0.96M NaClO,, 0.04M NaOH IO-' M HC10, IO-' M HClO, 0.1 M NaOH, 0.9 M NaClO, 0.1 M NaOH, 0.9 M NaCIO, 10-jM HC10, 10-3~ HCIO, 0.1 M NaOH, 0.9 M NaC10, 0.1 M NaOH, 0.9 M NaCIO, -
-
-
666 666 666 666 666 666 667 667 667 667 667 667 667 667 664 664 664 664 664
The ''N NMR (natural abundance) spectra of some Rh"' complexes with alkyldiamine and aza aromatic ligands display nitrogen resonances ca. 20 p.p.m. upfield from the free ligand for the alkyl complexes, and about 100 p.p.m. upfield for the aza aromatic ligandsF6' This large shift is presumed to be due to the influence of low-lying excited states of the ligands themselves and not to any unusual property of the complexes. No correlation of the chemical shift with the position of the ligands in the spectrochemical series could be found, and non-bonded magnetic interactions are presumed to be involved. Diastereoisomerism was observed for [Rh(1 ,2-diaminopropane),I3+, and evidence of two different ligand conformations for the complex ion was also observed.668 A variety of substituted ethylenediamine complexes have been resolved, and their ORD, CD and electronic spectra reported (Table 48), with assignments of absolute configurations based on a comparison of their CD spectrum with the CD spectrum of [Rh(en),13+.606 Complexes containing the optically active (R)-1 ,Zpropanediamine and (R,R)-2,3-butanediamine ligands, in which the ligands are locked in the A configuration by the bulky methyl groups, have also been r e s ~ l v e d . ~ * ~ ~ ~ ~ For [Rh{(R)-pn)ClJ and trans-[RhC(R)-pn),C1,1+,the optical activity must arise from the assymmetry induced by the conformation of the optically active ligand; the effect appears to be additive in these complexes with one and two chiral ligands respectively (Table 48). The A configuration is also assigned67oto the stable form (k,k,k) of (+),-[Rh( l,2-diaminocyclohexane),]3+ .671,672 Th e [Rh{( -t)-bn)J3+ ion (bn = 2,3-diaminobutane) has been prepared,673and all four diastereomers have been isolated and characterized by I3CNMR.674The NMR spectrum of the A,AJ form does not exhibit the broadened methy1 group resonance observed for the Co analog, even though the X-ray structure (as the Br- salt) shows a decreased intraring methyl group contact distance.674
48.6.2.3
Photochemistry of monodentate and bidentate amine complexes
The past 15 years have been exciting times for the study of Rh"' photochemistry. Photochemical ~ ~ ~mention ' ~ Rh"', but as transition metal reviews and texts of the late 1960s and early 1 9 7 0 ~hardly photochemistry grew in the 1970s, monographs and reviews chronicle the growth of Rh"' photochem i ~ t r y . 6 ' ~The ' ~ stability of the 3 + oxidation state and the inaccessibility of Rh" or Rh", the stereorigidity and inertness toward substitution of most Rh"' complexes in solution, and the photosensitivity of many Rh"' amines combined to make Rh"' photochemistry the most active Rh"' research area of the past 15 years. In general, the photochemistry of non-ammine complexes is discussed in the sections dealing with the specific compounds, but the amount of work presented on the photochemistry of simple monodentate and bidentate amine complexes requires this separate discussion.
Rhodium
98 1
Table 48 Electronic and CD Spectra o f some Rh"' Complexes Compound r'
ORD
bEJ
1
CD A&,,
R bsolure con$prution
Ref:
:
306 (251)
- 79
257 (246) 300 (250)
-
303 (252)
+ 154
255 (215) 300 (270)
f
[Rh{(R,R)-bn}J
300 (270)
+ 175
trans-[Rh{(R)-pn}, Cl2]+
405(75)
+ 158
[ W R ) - p n ) , l '"
+
I
Electronic (nm) (4
JWX
.?.,. .. . .
285 (120)
,
trans-[Rh{(R,R)-bn),Cl*]+ 405 (75)
+ 170
[Rh{(R)-Pn)Cl,I~
285 (120) 430 (70)
+ 188
[Rh{(R,R)-bn)CI,]-
350 (80) 430 (70)
+ 198
350 (82)
(i)
162
320 287 258 320 288 323 290 253 3 20 286 3 20 286 405 315 270 405 315 270 432 370 3 20 432 370 320
+ 2.0 -0.1 + 0.8 - 2.01 + 0.09 - 0.72 -t 1.25 - 0.5
606
-2.12
608
+ 0.61 -2.31 + 0.61
+ 0.76 -0.11 + 0.57 -0.12 -0.12 0.57 0.44 - 0.07 0.26 0.46 - 0.08 0.29
+ + + + +
608 606
608 608 608 608 608 608
The excited sfate
Electronic excitation of a low-spin, d 6 Rh"' amine shifts electron density to a metal-dominated orbital (e,) of n symmetry, which is antibonding with respect to the Rh-ligand bonds. This electron density can come from either a t2, orbital of 7~ symmetry (ligand field excitation) or from a ligand-dominated orbitd (ligand to metal charge transfer, or LMCT, excitation). Reduction or oxidation of Rh"' amines is difficult, so charge-transfer states are generally higher in energy than ligand field states and, for most complexes, they play little part in Rh"' photochemistry. With some exceptions, the quantum yield for photosubstitutions is independent of irradiation wavelength, suggesting that intersystem crossing to the lowest ligand field state occurs with unitary efficiency.6n5 Photophysical studies have confirmed that luminescence from Rh"' amine complexes generally occurs from the lowest energy ligand field triplet ~ t a t e . ~ " Co . ~ nsistent ~ ~ ~ ' with rapid population of the ligand field states, the rates of intersystem crossing are extremely rapid and have been estimated to 5 x 10" s-' (for ~is-[Rh(bipy),Cl,]+).~~~ to be in the range of 5 x 109s-' (for [Rh(NH3)5Br]2+)692 These findings all point to the generalization that the photochemistry of a Rh"' complex can be considered to be the chemistry of its lowest energy ligand field excited state. Even in rigid media at reduced temperatures, the lifetimes of the luminescing states are short, implying that the thermally equilibrated excited states are extremely reactive.bo93688691 With nanosecond and subnanosecond laser systems, the luminescence lifetimes under photochemically relevant conditions (fluid solution near room temperature) can now be m e a s ~ r e d . ~(Emission " ~ ~ ~ quantum yields are extremely weak under these conditions, and luminescence quantum efficiencies are estimated to be in the For [Rh(L),X]'+ (X = C1 or Br, L = NH, or ND,) and to lo-' cis- and trans-[Rh(en),Cl,]', the luminescent states have lifetimes in the range of 1-50 ns. These studies directly support the standard model, in which reactive and non-reactive paths compete for the transient excited state. For example, the rate of deactivation of [Rh(ND,),X]'+ is about half that of the undeuterated ions, consistent with the increased photochemical reactivity observed for the deuterated species.694It is interesting to note that for [Rh(cyclam)(CN),]+, the lifetime of the luminescing state under comparable conditions (326.1 K, 0.001 M aqueous HNO, solution) is 2.0ps, about three orders of magnitude longer than for the less constrained m i n e c ~ m p l e x e s This .~~ longevity is presumed to be due to the absence of a reactive decay mode. The nature of the photoreactive state, as opposed to the luminescing state, is still the source of some debate, as states other than the lowest triplet have been proposed to be important. Evidence that an excited singlet may be involved comes from time-resolved studies of the Iuminescence of some haloammine comnlexes. where a weak fluorescence. with a lifetime of about 100~s.was
982
Rhodium
observed.6Y2A nearby quintet state has also been mentioned695r701 and Endicott has questioned whether one should even consider t h i s photochemistry to be the reaction of a distinct electronic excited state.704He suggests that the photoinduced substitutions and isomerizations may be considered hot ground state processes, with the photoinduced substitutions and isomerizations, not observed upon thermal excitation, resulting when the relaxation trajectory of the distorted excited state goes to a thermally inaccessible region of the ground state free energy surface. Perhaps, he argues, one should focus on understanding the full ground state surface, not the excited state surfaces.704
(ii) Phrochernistry of [Rh(NH,),X]"+ ions Whatever the nature of the photoreactive state, photolysis of a Rh"' amine leads to increased electron density in the metal-centered, g* es orbitals. Ligand labilization (especially of strongly o-donating ligands), and subsequent solvolysis, is the anticipated (and observed) reaction. Photoinduced substitutions have now been reported for a large number of Rh"' amines, and some of the results are summarized in Table 49. The first quantitative photochemical study of a Rh"' amine was reported by Moggi,8who noted that both 254 nm (LMCT) and 365 nm (ligand field) excitation of [Rh(NH,)5C1]2+caused chloride labilization (equation 131). Other early reports include Basolo's study of the photoinduced stereoretentive halide aquation from [M(en),X,]+ (M = Rh, Ir; X = C1, Br, I),725and Broomhead's observation of chloride aquation from [RhCl,(phen),] f.726 While halide labilization dominates upon photolysis of [Rh(NH,), C1I2+,both bromo and ammine loss occur upon photolysis of the bromo analog (equation 132)6R5*707 and ammine is labilized from the iodo analog (equation 133).70gBiacetyl sensitization of the bromo complex quenches the biacetyl phosphorescence, but not the fluorescence,?O7consistent with a photoreactive triplet state. [Rh(NH3)5CI]2C+ H 2 0
5
+ C1-
[Rh(NH3),H20I3+
(131)
[Rh(NH3)sH20]3*+ Br-
[Rh(NH3)5Br]2++ H 2 0
3]3'
~
+
HCI % [Rh(en),(enH)Cll3+
(138)
The ability to measure luminescence lifetimes in fluid solution at ambient temperatures has allowed a much clearer picture of the events which lead to photoinduced substitutions. A recent flurry of activity, led by Ford and Skibsted, has allowed the determination of rate constants for aquations, as well as for radiative and non-radiative decay modes from photoproduced excited F~ r [Rh(NH,),X]*+ (X = C1, Brj and cis- and trans-[Rh(NH,),XY]"+ (X = C1-, Br-, StateS,667,697,703 H,O; Y = X, H,O, OH-) the rate constants for aquation of the excited states (at 25°C) range between 6 x lo6 to 3.1 x ~ O ' S - ' which , ~ ~ ~ represents an increase of 13-15 orders of magnitude over the rates of analogous reactions of the ground state complexes. For closely related complexes, the rate of labilization of X generally follows the order H,O > C1- > Br- > I-, perhaps, Ford suggests, due to the relative abilities of these ligands to form x-bonds with the &e; ( ~ ~excited 0 ~ ) states. The influence of Y on the rate of release of C1- or Br- follows the approximate order cis X > trans OH- w trans X > NH, zz cis OH-.703The rate of halide release is reversed in cis- and trans[Rh(tn),X,]+ (X = C1, Br) however, as the rate of aquation (and of non-radiative decay) is greater for the bromo than for the chloro c o ~ n p l e x e sThe . ~ ~ [Rh(tn),Br,]' excited states also display rates of aquation and non-radiative decay at least an order of magnitude greater than for the dibromotetraammines. These effects are thought to be due to crowding in the inner coordination sphere by the bulky Br and tn ligands. (X = C1, Br), and reduces, but does Hydroxide ion quenches luminescence from [R!~I(NH,),X]~~ not eliminate, the ligand labili~ations.~'~ For [Rh(NH,), BrI2+, ammine labilization is sharply reduced, but Br- loss is unaffected by the presence of OH-, leading Adamson to suggest that the two photoreactions originate from two distinct excited states; one which is luminescent, releases NH, and is quenched by hydroxide, and a second, non-luminescent state which releases Br- and is not affected by OH-.69sSuch a two-excited-state model, orginally proposed by Endicott707to account for ammine and bromide release from [Rh(NH,), Br]", contrasts sharply with the generally held view that Rh"' photochemistry originates from a single low-lying excited state (or several states in equilibrium with each other). Hydroxide also quenches NH, release from [Rh(NH,),I]'+, but increases the efficiency of I- release by a factor of 5.'02 Ford argues that the enhanced reactivity of 1- is more readily rationalized if the OH ion is not considered an 'innocent quencher', but as a reactant which chemically interacts with the photoproduced excited state. A proton transfer, to generate {[Rh(NH,),(NH,)X]+} *, which is comparable to the reactive intermediate proposed in the S, 1CB reaction of the ground state, is suggested.702The reactivity patterns of this new excited species would be unrelated to that of the normal excited state (present in acidic solution), accounting for hydroxide's ability to affect two reaction paths differently, without invoking a distinct excited state species for each reaction. The solvent cage can also influence photochemical reactivity, as studies with [Rh(NH,), C1I2+in various solvents showed that the identity of the photolabilized ligand, and the rates of radiative, non-radiative and reactive deactivations are solvent dependent. In polar media such as water and formamide, photoinduced C1- release dominates, but in less polar media (DMF, DMSO and MeOH), less capable of solvating the resulting C1- and Rh3+ ions, ammine substitution domin a t e ~ . ~ ~ ~ , ~ ~ ~ Ford and co-workers have also studied the luminescence and photochemistry of a variety of [Rh(NH3),LI3+ ions, where L is py, a substituted pyridine or an organ~nitrile."~~~'~ While both ligand field and n-n transitions are apparent in the absorbance spectra, luminescence originates from only the ligand field state. Despite the similarity of the ligand field absorption spectra for all the nitrile complexes, the luminescence spectra depend on the nature of L, and as the energy of the emission band decreases, the quantum yield for nitrile labilization increases. The variation in quantum yield as a function of wavelength again indicates that energy transfer from ligand centered x-x* states to ligand field states does not occur with unitary efficiency, but the lack of luminescence from the n-n* states suggests that other, non-radiative paths are available for the normally fluorescence ligands upon coordination to the R~I"'.''~
Rhodium
986
The photochemical reactivity of [Rh(NH,), (N3)]*+has proven especially interesting, for HCl solutions of the azidopentaammine ion undergo a ligand-centered reaction, releasing N, gas, and generating the chloramine complex (equation 139).731Minor products include [Rh(NH,),I3+, [Rh(NH,),(NH,OH)I3+ and [Rh(NH3),(H,0)]3+,and the ratio of products depends on the solution conditions and wavelength of irradiation. A coordinated nitrene mechanism, similar to that proposed for the thermal decomposition of Ru"'732 and Ir'11733azides was proposed,7Mand is represented in Scheme 22. The nitrene intermediate is a soft Lewis acid, and when scavenged by C1-, the chloramine is formed; the hydroxylamine results from reaction with H,O or C10;. The [Rh(NH,), (H,0)]3+ photoproduct was presumed to result from a redox process involving the formation of transient [Rh(NH3)5]2+and the azide (It is interesting to note that only the nitrene path was detected upon photolysis of [Ir(NH3)5(N3)]2t ,735 and the azide radical/redox path has been noted for the Co'" analog,736so a combination of these two paths for the Rh"' analog seemed a logical conclusion.) FIash photolysis studies detected a transient N:--linked dimer, [(NH,)5Rh-N-N-Rh(NH,),]4+,7'7further supporting the nitrene mechanism, but did not find any photoproduced azide radical,73scalling into question the redox path to [Rh(NH,),(H,0)]3+. Zink suggested739photoinduced heterolytic cleavage of the terminal N-N bond in the Rh-N -N-N moiety, releasing N- to the solution (which would be rapidly attacked by water or HC1 to generate NH,OH or NH,Cl), and forming the instantly aquated [Rh(NH3),(N2)l3+. While nicely accounting for the aquapentaammine product, Endicott and co-workers showed that virtually no NH,OH, NH,Cl, N3radical or N;- ion was generated in the photolyzed solution. While it seems an anticlimax, they concluded that the nitrene path is the only path available to [Rh(NH3)5(N3)]2+, and that the mysterious [Rh(NH3)5HZ0]3+ must be formed from secondary photolysis of the hydroxylamine or chloramine primary photoproduct^.^^^ [Rh(NHMN,)I2+
+ HC1
[Rh(NH3)dNH2C1)l3+ f N2
(1 39)
Scheme 22
(iii)
Stereochemical consequences and mechanism
The study of Rh"' photochemistry has been stimulated by numerous models which attempt to predict which of six ligands around an octahedrally coordinated metal center will be labilized upon ligand field photolysis. They range from sets of simple empirical rules to molecular-orbital-based theoretical s t ~ d i e s , ' ~ ' and ~ the success of Vanquickenborne's version743has made it the current the redox chemistry standard. With some exceptions (e.g, the nitrene route for the azido of the iod0,7~~ and the effects of various solvents on the chloropentaammine ion699)the photochemistry of the IRh(NH3),X]"+ions discussed in the previous section generally follow Vanquickenborne's model. One recent test of this model used the strong trans-labilizing influence of the cyanide ligand to generate trans-[Rh(NH3),(*NH,)Cw2+ from the reaction of trans-[Rh(NH,),(CN)CI]+ with "NH, .626 According to virtually all the models, photolysis of [Rh(NH,),CN]'+ should lead to labilization of an ammine in the plane cis to the coordinated CN-, and photolysis (at 254 and 313 nm) does lead to cis-[Rh(NH,),(H,0)CNJ2+. Virtually all (96% upon 313 nm irradiation) of the photoreleased ammine was unlabeled, confirming that photolysis of this complex leads to regiospecific labilization of an ammine in the equatorial plane.745 An interesting challenge to Vanquickenborne's model of regiospecific labilization has been ~ studied the photoinduced amine aquation from compresented by Muir and c o- w ~ r ke r s , 'who plexes of the form truns-[RhL4Cl,]+ where L is a heterocyclic amine (pyrazine, pyrazole, pyridine, imidazole, and substituted analogs). Unlike the ammine and ethylenediamine analogs, where chloride loss dominates, both amine and chloride are stereoretentively labilized from these com-
Rhodium
987
plexes, and the fraction of the reactive decay leading to amine labilization increases with the pK, of the free amine. An appealingly simple argument suggests that the weaker the amine base, the poorer a a donor it will be, and the more likely it is to be labilized in the photoproduced excited state.746 As it became clear that isomerization often accompanies photoinduced substitution, attempts to predict the stereochemical course of a given photochemical reaction also appeared, with the most successful being that of Vanquickenborne and C e u l e m a n ~In . ~ this ~ ~ model, represented in Scheme 23, the photoproduced excited state is presumed to undergo a dissociative loss of the photolabilized ligand, leaving a five-coordinate species in a square pyramidal (SP) geometry. This intermediate is then presumed to undero unimolecular rearrangement to its lowest energy structure, which for d6 metals is the SP configuration (the trigonal bipyramid arrangement was also considered, and is found to be the stabler geometry in d’, but not d6,systems). For a {[Rh(NH,),XJ”” )* species, two geometric isomers are possible for the SP configuration, and angular overlap calculations indicate that the stabler species will have the weaker u-donating ligand in the apical site, (That is, if X is a weaker u donor than NH,, excited state A* should be the dominant excited state, while if X is the stronger a donor, B* should prevail.) Nucleophilic attack, usually by solvent water, at the vacant coordination site of the equilibrated excited state complex completes the photoinduced aquation. trans
/5p
IM(NH,),XLI
CIS
(ammines omitted for clarity)
X
L
I,
X
OH* Scheme 23
This model has been tested in numerous studies by Ford (and his even more numerous coworkers), and it has a remarkable r e ~ o r d . Photoaquation/isomerization ~~~-~~~ studies have been presented for cis-[Rh(NH, j4XYln+(X = C1, Br; Y = H 2 0 , X),7’8trans-[Rh(NH,),(OH)v+ (Y = C1, Br, OH),M2.7’0 ~is-[Rh(NH,),C1,1+,~~~ trans-[Rh(NK3),(CN)Z]”+ (Z = NH3, OH),712cis- and trans[Rh(en),(NH,)(H,0)]3+,750 and cis- and trans-IRh(en),XY]”+ (X = C1, Br, I; Y = NH,, H,O or xj.637.721.723 each case, the stereochemical consequences of photolysis are consistent with Vanquickenborne’s model. In the study of cis-[Rh(NH,),C12]+,7’8 the HN0,-induced aquation of cis-[Rh(NH,),(N,)CI]+ (which should also generate a [Rh(NH3)4C1]2*species, but in the ground This implies electronic state) was shown to be stereoretentive, yielding c~s-[R~(NH,),(H,O)CI]~+. that the photoinduced cis* trans isomerization is an excited state process.718 Skibsted and co-workers have recently presented several studies which rigorously test these stereochemical models. Complexes of the form cis- and trans-[Rh(NN),XY]“+, (NN = en or tn (1,3-diaminopropane); X = H,O, OH; Y = H,O, OH), plus cis and rr~ns-[Rh(tn),(H,O)Z]~~ ( Z = C1, Br), display similar photochemistries: (a) cis to trans isomerization dominates for the diaqua and aquachloro ions (in which the H,O or halide, poorer a donors than en, occupy the axial site in the SP (square planar) intermediate); (b) tram to cis photoisomerization dominates for the hydroxoaqud species, as the OH- ion is a better 0 donor than the en nitrogens, and hence occupies These an equatorial site; and (c) no photochemistry is observed for the dihydroxo ions.626,667,7” results are summarized in Scheme 24 and Table 50. C 0 . C &FF*
Rhodium
988
Table SO Quantum Yields and p& Values for some [Rh(NN),X,]"+ Complexesa
en tn tn tn
en 2NH3
H*O H*O
c1 Br Br C1
< 0.002 0.10 0.058 0.025 < 0.001 0.064
0.27 0.29 0.538 0.55 0.95 0.523
0.61 1.0 b
0.048 0.033
-
4.47 4.39 6.39 6.46
-
-
b
-
-
6.34 6.15 7.53 7.51 -
1.91 8.20 -
~
8.24 8.20 -
-
-
-
-
626 124 667 667 637 716
a N u m k for ~ quantum yields and pK, values refer t o Scheme 24. bPhotolysis Leads to halide labilizationJM
~rans-[Kh(NN),(€I,O)X] " +
trans-[Rh(NN),(OH)X]("-')+
'
$8
,
'
6, 9,
c ~ . T - [ K ~ ( N N ) , ( H , O ) Xn +]
cis-[Rh(NN),(OH)X] (n-l)C
Scheme 24
The excited state intermediates proposed in the Vanquickenborne are coordinatively unsaturated and usually cationic, and should be powerful Lewis acids immersed in a strongly nucleophilic aqueous environment. Despite this, they are presumed to exist long enough to reach thermal equilibrium, with the stereochemistries of the final products independent of the labilized ligand. This hypothesis has recently been te~ted,'~~,~''as photolysis of cis- and trans[Rh(NH,),Cl,]+, and cis- and tr~ns-[Rh(NH~),(H,0)Cl]~+ should all lead to the same five-coordinate ([Rh(NH3),C1I2+}* intermediate, and should therefore all lead to the same photochemical product. Careful reinvestigation of the photochemistry of the trm-[Rh(NH,),C12]+,716the two cis complexe~,'~~ and a new study of trruas-[Rh(NH3)4(H20)Cl]2+7L6 showed that in all four cases the photoproduct is the same mixture of [Rh(NH,),(H,O)Cl]*+ isomers (17% cis and 83% trans; Figure 2). This strongly supports Vanquickenborne's stereochemical model for d 6 and suggests that the substitutional photochemistry of these Rh"' amines may indeed proceed via a purely dissociative mechanism, with a five-coordinate intermediate sufficiently stable to undergo unimolecular isomerization and reach thermal equilibrium between two similar SP structures. The limiting dissociative mechanism is also supported by studies of the effects of pressure on the X]= ~ +C1, Br), photoinduced aquation of cis- and rrans-[Rh(NH,),X,]+, C ~ ~ - [ R ~ ( NH , ) , ( H~ O)(X trans-[Rh(NH,),(OH)Cl]+,75' [Rh(L),XI*+ (L = NH,, ND,; X = C1, Br),752and [Rh(NH,),Xj"+ (X = NH,, I-, SO:-).711 In the tetraammine series, the volume of activation of photoinduced aquation varies between - 8.8 and + 9.3 cm3mol-'; in the pentaammine series halide loss has a negative volume of activation (between - 10.3 and -4.2cm3mol-'), while ammine loss has a positive AV* (+ 3.4 to 12.7cm3mol-'). The effect of pressure on the photochemistry of [Rh(NtJ,),C1]2+ in formamide, DMF or DMSO allowed an estimation of the volumes of activations for C1- labilization, NH, labilization, luminescent and non-radiative deactivation Com-
+
4 CI
I
OH2
f
*
CI
*
+ CI
I
17% cis and 83% trans
OH2
'+
[Rh (NH 3 ) 4 (H,O)Cl] (amrnlnes omitted for clarltyl
Figure 2 Photolysis of [Rh(NH,),CI,]+ and [Rh(NH,),(H,O)Cl]
Rhodium
989
parison of A V* values for reactive and non-reactive paths led to the suggestion of a strong coupling between the two processes.7s2After accounting for the expanded volume of the ligand field excited state, vanEldik and coworkers state that 'a collection of circumstantial evidence points to a limiting dissociative mechanism for the ligand field photosubstitution reactions of rhodium(1II) haloammine complexes in aqueous solution'.753
48.6.2.4 Polydentate aliphatic amine complexes
Aliphatic amines with three or more amine functions which form complexes with Rh"' are represented in (48). These can be subdivided into linear (trien, 2,3,2-tet, etc.), branched (tren) and cyclic (cyclam, cyclen, etc.) amines, and will be discussed in that order.
N
N
N
N
(48a) trien
N
N
N
N
N
N
(48b) 2,3,2-tet
N
N
(48c) 3,2,3-tet (48d) 1,1,7,7-tetraethyldien
n
iN> N N
(48e) tren
c: :3 W
(48f) cyclam
n
("L/%, N
(48g) cyclen
CN3 WN N
(48h) [ 91 aneN,
Two general synthetic paths to Rh"' polyamine complexes have emerged: extension of the procedure for the synthesis of [Rh(en),X,]+ 'I9 (reaction of RhC13*3H20or [RhCl$ with the HC1 salt of the appropriate amine), or the reaction of truns-[Rhpy,CI,]+ with the appropriate and later by Gillard and The [Rh(trien)ClJ+ ion was first prepared by Basolo and W i l k i n ~ o nThree . ~ ~ ~ geometric isomers are possible for octahedral complexes with linear tetradentates (49), and while isomeric assignment was not made in these initial studies, later work showed
Rhodium
990
Table 51 Electronic Spectra of some Polyamine Complexes of Rh"'
cis-a-[Rh(trien)CI,]Cl- H 2 0 cis-/I-[Rh(trien)Cl,]Cl-2H20 trans-[Rh(trien)Cl, ]C104 cis-a-[Rh(trien)Br,]+ cis-a-[Rh(trien)(N, ),I cis-a-[Rh(trien)I, ]+
. .,
1.'
+
cis-a-[Rh(trien)Cl(NCS)]+ cis-[Rh(trien)(H)Cl]+ trans-[Rh(2,3,2-te t)Cl,]+ cis-/I-[Rh(2,3,2-tet)C12]+ cruris- R , R-[Rh(3,2,3-tet)C121' [Rh(tren)Cl, ]NO, [Rh(tren)CI,]Cl [Rh(tren)C204]C10, H,O [Rh(tren)(N, ),IC1 [Rh(tren)(NO,), IN03 [Rh(tren)I,]I [Rh(tren)Br,]Br * 0.5H,O [Rh(tren)(H,O),]' ' [Rh(tren)(OH),]+ a-[Rh(tren)C1(H,O)I2+ /I-[Rh(tren)Cl(H,O)y a-[Rh(tren)Br(H,0)]2+ P-[Rh(tren)Br(H,0)l2+ +
352 (240), 288 (210) 350 (270), 300 (2290) 406 (128), 286 (490) 366 (306), 280sh (1330) 335sh (1130), 257 (12700) 370 (llOO), 295 (4600) -325, -300 300 (108), 255sh (140), 218 (-30000) 410, 293 348 (218), 295 (241) 417 (-75), -300 (- 190) 352 (155), 295 (180) 364 (321), 292 (330) 328 (290) 357 (320) 295 (800) 395sh (1350) 382 (260) 325, 272 332, 274 345, 287 335, 282 34s 357
756 756 756 756 756 756 756 757 754 758 759 619 760 76 1 76 1 761 76 1 761 762 762 762 762 762 762
-
that the cis-a isomer had been prepared. Several examples of [Rh(trien)X,]+ (X = C1-, Br-, I- and N;) have been prepared and the cis-a, cis-p and unstable trans isomers have been separated and identified on the basis of IR and UV/visible spectra (Table 51).'56 Reaction of [Rh(trien)ClJ+ (presumably the cis isomer) with BH; leads to a brown solution, said to contain the monohydrido cis-[Rh(trien)Cl(H)]+; analysis of the d-d bands suggested that H- belongs just above F- in the spectrochemical series.757 ' ~ ,a~n~a ~t i ~ n ~of' ~cis-a- and cis-/I-[Rh(trien)Cl,]+ and of Kinetic studies of the a q ~ a t i o n ~and ci.s-/1-[Rh(2,3,2-tet)C12]+763 have shown that, in the absence of light, substitutions occur with retention of amine configuration, and between 6&80 "C, the aquation rate constants are withn an order of magnitude, with the most reactive cis-,&[Rh(trien)Cl,]+, aquating about eight times as rapidly as the least reactive, cis-a-[Rh(trien)CI,]+. Activation parameters for these reactions, and reactions of other aliphatic polyamine Rh"' complexes, are given in Table 52. Photolysis of cis-~-[Rh(trien)Cl,]+and cis-/I-[Rh(2,3,2-tet)C12]+causes efficient release and isomerization, yielding a mixture of cis+ and trans-aquachloro products (equations 140 and 141).758The cis-a[Rh(trien)CI,] ion, however, is at least three orders of magnitude less photoreactive than either cis-p complex, as no photoinduced reaction could be observed, even after extended A similar reactivity pattern was noticed for the cis isomers of [Co(trien)Cl,]+, and steric constraints, induced by the configuration of the trien, were invoked to rationalize the photoinertness of cis-a-[M(trien)CI,]+ (M = Co or Rh)?66 (Interestingly, the cis-or-[Cr(trien)Cl,]' ion is quite A summation o f the photochemical reactivity of aliphatic polyamine Rh"' photo~ensitive.'~~) complexes is given in Table 53. &
Table 52 Activation Parameters for Reactions of some Aliphatic Polyamines of Rh"' Starting material
Reaction
AH* (kJmo1-')
Aquation Aquation Aquation C1 aquation C1 aquation Br aquation Br aquation C1 aquation Br aquation Decarboxylation
88 67 82 106 122 105 108 147 153 63
A S (J K-' mol-') - 71 - 113 - 84 24 65 20 17
+ + + +
+ 110 + 133 - 106
Ref.
763 163 763 764 764 764 764 764 764 765
99 1
Rhodium Table 53 Photochemical Behavior of some Aliphatic Polyamine Complexes of Rh"' Complex
Conditions
Products(s)
4
Re$
cis-a-[Rh(trien)CI,j+ cis-j-[Rh(trien)Cl,]+
0.1 N H2S04 0.1 N HzS04
[Rh(trien)Cl(H,O)]z+ [Rh(t~ien)Cl(H,O)]~+, 65% cis-j, 35% trans
< 10-4 0.21
655 758
cis-a-[Rh(trien)Cl(H,O)]'+
0.1 N H,S04 0.1 N H,S04 10% MeCN 10% MeCN 10% MeCN 10% MeCN 10% MeCN PH 1
-
< 10-4 0.22 0.01 1 0.37 0.03 0.32
758 758 768 768 768 768 768 163
ci~-j-[Rh(trien)CI(H,O)]~+ truns-[Rh(cyclam)C12] cis-[Rh(cyclam)Cl,]+ trans-[Rh(cyclam)Br,] cis-[Rh(cyclam)BrJ+ truns-[Rh(cyclam)I,]+ cis-j-[Rh(2,3,2-tet)C12~+ +
+
cis$-[ Rh(2,3,2-tet)CI(HzO)j2+ [Rh(tren)CI,]+ [Rh(tren)Br,]+ ~-[Rh(tren)Cl(H,O)]~+ a-[Rh(tren)Br(H,O)]'+ j-[Rh(tren)Cl(H,O)JZ+ j-[Rh(tren)Br(H,O)]'+
PH PH PH PH PH PH PH
1 1 1 1 1 1 1
trans[Rh( trien)CI(H, O)]'+ trans-[Rh(cy~lam)Cl(H~O)]~+ cis-[Rh(cyclam)Cl(H, O)]'+
rmans-[Rh(~yclam)Br(H,O)]~ +
cis-[ Rh(cyclam)Br(H, 0)I2
+
0.15
rranr-[Rh(cy~ldm)I(H,O)]~+ cis-[Rh(2,3,2-tel)C1(H20)]z+,
0.26
77% trow, 23% cis-p
1rans-[Rh(2,3,2-tet)C1(N,O)]'+
0.13 0.09 0.22 0.10 0.17 I- > Br- > C1- x N; > amine N > NCS-. Photolysis of trans-[Rh(cyclam)X,]+ and the cis-dichloro cation causes stereoretentive halide aquation (Table 53).768The electrical resistances of compressed samples of cis-[Rh(cyclam)Cl,](TCNQ) (TCNQ = the monoanion of tetracyanoquinoline) are 32 times that of the trans isomer.778For the more constrained tetramine 1,4,7,10-tetraazacyclodecane(cyclen; 48g) only the cis-[Rh(cyclen)Cl, ]+ ion has been characterized (as the dihydrate of the chloride salt).775 The coordination chemistry of the cyclic triamine 1,4,7-triazacyclononane ([9]aneN,; 48h) has been studied by Wieghardt et al. and has proven to be a rich area. In aqueous solution, [9]aneN3
Rhodium
993
Table 54 Electronic Spectra of some Cyclic Polyarninerhodiurn(II1) Complexes (nm) (4
Complex
[Rh(cyclam) X Y y + 354 (223), 299 (308), 207 (33 900) 406 (78), 310sh (80), 242sh (3300), 204 (37 100) 367 (243), 309 (871) 429 (97.3). 285 (2520), 235 (34 600) 407 (Kilo), 295sh (5300), 260sh (17 500), 228 (38 400) 515sh (64),466 (204), 353 (13 IOO), 275 (34500). 226 (22 800) 339sh (1250), 262 (11 800) 377 (892), 286 (12700), 208 (27 100) 293sh (1030) 320sh (535). 260sh (ZlOO), 213 (30800) 322 (1 1 lo), 244 (3630) 377 (882), 258 (3330) 363 (IOl), 276 (180) 385 ( 5 9 , 296sh (lot), 242sh (4420) 373 (113), 277sh (384) 468 (37), 403 (63), 310sh (106), 204 (25 600) 443 (153), 393 (158), 275 (5330), 230 (26 300) 494 (222), 341sh (763), 301sh (1290), 271 (2240), 230 (24 400) 357 (525), 256 (6320) 420sh (79), 363 (719, 261 (5640) 328 (251), 291 (192) 342 (118), 274 (222)
trans-1; cis-(N,): trans-(N3): cis-(N02): rruns-(NO2); cis-(NCS): trans-(NCS): trans-Cl(OH)+ rrans-CI(H,0)2+ truns-Br(OH)+ frans-Br(H20)" truns-l(OH)+ tram-I(H,O)'
Complex
+
trans-N, (OH)' trans-N, (H20)'+ cis-(OH)(H,0)2+ truns-(OH)(H20)"
XY trans-CIBr'
Jmox
(nm) (4
[Rh(cyclam) XY]"' 418 (89), 312sh (99, 26Osh (1950), 221 (32700) 493 (230), 445sh (138), 308 (3620), 245 (31 300) 382 (697), 270 (9390), 207 (23000) 368 (340), 252 (8080) 497 (151), 459 (152), 321 (&ZOO), 256 (33 900) 393 (698), 281 ( I 1900), 214 (21 400) 368 (359, 262 (8980) 465sh (283), 417 (654), 300 (10 IOO), 280sh (8970), 225 (20 900) 434 (393), 412 (413), 296 (4220), 274 (5660), 224 (30 600) 352 (%I), 270 (7330) 341 (IOI), 277 (163) 352 (65) 331 (226), 278 (202) 296 (249), 251 (230) 281 (465), 218 (1830)' 267 (270), 218 (162%" 365 (535), 302 (430) 296 (309), 244s.h" 297 (553), 252 (347)' 299 (511), 251 (360)c 294 (265), 249 (316)' 340 (670)' 340 (437)' 337 (418)' 340 (495)' 343 (430)' 340 (250), 280 (90)B
"[Rh(cyclam)XY]"" speclra from ref. 733. h L = ([9]aneN3). 'From ref. 774. From ref. 775. 'From ref. 776. 'From ref. 777.8From ref. 765.
is a tridentate ligand, and a series of doubly and triply bridged hydroxo, carbonato and acetato As summarized in Scheme 26, [Rh2([9]aneN,),(H,0),(pdimers have been ~haracterized.~~~,~~',~~~ OH)J4+ is deprotonated at pH values above 8.5 and the resulting conjugate base reacts with carbonate to give a carbonato-bridged dimer.775,779 A kinetic study of the decarboxylation (equation 142) has shown that both reactions exhibit an acidic dependence.775The X-ray structure of the diaquadi-p-hydroxo tetravalent cation (as the perchlorate salt) confirmed the di-p-hydroxo geme try.^^' Reaction of RhC1,-3H20 in ethanol with a methanolic solution of the N,N',N"-trimethyl substituted version of this cyclic triamine (Me3[9]aneN,)leads to a yellow solid, which, if refluxed in an alkaline perchlorate solution, gives yellow crystals of [Rh,(Me, [9]aneN3)2~-OH),](ClO,),. A preliminary X-ray structure of these crystals confirms the triply bridged hydroxo geometry, and gives an Rh-Rh distance of about 2.96 r
L
13+
O
r
12+
J
L = [9]aneN3
Sargeson and co-workers have synthesized, characterized and studied the chemistry of a variety of remarkable macrocyclic amine complexes of Rh"', in which the rhodium is virtually isolated
-
994 LRhLCI3]
Rhodium RhClp'3fizO
L
ZL
* [Rhl,]
34
AgCIO,
H LRh-0
3+
-RhL
1
co;
Scheme 26
within a cage defined by a six-coordinate aliphatic amine.7h' The syntheses are summarized in equation (143). Reaction of [Rh(en),I3+ with six equivalents of formaldehyde and two equivalents of ammonia in an alkaline (carbonate) aqueous solution leads to the basic cage complex [Rh(sep)13+. If nitromethane is used instead of ammonia, the [Rh(diN0,sar)j3' ion is formed; the nitrosyl functions can be reduced to amines with Zn/HCl. The compounds were characterized by UV/visible, IR and '€3, I3Cand Io3RhNMR; the electronic spectra are similar to that of the [Rh(en),]'+ parent, and the ' H decoupled I3C NMR shows resonances for two sets of six equivalent methylenes, consistent with D, symmetry. 73+ 7 5 +
+CHCH + ZMeNO,
Zn
(1-43)
__c
HCI
NH3 0
[Rh(diNO,sar)] 3 +
[Rh(diAMsarl12)i
'+
The effect of the cage on the one-electron reduction of the metal center has been studied in detail. Electrochemical reduction to [Rh(sep)12+was observed at - 1.8 V (vs. Ag/AgCl/O.l M LiCl) in acetone solution, but not in water; reduction is reversible, but further reduction to Rh metal does occur at - 2.2V. The Rh" thus formed is moderately stable, with a half-life of ca. 15ms at 20 "C. As rapid as this decay of the Rh" cage complex may seem, the one-electron reduction of Rh"' complexes is normally not reversible, and the Rh" cages do not dimerize, disproportionate or undergo further reduction to the Rh' complex. (While the Rh" state is stabilized by the cage, the photoproduced 'Texcited state of [Rh(sep)13+decays an order of magnitude more rapidly than the analogous state of [Rh(NH3)J3+;tliJ= 2.8 p s and 27 ,us, respectively.) Pulse radiolysis of [Rh(sep)13+ showed pseudo-first order quenching of the solvated electron (k = 1.4 x 101OM-'s-' at 25"C), producing [Rh(sep)lZf.The authors also comment on the remarkably regiospecific nature of the synthesis of [Rh(sep)13+,as 11 molecules must condense to form the 14-membered ring macrocycle,
Rhodium
995
and this reaction can reach 90% yields. If chiral [Rh(en),13+ is used, the resulting cage is also ~hiral.'~~
48.6.2.5
N-heterocycle complexes
( i ) Pyridine complexes
Numerous Rh"' complexes containing coordinated pyridine and substituted pyridines are known, but this area is dominated by the chemistry of the frans-[Rhpy,Cl,]+ cation. First characterized by J ~ r g e n s e n ,its ~ *chemistry ~ has been studied extensively by Gillard, Wilkinson and co-workers, and it has served as the parent for a host of derivatives. Attempts to replace the two chlorides by pyridine have been unsuccessful, and the hexapyridinerhodium(I1I) cation is unknown, although there are several reports of unsuccessful attempts to prepare it.781Reaction of trans-[Rhpy,Cl,]+ in refluxing pyridine leads to&!- and mt.r-[Rhpy,Cl,]; the addition of ethanol leads to a pink residue identified as a chloro-bridged polymer [Rhpy,C1,].78' Reaction of frans-[Rhpy4C1,js with Ag+ (to remove C1- ) presumably yields [Rhpy4(H,0),l3+, but this material does not pick up even one additional pyridine when refluxed in neat pyridine for four days!78'Early claims for [Rhpy,]X, and [Rhpy,X]X (X = C1 or Br)782were later shown to be due to the ubiquitous tran~-[Rhpy,X,]X.~~~'~~~ The standard preparation of trans-[Rhpy4C1,]+is due to De16pine,784-785 who noticed that reaction of pyridine with [RhCl,],- leads to mixtures of [Rhpy,Cl,] isomers, but that in the presence of primary or secondary alcohols, trans-[Rhpy,Cl,]Cl is formed. (The role of the alcohol in this reaction is discussed in more detail in Section 48.6.2.5.iii.) Far TR spectra (pyridine ring vibrations, Rh-N and Rh-CI stretches) of thefuc and mer isomers of [Rhpy,Cl,] confirmed the earlier isomer assignment.786 The trans-[Rhpy,Cl,]' ion is a convenient starting material, as the pyridines are readily replaced by other donors. Jsrgensen's ~bservation'~~ that [Rh(NH,), Cl]Cl, could be recrystallized from boiling pyridine, while NH, substitution of py in rrans-[Rhpy,Cl,]+ is facile, has been confirmed.78' The lability of the pyridine ligands was emphasized in a recent secondary ion mass spectroscopy (SIMS) study of solid trans-[Rhpy,Cl,]Cl- 5H,O, which indicated that successive pyridine labilization occurred (the ions [Rhpy,Cl,]+ for n = 4 , 3 , 2 and 1 were detected), while CI- was not lost from the parent; no evidence of [Rhpy,Cl]+ could be detected.'" Representative examples of the reactions of trans-[Rhpy,CI,]+ are given in Scheme 27 and Table 55. Pyridine volatility makes it an exceptionally convenient leaving group, as labilized pyridines can be steam distilled from the reaction flask. The neutral [Rhpy,(ox)X] complexes (presumably mer, X = C1 or Br) display an unusual synergistic solubility, as they are insoluble in both H,O and pyridine, but are relatively soluble in mixtures of the two solvents, reaching a maximum solubility in 1 : 1 mixtures of the solvent^.'^'
Scheme 27
I Rhodium
996
Table 55 Conditions for Reactions Cited in Scheme 27 Reaction 1 2
Rej:
Reactants
L = NH, L = N-methylimidazole N-N = en, pn N-N=phen Excess ethylenediamine Aqueous oxalate Refluxing pyridine L = substituted pyridine Dimethylglyoxime N4 = trien, 2,3,2-tet N, = 3,2,3-tet Aqueous CNConc. HCl
789 790 79 1 792 60 1 793 78 1 78 1 BO 1 754, 790 769 794 79 I
An extensive number of substituted pyridine analogs of trans-[Rhpy,Cl,]+ have been prepared via the path illustrated in equation (144). For the same reaction, if L = NH, or N-methylimidazole (miz), pentaamine complexes result. The [Rh(miz),Cl,]+ ion can be prepared by C1- anation of [Rh(miz),C112+in refluxing aqueous ethanol (pH 3). No reaction is observed for L = %-picoline, 2,4-lutidine or 2-aminopyridine, probably due to steric hindrance. The electronic spectra for all of the [RhL4X,]+ ions showed peaks at 409 and 439nm ( k 2 n m ) . Water adds, reversibly, to tram[Rh(4-formylpyridine),C12]+ as shown in equation (145); the analogous 3-formylpyridine complex is unreactive, and may be recovered unchanged from boiling water.795 RhX3.3H,O + 4L
3095
N,O
'
HY [RhL4X,1X
(144)
[RhL4X21Y
L = 3- or 4-substituted pyridine, 3,5-disubstituted pyridine, isoquinoline, pyrimidine, pyrazole or thiazole; X = C1, Br (and I for L = py); Y = univalent anion
trans-[Rh(N~ ! W 4 C I z l +
e HzO
trans-[Rh(N~ ; : 1 4 c l z l
+
(145)
OH
Halide substitution at trans-[Rhpy,Cl,]+ is possible, but refluxing truns-[Rhpy,Cl,]+ in HBr solution (4M) does not lead to [Rhpy,Br,]+, but trans-[Rhpy,Cl,](H, O,)Br,. An unusual fluorideassisted hydrolysis of trans-[Rhpy,X,] (X = C1, Br) rapidly leads to [Rhpy,X(OH)] k . Azide ion readily replaces C1- (and py) in [Rhpy4CI2]+,and the compounds [Rhpy,(N,)6-,]'"-3'+ (n = 2, 3, 4) have been Thiocyanate does not react with [Rhpy4C!l2]+, but NCO- and NO, each react to give incompletely characterized non-electrolyte~.~~~ Photolysis (436 and 254 nm) leads to both halide and py labilization from trans-[Rhpy,X,]+ (X = C1, Br).796The efficiencies of C1- and py release are comparable (upon d-d excitation at 436 nm), but are uncharacteristically low for photoinduced Rh-Cl bond cleavage. Bromide release dominates over py release upon 254 nm excitation of trans-[Rhpy,Brz]+. The photoreactions are presumed to proceed with retention of geometry.796 The trans-[Rhpy,Br,]+ ion has been the cationic host to an interesting bit of chemistry concerning the (HN206)-anion. The salt was first formulated as [Rhpy, Br,](NO,)(HNO,) by Poulen~?~' and it was later proposed, on the basis of a broad IR band near 800cm-', that the hydrogen dinitrate anion contains a symmetric H bond.798Thermogravimetric and mfferential thermal analysis showed that a molecule of nitric acid is released at cu. 200 0C.799A single-crystal X-ray analysis of t r a n s [Rhpy,CI,]~(NO,),] showed that the hydrogen dinitrate anion has two nitrates arranged such that four oxygens (two from each nitrate) form a slightly distorted tetrahedron, with the mean 0-0 distance about 3.06 A. The hydrogen atom was not found, but was presumed to be near the center of gravity of the oxygen tetrahedron.800The [As(Ph),][H(NO,),] salt also contains the hydrogen dinitrate anion, but in this case there is a linear, very short (0-0 distance of 2.45 A) hydrogen bond (53a); the anion of the Cs+ salt has a structure similar to that of the [Rhpy,Cl,]+ salt.80'A neutron diffraction study'" of trans-[Rhpy, Cl,][H(NO,),] revealed a disordered structure; the NO, moieties form a distorted tetrahedron of oxygen atoms, similar (but not identical) to that observed in the X-ray study,8°0where the hydrogen atoms are disordered between the two closest oxygen atoms (53b).The two sites are occupied with equal probability, so that the two non-coplanar nitrate groups may be considered to be bridged by two equivalent H bonds; the 0-H---0 moiety is bent with a +
Rhodium
997
bond angle near 167.5 '. The hydrogen dinitrate ion may, in fact, be a common ion in nitric acid solution, as Gillard and co-workers have prepared and analyzed the IR spectra of a variety of trans-[Rhpy,Cl,j+ analogs which all contain the [H(NO,),]- anion.803 The trans[Rhpy,Br2]Br.HBr*2H,0 crystalline salt has been proposed to contain the (H,02)+ cation with a symmetric 0. * - H -* -0hydrogen bond between two H 2 0 moieties.
.. '
Perhaps the most unusual, and potentially important, property of these pyridine-Rh"' complexes is their antibacterial activity. The activity is increased by substituting Br for C1 in the trans[Rhpy,X,]+ cation, and by adding lipophilic groups to the py ligand (e.g. Mepy, Etpy) and is decreased by adding polar groups (e.g. HOpy or H,Npy). The activity seems specific to the trans-[ML,X,]+ cation (M = Rh, but not Co or Ir; X = C1, L = py or substituted py, but not NH, or +en).8w
( i i ) Polypyridine complexes
While not yet as extensive as the chemistry reported for bipy and phen complexes of ruthenium, the chemistry of rhodium polypyridine complexes (especially their excited states) has generated much recent interest. The claim of Lehn and c o - w ~ r k e r that s ~ ~ excited ~ states of [Rh(bipy),13+are involved in the photoinduced generation of H, from water has sparked renewed efforts to understand the rich excited state chemistry of these complexes. Synthesis of polypyridine complexes of Rh"' generally involves replacing halide ligands with the chelated polypyridyls; fusion of RhCl, -3H2Q and bipy is reported to yield [Rh(bipy),]Cl, , and workup of the resulting solid in aqueous nitric acid gives an unreliable yield of [Rh(bipy),Cl,]NO3.*06 More convenient and reliable syntheses take advantage of the catalytic properties of mild reducing agents (see Section 48.6.2.4.iii) as the reaction of RhC1,- 3H,O with bipy (or phen) in ethanol/2-methoxyethanolsolution,m' or in ethanol with catalytic amounts of hydrazine,792gives [Rh(LL)2C12]+(or [Rh(phen),Cl,]') in high yields. An uncatalyzed reaction of [RhX6I3-(X ='Cl, Br) with phen leads to the monosubstituted [Rh{phen)&]- anion, as the (Hphen)+ salt, which can be further substituted to give [Rh(phen)zXz]+;'Kn,80s the chloride form of the latter complex has been * ~ ~ complexes of the series [Rh(bipy),(phen),-,]'+ (n = 0-3), resolved on a cellulose c o I ~ m n . Mixed have been prepared by Crosby and Elfring by bipy or phen substitutions of both chlorides in [Rh(N-N),C12]+ (N-N = bipy or phen)."' Scheme 28 summarizes some synthetic paths for the various polypyridine complexes of Rh"'.
[Rh(terpy),]Cl,
I
-
RhC13-3H20
I
I""
HCI(4)
[Rh(terpyhl X3
/
180°C
(phen-H)[ Rh(yhen)CI41
(X = Br-. I-, SCN-, CIOJ
H,O*
-
(H,O)IRh(~hen)Cl~l (NH,)[Rh(phen)Cl,]
Scheme 28
r Rhodium
998
The assignment of the geometric configuration for [Rh(bipy),X,]+ (and the phen analog) has been error prone and confusing, as electronic spectra,6n' acid adduct IR and Raman spectra,S08.811,812 I H NMR ,808 proposed mechanism of f o r m a t i ~ n , ~and ~ ~X-ray . ~ ' ~ powder patternsg1' have all been used to make (often conflicting) geometric assignments. It is now clear that these complexes, [Rh(N-N),X,]+ where (N-N) = bipy, phen or methyl or halo-substituted analogs; X = C1, Br, I, all share the cis geometry. The trans configuration is unknown for these polypyridyl complexes, while the cis configuration is unknown for the monodentate pyridine analogs [Rhpy,X,]+. Like the pyridine complexes, bis(bipy) complexes form acid adducts, and thermogravimetric analysis of [Rh(bipy),Cl,]Cl.HCI -2H,O, implies that the anion should be formulated as [(H,02)+(2C1-)]-.799 Recent interest in Rh polypyridyl complexes has centered on the behavior of their excited states; this is an active and complicated research area, and our understanding is less than complete. Discussion here will proceed as follows: (a) the photochemical properties; (b) the chemistry of the reduced polypyridyl complexes; and (c) the role of Rh"' polypyridyls as electron-transfer agents in the quenching of photoproduced excited states of [Ru(bipy),I3', and the photogeneration of H, from water. In aqueous solution at room temperature, cis-[Rh(N-N),X,]+ (N-N = bipy or phen, X = C1, Br, I) display deceptively simple photochemistry. Upon irradiation between 254 and 436 nm, all undergo stereoretentive halide loss, with the primary photoproduct being the corresponding [Rh(N--N),X(H, 0)]'+complex; minor Rh-N bond cleavage also occurs.'96 Halide release quantum yields are about an order of magnitude lower than those generally observed for haloamines of Rh"'. In a rigid environment at reduced temperature, these campiexes luminesce, and on the basis of lifetime measurements, a heavy-atom perturbation effect, and the large Stokes shifts, the luminescence has been assigned as a spin-forbidden phosphorescence.816This assignment was confirmed in a study of the luminescence at 77 K in an alcohol glass matrix (ethdnol-methanol, 4:l), in which intrinsic lifetimes, radiative rate constants, and quenching rate constants were determined. The intersystem crossing efficiency to the emitting triplet (a ligand-centered x-n* state) was found to be 'within 30% of unity', and a semi-empirical spin-orbit-coupling model was used to predict the intrinsic lifetimes of the excited states.*" DeArmond and Hillis have reported the absorption and emission spectra for several polypyridyl complexes of Rh"' (room temperature and at 77K in ethylene glycol-water 1:1, or ethanol-methanol 5:l glasses); their spectral results are given in Table 1 56."* The tris(bipy) and tris(phen) complexes show structured emission in the same region as the free ligands, implying that the luminescence is from a ligand-localized n-z* state, as suggested by Crosby.'" The bis(bipy) and bis(phen) cations showed broad, structureless emissions at low energy (with short lifetimes), consistent with Crosby's assignment of a &d,8" ' T I4 ' A ,phosphorescence.818 The intensity of the ligand-localized phosphorescence again suggests that the intersystem crossing efficiency for the tris-chelated complexes is near unity, but the lower intensity for the bis-chelated complexes suggests that population of the metal-centered 3T1state is less efficient.*I8Emission from these Rh complexes is generally more efficient than from the Ir analogs. Emission from the ligand-localized x-n* state of the tris-chelated complexes was further studied by Crosby, who prepared and showed that the luminescence (77 K, ethanol-methanol 4: 1 glasses) from [Rh(bipy),(phen),-,l3+ (n = &3), displayed long lifetimes (41, 24, 16 and 2.5ms for n = 0, 1, Table 56 Absorption and Emission Spectra of some [Rh(N--N)2C12]+ Complexes (77 K)*'*
5,,(nm)
Complcx [Rh(Ph@, IC13 IRhmPY),lC1, ci~-[Rh(phen)~Cl,]Cl cis-[Rh@ipy),Cl,]Cl Irr~n~-[Rh@y),Cl,]Cl
[Rh(en),lCI, [Rh(en),Cl,]CI
351 (30SO), 334 (4180), 314 (IOMO), 304 (20200), 459sh, 450, 279 (75 500) 321 (38 300), 307 (36300), 242 (40500) 385sh (981, 355 (3410), 337 (34SO), 321sh (5130), 298sh (21 loo), 275 (65 400) 382 (log), 342sh (2460), 313 (28800), 302 (24000) 465 (--3), 403 (81.5), 268 (8340), 262 (12500), 257 (13600) 35oSh, 296 (244), 253 (195) 469sh ( - 1.51, 400 (82.2), 284 (123)
Emission
Lifetime (s)
555sh. 48 1
4.84 x IO-*
521sh, 510, 483,450 709
2.21 x IO
4.15 x lo-'
704
4.73 x 10-5
649
5.2 x 10-4
sa8
2.1 x l o - 5 1.5 x 10-5
690
'
Rhodium
999
2 and 3, respectively), consistent with phosphorescence from a '(n-n*) excited state.8wFor n = 0 and 3, exponential decay of the phosphorescence was observed, but the mixed chelates (n = 1 and 2) showed non-exponential behavior. Crosby concludes that intraligand interaction is small in these complexes in both ground and excited states, and that the virtually isolated, ligand-localized, excited states decay independently, the apparent non-exponential decay resulting from the simultaneous, and unrelated, exponential decays from bipy- and phen-centered excited states. Supporting this model, Crosby notes that the absorption spectra of [Rh(bipy)(phen),],+ and [Rh(bipy),(phen)13+are virtually indistinguishable from the spectra of appropriate mixtures of the tris(bipy) and tris(phen) complexes.809 A study of [Rh(phen),13+confirmed that emission comes from a '(n-n*) state.819No emission was observed in fluid solution, but a long-lived excited state occurs in DMF at room temperature, as 290 nm irradiation of [Rh(phen),I3+ causes the characteristic 780 nm phosphorescence of *[Cr(CN),I3- (equation 146). The Sterm-Volmer constant of 3.0 x lo3M-' implies a lower limit for the excited state lifetime of ca. 0.30ms. Sensitization of biacetyl, and the oxidation of diphenylamine (in acetonitrile) and of 1,3,5trimethoxybenzene are also observed for *[Rh(phen)3]"+,implying that the 3(n-7c*) state of *[Rh(phen),l3+ is a powerful oxidant. A potential of 2.00 V is calculated for the *[Rh(phen),]3+/[Rh(phen),]3+ couple,819making the excited state of [Rh(bipy),13+a better oxidant than the Ru", Cr"' or Os" analogs. *[Rh(Phen)3J3+ -t [Cr(CN)6I3-
+
IRhWW313+
+
*[Cr(cN)6l3-
780nm
[Cr(CN),l31146)
In contrast, emission from [Rh(bipy),X, J' is from a metal-centered, '(rCd) state. The various low-lying excited states for [Rh(bipy),Cl,]+ are represented in Figure 3, with initial excitation into the '(n-n*) ligand state followed by relaxation and eventual phosphorescence from the '(d-d) state. The rise times for phosphoresence were reported to be 3 5 M 3 0 ns in room temperature solution and but these values were later found to be artifacts of the detection systern."l The emitting at 77 '(d-d) states of [Rh(bipy),X,]+ absorb at 580 (X = C1) and 550 nm (X = Br), and the lifetimes of their transient absorptions (measured by time-resolved absorption spectroscopy in air-saturated ethanol-methanol (4:l) solution at room temperature) were found to be 8411s (X = Cl) and 54ns (X = Br).82'(If the solutions are deoxygenated, the lifetimes increase by about a factor of five.) The presumed relaxation path is represented in equation (147), with the rate of internal conversion (IC) = 4 x 10" s-', followed by intersystem crossing (ISC), localized within the d-manifold, with the rate constant ca. 8 x 101'.82'
IC
('.-.*)
I(d4)
3 3(d-d)
(147)
A recent study"' of the effect of steric interactions on the luminescence of 2,Z'-bipyridyl complexes of Rh"' found luminescence from both ligand and metal-centered orbitals of [Rh(drnbpy),l3+ (dmbpy = 3,3'-dimethyL2,2'-bipyridyl). At 77 K the metal-centered luminescence dominates, but in
fluid solution the ligand-centered emission is enhanced. As in the unsubstituted analogs, ligandcentered luminescence dominates under all conditions for [Rh(dmbpy),Cl,]+. Luminescence from fluid solutions of [Rh(phen),13+ and [Rh(bipy),13+ is consistent with the 77 K behavior,8I6 as ligand-centered emission is observed.822 Closely tied to the photochemistry of these complexes is their behavior in the presence of reducing agents. Reduction of [Rh(bipy),13+with BH; or Zn amalgam was reported to yield [Rh(bipy)2]+.401 A 3I
r
Ligand-centered
E
: *
(d-d)
2-
v
MetaI -centered
P 0 C C
.-c0 .YI
I-
Phosphorescence
c
a 0-
Ground state
Figure 3 Low-lying excited states of [ R h ( b i ~ y ) ~ C l ~ ] +
1000
Rhodium
The kinetics of the reduction of [Rh(bipy),Cl,]+ in alkaline aqueous ethanol (under H,) revealed a two stage process: an initial induction period, during which time Rh' species formed, and a faster, autocatalytic region. The kinetics could be fit to the expression: d[Rh']/dt = k[Rh"'I2[Rh1]. The reaction rate was retarded by the addition of excess bipy, suggesting the suppression of a dissociation equilibrium, and the rate constant varies with [OH-], but not with [Cl-1. A five-step mechanism for the autocatalytic process was proposed, involving the formation (via an unspecified mechanism) of [Rh(bipy),]+, followed by the oxidative-addition of H, to [Rh(bipy),]+, and chloro-bridged dimeric and trimeric intermediate^."^ The electrochemical reduction of [Rh(bipy),Cl,]+ and [Rh(bipy)J3+ in room temperature acetonitrile solutions has been studied by DeArmond and co-workers and is represented in Scheme 29. For both complexes, they claim that the initial one-electron reduction is followed by a 'moderately' fast elimination of a ligand (Cl- or bipy); if the cyclic voltammetry scans are sufficiently rapid, this reduction is reversible.824After ligand labilization, the two paths merge, and there is evidence for two additional one-electron reductions, leading to [Rh(bipy),] and [Rh(bipy)J.
I.1
I
I IRh(bipy),l
Scheme 29
Studies, of the reaction of [Rh(bipy)J3+ with radiation-generated reducing radicals (e&, , C 0 , and MezCOH) have shown that, however appealing, this relatively simple picture is incomplete.825 These strong one-electron reductants quantitatively and rapidly (k = 109-10'0M- ' s-') generate [Rh(bipy)J'+. This ion has the following properties: (a) it has an absorption maximum at 485 nm ( E = 1000);{b) it slowly loses bipy ( k = 0.45 f 0.05 s-' at pH 3-10); (c) it is extremely air-sensitive, and reacts with Oz(k= 4.9 x 10' M - ' s-'); (d) in alkaline solution, [Rh(bipy)J2+ disproportionates (equation 1481, giving the red-violet [Rh(bipy),]+ ion and the starting complex; and (e) in very reduces [Rh(bipy),13+, resulting in a redox-catalyzed alkaline solutions (pH > lo), [Rh(bi~y)~]*+ chain of ligand labilizations.
+
[Rh(bi~y)~]'+ [Rh(bipy),]'+
[Rh(bipy),]+ f [Rh(bipy)~l~+
(148)
(red-violet) The two-electron reduction product, initially identified as '[Rh(bipy),]+' is a most mercurial at 518 nm (9500)] species, and Hoffman has identified four forms. (a) A red-violet, soluble form [La, identified as [Rh(bipy),(OH),J('-"'+. This may be dimeric, with OH- bridges. In neutral solution, the maximum absorbance shifts to 415 nm with a shoulder near 470nm. (b) A violet, insoluble form, identified as [Rh(bipy),X], for X = C1-, C10; ,etc. (c) A transient green form, observed when the red-violet species is acidified. This is presumably [Rh(bipy),(H,O),]+. (d) A colorless species formed in acidic solution, which is presumed to be a Rh"' hydride, e.g. [Rh(bipy),(H)I2+.
3
Rhodium
1001
At 'natural pH' (around 5), reduction of [Rh(bipy),j3+ with these radicals leads to H,, with an efficiency of about 25%. The dihydrogen precursor is presumed to be the colorless RH"'-hydride for [Rh(bipy),]+ is stable in an 0,free environment, and at pH 10 a suspension of [Rh(bipy),]ClO, is stable for at least three months, and does not produce any dihydr~gen..'~~ The reaction of radiolytically generated a-hydroxyalkyl radicals (RR'COH; R , R = H or Me) with [Rh(phen)J3+ has also been studied, with the emphasis being on the fate of the radicals; equation (149) represents the observed process.826An earlier ESR study of such reactions in ethanol glasses found a metal-containing radical for the [Rh(phen),13+ system, but not when the starting complex was [Rh(bipy)3]3'.827
[Rh(phen)J3+
+
A
RR'COH
adduct
[Rh(phen)3J2' +
products
-+
0 R&R
(149)
The importance of the reduced forms of [Rh(bipy),13+ is due to the fact that Rh"' polypyridyl complexes oxidatively quench the excited state formed upon visible light irradiation of [Ru(bipy),12+ (equation 150). Interest in this redox quenching dramatically increased when Lehn and co-workers demonstrated that aqueous solutions containing [R~(bipy)~]'+, RhC1,. 3H,O, K2PtCl, (or K,PtCl,), bipy and triethanolamine (TEOA) generated H, when the Ru complex was irradiated.805The role of the rhodium in this reaction has been the source of much discussion, as reduced forms of [Rh(bipy),13+ are thought to be the precursors to H, formation. In a detailed analysis of the mechanism of the H, generation, Lehn suggests that there are two catalytic cycles: the [R~(bipy)~]'+ cycle and the [Rh(bipy)J3+ cycle.828He conjectures that the [Rh(bipy),13+serves as a 'relay species', which mediates water reduction by intermediate storage of electrons. This process is thought to involve a series of one-electron transfers, involving [Rh(bipy),]+ and hydrido(bipy)rhodium species. A simplified representation of the relationship of the two catalytic cycles is shown in Scheme 30. Formation of H, requires two electrons, but the electrochemical experiments of DeArmondS2, showed that in acetonitrile the first two reduction potentials for [Rh(bipy),]'+ are close (- 0.86 and - 1.OO V), so that formation of the ubiquitous [Rh(bipy),]+ is facile.
+
*[R~(bipy)~]*'
[Rh(bipy)J3+
-
[Ru(bipy),I3+
+
[Rh(bipy),I2+
(150)
Ru
[ Keduced
-
El
..\'
form of Rh] -
Donor
H+
OX I DIZED PRODUCT
scheme 30
Lehn notes that photolysis of alkaline (pH > 10) solutions of [Ru(bipy),]'+ in the presence of [Rh(bipy)J3+ and TEOA (but no Pt) leads to the formation of the 520nm peak characteristic of [Rh(bipy),]+, suggesting that two-electron reduction is possible in this non-complementary system. They conclude that there are at least two paths possible within the Rh cycle, each involving a hydridorhodium(II1) species formed via the oxidative-addition of H, O+ (or H,O) to [Rh(bipy),]+ (equation 151). The two possible paths within the Rh catalytic cycle are presented in Scheme 31. Other workers have attempted to make sense of this complex set of reactions, and much of the work has concentrated on the nature of the transient [Rh(bipy),I2$, which is assumed to be the
[Ru(bipy!,l
3+
u
Scheme 31
primary product upon quenching the Ru excited state. No H, is generated in the absence of Pt salts, and it has been shown that the Pt prevents the disproportionation of the [Rh(bipy)J2+ (into Rh“’ and Rh’ species).s29A detailed study of the electrochemistry and quenching ability of a variety of RhL:+ ions (L = bipy, 4,4‘-Me, bipy, phen, 5-Clphen, 5-Brphen, 5-Phphen, 5-Mephen, 5,6Mezphen and 4,7-Me,phen) as well as [Rh(terpy)J3+ showed that all of these cations oxidatively quench the photoproduced *[Ru(bipy),J 2 + (equation 150). Detailed study of quenching by the [Rh(4,4’-Me,phen)J3+ ion suggested that the electron-transfer during the quenching is a facile and endergonic process.830Creutz and Sutin showed that a strong correlation exists between reduction potentials obtained in acetonitrile (where the one-electron reduction is q~asi-reversible)~~~ and those estimated in aqueous solution, implying that data collected in acetonitrile can be used to estimate aqueous behavior. They also argued against DeArmond’s interpretation (Scheme 29), and reversed their earlier conclusion,s29and concluded that the irritating irreversibility of the reduction of [Rh(bipy),I3+is due to rapid ligand iabilization from the two-electron reduction product (equations 152-154); they estimate that kfz 5 x 104s-’ in H,O at 25 “C. Pointing to the exceedingly rapid self-exchange between [RhL3j3+and [RhLJ2+ ( > 109M-’ s - I ) , they also suggest that the immediate quenching product is not a Rh” species. Rather, they suggest that ligand reduction occurs, generat-
1003
Rhodium
ing a species more appropriately designated as [Rh"'L2(L-)]2C. They recognize a few inconsistencies in this designation, and comment that our current view of the immediate quenching product [RhL3I2+contains some anomalies which need to be corrected.830A more detailed discussion of this interesting ion is presented el~ewhere.~~'
+ [ R h ( b i ~ y ) ~ ]+ ~ +e-
-
[ R h ( b i ~ y ) ~ ] ~ +e-
[Rh(bipy),]+
A
+
[Rh(bipy)J2+
(152)
[Rh(bipy),]+
(153)
+
[Rh(bi~y)~l* bipy
(1 54)
The current status of our understanding in this area has been succinctly summarized by Hoffman, who commented that . .reduction of [Rh(bipy)J3+ in aqueous solution yields a very rich chemistry. . . Both [Rh(bipy),]*+ and [Rh(bipy),]+ are involved in highly complex interlocking ligandlabilization, acid-base, redox, and aggregation reactions'.R25 I.
(iii) Catalysis of substitutions at Rh" Pyridine complexes of Rh"' have played a central role in the still poorIy understood area of catalyzed substitutions at Rh"' centers. Delepine first noted that the consecutive substitution of [RhCl6I3-by pyridine leads to the neutral, and insoluble, [Rhpy,Cl, 1, but that if the aqueous solvent includes some alcohol, pyridine substitution continues and trans-[Rhpy,Cl,] is the final product (equation 155).784Delipine showed that primary alcohols all had comparable effects on the rate of formation of [Rhpy4Cl2]', and that secondary alcohols were less active than primary alcohol^.^^^ Tertiary alcohols proved to be totally inactive, as are ether, dioxane or acetone. Poulenc had also reported that ethanol catalyzed substitutions at Rh"' [RhCl,j3-
fpy,
-cl-,
[RhtPY), c131w
'
alzol
[Rh(PY),C12IC1
(155)
Interest in this area was stimulated by a report from Basolo and co-workers,832who discussed possible mechanisms for the catalysis. They showed that the alcohols are true catalysts and are not consumed in the reaction (despite the detection of trace amounts of acetaldehyde as a reaction product), and that the substitutions could be catalyzed by a variety of reducing agents (e.g. Sn", BH;, N2H,, H3P02).The Co" ion and the [Rh,OAc,] dimer are both inactive, and while [Rh(CO)CI(PPh,),] and [Rh(CO)(SCN)(PPh,),j are inactive, the dimer [Rh(CO),Cl], is an extremely active catalyst. Kinetic studies were hampered by the evanescence of the catalyst, but Basolo did conclude that the reaction is first-order in Rh and catalyst, zero-order in pyridine (if in excess), and that C1- (as well as NO; and Cloy) inhibit the formation of trans-[Rhpy,CI,]+. In the presence of soft ligands, alkaline solutions of RhCI,.3H,O can be reduced to the Rh' state by alcohols.833 Basolo concluded that the Delepine catalyst involves a Rh' species, and suggested a mechanism analogous to Pt" catalysis of substitutions at Pt" centers (equations 156-1 58).834-835
-
[RhC15(H20))2Rh' HZO
+
[Rhpy,]+
+
4PY
+ [RhCl,(W,0)]2-
a
+ Rh'
tr~ns-[Rhpy,CI(OH2)]~+
Rh'
(156)
[RhbY)4l+
(157)
{H~O-Rhpy.+-Cl--RhCl,OH~)-
5
(158)
tran~-[Rhpy4Cl~]+
With general agreement that a reduced species must be catalyzing substitutions at Rh"', polarographic studies were used to investigate the nature of the reduced (and presumably catalytically active) species. Early work, using pyridine-supporting electrolyte, found the approximate reduction potentials, and provided evidence for a two-electron Gillard and co-workers used DC polarography a t a DME to examine many Rh'" complexes,839~840 and proposed a two-electron reduction, noting that complexes with aza aromatic ligands are reduced at significantly more positive potentials than their amine analogs. They suggested Rh' or Rh"'-H species as the primary reduction products. For py-containing complexes, a second reduction wave was observed, which was assigned to pyridine reduction; no second wave was observed for the ethylenediamine-containing ligands. For cis-cl-[Rh(trien)Cl,]+, however, two waves were observed, (E* = - 0.38 and -0.75V; both with n M 21, and monohydride and &hydride species were presumed to be the
I Rhodium
1004
product of these two reductions (equation 159). Reinvestigations3' of the previously studieds4' reduction of [Rh(H,edta)ClJ (initially thought to involve a three-electron reduction) indicated a similar two-wave system (n x 2 for each wave), presumably also leading to monohydride and dihydrido Rh"' species, although the products were not separated, and NMR signals from the resulting hydrides could not be detected. l e - , Et = - 0 75V 2e-, E ~ -o3av = [Rh(trWC121+ + H 2 0 , -"-, -OH-' [Rh(trien)C1(H)l+ + H 2 0 , -a-,-OH-' [Rh(trien)(H),]+ (1 59) The formation of a single Rh"' hydride was questioned in a detailed study of the polarography and coulometry of [Rhpy4Cl2]+.In an aqueous pyridine-pyridinium chloride-sodium chloride electrolyte (pH 5.3), this ion displays two distinctive types of polarographic beha~ior.'~'The 'normal' wave (El,,= - 0.43 V vs. SCEj is a diffusion-controlled, two-electron reduction at the DME to give an uncharacterized Rh' product; in solutions with high pyridine concentrations, an 'enhanced' reduction also occurs at the DME surface, which often masked the 'normal' reduction, leading to H+ consumption and the generation of hydrides. In large-scale reductions of fRhpy,Cl,]+, they observed the consumption of 1.4mol of H + per mol of Rh"' species reduced, which is not consistent with a single Rh"' hydride product. Their proposed sequence (equations 160-162) does not, however, clearly illuminate what must be a complex reaction ~equence.~" Rh"' Rh'
+
pyH+ xRh'
+ 2e-
-
Rh'
---*
+
-
Rh"'
+ yH+
H-
(normal wave)
+
py
(160)
(enhancedwave)
products, where v, >
(161)
(162)
x
Controlled-potential reduction of aqueous solutions of trans-{Rh(en), Cl,] in oxygen-free environments feads to the two-electron reduction product trara~-[Rih(en),(OH)(H)]+.~~~ In the process of reduction, 2 Fper Rh atom are consumed, but an intermediate with an absorption band at 338 nm (E = 9000) reached maximum concentration at ca. 0.9 Fper Rh. This oxygen-sensitive, diamagnetic intermediate does not catalyze substitutions at Rh"' centers, which suggests its formulation as the Rh" dimer [(H,O)(en), Rh-Rh(en)2(H,0)]4+ .843 Atomic absorption analysis of the one-electron reduced solution (reduced at a Hg electrode) revealed the presence of mercury, which led Anson and Gulens to question the Rh" dimer formulation and to propose that mercury leads to a Hg'+-bridged, Rh' dimer (equation 163). This same species, with the electronic spectra reported by Gillard for the Rh" dimer, could also be prepared by oxidizing tran~-[Rh(en),(H)(H,O)]~+ at mercury electrodes, and by mixing Hg2+ solutions with solutions of trans-[Rh(en), (H)(H,O)]*+. The formation of this trinuclear species can be regarded as a Lewis acid-base complex between Hg2+and the extremely basic [Rh(en),] .844 +
+
2[Rh(en),Cl,]+
+
Hg
+
2e-
+
{[Rh(en)2],Hg)4t + 4C1-
(163)
The two-electron reduction product, rvuns-[Rh(en),(OH)(H)]' displays a sharp ' H NMR signal at 30.6 T. The pseudo acid-base equilibrium, represented in equation (164), was proposed to explain the observation that this peak broadens as the pH is raised, disappearing completely by pH 13. [Rh(en),]+
+
H30f
e [Rh(en),(H)(H20)I2+ e
[Rh(en),(H)(OH)]+ + Hf
(164)
The formation of the hydridohydroxo ion can be considered to be the result of stereoretentive oxidative addition of H,O across the [Rh(en),]+. Acidic solutions of [Rh(en),(OH)(H)]+ are stable, even in the presence of O,, but in alkaline solution it reacts rapidly with oxygen, giving the hydroxoperoxo Rh"' monomer, trans-[Rh(en),(OH)(OOH)]+(equation 165). Interference from the secondary reaction (equation 166) complicates the overall stoichiometry of the reaction.475Concentration of this reaction mixture (iRh(en),(OH),]+ + [Rh(enj,(OH)(OOH)]+) in the presence of oxygen (pH 9-10> causes a color change to red and then to deep blue, with the formation of the superoxo-bridged Rh"' dimer, identified as [(H,O)(en), RhO,Rh(enh (H20)]5'."7' The transient red color is reminiscent of the electrochemical reduction of [Rh(NH,),(OHj(OOH)]+, which passes through a pink transient, after the consumption of 1 F per Rh, tentatively identified as the hydroxosuperoxo complex.845 [Rh(en),(OH)(H)]+ [Rh(en)2(OH)(OOH)I+
+
+ 0,
-
[Rh(en),(OH)(OOH)]+
/Wen),(OH)(H)l+
-+
WWen)2(OH)21'
( 165) (1 66)
Rhodium
'
.
1005
The catalytic properties of these reduced species have been actively studied by Gillard and co-~orkers.~ Among ' - ~ ~other ~ ~ ~things, they have shown that H2also initiates Delepine's reaction of [RhCl,]3-,R'6that O2dramatically impedes the catalyst,847and that halide substitution at trans~~ with [Rhpy,Cl,]+ (equation 167) is also catalyzed by primary and secondary a l ~ o h o l s . 'Consistent the pseudo acid-base equilibrium (equation 1641, the rate of catalyzed halide substitution is pH dependent, with little catalytic activity occurring in acidic solution^.'^^ There is an induction period for the reaction (the more alcohol in the solution the shorter the period), suggesting that deoxygenation of the solution must occur before the reduced catalyst can be effective. [Rhpy4Cl2]+f 2Br-
-
[Rhpy,Br,]'
+
2Cl-
(167)
The lack of alcohol catalysis of halide substitutions in the [Rh(en),X,]+ system has been attributed to the relative ease of reduction of the two Rh complexes (E,,, for trans-[Rhpy,Cl,]+ is - 0.39V, while it is -0.79V (vs. SCE) for irmw-[Rh(en),Cl,]+), accounting for their differing behavior toward mild reductants.'" Stronger reducing agents, such as BH; do catalyze halide exchange at [Rh(en),Cl,]+; [Rh(en),(OH)(H)]+ (but not [Rhpy,]+) is also an effective catalyst for halide exchange in neutral, deoxygenated solutions of [Rh(en),X,]+.850Kinetic evidence inconsistent with the mechanism proposed by Basolo has led to an alternative mechanism involving catalysis by .the poorly characterized Rh' species, and a [Rh'-X-Rh"']-bridged intermediate. Unlike the Pt'I-Pt'" system, where trans geometry is needed, BH; catalyzes bromide substitution at cis-a- and cis-P-[Rh(trien)Cl,]+, and the reactions proceed with retention of geometric ~ o n f i g u r a t i o nGillard .~~~ and Heaton have summarized the available mechanistic a r g u m e n t ~ , but ~ ~ ' the last shoe has yet to fall in this inadequately understood area. 4$.6.2.6
Hahitrile complexes
The first examples of halonitrile complexes were reported by Johnson et a/.:$' who .found that RhC13-3H,0 reacts readily when digsolved in an alkyl or aryl cyanide to form non-ionic complexes of the form [RhCl,(RCN),] (R = Me, Et, Ph or MeOCH,CH,). Electronic spectra all show a d-d band near 425nm, and a second band near 370nm (in nitromethane solution). Under the same conditions, the potentially bifunctional glutaronitrile (NC(CH,), CN) reacts with RhCl, to give an ill-characterized orange solid, which is presumed to be polymeric with bridging nitrile groups.852 Using 'less extreme conditions', Catsikis et al., prepared fac-[RhCl, (MeCN),], characterized by elemental analysis, electronic spectra (Table 57), and IR and proton NMR spectroscopy.8s5Assignment of geometry has been debated, as Johnson used far-IR spectra to assign a mer while Walton later suggested a fuc Gillard and co-workers used NMR, electronic and Raman spectroscopy to assign the mer geometry to a variety of [RhX,(RCN),] complexes (R = CD,, Et, PP, Pr', Ph or PhCH,).853 They found evidence for a fac isomer, but only for [RhCl,(MeCN),]. Anionic halonitrile complexes have been prepared by taking advantage of the insolubility of the (Et,N)+ salts. If RhCl,-3H20 is heated with @t,NjCl in a H,O/MeCN (111) solution, trans[Et4Nl[RhC14(MeCN),Jis isolated. If [RhCl,]'- is used as a starting material, and the electrolyte is added after heating, salts of the form M,[RhCl,(MeCN)] (M = Cs+, NH: or Et4N+)can be i~olated.'~'Some bromo analogs were similarly prepared.854Gillard prepared cationic complexes by the reaction of the neutral [Rh(MeCN),X,] (X = C1, Br) with AgNO, (or AgC10,) in acetonitrile Table 57 Electronic Spectra of some Halonitrile Complexes of Rh"'
852"
852" 853 852' 853 853
853 853 854 854b
8Mb
= Rhodium
1006
solution.g53Dissolution of this tetralus(acetonitri1e) cation in other nitrile solvents (RCN) leads to nitrile replacement, and is a general route to trans-[Rh(RCN),X,]+ cations.853The IR spectra of these complexes all showed the expected shift in the vCPN to higher energy, from 2257 cm-' in pure MeCN to around 23OOcm-' for the coordinated molecules. , :
i
48.6.2.7 Porphyrin and phthalocyanine complexes The chemistry of Rh"' complexes with porphyrin and phthalocyanine ligands is quite distinctive, as these rigidly planar, tetradentate amine ligands form compIexes via unique synthetic routes, and the chemistry of the resulting complexes differs substantially from that of complexes with less constrained ligands. A variety of porphyrin complexes of Rh'" have been characterized, and a list of the abbreviations used in this section is found in (54) and Table 58.
(54)
Fleischer was an early leader in this area, and found that when the dimeric Rh' complex [Rh(CO),CI], was mixed with mesoporphyrin IX diethyl ester (MePDEE) in glacial acetic acid, a weakly paramagnetic complex, formulated as [Rh(MePDEE)], was generated.859The source of the paramagnetism was not explained, and the coordination geometry about the Rh was not discussed. but the I R of the complex showed no N-H or C-0 stretches. The two-electron oxidation of a Rh' complex represents an unusual synthetic route to a Rh"' complex, but it has since become a general route to Rh"' porphyrins, and was quickly extended8m,861 to include TPP, TPyP and MePDEE analogues. The synthetic path is thoughtsm to pass through a [Rh'(porph)(CO)Cl] (presumably a dianion) intermediate, which is oxidized by 0, in solution to give the final Rh"' species. All of these porphyrin complexes are intensely colored, with the characteristic a, /3 and Soret bands near 550,520 and 400 nm respectively. Fleischer postulated that they are five coordinate (four porphyrin nitrogens and one coordinated H,O), with the sixth site available for substitution by other ligands. The source of the weak paramagnetism ( p x 0.4 BM)859was proposd to be due to the presence of a monomeric Rh" complex, [Rh"(TPP)], formed upon incomplete oxidation of the [Rh'(TPP)]Surprisingly, the proposed Rh" monomer did not bind O2 despite the known reactions of [Rh(NH3),I2' with 02,863 and the well-established reactions of Co" and Fe" porphyrin Table 58 Numbering Scheme for Substituted Porphyrins 3
Name
Meso Meso-DME HMP HMP-DME Deut Deut-DME OEP Etio
Mesoporphyrin IX Mesoporphyrin dimethyl ester Hematoporphyrin IX Hematoporphyrin dimethyl ester Deuteroporphyrin IX Deuteroporphyrin dimethyl ester Octaethylporphyrin Etioporphyrin I
TPP TPyP TPPS
Meso-substitutedporphyrins (sites 1-8 sites 9-12 tetraphenylporphyrin Phenyl tetrapyridinylporphyrin Pyridine tetra-p-sulfonatobenzeneporphyrin" C6H4SO;
'Kanion; other porphyrins listed PM' = CH,CH,COOMe. '
I
are
2
4
Abbreviation
5
Meso positions (9-12) contain H Me Et Me Et Me Et Me Et Me EOH Me EOH Me EOH Me EOH Me H Me H Me H Me H Et Et Et Et Me Et Me Et
2-anions; Me = Methyl,
Et
=
Ethyl,
Me Me Me Me Me Me Et Me
6
7
PH
PH
Me
PMc P" PMe PH PMc Et Et
PMc PH PMe PH PMe Et Me
Me Me Me Me Me Et Et
8
= H)
EOH = C(OW)HMe, PH = CH,CH,COOH,
1007
Rhodium
complexes with 02.Wayland and Newman argued against the existence of a monomeric Rh" porphyrin complex,BM and proposed that the only Rh" species present was a dimeric [Rh(porph)], with a Rh-Rh bond. The diamagnetic [Rh(OEP)), dimer, prepared by photolysis of [Rh(OEP)(H)], was convincingly characterized, and while it reacts as if it were a source of the [Rh"(OEP)] moiety, no stable Rh"-porphyrin monomers have yet been thoroughly characterized. The paramagnetism observed by Fleischer and James859*860,862 was attributed to [Rh(OEP)(O,)], which forms when [Rh(OEP)], is exposed to dioxygen. The complex was characterized by electronic, IR and EPR spectra, and formulated as a Rh"'-superoxo complex, with end-on bonding by the 0; ligand. The superoxo complex reacts with alkyl phosphines and phosphites,861and with piperidines6' to give 1:l adducts, and when warmed to 20 "C, its paramagnetism decreases, presumably due to the formation of the peroxo-bridged Rh"' dimer. Wayland has explored the chemistry of the dimeric [Rh(OEP)],, and found that it reacts with NO to generate [Rh(OEP)NO], which can be subsequently oxidized by 0, (in air) to give the nitrite (NO;) complex.865The same [Rh(OEP)(NO)] can be prepared by the reaction of NO with [Rh(OEP)Cl], although this reaction passes through a metastable intermediate, identified as [Rh(OEP)(C1)(NO)].86sSubsequent reaction with NO leads to the stable fih(OEP)(NO)] product. The intermediate is characterized as a 'n-cation radical' complex, containing an OEP- radical ligand, not a Rh". Support for the radical anion assignment comes from electrochemical oxidation of a variety of porphyrin-containing species, which revealed that the ease of removing the first two electrons from a porphyrin system is not strongly dependent on the cation bonded to the porphyrin (Table 59), implying that the electron is removed from a ligand-localized orbital which does not interact strongly with the metal. The second electron is also thought to come from a ligand-localized orbital, leaving a diamagnetic, n dication complex.865 An interesting series of substitution/redox reactions involving both dimeric Rh" and monomeric ~~ solution, H, reacts with [Rh(OEP)Clj Rh"' complexes of OEP has been r e p0 r t e d . 4~ *In, ~methanolic to generate [Rh(OEP)H] and HC1; borohydride reduction of [Rh(OEP)CI] leads to a species identified as [Rh'(OEP)]-, which forms the same hydrido species when acidified (vRhPH at 2220 cm-', ~ Dioxygen reacts with [Rh(OEF)Cl] to form the and when prepared with DC1, v ~ at ~1595- cm-'). dimeric [Rh(OEP)], (an unusual use of O2to reduce Rh"'); the Rh" dimer can also be formed by heating a degassed toluene or benzene solution of the hydrido complex. The reaction of the [Rh(OEP)], dimer with a variety of organic substrates leads to a series of organo-Rh"' (OEP) complexes. The reactions described in the past two paragraphs are summarized in Scheme 32. Single-crystal X-ray structures of several Rh" porphyrins have been reported. The structure of showed that the Rh"' fits nicely into the plane of the octahedral [Rh(Me,HN),(Etio)]Cl-2H2O porphyrin nitrogens, with an average Rh-N distance of 2.038 A;the axial dimethylamine nitrogens A square-pyramidal complex [Rh(OEP)(Me)] showed similar distanare 2.090 A from the ces around the metal, with the Rh in the plane of the porphyrin nitrogens, and the apical C atom 2.031 A from the (Ogoshi notes that this Rh-C bond is significantly shorter than the 2.081 A observed for the Rh-Me distance in another five-coordinate Rh"' complex [Rh(PPh,),(Me)(I),].868 The geometries of the two complexes are quite different, however, as the Rh is 0.25 A out of the plane established by the two P and the two I atoms, but is in the plane of the porphyrin nitrogen^.^^^ An early report by Fleischer$@describes the crystal structure of a paramagnetic (p = 1.95 BM) species, identified as [Rh(TPP)(Ph)(Cl)], which is said to have the 'usual planarity of the pyrrole rings and phenyl rings'. Despite its paramagnetism, the method of synthesis ([Rh(CO),Cl], is allowed to react with TPP in refluxing benzene) and the Rh-N and Rh-C distances (2.04 and 2.05 %L respectively) are reminiscent of Rh'" porphyrins, suggesting that this may be a radical-anion-Rh"' complex, not the Rh" species tentatively proposed by Fleischer."' Table 59 Oxidation Potentials" for some Rh"' Porphyrinss6'
First Complex
oxidation
H,TPP [Rh"' (TPP)CI] [Rh"'(TPP)(NO)]
0.97 0.98
H, OEP [Rh'" (OEP)Cl]
0.84 0.82 0.77
[Rh"'(OEP)(NO)]
0.94
Second oxidation 1.25 1.36 1.36 1.14 1.34
1.27
'Potentials are *0.01 V w. SCE, measured in CH,CI,.
I I
{ Rh"'(OEP)CI(NO)}
LNO
[ (OEP)RhkCH2Ph ]
r--
c [ Rh"'(OEP)02] (paramagnetic)
I
I Rh"'(OEP)(NO),I (paramagnetic)
1 : 1 adducts IRh'L'(OEP)]?Oz
(R = alkyl, piperidine)
(diamagnetic)
Scheme 32
Quantitative studies on the reactivity of Rh"' porphyrins have centered on substitutions (and proton exchanges) at the fifth and sixth coordination sites in [Rh(TPPS)(H,0)I3-; Ferraudi and co-workers have also presented a detailed study on the photophysics and photochemical reactivity of Rh"'-phthalocyanine complexes. Bailey studied the acid-base equilibrium (equation 168) for a variety of metalloporphyrins, and found a pKoHof 5.1 f 0.1 for [Rh(HMP)], while [Rh(TPPS)Jhad a p k H of 6.8 0.1g70(a pKoH of 7.5 had been reported earlier for the TPPS complex87').After analyzing the acid-base behavior of a variety of metalloporphyrins, Bailey found that the results could be most effectively interpreted in terms of a simple crystal field model, and argues that while the porphyrin ligands are certainly delocalized, this need not imply delocalized metal-ligand interactions. He reached the heartening conclusion that his 'observations strongly suggest that metalloporphyrin electronic structure . . . is much simpler than is generally appre~iated'.'~'
+
[ W P O V ~ ) ( H ~ O L I F= + [R~(~ov~)(H,O)(OH)] H+
(168)
The thermodynamics and kinetics of NCS-, CN-,871C1-, Br-, NCS- and I-872 substitutions at [Rh(TPPS)(H, O)] have been reported; the reactions involved are shown in equation (169), and the parameters determined are summarized in Table 60. Spectrophotometric titrations showed two inflection points as OH- is added to [Rh(TPPS)(H2O),l3-,and the consecutive PKA values (7.01 and 9.80 at 20°C) correspond to the PKA values for fuc- and mer-[RhCl,(H,O),], suggesting that the TPPS6- anion and 3 C1- ligands are comparable electron donors toward the Rh center.872The trends in the equilibrium constants (Table 60) imply that Rh"' is a soft (class B) acid in these complexes; the NCS- ion is presumed to be sulfur bonded, although no direct evidence is presented to support this assumption.
-. ti
H20
[Rh(TPPS)(L),]
5-
H20
L
L
,e
H2°
[Rh(TPPS)(H,O)Ll
4-
H20
K,
KA -H*
[Rh(TPPS)(H20)2] 3-
L
+
The substitutions follow the two-term rate expression: rate = (kl[L] k-,)[Rh3-]. As observed with Co and Cr porphyrins, the rate of ligand substitutions at the Rh"'-porphyrin moiety is much faster than for the comparable reactions of non-porphyrin complexes. For example, the rate of C1-
1009
Rhodium Table 60 Kinetic and Thermodynamic Parameters for Substitutions at [Rh(TPPS)(H,O),]+ a Anating anion
AH: (kJmol-I)
AS: (JK-'mol-')
c1
56.1 2.5 57.7 2 1.3 53.6 & 3.3 69.0 k 0.8 89.3 5 9.1 59.6 f 2.1
- 106 f 7.9 - 54.6 f 4.6 - 97.9 If: 16.5 -43.1 2.5 - 1 . 1 f 14.3 - 77.8 39.1
Br I NCS CN Proton exchanges OH
+
Re$
0.74+0.11 1.82 f 0.10 41.2 f 1.8 1.22(+ 0.04) x lo4 39 1880
< 0.03 < 0.03 < 0.03 3.68 & 0.42 -
872b 872 872 872 871' 874d
-
1.59( f 0.24) x 10- lo
-
3.55 x
6.3(+ 1.3) x IO-' ~
Kz
K",
Kh 9.67(& 0.12) x IO-' ~
Kl
I
~
385 384 383
~~~~~~~~
'Numbered reactions shown in equation (169). ref. 385, 0.10M H+,p = 1.0 (NaCI04) 'pH 4 7 (phthalate buffer), p (KSCN KNO,) p H 10 50 (borate buffer), p = 0 5 (NaCN + NaC10,).
+
= 0.5
P
anation of [Rh(TPPS)(H,0),I3- is lo7 and lo5 times faster than the rate of C1- attack on trans[RhCl,(H,O),~-, and tvan~-[Rh(en),(H,O),]~+, respectively. Based on the very negative AS* values observed, an associative interchange (I,) mechanism for these substitutions was suggested.872 (Pasternack and c o - ~ o r k e r s had ~ ' ~ earlier suggested that a porphyrin could be expected to strongly stabilize a five-coordinate geometry, suggesting that dissociative interchanges would be expected for substitutions at metalloporphyrins.) Studies on the effect of pressure on the rates of NCS- substitution at [Rh(TPPS)(H20),l3- support the dissociative rn0de1."~ The A V * of 8.8 cm3mol-' is similar to that observed for the Cr"I analog (7.4 cm3mol-'), but significantly smaller than observed for CO"' (15.4cm3mol-'); this parallels the trend to more negative AS* values for the Cr and Rh analogs. vanEldik conciudes that for [Rh(TPPS)(H,0),]3-, the 'experimentally observed values are in good agreement with those expected for an interchange mechanism in which the rate determining step is dissociatively activated'; noting the differences between the Co, Cr and Rh systems, he also states that 'the central metal atom seems to control the extent to which the porphyrin can modify the reactivity and mode of the substitution Numerous Rh' porphyrin complexes have been prepared (videsupra), and these have been used to obtain Rh"' porphyrins. Sodium ethoxide nucleophilically attacks the C atom of [Rh(TPP)Cl(CO)] (in methylene chloride/ethanol solutions), leading to orange-red crystals of a diamagnetic ethoxycarbonyl(TPP)Rh"' complex with a Rh-C d bond. Reaction of [Rh(TPP)Cl(CO)] with the basic LiNEt, in diethylamine leads to the analogous [Rh(TPP)C(O)NEt,], while no reaction of [Rh(TPP)Cl(CO)] is observed with the analogous NaSEt, Ogoshi and co-workers have studied the reactions of [Rh'(OEP)Cl] with a variety of reactive organic materials. As summarized in Scheme 33 this Rh' porphyrin undergoes oxidative eM,,."
[ (0EP)RhXl
A r*
[(OEP)Rh-(CH,)3COMel
scheme 33
1010
Rhodium
p-fluorophenyl cydopropy'/ketone
R
0
a
II
\
ethyl
PhCH
/ \
I
R
\/
O
alkylorvryl halldcs
N
N
Me \
/
R
C
(CO)zRh Rh(CO) bfomoacetate
R\ No
@
/ R'
I methyl
formate
R E = Me; R2 = E t Scheme 34
additions with an activated cyclopropane,s76alkyl halides,877organolithiums, alkenes and alkynes, c bond. While Rh' and Grignard~;'~'all but the Grignards lead to complexes with a Rh"'-C complexes generally catalyze the quadricyclane -.+ norbornadiene valence isomerization, the reaction of [Rh'(OEP)Cl] with quadricyclane leads to the cleavage of bnly one of the two cyclopropane rings, and the Rh"'(0EP) species represented in (55). Grigg and co-workers have studied a wide variety of oxidative additions at some dirhodium(1)porphyrin c o m p l e x e ~ . As ~ ~ summarized ~~~* in Scheme 34, the Rh"' products are generally monomeric, five-coordinate species with a Rh--C r bond.
A
(55)
Sperm whale apomyoglobin, reconstituted with Rh"', leads to stable Rh-myoglobin complexes which are inert to external anions (F-, OCN-, N; and CN-) and to reducing agents (dithionite, borohydride and d i h y d r ~ g e n )The . ~ ~mesoporphyrin-Rh"' ~ complex itself, however, is reactive, and resembles the native protein in some ways (its readily binds CN- ion. While the meso-Rh"'-Mb tight binding of the apoprotein and prosthetic group, metal-imidazole bonding, and NMR-determined protein conformation), the inertness of the meso-Rh"'-Mb to external anions leads to the supposition that there are no exchangeable aqua ligands coordinated to the Rh"', but that the rhodium binds to both the proximal and the distal imidazoles. This would make the Rh"' system a poor ferrimyoglobin model (in whch only the proximal histidine is bound to the metal), and a better model of an internal hemichrome. Access to the sixth coordination site is blocked in the reconstituted meso-Me-Rh"'-Mb, however, as a methyl group occupies the sixth site; the 'H NMR of the Me resonances are shifted 2 p.p.m. downfield upon incorporation into the myoglobin, due to the compression effect of the amino acid residues at the distal site. (This meso-Me-Rh"' -Mb is the first example of a reconstituted hemoprotein with an organometalloporphyrin.) Not surprisingly, the meso-Me-Rh"'-Mb is also inert toward substitution and reduction, but the effect of the coordinated methyl group is apparent in studies which compare the ability of meso -Rh"' and meso-Me-Rh"' to compete with Fe"' for coordination within the myoglobin. When meso-Rh"' is mixed with Fe"'-Mb (at a 4/1:Rh/Fe ratio, 3S°C,pH = 7.0, 24h), 90% of the Under resulting prosthetic protohemin in the native myoglobin is replaced by the meso-Rh"'. is incorporated, supporting the supposition similar conditions, only 15% of the meso-Me-Rh"'
101 1
Rhodium
'
'
that coordination by the distal imidazole occurs in the meso-Rh"'-Mb, and that this binding is blocked by the coordinated methyl, decreasing the stability of me~o-Me-Rh"'-Mb.~~~ Ferraudi and co-workers have presented detailed studies of the p h o t o ~ h e r n i c a l and ~~~~~~ photophysical b e h a v i ~ r * ~ of ~ *Rh(phtha1ocyanine) ~*' complexes. Like the porphyrins, the phthalocyanine dianion (ph, 56) is rigidly planar, with extensively delocalized intraligand bonding around four amine nitrogen donors. Photolysis (254 nm light) of Co"' phthalocyanines causes reduction of the metal and oxidation of an axially coordinated ligand, consistent with the population of a ligand-to-metal charge transfer excited state.88sIn contrast, ligand substitutions, without net redox chemistry, were observed when the isoelectronic [Rh(ph)X] (X = C1, Br, I) complexes were photolyzed under similar conditions. The simplicity of the net reaction belies the complexity observed when the reaction is studied by conventional and laser flash techniques. As Ferraudi states887'the photonic energy is degraded in a complex manner that involves photochemical, radiative and nonradiative paths'. The proposed mechanism for the photochemical path is represented in equations (170)-(177). In the first phase, electron transfer occurs, the anionic ligand is released, and a labile [Rh"(ph)] and a solvent radical are formed (equations 170-172); a Rh'"-(radical anion) species is also implicated. Reoxidation of the Rh" involves a complex set of reactions (equations 173-177). Equation (173) has a rate constant of about 10'OM-'s-', making the acetonitrile radical (CH,CN-) a poor candidate for the oxidation of the Rh" complex (equation 174). The ESR and the absorbance spectra of the RhI'I-ligand radical complex have been and the absorbance spectrum (Lx w 510 nm) is similar to that observed upon one-electron oxidation of uncoordinated phthalocyanine.888This radical-ligand complex is thought to result from population of the n.n* ligand-centered states involving the lone electron pairs of the bridging nitrogens of the ph ring.
[Rh"'(ph)(MeOH)X] f MeCN
+
[Rh"'(ph*H)(MeOH)]
[Rh"'Iph-H)(MeOH)X]
+
[Rh"'(ph)(MeOH)X] % MeCN
+ [Rh"(ph)] [Rh"'(ph*)(MeOH)X]+ + [Rh"(ph)] CH2CN.
CH,CN* [Rh"'(ph.)(MeOH)X]+
f
+
(1 70)
*
+ €I+ + X-
~Rh'"Cph.)(MeOH)X]~
[Rl1'~(ph.)(Me0H)X]- 1[Rh"(ph)] CH2CN-
+ CHzCN
-
% MeCN
MezCHOH
MezCHOH
-f
f
+ MeOH +
(172)
[Rh"'(ph.)(MeOH)X]+
(173)
[Rh"'(ph)(solvent)J+
[Rh"'(ph)(MeOH)X]
-
(171)
McCN
+
+
(1 74)
[Rh"'(ph)(solvent),]+
Me,COH
[Rh"'(ph)(MeOH)X]
+
(175) (176)
MqCOH
+
H+
(177)
In 0,-free solutions, [Rh(ph)(MeOH)Cl] luminesces (A, x 420nm, 4 = 1.3 x lop3)when excited with light of 400nm or less.866Excitation spectra show that the emitting state is well above the lowest n-n* state, making this an apparent violation of Kasha's rule. Similar luminescence is oberved for the CoI'I, Ru", AI1", and weakly observed for Cui' phthalocyanine complexes, and the luminescence is thought to originate from an upper-level, ligand-centered triplet. The action , spectrum for photoinduced hydrogen abstraction is independent of X in [Rh(ph)(MeOH)(X)] (X = C1, Br, I). The photosubstitutions are therefore proposed to originate from a ligand-centered 3n7c* state, while emission comes from a ligand-centered upper-level 3n-7c* state, with another, lower-lying n-n* state available. Ferraudi presents a detailed investigation of the time dependence C.O.C. + 4 G
1012
Rhodium
of the two 3n-71* states (emission and absorption studies), and of the relationship between the photoemissive and photoreactive states.ss8
48.6.2.8
Miscellaneous nitrogen iigands
The electronic and IR spectra of [Rh(NO,),I3- are consistent with the expected RhN, chromophore, and while ligand field bands were not observed, CT absorbances occur at 317 and 275 nm.889 Arylazo oximes (N-N) form [Rh(N-N),] complexes with Rh"' in which the arylazo oxime acts as a bidentate ligand (five-membered rings; 57).8w Reaction of Rh(NO,), -3H,O with the appropriate arylazo oxime in ethanol at pH 4 (with C0:- used to adjust pH) leads to a recipitate offncand mer-[Rh(N-N),]. The geometric isomers can be separated on alumina, and H NMR showed that approximately equivalent amounts of the fac and mer isomers formed (for R = Me or Pr and Ar = Ph). This represents an unexpected (and unexplained) enhancement of the population of the fac isomer, as statistically, a faclmer ratio of 113 would be expected; only the mer form was detected for [CO(N--N),].~~~ These rhodium complexes all display intense absorption bands near 480 and 320 nm, which are presumed to be intraligand bands.890
P
r-
1
(57) R = H,alkyl,aryl Ar = aryl
The tripyrazolylborate anion (RB(pz), , 58) acts as a tridentate ligand, and frequently results in complexes which are analogous to cyclopentadienyl complexes. The first Rh"'-RB(pz), complexes were prepared by the oxidative addition of X2 to [Rh,{RB(pz),)(CO),], and resulted in octahedral, monomeric complexes [Rh{RB(pz),)(CO)I,] (R = H, and, in low yield, for R = PZ)."~ For the tetrapyrazolyl species (R = pz) the NMR spectrum showed two, non-interconverting pyrazole environments in a 311 ratio, consistent with a tridentate configuration for the borate. The C-0 stretch is very high (2090 cm-'), consistent with the formal Rh"' designati01-1.~~~
---
(58) R = H , p z
Powell and co-workers have extensively studied the Rh"' chemistry of the hydrotripyrazolylborate anion (R = H).893They found that the reaction of equimolar amounts of Na[HB(pz),)] with RhC13*3H,O in refluxing methanol leads to two types of complexes, depending on reaction conditions. Dilute solutions and short reflux times (ca. 1h) yield chloro-bridged dimeric species (59a), while more concentrated solutions, refluxed for longer periods (ea. 4 h), generate the methanolsolvated monomer [Rh{HB(pz),}Cl, (MeOH)] (59b). These two species are convenient starting materials for a variety of reactions, and Powell reported over 60 derivatives.893The coordinated methanol can readily be replaced by other neutral monodentates (e.g. PR,, AsR,, py, NR,, RNC or CO), and reaction with the appropriate Ag or T1 salts gives [Rh{HB(pz),}YCl] (Y = acac, hfacac, Et,NCS2) and [Rh(HB(pz),)Y,L] Cy = Ac-, no L; Y = CF,CO;, L = H,O). Reduction of the monomer with H,/Et3N leads to the anionic hydride, [Rh{HB(pz),)Cl,(H)]-. Similar reactions were observed starting with the dimeric species.893 Schlemper and co-workers have reported the X-ray structures of the products of the reaction of RhC13 3H20 with a series of similar amine oximes (A-0; 60).The reaction product is intriguingly dependent on the length of the hydrocarbon chain linking the two amine oxime moieties, and
Rhodium,
1013
r’ ,
(59a)
(59b)
representations of the structures are shown in (61).894,895 For the longest hydrocarbon chain (n = 4), a binuclear species results, with both (A-O) ligands bridging between the two Rh centers (61a). At each end of this molecule there is a very strong hydrogen bond between the two oximes (0-0 distance is 2.387 A).g94With one less carbon (n = 3), the hydrocarbon chain is apparently not long enough to span the two Rh centers, as a monomeric tuans-[Rh(A-O)Cl,] com lex results (61b).895 Again, a strong hydrogen bond between the two oximes (0-0 distance 2.474 ) makes the (A-0) ligand virtually cyclic. For the even shorter hydrocarbon (n = 2), another dinuclear species is formed, but with a chloride and the two oxime moieties bridging the Rh centers (61d). There is no evidence of hydrogen bonding in this structure.894The last species studied has no hydrocarbon chain (in essence, n = 0), but the product has a structure similar to that observed when n = 3: a monomeric, trans-RhN4C1, chromophore, with a hydrogen bond between lhe cis oximes (0-0 distance of 2.459 A).sgs
w
(61a) n = 4
(6Ic) n = 0
(61b) n = 3
(61d) n = 2
1014
Rhodium
The single crystal X-ray structure of trans-methyliodo(d~uoro[3,3'-(trimethylenedinitrolo)bis(2pentanone oximato)]borate}rhodium(III) (62) shows that the Rh sits in the plane established by the four N atoms, with the I and the Me in the axial positions (2.813 A and 2.090 A, respectively). The two six-membered rings resemble a chair conformation, with the BF, bridge bent toward the axial methyl group, and the propylene bent toward the iodine atom.896 F
F
(62)
Complex s of the form [Rh(RBig),CI,]Cl (RBig = N'-(pchlor phenyl), N1-phen ,l, N'-(o-tolyl), N5-isoprop lbiguanide) have been prepared in which the biguanid is presumably ac ing as a N-N bidentate. ! ibstitutions in acidic soiution lead to [Rh(RBig)2X,]X for X = Br, I, NO2, CNS and N,, and oxidation of the dichloro species with NaCIO, is reported to lead to a Rh'" complex [Rh(R3ig),Cl2]C1, .897 Dimethylglyoxime (HON=C(Me)CC(Me)=NOH, abbreviated here as HDMG) forms severaE unbridged Rh" dimers (Section 48.5.2.l.i~)and has a rich chemistry with Co"' (the c o b a l ~ x i r n e s ~ ~ ~ ) ; examples of Rh"' complexes are less common, but interest in its Rh"' chemistry is growing. As in other metal systems, HDMG, or its monoanionic conjugate base (DMG), acts as a bidentate ligand, bonding through the two nitrogen atoms. Dwyer and Nyh01rn'~~ first reported the preparation of [Rh(DMG),Cl,]- ion (isolated as the H' salt), but this formulation was questioned by Gillard et who pointed to 0-H stretches in the IR spectrum, and suggested that while the [Rh(DMG),ClJ anion undoubtedly exists in solution, the solid state species is more accurately represented as [Rh(DMG)(HDMG)C1,].60'An early reportqmthat refluxing HDMG with trans[Rhpy,Cl,]Cl leads to trans-[Rh(DMG),(py)Cl] was also questioned by Gillardm' who found [Rhpy, CI,] as the major product under the cited conditions. In ethanolic solution in the presence of trace amounts of BH; they precipitated the [Rh(DMG),Cl,]- anion, either as the pyH+ or the [Rhpy,Cl,]+ salts. The chloropyridine adduct was eventually prepared by adding H,PO, to a hot suspension of pyH+ [Rh(DMG)2C1,]- in pyridine.601Powell has reported the syntheses of many [Rh(DMG),LX] complexes [L = PPh3, AsPh,, SbPh,; X = C1, Br, I (no SbPh,)], prepared by heating aqueous ethanolic solutions of RhC1,-3HZ0,L and HDMG in the presence of the appropriate potassium halide."' A similar route was used to prepare [Rh(DMG),(Cl)(ZMe,)] ( Z = P or As) and [Rh(DMG),(X)(thiourea)] (X = C1, Br).902 Single crystal X-ray structures of [Rh(DMG),(C1)(PPh,)],9°3 [Rh(DMG),(C1)(SbPh,)]904 and [Rh(DMG)(HDMG)(Cl)(PPh3)]C1CH,C1~Os show that these complexes all share a truns geometry with the rhodium in the plane established by the bidentate DMG ligands. This geometry is characteristic of bis(DMG) complexes (no cis Rh"' examples have been reported) as it allows hydrogen bonding between the oxime groups (43), so that the two DMG ligands may be considered to form one pseudo-macrocyclic dianion. X
I
Y (63)
Aqueous substitutions of [Rh(DMG),Cl,]-. have led to NH,[Rh(DMG),Cl(X)] (X = Br, I, NCS),
and
[Rh(DMG),(thiourea)C1],YW [Rh(DMG),(NH,)C1],907 [Rh(DMG),(H)(PPh3)],908 and
1015
Rhodium
'
I
. I
[Rh(DMG),(Cl)(PPh3)]?09 Reduction of Nyholm's [Rh(DMG),Cl,]- with BH; in alkaline H,O/MeOH (1:l) in the presence of Me1 leads to methylrhodoxime, trans-[Rh(DMG),(H,O)(Me)] (A, ( E ) at 400 (456), 329 (6300), 262 (7690), 225 (1 1 900) nm).89999'0 A recent kinetic study has shown that the methyl group strongly labilizes the trans site?" Anation of methylrhodoxime by N; ,SCN-, I-, py or thiourea is very rapid; about 100 times faster than anations at cobaloxime and six orders of magnitude faster than anations at [Rh(TPPS)(HzO),l3-, another complex with a trans(H,O)Rh(Me) moiety. Activation parameters (for thiourea anation AH* = 32.4 f 1.3 kJmol-', AS* = - 61 f 4.4 J K-' mol-') indicate that the lability is due to the low AH* values. The pK, of the coordinated water (9.78) does not suggest an unusually weak Rh-0 bond, so that the low AH* value must arise from a stabilization of the transition state. The anation rates are practically independent of the nature of the nucleophile, suggesting a dissociative process, but the very negative AS* values suggest a mechanism which is more associative than for the cobal~xirne.~~' Espenson has found that the rhodoxime behaves very much like the cobaloxime in its association with iron(II1) ion to form the bimetallic [Rh(DMG)(DMGFe)(H,O)Me]'+. Its formation constant (equation 178) is 68.8 k 4.0 (for cobaloxime, KF= 15.3 f O.g9l2).This unusual cation can be considered to be derived from the structure shown in (63), in which X = Me, Y = H 2 0 , and one of the hydrogen-bonded hydrogens is replaced by an Fe3+.The presence of the iron adduct is readily signalled by an intense absorption near 440nm ( E x 47000), and a comparison of the kinetic and thermodynamic parameters shows a strong similarity to the previously studied Fe3+ adducts of c o b a l ~ x i m e Even . ~ ~ ~ in the presence of 'forcing concentrations' of Fe3+,there was no evidence of incorporation of a second iron(II1) ion."' [Rh(DMG)2(H20)Me]+ Fe3+
e
[Rh(DMG)(DMGFe)(H,O)Me]'+ -t A+
(178)
Reduction of [Rh(DMG),Cl,]- in basic aqueous ethanol (to an unspecified Rh' complex) is autocatalyzed, with a rate expression of the form: rate = k[Rh111][Rh1].914 The rate of reduction is not affected by added chloride ion, but varies as the inverse fourth (!) power of [OH-]. A mechanism, involving deprotonation of the DMG ligands, and Cl--bridged Rh"'-Cl-Rh' intermediates, has been proposed.914
48.6.3 Complexes of Heavier Group V Ligands There is an extensive chemistry of tertiary phosphine rhodium(II1) complexes. However, there are comparatively few complexes of monodentate tertiary arsines, although the complexes of ditertiary arsines are more numerous. There are virtually no tertiary stibine complexes. The two main preparative routes to the complexes described in this section are: (i) direct reaction ,of the ligands with rhodium trichloride, which usually yields trichloro complexes; and (ii) oxidative addition to rhodium(1) tertiary phosphine complexes, which gives rise to more diverse products of the type [RhXYZ(PR,),]. Metathetical reactions on the Complexes prepared by either method (i) or (ii) have been used to prepare most of the remaining compounds.
48.6.3.1 Anionic complexes The anionic complexes containing phosphorus and arsenic ligands are shown in Table 61. It will be noted that there are no anionic complexes containing antimony ligands. Table 61 Anionic Rhodium(II1) Complexes Containing Tertiary Phosphine or Arsine Ligands Complex
Coior
M.p. ("C)
Ref
Yellow Lemon yellow Red-brown Orange-yellow Orange-yelIow Yellow-brown Pale yellow Yellow-orange
195" 9698 224-26 230-34 206 170"
915 916 916 916 916 916 917 917 917
Brown
Rhodium
1016
The most-studied anionic complex is [AsPh4][RhC14(PPh3),].It results from the reaction between tetraphenylarsonium chloride and tris(triphenylphosphine)rhodium(T) complexes. However, if trans-[RhCl(CO)(PPh,),] is selected as the starting material then the major product is [RhCl, (CO)pPh,), 3. The less stable tetramethylammonium salt can also be prepared from trans[RhCl(CO)(PPh,),] (equation 179).
fr~ns-[RhCl(CO)(PPh,)~l + (Me4N)Cl+ HCI
M5CO
(Me4N)[RhCl4(PPh3),1
(179)
The reactions of [ A s P ~ ~ ] [ R ~ C I , ( P P are ~ ~shown ) ~ ] in Scheme 35; both the chloro and triphenylphosphine ligands of the anion can be substituted by ligands of similar character. For the thiocyanato complex vc has been found to be 2105 crn-’?” Under a nitrogen atmosphere, neat dimethylphenylphosphine slowly displaces the PPh3 ligands from the complex. The anion [RhB,(PMe,Ph),]- has also been prepared in a 2% yield from RhCl,*3H,O and PMe2Ph?ls RhCI, . 3 H 2 0
[ RhCI(PPh, )3]
[PHMe,Phl [khCI,(PMe,Ph),]
[ AsPhql [Rh(NCS),(PPh,),]
i, [AsPh,]CI.HCI, Me,C0:’6
[AsPhd [RhBr4(PPh3)]
[ AsPh4j [ RhCI,(PMe,Ph),]
ii, PMe,Ph, (a) EtOH, (b) Me,C0;91s iii, LiNCS, Me2C0;916iv, LiBr, Me,C0;916v, PMe,Ph, N,, vi, [AsPh,]CI, EtOH’’’
Scheme 35 Preparation and reactions of anionic rhodium(II1) complexes
It is generally considered that the chloro ligands in the above complexes are not lrans to the tertiary phosphine ligands by virtue of vRh...,-, in their far IR spectra being above 300cm-‘. The PMe,Ph complex has a 3’PNMR spectrum consistent with trans tertiary phosphine ligands?’* The PMe,Ph ligands in the bis(dithiocarbonat0) complex K[Rh(S2CO),(PMe,Ph),] have been shown to be trans by X-ray cry~tallography.~’~ The rhodate(II1) salt was prepared by allowing RhCl,(PMe,Ph), to react with potassium O-ethyldithiocarbonate in refluxing If the thioarsine ligand (64) is allowed to react with aqueous rhodium trichloride then the tetrachlororhodate salt is formed. A similar reaction gives rise to the tetrabromo complex (equation 180). The tetraiodo complex can be prepared by addition of LiI to sodium hexachlororhodate(II1) and the ligand (equation 181).9’6
RhX3*3H,0+ (64) Na3[RhC1,]+ Lil
48.6.3.2
+
HX(aq)/EtOH
(64)
’
H 0 EtOH
[RhX*(64hl[RhX,(64)l [RhI,(64),][Rh14(64)]
[RRX, L] complexes
This is a rare Stoichiometry which can be achieved by the coordination of two bidentate anionic ligands. The most authentic examples of this stoichiometry have been prepared by oxidative
Rhodium
1017
addition to rhodium(I) complexes. Three bis( 1,3-diaryltriazenido) complexes have been prepared by oxidative addition of the appropriate diaryltriazene to RhCl(PPh,), under mild conditions (equation 182). [RhCI(PPh,),]
+ ArN3Ar
[RhCI(ArN,Ar),(PPh,),]
(182)
Ar = Ph, p-C,&Me, p-C6H4C123’
The reaction of RhCl(PPh), with pentane-2,4-dione (equation 183) is more complex, some An even trans-[RhCl(CO)(PPh,),] is formed as a result of carbonyl abstraction in a side rea~tion.~” more complex reaction results from the trapping by RhX(PPh3), (X = C1, Br) of the SO moiety produced in the decomposition of stilbene episulfoxide (65; equations 184). The dimeric product is unstable in the presence of PPh, and is reduced to the starting material (equation 185).921 PfiCH-CHZ
v 0
(65)
[RhC1(PPh3),] [RhX(PPh,),]
+
CHICIZ
(65)
+ acacH
+
[RhX(SO)(PPh,)],
[RhX(SO)(PPh,)]2
+
8PPh3
4
[RhCl(acac),(PPh,]
+
OPPh,
20PPh3
+ SPPh, + + 2SPPh3
PhCH=CHPh
(184)
(185)
An even more bizarre example of this stoichiometry is provided by the two dithiophosphorus complexes [Rh(S2PPh,)3PPh,] and [Rh{S,P(OEt),),PPh,]. These have been prepared from uncharacterized rhodium(I1) substrates by allowing the latter to react with PPh3 and the sodium salt of the dithioacid. In these two complexes the dithioanion trans to PPh, is monodentate. The same nominal stoichiometry is adopted by a few tetrafluoroborate salts of cations which contain similar bidentate thioanions; however, these cations are presumably five-coordinate having the formula [RhX,(PPh,)]+ (X = S2CNMe,; XH = 2-mercaptobenzothiazole, 2-mercaptopyridine, toluene-3-thiol). *”
48.6.3.3 [RhX,&,,] complexes Complexes adopting this stoichiometry are: (i) genuine five-coordinate complexes, or (ii) six-coordinate complexes containing either a bidentate anionic ligand, or (iii) an uncharacteristically bidentate neutral ligand. The final type of complex in this class is the six-coordinate binuclear Examples of the first three types of complex are: (i) complex such as [R~CI,(PBU,)~]~. [RhHCl,(PR,)2], (ii) [RhHz{S(NPh)CHMe2}(PPh,),], and (iii) [RhCl, { Me,As(o-C,H,OMe)),].
(i)
Five-coordinate complexes
The many hydrido and dihydrido complexes are a feature of this class. Their coordinative unsaturation undoubtedly arises from the high trans effect of their hydrido ligands. In this sense they can be regarded as the rhodium(II1) analogs of the [RhHLJ complexes (Section 48.4.2.1). A lesser number of five-coordinate complexes containing bulky ligands are also known. This latter type can be regarded as the rhodium(II1) analogs of the [RhXLJ complexes from which most can be obtained by suitable oxidative addition reactions. The dihydrido complexes (Table 62) can be obtained by the oxidative addition of molecular The tri(t-buty1)phosphine complexes hydrogen to rhodium(I) complexes (equation 186).’0*”9,922-926 can be prepared either from the chlororhodium(1) complex,923or rhodium trichl~ride.~~’ The former method seems more reliable since the latter reports the complex as a matt green substance, a color uncharacteristic of tertiary phosphine rhodium(II1) complexes. Indeed, Masters and Shaw report that the related tertiary phosphines PBu:R (R = Et, Pr) give green rhodium(I1) complexes in this reaction (see Section 48.5.2.1 The X-ray crystal structure of [RhH,Cl(PBu~),J(66)has been determined.923The space group r e ~ o r t e d for ~ ~ *TRhDJPPh,hl. P,,IC.differs from the C,/c of the former comolex.
Rhodium
1018
Table 62 Physical Properties of [RhH,XL,J Complexes ~
-
~~
M.p. ("C)
Color
Complex
-
Yellow Pale yellow Pale yellow Pale yellow Pale yellow Yellow Yellow Yellow Yellow Red
a
vcpN
rRh-H(p.p.m.)
14749 104-06 13642 93-96 168-72'
Red
-.
Red Orange Yellow Yellow
-
23.6 32.7 28.2, 21.5, 19.8 28.2, 21.5, 18.8
-
160-76 -
32.3 29.0, 22.1 27.9, 19.9
-,Y
H(cm-')
2090, 1940" 2085 2070 2055 2070, 2030 2220 2165, 2120b 2078, 2013 2177, 2057 2100,2040 2140, 2030 2115, 2000 2212, 2137 2051 -
Ref. 1I9 922 922 922 922 923 924 10, 925 IO 926 926 926 269 140 140
2110cm-'. bvRhpo 1560, 1528cm-'.'With decomposition
[RWPR3)31
+
Hz
R = Ph, X = CN,'I9C1, Br, I;"'
R
=
-
R
=
[RhHzX(PR,),l + PR, But, X = Cl4'j; R = Cy, X = C19";
(186)
pC,H.,Cl, X = CI, Br, 19'j
The dihydrido complex [RhW,Cl(PPh3),] is a very important intermediate in the homogeneous catalytic hydrogenation of alkenes." The monohydrido complexes (Table 63) can be made by the oxidative addition of HY species to rhodium(1) complexes (equation 187). Similar complexes can be obtained when bulky tertiary phosphines are allowed to react with alcoholic solutions of hydrated rhodium t r i ~ h l o r i d e ? ~ ~ * ~ ~ ~ PBu'
Table 63 Physical Properties of [RhHX,L,] Complexes Complex
[RhHCl?(PPh, )z ] OSCH, C1, IR~H'AG'CY,):I [RhHCI, (PBu'Pr, ),I [RhHCl,(FBu', Me),] [RhHCI,(PBu', Et),] [R~HCI,(PBU',P~)~] [RhHBr,~Bu'Pr,),] [RhHBr,(PBu', Me)?] [RhHBr, (PCYd2l [RhHC1(67 - H)(PPh,)z] [ u H C l ( a - H)(PPh,)zl [RhHBr(67 - H)(PPh,),] [RhHBr(68 - H)(PPh,),] [RhHCI(HS)(PPh,),] *O.SCH,Cl, [RhHCl(p-SC,H, Me)(PPh,),] [RhHC~~rWY3)21 [RhHCWCY,)Zl [fiHfJ, (ASPh3)21 [RhHC12(SbPh,h] a With
decomposilion.
Color
Yellow Orange
Red Red Red Khaki Dark red Dark red Orange Yellow Orange
Red Yellow Yellow
M.p. ("C) 116-18" 122-25" 171)-90" 15742" 17590" 161-71" 150-70" -
131-32 115-16 134-35 103-04 116-18" 160 -
-
7Rh-H
@.p.m.>
26.1 40.5 41.37 41.40 41 .IO 41.10 41.10
v , - ~ (cm-')
Re$
2160 1943 1945 1938 1940 1936 1946 1938
930, 931 932 2 69
-
2120 2080 2125 2080 2160 21 19 2069 2014
269 269 269 269 269 924 933 933 933 933 929 929 924 924 140 140
1019
Rhodium [RhX(PR,),] R
=
Ph, X
Y
=
+ HY
C1, Y
= (67),
=
[RhHXY(PR,),]
4
+ PR,
C19M,93’, CN, p-MeC6H4S929; X
=
(187) CI, Br,
(68)933; R = Cy, X = C1, Y = C1, Br, 1924
Usually the monohydrido species have the square pyramidal structure (69),268although the populous and important hydridosilylrhodium(II1) complexes have been shown to adopt the trigonal bipyramidal structure (70) in the solid state.’” H
(69)
(70)
The five-coordinate monohydrido complexes shown in Table 63 are less reactive and less important catalytically than their dihydrido analogs. One of the few reactions noted is the addition of pyridine to the tricyclohexylphosphine complexes to form six-coordinate products (equation 188).924 Six-coordinate complexes can also be obtained if the preparation of [RhH, Cl(PPh,),] is carried out in coordinating solvents.” [RhHC12(PCY,hI
+
-
PY
(lW
[RhHC1,(PCY,),PYJ
By contrast the silyl complexes are important catalysts in a variety of hydrosilylation reactions.” They can be prepared by the similar oxidative addition of hydrosilanes to rhodium(1) complexes, However, in the presence of both additional base and either directly or in solution (equation 189).935 triphenylphosphine a hydridorhodium(1) complex results (equation 190)?36 Alternatively in the presence of a large excess of hydrosilane the monohydrido complex is transformed into a dihydrido complex.922 [RhCl(ZPh,),]
+
SiHR,
+
[RhHCI(SiR,)(ZPh,),]
+
ZPh3
(189)
Z = P, As, Sb;X = Cl, Br, I; R = C1, OEt
[RhCI(PPh,),]
+ Et3SiH + EtjN
PPh3
[RhH(PPh,),]
+
Et,SiCI-NEt,
(190)
The complexes produced, although five-coordinate, often contain hydrosilane of crystallization. This additional hydrosilane is not associated with Despite the widespread use of such hydridosilyl complexes in hydrosilylation reactions, only a fraction of those employed have been isolated and characterized (Table 64). Solutions of the complexes are unstable due to loss of the silyl ligands, which are also readily displaced by other ligands. In the solid state the [RhHCl(SiX,)(PPh,),] complexes are more stable and only decompose upon heating in vucuo above 100 “C (equation 191).939 The thermal stability of the complexes increases with increasing electronegativity of groups bound to silicon, but decreases with increasing size of tertiary phosphine ligands. Some of the reactions of the silyl complexes are shown in Figure 4. 2[RhHCl(SiR,)(PPh,)2]
2SIHR3
+
+
[RhCl(PPh,),],
(191)
The analogous hydridogermyl complexes may be prepared similarly using hydro germane^.'^^^^' However, a 1Cfold excess of hydrogermane forms anionic complexes (equation 192). The germy1 complexes have a similar structure to their five-coordinate silyl analogs, and, despite being less extensively dissociated in solution, undergo many of the same reactions. Thus, they are decomposed by either carbon monoxide or ethylene (equation 193). [RhCl(PPh,),]
+
14GeHC1,
[RhHCl(GeX,)(PPh,),]
+
-+
L
[Ph,PH][Rh(GeC1,)6] +
trms-[RhCl(PPh,),L]
L = CO, C,H,
C.O.C.-LGG‘
+ H2 + HCl +
GeHX3
(192) (193)
Rhodium
1020 Table
64 Physical Properties of Five-coordinate Hydrido Silyl and Germy1 Complexes
Complex [RhHC1(SiMe3)(PPh3),y [RhHCl(SiEt,)(PPh,),] [RhHCl(SiPh, )(PPh, ),I [RhHCl{Si(OEt), )(PPh,),] [RhHCI(SiCl,)(PPh,),]" [RhHCl(SiClMe,)(PPh,),] [RhHCl(SiClEt, )(PPh, ),I [RhHCI(SiC1, Me)(PPh,),]" [RhHCl(SiCI, Et)(PPh,),]
[RhHCl(Si(OEt),}(PPh,Pr'),] [RhHCl(SiEt,)(PPh,Cy),] [RhHCl(Si(OEt), )(PPh,Cy),] [RMCl{Si(OEt), )(PPhCy,),] [RhHCI{Si(OEt), )(PPh, (p-C6H, Me)},] [RhHBr(SiEt,)(PPh,),] [RhHBr{Si(OEt), }(PPh, ),I [RhHBr(SSCl, )(PPh, ),I [RhHBr(SiClMe,)(PPh, ),I [RhHBr(SiClEt,)(PPh, ),I [RhHBr(SiCl, Me)(PPh,),] [RhHBr(SiCl, Et)(PPh,),] [RhHl{Si(OEt), }(PPh,),l [RhHI(SiCl, )(PPh,),]
[RhHC1{Si(UEt),}(AsPh3),] [RhHCl(SiCI,)(AsPh, [RhHCl{Si(OEt), }(SbPh,),] [RhHCl(SiCI, )(SbPh,),] [RhHCl(GeMe, )(FPh, ),I [RhHCl(GeEt,)(PPh,),] [RhHCl(GeCl,)(PPh, ),I [RhHCl(GeMe, )(AsPh, ), ] [RhHCl(GeEt, )(AsPh, ),I a
Color
hf.p.("C)
Yellow Yellow Yellow Yellow Bright yellow Yellow Yellow Bright yellow Bright yellow Yellow Yellow Yellow Yellow Yellow Yellow Bright yellow Orange-yellow Bright yellow Sright yellow Bright yellow Bright yellow
92-97 103-07 133-37 127-32 168-73 123-28 142-47 143-47 15843 126-32
Orange Orange Yellow
Yellow Gray Orange-yellow Yellow Yellow Orange-yellow Green Green
-
145-50b 132-36 122-28b 108-12 127-32 165-70 128-33 143-47 147-52 155-60 125-30 163-67 14247 178-82 152-57 183-88 90-94b 117-19b 148-52b 90-99 118-21b
~ ~ - ~ ( P . p . r n . ) vRh-&m-') -
24.30 24.8
-
24.45
23.2 -
2055 2020 2140 2060 2040 2060 21 10 2050 2130 I
-
~
24.6, 23.6 23.40 23.8 -
23.1 21.70 26.3 -
21.3
I
-
-
2070 2060 2120 2050 2130 2045 2130 2060 2050 2065 2080 2080 2080 2080, 2035 2107,2062 2123, 2098 2057, 2025 21 14, 2042
Ref:
937 937 937 937 937 937 937 937 937 938 938 938 938 938 937 935,939 937 937 937 937 937 937 937 937 937 937 937 940,941 940,941 940,941 940,941 940,941
Dichloromethane hemisolvate also known?39 With decomposition.
[ KhBrQi(OEt),l(PPhl)~]
t
Figure 4 Reactions of [RhHCl(SiX,)(PPh,),] complexes
Many rhodium(1) tertiary phosphine complexes react readily with molecular oxygen, although it is not always easy to characterize the products of these reaction^?^^,^ The complexes isolated are listed in Table 65. One problem is that coordinated oxygen oxidizes the tertiary phosphine ligands to form free tertiary phosphine oxide. Often the course of this reaction is solvent dependent; in ethanol the phosphine oxide is more likely to be formed than in hydrocarbon s ~ l v e n t s The . ~ ~ oxidation of ligands also occurs in the solid state reaction.942
Rhodium
1021
Table 65 Physical Properties of [RhXO,L,]Complexes
Complex r ,
Color
[RhCN(O,)(PPh,),l [RhCI(O, )(Pph3121 cis-[Rh(02)(S(NPh)(CNMe,)}(PPh,),] trans-[Rh(O,){ S(NPh)CNMe,)}(PPh,),] trans-[Rh(O,){ Me,N(CS)N(CS)NMe2}(PPh3)21 cis-[Rh(O,)(S, CNMe,)(PPh,),] truns-[lZh(O,)(&CNMe,)(PPh, ),I cis-[Rh(O,){Ph, P(O)(CS)NPh)}(PPh,)2]
v o ~ o ( c m-') L
Ref
9oD
930 119 942 942 942 942 942 942
Light brown
. 870
Green -
870 870 878 866 869
Ocher Ocher -
Green-yellow -
One of the best studies on the oxidation involves the rhodium(1) complex [(Ph,P),RhS(PPh) C=NPh]. These reactions are shown in Scheme 34. The N,N-dimethyldithiocarbamatocomplex merely undergoes oxidative addition.
Scheme 36 Reactions of [Rh(S(PPh,)CNMef(PPh,),] with 0, in the presence of PPh,
The reaction of dioxygen with [RhCl(PPh,),] has long been k n o ~ n ? ~The ~ cyano ~ " ~ analog ~ ~ reacts ~imilarly."~However, the product from the latter reaction may be six-coordinate with bridging dioxygen ligands since both the chloro complex and its dichloromethane solvate adopt structure (71)."6-947 The chlorobis(tripheny1phosphine) complex is not the sole possible product; under slightly different conditions [RhCl(O,)(PPh,),] is formed (see Section 4S.6.3.5). Most of the dioxygen complexes exhibit bands between 871 and 900 m-' in their IR spectra, which have been assigned to v ~ - ~ . There remain a number of other five-coordinate complexes whose physical properties are shown in Table 66. Of these complexes only the AsCy, complex has been prepared from a rhodium(II1) source (equation 194). The remainder have been prepared from oxidative addition reactions of [RhCl(PPh,),]. The arylazo complexes have structure (75),948whilst the sulfinato complexes are bound through
PPh3
cwl
s-s (73)
c1\
,NAr
/Cl
s-s
(74)
(75)
(ii)
Six-coordinate complexes
The complexes which contain a bidentate anion are listed in Table 67. Two preparative routes exist to these complexes. One involves oxidative addition of two monoanions to a rhodium(1) complex containing a bidentate anion. These are usually complexes containing a disulfur anion (equation 195).65 Dibenzoylmethane, in contrast to pentane-2,4-dione (see Section 48.6.3.2 above) adds in a similar fashion (equation 196).920 [Rh(S,PR,)(PPh,h]
+ R
[RhCI(PPh,),]
Iz =
+ (PhCO):CH?
[RhIz(SzPR,)(PPh,)~l
(195)
Cy, OPh --+
[RhCI,{(PhCO),CH}(PPh,),]
Table 67 Physical Properties of Six-coordinate [RhX3L,] Complexes Complex
Color
Yellow W H , (BH4)(PCY3),1 [RhH2(BH4)(PBut2Me)21 White Orange-red IRhH2 (PhN3Ph)(PPh3),I LRhH2 {(p-MeC6H4)2N,}(PPh3)21 Brown Green LRhHz {(P-ClC&4)2N3 J(PPh,),l [RhH, {(p-MeC6H4)N,(p-C6H,0Me)}(PPh3)2]Brown Yellow [RhHCI(CN)(PPh,),(WCN)] [RhC12 121 [RhBr,(NO1)(PMePh,),l [RhC12 {(PhCO)2CH)(PPh,),l [RhHC1(2-SC,H4PHPh)(PPh, j2] Yellow-brown ~rum-[RhCl,(S,CNMe,)(PMe,Ph),j Orange Orange cB,cis,cis-[RhCl,(S, PMe, )(PMq Ph),] cis,cis,cis-[RhCl,(S,PPh,)(PMe,Phk] Orange rner-[RhCl2(S,PMe2)(PMe,Ph),] Orange trans-[RhC1,(S2PMe,)(PMe2 Phh ] Orange !rans-[RhCl,(S,PPh,)(PMe, Phh] Orange trans-[RhCl2(S, COEt)(PMe2Ph)?] Orange cis-[Rh(S,CO)(S, COEt)(PMe, Ph), ] Yellow trans-[Rh(S, CO)(S,COEt)(PMe, Ph),] Yellow Brown [RNz(S,PCY,)(PPh3)21 Brown [RhI, {S2P(OPh)21(PPh,)21 [RhCl,(NO, )(AsMePh,),] [RhBr,(NO,)(AsMePh,),] I
a With
decomposition.
M p . ("C)
Ref.
-
924 924 231 23 I 23 1 23 1 929 953 953 920 954 915 915 915 915 915 915 915 915 915 65
144-47 1 0 6 11 142-45 107-14 18C90 -
25&55 2 18-20 20748 184-86 208- 10 14W2" 235-37" 228-29" 155-57 172-73" 150-53" -
-
65 953 953
Rhodium
1023
It is possible that the nitrato complexes [RhX,(NO,)(PMePh,),], prepared by the decidedly hazardous addition of concentrated nitric acid to ethanolic solutions of [RhX,H(PMePh,),], are also six-coordinate but the IR spectrum of the nitrato ligand reputedly gave no clue to its denticit y .953 Numerous complexes containing bidentate 1,3-diaryltriazenido ligands have been prepared by Robinson and his co-workers. These complexes have been prepared by allowing the 1,3-diarylor directly triazene to react with a rhodium(1) complex under mild conditions (equation 197)231,955 from RhC13-3H20?31The structure (76)has been deduced from the ,'P and high field ' H NMR ~pectra.'~' [RhH(PPh,),]
+
ArN, Ar
-
[RhH2(ArN~Ar)(PPh,),l
(197)
Ar = Ph, p-C6H,Me, p-C,H,Cl
Besides those complexes which contain bidentate anionic ligands there are also complexes which contain one bidentate neutral ligand. Very frequently this type of complex contains two identical neutral ligands one of which is monodentate and the second bidentate by virtue of complexing to rhodium through a second functional group on the ligand. The tertiary phosphine Ph,P(CH, COOEt) has been shown by Braunstein and his co-workers to form a rhodium(II1) complex when allowed to react with rhodium trihalides. The products are the facial isomers (77). This is in accordance with their NMR spectra which show 'J(P-P) = 23.5 Hz.956 c1
This behavior is in contrast to that of the bulky ligands Bu;P(CH,),COOEt (n = 2,3), which form rhodium(I1) complexes under these conditions,27sand the cyanophosphine PPh,(CH,CN), which forms mer-[RhCl,(PPh,(CH,CN)),].956 When hydrated rhodium trichloride is allowed to react with (78;Y = OMe, N M q ; Z = As), complexes of empirical formula [RhCl, (78),] are obtained. The orange-red dimethylaminophosHowever, one chloro phine complex has been shown to have the meridional structure (79).957-959 ligand is labile and can be replaced in the inner coordination sphere by the previously uncoordinated nitrogen atom of the second neutral ligand. This process is enhanced by allowing the complex to react with metal salts whose anions are of low coordinating power (Scheme 37).957
(78)
(79)
The dark red o-anisyl complex can be prepared similarly. It has also been shown to have a meridional structure by X-ray crystallography.*%' It too reacts with silver nitrate to form an ionic complex in which all four potential donor atoms of the two neutral ligands coordinate to rhodium.960
Rhodium
I024
NaBPh,
Scheme 37 Reactions of mer-[RhCI, {AsMe2(n-C,H,NMe,)},]957
The kinetics of the isomerization (equation 198) and analogous reactions have attracted much a t t e n t i ~ n . It ~ ~is~not - ~ ~surprising ~ that the rate of the forward reaction is increased in polar solvents.969
MezAsR -2. 7
C1
(198)
The antimony ligands (78; Y = OMe, NMe,; Z = Sb) also form [RhX,(78),] complexes when allowed to react with hydrated rhodium trihalides, except that rhodium tribromide and the anisyl ligand form the complex [RhBr3(78),]. These reddish complexes presumably have similar structures to those of their well-characterized arsenic analogs. Whilst there is no evidence that loss of a halo ligand occurs in 1,2-dichloroethane, silver nitrate displaces one halo ligand. Presumably the transdihaIo nitrate complex is This behavior is confined to antimony ligands of type (78). The three ligands Ph, Sb(o-MeOC,H,), MeSb(o-MeOC,H,), and Me, Sb(o-MeOC,H,) all form [RhC13L,] cornplexe~.~~'
(iii) Dinuclear complexes
These complexes are produced when rhodium trichloride is allowed to react with two, or less, equivalents of tertiary phosphine or arsine (equation 199). Their physical properties are listed in Table 68, and the complexes have been shown to adopt the non-centrosymmetric structure of the tributylphosphine This is in agreement with the dipole and 31PNMR
I
1025
Rhodium Table 68 Physical Properties of [RhX3L2I2Complexes Color
M.p. ("C)
ReJ
Red-orange Red-purple Orange-brown Red-brown Brown Red Red Yellow Red-brown Orangebrown
25&54 248-61" 18&92 172-80" 14650 205-10 220-25 27 I 1 68-7 1 240-56
114 972 972 972 972 114 114 973 972 972 974 975 972 976 976 976
Complex
-
I
225
Yellow Red-brown
248-55" 220" 244-46" 228-30"
Red Red Dark brown a With
decomposition.
complex. However, working up solutions of this complex causes it to disproportionate (equation 200).979 RhC13.3HIO -t 4ZR3 Z
=
P,
R3 =
----+
[RhCl,(ZR3)z]z
(199)
Et3, Pr3, Bu3, (C5H11)3,972 EtPh,973;Z = AS, R3 = Et?'' CH2 Ph974
-
+ rner-[RhCI, (PBu,),]
~ [ R ~ C I , ( P B U ~ ) ~ ] ~[Rh2C16(T'Bu3)3]
(200)
The trihalobridged structure (81) has been confirmed by X-ray c r y s t a l l ~ g r a p h yThe . ~ ~ disposition of each phosphorus atom trans to a different bridging chloro ligand is reminiscent of the structure of the [Rh2Cl5{Ph2P0)2H2]-anion (82)?*'
(81)
(82)
The trihalobridged species can also be obtained by allowing rhodium trichloride to react with 0.92 equivalents of tertiary phosphine. The trihalobridged complexes revert to dihalobridged complexes in the presence of more ligand (equation 201). [Rh,Cl,(PBu3)3]
+ PBu~ +
[KhCl3(PB~j)2]2
(201)
An unstable tri(perfluoropheny1)phosphinecomplex can be obtained by oxidative addition of chlorine to the dimeric rhodium(1) complex. The ethyldiphenylphosphinecomplex prepared in this way is more stable (equation 202). [RhCl(PRI1212
+ c1,
-
[RhCl, (PR, 1212
K3 = (CgF5)3,73EtPh2972
(202)
Rhodium
1026
Apart from the t r i e t h y l a r ~ i n and e ~ ~the ~ triben~ylarsine~~~ complexes the remaining tertiary arsine complex is formed by the ditertiary arsine 1,2-(Ph,As),C,H,; clearly this cannot adopt structure (80). However, the ditertiary stibine complexes976may well adopt structure (80), since their stoichiometry implies that only half the antimony atoms are coordinated to rhodium.
48.6.3.4 [RhH, XL,] and [RhHX, L 3 ] complexes The dihydrido complexes, despite their catalytic potential, are sparsely represented. It has been shown by X-ray photoelectron spectroscopy that the interaction of hydrogen with solid [RhCl(PPh,),] yields the complex [RhH2Cl(PPh3)3]+y82 This complex has also been detected in solutions containing excess PPh, and its NMR spectrum measured.983It was noted in Section 48.6.3.3 (i) above that [RhH,Cl(PPh,),] reacts with coordinating solvents to give complexes of this stoichiometry." The lability of the third triphenylphosphine ligand has also enabled the a-methylbenzylamine complex [RhH,Cl(PPh,),(PhCHMeNH,)] to be prepared and used as a chiral hydrogenation The smaller PEtPh, ligand is less labile and [RhH,Cl(PEtPh,),] can be prepared by oxidative addition of H, to [RhCl(PEtPh,),] in the presence of excess In the monohydrido complexes the neutral ligands adopt the mer configuration, thus giving rise to two isomeric forms. The first of these, designated a (83), has the hydrido ligand trans to an anionic ligand; the isomer (84) has the hydrido ligand trans to a neutral ligand. The physical properties of the two series are listed in Tables 69 and 70, respectively. L
L
L
L
(83)
(84)
It is not surprising that the a isomers are the more stable, given that the neutral ligands are invariably higher in the trans effect series than halogeno ligands. The isomerization from fl to c( isomers is accelerated by light or by heating, but it is inhibited by the presence of excess neutral ligand. It seems probable, therefore, that a dissociative mechanism i s involved.'s6 The ligand trans to the hydrido ligand influences the properties of the latter. The rhodiumhydrogen bond is weaker in the isomer than in the tl isomer, vRh .H being typically some 100cm-' lower in the latter (Tables 69 and 70). The 'HNMR spectra of the isomers are quantitatively similar, consisting of a pair of double triplets in both cases. However, in the spectra of the c1 isomers all the J(H-P) values are in the range 9-22Hz, whereas in the spectra of the p isomers the whole spectrum is displaced to lower fields and J(I--P) is about 200 Hz. This is indicative of the hydrido ligand being trans to the unique phosphorus ligand.gs6 The complexes have been prepared by several methods. When ethanolic KOH and rhodium Table 69 Physical Properties of a-[RhHX, L,] Complexes
[RhHCl, (PPh3)a 1 [RhHCI,(PMe,Ph),] [RhHCl, @ ,E t' Ph), J [RhHBr, (PEt, Ph),] [RhHCl,(PMePh,),] [RhHBr,(PMePh,),] [RhHCl,(PEtPh,),] [RhHBr,(PEtPh,),] [RhHI,(PEtPh,),] [RhHCl,(AsMePh,),] [RhHBr,(AsMePh,),] [RhHI,(AsMePh,),] [RhHCl,(AsPh,Pr),] [RhHBr,(AsPh,Pr),] [RhHI,(AsPh,Pr)J a
vRb-,,
Yellow Yellow-orange Yellow Yellow ~
Yellow Yellow -
Yellow Orange-yellow Orange-brown Yellow Orange-yellow Brown
160" 16M.S 14M5
2220
-
167-70 183-87 210' -
172-75 175-78 164 14548 141-45 127-29
1485cm-I. vRhPD 1483crK'. With decomposition.
-
2120 2110 2100 2077" 2073b 2058 2107 2107 2090
973 1 I4 114 973 984 984 973 973 973 985 985 985 986 986 986
Rhodium
1027
Table 70 Physical Propcrties of &[RhHX,L,] Complexes Complex
Color
M.p. ("C)
Jfm--H(m-'1
Re$
[RhHC12(PMe,),l [RhHC1,(PEt, 1 3 1 [RhHC12(PPh,),l [RhHCI, (PMe,Ph),] [RhHCl, (PMePh,),] [RhHCI,(PEtPh,),] [RhHBr, (PEtPh,),] [RhHI, (PEtPh:),] [RhHCI,(AsMePh, ),] [RhHCI, (PBu' Pr:),(CNMe)] [RhHCI, (PBu'Pr2),(NCMe)] [RhHC12(PButPr2)*PY1 [RhHCl,(fBu'Pr,), (P(OMe),}I
Yellow Yellow-orange Pale yellow Yellow
210-19 1w220 100" 2 I 5-20 15841 18&85
-
1 I4 114 973 114 984 973 973 973 984 269 269 269 269
a With
-
Yellow Yellow Orange Orange Orange Orange
-
-
1982 1964 1960
15841
-
124-31
-
99-105 122-26 85-100
-
decomposition
trichloride are allowed to react with PEtPh, at room temperature a-[RhHCl,(PEtPh,),] is formed.987 Piperidine/ethanol mixtures have also been used as the solvent.988Prolonged refluxing of the solvent results in carbonyl complexes being isolated. Phosphinic acid has been used as the source of the hydrido ligand in other reaction^.^^^^^^^ Zinc dust has also been used as the reducing agent.990 Alternatively, a-[RhHCl,(PMePh,),] has been synthesized by the oxidative addition of HCl to the rhodium(1) complex (equation 203).y86However, treatment of rhodium(II1) complexes with phosphinic acid yields the p isomers (equation 204).'86
+ HCI + c~-[RhHCl(PEtPh,)jl mev-[RhX,(ZMePh2),] + H,P02 + b-[RhHX2(ZMePh,),] [RhCl(PEtPhZ),]
Z
=
P, X = C1, Br; Z
(203) (204)
As, X = C1, Br, I
=
The tertiary arsine complexes were first prepared by Dwyer and Nyholm,'but, before the advent of NMR spectrometry, were erroneously considered to be rhodium(1I) complexes.989Further investigations have shown that in preparations starting from rhodium trihalides using a higher tertiary arsine: rhodium ratio and lower reaction temperatures favor the formation of the j isomer (equations 205 and 206).986 RhX,*3Hz0
+
RhX,-3H20
ErOH'H20
2AsEtPhz
+
,8ec
P
%-[RhHX,(AsELPh,)3]
5 &[RhHX2(AsEtPh2)J
3AsEtPh2
(205)
(206)
Few studies have been made of the reactions of this class of complex. The action of bases upon the complexes brings about reductive elimination of hydrogen halide (equation 207).973However, if this reaction is carried out in dichloromethane it appears that the hydrido ligand reduces the solvent since, although the base hydrochloride is formed, a rhodium(II1) complex results (equation 208).953 also produces hydrogen halide (equations 209 and 210). (The The action of bromine989or reaction with concentrated nitric acid has already been mentioned in Section 48.6.3.3.ii above.y53) ~t-[RhHCl*(PEtPh2)3] E-[RhHCl(PEtPh,),] [RhHBr(AsMePh,),] [RhHX,(ZMePh,),]
+
+ Br, + SO2
PhNHZ
UH2Cll
-
base
CI12C12
[RhCl(PEtPh*),]
(207)
[RhCl:,(PELPh:)3]
[RhBr,(AsMePh,),]
[RhX(ZMePh,),(SO,)]
+ HBr + HX
(208) (209)
(210)
X = C1, Br; Z = P, As
It has been shown that strongly coordinating ligands will displace one tertiary phosphine ligand from j complexes (equation 21 1). The products have been assigned structure (85).992 P-[RhHCl2(PButPr2),]
+
L
-+
fi-[RhHC12(PBu'Pr2)2L] + PBu'Prz
L = MeCN, MeNC, py, P(OMe),
(21 1 )
Rhodium
1028
YR3
PR, (85)
48.4.3.5
[RhX, L3] complexes
This is a populous class of complex: most known tertiary phosphines seem to have been utilized in preparing at least one complex of this type. Meridional isomers are normally the rule, but small tertiary phosphines are also capable of forming facial isomers. As might be expected, the structures of both isomers are essentially octahedral. However, even in the mer complexes, where steric repulsions between tertiary phosphines are minimized, there are slight deviations from orthogonafity. Further, in the mer complexes the length of the Rh-P bond trans to X is less than the length of the mutually trans Rh-P bonds. Both these features are apparent from the X-ray crystal structure of mer-[RhCl, (PEt,)3].991 The crystal structure of rner-[RhCL,(PBu;), {(POMe),}] has also been determined, and is in accordance with that predicted from its "P NMR spectrum.992The facial and meridional complexes that have been characterized are listed in Tables 71 and 72 respectively.
(86)
Both far TR and "P NMR spectrometry have been widely used to determine the configuration of the compIexes in solution. Generally mer complexes have three bands arising from rhodiumhalogen stretching, whilst thefac isomers only exhibit two such bands in their IR spectra. Such features are also apparent in the solid state Raman spectra of the complexes.998Early assignments of 1R94*949 or Ramang9*bands in the 300 cm- region to vRhpCl are probably erroneous, since bands in this region are found in the spectra of the analogous tribromo or iridium complexes. Goggin and Knight assign these bands in the spectra of [MX,(PMe,),] complexes as PC, asymmetric deformation modes.lW The P NMR spectra have been recorded with increasing precision with the passing years.918,9!?3.1DOl,lDOZ The work of Shaw's group is regarded as being more accurate since random noise decoupling was employed to enable certain Rh-P couplings in the meridional complexes to be determined.1002 The spectra of certain trimethylphosphine complexes have also been studied in some detail (see Table 73).1003,1004 Both 'HNMRIW5and EPRIW6spectrometry have been used to show that [RhCI, (PEt2Ph)3J adopts the meridional configuration. The X-ray photoelectron spectrum of fuc-[RhCI,(PMe,Ph),] Table 71 Physical Properties of fac-[RhX, (PR,),] Complexes Complex
[RhCl,(PHPhd3I [RhC13(PMe31 3 1 [RhCl, (PEt, 1 3 1 [RhCI, (PMe,Ph),] [RhCl, (PEt*Ph),j [RhCl, (PPhPr,),] [RhCI,(PBu,Ph),] [RhCl, { P(CH,0H)3}3] With decomposition.
Color
M p . ('C)
Ref.
Yellow Yellow-orange Yellow Yellow Yellow Yellow Yellow Yellow
I5ck70 16&70 160-70
112 114 114 114 993 993 993 994
21615
174-78 163-78 153-56 155-65"
Rhodium
1029
Table 72 Physical Properties of mer-[RhX, (PR,),] Complexes Color
M.p.vC)
Re$
Yellow-brown Orange Orange-red Orange Red Dark orange Orange-red Orange Red Ocher Yellow Dark yellow Orange Dark orange Dark maroon Pale yellow Bright yellow Bright yellow Orange Dark brown Yellow Orange Yellow Pale orange. Pale orange Orange Yellow Yellow Orange Orange Orange
150-70" 23W5 114-17 179-86" 1 W 3 40-50 139-42 30-35 200-10
112 114 972 972 972 114 972 114 114 995 114
Complex
hhC1; I@Me2Ph),]TRhC1, (NCOMPMe,Ph),l
-
15061 - 20 2 18-24 195-201 179-82 1 5 9 41 196-202 20614 206-24 17679 15845 13438 85-89 150-55" 138" 183-96 168-73 153-56 170-73 14W5 190-9 5 212-16 148-53 127-29 109-1 1 165-71 151-55" 14246 142-46"
b
Orange Orange Orange Orange Orange Orange Bright orange 'With demmposition.
114
972 918 918 918 918 918 918 918 918 918 918 918 918 972 993 993 918 918 136 996 994 994 994 994 997 972 972 972
[m]g - 56".
Table 73 NMR Spectral Data for [RhX,(PR,),] Complexes Complex
[RhC13(PMe3),l [RhC133@ E t' 13 1 [RhC13(PPr3131 [RhCI, (PBu,),l [RhCl, (PMe, Ph),] [RhCl, (PEt, Ph),] tRhC1, (PPhPr,),l [RhCl, (PBu,Ph),] [RhCI, (PMePh,),]
6P(1,3) (p.p.m.1
dP(2) bp.m.1
J/RkP(2,3)]
+8.6
- 7.6 - 20.0 - 20.0 - 14.2 - 14.9 - 14.7 - 15.1 - 4.4 - 3.1 - 17.5 - 17.1 - 12.7 - 12.7 - 12.7 - 12.8 3.4 2.8 20.0 - 19.9 - 16.0 - 15.9
82 84 83.8 84 83.8 84 83.6 86 84.6
- 4.3 - 4.3 1.1 +0.6 + 0.7 + 0.3
+
+ 5.5 + 5.0
- 3.9 - 3.9 0.8 + 0.4 0.7 +0.2 6.8
+ + +
+4.7
[RhCI, (PEtPh21 3 1
- 9.4 - 10.1
[RhCl, (PPh,Pr),] [RhCl, PBuPh, )3 1
-6.8 - 7.4
+ + -
(Hz)
84
84.0 85 84.2
84 84.1 86.0 85 84.8
85 85
Relative to 85% H 3 p 0 4 . In CH2CI, containing ca. 10% C,F6.
J[Rl-P(2)] (Hz)
103 109 112 115 111.9 114 112.5 I12 112.3 108 111.1 115 110.9 113 111.0 114.5 116 114 117
I20
J(P-PJ (Hz)
Frequency (MH-4
22 24.2 19 24.2 21 24.0 20.4 24.7
24.3" 24.3a 36.43b 24.3a 36.43b 24.3" 36.43b 24.3" 36.43b 24.3" 36.43b 24.3" 36.43b 24.3" 36.43b 24.3" 36.43b 24.3" 36.43b 24.3' 24.3"
13
23.2 20 23.1 19 22.9 24.5 14 22.8 14 18
Ref.
1001 1001
1002 1001 1002 1001 1002 1001
1002 1001 1002 1001 1002 1001 1002 1001 1002 1001 1002 1001 1001
1030
Rhodium
has been obtained and compared with those of similar complexes o f second and third transition series element^.'^' This technique has also been used to show that solid [RhCl(PPh,),] undergoes oxidative addition of hydrogen or oxygen to form [RhClY,(fPh,),] complexes (Y = H, O)?82 Perusal of Tables 71 and 72 shows the wide range of complexes that can be obtained. The essential problem is in selecting the correct preparative conditions for the isolation of the required complex in pure form. Many alternative products can be formed from the apparently simple reaction between hydrated rhodium trichloride and tertiary phosphines, some of which are shown in Figure 5. Accordingly many preparative methods have been devised for complexes of this stoichiometry, unfortunately none seem of very wide applicability, and many are specific for only one complex. Those intending their preparation should consult the methods given in the references of Tables 71 and 72, and should be cautioned that slight changes in the preparative methods employed invariably lead to other complexes. The facial isomers are obtained only from small tertiary phosphines, usually in poor yield. They are both less soluble and less stable than the meridional isomers. Comparatively little effort has been expended in investigating the reactions of the complexes, as opposed to the application of physical methods in determining their structures. However, in the meridional complexes the halo ligand trans to the tertiary phosphine is more labile than the other two and its replacement by other anionic ligands allows the preparation of [RhX,X’(PR,),] complexes from the trihalo complexes obtained from rhodium(II1) halides (equation 2 12). Prolonged reaction with salts of other anions results in all three halo ligands being replaced (equation 213).918
Y
=
rner-[RhCI,(ZMe,Ph),] X
=
+
y:?:
mer-[RhCI,(PMe,Ph),] -t NaS,Y
rner-[RhC1,(S2Y)(PMe2Ph),]
NaCl
(212)
CNMe?, PMeZ9l5
+
MIX
+
[RhX,(ZMe2Ph),]
(213)
Br, I, NCO, SCN, N3
It has been shown that the dioxygen complex [RhClO, (PPh3),], which X-ray crystallographyim reveals to have the structure (87),decomposes to dimeric [RhClO,(PPh,),], and OPPh, .*OO9 PPhs
I
Figure 5 Reactions of RhC1,.3H,O with tertiary phosphines
Rhodium
1031
The replacement of the tertiary phosphine ligands has seldom been attempted, and the [RhX3(PR3)2L]complexes have usually been prepared by addition of a neutral ligand to the coordinatively unsaturated [RhX, (PR, ),I complexes (see equations 188 and 21 1). Nevertheless, ditertiary phosphines replace the tertiary phosphine ligands in these complexes (equation 214).229 mer-[RhBr,(PBu,),]
+
Me2PCH2CH2PMe2
:o:z
c~~-[R~B~,(M~~PCH,CH,PM~,)~]B~ + 3PBu3
3
(214)
The tertiary arsine complexes (Table 74) are much less numerous than those containing tertiary phosphines. They are usually prepared by the interaction of tertiary arsines with rhodium trihalides. The poorer reducing properties of tertiary arsines make it much less likely that rhodium(1) complexes will be formed in this reaction. The halo ligands of these complexes can be replaced by other anionic Iigand~.''~One of the mutually trans tertiary arsine ligands can be replaeed by other monodentate ligands such as pyridine (equation 215). Two ligands can be replaced by bidentate nitrogen ligands (equation 216);*0'4 these latter products are discussed below in Section 48.6.3.6. mer-[RhX,(AsMe,Ph),] [RhX,(AsMe,Ph),]
+
bipy
+ +
py
+
mer-[RhX, (AsMe,Ph),py]
[RhX,(AsMe,Ph),(bipy)lX
(2 15)
+ AsMe,Ph
(216 )
One interesting reaction undergone by the tri(styry1)arsine complexes is the bromination of the C=C bond by bromine in CC1, (equation 217).''13 Similar behaviorg7'is exhibited by the few tertiary stibine complexes (Table 75) that have been isolated. Few physical properties of these complexes have been investigated, but the 12'SbMossbauer parameters for both rhodium(II1) complexes and the free ligands have been determined.1°16 [RhBr, (AsMe,(C, H4CH=CH2)),]
+
3BrZ
[RhBr, {AsMe, (C, &CHBrCH, Br)},]
Table 74 Physical Properties o f [RhX,(AsR,),] Complexes Complex
far-Isomers [RhC1,(AsEt,),l [RhCI, (AsMe,Ph),] mer-Isomers [RhCh (ASEL,1 3 1 [RhCI, (AsBu,),l [RhCI, (AsMe,Ph),] [RhBr,(AsMe, Ph),] [RhCl, Br(AsMqPh),] IRhC1, I(AsMe, Ph),] [RhCl, (AsMePh,),] [RhBr?(AsMePh, j,] [RhQ3 (AsPh,Pr),] [RhBr,(AsPh, Prj3] [RhCI; tAsMe,(l-C,,H,)M [RhBr, AsMez (1-ctoH~ 113 1 [RhBr, { AsMez(0-C, H,CH-CH, )}, ] [RhBr, {AsMe2(m-C,H4CH=CH2)),1 [RhBr, (AsMe,(p-C,H,CH=CH,)],] [RhBr, {AsMe, (0-C,H,CHBrCH, Br)},] [RhBr, {AsMe,(m-C,H,CHBrCH, Br)],] [RhBr, {AsMe,(p-C,H,CHBrCH,Br)),] [RhCI, (AsEtPh,),pyl [RhBr, (A~EtPh,)~py] [RhCl,(AsfrPh,),PYl [fiBr,(AsPrPh,)zPYl CfiCl,(AsBuPh,),~~l [RhBr,(AsBuPh,),pyl With decomposition.
Color
M p . ("C)
Ref.
Yellow Bright yellow
16673 198-206a
972 912
Red Red Orange-red Red Orange Blood red Orange Orange-red Orange Orange-red Orange Orange Orange Orange Orange Yellow Yellow Yellow Yeliow Yellow Yellow
111-13 11&22" 190-94" 192-200" 191-95 161-13 19 1-95 216-19
972 912 912 918 918 918 1010 IOIO
209- I2
1011 1011
188-90
209 214 13540 172"
10345" 1O5-1Oa 11G25" 238-39 255-56 247-49 252-54 196-97 184-86
1012 1012 1013 1013 1013 1013 1013 1013 1014 1014 1014 1014 1014 1014
-
(217)
Rhodium
1032
Table 75 Physical Propties of [RhX,(SbR,),] Complexes Complex
Color
Ref
[RhCl,(SbPh3),] [RhCI, Br(SbPh3),] [RhCI,I(SbPh,),] [RhCI, (NCS)(SbPh,),] [RhCl, (SnCl,)(SbPh,),] [RhCl, {Sb(o-C,H, OMe), I3I [RhBr, {Sb(o-C,H,OMe),}J [RhCI, {Sb(o-C,H,0Me)Me2],] [RhCI, { Sb(o-C, W, OMe)Ph,} [RhCl, {Sb(o-C, H,OMe), Ph},]
Orange Orange-brown Brown Orange Orange Dull orange Red-orange Yellow Orange-red Dull orange
1015 1015 1015 1015 1015 97 1 970 971 97 1 97 1
aM.p. = 158°C
48.6.3.6 [RRX, L,]X complexes The most numerous complexes of this general type are those containing two bidentate neutral ligands, which are discussed in Section 48.6.3.7 below. These can easily be obtained by the oxidative addition of two anionic ligands to the numerous [Rh(LL),]X complexes. There are, however, a considerable number of complexes which contain monodentate neutral ligands. These include the catalytically important [RhH, (PR,), (solv),]X comple~es,'~''-'~'~ and a number of dioxygen complexes.1020 The cationic dihydrido complexes (Table 76) were first discovered by Schrock and Osborn during their search for alternative homogeneous hydrogenation catalysts to the ubiquitous [RhC1(PPh,),].'0'7-'0'9 They found that if the cleavage of [RhCl(diene)], complexes by PPh, was carried out in polar solvents in the presence of a non-coordinating anion, [Rh(diene)(PPh,),E complexes were formed (equation 218). These complexes in turn underwent oxidative addition of dihydrogen to form rhodium(II1) complexes. During the latter reaction the dienes were reduced to alkanes and their place in the coordination sphere occupied by solvent molecules (equation 2 19). The stability of the dihydrido complex obtained depends markedly upon the tertiary phosphine present. Many complexes readily lose H, and revert to rhodium(1) complexes.t025 Since the dihydrido complexes catalyze the hydrogenation of alkenes by the dihydride route, and the rhodium(1) complexes by the alkene route, their stability and rates of formation are of considerable mechanistic It has also been shown that their stability is affected by the alkene present.'027 [RhCl(diene)]? [Rh(diene)(PPh,),]X
+
+
3H2
-
4PPh3 % [Rh(diene)(PPh&X
[RhH,(PPh,),(solvent),]X
(2 18)
+ alkane
(219)
Uniike the ligands in other normally kinetically inert rhodium(1II) complexes, these solvent ligands undergo rapid exchange in the dihydrido complexes with rates of 104s-' having been observed. The unusual rapidity of the exchange is due to the geometry of the cation in which the solvent molecules are trans to the hydrido ligands. However, the rate of exchange also depends upon the tertiary phosphine ligand present.'024 The solvent ligands can be displaced by bidentate ligands, such as bipy,lO" to form the complex
'M.p. = 79-81°C.bM.p. = 8C165"C. vRhPD = 1585, 1544cm-'.
Rhodium
*
'
1033
[RhH2(PPh,), (bipy)][ClO,], which had previously been prepared by the borohydride reduction of [RhRX(bipy),][CIO,] complexes in the presenoe of PPh, (equations 220 and 221). Its IR spectrum indicates that the hydrido ligands are cis.1ozgOther complexes of this type have been prepared by the oxidative addition of dihydrogen to [RhL,jX complexes. The hydrido ligands are also cis in these complexes. The dihydrido cation [RhH, (PPr',),(H,O),]+ is less well characterized but it has been used in the photochemical production of hydrogen (equation 222).'OZ9
[RhHz(PPr'3)2(H20)2]+ + H+
a
[RhH(PP*3)2(H20)3]2++ H2
(222)
There are two monohydrido complexes which have been prepared by the oxidative addition of hydrogen halides to [R~{P(OM~),},][BP~LJ (equation 223). Two other monohydrido complexes having hydrogen-bonded anions have been prepared by simultaneous substitution of and oxidative addition to dinuclear rhodium(1) complexes (equation 224). The anions give resonances at very low fields in the 'H NMR spectra of the complexes.'030The physical properties of the monohydrido complexes are listed in Table 77. [Rh(P(OMe),},][BPh,]
+
NaRw MeOH
HX
X
[RhClL21z+ PPh,(OMe) X
=
, , :z:c
=
b
IRhHX{P(OMe)3}41[BPh41f P(OMe)3
(223)
Br, I'a23
* [RhHCI{PPh2(OMe)},l~~H)
(224)
Cl, CF3C02
Yet other monohydrido complexes have been prepared by allowing a-[RhHX, (PEtPh,),] to react with ethanolic perchloric acid and a bidentate ligand (equation 22S).980Under similar conditions the p isomer forms dihalo complexes but these are better obtained from the trihalo complexes (equation 226).'"14 The coordinatively saturated complex [Rh{P(OMe),)J[BPh4] also forms dihalo complexes upon longer reaction with hydrogen halides (equation 227; cf. equation 223 above). The cis isomer is also the major product when bromine is allowed to react with the rhodium(1) complex (equation 228). The two isomers can be separated by fractional crystallization, the minor isomer being the least ~oluble."~~ HC104 [RhHX(ZEtPh,),(LL)][CIO,] a-[RhHX, (ZEtPh,),] + LL EtOH (225) Table 77 PhysicaI Properties of [RhHXL,]X Complexes
[RhHCl(PEtPh,), (bipy)][ClO, J
[RhHC1(PEtPh,),(phen)][C104] [RhHBr(PEtPh,),(bipy)][ClO,] [RhHBr(PEtPh,), (phen)]IClO,] [RhHCI{PPh,(OMe)},][(CF, C02)2HI
[RhHCI{PPh,(OMe)],j[PF,] [RhHCl{PPh, (OMe)j,][Cl, HI trans-[RhHBr{P(OMe), )4][BPh4]
trans-[RhHI{P(OMe),),][BPh,] [RhHCl(AsMePh,), (bipy)][ClO,] [RhHCl(AsMePh,), (phen)][CIO,] [RhHBr(AsMeP~,),(bipy)][C10,] [RhHCl(AsEtPh,), (bipy)][CIO,] [RhHBr(AsEtPh,), (bipy)][ClO,] [RhHCl(AsPh,Pr), (bipy)][ClO, J [RhHCl(AsPh,PrX @hen)][ClO,] [RhHBr(AsPh,Pr),(bipy)][CIO,] [RhHBr(AsPh,Pr), (phen)][ClO,] aNujol mull.
Yellow Yellow Yellow Yellow Pale yellow White White White -
Yellow Yellow Yellow Yellow
184-86 179-82 179-82 168-70 110 I10 IO0 129-30 133-34 212-15 16568 250-52 170-74 178-80 202-05
16M3 1S&89 235-37
23.80 23.33 28.81 23.63 __I
I
-
-
2O7Oa 2080 2073 207 1 2100 2100
1014 1014 1014 1014
2100
1030 1023 1023 1014 1014 1014 1014 1014 1014 1014 1014 1014
2072 2051" 2050 2060 2053 2045 2056 -
-
2053
1030 1030
Rhodium
1034
Table 78 Physical Properties of [RhX,L,]X’ Complexes CompIex
M.p. (“C)
Ref:
[RhCl, (PEtPh, )2 @iPY)l~C1041 [RhCl, (PEtPh,),(phen)][C104] [RhCl,(PMe,Ph),PYI~F,l [RhCI, (PMe,Ph), (NCC,F,)][PFJ [RhCl, (PMe,Ph),(p-NCC, F, CN), ,][PF,] [RhCl, (PMe,Ph)3 (OHdIFFA [RhC12(PMe2Ph),l[PF,I cis-[Rh(Br, { P(OMe), I4][BPh4lb Irans-[RhBr, {P(OMe), J,j[BPhJ cis-[RhI, {P(OMe),}, J[BPhJ [RhC1,(AsMePhZ)l(bipy)l[C1041 [RhCl, (AsMePh, ),(phen)][ClO,] [RhBr, (AsMePh, ),(bi~~)llc10,1 [RhBr, (AsMePh, ), (phen)][ClO,] [RhCl, (AsEtPh,), (bipy)][ClO,] [RhCl,(AsEtPh,), (phen)j[ClO,] [RhCl,(AsEtPh,)? (bipy)][CIO,]. 2H,O [RhCI, (AsEtPh,), (phen)J[C104].2H,O [RhBr, (AsEtPh, ),(b~PY)l[C1041 [RhBr, (AsEtPh, ), (phen)][C10,] [RhCl, (AsEtPh, )Z (bip~)lfBPh4] ,)~ [RhCl, ( A S E ~ P ~(phen)J[BPhJ
172-75 219-21 180-82 15941 172-74 120 21 15CS I
1014 1014 1031 1031 1031 1031 1031 1023 1023 1023 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014 1014
[RhC1,(AsPh,Pr),CblPy)I[C10,1
[RhCI, (AsPh,Pr)*(phen)][ClO,]
[RhBr,(AsPh,Pr),(bipy)][ClO,] [RhBr,(AsPh,Pr),(phen)][CIO,] [RhBr2(AsBuPh,),(bipy)][C10,]
147-50
I 5 1-53 154-56
269-73 169- 72 26345 17476
223-26 136-38 > 280 136-38 231-33 218-21 212-14 222-25 252-54 23840 200 05 255-57
25S57 212-14
[RhBr, (AsBuPh2), (phen)][C104] aOrange. White. ‘Yeliow-. HC104 EfOH
B-[R~HX(ZRI),If LL
+ HX [Rh{P(OMe),},I[BPh,l + Br,
[Rh{P(OMe),},][BPh,]
-
+
’ [R~X,(ZRI),(LL)I[C~O~I
cis-[RhX,(P(OMe)~),][BPh,]
[RhBr2{P(OMeh),IWh,I
+ P(OMe),
+
P(OMe)3
(226) (227)
(228)
The lability of the halo ligand trans to the tertiary phosphine in mer-[RhX,(PR,),] complexes has been exploited in the preparation of substituted cationic complexes (equation 229).’03’ [RhC13(PMe2Ph),] + L L
= py,
f AgPF,
2;:
[RhC12(pMe2Ph)3LI[PF61
(229)
PMezPh, HzO, CgFsCN, 0.5 P - N C C ~ F ~ C N
The dioxygen complexes [RhOz(ZMelPh),]X (Z = P, X = BPh,; Z = As, X = ClO,) contain They have been peroxo ligands. In the IR spectra of these complexes vopo is about 854~rn-’.’’’~’ shown to have structure (88) by X-ray crystall~graphy.”’~~~~~~~
2MezPh
(88)
There are many complexes containing one bidentate and two monodentate neutral ligands. A preliminary crystal study has been made on one of them.’034It may finally be noted that the oxidative addition of sundry reagents to rhodum(1) complexes is not an infallible preparative method for complexes of this type. Nitronium tetrduoroborate, for example, gives only a five-coordinate cation (equation 230).’O3’
Rhodium
1035
The addition of bromine or iodine to [RhL,]+ cations gives products whose solutions are poorly conducting. The corresponding iridium complex has been shown to be (89). The formation of polyhalogeno anions would explain why no corresponding complexes can be prepared by the oxidative addition of C1, .’036 I-
I
\
I
\I
48.6.3.7 [RhX, (LL)JX’ complexes
Complexes of this type can be prepared in three principal ways. The most obvious is by allowing hydrated rhodium trichloride to react with the bidentate ligand (equation 23 l).=’ Unfortunately this method gives rise to both cis and trans products. However, the reactions between rhodium(II1) halides and l,Zbis(diphenylphosphino)benzene (90)yield the trans product in the case of the chloride, whilst both the bromide and iodide form the cis p r o d u ~ t . ’ ” ~ RhC1,- 3H,O
+ 2Me2PCH2CH, PMe,
[RhCl, (Me2PCH2CH2PMe,),]Cl
-+
(23 1)
(90)
Pure samples of the 1,2-bis(dimethylphosphino)ethane complexes can be obtained by using the bidentate ligand to displace monodentate ligands from mer-rhodium(II1) complexes.228Again the larger halo ligands give rise to cis products (equations 232 and 233). mer-[RhCI,(PBu,),]
+
2Me2PCH2CH,PMe2
----$
truns-[RhC1,(Me2PCH2CH,PMe2),]C1 (232)
-, cis-[RhBr2(Me2PCHzCH,PMe2),]Br (233)
mer-IRhBr,(PBu,),I 4- 2Me,PCH,CH,PMez
The third preparative method is to oxidize rhodium(1) complexes of the type [Rh(LL)JX’ (equation 234). Thus, oxidation gives the other geometric isomer to that provided by ligand displacement (equations 232 and 233). [Rh(Me2PCH2CH2PMe2)2]C1
‘’’
cis-[RhClz(MezPCH,CH,PMe2),]Cl
(234)
However, although either chlorine or bromine can displace carbon monoxide from the five-coordinate rhodium(1) cation [Rh(CO){(Me, P),C, B,),j+, chlorine forms the cis isomer, whilst bromine forms the truns isomer (equations 235 and 236).Similar complexes can be prepared by metathetical reactions involving the ionic halide (equation 237).’”
+ Cl, [Rh(CO)(Me,PCH,CH,PMe,),]Cl + Br, [Rh(CO)(Me2PCH1CH,PMe2)2]CI
-
-+
cis-[RhC1,(Me,PCHzCH2PMez),]C1 + CO (235)
rruns-[RhBr,(MezPCH, CH2PMq)2jBr
+ CO (236)
truns-[RhC12(Me2PCH,CH2PMe,),]C1 + LiBr
+
trans-[RhC1z(Me2PCH,CH,PMe2)2]Br (237)
The range of products that can be obtained from oxidative addition reactions is illustrated by those of [Rh(diphos),]Cl (Figure 6). Other small molecules besides the halogens add oxidatively to [Rh(LL),]X complexes. Hydrogen adds reversibly to [Rh((Me2P),C,H,),]C1, to form a dihydrido complex (equation 238). The dihydrido complexes have generally been prepared by oxidative addition reactions; their physical properties are listed in Table 79. Hydrogen chloride, on the other hand, adds irreversibly to both four and five coordinate
Rhodium
1036
I
T
[ RhHCl(diph~s)~]Cl
I
{ RhHCl(diph~s)~][ BPh41
i, LL = dmpe, S,, CH C12;1M2 ii, LL = dmpe, X, (X = C1, Br)?" iii, LL = dmpe, HX (X = C1, Br)?*, iv, HC10,;214 V, LL = diphos, HCl$'4 vi, LL = diphos, NaBF.,, HCl,,, MeOH;223vii, LL = diphos, NH,PF,, O,, MeOH;'0'9 viii, NH,PF,;'M2 ix, X, (X = C1, Br);,,' x, NaBPh,, EtOH214 Figure 6 Oxidative addition and metathetical reactions of complexes containing ditertiary phosphines
[Rh(Me2PCH,CH,PMe,),]C1
+
H2
e [RhH2(MezPCH,CH2PMe),]C1
(238)
rhodium(1) complexes of this ligand (equation 239). Oxidative additions of hydrogen chloride to other rhodium(1) complexes can be reversed by addition of base to a solution of the resulting monohydrido complex (equation 240).223 [Rh(MezPCH2CH,PMez)21CI [Rh(LL),I[BF41
+
+ HCl
HC1
-*
[RhHCl(Me, PCHzCH2PMez),]C1
MeOH .EtJN,CDC,3L
[RhHCW-), I
W 4 I
(239) (2W
LL = CH,(PPh2),, CHMe(PPh,),, diphos, Ph2P(CHz)3PPh2
The other hydrogen halides add oxidatively to rhodium(1) complexes of ditertiary phosphines or arsines giving rise to numerous monohydrido complexes, whose physical properties are also listed in Table 79. However, it is possible to prepare certain monohydrido complexes from rhodium(II1) halides. One interesting reaction, carried out under an atmosphere of CO, gives rise to dicarbonyldichlororhodate(1) salts (equation 241).226 +
RhC13.3H20
+ 2LL
co, 78°C
C6H61E10H
[RhHCI(LL),][RhCl,(CO),]
(241)
LL = (19), (91), diars
Te t r a f l u o r ~ bo r a t etetraphenylborate ,~~~ and iodidezz7salts of a large range of rhodium(1) complex cations react oxidatively with dioxygen (equation 242). The physical properties of the dioxygen complexes are given in Table 80. The claim that a rhodium(lj species containing a ditertiary arsine
Rhodium
1037
Table 79 Physical Properties of Hydridorhodium(l1I) Complexes Containing Bidentate Ligands
>
-
M.P. ("GI
Color
Complex
cis-[RhH, {(Me,P),C,H,},~Cl Buff-white [RhH2 {(PhzP)(CH2)3 P P h d I 2 W 4 1 White White [RhH, (DIOP)zI[BF,I cis-[RhH2(91),]Cl cis-[RhH, (27),][pF6] Yellow trans-[RhHCl{(MqP),C2H, )JCl White trans-[RhHBr{(Me,P), C,H,},]CI Pale yellow trum-[RhHCl{(Ph,P),CH, J,][BPh,] Cream trans-[RhHCl (Ph,P),CH,},][BF,] Cream Cream trans-[RhHCI[(Ph, P)2CHMe), ][BF, ] ~ r ~ ~ , ~ - [ ~ h ~H4},][BF4] C l { ~ h ~ Cream P ) ~ ~ truns-[RhHCl{(Ph,P),(CH,), ),][BF4] Cream frans-[RhHCl{(Ph2P)z(CH2)3 )*IC1 Cream [RhHCW), 1PF.J [RhHCl(19),]C1 [RhHC1(19),][RhCI2 (CO), ] [RhHBr(l9),I[RhBr,(CO),l [RhHC1(91),]Cl [RhHBr(91),)Cl [RhHC1(91),I[RhCI,(COXl [RhHBr(91),][RhBr2(CO), ] [RhHCl(diars), ][pF,] Pale yellow cis-[RhHCl(27)JFF,Ih Yellow-orange cis-[RhHBr(27),]PF6j Orange cis-[RhHI(27)JPF6] Orange truns-[RhHC1(27X][PF~~ Yellow trans-[RhHBr(2~]~F,]' Yellow rrans-[RhHI(27)JPF6Y Yellow-green
-
-
86 153 16 o d 183-87 -
18.5 20.0 22.6 -
-
I
-
I
I
-
-
-
-
230' 239' 235d 183d 182d 202d
1900, 1870 2052 2080, 2020 1982, 1969 1982, 1981 2050 2030 2080 2078 2105,2099 2078 21 10 2090 2000 2078 2079 e 2052 2045 2000
I
-
-
Ref.
tRh--H(P.p.m.) v,-,(cm-')
-
25.9" 26.0b 249 -
29.3' 27.98 I
I I
-
I
-
-
28.17 27.07 25.07
2032 2024 2018
228, 229 1038 1038 239 24 1 228, 229 228 223 223 223 223 223 223 226 226 226 226 226,229 226, 229 226 226 226 24 1 24 1 24 1 24 I 24 1 24 1
aJ(RH) 16.1 Hz, J(P-H) 126Hz. bl(Rh-H) 16.2Hz,J(P-H) 12SHz. 'J(Rh-H) 18Hz, J(P-H) 11.5Hz. dWith decomposition. 1502cm-'. 'J(Rh-H) 11.3H.z. gJ(Rh-H) 11.9Hz. h0.25MezC0solvate.IO.5THF solvate. '0.7THF solvate.
e Y
~
dioxide ligand is formed in a reaction carried out under aerobic conditions is almost certainly erroneous. '04' [Rh(LL),]X -t
-
0 2
(242)
[RhO,(LL),]X
L L = CH2(PPh,)2, CHMe(PPh,),, Ph,P(CE12)3PPh2
H
\
/c=c
PhZAs
/
H
\ ASPh2
(91)
Table 80 Physical Properties of Dioxygen, Disulfur and Diselenium Rhodium(II1) Complexes containing Bidentate Ligands Complex
Color
M p . ("C)
vo-0
(m-' j
Ref:
~
-
~
1038
Rhodium
The [RhO,(diphos),][PF,] complex can easily be prepared (equation 243), but it loses oxygen upon dissolution in anaerobic methanol. In the solid state the dioxygen complex is sufficiently stable for its structure to have been determined by X-ray crystallography. The structure (92) is best described as trigonal bipyramidal with monodentate dioxygen,'03' which is in accordance with the NMR evidencez4' The similarity to a corresponding complex containing monodentate ligands (88) is readily apparent. Dioxygen reacts with solid [Rh{(Me,P),CzH4},]C1 to give a material exhibiting a band attributed to vo--o in its I R spectrum at 845 cm-', but satisfactory analytical results were not obtained?" NH PF
[Rh(diphos),]Cl
+ O2 ML:
[RhO,(diphos),](PF,)
(243)
(92)
The analogous [Rh{Ph,P(CH,)2SR),][BPh,] complexes also react with dioxygen (equation 244).*" Disulfur analogs of the dioxygen complexes can be prepared by the oxidative addition of S, fragments from cyclooctasulfur (equation 245).
[RhIPh,P(CH,)zSR}2I[BPh,l + 0,
-
[Rho, IPhzPICH,),SR),IIBP~l
(24)
Since the structure of [IrSe,(diphos),]+ is similar to (93), it may be assumed that the disulfuridorhodium(II1) cation has this structure. Indeed, such a structure has been proposed for the cation [RhS2{(Me2P)ZC2H4}2]+ on the basis of its IR spectrum, which has a band (attributed to vRh s) at 525 CM-' in both the chloride and hexafluorophosphate salts.'"
(93)
The dioxygen complexes undergo a number of interesting addition reactions with the lower oxides of non-metals that give rise to [RhXX(LL),]X" species containing coordinated oxoanions (equations 246 and 247).230 [Rho2{IMePhP)2C2H4}21PF61+ SO, [Rh02{(MePhP)2CzH, 3,1[PF,1
+
WO,
--
[Rh(O,SO, )((MePhP),C2H,},1[PF,3
(244)
[Rh(ONO& { ( M e P W K , H, l,I[PF,I
(247)
There remain a number of other complexes of this stoichiometry that have been prepared from rhodium(II1) halides; their cations usually contain halo ligands?35The number of these complexes has been increased by allowing them to undergo metathetical reactions with sundry metal salts (equations 248 and 249).2'4There are also a number of complexes that have been prepared from the oxidative addition of other compounds to rhodium(1) complexes. Among these are the alkyl or aryl sulfinato complexes that can be prepared by the oxidative addition of suitable sulfonyl chlorides to [Rh((MePhP),C,H,},][PF,] (equation 250). Whilst methane sulfonyl chloride gave the S-bonded adduct (vs4 (asym) 1214, 1194; vs4 (sym) 1035cm-'), the arene sulfonyl chlorides formed mixtures of 0-and S-sulfinates, the IR spectra of which showed an additional band in the region 104&1060 cm-' attributable to the latter isomer. Although no isomerization occurred in solution, :5e ~ r r ? p ? i o nof the S isomer in the solids slowly increased over a period of years.230
1039
+
RhC13.3H,0
2IButCH2),PC,H,P(CH2Bu')2
NH4PFs ELOH
(248)
[RhCh {(But CH, 1 2 pc2 H4P(CH2 But12 },I[PF,I
+
[RhHCl(diphos),]Cl
NaBPh,
[Rh{(MePhP)2C2H4}2][PF6] + RSOCl R
=
-
+
[RhHCl(dipho~)~][SPh,1
NaCl
(249)
[RhCl(RSO){(MePhP),C,H,),I[PF6]
(250)
Me, m-C6H4N02.p-CsH4Me, 2-CloH7
Since, in the main, complexes of this type are themselves derivatives of other rhodium complexes there seem to have been few investigations of their chemical reactions apart from the metathetical reactions outlined above. Apart from spectral studies there has been little interest shown in their physical properties, although it may be noted that the emission observed from [RhCl, &Ph2Z),C2H,),]C1 (2 = P, As) complexeshas been attributed to metal-localized phosphorescence.' The ditertiary phosphine complexes are listed in Table 81. The complexes of this stoichiometry containing ditertiary arsines probably outnumber those of tertiary phosphines. They have been prepared by the same general methods as the tertiary phosphine complexes, although unusually [RhCl, (diars),]Cl has been prepared from Na, [RhCl,]. Despite the relative antiquity of this method,Iw it has not been generally adopted (equation 251). The physical properties of the ditertiary arsine complexes are listed in Table 82. It may be noted that the IR spectra of the sulfinato complexes indicate that these are S-bonded like their phosphorus analogs.230 Na3[RhCl6] f diars
4
(25 1)
[RhCl,(diars),]CI
There are also several complexes containing ditertiary stibines of the type Ph2Sb(CW,), SbPhz, and there would probably be more were the 1,2-bis(diphenylantimono)ethane ligand known.Iw6 Ligands containing two different group V donor atoms also form complexes of this type. These include the ligands (94)IM5and (95).'037 There are also complexes formed by other ligands which contain but one group V donor atom. These include the hexafluorophosphate salt of (96),IM7and the anionic complexes of (a), which have been mentioned in Section 48.6.3.1
(94)
(95)
(96)
However, not all bidentate ligands form complexes of this stoichiometry. Although sulfur dioxide displaces CO from a binuclear rhodium(1) complex, the product (97) retains a rhodium-rhodium bond.'04*The complexes of (98) form primarily fac products when allowed to react with rhodium t r i c h l ~ r i d e . 'The ~ ~ strength of the rhodium-sulfur bond is also shown in the failure of diphos or Table 81 Physical Properties of [RhX,(LL),]X' Complexes Containing Ditertiary Phosphines ~~
Complex
cis-[RhCl, {(Me,P), C,H4},]C1 tram-[RhC1,{(Me2P),C2H4},]C1 trons-[RhC12{(Me,P),C2H4),]Br
trans-~hCI,((Me,P),C,H,},][BPh4] ciu-[RhBr2{(Me,P),C,H, j2]Br trans-[RhBr, ((MBP), C,H4},]C1 iRhC12 {(MePw)2C2H4}21[PF61 iRhBr2 {(MePw)2GH4 )21[PF61 [Rh12 t(MePhP)2C2H4)21[PF61 IRhS04 ((MePhP)2C2H4}ZI[PF61 [Rh(OH)(N0d{(MePhP)2C,H4 }21[PF61 trans-[RhC1(S02Me){ (MePhP),C, H4} ,][PF6 truns-[RhC1(m-O, SC,H4N02){(MePhP), C,H,}, IFFa] trans-[RhCl(p-0,SC, H4Me){(MePhP), C,H,},1PF6] trans-[RhC1(2-02SC,,H,){(MePhP),C, H4},][PF,] [Rhclz {(ButCH2)4P2C2H4h][PF61 [RhCI, (PhZPCH2 C02 Et)2][PF,] *With decomposition.
Color Yellow Yellow Yellow Pale yellow Yellow Dark yellow Pale yellow Bright yellow Brown Yellow Off-white Yellow Yellow Yellow Yellow Yellow -
M.p. ("C)
Ref:
330
229 229 229 229 229 229 230 230 230 230 230 230 230 230 230 235 1047
18&90 18-7
1SC-83 33435 33Y 301" 172" 203" 298" 232"
2Nb 225" 211-13 f
Rhodium
1040
Table 82 Physical Properties of [RhX,(LL),]x'
Complexes Containing Ditertiary Arsines
Color
M.p. ("C)
Ref
Yellow Yellow Yellow Pale yellow Pale yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow Yellow-orange Yellow-brown Yellow Light yellow Light orange Yellow Yellow Yellow-orange Brown Red-orange Dark red Dark red
> 330 > 330 > 330 3 IW 15f 195' 235'
230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 230 24 1 24 1 24 1 24 1 24 1
Complex r :
.
.,
181"
255" 255' 117' 20 1 195' 217 220' 194' 205' 210 309c 299F 290"
-
-
185 189' 213' 288" 238' 245'
aMe,CO hemisolvate. Me2C0 solvate. With decomposition.
Ph,PCH,CH,AsPh, to displace all the dithiophosphinato ligand despite demethylation of the latter occurring during the reaction (equation 252).'050
I
PhlP
\
/pph' CH2
RN=C
/
\
SH
PPh,
48.6.3.8 Complexes containing polydentate ligands Polydentate ligands form both rhodium(1) and rhodium(II1) complexes. Both oxidation states will be considered in this section since most of the rhodium(1) complexes readily undergo oxidative addition reactions. Bis(3-dipheny1phosphinopropyl)phenylphosphine (99) forms chloro, bromo and iodo rhodium(1) complexes (equations 253 and 254). The bromo complex can be obtained by addition of a large excess of LiBr to [RhCl(cod)], prior to addition of the neutral ligand.Io5'The iodo complex has also been prepared from [RhI(C8H,,)], .1051.10s2The halo complexes (100) have been shown to be square planar by X-ray ~rystallography.'~~'
I
Rkudium
[RhCl(cod)12 + (99) [RhCl(cod)]>
+ (99) X
=
-
Lix
1041 [RhC1(99)]
(253)
[RhX(99)]
(254)
Br, I
( 100)
(99)
A similar square planar complex (101) has been formed by the ligand PhP{ (CH2)3PCy,}2.The chloro complex has been prepared by synthesizing the ligand on a rhodium(1) template as shown in Scheme 38.'053The related tri(tertiary arsine) ligand PhAs((CH,), AsMe,), also forms a complex of similar structure to (101). The triarsine compIex can be formed by displacement of neutral ligands from other chIororhodium(1) complexes (equation 255).1054
Scheme 38 Template synthesis of [RhP{(CH,)3PCy2)2]
~
~
~
PhA6{(CHZ)gAsMeZ}Z ~ ~
[RhClAsPh{ ~ ~(CH,), ASM~,}~] 4 !
2
1
(255)
[RhCl(cod)],
The complexes readily undergo oxidative addition reactions but can easily add additional small neutral ligands to form five coordinate complexes (equation 256).Io5'The structure of the S-sulfur dioxide complex has been determined."56 The corresponding triarsine complex also forms a sulfur dioxide adduct. The chloro ligand can be replaced by non-coordinating anions (equation 257),'05' or in the case of the triarsine complex by carbon m~noxide.'~" The 31PNMR spectra of an extensive series of derivatives have been determined, but no preparative details have been given.'OS7The bis(diphenylphosphinosily1)amide anion functions as a tridentate ligand and displaces the majority of the monodentate tertiary phosphines from rhodium(1) complexes (equations 258 and 259).'05' [RhCI(W)] + L
-
[RhCl(!B)L]
(256)
L = SOL, BF3
[RhCl(99)] + NI&PF, [RhCl(PPh,),] [Rh(PMe,),]Cl
a
+ LiN(SiMe,CHzPPh2)2 + LiN(SiMezCH2PPh2)2
[Rh(MeCN)(!H)][PF,J
(257)
-+
[Rh{N(SiMe,CH,PPh,),)(PPh,)]
(258)
-+
[Rh{N(SiMe2CH,PPh2h)(PMe,)]
(259)
Chloro and iodo complexes of the quadridentate ligands (102), (103) and (104) have been prepared. No preparative details are available and the complexes are all too insoluble to confirm the speculation that they are trigonal bipyramidal.1059
~
~
+
I I042
Rhodium
9
0
(102)
(103)
\
r
? \
/' AsPh,
PPh2
PPh,
(104)
The rhodium(II1) complexes can be prepared either by oxidative addition to the correspondmg rhodium(1) complexes or by direct reaction of the ligands with rho&um(TTJ) salts. Normally the reducing properties of tertiary polyphosphines ensure that rhodium(T) complexes are formed; hence the rhodium(II1) complexes of these ligands have been prepared via oxidative addition reactions. However, the sterically hindered ligand (105) falls to reduce hydrated rhodium trichloride even when allowed to react with the latter in refluxing ethanol (equation 260).235 But
But
(105)
+
RhCl3*3H,O
(105)
-----*
[RhC13(105)]
(260)
The complexes of [PhP{(CH,),PPh,),] (99) readily undergo oxidative addition reactions (equations 261-263). Dioxygen forms a rhodium(II1) complex which reacts with sulfur dioxide to give a sulfato complex and with carbon monoxide to yield a carbonato complex (equations 264-266).'05' I,], which can be Similarly, the thermally stable dioxygen complex [R~CI(O,)P~AS((CH,)~ASM~, prepared by allowing dioxygen to react with the rhodium(1) complex, is a useful source of other rhodium(II1) complexes of this ligand (equation 267). Like the similar diphenylphosphino complex above it reacts with sulfur dioxide to form a yellow sulfato complex and with carbon monoxide to form a yellow-orange carbonato complex. Upon formation of these two complexes the characteristic v& band at 852cm-' disappears from its IR spectrum and is replaced by bands at 1255,1140 and 640 cm-' (bidentate sulfato ligand) or bands at 1650 and 1620 cm-' (bidentate carbonato ligand)."" [RhC1(99)]
[RhCl(99)]
+ H2 + HCI
[RhC1(99)]
+ Ss
tRhCl(9911
+
0 2
[Rh(O,)C1(99]] f SO, [Rh(O2)C1(99)] [RhClPhAs((CH,), AsMe,),]
[RhH,C1(99)]
(261)
[RhHCI,(W)]
(242)
[Rh(S2)Cl(99)]
(263)
[Rh(O,)fl(99)1
(264)
--*
+
-
-----*
+
+ CO + O2 +
--*
[Rh(SO,)ClW11
(265)
[Xh(CO,)C1(99)]
(246)
[Rh(02)C1PhAs((CH2),AsMe2}21
(267)
Tetrafluoroborate species also undergo two fragment oxidative addition reactions to the tritertiary phosphine complex (equation 268). The hydrido complex exhibits VRh-" at 2199cm-I. On heating the square pyramidallo@' diazonium complex (106) it is converted to the hydrido complex.1o52 The preparation of the hydrido complex from tetrafluoroboric acid is difficult to reproduce. It has been reported that the preparation from HBF4.Et20is successful only if ethanol is present. Under these conditions [RhHCl(L)(EtOH)][BF,]is formed. In THF solution the THF solvate is formed.'061
Rhodium
1043
Ph
I
/N N
1106)
-'
The halogens form trihaio complexes when allowed to react with [RhClL] (equation 269).los1The trichloro complex containing the tritertiary phosphine (107) can be prepared by the oxidative addition of chlorine to a rhodium(1) carbonyl complex (equation 270).'& tRhCl(W1
+
-
x 2
[RhX,(WI
(269)
X = C1, Br, I
"i" [RhCl(CO)(107)]
+
Cf, 5 [RhC13(107)]
(270)
The poorer reducing properties of tertiary arsines usually result in rhodium(II1) complexes being formed when these polydentate ligands are allowed to react with rhodium(lI1) salts. Thus, the tri(tertiary arsine) ligand (108) forms mixtures of facial and meridional rhodium(I1I) complexes (equations 271 and 272). The facial isomer is the more soluble of the two bromo complexes and it can be obtained from the mother liquors after the mer complex has crystallized out. Only thefac isomer of the iodo complex is known. It can be prepared by addition of lithium iodide to the reaction mixture (equations 273 and 274). The formation of this isomer is, perhaps, surprising on steric grounds, but if the preparation of the meridional complex is essayed by allowing lithium iodide to react with the mer-trichloro complex only one chloro ligand is replaced (equation 275). In acid solution the mer-tri(nitrat0) complex can be obtained from a metathetical reaction (equation 276). RhC13-3HzO
+
(108)
l2EH fuc-[RhCl3(108)] a+
RhCI,*3H20 + (108),::' RhBr,
-
+ (108)
RhCl,.3H20
+
HBr
rner-[RhBr,(108)]
+
fac- and mer-[RhBr3(108)]
(108)
+
+
LiI
mer-[RhC1,(108)]
mer-[RhCI, (loS)]
LiI
fac-[Rh13(108)]
-+
-+
mer-[RhCl,I(loS)]
(271) (272) (273) (274) (275)
HNO,
AgNO,
mer-[Rh(NO,), (108)]
(276)
The proposed structures of the two trichloro complexes are based upon their IR spectra. Bands due to vRh .-cIoccur at 332, 302 and 296cm-' in the spectrum of thefac isomer. The mer isomer has bands at 339, 307 and 262cm"-'. A rhodium(1II) perchlorate complex containing two molecules of the tritertiary arsine can also C.O.C.6 H H
Rhodium
1044
be prepared (equation 277).Io6' There is also a poorly characterized trichloro complex of the The hexa(tertiary phosphine) ligand (1 10) displaces neutral ligands, tridentate ligand (109).1063 includmg some that are quite strongly bound, from rhodium@)complexes, as in equation (278). The low value of 276cm-' for vRhPcI in the far-IR spectrum has led to the structure (111) being proposed for the product. The ligand (110) fails to reduce RhC1,.3H20 when allowed to react with it in ethanolic solution (equation 279). Further evidence for the ionic nature of the product (112) is provided by the reaction with NH,PF, (eqution 280).'064,'065
L
Rh(ClO,)? [RhCl(CO)(PPh3)2]
+
+
(108)
(110)
+
+
HClO,
[Rh(108),][C10,]3
[Rh2C12(110)]
+ (110) [Rh2C14(11O)]C12 + NH4PF6 2RhC13.3H20
Mixed N--O and N-S
118.6.4
48.6.4.1
N-0
+
+
[RhCl(PPh3)3]
(277)
+ (110)
(278)
[Rh2C&(110)]C12
(279)
[Rh2Q(11O)J[PF,]2
(280)
Ligand Complexes
ligand complexes
Surprisingly little work has been reported on tris(amino acid) complexes of Rh"'. Reaction of RhCl, 3H20 with sodium bicarbonate causes precipitation of Rh,03, which forms [R~(D-ala),]if heated immediately with D-alanine. Fractional crystallization allows separation of mer( - ) and mer(+) isomers, as well as some impure fac isomer.1066Reaction of the Li salt of DL-methionine (Li' MeSCH,CH,CH(NH,)COO- ) with anhydrous RhCl, in refluxing ethanol leads to the precipitation of [Rh(~~-methionine),], in which the methionines act as anionic bidentates, bonded through the 0 and N The solid state reaction of RhCl, with glycine is reported to lead to [Rh(gly),], characterized thermogravimetrically and by IR, UV and H NMR spectroscopy.lW8 Several [Rh(en),(amino acid)12+ complexes have been prepared and r e s o l ~ e d ~ (Section ~"~ 48.6.2.2.ii).6up"7 Early reports by MacNevin and co-workers1069-'071 indicated that Rh"' did not form complexes with ethyIenediaminetetraacetic acid (H,edta), although MacNevin finally did report indirect evidence of a Rh"'+dta intera~tion.'"~The first well-characterized complex was reported by D y e r and Garvan, who overcame the inertness of Rh"' by heating the H,edta with an aqueous solution of RhC1,-3H,O in an autoclave at 145-150 "C for four this pressurized 'sealed-tube' technique remains the standard route to aminocarboxylate. complexes of Rh"'. The resulting complex was shown to have a five-coordinate edta ligand with one uncoordinated carboxyl arm and a water coordinated at the sixth coordination site. The pK, of the free carboxyl group is 2.32 f 0.08, while the coordinated water has a p& of 9.15 f 0.02. The complex was resolved by precipitation of the ( -)-[Co(en),(NO,),]( -)-jRh(edta)(H,O)] diastereomer, followed by recrystallization of the Rh species as the Kf salt. Reaction of the monoaqua species with HCl or HBr leads to the compared tetradentate anions [Rh(H,edta)X,] - (X = C1, Br) and these complexes were
-
I
Rhodium
,
.
1045
to Co analogs'074and characterized by IR spectro~copy.'~~' Other workers have prepared and characterized a variety of NH: , K + and Na' ~ a l t s . ' ~NMR ' ~ ' ~ spectra ~~ were used to confirm that edta is pentadentate in strongly acidic solution, but that in a limited range (pH between 4 and 8), it is hexadentate, as [Rh(edta)(H,O)] reacts to form [Rh(edta)]-.'079A coordinated water is necessary, as 'H NMR showed that [Rh(edta)Cl]- remains unchanged between pH 1 and 9 (except for the deprotonation of the free carboxylate). Analysis of the NMR spectra also suggested that the sixth coordination site in these five-coordinate edta complexes is in the diamine plane.1o7gSolid salts of the six-coordiante [Rh(edta)]- anion have not been reported. Dwyer and Garvan also prepared and resolved the Rh"' complex of l-propylenediaminetetraacetic acid (pdta), and suggested it was a five-coordinate chelate, forming [Rh(pdta)(H20)]-.'080 Like the edta analog,'073resolved samples of the pdta complex rapidly racemize when exposed to light, but unlike the edta ion, the pdta complex spontaneously returns to the resolved form in the dark. This chromophoric behavior was not studied, but ascribed to the reversible formation of a diaqua species."** Wing questioned this, showing that photolysis does not change the acetate portion of the 'H NMR spectrum,"" clearly inconsistent with photoinduced aquation of an acetate arm of the pdta. He proposed that the mutarotation was due to the photoinduced formation of an unstable (axial methyl group) configuration as shown in (113); collapse to the stabler (equatorial methyl) configuration then occurs in the dark. The methyl group provides the driving force for this back-reaction, so the edta complex would not be expected to spontaneously re-resolve in the dark, consistent with observati~n.'~~~J~~~J~~'
An isomer of edta with two chiral centers, ethylenediaminedisuccinic acid (edds; 114) also forms a stable hexadentate complex with Rh1'1.'082 The ' H NMR spectrum of [Rh(S,S-edds)]- is virtually unchanged between pH 2 and 9, and the IR and NMR spectra are both consistent with a sixcoordinate geometry. At pH w 1, [Rh(S,S-edds)(H,O)] forms, and at pH x 10, the [Rh(S,Sedd~)(OW)]~anion was identified in solution.lo8* The X-ray structure of Cceedta complexes shows that the glycinate rings which are in the plane of the two amine groups (the G rings) are strained, while the out-of-piane rings (R rings) are not strained. For the larger Rh"', such a strain should be enhanced, leading Douglas and co-workers to study Rh"' complexes of a variety of aminopolycarboxylates with different chain length^.'^*^'^^ The 'sealed-tube technique' was used to react RhCl, 3 H 2 0 with ethylenediamine-N,N'-diacetic-N,N'-di-3-propionicacid (H,eddda; 115a) to form trans-0, - and trans-(O,O,)-Na[Rh(eddda)](119.1,~). Isomer assignment was made on the basis of ' H and I3C NMR, and IR spectra.'083The Rh"' complex with 1,3-propanediaminetetraacetate (1,3-pdta) is also hexadentate, and, based on CD spectra (Table 83), comparisons to Co analogs and to the CD spectrum of [Rh(S,S-edds)]-, ( -),-[Rh(l,3-pdta)].2H2O has been assigned the A configuration. The CD spectra are more intense, and broader than those of the Co"' and Cr"' analogs, as the low symmetry (C,)components are not resolved, presumably due to the larger spin-orbit coupling of the Rh.
-
0
II
HO-C-CHZ
11
I*
HO-C-C-N-CH~-CH~-N-~C-C-OH
I1 I
O
H
I
I
H
I
II
I
H2C-C
H
0
II
-OH
0 ( 1 14)
I
Nitrilotriacetate (NTA), N-methyliminodiacetate (MIDA) and iminodiacetate (IDA) (116) form a series of 1:1 and 1:2 (metaldigand) complexes with Rh"' (and Co"'), which have been characterized by ' H NMR and electronic spectra.'087On the basis of the NMR and a comparison to the Co analog,
1046
Rhodium
0
0 HO-c
II
II
-cH~-N-cH,-cH~-N-
I
I
HO-C-CCHZ-CH,
CH,-C-OH
HZC-CHZ-C-OH
II
II
0
0
0
( I 15b) trans-(05)
(115a)
( 1 15c) rrans-(O,O,)
an ill-characterized 1:1 adduct of NTA is proposed, [Rh(NTA)(H,O),], in which the NTA is a tetradentate. For all but one of the 1:2 adducts, the 'H NMR spectra show clean AB patterns, implying the truns+c geometry (117). The more soluble isomer of [Rh(TDA),]- shows two AB patterns, consistent with trans-mer geometry. The [Rh(NTA),]- ion has two free carboxylate arms (one per NTA), and the NMR shows that averaging of environments between the coordinated and uncoordinated acetates is not observed.'087 I 10 . 0 0.
1I
HO-C-CH,
>NR
HO -c -CH2
1I
0 (1 16) NTA, R = CH,COOH M I D A , R = Me JDA, R = H
cis
Table 83 Electronic and C D Spectra of some Aminepolycarboxylatorhodium(II1)Complexesa Complex * 378sh (405) 335 (745) 295sh (480) 380sh (390) 344 (565) 298sh (380) 375sh (215) 328 (565) 285sh (245) 358sh (284) 338sh (274) 293 (243)
--
-
mer-[HRh(NTAH),] 2H,O r~zer-[HRh(M1DA)~] .2H,O mer-[HRh(IDA),]*3H20 JcIc-@R~(IDA),] * 2HzO A-cis-a-[Rh(EDDP)(S-ala)]
-
400sh (* 26.5), 334.5 (154), -280~h 404 (25), 328 (149), 282 (24) 400sh (- 28), 328 (132.$), 284 (104.5) 362 (2741, 295 (237) 350 (196) 283 (232) 350 (188) 283 (214)
A-cis-a-[Rh(EDDP)(R,S-ala)]
350 (188) 283 (213)
A-cis-a-[Rh(EDDP)glyJ
360 (321) 290 (255)
348 (202) 283 (261) abbreviations given in text.
286 (+ 0.36) 393 (- 1.80) 345 ( f 2.84) 280 (- 0.55) 363 (- 2.29) 317 (4-2.74) 392 (- 1.03) 333 (+ 2.05) 294 (-0.65) 263 (+0.09)
1084 1084 1084 1085
1087 1087
-
A-cis-a-[Rh(EDDP)(R-ala)]
a Ligand
340 (-2.10)
1087
1087 360 (-5.15) 308 (+ 0.87) 270 ( -0.98) 360 (- 3.92) 308 (+0.89) 270 (- 0.76) 360 (- 4.66) 310 ( 0.94) 270 (- 0.94) 360 (-4.11) 308 (f0.71) 270 (- 1.44) 346 (+ 1.25) 306 (- 0.82)
+
1088
1088 1088 1088 1088
1047
Rhodium
M. E. Sheridan et al. have recently presented an interesting series of reports on the Rh"' complexes formed with the optically active tetradentate, ethylenediamine-N,N'-di-S-propionate, (3,s-eddp). 108s-109' Four geometric isomers of [Rh(s,S-eddp)X21"+are possible (1181, but for Rh"', only the A-cis-cr and i-cis-fi isomers could be detected (X, = 2C1-, n = - 1; ala-, n = 0; en, n = + 1). For [Co(S,S-eddp)(en)]+, three isomers were observed, with only the A-cis$ not formed.Io9*The electronic spectra for the two isomers of [Rh(S,S-eddp)(en)]+ are very similar, but the 'H NMR is diagnostic of isomeric configuration; the c1 isomer shows one methyl doublet and one H quartet on the propionate arm, while the fi form has dissimilar environments for the propionate arms, and shows two methyl doublets and two H quartets. Substitution of the en by two C1- ions occurs with retention of configuration in the A-cis-a case, and subsequent substitution of the chlorides by S-ala, R-ala and S,R-ala also occurs with retention of the A-cis-a c o n f i g u r a t i ~ n . l ~ ~ ~ * ' ~ ~ ~ Reaction of A-cis-fl-[Rh(S,S-eddp)Cl,]--with R-ala, S-ala or OH- leads to an unusual (but not isomerization to the A-cis-a configuration; even the Ag+u n p r e ~ e d e n t e d ' ~in ~ ~Co - ' ~systems) ~~ induced chloride hydrolysis of the p isomer occurred with isomerization of the S,S-eddp backbone to the A-cis-a configuration.
A-cis-a
A-cis-a
A-cis$
A-cis$
The dianion of dipicolinic acid (dipic) acts as a tridentate, forming Na[Rh(dipic),] and a series of complexes of the form [Rh(dipic)(N-O)]*nHZO (N-0 = an N and 0 donating bidentate, including the conjugate bases of picolinic acid, nicotinic acid, isonicotinic acid, glycine and aminophenol).lW7The bis complexes (119a) are diamagnetic monomers, characterized by UV/visible and IR spectra, magnetic susceptibility, and conductance measurements. The [Rh(dipic)(N-0)] complexes are presumed to be polymeric, with two oxygen atoms of a dipic carboxylate bonding to two different Rh centers, thus maintaining octahedral coordination of each Rh'" (119b). The complex [Rh(L),Cl,]CI (L = isonicotinic acid hydrazide), in which L acts as an N-0 bidentate, has been characterized by elemental analysis, conductance, magnetic susceptibility and electronic spectrurn.IW8
(119a)
(119b)
When a mixture of RhC13.3H20 with Zn dust is added to a hot pyridine solution containing NJV"'ethylenebis(sa1icylideneiminate)(the Schiff base salen), yellow crystals of [Rh(salen)py(Cl)] (120) form.'099If the Schiff base is mixed with the RhC13-3H20before adding the reducing agent, a pale yellow solid, formulated as [Rh(salen)py], precipitates. The lack of solubility and reactivity of this diamagnetic solid hindered identification, and both polymeric'W9and dimeric Rh11,4'3analogous to [Rh(DMG),(py)],, have been suggested. This same material could be prepared by reacting a methanolic solution of salenH, in Zn(Hg) with [Rhpy,Cl,]C1.'200 Reaction of [Rh(salen)Cl(py)] with Na(Hg) or BHb in the presence of organic halides leads to [Rh(salen)R(py)] (R = Me, Et, Pr",
I
Rhodium
1048
Pr', Bun, MeCO, CH,=CHCH,, anikine).1099 The [Rhpy4C12]+ion reacts with a variety of Schiff bases in boiling pyridine solution (in the presence of Zn dust and PF;), to generate a family of complexes of the form [Rh(SB)py,lPF, (121).'100 These all formed methyl adducts with Rh"'-Me bonds when reacted with methyl i0dide.4'~A similar reaction path ([Rh(py),Cl,]Cl+ Zn dust + Schiff base) was used to prepare [Rh(N-O)2py2]+ (122). In the absence of Zn dust, [Rh(py),Cl,]+ is unreactive toward Schiff bases, even when refluxed in pyridine for many hours. The best yields are obtained when equivalent amounts of Zn and Rh are
nv
(121)
48.6.4.2
N-S
B
=
1,2-phenylene 4-methyl-l,2-phenylene 4,5-dimethyl-l,2-phenylene ethylene (salen) 1,3-propylene 1,4-butylene
-I+
(1 22)
bidentate complexes
Cyclohexanone thiosemicarbazide (C5HIOC=N-N(H)C=SNH2 = HL) reacts with RhCl,.3H20 in refluxing ethanol to form the 3:l electrolyte [Rh(HL),]Cl,, in which the neutral semicarbazide is a sulfur- and nitrogen-bonded bidentate. The conjugate base also forms the non-electrolyte [Rh(L),] in which the anionic thiosemicarbazide is again an N- and S-bonded bidentate. These complexes were characterized by conductivity measurements, magnetic susceptibility (both diamagnetic), IR and electronic spectra."" Another N-S bidentate, 6-methyl-2-thiouracil, forms [Rh(L)JCl, salts (123a),'In2and IR analysis indicates that the thiouracil is bonded through the S and the N (at the 3 position), to form four-membered chelate rings, rather than the S and N-1 bonding observed for [M(L),I2+ (M = Pt or Pd). A brown diamagnetic solid, which analyzes as [Rh(tu),Cl] (tu = the monoanion of thiouracil, where Htu = C4H4N2S2; (123b), has been characterized by TR and electronic spectra. It is 'highly insoluble in common organic and inorganic solvents', and IR analysis suggests a polymeric structure, with bridging thiouracil and chloride ligands, with the thiouracil bonded through the N and S atoms, although a specific bonding geometry was not ~uggested."'~Reaction of RhC13*3H,O with 4-amino-3,5-mercapto-l,2,4-triazole (H, TZ) leads to two incompletely characterized complexes: a brick-red solid identified as [Rh(HTZ),Cl(H,O)] and an orange-red [Rh,(TZ)3]."04Both the HTZ- and TZ2- ions are presumably acting as. bidentates, andI weak IR bands near 330 and 4 3 0 m - ' are assigned as Rh-S and Rh-N stretches respectively, suggesting coordination through a S and a N ;om. H
\
-.
I
A0
'0
I t ; \
(1 23a)
(123b)
(124)
Thiosemicarbazidodiacetic acid W,L) (124) reacts with RhCl3*3H,O in ethanol in the presence of Lewis bases (B), forming [Rh(L)(B)Cl]-nH,O (B = OH-, py, a-picolone, PPh,, aniline); in the Examination of presence of bipy or phen (NN), the product is formulated as [R~I(L)(NN)]C~."~~ models and IR analysis suggest that the thiosemicarbazide diacetic acid acts as a dianionic tripodal tetradentate, bonding to the Rh through two 0, an N and an S atom.
I
Rhodim 48.6.5 48.6.5.1
1049
Oxygen Ligands
The hexaqua im
Early references to Rh perchlorate complexes were presumably the first reports of the [Rh(H,O),I3+ ion. When freshly precipitated Rh(OH), is reacted with one-third of the indicated molar quantity of perchloric acid, the resulting species in solution was identified as 'Rh(0H): ','Io6 and recrystallization of Rh(OH), from perchloric acid led to a solid identified as 'Rh(OH),C10~.1'07 More recent work"OBshow that reaction of RhC13*3H,0with conc. HClO, leads (after recrystallization) to 'yellow, light, fluffy needles' of [Rh(H,O),](ClO,), with a face-centered cubic lattice. Isotopic dilution studies revealed a hydration number of 5.9 (+0.4)."09 It is acidic in aqueous solution (at 64 cC,l'I K, = 4 x and at 25 "C,pK, = 3.3I1Io),and attempts to predict its acidity on the basis of optical basicities shows that outer-sphere solvent molecules significantly affect the experimentally determined pK, value."" The crystals are hygroscopic, and Forrester noted that replacement of the coordinated waters 'may easily tK made by other ligands'.''08 Harris and co-workers have extensively studied the kinetics of water substitutions in [Rh(Hzo),l3+(equation 281; = H20,'Io9 c1-,I1'' Br- 'lI3). A two-term rate law was found in all three cases, as was an inverse [H+] effect. Reaction of the substituting ligand with either accounts for the two-term rate law, while the [Rh(H20),I3+,or its conjugate base [Rh(H20)50H]2+, inverse [H+] effect occurs because the [Rh(H,0)OH]2+ is several orders of magnitude more reactive than the hexaaqua ion, making the base-catalyzed path the dominant route in all but the most acidic solutions. For C1 and Br anations, the monohalo-substituted species were not isolated, as rapid substitution of a second halide leads to the (presumably trans) [Rh(H20),X2]+ions. The kinetic trans effect was also cited to explain the reactivity of the [Rh(Hz0)50H]2+ion; for the [Rh(H,0),X]2+ series, the order of the lunetic trans effect induced by X was shown to be OH >> Br > Cl.'lL3Kinetic studies of the anation of [Rh(HzO),l3+ by NCS- (equation 281; X = NCS-) have been presented by Pilipenko and co-workers;1114-1116 at 80 "C, k,, = 0.104 M-' s-I, k,, = 36.8s-', AH* = 1 6 0 k J m 0 1 - ~ , ' ~K~= ~ ,2.74 x lo3."''
x
[Rh(HZO)J3'
48.6.5.2
+X e
[Rh(HzO)sX]"+
+ H20
(281)
The tris(oxalato) ion
the [Rh(ox),13- ion has an electronic consistent with First prepared by an octahedral Rhos chromophore. Interest in the chemistry of this ion was piqued by a suggestion, based on solid state broad-line NMR and dehydration data, that what had been called 'hydrated K,[Rh(ox)J was actually K6[Rh(~~)3][Rh(~~)2(oxH)(OH)] -8H,0,"21 with one of the six oxalates acting as a monodentate, and an OH- in the remaining coordination site. This was challenged by Gillard and coworkers,'"' who noted that aqueous solutions of [ R ~ ( o x ) ~are ] ~ -neutral, the NMR shows no rapidly exchanging protons and the Raman, electronic and CD spectra are virtually invariant between pH 2 and 11. Their differential thermal analysis also did not support the monodentate oxalate model."22 A single-crystal X-ray structure showed that in the solid state, K, [Rh(ox),] is a tris-chelated, distorted octahedral complex, but that the K+ ions are unusually Each K + ion is coordinated by seven H,O molecules, and sits directly close to the Rh center.1123 above a triangular plane of oxygens of the Rh coordination sphere only 3.48 A from the rhodium; the earlier suggestion1I2'of a monodentate oxalate ligand was readily explained in terms of this structure."23 The [Rh(ox),I3- ion was initially resolved as the strychnine salt,"" and more recently by precipitation of (-)[Co(en>,]( -)[Rh(o~),]."~~ The CD spectrum of ( +)-[Rh(ox),I3- in a crystal of NaMgAl(ox), * 9H,O is consistent with a trigonal structure in solution, again inconsistent with the monodentate model.'12sThe slow racemization of [Rh(ox),I3- was first noticed by Werner,"17and Ode11 et al. have studied the acid-catalyzed racemization;"26 at 44.6 "C in a 1 .O M HClO, solution, the rate of racemization is 5.03 x lO-'s-' with a AE* of 84kJmol-l. Ode11 did not suggest a racemization mechanism, but noted that the 'rate of racemization increases at a rate greater than the stoichiometric acidity'.Ikz6 Reaction of [Rh(ox),],- in refluxing perchloric acid leads to cis-[Rh(ox),(€€,O)zj-; subsequent substitutions lead to cis and trans isomers of [Rh(ox),X,]"- (X = H,O, C1-) which have been characterized by electronic and IR spectroscopy."" Exchange of labelled ox2- ion with [Rh(ox)$proceeds via a three-term kinetic expression, with a neutral path (k = 6 x 1W6s-' at 133.2 "C) as
Rhodium
1050
well as acid- (kH+= 1.5 x 10 'M-'S-') and base-catalyzed paths (koH-= 1.3 x 102M-1s-1),"2' Replacement of one oxalato ligand by two waters (equation 282) was shown to be acid catalyzed, and presumed to involve a rapid acid-base preequilibrium between the [Rh(ox),I3- and [Rh(ox),(mon~-ox)(H,O)]~-. A related study'I2*found AH* = 113 kJmol-I for equation (282). [R~(ox)~]'+ 2H20
4
[Rh(ox),(H,O),]-
+ ox2-
(282)
A series of reports by Milburn and coworker^^^^^^'^^^ has given a convincing model for the reactivity of [Rh(ox),I3-. The acid-catalyzed exchange of oxygen atoms (between solvent water and the coordinated oxalato ligands) has been studied for a variety of complexes ([M(ox),I3- for M = Co, Cr and for [Pt(ox)J2 ) and in each of these cases all 12 (or eight for Pt) oxygen atoms exchange at indistinguishable rates. For [Rh(ox),I3-, however, half of the oxygens exchange more rapidly than the other half.660The outer oxygens (the more rapidly exchanging, uncoordinated atoms) exchange at a rate similar to those of [Pt(ox),]'- and H ( o x ) - . ~ The ~ ' rates of inner-oxygen exchange and racemization (of [Rh(ox)J-) are also similar. A comprehensivestudy662of the kinetics of racemization and aquation of [Rh(ox),I3- showed that both racemization and aquation proceed by two-term rate laws: Rate of racemization Rate of aquation
=
=
{kr2[H+]+ k,,[H+l*}([Rh(o~)~]~-}
(k,,[H+]
+
k,,[H+ ]*}{[R~(ox)~]~-}
Activation parameters for these reactions, and for inner-oxygenexchange, are given in Table 84. The similarity in the rate of inner-oxygen exchange and the second-order racemization term (kr2) suggests parallel mechanisms. The racemization is significantly faster than the aquation (at 56 "C, when 50% of the ions have racemized, only 3% have aquated), and the rate order for the four processes is: outer-oxygen exchange > inner-oxygen exchange racemization > aquation. This is rationalized in Scheme 39 in which outer-oxygen exchange (steps 1 and 2) does not require ring-opening before exchange, and thus should not be very sensitive to the nature of the metal; the similarity of exchange rates for Rh"', Pt" and H+ oxalates supports this. Odell's observation"26that the rate of racemization increases at a rate greater than the stoichiometry is also rationalized by this scheme.662 The one-electron oxidation of [Rh(ox),I3--by Ce4+in sulfuric acid solution (1 M) is two orders of magnitude slower than the analogous reaction of [Cr(ox),13-, but there are many similaritie~."~~ The first step is presumed to be oxidation of coordinated oxalate to the ox- radical anion, followed by the formation of (presumably cis) [Rh(ox),(H,O),]-, Ce"' and two squivalents of C 0 2 . A transition state with an oxalate acting as a bridge between Rh and Ce (with bidentate linkages to both metals) was proposed, but is not supported by the large (positive) entropy of activation for the reaction (AS* = + 146 J K-' mol- I ; AH* = 136 kJ
48.6.5.3 Acetylacetonato Complexes The neutral [Rh(acac),] complex was first prepared by reacting Rh(NO,), with acetylacetone in an aqueous (pH 4) solution;"30 the [Rh(hfacac),] and [Rh(tfacac),] (hfacac = hexafluoroacetylacetonate; tfacac = trifluoroacetylacetonate) analogs were later prepared in a similar ~nanner."~' These complexes are extremely stable, as [Rh(acac),] can be recovered unchanged from boiling dilute acid, cold concentrated sulfuric acid, and from 10% caustic soda solutions. The pseudo aromaticity of the chelate rings is demonstrated by the NMR of the ring proton (4.42), and by the nitration (using hydrated Cu(NO,), 3H,O/acetic anhydride solutions) and monoformylation (using POCl, in dry DMF) of the chelated rings."31 The nitrated species is resistant to reduction,
-
Table 84 Activation Parameters for Reactions of [Rh(ox),]'Reaction
Ted
AH(kJmo1-')
AS(JK-'mol-l)
Ref.
k,,
97.5 k 6.3 112.1 f 6.3 97.9 f 8.4 106.7 f 8.4 108.4 f 8.4
-34.3 & I7 +24.3 17 -26.4 4 25 - 32.6 25 - 12.1 f 25
662 662 660 662 662
Racemization
kr3
Inner-Q exchange Aquation
kiL2
kd 'Labeled terms given in text.
+
scheme 39
as an acidic tin(I1) chloride solution leaves the nitro function unaltered, while reaction with Zn/HAc causes decomposition."3' IR studies of the adducts [Rh(acac),] -2(urea) and [Rh(acac),] 3(thiourea) indicate that the urea and thiourea are not directIy bonded to the rhodium, and that hydrogen bonding is important in the structure of the urea, but not the thiourea, adduct."" If the reaction between hydrated RhC1, and hfacac is carried out in dry ethanol, two different species (brown-yellow and red sublimates) of the composition (RhCl(hfacac),(H,O), } can be separated. While their insolubility makes study difficult, they were assumed to be the two possible geometric isomers of the doubly chloro-bridged dimer [(hfacac), Rh(p-Cl), Rh(hfaca~),]."~~ Electron impact ionization potentials of successively substituted acac complexes [Rh(acac),(3NO,acac),-,,I (n = I , 2 or 3) has shown that the first ionization potential is IinearIy related to the number of nitro substituents, increasing as the number of NO2- substituted acac ligands increases. The authors argue that such a monotonic relationship implies that ionization occurs from a metal-centered orbital, rather than from the highest filled orbital on any of the ligands."34Ionization From a metal-centered orbital is unexpected, as the lowest energy electronic transitions for these complexes have been assigned to ligand-centered nwc* transitions (unlike the metal-centered d-d transitions observed for the analogous Co"' and Cr'" complexe~),"~~ and luminescence of [Rh(acac),] occurs from the ligand dominated n* --* n t r a n ~ i t i o n . "(Rogerson ~~ does note that ionization from ligand orbitals could be rationalized if a delocalized ring model is used."34) Reaction of the dioxygen adduct of [RhCl(PPh,),] with Hacac in room temperature benzene leads to a hydroperoxo complex of Rh"', identified as [Rh(PPh,),(OOH)Cl(acac)]. In the presence of triphenylphosphine, triphenyl phosphite is released, with the formation of [Rh(PPh,),(OH)Cl(acac)], with trans phosphines (geometries were determined by 31PNMR).'"' Morris et al. have presented an interesting study of the ion dissociation reactions of the molecular ions of [Rh(acac)J and fluorinated analogs (125) (despite the article's title, which identifies Rh as rheni~m)."~'The ion reactions are surprisingly sensitive to changes in ligand composition. For [Rh(hfacac),] over 60% of the metal-containing ions are in the successively formed Rh(hfacac): , Rh(hfacac): and Rh(hfacac)' ions. The Rh(hfacac)+ ion is formally a Rh" species, and unlike the Co it is reduced further, generating numerous Rh' ions (e.g. Rh(CO)', RhCF: ). In sharp contrast, Rh(tfacac): is the base peak in the mass spectrum of the trifluoro analog [Rh(tfacac),] and undergoes virtually no reduction to Rh" or Rh' species. For the unfluorinated [Rh(acac),], an ion reaction pattern similar to that of [Rh(hfacac),] is observed.
-
Rh(-c:c2 0-cy
(125) C.O.C. 6 H H '
[Rh(acdc),] Y = X = Me [ R h ( t f i t c a ~ ) ~ ] X = Me, Y = CF3 [Rh(htacac),l X = Y = C'F3
1052
Rhodium
Unlike fluorinated acac complexes of most other metals, there is no evidence of F- migration to the metal in either of these fluorinated acac complexes of Rh"', but methyl migration does occur in the decomposition of [Rh(acac),]. Morris comments that Rh is the only example of the metals so far investigated that shows such sensitivity to changes in the ligand composition. When compared to their ruthenium analogs, the authors conclude that bonds to rhodium appear to be more easily broken and formed, an important aspect of the well-known catalytic activity of rhodium."3s Photolysis of [Rh(tfacac),] (tfacac is the unsymmetrically substituted 1, l ,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths; the relative efficiency of the two paths is dramatically solvent In cyclohexane, mer -,cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems 'strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of mer-[Rh(tfacac),] and suggests that the photoreactive states have considerable radical character'.Ila Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved.
48.6.5.4
Dioxygen complexes
Complexes containing the Rh-0, moiety are now well known, but there have been a few false starts and misunderstandings along the way. The first claim for an 0, complex of rhodium was made by Wilkinson and co-~orkers,"~' who reported that [Rh(H)(CN),(H,0)I2- reacted with O2to give the peroxide-bridged dimer [(H,O)(CN), Rh-02-Rh(CN)4 (HZ0)]". This was later disputed, and the reaction product identified as the protonated, peroxo monomer of Rh"', [Rh(0,H)(CN),(H,0)]2-.1142 The solid is diamagnetic, and displays 0-0 stretches at 839 and 825cm-', characteristic of an unsymmetric monomer; no such bands appear in the Co-peroxo dimer."". Reaction of [Rh(py)4C1,]C1. 5H20with O3 (in 0,) in alkaline ethanol leads to solutions which Addition of perchloric acid and recrystallizpass through yellow-orange to green to deep blue.1143 ation from conc. HC1 leads to deep blue crystals identified as the superoxo-bridged dimer . Variations on this theme are possible, as analogs have been [Cl(py), Rh-02-Rh(py),Cl](C10,), prepared in which the N-heterocycle is pyridine, 4-methylpyridine or picoline; if the reaction is run in base, chloride is replaced by OH- (or H,O, depending on the pH). The dimers are paramagnetic, and have low energy electronic transitions characteristic of 0;. They are strong one-electron oxidants in H 2 0 , and reaction with I- and Fe2+leads to the peroxo-bridged dimer; the superoxo species is not oxidized by MnO;, C1, or Ce4+, consistent with its assignment as a superoxo species. The analogous [Cl(bipy), Rh-0,-Rh(bipy),C1](ClO4), has been prepared by the reaction of [Rh(bipy),]ClO, with 0, and C1, in MeCN solution.Il4 The superoxo assignment is supported by an ESR spectrum showing S = 4with g factors virtually identical to those of the analogous Co"' complexes.1145 Photolysis of cis-[Rh(en),(NO,),]+ in the presence of 0, leads to a red species, identified as the monomeric superoxo cation [Rh(en),(NO,)(O,)]+, isolated as the chloride or nitrate Elemental analyses were not presented, but electronic spectra show a low-energy charge-transfer transition (485 nm) characteristic of the superoxo ligand, and the Raman spectrum shows an 0-0 stretch at 1052 cm- '. The cis isomer is much more reactive than the trans, and in the presence of Rh"' aqua species, the red-brown monomer reacts to form the familiar blue or purple dimers, [X(en), Rh (X = C1-, NO,, or H,O). The ESR spectra of a variety of these complexes are -0,-Rh(en),X]"+ consistent with the superoxo a~signment.''~' A molecular-orbital-based bonding model suggests that the unpaired electron is in a n* orbital located largely on the O,, consistent with the [Rh"'-O;] designation, and with the tendency of the monomeric species to Reaction of [Rh(CO),Cl], with tetraphenylporphyrin (TPP) in glacial acetic acid generates a material identified as '[Rh(TPP)]' and the authors state that they found 'no evidence . , . for the formation of oxygen complexes . . .'.862 This was questioned by Wayland and Newman*@who found that octaethylporphyrin (OEP) formed a monomeric Rh"' superoxo complex [Rh(OEP)(O,)]. Its spectral and chemical properties strongly support the superoxo designation; reexamination of the TPP complex implied that the previousiy identified Rh" complex [Rh(TPP)] is actually the Rh"' superoxo complex [Rh(TPP)(O,)]."@ Dioxygen (in air) reacts with the (presumably) Rh" dimers
1053
Rhodium
[Rh(L)],(L = the tetradentates, salen, salpn or saloph -see Section 48.6.4.1) to form paramagnetic (peE= 1 .SO BM at room temperature) hyperoxo-Rh"' c o r n p l e x e ~ ~ ~ ~
48.6.5.5 Miscellaneous oxygen ligands Solid rhodium sesquioxide, R h2 0 3 ,was first identified in 1927,'148and has the corundum struct ~ r e " ,(hexagonally ~ close packed 02-anions distorted by the presence of Rh3+cations in $ of the octahedral interstices). Each Rh is surrounded by six oxygens, three at 2.03 A and three at 2.07 A. An orthorhombic phase forms above 750 OC.'Iso There are two examples of chiral 0-0 chelates (camphor derivatives) which form neutral [Rh (0-O),] complexes. Reaction of RhC1,- 3 H 2 0 with the fl-keto aldehyde (+)-hydroxymethylenecamphorate (+hmc; 126a) in refluxing benzene leads to the two fuc diastereoisomers D-(-)[Rh( + hmc),] and L-( +)-[Rh( f h m ~ ) ~ ] separable , ' ' ~ ~ on an alumina column, and characterized by NMR, ORDjCD and UV/visible spectra. Unlike the Co"' analog, no mer isomers were detected. When RhC13-3H20is suspended in a hot, alkaiine solution of (+)-3-acetylcamphorate (+atc; 126b) for 20 h, four diastereomers (both A and A forms of both the mer andfuc isomers) of [Rh( +ate),] were separated via silica gel plate chromatography, and identified by NMR, ORD/CD and UV/ visible R \
(126) a: R b: R
= =
H, (+)-hydroxymethylenecamphorate (+ hmc) Me, (+)-3-acetylcamphorate(+ atc)
Several complexes with DMSO ligands have been prepared,'L53-1155 and IR spectra and the ease of formation of [Rh(DMSO),Cl,]- in C1- saturated aqueous solution led to the suggestion that [Rh(DMSO),](BF,), has four 0-bonded and two S-bonded DMSO ligands.'154An X-ray structure of [RhCl,(DMSO),] (prepared by DMSO substitution at truns-[Rh(DMSO),Cl,]-) shows that two of the DMSO ligands are S-bonded, while the third is bonded through its oxygen atom."" The chlorides in [RhCl, (DMSO),] lie along a meridional edge, with the two S-bonded DMSO ligands in mutually cis sites, in contrast to a calculation which indicated that mutual repulsion would prevent mutually cis S-bonded DMSO ligands.''56 A study of the thermodynamics and kinetics of the reaction between [Rh(H,O),I3+ and the NpO: ion (equation 283) found an equilibrium constant of 3.31 -t 0.06 at 25°C (AH = - 15.0 & 3.8 kJmol-I; A S = -42 & 13 J K - ' rn~l-l)."~'In the presence of excess Rh"' the reaction is pseudo first order in [NpO:] (AH* = 114 k 4 kJmol-I; AS* = 36 -t 4 J K-' mol-'). The reaction is also first order in F- ion, but the role of the fluoride ion is not understood. Fluoride ion does not catalyze water exchange at [Rh(H,0),I3+, and the rate of formation of [ONpORh(H,0)J4+ is twice as fast as the rate of H,O exchange at the hexaaqua ion, so it is unlikely that a dissociative ligand exchange is rate-determining in the dissociation of [ONpORh(H,0)j]4+.'157Mossbauer spectroscopy of the 237Npnucleus in the product shows that the Np is in the 5 + oxidation state, is not magnetically split by the Rh atom and that the neptunyl ion maintains its axial symmetry, supporting the suggestion that an oxygen atom of NpO: simply replaces a water in the inner coordination sphere of the Rh".''54 The presence of the actinide gives [ONpORh(H,0)s]4+ a rich electronic spectrum, but formation of the Np-0-Rh linkage hardly changes the IR and electronic spectra observed in the separate NpO: and [Rh(H,0),I3+ moieties. [Rh(H20)6]"
+
O-Np-O+
e
[O-Np-O--Rh(H,O),~+
+
HzO
(283)
In an effort to use [Rh(H,0),I3+ as a redox-stable, diamagnetic probe, its reactivity with a variety of polyphosphates has been e x p l ~ r e d . " ~Several ~ ~ " ~ ~complexes of the form [Rh(H,O),(PP)Y(PP = 2PO:-, HP,O:-, HP,Oi-, ADP or ATP) have been prepared, and characterized by electronic spectroscopy."60 The potential tridentate H2P3Q0(PPP) is reported to form [Rh(H,0)(PPP)]."60
1054 48.6.6
Rhodium
Sulfur and Heavier Group VI Ligands
48.6.6.1 Monodentate sulfur ligan& I . '
,
Alkyl and aryl sulfides coordinate to Rh"', Dwyer and Nyholm first reported that refluxing aqueous ethanolic solutions of RhCI,. 3H,O and diethyl sulfide leads to the neutral [RhCl, (Et2S),]."6' The tribromo and triiodo analogs were also reported; all three complexes are prone to dissociation of the sulfide ligand, as they smell of diethyl sulfide, and cryoscopic measurements in benzene consistently gave less than the theoretical molecular weight values. A study of their dissociation pressures showed that the stability of the Rh--S bond is greatest for the trichloro, and weakest for the triiodo They are diamagnetic monomers, electronic spectra (ethanol solutions) have been recorded, and X-ray powder patterns show that they are similar to the Ir analogs,"63 and thus were assigned the mer geometry."62 Reaction of [RhCl,(R,S),] (R = Me, Et, Pr) with bipyridyl has led tofuc- and mer-[Rh(bipy)(R,S)Cl,] (R = Pr), while reaction with pyridine has generatedfac and mer isomers of [Rhpy(R,S)2C13](R = Me, Et).1164 Walton prepared complexes of the form [RhX3(C4H,0S),](X = C1 or Br, and C&OS is the cyclic 1,Qthioxane) and characterized them by far-IR, IR, NMR and electronic spectro~copy."~~ The far-IR spectra imply a fac geometry. Initial loss of a labile sulfide group is thought to be the first step in the catalytic reactions of [RhCl, (EtzS)J. In a basic solvent (which promotes sulfide labilization) it catalyzes the homogeneous hydrogenation of various alkenic substrates.'L66 The kinetics of the hydrogenation of truns-cinnamic acid or maleic acid using [RhCl,L,] (L = EtzS or (PhCH,),S) as catalysts have been interpreted to suggest a path involving initial loss of a sulfide, followed by reaction of the coordinatively unsaturated Rh"' complex with HI and reduction to Rh'.''6"''6' The intermediacy of a RhI" hydride was also postdated. Initial dissociation of a NCS- ligand is also thought to lead to the electrochemically active = -0.36V vs. SCE) was observed in a [Rh(SCN)j]2- ion. One irreversible reduction wave polarographic study of Rh"' in aqueous NCS- solution, but the system was complicated by poisoning of the DME due to catalytic decomposition of SCN- and sulfate formation.''68 Refluxing RhC1,-3H20 with a variety of alkyl phenyl sulfides (PhSR for R = Me, Et, Pr",Bun) in methanol is a straightforward path to both isomeric forms of [RhC1,(PhSR)3].'169The mer forms have been characterized in all four cases, and thefac form was cleanly characterized for R = Et and Pr"; reaction of LiBr (in acetone) with mer-[RhCl,(PhSEt),] leads to mer-[RhBr,(PhSEt), 1. The trisulfide, MeC(CH,SEt), reacts with RhC13.3H20 in refluxing ethanol to give fac[RhCI,(MeC(CH,SEt), }I.'"'
48.6.6.2
Sulfur chelates
( i > Four-membered rings
Stimulated by an interest in determining the position of sulfur in the spectrochemical series, Jergensen prepared and studied the electronic spectra (Table 85) of [Rh(dtp),] and [Rh(dtc),] (dtp = (Et0)2PS; : dtc = Et2NCS;).''77 The complexes were prepared by heating an aqueous solution of RhCl,-3H20 with the sodium salt of the ligand; the addition of a small amount of ethanol 10 the solution (see Sectjon 48.6.2.5 .iii) allows the reaction to proceed at room temperature and yields a cleaner product. A series of metal dithiocarboxylates (RCS; ), including [Rh(dtb),], (NH,)[Rh(dtb),Cl,] and [Rh(dtpa),], has been characterized by magnetic measurements, molecular weight determinations and electronic Because of the strong intraligand bands, characteristic of dithiocarboxylates, they display rich electronic spectra (Table 85), but the low energy transitions could be assigned to the transitions expected for a high-field, d6 Rh"' center in an octahedral environment. The syntheses involve the straightforward reaction of Rh"' salts with the Na salt of the ligand, but of the numerous metals studied, Rh is unique in that the nature of the resulting (dtb) complex depends on the nature of the Rh"' starting material. If RhCl3-3H,O is used, [Rh(dtb),] forms, but if [RhC1,I3- reacts under virtually the same conditions, the product is [Rh(dtb)2C12]-."74 The N,N-disubstituted dithiocarbamate ligand (R2NCS; ) stabilizes high oxidation states of the some first row transition metals, and BF, oxidation of [Rh(Me,NCS,),] was reported to lead to the Rh" This was questioned by Hendrickson et ul., who showed that the product is actually the dimeric cation [Rh2(Me,NCS,),]+.1'79Characterization was by conductivities, IR and NMR The spectroscopy and by a single crystal X-ray structure of [RhZ(Me,NCS,),]BF4-0.5CH,Cl,.
Rhodium Table 85
1055
Electronic Spectra of some Sulfiu Complexes of Rhodium(II1)"
515sh, 290 305, 262 304 (26 400), 262 (20 200) 424sh, 34.6, 274, 219 422sh ( < lOO), 358sh (i 580), 285 (2910), 232 (1930) 490, 373, 260, 221 431 (loo), 354 (300), 266, 219 455, 366, 263, 227 460sh, 405 (692) 510sh (912), 472sh (19001, 391 (6600), 334 (1 6 200), 306 (I 7 400) 535 (1320), 510 (1410), 426 (3800), 360 (91 20), 328 (43 700) 420sh (30201, 382 (5370), 320sh (14500), 291sh (29 500), 260 (63 100) 526 (19000), -240 (-81000) 350 (2270), 320sh (1900), 252 (26950) 350 (1935), 255 (20 300) 370 (2180), 245 (24 150) 405 (2460), 320 (8550) 470 (1090), 410 (7801, 321 (12600), 273sh (18 MO), 258 (23200), 227sh (8000) 423, 282, 252: 226 *Ligand abbreviations in text. bDiffuse reflectanoe spectrum. 'In MeCN solution. solution. 'In CCI, solution.
1 I70 1171 1172 1 165b 1 165'
1165h 1165 1165 1173 1174d
I 174d 1174 1175e 1176 1176 1176 1176 1177 1177 CHC13
cation consists of a [Rh(Me,NCS,),]+ unit coordinated to an octahedral [Rh(Me,NCS,),] unit through two mutually cis sulfur atoms (127).The two Rh atoms have opposite absolute configurations, and the long Rh-Rh distance (3.556A) indicates that there is no significant metal-metal b~nding."~'The structure is similar to that of the Co"' analog,"8o but is in sharp contrast to the [Ru(Et,NCS,),]+ cation, which has two distinct types of bridging dithiocarbamates and a short Ru-Ru distance.llgl
c
(127)
Reaction of 'commercial rhodium trichloride' with HS2PF, (solvent unspecified) leads to a A diamagnetic, red-orange solid, it is monomeric in the gas sublimate identified as [Rh(S2PF2)3].i's2 phase. While one of the least reactive of the diffuorodithiophosphate metal complexes, it is rapidly hydrolyzed in air. IR analysis implies that the ligands are chelated. Red crystah of [Rh(S,PPh,),] can be isolated after refluxing rhodium trichloride and diphenyldithiophosphate in CHCl, /CCl, solution.'
(ii)
Five-membered rings
Complexes of the form [Rh(S-S),X,]Y (X = C1, Br; Y = C1, ClO,; 0.5S20,,PF, or BPh,), involving two different S-S bidentate [S-S = 2,5-dithiahexane (dth), MeSCH,CH,SMe or 1,2di(pheny1thio)ethane (PhSCH,CH,SPh)] have been characterized.II6' The methyi-substituted complexes [Rh(dth),X,]+ are water soluble and insoluble in organic solvents, while the phenyl derivatives are soluble in MeCN and MeNO,; electronic spectra imply both complexes have the trans geometry. An incompletely characterized, very insoluble complex of the composition [Rh(dth)CI,] was also isolated, and is assumed to be a dichloro-bridged dimer. These dithiahexane complexes
Rhodium
1056
have resisted spirited efforts (both chemical and electrochemical) to oxidize the rhodium center to the 4+ oxidation state. Both [Rh(dth),]BF, and [Rh(dth)Cl,], are unaffected by C1, in chloroform or by Ce4"; [Rh(dth)Cl,]- could not be oxidized by Cl, or by cyclic ~oltametry."'~ The dynamics of inversion at coordinated sulfur have been studied by variable temperature 'H NMR for a series of 2,Sdithiahexane complexes, including [Rh(dth)&]- (X = C1, Br).lLUAt room temperature in DMSO-d6,the [Rh(dth)ClJ ion displays two NMR peaks due to the SMe groups in the meso and the (i-) isomers; as the temperature increases, the peaks broaden and coalescence occurs at 100 "C. An interfering reaction occurs in DMSO (equation 284), resulting in dimeric [Rh2(dth),C1,j. Detailed interpretation of the NMR of this material was not done (five possible geometric isomers), but a trans geometry was assumed. The AG* of coalescence for [Rh(dth)XJ ( X = C1, Br) and [Rh(dth),Cl,] are identical, within experimental error (87.3, 85.5 and 85.3 kJ mol I , respectively)."84 +DMSO [Kh(dth)Cl,]- -c~ [Rh(dth)(DMSO)Cl,]
+
0.5[Rh~(dth),C&,]+ DMSO
(284)
The crystal structure of tris-(S-methylene-l,2-dithiolato)rhodium(I1I) ([Rh(S,C,H,]) shows it to be octahedral with no intraligand S-S interactions in the chelated ligands.'Is5
(iii)
Six-membered rings
The dithio derivative of acetylacetone (sacsac) forms a neutral, diamagnetic monomer with Rh"'. The synthesis of [Rh(sacsac),] requires the substitution of sulfur for oxygen in (presumably) coordinated acac ligands (the sacsac ligand has not been isolated in the monomeric state), and is accomplished by bubbling H,S through an ethanol/HCl solution of RhC1, 3H,O and acetylacetone at 0 0C."75The resulting [Rh(sacsac),]-O.S Me,CO, in the form of red needles, is stable up to 250 "C, and shows two 'H NMR signals in a 1:6 ratio (ring proton at 6.95 and the six methyl protons at 2.36 6). Unlike its Fe"', Ru"' and Os"' analogs, which all undergo a reversible one-electron reduction near 0 V (w. Ag/AgCl), [Rh(sacsac),] undergoes an irreversible two-electron reduction at - 1.05 V (vs. Ag/AgC1).'1s6A single crystal X-ray structure confirmed that the complex is monomeric, and that the large cavities in the lattice are suitable to hold the acetones of crystallization (the acetones were not found in the crystal structure, h~wever)."'~The complex is a distorted octahedron, with intraligand S-Rh-S angles near 97 ", and non-planar chelate rings. The unsymmetrically substituted dithioacac ligand, O-ethylthioacetothioacetate (O-Etsacsac), reacts with an ethanolic solution of RhC1,.3H20 to form [Rh(O-Etsacsac),]. The orange-red complex is kinetically stable, and its IR and far-IR spectra suggest an octahedral complex with three bidentate ligands in afuc geometry.'Is8The authors argue that the position of the methine proton in the NMR spectrum of [M(sacsac),] (or an O-Etsacsac analog) is not a valid measure of the degree of aromaticity in the chelate ring. A comparison of complexes for a variety of metals shows that the chemical shift of the methine proton is linearly related to the metal's d shell population, the molecular geometry, and the nature of the substituent, but is not a reliable measure of the pseudo aromaticity of the sacsac chelate
-
(iv)
Cyclic polysulJides
The cyclic, tetradentate thioether (TTP = 1,4,8,1l-tetrathiacyclotetradecane) reacts with RhX3*3H,0in boiling ethanol to generate [Rh(TTP)X,]X (X = C1, Br)."" If LiI or LiN02is added to the RhC1,.3H2O solution before the addition of the TTP, the corresponding diiodo or dinitro cations were formed. The complexes are 1: 1 electrolytes in MeNO,, and display IR and electronic spectra (Table 85) consistent with a cis geometry. The ligand field splitting of the sulfur ligands is comparable to the analogous macrocyclic amines. If the ethanol used in the preparation is not boiling, an insoluble material of the composition [Rh(TTP)C1],C12, forms, presumably a polymer or a chloro-bridged
48.6.6.3 Miscellaneous group V I Iigands The thiocyanate and selenocyanate ions (SCN- and SeCN-) react with RhCl,.3H20 to form Both the SCN- and the SeCN- are chalcogen-bonded, consistent with the soft character of the Rh"' in many complexes.
[RhXJ3
1057
Rhodim
Two different selenium-containing bidentates have been coordinated to Rh"' centers. Reaction of 1,Zdiselenocyanatoethane (Se,E = SeCNCH,CH,NCSe) with RhC13.3H20 in refluxing acetone/ ethanol (1: 1) solution generates a diamagnetic dimer [Rh,(Se,E),Cl,]. It is non-conducting in DMSO, and reacts with p-toluidine (L) in refluxing MEK to give the diamagnetic monomer [Rh(Se,E)(Cl),(L)] (equation 285)."*' IR spectra show the vCN of the SeCN- is hardly affected by coordination to the Rh*ll, implying that Se,E is acting as a bidentate selenium donor. Reaction of RhC1,. 3H,O with the dipotassium salt of I , 1-dicyano-2,Zdiselenoethene(KSe),C=C(CN), in aqueous solution containing (Ph,P)Cl causes precipitation of red 'semi-solid masses', identified as the monomeric [Ph,Pj,[Rh{Se,C,(CN),},], with an octahedral Rh"' surrounded by three diselenolate dianionic bidentates."YoConductance measurements indicate it is a 3: 1 electrolyte in acetone, but attempts to recrystallize this salt caused decomposition and the formation of solid diselenides.
Cl
CI
Collman and co-workers have presented synthetic routes to a variety of thiolato and selenido complexes of Rh"' with the heavy atoms covalently bonded in a linear array; one-dimensional, soluble polymers with a heavy-atom backbone surrounded by organic ligands -complexes which should have unusual electrical properties -are the ultimate Oxidative addition of diphenyl diselenide to the very reactive square planar Rh' complex [difluoro-3,3'-(trimethylenedinitrolo)bis(2pentanone oximato)borate]Rh' (abbreviated as [RhL4])"92,' 193 leads to t~ans-[RhL,(SePh),j.'~~~ A single-crystal X-ray structure shows the phenyl selenides are both bonded to the Rh"' through the Se atoms, and are truns to each other. Numerous complexes, including trans-[RhL,X,] (where X = HS, SGeEt,, SeGeEt, and the monothiol trans-[RhL,(H)(HS)], have also been characterized. ' 19' 48.6.7
Hexahalo and Aquahalo Complexes
First reported in 1951, K,[RhCl,] was prepared by mixing 'previoudy ignited NaCl' with finely ground Rh in a combustion furnace at 750 "C while C1, was passed through the mixture."95Work-up in aqueous HCl led to a solution which contained the [RhCl6I3-ion. This octahedral ion undergoes consecutive aquations in acidic aqueous solution (vide infra), but it is unusually photoinert for a Rh"' complex. The quantum yield for aquation of the low-energy ligand field band (518 nm) is 0.02, an order of magnitude less than the higher ligand field strength amine c ~ r n p l e x e s .Raman ~ ' ~ ~ spectral studies of the analogous [RhBr613-ion show that the ion has an octahedral ground state,ii97and resonance Raman and far IR studies of [RhBr,13- in AgBr crystals are consistent with the expected (cubic) ground state."98 The [trenH3l3' cation has been used to precipitate the [RhCl3X3I3-(X = C1, Br, I) and [RhBr613anions;"99 the electronic spectra are given in Table 86. If a solution of Na,[RhCl,] or RhC13.3H20 is extracted with a benzene solution of a long chain quaternary ammonium chloride, silica gel separation of the organic layer leads to (NXJ3[Rh2C19](X = Et or a C, to Cloaliphatic chain)."" Characterized by far IR and UV/visible spectra, the X-ray powder patterns show that [NEtJ, [Rh,Cl,] is isomorphous with its Cr'l' and thus consists of two pyramidally distorted RhC16 octahedra sharing a face of three C1 ligands. leads to a variety of aquachloro complexes of the form Hydration of [RhCl$; the spectra and reactivity of these ions have been thoroughly studied, as [Rh(H20),C16_n](n-3)+ aqueous solutions of RhCl3-3H20vary from yellow to various shades of red and brown. Approximate formation constants have been determinedim and synthetic routes to specific isomers have been reported. I2Oi The ubiquitous 'RhC13-3H20'appears to contain a variety of aquachloro species, as do solutions prepared by aquation of [RhCI,]'- or 'hydrous rhodium oxide'. Solutions of 'hydrated RhCl,', treated with base, perchloric acid and then with various amounts of LiCI exhibited Io3RhNMR signals which can be assigned to the 10 possible isomers of [Rh(H,0),C~-,]'"-3'+(n = &6).1206
Rhodium
1058
Table 86 Electronic Spectra of some Aquahalorhodium(II1) Complexes"
[R~(H,o),I~+ [Rh(H,0)5Cl]Z+ cis-[Rh(HzO)4Cl,]+ rruns-[Rh(H,O),C1,]+ mer-[Rh(H,O), Cl, ] f~c-[Rh(H,O),c1,I ci~-[Rh(H,O),Cl,1[Rh(HzO)zc14 1[Rh(H,O)CIS]Z[RhCl,]'[RhBr6l3[RhCl, Br,]'[RhCl3I,I3[Rh, C1J3 [Rh(H,O), Br]'+ ~
ci~-[Rh(H,O)~Br,l+ ~runs-[Rh(H,O)~Br,]+ mer-[Rh(H,O), b l [Rh(HzO)5112+
396 (62.0), 31 1 (67.4) 395 (57.9), 323 (32.5) 426 (50.4), 335 (50.0) 420 (50.4), 335 (52.4) 450 (- 68), 355 (- 72) 450 (64.9), 349 (49.5) 450 (-78), 350 (-60) 471 (77.1), 370 (71.6) 471 (85.2), 370 (71.6) 474 (68.3), 376 (93.5) 474 (78.4), 376 (99.6) 492 (101), 392 (113) 484 (99.6), 384 (78.2) 488 (72.0), 385 (54.1) 507 (72.8), 402 (73.4) 518 (111.5), 411 (93.8) 562, 441, 340 541, 457, 303 332 540 (98.0), 435 (308.8) 550sh (37), 454 (81), 350sh (81) 441 (70.6), 342 (70.6), 268 (582) 478 (122), 208 (34000) 483 (130), 256 (33000) 505 (155), 256 (16000), 213 (25000) 548 (98.4). 329 (567)
1200 1112 1200 460 1201 1200 1201 1200 1202 1200 1202 1203 1203 1200 1200 1200 1199 1199 1199 1204 460 460 1113 1113 1113 460
a Spectra recorded in acidic aqueous solution, except that of [Rh2C1,f-, which was recorded in benzene. Isomer assignment originally not given. Later assigned as tram isomer.'203
Commercially available samples (from three different suppliers) of 'RhCl,. 3H20' (in aqueous solution) initially displayed different '03Rh NMR spectra, with varying amounts of fuc- and rner-[Rh(HzO),Cl,], ~ i s - [ R h ( H ~ 0 ) ~ Cand l ~ ] +small amounts of trans-[Rh(H,O),Cl,]+ (the cations presumably as chloride salts).'200Overnight at 80 "C, all three solutions give identical '"Rh NMR spectra; solution composition was shown to be independent of the Rh"' concentration, implying that monomeric species dominate, and when heated in conc. HCI, all give [RhClJ' as the dominant species.'206 Similar aquahalo equilibria have been reported for aqueous hydrofluoric acid solutions of hexafluarorhodate(III), and "F NMR and electronic spectroscopy have been used to identify In the presence of C1- ion, aquachloro complexes species in the series, [RhF6~n(H20)nl("-3)+.1207 form, but there was no evidence of any mixed F-Cl species. (These are the only Rh"' complexes which claim to have a Rh-F bond.) All 10 isomers of the [RhBr,C1,-n13- series have been identified (Io3 Rh NMR) in solutions prepared by dissolving commercially available hydrated rhodium trichloride in various HC1 and HBr solutions.'*osThe lo3Rh resonances vary from 7985 p.p.m. for [RhC1,I3-, decreasing by about 150 p.p.m. for each bromide substituent, reaching 7077 p.p.m. for the hexabromo anion. Splitting for the expected cisltruns (n = 2 or 4) and merlfac isomers ( n = 3) was observed, although the claim that the observed 3/1 merlfac ratio is the statistically expected ratio1208 is not valid; statistical arguments would lead to a meriJhc ratio of 3/2. The single crystal X-ray structure of (NH,),[RhCl,(H,O)] shows it to have the expected octahedral structure, but the C1 trans to the water has the shortest Rh-CI distance, consistent with the known trans influence of the chloride ligand.'2wThe solid has significant intermolecular (Rh-0 -Ha -Cl-Rh) hydrogen bonding. Aqueous solutions of [Rh~H20)nC16~n](n-3)+ catalyze the hydration of acetylene under mild conditions."" The catalytic activity is reported to increase as the number of coordinated chlorides increases, up to five chlorides, with the aquapentachloro complex being the most active, and the hexachloro ion inactive. The immense field of Rh-catalyzed catalytic hydration of unsaturated organics has been covered elsewhere17and generally involves n interactions between Rh and C systems, and thus will not be covered here. Harris, Robb and co-workers have thoroughly studied the kinetics of ligand substitutions of the aquachlororhodium(1TI)~ ~ m p l e x e ~ . ' ~ ~ ~The ~ ~ *reactions ~ ~ ~ ' ~studied ' ~ - ~ are ~ ' summarized ~ in Scheme 40, and the activation parameters for the labeled reactions are recorded in Table 87. The stereochemical course of these reactions is dominated by the trans-labilizing effect of chloride ligands. For '
-
Rhodium "CI
[RhCl5XI3- + HzO (X = CI. Br, I , NO,. N,, NCS)
Table 87 Activation Parameters for the Reactions Shown in Scheme 40" Reuclion
(4 04 (4
(4 (4
(0 (g)
(h) (1)
ci> (k) (1)
(m) (n)
'
Type
A f f *(kJ mol- ')
AS* (J K-' mol-')
Ref:
Aquation Anation Aquation Anation Aquation Analion Anation Aquation Anation Aquation Anation Anation Anation Anation
105 (101 & 21) 71 (69 & 7) 108 (105 & 5) 92 (90 _+ 6) 129 6.3 1104 1 132 2 b 117 f 2
(+42 _+ 71) (-92 21) (+25 f 17) (+ 13 f 21) 79 21 38t4 150 & 8
121 I 121 1 1212 1212 1202 1202 1202 1202 I202 I202 I202 1202 1213 1112
137k3 112.5 1 143 3 (120 5) (137 k 10)
+
+ +
b 54 4 8 79 8 33 + 4 54 8 (38 f 17) (54 k 29)
*
aValues in parentheses determined from original data but analyzed with a different program.L2o2 Unparenthesized values from original references. bAt S O T , k = 2.1 x 10-4s-' but a temperature variation study was not reported.
example, aquation of IRhC15(Hz0)]2-leads to only ~is-[RhCl,(H,0)~]-,which undergoes subsequent aquation to yield only fuc-[RhCl,(H,O)J. With no chlorides trans to each other, sac[RhCl, (H,O),] is extremely resistant to further chloride hydrolysis. Similarly, anations by chloride ion, starting with either [Rh(H,0),I3- or fac-[Rh(H20),CI,], occur more readily at a site trans to a coordinated chloride.'202Such regiospecific substitutions only occur in strongly acidic solutions, where all of the coordinated waters are fully protonated. Since OH- has a rrans-labilizing influence comparable to C1-,'20*formation of hydroxo species dramatically alters this sequence; relevant acid dissociation constants are given in Table 88. The mechanisms of these reactions are discussed by Palmer and Harris, and after allowing for the inevitable uncertainty caused by ion pairing, they conclude that 'the evidence tends to favor a clear Additional support for a dissociative dissociative (five-coordinate intermediate) mechanism'.i202 mechanism in the substitutions of aquachlororhodium(II1) complexes comes from C1- isotopic exchange studies of [RhCl,]'- and [RhCI,(H,O)]*-, which suggested that the reactions proceed via
I 1 om
Rhodium Table 88 pK, Values of some Aquachlororhodium(lI1) ComplexeE; Complex
PK,
Ref.
[Rh(H2%13+ [Rh(H,O),C1I2+ cis-[Rh(H, O)4Cl,] trans-[Rh(H,O),ClJ+ fac-IRhW20), Cld mer-[Rh(H,O), C13J
3.40 4.86 k 0.01 5.69 rf: 0.03 6.0 k 0.1 7.31 40.03 6.96 0.04
I112 1201 I202 I202 I202 I202
+
+
initial aquation, followed by anation, rather than via the exchange path expected if an associative mechanism were operating.1212The rates of bromide and chloride anation of [Rh(H20),I3+ are virtually identical, further suggesting a common (Rh-0 bond cleavage) rate-determining step in both reactions."12,'113 Robb and co-workers have studied the kinetics of the anation of {Rh(H,O)Cl,l2- by a variety of monoanions (equation 286; X = C1, Br, I, NO,, N, or NCS).I2l3The rates of anation are relatively independent of the nature of the anating ligand ( k z 2.9 ( f 1) x 10-2 s-' at 35 "C), consistent with a dissociative mechanism. The rates of equilibration of the hexachloro, pentachloro and cis-tetrachloro anions were studied as a function of chloride concentration and the data were fitted to the kinetic expression: kobs= kaq+ k,,,[Cl-]. The effect of pressure (1-1500 bar) was also moni t o r ~ d ; ' ~for ' ~ an associative reaction, AY* should be negative (as low as - 18cm3mol-') and substrate independent, while a dissociative reaction should show a positive value (up to f 18 cm3mol-') and be sensitive to changes in the substrate. As shown in Table 89, the results are again consistent with a dissociative mechanism. In addition, the authors note that the A Y* values for the aquations and anations are similar to the known partial molar volumes of the leaving groups (C1- and H,O), which implies that the leaving groups are fully solvated in the transition A dissociative process is clearly indicated. [Rh(H,0)Cl,]2-
+
X-
e [Rh(X)Cl5j3- +
H,O
(286)
Not surprisingly, Hg" ion enhances the rate of C1- aquation from a variety of these complexes. Chan and Harris have presented a detailed kinetic study of the Hg"-induced aquation of fuc[Rh(H,O),CI,J, and the subsequently formed cis-[Rh(H,O),Cl,]+ and [Rh(H20)5C1]2+ions.'215For each of the reactions the data are consistent with a mechanism involving preequilibrium between the Rh-C1 substrate and the H$+ ion, followed by the rate determining cleavage of the Rh-C1Hg bond. For equation (287) (and the analogous reactions of jhc-[Rh(H,O),CI,] or cis[RhC1,(HzO),J+), the kinetic expression: kobs= kK[H$+]/(l K[H$+]) is observed. A double ') was used to determine values for K and k. For the reciprocal plot (kob:as a function of [HgZ+]fuc-trichloro, cis-dichloro and monochloro complexes, the values for k (s- I ) are 1.07 respectively. This decrease in rate and 8.9 (f1.8) x (f0.04)x lo-', 4.3 (k0.3) x constant parallels the rate decrease for the analogous uncatalyzed reactions. The corresponding K values (137 f 5, 10 k 1 and 3.0 0.6, respectively), are consistent with a coulombic interpretation, as K decreases as the positive charge on the Rh substrate increases. The association constants for these aquachloro complexes are surprisingly large when one considers that there is no identifiable The value of K is relatively Hg2+ complex formed when Hg2+ is added to [Rh(NH3),Cl]2+.12'6 independent of whether the Cl-Hg moiety is trans to a H,O or a C1-, but the rate of the subsequent reaction (k)is markedly increased by the trans labilizing influence of a CI- ligand. This observation rationalizes the rapid reaction of Hg2+ with mer-[Rh(H,O),Cl,], studied by Palmer and coworker~,'~'' who used the same reaction sequence and found the same kinetic scheme for tlus reaction, but found the association constant ( K ) for mer-[Rh(H,O),CI,] is 758 & 63, significantly
+
Table 89 Volumes of Activation of some Aquachlororhodium(II1) Ions'*' Substrate
Reaction"
[RhClJ[RhCl, (H*O)12[RhC15WI,O)I2cis-[RhCL(H,O), 1-
Aquation (a) Aquation (c) Anation (b) Anation (d)
A v* (a3 mol - I ) ~
a
Labeled reactions shown in Scheme 40.
'
~~
~
+21.5 k0.6
+ 14.3 + 0.5
+ 15.7 k 6.5 + 14.7 f 1.6
Rhodium
1061
larger than the 137 found for thefac i ~ o m e r . ' The ~ ' ~ H$+ and HgCl+ ions have comparable effects' on the reaction rates, but, not surprisingly, HgCd does not enhance C1- hydrolysis of these ..
+
[Rh(H20)5C1]2+
Hg2+
&
{Rh(HIO)ClHg}4+ A [Rh(H20)J3'
+
HgCP
(287)
The addition of trace amounts of Rh"' halides to photographic emulsions can marginally decrease the sensitivity, but dramatically increase the gradation of the emulsion, and is generally known as 'the rhodium effect'. The mechanism of the rhodium effect is not understood, but the available evidence suggests that the Rh"' is chemisorbed on the surface of the Ag halide grains,I2I8and that Studies the Rh"' exerts its effect in the formation of the latent image, not in the film de~elopment.'~'~ monitoring the effects of [RhX$ (X = C1, Br, CN, SCN), [Rh(bipy),Cl,]Q, [Rh(NH,),Cl]Cl, and [Rh(NH3),]C13,demonstrated that at least one halide (or pseudohalide) must be coordinated to the Rh'" for the rhodium effect to occur, suggesting a halide or pseudohalide bridge between Rh"' and Ag' centers is involved.'2B(Of the complexes listed above, only the hexaammine had no measurable effect on either the sensitivity or gradation of the film.) Clearly, study of the chemistry behind this effect deserves further attention.
48.7 RHODIUM(TV) COMPLEXES Beyond the 3 + oxidation state, rhodium forms a limited number of complexes in the 4+, 5+ and 6 + oxidation states. A recent review'22igives an excellent summary of the chemistry of the higher oxidation state chemistry of Rh (as well as Ru, Os, Ir, Pd and Pt). For the 4+ oxidation state, the hexafluoro, hexachloro and trioxo dianions are well characterized. The known neutral species include RhF, and some oxides. There are also scattered reports of Rh" complexes containing substituted biguanides and Schiff base chelates.
48.7.1 Anionic Complexes The [RhF,I2- anion has been prepared by several techniques, and a variety of cations have been used to precipitate it. Yellow crystals of M,[RhF,] were prepared by the direct reaction of F, with M,[RhCl,] (M = K, Rb, CS).''~' All three salts have room temperature magnetic moments charac= I .98 & 0.03 BM),"" consistent with a high field octahedral teristic of one unpaired electron (pCR environment (I&). Mixtures of NaCl and RhCl, were fluorinated with BrF, to yield Na,[RhF,] (pen= 2.0 BM at room t e m p e r a t ~ r e ) ;X-ray ' ~ ~ ~ powder patterns show that anhydrous Cs,[RhF,] is isomorphous with Na,PtF,, and that the earlier reported Na3[RhF,]'224is actually a mixture of NaF and Na,[RhF6].'223Other monovalent cations used to precipitate [RhF,]'- have included Li+ and NO+.'226Yellow or brown salts of divalent cations (Ca, Sr, Ba, Zn, Mg, Ni or Hg) have been prepared,'227and the unusual A$+ ion is stabilized in the black Ag[RhF,] sa1t.'228The peroxo salt, [O,][RhF,] has been prepared,'226and in NOF,, it reacts to give (NO),[RhF,]. This latter salt can also be prepared by the reaction of rhodium sponge with a mixture of gaseous NOF and F,; if a large excess of F, is used, the Rhv salt, (NO)[RhF,] is formed.1226 The peroxo and nitrosyl salts of [RhF,]'- have been characterized by Raman spectroscopy. The K and Rb salts of [RhF6I2-belong to the K2[GeF6](trigonal) structural type, while the Cs salt exists in both trigonal and hexagonal forms.'229 With the exception of the Sr and Ba salts, [RhF6I2-is quite sensitive to hydrolysis; a qualitative listing of rates of hydrolysis of group VIIIB hexafluoro dianions is PdF2- > RhFz- > RuFi- > PtFi- > IrFz- > OsFi-, with only the palladium analog more reactive.'230 The vibrational spectrum of the potassium salt has been measured (Rh-F stretch at 589cm-') and it is consistent with the expected octahedral geometry for [RhF6]2-.'23' Both solid state absorption spectra'232and diffuse reflectance spectra'233of [RhF,]'- salts show two weak, spinforbidden 'T,g+ 4T1g and 'T2*+ 4T2gtransitions at long wavelength (diffuse reflectance maxima at 862 and 625 nm). A variety of stronger, spin-allowed transitions occur at higher energy (521, 472, 362 and 305 nm). Several moderately strong bands below 2500 nm were assigned to transitions between spin-orbit components of the electronic ground The only other anionic Rh" halide complex, [RhCl,]'-, has been prepared by the oxidation of an The cesium salt is isomorphous with ice-cold aqueous suspension of Cs,[RhCl,] by C12or Ce1V.'234~'235 (NH,), [PtC1,]1234 and CS,[I~C~,].''~~ The reflectance spectrum of Cs, [RhCl,] has aroused interest, as
1062
Rhodium
Cs, [RhCl,] displays the lowest energy charge-transfer transition of the platinum metal hexachloride and the variation in magnetic dianions.Iz3'The Cs salt is paramagnetic (peH= 1.7 BM at 298 susceptibility as a function of t e m pe r a t ~ r e ' ~and ~ ' when diluted in a diamagnetic, isomorphous ~ ~ ]interpreted "~~ to imply a significant amount of Rh-Cl bonding in the lattice of C S ~ [ P ~ C was [RhClJ- ion. The dilution also confirmed earlier which showed it is exceedingly difficult to prepare pure Cs,[RhCl,], as at least one-third of the Rh atoms in each sample studied had been reduced to Rh"'. The [RhCl,]'- ion is very reactive in solution, and rapidly oxidizes coordinated chlorides to chlorine; this reaction has not been thoroughly studied, but 8 5 4 8 % of the anticipated C1, has been recovered, and the Rh-containing product is presumably an aquachlororhodium(TT1) anion.'238 Attempts to prepare salts other than cesium have not been successful,'237 presumably because of the rapid internal redox reaction which occurs in solution; the rapid precipitation of the insoluble Cs salt lessens this contamination. A ternperature-jump study of the outer-sphere reduction of [ R ~ I C ~(equation , ] ~ ~ 288) showed that at 10°C the equilibrium constant is 9.08 (p = 0.05 M) and the rate of both the forward and reverse reactions are approximately diffusion controlled (the lower limit of kro,is 2.5 x 109M-' s - ' ) . ' ~ ~ ~ [RhC1612-
+ [R~(phen),]~+F=
[RhC1,I3-
+
[Ru(phen)J3'
(288)
There are scattered reports of solid salts of Rh" oxyanions. Early descriptions of a number of uncharacterized 'rhodites' have not been confirmed in recent years; these rhodites, Na,O 8Rh02 and K,0*6RhO,, were prepared by heating M,[Rh(NO,),] (M = Na, K) in air at 440"C.'24' Heating Rh metal in Na,C03 in air leads to Na,[RhO,] with a sodium stannite type 1 a t t i ~ e .The I~~~ Li salt is also reported. The solid state structure of Sr,[RhO,] is similar to the well-characterized Sr4[Pt0,], and has a hexagonal lattice, with each rhodium octahedrally coordinated by six oxygen atoms, and linked in one-dimension through Sr The synthesis of this latter compound was not reported, and it was stated that the compound analyzed to be 'oxygen deficient', again suggesting Rh"' contamination. 48.7.2
Neutral Complexes
The reaction of anhydrous RhBr, with BrF, was reported to lead to a bromine-contaminated It is paramagnetic (peH= 1.1 BM sample (5.8% Br) of a purple-red solid identified as RhF4.1223,12z4 at room temperature).lm There is a more recent report of the preparation of RhF, by the direct reaction of F,(,, (2-3 atm, 250 "C) with Rh sponge.'245This material was shown to be paramagnetic, very susceptible to hydrolysis in air, and isomorphous with IrFI and PtF,. This latter RhF, was identified as 'brown', while the earlier reported material was identified as ' p ~ r p l e - r e d ' . 'Attempts ~~~ to reduce RhF, with SeF, lead to the formation of a pale pink solid identified as (RhF4),SeF,.'246 The nature of these adducts has not been reported, but the color calls to mind the pale pink, unstable RhF4-BrF, adduct reported by Sharpe.lu4 Hydrated Rho, has been prepared by the electrochemical or Cl, oxidation of Rh"' salts.'247Little solution chemistry has been reported for this species, but the hydrate is green in alkali, and blue in acetic acid solution; variation in the oxygen analyses suggests that the hydrate may be a peroxide. Thermal treatment causes decomposition of the oxide, not dehydrati~n."~'Anhydrous Rh'" oxide has been prepared by heating hydrated Rh,O, in oxygen,'248or by heating Rh(OH), or Rh(NO3),-6W,O in air at 450 0C-1249 When heated and squeezed (700 "C,3000 atm, 24 h) anhydrous Rho, forms an undistorted rutile structure, in which the Rh4+ion has an effective radius of 0.62 A (the effective radius of Pt4+ is 0.63 There is mass spectral evidence for R h o z , and electron impact measurements suggest an ionization potential of 10.0 eV.'2s1 Some uncharacterized Rh'" complexes, presumably with Rh-0 bonds, have been reported. A red solution of Rh" perchlorate has been claimed,'252and chemical and electrochemical oxidation of R h ,0 3 *H 20(or [Rh(H20)J3+) in a variety of mineral acids leads to a green solution, which presumably contains some Rh'" species.1z53
48.7.3
Miscellaneous Rh'" Complexes
Cyclic voltametry of the Rh"' complexes [Rh(R)(salen)py] (Et = Me, Pf,Pi;salen = dianion of bis(salicy1idene)ethylenediamine) displays reversible one-electron oxidations of the metal center. The ease of formation of the organometallic cation [Rh(R)(salen)py]+depends on the nature of the
I
Rhodium
'
1063
a-bonded alkyl ligand.'254Several complexes of Rh" (and Ir") with the bidentate N'-p-chlorophenyl-N'-isopropylbiguanide (L) have been reported. The magnetic moments of [Rh(L),]Cl,, [Rh(L),X,]C1, (X = NH3, py), and [Rh(L),Y,]Y, (Y = C1, NO,) are consistent with one unpaired electron (peE= 1 6 1 . 8 2 BM).Iz5'The [Rh(L'),CI,]Cl, (L' = N'-phenyl or N'-o-tolyl-NS-isopropylbiguanide) have been prepared by the NaClO, oxidation of the corresponding [Rh(L'),Cl,]Cl salt. "" Oxidation of [Rh(S2CNMe,),] with BF, was reported to lead to '[Rh(S,NCMe2)3BF,]', presumably a Rh" dithiocarbamate complex.12s7This was questioned by later work in which cyclic voltammetry was used to show that [Rh(S, CNMe,),] is irreversibly oxidized (in acetone/Et,NClO,) at a potential of 1.24V vs. Ag/AgCl. In dichloromethane or benzene, reaction of [Rh(SZCNMeZ),] with BF, leads to the binuclear Rh"' complex [Rh,(S2CNMe,),]BF,, not a Rh" monomer (Section 48.6.6.2)."79 Flcischer has reported"' that the reaction of [Rh(CO),Cl], with tetraphenylporphyrin in refluxing benzene leads to the paramagnetic [Rh(TPP)(Ph)(Cl)] molecule. This unusual species was formulated as an organometallic Rh'" complex, but the similarity of its structure and reactivity to other Rh"'-porphyrin complexes casts doubt on this oxidation state assignment (Section 48.6.2.7). Oxidation of the Rh" dimer [Rh,(O,CMe),] in air with an excess of the disodium salt of 1,l-dicyano-2,2-dithiolatoethylene[in the presence of (PPh,)Br or [NBu:]/I, (MX)] affords [M],[Rh(S,C=C(CN),),]; the same salts could be obtained starting from hydrated rhodium trichloride. While no ESR signal was detected at room temperature, a broad, featureless line was = 1 . 1 BM at room temperature). Volobserved at - 170 "C, and the anion is paramagnetic (pea tammetric studies showed two oxidation waves, the first of which is a reversible, one-electron oxidation, presumably yielding [Rh{S,C=C(CN), )J, but no reduction waves between 0.0 and 1.8 V could be detected.''s8 ~
48.8 RHODIUM(V) COMPLEXES
The complexes of RhVare limited to RhF,, RhF; , and a series of less thoroughly characterized oxide ions.
48.8.1 48.8.1.1
Anionic Complexes
[RhFJ
Bright yellow, moisture sensitive salts of M[RhF,] (M = Na, K, Rb, Cs) have been prepared by heating RhCl, and MCl in IrF, .Iz5' (The Cs+ salt was elsewhere reported to be 'red-brown'.lzm) The sodium salt appears to be the most stable, with a room temperature magnetic moment of 2.8 BM, consistent with a high field, tig electronic configuration for this octahedral cation.'260Considerable deviation from Curie behavior was noted for the Na+ salt, and attempts to prepare the Li+ salt were unsuccessful. Alkali and alkaline earth salts of RhF; have been prepared using ClF, as a vapor phase fluorinating agent.IZ6'Non-metal cation salts of RhF; , such as weF,]+, [KrF6]f,'2620'2 or NO+,1263 have been prepared; some vibrational data are available on the NO+ salt.'263
48.8.1.2
Oxyanions of
Rhv
Various ionic oxides of RhV have been reported to result from the oxidation of Rh"' salts by hypochlorite, hypobromite or BiO, .1261-1266 The color of the hypochlorite oxidation product is very pH sensitive, ranging through yellow-orange (pH 111, green (pH 8), blue (pH 6) and purple (pH 2). The blue material is anionic, and tentatively identified as Rho; (or RhCl; ), while the purple species (pH 2) is cationic, and tentatively identified as Rho: or Rho3+.The 5 + oxidation state assignment is based on the used of equimolar amounts of Rh"' and two-electron oxidants, such as OCl-.1265 Perbromate oxidation of RhC1;- (excess perbromate) leads to a violet solution at pH 11. Presumably a RhV'species, this violet solution gradually turns blue (Rh'). (Use of less OBr- leads to the yellow-orange Rh' species discussed above.) The blue RhVspecies is also unstable, and is reported to disproportionate (equation 289).IZ6'The most carefully characterized oxyanion of RhVis Rho:-. upon chemical oxidation of Rh"' salts, it has also been reported to First reported by Berzeliu~'~~' form upon anodic oxidation of Rh1''.'268In acihc solution the RhVoxide is reported to dispropor-
1064
Rhodium
tionate (equation 290), and its oxidation half-reaction (equation 291) has a standard potential of 1.87V vs. NHE.I2" 3Rhv + 2Rhv' ' .'
3[HRh04]*-
+
2H'
[HRh0412-
48.8.2
+
Rh"'
+
2[Rh04]'-
-+
H+
+
+
(289)
Rh(OH),
[Rho4]'-
+
H20
+ e-
(290) (29 1)
RbF,
The pentafluoride is a dark red, paramagnetic solid (pef = 2.93 3M at 298 K) prepared by the A single-crystal X-ray structure shows that solid reaction of solid RhF, and F, (6 atm) at 400 3C.1260 Each rhodium is coordinated RhF, has a tetrameric structure analogous to RuF, and IrFS(128).1270 by six fluorine atoms in an approximately octahedral arrangement. The terminal Rh-F distance averages 1.808 8,while the Rh-F distance is significantly longer (2.01 A) for the bridging fluorines. This compound is isostructural with the pentafluorides of Ru, Os and Ir.127'*1272 F
(128)
Numerous transition metals form [MF,], species, and there is an interesting correlation between the structure of the pentafluoride and the nature of the metal. All pentafluorides of the metals in Figure 7 have octahedral metal centers with bridging fluorines. Those in group A are tetrameric, bridges. Group B metals display polymeric with a square structure, featuring linear M-F-M chains, with M-F-M angles near 150 ', while group C metals have tetrameric structures with M-F-M angles near 135'. All have MF, units with close-packed fluorine atoms; group A display distorted C.C.P.lattices, with metals in 115 of the octahedral holes, while group C (which includes Rh) are h.c.p. For the left side of Figure 7, bonding can be rationalized with an ionic model, involving (MF:)(F-) units, while on the right side, covalent bonding is more appropriate, with two-electron/three-center bonds needed to account for the M-F-M bridges.'270 The mass spectrum of RhF, is not typical of a metal pentafluoride, and is relatively rich in ions containing more than one Rh atom.L273 For example, as the temperature increases, the concentration of RhlFg+ ion increases at the expense of monomeric RhF: and RhF; fragments.
48.9 RHODIUM(V1) COMPLEXES
The 6 f state is the highest formal oxidation state recorded for rhodium, and RhF6 is the only ' ~ be prepared carefully characterized example. Following the method used to prepare R u F ~ , ' it~ can
[\ A
E
C
Figure 7 Metals which form [MU,species having octahedral metal centers and bridging fluorines
Rhodium
1065
by burning rhodium metal in an atmosphere of F2/N2 in a cooled quartz reactor.'z75 Earlier workers1276 had attempted the direct oxidation of rhodium metal in hot ( S O M O O "C) F2, but found the major product was RhF, ,with a red-brown sublimate which contained some impurities of RhF, and RhF,. The highest Rh fluoride obtained using BrF, as the fluorinating reagent was RhF,.'277 One of the least stable of the metal hexafluorides, RhFn, is a black volatile solid (P,,oc = 49.5 Torr). The gas phase is deep red brown and mon~meric.'~'~ At room temperature it has a body-centered cubic lattice, and, like numerous other hexafluorides, it undergoes a phase change at reduced temperature. The low temperature phase has not been well defined, but is of lower symmetry, and is probably orthorhombic.1278For RhF, the phase change occurs at - 23 "C, which is comparable to the transition temperatures ("C)for other metal hexafluorides: Mo( - 36) Tc(- 19) Ru(- 30) Rh( 23) W(-20)
Re(-22)
Os(-21)
Tr(-11)
Pt(-11)
U(+25)
The electronic spectrum of RhF6 has not been reported, but ligand field calculations have been used to predict its The IR spectrum of RhF, has been reported,"" and it is consistent with the expected 0,symmetry. Like IrF,, the rhodium hexafluoride does not show the Jahn-Teller splittings of the Eg mode observed for the analogous Tc, Re and Os hexafluorides. Not surprisingly, RhF, is a strong oxidizing agent, and, as is its ruthenium analog, is capable of oxidizing Xe. 12*' Virtually everyone who works with RhF6 comments on its instability, however, ' which may account for the lack of studies dealing with its use as an oxidant. There are several reports of Rh"' oxides, but none of the compounds have been thoroughly characterized. There is a brief report of the existence of gas phase Rho3,with an ionization potential of 11.4 eV.I2" Differential thermal analysis indicates that Rho2 (in the rutile form) decomposes to Rho, (and presumably to Rh,,) at 850 "C.The trioxide then decomposes to Rh metal and oxygen at 1050°C.'250The most commonly reported Rh" oxide is the rhodate ion Rho:-. The potassium salt was first reported (in 1860) as the blue crystalline product of C1, oxidation of Rho, in concentrated KOH solutions.'283Brilliant blue solutions, presumed to contain the rhodate ion, have since been reported upon oxidation of alkaline solutions of Rh"' salts by hypochlorite,'284hypobromite'285and persulfates.'286Anodic oxidation of Rho, in dilute HClO, has also been reported to yield rhodate solutions. Little is known of the spectra or reactivity of this ion, but the Ba2+salt is reported to be paramagnetic, with a susceptibility characteristic of one unpaired electron,'286suggesting that the presumably tetrahedral rhodate ion may exist in the unusual low spin, $g configuration. 48.10 48.10.1
MISCELLANEOUS RHODIUM COMPLEXES Complexes of Mixed Oxidation State
There seem to be very few complexes in which there are two rhodium ions in different oxidation states. The best-authenticated examples are the [RhH,(PR3)2]2complexes [R = NMe,, P(OPf),]. The dimethylamino complex has been shown to have the structure (129) and contains both rhodium(1) and rhodi~m(III).'~*~ The other example is provided by the [Rh(02CMe),]+ ion in [Rh(O,CMe),],ClO, . This ion retains the classic lantern structure of the rhodium(I1) carboxylato complexes, but the salt contains both rhodium(1I) and rhodium(III).'2s8
48.10.2
Nitrosyl Complexes
Nitrosyl complexes are included in this section since, as Feltham and Enemark have noted, the predominantly covalent interactions between nitric oxide and transition metals make assignment of
I Rhodium
1066
charge to the nitrosyl ligand i r r e l e ~ a n t . ' ~ The ' ternary nitrosyl complexes [Rh(NO),X], can be prepared from the carbonyl halides,'2*'292 or from rhodium(1) cyclooctadiene complexes (equation 292).1293 The structures of these polymeric materials are uncertain. Griffiths has speculated that they are tetrameric cubanes rather than halo-bridged dimers.lZ9'
+
[RhClLZ], NO + [RhCI(NO)z], L = colZ9CbI292 , Cod1293
(292)
The reaction between rhodium trichloride and nitric oxide forms another ternary nitrosyl whose formulation is unknown.'294However, nitric oxide ligates rhodium(I1) acetate (equation 293).1295 The ternary complexes have been used to prepare other complexes, but these derivatives are commonly prepared from more accessible starting materials. [Rh(O,CMe),],
48.10.2.1
+ NO
[Rh(02CMe),(NO)lz
+
(293)
[Rh(NO)L,] complexes
X-Ray crystallography shows [Rh(NO) (PPh,),] to adopt the structure (130).1296 The bent NO group (mean Rh-N-0 = 156.7') is uncharacteristic of the nitrosyl ligand in {MNO)" complexes, which normally contain linear nitrosyl Three independent molecules exist within the unit cell. In each of these three molecules the Rh-N-0 plane makes a different angle with the vertical mirror plane containing a Rh-P bond. The electron diffraction study of gaseous [Rh(NO)(PF,),] shows it to have a nominally C, structure (131).1297 The C,,symmetry exhibited in the gas phase is retained in CCl, solution. I9F NMR spectrometry shows 4 F to be 3.8p.p.m. and coupling constants of 'J(P-F) 2,JP-F) = 1416Hz and 'J(Rh-F) = 18.0Hz have been deterNevertheless, it has been claimed that the mined. The approximate value for 'JP-P) was 85 H2.IZ9* results from X-ray photoelectron spectroscopy are more consistent with their formulation as Rh'NO- complexes.
+
0
I
N
I
Nitrosyltris(triphenylphosphine)rhodium(I) was first prepared, in admixture with [Rh(NO)C1,(PPh3),], by allowing excess triphenylphosphine to react with [Rh(NO),Cl] in tetrahydr ~ f u r a n . ~Since ' ~ ~ that time four other preparative methods, shown in Scheme 41, have been devised,199.201,202.130%1303 Of these methods that using N-methyl-N-nitrosotoluene-p-sulfonamide is the most ~ o n v e n i e n t . ' The ~ ~ dark ~ ~ ~crimson-red ' ~ ~ ~ ~ crystals of the complex start to decompose at 115 "C in air, but at 225 "C in a helium atmosphere.Ix0 Tertiary phosphine analogs have been prepared from [Rh(NOX Cl], .13" Nitrosyltris(phosphorus triflu0ride)rhodium has been prepared from both rhodium( - I) and rhodium(1) complexes (equations 294296).12y7 Their physical properties are listed in Table 90. The triphenylphosphine complex is quite reactive (Scheme 42). Both e l e c t r o n i ~ ' ~ and ' ~ 31PNMR spectrometry show the equilibrium constant for the reaction (297) to be of the order of 1 0 - 4 m ~ l d m - 3in dichloromethane, benzene1312.131 3 or THF,I3I3so the reactivity of the complex cannot arise from the facile loss of a triphenylphosphine ligand. The invariance of the line width of the doublet at - 48.8 p.p.m. ('J(RhP) = 175 Hz) with solvent suggests that the species is un~olvated.'~'~ This lack of dissociation makes the preparation of the bis(tripheny1phosphine) complex to appear erroneous.1314 [RhCWOhlx + PR3 R3
=
----*
-
J'h,(p-Cs&Me),,
K[Rh(PF3)A
+
NO
[Rh(NO)PR,),I
(294)
(p-GH,Fh, MePh [Rh(NO)(PF3)3I
(295)
1067
Rhodium
Ref: 202 1301 1301 1301 1301 1297 1052 1310 a
Under N,.
i, PPh,, Na/Hg, THF;'"P"301 ii, PPh,, NaBH,, p-MeC,H,S02N(NO)Me;199~20'*202 iii, PPh,, NO, Zn., THF;l3OZ iv, PPh,, EtOH;"" v, NaiHg, THF;"" vi, PPh,, N2H4l3O3 Scheme 41 Preparations of [Rh(NO)(PPh,),]
IRMNO)(PPh,)31 j
[F~(NO)~(diphos),]
[ Kh(NO){PhP(CH,CH zCHzPPh2 ) 2 } I
IRh(NO)(NOJ)Z(PPh3),1
[ Rh!N O W C S)z (PPI1 3 )z I
I
.
[ Rh(NO)CI(PhCO)(PPh,)zI
[Rh(NO)(ArN3Ar)(PPh3)31 [ Rh(NO)(SO,)(PPh&
I
_X [ Rh(NO)(SO, )(PPh, )2I
[ Rh(N0)2(PPh3 )z 1 IC 1041
+
i, (NSC1) CHC1,/CCl,;"05 ii, CHzC1;CHC1,;1306 CCL;'2"'" PhCI; CH -CHCH,C1;1)06 iii, I,; KgI,, - Hg;I3" iv, 02, RCOzH!" v, 4 M HN0,;'309 vi, HNCS, Clz;13Mvii, PhCOCI, C6H6;'30'\iii, AgClO,, C6H6;I5' ix, SO,, C6H,/C,Hl,;'3'" x, 02,C6 H6 .1310,13'1 , xi, (p-MeC,H,)zN,;231 xii, (99),C,H,, 8O"C;'"' xiii, [FeCl,(diphos)], THF;'sl xiv, [CoCl(PPh,),], C6H6;I5' xv, L = PPh,, [CoCJ(PPh,),], THF; L = diphos, [Co(dipho~),][ClO,]~,C,H,;"' xvi, [NiCl,(PPh,),], C&"I Scheme 42 Reactions of [lUt(NO)(PPh,),]
I
Rhodium
1068
[RhCI(PF3)2]2
+ PFJ f
NO
4
[Rh(NO)(PF,),]
(294)
[Rh(NO)(PPh,),l e [Rh(NO)(PPh,)J + PPh3 (297) The nitrosyl group is retained in the majority of the reactions. Usually two anionic ligands are added to form analogs of the important complex [Rh(NO)Cl,(PPh,),] (see Section 48.10.2.2). The complex has also been used to cataIyze homogeneous catalytic hydr0genati0n.l~'~ Other reactions include the nitrosylation of iron, cobalt or nickel complexes, and conversion to the cation [Rh(NO),(PPh,),]+ by allowing the complex to react with AgC10,.'5' One reaction in which the oxidation state of the metal remains unchanged is that with sulfur dioxide (equation 298).','O X-ray crystallography has shown that the SO, ligand binds through both sulfur and ~ x y g e n . ' ~ ' ~ ~ ' ~ ' ~ The sulfur dioxide complex (132) is easily oxidized by dioxygen in benzene solution to form the sulfato complex [Rh(NO)(S04)(PPh,)2].'3'03'316 The product distribution when '*02 is the reactant implies that an intermediate sulfur bound sulfato complex is f ~ m e d . ' ~ ' ~ [Rh(NO)(PPh3)3] + SOT
-
6 ' H6 iC7
I6
[WNO)(PPh,), (SO2)I
(298)
0
\N
(132)
Analogs of [Rh(NO)(PPh,),] containing t r i ( p - t ~ l y l ) - , * ~tri(p-flu~rophenyl)-'~~' ~.'~~~ or trilpchloropheny1)-phosphine"' have been prepared using N-methyl-N-nitrosotoluene-p-sulfonamide (equation 299);" or from [Rh(NO),Cl], .Irn1 Only one of their reactions has been investigated (equation 300). The tritertiary phosphine (107) displaces all three PPh3ligands when allowed to react with [Rh(NO)(PPh,),] to form the complex (133).I3O1The ,'P NMR spectra of (133) and related complexes have been ~ b t a i n e d . ' ~ ' ~ RhC13.3H20 f 10PR,
+ p-MeC6H4SO,N(NO)Me
EtOH
[Rh(NO)(PR3),]
(299)
R = pC,H,Me, P-C6H4F, p-CsH,CI [Kh(NO){P(p-C,H4Me), 14
+ PhCOCl
-
[Rh(NO)Cl,{P(p-C6H,Me),
121
(300)
NO
I
I
Me
48.10.2.2
[Rh(NO) X,L, J complexes
The most important complex of this type is [Rh(NO)Cl,(PPh,)J, which was first prepared in 1962: The tri(cyclohexy1)phosphineand triphenylarsine analogs were also prepared at this date. The
dichloro complexes were precipitated when excess neutral ligand was allowed to react with [Rh(NO),Cl], . The preparative routes available for [Rh(NO)Cl,(PPh,),] are summarized in Scheme 43. Again N-methyl-N-nitrosotoluene-p-sulfonamide is the most convenient source of the nitrosyl ligand.202,I323
Rhodium
1069
i, p-MeC,H,S0,N(NO)Me;Zoz,'323 N,, PPh,, EtOH;IZwii, NOCl, CH,Cl iii, Cl-;1321iv, HCI;'104v, NOCl;"" vi, PPh,, THF;'299vii, CH,C12;CHC13;'3M CCI4,129431306 . PhCI; CH -CHCH2CI;"dHC1, C6H6;I3"viii, HC1;'uL9J3" ix, H,O+, KN02; NO, CHCl,, H30+;''-x, p-MeC,H,SOZN(NO)Me20' Scheme 43 Preparations of [Rh(NO)CI,(PPh,),]
The dichlororhodium complex adopts the square pyramidal structure (134; L = PPh,). The plane.*324 The nitrosyl nitrosyl group is bent but the nitrogen and oxygen atoms lie on the P-Rh-P group in [Rh(NO)Br, {P(OPh), },I is also bent.'325Vibrational frequencies of the nitrosyl group in [Rh(NO)X,L,] compIexes are given in Table 91.
(134)
According to the 31 P NMR spectrum [Rh(NO)Cl,(PPh,),] is not dissociated in Many analogs of [Rh(NO)Cl,(PPh,),] are known since other group V ligands may replace PPh,, and other monodentate anions the chloro ligands. However, the latter are not derivatives of [Rh(NO)Cl,(PPh,),] since they have all been obtained from other sources. If nitrosyl bromide is allowed to react with hydrated rhodium trichloride and PPh, in ethanol, the mixed chlorobromo complex is obtained (equation 301).'303When [RhCI(PPh,),] is the substrate the dibromo complex is formed (equation 302). This complex can also be obtained from rhodium and PPh, , Replacement of PPh, by other tribromide, N-methyl-N-nitrosotoluene-p-sulfonamide tertiary phosphines permits analogous dibromo complexes to be prepared (cf. equation 299). Another source of the triphenylphosphine complex is [Rh(NO),Br],. The diiodo complex can be obtained from a similar reaction mixture containing LiI (equation 3O4).Im3 RhC13.3H20 + NOBr [RhCl(PPh,),] RhBr,
+
PR,
+
PPh,
+ NOBr
+ p-MeC,H,SOzN(NO)Me
[Rh(NO)ClBr(PPh,),]
(301)
-
(302)
[Rh(NO)Br2(PPh3h] [Rh(NO)Br,(FR,),I
R3 = Et,, P23,Bu3, Ph3, ( P - C ~ H ~ CMePhz ~)~,
(303)
I Rhodium
1070
Table 91 Physical Properties of [Rh(NO)X, L,] Complexes M,p. (“C)
Color
Complex
Yellow Green Green Green Brown Dark brown Light brown Light brown Pink Brown Fawn
v,
200-01‘
Red-brown Violet-brown
RhC13.3H20
‘
Re/:
(cm- )
1690 1645 1614 1639 1624 1632 1627 1665 1665 1665
-
1304 1304, 1309 127 127 127 127 127 1308 1308 1308 1323 1323 1323 202 1323 1323 1323 1303 1301 1323 1323 1323 1323 1293 1293 1293 1293 1293
-
-. -
1630 -
-
-
Green Brown Maroon Brown Brown Orange-brown Violet-brown Brown
-0
161& 1660 1608 -
-
-
-
211-15 23k38 230 259-60 249-50
+ PR3 + p-MeC6H,S02N(NO)Me
MI
1632 -
1632 1629 1626
[Rh(NO)I,(PR,),]
(304)
M = Li, R = Ph, MeC,H,, p-CsH4C1;M = K, R = Et, Bu
The bis(thiocyanat0) complex has been obtained by addition of KCNS to [Rh(NO),(PPh,),][c1O41.1304 The green chloro(nitr0) complex [Rh(NO)Cl(NO,)(PPh,),] has often been observed as an intermediate in the preparation of [Rh(NO)Cl, (PPh,),], but it has only been isolated when [RhCl(PPh,),] or trws-[RhC1(CO)(PPh3)2]have been used as the starting materials (equations 305-308).244131s-’325 [RhCl(PPh,),L]
+ N203
CHZC12
[Rh(NO)Cl(NO2)(PPh3),]
(305)
+ PPh, + [RhCl(CO)(PPh3)2]+ NO % [Rh(N0)C1(N02)(PPh3)2] + CO [RhCl(CO)(PPh,),] + NOCl [Rh(NO)Cl(NO,)(PPh,), J + CO
[RhCl(PPh,),]
+ 4NO
[Rh(NO)Cl(N02)(PPh3)2]
9
N20
(306)
(307) (308)
The TR spectrum of this complex usually shows two nitrosyl absorption bands, which have been ascribed to the presence of isomers. The bands which occur at 1508, 1309 and 816cm-’ arise from the nitro ligand and imply that it is N-bonded.I3” Whilst the dichloro complex (134)has trans chloro ligands,’“ the preparation of [Rh(NO)Cl(NO,)(diphos)], in which the anionic ligands must be c k , allows the possibility of geometrical isomerism in [Rh(NO)Cl(N0,)(PPh3),]. Cis anionic ligands have been found in the sulfato complex [Rh(NO)(SO,)(PPh,), J (135),’326and in the triphenyl phosphite complex [Rh(NO)Br, {P(OPh),)2].1327 However, the trifluoroacetato complex (136)has trans basal ligand^.'^'' If a green solution of [Rh(NO)Cl(NO,)(PPh,),] in benzene is allowed to stand, a brown complex is precipitated.244This may be the dinitrosyl complex which can be obtained from trans[RhCl(CO)(PPh,),] and nitric oxide when the reaction is carried out in carbon tetra~hloride’~~ or toluene’329solution. No corresponding dinitro complex is known, but two routes have been discovered to the dinitrato complex [Rh(NO)(NO,),(PPh,),] (equations 309 and 310).i330~’3”
Rhodium
1071
The most numerous complexes of this stoichiometry are the bis(carboxy1ato) complexes. They can be obtained from the reaction between [Rh(NO)Cl(NO,)(PPh,), J and carboxylic acids in aerated solution (equation 31 l).’27*1308 The IR spectra of the carboxylato ligands (Table 92) are consistent with their monodentate bonding shown in the structure (136).1328 [m(NO)(PPh,)31 4- RCOzH
R
=
~:m
’
[Rh(NO)(OCOR)2(PPh,)zl
(31 1)
Me, Et, p-C6H4Me,p-C6H,NO1,p-C6H4C1,127 CF3, C2F5,C6F51308
Unlike all the other complexes described in this subsection the bis{di(ptoly1)triazenido) complex (137) contains six-coordinate rhodium. It can be prepared from the reaction between the triazene and [Rh(NO)Cl(NO,)(PPh,),] (equation 312). The single methyl resonance at r 7.80 in its proton NMR spectrum is the reason for assigning it the symmetric structure.231
Table 92 IR Spectra of [Rh(NO)(O, CR),(PPh,),] Complexes’
-
‘Data from ref. 127.
- - -. -
- . .
.
.
.
1072
Rhodium
Many of the methods that have been used to prepare the [Rh(NO)X2(PR3),]complexes can be adapted to yield similar tertiary arsine complexes. For example, N-methyl-N-nitrosoto1uene-psulfonamide still functions as a source of the nitrosyl ligand in the presence of tertiary arsines (equations 313 and 314).
+ p-MeC,H, SO,N(NO)Me + AsPh, + [Rh(NO)Br,(AsPh,),] RhCl3.3H20 + p-MeC&SO,N(NO)Me + AsPh, A [Rh(NO)I,(AsPh,),] RhBr,
(313) (314)
Additionally all three dihalo complexes can be prepared from [Rh(NO)X,], species (equation 315).1293 The dichloro complex is also produced when AsPh, is allowed to react with [Rh(NO),CI] (equation 316).'299 [Rh(NO)X,], [Rh(NO), Cl],
+ AsPh, + AsPh3
+
[Rh(NO)X,(AsPh,),]
* [Rh(NO)Cl,(AsPh,),]
(3 15)
(316)
The action of nitrosyl chloridelm3or dinitrogen trioxidel3I8on RhC1,*3H20in the presence of AsPh, also yieIds the dichloro complex. There have been reports of the preparation of [Rh(NO)XL,] comp~exes. 1329,1332-1334 However, these complexes have only been prepared in the Soviet Union, and it has been claimed that their preparation cannot be repeated.'335
48.10.2.3
Cationic complexes
A number of cationic complexes have been prepared. The physical properties of these complexes are shown in Table 93. The first examples were prepared by allowing ammonia or other nitrogenous bases to react with [RhX(NO),], complexes (equation 317).1292
They are more usually prepared by the action of nitrosyl compounds on rhodium(1) complexes containing labile ligands.13361339 It has also proved possible in one instance to prepare this type of complex by anion metathesis (equations 318 to 320).'340
Table 93 Physical Properties of Cationic Nitrosyl Complexes
1073
Rhodium
There are two interesting reactions of [Rh(NO)(CO)(PPh3)2][BF4]2 that give rise to complexes of this type. The first can be classed as an esterification whilst the second is a disproportionation (equations 321 and 322).1341
The complexes containing organonitrile ligands themselves undergo a variety of reactions (Scheme 44). Whilst the structures of the five-coordinate cations remain a matter for speculation, the six-coordinate cation [Rh(NO)(MeCN), (PPh3)J2+has been shown to have the structure (138) by X-ray crystal10graphy.l~~
i, NOX (X = BF,, PF6);'337ii, NOPF6, MeCN;1336"3'iii, NOPF,, MeCN;1336 iv, NOPF,, MeCN;133b v, Bu'CN;'~~' vi, Na2S2CNEt,;'336 vii, d i p h o ~ ; 'viii, ? ~ ~PPh,;'336ix, Cl-;3336 x, PPh,;1336 xi, C1-;1336 xii, . C1-;'3Mxiii,1,2-(HO),C,R, (R, = H,, Br,, H3-4-N02,H3-4-CH0, H3-4-CO,Me);1342xiv, AgPF,, MeCN'3w
Scheme 44 Preparations and reactions of cationic rhodium nitrosyl complexes
t
\
C
\
Me
The crystal structure of the dinitrosyl cation (139) has been determi~~ed.'~" This cation can be prepared from other rhodium nitrosyl complexes {equations 323 and 324).151,13"
1074
Rhodium
The hexafluorophosphate analog can be prepared from [Rh(nbd)(PPh,), J[PF,] (equation 325). Bis-1 ,Z(diphenylphosphino)ethane displaces all ligands from the cation, but the reaction is obviously more complex since N,O and OPPh, are also produced (equation 326).234
-
[Rh(nbd)(PPh3)21~PF61f NO fRh(NO),(PPh3)2][PF6] + diphos
48.1 0.2.4
[Rh(No)2~Ph3)21[PF61
[Rh(diphos),][PF,]
---f
+
N20
+ OPPh, +
(325)
PPh,
(326)
Thionitrosyl complexes
There are a few thionitrosyl complexes of rhodium, many of which contain thionitrosyl bridges. They have been prepared by allowing PSCl], to react with sundry rhodium nitrosyl complexes (equations 327 and 328). [Rh(NO)(PPh,)3I
+
[WNOW, (ZPh3121
CCI4 CHC13
~sc113
f
z
PSCII3 =
' [Rh(NS)C12pPh3)12
CCI4 CHC13
' [Rh(NS)C&(ZPh3)12
(327) (328)
~ ~ , 1 3 0 51346
P
The dihalo complexes are believed to contain thionitrosyl bridges since the band in their IR spectra at CCI. 840cm-l disappears when the complexes are cleaved by excess neutral ligand (equation 329). In the latter complexes v I N . occurs at 1116-1 120 ~ m - ~ . ' ~ ~ ~ ~
48.10.3 Bimetallic Complexes
The complexes included in this section are those in which rhodium and at least one other metallic element are present. Normally the second metal is not bound to rhodium although complexes in which a metal-metal bond is supplemented by a bridging ligand are also included. It has been noted above that the weakly solvated cation [Rh(diphos)(MeOH),]+ can be obtained by hydrogenation of [Rh(diphos)(C,H,)][BF,]. If the former compound is allowed to react with an equimolar quantity of [IrH, (PPr\)21the di-p-hydrido complex (140) is formed (equation 330). The presence of base merely increases the yield. Only the heavy atoms have been located by X-ray ~rystallography.'~~~
i140)
(141)
Two related orange brown p-hydrido-p-chloro complexes (141) can also be obtained from [IrH,(PEt,),]. Further structural details are available. The "P NMR spectrum of (141) (PPh, = PEt,) has a doublet of doublets at 80p.p.m. and a set of four triplets at 58.8p.p.m. The latter group arises from the phosphorus atom bound to rhodium which is cis to the hydrido bridge.'"' [RhCl(PR3)212
+
RJP =
C7H8
[(RIP)?Rh(H)CIIr(PEt3)2l Et3P, 0.5 diphos
[IrH, (PEt,)?]
(331)
'
~
~
~
~
Rhodium
1075
Generation of the [Rh(diphos)(MeOH),]+ cation in the presence of mer-[IrH,(PEt,), J gives a dark green cation (142) that contains three hydrido bridges (equation 332). Again onIy the heavy atoms have been located by X-ray crystallography. The 'H NMR spectrum shows a multiplet at z 22.0 due to the hydrido ligands. As might be expected the 31PNMR spectrum is complicated, having multiplet peaks arising from the diphos ligand. The IR spectrum of the hydrido ligands shows peaks at 1800 and 1686cm-', whilst the corresponding trideuterio complex has peaks at 1300 and 1100~m-'.'~~~ &aBPh4 McOH
[Rh(diphos)(MeOH),]+ + mer-[IrH,(PEt,),]
' [(diphos)RhH3IrtPEt, )3][BPh4]
(332)
l i
I-
(142)
It is also possible to prepare bimetallic rhodium(1) complexes containing 'A-frame' ditertiary phosphine ligands. However, the majority of these complexes contain carbonyl ligands and have (143) upon heating in toluene decarbonylabeen dealt with in the companion v o l ~ m e .Nevertheless '~ tes and forms the bimetallic complex (144).I3jo
c1
L
'
(143)
Probably the most numerous bimetallic complexes of rhodium are those containing mercury. The complex ~-[RhHCl,(AsMePh,),] reacts with mercury(1I) halides,.'351 phenylmercury(I1) halides,'352 or rnercury(1) halide^'^''^''^^ to produce fuc-rhodium-mercury complexes (145) (equation 333). The physical properties of the products are shown in Table 94. C1
MePh,As
MePh, A s (1 45)
J-[RhHCl,(AsMePh,),]
+
HgX,
+
rner-[RhC1,(HgX)(AsMePh2),]
(333)
X = F1 C1, Br, I, MeCO,
Meridional rhodium(I1I) complexes also undergo reaction with mercury(1I) halides. Unlike the corresponding reaction with rhodium(1) complexes where a rhodium-mercury bond was formed,Is6 in these instances the tetrahedral [HgCl4I2-ion acts as a bidentate ligand (146; equation 334). Three bands in the far-IR spectrum of the chloro complex at 345, 310 and 242cm-' have been assigned to vRh c1 Hg, uHg cI and vRhPc, respectively. However, the reaction between the corresponding tertiary phosphine complex mer-[RhCl, (PMe,Ph),] and HgCl, also forms a polynuclear mercury complex (equation 335).1353 cnc
LII
Rhodium
1076
Table 94 Physical Properties of Rhodium-Mercury Compounds
Complex
Color
M.p.("C)
Ref
[ R W (HgWAsMePh, l31 [RhCl, (HgCl)(AsMePh,),] [RhCl, (HgBr)(AsMePh, ),I [RhCl, (HgI)(AsMePh,), ] [RhCl, (HgO, CMe)(AsMePh,),] [RhBrz(HgF)(AsMePhAl [RhBr, (HgCl)(AsMePh, ),I [RhBr,(HgBr)(AsMePh,),] [RhBr,(HgI)(AsMePh,),] [RhBr2(Hg02CMe)(AsMePh,),] [RhHgCl, (PMe, Ph),] [RhHgCl, (AsMe, Ph),]
Pale green Yellow Orange-yellow Orange Yellow Yellow Yellow Yellow-orange Orange Pale yellow Orange Red
195 205 165 155 I72 154 202 187
1351 1352 1352 1352 1352 1352 1352 1352 1352 1352 1353 1353
a With
148
I90 127-30 156-45"
decomposition
ZMe,Ph
mer-[RhC1,(AsMe,Ph),] rner-[RhC13(PMe,Ph)3]+ HgC1,
+
HgCl,
T z
[RhCl, (AsM~,P~)~(H~CI,)]
[Hg,CI,(PMe,Ph),]
+ [RhCI,(PMe,Ph),(HgCl,)]
(334) (335)
Similar bimetallic complexes to (146), except that they contain a bidentate, square planar, anionic ligand of the type [MCl,L]- (M = Pt, L = PEt,, PBu,, PBu'Pr,; M = Pd, L = PBu,) are believed to be formed as intermediates in ligand exchange reaction^.'^" Trihalo-bridged complexes (147) are formed in the reactions between [RuCl,(PPh,),J and mer[RhCl, L,] complexes (equation 336).135s-'3s6 K3P
/'iPR3
c1 \ Ru&1*J,tRh / Ph,P /
PR,
Lc,/ \ CI (147)
rner-[RhCl,(PR,),]
+
[RuCl,(PPh,),] + [(R,P),ClRhCI, RuCI(PPh,)(PR,)] R3 = Bu,, Ph,, Me,Ph, Et,Ph, Bu,Ph
(336)
The dihydridobis(cyclopentadieny1) complexes of both molybdenum and tungsten react with [RhH, (PPh,),(Me,CO),][PF,] to form deep brown complexes. Their proton NMR spectra indicate that the hydrido ligands are bound to both metals (148) since large coupling constants are observed (J(Rh-H) = 29Hz, J(W-H) = 107 Hz).When the tungsten compound is heated in acetone/D,O mixtures, deuterium is incorporated in the cyclopentadienyl ligand^.'^''
Rhodium
1077
48.1 1 REFERENCES . . r
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.
51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61.
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181. 182. 183. 184. 185, 186.
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187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199.
1079
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___
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c0.c. G I [ *
1084 459. 460. 461. 462. 463. 464. 465. 466. 467. 468. 469. 470. 471. 472. 473. 474. 475. 476. 477. 478. 479. 480. 481. 482. 483. 484. 485. 486. 487. 488. 489. 490. 491. 492. 493. 494. 495. 496. 497. 498. 499. 500. 501. 502. 503.
504. 505. 506. 507. 508. 509. 510. 511. 512. 513. 514. 515.
516. 517. 518. 519. 520. 521. 522. 523. 524. 525.
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I
Rhodium
.,
526. 527. 528. 529. 530. 531. 532. 533. 534. 535. 536. 537. 538. 539. $40.
541. 542. 543. 544. 545. 546. 547. 548. 549. 550. 551. 552. 553. 554. 555. 556. 557. 558. 559. 560. 561. 562. 563. 564. 565. 566. 567. 568. 569. 570. 571. 572. 573. 574. 575. 576. 577. 578. 579. 580.
581. 582. 583. 584. 585. 586. 587. 588. 589. 590. 591. 592. 593. 594. 595. 596.
1085
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1086 597. 598. 599. 600. 601. 602. 603. 604. 605. 606. 607. 608. 649. 610.
611. 612. 613. 614. 615. 616. 617. 618. 619. 620. 621. 622. 623. 624. 625. 626. 627. 628. 629. 630. 63 1. 632. 633. 634. 635. 636. 637. 638. 639. 640. 641. 642. 643. 644. 645. 646. 647. 648. 649. 650. 651. 652. 653. 654. 655. 656. 657. 658. 659. 660. 661. 662. 663. 664. 665. 666. 667. 668.
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I
Rhodium
,
669. 670. 671. 672. 673.
.
674. 675. 676.
,’
677. 678. 679. 680. 681. 682. 683. 684. 685. 686. 687. 688. 689. 690. 691. 692. 693. 694. 695. 696. 697. 698. 699. 700. 701. 702. 703. 704. 705. 706. 707. 708. 709. 710. 711. 712. 713. 714. 715. 716. 717. 718. 719. 720. 721. 722. 723. 724. 725. 726. 727. 728. 729. 730. 731. 732. 733. 734. 735. 736. 737. 738. 739.
1087
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Rhodim 808. 809. 810. 811. 812. 813. 814. 815. 816. 817. 818. 819. 820, 821. 822. 823. 824. 825. 826. 827. 828. 829. 830. 831. 832. 833. 834. 835. 836. 837. 838. 839. 840. 841. 842. 843. 844. 845. 846. 847. 848. 849. 850. 851. 852. 853. 854. 855. 856. 857. 858. 859. 860. 861. 862. 863. 864. 865. 866. 867. 868. 869. 870. 871. 872. 873. ,874. 875. 876. ’ 877. 878. 879.
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1090 880. 881. 882. 883. 884. 885. 886. 887. 888. 889. 890. 891. 892. 8Y3. 894. 895. 896. 897. 898. 899. 900. 901. 902. 903. 904. 905. 906. 907. 908. 909. 910. 911. 912. 913. 914. 915. 9I6. 917.
918. 919. 920. 921. 922. 923. 924. 925. 926. 927. 928. 929. 930. Y31. 932. 933. 934. 935. 936. 937. 938. 939. 940. 941. 942. 943. 944. 945. 946. 947. 948.
949.
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Rhodium
,’
.
950. 951. 952. 953. 954. 955. 956. 957. 958. 959. 960. 961. 962. 963. 964. 965. 966. 967. 968. 969. 970. 971. 972. 973. 974. 975. 976. 977. 978. 979. 980. 981. 982. 983. 984. 985. 986. 987. 988. 989. 990. 991. 992. 993. 994. 995. 996. 997. 998. 999. 1000. 1001. 1002.
1003. 1004. 1005. 1006. 1007. 1008. 1009. 1010. 1011. 1012. 1013. 1014. 1015. 1016. 1017. 1018.
1091
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1092 1019. 1020. 1021. 1022. 1023. 1024. 1025. 1026. 1027. 1028. 1029. 1030. 1031. 1032. 1033. 1034. 1035. 1036. 1037. 1038. 1039. 1040. 1041. 1042. 1043. 1044.
1045. 1046. 1047. 1048. 1049. 1050. 1051. 1052. 1053. 1054. 1055. 1056. 1057. 1058. 1059. 1060. 1061. 1062. 1063. 1064, 1065. 1066. 1067. 1068. 1069. 1070. 1071. 1072. 1073. 1074. 1075. 1076. 1077. 1078. 1079. 1080. 1081. 1082. 1083. 1084. 1085. 1086.
1087. 1088. 1089.
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Rhodium
r
1090. 1091. 1092. 1093. 1094. 1095. 1096. 1097. 1098. 1099. 1100. 1101. 1102. 1103. 1104. 1105. 1106. 1107. 1108. 1109. 1110. 11 11. 1112. 1113. 1114.
1115. 1116. 1117. 1118. 1119. 1120. 1121. 1122. 1123. 1124. 1125. 1126. 1127. 1128. 1129. 1130. 1131. 1132. 1133. 1134. 1135. 1136. 1137. 1138. 1139. 1140. 1141. 1142. 1143. 1144. 1145. 1146. 1147. 1148. 1149. 1150. 1151. 1152. 1153.
1093
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1094 1154. 1155. 1156. 1157. 1158. 1159. 1160. 1161. 1162. 1163. 1164. 1165. 1166. 1167. 1168. 1169. 1170. 1171. 1172, 1173. 1174. 1175. 1176. 1177. 1178. 1179. 1180. 1181. 1182. 1183. 1184. 1185. 1186. 1187. 1188. 1189. 1190. 1191. 1192. 1193. 1194. 1195. 1196. 1197. 1198. 1199. 1200. 1201. 1202. 1203. 1204. 1205. 1206. 1207. 1208. 1209. 1210. 1211. 1212. 1213. 1214. 1215. 1216. 1217. 1218. 1219. 1220. 1221. 1222. 1223.
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Rhodium
. .
1224. 1225. 1226. 1227. 1228. 1229. 1230. 1231. 1232. 1233. 1234. 1235. 1236. 1237. 1238. 1239. 1240. 1241. 1242. 1243. 1244. 1245. 1246. 1247. 1248. 1249. 1250. 1251. 1252. 1253. 1254. 1255. 1256. 1257. 1258. 1259. 1260. 1261. 1262. 1263. 1264. 1265. 1266. 1267. 1268. 1269. 1270. 1271. 1272. 1273. 1274. 1275. 1276. 1277. 1278. 1279. 1280. 1281. 1282. 1283. 1284. 1285. 1286. 1287. 1288. 1289. 1290.
1095
A. G. Sharpe, J . Chem. Soc., 1950, 3444. V. Wilhelm and R. Hoppe, Z. Anorg. Allg. Chem., 1974, 405,193. W, A. Sunder, A. L. Wayda, D. Distefano, W. E. Falconer and J. E. Griffiths, J. Fluorine Chem., 1979, 14, 299. V. Wilhelm and R. Hoppe, 2. Anorg. Allg. Chem., 1974,407, 13. B. Muller and R. Hoppe, Z. Anorg. Allg. Chem., 1972, 392, 37. D. Babel, Strucf. Bonding (Berlin), 1967, 3, 1. M. A. Hepworth, P. L. Robinson and G. J. Westland, J. Chem. SOC.,1954, 4269. D. H. Brown, D. R. RussellandD. W. A. Sharp, J. Chem. SOC.( A ) , 1966, 18. R. D. Peacock and D. W. A. Sharp, J. Chem. Soc., 1959, 2762. G. C. Allen, G. A. M. El-Sharkawy and K. D. Warren, Znorg. Chem., 1973, 12, 2231. F. P. Dwyer, R. S. Nyholm and L. E. Rogers, Proc. R. SOC.N . S .W., 1947, 81, 261. F. P. Dwyer and R.S. Nyholm, Nature (London), 1947, 160, 502. P. J. Cresswell, J. E. Fergusson, B. R. Penfold and D. E. Scaife, J. Chem. Soc., Dulron Trans., 1972, 254. C. K. Jsrgensen, Mal. Phys., 1961, 4, 231. I. Feldman, R. S. Nyholm and E. Watton, J. Chem. Soc., 1965, 4724. A. D. Westland, Can. J . Chem.. 1968, 46, 3591. P. Hurwilz and K. Kustin, Inorg. Chern., 1964, 3 , 823. E. Leidie, C.R. Hebd. Seances Acad. Sci., 1890, 111, 106: 1898, 127, 103. J. J. Scheer, A. E. VanArkel and R. D. Heyding, Can. J. Chem., 1955, 33, 683. J. J. Randall and L. Katz, Acta Crystallogr., 1959, 12, 519. R. S. Nyholm and A. G. Sharpe, J. Chem. SOC.,1952, 3579. P. R. Rao, A. Tressaud and N. Bartlett, J. Znorg. Nucl. Chem., Suppl., 1976, 23. M. L. Hair and P. L. Robinson, J. Chem. Soc., 1960, 3419. L. Wohler and K. F. A. Ewald, Z. Anorg. ANg. Chem., 1931, 201, 145. 0. Muller and R. Roy, J . Less-Common Met., 1968, 16, 129. G. Bayer and H. G. Wiedemann, Thermochirn. Acta, 1976, 15, 213. R. D. Shannon, So/fdState Commun., 1968, 6, 139. J. K. Nicholson and B. L. Shaw, Proc. Chem. Soc., London, 1963, 282. V. E. Kalinina and V. M. Lyakushina, Izv. Vys.Th. Uchebn. Zuved., Khim. Khim. Tekhnoi., 1976, 19, 1287 (Chem. Ahstr., 1977. 86, 11 376). G.Grube and E. Gu, Z. Elektrochem., 1937,43,397; G .Grube and K. H. Mayer, ihid., 1937,43,404; G. Grube and H. Autenricth, ibid., 1938, 44,296. I. Levitin, A. L. Sigan and M. E. Vol'pin, J . Chem. SoC., Chem. Commun., 1975, 469. C. Gheorghiu and L. Burdea, An. Univ. Bucuresti Chim., 1972, 21, 53 (Chem. Abstr., 1973, 79: 86962). C. Gheorghiu, Rev. Roum. Chim., 1968, 13, 1181 (Chem. Abstr., 1969, 70, 111248). L. R. Gahan and M. J. OConnor, J. Chem. SOL, Chem. Commun., 1974,68. N. G. Connelly and J. A. McCleverty, J . Chem. SOC.( A j , 1970, 1621. V. Wilhelm and R. Hoppe, J. Inorg. Nucl. Chem. Suppf., 1976, 113. J. H. Holloway, P. R. Rao and N. Bartlett, Chem. Comrnun., 1965, 306. D. D. Pastukhova, I. M. Cheremisina and S. V. Zemskov, Izv. Sib. Otd. Akud. Nauk SSSR, Ser. Khid. Nauk, 1972, 67 (Chcm. Absir., 1973, 78, 66411). V. B. Sokolov, Y. V. Drobyshevskii, V. N. Prusakov, A. V. Ryzhkhov and S.S . Khoroskev, Dokl. Akad. Nauk SSSR, 1976, 229, 641. W. A. Sunder, A. L. Wayda, D.Distefano, W. E. Falconer and J. E. Griffiths, J . Fiourine Chon.. 1979, 14, 299. G. H. Ayres, Anal. Chem., 1953, 25, 1622. F. Pantani, Tulunra, 1962, 9, 15. V. S. Syrokomskii and N. N. Proshenkova, Zh. Anal. Khim., 1947,2,247. J. Berzelius, Ann. Phdos., 1814,3, 252. G. Grube and H. Autenrieth, Z. Electrochem., 1938, 44, 296. S. Sarangiapani and J. E. Earley, J. Inorg. Nucl. Chem., 1981, 43, 2991. B. K. Morrell, A. Zalkin, A. Tressaud and N. Bartlett, Inorg. Chem., 1973, 12, 2640. J. H. Holloway, P. A. Rao and N. Bartlett, Chem. Commun., 1965, 306. N. Bartlett and D. H. Lohmann, J. Chem. SOC.,1964, 619. W. E. Falconer, G. R. Jones, W. A. Sunder, M. J. Vasile, A. A. Muenter, T. R. Dyke and W. Klemperer, J. Fluorine Chcm., 1974,4, 213. H. H. Claassen, H.Selig, J. G. Malm, G. L. Chernick and B. Weinstock, J. Am. Chem. Soc., 1961, 83,2390. C. L. Chemick, H. H. CIaassen and B. Weinstcxlk, J. Am. Chem. Soc., 1961,83, 3165. 0. Ruff and E. Ascher, 2. Anorg. Allg. Chem., 1929, 183, 193. A. G. Sharpe, J. Chern. Soc., 1950, 3444. S. Siege1 and D. A. Northrop, Inorx. Chem., 1966, 5, 2187. C. K. Jergensen, Aciu Chem. Scand., 1958, 12, 1539. B. Weinstock, H. H. Claassen and C. L. Chernick, J. Chem. Phys.. 1963,38, 1470. N. Bartlett, Angew. Chem., Znt. Ed. Engl., 1968, 7,433. J. G. Dillard and R. W. Kiser, J. Phys. Chem., 1965, 69, 3893. C. Claus, Bull. Acad. Petersburg, 1860, 2, 187. G. Grube and B. Gu, 2. Elektrochem., 1937, 43, 397. F. Pantani, Ric. Sci. Rend. Sez A , 1964, 4,41. A. Carrington and M. C. R. Symmons, Chem. Rev., 1963, 63, 445. E. B. Meier, R. R. Burch, E. L. Muetterties and V. W. Day, J. Am. Chem. Soc., 1982, 104,2261. M.Mosmer, T. Glowiak and J. J. Ziolkowski, Proc. 3rd Int. Semin. Cryst. Chem. Coord. Organomet. Compounds, 1911, 235. R. D. Feltham and J. H. Enemark, Top. Stereochern., 1981, 12, 155. W. Manchot and W. PAaum, Eer., 1927, 60, 2180.
I 1096 1291. 1292. 1293. 1294. 1295. 1296. 1297. 1298. 1299. 1300. 1301. 1302. 1303. 1304. 1305. 1306. 1307. 1308. 1309. 1310. 1311. 1312. 1313. 1314. 1315. 1316. 1317. 1318. 1319. 1320. 1321. 1322. 1323. 1324. 1325. 1326. 1327. 1328. 1329. 1330. 1331. 1332. 1333. 1334, 1335. 1336. 1337. 1338. 1339. 1340. 1341. 1342. 1343. 1344. 1345. 1346. 1347. 1348. 1349. 1350. 1351. 1352. 1353. 1354. 1355. 1356. 1357.
Rhodium W. P. Griffith, J. Lewis and G. Wilkinson, J. Chem. SOC.,1959, 1775.
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Commun., 1979, 1160.
49 Iridium NICK SERPONE and MARY ALAIN JAMIESON Concordia University, Montreal, Quebec, Canada INTRODUCTION IRIDIUM( - I) 49.2.1 Nitrogen Ligands 49.2.1.1 Nitrosyl ligands 49.2.2 Phosphorus Ligands 49.2.2.1 Phosphine ligands
1098 1099 1099 1099 1 loo 1 loo
IRIDIUM(0) 49.3.1 Nitrogen Ligand-s 49.3.1.1 Aliphatic amine 1igand.r 49.3.1.2 Nitrosyl ligands 49.3.2 Phosphorus Ligands 49.3.2.1 Phosphine ligands
1 loo 1100 1100
49.1 49.2
49.3
1100
1101 1101
IRIDIUMQ) 49.4.1 Group IV Ligands 49.4.1.1 Cyanide, isocyanide and fulminate ligands 49.4.1.2 Sn-containing ligands 49.4.2 Nitrogen Ligands 49.4.2.1 Ammonia and amine ligands 49.4.2.2 Nitrosyl ligands 49.4.2.3 Dinitrogen, azide, diazo, triazene. tetrazene and selenocyanate ligands 49.4.2.4 Polypyrazolylborate ligands 49.4.3 Phophorw and Arsenic Ligands 49.4.4 Oxygen Ligonds 28 49.4.4.1 BDikelonate, catecholate and semiquinone ligands 49.4.4.2 Bicarbonate and carboxylate ligands 49.4.5 Suvir Ligands 49.4.5.1 S,, CS,,thiocarbamate and related ligands 49.4.5.2 SO, and phosphine sulfide ligands 49.4.6 Halogens as Ligands 49.4.7 Hydrogen and Hydrides as Ligands 49.4.8 Multidentate Macrocyclic Ligands
1101 1101 1101 1103 1103 1103 1104 1105 1108 1108 1115 1115 1116
IRIDIUM(I1) 49.5.1 Nitrogen Ligands 49.5.1.1 Nitrosyl iigands 49.5.2 Phosphorus and Arsenic Ligan& 49.5.2.1 Phosphine ligands 49.5.2.2 Arsenic ligands 49.5.3 Su& Ligands 49.5.3.1 Mononuclear complexes 49.5.3.2 Dinuclear complexes 49.5.4 Halogens m Ligands 49.5.S Mixed Donor Atom Ligands 49.5.6 Multidentate Macrocyclic Ligands 49.5.6.1 Schif base ligands
1120 1120 1120
IRIDIUM(II1) 49.6.1 Mercury-containing Compounds as Ligands 49.6.2 Group IV Ligands 49.6.2.1 Cyanide, isocyanide and fulminate ligands 49.6.2.2 Si-, Ge- and Sn-containing ligands 49.6.3 Nitrogen Ligands 49.6.3.1 Ammonia and amine ligands 49.6.3.2 N-hereracyclic ligan&
1124 1124 1125 1125 1126 1128 1128 1130
49.4
'
49.5
1117 1117 1118 1119 1119 1120
1121 1121. 1121 1121 1121 1121 1123 1123 1124 1124
49.6
1097
I
1098
Iridium
49.6.3.3 Diazenato and diazo ligands 49.6.3.4 Azide ligands 49.6.3.5 Polypyrazolylborate ligands 49.6.4 Phosphorus, Arsenic and Antimony Ligands 49.6.4.1 Unidentate ligands 49.6.4.2 Polydentate ligands 49.6.5 Oxygen Ligands 49.6.5.1 Hydroxy ligands 49.6.5.2 Dioxygen ligands 49.4.S.3 EDiketonate, catecholute and semiquinone ligands 49.6.5.4 Carboxylate, chelating carboxylare, COj-, CG-, RCO; and oxalate ligands 49.6.5.5 Nitritenitro and nitrate ligands 49.6.6 Sulfur Ligundv 49.6.6.1 Sulfide ligands 49.6.6.2 Dithiocarbamate and CS,-based ligands 49.6.6.3 Other S-containing ligands 49.6.7 Selenium Ligands 49.6.8 Halogen as Ligands 49.6.9 Hydrogen and Hydrides as Ligands 49.6.10 Mixed Donor atom Ligands 49.6.10.1 Bidentate N-C ligands 49.6.10.2 Bidentate N-S ligands 49.6.10.3 Bidentate S-C ligands 49.6. I I Multidentate Macrocyclic Ligan&
1132 1133 1134 1134 €134 1135 1138 1138 1138 1140 1141
IRIDIUM(1V) 49.7.I Phosphorus und Arsenic Ligands 49.7.2 Oxygen Ligand,r 49.7.2.1 Hydroxy ligands 49.7.2.2 Oxalate ligands 49.7.2.3 Nitrato ligandv 49.7.3 Surfur Ligands 49.7.4 Halogens as Ligands
1155 1155 1 I56 1156 1156 1156 1157 1157
IRIDIUM(V) AND IRIDIUM(V1) 49.8.1 Iridium! V ) 49.8.2 Iridium ( V I )
1158
49.7
49.8
49.9 CATALYTIC ACTIVITY OF IRIDIUM COORDINATION COMPLEXES 49.10 AD INTEGRATUM 49.I0.l 49.10.2 49.10.3 49.10.4 49.10.5 49.10.6 49.10.7 49.10.8 49.IO .9 49.10.10
49.11
49.1
Nitrosyl Ligands Nitrogen Phosphorus Ligands Arsenic and Antimony Ligands Oxygen Ligands Boron Ligands Sulfur and Selenium Ligands Halide Ligands Hydride Ligands Catalyiic Activity of Iridium Coordination Complexes
REFERENCES
1143
1145 1145 1145 1146 1147 1148 1149 1 I52 1152 1154 1154 1155
1158 1158
1 I58 1161 1161 1161 1162 1164 1164 1165 1165 1166 1166 1166
1167
INTRODUCTION
The coordination chemistry of iridium is concerned primarily with the I, I11 and IV oxidation states; Complexes with iridium in the -I, 0, V and VI oxidation states are also known. Of these latter states, Ir-' and Tro are found in carbonylate anions, oligonuclear and substituted carbonyls, hydrides and ammines. Iridium(V) and (VI) complexes are predominantly hexahalide compounds. Diamagnetic iridium(1) complexes (d*1 form with n-acid ligands such as tertiary phosphines and arsines, carbonyls, nitrosyls and alkenes, as well as with hydride and halide ions. Several Ir' complexes undergo oxidative addition reactions to yield six-coordinate octahedral Ir"' species. Kinetically inert iridium(II1) complexes ( d 6 )are almost invariably low spin (t&) and six coordinate, though a few five- and seven-coordinate complexes have been reported. By and large, iridium(II1) complexes occur with 'soft' ligands, including CO, ER, (E = P, As, Sb), R,S, NO;, NH,, SCN-, SO:-, SO:- and X- (X = C1, Br, I). Additionally, there are numerous iridium(II1) hydride complexes, usually containing carbonyl, phosphine, arsine and/or stibene ligands. Neutral as well as anionic and cationic iridium(II1) complexes exist. A number of six-coordinate paramagnetic
I
1099
Iridium Table I
Oxidation States and Associated Coordination Numbers and Geometries of Iridium Complexes
Oxidation state
Coordination number
-I, d"
Four Four Four Four Four Four Four Five Four Five Fivc Five Six Six Six Six Six Six Six Seven Six
0, d'
I, d 8 11, d 7 111, d b
IV, d 5 V, d 4
VI, d'
Octahedral -
Octahedral Octahedral
[IrC&]*M[Ir(OH),], M = Zn, K2 CslIrF,1 [Ir(H),"(PPhEt,), W 6 1
iridium(1V) complexes ( d 5 )are known, in contrast to rhodium(1V). Stable IrTVcomplexes are formed with halogen, oxygen and nitrogen donors; these complexes are easily reduced to Ir"' with P, As and S donor ligands. Table 1 summarizes the coordination chemistry of iridium in terms of oxidation state and associated coordination numbers and coordination geometries. This chapter will not deal with iridium complexes in which the coordination chemistry of the iridiumxarbon bond is implicated inasmuch as Leigh and Richards' have very recently (1982) provided an excellent detailed review on such compounds; organoiridium' and iridium carbonyl3 complexes were also previously reviewed. Iridium complex chemistry has been reviewed (1980) along with rhodium: and in annual reviews.' Additionally, iridium complexes have been treated in 'Comprehensive Inorganic Chemistry'.6
49.2
IRIDIUM( -I)
49.2.1 Nitrogen Ligands 49.2.1.1 Nitrosyl iigands Several iridium( - I) nitrosyl complexes are known. Those with the general formula [Ir(NO),X(PPh,),] (X = C1, Br, I) are produced upon reduction of [Ir(NO),(PPh,),] with the appropriate lithium halide. IR spectral data for these complexes indicate bridging NO groups, as shown by the low N-0 frequencies [ Y(N-0) z 1540-1490 cm-'I.' Reaction of [Ir(NO),X(PPh,),] with NO results in the oxidation of NO according to reaction (l).' [~rIX)(N~),(PPh,),I4- 2NO
-
IIr(X)(NO,)(NO)(PPh,),l + N O
(1)
The d10complex [Ir(NO)(PPh,),] (I) has been prepared in a variety of ways, including (i) reaction of [Ir(NO),(PPh,),](C10,) with excess PPh,;7 (ii) reaction between [Ir(NO)(CO)(PPh,),] and excess PPh,: and (iii) the reduction of [IrCl,]. 3 H 2 0 or [Ir(Cl)(N,)(PPh,),] with sodium amalgam (or zinc metal) in the presence of PPh, and NO." An X-ray crystal structure determination of [Irinteraction was con(NO)(PPh,),] has shown a tetrahedral geometry about Ir. The Ir-N-0 sidered in terms of pr(PPh,),]- and [NO]+ moieties, wherein the NO group serves as a three-electron donor in a c-n synergistic interaction with Ir." The crystal structure of [Ir(NO)(PPh3),].C3H, has also been determined; presumably, this represents the first reported occurrence of unsubstituted cyclopropane in a crystal structure determined with X-rays." An ESCA study of [Ir(NO)(PPh,),] led to an O(1s)-N(1s) ionization potential difference of 131.9eV, indicative of the presence of a linear nitrosyl 1iga11d.I~Moreover, [Tr(NO)(PPh,),] undergoes oxidative additions with MeI, halogens and stoichiometric amounts of hydrogen halides to yield five-cbordinate complexes (2) with
1100
Iridium
tetragonal pyramidal structures in which the NO ligand is bent (see Scheme 1). However, reaction of [Ir(NO)(PPh,),] (1) with an excess of HBr and HC1 produces [Ir(NH?OH)(X),PPh,),], in which the nitrosyl ligand is converted to a bound hydroxylamine. Also, [Ir(NO)(PPh,),] undergoes protonation with non-complexingacids to yield hydride species.’ The [Ir(NO)(CO)(PPh,),] complex can be prepared via the treatment of [Ir(H)(CO)(PPh,),] with NO.’ The iridium( - I) dinitrosyl complex [Ir(NO)2(PR3)2]+has also been r e p ~ r t e d . ’ ~
r
1’
Zt
P
X-
# \ OHz
PPI13
L X = CI. B r : L = PPh, 0
/
/O
Scheme 1
The tetraazadiene complex [Ir(NO)(N-p-tolyl),(PPh,)] exhibits a conformational effect, and thereby a temperature dependent ‘ H NMR spectrum. Also, this complex readily forms a CO add~ct.’~
Phophorus Ligands
49.2.2
49.2.2.1
Phosphine ligands
The salt Na[Ir(CO),(PPh,)] reduces FJi(Cl),(PPh,),] to yield [Ir(CO),(PPh,)], with p i (CO),(PPh,),]. Reaction of the iridium( - I) complex with trans-pt(Cl),(py),] (py = pyridine) affords [M,(CO),(PPh,)(py)], where M, = Pt,, PtIr or 1r2.l6
49.3
IRIDIUM(0)
49.3.1
Nitrogen Ligands
49.3.1.1 Aliphatic amine ligands
Iridium(0) is present in carbonyl and substituted carbonyl Complexes, and perhaps in [1r(NH3)J, if indeed this species exists. Although the ammine complex [Ir(NH,),] has been reported,” the diamagnetism of this complex suggests an irridium(1) hydride species.
49.3.1.2
Nitrosyl ligands
The iridium(0) nitrosyl complexes [Ir(NO)(CO)(PPh,),]+ and [Ir(N03)2(NO)2(CO)(PPh3)2] have been synthesized via reaction of [Ir(CO),(PPh,),]+ with NO and N204, respectively. The reaction between [Ir(NO)(PPh,),] and R C 0 2 H reportedly yields {h(O,CR)(NO)(PPh,),], where R is CF, or C2F5.” The preparation of [Ir(NO)(PPh,)],O -C6H, (3) involves reaction of trans-[Ir(Cl)(CO)(PPh,),l and excess NaNO, under a nitrogen atmosphere. The complex (3) is diamagnetic and stable in air, but rapidly decomposes in solution on exposure to air.” The dinuclear oxygen-bridged complex [Tr(NO)(PPh,)],O appears to be a good candidate for a strong, considerably bent Ir-Ir bond.
Iridium
1101
SCF-X,-SW calculations on this complex have led to the conclusion that the NO ligands are crucial in stabilizing and in determining the exact nature of the substantially bent Ir-Ir bond (2.555 A). Contour maps suggest the complex to be closer to Ir'-NO" than to Iro-NO+, with square planar coordination about each Ir atom." The X-ray crystal structure of complex (3)23shows a bent Ir angle of 177" and an Ir-N bond -1r bond with the NO group n bonded to Ir, with an Ir-N-0 length of 1.76 8, suggesting that the Ir-N bond has some double bond character. Complex (3) does not undergo oxidative addition reactions with H,, Cl,, Br,, I2or HgX,. However, upon reaction with I,, HgCl, or HgBr,, oxidation of the Ir-Ir bond occurs without disruption of the oxygen bridge (reactions 2 and 3). Also, complex (3) reacts with tetracyanoethylene to form a I: 1 adduct, wherein the alkene inserts into the Ir-Ir bond with one carbon atom bonded to each Ir atom. With excess PPh,, complex (3) affords [Ir(NO)(PPh,),] and Ph, PO?' (3)
+
+ -
[Ir(X)(NO)(PPh,)]20f 4- HgX
HgX2 (3)
I>
tIr(0(NO)(PPh,)I,O
(2) (3)
Phosphorus Ligands
49.3.2
49.3.2.1 Phosphine Iigands Several series of dinuclear iridium(0) complexes with tertiary phosphine ligands are known. These include (a) [Ir(CO),(PR,)], where PR3 = PPry, PPrS, PBu;, PPh, and P(tolyl),; (b) [1r2(CO),(PR3),], where PR, = PPr;, PPh,, P(OPh), and P(tolyl),; (c) [Ir2(CO)5(PR3)3J,where PR, = PPh, or PPh,(tolyl); and (d) [II-~(CO),(PR~)~], where PR, = PPh, or P(OPh)3.24.25 The complex [1r2(CO),(PPh,),] (4) reacts with P(OPh), to yield [Ir,(CO),{P(OPh),),], from which [Ir,(CO), {P(OPh),},J is obtained on decomposition. Other reactions of these complexes are depicted in reactions (4)-(6).25The reduction of Fr(Cl)(CO)(PPh,),] under 20atm CO pressure in the presence of HNEt, affords [Ir,(C0)6(PPh3)2].26 (4) (4)
+
2PPh&-tolyl)l (4)
49.4
+ PPh :3iy '
+ so2
+
IIr2(CO)5(PPh3)31 CO
[I~,(CO)S(PP~~){PP~~(~-~O~Y~)}~I + PPh2 + CO
6 " '
'6*6
[ I ~ Z ( C O ) , ( P P ~ ~ ) ~ ( ~ - S Oco Z),~
(4) (5)
(6)
IRIDIWM(1)
The coordination chemistry of iridium(1) involves n-acid ligands. Both four- and five-coordinate species form. Iridium(1) complexes are invariably prepared via some form of reduction, either from analogous iridium(II1) complexes or from halide complexes in the presence of the complexing ligand, or via substitution.
49.4.1 49.4.1.1
Group lV Ligands Cyunide, isocyanide and fulminate ligmds
The yellow iridium(1) complex [Ir(CN)(CO)(PPh,),] has been prepared in two ways; (i) by the action of CN- on the [Ir(MeCN)(CO)(PPh,),]+ salt; or (ii) by treating [ir(Cl)(CO)(PPh,),] in a chloroform solution of PPh, with an anionic resin in the CN- form. The IR spectrum of the cyano analogue of Vaska's complex exhibits a v(CN) band at 2 1 2 O ~ m - ' . ~ ~ Transition metal complexes containing isocyanide ligands have been reviewed by Malatesta and BonatiZ8[{Ir(Cl)(CO)3}n]reacts with alkyl and aryl isocyanides (CNR) to give the iridium(1) isocyanide complexes [Ir(CNR),]+, where R = C6Hl,, p-MeC,H, or p-M&C,H,. IR spectra support a square planar structure for [Ir(CNR),J+. These complexes undergo trans oxidative addition with halogens (X,) to produce [Ir(X), (CNR),]' , with Me1 to yield [Ir(I)(Me)(CNR),]+ (R = C,H,,), and with 0, to give [Ir(CNR),(O,)Jf (R= C6Hll).The results indicate that the alkyl isocyanide complexes are more reactive toward oxidative addition than aryl isocyanide complexes.29 Electronic absorption and MCD spectroscopies have been employed to characterize [Ir(CNEt),]+ , which exhibits intense visible absorption assigned to MLCT (metal-to-ligand change transfer)
1102
Iridium
transitions from the occupied metal d orbitals to the lowest energy 71% CNEt orbital. The MLCT spectra have been interpreted in terms of a model which involves a single n* CNEt acceptor orbital and includes metal spin-orbit coupling in the MLCT excited states.30IR and electronic absorption spectra of [Ir(CNR),]+ (R = But) are indicative of normal monomeric Ir1/L4complex behaviour. However, [Ir(CNMe),]Cl exhibits a very different absorption spectrum, interpreted in terms of an oligomeric species [Ir(CNMe),];+ .3' Additionally photolysis of this [Ir(CNMe),];+ species reportedly affords [Er(CNMe),]+.31[Ir(CNR),]+ and [Ir(Cl)(CNR),(CO)] (R = C,H,) have also been prepared and ~haracterized.~' The complexes [Ir(CNR),]+ (R = aIkyl or aryl) readily undergo where X = Me, Pr, I, Y = I; X = MeCO, oxidative addition to obtain trans-[Ir(X)(Y)(CNR),]+, o-C,H,, HgCl, SnCl,, SnPh,, Y = C1; X = Y = Br; X = CN, Y = Br.33The reactivity and structures of some tetrakis(ary1 isocyanide)iridium(I) complexes have been investigated by Tanaka et The addition of HX to [Ir(CNR),(PPh,),]+ (R = alkyl or aryl) reportedly proceeds by displacing X- from the addendum YX to produce an ionic intermediate from which L is subsequently displaced by X- as depicted in Scheme 2.35 [Ir(L),]+
+ YX
-
[Ir(Y)(L)5]2+
+ X-
-+
[Ir(X)Y(L),]+
+ (L)
Scheme 2
X-Ray crystal and molecular structures for the stable five-coordinate iridium(1) isocyanide complex [Ir(CNMe)(dppe),]+ (5) reveal this complex to possess a trigonal bipyramidal geometry. The isocyanide ligand occupies an equatorial position and each dppe chelate bridges one axial and complexes (where one equatorial site.j6In contrast to [Ir(CNR), (PPh,),]+ ," the [Ir(CNR)(P-P),]' P-P = dppe, dppen and R = Me, p-MeC,H,) undergo oxidative addition of proton acids (including MeOH), halogens and HgC1, without loss of a ligand to yield [Ir(CNR)(P-P),YI2+ (Y = H, Cl, I, HgCl). However, [Ir(CNR)(P-P),]+ does not react with alkyl, allyl or acyl halides. The addition of proton acids yields cis products which, on heating, isomerize. Reaction of [Ir(CNR)(P-P),]+ with H, or 0, affords cis-[Ir(H),(P-P),]+ and [Ir(O,)(P-P),]+ , respe~tively.~~ t-Butyl isocyanide (CNBu') is known to react with the A-frame complex [Ir,(Cl)(CO)(P-P),]X (P-P = dppm, dpam; X = BPh; , BFL, PF; ) affording [I~,(C~)(CNBU~)(CO)~(P-P),]X, [Ir,(Cl)(CNBu'),(CO),(P-P),]X (P-P = dppm, X = BPh;; P-P = dpam, X = BF;) and [Ir2(CNBu'),(CO)(P-P),](BPh,),. These complexes were characterized by IR, ',C and j'P NMR spectroscopic techniques.j* Bidentate diisocyanocyclohexane ligands have been prepared. meso-l,3-Diisocyanocyclohexane reacts with [Ir(Ci)(~od)]~ to produce [Ir,(meso-1,3-C,Hl,N,),](C1), (6)for which a windmill structure has been propo~ed.~'
Iridium 49.4.1.2
1103
Sn-containing Iigands
The reduction of Vaska's complex, [Ir(Cl)(CO)(PPh,), 1, with sodium amalgam in THF yields a non-isolable anionic species (7;reaction 7). Upon addition of SnMe,Cl, SnPh, C1 or SnMe,Cl,, high yields of (8HlO),respectively, are obtained.40 Reaction of (9) with Br,, I, or mercury(I1) halides results in the cleavage of the Sn-Ir bond. The Ph,Ge-Ir analogue of (9) has also been prepared.,' t~ans-[Ir(Cl)(CO)(PPh,),]
Na[Ir(CO),(PPh,)]
(7)
(7)
(8)
(10)
(9)
The addition of anhydrous SnCl, and CIH2or C,Y, to trans-[Ir(CI)(CO)(PPh,),] yields [Ir(SnCl,)(C,H2)(CO)(PPh,),] and [Ir(SnCl,)(C,H,)(CO)(PPh,),]with structures (11) and (12), respectively. A five-coordinate structure was assigned to the intermediate product [Ir(Cl)(SnCl,)(CO)(PPh,)2] based on 1R spectral data. This five-coordinate complex reacts with H,, hydrocyanic acid and HX to yield stable products. Addition of HCl yields [Ir(H)(C1)(SnC13)(PPh,),].42 pPh,
PPtl,
PPtl,
The structure of the complex resulting from the reaction between [Ir(Cl)(CO)(PPh,),] and SnCl, has been discussed.43Recently, the existence of the Sn-Ir bond in the product was questioned.@ Based on ' I 9 Sn NMR data, the iridium(1) complexes from the reaction between [Ir(Cl)(CO)(PR,),] (PR, = P@-XC,H,),; X = H, F, C1, OMe) and SnCl, are isomers of composition [Ir(Cl)(SnCl2)(C0)(PR3)J; they contain bridging halogen ligands and Sn is not coordinated to Ir. When or reacted with HCl or H,, the expected iridium(I1I) complexes [Ir(H)(CI)(SnCI,)(CO)(PR,),] [Ir(H), (SnCl,)(CO)(PR,)] are obtained. These iridium(II1) complexes do contain an Sn-Ir bond.@ Reduction of Fr(X)(CO)(PPh,),] under the reaction conditions of reaction (7) to yield [Ir(CO),(PPh,)]-, followed by the addition of MX (M = SnCl,, SnMe,; X = CI), has yielded the dinuclear iridium(1) complex [{Ir(C0)3(PPh,)),M].4s
49.4.2
49.4.2.1
Nitrogen Ligands Ammonia and amine ligands
The reduction of [Ir(Br)(NH,),] with potassium in liquid ammonia purportedly yields the yellow iridium(0) complex [Ir(NH,),]." However, the diamagnetism of the product is suggestive of an iridium(1) hydride species, [Ir(H)(NH,),].
1104 49.4.2.2
Iridium Nitrosyl ligands
Transition metal nitrosyl complexes have been reviewed by Johnson and McCleverty.46The nitrosyl analogue of Vaska’s complex [Ir(Cl)(NO)(PPh,),]+ ,is as is [Ir(NO)(CO)(PPh,),].’ The latter complex is prepared by the action of nitric oxide on [Ir(H)(CO)(PPh,),]. [Ir(Cl)(NO)(PPh,),]+ has been obtainedg from the reaction of [Ir(H)(Cl)(NO)(PPh,)J with HC104, and undergoes several addition reactions to form five-coordinate nitrosyl complex cations as summarized in reactions (8) through (1 3).47 The carbonylation of complexes of the type [M(NO)(PPh,),] reportedly proceeds to mixed nitrosyl-carbonyl species (e.g. [M(NO)(CO)(PPh,),]). The kinetics of CO absorption by [Ir(NO)(PPh,),j in chlorobenzene to yield [Ir(NO)(CO)(PPh,j2] and PPh3 have been At constant pressure, kinetics first order in [Ir(NO)(PPh,),] were observed. Rate constants and activation parameters for CO absorption are consistent with the mechanism in reactions (14) and (15). [Ir(CI)(NO)(PPh,),]* + CO
-
+
[Ir(Cl)(NO)(CO)(PPh,),]+
(8)
[Ir(Cl)(NO)IPPh)31+
(9)
[Ir(CI)(NO)(PPh3),]++ PPh3 [Ir(C1)(NO)(PPh3)21++ dppe
[Ir(C1>(No>(dppe)(PPh3)~1+
[Ir(C1)(NO)(PPh3)z]f+ CI2
-
----)
+
[Ir(CI)(NO)(PPh3)21+ C1[Ir(CI)(NO)(PPh,),]+ + H 2 0 [Ir(NO)(PPh,),]
+
[Ir(Cl),(NO)(PPh,)zl+ [Ir(C1)z(NO)(PPh3)~l
[Ir(O~~)(Cl)~~O)(PPh~)~l+
t solvent e [Ir(NO)(PPh,),(solvent)J +
[Ir(NO)(PPh,),(solvent)] + CO
PPh,
[Ir(NO)(CO)(PPh,)2]+ solvent
----r
(10) (11)
(12)
(13) (14)
(1 5)
Several five-coordination iridiumu) nitrosyl complexes have been reported, including [Ir(X)2(NO)(PR,),] (X = C1, Br, I; PR, = phosphine) and [Ir(Cl)(NO)(PPh,),]+. The crystal structure of the latter reveals an Ir-N-0 angle of 124”. The coordination about Ir is tetragonal pyramidal with the NO ligand at the apex, and C1, CO and trans P atoms in the basal plane. The Ir-N distance is quite long (1.97A), suggestive of coordinated NO- with spz hybridization on the N atom (see 13).49The X-ray crystal structure of [Ir(I)(NO)(CO)(PPh,),](BF,)-C,Hhas , revealed it to be rather similar to [Tr(Cl)(NO)(CO)(PPh,),lf, with a distorted tetragonal pyramidal geometry about Ir. The angle is 125°.50The [Ir(NO)(phen)(PPh,),](PF,), Ir-N bond Iength is 1.89 A, while the Ir-N-0 complex contains an iridium(1) atom in a trigonal bipyramidal environment with the phen and NO ligands in equatorial positions. The NO ligand is present at NO+ .4
(13)
The complex [Tr(NO)(PPh,),($ -C,H,)](PF,) (14) can be prepared from the reaction of
[Ir(Cl)(NO)(PPh,),](PF,) and Sn(C3H5)4at 273 K in THF. Repetition of the synthesis employing [Ir(Cl)(NO)(PPh,),](BF,) afforded [Ir(NO)(PPh,),(q3 -C, H,)](3F4) *0.5H20(15). Though evidence for the linear bent equilibrium (14G 15) of metal nitrosyl complexes remains scarce, it has been postulated that [Ir(NO)(PPh,),(q3 -C, H5)]+ exhibits a facile, well-defined equilibrium. IR and H NMR spectroscopic studies suggest the presence of several dynamic processes in solution. An X-ray crystal structure of complex (15) confirmed a bent nitrosyl ligand. Additionally, [Ir(NO)(PPh,), (q3C,H,)]+ reacts with CO to yield acrolein oxime and [Ir(CO),(PPh,),]+, most probably via an intramolecular coupling process.” +
r
L
Iridium
.
1105
The dinitrosyl cation [Ir(NO),(PPh,),]+ (16)was synthesized and characterized by X-ray crystallography. A 3’P NMR study of complex (16) has suggested that phosphine exchange occurs primarily via an associative pathway.52The X-ray crystal and molecular structures of (16) have been determined.’, Reaction of complex (16) with PPh3yields [Ir(NO)(PPh,),] and NO+, though reaction with less bulky phosphines PR; affords [Ir(PR;),]+ .j2 Carbon monoxide reacts with complex (16) to give [Ir(CO),(PPh,),]+, C 0 2 and N,O; with tertiary phoshines, (PR,), R,PO and N,O are afforded. The carbon monoxide reaction was unexpected, the NO ligand being usually resistant to CO substitution. The proposed mechanism involves the prior formation of a five-coordinate adduct, [Ir(NO),(CO)(PPh,),]+ , which subsequently and rapidly undergoes oxygen transfer from a coordinated NO ligand to the combined CO, followed by an intramolecular attack of the other NO ligand on the Ir-N system thus produced.54 The X-ray crystal structure of [Ir(Cl)(NO)(PPh,)],O (17) reveals two identical square planar iridium(1) entities, linked by a common oxygen bridge.55[Ir(X)(NO)(PPh,)],O (X = C1, Br) reacts with excess PPh3 in CH, Cl, to yield [Ir(X),(NO)(PPh,),] and [Ir(NO)(PPh,),].2’
49.4.2.3 Dinitrogen, azide, diazo, triazene, tetruzette and selenocyanate ligundr
Dinitrogen complexes of the type [Ir(X)(N,)(PPh,),] (18) (X = C1, Br, I) have been synthesized via the mechanism illustrated in Scheme 3. This mechanism is supported by IR spectral and kinetic e~idence.’~ Furthermore, complex (18)’ forms the six-coordinate species (19) upon reaction with certain alkenes in which the N, ligand remains intact. However, reaction of (18) with PPh,, CO or aryl cyanates leads to N, di~placement.’~ In addition to the dinitrogen analogue of Vaska’s complex, [Ir(Cl)(N,)(PPh,),], impure samples of [1r(Cl)(N2)(PR3),],where PR, is PEtPh2, PBu“Ph,, PMe,Ph or PEt,Ph, have been obtained.” Oxidative addition of HBF,, CF,S03H or BuS0,H to trans[Ir(CI)(N,)(PPh,),] affords the reactive iridium(II1) complexes [Ir(H)(Cl)(L)(N,)(PPh,),] (L = FBF3, OSO,CF,, OS0,Bu). The tetrafluoroborate complex [Ir(H)(Cl)(FBF,)(N,)(PPh,),] reacts with valinate, diethyldithiocarbamate and PPh3 to give (20), (21)and (22), respecti~ely.~~ IIr(X)(CO)(PPh,), 1
+
ArC(O)N,
N=N
CO
Scheme 3
The treatment of [Ir(CI)(CO)(PPh,),] with an acid azide RC(0)N3 at 273 K has afforded complex (24), presumably through the 1,3-dipolar intermediate (23)which subsequently eliminates N-acyl isocyanate (Scheme 4). Complex (24) reacts with diethyl maleate (DEM) to give complex (25; see Scheme 4).5g
I
1106
Iridium H
PPh,
A complex with the formula [Ir(NO)(N,R,)(PPh,)] (26)(R = SOzC,H,Me) has been synthesized from [Ir(NO)(PPh,),] and p-toluenesulfonyl azide in benzene. The tetrazene complex (26)is believed to involve in solution an equilibrium between closed-ring (26b)and opened-ring forms (26a)and (26c).Structure (26b)is favoured in the solid state. Complex (26)reacts with gaseous HCI to yield [Ir(Cl),(NO)(PPh,)] and the corresponding amide (RNH,) and azide (RN,).I9
R (26a )
(26b)
(26~)
The neutral arenediazo iridium(1) complex [Ir(Cl)(N,C,Cl,)(PPh,),] (27)(N,C5C14= 2,3,4,5tetrachlorodiazocyclopentadiene)has been prepared via reaction (16). An X-ray crystal structure determination of complex (27)reveals a singly bent arenediazo 1igandqMFurther, complex (27) is somewhat similar to the cationic complex [Ir(Cl)(N,Ph)(PPh,),]" >1 in that both complexes form
1107
Iridium
~
five-coordinate adducts with phosphines and isocyanides. However, reactions with NOf , CO or N,Ph+ are quite different for the two iridium complexes. Complex (27) reacts with NO', CO, N,Ph+, PMe,, PMe,Ph, PMePh, or Bu'NC (= L) to give the five-coordinate species [Ir(Cl)(N,C,Cl,)(PPh,), (L)]' . Both complex (27) and [Ir(Cl)(N,Ph)(PPh,),]+ have the singly bent diazo linkage perpendicular to the P-P vector.",6' The related complexes [Ir(Cl)(N,C,X,)(PR,),] (X = C1, Br; R = Ph, p-FC,H,, p-MeC,H,) have been prepared, wherein iridium is in a square planar environment with mutually trans PR, ligands and a singly bent N,C5X4ligand. Complex (27) reacts with HCl to yield the six-coordinate iridium(II1) complex [Ir(H)(Cl)?(N, C5ClJ(PPh3)z].62A square planar environment for Ir has also been observed for [Ir(Cl)(N2C5Cl,)(PPh,),]. G H , , having mutually trans PPh, ligands. The neutral diazo ligand possesses a singly bent geometry in this complex; it appears that there is significant contribution from both canonical structures (28a) and (28b), though a greater contribution was expected from (28b).63
(28b)
(28a)
[Ir(CO),(PPh,)(p-SO,)], is known to react with p-substituted arenediazsnium salts, yielding = p-tolyl, p-methoxy, p-fluoro), in which a molecule of SO, is replaced by an arenediazonate molecule. The protonated derivatives of (29), {[Ir(CO),(PPh,)],(~-N=NHC,H,R)(p-S0,))2+, were obtained by treating (29) with HBF,. Reaction of (29) with alkali metal halides, MX, affords the diamagnetic complexes (lr(CO)(PPh,)],(pX)(C~-N,C,H,R)(CI-SO~)) (30), which contain halogen atoms (X) bridging the two Ir atoms as shown.M
{[Ir(C0)2(PPh,)]2(p-N,C,H, R)(p-SO,)}+ (29; R
II
NC,H,R (30)
McGlinchley and Stone65have reported the preparation and characterization ("F NMR and IR spectra) of [Ir(Cl)(CO)(NCF,CHFCF,)(PPh,),~, synthesized from [Ir(Cl)(CO)(PPh,),] and 2Hhexafluoropropyl azide in benzene. Reaction between [Ir(Cl)(N,)(PPh,),] and CF,CHFCF,N, has afforded the four-coordinate complex [Ir(Cl)(NCF,CHFCF,)(PPh,),] and molecular nitrogen.65 The crystal and molecular structures of [Ir(CO)(dtt)(PPh,),] (dtt = p-MeC4H,NN=NC6HpMep ) have been determined by counter data. In this square planar iridium(1) complex, the diaryltriazenido ligand (dtt) is unidentate and trans to the CO ligand.66 The five-coordinate iridium(1) selenocyanate complex trans-[Ir(CO)(NCSe)(Me2CO)(PPh,),] was prepared via metathesis with the corresponding chloro complex [Ir(Cl)(CO)(PPh,),]. The complex contains an N-bonded selenocyanate ligand.67trans-[lr(NCS)(CO)(PPh,),J has been prepared and contains an N-bonded thiocyanato ligand.67The iarge n-withdrawing character of the trans carbonyl ,ligand ita the thiocyanato and selenocyanato complexes probably favours N-bonding. The aminophosphine ligands (31a,b) ( c NPR,) are prepared from l-aza-4,1O-dithia-7-oxacyclododecane and chlorodiphenylphosphine or chlorodimethylphosphine. Complex (31) reacts with [Ir(C1)(CO)(cod)12to produce trans-[Ir(Cl)(CO)( c NPR,),].68
C.O.C. L J J
(31a) R = Ph (31b) R = Mc
1108
Iridium
49.4.2.4
1
:
.
Polypyrazolylborate ligan&
T r ~ f i m e n k ohas ~ ~reviewed the preparation of polypyrazolylborate ligands and the syntheses of metal complexes containing such ligands. An update has also appeared.70 [Ir(C1)(CO),], reacts with bispyrazolylborate anions to yield [Ir(CO), (H,B(pz),}] and [Ir(CO), {H,B(3,5-MeZpz)}](32) (pz = 1-pyrazolyl; 3,5-Me2pz= 3,5-dimethylpyraz0ly1).~’Displacement of a CO ligand by the a-accepting PPh, ligand affords [Ir(CO){H,B(pz),}(PPh,)]. These complexes were characterized by H NMR, IR and mass spectroscopy.
’
(32) R = H , M e
Phosphorus and Arsenic Ligands
49.4.3
The chemistry of iridium(1) Vaska-type complexes [Ir(A)(CO)(PPh,),] (A = various anions) has been re~iewed,’,~’ and therefore is not exhaustively dealt with here. These complexes are prepared by refluxing an iridium halide in a CO-releasingsolvent(alcoho1 or DMF), or by treating the iridium halide with CO, followed by PPh,. The analogous complexes [Ir(A)(CO)(ER,),], in which the phosphine ligands are replaced by other group Vb donor ligands are also well known. X-Ray crystal structure, dipole moment and ‘virtual coupiing’ studies have shown that the majority of complexes of the type [Ir(A)(CO)(L),] (L = PR, or ER,) possess the trans c ~ n f i g u r a t i o nFor . ~ ~complexes with bulky monodentate phosphine ligands (e.g. PR, = PMeBui), the trans configuration is maintained though there is restricted rotation about the Ir-P bonds as a result of the steric requirements of these ligands.74 However, the cis configuration was found for cis-[Ir(Cl) (CO)(Ph, ASCH=CHASP~,)],~’ @r(CO){N(R)C(Me)CHC(Me)CO}(PPh,)] (R = p-naphthyl, p-tolyl)?6 [Ir{NHC(CF,)=C(CMe=CH,)C(CF,)=NH)(CO)(ER,)] , (ER, = PPh,, ASP^,),^^ [Ir(OCRCOCR’)(CO)(PPh,)] (R = CF3,Ph, Me; R’ = Me),76and [Ir(Cl),(PMePh,),(R)] (R = Me, Et. Bradley and coworkers79have examined the electronic absorption spectra at 298 K of a series of 6 %planar complexes with the general formulae [Ir(A)(CO)(PPh,),] (A = OH, F, OC02H, ON02, Cl, OReO,, OClO,, N3, NCO, Br, NCS, NO,, I, NCSe, CN), [Ir(Cl)(CO)(ER,),] (ER, = P(oMeC6H4),, PCy,, PEtPh,, PPh,Bu, AsPh,, P(p-MeOC,HJ,), and [Ir(CO)(NCBH,)(PCy,),] (Cy = cyclohexyl). The visible spectra reveal three bands; the order of the derivatives noted above corresponds to decreasing energies of the longest-wavelength visible absorption bands and hence to the observed spectrochemical series orders. The two low-lying bands were assigned to the ‘ A ,--* A , , B2(,B1),and ‘ A ,+ B, transitions, respectively. Similar absorption spectra were obtained from the planar complexes [Ir(dppe),] and ~r(dppen),]+. The iridium(1) Vaska-type complexes trans-[Ir(X)(CO)(PPh,),] readily undergo oxidative addition with a wide variety of reagents to yield iridium(I1I) products.8’ Kinetic data for oxidative addition to trans-[Ir(Cl)(CO)(PPh, 12] are usually interpreted in terms of two limiting mechanisms: (i) a concerted process involving a relatively non-polar transition state, resulting in cis addition; and (ii) an SN2attack by iridium at an electrophilic centre in the substrate molecule involving a polar transition state and usually yielding a tmns With gaseous HX’ (X’ = C1, Br, F, HS, efc.), solid [Ir(X)(CO)(PPh,),] yields [Ir(H)(X)(X’)(CO)(PPh,),]the ; addition is presumably stereospecifically cis (H trans to X), based on observations that Ir-H stretching frequencies are sensitive to X and on the absence of H/CO vibrational coupling.” Cis addition of H2’, and 0,84 to trans[Ir(Cl)(CO)(PPh,),] has also been observed. The resulting stereochemistry of the iridium(II1) complexes formed as a result of oxidative addition of X,, HX, RX and acetyl halides to fruns[Ir(Cl)(CO)(PMe,Ph),] has been elucidated by NMR and far-IR ~pectroscopy.~’ Oxidative addition of acyl chlorides, allyl chlorides, methyl bromide and hydrogen halides to trans[Ir(X)(CO)(PEt,Ph),] (X = C1, Br, I) has been reported.86 Complexes of the type rrans[Ir(Cl)(CO)(PR,),] [PR, = PMe,(o-MeOC,H,), PMe,(p-MeOCJI,)] undergo oxidative addition with MeI, MeCOCl, PhCOC1, or CH,=CHCH2Cl (RX) to yield octahedral derivatives of the type depicted in (33). The reaction rates are faster than that where PR, = PMe,Ph; this has been I
,
+
Ir idiurn
'
.
1109
interpreted in terms of a direct electronic interaction between the methoxy oxygen of PR, and the Ir atom.87A related paperpgdescribes mechanistic studies on the oxidative addition of opticallyactive a-bromo esters {(RS,SR)-PhCHFCHBrC0,Et) to tralzs-[Ir(Cl)(CO)(PMe,),].NMR spectral data ('H, I3C, "F) were interpreted in terms of the reactions involving loss of stereochemistry at carbon and proceeding via a free radical mechanism, as shown by retarded rates being observed in the presence of electron scavengers. Proton NMR spectroscopy has been employed to monitor the stereochemical changes at carbon during the oxidative addition of alkyl halides to truns[Ir(Cl)(CO)(PR,),]. Complete loss of stereochemistry at carbon is observed. Two mechanistic pathways were postulated, one having characteristics consistent with a radical chain pathway as exhibited by several saturated alkyl halides, aryl and vinyl halides, and a-halo esters. By contrast, methyl, benzyl and allyl halides, and a-halo ethers react in a different manner, as shown by their insensitivity toward inhibitor^.'^ X
Ij
'
(33)
Acyl chlorides, RCOCI, react with [Ir(Cl)(PMePh,),], or with a solution containing [Ir(Cl)(cod)], and PMePh,, to yield the six-coordinate alkyl iridium(II1) complexes with mutually cis PMePh, groups, cis-[Ir(CI)(CO)(PMePh,),(R)] (R = Me, Et, Prn). In solution, complexes for which R = Et and Pr" rapidly equilibrate with the five-coordinate acyl iridium(II1) complex Fr(Cl), (COR)(PMePh,),], which also has mutually cis PMePh? ligands." tr~ns-[Ir(Cl)(C0)(PR~)~] and [Tr(Cl)(CO)(PR,),] (PR, = PMe,Ph, PEt,Ph) react with each other, presumably via a double-chlorobridged intermediate, and undergo rapid oxidative addition-reductive elimination. A mixture of truns-[Ir(Cl)(CO)(PMe, Ph),] and [Ir(Cl),(CO)(PEt,Ph),] was converted almost completely to truns[Ir(Cl)(CO)(PEt, Ph),] and [Ir(Cl),(CO)(PMe,Ph),] after 15min at 295 K in C6H6/C6D,.90The iridium(1) complex lJr(CO)(C,Ph)(PPh,),] (34) has been prepared from the reaction of [Ir(CO), (PPh3),]+ and phenylacetylene in acetone, presumably via the intermediate species '[Ir(CO),(HC,Ph)(PPh,),]+'. Complex (34) adds SO,, HgCl,, H,, O,, CF,C02H, MeCO,H, C,(COzMe),, Me1 and C1(CN)4via attack at the metal centre; in contrast, addition of HX and X, (X = C1, Br, I) proceeds by attack at both the metal centre and the alkyne linkage. Thus, addition of Br, yields [Ir(H)(Br)(CH=CBrPh)(CO)(PPh,),] and [I~(H)(B~)(CB~=CHP~I)(CO)(PP~,),~.~' Titration calorimetry has been employed to determine enthalpies of oxidative addition of RI (=Iz, HI, MeI, EtI, Pr"I, Pr'T, PhCH,I, PhC(O)T, MeC(O)I, (0-02C6C14),SO, and C,(CN),) to (X = C1, ER, = PMe3;92s93 ER, = PPh3, X = F, C1, complexes of the type tr~ns-[rr(X)(CO)(ER,)~] Br, I, NCS, N3; X = C1, ER, = P(OPh)3,PMe,Ph, PMePh,, PEt,Ph, PPh,Bu', PCy,, ASP^,).^ The enthalpy changes along with other thermochemical data were used to give a relative scale of Ir -R bond energies. The enthalpy of adduct formation between [Ir(Cl)(CO)(PPh,),] and SO, was also determined by GLC techniques, permitting tentative characterization of the donor in terms of the E and C m0dei.9~ The irreversible electrochemical oxidation of [Ir(X)(CO)(PR,),] (X = C1, Br, I; PR3 = PPh,, PPh,Et, PPhEt,, PEt3) on rotating Pt electrodes in Bu4NC104/CH,C1, reportedly proceeds at diffusion-controlled rates. In the redox addition process, atom transferability predominates over complex redox properties. Evidence indicates that the insoluble product of the one-electron oxidation of [Ir(X)(CO)(PR,),J is a dimeric iridium(I1) complex with an iridium-iridium bond.96 The reaction of [Ir(Cl)(CO)(PPh,),] with the hydroperoxide Bu'02H has yielded [Ir(Cl)(CO)(PP$),(Bu'O,H),] and a blue Ir/PPh,/Cl substance, along with CO, and OPPh,. It was proposed that the reaction might occur via coordination at a vacant site, with the possibility of oxygen transfer to oxidizable ligands!7 The displacement of MeCN in [Ir(CO)(PPh,),(MeCN)]+ by the coordinating anions A- (= CN-, C r C P h - , SH-, O,CCF;, 0,CH-, NCS-, NCO-, OH-, F- and ptoluenesulfinate) yields [Ir(A)(CO)(PPh,),], except where A- = CN- , and the five-coordinate [Ir(CN)(CO)(PPh,),] was ~btained.~' Oxidative addition of HCl to [Ir(Cl)(cod)(PEtPh,)] gives [Ir(H)(CI),(cod)(PEtPh,)] via the intermediate species [Ir(Cl),(cod)(PEtPh,)]- . Generally, the oxidation of the substrate precedes the nucleophilic attack of the anionic species; in this case, however, it was suggested that the oxidizing step is preceded by the nucleophilic attack affording the intermediate species.99[Ir(Cl)(cod)(PCy,)] has been prepared via addition of PCy, to [Tr(H)(Cl),(cod)],. In contrast, the
1110
Iridium
addition of PCy, to [1r(Cl)(cot),l2 (cot = cyclooctene) at 293 K in toluene affords [Ir(H),(C1){(P(C,H,)Cy,}(PCy,)], in which one cyclohexyl group has been dehydrogenated to cyclohexane, the double bond being coordinated to Ir. Under refluxing conditions, the four-coordinate complex [Ir(C1){P(c,H9)Cy2}(PCy,)]was formed. The syntheses of [Ir(Cl)(CO)(PCy,),] and [Ir(H),(Cl)(PCy,),] have also been reported.lWThe basic and bulky PCy, ligand is a potentially good ancillary ligand for a coordinatively unsaturated complex likely to react with small gas molecules. Complexes of the type [Ir(cod){P(p-RC,H,),},]A(R = C1, F, H, Me, OMe; A = ClO;, BPh;) have been prepared from [Ir(Cl)(cod)], and stoichiometric amounts of the appropriate phosphine, and undergo oxidative and displacement reactions. Thus, [Ir(COd)(P(p-RC6H4),}z] undergoes oxidative addition with Cl,, I, and MeT, and displacement of cod when reacted with CO or HZ.Io1 Transition metal complexes containing bulky tertiary phosphine ligands are expected to exhibit unusual physical and chemical properties. Complexes of the type tuans-[Tr(X)(CO)(PBu;R),] (35; X = C1, Br; R = Me, Et, Prn)have been prepared by passing CO through a solution of [IrC1,]2- (or [IrBr,]'- ) followed by phosphine addition.lo2A modifiFd method has been employed to prepare the 0-metailated iridium(1) complex [ir(CO){PBu~(C,H,O))(PBuk(C,H,O)Me-2)}] (36).'03Complexes of the type (35) exist as rotational conformers which interconvert at room temperature.', For complex (36), two rotational conformers are possible (36a,b) and thought to interconvert at room ternperat~re."~ Complex (36)can be demethylated to yield [I~(CO){PBu~(C6H,0)}(PBu~(C6H40H2)]], which on ,treatment with air yields CO, and the paramagnetic iridium(I1) complex frans[Ir(PBui(C, H,O)},] (37). Complex (37) has been characterized by X-ray structural analysis and magnetic measurements. Presumably, the formation of (37)proceeds by initial 0, uptake to form an 'end-on' species. Hydrogenation of complex (37) yields the iridium(II1) complex [Tr(H){PBu~(C,H,O)),] (M), which reacts with CO to yield complex (39), [Ir(H)(PBu~(C,H,O)},]. Complex (37)in benzene, when allowed to stand for 24 h, affords the C-metallated iridium(II1) complex [I;{PBu~(C,H,O)){PBu' (C6H,0)(CMe,i=H2)}] (4o).lo3 +
q-f I
O-Ir-
I
yrBUt 'But
But
co
I
0-h-
1
I
co
The bulky phosphine ligand PBu:(C-CPh) reacts with [IrCl,]'- in ethanol to give frans[Ir(Cl)(CO)(PBu~(C~CPh)},]. This iridium(1) complex further reacts with HC1 to yield [Ir(H)(Cl),(CO)(PBu:(C=CPh)},], with sodium propan-2-olate in propan-2-01 to yield and with sodium methoxide to give [Ir(CO)(PBu; [CH=C(O)Ph]){PBu:(C=CPh)}], [T&CO){PB$ [CH=C(O)Ph]} { PBu; [CH=C(OMe)Ph]}] .Io4 (0-Vinylpheny1)diphenylphosphine [o-CH,=CHC6H,PPh2, spp] and its arsenic analogue [o-CH,
I
Iridium
'
'
1111
=CHC,H,AsPh,, spas] (41) form five-coordinate complexes of iridium(1) with the general formula [Ir(Cl)(L)J (L = spp, spas). The cationic derivatives [Ir(L),A]+ (A = CO, PF,, PMePh,, py) have been obtained from Pr(Cl)(L),] and AgBF4, NH,PF, or NaBPh, in the presence of ligand A. All of these complexes are reportedly trigonal bipyramidal with the spp (or spas) ligands in axial sites. The vinyl groups are in equatorial sites and probably tie in the equatorial plane. Proton and 31P NMR data reveal the existence of two isomers (i) a 'symmetric' isomer having equivalent vinyl groups; and (ii) an 'unsymmetric' isomer having inequivalent vinyl groups. The isomerism reportedly arises from the different possible orientations of the vinyl groups with respect to each other.Io5
(41) spp E = P: spas. E = A s
The metal-containing phosphine [PPh,Fe(CO),(q-C,H,R)] (R = H, Me) reacts with [Ir(C8Hl,)(solvent),]+ to yield a variety of cationic mixed metal cluster species, including [Ir{Fe[Ir(FePPh,(CO),(Cp)},(solvent),I +, [Ir(CO),CFePPh,(CO),(Cp)),I+ PPh,(CO),(GP)),l+ [Ir(CO)(FePPh,(CO),(Cp),C1}], [Ir(X)(CO){FePPh,(CO),(Cp)},]+ (X = CN, Br, I), [Ir(Cl)(CO){FePPh,(CO),(q-C5H4Me)}2] and [Ir(I)(FePPh,(CO),(Cp)} ,I. The behaviour of the mixed metal cluster complexes toward CO and H, have also been reported.Io6 Several iridium(1) complexes containing the secondary phosphine ligands HPRz have been prepared. These include [Ir(HPR2)4]+,where HPR, is HPEt,, HPEtPh, HPPh?, HPPh(C,H,,) or and HPR,. IO7 Reactions of [Ir(HPR,),]+ with HP(C6Hl,)2, and were prepared from [Ir(Cl)(C8H14)]2 CO, H,and HC1 are summarized in reactions (17)-(19). IR spectral data of the dihydrido iridium(II1) complex (reaction 18) were interpreted in terms of a cis arrangement of the H ligands; this has been corroborated by ' H NMR. In contrast, IR and NMR studies of [Ir(H)(Cl)(HPR,),]+ (reaction 19) suggest a trans struct~re.''~trans-[Ir(Cl)(CO)(HPBu!J,] is formed from hydrated iridium(II1) chloride and PBui in DMF. This would appear to be the first example of a P-C bond cleavage of a tertiary phosphine resulting in the formation of a secondary phosphine complex.'" 9
2
[Ir(CO)(HFR2)4]f
-
[Ir(HPR,)41+
R, = Ph,,PhC,H,,
+
[Ir(I4%)41+
[Ir(HPR,),]+
+
H,
HCl
--
+
CO
+
[Ir(CO),(HPR,)J+ R2 =
MH)2(HPR2)41*
(17)
( C h W , ~ *
(18)
[Ir(H)(Cl)(HPR,),]
+
(19)
The cationic complexes [Ir(CO),(SbPh,),]+ and pr(CO),(PR,),]+ (R = Ph, C6Hll)have been synthesized from iridium carbonyl halide derivatives with halogen acceptors in the presence of C0.'O9 Several iridium(1) diphosphine complexes have been synthesized with the ligands Ph,PCH,CH,PPh, (dppe), Ph,PCH,PPh, (dppm), Ph,PC6H4PPh2(dpbe), Ph,PCH,CH,AsPh, (arphos), Ph,PCH=CHPPh, (dppen), Ph,AsCH,CH,AsPh, (dpae) and Ph,P(CH,),PPh2 ( n = 3, 4). Room temperature electronic absorption and MCD spectra of [lr(dppe),]+ and [Ir(dppen),]+ reveal intense UV and visible absorptions that have been attributed to MLCT transitions."' Excitation polarization of these two luminescent complexes in propan-1-01 at 108 K permitted detailed spectral assignments based on the spin-orbit coupling model for d 8 metal ions complexed and [Irwith x-acceptor ligands."' The absorption and excitation band energies of [Ir(P-€'),I+ (cod)(P-P)]+ (P-P = dppe, dppen, dpbe, arphos) are relatively insensitive to all ligands except cod. The red shift observed in complexes containing the cod ligand was attributed to destabilization of all the d orbitals due to closer approach of the diphosphine ligand to the Ir centre. The emission spectra and lifetimes of {Ir(P-P),]+ and [Ir(cod)(P-P)]+ have been interpreted in terms of a two-level spin-orbit split-triplet manifold.' The crystal and molecular structures of the five-coordinate complex [Ir(dppe), (CNMe)](ClO,) have revealed a somewhat distorted trigonal bipyramidal geometry about Ir, with each dppe ligand bridging an axial and an equatorial coordination site.Il3 The key structural parameters in this complex are the Ir-isocyanide ligand bond length and the angle in the trigonal plane trans to the unique ligand. A discussion of the effects of x-acceptor orbitals on substituents in the equatorial position (CNMe) was consistent with the finding that this iridium(1) complex exhibits the largest
1112
Iridium
P-Ir-P bond angle (120.36') when compared to complexes containing NO' and CO as the unique ligand. The fluxionality of this complex appears to involve a stereochemically non-rigid TBP structure. However, the geometry of [Ir(dppe), (CNMe)]' is similar to that found for several other dppe complexes of iridium, including [Ir(CO)(dppe),]+, [Ir(dppe),(O,)]+ and [Ir(dppe),(S,)] + . ' ' c 1 1 6 [Ir(dppe), (01)]+ reversibly binds CO in solution.83 The crystal structure of the CO adduct [Ir(CO)(dppe),]+ (42) reveals a distorted trigonal bipyramid with CO occupying an equatorial p~sition."~ HI, PP q P h ,
(42)
The four- and five-coordinate iridium(1) cations [Ir(dppe)*]+ and [Ir(CO)(dppe),]+ react with B,,H,,, BloHI3(C1-6)and (BloH13)-to yield a variety of ionic compounds, some of which are given in reactions (20) and (2l)."' Reaction of t-butyl isocyanide with the A-frame complex [Ir2[Ir2(Cl)(CO),(P-P),](X) (P-P = dppm, dpam) affords [ir,(CO),(P-P),(Bu'NC)](X), (Cl)(CO),(P-P),(Bu'NC),](X) (P-P = dppm, X = BPh,; P-P = dpam, X = BF,) and [Irz(CO)(P-P)2(ButNC)4](BPh,),, as characterized by IR and I3Cand 31 P NMR spectroscopies.'I8 Other neutral and cationic binuclear carbonyl complexes of iridium(I) containing dppm have been r e p ~ r t e d . "The ~ dppm ligand is excellent for stabilizing homo-bimetallic complexes with metametal bonds, A-frames, and eight-membered rings. The treatment of tralzs-[Ir(C1)(CO)(PPh3),]with [Pt(C~~C-p-tolyl),(~f-dppm),] in refluxing benzene has yielded [Pr(C=C-p-tolyl),(p-dppm), Ir(Cl)(CO)] (43) in > 80% yields. Also, [Ir(Cl)z(C8H14)4]reacts with ~t(C-CPh),(rf-dppm),] in benzene at 293 K to give [(PhC=C),Pt(p-dppm),Ir(Cl)] (44).120 Dialkynediphosphines of the type L = Bu',PCrC(CH,),CrCPBui (n = 4, 5 ) are known to react with Na,[IrCl,] and CO in hot ethanol to yield [Ir(H)(CI),(CO)(L)] (n = 5) which, when treated with NEt,, gives trans[ir(Cl)(CO)(L)].'21Alkynic groups promote the formation of large rings as a result of favourable conformational and entropy effects, The crystal and molecular structures of [Ir(Cl)(CO) (BU~PC=C(CH,)~C=CPBU\}]reveal a distorted planar coordination geometry about Ir with P atoms mutually trans.122 [Tr(d~~e)zI+ + B,,Hl?X
[Ir(CO)(dp~e)zl++ (NEt, H)[BloH131
(43)
-
x = w. 6-cI ' [It"(H)(Cl)(dppe),I[B,,H,,XI
[If ( C O ) ( ~ P P ~ )HI31 ~~[B + ~[Ir(Hh ~ (dppe)Zl[B, H
(20) (2 1)
(44)
The double bond present in Ph, PCH=CHPPh, (dppen) sometimes imparts different character to [Ir(dppen),]X and [Ir(dppen),YlX complexes (X = C1, Br, I, ClO,, BPh,; Y = CO, H,, 02,HC1, HI) than that present in the corresponding PhzPCHzCH2PPh2(dppe) complexes, The different behaviour was attributedIZ3to the presence of the double bond. The tendency of the dppen ligand to favour electron delocalization leads to a lowering of its basicity and to a n-bonding ability greater
I
Iridium
.
1113
than that of dppe. Thus, the dppen ligand makes less demand for dn electrons of Ir, and the back-donation involves mainly the carbonyl group in [Ir(CO)(dppen)2]+. (n > 9) bridge rhodium(1) Studies by Shaw et ai.'" have shown that the ligands Bu~P(CH,),PBu~ and palladium(I1) atoms, but chelate iridium(]) and platinum(I1) atoms in a truns manner. The diphosphine ligands Ph,P(CH,),PPh, ( n = 1 4 ) form trans carbonyl complexes of iridium(1) with the stoichiometry [{Ir(CI)(CO)(diphosphine)),J, in which the diphosphine (m = I, 3,4) bridges the metal atoms. Thus, the complex [Ir(CO){Ph,P(CH,)2PPh,}2][Ir(Cl),(CO)] forms with the structure (45).1'9b The intramolecular barriers to rearrangement of the trigonal bipyramidal complexes [Ir(CO){Ph,P(CH,),PPh, I,}+ decrease with decreasing ring size. This phenomenon has been suggested as a criterion for quantitative assessment of metal-to-ligand bonding.'"
(45)
The isolation of a series of iridium(1) complexes with the optically-active diop ligand (46) [diop = 4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl1,3-dioxolane] has been reported. Equivalent portions of diop and [Ir(Cl)(cod)], in ethanol yield [Ir(Cl)(diop)(cod)j*EtOH.126The X-ray crystal structure of this complex shows a distorted trigonal bipyramid in which diop serves as an apicakquatorial bidentate ligand. The structure is maintained in solution, as evidenced by 31 P NMR data in CDC13 at 223 K.I2' In acetone solution, reaction between [Ir(Cl)(cod)], and two or more equivalents of diop affords [Ir(diop),](BPh,), which reacts with CO to yield [Ir, (CO), (diop),I(BPh,), .lZ6
As well as serving as bidentate ligands, dppm, Bui PC-C(CH,),CrCPBu~ and Ph2P(CH2),PPh2 are also capable of bridging two iridium atoms. These ligands are incorporated in [Ir,(p-Cl)(pco)(co),(P-dPPm),l+ ~ ~ ~ , ~ ~ - ~ ~ ~ ~tIr~(Cl) - (CO ~){P~h,PI )CH2~),P~Ph,}~l2 ~ and ~ ~ 3
[I~(B~)(CO)(B~~PC~C(CH,),C~CPBU~}],.'~'~'~~
The sexidentate ligand P,P,P',P-tetrakis-(2-diphenylphosphinoethyl)-~,~'-diphospha-p-xylene (TDDX) forms, upon reaction with [Ir(Cl)(CO)(PPh,),], the binuclear diamagnetic non-ionic complex [Ir,(Cl),(TDDX)] (47). The TDDX complex is formed via complete displacement of CO and PPh3 from Vaska's The 'HNMR spectrum of complex (47) resembles that of [Ir(tripho~)(CO)](C1),'~~ in which the triphos coordinates three coordination sites in the square plane of the Ir ion. Thus, complex (47) is likely to be square planar with TDDX as a sexidentate donor binding three coordination positions on each Ir ion of the binuclear complex, as shown.IZ9The similar P,P,P,P-tetrakis-(2-diphenylarsinoethyl)-ol,a'-diphospha-p-xylene ligand (TDADX) (48) reacts with Vaska's complex in benzene to give [Ir(TDADX)](Cl). In this complex, the potentially sexidentate TDADX ligand appears to bind in a tetradentate manner through the As atoms. Reactions of [Ir(TDADX)](Cl) with H,, CO and NO are summarized in reactions (22>(24), respectively.l 3 ' The internally metallated iridium(1) complex (51) results from treatment of [Ir(CI)(PPh,),] with 1-Li-2R-1,2-Bl,CzH,,(R = Me, Ph) in ether. The reaction presumably proceeds by initial formation of a carborane iridium(1) complex (49),which is transformed to (51) via the intermediate species (50)
~
-
~
P
P
~
~
2
1114
Iridium
Ph,As
,AsPh2
(48)
+ H2 [Ir(TDADX)]Cl + CO [Ir(TDADX)]Cl + NO [Ir(TDADX)Cl
CHC13
* [Ir(H),(TDADX)]CI
CHC,3
* [Ir(CO)(TDADX)]Cl
(23)
[Ir(NO)(TDADXj]2'
(24)
-----,
by rapid intramolecular oxidative addition and reductive elimination (see Scheme 5).132*133 A similar reaction sequence was postulated for [Ir(PPh3)3(Me)].'34Complex (51) reacts with CO, H, and MeOH in solution according to reactions (25)-(27), respectively, depending on reaction conditions. Proton NMR and IR spectroscopy have been employed to derive the product stereochemi~tries.'~~
(49)
Scheme 5 (51)
+
CO
+
(51)
+ co
(51)
+
2c0
(51)
(51)
+
+
MeOH
[Ir(CO){P(C,H,)Ph,}(PPh,)] + PPh,
-
[I~(CO){P(C,H,)P~,)(PP~,),I
-
+
H,
[Ir(CO),(P(C6H4)Ph2j(PPh,)] fac-[Ir(H),(pph,),I
[Tr(H),{P(C~H,)Ph*~(PPhJ)21
(254
i25b) (254
(26) (27)
o-Ph2PC6H4CHkHCH(Me)C6H,PPh,-o(1 -bdpb) reacts with [Ir(Cl)(cod)], to yield [Ir(Cl)(1-bdpb)] (552), which exists as a mixture of isomers having differing conformations of the chelate ligand. Possible conformational interconversions have been reported.'36 Complex (52) undergoes irreversible oxidative addition at Ir with C1, and HC1, and reversible oxidative addition with H2.'36 The similar ligands o-Ph,PC,H,(CH,),C,H,PPh,-o (bdpbz) and o-Ph,AsC,H,(CH,),C,H, PPh,-o (bdabz) are found in [Ir(Cl)(bdpbz)(CO)] and [Ir(Cl)(bdabz)(CO)], respectively. The group Vb donor ligands span trans coordination sites. The bdpbz complex isomerizes in solution at room temperature by transferring a benzylic hydrogen atom to Ir and forming a chelate-Ir-carbon 0-bonded complex; the bdabz complex behaves in a similar fashion under more rigorous condition~.~~' The crystal structure of the square planar complex [Ir(Cl)(Cw)(Ph,PC,H,NMe,)] (53)has been determined. IR and "P NMR spectra for complexes (53), (54) and (55) have suggested that all three complexes possess the same stere~chernistry.'~~ It was also observed that these complexes are more effective than Vaska's complex for the isomerization of 1-hex-1-ene under hydrogenation conditions.' 3ri
1115
Iridium Me
Mb 'Me
(53)
Me
Me
(54)
49.4.4
49.4.4.1
Oxygen Ligands
FDikedonate, catecholate and smiquinone ligands
The syntheses and properties of [Ir(acac)($-C,Me,)] and [Ir(hfac)($-C,Me,)] (56) (acac = acetylacetonate, hfac = hexafluoroacetylacetonate) have been re~0rted.l~' These iridium(1) complexes are afforded upon reaction of [Ir(C1),(qs-C5Mej)], and the sodium salt of the appropriate acetylacetonate. Under more rigorous conditions, the bis-acac complex [Ir(acac),($-C,Me,)] (57) results, in which the two acac ligands are 00'- and C-bonded to iridium.
(56)
(57)
Complexes of the type in (58) have been prepared and their properties investigated by IR and H NMR spectroscopy.'40Proton NMR spectra reveal a regular variation of the chemical shifts of the alkene protons of the cod ligand with the electronic properties of the jl-diketonates. Analogous results are obtained from Ir-cod IR frequencies. Treatment of [Ir(C1)(C,H,>,]x (C,Hs = norbornadiene) with acetylacetone yields a species [Ir(acac)(C,H,),] (59) having two of the norbornadiene units half-linked together with ex@-trans-exo stereochemistry in a five-membered metallacycle. The crystal structure of complex (59) shows the acac ligand to be essentially planar.14'
R = R' = CF3
R R R R R R R
C.O.C. G J J '
= R' = Me = C F 3 . R ' = Me = R' = CMe3 = Me, R' = Ph = R' = Ph = Me, R' = Ph
CF3, R'
C&,S
1116
Iridium
IR,Rarnan and I3C NMR spectroscopic studies have been performed on various [Ir(acac)(L),] complexes (L = ethylene, propene, vinyl chloride, vinyl acetate, methyl acrylate, styrene) for the Also, the square planar iridium(1) elucidation of the bonding between Ir and the alkene 1iga11d.I~~ acetylacetonate complexes [Ir(LL)(L'),], where LL is a fl-diketonate and L' is CO or ethylene, have been studied by UVPES.143 The enthalpies of reaction of the crystalline [Ir(acac)(L),] complexes with gaseous CO (reaction 28) have been determined by differential scanning calorimetry. The enthalpies for the gaseous reaction have been derived from these results and Ir-L bond strengths estimated. [Ir(acac)(L)2l(s) + 2CO(g)
-
[Ir(a~ac)(CO)~l(s) + 2L(g)
(28)
IR studies on the photochemistry of the complexes [Ir(L)(CO),] (L = acac, hfac, tfac; tfac = trifluoroacetylacetonate) in inert (CH4.Ar) and reactive (CO, N2) gas matrices at 12 K have shown that [Ir(L)(CO)], [Ir(CO)(N,)] and [Ir(CO),(L')] (L' = unidentate form of L) are produced.14 The syntheses and characterization of nitrosyl catecholato complexes of the type in (60) have been r e ~ 0 r t e d . IThe ~ ~ addition reaction of PPh, to various [Ir(NO)(cat)(PPh,)] (catecholato) complexes to give [Ir(NO)(cat)(PPh,),] has been examined in order to determine if a reversible association of a neutral molecule to a potentially catalytic d 8 four-coordinate complex can be considered as a model of one of the steps involved in the overall catalytic process. The [Ir(N0)(1,2-0,C6Br,)(PPh3)] complex has been prepared and characterized. The Ir-0 bond length trans to the NO ligand is unusually short; this probably results from the bonding effect of having a strong n-acceptor frons to a strong 7r-don0r.I~~ Studies of the catalytic activity of [Ir(NO)(1,2-O,C.&)(PPh,)] and [Ir(NO)( 1,2-O2C6H4)(PPh,)]in the homogeneous hydrogenation of cyclic alkenes, dienes and trienes inhcate that the activity is related to the electron density on the central iridium atorn.l4* X = R = CI. B r . H X = H; R = NOz? CHO.
ol'" N
X
COzMe, CHZOH.
49.4.4.2 Bicarbonate and carboxyiate ligands The hydroxy complex trans-[Ir(OH)(CO)(PPh,),] reacts with C02 in ethanol to produce the monodentate bicarbonato complex [Ir(OCO, H)(CO)(PPh,),]. The rate of reaction is first order in complex and independent of C 0 2 and H 2 0 c o n c e n t r a t i ~ n s . l ~The ~*~ bicarbonato ~ complex reacts 149 with O,, affording [Ir(OC0,H)(CO)(0,)(PPh3)2]. [Ir(CO)(PPh,),(MeCN)] reacts with the peroxycarboxylic acids R C 0 , H in the presence of NEt, to yield [Ir(O,CR)(CO)(PPh,),] (61) (R = m-C1C6H4,Ph, Me). The alkyl peroxy complex (61) (R = m-ClC6H4)reacts with Me1 according to reaction (29). Reaction of Na0,COR (R' = mC1C6H4)with [Ir(CO)(PPh,),(MeCN)] + gives an inseparable mixture of [Ir(0,CR)(CO)(PPh3),], [Ir(O, CR)(CO)(O,)(PPh,),], [Ir(O,CR)(CO)(PPh,),] and [Ir(O,CR')(CO)(O,)(PPh,),]. However, reaction of Na0,Bu' with [Ir(CO)(PPh,),(MeCN)] or [Ir(F)(CO)(PPh,),] gave only [Ir(OH)(CO)(PPh,),].'" [Ir(X)(L)(L')] (L = CO, N,; L' = phosphine or arsine) complexes are known to react with RC0,H to yield [Ir(X)(O,CR),(L'),] (62) (X = Cl, R = Pn-C1C,H4, L' = PPh,, PPh,Me, AsPh,; X = C1, R = Ph, L' = PPh,, AsPh,; X = C1, R = Me, L' = PPh3;X = Br, R = m-ClC,H4, L = PPh,) and COz. The complex (62) possesses unidentate and bidentate carboxylate ligands, as illustrated. IS2 +
+
(61)
+
Me1
+
[~r(I)(OCOC,H,Cl-m)(CO)(PPh,)2(Me)l
(61)
(62)
(29)
Perfluorocarboxylic acids RFCOzH (RF= CF3, C,F,) are reported'53 to react with [Ir(NO)(PPh,),] in boiling acetone to give [Ir(NO)(OCORF)(PPh,),I(OCOR,). The formation of this salt presumably involves successive protonations at the metal centre followed by elimination of
I
Iridium
1117
dihydrogen and coordination of the- perfluorocarboxylate anion. A similar mechanism has been postulated for the reaction of [Ir(NO)(PPh,),] with HCl.154 Metathetical chlorine exchange employing Na[ON(CF,),] and [Ir(CO)(PPh,),(MeCN)]+ has yielded the iridium(1) complex trans-[Ir{ON(CF,), )(CO)(PPh,),]+ (63).This complex undergoes a wide variety of oxidative addition reactions, as revealed in Scheme 6.'55Complex (63)also reacts with the CF, N(o)CF,CF,N(o)CF, diradical to yield the seven-membered iridium(II1) metallacycle ON(CF,)(CF,CF,N(CF,)O}{ON(CF,),)(CO)(PPh,),], as given in the last reaction in Scheme
Scheme 6
49.4.5
49.4.5.1
Sulfur Ligands S,, CS,,tiriocarbamate and related ligands
An investigation of the side-on-bonded disulfur and diselenium complexes [Ir(P-P),(Y,)]+ (P -P = dppe, drnpe; Y = S , Se) has been reported.I5'An X-ray structure of pr(dppe),(Se,)]+ revealed it to have a structure very much similar to that of its dioxygen and disulfur analogues. The Se, group is side-on bonded to Ir at the equatorial positions of a distorted octahedron. The S, or Se, groups readily undergo reactions in which they are reduced. Thus, mercury or tertiary phosphines strip S or Se from the complex to form HgS (HgSe) or R3PS (R,PSe). [Ir(dppe),(S,)]+ is oxidized by periodate to [Ir{dppe),(S,O)]' and [Ir(dppe),(SO),]+ .I5' Also, [Ir(dppe), (SJ' is methylated at the S2 group by MeS0,F to yield [Ir(dppe),(S,Me)J+ The iridium(1) disulfide complexes [Ir{ S(CH, CH2SPh), )(cod)]+ and [Ir(Cl)(cod)(MeSCH,CH, SMe)] have been prepared from [Ir(Cl)(cod)],, PPh, and the sulfide.Im When [Ir(Cl)(CO)(PPh,),] is treated with CS,, a dark violet solution results and the addition of NaBPh, leads to the formation of dark violet crystals, which analyze as [Ir(CO)(CS,)(PPh,),](BPh,). It was formulated as an octahedral species with a n-bonded CS2ligand, as in (64). However, when attempts were made to alkylate [Ir(CO)(CS,)(PPh,),]+ to form the dithioester complex, which should have reacted with acids in an alternative route to the thiocarbonyl complex, the reactions proved unsuccessful. The complex turned out to be [Ir(CO)(S, CPPh,)(PPh,),]+ (65).[Ir(CO)(S,CPPh,)(PPh,),](BPh,) (65)exhibits a distorted trigonal bipyramidal arrangement of donor atoms about the central Ir atom. The (SaCPFh3)ligand behaves as a non-planar bidentate zwitterionic species Ph,P+ CS; ,in which the S atoms occupy an axial and an equatorial position. The CO ligand occupies the other axial position, while the two PPh, groups occupy equatorial sites.
1118
Iridium -1+
Very few examples of o-bonded CS2complexes are known in which the CS2ligand is bonded to iridium solely through an S atom. [Ir(CI)(PPh,),] reacts with CS2 to yield [Ir(PPh,),(a-CS,)(xCS,)](BPh,), which slowly decomposes at room temperature or reduced pressures resulting in the isolation of [Ir(PPh,),(n-CS,)]+ .163 The iridium(1) complex [Ir(PPh,),(CS)((r-CS,)(n-CS,)]+was initially formed upon reaction of trans-[lr(Cl)(CS)(PPh,),j with CS,, but the 0-bonded CS2 ligand was slowly lost to leave the PCS, complex [Tr(CS)(PPh,),(n-CS,)]+ The iridium(1) species [Ir(CO)(q2-CSNMe,)(PPh,)(SzCNMe,)](X) (66; X = C1, PF,) is formed upon reaction of [Ir(Cl)(CO)(PPh,),] with (MeNCS)2.165 Vaska-type complexes presumably react with Me2NCSCl in a similar manner to yield trans-[Ir(C1)(CO)(qZ-CSNMe),(PPh,)2]+. NM~?+
dC\
49.4.5.2 SO2and phosphine sul&de ligands
Levison and Robinson'66 have established that SOz reacts with [Ir(H)(CO)(PPh,),] to give
[Tr(H)(CO)(PPh,),(SO,)].A tautomeric equilibrium (reaction 30) involving a rapid hydride migration between Ir and the coordinated SOz ligand was postulated. A more recent inve~tigationl~' of the reaction of SO2with [Ir(H)(CO)(PPh,),] indicates the product to be [Ir(H)(CO)(PPh,),(SO,)j on the basis of 'H and 3'P NMR evidence. At low temperatures, a six-coordinate hydrido(su1fur dioxide) intermediate, [Ir(H)(CO)(PPh,),(SO,)],was proposed. The favoured structure for [Tr(H)(CO)(PPb,),(SO,)] is (67), although spectroscopic data also appear consistent with structure (68). No evidence (from NMR studies) was obtained in support of equilibrium (30) or for any fluxional process involving the coordination sphere of the five-coordinate d 8complex.'67The crystal and molecular structures of [Ir(CI)(CO)(PPh,),(SO,)](69) reveal a tetragonal pyramidal coordination geometry around Ir, with CO, C1 and trans P atoms in the base and the S of SO, at the apex. The 0 of the CO ligand, C1 and the two P atoms are coplanar, while the Ir-S02 moiety is non-planar. The Ir-S vector makes an angle of 31.6" with the normal to the SOz plane. The Ir -S interaction (2.49 A) is weak.'68
(67)
(68)
(69)
The four-coordinate iridium(1) complex [Ir(O, S-p-tolyl)(CO)(PPh,),] (70) (0,s-p-tolyl = p toluenesulfinate) has been prepared from [Ir(CO)(PPh,),(MeCN)] and sodium p-toluenesulfinate.16' This complex contains an 0-bonded sulfinate ligand, as indicated by IR spectral evidence!8 On addition of methyl iodide to (70), an iridium(II1) complex with 0-bonded sulfinate is formed. +
1119
Iridium
However, uptake of CO or 0, by (70) affords isomerization to S-bonded sulfinate and complex (71; reaction 3 1).y8*’69 The secondary phosphine sulfide Ph,HPS reacts with [Ir(Cl)(CO)(PPh,),] under mild conditions, giving [Ir(H)(CO)(PPh,),(SPPh,)] (72). 31 P NMR data have suggested that the SPPh, ligand is S-bonded to Ir. Complex (72) reactors with Me1 or Et1 to yield [Ir(H)‘(Cl)(I)(CO)(PPh,),],most probably by initial S-alkylation of the coordinated Ph,PS 0 OC-lr-0-S-R
+
II
L
-
OC,4%, TPh3 0 II
Ldi-i-R
PPh,
(71)
(70)
.
II
C1-Ir-CO
PPh,
PPh,
I
17SPPh2 (31)
(72)
Reaction of [{Ir(Cl)(CO), I,,] or [Ir(Cl)(cod)lzwith (C,H,,),PSSNH, or (PhO),PSSNH, results in the formation of [Ir(CO),(S-S)] (73) and [Ir(cod)(S-S)] (74) (S-S = (C6H,,)2PSS,(PhO),PSS), respectively. Complex (74) reacts with PPh, and CO according to reactions (32) and (33), respectively. The dicarbonyl complexes produced in reaction (33) react with PPh, and PEt, ( = L) according to reactions (34) and (35), respectively. The P,P’-dithiophosphate ligand of complex (75) remains bidentate and iridium is four coordinate. A structure for complex (76)was not reported; however, IR spectral data indicate 0,O’-dithiophosphate ligand behaves as a unidentate ligand. The different behaviour of the dicarbonyl complexes toward L could be rationalized in terms of electronic effects for which the {SSP(OPh),) ligand is expected to be a stronger n-acceptor than {SSP(C6HIl),}and therefore a stronger Ir-S bond is envisaged. This contrasts with the observed cleavage of the Ir -S bond in reaction (34). Complex (73) reacts with l,Zbis(diphenylphosphine)ethane (dppe) to produce [Ir(CO)(dppe),](S-S), in which the (S-S)- is a counterion. Also, complex (73)undergoes and oxidative trans addition with X, (=Cl,, Br,, I,) and Me1 to yield [Ir(X),(CO),(S-S)] [Ir(CO),(S-S)(MeI)], re~pective1y.I~~
+ PPh, [Ir(cod)(S-S)] + CO
[Ir(cod)(S-S)]
[Ir(C0)*(SSP(C,H,1)*)1+ L [Ir(CO)*(SSP(OPh), )I f L
+
-
4
[Ir(PPh,),(S-S)] [Ir(CO),(S-S)]
[~r(CO)(~){SSP(C,~,,~,~I
(32)
-
(33) (34)
(75)
Ir(CO)2(L)z{ SSP(OPh)2>I
4
(35)
(76)
49.4.6
Halogens as Ligands
The iridium(1) halide complexes [IrCI], [IrBr] and [IrI] are believed to exist. Heat treatment (to 1043K) of [IrCl,] causes sublimation of [IrCl], which decomposes above 1071 K and is insoluble in both alkaline and acidic media.I7’ [IrBr] may be obtained by heating the iridium(II1) bromide to 758 K in an HBr atmosphere. The monobromide complex is stable at lower temperatures, and is slightly soluble in water, in acidic and in alkaline media.’73[IrI] forms upon heating either [IrI,] or [Ir13]to 628 K in an HI atmosphere.”’
49.4.7 ’
Hydrogen and Hydrides as Ligands
The electrochemical reduction of a-[Ir(H)(Cl),(ER,),] (ER, = PPh,, AsPh,) in MeCN/C,H, solution has atforded [Ir(H)(PPh,),] and [Ir(H)(AsPh,),], respectively. The iridium(1) hydride complexes are stable as solids, but rapidly decompose in solution. Cryoscopic studies have suggested that in solution these complexes dissociate according to reaction (36). IR spectra and elemental analyses support the formulation as [Ir(H)(ER,),].’74 Related iridium(1) hydride compiexes may be synthesized via replacement of either H,175or HX’76by a phosphine ligand (reactions 37 and 38), or by phosphine ligand exchange (reaction 39).17’ Alternatively, complexes of this type can be prepared according to reactions (40)(-(43).14*164,1f8.*79 [Jr(H)(ER314 1 ~r(H),(SiR,)(CO)(PPh,),]
e
+ PR,
II4H>(ER31 31
&:
+
+
(ER,
[Ir(H)(CO)(PPh,),j 4- R3SiW
(36) (37)
1120
Iridium
[Ir(H),(CI)(CO)(PPh3),1+ PPh3
+ KOH
[Ir(H)(CO)(PPh,),l
[Ir(H), Pph3 )31 -k P(OPh)3
[Ir(H){P(OPhh1 4 1
+
+ PF3 + H2 -----, [Ir(C1)(C8Hl2)l2 + PrMgBr + PPh3 [IrCl,]
+
Cu
tIr(C1)(CO)(PPh,)21
+ NaEH4
[Ir(Cl)(CS)(PPh,),] + NaBH4
+ KCl + H 2 0
-
[Ir(H)(PF,),] [Ir(H)(PPh3),1
[Ir(H)(CO)*(PPh3 )21
PPh, -+ [Ir(H)(CS)(PPh,),]
(38) (39)
(40) (41)
(42) (43)
[Ir(H)(CO),(PPh,),] (from reaction 42) adopts a single structure in the solid state, but appears to exist as an equilibrium mixture of five-coordinate isomers in solution. The isomeric equilibrium is both solvent and temperature dependent.'" [Ir(H)(P(OPh), ),I (from reaction 39) undergoes a reversible ligand-metal hydrogen-transfer with hydrogen ehmination and metal-carbon bond formation. In this reaction, an aryl C-€3 bond in a donor ligand coordinated to Ir reacts with the Ir atom to form a metal-carbon bond; the hydrogen is transferred from the carbon atom to the Ir atom, forming a hydride ligand. This may subsequently be eliminated as H2.I8' Deuteration studies on rhodium analogues have shown that this intramolecular reaction involves an ortho C-H bond of the aromatic phosphorus donor ligand.Is2
Multidentate Macrocyclic Ligands
49.4.8
Few iridium(1) porphyrin complexes are known, owing to the difficulty in incorporating the iridium and the high resistance to reduction of the iridium(II1) complex to an iridium(1) complex. [Ir(Cl)(CO),],
+
OEPH,
-
pOEP[Ir(CO),],
+
[Ir(Cl)(CO)(OEP]
(44)
(77)
[Ir(Cl)(CO),],
+
AT-MeOEPH
+
AT-MeOEP[Ir(Cl)(CO),],
(45)
(78)
Complexes (77) and (78) (see reactions 44 and 45) have been synthesized via reaction of octaethylporphyrin (OEPH,) or N-methyloctaethylporphyrin (N-MeOEPH) with [Ir(Cl)(CO),],. The IR and visible spectra of (77) are quite similar to those of pOEP[Rh'(CO),], indicative of similar structures for the two complexes. No evidence for centrosymmetry in (77) was obtained, probably due to the reduction in molecular symmetry by coordination of three CO ligands to each Ir atom.'g3 Complex (78) is more readily formed than (77), probably the result of the distortion of the porphyrin ring due to N-alkylation of the pyrrolic N-H bond. The spectroscopic properties of (78) appear similar to those of N-MeOEP[Rh(Cl)(CO)2]2,'84 and thus similar structures for the I f and Rh' porphyrin complexes have been proposed, in which the two Ir atoms of the iridium dimer are bonded to the two adjacent nitrogen atoms of the porphyrinato core.183
49.5
IRIDIUM(I1)
Only a few complexes containing indium(I1) are known, most of which are of a special type as they are either dimeric and diamagnetic or contain non-innocent ligands. More likely, many reported iridium(I1) complexes are hydrides of iridium(II1) or iridium(II1) complexes containing a Q Ir-C bond rather than a supposed n Ir-C bond. Furthermore, iridium(I1) complexes, with a d7 electronic configuration, are expected to be paramagnetic.
49.5.1 49.5.1.1
Nitrogen Ligands Nitrosyl ligands
The iridium(I1) complex [Ir(Br)3(NO)(PPh,),] has been reported by Malatesta and coworkers.' A
+ I1 oxidation state assignment depends on the formalism used regarding the nitrosyl ligand. The complex is considered iridium(I1) if the NO Iigand is assigned as (NO)+.
Iridium 49.5.2
1121
Phosphorus and Arsenic Ligands
49.5.2.1
Phosphine rigands I
'
'
The syntheses and structures of trUns-[Ir(BU:PC,H,O),] and trans-[Ir{BuiPC6H,(OMe)O},] (79) have been reported by Mason and Thomas."' The iridium(I1) state is presumably stabilized by the 0 , P bidentate ligands. X-Ray analysis of these complexes reveal C, symmetry and planarity.
49.5.2.2
Arsenic ligands
Reaction between [Ir(H),(AsPh,),(CNC,H,Me-p)]and RC0,H (R = Me, Ar) results in the formation of the iridium(I1) complexes [Ir(O,CR), (AsPh,)(CNC,H,Me-p)] (SO), where R = Me, p-C1C6H,, p-MeOC,H,, 2,4,6-Me3C,H, and RC02 = pentane-2,gdionate. The complexes are paramagnetic, and, based on EPR spectra, have the structure depicted in (80). A tetragonally distorted octahedral geometry was proposed, for which the distortion increases with the ability of the planar ligands to delocalize the electronic charge of the iridium. Thus, complex (SO) for which R = Me is least distorted. The EPR spectra indicate that the arsine and isocyanide ligands are mutually trans.'86
1
CNC H,Me
49.5.3
49.5.3.1
Sulfur Ligands Mononuclear Complexes
Salts of [Ir(dppe),(q2-S,Me,)]2+ (81) were initially'59 prepared in 1979 via the alkylation of [Ir(dppe),(S,)]+ . The geometry about the perthio ligand in complex (81) was thought to resemble that found crystallographically in [Os(CO), (PPh,),(q2-S2Me)](C10,);187 that is the q2-S2Meligand should be structurally and electronically related to other derivatives of the S, ligand such as in [Ir(dppe),(S, O)]' Hoots and Rauchfuss'a8 have recently reported on the stereochemistry and reactivity of [I~-(dppe),(Z,R)]~~ (82; Z = S, Se; R = H, Me). In solution, [Ir(dppe),(Z,Me)]2+ exists as a mixture of two diastereomers in a 20:l (Z = S ) and 6:l (Z = Se) ratio, depending on the disposition of the methyl group relative to the dppe chelate rings. Figure 1 depicts the perspective views of the four stereoisomers of complex (82). The (A, S ) , (A, R) pair of enantiomers was thought to predominate, based on steric considerations. Complex (81) reacts with the nucleophiles PPhMe,, MeCN and CN- according to the reactions (46H48), respectively.'"
49.5.3.2
Dinucieav complexes
The dimeric iridium(II1) hydride [~Ir2(H)~(C0)4(PPh3)2}(S02)] (83) resdts from the reaction of [Ir2(CO),(PPh3),(p-SOz)j with H2."9' The SO, molecule may be considered formally as a diradical, sharing two electrons with Ir to form two covalent bonds; alternatively, SO, can be thought of as a two-electron acceptor from Ir to form the SO:- anion which bridges as a bidentate ligand. Either bridge systems include [Ir,(I),way, S is sp3 hybridized. Other complexes containing Ir-S-Ir (CO)z(SCMe,),(L)J (84),'90 [Ir(Co)(PR,)(~-.I-sBut)1~ (851, [Ir(H)(CO)(PR,)(~-SBu')l, (86),19' [Ir, (Br), (CO),(PPh, ),(TTN)] (TTN = tetrathionaphthalene) (W), lQ2 and [Ir,(CO),(SR), ] (SR = SPh, SBu') (88).193Complex (85; R = Me, Ph, NMe, OMe) reacts irreversibly with H2 to
Iridium
1122
(A S )
(A> R)
Figure 1 Perspective views of the four stereoisomers of [Ir(ZzR)(dppe),lz+(82)
produce complex (86). Elemental analyses, NMR and IR spectra, H, uptake measurements and molecular weight data reveal that the one-electron oxidative addition of H, results in the cleavage of the H-H bond, yielding complex (86), wherein each hydrogen atom is bound to a different Ir atom. The formally d 7 Ir" atoms in complex (86) are linked together with a single bond, in agreement with the diamagnetic behaviour of complex (86).I9IaThe reactivity of (86) towards protonation (H*) has been investigated, and reveals the formation of a dimeric monocationic species containing a two-electron three-centre bent lr-H-ir bond of the type ([I~(H>(CO)(PR,)(~-SBU~)]~(H))+ (R = Me, Ph, OMe). A bridging position for the added proton has been postulated on the basis of high-field 'HNMR and Ir-H stretching frequencies of the product complex and of its deuterated analogue {~r(D)(CO)(PR3)(p-S3u')],(H)}+ The oxidative addition of I, to [Ir2(CO),(L),(SCMe,),] (L = CO, PMe,, P(OMe),, PPhMe,) affords [Ir2(I),(CO),(L),(SCMe,)J (84)." Complex (87) is interesting: it is the first tetrathiolene complex in which TTN adopts a six-electron-donating bis-bidentate di(p-S) bridging configuration, rather than the usual four-electron-donating bis-didentate (p-TTN) bridging configuration. The [Ir,(CO),(SR),] complexes (88) possess a butterfly structure with intermolecular associations in the crystal. When treated with PR, ( = PMe,, P(OMe),, PPh,, P(NMe,),), the SBu' derivative yields [Irz(CO),(PR,),(SBu'),] and [Ir,(CO),(PR,),(SBu'),]. The latter complex reacts with dihydrogen to give [Tr2(H)2(C0)2(PR3)1(SBu')),]activation of H2 at one of the Ir atoms followed by intramolecular H transfer; protonation of this hydride species yields complex (88).'93
Iridium
1123
(84)
(83)
49.5.4
Halogens as Ligands
The existence of iridium(I1) halide complexes remains somewhat doubtful, although the preparation of both [IrBrJ and [IrIJ has been de~cribed."~ Heating [IrBr,] at 713 K in a stream of HBr reportedly yields the reddish-brown species [IrBr,], while [IrI,] was similarly obtained from the iridium(I1) iodide in HI at 403 K. EPR studies OR the product of electron irradiation of [1rCl6l3-in an NaCl matrix reveal a paramagnetic species formulated [lrC1,I4- . I g 4
49.5.5
Mixed Donor Atom Ligands
The stable iridium(I1) complex [I;{ PB~;(C,H,O)}{PBU;(C,H,OH))(CO)] has been synthesized by refluxing 2-methoxyphenyldi-t-butylphosphinewith [1rCl6l3-in propan-2-01under a CO atmosphere. Upon exposure to air, CO, evolution occurs, producing the paramagnetic iridium(I1) complex (89) and the iridium(II1) complex (W), for which X = H, OMe.I9' A dioxygen adduct of (89) (X = OMe) has been isolated.
The cyclometallated iridium(I1) complex [Ir(PBui (C,H,O)),] (91) is afforded when [Ir{PBu~(C,H,O)] (PBu\(C,H,OH)}(CO)] is exposed to air. Additional air exposure results in hydrogen abstraction from one of the Bu' groups, yielding a square pyramidal complex containing one cyclornetallated and one bicyclornetallated ligand. The complex (91) forms [Ir(H){PBu:(C,H,0))2] when treated with H2.'03
1124
Iridium
49.5.6 Mnltidentate Macrocyclic Ligands 49.5.6.1
Sch# base ligands
The reaction between [Ir1*1(H),(AsPh3),(CNC6H4Me-p)] and the quadridentate Schiff bases
NN’-ethyIenebis(salicy1ideneimine) (Hsalen) and NN‘-ethylenebis(acety1acetoneimine) (acacen) yield the iridium(I1) complexes {Ir(AsPh,)(salen)(CNC,H,Me-p)] and [Ir(AsPh,)(acacen)(CNC, H4Me-p)],respectively.’%
49.6 IRIDKJM(II1)
Iridium(I1J) complexes ( d 6 )are diamagnetic and low-spin (&). The coordination number of most known iridium(II1) complexes is six, though a few five- and seven-coordinate complexes have been reported. Iridium(II1) forms complexes with ‘soft’ ligands. The [Ir(H, O),]” ion is unknown; no solid iridium(II1) complexes with coordinated H 2 0 groups are known with the exception of a few aquachloro complexes.197
49.6.1 Mercury-containing Compounds as Ligands Complexes of the type [Ir(Cl)(HgY)(Y)(CO)(PPh,),] (Y = Cl, Br, I, OAc, CN, SCN) and [Ir(Br), (HgBr)(CO)(PPh,),] can be formed by the reaction of trws-[Ir(X)(CO)(PPh,),] with HgY,. Oxidation of these Ir-Hg complexes occurs with halogens (I2,Br,) and with gaseous HC1, as shown in reactions (49)-(51).19’Rapid substitution of the halide ions influences the nature of the halide in the reaction products; the halides preferentially coordinate to Ir in the order C1 > Br > 1.”’ The hydride complex produced in reaction (5 1) can also be obtained by oxidizing [Ir(Cl),(HgCl)(CO)(PPh,),] with H,. The physical and chemical properties of these Ir-Hg complexes clearly indicate that they contain a normal covalent Ir-Hg bond. Reaction of [Ir(Cl), (HgCl)(CO)(PPh,),] with PPh, in benzene results in the elimination of (PPh,),HgCl, to yield [Ir(Cl)(CO)(PPh,),].’48 [Ir(C1)(HgI)(I)(Co)(PPh,),l
+
I*
[Ir(Br)(Cl)(HgBr)(CO)(PPh,),] + Br, [Ir(Cl)*(HgC1)(CO)(PPh,),l
+
HCltg)
-
-
-+
[Ir(C1)(1)2(CO)(PPh,)21 + HgI2 [Ir(Br)2(Cl)(CO)(PPh~)21t HgBr,
w w -*~)
(CO)IPPh, )2 1
+
HgCl,
(49)
(50)
(51)
Crystal structure investigations of [Ir(Cl),(HgCI)(CO)((PPh,),] and [Ir(Br)(Cl)(WgBr)(CO)(PPh, ),I have suggested that the oxidative addition of HgC1, or HgBr, to trans-[Ir(Cl)(CO)(PPh,),] is predominantly trans, and the two complexes are isomorphous. The X ( = C1, Br) ligands are mutually cis, while the PPh3 ligands are mutually trans, as in (92).An SN2attack by iridium at an electrophilic centre in HgX, involving a polar transition state has been proposed.200
(92)
Oxidative addition reactions of HgCl, to [Ir(Cl)(NCF,CHFCF,)(PPh,),], and of diphenylethynylmercury or di-n-heptynylmercury to [Ir(Cl)(CO)(PPh,),] yield [Ir(Cl),(HgCl)(NCF,CHFCF,)(PPh,)J (93a)65and [Ir(Cl)(HgC=Y)(CO)(PPh,),(C=CY)] (Y = Ph, C,H,, ; 93b);O’ respectively. The structures of (93), as illustrated, were characterized by IR spectroscopy. Y
(93a) Y = C1
(93b) Y = C,H,, , CLHI
Iridium
'
.
1125
Electrolysis of [Ir(Cl)(CO)(PPh,),] on anodically polarized mercury electrodes in CH,Cl, leads to (94) and [{Ir(Cl)the formation of a product mixture of [{Ir(Cl)(CO)(PPh,),},Hg]2+ (CO)(PPh,),),Hg]2+ (95)as perchlorate salts when tetrabutylammonium perchlorate is employed as the supporting electrolyte. In the presence of tetraethylammonium nitrate, the product [{Ir(Cl)(CO)(PPh,), ]Hg(NO,),] (96) is obtained. Complexes (94)-(96) are converted to [Ir(Cl)(HgX,)(CO)(PPh,),] (X = C1, Br, I) on reaction with X-.'02
49.6.2 49.6.2.1
Group IV Ligands Cyanide, isocyanide and fulminate ligands
[Ir(CN),] is reportedly obtained by heating (NH& [Ir(CN),] at 603 K, decomposes at 633 K and exhibits v(CN) at 2202 and 2155cm-'. On addition of ammonia, [Ir(CN),] is converted to [Ir(CN), (NH,),] 1.~H,CI?~The hexacyano complex K, [Ir(CN),] is obtained on treating (NH4)3[IrCl,] with KCN, followed by crystallization from water.204Additionally, a series of complexes of the type M:"[Ir(CN),].xH,O, where M is Mn to Zn, are known."' [Ir(CN),(H2O)l2- is formed upon UV irradiation of [Ir(CN),I3- in aqueous solution. The aquapentacyano anion undergoes thermal anation by Cl-, Br-, I-, OH- and NCMe (= X), for which the [1r(CN),Xl3- anions were isolable in the solid form as [Co(NH3),I3+salts.206The lowest energy electronic absorption band in the UV spectra of [Ir(CN),X]'- has been assigned to the ' A , -,,E" and 'A, -+ 'E" transitions. The position of these bands established the order of increasing field strength of X as I- < Br- < C1- < H,O < OH- < NCMe. Reaction between [Ir(Cl)(CO)(PPh,),] and S(CN), gave [Ir(Cl)(CN)(NCS)(CO)(PPh,),] (97) via an S-thiocyanato intermediate. An X-ray crystal structure of (97) shows that the complex is essentially octahedral, and the N-C-S linkage is essentially linear. Of the nine S-thiocyanato isomers possible, the crystal structure and IR spectral results appear consistent with those in (97).207 [Ir(H)(C1)(CN)(CO)(PPh3),(98) ] and [Ir(Cl)(CN)(HgCN)(CO)(PPh,),]have been prepared from Assuming a cis addition the reaction of Vaska's complex with HCN and Hg(CN),, respectively.z08+209 of HCN, two possible structures of complex (98) are possible: (98a) and (9%). Proton NMR and IR data are best interpreted in terms of structure (98b).208
-
PPh, I
(97)
.
(98a)
(98b)
Those iridium(II1) isocyanide complexes obtained from oxidative addition to an iridium(1) isocyanide complex have been treated in Section 49.4.1.1. The iridium(I1I) complex [Ir(H),(AsPh,),(p-MeC,H,NC)] (99) is known to react with HX (or pseudohalides) to yield substitution products Pr(H)2(X)(AsPh,)(p-MeCsH,NC)] (X = C1, Br, I, N,).'" With carboxylic acids or pentane-2,4-dione, complex (99) yields the chelated complexes [Ir(L,)(AsPh,)(p-MeC,H,NC)] (Lz= carboxylate or pentane-2,4-dionate} in which Ir"' is reportedly reduced to Ir".'s6 Reactions (52)-(60) reveal the utility of complex (99) in synthesizing complexes with iridium in various oxidation states.'%
+ HOzCCH(OH)Me (99) + O-OHC6H4C02H + H02CCH(NH2)CH2Ph + (99) + HOC,H,OH-o (99) + C-NH~C~H~OH
[IfV{02CCH(NH)CH2Ph}2(AsPh3)(p-MeCsH,NC)] (54)
(99) f o-NH2C6H4C02H
(99)
(99)
4
~~(O,CCH(O)Me},(AsPh,)( p-MeC6H4NC)]
(52)
+
[Ir"(O-OC6H4C02)2(ASPh,)(p-MeC,H,NC)]
(53)
.--+
(99) + p-MeOC,H,CO,H
[IfV(o-OC,H,0)2(AsPh,)(p-MeC,H,NC)]
(55)
[IrJV(o-NHC6Hq0)*(AsPh~)(p-MeC6H,NC)]
(56)
iIr'"(o-NHC6&CO2)2 (AsPh, )(gMeC, HdNC)]
(57)
-+
[rrm(H)2(C1)(AsPh3)(pMeC6H4NCl]
(58)
-
Iridium
1126 (99) + tfac (99)
+
acac
-,
[Ir"'(H)?(tfac)(AsPh3)(p-MeC,H4WC)]
(59)
[Ir"(acac)(AsPh,)(p-MeC,H,NC)]
(60)
Beck and coworkers'" have reviewed transition metal complexes of the fulminate ion, CNO-, which bonds to the metal via the carbon atom. Fulminate complexes are, in general, similar to those with CN- ligands. An extensive series of stable, non-explosive fulminates has been examined, including [Ir(CNO),](AsPh,),. [Ir(CN0),I3- was prepared from hydrated iridium(TT1) chloride and Hg(CNO),, and characterized by I R
49.6.2.2
Si-,Ge- and Sn-containing ligands
Most iridium(II1) complexes containing Group IVb donor ligands are six coordinate and are more stable than their rhodium(II1) analogues. The complexes are prepared via oxidative addition to an iridium(1) complex. Thus, the facile reaction between Vaska's complex and silanes with electronegative substituents [R, SiH) gives an iridium(II1) complex with an Ir-Si bond. Reaction (61) occurs for R,SiH = (EtO),SiH, Cl,SiH, EtC1,SiH and PhC1,SiH.212Proton NMR and 1R
spectroscopy have been employed to elucidate the structure of the adducts formed between [Ir(X)(CO)(PEt,),] (X = C1, I) and MN, Q (M = Si, Q = H, C1, Br, I,+Me;M = Ge, Q = H, Cl, Br, I). The addition yields trans-[Ir(H)(MH,Q)(X)(CO)(PEt,),].When M = Si, the major product contains H trrans to Si; and when M = Ge, the major product contains H trans to X.'I3 Additionally, organosilanes and organogermanes (R, MH) add to trans-[Ir(Cl)(CO)(PPh,)zl to yield dihydrido complexes, wherein the expected Ir-MR, products are intermediates (see Scheme 7).2'4921592'6 Wans-lIr(CI)(CO)(PPh,), ] + R3MH
-
[Ir(H)(CO)(PPh,), I + RJCI
[ Ir(H)(CI)(MR,)(CO)(Pph,),
+R,MH
J
-
H
PW*
%]/
*MR3
Ph,P4I'H
co
Scheme 7
The kinetics of the oxidative addition of silicon hydrides to [Ir(dppe),]+ (reaction 62) have been investigatedm2I7 No conclusive results were obtained on whether reaction (62) proceeds via a concerted three-centre transition state or via a transition state analogous to that proposed for the Menschutkin reaction. The iridium(1) species trans-[Ir(X)(CO)(PEt,),] reacts with Y(SiH,), (Y = 0, S, Se) in benzene to yield [Ir(H)(I)(SiH,YSiH,)(CO)(PEt,),or ] [(Ir(H)(I)(SiH,)(CO)(PEt,), Reaction between trans-[Ir(X)(CO)(PEt,),] and P(SiH,), affords [Ir(H)(I){SiH,P(SiH,),}(co)(P& 1 2 [{Ir(H)(I)(siH2)2(COj(PEh h>2PSiH31 Or [{Ir(H)(I>(siH,)(Co)(PE)zt,1 3 PI, depending on the proportions of reactants employed. With N(SiH,),, tmns-[Ir(X)(CO)(PEt,),1 gives [Ir(H)(I){SiH,N(SiH,),}(CO)(PEt,),] and [{Ir(H)(I)(SiH,)(CO)(PEt,),),NSiH,]. The reaction products have all been characterized by ' H and 31PNMR spectroscopy. The oxidative addition reactions are similar to those between trans-[Ir(X)(CO)(PEt,),] and silyl halides,,', and similar stereochemistries have also been proposed.218
},v.
1 3
[Ir(dppe),]+
+ [Si(H)(Me), (EtO),_,l
-
[Ir(H)(dppe),{SiMe,(EtO)3- n 11'
(62)
Six-coordinate dihydrido complexes of the type [Ir(H),(GeR,)(CO)(PPh,),](R = C1, Me, Et) can be produced via Scheme 7. However, for R = Ph, the five-coordinate complex [Ir(H)(Cl)(GePh,)(CO)(PPh,)] is formed. For the complex where R = Me, crystal structural, ' H NMR and IR spectral data are consistent with structure (100).2'6These iridium(II1) complexes containing Ge donor ligands undergo several reactions, as summarized in Scheme 8.?16Furthermore, ethylene is hydrogenated by [Ir(H),(GeR, j(CO)(PPh,),] to yield the iridium(1) complex [Ir(GeR,)(CO)(PPh&j, as per Scheme 9.
Iridium
1127
H
+
Ge[CI)R3
Scheme 9
K,[IrBr,] reportedly reacts with SnBr, in aqueous HBr solution, from which The syntheses of (Me,N)[Ir(Br),(SnBr,),] was obtained upon addition of Me,NBr! [Ir(H)(Cl>Oo(PPh,131 [Ir(H)(Cl)(X)(Co)(PPh, 12 I, [Ir(D)(C1)(X)(Co)(Pphh,12 I, [Ir(H>*(XI(CO)(PPh,),] and [Ir(D)*(X)(CO)(PPh,),] (X = SnCl,) via oxidative addition of SnCI, to iridium(1) complexes have been described. SnC1, serves as a weak a-donor ligand ( i e . SnCI;), and exhibits a large trans effect due to its ability to form dn-dn bonds to the central Ir atom. TR and ' H NMR spectral data have been interpreted in terms of structure (101) for [Ir(H)(CI)(SnCl,)(CO)(PPh3),], although structure (102) cannot be The complexes [Ir(X)(SnR,)(CO)(PR;),] (X = Cl, Br, I; R = Me, Et, Ph; PR;= PPh,, PPh,Me, PPhEt2) have been prepared and characterized by H NMR and TR spectroscopy. The Ph, Sn-Ir complexes have been assigned the stereochemistry in (103), while the Et,Sn-Ir complexes are consistent with structure (104). Two isomers of the Me,Sn -1r complexes were isolable, and assigned to structures analogous to (103) and (104).220 I
H
H
1128
Iridium
( 104)
(103)
Intramolecular oxidation-reduction isomerism involving changes in formal oxidation state at the metal centre are rather uncommon in transition metal chemistry. However, from IR and ' H NMR spectral studies on [Ir(Cl)(SnCl,)(CO)(PPh,),] and [Ir(Cl)(SnC1,Me)(CO)(PPh3)2], it was concluded that each complex consists of two rapidly interconverting isomers. Further, the available data for the SnCl,-Jr complex are consistent with the isomerism arising from rapidly equilibrating oxidative addition and donor-adduct isomers (105a) (105b).220 SnCI,
SnCI4
(105a)
49.6.3
49.6.3.1
(105b)
Nitrogen Ligands Ammonia and amine ligands
The iridium(II1) pentaamine complexes [Ir(X)(NH,),]2f undergo octahedral substitution reactions in aqueous solution, similar to Co"' and Rh"' analogues.22'[Ir(X)(NH,),]2+ (X = NO,, C1, Br, I) undergoes aquation (reaction 63) with the reactivity increasing in the order X = C1 < Br < I < NO,; a linear free energy relationship was observed.222The aquation rates of [Ir(X)(NH,),]'+ are smaller than those of the cooresponding Rh"' complexes, consistent with considerations of crystal field theory. Additionally, [Ir(Br)(NH3),I2+ undergoes acid hydrolysis at 298 K with a rate constant k E 2 x lO-''s-'; E, = 113 kJ mol-' and ASt = - 59 J mol-' The rate constant k,, for the base-catalysed hydrogen exchange of [Ir(NH3)J3+at 298K in DzO (reaction 64) has been determined (kcx= 1.5 x 10-8M-'s-The chloride anation of [Ir(H2)(NH3)s]3+ is three times faster than the water exchange rate, and has been attributed to an associative interchange mechani~rn.~"
+ H20
e
+
7Ir-ND +
[Ir(X)(NH3>,lZ' Ir-NH
OD-
+ X-
[Ir(H20)(NH3),13+ OH-
(63) (64)
Iridium(II1) ammine complexes have been shown to be p h o t o a ~ t i v e Ligand . ~ ~ ~ ~field ~ ~ ~(LF) excitation (350 nm) of [Ir(N3)(NH3)J2+leads to efficient decomposition of N; with the formation of [Ir(NH2C1)(NH3)J3+and N2 in aqueous HC1.226The principal photoreactions observed on LF excitation (254 or 3 13 nm) of [Ir(NH,),j3+, [Ir(H20)(NH,),]3+ and [Ir(C1)(NH,),]2+ in aqueous solution at 298 K are summarized in reactions (65) and (66); [Ir(H,O)(NH,),I3+ is virtually unreactive under the conditions employed.227The photochemical properties of [Ir(X)(NH,),]2+ (X = C1, Br, I), trans-[Ir(I),(NH,),]+, trans-[Ir(I>(H,0)(NH,),12+ and [Ir(L)(NH3)J3' (L = H 2 0 , NH,, MeCN, PhCN) have been investigated in aqueous solution. LF photolysis leads to ligand photoaquation only, while irradiation of ligand-to-metal charge transfer (LMCT) absorption bands suggest the presence of a redox component as well as aquation. Quantum yields and product distribution are wavelength independent for LF excitation. Also, LF excitation is followed by efficient internal conversion/intersystem crossing to produce a common 'triplet' LF state.228Essentially, rhodium(IZ1) and iridiuq(II1) amine complexes behave in a parallel manner. [Ir(NH,),]'+
+
[Ir(Cl)(NH,),]'+
+ NH,+
(65)
W 2 0 3 [Ir(H2O)(NH,)J3+ + C1-
(66)
H30* % [Ir(NH,),(H,0)l3+
+
1129
Iridium
The synthesis and characterization of cis- and rrans-[Ir(X),(en),]+ (X = C1, Br, I) and trans[Ir(X)(Y)(en),]"+ (X = CI, Y = Br, I, HzO, OH, NCS, NOz, ONO; X = Y = NO3, N,; en = ethylenediamine) were reported by Bauer and B a ~ o l o , ~following '~ Kida'szMpaper on the synthesis of cis- and trans-[Ir(Cl), (en)J+. trans-[Ir(Cl>,(en),]+ can be prepared by a catalytic method employing H,PO,, while the cis isomer is obtained in a high ionic strength medium. Structural assignments were made on the basis of IR spectra and resolution of the cis isomer. Ligand substitution at trans-[Ir(X),(en),]+ occurs without stereochemical change, and with substitution rates independent of the concentration and nature of the incoming ligand and the ionic strength. The substitution mechanism involves an aqua intermediate, as illustrated in Scheme Aquation kinetics of complexes such as trans-[Ir(X), (en),] + were not directly accessible, the equilibrium in Scheme IO lying to the left. Table 2 shows various ligand substitution rates and activation parameters for tr~ns-[Ir(X)(en)~(L)]+. The base hydrolysis of cis- and trans-[Ir(CI),(en),]+ also proceeds with stereoretention.
Table 2 Rates of Ligand Substitution Reactions of trans-[Ir(X)(en)2(L)]+a
L CI Br I NO2
Br I I
Leaving group
k x 1 6 ( s - 1)b
c1
5.9
c1 CI c1 Br Br I
9.0 159 107
AS: (J K- I mol
AH$ (kJ mol-')
122.1 113.3 105.8
-25.1 - 37.6
- 16.7 -
I
14.5 I04 58
I)
I
- 37.6 - 20.9 - 12.5
113.7 113.3 120
Taken, in part, from ref. 231. bAt 378 K. a
The geometric structure of cis-[Ir(Cl),(en),]+ has been proved by resolution into its optical isomers.232Proton NMR studies have shown that the ammination of cis-[Ir(Cl),(en),]+ proceeds The absolute configurations of with retention of configuration to yield ~is-[Ir(en>~(NH,),]~+. optically-active cis-[Er(CI),(en),]+ and cis-IIr(en>,(NH,),I3+ have been inferred by comparing their electronic absorption and CD spectra with those of Co"' and Rh"' analogues; the A configuration has been proposed.233 The complexes trans-[Ir(x),(en),]+ (X = CI, Br, I) in aqueous solution undergo rapid photosubstitution to yield rr~ns-[Ir(X)(en),(H,O)]~'. LF excitation (350 nm) of trans-[Ir(Cl),(en),]+ affords photoaquation of C1- with a quantum yield of ca. 0.1 mol einstein-'.234 Irradiation into the LF bands of cis- and trans-[Ir(X)(Y)(en),]+ (X = Y = C1, Br, I; X = OH, Y = Cl) in aqueous solution leads to halide photolabilization. For X = Y = C1, the quantum yields are pH independent. In acidic solution, the drans dihalo complexes give trans-[Ir(X)(en),(H,0)]2+ with complete retention of configuration. But the cis analogues undergo simultaneous photoisomerization to yield a mixture of cis- and rra~ls-~Ir(x>(en),(H,O)]~+with the extent of photoisomerization following the order X = C1 < Br < I.235 The principal photoreactions observed for these complexes are consistent with theories236that propose ligand labilization from the lowest LF excited states being confined to the weak field axis. With an excess of Br- and I - , trans-[Ir(Cl),(en)z]+ reacts photochemically to yield trans-[Tr(Br),(en),]f and trans-[Ir(I),(en),]+, respectively.237 The photoaquations of X in trans-[Ir(X),(en),l+ provide a useful route for the rapid and clean preparation of various trans-[Ir(X)(Y)(en)J+ and trans-[Ir(X),(en),]' complexes (reactions 6770).229~237 The products obtained were those expected on the basis of the trans-effect behaviour in such systems.
-
2 truns-[Ir(Cl)(I)(en),]+ tra~ts-[rr(~~),(en),]+ truns-[~r(~), (en),] + rrans-[Ir(C1),(en),]+
-+
trans-[Ir(I),(en)z]+
(67)
truns-[Ir(Cl)(I)(en),]
(68)
truns-[Ir(oH),(en),l'
(69)
+
1130
Iridium trans-[Ir(X),(en),]+ trans-[Ir(X)(H,0)(en),jz+
*
hv H 0
2 tr~ns-[Ir(X)(H,O)(en),]~+
+
AgX(s)
tram-[Ir(X)(Y)(en),]+
(70)
The kinetics of chloride exchange in trans-IIr(Cl), ( t ~ i e n ) ] +(trien ~ ~ * = triethylenetetramine) show behaviour similar to that of cis- and trans-[Ir(Cl),(en),]+ .239 Also, the rate constant dependency on chloride concentration is similar for all three complexes. This dependency has been interpretedz3* either in terms of an ion pair or a dissociative mechanism, with the latter being more favourable. The bis-pyridino complexes trans- and cis-[Tr(Cl),(py),]- and trans-[Ir(Cl),(H,O)(py),] have been prepared and characterized. The thermal aquation of cis-[Ir(Cl),(py),] yields 5% trans product, and a trigonal bipyramidal intermediate species is postulated.x0 ~rans-[Ir(Cl),(py),]+~~”~~~ undergoes photoaquation of C1 at 350 and 254 nm irradiation. A significant amount of amine release also occurs during photolysis of this complex.243The photochemistry of several pyridine complexes has been reviewed by Balzani and Cara~siti;,~including (X = Cl,”7 B1-248), and cis[Ir(CI),(py)I2-,244 cis- and trans-[Ir(Clj,)py)2]-,245~246 trans-[Ir(X), and trans-[Ir(Cl),(py),]+ .249 ~
49.6.3.2
N-heterocyclic ligands
A proton NMR spectroscopic investigation of a complex originally thought to be [Ir(bipy)J3+
. (bipy = 2,2’-bipyridyl) has assigned the complex as cis-[Ir(Cl), (bipy)J+ ?M In agreement, Gillard
and HeatonZ5’have found that earlier reports of the syntheses of [Ir(phen),I3+ (phen = 1,lOphenanthroline) and [Ir(bipy),13+were erroneous. The preparation of cis-[1r(C1),(4,4‘-Ph2 bipyj,]’ , ciss-[Ir(Cl),(4,7-Cl,phen),]+ and cis[Ir(Cl),(bipy),j+ (4,4’-Ph, bipy = 4,4-diphenyl-2,2’-bipyridyl;4,7-Cl2phen = 4,7-dichloro‘l,lOphenanthroline) has been reported. The similarities between the UV spectra of the free ligands and the complexes led to an assignment of ligand-localized transitions in this region. Absorption bands in the visible region were assigned to charge-transfer transitions. All three complexes cxhibit luminescence at 19.5-21.1 kK at 77 K, ascribed to a charge-transfer (dn*)+. A , transition.=’ h i s s ion quantum yields, lifetimes and spectra, along with photosolvation yields, are reporttd for cis-[Ir(Cl),(bipy),]” and its deuterated analogue cis-[Ir(Cl),(bipy-ds),]+ in H,O,DMF, MeOkI and MeCN. Both charge-transfer and ligand-field excited states contribute to the decay properties in fluid media. Photosolvation rates are solvent dependent and thus consistent with either a dissociative or a dissociative interchange mechanism.253 and cis-[Ir(Cl),(phea1(4,7The luminescence decay curves of cis-Dr(Cl), (~hen)(5,6-Me,phen)]+~~~ = 5,6-dimethyl-l,lO-phenanthroline;4,7-Me2phen = 4,7-dimcthylM e , ~ h e n ) ] + ’ ~(5,6-Me2phen ~ 1,lO-phenanthroline) have been recorded as a function of excitation and/or emission wavelcngth. From emission wavelength studies, the total emission spectrum of the 5,6-Me2phen complex is decomposed into two component spectra, one arising from d?r* and the other from n?r* orbital parentages. Excitation wavelength studies have led to the proposal that upper states o! d?r* parentage feed the dn* emission while states of ?rn* parentage feed the zn* emission. The data obtained from emission wavelength studies of the 4,7-MeZphen complex best fit a model in which the luminescence occurs as a result of three sets of thermally non-equilibrated levels (lifetimes t = 6.0, 8.7 and 22 ps), arising from mixed dn*-nn*. dn*,and &d orbital parentages, respectively. A synthesis of [Ir(bi~y)~],+ has been reported by Flynn and D e r n a ~ . ~I3C ~ ‘ and proton KMR spectra have confirmed the tris-complex assignment. [Ir(bipy)J3 exhibits a phosphorescence at 22.3 kK with a mean lifetime of 80 ps in MeOH/EtOH glass at 77 K; luminescence was assigned as predominantly n-n* ligand phosphorescence. Gillard2j7has summarized many of the so-called ‘anomalies’ existing for transition metal polypyridyl complexes in aqueous solution, and postulated covalent hydrate formation to explain these ‘anomalies’. Nord and coworkers25shave questioned the validity, and indeed the premise upon which covalent hydration in polypyridyl complexes is based. The nature of the complex(es) containing three 2,2’-bipyridyls per iridium atom has been the since the report by Flynn and D e m a ~ ,of~ the ~ successful subject of a controversial debate259-263 preparation of the [Ir(bi~y)~],+ cation. This species was formulated as having all three bipyridyl ligands ligated to iridium(II1) via the nitrogen atoms, as demonstrated by I3C NMR spectroscopy (D3symmetry, five well-resolved I3C resonances). Later, Watts et identified another complex also containing three bipyridyl ligands per iridium atom but having distinct absorption and emission spectral properties, as well as different photophysical characteristics from [Tr(bipy),13+.The complex exhibits an emission in both acidic and basic media at lower energies than that for [Ir(bipy),13+,and
-
+
1131
Iridium
visible absorption bands in these media do not correspond with the tris complex. The complex was described as being comprised of iridium, two bidentate bipy ligands, one water molecule, and one monodentate bipy’, [Ir(bipy)2(H20)(bipy’)]3+(106a); this complex can be converted to the hydroxo form and isolated by treatment with base. The luminescence spectrum of complex (106a) arises primarily from interligand z-n* transitions of the bipy ligands, with some mixing from charge-transfer from iridium(II1) to bipy.261The luminescence lifetimes of complex (106a) and its hydroxo analogue, [Ir(bipy), (0H)(bipy’)l2+,along with their related emission quantum yields in H,O and D20,have been reported as well.259Together with the above information, the IR spectrum of (106a) is consistent with a formulation of the complex as the covalently hydrated species (106b); however, because there is no facile equilibrium between [1r(bipy)J3+ and the covalent hydrate [Ir(bipy)2(bipy*H20)]3* (106b), as revealed by the identical emission spectra, in both 0.1 M base and 0.1 M acid for [1r(bipy),l3’, the nature of the complex isolated by Watts et ai.259 cannot be (106b). Gillard and coworkers’62have argued ‘because there is no facile equilibrium, it is not to say that no equilibrium exists’, and explained the ‘puzzling’ properties of the complex in terms of the species (106b).
L
L ( 106a)
(106b)
Another source of controversy about the formulation of this iridium(II1) complex rests on its ’H and I3CNMR spectra.263The data from these spectra preclude the existence of structure (106b) and were taken as consistent with structure (106a).259A single crystal X-ray structural studyZ6’on the Cloy salt of this complex has demonstrated unequivocally that the species contains neither a monodentate nor a covalently hydrated bipyridyl ligand, at least in the solid state. All three bipyridyl ligands are chelated to iridium(III), but one bipy ligand is chelated via the nitrogen of one ring and the C-3 carbon of the other ring, reminiscent of o-metallated complexes. It has been proposed267that the can be understood in terms of, and are all consistent with, structure (107). Clearly, for complexes where monodentate or covalently hydrated species have been poshas tulated, renewed consideration must be given in the light of (107) or equivalent species. Furthermore, recent studies by Serpone reconsidered the H and I3CNMR data, as has Seddon+26y et ~ 7 1reveal . ~ ~ that ~ the nitrate salt of this cation contains two NO, units per iridium(III), consistent with structure (108). A very recent crystal structure determination of the red complex [Ir(bipyN,N’)2(bipy-C3,,’)I2+ (109) has revealed that each Tr atom is coordinated, in a distorted octahedron, to five nitrogen atoms and one carbon atom of the bipy ligands. In both N-bonded bipy rings and the C-bonded ring, the C(2)-N and C(6)-N distances are short, suggestive of rings 5, 7 and 13 being bonded to Ir via carbon (three I r ( b i ~ y ) , ~units + per asymmetric unit). Proton NMR and UV spectra of complex (109) were in accordance with the crystal structure analysis depicted in (109).271Also, Spellane et al.”’ have recently cited some ‘H and 13CNMR data for [Ir(Hbipy-C3, N’)(bipy-N,N’)J3+ in DMSO-d, which indicate the presence of a C-bonded ring to Ir. Experiments employing a lanthanide shift reagent confirmed the presence of a site of Lewis basicity on the carbon-bonded pyridine ring.272
.OH2 H’
/
1132
Iridium
49.6.3.3 Dinzenato and &azo ligands Transition metal complexes of aryldiazenato (ArN=N- ) and aryldiazene (ArN=NH) ligands serve as valuable models of the catalytic activation of N, and for its preliminary reduction to N,H,. Aryldiazenato and aryldiazene derivatives of trans-[Ir(Cl)(CO)(PPh,)J have been investigated by several groups.*73[Ir(X)(CO)(PPh,),] (X = C1, Br) reacts with arenediazonium tetrafluoroborates in the presence of LiCl to give [Ir(Cl)(X)(CO)(N=NAr)(PPh,),]. In the absence of LiCl, the cationic complex [{Ir(X)(CO)(N=Ar)(PPh,),},]+ is obtained. A crystal structural analysis of [Ir(Cl),(CO)(N-;NC,H,NO,-o)(PPh,),] confirmed the presence of a 'doubly-bent' aryldiazenato The N-ligand appears to exert a strong trans influence. The five-coordinate cation ligand (1 [Ir(Cl)(CO)(N=NC6H,F)(PPh3),]+ (111) also contains a doubly-bent ArN; ligand. Further addition of neutral or negative ligands affords six-coordinate iridium(1TT) complexes.275
( 1 11)
(110)
The reaction between [Ir(Cl)(CO)(PPh,),] and diazonium ions in the presence of ethanol or propan-2-01 affords the o-metallated diazene (112)276 and the iridium(II1) tetrazene complex (113).277 A mechanism for the synthesis of (113) has been l+
( 1 13)
( 1 12)
[Ir(Cl), (NH=NAr)(PPh,),] ' and [Ir(Cl), (NH=NAr)(PPh,),] have been synthesized from [Ir(H)(C1),(PPh,),].279Insertion of diazonium ions into mer-[Ir(H),(PPh,),] yields [Ir(H),(NH= NAr)(PPh3),]+; a preliminary X-ray investigation has suggested the structure (114).280The crystal and molecular structures of [Ir(H)(I)(NH=NC,H, Me-p)(PPh,),] (1 15) have been elucidated.'" More recently, it was shown that reaction between [Ir(H),(PPh,),] and p-substituted arenediazonium salts gives [Ir(H),(NH=NC,H,R)(PPh,),]+ at low temperatures (263 K) and the o-metal lated complexes [ir(H)(NH=N~,H3R)(PPh3),]+(116; R = F, OMe) at higher temperatures (3 13323 K). The o-metallated complexes (116) react with CO, PPh,, NaT and HC1 according to reactions (7 1-74), respectively.2s2 C6H3Me
I+
H\I,
1
%%,[**Ph3
4 b NA= N 'CC,H4N02
Ph3P
/Ii ,J PPh,
H
PPh,
(116) f CO
'".Q
-
(116) f PPh3
[ir(H)(CO)(NH=NCsH3 R)(PPh3),lf
+
[Ir(H)(NH=Nk6H,R)(PPh>)3]+
1133
Iridium
(73)
(116) + HCI
49.6.3.4
-3
[Ir(H)(CI),(NH=Nk,H,R)(PPh,),]
(74)
Azide ligands
The synthesis of the iridium(II1) azide complex (Bu:N), [Ir(N&] occurs via reaction (75). The visible and near-UV absorption and reflection spectra of this complex have been recorded; the assignment of bands in this and rhodium(II1) azide complexes has led to the location of the N; ligand in the spectrochemical and nephelauxetic series.283 Na,[IrCI,]* 12H20 t NaN3
H O/EtOH
(Bu:N)311r(N3)6]
(75)
trans-[Ir(N,)(en),]+ readily reacts with acid to liberate N, gas.229Subsequent investigation of this system suggested that the reaction occurs via a low-energy metal-nitrene (M-NH) pathway.284 [Ir(N3)(NH,),]2+ also liberates gaseous N, upon reaction with acid,285presumably by way of reaction (76). Kinetic studies were consistent with a nitrene intermediate pathway. Scheme 11 shows all of which are in agreement with a nitrene path; furthermore, some reactions of [Ir(N3)(NH3)5]2+, the coordinated nitrene serves as a powerful electrophile. Bas.010~~~ has given a brief review of the preparation and reactivity of transition metal nitrene complexes. Photolysis of Fr(N3)(NH,),I2+in aqueous HCl proceeds by reaction (77). A mechanism consistent with steady state and flash photolysis results is given in Scheme lLa4
+ 2H+ + X[Ir(N,)(NH,)J2+ + 2H+ + C1E1r(N3)(NH3),l2+
-
[Ir(NH3),(NH2X)l3+ + N,(g)
5 [Ir(NH,),(NH,C1)]3f
Scbeme 11
Scheme 13
+ N,(g)
(76)
(771
Iridium
1134
Zink286has applied a molecular orbital approach to photochemical assignments of excited states of iridium(II1) azide complexes. The observed products of 250-400 nm irradiation of [Ir(N,)(NH,),]Z+ are [Ir(NH,),(NH2C1)]3+and N,, the same products expected on the basis of populating the LL (ligand-localized n,, -+ n*) excited state. It was suggested, therefore, that the photoreaction originates in the LL excited state. The formation of a coordinated nitrene intermediate has been ascribed to a combination of LMCT and LL states rather than to the LL state alone.
49,6.3,5 Polypyrazolylborate ligands
From a structural point of view, the complex chemistry of the tripyrazolylborate anion frequently resembles that of cyclopentadienyl or pentamethylcyclopentadienyl complexes. The iridium(II1) tripyrazolylborate complexes [Ir(C1),(HB(3,5-Me2pz),}],, [Ir(C1),{HB(3,5-Me,pz),}(AsPhMe2)], [Ir(CF,C02)2{HB(3,5-Me,pz),}(H,0)] and [Ir(a~ac)(Cl){HB(3,5-Me~pz)~}] have been prepared and characterized by ' H NMR spectroscopy. None of these complexes exhibited a v(B--H) mode in the region of cu. 2500 cm-', as do their rhodium analogues.2s7
49.6.4 Phosphorus, Arsenic and Antimony Ligands 49.6.4.1
Unidentate ligands
Tridium(II1) phosphine complexes obtained via the oxidative addition to Vaska-type iridium(1) complexes have been discussed (Section 49.4.3). The stereochemistries of [Ir(X)n(ER,),(Me),-,] (X = Cl, Br, I; ER, = PMe,Ph, AsMe,Ph; n = 0, I , 2) have been assigned from the methyl resonance patterns of their IH NMR spectra.288Structural assignments of trans-[Ir(X),)(PMe,),], mer- and fuc-[Ir(X),(PMe,),j and trans-[Ir(X),(PMe,),] (X = C1, Br) have been made on the basis of IR and Raman [ < 800 cm-') spectral data; additionally, the significance of the Ir-P and Ir-C1 stretching frequencies has been discussed.289 Tri-t-butylphosphine reacts with hydrated iridium(II1) chlorides to yield [Ic(H),(C~)(PBU:),]?~ The two isomeric series, LX and D, of hydrido carbonyls of the type [Ir(H)(X)(CO)(ER,),] (X = C1, Br; ER, = AsMePh,, AsEtPh,) have been prepared. The structures of the a and p configurations are shown in (117a) and (117b), respecti~ely.~~' Other complexes containing unidentate As donor ligands have been prepared via oxidative addition and characterized, including [Ir(H), (AsPh3),(C0)]292 and [Ir(H)(X)(CO)(ER,), ] (ER, = AsPh,, A S M ~ ~ P Reaction ~ ) . ~ ~ ~between [Tr(H),(AsPh,),(CNC,H,Me-p)](118) and HX (or pseudohalides) leads to the substitution products [Ir(H), (AsPh,),(CNC,H,Me-p)Ol)l (X = C1, Br, I, N3).'10 With carboxylic acids and pentane-2,4dione, complex (118) yields the chelated complexes [Ir(L),(AsPh,)(CNC,H,Me-p)] (L = carboxylate or pentane-2,4-dionate), in which Ir"' is reportedly reduced to Ir".'86 Reactions (78H86) reveal the utility of complex (118) in synthesizing complexes with iridium in various oxidation states.'96 X
H
(117a)
+ acac + tfac
(118) (118) (118)
fIr"(a~ac)~(AsPh,)(CNC~H,Me-p)]
-+
[if"(H),(tfac)(AsPh,)(CNC,H,Me-p)]
-+
H02CC6H,0Me-p
+ HOzCCH(0H)Me (118) + HO2CC, HdOH-o + HO,CCH(NH,)CH,Ph
(118)
(118)
+
(117b)
(78) (79)
[Ir(H),(C1)(AsPh3)(CNC6H, Me-p)]
(80)
-+
[I?' { OzCCH(0)Me},(AsPh,)(CNC6 H,Me-p)]
(81)
----t
[I$" (OzCCsH40-0)z (ASPh,)(CNC6H, Me-p)]
(82)
+
----f
~Ir'V{02CCH(NH)CH2Ph)2(AsPh,)(CNC,H,Me-p)] (83)
1135
Iridium (1IS) -k HOC,H40H-o (118)
+
+
[Ir"(OC6 H,O-o),(AsPh,)(CNC, H,Me-p)]
HOC6H4NH2-o + [Ir" (OCsH4NH-o),(AsPh,)(CNC, H4Me-p)]
(84)
(85)
49.6.4.2 Polydentate ligands
Reaction of [Ir(H)(X),(PPh,),] (X = C1, 3r) and the potentially tridentate ligands (uPh2EChH4),E'Ph (TP: E = E = P; PDAS: E = AS, E = P; ASDP: E = P, E'= AS; TAS: E = E = As] has afforded complexes of the type [Ir(H)(X),(L)] (119), where L = TP, PDAS, ASDP or TAS. The syntheses and characterizaton of complexes (119) by 'Hand "P NMR spectroscopy have revealed the meridional isomer, as shown, to be preferred.294The tripodal ligand 1 ,1,1-tris(diphenylphosphinomethy1)ethane (MeC(CH,PPh,), = TDPME) reacts with pr(Cl)(CO)(PPh,),J in benzene to yield [Ir(Cl)(CO)(TDPME)], for which a trigonal bipyramidai structure (120) was tentatively assigned. The reaction of complex (120) with CO has been postulated as proceeding by way of Scheme 13 to give complex (121).295
(119)
(120)
TP: E = E' = P PDAS: E = As; E' = P ASDP: E = P; E' = As TAS: E = E' = As
Scheme 13
Reaction of [Ir(H)(Br)2(PPh3)3]with the tetradentate ligand tris(o-diphenylphosphinopheny1)phosphine, QP (1221, gives [Ir(H)(Br)(PPh, )(QP)#BPh,), which was assigned a seven-coordinate structure.296With the similar tetradentate ligands ASTP, PTAS and QAS (122), [Tr(H)(X),(PPh,),] (X = C1, Br) reacts to produce [Ir(PPh,)(ASTP)]+, [Ir(PPh,)(PTAS)] and [Ir(PPh,)(QAS)I +, re~pectively.~"Under the conditions employed [Ir(X)(PPh,),] (X = C1, Br) reacted with QP according to reaction (87), and [Tr(Cl)(PPh,)(cod)] reacted with QP to give [Ir(Cl)(QP)] and [Ir(PPh,)(QP)]+ . [Ir(Cl)(QP)] reacts with HC1 and C1, according to reactions (88) and (89), respecti~ely.,~~
W A S : E = AS: E' = P QAS: F, = E' = As
Iridium
1136
+ QP
[Ir(X)(PPh3),1
[Ir(Cl)(QP)I + HC1 [Ir(CI)(QP)l
+
-
[Ir(PPhdQP)l+ -t IIr@C)(QP)I
-+
4
(87)
[Ir(H)(CMQP)I'
(88)
[WClh(QP)I+
(89)
The trinuclear derivative [Ir, (Cl), {P, ( - Pf),},] results from the reaction between hydrated iridium(II1) chloride and the hexatertiary phosphine (Ph,PCH,CH,),FCH,CH,P(CH,CH,PPh,), (= P,( - Pf),) (123). Far-IR spectra for the trinuclear complex were interpreted in terms of three Cl atoms of each IrC1, moiety occupying meridional positions in an octahedron, and the Pz( - Pf), ligand bridging the three IrCl, groups. Also, bridging C1 atoms could be present.298 Ph
>
/Ph
P C HC ~ H ~
Ph
,cH2CH2P\ph
\
Ph\/PCHZCH2 /PCHzCH2P
\CH,CH,P\ APh
Ph
Ph
(123)
The bulky fl-ketophosphine ligands PBu:(CH,COPh) (Q) and PBui(CH,COBu') (Q') are known to behave as either (i) unidentate ligands through the P atom; (ii) bidentate ligands through the P atom and the keto group; or (iii) bidentate enolate ions. Iridium trichloride reacts with Q and Q', affording the six-coordinate complexes (124). Complex (124) reacts with NaNO, to give the fivecoordinate square-pyramidal hydride complexes (125), which take up CO to yield (126)?9' Carbonylation of Na[IrCl,] followed by the addition of Q yields (127) with an uncoordinated keto group. On treatment of complex (127) with NaOEt, a mixture of (126a) and (128) was reportedly formed. Proton and 31PNMR spectroscopic data were employed to elucidate the structures.299
(1243) R = Ph (124b) R = But
(125a) R = Ph (125b) R = But
(126a) R = Ph (126b) R = But
The cyclometallated chlorohydrido iridium(II1) complex (129) is obtained upon treatment of [Ir(Cl)(PPh,),] with organometallic derivatives -of either the unsubstituted 1,2- and 1,7-carborane (-2-H- 1,2-B,,C2H,,) or the C(7)-methyl- and -phenyl-substituted carborane (-7-R-1 ,7-B&,H10) (R = H, Me, Ph). Proton NMR spectral data revealed evidence that the H ligand is trans to C1 and mutually cis to three non-equivalent P nuclei.'33In MeCN, [Ir(PMe,Ph),]' reacts with H, to yield cis-[Ir(H),(PMe,Ph),J+; however, on refluxing for 6 h in the absence of H,, the complex fac[1r(H)(PMe,C6H,)(PMe,Ph),]+ (130) results.3w +Metallation in [Ir(H)(X),{P(OPh),} ,] (X = C1, Br) apparently proceeds via a reductive elimination of HX to produce [Ir(X){P(OPh), },I followed by an internal oxidative addition process. For the iodo analogue, [Ir(H)(I)2{P(OPh),},], a mixture
1137
.
of H, and HI is eliminated, which might suggest that a pathway which does not involve prior ~ ~ ' and Shaw302have reported on the o-metallageneration of an iridium(1) species is p l a ~ s i b l e . Duff tion of six-coordinate iridium(II1) dimethylnaphthylphosphine complexes, for which a 1,2-hydrometallation of coordinated ligand followed by H,elimination was postdated to account for o-metallation in a coordinatively saturated complex. The 8-methylquinoline derivative mqp (131) reacts with [Ir(Cl)(cot),]z and [2H,]-pyridine in hexane to produce [Ir(H)(Cl)(mqp-H3)(NC,D,),] (132), one of three possible isomers being illustrated.,03 Analogous complexes have been obtained An oxidative addition-reductive elimination sequence employing R,PCH,Ph (R = Bu', C6H11).3w was favoured for the o-metallation of the coordinatively unsaturated methyl iridium(II1) complexes [Ir(Cl)(X)(PPh ), (Me)] (X = C1, Br) t 0 yield [Ir(CI)(X) { PPh, H, )} (Pph,)].305
(e6
,
( 1 29)
( 1 30)
(131)
(132)
The treatment of mer-[Ir(Cl),(PMe,R),] (R = Ph, Me) with strong bases (e.g. LiNPr;, LiBu" or Li(CH,), Li) affords the three-membered ring metallacycles [Ir(Cl),(CH2PMeR)(PMe,R),] (133). The crystal structural data of complex (133) (R = Ph) prompted Al-Jibori et aL306to suggest that the base removes a proton from a methyl group of coordinated PMe,Ph to yield a carbanion, which subsequently attacks the metal with removal of the labile Cl ligand trans to PMe,Ph. The geometrical isomers (134a) and (134b), together with their corresponding enantiomers, were thought to be produced upon reaction of mer-[Ir(Cl){PMe(CH,Ph),},] and L ~ N P I - ~On ? ' ~exposure to excess dry HCl, complex (133) is rapidly and quantitatively converted to mer-[Ir(Cl), (PMe,R),]. When treated with Cl,, complex (133) (R = Ph) undergoes ring opening to give mer-[Ir(Cl),(PMe,Ph),(PMePhCH2C1)] (135). Complex (135) converts to the correspondingfac isomer (136) on exposure to visible radiation.306 I1
C'
cI1
Ph
/
PMePhCH,CI
Four- and five-membered ring metallacycles (e.g. 137) have been synthesized via reaction of tertiary aliphatic phosphines (e.g. PBuiPr", PBui Bu", PBui, PPI-~) with [1r(CI)(~ot),]~ in the presence of y-picoline or MeCN. The steric effects of the phosphines and the coligand were observed to determine the composition and the stereochemistry of the The reaction of the alkenic phosphine P(allyl)R, (R = But, C,H,,) with ~r(Cl)(cot),],in the presence of y-picoline produces the metallated complexes [Ik(H)(C1)(CH=CHCH,PR,)(R,PCHCH=CH,)(NC6H,)]. For the complex in which R = But and y-picoline was absent, the five-coordinate complex [I;(H)(Cl)(CH=CHCH,PR,)(R,PCHCH=CH,)] was obtained. The product stereochemistries
-
Iridium
1138
were inferrred from ' H and 31PNMR and IR spectral data.308Reaction between [Ir(Cl)(cot),], and m-FC,H4CH2P(C6H,l)2 affords two isomers in an 8: 1 molar ratio; NMR 13'Pand I9F)spectroscopic data have been interpreted in terms of structures (138a) and (138b), and a nucleophilic m e c h a n i ~ r n . ~ ~ c
( 1 38a)
(137)
-
(138b)
The tertiary phosphine PBu: (C,H,(OMe),-2,6) is known to react with iridium(II1) chloride in propan-2-01 to yield the five-coordinate hydride complex [Ir(H){OC,H3(OMe-3)(PBu:-2)},] (1391, which converts, in air, to the paramagnetic iridium(I1) species ~~(OC6H3(OMe-3)(PBu~-2)),1, which reversibly adds 0, and reacts with NO. The dioxygen adduct [Ir(0,){OC,H3(OMe-3)(PBu;2))J slowly converts to the coordinatively unsaturated C-cyclometallated iridium(II1) complex [TrtCH, CMe,PBu'C,H,(OMe-6)O) (OC,H, (OMe-3)PBuh-2))]. Complex (139) also reversibly adds pyridine and CO."y
49.6.5 Oxygen Ligands 49.6.5.1
Hydroxy ligands
[Ir(OH),] precipitates as a result of the reaction between K3prC16]and KOH under a CO, atmosphere. The product is usually colloidal, and ranges in colour from yellow-green to blue-black. Iridium(II1) hydroxo species (including [Ir(OH)6]3-and [Ir(OH),(H,O)]*- are thought to be present in alkaline solutions of Ir,03.3L0
49.6.5.2
Dioxygen ligands
Molecular oxygen adducts of transition metal Complexes are of interest and importance to catalytic processes and commercial oxidation processes, as well as being intermediates in oxidation reactions. Vaska3" has reviewed the nature of dioxygen bound to transition metal complexes. All known iridium dioxygen complexes possess the 'peroxo' structure (140). Experimental data reveal that the formation of covalent Ir-(0,) bonds on dioxygen addition to IrL, is accompanied by extensive redistribution of electrons, and the electron transfer is from the iridium to dioxygen. SCF-X-SW calculations on [1r(O2)(Ph3)J and [ I T ( P ~ ~ )indicate ~ ] ~ + 'peroxo'-metal b ~ n d i n g . ~ " +
Relative rates of 0, uptake by rrans-[Ir(X)(CO)(PPh,R),] (X = F, C1, Br, I; R = Ph, Et, Me) in CH,Clz follow the order R = Me > Et > Ph and X = I > Br > C1 > F.3'3If dioxygen addition is considered as an acid-base reaction (metal complex = Lewis base), then factors which increase the electron density at the metal centre should (in the absence of unfavourable steric effects) enhance the addition of 0, (reaction 90). The results suggest that as the basicity of the phosphine ligand increases and the electronegatively of X decreases, the rate of 0, addition also increases.313The X-ray crystal structure of [Ir(C1)(C0)(O2)(PPh,Et),]reveals a side-on coordination with the O2 centre, CO and Cl moieties defining the equatorial plane, or a distorted octahedral iridium(II1) .314 system with 0;-
Iridium
1139
The structures of [Ir(X)(CO)(O,)(PPh,),] (X = C1, I3IS3l7)have similar inner coordination spheres, the only significant difference being the considerably longer 0-0 internuclear distance in the iodo complex (1.51 8, for X = I vs. 1.301$ for X = Cl). Based on internuclear distance criteria, the chloro complex could be viewed as a ‘superoxo’ complex with iridium in the I1 oxidation state, and the iodo complex as a ‘peroxo’ complex with iridium in the I11 oxidation state. However, both complexes are diamagnetic, and other similarities between the two complexes leads to an iridium(II1) ‘peroxo’ assignment. CNDO-type molecular orbital calculations on these complexes have shown that the iridium ion has a formal charge of + 3, and that the 0, molecule coordinates to the metal ion through oxidative addition.318 Dioxygen addition to [Ir(A)(CO)(PPh,)J (A = F, OH, NCO, NCS, O,CH, 02CCF3,SH,C, CPh and p-toluenesulfinate) gives the iridium(1II j complexes [Ir(A)(CO)(0,)(PPh,),].3‘9The readily reversible 0, uptake by [Ir(SCN)(CO)(PPh,),] probably accounts for a previous report3I7 that [Ir(SCN)(CO)(0,)(PPh3)2] does not form. X-Ray crystal structure determinations of [Ir(dppe)2(0,)]+,320,321 and [Ir(O,)(PPhMe,),]+ 322 have implied that the 0-0 bond length is essentially independent of the metal and the basicity of the phosphine ligand. Analogous rhodium complexes exhibit similar structures.322A comparison320of the crystallographic data for [Ir(dppe),(O,)]+ and [Ir(X)(CO)(O,)(PPh,),] led to the following conclusions: (i) a short 0-0 bond length (1.30-1.42 8,) corresponds to reversible 0, uptake, while a long 0-0 bond (1.50-1.6258,) corresponds to irreversible O2 uptake; and (ii) increasing the donor strength of the phosphine ligand favours longer 0-0 bond lengths. Conclusion (ii) was and [Ir(0,)(PPhMe2),]+ determined to be invalid based on a comparison of [Ir(dppe)2(02)]+320*32’ (see above).322Conclusion (i) is invalidated inasmuch as the 0-0 bond iength (1.46A) in [Ir(Clj(CO)(O,)(PPh,Et),] would seem to indicate irreversible 0, uptake, though reversible 0, uptake was observed.”’ Furthermore, a redeterminationT2‘of the structure of Dr(dppe),(O,)]+ indicates that the unusually long 0-0 bond distance reported previously is actually an artifact caused by crystal decomposition during irradiati~n.~,’ The preparation of [Ir(X)(CO)(ER,),(O,)] (ER, = AsMePh,, AsEtPh,; X = C1, Br),Z9’and crystal structures of [Ir(O,)(PMe,Ph),]+, [Ir(PMe,Ph)4]+324and the PF, and ClO; salts of [Ir (dppm),(O,)]+ 325 have been reported. In the latter complex, identical 0-0 bond lengths (within experimental error) are observed, as well as similar geometries around iridium. Only different packings were observed for the PF; and ClO; salts of this complex. The conclusions to be drawn from dioxygen uptake reactions and crystal structure investigations of the resultant dioxygen complexes include: (i) the 0-0 bond length is constant, regardless of the metal or the other ligands present in the inner coordination sphere; (ii) the short and long 0-0 bond lengths observed are most likely artifacts caused by disorder and crystal decomposition; (iii) there is no correlation between 0-0 bond length and reversibility of 0, uptake; and (iv) 0, uptake can be viewed as an oxidation process, even if it is reversible. Kinetic measurements have indicated that 0, reacts more rapidly with the five-coordinate complex [Ir(I)(phen)(cod)] (141) than with the four-coordinate complex [Ir(phen)(cod)]+ (142) to yield’ [Ir(O,)(phen)(cod)]+ . The analysis was consistent with complex (141) being more susceptible to The reaction of complex (142; C1- salt) with O2in methanol was oxygenation than complex (142).326 investigated kinetically in the presence and absence of SCN- and I - . On addition of NaX (X = SCN, I), the five-coordinate complex pr(X)(phen)(cod)] (143) forms. An ‘end-on’ oxidative addition of 0, to complex (142) has been proposed (see Scheme 14).327
Scheme 14
An investigation of the thermal deoxygenation of solid [Ir(CI)(CO)(O,)(PPh,),] in nitrogen has revealed two rate-controlling steps, depending on the temperature. The steps are associated with the nucleation and growth of product, and a phase boundary mechanism.328 Complexes of the type [Tr(CO)(PPh,),(Ar)] undergo reaction with dioxygen to yield [IrC.O.C. L K K
~
1140
.I
Iridium
(CO)(O,(PPh,),(Ar)] wherein Ar = Ph, m-MeC,H, and pMeC,H,. However, complexes with reportedly due to steric shielding of the metal centre.329 o-substituted Ar ligands do not react with 02, The stoichiometric reaction of SO, with coordinated O2 to form coordinated sulfate appears to be general for dioxygen complexes. "0 isotope substitution and IR spectroscopy have been employed to study the reaction betwen [Ir(X)(CO)(O,)(PPh,),] (X = C!, Br, I) and SO2 (reaction 91). IR spectral results are consistent with a bidentate sulfate.ligand. The mechanism proposed for sulfate formation involves a peroxosulfate intermediate, that is insertion of SOzinto one of the Ir -0 bonds and bond formation between S and the leaving 0,with subsequent rearrangement of the peroxo-sulfite intermediate thus generated (see Scheme 15).330,33'
Scheme 15
49.6.5.3 ,&Diketonate, catecholate and semiquinone ligands The syntheses and stereochemical assignments of [Ir(H),(L)(PPh,),] (144) (L = acac, tfac, hfac) and [Ir(H)(Cl)(acac)(PPh,),] have been r e p ~ r t e d . ~ ~The ' . ' ~dihydride ~ complex (144) is prepared via reaction of [Ir(H),(PPh,),] with the appropriate j-diketone. TR spectral data for this complex are consistent with the pdiketonate ligands coordinated to Ir as bidentate ligands, and, combined with 'H NMR data, are consistent with structure (144). Both complexes have trans PPh, ligands. Reaction of the iridium(1) complex pr(acac)(C,H,),] with alkenic phosphines in the presence of y-picoline and acetonitrile produces the six-coordinate metaliated iridium(II1) complex [Ir(H)(L)(CH,CH=CH)(R,P)] (R,P = BuiP, (C6H,l)2P;L = y-picoline, MeCN). The ligand L is displaced by CO, CNR and P(OMe),?34 Reaction between allene and the iridium(1) complex [Ir(acac)(~-C,H,,),] at 195 K, followed by treatment with pyridine in heptane at 243 K, yields the iridium(II1) complex [Ir(acac)(C, H4)3(py)](145) with evolution of allene. The crystal structure of complex (145) reveals a distorted octahedral coordination geometry in which the acac ligand and an allene dimer serve as chelating ligands with a pseudo-fac configuration relative to each other. The coordination about Ir is completed by one of the double bonds of the allene molecule and by pyridine.335
PPh3 ( 1 44)
Transition metal complexes containing catecholate and semiquinone ligands have been reviewed by Pierpont and B ~ c h a n a n . ,The ~ ~ syntheses of catecholate adducts of platinum group metals by oxidative addition of benzoquinone (BQ) have been reported (reaction 92).337The coordinated catechol can then be oxidized to the semiquinone with silver ion (reaction 93). In reactions (92) and (93), L,M = [Ir(Cl)(CO)(PPh,),] and the benzoquinones (BQ) are shown in (146). The oxidative addition of phenanthrenequinone to [Ir(Cl)(CO)(PPh,),] has also been
1141
(93)
(146) X = CI. Br
On coordination to metals, the q u i n o n e 4 0 stretching frequency shifts at least 200-300 cm-', which infers a formal reduction to catechol with oxidation of the metal. The possibility of an intermediate radical anion coordination mode for o-benzoquinoneshas been d e m ~ n s t r a t e d by ~ ~the ' reversible chemical and electrochemical oxidation of reduced quinone complexes. ESR spectra of [Ir(Cl)(1,2-02C6C14)(CO)(PR3)2](PR, = PPh,, PMePh,) indicate that the unpaired electron is localized in the semiquinone ligand. The quinone coordination mode is strongly affected by the oxidizing ability of the quinone and the nature of the complex itself.
49.6.5.4
Carboxylate, chelating carboxylate C a -, CG-, RCO; and oxdate k a n d s
Iridium(]) complexes of the type trans-[Ir(X)(CO)(PR3),] (PR, = PEt,, PPh,, PPhMe,; X = C1, Br, I) undergo rapid oxidative addition with carboxylic acids RCOIH (R = €3, Me, CF,, Ph, 1-naphthyl) to yield the iridium(II1) complexes [Ir(H)(X)(O,CR)(CO)(PR,),] corresponding to both formal cis and trans addition of RC0,H to the Ir' complex. In solution, the carboxylato complexes undergo rapid anion exchange such that at equilibrium two additional hydride complexes, [Ir(H)(X),(CO)(PR,),] and [Ir(H)(O,CR)(CO)(PR,),], are present. For all the complexes, the phosphine ligands are mutually trans, the H and CO ligands are mutually cis, and 0,CR is unidentate.340The ease of protonation of the If complexes by RC02H to yield the Ir"' complexes has been observed341 to be dependent on the nature of PR,, yielding an order of ligand basicity PMe, > PMe,Ph > PMePh, > PPh,. [Ir(X)(ER&(L)] (L = N,, CO) is known to react with peroxycarboxylic acids, RC03H,affording the carboxylato complexes [Ir(X)(O,CR),(ER,),J (147; X = C1, R = 3-CIC6H,, ER, = PPh,, PPh,Me, AsPh,; X = C1, R = Ph, ER3 = PPh3, AsPh,; X = C1, R = Me, ER7 = PPh,; X = Br, R = 3-CIC,H4, ER, = PPh,) and COz. These complexes possess one unidentate carboxylate ligand and one bidentate ligand yielding a coordination number of six. The stereochemistry is revealed in (147). In contrast, reaction between [Ir(I)(CO)(PPh,),] and RC0,H has yieIded [Ir(O,CR),(PPh,),] (148; R = Ph, 3-CIC6H4),and a mixture of [Ir(O,CPh),(CO)(PPh,),] and [Ir(I),(CO)(PPh,),] with RCO, H = dibenzyl peroxide. A tentative stereochemical assignment for complex (148) has been shown.'52
(147)
(148)
The formation and decarboxylation kinetics of [Ir(OCO,)(NH,),]+ have been studied as a function of pressure, based on activation volumes for CO, uptake (AV* = -4.0cm3mol-') and decarboxylation (A V t = + 2.5 cm3mol-'), and partial molar volume measurements. The suggested mechanism for the formation and decarboxylation is given in Scheme 14. Bond formation during CO, uptake and bond breaking during decarboxylation were believed to be cu. 50% completed in the transition state (149) of these processes.342 The peroxocarbonato iridium complexes [ir(OCO,)(CO)(PPh,),(R)] (R = Me, Ph) have been synthesized from [Ir(CO)(O,)(PPh,),(R)] and liquid CO,, probably via external attack by CO, on
Iridium
1142
Scheme 16
NH3 (149)
coordinated 02.343 The reaction mechanism was thought to be similar to that proposed for the reaction of hexafluoroacetone with [Ir(C1)(CO)(O,)(PPh,),].wThe mechanism of the addition of hexafluoroacetone (CF, C(O)CF,) to ~r(X)(CO)(O,)(PPh,),] presumably involves direct nucleophilic attack of CF,C(O)(CF, on the coordinated 0, ligand to give the iridium(II1) ozonide product (151)via an ionic transition state (150;see Scheme 17).344
Scheme 17
The bis(alky1peroxy) iridium(I1i) complexes [Tr(X)(O, Bu'), (CO)(ER,),] (152; X = C1, ER, = PPh,, AsPh,, PPh,Me; X = Br, ER3 = PPh,, AsPh,) and [Ir(X)(O,CPhMe,),(CO)(ER,),] (153; X = Cl, ER, = PPh,, AsPh,; X = Br, ER, = PPh,) have been prepared via reaction of But02H or PhMeC0,H with the appropriate trans-[Ir(X)(CO)(ER,),] (X = C1, Br) complex. The analogous reaction with trans-[Ir(I)(CO)(PPh,),J affords [Ir(I),(O,CR)(CO)(PPh,),] (154) (R = But, PhMe,). The diiodide complex reveals IR and 'H NMR spectra interpreted in terms of the two PPh, ligands being mutually trans, and possessing either structure (154a) or (lHb).M5 Spectroscopic data on complexes (152)and (153)suggest the structure in (155).15'Treatment of (152) or (153) with organic acids R'C02H results in the substitution of only one of the alkylperoxy ligands, with formation of [Ir(X)(OCOR')(O,R)(CO)(PPh,),] (X = C1, R = But, R = CF,, CCI,, CHC1,. CO,H, cis-CH=CHCO,H, Ph, H; X = C1, R = CMe,Ph, R = CF,; X = Br, R = But, R' = CF,, HI."' The related complexes [Ir(X)(Y)(O,But)(CO)(PPh,),] (X = Y = C1, Br; X = C1, Y = Br, NO3) have been prepared via reaction of [I~(X)(O,BU')~(CO)(PP~,),] (X = C1, Br) with HC1, HBr or HNO, at 273K.M6
( 154a)
(1 54b)
(155)
-
Several iridium(II1) oxalato (ox) complexes have been reported, including K, [Ir(ox),], H,O, KH,[Ir(ox),] *4H,O, K3[Ir(C1)3(N02)(ox)]-2H20,M,[Ir(Cl),(ox)] (M = Na, Rb, Cs, NH,), M,[Ir(Cl),(0n)~]-nH,0,M,[Ir(Cl),(NO,),(ox)] -nH,O(M = Li, NayK, Rb, Cs, NH,) and H,[Ir(ox),] 'n-
Iridium
"
1143
H20.3'0The synthesis of K,[Ir(ox),] has been reported by several groups.347More recently, Flynn and D e m a ~ ,~have ' reported a simple high-yield preparation of this complex with a novel solvent extraction step. Various methods, including IR,Raman and 'H NMR spectroscopy, thermograviometry and pH invariance results, have been employed to determine the role played by water in the structures of crystalline transition metal oxalato complexes. For K, [Ir(ox),] * 4.5H20, a tris-chelated The structure with one water molecule hydrogen bonded in a unique fashion has been rates of racemization of F ~ ( O X ) have ~ ] ~ -been measured,349as well as the circular dichroism.350While K3[Ir(Cl)2(oxj2]exists as cis and trans isomers (cirr-[Ir(Cl),(ox),] 3H20 and fra~s-[Ir(Cl),(ox),]-4H,O) K, [Ir(ox)J *4.5H20 has been resolved into optical enantiomers which do not racemize in hot' water.35' Addition of the bis(trifluoromethy1)nitroxy radical [(CF,j 2 N 0-1 to [Ir(Cl)(CO)(ER,),] yields the complexes [Ir(C1)(CO)(ER3)2{ON(CF,),}] (ER, = PPh,, PPh2Me, AsPh,). Several iridium(II1) complexes containing the ON(CF,), ligand have been prepared via oxidative addition to [Ir(CO)(PPh,)2{ON(CF,),)].155 The stable diradical CF,N(6)CFzN(O)CF, reacts with trans-[Ir(Cl)(CO)(ER,),1 (ER, = PPh,, PPh,Me, AsPh,) and rrans-[Ir(CO)(PPh,), {ON(CF,), >] to form the seven-membered metallacyclic iridium(II1) complexes [~r{ON(CF3)CF2CF2N(CF3)O}(C1)(CO)(ER3)2] (156) and [~r{ON(CF,)CF,CF,N(CF,)O} {ON(CF,),)(CO)(PPh,),]. IR spectra have been interpreted in terms of structure (156) for the former complex; the iradacycle is presumably fluxional.IM CF3 p \ ,.o
tR3
c.1,
I
% ', p
rF2
oc4r\o
!F2
LR3 L ~ / CF3
49.6.5.5 Nitrito-nitro and nitrate ligands
Several iridium(II1) complexes containing the nitrito (NO,) ligand have been prepared via reaction of N,O, (or NO;) with an iridium(1) complex, as per reactions (94b(99).352 The nitrito ion [Jr(ONO)(NH,),]z+ rearranges to the nitro form [Ir(N0,)(NH,),]2+ in aqueous base according to equation (100),for which k, = 3.0 x 10- , s - ' and koH = 2.7 x 10--5dm3mol-'s-I . A base-catalyzed pathway was proposed3S3for the Co"', Rh"' and Ir"' complexes, as depict4 in Scheme 18. The temperature and pressure dependencies of the rates of linkage isomerization of pr(ONO)(NH,)5]2+ in aqueous solution indicate A V f = - 5.9 & 0.6cm3mol-', A H f = 95.3 f 1.3kJmol-' and ASt = - 1 1 f 4 JK-' mol-'.354The similarity between these results and those previously obtained for Rh"' and Co"' analogues355are indicative of an intramolecular mechanism.
(H,N),IrNH2
*+ ----- NO-2
-
I(H3N),1r-(NH2}l2+
[(H3N),Ir(X)]
2+
I
Seheme 18
(CO)(PPh,),] results from oxidative addition of NO; to [Ir(X)(CO)(PPh,),]. Several reaction pathways have been cited, including (i) reaction of [Ir(Cl)(CO)(PPh,)2] with concentrated HNO, in TH F at 313 K;359 (ii) reaction between [Ir(X)(CO)(PPh,),] (X = Cl, Br, N,, NCO, NCS) and (NH,),[Ce(NO,),], [Cu(NO,),] or [Fe(NO,),] in the presence of NO; at ambient temperatures;m or (iii) reaction of the dioxygen adduct of [Ir(CI)(CO)(PPh,),] with N02/N,0,.36’The related complex [Tr(C1)(NO3),(C0)(AsPh,),l was prepared via pathway (iii).361 The green iridium(II1) complex [Ir(NO,), (NO)(PPh,),] is formed by the action of ethanolic moiety has been proposed on HNO, on [Ir(NO)(PPh,),] or [Ir(NO)(CO)(PPh,),]. A bent Ir-N-0 the basis of the low v(N0) in the IR spectrum.358 Reaction of mer-[Ir(Cl),(ER,),] (ER, = PMe,Ph, PEt,Ph, AsMe,Ph) and AgNO, in acetone/ water solutions forms [Ir(Cl),(NO,)(ER,),], with the structure in (157a). In a similar fashion, [Ir(I)2(NO,)(PMe,Ph),] has been synthesized from mer-[Ir(I),(PMe,Ph)3].’62 Complexes of the type [Ir(Cl)(NO,)(X)(PPh,),] (X = C1, Br, I, N3,NCO, NCS) are prepared via electrophilic attack by 0, on the bent NO ligand in [Ir(C1)(X)(NO)(CO)(PPh3),]in benzene solution at ambient temperature, The rates of oxygenation decrease with X as I > Br > C1 > NCS > NCO > N3,363 which approximates the decreasing electron release by X. When [Tr(NO,),(NCO)(CO)(PPh,),] is allowed to react in EtOH/CH, Cl, solutions exposed to air, yellow crystals of [Ir(H)(NO,),(PPh, )J (157b) result. An X-ray crystal structural analysis of complex (157b) reveals the PPh, ligands as mutually trans, with one unidentate and one bidentate nitrate ligand (see structure 157b). Complex (157b) reacts with CO to yield [Ir(H)(NO,)*(CO)(PPh,),], in which both NO; ligands are ~ n i d e n t a t e .The ~ ~ related iridium(II1) cation [Ir(N03),(CO)(PMePh,),]+ (isolated as the BF; or C10; salt) has k e n synthesized by the reaction of NO,/N, O4with [Ir(CO)(O,)(PMePh,),]+ .365 Other iridium(II1) nitrato complexes which have been prepared are outlined in reactions (101)-(106).366 c1
I ,9.”’
PPh3
I 8P i ’
ONOz
K3E-Ir--ER3
I‘
O2NO-1r-O
I‘
H
R3E
C1
PPh3
0
/
1145
Iridium 49.6.6
Sulfur Ligands
49.6.6.1 Su&de ligands
Reaction between H,S and trans-[Ir(Cl)(CO)(PPh,),l in CH,Cl, leads to the formation of one of the first iridium(II1) complexes containing an SH ligand, namely [Ir(H)(Cl)(SH)(CO)(PPh,),l. This complex possesses a pseudooctahedral coordination geometry with trans PPh, ligands and trans H -1r-C1 and HS-Ir-CO arrangements. The oxidative addition of H,S to Vaska's complex is cis addition.367 [Ir(C1),(SEt2),][Ir(C1),(SEt2),]and fac-[Ir(CI),(SEt,),] can be prepared from an iridium(II1) halide and a monosulfide. The anion [Ir(Cl)4(SEt,),]- can be oxidized to trans-[Ir(Cl)4(SEt,),].368 Irradiation offac-[Ir(Cl),(SEt,),] gives [Ir2(Cl]6(SEtJ4],which exists in two isomeric forms.16' Also, the complexes [Ir(Cl),(SEt,),-,(py),,](n = 1, 2), [Ir(Cl),(SMe,),] and [Tr2(C1)S(SR2)4] (R = Me, Et) have been reportedim though not well characterized. Photolysis (d-d excitation) of mer[Ir(Cl)3(SEt2)3]in benzene or toluene solution yields a mixture of [Ir,(C1),(p-C1)2(SEt,)4] (158) and [Ir2(Cl)s(p-C1)(p-SEt,)(SEt,)3] (159).369The unusual bridging SEt, group in (159) was identified by the absence of rapid inversion at the tetrahedrally coordinated S atom in the ' H NMR spectrum. The crystal structures of complexes (158)370and (159)37'have been determined.
(159)
( 1 58)
Hydrogen sulfide, 4-methylbenzenethiol and Cmethylbenzene-1,2-dithiol add to truns[Ir(Cl)(CO)(PPh,),] to yield the iridium(II1) complex [Ir(H)(Cl)(CO)(PPh,),(SR)],wherein SR is SH, SC6H4Meor SC,H,MeSH, respectively. Spectroscopic data are consistent with the structure (160) with CO and SR groups trans to each other.372 H
49.6.6.2 Dithiocarbamate and CS,-based ligands
The X-ray crystal and molecular structures of the iridium(II1) dithiocarbamate complex
[Ir(S2CNC4H,)3](C6HS)0.5 shows a trigonally-distorted octahedral metal-ligand coordination sphere, A comparison with analogous Fe"' and Cr"' complexes indicates that the indium complex is closest to octahedral in its en~ironment.~'~ The dithiocarbamato and 0-dithiocarbarnato derivatives [Ir(H),(S-S)(PPh,),] (S-S = R2NCS;, ROCS; ;R = Me, Et) were prepared by treating the appropriate chloro or carboxylato complex with sodium salts of the S-S ligands. [Ir(S,CNMe,), (CO)(PPh,),]' formed upon reaction of Vaska's complex with tetramethylthiuram disulfide; the assigned stereochemistry is that illustrated in (161).The stereochemistry assigned to [Ir(H),(S-S)(PPh,),] is depicted in (162). IT( and NMR spectroscopy have confirmed the presence of a substantial barrier to rotation about the S2C-NR, bond in (161), but nothing comparable was observed for the S,C-OR bond in (162).374
I
s,f S ph, B *"'
sir\co L S (161)
s,
f'":"
QCH PPh,
162)
1146
Iridium
The iridium(II1) hydrides truns-[Ir(H)(Cl),(PPh,),] and mer-[Ir(H),(PPh,),] react with CS, to yield the dithioformato complexes [Ir(Cl),(S,CH)(PPh,),] (163) and [Ir(H),(S,CH)(PPh,),j (la), respectively. Both complexes exhibit I R absorption at ca. 1215-1235 and 900-930 cm-' , attributable to V(S,CH). Proton NMR spectra established the stereochemistry of these complexes, as shown in (163) and (164).375,376 The equivalent coupling of the dithioformate proton to two ' H nuclei (as in 164) clearly favours the symmetrical structure for the dithiofonnate ligands, as opposed to any asymmetric alternatives. CI
( 1 63)
PPh,
( 164)
Insertion of CS, into [Ir(H)(CO)(PPh,),] has been monitored by NMR spectroscopy. A definitive assignment of the product, either IIr(S,CH)(CO)(PPh,),] or [Ir(CS, H)(CO)(PPh,),], could not be made owing to the lack of documentation on the SH proton chemical shift. However, the observed signal at 5 = 5.0 p.p.m. may be indicative of [Ir(CS,H)(CO)(PPh, j2]?" Carbon disulfide reacts with [Tr(H)(CO),(PPh,),] to yield an iridium(II1) species containing two CS2 molecules bonded to each Ir atom. The proposed formulations for this species included [Ir(SSCH)(COj(PPh,), (z-CS,)] and [Ir(CSSH)(CO)(PPh,),(.n-CS,II.Here again, no definitive structural assignment was put forward as no NMR signal was observed nor an analysis reported.377 However, the dithioformato ligand is most likely unidentate in either case. It has been proposed37s that the complex obtained by the addition of CS, to [Ir(I)(CO)(PPh,)2] has the orientation isomers of configuration (165) and (166). However, Deeming and Shad7' have suggested that [Ir(Cl)(CO)(PMe,Ph),(CS,)] exists as only one of these isomers in solution, as we11 as in the solid state.
49.6.6.3
Other S-containing ligands
The first sulfenato and alkylsuifenato complexes of iridium(iI1) were prepared by George and Watkin~.~" The sulfenato complexes [Ir(C1),(MeSO)(CO)(PR3),](167; PR, = PPh,, PPh, Me) were prepared from [Ir(Cl)(CO)(PR,),] and MeS(0)Cl; the alkylsulfenato complexes [Ir(Cl),(ROSCd)(CO)(PR3)J (168; PR3 = PPh,, R = Me, Et; PR3 = PMe,Ph, R = Me) were obtained via reaction of [Ir(CI)(CO)(PR,),] and the appropriate ROS(0)CI. These complexes do not rearrange to yield Me),] sulfinato complexes. The preparation of the sulfinato complex [Ir(CI)2(MeS0,)(CO)(PPh, (169) and the thio complex [Ir(CI),(SC,H,Me-p)(CO)(PR,),] (170; PR, = PPh,, PPh2Me)have also been reported."' Their stereochemistries were assigned on the basis of 'H NMR and IR spectral data. Oxidative-elimination products of the type [Ir(X)(SC,H4Y){SC,H,(N02)2}(CO)(PPh,)]2 (171) were prepared from trans-[Ir(X)(CO)(PPh,),] (X = C1, Br, I) and a series of 2,4-dinitrophenyl CY-phenyl disulfides (Y = NO,, Br, F, H, Me, MeO; reaction 107). The S-S bond in the unsymmetrical disulfide ArS-SAr' is cleaved by Vaska's complex, and the resulting fragments then coordinate to Ir to form a dimeric iridium(II1) complex, as depicted in Scheme 19.38'
Iridium 2[Ir(X)(CO)(PPh3),] f 2GrS-SAr'
-
1147
[Ir(X)(CO)(PPh,)(SAr)(SAr')],
+
2PPh3
(107)
PPh,
PPh,
(171)
Scheme 19
The oxidative addition of sulfonyl chlorides [RSO, Cl] to Vaska's complex gives the iridium(II1) sulfinate complex [Ir(Cl),(RSO,)(CO)(PPh,),](172; R = Me, Et, Pr', p-MeC6H4,p-C1C6H4,p N0,C6H4) in which the SO, group is bound to Ir through the S atom. A tentative structure (172) was suggested based on 'H NMR spectra1 ir~terpretation.~'~ From the addition of RSO, to [Ir(Cl)(CO)(PMe,Ph),], IIr(Cl),(RSO,)(CO)(PMe,Ph),] is obtained, with stereochemical arrangements analogous to (172).379 R I
o=s=o
(
172)
The organoperthio group RSS- is capable of complexing with truns-[Ir(C1)(CO)(PPh3)2] to form [rr(cl),(ssc6F,)(CO)(PPh,),].~87The reaction is analogous to the addition of RSH,"' RSSR381and RSC1380to square planar iridium(1) complexes. The monomeric iridium(II1) complexes [Ir(S-S), (q-CsMe,)] (S-S = S,CNMe,, S,PMe,) have been prepared via reaction of [Ir(Cl),(q-C,Me,)], and excess dithioacid anion S-S- .Analytical and IR, 'H,I3C and 3'PNMR spectroscopic results were interpreted in terms of the occurrence of uniand bi-dentate dithioacid exchange, probably via a dissociatively controlled intramolecular mechanism.384Another report3*'on the preparation of [Ir(S,CNMe,),(q-C,Me,)] suggests that the complex contains one uni- and one bi-dentate dithio ligand. The preparation of [Ir{S,C,(CN),}(q-C,Me,)] was also rep~rted.~"Complexes of the type [Ir(H),(Rdtc)(ER,),] (173) [ER, = PPh,, AsPh,; Rdtc = piperidine dithiocarbamate (Pipdtc), piperazine dithiocarbamate (Pzdtc), N-methylpiperazine dithiocarbamate (Me-Pzdtc), morpholine dithiocarbamate (Timdtc)] have been prepared and characterized by IR and 'H NMR spectroscopy. The stereochemistry proposed for these complexes is shown in (173)."'
PPh3
(173)
Selenium Ligands
49.6.7
The electrochemical behaviour of the iridium(TI1) complexes [Ir(dppe),(L,)]+ (L, = Se,, S,, 0,) has been investigated,3sRrevealing the dissociation of L,s consequent to the addition of one electron to the antibonding orbital of the n: component of the Dewar-Chat-Duncanson model for ML, bonding. The mechanism for the electrolytic reduction is given in Scheme 20. The progression of the C 0 C' 4-
KK'
Iridium
1148
first reduction wave (A) to more negative potential for L, = Se, + S, -,O2was taken to be indicative of the n-back-bonding interaction enhancing in the same direction; thus, a stepwise destabilization of the lowest unoccupied molecular orbital follows the same order. Two iridium complexes containing Se donor ligands have been prepared via oxidative addition of carbon diselenide and areneselenols to tr~ns-[Ir(Cl)(CO)(PPh~)~]?~~ As such, addition of CSe, to Vaska's complex produced [Ir(Cl)(CO)(CSe,)(PPh,),] (174), while addition of p-MeC,H,SeH resulted in [Ir(H)(Cl)(CO)(PPh,),(SeC,H,Me-p)] (175). The IR spectrum of complex (174) exhibits a low v(1r-C1) value, suggesting that the C1 ligand is not trms to CO. However, the IR data are consistent with the configuration in (174), and are analogous to those reported for [Ir(Cl)(CO)(CS2)(PPh2Me),].379 The 'H NMR spectrum of complex (175) is consistent with a structure in which the two PPh, ligands are cis to the hydride ligand and arms to each other. The suggested configuration of [Ir(H)(Cl)(CO)(SeC,H,Me-p)(PPh,),] is that given in (175), based on 'H NMR and IR spectral interpretation .789
Scheme 20
(174)
(175)
[Ir(Cl)(CSe)(PPh,),] readily undergoes oxidative addition with Cl,, 0, and AgC10, in MeCN to yield [Ir(Cl),(CSe)(PPh,),], [Ir(Cl)(CSe)(O,)(PPh,),] and [Ir(CSe)(PPh,),(MeCN)](ClO,),respectively. However, reaction of [Ir(Cl)(CSe)(PPh,),] with BH; and PPh3 affords the Ir-Se complex [Ir{H),(SeMe)(PPh,),]. Also, reaction between [Ir(Cl)(CSe)(PPh,),j and CSe, gives the complex shown in (176), which on reaction with Me1 leads to complex (177).390 :eMc
PPh3
f"".
*+, ,."'
C',
[
sccd Irc+'
I
I%%,
1: ,c
I &-\,. \.':.
S-j,[',G
I
PPh3 Se
PPh3
C
Ij
SeMe ( 1 77)
( 1 76)
The seleninato iridium(II1) complexes [Ir(H), (XPhSeO,)(PPh,),j (X = H, p-C1, m-C1, m-Br, rn-NO,) have been synthesized from [Ir(H),(PPh,),] and the appropriate sodium benzeneseleninate salt. The monomeric diamagnetic complexes have the hydride ligands in a cis arrangement and the RSeOl ligand coordinated through simultaneous 0, Se bonding, as in structure (178).3''
(178)
49.6.8
Halogens as Ligands
Iridium(II1) fluoride is prepared by reducing [IrF,] with metallic iridium at 323 K. [IrF,] is black, possesses a hexagonal close-packed structure and is relatively inert toward !
1149
Iridium
The red octahedral complex [TrCl,] results from direct chlorination of metallic iridium at 773 K;," two foms of [TrCI,] are known, both water insoluble.392The dark green hydrate, [IrCl,]*3H20,is obtained upon heating the oxychloride [Ir(Cl),(OH)] -3H20in HC1.343 have recently reviewed the various preparative routes and structural Ferguson and chemistry of iridium(II1) chloro and bromo complexes. In general, iridium(II1) chloro complexes of the type M3[IrC16]-12H20(M = Li, Na), M,[IrC1,]-H20 (M = K, Rb, Cs, NH,), M,[IrCl,] (M = Na, K, NH,) and M2[Ir(C1)s(H20)](M = K, Rb, Cs, NH,) are prepared via the reduction of M,[IrCl,] by reagents such as Fe2+,39s ethanedioate ( ~ x a l a t e ) , ~ ~or" ~hydrogen ~' ~ulfide.~~~,'"''' Factors affecting product composition include chloride ion concentration, choice of M, method of recrystallization and solution age. The crystal structures of iridium(II1) chloro complexes depend on M as well as the presence or absence of water of crystallization. Single-crystal X-ray structures of K,[IrCl,], K,[IrCI,] *H,O and (NH,),[IrC16] *H,O reveal them to be isostructural with their rhodium analogues, thou& each complex belongs to a different space The aquation of [IrC&]'- yields [1r(C1)5(H2)]2-,[1r(C1j4(H,0),]~and [Tr(C1),(H,0),].401 The activation entropy associated with this reaction was suggestive of a dissociative mechanism. In the presence of added salts, the calculated activation entropies were attributed to the orienting effect of the cation (e.g. Li+) on the water adjacent to the [1rCl6]'- anion.402Irradiation of K,[IrCl,] doped into NaCl reportedly yields [IrC1,l4-, based on ESR data.403 The reddish-brown [IrBr,] is also known, and thought to be formed upon treatment of '[IrBr,]' with bromine.393More likely, however, is that [IrBr,] is grey-brown in colour and obtained via direct bromination of metallic iridium at 843 K under 8 atm pressure.404Additionally, the hydrated complexes [IrBr,]*H,O, [IrBr,] -3HBr.3H2O and [IrBr,] *Hpr-H,O are thought to exist.310 Iridium(II1) bromo complexes of the type M,DrBr,], M,[IrBr,] 12H,O, M,[IrBr,] *H,O and [Co(NH,),][IrBr,] have been cited. They are, in general, prepared via reduction of [IrBr6]2-,3'o treatment of [Ir(OH),] with HBr?" or treatment of the iridium(II1j chloro analogue with HBr.394,w,4"7 The hydrated complexes K,[IrBr,] .4H,Oa5 and KJIrBr,] .3H,OmSare to actually be (H30)K,[IrBr,3,.9H,O. A single-crystal X-ray structure determination of Rb,[IrBr,] .H,O has shown it to be isostructural with (NH,),[IrCl,] .H,0.394bX-Ray powder photographs, along with comparative analyses, suggest that the complexes [cO(NH,)6][Irx6] (X = C1, Br) crystallize in the cubic space group Pa3.394b Preliminary single-crystal X-ray on (NH,),[IrBr,] -H,O, Cs,[IrBr,j *HzOand (H,O)Ks[IrBr6],.9H20, as well as Rb,[IrBr,].H,O, have revealed the [IrX,I3- anions to be distorted as a consequence of the non-spherical packing of the cations and, in some cases, of the water molecules surrounding the anions. The reaction between Ir203and HI presumably gives [IrT,], while [IrIJ- can be synthesized from Additionally, the hydrated complex [IrI,] 3H,O has k e n r e p ~ r t e d . ~ " [TrT,] -3H,O and KI.310*401 )~= ] Rb, Cs, Me,N) have The dinuclear complexes M3[Ir2(C1)9](M = Rb, Cs) and M , [ I T ~ ( B ~(M recently been prepared. Heating M2[Ir(Cl)5(H20)]under vacuum to 623 K, followed by cooling and treating with water, led to the isolation of impure samples of M3[Ir2(C1)9].The M3[Ir2(Br),] complexes result from treatment of M, [IrCl,] .H,O or M, [Ir(Cl)s(HzO)] with HBr.394b
-
49.6.9 Hydrogen and Hydrides as Ligands The five-coordinate iridium(II1) dihydrates [Ir(Hjz(Cl)(PR3)2](PR, = PBu;, PBuiPh) have been synthesized from IrCl,.xH,O and PR,, and have the structure depicted in (179). Reactions of complex (179) with Nal, CO and H, are summarized in reactions (lOXF(1 10).290b Hydride complexes of the type [Tr(H),(CI),-,(PRBu~),] (n = 1, R = Me; n = 2, R = But) react with NaBH, to yield the non-fluxional complexes [Ir(H), ( B H , ) ( P R B u ~ ) , ] . In ~ ~toluene, ~ [Ir(Cl)(N,)(PPh,),] and HC1 react to yield [Ir(H)(Cl)2(PPh,),]; in ethanol, however, CO is abstracted from the solvent affording [Ir(H)(Cl),(CO)(PPh,), j EtOH.409Reaction between [Ir(Cl)(cod)], and HCN produced the unusual five-coordinate species [ir(H)(Cl)(CN)(~od)].~'*
-
Iridium
1150
Oxidative addition of H, to iridium(1) complexes yields a variety of six-coordinate iridium(II1) dihydrides. Dihydrogen reacts with [Ir(Cl)(CO)(PPh,),] in benzene at 298 K to produce [Ir(H)>(C1)(CO)(PPh,),];411the reaction is reversible and a kinetic investigation has been rep~rted.~" [Ir(Cl)(PPh,),J reacts with H, at 298 K and atmospheric pressure to give [Ir(H)2(Cl)(PPh,),].4'3The reversibly reacts with H,, affording four-coordinate iridium(1) complex [Ir(dppe),] [W W 2 (dppe),l+ .414 The addition of hydrogen halides (ens. HCI) to iridium(1) complexes also affords iridium(II1) hydrides. The stereochemistry of these additions to [Tr(Cl)(CO)(PPh,)2]414b and related complexes has been studied in some In the solid state or in non-polar solvents cis adducts are formed, while the use of polar solvents gives mixtures of cis and trans adducts. Reaction between [Ir(Cl)(PPh,),] and HCl gives [Ir(H)(Cl),(PPh,),]; the corresponding arsine and stibene complexes are sirnilarb obtained.413 [Ir(dppe),]+ undergoes oxidative addition with HCl to yield [Ir(H)(Cl)(dppe),]+ .414 Oxidative addition of HCN to [Ir(Cl)(CO)(PPh,),] gives [Ir(H)(CI)(CN)(CO)(PPh,),].418 Additionally, pentafluorobenzenethiol oxidatively adds to [Ir(Ph)(CO)(PPh,),] to produce [Ir(H)(Ph),(CO)(PPh,),], and to [Ir(Cl)(CO)(PPh,),] under mild conditions to give the same p r o d u ~ t . ~ " Five-coordinate iridium(1) complexes may react with hydrogen halides (HX) to eliminate a ligand and form Ir-H and Ir-X bonds. Both [Ir(CO)(PMe,Ph),]+ and [lr(CO),(PMe,Ph),]+ react with Dimethylphenylarsine is displaced from [Ir(CO)(AsHCl to yield [Ir(H)(Cl)(CO)(PMe,Ph),]+."'" MezPh),(PMe,Ph),]+ by HCI, yielding [Ir(H)(CI)(AsMe,Ph)(CO)(PMe,Ph),]+?20 Thiophenol reacts with Fr(Cl)(CO)(PPh,),], from which [Ir(H)(Cl)(Ph)(PPh,),] is 0btained.4'~ Substituted thiophenols react similarly with Vaska's complex.410-419.421 Salts of [Ir(H),(CO),(ER,),]+ (ER, = tertiary phosphine or arsine) have been prepared via reversible hydrogen displacement of CO from [ W O ) , (J3R3),1.422 The preparation and interconversion of two isomeric iridium(1II) trihydrides, fac- and mer[Ir(H),(CO)(PPh,),] (180), have been studied. Under a hydrogen atmosphere, both isomers undergo spontaneous interconversion to a dynamic equilibrium. The kinetic data obtained from the interconversion and H, displacement from both isomers by PPh, suggest that the interconversion process occurs via a reversible reductive elimination-oxidation sequence. Both processes are believed to involve the intermediate species [Ir(H)(CO)(PPh,),] (181). It has further been postulated that the interconversion process proceeds by slow unimolecular loss of H2 (reductive elimination), followed by a rapid readdition of H, to the coordinatively unsaturated intermediate species (181), as depicted in Scheme 21.423 +
'i
Ph3P%+ ++ Ph,P-Ir-H
I
H'
co
&
IIr(H)(CO)(PPh3),l + H2
e
7
Ph3$.+e+ OC-Ir-H
I
H'PPh,
(1x1)
(180)
Scheme 21
The addition of Na,[IrCl,] to a hot ethanolic solution of PPh3followed by NaBH, addition gives mer-[Ir(H),(PPh,),] on coo1ing;fac-[Ir(H),(PPh,),] is obtained by evaporating the filtrate obtained under reduced pressure.424The structure of mer-[Ir(H),(PPh,),] 0.5C6H6shows a very distorted octahedral coordination geometry about Ir with three adjacent sites occupied by P atoms.425Both fac- and mer-[Ir(H), (PPh,),] react with carboxylic acids in boiling 2-methoxyethanol to give [Ir(H),(02CR)(PPh,),] (R = Me, p-tolyl, p-C1C6H,, p-N0,C,H4).426 The oxidative addition of HBF,, CF,S03H and BuS0,H to trans-[Ir(Cl)(L)(PPh,),] (L = CO, N2) has afforded the highly reactive iridium(II1) complexes [Ir(H)(Cl)(X)(L)(PPh,),](X = BF4, CF,SO,, BuSO,) (182), in which X can easily be substituted by 0 and K donors. The stereochemistry of the hydride complex (182) is The dimeric hydrido complex [Tr(H)(X),(CO)(PR,)], (X = C1, Br; PR, = PEt,, PPh,) has been synthesized by carbonylation of iridium halides followed by phosphine addition; the complex
-
Iridium
1151
contains halide bridges:" Hydride bridges are present in [Ir2(p-H)3(H)Z{PPh3)4](PF6) (183) for which X-ray structural, IR and 'HNMR s ectral data have been interpreted in terms of the presence of an iridium-iridium triple bond (2.518 )>"
w
H
( 1 82)
(183)
The trimeric complex [{ Ir(H)2(PCy,)3(C,H,N)},(p-H)]2+ (184; N = Pyridine) has been characterized by X-ray crystallography, IR and 'H NMR spectroscopy, and found to contain a tricoordinate hydrogen ligand within a triangle of indium at0ms.4'~The tetranuclear complex anion [Ir4(H)(CO)l,]- has been isolated as the tetraalkylammonium salt by treating [Ir4(CO)12]with K2C03and CO in The dihydride species [Ir4(H)2(CO)II] was produced upon treatment of the anion with acetic acid:,' In concentrated H,SO,, slow dissolution of [IL,(CO),~]gives rise to a non-isolable solution thought to contain [Ir4(H)Z(C0)1Z].2+ .431 The dihydrido anion [Ir4(H)2(C0)10]2p (185) was synthesized in two stages: [Ir4(CO)lZ]reacts with ROp in ROH (R = Me, Et) to yield [lr,(CO,R)(CO),,]-, which, when added to alkaline hydroxide, gives the diani~n.,~'The X-ray crystal structure of complex (185) is quite similar to that of [Rh,(CO),,], wherein the iridium atoms form a tetrahedron and there are three bridging CO ligands. Inasmuch as two terminal CO ligands are missing from the basal plane, the hydride ligands were suggested to occupy these positions. The two Ir-Ir bonds trans to the H ligands are longer (2.802 A) than the remaining Ir-Ir bonds (2.71 A). The presence of hydride ligands was also confirmed by the hydride NMR resonance (6= - 16.15p.p.m. relative to SiMe,) observed in the 'H NMR spectrum of the dianion (185) in CD,CN.432
( 1 84)
(185)
The hydrido-bridged dinuclear complex [(PR,),Rh'(p,-H)(p2-Cl)I~1'(H),(PEt, ),I (186a; PR, = PEt3, or 2PR3 = dppe) was produced upon treatment of [IT(H)~(PE~~),] wrth [Rh(y Cl), (PR3)J2. An X-ray crystal structure determination reveals a distorted octahedral geometry about Ir and a distorted square planar geometry around Rh.433The similar complex [(dppe)Rh1(p2H)21r1'1(H)2(PPIJ3)2] has been prepared and characterized by X-ray crystal structure and NMR analyses. Its structure is similar to that of complex (186a).434Another hydrido-bridged dinuclear has the structure shown in complex, [(dppe)Rh(~i~-H)~Ir(PEt~)~]+,
PEt, ( 1 86a)
( 1 86b)
I152
Iridium
Reaction between [Ir(H),(OCMe2)2@Ph,),]+ and [M(H),(Cp),] (M = Mo, W) has yielded
[(PPh3)2(H)Ir(p-H)2(q-a:l-5-q-C,H,)M(Cp)J+. Intermediate formation of [(PPh,),(H),Ir&H)>M(Cp),]+ was reportedly observed, and kinetic parameters were obtained for the C-H insertion reaction. Reversible hydride bridge cleavage for M = W occurs with pyridine and acetonitrile!36 The hydrido-bridged bimetallic complexes (187)-(190) have been synthesized from trans[Pt(L),(MeOH)(R)]+ and [Ir(H)5(L')z].Complex (187) reacts with CO at room temperature to yield [Ir(H),(CO)(PEt,),]+. Reaction of complex (187) with ethylene gives (19Ob); complex (190) reacts with H, at moderate temperatures and pressures to produce [(PEt,),Pt(p-H),Ir(H),(PEt,),]+.437
r
L
l+
(187a) R = H, L = L' = PEt,
(187b) R = H, L = L' = PEt,
(188a) R = Ph, L = L.' = PEt3
(188b) R = Ph, L = L' = PEt,
(189a) R = H, L = PEt,, L' = PPr,
(190b) R = Et. L = L' = PEt,
(190a) R = Et, L = L' = PEt,
49.6.10 Mixed Donor Atom Ligands 49.6.10.1
Mentate N-C ligands
Organometallic intramolecular coordination complexes consist of those which have at least one M-C bond and at least one group forming an intramolecular coordination bond. Iridium(1II) complexes containing an N donor intramolecular coordination bond are well known, and conform to the five-membered ring structure theory first proposed by Matsuda et al:438organometallic intramolecular coordination complexes tend to form five-membered ring structures. Transition metal organometallic intramolecular coordination complexes containing an N donor ligand have been reviewed by Omae.439 Reactions of trans-[Ir(Cl)(N,)(PPh,),] with benzylideneamines have afforded the o-metallated complexes (191) for X = CH, N (see reaction 112).440Whereas square planar palladium(I1) benzylidenearnine complexes are formed via a substitution reaction of she metal with the hydrogen at the ortho position, octahedral iridium(II1) analogues are more probably formed by an insertion reaction arising from the affinity of the Ir atom to the H atoms (reaction 112). The insertion mechanism involves g ( N ) coordination of the imine ligand, which brings an ortho or alkenic CH group close to the Ir atom, followed by oxidative addition of the ortho or alkenic CH group to the Ir atom. Treatment of complex (191) with AgC10, and then with C,H,,NC or CO (= L) gives rise to the formation of complex (192) with the metallacyclic ring remaining intact (see reaction 113)."' CI
(191)
h3
_H
Iridium
1153
Five-membered cyclometallated complexes similar to (191) are produced from the reaction between alkenic imines and iridium complexes. The reaction of alkenic imines with [Ir(Cl)(C8H,4)2]2 in the presence of cyclohexylphosphine yields complex (193),"* which, when treated with Cl,, undergoes substitution without Ir-C bond rupture to give (194; reaction 114)."' Azobenzene or its derivatives react with ~r(Cl)(C8H14),], in the presence of PR, (R=Ph, cyclohexyl) according to reaction (1 12) to yield the cyclometallated complex (195)."',"''2
(195) R = Ph, Mc,tolyl, xylyl: R' = H, Me
Two products are formed via the reaction of trans-[Ir(Cl)(CO)(PPh,),] with p-fluorobenzenediazonium t e t r a f l u ~ r ob o r a t e . " One ~ ~ ~ is a red, air-stable, diamagnetic tetrazene complex,"53M6 while the other is the yellow, air-stable diamagnetic complex (196). The X-ray crystal structure of the acetone solvate of complex (196) has been determined. Various substituted arene diazonium ions react with [Ir(Cl)(CO)(PPh,),] and its analogues in the presence of lithium chloride, benzene/ethanol, or benzene/propan-2-01to yield o-metaliated aryldiazene complexes (197) (where X = C1, Br, The complex F, I, OC10,; R = H, F, Br, C1, Me, CF,, NH,, NOz, OMe; and Y = BF;, C10;)."7a (197) may undergo deprotonation, affording an aryldiazenato complex (198), as well as hydrogenation in the presence of a palladium catalyst to produce an o-metallated arylhydrazine complex (199). The X-ray crystal structure of (196) and (197), dichloro(4-methoxyphenyldii~nide)-C~,N~-bis(tripheny1phosphine)iridium-chloroform (1: I), has been determined. The indium atom displays a distorted octahedral coordination and is bonded to the terminal nitrogen atom.M9
r F
7
1'
R
(197)
( 196)
l+
R
(198)
(199)
An insertion product is obtained upon reaction of [ I T ( H ) , ( P P ~ ~with ) ~ ] p-anisolediazonium tetrafluoroborate, and undergoes o-metallation to yield [Ir(H)(NHNC,H,OMe)(PPh,),] (BF,) (200).450The neutral complex [Ir(H)(X)(NHNC,H,OMe)(PPh,),] (201) results from the action of hglides X- on complex (200). The reaction of complex (201) with halogens, X,, affords [Ir(X), (NHNC,H, OMe)(PPh,),]. The syntheses of phenyldiimide complexes have been reported by Toniolo and c0workers.4~'One example is the o-metallation reaction of the aryldiazenato iridium(II1) complex (202) with phenylacetylene (reaction l 15) to yield complex (203). +
[Ir(CI)(CO)(N,C6H4R-p)(PPh,),](BF,)
+ HC=CH
EtOH or C6H&cetone
(202)
IIr(Cl)(CO)(NH=NC, H,R-p)(PPh3)2(C_CPh)]' (BFJ
(2031
b
(1 15)
1154
Iridium
The reaction of Na, [IrCl,] in 2-methoxyethanol with benzo[h]quinoline (bhqH; 204) yields [Ir(Cl)(bhq),], (205), in which the bhq ligand is coordinated at N-1 and C-10 atoms. Proton NMR and IR spectra support a structure similar to that of the rhodium(II1) analogue (205). Complex (205) reacts with L (= PBu;, SEt,) to give [Ir(Cl)(bhq),(L)] (206).452 P
(205) M = Rh, Ir
(204)
49.6.10.2
(206)
Bidentate N-S ligands
Alkyl and aryl isothiocyantes, RCNS, parallel CS, in their capability to insert into M-H bonds The and yield products having the N-alkyl- or N-aryl-formamide chelate ligand RN=CH=S. iridium(iI1) complexes [Ir(X),(RN=CH-S)(PPh,)2] (207; X = C1, Br; R = Me, Et, Ph) can be prepared from trans-[Ir(H)(X), (PPh3)J and the appropriate alkyl or aryl isothiocyanate. Two isomeric forms of (207) were deemed plausible. with (207a) the preferred stereochemistry for the alkyl products and (207b) for the aryl products.4s3 PPh3
X
PPh,
(20%)
(207a)
49.6.10.3
Bidentate S-C ligands
The X-ray crystal structure of [Ir(CO)(PPh3)(q2-SCNMe,),]+ (as the BF, salt) reveals that both IrSCNMe, units are virtually planar and coordinated via C and S, with bond distances comparable to other q2-SCNMe2complexes. The coordination geometry about Ir is a distorted ~ctahedron.~" IR spectra on the products of reactions (1 16) and (1 17) have also been interpreted in terms of q2 coordination of the SCNMe, ligand, via C and S.4s4Interestingly, the product of reaction (1 17), trans-~r(C1)(CO)(PPh,),(q2-SCNMe),)] *, has been reported4" to be formed in the absence of additional C1-; whereas, in the presence of C1-, [Ir(Cl),(CO)(PPh,),(q' -SCNEt)] was formed.3n Thus, it would appear that SCNMe, is capable of q2coordination via displacement of C1- from Ir"', whereas SCNMe is not. The reaction of [Ir(H)(CO)(PPh,),] with Me,NC(S)Cl results in immediate that exists in two precipitation of an intermediate complex, [Ir(H)(CO)(PPh,),(q2-SCNMe,)]", isomeric forms (208a) and (208b). Reaction of complex (208) with Et3N in C,H, affords complex (209), [Ir(CO)(PPh,)(pSCNMe,)l,, as in reaction (1 1X).454 In the complex analogous to (2081, [Ir(H)(Cl)(CO)(PPh,),(q'-SCNMe)], y ~ coordination ' of SCNMe is present and C1- is not displaced from the coordination sphere.455 [Ir(C1),(PPh3),] + Me,NC(S)CI 3 cis-[Ir(Cl),(q2-SCNMe2)(PPh3),] (1 16) [Ir(CI)(CO)(PPh,),] seee*
/
NMe2
""m ,."'
+
l+
(208a)
r
3
4iL
/
NMe2
~~u~~-[I~(C~)(~~-SCNM~,)(CO)(PP~~),]+ (1 17)
l+
rh:H *++ P
+
blibH PPh,
Me2WC(S)C1
oc'\ M,ce-s,z h If,N,
7
(118) NMe,
Ph,P
PP~,
(208b)
3
LH,
(209)
Iridium
1155
49.6.1 1 Multidentate Macrocyclic Ligands
'
.
The iridium(II1) octaethylporphyrin (OEP) complexes (210)-(212) have been prepared and characterized spectroscopically by Ogoshi et al.lg3(reactions 119-121). Scheme 22 depicts various reactions of complexes (210)and (212)to yield hydrido- (213)and organo-iridium(II1) (214k(217) porphyrin complexes. The preparation and influence of axial ligands on the central iridium atom of several iridium(II1) octaethylporphyrin complexes has been reported. The complexes investigated include [Ir(X)(CO)(OEP)] (218;X = C1, Br, ClO,, CN, BF,, Me) and [Ir(L)(OEP)(Me)] (219; L = no ligand, N-methylimidazole (mim), pyridine, NH,, CN). For the carbonyl complexes (218), the CO stretching frequencies and absorption peak wavenumbers varied in the order Me-, Br- z C1-, ClO; z BF,, that is in the order of decreasing electron-donating tendency of the axial ligand X. Proton NMR studies were employed to determine the cis and trans influence of the porphyrin ligand. A linear correlation between the chemical shift of the mem protons (cis influence) 1.3V has been observed. These and the reversible redox potential (1r1/Ir1")in the range + 0.4 to iresults were explained456in terms of the change of electron density caused by chelation of the axial ligands depending on their electron-donating tendency; the order to this tendency varies for complexes (218) and (219) as: CO < no ligand < py < NH, < mim < CN- < C10; NN BF; < C1- NN Br- < Me-.
Scheme 22
The reaction of meso-porphyrin IX diethyl ester with [Ir(Cl)(CO),], or [Ir(Cl)(CO)(cot),] yields the iridium(II1) porphyrin derivative [Ir(CO)(HePDEE)] (HePDEE = hematoporphyrin diethyl ester).457This complex, along with the dimethyl ester derivative, has been characterized by electronic, IR, electron spin resonance and mass spectroscopic techniques as well as magnetic susceptibility measurements. The iridium complexes differ from their rhodium analogues in that they retain a CO ligand in the coordination shell.
49.7 IRIDIUMUV) 49.7.1 Phosphorus and Arsenic Ligands The paramagnetic iridium(1V) complexes trans-[Ir(Cl),(ER,),] (ER, = tertiary phosphine or arsine) have been prepared via chlorine oxidation of (ER,H)[Ir(Cl),(ER,),].458 These iridium(1V) complexes are considered attractive as potential oxidizing agents inasmuch as they are soluble in a variety of organic solvents and thus as oxidizing agents of organic molecules and organometallic complexes!s9 Chlorine oxidation of [ I r ( Q (PMe,),]- in CHCl, yields drans-[Ir(Cl),(PMe,),], for which IR spectra were interpreted in terms of irans C1 ligands.460The electronic and MCD spectra of the D4*iridium(1V) complexes trans-[Ir(CI),(ER,),] (Er, = PMe, Ph, PEt,Ph, AsPG) have re-
1156
lridium
vealed LMCT bands occurring at exceptionally low energies. These transition energies aid in the approximation of the bonding orbitals of ER3 with respect to the metal; and from these energies, correlations were made between the energy of the ligand CJ orbital and the Ir-ER, bond stability. The LMCT transition parallels the reducing power of ER3.&’The trm-[Ir(Cl),(PMe,Ph),] complex .is reduced to truns-[Ir(Cl),(PMe,Ph),]- at 0.90V (vs. Ag-AgCl) in MeCN. The utility of this complex as an oxidizing agent with fM(Cp),] (M = Ni, Fe), [Pt(C,H,)(PPh,),] and trans[Pd(Cl),(AsMe,),] has been investigated!62 For example, with [Pt(C,H,)(PPh,),], the iridium(II1)platinum(I1) complex [(Cl), (PMe,Ph),Ir(p-Cl), Pt(PPh,),]+ is f0rmed.4’~ The iridium(1V) complexes [Ir(RCO,),(ER,)(RNC)] (ER, = AsPh,; RNC = pMeC,H,NC; R’ = MeCH(O)CO,, o-OC,H,CO,, PhCH,CH(NH)C02, eNHC,H,CO,), [Ir(&C6&O)7(ER,)(RNC)] and [Ir(o-NHC,H,O),(ER,)(RNC)] have been prepared from the iridium(II1) com19‘ plex [Ir(W),(AsPh,),(p-MeC,H,NC)]. On treating [Ir(ER,),]+ (ER, = P(OMe)Ph,, PMePh,) with excess I, in CH,Cl, or acetone, a complex of stoichiometry [Ir(I), (ER,),]Is obtained. An X-ray structure determination of the complex of [Ir(I),(PMePh,),] established this compound as a salt of formula [Ir2(I)@ MePh,),]’ (I);, as depicted in (220).463The coordination about Ir is a distorted octahedron, and the complex has a triple I bridge with three I atoms shared by both Ir atoms. There is no Ir-Ir bond (> 3.5 fQ.463
’
r
L
49.7.2
J
Oxygen Ligands
49.7.2.1 Hydroxy ligands
The complex K,[Ir(OH),] can be synthesized by treating Na,[IrC16] with KOH, and has been characterized by absorption spectroscopy, thermogravimetric analysis and voltage-current curves.- CdfIr(OH),] possesses a tetragonal structure and serves as a magnetically dilute insulator, while Zn[Ir(OH),] has a cubic structure and exhibits an anomalous magnetic behaviour.&’
49.7.2.2
Oxalate ligands
The oxidation of [Ir(ox>,l3- by cerium(IV) in aqueous acidic sulfate or perchlorate media has given [Ir(ox)J2-. That the product is indeed [1r(ox),l2- and not [Ir”’(ox),(C,O; .)I2- finds support from the slow reaction observed in the presence of excess cerium(1V); free or coordinated oxalate radical anion would be expected to react rapidly with Ce’”. In solution, [Ir(ox),I2- undergoes slow pseudo-first-order reaction to yield the highly reactive intermediate [If1’(ox),(C20; -)I2-. 49.7.2.3 Nitrato ligands
The iridium(1V) nitrato complexes M2pr(N03)6] (M = K, Rb, Cs), isomorphous with K2[l?t(N03)6]and possessing magnetic moments consistent with a d Selectronic configuration, are attained from the reaction of M,[IrBr,] with N2OS.ESR spectra at 77 K reveal extensive x bonding The ESR spectrum of K,[Ir(NO,),] in K,[Pr(NO,),] was found between Ir and the NO, !igand~.*~ to be isotropic at 77 K, indicating that any T, ground state Jahn-Teller term must be dynamic. Considerable Md + LX delocalization was shown by the hyperfine constants. The presence of symmetrically bidentate NO, groups in K,[II-(NO,)~]was supported by TR, Raman and ESR spectroscopic analyses!68 The solution properties, IR spectra, magnetic properties and X-ray powder diffraction data for M2[Ir(N03),1 have been reported more recently.469
Iridium 49.7.3
'
.
1157
Sulfur Ligands
Chlorine oxidation of the iridium(II1) anions [1r(C1)5(SPh2)I2-,[Ir(Cl),(SPh,)]-, cis- and trans[1r(Cl),(SMe2),]- and [Ir(Cl)4(L)]p(L = RS(CH2),SR, RSCH=CHSR, 1,2-C6H4(SR)2;R = Me, Ph; n = 2, 3) has led to the formation of the corresponding iridium(1V) complexes. The Ir" complexes [1r(C1)4(L)] have a cis-octahedral geometry. All complexes have been characterized on the basis of IR, Raman and electronic absorption spectra, as well as 'H NMR spectra?"
49.7.4
Halogens as Ligands
The TV oxidation state is the most stable for iridium halogeno complexes. The preparation of [IrF4] has been reported,47'though the actual product is probably [IrF,]. A subsequent paper472 reported the formation of [IrF,] from [IrF,] and iridium metal; on heating [IrF,] disproportionates to [IrF,] and [IrF,]. Additionally, the formation of [IrF,] in high yields (95-100%) was reportedly obtained upon hydrogen reduction of [IrF,] or [IrF,]. Polyisotopic mass spectra, visible and IR spectra, as well as chemical analyses and X-ray powder patterns support the proposed composition.473(NO),[IrF,] has been synthesized by the action of NOF and F, on metallic iridium, and analyzed by resonance Raman spectro~copy:~~ Hexachloroiridates(1V) may be prepared by (i) chlorinating a mixture of iridium powder and an alkali metal chloride; (ii) in solution, by the addition of an alkali metal chloride to a suspension of hydrous IrO, in aqueous HCl;475or (iii) by the oxidation of [IrCl6I3- by O2 in acidic media.47M78 Oxidation by 0,probably occurs via reaction (122), with Eo = 1- 0.21 V.I4Fine479has suggested that the reverse of reaction (1 22) occurs in alkaline media. The black crystalline sodium salt Na, [IrCl,] is water soluble, and is the usual starting material for the synthesis of other iridium(1V) coordination complexes. The resonance Raman spectrum of (NBu,), [Ircl,] reveals anomalous polarization of all bands; this has been ascribed to Jahn-Teller distortions present in the excited electronic The single-crystal electronic spectra of various [IrCl,]'- salts have also been reportedM0 4[IrC1,I3-
+ 0,+ H+
-+
4[IrC1,I2-
+ 2H20
(122)
Reduction of @C1,l2- by a variety of agents has been cited. The reduction of [IrX,]'- (Cl, Br) by 2-thiouracil and 2-thiopyrimidine follows second-order rate laws, first-order with respect to both the concentration of the oxidant and the reductant. A pH-rate profile as well as activation parameters have been presented, and a free radical mechanism proposed.4s' Reduction in aqueous alcoholic solution by aquated electrons, hydrogen and alkyl radicals was investigated by pulse radiolysis, and found to result in the production of [IrC1,]3-.482A kinetic examination of reaction (123) in aqueous solution shows that this outer-sphere electron transfer reaction proceeds by parallel paths first and second order in substrate (SCN-)."3 The kinetics of the oxidation of hydrazine by [Ir(Cl)4(H,0)2], [Ir(C1),(H2O)]-, [IrCl6I2-, as well as [IrBr,]'-, in aqueous acidic perchlorate solution have been 6[IrC&I2- + SCN-
+ 4H20
+
6prC16]3-
+
Sod2- + CN-
+
8H+
(123)
The j-j model has been employed to assign ligand-to-metal charge-transfer bands in the absorption and MCD spectra of (NBu,), { trans-[Ir(Cl),(Br),]) and (NEt4)2{ trans-[Ir(Br),(C1)2]).485 Irradiation of CT or 6 d absorption bands of [IrCl,]'- results in substitution and redox processes.486 Irradiation at 254 nm (CT bands) produces [Ir(Cl)5(H20)]-and [Ir(Cl)5(HzO)12-;by contrast, d-d band irradiation yields only [tr(Cl),(H20)]-, except in the presence of added CI- when [IrC1,I3- is formed. Salts of [IrBr612-are obtained by repeated treatments of the corresponding chloro complexes with HBr. The reaction between [IrBr,]*- and Cr" proceeds, at least in part, through an inner-sphere redox mechanism in two stages. Initially, Ir" and Cr" react via parallel inner- and outer-sphere paths to give [(Br),Ir"'BrCr"'(H,O),], and [IrBr6I3- and [Cr(H,O),I3+, respectively. Subsequently, the binuclear species dissociates via a parallel bond-rupture process to yield [Ir(Br)5(HzO)]2- and [Cr(Br)(H,0)5]2+?ST The reaction between [IrC1,l2- and Cr" also proceeds through parallel innerand outer-sphere reactions.488A kinetic investigation of reaction (124) in aqueous solution reveals a similar mechanism to that exhibited in reaction ( 123).483 2[IrBr,12-
+
21-
+
2[IrBr,I3-
+
I,
( 124)
Iridium
1158 49.8 49.8.1
IRIDIUM(V) AND IRIDIUM(V1) Iridium(V)
Very few iridium(V) ( d 4 )complexes are known; for those known, hydride or halogen ligands are usually present. The iridium(V) fluoro complex [IrF,] has been prepared via reduction of [IrF,] with silicon or d i h y d r ~ g e nIn . ~the ~ ~gas phase, [IrF,] possesses D3,,symmetry,489while in the solid state, a fluoridebridged polymeric structure with bridges mutually cis is indicated on the basis of IR4', and 19FNMR s t ~ d i e s . 4M[IrF6] ~~ (M = alkali metal) has been obtained from the reaction of [lrBr3], KBr and BrF,."91 (NO)[IrF6], [SeF,][IrF,] and [SeF,][IrF,], have also been reported.492 The pentahydride complexes [Ir(H), (PR3)J (PR, = PMe3,4Y3 PEt2Ph494) have been prepared and characterized by IR and NMR spectroscopy. [Ir(H),(PMe,),] exhibits one of the most intense Ir -H absorptions at 1920cm-'. The "P NMR spectrum of this complex established the presence of five magnetically equivalent hydride The IR and 'H NMR spectra of [Ir(H),(PEt,Ph),] have also been reported.494
49.8.2
Iridium(V1)
Direct fluorination of metallic iridium at 543K has produced the rather unstable, yellow d 3 complex [IrF,], which is isomorphous with its osmium(V1) analogue. [IrF,] tends toward dissociation, yielding fluorine and lQwer iridium fluorides. The octahedral [IrF,] species decomposes in water; this is accompanied by ozone liberation. The magnetic moment of [IrF,] is pefr = 3.3 BM at 300 K.492b
49.9
CATALYTIC ACTIVITY OF IRIDIUM COORDINATION COMPLEXES
Iridium coordination complexes are known to serve as homogeneous catalysts in a wide variety of reactions, including hydrogenation of C-C multiple bonds, isomerization of alkenes, dienes and alkynes, and oxidation of various organic substrates. In general, the iridium complex catalysts are less active than their rhodium analogues, presumably due to the increased activation energy required for substance coordination as well as the rearrangement within the coordination sphere. IridiumII) complexes are well suited for hydrogenation catalysis owing to their ready capability to undergo addition. Vaska-type complexes, pr(X)(CO)(PPh,),], iridium(1) and iridium(II1) hydride complexes have been shown to catalyze such reactions efficiently; the substrates catalytically hydrogenated by these complexes are collected in Table 3. The isomerization of alkenes and dienes, as well as the oxidation of several organic substrates, are catalyzed by iridium complexes in homogeneous solution; these are summarized in Tables 4 and 5 , respectively. Several other types of reactions are catalyzed by iridium complexes in 'homogeneous solution as well; among them are ring-opening, disproportionation, deuterium exchange, cyclotrimerization, SO, elimination, C-H bond activation, decarbonylation (and carbonylation) and hydrogen transfer processes. (221) In the presence of catalytic amounts of [Ir(Cl)(CO)(PPh,),], exo-tricyclo[3.2.1.0234]oct-6-ene rearranges selectively and quantitatively to 5-methylenebicyclo[2.2.Ilhept-Zene (222) upon heating ~ ~ ~similar conditions, exo,exo-tetracvat 403 K in benzene (reaction 1 2 5 ~Under clo[3.3.1 .02".0678]nonane(223) rearranges to exo-6-methyIenetricyclo[3.2.1 .02,4 joctene (224) (re&tion 126).495
1159
Iridium Table 3 Hydrogenation Catalysts c a t alp1
Suhrrate
Ketones Alkenes Alkenes, alkynes Maleic acid Alkenes Acid aters Unsaturated substrates Unsaturated substrates Unsaturated substrates Unsaturated substrates a-Alkenes, hex-1-ene a-Alkenes, hex- I-ene Ethylene, acetylene Butadiene Dimethyl maleate Cycloocta- I &diene, hexa- 1,3-diene, norbornadiene, butyraldehyde, alkynes 4-t-Butykyclohexanone Diphenylacety lene Hept-1-ene Hex- I-ene Hex- I-cne Hex- I-ene Cnd
Cyclohexenes Cyclohexenes Alkenes, alkynes
Refs. 514
500,515 516 517 518-521 522 523 523 5 23 523 524
524 525, 526
527 528 529,530
531 5 32 533 534 534 534 535 536 536 537
Table 4 Isomerization Catalysts Catalyst
Substrule
Refs.
The disproportionation of cyclohexa- 1,6diene to benzene and cyclohexane is catalyzed by [Ir(X)(CO)(PPh,),] (X = C1, I), [Ir(Cl)(CO)(AsPh,),] and [Ir(H),(Cl)(CO)(PPh,),]~% The iridium(1) complex [Ir(cod)(MeCN),j+ reportedly catalyzes both the disproportionation of cyclohexa- 1,3diene to benzene and cyclohexene, as well as the isomerization of cyclohexa-l,6diene to the conjugated isomer.4Y7[Ir(Cl)2(C5Me;)lz catalyzes the disproportionation of acetaIdehyde to acetic
1160
Iridium
acid and ethanol in water in the absence of base.498The disproportionation of PhMeHz is also catalyzed by Vaska's complex.4g9 Barefield et report that [Ir(H),(ER,),] (ER, = PPhEt,, PEt,, PMe,) catalyzes the exchange between D, and benzene, probably via oxidative addition of the C-H bond to a coordinately unsaturated intermediate species. Also, Vaska's complex catalyzes the exchange between D, and C2H4,concomitantly with hydr~genation.~"" [Ir(Cl)(N,)(PPh,),] reacts with dialkyl acetylenedicarboxylates to form complex (225) via N, displacement. The complex (225) may react with an alkyne at 383 K to yield the iradacyclopentadiene complex (226) and then hexasubstituted benzenes (Scheme 23)."'
R
Scheme 23
The elimination of sulfur dioxide from arenesulfonyl chlorides is catalyzed by Vaska's complex,5o2 while this same iridium complex apparently catalyzes the aromatization of various 1,Cdihydro derivatives of aromatic hydrocarbon^.^'^ The diphosphine complexes [Ir(dppe),]+ and [Ir(dppe)(cod)]+ reportedly catalyze the decarbonylation of aldehyde^.^^^^^^^ Methanol carbonylation is catalyzed by Pr(Cl),] * H 2 0to yield acetic acid. The active species is thought to be an acetyl iridium(II1) species, wherein the rate-determining step involves electrophilic attack of this species on methanol. The effects of added iodide ion on reaction rates have also been in~estigated.~'~ Hydrogen transfer reactions are catalyzed by several iridium complexes, including the dimethyl sulfoxide (DMSO) complexes cis- and trans-[Tr(CI),(DMSO),] -, [lr(Cl),(DMSO),] and [Tr(H)(C1)2(DMS0)3],507 as well as the cyclooctadiene (cod) complexes [ I r ( C l ) ( c ~ d ) ] ,and ~ ~ ~[Ir(3,4,7,8Me4phen)(cod)]+ and trans-[Ir(C1)(CO)(PPh,),l.510Vaska's complex catalyzes the conversion of p-methoxybenzoyl chloride to the corresponding a1dehyde.'l0 The dimethyl sulfoxide iridium(II1) complexes catalyze hydrogen transfer from propan-2-01 to unhindered cyclohexanones to yield cyclohexanols,507while the cod complexes serve as catalysts in the transfer of hydrogen from propan-2-01 to alkenes, ketones508and ct,fl-unsaturated ketones.509 Additionally, [Ir(X)(CO)(PPh,),] (X = C1, Br) catalyzes the cleavage of tetramethyl- 1,2dioxetane (tmd), presumably via reactions depicted in Scheme 24. *
Scheme 24
The redistribution of siloxanes has been observed5" to be catalyzed by Vaska's complex. The water-gas shift reaction is homogeneously catalyzed by iridium complexes, including [Ir(L), (cod)]+ where (L)2 is (PMePh,),, (PPh,),, dppe, phen, 4,7-Ph2phen, 4,7-Me2phen, 3,4,7,8-Me4phen or phenS (phenS = 4,7-diphenyl-l ,IO-phenanthrolinesulfonate). The catalytic activity increases for those catalysts with bidentate (L,) ligands and is sensitive to temperat~re.~"
-
Iridium 49.10
1161
AD INTEGRATUM
The activity in the chemistry of iridium remains unabated. Since the writing of this chapter, several hundred papers have been published that are germane. These have been integrated below in terms of ligands bound to Ir.
49.10.1
Nitrosyl Ligands
The iridium(1) nitrosyl complex [Ir(NO)(PPh,),] reacts with perfluorocarboxylic acids RCO,H in the presence and absence of 0, to yield [Ir(NO)(O,CR),(PPh,),]. A crystal structure determination of the trifluoroacetate derivative [Ir(NO)(O, CCF,),(PPh,),] reveals an essentially trigonal bipyramidal coordination about Ir, with an equatorial linear NO ligand, unidentate carboxylate ligands NMR spectroscopy and gas uptake/evolution measurements have eluand axial PPh, ligands.548 cidated the role of O2 in the conversion of [Ir(NO)(PPh,),] to [Ir(NO)(02CCF3)2(PPh3)2].sg The reaction of [Ir(NO)(dppn)(PPh,),](PF, j2 or [Ir(X)(NO)(dppn)(PPh3),1(PF,) (dppn = bis(2’pyridy1)pyridazine; X = C1, Br, I) with ~d(Clj,(PhCN),] or with [Pt(I),(cod = cyclooctadiene) IR and 31PNMR gives mixed metal binuclear species, e.g. [Ir(NO)(dppn)(PPh,),(PdCl,)](PF,),. spectroscopic results and conductivity measurements suggest a structure possessing a bridging NO, i.e. [Ir-N(0)-Pd].5’0
49.10.2
Nitrogen
The compounds [Ir(cod)L,](BF,) have been prepared for which L is benzonitrile (bzn), phenylacetonitrile (PhCH, CN), 4-methoxybenzonitrile (CMeObzn) and 2-chlorobenzonitrile (2-Clb~n).~” IR spectra are consistent with the Coordination of the benzonitrile ligand via the free electron pair of the nitrogen atom. Also, the complexes [Ir(cod)L,]A, where L = 2-methylpyridine, 2-ethylpyridine, 5-nitro-6-methylquinoline,quinoline, 4,7-dichloroquinoline and [Ir(cod)(L-L)]A, where L is succinonitrile, 8-aminoquinoline, ethylenediamine, 1,2-diphenylethylenediamine, and N,N,N’,N‘ptetramethylethylenediamine,have also been synthesized.’” Nitriles, Na(RCN),, add to azide ligands of [Irm,)(CO)(PPh,),] to give the 5-R-tetraazolato complexes [Ir(CO)(N,CR)(PPh,),] (R = Me, CF,).S52Amino-, phosphino- and thionitriles also add, via, 1,3-~ycloaddition,to azide ligands of [Ir(N,)(CO)(PPh,),] to give corresponding 5-R-tetraazolato complexes.’” The dinuclear iridium(1) complex [Ir(CO)(PPh,)(p-pz)IZ(pzH = pyrazolyl, C3N,H,) is afforded via reaction between [Ir(Cl(CO)(PPh,),1 and Napz.jj4 The analogous complex [Ir(cod)(p-pz)], is obtained by reaction of [IrCI(cod)], with pzH, followed by CO and PPh, treatment. The product reacts with I, or Br, to form [IrX(CO)(PPh,)(p-pz)], (X = I, Br), and with C1, to give [Ir(Cl)(CO)(PPh,)(4-Clpz)],.555 The synthesis and characterization of a number of iridium(1) complexes containing the bidentate, chelating pyrazolylgallate ligand MqGapz has been reported.556 Crystal structural studies of [Ir(cod)(Me,Gapz)], [Ir(CO), (p-pz)], and [Ir(CO),(p-3,5-Me2pz)] have confirmed the expected boat conformations for the six-membered M-(N--N),-Ir rings (M = Ir, Ga). Oxidative addition of Me1 to the iridium(1) complex [Ir(p2-C8H,,)(N(SiMe2CH2PR2)2}] (R = Ph, Pr’) yields monomeric, five-coordinate complexes of the type [Ir(I)(Me){N(SiMe,CH2PR,), )I. Spectroscopic and crystallographic measurements reveal the complexes to have a square pyramidal geometry with the methyl ligand in an apical position. In the presence of H2, these complexes rapidly form the iridium(II1) amine hydrides [ir(I)(H)(Me)[NH(SiMe,CH,PR,),}] with a stereochemistry corresponding to an overall trans addition of H2.’It appears that H, activation by these iridium complexes occurs via an oxidative additionlreductive transfer pathway rather then a direct heterolytic cleavage of H,. [Ir(Cl), (PPh,)J reacts with NaN, in alcoholic solution giving ~is-[1r(H)~(N,)(F’Ph~),1; on exposure to light, the product releases H, and [Ir(N,)(PPh,),] in a reversible manner. The crystal structure of the &hydride shows one H ligand is trans to N, and the other H ligand is trans to a PPh, ligand.’” Base hydrolysis of cis-[Ir(Cl),(en),](Cl) gives cis-[1r(0H)(en),(H20)](S20,) in 80% yields. By contrast, a 3:2 mixture of cis- and trans-[Ir(OH)(en),(H,O)](ClO,), is obtained from the base as indicated by chemical analyses, electronic spectra and hydrolysis of trans-[Ir(Cl),(en),](Cl), w7 and potentiometric titrations. The concentration acidity constants are: K,,= 10-6.290*0
I Iridium
1162
= 1 0 - s o ~ 7 ~ 0 0 0 9moll-' for the cis isomer, and K,, = 10 4798k0004 and K a2 = 10-7~u56*0010 moll a2 for the trans isomer.559The properties of the iridium(II1) species [Ir(en),],[Cu,C1,]C1,~2H2O were characterized by EPR spectroscopy, magnetic susceptibility measurements and X-ray diffractometry. The crystal structure suggests layers of magnetically inequivalent Cu, Cli- dimers with antiferromagnetic intradimer coupling. The dimers appear well separated by large [Ir(en),P+ dimagnetic complexes.560 Complexes of the type [Ir(H),L(PPh,),]A ( L = 8-methylquinoline, caffeine) each posses a C -H---Ir bridge, structurally characterized for the methylquinoline case where A = SbF,. On the reaction trajectory, the authors propose that for a given metal basis of the C-H + M -+ C-M-H species capable of breaking C-H bonds,s6'conformational and steric effects will play an important role in determining whether cyclometallation or attack on an external substrate (e.g. alkane) will occur. Moreover, it would seem that sterically uncongested metal systems will favour the occurrence of alkane activation, rather than cycl~metallation.56' The triply o-metallated cornplexfa~-[Ir(ppy)~] (ppy = 2-phenyl pyridine) has been characterized in its ground and luminescent excited states.s62The dichloro-bridged dimers [1r(Cl)LJ2 (L = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq)] were investigated by ',C and IH NMR and by absorption and luminescence spectroscopy. The NMR results confirm previous formulation of the complex as dichloro-bridged o-metallated dimers in halocarbon solvents, but indicate they are cleaved to monomeric species of the type [Ir(Cl)L,(S)] (S = solvent, e.g. DMF) in ligating solvents S . Comparison of the results suggests that the o-metallated ligands are considerably higher than bipy or phen in the spectrochemical series.563The observations are consistent with the synergistic function of the o-metallated ligands as both strong c donors and 7~ acceptors. cis-[Ir(bipy),(OSO,CF,),](CF,SO,) has been isolated and employed in the synthesis of new hydrido coinplexes of iridium(II1): cis-[Ir(H)(bipy),(PPh,)](PF,), and .ci~-[Ir(H),(bipy)~](PF,). Additionally, an improved high-yield synthesis of [Ir(bipy)J(PF& has been reported.5a The two bis bipyridyl complexes have interesting spectral and electrochemical properties. Ohashi and Nakamura565have obtained the emission decay curves of cis-[Ir(CI),(bipy),](Cl), cis-[Ir(Cl),(phen),](Cl), cis-[Ir(Cl),(4,7-Me2phen),](C1) and cis-[Ir(C1),(5,6-Me2phen),](C1). The spectra exhibit a dependency on the solvent character of the DMF/H,O mixed solvents at 77 K and at 298 K. It appears there are at least two kinds of emitting states for these species in DMF/H,O solvents. The iridium(II1) bipyridyl complex [Ir(C3,N'-bipy)(bipy),]'+ is an attractive photosensitizer in aqueous bromide solutions. Based on product analysis, steady-state emission and pulsed laser experiments, the photosensitization proceeds along a mechanism that involves two exciplexes and the transients Br; and H0,.566 The complexes of p-dimethylaminoanil and of 3-acetyl-4-hydroxycoumaringlyoxal with some third row transition metal ions (Ir3+,Pr4', U@+) have been produced, and [Ir(ClgHls04N,),](Cl) has been characterized by IR spectroscopy and magnetic The Lhermally stable, Schiff base complexes [M(PPD)(Cl,]-yH,O (M = Ir, Pt; x = 4;PPD = pMe,C, H,CH=N(CH,),NH,) have been prepared; characterization by IR spectroscopy shows the dimethylamino and free amino groups to be the coordinating sites. Further characterization by magnetic moment measurements and NMR and electronic spectral data indicates the iridium(IV) complex to be octahedral.s68
49.10.3
Phosphorus Ligands
The complexes trans-[Ir(Cl)L(P(CHMe,),),] (L = CO, py, MeCN) were prepared from pr(Cl)(cod),], via the intermediate [Ir(Cl)(P(CHMe,),},] complex. trans-[Tr(Cl)(CO){P(CHMe,), 12] reacts with LiC,Ph to give trans-[Ir(C,Ph)(CO){P(CHMe2)3},], which, on treatment with HX, yields [Ir(H)(X)C,Ph(CO){P(CHMe,),),] (X = CI, CF,CO,). trans-[Ir(CI)(N,)(P(CHMe,),),] is obtained from the carbonyl complex and 2-furfuryl azide.569Some new mono- and di-nuclear iridium(1) complexes containing the bzs(tertiary phosphine) ligands Ph,P(CH2),PPh, (n = 2, dppe; n = 3, dpppj have been prepared, and the formation of iridium(II1j hydrides by H, oxidative addition has been studied by Fisher and Eisenberg.'" The dinuclear complexes [Ir(X)(CO)(dppp)], (X = Br, I) possess trans phosphine donors, with the dppp ligands bridging the iridium(1) centre. All the dppp complexes are mononuclear, with dppp serving as a chelating ligand. The complexes [M(CN),(dppmP),] (M = Pt, Pd), when treated with [Ir(CI)(CO),(NH,C,H,Me-p)] or trans[lr(Cl)(CO)(PPh,),], give the heterometallics [Ir(C1)(CO)(p-dppm)2M(CN),].571 The treatment of [Ir(CNB'),(p-dppm)],(Cl), with dppm gives labile systems at 293 K. At 213 K, "P-'H NMR spectroscopy reveals complete conversion to the tris-monodentate dppm complexes
Iridium
1163
(Ir(CNBu‘),(dppmP),](C1).Reaction of [Ir(Cl)(cod),], with four equivalents of dppm, six equivalents of ButCN, and two equivalents of [Au(Cl)(PPh,)] yields the fluxional molecule [Ir(CNBu‘),(pdppm), Au](Cl),; however, reaction between [Ir(CO)(dpprnPP’),](Cl) and IAg(CI)(CNBu‘)] affords ’
.
[Ir(CNBut)(CO)(p-dppm)2Ag(Cl)].s7~ Some novel five-coordinate iridium(1) complexes of the type [Ir(CO)(chel-P,)(R)] (chelP, = PhP(CH,CH,CH,PPh,),, R = CH,CMe,, CH,SiMe,; chel-P, = MeP(CH,CH,CH,PPh,),, R = CH,SiMe,; chel-P, = PhP(CH,CH,PPh,),, R = CH2SiMe,, 4-MeC,H4) have been synthesized from [Ir(CO)(PPh,),(R)] and the respective triphosphine. ” P NMR studies show these complexes to be stereochemically rigid at normal temperature^.^" Reaction between [Ir(q4-cod)(q3X-Ray 2-XC3H4)](X = Me, H) and di-t-butylphosphine gives [Ir(q4-~~d)(q3-C3H5){PH(But)z}]. crystallographic investigations suggest that where X = Me the complex possesses the square pyramidal geometry with the phosphine ligand in an axial In the complex [ir(y2-o-BrC6H,PPh,)(cod)](SbF,), o-BrC6H4PPh,chelates to Ir via P and the Br bound to the aromatic ring. The complex reacts with C1- to give [Ir(Cl)(q’-BrC,H,PPh,)(cod)], with MeCN to produce [Ir(q’-BrC,H,PPh,)(cod)(MeCN)](SbF,), and with H2 to yield [1r(H),($-o-BrC,H,~Ph,)(~od)(SbF,).~~~ Reaction between Vaska’s complex and the phosphinethiol chelate (dipheny1phosphino)ethanethiol(PSH) gives [Ir(H)(CO)(PS)(PSH)](Cl).Two 576 equivalents of o-(dipheny1phosphino)benzoylpinacolone (HacacP) react with [Ir(Cl)(CO),(pNH,C,H,Me)] giving [Ir(Cl)(CO)(HacacP),], which undergoes smooth metallation with copper(I1) carboxylates to give Dr(Cl)(CO){Cu(acacO), 11.The trans-[Ir(Cl)(CO){Zn(acacP),)I complex can be synthesized by treating ZnEt, with two equivalents of HacacP followed by [Ir(Cl)(CO),@H,NC,H,Me)]. EPR data for the Ir-Cu complex indicate the formation of an enolate bridge between Ir and CU.’?~ The solid state structure of [1r(Cl)(PF3)J2consists of infinite zigzag chains of Ir atoms with short inter- and intra-molecular Ir---Ir contacts. By contrast, related complexes containing PF,NMe,, e.g. [Ir(Cl)(PF,NMe,),] and [Ir(Cl)(PFNMe,),],, do not appear to have extended metal-metal interaction^.^^' ”P spin-lattice relaxation times TI, and ,lP-’H nuclear Overhauser enhancement values for CDC1, solutions of the compounds P(C,H,-p-X),, [Ir(Cl)(CO)(P(C,H,-pX),},] (X = H, Me, F, C1, OMe) and [AU(c1){P(c,H,-p-x),}] (X = H, OMe) have been determined.579For the uncoordinated tertiary phosphines TI = 26-28 s, except for X = OMe, where TI = 14 s. For the iridium(1) complexes, T, = 4.4-8.7 s, whereas for the gold(I) compounds, TI = 15.7 s (H) and 11.3 s (OMe). Differing relaxation mechanisms within each cIass of compound seem to be indicated by the data.579 Vaska’s complex reacts with F-t-butyl hypochiorite to yield a complex tentatively postulated as [Ir(Cl)(CO)\(CF,), CO),(L)J, where L presumably is a complex ligand containing (CF,), CO and C1 groups.58 Stang and coworkersSR’have determined the kH/k,, (= secondary kinetic deuterium isotope effect) values for the reaction of [Ir(Cl)(CO)(PPh,),] with Me1 (kH/kD3= 0.93) and MeO= 1.1 3) at 308 K in toluene. The values are consistent with a Menschutkin-type SN2 S02CF3(kH/kD3 displacement process. Reaction of Vaska’s complex with LiW(CO),(PPh,)] yields the dinuclear complex [IrW(p-PPh,)(CO),(PPh,),]. Also, reaction between Vaska’s complex and LiW(CO),(PPh, H)(PPh,)] gives [IrW(H)(CO),(p-PPh,),(PPh,),], characterized by X-ray diffractometry. The hexacarbonyl complex above adds H, and CO at the Ir centre, while the pentacarbonyl complex reacts with LiPh and LiMe to yield the acyl-halide derivatives [Li(THF),][IrW(H)(CO),(pPPh,),(PPh,)C(O)Rj (R = Ph, Me) with the acyl ligand terminally bound to W and the hydride to Ir.582PH, adds oxidatively in toluene to trcans-~r(X)(XO)(PEt,),](X = Br, C1) to give [Ir(X)(H)(CO)(PEt,),(PH,)]. The [Ir(X)(H)(CO)(PEt,)?(AsH2)]complexes have also been isolated. The complexes were characterized by ‘ H and j’P NMR spectroscopy. [Ir(H)(CO)(PEt,),(PH2)(PH3)]e is produced upon reaction of trans-[Ir(X)(CO)(PEt3),]and PH3 in CH2C12at 180K, followed by internal oxidative addition at greater than 230 K.583 Dimethyl- and diethyl-phosphine oxide, methylphosphonite, and ethylmethylphosphonite all add oxidatively to [Ir(Cl>(MezPhP),] to produce hydrido(dialky1phosphinito) or alkyl(alky1phosphinito) iridium(II1) complexes, [Ir(Cl)(H)(PPhMe,), {P(O)RR}]+. The nature of these species was ascertained by ‘H and 3LP NMR.584[Re(H),(q4-C5H6)(PPh,),] reacts with [Ir(Br)(CO)(dppe)] in C6D6 to give [Re(H),(Cp)(PPh,),] and [Ir(Br)(H), (CO)(dppe)]. The isomer of the iridium complex formed exclusively is the thermodynamically favoured isomer having H trans to P and Br. The driving force for the dehydrogenation of the Re complex by the Ir complex is attributed to the stronger M-H bond for iridium and the greater thermodynamic stability of the dehydrogenated product [Re(H), (CP)(PPh, Homobimetallk iridium(1) complexes containing the binucleating PNNP ligand undergo oxidative addition and reductive elimination reactions with acetyl chloride and methyl iodide. Thus,
1164
Iridium
[Ir, (p-PPh,)(CO)(PNNP)] reacts in two successive irreversible oxidative addition steps with either acetyl chloride or methyl iodide to give the diacyl [Ir2(p-PPh2)(CO)z(PNNP)(MeC0)2 (Cl),] and dimethyl [Ir, (p-PPh,)(CO),(PNNP)(Me),(I),]complexes, respectively, in which iridium is Ir'1'.586 The hydrido-bridged dinuclear complexes [Ir(Cl)(L), (p-H)(p-Cl)Rh(dppe)]+can be prepared by (L = PMe,Ph, PEt,Ph, PEt,, dppe). reacting [Rh(acetone),(dppe)]+ with mer,truns-[Ir(Cl),(H)(L),] The X-ray crystal structure of the dinuclear complex shows a distorted octahedral coordination around Ir. In a similar manner, one can produce the isoelectronic complex [Ir(H)(PEt,), (p-Cl)(p[Ir(H),(PEt,),] reacts with complexes conH)Rh(dppe)](BF,) from rner,~is-[Ir(Cl)(H),(PEt,),].~~~ taining the 'RhClL,' moiety (L = PEt, or L, = dppe) to give [Ir(H),(PEt,),(p-Cl)(p-H)Rh(L),].The crystal structure of [Ir(H),(PEt,),(p-CI)(p-H)Rh(PEt,),] indicates a distorted octahedral geometry about Ir, with the bridging chlorine atom asymmetrically bonded to both Ir and Rh.588 Reaction of Vaska's complex with the functionalized silanes PPh, CH, CH2SiRR"H ( R = R" = Me, Ph; R = Me, R" = Ph; R' = Me, Ph, R = H) yields the iridium(II1) adducts [Ir(CI)(nH)(CO)(PPh,CH,CH,SiRR")(PPh,)](n = 1, 2). The complexes are chiral at Ir; however, while the complexes with R = R = Me and R' = R = Ph are enantiomeric, those with R' = Me, R = Ph, R = Me, R = H, and R = Ph, R = H are diastere~meric.~'~ The five-coordinate iridium(II1) complex [Ir(Cl)(PPh,CH,CH, SiMe,),], prepared via reaction of [Ir(Cl)(cod)], and excess PPh,CH,CH,SiMe,H in THF solution, has been characterized by IR and ' H NMR spectroscopy and by X-ray diffractometry. The complex reacts with NaX (X = Br, I) in acetone to give [Ir(X)(PPh,CH, CH, Auburn et a!."' have described the synthesis of a chelate-silyl iridium(1) complex from iridium(II1) via reductive elimination. Thus, photolysis of [I;(H),(CO)(PPh,CH,CH,SiMe,)(PPh,)] evacuated in THF gives < 30% yield of [Ir(CO)(PPh,(CH,CH,SiMe,)(PPh,)].In a CO atmosphere, the same reaction gives > 80% yield of the same product. Crystal structurai studies of the iridium@)complex show a five-coordinate Ir atom with Si in an axial position trans to PPh,, and two CO ligands in the equatorial plane.59' Rhodes and coworkers592have reported on the unexpected results of reaction between Cu' and the mer andfuc isomers of [Ir(H)3(P)3],where P is PMe,Ph. 'H NMR spectroscopy has identified [{rner-Ir(H),(P),},CU](PF,) as the product of reaction between [CU(hkCN),](PF,) and two equivalents of mer-[Ir(H),(P),] in THF. In the addition of [Cu(MeCN),(PF,) to [vueIr(H),(P),},Cu](PF,), there is no conversion of thefuc to the mer isomer in MeCN at 298K over several hours. The Ag' adduct of f~c-[Ir(H),(P)~1 has also been synthesized.592The stoichiometric protonation of fac-[Ir(H), (PX] in CDCl, using HBF,.Et,O shows complete conversion to [Ir(H),(P),]+ . The latter possess a pentagonal bipyramidal structure at 193K. Protonation of mer-[Ir(H), (P)J yields an identical product. Although spectroscopically undetectable, Hz elimination from [Ir(H),(P),]+ would seem to give rise to [Ir(H)2(L)(P)3]+(L = N,, CO, MeCN). In essence, what has occurred is reductive elimination (Ir" to Ir"1).593An investigation of the Cuf fuc[Ir(H),(PMe,Ph),] system at 1r:Cu stoichiometries of less than 2:l has led to evidence of an unexpected yet isolable aggregate. At an 1r:Cu ratio of 2:3, the product is consistent with the fOlXNllatiOn Pr2CU3 (H),(MecN),(PMe,Ph),](PF,), .594 have The X-ray crystal structure of [Ir(Cl), (C,H5)(CO)(PMePh,),]595and [Ir4(CO),(PMe,Ph)4]596 been determined. 7
49.10.4
Arsenic and Antimony Ligands
Pentacoordinate iridium(1) complexes of the general formula [Ir(Cl)L,(TFB)] or [Ir(Cl)(LL)(TFB)] (TFB = tetrafluorobenzobicyclo[2.2.2]octatriene; L = PPh,, AsPh,, SbPh,; LL = dppe, bipy, phen) have been synthesized from [Ir(Cl)(TFB),] and the appropriate ligand.597 Bis(diphenylphosphinomethy1)phenyl arsine, dpma, reacts with [Ir(Cl)(CO)(p-toluidine)]to give [Ir, (CI),(CO),(p-dpma),]. The latter dimeric complex subsequently reacts with bis(benzonitri1e)palladium(1I) chloride in CH,C12 to obtain [Ir,Pd(Cl),(CO),(p-dpma),](BPh,). The dpma ligand is sufficiently flexible for it to accommodate a range of metal-metal interactions within the polynuclear units.598 49.10.5
Oxygen Ligands
Homo-and hetero-dinuclear bis(di-p-amide or -oxo)iridium(I) complexes of formula [MM'kL)(cod),] (L = 1,8-(NH,),naphthalene, 1&(O),naphthalene, 2,3-(O),naphthalene) have been synthesized and characterized by IR spectroscopy.599Two different routes to the preparation of the first
Iridium
1165
iridium(1) carbon-bonded diketonate complex, [Ir(acac-C')(cod)(phen)], have been reported.600The crystal structure of this complex reveals pentacoordinate iridium in a distorted pyramidal stereochemistry. The synthesis and characterization of binuclear complexes containing the flexible bridging bis(2,4-pentanedionato) ligand have been cited.60'Thus, [Ir(cod)], {xyl(acac), } j is afforded upon reaction of [Ir(p-Cl)(cod)], with p-silylenebis[3-(2,4-pentanedione)],xyl(Hacac),. The methylene complexes [Ir(I)(CO)(=CH,)(PPh,),] and [Os(Cl)(NO)(=CH,)(PPh,),] form 1:1 adducts with SO,, and an X-ray crystal structure determination of the osmium complex shows the sulfene fragment in [Os(Cl)(NO){CH, S(O)O)(PPh,),] to be C,O-bound forming a non-planar, fourmembered ring with the S atom having pyramidal geometry.602
49.10.6 Boron Ligands Reaction between K[B4H9]and trans-[Ir(C1)(CO)(PMe2Ph),]in the presence of an additional In the crystalline state, this iridapentaborane equivalent of K H gives [Ir(q4-B4H9)(CO)(PMezPh)2]. consists of an open five-atom cluster in which the [Ir(CO)(PMe,Ph),] fragment occupies an apical vertex site and is bound to the four boron atoms of the (q4-B4H9)ligand.603Li[CB,H,,] reacts with [Ir(CI)(PPh,),] in diethyl ether to produce several products, including closo-[1-PPh3)-2,2,2H(PPh,),-2,10-lrCB,Hs]. An X-ray diffraction study of this closo complex shows the ten-vertex cage to have bicapped Archimedean square antiprismatic geometry, with Ir in an equatorial position. The apical sites are occupied by the carbon, and a boron covalently bonded to the P of a PPh, ligand.604 NMR spectroscopy characterized the aruchno-6,9-dimetalladecarboranecomplex [(PMe,),PtB8HloIr(H)(PMe,),(CO)], obtained on deprotonation of an arachrso~-iridanoboranewith KH followed by reaction with cis-[Pt(Cl),(PMe,),]. Mild thermolysis or UV photolysis of the urachno-4iridanoborane produces the corresponding nirdo-2-iridanoborane. This is the first nido nine-vertex metalloborane to be structurally characterized. The urachno -,nido transformation, occurring via dihydrogen loss, follows first-order kineticsm5
49.10.7 Sulfur and Selenium Ligands Claver and coworkersm6have prepared the cationic iridium(1) complexes ~r(cod)(p-L)],(C104), and [Ir(~od){p-(L-L))]~,where L is SMe,, SEt,, tetrahydrothiophene (tht), or trimethylene sulfide (tms), and (L-L) is 1,4dithiacyclohexane (dt), (SBU')~(CH,)~ (tmdto), or (SMe),. These same authors have also isolated the mononuclear complexes [Ir(cod)(L),](ClO,) (L = tht, tms) and [Ir(cod)(L-L)j(ClO,) (E-L = dth).607These react with H2 to give the corresponding iridium(II1) dihydride complexes, with [Ir(cod)(L),]+ (L = tht, tms) react with one or two moles of PPh, to yield the pentacoordinate derivatives [Tr(cod)(L),(PPh,)]+ or [Ir(cod)(L)(PPh,),]'. The thiolato-bridged dinuclear iridium(1) complexes [Ir(CO)(L)(p-SBu' )I2 react with dihalomethane to produce [Ir(X)(CO)(p-SBu')],(p-CH,)(X = I, L = CO, P(OMe),, PPh3, PPh,Me, PMe,; X = Br, L = PPh,). A crystal structure determination of [Ir(I)(CO)(p-SBu')(P(OMe), )12(p-CH2) reveals Ir to be octahedrally coordinated to one CO, one P(OMe),, one I, two bridging S atoms of the thiolato ligands, and the bridging C atom of the CH, group. The phosphite ligands are mutually trans, as are the carbonyl ligands.608[Ir(dppe),(S,O)]+ reacts with MeOS0,F to yield [Ir(dppe),(q2S20Me)]2+twhich has been characterized by X-ray crystallography, and which can be converted to [Ir(dppe)(MeCN)(q'-SOMe)]'+ upon reaction with two equivalents of MeCN.# Teo and Snyderhave elucidated the crystal structure of [Ir(X)(CO)(PPh,),(TCTTN)]*C,H,- MeCN (X = Cl, €3; TCTTN = tetrachlorotetrathionaphthalene,Cl0Cl4S4).The Ir atom is octahedrally coordinated to two trans PPh, ligands, two S atoms from TCTTN, one CO ligand and one X ligand. obtained via reaction of Crystallographic studies have characterized [Ir(Cl)(H)(CO)(PPh,),(SH)], Vaska's complex with H2S in CH2C1, solutions.611 The stepwise oxidation of [Ir(dppe),(q*-S,)](PF,) with m-chloroperbenzoic acid gives [Ir(dppe),(q2-SzO)](PF,) and [Ir(dppe),(q2-S, o,]fPF,). Oxidation of [Ir(dppe),(v12-Sez)](PF,) with peracetic acid affords [Ir(dppe),(q*-Se,O)](PF,). Though the preparation of q2- or q2-OS0 complexes via selective sulfur-abstraction reactions is not possible, the unsymmetrical dichalcogenide complex [Ir(dppe), (q2-!Be)]+ can be prepared from [Ir(dppe),](Cl) and [Ti(Cp), S, Se,-, (x x 3).612 Complete reduction of the selenocarbonyl ligand of [Ir(Cl)(CSe)(PPh,),] by NaBH., in the presence of PPh, produces [Tr(H),(PPh,), (SeMe)]. Additionally, CSe, reacts with [Ir(CI)(CSe)(PPh,),] to form the iridium(II1) adduct [Ir(Cl)(q2-CSe,)(CSe)(PPh,),].613
49.10.8 Halide Ligands
[Ir(Cl)6(H)2]can be prepared via the electrochemical dissolution of powdered iridium in HCI. The rate of dissolution was studied as a function of current density. HC1 solution concentration, temperature, and as a function of the number of bipolar electrodes.614The first C1-containing compound of iridium(V1 has been isolated from reaction between [Ir(Cl)6]'- and BrF, in liquid H F giving [Ir(Cl)6-,,(F),,]- ( n = 1-6). The deep red Cs[Ir(Cl)(F),j has been characterized by IR and Raman
49.10.9 Hydride Ligands
Oxidative addition olH2to the iridium(1) chelates [Ir(X)(CO)(dppe)]'" (n = 0, X = CI, Brl I, CN, H; y1 = 1, X PPh,, dppe) proceeds with > 99% stereoselectivily tu yield a cis-dihydride product with one H trarls to P(dppe) and the other H trans to CO. The addition reaction is controlled by electronic factors of the CQ and X ligands. Where X is C1, Br or I, the stereoselectivity has a kinetic origin with subsequent conversion of the initially formed isomer to the more stable cis-dihydride isomer. The basis for the electronic factor is attributable either to (i) enhanced overlap in the n-back-bonding interaction between metal d and o*(H,) orbitals; or (ii) preferred bonding of one set of trans ligands as the reaction proceeds.616[Ir(H)(CO),(PPh,),] reacts with the coordinatively unsaturated dihydride cluster [Os,(p-H),(CO),,J to yield [Os,(H),(CO), {Ir(PPh,))]. The X-ray crystal structure of the product reveals that the Ir atom caps the Os, triangle to form a distorted tetrahedral metal f r a m e w ~ r k . ~Reaction '~ between an acetone solution of [lrAu,(NO,)(PPh,),](BF,) and 1 atm H, yields the hydride cluster complex [lrAu,(H),(PPh,),](BF,). The two hydride ligands in this complex could not be located by X-ray analysis; they are believed to be bridging-Ir-axial-Au bonds. NMR spectral data support this assignment, though a terminal Ir -H bonding description also appears plausible."* Trimethylphosphine reacts with [Ir(C,Me,)],(p-H), A (A = PF6, BF,) to give the monokydridebridged dimer {Ir(H)(C,Me,}(PMe,}]2(p-H)A.The product for which A = BF4 reacts with a third equivalent of PMe, at elevated temperatures to produce the monomeric species [Tr(H),(C Me,)(PMe,)] and [Ir(H)(C,Me,)(PMe,)2](BF4). Deprotonation of [Ir(H),(C,Me,)(PMe,)] with LiBuYpmdeta. leads to [Ir(H)(C,Me,)(PMe,)][Li(pmdeta),],which reacts with alkylating agents to yield complexes of the type [Ir(H)(C,Me,)(PMe,)(R)] (R = LiBH, reacts with [Ir 373. Dalton Trans., 1977, 677. A. B. Gilchrist and D. Sutton, J . Chern. SOC., P. L. Bellon, G. Caglio, M. Manassero and M. Sansoni, J . Chem. Snc., Dalton Trans., 1974, 897. M. Angoletta, L. Malatesta and P. L. Bellon, J . Organomet. Chem., 1976, 114, 219. (a) L. Toniolo, J. A. McGinnety, T. Boschi and G. Deganello, Inorg. Chim. Acta, 1974, 11, 143; (b) U. Croatto, L. Toniolo, A. Immirzi and G. Gombieri, J . Organomet. Chem., 1975. 102, C31; (c) L. Toniolo, Chem. h d . flondon). 1976, 30; (d) L. Toniolo and D. Leonesi, J. Organomet. Chem., 1976, 113, C73. M. Nonoyama, Bull. Chem. SOC. Jpn., 1974, 47,767. S. D. Robinson and A. Sahajpal, h o g . Chem., 1977, 16, 2722. A. W. Gal, H. P. M. M. Ambrosius, A. F. M. J. Van dcr Ploeg and W. P.Bosman, J . Organornet. 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,
I
484, 485. 486. 487. 488 489. 490. 491. 492.
.
493. 494. 495. 496, 497. 498. 499. 500. 501. 502. 503. 504. 505. 506 507 508 509 510 51 1 512. 513. 514 515. 516. 517. 518. 519 520. 52 I 522 523
524 525 526 527 528 529 530 53 1 532 533 534 535 536 537 538. 539. 540. 541. 542. 543. 544 545. 546. 547. 548
1175
D. F. C. Morris and T. J. Ritter, J. Chem. Soc., Dalton Trans., 1980, 216. S . B. Piepho, W. H. Inskeep, P. N. Shatz, W. Preetz and H. Homberg, Mol. Phys., 1975, 30, 1569. L. Moggi, G. Varani, M. F. Manfrin and V. Balzani, Inorg. Chim. Acta, 1970, 4, 335. W. S. Melvin and A. Haim, Inorg. Chem., 197, 16, 2016. R. N. F. Thorneley and A. G. Sykes, J. Chem. Soc., 1970, 232. W. E. Falconer, G . R.Jones, W. A. Sunder, M. J. Vasile. A. A. Meunter, T. R. Dyke and W. Klemperer, J. Fluorine Chem.. 1974,4, 213. T. Cyr, Can. J. Spectrosc., 1974, 19, 136. M. A. Hepworth, P. L. Robinson and G. J. Westland, J . Chem. Soc., 1954, 4269; 1958, 611. (a) N. Bartlett, S. P. Beaton and N. K. Jha, Chem. Commun., 1966. 168; (b) P. L. Robinson and G . J. Westland, J . Chem. SOC.,1956, 4481. E. K. Bareficld, G . W.Parshall and F. N. Tebbe, J . Am. Chem. Soc., 1970, 92, 5234. B. E. Mann, C. Masters and B. L. Shaw, J . Chem. Soc. ( n j , 1970, 703. H. C. Vogler, H. Hugeveen and M. M. P. Gaasheek, J . Am. Chem. Soc., 1969, 91, 2137. J . E. Lyons, Chem. Commun., 1969, 564. M. Green, T. A. Kuc and S . H. Taylor, Chem. Commun., 1970, 1553. J. Cook. J. E. Hamlin, A. Nutton and P. M. Maitlis, J . Chem. Soc., Chem. Commun., 1980, 144. M. D. Curtis and P. S . Epstein, Adv. Organomet. Chem., 1981, 19, 213. G . G. Eberhardt and L. Vaska, J. Catal., 1967, 8, 183. J. P. Collman, J. W. Kang, W. F. Little and M . F. Sullivan, Inorg. Chem., 1968, 7 , 1298. J. Blum and G . Scharf, J. Org. Chem., 1970, 35, 1895. J. Blum and S . Biger, Tetrahedron Lett., 1970, 1825. D. H. Doughty and L. H. Pignolet, J . Am. Chem. Sue., 1978, 100,7083. D. H. Doughty, M . F. McGuiggan, H. Wang and L. H. hgnolet, 'Fundamental Research in Homogeneous Catalysis', Plenum, New York, 1977, vol. 3. T. Mizoroki, T. Matsumoto and A. Ozaki, Bull. Chem. Srrc. Jpn., 1979, 52,479. (a) Y. M. Y. Haddad, H. B. Henbest and J. Trocha-Grimshaw, J . Chem. Soc., Perkins Truns. 1 , 1974, 592; (b) Y. M. Y. Haddad, H.B. Henbest, J. Husbands, T. R. B. 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1176
Iridium
549. E. B. Boyar, D. S. Moore. S. D. Robinson, B. R. James, M. Preece and I. Thorburn, J , Chem. Soc., Dalton Trans., 1985, 6ij. 550. M. Ghedini and F. Neve. J. Chem. Soc.. Dalton Trans.. 1984. 1417. 551. R. Uson, L. A. Oro, D. Carmona and M. A. Esteruelas, Inorg. Chim. Acta, 1983, 73, 275. 552. P. H. Kreutzer, J. C. Weis, H. Bock, J. Erbe and W. Beck, Chem. Ber.. 1983, 116,2691. 553. J. Erbe and W. Beck, Chem. Ber., 1983, 116,3867. 554. K. A. Beveridge, G. W. Bushnell, K. R. Dixon, D. T. Eadie, S. R. Stobart, J. L. Atwood and M. J. Zaworotko, J. Am. Chem. Soc., 1982, 104, 920. 555. J. L. Atwood, K. A. Beveridge, G. W. Bushnell, K. R. Dixon, D. T. Eadie, S. R. Stobart and M. J. Zaworotko, Inorg. Chem., 1984, 23,4050. 556. S . Nussbaum, S. J. Rettig, A. Storr and J. Trotter, Can. J. Chem., 1985, 63, 692. 557. M. D. Fryzuk, P. A. MacNeil and S. J. Rettig, Organomerallics, 1985, 4, 1145. 55x. 1. Walker and J. Straehle, Z. AnorR. A&. Chem.. 19x3, 506, 13. 559, F. Galsboel and B. S. Rasmussen, A c f a Chem. Scand., Ser. A , 1984, 38, 141. 560. S. K. Hoffrnann, D. J. Hodgson and W.E. Hatfield, Inorg. Chem., 1985. 24, 1194. 561. R . H. Crabtree, E. M. Holt, M . Lavin and S. M. Morehouse, Inorg. Chem., 1985, 24, 1986. 562. K. A. King, P. J. Spellane and R. J. Watts, J. Am. Chem. Soc., 1985, 107, 1431. 563. S. Sprouse, K. A. King, P. J. Spellane and R. J. Watts, J. Am. Chem. Soc., 1984, 106, 6647. 564. B. P. Sullivan and T. J. Meyer, J. Chem. Soc., Chem. Commun., 1984, 403. 565. Y. Ohashi and J. Nakamura, Chem. P h p . Lett., 1984, 109, 301. 566. A. Slama-Schwok, S. Gershuni, J. Rahani, H. Cohen and D. Meyerstein, J. Phys. Chem., 1985, 89,2460. 567. V. K. Rastogi, J. P. Goel, B. S. Tyagi, R. C. Saxena and R. K. Kela, J. Indian Chem. SOC.,1983, 60, 783. 568. S. M. Ali, K. Iftikhar and N. Ahmad, Synth. React. Inorg. Metal-Org. Chem., 1984, 14, 1031. 569. H. Werner and A. Hoehn, 2. Naturforsch., Ted B, 1984, 39, 1505. 570. B. J. Fisher and R. Eisenberg, Inorg. Chem., 1984, 23, 3216. 571. F. S. M. Hassan, D. P. Markham, P. G. Pringle and B. L. Shaw, J. Chew. Soc.. Dalron Trans., 1985, 279. 572. C. R. Langrick and B. L. Shaw, J. Chem. SOC.,Dalton Trans., 1985, 511. 573. E. Arptlc and L. Dahlenburg, J. Organornet. Chem., 1984, 277, 127. 574. 6. D. Murray and P. P. Power, Organomeaallics, 1984, 3, 1199. 575. M. J. Burk, R. H.Crabtree and E. M . Holt? Organometallics, 1984, 3, 638. 576. D. W. Stephan, Inorg. Chem., 1984, 23,2207. 577* D. A. Wrobleski, C. S. Day, B. A. Goodman and T. B. Rauchfuss, J. Am. Chem. Soc., 1984, 106, 5464. 578. P.B. Hitchcock, S. Morton and J. F. Nixon, J. Chem. Soc., Chem. Commrm., 1984, 603. 579. S. Jam-Buerli and P. S. Pregosin, Magn. Reson. Chem., 1985, 23, 198. 580. J. M. Canich, G. L. Gard and J. M. Shreeve. Inorg. Chem., 1984, 23, 441. 581. P. J. Stang, M. D. Schiavelli and H. K. Chenault, Organometallics, 1984, 3, I133. 582. M. J. Breen, P. M. Shulman, G. L. Geoffroy, A. L. Rheingold and W. C. Fultz, Organometallics, 1984, 3, 782. 583. E. A. V. Ebsworth and R. Mayo, Angew. Chem., 1985, 97, 65. 584. T. R. B. Mitchell, J. Organomet. Chem., 1984, 270, 245. 585. W. D. Jones and J. A. Maguire, J. Am. Chem. Soc., 1985, 107,4544. 586. T. G. Schenck, C. R. C. Milne, J . R.Sawyer and B. Bosnich, Inorg. Chem..1985, 24,2338. 587. H. Lehner, D. Matt, A. Togni, R. Houvenol, L. M. Venanzi and A. Albinati, Inorg. Chem., 1984,23,4254. 588. A. Albinati, H. Lehner and L. M. Venanzi, Inorg. Chem., 1985, 23, 1483. 5 ~ 9 . M .J. Auburn, R. D. Holmes-Smith and S. R.Stobart, J. Am. Chem. Soc., 1984, 106, 1314. 590. M. J. Auburn and S. R. Stobart, Inorg. Chem., 1985, 24, 318. 591. M. J. Auburn, S. L. Grundy, S. R. Stohart and M. J. ZdwOrOtkO, J. Am. Chem. Soc., 1985, 107, 266. 592. L. F. Rhodes, J. C. Huffman and K. G. Caulton, J. Am. Chem. Soc., 1984, 106,6874. 593. L. F. Rhodes and K. G. Caulton, J. Am. Chem. Soc., 1985, 107, 259. 594. L. F. Rhodes, J. C. Huffman and K. G. Caulton, J . Am. Chem. Soc., 1985, 107, 1759. 595 P. N. Swepston, N. L. Jones, J. A. Ibers, M. Shang, J. Huang and J. Lul Acta Crystallogr., Sect. C, 1984, 40, 39. 596 A. J. Blake and A. G. Osborne, J. Organornet. Chem., 1984,260,227. 597 R. Uson, L. A. Oro, D. Carmona and M. A. Esteruelas, J. Organomet. Chem., 1984,263, 109. 598 A. L. Balch: L. A. Fossett, M. M. Olmstead, D. E. Oram and P. E. Reedy, J. Am. Chem. Soc., 1985, 107, 5272. 599 L. A. Oro, M. J. Fernandez, J. Modrego and J. M. Lopez, J. Organomet. Chem., 1985, 287,409. 600 L. A. Oro, D. Carmona, M. A. Esteruelas, C. Foces-Foces and F. H. Cano, J. Organomet. Chem., 1983, 258,357. 60 1 B. C. Whitmorc and R. Eisenberg, Inorg. Chem., 1984, 23, 1697. 602. W. R. Roper, J. M. Watcrs and A. H. Wright, J. Organomet. Chem., 1984, 276, C13. 603. S. K.Boocock, M. A. Toft, K. E. Inkrott, L . Y. Hsu, J. C. Huffman, K.Folting and S. G. Shore, Inorg. C k m . , 1984, 23, 3084. 604. N. W. Alcock, J. G. Taylor and M. G. H. Wallbridge, J. Chem. Soc., Chem. Commun., 1983, 1168. 605. J. Bould, N. N. Greenwood and J. D. Kennedy, J. Chem. SOC.,Dalton Trans., 1984, 2477. 606. C. Claver, J. C. Rodriguez and A. Ruiz, J. Organomet. Chem., 1983,251,369. 607. J. C. Rodriguez, C. Claver and A. Ruu,J. Organomet. Chem.. 1984, 272, C67. 608. M. El Amane, A. Maisonnat, F. Dahan, R. Pince and R. Poilblanc, Organomerallics, 1985, 4, 773. 609. J. E. Hoots, T. B. Rauchfuss and S. R. Wilson, J. Chem. Soc., Chem. Commun., 1983, 1226. 610. 3. K. Teo and P. A. Snyder-Robinson, Inorg. Chem., 1984, 23, 32. 611. A. M. Mueting, P. Boyle and L. H. Pignolet, Inorg. Chem., 1984, 23, 44. 612. J. E. Hoots, D. A. Lesch and T. B. Rauchfuss, Inorg. Chem., 1984,23,3130. 613. W.R. Roper and K. G. Town, J. Organomel. Chem.. 1983, 252, C97. 614. R. T. Khleborodova, P. V. Seliverstov, A. N. Novikova and A. I. Ryazanov, Tr. IRfiA, 1984, 46,28. 615. W. E’reelz and D. Tensfeldt, 2. Anorg. A & Chem., 1985, 522, 7. I 616. C. E. Johnson and R. Eisenberg, 1.Am. Chern. SOC.,1985, 107, 3148. 617. B. F. G. Johnson, J. Lewis, P. R. Raithby, S. N. Azmdn, B. Syed-Mustaffa, M. I. Taylor, K. H,Whitrnire and W. I Clegg, J. Chem. Soc., Dalton Trans., 1984. 2111.
i
I
Iridium 618, 619. 620. 621. 622. 623. 624. 625. 626. 627. 628. 629.
630. 631. 632. 633. 634.
635. 636. 637. 638.
1177
A. L. Casalnuovo, J. A Casalnuovo, P. V. Nilsson and L. H. Pignolet, Inorg Chem., 1985, 24, 2554. T. M. Gilbert and R. G. Bergman, J . Am. Chem Soc., 1985, 107, 3502. T. M. Gilbert, F. J. Hollander and R. G. Bergman, J. Am. Chem. Sor., 1985, 107, 3508. R. H. Crabtree, G. G. Hlatky, C. A. Parnell, B. E. Segmueller and R. J. Uriarte, Inorg. Chem.. 1984, 23, 354. G. B. Robertson and P. A. Tucker, Aust. J . Chem.. 1984,37,257. M. J. Mockford and W.P. Griffith, J. Chem. Soc., Dalton Trans., 1985, 717. M. J. Fernandez and P. M. Maitlis, J. Chem. Soc., Dalron Trans., 1984, 2063. L. A. Oro, J. A. Cabeza, C. Cativiela, M. D. D i u de Villegas and E. Melendez, J. Chem. Soc., Chem. Comrnun., 1983, 1383. D. A. Evans and M. M. Morrissey, Tetrahedron Left., 1984, 25, 4637. S. H. Park, H. K. Park and C. S . Chin, Inorg. Chem, 1985. 24, 1120. M. M. Khan, B. T. Khan and K. Nazeeruddin, 1. MOL Catal., 1984, 26, 207. V. E. Kalinina, Yu.A. Zhukov and T. N. Koroleva, Vopr. Kinet. Katal., 1982, 25. P. K. Saiprakash, K. C.Rajanna, Y. U.Devi, C. H.N Rao, Indian. J. Chem., Secr, A , 1984,23,650. M. Visintin, R. Spogliarich, J, Kaspar and M. Graziani, J. Mol. Catal., 1984, 24, 277. E. Farnetti, F. Vinzi and G. Mestrom, J. Mol. CataZ., 1984, 24, 147. H. Felkin, T. Fillebeen-Khan, Y. Gault, R. Holmes-Smith and J. Zakrzewski, Tetrahedron Leu., 1984, 25, 1279. F. Vinzi, E. Farnetti, G. Zassinovich and G. Mestroni, Congr. Naz. Chim. Inorg. [Atti], 15rh, 1982, 197; SOC.Chim. Ital., Div. Chim. Inorg. Bari, Italy. L. A. Oro, M. T. Pinillos, M. Roy0 and E. Pastor, J. Chem. Res. ( S ) , 1984, 206. V. 0. Reikhefel'd and A. V. Nikitin, Deposited Doc.,VINITI 4717-84, 1984, 26 pp. M. Roeper, E. 0. Elvevoll and M. Luetgendorf, Erdoel Kohle, Erdgas, Petrochem., 1985, 38,38. G. A. Abakumov, I. L. Knyazeva, N. N. Vavilina, L. V. Gorbunova and S. V. Zhivtsova, Izv. Akad. Nauk SSSR, Ser. Khim., 1984, 1352.
Iron(ll) and Lower States PELHAM N. HAWKER Catalytic Systems Division, Johnsun Matthey PLC, Ro yston. UK
'
and MARTYN V. TWlGG IC1 Chemicals and Polymers Group, Billingham, UK ~~~
~
~
44.1.1 GENERAL SURVEY 44.1.1.1 Introduction 44.1.1.2 Iron Complexes in Analysis 44.1.1.3 Biological Sysrems 44.1.1.4 Organometallic Chemistry 44.1.IS Iron M8mbauer Spectroscopy 44.1.1.6 Owidation States and Coordination Numbers 44.1.1.6.1 Low oxidation slates 44.1.I .6.2 Iron ( I I j 44.1.1.6.3 Irort(IIIj 44.1.1.6.4 Iron(II)-iron(III) redox behaviour 44.1.16.5 Mixed iron(IIj-iron(IIlj complexes 44.1.1.6.6 Higher oxidation states
1180 1180 1180 1180 1181
44.1.2 NITRIC OXIDE COMPLEXES
1187
44.1.2.1 44.1.2.2 44.1.2.3 44.1.2.4 44.1.2.5 44.1.2.6 44.1.2.7
1181 1182 1184 1184 1185 1185 1187 1187
Introduction Simple and Binary Nitrosyls Nitrosyl Carbonyls The Nitroprusside Ton Complexes Coniainirlg Nitric Oxide and Sulfur Donors Nitrosyl Hulidex NO Ada'ucts of Complexes Containing Polydenfote Ligands
1187 1188 1188 1189 1191
1193 1194 1195 1195 1195 1196
44.1.3 IRON(0) AND LOWER OXIDATION STATES 44.1.3.1 Introduction 44.1.3.2 Nitrogen Ligands 44.1.3.3 Phosphorus Ligan& 44.1.4 IRON(I) 44.1.4.1 Introduction 44.1.4.2 Carbon Monoxide and Arsine Ligands 44.1.4.3 Phosphoru Ligand 44.1.4.4 Cyanide Complexes 44.1.4.5 Macrocyclic Ligands 44.1.4.6 Organometallic Complexes 44.1.4.7 Nitric Oxide Complexes
1197 1197 1197 1198 1201
44.1.5 IRON(I1) 44.1.5.1 Introduction 44.1.5.2 Cyanide Complexes 44.1.5.2.1 The hexacyano complex 44.1~ 2 . 2Pentacyano complexes 44.1.5.2.3 Reactions of [Fe(CN),L]'44.1.5.2.4 Prussian blue 44 .I 3.2.5 Isocyanide complexes
1203 1203
1201 1203 1203
1204 1204 1205 1205 1206 1208 1209 1210 1210 1211
44.1.5.3 Alkyl Cyanides 44.1.5.4 Simple Amines 44.1.5.4.1 Saturated and primary aromatic amines 44.1S.4.2 I,&Napkthyridine
C.O.C. &LL*
1179
11x0
Iron(I1) and Lower States
44.1.5.5 Monodentate Aromatic Amines 44.1.5.6 Diimines and Related Ligands 44.1.5.6.1 Introduction 44.1 ~ 6 . 22.2’-Bipyridine and 1.10-phenanthroline 44.1~ 6 . 32,2’:6‘,2”-Terpyridine 44.1.5.6.4 Other diimines and related ligands 44.1.5.7 Dioximes 44.1 S.8 Phosphorus and Arsenic Donors 44.1.5.8.1 Monodentate ligands 44.1.5.8.2 Polydentate ligands 44.1.5.9 Oxygen Donors 44.1.S.9.1 Water 44.I.5.9.2 Acetylacetonate 44.1 S.9.3 Pyridine N-oxide 44.1.5.9.4 Miscellaneous oxygen donors 44.I.5.9.5 Mixed donor ligands containing oxygen 44.1.5.10 SuSfur Donors 44.1.5.10.1 Binary sulfides 44.1.5.10.2 Iron sulfur clusters 44.1 5 1 0 . 3 Dithio ligands 44.1.5.10.4 Mixed sulfur nitrogen donors 44.I S.1I Halides 44.1.5.1 I .I Fluorides 44.1.5.11.2 Chlorides 44.1.5.1 I .3 Bromides 44.1.5.11.4 Iodiuks 44.1.5.12 N Donor Mocrocycles 44.1.5.12.1 Introduction 44.1S.12.2 Saturated macrocycles 44.1.5.12.3 Partially unsaturated macrocycles 44.1.5.12.4 Phthalocyanine 44.1.5.12.5 Porphyrins
,
44.1.6 REFERENCES
1212 1214 1214 1216 1222 1222 1231 1233 1233 1234
1235 1236 1236 1231 1237 1238 1238 I 240 1240 1245 1247 1249 1249 1 249 1250 1250 1250 1250 1250 1252 1260 1266 1270
44.1.1 GENERAL SURVEY 44.1.1.1 Introduction Iron is the most abundant transition metal in the earth’s crust, occurring to the extent of some 5% in igneous rocks.’ It is thought metallic iron is a major component of the earth’s core.’Iron has
a rich coordination chemistry and many of its complexes find use in practical applications.
44.1.1.2 Iron Complexes in Analysis The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic ‘wet’ and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(II1) in aqueous solution include the formation of ‘Prussian blue’ as a result of reaction with [Fe(CN),I4-, and the formation of the intensely red-coloured [Fe(H,O), SCN]” on reaction with thiocyanate ion4 Iron(l1) forms particularly stable red tris chelates with qd-diimines such as 1,lO-phenanthroline or 2,T-bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.’ In gravimetric estimations, iron(II1) can be precipitated as the insoluble 8hydroxyquinoline or a-nitroso-8-naphthol complex which is then ignited to Fe,O, .6 In many situations the levels of free [Fe(H, O),]” may be controlled through complex formation by addition of edta.
44.1.1.3 Biological Systems I
The versatility of the chemistry of iron is reflected by the variety of roles it plays in biological systems where it is frequently found at the active centres involved in processes such as oxygen and
Iron(1l) and Lower States
1181
electron transport in nitrogenase, many oxidases and in metalloenzymes such as hydrogenases and reductases. While sustained research over the last two decades has provided much detailed information about the coordination environment of iron in several important natural systems (e.g. haemoglobin, ferredoxins and cytochromes), there are still many cases in which the exact location of the iron is not known.
44.1.1.4
Organometallic Chemistry
There have been rapid developments in the organometallic chemistry of iron and during recent years a huge amount of work has been published in this area, as can be judged from there being some 2000 references concerned with iron compounds in the companion work ‘Comprehensive Organometallic Chemist~y’.~ As a result of the effort that has gone into the biological and organometallic areas, the basic coordination chemistry of iron has been rather overshadowed and there have been relatively few reviews or books published on the subject in recent years.
44.1.1.5
Iron Mbssbauer Spectroscopy
There are few direct spectroscopic probes for iron; for instance, in most complexes charge transfer bands obscure the d-d transitions and magnetic measurements cannot provide information on low-spin iron(I1) complexes. Therefore when the Mossbauer technique became generally available in the late 1960s, it was widely used for the study of iron complexes. The effect’ arises from the resonant absorption of y-rays by the 57Fenucleus, which has a natural abundance’ of 2%. When the y-ray source and sample nuclei are in different environments, the shift in the energy of resonant absorption is expressed as a velocity relative to some arbitrary zero such as stainless steel or Na,[Fe(CN),NO] *2H20.Whilst the value of the shift is temperature dependent, it is also affected by the electronic environment. The chemical or isomer shift (6) arises from the interaction of electrons with the 57Fenucleus. This interaction is strong for electrons with zero angular momentum, that is s electrons, and 6 is a linear function of s electron density at the nucleus. The contribution of the outer s orbitals make 6 sensitive to the chemical environment. In high-spin complexes 6 depends markedly on oxidation state, and Mossbauer spectroscopy may be used to indicate the oxidation state of high-spin iron complexes. At room temperature 6 varies from + 1.O to + 1.6 rnrn s- for high-spin iron(1I) complexes, and from + 0.45 to + 0.75 mrn s-‘ for high-spin iron(II1) complexes (Figure 1). The chemical shifts of high-spin complexes also depend on the nature of the bound ligands. For example, the effect of ligand electronegativity on 6 is in the same order as the nephelauxetic series.“ Such large effects are not seen in low-spin complexes. Both iron(I1) and iron(IT1) low-spin complexes have 6 values between 0.0 and + 0.3mm s-’ (relative to Na2[Fe(CN),NO].2H,0) illustrating the importance of back donation.“ Indeed, n bonding effects appear to be the main cause of chemical shift variation in low-spin complexes. Bancroft and coworkers derived ‘partial isomer shifts’ (PIS) for a number of ligands, which allow the chemical shift to be calculated for a range of low-spin iron(I1) complexes.12As expected the values of PIS are in the same order as the spectrochemical series. The Mossbauer effect has its origin in the nuclear I = 3 to I = 2 spin transition. This transition is split by the electric quadrupole interaction to give a two line spectrum. Figure 2 illustrates the relevant transitions together with the six line spectrum which results from the simultaneous interaction of a magnetic field. The electric field gradient at the iron nucleus can arise from both asymmetric occupation of the d orbitals and asymmetry of the ligand field. There are therefore a number of factors affecting the quadrupole splitting energy AEQ. In high-spin d’ compkxes where the coordinated ligands are identical, the symmetric population of the d levels normally results in very small values for AEo. In contrast, high-spin d6 centres with one spin-paired 3d orbital have sufficient field gradient at the nucleus for a two line Mossbauer spectrum to be observed. Low-spin d6 complexes have a symmetrical configuration and, for example, [Fe(CN),I4- displays a single line spectrum, whilst the incomplete t2g set in low-spin d S iron(II1) creates a field gradient which induces splitting similar to that observed in high-spin iron(I1) complexes. In complexes where the ligand environment is not symmetrical, a field gradient is created and electric quadrupole splitting is observed. There are many low-spin iron(I1) complexes which show two line spectra and a range of compounds of the type [FeA,B,], [FeA,B] and [FeB,A,] have been investigated.I6 Theoretically the relation in equation (1) would be expected and this has been
-
’
I
iron(II) and Lower States
1182
High-spin (iron III
High-spi n (iron IIt)
Low-spin complexes
Figure 1 Variation of chemical shift (6) at 25 "C with oxidation state for low-spin cyanide and high-spin chloridcs. Data From refs. 13- 15
A
0
C
0
Figure 2 Schematic description of some "Fe Mossbauer spectra showing original nuclear energy levels (A) slightly perturbed through interaction with S electron density (B) giving rise to a chemical shift (6) resulting from quadrupole interactions (C) and magnetic splitting of the pound and excited states (D). Arrows indicate transitions responsible for one, two and six line Mossbauer spectra
observed within the limits of experimental error. Thus observed AEQ values may support structural assignments, as well as helping to define oxidation state and spin state. Much of the chemically important information to be derived from Mossbauer spectroscopy is obtained from the chemical shift and the quadrupole splitting energy. Additional information can be obtained by measuring the spectrum of the sample in a magnetic field which further removes nuclear spin degeneracy" and often results in a six line spectrum. AE, (trans) = 2AEQ (cis)
(1)
44.1.1.6 Oxidation States and Coordination Numbers
Iron compounds are known in which the oxidation state of the metal ranges from - I1 (d") to
I
Iron ( I I ) and Lower States
1183
VI ( d 2 ) , and the examples in Table 1 illustrate the kinds of complexes formed and their stereochemistries,.Several of these oxidation states are not well characterized; the - I and V states are rare, while I is not common. The most frequently encountered oxidation states in complexes of a conventional coordination type (particularly in aqueous media) are I1 ( d 6 )and I11 (d’): the ferrous and ferric states respectively. The preferred nomenclature, and that used here, indicates these oxidation states as iron(I1) and iron(II1).
’,
Table 1 Selected Examples of Iron Complexes Illustrating the Range of Oxidation States and Coordination Geometries
Suidarion stare - I1
(d”)
Coordination number
Complex
Coordination geometry
Tetrahedral
4
Trigonal bipyramidal Square pyramidal Tetrahedral Trigonal bipyramidal square pyramidal Octahedral Pentagonal bipyramidal Dodecahedral Trigonal Tetrahedral Square pyramidal Trigonal bip yrimidal Octahedral Pentagonal bipyramidal Dodecahedral Tetrahedral
5
111 (d’)
5
[Fe(diars), Cl,]” Fa: -
Octahedral Tetrahedral Tetrahedral
,
Cvmenrs Uncommon oxidation state, but familiar as Collman’s reagent Na, [Fe(CO),] and other carbonyl anions Uncommon oxidation state in classical complexes but extensively found in organometallic compounds Very few paramagnetic irono) complexes have been reported Common oxidation state in aqueous solution. Stabilized by high-field ligands, otherwise easily oxidized to iron(II1). Octahedral geometry is most common.
R.J 1
2 3
4 5
9 Most iron(II1) complexes are octahedral but tetrahedral and square pyramidal geometries are also important. In aqueous media, Fe3+ readily hydrolyzes and bridged dimeric species are common. Low-spin complexes are only obtained with the strongest field ligands
10
11 12
13 14 15
lron(1V) is not common; known in mixed oxidcs. Most easily prepared complexes are Lhose with dithiocarbamate ligands Only known in the sotid oxides M,Fe04 and not well characterized Blue FeOi- is very strongly oxidizing, finds uses in organic chemistry
ipe = 1,2-bis(diphenyphosphino)ethane. 3-naph = 1,8-naphthyridine. : = dithiocarbamate. PAP = macrocycle from 2,9-di(l -methylhydrazino)-l ,IO-phenanthroline and 2,6-diacetylpyridine. R. G. Teller, R. G. Finke, J. P. Collman, H. B. Chin and R. Bau, J. Am. Chem. Soc., 1977,99, 1104. A. F. Clifford and A. K. Mukherjee, Inorg. Chem., 1963, 2, 151. P. Giannocaro and A. Sacco, Inurg. S w t h . , 1977, 17, 71. L. H. Pignolet, D. Forster and WAD. Horrocks, Inorg. Chem., 1968, 7, 828. M , DiVaira and P. L. Orioli. Acta Crystallogr., Sect. E, 1968, 24, 1269. A. M. Brodie, S. H.Hunter, G. A. Rodley and C. J. Wilkins, Inorg. Chim. Aatu. 1968, 2, 195. L. Johansson, M. Molund and A. Oskarsson, Inorg. Chim, Aclu, 1978, 31, 117. M. M. Bishop, J. Lewis, T. D. ODondghue. P. R. Raithby and J. N. Ramsden, J . Chem. Soc., Dahon Truns., 1980. 1390. P. Singh, A. Clearfield and I. Bernal, J. Coord. Chem.. 1971, 1, 29. W. Buerger and U. Wannagat, Monutsh. Chem., 1963, 94, 1007. 5. Zaslow and R. E. Rundle, J. Phys. Chem., 1957, 61, 490. . R. L. Martin and A. €5. White, Inorg. Chem., 1967, 6, 712. 1. S. Wood and J. Drummond, Chem. Commun.. 1969, 1373. 1. C. Bailar and E. M. Jones,Inorg. Sq’nth., 1939, 1, 35. M. D. Lind, M. J. Hamor, T.A. Hamor and J. L. Hoard, Inorg. Chem., 1964, 3, 34. V. Logan, T.J. King, A. J. Morns and S. C. Wallwork, J. Chem. Sac. ( D ) . 1971, 554. 3. K. Bauer and H. G. Tennanr, J . Am. Chem. SOC.,1972,94, 2512. 3. S. F. Hazeldean, R. S. Nyholm and R. V. Parish, J. Chem. Soc. ( A i . 1966, 162. N. A. Levason and C. A. McAuliffe, Coord. Chem. Rev.. 1974, 1 2 , 151. i. J. Audette and J. W. Quail, Inorg. Chem.. 1972, 11, 1904.
16 17
18 19
20
Iron(lI) and Lower States
1184 44.1.1.6.1
Low oxidation states
As expected, the lower oxidation states of iron are stabilized by iz acceptor ligands such as phosphines and carbon monoxide, and there is a large volume of ‘organometallic’ chemistry involving what are formally iron(0) centres.’ There are very few examples of iron in the oxidation states - I1 (d’) and - I (d’). While the - I1 oxidation state is rare, the pale yellow compound Na,[Fe(CO),] (best prepared by reducing [Fe(CO),] with sodium amalgam in THF) and other derivatives of [Fe(CO),]’- are familiar because they are being increasingly used in the synthesis of aldehydes and ketone~.”,’~ The [Fe(CO),]’- anion is isoelectronic and isostructural” with tetrahedral [Ni(CO),]. Many iron(0) compounds are known, most of which are best regarded as organometallic compound^.^ However, species such as [Fe(CO,)L,] (L = PPh,, AsPh, etc.) are considered in this chapter. Reduction of conventional iron coordination complexes can yield species containing iron(0). For instance, alkali metal reduction” of red [Fe(bipy),I2+ affords black [Fe(bipy),], but care is necessary when assigning oxidation states in such complexes without complete spectroscopic characterization, since reduction of the ligand (rather than at the metal) may take place where extensively delocalized ligand systems are involved such as with the macrocyclic porphyrins” and phthalocyanine^.'^ While not common, a number of iron(1) complexes have been isolated and characterized, although as might be expected, disproportionation to the more stable iron(0) and iron(I1) will take place in many situations. Several iron(1) complexes with phosphine ligands such as the trigonal bipyramidal [FeCl(dppe),] have been reported. Alkali metal reduction of iron(I1) phthalocyanine ([FePc]) gives [FePcI- that contains iron(1). More highly reduced species may also contain iron in the same oxidation state with additional electrons being localized on the ligand.23Several nitric oxide complexes are considered to contain iron(1) centres, including the coloured species [Fe(H,O),NO] of the nitrate ‘brown ring test’. Assignment of oxidation state in these complexes is often not straightforward, and accordingly it has not been firmly established in many instances. Therefore, in this chapter nitric oxide complexes have been grouped together rather than divided according to oxidation state. A number of more or less transient iron(1) complexes have also been generated using electrochemical or radiolysis techniques and characterized in situ by spectroscopic methods.
44.1.1 h.2
Iron (ZZ)
An extremely large number and wide range of iron(I1) and iron(TT1) complexes are known. These dh and d’ configurations represent the most important oxidation states of iron. Most iron(1T) complexes have an octahedral geometry, although there are examples of four-, five- and even eight-coordination. Salts of tetrahedra1 [FeC1,I2- are readily obtained with large cations;24other tetrahedral complexes include [Fe(OPPh,),12 +,[Fe(OP(NMe2),},]2Cand the iron(I1) centre in the rubredoxins with four sulfur donors. Some tetrahedral rubredoxin model compounds have also been ~haracterized.~~ Square planar iron(I1) complexes are formed with tetradentate nitrogen donor macrocycles of which the most familiar are phthalocyanine and the porphyrins, to which in recent years have been added a large number of additional ‘synthetic’ macrocycles?6 The magnetic properties of some of these complexes indicate the relatively unusual intermediate spin (S = 1) ground state. There is a strong tendency for all of these compounds to acquire two axial ligands (usually nitrogen donors) and become six-coordinate and low-spin. Carbon monoxide and dioxygen adducts are also formed with these complexesz7and it is now understood how, via kinetic or steric effects, to inhibit oxidation to p-oxo iron(II1) species so mimicking the behaviour of haemoglobins and related oxygen carriers. Numerous five-coordinate high-spin iron(I1) complexes, with tetradentate nitrogen donor macrocyclic ligands, have been reported in which the fifth coordination site is occupied by a halide ion. Other five-coordinate complexes include [Fe(terpy)X,] (refs. 28-30) and examples with tripod ligands, such as [Fe{N(CH,CH,PPh,), 1x1’. High-spin and low-spin complexes with tripod ligands are known:’ as well as those displaying spin cross-over. The iron(I1) ’4 ground state is split by octahedral fields into ’T, and ’Eg states. High-spin octahedral iron(I1) complexes have magnetic moments typically about 5.2 BM and show broad 5T2, to ’E, transitions3’ occurring in the visiblelnear-IR region of the spectrum. Thus salts of the familiar [Fe(H20)6]’+ion are pale green in solution, with an absorption peak at 10400 cm-’ and a shoulder at 8300cm-’ that may result from a Jahn-Teller effect.32Most simple iron(I1) salts are susceptible
-
I
Iron(ll) and Lower States
’
1185
to air oxidation, a process catalyzed by copper(I1) and palladium(I1) that has been extensively studied.” With very strong-field ligands, like cyanide ion and cr,cr’-diimines, spin pairing can take place with the result that complexes such as [Fe(phen), (CN),] are essentiaily diamagnetic, although some temperature independent paramagnetism, amounting to rather less than 1 BM, is commonly r e p ~ r t e d . ~These ~ . ~ ’ low-spin complexes are usually intensely coloured (typically red or purple) due to strong metal-ligand charge transfer36bands that obscure d-d transitions. An important feature of low-spin octahedral iron(I1) complexes, which is in accord with crystal field stabilization energy consideration^,^^ is their stability and kinetic inertness. As a result of this, though not as extensive as that of the isoelectronic, highly inert cobalt(II1) centre, a considerable amount of work on their preparation and the kinetics and mechanisms of their reactions has been accumulated. While [Fe(phen),(CN),] is low-spin, most complexes of the type [Fe(phen),X,] are high-spin. However, by appropriate choice of the anion X- ,it is possible to obtain complexes having strongly ternperaturedependent magnetic moments that cross over from being low-spin to high-spin as the temperature is increased. A number of other iron(I1) complexes display similar b e h a ~ i o u r Moreover, .~~ complexes such as [Fe(phen),(ox)] have two unpaired electrons,39an unusual situation for a d6 centre that results from deviation from strict octahedral symmetry. Although very high coordination numbers are rare for iron(II), eight-coordination is well characterized in the red-orange high-spin compIex with I ,&naphthyridine, [Fe(naph),]” , which in the perchlorate salt has a distorted dodecahedral geometry.40A1 Seven-coordination is more common than eight-coordination, and is known in several macrocyclic complexes.42
44.1.1.6.3
Zron(ZZ1)
As with iron(II), most iron(II1) complexes are octahedral; however, four- (tetrahedral) and five-coordination are also well known (see Table 1). There are also examples of higher coordination number that include the relatively uncommon seven-coordination found in some iron(II1) nitrogen macrocycle complexes43and in [Fe(edta)(H,O)]- (ref. 44). Eight-coordination is exemplified4’ by [Fe(NO,),]-. Iron(II1) has a strong affinity for oxygen ligands and a marked tendency to form oxo-bridged complexes. With p o x 0 bridges, as for instance in [{Fe(salen)},O] (ref. 46) and [(FeC1,),0]2- (ref. 47) that contain two five- and two four-coordinate centres respectively, there is strong antiferromagnetic coupling that results in low and temperature dependent magnetic moments. Monomeric five-coordinate complexes such as [Fe(S2CNR2),X] havea three unpaired electrons. A large number of iron(II1) complexes with sulfur donors are known. In most octahedrd monomeric complexes, iron(II1) is high-spin, for instance [Fe(acac),], and these complexes have magnetic moments close to the expected spin-free value. Complexes containing very high field ligands, as in [Fe(CN)6I3’-and [Fe(phen),13+,are low-spin and they display higher than predicted room temperature magnetic moments due to large orbital contributions. As expected in this situation, the observed magnetic moment decreases as the temperature is lowered. Like low-spin iron(I1) complexes, low-spin iron(II1) centres are significantly more kinetically inert than their high-spin analogues. As previously noted, spin cross-over behaviour is well established in iron(I1) complexes and there are also examples involving iron(II1) species. For example, a variety of iron(I1I) dithiocarbamate complexes of the type [Fe(S,CNR,),J having a trigonally distorted octahedral geometry display high-spin/low-spin cross-over b e h v i o ~ r . ~Little ~ , ’ ~is known about the spin forbidden d-d transitions of iron(II1) complexes due to near-UV charge transfer bands that ‘tail’ into the visible part of the spectrum. Even simple iron(II1) salts in aqueous media are yellow.
44.1.1.6.4
Zron(ZZ)-iron(ZZZ) redox behaviour
The [Fe(H,O),]*+ cation is oxidized by molecular oxygen to [Fe(H20)6]3+.The usually quoted potential for the Fe3+”+couple is + 0.77 V, but the latest discussions’gives + 0.738 V. The standard electrode potential is strongly influenced by the nature of the ligands involved, and a wide range of Eo values is covered depending on the complex concerned. Indeed, it is this ability of iron to span a wide range of redox potential that is exploited in a number of biological systems. Figure 3 illustrates the standard electrode potentials of some typical complexes. In general the high field a,a’-diimine ligands stabilize iron(I1) relative to the aqua cations when a low-spin complex is formed,
rron(Ir) and Lower States
1186
an$ the importance of back-bonding in these complexes is emphasized by the significant stabilizing effect of electron withdrawing substituents in the ligand.
Simple t&lXphen)J"+ Ferrocene complexes [ F e C b i p y l ~ ~ ) , _ ~ ] ~ ~ F e O
Lr5
"
[Feloo)(OH&]
Haem
*+ derivatives
Non-haem derhdiws
Iron (II)stabilized +I2
5Me
3,4,7,€+Me, +OB
(bipyJ,(CNI,
11
I
+04
0
t
3oxin0 te %ximote-
Iron (m)stabilized -04
Figure 3 Standard oxidation potential for iron complexes in aqueous solution
The anionic cyano iron(TI1) complexes [Fe(CN),I3 and [Fe(bipy)(CN),]- are more stable than the iron(IT) forms relative to the aqua cations. This is at variance with the expectation of the iron(I1)xyanide bond having greater strength than the iron(III)-cyanide bond. It appears that in these instances the electrode potentials are influenced by the large negative entropies of hydration of the anionic iron(I1) complexes, and it is this that destabilizes them with respect to the higher oxidation ~ t a t e . That ~ ~ . is, ~ ~the higher bond strength (enthalpy) of the iron(I1) complex is outweighed by a hydration (entropy) term which no doubt involves structure-forming hydrogen bonding between the cyanide nitrogen atoms and the solvent water. In keeping with this proposal, the iron(II1) oxidation state becomes progressively more stable along the series [Fe(bipy)2(CN),]0'+, [Fe(bipy)(CN),I2-'- and [Fe(CN),]4-'3- as the number of cyanide ligands increases. In the solid state, iron complexes can display interesting internal redox behaviour as a result of applying high pressure.s4 Reduction of iron(II1) to iron(I1) on the application of pressure is well documented and is usually monitored by Mossbauer or optical spectroscopy. For instance, conversion to iron(I1) in [Fe(acac),] proceeds continuously with pressure over the range above 30 kbar,
Iron(il) and Lower States
1187
but does not go to completion. After the pressure is released and mechanical strain removed by grinding the sample, iron(ll1) is reformed. Zn examples of this kind it appears it is the ligand that is being oxidized. Application of high pressure to solid complexes that are high-spin, but close to the cross-over point, can cause them to go low-spin (e.g. [Fe(phen),(SCN),]) but there are examples where application of pressure causes a low-spin complex to become high-spin. Thus at pressures approaching 200 kbar low-spin [Fe(CN),I4- becomes high-spin, and [Fe(phen),12+similarly displays a Mossbauer spectrum characteristic of high-spin iron(I1). Clearly, in order to interpret these results the overall total volume for the different forms has to be considered, as well as the spin-transition energetics.
44.1.1.6.5
Mixed irorr(ZI)-iron(ZZI) complexes
Both delocalized and localized Fe"/Fe'[' mixed valence complexes are known, the most comprehensively investigated being Prussian blue which has the limiting formula Fe:"[Fe11(CN)6], * 14H20.The single crystal structure has been reported:' and the degree of valence delocalization has been extensively probed.56Recently, thin films of insoluble Prussian blue have been intensively studied because of their electrochromic behaviour; electrochemical oxidation gives continuous mixed valence composition (up to complete oxidation) while reduction to Prussian white A number of other delocalized systems have been studied takes place cleanly at a critical p~tential.~' involving aromatic bridging ligands. In contrast, the mixed valence compound Fe,F, -2H,O is a nonmolecular solid with a three-dimensional network structure with distinct Fe" and Fe"' coordination sites, in which the water is bound to the Fe" centres.58
44.1.1.6.6 Higher oxidation states The IV, V and VI oxidation states of iron are all known in oxyanions. However, relatively few well-characterized complexes with iron in an oxidation state higher than three are known. A review59 concerned with these oxidation states was published in 1974. Complexes containing iron(1V) are not large in number. This appears to be the highest oxidation state of iron in the catalases,6' and a considerable body of information has been collected on the strongly oxidizing iron(1V) porphyrins.61*62 The green/black low-spin d4 [Fe(diars),XJ2+ (X = C1, Br) obtained by oxidation of [Fe(diars),X,]' with concentrated nitric appears to have a trans configuration and is not very stable. More recent examples of well-characterized (X-ray) iron(1V) species involve dithiocarbamate ligand^.^^,^^ For example, low-spin [Fe(S,CNEt,),]+ is obtained by merely treating the corresponding iron(II1) complex with an excess of i ~ d i n e . ~The . ~ ' interpretation of the Mosfbauer spectra of these complexes is based on that applied to d5 and d6 systems.68An organometallic compound of the type [FeRJ has been reported69with R being the particularly favourable I-norbornyl group, and this could be thought of as being an iron(1V) derivative. This 'oxidation state' is, however, rare in organometallic compounds. The + 5 oxidation state is not well defined and seems only to exist in M,FeO, (M = alkali metal), but even these compounds have not been obtained pure.70The + 6 oxidation state is more familiar. Deep red/purple saits, containing discrete tetrahedral FeOi- units, are obtained by anodic oxidation, or the hypochlorite oxidation of freshly precipitated iron(I1I) hydroxide." Water soluble potassium ferrate(V1) (KZFe04)is fairly stable in basic solution, but gives iron(IT1) and oxygen in neutral or acid solution. It is more strongly oxidizing than permanganate and has some application in organic oxidations. Other than these oxyanions there appear to be no stable coordination complexes of iron(V) or iron(V1).
44.1.2
NITRIC OXIDE COMPLEXES
I
44.1.2.1 Introduction The nitric oxide molecule shows many similarities to carbon monoxide in its ability to form complexes with transition metals. Nitric oxide has an extra electron, occupying a n* antibonding orbital, which is relatively easily lost. In the case of terminally bound NO, simple MO theory predicts that whilst M-NO' will be linear, M-NO- may be a 'bent' bond. The potential thus
Iron (11) and Lower States
1188
exists to establish the formal oxidation state of the metal centre in nitric oxide complexes by examining their IR spectra. Although well-authenticated examples of bent M-NO bonds exist:’ they are rare and there have been numerous discussions of the bonding in transition metal nitrosyl comple~es.’~ Iron ’ nitrosyl complexes have not been classified by oxidation state but are discussed here separately.
44.1.2.2
Simple and Binary Nitrosyls
Reports in the older literature suggested the existence of [Fe(NO),]. However, this is likely to be a dimeric compound8’ containing a bridging hyponitrite ligand (1). One electron reduction of the complex {Fe(NO), Cl], , chemicallys3or electrochemically,s4leads to a species which acts as a catalyst for the cyclodimerization of dienes. This has been described as ‘Fe(NO),’, which probably exists in THF solution as [Fe(NO),(THF),]. Aqueous solutions of ferrous salts absorb nitric oxide to give the familiar colour of the ‘brown ring test’, which is duess to the complex ion [Fe(H,0),N0]2+.
44.1.2.3 (ii
Nitrosyl Carbonyls
iFe‘e(COi2( N O ) , ]
Several preparations of dicarbonyl dinitrosyl iron have been reported including treatment of [Fe,(CO),,] or [Fe,(CO),] with nitric 0xide,~~7~’ acidification of a mixture of [HFe(CO),]- and nitrite ion,” reaction of iron pentacarbonyl with nitrosyl chloride’’ and acidification of a mixture of [Fe(CO),NO]- and nitrite iong0as in equations (2) and (3). [Fe(CO),]
Na[Fe(CO),NO]
+ NaN02 + 2NaOH % Na[Fe(CO),NO] + CO + Na,CO, + H 2 0 + NaN02 + 2MeC0,H + [Fe(C0)2(NO),] + CO -k 2MeC0,Na i-H,O
(2) (3)
Red crystalline [Fe(CO), (NO),] has a distorted tetrahedral geometry. It is somewhat air sensitive, melts a t 18 “Cand is readily transferred in the vapour phase in a vacuum system. The IR spectrum in cyclohexane solution contains strong carbonyl bands at 2083 and 2034 cm-’ and strong nitrosyl bands at 1810 and 1756 cm-I. A full IR assignment has been reported?’ [Fe(CO),(NO),] reacts with a wide variety of ligands with preferential CO displacement; both monomeric and dinuclear complexes result. A kinetic study of CO displacement reactions of [Fe(CO), (NO),] by phosphines, phosphites and triphenylarsine supports92a bimolecular displacement mechanism where the entering ligand attacks the ‘soft’ iron atom to give a five-coordinate transition state. The substitution can be catalyzed by various hard ba~es.9~ This is thought to occur via attack on a carbonyl carbon atom which destabilizes the M-CO bond so making carbonyl a better leaving group.
( i i ) The [FeCCO), NO J - anion
The [Fe(CO),NO]- anion is formed by the reaction of [Fe(CO),] with nitrite ion in methanol, and isolated as [Hg(Fe(CO),NO},] by reaction with aqueous mercurous ~ y a n i d ebut ~ , many ~ ~ other salts are also k n ~ w n . The ~ ~ anion ~ ’ can be conveniently prepared in high yieldg8as the bright yellow salt [(Ph,P),N]pe(CO),NO] by the reaction of [(Ph,P),N](NO,) with [Fe(CO),] in THF (vco, 1982m, 1876s; vNo, 1650mcm-I). IR studies suggest that in the mercury derivative the three metals are bonded in a linear fashion, each iron atom having trigonal bipyramidal ge~metry.~’Carbon in benzene at room temperature to give monoxide is readily displaced by IC acceptor [Hg{Fe(CO),(NO)L},] (L = R,P, R,As, R,Sb, (RO),P) in which the geometry of the iron and mercury atoms remains unchanged.ID’In the case of reaction with pyridine (or other N bases and isocyanides) however,”’ disproportionation yields a mixture of [Fe(py),][Fe(CO), NO] and
Iron(II) and Lower States
1189
[Fe(py),(NO),]. Interestingly, [Hg(Fe(CO), NO),] reacts slowly with tris(dimethy1amino)phospine (Tdp) in refluxing benzeneY4to give first partial then complete substitution of CO (equation 4). Mercury iron nitrosyl complexes are the basis for two route^"^-'^ to iron-tin, iron-germanium and iron-lead bonds. These are illustrated in equations ( 5 ) and (6).
[Hg(Fe(CO),(NO)(PPh,)},]
.
+
+
[Cl, SnFe(CO)2(NO)(PPh3)]
2SnCh
HgCl,
(5)
The [Fe(CO),NO]- anion reacts with LAuX (L = (MeO),P, Me,P, Ph,P, (p-ClC,H,),P) to give complexes [LAuFe(CO),NO] containing gold-iron bonds"' which undergo CO substitution reactions in a fashion similar to [Hg(Fe(CO),NO),] as in equation (7), where L = (PhO),P, Ph,P. Conductimetric titration has been employed to studylo6the reduction of [Fe(CO),NO]- through to [Fe(CO),N0I5- although the oxidation state of the metal at any particular stage is uncertain. [Ph,PAuFe(CO),(NO)] -t L
44.1.2.4
2$Gk,
[Ph,PAuFe(CO),(NO)LJ
(7)
The Nitroprusside Ion
Sodium nitroprusside (Na,[Fe(CN),NO] *2H,O), possibly the best known of all nitrosyl complexes,'O' is made either by the action of nitric acid (equation 8) or sodium nitrite (equations 9 and 10) on the hexacyanoferrate(I1) anion. The equilibria (9) and (10) are driven to the right by the addition of barium chloride to the reaction mixture and by removing HCN in a stream of CO, passing through the hot solution. The overaIl process'08is shown in equation (1 1).
-
+ C02 f NH: IFe(CN)J4- + NO; e [Fe(CN)SN02]4- + CN[Fe(CN),NO,l4- + H 2 0 S [Fe(CN),NO]'- + 2 0 H -
[Fe(CN),I4-
2[Fe(CN),I4-
+ 2NO; +
+
4W4 -t NO;
3Bazf
+
3C0,
(8)
[Fe(CN),N0l2-
+ H,O
+
2[Fe(CN),N0I2-
+ 2HCN +
(9) (10)
3BaC0, (1 1)
Red crystals of sodium nitroprusside are readily soluble in water and ethanol; aqueous solutions undergo a complex decomposition process in light.'" Low-spin (diamagnetic) [Fe(CN),N0I2-, as the sodium, iron(II), copper(I1) and zinc(I1) salts, shows considerable conversion to the high-spin state at 20-1 10 "C under high pressure."' The nitroprusside anion has an N-0 bond length of 1.13 1$,an Fe-NO angle of 180 O and an NO stretching frequency of 1947cm-'indicating an NO+ complex of iron(I1) with extensive a bonding. However, the Mossbauer spectrum and shortness of the Fe-N bond (1.63A) argue"' for a less positively charged NO ligand and indeed the X-ray photoelectron spectrum"z indicates a positive charge on NO of approximatery 0.35.One-electron reduction of [Fe(CN),NOl2- by sodium in liquid arnrn~nia,"~ or a variety of radicals formed in aqueous solution in radiolysis experiment^,"^ results in the formation of [Fe(CN),N0l3-, which undergoes a first order decay with loss of cyanide to give [Fe(CN),NO]'-. Both the tetragonal pyramidal structure''5 of [Fe(CN),NO]'- and the likelihood that it is a &cyanide which is lost in both this transformation and reactions involving nucleophilic attack on nitroprusside are predicted from INDO molecular obital calculationsIl6based on recent structure determinations. Multielectron
+
1190
Iron(li) and Lower States
electrochemical reduction in nonaqueous solvents results' l 7 in the formation of [Fe(CN),NH, OH]-. The nitroprusside ion is attacked by hydroxide to yield the corresponding nitro compound [Fe(CN),NO,l4-. The rate of formation of [Fe(CN),NO2I4-is first order in OH- and nitroprusside concentrations"' and electrochemical studies suggestLLg the formation of the transient intermediate [Fe(CN),(N(=0)OH)I3- (equations 12 and 13). [Fe(CN),NO]'[Fe(CN),NO,HI3-
+
+
OH-
OH-
-
% [Fe(CN),NOzH]3-
+
[Fe(CN),NOJ-
(12)
(13)
H20
The yellow solid Na,[Fe(CN),NO,] is preparedL2'by the reaction of concentrated aqueous NaOH with a methanolic solution of sodium nitroprusside at 2 "C. The kinetics of the hydrolysis equilibrium (14) have been investigated."* [Fe(CN),H,0I3- is also the usual product in the reaction of nitroprusside ion with ketones, and molecules containing active methylene groups, in alkaiine solution.lWThe reaction with acetone has been studied in some detail'" and the overall reaction in Scheme 1 proposed. Salts of the type (2) shown in Scheme 1 can be isolated. The red ion [Fe(CN),N(O)CHCOPhI4- formed in the reaction of nitroprusside with acetophenone can be protonatedI2*to yield a blue, isolable compiex thought to have structure (3). [Fe(CN),NO2l4-
+ H 2 0 ==
[Fe(CN),H20I3-
+ NO;
e,
[Fe(CN),H20I3- + bleCOCH=NOH
CHCOMe
(14)
l4;
H+
Scheme I ,O.*.+*H, (EiC),Fe -N
3-
...-cr] Ph
(3)
Colour reactions of the nitroprusside ion such as these have been known for many years.Iz3Most of these reactions involve nucleophilic attack on the coordinated NO group resulting in either displacement of NO or the formation of an unusual ligand bound to the iron. In the Gmelin test for SH- a purple-violet colour is formed. This has been r e p ~ r t e d " as ~ being due to the ion [Fe(CN),NOSI4-; however this is probably an oversimplification'25and no definite structural assignment has been made. Na, Se and Na,Te react with nitroprusside to give'26 respectively blue-green and black colours. Alkaline solutions of mercaptans react with nitroprusside to form purple-red c01ours'~~ and thus this reaction has been used to identify cysteine in mildly basic solution.'*' Thiourea and its derivatives react with nitroprus~ide'~~ to give red coloured adducts which, in the case of thiourea, slowly becomes blue. The structures of these adducts are uncertain but the final blue colourL3' may be due to [Fe(CN),(NCS)I3-, which can be produced by the photolysis of nitroprusside ion in the presence of aqueous thiocyanate, or by acidification of alkaline solutions of [Fe(CN),NO2I4- containing thiocyanate. The reaction path shown in Scheme 2 has been proposed.13' from orange Whilst crystals of [Fe(CN),(NCS)][Bu,N], are orange, the colours of the salt varyLo7 in acetone, to purple in ethanol and blue or purple in aqueous solutions. The analagous selenium compound [Fe(CN),(NCSe)][Bu,EJI, is in the reaction of Na,[Fe(CN),NH,] with n-butyl ammonium chloride and KNCSe in acetone.
Iron(lI) and Lower States
a
[Fe(CN)5N012- + H20
r :
[Fe(CN)SH2013-+ NO+ I
-
[Fe(CN),NO2I4- + KCS-
1191
Pes-
[Fe(CN),NCS14- + NO; k N 0 .
[ Fe(CN),NCSI3-
Scheme 2
Nitroprusside ion reacts with ammonia'33at pH 11.612.5 to give [Fe(CN),NH3l3- and nitrite as in equation (15); with higher concentrations of ammonia nitrogen is evolved, presumably following attack on the NO ligand by ammonia. Similar behaviour is observed with primary aliphatic amines.'34In liquid ammonia nitroprusside is attacked by NaNHPh to give Na,[Fe(CN), {(N-0) NPh}], whereas only Na, [Fe(CN),NH,] is isolated'35following the reaction with NaNRR.
Sodium nitroprusside is a rapidly acting peripheral vasodilator which has been k n o ~ n ' ~ ~for ,'~' some time to lower blood pressure swiftly in emergencies. Whilst useful for controlling blood pressure during surgery, only low doses have been recommended because sodium nitroprusside apparently forms free cyanide in vivo. The concentrations of CN- measured in the past may, due to photodecomposition during lengthy colorimetric analysis. however, have been an artefactL3* The reactions of sodium nitroprusside with amines have been in~estigated'~~ as part of a study to identify its mode of biological action.
44.1.2.5
Complexes Containing Nitric Oxide and Sulfur Donors
The most familiar complexes in this class are Roussin's red and black saltslmobtained as a mixture from the reaction of iron(TI1) sulfide with nitric oxide and an alkali metal ~u1fide.I~' The dimeric and diamagnetic red salt, K,[Fe(NO),S], gives rise to a series of stable esters [Fe(N0)2SR]2(R = Me, Et, Ph etc.) which are also diamagnetic, dark red and soluble in organic solvents. Some preparative routes14*to the red salt and its esters are shown in Scheme 3.
-
/ \ K2 [Fe(N0l2S1
RSH
[ ~ ~ ( ~ O ) a ]
NO/rH,Cl a
K.S
[Fe(N0)211
~
'r-*
[Fe(KO)2SR1
or UaNO,
RSH
/NnoH,i.tDH
IFt'lSK)(CO),I,
Fe(OH), + RSH
[Ft.(NO)z(CNRhJ
/
IF4NO)>(CO)zl " A . R. Butler, C. Glidewell and J. McGinnis. Inorg. Chim. Acta, 1982, 64, L77.
Scheme 3
The structure of the red ethyl ester143(4) has two iron and two sulfur atoms in a planar configuration, the iron atoms being approximately tetrahedrally coordinated by two bridging sulfur and two nitrosyl ligands. The angle at sulfur is 73.7 ', the Fe-Fe distance 2.72 A and the N-Fe-N bond angle 1 1 7 O . The X-ray structure'43reveals the presence of only the C,,isomer whereas NMR measurements in d8-toluene show both C,,and C,, forms.'a The activation energies for the interconversion of the two forms (typically AG# = 75.3 kJmol-') have been derived for a variety of alkyl substituents.
1192
rron(i1) and Lower States
The black salts M[Fe,S,(NO),] (M = Na, K, Cs, AsPh,) are classically preparedk4'from NaNO,, KHS and iron(I1) sulfate. They may also be prepared in good yield'44 by nitrosylation of [Fe,S2(C0),], [Fe, s2(cO)6]*- or [Fe,S2(C0),] by reaction with sodium nitrite in a refluxing solution of sodium hydroxide in aqueous ethanol. The structure (5) consists of a trigonal pyramid of iron atoms with triply-bridged sulfur atoms and iron-iron bonds between the apical Fe-NO group and three basal Fe(NO), groups. The three Fe(NO), groups are also joined by the triply bridging sulfur atoms. An early structural found the Fe-N bond length shorter in the apical group. More recent work'46on the AsPh: salt, prepared by Na/Hg reduction of [Fe,(NO),(p,-S),] in THF, has however shown that all Fe-N and N-0 bond lengths in the black monoanion are essentially the same. This observation prompted a reexamination of the bonding in both the red and black salts in comparison with the closely related [Fe,(NO)4(p3-S)4]clusters. The black salt can be formally constructed14 from the dimensionally similar groupings p r e ~ e n t ' ~ ' ,in ' ~ the red salt and in [Fe,(NO),(p, -S)J, The bonding models developed for these two molecules suggest that the bonding in the monoanion of the black salt can be analagously described under C,,symmetry. NO I
/ Et
ON
NO (5)
(4)
The preparation of the five-coordinate iron nitrosyl dithiocarbamate complexes [Fe(NO)(S2CNR2),] was first reported'49 by Cambi and Cagnasso. These compounds may be preparedt5' by reacting a solution of a divalent iron salt and NaS2CNR, with NO in methanol. Crystal structures of [Fe(NO)(S,CNR,),] (R = Et, (6), at room temperature"' and subsequently for R = Me at - 80 "C) showIs2a square pyramidal geometry with an approximately linear apicaI iron nitrosy1 group (Fe-N-0 = 174" and 170 respectively) in contrast to the bent (M-N-0 = 135 ") metal nitrosyl group in the isomorphous complex'53 [ C O ( N O) ( S ~ C NM ~ ~ Both ) ~ ] . iron complexes are paramagnetic. ESR and Mossbauer spectra of [Fe(N0)(S,CNR2),1 have been interpreted'5"156in terms of a formal iron(1) species in which the unpaired electron resides in the iron dr2 orbital. This conclusion is supported'" by a study of 14 iron nitrosyl dithiocarbamate complexes with differing organic substituents. The pK, of the corresponding dialkylammonium ion was shown to be an effective measure of the mesomeric effect of the -NR, groups of the dithiocarbamate ligand which in turn affects vNo and the Fe-N bond length.
P (6)
Green-black five-coordinate [FeNO(S,CNR,),] is convertel into the dark brown six-coordinate molecule [Fe(NO),(S, CNR2)J by action of NO in chloroform. Subsequent investigation,'59however, has failed to confirm this although some evidence for the existence of the molecule as an intermediate was gained from 14N/15Nexchange studies. Direct oxidation of [Fe(NO)(S,CNMe,),] at room temperature'@ leads to cis-[Fe(NO)(S,CNMe2)2X](X = Br, I, NO2) whilst at - 78 "C trans-[Fe(NO)(S,CNMe,),(NO,)] is formed.lS9In this case, the trans isomer is the kinetic product which isomerizes at 5°C to the thermodynamically favourable cis isomer. Trans products are apparently favoured by ligands such as MeCN and -NCS- which are relatively weak CT donors and poor 7c acceptors, Ligands which have either strong 0 donor (and strong R acceptor) properties such as MeNC and NO;, or which are able directly to donate from large R orbitals into vacant cis n* orbitals (such as halides) favour a cis c ~ n f i g u r a t i o n . ' ~~ In the cis nitro compound, NO and NO; undergo16' a novel intramolecular exchange reaction. The cis octahedral geometry of the halogeno derivative [Fe(NO)(S,CNEt,),I] has been confirmed'6zby
Irom(l1) and Lower States
1193
magnetically perturbed Mossbauer spectroscopy at 4.2 K. Whilst the bis-nitrosyl bis-dithiocarbamate complex has not been substantiated, an analagous complex ck--[Fe(NO),(Sacsac),] (Sacsac = dithioacetylacetonate) has been reported'63 (vNo = 1765, 1790cm-I). Other examples of the stabilization of an iron nitrosyl complex by chelating sulfur ligands are the bis-dithiolene complexes'&2[Fe(NO)(S,C,R,),]Z (2 = 0, - 1, - 2, -,3; S G R , = disubstituted cis-ethylene-l,2-dithiolates, tetrachlorobenzene-1,2-dithiolate,toluene-3,4-dithiolate).These complexes have square pyramidal geometry, analagous to the iron nitrosyl bisdithiocarbamate complexes discussed above, as exemplified in a crystal structure'65 of [Fe(NO)(S,C,(CN), ),I (7).The Fe-N-0 bond angle (measured at room temperature) is reported to be I68 '. These complexes undergo a series of one-electron transfer reactionsIa which have been investigated voltammetrically. The half-wave potential for each redox step is dependent on the dithiolene ligand, becoming more as the ligand becomes more electron withdrawing; the order positive S,C,(CN), > S2C,(CF,), =- s2c6cl4 > S,C6H3Me > S2C2Ph, has been observed.'" The effect is also seen in substituted diaryldithiolenes,'" half-wave potentials becoming more positive as substituents on the aryl group become more electron withdrawing. The complexes [Fe(NO)(S2C2R2),] undergo ligand exchange reactions, [Fe(NO){S2Cl&N)Z}(S2C2Phz)]- for example resulting167from mixing the two corresponding bis-dithiolene monoanionic complexes.
P
N I
(7)
In the formation of [Fe(NO)(S,C,Ph,),] by the reaction between [Fe(S,C,Ph,),], and NO in cold chloroform, a purple complex'68 [Fe,(NO), (S,C,Ph,),] -CHCl, can also be obtained. The monoanion can be isolated as a green solid by reaction of the neutral complex with NaBH,. The Mossbauer spectrum of the neutral complexlsgshows that the two iron atoms are not equivalent, possibly due to interaction with the chloroform molecule. Interesting structure-related effects are also ob~erved''~ in the complexes [Fe(NO)(S,CO, Me,)(P,P)] (P,P = diphos, 2PPh,). ESR spectra indicate that the unpaired electron may reside in different metal orbitals depending on the steric influence of the phospine ligands on the basic square pyramidal geometry of the complexes.
44.1.2.6 Nitrosyl Halides [Fe(NO),X] and [Fe(NO),X], (X = C1, Br, I) are known, although the former is highly unstable. [Fe(NO),Cl] is prepared"' by the action of NO on FeCl, at 70 "C in the presence of iron powder; the bromide and iodide analogs are prepared'72from their respective tetracarbonyl halides as in equation (16). The more stable diamagnetic halides [Fe(NO),X], are nonpolar and dissolve in organic solvents. The iodide, which has been studied in the most detail, may be prepared by direct reaction of NO with FeI, at elevated temperature, from [Fe(NO),(CO),j by carbon monoxide displacement'" with iodine or by the room temperature reactioni7,of NO with FeI, in acetone in the presence of iron powder. The dimeric nature of [Fe(NO),X], has been confirmed by a crystal structure determination of the iodide The iron atoms are essentially tetrahedrally coordinated with iodine bridges. Although the bond is long (3.05 A) there is a considerable iron-iron interaction. Both types of halide react with a variety of ligands to form monomeric complexes [Fe(NO),LX]. [Fe(CO),XI2
+ Fe + 6NO -,
2Fe(N0)3X
+
4CO
(16)
= Iron(TI) and Lower States
1194
[XFe(N0l2L1 [Fe(NOj,L I'
[ Fe(NO),diphos]
[ClFeNO( das),]+ IFc(NO)ZLZI
iF e ( N 0 h LA1 tXFe(NO), L 1
" L = PPh,, AsPh,, SbPh,, C,H,,N; W. Hieber and R. Kramolowsky, Z . Naturforsch., TeilB, 1961, 16, 555. W. Hieber and R. Kramolowsky, 2. Anorg. Allg. Chem., 1963, 321, 94. W. Beck and K. Lottes, Chem. Ber., 1965, 98, 2651. bW. Hieber and G . Neumair, Z . Anorg. Allg. Chem., 1966, 342, 93. 'L2 = (Ph2P)?,(Ph,As),; W. Hieber and R. Kummer, Z . Anorg. ANg. Chem., 1966, 344,292. L = acac, amino acid, diamine, thioamide, o-(SH)(NH,)C,H,; W. Hieber and H. Fuhrling, Z . Anorg. Allg. Chem., 1971, 381,235; W.Hieber and H. Fuhrling, Z . Anorg. Allg. Chem., 1970,377,29; W. Hieber and K. Kaiser, Z . Anorg. Allg. Chem., 1968, 358, 271. "The bromo and iodo derivatives [XFeNO(das)?]+ were not prepared from the nitrosyl halide dimer. W. Silverthorn and R. D. Feltham, Inorg. Chem., 1967, 6, 1662. Scheme 4
Some reactions of [Fe(NO),X], are illustrated in Scheme 4. The complexes [Fe(NO),X,] and [FeNOXJ are also known. [Fe(NO),X,]~(PPh,),] is ~ r e p a r e d "by ~ the reaction of alkyl halides with [Fe(CO),NO][N(PPh,),] (X = C1, Br or I). Reaction of the carbonyl nitrosyl with halogens affords'75[FeX,(NO)][N(PPh,),] (X = C1, Br) or [FeX,(NO),][N(PPh,),] (X = I).
44.1.2.7
NO Adducts of Complexes Containing Polydentate Ligands
There are few examples of simple nitrosyl complexes containing 0 N donor ligands. The Fe-NO moiety requires to be stabilized by appreciable d--71 interaction and this is at odds with the classically 'hard' nature of N donor bases. In cases where iron nitrosyls are stabilized in coordination with a nitrogen (or oxygen) donor ligand, there is usually a degree of n. acceptor character associated as, for example, with delocalized planar Nitric oxide reacts with solid p-iron phthal~cyanine"~ to yield a 1: 1 adduct (from which NO is readiiy displaced by pyridine). A similar route has been used to prepare nitrosylrnyogl~bin'~~ and nitro~ylhaemoglobin"~ for single crystal ESR studies which have been interpreted on the basis of a bent metal-nitrosyl bond. Iron nitrosyl complexes of tetraphenylporphyrin (TPP) have been investigated as possible models for the haem protein systems. The five-coordinate'sOcomplex [Fe(NO)TPP] has an Fe-NO bond angle of 151 O, whereas'*' in [Fe(NO)(TPP)(1-MeIm)] (MeIm = 1-methylimidazok) it is 142 '. The analogous complex [Fe@TO)(TPP)(4-MePip)] (CMePip = 4-methylpiperidine) was found to crystallize in two distinct forms, with and without chloroform solvate,lS2in which the Fe-NO angles are 138.5 and 143.7 respectively. A structural correlation has been noted'" between the stretching frequency of the coordinated nitrosyl and the length of the trans Fe-N bond. Nitrosyl (meso-2,3,7,8,12,13,17,18-dctaethy1-5-nitroporphyrinato)iron(II) has been shown''3 to dimerize at high concentration at 9 K. This observation implies an explanation of the ESR spectrum of the NO-haem-salicylate system, which is in turn a model of the observation that the interaction of oxyhaemoglobin with salicylate causes enhanced oxygen uptake in the presence of a substrate such as ascorbate. An interesting exampie of an iron nitrosyl complex with an N donor ligand without a delocalized 71 system is providedlS4 by the complex [Fe(TMC)(N0)I2+ (9) (TMC = 1,4,8,11-tetramethyl-l,4,8,11-tetraazacyclotetradecane). The complex is stabilized with considerable distortion from tetragonal pyramidal towards trigonal bipyramidal geometry. Nitric oxide reacts with an ethanolic solution of [Fe(salen)] [salen = N,N'-ethylenebis(salicylideniminato)] at room temperature to give the black complex [Fe(salen)NO]. This compound e x h i b i t P an S = to S = 3 spin cross-over at 175 K. The corresponding complex of the 5-chlorosalen ligand [Fe(5-Cl-salen)NO], however, shows no sharp spin cross-over transition but a steady decrease in magnetic moment from 3.8 to 0.5 BM over the temperature range 4100 K, indicativeix4
IronCII) and Lower
States
1195
of an antiferromagnetic interaction and this implies a different lattice arrangement than in [Fe(salen)NO], possibly involving dimer formation. [Fe(TMC)NO] also exhibits an S = to S = 12 spin transition although without a sharp transition temperature. Interestingly the S = $ to S = interconversion rate is considerably slower in [Fe(salen)NO]. The spin state equilibria have also been described for [Fe(salphen)NO] (salphen = N,IV'-o-phenylenebis(sa1icylideniminato))and here as in [Fe(salen)NO] hysteresis accompanies the spin cross-over at around 175 K. Indeed the existence of S = + t o S = spin equilibria are a feature of iron nitrosyl complexes of quadridentate Schiff bases and analogous compounds. For example the complex [Fe(J-ph)NO] (10) shows a sharp transitionls7 in peT from 2.0BM at 90K to 3.2BM at 70K whilst [Fe(bza-ph)NO] (11) shows187a gradual transition in per from 2.1 BM at 90 K to 2.6 BM at 300 K. Recently iron nitrosyl complexes of the bidentate ligand Q (Q = 8-quinolinolate) have been isolated.'" [Fe(S-chloro-Q), (NO)(DMF)] and [Fe(2-methyl-Q)(NO),] have magnetic moments of 4.21 BM and 1.97 BM respectively, whilst peR.for [Fe(Q),NO], varies'8sfrom 4.4 at 285 K to 4.0 BM at 83 K.
+
Ac
(1 1)
44.1.3 IRON(0) AND LOWER OXIDATION STATES 44.1.3.1 Introduction The zero and lower oxidation states are relatively unimportant in the classical coordination chemistry of iron. With increased electron density on the iron, d-d and d-x interactions are particularly important, and systems with 7c acceptor orbitals are the dominant ligand species. Amongst the most common ligands encountered are carbon monoxide, phosphines, phosphites and unsaturated hydrocarbons. There are, however, a few relatively well-characterized iron(0) complexes derived from the reduction of higher oxidation state complexes containing N donor ligands possessing delocalized n: systems.
44.1.3.2 Nitrogen Ligands Herzog and c o - w o r k e r ~ lreduced ~~ [Fe(bipy),]'+ to the zerovalent complex with Lizbipy. Reduction of FeCI, in the presence of 2,2'-bipyridine also results in the formation of [Fe(bipy),] as in equation (17). The black crystalline complex [Fe(bipy),] dissolves in THF, dioxane or benzene to give red-violet solutions, while in protic solvents such as water, dilute acid or alcohols the complex is oxidized to [Fe(bipy),]*+ with liberation of hydrogen. [Fe(bipy),] decomposes at 145 "C in vacuo to iron and 2,2'-bipyridine. The compound appears to be paramagneti~'~' although its magnetic susceptibility has not been reported since the measurement is distorted by traces of metallic iron. Conductance and ESR measurements (gav= 2.075) are in agreementlgOwith a dSconfiguration with
lron(Ii) and Lower States
1196
neutral ligands. If an excess of reducing agent is sed'^'*''* a species is generated which apparently contains iron( - 1) as in equation (1 8). However, ESR evidence suggests that the unpaired electrons in Na[Fe(bipy),] occupy molecular orbitals composed mainly of ligand orbitals. A similar reduction route can be used to prepare [Fe(terpy),] and related iron(0) c0mp1exes.l~~ [Fe(bipy),] and coordinatively unsaturated [Fe(bipy),] can also be prepared194by the thermai decomposition of the alkyl complex [Et,Fe(bipy),]; the tris derivative is formed when the decomposition takes place in the presence of free 2,2’-bipyridine. 2FeC12
+
3Nabipy
6bipy
+
+
3Li2bipy
[Fe(bipy),]Cl,
--f
2[Fe(bipy),] Na[Fe(bipy),]
+
-+
4LiC1
3bipy
+
+
3bipy
(17)
3NaCl
(18)
The reduction of the classic iron(I1) tris a,”-diimine chelates has also been investigated electroc h e r n i ~ a I 1 y . ~Whilst ~ ~ ~ ’the ~ ~ first two reduction steps to the one and zero oxidation states are well understood, the nature of further reduced species is unclear. Such studies, however, do appear to demonstrate a limit to which increased electron density at the iron can be supported before charge becomes resident on the ligands. In the case of [Fe(bipy),] the increased electron density at iron must be stabilized by d-nback-donation. The structure of [Fe,(CO), (bipy)] demonstrate^'^^ that 2,2’-bipyridine is a poorer n acceptor than two carbon monoxide ligands. Substitution of two carbon monoxide ligands in [Fe,(CO),] by bipy leads to this complex which has one not three bridging CO groups. This is explained”’ in that bridging CO is a poorer x acid than terminal CO. The structure generated has a reduced number of bridging CO (increased number of terminal CO) ligands to make up for the poorer TC acidity of bipyridine. Whilst the TC acceptor properties of 2,2’-bipyridine represent a limit to the formation of less than zerovalent complexes with neutral ligands, the structure and spectra of [($-toluene)Fe(bipy)] clearly d e m o n ~ t r a t ethat ’ ~ ~ 2,2’-bipyridine does have considerable ability to accept n electron density from iron(0). The strength of the interaction is apparent from the Fe-N distance of 1.98, and the inertness of the toluene ligand which is only displaced by a very strong n acid such as dppe.
44.1.3.3
Phosphorus Ligands
Commonly, many iron(0) compounds containing phosphorus donor ligands are thought of as ‘organometallic’ rather than coordination complexes. Nonetheless, a brief discussion of some of their mare important structural features is appropriate. Iron carbonyls react themally’’93’mor photochemically20‘with PPh, to yield a mixture of [Fe(CO),PP$] and [Fe(CO),(PPh,),]. Especially bulky phosphines such as P(Bu‘), or P(EMe,), (E = Ge, Si, Sn) produceN2only [Fe(CO),PR,]. [Fe(CO),(PR,),] are stable pale yellow complexes which can also be obtained from a variety of organometallic compounds containing the Fe(CO), moiety. IR and Mossbauer spectra of [Fe(CO), (PR,),]are consistent with a trigonal bipyramidal structure and these complexes are only known as the trans isomers. [Fe(CO),] reacts with bidentate phosphines to give mononuclear products containing mono and bidentate phosphine ligands, and binuclear complexes containing bridging phosphine ligands. The complex Fe(C0), dmpe] (dmpe = 1,2-dimethylphospinoethane) has a geometry intermediate between square pyramidal and trigonal bipyramidal because of the small ligand ‘bite’. The iron atoms in the dinuclear bridged complex (12) are coordinated in a trigonal bipyramidal geometry with the phosphorus ligands occupying cis axial positions. Me,P
n
PMez
t
I
co
co (12)
The carbonyl ligands in [Fe(CO), (PR,),] compounds are rather inert towards substitution, but these compounds do find use as starting materials for a number of complexes via an oxidative route (vide iafra). Although [Fe(CO), (PPh,),] is only sparingly soluble in most common organic solvents,
Iron(lI} and Lower States
d
1197
it is rather more soluble in concentrated sulfuric acid giving a yellow solution shown by NMR spectroscopy to contain a protonated species. Careful halogenation of [Fe(CO), (PR,),] affords'" [Fe(CO),(PR,)X,]. The controlled one-electron oxidation of [Fe(CO),(L)J complexes generates deep green iron(1) complexes, some of which can be isolated and are useful in the preparation of novel iron(I1) complexes. These reactions are discussed in Section 44.1.4.3. trans-[Fe(CO), Lz] complexes react with mercury(I1) halides to form adducts (L = PPh, adduct is 1 : 1 , L = P(OPh), adduct is 1:2). Carbon monoxide is displaced photochemically from [Fe(CO),(PEt,),] by sulfur dioxide to give [Fe(CO),(PEt,),(SO,)], which undergoes oxidativezo4elimination with HI to yield [Fe(CO),(PEt,),I,]. The analogous complex [Fe(CO),(Ph,),I,] has been shownZoSto contain cis iodides, cis carbonyls and trans phosphines. The analogous phosphite complexes [Fe(CO), {P(OR), ]zX,], however, are thought2&to have cis carbonyl, cis phosphite and cis halides for X = I but trans halides for X = Br. Higher degrees of carbonyl substitution by phosphorus donor ligands in iron(0) complexes may be obtained by reduction of the corresponding halide in the presence of free ligand, or by direct synthesis using iron atoms. Thus reduction of [Fe(dmpe),Cl,] by sodium naphthalenide in THF yields2'' red-brown air sensitive [Fe(dmpe),]. Sodium amalgam reduction of [Fe{P(OMe), 1, Cl,] in the presence of P(OMe), giveszo8the very air sensitive yellow complex [Fe(P(OMe), Is]. This complex reacts with carbon monoxide in ether to yield the remaining two memhrs of the series [Fe(CO), {P(OMe),) 5 - x ] , namely [Fe(CO)(P(OMe), },I and [Fe(CO), {P(OMe),},]. Reduction of [FeCl,(PMe,),] with sodium amalgam in THF in the presence of excess PMe, under argon gives [Fe(PMe,),] (13) in which a hydride transfer has taken place to yieldz09~z10 an octahedral iron(I1) species. H
Me3P (13)
Iron pentacarbonyl reacts with PF, a t elevated temperature and pressure giving the series of complexes [Fe(CO),-JF'F,),]. [Fe(PF,),] may also be made by the direct reaction2" of iron atoms with PF3 at low temperature. This route enables the novel binuclear complex [(PF,), Fe(PF2)2Fe(PF,),], not available by other means, to be obtained. The metal atom technique2l2 has been extended to the synthesis of a number of dS iron c ~ m p l e x e s ~ " ~with '~ phosphorus donor ligands. In the low temperature reaCtion2I6of iron atoms with cyctoocta-l,5-diene (1,5-cod), the very temperature-sensitive and highly reactive compound [Fe(l,kcod),] is formed. This compound reacts with PMe, to yield [Fe(l,S-cod)(PMe,),] but with P(OMe), to yield [Fe( 1,3cod){P(OMe), },I. [Fe(1,5-c0d)~]reacts with dppe in ether/methylcyclohexaneat - 20 "C to yield, presumably, [Fe(dppe),], which decomposes above 0 "C but reactsZl6with nitrogen to give the five-coordinate dinitrogen complex [Fe(dppe),N,].
44.1.4
44.1.4.1
IRON(1) Introduction
Although isoelectronic with cobalt(II), d7iron(1) complexes show a marked instability, particularly with respect to disproportionation to iron(0) and iron(I1) complexes. Accordingly few iron(1) compounds have been isolated and characterized. It is therefore not appropriate to group iron(1) complexes by the ligand classification used for other complexes. The scheme which has been adopted is to discuss reports of iron(1) complexes under the headings of: carbon monoxide and arsine ligands, phosphorus-containing ligands, cyanide ligands, complexes of macrocyclic ligands and organometallic complexes.
44.1.4.2
Carbon Monoxide and Arsine Ligands
+
There are several examples of iron(1) complexes in which the 1 oxidation state is stabilized by coordination of carbon monoxide or arsine ligands. In an early paper217Hieber and Lagally
1198
Iron(II) and Lower States
described the formation of [Fe(CO),q by heating [Fe(CO),I,] in a stream of C 0 2as in equation (19). Careful oxidation of [Fe(CO),pdma] with iodine in benzenelnitrobenzene (equation 20) under nitrogen has been reported’l6 to give brown yellow [Fe(CO),pdrna]+I- (pdma = o-phenylenebisdimethylarsine). [Fe(C0)21,j [Fe(CO),pdma]
+
-
$I2
[Fe(CO),I] +
+ 41,
[Fe(CO),pdma]- I
Amongst dimeric organometallic compounds which formally contain iron(1) is the well-known”’ diamagnetic dimeric carbonyl complcx [Fe(q-C,H,)(CO),], . Recently a series of paramagnetic monomeric iron carbonyl hydrides has been generated2,’ by UV irradiation of dilute solutions of [Fe(CO),] in pentane under hydrogen at low temperatures. The iron(1) species [HFe(CO),] was characterized by ESR spectroscopy (g = 2.0120). One-electron electrochemical oxidation o f [Fe(CO),] in trifluoroacetic acid is reported=’ to yield transient [Fe(CO)J+ (half-life = 50-500 ms). The naked ion Fe+ has been observed in ion cyclotron resonance studies of its interaction with alkanes.’= Fe’ was generated in the gas phase by electron impact on [Fe(CO),].
44.1.4.3 Phosphorus Ligands
In much of the reportcd chemistry of iron(1) the low oxidation state is stabilized by coordination to phosphorus-containing ligands. Thus, Sacco and co-workers preparedu3the paramagnetic iron(1) hydride [FeH(dppe),] (dppe = 1,2-bis(dipbenylphosphino)ethane) by the sodium reduction of [FeHCl(dppe),] in benzene under nitrogen as in equation (21) and also by its reaction with [Co(N,)(PEt,Ph),]Na in THF. Deep red crystalline [FeH(dppe),] is soluble in benzene and toluene but insoluble in pentane. The compound is paramagnetic both in the solid state and in solution and with obeys the Curie-Weiss law between 97 K and 297K (0 = 14.5 ”). It is [CoH(dppe),] and shows a strong ESR signal at room temperature in toluene solution centred225at g = 2.085. Under a nitrogen atmosphere [FeH(dppe),] reacts with trityl salts (Ph,CX; X = BF,, ClO,) in benzene to yield [FeH(N,)(dppe),]X as in equation (22) and with stoichiometric amounts of HC1 or HC104 in ethanol to yield respectively [FeHCl(dppe),] and [FeH(N,)(dppe),]+ClO; . The reaction with acids appears to proceed via the oxidative addition of a proton to give an unstable iron(II1) dihydride [FeH,(dppe),]+ . With a large excess of hydrochloric acid at 80 “C [FeCl,(dppe),] is formed. The reduction of [FeCl,(dppe),] with stoichiometric amounts of sodium in the presence of free phosphine as in equation (23) yields the analogous iron(1) halide [FeCl(dppe),]. [FeHCl(dppe),]
+
Na
[FeCl,(dppe),]
+
Na
-
--+
[FeH(dpp~)~] + NaCl
[FeCl(dppe),j
+
NaCl
(21)
(23)
The red crystals of this complex have a bulk magnetic moment of 2.11 BM (0 = 8 K) and in toluene solution a strong ESR signal with a partially resolved hyperfine structure226is centred at g = 2.075. The complexes [FeX(dppe),] (X = Br, 1) have also been prepared. Reaction of [Fe(dppe),] with AlEt,, AlEt,C1 or Al(Bu’),H affords [Fe(dppe),]*X (X- = AIH,R; or AlC1,R; ). These four-coordinate iron(1) complexes react with ligands L to give the five-coordinate cations [FeL(dppe),]+ (L = C,H,, CO, N,, MeCN).227The ESR spectra of a range of iron(1) bis-dppe complexes have been measured; the reported g values (Table 2) are sensitive to both coordination number and the nature of the additional ligand. On the basis of their ESR spectra the complexes [FeH(dppe),] and [FeCl(dppe),] have been ‘assigned respectively square pyramidal and trigonal bipyramidal coordination ge~metries.”~ In deaerated methanolic solution, iron(1I) hydrido complexes of the type [FeH(L)(dppe),]+ (L = MeCN, MeCH,CN, CH,=CHCN, PhCN and g-MeC,H,CN) are reduced to [FeH(L)(dppe),] by solvated electrons produced by y-radiolysis of the solvent.*,*In pulsed radiolysis studies [FeH(L)dppe,] decays to five-coordinate [FeH(dppe),] (half-life < 1 s). In continuous radiolysis however, [FeH(dppe),] is not the final product and here as in its reaction with acids it is suspected that oxidative addition of a proton yields an unstable iron(II1) dihydride which decom-
Iron(l1) and Lower States
1199
Table 2 ESR g values lor dppe Complexes of Iron(1)it Complex
go
2.0856 2.0768 2.0900 2.1068 2.085 2.0515 2.0981 2.21
,
'dppe = 1,2-his(diphenylphosphino)ethane; data from M. Gargano, P. Giaiinoccaro, M. Rossi? G. Vassapollo and A. Sacco, J. Chem. Soc.. Dalton Trans., 1975, 9. ,
poses to hydrogen and [Fe(dppe),]+. Further oxidation yields [FeH(dppe)z]2+which is itself reduced by the iron(1) species [FeH(dppe),] to [FeH(dppe),]'. This coordinatively unsaturated species then binds the neutral ligand present in solution and so regenerates the starting complex.228Pulsed radiolysis studies of the complexes [FeH(L)(dppe),]+ (L = P(OMe),, CO) in deaerated methanol solution showed the existence of a novel five-coordinate iron(1) intermediate [FeH(L)(dppe),] in which one dppe is monodentate. Ring closure by dppe with elimination of L results in the familiar229 iron(1) complex [FeH(dppe),]. These processes are outlined in Scheme 5. Interestingly, pulse radiolysis techniques have also been used to generate transient iron(1) species in aqueous solution.230 pulsed 7 radiolysis
l FeH(L)(d w M +
e9
[FeH(L)(dwe)21
-
continuous y radiolysis
[FeH(dppe),l + L
MeOH:
Scheme 5
Cyclic voltammetric studies of [Fe(CO),L,] {L = PPh,, PMePh,, P(NMe,),, P(OPh),; L, = dppe, dppm (dppm = bis(dipheny1phosphino)methane)) in dichloromethane showed reversOxidation ible one-electron oxidation to [Fe(CO),L,]+ ,which was identified spectroscopically.2"~212 of [Fe(CO), (AsPh,),] is apparently not reversible but the corresponding iron(1) cation was identificd by ESR and IR spectroscopy. The complexes [Fe(CO),L,]+ are generated chemically as deep green solutions23'by the oxidation of [Fe(CO),L,] with [N(C,H,Br-p),]' PF; in dichloromethane (formed by the reaction between N(C,H,Br-p), and AgPF, followed by filtration to remove metallic silver) (equation 24). The salt [Fe(CO),(PPh,),]+ PF; was isolated as the hemidichloromethane solvate, which is soluble in polar solvents. Reaction with halogens (X2, X = C1, Br, I; 1 : 1 ratio Fe:X) in dichloromethane (equation 25), is rapid to give [FeX(CO), (PPh,),] + . With halide ions [Fe(CO), (PPh,),]+ (1:1 molar ratio) forms the paramagnetic intermediate [FeX(CO), (PPh,)J which disproportionates to give [Fe(CO),(PPh,),] and [FeX,(CO),-,(PPh,),], (X = I, n = 1; X = Br, n = 1, 2: X = C1, n = 2). The iron(1) species [FeX(CO),(PPh,),] is an intermediate in the oxidative elimination reactions of [Fe(CO),(PPh,),] with halogen. It is not clear whether this species contains an Fe-X bond or a halogen-acyl [FWO), L21
+
[Fe(CO,)L,]+
N ( G H4 Br-P):
+
X2
--+
-
[Fe(CO),L1+
[FeX(CO),L]'
+ X-
(24) (25)
The electrochemical reduction of a series of cationic iron(I1) hydrido complexes trans[FeH(L)(d~pe)~]+ (L = N,, C,H,N, PhCN, CH,CHCN, MeCN, P(OMe),, P(OEt),, CO) has been shown to proceed via transient d7 iron(1) species which lose one of the neutral ligands. Further reduction of the resulting five-coordinate intermediates yields stable d8 iron(0) anionic hydrides.*" The oxidative substitution and radical coupling reactions of [Fe(CO),(PPh,),]+ have led to a number of unusual iron(0) and iron(I1) complexes, e.g. equation (2S), which cannot be obtained234 from its diamagnetic precursor [Fe(CO),(PPh,),]. The iron(1) species is relatively unstable, probably due to poor charge delocalization by the phosphine ligands. The presence of an imidazolidinylidene
1200
Iron (11) and Lower States
' 2N0 3 I
Iron (11) una!Lower States
1201
ligand allows more efficient charge de10calization.~’~ SevFral carbenecarboykiron(1) complexes have been isolated by oxidation of [Fe(CO),(L)(L’)] (L= :CN(Me)CH,CH,NMe; L‘ = PPh,, PEt,, +dppe, P(OPh),) using a variety of agents (I,, AgBF,, AlC1, ,MeI, PhSCC1, TCNE, AgCN, AgPF,, AgSbF,) in T H F or dichloromethane under argon. Nine crystalline complexes have been reported, and these are listed in Table 3 together with some of their properties. The iron(1) salts are susceptible to neutral ligand exchange reactions, in particular CO displacement as for example in equation (26). When a THF solution of an allyl iron halide [Fe(q-C,H,)(CO),L(X)] (X = Br or I; L‘ = CO, phosphine or the carbene ligand L above) is stirred with the electron rich alkene L2,the neutral iron(1) complexes [Fe[q-C,H, )(CO), L ] are formed (equation 27). These green complexes have been characterized spectroscopically in THF solution and data are given in Table 4. IFC(CO)~ (L),I[BFJ
=
(26)
[;g) Ij: R
L2
[F~(CO>, (L), ( P P ~)I[BF, , I
THz:ccb
R
R
,L =
R
R
R
L’ = co, YR3, L Table 4 1R and ESR Spectroscopic Data for THF Solutions of the Complexes [Fe(&H,)(CO),L]
L
co PMezPh PPh, PEt, L ~~~
gm
2.045 0.048 2.050 2.046 2.05 1
a
(”P)(m73 0.72 1.69 1.15
v ( C 0 ) (an-’)
2049, 1957, 1956, 1947, 1968,
1065 1892 1893 1889 1907
~
M. F.Lappert, J J. MacQuitty and P.L. Pye, J. Chew. Soc., Dalton Trans, 1981. 1583.
44.1.4.4
Cyanide Complexes
Treadwell and Huber reported236the electrolytic reduction of aqueous K,[Fe(CN),] in the presence of excess KCN yielding a colourless solution which reduces one mole of [Fe(CN)6]3-. This was interpreted as indicating the formation of an iron(1) cyano complex, although the possible formation of a hydrido species such as [Fe(CN),w3- has not been ruled out by subsequent chemical The complex ions [Fe(CN),HI4- and [Fe(CN),I4- have been observed in pulse radiolysis studies of aqueous hexacyanoferrate(I1) s o l ~ t i o n s . y-Irradiation ~~* of single crystals of alkali halides doped with hexacyanoferrate(I1) ions have yielded ESR spectra characteristic of a number of iron(1) s p e ~ i e s . ~ These ” ~ ~ ~ include ~ [Fe(CN)J4-, [Fe(CN),(H,O)l4-, [Fe(CN),Cl]’-, [Fe(CN),C1,I5-, [Fe(CN),Br]’- and [Fe(CN),BrJ-. The spectra of the pentacyano species contain some evidence for a ‘bent’ cyanide ligand.239
44.1.4.5
Macrocyclic Ligands
There have been a number of reports of iron(I>complexes generated by the reduction of iron phthalocyanine compounds. The assignment of metal oxidation state is complicated by the ability of the phthalocyanine ring to undergo reduction. Progressive reduction of iron(I1) phthalocyanine with sodium in THF first yields a purple solution which shows a strong ESR spectrum centred at g = 2.01, and on further reduction the solution turns blue and the ESR signal collapses. A final reduction stage yields a green solution whose ESR is a single isotropic line at g = 2.003. Subsequently further electrochemical investigation of this sequence identified a fourth (earlier) stage
1202
Iron(11l) und L o w w Skntes
in the sequential reduction tho.ugh this slage could not be reproduced using Na/THF.24' These authors postulate the sequence shown in Schcmc 6. [Fr"pz]
--%
green
[Fel'pc] pink
[Fe'pcI2-
A
e_
jFe' pc j3-
h lue-I: i o 1et
purple
[F;e1pcl4green- blue
Scheme 6
The redox couple [Fe"pc]/[Felpc] is sensitive to solvent since the extent of 71 back-donation to phthalocyanine is influenced by the electron donating ability of axial solvent ligands. Thus the iron(l1) complex is more easily reduced ir. solvent systems where 71 hack-donation is minimized. The ~~ case of reduction of [Fe"pc] lies in the sequencc pyridine > DMSO > DMF as s 0 1 v e n k ~By contrast the Fe"/Fe' redox potentials of iron(T1)porphyrin complexes show little solvent dependence because their capability for n back-donation is relatively ~ 1 - d . ~ ~ ~ Thc iron(1) anion [Fc(TPP)] (TPP = tetraphenylporphine dianion) is producedz4' by the reduction of p-oxo-bis(tetraphenylporphineiron(II1)) with NaiHg in THF. The product appcars to exist in different spin states in T H F solution (20C-300 K) and frozen THF matrix at 77 K. A recent study, however," is unequivocal about the 6' Fe' nature of the complex in the presence of pyridine. Carbon monoxide reacts with [Fe(TPP)] to form a five-coordinate complex [Fe(TPP>CO],which can be reduced electrochemically to the corresponding iron(1) species from which, however,245CO spontaneously dissociates. The Fe-CO in:cractisn is stabilized by the. presence of hydrocarbon chains bound by amide linkagcs to the ortho position of the TPP phenyi rings. Carbon monoxide adducts of iron(1) complcxes of a. number of these 'superstructured* porphyrins have been rcported.'45 The chemistry of these high!y reduced species is of raievance to undcrstanding"' the reactions of cytochromc P-450 and the peroxidases. Dlli
~I:c."'!;~pcl] ?-
ii \
2 [r:CI'(tspe)]4-
uII;_ 0;
01
[ Fr1(tspc5-)J 6-
not ESR detectable
20% pyridinc
j1;e"'itspc)
13-39.
ZO!?, DhIl
[Fc"'[:tsl:r)l'
5
[I:e'(tspc)]
ESR iIi.l.rcLal>lr Scheme 7
Deep blue jiron(III)(tspc)] (tspc = 3,lO: 17,24-tetrasulfonatophthalocyanine) is reduced in aqueous solutions to give two iron(]) tspc specie^'^^.'^^ as shown in Scheme 7. Stoichiometric sodium of [Fe"(salen)] in THF affords high-spin d7 [Fe(salen)]-. At room temperature this complex has a magnetic moment of 3.7BM and the Curie-Weiss law is obeyed down to 90K (8 = - 46 '). The sodium salt is freely soluble in THF forming air and moisture sensitive deep grew solutions. The [Fe(salen)]- anion is a strong nucleophile reacting, as in equation (28) for instance, with benzyl chloride at - 60 "C to give the corresponding iron(III) benzyl derivative249which is high-spin. [Fc(~alen)]'-+ PhCH,CI
[PhCH,Fe(sulen)]
+
CI-
(28)
Controlled potential electrolysis olthe iron(I1) complex [Fe(L)(MeCN),]'+ (L = Me,[14]1,3,8,10tetraeneN,* see Section 44.1.5.12.3)in acetonitrile at the first reduction plateau gives"" the purple iron(1) cation [Fe(L)]+,isolable in high yield as the CF,SO; salt. The ESR spectrum in frozen acetonitrile is characteristic of low-spin d' iron(1) having a rhombic distortion from octahedral geometry. The effective magnetic moment measured by an NMR technique is 2.3 BM. A second quasi-reversible reduction step yields the analogous iron(0) complex; hovvever, repeated scanning o f this redos system results in hydride abstraction from the supporting electrolyte Bu,"NBF, to form a second iron(]) complex [FeH(L)(MeCN)]. The purple-black product also shows the appropriate ESR spectrum for low-spin d7 iron(1) (pef = 2.1 BM). An Fe--H stretching vibration is observed as a sharp IR absorbtion at I890 cm-'. The iron@)alkyl and aryl derivatives [FeR(,L)]result2s9from the sequential reaction of the ircn(1I) complex [Fe(L)CI] with two equivalcnts of the appropriate lithium alkyl or aryl as in equations (29) and (30). ESR and magnetic. data arc again consistcnt with +
Iron(II) and Lower States
I203
d7 iron(1) and the unpaired electron occupies a molecular orbital which is predominantly metal ion in character. [Fe(L)Cl]+ -t LiR [Fe(L)Cl]
44.1.4.6
+
+
LiR
[Fe(L)CI] -k Li' +
[Fe(L)R]
+
+ +R-R
(29)
LiCl
(30)
Organometallic Complexes
The oxidation state of + 1 in iron chemistry is interesting for its rarity and so it is appropriate to include a description of some compounds which would not classically be regarded as coordination complexes. The iron(1) complexes [(15-C,~S)Fe(f16-C6Meb)], [(qs-CSHS)Fe(q6-C,Et,)]and [($C, Me,)Fe(y6-C, Me,)] are obtained by Na/Hg reduction of the corresponding cationic comat 20 "C. The parent compound [($-C,HS)Fe($-C6H6)] is highly unstable with p o u n d ~in ~ ~QME ' respect to dimerization through the arene ring which is preceded by intramolecular electron transfer to an arene carbon atom. A catalytically active species containing iron(1) has been p o s t ~ l a t e din~the ~ ~cross ~ ~ coupling ~~ of 1 -bromopropene with a variety of alkylmagnesium bromides in the presence of a reagent derived from the reduction of tris(dibenzoylmethido)iron(III). At intense ESR signal centred at g = 2.08 is observed on treating [Fe(DBM),] with excess ethylmagnesium bromide in THF at -40°C. A similar spectrum is observed on treating FeCl, and [Fe(acac),] in this way. The existence of an iron(1) intermediate has been postulated in certain systems studied for the fixation of molecular nitrogen. Thus, an ESR signal at g = 2.01 observedzs4on mixing FeC1, and LiPh in ether was assigned to d7 iron(1).
44.1.4.7
Nitric Oxide Complexes
One of the earliest assignments of an oxidation state of + 1 for an iron complex was that of Wilkinson and co-workersgSbased on the magnetic susceptibility, IR and absorbtion spectra of [Fe(H,0),N0]2+. The potential for nitric oxide to act as either a two or three electron donor in its coordination to transition metal ions, however, has led to much controversy. For this reason iron nitrosyl complexes are discussed separately in Section 44.1.2.
44.1.5
44.1.5.1
I
IRON(I1)
Introduction
For many years the coordination chemistry of iron(I1) was overshadowed by the Werner complexes of its isoelectronic neighbour cobalt(II1). Compared with cobalt(II1) a substantially higher ligand field is needed to produce low-spin iron(I1) complexes; once formed, however, they are usually thermodynamically very stable and kinetically inert. Complexes of this type are easily prepared, and a number have been known for a long time. Classic examples include mixed oxidation state cyanides (Prussian blues) and tris-(cl,d-diimine) complexes ('ferroins'). More recent examples include complexes containing phosphorus and sulfur donors. With weaker field ligands oxidation to iron(IT1) species is a common preparative problem, and no doubt this has been one of the reasons for the slow early development of what has become such a rich area. In complexes having intermediate ligand field srrengths thermally induced spin transitions become possible and these have been extensively investigated. With strong field tetradentate macrocyclic ligands the relatively unusual S = 1 spin state is often observed. A major objective has been to devise complexes that mimic the behaviour of the natural iron(I1) oxygen carrying proteins, and a range of synthetic porphyrin complexes are now available that do this. Another important biologically relevant area is that of the ironhlfide clusters that provide convincing models for several electron transfer systems. A notable feature of the literature concerned with iron chemistry is that while a number of excellent reviews are available that deal with specific topics, there are none that deal with the whole coordination chemistry of iron(I1). The present review first deals with cyanide complexes followed by complexes containing simple monodentate nitrogen donors. The huge amount of work published C.O.C. & M M
Iron(l1) and Lower States
1204
on diimine and related complexes is covered in several subsections before considering complexes with phosphorus, oxygen and sulfur donors. Then halide complexes are discussed before finally considering the N donor macrocyclic systems.
44.1.5.2 44.1.5.2.1
Cyanide Complexes The hexucyuno complex
The ,exceptionally high affinity255of iron(I1) for cyanide ion is reflected in the heat of f ~ r m a t i o n ~of ~ ~the . ~[Fe(CN),I4'~ anion (equation 3 1). The stability of this, the ferrocyanide ion, of its iron(II1) salt, Prussian blue, possibly the first isolated is illustrated by the nature and history258 coordination complex, which is discussed in Section 44.1 5 2 . 4 . [Fe(H20)6]2+
+
6 0 -
4
[Fe(CN),l4-
+
6H20
AI$ = - 358.9kJmol-'
(31)
The addition of aqueous cyanide to a solution containing an excess of an iron(I1) salt in the precipitation of an impure iron cyanide. On the other hand, the addition of excess cyanide to iron(I1) solutions rapidly gives the hexacyanoferrate(I1) ion. The rate of uptake of the sixth cyanide ion is significantly slower than for the previous five due to low-spin [Fe(CN),Hz0I3- being kinetically inert. This second order stage of the reaction has2@' an activation energy of 104kJ mol-' and an equilibrium constant of 10'. In fact, these data relate to the formation of [Fe(CN),I4- from both [Fe(CN),H20I3-, itself very stable, and [Fez(CN),,,l6-, both of which are present in the reacting solution, The inner sphere reduction of [Fe(CN),I3- with a variety of mild reducing agents readily forms [Fe(CN),I4-. Such reactions may be mechanistically complex, as for example in the reduction with sulfite ion which involves26ian intermediate formed from [Fe(CN),I3- and SO; radical anion as shown in Scheme 8. The octahedral geometry of the ferrocyanide ion has been established262in K,[Fe(CN),] 3Hz0. When ammonium hexacyanoferrate(I1) is heated to 320 "C in vucuo, a pale green substance of empirica1 formula 'Fe(CN),' results about which little is known. [Fe(CN)613- + SO:- -+ [Fe(CN),I4- + SO: N
-
[Fe(CN),]+ + SOj [Fe(CN),(CN-S0,)14-
-
-
[Fe(CN),(CN-S03)14-
+ H20
[Fe(CN),I4- + SO:- t 2H*
Scheme 8
With the exception of the barium salt, which is only sparingly soluble, the ferrocyanides of the alkali and alkaline earth metals are easily soluble in water though not in alcohols. All these soluble salts separate from aqueous solution as well-defined pale yellow crystals. Large numbers of sparingly soluble salts of general formula K,M" [Fe(CN),] and KM"' [Fe(CN),] are obtained by precipitation, as are the salts of most transition metal ions. These have polymeric structures involving M-N=C-Fe coordination. In some cases linkage isomerization has been demonstrated, for example KFeCr(CN), is less stable than KCrFe(CN), which has been shownz63to contain Cr-NEC-Fe linkages. Many of the insoluble transition metal salts of [Fe(CN),r- are useful ion exchange materials and the copper(I1) salt has a classic use as a semipermeablemembrane in the measurement of osmotic pressure. The alkali metal salts may also be made via the acid [&Fe(CN),l, which is obtained in aqueous solution by ion exchange. This strong tetrabasic acid can be precipitated as an ether addition compound by adding ether to a strongly acid solution of [Fe(CN),I4-. Subsequent removal of the ether is readily accomplished to afford [H,Fe(CN),J as a white powder which slowly becomes blue in moist air and is soluble in water and ethanol. The protons have been shown264,26s to be bound to the nitrogen atoms of the CN groups. The acid dissolves without decomposition in liquid hydrogen fluoride2,, to give [Fe(CNH),]'+ . An X-ray of [Fe(NCD),I2+as the [FeCl,]- salt reveals tetrahedral [FeCl,]- and octahedral crystal -Cl-Fe"' deuterium bonds. The rigidity [Fe(NCD),I2' moieties joined together by Fe"-NCD* of the linear N-C-D group results in some tetragonal distortion of the FeN, octahedron. The acid M4Fe(CN),] decomposes268on heating in various atmospheres to form Prussian blue type compounds. Both the acid and the potassium salt react269with BF, to give adducts whose IR and Mossbauer spectra are consistent with the Lewis acid bound to the nitrogen atoms of the cyanide as in equations (32) and (33).
I
--
Iron(1l) and Lower States [Fe(CN),(CNH),I K[Fe(CN),]
+ 2BF3 + 6BF3
1205
[Fe(CNH),(CN. BF,),]
(32)
K4[Fe(CN-BF3),1
(33)
The kinetic inertness of [Fe(CN),I4- is typified by the extremely slow thermal displacement of CN- by a,d-diimines. Such reactions are however catalyzedz7’by a variety of metal ions such as Hgz+ and the stability constants for some of the intermediate heteropolynuclear complexes have been determined.”‘
44.1.5.2.2
Pendacyano complexes
On a small scale, photolysis of [Fe(CN>,j4- in aqueous solution^^"^^^^ affords some [Fe(CN),H20I3-. When the reaction is carried out in the presence of nitrosobenzene the loss of cyanide is accelerated resulting in the formation of the violet PhNO complex. The rate of this process is also enhanced by the presence of certain metal ions, for example H$+ or Pb2+. Larger scale preparations usually involvez74reductive removal of NO from the nitroprusside ion in the absence of potential donor ligands other than water. Thus the reaction of [Fe(CN),NOIZ- with hydroxylamine in the presence of sodium carbonate yields a yellow deliquescent powder formulated as Na,[Fe(CN),H,O]. The violet PhNO complex [Fe(CN),PhN0I3-, which has I.,,? = 528 nm in aqueous solution, is formed when this salt reacts with nitrosobenzene. In common wtth many other [Fe(CN), LI3- ions, reaction275with an excess of cyanide yields [Fe(CN),I4-. Solutions resulting from the preparation of [Fe(CN),H,0I3- by reducing nitroprusside ion contain dimeric species, as indeed may solutions re~ulting”~ from the photolysis of [Fe(CN),I4-. Two dimers have been identified, as [Fe,(CN),,J6- and [Fez(CN),,]7-.The structure proposed for [Fez(CN),o]6- requires’” two non-linear cyano bridges. A single equivalent oxidation of = 1200nm; E = 104) and a second equiv[Fez(CN),o]6-yields a green complex [Fe,(CN),J- (A, alent oxidation gives a purple ion [Fe2(CN),,I4- (,I, = ,, 560nm; = E = 1600). The UV and visible s p e ~ t r a ’ ~of ~ , [Fez(CN),,]6’~~ and [Fe2(CN),o]4-are similar to those of the single bridged dinuclear complexes [Fez(CN),,j7- and [Fe(CN),,]’-. The structure of the dimeric species derived from [Fe(CN),HZ0l3-has not been fully resolved despite recent work. Structures (14) and (15) have been proposed containing one or two cyanide bridges respectively.
(14)
(1 5)
Competition ratios for the reaction of the dimer with several nucleophiles have been determined280 and the similarity between the ratios for the dimer and for [Fe(CN),H,O]’- suggests the presence of an aqua ligand in the former. [Fe(CN),H,0I3- is also generated by dissociation of [Fe(CN),NH,j3- or [Fe(CN),SO$ in aqueous solution; it then equilibrates with the dimeric species. In the case of the ammine complex277the dimerization step is rate determining in acidic solution (rate constant: 1.1 dm3mol-’s-’ at 298 K) whilst in alkaline solution it is the reaction of [Fe(CN),H,0I3- with another [Fe(CN),NH3l3- which is the slowest step. [Fe(CN),H20I3- also reacts278with a variety of [M(CN)J- complexes to give bridged dimeric species [FeM(CN),,]”- (e.g. M = Fe, m = 7, M = Co, m = 6). The formation of binuclear species also takes place through bound organonitriles, for example [Fe(CN>,H2Ol3’-reactszs3with [CO(NH,),L]~+(L = 3- or 4cyanopyridine) to form [(NC), Fe(y-L)Co(NH,),] which then undergoes intramolecular electron transfer to give iron(lI1). The ease with which oligomeric species are formed in this system has probably contributed to the difficulty in fully characterizing the products. In the absence of oxygen [Fe(CN),H,0I3- decomposeszwon heating to give [Fe(CN),I4- and [Fe(HzO),I2+.
44.1.5.2.3
Reactions of [Fe(CN),LI3-
Useful starting materials for the synthesis of [Fe(CN), LI3- are the aquapentacyanoferrate(I1)ion [Fe(CN),H20 j 3 - and the amminepentacyanoferrate(I1) ion [Fe(CN),NH3I3-. These react with a varietv of ligands with displacement of H,O or NH, bv L to give TFe(CN),L13- (ex. CO displaces
I206
Iran(1I) and Lower Stutes
NH, to give [Fe(CN),CQ]”). [Fe(CN),NH,I3- is best prepared285from the nitroprusside ion by standing in a solution of concentrated ammonia buffered with sodium acetate as in equation (34). [Fe(CN),NO]’[Fe(CN),NO]’-
+ 2NH3 + OH-
+ 2RNH, + OH-
-+
-+
[Fe(CN),NH,I3[Fe(CN),RNH2r-
+
N2
+
+ N? +
H20
ROH
(34)
+
H2
(35)
Nitroprusside may also be reduced with Na/Hg to give [Fe(CN),NHJ-. Complexes containing primary amines can also be from the nitroprusside ion in the presence of sodium acetate with the formation of nitrogen and the corresponding alcohol as in equation (35). The ammonia ligand in [Fe(CN)5NH3]3-is rapidly hydrolyzed (ti 40 s, 25 ‘C). If the reaction is carried out in the presence of a small amount of ascorbic acid, to prevent aerial oxidation to iron(III), this represents a convenicnt route to [Fe(CN),H,O]’- . The kinetics of ligand dissociation from pentacyanoferrates(II), [Fe(CN), L]”-, have been extensively investigated. These reactions proceed through a limiting dissociative mechanism. Variation of the rate constant with different incoming ligands has been reported in relatively few cases. The dependence of reactivity on the nature of the leaving ligand, however, has been examined in detail. Whilst charge effects are small, solvation effects are important. The strength of the iron-ligand bond is paramount in determining dissociation rates and attempts have been made to assess the CJ and .n bonding contributions from metal-ligand charge-transfer bands and ligand pK, values. Table 5 contains data for the formation of [Fe(CN), L]n- complexes from [Fe(CN),H,0]3p and Table 6 contains kinetic parameters for dissociation of L from [Fe(CN),L]” complexes. Photolysis of six-coordinate [Fe(CN),enI3 results in the loss of one cyanide to givezx7the chelated complex [Fe(CN),enI2- in high quantum yield. With ligands such as the cyanopyridines that have two donor sites [Fe(CN), L]”- complexes have been observed to exhibit coordination isomerization. Several such compounds have been ~ t u d i e d ~and * ~ interestingly ~~*~ some of the isomerizations involve an intramolecular mechanism.
-
Table 5 Kinetic Data for Formation of [Fe(CN),L]”- from [Fe(CN),(OH,)]’- and L
k, (2S”C) L
Ethane-l,2-diamine Elhanc-1,2-diamineH ’. Hydrazinc Hydrazinium+ u 2 -
B
AAlAA”
a
3-Acetylpyridine 4-Acetylpyridine Quinoxaline Thiourea Thioacetamide Dithiooxamide Benzotriazole Carbon monoxide AA
=
(M-ls-l)
ASf (J K-‘ mol-’)
Re$ 1 1
230 410 400 250
9 18-30 26370 473b 471b 425 194 27 1 460 330 310
aminti acids in dianionic (Glu:
b A t 297.2 K 1. J. A. Olabe and
AHf (kJmol-’)
. Ty?
5&74
--14 to
64
79
67 64 60 65 65 60 63
33 25 1 21 22 6 15
+21
1 1 2 2 2 3 3 4 5 5 5 6, 7 8
1, monoanionic and zwiLterionic f u m s
L.A. Gentil, Trnmiiion Mer. C h m . (Weinheim. Ger.), 1983, 8, 65. 2. H. E. Toma, A. A. Batista and H. B. Gray, J. Am. Client. Soc., 1982, 104, 7509. 3. M. I. Bhndamer, A. Burgess, J. W. Morecom and R. Sherry, Transition Mef. Claem. (Weinheim, Ger.), 1983, 8, 354.
4. A. L. Coelho, H. E. Toma and J. M. Malin, Inorg. Client.. 1983, 22, 2703. 5. H . E. Toma and M. S. Takasugi, Pol-yhedron, 1982, 1, 429. 6. H. E. Toma, E. Giesbrecht and R. L. E. Rojas, Can. J . Chem., 1983, 61, 2520. 7. H. E. Toma, E. Giesbrecht and R. L. E. Rojas, Quim. Nova, 1983, 6, 72. 8. H. E. Toma, N. M. Moroi and N. Y.M. Iha, An. Acad. Bras. Cienc., 1982. 54. 315.
44.1.5.2.4
Prussian blue
Reaction of an excess of iron(I1) salt with [Fe(CN),13- or an excess of an iron(TT1) salt with [Fe(CN)& produces voluminous deep blue precipitates formerly known as Turnbull’s blue and
I
Iron(I1) and Lower Stutes
1207
Table 6 Kinetic Data for Dissociation of L from [Fe(CN),LJ"L
ASf
103k (25°C) (s-')
AH" (kJmol-')
17.5 0.48 0.29 0.67 0.57 1.42 0.71 0.90 1.78 1.6 50 4.4 2.7-26 8.8 6.2 1.1 , 0.86 1.61 2.2 1.o 0.77 0.42 9.0 1.3 620 0.075 12.60 27.1 46 3.7 9.0 10-5
94 116 114
33 82 70
96 97
21 17 53 76 57 94 27 28-5 1 25 5 46 59 75 16 50 44 59 70 8 17 46 21 64 70 19 11
'
(J K - mol-'
AV+
1
(cm3mol-')
Ref
I:.
. .
Ammonia Nicotinate Isonicotinate Methyl isonicotinate Ethyl isonicotinate Nicotinic acid Isonicotinic acid Nicotinamide Pyridylpyridinium Hydrazine H ydrazininum 1,2-Dimethylethane-l,2-diamine Amino acids Piperidine Pyridine 3-Acetylpyridine 4-Acetylpyridine 3-Cyanopyridine 4-Cyanopyridine 2-Methylpyrazine Pyrazine Pyrazine N-oxide Pyrimidine Quinoxaline Dimethyl sulfoxide Thiourea Thioacetamide Dithiooxamide N- 1-benzotriazole N-2-benzotriazole Carbon monoxide
I06
I10 I07 1ox
94 92-99 94 88 104 106 113 93 tD5 I14 110 111 91 82 110 88 106 108 93 89
1
2 2 3 3 2 2 2 2 4 4 5 6 7 8 1
20.6 20.6 10.4
9 9 10 10 10' 1 2 11 11 12 13 13 13 14 14
1 . H. E. Toma and J. M. Malin, Inorg. Chem., 1973, 12, 1039. 2. P. J. Morando and M . E. Blesa, J . Chem. Soc.. Dalton Truns., 1982, 2147. 3. N. D. Lis de Kat7 and N. E. Katr, Monatsh. Chem., 1982, 113, 745. 4. J. A. Olabe and L. A . Gentil, Transition Met. Chern. ( W e d w i m , G P ~ . 1983, ), 8, 65. 5. D. 8 . Soria, M. del V. Hidalgo and N. E. Katz, A Chern. Soc.. Ddlon Trans., 1982, 1555. 6. H. E. Toma, A. A. Batista and H. B. Gray, J . Am. Chern. Sac., 1982, 1M3 7509. 7. J. A. Salas, M . Kate and N . E. Katz, J. Solution Chew., 1983, 12, 115. 8. G . C . Pedrosa, J. A. Safas, M. Katz and N. E. Katz, J. Courd. Chem., 1983, 12, 145. Y. N . Y. M. Iha and H. E. Toma- An. Accad. Bras. Cienc.. 1982, 54, 491. IO. M. J. Blandamer, A. Burgess, J . W. Morecom and R. Sherry, Transition Met. Chem. {Weinheirn, Cer.), 1983, 8, 354 1 1 . A. L. Coelho, H. E. Toma and J. M. Malin, Inorg. Chem.. 1983, 22, 2703. 12. H. E. Toma, J. M. Malin and E. Giesbrecht, Inorg. Chem., 1973, 12, 2084. 13. H. E. Toma and M. S. Takasugi, Po/yhedron, 1982, 1, 429. 14. H. E. Toma, E. Giesbrecht and R. L. E. Rojas, Can. J. Chem., 1983. 61, 2520.
Prussian blue (PB) respectively. They are identical mixed oxidation state polymeric complexes having the idealized composition Fe, [Fe(CN),j, , i.e. iron(II1) ferrocyanide. If precipitation is rapid in the presence of potassium ions, these are incorporated with the formation of a so-called 'soluble PB' that is readily peptized. Prussian blue is an historically important salt which is amongst the first identified coordination compounds. Its early isolation is due in part to the especially strong affinity of iron(I1) for cyanide and to the stability of the resultipg complexes. Small almost black crystals of PB, free from alkali metal ions, separate slowly when FeCl, reactszw with [H,Fe(CN)J in 10 M HCl (approximately ten weeks). X-Ray and neutron (powder) diffraction studies291confirm that the structure (analogous to other polynuclear cyanides) consists of a cubic Fe"'NCFe" framework where a quarter of the iron(II1) sites are vacant. The average c o m p o ~ i t o n of * ~the ~ pseudooctahedral coordination sphere of iron(I1I) is FeN, Ol.5. In the ideal stoichiometry of Fe,[Fe(CN),], * 14H20, six of the water molecules occupy coordination sites around the iron(II1) atoms whilst eight occupy uncoordinated interstitial sites. Recent studies employing DTA and NMR techniques a ~ p e a 8 to ~ ' show a third type of water molecule in the Prussian blue structure. These conclude that of the eight uncoordinated water molecules four are 'lattice' water molecules in the w o n d coordination sphere and four are 'zeolitic' water molecules in the third coordination sphere of the paramagnetic iron(I1) centres. Mossbauer and magnetic measurements
,
Iron(lI) and Lower States
1208
on a series of Prussian blue analogues, MP[MB(CN)6]k*mHz0, show2%that the ions bound to the carbon end of the cyanides (MB)are low-spin and all ions in MAsites are high-spin. As in Prussian blue itself the analogues are characterized by well-defined octahedral cyanide sites, extensive water of hydration and variations in the MAsites. Individual iron atoms in Prussian blue appear to have well-defined electronic configurations with lifetimes of around lo-' seconds. An interpretati~n,~~ of the electronic spectrum suggests that the blue colour arises from electron transfer between the [Fe(CN)$ and iron(II1) ions and that the relevant electrons reside on the ferrocyanide moiety in the ground state. Vacuum pyrolysis of Prussian blue can result in valence reversal to give296iron(I1) ferricyanide, which has also been in aqueous solution, the complex previously thought to have been 'Turnbull's blue'. The redox chemistry of the Prussian blue family (Table 7) has attracted considerable attention. The generation of thin films of Prussian bluezg8has led to studies of its mediation in electron transfer reactions299and of the electrochemical processes300~30' involved in its deposition and redox reactions. This work has been spurred by its electrochromic properties which have been used in prototype electronic display devices based, for example,302on Prussian blue modified SnO, electrodes. A recent review303deals with the electrochemistry of electrodes modified by depositing thin films of PB and related compounds on them. Interestingly, true Prussian blue is somewhat difficult to process and modern 'iron blue' pigments such as Milori blue are derived from the oxidation of 'Berlin white' {Fe(NH,), [Fe(CN),]} to give iron(II1) ammonium ferrocyanides. Table 7 The Prussian Blue Family
Trivial name
Idealized formula
Soluble Prussian blue Insoluble Prussian blue Prussian brown Berlin green Prussian green ' Prussian white Everitts salt
Ref
KFe"' [Fe"(CN),] ~ $ 1 1 F II 4 [ e (CN),I,
1 1
Fe"'[Fe"' (CN),]
2, 3, 4
KFe,"'[Fe"(CN),][Fe"'(CN),j,
2, 3 2
K2Fe"[Fe"(CN),]
I . D. Davidson, J . Chem. Educ., 1937, 14, 211. 2. J. F. De Wet and R. Rolle, Z. Anorg. Allg. Chem., 1965, 336, 96. 3. P. Ellis, M. EckhoKand V. D. Neff, J . Phys. Chem., 1981, 85, 1225. 4. N. V. Sidgwick, 'The Chemicat Elements and Thcir Compounds', Oxford Univcrsity Press, London, 1958, 1339.
44.1S.2.5
Isocyanide complexes
Organic isocyanides react directly with iron(I1) salts to form complexes containing no more than four isocyanide ligands. Thus FeX, reacts in alcoholic solution to givem the stable diamagnetic complexes [FeX,(CNR),] (X = C1, Br, I, SCN; R = alkyl, aryl). Both cis305and trans isomersm exist and these can easily be separated by chromatography on alumina. In some cases the cislrruns ratio is determined by the temperature of the reactant solution.M7The addition of isocyanides to Fe(ClO,), in the absence of solvent yields diamagnetic [Fe(CNR), ](ClO4),, which apparently contains four-coordinate iron(I1). A wide range of iron(I1) isocyanide complexes is obtained by the alkylation of the corresponding low-spin cyanides. As early as 1888 Freund308 synthesized [Fe(CN), (CNEt),] by the alkylation of silver ferrocyanide with ethyl iodide (equation 36). A&[Fe(CN),I
+
4EtI
+
[Fe(CN),(CNEt),]
+ 4AgI
(36)
This and related reactions were studied by Hartley309,310 who obtained3" some [Fe(CNMe),] by methylation of Ag, [Fe(CN),] with MeL Originally these complexes were incorrectly formulated and were not recognized as coordination compounds until 1933. An early X-ray structure determination was reported3', in 1945 for [Fe(CNMe),]C1,. Transalkylation of iron(1I) isocyanide complexes can be acc~mplished~'~ by reacting the CNR complex with alkyl halide R'X that has a higher boiling point than RX which is continuously distilled from the reaction mixture. The reaction is general for aliphatic iron(1I) isocyanide complexes and yields up to 80% are possible although usually several products are formed which require chromatographic separation. Although [Fe(CNMe)J+ results from the reaction,', of ferrocyanide with dimethyl sulfate as in equation (37), the product of the reaction between diazomethane and ferrocyanic acid3'' is [Fe(CN),(CNMe),] as shown in equation
Iron(I1) and Lower Stares
1209
(38). On heating (150 "C, 10h) [Fe(CNMe),]Cl, is converted3I6to a mixture of cis- and trans[FeCl,(CNMe),]. Somewhat surprisingly, [H,Fc(CN)~]reacts with aliphatic alcohols to form [Fe(CN),(CNR),] and other complexes. Intermediate species formed during this reaction can polymerize with the evolution of hydrogen cyanide. Addition of HCN displaces isocyanide, however, and so constitutes a catalytic cycle for the synthesis313of RNC from HCN and ROH.
+ 3Me2S04 [H4Fe(CN),] + 4CH,N, -, [Fe(CN),I4-
-+
[Fe(CNMe),]''
+
[Fe(CN),(CNMe),]
3SO;-
(37)
+ 4N,
(38)
Controlled alkylation of [Fe(CN),(CNMe),] with one equivalent of Me1 (80"C, 4 h) affords [Fe(CN)(CNMe),]I in 70% yield.3" Iron(I1) complexes containing more than four isocyanide ligands can also be obtained by the reaction of free ligand with an iron compound in another oxidation state. Thus [Fe(CNMe),][FeCl,], results318from the room temperature reaction between FeCl, and CNMe. The cation [Fe(H)(CNBu'),]+ is obtained by protonation of Fe(CNBu'),] which is formed3'' by sodium amalgam reduction of FeBr, in the presence of an excess of the isocyanide (equation 39). The structure of [Fe(CNBu'),] differs from the ideal trigonal bipyramidal configuration, and the equatorial ligands are non-linear,,' having a CNC angle of 135" as in [FeCl,(CNMe),] for which an angle of 166 was reported.,', In contrast, the reported3" CNC angle in [Fe(CNMe)6]2+is 173 '.
-
FeBr,
[Fe(CNR),]
3 [Fe(H)(CNR),]BF,
(39)
Hydrazine reacts with [Fe(CNMe)J2+ to form the chelatf:(16) in which addition to two adjacent isocyanide ligands has taken place.321Methylamine reactP2 in a related way to give (17), while reaction with ammonia results in the formation323of a monodentate carbene complex (18). Methylation of [Fe(CN),(phen),] with Me, SO, yields [Fe(CNMe,)(phen),1,26 which reacts with hydrazine to give323the tris chelate (19) in a similar fashion to the formation of (16). Amongst the aromatic isocyanide iron(I1) Complexes, those containing benzyl isocyanide are the best studied. Coordination influences the reactivity of the ligand resulting3" in rapid nitration which, ~,~~ effect ~ may in common with sulfonation and bromination, takes place at the para p ~ s i t i o n ? ' This result from anchimeric assistance as shown in structure (20). This argument is supported by the effect being destroyed in the presence of a strong Lewis acid which prevents the participation of the nitrogen lone pair. Aliphatic aldehydes in 96% sulfuric acid alkylate the aromatic ring to give substituted benzyl alcohols, which subsequently react to form polymeric rnaterial~.~'~ Me-N-H HN
4
&*yFe(CNM
MeNH e)4
4
MeNH
A
HqFe(Phen)z HN
MeNYFe(CNMe)4 MeHN H2N>Fe(CNMe)4
Me-N-H
44.1.5.3
MeNH
MeNH
N=C=Fe(L)5
6
Alkyl Cyanides
Metallic iron reacts with nitrosyl tetrafluoroborate in acetonitrile to yield3" off-white [Fe(NCMe),](BF,), (pefi= 5.2 BM). The [Fe(NCMe),]'+ cation is readily formed when iron is oxidized by a number of strong oxidizing agents in rigorously anhydrous and oxygen free acetonitrile. Early workers investigated the use of chlorine, bromine and iodine3'* as oxidizing agents, whilst more recent has employed high oxidation fluorides such as WF,, MoF, or PF, . Oxidation of [Fe(NCMe),]'+ with chlorine in acetonitrile yields [Fe(NCMe),][FeCl,], . This salt and its bromine analog are reduced3'* by refluxing over iron wire to give complexes with the stoichiometry [FeX,(NCMe),]. Some properties of these complexes are given in Table 8. The hexakis acetonitrile iron(I1) cation reacts with P(OMe), [but not under similar conditions with PF, or P(OPh),] in acetonitrile solutions329with stepwise replacement of MeCN by P(OMe), . The final species obtained at room temperature is [Fe(NCMe){P(OMe),),][PF,],; four intermediate cations can be positively identified by NMR spectroscopy and a fifth tentatively assigned. Formation of [Fe(NCMe),P(OMe),12+ is very rapid330and cross-over from high- to low-spin iron(I1) probably
1210
Iron(1I) and Lower Stutes
occurs with the next step, i.e. formation of [Fe(NCMe), {P(OMe),)J2+, which is thought to have a cis configuration. This red species creates the initial colour seen on mixing P(OMe), with an acetonitrile solution of [Fe(NCMe)J'+, Subsequent substitutions are far slower and give rise to colour ranges from red to orange and finally yellow on standing for several days. As expected the crystal structure of [Fe(NCMe),][FeCl,], reveals octahedral [Fe(NCMe),]'+ cations and tetrahedral [FeClJ anion^.^^',^^* Table 8 Properties of selected MeCN Complexes" Compound
Colour
[Feci,(NCMe),] [FeBr,(NCMe),] [Fe(NCMe)6l[FeCh1, [Fe(NCMc)61[FcBr412 [Fe(NCMe),l[FeI,I,
Off-white Brown Yellow Red Dark green
(BM)
Yoh,
_.
5.92 5.89 5 48
IR (cm-') 2275, 2285 -.
.
2270: 2290 2285, 2305 2270, 2290
a B . J. Hathaway and D. G. Holah, J. Chem. Soc., 1964, 2408
44.1.5.4 Simple Amines 44.1.5.4.1
Saturated and primary aromatic amines
Although not commonly encountered because of their facile oxidation to brown compounds of uncertain composition, iron(I1) hexaammine salts can be prepared333under carefully controlled conditions. Thus, addition of ammonia to freshly prepared FeBr, under an atmosphere of hydrogen gives a green precipitate which is converted with excess ammonia to light blue-grey crystals of [Fe(NH,),]BrZ. Principal IR absorbances are 1595m, 1159vs, 625vs cm-' (NH,) and 315scm-' (Fe -N).334 In common with other + 2 transition metal ions, iron(I1) forms stable complexes with a wide range of simple amines, and these are generally high-spin and sensitive towards oxidation. Iron(I1) chloride reacts directly335with excess anhydrous primary amines (NH,R, R = Me, Et, Pr" and Suo) to form the complexes [Fe(NH,R),Cl,]. Interestingly, FeCl, is reduced335by primary amines to form the same white air sensitive microcrystalline solids, which are thought to have tetrahedral geometry. Complexes of stoichiometry [Fe(SO,)(L),] are formed"' by the direct reaction of iron(I1) sulfate and aniline or para-toluidine. The unusual monodentate, monoprotonated ditertiary amine ligand dabcoH+ (21) reacts"' with FeX, in anhydrous ethanol to yield complexes [FeX,(dabcoH * )JX which have trigonal bipyramidal geometry (22), unusual33sfor high-spin complexes of monodentate ligands (X = C1, pef = 5.21 BM; X = Br, pefi= 5.13 BM). Hydrogen bonding and electrostatic forces play an important part in directing the stereochemistry of these compounds. Other workers339 using the same method have obtained [FeCl,(dabco)(dabcoH-)] (23), which upon heating (184 "C) loses dabco to form tetrahedral [FeCl, (dabcoH+)]. Under the appropriate condition^^^ this complex may be formed from the reaction between FeCl, and dabcoH+ in the presence of C1- . Under carefully controlled conditions the less stable [FeX, (dabcoMe' )(H20)] and [FeX, (dabcoMe')(NH,)] (X = C1, Br) can also be obtained.341In contrast, with FeI, the four-coordinate . ~ ~ the analagous monotertiary pseudotetrahedral [FeI,(dabcoH+ )] is the only species d e t e ~ t e dWith amineJ4*quinuclidine (24) in which electrostatic hydrogen bonding effects are essentially absent, only pseudotetrahedral [FeCl, (quin),] is formed.
3
Iron(IZ) and Lower States
121 1
Unlike the reactions of pyridine and other tertiary amines with [Fe(CO),] that give products such as [Fepy,][Fe,(CO),,], primary and secondary amines under anhydrous conditions form adducts prior to reaction giving [Fe(CO),(amine)]. In some instances two amine molecules are invovled in adduct formation and one of these is thought to attack a CO ligand whilst the other is hydrogen bonded to the c o m p l e ~ . ~ ~ ~ , ~ ~ Iron(I1) forms345stable complexes with a range of polyethyleneamine and related polydentate amines. There has been considerable research effort3&devoted to the measurement of the thermodynamics of formation of these complexes in aqueous solution. Thus, for example, ethylenediamineM7forms mono, bis and tris complexes in aqueous media with successive enthalpies of reaction being 21.1,43.5 and 66.3 kJ mol-‘ respectively. Similar thermochemicalmeasurements have been made on a number of other polydentate amines, for example diethylenetriamine,” 2,2’,2”-triatrieth~lenetetramine,~~’ tetraeth~lenepentamine~~‘ and N,N,N*,N’-tetra(2min~triethylarnine,~~~ amin~ethyl)ethylenediamine.~~~ Many aliphatic polydentate amines form stable iron(I1) octahedral complexes and in some instances these are obtained directly from Fe”’ precursors. Thus anhydrous ethylenediamine or propylenediamine react with FeCl, to give octahedral [Fe(en),]CI, and [Fe(pen),]Cl, respectively.353 In contrast bis(2-dimethylaminoethy1)methylamine r e a c t P with FeCl, in hot butanol to yield amethyst, air sensitive crystals of high-spin (pem= 5.2 BM) five-coordinate [Fe(dien)Cl,]. A number of similar iron(I1) complexes have been isolated with tri- or tetra-dentate For example, the trigonal bipyramidal coordination geometry (25) of [Fe(Me,tren)Br]Br (Me, tren = tris(2dimethylaminoethy1)amine)has been confirmed357by an X-ray crystal structure determination.
Interestingly an attempt358to measure the heat of formation of the nonmethylated propyl derivative (3,3’,3”-triaminotripropylamine)in aqueous solution failed because of hydroxide precipitation in advance of any complex formation. Stability constants for a range of metal(I1) ions with Me,tren and Me,dien in aqueous solution have been interpreted355as showing that sixcoordinate octahedral geometry is the preferred configuration for iron(1I); presumably fivecoordination results from steric factors. When possible, the donor atoms of these polydentate ligands are compressed into octahedral symmetry. Thus tetren (tetraethylenepentamine) reacts with FeSO, under acid conditions to form a solution which when alkalized to pH 8.5 takes up carbon monoxide to form359six-coordinate [Fe(tetren)C0I2+.A large number of high-spin six-coordinate having picrate couniron(I1) complexes of di-and tri-dentate polyamines have been ~ynthesized~~’ terions.
44.1.5.4.2 1,8-NapRthyridine
When a solution of l,%naphthyridine (napy) in ethyl acetate is added to [Fe(H,O),](ClO,), in refluxing 2,2-dimethoxypropane/ethanol,a red-orange product may be isolated36’which has the stoichiometry [Fe(napy),](ClO,), (26). The complex has been shown362 to possess an eightcoordinate iron(l1) centre with distorted dodecahedral geometry. Eight-coordination is rare in transition metal complexes having a more than half filled d shell and in this case gives rise to unusual Mossbauer e f f e ~ t s , ~for ~ ~example . ~ ~ , an extremely high quadrupole splitting of 4.49 mm s-’ (at 77 K). The four-membered chelate ring having a ‘bite’ of 2.2 A is maintained in iron(I1) complexes of substituted naphthyridines (2,7-dimethyl- l,&naphthyridine = dmnapy; 2-methyl- 1,8naphthyridine = mnapy). However, the markedly lower quadrupole splittings observed in [Fe(mnapy),](ClO,), and [Fe(dmnapy),J(C104), are interpreted365as being derived from an octahedral coordination geometry.
C.0.C4 - M M ‘
1212 44.1.5.5
Iron(lI) md Lower States Monodentate Aromatic Amines
Although pyridine has relatively weak coordinating properties,366a very large number of iron complexes containing one or more pyridine ligands are known.367In dilute aqueous solution it appears that only two molecules of pyridine are bound to Fei: with formation constants3a of log,,Kl = 0.6 and log,,/?, = 0.9. Increasing the pyridine concentration decreases the dielectric constant of the medium so favouring the formation of inner-sphere complexes with anions. Thus in concentrated solutions [FeX,py,] complexes are obtained (see below). Steric effects are not an important factor here since the [Fepy612+cation can be formed in the absence of other potential ligands. The reaction of [Fe(CO),j and [Fe, (CO),,] with pyridine was first investigated by Hieber and coworkers in 1930, and the products formulated as [Fe,(CO),py,] and [Fe(CO),py] respcct i v e l ~ . ~ Subsequent ~ ~ . ~ ' ~ work showed that only one complex salt is formed in these reactions, [Fepy$+ [Fe4(CO),,]Z- having a magnetic moment in keeping with a high-spin iron(I1) centre (5.47 BM).37'+372 The X-ray crystal structure of this salt that in the [Fepy6J2+cation (27)the pyridine ligands occupy three mutually perpendicular planes with each pair of trans pyridine rings being coplanar. The [Fepy612+cation is also obtained from reaction of [C4F7Fe(py),(CO),I] with pyridine.374
Yellow trans-[FeCl,py,] is precipitated3'$when a saturated aqueous solution of FeCl, is added to an excess of pyridine in the absence of air. This complex is i s o t ~ p i c with ~ ' ~ the analogous cobalt and nickel complexes known to have trans configurations. Being significantly more stable towards oxidation than simple iron(I1) salts, and since [FeCl,py,] has moderate solubility in common organic solvents, it is a convenient source of iron(1I) for the preparation of other complexes. When heated under nitrogen solid trans-[FeCl, pya]loses pyridine, successively forming three polymeric pyridine complexes .377 Related complexes e.g. [FeCl, (4-Mepy),], [Fe(NCS), py,]) form only a single welldefined intermediate phase containing two pyridine ligands as shown in Scheme 9. [FeX,(H,0)4] + 4Y-py
-
truns-[FeX20'-py)~l ~
e
~
1
~
.
t Scheme 9
The enthalpy for loss of two molecules of pyridine from [FeCI,py,] has been estimated3" to be 113.0 & 1.7 kJmol-' and 128.0 f 1.3 kJmol-' for loss of the remaining two pyridine molecules to form FeC1,. trans-[FeX,(Y-py),] complexes containing a wide range of anions and substituted & ~ *main ~ features are strong pyridines have been prepared'79 and their IR spectra r e p ~ r t e d . ~ *The sharp bands characteristic of coordinated pyridine. The complexes trans-[FeI,py,] 2py and f r m [Fe(NCO),py,] -2py display bands due to coordinated and free pyridine and are therefore formulated as written.383All of these complexes have absorptions in the near IR assignable to &d transitions, and splitting of the 'Eg level is in accord with a tetragonally distorted octahedral structure.384Magnetic properties and Mossbauer spectra of these complexes have been interpreted on this b a ~ i ~Only . ~ the ~ splitting ~ , ~ ~of~the eRorbitals can be obtained from the electronic spectra,
IronlII) and Lower States
1213
but from measurement of AEQ in the Mossbauer spectra the splitting of the i2, orbitals has been estimated386for [FeX,L,] where X = C1, Br, I, NCS; L = py, 4-Mepy, isoquinoline. Table 9 lists properties of a number of [FeX,L,] complexes. The complex [Fe(NCS),py,] can be obtained in to be trans and cis isomers. However, their forms that have been claimed38Y and magnetic moments are similar3wand Mossbauer spectroscopy indicates they are not different isomers.391-392 X-Ray ~ r ys t a l l og r a p h yconfirms ~ ~ ~ that the yellow complex has the trans configuration with N-bound thiocyanate ligands. It appears the black form contains small amounts of Fe"' impurity, which is readily formed since the Fe3+/Fe2+and (SCN),/SCN- redox potentials are similar. In keeping with this explanation solutions of yellow truns-[Fe(NCS),py,] do not darken, even over long periods under oxygen-free condition^^^^^^^^ and dark violet solutions formed by exposure to air are reconverted to the yellow form by sulfur dioxide.3g2 Table 9 Solid State Magnetic and Spectroscopic Properties of some rrun.r-[FeX,py, J Complexes Complex
Absorption rnax.' (cm-')
Magnetic momenls2 (3M)
[FeCl, PY4 1 [FeBr, PY4 1 [FeI,py41 [FeI,py41*2PY [Fe(NCS), PY4l [Fe(NCSe),py41 [Fe(NW*PY,l [Fe(NCO),PY,l. 2PY
LO 300, 8550sh 10650, 7750 11 2002 I1 0002 11 250, 9900sh 11 200, 9900sh 12 200, 9000' 12 200,90002
5.34 5.28 5.90 5.64 5.47
3.14 2.52 0.65 0.83 1.52
1.13 1.16 1.25 1.17 1.21
5.13 5.20
2.52 2.59
1.24 1.24
N
--
Mfissbauer parameler? AEp(mms-') &(mrns-')
'
D. M . L. Goodgame, M.G d g a m c , M . A. Hitchman and M. J. Weeks, Inorg. Chem., 1966,S, 635. * J . F. Duncan, R. M . Golding and K.F. Mok, J. Inorg. Nucl. Chem., 1966, 28, 11 14.
As with pyridine, fl-picoline, y-picoline and isoquinoline, complexes of the type [FeX, (amine),] (X = Cl, Br, r) are obtained by adding a slight excess of the free amine to an aqueous solution of the iron(I1) halide.385*394 In contrast the less basic ligands 4-cyanopyridine and 3,5-dichloropyridine, which are solids and therefore have to be added as an alcoholic solution, afford3" complexes of the type [FeX,(ligand),] (X = C1, Br). With X = NCO, [Fe(NCO),(4-CNpy),j and [Fe(NC0),(4CNpy),] have k e n isolated as have the corresponding 3-cyanopyridine complexes. It appears that the bis complexes have polymeric tetragonal structures.39sSterically crowded quinoline forms tetrahedral complexes, [FeX,(quinoline),] (X = C1, Br, I, NCS) whose Mossbauer, electronic and A~ high-spin IR spectra have been extensively i n ~ e s t i g a t e d . ~ ~ , ~ ~ ~ hydrated 6-amino-2-methylquinoline sulfate iron(1I) complex has been isolated that is thought to be octahedral and monomeric with a monodentate sulfate ligand.398 Another sterically demanding ligand, 2-methylimidazole (ZMeimid), forms brown tetrahedral complexes [Fe(2-Meimid),]X2 (X = ClO; , BF; ) that are not well characterized, while with halides neutral complexes are obtained.399The Complexes [FeX,(Me,imid),] (X = C1, Br) are formed'" with the equally bulky 1,2-dimethylirnidazole (Me,imid). However, when the anion is thiocyanate the tetragonal complex [Fe(NCS), (Me,imid),] is obtained.m In contrast to the behaviour of the more sterically hindered pyridine, imidazole itself401,402 and its N-alkyl derivativesmm5 readily form yellow/orange salts of the type [Fe(imid),]'+ that are easily isolated with anions such as NO;, Br-, I- and C10;. The physical properties of these solid high-spin air sensitive complexes are well documented (Table lo), and in methanol solution proton NMR isotropic shifts of [Fe(imid),](CIO,), indicate the cation has 0,.symmetry.406 Several simple iron(1I) complexes containing pyrazoles (pz) have been prepared, usually from ethanol solutions dehydrated with a small addition of triethyl orthoformate. Thus light yellow [Fe(pz),]X, (X = BF; , C10;) are high-spin complexesw' with room temperature magnetic moments of 5.4 BM. With the tautomeric ligand 3(5)-methylpyrazole, (28) (Mepz), tetragonal complexes of the type [Fe(Mepz),X,] (X = C1, Br, I) are formeda6 with magnetic moments 5.6 BM. The complex formulated as Fe(Mepz\,(ClO,), that has a room temperature magnetic moment of 5.43 BM undoubtedly contains the [Fe(Mepz),]'+ c a t i ~ n . ~ Table ' 10 lists magnetic and Mossbauer parameters for several monodentate aromatic nitrogen donor complexes. Several iron(I1) triazole complexes are known and the unsubstituted ligand forms bridged species via the 2- and 4-nitrogen atoms. Addition of 1,2,4-triazole (29) (trz) to a slightly acidified solution of FeCl, and NH,SCN in the presence of SO, [to prevent formation of iron(III)] affords pale green crystals of [Fe(NCS), ( t r ~ ) ~This ] . polymeric complex has an interesting layer structurea8 containing somewhat distorted N, octahedra, with the layers separated by hydrogen-bonded NCS" groups. N
1214
rron(IK) and Lower States Table 10 Selected Iron(11) Complexes Containing Monodentate Aromatic Nitrogen Donors Miissbauer parameters 6 AEQ
Complex
CoIour
pef (BM)
[Fe(irnid),](ClO,), ~e(Meimid),](CIO,), [Fe(Pz),l(C10,)2 [FeCMpy),l [FeCI, (isoquinoline),] [Fe(2-Meimid),](C104), [FeCl2(Me, imid), J [FeBr,(Me, imid),] [FcCl,(quinoline),]
Yellow Yellow Yellow Yellow Yellow
5.59 5.72 5.55 5.34 5.30
I .3w 1.13b 0.85'
1.19 1.22 3.14 3.18 3.19
5.20
1.15" 1.1 I" 0.87"
3.19 3.32 2.72
Brown Yellow Yellow Yellow
1.30"
Ref.
Chemical shifts relative 10: "sodium nitroprusside; no1 reported; '"Co-Pd source; dnatural iron I. J. R d i j k , Red. Trnv. Chim. PQ~.T-Bos. 1971, 90, 1285; C . D. Burbridge and D. M. L. Goodgame, Inorg. Chim. Acta, 1970, 4, 231. 2. J. Reedijk, Inorg. Chim. Acta, 1969, 3, 517. 3. J. F. Duncan, R. M. Golding and K. F. Mok, J . Inorg. Nucl. Chem.. 1966, 28, 1114. 4. J. Reedijk, Red. Trav. Chim. Pays-Bas. 1972, 91,507. 5. J. Reedijk, J . Inorg. Nucl. Chem.. 1973, 35, 239. 6. C. Burbridge, D. M. L. Goodgame and M. Goodgame, J . Chem. SOC.( A ) , 1967, 349. 7. C. D. Burbridge and D. M. L. Goodgame, J . Chem. SOC.( A ) , 1968, 1074.
(28)
(29)
Magnetic properties are anisotropic with antiferromagnetic intralayer e x ~ h a n g eIn . ~contrast, ~ with the sterically hindered 4-substituted derivatives (particularly t-butyl) the ligand is forced to be monodentate or bidentate via the 1,Znitrogen atoms, and an asymmetrical dinuclear complex, is formed410athat is readily oxidized to iron(II1). Protonation of the [Fe(NCS), ( B ~ t r z )*H,O, ~] 2,6-dimethylpyrazine complex [Fe(CN), (dmpz)13- brings about dramatic spectral changes associated with the metal to ligand charge transfer. This shifts from 440 nm to 600 nm, and the pK, of the coordinated ligand (30) is 2.80 compared with 1.9 for the free ligand!lob
AM' 7Me
44.1.5.6
Diimines and Related Ligands
44.1.5.6.1 Introduction Good u donor properties and appropriate n back-bonding abilities, supplemented by the chelate effect, result in most a,&'-diiminesbeing high-field bidentate ligands which, when steric effects do not interfere as in, for example, 2,9-dirnethylphenanthr01ine~''~~~~ are comparable in strength with cyanide ion. Consequently most complexes of the type [Fe(a,a'-diimine),]*+ are low-spin and essentially diamagnetic, displaying but a small temperature independent paramagnetism of 1 BM. Metal-to-ligand charge transfer results in character is ti^^'^,^^^ intense absorption in the visible region of the spectrum ( E lo4), and their magnetic circular dichroism spectra have been studied4" down to 4.2 K. Since the tris complexes are readiiy formed in dilute solution (with extremely large stability constants- see Table 1 1) they have been used extensively for the spectrophotometric determination of iron.'16 Another analytical application is the use of large [Fe(cl,a'-diimine),]*+ cations to precipitate anions in gravimetric procedures, These complexes have also found application as reversible high potential (- 1.1 volt) redox indicators since in solution the iron(II1) complexes are virtually colourless in comparison with intensely coloured iron(I1) speciesn4" The redox potentials of a very wide range of substituted [Fe(LL),]3+'2+complexes in aqueous
-
N
Iron(II) and Lower States
1215
Table 11 Absorption Maxima, Extinction Coefficients and Stability Constants of Substitued [Fe(bipy),]’+ and [Fe(phen),]’+ Complexes in Aqueous Solution ______~
510
1.11 x 104
4,7-Me,phen S,&Me,phen S,X-Ph,phen 4,7-Ph2phen 5-Phphen 5-NH2phen 5-C1p hen 5-Brphen 5-Fphen 5-N02phen 4,7-(HO),phen 4,7-(MeO), phen 4,7-(Ph0), phen 5,6-(MeO),phen 3-S0, Hphen 5-S03phen
512 496 SI2 520 525 533 522 524 512 515 509 510 520 500 500 518 517 512
1.22 1.15 x 1.40 x 1.26 x 7.02 x 2.24 x 1.27 x 1.21 x 1.17 x 1.25 x 1.20 x 1.15 x 1.48 x 1.13 x 1.47 x 1.20 x 1.08 x 1.22
biw 4,4‘-Me, bipy 4,4‘-Ph, bipy 4,4-(C02H), bipy 4,4‘-(NH,),bipy 4,4‘-C1, bipy
522 528 552 540 569 532
8.70 9.30 2.11 1.48 1.36 8.30
Phen 2-Mephen 5-Mephen 3,8-Me2phen
21.2 10.8 21.9
104 104
23.1 23.0
io4 104 103 104 io4 104 io4 104 io4 lo4 io4 lo4 lo4 104 lo4 io4
21.8* 21.1 19.7 17.8
17.4
x io3 x lo7
x io4 x io4 x io4
103
*In 10% ethanol A. Schilt, ‘Analytical Applications of 1 , IO-Phenathroline and Related Compounds’, Pergamon, Oxford, 1969. b W . A. E. McBryde, ‘A Critical Review of Equilibrium Data for Proton and Metal Complexes of 1,lO-Phenanthroline, 2?2’-Bipyridine and Related Compounds’, Pergaman, Oxford, 1978. ._
a A.
media have been reported4’*but there is little data available on the effect of the Preparatively the iron(II1) complexes are easily obtained from the corresponding iron(I1) cations using cerium(1V) in acid solution. The optically active iron(II1) complexes can be prepared from the corresponding optically active iron(T1) complexes. They racemize slowly in the solid state, but only take a few minutes in solution.420 As expected from crystal field consideration^^^' the low-spin iron(1I) complexes are substitution inert, and thermodynamically very stable with respect to Fe2+and the free ligand; overall formation The consecutive formation constants for iron(I1) 2,2’constants (p3) can be as large as bipyridine and 1,lO-phenanthroline complexes derived by Irving and Mellor4= are not in the usual order K , > K2 > IC3,but rather K , > K2 -=4K3 (Table 12) and this was attributed to spin pairing on addition of the third ligand since [Fe(H,0),(LL)J2+ and [Fe(H,O),(LL)]’+ will be high-spin by analogy with the corresponding chloro complexes. While this behaviour is consistent with enhanced stability of the low-spin tris complexes, Brisbin and M ~ B r y d have e ~ ~cast ~ considerable doubt on the numerical significance of the reported K2 values. There is no doubt, however, that in these systems Kl -g K2K 3 .With 2-methyl-l ,lo-phenanthroline steric hindrance is responsible for the tris complex being high-spin and the more usual order Kl > K2 > K3 is observed.424 Table 12 Successive Formation Constants for lron(I1) 2,T-Bipyridine and 1,lOPhenantfiroline Complexes” Complex
KI
K*
K3
k ! O B3
2,2’-Bipyridine 1,IO-Phenanthroline
1.6 x 104 7.2 x 105
5.0 x 103 1.8 x 105
3.5 x 109 1.1 x 1O’O
17.5 21.1
‘AI 25.0”C, H. Irving and D. H. Mellor, J. Chem. Suc., 1962, 5222; 1962, 5237. See also criticism of K, values: D. A. Brisbin and W. A. E. McBryde, Can. J . Chem., 1963,41, 1135.
1216
Iron(lI) and Lower States [Fe(H,0)6]2+
+
[Fe(H20),(LL)12'
KI
LL
e [Fe(H20)4(LL)]Z' + K2
+
LL F [Fe(H2O),(LL),I2+
[Fe(H20)2(LL),]2'
+
2H20
(4)
+ 2H20
(41)
2H20
(42)
4
LL
e [Fe(LLX]*' +
Over recent years there have been a number of publications concerned with formation constants of [Fe(r,or'-diimine),]'+ complexes at various temperatures. Consequently additional AH O and ASo values are now available (Table 13),425-"28 and some data concerning mixed solvents have also been r e p ~ r t e d . ~ Most ~ ~ . of ~ ~the~ substitution " reactions of the tris ligand, low-spin, intensely coloured complexes proceed at rates conveniently monitored by conventional spectrophotometric techniques, and a considerable body of literature dealing with these kinetic and mechanistic aspects has been published. The most important a,&'-diimine ligands are 2,2'-bipyridine and 1,lo-phenanthroline, and their iron(I1) complexes are dealt with first before considering complexes of other a,a'-diimines. Table 13 ThermodynamicParameters for Formation of Selected [Fe(a,a'-diimine),y+Complexes from Fez+and Ligand Ligand
phen 5-N02phen 5,6-Mqphen biPY
-AGO (kJmol-')
- A H " (kJmol-')
118.1 92.2 125.2 93.3
130.7 f 1.1 136.9 f 3.8 128.5 4 2.8
-AS" (JK-'mol-')
Ref:
41.7 3.4 127.2 f 11.8 10.9 f 14.6 98.7 5.8
+
128.7 & 2.0
1
1 2 1
1. R.D. Alexander, D. H. Buisson. A. W. L. Dudeney and R. 3. Irving, J. Chem. Sac., F ~ r ~ d aTruans. y I , 1978.74, 1081. 2. S. C. Lahiri and S. Aditya, J . Inorg. Nucl. Chem., 1968, 30,2487.
44.1.5.6.2
(i)
2,2'-Bipyridine and IJ0-phenanthroiine
Tris complexes
The intensely red cation [Fe(bipy)J'+ was first described by B l a in ~ ~1888 ~ who ~ obtained 2,2'-bipyridine (31) by pyrolyzing copper(I1) picolinate. Ten years later Blau reported433the synthesis of I , 10-phenanthroline (32)and showed that it forms a similar tris complex with iron(I1). He also noted some of the redox properties of these complexes. It was not until 1931 that they were In 1912 Werner demonstrated the introduced as reversible high potential redox octahedral geometry of [Fe(bipy),]'+ by the successful resolution of its optical isomers.435More recent studies showed that complexes related to [Fe(phen),]'* have biological a ~ t i v i t y . 4Solutions ~~ containing [Fe(bipy),]'+ or [Fe(phen),I2+ are obtained by addition of a slight excess of the ligand in ethanol, methanol or acetone to an aqueous solution of an iron(I1) salt. The red solutions so obtained are indefinitely stable at pH 7; addition of a concentrated solution containing a large anion such as C10; , Br- or I- precipitates the cations, and the complex salts can be recrystallized from organic solvents. However, when the anion (X) is a moderately good ligand, complexes of the type [Fe(NN),X,] can result on recrystallization (see below). The small ligand bite in [Fe(phen),j'+ results in a slight compression along the pseudo-three-fold axis as well some twist distortion from Ohg e ~ r n e t r y .In ~ ~[Fe(phen),]I, ~ . ~ ~ ~ . 2 H 2 0each complex is surrounded by four others with one of the ligands protruding into the 'pockets' of the neighbouring c o m p l e x e ~ ~ ~ ~
-
(31)
(32)
Optically active [Fe(phen),]'+ and [Fe(bipy),12+ are readily obtained (via the antimony dtartrate439).Racemization in the solid-state is very slow and available kinetic results have been discussed in a recent review.w The volume of activation for racemization of crystalline 0.1 cm3mol-' at room temperature. The [Fe(phen),J(ClO,), has been to be -0.9 volume of activation for racemization in solution is positive and large (14.2 & 0.3cm3molL' in
Iran(11) and Lower States
1217
0.01 M H W 3 ) and similar to that for dissociative acid aquatioqW possibly implying a common mechanism. However, the rate of racemization is some nine times faster than dissociation, arguing against a common mechanism (see below). Mechanisms involving excitation of the low-spin iron(I1) centre to the high-spin 'T2 state, and twist processes have been considered for the racemization Very recently it has been process, which is enhanced by the presence of organic c~solvent."~~"~ reportedM6" that the iron(I1) complex of sulfonated 4,7-diphenyl- 1, IO-phenanthroline, [Fe{(S0,Ph),phen),]4-, dissociates and racemizes at the same rate in water (15-35 "C) but the addition of a cosolvent affects these processes differently. In all but extremely dilute solutions of mineral acid, protonation of free ligand causes [Fe(phen),12' , [Fe(bipy),]'+ and related complexes to dissociate in an attempt to restore equilibrium conditions as in equation (43).As a result the intensely coloured solutions fade, obeying simple first order kinetics and become colourless over a period of minutes to hours depending on temperature and the complex concerned. Being easily monitoredu7 these reactions have been the subject of numerous kinetic studies. Dissociation can also be induced by the presence of a large amount of metal ion such as H$+ that forms a relatively stable colourless complex with the a,a'-diimine ligand.444*448 Dissociation also takes place when a high concentration of a ligand is added that complexes with the iron and successfully competes with the a,a'-diimine. The latter approach is particularly effective with polycarboxylates under mildly oxidizing conditions (e.g. with air), when iron(II1) polycarboxylates result.u9 First order rate constants for the acid aquation of [Fe(phen),]'+ and tris complexes of substituted 1,lO-phenanthrolines not having ionizable groups only vary slightly (decreasing) with acid concentration. In keeping with a dissociative mechanism, activation energies are as high as 125 kJ mol-' and entropies of activation are large and positive (Table 14).450Variations in aquation rate for complexes such as [Fe(NH2phen),I2+with acid concentration are attributed to protonation equilibria involving the substituen t.45' Table 14 Rate Constants and Activation Parameters for Aqueous Solutions of Complexes of the Type [Fe(Xphen),]** in 1 M Sulfuric Acid
'
X
b "C (s - '1
AHf (kJ mol-')
A S # (J K - mol-' )
7.3 x 4.9 x 10-4 2.2 x
122.5 117.0 116.2
+ 87.8
S-NO, 4,7-Me,
H
+ 74.8 + 54.3
A V (~m~m0l-l) f
15.4 f 0.4
+ 17.9 f 0.3
+11.6 & 0.6
Standard deviations associated with AH' are k0.8 kJmol-I and with AS+ f 3 JK-'mol-' in each case. f.-M. Luck D. R. Stranks and I. Burgess, J Chem. SOC.,Dalton Tram., 1975,245; J. Burgess and R. H. Prince, J. Chem. Soc., 1963, 5762.
[Fe(LL),]'+
+
3H+
-
Fez
+
3LLH'
(43)
The behaviour of [Fe(bipy),I2' towards acid aquation is not the same as [Fe(phen),]'+ . In the presence of sufficientmineral acid [Fe(bipy),]'+ dissociates, the rate of aquation increasing with acid concentration until a rate limit is reached4" at 2N. This is i n t e r ~ r e t e d ~ in ' ~ ,terms ~ ~ of a mechanism involving species containing monodentate protonated 2,Y-bipyridine as illustrated in Scheme 10, rather than the original suggestion455of a protonated intermediate with six iron-nitrogen bonds (although related species have been claimed456).The 1,lO-phenanthroline ligand appears too rigid to easily form monodentate protonated species so the acid dependent path (k3[H+] in Scheme 10) is not available for [Fe(phen),l2+,It has been deduced that the only low-spin species in Scheme 10 is [Fe(bipy)J2" itself!57 This scheme does not, however, account for all of the reported observations. In concentrated hydrochloric acid [Fe(bipy),]'+ dissociate^^'^ but in other concentrated acids the situation is more complex.459In fact it is difficult to obtain the iron(I1) complexes in concentrated solutions of nitric or sulfuric acids due to oxidation to iron(III), and the overall pattern of behaviour may be related to the activity of water in the concentrated acids.460In this connection it is interesting to note that rates of dissociation are significantly less in D,O/HCl than in H20/HCl soIutions!6' A considerable amount of work, notably by Burgess,462has made use of these dissociation reactions as a probe for structure in mixed solvent systems. Both [Fe(phen),12+ and [Fe(bipy),12+ and their substituted derivatives undego relatively rapid reactions with strong nucleophiles such as OH-, CN- and N; to give nucleophile dependent products. With cyanide ion the low-spin first formed product463is [Fe(CN),(LL),], which after long reaction periods is converted4" to [Fe(CN),(LL)]'- as in equation (44).These cyanide complexes show strong solvatochrornatic charge-transfer bands.46M7For instance the absorption maximum of [Fe(CN),(bipy),] in the visible region in water is 515 nm while in pyridine it is 629 nm;there are also
-
1218
Iron(I1) and Lower States
Scheme 10
correspondingly large changes in extinction coefficient. Reaction of fFe(LL),I2+ complexes with hydroxide ion leads to complete displacement of the ligand from the iron and formation of iron(II1) hydroxidew as in equation (4.5). It appears that formation of highly insoluble iron(II1) hydroxide provides the thermodynamic driving force for the overall reaction. Addition of edta to chelate the iron(II1) prevents precipitation of FelOH), while not inhibiting the reaction. In the absence of oxygen there is no reaction. The effect of micelles on the aquation and base hydrolysis of [Fe(phen),12+has been reported.469Reactions with both cyanide and hydroxide ions under pseudo first order conditions follow the second order rate law shown in equation (46). At very high nucleophile concentrations higher orders in nucleophile have been reported470(e.g. k,p-1' and k4B-l3),and it seems likely these result from specific ion effects. The k , term is assigned to ligand independent rate determining dissociation of the complex, and the k2 term to associative reaction with the The latter second order path is usually dominant and surprisingly is associated with a positive volume of activation."' Since there are no ionizable hydrogens present, a conjugate base mechanism is not possible, so the observed kinetic behaviour might result from direct nucleophilic attack on the metal centre, or perhaps through a mechanism involving ion-pairs (see below). Support for the second order reactions involving attack on the iron centre comes from the reaction of optically active [Fe(phen)J2+ with CN- which gives47s some inverted [Fe(CN),(phen),]. Both retention and inversion paths are involved and although interpretation of the kinetic data is not straightforward, it is concluded that the optical inversions are true chemical i n v e r s i o n ~ ? ~Curiously, ~ , ~ ~ ~ ~ however, the reaction of optically active [Fe(bipy),]'+ with CNinvolves some retention of activity4" with a greater dependence on [CN-] than is the case for inversion of [Fe(phen),12+.An alternative, and interesting, mechanism involving an initial reaction of the nucleophile with the coordinated a$-diimine ligand was postulated478by Gillard in 1975. By analogy with reaction of some quaternary nitrogen heterocyclic c o m p o ~ n d s ~it~was ~ , ~suggested *~ the second order path corresponds to nucleophilic attack on the coordinated (Le. quaternary) ligand at a position close to the metal which is followed by rapid transfer of the nucleophile to the metal. [Fe(LL)3]2-
+
OH-/02
* Fe(OH)3 + 3LL
(45)
It was further suggested that in some situations water might add across the C=N bond (covalent hydration) with subsequent deprotonation in alkali leading to formation of a pseudo base that rapidly undergoes intramolecular shift of the OH from ligand to metal. These suggestions stimulated much work both by Gillard's group and others, most of which has been reviewed:" but there remains disagreement concerning the importance of covalent hydration and pseudo base formation in reactions of iron(I1) a,a'-diimine complexes and also complexes of other rnetal~.~*~"*~ It appears that the best evidence for nucleophilic attack on the ligand in iron(I1) 2,2'-bipyridine and 1,lOphenanthroline complexes involves 5-nitro- I, 10-phenanthroline. However, this results from Meisenheimer complex formation with attack taking place on the 6-position, which is remote from the iron centre rather than adjacent to it.484,485
Iron(I1) and Lower States
1219
The participation of reaction paths involving ion pairs has been proposed to explain the second order behaviour of IFe(LL)J2+ complexes with OH- and CN- . Margerum first suggested4*‘the [Fe(phen),]*+-nOH- ion pairs are likely to dissociate at rates different from [Fe(phen),]’+ itself, and pointed out that other anions such as C1-, F-, NO;, (210; etc. might be expected to form ion pairs as readily as OH-, though none of these ions significantly enhance the rate of dissociation in aqueous media.145*487 This could be due to a number of factors including hydration/steric effects and unfavourable competition with dipolar interactions. Ion pairing of [Fe(LL)J2+ cations with a range of common anions in nonaqueous media is well established and in some instances hydrophobic effects are operative.488They have been used over a number of years as a means of concentrating these complex cations (via solvent extraction into an immiscible liquid) in spectrophotometric determinations.489494 There are reports of enhanced rates of acid solvolysis of [Fe(bipy)J2+ and [Fe(phen),]*+ by common anions in dimethyl and methanol.4MIn the latter solvent the effectiveness in accelerating the rate is C1- Br- MeC6H4SO; > HSO; NO; > C104 . In DMS?, acid solvolysis of [Fe(phen),12+is dramatically increased by C1- (added in the form of LiCl or Et,NCH,PhCl- ) and this effect is inhibited by HSO; . These observations were quantitatively accounted for by the competitive ion-pair formation mechanism shown in Scheme 11 involving a reactive ion pair with C1- and a more inert [Fe(phen),]’+ *HSOy ion pair:97
-
+
[Fe(phen),]’+
K:
+ C1-
[Fe(phcn),lZ+*HSO;
iI
-
[Fe(phen)3]2f-C1-
Fez+ +- Hphen’
Scheme I 1
Similar ion-pair Competition has been reported for the reaction of CN- with [Fe(phen),]’+ in water where addition of I- inhibits the reaction by formation of a relatively inert iodide ion pair!9* In this instance there is independent evidence for ion-pair formation with iodide.499There is, however, criticism of ion-pair mechanisms although the observed kinetic behaviour is not in accord with the proposed’OO alternative mechanism involving a preequilibrium to form a bis chelate. A recent lengthy pape?’’ dealing with reaction of several [Fe(LL),]’+ complexes with CN- highlights the importance of other effects in some instances. [Fe(LL)7]2+(LL = phen, bipy), in the presence of long chain alkyl sulfonates, are readily extracted into chloroform,502and dissociation and racemization rates are enhanced via specific interaction^.^'^
(ii) Bis complexes
Complexes of the type [FeX,(LL),] (LL = phen, bipy) where X is a unidentate anionic ligand have been studied extensivelyswbecause they display a range of spin states depending on the nature of X. [Fe(CN),(LL),] complexes are low-spin ( ‘ A l ) and kinetically inert. Weak field ligands (X-) result in high-spin complexes (’T2),,while with some ligands, notably NCS- and NCSe-, the ’T2to 1 A , spin transition can be observed in the form of abrupt changes in magnetic and spectral properties over a fairly narrow temperature range.’” Low-spin cis-[Fe(CN),(phen), j is obtained from the reaction of [Fe(phen)J’+ with CN- in aqueous solution. There has been controversy over its stereochemistry with some IR data suggesting cis and trans isomers exist, but Mossbauer experiments506indicate the existence of a single isomer (AEQ = 0.58 mm s- I , S = 0.27 relative to stainless steel). The cis complex is only very slightly soluble in water, but readily extractable with chloroform or nitrobenzene and as noted previously it exhibits remarkable solvarochromic properties (Table 15). In aqueous solution it is orange and in chloroform or nitrobenzene it is violet. The reaction of [Fe(phen)J’+ with CN- has been recommended for the selective and sensitive determination of cyanide.s07If excess [Fe(phen),]’+ is present only [Fe(CN),(phen),] is formed, with excess CN- reaction to give [Fe(CN),(phen)12- is so extremely slow at room temperature that it does not interfere. The cyanide ligands in [Fe(CN),(LL),j (LL = phen, bipy) are sensitive towards protonation: successive protonation of both cyanide ligands causes the colour of the complex to change from violet through orange-red to yellow.508~”9 Protonation in liquid hydrogen fluoride has also been examined in detail,”’ and a cis configuration confirmed for the diprotonated cation. Both
+
Iron(l1) and Lower States
1220
Table 15 Wavelengths (nm) of Maximum Absorption of [Fe(LL),(CN)J Complexes in Various Solvents Solvent
[Fe(bipy), ( C N ) , I
[Wphen), (CN),I
IFe(4,7-Me2phen),( C N ) , ]
Water MeOH EtOH Pf OH BunOH MeCN DMF
515a
516
505
5 w
545
568' 512
551 566 510
534 548 555 551
596 614
588 605
625
607
619
618
574 604 619 626 629d
Me,CO
c5 HS N
Extinction coefficient: " 5 . 5 x IO3, b6.4 x Id. '6.9 x lo3, d8.6 x Id. A. A. Schilt, J . Am. Chem. Suc., 1960, 82, 3000; J. Burgess, Spactruchtm. Acto, Part A , 1970, 26, 1369.
[Fe(CN), (phen),] and the corresponding bipy complex can be oxidized to give iron(II1) comple~es.~'' They are useful as indicators for titration of weak bases in nonaqueous solvents, and also for redox titrations. The reaction of [Fe(CN), (bipy),] with hydroxide ion follows a first-order rate law with rates independent of [OH-] to give, presumably, hydrated iron(II1) oxide via air oxidat i ~ n . ~Other ' * low-spin complexes of the [FeX,(LL),] (LL = phen, bipy) typeSware with X- = NO; and CNO-, while most others are high-spin. In this respect the halide complexes are typical with magnetic moments of 5.3 BM at room temperature. They are obtained from reaction of the anhydrous iron(1l) salt with the diimine in a low polarity solvent,513or from the corresponding tris complex by extraction or refluxing with hot organic solvent, or pyrolysis in the absence of oxygen. Thermal decomposition of the tris complex as in equation (47) achieved by refluxing or extracting with a hot solvent such as chloroform, benzene, or methylcyclohexane, can be satisfactory although long reaction times are necessary in some instance^.^'^^^'^ This procedure does not make use of water or methanol, because in these solvents most of the high-spin bis ligand species are not stable. They usually dissociate forming the tris complex according to equation (48), although there are instances where, depending on the nature of X- , recrystallization from methanol or ethanol affords the bis complex.
-
[Fe(LL),]X, 3[FeX,(phen),]
-+
-
[Fe(LL),X,]
2[Fe(phen)Jz+
+
+ LL Fe2+
(47)
+
6X-
(48)
Japanese have reexamined the pyrolysis of [Fe(bipy),]Cl, .5HzO, a means of making [FeX, (LL),] complexes originally investigated by Germans16and later by American Using Mossbauer spectroscopy they found5" [FeCl,(bipy),] is obtained in the temperature range 140-200 "C with dissociation of the bipy ligand taking place at 140 "C before dehydration is complete. However volatilization of the dissociated ligand from the solid proceeds only slowly below 200 "C. The room temperature Mossbauer parameters of [FeCl,(bipy),] (6 = 1.01mm s-' , AE, =. 2.98 mm s-') obtained in this way are the same as previously rep~rted~'"~'for [FeCl,(bipy),] made in acid aqueous solution from the ligand and Fez+. While most [FeX,(LL),] (LL = phen, bipy) complexes are high-spin ('T,) and a few low-spin ( ' A , ) , some, such as [Fe(ox)(phen),] (ox = oxalate) and [FeF,(phen),], are ~ l a i m e d to ~ 'be ~~ of~ ~ ~ intermediate spin. Of particular interest are those complexes (notably when X - = NCS-, NCSe-) in which the high-spin to low-spin transition can be observed in the form of an abrupt change in magnetic and spectral properties over a fairly narrow temperature range.504,505 These complexes can be obtained from the red precipitates formed when aqueous solutions of Fez+ and the anion are treated with the calculated amount of ligand. These red hydrated compounds on dehydration afford, for example,523purple cis-[Fe(NCS), @hen),]. The red compound containing phen dissolves in solvents such as dimethyl sulfoxide and nitromethane, but not in diethyl ether or dioxane. In acetonitrile rapid isomerization takes places24to give the insoluble purple cis-[Fe(NCS),(phen),]. This behaviour is consistent with the red compound being [Fe(phen),],[Fe(NCS),](NCS), .nH,O and this formulation is supported by Mossbauer and IR spectra as well as other measurement^.^" Equations (49) and (50) represent the process taking place. At room temperature the magnetic
-
3Fe2+
+
6SCN-
+
6phen
----*
[Fe(phen),],[Fe(NCS),](NCS),
-
[Fe(phen),J,[Fe(NCS),J(NCS)2 3 cis-[Fe(NCS),(phen),]
(49) (50)
Iron(I1) arad Lower States
1221
moments of [Fe(NCS), hen)^], [Fe(NCSe), (phen),] and [Fe(NCS), (bipy),] are in the range characteristic of high-spin complexes (4.9 to 5.2 BM) but at lower temperatures abruptly decrease (e.g. in [FeX,(phen),] at 174K for X- = NCS-; at 232K for X- = NCSe-). The observed magnetic moments tend to approach values more typical of low-spin complexes at 77K. The existence of several polymorphs complicates the situation, but spectroscopic studies, and in particular Mossbauer data (Table 16) conclusively show that the transition involves a change from the ’T2to ‘A, spin state. However, a simple model based on a ‘pure’ Boltzmann distribution over a fixed set of energy levels is an inadequate interpretation of the experimental results. It seems likely that solid state effects such as domain size have an important role in this behaviour. Thus samples of [Fe(phen),(NCS),], prepared by precipitation in methanol and by acetone extraction of [Fe(phen),](NCS), , show a number of differences resulting from different crystal size and quality. The spin transition in the sample obtained from methanol is gradual while that of the extracted sample is abrupt. Their thermal behaviour (DTA) and temperature dependent X-ray diffraction patterns also show significant difference^.'^' Table 16 Mossbauer Data for Complexes of the Type [Fe(LL), X,] (LL = phen, bipy) Displaying Anomalous Temperature Dependent Magnetic Moments Complex
[Wphen), (NCSM [Fe@hen),(NCS),lb [WPhen),(NCS),lb [Wphenh (NCSehl [Fe(ClphenMNCSXI [Fe(MePhe~),(NCSl,l EFeWO, phen), (NCSM Fe(Me2phen), (NCS), I [WbiPY),(NCShI
Liquid nitrogen temperature 6 (mms-’) hE, (mms-’) 0.37 0.46 0.42 0.35 -
0.34 0.36
0.34 0.36 0.33
0.18 0.30 0.4t 0.34 0.46 0.50
Room iemperature AEQ( m s - ’ )
6 (mms-’) 0.98 1.01 0.94 I .03 -
0.97 1.06
2.67 2.71 2.64 2.52 2.73 2.55 2.67 2.59 2.18
Re$ 1
2 2 1 3
3 3 4 5
a From
methanol. bAcetone extracted [Fe(phen)JNCS), . 1. E. Konig and K. Madeja, Inorg. Cbem., 1967, 6, 48. 2. P. Ganguli, P. Gutlich, E. W. Muller and W. Irler, J . Chem. Sac., Dalton Trans., 1981, 441. 3. A. J. Cunningham, J. E. Fergusson, H. K. J. Powell, E. Sim and H. Wong, J . Chem. Soc., Dalfan Tmns., 1972, 2155. 4. E. Konig, G. Ritter and B. Kanellakopulos, J . Phys. Chem., Solid Stare P h p . , 1974, 7 , 2681. 5. E. Komg, K. Madeja and K. I. Watson, J . Am. Cbem. Sot., 1968, 90, 1146.
(iii) Mono complexes In solution yellow iron(I1) species containing one a,a’-diimine ligand can be obtaineds26from an excess of Fe2+ in an acidified solution of the ligand (which controls the amount of available free ligand via protonation). Formation constants for many such complexes of substituted 1,lophenanthrolines have been reported and tab~lated.4’~ The rate of formation of [Fe(phen)(H20),I2+ is rapid and has a very unfavourable entropy of activation (E, = 53.6kJmol-’, AS# = - 66.9 J K-I Kinetics of the corresponding reactions with 2,2’-bipyridine have also been reported.528Reaction of [FeF,py,] with 1,lO-phenthroline in pyridine affords violet [FeF,(phen)] (4.4BM) which is no doubt polymeric.5i3Other dihalo complexes can be obtained by pyrolysis of the corresponding tris complexes. From [Fe(bipy),]Cl,. 5 H 2 0two [FeCl,(bipy)] isomers have been characterized by Mossbauer spectroscopy.s15The orange isomer originally thought to be monomeric and possibly tetrahedral (6 = 0.93 mrn s-’; AEp = 3.58 mm s-’ at 293 K) is obtained from the first formed [FeC12(bipy),], on further heating it is converted to a rose-red polymeric form. This is similar to that prepared from FeCl, and the ligand in hydrochloric acid?’’ for which the Mossbauer data are consistent with the presence of polymeric, chloride-bridged, octahedral highspin iron(II), as are IR spectra and magnetic moments. It has been suggested the corresponding 5,5’-dimethyl-2,2‘-bipyridinecomplex is probably a dimer containing fivecoordinate i r ~ n ( I I ) . ~ ” Very recently, however, the orange isomer of IFe(bipy)Cl,] has been obtained from ethanol solutions of FeCl2.4H2O and a deficiency of bipy,. and on the basis of low temperature magnetic data structure (33) comprising a chain of five-coordinate iron(I1) centres with single chloro bridges was proposed.520 The [Fe(CN),(LL)]’’ (LL = bipy, phen) complexes have not been extensively studied. They are low-spin, and the proton NMR spectra of the complexes with LL = bipy and 5,5‘-Me,bipy are consistent with the expected two-fold symmetry of the diimine. With LL = phen an unexpected
I
Iron(II) and Lower States
1222
inequivalence has been observed that was postulated to be due to addition of water to an imine double bond.529Like [Fe(CN),(LL),], the anionic mono diimine complexes exhibit solvatochromatic effects. [Fe(CN),(LL)]'- complexes can be obtained by prolonged (days) reaction of [Fe(LL),]*+ with cyanide ion, but a better route is from [Fe(CN),I4-, which reacts with bipy in the presence of a deficiency of Hg2+ to form predominantly [Fe(CN),(bipy)J2-. With an excess of Hg2+ only [Fe(bipy),]'+ is formed (Scheme 12).530The pyrolysis of [Fe(CN), (LL)]'- complexes, including the diprotonated species, has been studied extensively by Hungarian w ~ r k e r s . ~ ~ ' - ~ ~ ~
bipy XSHg"
44.1 S.6.3
I
rapid
C N - , days
2,2':6',2"- Terpyridine
The terdentate ligand 2,2':6',2"-terpyridine, terpy (M), forms mono and bis complexes with iron(I1). On the basis of a comparison of X-ray powder data it was suggested534that [Fe(terpy)X,] (X = Br, I) has a five-coordinate structure similar to that of [Zn(terpy)Cl,]. In spite of early report^'^^.'^^ claiming that the chloro complex is isomorphous with the zinc analogue, its X-ray powder pattern is different and it has been reformulated537as [Fe(terpy>,][FeCl,], which is in accord with an observed538magnetic moment of 4.60 BM, well below the spin-only value.
(34)
Fe2,f in the presence of an excess of terpy forms539,s40 the very stable low-spin complex [Fe(terpy),j2+,for which log b2 = 20.4, AH = - 113kJmol-I and AS = - 13J K-' mol-'. The rate of ligand exchange is slow and has been examined using tritium-labelled t e r p ~ . In ~ ~acid ' solution, ligand dissociation takes places4*and the observed kinetic behaviour as a function of acid concentration is complex.543Reaction of [Fe(terpy)2]2' with CN- follows second order kinetics to give [Fe(terpy)(CN),]- whose maximum absorption in the visible region (metal-ligand ct) varies markedly with the nature of the ~olvent.~"Oxidation of [Fe(terpy)JZt yields the iron(II1) complex (- I . 1 V), while controlled reduction affords the -t 1 and 0 formal oxidation state^.^,^^
44.1.5.6.4
Other dimines and related ligands
The basic iron(I1) a-diimine five-membered unsaturated chelate ring (35) is a key unit in iron chemistry. In this review the classic ligands important in the early development of the area have been allocated separate sections: 2,2'-bipyridine and 1,IO-phenanthroline (44.1.5.6.2), and 2,2':6',2"terpyridine (44.1.5.6.3). The closely related dioximes are discussed in Section 44.1.5.7, while the many macrocyclic systems containing such linkages are covered in later sections. Once it was appreciated that only the basic structure (35) rather than a fully aromatic ligand is necessary to obtain stable, highly coloured (metal-to-ligand charge tran~fe?'~)complexes, many ligands containing this moiety were synthesized and their iron(I1) complexes prepared. Some of the most readily
-
Iron(1I) and Lower States
1223
accessible complexes are obtained via in situ Schiff base formation, and several useful reviews concerned with them are available.546b546" These species are discussed first, before dealing with complexes of fully aromatic ligands. Most of these complexes are red or purple, and many with six nitrogen donors are low-spin and not of particular interest unless they display unusual ligand reactivity or because they have been the subject of kinetic studies. Several of these complexes can be obtained via oxidation/dehydrogenation of the corresponding saturated complex and this illustrates the thermodynamic stability of (35) in low-spin complexes. There has also been considerable interest in those complexes that exhibit spin transitions as a result of steric or electronic effects causing a weakening of the ligand field strength.
(35)
Blau reported547in 1898 that the saturated or&-dipiperidine (36a) forms a purple iron(I1) complex. This surprising finding was later shown by K r ~ m h o l to z ~be ~ ~due to air oxidation of the ligand in the presence of Fez+to give the tris chelate of (36b). He also demonstrated the template reaction of methylamine with glyoxal or biacetyl in the presence of Fez+ that affords red tris complexes of the diimines (37a) and (37b).Although the free ligands are unstable, these complexes are very stable with respect to dissociation in acid but rather less so towards alkali. Their visible spectra are similar to those of [Fe(bipy),]'+ and [Fe(phen)J'+. For IFe(37b)J2+, A,,, = 568 ( E = i .07 x lo4) and as with related complexes, on the high frequency side of this electronic band there are shoulders. These have been assigned to metal-to-ligand charge transfer,s47while a later Raman study indicated they are largely due to vibronic transition^.^^ The range of complexes obtainable via this template synthesis is illustrated by the selected examples in equation (51). The isomer shifts of the tris complexes in their Mossbauer spectra''' do not differ greatly (8 - 0.06mms-' at 298 K relative to 57Co/Cu)and the quadrupole splitting values fall within the range 0.5 to 0.6mm s-' except for complexes with phenyl groups. These have AEQ 0.9 mm s-', that may result from steric effects or increased acceptor ability. The former is favoured. The tris chelate derived from glyoxal and methylamine, [Fe(37a),12+,has been resolved and its racemization kinetics st~died.'~'Racemization involves both intramolecular twist and bond breaking paths (- 70% via twist at 69.4"C). The activation enthalpy for bond breaking, as with [Fe(bipy),]'+, is lower than that for twist racemization. In contrast with the more rigid [Fe(phen),j2+, the activation enthalpy for dissociative bond breaking is larger than that for racemization. N
-
m ' Q 4 J A=A \
H H'
MeN
(36a)
(36b)
RCOCOR'
+
NMe
(37a)
MkM"
MeN
NMe
(37b)
-
MeN. "inR' , NMe Fe'l, R = H; R' = H , Me, Ph R =Me; R' = M e , Et, Ph RR' = (CH216
2MeNH2
Fez-
Calculations553 on complex (38) confirm metal-to-ligand back-donation is important and indicate that HOMOS in complexes of this type have considerable ligand character. The latter result is relevant to the behaviour of related complexes upon oxidation. Oxidation of tris{(glyoxal)bis(methylimine)}iron(II), [Fe(37a),I2+, by cerium(1V) is not like the reaction involving phen or bipy that simply gives the corresponding iron(II1) complexes. Here oxidation of the coordinated ligand takes place to give two or more products including complex (39). It appears554 that this reaction does not proceed via direct attack on the ligand but rather at the metal centre giving an intermediate iron(I1I) complex, which decomposes to give several products. Electrochemical and spectrophotometric monitoring of the reaction in 4.07 M H,SO, led to evidence55ssupporting a mechanism in which intramolecular electron transfer in the first formed iron(TI1) complex is
1224
Iron(II) and Lower States
assisted through nucleophilic attack by water on the ligand. This proton abstraction takes place at Me or CH, explaining the observed ligand oxidized products. However, these radicals may react with water at the same time reducing a molecule of [Fe(37a)J3'. The basic correctness of this complicated mechanistic scheme was confirmed by electrochemical studies5%that yielded values of the rate constants for some of the faster processes. As acidity is increased the thermodynamic stability of the iron(II1) complex increases with respect to iron(II), and in strong sulfuric acid (> 10 M) [Fe(37a)3]3+does not undergo ligand oxidation. Under these conditions activation for the Cerv oxidation are AH' = 43.9 f 4.2kJmol-' and A S # = - 33.5 & 8.4J K-' mol-'.
(38)
(39)
In acetonitrile electrochemical reduction gives iron(1) and iron(0) species.ss7bIn this solvent ligand oxidation reactions are almost absent, but they are not completely avoided due to the presence of small amounts of water."* This problem was overcome by using a totally anhydrous low temperature (25 "C)molten salt comprising aluminum chloride and ethylpyridinium bromide (2: 1 mole ratio). In this medium, reversible one-electron electrochemical oxidations take placessg with [Fe(37a)J2+, /Fe(37b)J2+ and a number of related complexes.s" Here the iron(IT1) form is thermodynamically more stable than is iron(II), whereas in aqueous solutions the reverse is true. Addition of [Fe(37a),I2+ to the Feigf -catalyzed decomposition of hydrogen peroxide inhibits oxygen evolution by competing with Fei: for HO; radical^.'^' Thus the iron(I1) complex is a trap for HO;, itself becoming oxidized in regenerating HzO,. In acid solutions [Fe(37b),I2+ undergoes autooxidation through a free radical process. The reaction is first order in complex and half order in oxygen, suggesting protonation is involved in a preequilibrium step. The main product results form the oxidation of C-methyl to a hydroperoxide, and oxidation only takes place when a C-methyl or C-methylene group is present. Dihydrazones of a-diketones can form low-spin tris complexes with iron(I1). Thus the complex with biacetyldihydrazone (Ma) is well characterized and related tris complexes derived from bend and glyoxal are also l ~ w - s p i n , ' while ~ ~ , ~bis ~ complexes of the type [FeX,(NN), J are paramagnetics6' with magnetic moments of 3 BM. These unusual values might be indicative of the complexes being incorrectly formulated. The bidentate and tridentate hydrazones (4Ob) and (40c) derived from pyridine aldehydes form low-spin tris and bis complexes respectively. However, considerably more work has been carried out on the related Schiff bases (41) obtained from pyridine aldehydess7 or ketoness6' and a primary amine (that can be aliphatic or aromatic). In both cases the resulting Schiff bases acts as a bidentate ligand and the low-spin tris iron(1I) complexes are deep purple. They are prepared by adding a solution of an iron(I1) salt to a mixture of a pyridine-2-carbaldehyde (or ketone) and the amine. This facile template synthesis can be used to prepare the free ligand via subsequent demetallation with edta. This is conveniently carried out in aqueous methanol, but if aqueous acetone is used, addition of acetone to the C=N bond takes place,s69as shown in equation (52). When the complexes are formed in the presence of halide, green bis complexes [FeX,(NN),] result that are high-~pin.'~''Extraction of these compounds with refluxing chloroform or cyclohexane yields mono complexes [Fe(")X,] that undoubtedly contain halide bridges.
-
Acid dissociation of several iron(I1) tris complexes of (41) has been well studied. For R = H, R' = alkyl or phenyl the variation of rate with acid concentration is like that for [Fe(phen),]'+. This is surprising since these ligands might be expected to be flexible, and so form a monoprotonated intermediate in the way [Fe(bipy)J2+ does. Perhaps this is due to the partially
Iron(II) apld Lower States
1225
dissociated intermediate having a very short life. Substitution in the phenyl ring does affect the dissociation rate in a way that can be correlated with Hammett constants. In contrast to the behaviour of most of these complexes, those with ligands derived from nitroanilines or fluoroaniline dissociate at rates that depend on acid concentration. Moreover, the results consistent with a rate law containing an [acidI2term that has been interpreted in terms of a rapid protonation preequilibrium, or hydrogen bonding with H 3 0 + .With R = Ph isomeric complexes are obtained that dissociate at different rates,s68and similar effects have been with R = H, R = Me, Pr; and R = R = Me.573These and related complexes react with hydroxide ion574and cyanide ion in second order reactions. Iron(I1I) hydroxide is the product of the former and dicyano complexes of the latter. Like [Fe(CN), (phen),] the analogous Schiff base complexes are low-spin, and display strong charge-transfer spectra that are very solvent de~endent.~""Reaction of [Fe(41)J2+ (R = Fh, R = 3,4-dimethylphenyl) with hydroxide ion has A V 11 cm3mol-', and this increases markedly as the solvent is made increasingly methanol rich highlighting the role of solvation.575b Complexes of the [Fe(41),j2+ type react with hydrogen peroxide'" at a rate independent of [H202]that is limited by ligand dissociation. With peroxodisulfate in some instances two parallel reactions take place, one being independent, and the other dependent on the oxidant concentration. Thus, for R = R = Ph a single term rate law applies, whereas when R = H, R = Ph there is a two term rate law.576Oxidation of [Fe(41)j2' (R = R = Me) with the more powerful cerium(1V) is more complicated, and the reaction depends on acidity. Oxidation of the metal is followed by intramolecular reduction back to iron(I1) with concomitant radical formation at the ligand that is assisted by water and retarded by Under appropriate conditions the major product is [Fe(41),I2+ with R = CHO, R' = Me. A large number of polydentate Schiff base ligands have been reported, and of particular interest are the iron(I1) complexes of ligands such as (42) (R = H, Ph) because they are low-spin, even though they contain only two diimine units.57sThe rate of acid aquation of [Fe(42)I2' is slow, but this depends on acid concentration. Reaction with hydroxide ion is second order, as is that with cyanide ion. The product of the last reaction is not [Fe(CN),l4-, but [Fe(CN),(42)]'- that has only one diimine bound to the metal. Related ligands have been prepared and much of the early work has been reviewed.546b546"
PZnE
-
(42)
The tridentate ligands (43a) and (43b) form low-spin bis ligand complexes. The kinetics of several reactions of these, and related complexes have been studied.579" The cisltrans isomers (43c) and (43d) of pyridine-2-carbaldehyde 2-pyridylhydrazone can be isolated and both form iron(I1) comp l e ~ e s . "A ~ ~tris complex is obtained with (43d). In water this is yellow, and when an acid solution is boiled it is converted to the bis complex [Fe(43c)J2+.This complex is exceptionally stable (log p2 = 16.7) and it is acidic so that in alkaline solution it forms a neutral ~ o r n p l e x . The 5 ~ ~hexadentate ~ ligand (43e) forms a well-characterized purple low-spin complex,579dbut substitution of methyl groups in the pyridine rings adjacent to the nitrogen atoms results in a complex displaying spin c r o ~ s - o v e rThe . ~ ~dynamics ~~ of this spin change have been monitored using Iaser temperature-jump methods.s97f Iron(I1) Complexes containing saturated amines can undergo oxidative dehydrogenation to give thermodynamically more stable imine complexes. Mention has been made earlier in this section about the behaviour of a,a'-dipiperidine in this respect.548One of the most well-studied systems involves yellow diamagnetic 1,2-diamine complexes of the type [Fe(CN),(diamine)12-. Upon oxidation in alkaline solution these give red diimine complexes as in equation (53). The reaction goes via an iron(1II) intermediate that can be isolated under acidic conditions. In the presence of base this gives complexes containing mono- and di-imine linkages.580The electrochemical and ferricyanide
Iron(II) and Lower States
1226
N\
NH
oxidation of the ethylenediamine complex [Fe(CN), (en)]’- has been studied in detail.58’Reaction of the first formed low-spin iron(II1) complex with OH- leads to NH, deprotonation that is followed by internal electron transfer giving iron(I1) with a NH radical, that forms the Fe”=NH linkage on further oxidation. Tetracyano complexes containing optically active diamines have been prepared,5s2 and their CD spectra recorded. Treatment with basic hydrogen peroxide gives the expected red diimine complexes. A large amount of work has been done on iron(I1) complexes containing 2-aminomethylpyridine (a-picolylamine) (44a) and its derivatives. The tetracyano complex [Fe(CN),(44a)I2 is smoothly oxidized to the imine complex (44b)that is quite stable in mildly acid In alkaline solution further oxidation gives the corresponding oxime (44c) complex together with uncharacterized decomposition products. Reaction of (Ma) and sodium pyruvate with Fe;: does not give the expected complex containing (44d)because isomerization to the conjugated imine takes place that is followed by transamination with another molecule of (44a) to give the complex containing the tridentate ligand (44e)and alanine.^^^ The related sulfur ligand (440forms a low-spin bis chelate,585a whereas the tris complex with (44g) has a magnetic moment of 2.8 BM. The mixed donor ligand (44h) forms a low-spin bis complex, and high-spin complexes of the type [FeX, (44b)l that may be f i v e - c o ~ r d i n a t e 2,2’-Dipyridylmethane .~~~~ (44i) appears586to form only high-spin iron(I1) complexes but these are d the type [FeC12(44i),]that has pCf = 5.3 BM. 2-Aminomethylpyridine (Ma) itself forms [Fe(44a),I2+,which exists in different colours and spin forms. AS a methanol soivate, the high-spin chloride salt adopts the mer configuration whereas it is a fuc isomer as the dihydrate (due to hydrogen bonding? that is low-~pin.~*’~ The difference in Fe-N bond lengths in these complexes is large (-0.19 ). The alcohol solvates of the bromide and chloride salts display sharp spin transitions to low-spin forms in the region 13G110 K, but removal of solvent molecules causes the transitions to become very gradual.587b The iodide salt contains two isomers in the lattice: the mer isomer has Fe-N = 2.20 A and thefac form (with Fe-N = 2.06 A) exists as a mixture of interconverting high- and low-spin species at room temperature.587cAt room temperature the ethanol solvate of the chloride salt is yellow, but at liquid nitrogen temperature it is red. X-Ray structure work carried out at 298, 150 and 90K shows the average Fe-N bond bite distances in the high- and low-spin forms are 2.195 A and 2.013 A respectively. The N-Fe--N angle increases by about 6 ” on going from high- to low-spin, and there are small changes in N-C distance consistent with increased d-71 interaction.587dAs suggested by Mossbauer the disorder of the alcohol molecules also varies with the spin transition, and their ordering may act as a trigger for the spin transition.
1
Iron(i1) and Lower States
1227
Danish workers have synthesized several polydentate ligands derived from (Ma) that illustrate the sensitivity of the spin transition to small ligand modifications and indicate that the chelate effect may be as important as conjugation in the ligand. Thus, with (44j; n = 2 , 3 and R' = R2 = H) low-spin complexes of the type [Fe(NCS),(44j)] are obtained, but if R' or R2 is methyl, high-spin complexes , ~is~ the ~ ~ complex result.s88aThe bis complex of the tridentate ligand (44k) is red and l o w - ~ p i nas of the hexadentate ligand (441) when n = 3 and R = H, but high-spin for n = 2, R = Me, while complexes of closely related ligands display magnetic moments that are strongly temperature de~endent."~" Complexes containing the tetradentate ligand ( 4 4 4 similarly display a variety of spin behavio~r.~ With * ~ ~n = 1, R' = H, R2 = Me [Fe(NCS),(44m)] is high-spin whereas spin transition is observed when n = 1, R' = H,Me, R2 = H. T'riaza macrocycles ( a n ) with pendent pyridylmethyl groups making them hexadentate have been reported.58seWhen n = m = 2 the pseudooctahedral [Fe(44n)I2+is low-spin, but the complex with the somewhat expanded ligand having n = m = 3 the complex is high-spin.
R'
OR1
'
k - (CH*Iq-- N,
R2
RZ'
(44j)
hn(II,J and Lower States
1228
Ligands of the type (45a; X = NH, S, 0) form a bridge between simple cyclic diimines and the fully aromatic ligands containing two heteroatoms. The tris complex with X = S is low-spin and the kinetics of its ligand dissociation reactions involving acid, hydroxide, peroxydisulfate, and heavy metals such as Ag+, Hg2+ have been studied. Interestingly there is no catalysis by the heavy metals.5s9aThe mixed ligand complex [Fe(NCS),(45a),] (X = SI is high-spin at room temperature, but when the temperature is lowered it undergoes spin c r o ~ s - o v e r .The ~ * ~tris ~ complex with X = NH displays a spin transition at the relatively high temperature of 310K, and its Mossbauer spectrum at 293 K has a poorly resolved quadrupole-split doublet centred at 0.29 mm s-’ (relative to natural iron) attributable to the major low-spin component. At 77 K only the resonance due to this form is The rapid (stopped-flow) reaction of [Fe(45a),I2+ X = 0 (A,,, = 500, E = 1.6 x IO3) with OH- has been reported to involve attack by OH- on the coordinated Ligand followed by ligand decomposition and finally precipitation of Fe(OH),. The first stage is a preequilibrium for which thermodynamic data have been pre~ented.~”‘The related tris complex of 2,2’-bipyrazine (45b) behaves similarly, and reaction rates for this and related complexes have been compared with charge densities based on INDO molecular orbital calculations,589c In contrast the tris complex with (45c; X = NH) is h i g h - ~ pi n ”down ~ ~ to 115 K, and when X = 0 the complex remains high-spin over the complete temperature range inve~tigated.4~~‘ In acetonitrile \Fe(45c)J2+ reacts with oxygen to form water and an iron(II1) complex in which one of the ligands is deprotonated. The mechanism of this reaction is not fully resolved, but it is thought an intermediate containing coordinated oxygen may be involved.589g
-
-
Octahedral tris ligand complexes containing the pyridylimidazole ligand (45d) or related ligands (45e) and (450 display spin cross-over behaviour that in most instances is sensitive to both the nature of the anion and the number of molecules of water of crystallization. Several groups have e ~ a m i n e d ~the~ deep ” ~ ~red hydrated [Fe(45d),](C104), that is high-spin at around room temperature, but whose magnetic moment gradually decreases as the temperature is lowered until at about 100 K it rapidly falls to 1 BM. As in other spin cross-over systems this behaviour cannot be described adequately by a simple Boltzmann distribution with a fixed energy difference.5wb
-
Mossbauer spectra recorded59kdown to 4.2 K readily distinguish between the low-spin and highspin forms. [Fe(45e),](C104), exists in dark blue and dark purple forms.590bThe former is low-spin and the latter high-spin at room temperature (5.25 BM at 295 K). The magnetic moment of the high-spin complex decreases rapidly at around 120 K . The complex [Fe(45f),I2+ was first investigated for possible use in the spectrophotometric determination of iron, and although its extinction coefficient is not particularly large, it has the advantage of being extractable into isoamyl alcoh01,’~ This complex is expected to have a relatively stable high-spin form as a result of steric effects, and in fact with some of its salts it is not possible to characterize the low-spin ground state magnetially.'^^^^ Mossbauer spectra show the coexistence of high- and low-spin forms that have a mer configuration, indicating that there is a substantial difference between the coordinated pyridine and benzimidazole nitrogen atoms. Rather surprisingly the bis diimine complex [Fe(NCS),(45f),] is fully high-spin down to 4.2 K (ref. 590g). The dynamics of the spin equilibria in solution for [Fe(45d),I2+ and [Fe(451)J2+ have been investigated using laser Raman temperature-j~mp’~~~ and photochemical perturbation with a Q-switched laser. Recently volumes of activation have been determined for both forward and reverse processes. Both complexes show a marked solvent dependence for AV’ (high + low), but AV’ (low + high) is not very sensitive to the nature of the solvent. The former are negative, as are corresponding values of A S z , in agreement with there being substantial Fe -N bond contraction in the activation ~tep.~~”,’~OJ
Iron(II) and Lower States
1229
Just as bidentate ligands have been formed by attachment of an imine containing group, or other suitable donor, to the 2-position of a monodentate pyridine, so tridentate ligands have been obtained from 1,lo-phenanthroline. Thus, the 2-amidoxime (46a) has a fairly strong ligand field arid [Fe(46a)J2+ is l o w - ~ p i n , but ~ ~ lthe ~ spin cross-over point is approached in the bis ligand complexes of the hydrazones (46b). Magnetic properties of these complexes depend on the nature of R1and R2,and are most pronounced when R1= RZ= Ph. This complex has a sharp spin change near room Similar and this is associated with X-ray powder diffraction pattern steric effects operate in the bis complex of the 2-carbaldehyde imines (46c). With R = But the complex is high-spin and there is a gradual spin cross-over as the temperature is lowered, but with almost all other R groups the complex is low-spin at room temperat~re.5”~
Several 1,lO-phenanthrokne derivatives with a five-membered heterocyclic group in the 2-position have been prepared, and most of their bis iron(I1) complexes display temperature dependent behaviour. Salts of [Fe(47a)2]2+are high-spin and they undergo a spin change at low temperature, but this is so gradual it seems likely that solid-state effects are involved.s92”However, no changes in the powder X-ray diffraction pattern are evident. The bis ligand complex of the imidazoline derivative (47b) is predominantly low-spin at room temperature ( 1.8 BM) but the magnetic moment gradually increases as the temperature is raised.592bBoth salts of the benzimidazole analogue complex [Fe(47c),12+ and its neutral deprotonated form, are low-spin at room temperature, and their magnetic moments only increase slightly at higher temperatures. A number of complexes of related ligands with 2-thiazole groups have been prepared and these usually display temperature dependent magnetic moments. At first sight however, it is rather surprising that Fe(47d),I2’ is high-spin over the entire experimental temperature range (83-363 K). This behaviour is attributed to the steric effect of the uncoordinated pyridyl group.592c N
q Nf,
H
Most heterocyclic tridentate ligands related to terpyridine (Section 44.1 5 6 . 3 ) have a smaller ligand field, and so salts of the bright red [Fe(48a)]’+ are high-spin (- 5.5 BM) at room temperature. 100K the colour Their magnetic moments decrease only gradually to 3.3 BM at 80K. At changes to intense purple, and the reason for the high limiting magnetic moment might be due to only half of the molecules becoming l ~ w - s p i n . ~Introduction ’~ of a methyl group adjacent to one ~ ~ ~dibenof the pyridine nitrogen atoms results in the bis complex becoming fully h i g h - ~ p i n . ’The forms reddish purple [Fe(48b),I2+ that is high-spin at room temperature (4.49 BM zothiazole (ab) at 3 13K). The magnetic moment decreases as the temperature is lowered but some paramagnetism in alcohol the black remains at 83 K (2.23 BM). When a solution of FeBr, is added to one of (ab) crystalline salt [Fe(48b),][FeBr4]Bris ~btained,’~” in which it was deduced the magnetic moment of the iron(I1) cation only varies slightly with temperature. The tripyridyl-s-triazine ligand (49) is also tridentate and forms a very stable low-spin bis complex with iron(I1) for which log p2 = I 1.05 at 23 “C(ref. 594a). It proved possible to separate the stepwise
-
-
1230
IrorZ(i1) and Lower States
(48b)
(48a)
formation constants, and the value of K, (log K2 = 6.26) is significantly larger than Kl (log Kl = 4.81): in keeping with the mono complex (presumably [Fe(49)(H,0)J2+) being high-spin. The kinetics of formation and dissociation of [Fe(49),I2 have been studied'94band surprisingly it appears that addition of the first ligand to [Fe(H,0),]26 is rate determining, while as expected, formation of the mono complex is rate determining in the dissociation. As expected for dissociation of a basic for [Fe(terpy), J 2 + , the acid dissociation kinetics are cornplextridentate ligand, and as particularly so because of the uncoordinated basic nitrogen sites. Reaction of [Fe(49),]*+ with ~ ~is' . ~ ~ hydroxide ion is first order in both complex and hydroxide, and it has been s u g g e ~ t e d ~this due to a rapid preequilibrium with CN- or OH- which is followed by ligand dissociation. The derived activation parameters support this proposal. Electrochemical reduction of [Fe(49),I2+ affords neutral [Fe(49),], that has been suggested595contains anionic ligands. +
The unsymmetrical 1,2,4-triazine ring is present in several ligands recommended for use in the a water soluble reagent spectrophotometric determination of iron. Sulfonation of (50a) that is bidentate forming a tris diimine complex with b3 4.9 x lOI5, for which an extinction = 652 nm) has been reported.597 The course of its reaction with coefficient as high as 2.86 x lo4(A, hydroxide ion proceeds598"via an intermediate (formed by some ligand reaction) that in turn undergoes dissociation. Reaction with cyanide ion follows a simple pattern in water,598h but this is not the case in aqueous DMSO. Here spectral evidence suggests cyanide ion attacks one ligand, while parallel ligand displacements take place.598c The related sulfonated ligand (50b) is claimed599a to be the most sensitive chromogen for iron(I1) with E = 3.22 x lo4 (Lma = 565 nm). The presence of the sulfonic acid groups has little effect on the absorption maximum or extinction coefficient of the iron(I1) tris ligand complex.599b Reaction of [Fe(SOb),]"- with cyanide ion is complex, there being several intermediate species. The tridentate derivative of 1,IO-phenanthroline with a 2-( 1,2,4triazine) ring (51) forms a deep purple low-spin complex [Fe(51),I2' .600
-
SO; I
Iron(1I) and Lower States 44.1.5.7
1231
Dioxirnes
In the absence of a reducing agent and strong axial ligands, [Fe(DMG),] (DMG = dimethylglyoxime) is unstable. Hydrazine can fulfil both of these roles, and red [Fe(DMG),(N,H,),] 525 nm, E lo4)is an air stable low-spin complex.601 The structure of the analogous [Fe(DMG),(imid),] (52) has been determinedm2by X-ray diffraction; the two DMG ligands are planar and the six nitrogen atoms bound to the low-spin iron(I1) centre exhibit the expected tetragonal distortion from octahedral symmetry. Bis-methylimidazole complexes of iron(I1) dimethylglyoxime systems have been synthesized603with varying degrees of ring closure that extend to macrocycles and in such complexes one axial imidazole ligand may be substituted by either carbon monoxide or knzyl isocyanide. For the complexes [Fe(53)(Meimid)(CO)r+ the order of CO lability is: DMGH < DOHpn < DOHen < TIM < TAAB, I4aneN, < porphyrin, phthalocyanine .< ISaneN,.
-
DMGH: X = Y = O.HO DOHpn: X = (CH,), Y = O.HO DOHen: X = (CIH,), '1' = O.HO TIM : X = Y = (CH,),
hid
(53)
Increasing net positive charge on the iron results in increased lability of the 7~ acceptor carbon monoxide ligand. The cationic complexes of TIM, TAAB and 14aneN, show very similar CO dissociation rates despite large variations in ring size and ligand unsaturation. The larger rings achieve the environment needed to stabilize low-spin iron(I1) by puckering to avoid conformational strain. There is an underlying trend with ring size however as shown by the sharp increase in CO dissocation rate for the ISaneN, complex. The structure of the 1,Zcyclohexanedione dioxime (niox) complex, [Fe(niox), (imid),] (54), which is morc soluble than the analogous DMG complex, has also been determined,6@' but because of disorder in the cyclohexyl rings some of the important structural parameters were ill defined. However, the Mossbauer spectra of [Fe(niox)2LJ (L = py, imid, NH,, BuNH,) and related complexes have been studied in detail and are consistent with considerable tetragonal distortion.b0s,6"h [Fe(niox),(imid),] reacts with CN- to give [Fe(niox),(CN),]*-, and by using only one equivalent of CN- [Fe(niox),(CN)(irnid)]- can be obtained.605Reaction with CO affords [Fe(niox),(imid)(CO)] as in Scheme 13. imid
[Fe(niox),(imid),]
A
[Fe(niox),(CN-)(irnia)l
c?l-_
Scheme 13
1232
Iron(1I) and Lower States
Iron(I1) complexes of monooximes derived from pyrazole-4,5-dione have been reported.@"These are high-spin with the iron coordinated to oxygen donors. The preparation has been reporteda8 of oligomeric iron(I1) tetraoxime complexes having molecular weights between 2600 and 3 100. These oximes have aliphatic backbones which are very flexible and as a result the FeN, units appear to be independent of one another with protons a or jl to the oxime groups undergoing no shift upon complexation. Other related oxime complexes are discussed in a review.609However, of more interest are the encapsulated complexes obtained by capping oxime oxygen atoms of the, formally intermediate, tris oxime complexes with suitable groups. Thus reaction6'*of FeC1, with dimethylglyoxime dioxime (R' = Ph) and boric acid in refluxing alcohol (R' = Me) or 1,2-diphenylethane-172-dione (ROH,R = H, Me, Et, Pr', Bun) affords red low-spin complexes (55). No doubt these complexes, like their Co"' analogues, approach a trigonal prismatic configuration.61',612Russian workers613 obtained the corresponding 1,Z-cyclohexanedione dioxime complex capped with BF by using BF3 in acetonitrile in place of boric acid in alcohol. Alcoholysis of this complex affords the BOR derivatives. Experimental methods for the substitution of a wide variety of groups have subsequently been d e ~ e l o p e d . ~ ' ~
OR (55)
A correlation has been established between the iron(II)/iron(III) redox couple of such complexes and the substituent on the apical boron atoms; no corresponding trend in "B NMR shifts has been observed, however. The related dark red low-spin, non-octahedral complex (56) is obtained615by encapsulation of the preformed hexadentate ligand with BF, . X-Ray crystallography reveals616that the coordination geometry deviates by approximately 2 1.5 O from the ideal trigonal prismatic configuration in this complex which has a formally 'chiral' iron centre. The analogous complex [Fe(py3TPN)I2+is closer to an idealized octahedral geometry6I7having an 'angle of twist' of 17 ".An analysis6" of a number of crystal structures suggests that geometry may be rationalized on the basis that ligand rigidity favours trigonal prismatic coordination. Amongst the most 'octahedral' of such complexes is [Fe(py, tren)I2+which shows a high e low spin transition both in the solid state and in solution.618A variable temperature X-ray structure analysis shows that steric interactions between methyl groups and neighbouring pyridine rings is responsible for variations in magnetic behaviour as the number of methyl groups varies. Additionally, it was noted that the Fe-N bond distance contracted by approximately 0.12 8, in the solid state transition from high-spin (p& = 5.0 BM, 300 K) to intermediate-spin Geff= 2.3 BM, 205 K).
Iron(l1) and Lower States 44.1.5.8
44.1.5.8.1
1233
Phosphorus and Arsenic Donors
Momdentate ligands
A considerable number of stable organometallic iron(0) compounds containing phosphorus donors are known, but comparatively few with arsenic donors have been reported. In contrast many simple iron(I1) phosphine complexes tend to be unstable, and undergo dissociation in solution. It appears that phosphites (notably trimethyl phosphite) tend to form more stable simple iron(I1) complexes than do the corresponding phosphines. However, with polydentate phosphines more stable complexes result, and five-coordination is relatively common. Complexes in several spin states, including some displaying spin-equilibrium behaviour, are known. Diamagnetic octahedral complexes of the type [FeX, (PP),] are well characterized, and interesting bis carbonyl complexes containing monodentate or polydentate phosphorus donors are obtained upon reaction with carbon monoxide under surprisingly mild conditions. Few iron(I1) complexes containing arsenic donors have been well characterized. Colourless needles, assumed to be [FeCl, (PEt,),], have been isolated6lqfrom FeC1, and the phosphine in benzene. In ethanol, apparently, there is no reaction620which illustrates the relatively poor affinity of iron(I1) for trialkylphosphines. Refluxing anhydrous FeX, with a tertiary phenylphospine PR, (R, = Ph,, Ph,Et, PhEt,) in benzene affords6I9the high-spin complexes [FeX,(PR,),] for which magnetic moments over the range 4.8-5.3 BM have been reported; this and the electronic spectrum of [FeBr,(PPh,),] are consistent with a tetrahedral configuration.62'Iron(I1) halides react with the secondary phosphines HPR, (R2= MeFh, EtPh, Et2) in anhydrous ethanol to form complexes of the type [FeX(HPR,),]+ that have been isolated as their BF7 and PF; salts.622These red/violet complexes are paramagnetic (- 3 BM) with triplet ground states. Jn salution, some phosphine dissociation takes place to form tetrahedral complexes. With carbon monoxide at room temperature bis carbonyl complexes are formed as in equation (54); they are yellow and appear to have trans-carbonyl ligands in solution. 'Fe(PF6)Z' and Fe(BF4),.6H,O react with an excess of secondary phosphine (HPR,, R, as above) in 2-propanol to give yellow or yellow/brown [Fe(HPR,),]X, (X = PF,, BF,). These complexes are paramagnetic with peffin the range 1.32.5 BM, but the presence of paramagnetic impurities makes it difficult to establish the actual ground state. Reaction of [Fe(HPR,),](PF,), with carbon monoxide takes place in two stages:,,, addition of the first CO is rapid with subsequent phosphine substitution being relatively slow to give diamagnetic trans-[Fe(HPR,),(CO),] as in equation (55). lFeX(HPR2)4]+ 4- 2CO Fe(HPR2)51a+
00
+
[FeX(CO),(HPR,),]+
+
HPR,
(54)
+
[Fe(HPR2)5C012+ % trans-[Fe(HPR,),(C0)2]2+ HPR,
(55)
While the secondary phosphine PCy,H forms the colourless tetrahedral complex [FeCl,(PCy,H),] (5.12BM) with FeCl,, the complex [FeCl,(PEt,H),] is red with a magnetic moment of 3.61 BM, which is claimed to be consistent with a square planar structure.623With FeCl, or FeBr, and the primary phosphine PPhH,, the orange/red square planar complex [Fe(PPhH,),]X, can be f ~ r m e d , ~but " PCyH, is said to be anomalous in forming [FeBr,(PCyH,),] (3.48 BM), while with FeCl, square planar [FeCl,(PCyH,),] r e ~ U l t sThe . ~ ~phosphides ~ [Fe(PCy,),] and Li[Fe(PCy,),] are formed from the reaction626of FeBr, withLiPCy, . Carbon monoxide reacts with mixed alkyl/aryl phosphine iron(I1) complexes (but not those of PPh,) to give well-defined high-spin complexes of the type [FeCI,(PR,), (CO)1].6'9 The IR spectra of [FeX,(PEt,),(CO),] (X = NCS, C1, Br, I) have been recorded.627Carbonylation of a mixture of FeCl, and diphos in benzene affords628[FeCl, (diphos)(CO), 1, which on the basis of its IR spectrum has cis CO ligands. However, when THF is the solvent a different isomer results that has trans CO A wide ligands, and refluxing the cis isomer in acetone causes it to isomerize to the trans form.628 range of low-spin iron(I1) complexes of the type [FeX,(CO),P,] have been reported and charac.~~ available P(OMe), is a good ligand due to terized in which P = phosphine or p h o ~ p h i t e Readily the low steric requirements associated with strong r~ donor and n acceptor properties. In many respects, P(OMe), mimics the behaviour of isocyanides, and many low-spin iron(I1) complexes containing CO and P(OMe), have been reported.630Reaction of FeC1, with P(OMe), in benzene or methanol produces an uncharacterized, readily oxidized mixture which may contain [Fe{P(OMe),},]2f, [FeCl,{P(OMe),},] and other products. However, Fecil and P(OMe), with SnCI, react in stoichiometric amounts to give cis-[Fe(SnCl,)CI{P(OMe),}J,which reacts with NaBPh, in methanol to give [Fe(SnCl,){P(OMe),),j as a yellow precipitate.6mIn the presence of NaBH,, reaction of FeCl, with an excess of P(OMe), affords a high yield of sightly soluble off-white
1234
Iron(I1) and Lower States
material said to be [Fe{P(OMe),],], but complete analytical results supporting this formulation were not In contrast, sodium amalgam reduction of [FeCI, {P(OMe),},] in the presence of free P(OMe), affords208yellow crystals of {Fe{P(OMe), (see Section 44.1.3.3) that reacts with NH,PF, according to equation (56). On the NMR timescale the hydride so formed is kinetically inert with respect to exchange reactions, but reaction with CF, COzH gives hydrogen and ostensibly [Fe{P(OMe),},I2+ that was ~haracterized~~' by NMR. It also reacts," with Me1 to give [FeMe{P(OMe), )J+.
Is]
.-
[Fe{P(OMe),},] FeCl,
+
+
NaBH,(CN)
NH4PF,
+
+
[FeH{P(OMe),},]PF,
+
(56)
NH,
trans-[FeIBH,CN),{P(OR),}4]
P(OR),
(57)
Reaction of hydrated FeCl, with excess Na(BH,CN) and P(OR), (R = Me, Et) in methanol as in equation (57). However, a mixture of cis and gives63'low-spin trans-[Fe(BH3CN)I{P(OR),}4] trans isomers resulted when the iron was obtained by electrolytic dissolution in acetonitrile, and it appears both solvent effects and the nature of the phosphite are important in determining the isomer ratio. Diethylphenyl phosphonite (P(OEt),Ph) reacts632with FeC1, in ethanol to give wellcharacterized, light yellow, diamagnetic cis-[FeH, {P(OEt),Ph},] as in equation (58). The product has Fe-H distances of 1.51 A, with a geometry intermediate between octahedral and tetrahedral. It is indefinitely stable under argon or nitrogen, but the crystals turn brown after a few days exposure to air. The related, though less well-characterized, trimethyl phosphite dihydride complex [FeH, {P(OMe),},], as well as a tetrahydride, have also been described.210 FeCI, 44.1 -5.8.2
+ P(OEt)lPh
BlI+LOH
cix-[FeH, {P(OEt):Ph),]
(58)
Polydentate ligands
Many iron complexes having interesting structures and reactions are formed with polydentate arsenic and phosphorus ligands.633However, in the present context the coordination chemistry of the arsines is considerably less extensive than that of the phosphines, and there are many examples when it is not possible to prepare the corresponding arsine analogue of a well-characterized phosphine complex. The most well-known ditertiary arsine is o-phenylenebis(dimethylarsine), (diars) and the complexes [FeX,(CO),(diars)] (X = Rr, I) and [FeT2(diars)]are readily obtained by Nitric acid oxidatio# of [FeCl,diars] affords halogen oxidation of appropriate iron(0) the iron(1V) complex [FeCl, (diars)]'+ . Iron powder reacts with o-phenylenebis(diethy1phosphine) (C,H,(PEt,),) under an atmosphere of hydrogen at 200 "C to give434small amounts of the dihydride druns-[FeHCl(depe),l (depe = 1,2trans-[FeH,{ C,H4 (PEt,), }]. The monohydride bis(diethy1phosphino)ethane) reacts with NaBPh, in acetone under nitrogen to give the relatively stable cationic dinitrogen complex trans-[FeH(N,)(depe),]' , as in equation (59), which was isolated as the BPh; ~alt.6~' The same reaction carried out in the presence of a wide range of ligands (L), including phosphites, affords639the complexes trans-[FeHL(depe),]- . Mossbauer spectra obtained at 4.2 K in a high applied magnetic field permit discussion of the bonding in these complexes. It is concludedw that the instability of N, complexes is associated mostly with the weak cr donor properties of N2. Even though the related complex trans-[Fe(q2-H,)(H)(diphos),l+is stable to loss of hydrogen up to S O T , it must be stored under hydrogen or argon since it slowly reacts@' with molecular nitrogen to give trans-[FeH(N,)(diphos),]+ . The reactant v2-H, complex undergoes intramolecular exchange of terminal hydride with the hydrogens of the q2-H2ligand,Mzfor which the activation parameters are A H # = 58.2 & 2.9kJmol-' and AS' = -4.2 f 12.6 J K ' m o l - ' . The X-ray structure of trans-[Fe(y2-H,)(H)(diphos),]BF, has been determined.", Reaction of this cornplex with good donor ligands gives complexes of the type trans-[FeH(L)(diphos),]' , like those of depe previously discussed. The bidentate phosphines R,PCH,CH2PRz (R = Me, Et) readily form Octahedral low-spin trans dichloro complexes, trans-[FeCl, (R,PCH,CH,PR,),]. They have been the subject of high precision X-ray structure determinationsM3that show Fe-P distances (- 2.2 A) are dramatically shorter than in related high-spin complexes. For example, the polytertiary phosphine (Ph2PCH,CH,)zPPh, acts as a bidentate ligand in the distorted octahedral [FeCl,{(Ph,PCH,CH,),PPh,),] that is high-spin with Fe-P distances@', in the range 2.66-2.71 A. The tripod ligand (57) reacts with iron(I1) salts in the presence of NaBH, to give the structuralIy novel diamagnetic binuclear iron(I1) complex (58), which contains two octahedra sharing a face trans-[FeHCl(depe)J
+
NaBPh,",
+ truns-[FeH(N,)(depe)JBPh4
+ NaCl
(59)
u i r o n ( l i ) and Lower States
1235
having three hydride ligands at the edges,M4Thus, the iron centres are bridged by three hydrides. As expected, treatment with hot acid gives hydrogen and liberation of the tripod ligand. No complex was isolated when the corresponding arsenic ligand was used. Similarly the arsenic analogue of the tetrateritary phosphine (59) does not appearM5to complex with iron(I1). Reaction of (59) itself with anhydrous FeC1, in ethanol gives an intense purple solution containing five-coordinate [FeC1(59)]+, and similar complexes are formed with other halides that have magnetic moments of 3.1 BM. With NCS- and CN- , however, six-coordinate diamagnetic complexes of the type [FeX, (59)] are obtained .M5
-
(57)
(58)
(59)
In contrast, the more flexible tripod ligand P(CH,CH,CH,PMe,), reacts rapidly with anhydrous iron(I1) halides to give deeply coloured solutions which afford violet crystals of [FeX,P(CH, CH,CH,PMe,),] that are diamagnetic and non-electrolytes.M6X-Ray structure determination confirms these complexes have cis halides with a slightly distorted octahedral geometry, rather than the five-coordinate trigonal bipyramidal structuresM6of the cationic complexes formed with (59) and P(CH,CH,PPh,),. The reason for the change in geometry from five- to sixcoordination for the halide complexes on going from the more rigid ligands to P(CH,CH,CH,PMe,), is no doubt associated with the flexibility of the C, chains that provides a larger chelate bite angle {(59) has only 84 & 2“) and enhanced Fe-P overIap. Like ligand (59), P(CH, CH2PPh2)3 formsM7 a diamagnetic six-coordinate dithiocyanate complex [Fe(NCS),P(CH,CH,PPh,),]. In contrast to the triplet ground state of the five-coordinate halide complexes of the tripod ligand (59), the corresponding complexes of the linear tetradentate ligand (60),[FeX(60)]+ (X = C1, Br, I; isolated as their BPh; salts) have room temperature magnetic moments between those expected for S = 0 and S = 1. Spin equilibria between a singlet groundstate and low-lying triplet state could be responsible for the non-Curie-Weiss temperature beha~iour.~’ An X-ray structure determination of the five-coordinate bromo complex shows it to have distorted trigonal bipyramidal geometry.618As with ligand (59), the dithiocyanato complex . ~ ’ mixed donor ligand 2,6-bis(2[Fe(NCS),{60)] is diamagnetic and presumably ~ ~ t a h e d r a l The diphenylphosphinoethy1)pyridine(61) forms a series of 1 : 1 five-coordinate complexes with iron(I1) halides.M9The yellow chloride and bromide complexes [Fe(Gl)XY], (X = Y = C1, Br) are high-spin (- 5.35 BM) and obey the Curie-Weiss law over the entire temperature range studied (93-293 K). However, the corresponding brown diiodo complex (X = Y = I) and the red mixed iodothiocyanate (X = I, Y = NCS) show anomalous behaviour with their magnetic moments abruptly falling at low temperatures, but even at the lowest temperature employed (93 K) neither complex completely reached the low-spin
44.1.5.9
Oxygen Donors
With simple oxygen donor ligands, iron(I1) complexes are high-spin and usually easily oxidized by air to iron(II1) species. The number of complexes studied containing oxygen donors is much less than those having nitrogen donors. However, some mixed donor Complexes involving oxygen are particularly important, and the most notable of these are the iron(I1) nitrogen macrocycles which bind dioxygen. These are considered in the section concerned with porphyrins (44.15 1 2 . 5 ) . C.O.C. &-NN
1236 44.1.5.9.1
Iron (II) and Lower States
Water
Dissolution of iron in dilute mineral acid affords [Fe(H,O),]'+. Although thermodynamically unstable towards air oxidation, the process is slow unless a strong oxidizing agent is used. In alkaline solution however, iron(I1) is strongly reducing and is readily oxidized by atmospheric oxygen, and there is evidence that it will even reduce N, to ammonia.6s0Iron(I1) oxide species are also active in the photoassisted reaction of water with nitrogen to give ammonia.6s'White Fe(OH), is much more soluble that its Fe"' counterpart and in the presence of air darkens rapidly through oxidation. Solutions of the [Fe(H,0),]2+ are only slightly acidic (equation SO), so that iron(I1) carbonate precipitates on addition of Na,CU, and no carbon dioxide is evolved. Aqueous solutions of [Fe(H,O),I2+ are pale blue-green and this octahedral cation is present in many hydrated iron(I1) salts such as the sulfate and the perchlorate. In the double salt (NH,),SO4~FeSO,*6H,O(Mohr's salt or Tutton's salt), which is rather more resistant to oxidation than the simple sulfate, the octahedron has principally a tetragonal distortion whilst in FeSiF, 6 H 2 0the hexaaqua complex has a trigonal distortion.652In natural waters, iron is mostly present as insoluble iron(II1) hydroxide. However, decomposition of inorganic matter, particularly in subsurface waters, can reduce iron(II1) to iron(I1) (lowering pH), whose hydroxide is relatively soluble, and this phenomenon can cause difficulties in water purification. The common method for removing iron from domestic water supplies involves aeration, which increases the pH by removing CO, and provides enough oxygen for the oxidation of iron(I1) to iron(I1I) as shown in equation (61). The kinetics of this oxidation have been studied by several groups and a number of different rate equations p r ~ p o s e d The .~~~~~ reaction is catalyzed by copper(I1) and it is particularly rapid in the presence of aquapalladium(I1) ions.33The oxidation of [Fe(H, O);+] by molecular oxygen in perchloric acid solution yields a green intermediate complex having an absorption band at 337nm. This green complex consists of one iron(I1) and three iron(II1) octahedra linked by oxo bridges.662
-
+ HzO [Fe(H,O),(OH)]+ + H 3 0 + K1 = 2Fezf + 5H20+ +02+ Fe(OH),,,, + 4H'
[Fe(H20)612+
44.1.5.9.2
(601 (61)
Aeetylacetonate
In the presence of piperidine, trans-peC1, (H, O),] reacts with acetylacetone in degassed water under nitrogen, to formM3hydrated bis(2,4-pentanedionato)iron(II). T h s classic neutral coordination complex is, however, coordinatively unsaturated and has an unusual tetrameric structure. X-Ray crystallography664reveals that the structure (62) consists of a pair of asymmetric Fe, units linked by rather long Fe-C bonds (2.785 A). The acac- ligand which contains the bonding cacbon atom also has an oxygen atom which bridges two iron atoms within the asymmetric unit. It is therefore somewhat unusual in having four coordination bonds to three separate metal atoms. The zero field Mossbauer spectrum of the tetramer consists665of two averlapped quadrupole doublets representing the two metal environments. The hyperfine splitting observed between 16.0 and 1.67 K, low temperature susceptibility666data (approaching the spin only value at 4.2K), and the tetrameric structure indicate the unusual phenomenon of slow paramagnetic relaxation.
(62) [Fe4acac81
Magnetic data for other iron(I1) P-diketonates, e.g. 1-(3-pyridyl)-l,3-butanedione and 44trifluoro-l-(3-pyridyl)-1,3-butanedione, indicate667the complexes to be polymeric. These com-
Iron(1I) and Lower States
1237
pounds are all highly air-sensitive, although the hexafluoroacetylacetone complex [Fe(F,acac),(H,o),] is somewhat more stable.668[Fe(acac),] itself reacts with a variety of bases to form hexacoordinate a d d u c t ~ . ~Where ~ ~ , ~the ~ ' base is a heterocyclic diamine such as 4,4'-bipyridine, some polymeric products are observed and the 1 :1 adducts are highly stable.671The polymeric species can be oxidized with iodine to give672a mixture of iron(I1) and iron(II1) centres and the ratio of the oxidation states is characterizable by Mossbauer spectroscopy. The choice of diamine, and in particular its degree of electron delocalization, has a marked influence on the conductivity of the polymeric materials. This allows a strategy for the accurate control of the conductivity of a low dimensional coating material. 44.1.5.9.3
Pyridilre N-oxide
dark red crystals of [Fe(H,O),](ClO,), reacts with pyridine N-oxide in methanol to [Fe(pyO),](ClO,), ,whose IR spectrum has been analyzed in some detai1.674*675 Analogous complexes are formed with a variety676of substituted pyridine N-oxides, for example 2-, 3- and 4-cyanopyridine N-oxides form677octahedral hexakis complexes. A number of complexes containing bidentate oxygen donors have been reported; the oxime of pyrazoledione, for example, forms678a bis iron(I1) complex. Thus, iron(I1) ammonium sulfate reacts with (63) to form the turquoise complex [Fe(63), .H,O], whose magnetic moment decreases with temperature; it has a large negative Weiss constant and is almost certainly polymeric. The complex reacts with pyridine to form [Fe(63),(py),], whose magnetic behaviour is the opposite of the starting material suggesting it is a monomeric pseudooctahedral c0rnplex.6~~ Iron(I1) oxime complexes with the basic coordination illustrated in structure (64) occur in the green biological pigment ferroverdin.679The reaction between [Fe(H,0),I3+ and catechol provides a series of complexes whose iron(II)/iron(III) redox reactions have been investigated680by Mossbauer spectroscopy as a model system for the microbial transport of iron by enterobactins. In contrast, hydrated iron(I1) perchlorate reacts with a number of phosphate esters to yield stable high-spin five-coordinate complexes. Thus the perchlorate hydrate, dehydrated by treatment with triethyl orthoformate, reacts with dimethyl-methylphosphonate (DMMP) to form an oil which when extracted with ether yields buff [Fe(DMMP),](ClO,), (pe8= 5.14 BM).68' In an analagous preparation trimethyl phosphate (TMP) yields first the monohydrate [Fe(TMP),H,Oj(ClO,),, which is readily dehydrated682to the five-coordinate complex, which is stable in air (kr= 5.12BM). In anhydrous acetone the iron(I1) salt also reacts with trimethylphosphine oxide or trimethylarsine oxide to yield683,684 high-spin five-coordinate complexes [FeL4(C10,)](C10,) (L = Me,AsO, pen = 4.94BM). In the case of the more sterically bulky ligand diphenylmethylarsine oxide, a compound with the stoichiometry FeL,(C10,), was formed, but the IR spectrum685did not show the presence of coordinated perchlorate ion. Indeed, the electronic spectrum and magnetic data point to an essentially tetrahedral structure.bB6A four-coordinate tetrahedral structure is also observed6s7for the complex [Fe(HMPA),](ClO,), (HMPA = hexamethylphosphoramide) (peR= 5.19 BM). The neutral four-coordinate complex [Fe(TPO),Cl,] is formed when Feci, is refluxed in an acetone solution of triphenylphosphine oxide under nitrogen.688
0,
44.1 S.9.4
,N=O Fe
Miscellaneous oxygen donors
When an iron(I1) salt is added to an aqueous solution of iron(II1) citrate, the greenish yellow colour intensifies and a mixed valence complex containing one iron(I1) and three iron(II1) centres together with one citrate ion has been de~cribed.6~~ In aqueous solution iron(I1) reacts with oxalic acid to form a precipitate having a stoichiometry of FeC,O,- 2H,O, for which two crystalline habits have been identified.6wIn methanol, a monomethanolate complex with stoichiometry [FeC,O,(MeOH)H,O) is formed, the hexacoordinate iron(I1) has a symmetric tetragonal distortion in the
I
Jron(II) and Lower States
1238
hydrate but the position of the iron atom is not symmetric in the latter complex.69’The perchlorate salt of hexaaqua iron(I1) reacts692with DMSO in acetone to give the pale green complex [Fe(DMSO),](ClO,), , which explodes on heating. Iron(I1) halides react with a variety of ethers (THF, 1,Zdimethoxyethane, 1,Cdioxane) to yield adducts which have been studied by electronic and IR spectroscopy, and magnetic meth0ds.6~~ Decomposition of FeS04.6H,0 on heating involves stepwise loss of water, and when heated in air, anhydrous FeSO, gives Fe(OH)SO, that decomposes to give Fe,O(SO,),.
44.1.5.9.5
Mixed donor ligands containing oxygen
This class of complex comprises mostly mixed 0 and N donors. Bidentate donors such as the Schiff base derived695from salicylaldehyde and aniline (65) form unusually stable complexes with iron(I1j. In the case of 1,lO-phenanthroline N - ~ x i d e ~however, ’~ the iron(I1) complex [Fe(phenNO),](ClO,), is high-spin bCE = 5.47 BM) in contrast to [Fe(phen),12+(Section 44.1.5.6). Amino acids are N,O donors and form a series of iron(I1) complexes.697They are high-spin, hexacoordmate and are thought to have extended carboxylate-bridged structures. Interestingly, the sulfur-containing amino acid methionine is also a bidentate N,O donor to iron(I1). Here again, a carboxylate-bridged polymer structure has been proposed.698A low-spin iron(I1) complex is formed with the violurate ion (66)and X-ray structural analysis699shows [FeV,](NH,) to havefac geometry similar to ferroverdin (67), a tris-nitrosophenyl iron(I1) complex.7MFerroverdin was the first naturally occurring nitrosophenyl complex observed. Early reports on the extremely air sensitive Schiff base complex of iron(T1) (65 and 68) have in some cases been shown7”’to have involved oxidation products. Their Mossbauer and electronic spectra show (for closely related systems) a linear relation between isomer shift and quadrupole splitting. The temperature dependence of the splitting implies7” a large axial distortion in these six-coordinate complexes. Temperature dependence has also been observed7” for at least one quinone adduct of [Fe(salen)], [Fe(salen),(C,H,O,)] (salen = 68, R = (CH),). However, the Curie-Weiss behaviour and magnetic susceptibility of this complex (peR= 5.76 BM, 297 K) suggest that it is in fact d 5 high-spin iron(II1). Of greater interest however, is the nitrosyl adduct of [Fe(salen)]. This complex has been observed703to have a relatively rare S = 3 to S = spin cross-over near 175 K. An X-ray crystal structure704has established a square pyramidal geometry relatively undistorted towards trigonal bipyramidal, unlike the macrocyclic complex [Fe(Me, 14aneN,)NO].705The latter complex has an essentially linear NO group, whilst in the salen complex it is bent. These features persist through the spin transition.706There are a series of ligands known as ‘strati-Schiff bases’ which form complexes of i r ~ n ( l I ) . ~These ’ ~ ligands are being explored as potential models for enzyme systems in which metal coordination sites are ‘stacked‘. Tridentate mixed 0 , N donors include (69) and (70)708and the sulfur-containing system TAN. A crystal structure709of [Fe(TAN),] (71) shows the two ligands to be nearly planar with the two planes essentially perpendicular to one another and the ligands in mer coordination. The tetradentate O,N, ligand H,acacen forms a dimeric high-spin iron(I1) complex [Fe(acacen),] (per= 4.84 BM, 294 K). An X-ray structure has confirmed7” that the pentacoordination of iron(I1) is maintained on reaction with EtONa when a tetranuclear Fe,Na, complex is formed in which the iron atoms are doubly bridged by EtONa (72). A five-coordinate complex is also obtained with the terdentate ligand7” Me,daeo, [Fe(Me,daeo)Cl,] (73), peE= 5.26 BM, 295 K, and the quinquedentate ligand’’, bis(sa1icylidinimine-3-propy1)amine(SALRDPT, 74). This and related complexes have been discussed as model systems for oxygen transport and a ~ t i v a t i o n . ~ ’ ~ The pentadentate N 3 0 z ligand 2,6-diacetylpyridenebissemicarbazone forms a seven-coordinate iron(I1) complex having axial chloride and water ligands. The ligand is formed in situ as an alcohol solution of diacetylpyridine and FeC1, is treated with semicarbazide hydrochloride. X-Ray structural analy~is”~ shows the ligand to be planar. The unusual planar hexadentate N 4 0 2ligand (75) forms71‘ binuclear iron(I1) complexes [Fe, (75)L,]’+ (L = py, im, Meim). These high-spin complexes (L = Meim, peR= 4.71 BM, 298 K) show a marked antiferromagnetic exchange interaction. Analysis shows that there is little difference in the magnitude of this interaction between the six-coordinate and comparable five-coordinate complexes.
+
44.1.5.10
Sulfur Donors
Iron-sulfur chemistry has developed over recent years to become a very rich area. A feature of many complexes is their redox behaviour, with more than one oxidation state being important. This
Zron(iI) and Lower Slates
Q PN
.?>-
A*
1239
Na
0
so
9
HO
R' R
N
'
NYR"
section is mainly concerned with iron(I1) complexes but inevitably some iron(II1) complexes are discussed (notably in mixed oxidation state clusters). Similarly, some iron(I1) sulfur donor complexes are included in Section 44.2.5, which deals with higher oxidation states, and accordingly readers should consult both sections. Those sulfur complexes containing nitric oxide are contained in Section 44.1.2.5. With the recent availability of 1,4,7-trithiacyclononane (planes,) via an elegant template ~ynthesis"~ there has been inerest in comparing its complexing ability with that of the surprisingly high-field ligand [9]aneN, that forms an octahedral low-spin iron(1I) complex.717Purple [Fe([9]aneS3),]'* is also low-spin and its stability is such that it is obtained when Fe" and Fe"' salts are reacted with the free ligand. Its electronic spectrum exhibits two d-d transitions typical of Fe" in an octahedral ligand field, that are not obscured by intense charge-transfer bands. X-Ray structure determination718confirmed an octahedral geometry shown in (76).
Iron (TI) ond Lower Stutes
1240 44.1 S.10.1 Binary su$des
Iron(I1) sulfide is ordinarily dark grey to brown, but when very pure it has been reported to form almost colourless It is quite insoluble in water, and its reaction with oxygen is exothermic to give Fez0; or Fe,O, depending on conditions. Rarely is the iron to sulfur ratio unity and formulae ranging from Fe, S, to Fe, I S,, have been assigned to these iron deficient non-stoichiometric phases. There are a number of iron(I1) sulfide minerals which contain an approximately octahedral environment of sulfur atoms. Troilite, stoichiometric FeS, is hexagonal with the NiAs superstructure’” having Fe--S bond lengths7*’ranging from 2.38 to 2.72 A with an average of 2.5 A. Pyrrhotite Fe,-,S exists in hexagonal and monoclinic forms, with the Fe-S bond lengths7” in the latter ranging from 2.37 A to 2.67 A.The cubic disulfide pyritc FeS2contains a regular FeS, octahedron marcasite, a tetragonal polymorph of FeS, has an octahedral with Fe-S bond lengths of 2.26 geometry distorted by elongation along the c axis. In both pyrite and marcasite the anions are disulfide (Si-) and not sulfide (S2-). Troilite (FeS) and pyrrhotite (Fe,_,S) are high-spin, but pyrite Application of high (FeS,) is low-spin and each have been studied by Mossbauer spectr~scopy.”~ pressure converts the high-spin forms to low-spin, and the transition is reversed when the applied pressure is r e m ~ v e d . ~ ~ ~ . ’ * ~ Molecular orbital/energy band calculations have been performed for a range of iron(f1) sulfides7?7728 and there is interest in the band-like valence-electron states in FeS,729and also in their bulk and surface properties typified by studies using Auger and X-ray ab~orption.’~’-’~~ The extensive work in this area published during the period 196G1969 is contained in an excellent review by Ward.734Unfortunately, discussion about the fascinating tricyclic iron(II1) are outside polysulfide anion [Fe,S,,]’ - 7 3 5 and its reactions with, for example, activated of the scope of [his section.
-
44.1.5.10.2
Iron surfur clusters
Tetrahedral iron sulfide species that can have two or more charge states are involved in many electron transfer proteins.737The core structures that have been identified in proteins include the single iron centres of rubredoxin, and the two iron/two sulfide and four ironjfour sulfide centres of various ferredoxins. A wide range of model complexes have been prepared and characterized, and the results of this work, as well as that on the natural system, have been extensively re~iewed.’”~’~ Trimethylarsine sulfide forms74’ two types of tetrahedral iron(I1) complexes [Fe(SAsMe,),](ClO,), and [FeX,(SAsMe, (X = Cl, Br), but one of the first structurally wellcharactcrized tilrahedral iron([[) sulfur coinplcxes was [Fe(SPMe,NPMe, s),] (77) that has a magnetic moment of 5.25 BM in thc solid stak and 4.99 BM in dichloromethane solution.746The bond angles from measured’4’ Fe--S bond lengths r a n g from 2.339 to 2.380 A and the S-Fe-S 100.5 ‘to 1 14.9 Simpler synthetic analogues of the single iron centre of bacterial rubredoxins have been prepared using PhS-, i.e. [Fe(SPh),]’-, but the best characterized species involve the oxylyldithioiate ligand (78). Discrete tetrahedral complexes of both iron(I1) and iron(II1) have been isolated.748Thus reaction of [FeClJ- with excess o-xyl(SH), affords orange [Fe(S,-o-xyl),]’whereas use of a 2: 1 ligand to metal ratio gives the dimer [Fe,(S,-o-xyl)J-, which on treatment with excess thiol is converted to the monomer. Controlled aerial oxidation of [Fe(S-o-xyl),]*- affords the iron(I1I) complex [Fe(S,-o-xyl),]- , which is readily isolated as the Et4Nf salt. The structures of both complexes have been determined by X-ray The mean Fe”-S bond length is 2.356& which is some 0.098, longer than that of the Fe”’ monomer. @.
~
\
Iron(I1) and Lower Sates
1241
The oxidation state of the iron centres within a sulfide cluster can be the same or formally inequivalent. However, in all oxidation states antiferromagnetic spin coupling can take place directly or via bridging sulfur atoms. The [Fe,S, j core is present in spinach ferredoxin, and a number of model complexes of the type [Fe,S,(SR),]*- containing iron(II1) have been prepared and fully characterized. In particular salts of [Fe, S2(S,-~-xyl),]~-and [Fe, S,(S-p-t01),]~- (prepared from FeCl,, NaSH and ArSH in the presence of NaOMe) have been the subject of high resolution X-ray structure determination^.'^^,^^^ In this oxidized form the iron(II1) centres are in an approximately tetrahedral environment of sulfur ligands, but at low temperatures the complexes are diamagnetic and display no ESR signals due to antiferromagnetic coupling. At room temperature, population of the S = 1 level can give rise to some paramagnetism. One-electron reduction of fFe2S2(SR),J2produces inequivalent iron centres on the Mossbauer timescale. The isomer shifts correspond to the presence of Fe" and Fe"' centres that are both high-spin and antiferromagnetically coupled. Work on the corresponding reduced natural spinach ferredoxin continue^,^" and its chemistry parallels that of these model complexes. In contrast however, under conditions used to prepare [Fe,S,(S oxy1),I2-, 1,2-ethanedithiol forms a black iron(II1) dimer, [Fe,(edt),12- , that contains no s ~ l f i d ~ ~ ~ ~ ~ Moreover, the iron environment is not tetrahedral, but consists of five sulfur atoms with a distorted trigonal bipyramidal geometry.753Although iron sulfur complexes are usually kinetically labile with respect to exchange, [Fe, (edt),I2- fails either to undergo thiolate ligand substitution reactions or to incorporate sulfide ion. Unlike the sulfide-containing dimers that undergo two one-electron reductions, [Fe,(edt),]*- exhibits a single two-electron reduction.752This difference may reflect the relatively long Fe-Fe distance in [Fe,(edt),]*-. A considerable number of ferredoxins are known that contain one or more [Fe,S,] clusters. At low temperatures these are diagmagnetic in the oxidized form but weak ESR signals can often be detected and the reduced forms display strong ESR signals. The structural features of the cubanelike [Fe4S4]core have been well defined in several proteins; and a series of thermodynamically stable model complexes of the type [Fe, S4(SR),I2- have been prepared and thoroughly investigated.7399740 In the natural systems SR is a cysteine ligand. Three overall charge levels are accessible as indicated in equation (62). Fe4S4(SR),l3'3Fe"lFe""
e [Fe4S4(SR).J2- e
[Fe,S,(SR),]-
(62)
'1 Fe" 3Fe"' '
'2Fe"2Fe1"'
It appears that no single protein in aqueous media will undergo reversible redox reactions between all three oxidation levels. The '3Fe" lFe""/'2Fe" 2Fe"" couple is found in many bacterial proteins, while the '2Fe"2Fe""/'lFe" 3Fe"" couple is only found in the so-called high-potential iron protein from photosynthetic bacteria. In marked contrast to the dimeric 'Fe" Fe"" complexes that display two pairs of doublets in their Mossbauer spectra that are typical of isolated iron(1I) and iron(II1) centres in tetrahedral environments, the Mossbauer spectra of [Fe,S,(SR),]'- complexes show them to be largely valence delocalized.754Only limited spin delocalization, which is more readily detected, is present in the paramagnetic complexes [Fe4S,(SR),I3- and [Fe,S,(SR),]- . Simple molecular orbital descriptions of these complexes have been presented.755Preparatively they are easily obtained from FeCl,, NaSR, NaSH and MeONa in methanol. Usually the reaction is carried out in two FeCl, is reacted with NaSR to give an insoluble (if R = aryl) polymeric dark green sulfide, which is then treated with NaHS/MeONa to give the corresponding disulfide and the desired mixed oxidation state cluster Na2[Fe4S4(SR),].The overall reaction is illustrated in equation (63). A wide variety of thiols have been used in these reactions including p-mercaptopropionic acid (R = CH2CH2C0,H), which affords757the water soluble [Fe,S, {S(CH2)2COz}4]6-, which has a half-wave potential75s(0.58 V vs. SCE) similar to that of natural ferredoxin. 4FeC1,
+
6NaSR
+ 4NaSH + 4NaOMe -,
Na2[Fe4S4(SR),] + RSSR
+ 4MeOH +
I2NaCI (63)
Thiolate exchange reactions of the dimeric iron(I1I) and the iron(II)/iron(III) cubane tetramers, [Fe$,(SR),]'and [Fe4S4(SR),]2-, have been s t ~ d i e d in~ some ~ , ~ detail ~ ~ and provide routes to several related complexes. Exchange takes place with other thiols (RSH) as in equation (64). [Fe,S,(SR),]*-
+ mRSH
--+ [Fe,S,(SR),_,(SR),]*-
+
mRSH
n = 2, 4;m = 0 4
(64)
1242
Iron(l1) and Lower Stutes
Phenols also react to give the corresponding complexes, e.g. [F~,S,(OP~I),]~-, salts of which have been isolated.760With benzoyl chlorides the fully substituted clusters [Fe,S,Cl,]'- (n = 2, 4) are obtained, which have been structurally The tetramer [Fe4S4C1,]2- and related iron(I1)-containing cubane complexes are discussed in more detail later in this section. Since the initial work on [Fe,S,(SR),]'- clusters, spectrophotometric and NMR studies have provided further insight into routes to their formation, and highlighted the importance of solvent as in equation effects in these reactions. The route from FeClJRS- and HS- may be ~urnrnarized'~~ (63). With methanol or acetonitrile as solvent, in the absence of HS-, and with the ratio PhS-:Fe = 3 5 1 reaction (65) takes place to give the insoluble adamantane-like7" iron(II) cluster' [Fe4(SPh),,12- (79) in high yield. This is oxidized by elemental sulfur in a surprisingly smooth reaction, in view of the degree of reorganization involved, to give the '2Fe"2Fe"1' cubane complex [Fe,S,(SPh),]'- according to equation (66). 4FeC1,
+
-
14PhS-
[Fe,(SPh)lo]z-
+ +S8
[Fe,(SPh),,]'-
f
2PhSSPh
[Fe4S,(SPh),I2-
--t
+
+
12C1-
(65)
PhSSPh
(66)
1 2-
SPh
I
'SPh
However, in acetonitrile with the ratio PhS-: Fe < 5: 1, the mononuclear tetrahedral iron complex [Fe(SPh),12- is formed first, which reacts with sulfur to form the iron(II1) dimer [Fe,S,(SPh),]Zaccording to equations (67) and (68). No further reaction takes place, but addition of methanol facilitates the reductive elimination of PhSSPh from the dimer to give the '2Fe" 2Fe"" cubane complex as in equation (69). Although this series of reactions has been written for PhS' it appears similar reactions take place with alkyl as well as aryl thiolates. 2FeC1,
+
2[Fe(SPh)4]z-
10PhS-
+
$3,
+ PhSSPh $- 6CI[Fe2S2(SPh),12- + PhSSPh + 2 P h S [Fe,S,(SPh),]*- + PhSSPh + 2PhS-
-
4
2[FeZSz(SPh),l2-
2[Fe(SPh),12-
(67) (68)
(69)
The convenient route to [Fe,S4(SR),]2-y using FeCl,, RS- and sulfur, depends on thiolate reducing half of the iron(II1) to iron(II), and all of the sulfur to sulfide. As a result more thiolate is required than with procedures starting with sulfide ion. This requirement can be reduced by using an iron(1I) salt765rather than iron(1II). The overall reaction is then as in equation (70). 4FeC1,
+
4s
+
10RS-
+
[Fe,S4(SR)J2- -t SCI-
RS\ S-Fe
R
sS-Mo-S-Mo-S / \s/
\ Fe-S
Ri
n
n
+
3RSSR
/
SR
Fe-S
\% / S-Ft \
SR
(70)
i3-
Iron(ll) and Lower States
n
‘SR
1243
RS
A further oxidative addition reaction of [Fe(SPh>,l2- is that with organic trisulfides to give [Fe,S,(SPh),]*- in high yield.766[Fe(SPh),]’- also reacts with additional FeC1, or FeBr, (1 :2.4molar ratio) to give the paramagnetic clusters [Fe,(SPh)6X,]2- (80), which have been isolated as their PPh: salts. The X-ray structure of [Fe,(SPh),Cl,](PPh,), a nearly regular tetrahedron of iron atoms within a slightly irregular octahedron of bridging sulfur atoms in an adamantane type of cage. The mean Fe-Fe and Fe-S bond distances are 3.94 8, and 2.36 A respectively. A series of novel bridged cluster complexes result from the reaction of MoS:- with RS- and FeCl, in methanolic solution containing quaternary ammonium cations. Depending on reaction conditions, yields of three such complexes as their R,N” salts can be o p t i m i ~ e d : ~ ~[MO,F~,S,(SR),I~~-~~” (80, [M02Fe6Ss(SR),]3- (82) and [Mo,F~,S,(SR),,]~-’~-(83). The first complexes of this type were reported in 1978 by two research groups simultaneously; these were [Mo, Fe,S,(SEt),13- 767 and ] ~ - are similar to those in [Mo,F~,S,(SP~),]~.771.772 The iron atoms in the [ M o ~ F ~ ~ S , ( S R ) ,anions the [Fe,S,(SR),]’- clusters and their Mossbauer spectra are consistent with extensive delocalization over the metal centres. Tungsten analogues have been prepared, and the properties of these ~ ~ ~ ~ ~the ~ most extensively investicomplexes are similar to those containing m o l y b d e n ~ r n . 7Amongst gated complexes in this series are (82) and (83), and analogues containing bridging methoxide ligands have also been isolated.773Equations (71) and (72) give the overall stoichiometries for formation of structures (82) and (83) respectively. The first identifiable complex formed in these reactions is the sulfur-bridged dimer [(RS), Fe(S),MoSJ2, which is well c h a r a c t e r i ~ e d . ~ ~A~ . ’ ~ ~ growing number of related trimers containing Fe,Mo or FeMo, centres with bridging sulfides have been reported. Much of the work on these and their tungsten counterparts has been reviewed by Couco~vanis.~~~ 2MoS:2MoS:-
+ 6FeC1, + 17RS- ++ 7FeC1, t 2 0 R S - d
+ + 18C1[ M O , F ~ , S , ( S R ) ~ ~+ ] ~ -4RSSR f 21C1[ M o ~ F ~ & ( S R ) , ] ~ - 4RSSR
(71) (72)
A number of iron sulfide/nitrosyl complexes are known; these include [Fe, S,(NO),]*-, [Fez(SR),(NO),] and the black tetramer [Fe,S,(NO),]- (Roussin’s salts), as well as neutral [Fe,S,(NO),]. All these are diamagnetic and are discussed in Section 44.1.2.5, which is concerned with iron nitrosyl complexes. A variety of other iron/sulfide clusters are known and their range is illustrated by the following examples. Amongst the first Fe, S4 complexes to be structurally well A characterized was [Cp,Fe,S,] obtained from reaction of [Cp, Fe,(CO),] with elemental related complex is [Cp, Fe, S2(SEt)J, obtained from reaction of [Cp, Fe,(CO),] with ethyl polysulfide in refluxing methylcy~lohexane.~~~ This dark green diamagnetic complex contains an unusual planar FeSSFe bridge,’” and its electrochemical behaviour suggests the diamagnetism results from a unique electron pair coupling.78’The planarity of the FeSSFe bridge is retained on one-electron oxidation to [Cp2Fe,S,(SEt),]+ and following further one-electron oxidation (nonreversible) [Fe, Cp,(NCCH, ), (SEt),]” was isolated.78‘ As previously noted, benzoyl chloride reacts with [Fe,S,(SR),]’- to give [Fe,S,C1,]2 ,which has bridging sulfides and terminal chloride ligands. Salts of this black anion are air sensitive, and its cubane core structure is similar to that of the thiolate precursor. There is a departure from ideal cubic symmetry with the Fe, portion being essentially tetrahedral.762Treatment of [Fe,S,Cl,]*- with sodium iodide affords the iodo complex [Fe,S,I,]2-. The bromo anaiogue is obtained in the same way, but not the fluoro complex.761Electrochemical reduction of the dimer [Fe,S,Cl,]Z- gives76’the corresponding tetramer as in equation (73). A similar scheme accounts for the behaviour of the analogous thiolate complexes. Their reduced dimers are sufficiently stable for the 3 - /4 - process involving dimers to be observed together with that for the 3 - /4 - and 2 - 13 - tetramer^.^" A range of mixed halide/diethyldithiocarbamate (Et2dtc)cubane clusters have been prepared783*784 from [Fe,S, Cl4I2-, PhS- and Et,dtc- via the series of reactions shown in Scheme 14. 2[Fe,S,CI4]*C O C . 4 - NN’
e-
F~CP~I’
[Fe,S,Cl,]’-
+ 4C1-
(73)
I 1244
iron(J1) and Lower States [Fe4S4(SPh),CIzJZ-
Scheme 14
Structures of the complexes [F~,S,(E~,~~C),X,-,]~(X = C1-, PhS-; n = 1, 2) have been determined784and each have the familiar iron/sulfur core. There are relatively long distances between the five-coordinate iron atoms bound to Et,dtc ligands, as for example in [Fe& C1, (Et,dtc),]'- (84).
7'
(84)
A number of large and structurally interesting iron sulfide/halide clusters have been isolated and characterized. Refluxing a mixture of iron, sulfur and Et,NI in THF for 24 hours7*'gives a moderate yield of (Et4N)Z[Fe6S61,]in the form of small black platelets, as in equation (74). The formal oxidation state of the iron is +2.67, that is '4Fe1"2Fe"'.This reacts further with iron and 12/T- in dichloromethane over a protracted period to give (Et4N)3[Fe,SsIs]according to equation (75). The eight iron atoms in this complex (85) form an almost perfect cube with terminal iodide ligands and p,-bonded sulfides on each face.786A Fe, cluster with triply bridging sulfide ligands is obtained by reaction of Fe(BF,), and PEt, (1:3 molar ratio) with H,S in an acetone/ethanol mixture. After several days in air the green solution becomes black, and addition of NaBPh, deposits crystals of [Fe,S, (PEt, )6](BPh4)2.This iron(II1) cationic complex (86) contains an octahedral arrangement of iron atoms with terminal phosphine PEt,
I
-
Zron(I1) and Luwer States
1245
+ 6s + 2 4 f 21- -+ [Pe,S61,]2[Fe6SsI,]'- + 2Fe f 1- + :I2 --+ [Fe,S6I,I3-
(74)
6Fe
(75)
I
44.1.5.10.3
Dithio ligands
This section is concerned with l,l-, 1,2- and 1,3-dithio chelates typified by dithiocarbamates (87, the dithiolenes (88) and dithoacetylacetonates (89) together with the [FeX,]'- dithiolinium salts. All of these iron complexes are of interest because of both their novel electronic strudures and redox processes involving high and low oxidation states. Relatively few monothiocarbamate transition metal complexes have been prepared and the limited chemistry of polymeric [Fe(SOCNMe,),] is discussed in a recent review+''* In contrast, many transition metal dithiocarbamates are known and their chemistry has recently been reviewed.''' Very large quadrupole splittings ( 4.2 mm s-') in the Mossbauer spectra of [Fe(R,dtc),] complexes indicate a dimeric structure in the solid state and magnetic susceptibility measurements show antiferromagnetic coupling (Ntel temperature = 1 IO K).m-791 The X-ray structure of [Fe(Et,dtc),] (90) confirms it is dimeric and shows the two dithiocarbamate ligands; each bridges one axial and one equatorial position of the trigonal bipyramidal geometry around the iron atoms.792Generally, in solution these iron(II) complexes are complex [Fe(R,dtc),]. In the extremely air sensitive being rapidly o ~ i d i z e d ~ to ~ l the - ~ ~iron(II1) ~ presence of excess R,dtc- the tris complex {Fe(R,dtc),]- is formed.794This is also air sensitive and its stability depends on the cation. Oxidation of the iron(II1) complex in benzene by air795or iodine796-797 affords the corresponding iron(1V) complex. Electrochemical reduction of the iron(II1) complex [Fe(R,dtc),] in acetone and acetonitrile using platinum electrodes givesT9'the high-spin tris iron(I1) complex via a process indicated by cyclic voltammetry to be reversible. Measurements obtained during controlled potential electrolysis, however, indicate that there is some loss of ligand on reduction. With mercury electrodes additional reduction reactions have been o b ~ e r v e d . ~ ~ ~ ~ ~ ~
-
(87)
(88)
(84)
(90)
[Fe(R,dtc)J is formed first, followed by loss of R,dtc- and sequential reduction to give' [Fe(R,dtc),]- and [Fe(R,dtc),12- as shown in Scheme 15. In the presence of bipy, reduction of [Fe(R,dtc),] givessm the well-characterized iron(I1) mixed ligand complex [Fe(R,dtc),(bipy)], and this process can generate bipy- radical anion. The iron(1) complex [Fe(CO),(PPh,),] * undergoes"' oxidative substitution with R,dtc- (and - S,CPPh,) to give the cationic dicarbonyl complexes cis~rans-[Fe(CO),(PPh,),(S, CNR,)]- . The same complex is obtained with Me,NC(S)SS(S)NMe, . Golden yellow [Fe(Me,dtc),(CO),] is obtained from the reaction of [FeBr,(CO)4] with Me,dtc- in acetone.802The UV photoelectron spectra of several [Fe(R,dtc),(CO),] complexes have been examined.803In the vapour phase CO is lost to give [Fe(R,dtc),]. The analogous cyclopentamethylene dtc dicarbonyl complex is the product from addition of the corresponding disulfide to [Fe(CO),] in THF. In crystals of this complex analyzed by X-ray crystallography,804only one of the possible structural isomers was present, resulting from the relative orientation of the chair-form piperidine rings. [ Fe(R2dtclbip y 1
ee
[ Fe(R2dtc)bipy I -
It.Scheme 15
Iron(lI,i and Lower States
1246
Whilst little information is available concerning iron(I1) xanthates, the preparations of [Fe(S,COEt),] and [Fe(S,COEt),]- have been describedsnsand addition of potassium xanthate to a solution of FeCl, and 1,lO-phenanthroline (2: 1 : 1 ratio) yields [Fe(S,COEt),(phen)]. If 1,lOphenanthroline is replaced by 2,2'-bipyridine, two crystalline modifications of [Fe(S,COEt),(bipy)] are f~rrned.~" Pyridine reacts with iron(II1) xanthates to formso6high-spin iron(I1) complexes as in equation (76). 2[Fe(S2COR),]
+ 4py
-
2[Fe(S,COR),py,]
+
ROC(S)SSC(S)OR
(76)
The thioxanthate complex [Fe(S, CSEt),]- is obtained from reaction of trans-[FeCl,(H,O),] with potassium ethylthioxanthate in methanol or THF. Initially, a violetlred-brown solution is obtained although this can become bluish-green at high ligand-to-metal ratios. However, addition of R,N+ yielded only one product, namely the tetraalkylammoniurn salt of [Fe(S,CSEt),]- . An X-ray crystal structure analysis"' shows the FeS, core to be distorted from a regular octahedron towards a trigonal prism with an average Fe-S distance of 2.52A. The blue colour of dilute solutions of Et,N[Fe(S,CSEt),] in acetonitrile fades over a period of a few days at 10 "C to give some brown material and black crystals of an unusual iron(II1) trithiocarbamate (Et4N), [Fe(SEt)(S,CSEt)(CS,)],. This complex has a relatively short Fe-Fe distance. The iron atoms are bridgeds0' by two ethylthio and two ethylthioxanthate ligands. The very reactive high-spin complex [Fe(S2PF,),j is formedso9in the absence of air and water from iron metal or FeC1, with anhydrous HS1PF, . Oxidation states higher than + 2 are favoured by the presence of dithiolene ligands and these complexes are discussed in Sections 44.2.5.2 and 44.3. In the present section, discussion is restricted to the reaction of [Fe(Et,dtc),] with S,C,(CF,),. In THF solution in the absence of air this reaction yields the complex [Fe(Et,dtc),(S,C(CF,),}] as in equation (77). The geometry of this complex is slightly distorted"' from an octahedron towards a trigonal prism. As is frequently the case with complexes of this type, the assignment of oxidation state to the metal ion is somewhat ambiguous. Nonetheless, there is a high degree of Fe" character associated with the structural data. Magnetic data for both the solid complex and in solution are consistent with a singlet/triplet spin equilibrium. In solution two stereochemical rearrangements are observed*" on the NMR timescale: inversion (AH' 37.7 kJmol-') and C-N bond rotation (AH' 66.9 kJmol-I). In electrochemical experiments in acetonitrile solution, two reduction and one oxidation steps have been observed.*I2 [WEt&),l
+
S2Cl(CF3),
-
-
[Fe(Etzdtc)z{S,C,(CF,), 11
(77)
The complex [Fe(SPMe2NPMe,S),] containing an unusual 1,3-dithio ligand has a tetrahedral structure, and is discussed in Section 44.1.5.10.2. Early report^*''^^'^ of attempts to prepare complexes of dithioacetylacetone describe a reaction mixture of Fe", acetylacetone, HCl and H,S in ethanol, which somewhat surprisingly yielded a deep purple 3,5-dimethyl- 1,Zdithiolinium salt (91) of the [FeCl,]'- anion. Simple [FeX,]'- salts are pale cream and dithiolinium cations have no appreciable absorption in the visible region. Thus many workers'" suspected the presence of Fe-S bonds. The discrete [FeC14]'- ion has, however, been demonstrated in an X-ray structure The intense colour of this and related salts is due to a broad absorption (around 20 000 cm-' ) arising from charge transfer between the sulfur-containing cation and the reducing tetrachloroferrate anion. The structure of (91),[FeCl,] contains several potential S * * * C1 interactions. Each cation participates in three relatively short S-CI contacts which represent a ready route for charge transfer. A variety of substituted 1 ,2-dithiolinium tetrachloroferrate(T1) salts have been characterized, and their charge-transfer behaviour rationalized in terms of a combination of steric and electronic effects. Chemical reduction of (91),[FeCl,] (e.g. with NaBH, or Na,S,O,) givesRI7a moderate yield of the low-spin neutral iron(II1) dithio-4-diketone complex [Fe(C, H,S,),]. Me*Me
s-s (91)
Red-brown crystals of this complex dissolve in chloroform and are reduced further, on shaking the solution with aqueous Na,S, O,, to give the bis-dithio-4-diketone iron(I1) complex [Fe(C,H,S,),], which is very sensitive to reoxidation. A similar complex is postulated by Japanese workers,R'8 perchlorate in the presence of resulting from lhe reduction of 3-methyl-5-phenyl-l,2-dithiolium Fez' . The exact structure is unknown but is thought to be as shown in (92). [FeCl,]*- f o r r n ~ "a~
-
Iron(II) and Lower States
1247
related charge-transfer complex with the dication of N,N'-dimethyl-4,4'-bipyridine(paraquat). EPR spectroscopy shows the presence of some completely reduced radical cation pq' although the Mossbauer spectrum indicates that the majority of iron is present as iron(I1) in the complex. The X-ray powder diffraction pattern is very similar to that for an equivalent cobalt salt whose X-ray crystal structure showss2' the paraquat cation to be planar.
44.1.5.10.4
Mixed suwur nitrogen donors
A variety of iron(T1) complexes with sulfur and nitrogen donors are known. Many have unusual coordination geometries and interesting magnetic behaviour. The iron(1I) complexes of (93) with stoichiometry [Fe(93)Y2](Y = C1, Br, I, NCS) are high-spin and their magnetic moments, in the range 5.3-5.4 BM, are consistent with octahedra1 coordination. The halide complexes are orange or yellow, whilst the thiocyanate is dark green. The complexes dissociate in nitromethane and reaction (78) has been proposedR2' for which the equilibrium constant increases in the order C l- NCS < Br iI. [Fe(93)Y2]
+ CN3N0,
--f
[Fe(93)Y(CH,N0,)ItY-
(78)
In contrast, the more delocalized hexadentate ligand (94), which provides an S,N, donor set, formss2' a low-spin iron(I1) complex. Here, the higher degree of conjugation demands that the geometry of each NNS sequence must at least approximate to being planar with the result that only two enantiomeric forms are possible. The stability of this iron(I1) complex is illustrated by its formation from iron(II1) salts. If excess FeC1, is used, the product is blue [Fe(94)1[FeC14],,whereas excess FeCl, affords [Fe(94)][FeC14].
(93)
(94)
The Schiff base condensation of 2,6-diacetylpyridine with 4,7-dithiadecane-1,IO-diamine yields the macrocyclic ligand (99, which has an S,N, donor set. The six-coordinate complexes [Fe(95)Ylf can be high- or low-spin depending on the nature of Y, for example magnetic moments for Y = C1, Br, I and NCS are 5.2, 5.3, 0.3 and 0.6 respectively. Three distinct structural modifications of the stoichiometry Fe(95)(NCS), have been characterized in the solid state. In acetonitrile solution, however, there appears to be a common structure. Two of the solid state structures are high-spin and the other low-spin; no doubt these involve different conformations of the macrocyclic ligand. X-Ray crystal structure analysis of the low-spin complex the macrocycle wrapped around the iron(I1) centre as a pentadentate ligand with a nitrogen bound NCS- group completing a distorted octahedral geometry. The neutral bis complexes of the linear aliphatic ligands (96) and (97) are both high-spin and have slightly distorted trigonal bipyramidal structures involving dimercaptobridged dimers. The extent of distortion from the trigonal bipyramidal dimer structure (98) is greater for the ligand (96) (which is the less flexible of the two) than for (97). The two iron atoms are sufficiently close (3.2 A) for significant antiferromagnetic exchange to occur by direct overlap of irongz4orbitals. Because of the closeness of the participating nuclei and the orbital dependence of the effect, it is possible to evaluateR25electron pair exchange parameters for the temperature dependence of the magnetic susceptibility of the complexes [Fe(96)], and [Fe(97)I2. Neutralization of an alcoholic solution of o-aminobenzenethiol (99) with a slight deficiency of
h n ( l Z ) and Lower States
1248
NaOH in the presence of FeSO, affords a pale yellow precipitate of [Fe(SC,H,NH,),]. It is insoluble in most common solvents but slightly soluble in DMF. This iron(1I) complex is not well characterized; it is antiferromagnetic with a well-defined NCel point of 138 K and a magnetic moment of 3.95BM (rather low due to exchange). The reflectance spectrum of the complex indicates sixcoordination and it may be assumed that its structure is polymeric and involves sulfur bridges. Ethylenediamine reacts with bis(thiosalicylideniminato)iron(II) to give”’ (loo), which has a magnetic moment of 1.9BM. The well-known oxygen analogue of this complex, [Fe(salen)], has in contrast a spin-only value of 4.77 BM.”’ The low temperature Mossbauer spectrum of (100) is compatible with the intermediate S = 1 spin stare, while the temperature dependence of its magnetic moment between 4.2 K and 300 K is consistent with coupling in a polymeric structure. The complex reacts with oxygen in pyridine at room temperature, affording the corresponding iron(II1) p o x 0 dimer. The room temperature UV spectrum of (100) in pyridine is similar to that in dichloromethane, but at - 23 “C a dramatic change from blue-black to red (A = 508 nm) takes place. This has been ascribeds2’ to pyridine coordination. When carbon monoxide is passed into such a solution it also becomes red and the complex [Fe(100)(CO)(py)] is formed. This has been charac, ~ ~ ~ shows the pyridine and carbon monoxide terized by X-ray crystal structure d e t e r m i n a t i ~ nwhich ligands to be trans to each other. A variety of complexes have been prepared”’ with ligands formally similar to (100) but with the CH,CH, moiety exchanged for different groups. Some of these also have the unusual S = 1 spin state.831In contrast the neutral bis-ligand complexes of the bidentate (101) are high-spin (magnetic moments approximately 5.1 EM)’’’ and the assignment of an S = 2 spin state is supported by Mossbauer data.831
qFe,p oz, -N/
(99)
W \N( 100)
m a (101)
The quadrupole splitting in the Mossbauer spectrum of the thiosemicarbazide complex [Fe(NH2NHCSNH2)2]S04 is particularly largeE3’(4.65mm $ - I ) and an X-ray crystal structure an unusual chain-like polymer formed by SO:- bridges between cis- and trans[Fe(NH2NHCSNHZ),]’+units. These are thus two different iron environments and the Mossbauer spectrum observed is less complex than expected. The origin of the large quadrupole splitting remains unclear. A vanillin thiosemicarbazone iron(I1) complex (102) has been reported834and its fungicidal properties investigated. Cysteine reacts with Fez+ under acidic or neutral conditions to give a light brown complex of 1:1 stoichiometry. This complex probably involves bonding through the carboxylate oxygen atoms. If, however, the reaction is carried out in alkaline solution, a 1:2 (FeL,) complex is formed. Under the same conditions and in the presence of carbon monoxide, a red dicarbonyl [FeL,(CO),] results. The IR and Raman spectra of these complexes have k e n recorded.835The [FeL,] complex has a polymeric structures36with antiferromagnetic co~pIing8~’ which probably occurs via thiolate bridging groups. The Mossbauer spectra of these complexes have also been reported;838however, their structures are not known with any certainty.
Zron(ZZ) and Lower States HN-C-NHt
H Y e
I
cH = N \ /
/ \
I1
NH2-C-
44.1.5.11.1
I
a i='"
HzO-Fe-H,O
S
44.1.5.11
1249
OH
OMe
NH
Halides Fluorides
Iron difluoride is an off-white sparingly soluble material, which may be prepared by the reaction dehydration of the tetrahydrate,',' displacementw2of chlorine in FeCl, of metallic iron with HF,839*840 by HF, reduction of FeF,.xH,0843or reaction of Fe203with carbon and HF.841It has been shownw to be monomeric in the gas phase between 692 "C and 876 "C. The solid has the rutile structure in which FeF, octahedra are tetragonally distorted with iron-fluorine bond distancesMsof 1.99 A ( x 2) and 2.128 ( x 4). Structural transformations have been observed at very high pressures.846Above loOK, FeF, has a magnetic moment of 5.56BM. Iron difluoride tetrahydrate is a white solid which browns in air. It may be prepared by dissolving is described metallic iron in warm hydrofluoric acid and precipitating with ethanol. The as an array of discrete [FeF,(H,O),] octahedra in which H,O and F- are randomly distributed over 12 possible sites, each site being occupied by an average of 30 and +F-. A saturated aqueous solution of the hydrate formss4 a yellow precipitate of composition Fe2FS*7H,0on standing in air for several days. This complex is dehydrated at 100°C in nitrogen to give FqF,*2H,O and completely dehydrated at 180 "C to give the grey mixed oxidation state salt Fe,F,. The structure of the hydrate contains distinct Fe" and Fe"' centres.849
44.1.5.11.2
Chlorides
Iron dichloride is a hygroscopic white solid readily soluble in water and ethanol. It may be prepared by reaction of metallic iron with HCI at red heat,*" under mild conditions in THF"' or methan01,"~ or with a mixture of the acid and chlorine at 700 "C. The trichloride decomposesss4at 300°C in vucuo or may be reduced for example with H,842or H2SsS5to yield FeC1,. Thermal decomposition of FeCl, aids purification of FeCl, by vacuum sublimation. Anhydrous FeCl, may also be prepared by dehydration of [FeC12(H,0),] and the reaction of metallic iron with CC14.857 Whilst the vapour of FeC1, is monomeric (v = 492 cm-' ) at lower temperatures,8" an increasing is present as the temperature is raised. Crystalline FeC1, has proportion of dimer (v = 430cm-1)858~859 the CdC1, structureBmunder normal conditions. The packing geometry of the FeCl, octahedra appears to be dependent on the mode of crystallization. At high pressures structural changes occur; a magnetically observable change to a Cd12structure is seens6' at low temperatures at 2 kbar pressure and the structure undergoes a similar modificationB6' at 5.7 kbar at room temperature. The anhydrous dichloride has a magnetic moment of pcA= 5.87 BM at 300 K. FeC1, reacts readily with a wide variety of ligands to form [FeCl,L,,] complexes, many of which are discussed elsewhere in this chapter. Also of interest is the tetrahedral anion [FeCl,]'- found in a number of complexes. Tetrachloroferrates may be prepared by the reaction of alkali metal chlorides with FeCl, in the melts63or, for example, the reaction of triphenylmethylarsonium chloride or tetraethylammonium chloride with FeCl, in deoxygenated ethanol.8a The tetrachIoroferrate(I1) anion is less common than its iron(II1) counterpart +and there have been few structural determinations. The geometry of [FeC1,I2- is frequently complicated by interaction with other species, as for example in the case of the complex {[Fe($-CsH,)(CO),],SbC1),[FeC1,] CH,Cl, in which hydrogen bonding between [FeC1,I2- and chloroform is thoughts6' to cause a significant distortion from perfect tetrahedral symmetry. In ~Me,],[FeCl,], which is isostructural with the analogous cobalt, nickel and zinc salts which have
Iron(II) and Lower States
1250
no close interactions, the anion is nearer to ideal tetrahedral symmetry.866Interestingly the bond lengths (Fe-Cl = 2.29A) in the iron(I1) anion are 0.1 1 A longer than those found in the iron(II1) anion [FeClJ. This is in contrast to the octahedral case where, for equivalent spin states, bond lengths differs66by about 0.04 A between the iron(I1) and iron(II1) oxidation states. This difference has important implications in the study of iron-sulfur proteins where oxidation state changes in tetrahedral iron anions are frequently observed. The interesting charge transfer observed between [FeCl4I2- and dithiolium cations is discussed in Section 44.1.5.10.3. A number of other complex binary iron chlorides exist, often poorly characterized. The [FeCl6I4-anion is found as the mineral rinneitese7in potash deposits. The tetrahydrate [FeCl, (H20)4]may be prepared868by dissolving metallic iron in aqueous HCl and crystallizing the product at room temperature. The distorted octahedral structure”’ has trans chloride ligands and there is extensive hydrogen bonding in the crystal lattice (Fe-Cl = 2.38 A; Fe-0 = 2.09, 2.5 A). Crystallization at 75 O yields the dihydrate,870which can also be prepared by careful dehydration of the tetrahydrate. The structure of FeCI, .2H,O consists of chains of [FeCI,(H,O),]*- octahedra with trans aqua ligands which are linked by two bridging chlorine atoms870(Fe-0 = 2.075 A; Fe-Cl = 2.488, 2.542 A).
44.1.5.11.3 Bromides
Iron dibromide is a pale yellow-to-brown hygroscopic solid that is soluble in water, ethanol, ether and acetonitrile. It is prepared8” by the reaction between metallic iron and bromine in a flow tube at 200°C. FeBr, is purified from FeBr3 by sublimation in nitrogen or under vacuum. Other preparative routes include the reaction of metallic iron with HBr in methanol,852hydrobromination of Fe203872 in a flow tube at 200 “C-350 “C and dehydration873of [FeBr,(H,O),]. FeBr, crystallizes in a layer lattice of the CdI, type873and has a magnetic moment of 5.71 BM at 295 K. Like the chloride, FeBr, readily forms adducts with a wide range of donor molecules including amines, pyridines, phosphines, arsines and oxygen donor ligands. A series of hydrates may be crystallized from aqueous solution at different temperature^,'^^ a 9-hydrate below - 29.3 “C, a 6-hydrate’” at room temperature, a tetrahydrate above 49 “C and a dihydrate above 83 “C.
44.1.5.11.4 Iodides
FeI, is a red-black crystalline solid which is hygroscopic, very soluble in water and also soluble from elemental iron and iodine because of the in ethanol and ether. It may easily be prepareds7M78 non-existence of the triiodide. Anhydrous FeT, has the CdI, structure879and a magnetic moment of 5.75 BM at 295 K. As with the dibromide, a series of hydrates are obtainable from aqueous solutions of FeI,; for example, the green tetrahydrate results from evaporation at room temperature. FeI, also forms a wide range of adducts with suitable donor ligands. Recent work has identified significant advantages877in the use of high purity FeI, in discharge lamps.
44.1.5.12
44.1.5.12.1
N Donor Macrocycles lntroduction
There is a particularly extensive and rich coordination chemistry associated with iron(I1) N donor macrocycles. The coordination chemistry of the biologically important iron porphyrin complexes has been of interest since the classic studies of Fischer, and over recent years there has been a resurgence of work on these and also on a wide range of related synthetic macrocyclic complexes. This section concentrates on their coordination chemistry, and where appropriate highlights enhanced ligand reactivity specifically induced by the iron(I1) centre. First saturated ligands are discussed and then the unsaturated systems, with the particularly well-studied porphyrins and phthalocyanines being dealt with in separate subsections.
44.1.5.12.2
Saturated macrocycles
This section is concerned with complexes formed by macrocycles which are effectively polydentate (103) (1,4,8,11ethylene or propylene amines. The prototype ligand is ‘cyclam’ or [14]aneN~*o,881 1
1251
Iron(1I) and Lower Stutes
tetraazacyclotetradecane). Thus, when FeCl, -4H,0 is added to a warm solution of Me,[ 14]aneN4 (104) in DMF, a tan precipitate of [Fe(Me6[14]aneN,)C1,] formsss2on cooling to room temperature. A similar complex with axial acetonitrile ligands resultsss3from the reaction of Me6[14]aneN, with iron(I1) acetate or perchlorate in anhydrous acetonitrile. This complex is low-spin, highly air sensitive and, in acetonitrile solution, is progressively oxidized to yield the diimine complex [ F e ( ~ e ,14]1,3,8,10-tetraeneN4)]*+ [ (105). NCMe
T*+
\
Reactions of [Fe(hfpd)(H,O),] (hfpd = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato) with (104) and related macrocycles in rigorously anaerobic conditions affordsw complexes of the type [Fe(L)(hfpd)] (L = 104). Mossbauer spectroscopy indicates six-coordinate hgh-spin iron(II), but the nature of the bonding of the axial ligands is not fully resolved. TMC (tetramethylcyclam), the tetra-N-methyl derivative of (103), reactsss5readily with [Fe(NCMe),](BF,), in acetonitrile to give the five-coordinate high-spin iron(I1) complex [Fe(TMC)(NCMe)](BF,)2. Substitution of the coordinated acetonitrile yields [Fe(TMC)X]+ (X = Br, C1, NCS- ,N; ), which are also five-coordinate. All these complexes react in the solid state with moist ambient air to give dark red-brown compounds which are reportedss5to be six-coordinate S = 1 iron(I1) species, having a water molecule in the sixth coordination position. Complex (105) shows three well-defined reversible electrochemical reduction steps,88'corresponding to Fe', Feoand Fe- ' oxidation states, whereas in acetonitrile [Fe(Me, [ 14]aneN,)(MeCN),]2f reduces irreversibly. The saturated nitrogen atoms in such ligands are relatively 'hard' but complexes with strong field axial ligands tend to be low-spin. Thus when the axial acetonitrile ligands in purple, low-spin [Fe(Me,[ 14]aneN,)(MeCN),](BF,), are a powder-blue high-spin species [Fe(Me,[ 14]aneN,)(BF,)](BF,) containing removed in vacu0*s6~887 coordinated BF; is formed. A series of complexes [Fe(Me,[ 14]aneN4)X,]have been synthesized.887 In the order of the spectrochemical series, low-spin complexes were observed for X = CN- , NO; (and MeCN as BF; and PF; salts), high-spin Complexes for X = MeCO; ('&& = 5.7 BM) CI(5.95), Br- (5.47), I- (5.37) and BF, (5.7, five-coordinate). The X = NCS- complex exists in a mixture of spin states having a room temperature magnetic moment of 1.5 BM and a weak ESR signal at g = 2.285. The stabilization of iron(I1) in complexes having predominantly saturated nitrogen donor ligands is unusual, and may be ascribed to the conformation of the macrocycle, which maintains the planarity of the donor set, and hence a relatively high ligand field strength. Significantly increasing ligand field strength, however, results in the formation of five-coordinate high-spin complexes, probably because the metal ion is then too large to fit inside the plane of the high-field ligand. Thus, with relatively weak-field axial ligands such as halide, the tetraimine (106) formsss8a five-coordinate complex [Fe(Me, [ 1411,4,8,11-tetrdeneN4)(C1)](C10,).Mossbmer studies have shown"' that the n-accepting power of the tetradentate macrocyclic amines increases with the degree of unsaturation and that bonding to iron(I1) is especially important for ligands containing the a-diimine moiety. Iron(I1) complexes of a range of saturated macrocyclic amines of varying ring size, i.e. [13]aneN4(lCn), [15]aneN, (108) and [16]aneN4(109), have been synthesizedsg0in addition to those of the non-methylated ligand cyclam [14]aneN, (103). The ligand field strength of the macrocycle varies markedly with ring size, being greater in smaller rings. This difference is demonstratedSwby the spin states of tetragonal iron(I1) complexes with rings of differing size (Table 17). Interestingly, the presence of methyl substituents on the saturated tetraaza macrocycles also affects the properties of their iron(I1) complexes. Whilst [Fe(Me,[ 14]aneN4)(NCS),] shows a ' A , , ~ 5 T z , spin transition, [Fe([14]aneN4)(NCS),] is low-spin. Furthermore, [Fe([l4]aneN4)(MeCN),](PF,), does not lose MeCN to form a five-coordinate species which is in contrast to the analogous complex [Fe(Me6{14]aneN4)(MeCN),](BF,), . A further example of the
I
Iron(1I) and Lower States
1252
Table 17 Physical Properties' of peX,(b])aneN.,)] Complexes withy
X= II31ane
[14]ane
[15]ane
II6Jane
Colour PdBM) 6(mms-l) AEQ(mms-l) Colour &ABM) 6 (mms-I) A&, (mm s-I) Colour Pew (BM) 6 (mm s - I ) AEQ(mms-l) Colour PetI (BM) 6 (mms-'f A& (mm s-' )
MeCN'
Not isolable Purple 0.43 0.57 0.66 white 5.52 1.14 I .29 White 5.55
1.17 1.38
NCSWhite' 5.42 1.12 1.92 Blue 0.72 0.62 0.35 White 5.32 1.20 1.22 White 5.52 1.25 1.97
=
NO;
Red 0.57 0.49 0.67 Magenta' 3.34 0.33 < 0.40 Orange' 5.53 1.05 1.91
13, 14, 15 and 16
CNOrange 0.76 0.40 0.79 Orange 0.77 0.44 0.94 Lavender 0.69
0.48 1.44 Not isolable
Magnetic moments measured at 25 "C, Mossbauer parameters relative to stainless steel. 1. D. D. Watkins, D. P. Riley, J . A. Stone and D. A. Busch, Znorg. Chern., 1976, 15, 387. 2. As PF; salt. 3. High-spin state indicates N C Y are coordinated czs. 4. Stoichiometry [Fe[lS]aneN,(NO,)]+, severe distortion from octahedral symmelry results in a triplet ground state. 5. Compound has stoichiometry [Fe([16]aneN,)(N02)]N02.
effect of ring size is given"' by the reversible uptake of carbon monoxide by low-spin [Fe([14]aneN4)(MeCN)2]2$ and high-spin [Fe([15]aneN4)(MeCN),]2+.The Latter complex becomes low-spin on substitution of one MeCN by CO, which was the first example of a non-haem complex changing spin state on binding CO. The relatively weaker ligand field of [15]aneN4and the proximity of the CO complex to the spin cross-over contributes to a 2000-fold increasesg1in the lability of CO in [Fe([15]aneN4)(CH3CN)(CO)]** over the analogous [14]aneN, complex. The macrocyclic triamine [9]aneN3(110) can only coordinate 'facially', in contrast with common open chain polyamines and macrocycles of larger ring size. ReactionEg2of the free ligand with FeCl, leads to the precipitation of blue [Fe([9]aneN3),]C1,, which may easily be converted to the bromide. Surprisingly, this complex is diamagnetic and stable in aqueous solution although air sensitive. The formal redox potential at 3 "C in 0.1 M KC1 is + 0.13 V. Thus the iron(II1) complex is but a weak oxidant.
44.1.5.12.3
Partially unsaturated macrocycles
Interest in complexes of new synthetic macrocyclic ligands (and especially amines) was sparked by the discovery of the so-called Curtis ligands (111) first observed893as the product from the
Iron(II/ and Lower States
S
1253
reaction of tris(ethylenediamine)nickel(II) with acetone. The ligand (111) (Me6[14]dieneN4) stable, high-spin complexes of the type [Fe(Me,[14]dieneN4)X](C104) with X = Cl, Br, I. An X-ray crystal structure determination895shows five-coordinate iron(I1) in [Fe(Me,[ 14)dieneN4)C1]I.The macrocycle however is not planar but adopts a puckered configuration. Steric constraints prevent the high-spin iron(I1) from fitting inside a plane containing the four nitrogen atoms and the iron sits slightly proud of the ring as illustrated in structure (112) with one axial chloride. The ligand is too rigid to allow coordination of a second cis halide. The d-d electronic spectrum of this complex in solution is the same as the solid-state spectrum and this, together with the fact that the iron(I1) centre remains high-spin, indicates that the unusual five-coordinate structure persists in solution. A related series of complexes have been prepared,896 which show that the spin state and coordination number of the iron are dependent on the nonmacrocyclic ligand (Table 18). The acetonitrile complex [Fe(Me,[l4]dier1eN,)(MeCN),]~+ exists as two distinct isomers (which have different IR, visible and NMR spectra) that are interconvertible and derive from the meso and racemic forms of the asymmetric nitrogen atoms in the macrocyclic ligand. Interestingly, in the case of [Fe(Me,~14]dieneN4)(phen)](C104)~the phen ligand is constrained to coordinate in a cis fashion if it is to be bidentate. This complex shows a thermal ' A l g F S T Zspin p equiIibrium. The UV/visible spectrum contains mainly strong charge transfer bands, and as the marked dependence of magnetic moment on temperature suggests, spectral characteristics also change with temperature. Thus at room temperature the complex is purpleblack, whilst at 150 "C it undergoes a reversible change to a pink-red colour.
I
i'
2.30619
Table 18 Some Properties of Fe" Complexes of the Macrocycle L (L = 5,7,7,12,14,l4-hexamethyl-l,4,8,1l-tetraazacyclotetradeca-4,11diene( 11 1))
fief
(r.1.)
Complex
Colour
(BM)
[FeL(MeCN),](ClO& [FeL(NCS)*I FeL(MeCN),II, FeL(MeCN)(imidazole)](BPh,), [FeL@hen)l(C104)z [FeL(Cl)l(C104) lFeL(Br)l(CQ) 'FeL(I)](CIO,) {FeL}20H](C10,)1~H,0 FeL(I),I
Pink4 Brown Dark red Red-brown Purple-black2 Off white Off white Pale yellow-green White Brown
0.4 0.5 0.62 0.42 3.70' 5.50
Electronic spectra (molar extinction coeficient) (W Solveni
A, (in MeNO?) (cm2ohm-' mol-' )
~~~
.-
- -. -
5.11
5.15 Ref. 3 5.25
199 8 244 170 189 95 106 93 90
19.5 (85), 29.1 (2400) 19.0w, 23.25, 25.0s 19.6 (Sl), 29.4 (5200) 20.0 (70), 24.9s, 31.7s
MeCN
--
MeNO, MeNO, MeN02 Solid MeOH
4.2 (7). 12.2 (5) 5 (-, 3,12.2 (4.9) 4.5 (- 4), 12.2 (4) 5.12w, 12.2w 12.2 (13)
solid MeCN MeCN
- .
I . V. L. Geodken, P.H.Merrell and D.H.Busch, J. Am. Chem. Soc.. 1972,94, 3397. 1. Undergoes a reversible colour change to pink-red at 150°C. i,A value of 4.85 BM reported but the authors suspected some contamination, I Two forms with different solubilities were reported. 5. A strongly temperature dependent zrnis observed.
These complexes are very sensitive and many decompose rapidly on exposure to air. Controlled reaction of low-spin six-coordinate complexes with oxygen, however, can lead to oxidative dehydrogenation of the ligand forming iron(I1) complexes of tetraaza macro cycle^^^^ which in some cases are isolable as the free ligand (Scheme 16). The high-spin five-coordinate halide complexes [Fe(Me,[ 14]dieneN,)%+ react with controlled amounts of oxygen to yields3' intense blue precipitates in which oxidative dehydrogenation of the ligand is apparently accompanied by the tautomerization of the P-diimine to the a-diimine. This tautomerization also occurs to some extent during the oxidation of low-spin six-coordinate complexes with strong field axial ligands such as CN-, NCS- or imidazole.
Iron(I1) and Lower States
1254
I
-
0,
.
MeCN
H'-
trace
H2O
\
\
NrMe
NCMc
NCMe .4
NC'Mr \
NCMe
Scheme 16
Oxidative dehydrogenation of the fully saturated 14-membered tetraaza macrocycle iron(I1) complex affords different products. Thus when [Fe(Me,[ 14]aneN4)(MeCN),](BF4),is oxidized in acetonitrile solution, the resulting complex containss9' a macrocyclic tetraazatriimine ligand with one a-diimine group. If the BF; salt is exchanged with NH,PF,, the resulting red-violet complex is further oxidized in basic acetone with the solution becoming deep blue. Extraction of the resulting mixture with chloroform, followed by acidification in acetonitrile and crystallization from ethanol saturated with NH4PF6,yields the violet complex of the corresponding tetraazatetraimine having two a-diimine groups as shown in Scheme 17. The axial acetonitrile ligands in the triimine complex A in Scheme 16 are readily exchanged89K to give highly air sensitive products which are dehydrogenated by molecular oxygen to the corresponding tetraimine complexes (L = imidazole, SCN- , NO,, CN-, CNH, CNMe and C1- ). NCMe
NCMe
(PF6)2
\
NCMe
SCMe Scheme 17
The analogous complexes [Fe(TIM)L,](PF,), (TIM = 113, R = Me; L = MeCN, imidazole) may be synthesizeds99from iron dichlorideltin dichloride and the reaction product of 2,3-butanedione and 1,3-diaminopropane in methanol followed by addition of acetonitrile. The X-ray crystal structure of [Fe(TIM)(MeCN),](PF,), has been determinedm and the iron shown to be coplanar with the ligand. Both acetonitrile ligands in the burgundy-red low-spin complex may be exchanged for imidazole, or a single acetonitrile exchange for CO. Substituents on the macrocyclic ligand influence the redox potential"' such that the iron(II1) oxidation state is stabilized with increasing 0 donor strength of the ring. The iron(1I) oxidation state is stabilized with increasing a acceptor ability. An [Fe(TIM)L2I2+complex forms the starting point for the synthesisgo2of novel bimetallic
Iron(l1) and Lower States
1255
I'I
complex (114) in which an imidazole group bridges Cu" and Fe" atoms. It is thought such a system might mimic the interaction between the haem a3 centre and the Cu(P) protein compounds of cytochrome oxidase. The kinetics of axial acetonitrile substitution by N-methylimidazole have been for the complexes [Fe(TIM)L,](PF,), (R = Me, p-MeOC,H,, p-MeC,H4, Ph; in~estigated'"~,~" L = MeCN). The reactions proceed via a dissociative mechanism but whether the loss of the first or second acetonitrile is the rate determining step is the subject of discussion. Activation parameters for this process are given in Table 19 together with data for the same reaction of complexes with two related macrocycles. For these complexes there is a linear free energy relationship between ASZ and A H Z but data for the equivalent complexes with ligands (106) and (121) (Table 19) are not within this correlation. However, there appears to be a relationshipgo5 between AH$ (or k,)and the donor ability of the macrocyclic ligand as measured by the half-wave redox potentials of the complexes.
Table 19 Activation Parameters for Loss of Acetonitrile from trans-[FeL(MeCN),y+ in Acetone
EliZvs. SCE (V)
L ~~~~~
(113), R = pMePh (113), R = Ph (121) (113), R = p-MeOPh (1W (1131, R = Me
~
~
~
1.22 1.25 1.18 1.12 0.89 0.97
~~
AHf (kJ mol-')
A S # (J K-'mol-')
Ref:
91.2 1.3 90.8 8.8 82.4 f 4.2 81.6 & 15.9 14.5 6.1 67.8 10.5
73.2 66.9 91.2 49.0 58.2 22.2
2 1 3 2 3 1
~
+
1. D. E. Hamilton, T. J Lewis and N. K. Kildahl, Inorg. Chem.. 1979,18,3364. 2. N. K.Kildahl, T. I. Lewis and G.Antonopoulos, Inorg. Chcm., 1981, 20, 3952. 3. N. K. Kildahl, K.J. Balkus and M. J. Flynn, Itzorg. Chew, 1983, 22, 589.
Iron(I1) complexes of the dianion of the macrocycle Ph2[14]tetraeneN4 (115) have been prepared906by the convenient reaction of anhydrous iron(I1) acetate with the free ligand in DMF. The iron(I1) complex of the analogous Ph2[l6]tetraeneN, ligand is prepared in the same way but the preparation of the complex of the macrocycle of intermediate ring size Ph, [15]tetraeneN4requires the presence of two equivalents of pyridine to assist in incorporation of the metal. Interestingly, these complexes are well-defined examples of intermediate spin state ( S = 1) and have reduction potentials (in DMF at 25 "C) in the range - 3.2 to - 3.7 V. Oxidation of these complexes with Br, in chloroform affords the equivalent iron(II1) complexes and reaction with diphenyl sulfide (with a catalytic amount of PhSH) gives the iron(II1) benzene thiolate complexes in good yield.
I
1254
Iron(II) and Lower States
(115)
The macrocyclic ligands (116) may be synthesized by a modified Jaeger method (Ni“ template synthesis). These ligands react differently according to their ring size to form’” complexes of iron(I1). When the 14-membered ligands react with [Fe(NCMe),]’+, a complex containing axial trans acetonitrile groups and a bis-a-diimine macrocyclic ligand (117) is formed. In the corresponding experiment with the 15- and 1&membered ligands however, acetonitrile molecules undergo an electrophilic reaction with the apical carbon atoms of the six-membered B-diimine moieties in the complex. The resulting compound is an iron(I1) complex of a partially cyclized bis-b-triimine of (118) (R = Me, R‘ = H, hexadentate ligand (118). The X-ray crystal X = Y = (CH,),) shows the six imine nitrogen donors to form a slightly distorted octahedron. The double bonds are localized (C-N average = 1.24 A) and the macrocycle is folded to accommodate cis coordination of the iminoethyl groups. NCMe
1’’
Both complexes (117) and (118) may be deprotonated (e.g. complex (117) by reaction with triethylamine in acetonitrile, complex (118) by reaction with ButOK in Et20)to yield”’ novel square planar complexes of the corresponding dianionic tetraaza macrocyclic ligands. These unusual complexes exist in an intermediate S = 1 ground state. Subsequent reprotonation of the square planar complex of the 14-membered ring system [Fe(Me,[ 14]tetraeneato(2 -)N4)] in acetonitrile regenerates complex (117). However, similar treatment of [Fe(Me,[l Sltetraeneato(2 -)N4)] yields a trans acetonitrile pentaimine iron(I1) complex (119), whilst in the case of [Fe(Me2[16]tetraeneato(2-)N4)] the trans hexaimine analog of complex (118) is formed. Similar reactions in the presence of pyridine generate the expected trans pyridine tetraaza macrocyclic iron(I1) complexes. Novel iron(II1) complexes are generated’’’ by the oxidation of the square planar tetraaza macrocyclic dianion iron(I1) species. The X-ray crystal structure’” of the square planar complex of the dianion (120) shows that the ligand has a saddle shape, resulting from interaction between the methyl groups and the benzene rings. The structure of the free rieutral ligand is remarkably similar to its structure in the complex. The average Fe-N bond distance is 1.92A (noticeably shorter than in equivalent S = 0 or S = 1 iron(I1) porphyrin complexes) and the ligand conformation helps to place the iron atom 0.1 14 A above the plane of the donor set. Iron(I1) complexes of this ligand readily form’” monocarbonyl complexes, with or without a second axial ligand. The geometry of the ligand allows for geometric isomers to be formed for complexes [Fe(L)(CO)(L)] (L = 120, L = amine etc.) depending upon which side of the ligand CO is coordinated to iron. For both L = NH2NH2and the five-coordinate complex [Fe(L)CO],the CO ligand is located on the side of the macrocycle toward which the planar benzene rings are inclined. (This orientation is also observed9I3in the case of the iron(II1) complex [Fe(L)Cl], L = 120). In the case where L = py however, an X-ray crystal structure confirms912that
7
Iron(l1) m d Lower States
1257
the CO is coordinated on the opposite side of the ligand. It is thought that this isomer may be more stable because steric interactions between the a-hydrogen atoms of pyridine and the macrocyclic ring are thus minimized. Kron(1K) complexes of the tetraaza macrocycle (121) are reportedly formed914in low yield from the tetramerization of o-aminobenzaldehyde. ,
I
‘Template condensation’ has been used to obtain other fully conjugated macrocycles as their iron(I1) complexes shown in Scheme 18. The reaction takes place readily over 20 minutes with the dropwise addition of hydrazine to a solution of 2,6-diacetylpyridine and Fe(C104),.6H,0 in acetonitrile. The product separates as an intense blue-green pre~ipitate.9~~ Other six-coordinate low-spin complexes of these ligands are obtained with a variety of other axial ligands (NO;, CO, C1-, Br-, NCS-, NT), An X-ray crystal structure of complex A (Scheme 18) show^^'^ that the macrocycle ligand is planar. The iron-nitrogen distances are somewhat compressed (1.899 A and 1A92 A) and the bond lengths of the macrocycle ring show a conjugated rather than an aromatic structure. The electronic spectrum argues for considerable delocalization, beyond that which would be expected based solely on the a-diimine linkages present in the molecule. The planarity of the macrocycle can, however, be disturbed9” by the coordination of a bidentate ligand such as phen or bipy. In this case the ‘puckering’ of the ring causes the electronic spectrum to revert toward that characteristic of a tis-&,a’-diimine complex. The macrocycle (120) results from the template condensation of o-phenylenediamine with pentane-2,4-dione on nickeI(I1) after Jaeger. The hydrochloride salt of (120) (obtained free of Ni after treatment with HC1) mono carbon monoxide complexes [Fe(L)(base)CO] (L = 120, base = MeCN, py, 4-picoline, hydrazine) on reaction with anhydrous iron(1I) salts under an atmosphere of carbon monoxide. An X-ray crystal structure of [Fe(L)(NH,NH,)CO] shows9I6that, here too, the macrocycle is slightly saddle shaped (non-planar) as a result of steric interactions (vide The carbon monoxide islinear and shows a CO stretching frequency for a series of such complexes (193&1940cm-’) slightly lower than that of carbon monoxyhaemoglobin (1951 cm-’).
. NCMe
\
NCMe
A Scheme 18
The ability to exchange small ligand molecules in tetraaza macrocyclic complexes of iron(I1) is relevant to the well-known reversible binding of molecular oxygen to haemoglobin?” The first such reversible binding of O2to an iron(I1) complex was observed in 1973. Previously molecular oxygen was bound to iron(I1) complexes irreversibly, usually with oxidation to iron(II1) which was thought to proceed via the formation of an 0,-bridged dinuclear intermediate. The complex (122) was designed’’’ to have a steric cage which would allow entry to O2but effectively prevent sufficiently close approach on the part of another iron(I1) centre to form the bridged intermediate. The dark
ZWPI (11) rrnd Lower Statl?s
1258
blue-black complex (122) reacts with one molecular equivalent of oxygen in THF/dimethoxyethane containing 4% pyridine to form a cherry red solution. When the complex was dissolved in toluene/ 1% pyridine and oxygen take-up was monitored by optical spectroscopy, it was found9'* to be reversible over a number of freeze-thaw cycles. An X-ray crystal of complex (122) shows the iron coordination geometry to be essentially square planar with two relatively short, Fe--N bond lengths (1.846 and 1.826A). The short bonds are probably due in part to the constricting nature of the macrocycle, which is itself slightly puckered. The deep 'pocket', which effectively prevents the formation of the dinuclear species, is evident. This has subsequently been shown to be a useful feature (though not a necessary one) for reversible oxygen uptake as is demonstrated by this behaviour in, Tor example, tetraphenylporphyriniron(l1) (see Section 44.1.5.1 2.5). Iron(l1) complexes of the ligands (123) and (124) have been prepared. Thc pyridine ring in (123) grcaily stabilizes the reversible formation of a pink-violet osygcn adduct formulated92oas [({FcL]'+ )?O:](L = 123) in aqueous solution. By contrast thc complex of the fully saturated ring (124) yields a very short lived oxygen iron(T1) adduct and autooxidalion is very rapid.
(123)
(1 22)
(124)
There are a series of macrocyclic ligands incorporating the pyridine moiety. The 14-membered macrocycle (125) forms"2' a series of high-spin five-coordinate iron(T1) complexes [Fe(L)X]' (L = 125; X = C1, Br, I). The corrcsponding complcxcs where the monvdectate ligands are azide or bidentate acetate are also high-spin but six-coordinate whilst the dithiocyanate complex is low-spin and six-coordinatc (Table 20). The coordination numbcrs wcre identified on the basis of Mijssbauer spectroscopy. I,a.rge AEQ values in five-coordinate complexes of iron( 11) are thought lo arise from a combinatien of the nonspherical electron distribution about the iron nucleus and the dissymmetry of the ligand field. The assignment of a ' B 2rather than a 5 Eground state (derived from the tetragonal distortion of an octahedral field) to iron in these complexes is supporled by a magnelically perturbed/Mossbauer on the five-coordinate high-spin complex [Fe(Me6[l4]dieneN,)C1]ClO,. The AEQ value for the acetate complex is similar to that found for high-spin six-coordinate iron(I1) and it is assumed that the bidentate acetate ion causes some folding of the macrocycle. AEQ values for the azide and thiocyanate complexes are consistent with high-spin six-coordinate and low-spin six-coordinate iron(I1) complexes respectively. Examples of seven-coordination in pyridine-containing macrocycles have also been osberved. Thus the iron(TT) complexes [Fe(L)X,].xH,O (L = 1.26; X = halide or pseudohalide) are when the corresponding seven-coordinate iron@I I ) coinplexes are reduced with aqueous sodium dithionite in the presence of excess NaX or by the direct template synthesis with 2,6-diacetylpyridine and the corresponding tetramine in the presence of iron(l1). The triimine moicty givcs rise to a strong
( I 25)
(1261
Iron(I1) and Lower States
1259
1260
I m n ( l l / and Lower States
visible absorption derived froin metal-to-ligand charge transfer; the complexes are deep blue or blue-green. An X-ray crystal structuregz4of the iron(11) dithiocyanate complexes of thc 15- and 16-membered macrocycles (126) confirms seven-coordination with a distorted pentagonal bipyramidal geometry. The thiocyanate ligands occupy the axial positions and the macrocycle is somewhat puckered. Replacement of the anionic thiocyanate ligands with neutral water molecules makes almost no difference925to the structure. The substitution of 3,6-dioxaoctane- 1,8-diamine for 3,6-diazaoctane-1,s-diamine in a template synthesis with 2.6-diacetylpyridine on iron(I1) yields the mixed donor macrocycle (127). This molecule also a seven-coordinate iron(I1) complex [Fe(L)(NCS),] (L = 127). This blue high-spin complex (room temperature magnetic moment = 5.44 BM) exhibits an electronic spectrum and electrochemical behaviour which indicate that the complex of the mixed donor oxygencontaining macrocycle stablizes the 2 oxidation state to a greater extent than the same complex of the corresponding macrocyclic pentarnine. The seven-coordinate high-spin iron(T1) complex [Fe(L)(H20)2](L = 128) of the related macrocycle (128) is formed by thc condensation of 2,9-di(1-methy1hydrazino)-I , 10-phenanthroline with 2,6-diacetylpyridine in the presence of fresh FeCl2.4H,O. The X-ray crystal structure92' of this complex clearly shows that the iron(I1) atom is coplanar with the macrocyclic donor set.
+
The Schiff base condensation of 2,6-diacetylpyridine with 4,7-dithiadecane-1,lO-diamine yields the similar NISImacrocycle (129). This ligand is able to form complexes with iron(I1) but its mode of coordination adapts to the monodentate coligand. Thus X-ray crystal structuresq2'show that in [Fe(L)(NCS)]+ and [Fe(L)(MeOH)CI]+ (L= 129) the iron atom has a distorted octahedral geometry. In the first complex the macrocycle is quinquedentate and wraps around the iron atom. In the second complex the macrocycle is quadridentate with one sulfur atom wcakly coordinatcd and one free. A compound with the formulation [Fe(L)(NCS),] (L = 129)has been observedg2'and evidently exists in three different forms (from IR, Mossbauer and magnetic data). Although X-ray structural data are not available on these isomers it is likely that they also reflect different ligand conformations of the N, S2 macrocycle.
44.1.5.12.4
Phthalocyanine
Iron(1I) phthalocyanine (130) was originally d i s c ~ v e r e d by ~ ~chance ~ ~ ~ ' during ~ the manufacture of phthalimide at the Grangemouth works of Scottish Dyes Lid (subsequently part of IC1 plc). Commercially, phthalocyanines are an important class of bluc/green pigments and they arc also used in some dyestuffs, having very good colour intensity with excellent light fastness as well as thermal stability and chemical inertness?'-934 The iron(T1) complex can be obtained by the heterogeneous reaction of iron salts with the parent macrocycle(metal-free)phthalocyanine935~936 (PcH2)in a high boiling basic solvent as in equation (79), or from the metal exchange reaction of Fez' with Li,Pc in acetone937(equation SO). However, the best laboratory procedure makes use of the reductive cyclization of phthalonitrile with very finely divided iron generated from [Fe(CO),] as in equation Good yields of pure product are (81) in a high boiling solvent such as l-chl~ronaphthalene.~~~ obtained over about an hour, and extensive washing with organic solvent removes remaining reactants and by-products. The same procedure enables the perfluoro derivative to be obtained from tetrafluorophthalonitrile (equation 82). Unlike the unsubstituted compound, the perfluoro derivative is soluble in several organic solvents, including acetone, to give intensely deep blue solution^."^
Iron(ll) m d Lower States
PcH2 PcLi,
+ Fe2+ + Fez+ +
+ +
1261
+ 2Hf [FePc] + 2Li+ [FePc]
[Fe(CO)J
-+
[FePc]
(79) (80)
+ KO
P Solid phthalocyanine complexes of the first row transition metals exist in at least two wellcharacterized forms (a and 8) that differ in the relative orientation of the macrocycle plane^.^^.^^^ In the iron(I1) complex, the resulting different intermolecular interactions give rise to differences in their Mossbauer ~pectra.9~’ The most stable p-form is obtained as long purple needles by slow high-vacuum sublimation and the physical properties of this material have been studied in detail by many workers. Several groups have examined the magnetic properties of [FePc] and although all the measurements confirm the S = 1 intermediate spin state, there is some disagreement about the electronic configuration. One of the more exact Mossbauer determinations on a powdered sample ~ ~ other , ~ over the temperature range 4.2-100 K was interpreted in terms of a ’Egground ~ t a t e ,while low temperature magnetic measurements are consistent with the 3B2gground state.%’ However, the directly determined electron density distribution mapped from a set of accurate X-ray reflections ~ , ~study ~’ collected at 100 K indicates ’Egas being the major contributor to the ground ~ t a t e . ~This also shows a deficiency of iron electron density along the axis perpendicular to the molecular plane, with an excess density along the diagonals of the Fe-N(pyrro1e) directions. Magnetic circular dichroism spectra of [FePc] in dichlorobenzene could support a 3A, ground statew8and bands at 714nm and 800nm were assigned to metal-ligand charge transfer, although such spectra could be associated with protonated specie^?^^.^^' Nevertheless, it may be possible that the ground state is not the same in the solid as in poorly coordinating solvents. Iron(I1) phthalqanine is insoluble in most non-coordinating organic solvents but it does dissolve in very strong acids such as sulfuric and chlorosulfonic acids due to protonation of the basic bridging aza nitrogen a t o r n~ ! ~ ’ *Although ~~~ not soluble in hot hydrochloric acid, a reaction occurs which produces a dark material originally called953‘chloroferric phthalocyanine’ and formulated and even from gaseous hydrogen [CIFePc]. Since this material is formed in the absence of it appears likely that it is a hydrochloride of iron(I1) phthalocyanine (FePcH+Cl- ) in which a bridging aza nitrogen atom is protonated. There is, however, evidence956indicating that when the reaction with hydrochloric acid is carried out in air some iron(II1) species are formed, and it is possible that mixtures of protonated iron(I1) complexes together with p-oxo iron(II1) dimers can be formed (see below). In warm concentrated nitric acid the first formed, purple, cationic radical species degrade to phthalimide and ammonium ion (equation 83). Reaction of [FePc] with thionyl chloride produces [FePcCl,], as in equation (84), which is considered to contain iron(II1) and the oxidized macrocycle Pc-, with chloride ions in the axial p o sit i o n ~ .~ ’ ~The ,~ ~overall ’ oxidation level is confirmed by oxidative titration and the observed magnetic moment (3. I BM) indicates a low-spin S = $ state. Considerable care is needed to exclude
Tron(II) and Loww States
I262
c1 I
traces of water during the preparation of [FcPcCl,] to avoid formalion of the hydrochloride. However. once formed [FePcCI,] appears stable towards water?'h Although a-[EePc] does not re.act with nitric oxide, the fl form after a long period at room temperature forms [FePcNO], which is quite stable. 1:7.95$ [FePcNO] reacts with pyridine forming [FePcpy,] and when heated at 250 "C [FePc] is reformed. The IR spectrum 1737cm-') suggests it should be regarded as an Fe"'-NOcomplex. Addition of ligands in the axial positions of [FePc] takes place readily in solution to form six-coordinate low-spin complexes. Thus, [FePc] dissolves in pyridine to form a deep green complex [FePcpy,] that is easily isolated and has moderate solubility in sohents such as chloroform and t o l ~ e n e . ' Interesting ~ ~ . ~ ~ ~ one-dimensional polymeric bridged complexes (,131)result when bidentate In solution [FePcpy,] reacts with aromatic amines such as pyrazine or 4,4'-bipyridine are u~ed.'~':~'' C3.l to form [FePcpy(CO)],"' as shown in equation ( 8 9 , and diamagnetic red-violet crystals of [FePc(DMF')(CO)]are formed from phthalonitrile and [Fe(CO)5]at modcrate temperatures'" in DMF, as in equation (86). (viNo
[FePcpyll \Fe(CO),]
+
-
+
CO
--
4n-CxH,(CN),
+ py
(85)
[FePc(DMF)(CO)]
(86)
[FePcpy(CO)] DMP
[FePc(DMF)(CO)] is only stabie in air for a few minutes, but it is stable under an atmosphere of carbon monoxide. In a vacuum at 80 "C it decomposes to a mixture of 2- and B-[FePc]. Tn DMSO, [FePc(DMSO),] reactsYb5 with a large excess of carbon monoxide to form [FePc(DMSO)(CO)j at a rate given by kobs= k,[CO]+ k, with an equilibrium constant of 1 x 104dm3mol~' at 20 'C. Related monocarbonyl complexes with iV,N-diethylacetamide. pyridine, piperidine. hexamethylphosphoramide and Ph,PO trans ligands have been reported.966Exposure of [FePcj in a hydrocarbon solvent to carbon monoxide at high pressure affords [FePc(CO),]. Other mixed ligand complexes of the type [FePcLL'j containing a range of ligands have also been isolated or characterized in solution (Table 21)?67-g69The structure of [FePc(4-Mepy)?] has been determined970b X-ray methods and an FcPN(4-Mepy) distance of 2.00 A reported. A similar bond length of 1.94 was derivcd for the axial Fc---N in [FcPc(BuNH,),] from thc induced shift of NMR proton resonances.g" These structural determinations are in agreement with Mossbauer spectra of [EePcL,] Relatively large quadrupole splitting (Table 22) is in accord with a tetragonal structure with the in-plane ligand field being greater than the axial field. Moreover, with acceptor ligands the TC interaction is generally only weak. In the essentially diamagnetic [FePc(DMSO),] (0.74 BM) the iron is coordinated to the suIfur atoms of the axial DMSO ligands.975The in-plane Fe--N bond distances are similar to those in [FePc(4-Mepy)J and [FePc] itself, while the Fe-S distance of 2.3 1 A suggests there is very little back-donation. The thermodynamics and mechanisms of axial ligand exchange in [FePcLL'] complexes shown in equations (87) and (88) have been extensively studied using visible spectroscopy. For L' = substituted pyridines (L = DMSO) the binding strength order is 4-NH2py > 4-Mepy > pyridine > 4MeCOpy > 4-CNpy, which follows the Lewis basicity of the ligands and highlights the importance of u bonding.97hWith nitrogen ligands of different types the order is imidazole > piperidine > pyridine indicating that n bonding must be important with the relatively poorly basic imidazole. But
-
K
Iron(l1) and Lower States
1263
Table 21 Selected Complexes of the Type [FePcLL] Characterized in the Solid State or in Solution
L
L' ~
hid 4(5)-Me imid I-Me imid Bu"NH, PiP CM~PY DMSO Bu'NC Hex"NC PhNC 2,6-Me2C,H3NC PhCH,NC PhCH, NC PhCH,NC PhCH, NC PhCH,NC PhNO p(oph)3 p-MeC,H,NO C,H,,NC P(OEt), PBu,
co
co co
co co
~
Rej ~~
imid 4(5)-Me imid 1 -Meimid BunNH, PiP 4-MepY DMSO Bu'NC AexCNC PhNC 2,6-Me2C,H,NC PhCH,NC PhCH,NC PY
P'P
1-Meimid BunNH, BunNH, 1-Meimid 1-Meimid P(OEt), PBu,
DMF
PY
PIP Ph,PO
co
~
1 1 1 2
3 4 5
6 6
6 6 6 1 1 1 1
8 8 8 8 8 9 10 11 11 11 11
I. H.Giesemann, J . Prokr. Chem., 1956, 4, 169. 2. J. E. Maskasky, J. R. Mooney and M. E. Kenney, J . Am. Chem. SOC.,1972, 94, 2132. 3. D. W. Dale, R. J. P. Williams, P. R. Edwards and C. E. Johnson, Trans. Faraday SOC,1968, 64, 620. 4. T. Kohayashi, F. Kurokawa, T. Ashida, N. Uyeda and E. Suito, Chem. Commwn., 1971. 1631. 5. F. Calderazzo, F. Prtmpaloni, D. Vitali, I. Collanti, G. Dessy and V. Fares, J . Chem. Soc., Dalron Trans., 1980, 1965. 6. U. Keppelcr, W. Kohel, H. U. Siehl and M. Hanack, Cham. Ber., 1985, 118, 2095. 7. D. V. Stynes, J. Am. Chem. Soc.. 1974,96, 5943. 8. J. J. Watkins and A. L. Balch, Inorg. Cliem., 1975, 14,2720. 9. D. A. Sweigart, J . Chem. Soc.. Dalton Trans., 1976, 1476. 10. F. Calderazzo, D. Vitali, G. Psmpaloni and I. Collanati, J . Chem. SOC.,Chem. Commun., 1979, 221. 11. F. Calderazzo. S. Frediani. B. R. James. G. PamDaloni. K. J. Reimer, J. R. Sams, A. M. Sera and D. Vitali, Inorg. Chem.,
1982, 21, 2302.
with carbon monoxide, bonding is considerably weaker963than in iron(I1) porphyrin complexes where .n bonding is generally considered important. As expected, steric effects in [FePcLL'] complexes are important. Thus 2-methylpyridine and 2-methylimidazole only form very weakly bound complexes. This may indicate that displacement of the iron from the macrocycle core is more difficult with phthalocyanine than porphyrins. The rates of ligand exchange reactions in [FePcLL'] are moderately fast and must be studied by rapid mixing techniques such as stopped These reactions (equations 84 and 90) are dissociative in nature involving the coordinatively unsaturated [FePcL]. As evidenced by the ratio of the rate constants k- I /k2the intermediate does discriminate between ligands, again suggesting the geometry to be square pyramidal with the iron centre in the macrocycle plane. Thus no spin change is required, which would be the case if the metal moved out of the plane. Depending on the relative values of the rate constants involved, this mechanism can result in reaction rates that are independent of the concentration of the incoming ligand [L], (k2[L] > k-,fL']) or first order in [ L ] (k-,[L'] > k,[L"j) and examples of both kinds of behaviour The rates of these reactions are significantly slower than the corresponding have been rep~rted.~'~-'~' reactions of porphyrin complexes, which may be due to a higher charge on the iron atom, or the phthalocyanine being more rigid than the porphyrin ring system. Reaction of [FePc] with sodium metal or dilithium benzophenone in THF affords reduced solvated species of the type M,[FePc].mTHF (n = 1 4 ) . Magnetic and ESR measurements
iron(1i) and Lower States
1264
Table 22 Mossbauer Parameters* for Iron(I1) Phthalocyanineand Selected Derivatives at Room and Liquid Nitrogen Temperatures
Complex [FePc]* *
IFePcpy, 1
Temp ("C)
AEp (mms-I)
6 (mms-')
Ref.
25 22 20 - 162 - 185 - 196 22
2.58 2.67 2.64 2.60 2.64 2.71
0.63 0.68 0.66 0.74 0.64 0.77 0.53 0.62 0.54
1 2 3 1 3 2 2 2 2 2 1 1 1
2.05 1.89
- 196
[FePcim,] Lit FePc] LiJFePc] Li,[FePc] Li, [FePc]
22
1.79 1.71 3.24 3.25 1.86 1.87 1.77 1.78 1.65 1.66
- 196 26 - 153 25 - 160 25 - 163 25 - 160
0.63 0.6 I
0.69 0.52 0.61 0.58 0.66 0.61 0.71
1 1 1 1
Relative to Na?[Fe(CN),NO].ZH,O
**For results at high temperatures see: J. Blomquist and L. C. Moberg, Phys. Scr., 1974,9,350; and for results at 4.2 K in applied magnetic fields see ref. 3 and B. W. Dab, R.I. P. Williams, P. R. Edwards and C. E. Johnson, J . Chem. Phys., 1968, 49, 3445; T. H. Moss and A. B. Robinson, Inorg. Chem., 1968, 7. 1692. 1. E. Fluck and R. Taube, Develop. Appl. Spec.. 1970,8, 244, 2. A. Hudson and H. J. Whirfield, fnorg. Chem.. 1947, 6, 1120. 3. I. Dezsi, A. Balms, B. Molnar, V. D. Gorobchenko and I. I. Lukashevich, J. Inorg. Nucl. Chem.. 1969,31, 1661.
[FePcL,] [FePcLL']
+
L
KI e
[FePcLL]
+L e K2
[FePcLL]
kl
e k-
[FePcL;]
+L +L
[FePcLl 4- L
I
[FePcLj
+
k2
L"
't_2
[FePcLL"]
s ~ g g e s t ~the ~ ~first . ~ ~formed ' product contains iron(1) and the more highly reduced compounds iron(0) with additional electrons being located on the macrocycle ligand. However, electronic spectra were subsequently interpretedgg3in terms of only iron@)centres being present. The reduced compounds undergo oxidative addition reactions with organic halides to produce interesting organometallic iron species982924' and this work has recently been r e v i e ~ e d . ' ~A recent electrochemical/ESR study of [FePcj indicates the first reduction product (purple) is a five-coordinate iron(1) species. The voltage between the first and second reduced species is significantly solvent dependent ( - 0.2 V in pyridine and 0.6V in dma), and again it is concluded the second product is an iron(1) complex of the reduced ligand Pc3-, but not all of the earlier observations are completely explained.2"
-
Q R
I
Iron(I1) and Lower States
1265
Among the water soluble derivatives of [FePc] the tetrasulfonated complex (132; R = SO;) is the most studied although many aspects of its chemistry are not completely resolved. Direct sulfonation of [FePc] is not clean?*' and it is better to start with Csulfophthalic acid in a cyclization procedure. Weber and B ~ s c hformulated ~ ~ ~ their product obtained in this way as the iron(I1) complex Na[FePc(SO,H), (SO;)]-3H,0, but other workers favour a mixture of iron(II1) complexe~,9~~ Na,[FePc(SO; )4].7H20 and Na4[FePc(SO;),(OH)].2H20.In solution a magnetic moment of 1.8 BM was reported and that of the solid 3.0 BM.996In solution, as with other phthalocyanines,9" dimerization takes place, but here perhaps involving p-oxo Fe"' dimers. When Fez+ is added to a neutral solution of the tetrasulfonated metal-free ligand {H,Pc(SO, HI43 a green solution is formed989that has a visible spectrum characteristic of a first row transition metal complex (A, 671 nm), with a strong absorption at 425 nm similar to well-characterized [FePcL,] complexes. At pH = 4.5 this reacts with oxygen according to the 1:2 (0, to Fe) stoichiometry shown in equations (91) and (92). New bands at 637 nm and 344 nm due to the oxygenated complex disappear when the solution is purged with nitrogen at 70°C. These resultsge9favour the participation of an iron(I1) rather than an iron(II1) complex and it is of interest that it is possible to obtain reconstituted haemoglobins with tetrasulfonated iron phthalocyanine in the place of haem,Pg0and furthermore that these complexes reversibly combine with oxygen. Continuous UV irradiation of sulfonated iron phthalocyanine in solution causes low quantum yield degradation of the macrocycle with liberation of Fe3+,but in the presence of 2-propanol iron(1) species result.99' [F~Pc(SO;),(H,O),]~[FePc(SO;
)4
+
O2
(H2OXO2)I4-
e [F~PC(SO;),(H~O)(O~)]~+ HzO
+ [FePc(SO;
)4
[(SO; ),PcFe(H, O)(O,)(H, O)FePc(SO;
(91)
(H2 OM4- S
+ H, 0
>,I8-
(92)
The nature of products resulting from the reaction of oxygen with [FePc] itself has only recently been clarified. In 1965 Weber and Busch reported that the solid sulfonated complex reacts reversibly with oxygen?E6The rigorously dried complex in the absence of oxygen displayed a magnetic moment of 4.80BM, but only 3.08 BM in air. In contrast to the reversible oxygen carrying behaviour in solution discussed Weber and Busch found the complex was oxidized in aqueous solut i ~ n Reexamination . ~ ~ ~ of this system also led to the conclusion that irreversible oxidation takes place?92While very dilute solutions ( lo-' M) of [FePc] in DMSO slowly fade at a rate proportional to the concentration of oxygen,977*978 more concentrated solutions form a precipitatew3on standing which was subsequently identified as a Fe"* p-oxo dimer.w4In sulfuric acid, a kinetic study showedw5 the initial reaction of [FePc] with oxygen is reversible and the species formed decomposes irreversibly with formation of Fe'+ and oxidation of the macrocycle. Two crystalline forms of the p o x 0 dimer have been claimed on the basis of X-ray powder patterns and magnetic properties?96but the results of Mossbauer experiments indicated997that one of these is in fact a mixture of two species -perhaps a small amount of the p-oxo dimer with an unidentified low-spin iron(I1) complex. The quadrupole splitting for [(FePc),O] is the smallest so far reported for a p-oxo-bridged iron dimeric species. ESR data have also been interpreted998in terms of only one species. However, other evidence has very recently been presented999for the presence of two compounds. Preparatively, solid [(FePc),O] is best obtained by the interaction of oxygen with [FePc] suspended in DMF or other solvents. The soluble substituted complex [FePcRJ (R = dodecylsulfonamide)forms [(FePcR,),O], which when irradiated with visible light is photoreduced to the iron(I1) complex.'m Reaction with bases such as imidazole similarly causes reduction, and bis-base iron complexes are formed.1000 [(FePc),O] itself oxidizes PPh, to OPPh, in the presence of pyridine under mild conditions100'as in equation (93), and [FePcj is a catalyst for PPh, oxidation at room temperature if high pressures of oxygen are used.'"' ry
[(FePc),O]
+
PPh3 PY. 2[FePcpy2]
+
OPPh)
(93)
Reaction of [(FePc), 01 or [FePc] with NaN, in 1-chloronaphthalene affordsIm3the highly insoluble, but stable, nitrido-bridged complex [(FePc),Nj, which is more resistant to reduction than [(FePc), 01. Oxidation of [(FePc),Nl by strong oxidizing agents causes one-electron oxidation, and a PF; salt can be crystallized in the presence of pyridine: [(pyFePc)2M'PF;I The electrochemistry of both [(FePc),O] and [Fe(Pc),N] in pyridine has been examined in considerable detai1.Iw A range of six-coordinate iron(Ii1) phthalocyanine complexes of the type [FePcL,]- (L = OH-, OPh-, NCO-, NCS-, N;, CN-) have been r e p ~ r t e d ' ~ *that ' ~ ~are low-spin with magnetic moments 2.05-2.49 BM. With halide and oxygen donor ligands five-coordinate complexes are formed [FePcX]
I Iron ( ] I ) and Lower States
1266
(X = Cl, Br, I, RCO,, RSO,) that have magnetic moments 3.9-4.53 BM indicative of S = 3 or mixed spin states ( S = t , 5). Work on iron(I1I) phthalocyanine complexes has been hampered by their intractable nature and the use of derivatives soluble in organic solvents is likely to facilitate characterization. Recent examples of such ligands include a phthalocyanine having a long chain ester substituent (132) with R = O,C(CH,),Me. On the basis of magnetic (4.4BM in CH,Cl, and 5.OBM in CHC1,) and ESR results, the iron complex is assigned the + 3 oxidation state. The mono-imidazole complex is also high-spin, but the bis-imidazole species is low spin. The formation constants for this process have K , K2.1008.1a09 The iron(II1) complex of (132) (R = O,C(CW,),Me) and related phthalocyanines appears'o1oto coexist in high- and low-spin forms. Much needed X-ray structural data on iron(II1) phthalocyanine complexes is now becoming available, for instance the structure of [FePc(CN),]- has been reported."" Others are the products of [FePc] oxidation by iodine in hot chloronaphthalene: [FePcT]and a dimer [FePcCl],I,. The former is a one dimensional molecular conductor that is best formdated as [FePc]Il3+$(I;) in which there are I; chains. The dimer has a structure in which the chlorine atoms have been abstracted from the solvent. In this five-coordinate iron(II1) complex, the iron atom is 0.3 A above the plane of the inner nitrogens and the phenylene rings are slightly bent away from the iron
+
44.1 S.12.5
Porphyrins
The iron porphyrins and related compounds constitute an extremely important class of coordination complex due to their chemical behaviour and involvement in a number of vital biological systems. Over recent years a vast amount of work on them has been published. Chapter 21.1 deals with the general coordination chemistry of metal porphyrins, hydroporphyrins, azaporphyrins, phthalocyanines, corroles, and corrins. Low oxidation state iron porphyrin complexes are discussed in Section 44.1.4.5 and those containing nitric oxide in Section 44.1.4.7, while a later section in this chapter (44.2.9.2) is mainly concerned with iron(II1) and higher oxidation state porphyrin complexes. Inevitably however, a considerable amount of information on iron(I1) complexes is contained in that section as well as in Chapter 21. I. Therefore in order to prevent excessive duplication, the present section is restricted to highlighting some of the more important aspects of the coordination chemistry of the iron(I1) porphyrins while the related unusually stable phthalocyanine complexes are discussed in the previous section. Much of the older work (including some useful practical procedures) concerned with natural iron porphyrins is contained in Falk's classic book 'Porphyrins and Metalloporphyrins', which was published in 1964.'013gubsequently an enlarged multiauthor edition appearedI0l4 that covered the literature to the end of 1974. Since then a number of reviews concerned with iron porphyrins have been published. '"15-1022 The most important recent books are the seven volumes entitled 'The Porphyrins' edited by Dolphin,"" and 'Iron Porphyrins' edited by Lever and Gray,l*16 while Berezin's is a useful guide to some of the less familiar Russian literature. The most widely studied natural complexes are those of protoporphyrin IXwhose chloro iron(II1) complex is readily available. This is often used as its dimethyl ester which is also commercially available and is known as hemin ester (133) and which is soluble in organic solvents. However, this and related compounds suffer irreversible photodegradation in the presence of oxygen, while water soluble derivatives are susceptible to acid-catalyzed hydration of the reactive vinyl substituents (the so-called mesoporphyrin has ethyl groups instead of vinyl groups). Moreover, until comparatively recently the exact nature of the species present in aqueous solutions was often poorly defined. Therefore the use of more stable and better characterized synthetic porphyrins in nonaqueous media has usually been preferred. The iron complexes of octaethylporphyrin (134) (OEP), 5,10,15,20tetraphenylporphyrin (135) (TPP)and its derivatives are among the most widely studied, with TPP complexes being the most popular. A further problem with many metalloporphyrins is that they dimerize or aggregate in solution. The extent and type of aggregation depend on several parameters, but in general TPP complexes do not show these effects as markedly as natural porphyrins.'023Purple H,TPP and its derivatives are prepared'024by the often vigorous reaction of pyrrole with benzaldehyde or another aromatic aldehyde. Although simple to perform, these reactions afford only moderate yields of product that may be freed from chlorin by chromatography on alumina. It is usual to insert iron(II1) into the preformed macrocycle using acetic acid or DMF as solvent. Reduction of [XFeTPP] (typical X = C1) by chromium(II),'025borohydride"'' etc. gives the iron(I1) from the free ligand via reaction with freshly prepared iron(1I) complex. This can also be obtained1a27 propionate in propionic acid. Solid [FeTPP] is best purified by vacuum sublimation, which gives lustrous dark blue crystals; in noncoordinating solvent it is air sensitive.'028The solid-state
I
Iron(l1) and Lower States
1267
Ph (133)
(135)
(134)
structure'029of [FeTPP] crystallized from benzenelethano1 has Fe-N bond lengths (1.97 A) characteristic of an intermediate S = 1 spin state that is consistent with a room temperature magnetic moment of 4.4 BM and low temperature Mossbauer data obtained in large magnetic fields. The structure has a slightly ruffled core with the phenyl rings rotated 10 from being perpendicular to the porphyrin core. Other iron(l1) four-coordinate porphyrins also have S = 1. The magnetic behaviour of readily purified [FeTPP] has been thoroughly investigated and this work has recently been re~iewed."~'The structure of iron(I1) octaethylporphyrin has been rep~rted"~' and compared with that of the corresponding chlorin. Both, like [FeTPP], have intermediate S = 1 spin states. Overall, however, there is surprisingly little published magnetic data for iron(I1) porphyrins compared with that available for the iron(I11) complexes. Like related complexes, [FeTPP] reacts with many nitrogen bases to give low-spin bis-base species. The stability of these complexes is such that even refluxing iron(II1) [ClFeTPP] in piperidine affords'n32[Fe(TPP)Cpip),] via a poorly understood process which may invoke an axial radical ligand.'033,'034 In the presence of pyridine addition of OH- also induces r e d ~ c t i o n . "This ~ ~ may involve addition of OH- to coordinated pyridine.'n36The structure of [Fe(TPP)(pip),] has been determinedIo3' by X-ray methods and the rather long Fe-N(pip) bond distance of 2.127 A attributed to steric interaction involving the piperidine NH. With bases having even stronger steric effects, such as 2-methylimidazole, mono-base high-spin five-coordinate complexes result'038that in this instance have a significantly shorter Fe-N(base) distance of 2.014w. In this complex the iron centre is displaced from the porphyrin core by about 0.5 A, and there is also substantial doming of the porphyrin core it~e1f.I"~~ The molecular packing of [Fe(TPP)(ZMeimid)] has been examined in considerable detailIw0The iron is similarly displaced from the macrocycle in [(ON)Fe(TPP)] (about 0.2 A), which has a bent NO and is best described as a low-spin iron(l1) c ~ m p l e x . ' ~ ~Nitric ' ~ ' " ~oxide complexes of this type are capable of binding'o43an axial nitrogen or sulfur ligand so becoming six-coordinate. The Mossbauer spectra of several five-coordinate protoporphyrin IX iron(I1) complexes in frozen aqueous solutions displayIw large quadrupole splittings (4.0 to 4.4mm s-I). The largest value is with acetone, but this complex is incompletely formed. The splitting order for the more stable complexes is F- > catechol < phenol. In dichloroethylene the formation constant for the reactionlo4' of F- with [Fe(TPP)] is 600 M-' . In organic solvents such as chloroform and benzene, iron(1I) porphyrins are free of axial ligands and there have been several studies concerned with the thermodynamics of binding axial nitrogen bases, both in non-coordinating and donor solvents that are ligand replacement reactions (equations 94 and 95). The results of some early concerned with measuring stability constants for these reactions may be in error due to incomplete reduction of iron(JT1) by hydrazine monohydrate. The successive equilibrium con1.5 x I03M ' and s t a n t ~ ' " ~for binding pyridine to [Fe(TPP)] in benzene are Kl K2 1.9 x 104M-' respectively. The larger value for K, is in keeping with the monopyridine complex being high-spin and the extra stability of the final product being associated with a transition to the low-spin state. In this instance the monopyridine intermediate is five-coordinate and may have a structure similar to that of [Fe(TPP)(2-Meimid)]with the iron centre displaced from the porphyrin plane. The redox self exchange rates for [Fe(por)B,] complexes are rapid and rate constants for this process {Fe"'/Fe*'} do not vary in a regular fashion with steric hindrance on the periphery of the it is t h o ~ g h t ' ~ ~electron ~ , ' " ~ transfer takes place via the edge of the macrom a c r ~ c y c l e ,although '~~~
-
-
O
-
-
-
[Fe(por)] + B [Fe(pot)Bl C.O.C. 4--00
+
B
-
-+
[Fe(por)B]
(94)
[Fe(por)B,]
(95)
I Iron(U) and Lower States
1268
cycle in natural systems such as cytochromes.'O'O~'O'' The rates of thermal ligand substitution reaction are within the stopped flow range'053but photodissociation is extremely rapid and it appears only one ligand is released on excitation of the six-coordinate complex,'054in the primary process. The overall recombination kinetics can be complex due to spontaneous dissociation of the first formed five-coordinate complex. Cyanide ion binds stronglyloS6 to iron(I1) porphyrins to give complexes typified by [Fe(por)(CN),I2- that are low-spin. In DMSO the intermediate complex [Fe(por)(CN)(DMSO)]appears relatively stable. The self exchange rate for [Fe(TPP)(CN),I2-'- in DMSO is 5.8 x IO7M - ' s - ' (27 "C) while cyanide exchange is relatively slow. The bis(isocyanide) complex [Fe(TPP)(Bu'NC),] has Fe-C bond distances of 1.90A and the Fe-C-N angle is significantly less than 180 ', which may result from packing effects.10sn Sulfur donors may also act as axial ligands; the X-ray structure analysis of the thioether complex [Fe(TPP)(THT),] shows an Fe-S distance of 2.338 A, very similar to that in related iron(II1) system^.^^^^,'^^ With weak field axial oxygen donor ligands iron(I1) porphyrins are usually high-spin, kinetically labile and susceptible to air oxidation (see below). In THF it appears [FeTPP] is mainly in the form of the five-coordinate species [Fe(TPP)(THF)].'O"' However, it is possible to crystallize [Fe(TPP)(THF),] and, as might be expected, it containsIM' substantially longer Fe-0 bonds (2.31 A) than do related low-spin complexes. Both bis-phosphite and bis-phosphine iron(I1) porphyrins are low-spin, and Mossbauer data for a series of these complexes have been interpretedIM2in terms of changes of the axial ligands causing substantial changes in the iron-porphyrin bonding. Thus the macrocycle ligand tends to act as an electron ink.'^^-'^^ Aliphatic hydroxylamines react with [Fe(TPP)CI] in the presence of pyridine to give'065the corresponding iron(I1) pyridine nitroso complex. This reaction is shown in equation (96) for isopropylhydroxylamine, and treatment of the product with additional pyridine liberates acetone oxime with formation of [Fe(TPP)(py),] as in equation (97). [Fe(TPP)Cl]
+
Me, CHNOH/py
[Fe(TPP)(py)(Me2CHNO)]
+ py
-
--*
[Fe(TPP)(py)(Me,CHNO)] [Fe(TPP)(pyhl
+ Me,CNOH
(996) 197)
Carbon monoxide reacts'066with [Fe(TPP)] in toluene to give apparently low-spin [Fe(TPP)CO]and [Fe(TPP)(CO),] with successive equilibrium constants of 6.6 x and 140 respectively at 20°C. Similar values have been reported for reaction of carbon monoxide with natural porph~ins.'ozs~'067~'M* Simple bis-nitrogen base iron(I1) porphyrins in noncoordinating solvents react with carbon monoxide as in equation (98) to give monocarbonyl complexes. The thermodynamics in detail as well as for more complex sterically and kinetics for por = TPP have been hindered porphyrins (see below). Considerable attention has been given to the reversible oxygenation of iron(I1) porphyrins due to the importance of haemoglobin and related natural syst e m ~ . ' ~(For ' ~ an excellent review of the non-haem oxygen carrier hemerythrin, see ref. 1071). The unliganded iron(I1kporphyrin complexes, in the absence of a reducing agent, react with oxygen to and intermediate form iron(II1) po x 0 dimers. The dimer [(FeTPP), 01 is now well chara~terized'~'~ adducts of the type [Fe(TPP)O,] have been observed'073using matrix isolation techniques. In the presence of excess nitrogen base there is no reaction with oxygen, but the simple bis complexes [Fe(por)L,] in dichloromethane at low temperatures (- 79 "C)undergo oxygenation according to equation (99) that is reversed when the solution is purged with nitrogen. Solvent effects are important in this process. Thus in contrast to the behaviour in dichloromethane, oxygenation of [Fe(TPP)py,] is not complete in toluene under 1 atm oxygen pressure.'064 [Fc(por)LJ f CO
-
[Fe(por)(CO)LI
+
L
(98)
The rate determining step in these reactions with oxygen or carbon monoxide is dissociative loss of nitrogenous base from [Fe(por)L, J to generate a five-coordinate intermediate that rapidly reacts with either oxygen or carbon monoxide to generate the product. The five-coordinate intermediate shows no marked kinetic preference for nitrogen base or oxygen, reflecting the high reactivity of this species. However, in the presence of oxygen and carbon monoxide the five-coordinate intermediate has it kinetic preference for oxygen, although this first formed species subsequently converts to the thermodynamically favoured carbonyl The displacement of oxygen from [Fe(TPP)(L)(O,)] is also diss~ciative''~~with the kinetic trans effect of L being pyridine > piperidine 9 1 -methylimida.zole. However, as solutions of the oxygenated complexes are
I
Iron (]I) and Lower States
r' .
1269
warmed to room temperature, oxidation rather than oxygenation becomes increasingly important. inhibits p-oxo Embedding the porphyrin in a polymer r n a t r i ~ ' ~or ~ ~attachment . ' ~ ~ ~ to a dimer formation by physical separation of the iron centres and can yield systems capable of reversibly binding oxygen. Steric protection of the oxygen binding site itself can also inhibit the reactions that result in formation of iron(II1) p-oxo dimers. Examples af such complexes are the 'capped' porphyrins (136),'07&'080'picket fence' porphyrins (137),Io8''basket handle' porphyrins (138)'082and the 'cyclophane hemes' (139)Ios3that have been extensively studied in connection with the biological iron(I1) oxygen carriers myogIobin and haemoglobin.
7" +
co
Me
R'
(138)
(139)
Reversible oxygenation of these complexes in solution usually requires the presence of an added axial ligand such as pyridine or 1-methylimidazolewhich coordinates to the unprotected face of the porphyrin, and so stops p-oxo-dimer formation taking place from this side. Thiolate can also fulfil this as it does in cytochrome P-450.This fifth ligand site can be conveniently occupied by a nitrogen donor covalently bound to the macrocycle so effectively making the ligand quinquedentate. Complexes of this type include the 'tail base' or 'chelate heme' (140),'088,'089while those that involve two links to the base forming a more rigid system are typified by the 'double strapped' (141)1090 and 'hanging base' (142) complexes.La9'-'093 Rigidity of the strapping groups can favour four- or five-coordination so eliminating the undesired oxidation reactions, but if too rigid, motion of the iron(I1) centre can be inhibited resulting in decreased affinity for molecular oxygen."
I
The oxygen-binding properties of model complexes are similar to those of myoglobiE and haemoglobin, although their affinity for carbon monoxide is much less than it is for most model complexes. It appear^'^^^.''^^ this is due to steric constaints in the natural oxygen carriers that demand a bent or tilted geometry of the diatomic ligand. This is satisfactory for the inherently benr Fe-0-0 group, whereas the linear Fe-CEO group is significantly destabilized. This 'distalside' stcric hindrancc involves imidazole and isopropyl groups in the globin chain. Similar effec.ts havc been observed with some sterically demanding model complexes.'UY7 The rates of reaction o f carbon monoxide with a range or five-coordinate iroii(l1) porphyrin species have""' reduced equilibriuru constants for systems with severe steric emects. This is caused by slower reaction wilh CO rather Than a change in the rate ol' dissociation Goni the six-coordinatc complex. However, a variety of quite subtle electronic and steric effects may be operative in these processes. including free movement o f the iroE (and if appropriate d0min.g or ruffling of the macrocycle) when forming the five-coordinate intermediate, polarity and specific interactions such as hydrogen bonding at or near the binding site, as well as the electronic behaviour of the porphyrin and the influence of solvation. The importance of polarity is demonstrated by an amide strapped complex that has an affinity for oxygen an order of magnitude greater than that of the ether analogue,i099presumzbly due to enhanced stabilization of the charged {Fer" ( 0 2 )} -contribution to the oxygenated species. However, care in interpretation in such siluations is needed because quite small steric changes can also be important. The nature of lhc frons axial base may also infl.uence the 0, affinity of iron(T1) porphyrins. Early report^^^'^"'^^ siiggested pyridine produces a weaker ironjO, interaction than does imidazole. This could he due to the decreased 71 basicity of pyridine compared with that of With various 'capped' porphyrins the reverse is trueL1"and it has been concluded"" that. generally, substitution of pyridine for imidazole does not have large effects on carbon monoxide affinities. However, the relative differences induced by changing the base differ""4 for binding CO and 0,. Clearly care is needed when trying to separate the importance of small electronic and steric changes in these systems. Often in these studies different solvents have been used. It has been shows"" for bath an unprotected and a bis-pocket porphyrin'Io6 whose 1,2-dimethylimidazole adduct has moderate affinity for reversible oxygenation, that solvent polarity enhances oxygen affinity but decreases carbon monoxide affinity. Here AG for oxygenation correlates well with empirical solvenr parameters such as 71". Earlier, it was noted'''7 that TC donor/acceptor interactions involving the macrocycle do not influence carbon inonoxide binding rates. As expected, the very large binding constants for nitric oxide to a series of iron(,TI) 'capped' porphyrins to form [I'c(por)(B)(NO)] (with bent Fe-N-0) correlate well with oxygenation data implying similar steric cf€ccts.'"* In thc absence of an axial base [Fe(por)(NO)]reacts with cxccss nitric oxide to give a producr formulatcd'i"9as [Fe(por)(YO I )(NO, )] as shown in eqilation (100). [Fe(por)(NO)]
- 3N0
+-
[Fe(por)(NO)(NO,)j
+
N,O
( 100)
Several iron porphyrin carbenes are known'"' that are obtained by the general reaction of the chloro iron(II1) complex with RCXj (X = Cl, Br) in the presence of iron powder or sodium dithionite as in equation (101). The air sensitivity of the product depends on the nature""."" of R. Formally these complexes could bc dcscribed as iron(l1) species. They are diamagnetic, and can bond an additional axial ligand. Thc structure of the dichlorocarbene complex [Fc(TPP)(CClZ)(H,0)]has been determined by X-ray methods,"" which showed a short Fe--C distance of 1.83A. This complex is reactive and, for instance, with primary amines a coordinated
Iron(II) and Lower States
1271
isocyanide results. A novel bridged, or ‘double carbene’ [{Fe(TPP)},C] has been obtained from the use of CBr, in the reductive procedure shown in equation (101). Alkyl radicals react with iron(I1) porphyrins to form [Fe(por)(R)]. Methyl radicals are captured rapidly, but chloromethyl radicals react slowly and it has been ~uggested”’~ this is a possible cause for the toxicity of chloromethyl compounds. Reaction of chloro iron(II1) complexes with Grignard reagents also affords alkyl derivatives. Chemical oxidation of the phenyl complex [Fe(por)(Ph)] causes migration of the aryl group onto a pyrrolic nitrogen to form an iron(II1) N-substituted complex. Reduction induces the reverse reaction, while the metal free N-substituted porphyrin is obtained on d e m e t a l l a t i ~ n . ” ~ ~ . ’ ’ ~ ~ These transformations may be viewed as initial oxidation of the metal centre t o Fe”, with the reverse reduction involving oxidative addition of NPh to an iron(1) centre. These and other reactions of an organometallic nature are the subject of a recent review.””
Acknowledgement
Thanks are due to all of those who provided reprints or preprints, and to J. Burgess and the late S. M. Nelson for information and helpful discussions, 44.1.6 I. 2.
3. 4. 5. 6.
7. 8.
9. 10.
11. 12. 13. 14. 15. 16. 17. 18.
19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.
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I
I274 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. I Xh.
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1276 313. 314. 315. 316. 317, 318. 319. 320. 321. 322. 323. 324. 325. 326. 327. 328. 329. 330. 331. 332. 333. 334. 335. 336. 337. 338. 339. 340. 341. 342. 343. 344. 345. 346. 347. 348. 349, 350. 351. 352. 353. 354. 355. 356. 357. 358. 359. 360. 361. 362. 363. 364. 365. 366. 367, 368. 369. 370. 371. 372. 373. 374. 375. 376. 377. 378. 379. 380. 381. 382. 383.
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384. 385. 386. 387. 388. 389. 390. 391. 392. 393. 394. 39s. 396. 397. 398. 399. 400. 401. 402. 403. 404. 405. 406. 407. 408. 409. 410.
I
4
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1278
Irort(II) and Lower States
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t
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1279
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iron(I1j und Lower States
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Subject Index Acetic acid, ethylenediaminetetra-, 253 Alkenes oxidation osmium tetroxide, 590 Alkynes reactions osmium tetroxide, 590 Amino acids reactions osmium tetroxide, 590 Analysis iron complexes, 1180 Aromatization iridium catalysts, 1160 Benzaldehyde, o-aminoself-condensation, 257 Bonds iron-germanium, 1189 iron-lead, 1189 iron-tin, 1189 rhodium-rhodium, 903 cleavage, 950 Catalases, 263,264 Catechol dioxygenases, 230
Catechols reactions osmium tetroxide, 593 Chloromethyl compounds toxicity, 1270 Cobalt complexes, 635-882 ADP, 760 amides, 682 arsenates, 774 arsenic ligands, 767-775 ATP, 760 bipyridyl, 69 1 bis(dithiolates), 876 carboxylates, 790 cyanates, 679 cyanides reduction, 646 disulfides, 829 l,Z-dithiolates, 871 1,3-dithiolenes, 880 nitriles, 674 nitrito, 666 nitro, 666 nitrosyl, 657 bonding, 663 reactivity, 664 structure, 663 synthesis, 657 oxygen ligands, 775-829 peptides, 682 peroxo, 775,784,785 phenanthroline, 691 phosphinates, 767 phosphine oxides, 767 phosphines, 701
dinitrogen, 709 hydrides, 704 nitrosyl, 71 1 phosphinites, 746 phosphites, 704,746,766 phosphoamidates, 766 phosphonates, 766 phosphonites, 746 phosphoramides, 767 phosphorus ligands, 699-767 properties, 637 pyrophosphates, 760 reactions, 637 selenates, 880 selenites, 880 selenium ligands, 880 selenocyanates, 679 selenophosphinates, 869 silyl, 655 structure, 637 sulfenates, 833 sulfides, 829,849 sulfinates, 833 sulfur ligands, 829-880 tram effect, 856 superoxo, 775,776,781 synthesis, 637 terpyridyl, 691 thiocyanates, 679 thiolates, 833 metal binding, 847 thionitro, 858 thionitrosyl, 858 thiophosphinates, 869 triphosphates, 760 tris(l,2-dithiolates), 876 ureas, 682 Cobalt(-I) complexes arsenic ligands, 769 bipyridyl, 691 cyanides, 646 phenanthroline, 691 phosphines bidentate, 728 monodentate, 718 multidentate, 738 tridentate, 738 phosphinites, 747 phosphites, 747 phosphonites, 747 terpyridyl, 691 Cobalt(0) complexes arsenic ligands, 769 bipyridyl, 691 carbon disulfide, 646 cyanides, 646 phenanthroline, 691 phosphines, 718 bidentate, 728 multidentatc, 738 tridentate, 738
1289
Subject Index
1290 phosphinites, 747 phosphites, 747 phosphonites, 747 terpyridyl, 691 Cobalt(I) complexes arsenic ligands, 769 bipyridyl, 691 carbon dioxide, 637 cyanides, 647 formato, 791 phenanthrolinc, 691 phosphines, 722 bidentate, 735 multidentate, 744 tridentate? 744 phosphinites. 747 phosphites, 747 phosphonites, 747 terpyridyl, 691 Cobalt(I1) complexes arsenic ligands, 769 bipyridyl, 691 carbonates, 811 cyanides, M8 dithiocarbonatcs 868 ethers, 828 imidazole, 693 phenanthroline, 691 phosphines, 724 bidentate, 736 multidentate, 745 tridentate, 745 phosphinites. 749 phosphites; 749 phosphonites. 749 purine: 683 pyrazine: 683 pyrazole, 693 pyridine, 683 selenenates, 881 seleninates, 881 selenolates, 881 selenophosphinates, 870 sulfides, 849 terpyridyll 691 tetrazole, 693 thiophosphinates. 870 thiosernicarbazones 857 triazine, 683 triazole, 693 trithiocarbonates. 868 Cobalt(II1) complexes alkenedithiolates, 869 alkyl monocarboxylates, 792 amides, 682 arsenic ligands, 773 aryl monocarboxylates, 792 bipyridyl, 692 carbonates, 811 carboxylates quadridentate, 803 quinquedentate. 808 sexidentate, 808 synthesis, 790 tridentate: 803 chlorites, 825 chromates, 825 citrates, 803 cyanates, 679 cyanides, 652 dicarboxylates, 800 disulfides, 855 1,l-dithio acids, 860 ~
~
dithiocarbamatesl 864 dilhiocarbimates, 869 dithiocarboxylates, 864 1,l-dithiolates, 860.868.869 formato, 791 hydroxycarboxylatesI 802 imidazole, 697 iodates, 825 iodine polyoxyanions: 826 isothiocyanates, 858 malatcs, 803 molybdates, 825 molybdenum polyoxyanions,826 niobium polyoxyanions, 826 nitriles, 674 orthophosphates, 750 halides, 753 oxyanions, 817 peptides, 682 perchlorates, 825 perrhenates, 825 phenanthroline?692 phosphates, 750 phosphines, 726 bidcntatc, 736 phosphinites, 749 phosphites, 749 phosphonites, 749 purine, 687 pyrazine, 687 pyrazole, 697 pyridine, 687 salicylates, 802 selenates, 817 selenenates, 881 seleninates, 881 selenites, 820 selenocyanates, 679 selenolates, 881 selenoph(isphinates. 871 sulfamides, 858 sulfates, 817 sulfenates, 83s mixcd bidentates, 83Y monodentate, 835 quadridentate, 847 sexidentate, 847 tridentate, 847 tris(bidentate), 837 sulfides, 849 bidentate, 850 multidentate, 852 sulfinates, 835 mixed bidentdtes, 839 monodentate, 835 quadridentate, 847 scxidentate, 847 tridentate, 847 tris(hidentate)>837 sulfites, 820 sulfonamides: 858 sulfones, 849 bidentate, 850 multidentate, 852 sulfoxides, 849 bidentate, 850 multidentate, 852 tartrates, 803 tclluratcs, 817 terpyridyl, 692 tetrazole, 697 thiocyanates, 679 thiolates, 835
Subject Index mixcd bidenlates, 839 monodcntatc, 835 quadridentate, 847 sexidentate, 847 tridentate, 847 tris(bidentate). 837 thiophosphinates, 871 thioselenocarbamatesI 864 thiosemicarbazides, 857 thiosulfates, 817 thioxanthates, 866 trialkylphosphine dithio acids, 867 triazine, 687 triazole, 697 tungstates, 825 tungsten polyoxyanions, 826 ureas, 682 xanthates, 866 Cyclophane hemes iron complexes, 1269 Cytochrome b , 263 Cytochrome c , 263 Cytochrome P-450,263,264,1269 Cytochromes, 259,263,1268 Decarbonylation iridium catalysts, ll@l Despujolsite, 104 Dienes reactions osmium tetroxide, 590 1,2-Dioxygenase, 232 Dirhenates, nonahalo-, 176 Dirhenium heptoxide, 197 Disproportionation iridium catalysts, 1159 Electron transport heme proteins. 263 Entcrobactin iron complcxcs. 230 Ferrates, 266,267 Ferrates, hexacyano-, 220, 1204 Ferredoxins, 237,238, 7240, 1241 2-iron, 235 Ferrichrome A, 233.234 Ferrichromes, 233 Ferrioxamines, 233 Ferroin, 1203 Ferroverdin, 1238 Glycols reactions osmium tetroxide, 59C Heme proteins, 262 electron transport, 263 Hemerythrin, 253 Hemin ester, 1266 Hemoglobin, 262? 1257,1269 artificial, 100 Hemoglobin, nitrosyl-. 1194 Hemoproteins, 259 High-potential proteins, 238 Horseradish peroxidase, 265 Hydrogenation cobalt hydrido phosphorus complexes, 707 iridium catalysts, 1158 Hydrogen transfer iridium catalysts, 116U Iridium complexes, 1097-1187
antimony ligands, 1164 arsenic ligands, 1'164 boron ligands, 1165 catalysts, 1158, 1160 halides, 1166 hydrides, 1166 nitrogen, 1161 nitrosyls, 1161 oxygen ligands, 1164 phosphorus ligands, 1162 selenium ligands, 1165 sulfur ligands, 3 165 Iridium( -I) complexes, 1099 nitrogen ligands, 1099 nitrosyls, 1099 phosphines, 1100 phosphorus ligands, 1100 Iridium(0) complexes, 11 00 aliphatic amines, 1100 nitrogen ligands, 1100 nirrosyls, 1100 phosphines, 1101 phosphorus ligands, 1101 Iridium(1) complexes, 1101-1120 amines, 1103 ammines, 1103 arsenic ligands, 1108 azides, 1105 bicarbonates, 1'1I6 carhon disulfide, 1117 carboxylates, 1116 catecholates, 1115 cyanides, 1101 diazo compounds, 1105 P-diketonates, 1115 dkitrogen, 1105 disulfur, 1117 fulminates, 1101 halides, 1119 hydrides, 1119 hydrogen, 1119 isocyanjdes, 1101 multidentate macrocyclic ligands. 1120 nitrogen ligands, 1103 nitrosyl catecholates, 1I16 nitrosyls, 1104 oxygen ligands, 1115 phosphine sulfides, 1118 phosphorus ligands, 1108 polypyrazolyl borates, 1108 porphyrins, 1120 selenocyanates, I105 semiquinones, 1115 sulfur dioxide, 1118 sulfur ligands, 11 17 tetrazcncs, 1105 thiocarbamates, 1117 tin compounds, 1103 triazenes, 1105 Vaska-type complexes electrochemical oxidation, 1109 oxidative addition, 1108 Iridium(I1) complexes, 1120 arsenic ligands, 1121 halides, 1123 mixed donor atom ligands, 1123 multidentate macrocyclic ligands, 1124 nitrogen ligands, 1120 nitrosyls, 1120 phosphines, 1121 phosphorus ligands, 1121 Schiff bases, 1124 sulfur ligands, 1121
1291
I
1292 Iridium(II1) complexes, 1124-1155 alkenic imines, 1153 alkyl isothiocyanates, 1154 alkylperoxy, 1142 amines, 1128 ammines, 1128 antimony ligands, 1134 areneselenols, 1148 arsenic ligands, 1134 aryl isothiocyanates, 1154 azides, 1133 benzofhJquinoline, 1154 benzylidenearnines, 1152 2,2'-bipyridyl, 1130 bromides, 1149 carbonates, 1141 carbon diselenides, 1148 carbon disulfides, 1145 carboxylates, 1141 catecholates, 1140 chlorides, 1149 cyanides, 1125 diazenato, 1132 diazo compounds, 1132 b-diketonates, 1140 dioxygen, 1138 dithiocarbamates, 1145 fluorides, 1148 fulminates, 1125 germanium compounds, 1126 hydrides, 1149 hydrogen, 1149 hydroxy, 1138 iodides, 1149 isocyanides. 1125 mercury compounds, 1124 mixed donor atoms, 1152 multidentate macrocyclic ligands, 1155 nitrates, 1143 nitritenitro, 1143 nitrogen heterocycles, 1130 nitrogen ligands, 1128 actacthylporphyrin, 1155 oxalato, 1142 oxygen ligands, 2138 peroxocarbonates, 1141 peroxycarboxylates, 1141 1,lO-phenanthroline, 1130 phenyldiimides, 1153 phosphines, 1134 phosphorus ligands, 1134 polypyrazolylborates, 1134 selenium ligands, 1147 selenolato, 1148 semiquinones, 1140 silicon compounds, 1126 sulfenato, 1146 sulfides, 1145 sulfmato, 1146 sulfur ligands, 1145 tetrazenes, 1153 tin compounds, 1126 Iridium(1V) complexes, 1155 arsenic ligands, 1155 bromides, 1157 chlorides, 1157 halides, 1157 hydroxy, 1156 nitrates, 1156 oxalates, I156 oxygen ligands. 1156 phosphorus ligands, 1155 sulfur ligands, 1157
Subject Index Kridium(V) complexes, 1158 fluorides, 1158 Iridium(V1) complexes, I158 Iron complexes acetonitrile, 1210 analysis, 1180 biological systems, 1180 coordination geometries, 1183 coordination numbers, 1182-1187 dinitrosyldicarbonyl, 1188 Mossbauer spectroscopy, 1181 nitric oxide, 1187-1195 nitrosyls binary, 1188 bis( dithiolene), 1193 carbonyl, 1188 dithiocarbamateb, 1192 halides, 1193 iodide, 1193 polydentate ligands. 1194 tetraphenylporphyrin, 1194 oxidation states, 1182-1187 pyridine, 1213 tricarbonylnitrosyl anion, 1188 Iron(-11) complexes, 1184 Iron(-I) complexes, 1184 Iron(0) complexes, 1184,1195 2,2'-bipyridine, 1195 1,2-dimethylphosphinoethane,1197 nitrogen ligands, 1195 phosphites, 1197 phosphorus ligands, 1196 trifluorophosphine 1197 triphenylphosphine, 1196 Iron(1) complexes, 1184,1197-1203 alkyl, 1202 arsines, 1197 aryl, 1202 1,2-bis(diphenylphasphino)ethane, 1198, 1199 carbenes ESR, 1200 carbon monoxide, 1197 cyanide, 1201 macrocyclic ligands, 1201 nitric oxide, 1203 phosphorus ligands, 1198 phthalocyanine, 1201 tetraphenylporphyrin. 1202 3,10,17,24-tetrasulfonatophthalocyanine,1202 tricarbonylbis(tripheny1phosphine) oxidative substitution, 1245 Iron(I1) complexes, 1179-1270 acetonitrile Mossbauer spectra, 1255 acetylacetonate, 1236 a-alanine, 1226 alkyl cyanides, 1209 amines, 1210 aromatic, 1210 hexamine, 1210 monodentate aromatic, 1212 polyethylenearnine, 1211 pyridine, 1211 amino acids, 1238 o-aminobenzenethiol, 1247 2-aminomethylpyridine, 1226 6-amino-2-methylquinoline, 1213 amminepentacyanohydrolysis. 1206 arsenic ligands, 1233 monodentate, 1233 polydentate, 1234
Subject Index benzimidazole, 1228,1229 a',a'-bipiperidinc, 1223,1225 bipyridine, 12151222 cyanide, 1219 Mossbauer spectra, 1221 spectra, 1215 thermodynamic properties, 1220 bis(2,2'-bipyridine), 1219 1,2-bisdiethylphosphinoethane, 1234 bis(2-dimethyIaminoethyI)methylamine, 1211 2,6-bis(2-diphenylphosphinoethyl)pyridine,1235 bis(ethy1 dithiocarbonate), 1246 bis(methylimidawle), 1231 bis(1,lO-phenanthroline), 1219 bis(thiosalicylideneiminato), 1248 bromide, 1250 Curtis ligands, 1252 capped porphyrins nitric oxide binding, 1270 chloride, 1249 citrate, 1237 cubanes, 1242 cyanide, 1204-1209 3-cyanopyridine, 1213 4-cyanopyridine, 1213 2-cyanopyridine N-oxide, 1237 3-cyanopyridine N-oxide ,1237 4-cyanopyridine N-oxide, 1237 cyclam, 1250 1,2-~yclohexanedionedioxime, 1231,1232 cysteine, 1248 2,6-diacetylpyridinebis(semicarbazone),1238 dibenzothiazole, 1229 3,5-dichloropyridine, 1213 diimines, 1214-1230 formation constant, 1216 P-diketonates, 1236 a-diketone dihydrazones, 1224 1,2-dimethoxyeihane, I238 l,l'-dimethyl-4,4'-bipyridine, 1247 5,5'-dimethyl-2,2'-bipyridine,1221 3,S-dimethyl-l,2-ditbiolinium, 1246 dimethylglyoxlme, 1231 1,2-dimethylimidazole, 1213 2,7-dimethyl-1 $-naphthyridine, 1211 2,9-dimethyl-l,lO-phenanthroline, 1214
4,7-dimethyl-l,lO-phenanthroline spectra, 1220 2,6-dimethylpyrazine, 1214 3,6-dioxaoctane-1 &diamine, 1260 1,4-dioxane, 1238 dioxines, 1222, 1231
4,7-diphenyi-l,l0-phenanthroline sulfonated, 1217 2,2'-dipyridylmethane, 1226 dithio, 1245,1246 dithioacetylacetonates, 1245 dithiocarbamates, 1245 dithiolenes, 1245, 1246 1,2-dithiolinium, 1246 ethylenediamine, 1211 oxidation, 1226 fluoride, 1249 geometry, 1184 halides, 1249 hexacyano, 1204 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, 1251 hydrazine, 1231 imidazole, 1213 imidazoline, 1229 iodide, 1250 iron(1lI) complexes redox behaviour, 1185
1293
isocyanide, 1208 alkylation, 1209 aromatic, 1209 transalkylation, 1208 isoquinoline, 1213 macrocyclic ligands loss, activation parameters, 1259 methionine, 1238 2-methylimidazole, 1213 2-methyl-l,6-naphthyridine,1211 2-methyl-1 ,lO-phenanthroline, 1215 mixed donor ligands oxygen containing, 1236 mixed iron(II1) complexes, 1187 mixed 0 , N donors tridentate, 1238 mixed S,N donors, 1247 monodentate aromatic nitrogen donors, 1214 monothiocarbamates, 1245 1$-naphthyridine, 1211 N donor macrocycles, 12561270 saturated, 1250 nitrosyl
meso-2,3,7,8,12,13,27,28-octaethyl-5nitroporphyrinato, 1194 oxygen donors, 1235 partially unsaturated N donor macrocycles, 1252-1260 pentacyano, 1205 photolysis, 1206 reactions, 1205 1.lo-phenanthroline, 1215-1222,1229 cyanide, 1219 formation constant, 1215 spectra, 1215,1220, 1221 thermodynamic properties, 1217 1,lO-phenanthroline N-oxide, 1238 phenols, 1242 o-phenylenebis( dimethylarsine), 1234 phosphines, 1233 carbon monoxide reactions, 1233 phosphites, 1233 phosphorus donor, 1233 monodentate, 1233 polydcntate, 1234 phthalocyanine, 12661266 Mdssbauer spectra, 1264 P-picoline, 1213 y-picoline, 1213 porphyrins, 1203,1266 bis( phosphine), 1268 bis(phosphite), 1268 carbenes, 1270 dichlorocarbenes, 1270 double carbenes, 1270 reversible oxidation, 1268 propylenediamine, 1211 pyrazole, 1213 pyrazoledione oxide, 1237 pyrazole-4,S-dione oximes, 1232 pyridine, 1212 pyridine-2-carbaldehyde 2-pyridylhydrazone, 1225 pyridine macrocycles seven-coordination, 1258 pyridinenitroso, 1268 pyridine N-oxide, 1237 pyridylimidazole, '1228 quinoline, 1213 Schiff bases, 1233,1238,1247 polydentate, 1225 strati-Schiff bases, 1236 sulfidehitrosyl, 1243 suifides
1294
Subject Index
binary, 1240 sulfur donors, 1238-1248 terpyridine, 1222, 1229 1,4.8,1I-tetraazacyclotetradecane, 1251 tetraazamacrocyclic, 1257 tetraazatetramine. 1254 tetraazatriirnine. 1254 tetraethylenepentamine, 1211 tetrahydrofuran, 1238 tetraphenylporphyrin, 1258 2-thiazole. 1229 thiocyanate, 1247 thiols. 1241 thioseiiiicarbazides, 1248 thioxanthates, 1246 3,3‘,3”-lriaminotripropylamine, 1211 1,2,4-tnazine, 1230 triazole. 1213 trimethylarsine sulfide, 1240 tripyridyl-s-triazine. 1229 tris-u,a’-diimines reduction, 1196 tris(2,2’-bipyridine), 1216 pyrolysis. 1220 tris(ethy1 dithiocarbonate), 1246 tris(1 10-phcnanthroline), 1216 t.risulfides, 1243 I .4,7-t rithiacycloiionane 1239 vanillin lhiosemicarbazotie, 1248 violurate, 1238 water, ,1236 xanthates, 1246 o-xylyldithiolate, 1240 Iron(II1) complexes, 217-267 alcoholates, 227 amides, 227 amino acids, 253 aminocarhoxylates, 253 aqua, 226 arsenic ligands. 226 arsine oxide, 228 azido, 222 bcnzohydroxamato, 234 2,2‘-bi-2-imidazoliiic, 224 2,2’-bipyridine. 223 N,N’-his(2-pyridylmrthyl)ethylent.dianline, 225 bis( triinethylsilgl)ainine, 222 2,2‘-bi-1,213.4-tetrahydropyrimidine, 224 carbon ligands. 220 carboxylates, 728 catecholates. 230 catechols, 230 chlorides, 247 clusters, 229 colour. 219 cyanides, 220 1,2-diamiaoethane, 223 1,l-dicyanocrhylene-2,2-dithiol~tc,245 dikctones. 229 di-2-pyridylamineI 225 diselenocarbarnates, 243 dithioacetylacetates, 246 dithiocarbonates, 243 dithio-p-diketones, 1246 dithiooxalates, 246 dithiophosphinates, 243 1,2-ethanedioll 1241 ethers, 227 fluorides, 247 halides, 247 hydrolysis. 218 hydrolytic polymerization, 228 hydroxamates, 230 I
hydroxamic acids- 233 iron(I1) complexes redox behaviox, 1185 iron-sulfur cluster compounds, 234 macrocyclic ligands, 255-265 magnetic mom+, 21s 2-methyl-8-quinolino1 electrochemistry, 233 mixed iron(I1) complexes, 1187 nitrogen ligands, 222-226 Mossbauer spectra, 223 oxygen ligands, 226-234 1,10-phen;inthroline, 223 phenols, ’230 N , W ( 1,2-phenylene)his(salicylidenimatil),232 phosphine N-oxides, 228 phosphorus ligands, 226 pyridine, 222 pyridine N-oxide. 227 2-(2’-pyridyl)imidaole, 225 8-quinolinol electrochemistry, 233 quinones, 230 Schiff bases, 226,219 spin-crossover, 252 salicylaldiminc, 249 serniquinones, 232 sulfido, 234 sulfoxides, 227 sulfur ligands, 234-247 tcrpyridyl, 223,225 1,4,8,11-tetraazacyclotetradecane,256 2,2‘:6’,2”:6”,2”‘-tetrapyndyl, 225 thiocarbamates, 247 thiohydroxarnic acids. 233.234 thiolato, 234 thioxanthates, 243 N,N‘,W‘-triacetylfusarinine,234 tri-2-pyridylamitit:. 225 1,3,5-tris(2,3-dihydro~yb~n~oylaminomethyl)ben~e~~~ 231 xanthates, 243 Iron(lV) complexes. 266 1 I87 Iron(V) complexes, 266, ‘I 187 Iron(V1) complcxes, 266, 1187 Iron compounds Iron proteins non-heme, 234 Iron-sulfur cluster compounds, 1203 dinuclear, 235 electrochemistry 236 hexanuclear, 241 mononuclear, 235 tetranuclear, 238 trinuclear, 237 Isomerization iridium catalysts, I158 ~
Jouravskitc, 104 Lactoferrin. 231 Manganates, hexachloro-. 58 Manganates, tetraalkyl-. 13 Manganates, tetrahalo-. 59 Manganates, tetraoxo-. 109 Manganates, trihalo-. 59 Manganese complexes. 1-11 1 carbonyl-nitrosyl, 7 cyanc+nitrasyl. 8 electronic spectra, 4 lability, 6 magnetic moments, 4
Subject Index nitrogen uses, 30 NMR, 6 oxidation states, 7 Manganese( -I) complexes, 7 Manganese(0) complexes, 7 cyanides, 7 monocyano nitrosyl-carbonyls, 8 nitrosyls, 7 Manganese(1) complexes, &9 cyanides, 8 cyano-carbonyls, 8 dinitrosyl, 8 tetrahydroalaiie, 9 Mangaoese(1l) complexes, 3, (+82 acctylacetonates, 48 acetylides, 14 alcohols, 37 alkoxides, 37 alkyl phosphines. 13 alkyls, 12 amides, 15, 16 amine oxides, 39 amines, 16 amino acids, 43,62 sulfur containing, 70 ammines, 15 antimony ligands, 3'1-34 arsenates, 45,56 arsenic ligands, 31-34 arsenic oxides. 39 aryls, 12, 14 azides, 22 binary alkyls, 13 bipyridyl, 24,25 anions, 25 2,2'-bipyridyl dioxide. 51 biquinolyl , 2 4 bis(2-aminoethylamine) 26 bismuth ligands, 31-31 his(2-pyridylinethyl)amine. 26 bis(salicylidcne)ethyIenediamine,66 biuret, 62 borates, 41,42 bromides, 58,59 4-butyllactarn,39 carbonates, 41.42 carbonyl compounds, 38,39 carboxylates, 43 catechol, 48 chlorides, 56,57 chlorophosphates, 45 coordination geometries, 10 coordination polyhedra, 10 crown ethers. 52,80 cyanates, 22 cyanides, 11 cyano-nitrosvls. 12 cytosinc, 61,62 1,2-diaminoethane, 23 dicarhoxylates, 49 diethers, 47 diethylurea, 39 diketones, 48.49 1,2-dimethoxyethane,47 dimethylacetamide 39 dimethylurea, 39 diols, 47 dioxane, 38, 47 dioxygen, 41,73 diphenols, 47 dithiocacodyla tes 54 dithiocarbarnatcs, 53 I
lJ-dithioethane, 53 1,l-dithiolates, 53,54 1,2-dithiolatcs, 53 dithiolenes, 54 dithiooxalates, 51 dithiophosphinates, 54, electronic spectra, 11 ESR, 6 ethers, 38 ethylenediaminetetraacetic acid, 69 ethylxanthate, 23 fluorides, 56 fluorophosphates, 45 formamide, 39 fulminates, 'I4 halides, 5-60 fluid phase, 60 hemoglobin, 73 hexaaqua bond distances, 3 hexafluorophosphates, 60 hexafluorosilicates, 60 hexamethylenetetramine, 16 hydrazine, 17 hydrides, 60 hydrogen ligands, 60 hydroxides, 3 5 , 3 6 hydroxycarhoxylates, 49: 51 hydroxylamine, 17,BZ 8-hydroxyquinolinc, 64 hydroxyquinones, 48 imidazoles, 19,20 iodates, 47 iodides, 58,59 lability, 10 Iactarns, 39 macrocyclic ligands, 72-82 magnetic moments, 5 maleic acid, 50 malonatcs, 50 mixed donors macrocycles, 80 rnolybdatcs, 47 monocarboxylates, 43 l,S-naphthyridine, 25 niobates, 47 nitrates, 44 nitriles, 21 nitrites, 22, 44 nitrogen donors macrocycles, 76 nitrogen-oxygen ligands bidentate, 61-64 octadcntatc, 69 quadridentate, 66 quinquedentate, 69 sexidentate, 69 terdentate, 65 nilrogcn-sullur ligands. 70 nitrosyls, 21 oxalates, 50 oxides, 35,3h oxyanions, 41-47 oxygen ligands, 3 4 5 2 macrocycles, 80 oxygen-sulfur, 71 perchlorates, 46 phenanthroline, 24 phenoxides, 37 phrnylenediamines. 23 phosphates, 45 phosphates, 45 phosphinites, 45
1295
I
1296 phosphonatas, 45 phosphoramide, 52 phosphorus ligands. 31-34 phosphorus oxides, 39 phthalocyanine. 75 porphyrins, 72 primary amines, 16 propionarnide, 39 pyrazoles. 19,20 pyridine oxides, 39 pyridines, 18 pyridylquinolinc, 24 quinolines, 18 Schiff bases, h 1 salicylaldirnines, 21,63 salicylates, 51 secondary amines, 16 selenates, 46 selenites, 46 selenium oxides? 39 selenocyanates, 22 silicates, 41, 42 spin states. 10 sulfates, 46 sulfides, 53 sulfinates, 46 sulfoacetic acid, 57 sulfonates, 46 sulfoxides, 39 sulfur ligands, 53 sulfur oxides, 39 tclluratcs, 46 tellurites, 46 terpyridyl trioxide, 52 tertiary amines. 16 tertiary arsines, 31 tertiary phosphines, 31 tertiary stibines, 32 tetrafluoroborates, 60 tetrahydrofuran, 38 thiazyltrifluorides, 22 thiocarbonyls, 54 thiocyanates, 22 thiolates, 53 thiophenolates, 53 triazoles, 14,20 tricyanomethanide, 14 tropolones, 48 tungstates, 47 urea, 39 vanadates, 47 water, 35 xanthates, 54 Manganese(II1) complexes, 82-102 acetylacetonates, 89 alcohols, 89 aminotroponirninatcs, 85 arsenates, 87 biguanides, 85 bipyridyl, 84 bromides. 92 carbon ligands, 83 carbonyl compounds, 90 carboxylates. 88 catechol, 89 chlorides, 91 cyanides. 83 cyclarn, 100 dithiocarbamates, 91 l,Zdithioethane, YO ethers, 90 fluorides, 91 halides, 91
Subject Index hexafluorides, 92 hydrides, 93 hydrogen ligands, 93 hydroxides, 85,86 hydroxycarboxylates ,89 8-hydroxyquinoline, 90 iodates, 88 iodides, 92 macrocyclic ligands, 97-102 magnetic moments, 5 meso-a,a,a,a-te trakis (0-nicotinamidophenyI)porpbyrin, 98 mixcd nitrogen-oxygen donors bidentate, 93 quadridentate, 94 quinquedentate, 96 septadentate, 96 sexidentate, 96 terdentate, 93 mixed nitrogen-oxygen-sulfur donors, 96 mixed oxygen-sulfur donors, 96 nitrates, 84,87 nitrogen ligands, 84 oxalates, 89 oxides, 85,86 oxyanions, 87 oxygcn ligands, 8 . W O phenanthroline, 84 phenols, 89 phosphates, 87 phosphorus ligands, 84 phthalocyanines, 99 electrochemistry, 99 ESR, 100 oxygenation, 99 polyhydroxycarboxylates, 89 porphyrins, 97 magnetic susceptibility, 97 redox potentials, 97 visible spectra, 97 X-ray structures, 97 pyridinc, 84 pyridine oxides, 90 Schiff bascs, 93 salicylaldehydc, 85 salicylates, 89 selenites, 88 semiquinones, 89 sulfates, 87 sulfides, 90 sulfur ligands, 90 thiols, 90 troponiminates, 90 urea, 90 water, 85 Manganese@') complexes, 102-109 alkyls, 103 bipyridyl, 103 bipyridyl oxides, 106 carbon ligands, 102 catechol, 106 chlorides, 108 cyanides, 83,102 dialkyldithiocarbamates, 106 dithiolates, 106 dithiolenes, 107 fluorides, 107 halides, 107 hydroxides, 104 iodates, 105 macrocyclic ligands, 108 magnetic moments, 5 nitrogen ligands, 103
Subject Index oxalates, 105 oxides, 104 oxyanions, 105 oxygen ligands, 104 periodates, 105 peroxo, 105 phosphates, 105 phosphines, 104 phosphorus ligands, 103 phthalocyanines, 109 polyhydroxy ligands. 106 porphyrins, 108 salicylaldiminates, 108 selenites, 105 sorbitol, 106 sulfates, 105 sulfur ligands, 106 tellurates, 105 terpyridyl oxides, 106 water, 104 Manganese(V1 complexes, 109-1 11 nitrites, 110 porphodimethines. 111 porphyrins, 110 Manganese(V1) Complexes, 109-111 Manganese(VI1) complexes, 109-11 1 Marcasite, 1240 Mercury compounds ruthenium complexes, 280 Mesoperrhenates, 198 Mesoporphyrin iron complexes, 1266 Methemerythrin, 254 Molybdenum-iron-sulfur complexes, 241 Mossbauer spectroscopy iron, 1181 Myoglobin, 263, 1269 Myoglobin, nitrosyl-, 1194 Nitroprussides, 1189 reduction, 1206 Nucleic acids reactions osmium tetroxide, 591 Nucleosides reactions osmium tetroxide, 591 Nucleotides reactions osmium tetroxide, 591 Organoiron, 1181 Organoiron(1) complexes, 1203 Orthoperrhenates, 198 Osmates dioxo, 581 fluorohah, 610 Osmium coordination numbers, 523 history, 522 oxidation states, 522 reviews, 522 Osmium complexes, 519-619 alkaloids, 569 alkoxo, 596 amido, 557 amines, 557 bidentate, 530 monodentate, 530 amino acids, 602
8-amino-7-hydroxy-4-methylcoumarin, 569 2-aminomethylpyridine, 534 ammines, 52s
aqua, 529 binuclear, 530 carbonyl, 529 halo, 529 hydroxo, 529 phosphines, 529 . substituted, 529 triflato, 529 unsubstituted, 528 antimony ligands, 569-579 aqua, 579 arginine, 602 arsenic ligands, 569-579 arsines bidentate, 578 polydentate, 579 azido, 565 benzamido, 616 benzotriazole, 568 biguanides, 567 binuclear dinitrogen-bridged, 556 oxo-bridged, 593 bipyridyl, 537 cyclic voltammetry, 541 differential pulse polarography, 541 dimers, 541 electronic spectra, 540 polymers, 542 redox properties, 540 spectroscopy, 540 surface-enhanced Raman spectra, 540 2,2'-bipyrimidinyl537 2,2'-biquinolinyl, 542 bis(2,6-di(2'-quinolyl)pyridine), 544 boranes, 616 bromo, 613 2,3-butanediimine, 533 carbohydrates, 602 carboxylates, 600 catecholates, 597 chloro, 613 cyanides, 525 dialkyl sulfides, 603 diazenide, 551 diethylselenocarbamato, 609 diisopropyldithiophosphate,608 P-diketonates, 596
4-p-dimethylaminoanilino-3-penten-2-one, 569 dimethyl sulfoxide, 602 dinitrogen, 554 dioxygen, 596 disutfur, 603 dithioacetylacetonates, 4Q6 dithiocarbamato, 604 dithiocarboxylates, 607 dithiolene, 606 ethylenediamine, 530 deprotonated, 531 fluorides, 609 fulminates, 526 glycine, 602 Group IV donors, 524,525 Group V donors, 524 Group VI donors, 524 Group VI1 donors, 524 halogen ligands, 609-615 heteronuclear dinitrogen-bridged, 556 histidine, 602 homogeneous catalysis, 524 hydrazine, 556 hydrides, 615
1297
I 1298 hydrogen, 615 hydrosulfidcs, 6UO. 603 hydroxo, 579 hydroxybenzamido, 616 hydroxylamine, 557 2-hydroxypyridine, 534 irnidazolidine-2-selones, 609 imidazolidine-2-thiones, 607 imido. 557 iodo, 613 isonicotinamide, 534 isonicotinic acid, 534 kctoncs, 598 lysine, 602 mercury ligands, 525 N heterocycles; 533-544
nitrato: 598 nitrido. 56cC565 nitriies, 566 nitro. 598 nitrogen ligands, 527-569 nitrosyl, 54.1-551 ammines. 546 arsines. 547 azido?546 cyano. 546 halidcs. 546 nitro. 54h phosphines, 537 stihines. ,517 X-ray structure, 518 octaethvlporphvriti, 617 oxo, 579-596 alkenes. 585. 587 alkynes, 58: dienes. 586,587 diols. 585 trienes, 5% oxo esters nitrogenous bases, 585 oxyamino? 55 1 oxygcti-containing ligands, 579-603 pcroxo. 596 1,~O-pht.nanrhriilinz.537 electronic spectra, 540 redox properties? 540 spectroscopy. 540 2-(phenylazo)pyridine, 535 phosphates. 599 phosphineaminato. 568 phosphineiminato, 568 phosphines bidentatr, 578 halo, 572 polydcntate. 57Y phosphorus ligands, 569 phosphorus trihalidcj, 575 phthalocyanine, 618 picoline 534 piperidine. 535 polymers, 524 polypyridyl. 537 porphyrin. 617 proline. 602 pseudohalides. 565 pyrazine. 535 pyrazine-bridged binuclear. 536 pyridazine. 535 pyridinc, 333 2-(2’pyridyl )quinoline, 542 pyrimidine: 5.15 sclcnido. 609
Suhjecr Index selenium-containingligands. 609 selenoether, 609 strychnine, 569 sulfamato, 616 sulfates, 599 sulfides, 599, 603 sulfur-containing ligands, 603-608 sulfur dioxide, 606 superoxo, 596 tellurium-conlaining ligands, 609 trrpyridyl, 542 tertiary ai-sincs. 576 halo. 576 hydrido, 576 tcrtiirry phosphatesl 575 tcrtiary pliosphincsl 57C-579 clihyclrido, 570 hexahydridu. 571 hydrides, 570 monohydrido. 570 pentahydrido. 571 polynuclear hydrides. 572 tetrahydrido. 571 trihydrido, 571 tertiary stibines, 577 tetrahydroborates, 616 thiazolidincthiones, h07 ?-tliionniidopyridine, 6 7 tIiiocarham;iees, OW thiocarboxylatcs. 607 thiocyanato, 565 thio-fi-diketonates, 606 thioethers, 603 thiolates. 607 thiones, 607 thionitrosyl, 552 thionitrosyl chloride! 552 thiourea, 608 tin, 526 triflates, 600 trithiocarbamaro, 604 ti-opolonaks, 597 rl-tuhociiramine, 544 C)srniurii(-Il) complexes phosphorus trihalidz. 575 Osmium(0) complexes phosphorus trihalide. 576 tertiary phosphates, 575 Osmium(1) complexes cyanides, 525 Osmium(I1) complexes ammines halo, 529 biguanidcs, 567 binuclrar oxo-bridged, 594 hipyridyl, 538 spectroscopy; 538 bis(thionitrosy1) 552 cyanides, 525 inelastic clcctron-tuncliing spectroscopy, 525 vibrational spectra. 525 P-diketonates. 597 dinitrogen amines, 553 ammines, 554 arsines, 555 phosphines, 555 etliylenrdiarnint.. 531 deprt)tonatedl 532 tiionotliionitros~l,553 nitriles, 567 octacthylporplivrin: 617 I
~
Subject h d e x 1,lO-phenanthroline 538 spectroscopy, 538 phosphines halo, 572 phosphorus trihalides, 576 phthalocyanine, 618 pseudohalides, 566 pyrazines, 536 pyridazines, 536 pyridines, 533 pyrimidines, 536 sulfides, 599 terpyridyl, 542 redox properties, 543 spectroscopy, 542 tertiary phosphates, 575 thiourea, 608 Osmiurn(1KK) complexes ammines halo, 529 benzotriazole, 568 biguanides, 568 binuclear oxo-bridged, 594 bipyridyl, 538,541 electronic spectra. 541 electron transfer, 539 polarography, 539 scdox properties, 539,541 spectroscopy, 538 cyanides, 526 P-diketonates, 597 dini trogen amines, 555 ammines, 555 dithiocarbamates. 604 ethylenediamine, 531 deprotonated, 532 halides, 613 nitriles, 567 octaethylporphyrin 618 1,10-phenanthrolinc, 538,541 electronic spectra, 541 electron transfcr, 539 polarography, 539 redox properties, 539.541 spectroscopy, 538 phosphines halo, 573 phosphorus trihalides?576 phthalocyanine, 619 pseudohalides, 566 pyrazines, 536 pyridazines. 536 pyridines, 534 pyrimidincs, 536 sulfides, 599 terpyridyl, 543 redox properties, 513 tertiary phosphates, 575 thioethers, 603 thiolates, 607 thionitrosyl, 553 thiourea, 608 Osmium(1V) complexes ammines halo, 529 hipyridyl, 539,511 cyanides, 526 P-diketonates, 597 dinitrogen amines, 555 ammines, 555 I
C.O.C. L P P
dithiocarbamates, 604 ethylenediamine, 531 deprotonated, 532 fluorides, 609,610 halides, 613 nitrido, 560 binuclear, 563 bonding, 564 polynuclear, 565 structure, 564 trinuclear, 564 octaethylporphyrin, 618 oxo, 580 y-oxo, SY4
1,IO-phcnanthroline, 53Y, 541 phthalocyanine, 619 pseudohalides, 566 pyridines, 534 sulfides, 599 terpyridyl, 543 tertiary phosphates, 575 thioethers, 604 thiolates, 607 tiiazenido, 564 OsmiumrV) complexes bipyridyl, 541 ethylenediamine dcprotonated, 533 fluoro, 611 halides, 61.5 imino, 558 nitrido, 560
oxo;580,595 1,IO-phenanthroline, 541 terpyridyl, 543 Osmium(V1) complexes biguanides, 568 binuclear oxo-bridged, 594 halo, 594 bipyridyl, 541 cyanidcs, 526 dioxo, 581 ethylenediamine, 531 deprotonated, 533 fluorides, 611 hexaoxo, 580 imino, 558 nitrido, 560 halo, 561 octaethylporphyrin, 618 oxo, 580,595 oxo esters, 584 pentaoxo, 580 l,10-phenanthroline, 541 phthalocyanine, 61Y sulfides, 599 terpyridyl, 543 tetraoxo, 580 thiourea, 608 trioxo, 580 Osmium(V11) complcxcs bisimido, 559 fluorides, 612 hexaoxo, 588 imido, 558 monooxo, 588 oxo halo, 596 pentaoxo, 588 terrakisimido, 560 tetraoxo, 588 trisimido, 560 Osmium(VII1) complexes
1299
I Subject lndex
I300 dioxo 593 fluorides, 612 hexaoxo? 588 monooxo, 593 nitrido, 562 oxo, 588 aqua. 59 1 halo, 592 hydroxo, 591+596 pentaoxo? 588 tetraoxo, 588 trioxo. 593 Osmium tetroxidc, 588 adducts nitrogen donors. SO2 biological ti5sue, 591 electrochemistry 589 inIrared spectra. 589 oxidation alkenes. 590 polarography, 589 preparation. 589 properties. 589 reactions. 590 solutions, 589 spcctroscopy, 589 Osmyl complcxcs cyano. 583 halo, 583 nitrogen donor ligands, 583 oxo, 584 halides, 583 oxy, 583 thioether. 581 Oxidation iridium catalysrs. 1158 Oxyhemerythrin. 253 ~
I
Peptides rcactions osmiurti tciroxidc, 590 Permanganates: 109 Fcrovskiks, 196 Peroxidases. 263.264 Perrhenates. 127. 197 Perrhenic acid: 198 Phosphinodithionates. 604 Phospholipids biological tissue osmium tetroxide, 591 Phthalocyanine, chloroferric, 1261 Porphyrin. octaethyliron complexes. 260,1266, 1267 Porphyrin, 5.1.0,15,2O-tetraphenyiiron complexes, 260, 1266 Porphyrins basket handle, 1269 cappcd iron cornplexcs, 1269 iron complexes. 259 iron(1l) compIexes cyanide ion binding, 1268 picket fence iron complexes, 1269 Proteins reactions osmium tetroxide. 590 Protocatechuate 3.4-dioxygenase, 232 Protoporphyrin t>i iron(I1) complexes, 1266,1267 Prussian blue, 221. 1203, 1206, 1208 Pyridine. 7,6-diacetylcondensation with linear polyamines, 258
Pyrite, 1240 Pyrotite, 1240 Pyrutite, 1240 Rhenates, hexahalomagnetic properties, 173 polarography, 173 spectra, 172 substitution reactions, I73 Rhenium complexes, 125 arsines, 162 dinuclear, 128 mononuclear oxidation states, 127 nitrosyl, 2072-aminobcnzcnethiolI 202 tris(2,2’-hipyridinc). 202 tris( 1,lO-phenanthroline), 202 phosphines, 162 trinuclear, 128 Rhenium(0) complexes. 128 Rhenium(1) complexes, 178-135 alkyl isocyanides. 128 aryl isocyanides. 128 cyanides, 128 dinitrogen, 130 dinuclear dinitrogen, transition metals, .I33 tiiixcd dinitrogen-phnsphjnes mononuclear, 129 mixed metal dinuclear. 132 trinuclear, 132 phosphines, 133 phosphites, 135 Rhenium@) complexes. 135-143 dinuclear hidentate arsines. 139 hidentate phosphines, 139 hydrides, 132 nionodentate phosphincs, 136 .Re2‘+, 136 Re2s+,130 sulfur, 142 mononuclear, 135.137 alkyl isocyanides, 135 arsines, 136 aryl isocyanides, 135 cyanides, 135 phosphines, 136 trinuclear, 142 Rhenium(II1) complexes. 143-164 cyanides, 143 alkyl isocyanides, 143 arnidcs, 144 amidines, 146 amines, 144 arsines, 147 aryl isocyanides, 143 cyanates, 145 diazines, 144 diethyldicarbamato, 150 P-diketones. 149 dinitrogen, 144 N heterocycles, 144 nitriles, 146 oxygen compounds. 149 phosphines, 117 phosphitcs: 147 stihines, 147 sulfur compounds. 139 thiocyanates, 145 thiourea, 150
7 Subject Index triazines, 144 dinuclear alkyl carboxylates, 158 amidines, 156 arsines, 157 aryl carboxylates, 158 halides, 154 hydrides, 150 mixed donor atom ligands, 153,160 N heterocycles, 156 phosphines, 157 ReZ6+,154-160 selenocyanates, 156 sulfates, 158 sulfur ligands, 160 thiocyanates, 156 hexanuclear, 164 sulfides, 164 tellurides, 164 moIllonuclear , 143-154 hydrides, 150, 151 trinuclear alkyl isocyanides, 161 amines, 161 ammines, 161 arsines, 162 aryl isocyanides, 161 azides, 161 cyanides, 161 dinitrogen, transition metals, 133 halides, 163 nitriles, 161 nitrogen compounds, 161 oxygen compounds, 162 phosphines, 162 Re39+,160-164 selenium compounds, 163 sulfur compounds, 163 thiocyanates, 161 Rhenium(1V) complexes, 165-177 dinuclear, 176 mononuclear, 165116 acetylacetonates, 170 alkoxides, 170 alkyl isocyanides, 165 amidates, 167 ammonia, 166 arsines, 168 cyanates, 166 cyanides, 165 diazines, 166 hydrides, 174 imides, 166 mixed donor atom ligands, 175 mixed oxide-halides, 171 N heterocycles, 166 nitriles, 167 nitromethane, 171 oxygen compounds, 170 phosphines, 168 pyridines, 166 [ReCI,L]-, 172 [Rex,( OH)]2 - , 172 [ReXJ-, 172 spectroscopy, 172 stability, 172 structure, 172 stibines, 168 sulfites, 171 sulfur compounds, 170 thiocyanates, 166 water, 170
Rhenium(V) complexes, 177 2-aminobenzenethiol, 188, 192 2,Z’-bipyridyl, 187 4-chloro-2-aminobenzenethiol, 192 cyanides, 187 dialkyldithiocarbamates, 188 1,2-ethanedithiol, 192 ethylenediamine, 187 halides, 192 hexacyanates, 193 hydrides, 192 imides, 188 lanthanides, 192 mononuclear acetonitrilc. 179 amines, 177,186 arsines, 179 aryl isocyanideT,l77,185 1,2-bis(ethylthio)ethane, 182 cyanides, 177,185 dimethyl sulfoxide, 187 dioxane, 182 dithiocarbamates, 183 NN-ethylenethiourea, 182 halides, 183 macrocycles, 184 mercaptothioacetic acid, 183 mixed donor atom ligands, 184 N hcterocycles, 177,186 octaethylporphyrin, 185 1,4-oxathiane, 182 oxygen compounds, 181 phosphines, 179,186 phosphites, 179 [Re013 + , 177-1 85 [Reoz]+,185 Schiff bases, 184 stibines, 179 sulfur compounds, 181 thiocyanates, 179,187 thiols, 182 thiourea, 182,187 vinyl amides, 185 N hetcrocyclcs, 187 nitrides, 188 octaethylporphyrin, 188 oxo-bridged mixed fluoro-hydrox&oxalato, 188 p-oxo-bridged [ReZO3l4+,187 oxygen compounds, 192 polymolybdates, 192 polytungstates, 192 pyridine, 187 Schiff bases, 188 sulfides, 192 Rhenium(V1) complexcs, 194 alkoxides, 196 amides, 194 amines, 199 carboxylates, 199 dimethylformamide, 199 dioxane, 198 halides, 195, 199 2-hydroxypyridine, 199 imides, 194 mixed sulfur-nitrogen compounds, 196 N heterocycles, 199 nitrides, 194 oxide halides, 195 oxoanions, 196 pyridine, 199 sulfates, 198 sulfur compounds, 196 tellurates, 198
1301
I
Rhenium(VI1) complcxcs, 196202 amides, 196 deuteridcs, 201 halides, M! hydrides, 201 imides. I96 nitrides, 196 thiols, 201 Rhodium occurrence, 902 Rhodium. bcnzonitrilcpcntaainminc-his(diphenylphosphint,)-l-prllpanol~-
t.ctr atluo rnlmra te, 928 Rhodium, b i s ( d i ~ l ~ e ~ ~ y l p h n s p l i i t i o ~ t!JOh lia~i~.~-~ chloride. 976 perchlorate structure. 928 Rhodium. bis(vicylidenebis(dipheny1phosphinc) 1perchloratel 928 Rhodium. brornotris(tripheny1phosphinc)structure. 917 Rhodium, chlorotetrakis(difluoro(dimethylamino)phosphincj-, 924 12hodinni. chlorotctrakis(trifluoro(dicthy1amino)plicnylpliospliine)-, 913
NMR, 918 solutiun chemistry, 918 structurc, 917 thermal decomposition, 918 X-ray cnision spectrum, 918 Rhodium. dicpnotris(dipheny1phosphinoethane)di-. 928 Rhodium. hesakisi trifluorophosphine)di-, 905 Rhodium. hydridobis( dipheny1phosphinoethane)-.924 Rhodium. hydridotetrakis((butylidynetrimethy1ene jphosphite).. 921 Rhodium, hq'd:idotetrakis(niethyldiphenS.lphosph~ne I-, 924
Rhodium liydritic,rcti-akis(t.tiaryl phosphite j - , 922 Rhodium, liydridntetrakis(triet1iyl phosphite)-, 92 1 Rhodium, h~dridoterrakis(trifluorophosphiii~~j-. Y71 Rhodium, hydridotctrakis(trimcthylphos~hincl-,923 Rhodium, hydridotctrakis(triphcny1phosphinc)-.921. 24' Rhodium, hydridotetrakis(triphcny1 phosphite)-, 973 Rhodium, h~di-ido(ti-iRuoi-oi7hosphiiie)tri~(tiiphenylphusphiiie)-, 92 1 Rhodium, hydrido(tripheny1arsine)tris(tripheny1phosphine)-, 921,923 Rhodium, hydridotris(tripheny1phosphinej-,916 preparation. 916 Rhodium. iodotetrakis(difluoro(diethy1aminojphosphine)-. 924 Rhodium. pentaammineurealinkage isomerism, 961 Rhodium, pentakis(trialkyl phosphite), 929 Rhodiuin. rctrakis(acctat0)diantitumor propcrtics. 936 ozonizatirm 051 Rhodium. reti-akis(trimetliylpliospliine.)reactions- 926 Rhodium carboxylates, 903 chemotherapy. 903 Rhodium complexes, 901 applications. 403 bimetallic. 1074, 1076 cationic, 1072.1074 chloronitrosylnitrobis(triphcnylphosphine). 1070 dichloronit.rosylhis(triphenylphosphine).'I 068 mcrcury, 1075 rniscellaneou%.1065, 1076 mixed oxidation statc, 106S molyhdenuni, 1076 ~
nitrosyl, 10hS. 1074 nitrosylhis(car-boIS:1ato) 1071 nitrosylbis(di-(p-rolyl)triazenido),1071 nitrosylbis(tertiarg phosphine), 1072 nitrosyldinitratobis(triphenylphosphine), 1070 nitrosylsulfdtobis(triphenylphosphine),1070 nitrosyltris(rriphenylphosphine), 1066 photosensitivity. 903 ruthenium, 1076 thionitrosyl. 1074 tungsten, 1076 Rhodium( - I ) complexes, 904, 005 Rhodiutn(0) complcaes, 90.5, YO6 lripherlylphospilirlc, YO5 Rhodium( I ) coniplescs. 906929 anionic, 906 bidentatc anions N-methylsalicylaldiminato, 908 peiitanc-2,4-dionato, 908 trifluoroacetato. 908 bridged, 908-91 1 dinitrogen, 919 binuclear, 920 four-coordinarc anions, 925 hydrido, 921 hydrido dinitmgcn. 920 piI aconrdin ate anions, 929 three-coordinate. 906. 907 bis(tricyc1ohexylphosphine). 906 ~
bis(trimcthyIsilyl)amidol 907 bromo, 907 dinitrogen, 907 fluoro, 907 iodo, 907 sulfur dioxide. 907
Rhodium(I1) cornpleses. 93G952 acetates dimeric?934 4-;iminci-S-(aminrimrthyl)~~~~1~t~1ylpyri1~~icii11e, 937
1icarbnn;itcs dimeric, 938 carbonates dirncric, 938 carbon bondcd adducts. Y38 carboxylate bridged dimeric, 941 dihydrogen phosphates dimeric, 938 dimeric, 933-952 asymmetrically bridged, 939, 940 bonding, 944
catalytic activity. 950 electrochemislry, 9-18 electronic structure- 944 isonitriles, '143 rnixcd bridgs, 940; 9J2 o x o briclgcd, 933 oxypyridine hriclged 939 reactivity, 947.951 Schiff bases. 943 steric crowding. 936 structure, 934,939 substitutions, 9-17 synthesis, 934,939 without bridging ligands, 942,944 dimethyl sulfoxide dimeric, 937 fluorinated carboxylates dirncric, 937 monomcric, 93G-933 tertiary arsine. 933
Subject lndex tertiary phosphine, 932 tertiary stibine., 933 I $-naphthyridine bridged dimeric, 941 N donor adducts dimeric, 938 nitrosyl dimeric, 938 polynuclear, 951,952 salicylates dimeric, 936 sulfates dimeric, 938 Rhodium(II1) complexes, 952 acetylacetone, 10.50 (+)-3-acctylcamphorate, 1053 acyl, 953 allylamine, 964 amine oximes, 1012 amines, 963 excited state, 981 stereochemistry, 986 amino acids, 971 4-amino-3,5-mercapto-l,2,4-triazole, 1048 aquacarboxylatobis(e thylenediamine) rrans-, 978 aqua halo, 1057 arylazooximes, ‘I 012 azidopentaammine photochemistry, 985 bidentate aliphaiic amines, 965 bidentate amines photochemistry, 980 biguanide, 1014 bipy ridy 1 electrochemical reduction, 1000 photochemistry, 998 reduction, 999 2,2’-bipyridyl luminescence, 999 N,N’-bis(3-aminopropyl)ethylenediamine stereochemistry, 9Y1 bis(bidentatc phospkincs): 1035,1039 disullur, 1038 oxygen, 1036 bis(bidentate stibines), 1039 bis(3-diphenylphosphinopropyl)phenylphosphine 1040 bis(ethylenediamine), 966 cis-, 961 NMR, 971 substitutions, 972 trans-, 967 X-ray structures, 971. bis( l-(phenyIphosphine)-3-(diphenylphosphine)propyl, 1042 carboxylatobis(e thylenediamine) decarboxylation, 917 chlorodioxygenbis( l-(phenylarsino)-3(dimethy1arsino)propyl-, 1042 crystal structure, 1028 IR spectroscopy, 1128 NMR, 1028 cyano, 952 cyclic polysulfides, 1056 cyclohexanehexanonethiosemicarbazide,1048 dichloro(N,N’-bis(2-aminoethyl)-l,3-propanediamine), 990 dichlorobis(bipyridyI), 997 dichlorotetrapyridine, 995 dichlorotriethylenediamine, 989 l,l-dicyano-2,2-diselenoethylene, 1057 difluorodithio phosphates, 1055
dihalobis(ethy1enediamine)
1303
ammoniation, 974 dihydridobis(bidentate phosphines), 1035 dihydridotetrakis(phosphine), 1032 dihydridotriphosphine, 1026>1027 dimethylformamide, 953 dimethyl glyoxime, 1014 dimethyl sulfoxide, 1053 dioxygen, 1052 1i?-diphenyIthioethane, 1055 dipicolinic acid, 1047 1,2-diselenocyanatocthanc,11156 2,5-dithiahexane, 1055 dithiuacetylacetone, 10% dithiocarbamates, 1054 dithiocarboxylates, 1054 N.W-cthylcncbis(salicylidineimine), 1U47 ethylencdiaminc, 965 ethylenediamine-N,W -diacetic-N,iC”-di-3-propionic acid, 1045 ethylenediamine-N,N’-di-S-propionate, 1047 ethylenediaminedisuccinic acid, 1045 ethylenediaminetetraacetic acid 1045 o-ethylthioacetothioacetic acid, 1056 Group IV ligands, 952 Group VI ligands, 1054 halo, 1057 halonitriles, 1005 hexaammine photochcmistry, 984 hexaaqua, 1049 hexahromo, 1057 hexachloro, 1057 hexafluoro, 1058 hexafluoroacetylacetonate, 1050 hexanitro, 1012 hydridobis(bidentate phosphines)? 1036 hydridotriphosphine, 1026, 1027 (+)-hydroxymethylenecarnphorate 1053 iminodiacetic acid, 1045 mesoporphyrin, 1010 N-metbyliminodiacetic acid, 1045 6-mcthyl-2-thiouraci1, 1U48 mixed nitrogen-oxygcn ligands, 1044 mixed nitrogen-sulfur ligands, 1044 monodentate amines, 953 photochemistry, 980 myoglobin, 1010 nitrilotriacetic acid, 1045 nitrito pentaammines isomerization, 961 nitrogen ligands, 953: 1015 octahedral, 903 oxygen ligands, 1049 pen taamminechloro photochemistry, 982 solvcnt cagc, 985 pentaammineiodo photochemistry, 982 pentaammine nitriles hydrolysis, 962 pentaammines, 953 anation, 956 volume of activation, 958 aquation, 956 characterization, 953 luminescence, 985 photochemistry, 982 reactions, 956 substitution reactions, 959 synthesis, 953 phosphines dinuclear, 1024, 1026 phthalocyanines, 1006, 1012 I
I
1304 polydentate aliphatic amines, 989 polydentate phosphines, 1040,1044 polypyridine, 997 porphyrins, 1006>1012 1,3-propanediarninetetraaceticacid, 1045 1-propylidenediarninetetraaceticacid, 1045 pyridine, 995 antibacterial activity, 995 substitution, catalysis, 1003 selenido, 1057 selenocyanates, 1056 stereochemistry, 902 structure, 1028 sulfides, 1054 sulfur ligands, 1054 five-membered rings, 1055 four-membered rings, 1054,1055 monodentate, 1054 six-membered rings, 1056 tertiary arsines, 1027 tetraamminediiodo photochemistry, 982 1,4,8,1l-tetraazatetradecane, 992 tetrakis(phosphine), 1032 1,4,8,11-tetrathiacyclotetradecane,1056 thiocyanates, 1056 thiolato, 1057 thiosemicarbazidediacetic acid, 1048 1,4,7-tnazacyclononane, Y92 trichlorostannyl, 953 trifluoroacetylacetonate, 1050 tripyrazolylborate, 1012 tris(alanine), 1044 tris(amino acids), 1044 tris(2-aminoethyl)arnines, 991 tris(bipyridyl), 997 tris(methionine), 1044 tris(oxalato), 1049 tris(S-methylene-2-dithiolato) crystal structure, 1056 tris(tertiary arsines). 1031 tris(tertiary phosphines), 1028 tris(tertiary stihines), 1031 Rhodium(1V) complexes, 1061,1063 anionic, 1061: 1062 hexachloro, 1061 hexalluoro, 1061 halo neutral, 1062 tetrafluoro, 1062 Rhodium(V) complexes, 1063 anionic, 1063 hexafluoro. 1063 oxyanions, 1063 pentafluoride, 1064 Rhodium(VT) complexes, 1064 hexafluoro, 1064,1065 Rhodium effect photography, 106’1 Rhodium oxides, 1065 hydrated, 1062 Rhodium sesquioxide, 1053 Rhodoxime, 1015 anionic arsenic ligands, 1015 phosphorus ligands, 1015 six-coordinate, 1022,1024 bis(phosphine), 1017 five-coordinate, 1017,1021,1026 hydridogermyl, 1019 hydrosilylation, 1019 silyl, 1019 chlorotris( triphenylphosphine)
Subject Index reaction with dioxygen, 1021 1,3-diaryltriazenido, 1023 ethyl (triptienylphosphino)acetate,1023 nitrato, 1023 tertiary arsine, 1015,1044 tertiary phosphine, 1015, 1044 triphenylphosphine, 1016,1017 Roussin’s black, 1191 Roussin’s red, 1191 Roussin’s salts, 240, 1243 Rubredoxins, 1240 bacterial, 1240 Ruthenium properties, 279 Ruthenium complexes, 271 acetamide, 466 amines binuclear, 325 halides, 324 nitrogen donor ligands, 322 oxygen donor ligands, 323 amino acids, 465 ammines, 289 alkenes, 291 amines, 292,311,321 amino acids, 310 amino esters, 310 binuclear, 311 carbonyl, 291 carboxylates, 306 cyanates, 301 cyanides, 291 dinitrogen, 299,317 halide bridged, 319 halides, 308 heterocycles, 312 metalloflavins, 310 monodentate heterocycles, 292 mononuclear, 321 nitriles, 301, 312 nitrogen donor bridged, 311,320 nitrosyls, 297 nitrous oxide, 301 oxygen donor bridged, 318,320 polynuclear, 320 sulfoxides, 306 sulfur donor bridged, 318 sulfur donor ligands, 307 thiocyanates, 301 trinuclear, 320 antimony ligands, 366-420 aqua, 420 arsenic ligands, 366-420 carboxylates, 425 cyanides, 281 dicarboxylates, 429 diimines, 363 diothiocarboxylates, 436 dipicolinic acid, 465 dithiocarbamates, 432 dithio-b-diketonates, 436 dithiolenes, 436 dithiophosphato, 436 dithiophosphinato, 436 ethylenediaminetetraacetic acid, 466 fulminates, 281 germanium-containing ligands, 287 Group IV, 281-289 halogens, 440-450 heterocycles, 325 bidentate, 327 binuclear, 357 carbon donor ligands, 326
I
Subject Index carbonyl donor ligands. 356 halide-bridged, 361 halides, 327,357 mixed donor ligands, 353 mononuclear, 325 nitrogen donor bridged, 357 nitrogen donor ligands, 326,356 oxygen donor bridged, 360 oxygen donor ligands, 327,357 polynuclear, 357 sulfur donor bridged, 360 sulfur donor ligands. 357 tridentate, 354 heterocyclic carbon donor ligands, 345 carbonyls, 345 cyanides, 345 nitrogen donor ligands, 347 nitrosyls ,348 oxygen donor ligands, 350 tertiary amines, 347 hydrazines, 363 hydrides, 450 catalysis, 463 hydrogen, 450 hydroxy, 421 8-hydroxyquinolines ,465 isonitroso ketones, 464 ketoenolates, 423 ketoximes, 464 macrocyclic, 469 mercury ligands, 280 metallothioanions, 432 6-methyl-2-pyridinolates, 466 monothio-0-diketonates, 436 nitrides, 364 nitriles, 365 nitrogen-carbon ligands, 468 nitrogen ligands, 28S364 nitrogcn-oxygen ligands, 463 nitrogen-sulfur ligands, 467 nitrosyls, 361 halides, 362 nitrogen donor ligands, 361 oxygen donor ligands, 362 sulfur donor ligands, 362 oximes, 464 oxo, 421 oxygen ligands, 420-429 phosphines acetone, 399 acyl, 386 alcohols, 399 alkenes, 390 alkyl, 386 amines, 391 ammines, 391 aryl, 386 azido, 393 bidentate N heterocycles, 391 carbamates, 403 carbamoyl complexes, 391 carbenes, 384 carbimides, 384 carbonates, 401,403 carbon donor ligands, 380 carbonyl halides, 380 carbonyls, 370 carbonyl selenides, 383 carbonyl sulfides, 383 carboxylato, 402 cyclomctalated, 388 diazines, 394
diazinido, 394 dimethylacetamide, 399 dimethylformamide, 399 dimethyl sulfoxide, 408 dinitrogen, 393 dioxygen, 401 dithiocarbamato, 405 dithiocarbonato, 405 dithiocarbonimidato, 405 dithiocarboxylato, 406 dithiolene, 406 dithiophosphato, 405 dithiophosphinato, 404 formamido, 395 formamidinato, 395 formazan, 395 hydrazines, 391 hydroxo, 399 isocyanides, 370,384 ketoenolates, 400 monodentate N heterocycles. 391 monothiocarbamato, 407 monothiocarbonato, 407 monothiocarboxylato, 407 nitrato, 401 nitriles, 396 nitrogen donor ligands, 391 nitrosyls, 367,393 oximes, 397 oxygen donor ligands, 399 perchlorato, 401 phosphorus-nitrogen ligands, 408 phosphorus-oxygen ligands, 408 phosphorus-sulfur ligands. 409 quinones, 400 Schiff bases, 397 selenocyanates, 396 sulfato, 401 sulfido, 404 sulfur dioxide, 408 sulfur donor ligands, 404 tetrahydrofuran, 399 thiocyanates, 396 thioformamido, 39.5 thiolato, 404 thionitrosyls, 393 triazenido, 395 tridentate N heterocycles, 391 urylene, 395 water, 399 phosphorus ligands, 366-420 phthalocyanines, 474 porphyrins, 469 Schiff bases, 463 selenium ligands, 439 silicon ligands, 284 carbonyl, 284 carbonyl, phosphine, 285 tertiary arsines, 287 tertiary phosphines. 287 sulfido, 430 sulfoxides, 437-439 sulfur donor ligands halides, 352 oxygen donor ligands, 352 sulfur ligands, 430 sulfur-oxygen ligands, 468 tellurium ligands, 439 tetraaza macrocycles, 475 tetrathia macrocycles, 477 thiazoles, 430 thiocyanates, 365 thioethers, 432
I I306 thiolato. 430 thiourea. 437 tin-containing ligands, 287 triaza macrocycles. 477 triazine 1-oxide, 467 Ruthenium(0: complexes, 27') binuclear phosphines. 372 hydrogcn. 450 mononuclear phosphincs. .36b phosphines carbonyl, 373 nitrosyl, 372 tctranuclcar phosphines. 372 trinuclear phosphines, 372 RutheniumiIj complexes, 279 ammines dinitrogen. 299 binuclear phosphines. 374 carbonyl halides, 440 cyanides, 281 halides, 4 0 hydrides. 450 monunuclcnr phosphinrs, 374 teti-anuclear phosphines, 374 Rutheniur~i(II)complexes, 279 amines halides. 324 mononuclear. 321 ammines, 289 bidentate heterocycles, 297 carboxylates. 306 dinitrogen, 299 halidcs, 305 hydroxy, 304 numxicnratc hctcrocyclcs, 292 nitriles. 301 nitrosyls. 297 nitrous oxide. 301 sulfoxides. 306 aqua. 420 aqua halides. 442 binuclear hydrido, I 6 1 carbonyl bromides. 142 carbonyl chlorides, 142 carbonyl fluorides, 442 carbonyl halides: 442 anionic?443 carbonyl iodides. 412 cyanides. 281 ammines, 262 arsines, 182 binuclear. 2.83 bipyridyl: 282 nitrogen donor ligands, 282 phosphines. 262 pyrazine, 252 halides. 411 heterocyclic oxygen donor ligands, 350 lead-containing ligands, 289 mixed valence. 415 mononnclear anionic. 460 antimony donors. 379 arsincs, 379
cyclometallated. 455 dihydrido. 450 hydrides, 450 monohydrido, 452.4% phosphines. 376 nitriles, 365 polydentatc arsines, 380 phosphincs. 380 stihines, 380 pi'lynuclealhydrido, 401 sulfur donor ligands h..d 1-1cles. . 352 tclranuclcar hydrido, 461 thiocyanates, 365 tin-containing ligands carbonyl. 288 carbonyl halide. 288 halides, 288 trinuclear hydrido, 461 Ruthenium(II1) complexes. 279 am ines halides, 324 m~inon~icIear, 322 arnrnincs, 290 hide t i tate hctcmcyclss. 297 carboxylatcs. 306 dinitrogen, 301 halides, 308 hydroxy, 304,305 imines, 3U2 monodentate heterocycles, 296 nitrilcs, 302 sulfoxides, 307 aqua, 42'1 aqua halides, 445 biriuclcer arsines, 417 phiisphines, 410: 117: 416 plydentate phosphine bridges, 41 4 bromides, 414 carbonyl haiidzs. 446 chlorides. 444 cyanides, 283 fluorides, 443 halides, 443 anionic, 444 heterocyclic oxygen donor ligands. 351 hydroxyl halides, 345 iodidcs. 441 mixed valcnce, 415 rnononuclt.ar arsines, 416 phosphines, 416 stibines, 416 nitriles, 366 phosphines double halide bridged, 410 triple halide bridged. 410 polynuclear arsines, 117 phosphines, 41[>,417 sulfur donor ligands halidcs, 353 tetranuclear phosphincs, 410 thiocyanates, 366
I
Subject Index trinuclear phosphines, 410,418 Ruthenium(1V) complexes, 279 ammines, 291 monodentate heterocycles, 297 aqua halides, 418 cyanides, 284 halides, 446 anionic, 447 neutral, 446 oxo halides, 448 heterocyclic oxygen donor ligands, 352 hydroxo halides, 448 oxo, 421 phosphines, 419 sulfur donor ligands halides, 353 Ruthenium(V) complexes, 280 ammines, 291 cyanides, 284 halides, 448 heterocyclic oxygen donor ligands, 352 oxo, 421 Ruthenium(V1) complexes. 280 halides, 449 heterocyclic oxygen dunor ligands, 352 oxo, 422 Ruthenium(VI1) complexes, 280 cyanides, 284
C.O.C. &PP*
1307
heterocyclic oxygen donor ligands, 352 oxo, 422 Ruthenium(VII1) complexes, 280 halides, 450 heterocyclic oxygen donor ligands, 352 oxo. 423 Ruthenium purple, 281 Siderophores, 230,233 Siloxanes redistribution iridium complex catalysts. 1160 2,2',2-l'erpyridyl, 26 2.2'-iminohis(ethyleneiminoethylamine),29 2 2 ' ,2"-propylidynetris(methyleneiminoethylamine), 30 triethylenetetramine, 27 tris(2-dimethylaminoethy1)amine.29 Thioperrhenates, 200 Trienes reactions osmium tetroxidc, 590 Trolite, 1240 Turnhull's hluc, 221, 1206 Volume ol activation pcntaammine rhodium(lII), 958 Water-gas shift reaction iridium complex catalysts, 1160
Formula Index Ru(CO),(AsPh,), 371 ASRUC~~H~~CI~P { RuCI,(AsPh,)(PPh,)), ,379 AsRuC~&I~ICIN, [RuCI(bipy),( As Ph,)] +,392 AszC'JCz6HzzIz CoIz(PhzAsCH=CHASPb,), 769 ASZCOCZ~HZ~C~~ CoClz(Ph,AsCH,CHzASPh,), 769 ASZGC~~H~OIZ CoIz(AsPh3)Z, 769 AszCOC~OH~ON~ [Co(CN),(AsPh3)Z]-, 773 AszCo,C,&,d.i06Pz Co { Co{As( O)MePh}(HDMG)(PRu3)}z, 775 As2FeC6Hi8CI2S2 Co(HDMG),(AsMez),774 ASCOC~~H~~CIN~O~ FeC12(SAsMe,), , 1240 AszFeCloH16C1z CoCl{As(OH)MePh)(HDMG)2,774 FtCl,(diars), 1234 A S COC~~ HI~ NZ CoH(Nz)(ASPh,), 769 AszFeC1oHd2 FeI,(diars), 1234 AsCoCZ,H,sNO, AszFeClzH16BrzOz 660 Co(NO)(CO),(AsPh,), ._ FeBr,(CO),( diars), 1234 AsCoC,,H4,N~04P Co(HDMG),(PBu,)(AsMePh), 775 AS2FeCi3H1603 [Fe(CO),( diars)]+, 1198 AsCoC,,Hd20 [cO{AS(~~H,ASP~Z-~),}(C~)]+, 769 As,FeC39H3003 Fe(CO)3(AsPh3)2,1199 AsCoFeMoWC,H$ AszIrC,7Hz,CI0 CoFeMoWS(AsMc,). 832 ICl(CO)(Ph,AsCH=CNAsPh,), 1108 ASCOH&504 AszIrC37H30C1Nz07 [CO(OASO~H)(NH,),]*, 775 lrCl(N03)2(CO)(AsPh3)21 1114 AsCoHlsN504 As~I~C~~H~OCIO [C~(OASO,H,)(NH,),]~',775 I ~ C I ( C O ) ( A S P ~1159 ~)~, AsIrC,,H,,CIOP, [IrHC1(AsMe,Ph)(C0)(PMe2Ph),]+,I150 AszIG~H330 IrH,(AsPh3),(CO), 1134 AsIrC30HzsN04 AS~I~C~~H~~CI Ir(OAc),(AsPh3) (CNC6H4Me-4),1121 IrC1(2-CHz=CHC6H4AsPhz)z.1111 Ir(AsPh3){MeCOCHC(Me)=NCHz},(CNC6H4Me-4), AszIrC+,H3,C1N 1124 IrH2C1(AsPh3),(4-MeC6H4NC), 1125, 1134 A s ~ I ~ C ~ ~ H ~ ~ N A s I ~ C ~ ~ H ~ ~ N ~ ~ ~ Ir(AsPh,)(salen)(CNC6H4Me-4), 1124 IrH3(AsPh3)2(4-MeC6H4NC), 1125,1134,1156 AsIrC7,H5~P, ASzIrC&44~6; [Ir(PPh3){As(C6H4PPhz-2)3}] +, 1135 Ir(aca~)(AsPh~)~(4-MeC~kl~NC), 1126 As2MnCl2HI6Br2O2 AsMnCRH,,Br,N MnBr,(Cdj2(diars), 31 MnBr~(2-hzNC6H4AsMe,), 34 AsMnCI3Hi3C1O8 AS~M~C~~H~~B~O, [MnBr(C0)3(AsMe2Ph)2]+,31 [Mn(CI04)(Ph2MeAs04)]+,40 AsMnC16Hz4Nz &&'fnC36H3oCl3O~ [ M ~ I ( ~ - H , N C ~ & , A S M ~34 ,)~]~~, MnC13(Ph3AsO)Z,90 AsMnCZ7Hz5N4 AsZMnFz4N4S4 Mn(2-(MezAs)C6H4N=CPhC(Ph)=NNHpyridyl-2), M ~ ( A s F ~ ) ~ ( N S22,W F~)~, 34 ASzOSC16HzzC14 0 s ( A ~ M e ~ P h ) ~577 Cl~, AsMnzH05 As~OSC~~H~~CI~ Mnz(OH)As04, 45 AsReCz6H24CL4P 0 ~ ( A s P r , ) ~ C577 l~, ReCl,(arphos), 169 A~zO~C25HzzC14 AsR~C~HI~CI~S Os(dpam)CI,, 579 [RhCI4(2-MeSC6H~AsMe,)1-,1015 As2OG4HdW' AsRhC7ZHblP3 OS(PPh(CH2CH2AsPh,),} CI3, 579 RhH(PPh,),(AsPh,), 921,924 A s ~ O S C ~ ~ H ~ ~ B ~ ~ AsRuCZZHlS04 0 s ( A ~ P h ~ ) ~ B577 r,,
ASC~ZCIZH~~N~~~SZ [{Co(en),(02CCH,s)}~Ad"',848 AgIrC56H,3CINOP, Ir(CNBu')(CO)( p-dppm),AgCI, 1163 AhhCI2H3,P4 Mn( AIH4)(MezPCH2CH2PMez)2, 9,32 A IOsC33H 54C12N 2 P 3 Os(NNAIMe3)(PEt2Ph)3C1z, 555 Al~MnC24H560~ Mn{A1(OPr')4}z, 37 A12MnHs M ~ I ( A I H ~61 )~, AsBIrC23H&&N6 IrCI,{ HB(fiN=CMeCH=CMe),}(AsPhMe,), 1134 AsCoC,nHwNaOa __ __ .
1309
I
Formula Index
1310 AsZOSC,~H,,CINO Os(NO)CI(AsPh,),, 550 A S ~ O S CI?,CI,NP ~~I Os(NPPhMe,)C13(AsPh3)2,568 As2ReC8H14CLF4
ReC1,(Me,AsCH{C(AsMe2)CF2CHF2}, 170 As2ReC9Hl2QF6 ReCI4(Me2A&=C(AsMe2)CF2CF2~F2) 170 As2ReCIOHl&130 ReOCl,(diars), 181 AS~RCC~OH~~CI~ ReCI4(diars): 170 As,KeC,,,H33C13 KeQ,3(AsEt,l'h),, 147 ~
As,ReC',,H,,C:I,P,
RcCL4(arphosj2 . 169 As2ReC,,H,,P2
ReH3(PPh3)2(Ph2AsCH2CH2AsPh2), 150 0 11HS, ~ A S ~ R ~ ~& C rI3 ~ Re3Br3(As04)2(DMS0)3, 162 As~R~CZOH~~CI~N~
[RhC1,(2-Me2NC6H,AsMe2)2]t, 1023 As2RhC2oH32CIJNl RhCI,(?-Mr2NC,H,AsMe2)2, 1023 A~2RbC2.1H27CliN RhCt,(AxMe,Ph),(py), 1031 As2RhC26H26C12N03 Kh(:l,~Nn,)(AsMePh,),, 1022 As,RhC,,H, jC:ISN RhCI,(AsEtPh,),(py), 1031 As,RhC,,H,,Cl,NO Rh(NO)Ct2(AsPh3),,1070, 1072 As2RhC,,H3J2NO Rh(NO)I,(AsPh,), . 1072 AS2RhC36H31Ci2 RhHCl,(AsPh,),, 1018 A~2RhC36H32Cl RhH,C1( ASPh,),, 1018 As,RhC,,H,4C12N, [KihC1,(AsMePhl),( bipy)]+, 1034 As,KhC,,H,,CIN, I033 [ RhHCI(AsMePhl),(bipy)]+, As2RhC,,H&I, RhCI,{As(C,H, 1022 As2RhC38H34C12N2 [RhCl,(AsMePh,),(phen)] ' , 1034 As,RhC3sH35C1N2 [RhHC1(AsMePh2),(phen)]+, 1033 A~2RhCzsHjsClzN2 [RhC12(AsEtPh2),(bipy)]+,1034 As2RhC38H39CIN2 [RhHCI(A~€tPh,)~(bipy)]+, 1033 As2RhC40H36Y,0 Illh(NC))(RiIeCN)2(AsPhj)2]2+, 1072 As,RhC,,H,,C:I,N, [RhCl1(AsCtPh2),(I.'hcn)l', 1034 As,RhC,,,H4zCl,h'2 [RhCl1(AsPh2Pr),(bipy)]'~, 1034 AS2RhC40Hj3CINz [RhHCI(AsPh,Pr),(bipy)]+, 1033 As#hC42Hd2C12N, [RhCI,(AsPh,Pr),(phen)]+, 1034 As2RhC42H,3CIN, [RhHC1(AsPhZPr),(phen)]+, 1033 As2RhC4,H46BrlY2 [RhBr,(AsBuPhZ),(bipy)]+, 1034 As2RhC44H46BrzN2 [RhRr,(AsBuPh,),(plieti)]+, 1034 As,RhGeC391140CI RhHCI(C chic,) (AsPh,), , 1020 As,RhSiC,6H,,C14 RhHC1(SiC1,)(AsPh3)2,1020 As,RhSiC,,H,,C10,
RhHCI{Si(OEt),}(AsPh,),!1020 As2RuC3,jH3"ClNO RuCI(NO)(AsPh,),, 368,394 As~RuC~~H~CI~ {RuClz(AsPhj),},,, 379 As2RuC36H30C1,N02 RuCI~(NO)(ASP~~)(OASP~;). 394 AS2RuC36H,oCI,NP, RuC13N(PPh3)2. 119 As~RuC~~H~~BT~ RuHBr2(AsPh3),. 461.462 As2RilC37H,4ClaC) KuCI,(AsPh,),(MeOH). 379,416 As2KuC:38H3,,Br30S RuBr?(AsPh,),(I)MSO), 479 As2RuC401.143C1UlP2SI RuHCI(DMSO)~(ASP~,),, 158 AS~RUC~~H~~N~O~S, [ R U H ( N ~ ) ( D M S O ) ~ ( A S P459 ~~~~]+, As2RuC4,H3,F60P& Ru{S&dCF&I ( C O ) ( A ~ P ~ ~ 406 )Z, As~RuC~~H~~CIP, RuHC1(PPh3j2(Ph2AsCH2CH2AsPh2), 453 AS~RLIC~,H~~Y~ [Ru(oc.laelhyIporphyrin)(~~Ph~)~]+, 471 As2Ru2C6,H,,CIti [liuC:12{Ar(C6H4Me-4),)(@),KuC:I(As(C.1 I~hl~-1)~)~]~-, 415 As3CoC17H2Jlj CoC13{MeAs(C,TT,AsMr2-2),),774 As~COC~~H~~O~
[CO{M~C(CH,A~P~~),)(CO)~]~, 769,773 As~COCS~H~~ CoH,(AsPh,),, 769 A~3Co2C24H23F603
CO~{C~(CF,),)(CO)~{M~A~(C~H~ASM~,-~),}, 769 As3HgRhC24H33C15 RhC1,(AsMeZPh),(HgC12),1076 As,H~R~C~~H~,CI,F RhCl,(ligF)(Ashlelefh,)3, 1076 As,HgRhC,,H,,C:I, RhCI,(H~CI)(As~lcPIi,),,1 076 As,HglihC,,H,,CI1O, RhCI,(HgOAc)(AsMePh,),, 1076 A s ~ I ~ C ~ ~ H ~ ~ P ~ [Ir(PPh3){P(C6H1A~Ph2-2)3)] , 1135 As~OSCZ~H~~CI, Os(AsMe,Ph)jC13, 577 ASSOSC~~H~~CI~ Os(AsPr,),CI,, 577 As~OSC~ZH~~ Os(AsEtPh,),H,. 576 AS30SC6&&12h2 Os{As(C:,I ~ ~ M e - ~ ) 3 ~ , C : 1 2556 (~21~~), As~RcC~~IIS~ ReHs(AsEtPh1j3,192 As3RhC.16H,9 RhCI { PhAs(CH,CH,CH,AsMc,),), 1041 AS>KhC16H2gCIO? Rh(02)C1{PhAs(CH,CH2CH2AsMeZ),), 1042 As3RhCI7Hz3Br3 RhBr3{MeAs(C6H4AsMe,-2),),1043 As3RhC17H23C13 RhCI3{ MeAs( C6H4AsMe2-2),,1043 As3RhCl7Hz3N3O9 Rh(NO,), (M ~ A S(C , H , A SM ~ , -~ 1043 )~ } , As3RhClsH&I3 RhCI3(AsEtJ3, 1031 As3RhC~4H3.3C1, RhC1,(AsMe,Ph),l 1031 As3RhC30H42Br3 KhBr, { AsMe,(CH,=CHPh) j 3 , 103I AS~R~C~~H,,~CI~
I
Formula Index RhC13(AsSu,)3, 1031 AS,R~C,~H~~B~, RhBr,(AsMePh,),, 1027 A~3RhC39H39Ci3 RhCl,(AsMePh,),, 1031 As3RhC39H4nBr RhHBr(AsMePh,),, 1027 A~3RhC39HjOC12 RhHCI,(AsMePh,),. 1026 As3RhC45H51C13 RhCl,(AsPh,Pr),, 1031 As,RhC,,H,,CI, RhHCI,(AsPh,Pr), 1026 As3RhC,2H61P RhH(AsPh,),(PPh,), 924 As3RuCZ4HJ3Cl3 RuCI3(AsMe2Ph), .379 As~RuC~~H~~CIZNO R ~ C I , ( N O ) ( A S M ~ P ~374 ,)~, AS~RUC~~HSZNZO~S~
1311
AS~COC~~H~~CIZ
[CoC1,{{Mc2As(CH,)3AsPhCH,},}]~7 773,784 AsXG4H3a02 [Co(O,) { { MezAs(CH2)3AsPhCHz}2} I+, 773,784 AS4COC24H4202 [Co{ { M ~ , A s ( C H ~ ) ~ A S P ~ C H (H,0)2]3+, ,}~) 784 As~COC~,,H~~B~NO [Co(NO)Br(diars),]+, 666 AS~COC~~H~~CINO [Co(NO)Cl(diars),]+, 658 [Co(NO)Cl(diars),]"-: 658 As4CoC,,H64N0 [Co(NO)(diars),lZ'.,658,hhl,tiCfl AS~COC~~H~~N~OS [Co(N0)(NCS)(diar~),]+~ 661 664 As~COC~~H~~NS (CO(NCS)(P~,ASCH=CHASP~&]+~ 769 ASqCOC72H6.1 [CoH(AsPh,),]-, 769 As4FeC12H36C108 [Fe(OAsMe3),(C10,)]', 1183,1237 AS~F~CI~H~~S~ [Fe(SAsMe,),]*+, 1240 As4FeC,sH,6Cl, [ F C { A S ( C H ~ C H ~ C H ~ A ~ M ~ ~226 )~}C~,]+, As4FeCznH32Clz ~ F e ( d i a r ~ ) ~ C l11x3, ~ ] ~ +1187 , As41rC64H64NOP2 ~Ir(NO){1,4-{(Ph2AsCH,C.H,),PCH,)2C,H4}]+, 11 14 AS41rC64H64PZ [Ir(l,4-{,(PhzAsCHZCH2)zPCHz}2C6H4)] ' . 1113 ~
[RuH(N2)(DMSO)z(AsMePh2)3J+, 459 As~RuC~~H~SCI~O~ RuCl,(AsPh,),( 0,),416 As~RuC~~H~SC~~ RuCI,(AsPh,),. 379.416 As3RuCs4H45C130 RuCI3(AsPh3),(OAsPh3), 416 AS,Ru2C,,H63CI, [RuZC16{As(C6H,Me-4)~}~] .415 AS4COCi2Hm [Co(Me,AsCH=CHAsMe,),If ,769 A s ~ C O C ~ ~ H ~ ~ B ~ ~ [COB~,(M~,ASCH=CHASM~~)~]+, 773 [IrH,{1,4-{(Ph,AsCH2CH2)2YCHz}~C6H4}]+, 1114 AS~COCIZH~~CI As41rC6sH640P2 [CoCl(Me,AsCH=CHAsMe,),]+, 769 [Ir(CO){ 1, ~ - { ( P ~ , A S C H ~ C H , ) ~ P C ~ ~ } , C 11~14H ~ } ] + , As4IrC7,HS7P A s~COCIZH~Z ~Co(Me,AsCH,CH2AsMe2),]+.769 [I~(PP~,)(AS(C~H~ASP~,-~)~}]-, 1135 Ai4C~C18H34Br2 AS&C,ZH~IP~ ~ C ~ H ~ } ]IrH(AsPh,),, +, [CoBr,{ 1, ~ - ( M C ~ A S C H ~ C H , C H ~ A ~ M ~ ) 774 1119 A~,CoC,sHw02 A~4MnC72116nC108 [Co(0;) { I ,2-(Me2A sC1~?CHzCII2AsMe)~C6I-I4}]774 [Mn(C104)(Ph3AsO),J'I 47 As~COC~~H~~B~ As~OSC~~H~~CINO CoHBr{1,2-(Me,AsCH2CH2CH2AsMe),ChH4}]+,774 ~Os(NO)(diars),Cl~z',547 As40sC2,,H3,CIN2 A~~COGSH,~, [CoH, { 1 ,2-(Me,AsCH,CR,CH,AsMe),C6H,}]+.774 {Os(N,)(diars),Cl]+, 555,565 As,OSC,~H,~CIN~ As4CoGnH32 [Co(diars),J2+>772 Os(N,)(diars),CI, 565 A s~COC&~,B~ NO As40sC2,,H,,C1, [CoBr(NO)(diars),j 773 [ O s ( d i a r ~ ) ~ C l 579 ~]~+, AS~COC,,H&I, As40sCzzH3zNzSz [CoCl,( diars),] 773 Os(diars),(SCN),, 566 As~COCZOH~ZIZ As~OSC~OH~~N~ [CoI,(diars),]+, 773 [Os(bipy)(diar~),]~+, 540 As~COC,,H~,NO AS40SC40H62 [C~(NO)(diars)~]+, 773 Os(AsEtZPh)4HZ, 576 As~COCZOH~~O~ ASJOSC52H44NZS2 [C~(O,)(diars)~l+, 773,783 Os(dpam),(SCN),, 566 As,CoC,,H,,O&l, As~OSC~~H~~CI, Co(OCl,),(diars), , 772 Os(As( C6H4AsPh2-2)3) CL, 579 As4CoC,,H3,C1 ASJOSC~~~I~,$%~~ [CoHCl(diars),J+ ,773 Os(AsPh,)4Br,, 577 As4CoCZnH34 AslPh&,H45Cl30 [CoH,(diars),j+, 773,784 RhzC13(~-H)(~-C0)(d~am)2, 941 AS~COC~OH~~O As4ReCzoH3,Clz 773 [CoH(H,O)(diar~)~]~* [ReClz(diars),]+, 162 As4ReC,,H3,C14 As4CoCznH3602 [C~(H,O),(diars),]~+,772,784 (ReCl,(diars),]+, 193 As4ReC,,H3,CI9 AGoCziH3@3 [Co(C03)(diars),] ,773 Re,Cl,(diars),, 162 AS4C0C22H32N2SZ As& GoH, zC1z Co(NCS),(diars),, 769 RcCl,(diars),, 136 [Rr,Cl,(diars),l' , 148 As4CoCzzHdL [CoMe,(diars),]', 773 AsqReC52HS1 ~~
~
~~
+
+
~
+~
I
+
~
I
1312
Formula Zndex
ReH3(PhzAsCHzCH,AsPhz),, 150 A s ~ R ~ ~ C ~ ~ H ~ ~ Re2H8(AsPh3)4r174 As~R~~C~~H,&~~P~ Re3C19{(Ph2AsCH2CH2)2PPh},,162 AS~R~C~OH~ZCINO~ [RhCI(NO,)(diars),]+ , 1040 AS~R~CZOH&~Z [RhCiz(diars),]+. 1039, 1040 As4RhCZOH3204S 1040 [Rh(SO,)(diar~)~]+, As4RhC2,,H3,C1 [RhHCI( diars),] -, 1037 As~R~C,,H&IO~S [RhCI(O,SMe)(diars),]+, 1040 AS~R~CZ~H~~C~NO~S [RhCL(4-O2SC6H,NO2)(diars),]+, 1040 As4RhCZ6H37CIOZS [RhCl(O,SPh)(diars),]-, 1040 As~R~C~~H~~C~OZS [RhCI(4-0,SC6H,Me)(diars),1+, 1040 As~R~CSZH~~SZ [RhS2(PhzAsCH=CHAsPhz)Z]+, 1037 As4RhCszH45Cl [RhHCl(PhZASCH=CI IAsPhz)z]+, 1037 AS4RhCszH46 [ R ~ H ~ ( P ~ ~ A s C H = C H A S1037 P~J~]+, AS~R~C&~~CIOZS [RhCl(OzSMe)(PhzAsCH==CHAsPhz)2]+, 1040 AS~R~C~VH~QCIO~S [RhC1( OzSPh) (Ph,AsCH=CHAsPhZ)z]+, 1040 As4RhC5,H5,C1O2S
[RhC1(4-02SC6H4Me)(PhZA~CH2CHZA~Ph2)z] +
AS~R~~CZ~&OC~S { RhC13(AsEt3)2}2 , 1025 A~4RhzCS1H~C120 Rh,CI,( p-CO) (dpam),. 941 As4RhzC~z&CLIzO~ Rh2(C0)2C1212(dpam)2,941 A~4RhzCsJLC1202 Rh,(CO)2CI,(dparn),, 941 AS4RhZC68H64C12
{RhCl{ 1 3 {(4-MeC6H4),As)2C6H4}}2,913 Gs~RuC~~H~~C~N [RuCl(NH,)(diars),]+, 391 AS~RUC~~H~~CINO [RuC1(NO)(diar~),]~-,394 As~RuC~OH~~C~NO~ RuCI(N 0,)(dia rs), , 394 As~RuC~OH~~CIN~ [RuCl(N,)(diars),]+, 391,394 AS~RUCZ~H~ZC~N~ RuCl(N,)(diars),, 394 As~RuCZOH&~Z RuCl,(diarsj,, 379 [ R ~ C l ~ ( d i a r s ) ~418 ]+, AS~RUC~~H~JNO [R~I(NO)(d ia r s) ~]394 ~+. AS4RUCzoHd6 Ru(N,),(diars),, 394 As~RuC,~H&I RuCIMe(diars), 386 As~RuC~QHJ'ZS~ Ru(S2PMe2)z(diars)z, 405 As~RuC~~H&I~ RuCl,(AsMe,Ph),, 379 As~RuC,~H~~CI$~O RuCI,(NO)(dpam),, 393 AS~RUC~~H,,CI~ RuCL2(Ph,AsCH=CHAsPh,),, 379 As4RuCs,H,,C120, RuCI,(CO),(dparn),, 383 As~RuC&,,CI
F O F W J index U~~
1313
BFeClsH9FN603P
[Fe{FB(ON=CC,H,N),P)]2+,1232 BI~AsC~~H~~C~~N~ IrC12{HB(&N=CMeCH=~Me)3} (AsPhMe,), 1134 B1rC8H8N4O2 Ir(CO),{H,B(&N=CHCH=~H)z}, 1108 BI~CI~HI~N~OZ Ir(CO),{ H,B(&N=CMeCH=CMe),} , 1108 BIrC1sH,,PT 1rHJ(BH4j(PBut2Me),, 1149 BIrCigHz7F6N605 1134 Ir(O,CCF,),{ HB(fiN=CMeCH=~Me)3}(H20), BIrC20H,,C1N60, Irlacac~CIlHBf&N=CMeCH=cMcL). 1134
Mn(BH4I3,93 B4IrG7H350P~
Ir(q4-B4H9)(CO)(PPhzMe),, 1165
56FeC6FisNs [Fe(CNBF3),I4-, 1205 BSIrCS5H5Z3
2,10-IrCB81-IS-l-PPh3-2,2.2-H(PPh3)2, 1165 BRIrPtCg3H4,0P4 PtBsH,,IrH(CO)(PMe,),, 1165 B10°SC10H4002P2
OsB10H8(PMe2Ph)2(OEt)2,616 ~ I O ~ S Z C ~ S H ~ I C ~ ~ ~ ~ P ~ Os,Cl,(PPh3)2(PMe2Ph),B,,,H8(OEt)2, 616 &o R U C S~ H ~ IN Z P~ S RUH,(N,B,~H,SM~,)(PP~~)~, 4Sl
Ba2Fe04,266 (Re(NBBr,)Br,]-, 194 BReCZ7HZ5Cl2N6P
ReCl2(PPhll{HB(flN=CHCH=kHIlI.154 I-,
BRhC@l,CIiN6
I
[Rh{HB(NN=CHCH=CH),)C12HI-, 1012 BRhC1oHloIzN6O R h ( H B ( ~ N = C H C H ~ H ),., , } .( C O. )-I ,1012 , BRh6&;4C12N60 Rh(HB~&N=CHCHdHLMX(MeOH~. ,", ,, 1012 BRhC14H;5FZIN402 RhMeI F kON=CEtCMe=N(CH,),N=CEtCMe= 1014 BRhCZ1Hz6P' RhH(BH4){P(C,H4Me-2),], 932 - \
k,.
BRhC30H3806P2
[Rh(BPh,){P(OMe),),, 930 BR~C~~H~ZPZ 1022 R~H~(BH,){P(C,H,I)~}~, BRhC4,H3,C10 P RhHC1(2-C&&BH)(PPh3),, 1018 BRuCHlsN6 [Ru(BH,CN)(NH,),]+, 282 [Ru(BH3CN)(NH3),IZ+,282 BRUC,H~~P~ RuH(@-BH.,)(PM~~)~, 454 BRuCisHisNs [Ru{B(&N=CHCH=&3)4) (C&)]+, 356 BRuCmHnP, RuH(llZ-BHal(PMePhn)a, 454
Rug(BH4)( CO) { P(C6H, 2. 457 BR~CjgH30N02P2 RuH( BH,CN) (CO),(S-Ph-dibenzophosphole), ,457 BRG4H5oP3 RuH(BH,)(PPh,),, 454 BRUC~~H~OOP~ RuH(BH,)(CO)(PPh,),. 457 B2FeC6H4F3N6 Fe(CNH)4(CNBF3)2,1205 B2FeC14H42N2012P4 Fe(BH,CNjz{P(OMe)3}4, 1234 B21r2C30H44C14N12 {IrCl,{HB(NN=CMeCH=&Me),j j2, 1134
CoAsCjnH2oN404 Co( HDMG),(AsMe,), 774 CoAsC,,H,,CIN,O, cOCl{ As(OH)MePH)(HDMG),, 774 CoAsC&1,N, CoH(N,)(AsPh,), 769 CoAs~,H,,NO, Co(NO)(CO),(AsPh,), 660 COASC~~H~~N~O~P Co(HDMG),(PBu,)(AsMePh), 775 CoAsFeMoWCzH6S CoFeMoWS(AsMe,), 832 COASH,,N.O, [Co(OAs03H)(NH3),]+, 775 CoAsH,,N,O, [CO(OASO,H,)(NH~)~]~+, 775 CoAs2C26H2212
Co12(Ph2AsCH=CHAsPh2),769
CoAszGoH3oN4 [CO(CN)~(ASP~~),]-, 773 COAS~C~~H~~CI~
C O C ~ , { M ~ A ~ ( C ~ H ~ A774 SM~~-~)~}, COAS~C~~H~~OZ [Co{MeC(CHzAsPhz),}(CO),]+, 769,773 CoA G s J L s CoH,(AsPh,),, 769 COAS4CizHm [Co(Me2AsCH=CHAsMe,),1+, 769 CoAsrC12HZ8Br, [COB~~(M~~ASCH=CHASM 773 ~~)~]+, QAS~CI~H~Z [Co(Me,AsCH,CH,AsMe,),]+, 769 C O A S ~ C ~ ~ H ~ ~ B ~ ~ [CoBr,{ 1,2-(MezAsCH2CHzCH2AsMe)zC6H4}]+, 774 COAS4C18H3402 [Co(O,) { 1,2-(MezAsCH2CHzCHzAsMe)zC6H4}]+, 774 C ~ A S ~ C ~ ~ H ~ ~ B ~ CoHBr{ 1,2-(MezAsCHiCH2CH2AsMe)2C6H4}]+, 774 COAS4CisH36 [CoH,{ 1,2-(MezAsCH2CH2CH2AsMe)2C6H4}]+, 774 CuAs4C20H32
[ C ~ ( d i a r s ) ~ ]772 ~*, GJAS~C~~H~~B~NO [CoBr(NO)(diars),]+, 773 COAS4CZOH3ZC12
I
1314
Formula Itidex
[C'o(C'O,)(NI 1 3 ) j ] 2 * . 813 [CoCl,(diiirs)zj+ 773 [ Co(OCO,)(NH,),]-, 790.8 14 COAS~C~~H~~CI~OS CoCH,,N, Co(OC10,)2(diars)z. 772 [Co(CN)(NH,),]'+. 650,654 COAS~C~OHJ~ [CoI, (diars),] 773 CoCWi,N60 [Co(NCO)(NH,)5]2+.677. 679 COAS,C,~H,,NO CoCH, , 1235 Fe(terpy)2, 1196 [Fe(terpy)#++.225. 1222 FeC33H30N12 [Fe(2-pyG=NNHpy-2)3]2L,1225 [Fe(terpy)#+. 223.225 F~C~~HI~NIU FeC3~H22N802 [Fc(phthalacyanine)(CN),] -,1266 , 1229 [Fc { 2-phenc=NUC( Mc)=N H ) beC~H16Ne.02 FCC3nH&186 Fe(phlhalooyaninc)(C0)2, 1262 Fe( bipy),]C12. 1220 FcC34i132N6 FeCqnHz4N6 ' 1229 [Fe(2-phenC=NB~')~]~ Ferhipy),. 1184. 1195, 1196 [Fe(bipy),] . 1196 FeCq5Hz2Nx0,S Fe(phthalocyanrne)(DMSO)(CO), 1262 [Fe(hipy),j2'. 1195, 1215-1219, 1222 [Fe(bipy),)j+, 223 FeC36H21N906 [Fe(5-O2Nphen),I2+. 1216.1217 Fe(CN),(J.7-Me2phen),, 1220 FeC3,HZ3N902 F ~ C ~ O H ~ ~ S , Fe(phthalocyanine)(DMF)(CO), 1262 Fe(NCS)2(Me2phen)2,1221 FeC36Hd6 FeC3oH2ds [Fe(~hen)~],'.1183, 1187. 1215-1219 [Fe(2-phenc=NCH,CH2P(TH)2]2+,1229 [ F e ( ~ h e n ) ~ ]223.1185 ~+, FeC30H26W14 Fe(O&=CCO NPhN=c Me)2(py)2,1237 FeC36Hz4N603 IFe(phen N - o ~ i d e ) ~ ]1238 ~+ FeCXnH,,N, FeC36iT24N12 [Fe(2,7-d1methyl-l.8-naphthyridine),12+, 1212 [Fe{2,4,6-py),-s-triazitie}~]*-, 1230 [Fe(py),JZ ' 121 I , 1212 FC'&H30Y606 F e C J L7Ns [Fc(H2Nphen),j2-, 1217 [Fe(py N o ~ i d e )',~ 1237 ]~ I - .
.
I _
li
I
1333
.FormulaIndex Fe(tetraphenylporphyrin)O,, 1268 [Fe(tetraphenylporphyrin)(02)]~.261 FeC44H28N50 Fe(tetraphenylporphyrin)(NO). 1194, 1267 FeC44H33N402 [Fe(tetraphenylporphyrir~)(H,O)~]-. 261 FeC44H4zNzP4SZ Fe(NCS)z{(PhzPCH2CHzPPhCH2),},1235 FeC4,H2,N40 Fe(tetraphenylporphyrin)(CO). 1202,1268 FcC~~H~~CI~N~O Fe( tetraphcnylporpbyrin)(CCI& HzO), 1270 FeC4sHdqO 1 2
[Fe(2-pyC=NC6H.& H)312+,1228 FeCdLC12N4 FeCl,(isoquinoline)4, 1214 FeC36H28N80zS2 Fe(phthalocyanine)(DMSO)2, 1262 FeC36H30BrzP~ FeBr,(PPh3)z. 1183, 1233 FeC36H30C1~0J'~ Fe(OPPh3)&, 1237 FeC3J13& [Fe(2-methyl-l .H-naphthyr~cline),]~+, 1211 FGJLC1N404 [Fe(octaethy1porphyrin)(ClO4)], 261
[Fe(4-O-3-ONC,H,CO2C,H,CH=CH,-4j3] , 1238
FeC36H46NG03
Fe( NO)(rnesu-2,3,7,8,12,13,17,18-octaethyl-Snitroporphyrinato), 1194 FcC,," IFe-
FGdL03S3 Fe( PhC(S)CHC(O)Ph), .247 FcCGHJ~O~ Fc(PhCOCHCOPh),, 1203 FeC46Hz8N40~ Fe(tetraphenylporphynn)(CO),, 1268 FeCdLsN6 [Fe(tetraphenylporphyrin)(CN) ~. 260 [Fe(tetraphenylp~rphyrin)(CN)~]'-. 1268 FG6€I4JW"SZ Fe(NCS)z{P(C€12CII,PPh2)3}.1275 FeC4,HR4N76 Fe(N0 tetra henylporphyrin)(MeNm= M H j , 1194 FeC4JW%
-
1 1
FeC38H14N8S2 [Fe(phen)3](NCS)2, 1221 F~C~SHZ~NIZ Fe(phthalucyanine)(H~CH=NCH=cH),, 1264 F ~ G ~ H ~ o I z W ~ Fe(CO)Z{P(OPh)3]212,1197 FeC38H32N406 Fe(salen)z(quinone), 1238 FeC38H38N6
[Fe(fi=CPhCPh=N(CH,),N=OPhCPh=N(~H,),}
(MeCN)2]2+.1255 FeC3,H30C103P2 FeCI(CO),(PPh3)2, 1199 [FeCl(CO),(PPh,),l+, 1199 FeC39H3003P2
Fe(CO)3(PPh3)2,1196.1199,124S [Fe(CO)3(PPh3)21I , 1199 FeC39H3003p, [Fe(CO),(PPh,),]+. 1183, 1199 FeC39H3009P2 Fe(CO),{ P( OPh)3}z, 1199 FeC40H38N10 Fe(phtha1ocyanine) (BuNHZ),, 1262 FeC40H5204 [Fe(OC6HMe4-2,3,5.6)41-. 230 . . FeC40H56N40z
[Fe(~ctaethylporphyrin)(EtOH)~]+,261 FeCdLOHPn FeH,{ P(OEt),Ph},, I234 FeC42H26N10 Fe(phthalocyaninc)(py),. 1262.1264 FGzH~~N, [Fe(5,6-Me2phen)3]2f,1216 [Fe(4,7-Me,phen)J2 , 1217 FeC42H4ZC1P4 [FeCI{(PhzPCH,CH2PPhCH2)z}] ' , 1235 FeC44H2sCIN4 Fe(tetrapheny1porphyrin)CL 261,1267,1268 [Fe(tetraphenylporphyrin)Cl] ,265 FeC44Hz8CIN404 [Fe(tetraphenvlporphyrin)(C104), 261 . . . . FeC44Hz8F;N4 [Fe(tetraphenylporphyrin)F,1-, 260 FcC4.iHmN4 Fe(tetraphenylporphy-rin),1266-1268 [Fe(tetraphenylporphyrin)] , 1202 FeC44Hd402 +
Fe(tetraphenylporphyrin)(fl=CMcNHCH=cH), 126P FeC48H36N40 Fe(tetraphenylporphyrin)(THF), 1268 FeCdpH,,NAO, -~ - [Fe(tetraphenylporphyrin)(EtOH),lf ,260 FeC4BH54Ns Fe{ ~~;J=C(C~H~,)NN=(C~~H~~)=?JN=C(C~H~~)NN= (Cq.H,n)=N}. -, , 1258 FeC49H,lN~0 Fe(NO)(tetra hen 1 or hvrin){HN-H2)J, 1194 FeC50H34N8 [Fe(2-phenC=NNPh2),12+. 1229 FeC&d8 rFe(tetrapheny1porphyrin) (imidazolc),] '. 260 .. . . FeCC,2H40N& Fe(retraphenylporphyrin)(py)(ONCHMe,), 1268 FeCSJL4N4O2 Fe(tetraphenylporphyrin)(THF),, 1268 FeC5zH44N4S~ Fe(tetraphenylporphyrin){=H, 2 , 1268 [Fe(tetraphenylporphyrin){ (CH,), H2}2]+,260 FeCS2H48C1P4 FeCl(dppe),, 1184,1198.1199 FeCs,H,,CIzP4 FcCl,(dppc),, 1198 FeCs2H4sN202P4 Fe(NO),(dppe),, 1067 FeC52H48N2P4 Fe(Nz)(dppe)z,1197 FeC5,H4& _I
-
F-$
Fe(tetraphenylporphyrin)-(HfiCH2CH,NHCH,~Hz)2, 1267 FeC52H4sP4 Fe(dppe),, 1197 [Fe(dppe),]+, 1198, 1199 FeCS2H4&1P4 FeHCl(dppe),, 1198 FcCdLN,P, [FcH(N,)(dppe),]+, 1198, 1234 F~CSJLP~ FeH(dppe),, 1183,119X.1199 [FeH(dppe),]+, 1199 Fe C 5 2 H ~ ~ P4
Formula Index
1334 [FeH,( dppe),]'. 1198 FeC5,H5,P4 [Fe(rl,-H,)(tr)(dppe),]t, 1234 FeC53H480P4 [Fe(CO)Idppe)21+,1199 FeCS3H4@P4 [FeH(CO)(dppe),] , 1199 F ~ C S ~ Ho ~ ~ W [Fe{2-phen&NN=CPhC(Ph)=& }J2+, 1230 FeCs.&sN6 Fe(tetraphenylp~rphyrin)(py)~,1268 [Fe(tetraphenylparphyrin)(py),]+, 260 FeG4H42ClP4 [FeCI{P(C,HJ'P h2-2)3) I+, 1235 FeCs&& Fe(tetraphenylporphyrin)(Bu'NCjJ, 1268 +
[FeH { P(OMe), } ( d p ~ e ) ~, ]1199 FeCS6H42N2P4 +
Fe(CN)2{P(C6H4PPh2-2)3}, 1235
FeCS8H&12N4
[Fe(tetraphenylp0ryhyrin)(C==C(C,H,C1-4),)]~~ 265 FeC60H52N~Os Fe,(fi=CHC,H,NCH(OMc C6H4N= CHC,H,NCH(OMe) ,H4)0, 258 FeC68H66ClzPs Feci2(PhP(CH,CI 12PPh2)2}2, 1234 FeC&42N6%& [Fe{(0,SC,H4)2 phen},I4-, 1217 FeC72H6004P4 [Fe(OPPh3),I2+,1184 FeCdh~6N408 Fe{ {Me(CH,),CO,),phthalocyanine}, 1266 FeCIN,O, Fe(NO),CI. 1193 FeCI2HsO4 FeCIZ(H,O),; 1250 FeCl,N,O, IFc(NO),Cl21', 1194 _" FeC13N0 [FeCI,(HO jL. 1194 FeCI, [FeCI,]-, 218.227.247,248, 1183 [FeCL]?, 1182 [FeC4I2-*1182,1184, 1246, 1249 FeCl, [FeC1,I3-, 237.248 IFeC1,14-. 1250 FeCoCSHlsNl2
Fe(PF,),, 1197 FeGeC3XH1DNOIP
Fe(GePh,)(CO)2(NO){P(OPh)3j,1189 FeHOSS Fe( OH)(SO,), 1238 FeH,CI,O [FeCI,(H20)]Z-,248 FeH,F,O [ F e ~ ( H , 0 ) ] 2 -217 . FeH8CI2O4 [ F c ( H ~ O ) ~ C I ~227,247 ]-, FeHloCIOS 227 [Fc(l120)5CIj2+, Fel I l o N 0 6 Fe(H,OjS(NO), 1184 [Fe(1I2O),(NO)]*+,1188,1203 FeH1206 [Fc(H20),J2+,1184.1185.1204. 1236 [Fe(H20),l3+,218.226.1180, 1185 FeH15N60 [Fe(NH,),(PiO)]*+.222 FeH18N6 [Fe(NH,),l2+, 1210 FeIN303 Fe(NO),I, 1193 FeI,N,O, Fe(NO),I,, 1194 Fe1,NO Fe13(NO), 1194 Fe Li C, ,l-l&
F~I,~(OCHBU',)~(HC)CHB~',), 227 FeMo,S, [ M O , F ~ S , ] 242 ~~, FeN4012 [Fe(NO,),]-, 229,1183,1185 FeN15 [Fe(N,),I2-, 222,1183 Fe0, [FeO,]*-, 267,1183,1187 [Fe0413-, 267,1183 FeOsC4H,,N4O7 [ F ~ ( H , O ) , 0 0 s O ( e n ) ~.583 ]~' FeRhC9H19N405 [Rh(HDMG)(HDhlG-Fe)(H,0)Me]2C, I015 FeRuCJ 118Cl,0,P2 RuCl(FeCl,)(CO),(P.Me,),, 382 FeRuC9H19N12
[Fe(CN),(p-&=CHCH=NCH=cH)Ru(NH,),]-. 319 FeRuCl1H2,N6 [Ru(NH,),(FeCN)12+.317
[Co(NH,),(&CH=NCH=cH)Fe(CN),]-. 699 FeCoGH19N,,
[CO(NH,),(~~=CZICH=NCH=CH)FC(CN)~] -,690 FeCoCllNll [Co(CN),(pCN)Fc(CN),13 -,650 FeCoC,,H,,N,OP,
[Fe(CN)~(~-CN)CO(CN),(PEt,),olj FeCoMoC1,H,O,S CoFeMoS(CO),Cp, 832 FeCoMoC,2H200,S CoFeMoS(CO),Cp(PMePrPh), 832 FeCrCbN, [CrFe(CN)J. 1204 FeF4P2S4 Fe(S,PF,),, 1216 FeF, [FeF6l3-, 218,247 FeFLSi FeSiF,, 1236 FeFlSP5
~
647
Fe-{N(SiMe&}3, 222, 1183 FeSnC1~H&1.&2P4 Fe(SnC1,)CI{P(OMej3),. 1233 FeSnClSH4sC1301SP5 Fe(SnC1,) { P( OMej3},. 1233 FeSnC20HlSCL3N03P Fe( SnCI,)(CO),(NO)(FPh,), 1189 FeSr204 Sr2Fe0,, 266 Fe,C2Ii,N,O4S, {Fe(NO),(SMe)>,, 1191 Fe2C4Hl0N4O4S2 {Fe(NO),(SEt)},. 1191 Fe&N6
1335
Formula index [Fe{Fe(CN),}l2-, 221 Fe&Hi6& [Fe,(SCHzCH,S)4]2-. 1241 FezCsHzoS6 [Fe,S,( SEt)4]z-, 238 FezCioH3Nii [Fez(CN)io(NH3)]4-. 221 FezCioNio [Fez(CN)io]4-,1205 [Fez(CN)io]5-, 1205 [Fez(CN)io]b-,1204, 1205 FezCiiNii 12U5 [Fez(CN) FezCizHioN404S2 {Fe(NO)Z(SPh)}z,1191 FeZCi4H~004 {FeCp(CO)z}2.1198 FeZC14Hi4N20i~
Fe(Hz0){2,6-~~(C02)2}(u-OH)zFe(HzO) ( 26-
[FeZ(~=CI-IC6H4N=CHC6H4N=CHC6H4N= m 6 H 4 ) z 0 ] 4 +257 , .
.
Fe;C6iH32Ni60 {Fe(phthalocyanine)}zO,, 262,1265 Fe2C64H3zNi7 { Fe(phthalocyanine),}N, 1265 Fe2C74H42N19 [(Fe(phthalocyanine)(py)}zK]+. 1265 FeZC82H81P6
{Fe(triphos)},(p-H),, 1234 Fe,C,,H,,N,O {Fc(tetraphenylporphyrin)}20. 261. 1268 [Fe,(tctraphcnylporphyrin)zO]-. 266 [Fe,(tetraphenylp~rphyrin)~O]~+~ 266 Fe~Cs&6N9 {Fe(tetraphenylporphyrin)}zN. 261 FeZCS9H56N8
{Fe(tetraphenylporph)rin)),C. . . . Fe;Cl2S2 [FezS2C1z]2~, 1243 FezCt4Sz [FezSzC14]z-,236 Fe2CI6 Fe2C16,248 Fe2C160 [ F ~ C 1 ~ 0 F e C 1 ~248 1~-, [(FCC~,),O]~-.1185 [Fez{(0zCCH2)2NCI12CH2N(OH)CHzCOz}zO]2 ,253 FezOC16,225 FezCibH3&S4 Fe2Fj { F C { ( S C H ~ C H ~ N M ~ C)?,H ~1247 )~) Fe2F5,247, 1187, 1249 Fe2FzzC8 ~, (Fe(PFd3 1z ( ~ - P F ~11) 97 FezH4FsOz FezFs.2Hz0,1249 Fe2H14F507 Fe2Fs(HZ0)7,247 FezHlsOlo
PY(COZ)Z}, 228 FeZC14Hi4S4 [FezS2(4-SC6H4Me),],-, 1241 Fe2Ci4HZOS4 FezSz(SEt),Cp2, 1243 [FezSz(SEt)zCpz]+,1243 FezCihHi6S6 [FczSz{1, ~ - ( S C H ~ ) Z C ~ I ~ 1241 ~}Z]~-~ IS FeZCi6HZ2N40
1270
[Fe(HzO)4(p-OH)zFc(H20)4]4~, 228 Fe*J%C6NZ08 253 Hg{Fe(CO),(NO)j2, 1189 [Fez{(0,CCHZ)zNCH2CHZN(CH2C0,H)z},0]4-, FeZC24H16C1ZN40 ISC~OHH~ONZO~PZ Hg { Fe( CO)z(NO)(PPh,) ] z, 1189 FezO(phen),CIZ, 223 Fe2HgC40H30NzOl~~~ Hg{Fe(CO),(NOj{P(OPh),)}a, 1189 Fe21rC3sH30104P2 IrI{FePPh,(CO)zCp}z. I111 Fe21rC38H3004P~ [Ir{FePPhz(CO)zCp},]+, 11I1 Fe21rC39H30Br05P2 1111 [ IrBr(C0) { FePPhz(CO)zCp} [ {Fe{2,6-py(CHNHCH2CHzNHCHz)zCHz)zCHz}}z02]4i FezIrC39H30C105Pz 1258 [Ir(CO){FePPhz(CO)2Cp}zCl]. Ill1 FeZIrC4iH3~07P2 [Ir(CO),{FePPh,(CO),Cp}2~+3 11I 1 FeZMoCl4S4 [MoFe,S4C1412 ,242 F c & ~ ~ ~ S ~ [{Fe(NO)zS}z]Z-,1141,1243 {Fe(salen)},O, 251, 1185 Fe,Na,CqnHF7N407 [Fez(salen)zO]’. 266 F~ZC~ZKZN~O~ Fe,Na;{ { MeCOCHC(Me)=NCHz}Z}z(OEt)(THF)z, {Fe(salen-H)(OH)}z.252 1238 FezC36Hz4N606 Fez05S FezO(S04),1238 { Fe(8-q~inolinolare)~(NO)) z, 1195 Fe2C40H3~N~ 03 Fe2RuC8Hi8C1802P2 [Fez(2,2‘:6’,2”:6“,2”’-tetrapyridyl)z(HzO)20]4+.226 Ru(FeC14)z(CO)z(PMe3)z,382 FeZC42H30NZOZSh FeZR~2C60H62C14P6 .1 ‘-diphosphaferocene)},, 414 {RuCl2(3,3’,4,4’-Me4-1 Fe2(NO)z(SzC,Ph,)3, 1193 FeZC5ZH60Ni203 Fe& [ E e Z { ~ 2 - p y ~ ~ ~ ~ ~ ~ ~ H ~ C H Z ~ ~ ~ H z ~ ~ -[FyzS1ZI22 j z } z (, 1240 HzO)~O14~, 220 Fc&Hz& ~
1336
Formula Index Fe 11 Si, FellSlz, 1240 GaMnClIH2,NSO3 Mn(3,5-dimethylpyrazole)(NO),( Me,NCH2CH,0GaMe,), 9 GeAszRhC,9H40C1 RhHCl(GeMe,)( AsPh3),. 1020 GeFeC38H30N06P Fe(GeYh3)(CO)2(NO){Y(OPh),}, 1189 GelrC37H31C:IOP I ~ H C ~ ( G C P I I ~ ) ( C O ) ( P1126 P~~), GeTrC4,,H4 OPz IrHz(GeMe,)(CO)(PPh3)2, 1 126 GCR~C~~H~TC~~P~ RhHCI( GeCI3)(PPh3)>,1020 GeRhC39H,oClP2 RhHC1(GeMe3)(PPh&, 1020 GeRuC7H904 [Ru(GeMe,)(CO),]-, 287 Ge,RuC4C1604 Ru(GeCl,),(CO),, 287 Ge2RuCloH1804 Ru(GeMe3)2(CC))4.287 Ge2RuCl2HI8O6 Ru2(p-GeMe2),(CO),. 287 Ge,RuC,,H,,O,P, Ru(GcMc3)2(C0)2(PPh,)2: 287 GezRu2Cl4HlsOs { Ru(GeMe,)(CO),},. 287 G c ~ R u ~ C I ~ ~ H ~ ~ ~ ~ (Ru( p-GeMe,)(CO),) 287 Ge4Ru2C1GH3006 { Ru(v-GeMe2)(GeMe,)(CO),},, 287 Ge6RhCl18 [Rh(GeCI,),]-, 1019
;.
Fe&S, [Fe4S4C14j2-,236240,1242-1244
H~As~R~CZ~H~~CI~ RhC13(AsMe2Ph)3(HgC12):1076 HgAs&hCwHwCLF RhC12(HgF)(AsMePh,),, 1076 HgAs3RhC39H3PC13 KhC12(HgC1)(,AsMePh,),. 1076 I IgAs,KhC.~,,H,,CI,O, RhCl,( I-IgOAc) ( A r; Me Ph 2 ) , 1O76 HgC4N4Se4 IHg(SeCN),)'-, 681
Formula Index
1337
[Ir(CO){1,4-{(Ph2AsCH2CH2)2PCH2}2C6H4}] ', 1I14 IrAs4C72H57P [Ir(PPh3){AsfC6H,AsPh,-2),}]+, 1135 ~~AS~C~ZH~IP~ IrH(AsPh,),, 1119 I ~ A u C ~ ~ H ~ ~ N ~ P ~ [Ir (CNBU')~ (p-dppm),Au] 2+, 1153 I~AU~CIOSH~~P~ [IrA ~ ~ H , (p p h , )~ 11166 +, IrBC8H8N40, I~~CO),(H,B(AN=CHCH=~H),). 1io8 1rBC12H16N402
lr(CO),{H,B(AN=CMeCH=CMe),}, 1108 IrBCl9HZ7F~N6O5 Ir(O,CCF,),{ HB(fiN=CMcCH=~Me),}(H,O), 1134 kBCzoHz9C1N60z,1134 I~(~c~~)CI{HB(~N=CM~CH=(?M~)~), 1134 IrBC36H31ClF4N2P2 IrHCl(FBF,)(N,)(PPh,'),, 1105 '
IrB4CZ7H3,0P2 Ir(q4-B4H9)(CO)(PPh,Me)2r1165 I rH 8(355H54P3 2 , 10-lrCH8H8-1-PPhS-2,2,2-H(PPh3)Z, 1165 IrB8PtC13H470P4 PtBBHloIrH(CO)(PMe3)4, 1165 IrBrzC14 [IrC14Br21Z-,1157 IrBr4Clz IrjCNBu')(CO)( L-dppm),AgCl, 1163 [IrBr,ClJ-, 11.57 IrBr6 IrAsBCZ3H3Cl2N6 IrCl,{ HB(fiN=CMeCH=cMe),} (A,PhMe,), 1135 [IrBr,]'-, 1157 [IrBr613-, 1149,1157 I~ASC~~H~~CIOP~ [IrHC1(AsMeZPh)(CO)(PMe,Ph),] , 1150 IrCH15N503 IrAsC,,,H,,NO, [Ir(NH3)4C03)l~'960 Ir(OAc),(AsPh,)(CNC,H,Me-4), I121 [Ir(OCO,) (NH&]+, 1141 IrC2C1,NOh Ii.AsC38H40N302 Ir(AbPh3){MeCOCHC(Me)=NCH2},(CNC6H4Me-4), IrC.l,( NO,)( C,O,)]"-, 1 142 Ir&C1,NO6 1124 [IrCI,(N0,)(C,04)]3-, 1142 TrAsC42F136N302 IrCZC1404 Ir(AsPh3)(salen)(CNC6H4Me-4), 1124 [IrC14(CZ04)]3-,1142 IrAsC7,H,,P4 IrC3H6Ns [Ir(PPh,) { A s ( C ~ H ~ P P ~ ,1'- ,~1135 )~) Ir(CN)3(NH3)2,1125 IrAsZC27H22CI0 IrC,N, IrCI(CO)(Ph,AsCH=CHAsPh,), 1108 Ir(CN)3, 1125 I ~ A S ~ C ~ ~ H ~ ~ C ~ N ~ O ~ IrC4ClZ08 IrCl(N03)2(CO)(AsPh3)2.1144 [IrC12(Cz04)2]3-,1142 IrAs2C37H30CI0 IrC,H,,Cl,S, IrCI(CO)(AsPh,),, 1159 IrCI3(SMeZ),,1145 ITAs,C,~H~~O IrH,(AsPh,),(CO). 1134 IrC4Hl2CI4O2Sz [IrC14(DMS0)2]-, 1160 IrAs2C40H74C1 IrC,H&l4S2 IrC1(2-CH2=CHC,H,AsPh,) 1111 [IrC14(SMe2)2]-,1157 ITAs~C~~H~~CIN IrHzC1(AsPh3),(4-MeC6H4Ncjr 1125, 1134 IK4H16ClIN4 [IrCII(en)J', 1129 lrAs2C4,H4nN IrC4H&12N4 IrH3(AsPh3),(4-MeC6H4NC), 1125, 1134, 1156 (IrClz(en)z]t, 1129, 1161 I ~ A S ~ C ~ ~ H ~ ~ N O ~ Ir( acac)(AsPh3),(4-MeC6H4NC), 1126 ITC~HISNIO [Ir(N3)2(en)zl', 1133 I ~ A s ~ C ~ ~ H ~ ~ P ~ XrC4H18CIN40 1135 [Ir(PPh3){ P(C6H4AsPh2-2)3}]+, [IrCl(en),(HZO)]z', 1129 ITAS~C~~H~~NOP~ IrC4H18N40, [Ir(NO){l ,4-{(PhzAsCH2CH2)2PCHz)2C6H4}~+, 11 14 IIr(OH),(en),]+, 1129 IrAs4C64H64P2 [Ir { 1,4-{ (Ph2AsCH2CH2)2PC112) &H4}] , 1113 IrC4H1&02 [Ir(OH)(en),(H,O)]", 1161 IrA s4CJM'z [IrH,{ l,4-{(Ph,AsCH,CH2),PCH,},C,H,}]+.1114 1GH22N6 IrAs,C,,H,,OP, [ I T ( ~ ~ ) ~ ( N H 1124 ~)~I+, +
+
C.O.C.4-cjQ'
Forrnnlu Index
1338 . .
[Ir(CN),(H)I3-, 1166 IrC5H2N,0 [Ir(CNj,(H20j]2-. 1125 IrC5HjC1,N [1rClj(py)l2-, 1130 IrC6HI7CIZOjS3 IrHC12(DSbf\i10)3.1160 IrC6HlBClF,N,P, IrCI(PF,NMc,),. 1163 II-C~H.,~ CI~N,
[IrC1Z{(H,NCH2CH,NHCH,)Z)] ' , I 131)
IrC6HlaCI,O,S> IrCIJ 1) MSOj3 'I I h0 1 ~ ~ ~ 1 1 ~ IrCI,(PMe3),, 1134. 1155 IrC6HI9CIZO3.S3 IrHC!z(DMSO)3, 1159 IrC6HZ3P, IrH5(PMejj2.1158, 1160 IrC6H24Nb [Ir(en),]+? 1162 IrC6N6 [Ir(CN)J3-, I125 IrChN606 [Ir(CNOj,1.7 , 1126 ~
~
~
1
~
~
~
II-c:6c),2
[lr(C204)3]2-7 1150
[Ir(C,04)3]3-. 1112, 1156 IrC7H7K202 Ir( CO),(p-r;"=CMeCH=cMe), IrC7H70, Ir(acac)(CO),. 1116 IrC7HI8ClOP2 IrCl(CO)(PbIe,), . 1109
1161
IrCI,(PMe,),, 1134 IrCI3Hl9O2 Ir( acac)(cod) , 11I5 IrCI3Hz5P [IrH(C,Me,)(PMe,)]'. 1166 IrCi,Hzd' IrH,(CSMe5)(PMe3): 1166 IrC,,H33CIOP3 IrHCI(COj(PEt3)2(PH2),1163 1rC13H3bOP4 [ 1rH(CO)(P~t,jz(P1I2) (PH,)] +,1163 IrC.,,H,,,O,PS, Tr(CO),{SSP(OPl~),}.1119 1rCl4HjSNLS2 Ir{S~C2(CN),}(1-C5Me5!,1147 I rC ti zzC) ,1S' Ir(CC)),{SSP(C:,tJ,,),) 11 14: IrC,SHISC13N, IrC13(py)3,1130 I~CISHMF~O, Ir(hfacac)(q5-CsMe5j. I1 15 IrC,,HzzOz Ir( acac)(~f-C,Me,). 1115 IrC,,H,,N3S' Ir(S2CNC4H,), , 1145 IrC,,HzzC14P, IrCl,(PMe,Ph),, 1156 [IrC14(PMc2Ph)2]-,1156 IrCI6HL7N2S4 1i-(S,CNMt",),(11-CjMCI). 1 147 Ti-Cl6H3,P2 [IrH(C,Me,)(PMe,),]', 1166 I~CI~H~OC~S~ [IrC12(SEtz)4]i,1145 IrC17H,,C1NOP IrC1(CO)(Ph,PCH2CH2NMez), 1115 IrC17H,,C10P, IrCI(CO)(PMe,Ph),. 1108 IrC,,H,,CI3OPZ IrCI,(CO)(PMe,Phj,, 1109 IrCl7H,,C:IC)P, IrCl(C0)(HPnui,j,, 1.1 11 IrC sHzzC10P2 IrCI(CO)(Ph,PCH2CH3CH2PMc,), 1115 IrC,,EI,,CIOP,S, IrCI(CO)(PMe,Ph),(CS2), 1146 IrCl,Hz,03 Ir(OAc)(cod),, 1159 IrC18H2sCI,P IrHCl,(cod)(PEtPh,)I 1109 IrClBH4zC10P, IrCI(COj(PPr'3)2. 1162 IrC18H47C1N2P, IrCI(N 2 ) (Pl'r'& 1162 IrC:,8H47P2 IrH,-(PPr',),, 1 107
IrC,,H1,C1,S [TrCl,(SPh,)l.-. 1157 IrC ,,Hl,,CI,S
Ir(q4~cod)(q3-C,Hg)(PHBut,), 1163 IrC,ZH1yCl&
Ir(4-MeZNC,H,CH=NCHzCH,CH,NH,)CI,.1162 IrClzHzoN4 [I$C%Et)4]-. 1101 IrCl2HZ2CIS2 IrCl(cod)(McSCH,CH,SMc). 1117 IrC12H30C13S3 IrCI,(SEt2)3 1145 IIC,,IT;,P2 TIH;(PLt,)2. 1151 IrCI2l1&I2P4
IrC1yH4,0P3 [IrH,(CO)(PEt3)3]f. 1152 IrCZoH16C1,N4 [IrCl,(b~py)~]. 1130. 1162 IrCZ0Hl8N4 [IrH,(bipy),]-, 1162 IrC,oH,oC121\;4 [IrC12(~y)41+, 1130 TrC,,FI,,IN2 IrI(phen)(cod). I139 IrC,,&,NL [Ir(phcti)(cod)]'. I139 IrC20H20N202 +
Formula Index [Ir(O,(phen)(cod)]+ , 1139 IrC20H2904 I r ( a c a ~ ) ~ ( q ~ - C , M1115 e~), IrCzoH35P~ IrHS(PEt2Ph)2,1158 IrCZ0H36N4 [Ir(CNBU')~] +,1102 IrC21H1s03P L-(COMPPhdp, 1100, 1103 IrC2,Hz0CINOP IrC1(CO)(2-Ph2PC6H4NMe2), 1114 IrC,,HlnC1OP2 IrCl(CO)(PEt,Ph),, 1108 IrC21H30C130P2 IrCl,(CO)(PEt,Ph),, 1109 ~TCZZHI~F~N~~~SZ [Ir(bipy)2(OS02CF3)2]+,1162 IrC22H46ClP2 I_ I"
1339
IrCl(CO)(PMcPh,),( Me), 1109 IrC2sH&IZ03P2S
IrCI2(MeSO2)(CO)(PPh,Me)~, 1146
'IrHC1(CH=CHCH2PBut2)(But2PCH2CH=CH), 1137 IrC23H26C1NP2S2 IrHC1(PPh3)2(S2CNEt2),1106 1116
'1116 IrC24H28N2
[Ir(3,4,7,8-Me4phen)(cod)]*, 1160 IrC24H30S3 Ir{ S(CH2CH2SPh),}(cod)]+, 1117 IrC24H32C12P3 frCl,(CH,PMePh)(PMe2Ph)2, 1137 . . 1r~~~H&l4P3 IrC13(PMezPh)Z(PMePhCHzCl),1137 IrC24H33C12N03P3 IrC1,(N03)(PMe2Ph)3, 1144 I~c,,H;,cI,P~' IrCl,(PMe,Ph),, 1137 IrCz4Hd3 IrH,(PMe,Ph), , 1164 IrCZ4Hd'3 [IrH4(PMezPh),]+. 1164
TrC H&lNP, ~~~Cl(~H=CHCH,~Bu'z)(But2PC~2CH=CH,)(4Mepy), 1137 IrCz3H30C103P~ IrCl(CO)(0,)(PPh,Et)2, 1138 IIC~~H~ONOPZS [Ir(dppe)(MeCN)(q1-SOMe)12f, 1165 IrC2~H30'6PZS~
IrH(CO)(SCH2CH2PPh2)(HSCH2CH2PPh2),1163 IrC,0H,,Cl,0P2 1109 I~Cl,~COEt)(PMePh2)2~
IrH2C1(PBu'3)2,1134 IrC25H21ClN30 IrCl(CNC,H,),(CO), 1102 IrCz5HZ3N40P , [ i ; ( b i ~ y ) ~ ] ~1130, + , 1162 Ir(C0) {H,B(fiN=CHCH=CH),}(PPh,), 1108 TIC~OH~~N~O IrCZ;H2;NZOPS2 [Ir(bipy),(OH)I2+, 1131 [Ir(CO)(PPh3)(q2-SCNMez)2]+, 1154 IrC3oHz6N60 IrCz5H27N20PS3 [Ir(bipy)3(H,0)]3+,1131 II~(CO)(TI~-CSNM~,)(PP~~)(S~CNM~~)]+, 1118 IrCJoH,,C1,OPz ~rc ,sk2,Nzbz IrC1z(C3HS)(CO)(PMePh2)2, 1164 Ir( acac)(cod)(phen) , 1165 IrC10 4 6c12p 3 IrC25H34C10P3 IrHCl,(PEt,Ph),, 1159 [IrHCl(CO)(PMe2Ph)3]+,I150 IrC3oH47OP2 Ir~26HzoClzN4' Ir( C,Ph)( CO)(PPr',),, 1 162 [IrCl,(phen)(S ,6-Me2phen)ltI 1130 IrC30H4703Pz IrC,,H,,BrClP 'Ir(CO){PBut,(C6H4d-2)} {PBut2(C6H40Me-2)},1110 Ie3oH4704Pz kr{ CH2CMe2PButC6H3(OMe-6)0)(OC.&3(0Me-3)(PBu'Z-~)},1138
1115 Ir(a~ac)(nbd)~, IrC,6H45CIP I I C I ( C O ~ ) ( P ( C ~ Hl~l O~Y) ~ ) ,
IrC30H4904P2 mC6H3(OMe-3)(PBu'2-2)}2,1138
1340
Fomula lnilex IrH(octac~tivlporph~rirr). 1155
IrBr(YO)2[~PPh3)2. I159 IrC3,H3,Br3h'OP2 IrBr,(NO)(PPh,),, 1120 IrC,,H30CINOP, [IrCl(NO)(PPh,),] ' , 1104 IrC,,H,oC.lK'ZOLP2 Ir(NOj,Cl!t'rli,)L, lWi) IrC361.13,,C1N2PZ IrCI(N2)(FPI~3)2, 11115. 1.106, 1.159. 116U IrC:,,H,,CI,NOP IrCIZ(NO)(PPh,j. 1106 IrC,,H,,CI,YOP, IrC12(Y0)(PPhzjz.1104 IrC36H,,C!2N03P2 IrCIZ[U0,) (PPhijL. 1144 IrC,,H,,,C13%OP2 [IrC13(NO)(PPh3),]', 1104 IrC36H3UK,02P2 [Ir(NO)Z(PPh,)2]'-,1105, 1159 IrC,,H&,OSPL lr(K03j~[f'l'h3:)2. 1143 IrC.36113,)Nj07P2 Ir(~03)2(NU)(PPli,),,I I44 IrC36H3.,clsoP2 IrH(NO)C!(PPh3)2, 1100 IrC36H3,C12P2 IrHC12(PPh3')2.1135. 1149 IrC36H,,N0,PI [Ir(OH)(NO)(PPh,j2] ' , 1104 IrC36H3,N,0,P2 IrH(N031,(PPh3),, 1144 IrC36H1,C13NOP2 Ir(YH,OHjCl,(PPh,),, 1100 IrC36H44r N Irl (octatthylporphyrin) , 1 155 lr(.Y3bl l,JJ4
[ ~ r ( o c t a c t h y ~ ~ ~ o I p h 4 . r1155 in)l~, 1rG6H4&
IrC37H290P2 Ir(CO)(C6H,PPh2)(PPh,). 1114 IrC37H;oCIN02P2 [IrCI(NO)(CO)!PPh,),l-. 1104 IrC3,H3,CINdM', IrCI(NO, J(N 0 3 ) ( C 0l(PPh3)]., 1143, 1144 I K , , l I,,,CINzO-~Pt J rC?I(NOj)?( CO)(PP h3'):. 'I I 43 IIC~~H~~C'IOP~ IrCI(CO)(PPhj),: lli15, :I(& 1109, 1138, 1163 IrC3,H,,C1O2P2Se TrCI(CSe)(0,)(PPh,j2, 1148 1rC3,H3,CIW'2 IrCl(C0)(O2)(PPh&. 1139. 1140, 1159 IrC,,H,,CIO3P2S IrCI(CO)(PPh,),(SO,). 1118 IrC,,H,,CIO,P,S IrCI(C0)(0,S02)(PPh,)2. 1140 IrC:,7H,,C:11'2Se Ir(.?I((ISe)(t'Ph,),. 1148. 1165 IrC37H30C?IP2Sc2 I ~ C I ( C S ~ , ) ( P P I114s ~~,I~, IrC.37H&130P2 IrCI,(CO)(PPh,):, 1117 IrC3,H3,JCIJ'2Se IrC1,(CSe)(PPhz)z,114X IrC3,H30102P2 [IrI(NO)(CO)(PPh,)Z]+.1104 IrC37H30130P2 IrI,(CO)(PPh3)2. 1141 IrC3,H,,N0,P, Tr(NO)(CO)(PPh3)3.i100, 1104 [ I I - ( N O ) ( ~ O I ( P P ~ , )1~In0 ]+. IrC,,HJ'J30P> Ir( N,J(CO j ( PPh,,),. 1 1hl TrT37H,30N30J'2 rr(No,)(l.ro,),(Co)iPPh,),, 1143, 1144 Ir(NC)3),J~~))2(CO)(PPh3)1, 1100 IrC3,1 J31CIIOP2 IrHCII(COj(PPh,;I,. I 1 19 TrC3,H3,C120P2 IrHCl2(COj(PPh3j2.1149 IrC3,H31C12P2S2 IrCI,(S,CH) (PPh,) I 1146 1rC37H3N206P2 IrFY(NO,)(~O,j~CO)(PPh,)2,1143, 1144 ~
TrC37H31N207P2
IrH(N03)Z(C0)(PPh3)Z. 1144 IrC37H310ZP2 Ir(OH)(CO)(PPhi)2,1I16 II-C.?~,H,,O~P~S 11-1l ( ( . ~ ~ ) ) ( l ' l ' h 3 ~ 1 ~ S11 ( ~18 ,)~ TrC,,H3,CIOP2 IrH,CI( CO)(PPh, j,. 1150, 1159 IrC3,H3,C1OP2S IrHCl(SH)(CO)(PPh,),, 1145,1165 IrC37H33C12PZ IrC1,(PPh3)2(.\.~fe):I137 IrC37H331NOP2 IrMe(NO)I(PPh,j,. 1100 IrC37H330Pz IrH,(CO)(PPh,),, 1150 1G7HJ3PZS2 IrH2(S,CH)(PFh3),, 1146 IrC,,l I,,CIN,O IrCI(CC))(oc!aethviporphyrin), 1120, 1155
160.
Formula Index
II'C~~H~~CIOPZ IrC1(CO){P(C6H11)3)zr 1110 IrC3sHzsOzPz Ir(C0)z(C6H4PPh2j(PPh3). 1114 IrC38H30C102P2 TrC1(CO)z(PPh3js,1159 TrC38H30CIOP2Se2 IrCI(CO)(CSe,)( PPh&, 1 148 IrC78H,,,C1P,Se2 IrCI(q,-CSc,)(CSe) (PPh,), 1148, 1165 1rC38H30F3N03PZ
Ir(0,CCF3)(NO)(PPh3)2, 11110, 1116 IrC38H3010PZ IrI(SCS)(CO)(PPh,),. 1146 IrC38H30NOP2 Ir( CN)(CO) (PPh3)2,1101 IrC38H30NOP2S Ir(NCS)(CO)(PPh,),, 1107 Ir(SCN)(CO)(PPh,),, 1139 1rC38H3002PZ
[Ir(CO)z(PPh3)2]7,1111, 1167 IrC3nHqoP2S3 [Ir(CS)(PPh,),(n-CS,)j+, I 1 18 IrC3sHJlClNOPl IrHCI(CN)( CO)(PP1i3)2,1125,1150 IrC38H310P,S2 Ir(CS,H)(CO)(PPh,),, 1116 Ir(S,CH)(CO)(PPh,),, 1116 IrC38H310~P~ IrH(CO),( PPh,),. 1120 1rG8H3@4P~ Ir(OCO,H)(CO)(PPh,)z. 1116 1rC38H3106PZ
Ir(OC0,H)(CO)(02 )(PPh,),, 1116 IrC3*H3,N4P IIrH(hipy),(PPh3)]Z+,1 162 IrC,8H33C1202P2S IrCI,(McSO)(CO)(PPh,), , 1146 TrC,8H3,CI,0,P,S IrC1,(MeSOz)(C0)(PPh3),, 1146, 1147 IrC39H30C1F6NOZPz IrCI(CO)(PPh,),{ON(CF,),) , 1143 IrC39H30CINZOP2S IrCl(CN)(NCS)(CO)(PPh3)z, 1125 IrC39H30F6NOZP2 [Ir{ON(CF3),}(CO)(PPh3),J+. 1117 IrC39H30FhN04PZ Ir{ON(CF3)Z}(CO)(02)(PPh3),, 1117 IrC39H3003PZ [Ir(C0)3(PPh3j2]', 1105, 1159 IrC39H30PzS5 [Ir(PPh3)z(CS)(o-CS,)(n-CS,)I+, 1 I 18 IrC3,H,,C1F,NP, IrCI(NCF,CHFCF,)(PPh,),, 1107 IrC39H310P~S4 Ir(CSSH)(CO)(PPh3)z(s-CS,),1146 Ir(SSCH)(CO)(PPh,),(s-CS,), 1146 IrC39H32C10P2
IrC1(2-PhzPC6H4CH2CHzC6H4PPhz-2)(CO), 1114 IrC?9H,,NOP, [Ir(CO)(PPh,),(MeCN)If, 1109 IrC3,H3,NP,Se [Ir(CSe)(PPh,),(MeCN)]+ , 1148 IrC39H3303P2 Ir(OAc)(CO)(PPh,),, 1116 I~CJ~HJ~O~'~ Ir(OCOJ)(CO)(PPh,),(Me),1141
1341
ItC&34CINOPZS IrHCl(CO)(rl l-SCNMe) (PPh&. 1151 ITC~~H~~NOP~ [1r(NO)(PPh3),(q3-C3H5]+.1104 IrC39H36C12NSP2 IrClz(+SCNMeZ)(PPhy)2, 1154 IrC39H39CIP3 IrCl(PMePh,),, 1109 IrC39H44C10ZPZ IrCl(diop)(cod), 1113 I~C~OH~OF~N~SPZ tr(N0)(02CCF3)Z(PPh,)2,1161 IrC40H3,CIF6NOP2 IrCI(CO)(NCF,CHFCF,)(PPh,), 1 1117 IrC4,~H33F61N0J'~ IrI(Me){ON(CF3),}(CO)(PPh3),,1117 IrC40H34C1PZ IrC1(2-CHZ=CHC6H4PPh&,1111.1I14 I~C~OH~~C~NOP~S [1rCl(CO)(qZ-CSNMe2)(PPh3j2]', 1118 [IrCI(CO)(qz-SCNMe2)(PPh,)2]+, 1154 1rC40H3612P2Se3 IrI,(MeSeCSeCSeMe)(PPh,),. 1148 IrC40H37NOP2S [IrH(C0)(q2-SCNMe,)(PPh,),]+1154 IrC4~H3&d', [IrH2(MeCN),(PPh,),] , 1166 IrC40H39N20,P3 [Ir(No, jz(C6)(PMePhz),], I 134 I C iH ~ C ~ F I O N K M ' ~
~r{ON(CF,)CF~CF2N(CF3)~}CI(CO)(PPh3),, 1143 ITC~~H&I,N,PZ Cl} (PPh3),, 1106 IrCl{N,m(Cl)=CClC(Ck)& I~C~~H~IC~~N~PZ IrHCI, {N&C(Cl)=CCIC(Cl)=~ C1) (PPh,),, 1107 I~C~IH~IF~OSPZ IrH(02CCF3),(CO)(PPh3),, 1117 I~C~IH~~NOZP& Ir(CO)(NCSe)(Me,CO)(PPh,)2.1107 IrC,,H,8C10,P, IrHCl(acac)(PPh,),, 1140 IrH,(acac)(PPh,),, 1140 IrC41H41C1N02PZ
IrHC1(PPh3)Z(0zCCHPrNH2), 1106 TrC42H35C1NZPZ IIrCl(N,Ph)(PPh,),l+, 1106 ~Ir642H35FNZPZ [IrH(HN=NC6H3F)(PPh3),1', 1132 IrCJ2Hq6CIP,S IrHC1(SPh)(PPh3)z,1150 I~GzHwFN~'z [ IrHZ(HN=NChH4F)(PPh3)2] 1132 IrC42H39C10P3 IrCI(CO)(triphos) , 1135 IrC421-139C1203P2 II-CI,(O,CB~~)(CO)(PP~,),, 1 112
+.
I~C~ZH~~OZPZ [IrH,(PPh3),(MezCO),]', 1166 IrC43H3oClzF50PzS2 IrCl2(SSC6F5)(CO)(PPh3),, 1147 1G3H30C1503 IrCI(1,2-0,C6C1,)(CO)(PPh3),, 1141
(PPh3)2, 1117 IrC43H34CIFN20P2
Formula Index
1342
IrC H ,ci,N OP, *,H30Me-4)(PPh,),,
1153
IrC,,H,,ClN,CIP, hHCI(NHNk,H,Ohle)(PPh,),, 1153 IrC4,H,,IN,P2 1r~I(HN=NC6H,Me-4)(PPh,),, 1132 IrC43H3902P3 [Ir(CO),(triphos)]+. 1135 IrC4,H42N20P2S4 [Ir(S2CNMe2)1(CO) (PPh,),]+, 1145
1
+.
Ir(O,CC6H4CI-3)(CO)(PPh3),, 1116 IrC44H37C120P2S
IrCI,(SC6H4Me-4)(CO)(PPh3),, 1146 IrC44H3703PZS Ir(0*SC6H,Me-4j(CO)( I'Ph3),, 111.8 IrC44H38CIOP2Se IrHCl(C0)(PPh2),(SeC,H,Me-4), 1 148 IrC,H,,CINOP, [IrCl(NO)(dppe)(PPh,)] ", 1104 IrC44H4ZC1NZ04P2 IrCI(N2)(PPh3)2(Et0,CCHCHCHC0zEt),1106 IrC44H42N202P2SZ { Ir(CO)( U-SCNMe,)(PPh,)} ,, 1154 IrC44H4z p3 [Ir(cod)(PPh,),]+, 1110 IrC,,HS6N4 Ir(cod)(octaethylporphyrin), 1155 IrC45H350P2 Tr(CO)(CIPh)(PPh3)2, 1109 IrC45H37C103P2 Ir(O,CC6H,C1-3)(CO)(PPh,),(Me), 1116 IrC4sH3,C120J'2 Ira,(CO)(PPh,),(COC~I,Ph), 1117 IrC45H4ZC1NP2 IrHCl(MeK=CHC6fI4Me-4)(PPh3),, I 'I52 IrC4,H5,CI0,P, 1rCI(CO)(PPh3),(Bu'OOH),, 1109 IrC46H.13N50P Ir(NO)(NC6H4Me-4),(PPh,), 1100 IrC4,H30Cl,0PzS4 IrCl(COI(PPh;l(tstrachlorotetrathionaDhthalene). 1165 I T C ~ ~ H ~ ~ ~ ~ P ~ IrWH(C0)4(p-PPh2)2(PPh3)(OAc), 1163 IrC48H38N30P2 [Ir(NO)(phen)(PPh3)2]2' , 1104 ITC48H44P4 -I
[Ir(Ph,PCH=CHPPh,),]+, 11 1 1 , 1112 IrC5zH48N204P4 [Ir(NOz),(~pp~)zl+. 1143 IrCszH4&P&z [Ir(dppe),(qz-S20)]-. i117, 1121, 1165 IrC52H480P4SeZ [Ir(dppe),(q2-Se,O)]+, 1165 IrC52H4802P4 [Ir(dppe)z(Oz)]+.1112. 1139 IrC5,H4,02P4S, [Ir(dppe),(q2-S,0,)]+. 11 17, 1165 IrCSZH4,P4 [Ir(dppc),]', 1111. 1112, 1160 IrC',,H4,P4SSr [Ii(dppe),(q2-SSe)] 1165 IrC5LH48P4S2 [Ir(dppe),(S,)]+, 1112. 1117. 1147 IrC5,H4,P4Se2 [IrSe,(dppe),]+. 1038 IrC,ZH,9CIP4 [IrHCI( dppe),] 1150 IrC52H50P4 [IrHz(dppe)ll+, 1150 IrCS2Hs2I5P4 { IrI(PMePh,),) 2 ( ~ - 1 ) 103s ~. XrCS3H440P4 [Ir(CO)(Ph2PCH=CHT'Ph2)2]t, 1113 IrC:SqH4,0,1'2 Tr(OzCPh)3(PPh3)2,1131 IrC53H480P4 [Ir( CO)(dppe),]+ , 1112,1113 1rC53H510P4S2 [Ir(dppc),(q2-SzOMe)]?'. 1165 IrC53H51P4S2 [Ir(dppe),(x2-SzMe]+, 1117 [Ir(dppe),(qz-S1hie)]2+, 1121 IrCS4H4,C1P4 IrC1{P(C,H4PPh2-2),}, 1135 IrC54H42W'4 [IrC~,{P(C,H,PPh,-2),)1-, 1136 IrCF4H43C1P4 [IrHCI{P(C,H4PPh,-2),)1-, 1136 IrC54H44P3 Ir(C,H4PPh2)(PPhJ)2. 1114 lrC54H45ClNOl', [IrC'I(No)(I'Ph,l,j-, 1 104 IrCS41145ClC)J'3 IrCl{P(OPh);},, 1136 IrC54H45C1P3 ~rHCI(Ph,P~,H,)(PPh,),, 1137 IrcI(PPh,j,, 111i, ilj9'IrC54H45NOP3 Ir(NO)(PPh3)3, 109!k1101, 1104,1105 IrC54H45N3P3 Ir(N3)(PPh3j3,1161 IrCs4H4sO7P2 Ir(CI,c:Ph),(CC))(PPh,),, 1141 IrCS4H4&IP3 IrHCI(PPh3)3. 1106 I~C~+H~~CIZO,P, lrHCI,{P(OPh),),l 1136 I~C~JH~~C~ZP~ IrHC12(PPh,)3, 1150 ITC54H46NOP3 IrH(NO)(PPh&, 1100 I~CS~H~J', IrH,( C6H4PPhZ)(PPh3) 1114 IrC5,H4,C1P3 IrH,Cl(PPh,),, 1150 I~C54H47NOZP3 Ir(No)(PPh,),(tr,O), 1100 IrCS4H4,N3P3 IrH2(N3)(PPh3)3,1161
\
IrH( SC6F5j2(CO)(PPh3),, 1150 IrC49H3gFZN40P2
.
[Ir(C0)(FC6H,NN=NNC6H4F)(PPh3),]1132 IrC49b1410P3S IrH(CO)(PPh,),(SPPh,), 1119 IrC.7oH4402P4 [Ir~dppm),(OJl+, 1139 IrCS1HZ9N30PZ Ir(C0)(4-MeC6H4NN=NC,H,Me-4)( PPh&. 1107 IrC52H44P4 +
1
Formula Index IG4H48P3 IrH3(PPh3)3,1114,1150,1159 IrC.,H.,NPA -[Ir(CNMe)(dppe)2]+,1102, 1111 I~CS~HSW"SZ [Ir(dp~e)~ (ScN) (SMe)]+, 1122 IrCF,H440P, Ii(C0) (CiH4PPh (PPh, )2,1114 IrC55H45P3S~ [Ir(PPh,),(n-CS,)l+, 1118 ~, IrC5,H4,0P3' IrH(CO)(PPh,),, 1119,1159 IrC&a,PqS IiH(CSr(PPh3)3, 1120 IrC5,H,oP3Se IrH,( PPh3)3(SeMe), 1148,1165 IrCs5HS4NP4S [Ir(dppe),(CNMe)(SMe)l2+, 1122 IrCs6Hs0zP3 I ~ H , ( O A C ) ( P P ~1150 ~)~, IrC56H45NOP3 Ir(CN)(CO)(PPh,), , 1109 I~CS~H~S~P~SZ [Ir(CO)(S2CPPh3)(PPh3)2]+, 1117 IrC56H45P84 [Ir(PPh3)l(o-CS2)(x-CS,)] ' , 1118 IrCS7H5,NOP3 I r ( N O ) ( P P h , ) , ( ~ H , ) , 1099 IrC60H52N302P3 [IrH2(HN=NC6~N0z)(PPh3)3]+, 1132 I -
IrCssHs4N~P6 [Ir(CNBu')z(dppm)3]+, 1163 IrCIF, [IrC1F5]2-, 1164 IrCL
1343
IrHClaO IrCI2(0H), 1149 lrHF,zP4 IrH(PF3)4,1120 IrHzBr50 [IrBrs(HzO)]2-, 1157 IrH2C1,0 [IrC15(H20)]-, 1157 [IrCls(Hz0)]2-, 1149, 1157 I~H~CI, IrC16H2, 1166 IrH4CI4O2 IrCl.,(Hz0)2, 1157 [IrCL,(H20)z]--,1149 IrH6C130, IrCl,(H20)3, 1149 .I
[iri(H;O)(NH,),I2+, 1128 IrH,,BrN, [IrBr(NH3)s]2+,1128 IrHl XIN, [kl(Ni-13)5]", 1128 IrH1SN5 Ir(NH,L. 1100 IrHl~N60~' [Ir (NO2)(NH3)5]2 + , 1143 [Ir(ONO)(NH3)5]Z+,1143 IrH15N8 [Ir(N3)(NH3),]2+, 1128, 1133 IrH,.CIN, [ir(NH;CI)(NH3),]3+, 1128, 1133 IrH,,N, [IrHZ(NH,),]3++, 1128 IrHl ,N60 [fi(H,O) (NH3)J3+, 958, 1128 IrH,eN, [Ir(Nn3)6]3+, 1128 IrH&,O [Ir(NHz0H)(NH3),]3C,1133 IrHgC37H30BrZC10PZ IrBrC1(HgBr)(CO)(PPh3)z,1124 IrHgC37H30~INz0P2 { IrCl(CO)(PPh3)2}Hg(N03)z, 1125 IrHgC37H30C130P2 IICI(H~CI~)(CO)(PP~~)Z, 1125 IrCIz(HgCI)(CO)(PPh,)2, 1124, 1127 IrHgC3yH&1NZOPZ I rC1(CN) (HgCN)(CO)( PP h3 ) 2r 1125 TrHgC39H31C13F6NP2
IrC12(HgCI)(NCF2CHFCF3)(PPh3)2, 1124 IrHgC53H40CIOP,
IrC1(HgC=CPh)(CO)(PPh3),(C=CPh),1124 IrFezC38H30104PZ IrI{FePPh2(CO)zCp},, 1111 IrFe2C38H300.J'~ [Ir{FePPh2(CO)2Cp}2]+,1111
[IrBr(CO){FePPh2(CO)zCp}2]+, 1111 IrFezC39H30C105P2 [Ir(CO) { FePPha(CO)2Cp}ZC1,1111 IrFezC41H3~07P~ [Ir(CO)3{FePPh2(CO)2Cp}2]', 1111 IrGeC1,H,,CIOP IrHCi(GkPh3)(CO)(PPh3), 1126 IrGeC40H410PZ IrH2(GeMe3)(CO)(PPh,),, 1126
IrHg3CliiH9oC1303P6 ', 1125 [{IrCI(CO)(PPh3)2}3Hg]2 IrHg4C148H120C1404Ps [ { IrCl( CO)(PPh,),} ,HglZ+. 1125 IrI, [1r1,13-, 1149 IrN6018 [Ir(NO3),I2-, 1156 [Ir(N03)6]3-, 1143 IrNls [1r(~4,13+,1133 Irb s 3 ~ C ~ ~ H , , 0 , , P OS,H,(CO),~{Ir(PPh,)}, 1166 IrPdCsoH40C12NsOPz
Forntrnla 1ndc.x
1344
[Ir(NO){(2'-p~),pqr1clazir~e}(FPli,),(PdC1,)]~+. 116: IrPtC,,H,,P, [PtH( PEt,) (k1-H)21rH(PEt3)3] , 1152 [Pt(PEt3)2(p-H),IrH,( PEt,)?] . 1152 IrPtC,,H&I,P, [IrC12(PMezPh)2(u-C12)Pt(PPh3),] ' , 1156 IrPtC,3H44C1N20P4 IrCl(CO)(u-dppm),Pt(CN),,1162 IrPtC6,H,+C1P, Pt(C:~CPh)li~-dppm),lrCI, 1112 IrPtC&l 1,8CIOP4 Pt(C=C:C:,H,Me-3),( y~dl.'pi"),lrC:I(C:C)). I I I 2 IrKhC,,Hh3C1P., IrH~(PEt.,'),~!~-C~lj(~~-l I)Rh(PEt,),, 1074. 1151. I164 lrRhC44H7J', Kh(dppe)(p-Hj21rH,[PPr',),, 1074 Kh(dppz)(rLa-Hj21rHZ(PPr':~),,1 151 IrRhC,,H,,CIP5 [IrH(PEt,),(g-Cl)( p-H)Rh(dppe)]+, 1 164 IrRhC,,H7,P5 [Rh( dppe) ( p H ) Ir( PE t 3 ) 3 ] , 1075, 1151 IrSb,C,6H4 jO, [Ir(CO),(SbPh,),]+, 1111 1rSiClGH3>C! +
+
+
IrHI{ SiH,N(SiH3j,}(CO)(PEt3),, 1126 IrSi3C13H,,IOP3 IrHI{ SiH2P(SiH3),}:CO)(PEt,),,1126 TrSnC2,H,,0;P Ir(CO')3(SnM~3!(PPh,),1103 IrSnC36H31C.14P2 IrHCi(SnCt,!(PPh,j,, I103 IrSnCJ,II?2C:I,C)1', IrH2(SnC13)(~'Tl)( l"Ph3)2, 1103 IrSnC37H30C130P, Ti-CI(SnCI,)(CO)(PPh3),, 1 103 IrSnC37H,,CIjOP2 IrC1(SnC1,)(CO)(PPh3),, 1128 IrSnC,,H3,Cl40P, IrHC!(SnC131(COj(PPh,)2, 1103, 1127 IrSnC3,H3,C1,0P, IrH2(SnCl,j(C0)(PPh3),. i127 IrSnC,8Hj3C140?2 IrCI(SnC1,Me)( CO)(PPh3)2,1128 IrSnC39H3003P Tr(C:C)),(SnT'h3)(PPhJ), 1103 IrSnC3,t-13,C:l?0P, Ir(SriU1,) ( C J I,)(CC))(PPh,), , 1103 IrSnC,,H3,C130P, Ir(SnC13)(C,2H~)(C(~)(pph,),, 1103 IrSnCS4H46C14P, IrHC1(SnCl3)(PPh2),. 1127 IrSn,Brlo [IrBr,(SnBr,),]-. 1127 IrWC&jH42P2
[IrH(PPh,)2(u-H),(11-o:l-5-rl-C,H,WCp]-. 1152 IrWC54H40W3 IrW(u-PPh,)( CO),(PPh3)2, 1163 1rWC66Hj105?4
IrWH(CO),(u-PPh,)2(PPh,),,1'163 Ir2As6C80H72C1202 Ir2C12(CO)2{ ' P ~ A S ( C H ~ A ~ ?,P I1164 ~~)~) Ir,As,PdUsu~K,:C:I3(32 IIrlPdC1,(CC)),{PliA5(CH,AsPh,),) ,I-, 1164
Formula Index
1345
[{IrH(CO)(PPh,)(p-SBu')),H]+, 1122 { Co(Pr,salen)( COz)K(TtiF)),, 638 I~~C~~H~~NZO~PZS [ { Ir(C0)2(PPh3)}2(p-N,C,II4Me-4)(p-SO,)]', 1107 La4Rc2010 La4Re2010,177 h2C47H38N206P21j [ {Ir(Co)2(PPh,)},(~-~=N~~6H4Me-4)(lr-S0,)]2+, La6Rr,OIs 1107 La6Re4OI8,177 LiFeC45Hy60, I ~ z C ~ ~ H ~ ~ B ~ Z ~ Z P ~ S ~ IrzBrz(CO)2(PPh3),(tetrathionaphthalene), 1121 FeLi(OCHBu'2)4(HOCHBut2). 227 Li2RuC4,H6,,02P4 I~ZC~~H~SO~PZSZ Ir2(C0)4(PPh3)z(SBu')z,1122 RuHMe(PPh3),(LiMeOEt2j,, 454 Ln4Re2OI1 I~ZCS~H~~OZP~S Ln4Re,011, 192 Ir*(p-S)(C0)z(~dPPm)z, 1159 Ln6Re4018 IrzC53H44C103P4 Ln,Re,O,,, 192 [Ir,Cl(Co),(dppm),l', 1112 Ir2C53H44C12Q3P4 I r ~ ( p - C l ) ( ~ ~ - C O ) ( C O ) ~ ( ~ - d pI I13 pm)~l~~ Mn(A1H,)(Me2PCI-12C132PMe1)3132 Ir,C3,H,,CQP4 [Ir,(p-Cl)(p-CO) (CO),(p-dppm)2]+, I 113 MnAkC12H36P4 Mn(AlH4)(MezPCH2CH2PMe2)2, 9 IrzC56H52C1202P4 {IrCl(CO) {Ph2P(CH2)2CH2)},, 1113 MnA12C24H5608 Ir2C58H53C1N03P4 Mn { Al( OPr'),} z , 37 [Ir2C1(CNBu')(C0)3(dppm)2]+, 1102 MnAlzHs Mn(A1H4),, 61 IrzC59H4504'3 Ir2(CO),(PPh3)3. 1101 MnAsCsHlzBrzN MnBr,(2-HzNC,H,AsMe,), 34 1r2C62H6ZC1N202P4 [Ir,CI( CNBu') 2(CO)2(dppm),]+, 1102 MnAsC, 3Hl,C10, [Mn(ClO,)(Ph,McAsO,)]' , 4 0 Ir2C dL4ChP6 IrzClz{~ , ~ - ( P ~ ~ P C H ~ C H ~ ) ~ P C H : !11 )Z 13C ~ H , } , MnAsCItiHZ4N2 [Mu(Z- t-12NC6H4AsMez)2]2f, 33 1r2C71H80N40P1 [Ir,(CNB~')~[CO)(dppm)zjz', I102 MnAsC2,Hz5N4 Mn{ 2-(Me,As)C,H4N=CPhC(Fh)=NNHpyridyl-2}, llZC72H65P4 34 [Irz(p-H)JH2(PPh3)4] ' 1151 MnAs2C12H.,6BrL02 Ir2C76&004p4 Ir,(CO),(PPh3),, 1101 MnBr,(CO),(diars), 31 MnAs2C1yH22Br03 IrzC76H60016P4 Ir2(C0)4{P(OPh),) 4. 1101 [MnBr(CO)3(AsMezPh),]+, 31 MnAs2C36H30C1302 IrzC9sH96O8P6 [Ir2(CO)2(d~op)s]2*. 1113 MnCI,(Ph,AsO),, 90 MnAs2FZ4N4S4 IrzClzF1zP4 { IrCl(PF3)2},, 1163 M ~ ( A S F ~ ) , ( N S F22,m ~)~, 1\/InAs4C72H60C108 Ir2C19 [Mn(C104)(Ph3As0)4]+,47 [IrzCI9l3-, 1149 MnAs5C151145C)5 I~~CU~CS~H~INBP~ [Ir,Cu3H,(MeCN),(PMe,Ph),j3 ' , 1164 [MII(M~~ASO ' ,)40~ ] ~ Ir2Si3C~6H69IzN02P4 MI I BO ~ {IrHl(SiH2)(C0)(PEt3),),(NSiH,), 1126 IMnBO,l-, 42 lr2Si5C26H7J2U2P5 MnBz04 MnB2O4,42 { IrHI(SiH2)2(CO)(PEt3)2}Z(PS~H3), 1126 IrZSnC42H30CIZ06PZ MnBzOs { Ir( CO),( PPh,)) (SnCI,), 1103 MnB205,42 MnB407 1r2SnC44H3606PZ { Ir(CO),(PPh,)},(Sn&), 1103 MnB407,42 MnB6010 1r3C116H120C19P12 Ir3CI9{(Ph2PCH2CH2)2PCH,CH2P(CHzCH2PPh2)2}2MnB6OI0,42 1136 ,MnBr,CI, [Mnl31-~Cl~]~-, 59 1151 MnBrJ, [MnBr,I,]*- , 59 Mn13r4 [h111Br,]~-,11,58 MnBq [MnBr6I4-, 59 MnCH4CI2N2S MnC12(HZNCSNHZ), 54 MnCN304 M~I(CO)(NO)~, 7 Ir4Cl~HzOl~ 1151 MnCN403 [Ir4Hz(CO),2]ZC, [Mn(CN)(N0)31-, 8 IrK 4~H4408P4 Ir4(C0)8(PMe2Ph)4,1164 MnC0,P [Mn(C03)(P04)13-, 41 K C O C ~ 7~4NH2 0 2 MnCZH3N403 Co (salen)K, 637 Mn(CNMe)(NO),,8 MnC2H5N305S KC0C27H34N205
.
.
Formulu Index
1346 Mn(S04)(H20)(triazole), 21 MnC,NO, [M~cO),W) ,I - , 7 MnC,N,O, [Mn(CZ04)(rYT02),]2-,50 MnC2N,02 [M;(CN),(N0),J3-, 7 MnC206 [Mn(CO,),]z-. 41 MnC20,SZ [Mn(c204)(szo3)lz , S O MnC,H20, Mn{(l3,C),CH,), SO MnC,N3 Mn(CN),, 83.102 [Mn(CN),]-, 11, 12 MnC4H206 [Mn{02CCH(0)CH(O)COz)]z , 5 I MnC,R,O, Mn(matate), 51 MnC4H5N0, Mn{HN(CH,CO,),}, 66 MnC,&O6 Mn(glycollate)l, 51 MnC4H8N204 Mn(H2NCH2C0&. h3 MnCdHgS4 [Mn(SCHzCH2S)2]2 .53,’40 MnC4HloCl2N,0, MnCI2(H2NCONHCONH2),, 62 MnC4HloN20BS MnS04(H2NCH2C02H)2,62 MnC4HloN4010 Mn(NO3),(H2NCH,C0,H),, 62 MnC4H12 [MnMe,IZ-, 13 MnC4H14N4 Mn(C2HMNH3I4,14 M~C~HI~N~OJ‘Z Mn{02P(CH2NH2)2)2, 64 MnC4N05 Mn(CO),(NO). 7 MnC,N,O, [Mn(CN)(CO)WJ)J-, 8 MnC4N404 [Mn(NC0),I2-. 22 MnC4N,S, [Mn(NCS),I2-, 22 MnC4N,Se4 [Mn(NCSe),l2-. 22 MnC,NSO [Mn(CN),(NO)]“, 8 MnC,04S4 [Mn(SzC20z)2]2-.5 1 MnC,O, [Mn(C2O4hI2-,50 MnC5HNS0 [Mn(CN),(OH)I4- 11 [M~ (CN)S( OH) I ~8 MnC5H3N204 Mn(CNMe)(CO),(NO), 8 MnC5&CI2NO MnCl2(2-pyridone), 61 MnC5HSCl2N MnClz(py>, 18 MnC,H,,NO, [ M n ( H ~ H C 0 , H ) ( H , 0 ) 4 ] z f ,61 MnCSN, [Mn(CN),I3 MnC5N,SS [MII(NCS)~]~-, 22 MnC,N,O [Mn(CN),(NO)]’-, 12,83 I
[Mn(CN),(NO)J3-, 12 MnC,N,O, [Mn(CNjS(NO2)]”. 12,83 MIlC6H4N6 Mn(CNH),( CN)z, 12 MnC6H4014 [Mn(Cz04)3(Hz0)21-.89 MnC6H6N8S2 Mn(NCS),(triazole)z. 21 MnC6H8CI,N, MnCI,( pyrazolc)?. 20 MnC,H806 Mn(i.-ldclate)2, 5 I MnCJIYN6O6 [Mn(ON=C‘Meh’O),] .63 MnC,H ,NL02S2 Mn(NCS),(EtOH),. 27 MnC,H,& Mn(OAc),(H20)2. XS MnC6H18BrP*’zOBP2 MnBr(NO)z{P(Ohle)3)2.9 MnC6HisI,P, Mn13(PMe3),. 85 MnC6H24N6
(Mn(CN)ij4-, 5, 7.11 [Mn(CN)615-, 8 MnC6N6S6 [Mn(NCS),]“. 11.22 MnC,N& [Mn(NCSe)6]4-. 22 MnC6N,06 [MnL(N3)7(CO)hl+. 23 MnC,O 1L [Mn(Cz04)3]3-.89 MnC‘,I I, [Mn(C‘,Il),jz~, 14 MnCBH,,Cl2N6O, MnCl,(cytosine),, 62 MnCsHIlI,P MnI,(PMe,Ph), 32.59 MnC8Hl4C1,N,O2 MnC1,(8-quinol1nol)~.61 MnC8H,,Ci2O2 MnCl2(THF),, 38 MnC8HlbCl4OBPZ Mn(PO2Cl,),(Et0Ac),. 39 MnC,H ,& Mn(OBu‘),, 37 MnC8N4S4 2 - , 54 [Mn {S2C2(CN)z}21 MnCBN50S, [ M n ~ S z C z ( C N ) , ~ z ~ N 054 )12~, MnC8O4S4 [Mn{Sc=C(S)COcO}z]z-, 53 MnC~Hi503S6 [Mn(S2COEt),]-, 55 MnC9H,,CI,N,03 MnCl,(H,NCOEt),. 39 MnC,H,,BrzN,O, MnBr2(MeNHC03HMe),, 39 MnCJ I,CI,N,OZ Mn(bipy dioxide)C1,. 51 MnC,,H,CI,N,
Formula Index
1347
M~CI~HRO~ Mn(2-HOC,,H4C0z)2,51 MnC,,Hl0O4 Mn(tropolonate),, 48 MnCl4Hl2Br2NZ0
MnBrz(HzO){2-(2-pyridyl)quinoline}. 24 MnC14H1sN208
[Mn(02CCHz)2N~H(CHz)4~HN(CH2C02)z]-, 96
__
MnC, AHTAP, "_ Mn{2-C6H4(CHz),}(Me,PCH2CH2PMe2),14 MnC14H37P4 MnH(~IzC~CH2)(Me2PCH~CH~PMe2),, 93
Mn(acac),, 5, 89 MnCiSHzzCIN4 MnCl{HN CH N=CMe-2,6-pyC(Me)= &H,J, 77 M~C,&Z,CLNS
MnC10H24N4
[MnCI, H~CH,CH,NH(CH,),NHCH,CHZb H Z 1 3 ) .. ,+> MnCloHzsP2 MnMe4(MezPCH2CH2PMe2),34,103 M~CI~H~~PZ M t ~ M e ~ ( p M e103 ~)~, MnCllH14N04S Mn(acac),(NCS), 89 MnC12F,,O, [Mn{(CF3)zCOC02)?.j3-I 89 MnCIZHsBrZN4SZ M I I ( N C S ) ~ ( ~ - B ~ -19 ~~),, MnC l~HI.W L Mn{2-OC,H,CH=N(CH,),O~~OAc), 93 . ~. MnC,;H140, Mn{ O,CC(Me)=C(Me)CO,H},, SO MnC12HlsC12N6 MnClz(2-Me-imidazole),, 19 MnC12H2Z014
MnClSHz3CLN50~ Mn(CI0, H h CH )zN=CMe-2,6-pyC(Me)= W H d z ) , 78 M~CISH~~N~S~ Mn(S2C$lCH2CH20CHz~H2)3, 91 ' MnCi~Hz40.5 (Mn(aca~H),]~+, 48
,
MnC15H30N3S6
Mn(SZCNEtz),, 91 MnC1~HloO~S4 [Mn(SPh),{ Sc=C(S)COt 0}l2-, 53 MnC16HizN606 Mn(NO,),( l,B-naphthyridinc),. 25 MnC16H14N20Z
Mn(salen), 5,11,66 M~CMHI~NZOZ Mn(2-OC6H4CH=NMe),, 64 MnC16HlsNz0zS4 Mn(S,COEt),(bipy). 55 MnC16&&zNs MnBrz(5-methylpyrazole),, 19 M ~ C M H ~ Z B ~ ~ ~ S Mn( 12-crown-4),(Br3),, 80 M~CI~H~~C~ZN~ [MnCl HNCH CH NHCMe2CH,CWMeNHCH,CH,-HMe)l+'. _ _ 100 MnC,,H,4NZOSi4 Mn{N(SiMe,),},(THF), 17 MnCI6Hd4Si4 [Mn(CH2SiMe3)4]2, 13 MnCi7Hi7Nz03 MnC(2-OC,H4CH=NCHd,CH2)(OH). -,-,. , 95 ~~
,N=CMe-2,6-pyC( Me)=
1348
Foriniilu l m k x
[Mn(S2C6C14)3]Z-.107 MnC,,H,,N,02 Mn(8-q~inolinolate~~. 64 MnCI8Hl2N3o6 Mn(2-pgC02),, 93 MnClaH1~N206 [Mn(2-OC6H4CH=NCH2C02)21-, 94 MnCl8HI7N20, Mn(salen)(OAc). 5 , 67,95 MnC rRH,sCIYI,P [Mn { (2-pqridylCH,) 3P}Cl]* , 3 4 MnC& ,&O& Mn(S,COEtj,(phcn), 55 MnC,,HZZN402 Mn(sdkn)(en), 66 MnC1&L4% Mn(2-CH,C6H4NMe2)2,14
Mn(C"O)Z(phen),, 11 MIlC26H20N202 Mn(2-OC6H4CH=NP ti) 2 . 64 M~C~~HXN~SZ Mn(NCS j2(5-Me-py), , 19 MnC26H44P2 Mn(CH2CMe2Ph)2(PhIe3)2, 13 MG7Hl8N3O3 Mnl8-quinol1nolate),. ,~63. 93 MnC28H;,N5 Mn{2-C,H4(N==CMeCH=CMeN),C,H,-2} (NEt,), 77 MnC2,Wd4 Mn(norbomyl),, 103 .MnC30H22N606 [Mn(terpy triiixide)2]z"~,5'1, 52. '106 MnC130H24N6 [Mn(bipyj3l2', 23-25 Mn(bipy),, 25 [Mn(bipy),]-, 25 [Mn(bipy),14-, 25 M~C~OHZ~NSO~ [Mn(bipy dioxide),]" . 106 [Mn(bipy dioxide),13-. 90 MnC.30H3i,N6
[Mn(PY)61Zi, MnC3,,H4,P MnPh2(P(C,H,,)~),13, 33 MnC3,H16CIN8 MnC.~l(phthal(icyanii~e). 49 MIIC,~H,,N, Mn(phtha1ocyanine). 5 , 75,99 MnC,,H., sNxO MnO(phthalocyanine), 5 MnC32H16N80~ Mn(phthalocyanine)02: 76 MnC32H24N8 [Mn( 1&naphthyridine),]*+, 25 MnC32H26N602 [Mn{(2-pyrldyl)3COH} 66 . [Mn{(2-pyridyl)3COH}z]2+ 27 MnCdh4N7 [Mn( { (2-pyridylCH2),NCH,} ', 30 MtlC'35H19Nlo M t i ( p h t h a l o c ~ ~ a n i i i e ) ( H ~ ~ 76 ~~N~~~1), MnCd1Z.iNb [Mn(p hcn ),] 23 MnC36H30C1302P2 MnCl,(Ph3P0)2, 90 MnC36H30P3 [Mn(PPh,),]-, 34 MnC36H44N402 Mn(octaethylporphyrin)O,, 73 MnC3,H30C1N02P,S MnCl(NCS)(Ph3PO)2.41 I
~
+ ~
MnC,;H,,CiN, ' MnCl{ 2-NC6H4N=CMeCH=CMeN-2-C6H,N= CMeCH=cMej. 101
M11C4,I 12,N,,, Mn(p1ithalocyanine) (py),, 75 MnC4zH6006 [Mn(3,5-Biit2-1,2-02C6H,),]*-, 106 MnC4,HZ8C1N4 MnCl(tetrapheny1porphyrin). 72, 97, 98 MnC44H28N~ Mn(tetraphenylporphynn), 72 [Mn(tetraphenylporphyrin)]+,99 MnC4,HZ8N402 Mn(tetraphenylporphyrin)O,, 73 MnC4,H2,NS MnN(tetraphenylporphyrin), 110 MnC+J+&t Mn(N,)(tetraphenylpn~hyrin),Y8
Formula index MnC44H26N10
Mn(N,),(tetraphenylporphyrin), 108 MnC46H34N402 Mn(OMe),(tetraphenylporphyrin), 108 MnC46H36N40z [Mn(tetraphenylporphyrin)(hleOH),]+, 97 MnC47H31N6 Mn(tetraphenylporphyrin)(imidazolate), 97
.
. '
[Mn(PPh,),I2-, 34 MnC48H46NZP4 [Mn(PPh2)4(NH3)2]2-.34 MnC49H33C1N5 MnCl(tetraphenylporphyrin)(py), 97 MIIC,~H,,& Mn(tetraphenylporphyrin)(py), 73 MnCS0H3,N8 [Mn(tetraphenylporphyrin)(imidazolate),l , 9 7 MnCszH36N303 M n(2-0C6H4CH=N CH2Ph) 3 93 MnCS,II,,C:IZP, [MnC12(diphos)2]+,85 [MnCl,(diph~s),]~*,104 MnCS4H4dW" [Mn(2-pyridy1CHzPPh2)3]2~,34 MnC56H48C12P4 MnCl,(diphos),, 32 [MnCl,(diphos),]+, 32 [MnCl,(diph~s),]~+, 32 M~C~SH~~NIZO~ Mn{meso-a,a,a,a-tetrakis(2nicotinamidopheny1)porphyrin) ,98 MnC7, ti6,, IO, P4 [MnI(Ph3PO),]', 40 MnC7dL2N 6 0 { Mn(phthalocyanine)(py)) ,O, 5,76 MnC10, Mn03CI, 110 MnC106S Mn03S03CI, 110 MnC1, MnCI,, 56 MnClz12 [MnC1212]Z-,59 MnCI2O2 MnO,Cl,, 110 MnClzOs MIl(C104)z, 47 MnCI, MnCl,, 91 [MnCI,]-, 57 MnC1,O MnOCI,, '1.10 MnCI, MnCI,, 108 [MnC1412-, 5, 11.55,58 MnC1404P2 Mn(ClzP02)2,45 MnCI5 [MnClsJ2-, 91,92 MnC1, [MnC16]2-, 108 [MnC1,13-, 91 [MnCl6I4- 58 MnCoC,9H440zP,S, Co(triphos)(~-CSa)Mn(CO),Cp,646 MnFO,
MnO,F, 110 MnF0,P Mn(PO,F), 45 MnF, MnF,, 56 MnF, MnF,, 91 MnF, MnF,, 107 [MnF,]-, 91 MnF, [MnF,]-, 1U7 [hlnF,]", 91 MnF, [MnF6IZ-, 5,107 [MnFJ-, 91 [MnF6I4-, 56 MnGaC11H23N503 Mn(3,S-dimethylpyrazo1e)(NO)Z(Me2NCHZCH,0GaMez),9 MnHClO Mn(OH)Cl, 35 MnHI MnHI, 60 MnHO, MnO(OH), 86 MnH0,P MnHPO,, 45 M n l j O*P MnHP04. 45 MnHORSe3 MnH(SeO,)(SeZOs),88 MnH, MnH,. 60 MnHzOz Mn(OH),, 35 MnH, MnH,. 93 hhH306Pz MnH2P206,87 MnH,F40z [MIlF,(IT,O)ZI-, 92
MnH& Mn(NH&, 16 MnlI,O, [Mn(OH),I2-, 35 !v1nH,O6P2 Mn(H2P04)2,45 MnH& ~ M I I ( O H ) ~ ] 104 ~-, [Mn(OH)6]3-, 86 [Mn(OH),],-, 35 MnH60ziP6 IMn(H,Pz07)313-, 87 MnHsCl2N4 MnClz(NZH4)rt 17 MnHsN4 [Mn(N14z)4]2-, 16 MnHl1U6 [M~(OH)(HZO)SI',35 MnH12C12N4 MnClz(NH3),, 15 hlnH1,O6 [Mn(Hz0)6]2+,11,35,60 [ M n ( H ~ 0 ) 6 ] ~85 +, MnHlsN6 [Mn(NH,)6]'+, 15 MnHgC4N4S4 HgMn(SCN),, 22 Mn12 MnI,,59 hlnI,O, Mn(IO,),, 46,47
1349
1350 Mnl, [MI&] ,59 Mn13018 [Mn(IO,),]" . 105 MnI, [MnI4Izp,11.59 Mn15015 [Mn(I03),12-. 88 MnW [Mn(I03),12-, 105 MnK,0,S2 K2Mn(S04)2,36 MiiN,O, Mn(NW 2, 44 MnNz06 MII(NO,)~.44.87 MnN408 [Mn(N0,),I2-. W MnN4OI2 [Mn(NO,),]-. 87 [Mn(NO,),]'-. 44 MnN5010 [Mn(N0,)5]3p, 44 MnN, Mn(N&, 23 MnNlz [Mn(Nd412 ,23 MnNa,0BSe2 Na,Mn(SeO,),. 16 MnO, MnO,. 102.104 Mn03S MnSO,, 46 Mn0,Se MnSeO,, 46 Mn03Si MnSiO,, 41 MnO,Te MnTcO,. 46 Mn04 [MnO,l-, 109,525 [Mn0,l2-. 109 [Mn04]?-. 109 [MnO$ , 104 Mn04P MnPO,. 87 MnO,S MnS04, 46 Mn0,Se MnSeOl, 16 MnO, [MnO5I3-. 109 Mn0,Se2 MnSe105, 46 MnO& [Mn(SO,),1'-.46 Mn06Sc, Mn(ScO,),. 16, 102. 1U5 [Mn(SeO&l ,88 MnO, [Mn(O2),0l4-, 105 Mn07P2 [MnP20,]-. 87 MnO, [Mn(OZ),l4-. 105 Mn08P2 [Mn(P0,),I2-, 105 [Mn(P0,)z]3- 87 MnO& [Mn(SO,)*I-, 87 [Mn(SO&l+, 86 Mn09P3 Mn(P03),, 87
Formula Index MnOi,Te3 [Mn(Te06),]14-. 105 MnRuC54H,006Pz
RuMn(p-PPhz)(CO)6(PPh3)2r 373 M~RUC~~H~~CIO~P~ RuMnCI(p-dppm),(p-CO),(CO),, 373 MnS MnS, 53 MnWI1H20d' [PMnW, 1039(OHd15-,47 MnW 1,H206ZP, [P2MnW17061(0H2)ls-.47 Mn,AsHOS Mnz(OH)As04, 15 Mn,BH414 Mn,(BH4)14, 61 Mn2C4N804 [Mn2(W4(N0)4I4-.7 Mn2C,Hl8CI4O3 Mn2C14(EtOH)3.37 Mn2C6012 [Mn2(C204)3l2-, 50 MnzCsHi6Ss 90 [Mn2(SCH2CH2S)4]2-. Mn2CxOIx [MnzO2(CeO4),]'-. 105 ~hC1oH12N2Os
Milz{(C)2CCH2)2NCH2CH2N(CH2C02)2}, 70 Mn2CloNlo0 [Mn,O( CN) -.83 MnZC18H27N906 [Mn,Cl(isonicotinyl hydraz~de),(H,O),]~+,64 Mn2C19Hz5N19S4 Mn2(NCS),(4-metriazole),, 21 M~~CZOH~~N~OI~ Mni{(02CCH2)zNCH,CHzN(CH2C02)(CH,COzH)],. 70 MnzCz4Hz6Nl, Mn,(iidozolate),(imidazole),, 21 Mn2C24H72N4SiX Mn,{N(SiMc,),),, 17 Mn2Ci;H4,N2OR . Mnz(acac),(H,NCH2CH=CH,),, 49 MnGnH4hNz08 MnJ(02'CButj4(py)2.43 Mn2C3ZH~8N408 {Mn(salen)O},O,, 67 Mn2C36Hz4C14N4 Mn,CI4(2,2'-biquinolyl),, 24 MnZC40H32N802 103 Mnz02(bi~y)4, [Mn202(bipy)4]3', 84 M~~C~SH~ZN~OZ Mn,O,(phen),, 103 Mn2C64HxNd (Mn(phtha1ocyaninc)) * O ,76 Mn2C7,H,,N.,,0 {Mn(phthalocyanine)(py)}20, 4Y Mn,Cs8H56Ni40 {Mn(N,)(tetraphenylp~rphyrin)]~O, 98, 108 MnZC1O4P MnZCIPO4,45 Mn2CI7 [MnzCI7l3-, 58 Mn,ErH, MnzErH4,60 Mn2ErH4.6 MnzErH4.6r60 Mn2Fs Mn,F,, 91 Mn2F9 [MnJ,] , 107 MI1,H,C103
I Formula Index
Mn4Ci;H6iNl O6
[Mn406(Hd(CH,CH,NH)2CH,~H,},]4+, 103
1351
Na2Fe2C34H57N407 Fe,Na,{ { MeCOCHC(Me)=NCH2}z}2(OEt)(THF)2, 1238 Na2MnOsSez Na2Mn(Se04)2,46 NazMn4Ol5SS Na2Mn4(S03)5,46 NbeCoCH3N022 [CO(N~,O,~)(CO~)(NH,)I~-, 826 NiC54H45CINOP3 Ni(NO)CI(PPh,),, 1067 NPOz [Np02]+,1053 NPRhH1oO.I [Rh(H20),0Np0]4+,1053 OSAIC&~~C~~N~P~ O S ( N N A I M ~ ~ ) ( P E ~ ~555 P~)~C~~, O SA S&~ H Z Z C ~ ~ 0 s ( A ~ M e ~ P h ) ~577 Cl~, OSASZC~~H~~C~~ O S ( A S P ~ ~ )577 ~C~~, OSAS~CZ~H,~C& Os(dpam)Cl,, 579 OSAS~C~~H~~C~~P O S { P P ~ ( C W ~ C H ~ A S PCl3~ , )579 ~} OsAsZC36H30Br4 Os(AsPh&Br,, 577 OSAS~C~&I~,-,CINO Os(NO)Cl(AsPh3)z, 550 OSAS~CZ~H~~C~~ Os(AsMe2Ph),C13, 577 OSAS3C27H63C13 Os(AsPr,),CI,, 577 OSAS~C~ZH~~ Os(A~EtPh2)3H4,576 OSAS~C~~H~~C~ZN~ O S ( A S ( C , H , M ~ - ~ ) ~ } ~ C I ~ (556 N~H~), OSAS~H~ZC~~H~~CI~ Os(AsPhj)jCI.2HgClz, 525 O SA S~ C ~ O H ~ ~ C IN O [Os(NO) (d i a r~ )~ Cz+,l ] 5 47 OSAS~C~~H~~CINZ
1352
Formula Index [ O ~ N ( C z 0 4 ) d f W ) l ' 562 OsC4HzN,0, [OS(CN),(OH),]~-, 526 OSC~HZN~O [OsN(CN),(H,O)]-. 525,562 OSC,H*NzO6 O~Oz(Gly-0)2,583 OSC~H~OS O~O(OzCzH4)2,585 OsC4Hs06 [OsOz(OzCzH,)Z]*-. 585 OSC~H~NO, OsO,(NBu'), 558 OsC,Il ,,f21z02Se2 OsO,Clz(MeSeC:FizCJI,SeMe), 609 OsC4HlzCI2OZSz OSO,CI~(SMC,)~, 584 OGHizNioOz Os02 { HN=C( NHZ)NC( NHz)=NH} 2,568 OsC,HizO6 10s0,(OMe),12-. 580,582,585,596 I
OSC~H~~NO~S' ~
[Os(SCHzCHzNMe2)(OH)4]-,607 OsCoHi a N q n 0 7 [OsO;{ HN=C(NH,)NHC(NHz)=NH}
z1+,
568
[OSOZ{HN=C(NH,)NHC(NH,)=NH},]~', 568 OSCdH1dN 110 [OsC)N{~ N = C ( N H I ) N f - I C ( N T i Z ) ~ N ~ ~ } , I '463 -, OSC~H,~CIN~
IOS($J==CHCH=NCH=CH)( NH,),cI]?+, 535 OsCJH 16CIZN,
[Os(d,O,)(CO)Cl,]z-, 601 OsC4C112N2 [OS{ NC(CC~~)NCCICCIJ}CI~], 558 OsCdHzNOq
[oS(py)cl,]-, 534 OsCsHZoN6 [ O S ( ~ ~ ) ( N H , ) ~533.534 ]~', [Os(p~)(NHd,j'-, 534 OSCsN, [Os(CN),I4-, 525 OsC,NbO [Os(NO)(CN),12-. 546 OsC5NhOS5 [OS(NO)(NCS),]~-,548,566 OSC~N,S~ [Os(NS)(NCS),]*-. 553, Sh2,566 OSC&N$ [OS(H~NCSNH,)(CN)~]*-. 608 OSC~H~N~OS [Os(NO)(CN),(HZNCSNH,)Jz-, 608 OSC6H40io [OSOZ(O,CCH~CO~),]~-, 582 OSC~HSC~~NO [os(py)(co)cI,1- 534 OsC6H8H208 Os(en)(C,04),, 531 OSChllQOH 1
Fbrmulu index [OsO,(OAc),]-, 582,585,600 0SC6H1006S2
[Os(OH),{ O,CCH(S)Me} J 2 - , h 17 OSC6HizCIzNzS4 Os(SzCNMez)zC12,605 OSC,Hi,N,O [O~(N~)Z(NH~)~(~-PYCONHZ)]~+, 555 OsC6Hl8CI3O3S3 Os(DMSO),CI,, 602 OSC6Hi8CI,O6P, Os{ P(OMe)3}2C14,575 OSC~HZ~N~ IOs(H2NCH2CH2NH)31 ' .532 ~. OiC,H2,N, [Os(HN=CHC€I,NH2)( C I ~ ) ~ ] ' + ,532 . [Os(H2NCH2CH2NH),(cn)] + 532 [Os(H2NCH,CHzNH),(en)]2+. 532 OSC6Hz3~6 [Os(HzNCHzCH2NH)(en)3]2+.530,532 [ O S ( H ~ N C H ~ C H ~ N H ) (532 ~~)~]~+, [Os(H2NCHZCHzNH),(en)]2+,531 OSCgH2412N6 [ O s ( e r ~ ) ~ I ~531 ]~+, OSCgHMN6 [Os(en),]*+, 531 [Os(en),13 ' ,530.532. 532 OsC,Ij,4N;zS, [Os(H,NCSNH2),IZC,608 I O S ( H , N C S N H..~ ',608 )~~~ osc,rj, [0s(CN),J3-, 525,526 [Os(CN),I4-, 525,526 OSC,N,&, [OS(NCS)6]3-, 566 OSC~H~~
