Advances in Inorganic Chemistry Volume 46 (Including Bioinorganic Studies)

Advances in INORGANIC CHEM1STRY Volume 46 ADVISORY BOARD 1. Bertini D. M. P. Mingos, FRS Universita degli Stud; d...

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Advances in INORGANIC CHEM1STRY

Volume 46

ADVISORY BOARD

1. Bertini

D. M. P. Mingos, FRS

Universita degli Stud; di Firenze Florence, ltaly

lmperial College of Science, Technology, and Medicine London, United Kingdom

A. H. Cowley, FRS

J. Reedijk

University of Texas Austin, Texas, USA

H. B. Gray California Institute of Technology Pasadena, California, USA

M. L. H. Green, FRS University of Oxford Oxford, United Kingdom

0. Kahn lnsfitut de Chimie de la Matiere Condensee de Bordeaux Pessac, France

Andre

E. Merbach

lnsfitut de Chimie Minerale et Analyfique Universite de Lausanne Lausanne, Switzerland

Leiden University Leiden, The Netherlands

A. M. Sargeson, FRS The Australian National University Canberra, Australia

Y. Sasaki Hokkaido University Sapporo, Japan

D.

F. Shriver

Northwestern University Evanston, Illinois, USA

R. van Eldik Universitat Erlangen-Numberg Erlangen, Germany

K. Wieghardt Max-flanck lnstitut Mulheim, Germany

Advances in

INORGANIC CHEMISTRY Including Bioinorganic Studies EDITED BY A. G. Sykes Department of Chemistry The University of Newcastle Newcastle upon Tyne United Kingdom

VOLUME 46

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ACADEMIC PRESS

San Diego London Boston New York Sydney Tokyo Toronto

This book is printed on acid-free paper.

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Copyright 0 1999 by ACADEMIC PRESS All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the Publisher. The appearance of the code at the bottom of the first page of a chapter in this book indicates the Publisher’s consent that copies of the chapter may be made for personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. (222 Rosewood Drive, Danvers, Massachusetts 01923), for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. Copy fees for pre-1999 chapters are as shown on the title pages. If no fee code appears on the title page, the copy fee is the same as for current chapters. 0898-8838/99 $25.00

Academic Press u division of Hurcouri Bruce & Company

525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.apnet.com Academic Press 24-28 Oval Road, London NW I 7DX, UK http://www.hbuk.co.uk/ap/ International Standard Book Number: 0-12-023646-X PRINTED IN THE UNITED STATES OF AMEIUCA 98 99 0 0 0 1 02 0 3 B B 9 8 7 6

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3 2

1

CONTENTS The Octahedral MsY8and M6Y,* Clusters of Group 4 and 5 Transition Metals NICHOLAS PKOKOPLIK AND D. F. SHRIVER 1. Introduction . . 11. Group 6 . 111. Group 5 Metal Halide Clusters . IV. Materials Chemistry Derived from Soluble Metal Halide Clusters References .

. .

. .

.

1 3 24 35 44

Recent Advances in Noble-Gas Chemistry JOHN H. HOLLOWAYA N D ERICG. H o w I. Introduction

.

Recent Review Literature . The Possibility of Argon Chemistry . Krypton Chemistry . Xenon Chemistry . VI. Radon Chemistry . References .

11. 111. TV. V.

. .

.

. . .

. .

51 53 54 55 61 91 93

Coming to Grips with Reactive Intermediates ANTHONY J. DOWNSA N D TIMOTHY M. GREENE Introduction . Reaction Intermediates: Nerve Centers of Chemical Reactions . Experimental Characterization of Reaction Intermediates: Retardation Experimental Characterization of Reaction Intermediates: Time-Resolved Methods V. Experimental Characterization of Reaction Intermediates: Flow and Other Methods . VI. Conclusions . . References

I. 11. 111. IV.

V

. .

101 106 107

.

136

.

155 163 166

,

.

.

vi

CONTENTS

Toward the Construction of Functional Solid-State Supramolecular Metal Complexes Containing Copper(1) and Silver(1) MEGUMU MUNAKATA, LIANGPINGWU, AND TAKAYOSHI KURODA-SOWA I. 11. 111. IV. V.

VI. VII. VIII. IX.

. Introduction Helical Frameworks . S----SContact-Assembled Frameworks . Hexagonal Frameworks and Graphite-like Structures Hydrogen-Bond-Assembled Frameworks . a-a-Interaction-Assembled Frameworks . Diamondoid Frameworks . Other Frameworks Based on Covalent Bonds , Concluding Remarks . . References .

.

. . .

. . . . . . .

174 176 192 204 219 228 240 251 292 293

Manganese Redox Enzymes and Model Systems: Properties, Structures, and Reactivity NEILA. LAW,M. TYLERCAUDLE, AND VINCENT L. PECORARO I. Introduction . 11. The Enzymes . 111. Structural Models IV. Physical Properties V. Reactivity . VI. Conclusion . References .

.

.

. . . . . . .

305 310 343 379 393 424 425

.

441 443 456 470 485 485

Calcium-Binding Proteins BRYANE. FINNAND TORBJORN DRAKENBERG I. 11. 111. IV. V.

Introduction . Intracellular EF-Hand Calcium-Binding Proteins . Calcium-Mediated Membrane-Binding Proteins . Extracellular Calcium-Binding Proteins , Summary . References .

. .

. . .

vii

CONTENTS

Leghemoglobin: Properties and Reactions MICHAEL J.

D A V I E S , CHRISTEL M A T H I E U , A N D

. I. Introduction 11. Structure . 111. Biological Localization . IV. Reactions with Different Molecules . V. Oxidation of Fe(I1) Leghemoglobin . VI. Oxidation of Fe(II1) Leghemoglobin . VII. Reduction of Fe(IV)=O Leghemoglobin VIII. Reduction of Fe(II1) Leghemoglobin . Reactions of Globin-Derived Radicals References .

ALAINPUPPO 495 497 499 501 507 511 519 524 527 538

. . .

Ix.

INDEX . CONTENTS OF PREVIOUS VOLUMES

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543 555

This Page Intention ally Left Blank

ADVANCES IN INORGANIC CHEMISTRY, VO1.

46

THE OCTAHEDRAL M6Y8 AND M6Y12 CLUSTERS OF GROUP 4 AND 5 TRANSITION METALS NICHOLAS PROKOPUK and D. F. SHRIVER Department of Chemistry, Northwestern University, Evanston, Illinois 60208

I. Introduction 11. Group 6 A. Synthesis of Group 6 Clusters B. Axial Ligand Chemistry C. Inner Ligand Chemistry D. Redox Chemistry and Photophysics of the Group 6 Metal Halide Clusters E. Molecular and Electronic Structure of the Group 6 Metal Halid Clusters 111. Group 5 Metal Halide Clusters A. Synthesis of Cluster Core B. Redox Chemistry of the Group 5 Clusters C. Ligand Substitution D. Electronic and Molecular Structure E. Niobium Iodide Clusters {Nb,I,}"+ IV. Materials Chemistry Derived from Soluble Metal Halide Clusters A. Higher Nuclearity Clusters B. Supported Cluster Materials C. Charge-Transfer Salt Complexes D. Extended Solids E. Chemically Modified Surfaces References

I. Introduction

The early transition metal halide clusters (MfiY8}"+(M = Mo, W; Y = C1, Br, I; M = Nb; Y = I) and (MfiYl2}"+(M = Nb, Ta; Y = C1, Br) are the basis for a diverse solid-state chemistry in which cluster fragments are interconnected by halide bridges. Discrete clusters containing the {MsYs}"+and {MfiYln}"+ units are obtained from these materials by addition of ligands that disrupt the halide bridges. Control of the coordination environment about the metal octahedron in the soluble clusters provides control of the chemical and physical properties 1 Copyright C 1999 bv Academic Press All lights 01 reproduction in any farm reserved os9s-smwY $25 on

2

PROKOPUK AND SHRIVER

of the cluster. In addition, the discrete clusters provide building blocks for the construction of new types of extended cluster arrays and cluster based materials. Octahedral clusters of the electropositive metals, groups 3 to 7, are stabilized by a-donor ligands such as halides, chalcogenides, and alkoxides, but the majority accessible to solution chemistry are the halide complexes. These highly symmetric and aesthetically pleasing {M6Y8}"' and {M6Y12}n+clusters contain a robust core of six metal atoms and either 8 face-capping {M,Y8}"+or 12 edge-bridging {M6Y12}"+ halide ligands Y. An additional 6 ligands X are terminally bound, one to each metal center, completing the structures [M6Y8Y61n-6z and [M6Y12&61n-6Z (Fig. 1).In the present discussion it is convenient to refer t o the charge of only the {M6Y8}"+ or {M6Y,2}"+ unit instead of referring to the entire cluster and its full complement of axial ligands. This notation is especially useful when there is ambiguity as to the type and charge of the ligands occupying the axial sites or when the properties of the cluster core are independent of the axial ligands. Nomenclature developed by Schafer and von Schnering (1) denotes the bridging ligands as inner (superscript i) and the terminal ligands outer (from ausser) or axial (superscript a); thus, [Mo&~,,]~is noted [Mo6Cl!!Ckjl". Soluble octahedral clusters {M6Y8}ntand {M6Y12}n+ with inner halide ligands have been reported for Nb, Ta, Mo, and W. Molybdenum and tungsten are observed predominately in the {M,Y,}"+ geometry, and the group 5 metals are found with the {M6Y12}nt arrangement. One notable exception is the niobium iodide clusters such as "b618(NH2CH3)6]in which the iodide ligands occupy face-capping sites on the Nb, octahedron (2). Solid compounds with {M6Ys}"+or (M6Yl2)"' units and halide ligands have been observed for other metals, including those of the rare earth

0

0

A

B

OM

QY

OX

FIG. 1. Structures of [MQY&l' A and [M6Y1&10B.

OCTAHEDRAL CLUSTERS OF TRANSITION METALS

3

Am Cm Rk Cf Es Fm Md No Lr FIG.2. Periodic table of metals (shaded) found in {MM,Y,P'or {M,Y,J"+ geometry.

and late d-block elements (Fig. 2). Extensive intercluster bridging has confined the chemistry of the majority of these compounds to the condensed phases. By contrast, the weak intercluster bridging in the group 5 and 6 derivatives provides solid-state materials that are easily dissolved into molecular cluster species. Subsequent solution chemistry has revealed rich photophysical and redox properties for these compounds. The relationship between the ligation of the group 5 and 6 metal halide clusters and their chemical and physical properties, as well as the subsequent materials chemistry that has evolved from this association, is the subject of this review. The solid-state and solution chemistry of the related zirconium halide clusters {Zr,Y,,}"+, which occur with interstitial atoms occupying the center of the metal octahedron or hydrides ligands bridging the octahedron faces, is relatively new and has been the subject of a number of reviews ( 3 , 4 ) .

11. Group 6

Recent interest in halide clusters containing the {M,Y8}"+ unit stems in part from its structural relationship to the superconducting Chevral phases PbMosQs (Q = S, Se, Te) in which eight chalcogenide ligands Q occupy the face-capping positions ( 5 , 6). Early development of the ligand substitution chemistry of the group 6 clusters had led to recent applications of these compounds to catalysis and materials

4


chemistry. Molybdenum(I1) chloride was discovered over 100 years ago (71,and the related cluster M O ~ C ~ ~ ( O H ) ~was ( H one ~ O )of~ the first complexes characterized by X-ray diffraction (8).Renewed interest in the substitution chemistry of the inner ligands arises from the promise of generating {MO&g}2- (& = s, Se, Te) units in solution, which are potential precursors to Chevral phases. Also of current interest is the discovery of the long-lived excited state exhibited by the molybdenum and tungsten clusters (9, 10). A. SYNTHESIS OF GROUP6 CLUSTERS Early synthetic procedures for the group 6 metal halide clusters relied on disproportionation reactions of the intermediate halides, MoC13,MoBr3, WCl,, and WBr4, at elevated temperatures to produce the clusters MsXlz(11,12). These reactions suffered from inherently poor yields and the need to synthesize and isolate the reactive intermediate metal halide. McCarley and co-workers ( 1 3 ) introduced the use of sodium tetrahaloaluminate melts to reduce the halides MoX5 and WX, (X = C1, Br) with aluminum at lower temperatures: 6MoC15+ 6Al- Mo6Cllz+ 6AlC1, 6WCls + 8Al+ W&!l,

+ 8AlC1,.

(1) (2)

Near quantitative yields are achieved for the molybdenum halides, but substantially lower yields (50%) are obtained in the synthesis of the tungsten clusters, which require higher temperatures and the use of Vycor reaction vessels. A higher yield low-temperature synthesis was devised by McCarley and co-workers for W6C1,2using iron as the reducing agent (14). The chemical reduction of the higher Mo and W halides provides good yields of the octahedral clusters, but the mechanism is obscure. By contrast, chemical oxidation of zero-valent Mo and W leads t o the bromo and iodo cluster species in poor yields but provides considerable insight into the formation of these cluster compounds. The reaction of the hexacarbonyls Mo(CO)$or W(C0l6 with Iz at moderately low temperatures produces a mixture of metal halide phases (15, 16). In the reaction W(CO), with Iz, lower nuclearity clusters have been isolated as reaction intermediates that lead to w6 species. The tri-, tetra-, and pentanuclear tungsten iodide species are obtained from W(CO)s and I2 by varying reaction times and temperatures, and the

OCTAHEDRAL. CLUSTERS OF TRANSITION METALS

5

following reaction sequence was proposed by Franolic, Long, and Holm:

The lower nuclearity clusters [ M O ~ C I ~were ~ I ~isolated from MoClSin AlCl,/KCl/BiCl,/Bi melts that led to the hexanuclear species ( 17). This is the only intermediate isolated from reaction mixture that pro~ } ~ Solution + chemistry has yielded Moq, Mag, duces the { M O ~ C ~core. and M o ~halide species from mononuclear molybdenum complexes, suggesting that a nucleation process similar to that proposed for the tungsten systems may take place during the formation of hexanuclear molybdenum clusters ( 18-22). Extraction of the cluster from the products of the solid-state reactions with HX yields the hydronium salts of the molecular species (H30),(M&&).6H,0, which are converted into the polymeric material M 6 m & under vacuum at 200°C (12, 23). The structure of M&X$X&, also designated MGX12, consists of a {M6&}4+ core with two terminal axial ligands X2” and four axial ligands bridged to neighboring clusters Xjp2(11)(Fig. 3). Both M a l zand (H3O),(M&&).6H20 may serve as precursors to new molecular species containing the metal halide cluster core (MO&}~’. B. AXIALLIGAND CHEMISTRY The ligand substitution chemistry of the axial ligands of the group 6 clusters is considerably more developed than the substitution chem-

FIG.3. Structure of Mo,C1,C12Cl,, . Reprinted with permission from Ref. (130).

6


istry of the inner ligands. This is due largely to the robust nature of the {M6Y8}*+core, which requires substantially harsher conditions to displace the inner ligands. The increased lability of the axial ligands enables the coordination environment about the {M6Y8}4t framework to be altered without disrupting the metal octahedron on the Y8 cube.

1. Mechanism of Ligand Exchange Kinetic studies of the ligand exchange reaction

were performed by Sheldon with 36C1-labeled[Mo6C18C1612in aqueous solutions. The rate-determining step was found to be first order with respect to the molybdenum cluster and independent of halide concentration, and hence the aquation step

was proposed as rate limiting (24). Schafer found similar results with the exchange reactions of [Mo6Cl8Cl6I2and [w6c18c1612-with Br- and I- (25).Two significant findings from Schafer’s work are the equivalency of the six axial positions and the rate dependence on the metal itself, which show that reactions of molybdenum clusters proceed faster than those of the analogous tungsten complexes. Preetz substantiated the equivalency of the axial positions for the ligands F-, C1-, Br-, I-, and NCS- by monitoring the 19Fand 15Nand 95MoNMR spectra of the ligand exchange reactions

and

Reaction rates were found to increase with the series X = C1 < Br < I < SCN < F, equilibration of the thiocyanate cluster was achieved in 10 h, and 2-3 days were required for the formation of the fluoride cluster at room temperature (26,271. All the clusters [ M O ~ C ~ ~ F ~ - ~ X I ~ and [Mo&~,(NCS)~-,X,]~including structural isomers Ke., mer or fac of [Mo&18F3X3I2> were identified by NMR. In all cases a statistical


7

distribution of all possible compounds, inclusive of the ratios for the structural isomers, was obtained. Importantly, no evidence of isomersteering electronic effects was observed. 2. Synthesis of Molecular {M6Y8}Clusters

Various types of neutral ligands, including nitrogen donors (12,23, 28, 29), oxygen donors (12, 28, 30, 311, phosphines (28, 32, 331, and solvent molecules (28, 34, 351, disrupt the extended MosCllz and WsCllzsolid structure. Under harsher conditions terminal chloride ligands of MosCllzare displaced by neutral donor ligand, L:

For instance, addition of PPh3 to MosCllz yields M o , C ~ , C ~ , ( P Pat ~~)~ room temperature, but it is necessary to reflux MosCllzwith an excess of PPh3 in tetrahydrofuran to produce the triphosphine cluster [ M O ~ C ~ ~ C ~ ~ ((28). P P ~Similar ~ ) ~ I Cexperiments ~ by Walton and coworkers with PEt3 and Mo6ClIzin refluxing ethanol resulted in the reduced cluster M o ~ C ~ , C ~ , ( P Ewhich ~ , ) ~ , contains a {Mo,C~,}~+ core. Magnetic susceptibility measurements conducted on M o , C ~ , C ~ , ( P E ~ ~ ) ~ indicate that the cluster is diamagnetic, a result inconsistent with the expected odd number of electrons in the {Mo,C~~}~+ core. The compound was subsequently reformulated as the reduced species [ M O & ~ ~ ( P E ~ ~which ) ~ ] ~ + ,crystallizes with the counteranion [Mo6Cl8Cl6l2-(321. The introduction of two triphenyl phosphine ligands increases the lability of the remaining chloride ligand in the biphosphine cluster Mo6C18C1,(PPh3),. Both ethanol and pyridine dissolve Mo6Cl12,forming the solvent complexes Mo6C18Cl,(solvent)z,but dissolution of the biphosphine cluster M O ~ C ~ , C ~ , ( Pin P ~these , ) ~ solvents to leads to the species [Mo,C1,C1z(PPh3)2(HOEt)212+ and [MosCl&12(PPhs)A py)z12+ (281. Neutral clusters with the formula Mo6ClsCl4L2can exist as cis or trans isomers. Differences in polarity of the cis and trans structures were employed by Saito and co-workers to separate the isomers of [Mo6C1&1,(P(n-C3H7)3]2by chromatography to produce 3 and 15% yields of the cis and trans respectively ( 3 3 ) .The less polar trans isomers of MO&!~&~~(PR~)~ (R = n-C,H9, n-C,HI1) have also been isolated in greater than 40% yields (Fig. 4). Shifts in the 31PNMR spectra reveal a preference for the formation of the all-trans isomer. DiSalvo and co-workers effected the isomerization of trans-MosC18C14[P(nC5Hl113]2into ~ ~ ~ - M O ~ C ~ ~ C ~ by, immersing [ P ( C ~ Hthe ~ )trans-P(n~~~ C,H,1)3 cluster in a THF solution of P(C,H,), (Fig. 5). A 95% conversion

8


ii

FIG.4. Structure of tralzs-MofiClsC1,tP(n-C4Hs)r12.

to the cis isomer was obtained (36).Alkylation of the trans phosphine clusters with trialkylaluminum reagents AIR; (R’ = CH, , CzHs, n-C3H7,n-C4H9,and n-C6HI3)leads to the replacement of two axial chloride ligands with R groups, M O & ~ & ~ ~ ( P (R’Iz R , ) ~(Fig. 6). Additional AIR; reacts with M O ~ C ~ ~ C ~ ~ ( P R ,producing ) , ( R ’ ) ~ the trialkyl M O ~ C ~ ~ C ~ ( P R ~and ) ~ (both R ’ ) mer ~ , and fac isomers were shown to be present by 31PNMR. These organometallic derivatives are the only examples in which carbon atoms are directly bound to the metal atoms of the { M o ~ C ~framework. ~}~+ Addition of two equivalents of sodium methoxide to Mo6ClI2in methanol leads to [ M O ~ C ~ ~ C ~ ~ ( O(37). C H Single-crystal ~)~]~X-ray diffraction reveals that the methoxide ligands occupy axial positions in a trans geometry (Fig. 7). However, it is not clear whether the trans

FIG.5. Structure of cis-MofiC1,C1,[P(C,H,),l,.


9

FIG.6. Structure of all-trans-Mo~Cl,CI,(C2H,),[P(C,H,),I,.

isomer is formed exclusively or only crystallizes preferentially because neither solution NMR nor X-ray powder pattern data on the bulk material was reported. In contrast to the bisphosphine clusters MO,$~~C~,(PR,), , in which both electronic and steric factors may affect the cisltrans ratio of the products, only electronic effects are likely to influence the formation of the trans isomer of [Mo,Cl,Cl,(OCH,),]”. The trans isomer of [ M O ~ C ~ ~ C was ~ ~ Bprepared ~ ~ ] ” from crystalline MoGClHC14(H20)2 in a heterogeneous reaction with Br-. Homogeneous mixtures of Mo6C1RC1,(H20)2and LiBrl[(CfiH5)4AslBrlead to bromide-rich clusters [(CfiH5)4As12[Mo6C18Cll,yBr4.81 after only 30 min (35).

FIG.7. Structure of trans-[M0,CI,C1,(0CH,,),]~

10

PROKOPUK AND S H R I W R

Mixed halo clusters (H30)2M6C&Y6 (M = Mo, W; Y = F, Br, I) have been prepared from the aqueous acids HY and Mo6Cllz(12, 27, 32, 35). It is likely the substitution results from an excess of the acid, HX, rather than the relative affinities of the { M O ~ Ccore ~ ~ for } ~ the ~ halide, X-, ions. Dehydration of (H30)z[M6C18Y6] leads to the mixed ligand bridged systems Mo6C18YzY41a, which are analogous in structure and reactivity to MoGCllZin that addition of 2L produces Mo6Cl8Y4L2. One of the first comparative studies of the mixed halide clusters MosC18YzY41z (Y = C1, Br, and I) revealed that the axial iodide ligands of M O ~ C ~ , are I ~ Imore ~ , ~ susceptible to displacement by nitrobenzene than the terminal bromide or chloride lands of Mo6C18YzY41z (Y = C1, Br) ( 2 3 ) .Dissolution of Mo6C181z141z in nitrobenzene produces and the bromo and chloro derivatives yield bisnitrobenzene complexes M O ~ C ~ , Y ~ ( N ~ ~ C ~ H ~ ) ~ . Strongly basic anions such as hydroxide and alkoxide easily displace the axial chloride ligands of [Mo6C18C1612-;thus, titration of [MO~C~,C~,]~with OH- initially precipitates the tetrahydroxo cluster M O ~ C ~ ~ ( O H ) ~14H20, ( H ~ O )which redissolves as the hexahydroxo cluster [MO&l&OH)6]2-with increasing pH (23). Similarly, when Mo6Cllz is refluxed with sodium alkoxides in alcohol solution, the alkoxide substitutes for all of the axial chloride ligands (35, 3 9 ) . Methoxide, ethoxide, and pentafluorophenoxide clusters [MO~C~,(OR)~]~have also been generated by this method (35, 38). Increased lability of the axial positions has been achieved by introducing weakly coordinating ligands such as triflate (trifluoromethanesulfonate) (40, 41 1, tetrafluoroborate (26, 42-45), nitrate (43, 46, 4 7 ) tosylate (401, trifluoroacetate (40,481,and perchlorate ( 3 4 ) .(Caution: perchlorate complexes are hazardously explosive.) Reactions of the chloro cluster [Mo6C18C1612with the silver salt or the protonated form of the weakly coordinating base B- generate the reactive species [MO&18B6]2-:

Only the axial chloride ligands were abstracted by the silver salts or acids, leaving the inner ligands and the (MO,C~,}~~ core intact. The weakly coordinating ligands are easily displaced by anionic, X-, or neutral ligands, L, generating the hexasubstituted clusters Cotton and Curtis first employed this [Mo6C1&l2- or [Mo~C~,L,]~+. strategy with AgC104 to prepare the first tetracationic clusters with


11

the ( M O ~ C ~ ~unit, } ~ ' [ M ~ ~ C l ~ ( s o l v e n t ) ~ l ( C(solvent l O ~ ) ~ = DMF or DMSO) (34). These compounds demonstrate the utility of using weakly coordinating ions to alter the coordination environments about the { M O ~ C ~core, ~ } ~but + they are potentially explosive. The triflate derivative [ M O ~ C ~ ~ ( O S O ~(Fig. C F 81, ~ ~ has ~ I ~been used extensively in our laboratory to prepare new cluster derivatives with neutral ligands not previously observed to bind to the ( M O ~ C ~core ~}~' in all six axial positions. Limits of the substitutional lability of the triflate ligand of [Mo~C~,(OSO,CF~),~~have been explored with 19F NMR (49). Diagnostic shifts in the 19FNMR signal of coordinated and uncoordinated (free) OSOzCF3- may be employed to monitor the displacement reactions. As expected, only one peak is observed in the 19F NMR spectrum of [Mo6C18( OSOzCF3)6]"in noncoordinating methylene chloride. Two peaks are observed with THF, acetonitrile, and acetone, suggesting incomplete substitution of the triflate ligand by solvent molecules. A single peak is observed for [MO&18(OSO$F3)6]2- in both DMSO and methanol, indicating that the triflate ligands are completely displaced in these solvents (49). Titration of [Mo6C18 (OS02CF3)6]2in methylene chloride with phosphine ligands PR3 (R = Et, t-butyl, Ph) generates all possible species [MosCls (OS0zCF3)6-x(PR3),1'2-"'( 0 5 x 5 6) as determined by 31PNMR. Unlike the ligand exchange reactions between [MO&18F612- and [Mo6C18Y612(Y = C1, Br, I, and NCS), in which a statistical distribution of products was formed (26,271, the x = 2 species is disfavored in the phosphine titration reactions. This may be due in part to the polar CH,Cl, solvent, which may disfavor the neutral species. Interestingly species

13 FIG.8. Structure of ~Mo6Cl8(OSO,CF3),]'

12

PROKOPUK AND SHRrVER

with x < 6 persist in the presence of excess of the phosphines P[C(CH3)3]S and P(C2H5)3( 4 9 ) .It is unclear whether steric or electronic factors prevent full substitution of the triflate ligands by the phosphines. The acetonitrile ligated cluster [ M O & ~ ~ ( N C C His ~ )an ~ ]even ~ + more labile cluster than DiSalvo and co-workers prepared the hexaacetonitrile complex as the [SbCl,]- salt by treating [ B U , N ] ~ [ M O , C ~ , ( ~ S O ~ C Fwith ~ ) ~ ] KSH to generate [Bu4N12 [MO~C~~(SH which ) ~ I , reacts with SbCl, in CH3CN to produce [ M O ~ C ~ ~ ( N C C H ~ ) in ~ ] less [ S ~than C ~ ~5% ] ~ yield (50).Some decomposition of the { M O ~ C ~core ~ } ~ 'is . required to provide the sixth chlorine atom for the formation of the [SbCl,]- counteranion. A similar abstraction of inner chloride ligands may have occurred in the alkylation reactions of the phosphine clusters with trialkyl aluminum reagents studied by Saito ( 3 3 ) . The [Mo&~&NCCH&~~+ cation also can be prepared in near quantitative yield by reaction of the azide ligands of [ M O ~ C ~ & Nwith ~ ) ~NO' ] ~ - in acetonitrile; however, the CH3CN ligands are removed under vacuum and the BF4- ions then bridge molybdenum centers to fill the coordination sphere of the {Mo~C~,}~+ units ( 4 9 ) . The methoxide derivatives [MO&~&OCHS),#- and [ M o , C ~ & ~ ~ (OCHS)212provide an alternative strategy for generating new clusters with the { M o & ~ ~ core } ~ + (51-55). The strongly basic OCH,- ions are easily abstracted by Bronsted acids, liberating methanol. Nannelli et al. (54)first demonstrated the lability of the methoxide ligands with metathesis reactions between [Mo~C~,(OCH,)~]~and phenol:

The trans isomer of [Mo,C~,(OCH,A~)~]~(Ar = g-anthracenemethano11 was prepared from trUnS-[MOsC18C14(0CH3)212- (Fig. 91, indicating that little or no isomerization or ligand exchange between clusters occurs (37).This strategy is convenient in that readily available proton sources including alcohols, phenols, thiols, and carboxylic acids can be used, and the only by-products are solvent molecules. Other organic ligands with pK, values comparable to phenol, such as sulfanamides, benzamide, pentafluoroanaline, acetylacetonate, and nitromethane, do not undergo the expected metathesis reactions with [MO&l~(OCH&12-(56). Interestingly, the hexaphenoxide derivative [ M O ~ C ~ ~ ( O C reacts ~ H ~ ) with ~ ] ~ -benzoic acid to produce the benzoate cluster:


13

e FIG.9. Structure of trans-lMo,Cl,CI,(OCH,C, ,HqI2lL

However, no reaction is observed between [Mo&!~~(OC~H,),]~and sodium benzoate, suggesting that formation of the alcohol or phenol is the driving force for these reactions (56). C. INNERLIGANDCHEMISTRY 1. Mechanism of Ligand Exchange

Substitution of the inner ligands of {MO&!~~}~+ and {WtiC18}4+ clusters requires more forcing conditions than those used t o exchange axial ligands. Neither boiling aqua regia nor fuming sulfuric acid affect the {Mo6ClXJ4+ core (57). Basic solutions, however, disrupt the cubic array of inner ligands. The inner halide ligands of [ M O ~ C ~ ~ ( O Hand )~I” [Mo,Br,(OH),I2- are susceptible to partial displacement by hydroxide ions, [MO&-,(OH),(OH)~I’~. Kinetic studies of the substitution of the inner chloride and bromide ligands by OH- reveal that the process occurs with a second-order rate law, which is first order with respect to both cluster and the hydroxide ion. Interestingly, substitution of by OH- was found to be autothe bromide ligands of [Mo6BrH(OH),I2 catalytic, suggesting that steric factors play a role in displacement of the bromides (58,59).

2. Inner Ligand Exchange

As implied in the previous discussion, the inner ligands have lower lability than their outer counterparts, but full substitution of the in-

14

PROKOPUK AND SHRTVER

ner chloride ligands by methoxide ions can be effected by boiling solutions of sodium methoxide and MosCllzto dryness. The resulting cluster, Naz[Mos(OCH3),(0CH3)6], which has face-capping methoxide ions, is pyrophoric (39, 60). Briickner et al. prepared mixed inner ligand species by heating the mixed halide clusters MO&l&&/z (X = Br, I) to 400°C (44, 45). The resulting mixtures of clusters were converted to the fluoride derivatives [Bu4NIz[Mos(Y,C1,-,)F6] and analyzed by NMR spectroscopy. Deconvolution of the 1D and 2D 19FNMR spectra revealed a mixture of products and structural isomers, all of which were identified in the NMR spectra. A statistical distribution of products is obtained from A contrasting result was observed when Mo6Cl~BrzBr41z. was heated to produce the z = 3 and 4 species as the most abundant species. Complexes containing iodide ligands in close proximity are favored. Complete exchange of both inner and outer chloride ligands of M&1&1zC14/~ (M = Mo, W)occurs in fused salt mixtures of LWKX (X = Br, I) (12, 61). The similarity between the structure of {MosC18}4+ and the MosQs (Q = chalcogenide) unit of the superconducting Chevral phases has stimulated interest in the substitution chemistry of the inner ligands and, in particular, substitution of the inner chloride ligands by a chalcogenide. A number of mixed halide-chalcogenide clusters {Mo6(Y8-,&)}(X = C1, Br, I; Q = S, Se, Te; z = 1, 3, 4, 6, 8) have been prepared from Mo6C18C1zC141z and the elemental chalcogenide at elevated temperatures (9OOOC) (62-64). The clusters are linked into extended arrays through both inner and outer bridging ligands {Mo6(Y,-,Qz)}(Fig. 10). Diamagnetic and dielectric properties of these solids reveal insulator to metal to superconductor transitions upon increasing chalcogenide content, halide size, and intercluster bridging.

84 FIG.10. Intercluster bridging between {MQYJ”’ clusters. The vertices of the cube represent the inner ligands Y.


15

Lower temperature methods for introducing chalcogenides into the face-capping positions of MosCllzwere developed in McCarley's laboratory, where MosClle in pyridine was refluxed with NaSH. The resulting soluble mixed-capped cluster (pyH),[Mo,(C1,S)Clsl was obtained in 41% yield (65). Saito used similar conditions followed by extensive chromatographic separation to prepare the mixed halidechalcogenide clusters [Mo6(Y7Q)&I3-(X = Y = C1, Br; Q = S, Se) and the disubstituted species [ M O , ( C ~ ~ S ~ ~ ) C(66, ~ ~ I67). " ' ~ -The chemistry of these interesting chalcogenide clusters has not been pursued. The observation that chromatography of the clusters with a chloride-deficient eluent produces the aqua complex [Mo,(C~,Q)(H,O),~~+ indicates that the axial chloro ligands of the chalcogenide-doped clusters are more labile than those of the all-chloro complex [Mo6C18C1612-. Tungsten clusters with inner halide-chalcogenide ligands have been prepared by the reaction of W6Cllzand AzSe (A = Na, K) (68). When W6Cl12and a n alkali metal chalcogenide are refluxed in toluene with triethyl phosphine, clusters with mixed ligands are produced, [Ws(Se,C18-,)(PEt,),]1t ( z = 7.0, A = Na; z = 6.4, A = K). Slight changes in reaction conditions or chalcogenide source significantly affect the halide/chalcogenide ratio of the substituted product. Full substitution of the inner chloride ligands of both MosCllz and W,Clle by chalcogenides produce molecular clusters with the {M6Q8}n+ cores. When W,Cl,, and elemental sulfur are heated to 300"C, a n uncharacterized cluster results, which reacts with magnesium in the presence of triethylphosphine to generate W6S8(PEt3),in 10% yield (69). Better yields for {MsS8}"+(M = Mo, W) are obtained when M,Cl12,NaSH, and NaOBu are refluxed in pyridine. In these reactions the NaOBu serves as a proton abstractor and the chalcogenide clusters are isolated as the pyridine adducts ( 1 4 , 70, 71). The chemistry and structural variations of the group 6 metal chalcogenide clusters were recently reviewed in this series, and the reader is referred to this publication for further information on these compounds (72).

D. Redox Chemistry and Photophysics of the Group 6 Metal Halide Clusters The robust nature of the (MO,C~~}~+ core in combination with its photophysical and redox properties is ideal for solar energy storage (73). Voltammetry on [Mo6Cl8Cl6I2in acetonitrile reveals two l-electron steps at - 1.53 and 1.56 V (vs SCE), corresponding to generation of the oxidized and reduced species [Mo6C1&!ls]'- and [Mo6Cl8Cl,l3-, respectively ( 74). Electrolysis of [Mo6ClsCl6l2-a t potentials more posi-

16


tive than that of the 2-/1- couple generates the oxidized cluster [Mo6C18C16]'-,with no evidence for decomposition or any new electroactive species (75). Coulometric measurements are consistent with quantitative one-electron oxidation of the cluster; however, the oxidized species [Mo6C18C16]'with a {MO,C~,)~' core was not isolated (10). Voltammetry experiments on [Mo6Cl8Clsl2in basic chloroaluminate melts reveal a reversible oxidation couple corresponding to generation of [Mo6C1,C161'-. In contrast t o the voltammetry experiments in acetonitrile, the [ M O ~ C ~ ~ C ~ ~ ] ~ ~couple / [ MinO the ~ Cionic ~ , Cmelt ~ ~is]chem~~ ically irreversible, presumably due to dissociation of axial chloride ligands (76). The oxidized clusters [MsY&]l- (M = Mo, W; Y = C1, Br, core have been generated electroI; X = C1, Br, I) with the chemically in organic electrolytes (Table I) (10,77, 78). The tungsten clusters [wsY&]'- (X, Y = C1, Br) have been produced by chemical oxidation of [W6Y&l2- with NO+ (79). Oxidation of WsBr12 with liquid bromine in a net two-electron process, and the extended structure, WSBr8Br4(Br4)2,2, results consisting of {W6Br8}6+ units bridged in a linear array by (Br4Y anions (80, 81).The rearrangement of inner halide ligands to edge-bridging positions takes place with w&l&12c14/2 at elevated temperatures in the presence of chlorine (821. The resulting core {w&l12}6+contains 12 edge-bridging chloride ligands, and 6 axial chlorides complete the structure of W&ll&l6. A n alternative route to w6c1&16 employs octachlorocyclopentene, C5C18,as both the oxidant and the chloride source (83). Oxidation of M o ~ C ~ ~ C ~by, C C,C& ~ ~ ,leads , to M0,$1&13, which can be converted to the tetraethylammonium salt of [Mo6Cl12C1613(83). The clusters w6c1&16 and [Mo6Cl1,Clsl3-are the only known examples of TABLE I REDOX POTENTIALSO

FOR

1.60

1.38 1.14 0.99 0.93 0.80

0.57 0.56

SOME{MBY,P+CLUSTERS

- 1.56

I0 10 10 78 77 78 78 78

All values were obtained from voltammetry experiments in CH,CN and are referenced t o SCE.


17

group 6 metal halide clusters adopting the {MHY1l}’lt geometry. Conversion of the {M6YR}4+ core to {M,Y,,}”+has only been observed in these reactions, and no examples are known in which the reverse transformation from {M6Y12}”T to {M6Y8}4+ occurs. Clusters containing chalcogenide ligands {Mo6(ClR-2Q2)}f1+ are somewhat more substitutionally labile than the all-halo complexes and undergo oxidation at potentials about 0.87 V less positive than that needed to oxidize the parent perhalo cluster (Table 1). The oxidation potentials of the chalcogenide-substituted clusters change very little with the identity of the chalcogenide, but significant solvent dependence is observed for the chalcogen-containing clusters. The cyclic voltammograms of [ M O , ( C ~ ~ S ) Cin~ ~the ~ ” noncoordinating solvent methylene chloride reveal a quasi-reversible wave corresponding to reduction of the cluster a t 0.14 V vs. the ferrocenelferrocenium couple. In acetonitrile this wave is shifted to 0.28 V, indicating that partial substitution of the axial ligands by CH3CN has occurred. New waves grow in upon changing the potential on acetonitrile solutions of [Mo6(C17S)C161”,suggesting that further changes in the coordination environment of the {Mo,(C17S)}”+core take place upon switching the oxidation state of the cluster (66, 84). In the early 1980s Gray and co-workers discovered an unusually long-lived excited-state lifetime, 180 p s , for the cluster anion [Mo&~,C~,]~-. The emission spectrum is substantially red shifted from the absorption bands (9) (Fig. 11).This is one of the longest observed lifetimes of any purely inorganic compound. Shifts in the near-UV absorption band upon variation of the Y ligands in { M o ~ Y ~and }~+ {W6Y8}4were initially taken as an indication that the transition originates from an (inner) ligand-to-metal charge transfer; however, the

,/-

FIG. 11. Absorption (full) and emission (dashed) spectra of IBu,NIL[Mo,C1&l,,I in CH,Cl,.

18


independence of the emission spectra of the {Mo&}~+unit on changing the identity of inner ligands suggests that the transition responsible for radiative decay is between purely metal-based orbitals (10,77, 85, 86). The {W6Y8}4+clusters display emission spectra that vary only slightly with changes in the inner and outer ligands. A summary of the photophysical data for various molybdenum and tungsten clusters is given in Table 11. The low rate constants for the red-shifted radiative decay are attributed to a spin- and symmetry-forbidden character of the transition. Also important to the low rate constants is the absence of high-energy vibrational modes capable of deactivating the excited state. Additionally, the halide ligands appear to shield the Mo6 and w6 core and thereby inhibit energy transfer t o solvent molecules. From the photophysical and electrochemical data on [ M O ~ C ~ & ~ ~ ] ~ - , a modified Latimer diagram can be constructed relating the excited cluster [ M o ~ C ~ , C ~to ~ ~the " * oxidized and reduced ground states ( 7 4 ) (Fig. 12). When [Mo6C1&l6I2-*is quenched with an electron donor such as benzoquinone, [ M O ~ C ~ , C ~is~ ] generated. ~Similarly, [Mo6C18C1612-* is quenched by the electron acceptor phenothiazine to produce [M0,C1&1~1'-.The long excited-state lifetime of [Mo6C1&1612-* and the strong reducing and oxidizing nature of the electronically TABLE I1

PHOTOPHYSICAL DATA^ OF {M6Yap+CLUSTERS Cluster

A,,~,,,

805

825

880 814 802 766 758 752 701 698 698

7(ps)

180 140 86 190 110 71 84 2.2 4.4 5.6 15 15 19 16 27 30

k , (103 s-1)

k , (103 s-1)

1.1

4.5

2.1

7.0

1.9 13 10 23 10 8 17 11 11 13

Values are reported with acetonitrile solutions of cluster.

10 650 490 310 93 59 50 89 34 20

Refs. 10, 85 89 89 89 10, 85 89 85 10, 77, 89 85, 89 77, 89 77, 89 77, 84 89 77, 89 77, 89 77, 84


pIO,CI,CI,]'~-

- I .J3

t1.30

19

vlo,CI,CI,]'

FIG.12. Latimer diagram of [Mo6C1,C1~I". Excited state energy in eV; electrode potentials estimated vs. SCE in CH,CN.

quenched clusters should be useful for converting solar energy to chemical energy ( 73). The potentials in the excited-state Latimer diagram demonstrate that the annihilation reactions of [Mo6C18C1613and [Mo6C18C161'-liberate enough energy to generate [Mo6Cl8Cl6l"* and electrogenerated chemiluminescence (ECL). This phenomenon has been used extensively to study electron transfer reactions between the excited states of a broad range of the halide clusters (M = Mo, W; X = C1, Br, I; Y = C1, Br, I) in combination with both electron donors and acceptors (78, 87, 88). Chemical oxidation of benzyl alcohol and 2-propanol by electrochemically generated [Mo6ClsC1611-has been demonstrated; however, photochemically generated [MosC18C161'~ is less effective for the oxidation of alcohols (75). The excited cluster [Mo6C18C1612-* is quenched by molecular oxygen through an energy transfer mechanism to generate singlet oxygen, which was identified by reaction with l-methylcyclohexene (891. E. MOLECULAR AND ELECTRONIC STRUCTURE OF THE GROUP 6 METAL HALIDE CLUSTERS 1. Electronic Structure Electronic structure calculations carried out at a variety of levels on these octahedral metal-halide clusters vary in detail (90-104). Our view of the metal-based orbitals responsible for metal-metal bonding in the {M6Y8}4+ unit has evolved very little since Cotton and Haas' semiempirical treatment in 1964 (90). Undoubtedly the importance of symmetry in the calculations has led to the consistent description of the bonding scheme. Only a brief summary emphasizing the relation between the proposed models to the physical and chemical properties of the clusters is presented here. For a more detailed discussion of the bonding numerous reviews are available ( 105-1091.

20

PROKOPUK AND SHRIWR

Orbitals responsible for the metal-metal bonding in {M6Y8}4+ have t2", and eg symmetry and are directed along the edges of alg,tlu,tZg, the M6 framework, generating 12 two-electron bonds. A total of 24 cluster bonding electrons, CBE (CBE number is equal to the number of valence electrons of the metal atoms minus the number of anionic ligands and the charge of the cluster) fill the bonding orbitals, creating "closed shell" diamagnetic compounds (Fig. 13). ESR measurements are consistent with the proposed diamagnetism of [M6Y8&I2(M = Mo, W) clusters and paramagnetism of the oxidized species CM~Y&l*-(10, 79). Some dispute persists as to the size of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap in the closed-shell 24-electron species. This aspect of the molecular orbital diagram is of particular interest for its implications on the photophysical properties of the clusters. Electrochemical measurements are consistent with a large, 2- to 3-eV, separation; however, the exact nature of the excited state remains uncertain. 2. Molecular Structure These highly symmetric compounds provide aesthetically pleasing structures with near perfect Mo6and octahedrons that expand and contract to accommodate different coordination environments. Comprehensive studies of the structures of the halide clusters [Mo6Y&I2-

FIG.13. A molecular orbital diagram for {M,Y,}4+ based on a n Extended Hiickel calculation, adapted from reference (108).

21


(Y = C1, Br, I; X = F, C1, Br, I) by Preetz reveals that both electronic and steric effects of the inner and outer ligands influence the Mo-Mo, Mo-Y, and Mo-X bond lengths (42, 43, 110). For a given {Mo,Y,}~+ (Y = C1, Br, I) core, the Mo-Mo bond length increases with the axial ligands series X = F < C1 < Br < I (Table 111). Expansion of the Mo6 octahedron is attributed to changes in the electronegativity of the axial ligands. The 95Mo NMR spectra of the various halide clusters reveal that resonances occur at the lowest field for complexes having six axial fluoride ligands. A high field shift occurs with increasing polarizability of the axial ligands, indicating that the electron density on the metal centers increases with decreasing electronegativity of the outer ligands. Because this increased electron density coincides with an expansion of the Mo6 octahedron, it is reasonable to assume that the added electron density increases occupation of the antibonding orbitals of the Mo6 framework. In addition to increasing the electron density on the metal centers, changes in the identity of the axial ligands X from F to I also result in a reduced HOMO-LUMO gap as the ligand field strength of X decreases. The 95MoNMR shift of the halide clusters varies linearly with the electronegativity of the axial ligands (110), and a similar relationship exists between the 95Mo NMR shift and the Mo-Mo bond length (Fig. 14). TABLE I11 SELECTED BONDLENGTHS" OF SOME LMo6Y8&]*-CLUSTERS Cluster

d(Mo-Mo)

2.593 [Mo,CI,F,I" [MofiC1$ls12 2.602 [ M O ~ C I ~ B ~ ~ ~2.604 ~ ~ [Mo~CI,IJ2.615 [MotiBr,F61*2.618 [MosBrxClsl" 2.636 [MotiBr8Brtil" 2.640 [MotiBr,I,l" 2.644 [Mo&F,I" 2.650 [Mo,I,CI,]'~ 2.655 [MotiIRBrhlZ. 2.670 [Mo&I,I'2.675 IMos(CI$)CIJ 2.609 [MoB(C17Se)ClJ3 2.616 [Mo6(Cl6Se,)C1,1" 2.610 ~

" All distances are in A.

d(Mo-Y)

d(Mo-X)

d(Y-Y)

d(Y-X)

Ref.

2.488 2.469 2.465 2.466 2.622 2.604 2.600 2.598 2.798 2.775 2.775 2.767 2.479" 2.501b 2.513"

1.993 2.420 2.565 2.788 2.012 2.451 2.491 2.815 2.008 2.460 2.616 2.846 2.468 2.471 2.496

3.517 3.489 3.483 3.484 3.710 3.680 3.674 3.673 4.006 3.923 3.924 3.911

3.234 3.523 3.629 3.801 3.305 3.591 3.675 3.854 3.394 3.656 3.764 3.923

42 42 42 42 43 43 43 43

3.559

3.591

'' Chalcogens are disordered over the capping positions.

110

43 43 43 66 66 67

22


X.

m

r .

I

Average Mo-Mo bond length I Angstroms

FIG.14. Plot of 95Mo NMR shift vs. average Mo-Mo bond length. Data are taken from references (43, 66).

The expansion of the Mo6 octahedron correlates with contraction of the cubic array of the eight inner ligands and shortening of the Mo-Y bonds. The net result of these distortions is a deviation in the planarity of the Mo-Y4 units on the face of the Y8 cube (Fig. 15). The Mo atoms of the clusters [MO,C~~&]~(X = F, C1, Br, I) protrude outside the halide cube with progressively increasing distances as the axial ligand is changed from F through I. At the other extreme, the metal atoms lie within the Is cube and approach the Y4 face as X increases in size for [Mo61&12-. The [Mo6Brs&12-clusters exhibit intermediate structures, with the metal atoms of [Mo6BrsF612inside the halide cube and those of [Mo6BrsBr6I2and [ M O ~ B ~ &outside. ]~With the ex~ ] ~ [- M O ~ B ~ ~ overlap F ~ ] ~ - of , the inner and ception of [ M O & ~ ~ Fand outer ligands induces a strain within the cluster framework, and this strain becomes more pronounced as the size of either X or Y increases. The strain may account for the increased reactivity in clusters with larger ligands (26, 27).

tI

FIG. 15. Distortions of Y8 cube and X, octahedron with changes in X or Y.


23

Changes in the inner ligands Y of the clusters [M6Y8X,12from C1 to I results in an expanded Ms octahedron and a Y8 cube. Substitution of chalcogenide atoms for the inner halide ligands has only a modest effect on the average Mo-Y and Mo-Mo bond length (Table III), with the chalcogenide ligands distributed among two or more face-capping sites. However, the terminal chloride-molybdenum bond lengths are elongated compared t o the all-halide clusters. These bond length changes have been attributed to decreased electrostatic attraction between the axial chloride ligands and the less positive halidechalcogenide cores { M o ~ Y ~ - , Q ~ ) ~ ~ ~ ~ ’ + . The pseudo-halide derivatives [Mo&YJ- (X = C1, Br; Y = N3 ( 4 6 ) , NCO (471, NCS (X = C1 only) (40, 111, 1121, and NCSe ( 4 7 ) ) have been reported and crystallographically characterized. Some discrepancies exist in the assignment of the disposition of the ambidentate ligands. For instance, both N- and O-bound cyanate are indicated by 14NNMR in tetrachloroethane; however, crystallographic studies are consistent with N-bound cyanate ligands. Selenocyanate is found to be exclusively N-bound by I4N NMR, IR, and X-ray crystallography. Thiocyanate ligands of most [ M O ~ C ~ ~ ( S C N salts )~I~ are - assigned as N-bound to the cluster, with the exception of K2[Mo6C18(SCN)61, which has been identified as S-bound in both the solid state and solution. Interestingly, structures of the cluster anions [Mo6Br8(N3),1”vary with the type of counterion, including relatively noninteracting [Bu4N]+and [Ph3P=N=PPh3]+.The cluster anions of these salts have regular octahedral geometries; however, the Mo-Mo and Mo-Br bonds vary by as much as 8% between the two structures. Several vibrational studies of the [MsY,&12- clusters have been reported (12, 53, 113-117). The structures of these compounds led Hogue and McCarley to predict that extensive mixing of the symmetry coordinates in the normal modes would prevent the application of group-frequency concepts ( 1 2 ) ;however, diagnostic M-M, M-X, M-Y, and X-M-Y stretching frequencies have been identified, and they find use in the characterization of the clusters. Selection rules for the high-symmetry (0,)halide clusters [M6YM&I2-(Y = C1, Br, I; X = F, C1, Br, I) predict a total of ten Raman active (3A,, + 3E, + 4Tz,) and five IR (5T1,) active modes. The three totally symmetric (Alg)bands, which consist of the breathing mode of the M6octahedron, the YMcube, and the terminal X , ligands, are virtually unmixed and thus provide diagnostic features that are easily identified by their totally polarized Raman bands. Decreased symmetry in the environment about the MI; core, owing to site symmetry in the solid or the presence of axial ligands having lower symmetry, does not prevent application of groupfrequency concepts.

24


Pyridine N-oxide derivatives, M O ~ C ~ , C ~ ~ ( O N C ,have H ~ Rbeen ) ~ , prepared, and the v(Mo-0) band was assigned around 1200-1270 cm-'. The v(M-0) frequencies correlated with the Hammett constant (T of the R group. As the R group becomes more electron withdrawing (or increases in u),the v(Mo-0) stretching frequency decreases. This observation contrasts with trends in the v(M-0) frequency with latetransition metal complexes such as Ni, Co, and Cu, in which the M-0 vibrational energy increases with the electron-withdrawing nature of the R group and indicates that n-bonding is unimportant in the axial ligand-metal bonds (31). Oxidation of the {M6Y8}4tcore removes electrons from the bonding orbitals of the M6framework, causing an expansion of the metal octahedron. Few of these oxidized clusters have been isolated and structurally characterized. The EPR spectrum of [Mo6C1&1,]'~indicates that the compound is distorted along one axis; however, single-crystal X-ray data is not available to confirm this distortion. The EPR spectrum of the oxidized tungsten cluster [W6Br8Br6l1- contrasts with the spectrum of [Mo6C1&1,]'- in that the values are consistent with an undistorted octahedron, which has been verified by the crystal structure of [(Ph3P)2Nl[W6Br8Br61 (10, 79).

111. Group 5 Metal Halide Clusters

Clusters containing the {Ta&112}2t and {Nb6C112}2t cores have been known for nearly as long as those of the group 6 metals, and their structures were first reported by Linus Pauling (118).Despite the long history of the group 5 metal halide clusters {MsY12}"+(M = Nb, Ta) the ligand substitution chemistry is not as well developed as that of the analogous group 6 metal compounds {M6Y8}4+ (M = Mo, W), because of the extensive redox chemistry associated with the {M6Y1,}"+ core. Five different oxidation states have been observed for the Nb and Ta cores, with compounds in the n = 2-4 states being most common. The n = 1 and n = 5 states are electrochemically accessible; however, clusters in these oxidation states have not been isolated. As the average oxidation state changes for the metals in the cluster core (M6Y,,}"', the affinity for the axial ligands is altered. In the more electron-rich (reduced) state, the clusters have higher affinity for IT-acid ligands such as phosphines, and in the more electron-deficient state, the core has a greater affinity for n-donor ligands such as chloride. Thus, the stability of a particular oxidation state is altered by the coordination environment about the {M6Y,,}"+core.


25

A. SYNTHESIS OF CLUSTER CORE clusters of niobium and tanThe most convenient route to {M6Y12}2+ talum is the conproportionation of the pentahalides NbY5 and TaY5 (Y = C1, Br) with excess of the metal in molten alkali halide: 20NaY + 14MY5+ 16M + 5Na4M,Y18.

(13)

Formation of lower nuclearity compounds can be avoided by using a 10 to 7 ratio of NaY to MY5 (119).The conproportionation reaction is convenient for the synthesis of the group 5 metal clusters because it allows the use of glass reaction vessels as opposed to electron-beamsealed metal tubes, it provides almost quantitative yield of cluster (in excess of go%), and it requires only 24 h for the reaction to reach completion, as opposed to 4-6 days. Work-up of the product in aqueous acid H Y solutions with small amounts of stannous chloride to prevent oxidation produces M6Y14.8H20. Unlike the group 6 polymeric materials MsYlz, M,Y14*8H,O consists of discrete molecules with a {M6Y12}+core coordinated by two halide ligands and four water molecules, and is accurately described as MsH12Y2(Hz0)4.4H20. The soluble tertiary salts &Nb6Y12Yshave been used as precursors to other cluster compounds with the {Nb6YI2)a+ core, with limited success (120, 121). B. REDOX CHEMISTRY OF THE GROUP 5 CLUSTERS Initial studies on the redox properties of the {M6Y12}n+ clusters were reported on poorly defined complexes simply noted as [TasC1,,]2’ or [NbsCIJt (1221. Aqueous cluster solutions were prepared from the sulfate salts Ta6C1,,S04 and Nb6C1,,SO, with various electrolytes. Some combination of water molecules and anions fills the coordination sphere of the {M6Y12}”+cluster. Despite these ambiguities, the oxidized compounds {M6Y12}4.C were obtained with various chemical oxidants by a two-electron transfer process (123): {M6Y12}2+ + 20x -+ {MsY12}4+ + 2Red.

(14)

Subsequent investigations ( 124-126) revealed an equilibrium between the intermediate {MfiY12}3+ state and the {M6Y,2}2+and {M6Y12}4 clusters, +

26


and a slow disproportionation of the oxidized { T E ~ ~ Y (Y~ =z }C1, ~ ~Br) cluster to mononuclear Ta(V) complexes and the reduced clusters {Ta6Y12}2tand {Ta6Y1~}~'. Cyclic voltammetry studies (127) on {TasBr12}2treveal two reversible one-electron oxidations corresponding to generation of the n = 3 and n = 4 states,

* -e

-e

{Ta6Br1z}2t

te

{Ta6Br12}3+ {Ta6Br12}4t, +e

(16)

suggesting only a minimal structural rearrangement upon changing oxidation state. Contrasting results were obtained by Hussey and coworkers (128, 129) who investigated the cyclic voltammetry response of [TasCllzC16]'6-n)in acetonitrile. Three 1-electron transfer processes were observed, corresponding to generation of the n = 2-5 species. The reduced cluster [Ta6C11zC16]4undergoes facile substitution by an acetonitrile molecule, generating [Ta6CllZClS(CH3CN)l3(Fig. 16), which is detected in the voltammogram along with the liberated chloride ion. Noncoordinating electrolytes in methylene chloride prevent the loss of the chloride ligand, and the redox couples are chemically reversible (Fig. 16). Interestingly, [TasC11S(OSOzCF3)6]2undergoes two reversible one-electron reductions in acetonitrile to generate [TasC11z(0S02CF3)s13-and [TasCl,~(oso,CF,),]~-, with no evidence for

-1.5

0.5 1.0 1.5 Potential in V vs. Ag/AgCI

-1.0 -0.5 0.0

FIG. 16. Cyclic voltammogram of [Bu,Nl,[TasC1,,Clsl in (A) CHZC1, and 0.1 M [Bu,NlBF,. Sweep rate is 0.10V/s.

(B)CH,CN


27

ligand displacement on the cyclic voltammetry time scale. Presumably the strong (T- and n-donor properties of the chloride ions weaken the Ta-C1” bonds of [Ta6Cl12C1614-, making the chloride ligands susceptible to displacement. In contrast, the reduced {TasC1,2}2+core of [TasC112(OS02CF3)6]4is stabilized by the weak donor properties of the triflate ligands. In acidic chloroaluminate melts, the redox potentials of the {Ta&112}2+’3+and {Ta&!112}3+14+couples are anodically shifted by one volt compared to values obtained in acetonitrile electrolytes, suggesting that the majority of the axial chloride ligands have been replaced by [AlClJ ions (129).A similar shift in the redox potentials of couples is observed when changing the {Nb6C112}2fi3t and {Nb6C112}3+14 from a basic chloroaluminate melt to an acidic one. In C1--rich molten salts, the highly reduced cluster [Nb6c112c1615with a {Nb6clI2}’+core is accessible (128),but this species is not accessible in acidic melts or acetonitrile (130).At the other extreme, the highly oxidized cluster [Nb6C112Cls]1with a {Nb6C112}5+ core is generated at 1.76 V vs. SCE in acetonitrile electrolytes. +

C. LIGANDSUBSTITUTION The ligand substitution chemistry of the “b6Y12&I‘6-n’- and [Ta6Y12&](6-’1)clusters is complicated by disproportionation reactions and dependence of the ligand affinity on the oxidation state of the cluster. Unless measures are taken to control chemical potentials, unwanted redox reactions can produce mixtures of partially substituted clusters in a mixture of oxidation states. Consequently, no strategies for exchanging all six axial ligands with a wide range of donor groups exist. Instead, a number of less versatile methods have been developed that introduce a limited range of ligands a t the axial positions and provide moderate control of the redox chemistry. The axial ligands of &Nb6YrLY6(A = alkali metal; Y = C1, Br) undergo simple metathesis with azide and thiocyanate. Dissolution of &NbsYI2Y6in alcohol solutions of N,: or NCS- produces the crystalline compounds &NbsYl2& (X = NB, NCS) (131-133). Long reaction times, on the order of weeks, are required when the A4Nb6Y12Y6 clusters are used as precursors to new cluster compounds. Also, the relative affinity of the {M6Y12}‘+ core for different anionic and neutral ligands is unknown and may restrict the types of clusters accessible from &Nb6Y12Y6. The hydrated complexes M6Yl2Y2(H2O),-4H20 (M6Y14) provide more versatile precursors t o new {M6Y12}’+ (M = Nb, Ta) derivatives. The

28


aqua ligands are slowly displaced by neutral L or anionic X- donor ligands, generating M6YlzYzL4or [M6Y12Y2&l4-(31, 134-1361. The range of ligands amenable to this strategy has not been delineated, but if care is taken to prevent oxidation of the cluster, a variety of 8-acids are likely to bind to the reduced {M6Ylz}2+ core. Saito and coworkers used chromatography to isolate the cis and trans isomers of the phosphine complexes [M6C112C1z(PR3)4] in moderate yields (1337%) (135) (Fig. 17). Also isolated in the separation procedure was the trisubstituted cluster [Ta6C112C13(PEt3)31, indicating that substitution of the water molecules of Ta6Cll,C1,(H20),.4H20is more complex than first proposed. Repeated addition of aliphatic alcohols ROH to M6Y12Yz(H20)4.4Hz0 (M = Nb, Ta; Y = C1, Br) under an oxygen-free atmosphere eventually yields [M6Ylz(ROH)61X2 (137).Interestingly, 'H NMR and conductivity measurements indicate that the axial halide ligands (especially bromide) are displaced by methanol before the axial water ligands (138). Under anaerobic conditions [M6Ylz~OCH3)z(HOCHz)41 can be obtained from methanolic solutions of M6YlzY2(Hz0)4.4Hz0 and two equivalents of sodium methoxide (139). Care must be taken to exclude air from these reaction mixtures to avoid unwanted oxidation of the cluster. In basic solutions the {M6Y12}'+ core is rapidly oxidized upon exposure to air. Presumably the strong u- and 8-donor properties of hydroxide and alkoxide ligands increase electron density on the

Q

FIG.17. Structure of t r a n ~ - T ~ C l ~ , C l ~ ~ P ( C ~ H ~ ) ~ 1 ~ .


29

{M6Yl,}2rcore, making the clusters [M6Y,2(OR)s14(R = H, CHd susceptible to oxidation. Titration of M6YIzYa(Hz0)4 with OH- precipitates the neutral complex M,Y,,( OH),(H,O), . 6Hz0under Na (140).Isolation of M6Y,,(OH),(H20)4.6H,0indicates that the axial chloride ligands are displaced in preference to the axial H,O ligands. Subsequent addition of OH- yields &M,Y,,(OH), . In oxygen-containing atmospheres, MfiY12 (OH),(H,O), precipitates and then redissolves as A2MsY,2(OH)fi with increasing pH ( 141 1. The methoxide clusters M,Y,2(OCH3),(HOCH3)4 and A,M,Y,,(OCH,), have been prepared in a similar fashion (139) (Fig. 18). In acid environments the {M6Y12}2+ core of MsY12Y2(H,0)4.4Hz0 is depending on the metal, inner oxidized to either {M6Y,2}3'or {M6Y12}4t to ligand, and acid source. Hydrochloric acid oxidizes TafiC112Cl,(H20)4 Ta6C1,,C1,(H,O), with a {Ta6C11,}4+core (142); however, NbsCllzCl~ core (143, 144). (HzO), is oxidized to [Nb6C1&1,1S- with a {Nb6C112}3+ Synthesis of the two-electron oxidized cluster [Nb6CllzCls12~ requires (143, 145). chlorine gas to oxidize Nb,ClIaCla(H~O), Introduction of the weakly coordinating triflate ion at the axial positions of the {M6y12}"+cluster is accomplished by the reactions [ B U ~ N I J ~ " ~ C+~xsHOSO2CFS ~~C~~] + [Bu4N1~[Ta,C1,,(0S0~CF:~~~1 + 6HCl(g). (17) The resulting cluster is an electron deficient species (146) (Fig. 19). The niobium analog [Bu4Nls[Nb,Cl,a(OS0,CF3)61has also been prepared. Cyclic voltammetry on solutions of [Bu4Nla[Ta6Cllz(OSOzCFB)61 reveals two 1-electron transfers a t 0.89 V and 0.29 V vs. AgIAgC1, corresponding to generation of [Ta,Cl,,(OSOaCF,),]3- and [Ta6Clln (OS02CFS)fi]4-, respectively. These reduction potentials are among the

FIG.18. Structure of [Ta,C1,,(OCH,),JL .

30

PROKOPUK AND SHRIWR

FIG. 19. Structure of [Ta,Cl,,(OSO,CF,),IZ-

most positive observed for the {TasC11z}4+ and {TagC11~)~+ clusters and attest to the poor donor strength of the triflate ion. The strong oxidizO~CF,)~ the ] simple meing character of [ B U ~ N ] ~ [ T ~ , C ~ ~ ~ ( O Sprevents tathesis reactions with anionic ligands X- such as C1-, Br-, I-, NCS-, and CN- that are observed with the analogous [Bu4Nlz[MosCls (oso2cF,),] (40). Instead, a complex redox process accompanies the substitution reactions and reduced clusters [Bu4N13[Ta6Cl12&I(X = C1, Br, I, NCS) are obtained. A slight excess of X- is required for complete substitution of the triflate ligands, and the reaction rate increases with the ligand series C1- < Br- < NCS- < I- < CN-. This trend does not follow the basicity or polarizability of the ligand, so other factors, possibly oxidation potentials of X-,influence the redox/ substitution process. In fact the reduced cluster [Ta&Il2(OSO2CF3),I3is observed spectroscopically as an intermediate. Reduction of [Ta6C11z(OSOzCF3)s]2by C1- (1.29 V vs. Ag/AgCl) would not be predicted from redox potentials; however, refluxing methylene chloride solutions of [ B u , ~ ] z [ ~ a 6 ~ 1 1 z ( ~and ~ ~ z[Bu,NICl ~ F 3 ) 6 yields ] the reduced product [ B U ~ N ] ~ [ T ~ , C ~ ,It ~ Cis~likely ,]. that oxidation of C1- is promoted by loss of Clz due to a slow stream of Nz passed over the reaction mixture. In mixtures of [BU~N],[T~,C~~~(~S~~CF,),I and the more reducing ligands I-, Br-, and NCS-, the reduced cluster core {TasC112}2+ is detected spectroscopically and the proposed reaction/substitution scheme in Fig. 20 is proposed. Surprisingly, the simple metathesis reaction


31

FIG.20. Reaction scheme of [TasCl~~(OSOzCF3),12with X

is not observed for any anionic, X-, or neutral, L, ligands including phosphine oxides.

D. ELECTRONIC AND MOLECULAR STRUCTURE Numerous electronic structure calculations on the {MfiY12}n+ cluster have been performed at various levels (90, 91, 93, 94, 96, 98, 104). The molecular orbital structure obtained from extended Huckel calculations indicates that the eight metal-based orbitals responsible for M-M bonding in the {MfiY12}n+ unit have alg,tZg, tlu,and a2,, symmetry (147)(Fig. 21). In contrast to the directionality of the bonding orbitals in {M6Ys}4+ (see Section II.E.l), the bonding orbitals of {M6Ylz}"+are directed along the faces of the metal octahedron, creating 8 threecenter, two-electron bonds. The number of cluster-bonding electrons available to fill the eight bonding orbitals is equal to the number of valence electrons of the metals minus the number of anionic ligands and the charge of the cluster. Thus, [Nb6Cl12C16]4is electron precise, with 16 CBE [30 - 18 - (-4)1, and [Nb6Cll2Cl6l4is electron deficient with 14 CBE. The exact bonding or antibonding character of the a2,,level differs among the various calculations depending on how the contribution of the axial ligands to the molecular orbitals is considered. Oxidation of the {Nb6cll2}" core to {NbfiC112}4+ corresponds to depopulation of the

32


FIG.21. A molecular orbital diagram for {M,Y,,j"+ based on an extended Hiickel calculation, adapted from reference (108).

a2ulevel; thus, the aZulevel is the HOMO or LUMO of the n = 2 or n = 4 clusters respectively. Magnetic susceptibility measurements and ESR spectra of the clusters with n = 2-4 confirm the assigned aZusymmetry of the HOMO/LUMO (148).Hyperfine splitting of the unpaired electron of [Nb6Y&I3- delocalized over the six niobium centers (spin 9/2) leads to impressively split ESR spectra (Fig. 22). X-ray

l

2.8

.

I

3.0

.

I

3.2

.

1

.

3.4

I

3.6

.

I

3.8

.

I

4.0

.

2

kG FIG.22. EPR spectrum of [Bu4NI~,[NbsCll2BrGI in CHaClaa t room temperature.

OCTAHEDRAL, CLUSTERS OF TRANSITION METALS

33

crystal structure determinations for the cluster series [NbsCl12Clsl(n-6)reveal that the average Nb-Nb bond length decreases from 3.029 to 2.899 A as the oxidation state on the {NbsC112}'L+ core is lowered from n = 4 to n = 2 (149). This trend is consistent with the assignment of the aZulevel as predominately bonding in character for the all-chloro clusters. The high energy of the a2"orbital explains the relative stability of the oxidized clusters {M6Y12}3+ and {M6Y12}4+. The average oxidation state of the metal atoms ranges from 2.33 ( n = 2) to 2.66 ( n = 4). Numerous UV-vis absorption studies on the Nb and Ta clusters have been reported; however, despite the existence of one-to-one correspondence in the spectra of the two metals, there is little agreement on the assignments for the metal-based transitions (135, 150-152). Solutions of both {TasY12}4+ and {Ta6Y,2}3+ are orange-brown in color, and the niobium analogs {Nb6Y,2}4t and {NbsY12}3t are yellow-brown in solution. The reduced states {Nb6Y12}2+ and {TasY~z}~~ are olive green and green, respectively. Attempts to assign the electronic transitions of the clusters have been complicated by the similarity in the spectra of {M6Y12}3+ and {M6Y12}4+ clusters, which makes identifying mixtures of clusters in these oxidation states difficult. Additionally the {MsY12}2-i state is intensely colored and therefore easily obscures the spectra of the oxidized clusters.

E. NIOBIUMIODIDE CLUSTERS {NbsIs}"+ Hexanuclear niobium iodide clusters {Nb61B}f1+ are the lone exception to the usual edge-bridging halide geometry adopted by the group 5 metal halide clusters. With eight face-capping iodide ligands, the {NbsIs}"+core is isostructural with the group 6 metal halide clusters {MO~C~ and ~ }{W6Clg}4+ ~+ (153).The similarity in the metal-ligand configurations permits the use of the molecular orbital structure derived for the group 6 metal halides {M6Y8}"+ to be applied to the {Nb61R}n+ unit. The Nb61B13 is extremely electron deficient, with 19 CBE available for 12 metal-metal bonding orbitals. This unique electronic structure gives rise to the distinctive chemical and physical properties of { N ~ ~ I J J I + . The binary phase Nbs1813is prepared (154) from the decomposition of Nb318at 950°C:

34

PROKOPUK AND SHRIWR

Increased yields are obtained when Niobium metal is added to the reaction mixture to reduce the NbI, by-product: 11NbI4 + 13Nb+ 4Nb&I3.

(20)

The addition of CsI to the reaction mixture produces the reduced cluster CsNb61813, with 20 CBE (155). The electron deficiency of Nb61813and csNb61813 allows these compounds to absorb hydrogen in the solid state at 300°C and atmospheric pressure, generating HNb61813and CsHNb61813( 155, 156). Low-temperature neutron diffraction studies on Nb61813, HNbs1813, and DNb61813 reveal that the hydrogen atom is slightly displaced from the center of gravity of the surrounding Nb6 octahedron (157). The size of the displacement is enough to accommodate multiple hydrogen atoms, suggesting that more than one interstitial hydrogen may reside in the cage in high-temperature phases of HNb61813.Adsorption of hydrogen into the interstitial position is unusual, and this is the only case in which an interstitial atom is implanted in a preexisting M6 octahedron of a metal halide cluster. The uptake of hydrogen by Nb61813increases the number of CBE from 19 to 20, and thus a decrease in the average Nb-Nb bond length is expected. However, single-crystal X-ray diffraction studies of Nbs1813and HNb61813reveal a slight expansion of the Nb6 cage of HNb61813compared t o the intersticontial free cluster (158) (Table Iv). By contrast, Zr6HCl12(EtNH2)6 tains interstitial hydrogen atoms but an average Zr-Zr bond length is -0.3 A shorter than the interstitial free [zr&1&C16I2-, which has a comparable CBE count (both at 13) (159, 160). Low-temperature magnetic susceptibility measurements on Nb61813 and HNb61813 reveal ground spin states of S = 1/2 and S = 0, respecTABLE IV NIOBIUM-NIOBIUM BONDDISTANCES IN SOME {Nb6I8P+CLUSTERS Cluster

CBE

d(Nb-Nb)(AP

Ref.

Average Nb-Nb bond lengths are reported with crystals at room temperature except HNb61813(211 K).


35

tively. At higher temperatures the former has a ground state S = 3/2 and the latter S = 1 (161).The change in spin states is not due to simple transitions from the ground to excited state but a coupled electronic and structural phase change (158,161, 162). Heat capacity, X-ray diffraction, and magnetic susceptibility measurements indicate that a second-order or higher phase change occurs at 274 K for Nb61813 and 324 K for HNb61813.The structural deformation, a rotation of one Nb, face of the octahedron by 7", is accompanied by the crossing of electronic levels, which leads to a reduction of the spin degeneracy. This spin-crossover transition is made possible by competition between the effects of a spin flip and changes in the molecular orbital levels that result from the cluster distortion (163). Two discrete molecular compounds containing {Nb618}"+ units were obtained from reactions of Nb61813and NH,R (R = CH, , C,H,) (2).The amines displace all the bridging axial iodide ligands, generating the neutral complexes [Nb618(NH,R)6].These clusters are electron-rich with 22 CBE compared to the 19- and 20-CBE {Nb618}n+ cores obtained from solid-state reactions. Consequently, the increased number of electrons available for cluster bonding orbitals results in shorter Nb-Nb distances (Table IV). Magnetic susceptibility measurements indicate that [Nb618(NH2CH3)6] is diamagnetic. This observation is contrary to the prediction of a half-filled e, HOMO for a 22-CBE species with a {M6Y8}"+ geometry.

IV. Materials Chemistry Derived from Soluble Metal Halide Clusters

A. HIGHER NUCLEARITY CLUSTERS Mixed metal clusters have been generated by reaction of organometallic species with ambidentate cyanide ligands and [Mo6C18 (OSO&F,)6]2- (411. Both [CpMn(C0)2CNl- and CpRu(PPh,),CN displace the triflate ligands of [ M o ~ C ~ ~ ( O ~ O ~ forming C F , ) ~ ]the ~ ~ ,12 metal clusters { M o ~ C ~ ~ [ ( ~ - C N ) M ~ ( C O )and ~ C ~{Mo6C18[(p-CN) ]~}~R U ( P P ~ ~ ) ~ Crespectively. ~ ] ~ } ~ + , Single-crystal X-ray structure determinations on (PPN)2{Mo6C18[(p-CN)Mn(C0)2Cp]6} reveals that the [CpMn(C0)2CNl-fragment is coordinated to the {MO,C~,)~+ core via bridging cyanide ligands (Fig. 23). Interestingly, the electronic spectra of these mixed metal clusters show an extremely intense absorbance that most likely corresponds to a charge transfer band [possibly intervalence charge transfer from the CpMn(C0)2CN- ligandl. The intensity of this band cannot be accounted for from a simple com-

36


FIG.23. Structure of { M O ~ C ~ ~ [ ( ~ - C N ) C ~ M ~ ( C O ) ~ I ~ } ” .

bination of the absorbencies of the manganese centers and the { M O ~ C ~core; ~ } ~ ’ however, “intensity-stealing” between a Mo6-based d-d“ transition and a charge transfer band may lead to the increased absorption (Fig. 24). Cyclic voltammetry on [ B U ~ N ] ~ { M O ~ C ~ , [ ( ~ - C N ) M ~ (reveals C~)~C~]G) a broad redox wave assigned to the oxidation of the pendant manganese centers. This wave is anodically shifted 0.26 V from the “free” ligand/metal complex and is consistent with reduced electron density on the C ~ M n ( c 0fragment. )~ The breadth of the redox wave is attributed to communication between the manganese and molybdenum atoms through the bridging cyanide ligands. Chemical oxidation of ( M O ~ C ~ , [ ( ~ - C N ) M ~ ( C O ) ~with C ~ ]NO+ , ) ~ - results in clean substitution of one of the carbonyls of the manganese fragment by NO’, generating (Mo6C1,[(~-CN)Mn(CO)(NO)Cp],)4+.

250

300

350

400

450

500

550

600

t

wavelength I nm

FIG. 24. Absorption spectra of (A) [ B U ~ N I [ C ~ M ~ ( C O ) ~ C(B) N I , [Bu,NlZ[Mo6CI, in CH2Clz. (OSOZCFJ)61, and (C) [Bu4NlZ{MosC181(~-CN)CpMn(CO)z16}


37

Reaction of NazMo6C18( OCH3)s with the carboxylic acid H02CC5H4 FeCp produces the organometallic derivative Na2M06C18(02CC5H4 ~}~' FeCp), (55). The iron centers are attached to the { M O ~ C ~core through the carboxylate moieties on the Cp ring (Fig. 25). In contrast to voltammograms of {M0~Cl~[(~-CN)MnfC0)~Cpl,f2, cyclic voltammograms of Na2MofiC18(OzCCsH4FeCp)6 in DMF display a single redox wave shifted 0.10 V from that of the acid H02CC5H4FeCp,indicating that the iron centers are independent and equivalent. Cyclic voltin DMSO reveals a chemically ammetry on NazMosC18(02CC5H4FeCp)fi irreversible oxidation of the iron centers. A reduction wave is observed substantially shifted to negative potentials. This chemical irreversibility is attributed to reduction of uncoordinated ferrocenium carboxylate. A decrease in basicity of the carboxylate moiety upon oxidation of the pendant ferrocenyl group should make this oxidized ligand a weaker donor, which is susceptible to dissociation. Presumably, vacant coordination sites of {Mo,C~,}~+ are taken up by solvent molecules.

B. SUPPORTED CLUSTER MATERIALS A number of strategies have been developed to generate composite materials by the incorporation of the { M O ~ C ~unit ~ } ~into ' a host ma-

38


trix. Coordinating polymers such as poly(4-vinylpyridine) (PVP) provide sites to bind the { M o & ~ ~ units } ~ + directly to the polymer backto bone. Thus, the addition of Mo6Cl12or [Bu4NI2[Mo6C1,(0SO~CFg)61 PVP in ethanol secures the {MO~C~,}~+ unit to the pyridine moieties of the polymer. Because the { M O ~ C ~unit ~ } ~acts + as a cross-linking agent, the nature of the axial ligands on { M O , C ~influences ~}~~ the physical properties of the cluster/polymer composite material ( 164 ). Dissolution of MosCllzin ethanol generates M O , C ~ ~ C ~ ~ ( Ewith ~OH two ) ~ labile , axial sites capable of coordinating pyridine. In contrast [Bu4N12 [MO&1,(OSO&F&] has six sites available for pyridine to bind and thus provides a more effective cross-linker. The number of accessible coordination sites is reflected in the glass transition temperature, T,, of the resulting material. Adsorption of Mo&l12 and [Bu4Nlz[Mo&18 (OS02CF3),]into PVP in a ratio that provides equal donor groups t o cluster binding sites results in composite materials with a higher T, than those derived from Mo6ClI2.Immobilization of the { M O ~ C ~core ~}~+ by the PVP does not drastically alter the emission spectrum of the { M O ~ C ~core; ~ } ~ however, + increasing the number of pyridine groups coordinated to the cluster lowers the excited-state lifetime (164).The photocatalytic properties of these materials have been demonstrated, and singlet oxygen generated by quenching the excited cluster with O2 has been used to oxidize alkenes (165). DiSalvo and co-workers developed a monomer in solvent strategy for preparing monodispersed { M O ~ C ~units , } ~ ~in a polymer matrix in a coordinating sol(166).By dissolving vent with a polymerizable functional group such as N-vinylimidazole (NVI), a monomer unit containing the {MO,C~~}~+ unit is generated, [Mo,C~,(NVI)~](OSO,CF,),. The low solubility of these tetracationic clusters is circumvented by using the monomer unit, in this case NVI, as the solvent. Polymerization of the cluster/monomer solution produces a monodispersed cluster in an organic matrix. In addition t o solvating the cluster cations, the uncoordinated monomer/solvent dilutes the highly cross-linking clusters. By adjusting the cluster concentration in the monomer/solvent, the degree of cross-linking can be controlled. Oxide supports have also been used to immobilize {Mo,Cl8}*+clusters with various degrees of success. Basic and acidic silica adsorb [BU,N]~[MO,C~,(OSO~CF,),] (167).Both electro[ B U ~ N ] ~ [ M O &and ~~C ~~] static and covalent binding of the {M~,cl,}~+ core to the silica surface can be obtained with proper choice of cluster, solvent, and silica treatment. The photophysical and photocatalytic properties of the cluster are retained on the Si02support (168).


39

Intercalation of the aqua complex [Mo&~~(H@),]~+ into layered sodium montmorillonite creates a pillared structure (169). The cluster cations increase the d-layer spacing from 12.5 A for sodium montmorillonite to 16.6 A for the intercalated composite material. The driving force for the intercalation process is attributed to ion exchange with the Na' of the montmorillonite; however, the amount of cluster adsorbed by the layered clay is larger than expected based on the sodium content. Similar observations have been made for the intercalation of {Nb6C112}2+'3+ and {Ta6C112}2+ into sodium montmorillonite, which results in an increase in d-layer spacing and adsorption of excess cluster (170). The axial ligands attached to the intercalated group 5 clusters were not unambiguously identified, thus the driving force for intercalation is unclear.

C. CHARGE-TRANSFER SALT COMPLEXES Organic-inorganic charge-transfer salt complexes containing radical organic cations such as tetrathiafulvalenium (TTF)' or tetramethyltetrathiafulvalenium (TMTTF)' and anionic clusters [M6C18C1612and [M6Cl12C16]'6~"'as counterions have been electrocrystallized. Other organic-inorganic salt complexes have been prepared from these organic radicals with smaller anions such as BF;, PF,, ReO;, and SbF, ( 171). Interactions between the organic radical cations, especially overlap of the HOMO of neighboring cations, can produce semiconducting, metallic, or superconducting properties in the molecular salt compounds. Using large cluster anions changes the anion/ cation volume ratio, thereby forcing the cation radicals into new packing arrangements. Also, the use of radical cluster anions creates the possibility of anion-cation spin interactions. Single crystals of the first charge-transfer salt complex containing a hexanuclear metal halide cluster, (TMTTF)2[Mo6C18C16], were prepared from TMTTF and [Et4N]2[Mo6C18C18] (172). The structure of (TMTTF)2[Mo6C18C16] consists of cation dimers with the anionic metal clusters in a distorted CsCl structure (Fig. 26). The CsCl arrangement is likely a consequence of the comparable volumes of the cation $mer and [MO~C~,C~,]~-. Interdimer distances (S...S distance of 4.87 A) are quite large, resulting in an insulating material. As expected no ESR signal could be obtained because the cation radicals are paired. The tetrakis(methy1thio)tetrathiafulvalenium (CH,S),TTF+ salt of [Mo6C18(NCs)6]2-was prepared in the same manner as (TMTTF)z[MosClsCls]and has the same CsC1-based structure with isolated dimers of (CH,S),TTF' (111). However, unlike

40


FIG.26. Unit cell of [TMTTF12[Mo6C18C161.

(TMTTF)2[Mo6C18C161,the cation dimers of [(CH3SI4TTFl2 [Mo6C1,(NCS),]interact with the axial ligands of the cluster. Sulfursulfur distances separating the (CH3S),TTFt dimer and [Mo&~,(NCS)~]~are 3.793 and 3.755 These cluster-cation interactions stabilize a slipped planar configuration in the cation dimer, which favors a mobile triplet exciton at room temperature. In effect, the triplet state cation dimers are coupled through the [Mo6C1,(NCS),l2-cluster . Charge-transfer salt complexes containing the ESR-active [NbfiCll2Cll2I3cluster with the organic cation radicals TTFt and TMTTF' were prepared by electrocrystallization from CH3CN/CH2CI solutions (130, 173). The TTF' salt, which was isolated as (TTF)2(Bu4N)[Nb6C112Cllz].CH3CN, contains isolated TTF' dimers. At higher temperatures the triplet state of the cation dimer is populated and interacts with the unpaired electron localized on the {NbfiCL12}3' core. In contrast, the TMTTFt salt as well as the selenium analog TMTSF were isolated as (TMTTF),[Nb6ClI2Cll2] .0.5CH2C1, containing a [(TMTTF),I3' cation and neutral TMTTF molecule. The TMTTF units in the [(TMTTF)J3' cation are stacked into infinite layers surrounded by [Nb6C112C11213anions (Fig. 27). The fifth TMTTF molecule is neutral and orthogonal to the cationic TMTTF species. Four partially oxidized TMTTF molecules are present. The HOMO of the organic stack is 5/8 filled (5 electrons in 4 orbitals), and metallic properties are expected. Conductivity measurements, however, indicate that the hybrid material is semiconducting, with band gaps of 0.16 and 0.20 eV for the TMTTF and TMTSF, respectively. Inequivalency in

A.


41

p

FIG. 27. Unit cell of [TMTTFl,[Nb,Cl,,Cl,]; two clusters a t corners of the unit cell are omitted for clarity. Reprinted with permission from Ref. (130).

the structures of the stacked cations is invoked to explain the electron localization and the resulting semiconducting properties. A single ESR signal is obtained for both (TMTTF),[Nb6Cl1,C1,,1 and (TMTSF)S[NbsC112C112], which indicates mixing of the localized spins of the cluster and cation.

D. EXTENDED SOLIDS The most notable example of a covalently linked extended solid generated from a soluble precursor is Prussian blue (174). A number of strategies for assembling discrete mononuclear complexes into ex-

42


tended solids have been adapted to the formation of an extended metal halide cluster network. Interactions between counter cations and the ligands of the metal halide clusters often result in solids with bonding in one, two, or three dimensions. Hydrogen bonding between ligands with donor or acceptor groups and water molecules in the crystal lattice also provides intercluster linkage. Alkali metal salts of [M6Y&12- and [MsY12&l'6-n'often contain cation-cluster interactions that fill the coordination sphere of the alkali metal cation. When ligands from neighboring clusters coordinate to a common cation, an extended network results. A recent example is in the ferrocenecarboxylate derivative, Na2[Mo6Cls(02CCSH4Fecp)6], which the ferrocenyl moieties are connected to the { M O ~ C ~core ~}~+ through the carboxylate groups (55).The multidentate properties of the carboxylate group enable the ligand to coordinate to the cluster and the sodium counterions. Two carboxylate ligands from one cluster are linked to a sodium counterion, and a third ligand from an adjacent cluster anion plus a methanol molecule fill the coordination sphere of the cation. These interactions create a one-dimensional array of [ M O ~ C ~ ~ ( O ~ C C S H ~anions F ~ C ~(Fig. ) ~ ]28). ~Crystals of the pseudohalide complexes AM6Y12& (A = K, Rb, Cs; Y = C1, Br; X = NB,NCS) produced from aqueous solutions incorporate water molecules in the unit cell (131-133). These water molecules as well as the inner halide and the pseudohalide ligands coordinate the alkali cations, resulting in an extended solid with a threedimensional array. Interestingly both the a- and P-nitrogens of the azide ligand bind the cations, whereas only the sulfur atoms of the thiocyanate groups coordinate to the alkali metals. In addition to cation bridges, water molecules connect the azide ligands of adjacent clusters via hydrogen bonding.

FIG.28. Display of the linear chain of {Mo&&}~+ units connected through bridging carboxylate ligands and sodium ions of Naz[Mo6Cls(OzCC6H4Fecp)61. The ferrocenyl moieties of the carboxylate groups are omitted for clarity.

43


Extensive hydrogen bonding has been observed in many structures containing {M6Y12}n+units with OH-, HzO, or CHBOHligands in the axial sites. The axial ligands act as both proton donors and acceptors. Hydrogen bonding interactions between the axial ligands and solvent molecules of th‘e crystal lattice create three-dimensional cluster systems. A few structures containing extended networks of clusters linked via hydrogen bonds are listed in Table V. It should be noted that the hydrogens were not located in the structures but inferred from short donor-acceptor distances. Two compounds, (Me,N) (175) and [Nb6Brlz(HzO)61[HgBr41 12Hz0(1761, pos[Ta6Cllz(Hz0)61Br4 sess hydrogen bonds between water donors and bromide acceptors. DC electrical measurements indicate that [Nb6Br12(Hz0)61[HgBr41. 12H20has semiconducting properties; however, the origin of this conductivity is not completely understood.

E. CHEMICALLY MODIFIED SURFACES The affinity of the cluster anions [ B U , N ] ~ “ ~ ~ C ~ & ] ~ =-C1, ( X Br, I) for gold and silver surfaces is determined by the identity of the axial ligand X (177). Only [Nb6CllzBr613adsorbs in a monolayer fashion on gold as determined by quartz crystal microgravimetry, X-ray photoelectron spectroscopy, and cyclic voltammetry. Redox potentials for “b6Cl12Br613-adsorbed to gold are anodically shifted 0.15 V relative to the cluster in solution. This shift is consistent with the transfer of electron density from the cluster to the metal surface. By contrast no TABLE V HYDROGEN-BONDED CLUSTER COMPLEXES Cluster

H Bonding

Ref. 175 176 176 179 179 180 181 181 181

Bromide ligands from both the cluster cation and the anion partake in hydrogen bonding. Cation and anion are involved in hydrogen bonding.

44


sign of niobium or chlorine is observed in the X P S spectrum of gold surfaces that were exposed to [Bu~N]3[Nb6C112C~613~. At the other extreme, solutions of [Bu,N]3[Nb6C11zC161 completely dissolve gold films. The increased reactivity of the clusters with the heavier halides parallels the interaction of gold with the free halide ions., where C1- adsorbs the weakest and I- the strongest (178). Of the three clusters only [Nb6C1,2C16]3-forms a monolayer on silver surfaces. Both [Nb6CllzBr6]3and [Nb&llz16]3- dissolve silver. These findings demonstrate that halide ligands should be versatile anchors for immobilizing many types of metal complexes to metal surfaces. Pyridine groups immobilized on gold surfaces through strong goldsulfur interactions displace the triflate ligands of [Ta6Cl12 (OS02CF3)fi14and result in a immobilized monolayer of cluster on the gold surface. XPS reveals that an average of four triflate ions are replaced by the surface-bound pyridines. Dilution of the pyridine surface with coadsorbed thiolphenol, a noncoordinating moiety, reduces the amount of cluster adsorbed to the surface and the average number of triflate ligands displaced by surface pyridine. The number of triflate ligands remaining on the surface-bound cluster may be adjusted with the thiolphenol/mercaptopyridine ratio. The different coordination environments about the {Ta&112)"t core in the resulting material have been identified with differential pulse voltammetry. As the number of coordinating pyridine ligands coordinated to the {TafiC112}'1+ core decreases, the oxidation potentials of the clusters are cathodically shifted. The shift reflects the n--acid character of the pyridine ligand.

ACKNOWLEDGEMENTS We appreciate the many contributions of former graduate students and senior collaborators, who have contributed to our research in the area of inorganic metal cluster chemistry: Laura Robinson, Dean Johnston, Joseph Hupp, and Donald Ellis. Our research in this area was supported by the National Science Foundation, inorganic chemistry program and the NSF supported Materials Research Center a t Northwestern University.

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ADVANCES IN INORGANIC CHEMISTRY, VOL.

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RECENT ADVANCES IN NOBLE-GAS CHEMISTRY JOHN H. HOLLOWAY and ERIC G. HOPE Department of Chemistty, The University of Leicester, Leicester LEI 7RH, United Kingdom

I. Introduction 11. Recent Review Literature 111. The Possibility of Argon Chemistry Krypton Chemistry V. Xenon Chemistry A. Core Chemistry and Recent Advances B. Reactions of the Xenon Fluorides VI . Radon Chemistry References

rv.

I. Introduction

Until "XePtF," was reported in 1962 (I),the only compounds containing noble-gas elements in combination with other elements were weakly bonded species of two main types: (i) gaseous cationic or excited-state species observed by spectroscopic means and (ii)species in which the noble-gas atoms are adsorbed, enclathrated, or encapsulated on or within the lattices of other molecules. The former includes mostly excited-state diatomic molecules containing like or unlike noble-gas atoms, or diatomics containing oxygen, nitrogen, or metal atoms in combination with a noble-gas atom, a wealth of van der Waals-bonded dimers and trimers, diatomic cationic species containing like or unlike noble gases, or noble-gas hydrides. These were briefly reviewed in 1968 (2). The latter consists mainly of clathrates in which the noble-gas atoms lie at the interstices of crystalline cages formed by water, organic molecules, or double hydrates involving both organic and water molecules [e.g., species such as R.2Ng.17H20 (R = CH,COOH, CH2C12,CHC1, or CCll, and Ng = Ar, Kr, or Xe)l (2-41, or where noble-gas encapsulation in zeolites occurs ( 4 ) . Subsequent to the discovery of the xenon-platinum hexafluoride 51 Copyright c) 1999 by Academic P r a s All rights of reproduction In a n y form reserved 0898-8838/99 525 00

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HOLLOWAY AND HOPE

complex, simple binary fluorides of xenon (XeF,, XeF,, and XeF,), krypton (KrF2),and radon (probably RnF2),have been prepared (2-41, and the general chemistry of these species has been well established and developed. Compounds isolable at ordinary temperatures have been obtained only for the heavier noble gases, and the only stable compounds that can be made by direct synthesis are the fluorides. Most of the compounds that have been obtained consist of the noble gas combined with the most electronegative elements, fluorine and oxygen. However, compounds in which the noble gases are bonded to nitrogen, carbon, or chlorine have been characterized, and there is a growing and interesting chemistry evolving around some of these species. The chemistry of xenon is undoubtedly the most extensive, xenon occurring in oxidation states +2 to +8, and stable species have been isolated for each of these. For krypton and radon, the compounds found so far are all in oxidation state +2 and, until quite recently, all of the known species involved only krypton-to-fluorine or radon-tofluorine bonds. The most sensitive detection methods have provided no definitive evidence for a stable argon compound. The inert status for helium and neon remains unchallenged. The chemistry of xenon stems from the reactions of the binary fluorides and falls into five main categories: 1. reactions in which the xenon fluoride acts as a fluorinating agent, 2. hydrolysis reactions yielding oxide fluorides, oxides, and oxygencontaining salts, 3. fluoridelanion metathesis reactions between the fluoride and an anhydrous acid, 4. reactions with fluoride ion acceptors, yielding fluorocations, and 5. reactions with fluoride ion donors, yielding fluoroanions. Krypton chemistry parallels that of xenon but is, in general, more limited. The bonds between krypton and other elements, even the highly electronegative fluorine and oxygen atoms, are weaker than those with xenon. The outcome is that although krypton-fluoride cationic species are known, Kr-0, Kr-N, and Kr-C bonds are difficult to obtain and, where they exist, are unstable. On the other hand, the weakness of the Kr-F bond, both in neutral KrF, and in the cations [KrFI' and [Kr,F,I+, provides a means of carrying out useful low-temperature fluorinations, which have been exploited in producing novel species that are often inaccessible using elemental fluorine. Radon apparently forms a difluoride and some cationic complexes. However, the evidence is based exclusively on radiochemical tracer

RECENT ADVANCES IN NOBLE-GAS CHEMISTRY

53

experiments because radon has no stable isotopes, and the work is made even more difficult by radon’s intense radioactivity, which not only makes it hazardous to manipulate but also leads to decomposition of the reagents involved. A wealth of reviews covering various aspects of noble-gas chemistry were written up to the end of 1979 and some of those written in English are listed in reference ( 5 ) .In addition to the books ( 2 ,3 ) and an extensive and authoritative book chapter ( 4 )already mentioned, three other books appeared in the early days of noble-gas chemistry, and these are listed in reference ( 6 ) .There is also a valuable and authoritative bibliography covering the literature in the period 1962 to 1977 (7). Because the main features of noble-gas chemistry are now well documented to the extent that they appear in all undergraduate inorganic textbooks, this review concerns itself only with the new chemistry that has evolved since 1979, and an effort has been made to pick out the main features of this work and set it within the context of the chemistry established up until that time.

II. Recent Review Literature

Since 1979, there have been a number of reviews of note. Among them is an interesting summary of the history of the discovery of noble-gas chemistry in which the reasons for the failure of Yost and Kaye t o obtain noble-gas compounds in the 1930s are examined. The article also speculates whether there were others who envisioned noble-gas reactivity and discusses work that took place before Bartlett’s key experiments (8).Other general summaries of noble-gas chemistry were published in celebration of the 25th anniversary of the discovery of chemical activity of xenon, a t the 100th anniversary of the discovery of fluorine (9) and very recently in a new “Encyclopaedia of Inorganic Chemistry” (10).A review covering aspects of the chemistry of xenon, and especially its reactions with pentafluoro-oxoselenate and pentafluoro-oxotellurate groups, was published a t the beginning of the review period ( l l ) ,and a detailed and comprehensive academic review of cationic and anionic complexes of the noble gases was published in 1984 (12).An important development in noble-gas chemistry over the last 15 years has been the synthesis of compounds containing stable bonds between noble gases and elements other than fluorine and oxygen. This was foreseen in an article in 1982, which traced

54

HOLLOWAY AND HOPE

then-recent developments and discussed Xe-N bonding as well as the fragmentary evidence for Xe-C and Xe-B bonds (13). An authoritative review on the chemistry of radon, which includes the formation of clathrate compounds, the simple and complex fluorides, ionic radon in solution, redox properties, and the various unsuccessful efforts to prepare an oxide and halides of radon other than the fluoride, was published by the most significant contributor to this field in 1983 (14). A review article on van der Waals complexes of metal-atom noblegas complexes was published in 1995 (15).

111. The Possibility of Argon Chemistry

The existence of krypton and xenon compounds inevitably raises the question as to whether noble-gas chemistry might be extended to argon. The increasing range of krypton compounds that have been synthesized over the last 10 years, and in particular the recent observation of Kr-0 and Kr-N species, will undoubtedly further stimulate interest in this area. Although some years ago ab initio calculations on [HeFI' and [NeF]+ indicated that the ground states of these molecules are unstable, those for [ArF]' implied a stable ground state and suggested that a likely complex to be synthesized would be [ArFI+[PtF,I- (16). Evidence from photoelectron spectroscopy did indeed produce evidence of [ArFI+ and provided a measure of its dissociation energy {D,([ArFI+)2 1.655 eV) but confirmed instability for the helium and neon analogs (17). More recently new ab initio calculations at the MP4C3DTQY6-311G(2df, 2pd)//MP2/6-31G(d,p) level predict that the best counterions for the stabilization of [ArF]+appear to be [AuFJ or [SbFJ (18). High-level calculations also predict that [HC=N-ArFl+ should be stable with a binding energy of 160 kJ.mol-' which is closely similar to that of the recently observed krypton analog [HC= N-KrFl+ (157 k J . mol-') (19).Interesting work, in which electronegativity has been redefined and adjustments to the well-established electronegativity scales have been made, has demonstrated close agreement between the four scales (Allred and Rochow, Sanderson, Mulliken, and Pauling) and that the values for argon and krypton are similar (20).All of this suggests that more effort to prepare an argon fluoride complex might be worthwhile.


55

IV. Krypton Chemistry

The chemistry of krypton is less extensive than that of xenon, and the well-established characteristics, summarized in detail in the book chapters in 1973 by Bartlett and Sladky ( 4 ) and in 1984 by Selig and Holloway (12) are dealt with in outline here, but the new developments since 1979 are referenced in detail. The only stable binary fluoride of krypton is the difluoride, KrF2, and all the known chemistry derives from this molecule. Early reports of the preparation of KrF4have not been substantiated, and no simple stable oxides or oxide fluorides have been isolated. However, small amounts of the violet free radical, KrF, have been observed following y-irradiation of KrFz (2-4). There has also now been direct observation of [KrO,]' ( n = 1,2) and [&OH]' by Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry, for which ab initio theoretical calculations suggest that the two species are covalently bonded with bond dissociation enthalpies of 237.7 and 169.5 k J . mol-' respectively (211. The volatile, colorless crystalline difluoride was prepared initially by subjecting mixtures of krypton and fluorine at low temperature and pressure to an electric discharge or by irradiation of liquid krypton/fluorine mixtures with UV light (22). The latter is a more rapid and convenient way of preparing KrFz than the earlier discharge method, and it has since been found that laser as well as UV photolysis is also effective (23). These photolytic methods are particularly useful for preparing small quantities (22,231. Because molecular fluorine is not significantly dissociated until 500-600°C, the preparation of thermally unstable compounds such as KrFz by a thermal method did not seem to be an option. However, in 1976, Soviet workers reported the preparation of KrFz using catalytic-thermal generation of atomic fluorine from molecular fluorine (24). The nonequilibrium conditions necessary were created by generating atomic fluorine on the surface of a nickel wire at 600-700°C and allowing the atomic fluorine to react with krypton in a low-temperature zone close to the reactor wall a t -196°C. The rate of production is high (-6 g/h) compared with other preparative methods. The reaction method and operating conditions of another high-yield, thermal catalytic method has also recently been discussed (25). Krypton difluoride has now been shown to exist in two crystallographic forms, a low-temperature ( - 196°C) phase, referred to as a-KrFz, and a high-temperature (-78°C) phase, /3-KrF2(23). Study of the vibrational spectra has shown that the X-ray structure reported

56

HOLLOWAY AND HOPE

in 1972 (261, which contains two linear KrFz molecules per tetragonal unit cell (space group P4,lmnrn) aligned in planes perpendicular to the tetrad axis, is that of P-KrF,. Raman data suggests that the a-KrFpcrystallizes in the same space group as XeF, (271, but a definitive conclusion must await an X-ray structural investigation of the krypton species at low temperature. The adducts of KrF, are analogous to those of XeF, and until quite recently were limited to cationic derivatives, [KrFl', [KrFl' .xKrF2, or [Kr2F3]+. KrFz, of Lewis acid pentafluorides (12,281 or the [KrpF31+ related but somewhat less powerfully Lewis acidic metal oxide tetrafluorides (12,28,29). More recent reports have included the synthesis of [KrF]'[MF,J (M = As, Sb) in anhydrous HF (30), the preparation of [K~F]'[AS~F~~]for the first time, and the observation of two solidstate forms of [KrFl+[SbpFJ (23), providing only the second example of polymorphism in krypton complex chemistry {cf. [KrFl+[AsF61(31I}. The cationic complex of krypton difluoride, [Kr,F31+.xKrFz. [BF41-,incorporating the [BF4]-anion, has been prepared for the first time from the reaction of KrFz with BF3at -40°C in HF. The presence of HF to help bring about the ionization was shown to be essential in this preparation because the complex is not produced a t temperatures as low as -78°C when KrFz and BF3 are brought directly together (30). The use of anhydrous HF has also provided a means of synthesizing two new metal fluoride adducts, 2KrFzsMnF4 and KrFz.MnF4, from the combination of KrFz and MnF, in HF solvent (32). The 2 : 1 adduct decomposes in dynamic vacuum at -45"C, yielding KrFz and the 1: 1 adduct, which itself is stable up to -25°C. Because neither species provided a Raman spectrum, their exact nature is uncertain, but it Seems likely that they are molecular adducts rather than complexes with cationidanionic character. In general, the component molecules of all of the cationidanionic complexes are linked together by fluorine bridges, which have a considerable degree of ionic character. However, Raman spectra of the complexes derived from the metal oxide tetrafluorides indicate that they are best formulated as essentially covalent structures (12). The spectra of the [KrpF31'cation correlate well with those associated with the analogous xenon species, but additional peaks, which have no equivalents in the xenon spectra, indicate that, unlike [XezF3It,the krypton cation is unsymmetrical (Fig. 1)with one short and strong bond, %,-Fa, a weaker and longer bond, Krb-F,, and two bridging bonds, Krh----Fhand Kr,----Fb,which are also of different lengths and strengths (31). Alternatively, the cation may be regarded as a dis-


57

FIG.1. The structure of IKr2F1]'

torted KrFz molecule fluorine bridged to a [KrFI' cation, [KrFz. KrF]' (33). As in the case of xenon, one of the most interesting developments in krypton chemistry has been the identification of bonds between noble-gas atoms and elements other than fluorine. The first example of such a species containing a Kr-0 bond, Kr(OTeF5In,has been prepared by the reaction of krypton difluoride with B(OTeF& a t -90 to -112°C in S02C1F solvent (341, the thermally unstable product decomposing to species analogous to those found in the decomposition of Xe(OTeF&. The high solubility of B(OTeF& relative t o KrFz in SOzCIF at the low temperatures required for the stabilization of Kr(OTeF& is thought to serve to maintain B(OTeF& in excess and to prevent the formation of FKr(OTeF5),which was not observed ( 3 4 ) . Perhaps even more remarkable is the extension of krypton chemistry to include Kr-N-bonded species in the cations [HCrN-KrF]' ( 3 5 )and [R,C=N-KrFl+ (Rf = CF:,l-.In HF solution at -23"C, however, the product is [Me2SFlt (196).A similar reaction has also been observed in the combination of (C&?s)aSwith [XeFl+[MFJ (M = As, Sb) in HF, which yields deep violet crystals of An X-ray crystal structure determination of the l(C,F,),SFl'[MFJ. antimony derivative has shown that the cations and anions are connected via interionic fluorine bridges to produce an infinite chain. An-


87

other feature is a short S-F bond distance [158.4(3) pm] (197). The conversion of CH3S-- to CH,FS- in methionine and methionylglycine derivatives has also been achieved using XeFa in solution in MeCN a t or below room temperature (198). Oxidation of MeSCN or S(CN), t o [Me(NC>SFl+and [(NC),SFl+ respectively using [XeFI+[MFJ (M = As, Sb) has also been reported (199). The oxidative fluorination of (CF,),C=NSNCO by XeF, gives rise to two isomeric sulfur(IV1 compounds, (CF3),CF -N =S(F)NCO and ( CF3I2CFN=S =NC(0)F. The third possible isomeric product of 1,3 addition, (CF,),C=N-S(F)= NC(O)F, was not observed (200). The oxidative fluorination of sulfur(IV) compounds such as diphenyl sulfoxide has been shown to occur under mild conditions with XeF, and catalytic amounts of chloride ion. In the Ph$O case the product is Ph,S(O)F, , which is formed in essentially quantitative yield within a few minutes. The chloride ion reacts with XeF, to produce fluoride ion, and a mechanism is proposed that involves fluorosulfur(1V) anions and fluorosulfur(V) radicals (2011. Reactions of MezSe with XeF2 in trichlorofluoromethane proceed smoothly to give MezSeFz(190). Oxidative coupling of R2Se2(R = Ph, biphenyl, alkyl) to unsymmetrical disubstituted acetylenes and cycloalkynes using xenon difluoride has been shown to yield the vicinal (E)-fluoroalkenyl selenides in high yield. Alternatively, the PhSeF equivalent can be formed from PhSeSiMe:i and XeF, (202). Although PhSeCl and PhSeBr are commercially available sources of selenic electrophiles, PhSeF, which is too labile to be isolated, can be generated in situ from PhSeSePh with XeF, (203). This has been used to generate the novel phosphaalkenes from phosphaacetylenes. Reaction of XeFz with Ph,Te gives PhzTeF2(189).In related studies PhTeF3 or PhTeTePh gave PhTeF5, PhzTeFz or PhsTeFBgave PhzTeF,, Ph3TeF or Ph3TeCl gave Ph3TeF3 (with small amounts of Ph3TeF,Cl in the case of Ph3TeC1),and Ph4Te gave Ph,TeF2 (204). The latter has subsequently been characterized (205). In a study on the synthesis of tetrakis(perfluoroalky1) tellurium species, Te(Rf)4,the compound Te(CF3), was obtained from the reaction of Te(CF3),ClZwith Cd(CF,), . Reaction of this compound with fluoride ion acceptors gives rise to the complex cation [Te(CF:3)31i,and with certain fluorides the complex anion [Te(CF3l4F1-is formed. However, with XeF, it appears to be oxidized to (CF3I4TeF2(206).The related compound R,TeF, (R = CH3)was prepared similarly by the reaction of TeR4 with XeF2 (207). As part of a study of the Lewis acid behavior of [Te(OTeF,),l toward [OTeF,l-, higher concentrations of the [FTe(OTeF,),]- anion in solution in

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HOLLOWAY AND HOPE

SOzCIF were generated by the interaction of XeF, with [N(CHs),l+ [Te(OTeF,),I- at -50°C according to Eq. (30) (51): 2[Te(OTeF5)J

+ XeFz

S0,ClF -50°C

2[FTe(OTeF5),I-+ Xe(OTeF&. (30)

Liquid bromine reacts with [XeF]+[MF6]-(M = As, Sb) at room temperature in a complex reaction sequence that gives BrS+[MF6]-,

and its crystal structure has been determined (208). The reaction between HI and [XeFIt[AsF6]-has been shown to give as the final products of reaction. No experiXe, HF, and [I,12'[AsFfi1~ mental evidence for [XeIl' was found in the reaction products, and theoretical calculations show that such a species is bound only in the 'C singlet state and, after interconversion into the unbound triplet state, would immediately dissociate into xenon ('S) and I'(3P) (209). In synthetic and structural studies of a number of anions the [IF,]-, [BrFJ, and [IF4]- salts were synthesized according to Eqs. (32) to (34) (210):

CsBr + XeF,

Cs+[BrF,]- + Xe

The chemistry of the interaction of XeFz with alkyl and aryl iodides has been further extended by investigations of the reactions with CF3CH21,3,5-ClzC6H31,and 2-CF3CfiH41. In each case the alkyl(ary1) iodine difluoride was obtained (189). In related reactions between MeX (X = C1, Br, or I) and XeFz, the reactions proceed smoothly, the rate of reaction reflecting the basicity of the substrate, and in the case of MeI, MeIFz is formed. With MeCl and MeBr, however, MeF is the product (190). Although difluoroiodo compounds have been used as


89

synthetic intermediates, none had been isolated until treatment of 4-iodotricyclene with excess of XeFz in CC1, produced the difluoro derivative [Eq. (35)]as a pale yellow, waxy solid. This is stable in air for a few hours and indefinitely in solution under a n inert atmosphere (2111:

I’

IF,

b. Reactions with Transition-Metal, Lanthanide, a n d Actinide Species The reaction of chromyl fluoride with XeFz a t temperatures up to 278°C is a high-yield route to CrOF3 that is superior to other routes (52). Reaction with CoFz, CoClZ, or metallic cobalt have all been shown to yield CoF,, and DTA data have provided information on the temperatures of initiation of the reactions established ( 4 6 ) .The mild fluorinating ability of XeFz in solution has been exploited in the synthesis of Ir(CO)C1z(PEt3)z(P’F4), whose I9F and NMR spectra clearly showed the presence of the Ir-P’F, group (212):

It was also found to fluorinate the five-coordinate carbonyl complex of Ir(I), [Ir(C0)3(PEt3)z]+, to yield the first fluoroacyl derivative, Ir(CO)2 F(COF)(PEt,), (213). Xenon difluoride also reacts with [Pt(NH3),1C12 in the solid state to give t r a n ~ - [ P t ( N H ~ ) ~ F ~but l C lin ~ , solution in MeCN this is converted to t r a n ~ - [ P t ( N H ~ ) ~ C(214 l ~ ] ).F ~ The analogous oxidation of [Pt(py)4]2+[BF41f gives the crystallographically characterized [PtFz(py)412+[BF41; (215). It was established in the 1970s that XeFz in solution was a valuable means of introducing fluorine oxidatively into transition carbonyl compounds to produce carbonyl fluorides (216, 21 7). Recently, there has been renewed activity in this area, supported by the improvements in multinuclear Fourier-Transform NMR techniques. Among

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HOLLOWAY AND HOPE

the more recent papers is a report of a reaction of XeF, with [Re2(CO),,] in perfluoro-l,3-dimethylcyclohexanethat concludes that the product is Re(C0)3F2(218)even though the properties of the compound produced are clearly those of Re(CO)SF.ReF5 (217). Considerable progress has been made with oxidative fluorination of ruthenium and osmium carbonyls and their derivatives. Thus, [Ru3(CO),,I with XeF, in anhydrous H F gave, in solution, cis-[RuFz(CO),l as the main product along with small amounts of [RuF(CO)J+, [ R U ~ F ~ ( C O ) ~ I , [{RuF(CO),}~(~-F)I',mer- and fuc-[RuF3(CO)J, [{RuF~(CO)~},I, and [{RuF,(CO)3}(p-F){Ru(C0)5}l +,all of which have been characterized by 13C,I9F and 13C-{19F}NMR spectroscopies. Removal of the HF solvent yielded tetrameric [{RuF,(CO)~}~] (219). Similarly, the equivalent reaction with [ O S ~ ( C O )produced, ~~] in solution, a range of mono- and dinuclear osmium carbonyl fluoride species, with c i s - [ 0 ~ ( C O ) ~ Fas~ l the major product and [Os(CO),F]+,[ O S ~ ( C O ) ~ F and ~ I ,[ O S ~ ( C O ) ~ Fas ~]+ minor products. The main product is stable in solution, but removal of the solvent results in CO loss to give the fluoride-bridged tetrameric analog to the ruthenium tricarbonyl difluoride, [{OS(CO)~F,}],(220). The mechanism of the oxidation of low-valent ruthenium and osmium complexes has also been elucidated by a study of the reaction of XeF, with [M(CO),(PPh3),] (M = Ru, 0s) in dichloromethane (221). Fluoroacyl complexes of ruthenium and osmium, [M(COF)(C0)2 F(PPh3)21(M = Ru, 0s) were identified as intermediates in the oxidative addition of XeF, in solution, and the mechanism, probed by addition of BF3, has been shown to involve oxidation of the metal center by [XeFI' to give a monofluorinated cation. Subsequent nucleophilic attack by F- at the coordinated CO with CO elimination gives, finally, [OC-6-131[M(CO),F2(PPh3)21. In the ruthenium case, the ligand arrangement has been confirmed by a single-crystal X-ray crystallographic study (2211. Controlled sequential reaction of [Ir4(C0>121 with XeF2 in anhydrous H F a t low temperature yielded, initially, fac- and mer-[Ir(CO)3FS], and, ultimately, IrF, . Removal of the solvent at the initial stage of the reaction results in the precipitation of f~c-[1r(CO)~F~l, which contains predominantly u-bonded carbonyl ligands with high CO-stretching frequencies (222). Xenon difluoride has been shown to react with organomercury compounds, RzHg (R = PhC=C, p-MeOC6H4,p-Me2NC6H4,p-Et02CC6H4, and PhCH2), with cleavage of the C-Hg bond t o give RHgF or RHgFI HgF, mixtures. When R = benzyl, the RHgF formed undergoes fast demercurization. From the composition of the reaction mixtures it has been established that the reactions involve free radicals. The absence


91

of fluorinated organomercury products is evidence that XeF, reacts more readily with the C-Hg bond than with C-H or C-C bonds. With HgX, (X = C1, Br, I) under mild conditions, HgF2, halogen gas, and xenon are produced (223). The xenon fluorides, XeF2, XeF,, and XeF6, have all been used in preparations of some lanthanum tetrafluorides, LnF4 (Ln = Ce, Pr, or Tb) and their heptafluoro complexes, Cs3LnF7(Ln = Ce, Pr, Tb, Dy, or Tm) (224). Detailed examination of the reaction of CssLnCl6 with XeF, a t 100-400°C has shown that CsyLnF7(Ln = Ce, Pr, Tb, Nd, or Dy) species are formed, but for E r and Yb under the same conditions only the lanthanum(II1) species CssLnFs (Ln = E r or Yb) are produced (224). In the actinide field, stopped-flow spectroscopy has been used to study the oxidation of plutonium(II1) to plutonium(1V) in 1M HClO, by XeFz. Two moles of Pu(II1)are oxidized per mole of XeF, consumed, and there is no evidence for oxidation to PuW) or Pu(V1). Rate constants and activation parameters have been obtained and a mechanism postulated that invokes a sequence of 2 one-electron oxidation steps (225). The oxidation of americium by a range of xenoncontaining oxidants in KHCO,, KHCO, and K2C0,, and in K&O, solutions, has been studied (226,227).Xenon difluoride and xenon trioxide convert AmlIII) to Am(IV) in solution (2261, whereas NaJXeOJ oxidizes Am(II1) to Am(IV), A m W , and Am(V1) mixtures unIess the perxenate is in excess, when Am(V1) is produced (226,227). The trioxide, Xe03, reacts with Am(II1) only under UV irradiation (227).

VI. Radon Chemistry

The chemistry of radon (11) was outlined by Stein in 1983 (14). Since then, evidence for radon in a higher oxidation state (RnF, or RnF6) and a radon oxide (RnO,) has been claimed and disputed, and the ions, [HRnOJ', [HRnOJ, and [RnOSFl have been reported. Soon after the initial discovery of xenon compounds in 1962, a lowvolatility fluoride was detected in the product of the combination of radon with fluorine a t 400°C (226). It was also shown that 'L22Rnis oxidized by chlorine and bromine fluorides, IF7 (227-230) or [NiFsl" (227-229) in H F to give stable solutions of radon fluoride, and that radon can be collected as a solid by oxidation of the gas with liquid BrF3 or the solid complexes [C1F2]+[SbFsl-,[BrFJ'[SbFJ, [BrF,]' [Sb2F113-,[IF41:'+3[SbF61-, and [BrF2I'[BiFJ (231-232). Electromigration studies suggested that in the solution studies the radon is pres-

92

HOLLOWAY AND HOPE

ent as [RnF]' or [RnI2+(228, 229). Such ionic formulations were also favored in relativistic quantum mechanical calculations (233). This has been borne out further by more recent chemistry in which solutions of cationic radon have been prepared by oxidation of elemental radon with halogen fluorides and shown t o displace hydrogen, sodium, and potassium in solid materials such as Nafion resins (H+ and K' forms), potassium hexafluorophosphate, and sodium hexafluoroantimonate. It has also been shown that radon in this state can be quantitatively recovered by ion exchange and eluted with bromine trifluoride in sulfuryl chloride (234). By comparison of the properties of the products from all of these reactions with the known properties of the fluoride and fluoride complexes of krypton and xenon, it is almost certain that radon forms a difluoride, RnFz, and derivatives such as [RnFI+[SbFJ, [RnF]+[TaF6]-,and [RnF]+[BiF6]-,which are direct analogs of krypton and xenon species (235). This has been further strengthened by the observation of the reaction of radon with solid [02]+[SbF61-a t room temperature, which yields gaseous oxygen and an involatile radon compound (236). Comparison with xenon chemistry suggests the reaction

This was further substantiated by reactions of radon with fluoronitrogen salts and halogen-fluoride metal salts, and the development of a method of collecting radon from air using [o21+[SbF6l-or [IFJ+[SbFJ (237). Hydrolysis of radon fluoride with water yields radon, oxygen, and HF. This parallels the reactions of KrFz and XeFz and is in marked contrast to the reactions of XeF4 and XeF6, in which some xenon is retained in solution as Xe(V1). It also coprecipitates with XeFz from halogen fluoride solutions and complexes with XeFzbut not XeF4(238, 239) all of which points to the fluoride being RnFz. Claims by Russian workers that a higher fluoride of radon, RnF4 or RnF6, can be prepared in tracer experiments by heating radon, xenon, fluorine, bromine pentafluoride, and either sodium fluoride or nickel fluoride, and converted to RnOs by hydrolysis (240) appeared to others (235) to be due to the precipitation of radon as a solid complex, which is probably [RnF]2+[NiF612-. However, the precipitation of CsXeOBF from aqueous solutions results in the coprecipitation of radon, and this has been taken by the Russian group as confirmation that RnOB is the product of hydrolysis of the fluoride formed (241). Furthermore,


93

it has been suggested that the failure of Stein to observe the highoxidation-state [RnO3F1-species was probably due t o high F- concentration (242).More recently, ultracentrifugation of hydrolyzed radon solutions, coprecipitation studies, and kinetic data for the decomposition of the solution species have been interpreted in terms of it being RnO, (243). Recent electromigration studies have also been used to suggest that in acidic aqueous solutions (pH > 5) cationic, [HRnOJ’, and anionic, [HRnOJ, forms of radon are produced, the validity of the electromigration method having been established using xenon(V1) (244).

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192. Naumann, D.; Nowicki, G.; Sassen, K.-J. 2. An,org. Allg. Chem. 1997, 623, 1183. 193. Lewe, T.; Naumann D.; Nowicki, G.; Schneider, H.; Tyrra, W. 2. Anorg. Allg. Ch,em. 1997, 623, 122. 194. Minkwitz, R.; Nowicki, G. Angew Chem., Znt. Ed. Engl. 1990,29, 688. 195. Patrick, T. B.; Nadji, S. J . Fluorine Chem. 1988, 39, 415. 196. Forster, A. M.; Downs, A. J. J. Chem. Sac., Dalton Trans. 1984, 2827. 197. Minkwitz, R.; Nowicki, G.; Preut, H. 2. Anorg. Allg. Chem. 1992, 611, 23. 198. Janzen, A. F.; Wang, P. M. C . ; Lenire, A. E. J . Fluorine Chem. 1983,22, 557. 199. Minkwitz, R.; Nowicki, J.; Jahnkow, B.; Koch, M. 2. Anorg. Allg. Chem. 1991, 596, 77. 200. Steinbeisser, H.; Mews, R. J. Fluorine Chem. 1981, 17, 505. 201. Janzen, A. F.; Ou, X. J . Fluorine Chem. 1995, 71, 205. 202. Poleschner, H.; Heydenreich, M.; Spindler K.; Haufe, G. Synthesis 1994, 1043. 203. Laahi, K. K.; Fiedler, W.; Regitz, M. J. Chem. Soc., Chem. Commun. 1997, 1641. 204. Alam, K.; Janzen, A. F. J . Fluorine Chem. 1985,27, 467. 205. Minoura, M.; Sagami, T.; Akiba, K.-Y.; Modrakowski, C.; Sudau, A,; Seppelt, K.; Wallenhauer, S. Angew. Chem., Int. Ed. Engl. 1996,35, 2660. 206. Naumann, D.; Butler, H.; Fischer, J.; Hanke, J.; Mogias, J.; Wilkes, B. 2. Anorg. Allg. Chem. 1992, 608, 69. 207. Ahmed, L.; Morrison, J . A. J. Am. Chem. SOC.1990, 112, 7411. 208. Hartl, H.; Nowicki, J.; Minkwitz, R. Angew Chern., Int. Ed. Engl. 1991, 30, 328. 209. Klapotke, T. M. Heteroatom Chem. 1997,8, 473. 210. Zhang, X.; Seppelt, K. 2. Anorg. Allg. Chern. 1997, 623, 491. 211. Bradley, G. W.; Holloway, J . H.; Koh, H. J.; Morris, D. G.; Watson, P. J . J. Chem. SOC., Perkin Trans. 1 1992,3001. 212. Ebsworth, E. A. V.; Holloway, J . H.; Pilkington, N. J.; Rankin, D. W. H. Angew. Chem., Int. Ed. Engl. 1984,23, 630. 213. Ebsworth, E. A. V.; Robertson, N.; Yellowlees, L. J. J . Chem. SOC., Dalton Trans. 1993, 1031. 214. Zheligovskaya, N. N.; Kiselev, Yu. M.; Krasovskaya, E. P. Koord. Khim. 1980, 6, 1080. 215. Drews, H. H.; Preetz, W. 2. Anorg. Allg. Chem. 1997, 623, 509. 216. (a) Bruce, D. M.; Holloway, J . H.; Russell, D. R. J . Chern. SOC., Chem. Cornmun. 1973, 321; (b) Hewitt, A. J.; Holloway, J . H.; Peacock, R. D.; Raynor, J . B.; Wilson, I. L. J. Chem. Soc., Dalton Trans. 1976, 579; (c) Bruce, D. M.; Holloway, J . H.; Russell, D. R. J. Chem. Soc., Dalton Trans. 1978, 1627. 217. Bruce, D. M.; Hewitt, A. J.; Holloway, J. H., Peacock, R. D.; Wilson, I. L. J . Chem. Sac., Dalton Trans. 1976, 2230. 218. Misra, S. N. Indian J . Chem. 1979, 178, 101. 219. Coleman, K. S.; Holloway, J . H.; Hope, E. G. J. Chern. SOC., Dalton Trans. 1997, 1713. 220. Brewer, S. A,; Holloway, J . H.; Hope, E. G. J . Chem. Soc., Dalton Trans. 1994, 1067. 221. Brewer, S. A,; Coleman, K. S.; Fawcett, J.; Holloway, J. H.; Hope, E. G.; Russell, D. R.; Watson, P. G. J. Chem. Soc., Dalton Trans. 1995, 1073. 222. Brewer, S. A.; Brisdon, A. K.; Holloway, J. H.; Hope, E. G.; Peck, L. A.; Watson, P. G. J. Chem. Soc., Dalton Trans. 1995, 2943. 223. Butin, K. P.; Kiselev, Yu. M.; Magdesieva, T. V.; Reutov, 0. A,, J . Organomet. Chem. 1982,235, 127.

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224. Kiselev, Yu. M.; Goryachenkov, S. A.; Martynenko, L. I. Russ. J . Znorg. Chem. (Engl. Transl.) 1984,29, 38. 225. Cook, R. L.; Woods, M.; Sullivan, J. C.; Appelman, E. H. Znorg. Chem. 1989, 28, 3349. 226. Fields, P. R.; Stein, L.; Zirin, M. H. J. Am. Chem. SOC.1962,84, 4164. 227. Stein, L. J. Am. Chem. Soc. 1969,91, 5396. 228. Stein, L. Yale Scientific Magazine 1970, 44, (8), 2. 229. Stein, L. Science 1970, 168, 362. 230. Stein, L. U.S.P. 3 660 300, 197. 231. Stein, L. Science 1972, 175, 1463. 232. Stein, L. J. Znorg. Nucl. Chem. 1973,35, 39. 233. Pitzer, K. S. J . Chem. Soc., Chem. Commun. 1975, 760. 234. Stein, L. J. Chem. Soc., Chem. Commun. 1985, 1631. 235. Stein, L. Znorg. Chem. 1984,23, 3670. 236. Stein, L. Nature (London) 1973,243, 30. 237. Stein, L.; Hohorst, F. A. Enuiron. Sci. Technol. 1982, 16, (7), 419. 238. Nefedov, V. D.; Torpova, M. A.; Avrorin, V. V.; Dudkin, B. N. Radiokhimiya 1971, 13, 916. 239. Avrorin, V. V.; Nefedov, V. D.; Toropova, M. A. Radiokhimiya 1974, 16, (21, 261. 240. Avrorin, V. V.; Krasikova, R. N.; Nefedov, V. D., Toropova, M. A. Radiokhimiya 1981, 23, (6), 879. 241. Avrorin, V. V.; Krasikova, R. N.; Nefedov, V. D.; Toropova, M. A. Radiokhimiya 1986,27, (4), 511. 242. Avrorin, V. V.; Krasikova, R. N.; Nefedov, V. D.; Toropova, M. A. Radiokhimiya 1987,29, (31, 426. 243. Avrorin, V. V.; Krasikova, R. N.; Nefedov, V. D.; Toropova, M. A. Radiokhimiya 1989,31, (21, 124. 244. Avrorin, V. V.; Krasikova, R. N.; Nefedov, V. D.; Toropova, M. A. Radiokhimiya 1989,31, (61, 63.

ADVANCES IN INORGANIC CHEMISTRY, VOL. 46

COMING TO GRIPS WITH REACTIVE INTERMEDIATES ANTHONY J. DOWNS and TIMOTHY M. GREENE Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, United Kingdom

It i s a capital mistake to theorise before one has data. Inseizsibly one begins to twist facts to suit theories, instead of theories to suit facts. A. Conan Doyle, “A Scandal in Bohemia”

I. Introduction II. Reaction Intermediates: Nerve Centers of Chemical Reactions 111. Experimental Characterization of Reaction Intermediates: Retardation A. Gas-Phase Studies a t Low Pressure B. Gas-Phase Studies in Supersonic Jets C. Trapping and Matrix Isolation D. Solution Studies a t Low Temperatures IV. Experimental Characterization of Reaction Intermediates: Time-Resolved Methods V. Experimental Characterization of Reaction Intermediates: Flow and Other Methods VI. Conclusions References

I . Introduction

Chemistry is the science of change. It is ironic then that, although we have become so knowledgeable about the properties of reactants and products-about the beginnings and ends of chemical journeys-we remain more often than not unsure of just how those journeys are made. Our modern armory of sophisticated spectroscopic and diffraction methods provides every expectation of a direct hit on the ‘‘fixed” objects represented by reactants and products, but is seldom 101 Copyright ID1999 by Academic Presa All rights of reproductiun in any form reserved

O R ~ X - H ~ ~$25 R Moo ~

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equipped to target the shadowy, moving entity presented by the process of their interconversion. How the reaction rate varies with concentration, temperature, and other conditions may give vital clues but rarely establishes with any certainty the mechanism of what may be, at least superficially, quite a simple homogeneous reaction. Elaborating the mechanism of a reaction is an altogether more complicated undertaking than determining the structures of long-lived reagents or products in that we are seeking ultimately a detailed description of the way in which the structure and bonding of the reagents change with time in each of the several individual acts that normally make up a chemical change. In general terms, a complete account of any such mechanism is going to require knowledge of no less than four aspects (1-3): 1. subdivision of the reaction into its individual steps or equilibria; 2. characterization of each intermediate species in terms of its composition, structure, energy states, and lifetime; 3. a description of the transition state appropriate t o each elementary reaction step, again with reference to composition, stereochemistry, and energetics; and 4. a complete specification of the processes leading to and from each transition state in relation to the geometries and energy levels (mainly electronic and vibrational) of the reactants, intermediates, and products in their ground and excited states. With reactions occurring as they usually do in solution, the role of the solvent at each stage needs to be realistically assessed. The history of kinetic studies is littered with the wrecks of mechanistic hypotheses that ultimately foundered on the unsuspected reefs of solvent mediation. It can come as no surprise, then, that a comprehensive mechanistic description is still well beyond the reach of present techniques, both practical and theoretical, except in a few cases, and those mostly confined to the simplest of systems. More often than not we are obliged to piece together a mechanism that is consistent with all the available facts, both kinetic and nonkinetic; very rarely do we have compelling and unambiguous evidence of one particular mechanism. Nothing could be much simpler, it might be thought, than the exchange that the neutral metal hexacarbonyl CI-(CO)~undergoes with free CO in both the solution and gas phases, particularly as it proceeds in accordance with a rate law of the form

COMING TO GRIPS WITH REACTIVE INTERMEDIATES

103

The solvent having little effect on the activation parameters, it is reasonable to assume that a dissociative mechanism is a t work, with rate-determining fission of a Cr-CO bond being followed by rapid attack a t the resulting pentacoordinated intermediate Cr(CO), ( 4 - 6 ) . Early kinetic studies on substitution of CO by another two-electron ligand L, Cr(CO16+ L

-

Cr(C0)SL + CO

(1)

e.g., L = amine, phosphine, or phosphite, appeared also ( 4 - 6 ) to point to a simple dissociative mechanism, again with Cr(CO), as a common intermediate. However, investigations extending over a much wider concentration range have now shown that a two-term rate law is commonly applicable, uzz.

The activation parameters AH$ and AS$ characterizing the kl term are very similar to those associated with CO exchange, whereas AH2$ for the k 2 term is significantly smaller than AHl$, and AS2$, being negative, differs not only in magnitude but also in sign from A S l $ The most plausible interpretation of these and related experiments is that substitution occurs via two competing pathways, one involving a dissociative mechanism closely akin to that envisaged for CO exchange (see Fig. 11, and the other a “dissociative interchange” (Id)

Reaction coordinate

-

FIG.1. Hypothetical free energy profile for the substitution reaction Cr(CO)6+ L + Cr(CO)SL + CO on the assumption that it proceeds via a dissociative mechanism.

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DOWNS AND GREENE

process in which there is relatively little bond-making in the transition state. How then are we to picture this Id process? Perhaps L forms encounter complexes in which it occupies an outer-sphere site of a solvated Cr(CO)5fragment. There are other possible explanations, though, and we are now on the "tossing sea" of conjecture, however well informed, rather than the firm ground of incontestable result or inference. Still more problematic is the apparent migratory insertion of nitric oxide into transition metal-carbon bonds, an important reaction in metal nitrosyl complexes and one that may be relevant to biochemical reactions (7). On the evidence of isotopic labeling and kinetic experiments, the insertion of NO into the Co-CH3 bond of the (cyclopentadieny1)cobalt complex ( v5-C5H5)Co(NO)(CH3), which occurs in Reaction ( 3 ) ,

is intramolecular, and the experimental results are consistent with, but do not prove, a mechanism involving methyl migration, followed by phosphine addition (see Scheme 1, lower path) (8).This has the attraction of resembling the mechanism generally agreed to hold for CO insertion in a reaction such as MII(CO)~CH~ + CO + Mn(CO)5[C(0)CH31

(4)

and which is a critical step in many important carbon-carbon bondforming processes mediated by homogeneous transition-metal cata-

04"

R

L-PR,

SCHEME 1. Alternative pathways for the migratory insertion reaction of (q5-C5H6)Co(NO)(CH3)in the presence of a phosphine (8).


105

lysts. Still, the worm of doubt may turn. For is not NO a more versatile ligand than CO, and can it not, for example, function as either a 3e- ligand or a le- ligand? Thus, addition of a phosphine molecule accommodated by a change in the ligation of the NO to give a bent Co-N-0 unit offers another reaction pathway, as shown by the upper route in Scheme 1. There is persuasive evidence that such a change affords a low-energy channel in other reactions (81,and it is not obvious why it should not do so to effect NO insertion. In the absence of any definitive experimental information on this score, quantum chemical calculations have been carried out to explore each of the two possible pathways ( 8 ) . The geometries and energies of the reactants, intermediates, transition states, and products have been determined on the basis of Density Functional Theory (DFT), with the results illustrated in Fig. 2. Hence it emerges that the mechanism in which NO insertion precedes phosphine association incurs activation barriers not exceeding 84 kJ.mol-' and is therefore favored over the mechanism involving insertion after phosphine addition, which is opposed by appreciably higher barriers (80-218 kJ.mol-'1. This pleasing endorsement by theory of the mechanism deduced by experiment may seem to settle the issue, but the theory has not been able to budget for the effects of solvation. Still, we cannot be entirely sure of our ground, and about this particular reaction the last word has surely not been said. The preceding examples of CO exchange and NO insertion reactions have to do with systems that have attracted considerable experimental and theoretical attention. Accordingly, although there are fundamental questions still to be answered, some mechanistic features at least are not in doubt. For most of the myriad chemical reactions that

FIG. 2. The calculated profiles of the potential energy surfaces (at the DFT-B3LYP level) for the insertion reaction ( q5-C5H5)Co(NO)(CH3) + phosphine ($-C5H,)Co(phosphine)[N(0)CHplon the assumption (a) of an intramolecular rate-determining mechanism, or (b) of an associative mechanism (8). --.f

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are known to occur, though, we have neither the facts nor the theory to argue the mechanistic case, and analogy and intuition may well be the sole guiding principles. Hence we may be spared the great tragedy of science-that is, the “slaying of a beautiful hypothesis by an ugly fact”-but, without a secure understanding of mechanism, we are hostages to experience, empiricism, and prejudice when it comes to devising the most efficient, safe, and reliable way of engineering a desired chemical change.

II. Reaction Intermediates: Nerve Centers of Chemical Reactions

Crucial to any problem of mechanism must be the part played by any intermediate compound, for example (i) CI-(CO)~ in the exchange reactions of Cr(CO)5L (L = CO or other ligands) and (ii) either in Reaction (3). (C5H5)Co[N(0)CH3] or (C5H5)Co(phosphine)(NO)(CH3) Indeed, were it possible to detect, identify, and characterize the relevant intermediates, the primary issue of which path is taken in Reaction (3) (see Scheme 1) could be settled beyond peradventure. More generally, the intermediates may be open-shell molecules; they may be radicals; they may involve elements in unusual oxidation states or with unusual coordination geometries or partners. However, until we know what these intermediates are and the part they play, we cannot begin to comprehend the mechanism of any given reaction. As a start, we need to be sure of not only the identities but also the molecular and electronic structures of the intermediates. Such properties cannot necessarily be deduced from the properties of stable molecules and may well challenge conventional principles of bonding. This is all very well, but how are we to cope with intermediates that have but a fleeting existence under normal conditions? At a vapor pressure of 1 torr and in the presence of CO, for example, Cr(COI5is found to have a lifetime in the order of one-millionth of a second at room temperature (9). To counter the all too brief stay normally enjoyed by a molecule such as Cr(CO)5,the experimenter has three main options. One way is to slow down or inhibit the reactions disposing of the molecule. Another is to record a spectroscopic autograph, in absorption or emission, within the lifetime of the transient and so monitor its temporal fate (as in “flash photolysis”). The third is to resort to a flow method, depending on continuous generation of the intermediate in a fluid traveling at a uniform rate along a tube; under the right conditions, decay of the intermediate gives rise to a steady-state concentration


107

that varies as a function of the distance traveled along the tube, and that may be analyzed by essentially “static” spectroscopic measurements. Each method has its strengths and weaknesses. None is able individually to deliver all the information we are likely to require for a proper appreciation of how the intermediate fits into the overall mechanism of a given reaction. If we are to realize that ambition, we must be looking therefore to exploit not one but several methods. To gain a better idea of the three strategies for characterizing short-lived intermediates-namely, retardation, time-resolved, or flow methods-we turn next to a brief survey of how they work in practice and of just what light they can or cannot shed on a particular intermediate. How mechanistic studies have been advanced in this way will be illustrated in the course of the account.

HI. Experimental Characterization of Reaction Intermediates: Retardation

There are various ways of slowing down or even suppressing totally the reactions of a molecule. In principle, we may limit its collisions with other molecules by confining it to the gas phase and there minimizing its pressure; we may minimize the thermal energy to which it has access by lowering its temperature; or we may trap it with a suitable substrate. The best policy in practice may well be either simultaneously or in separate experiments to apply more than one of these restraints.

A. GAS-PHASE STUDIES AT Low PRESSURE The gas phase at low pressures is the best medium for detailed structural and spectroscopic studies of very simple, robustly bound molecules such as AIH, SiO, or C10 (10). Generated typically by thermal or discharge reactions, these may survive a t partial pressures high enough and for times long enough to be interrogated by their electronic emission, laser-induced fluorescence, microwave, or infrared absorption spectra, such studies being greatly facilitated by the development of tunable lasers and double resonance techniques. Hence an immense effort has been invested over the years in measuring and analyzing the electronic spectra of stable and unstable diatomic molecules, to give a rich return on information about the bond lengths and vibrational and electronic properties that characterize not only their electronic ground states but also their excited states.

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A good example is the molecule SiO, a cosmic precursor to silicon oxide chemistry as we now know it on earth (11).In more recent times, too, this molecule has assumed considerable significance because of its relevance to oxidation reactions taking place at the surfaces of silicon wafers and to the creation of antireflection coatings on these and other solid-state devices. Unlike its more familiar counterpart carbon monoxide, SiO is normally quick to aggregate and disproportionate [Reaction (5)] at temperatures below 1000°C: 2SiO

Si + SiO,.

(5)

Stability is not a problem at high temperatures, though, and Reaction (5) can be reversed to produce SiO vapor at a concentration sufficient for microwave absorption measurements by heating a homogeneous mixture of silicon and silicon dioxide to 1350°C in a ceramic reaction vessel, which also serves as the spectroscopic sample chamber (12). Alternatively, excited SiO molecules, produced by the action of a discharge on a gaseous mixture including, say, Sic&and O,, can be characterized by their electronic emission spectrum. It has taken 10 to 20 years to unravel all the features of the electronic spectrum extending from the near- to the vacuum-ultraviolet and assign the different band systems, but we now have a relatively full knowledge of the molecule in its electronic ground and numerous excited states ( 1 3 ) . Rather easier to decipher is the microwave spectrum of the molecule in its electronic ground state, whence a wide range of vibrational and rotational states have been accurately detailed (111. These and other studies give every confidence that we are dealing with a z*Si160 molecule having the following equilibrium parameters: re = 150.9739 pm, we = 1241.56 cm-', and pe = 3.0882 D (13). Some of the microwave lines have served as distinctive signatures by which SiO has been recognized in astronomical sources, and SiO masers have relayed important information about both the conditions in circumstellar shells and the mechanisms of mass loss. Solid boron nitride is a ceramic material of some consequence that can be formed at high temperatures by the reaction of boron atoms with Nz or NH3. A likely intermediary in its formation is another high-temperature molecule BN, first observed through its electronic emission spectrum nearly 60 years ago but the properties of which have emerged only slowly (14). Identification of the several low-lying electronic states of the molecule is a major problem and only very recently has it become clear that the ground state is not (as with the isoelectronic Cz molecule) but 311 (15).The A311,-X311itransition,

lz+

109


the equivalent of the celebrated Swann bands of C,, can be excited in emission by the action of a microwave discharge on traces of BCls and N2 entrained in a helium gas stream. Analysis of the well-resolved rotational lines of the 0-0 and 1-0 bands yields the following properties for the ground state of llB1*N:re = 132.9 pm and o,= 1514.6 cm-’ (13, 16). The molecule C10 is another reactive species that has attracted still more attention. It plays a significant role in the destruction of stratospheric ozone through catalytic cycles involving heterogeneous and homogeneous reactions, as in Eq. (6) ( 1 7 ) :

c1 f 0:3+ c10 + 0 2

I

(6)

C10+0-+C1+0~.

Current wisdom can be summarized in the flow chart of Fig. 3. More specifically, the C10 radical is involved in several possible kinetic mechanisms linking global release of chlorofluorocarbons (CFCs) to the Antarctic ozone loss during each austral spring ( 18).An accurate characterization of the spectroscopic and other properties of C10 is therefore vital, not least as a prelude to tracing the crucial correlation between CFCs, C10, and 0,. C10 is short-lived a t the high molecular concentrations characteristic of the condensed phases but can be generated a t low pressures in the gas phase, for example by the action of

nv

I

I

I

LIU

I

FIG. 3. Chemical cycles affecting the formation and decay of chlorine oxide trace species in the earth’s atmosphere (reproduced with permission from Wayne, R. P. “Chemistry of Atmospheres,” 2nd ed., p. 137, Clarendon Press: Oxford, 1991).

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DOWNS AND GREENE

a microwave discharge or a short burst of radiation on a mixture of Clz and 02.On the evidence of a detailed analysis of the A211i-X211i band system as measured both in absorption and in emission at high resolution, of the infrared spectrum also measured at high resolution, and of the microwave spectrum, the 35Cl'60molecule has a 211 ground state with re = 156.960 pm, we = 853.724 cm-l, pe = 1.24 D, and D;= 265.4 k J * mol-l(I3, 19). Of its reactivity, though, the properties give the barest hint. Simple polyatomic molecules such as BH2 (201, BH, (211, and HBNH (22) may be no less amenable to high-resolution spectroscopic studies. Commonly invoked as an intermediate in reactions of boron hydrides, the BH3 molecule is made elusive by its rapid dimerization to diborane. It is produced, nevertheless, by the action of a dc discharge on a gas stream composed of diborane and helium flowing through a multipass absorption cell; here it can be recognised by its vz (a;) and v3 ( e ' ) vibrational bands as recorded with a high-resolution FT-IR spectrometer (21). Detailed analysis of the rotational structure associated with the bands fixes a regular planar DBhstructure for BH, in its ground electronic state with r&B-H) = 119.001 pm. Another discharge reaction, this time involving diborane and ammonia, generates the species HBNH with a lifetime of a few hundred ms under normal conditions. That this is a linear molecule is made clear by the rotational structure of the v3 band (corresponding to the BN stretching fundamental) exposed by an IR diode laser spectrometer (22);the observed rotational constants imply a BN bond length (123.81 pm) appreciably shorter than those in H3BNH3,HzBNHz,and even BN itself (165.76, 140.3, and 132.9 pm, respectively). In principle, electron diffraction is another tool capable of giving precise information about the structure of a gaseous intermediate, even when this is quite a complicated molecule (23). In practice, though, there are numerous complications liable to militate against electron diffraction as the sole agency of characterization. Chief among these is the low resolution of the electron-scattering pattern of a molecule. As a result it is seldom easy to discover much about the structure-sometimes even the identity-of a specific molecule in a gaseous mixture. To resolve an otherwise severely underdetermined problem, we need independent testimony about the molecules making up the mixture, for example by reference to the mass, rotational, or vibrational spectrum of the sample or to the results of quantum chemical calculations (24). A good example of this strategy is provided by a careful study of the thermal decomposition of trimethylamine vapor (25).Mass spectrometric analysis of the vapor stream shows that de-


111

composition proceeds at 515°C primarily in accordance with Eq. (7) to give methane and the short-lived compound CH3-N=CH2:

Analysis of the electron-diffraction pattern of the reaction mixture implies the following mole fractions: (CH&N, 0.63; CHI, 0.18; and CH3-N=CH2, 0.19. The structure of the CH3-N=CH2 molecule has then been determined by a joint analysis of the electron-diffraction pattern and the rotational constants (derived from the microwave spectrum), giving parameters [rZ(N=C)= 127.9 pm, rZ(N-C) = 145.8 pm, and LC-N=C = 116.6"l very close to those calcylated by ab znitio methods. Similarly, pyrolysis of propylenimine, HNCH2CHCH3, a t 470°C results in complete decomposition to give the short-lived species syn- and antz-CH3-NH-CH=CH2 and truns-CH,-N=CHCH, (26);geometrical structures and mole fractions of all three products have been deduced by analyzing the electron diffraction pattern of the vapor in concert with the properties implied by quantum chemical calculations. How the proportions of the transients vary with temperature sheds significant light on the reaction path taken by the thermal rearrangement. Life is altogether simpler with only one molecular species in the vapor, and electron diffraction is well established as a primary method of characterizing high-temperature molecules such as MgC12 (27) or C,, (28). In general, however, the optimum conditions can be met only for a relatively long-lived intermediate such as S2F4.This is formed by the dimerization of SF, on the winding reaction path that starts with SClz and a metal fluoride and affords the best known route to SF4, represented mainly by the overall reaction, 3SC12 + 4MF + SF4 + S&1, (e.g., M

=

+ 4MC1.

(8)

Na or K)

The labile molecule has required the combined forces of electron diffraction, microwave, vibrational, and "F NMR spectroscopy, and ab initio calculations before yielding the secrets of its remarkable, unsymmetrical structure, I (29);this can be viewed as a trigonal bipyramid centered on one sulfur atom with a lone pair of electrons, the SF group, and one fluorine atom occupying the equatorial sites. Electrondiffraction studies have also contributed to our understanding of how

112

DOWNS AND GREENE

I

F

1

gaseous organometallic compounds can, through their decomposition, furnish solid materials with useful electronic or other properties. For example, thermal decomposition of t-butylgallium sulfide, [(But)GaS14, results in the growth of a new cubic phase of Gas. Not only is this material ideal for the electronic passivation of GaAs surfaces, it has a large band gap, which makes it suitable as the insulating “gate” layer in certain transistor devices. What is striking about the structure of the gaseous precursor, 11, as determined by electron diffraction, is the presence of a distorted cubane-like Gads4core with a t-butyl group bound to each gallium atom and T symmetry overall (30).Irradiation of the precursor with the output of a UV excimer laser gives rise to the photofragments (But),Ga4S4(where x = 0-31, which can be detected by their time-of-flight mass spectra following ionization by a second UV excimer laser. It appears, then, that fragmentation proceeds primarily by loss of the organic substituents with retention of the Ga4S4core, clearly suggesting that growth of cubic Gas depends on the oligomerization of preformed Ga4S4fragments.

Problems of analysis apart, experiments of this sort can at their best provide a remarkably full picture of a gaseous molecule, at least with regard to its physical character (10). By deliberately seeking to


113

frustrate any chemical reactions, however, we are naturally denying ourselves any detailed chemical and, in particular, kinetic information. There are other limitations, too. The conditions are far removed in terms of molecular concentrations, as well as environment, from those normally prevailing in the condensed phases. For reasons of rotational congestion, moreover, high-resolution spectroscopic measurements cannot usefully be applied to molecules containing more than a few atoms, and for reasons of pressure and temperature such experiments may lose sight of molecules incorporating weak bonds. In fact, weakly bound molecules commonly feature as the initial products when two reagent molecules interact. These problems lead naturally t o the question, How can the power and precision of modern high-resolution spectroscopic methods be brought to bear on larger andlor weakly bound transients? Fast freezing in one form or another is the obvious response.

B. GAS-PHASE STUDIES IN SUPERSONIC JETS When a noble gas seeded with a small amount of other molecules expands through a nozzle into a vacuum, rapid adiabatic cooling occurs, provided that the mean free path of the molecules in the unexpanded gas is short compared with the nozzle diameter. Within only a few nozzle diameters (usually ca. 5 mm), the molecules in the jet of gas emerging from the nozzle achieve a very narrow distribution of speeds in the jet direction, clustered around the supersonic value of ca. 5 x lo4 cm . s-l, and have nearly zero relative speed in directions transverse to this. Consequently, within 5 to 10 p s of emerging from the nozzle into the vacuum, the molecules are in an essentially collisionless state and have a velocity distribution in the transverse direction corresponding to temperatures of a few Kelvin. A similar effective temperature is also achieved through cooling of the rotational and vibrational degrees of freedom. If we can arrange for the intermediate to be formed within the noble-gas stream just before it emerges from the nozzle, freezing times in the order of microseconds can be attained, and thereafter the intermediate is effectively isolated, having no mechanism, either unimolecular or bimolecular, by which to react or decompose. One experimental arrangement, illustrated in Fig. 4, consists of two concentric, coterminal tubes (31-33). One reagent admixed with an excess of noble gas is delivered in short pulses via the outer tube while the second reagent (typically also diluted with noble gas) flows continuously through the central tube. The two reagents

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DOWNS AND GREENE

Series 9 solenoid valve

Teflon outer tube pulse of F2-Ar from solenoid valve 0.3 mrn 1.d. glass capillary

FIG.4. The fast-mixing nozzle used to observe the rotational spectrum of the gaseous complex H,N----F,[reproduced with permission from (33),p. 1121.

meet and mix in the roughly cylindrical interface between the concentric flows as they simultaneously expand. Any intermediate formed here rapidly undergoes collisionless expansion, is then effectively frozen, and may be investigated by such techniques as microwave, infrared diode-laser, laser-induced fluorescence, or photoionization spectroscopy. For characterizing a dipolar molecule in its electronic ground state, few methods are more instructive than pulsed-nozzle Fourier-transform microwave spectroscopy (32). As illustrated schematically in Fig. 5, a short pulse of microwave radiation directed at the gas pulse excites a rotational transition in the species of interest; subsequently the rotationally excited molecules reemit radiation, which is detected. This technique provides a remarkably sensitive probe for transients, the properties of which can be specified with all the precision and detail peculiar to rotational spectroscopy only microseconds after their production. In relation to a weakly bound adduct A----Bformed by two molecular reagents A and B, for example, we may draw on the rotational spectrum to determine such salient molecular properties as symmetry, radial and angular geometry, the intermolecular stretching force constant and internal dynamics, the electric charge distribution, and the electric dipole and quadrupole moments of A----B(see Table I). The mechanism of the addition of molecular chlorine to an alkene


115

Pulsed nozzle

Fabry-Plrot mirrors

FIG. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency v,,, falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1p s ) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2ii: 100 p s ) compared with the relatively intense exciting pulse (half-life in the cavity T~ = 0.1 ps). If detection is delayed until ca. 2 ps after the polarization, the exciting pulse has diminished in intensity by a factor of ca. lo6 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at v, is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector a t the beat frequency 0 - I),,, as a function of time, as in NMR spectroscopy; Fourier transformation leads to the frequency spectrum [reproduced with permission from (31),p. 5631.

has been the subject of much investigation and discussion ( 3 4 ) .In a fairly polar solvent and in the dark, the reaction has been shown to be of first order in both components, indicating that the entity responsible for electrophilic attack on the alkene is the dihalogen molecule. There is a general consensus that the mechanism, as represented by Eq. (9), involves a pre-equilibrium molecular association of the alkene and C12to give the adduct 111: alkene

+ C12+ alkene----C12+ alkeneC1' + C1-. I11

Iv

(9)

116

DOWNS AND GREENE

TABLE I MEASURABLE SPECTROSCOPIC AND MOLECULAR PROPERTIES OF GASEOUS A-B MOLECULES (32) ACCESSIBLE THROUGH GROUND-STATE ROTATIONAL SPECTRA Spectroscopic parameter

Molecular property of A----B

Form of the spectrum

Symmetry

Rotational constants, -40, &, GI

Quantitative determination of radial and angular geometry; nature of intermolecular binding Quadratic force constant for A----Bstretching, k,, Relate to electric field gradient qi, a t a nucleus X having a nonzero electric quadrupole moment Electric dipole moment of A....B Molecular g factor and electric quadrupole moment of A----B

Centrifugal distortion constant, DJor AJ Hyperfine coupling constants, x,, and D,, Stark effect Zeeman effect

Comment/example Spectral pattern varies for linear, symmetric-top, and asymmetric-top species, e.g., F2----NH3 is a symmetric top (35). (34) and e.g., CzH4----C12 F,----NH,(35)

DJis roughly proportional t o k,,. qig depends on the detailed

electric charge distribution within A----B. (36) e.g., HC’sNN----H”6Cl

e.g., Ar----HCI(37)

This intermediate then ionizes in the rate-controlling step to give a cation, IV,which reacts rapidly with Cl-. For many years, however, the precise nature of the pre-equilibrium complex I11 has been a matter of speculation. It is clearly desirable to establish with certainty and precision the existence of any such complex. Mixing ethene and molecular chlorine in an apparatus of the type described gives rise to a new ground-state rotational spectrum attributable to the complex CzH4----Cl2. Analysis of the spectrum indicates that this complex has the Czugeometry illustrated in Fig. 6(a), with the Clz molecule lying along the C2 axis of ethene that is perpendicular to the molecular plane. The weakness of the interaction between the molecules is made abundantly clear by the following features (34): the rotational constant A, is only slightly larger than the constant C, of free ethene; the inner C1 atom is relatively remote from the center of the ethene C=C bond (estimated distance 312.8 pm); at 5.9 N m-l the intermolecular stretching force constant, k,,,deduced from the centrifugal distortion

-


117

constant is quite minuscule; and the nuclear quadrupole coupling constants xgg(35Cl) undergo quite minor changes, implying that the cylindrical symmetry of the electric charge distribution of Clz is only slightly perturbed on complex formation. Hence, there can be little doubt that the intermediate is formally a b7i.w complex of the weak, outer type, according to Mulliken's classification. More complicated and less easily controlled is the reaction between ammonia and fluorine gases, which under normal conditions of temperature and pressure results in a spectacular flame and the formation of NF, in small yield. Despite the prodigious reactivity of molecular fluorine, it has proved possible through the device of fast mixing and cooling of the gases in a supersonic jet to detect and characterize the weakly bound prereactive intermediate H3N----F2on the basis of its ground-state rotational spectrum (35). The results attest to the C3rstructure illustrated in Fig. 6(b) with a long intermolecular contact, r(N----F),of 270.8 pm and a stretching force constant k,,of no more than 4.7 N.m-'. By contrast, the corresponding complex H,N----ClFfeatures an appreciably stronger intermolecular bond, with r(N----Cl)= 237.6 pm and k,,= 34.3 Nem-', pointing to a small but significant transfer of charge in the sense H3N----Cl+F(38). It is not just loosely bound complexes that can be specified with profit by spectroscopic analysis of a supersonic jet. Of this there is no more thrilling example than the transient organometallic radical VCH (39),formed when vanadium atoms generated by laser ablation are entrained in a pulse of high-pressure helium containing 5-10% CH, prior to expansion to give a supersonic, cooled molecular beam. High-resolution studies of the molecular fluorescence near 800 nm excited by a tunable probe laser reveal extensive vibrational and

I

c'

P-------

FIG.6. Structures of the gaseous prereactive intermediates (a) C2H4----C12 and (b) H:jN----F,.

118

DOWNS AND GREENE

rotational detail. Hyperfine patterns characteristic of a nucleus with spin I = 7/2 confirm that the carrier contains a vanadium atom; the subband structures are those of a linear molecule; the integer rotational quantum numbers point to an even number of unpaired electrons; and the presence of hydrogen is borne out by experiments with CD4, which give rise to the corresponding bands of VCD. The ground state of the molecule can be identified unambiguously as 3A, and the associated rotational constants imply the following dimensions for the substitution structure: rs(V=C) = 170.25 and r,(C-H) = 108.0 pm. Hence, we now have substantive experimental data for an exemplar of the simplest possible type of metal carbyne complex, making clear inter alia that the V=C stretching mode is at 838 cm-', and not near 1300 cm-l as has been suggested for related species. Even more challenging are some of the neutral and charged species that are created and destroyed within an electric discharge. Yet, by subjecting a pulsed molecular beam to the combined actions of supersonic expansion and a corona discharge (on the downstream side of the nozzle), it has been possible to detect and study short-lived intermediates as varied as OH, NB , and CH3(40). Optical spectroscopy has been the traditional source of information, but major contributions to our limited knowledge of the highly complex chemistry of plasmas have come recently from mass, microwave, and infrared laser spectroscopies. Lifetime is not the only problem, for even within the discharge the concentrations of the relevant species are often very small. This has stimulated the development of special modulation techniques to improve both selectivity and sensitivity (40). Use of an ac discharge gives, for example, Doppler shifts of spectroscopic frequencies due to molecular ions, which vary with the discharge frequency vf, and state populations of any transient species, which vary with twice this frequency. When the spectroscopic detection system is referenced to the frequency ur or 2 u f , the resulting demodulated signal consists only of what are velocity-modulated or population-modulated lines, respectively. Transients such as OH and CH3are no mere scientific curiositieswill-o'-the-wisps peculiar t o the elaborate combination of discharge, supersonic expansion, and spectroscopic subterfuge. Some of these molecules play vital roles in regimes seemingly quite remote from plasma chemistry. For example, the OH radical is crucially significant as an oxidizing initiator in combustion and in the troposphere, where its natural concentration is liable to be seriously augmented as a re-


119

sult of pollution (17). Two lines of investigation must suffice to demonstrate the scope of modern discharge experiments. 1. The molecules HCCCS and HCCCCS have been characterized by their microwave spectra following their generation by the action of a pulsed discharge on a supersonic molecular beam of argon containing traces of C2H2and CS2(411. The microwave frequency, swept in small steps of a few MHz, is fed to a Fabry-Perot cavity, which is maintained at the resonance frequency by synchronous length adjustment so as to retain the high Q value of the cavity. This feature, combined with cooling t o rotational temperatures near 1-2 K, enhances greatly the sensitivity of the experiment. The resulting high-resolution spectra are noteworthy for their disclosure that both HCCCS and HCCCCS, in common with HCCS, are linear in their electronic ground states (TI) and so contrast with molecules in the analogous family HC,,O ( n = 2-41, all of which are terminated by angular C-C-H units (41).That molecules of both series are created in discharge reactions is of more than terrestrial note, beceuse they are potential players of some significance on the interstellar scene. 2. Silane discharges are of practical use as sources of thin-surface films. Infrared diode laser spectroscopy with Zeeman modulation has been exploited to detect and analyze the bending fundamental v2 of the pyramidal SiH3 radical (42), which appears t o be a primary component of silane plasmas and is likely therefore to be a key intermediate in chemical vapor deposition processes originating in silane. Analysis of the results gives r(Si-H) = 146.8 pm and LH-Si-H = 110.5'. Other discharge products include SiHz and the novel molecule SieHz, which is predominant in a low-pressure silane/argon plasma on the evidence of mass spectrometric measurements. With a low-power plasma cooled at liquid nitrogen temperature, Si2Hecan be detected by its submillimeter-wave rotational spectrum. The rotational lines are unaffected by a confining magnetic field and so cannot arise from a paramagnetic or ionic species. Moreover, they correspond to the pattern expected for a near-prolate symmetric top with ( B + C)/2 = ca. 7.2 GHz, a value close to what would be expected for a molecule containing two Si atoms, and adjacent lines exhibit a 3 : 1 intensity alternation consistent with the nuclear spin statistics associated with two equivalent protons. Putting this and other evidence together leads to the double hydrogen-bridged butterfly structure depicted in Fig. 7(a), quite unlike the familiar linear configuration of CzH2( 4 3 ) .Confirmation that this is indeed the most stable conformer of SizH2comes from

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DOWNS AND GREENE

FIG.7. Structures of the molecule SizHzpresent in a silane/argon plasma (a) i n its ground electronic state and (b) in an excited state. Dimensions are those determined by analysis of the high-resolution rotational spectra (43, 45).

ab initio quantum chemical calculations (441,which also predict, however, the existence of two other stable conformers, including one having the monobridged structure shown in Fig. 7(b) and lying only about 36 kJ-mol-' above the global minimum. It has been a triumph for theory that such a conformer should now have been detected and characterized experimentally following closer scrutiny of the millimeter- and submillimeter-wave rotational spectrum associated with the products of the silane/argon plasma ( 4 5 ) .The rotational constants of the normal and perdeuterated versions of the molecule afford an ro structure with the dimensions indicated in Fig. 7(b) and in tellingly close agreement with those predicted by the ab initio calculations. Interestingly, the molecule, which is estimated to have a lifetime not exceeding 10 ms, displays the shortest Si-Si bond to be observed to date. Nor is this the end of the tale, for the heteronuclear species SiCHz, produced by striking an electric discharge in a high-pressure argon pulse seeded with tetramethylsilane vapor, has for its ground state the silylidene structure Si=CH2. The molecule has been characterized not only by its microwave spectrum but also by its distinctive, intense laser-induced fluorescence spectrum (461, which consists of

COMING TO GRIPS WITH REACTM? INTERMEDIATES

121

15-20 vibronic bands in the 300- to 342-nm region; the rotational structure of the bands, clearly defined at high resolution, features a central subband that is weaker than the outer ones, a necessary consequence of the nuclear statistical weights if there are two equivalent hydrogen atoms. The rotational constants of SicHz and SiCDz have been used to deduce the following partial substitution parameters for the Czu ground-state structure: rs(C-H) = 109.9 and rsfC=Si) = 170.6 pm, and LH-C-H = 114.4". Than SizHz and SiCH2 we could have no better demonstration of how remote open-shell reactive intermediates may be in their molecular and electronic structures from the familiar world of long-lived, closed-shell molecules with their apparent obedience to conventional bonding principles. Chemical studies of plasmas are still only in their infancy, and more new and exotic intermediates will surely come to light. Whatever we may make of their structures and bonding, they will have to be accommodated by any mechanistic proposal that seeks realistically to account for the reactions taking place in the discharge (47). Then felt I like some watcher of the skies When a new planet swims into his ken . .

C. TRAPPING AND

MATRIX

.

ISOLATION

One way of catching and identifying a reactive transient is to intercept it with a reactive substrate so as to produce a known product that has a distinctive spectroscopic signature. For example, hydrogen atoms formed in the course of a reaction may be effectively scavenged by carbon monoxide to form the well-authenticated radical HCO, which can be identified unambiguously under appropriate conditions by its infrared or ESR spectrum (48).In a similar vein, ground-state sulfur atoms may be scavenged by dioxygen and detected indirectly by the infrared absorption or UV emission spectrum of the SO, molecules thus generated; by contrast, sulfur atoms in the excited 'D state insert characteristically into the C-H bonds of alkanes (48). Some of the first evidence pointing to the formation of the trihydrides of aluminium and gallium came similarly from trapping experiments, this time with trimethylamine to form the known, relatively stable adducts (Me3N),,MH3(M = Al or Ga; n = 1 or 2) ( 4 9 ) .With molecules as with atoms, it may be possible on the basis of suitable trapping

122

DOWNS AND GREENE

experiments to determine whether the intermediate is formed in its electronic ground state or in an excited state. “Singlet” dioxygen, that is, the 0, molecule in its excited lh, electronic state, results from the stoichiometric oxidation of hydrogen peroxide by hypochlorous acid: HOOH + HOCl + lo2+ H20 + HC1.

(10)

It has a lifetime of only 2 ps in aqueous solution, but it may be intercepted and thereby distinguished from ground-state dioxygen by its facile addition to a conjugated molecule such as butadiene (50):

An alternative and more direct strategy involves matrix isolation (51-53). Here the trick is to catch and hold the intermediate by embedding it in a rigid, inert host such as solid argon (the matrix) at low temperatures (4-50 K). Hence, our fugitive is at once isolated and effectively immobilized. To some extent the experiments imitate nature, for the mineral lapis lazuli owes its beautiful blue color to the highly reactive radical anion S , , which is entrapped in an aluminosilicate matrix. They also improve upon nature with the choice of a transparent, weakly interacting host such as a frozen noble gas at low temperatures, in place of the polar aluminosilicate, which is opaque to broad regions of electromagnetic radiation. However short-lived the transient may be under normal conditions, its lifetime can be extended almost indefinitely by matrix isolation, and we can appeal to a variety of spectroscopic techniques-notably vibrational, electronic, and ESR methods-to follow what is going on in the matrix and to detect, identify, and characterize the trapped species. The method has great advantages. Spectroscopic measurements can be made at leisure ai., therefore in detail, and several different methods can be applied to interrogate a particular sample. Moreover, the infrared spectrum, which has been the principal agent of detection and analysis of matrix samples, is usually comparatively simple, with sharp linelike features marking the different transitions (and reflecting the conditions of low temperature, relatively isotropic, weakly interacting environment, and inhibition of rotation). Not only can vibrational frequencies meaningfully be measured to -+ 0.1 cm-


123

in many cases, giving values close to those of the gaseous molecule, but the linelike character of the bands bids fair to the study of isotopic effects, frequently the most compelling means of determining the stoichiometry and geometry of the trapped molecule. On the other hand, there is also a price to be paid. Thus, inhibition of rotation means that we lose precise information about the dimensions, vibrationh-otation, distortion, and charge distribution of the isolated molecule. In addition, various complications can arise from the weak but significant perturbations imposed on the trapped species by the cage it inhabits, which is likely to vary in size, geometry, and composition within a given matrix; as a result, specific spectroscopic transitions do not always appear as sharp bands but sometimes as multiplets or relatively broad bands. The combination of rapid quenching with the rigidity of the resulting condensate is also liable to lead to the trapping of different conformers of a particular molecule. In practice no less than principle, the performance of a typical matrix-isolation experiment is quite simple. As illustrated in Fig. 8, a gas mixture is deposited under controlled, high-vacuum conditions on a cold surface the nature of which is dictated by the type of spectroscopic measurement to be made on the sample (51-53). The species of interest may be formed in the gas phase (by pyrolysis or in a discharge) and then quenched rapidly with a n excess of the matrix gas; sometimes it is formed during the cocondensation of two reagents; commonly it is generated in situ, usually by irradiation of an appropriate matrix-isolated precursor. As noted previously, identification of a novel or unfamiliar captive molecule is accomplished more often than not by reference to its infrared spectrum, with the following steps being typical of the way in which the experiment might proceed. 1. First, the number of distinct species present in the matrix has to be established; this is done by allocating the infrared absorptions to the appropriate species on the evidence of how the spectrum varies with changes in the experimental conditions. 2. We need then to assess the number, frequencies, and intensities of all the absorptions associated with a given molecule before drawing on the evidence of distinctive features due to specific groups, analogies with known compounds, or the vibrational selection rules in order to formulate its likely identity. 3. Evidence of identity is greatly strengthened if it is possible to devise a n alternative route to the molecule in question starting from a different precursor. This strategy is well illustrated by the 16-electron

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DOWNS AND GREENE

Inlet for gaseous sample

Refrigerant

Inlet for matrix gas

(4-20 K)

High-vacuum pumping system

Valve

Photolysing radiation Quartz window

Infrared spectrometer beam

matrix support (C s I window)

Sample inlet


hv

125

Ar matrix

FIG.9. Different routes leading to the unsaturated intermediate tungstenocene, (q5C5H5)2W[reproduced with permission from Downs, A. J.; Hawkins, M. Adv. Infrared and Raman Spectrosc. 1983, 10, 61.

intermediate “tungstenocene,” ( $-C6H5),W ( 5 4 ) , which has been shown by its infrared spectrum to be the common product of the matrix reactions set out in Fig. 9. 4. Annealing or photolysis of the matrix may also be instructive for the evidence of reactions such as ( 5 5 )

2BH3

Ar matnx 27

K

BPHB.

(12)

FIG.8. ( a ) Schematic representation of a typical matrix-isolation assembly. (b) Schematic plan of a matrix-isolation assembly suitable for infrared (transmission) measurements and for photolysis experiments [reproduced with permission from Almond, M. J.; Downs, A. J. Adv. Spectrosc. 1989, 17, 31.

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DOWNS AND GREENE

5. Most revealing of all is usually the response of the spectrum to changes in the isotopic composition of the molecule. If the sample is prepared in such a way as to produce a mixture of isotopomers, then it is often possible to deduce unequivocally the stoichiometry and geometry of the molecule from the number, frequencies, and relative intensities of the infrared absorptions. With sufficient information, we may even be in a position t o estimate one or more interbond angles, if these are not already fixed by the symmetry of the molecule. In the case of metal carbonyls, the vibrational problem can be simplified by concentrating on the restricted region of the infrared spectrum near 2000 cm-I, corresponding to C-0 stretching vibrations, v(C-01, which can be successfully analyzed independently of the other fundamental vibrations through the use of an energy-factored force field. For each of several possible models of a particular metal carbonyl fragment M(CO), , it is then comparatively easy to estimate the frequencies and relative intensities of the v(C-0) infrared bands for the family of isotopomers M(12C0),(13CO),-, (x = 0-n) and so determine which model gives the best account of the observed spectrum. Figure 10 shows the results of applying this procedure (i) to Fe(C0I4,the primary product of ultraviolet photolysis of Fe(CO), (56), and (ii) to O,MO(CO)~,a prominent intermediate in the train of reactions induced by ultraviolet photolysis of Mo(CO16 in the presence of 0, (57). Hence, there is little doubt that Fe(C0I4 has the CZustructure V or that 02Mo(CO)4 has the tram configuration VI with a square planar Mo(CO)~ fragment.

V

VI

6. The greatly enhanced sophistication of modern quantum chemical calculations means that we are now able to predict with considerable confidence the equilibrium structure of a molecule, the harmonic frequencies of its normal modes, and the intensities of the relevant features in infrared absorption. Accordingly, it has become quite commonplace with matrix as with other studies to test the inferences drawn from experiments against the results of appropriate ab initio or DFT analyses. How closely theory matches experiment is shown

127

COMING TO GRIPS WITH REACTIVE INTERMEDIATES r

1

I

(i i)

A a

?

I

I

I

a 2110

2080 cm-1

2675

2050 2000

19j5

1950

cm-1

FIG.10. (i) IR Spectra showing how the structure of matrix-isolated Fe(CO), was established: (a) observed spectrum of Fe(CO)l with partial W O enrichment isolated in a solid SF,, matrix a t 20 K [bands shown in dotted lines are due to residual Fe(CO),, precursorl; tb) spectrum calculated for the C2,,geometry, V, which gives the best agreement with the observed spectrum; (c) spectrum calculated for a C:+,geometry which provides a poor match to the observed spectrum 156). tii) IR Spectra showing how the structure of trans-0,Mo(CO)4was established: ( a ) observed spectrum of trans02Mo(C0)4with partial W O enrichment isolated in a solid CH1 matrix (bands shown in dotted lines are due to photoejected '.'CO); (b) spectrum calculated for a planar Mo (CO), fragment with Dlh symmetry, VI (57).

for the case of matrix-isolated MezAINHzformed in situ by the elimination of CH, from the adduct Me& * NH3 through irradiation with light a t wavelengths near 210 nm (see Fig. 11)(58). In relation to mechanistic enquiries, the principal strength of matrix isolation lies in the identification and specification of real or potential reaction intermediates. From such studies has come, for example, our first sighting of unsaturated molecules such as Fe(C01, (561, Fe2(CO), (591,Cr(CO)5(601, (q5-C5H&W(54),and Ru(dmpeh (dmpe = MezPCHzCH2PMe2)(61). The equilibrium structures of the ground-

2

128

DOWNS AND GREENE

X

1

3400

,

3000

I

1

1600

I

,

,

1000

,

,

I

400

FIG.11. IR Evidence for the formation of MezAINHaby W irradiation (A = 210 nm) of the complex Me&l.NH3: (a) spectrum calculated for Me2AlNH, a t the RMPZ(fc)/ 6-31G* level of theory; (b) difference IR spectrum after irradiation of Me&. NH3 (decreasing absorptions point downward; increasing absorptions point upward). x indicates CH,, y indicates NH,, and z indicates H2 0 (impurity) [reproduced with permission from (58),p. 63721.

state molecules appear to be independent of the matrix material (provided that there is no direct reaction with the matrix), and the case histories of numerous molecules affirm that the same structures persist in the fluid phases. About the electronic and photochemical properties of such molecules there is much also to be gleaned through matrix isolation, so there can be little doubt, for example, that Fe(CO)*has a triplet electronic ground state or that Cr(COI5as initially generated by photolysis of Cr(CO)6is in a vibrationally or possibly electronically excited state. Some clear sign of the reactivity of the molecule may be found. In this regard, few experiments are more striking or more significant than those in which the visible absorption spectrum of Cr(CO)5is found to vary dramatically according to the nature of the matrix in which the molecule is trapped (60).Hence we


129

learn just what an extraordinarily powerful Lewis acid Cr(CO&is, for the unsaturated molecule, which has a square pyramidal (C4")structure, evidently interacts to a n appreciable extent through the vacant, sixth coordination site even with a matrix molecule such as CH4,Xe, or Ar that has few claims to be regarded as a normal ligand. Experiments with mixed matrices have shown that the token ligand is interchanged by irradiating selectively into the characteristic electronic absorption band a t lowest energy. Because the molecules are not free to rotate, it is possible to select those in certain orientations by the use of plane-polarized photolyzing radiation and so generate products in orientations that can be analyzed spectroscopically, also with the aid of polarized light. Hence, the interchange reactions are found to occur by expulsion of one token ligand, followed by a Berry pseudorotation, and uptake of another token ligand (Fig. 12). There is still much that we do not know about Cr(CO)5and its photochemistry, but these matrix studies have surely opened our eyes to the nature of this particular intermediate and the part it plays in the supposedly simple substitution reactions of Cr(CO), . However, matrix studies are limited ultimately by the low mobility of the caged species, allied to the narrow temperature range over which they can be held. The very immobilization that helps to preserve a highly labile molecule in a solid matrix becomes a n Achilles' heel when we wish to explore its reactions. The restraining effect of the matrix cage is much in evidence in the way photodissociation or photoisomerization reactions are immediately arrested and in the way bimolecular reactions induced by photolysis are normally restricted to neighboring pairs of molecules. Reaction channels that are important in the mobile fluid phases may thus be debarred, whereas other channels of minor importance in the fluid phases may well be favored. For example, photolysis of the recently identified chlorine oxide ClC10, (formally a dimer of C10) results in isomerization to ClOClO and ClOOCl when the molecules are confined to a matrix, but yields Clz and O2 as the only detectable products in the gas phase (62).We have already considered the scope that exists for recognizing and characterizing reactive intermediates in the gas phase. What options are there for extending our investigations to the liquid phase and to solutions?

D. SOLUTION STUDIES AT Low TEMPERATURES Various problems are inherent in the solution state when it comes to tracking reactive intermediates. First, the solvent may be a far

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Xe

FIG.12. Schematic representation of the photochemical behavior of the Cr(CO)Sfragment in a mixed Ne/Xe matrix a t low temperatures. * represents the Cq. fragment in the excited 'E state [reproduced with permission from (60),p. 1361.

from innocent spectator; second, as with matrix samples, rotational transitions are not a feasible source of information; third, the spectroscopic features due to a solute molecule are appreciably broader than those of the same molecule in a solid, inert matrix, so that isotopic effects are much harder to evaluate; and last, most solvents absorb strongly in many regions of the electromagnetic spectrum, thereby obscuring our vision of the solute species in these regions. Nevertheless, the simple expedient of cooling a solution may be sufficient to extend the lifetime of an intermediate to the point at which it can be detected by conventional spectroscopic means. A good example is the q1-C2H3)(q2-C2H,)H (VII). This is vinyl hydride complex ( q5-C5H5)Ir(


13 1

VII

one of the intermediates formed by photolysis of the corresponding bidethenel complex ($-C5H5)Ir($-C,H,), with light having wavelengths >290 nm (63).The photoinsertion into a C-H bond of a coordinated ethene molecule can be engineered simply by photolyzing a frozen [2H81tolueneglass at 77 K, and the vinyl hydride can be recognized definitively by thawing the glass and measuring the 'H and I3C NMR spectra of the resulting solution at ca. 230 K, raising the temperature to 273 K results in slow decomposition, with reversion to the starting material. There is much to be gained from the use of a very weakly coordinating solvent such as a liquefied or even supercritical noble gas (64,651. The noble gases may each have a very narrow liquid range a t low pressure, but by working at pressures of 20 bars or more it is possible to span the entire range from 77 K to ambient temperatures with liquid argon, krypton, and xenon. I11 adapted though these liquids may be for the dissolution of polar compounds, they are capable of dissolving more or less freely many organic and organometallic materials as well as gases such as H2, N,, GO, and CHI, and analysis of the resulting solutions is hugely facilitated by the absence of solvent absorption in the infrared, visible, and normal ultraviolet regions, with the result that much longer pathlengths are admissible than for conventional solution studies. Accordingly, we can draw on a range of conventional spectroscopic methods to examine the solutions. Bandwidths may be appreciably greater than for matrix spectra, but problems of matrix site effects are eliminated and the bands are regular and symmetrical in shape. Moreover, the solutions give much flatter spectroscopic baselines than do matrices; allied to the advantage of the long pathlength, this increases considerably the capacity to detect weak bands. More fundamentally still, and in contrast with matrix studies, solution-based measurements provide the opportunity of exploring the kinetics and positions of equilibrium of reactions under thermal control. At the same time, we lose sight of some intermediates revealed by matrix isolation or other means that do not survive

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in solution, at least long enough to be detected by normal means. Solution experiments may thus succeed in recording the fate of a labile, coordinatively saturated organometallic molecule such as CI-(CO)~(N~) but not of a coordinatively, unsaturated one such as CI$CO)~. Figure 13 shows the design of one such cell as used by Poliakoff and Turner and their group at Nottingham for experiments of this sort (64). The cell is cooled by liquid nitrogen, the flow of which is pulsed to vary the temperature. An important feature not illustrated in the figure is the presence within the cell of a microscale magnetic stirrer “flea”; hence, even solutions that are strongly absorbing in the ultraviolet or visible regions can be made to react photochemically because the portion of solution immediately adjacent to the irradiation window is constantly being renewed. For photochemically generated compounds that have lifetimes in the order of seconds, it may be

FIG.13. Schematic view of a variable temperature cold cell used for infrared studies of liquid noble-gas solutions. Cooling is achieved with a pulsed flow of liquid N2 (LN2) controlled by the output from one of the two thermocouples T so as to stabilize the temperature. The whole cell fits into a vacuum jacket (not illustrated), which is pumped through the tube V in the top flange of the cell. The solution under study can be passed through the cell from a room temperature reservoir via the two tubes marked In and Out [reproduced with permission from (811, p. 5551.


133

necessary to record the spectrum while the solution is being irradiated. Some idea of the compass of such studies may be gained from the case of the dihydrogen complex Cr(CO)S(q2-H,),which for all its 18electron configuration, is too labile to be isolated or spotted by ordinary methods of attack. The complex can be detected following photolysis of Cr(CO)6 or Cr(COI5(NH3)in an H2-doped matrix, but the matrix testimony has to rely principally on v ( C - 0 ) bands in the infrared spectrum; the broad v(H-H) feature near 3000 cm-' is some 20 to 30 times weaker than the least intense of the v(C-0) bands and, not surprisingly, is barely visible above irregularities in the baseline. These findings have been largely eclipsed, though, by the results of experiments involving photolysis of Cr(CO&dissolved in liquid xenon under a pressure of H2. Using these conditions, the Nottingham group has succeeded in detecting a weak, broad infrared absorption fulfilling all the requirements for assignment to the v(H-H) mode of the coordinated H2 moiety (66).The absorption comes a t 3030 cm-' when Hz is the ligand, shifting to 2241 cm-' when Hz gives way to Dz. On the other hand, similar experiments starting from the bis(ethene) complex ( q5-C5H5)Rh(q2-CzH,)z reveal the formation not of a dihydrogen complex but of the unstable dihydrido derivative (q5-C5H5)Rh($-CzHJ(H)z (67). Ultraviolet irradiation of a liquid xenon solution of Cr(CO)6without any other agent generates a new species that, with a half-life of about 2 s at 175 K, is sufficiently long-lived to be detected by conventional infrared measurements. The species has d C - 0 ) absorptions very close in frequency and intensity pattern to those assigned to the matrix-isolated species Cr(CO)5----Xe (64).Its identity can be checked, first by deliberately doping the solution with a range of likely impurities in order to rule out the possibility of its being some other compound of the type Cr(CO)5L (L = Nz, H20, etc.), and second, by carrying out experiments with Cr(CO)6 dissolved in pure liquid krypton and in liquid krypton doped with 5% xenon. Hence, the realization is dawning that the coordination of Cr(COI5 by xenon involves not a generalized solvation but a specific interaction with the formation of a Cr-Xe bond of appreciable strength. Similarly, the transient Ni(CO),(N,) has been identified following its generation by photolysis of Ni(CO), in liquid krypton doped with Nz a t 114 K. Ultraviolet irradiation establishes a steady-state concentration of Ni(CO),(N,); as soon as the photolysis source is switched off, the dinitrogen complex begins to decay, reacting thermally with CO to regenerate Ni(CO), . The kinetics of the decay reaction,

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have been investigated in some detail over the temperature range 112-127 K (68). The rate shows a first-order dependence on the concentration of Ni(CO),(Nz)and a more complex dependence on the concentrations of CO and Nz. Hence, it appears that the reaction proceeds via two simultaneous paths, one dissociative and the other probably associative. An estimate of about 42 t- 4 k J mol-' for AH$ for the dissociative path gives a measure of the Ni-N2 bond dissociation enthalpy in Ni(CO),(NZ). In an extension of the technique we can now trade on the remarkable properties of supercritical fluids (64, 69). These combine a number of properties of gases and liquids; like gases they are highly compressible and are completely miscible with Hz or Nz, but like liquids they are relatively dense and dissolve a wide range of nonionic solids. In fact, most materials are considerably more soluble in supercritical xenon than in liquid xenon, despite the fact that the density of the supercritical material at the critical point is only about one-third that of liquid xenon. Moreover, the solubility of any compound in a supercritical fluid increases with increasing pressure at a constant temperature, with the result that the solvating power of the medium can be tuned by varying the pressure. It is the superior solubility of compounds that gives supercritical solvents a major advantage over their normal liquid counterparts and particularly in reactions involving gases such as Hz or Nz. Thus, for a given pressure of gas, the effective concentrations of Hz in a supercritical solution can be nearly an order of magnitude higher than in a liquid solution. As a result, it is altogether easier to detect, preserve, and even synthesize known or potential reaction intermediates. Previously unknown dihydrogen and dinitrogen complexes, which would normally decay rapidly at ambient temperatures, have thus been formed in supercritical xenon, where they are sufficiently robust at room temperature to be characterized quite normally by their infrared spectra (69, 70). Kinetic measurements between 283 and 353 K show that (r16-C6HSMe)Cr(CO)z(772-Hz) reacts with CO in accordance with Eq. (141, with an activation enthalpy of 70 t- 5 k J . mol-', which may correspond to the Cr-H2 bond dissociation enthalpy :

+

+

( 776-CGHsMe)Cr(CO)z(~-H2) CO -+ (q6-C6HsMe)Cr(CO), H2.

(14)

This conclusion finds support from more recent studies, also at high pressure but in n-heptane solution and using photoacoustic calorime-

135


try to measure the enthalpy change of Reaction (15) (711, M(CO)G + L + M(CO),L + CO

(15)

(M = Cr or Mo; L = n-heptane, H2or N2); the Cr-H2 and Cr-N2 bond enthalpies are estimated in this case to be 78 and 81 kJ . mol-', respectively. Figure 14 illustrates the region of the infrared spectrum associated with z4N-N) modes for a supercritical xenon solution of the rhenium carbonyl ( T ~ - C ~ H ~ ) R ~under ( C O )a~pressure of NZ (ca. 90 bars) and shows how this changes with time on photolysis with ultraviolet light (70). The spectrum bears clear witness to the sequential formation of C~H ) ,, ) R ~ ( C O )and ( N ~(q5-C5H5))~, the species ( T ~ - C ~ H ~ ) R ~ ( C O( T) ~ (- N Re(N2I3a t room temperature, and the last two turn out to be surprisingly stable under these conditions. In suitable circumstances, solutions in supercritical solvents are amenable even to NMR measurements (72). Supercritical fluids are thus affording a new dimension and are giving direct access to new chemistry involving reactive intermediates, as well as a new approach to problems of analysis, synthesis, and materials processing.

0.30

0.15

0 2160

2130

2100

2070 v/cm-'

FIG.14. IR spectra showing the sequential formation of ( v ~ - C ~ H ~ ) R ~ ( C O (dark )~(N~) peaks), (7)"-CSH5)Re(CO)(Nz)z (medium peaks), and ( T ~ - C ~ H ~ ) R (light ~ ( Npeaks ~ ) ~ ) from ($-C,H5)Re(CO)3 and Nz in supercritical xenon solution [reproduced with permission from Poliakoff, M.; Howdle, S. Chem. Br. 1995, 31, 1201.

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DOWNS AND GREENE

IV. Experimental Characterization of Reaction Intermediates: Time-Resolved Methods

To retard or suppress chemical reactions it has been necessary to work under rather unusual conditions. It is only natural to want to know about the behavior of intermediates under more normal conditions. However, if the species are highly reactive, detection and monitoring of their fate demand very rapid spectroscopic measurements. From the more or less conventional type of spectroscopic analysis that has been the mainstay of our investigation so far we pass to methods that can be adapted to real-time measurements and in which spectral evolution or decay can be monitored at intervals sometimes as short as femtoseconds. It will soon be apparent?though, that the knowledge about the kinetic correlation between intermediates, reagents, and products thus gained can only be at the expense of detailed knowledge about the physical properties of the molecules-sometimes even their very identities. The function of the gas-phase, supersonic-jet, matrixisolation, and low-temperature solution techniques described in the preceding sections is in the provision of a “spectral library” of more or less well-authenticated reaction intermediates, whether real or model ones; this library is going to prove essential to the interpretation of many time-resolved experiments. The most popular procedure involves flash photolysis or, in the terms of laser technology, pump-and-probe experiments (73-76). Flash photolysis was developed by Norrish and Porter in Cambridge in the 1940s and earned them the Nobel Prize in 1967. The basis of the technique is simplicity itself. A pulse of light is used to generate the transient, typically in a fluid sample at or near ambient temperatures, and a spectroscopic record of the sample in absorption or emission is then kept as a function oftime. The time-scales of the reactions that can be studied are limited by the duration of the initiating light pulse. The flash lamps used by Norrish and Porter, which give pulse durations of several milliseconds, have now been largely superseded by high-powered lasers giving pulse durations of nanoseconds or less. By means of a process called mode-locking it is possible even to generate laser pulses lasting only a few picoseconds or less; the world record is currently reported to be 4 femtoseconds. Such lasers offer a means of tracking very fast kinetic phenomena, such as the isomerization of an electronically excited alkene, the rotational motion of a large molecule in solution, or some of the fast processes occurring in photosynthesis. Although these ultrafast processes (77) are not “reactions” in the sense that we have discussed so far, they are none-


137

theless important in the contributions they may make to overall reactions. An alternative technique for the rapid generation of transient species is that of pulse radiolysis triggered not by a light flash but by a short pulse (10-9-10-6 s) of high-energy electrons (1-5 MeV); this approach has been employed primarily for studies of liquid phase kinetics and especially of the solvated electron, but it is a very flexible method with applications also for gas phase studies (78). Representative of this method of attack is the formation of the peroxynitrate anion 02NOO- less than 2 ms after pulse irradiation of aerated aqueous solutions containing nitrate and formate a t relatively low concentrations (79). The anion, which can be detected and monitored by its UV absorption near 290 nm, is a strong oxidizing agent notable for its potential mediation in some of the reactions that make living systems vulnerable to nitrogen dioxide, one of the most toxic components of polluted atmospheres. It is of some consequence therefore that studies of several oxidation reactions should reveal two distinct pathways by which the peroxynitrate decays, one proceeding through the parent acid OzNOOH and the other probably evolving through secondary reactive intermediates derived from the decomposition of the peroxynitrate. Once the transient species has been formed, it has to be monitored by some form of kinetic spectroscopy, typically with ultraviolet-visible absorption or emission, infrared (time-resolved infrared or TRIR) (74), or resonance Raman (time-resolved resonance Raman or TR3) (80) methods of detection. The transient is usually tracked by a probe beam a t a single characteristic frequency, thereby giving direct access to the kinetic dimension. Spectra can then be built up point by point, if necessary, with a n appropriate change of probe frequency for each point, although improvements in the sensitivity of multichannel detectors may be expected to lead increasingly to the replacement of the laborious point-by-point method by full two-dimensional methods of spectroscopic assay (that is, with both spectral and kinetic dimensions). Ultraviolet-visible absorption has traditionally been the basis of detection in flash photolysis experiments. It offers a number of advantages in sensitivity and efficiency and has certainly delivered much vital information about the reactivity of transient species. On the other hand, the ultraviolet-visible absorption bands characteristic of any but the simplest molecules tend to be broad and relatively uninformative as regards identity and structure, and so we may run into problems not only with the overlap of absorptions due to different

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DOWNS AND GREENE

species, but also with deciding on the nature of the carrier of a particular absorption spectrum. Altogether more discriminating, albeit less sensitive, are the TRIR methods that have been successfully developed in the past 15 years or so (74, 81). With the sort of apparatus depicted schematically in Fig. 15, it has been possible to draw on the spectral library built up from matrix isolation and other experiments in the “slow” regime to identify transients formed in the gas phase or in solution at ambient temperatures, as well as investigating their kinetic behavior. Just what can be achieved in this way will be demonstrated by some examples representative of recent studies. As a start, we return to that remarkable Lewis acid Cr(COI5.Flash photolysis experiments have been carried out with Cr(CO)6either in solution or in the gas phase and with either infrared or visible probes (9,601. For example, photolysis of gaseous Cr(CO)6with the output of an XeF laser yields a predominant transient that can be tracked by a IR spectrometer

n

HeNe alignment laser

Sample cell

Oscilloscope Pulse generator

\

0.-

Micrmmputer

Detector

Transient digitiser

FIG.15. Schematic representation of the time-resolved infrared (TRIR) flash photolysis apparatus used a t Nottingham. The UV pulse laser generates transient species; the continuous IR laser monitors the change in transmission a t a particular IR frequency, producing a trace showing the IR absorbance as a function of time. The experiment is repeated a t different IR frequencies so that a complete IR spectrum of the transient can he built up [reproduced with permission from (97),p. 1031.


139

line-tunable, liquid-nitrogen-cooled CO laser, changes in intensity of the transmitted infrared light being recorded by a high-speed InSb detector. The v(C-0) bands clearly imply that the species is “naked” Cr(CO), (freed a t last from any token ligand) with the same square pyramidal geometry as the molecule identified previously by matrix isolation (see Section 111,C).The identification is confirmed by the decay of the transient a t a rate matching the rate of reappearance of Cr(CO)6.The rate constant for the reaction of Cr(CO), with CO a t 293 K comes out to be 1.5 t 0.3 X 10’”dm3-mol I . 5 - l . Similar studies of the reaction of Fe(C0I4with CO give a rate constant about two orders of magnitude smaller (56).This difference can be rationalized, a t least in part, with the realization that Fe(CO), has a triplet ground state whereas Cr(CO), almost certainly has a singlet ground state. Closer examination shows that Cr(CO), is generated within 1 picosecond of light absorption and that it is formed in vibrationally excited states that relax over tens if not hundreds of picoseconds (60).These details emerge not only from time-resolved ultraviolet-visible studies but also from analogous TR3 experiments with cyclohexane solutions in which two 5-ps pulses a t A = 266 nm are employed; the first dissociates Cr(CO)6,and the second probes the resonance Raman spectrum of the photoproduct Cr(CO),. Hence, it appears that the naked intermediate, albeit vibrationally excited, can exist for a few picoseconds before taking up a solvent molecule. Extension of these studies to supercritical fluids provides a way of making TRIR measurements on intermediates incorporating a bond between a metal center and a weakly coordinating ligand such as a noble gas or CO,. Hence, for example, a series of organometallic noble-gas compounds, W(CO),(Ar), M(CO),(Kr), and M(CO),(Xe) (M = Cr, Mo, or W), have been detected transiently in fluid solution a t room temperature (82).The second-order rate constants for the reaction of M(CO15Lwith CO show that the reactivity for a given metal M varies in the order L = Kr > Xe = C 0 2 , whereas the reactivity for a given ligand L varies in the order M = Cr = Mo > W. The reaction of W(CO),(Xe) with CO has an enthalpy of activation of 34 k J . mol-’, representing a lower limit for the W-Xe bond enthalpy. The bond is therefore twice as strong as a conventional hydrogen bond, and its mechanistic impact cannot be ignored. The photochemistry of the binuclear iron carbonyl [(q5-C5H5)Fe( C O ) z ] 2VIII, , provides another outstanding example of the interplay between time-resolved and alternative strategies (76). According to the infrared spectrum of this compound in a suitable matrix a t low temperatures (83), ultraviolet photolysis results in the elimination of free CO, but the pronounced cage effect of the medium entirely sup-

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DOWNS AND GREENE

presses the separation of (q5-C5H5)Fe( C0lz radicals, X. Experiments with a l3C0-labeled sample demonstrate that the iron-containing product formed by CO loss has a binuclear framework incorporating three CO bridges, M: 0

IX

VIII

X

Pulsed laser photolysis (at A = 308 nm) of VIII in an alkane solution at room temperature can be shown by TRIR measurements to generate not one but two metal carbonyl species with quite different kinetic properties (84).One transient displaying two infrared bands characteristic of the u(C-0) modes of terminal CO groups decays rapidly by second-order kinetics (with kz = 5 X lo9 dm3.mo1-l. s-') to regenerate the precursor within about 50 ps. The infrared spectrum of this species, allied to its other properties, makes clear that it is the radical transient is the triply CO-bridged ( Q ~ - C ~ H ~ ) F ~ X. ( C The O ) ~ second , complex M,easily recognizable by the close resemblance its infrared spectrum bears to that of the matrix-isolated species. In the absence of added CO, this has a half-life of about 0.6 s-that is, some four orders of magnitude longer than that of the radical. IX reacts with CO, and its decay as well as the regeneration of VIII can also be monitored by TRIR measurements. Similar experiments with another ~ ( CLO=) CH3CN ~L substrate L to form the product ( Q ~ - C ~ H ~ ) ~ F ~[e.g., or P(OMe)3, see Fig. 161 reveal the participation of both intermediates M and X through parallel reaction paths. The reactivity of the radical X may thus be gauged by rate constants close to the diffusioncontrolled limit. For another string to our spectroscopic bow we may also turn to Raman scattering, using a single visible laser to fulfil the dual functions of pump and probe, and detecting the emitted light by a suitable optical multichannel analyser. Cyclohexane solutions of [( Q ~ - C ~ H ~ ) F ~ and ( C Oits) ~permethylated ]~ analog [ ( Q ~ - C ~ M ~ ~ ) F ~ ( C O ) ~ I show resonance Raman behaviors correlating with the lower energy


141

U

C

0

n

a

el

1

1950

L

I

1850

I

1750

WO"e""lllber/Clll-'

FIG. 16. Time-resolved infrared spectrum obtained after UV flash photolysis of I($CSH5)Fe(CO)211,VIII (A), and MeCN in cyclohexane solution at 25°C. The bands ar e labeled thus: B, ($-C5H5)Fe(CO)2,X C, ($-C5H5),Fe,(p-CO)n,M;and D (r)"-CSH&FeL(CO).I(MeCN).The first three spectra correspond to the duration of the firing of the UV flash lamp, and subsequent spectra are shown a t intervals of 10 p s . The negative peaks in the first spectrum are due to material destroyed by t h e flash; these have been omitted from the subsequent traces to avoid undue confusion [reproduced with permission from Dixon, A. J.; Healy, M. A,; Poliakoff, M.; Turner, J. J. J. Chem. Soc., Chem.. Commun. 1986, 9941.

electronic absorption bands, thereby shedding light on the molecular orbitals involved in the electronic transitions (85). By the simple device of increasing the incident laser power it is possible to observe additional rich resonance Raman spectra that must originate in the binuclear transients (775-CSRS)zFez(~-CO)~, where R = H (MIor Me.

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With its different requirements, the Raman effect gives access to vibrational transitions that may well be lost to view in infrared absorption. Time-resolved Raman spectroscopy, both spontaneous and coherent, has developed apace in the past two decades (86). As a means of pursuing short-lived intermediates, spontaneous resonance Raman scattering has proved particularly instructive, partly because of its high selectivity, partly because of its enhanced sensitivity. Since studies of this sort were initiated in 1976, the time resolution has improved from microseconds to subpicoseconds. The method has really come into its own, notably in the hands of Kitagawa and his colleagues at Okazaki (87), in the investigation of heme proteins and of how they bind dioxygen and other ligands. For example, the reaction of dioxygen with the reduced enzyme cytochrome c oxidase (CcO), formed by CO photodissociation in aqueous solution, has been followed by TR3 measurements (87), which reveal within 10 ms the growth and decay of several intermediates identifiable by emissions due to 0-0 or Fe-0 stretching vibrations in the region 300-850 cm-*. The origins of the bands have been traced by experiments with "0enriched samples of 02,and hence it can be shown that the O2 binds initially to the Feassite in CcO in an end-on fashion with an Fe-0-0 bond angle close to 120". Studies with picosecond time resolution witness within 5 ps of photolysis of the CO-bound precursor an intermediate believed t o be the high-spin, five-coordinate heme Fe:; to which a histidine (His) is ligated; this is indicated by the growth of a band at 220 cm-' attributable to the Fe-His vibration (88). Intense interest in how carbon-hydrogen bond activation may be engineered has led to detailed studies of several transition-metal compounds known to compass reactions of this type, which are sometimes initiated thermally, but more often photochemically. Two such compounds are the iron and ruthenium dihydrides M(dmpe)zHz(M = Fe or Ru; dmpe = Me2PCH2CH2PMe2), which react with alkanes RH under irradiation in accordance with Eq. (17) (61, 76):


143

The ruthenium compound gives an object lesson in the merits of combining matrix with time-resolved solution studies. Photochemical reductive elimination of Hz may be followed by the decay of the characteristic v(Ru-H) bands in the infrared spectrum or by the growth of three intense visible bands at ca. 740, 540, and 460 nm. Although the product lacks characteristic infrared features, the circumstances of its formation and its response to matrix dopants imply that it is the 16-electron species Ru(dmpe):,, presumably with a square planar skeleton. Flash photolysis of a cyclohexane solution of the dihydride with a pulsed laser yields a transient with a visible spectrum remarkably like that of the matrix photoproduct. The transient signal decays by second-order kinetics over ca. 80 p s as the precursor is regenerated. Addition of H2 even in very low concentration increases the rate of decay, causing it to become pseudo-first order; under these conditions the reaction of the intermediate with H2 is essentially diffusion controlled, being opposed by a minimal activation barrier. The corresponding iron compound Fe(dmpe)2H, cannot be vaporized without decomposition and so does not lend itself to matrix isolation, but flash photolysis of a n alkane solution at room temperature g v e s a transient Fe(dmpeIz differing conspicuously from its ruthenium counterpart. For one thing, its electronic absorption spectrum shows but a single band a t low energy, and that not in the visible but in the nearultraviolet region. The reactivities are quite different too: both intermediates add substrates to form products of the type M(dmpeIzL, where L = CO, C2H4,or PMe3, as well as reacting with Hz, certain hydrocarbons, or EtySiH to give insertion products of the type M(dmpe12(X)(H)(X = H, organic group, or Eta&), with the kinetic results summarized in Fig. 17. The reactions with Hz, EtsSiH and alkenes are slower by at least two orders of magnitude for Fe(dmpeIz than for Ru(dmpeIz,whereas alkanes and arenes find Fe(drnpe&much the more reactive of the two. One plausible explanation of the spectral and kinetic differences is that the intermediates have different structures, with Fe(dmpe12adopting a “butterfly” configuration unlike the square planar form of its ruthenium analog, although we lack definitive evidence on this point. The reduced rate of the back-reaction with H2 to regenerate the dihydride precursor must be one of the key factors making Fe(dmpe)z more reactive than Ru(dmpeIz toward C-H insertion reactions, particularly with alkanes and arenes. Similar experiments involving photolysis of the ruthenium hydride place Ru(PP3)Hz,in which the two bidentate dmpe ligands have given place to the tetradentate phosphine PPy= P(CHzCH2PPh2),, reveal the

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'0

--..co

..HZ

-co

-Et&H

2cyclopantene 6. 109 k2 4 -

2 -

-benzene. max -cyclopentene -EtjSIH H2

-pentane -cyclohexane

0

FIG. 17. Schematic comparison of the intermediates Ru(dmpe)a and Fe(dmpe):, contrasting their UV-visible spectra and reactivities; the (dmpe = MeaPCH2CHaPMe2) latter are expressed as log k a where k a is the second-order rate constant for reaction with the substrate a t room temperature deduced by laser flash photolysis [reproduced with permission from (611, p. 3641.

transient intermediate Ru(PP3) (89). Here the PP3 ligand obliges the molecule to adopt not a square planar RuP4 skeleton but a pyramidal (XI) or butterfly one (XII), and it is noteworthy that the UV-visible

XI

XI1

spectrum resembles that of Fe(dmpeI2with but a single absorption a t 395 nm. In its reactivity, moreover, Ru(PP3) is quite different from R ~ ( d m p e )being ~, much slower, for example, to react with H2. The


145

corresponding osmium intermediate, which has a similar structure on the evidence of its W-visible spectrum, does not even react with H2 but inserts into the C-H bonds of alkanes with rate constants in the order of lo5 dm” mol-’. s at 295 K. Hence, it appears that the enforced nonplanar geometry effectively prepares Ru(PP3),and more especially Os(PP3),for C-H activation (89). In the first application of ultrafast TRIR spectroscopy (that is, with time-scales of to IO-’s), reductive elimination and oxidative addition of H2 have been traced following flash photolysis of the related ruthenium carbonyl dihydride complex Ru(PPh,),(CO)H, in benzene solution (90). This precursor is of particular note because it is known to catalyze insertion of alkenes into C-H bonds a t the unsaturated carbon center of alkenes or arenes in a position p to a carbonyl group. The course of events has been monitored from excitation with an ultrafast UV laser (giving pulses a t A = 304 nm with a n energy of 1-2 pJ, a full width of 4 ps a t half of maximum duration, and a repetition rate of 1.05 kHz) to 2000 ps after the laser pulse using the v(C-0) mode as a reporter, IR absorption measurements being made with a CO laser or with a diode laser via a detection system employing “up-conversion.” Photolysis results in reductive elimination of H2 and the formation of an unsaturated transient, Ru(PPh3)&CO), which can be identified unequivocally on the basis of its rapid buildup, the kinetics of its reaction with H2, the partial regeneration of starting material, and the frequency of its v(C-0) band. Moreover, the photoelimination step-involving H-H bond formation, Ru-H bond cleavage, and any reorganization of the coordination geometry at ruthenium-is found to be complete within 6 ps. Nor has the speed limit yet been reached, for there are even ways of clocking chemical events a t the femtosecond level. For example, Ahmed Zewail and his colleagues a t the California Institute of Technology have developed a novel approach relying on ultrafast lasers giving femtosecond pulses to study in real time the transition-state dynamics of charge-transfer and other reactions (91, 92). The entire molecular system is prepared on a reactive potential energy surface and in a well-defined impact geometry. A femtosecond light pulse induces a charge-transfer transition, and the dynamics of the ensuing events are then followed by interrogating the transition state or the reaction products; this is done with probe femtosecond light pulses causing photoionization and mass spectrometric sensing of the resulting fragments. The method is well illustrated by the dissociative charge-transfer reaction of the benzene-iodine complex (91 ):

146

DOWNS AND GREENE

rcnc tniit

transition state

products

The transition state is directly accessed by excitation into the charge-transfer absorption band of the complex formed initially by expanding a gas mixture containing benzene, iodine, and helium through a pulsed valve. The resulting molecular beam is skimmed and intersected by the femtosecond laser pulses in the acceleration region of a linear time-of-flight mass spectrometer. The pulses of the femtosecond pump-laser (at A = 275 nm) initiate the reaction and define t = 0. The probe pulses (at A = 304.7 nm) then detect the free iodine atoms through a resonantly enhanced multiphoton ionization (REMPI) process. Transient I atom growth is measured by monitoring the It signal as a function of the pump-probe delay. In addition, by using linearly polarized pump pulses and having provision for varying the orientation of the plane of polarization with respect to the axis of the mass spectrometer, it is possible to analyze the velocity distributions of the I atoms. Hence, microscopic elucidation of the reaction dynamics and mechanism becomes a practical proposition. We are able to witness the evolution and decay of the transition state and to show that dissociation with the formation of I atoms occurs via one of two exit channels. One (ionic) follows the ionic potential of the charge-transfer state CsH& to produce c6Hg.1- + I; the other (neutral) involves intermolecular electron transfer through coupling of the transition state to neutral, locally excited, iodine-repulsive states, and in this case the products are C6Hs.I + I. With a timescale of 250 fs, the neutral process is ultrafast, outstripping the ionic process, which proceeds at a leisurely 800 fs or so. The kinetic energy characteristics of the I atoms even go so far as to reveal a near-axial geometry for the transition state. A similar approach has been adopted to show that the two-center elimination reaction (19) is a two-step, nonconcerted process evolving via the intermediate CzFJ (92):


147

Here the time-scales for the breaking of the two C-I bonds differ by two orders of magnitude: the primary step takes about 200 fs, whereas the secondary step occupies 25 ps. The speed of the first step reflects the repulsive force in the C-I bond caused by the promotion of a nonbonding electron from the highest occupied molecular orbital (HOMO) to the a * lowest unoccupied molecular orbital (LUMO), whereas the second elimination is governed by the energy redistribution attending the concerted fission of a C-I a-bond and making of a C-C .rr-bond. The high-resolution spectroscopic methods involving, for example, infrared diode lasers that have been successfully applied to the detailing of gaseous transients have also been adapted to the time domain (10). High temporal resolution cannot of course be reconciled with high spectral resolution, but measurements made a t even millisecond or microsecond intervals can still provide invaluable information about many reaction systems, with the bonus of unambiguous identification of intermediate species made possible by the high spectral definition. Reactions have been induced typically by irradiation with a high-power source, such as a n excimer or CO, laser, or on occasion by pulse radiolysis, and the subsequent course of events has been monitored by a suitable high-resolution spectroscopic probe. For example, Curl’s group a t Rice University in Houston, Texas has investigated the reaction between the radicals NH, and NO, which appears to proceed via more than one channel (93):

NH,

+ NO

P

N, t H,O

(a)

H + N,+OH

(b)

HN,(?) + OH

(c).

(20)

This is of considerable interest because of the key role of the reaction in the thermal “de-NO,” process involving the catalyzed reaction of ammonia with nitrogen oxides; hence, in principle, the gaseous emissions from industrial plants and power stations can be purged of the

148

DOWNS AND GREENE

noxious oxides, which are known to be major sources of smog and acid rain. Reaction (20) can be initiated by excimer laser photolysis of ammonia in the presence of NO and monitored by comparing the intensity of the infrared absorption due to NH3 with that due to NH, as functions of time, and also with those due to OH and H,O. Hence, the branching ratio of the OH channel or channels has been determined at different temperatures, with results suggesting that an additional channel may be implicated. Replacing the probe laser of these studies with electron pulses offers a means of imaging structural changes by electron diffraction, but observation of chemical changes in this way is nontrivial, and only recently have the odds against the experiment been significantly shortened (94-96). The essential experimental principles of “stroboscopic” electron diffraction were first devised more than a decade ago, with a pulsed electron beam being generated by photoemission with millisecond to picosecond time resolution. Major advances have come with the application of online data-recording techniques, involving either photodiode array or charge-coupled device (CCD) detection. Quite apart from the low intensity of coherent molecular scattering compared with incoherent background scattering, however, numerous problems remain to be solved. How, for example, is one to determine in situ the zero of time in a chemical change? And how is one to take account of molecular ensembles in nonequilibrium (that is, non-Boltzmann) vibrational distributions? In fact, a temporal resolution of 15 ns has been achieved with a pulsed laser-driven electron source. Hence, the structural and vibrational kinetics of CS have been explored during the first 120 ns following the photodissociation of CS, at A = 193 nm (94):

The observed changes of vibrational population with time have been interpreted on the basis of inelastic collisions of CS with sulfur atoms in the electronically excited ‘D state. Further hazards await any attempts to probe picosecond or subpicosecond changes by electron diffraction: space-charge effects broaden the electron pulse-width, and temporal overlap of the initiating photon pulse and the probe electron pulse must somehow be established. For ultrafast studies it is necessary then (i> to measure the electron pulse-width, (ii) to have the means of accurate clocking of the reaction, and (iii) to be able to detect single electrons (so as to reduce the electron flux and minimize space-


149

charge broadening). Zewail and his group have risen to this challenge by developing the new apparatus shown in Fig. 18 (96).This consists of a femtosecond laser, an ultrafast electron gun, a free-jet expansion source, and a newly designed two-dimensional single-electron detec-

Thermoelectric

I Fibre-optic raper

FIG. 18. ( a ) Ultrafast electron diffraction apparatus consisting of a n electron gun chamber, a diffraction chamber, and a detector chamber. Two fs laser pulses are used, one to initiate the chemical change and the second to generate the electron pulse. (b) Detector system: incident electrons either directly bombard a small CCD or strike a phosphor-coated fused fiber-optic window. Light emitted from the phosphor is amplified by an image intensifier and brought to a scientific-grade CCD. Both CCDs are thermop. 1601. electrically cooled [reproduced with permission from (96),

150

DOWNS AND GREENE

I

0

1

3

2

r

4

5

(A)

FIG.19. Radial distribution functions f ( r )and Af(r) derived from the molecular scattering curves for gaseous CHzIza t different delay times between the photodissociating fs laser pulse (at A = 310 nm) and the ps electron pulse (pulse-width 15 ps). The corresponding theoretical f ( r ) curve for CH212is supeorimposedon the -20-ps data set. The changes observed are a t r = ca. 2.0 and 3.5 A corresponding to the C-I and I----I internuclear spacings, respectively [reproduced with permission from (96), p. 1611.

tion system. Femtosecond laser pulses are created from a collidingpulse mode-locked ring dye laser, the output from which is directed through a four-stage pulsed dye amplifier with no pulse compression (620 nm, 2-3 mJ, 30 Hz, 300 fs). To initiate the reaction, 95% of this beam is doubled (310 nm, ca. 250 pJ);the remainder (also doubled) is focused on a back-illuminated, negatively biased photocathode to generate the electron pulses. Critical to the success of the experiment is the detector, a two-dimensional CCD operating in a direct electron bombardment mode. Hence, a temporal resolution in the picosecond range can be realised. The first such studies have monitored the photodissociation of CHJz a t A = 310 nm; the molecular scattering and radial distribution functions vary significantly with time (see Fig. 19, for example) in a manner wholly consistent with Reaction (22): CH&&

CHzI -t I.

(22)


151

A more detailed quantitative analysis may be expected to elucidate the structure of the CHJ intermediate. Modulation and difference detection techniques should provide a way of suppressing the background scattering from unreactive species, thereby enhancing the precision with which structure changes can be evaluated. To understand the whole of a photochemical pathway requires a knowledge of the excited electronic state or states (75, 97). In addition, some thermally activated processes proceed through the formation of intermediates or of products in excited states (witness, for example, the formation of singlet dioxygen). However, inorganic and organometallic molecules in these states are generally far too shortlived to be characterized directly by conventional spectroscopic observations, and there is no physical device open to us that will significantly extend their lifetimes. Figure 20 illustrates schematically a simple system involving just the ground and one excited state, and

f

Exated sfafe

I

2.

P

c

w

Resonance

Raman

Qk (distortioncoordinate)

FIG.20. Schematic representation of the potential energy curves for the ground and an excited electronic state of a molecule and some of the spectroscopic connections that can be made between them [reproduced with permission from (7.9, p. 1191.

152

DOWNS AND GREENE

some of the spectroscopic connections that can be made between them. In principle, the optical absorption and excitation spectra provide a means of probing the vibrational levels of the excited state, whereas the emission spectrum reports not only on the vibrational levels of the ground state but also on the lifetime of the excited state. The intensities of the vibrational components of these spectra contain the seeds of information about the distortion the molecule experiences with the switch from the ground to the excited state. Unfortunately, though, relatively few molecules, and then at low temperatures, afford the sort of fine structure needed to make this a realistic proposition. The metallocene intermediates (q5-C5H5)zM (M = W or Re) provide two cases in point, well-resolved vibrational progressions in the optical spectra of the matrix-isolated species admitting meaningful estimates of the changes in the metal-to-ring-centroid distance accompanying the electronic transition at lowest energy (61, 76).Resonance Raman spectra also give access to similar structural information. The case of the binuclear iron carbonyls (q5-C5R5)2Fez(C0)4 (R = H or Me) has already been alluded to (85), and similar studies have shown that the W-N and trans C-0 bonds of W(CO)5(pyridine)are extended by 18 and 12 pm, respectively, on excitation to the photoactive state (75). However, none of these methods involves probing of the excited state in real time, with the advantages this would bring to its structural and temporal delineation. One way of observing directly the vibrational spectrum of a molecule in an excited state is to engage TR3 measurements (801, as with W(CO),(diimine) complexes (diimine = 2,2'-bipyridine or related species) in the tungsten-todiimine charge-transfer excited state at lowest energy (98). The totally symmetric cis-carbonyl v ( C - 0 ) mode has a frequency about 50 cm-' higher than it does in the ground state, implying some reduction of metal-carbonyl a-back-bonding in the excited state. For all its merits, however, this approach is somewhat limited in its application, and there are real attractions in being able to record the infrared spectrum of a molecule in an excited electronic state. The new developments in TRIR spectroscopy now make this possible, at least in suitable cases. Thus, fast TRIR measurements are able to detect and monitor the lower metal-to-ligand charge-transfer state (MLCT where L = 4-cyanopyridine) of the tungsten pentacarbonyl complex W(C0)5(4-cyanopyridine),XI11 (99). Excitation causes the v(C-0) infrared bands to shift to higher frequency, confirming that the metal center is oxidized in the excited state. This state can be observed to decay, partly reverting to the ground state and partly with dissocia-


153

0

XI11

tion to form solvated W(CO)5and 4-cyanopyridine. The second reaction channel depends on rapid thermal equilibration between the MLCT and a second excited state, which is metal-centered or ligand field (LF) in nature, and it is this state that is dissociative with respect to separation of the 4-cyanopyridine ligand. confirmation of this comes from the finding that the yield of W(CO), varies with temperature, giving an estimate of some 4000 cm-' for the energy gap separating the lowest MLCT and LF states (see Fig. 21). This example illustrates the sort of subtle details that time-resolved studies are now able to chart with regard to excited states and the chemical processes they beget. About reactions occurring in the solid state there is not generally the same urgency that distinguishes so much of the chemistry of the more mobile phases. There may not be the same call for ultrafast LF

FIG.21. Scheme showing the low-energy states of the photosystem W(C0),(4-CNpyr) (4-CNpyr = 4-cyanopyridine).k , and k2 represent the sums of the radiative and nonradiative rate constants for the decay of the levels 1 and 2, respectively; kd is the rate constant for photodissociation to W(CO), and 4-CNpyr [reproduced with permission from (99), p. 35461.

154

DOWNS AND CREENE

methods of detection and analysis, but the ability to follow the evolution of a solid-state transformation is no less desirable for a proper kinetic and mechanistic understanding of the change. One such change currently attracting much interest is intercalation whereby mobile guest species enter a crystalline host lattice that contains an interconnected system of empty lattice sites (100). The staging of intercalation in Ago,,,TiS2and Hg,TiS2 has been effectively witnessed, for example, as a function of time by high-resolution transmission electron microscopy. There has been developed, in addition, a reaction cell that enables intercalation reactions to be monitored in situ using real-time X-ray diffraction techniques. Exploiting the high flux, white X-ray radiation from a suitable synchrotron radiation source and energy-dispersive diffraction techniques permits a large energy window of the powder X-ray diffraction spectrum to be recorded simultaneously at a single fixed detector angle, with acquisition times in the order of seconds. Figure 22 illustrates strikingly (100) the progressive intercalation of the electron-rich metallocene (T~’-C,H,),CO into a tin disulfide host. Still more dramatic is the very recent tour de force in protein science in which a group led by Michael Wulff and Keith Moffat at Chicago has virtually “watched” a protein function (101). Working at the European Synchrotron Radiation Facility, this team has devised a way of collecting pulsed Laue X-ray diffraction patterns with nanosecond time resolution. Hence, it has been possible to trace the structural changes that accompany the process of heme

FIG.22. A stack plot showing the evolution of the energy dispersive X-ray diffraction spectrum of SnS2 following the injection of (q5-C,H,),Co. Each spectrum took 10 s to record. The signals a t 25 and 29 keV arise from resonances from the tin K., and & core electrons and have been used to normalize the other signals [reproduced with permission from (IOO),p. 1831.


155

and protein relaxation brought about first by photodissociation and then by rebinding of the carbon monoxide complex of myoglobin.

V. Experimental Characterization of Reaction Intermediates: Flow and Other Methods

The basis of flow methods has been outlined earlier. We have to find a way of generating our intermediate continuously in a flowing system-sometimes by mixing gases or solutions, sometimes by the action of a discharge-in a way that can be matched to its kinetic properties, the flow rate, and the time-scale of the spectroscopic method of detection. Even here it may be necessary to keep to a minimum the time taken to record the spectrum. In the past much of the work with flowing gases relied on photographic recording of electronic emission or absorption spectra, but only for the smallest molecules are these likely to give detailed information about the vibrational and other properties of any intermediates. By contrast, current laser techniques allow much more subtle interrogation of transient molecules, both small and large, in flow systems, leading sometimes to the identification and precise characterization of these molecules ( l o ) ,as well as reporting on the kinetics of the reactions that feed upon them. The sort of apparatus that has been used for the study of a gaseous reaction initiated by a discharge is shown schematically in Fig. 23 (73).The time between initiation and reactant or product detection at some point downstream in the flow tube can be calculated if the velocity of the gas mixture is known. Detection and monitoring of the relevant species can be effected by various spectroscopic techniques, usually mass spectrometry, resonance fluorescence, laser-induced fluorescence, ESR, or laser magnetic resonance; high-resolution techniques such as microwave and infrared diode laser spectroscopy are also compatible with the analysis of reactive gas flows, although they lack the sensitivity of the more widely used methods which are better suited to kinetic studies. Resonance fluorescence is a well-tried method for tracking the concentrations of atomic transients (e.g., H, N, 0, F, C1, or Br), whereas laser-induced fluorescence (LIF) offers greater sensitivity and applicability, especially for molecular transients (e.g., OH, CN, or CH90), at the cost of greater experimental complexity. Laser magnetic resonance (LMR) employs a CO, laser to produce a variety of sharp rovibrational lines to one Qf which a rovibrational line of a radical may often be tuned by the application of

156

DOWNS AND GREENE

To

r able ctor

Excimer laser I

Trigger pulse

FIG.23. Discharge flow apparatus used to study reactions of OH radicals with detection by laser-induced fluorescence. The OH radicals are generated in situ by the reaction between H atoms (themselves formed by the action of a microwave discharge on a n H2/Hemixture) and NOz. The distance between the points of initiation and detection is vaned by moving the inlet tube [reproduced with permission from (73),p. 301.

a magnetic field. The biradical methylene is generated in its triplet electronic ground state by the reaction of oxygen atoms with ketene:

0 + CHz=C=O + 3CH, + O=C=O.

(23)

At slightly higher energies there exists a singlet state. Whereas singlet methylene can be detected by LIF, only the triplet state responds to LMR. Hence, it will be evident that the detection techniques are often complementary, and that no one technique is ideal for all transients. The reaction of 3CHzwith ethene,

3CHz+ CHz=CHz+ CH3CH=CHz,

(24)

157


has been studied by monitoring the LMR signal as a function of the distance between the mixing point and the laser detector with a range of different ethene concentrations; on this basis the relevant kinetic parameters have been evaluated (731. Flow methods have found frequent application in the direct study of C10 radicals and related species noteworthy as sources and reservoirs of the halogens in the atmosphere (17).In a typical experiment an inert carrier gas is pumped at pressures near 1-10 torr down a glass or metal flow tube; the first reactant may be added at a point upstream to the carrier flow, and the second reactant is added subsequently. The evolution of concentration with distance after mixing is then followed. Hence, for example, the reactions of C10 generated in a discharge-flow system have been explored, and mass spectrometric measurements have been used to determine the overall rate constant for its decay by the second-order process [Eq. (25a)l (102);resonance fluorescence associated with C1 atoms indicates that Reaction (25b) is the major pathway by which C10 decays. Mass spectrometric detection has also been used to determine the relative importance of the two channels [Eq. (25c)l followed by the reaction between C10 and the NO3 radical (103), whereas ESR detection of the C10 provides a means of exploring the kinetics of the reaction with HN03, which probably proceeds according to Eq. (25d), and so allows this to be discounted as having a significant role in the stratosphere (104).

+ C10 + C10 C10 + NO, C10 + HNO,

C10 C10

+=---

all products c1 + ClOO ClOO + NO2 OClO + NO2 HCl + NO2 + 0,. (d)

z twists around in which the ligand, 6,6””-dimethyl-quinquepyridine, the dinuclear core, with two nitrogens coordinated to one Ag and the other three nitrogens to the second Ag, adopting the usual [2 + 31

COPPERW AND SILVERU) SUPRAMOLECULAR METAL COMPLEXES

189

mode ( 4 3 ) .As expected, both silver(1) ions are five-coordinated with a flattened and distorted trigonal bipyramidal geometry. Interestingly, another quinquepyridine derivative with two further phenyl groups a t the 4‘ and 4”’positions forms a [4 + 41 helicate in [Ag{(MePh)zL15}zl (C10&, in which (MePh)zL15actually acts as a tetradentate ligand and the central pyridyl ring plays the role of a rigid spacer rather than a donor ( 4 3 ) . Because spacers linking the oligopyridine units have a notable impact on the versatility of the compounds in accommodating metal ions with different sizes and different geometries, one of the approaches for rational design of helical structures is to use aliphatic ethylene or aromatic 1,3-phenylene groups as spacers (3 6 ). For example, Lls with 173-phenylenea s a spacer linking two terpyridine units exhibits essentially a pentadentate coordination mode in [Ag2(L16)21(PF,)z ( 4 4 ) .The crystal structure reveals a [5 + 51 doublehelical bimetallic silver complex in which each Ag(1) adopts a n irregular five coordination. Terpyridine is the simplest oligopyridine capable of forming a double-stranded helicate. Copper(1) complexes with terpyridine derivatives have been reported. In [Cuz(L17)z1(PF6)2 ( 4 5 ) and [CUZ&~ZI(PF& (461, the ligands have essentially distributed themselves to present bidentate domains to one metal center and a single pyridine donor to the other, to give a [4 + 21 double helicate. The two copper atoms involve different coordination environments, one with a distorted tetrahedral geometry and the second in a approximately linear twocoordinate environment (Fig. 10). Dinuclear double helicates are too numerous to be fully discussed here. In contrast, tri, tetra, penta-, and polynuclear double-helical complexes of copper(1) and silver(1) appeared only in a limited num-

FIG.10. Schematic representation of the two copper atoms involving different coordination environments in a [4 + 21 double helicate.

190

MUNAKATA, WU, AND KURODA-SOWA

ber. A ligand with three bipyridiyl binding sites such as Llo can bind up to three cations, giving a trinuclear helical complex. Trinuclear double helicates [Cu3(Llo)zl(PF,)3 (47)and [Ag3(Lzo)21(CF3S03)3 (48) were prepared by treatment of sexipyridine and the relevant metal salts. Structure analysis and molecular modeling studies indicate that the two ligand groups fit three metal ions inside the doublehelical array by twisting around the helical axis [Fig. 2(d)]. Each metal center is coordinated to a pair of adjacent pyridine residues from each ligand in a distorted tetrahedral environment. The functionalized oligopyridines of Lz0with n = 4 and 5 formed tetra- and pentanuclear double helicates, respectively, with Ag(1) (48) and Cu(1) (49).Their overall structural features are analogous to those of the trinuclear double helicates, but their total length is estimated to be as long as around 22 A and 27 A, respectively. These self-organized nanostructures promise potential applications in the field of functional nanoscale species and molecular devices. Discrete infinite double helices are quite rare in inorganic and coordination chemistry [Fig. 2(e)l. Ciani has reported that reaction of Lzl with Ag' salts in the ratio 2 : 1gave completely different and noteworthy products, one of which is the infinite double-helical coordination polymer [Ag(Lz1)ICF3SO3 (50). The structure shows the balanced packing of left-handed and right-handed double helices of cationic -Ag-L-Ag-Lchains (Fig. 11). The period of the helices is and the two strands are bridged by weak Ag-Ag aurophlic 21.1 ( C P ~ - ~interactions. '~) The copper(1) complex [ C U ~ ( L ~ ~ ) ~ Ihas (C~O~)~ been prepared and structurally characterized (51). The basic dinuclear unit has a structure similar to that observed in [Cuz~L14~zlz+ (40). The ligands bind in a bis-monodentate coordination mode and form the strands of the helix that twist around the helical axis on

A,

FIG.11. Space-filling model of the infinite double helix of [Ag(L,,)ICF,SO,. (From Fig. 2 in Carlucci, L.; Ciani, G.; W. v. Gudenberg, D.; Proserpio, D. M. Inorg. Chern. 1997,36, 3812.)

COPPER(1) AND SILVERU) SUPRAMOLECULAR METAL COMPLEXES

19 1

which the copper ions lie. Each Cu(1) center is nominally linear coordinated by two imidazole units, and the interactions with the two bridging pyridines are weak. The double-helical subunits stack together in parallel columns of constant but different helicity, leading to infinite double-helical columns. Chiral helical complexes are of interest in connection with the manifestation of functions such as optical activity, molecular recognition, and enantioselective catalysis (52-56). With spirobisindanol and dimethylbiphenic acid as chiral templates, Siege1 has successfully obtained bipyridine based double helicates containing up to three copper(1) ions by enanthoselective synthesis (57). The spectroscopic results confirmed the complex as a single enantiomer of the head-tohead isomer whose stereochemistry is controlled by the single stereoelement of the template throughout a span of 20 A, demonstrating the ability to transmit stereochemical information over nanometer distances in the compounds. Chiral double-helical structures of copper(I1)-L- and -D-arginine complexes with aromatic dicarboxylates have been reported, although the corresponding copper(1) species are not known (58). These crystals reveal that the complex with L-Arg forms a right-handed helix, whereas the complex involving D - h g gives a left-handed helix [Fig. 2(r?l. This indicates that the handedness of the double helices in the solid state is governed by ligand chirality. These and other synthetic routes open new ways for development of enantioselective reagents and may contribute to the understanding of spontaneous aggregation of conjugated helical molecules in biological systems (59, 60).

C. TRIPLE-HELICAL COMPLEXES There is only one example of a triple helix containing Ag(I), reported recently by Williams and his co-workers (61). The colorless crystals [Ag3(L23)51(BF4)3 were obtained by diffusion of benzene into a solution of the compound in acetonitrile. The structure consists of an equilateral triangle of silver ions with the ligands bridging the sides of the triangle [Fig. 2(g)l. The silver ion is not in a strictly linear environment, with an N-Ag-N angle of 153.3(6)”. Each ligand binds to one metal from below the plane of the silver atoms and to a second metal from above the plane. The structure may be considered as a triple helix in which the ligands wrap around the threefold axis and are held in place by coordination to the Ag, triangle.

192


Ill. S - - 4Contact-Assembled Frameworks

Some sulfur-rich dithiolates and the tetrathiafulvalene (TTFI-based species involved in the following discussion are listed in Fig. 12. Following the observation that TTF (L2&forms a stable radical when treated with chlorine (62) and the discovery that a partial charge transfer between separately stacked donor and acceptor molecules leads to the first organic metal, TTF * TCNQ (tetracyanoquinodimethane) (63), the use of sulfur-containing molecules as precursors for synthesis of conductive or superconductive materials has been of unabated interest for over two decades. Because the large sulfur atomic orbitals are capable of promoting effective intermolecular overlap in the organic metals, partial oxidation of TTF and its substituted derivatives, when coupled with suitable electron acceptors, can give rise to low-dimensional arrays of these donors through sulfur-sulfur molecular interactions, providing an effective pathway for electronic conduction. A tremendous number of studies have been carried out on the

L27

M e 0 2 C f ~ s Me02C

s

LZ8

s-

'Xs F3c1sNcxs S

S

F&

L29

S'

NC

S

L30

L31

.=(Ise-

0

Se'

S

0 L32

L33

L36

L34

L35

lTC,-lTF

FIG. 12. Sulfur-rich dithiolates and TTF derivatives.

L38

COPPER(I) AND SILVERU) SUPRAMOLECULAR METAL COMPLEXES

193

solid-state conductive properties of the TTF . TCNQ-like organic and inorganic metals by modifications of TTF (64-66). By contrast, the study of TTF-based species as building blocks in supramolecular chemistry has attracted attention only recently (67). With a preference for a soft donor such as the sulfur atom, both silver(1) and copper(1) readily form metal complexes with the sulfur-rich TTF-based organic molecule, with a high possibility of generating one- two- and three-dimensional supramolecular and macromolecular system by the covalent bonds as well as by S----Scontacts. In general, the S----Scontacts considered to be effective in sulfur-rich conductive compoundso refer to those having interplanar S----S separations a t 3.35-3.75 A, close to the sum of two sulfur-atom van der Waals radii (3.60A). This section focuses on construction of the supramolecular frameworks through S----Scontacts based on dmit, BEDT-TTF, TTC,TTF, C5H4S,and the related species.

A. dmit

AND THE

RELATED LIGANDS

The sulfur-rich 1,3-dithiole-2-thione-4,5-dithiolate (dmit) (LZ5)has received considerable attention. Using square planar coordinating metal ions such as Ni, Pd, and Pt, many electrically conducting materials derived from planar metal complexes [M(dmit)J- have been synthesized, and some of them have been found to undergo a superconducting transition at low temperature by applying high pressure. The nonplanar bulky metal complexes [M(drnit),l"- (M = Fe, Re, Mo, or W) have received equal attention. Numerous papers and reviews have focused on the study of the role of the intermolecular and intramolecular interactions in determining the electrical properties in the system (68-83). As a n example, Fig. 13 shows a uniform segregated stacking arrangement of the constituent Ni(dmitI2 units with the open-shell 7r-acceptor TTF and the closed-shell cation (CH3I4Nin the superconductors (TTF)[Ni(drnit)& (84) and [(CH,),N][Ni(dmit)zlz(851, respectively, where close intermolecular S----Scontacts occur between units in adjacent stacks leading to a n infinite network. Furthermore, extensive studies have also been carried out by using other ligands resembling dmit, such as L26-L33 (86-92). Along the same line, the selenium analogs (L34and L35)are also of much interest, because even more effective molecular interactions are expected to occur owing to Se having more extended orbitals than sulfur (93,941.

Although a number of Cu(I1) complexes with the dmit-like ligands have been reported in which the metal ion involves a significant dis-

194


0

b

(b) FIG. 13. Molecular packing of superconductors (TTF)[Ni(Lp)& (a) and [(CHAN] [Ni(La&lz(b). (From Figs. 5 and 6 in Bousseau, M.; Valade, L.; Legros, J.-P.; Cassoux, P.; Garbauskas, M.; Interrante, L. V. J. Am. Chem. SOC.1986, 108, 1908, and Figs. 1 and 4 in Kim, H.; Kobayashi, A.; Sasaki, Y.; Kato, R.; Kobayashi, H. Chem. Lett. 1987, 1799.)

COPPERW AND SILVERU) SUPRAMOLECULARMETAL COMPLEXES

195

tortion from planar to tetrahedral (73, 83, 89, 931, few nonplanar Cu(1) or Ag(1) metal complexes of dmit are known. The most interesting of these is [mpy]z[Cu4(dmit)31, prepared, like its Cu(I1) analog, by a route of direct reaction of coppert11 salt and Nazdmit in the presence of excess of [mpylI (95). The structure contains dimerized anion units, each consisting of a tetranuclear Cuds6cluster. Each Cu(1) ion involves a distorted tetrahedral coordination comprised of four dmitsulfur atoms. The dimeic units further interact with each other through close sulfur-sulfur contacts to form a two-dimensional molecular interaction sheet. The cations are located between the anion sheets. B. BEDT-TTF BEDT-TTF (L36) is another electron donor molecule and in combination with inorganic anions has provided several air-stable superconductors at low temperature (96-99). In fact, both dmit and L36 superconductors have the characters of (i) good orbital overlap between extended r-electron systems (S----Scontacts) and (ii) partial filling of the conduction band through either partial oxidation or partial reduction. Here again the radical cation salts of square planar transition metal complexes dominate, and they feature S----S interactions between planar L36'+ molecules leading to segregated columns of donors. On the other hand, the reported copper(1) and silver(1) coordination complexes of L36 are scarce. They include (LSt)Cu2Br3,prepared by a redox reaction between L36 and [CuBr4I2-(100).The most unique feature of the complex is that the L36 molecules are not stacked in a column but are coordinated to the tetrahedral copper(1) centers in a -Cu-Br-Cu-(p-Br)z-Cuchain (Fig. 14). Due to lack of close S----Scontacts between radical cations, the complex shows low conductivity .

FIG.14. Structure of (L,+)[Cu2Br31.

196


C. TTC,-TTF SYSTEMS Several copper(1) and silver(1) complexes with tetrakis(alky1thio) tetrathiafulvalene (TTC,-TTF) (L3J have appeared in the literature, and various S----Scontact-assembled frameworks have been observed. L37 contains a central TTF (L2J r-system, which keeps the electrondonor property. As single-component organic semiconductors with low electrical conductivity, their physical properties have been widely investigated. They can be easily oxidized to the stable radical cation; accordingly, their chemistry is dominated by electrotransfer processes. They react with organic electron acceptors such as TCNQ and HCBD (hexacyanobutadiene), and inorganic oxidants to form partially or completely oxidized materials containing L37’+, where p ranges from about 0.6 to 2.0 (101-104).In most of these chargetransfer compounds the donors and acceptors are stacked in segregated columns. Construction of two- and three-dimensional coordination polymers using the diversity of copper halide frameworks is an area of considerable importance. Besides its electron-donor character, Lg7possesses alkylthio groups, which have coordination ability to metal ions through the sulfur atoms. This gives Lg7species a unique ability in linking metal ions to form coordination polymer structures. Six Lg7 complexes with copper(1) halides, [(CUX)~(L~,)] (where n = 1 or 2 and X = C1-, Br-, or I-), have been structurally characterized (105-107). The syntheses of these compounds are straightforward, usually carried out by direct reaction of copper(1)halide and Lg7in a nonaqueous solution. The structural determination of these complexes reveals three different copper halide frames, namely rhomb, helix, and zigzag chain (Table 11).Although the metal ions in all cases involve a tetrahedral coordination comprising two halogen atoms and two sulfur atoms of the Lg7moiety, with a certain degree of distortion, which in turn acts as a bidbidentatel linker between the two metal centers through alkyl thioether groups, the resulting compounds often exhibit for exnovel structures dramatically changed by the halogen. In LgTa, ample (1051,the chloride complex consists of novel two-dimensional sheets of L37a molecules arranged between zigzag frames of {CuCl}, . Within the two-dimensional sheet no significant S----Scontacts are observed because the long Cu----Cuseparation of 5.78 A on the same side of the CuCl zigzag frame gives a long interplanar spacing (4.90 A) of Lg7,,. However, between the parallel sheets there are close intergiving a three-dimensional netmolecular S----Scontacts of 3.53 work (Fig. 15). By contrast, a novel framework of {CuBr},, helixes

A,

COPPER(I) AND SILVER^ SUPRAMOLECULAR METAL COMPLEXES

197

TABLE I1 BONDANGLESAND CuX FRAMEWOKKS I N TTC,,-TTF(Ls,) COMPLEXES OF COPPEKU) HALIDE 01

P

Y

6

113.4 116.0" 109.1" 112.6"

113.1" 111.8 113.6 106.5

88.8 89.9 91.1"

116.1 116.1

116.8 112.5

111.8

113.2

CuX framework

Ref.

rhomb rhomb rhomb rhomb

105 107 106

87.1 86.9

helix helix

105 106

90.9

zigzag chain

105

108

Average value.

< =c,X

-c u

X

rhomb

infinite helix

zigzag chain

which are further linked to each other by coordination of L37ato the metal was observed in the bromide complex. The L37amolecules are stacked together, with the dihedral angle between the neighboring molecules being essentially zero on the stacking axis. The unique feature of the structure is that the {CuBr}, helixes orientate alternately in the reverse direction and the L37amolecular stacks are mutually perpendicular to each other along the c-axis. The S----S contacts of 3.68 A between the stacked L37amolecules lead to a two-dimensional framework. The iodide complex contains an infinite chain of the bent LS7, molecules perpendicularly bridged on the methyl thio2ther groups by rhomboid CuzIzmoieties. The S----Sdistances of 3.75 A between the chains indicate weak interaction present, associating the adjacent chains of -L-Cfi-Ltogether, giving a two-dimensional network. Similar results have been observed in the corresponding LS7,, complexes (106, 1071, in which the chloride complex involves helical

198


3.53.i

GI

!

12Dsheet

I

-

I uc’ --uc

II

CI I

C-ICU-

I

I

I

FIG.15. Schematic views of different frames of CuX in [(CuX),(L,,,)I complexes.

{CuCl}, frames whereas the bromide and the iodide both contain CuzXzrhombs. Bond angles of copper(1) halide complexes with LSmare summarized in Table 11. It can be seen that the main factor determining the framework of the CuX unit seems to be connected with the Cu-X-Cu angle (a).As a increases smoothly from 66.6 to 1 1 1 3 , the framework changes from rhomb to zigzag chain through infinite helix, whereas other angles do not show such correlation. The L37a+ cation coordination complex (L37,)[CuBrz1zalso falls nicely into this category (108). As a comparison, it is instructive to briefly mention the corresponding LS7b+complexes of copper(I1) halide (L3&[Cu2&1 (where X = C1or Br-) prepared by oxidation of L3%with CuC12 or CuBrz (109). Unlike the copper(1) halide complexes with LS7bridging between two metal atoms, both structures consist of two segregated stacks of L3%+ donors and CU&~- acceptors associated via short S----Scontacts (Fig.

COPPER(1) AND SILVER(1) SUPRAMOLECULAR METAL COMPLEXES

199

FIG. 16. Packing diagram of (L,,)[Cu,CI,I showing S----Sand S----Cl.(From Fig. 1 and Fig. 2 in Wu, L. P.; Gan, X.; Munakata, M.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y. Mol. Cryst. Liq. Cryst. 1996,285, 75.)

16). The donors face to each other to assume a dimeric structure, with close intradimeric S - - 4conducts of 3.43-3.52 A. Consistent with the absence of significant interdimeric S----S interactions, the two compounds exhibit low electric conductivity. The diversity of copper halide frameworks, affected by several elements such as the metal ions, coordination of the halogen atoms, and the stereofactors of the ligand, makes it possible to design two- and three-dimensional coordination polymers. Copper(1) perchlorate and tetrafluoroborate salts have also been found to form coordination compounds with the neutral ligands L3, (n = 2 or 3), but the structures vary little due to nonparticipation of the counteranions C10,- and BF,- for their weak coordinating ability (110, 111). In fact, they are 1:1 metal-ligand compounds in which

200


each Cu(1) ion is tetrahedrally coordinated by four sulfur atoms from the bridging Lg7molecules to form infinite polymeric chains separated from each other by the anions. The shortest S----Sdistances between suggesting negligible the adjacent chains in all cases are over 3.8 S----Sinteractions in the system. Same conclusion can be drawn from the Ls7, complex [Ag(L3,,)(03SCF3)I(1121, in which each Ag(1) ion is six-coordinated by four sulfur atoms from the bridging Lg7*and two oxygen atoms from the bidentate 03SCF3anion, resulting in a structure of two interwoven polymericochains. The closest S----Sdistance between the two chains is 4.00 A, precluding any significant S----S contacts present. Apart from the varieties of interesting structures observed in the Lg7system, it is worth noting the preparation of these complexes and the conductivity of the Iz-doped species. As mentioned previously, these complexes are easily prepared without elaborate synthetic procedures or particular precautions. However, successful preparation of the single crystals of these complexes largely depends on the reaction temperature. Again take Lg7,, for example. The preparation was carried out by allowing the mixture solution of CuX salt and L37ato stand in a water bath at a constant temperature of 70, 55, and 23°C for the chloride, bromide, and iodide, respectively (105).The same reactions performed at other nondesignated temperatures may lead to a mixture of precipitate and tiny single crystals unsuitable for X-ray analysis. This is also true for the preparation of the corresponding L,,, complexes (106,107).Iodine oxidation is a viable synthetic route to highly conducting mixed-valence materials. Although copper(1) and < silver(1) complexes with neutral Lg7 ligands are insulators S . cm-') at room temperature, their iodine-doped products behave as semiconductors, with conductivities increasing in the order [(CUC~)Z(L~~)I < [(CuBr),(L,,)I < [ ( C U I ) ~ ( L(Fig. ~ ~ ) I17). A partial oxidation can be assumed to occur in the 12-dopedspecies and the mixedvalence ligand-ligand interactions might cause electrical conduction pathways.

A,

4,5-Ethylenedithio-l,3-dithiole-2-thione (C5H,S5)(Lss)is well known as an electron donor (113, 114).It is also a derivative of dmit and has a structure similar to half of LSs. The structure determination of its metal complexes has demonstrated the unique coordination versatility of Lg8(Table 111). It can act as a monodentate, bidentate, or even


201

-1.5

-1.7 -2.1

E

-

%? D

-2.5

-rn 0

lrdoping

.-c>i .-

12-doping

c)

0

3

'0 C

8

-3.6 -

-3.5

orange crystal

= CIg

Bry

I

orange crystal

orange crystal

L37a = tetrakis(methy1thio)tetrathiafulvalene

FIG.17. Preparation scheme and conductivity of [(CuX)2(Ls~,)l.

tridentate ligand, binding one and bridging two or three metal ions through the thiocarbonyl group alone or by both thiocarbonyl and thioether groups. Up to now, three silver(1) and one copper(1) complexes of L38have been synthesized and crystallographically characterized. They display a variety of one- to three-dimensional frameworks based on coordination bonds as well as intermolecular S----S contacts. Reaction of AgC104.H20 and L,, in acetonitrile gave red brick crystals of {[Ag(L38)31C104~CH3CN}2 (115).The compound has a dimeric structure

202


TABLE 111 COORDINATION MODESOF C5H4S5(LS), THE METALCOMPLEX STRUCTURE AND THE CONDUCTIVITY OF THE I 2 - D 0 pPRODUCTS ~~ Mode"

Structure of the complex

Conductivity S . cm-'

Ref.

[Agz(Lss)J(C104)~ I, (11) 111 [Ag(Lsd,NO,I [ A ~ ( L s ~ ) C F ~ S O ~ IV I I, I11 [CuaI4(Lsa)al

dimer infinite dinuclear chain 2-D sheet infinite tetranuclear chain

6.9 X 3.0 x 10-5 1.5 x 10-4 2.2 x

115 116 115 117

Complex

_______

~

Coordination modes of C5H4S5.

n s

g

sKs S

IM

mode I

A

kS-" sHs-M A

A

sKs

sKs

s)-+s

sKs JSI

M M mode I1

S

IM mode 111

JSI

M M mode IV

with two nonequivalent intercoordinated quasi-trigonal [Ag(L3&1+ units, which are further assembled by very close S--------S contacts of 3.284(4)-3.569(2) to form a one-dimensional chain structure (Fig. 18).Replacement of the anion by triflate in preparation gave a poly-

A

FIG.18. Schematic representation of S----Scontacts in {[Ag(LS)31C104. CH,CN}, .


203

nuclear complex with a different stoichiometry, [Ag(L,,)CF,SO,I ( I 15). The structure contains two crystallographically independent five-coordinate silver(1) ions, very rare stereochemistry for Ag(1). Both the L3, group and the CF,SO, ion bridge between two Ag atoms, forming a one-dimensional chain along the c-axis. All the ligand groups are approximately perpendicular to the chain, and between the c h a i p there are several close S----Scontacts (the shortest one being 3.262 A), resulting in a two-dimensional polymer structure (Fig. 19). The third silver(1) complex belonging to this family is [Ag(L,,),NO31 (116).Each L,, displays a bidentate fashion through both thiocarbonyl sulfur and thioether sulfur atoms bridging two octahedral silver atoms, giving a n infinite chain. The strong S----Sinteractions with a distance of 3.41 A between two nearby chains along the b-axis generate a twodimensional network. The corresponding copper(1) complex [ C U ~ I ~ ( L ,was ~ ) ~prepared I by reaction of L3, and CuI in acetonitrile (117). The structure consists of stepped Cu414clusters bridged by L3, groups, and the resultant polymeric chains are further assembled by short interchain S----Scontacts (3.257 A)to form a three-dimensional network (Fig. 20). Unusually short S----S contacts have been noticed among theo L38 complexes; for example, 3.23 A in (TTF)[Pt(L3&I3 and 3.26 A in (TTF)[Pd(L3d212,[Ag(L3dCF3S031, and [ C U ~ I ~ ( L(115). ~ ~ ) ~ The I extremely short S----Scontacts are attributable to metal-metal bonding in the first two cases, and to the coordination bond linkage as well as the effect of molecular packing in the rest. The copper(1) and silver(1) complexes of L3, were partially oxidized by iodine doping, and the electrical resistivity of the compressed pellet of the compounds as measured by the conventional two-probe technique. Like those of L3,, they are nonconducting a t room temperature but display semiconduc-

FIG. 19. Schematic representation of S----Scontact assembled network in IAg(Ls8) (CF8OdI.

204


54

D: S9...S9' (3.62A)

FIG.20. Structure of rCu4I4(L8&1.

tivity after Iz doping, with conductivities of 10-4-10-5 Sacm- (Table 111).

IV. Hexagonal Frameworks and Graphite-like Structures

The scaffolding and related microporous materials are receiving widespread interdisciplinary attention, and the borders between chemistry, physics, and materials science are therefore vague and

COPPER^ AND SILVER^ SUPRAMOLECULAR METAL COMPLEXES

205

subjective. Several general reviews and textbooks have provided a comprehensive overview of the chemistry and structural properties of these materials (118-120). The literature relating to assembly of copper(1) and silver(1) complexes into network structures with a graphite-related lattice has reflected a general interest and advancement in our understanding of the fundamental chemistry and processing characteristics of the system and its potential applications as molecular sieves, catalysts, and optical materials. This section is concerned with construction of hexagonal and graphite-like lattices regulated by the metal-ion stereochemistry, the ligand, and the counteranions, placing emphasis on the nitrogen-containing systems such as pyrazine, phenazine, 2,1,3-benzothiadiazoles,and quinoxaline (quin) (Fig. 21). Metal cyanide will be treated in a cursory manner, leaving the readers to consult recent literature and reviews.

A. METALCYANIDE-REGULATED BY THE METAL ION Metal cyanide, which has received little attention for years in many texts concerned with inorganic structures, is now found to possess the most outstanding feature as a building block for infinite polymeric frameworks. Some rather elegant metal cyanide structures with extended frameworks have recently been discovered (121-128). The synthetic strategy is based on the combination of a diatomic bifunctional rodlike CN- ion with a considerable preference for binding metals at each end in a linear fashion together with modification of

L39

(Pz)

L40

L43

L 41

L45

L44

L46 (phz)

L42

L47

(bW

L48

FIG.21. Some nitrogen-containing ligands in copper(1) and silver(1) complexes having hexagonal frameworks.

206


the frameworks by metal ions favoring different stereochemistries. The crystal structures of [N(CH,),I[Cu'Zn"(CN)~]and two isomorphous compounds, Zn(CNI2 and Cd(CNI2,have shown that the tetrahedral metal ions favor formation of interpenetrated diamondlike frameworks (127). A three-dimensional honeycomb structure containing hexagonal channels has been observed in complexes consisting of simple cyano-cadmium frameworks in which the metal centers are of two types: octahedral and distorted five-coordinate or octahedral and tetrahedral (122,128).By replacing CN bridges in multidimensional CNlinked octahedral cadmium complexes with linear NC-Ag-CN or NC-Ag(CN)-Ag-CN units, Iwamoto has successfully obtained two cadmium-silver complexes with double and triple interpenetrating frameworks (123). An infinite three-dimensional framework related to PtS has been observed in [NMe,l[CuPt(CN),], in which tetrahedral Cu(1) and the square planar Pt(I1) centers are linked together by CN rods, generating two mutually perpendicular and equivalent sets of hexagonal channels of large cross section together with large empty square channels perpendicular to the hexagonal channels as shown in Fig. 22 (121).

B. PYRAZINE SYSTEMS-REGULATED BY SUBSTITUENTS Pyrazine (LS9)and substituted pyrazines have long been known to act as em-didentate ligands to linearly bridge metal ions, generating oligomeric and polymeric metal complexes with infinite chain and

FIG. 22. Extended framework containing hexagonal and square channels observed i n [CuPt(CN)J:-. (From Fig. 2 in Gable, R. W.; Hoskins, B. F.; Robson, R. J. Chem. SOC. Chem. Commun. 1990, 762.)

207


pleated sheet structures, double and triple interpenetrating frameworks, and interwoven honeycomb architecture (123, 129-132 ). The essential features of some structurally characterized copper(1)and silver(1) complexes with pyrazine and its derivatives including quinoxaline and phenazine are summarized in Table IV. Among them those containing two-dimensional six-membered rings of metal ions linked by LBSbridges are rather common and thus have received considerable attention recently. From the point of view of rational synthesis, it would be possible to obtain polymeric graphite-related cationic lattices in which the three-coordinate metal ions are bonded only to bridging LBSligands by utilizing noncoordinating anions as the counterions in the synthesis of LSs complexes. However, owing to the effects of substituents, the six-membered rings obtained differ in detail from complex to complex in terms of distortion of planarity, which can be grouped under four heads as shown in Fig. 23. It has been suggested that more than two substituents on a pyrazine ring would hinder four coordination of the metal ion, giving infinite zigzag chains or sheets of the cross-linked chains as observed in disubstituted pyrazine systems (135).Therefore, controlling spatial factors plays a key role in building infinite polymeric structures, and the modification of linking ligands by substituents on pyrazine is a practical method for the synthesis of designed copper(1) polymers. Slightly distorted hexagonal frameworks defined by six LBS-bridged copper(1)ions are observed in [CU~(L,,,)~(PF,)~I (L40= 2,5-dimethylpyrTABLE IV Cu(1)AND Ac(1) COMPLEXESWITH PYRAZINE AND ITS DERIVATIVES Compound

CN"

Frameworks

Ref. ~

[Cuz(L,o)dPFs)zl four three [cuz(L41)31(c1o4)2 [Cu2(~~31(c104~, three [Cuz(Lsd31SiFs three ~ C ~ Z ( L ~ , ~ ~ I ( C I O , ~ ~ ( M four, ~ ~ C five O)~ [C~,(LS~)~(CH~CN)ZI(PF~)Z four four [CUZ(L,)4 sl(ClO4)z four [cu(L,o)zlPF~ [Cu,(4,)31(C104)z three, two ~cuz~L4~~31~c1o~~1CIo4 four [Agz(La)(NO3)zI three [Agz(LddBFdz three

2-D hexagonal 2-D hexagonal 2-D hexagonal 2-D hexagonal 2-D hexagonal 2-D hexagonal 2-D hexagonal, square 3-D adamantane linear 3-D adamantane 2-D hexagonal 2-D hexagonal, 3-D interpenetrating nets

' CN, coordination numbers of the metal ions.

133 134 135 136 29 137 135 133 137 132 138 130

208


FIG.23. Cu(1) and Ag(1) complexes of pyrazine and the derivatives having hexagonal frameworks: (a) slightly distorted hexagon; (b) distorted hexagon and square; (c) zigzag 2-D sheet; (d) ternary complex. The molecule in the box represents the ligand involved.

azine) (133), [ C U ~ ( L ~ ~ ) ~ ] (L41 ) ( C ~=O2,3-dimethylpyrazine) ~)~ (1341, [ C U ~ ( L ~ ~ ) ; J ((L42 C ~ O=~2,6-dimethylpyrazine) )~ (135), [Cu2(L3&lSiF6 (136),and [ C U ~ ( L ~ , ) , ] ( M ~ (L43 ~ C O=) ~pyrazineamide) (29).Structural studies have established trigonal environments for the metal. These complexes show an infinite sheet structure of macrocations with uninteracting or weakly interacting counterions. If the interaction between the metal and the anion is ignored, the Cu(1) ion in these com-

COPPER(1) AND SILVER(I1 SUPRAMOLECULAR METAL COMPLEXES

209

plexes is essentially in a three-coordinate trigonal planar geometry and the pyrazine ligands bridge adjacent trigonal metal centers to form a graphite-like lattice, Fig. 23(a). Thompson has successfully isolated two Cu(1) complexes from the same reaction mixture of an ethanolic solution of Cu(PF& and L 4 0 , [Cu2(L40)31(PF,)2 and [ C U ( L ~ ~ ) , I P F ~ (133).The structure of the former is made up of parallel puckered layers of the cation, and within each layer the copper(1) ions are linked by L40,defining hexagons. PF, anions are located between the layers and interact very weakly with the copper(1) ions through one F atom, leaving the Cu(1)ions essentially three coordinate. Except for the puckering of the layers, the structure is reminiscent of that of the intercalated graphite. By contrast, each metal ion in the latter case is bonded to four rather than three L40 molecules, with an angularly distorted CuN4chromophore. As a result, the structure exhibits a diamondoid framework of copper(1)ions linked by the rodlike L 4 0 ligands, and the PF; anions are located in cavities within the lattice. Likewise, the reaction of Cu(C104)2with L41 in an aqueous solution yields an air-stable complex, [ C U ~ ( L ~ ~ ) ~ (whose C ~ O ~structure ) ~ ] , is also found to contain a similar two-dimensional graphite lattice of tricoordinate copper(1) atoms bridged by L1lligands (134).Although the exact course for reduction of copper(I1) complexes with nonchelating ligands is not clear, the most plausible explanation lies in tetrahedral distortion of the square planar copper(I1) species by crowding effects of the methyl groups, and consequently, such reduction process is dependent on the substituents, temperature, and solvents (233, 134 ). On the other hand, copper(1)complexes of L39can also be prepared by direct reaction of copper(1) salts, [CU(CH,CN)~]X, and L3, ligands in nonaqueous solutions, such as [CU,(L~~),]S~F, (136).Here again the L3, ligands bridge adjacent trigonal Cu' centers to form hexagonal frameworks with bridged copper cations 6.936 and 6.685 A apart. The SiFi- anions are located in channels and are hydrogen bonded to L3, hydrogen atoms. The most significant feature of the complex is that the hexagonal frameworks are interwoven as shown in Fig. 24. The three-coordinate copper(1)ions can also be linked by the disubstituted Ls9on the 2- and 6-positions as observed in [ C U ~ ( L ~ ~ ) ~ , I ( C ~ O ~ ) , (135).The graphite-like lattice is constructed by the alternating arrangement of the two crystallographically independent three-coordinate copper atoms. The hexagons generated by the Cufi units are slightly distorted due to compression, and the mutually confronted Lea molecules on the sides of the hexagon are sitting out of the plane and parallel to each other, providing a large cavity. It is surprising that neither the anion nor the solvent molecule is included in the cavity,

2 10


.......

I................._

i cuqubcu = 1 1

0

LStl C(

:i

/CU-dcu,

u

f c-cu*k

...........................

FIG.24. Schematic view of the interwoven honeycomb grids in [ C U ~ ( L ~ ~ ) (From ~IS~F~. Fig. 4 in MacGillvray, L. R.; Subramanian, S.; Zaworotko, M. J. J. Chem. SOC.Chem. Commun. 1994, 1325.)

they rather form acetone-perchlorate layers intercalated between the copper sheets. Formation of trigonal copper(1) polymers with dimethyl-substituted pyrazines is presumably due to the two substituents on the L3,ring, which hinder the formation of tetrahedral coordination of the metal. No examples of hexagonal frameworks are found to be built up by tetrahedral copper(1) ions. This is understandable because formation of graphite-like sheets would require Cu6motifs on a plane with each metal center linked to three neighbors. Therefore, trigonal, trigonal pyramidal, and trigonal bipyramidal stereochemistries are necessary. The recently characterized copper(1)complex [ C U Z ( L ~ ~ ) ~ ~ ( C ~ ~ ~ ) ~ ( provides the first example of mixed four- and five-coordinate copper(1) ions composing such a hexagon (29). As illustrated in Fig. 25, the cation contains two crystallographically independent copper(1) ions; one involves a distorted trigonal pyramidal CuN30 core composed of the terminal pyrazine nitrogens of three different L43molecules and an axial caboxamide oxygen, and the second involves a distorted trigonal bipyramidal structure with two, rather than one, carboxamide molecule exhiboxygen atoms occupying the apical positions. Each L43


2 11

cu FIG.25. Molecular structure of [ C U ~ ( L ~ ~ ) ~ I ( C I O ~ )in~ which ( M ~ ~the C Ofour) ~ an d fivecoordinate Cu(1) ions are bridged by the tridentate La groups (a) giving a hexagonal channel structure with pores open to accommodate (2104 ions and acetone molecules (b). (From Figs. 1 and 2 in Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Moriwaki, K.; Kitagawa, S. Znorg. Chem. 1997,36, 5416.)

its a tridentate coordination mode, bridging two metal centers, giving apparently hexagonal channels with pores open to accommodate the anions and acetone molecules as guests. The complexes falling into other groups as shown in Fig. 23(b)-(d), are significantly less numerous. The chlorine-substituted pyrazine is

212


reported to form a coordination polymer containing both distorted hexagonal and square frameworks. The reaction of 2-chloropyrazine (La) with [Cu(CzH4)C1O4] in acetone yielded orange brick crystals, . Structural studies have shown both copper(1) ions [CU,(L~~),,I(C~O~), involved tetrahedral environments (135).As L3g and the dimethylsubstituted derivatives do, each L, molecule exhibits a bidentate coordination bridging two metal centers forming a square planar Cu4 framework. These square planes are linked to each other by sharing the edges to afford one dimensional ribbons. The adjacent ribbons are further bridged by L44,leading to two-dimensional pleated hexagonal Cue sheets (Fig. 26). The results further demonstrate that in metal complexes of LSgderivatives, the substituents on pyrazine can greatly

L

Cu(.

FIG. 26. Schematic drawing of infinite sheets in [CU~(L&](C~O~)~. Copper atoms and bridging Ld4molecules are denoted by open circles and solid lines, respectively. Each pyrazine ring is distinguished by alphabetical label. (From Fig. 2 in Kitagawa, S.; Kawata, S.; Kondo, M.; Nozaka, Y.; Munakata, M. Bull. Chem. SOC.Jpn. 1993, 66, 3387.1

COPPERU) AND SILVER(J) SUPRAMOLECULAR METAL COMPLEXES

2 13

affect the stereochemistry of the metal, which in turn plays a key role in determining the framework of the structure. The copper(1) complexes with pyrazine and tetramethylpyrazine (LQ5) may be the best examples to elucidate the steric effects imposed on the substituted pyrazine toward rational synthesis of copper pyrazine polymeric complexes in the single-crystal phase (137).The single crystals of both [ C U ~ ( L ~ ~ ) ~ ( C H S C Nand ) ~ ~[( C P FU~~)(~L ~ ~ ) ~ I ( C ~ O ~ ) were prepared by reaction of the corresponding ligand and the copper(1) salt in acetone. In contrast to the disubstituted pyrazine complexes, in which the metal ions in most cases are found to be in a trigonal environments, the geometry around the copper atom in [CU~(L,~),(CH~CN)~I(PF,), is a distorted tetrahedron comprising three L30 nitrogens and one bent bonding of CH,CN. Consequently, the hexanuclear unit Cu6 present in the infinite cationic sheet is in a chair-type cyclohexane-like framework [Fig. 23(c)l. On the other hand, the L,,-bridged copper(1) complex [ C U ~ ( L ~ ~ ) ~ Icontains ( C ~ O ~a) ~ linear chain polymeric framework. The two copper atoms in the unit cell have different coordination environments, one with trigonal planar and the other with perfect linear geometry. The two-coordinate Cu(1) ions form a linear chain framework, and the trigonal ions are attached to the chain just like a pendant (Fig. 27). The fact that a hexagonal or cyclohexane-like framework of copper atoms is not present in the cation is due to the bulky Lq5ligands. The four methyl groups on all the substitutable positions of pyrazine give rise to steric constraints and, as a result, prevent formation of a hexagonal ring unit and give a linear link as a less steric form.

y-N\ 112.1"

cu\

cu

I

I

112.10

C"

I

C"

\cu'cu\cu~c"\cu~

I

I

I

5" 7" 1"

FIG. 27. Schematic views of the chairlike hexagonal framework in [Cu2(L,,), (CH,CN),I(PF& (a) and a n infinite chain structure in [Cu2(LQ)31(C104)2 (b). (From Fig. lb in Kitagawa, S.; Munakata, M.; Tanimura, T. Znorg. Chem. 1992,3I,1714.)

214


C. PHENAZINE AND BENZOTHIADIAZOLE SYSTEMS-REGULATED BY COUNTERANIONS

Like pyrazine, phenazine (La), 2,1,3-benzothiadiazoles (L4,), and quinoxaline (L4&are of interest in the construction of the molecular assembly of the metal complexes because they can also act as bridging ligands linking two metal centers (138, 139). Phenazine formed 1: 1 donor-acceptor complexes with PMDA (pyromellitic dianhydride) and TCNQ, whose structures show alternate stackings of donor and acceptor molecules (140, 141 ). Several copper(1) and silver(1) complexes of La have been prepared and their structures determined, and among these silver(1) nitrate, [Ag2(L4,)(NO,),],is of particular interest (138).The compound was prepared by reaction of AgNO, and L46 in methanol at ambient temperature. In this complex each silver atom is coordinated to two nitrate anions and one L46 molecule in a distorted trigonal geometry, and each L46 molecule in turn links two trigonal silver ions as expected, Fig. 23(d). The most remarkable feature of the ternary complex is that the counteranion NO, acts as a spacer linking two L16-bridged Ag atoms, leading to a framework of sixmembered rings of silver atoms extending in the direction of the band e-axes (Fig. 28). The Ag-0 bond distances of 2.432(6) and 2.569(7) A are in the range of 2.367 to 2.689 A for Ag(1) complexes with nitrate, suggesting fairly strong interaction between the metal and the nitrate ion. The six Ag atoms on a two-dimensional sheet lie

-.

0

I3----jJ

c

FIG. 28. Hexagonal framework in [Ag,(L,,)(NO,),I is constructed by six metal ions interconnected by L, and NO;. (From Fig. 10 in Munakata, M.; Kitagawa, S.; Ujimaru, N.; Nakamura, M.; Maekawa, M.; Matsuda, H. Inorg. Chem. 1993,32,826.)

COPPER(I) AND SILVER(I) SUPRAMOLECULAR METAL COMPLEXES

2 15

approximately on the same plane, and each sheet is weakly linked to its neighbors by the interaction of Ag and the third oxygen atom of the nitrate anion with an Ag-0 distance of 2.669(7) A, giving a threedimensional structure composed of the two-dimensional sheets. By contrast, in the corresponding silver perchlorate complex of L46, [Ag(L4&1O4, the perchlorate ions are not coordinated, and subsequently the complex is composed of infinite -Ag - -Ag - linear chains along the a-axis and a a-a interaction of the L6 aromatic rings between the chains. It is informative enough that in the mononuclear copper(1) nitrate complex of L 4 6 , [ C U ( L ~ ) ~ ( N Othe ~)], Cu(1) ion is coordinated to two terminal L46 molecules and chelated by two oxygen atoms of the nitrate in a distorted tetrahedral environment. The structural differences between the copper(1) nitrate and silver(1) nitrate complexes of L46 are obviously due to the pronounced tendency of copper(1) ion for a tetrahedral coordination. Therefore, in the metal-phenazine system the counteranion as well as metal ion stereochemistry play an important role in regulation of the molecular assembly into different frameworks and stoichiometries. Three copper(1) complexes of L4, have been reported and it reveals that the frameworks in the compounds are regulated by the counteranions as shown in Fig. 29 (139).Because perchlorate ion has weak

1

X=CI04’

FIG.29. Regulation of the six-membered ring structures by anions in a n L4, system.

2 16


coordination ability, in [ C U ~ ( L ~ ~ ) ~ ( C ~one O ~ C10; ) ] C ~isO noncoordi~ nating and the other binds weakly with only one copper ion rather than as a interconnecting ligand. This leaves the two copper atoms in different coordination environments, tetrahedral and trigonal geometry, respectively. As a result, the complex is composed of a [cu&L47)G]6+framework containing a six-membered ring of copper atoms interconnected only by L47molecules (Fig. 30). Copper atoms in the two-dimensional sheet are arrang$d in a staggered conformation with a maximum deviation of 0.50 A from the mean plane. Replacement of copper(1) perchlorate by copper(1) hexafluorophosphate in reaction with L47isolated another polymeric complex with the for-

FIG.30. The six-membered ring of copper atoms in [ C U ~ ( L ~ , ) ~ ( C ~ O ~is) composed IC~O~ of [CU,(L~,)~]~+ unit. The coordination of perchlorate is not shown.

COPPERU) AND SILVERO) SUPRAMOLECULAR METAL COMPLEXES

217

mula [CU(L~~)(HPO~F)]. In this complex each Cu(1) ion is coordinated to two nitrogen atoms of different L47ligands and two oxygen atoms of different HP03F- ions in a distorted geometry. An important feature of the structure is that the HPO& anion acts as a n interconnecting ligand between two Cu atoms and takes part along with L47 in formation of the significantly distorted hexagonal framework composed of [C~(L47)4(HP03F)412+ units (Fig. 31). The copper atoms in the two-dimensional sheet are arranged in a ladder pattern in the acplane and a shallow roof pattern in the ab-plane. As discussed earlier, nitrate ion is expected to change the framework of the six-membered rings of copper atoms because it often functions as a bridging ligand. The copper(1) nitrate complex of L47[Cu(L4,)(N03)Iwas prepared by reduction of copper(I1) nitrate trihydrate under an ethylene atmosphere followed by reaction with L47in THF. In the complex the tetrahedral coordination of each copper ion is achieved by two L47ligands

'1-

\

,

FIG.31. The anion HPO$ acts as an interconnecting ligand between two Cu atoms (only the coordinating oxygen atom is shown) and takes part in formation of the hexagonal framework in ICu(L,,)(HPO,F)J.

218


FIG.32. Top (a) and side (b) views of the stacks of L4, molecules within and between 2-D sheets in rCu(L4,)(N03)1.(From Fig. 6 in Munakata, M.; Kuroda-Sowa, T.; Maekawa, M.; Nakamura, M.; Akiyama, S.; Kitagawa, S. Inorg. Chem. 1994,33, 1284.)

and two nitrate anions (Fig. 32). The six-membered rings of metal ions interconnected by L4, and NO, are extremely distorted, and are in fact in a chair form, with the ma5imum deviation of the Cu atom from the mean plane being 2.1-2.4 A. Examined along the c- and baxis, the copper atoms in the sheet are arranged in a ladder and a roof pattern, respectively. In addition, the shortest interplanar spacing distanFes of L4, moeties within and between the sheets are 3.30 and 3.39 A, respectively. Consistent with the observed V-v interactions in the system, the complex in powder form shows semiconductivity, with electric conductivity cr being 10-6.3S . cm-'.

COPPERU) AND SILVERW SUPRAMOLECULAR METAL COMPLEXES

2 19

It is proposed that a combination of three- and four-coordinate metal ions and bridging pyrazine and pyrazole molecules is highly preferred for construction of a two-dimensional sheet structure composed of six-membered rings of metal ions interconnected by aromatic ligands. The participation of suitable counteranions in coordination environments of the metal ions can regulate the frame of the ring, which governs the orientation of the aromatic ligands and the stack of the aromatic ligand within and between the sheets (139). V. Hydrogen-Bond-Assembled Frameworks

Self-associated intermolecular and intramolecular hydrogen-bonding assemblies have captured the attention of many research groups involved in supramolecular chemistry, molecular recognition, and crystal engineering because suitable matching of hydrogen bond donors and acceptors in number and orientation frequently leads to formation of new molecular aggregates (142-167). Hydrogen bonds are of paramount importance in biochemistry for determining the secondary structure of proteins and binding substrates to enzymes, receptors, and carriers. They are weaker than conventional covalent bonds but stronger than van der Waals interactions. Although there is no universal agreement on the best description of the nature of the forces in the hydrogen bond, it is generally accepted that hydrogen bonding, X-H----Y, occurs only between a hydrogen atom bound to an electronegative atom X and another atom Y that is also highly electronegative and has one or more lone pairs, enabling it to act as a base (142). The concept of hydrogen bonding was originally used to explain physical properties of simple organic and inorganic compounds such as abnormally high boiling points and heats of vaporization, but in recent years it has been noted that the introduction of a hydrogen-bonding interaction between ligands in transition-metal complexes is an indispensable tool for creating a variety of molecular architectures in a predictable fashion via self-assembly and molecular recognition. Several important reviews related to this field now are available ( 142-1 4 7 1. With the combination of the covalent bond forming capability of the metal ion and the inherent capability of the ligand for formation of complementary hydrogen bonds, a diversity of H-bonded structures has been obtained, including intramolecular hydrogen-bonded monomers (148), intermolecular hydrogen-bonded dimers, tetramers (149-151 ) and polymers having one-dimensional chain (152), two-

220


dimensional sheet (153-158) and three-dimensional networks (159163). The most commonly encountered hydrogen-bonding interactions in H-bonded supramolecular frameworks are 0 -H----0 and N -H----0.Additionally, some weak hydrogen bond interactions such as C-H--0, C-H----Cl and even X-H----arene are also reported (146).The interaction may involve only one hydrogen bond donor and one acceptor, may be bifurcated or trifurcated, and may occur between atoms, molecules, or ions (142).In this respect, the bifunctional ligands should contain the simple functional groups such as carboxyl, amide, amino, cyano, and pyridone. Recent reports have shown that hydrogen bonds can occur in the system between ligand functional groups and some inorganic anions such as BF,, C104, SO$-, and halides, and even water and ammonia molecules (146, 148, 158). The ligands involved in the following discussion of the hydrogen-bonded complexes of Cu and Ag are listed in Fig. 33. Mingos has shown in his recent review how the application of molecular recognition principles based on triple hydrogen bonding resulted in the crystal engineering of aggregates based on ligands that can simultaneously form stable and inert metal-ligand bonds and have recognition sites for complementary arrangements of hydrogenbond donors and acceptors (144),which has been excellently demonstrated by self-assembly of the copper(I1) complex [Cu(L,,), . 2 melamine] (HL4, = 5-(2-pyridylmethylene)-hydantoin)in which L4, involves simultaneous coordination with Cu and an ADA=DAD (A = hydrogen-bond acceptor, D = hydrogen-bond donor) triple hydrogenbonding arrangement with melamine molecules (Fig. 34)(156). Among extended polymeric structures, two-dimensional hydrogenbonded frameworks are enormously popular, and the resulting assemblies resemble practically crinkled tapes (153,154)and infinite sheets (155-158). Smith and co-workers have structurally characterized a

b 2

HL53

L54

FIG.33. List of ligands in copper(1) and silver(1) complexes having hydrogen-bonding networks.


221

FIG. 34. Hydrogen-bonding networks in [Cu(L4&.2 melamine]. (From Fig. 2 in Chowdhry, M. M.; Mingos, D. M. P.; White, A. J. P.; Williams, D. J. J. Chem. SOC., Chem. Commun. 1996,899.)

number of silver(1) carboxylates, and among these phthalate and trimesate (LEO) have been found to form hydrogen-bonded polymers involving the ammonia molecule (155). The silver(1) complex [Ag2(phthalate)(NH3)21 contains a phthalate-bridged dimeric unit in which each Ag atom is coordinated to one ammonia molecule and a single phthalate carboxylate oxygen in an essentially linear fashion. All three amine hydrogens are involved in intermolecular hydrogenbonding interactions, giving a two-dimensional hydrogen-bonded sheet structure extending across the bc-direction of the cell (Fig. 35). The adjacent sheets are further linked by hydrogen bonds via the uncoordinated carboxyl oxygens. In the corresponding trimesate complex, [NH,l[Ags(L5,),(NH,)z(H20)21 . HzO, a two-dimensional sheet framework is based on a pseudo-centrosymmetric S-type trimer unit linked by the carboxylate groups of two independent trimesate residues, and the hydrogen bonds between the uncoordinated carboxylate oxygens and amine and water molecules stabilize the structure. Likewise, in the copper(I1) complex of 4,4’-bipyridine ( L d , the coordinating water molecule bridges between the metal center and uncoordinated LS1,affording two-dimensional rectangular grid sheets (158). The structural determination of the Cu(1) complex of 2-cyanoguanidine (LS2),[{Cu~L51)2},~L51)][BF~l,~ MeCN, reveals that the structure contains a dinuclear cation in which two T-shaped copper(1) centers are bridged by L51and terminally coordinated by two L5%molecules

222


FIG. 35. Hydrogen-bonding networks in [Ag,(phthalate)(NH&I. (From Fig. 2 in Smith, G.; Reddy, A. N.; Byriel, K. A.; Kennard, C. H. L. J. Chem. Soc., Dalton Trans. 1995, 3565.)

(157).An extensive hydrogen-bonding network between BF, ions and LS2molecules gave a two-dimensional sheet structure as shown in Fig. 36. It is intresting to note that all four F atoms for each anion are involved in hydrogen bonding, three of which are sitting within the two-dimensional sheets and the fourth bridging the sheets. The principles of molecular recognition can also be used to assemble three-dimensional structures, but additional requirements on the number and orientation of the complementary hydrogen bonding motifs must be met (159-163). Nakasuiji has selected biimidazole, pterine, lumazine, and glycoxime as the target ligands because their chelating ability to a metal element and multi-H-bonding sites, and has constructed a number of three-dimensional hydrogen-bonded networks of metal ions such as Ni(I1) and Cu(I1) (160, 161). Molecules in which the motifs are related by Td or even S4symmetry are strong potential candidates for the engineering of a three-dimensional network exhibiting microchannels or diamondoid structures (146). The system we have worked with are literally double hydrogen bonding of the ligands combined with tetrahedral copper(1) ion in an attempt to design unprecedented two-dimensional and three-dimensional molecular architectures.

A. THREE-DIMENSIONAL SUPRAMOLECULAR Cu(1) COMPLEXES WITH CHANNELS (HL53) The bifunctional ligand 3-cyano-6-methyl-2(1H)-pyridinone as shown in Fig. 33, possesses both a coordination group (CN) and a


223

FIG.36. Two-dimensional sheet structure of [(Cu(Lsz)2}~(L~,)11BF112 MeCN showing hydrogen-bonding associations. (From Fig. 2 in Batsanov, A. S.;Begley, M. J.; Hubberstey, P.; Stroud, J. J . Chem. Soc., Dalton Trans. 1996, 1947.)

hydrogen-bonding site (pyridone), and it would be a suitable candidate for the formation of H-bonded metal complex supramolecules. The second reason for choosing it in the study is its relatively small size, which would reinforce the unidentate coordination of each ligand to the tetrahedral copper(1) ion without steric hindrance. The reactions of copper(1) salts with HLssin acetone have isolated four coordination polymers, [Cu(HL,,),IX, where X = ClO,, BF; , PF;, and CFs SO, (164). Although each structure contains a three-dimensional framework of tetrahedral CuNj centers linked by intermolecular hydrogen bonds through pyridone N and 0 atoms in a head-to-tail mode, different patterns of hydrogen bonding give rise to two types of different frameworks-namely, square channel and superadamantane networks-depending on the kinds of counteranions. In the complexes with relatively small anions, perchlorate and tetrafluroborate, each HL53molecule is hydrogen bonded to two adjacent others through pyridone N and 0 atoms; that is, each I C U ( H L ~ ~ ) entity ~ I X is connected to eight neighboring counterparts as shown in Fig. 37. The dihedral

FIG.37. Part of the hydrogen-bonded structure (a), packing diagram (b), and spacefilling model (c) of [Cu(HL,),]CIO,. (From Figs. 2 and 3 in Munakata, M.; Wu,L. P.; Yamarnoto, M.; Kuroda-Sowa, T.; Maekawa, M. J.Am. Chem. SOC.1998, 118, 3117.)


225

angle between the hydrogen-bonded HLS3groups is 83.44" and 84.98" for the perchlorate and the tetrafluroborate, respectively. This results in four [Cu(HL,,),IX monomers interconnected through hydrogen bonds, forming a layer with an open cavity occupied by the counteranion. The overall structure is made of such layers, each consisting of an infinite square array of copper atoms coordinated with HLs3groups and bridged by hydrogen bonds, leading to the cross-linking of a given layer with those immediately above and below it. A square channel framework is generated with a relatively small size of cavities (ca. 12.1 A in diameter) filled with guest anions. Let us name such hydrogen bonding as A type, illustrated schematically in Fig. 38. The hydrogen bonding network in [CU(L,~)(HL,,),Iis also found to belong to this type (165). When the small anion is changed to the drastically larger one, PF; or CF,SOi, the cavities in the square channel lattice for the perchlorate and tetrafluroborate cannot be effectively adapted for large guest ions only by elongating the hydrogen bond distances. If the stoichiometry is to remain the same, one way for the system to respond and avoid this destablizing repulsion is to rearrange the hydrogenbonding mode as B type, that is, each HLS3moiety in the monomeric unit of [Cu(HL,,),]X is head-to-tail hydrogen bonded to only one adjacent HL,, molecule, rather than two, with a rather smaller dihedral angle (0"-11.43"). Thus, one [CU(HL,,)~IXentity is connected to four adjacent others, and these repeating units form a three-dimensional adamantane architecture of metal atoms (Fig. 39). The diamondoid frameworks are stacked with each other in such way that all the copper centers are found on lines parallel to the c-axis, which flefines enormous linear chambers with larger open cavities (ca. 13.3 A in diameter), that just fit the large anions PF; and CF3SOi. To show how the assembling process would respond in the system

I

0-1 1 '

FIG.38. Schematic presentation of two types of' hydrogen bonds in HL, complexes.

226


t

t

CU

FIG.39. Part of the hydrogen-bonded structure (a), packing diagram (b), and perspective view of diamondoid framework (c) in [Cu(HL,)JPF,.


227

when H-bonding interaction is absent, the structures of two copper(1) complexes of L53were determined (165, 166). It reveals a completely different stoichiometry and framework in both [c~g(L63)4]c104and [Cu,o(L,3),](BF,)2, in which each deprotonated LS3monoanion employs all its three functional groups bridging three rather than two copper atoms, forming a supramolecular channel architecture containing a Cud cluster (Fig. 40).These findings suggest that the bifunctional ligand group m e 3 possesses unique ability for the molecular recognition, which has decisive influence on crystal engineering of coordination polymers. Modification of the hydrogen-bonding mode and distances as well as the direction of H-bonding sites at molecular level can effectively facilitate transition of one framework to the other.

FIG.40. Part of the molecular structure (a) and extended framework (b) in [Cu,(L,,),I

c10,.

228


B. HYDROGEN-BONDINGAND ~-~-STACKING-~SEMBLED Cu(1) COMPLEXES The second bifunctional ligand selected to construct the metal architecture is 2-hydroxyquinoxaline (Lu), because it also possesses both N-coordination sites and potential double H-bonding sites (CN=COH or CNHC=O if it undergoes tautomerization). The reaction of copper(1) perchlorate and Ls4under an atmosphere of ethylene gave the complex [ C U ( L ~ ~ ) ~ ( C ~ H ~ whose ) ] C ~structure O~, was determined by X-ray analysis (167). Its unique feature is the cooperative effect of hydrogen-bonding and aromatic-stacking interactions simultaneously present in the system. The metal center is in a trigonal planar geometry achieved by coordination to one ethylene and two Ls4 molecules. The cation maintains the fundamental feature of the free ligand in the sense that two LE4molecules are linked to each other by head-to-tail double hydrogen bonds, resulting in formation of an infinite zigzag chain (Fig. 41). The IR spectrum of the complex shows low v(NH) stretching frequency in the region 2950-3050 cm-', consistent with the short NH----0hydrogen bonds observed. Between the adjacent chains the H-bond assembled La4moleculeoplanes are stacked to each other, with the average distance of 3.30 A. This gives a unique two-dimensional cooperating structure stabilized by hydrogen bonding and r-r interactions as well as covalent bonds, reminiscent of a proton-electron transition (PET) system. It is worth mentioning that ethylene also plays an important role in the construction of the resultant architecture as a spacer because of its relatively small size, which reduces the packing volume, ensuring the effective stacking of the aromatic planes.

VI. a-a-lnteraction-Assembled Frameworks

A. INTERMOLECULAR n-n INTERACTION IN DISCRETE COORDINATION COMPOUNDS Planar coordination compounds with aromatic ligands (Fig. 43,especially those having extended 7~ systems, show r-r interaction in solid states. In alkene or alkyne n-bonded Cu(1) complexes, in-plane coordination of a C =C or C = C bond to trigonal planar Cu(1) centers often leads to planar molecular conformations (167-1 72). The infinite n-r stacking columns are confirmed in the 2,2'-bipyridine (Lss) com-


229

FIG. 41. Packing diagram of LCu(L,)e(C2H,)IC104; side view showing H-bonded networks (a) and top view showing aromatic stacking interactions. (From Fig. 2 in Dai, J.; Yamamoto, M.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Munakata, M. Inorg. Chern. 1997,36,2688.)

plex [CU(L~~)(C~H,)]C~O, (169)and the 1,lO-phenanthroline (LW)complex [ C U ( L ~ ~ ) ( C ~ H C O ~ E(1 ~ )71 ]C O ~ nearest carbon-to-carbon ) ~ with distances of 3.31 and 3.37 (172,1731,respectively, and dimer forma(171) tion through n-rr interaction can be seen in [Cu(LW)(CZHz)IC1O4 [the nearest carbon-to-carbon distances are 3.42 ( 172)l. Flat conformations are also observed in cationic parts of dimeric Cu(I) and Ag(1) compounds bridged by two l&naphthyridine ligands, (1741. The former comLS7, [ C U ( L ~ ~ ) ] ~ ( C and ~ O[Ag(L67)12(C10,)2 ~)~ pound forms a dimer through n-n interaction of the neighboring L67

A

A

230


L56

L55

ae L59

L61

L62

Me.

Me

L63

L65

*@

\ /

L67 L68

L69

L66

ip /

L70

Me

@ \ /

L71

L72

L73

L74

FIG.42. Ligands that appear in Sections VI and VII.

molecules; the latter shows one-dimensional infinite :tacking column of the cationic part with interplane distance of 3.40 A (172) as shown in Fig. 43. Another flat dimer, [Cu(L5,)l2(175), also shows a onedimensional a-a stacking column, with interplanar separation of 3.41

A (172).

Not only planar compounds but also nonplanar ones show a-a stacking interaction. In the crystal structure of CUI(L& (176), two


23 1

3 L -3 -c 3.40 A

Co=-o; 3 - J - 3

Ag

napy

FIG.43. A crystal-packing view of [Ag(L5,)1~(C104)2 showing r-n interaction between planar cationic parts.

2,6-dimethylpyridines Lse and a n iodine coordinate to a Cu(1) ion in T-shaped trigonal geometry, and the dihedral angles between two ligand planes and the CuINp plane are 87.6 and 71.2", indicating nonplanar conformation. However, one-dimensional n-n stacking can be easily recognized in Fig. 44, with a nearest carbon-to-carbon distance of 3.447 A. Changing the counteranion from an iodide to a perchlorate ion gives two-coordinated copper(1) or silver(I) compounds. Whereas planar cationic parts of [M(Lsg),1X(M = Cu, Ag; X = ClO, , NO,) (177) with two-coordinated M(1) centers show only weak n-n interytion, with the nearest carbon-to-carbon distances being 3.59 to 3.66 A, one of the polymorphs of [ C U ( L ~ ~ ) ~ I(178), C ~ Owith ~ a dihedral angle between two LS9ligands of 5 6 2 , shows a close n-n contact of 3.33 A (172) between two Ls9ligands residing in adjacent molecules. We can see not a one-dimensional n-n stacking column but a n-n-interactionassembled one-dimensional structure like ----Ls9 - Cu -Lse---L59- Cu-L59----. Although a trigonal planar geometry is maintained around a Cu(1) center in the ethylene-coordinated complex [Cu(L54)z(CzH4)IC104 (1671, the two Ls4 molecules coordinated to the Cu(1) ion have a dihedral angle of 102.5", giving a butterfly structure. Repetitions of double hydrogen bonds between two Ls4ligands in neighboring complexes form an infinite zigzag chain (Fig. 41). Additionally, a n-n interaction between two LMligands residing in neighboring chains (the interplanar separation of 3.30 A) forms a two-dimensional structure as shown in Fig. 41. This gives a unique two-dimensional cooperating structure, which is a fundamental characteristic of proton-electron transfer (PET) systems (179, 180).

232


I

3.447 A

c sinp

FIG.44. A crystal-packing view of CuI(L6& showing r-a interaction between L60 ligands. (From Fig. 5 in Healy, P. C.; Pakawatchai, C.; White, A. H. J. Chem. SOC., Dalton Trans. 1983, 1917.)

The copper(1) ion in [ C U ( L ~ ) ~ ] CMeOH ~ O ~ (181) has a distorted trigonal geometry with three nitrogen atoms of three 1-aminopyrenes (Lm).Two of them are parallel to each other, whereas the rest lie approximately perpendicular to the other two as shown in the molecularpacking view (Fig. 45). Intra- and intermolecular n-n interactions of the former two form a one-dimtnsional stacking column with interplane separations of 3.40-3.43 A. Moreover, intercolumn n-n interactions between LBOligands with an interplane separation of 3.52 A result in the formation of a two-dimensional structure. The copper(1) ion in [Cu(L&H,0)IC1O4 (138)has a distorted trigonal geometry with coordination of two nitrogen atoms of two terminal 9

COPPERU) AND SILVERW SUPRAMOLECULAR METAL COMPLEXES

233

FIG. 45. Molecular packing view of [Cu(L,)JClO,. MeOH. (From Fig. 2 in Munakata, M.; Dia, J.;Maekawa, M.; Kuroda-Sowa, T.; Fukui, J. J . Chem. SOC.,Chem. Cornmun. 1994, 2331.)

L4 molecules and a water oxygen. The coordinated L4 molecules form an alternating r-n- st9cking column, with the nearest intermolecular separation being 3.46 A. Because basicity of acridine is stronger than that of La (1821, adding a solution of L61 in methanol to a solution containing copper(1) and L4 gives a discrete complex, [ C U ( L ~ ~ ) , N O , IH(,L O)~,,~ ~(183).The copper(1) ion is trigonally coordinated by the two nitrogen atoms of L, and one of the oxygen atoms of the NO; anion. Two coordinated L61 molecules are not parallel to each other at a dihedral angle of 63.6", and each has a r-n- interaction with an adjacent Ll molecule coordinated to another copper atom (Fig. 46). The interplane separations of these intermolecular r-r interactions are 3.41 and 3.49 A. A metalfree L4 is sandwiched between L61 ligands, forming an infinite columnar stacking along the a-axis with a L46-L61-]LB, repeating unit. The closest C----Cdistance of 3.39(1)A indicates a significant n--r interaction between L4 and 4 1 . An L46-bridgedCu(1) dimer complex, [CU,(L~,)~(M~OH),I(L~~)(PF,), (138) shows an alternate r-r stacking of coordinated and uncoordinated La. Two copper(1) ions are crystallographically the same: a T-shaped three-coordination of a methanol and terminal and bridging La with a remarkably large N(termina1 L&Cu-N(bridging La) angle of 156.1". A metal-free L4 molecule is located near the bridging L4, with a dihedral angle of 7.6"and with a nearest carbon-to-carbon sepa-

234


FIG. 46. Molecular views of [Cu(L6&N03](La.H 2 0 ) , ,indicating r-r interaction between Lp6and La1 (a) and between two LBLligands (b). (From Fig. 4 in Kuroda-Sowa, T.; Munakata, M.; Matsuda, H.; Akiyama, S.; Maekawa, M. J. Chern. Soc., Dalton Trans. 1995, 2201.)

ration of 3.42A, indicating a significant ?T-T interaction between them. Molecular packing along the c-axis reveals one-dimensional T-?T stacking of bridging and metal-free La alternately.

B. INTERAND INTRAPOLYMER r-rr INTERACTION One-dimensional polymeric structures are observed in copper(1) complexes with 2,9-dimethyl-l,lO-phenanthroline, [CU(L~~)(CN)] and [CU(L62)(NCS)](184). Cn- or NCS- acts as a bridging ligand, and L62 chelates to a copper(1) ion to block two of four coordination sites of the tetrahedral center, resulting in the infinite zigzag chain. In both compounds, zigzag chains are connected through T-?T interaction between L62 molecules residing in neighboring chains, which form two-

COPPERU) AND SILVER(1) SUPRAMOLECULAR METAL COMPLEXES

235

dimensional polymeric structures. Similar two-dimensional network formations are also observed in the trinuclear copper(1) complex with 2,2’-biquinoline (La) [CU,(L,)~(CN),] (1851, in which tetrahedral copper(1) ions coordinated by chelating L63 ligands are bridged alternately by Cu(CN)i and CN-. When La is used as a bridging ligand to construct coordination polymer compounds, interpolymer T-T interactions are often observed. A one-dimensional zigzag chain composed of bridging L,, and a tetrahedral copper(1) ion can be seen in [CU(L,~)(NO~)] (183) (Fig. 47). The copper(1) ion is coordinated by the two nitrogen atoms of the two L46 molecules and by the two oxygen atoms of the NO, anion.

FIG.47. Top (a) and side views (b) of the crystal packing of [Cu(L,,)(N03)]showing the La columnar stacks. (From Fig. 2 in Kuroda-Sowa, T.; Munakata, M.; Matsuda, H.; Akiyama, S.; Maekawa, M. J. Chem. SOC.,Dalton Trans. 1995, 2201.)

236


Two L46 molecules coordinated to the same copper atom are inclined to each other at a dihedral angle of 82.2'. As can be seen in Fig. 47, the L46 molecules are stackFd in the b-axis direction, with an interplane separation of 3.47 A. Thus, the compound consists of onedimensional zigzag chains of copper atoms and L46 molecules, interconnected through L4&n-n interactions, resulting in a two-dimensional interaction. A similar zigzag chain composed of bridging 4 6 and a trigonal planar copper(1) ion is observed in [cu(L46) (MeCN)],(L,)(PF,), (La : pyrene) (183).The L46 molecules are stacked in the c-axis direction with an interplane separation of 3.47 A. Thus, the compound consists of one-dimensional zigzag chains of copper atoms and L46 molecules interconnected through L, n-n interactions, resulting in a two-dimensional interaction. A series of halogen-bridged ~ o p p e r ( I ) - L compounds, ~~ [CU~(~-X)~ (L46)l (X = I, Br, or C1) (1861, show one-dimensional (X = I) or twodimensional (X = Br or C1) polymer frameworks. CuzIzrhomboids in the iodide compound are connected by bridging L, ligands through trigonal planar Cu(I) centers, forming an infinite straight chain. The dihedral angle of the rhomboid and L46 plane is 75.28'. The n-n interaction between L46 molecules of adjacent chains (interplanar distances of 3.46 A) gives the complex two-dimensional structure. On the other hand, both the bromide and the chloride compounds have almost same structure: CuX infinite stairs bridged by L , through coordination to distorted tetrahedral (Cu(1) ions, forming a two-dimensional network structure as shown in Fig. 48 for the bromide compound. Intrasheet r-n interactions betwen LlS molecules are also observed in both compounds, with interplanar distances of 3.40 and 3.36 A for the bromide and the chloride compounds, respectively. In the solid-state 13C NMR spectra of these compounds, the increase of the upfield shifts of the resonances assigned to the quaternary carbon atoms of L46 upon coordination (-2.4, -2.9, and -3.9 ppm for the iodide, the bromide, and the chloride compounds, respectively) are well correlated to the decrea!e in the interplanar distances of L 4 6 molecules (3.46, 3.40, and 3.36 A, respectively). Not zigzag but completely straight one-dimensional chains can be seen in [Ag(L46)I(c104) (138),in which the Ag(1) ion has a linear two coordination of two nitrogen atoms of bridging L46 ligands. All the L 4 6 molecules are parallel, as shown in Fig. 49, which results in the formation of a one-dimensional chain structure like a flat ribbon. The shortest intermolecular distance is 3.36 indicating significant T-n interaction, though the overlap between them is not so large. Because one chain interacts with four neighboring chains through n-n interac-

A,

COPPER(1)AND SILVER(1) SUPRAMOLECULAR METAL COMPLEXES

237

FIG.48. Top (a)and side views (b) of the packing arrangement of ICu2(pBr)2(p-L4B)I. (From Fig. 4 in Munakata, M.; Kuroda-Sowa, T.; Maekawa, M.; Honda, A.; Kitagawa, S. J. Chem. SOC.,Dalton Trans. 1994, 2771.)

tions, this system has a three-dimensional network assembled by TI-n interaction. A two-dimensional sheetlike structure in [Ag2(L,,J(N03),I (138) is composed of bridging L46 and bridging nitrate anions coordinated to trigonal Ag(1) ions, forming six-membered rings of Ag(1) ions. The significant n-n interaction between two L46 molecules in the neighboring sheets (the shortest intermolecular distance of 3.34 A) indicates the formation of a a-n-interaction-assembled three-dimensional network structure. Perylene molecules &4), having an extended n-system, easily form a-n interaction. Four peripheral C=C moieties of Lfi9coordinate to

238


FIG.49. Molecular structure (a) and a perspective view of the packing arrangement (b) of [Ag(Le)l(CIOa).(From Figs. 7 and 8 in Munakata, M.; Kitagawa, S.;Ujimaru, N.; Nakamura, M.; Maekawa, M.; Matsuda, H. Znorg. Chern. 1993,32,826.)

four Ag(1) ions in $-fashion in [Ag2(L6s)(C104)2] (187).The silver(1) ion has a distorted tetrahedral geometry with two C=C groups of two L69 molecules and two oxygen atoms of two perchlorate anions. The resulting two-dimensional sheet shows a W-type wavy conformation 5s shown in Fig. 50, which enables effective n-n interaction (3.31 A) between L@ molecules in adjacent sheets. Thus, the n-n interaction promotes the network dimensionality from two to three dimensions. Chair-formed six-membered rings of Cu(1) ions in [Cu(L4,)(NO,)I (139)form a two-dimensional sheet with a certain thickness, in which each Cu(1) ion is bridged by two L4, and two nitrate anions. Because L4, molecules are almost parallel to the two-dimensional sheet, both intra- and intersheet n-n interaction can be seen, with inter-L4, separations of 3.30 and 3.39 A respectively. The latter interaction enables the three-dimensional network structure assembled through n-n interaction.

COPPERU) AND SILVERU) SUPRAMOLECULAR METAL COMPLEXES

239

(4

FIG. 50. W-type wavy conformations of two-dimensional sheets in IAg,(LBB)(C104)ZI. (From Fig. 2 in Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Sugimoto, K. Znorg. Chern. 1997,36,4903.)

240


VII. Diamondoid Frameworks

We can imagine a formation of a cubic or a hexagonal diamondrelated framework (Fig. 51) from a combination of a tetrahedral metal ion and a rodlike bridging ligand. Although both types of frameworks are found in minerals or polymorphs of ice (1881, so far as we know, only the former type of framework is known in coordination compounds. Hereafter, we use the terms “diamond” or “diamondoid framework” to mean cubic diamond-related framework. As Hoskins and Robson have proposed (1271, a compound having a diamondoid framework, if it can have a large cavities or channels inside, can offer a number of features of potential interest such as molecular sieve properties, heterogeneous catalytic properties, and mechanically strong materials with an unusually low density. Additionally, T-T interaction between aromatic bridging ligands often controls the degree of interpenetration of diamondoid frameworks and sometimes gives electronic conducting material. In this section, we will focus on coordination compounds with diamondoid frameworks, especially containing copper(1) and silver(1) ions. These ions having d10 electronic configuration are suitable for a tetrahedral metal center in a diamondoid framework. Table V lists the coordination polymers having diamondoid frameworks reported so far, together with other coordination polymers having related frameworks.

A. BRIDGED BY PYRIDINEOR

PYRAZINE

DERIVATIVES

Pyrazine, LS8,and its derivatives are the shortest bridging ligands (133)has a tetrahenext to cyanide. The copper(1)ion in [CU(L~~),IPF, dral geometry, with coordination of four N atoms of Ld0.The other end of the N atoms of each L40 coordinate to other copper(1) ions re-

%%

FIG.51. Cubic (left) and hexagonal (right) diamond-related frameworks.

COPPER^ AND SILVER^ SUPRAMOLECULAR METAL, COMPLEXES

241

TABLE V COORDINATION POLYMERS HAVING DIAMONDOID AND RELATED FRAMEWORKS Compound

Bridged M~-;-M Degree of distance (A) interpenetration

Remarks

5.03 5.11 5.46 6.99 8.856" 8.9 5.04, 9.54 9.93 ca. 10 10.90 11.16 11.6 11.76 11.9 12.09 12.76 13.5 13.55 16.4 17.0

Ref. 127 127,204,205 127,204,205

I33 127 71-71

71-71

8-8

8-71

71-77 71-8

193 206 190 208 207 136 190 4 195, 196

194 197, 798 164

8-71

192

71-71

201,202 203

71-8

Zinc-blende structure.

', Cu----Cudistance.

sulting in the formation of a diamondoid framework (Fig. 52). The shortness of the bridged Cu----Cuseparation of 6.99 A together with the presence of the PF; counteranion and methyl groups in L4,,prevent the interpenetration of any other framework. Although coordination chemistry of Ag(1) and L39(130, 289) revealed several compounds having interesting three-dimensional observed such as cw-ThSi2 or ReOs types, no diamondoid framework has been observed in this system. When 4,4'-bipyridine, Leo,is used instead of L4,,fourfold interpenetrated diamondoid frameworks are obtained for both Cu(1) and Ag(1) ions. The Cu(1) compound [Cu(L&lPF6 1136) exists as four independent concatenated diamondoid frameworks with Cu----Cuseparations of 11.16 A. The Cu(1) centers occupy crysta!lographic 4 positions and hence all Cu-N bonds are identical, 2.034 A. The PF; counteranions

242


FIG.52. A single diamondoid framework in [Cu(L&lPFs. For clarity, the PF, anions are omitted.

occupy channels that are parallel to the c-axis and they sit on fourfold (190) also crystallographic axes. The Ag(1) analog [Ag(LBo)~CF3So3 contains four interpenetrating diamondoid frameworks (Ag----Agintraframe separations of ca. 11.6 A). The Ag(1) cations display a distorted tetrahedral geometry, with Ag-N contacts different for the two Interconnection of independent LWligands (mean 2.270 vs. 2.380 trigonal Cu(I) centers through LBO was achieved by hydrothermal synthesis (191). The resultant crystalline compound [cU(L60)1.5]* NOs(H20),,26 shows sixfold interpenetrated a-ThSi2-typeframeworks. By lengthening the linking bipyridyl ligand via insertion of a trans C=C double bond between the pyridyl units, the length of the resultant briding ligand, 1,2-truns-(Cpyridyl)ethene (L70), is extended by about 2.4 A and the interpenetration in diamondoid frameworks is also changed. The Locompound with Cu(I), [ C U ( L ~ ~ ) ~ ~ B F ~ ( O . ~ C H ~ C ~ (192),has fivefold interpenetrated diamondoid frameworks. The Cu----Cu distances bridged by L70 range from 13.33 to 13.82 A to create large cavities within the diamondoid framework as shown in Fig. 53. All five independent frameworks are polycatenated with channels throughout the structure: these channels accommodate BF; counteranions and CH2C12solvent molecules. Among the bipyridine derivatives, L64 can also act as a linear bridging ligand if the transoid conformation is maintained. [Cu(L,&]X (X = BF4, PF,) (1931, having a twofold interpenetrated diamondoid framework, can be obtained as yellow-green triangular crystals. The

A).


243

FIG. 53. View of the structure of [Cu(L,,),JBF,. (From Fig. 2 in Blake, A. J.;Champness, N. R.; Chung, s. s. M.; Li, W.-s.; Schroder, M. J. Chem. Soc., Chem. Commun. 1997,1005.)

LM has surely a transoid conformation, but the rings twist out of plane relative to each other by about 30". In addition, the Cu-N bond is bent out of the plane of the aromatic ring by 19,leading to a closing of the Cu-L-Cu angle to 134". Bridged by this large ligand, the resulting framework contains copper atoms separated by 8.9 A and significant void volume. The extra space is occupied by an identical network forming a twofold interpenetrated structure (Fig. 54).

B. BRIDGED BY BISNITRILE LIGANDS Nitrile compounds are generally accepted to be weaker donors than pyridine derivatives; still, they can coordinate to d'O metal ions. A bisnitrile compound, if the two CN groups have opposite directions, can act as a rodlike bridging ligand in the construction of diamondoid frameworks. Although directions of two CN groups in alkyldicarbonitrile, NC(CH,),CH, are not fixed a priori due to free rotation around methylene carbons, a Cu(1)-adiponitrile ( n = 4) complex, [CU(NC(CH~)~

244


FIG.54. A schematic illustration of twofold interpenetrated diamondoid frameworks of [Cu(L&lX (X = BF,, PFB).The spheres represent the copper atoms, and the twisted bipyridine ligands are shown as the bet cylinders linking the copper centers together. (From Fig. 2 in Lopez, S.; Kahraman, M.; Harmata, M.; Keller, S. W. Znorg. Chem. 1997,36, 6138.)

CN),INO, (1941, shows a diamondoid framework. In the crystal, four methylene groups of the adiponitrile form a planar zigzag chain and the terminal CN groups align almost antiparallel, which allows the adiponitrile to behave as a linear bridging ligand. The bridged Cu----Cudistance of 12.09 A together with the small size of the counteranion (NOi3)enable it to show sixfold interpenetration of independent diamondoid frameworks. Terephthalonitrile, L66, having two CN groups at the 1- and 4-positions in a benzene ring, is a good candidate for a rodlike bridging ligand for d'" metal coordination polymers. When the aceonitrile liare substituted by L65, the crystals formed gands of [CU(CH~CN)~]BF~ have the composition [Cu(L6,),]BF4(4). Diamondoid framgworks are indeed formed (the bridged Cu----Cudistance of 11.76 A), but the structure contains fivefold interpenetrated independent frameworks. Channels of rhombic cross section are generated in which the BF, ions are located. Methyl substitution of La6 causes a drastic change in the interpenetration of the diamondoid frameworks. Crystals of [Cu(L,,)21X(L66)(THF)(X = BF4, C104) (195, 196) were obtained by solution into the corresponding copper(1)soluslow diffusion of the LBB tion. X-ray crystal structure analysis revealed that both compounds contain threefold interpenetrated diamondoid frameworks as shown in Fig. 55. Because the bridged Cu----Cudistance in these compounds (11.9 A) is almost same as that bridged by L65 in Cu(L&BF4 (41, the lesser interpenetration in LBBcomplexes should be caused by the bulkiness of methyl groups in L66. This is also strongly correlated to the incorporation of guest molecules, a metal-free LBB,and THF. The metal-free dmtpn incorporated in the cu(L& lattice participates in the formation of a T-T stacking column (discussed later).

COPPER(I) AND SILVERW SUPRAMOLECULAR METAL COMPLEXES

245

FIG. 55. A threefold interpenetrated diamondoid framework in [ C U ( L ~ ) ~ I X ( L ~ ) (THF). For clarity, the ligands are represented by lines and the PF, molecules are omitted.

Among the compounds having diamondoid frameworks listed in Table V, the copper complex with 2,5-dimethyl-NJV’-dicyanoquinonediimine [Cu(L7,),I (197, 1981, has a quite interesting conducting property. Recent developments on this and related compounds show other interesting features including metal-insulator-metal transition (reentrant behavior) (199),three-dimensional Fermi surface character (2001, and so on. The structure of [Cu(L,J21shows sevenfold interpenetrated diamondoid frameworks with strong n-n interaction (the interplanar distances between the neighboring L71 molecules are 3.13.2 A). It should be noted that not even-number-fold but sevenfold interpretation in this class of complexes probably disturbs the dimer formation between adjacent LT1molecules, which contribute to the aforementioned conducting behavior. A ninefold interpenetration of diamondoid frameworks, the highest degree thus far reported, is observed in a series of Ag(1) complexes with 4,4’-biphenyldicarbonitrile,L72 (201, 202). Crystallization of L72 with AgX (X = PF6,AsF6, SbF,) (1: 1molar ratio) by heating and slow cooling from ethanol produces yellow crystals of [Ag(L72)21X (X = PF,, AsF6, SbF,). All of these structures show ninefold interpenetrated diamondoid frameworks. This highest degree of interpenetration is a result of the rather large ligand used (the bridged M----Mdistance of 16.4A). The large amount of void volume created by a single network is filled by eight identical nets (Fig. 56).These nets consist of parallel ligands offset along the long axjs and display n-n stacking at a planeto-plane distance of 3.4to 3.6A. Columns of counterions are revealed

246


FIG. 56. A ninefold interpenetation of diamondoid frameworks of [Ag(L&IX (X = PF,, AsFs, SbF,). For clarity, the L,, are represented by lines and the PF, are omitted. (From Fig. 1 in Hirsch, K. A,; Wilson, S. R.; Moore, J. S. Chem. Eur. J. 1997, 3, 765.)

down the fourfold c-axis. A packing model for this system proposed by Moore et ul. (202) explains a relationship between the degree of the interpretation and the bridged M----Mdistance. Their model also explains the effect of the anion size on the height of adamantoid cages in a series of compounds [Ag(L&IX (X = PF6,AsF6, SbF6). The longest M----Mdistance in a diamondoid framework is observed in [Ag(L,3)21(C104)H20 (203).The trunsoid conformation of 3,3'-dicyanodiphenylacetylene, L73,bridges two Ag(1) ions separated by 17.0 A. The obtained structure shows eightfold interpenetrated diamondoid frameworks (Fig. 57). A majority of the large amount of void space created in a single diamondoid framework is filled through interpenetration, resulting in an eightfold diamondoid network. The remaining space within the lattice is filled by perchlorate ions and water. Interpenetration in this structure is mediated by T-T stacking of LT3(discussed later).

C. OTHERBRIDGING LIGANDS One of the simplest bridging ligands in constructing diamondoid frameworks is a cyanide, CN-. It is well known that Zn(I1) and Cd(I1)

COPPER(1jAND SILVER(1)SUPRAMOLECULAR METAL COMPLEXES

247

FIG. 57. Diamondoid framework in [Ag(L~:3~dI(C101)H20 representing 7-n stacking along the maxis. (From Fig. 8 in Hirsch, K. A.; Wilson, S. R.; Moore, J. S. Inorg. Chem. 1997, 36, 2960.)

ions bridged by cyanide form doubly interpenetrated diamondoid frameworks of M(CN12(127, 204, 205). A partial replacement of the metal site by a Cu(U ion needs incorporation of countercations due to the charge neutrality, as exemplified in the case of NMe,[CuZn(CN),I (127). The incorporated NMe4 cation prevents the interpenetration of another network, resulting in the formation of a single framework, although the framework is no longer a diamondoid but a zinc blende structure. A detailed structural analysis concluded that carbon atoms in cyano groups coordinate to Cu(I) centers to form Cu-CN-Zn rods. A similar zinc blende structure is also seen in [Cu(C(CGH4CN),IBF4 .xC6H5N02 (x = 7.7) (125, 1271, in which C6H4CN can be regarded as a bridging ligand between Cu(1) and C" centers. The large void space created by the bridging ligand is filled by a counteranion and solvent molecules. A bridging ligand having mixed functional groups also affords a diamond-related framework, 4-Cyanopyridine, LG7,having both a cyano group in one end and a pyridine nitrogen in the other, is such a typical asymmetric bridging ligand. Cuboidal crystals of [Ag(L6,)21BF, (190)were obtained by slow diffusion of an ethanolic solution of AgBF, placed over LG7dissolved in THF (molar ratio 1:2). The fourfold interpenetrated diamoadoid frameworks have a Ag----Agintraframe separation of 9.93 A. The Ag(1) ions, lying on mm special positions, exhibit markedly flattened tetrahedral geometry, with

248


N(py)-Ag-N(py), N(cn)-Ag-N(cn), and N(py)-Ag-N(cn) angles of 127.7(5), 126.5(5), and 101.45(12Y, respectively. These local distortions result in a global compression of the diamondoi! framework along c. The Ag-N bond lengths have values of 2.270(6) A [Ag-N(py)l and 2.350(10) A [Ag-N(cn)]. The presence of iso-oriented anisobidentate (with donor ends differing both in steric need and in basic character) ligands gives rise for [Ag(b7)JBF4to a polar axis (c). A T-T stacking column of aromatic ring! could be observed along the c-axis (interplanar distance of 4.0 A). Le7 also forms a diamond-related framework with CuCN. A Cu(1) ion in [CuCN(L,,)I (206) is coordinated by one carbon atom and three different types of nitrogen atoms. The structure was first interpreted as triply interpenetrated threedimensional polymers with one-dimensional zigzag chains of CuCN bridged by Le7 and has recently been reappraised to have diamondrelated networks (190). 2,7-Diazapyrene (LT4),having an extended T-system, also produces a Cu(1) coordination polymer, [Cu(L7,),1PF6(207), which shows threefold interpenetrated diamondoid frameworks. The spaces within each diamondoid framework in [Cu(L,,),IPF, are filled by two other diamondoid frameworks, and the three interpenetrating arrays are related to each other by a 90"rotation (Fig. 58). This form of interpenetration contrasts with that observed for other diamondoid frameworks appearing in Table V, which are related t o each other by simple translation. Adjacent frameworks interact with each other via face-to-face T-T interactions between L7, molecules, which are arranged so that the N----Naxes of the ligand are at 90" to one another. Because of the greater lateral steric bulk of L,,, the number of interpenetrating frameworks of three is smaller than other bridging ligands, giving similar M----Mdistances (Table V). This represents the identification of another factor controlling the degree of interpenetration in diamondoid frameworks. Although HL63 has both a CN group and a pyridone group, a dimerization of two HL53molecules through head-to-tail hydrogen bonds between the pyridone groups brings a symmetric rodlike bridging li~IX = PFs, CF,SOJ (164), each gand. In the crystals of [ C U ( H L ~ ~ )(X HL63 moiety in the monomeric unit of [ C U ( H L ~ ~ )is~ head-to-tail IX hydrogen-bonded to one adjacent HL53 molecule. Thus, one [Cu(HL,,),IX entity is connected to four adjacent others, and these repeating units form a three-dimensional diamondoid framework. The structure exists as fourfold interpenetrated diamondoid frameworks with intraframe Cu----Cuseparations of 13.5 A in both cases. The diamondoid frameworks in both complexes are stacked with each other in such way that all the copper centers are found on lines parallel to the


249

FIG. 58. Threefold interpenetrated diamondoid frameworks observed in [Cu(L,,),] PF,. (From Fig. 1 in Blake, A. J.; Champness, N. R.; Khlobystov, A. N.; Lemenovskii, D. A,; Li, W.-S.; Schroder, M. J. Chern. Soc., Chern. Cornrnun. 1997, 1339.)

c-axis, which defines enormous linear chambers. The counteranions PF6and CF3S03occupy channels that run along the fourfold crystallographic axes. Not a single metal ion but a tetranuclear silver cluster plays a role of a tetrahedral center in a diamondoid framework in [Ag2(02C(CH2)2C02)I (208). The compound consists of planar tetrasilver clusters bridged by succinate anions as shown in Fig. 59. The Ag, cluster exhibits pseudo-S, symmetry and is linked though its four ligands to four neighboring clusters, resulting in the formation of triply interpenetrated diamondoid frameworks. This is the first example of a three-dimensional coordination polymer containing metal clusters.

D.

T-T

INTERACTION IN DIAMONDOID FMEWORKS

Interpenetration in diamondoid frameworks is often mediated by interaction between aromatic bridging ligands (Table V). A pack-

T-T

250


FIG. 59. A view of the Ag, cluster and the surrounding ligands in (CHz)zCOz)I. (From Fig. 1in Michaelides, A.; Kiritsis, V.; Skoulika, S.; Aubry, Chem., Znt. Ed. Engl. 1993,32, 1495.)

ing model for such systems proposed by Moore et al. (202) explains a relationship between the degree of the interpenetration and the bridged M----Mdistance, and also explains why two polymorphs can be obtained in [Ag(L7,),1AsFs(202). Face-to-face n--n--stacking interactions between Fets of L70 ligands in [ C U ( L ~ ~ ) ~ ~ B F ~ ( O(192) .~CH (3.659 ~ C ~and ~ ) 3.855 A) control the separations between adjacent diamondoid frameworks. The a-n--stacking interaction is clearly important in the overall control of this extended structure, and the presence of this interaction is along the direction in which the long-range structure is formed during crystallization. In case of [Cu(L&IX(L,)(THF) (X = BF4, C10,) (195, 1961,metalfree and coordinated LBB ligands stack alternately; with a nearest carbon-to-carbon separation of 3.37(2) and 3.51(2) A, to form a one-dimensional n--n-stacking column as shown in Fig. 55.

COPPERU) AND SILVER^ SUPRAMOLECULAR METAL COMPLEXES

25 1

r-a-Stacking columns in [Ag(L7,),I(C104)H,0(203)occur along both the a- and b-directions, with a plane-to-plane distance of 3.51 A. Along a , adjacent ligands in the stack are flipped 180" relative to one another such that nitrile groups are not overlaid (Fig. 57). The stacking along b is unusual in that a large offset angle causes a LT3molecule to stack between two fragments each consisting of two halves of L7, coordinated to silver(1).To allow stacking in two directions, mutually perpendicular stacks organize in a!ternating layers. A close interplane separation of 3.38 A between adjacent HL,, molecules in [Cu(HLC3),]X(X = PFs, CF,SO,) (164) suggests the presence of strong r-r interactions betwten pyridone rings. This separation is slightly shorter than the 3.40 A observed in the copper(I)-LGOcomplex (186).

VIII. Other Frameworks Based on Covalent Bonds

Apart from several of the classes of intermolecular forces based in supramolecules described earlier, coordination polymers with extended frameworks based mainly on covalent bond forces represent another well characterized class of transition metal supramolecules. One might anticipate that application of the strategies that have worked so well for the preparation of copper(1) and silver(1) coordination polymers with a diversity of novel frameworks based on intermolecular forces would allow a similar easy access to formation of the covalent-force-based frameworks by the selection of suitable ligands and spacers and the introduction of flexibilizing groups, either in the polymer backbone or in the side-group structure (Fig. 60). Recent publications did reveal several such notable examples of infinite chains, two-dimensional sheets, and three-dimensional networks formed by metal cations linked through multidentate organic ligands.

A. INFINITE-CHAIN STRUCTURES 1. Light-Induced Crystal Oscillation In addition to their novel frameworks, low-dimensional metalcontaining coordination polymers are of interest with respect to their electrical, magnetic, and optical properties, and possibly their catalytic behavior. For example, the study of photochromic compounds, which undergo thermal irreversible and fatigue-resistant photochro-

252


L75

E

Me

1 b

Me-A-C-C-

-Me HZ HZ Me

c) L7Q

L78

181

180

L82

L86

N-

N L89

L90

191

192

FIG.60. List of ligands in copper(1) and silver(1) complexes with linear chain, 2-D and 3-D networks.

COPPERU) AND SILVER(I) SUPRAMOLECULAR METAL COMPLEXES

L99

L98

A c:v3

253

Lg7

H3

LlOl

mic reactions, is one of the key points in the current revival of interest in designing light-triggered molecular and supramolecular devices (209).Recent work by Munakata and co-workers has yielded a range of interesting and well-characterized materials with intriguing prop-

254


erties (210, 211). Reaction of copper(1) perchlorate solution with cis1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (L75) yielded single crystals of [ C U ( L , ~ ) ~ ] C the ~ O ~structure , of which is composed of noninteracting anions and macrocations in which each metal center is coordinated with one CN group of the four dithienylethene molecules and each L,s in turn bridges two copper(1) ions with two cyano groups, leading to an infinite network of metal cations as illustrated in Fig. 61(a). The most striking feature of the complex is that it shows reversible ring-closed and ring-open transformation under different wavelengths of light when probed by both electrical and NMR spectroscopy. This cycle can be repeated many times, indicating that the reversible cyclization reaction takes place in the crystalline state. The light-induced crystal oscillation is suggested to be one possible pathway to account for the approach and rotation of the two thiophene rings as shown in Fig. 61(b). Such a system may be used as an optical memory. 2. Organometallic Polymers

Several organometallic polymers containing copper(1) and silver(1) ions have been reported that, by definition, involve unanimously direct interaction between the carbon atoms of the ligand and the metal ions. The organic ligands studied include alkynyls, isonitriles, and a-conjugated systems. Addition of AgC104 to truns-[Pt(C= CH)2 (PMezPh)J gave a 1 : 1adduct of [PtAg(C104)(C=CH)2(PMe2Ph)2] (212) in which the trans coordination of the Pt is maintained and the Ag atom is coordinated on the same face of each Pt coordination plane, forming a zigzag chain with a perchlorate ion in each cavity along the chain as shown in Fig. 62. Recently Bertrand reported the first bis(carbene1-silver polymer [Ag(L&F3S03 obtained at -30°C from an acetonitrilelether solution of 1,2,4-triazole-3,5-diylidene (L76) and AgCF3S03 (213). In the cation each ligand links two linear silver atoms with C-Ag-C bond angles of 175-180",and due to the alternation in the orientation of the five-membered rings, the complex is regarded as a one-dimensional polymer in which all the rings are coplanar. Despite the fact that silver(1)-aromatic complexes of benzene, cyclophane, indene, acenaphthene, naphthalene, and anthracene have been reported, the corresponding organometallic polymers of pyrene (Led and perylene (L6g) have been crystallographically characterized only recently (187).X-ray structure determination of the complex with pyrene [Ag2(Lm)(ClOJ21 reveals that it exists in the solid state as an

255

COPPERW AND SILVERU) SUPRAMOLECULAR METAL COMPLEXES

*

405nm

550nm

twisted ring-open form

coplanar ring-closed form

Light-induced crystal oscillation

FIG.61. Schematic presentation of chain structure (a) and the light-induced crystal oscillation model (b) in [ C U ( L , ~ ) ~ ~ C ~ O ~ .

arene-linked polymer of dimers as shown in Fig. 63. Within the dimer, there are two independent Ag(1) ions coupled by one perchlorate-oxygen bridging with Ag(l)----Ag(2) separation of 4.39 Each pyrene moiety exhibits a tetra-q2-coordination fashion sequentially bridging four metal centers, resulting in a polymeric W-type sandwich

A.

256


L L

PMezPh

FIG. 62. Structure of the linear [PtAg(C104)(C-CH)2(PMe2Ph),3.(From Fig. 3 in Yamazaki, S.; Deeming, A,; Speel, D. M.; Hibbs, D. E.; Hursthouse, M. B.; Malik, K.M. A. J . Chem. SOC.,Chern. Commun. 1997, 177.)

of alternating aromatic and silver(1)perchlorate groups running along the b-axis as shown in Fig. 63(b). Several research groups have used isonitriles to prepare organometallic polymers (214,215).End-on coordination at the terminal carbons and linear geometry at the nitrogens impart to these molecular structures the characteristic of association of metal complexes into di-, tri-, and even polymeric species. Harvey has recently reported a series of new organosilver polymers of 1,8-diisocyano-p-methane (LT7), [Ag(L,,),]X (X = BF;, NO, or ClO;), with tubular structures in the solid state (214). Each ligand group bridges two tetrahedral silver atoms, forming zigzag chains that crystallize side by side separated by two rows of the counteranions (Fig. 64). Before this study two series of organosilver polymers with 2,5-dimethyl-2’,5‘-diisocyanohexane (L78)had been reported (215). Complexes of the first series contain one diisonitrile per metal with the formula [Ag(L78)IX(X = BF;, PF;, or NO;). The structures contain infinite chains of silver atoms alternating with bridging ligands. The second series crystallized as [Ag,(L78)31X2, and again, the extended trans-conformer of L7*alternating with Ag atoms generates infinite chains. However, the unique structural difference lies in a adjacent pairs of chains cross-linked by extra ligand groups, leading to a ladderlike pattern.


257

FIG.63. The W-type sandwich arrangement of an arene-linked polymer of dimers in [Agl(L,)(C104)2].(From Fig. 1 in Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Sugimoto, K. horg. Chem. 1997,36,4903.1

258


0

J

FIG. 64. Structure of the polymeric chain in [Ag(L&]NO,. (From Fig. 1 in Fortin, D.; Drouin, M.; Turcotte, M.; Harvey, P. D. J. Am. Chem. SOC.1997,119, 531.)

3. Heterometallic Cluster Chain Heterometallic polymeric cluster compounds have received considerable attention due to their useful properties, which are derived from low-dimensional structures. Haushalter and co-workers recently initiated a study designed to determine the possibility of introducing unpaired electrons to closed-shell, low-dimensional Zintl-phase materials by suitable structural modifications or elemental substitutions, with the aim of increasing their electrical conductivity (216).Figure 65 shows the structure of one such example, (Et4N)4[A~(Agl-xA~,)2 Sn2Te9],which contains a semiconductive B-A-wide, one-dimensional chain composed of four different elements surrounded by insulating organic material. The magnetic susceptibility of the compound [Fig.

FIG. 65. Structure of the 1-D polymeric chain [ A U ( A ~ , _ ~ ~ ) (From ~ S ~ Fig. ~ T 1 ~ ~ I ~ ~ . in Dhingra, S. S.; Seo, D.-K.; Kowach, G . R.; Kremer, R. K.; Shreeve-Keyer, J. L.; Haushalter, R. C.; Whangbo, M.-H. Angew. Chem., Znt. Ed. Engl. 1997,36,1087.)


259

66(a)l indicates that the chain is diamagnetic in nature and is nearly independent of temperature down to 4 K. Both resistivity data obtained by the microwave cavity perturbation technique and optical diffuse reflectance measurements as shown in Fig. 66(b) and ( c ) respectively, support the claim that the compound is a semiconductor and the delocalized one-dimensional band structure undergoes a Peierls distortion above room temperature. Heterometallic polymeric cluster compounds [W4Ag5S161[M(dmf),l (M = Nd or La) derived from tetrathiotunstate and silver(1) have been reported (217). The onedimensional polymeric anion can be regarded as octanuclear cyclic cluster fragments, [WAg4SI6l4-, linked through Ag' ions as shown in Fig. 67. The significantly short W-Ag bond length of 2.928(1) A is observed in the analogous compound [Ag4WS41(NH4), suggesting that Ag- > WS, electron delocalization and stronger continuous metalmetal interactions are present in this type of heterometallic chains. Reaction of (NEt4),MS4(M = Mo or W) with CuCl and KSCN (or NH4SCN)afforded a set of mixed metal-sulf'ur compounds containing infinite anionic chains Cu,(NCS),MS:- and (CuNCS),WSP- or twodimensional polymeric dianions (CuNCS),MSi- (218). Short W----Cu distances ranging from 2.62 to 2.70 A are observed in these compounds. Incorporation of polyoxoanions into one-dimensional coordination polymer matrices, either through direct condensation to form oxo-bridged arrays of clusters or through transition-metal coordination compounds acting as inorganic bridging ligands, provides a route to composite organic/inorganic materials. In this respect, two composite compounds containing Cu(1) complex ions of 4,4'-bipyridine (L51), 8H20, have recently been pre[CU(L51)4MO@26]and [CU(L~,)~MO,,O~,~ pared by intercalation of molybdenum oxide clusters into the void space in the Cu-LSl subunits (219). By comparison, in a new mixedligand C d I ) polymer containing LS1,[(PPh3)2Cu2C12(L51)1, a different lattice appeared (220). This structure consists of (PPh3)2Cu2C12 units that are bridged by L51 (Fig. 68). The significance of these studies is that members of the vast family of polyoxoanion clusters may be employed as structural motifs to fill tunable void volumes created by extended cationic frameworks, and it would appear that this synthetic approach provides a method for structural modification of metal oxide and, consequently, tuning of electronic, magnetic, and optical properties of the oxide phases. 4. Mixed-Valence Copper Complex with Chain Structure

The mixed-valence copper complexes with polymeric chain structures have been described. Reaction of the cyclic thioether ligand tet-

260


a

I

R I%

COPPER(I) AND SILVER(I) SUPRAMOLECULAR METAL COMPLEXES

261

FIG. 67. Structure of the l-D polymeric anion [W&g4Sl6l4-.(From Fig. 1 in Huang,

Q.; Wu, X.; Wang, Q.; Sheng, T.; Lu, J. Angew. Chem., Int. Ed. Engl. 1996,35,868.)

rahydrothiophene (LT9)with CuClz.2H20 in acetone yielded the polymeric, mixed-valence complex [ C U ~ ~ C U ~ ~ ( (221 L , ~).) ~The C ~structure ~I contains two distinct types of copper atoms: The divalent copper atoms involve an intemediate geometry between square planar and tetrahedral, comprising four chlorine atoms, and the monovalent copper atoms have distorted tetrahedral SzC12donor sets. The magnetic susceptibility data, ESR, and electronic reflectance spectra for the compound indicate that no intervalence interactions occurred between two Cu(1) and Cu(I1) sites.

5. Diradical Ligand Complex with Chain Structure Polymeric transition metal complexes of organic radical ligands have received recent attention in magnetic systems. The ligands concerned include the diradical 1,1',5,5'-tetramethyl-6,6'-dioxobis(verdazyl) (LBO). Reactions of LBO with copper(1)halides in acetonitrile or copper(I1) halides in methanol gave copper(1) complex [Cu2X2(L8,)I(2221, whose structure is found to be a polymeric chain with the tetrahedral copper atoms alternately bridged by two halide ions and two verdazyl diradicals [Fig. 69(a)l. Magnetic susceptibility measurements indicate that the spins couple in one-dimensional chains with alternating exchange parameters [Fig. 69(b)]. Although the intraligand electronic exchange is still antiferromagnetic in the system, the triple excited state is considerably stabilized compared with the free ligand as a result of an increasingly important superexchange through the copFIG.66. Physical properties of [Au(Ag,-,Au,),Sn~Te,l' . (a) Temperature dependence of the magnetic susceptibility ,y; (b) temperature-dependent resistivity p ; (c) optical diffuse reflectance R versus wavelength A of incident light. (From Fig. 4 in Dhingra, S. S.; Seo, D.-K.; Kowach, G . R.; Kremer, R. K.; Shreeve-Keyer, J. L.; Haushalter, R. C.; Whangbo, M.-H. Angew. Chem., Int. Ed. Engl. 1997,36,1087.)

262


FIG.68. View of the [(PPh,)zCu,Clz(L,,)]chain. (From Fig. 2 in Lu, J.;Crisci, G.; Niu, T.; Jacobson, A. J. Znorg. Chem. 1997,36,5140.)

Temperature IK

FIG. 69. View of the chain structure (a) and temperature dependence of magnetic susceptibility (b) in [CuzXz(L,)I. (From Fig. 3 and Fig. 7 in Brook, D. J. R.; Lynch, V. Conklin, B.; Fox, M. A. J. Am. Chem. SOC.1997, 119, 5155.)


263

per(1) center. Another example of coordination of diradical with copper metal is reported by Oshio in a copper(I1) complex of imino nitroxide [CuYL8JI(PF& obtained from the reaction of [Cu1(CH3CN),I PF6 with LSl in methanol (223), where HLBl is 2-(1‘-oxy-4’,4’,5’, 5’-tetramethyl -4’,5’ - dihydro - 1’H-imidazole- 2‘- yl)- 6 - ( 1”- oxyl- 4,4”,5”, 5”-tetramethyl-4,5”-dihydro-1”H-imidazole-2”-yl)pyridine. In this complex the square planar copper(I1) ions are bridged by the iminohydroxyamino anions to form a one-dimensional helical structure. Here again, the magnetic susceptibility data revealed antiferromagnetic interactions. From the investigation of the coordination chemistry of these unusual diradicals it can be expected that the bridging ligands LBOand the polypyridyl type of imino nitroxide hold great promise as building components for future design and construction of polyradical magnetic materials.

6. Carboxylate Ligand Complexes Although the silver(1) ion is regarded as a typical soft Lewis acid, it forms a variety of coordination compounds with carboxylate ligands. Mak classified the coordination modes of Ag carboxylates into four types, most of which are constructed from either dimeric units or polymeric networks of dimeric subunits (224). The same author prepared and characterized two polymeric silver(1) complexes of betaine, ~ A g z ~ M e ~ N C H z C O O ~ z ~ H zand O ~ z [Agz(C~H~NCH~COO),(C10~)21 ~NO~~~l (224).The two complexes are structurally similar, both consisting of centrosymmetric carboxylato-bridged Ag,(carboxylato-O,0’)2 dimers, which extend into a stairlike polymer through the coordination of each metal center by a carboxylate oxygen atom from an adjacent dimer. It is worth noting that both polymers are found to involve very short intradimer Ag----Agdistances of 2.898(1) and 2.814(2) respectively, compared with other dinuclear silver(1) carboxylates.

A,

7. Thio-Ligand Complexes Although both copper(1) and silver(1) metal ions are expected to readily form coordination compounds with thio ligands by covalent bonds, few polymeric structures for thioether crown complexes have been reported. Among these few examples, two silver(1) complexes with thioether macrocyclic ligands are particularly interesting (225). Reaction of [24]aneS8(Lag)with 2 molar equivalents of AgCF3S03gave a one-dimensional polymer, [Agz(L82)(CF3S03)z(MeCN)21, in which each Ag atom is coordinated to four S-donors in a distorted tetrahedral geometry. As illustrated in Fig. 70, the four S-donors come from two different ligand molecules to generate an infinite ladder polymer

264


0 0

0 0 0 0 FIG.70. View of the polymeric chain in [Ag2(L8,)I2+. (From Fig. 1 in Blake, A. J.; Li, W.-S.; Lippolis, V.; Schroder, M. J . Chem. Sac., Dalton Trans. 1997,1943.)

along the b-axis. A similar reaction with the crown thioether containing four S-donors, [16]aneS4(LES),gave a three-dimensional polymer, [Ag(LEs)(BF4)1, in which each Ag(1) ion is coordinated by four symmetry-equivalent S-donors in a tetrahedral geometry, and each ligand group in turn bridges four different Ag atoms. The work illustrates the potential of thioether crowns as building blocks for the synthesis of inorganic architectures using their ezo-orientated S-donors. The rodlike ligand 2,l l-dithia[3,3lparacyclophane (Lm)is found to be a particularly versatile organic ligand for the formation of polymeric chains in the presence of bridging anions (226, 227). The arrangement of such chains in the lattice is controlled and modified by the metal ions and coordination of a suitable anion. Two copper(1)and one silver(1) complex of LE4have been reported (226), in one of which, [ C U ~ B ~ ~ ( L ~ ) ( M the ~CN two ) ~Br ] , atoms bridge pairs of Cu(1) ions to form a rhombic CuBrCuBr ring as shown in Fig. 71 and each ligand molecule links two separate metal cations on each side through two sulfur atoms, resulting in a one-dimensional polymeric chain running parallel to a diagonal axis of the triclinic cell. Silver(1) complexes of 1,4-thioxane (L,) have been reported (228). Both [Ag(L,)(CF,SO,)I and [Ag2(L86)(CF3S03)21 were isolated from the mixture of Lg5 and silver triflate at ambient temperature in a 1: 1 mixture of dichloromethane and acetonitrile. The former involves silver(1) cations tetrahedrally coordinated by two sulfur atoms of two Lgg molecules and the oxygen atoms of two bridging triflate anions, giving in the crystal an infinite one-dimensional chain with bridging only through the triflate moieties. Although the immediate coordination environment around the silver(1) ion in the latter complex is similar t o that in the former, bridging between Ag' cations occurs via the participation of both 1,4-thioxane sulfur and triflate oxygen atoms, leading to a two-dimensional lattice.

COPPERU) AND SILVER^ SUPRAMOLECULAR METAL COMPLEXES

265

FIG. 71. View of the infinite chains in [CuzBrz(Llcl)(MeCN),l. (From Fig. 1 in Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Nakagawa, S. J. Chem. Soc., Dalton Trans. 1996,1525.)

Silver(1) complexes of the nonchelating dithioether ligand have been described (229). Crystallographic studies of [Ag(MeSCH&H, CH2SMe)lBF4revealed a chain structure involving trigonal (p,-S),,S ligation with one S donor bridging two adjacent Ag cations while another S donor of the dithioether ligand is nonbridging. In contrast, revealed monodenX-ray analysis of [Ag(PhSCH2CH2CH2SPH)21BF4 tate S ligation of four distinct dithiother ligands to the Ag atom, giving an approximately tetrahedral geometry at the metal ion, with the second S donor of each ligand linking to four other Ag' ions, hence generating a three-dimensional infinite lattice. Due to the biological relevance of copper-sulfur coordination compounds to metalloproteins, the copper(1) complex of thiourea, ICU{SC(NHCH~)~)~INO~, has been prepared from an aqueous solution of copper(I1) nitrate and N,"-dimethylthiourea (230).The structure contains substantially distorted CuS, tetrahedra, each being linked by common edges with two neighbors, resulting in infinite isolated chains parallel to the c-axis of the cell. The nitrate anions are not coordinated to the silver(1) ions, but associate the individual chains of edge-sharing CuS, tetrahedra together. 8. Other Nitrogen-Containing Ligand and Mixed-Ligand Complexes

Mingos and co-workers have recently reported several novel silver(1) complexes with simple polydentate acyclic nitrogen-donor ligands such as diethylenetriamine (dien) and tris(2-aminoethy1)amine

266


(LE6)(231). The dien complex [Ag(dien)lPF, is slightly light-sensitive, but may be stored indefinitely under nitrogen at -30°C in the dark. The complex consists of two crystallographically independent silver atoms and two nonequivalent dien ligands, which link adjacent T-shaped silver centers to form a catena structure. The tren complex [Ag(LBs)]PF6 also has a zigzag chain structure and possesses similar structural features except that the two silver(1) ions involve distorted tetrahedral geometries and tren acts essentially as a tetradentate ligand. Luminescent coordination compounds continue t o attract considerable attention. Zink recently reported a new mixed-ligand copper(1) polymer that shows interesting photoluminescence (232). The complex [CuCl(L,,)Ph,P] consists of a one-dimensional chain lattice of metal ions bridged by both C1- ions and pyrazine molecules. The compound shows conductivity of less than lo-* S cm-'. The absorption spectrum of the complex shows a band at 495 nm, which could be interpreted as the promotion of an electron from the valence band to the conduction band. On the basis of resonance Raman spectra, the lowest excited state in the polymer is assigned to the Cu(1)-topyrazine metal-to-ligand charge-transfer excited state. In the previous section, l-cyanoguanidine (L5,) was quoted as an example of the multifunctional ligands for the formation of H-bonded frameworks (157). Because this molecule can act either as a monodentate ligand (nitrile N donor) or as a bidentate bridging ligand (nitrile and amino N donor) a number of 1 : l and 1 : 2 l-cyanoguanidinecopper(1) halide adducts have been reported (233). The complex [CuX(L5,)I,where X = C1- or Br-, is characterized by two mutually perpendicular chains of [CuX(L,,)], and (CuXi), joined at a common halogen. Within these two individual chains the copper atoms are found to involve different distorted tetrahedral and trigonal planar stereochemistries, respectively. Among the mixed-ligand-complexed copper(1) complexes, the one formed by pyrazine (Lso) and 3-methylpyridazine (L8,) deserves to be mentioned (234). The compound [cuZ(L8,)Z(LS~>~1(c104)2 was prepared by the stepwise reaction of copper(1)with two ligands. The two copper centers involve different coordination environments, one in a threecoordinate Y-shaped form and the other in a typical four-coordinate tetrahedral form. The structure consists of a tetracopper unit bridged alternately by Lg, and LDBgroups (Fig. 72). In addition, the cyclic voltammogram of the complex reveals two reversible redox couples with Ellz = +0.22 and +0.53, which are ascribed to the Cu'Cu'/Cu'Cu" a


0

4-coordinate copper

0 3-coordinate copper

+

267

inUaunit prz

nmpz

FIG.72. Structure of an infinite cationic chain in [Cu2(L,,),(L,,),I(C10,),. (From Fig. 1 and Fig. 2 in Kitagawa, S.; Munakata, M.; Tanimura, T. Chern. Lett. 1991, 623.)

and CurCu"/CurrCu" reactions of the closely contacting dicopper moiety, respectively. In a n attempt to use dicopper(1) complex unit as building block for synthesis of metal-containing polymers, Kitagawa reported his interesting results in synthesis and structural characterization of a n infinite stair-type chain compound (235).The complex of thiochrome (L88) [CU~(L,&I(C~O,)~ was prepared by reduction of copper(I1) perchlorate followed by reaction with the ligand. The structure contains a dinuclear cationic unit in which the two T-shaped metal ions are doubly bridged by two thiochrome molecules with extremely short Cu----Cu

268


A.

contact of 2.476(3) The dimeric units are mutually linked by the 2-hydroxyl group of the coordinated ligand group, giving a novel stairtype chain. The cyclic voltammogram in the solid state gives a single quasi-reversible wave, indicating that the dicopper site undergoes redox reactions. Coordination compounds of biheteroaromatic ligands have been extensively studied (236). The nitrogen-rich ligand, 3,5-bis(2-pyridyl) pyrazole (Lee),in which the pyrazole ring is attached to bulk nitrogen heterocycles a t the 3-and 5-positions, belongs to this class. Polymeric silver(I) complexes containing the uninegative pyrazolate ion had been known for over a century when the formation of an insoluble silver pyrazolate salt Ag(pz) was reported; however, its structure has never been crystallographically established (237). Fortunately, the structure of the polymeric silver(1) complex of the pyrazole derivative ligand Leuhas been determined recently (238).Along with it, Munakata has also reported a copper(I1) and another silver(1) complex with Lsa. The reaction of Leuand AgC10, in different solvents at various molar ratios surprisingly leads to the isolation of two totally different - 2MezC0, and one silver(1) complexes, one dimer, [Ag2(L8u)41(C104)z polymer, [Ag(L,,)]ClO,. In the polymeric structure the coordination around the metal ion involves a distorted trigonal planar core, AgN3, and each LSl molecule bridges two metal centers, giving an extended zigzag chain as shown in Fig. 73. Between the chains there is no effective interaction present due t o the presence of anion column separating the cationic chains. Like Lee,hexamethylenetetraamine (LW)is also a potentially tetradentate ligand. Two silver(1) coordination polymers with this polydentate N-donor base have been reported, and one involves a one-dimensional lattice (239). Reaction of AgSbF, and Luoin a 1: 1 molar ratio in ethanol-CHuClz isolated the compound [Ag(Luo)lSbF,.HzO. The structure contains a novel type of ribbon formed by hexagonal meshes of alternate Ag' ions and Lw,molecules. The water molecules coordinated to the silver ions form O-H----N hydrogen bonds with the bo groups of adjacent ribbons, which generate a three-dimensional network of unprecedented topology (Fig. 74). In terms of Pearson's hard-soft acid-base principle, the soft Lewis acid copper(1)ion is not compatible with the hard base HzO molecule, but a recent article provided the first example of a copper(1)-water bond (240).The novel copper(1) complex with 2,3-diphenylquinoxaline (Ld, [Cu(LUJH~O)IC10,,is diamagnetic and indefinitely stable in air in the solid state. Its structure consists of an infinite polymeric cationic chain in which adjacent metal centers are bridged by the aro-

COPPERU) AND SILVERII) SUPRAMOLECULAR METAL COMPLEXES

269

FIG.73. View of the chain structure in [Ag(L,)]-. The dashed line shows the contact between the nearest silver atoms, Ag----Ag5.67 A. (From Fig. 3 in Munakata, M.; Wu, L. P.; Yamamoto, M.; Kuroda-Sowa, T.; Maekawa, M.; Kawata, S.; Kitagawa, S. J . Chem. SOC.,Dalton Trans. 1995, 4099.)

matic nitroeens of the bidentate ligand of bl.The Cu-0 bond length of 2.154(6) A is longer than the calculated distance of 1.99 A based on the bond-valence-sum models. Therefore, the coordination of the copper is best described as distorted trigonal planar CuNIO with a weak interaction with the water molecule.

Q

Q

FIG. 74. One-dimensional ribbon in [Ag(L,,)lSbFs. H,O. (From Fig. 2 in Bertelli, M.; Carlucci, L.; Ciani, G.; Proserpio, D. M.; Sironi, A. J. Muter. Chem. 1997, 7, 1271.)

270


Several copper(1) and silver(1) complexes with trans-1,2-bis(2-pyridy1)ethylene (Lo,) have been reported (241). In [M(L,dIX, where M = Cu or Ag and X = PF, or ClO;, a distorted linear coordination geometry completed by the nitrogen atoms of two different ligands is observed for both Cu' and Ag' ions. Both complexes have a similar chain structure, and they only differ in the mode of polymerization, being of rectangular and triangular wave chain types, respectively (Fig. 75). Additionally, the pyridine ring contact between the adjacent chains in the copper complex is 3.46 indicative of r-r interactions present. In another copper(1) complex, [Cu(Lo,)(CO)(CH,CN)]PF,,an infinite

A,

b

i (b)

FIG. 75. ORTEP views of the cationic chains in [Cu(L,)IPF6 (a) and tAg(L)1C1O4 (b). (From Fig. 1 in Powell, J.; Horvath, M. J.; Lough, A. J. Chern. SOC.,Dalton Trans. 1996,1669.)


271

one-dimensional chain based on bridging of Lg2between two metal ions is also observed. However, with additional coordination of carbony1 and acetonitrile, each metal center involves a tetrahedral core CUN~N'C. Coordination compounds containing simple alkyl and aryl halides have been structurally characterized (242, 243). Novel examples include silver(1)-iodocarbon complexes obtained from the reaction of AgPFs and AgN03 with diiodomethane, 1,3-diiodopropane, and simple aryl iodides (243).The compound [Ag(NO,)(l,2-12C6H4)lis composed of spiral chains of [Ag(p-N03)I, as shown in Fig. 76, in which the nitrate uses only one oxygen atom to bridge two silver ions. In [Ag{I(CH2)31}al PF6,[Ag(ICH21)21PF6, [Ag(l,2-I2C6H4),lPF6, and [Ag(l,2-BrIC6H4)41PF~, the structure consists of tetrahedrally coordinated Ag' ions and bridging iodocarbon ligands, giving rise to a chain polymer array.

STRUCTURES B. TWO-DIMENSIONAL 1 . Polycatenane and Polyrotaxane Complexes

Catenanes are a group of interlocked or catenated molecules, and rotaxanes are a class of molecules in which a dumbbell-shaped compo-

FIG.76. Structure of the spiral chains in IAg(N03)(1,2-I,C,H,)].(From Fig. 8 in Powell, J.; Horvath, M. J.; Lough, A. J. Chem. SOC.,Dalton Trans. 1996, 1669.)

272


nent is encircled by a macrocycle. The efficient synthesis of such systems fascinated and inspired many scientists in the 1960s and 1970s (244). Due to the introduction of metal templating and supramolecular approaches for molecular threading and interlocking, this area has experienced a renaissance in recent years (245,246).Multicomponent polycatenane and polyrotaxane molecular systems incorporating metal-ion templates represent a particularly interesting class of supramolecular species that might exhibit intriguing chemical topology and display some fascinating electronic, optical, and magnetic properties (247-251 ). Schroder and co-workers have successfully isolated a unique polycatenated undulating molecular ladder that forms interwoven twodimensional sheets (252). The reaction of [CU(M~CN)~]PF~ with 1,4bis(4-pyridy1)butadiyne (Lg3)in MeCN-CH2C12 yielded the complex [ C U ~ ( M ~ C N ) ~ ( L ~ ~ )The ~ ] (compound P F ~ ) ~ . exists as a network of molecular ladders in which the two independent Cu' centers are each coordinated in a tetrahedral geometry t o three LBsgroups and one MeCN molecule. The lattices are polycatenated to give a remarkable twodimensional layer structure (Fig. 77). The sheets of interwoven molecules are separated by PF, counteranions and solvent molecules. The structure is further stabilized by the 7r-v interactions between adjacent, symmetry-related ladders. Robson has constructed a two-dimensional polyrotaxane sheet from the rodlike ligand 1,4-bis(imidazol-l-yl-methyl)benzene(I&,) (253). The compound [Ag2(L,)3(N03)21 was obtained by reaction of LB4and silver nitrate in aqueous methanol. The coordination polymer consists of one-dimensional chains in each of which the metal center is located

& -l

L93

FIG. 77. View of the polycatenated sheet structure in [Cu2(MeCN),(L,),I(PF,), . (From Fig. 3 in Blake, A. J.; Champness, N. R.; Khlobystov, A.; Lemenovskii, D. A.; Li, W.-S.; Schroder, M. J . Chem. SOC.,Chem. Cornmun. 1997, 2027.)

COPPER(1) AND SILVER(I1 SUPRAMOLECULAR METAL COMPLEXES

273

within the plane of the N3 donor set. The individual chains are associated t o produce the polyrotaxane sheets as shown in Fig. 78, in which the LDpmoiety provides both the rodlike segments and two half-loops for each of the rings. K m and Whang have constructed a polyrotaxane containing cyclic beads threaded on two-dimensional coordination polymer networks (254). The compound [Agz(L9s)a~cucurbituril)31(N0,),~ 40H20, where LDs = N,N’-bis(4-pyridylmethyl)-1,4-diaminobutane dihydronitrate and cucurbituril = C36H36N24012, was prepared by the route shown in Fig. 79. The compound is a polyrotaxane in which the cyclic beads of cucurbituril are threaded on a two-dimensional coordination polymer network. The two-dimensional polyrotaxane network forms layers stacked on each other, which in turn fully interlock with themselves. 2. Two-Dimensional Network Comprising Copper Clusters Copper(1) thiolate complexes show remarkable diversity of structure, and the most common structural unit in the known thiolate is a three-coordinate copper atom bonded to p2-bridging thiolates (15). Parish has recently reported a novel compound in which the thiolate sulfur atom involves unusual four-way bridging (2551. The compound with composition [ C U , ~ C ~ ~ ~HzO ( S R(R ) ~=I CHzCHzNH3) ~ was obtained by reaction of CuCl and cysteamine hydrochloride in an aqueous solu-

FIG.78. A 2-D polyrotaxane network in [Ag,(L94),3(N03)21. (From Insertion Fig. V and Fig. 2 in Hoskins, B. F.; Robson, R.; Slizys, D. A. J.Am. Chem. SOC.1997, 119, 2952.)

274


FIG.79. Construction scheme for the 2-D polyrotaxane network of [Ag,(L&(cucurbi40Hz0. t~rilhI(N0~ ) ~ . (From Scheme 1in Whang, D.; Kim, K. J. Am. Chern. SOC.1997, 119, 451.)

tion. X-ray analysis revealed a polymeric structure containing a centrosymmetric Culz cubo-octahedron unit. A regular octahedron of sulfur atoms, bearing organic chains pointing outward, has each edge bridged by a CuCl unit. There are thus three mutually perpendicular, planar, eight-membered [(CuCl)(p2-SR)],rings intersecting at the sulfur atoms, giving a gimba-like “Atlas-sphere” configuration (Fig. 80). The Cu12C112(SR)6 clusters are further linked to form two-dimensional sheets with different linkages in the two directions. 3. Two-Dimensional Network Containing Hexagonal Meshes and a Zeolite-like Framework

Ciani and co-workers have reported the self-assembly of two remarkable polymeric networks based on an anionic acetonyl derivative of tetracyanoethylene (256).The attempts to obtain novel extended frameworks of Ag ions bridged by the neutral tetracyanoethylene


275

FIG.80. Structure of the basic [Cu13C113(SR)61 unit and the layers of clusters forming sheets. (From Fig. 1 in Parish, R. V.; Salehi, Z.; Pritchard, R. G. Angeru. Chern., Int. Ed. Engl. 1997, 36, 251.)

have led to the unexpected formation of [Ag(L,)I, where L, = 1,1,2,2tetracyanopentan-4-one-1-ide. The structure consists of twodimensional undulated nets of hexagonal meshes formed by alternate three-coordinate silver ions and tridentate anion LBs(Fig. 81). Two independent nets of this type interpenetrate to give a layer structure. The targeted synthesis of [ C U ( M ~ C N ) ~ ]with P F ~the preformed conju-

276


FIG.81. View of the twofold interpenetrated layer of [Ag(L,)l. (From Fig. 1 in Carlucci, L.; Ciani, G.; Proserpio, D. M.; Sironi, A. Angew. Chem., Znt. Ed. Engl. 1996, 35, 1088.)

gated acid of LBBin acetone gave the corresponding copper(1) polymer [Cu(LBB)I. Me2C0.Although the compound holds the same stoichiometry as the silver complex, its structure consists of a three-dimensional neutral network of four-coordinate copper(1)ions linked by anions LW. The framework resembles a honeycomb with large channels that include guest acetone molecules. The large difference between the two structures can be related t o the higher tendency of Cu' ions over Ag' ions to achieve a tetrahedral coordination. Therefore, by using this methodology it should be possible to generate other topologies with different metal ions and/or with similar polycyano anions. The assembly of a long-range solid-state superstructure with a zeolite-like framework has been exploited by a number of research groups (128,227).Recently, it has proven possible to synthesize coordination networks with microporous structures that maintain crystal integrity upon loss of guest species. Such zeolite-like behavior of a coordination network represents a new challenge for chemists to look for new class of microporous substances. Reaction of silver triflate and the tritopic ligand 1,3,5-tris(3-ethynylbenzonitrile)benzene(Lu,) yielded solid [Ag(L,,)(CF,SO,)]. 2C6H6 (257).The compound contains a two-dimensional network in which the coordination geometry around silver is trigonal pyramidal, with three nitriles of the network in the basal plane and a trilate counterion in the apical position as shown in Fig. 82. The sheets are stacked, creating a channel structure in which solvent benzene molecules reside in the micropores. The com-

COPPERU) AND SILVERU) SUPRAMOLECULAR METAL COMPLEXES A

277

t

FIG. 82. Structure and coordination network in [Ag(L,,)(CF,SO,)I.(From Fig. 1 in Venkataraman, D.; Gardner, G. B.; Lee, S.; Moore, J. S. J.Am. Chem. SOC.1996,117, 11600.)

pound shows stability toward partial loss of inclusion molecules. The TGA trace of the compound reveals two discrete mass losses at 110 and 145°C (Fig. €431, corresponding to the removal of distorted benzene molecules. At 145”C, a solid-to-solid phase transition occurs concomitant with the loss of the remaining benzene molecules. DSC and optical microscopy indicate no evidence of a phase change associated with the first mass loss. Further observations demonstrate that the sample can reversibly remove and reabsorb benzene molecules without collapse of the channel network analogous to that of zeolite-type materials. 4. Two-Dimensional Network Containing an Undulated Wave

Layer Structure In the Section VIILA, “Infinite-Chain Structures,” we noted that Mak et al. characterized a number of carboxylato-bridged silver(1) complexes with chain structures (224). The same research group has

278


temperature ("C)

FIG.83. Overlay of TGA and DSC traces for [Ag(L,,)(CF,SO,)]. (From Fig. 2 in Venkataraman, D.; Gardner, G. B.; Lee, S.; Moore, J. S. J.Am. Chem. SOC.1996,117, 11600.)

also employed dicarboxylate-like ligands rneso-2,5-bis(trimethylammonio)adipate (Log)and rneso-2,5-bis(pyridinio)adipate(L,) to synthesize silver(1) complexes with two-dimensional networks (258).Structure analysis revealed that in each of the four complexes [Agp(LQe)l(C104)z, [Agz(LQ9~I(C104)z, [Agz(L~g)l(NO,),,and [Ag~(Lw)l (NO,), , a dimeric unit is involved in which each pair of adjacent metal atoms is doubly bridged by coplanar syn-syn p-carboxylate-0,O' groups. The dimeric subunits are extended into a step polymer through the linkage of each metal center to a carboxylate group of an adjacent dimer. The basic coordination environment for the metal is completed by three carboxylato oxygen atoms in a T-shaped geometry, if the interaction between the anion and the metal center is ignored, as occurred in two perchlorate complexes. Because the nitrate anion functions as a unidentate and 0,O'-bridging mode in the third and the last complex, respectively, it leads correspondingly to a distorted tetrahedral and unusual square pyramidal five-coordination at Ag' ion. Figure 84 shows the structure and packing drawing of the layer structure of the five-coordinate silver complex. All compounds exhibit short Ag----Agseparation in the dimeric unit, ranging from 2.794(1) A to 2.878(2) A, substantially smaller than that in metallic silver (2.89A). It is debatable whether these distances are a result of the bridging ligand or suggestive of strong metal-metal interactions present. Crown thioether can not only form a one-dimensional chain structure, it can also form two-dimensional sheet frameworks (225).The crown thioether ligand (OH),[lGlaneS, (Llm)reacted respectively with silver nitrate and silver acetate, giving two polymeric silver(1) com-


279

FIG.84. Structure skeleton of the asymmetric unit (a) and packing drawing of the layer structure (b) in [Ag2(L,)(NOa)pl.(From Fig. 5 in Wu, D.-D.; Mak, T. C. W. J. C h e n . Soc., Dalton Trans. 1995, 2671.)

plexes, [Ag(Lloo)lOzCMe and [Ag(Lloo)lN03 (259).The two compounds are not isostructural, but their structures and dimensionality seem to be controlled by change of the anions. In the complex with silver acetate, each metal center is bound to four thioether sulfur atoms in a rather distorted tetrahedral fashion. The silver atoms are in a twodimensional sheet arrangement interconnected by the macrocycle in which two crystallographically independent Ag' ions are stacked alternately along a diagonal to the a- and c-axes of the unit cell (Fig. 85). By contrast, the complex with silver nitrate consists of a three-

280


-C

b

FIG.85. Packing diagram for [Ag(LlOo)lO2CMe with dashed lines showing the hydrogen bonds. (From Fig. 4 in Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M. J. Chern. SOC.,Dalton Trans. 1995,3215.)

dimensional tetrahedral network linked by an exodentate coordination of the thioether.

5. Conductive r-Complex Although the silver(1)-pyrene complex [Agz(Lee)(CIO&]is found to display a chain structure as already mentioned, a similar complex with perylene, [Agz(L6s)(C104)z], consists of a two-dimensional framework of the metal ions bridged by the bidentate counterions and the tetra-$-arene groups shown in Fig. 50 (187). Such two-dimensional sheets are further connected by the superpose! intersheet aromatic rr-r stackings at an average distance of 3.31 A, generating a threedimensional network structure. The physicochemical property measurements of two complexes show that a t room temperature no strong ESR spectrum was observed for each case, but the light-irradiated samples show a characteristic g value attributable to the aromatic radicals, suggesting that upon irradiation electron transfer partially takes place from the aromatic donor to the silver (I) ion, giving an organic radical cation and silver(0). In addition, both compounds are electrically nonconducting, whereas their Iz-doped samples display semiconducting behavior at ambient temperature, which is presumed

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to connect mainly with the nonintegral formal oxidation state of the crystallized organic radicals. These conducting radical cation complexes might constitute new functional materials among lowdimensional molecular solids similar to polymetallocenes and multidecker metallacarboranes. A further example of a silver(1) n-complex with a two-dimensional sheet framework is found in a silver(& (243). The solid-state iodocarbon complex, [Ag2(02PF2),(p-IC6H4Me)l structure of the compound is composed of an infinite two-dimensional sheet array of [Ag(O,PFz)l, in which each p-iodotoluene bonds to two Ag' ions via one I-Ag bond and an $-arene bond involving two of the carbons not bonded to I or Me. 6. Inorganic Polymers

The development of inorganic polymer chemistry based on incorporation of transition metals into a polymer main chain has resulted in diverse arrays of interesting structures in recent years. These polymers are expected to constitute a new type of inorganic functional material. It is not the aim in this section to undertake a comprehensive treatment of this field; for this, the reader is directed to the recent reviews and the references herein (260,261).Here we just list a few examples of inorganic polymers that incorporate copper(1) or silver(1) ions with two-dimensional sheet frameworks. Pfitzner has carried out a systematic investigation on new adducts of copper(1) halides with neutral chalcogen fragments or polychalcogenide anions (262).One of them, (CuI)&uzTeSy,was found to contain layers of the complex thioanion [TeSJ- embedded between layers of iodide ions. Copper atoms are distributed in the arrangement of iodine and sulfur atoms. Kanatzidis and Zhang have utilized sulfur-rich mixed polysulfide/ telluride fluxes in combination with transition metals to synthesize novel solid-state materials (263). Thus, six new quaternary copper(1) and silver(1) compounds with composition AMTeS3,where A = K, Rb, or Cs and M = Cu or Ag, were prepared. The structure consists of anionic [MTeSJt- layers and charge-compensating alkali ions between the layers. Each layer is composed of tetrahedrally coordinated Cu' or Ag' centers and trigonal pyramidal TeSi- units, joined together via bridging S atoms. Similar low-dimensional quaternary compounds, KCu,AsS3 and KCu4AsS4,have also been reported, and a unique layered framework was observed in both compounds in which the Cu(1) ions are linked in a complex manner by a series of trigonal ASS:- groups or ASS:- groups as well as S2-ions (264).

282


7. Inorganic Grid and Heterobimetallic Aggregates Inspired by the idea that an n-topic ligand would form an n X n inorganic grid based on n2metal ions and 2n ligand components, Lehn and co-workers have reported the self-assembly of the inorganic 3 X 3 grid consisting of an array of nine silver(1) metal ions and six tritopic ligands (265). The complex [Ag,(Llol)sl(CFBSO~)9, where Llol = 6,6’-bis[2-(6-methylpyridyl)]-3,3’-bipyridazine, contains an arrangement of a 3 X 3 grid of nine Ag’ ions chelated by six bidentate ligand molecules as shown in Fig. 86. Such an architecture opens the way to a whole family of polynuclear inorganic m X n grids, which represent a new facet in the controlled arrangement of metal ions into specific arrays and patterns. Among the previously discussed heterobimetallic aggregates of copper(1) with thiotungstates and thiomolybdates, those containing the two-dimensional polymeric dianions (CuNCS),MSq- are noteworthy (218). X-ray diffraction results show that four edges of the tetrahedral MSq- (M = Mo or W) core are coordinated by copper atoms forming WS4Cu4 aggregates linked by eight-membered rings of - CU(NCS)~CU-(Fig. 87). 8. Two-Dimensional Sheets Containing Cu6Hexagons

The rodlike ligand La was also found to form a polymeric complex with copper(1) (2261, but the complex obtained by reaction of La with 1 molar equivalent of CuI in thf, [Cu2I2(La),1.thf, is surprisingly not isostructural of the corresponding complex of the bromide. Instead, it

FIG. 86. Crystal structure of the self-assembled 3 X 3 inorganic grid [Ags(L,,,),] (CF,SO,),. (From Fig. 3 in Baxter, P. N. W.; Lehn, J.-M.; Fischer, J.: Youinou, M.-T. Angew. Chem., Znt. Ed. Engl. 1994,33,2284.)


283

6

FIG. 87. Perspective view of zig-zag anionic chains in (CuNCS),MS; . (From Fig. 2 in Manoli, J. M.; Potvin, C.; Secheresse, F.; Marzak, S. Inorg. Chem. Actu 1988, 150, 257.)

is composed of a two-dimensional framework in which a large ring containing six copper atoms includes a thf molecule as guest inside the open cavity (Fig. 88). As in the bromide, the rhombic CuICuI ring formed by bridging the two iodide ioas between pairs of copper atoms leads to a Cu----Cudistance of 3.18 A and an I----Idistance of 4.30 A. Each L, fragment bridges two separate metal cations through two sulfur donors, resulting in a two-dimensional sheet arrangement of copper(1) ions. The framework contains six-membered rings of Cu6 hexagons incorporating the thf molecule in the open cavity. In contrast to the formation of the rhombic ring by copper(1)halide ions, the one-nitrate oxygen bridging two metal ions in the silver(1) complex [Ag(L,,)(NO,)l yielded a unique three-dimensional channel framework of silver(1) ions (226). For these reasons, both metal ions and counteranions are considered to be the fundamental factors controlling crystallization and variation of frameworks.

STRUCTURES C. THREE-DIMENSIONAL 1. Construction of Channeled Frameworks

The rational design of multidimensional coordination compounds incorporating large cavities or channels capable of hosting small mol-

2 84


(b)

FIG.88. Molecular packing diagram of complex [Cu212(L&I. thf (a) and a view of the 2-D sheet consisting of Cue hexagons (b) where only the metal centers are presented as open circles. (From Fig. 3 in Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Nakagawa, S. J. Chen. Soc., Dalton Trans. 1996, 1525.)

ecules is currently recognized as one of the most important issues in the synthesis of functional materials for molecular sieves, shapeselective catalysis, ion exchangers, and many other applications ( 4 , 266-270). The approach generally employs rigid and highly symmetric organic units as building blocks via intermolecular forces such as

COPPER^ AND SILVER^ SUPRAMOLECULAR METAL COMPLEXES

285

hydrogen bonds (159-163) or coordination to metals. Such zeolite-like behavior of the coordination network in copper(1) and silver(1) complexes is not limited to two-dimensional structures (257). In fact, the majority of examples are found in three-dimensional copper and silver complexes of nitrogen ligands derived from organocyanides (121-128, 271 1, 4,4’-bipyridyl, and hexamethylenetetraamine. The simple ligand 4,4’-bipyridyl has been chosen by many groups as a rod linking together metal centers to give extended solids with diverse topologies (272-275). Reaction of AgNO, and L51 in MeCN gave the silver(1) polymer [Ag(L,,)INO, (273,274j. The crystal structure reveals a triple interpenetrated cationic network schematically shown in Fig. 89, each consisting of linear [Ag(L51)lru chains cross-linked by Ag-Ag bonds of 2.970(2)A. The resultant open framework contains large interconnected cavities that form stairlike microchannels of approximate cross section 6 X 23 A. Rather small channels are observed in the corresponding complex of copper(1) chloride, [Cu(LS1)C1], obtained by diffusing solutions of CuCl and L5,into ethylene glycol (275). The structure contains chloro-bridged Cu’ dimers, which form a sheet framework with 4,4’-bipyridyl molecules. The overall structure thus formed is a neutral three-dimensional framework, in which the interpenetrating two-dimensional sheets fill yp most of the pore space with channels of only small diameter (2 X 4 A) remaining open, too small to accomodate acetonitrile or DMSO solvent molecules as guests in the crystal. Paralleling the approach used in supramolecular organic chemistry of adamantyl templates with different synthons (266-268), Ciani and

FIG.89. Schematic representation of the 3-D framework in [Ag(Lsl)lNO:,,with circles representing Ag cations, filled bars representing L51 and empty bars representing Ag-Ag bonds. (From Scheme 1-D in Robinson, F.; Zaworotko, M. J . Chem. Soc. Cheni. Commun. 1995, 2413.)

286


co-workers have developed a scheme using the potentially tetradentate ligand hexamethylenetetraamine (L,) for the preparation of supertetrahedral networks with metallic synthons (239,276-278). They employed reaction of Lm with different silver(1) salts of noncoordinating anions and isolated a number of polymeric silver complexes with three-dimensional frameworks. The first of this series, [Ag(L,)I PF6.HzO,was reported in 1995 (276). The compound was obtained by slow evaporation of an ethanolic solution of AgPF6 layered on a solution of L, in CHzClz.The structure contains a molecular-based framework topologically related t o the three-dimensional, three-connected cubic net of highest symmetry as schematically shown in Fig. 90. The PF; anions and the guest water molecules occupy the large octagonal channels and form an extended hydrogen-bonding network. Similar reactions under different conditions afforded three other compounds. The structure of [Agll(L,)6](PF6)l,.14Hz0 consists of an open threedimensional cationic frame, with all the Lm molecules acting as tetradentate ligands while nine silver cations are biconnected and two are triconnected to the ligands per formula unit (239). The (3,4)-connected network schematically shown in Fig. 91 is composed of triconnected (silver ions) and tetraconnected (L,) centers in the ratio 1: 3. Such connection generates two types of parallel channels to host the anions and the water molecules. The compound [Ag5(LW))6](PF6)5. 3CHzC12is - 3EtOH gives a threean oligomer, whereas [Ag,(Lso)(HzO)l(PF6)4 dimensional network with large cavities and channels of the hexagonal sections, including anions and solvent molecules as guest species (277). in a different molar ratio yielded two Reaction of AgC104 and LBO polymeric complexes (278). The crystal structure of [Ag(L,)IClO, consists of two-dimensional infinite layers of hexagonal meshes formed by alternate triconnected silver ions and L, molecules. The structure

FIG.90. A schematic view of the 3-D network in [Ag(L,,)IPF,. HzO. (From Chart 1 in Carlucci, L.; Ciani, G.; Proserpio,D. M.; Sironi, A. J. Am. Chem. Soc., 1995,117, 12861.)


287

FIG. 91. A schematic view of the 3-D network in [Agll(Leo)sl(PFs)ll~ 14H20. (From Fig. 7 in Bertelli, M.; Carlucci, L.; Ciani, G.; Proserpio, D. M.; Sironi, A. J. Muter. Chem. 1997, 7, 1271.)

of [Ag3(Lso)21(C104)3. 2H20 contains an open three-dimensional cationic network formed by [Ag(Lso)]hexagonal layers, which are joined by biconnected silver ions (Fig. 92). Thus, the three-dimensional framework is a (3,4)-connected net comprising triconnected and tetraconnected centers in the ratio 1: 1. Guest water molecules occupy the large hexagonal channels and interact with the silver ions of the layers. Thermal analysis has shown that these water molecules can be reversibly removed from the crystals by thermal activation. Another highly symmetrical coordination polymer was formed in the reaction of [CU(CH,CN)~]C~O, and 2,4,6-tri(4-pyridyl)-1,3,5-triazine (Lloz).The compound formulated as has a cu-

FIG.92. A schematic view of the 3-D network in [Ag3(Ld21(C10,)32Hz0. (From Fig. 2 in Carlucci, L.; Ciani, G.; v. Gudenberg, D. W.; Proserpio, D. M.; Sironi, A. J . Chem. SOC.,Chem. Commun. 1997, 631.)

288


bic (3,4)-connectednet with large cavities (279).All the ligand groups are equivalent and are attached to three copper centers at the corners of an equilateral triangle, and each copper ion is coordinated by four pyridine donors in a distorted tetrahedral arrangement. This results in an infinite (3,4)-connected three-dimensional network containing large octahedral chambers of diameter 18.241(4) A. Each chamber is connected by its six copper vertices to six others whose centers are arranged octahedrally around, generating an infinite cubic collection of chambers (Fig. 93). Reid has extended the investigation of channeled frameworks to the methylene-bridged bidentate ligands MeECHzEMe (E = S, Se, or Te) on the expectation that such ligands would disfavor chelation and might therefore promote formation of ordered extended networks (280).X-ray diffraction studies show that in [ C U ( M ~ S ~ C H ~ S ~ M ~ ) ~ ] P F each copper ion is tetrahedrally coordinated to one Se donor of four diselenoether ligands and the second Se donor of each ligand is ligated to a different adjacent Cu' ion to give an infinite threedimensional cationic network (Fig. 94). The network contains cylindrical channels of diameter ca. 12.5 A incorporating the PF, counteranions. The silver(1) complex [Ag,(MeSeCHpSeMe)*l(BF,),contains a similar three-dimensional lattice with rectangular-shaped channels of cross section ca. 12.6 X 4.1 A to fill in the anions. A large number of host-guest structures are known, but a reversible guest exchange or chemical transformation of the guest inside a coordination polymer framework has been described for only a few (257, 281-283). In most cases the integrity of the inclusion lattice cannot be effectively maintained in the absence of guest molecules

L102

FIG. 93. Schematic representation of two interpenetrating, identical frameworks in [ C U ~ ( L ~ ~ ~ ) ~ I(From ( C ~ OFig. ~ ) 3~ .in Abrahams, B. F.; Batten, S. R.; Hamit, H.; Hoskins, B. F.; Robson, R. Angew. Chern., Znt. Ed. Engl. 1996,35, 1690.)

COPPERU) AND SILVERU) SUPRAMOLECULARMETAL COMPLEXES

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FIG. 94. View of the 3-D network in [ C U ( M ~ S ~ C H ~ S ~ M(From ~)~]P Fig. F ~1. in Black,

J. R.: Champness, N. R.: Levason, W.; Reid, G. Inorg. Chem. 1996,35,4432.)

(166).By using the trigonal ligands, 1,3,5-tricyanobenzene (L103) and 1,3,5-tris(4-ethynylbenzonitrile)benzene (LlO4),to make networks joined at the vertices by metal ions, Moore and Lee have obtained two silver(1) complexes [Ag(Llo3)(CF,S03)I and [Ag(Llo4)(CF3S03)I by mixing the constituent molecules in benzene (281, 282). The crystal structure of the compound with L103 consists of honeycomb sheets based on alternating LIo3and the trigonalaAg’ units. The honeycomb sheets create cavities of diameter 10.03 A to accommodate CF3SOi ions which are weakly bound to the silver at the axial position of a trigonal pyramid. By contrast, the complex with tbe larger ligand L104 generates larger channels of diameter 15 X 22 A and consequently involves interpenetration of lattices (Fig. 95). The three-dimensional (3,3)-connected net is based on the end-on coordination of L104 to the trigonal pyramidal silver cations. The interpretation of the networks generates hexagonal channels in which the benzene molecules from the crystallization process are located. Investigation of exchange properties within a host crystal were carried out in solution and in vapor with a variety of guest molecules. The process was monitored by powder X-ray diffraction and thermogravimetric analyse (TGAYdifferential scanning calorimetry (DSC) studies, and the results show that the host adduct can be repeatedly voided of and filled with guest molecules without the destruction of the channel framework. 2. Feldspar Structure Without Void Space The term “mineralomimetic chemistry” was proposed to denote the chemistry of the build-up of mineral-like structures using materials

290


FIG.95. View of [Ag(L,,)(CF,S03)1down the a-axis with plane waves corresponding to the 002 reflection highlighted. Triflate anions are omitted. (From Fig. lc in Gardner, G. B.; Kiang, Y.-H.; Lee, S.;Asgaonkar, A.; Venkataraman, D. J. Am. Chem. SOC.1996, 118, 6946.)

that never give stable minerals in nature (270). The topologic similarities observed between the well-defined coordination geometries and in natural minerals prompted many chemists to mimic the structures of simple minerals (121). The copper(1) complex of pyrimidine (LlOs), [Cu(LlO,),IBF5,was found to have a three-dimensional structure related to that of the natural mineral feldspar (284). The complex, obtained by reaction of [Cu(CH3CN)JBF4and pyrimidine, contains two independent copper centers in the unit cell, each coordinated in a slightly distorted tetrahedral geometry by four nitrogen atoms from four different ligand molecules. Each Llosin turn bridges two copper ions, leading to a three-dimensional framework. The structure is related to that of feldspar in such that the corner-shared network consists of eight-rings of tetrahedral connected together with four-rings to form elliptical channels (Fig. 96). Due to interpenetration of neighboring layers, such channels are almost completely constricted, leaving no void space in the structure. This phenomenon has been previously described in [Cu(Lsl)C1],where the interpenetrating of the two-dimensional sheets resulted in small channels (275). Thus, it would be possible to increase the volume of the channels by lengthening the spacer ligands that connect the metal centers.


291

b

FIG.96. Polyhedral packing diagram of [Cu(L,,,),IBF, (a) and schematic view of the feldspar structure viewed down the a-axis (b). (From Fig. 2 in Keller, S. W. Angew. Chem., Znt. Ed. Engl. 1997,36,247.)

3. Channel Structure Through Siluer(l)-Promoted in Situ Ligand Synthesis Successful construction of multidimensional frameworks largely relies on ligand design to suit different geometries and coordination numbers of the metal ions. A recent report on isolation of an unexpected three-dimensional metal complex by in situ cyclization of the ligand aroused tremendous interest among ligand designers (285). Reaction of 1,2-trans-(4-pyridyl)ethene(L7J and AgBF, in MeCN and CHzClzin the presence of light yielded [Ag(Llw)IBF4,where Llw = 1,2,3,4-tetrakis(4-pyridyl)cyclobutane.The complex exists as a threedimensional cationic polymer in which each Ag' ion is coordinated to one pyridyl group of four different Lloagroups. Each Llw bridges four tetrahedral metal centers leading t o a three-dimensional array of silver atoms containing channels to fill in the BF; anions and MeCN solvent molecules (Fig. 97). N

AgBF,

2

MeCN/CH,CI,

70

L 106

292


FIG.97. View of the polymeric array formed by [Ag(Llos)l+ illustrating the channels formed. (From Fig. 3 in Blake, A. J.; Champness, N. R.: Chung, S.S.M.; Li, W.-S.; SchrGder, M. J. Chem. SOC.Chem. Commun. 1997, 1675.)

X-ray structural determination results revealed that the ligand L,,, had undergone a [2 + 21 cyclization to form a tetrapyridyl-substituted cyclobutane ring. The light-induced dimerization is found to be promoted by the presence of Ag' ions. Although the detailed mechanism of the cyclodimerization reaction is not clear, such in situ formation of the ligand nevertheless represents a new approach to future inorganic crystal engineering. IX. Concluding Remarks

The design and synthesis of new functional polymeric coordination complexes through intermolecular interactions or coordination bonds is a continuing challenge. Our aim in this review has been to show what this development has achieved recently in copper(1)and silver(1) chemistry. Throughout we have employed relatively straightforward ideas covering the latest references and reviews. The concept of supramolecular chemistry is not new, but in order to keep this review within bounds, we have had to severely restrict the number of references. Most of these can be traced via a recent comprehensive review (286).In the present review we have tried to emphasize three aspects toward the construction of functional solid-state supramolecular metal complexes, illustrating them by reference to a variety of topological systems. The first point is that the coordination number and stereochemistry of metal ions play an important role in construction of multidimensional networks. Most polymeric frameworks encoun-

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tered, such as diamondoid, are based upon linear, trigonal, and tetrahedral metal templates. Therefore, the 4-coordinate tetrahedral copper(1) ion and the 2-4-coordination of silver(1) ion are likely to be the best candidates to produce diverse architectures. Second, designing ligands is an important step in making new functional solids. We must realize that the study we are undertaking today is not just limited to appreciation of novel frameworks for the aesthetics that the system can offer. We have already explored a number of functional ligands such as the conductive molecule TTF and derivatives, the photochromic compound cisl,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl) ethane (LT5),and the linear exodentate spacer 4,4’-bipyridine. We also have noted that in the metallic conductive compound [Cu(L,,),], matching of metal d-orbitals with n-orbitals of ligand may be responsible for the observed extremely high electrical conductivity (197200). Thus, in striving for functional coordination materials with potential applications as inorganic devices, the search for functional ligands constitutes a new challenge. Finally, X-ray diffraction in combination with other physical methods has proven to be the dominant means of structure determination of inorganic polymers. To obtain single crystals in the polymeric phase, the art of synthesis must undergo a constant modification. On the other hand, application of newer experimental techniques for structural analysis of the powder sample may provide further advances in the study of polymeric structures. The structural chemistry of coordination compounds continues to pose challenging problems, but a combination of physical methods and revolutionized synthetic techniques should clarify the picture and provide a real opportunity for those involved in this research. It should be encouraging. Such a prospect provides a major impetus to discover other coordination polymers with unexpected frameworks.

REFERENCES Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988,27, 89. Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990,29, 1304. Vogtle, F. “Supramolecular Chemistry”; Wiley: Chichester, 1993. Robson, R.; Abrahams, B. F.; Batten, S. R.; Gable, R. W.; Hoskins, B. F.; Liu, J. In “Supramolecular Architecture”; Bein, T., Ed.; American Chemical Society: Washington, DC, 1992; Chapter 19; p. 256. 5. “Comprehensive Supramolecular Chemistry”; Lehn, J.-M., Ed.; Pergamon Press: Oxford, 1995; Vol. 9.

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ADVANCES IN INORGANIC CHEMISTRY, VOL. 46

MANGANESE REDOX ENZYMES AND MODEL SYSTEMS: PROPERTIES, STRUCTURES, AND REACTIVITY NEIL A. LAW, M. TYLER CAUDLE, and VINCENT L. PECORARO Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109

I. Introduction 11. The Enzymes A. Manganese Superoxide Dismutase, MnSOD B. Manganese(I1) Dioxygenase C. Manganese Peroxidase, MnP D. Manganese Ribonucleotide Reductase, MnRR E. Manganese Catalase F. Binuclear Manganese Enzymes: A Comparison of Three Enzymes G . The Oxygen-Evolving Complex, OEC 111. Structural Models A. Mononuclear B. Binuclear C. Trinuclear D. Tetranuclear IV. Physical Properties A. Electronic Spectroscopy B. Magnetism C . EPR Spectroscopy D. X-Ray Absorption Spectroscopy V. Reactivity A. Organic Transformations B. Enzyme Model Systems VI. Conclusion References

I . Introduction

Manganese is one of several first-row transition elements that have been employed by biological systems to assist in varied metabolic and structural roles. Manganese is used to give structural support to pro305 Copyiiglit 1999 by Academic P I F S ~ All ilghla of repiudurtion i n any form reserved ORSR-RR JRi9Y $25 00

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LAW, CAUDLE, AND PECORARO

teins and is a cofactor in chemical transformations that include hydrolytic and redox reactions. Perhaps the best known function, and the one of greatest importance to aerobic life, is in the oxygen-evolving complex, which oxidizes water to dioxygen during photosynthesis. Some representative reactions catalyzed by manganese-containing enzymes are shown in Scheme 1. Oxygen Evolving Complex 2 HzO

*4Hf+

4e-

+

O2

Manganese Catalase

Manganese Superoxide Dismutase

Arginase arginine

ornithine+ urea

Manganese RibonucleotideReductme

*

RNA

DNA

SCHEME 1.

Several enzymes are known that naturally contain manganese. Most of these employ manganese in nonredox roles (I),for example conducting hydrolytic reactions, a Lewis acid role, or utilizing Mn" to provide structural support to a protein so that it may maintain a specific conformation. For example, pyruvate carboxylase binds Mn" tightly, but the Mn" is not involved in the enzyme's catalytic process, thus playing a structural role (2). Several binuclear metallohydrolases are known that contain a binuclear Mni' site (3-5). Many of these may be categorized as phosphohydrolases, hydrolyzing phosphate groups from a substrate molecule. Hydrolases are often activated by more than one MI1 element, although at least one is specific for manganese. One particularly key metallohydrolase that contains manganese is the enzyme arginase (4-7), an enzyme for which a crystal structure

MANGANESE REDOX ENZYMES AND MODEL SYSTEMS

307

at 2.1-A resolution has recently been solved for the arginase enzyme from rat liver (Fig. 1) (8). This enzyme catalyzes the hydrolytic conversion of arginine to urea and ornithine. Arginase is found in a great variety of organisms for the removal of nitrogen-containing wastes and is therefore integral to the biological cycle of nitrogen. Many of the binuclear hydrolases may be active with metals other than Mn at their active sites, but arginase has been shown to specifically require manganese to be fully functional ( 5 ) .The active site contains a binuclear Mnp dimer bridged by a bidentate carboxylate and two p2-"O" donor bridges, a monodentate carboxylate, and a solvent molecule derived hydroxide (or perhaps water a t lower pH, the crystals were grown at pH 8.5). The catalysis of this reaction involves a bridging p-OH unit, which is formed according to a mechanistic proposal by loss of a proton from a water molecule upon binding of substrate. This hydroxide is believed to be ideally situated for forming the intermediates required for arginase hydrolysis (8). Finally, arginase has recently been reported to exhibit some redox activity, catalyzing the disproportionation of hydrogen peroxide and peroxidase activity (discussed later) (9,10). Manganese(I1) ions may also be employed in the hydrolases that compose the ribonuclease H domain of reverse transcriptases ( 3 , 4 ) . Many of these enzymes, which may employ either Mn" or Mg", are found in a variety of organisms where they may or may not be essential (e.g., Escherichia coli) ( 3 , 4 ) . This class of enzymes catalyzes the hydrolysis of DNA-RNA hybrids. However, retroviral reverse transcriptases are critical for the replication of retroviruses, and Mn" may be the required cofactor for these ribonuclease hydrolases to function ( 3 , 4 ) . One example of this family of hydrolase enzymes is the RNase H enzyme from HIV-I. This enzyme has been crystallographically characterized ( 1 1 ) . In the crystal structure at 2.4-A resolution, the two Mn" ions are separated by a distance of about 4 A. They are bound to carboxylate residues that are located near the surface of the enzyme. One of these carboxylates bridges the two manganese

FIG.1. Structure of the active site of the rat liver arginase enzyme based on (8).

308


ions in a motif that is now recognized as a common structural element in many binuclear metalloenzyme active sites (discussed later). Some controversy with respect to the active sites of ribonuclease enzymes does exist, with some reports suggesting that there may be only one metal ion present (or perhaps required) in the active enzyme (12, 13). Concanavalin A, a lectin, is another interesting nonredox manganese-containing enzyme that is found in various forms in plants and animals (14, 15). Lectins are a class of enzymes known to bind small carbohydrate molecules (16, 17). The active site of crystallographically characterized concanavalin A enzyme shows a six-coordinate Mn" bridged via two carboxylate-bearing protein residues to an approximately seven-coordinate Ca" ion (Fig. 2) (14,015,18).The calcium and manganese ions are separated by about 4.2 A. Concanavalin A is an interesting case of a mixed metal active site in a biological system. If the Call is replaced by another Mn" ion, an EPR spectrum consistent with a weakly coupled MnH system is observed (19, 20). Newer crystal structures of concanavalin A with the substrate methyl-a-Dmannopyranoside have been reported. The more recent structure in 1994 has a resolution of 2.0 A. These structures show that methyl-an-mannopyranoside is bound near the metal binding site, about 8.7 A from the calcium and about 12.8 A from manganese (21). The 1994 structure also indicates that a water bound to the calcium may be involved in hydrogen bonding to the carboxylate-bearing protein residues, which are in turn hydrogen bond t o the methyl-a-D-mannopyranoside (18). Finally, a new crystal structure for concanavalin A at 0.94-A resolution has been solved (22).

H

N FIG.2. Structure of the manganese and calcium dimer in concanavalin A, based on ( 1 4 , 15, 18,221.


309

Redox chemistry is another facet of the biological chemistry of manganese, and various enzymes employ manganese a t mono-, bi-, and tetranuclear sites. Three mononuclear manganese enzymes are currently known that catalyze redox-based chemical transformations. These are manganese superoxide dismutase, manganese peroxidase, and manganese” dioxygenase. Catalase and the manganese ribonucleotide reductase are two known manganese enzymes that undertake redox transformations that utilize a binuclear manganese site, and a tetranuclear cluster of four manganese is found in the oxygen evolving complex (OEC) of Photosystem I1 (PS 111, the site where water is oxidized during the photosynthetic process to yield dioxygen. The focus of this review will be on mono- and multinuclear redox active manganese systems, and in particular, the relation of bi-, tri-, and tetranuclear manganese complexes as models for the multinuclear manganese-containing enzymes. Aspects of the structurally characterized compounds, spectroscopic features of synthetic compounds and magnetochemistry will also be discussed. The reactivity of complexes will also be addressed, with respect to epoxidation and catalase models, for example. Throughout, discussion will relate our current knowledge of manganese chemistry to the broader area of biological systems. Before addressing the synthetic complexes and their properties in detail, a brief overview of some of the redoxactive manganese enzymes will be presented. Finally, several reviews of manganese coordination complex chemistry have appeared in recent years. Among these are reviews by Wieghardt, Pecoraro and co-workers, Vincent and Christou, Brudvig and Crabtree, and Dismukes and co-workers, which have focused on a wide range of model systems, manganese-dioxygen interactions, mechanistic considerations of reactivity studies, and single systems such as the OEC (5, 23-50). The 1992 book “Manganese Redox Enzymes” (28) also contains much relevant information about biological manganese chemistry by a variety of researchers. A 1996 issue of Chemistry Reviews focusing on bioinorganic chemistry contains several reviews that are pertinent to the biological role of manganese (3, 33, 37). In addition, several reviews aimed solely at the OEC are also available, including reviews by Debus (42) and the recent book “Oxygenic Photosynthesis: The Light Reactions’’ edited by Ort and Yocum (48). Finally, the Proceedings of the Photosynthesis Conference, held periodically, presents short updates from research groups around the world as they work to comprehend this intricate system (51).

310


II. The Enzymes

A. MANGANESE SUPEROXIDE DISMUTASE, MnSOD Manganese superoxide dismutase (MnSOD) is a redox-active manganese enzyme that employs a mononuclear manganese ion at its active site. Discovered in 1970 (52),this enzyme catalyzes the dismutation of superoxide (HOz), to dioxygen and hydrogen peroxide, as shown in Scheme 2. Superoxide is the radical, one-electron-reduced Mn3+ Mn2+

+ +

-

HOP' HOP' + H+

Mn2+ + O2 + H' Mn3+ + H202

SCHEME 2.

form of dioxygen. MnSOD is found in a broad range of species that run the gamut from bacteria to humans (53).It is a member of one of the two families of superoxide dismutase enzymes, the first of which is the family that contains MnSOD and the analogous iron superoxide dismutase, FeSOD. A high degree of sequence homology exists between the two types of enzymes, and the active sites of both enzymes employ the same types and orientations of protein residues to generate a trigonal bipyramidal geometry about the active site metal (54611. The second family contains the structurally distinct copper/zinc superoxide dismutase, CuZnSOD (62, 63). Recent reports have indicated that a new group of nickel superoxide dismutase enzymes might also exist (64). Superoxide and superoxide dismutases have been the subject of a variety of reviews (5,29, 53, 65). Superoxide, H02, is produced naturally by several enzymes and was first recognized as an intermediate of aerobic respiration in 1968 (62, 66-68). Although it may be utilized beneficially, for example by macrophages t o fight an infection (691,it is generally not a benign byproduct. Therefore, SODS are critical to aerobic life forms, in which they perform the important role of protecting an organism from the deleterious effects of superoxide. Although HOz may not be the direct cause of oxidative damage to an organism due to its relatively short life span (self dismutation occurs at a rate of 2.0-3.2 X lo5 M-' . s - l ) (521, it is sufficiently long-lived to allow it to engender chemical compounds such as hydrogen peroxide or hydroxyl radical, HO., which readily cause oxidative damage to cellular components (70). Another aspect of superoxide chemistry is the growing research into peroxynitrite (71-741, which can be formed from nitrite and superoxide. Thus, superoxide has been implicated as a participant in a vari-


311

ety of destructive processes affecting DNA, cell membranes, arteriosclerosis and the damage that is caused to cells deprived of oxygen, ischemia, during a stroke or heart attack due to an increased output of superoxide once oxygen is restored to the affected cells and their enzymes (69, 71, 72). MnSOD is often found as a homodimer or homotetramer, with approximately one manganese per monomer subunit (53).A few of these enzymes have been crystallographically characterized, including the enzyme from human mitochondria (75) and the enzyme from the bacterium Thermus thermophilus (76).The active sites of these enzymes have been shown to contain a single manganese bound by three histidines, one aspartate, and either a water or hydroxide in a trigonal bipyramidal geometry as shown in Fig. 3, based on the T. thermophiZus structure (76).In this structure, two of the histidine residues and the aspartate form the equatorial plane, and the third histidine and waterhydroxide occupy the axial positions. Other structures of this enzyme with azide in the active site indicate that the coordination number of the manganese may be expanded to accommodate a sixth ligand, for example, superoxide during turnover (58). MnSOD catalyzes the disrnutation of HOz into dioxygen and hydrogen peroxide. As a redox enzyme, it shuttles between the MnT'and the Mn"' oxidation states (77).This process has been studied, and the enzyme has been shown to catalyze this reaction at a rate of 1-2.2 X 109 M-I.s-I , which is at the diffusion limit (52). A representation of the proposed catalytic cycle is shown in Scheme 3 (78). Several studies on MnSOD kinetics have been published (78-82). Early studies on the reaction of MnSOD from Escherichia coli and Bacillus thermophilus indicated evidence for a four-step process involving two fast and

FIG.3. Structure of the active site of manganese superoxide dismutase, based on (76).

3 12


two slower steps (79, 80). Later studies utilizing stopped-flow techniques refined this further. Saturation kinetics were observed for the T. thermophilus MnSOD, and the data showed that there were three phases to this reaction: a fast “burst” phase of quick HOz dismutation, a slower second phase, and a final fast phase (78).A “dead-end form of the enzyme was implicated to account for the slow phase (78). This has been formulated as a side-on-bound MnIII-peroxo species based on spectroscopic similarities to manganese model complexes with side-bn-bound-peroxy groups (83). Such phases are not observed for FeSOD (84).

+

H02

t

Mn3+SOD-Oi

Mn2+SOD

“Dead-End”Complex

SCHEME 3.

B. MANGANESE(II) DIOXYGENASE There now exist three reported Mn(I1) dioxygenase enzymes. The most extensively studied of these is the Mn(I1) dioxygenase from Arthrobacter globoformis CM-2 (85).The first Mn(I1) dioxygenase was reported in 1981 from Bacillus breuis (86).Dioxygenases catalyze the incorporation of dioxygen into an organic substrate (87). In this case, the substrate is an aromatic compound that contains a catechol cisdiol structure. These reactions are important in that they catalyze the degradation of aromatic compounds to simpler carbon components for action by other enzymes. In addition, the ability of dioxygenase enzymes to initiate the decomposition of aromatic compounds may be useful in the bioremediation of sites contaminated with aromatic organics of this type (88). Most of the known dioxygenases contain iron, and these enzymes tend to be very specific in their function (87, 89). For example, the


313

Fe(II1) dioxygenases have generally been shown to be intradiol dioxygenases, meaning that they cleave the aromatic ring between the cisdiols. The Fe(I1) dioxygenases are classified as extradiol dioxygenases and cleave to the aromatic ring to one side of the diol moiety. Substrate specificity is also exhibited to various degrees, with many dioxygenases preferentially cleaving one substrate over others (89). The Mn(I1) dioxygenases characterized to date belong to the class of extradiol dioxygenases (86, 90). Although none of the Mn(I1) dioxygenases have been structurally characterized, a recent structure has appeared for an Fe(I1) dioxygenase (91).Whereas the Fe"' enzymes have phenolate and histidine ligands, the Fe" was bound to the protein by two histidines and a glutamate, with two waters completing the first coordination sphere. The geometry about the iron atom is square pyramidal, with one of the histidines occupying the axial position (Fig. 4).Finally, some evidence for an Fe"/Mn" dioxygenase correspondence similar to that observed for MnSOD/FeSOD might also exist, as evidenced by the conserved residues that form the metal binding sites of these dioxygenases (92). Thus, this manganese enzyme may have a structure similar to the iron site. The details of the reactions of the iron dioxygenases have been extensively probed for the Fe(II1) enzymes and more recently for the Fe(I1) dioxygenases (87,891. These two groups of enzymes exhibit different catalytic processes. For both of these types of enzymes, though, it has been shown that although the reaction catalyzed is redox in nature, the central metal does not cycle between discrete oxidation states during the reaction. A resonance form of an intermediate, however, has been proposed to exist briefly as Fe"' for the Fe(I1) dioxygenases. A recent proposal for the catalytic cycle of an Fe(I1) dioxygenase is presented in Scheme 4 (89, 93). The catechol substrate is thought to bind as a monoanion to the central metal ion followed by the coordi-

FIG.4. Structure of the active site of an extradiol iron(I1) dioxygenase, based on (91).

314


SCHEME 4. A proposed mechanism for extradiol Fe(I1)dioxygenases. [Adapted with permission from (89).Copyright 1996 the American Chemical Society.]

nation of a dioxygen molecule to the metal, which might be formally considered to be a superoxide bound t o an Fe”’. It is believed that substrate binding activates the metal center for binding dioxygen, based on the increased affinity for binding nitrosyl to the active site iron in the presence of substrate (89). Attack of the “superoxide group” on the carbon adjacent to the catechol moiety followed by rearrangement to give the product and the product’s release complete the catalytic cycle. This mechanism contrasts with the Fe(II1) dioxygenase mechanism, wherein the iron activates the substrate for direct attack of dioxygen on the aromatic ring of the substrate. The reaction rate of the manganese containing A. globoformis enzyme, with its primary substrate 3,4-dihydroxyphenylacetate, has been measured, yielding values of kcat= 1400 min-l, and K, = 7 pM (85).It is likely


315

that a similar mechanism exists between the Fe" and Mn" enzymes (89,90). 1. Modeling Features, MnSOD and Mn(II) Dioxygenase These two enzymes present a unique challenge for the synthesis of small-molecule model complexes. First, the MnSOD has been shown to have an active site with a trigonal bipyramidal geometry about the manganese ion, whereas the Mn(I1) dioxygenase most likely contains a square pyramidal geometry about the manganese ion in accord with the Fe(I1) dioxygenase structure. Few complexes of manganese in a trigonal bipyramidal geometry are known, and these are predominately of Mn", whereas the MnSOD enzyme has been crystallographically characterized with a trigonal bipyramidal geometry about the manganese in both the Mn" and Mn"' oxidation states. In the case of the Mn(I1) dioxygenase, the structure is probably square pyramidal by analogy with the Fe(I1) dioxygenase structure. Square pyramidal complexes are the more common form for five-coordinate manganese complexes, but this geometry is most often observed for the Mnl" oxidation state due to the presence of the pseudo-Jahn-Teller distortion axis, and not Mn". The reactivity of model compounds is also key to gaining a better understanding of an enzyme. The MnSOD presents a number of challenges, the first of which is catalytic dismutation of superoxide. Many Mn compounds are known to interact (discussed later) with superoxide, but few have been shown to be catalytic in nature. The unique characteristics of the catalytic cycle, with the "dead-end" complex, also provides challenges for synthesis. Further, there has been some interest in recent years in the use of manganese complexes as pharmaceutical superoxide dismutase agents to aid in reducing cellular damage by superoxide, for example in the wake of a heart attack, which opens questions of the biological compatibility interactions of the complexes and their solution behavior at physiological pH values.

C . MANGANESE PEROXIDASE, MnP Manganese peroxidase (MnP) is an unique enzyme in many respects. It is an extracellular enzyme that involves a heme protoporphyrin IX for the oxidation of Mn" to Mn"' (94,951. A crystal structure at 2.06-A resolution of the manganese peroxidase from the white rot basidiomycete Phanerocaete chrysosporium, which utilizes this enzyme to degrade lignin, appeared in 1994 (96).The active site (Fig. 5) and the overall structure are quite similar to lignin peroxidase (Lip),

3 16

LAW, CAUDLE, AND PECOFURO

FIG.5. Structure of the active site of MnP. The left half is a side-on view of the active site showing the protoporphyrin M, the bound proximal histidine, and the water and histidine poised on the opposite face. The right half is a “top”-down view showing the heme and the bound Mn”, and the additional protein residues that act as ligands to Mn”. [Adapted with permission from (96).Copyright 1994, The American Society for Biochemistry and Molecular Biology.]

an enzyme with which MnP shares a significant sequence homology (96). The white rot fungi are some of the only species known that are able to degrade lignin (97-99). Furthermore, MnP itself is one of only two, or perhaps three, enzymes that are known to degrade lignin. The related Lips are the other lignin-degrading enzymes that have been known since the early 1980s (98, 99). Meanwhile, a few more recent reports have suggested that some fungi may also utilize a laccase in addition to MnP and Lip to facilitate lignin degradation (100, 101). The MnP and Lip enzymes catalyze the biodegradation of lignin to phenolic residues that can then be further decomposed. Lignin, found predominantly in plant cell walls, is a random three-dimensional polymeric material composed of phenolic residues that is thought to be the second most common biopolymer after cellulose. Lignin comprises a significant portion of woody plants, with lignin content values ranging from 15 to 36%(97-99,102). Lignin is also found in a variety of other vascular plants. The major role of lignin in the plant is to provide support and protection for plant tissue (98, 99). The lignindegrading peroxidases act on lignin by initiating one-electron oxidations that lead to the breakdown of the linkages that connect the building blocks of this material. It has been shown that the decomposition of cellulose is retarded by lignin, probably as a result of lignin


317

interfering with access to the cellulose (98, 99). [As has been pointed out, the importance of the degradation of lignin is underscored by the extensive amounts of carbon employed in lignin and cellulose manufacture by plants and the interplay that this may have with respect to the cycling of carbon on the globe (97-991.1 Thus, the Mn-catalyzed degradation of lignin may be designed to allow the fungus access to the cellulose (98, 99). MnP functions by the oxidation of the heme by hydrogen peroxide to form compound I, Ferv=O(P+')(Scheme 5). (95, 103-106). The oxidized heme then oxidizes a Mn" that is bound to at least one of the

oxalate

SCHEME 5.

two propionate arms of the heme group in a site located near the edge of the enzyme, an observation based on the recently solved crystal structure of the P. chrysosporium enzyme (96).This forms a Mn"' ion and Compound 11, Fe(IV)=O. The Mn"' then diffuses away from the enzyme to oxidize lignin. In this way, the enzyme can work on the random structured polymer, where structural specificity may be a problem for an enzyme and where Mn"' may diffuse more easily into the substrate than a larger enzyme would be able to do. The Mn"' initiates a one-electron oxidation, most likely forming an initial phenoxy radical cation, that triggers a cascade of oxidations that eventually break down the linkages between the lignin subunits. Compound I1 is then reduced to the resting Fe"' oxidation state by oxidizing a second equivalent of Mn". This step absolutely requires manganese, preferentially a Mn"-oxalate complex (1051, unlike the first step, which will function with substrates other than manganese (103).A new addition to the chemistry of lignin peroxidases is the observation that Lips are also capable of oxidizing Mn" to Mn"' in the presence of oxalate (107). Thus, some Lips may also act as a manganese peroxidase. Oxalate and similar small molecule chelators play a key role in the

3 18


function of MnP (104-109). Oxalate and malonate have been observed to be excreted in active MnP-producing cultures (104), and they have been shown to promote enzyme activity (105, 106, 108, 109). One of the roles proposed for the small molecules is to aid in the diffusion process of Mn"' and to stabilize the Mn"' from disproportionating to Mn" and MntVso that it may reach the target lignin and initiate the depolymerization of lignin (95, 103). It is not known whether oxalate binds with manganese to the enzyme during the reduction of Compound I to Compound I1 or whether a small molecule is taken up to assist departure of Mn"' from the enzyme. No evidence for oxalate or other small molecules bound to manganese were observed in the crystal structure other than water. It is known, however, that without the oxalate or a related small chelating molecule, turnover is reduced (104-106). Furthermore, studies suggest that the manganese required for the reduction of Compound I1 must bind with the exogenous ligand oxalate (105, 106). Experiments with a-hydroxy acids such as lactic acid or small organic acids such as malonic acid, showed that such compounds also appeared to promote the activity of MnP (95, 103). However, related compounds that could form stronger complexes with manganese were found to inhibit the enzyme, presumably by out-competing the enzyme system for binding Mn". It was proposed that these compounds might facilitate the diffusion of Mn from the enzyme and in that way lead to enhanced reactivity. Roles for oxalate beyond the reduction of Compound I1 (e.g., assisting in the diffusion of Mn"') remain controversial (104-107, 109). Oxalate is known to be decarboxylated in the presence of Mn"', so it may not be utilized in Mn"' diffusion, although that may be a role for the malonate. One other proposal for the role of oxalate was the reduction of MnIV0 2 which , is known to form near these enzymes, to return the manganese to a functional oxidation state. Some model work has addressed these points, and although unstable, such Mn'II-oxalate complexes may have lifetimes sufficient for them to reach the substrate and initiate oxidation before they would completely decompose. Finally, there is a functional similarity between the MnP and Lip enzymes (110). Lip has been shown to be able to oxidize Mn" (107). Further, it has been shown that a veratryl alcohol is produced in conjunction with Lip, and it has been suggested that this compound may serve Lip in a capacity similar to that served by Mn" in MnP (109).

Functional mimic systems for the MnP enzyme will be discussed in Section V, "Reactivity." An example of monomeric complexes prepared with a-hydroxy acids as functional mimics is presented in Section III.A.3 on monomeric structures.


3 19

D. MANGANESE RIBONUCLEOTIDE REDUCTASE, MnRR The conversion of ribonucleotides into deoxyribonucleotides for the synthesis of DNA is key to the survival of any DNA-based life form. This process is completed by one of a variety of ribonucleotide reductases that reduce the furanose ring of a ribonucleic diphosphate acid monomer by replacing a hydroxyl functional group at the 2'-position with a hydrogen (Scheme 6) to generate the deoxyribonucleic acid Phosphates.

Phosphates.

* HO

HO

OH

RNA

H

DNA

SCHEME 6

monomers necessary for DNA synthesis ( 111 -1 14). Ribonucleotide reductases have been identified with a wide variety of protein structures and active site compositions. Many are known that contain a diiron core (in the oxidized form of FeRR, the active site contains a n FeB' dimer bridged by a p-0x0 and a p-carboxylato), a stable tyrosine radical, and potentially redox active thiols in the form of cysteines a t the active site ( 1 1 4 , 115). The role of the diiron dimer is to generate the stable tyrosyl radical, which in turn initiates a transient thiyl radical at the active site (114, 115). Two other classes of ribonucleotide reductases have also been identified; one is based on adenosylcobalamin (114, 116) and the other, which is found in strictly anaerobic microorganisms, likely utilizes a glycyl radical in conjunction with S-adenosylmethionine to reduce RNA. These enzymes have been grouped as Class I, Class 11, and Class I11 ribonucleotide reductases, respectively ( 116). A fourth segment of these enzymes is represented by the manganese-based ribonucleotide reductases, which probably function in a manner similar to the iron ribonucleotide reductases (discussed later) and, therefore, may fall under the Class I umbrella (116).The MnRR enzyme is a bacterial enzyme, and is widely distributed among the Coryneform and related bacteria ( 1 1 1 , 1 1 7 , 118). Several members of this group of bacteria have been shown to absolutely require manganese for the transformation of RNA into DNA. The MnRR enzyme from Corynebucterium (formerly Brevibacterium) ammoniugenes has been the most studied member of this group ( 1 1 1 ) . The FeRR system has been extensively studied (112, 114). For this type of ribonucleotide reductase, the enzyme is comprised of two subunits, known as R1 and R2 (or occasionally B1 and B2). The R1 and

320

LAW,CAUDLE, AND PECORARO

R2 subunits are each comprised of a homodimer of the a- and p-polypeptides, respectively (112, 114), and both the R1 and R2 subunits have been crystallographically characterized (119-122). The active site for RNA reduction is located on the R 1 subunit. The oxidized diiron site and the stable tyrosine radical are located in the R2 subunit. The diiron site is comprised of two Fe"' ions,obridged by an 0x0 and a carboxylate, with an Fe-Fe distance of 3.3 A. The reduction of RNA begins with the abstraction of a hydrogen atom from the 3'position of the furanose ring by a transient thiyl radical. This is believed to be generated via an electron-transfer mechanism that allows the tyrosyl radical to form a thiyl radical from one of the cysteines located at the active site. In addition to that cysteine, a pair of redoxactive cysteines is located at the active site. This pair of cysteines is then proposed to be oxidized to a disulfide-linked moiety in conjunction with substrate reduction. In the first step, a hydrogen atom is abstracted by the thiyl radical from the 3'-position on the furanose ring. A proton from one of the redox-active cysteines is transferred to the hydroxyl group of the 2'-position to form a water-molecule-leaving group. Completion of the oxidation to the disulfide and the regeneration of the initial thiyl radical by H-atom transfer to the 3'-position of the furanose yields the DNA monomer product (Scheme 7 ) (114, 115). The disulfide linkage is then believed to be reduced by another enzyme system to regenerate the active site (115).For several of the MnRR systems, a thioredoxin/NADPH-dependent thioredoxin reductase system is thought to complete this phase of the process (123). Although an X-ray crystal structure of MnRR has not been solved to date, a structure with Mn substituted into the iron ribonucleotide reductase has been reported (Fig. 6) (124). In this case, the two manganese(II1) ions were bridged by two carboxylates from protein residues, but without the 0x0 bridge postulated for the active Fe;" form of the FeRR enzyme. This inactive enzyme with manganese was proposed as a model for the inactive Fe;' form of the iron enzyme and is

FIG.6. Structure of the Mn"-substituted FeRR enzyme, based on (124).


32 1

Phosphates,

DNA

Phosphates,

HO H20'

/

Phosphates,

O O M S E

H2O Phosphates,

o ) W "

HO

+

HO

SCHEME 7. A partial proposed mechanism for the functioning of iron ribonucleotide reductase. Manganese ribonucleotide reductase may function in a similar manner. [Adapted with permission from (113)and ( 1 1 1 ) . Copyright 1994, Current Biology Publishers, and Copyright 1994 Marcel-Dekker, Inc., respectively.]

quite similar to the structure of the reduced Fekr active site which has since been crystallographically determined. Based on UV-visible spectroscopy of MnRR and the similarity that this spectrum bears to that observed for catalase and to model 0x0-, carboxylato-bridged Mn"' dimer complexes, the active site structure of MnRR is proposed to include the Mn-0-Mn structural motif ( 1 1 1 ) . This would also be similar to the active site structures observed for FeRR. This proposal is

322


further bolstered by the recent crystal structure of the catalase from the bacterium Thermus thermophilus (125) (discussed later), which generally corroborates the postulated catalase p-carboxylato-, p-oxobridged Mn dimer motif for the catalase active site structure. In addition, the assignment of a Mnf’ oxidation state is supported by the EPR silence of this enzyme with respect to the observation of any signal arising from a manganese dimer (111). MnRR converts all four RNA bases into their corresponding DNA products (111, 123), and the overall enzyme is considered to be structurally similar to FeRR ( 1 1 1 ) . For example, MnRR is comprised of R1 and R2 subunits that are analogous to those of FeRR. A mechanistic similarity between these two systems, however, was not certain. No radical signal equivalent to the FeRR tyrosyl radical had been observed for MnRR prior to 1996 (126).One tantalizing piece of evidence was the hydroxyurea sensitivity of MnRR, a trait also observed for FeRR. Hydroxyurea is known to quench the FeRR tyrosyl radical (118). In the meantime, proposals for this lack of a radical signal had been put forward. One proposal suggested that the coupling of a stable protein radical to the Mn of MnRR might account for the lack of any observed radical by EPR in MnRR (111).Another suggested that the manganese dimer might engage more directly in the RNA-to-DNA conversion process, perhaps by abstracting the initial hydrogen atom itself to initiate the reduction process, in a role distinctly different from that proposed for the diiron site of FeRR (111). Proposals that mirror FeRR, however, may now, in light of the radical signal that has recently been observed in the MnRR R2 subunit, be reasonably considered (126).Unlike the relatively stable FeRR enzyme radical, though, the MnRR radical only has a half-life of 1.5 h. It was only after developing a “fast” isolation technique for this subunit that Auling and co-workers were able to observe this radical signal in the EPR. By analogy, this radical is proposed to be located on tyrosine residue. Finally, the presence and concentration of the radical signal were shown to correlate with the activity of an MnRR sample (126). Otherwise, little is known about the exact identity or proximity of this radical to either the manganese dimer or the substrate binding sites. E. MANGANESE CATALASE Catalase is yet another manganese-containing enzyme that interacts with dioxygen or its reduced forms. The role of this manganese enzyme is to protect the organism from oxidative damage that may be caused or initiated by the presence of hydrogen peroxide, such as in the interaction of hydrogen peroxide with Fe” in Fenton-like chem-


323

istry to generate H0’63. To date, manganese-containing catalases have only been found in bacteria-for example, the enzymes from Lactobacillus plantarum (127, 1281, Thermoleophilum album (129), and T. thermophilus (130, 131). Several reviews with respect to the catalase itself and the modeling chemistry of this enzyme have appeared in recent years (3,5 , 2 3 , 2 5 , 2 6 , 2 9 ,30, 38). The catalase enzyme that utilizes manganese instead of a heme group to catalyze the disproportionation of hydrogen peroxide t o dioxygen and water was known for several years before the presence of manganese was confirmed in 1983 (128). A low-resolution crystal structure in 1985 indicated that the enzyme from T. thermophilus contained a binuclear manganese site, with a Mn-Mn separation of about 3.6 A (130, 131). This was further supported by evidence from optical spectroscopy that suggested that the active site contained two manganese ions that were bridged by one 0x0, p-02-, and at least one carboxylate ligand based on spectroscopic comparisons to 0x0- and carboxylato-bridged manganese model complexes (30, 132, 133). Extended X-ray absorption fine structure (EXAFS) studies of this enzyme suggested that the first coordination sphere about the manganese of the catalase consisted of O/N donors (134, 135). The presence of the dimer was further bolstered by the observation of EPR spectra consistent with a coupled manganese dimer, for example the Mn”’Mn” EPR spectrum of the “superoxidized” enzyme (136).EXAFS showed that this “superoxidized” catalase has a Mn-Mn vector of 2.7 A, consistent with a Mnz(p2-O2- core. Very recent reports of crystal structures at resolutions of 1.6 A for the reduced, Mnf, and 1.4 A for the oxidized, Mn$, forms of the enzyme from T. thermophilus confirm much of what had been proposed for the structure of the catalase’s active site (125).A representation of these results is shown in Fig. 7. The active site assembly contains two five- or six-coordinate

FIG. 7. Structure of the active site of the 2’. fherrnophilus catalase enzyme, based on (125).

324


manganese ions bridged by one carboxylate from a protein residue, one water molecule, and a hydroxide ion in the reduced enzyme. The hydroxide is suggested to be converted to an 0x0 bridge in the oxidized form of the enzyme. One additional water molecule, carboxylates, and imidazoles complete the first coordination spheres of the Mn ions. The geometry about both Mn ions has been describtd in terms of square pyramidal (125). The Mn-Mn vectors are 3.18 A in the reduced form and 3.14 A for the oxidized form of the enzyme. Finally, this binuclear manganese site is bound within a bundle of four a-helices. The T. thermophilus enzyme itself is a homohexamer, composed of six identical subunits (125, 131). During its catalytic cycle, the Mn catalases shuttle between the MnP and the Mn$ oxidation states in 2 two-electron reactions (137). Overall this process converts two hydrogen peroxide molecules into one molecule of dioxygen and two water molecules. A proposal for the catalytic cycle of catalase is presented in Scheme 8 (23). When the

, Y o

o r 0

SCHEME 8. A proposed mechanism for the disproportionation of hydrogen peroxide by manganese catalase, based on the mechanism proposed by Penner-Hahn in 1992, and the 2'. thernophilus catalase crystal structure. [Adapted with permission from (24). Copyright 1992 WILEY-VCH Verlag.]

T. thermophilus enzyme is isolated, a mixture of enzyme oxidation states including MnH, MnIIMnIII, MniI', and Mn1"MnWis observed (138, 139). The MnIIMn"' and Mn"'MnTVoxidation states are catalytically inactive but can be reduced to active oxidation states, Mn"Mn" or Mn'IIMnIII, by hydroxylamine (23). The Mnl*'MnTV oxidation state is quite intriguing because it is thought to contain a bis-(p2-02-),core which previously has been implicated in the oxygen-evolving complex

MANGANESE REDUX ENZYMES AND MODEL SYSTEMS

325

(23). The products of the "superoxidized" state may be explained if

one considers the possibility that occasionally during turnover an active site at Mnil' is reduced by only one electron to Mn"Mn"'. The following two-electron oxidation step would then yield the Mn"'Mn'" form of the enzyme, which is only slowly reduced back to an active Mn!" form. Such chemistry was shown to be viable by the introduction of the one-electron reductant hydroxylamine to catalase followed by the addition of hydrogen peroxide (23). These catalases exhibit saturation kinetics and complete the disproportionation of hydrogen peroxide at a rate of 5.75 X lo5 M-l-s-' for L. plantarum (231, 1.7 X lo6 M-. s-l for T. album (129) and 3.2 X lo6 M-'. s-' for T.thermophilus (131, 140). Finally, the catalase enzymes have been shown to be inhibited by azide, chloride, fluoride, and thiocyanate (23). In the case of fluoride inhibition, there is significant controversy both for the number of fluorides that are bound and whether or not the anion acts as a terminal ligand or a bridging p2-F- species. Numerous model complexes that model one or more traits of the catalase enzyme have been prepared over the years (discussed later). Several systems that mimic the function of this enzyme have been reported, and a selection of these will be discussed in Section V, "Reactivity." Other models with respect to structures or physical properties of the catalase enzyme will be presented in the ensuing sections of this review.

F. BINUCLEAR MANGANESE ENZYMES: A COMPARISON OF THREE ENZYMES One of the striking features of the two redox-active binuclear enzymes described earlier, MnRR and catalase, and the hydrolytically active enzyme arginase is the apparent similarity of their active site structures. Yet all three exhibit vastly different reactivities. Both the MnRR and catalase exhibit UV-vis spectra consistent with carboxylato- and 0x0-, hydroxo-, or aquo-bridged cores. This would indicate that both have similar Mn-Mn core structures. An EXAFS study by Stemmler, et al. (9) compared the active sites of these three enzymes. Those studies suggested that arginase and the iron ribonucleotide reductase substituted with Mn had similar structures that tended to be six coordinate. Their data for catalase suggested that the manganese of this enzyme were likely five coordinate. No crystal structure for MnRR has been solved to allow for a specific identification of ligands or a direct comparison of distance parameters and ligand orientations. However, it is possible to compare the arginase (8)and catalase crystal structures in this manner (125). Other comparisons have ap-

326


peared in the literature (9, 10).These environments are quite similar, as can be seen from Figs. 1 and 7, and the potential correlation between the two sites is strengthened by the low level of catalase activity exhibited by arginase (9, 10). The similarity of the bridging motifs and protein residues as ligands is striking. Both have enzyme-active sites that contain dimers of Mn ions bridged by a bidentate carboxylate and two ,uZ-"O"donor bridges: a monodentate carboxylate and a hydroxide (crystals grown at pH) in arginase, and a water molecule and hydroxide in the reduced catalase. The remaining ligands to the metal ions on each side of the dimer are identical in terms of type, carboxylates, and imidazoles, and in terms of their general positions about the manganese dimer. In the case of arginase, one Mn ion is five coordinate, the other is six coordinate due to a bidentate carboxylate. The catalase contains a water in the area of what would be the corresponding open site on the five-coordinate arginase Mn, and the carboxylate donor on the second manganese is monod$ntate instead of bidentate. The Mn-Mn vector for arginase is 3.3 A compared to 3.14 and 3.18 for the MnH and Mnil' oxidation states of catalase. Presumably, the native MnRR will also have a Mn-Mn vector in the 3.0- to 3.5-A range. The broader structure about the Mn cores of these sites of the arginase and catalase enzymes, however, does vary considerably from one enzyme to the other. Both of these enzymes are located within hydrophobic pockets. However, in the case of the active-site structure of catalase, there is an additional glutamate that may hydrogen bond to two of the imidazoles that are bound to the manganese and a lysine NH3+group is also hydrogen bonded to a manganese ligand carboxylate (125). In arginase, there are only two other polar residues reported to be near the active site beyond those that are bound to manganese (8). One of these is a histidine imidazole, which may function in a proton shuttle capacity. The other is a glutamate, which probably salt-bridges to the guanidinium functional group of the arginine substrate. The protein structures near the active sites are also quite different. In catalase, theobinuclear active site is located within a fourhelix bundle about 18 A from the surface at the bottom of a channel. Overall, the catalase enzyme is composed mostly of helical structural motifs. Meanwhile, the arginase enzyme's active site is not located within a four-helix bundle, and this enzyme contains several helix and P-Fheet structures. The active site of arginase sits at the bottom of 15-A-deep cleft that is located within an area of turns and loops between helices and P-sheets. The reactivity of catalase enzymes and rat liver arginase may also now be directly compared. Arginase has recently been reported to dis-

A

A


327

proportionate hydrogen peroxide, albeit much less effectively than the actual manganese catalases (in a buffered solution a t pH 9.5) (9, 10), albeit much less effective than the actual manganese catalases, exhibiting a rate of reaction on the order of 30 s - I versus 200,000 5-l for L. plantarum (10).It was also reported that evidence for the manganesesubstituted iron ribonucleotide reductase exhibiting a low level of catalase activity might also exist (9).No "superoxidized form of arginase could be formed in the presence of hydrogen peroxide and hydroxylamine, but it did show hydroxylamine inhibition. Another difference between these systems was a lowered sensitivity of arginase to the inhibitory effects of halides (10). A final point of comparison is the reported peroxidase activity of arginase. The catalase from T. album has been shown to complete a peroxidase reaction on p-phenylenediamine (129). Arginase was shown to perform peroxidase activity on this compound and o-phenylenediamine (10).Thus, as one might expect from their structural similarities, these enzymes can in some cases catalyze similar chemistry. Thus, despite the initial similarities of the immediate active site dimers, a broader inspection of the enzyme begins to suggest potential explanations for the differences in their reactivity. For example, the existence and type of residues near the active site, with respect to acidhase chemistry, that may attenuate the overall reactivity of the core will play a role. The types of bridging p"0" moieties, monodenate carboxylate and hydroxide in arginase versus hydroxide and water in catalase, probably play a significant role. The hydroxide and the proposed 0x0 (or perhaps two hydroxides) of the MniI' form of the enzyme may be better able to support the higher oxidation state on manganese versus the carboxylatehydroxide bridges of arginase. The two five-coordinate manganese of catalase may play a role, too, if rearrangement were required for the activation of arginase. Overall, these enzymes exemplify a growing theme in bioinorganic chemistry, that of nature utilizing an active site structural motif in more than one enzyme to catalyze quite similar or dissimilar chemistries. Such reapplication of a structural motif has been observed in the recurring themes of binuclear iron ( 1 4 1 ) ,for example. For manganese, it appears that a general binuclear manganese active site design has been reapplied three times in separate settings for at least three different purposes.

G. THEOXYGEN-EVOLVING COMPLEX, OEC Dioxygen is important to all aerobic life forms. It is required for respiration and, therefore, the release of energy required to sustain

328


biological processes. Dioxygen is a by-product of photosynthesis and is produced by the oxidation of two molecules of water to dioxygen in conjunction with the release of four equivalents of protons and four equivalents of electrons, which will be processed into reducing equivalents suitable for eventual carbohydrate biosynthesis. The catalytic site for water oxidation is known as the oxygen-evolving complex (OEC) of Photosystem I1 (PS 11),located in the chloroplasts of green plants and algae (42, 48). The OEC, sometimes referred to as the WOC, water oxidation complex, or water oxidase, contains a group of four manganese ions at its active site (42).Calcium and chloride are also required for the proper function of the OEC (35).Part of a complex system of several polypeptides and cofactors, which are membrane-bound in the thylakoid membrane of green plants, the OEC’s role is to provide the reducing equivalents necessary to rereduce a specific photoexcited chlorophyll a molecule that initiates the chain of events leading to the production of reducing equivalents. The polypeptides of PS I1 and those surrounding the OEC serve in various roles, many of which have been probed and elucidated ( 4 2 , 4 8 , 5 1 ) .Some of these roles relate directly to the manganese cluster of the OEC and the other inorganic cofactors. Based on the wealth of research in this field related to PS 11, a gross structural similarity of green plant and algal PS 11s is believed to exist with that of the crystallographically characterized (142) bacterial (Rhodopseudomonas) PS 11s (42, 143) that do not employ the OEC. The commonalities between these systems has led to a proposed orientation of the various components of PS I1 (Fig. 8) (24).The OEC is located near the “base” of two polypeptides known as D1 and D2 and is surrounded and stabilized by several additional polypeptide units (42, 144).

+

4

FIG.8. A scheme of the generally proposed layout of the components of Photosystem 11. The polypeptides are labeled, too. The electron flow in PS I1 follows the arrows shown. [Reproduced with permission from (24).Copyright 1998 Springer-Verlag.1


329

For more detailed discussions of the biochemical aspects of the PS 11, such as the role of the various polypeptides or differences between the PS IIs, the reader is directed to one of several reviews and compilations. The OEC and PS I1 have been extensively reviewed over the past several years. The review by Debus in 1992 sums up much of the knowledge obtained on PS I1 to that point (42). The Photosynthetic Congress, held every few years, has produced several volumes of material from a broad spectrum of researchers covering the whole of photosynthesis with current results to the time of that Congress (51). A 1996 book edited by Ort and Yocum (48) contains several chapters covering various aspects of photosynthesis, from PS I1 to PS I. Manganese modeling chemistry has also been extensively covered by reviews (24,26-31,34, 37, 45,50).

The OEC is involved in the earliest stages of the overall photosynthetic process, which produces NADPH for carbohydrate biosynthesis. Overall, the photosynthetic system is composed of two halves, PS I1 and PS I (48). PS I1 provides reducing equivalents that are transferred to PS I and are eventually converted into the reducing energy of NADPH. Both PS I and PS I1 absorb light energy to initiate their respective electron flows. The protons released during these steps are likely utilized in the generation of a proton gradient for the synthesis of ATP, which is also required for carbohydrate biosynthesis (48). It has been established that the OEC contains a tetranuclear cluster of manganese ions, and that it requires at least one calcium ion (39, 145, 146) and at least one chloride ion (147, 148) to complete its catalytic cycle. This catalytic cycle involves four successive oxidations of the OEC (42, 48) to eventually yield a highly oxidized system that then completes the oxidation of two molecules of water with the concomitant release of dioxygen and the return of the OEC to an overall oxidation state four levels lower. The sequential oxidation process starts when light energy is harvested by antennae chlorophyll molecules, which transfer this energy to the Psso chlorophyll a molecule of the reaction center. This photoexcited P680 then reduces Pheophytin A to give a charge-separated state of Ps+Boand Pheo-. Pheophytin A then reduces a bound plastoquinone molecule, QA, which then reduces a dissociable plastoquinone molecule, known as QB. Once Q B has been doubly reduced and doubly protonated, it departs to carry the collected reducing equivalents further into the photosynthetic system (Fig. 8). Once oxidized, Psso+ needs to be “reset” so that new reducing equivalents may be initiated, and to prevent charge recombination, or back-reactions, from occurring. This is accomplished by oxidizing the OEC. This oxidation is mediated by a particular tyrosine residue, known as Tyrosine Z, Y;,residue 161 of the D1 polypeptide (149,150).

330


This tyrosine is initially oxidized by P&, and then it in turn oxidizes the OEC by one electron. In this manner, the OEC is oxidized successively four times, whereupon dioxygen release occurs, the OEC returns to its initial oxidation state, and the successive oxidation cycle begins anew. Four protons are generated in conjunction with the electron withdrawal and dioxygen release, and these are likely utilized in the generation of a proton gradient for ATP synthesis (42, 144). The cycle of four progressive oxidations followed by release of dioxygen with the concomitant reduction of the manganese center of the OEC has been termed the S cycle, or S clock (151-153) (Scheme 9). This 2 H20

/

SCHEME 9.

clock is a five-stage cycle that proceeds from So to S4, with So being the most reduced form of the enzyme whereas S4represents the most oxidized form. Dioxygen release occurs during the S3-S4-So phase of the cycle, presumably once S4 is reached (152, 153). S , is relatively unstable and rapidly returns to the So state (46).The initial evidence for the S cycle was reported by Joliot (151).Those experiments were reproduced and reinterpreted by Kok and co-workers (153), who showed that, for photosynthetic centers given a sequence of single flashes of light, a burst of dioxygen occurred with the third flash of light and a burst of dioxygen then occurred with every fourth flash thereafter (Fig. 9). The initial burst came on the third flash, indicating that PS 11s stored in the dark will equilibrate t o the S1 state, which is one step oxidized from the OEC’s lowest overall oxidation state. Further research has tied at least some S-state progressions to an increase in the overall oxidation of the manganese cluster of the OEC (discussed later). In the absence of crystallographic data, all of the structural infor-

33 1


2.4

30 FLASH NUMBER

n

-I

wh

1.6

-

0" Q

w

N

= 0.0 -

J

4

p: 0

0 2 EVOLUTION IN FLASHING LIGHT

z

1

0

6

I

1

12 18 FLASH NUMBER

I

I

24

30

FIG.9. The Four Flash experiment. The peaks in the plot represent the dioxygen bursts that occur with every fourth flash of light administered to a PS I1 sample. This cycle peaks on the third flash due to the resting state of the enzyme being a t S , . The process is eventually dampened by double hits of light leading, for example, to an inhomogenous sample with respect t o the S state. [Reproduced with permission from ( 4 6 ) . Chapter 9. Copyright 1996 Kluwer Academic Publishers.]

mation known about the OEC has been gathered by the use of spectroscopy (24,28,33,42,46). The use of X-ray absorption spectroscopy ( U S ) has provided some particularly useful quantitative structural information. EXAFS (extended X-ray absorption fine structure) is an X-ray absorption spectroscopic technique that provides distance information and some idea about the nature of the first coordination sphere of the element in question. EXAFS studies have shown that the OEC manganese ions are present in a predominantly O/N-donor coordination environment (24, 154). Furthermore, EXAFS data show that there exists at least one Mn-Mn vector, and prob%bly two, of 2.7 A, and a third significant vector is at a distance of 3.3 A (155-159, 477), which may result from a Mn-Mn or Mn-Ca interaction (discussed later). EPR spectroscopy has been another extremely important means for exploring the OEC (24, 28, 42, 46). The S, state is readily prepared

332


by dark-adapting preparations of PS IIs, from which Sz is achieved by flashing the sample with one pulse of light (42).The Sz state exhibits at least two distinct EPR signals, which have been the focus of extensive research and controversy, too. The first, the multiline, is a multiline signal centered around g = 2 (160, 161), and the second, which depends on sample preparation technique and conditions, appears a t g = 4.1 (162-166). These EPR signals may be interconverted, and are shown in Fig. 10. The multiline signal is indicative of magnetically coupled manganese ions, with at least 18-20 resolved lines being observed. It also indicates that at least two of the manganese are involved in a multinuclear cluster. Based on the expected manganese oxidation states, and more lines than the 12-16 traditionally observed for a MnlIIMnNdimer, a cluster of at least three manganese ions is probable (46).The second EPR signal at first appeared similar to that generated by an isolated MnN, leading to trinuclear/mononuclear proposals for the structure of the OEC. Later EPR evidence showed that this signal did not result from a monomeric MnIVion, but rather most likely arose from a multinuclear manganese structure itself (167, 168). In ammonia-treated, oriented PS I1 samples, a multiline signal was observed in the g = 4.1 EPR spectrum. Further-

'...,.,..,....(,...,...,,....,.... lo00

ZOO0

3000 Maptic Field (G)

....,' 40oo

FIG.10. The g = 4.1 EPR signal from the SS state of the OEC (left) and the g = 2 multiline signal (right). [Reproduced with permission from (46), Chapter 9. Copyright 1996 Kluwer Academic Publishers.]


333

more, another study on this signal suggested that it arose from an S = 512 ground state (1691,which would be inconsistent with a Mn" ion, but not a multinuclear array of manganese. The appearance of the multiline EPR signal upon going from S1 to 9, also supports the existence of metal-centered oxidation (160, 161). For a more detailed discussion on these two signals and the conditions affecting their respective formation, the reader is again directed to one of the various reviews and compilations (42, 46). In 1997, new EPR data on the So state was reported by Messinger and co-workers (170, 171). Until these reports, no EPR spectra had been recorded for this S state. The first set of experiments took darkadapted PS 11s to Soby the addition of reductants (171). This S state is labeled S$ to account for potential alterations to the manganese structure. A multiline signal EPR signal was observed for this S$ state that exhibits 24-26 lines centered near g = 2 [Fig. ll(a)l. This signal is wider than the S , multiline and exhibits more lines, many of which do not line up with transitions observed in the S , multiline. The authors suggest that this added breadth is a result of the lower overall oxidation state of the manganese ions composing Soand that this may be indicative of the presence of a Mn"Mn"' couple based on model complex data. A second set of experiments reported by Messinger, Sauer, Klein, and co-workers (170) took dark-adapted samples through one S-state cycle to poise them at So.This preparation also presented a multiline signal, which did not appear to differ significantly from that observed for S$ [Fig. ll(b)]. For both of these spectra, the presence of methanol (0.5-1.5%) in the sample was required to observe the multiline signals. Without methanol, a broad EPR signal is still observed for the So preparation. The authors theorize that methanol may be binding to the manganese cluster and that this facilitates the observed spectra, perhaps by altering the coupling of the manganese ions. Simulations conducted to ascertain what the possible oxidation states of the manganese ions in So might be have not yet clarified this point, because the overall oxidation states Mn"Mnin, Mn"Mn1I1MniV,or Mni*'MnIVall gave similar spectral widths consistent with the data. However, the simulations did suggest that neither binuclear species nor a tetranuclear cluster arranged as MnI1Mnw/ MnlIIMnIVwould not yield comparable spectral widths. Also in 1997, Styring and co-workers (172) reported a multiline signal for So.Their study showed that by administering light flashes to PS I1 samples, they could produce a multiline signal that they believed to arise from Soafter 3 flashes from S z samples. This spectrum is also wider than that observed for S2, similar to that reported earlier. A second se-

334


A 3 2.8 2.6 2.4

g value 2

22

1.8

1.6

at

B I

I

.

250

I

I

,

l

.

300

,

.

.

,

.

350

Field (mT)

.

.

.

,

.

400

p=2

L

1900

2700

3500

4300

5100

Magnetic Field (Gauss)

FIG. 11. Sa and SoEPR signals. (a) EPR Spectrum of the Sg state. The top spectrum, A, represents the S-l state that was formed by reducing PS I1 samples with hydrazine. This was illuminated to yield a new multiline signal, C. B is the spectrum of the Sz state multiline from control experiments. Details of sample preparation may be found in the references provided. (b) A and B. Difference spectra of prepared So minus residual S1 signal. B. For comparison, the Sz multiline under the same conditions. Note that So is a broader signal and is shifted with respect to the Sz signal. C. Sospectrum without added methanol. D. Sg*Spectrum produced by reducing S1samples with hydroxylamine. E. Simulated spectrum of So. [(a) and (b) reproduced with permission from (171) and ( 170), respectively. Copyright 1997 the American Chemical Society, respectively.]

quence of light flashes regenerated the So signal. They also utilized methanol in their samples. EPR signals have also been reported for the S, state (173). A very intriguing article (174) has recently appeared in which Britt, Debus, and co-workers showed, via the application of parallel polarization EPR studies, that a multiline EPR signal can be observed for the S1 state (Fig. 12). When illuminated, the amplitude of this signal was “greatly reduced,” and a conventional perpendicular polarization g = 2 multiline signal consistent with the formation of the S, state was

335


(a) Parallcl Polarization

?P

I

I

zoo

I

ooo

I

I

1

1MM

1200

1

800 Magnetic Field (G)

600

(b) Perpendicular Polarizatlon

Illuminated minus Dark

9# 1

1

2s00

Moo

1

3500 Magnetic Field (G)

I

I

4000

4500

FIG.12. S, multiline signal. In (a) the spectrum (parallel polarization EPR) labeled Dark represents a dark-adapted PS I1 sample; thus, it is in the S, state. The second trace represents the sample following illumination with light to generate S2. The top, Dark, spectrum minus the middle, Illuminated, spectrum yields the third spectrum in (a). This represents a well-resolved multiline signal arising from a “multinuclear exchange coupled paramagnetic Mn cluster” (174) in the S, state of the OEC. If the subtraction is reversed and the spectra are recorded using perpendicular polarization, then when the Dark spectrum is subtracted from the Illuminated spectrum, an S2 multiline signal is readily observed (b). [Reproduced with permission from (174). Copyright 1998 the American Chemical Society.]

336


observed. This S, multiline signal is consistent with the interaction of multinuclear clusters of manganese ions, and these authors proposed that it arises from a cluster of manganese ions. This proposal would minimally require that a trimeric cluster of coupled manganese ions exists, versus an arrangement invoking two distinct dimers, one a M$' dimer and the other a Mny, an arrangement based on the assumed overall integer spin of this S state. Furthermore, because Sl is expected to be integer spin, a trimer/monomer motif would suggest that the monomeric manganese should be in the Mn"' oxidation state. If that were true, then a six-line spectrum consistent with a Mn"' should be observable under the conditions employed to observe this new S1multiline signal. The lack of such a signal is suggestive that this new multiline signal actually arises from a tetranuclear cluster of manganese ions. Application of the X-ray Absorption Spectroscopy (XAS) to the observation of Mn K-edge data (24, 32,33, 156, 1751, via X-ray Absorption Near Edge Structure (XANES), has provided insight into the overall oxidation state of various S states. The inflection point of the edge shifts for the various oxidation states of manganese (24, 33). Studies of the OEC show changes in edge energies consistent with an increase in the overall oxidation state of the manganese cluster. Thus, at least some of the oxidations of the OEC occur on the tetranuclear cluster of manganese ions. Numerous XAS studies in conjunction with EPR data have lead to proposals of the oxidation levels of the OEC S states (24, 33, 42, 46). Two of these proposals are presented in Scheme 10. The S-state oxidation levels of the manganese are not s-state

so

s, s2

s, s.4

IIInIInIIIiv IrinIinvnv

riinvnvm rvnvnvnv tvnvnvN

IIIIIIIIIIIIIV IIInIInvm IIIIIVIIVIIV

IIIIIIIIVIIV

nmnvnv IVIIVIIVIIVY I. SCHEME 10.

clear-cut, and some controversy does remain ( 2 4 , 3 3 , 4 2 , 4 6 ) .Much of this controversy revolves around defining oxidation states for the S states from spectroscopic techniques. For example, the transitions from So to Sl and S1 to S2 have strong evidence for manganese-based oxidation state changes. However, the nature of the S2to S, transition is quite controversial because there is no clear-cut alteration to the observed XANES edge energies. Proposals to explain this have invoked oxidized protein residues or structural rearrangements of the


337

tetranuclear cluster (176). The imidazole oxidation that had been suggested, however, is unlikely. The signals that were attributed to the oxidized imidazole have now been shown to arise from Y; (177, 178). In the final S , to S4to So phase, little is certain due to the instability of the S, and the transient nature of the Sqstate. Again the oxidation state controversy is centered on whether oxidation occurs on a protein residue, such as Y;, or on a manganese of the OEC. The results of the XAS and EPR data, combined with knowledge from model complexes, (discussed later) allow for the formulation of structural models of the OEC. Many manganese compounds have been proposed as structural models for the OEC, ranging from butterfly arrangements to adamantanes to cubanes (26,27,29-34,50). Most of those proposals, however, are inconsistent with the known structural data for the OEC. Currently the most widely favored model is the “dimer of dimers” model (Scheme 111, first suggested by Klein,

SCHEME 11.

Sauer, and co-workers (32, 154). This model proposes that two manganese dimers that interact with one another exist in the OEC. The dimers are suggested to contain Mnz(pz-02-)2 cores and t o be bridged to one another by 0x0 and carboxylato units. This model is supported by several pieces of current data on the OEC. First, the 2.7-w Mn-Mn vectors of the OEC are most consistent with crystallographically characterized dimeric manganese model complexes that contain Mnz(pz-02-)z cores, and two such vectors are observed for the OEC. Second, such a model would provide the potential for magnetically coupled Mn-Mn interactions to occur. Bridging two Mn2(pz-OZIz cores ) would proto one another by carboxylates and a single ( p 2 - 0 2 -unit vide for an approximately 3.3-w distance, a formulation consistent with known model compounds (discussed later). Furthermore, the

338


EPR data from So, S , , and S2indicate that there are coupled Mn ions. In particular, the multiline signals are suggestive of coupled manganese structures that require clusters of greater than two manganese ions. The related Electron Nuclear Double Resonance (ESEEM) and Electron Spin Echo Envelope Modulation (ENDOR) spectroscopies have also proven useful in elucidating information about protein residues near the OEC. One of these residues is the important protein radical Tyrosine Z, Y;, which is usually found in an oxidized radical form (42).Study of this protein residue via ENDOR and ESEEM spectroscopies has been enlightening. The presence and protein residue associated with this radical signal was defined several years ago, and Y; was originally assigned the role of an electron-transfer mediator for the electrons moving between the OEC and (150, 179). More recent work has been modifying this view. Tyrosine Z has been shown by Britt and co-workers to be located in close proximity to the manganese cluster, probably within 5 A (180).Probes of the nature of this tyrosyl radical by Babcock’s group have suggested that this tyrosine may play a more active role than solely to act in an electron-transfer capacity (181,182).It does not bear the hallmarks of a protein residue whose sole purpose is to participate in an electron-transfer chain-for example, it does not retain a nearly rigid orientation with respect to its surroundings. Other similarities between this redox-active tyrosine and other metalloenzyme systems that employ radical components suggested that this tyrosine might also act in such as role (47, 183). These observations have lead to recent proposals by Britt and more intimately in the water oxidation mechBabcock that involve Y;, anism (182, 184,185).Instead of merely acting as an electron transfer agent between the OEC and P&, it is now proposed to perform a hydrogen atom abstraction. New models for the function of the OEC have been formulated based on this proposal (discussed later). The role of Yz is still not settled, of course, as evidenced by a recent article arguing against a hydrogen atom abstraction role (186).Finally, spectroscopic studies have also focused on identifying potential protein residues that might be ligands to the OEC. Through ESEEM spectroscopy, the presence of at least one imidazole ligand to the OEC manganese cluster has been established (187). Studies involving calcium and chloride have shown these ions to be critical t o the proper functioning of the OEC (35, 146, 1881, although their exact roles have yet to be elucidated. Without either of these, dioxygen evolution is halted and the OEC cannot complete one full cycle. It has recently been reported, for example, that the OEC can reach Sz without chloride, but cannot be further oxidized to S , o r pro-


339

ceed from S3 to S, without the addition of chloride (188). There has been speculation that the chloride’s role may be to prevent premature loss of oxidizing equivalents, perhaps by preventing the premature loss of oxidation equivalents in the formation of hydrogen peroxide (189).Another suggestion may relate chloride to substrate binding, an idea that has received some support from small-molecule-binding studies, which indicate that there is a site for a small molecule such as amnonia to bind to the OEC in a process that displaces chloride (35, 190). Calcium may be bound near the manganese and has been proposed to be the source for the Mn-X 3.3-A distance observed by EXAFS (191). One possible role for the calcium may be to act as a water-binding site from which water is transferred at some point to one of the four Mn ions of the OEC (192, 193). Various researchers have studied the effects of substituting for these critical ions. Only strontium has been successfully inserted in place of calcium with retention of any level of function, albeit severely attenuated (35, 194). Substitution for chloride is less sensitive. The addition of bromide does not appreciably alter the ability of the OEC to generate dioxygen (35,421. The OEC is also functional, but at reduced levels in the presence of iodide and nitrate. Fluoride strongly inhibits the OEC and has been suggested to alter the structure of the manganese cluster structure based on EXAFS data (195). Amines also replace the chloride and inhibit the function of the OEC (35, 190). Ammonia is a particularly interesting case in this regard, in that it appears to bind at two different sites within the OEC, suggesting that there may exist a second chloride-inaccessible site, which has been proposed to be a waterbinding site to a manganese (35, 196). Water is of course key to this whole process. Water-binding studies have been attempted to establish when the two water molecules that are oxidized to dioxygen become bound to the OEC and whether they are bound directly to manganese. Early water-binding studies suggested that the water converted to dioxygen is still exchangeable until at least S, . From isotopic labeling studies, it appears that the waters that are oxidized are not bound or highly restricted with respect to exchange until the late S states (197). A recent water-binding study suggested that there were two waters in exchange with the bulk water, but that these waters were exchanging at different rates (198). One proposal put forward to explain this observation was the existence of an Mn=O moiety. A review based on the thermodynamics of water oxidation was published in 1996 (37). Pulling the data together again, a larger picture of some to the structural and reactivity details of the OEC begins to emerge. The new hydrogen atom abstraction role proposed for Yz coupled with the

340


dimer of dimers model has led to some intriguing new hypotheses for the potential manner in which the OEC functions (46, 47, 177, 178, 181,182,184,185).A synthesis of these new concepts is presented in Scheme 12 (182). If the OEC functions in the newly suggested manner, then it would join a growing class of enzymes that utilize protein a', H

-!-

I

7kl+ Y',

-7Y-',

't.1"

SCHEME 12. A mechanism for dioxygen evolution by the OEC as proposed by Babcock that employs the hydrogen atom abstraction concept. [Reproduced with permission from (182). Copyright 1998, the American Chemical Society.]

residue radicals to complete their catalytic processes (47, 183). The dimer of dimers structural model has been modified t o include hydrogen atom abstraction from a water bound to a manganese by Y ; , which is equivalent to proton loss being coupled to the stepwise oxidation of the OEC. This proposal eventually leads to the generation of a high-valent manganese (IV)or (V) with an 0x0 bound, Mn=O, that then combines with a second water-derived oxygen atom bound to a nearby manganese to form dioxygen during the S4-So step. Another feature of these new models is the retention of charge neutrality throughout the process. Scheme 13 shows the Babcock proposal as modified by our group. For any functional proposal to gain validity, one needs to consider whether or not the energetics of the process involved would be favorable. Recent studies by Pecoraro and co-workers suggest that the energies required for this process to occur are likely to be favorable (discussed later).


+'%

2 H,O+ CI-

t

341

\

e- ,H+ YZ'

/e-.H+

s2

SCHEME 13. A mechanism for dioxygen evolution by the OEC involving hydrogen atom abstraction a8 proposed by Pecoraro.

The OEC provides perhaps the most complicated manganese enzyme system yet known and thus is rich with possibilities for the exploration of manganese chemistry. Overall, the OEC and PS I1 are among the more intricate enzymatic systems that are currently being explored. Structural models, spectroscopic models, and reactivity model complexes are all integral to insights that have been attained and those yet to be gained. The following paragraphs detail some of the work in this area. Models related to water oxidation are presented in the Section V, "Reactivity." A selection of the newer multinuclear complexes will be discussed in Sections 1II.B to 1II.D. Again, many reviews have dealt with model complexes for the OEC (29-31, 34, 45, 50). Many bioinorganic chemists have prepared speculative structural models over the past several years to gain a n insight into this system-for example, with respect to possible arrangement of manganese ions-or to provide characterized materials as reference compounds for spectroscopic examinations. The agreement between EXAFS data and the Mn-Mn vector of a Mn&-O), core is one prime example of structural modeling chemistry and spectroscopy combining to elucidate critical data on this system (discussed earlier). Multinuclear clusters are always of interest in this field, and several have been prepared (28-31, 34, 45). The adamantane and cubane structures led to proposals for the possible oxidation of water to dioxygen

342


that involved rearrangement of a manganese cubane to adamantane upon incorporation of water molecules, and another proposal invoked a butterfly structure that was converted to a cubane during the catalytic cycle (199, 200). These proposals, among others, have been extensively reviewed, and the reader is directed to one of the referenced sources noted earlier. New examples of dimers of alkyl-bridged Mn2(pn-O),cores have been prepared and structurally characterized in recent years (discussed later). A few of these also include waters bound to the manganese. One of the more interesting tetranuclear manganese structures in this class of compounds was reported in 1992 by Armstrong and co-workers, who then further probed its complicated magnetic couplings (201). A variety of other complexes have been prepared that have proven useful in the area of the spectroscopic understanding of the OEC. Many have been used as models for XAS studies to establish what is to be expected from the various oxidation states of Mn. Others have been probed to gain a better understanding of the interactions between manganese nuclei-for example, most Mn"'MnTVcomplexes that are known exhibit 16-line EPR signals (140). However, the Mn"' Mn'" complex prepared with the 2-OHsalpn ligand (202) only exhibits a 12-line spectrum. Another area that model complexes may help to address is the effects of water binding to a manganese cluster. Small molecules such as water can be bound t o the complexes derived from 2-OHsalpn, and these have been probed by ENDOR spectroscopy to ascertain what perturbations might arise from that bound molecule versus the complex without such an additional ligand (203). The energetics of water oxidation is another key point to address. This topic includes the interesting question of how the oxidizing equivalents are stored in the OEC without being released too soon, and how the requisite 3.6 V versus NHE for water oxidation can be built up when Y; is thought to maximally be able t o provide oxidations at a level of 1.1 V per oxidation (41).In addition, the proposal of hydrogen atom abstraction has led t o the exploration of how the homolytic bond dissociation energy of a water bound to a manganese dimer might be altered from the values observed for an individual water molecule in solution. To abstract a hydrogen atom from water would require more energy than is available to Y; (Table I). However, Pecoraro and co-workers have shown that the AG value for removing a proton from a protonated p-0x0 bridge is within a reasonable range for the Y; to be able to abstract a hydrogen atom (Table I) (204,205). These studies employ a concept worked out by Mayer for the oxidation of, for example, toluene by permanganate (206). Studies of water

343


TABLE I HYDROGEN BONDDrssOCiArION ENERGIES" Complex

HBDE (kcal/mol)

Reference

86 119 76 77 79 84

457 458 204 204 204 204,459,460

89 94 85 89 82 86

205 205 205 205 205 205

Tyrosine Water IMn'"(salpn)(p-0)I2 LMn'"(3,5-di-C1-salpn)(p-O)12 [MniV(3,5-di-(N02)salpn)(p-0)12 [Mniii'TV(bpy)(p-O)l~+ LMn$"(2-OH(3,5-diClsal )pn),(OHJl [Mn'i'MnTY(2-OH(3,5-d~Clsal)pnJ2(OHL)1' IMn~"(2-OHsalpn),(OH2)] IMn'1iMniV(2-OHsalpn)2(OH,)1 LMnYi(2-OH(3,5-di-t-Bu~alJpn)~(OH~)l

~Mn"iMn"'(2-OH(3,5-di-t-Busal)pn)2(OH2)l

For complete details, including complex pK,'s and electrochemical reduction potentials, the reader is directed to (204)and (205).

bound to complexes derived from the 2-OHsalpn ligand show that hydrogen atom abstraction from these complexes is also in a reasonable energy range, theoretically, for H-atom abstraction by Y; (205). In addition, the normally observed increase in reduction potential of this complex with an hydroxide bound is attenuated. This also suggests that if H-atom abstraction is occurring in the OEC, this might favorably affect the redox potential of the manganese dimer to which that water is bound, further stabilizing the system against early loss of its oxidizing equivalents before water oxidation can occur and thus allowing the potential for oxidation of the manganese cluster to remain in a range accessible to Y;. A report of quantum chemical calculations in 1997 supports these data (207). Other topics of importance to the OEC will be discussed in greater detail later. Water oxidation models will be addressed in Section V, "Reactivity," and models for the catalase-like reaction of the OEC will be addressed in Section V.B.3., "Catalase."

111. Structural Models

Manganese redox enzymes exhibit a range of structural motifs involving mononuclear or bi- and tetranuclear aggregates of manganese ions. There has been a great deal of research in the area of structur-

344


ally modeling the active sites of manganese enzymes such as the MnSOD, catalase, and the OEC. These small-active-site analogs have been prepared in the hope that structural mimics will also mimic (i) the spectroscopy and (ii) the reactivity of the biological center. Some success has been achieved in modeling the reactivity of catalase and SOD with these models, but there is no structural analog for the OEC that exhibits water-oxidizing activity. The following is an overview of additions in the field of manganese chemistry since the superior review by Wieghardt in 1989 (30).This section will provide a compendium of the structure types that have appeared in the literature over the past eight years but is not meant to be an exhaustive recapitulation of every structure that has been generated during that time.

A. MONONUCLEAR 1. M n (II)

Mononuclear complexes of manganese are quite common and have been prepared in all of the currently recognized biologically relevant oxidation states of manganese, II-V. Mononuclear Mn" complexes exhibit a range of polyhedra about the metal, with coordination numbers of 5-8 dominating. The lack of coordination number specificity is understandable, as this d6 ion does not, in theory, exhibit specific geometric preferences. Numerous complexes have been prepared with predominantly N,O,-donor atom sets, which are often augmented by halide. or pseudohalide ligands and the occasional sulfur donor. In general, this represents the known biologically relevant donor atoms to manganese. Several of these complexes have been generated with the target of mimicking or modeling MnSOD. A variety of five-coordinate complexes of Mn" have been prepared. Crystallographically characterized Mn complexes with a trigonal bipyramidal geometry are still relatively rare, however. Kitajima and co-workers (208) reported two trigonal bipyramidal complexes utilizing hydrotris(3,5-diisopropylpyrazo1-l-yl)borate,HB-i-Pr, a facially binding tridentate ligand (209,210)(Fig. 13).The coordination sphere of the first complex was completed by a bidenate benzoate; the other complex contains a monodentate benzoate and one equivalent of 3,5diisopropylpyrazole. Another variation that has recently appeared is composed of tbp complexes of Mn" with tris(benzimidazoy1-2-methyl)amine bound t o the manganese (211,212) (Fig. 14).The tridbenzimidazoyl-2-methyllamine is a tetradentate tripodal amine ligand that provides four N-donor atoms. The first coordination sphere of these complexes was completed by a chloride. However, these products are


345

FIG. 13. [Mn~"~hydrotris(3,5-diisopropylpyrazo1-l-yl)borate)(3,5-diisopropylpyrazole~ (benzoate)] perchlorate. [Reproduced with permission from (208). Copyright 1993 the American Chemical Society.1

not purely five-coordinate compounds. The crystal structure of each bears both a five-coordinate form with one C1- bound to the central Mn and one C1- counterion, and a six-coordinate complex with two chlorides bound to the central Mn (Fig. 14). Another five-coordinate complex appeared in 1990 (213) with a single bis(benzyimidazolyl-1methy1)amine ligand and two acetate molecules (Fig. 15). The amine ligand binds meridionally, with the two acetate ions completing the equatorial plane of the structure. A few Mn" complexes with square pyramidal geometry were also reported. In one case, the basal plane is occupied by two dihydrobis(pyrazo1-l-y1)borate ligands with an axial chloride (Fig. 16) (214).In another, the axial position is occupied by an axial THF molecule with two THF molecules and two phenoxy donor groups in the basal plane (215). The greatest number of complexes characterized are those that are six coordinate. One general structure type involves multidentate li-

FIG.14. Crystal structure (left) of [MnYtris(benzimidazoyl-2-methyl)amino)C1It, the trigonal bipyramidal complex. Crystal structure (right) of the neutral six-coordinate [Mn"(tris(benzimidazoyl-2-methyl)amine)C121. [Reproduced with permission from (211) and (2121, respectively. Copyright 1997 the American Chemical Society and copyright 1995 Elsevier Science, respectively.]

346

FIG. mission


with per-

gands that fill four of the six available coordination sites, allowing monodentate ligands such as halides or water to occupy the two remaining positions in the first coordination sphere. In complexes that involve bidentate ligands such as phenanthroline, bipyridine, or acetylacetone, which often do not bind opposite to each other in the “equatorial” plane of these compounds, a structural type in which the two remaining coordination sites are cis to one another is promoted. One example of these bis bidentate chelate complexes is the complex (ditriflatolbis(bipyridin0) Mn” (216) and bis (phenanthrolino)(dithiocyanato) Mn” (Fig. 17) (217). One of the more unique examples in this group involves a fused-ring system, which provides a tetradentate N4donor ligand that occupies three coordination sites on one face and one addition site on the other. The two remaining cis positions were filled by bromides and chlorides, ultimately leading to distorted octahedral structures (Fig. 18) (218).This group also includes complexes derived from the tris(benzimidazoy1-2-methy1)amine ligand mentioned earlier (Fig. 14) (211,212). When the chelating ligands are able to occupy the “equatorial plane” of a Mn” complex at the same time, the monodentate ligands

FIG. 16. Square pyramidal [Mnl’(dihydorobis(pyrazol-lyl)borate)C1]’. [Reproduced with permission from (214).Copyright 1990 the Royal Society of Chemistry.]


347

S FIG. 17. IMn"(phenanthr~line),(SCN)~].[Reproduced with permission from (217). Copyright 1993 International Union of Crystallography.]

will bind to the central Mn" ion on the remaining two trans "axial" positions. Variations on this theme include charged chelating ligands with a pair of neutral ligands axial or equatorial neutral chelates, such as a tetraazamacrocyle, for example, with charged axial ligands, such as halide ions (Fig. 19) (219). Yet another variation found for these complexes is one in which all of the ligands about the central Mn" are neutral, for example the complex [bis(bis(2-pyridylmethyl) amino)MnI1l2+(Fig. 19) (220). A less common but rapidly growing family of Mn" complexes are those that are seven coordinate. These complexes may be built up from a variety of ligand systems that range from multiple ligands to a single septadentate ligand. Some of these systems have been tested as MnSOD mimics. Most of the seven-coordinate complexes that were structurally characterized in recent years have adopted a pentagonal bipyramidal geometry. One motif is based on a pentadentate macrocylic ligand with two unidentate ligands in the axial positions. One particular family of pentagonal bipyramidal complexes was prepared by Riley and co-workers from the macrocyclic ligand 1,4,7,10,13-pen-

FIG. 18. Mn"(2,5,8,10,13,16-hexaazapentacyclo~8.6.l,1'~5.09~'H.O'"~'7]o~tadecane)Cl~. [Reproduced with permission from (218). Copyright 1989 Verlag der Zeitschriften fur Naturforschung.]

348


FIG. 19. [Mn”(l,4,7,10-tetraazadodecane)C12], left, and [Mn”(bis(2-pyridylmethyl) amine)z]2+, right. [Reproduced with permission from (219)and (2201,respectively. Copyright 1996 International Union of Crystallography and Copyright 1992 American Chemical Society, respectively.]

FIG. 20. Two seven-coordinate Mn” complexes that have been tested as MnSOD mimics. [Mn11(1,4,7,10,13-pentaazacyclopentadecane)Cl~] (left) and [Mn”(trans-2,3eyclohexano-1,4,7,10,13-pentaazacyclopentadecane)Clz](right). [Reproduced with per mission from (221) and (222). Copyright 1994 and 1996 the American Chemical Society, respectively.]

0

0

0

FIG.21. [Mn”(tris(ethan~l)arnine)~]. [Reproduced with permission from (217). Copyright 1993 Verlag der Zeitschriften fur Naturforschung.]


349

N

FIG.22. [Mn"(tris(benzimidazoyl-2-methyl)amino)(N0,)Z1. [Reproduced with permission from (211).Copyright 1997 the American Chemical Society.]

taazacyclopentadecane (221, 222) (Fig. 20). The first coordination sphere in these complexes is completed by two axial chlorides. This work was specifically focused on the development of MnSOD mimics, and these workers have generated a wide range of complexes by preparing a series of ligands in which the macrocyclic ligand was modified by additions to the ring's carbon skeleton (222). Another example involves a modified 15-C-5 ligand with axial di-t-butylnaphthylsulfonates (223),and a related complex has been prepared with two axial trifluoromethylsulfonates (224). Pentagonal bipyramidal complexes do not, however, require macrocyles to form. Other pentadentate but open-ended ligands have also proved to be well suited for the generation of seven-coordinate complexes (225-227). Figure 21 shows an example of a complex with an MnLz motif, wherein one tris(ethano1) amine ligand binds as a tetradentate ligand, while the other binds as a tridentate ligand with one unbound ethanol arm. Another example of CN 7 is that of the tetradentate ligand tris(benzimidazoy1-2-methyl) amine and Mn" (Fig. 22) (211), wherein the other three binding sites are occupied by one monodentate nitrite and one bidentate nitrite molecule. A final example is provided by the complex of Mn" with quaterypyridine, one water, and a bidentate acetate (Fig. 23) (226).

FIG. 23. [Mn"(quaterpyridine)(OAc)(H,O)l+.[Reproduced with permission from (226).Copyright 1993 Elsevier Science.]

350


FIG.24. [Mn~'(1,4,7,10,13,16,19-septaazacyclouneicosane)l~+. [Reproduced with permission from (228). Copyright 1990 the American Chemical Society.]

Two seven-coordinate complexes were attained by utilizing single septadentate ligands. The first involved 1,4,7,10,13,16,19-~eptaazacyclouneicosane, but no specific geometric orientation about the Mn" was reported (Fig. 24) (228). The second structure is of a monocapped trigonal antiprism, prepared by using the septadentate ligand formed by Schiff-base condensation from tris(2-aminoethy1)amine and 2-pyridinecarboxaldehyde t o give tris(2-aminoethy1)amine-pendant pyridyl-2-methyl functional groups (Fig. 25) (229). This complex was tested for MnSOD activity (discussed later). A final structural motif for Mn" is represented by a few eight coordinate structures. One of these was a product of tetraazacyclododecane with pyrazolylyl-l-methyl groups appended to the ring nitrogens

FIG.25. [Mn"(tpaa)12+.[Reproduced with permission from (229). Copyright 1996 the American Chemical Society.]


351

FIG.26. [Mni'~1,4,7,10-(l-pyrazolylmethyl~tetraazacyclododecane)]2+ (left) and [Mn" (tris(2-pyridylmethyl)amine)12+(right). [Reproduced with permission from (230) and (232).Copyright 1992 the Royal Society of Chemistry and copyright 1993 Elsevier Science, respectively.]

(Fig. 26) (230).This led to a Mn" complex having a square prismatic geometry. The second complex is composed of a cryptate ligand that yields a complex with a cubic symmetry (Fig. 27) (231).The third structure is built up from two tetradentate tripodal amine ligands about the Mn" ions (Fig. 26) (232). One final note is of interest. Most of the structurally characterized and otherwise isolated complexes of Mn" tend to be high spin. The structure of at least one low-spin complex was reported during the past few years. This is a six-coordinate complex composed of two methyl-4,5-dihydro-5-{[imino(methylthio)methyllazo}-3,5-dimethyl1H-pyrazole-1-carboximidothioatoligands, which provide an N, first coordination sphere via one aza-nitrogen and two carboximido-nitrogens (Fig. 28) (233).

FIG.27. [Mnii([2.2.2]cryptand)]'+ [2.2.2]cryptand = 4,7,13,16,21,24-hexaoxa-l,lO-diazabicyclo[8.8.8]hexaxosane. [Reproduced with permission from (231).Copyright 1992 Wiley-VCH Verlag.]

352

LAW, CAUDLE, AND PECOFURO

I

FIG.28. Low-spin [Mn"~methyl-4,5-dihydro-5-{~imino~methylt~o~methyllazo}-3,5-dimethyl-lH-pyrazole-l-carboximidothioate)1. [Reproduced with permission from (233). Copyright 1993 the Royal Chemical Society.] 2. Mn(III)

Five- and six-coordinate structures dominate the crystallographically characterized structures reported for Mn"'. Here the pseudoJahn-Teller axis of this d4ion plays a role in the overall structure of the Mn"' coordination complex, so these complexes tend to adopt either a square pyramidal or octahedral geometry. Most of the five-coordinate complexes that have been structurally characterized are found to adopt a square pyramidal geometry. A prime example of such products are the myriad of Mn"' complexes that utilize a tetradentate Schiff-base ligand (234),for example H2-Salpn or H2Salen,with a fifth donor in the axial position t o complete a square pyramidal structure. Such structures are also observed in the Mn"' porphyrin complexes with a single axial ligand (470). This structure ably supports the pseudo-Jahn-Teller axis along the axial, or z, axis. Specific examples of this group of complexes are the complexes utilized for exploring epoxidation chemistry (235).One of these complexes is shown in Fig. 29 (236),with a structure similar to the other members of this type of complex (237).For this type of complex, the Mn'" ion tends to be located out of the equatorial plane toward the axial ligand. Another area of Mn"' complexes are those that utilize a variety of porphyrins as ligands. Five-coordinate complexes were not limited to the square pyramidal geometry. At least two reports of structurally characterized trigonal bipyramidal complexes have appeared in the literature in recent years. The first is of a complex in which the Mn"' is surrounded by an 0, first coordination sphere composed of two bidentate 2,2'-biphenoxide ligands and one monodentate 2,2'-biphenoxide to create a distorted trigonal bipyramidal geometry (Fig. 30) (238).The


353

FIG. 29. {Mn1''[3,5-~di-t-buty1~Sa1icylidenimino-~~~)-(1,2-diaminocyclohexane)l}. [Reproduced with permission from (236).Copyright 1996 Wiley-VCH Verlag.]

second is a very intriguing structure with a tegradentate tripodal amine ligand (239).Three of the five donor atoms are nitrogens from amide functional groups (Fig. 31). The unique feature of this complex is the hydroxide bound to the Mn"' ion. In general, structurally characterized complexes of manganese with hydroxide tend to be rare. Six-coordinate complexes of Mn"' are also quite common. Many of these, again, were prepared with tetradentate Schiff-base ligands that occupy the equatorial plane of the complex while two other ligands bind to the central manganese ion on the remaining trans axial positions (234, 240). Structures with Mn"' were not, of course, exclusively prepared with tetradentate Schiff bases or similar ligands, however (241). A unique six-coordinate structure was reported in 1992 for a MnIII-OH complex, another rare example of a structurally characterized Md'I-OH complex (Fig. 31)(242).

FIG. 30. [Mni"(2,2'-bisphenoxide)(2,2'-bisphenoxideH)l.[Reproduced with permission from (238).Copyright 1991 the American Chemical Society.]

354


od FIG. 31. Rare examples of Mn"'-OH-complexes: [Mn"'(tris(cyclopropylcarbanoylmethyl)amine)OH]- (left) and [Mn'11(bis(2-hydroxy-5-nitrobenzoiminopropyl)methylamine)OH]. [Reproduced with permission from (239) and (242). Copyright 1997 and 1992 the Royal Society of Chemistry, respectively.]

The structure of one rather key six-coordinate Mn"' complex in this area was reported by Kitajima and co-workers in 1994. This complex is composed of the hydrotris(3,5-diisopropylpyrazol-l-yl)borateligand, one monodentate 3,5-diisopropylpyrazole,and a side-on-bound peroxide molecule (Fig. 32) (243).This structure is important with respect to the question of the "dead-end" complex that is proposed to occur during turnover in MnSOD. It is also a rare example of a structurally characterized Mn-peroxide moiety. One of the key features of this system is the two differing structures that were isolated. In the second structure (Fig. 32), the peroxide is presumed to be hydrogen bonding to the extra pyrozole ligand, based on the shortened interaction between the hydrogen on the nitrogen of the pyrazole ring and the oxygens of the peroxide. 3. M n ( W

A less common but growing class of complexes are the mononuclear complexes of MnW.The structurally characterized MnW,d3,complexes tend to adopt six-coordinate structures that are octahedral in nature. Typically, these MnWstructures require charged ligands to support the higher oxidation states. This is also true of the Mn" complexes. In most of these complexes, phenolato, alkoxo, or amido donors have been employed to stabilize these higher oxidation states. Among these are the complexes of MnTVsalpn(acac)(PF6)(244) (Fig. 33), which is octahedral with the salpn2- ligand in a cis-beta conformation, and a MnW(salpn)C12 (245)complex with an equatorial salpn2- and two axial chlorides (Fig. 33). An example of an O6 first coordination sphere is


355

FIG. 32. [Mn~"(hydrotris(3,5-diisopropylpyrazol-l-yl)borate)(3,5-diisopropylpyrazole~ peroxide]+ Two different structures were grown at low temperatures. In one structure (bottom) there is a hydrogen-bonding interaction between the 3,5-diisopropylpyrazole ligand and the peroxide moiety. The hydrogen-bonding interactions are indicated in the expanded figures to the right. This is a n important complex with respect to modeling manganese redox enzymes. [Reproduced with permission from (243). Copyright 1994 the American Chemical Society.]

356


FIG. 33. [MnTY(salpn)(acac)](PF6) (left) and MnN(salpn)C12(right). [Reproduced with permission from (244)and (245).Copyright 1991 the American Chemical Society and copyright 1995 the Royal Society of Chemistry, respectively.]

provided by the Mnw complexes with a-hydroxy acids that were prepared to model manganese peroxidases (Fig. 34) (246).A unique hexadentate ligand, based on octamethyltetraamine with salicylic acid moieties appended to the terminal nitrogens to form amide functionalities, produces the mononuclear Mn'" complex shown in Fig. 35 (247).

The macrocycle 1,4,7-triazacyclononane (2481,tacn, has been a popular ligand in the manganese community in recent years. Two interesting complexes based on this ligand with N303first coordination spheres have been reported. One Mn" example is the MnTVtacn (OCH3I3PF6 complex (249),which has been suggested as able to epoxidize olefins. This complex adopts a structure with a facial array of methoxides due to the nature of the other, facially binding tacn-based ligand. The second is one in which the alkoxides are appended by alkyl arms to the tacn ligand, similar to the structure in Fig. 36. That structure is represented by the complex with an N6 first coordination

FIG. 34. [MnN(2-hydroxyethylbutyric acid)J. [Reproduced with permission from (246).Copyright 1991 the American Chemical Society.]


357

FIG.35. ~Mn'V~l,l0-bis(salicylamido)hexamethylenetetraamine~]. [Reproduced with permission from (247).Copyright 1992 the American Chemical Society.]

sphere and the triply deprotonated ligand 1,4,7-tris(o-aminobenzyl)1,4,7-triazacyclononane (Fig. 36) (250). 4. MnW)

Structurally characterized Mn" complexes are rarer still. The new examples of mononuclear manganese complexes in this oxidation

FIG. 36. [Mn'v(1,4,7-tris(o-aminobenzyl~-1,4,7-triazacyclononane~l+. [Reproduced with permission from (250).Copyright 1995 the American Chemical Society.]

358


FIG.37. [MnV~N,N'-bis~salicylideneimino~-2,4-dimethyl-2,4-butanediamine~~N)]. [Reproduced with permission from (251).Copyright 1996 the American Chemical Society.]

state have been of nitrodo, N3-, and Mn-oxo derivatives. Most of these adopt a square pyramidal geometry with the organic ligand occupying the axial plane while the 0x0 or nitrido group occupies an axial site (251-253). A manganese-nitrido complex with this orientation appeared with the ligand sale+ and the same ligand with all of the ethylene backbone's protons replaced by methyl groups, saltme+ (Fig. 37) (251). Both the MnV=O group and the MnV=N groups were first characterized in porphyrin systems, but only the MnV=N has been structurally characterized (Fig. 38) (254). More recently, two MnV=O complexes have been crystallographically characterized (255-257), for example the complex shown in Fig. 39. A unique feature of all of these MnVcomplexes is the short Mn-nitrido and Mn-0 bonds. The Mn-N bonds are on the order of 1.50-1.54 A, and the Mn-oxo bonds have been determined to be about 1.54-1.55 A. Most of these complexes are square pyramidal, but some variation is now known. A recent report of a reputed trigonal bipyramidal Mnv-nitrido complex composed of two bidentate ligands has appeared (2531,and a six-coordinate MnVcomplex was produced by Wieghardt and co-workers in 1996 (258) (Fig. 40) that employs a trimethyl tacn ligand, one acetylacetonate ligand, and a terminal nitrido. The Mn-N (nitrido) distance in this complex is 1.518 A. One nitrogen donor atom from the tacn ligand occupies the axial position opposite t o the nitrido at

FIG. 38. [MnV(OEP)(N)].[Reproduced with permission from (254). Copyright 1983 the American Chemical Society.]


359

I

FIG.39. [MnV(LKO)I-.[Reproduced with permission from (256).Copyright 1990 the American Chemical Society.]

an elongated distance of 2.301 8 compared to the other two N donors from this ligand at 2.055 8 and 2.073 8. The structures of both of these types of Mn" complexes, Mn"=O and Mn"-N, are important structural models for epoxidation and aziridination reagents, and for the OEC with respect to the possible formation of a Mn"=O during turnover.

B. BINUCLEAR The majority of model compounds synthesized for studying the biomimetic chemistry of manganese are binuclear complexes, due to the

FIG.40. [MnV(Me3tacn)(acac)N]. [Reproduced with permission from (258). Copyright 1996 the American Chemical Society.]

360


relative ease and reproducibility of their preparation when compared with complexes of higher nuclearity. There are a number of binuclear manganese enzymes for which binuclear complexes can function as direct structural models (discussed earlier). Furthermore, higher nuclearity clusters in model chemistry and biological chemistry can often be described as assemblies of well-characterized binuclear units. Within the myriad of binuclear manganese complexes synthesized to date, a few core structural motifs appear with regularity. Each core is distinguished by a characteristic Mn-Mn distance that is relatively invariant with changes in the supporting ligands. As such, the Mn-Mn distance, where it is known, is often used as evidence of a particular core structure when a full crystal structure is not available. This is often the case in the biological systems themselves, which have been studied by X-ray absorption spectroscopy and from which Mn-Mn distances are determined even in cases where no crystal structure is available. Table I1 lists the binuclear manganese complexes whose cores have been structurally characterized along with their respective Mn-Mn separations. Although a multitude of dimanganese complexes have been prepared within each core structure, we will restrict the discussion in this section to those that have been structurally characterized by X-ray crystallography or X-ray absorption spectroscopy. TABLE I1 GENERAL TRENDSIN MN-MN DISTANCES OF SOME COMMON BRIDGING MOTIFS Bridging motif

Oxidation state Mn-Mn Distance (8) 2.8 2.6-2.7 2.6-2.7 3.2-3.3 3.1-3.4 3.1-3.2 3.25 3.6 3.3 3.1-3.6 2.6 3.4-3.5 3.5-3.6

References are given in text or are in (280).


36 1

The simplest binuclear structure possesses two manganese ions bridged by a single 0x0 or hydroxo ligand. Because dioxo- and dihydroxo-bridged manganese dimers tend to be thermodynamically stable, the mono-oxo and monohydroxo complexes are isolated and characterized only with sterically demanding ligands that do not provide two labile manganese coordination sites in a cis orientation. Only three complexes with biologically relevant ligands have been structurally characterized, these have two manganese bridged by a single hydroxide. The only nonporphyrin manganese dimer bridged by a single hydroxo linkage is the binuclear complex of the macrocyclic ligand tpictn (Fig. 41) (259).The Mn-Mn separation in this highly supported structure is 3.6 8, with a Mn-O-Mn angle of 126.8'. Mn2(tpictn) (p-OH) is a special case, however, in that the Mn,(p-OH) core is supported by a macrocyclic ligand that spans both Mn" ions. The only unsupported complexes having a single hydroxide bridge are the Mn"' porphyrin complexes [Mn"'(OEP)],(p-OH) (260) and [Mn*1*(TPP)12 (p-OH) (261) (Fig. 42). These complexes exhibit Mn-O-Mn angles of 152.7' and 160.4, respectively, consistent with the bridging hydroxo group. The more linear geometry of the TPP derivative is probably due to steric demands of the pendant phenyl groups, resulting in a longer Mn-Mn distance, 3.993 8, for this complex than for the OEP derivative. [Mn"'(5-NOzsaldien)lz(p-O) (262) has a single oxide ligand bridging two Mn"' ions and has a Mn-Mn distance of 3.490 8 (Fig.

Fic. 41. [Mnb'(tpictn)(p-OH)l. [Reproduced with permission from (259). Copyright 1995 the Royal Society of Chemistry.1

362


Q FIG.42. [Mn"'(OEP)],(p-OH)+ (left) and [Mn"(TPP)N3lZ(p-O)(right). [Reproduced with permission from (261)and (265).Copyright 1996 the American Chemical Society and copyright 1981 the Royal Society of Chemistry, respectively.]

43). The hydrotris[(3,5-diisopropyl)pyrazolyllborate derivative [Mn"' (HB(3,5-iPr2pz)2(3-iPr0-5-iPrpz))12(p-O) (263) (Fig. 43),has a similar Mn-Mn distance, 3.53 A, and a more linear Mn-0-Mn geometry explained by steric demands of the hindered HB(3,5-iPrzpz)2(3-iPrO-5iPrpz) ligand. The MnTV complex with tetraphenyl porphyrin forms an 0x0-bridged complex, [MnW(TPP)N3l2(p-O), with a nearly linear Mn0-Mn angle (264,265) (Fig. 42). Analogs of p-OH-bridged complexes having a single p-alkoxo ligand are known as well. In fact, when analogous p-OH and p-OR bridged complexes are structurally characterized, the structural parameters are often quite similar. At this time the only structurally characterized complexes having a single p-OR ligand involve the highly supported 2-OHsalpn ligand motif (266-268) For example, [Mn1Y2-OH-

FIG. 43. [Mn1'1(5-N02saldien)]2(p-O)and [Mn"'(HB(3,5-iPr2pz)2(3-iPrO-5-iPrpz))lz (p-0).[Reproduced with permission from (262)and (271).Copyright 1990 and 1991 the

American Chemical Society, respectively.]


363

FIG. 44. [Mn'"(2-OH-(5-Clsal)pn)],(CH30H) and [Mn"'Mnw(2-OH-(3,5-(Cl)zsal)pn)2

(THF)]'. [Reproduced with permission from (270).Copyright 1992 the American Chemical Society.]

(5-Clsal)pn)lZ(CH30H)(Fig. 44) has a Mn-Mn distance of 3.808' and a Mn-O-Mn angle of 129" (269), within the range expected for Mn"12(p-OH) complexes (discussed earlier). The analogous mixedvalent [Mn"'Mn" (2-OH-(3,5-(Cl)zsal~pn)2(THF)lt complex also has a

364


single alkoxide bridge (270) but with a shorter Mn-Mn distance of 3.65 8, and a Mn-O-Mn angle of 126.7"(Fig. 44). Bis-p-oxo dimanganese complexes are the most prevalent structurally characterized dimanganese motif in the literature. A smaller number of bis-p-hydroxo- and bis-p-alkoxo-bridged analogs are also known. Because of the prevalence of similar structures differing only in the supporting ligands, binuclear manganese bis-p-0x0, bis-p-hydroxo, and bis-p-alkoxo complexes are most conveniently considered as discrete manganese cores in which the supporting ligands play a stabilizing role. This view is supported by the remarkable similarity in core structure among complexes having very different ligands. We will begin by discussing lower valent states and continue with increasing oxidation state of the manganese in the binuclear cluster. [Mn11z(HB(3,5-iPrzpz)3)]2(0H)z, with a Mn-Mn distance of 3.314A, is to date the only crystallographically characterized binuclear manganese complex with two hydroxo bridges (271) (Fig. 45). When the bis-p-oxo complex this complex is oxidized by Mn04- or 02, [Mn"'tHB(3,5-iPrzpz>3)l~~O)z is formed (Fig. 45).This oxidized complex has a much shorter Mn-Mn distance of 2.696 8, between two fivecoordinate manganese"' ions. The six-coordinate complex [Mn"'(bispic-

b FIG.45. [Mn~'(HB(3,5-iPr,p~)a)l~(OH)~ (top) and [Mn'1'(HB(3,5-iPrzpz)3)12(0)2 (bottom). [Reproduced with permission from (271).Copyright 1991 the American Chemical Society.]


365

FIG.46. [MnT"(bispicMepen)l~(p-Oj. [Reproduced with permission from (272). Copyright 1994 the American Chemical Society.]

Me2en)12(p-O) (Fig. 461, has nearly the same Mn-Mn distance, 2.699 elongation axis common in Mn"' ions does not include a bridging ligand (272). No bis-p-hydroxo dimanganese(II1) cores have been structurally characterized, but the analogous bis-p-methoxo dimanganese(II1) complex [Mn"'(salpn)], (p-OCH,), (Fig. 47) has been prepared and exhibits a 3.192 A Mn-Mn distance (244). The longer Mn-Mn distance for the alkoxide-bridged complex reflects the Jahn-Teller elongation axis on each of the symmetry-related manganese ions, which includes a bridging alkoxide ligand. The supported [Mn"'(2-OHsalpn)12(Fig. 48) has two alkoxide bridges as well, but with slightly longer Mn-Mn separation due to the highly supported ligand structure (273,274). Mixed-valent dimanganese(II1,IV) complexes with the Mn"'Mn'" (~-0 core )~ have structural parameters very similar to the corresponding MII?(~-O)~ complexes, with Mn-Mn distances in the 2.7-A range. In spite of the similarity to homovalent cores, the prototypical [Mn"' (276) comMn1v(bpy)4(p-0),13+ (275) and [Mn"1Mn'V(phen),(p-O)213+

A, because the Jahn-Teller

FIG.47. IMn"i(3,5-diC1-salpn)(p-OCH,)I,. [Reproduced with permission from (244 j. Copyright 1991 American Chemical Society.]

366


FIG.48. [Mn"'(2-OH(5-NOz)salpn)]z. [Reproducedwith permission from (273).Copyright 1993 the American Chemical Society.]

plexes are valence localized in the solid state, with one manganese exhibiting the axial elongation associated with isolated Mn"' centers and the other having more octahedral geometry consistent with Mnrv complexes exhibit valence localization (Fig. 49). Other Mn111Mn1V(p-0)2 as well (272,277,278). The only example of a mixed-valent Mn1I1MnW (2791, which bis-p-alkoxo-bridged complex is [Mn111MnTV(2-OHsalpn)zl+

FIG. 49. [Mnr1LMnw(bpy)4(p-O)2]3t. [Reproduced with permission from (275). Copyright 1995 the American Chemical Society.]


367

FIG. 50. [Mn'V(salpn)]z(p-O)z.[Reproduced with permission from (244). Copyright 1991 the American Chemical Society.1

has a Mn-Mn separation of 3.3 A, substantially longer than for bisp-0x0 complexes in the same oxidation state. The higher valent Mnr(p-O), complexes have been prepared with many supportin ligands, but all show the characteristic Mn-Mn distance of 2.7 (34).The prototypical complex [Mn'Ysalpn)lz(p-O)~ is shown in Fig. 50 (280, 281). Each successive protonation of [Mn [MnTV(salpn)l ,(p-O)(p-OH)' and [MnW(salpn)l, ( ~ a l p n ) ] ~ ( p - to O )give ~ (p-OH),t was shown by X-ray absorption spectroscopy to result in an increase in the Mn-Mn distance of 0.10 A (282, 283). The structural effect of protonation on the Mniv(p-O)zcore is considerably less pronounced than alkylation of the corresponding MnY1(p-0)2complexes due to rearrangement of the Jahn-Teller axis in the Mn"' case. A new structure type for Mnbv(p-O)zSchiff-base complexes was reported for the complex [MnlV(salen)]z(p-O)z ligand (234, 284,285). In these complexes (Fig. 51), the salen ligand bridges the two manganese ions.

1

FIG.51. [Mn1v(salen)]2(~-0)2. [Reproduced with permission from (284). Copyright 1998 Elsevier Science.]

368


FIG.52. ([Mn'"(tacn)ll(~-O)~(~-Ol)} (left) and [Mn'"(ta~n)l&-O)~ (right). [Reproduced with permission from (286)and (287). Copyright 1988 and 1990 the American Chemical Society, respectively.]

There are two unique examples of triply bridged Mny dimers that deserve comment at this point. The first is the [MnTV(tacn)lz(p-O), com lex with three bridging 0x0 ligands and a Mn-Mn distance of 2.3 (Fig. 52) the shortest Mn-Mn distance of any binuclear complex t o date (286). The second is {[Mn'v(tacn)lz(p-O)~~p-Oz)} (Fig. 52), which is the only example of a binuclear Mn complex having a p-1,2 bridging peroxide ligand and a Mn-Mn distance of 2.531 A (287). These two complexes may be considered as MniYp-O), cores with either an additional p-0x0 or the p-1,2-peroxo added as a third ligand. Because of the additional ligand, the MniV(p-O),core is puckered in contrast to simple MnY(p-O):!cores. The importance of the latter structure is that it may be the best model to date for the initial manganeseoxygen species formed immediately upon 0-0 bond formation in the critical S4-So transition in photosynthetic oxygen evolution. There is a large and important class of binuclear manganese complexes having at least one p-carboxylate bridge in conjunction with one or two p-0x0 bridges. This class of p-oxo-p-carboxylato complexes is important not just in the context of binuclear manganese enzymes such as catalase [which has recently been shown to contain such a core (12511, but as analogs for iron oxo-carboxylato complexes, which are very important in the chemistry of nonheme iron redox enzymes. The simplest example in this class of manganese compounds is represented by the p-oxo-p-carboxylato system [Mn"'(bispicen)lz(pU-O)(p~,~OAc) with a Mn-Mn separation of 3.28 A (288) (Fig. 53). This core motif has been structurally characterized only in the dimanganese(II1) oxidation state (289). Although no examples of crystallographically characterized Mn;I(p-OH)(OAc) cores exist, the topologically similar MniII(salampn)(OAc)complex (290) has a MniII(p-OR)

w


369

FIG. 53. [Mn"'(bi~picen)],(~-O)(~,,~-OAc) (left) and [Mn"'(tmima)]z(~-O)(~,,~-OAcj (right). [Reproduced with permission from (288) and (289). Copyright 1994 and 1993 the American Chemical Society, respectively.]

(OAc) core with a Mn-Mn separation of 3.55 A. The catalase mimic Mni1(2-OHbenzimpn)(OAc)(Fig. 541,has the same core in the lower oxidation state (291) with a nearly identical Mn-Mn separation, 3.54 A. The more common members of the class of p-0x0-p-carboxylato complexes have three bridging ligands instead of two. The Mn,(p-O), (p-RC0,) core may be considered as a Mnz(p-O), core with a carboxylate spanning the Mn-Mn vector (292-294). However, the presence of the additional carboxylate bridge induces the Mn,(p-O), unit to be (Fig. 55) has a bent. For example, [Mn~~bpy)(H,0~12(p-O),~p,,~-OAc~3+ dihedral angle between MnYpO), planes of 162" and a Mn-Mn distance of 2.64 A (295),which is somewhat shorter than Mn-Mn separations in typical Mnr(p-O), cores. Reduced derivatives containing core have similar structural parameters the Mn"'Mnrv(p-0),(p~,~-OAc)

FIG.54. Mn~Y2-OHbenzimpn)(OAcj.[Reproduced with permission from (291). Copyright 1994 the American Chemical Society.]

370


P

FIG. 55. [Mn'v(bpy)(H,0)],(~-O)~(~~,3-OA~)3+. [Reproduced with permission from (295). Copyright 1994 the American Chemical Society.]

(296). No bis-p-hydroxo-p-carboxylato cores have been isolated. complexes However, several examples of Mn~1'(p-OR)2(p1,3-OAc) have been prepared and characterized (266). Mn$'(2-OHsalpn) (MeOH)2(p-OMe)(pl,3-OA~) is typical, with a Mn-Mn separation of 2.9 8 (266). The p-oxo-bis-p-carboxylato structure type, Mn,(p-O)(p-OAc),with two syn-syn carboxylate groups and a single p-0x0 ligand has been structurally characterized with a number of supporting ligands, but only in the dimanganese"' oxidation state. The Mn-Mn distance in these compounds is in the range of 3.1-3.2 8, as would be expected for the single 0x0 bridge. This core topology is particularly interesting because it is the only p-oxo-p-carboxylato motif to have been isolated in three different protonation states: Mn;"(p-O)(p-OAc),, Mn"Mn"' (p-OH)(p-OAc),, and Mnf(p-OHz)(p-OAc)z(297-299). Representative members of this series are illustrated in Fig. 56. It is noteworthy that each protonation is accompanied by a decrease in oxidation state and a consistent increase in Mn-Mn separation from 3.1 to 3.4 to 3.6-3.8 8. This is consistent with the decreased donor ability of the single-atom bridge with each successive protonation, which promotes longer bond lengths as well as stabilizing the lower dimer oxidation states. Dimanganese model complexes having only carboxylate bridges are known primarily in the lower manganese oxidation states, and complexes supported by only a single p-carboxylate unit are particularly rare (300-302). However, there are examples of dimanganese(I1) complexes bridged by a single carboxylate. Mn;*(RCO2)core has the bridging carboxylate in a syn-anti configuration with a long Mn-Mn distance of 5.6 8, Scheme 14,and complexes in which the carboxylate bridge is in the anti-anti configuration have exceptionally long 5.9to 6.0-8 Mn separations. Bis-p-carboxylato complexes, Mnr*(p-RC02)2,


371

FIG.56. Examples of MnZ(p-O)(p-RC02)2 cores with varying protonation of the "p-0" bridge. Upper left: [Mn"(N,N,N',N'-tetramethyl~ethylenediamine)~IZ(0Ac)~(p-OAc),(pOHZ). Right: {IMn"Mn"'(Me,tacn)Z1(p-(CH,),COO)z(p-OH)}+Z. Lower left: [Mn"' (bipy)CIZlZ(p-OA~)Z(p-O). [Reproduced with permission from (472),(298), and (473),respectively. Copyright 1992 American Chemical Society, copyright 1996 WILEY-VCH Verlag, and copyright 1993 American Chemical Society, respectively.]

have much shorter 4.3-A Mn-Mn separations with a syn-syn configuration for both p-carboxylate groups. Addition of a third carboxylate group, Mn11(p-RC02)3, yields a Mn-Mn distance of 3.6-4.0 A (303). Mn

Mn

\

0d,

R

syn-syn

Mn

Mn- O y O- Mn

\

0y o ' M n R syn-anti

R anti-anti

SCHEME 14.

Other unsupported Mnf'(p-OR)2 cores have Mn-Mn separations of 3.0-3.2 A. Complexes having one supported p-alkoxo and one unsup-

372


ported p-methoxo ligand can have Mn-Mn distances of 3.0 A or lower (266,267). Alkoxo-p-carboxylato complexes are topologically similar to their bis-p-0x0 analogs, however, the Mn-Mn distances tend to be considerably longer in the p-alkoxo complexes than in corresponding p-0x0 derivatives. For example, p-alkoxo-p-carboxylato complexes, Mnp (p-OR)(p-RCO,), have typical Mn-Mn seearations of about 3.5 A. This is considerably longer than the 3.3-A separation observed for Mni1'(p-O)(p-RCOz). Mnz(p-OR),( -RC02)complexes have Mn-Mn distances ranging from 2.9 to 3.0 which is about 0.40 longer than the oxo-carboxylato-bridged analog. Carboxylato-bis-p-alkoxo complexes having the MniYp-OR)(p-RCO,), core have even longer 3.3- to 3.4-A separations (304)which extend to as much as 3.5 A upon oxidation to the MnllMnll'(p-OR)(p-RCOz)zoxidation level (305, 306). This compares favorably with 3.4 A for the MniI(p-OH)(p-RCO,)(297,298) complex recently prepared, demonstrating that alkylation of p-oxobridged dimanganese cores is a good structural analog for protonation. p-Phenoxide-bridged dimanganese clusters are structurally similar to their alkoxide-bridged counterparts (304-306). However, an additional factor not present in alkoxo-bridged complexes is the possibility of rotation about the Ar-0 bond in phenoxo complexes (238, 307, 308). Because the donor ability of the phenoxide ligand is dependent on the orientation of the metal ion with respect to the aromatic plane, we might expect that this would be manifested in structural changes upon Ar-0 rotation in p-phenoxide-bridged manganese dimers. In fact, when Mn!Yp-ArO), dimers are examined, it is observed that as the dihedral angle between the MnzOzplane is increased from nearly zero to nearly go", the Mn-Mn distance increases from 3.26 .k (304) to 3.42 A (307).This is consistent with the phenolate oxygen being a weaker donor when the metal ion acceptor is oriented perpendicularly to the aromatic plane. There are a limited number of N-bridged or imidazole-bridged dimanganese complexes with biologically relevant supporting ligands (3091, consisting of azide-bridged complexes. Azide ligation is often used as a model for peroxide ligation to manganese complexes due to the greater stability of azide complexes, which facilitates structural characterization. h i d e may bridge either in a p-1,l fashion, yielding Mn-Mn separations of 3.4-3.5 & or as an p-1,3 bridge to give a 3.5-A separation (310, 311). With respect to imidazole-bridged complexes, a complex with the ligand DCBI, 4,5-dicarboxyimidazole, was with a Mn-Mn recently reported, [Mn'"(3,5-di-t-b~tylsalpn)]~(DCBI), separation of 6.18 A (Fig. 57) (312).

[

A


373

FIG. 57. [Mnrv(3,5-di-t-busalpn)I,DCBI. [Reproduced with permission from (312). Copyright 1997 the American Chemical Society.]

C . TRINUCLEAR Trimanganese clusters (and clusters of higher nuclearity) can usually be considered as assemblies consisting of the binuclear units discussed in the previous section. There are no known trimanganese enzymes. However, it has been proposed in the past that the fourmanganese active site in the OEC may actually be composed of a trinuclear and mononuclear manganese site (discussed earlier). The only trimanganese complexes having only p-0x0 bridges between the manganese ions are those of the Mn3(p-0I4class (292, 313, 314). These consist of a MnZ(p-O),core that is spanned by a Mn(p-O), unit (Fig. 58). The topology of these complexes is reminiscent of the Mnz

FIG. 58. Two examples of trimeric structures with Mn0,-bridged Mn2(p-0)2cores. [Reproduced with permission from (292)and (313).Copyright 1992 and 1990 the American Chemical Society, respectively.]

374


0

0

N N

N

N 6 N FIG.59. The trimeric (w3-O)(p-peroxocomplex). The other ligands are acetates and diethylenetriamine. [Reproducedwith permission from (317). Copyright 1988 the American Chemical Society.]

(p-0)2(p-RCOz)core, with the Mn(p-O), unit replacing the RCOz unit of the latter. There are several reported structures of the Mn3(p3-O)unit supported by additional p-carboxylate bridges (315, 316). The Mni11(p3O)(MeC02)6complex has each manganese ion bridged by two carboxylate and one p3-0x0group structurally analogous to the Mndp-0) (p-RCO,), complexes. The Mn-Mn separation in Mni1'(p3-O)(MeC0z)6 3.35 8, is longer than in Mnz(p-O)(RCO,),and is more reminiscent of the distance in the Mn2(p-OH)(p-RCOz),complexes. A unique member of this class of p 3 - 0 ~ 0trimanganese complexes has a single pl,z-peroxo ligand bridging two of the manganese ions (317) (Fig. 59). Another has been produced with a ps-OH and a bridging catecholate (318). Trinuclear manganese motifs are common among carboxylate comcore has found been with variplexes. The Mn~11Mn11(p-OR)z(p-RC02)4 ous Schiff-base-containing supporting ligands (269, 319, 469). These cores are characterized by two terminal Mn" ions, which "encapsulate" a central Mn"' ion with Mn-Mn neighboring distances of 3.5 8, which is consistent with the MnllMnlll(p-OR)(p-RCOz)zdimanganese cores having an identical bridging motif. The p-hydroxo-bridged analog, MnH'Mn"(p-OH),(p-RCO,), , has a similar structure with a somewhat shorter 3.4-A Mn-Mn separation (Fig. 60) (320). Univalent Mnil trimers having only bridging carboxylate units, Mni1(p-RC02)6, exhibit a Mn-Mn bridging motif in which one of the three bridging carboxylate undergoes a carboxylate shift in which one of the carboxylate oxygens bridges two Mn" ions (Fig. 61) (321). Therefore, these core units are more precisely defined as Mn&1(pl,3-RC02)4(pl,l-RCO~)


375

FIG.60. A Mn"iNnli/Mn'ii trimer with a Mn~"Mn'1(p-OH)2(RCOs)4 motif. [Reproduced with permission from (320).Copyright 1994 the American Chemical Society.]

with a characteristic Mn-Mn separation of 3.5-3.6 A. The p-phenolato-bridged trimanganese(I1) complexes exhibit a similar carboxylcore with interate shift, giving the Mn~l(ArO)a(pl,~-RCO,),(pl,,-RCO,), manganese separations of 3.27 A (Fig. 62) (322).

FIG.61. Mn" trimer with a Mni'(RC02)tibridging motif that includes bridging p,,.Icarboxylates and pl,,-carboxylates. [Reproduced with permission from (321). Copyright 1990 Wiley-VCH Verlag.]

376


-N

FIG. 62. Mn~1(ArO)2(~Ll,s-RC02)2(CL1,~-RC02)2 core. [MnP(5-NOzsalim)2(OAc)ll. [Reproduced with permission from (322).Copyright 1995 the American Chemical Society.]

D. TETRANUCLEAR As with binuclear manganese complexes, several recurring tetramanganese core structures have been synthesized with different supporting ligand sets (44, 323, 324). The simplest tetranuclear manganese cluster is the cubane core, Mn404, which has tetrahedral symmetry in the absence of distortion. A generalized scheme of a cubane core is shown in Scheme 15. This core is arranged in such a way that two 0x0 bridges exist between any two manganese ions so that all of the manganese ions are equivalent. A number of mixed-valent distorted cubane core structures are known in which the manganese distances are not all equivalent (325,326).This core was proposed in the past as a model for the manganese cofactor in photosynthetic oxygen evolution, so it has been synthesized with a number of supporting ligand architectures. One of the 0x0 groups in the cubane core can be replaced with a carboxylate donor to form the asymmetric Mn403(PhCOJ complex, in which the four manganese are no longer equivalent and Mn-Mn distances of 2.8 and 3.3 A are observed. This complex is unique among tetramanganese cores in exhibiting pl,land p1,3 bridging by a single carboxylate group. An 0x0 group may also be replaced by a p3-chloride ligand to yield the distorted cubane, MII,(O)~C~ (Fig. 63) (327,328).In this mixed-valent Mnil'MnTV complex the three Mn11'Mnw02 faces have Mn-Mn separations of 2.8 A, as expected for bis-p-oxo-bridged Mn"'MnW complexes. However, the MniI' (O)(Cl) faces have an average Mn-Mn distance of 3.3 A. Alkoxidecontaining cubane analogs (329),Mn,(OR),, have also been prepared and shown to be topologically similar to the conventional oxide-containing cores with somewhat longer Mn separations.

377


Mn40nX, Core Structures

0-

MnM ,/n0 -

0-Mn-

0 Adamantane

0

'Fused-Open' Cubane

Ih

0

Mn-

1

\O+Mn 0- -Mn

\I \I Mn-

0

A Selection of Cubane Core Structural Types

'Butterfly" Type Cores

SCHEME 15

Addition of two oxide donors yields the adamantane complexes, MQO,, which also have 0x0 bridges separating all of the manganese (Scheme 15) (330). The difference between the cubane and adamantane core is the presence of two oxygen atoms, leading to the proposition that an adamantane-to-cubane conversion in the OEC may lead to oxygen evolution (31, 199, 200). Another Mn406cluster tetranuclear cluster is the "fused open cubane," in which a Mn406 cluster is formed that is the equivalent of two face-sharing Mn304 cubanes, with one manganese site vacant (329) (Scheme 15). Loss of two oxide bridges from a cubane core yields the Mn402core complexes, commonly called butterfly clusters, which have an MnzOzcore as their foundation and two p3-0x0 groups connecting to the two pendant Mn ions lying above the MnzOzplane (Scheme 15) (331, 332). A centrosymmetric Mn402core has also been observed as well. In these complexes, two p3-0x0groups from MnzOzbridge to two additional manga-

378


FIG.63. A manganese cubane cluster with a corner oxygen atom replaced by a chloride. [Reproduced with permission from (327). Copyright 1993 the American Chemical Society.]

nese ions, but they are now disposed on opposite faces of the MnzOz plane. There are a number of tetramanganese clusters that cannot be considered as merely distorted members of the basic classes, but that may also be relevant in biological manganese chemistry. For example, a highly symmetric tetramanganese complex is formed with the ligand 2-OHpicpn (333).This complex, an efficient catalyst for the catalase-like disproportionation of hydrogen peroxide, is an Mn4L4structure in which each pair of manganese ions is bridged by a single alkoxide bridge covalently attached to the picpn unit (Fig. 64). Each manganese is separated from its neighbor by 3.7 A. In this complex all of the Mn-Mn linkages are identical, so it is not precisely a dimerof-dimers model. However, the complex M r ~ ~ O ~ ( t p h pcontains n ) ~ + two MnzOzcores cofacially oriented and linked by two alkoxide bridges covalently bound to the tphpn ligand (334)(Fig. 64). In this case, two different Mn-Mn linkages support the cluster, and this is accurately considered as a dimer-of-dimers structure. A variation on this theme is the dimer-of-dimers complex [Mn,O,( tmdp)]%+, which also has two MnzOzdimers, but in this case linked by methylene spacers of the tmdp ligand. This complex has a rather short 2.6-A Mn-Mn distance across the dioxo core, but a long 5.9-A Mn-Mn distance across the methylene spacer connecting the two dimers (335)(Fig. 65).


379

FIG.64. Two tetramers. Left, is the [Mn,0,(tphpn)214atetramer. Right, [Mni1(2-OH picpn)J tphpn = N,N ,N',N,-tetrakis~pyridylmethyl~-1,3-diaminopropan-2-01. 20Hpicpn is the Schiff-base condensation product of pyridine-2-carboxaldehydewith 1,3-diaminopropan-2-01, [Reproduced with permission from (334)and (333),respectively. Copyright 1991 and 1996 the American Chemical Society, respectively.]

A number of chainlike tetramanganese complexes may also be relevant to biological manganese chemistry. The Mn,O,(bpy)i+ complex (336) has the same core stoichiometry as the adamantane complexes (Fig. 66). However, this complex contains four Mn" ions in a linear configuration in which each pair of neighboring manganese ions is bridged by two 0x0 bridges with a Mn-Mn distance of 2.7 A. Longer Mn-Mn distances of 4.9 and 6.4 A are observed between the 1,3 and 1,4 manganese ions. A topologically similar complex, Mn4(2-OHPh PhenI6, was prepared having six phenoxide instead of oxide bridges (337).The Mn-l-Mn-2 distance here is 3.4 A, characteristic of dimanganese diphenoxide complexes, in which the Ar dihedral angle with the Mn202plane is nearly 90". Mn902(tphpn)z(Hz0)2(CF~SO~~~+ has a Mn,(0)2(RO)2 core with two manganese linked to a Mnz02 core through alkoxo bridges (Fig. 67) (338). IV. Physical Properties

Hydrated manganese in acidic aqueous solution exists in two oxidation states, Mn" and Mn"], with a potential of + 1.51 volts separating them (339). Therefore, Mn"' in solution is a strong oxidant. This

380


FIG.65. A dimer of dimers. Two ( p - 0 2 - ) ,bridged Mn"'MnN dimers are bridged to one another via the ligand. Water molecules are also bound to the manganese of this complex. [Reproduced with permission from (335).Copyright 1994 the Chemical Society of Japan.]

arises from the strong driving force to form the stable half-filled d5 subshell of Mn" from the d4 Mn"' ion. The MnWstate, which is a d3 ion, exists as the dioxide MnOz or as oligomeric forms thereof. Permanganate ion, having a do Mnwlcenter, is diamagnetic. Mononuclear metal-ligand complexes with the Mn"-MnV oxidation states are all known provided suitable chelating ligands are provided, and polynuclear complexes with Mn1I-MnWare known as well. Consequently, to understand the spectroscopy of polynuclear complexes, it is necessary to begin by examination of the electronic properties of


381

FIG.66. The first coordination sphere of the complex Mn106(bpy)Q+ showing the core structure. Matching subscripts on the nitrogens designate that they originated from the same bipy molecule. [Reproduced with permission from (336). Copyright 1994 the American Chemical Society.]

manganese ions in these oxidation states. Mn" in an octahedral ligand field has a ground state that has spin S = 5/2 but is an orbital singlet. Therefore, no spin-orbit coupling arises from the ground state of Mn", and S is a good quantum number for describing

FIG.67. The linear chain structure with the tphpn ligand. The two inner manganese are involved in MnzOzcore and are then bridged to the outer two manganese ions by ligand alkoxides. [Reproduced with permission from (338).Copyright 1990 the American Chemical Society.]

382


the spin levels of this ion. The same is true for the d3 MnIVion, which has an orbital singlet 4A2ground state in an octahedral field. However, the ground state of the octahedral d4 Mn"' ion is the orbital doublet 5E, which gives rise to substantial spin-orbit coupling, L S. Therefore, S is not a good quantum number for describing Mn"' electronic properties, and the value L + S is used. The MnVcomplexes thus far isolated are square pyramidal and diamagnetic. This suggests a very strong axial splitting in the t2gorbital set leading to a singlet 'A ground state in these complexes.

A. ELECTRONIC SPECTROSCOPY Because the most intense bands in the visible spectroscopy of manganese compounds are charge-transfer bands involving the bound ligands (i.e., metal-to-ligand or ligand-to-metal charge transfer), the visible spectra of polynuclear complexes tend to be very similar to those of mononuclear complexes with the same ligands. Consequently, little information on the structure of polynuclear complexes is contained in their electronic spectra. Exceptions to this rule are certain localized mixed-valence complexes that exhibit intervalence charge transfer bands (ITS)formally involving electron transfer between nonequivalent metal sites in the complex. Because this can only occur in polynuclear complexes that are valence localized in the ground state, valence-delocalized complexes cannot exhibit IT bands. An example of an IT transition at 1170 nm ( E = 270 M-'.cm-l) is found in the [Mn11Mn111salamp21complex, whose structure is shown in Fig. 68,

FIG.68. Core structure of [Mn"Mn"'(~almp)~]., a diagram of the ligand and the binding motif for one ligand to the manganese are shown above the core structure. [Reproduced with permission from (308).Copyright 1991 the American Chemical Society.]


383

along with a representation of the ligand (308).The appearance of IT bands is rationalized based on Marcus theory. Electron transfer between two coupled metal centers, MAand MB, is rapid compared to molecular vibrations, resulting in a low-lying excited state in which an electron formerly residing in an orbital of predominantly MAcharacter has been promoted to an orbital of predominantly MB character. This excited state may relax via a nonradiative mechanism wherein the ligand environment adjusts along a vibrational normal mode to return to the ground state configuration. Therefore, the energy and lifetime of IT excited states can be strongly influenced by the vibrational structure of the polynuclear complex. UV-vis spectroscopy proved particularly enlightening with respect to the catalase system (23, 30, 132). The spectra of model complexes bridged by carboxylate and 0x0 bridges were quite similar to those observed for the catalase. This led researchers to the proposal that such a motif might also be present, a hypothesis that has been borne out by the crystal structure of the catalase (125). B. MAGNETISM Temperature-dependent magnetic susceptibility is often used in the study of polynuclear metal complexes to determine the magnitude of spin exchange energetics between two metal centers in the same molecule (340, 341). This magnitude is called J and is a measure of the energy difference between successively higher molecular spin states when a molecule is placed in a magnetic field. The temperature dependence of the molar magnetic susceptibility x,,, arises from two sources. The first is zero-field splitting, which is measured by D and is observed in mononuclear complexes as well as polynuclear complexes. The primary factor contributing to D in manganese complexes is coupling of the orbital angular momentum and spin angular momentum vectors, and is defined by L . S. Octahedral Mn" has a t&eE configuration with a 6A1ground state, and MnN has a t!,e: configuration with a 4Azground state. Because both of these ground states are orbitally singlet and have no net orbital angular momentum, D will typically be on the order of 0.10 cm-' or less in complexes of these ions (339).Therefore, in polynuclear complexes of these ions, S is a good quantum number and the magnetic exchange interaction between the ions can be described by using a spin-only Hamiltonian. However, Mn"' has a t!,e; configuration with a 5E ground state, which is an orbital doublet that will have a substantial contribution to the overall magnetic moment arising from the net orbital angular mo-

384


mentum. As a result, Mn"' systems may have D on the order of a few cm-'. For such a system, S is not a good quantum number to describe the energy of the state, and a spin-only description of the magnetic exchange may not apply. The low-temperature susceptibility of Mn"' complexes are often dominated by the thermal depopulation of the higher spin states giving a diamagnetic ground state at low temperature. The second source for the temperature dependence of xm is spinspin (S1* S z )coupling or spin exchange, which is unique to polynuclear complexes. S1.Sz coupling arises when the spins on two metal centers, MI and M2, are correlated. The two spins may be coupled antiferromagnetically so that the ground spin state S for the system is S = S , - Sz. On the other hand, the spins may be coupled ferromagnetically to give a ground spin state S = Sl + S z . We will discuss S1* S z coupling in the context of binuclear systems with the understanding that the general principles applied in the binuclear case also apply to higher polynuclear complexes. Characterized MnH systems are typically weakly coupled, with 5 values on the order of 10 cm-' or less (30).The orbitally single 'Al ion is therefore a good example of a system whose temperature-dependent magnetic susceptibility is well treated by the spin-only formalism due to the lack of L .S coupling from the Mn" ground state. The spin-only Hamiltonian is given by H = SIJSzwhere 5 is the spincoupling constant with units of energy and Sl and Sz are the spin operators. The eigenvalues E for the system HW = EW are given by E = J [ S ( S + 1) - Sl(Sl+ 1) - Sz(S2+ l)]and the spin energy levels are given by 0, 2J,6J, 125,205, and 305. The temperature-dependent magnetic susceptibility is then a result of the depopulation of higher spin states as the temperature is decreased, according to the Boltzmann distribution of states, i.e., P, = A,exp(E,/kT)LU,exp(E,/kT), where En and A, are the energy and degeneracy of state n and P, is its relative population. For an antiferromagnetically coupled system, the ground state is diamagnetic and the magnetic moment should approach zero as the temperature is lowered. Ferromagnetic coupling gives rise to a paramagnetic S = 5 ground state so that the low-temperature magnetic moment will approach 11 BM per binuclear complex. Binuclear Mn'" complexes may also be treated by a spin-only Hamiltonian with eigenvalues E = 0, 25, 6J, and 125. The ground state is also a spin singlet in this case, but the high-temperature moment should approach 5.4 BMhinuclear complex due to equivalent population of all of the spin states.


385

The Mn"' ion has an 5E ground state that is orbitally nondegenerate. Even so, the spin-only contribution to the temperature-dependent magnetic susceptibility in strongly coupled Mn"MnT", Mn"' 2 , and Mn"'MnIV complexes may be treated in the same way as for the Mnil case discussed earlier. For cases in which J 4 D, D can be considered as a perturbation on the spin-only energy levels and the spin-only formalism is usually adequate to explain the magnetic data and to calculate J . For systems with extremely weak coupling (i.e., J < D,J may be considered as a perturbation on the D-split levels, and the magnetism will be dominated by D. However, when J and D effects are of the same order of magnitude, treatment of the temperaturedependent susceptibility is complicated by the fact that neither S nor L is a good quantum number for describing the spin exchange. A full treatment of magnetic exchange involving both spin and orbital contributions is quite involved and beyond the scope of this discussion (340, 341). Great care must be exercised in the treatment of data on weakly coupled systems containing Mn"' in order to accurately assign which effect is responsible for the observed temperature-dependent magnetic data. In many cases, one cannot uniquely fit the magnetic data to a single set of J , D, and g values.

C. EPR SPECTROSCOPY Electron paramagnetic resonance spectroscopy is one of the primary tools in studying the electronic structure of polynuclear complexes (341). Whereas magnetic susceptibility studies are capable of detecting electronic interactions as small as a wavenumber (discussed earlier), the EPR spectrum of a polynuclear complex may be sensitive to intramolecular exchange couplings as small as 0.001 cm-l even at room temperature. Additionally, the 55Mnnucleus has a nuclear spin ( I ) of 5/2. For a mononuclear system, six hypefine lines are predicted. However, for a multinuclear system containing n Mn ions, as many as 6" lines may be observed. This gives rise to a rich EPR spectroscopy among polynuclear manganese complexes, which has been used extensively in the study of biological manganese systems and model complexes. Mononuclear Mn" and Mn" complexes exhibit characteristic EPR spectra that are, for the most part, insensitive to the ligand environment around the metal. However, the EPR spectra of polynuclear complexes are strongly dependent on the metal ligation environment, the nature of bridging ligands, and the degree to which the metal

386


ions are coupled. We will examine some of the relevant binuclear and polynuclear cases in more detail.

1. Binuclear Complexes Mononuclear Mn" complexes almost always exhibit a single derivative signal with a crossover at g = 2 and often having a well-defined six-line Mn hyperfine splitting superimposed on it, as expected from an S = 5/2 spin system with only slight spin-orbit coupling. However, two antiferromagnetically coupled Mn" ions coupled antiferromagnetically would be expected to be EPR silent at low temperature owing to the ground-state spin singlet. Binuclear MnP complexes often do exhibit an EPR spectrum due to thermal population of the S = 1 through S = 5 spin states. Although these are non-Kramer's systems with integer spin, which often do not show X-band EPR spectra in mononuclear complexes due to zero-field effects, such effects are minimal in Mn" due to the 6A1ground state, and there are a number of examples of EPR spectra of Mn" dimers (279, 438, 439, 474, 476). Binuclear mixed-valent Mn'lMnlll complexes nearly always exhibit an EPR signal because the ground state and all thermal excited spin states are Kramer's spin systems. The form of the spectrum depends strongly on the magnitude of J and on the zero-field splitting imparted by the Mn"' ion. Strongly coupled systems typically exhibit a signal at g = 2 with a multiline hyperfine splitting from two Mn ions; more weakly coupled systems show signals withg > 2 due to transitions involving zero-field split levels (305, 475). Unlike Mnil dimeric systems, MnH' complexes almost never show EPR spectra in the X-band region due to zero-field splitting that puts allowed transitions out of range of X-band energy (9.5 GHz). However, strongly-coupled ( J = 100 cm-' ) mixed-valence Mn"'Mn'' systems show very characteristic EPR spectra at g = 2, consisting of between 12 and 19 well-resolved Mn hyperfine lines, consistent with an S = 1/2 ground state. A typical such spectrum, for M"'MnIV (p-O),L,, is shown in Fig. 69. Although 36 lines are theoretically possible for two coupled manganese ions, between 12 and 19 lines are usually observed in practice due to overlap of some of the hyperfine lines. The g = 2 signal is the only one typically observed for strongly coupled (~-0 complexes )~ (140), but Mn"'Mn" systems such as the Mn1I1MnW ( J = 10 cm-') may also exmore weakly coupled Mn"'MnTVsystems a lower-field signal at higher temperatures that is associated hibit with thermal population of a zero-field split S = 312 excited state (202), a feature also often observed in the EPR spectra of mononuclear MnIVcomplexes, which are also S = 3/2 ions (140).


11 2200

I

I

2600

I

I

3000

I

I

3400

I

I

3800

I

387

I

4200

G(H4 FIG. 69. A Mn"'Mn'" exchange coupled EPR spectrum, from [Mn"'Mn"(bpza)p (p-O)2]3'.bpza = 2-(2'-pyridyl)benzimidazole.LReproduced with permission from (465). Copyright 1994 Elsevier Science.]

Strongly coupled binuclear Mn;" complexes have never been shown to exhibit X-band EPR spectra due to the low-energy singlet ground state. Because stabilization of the high-valent Mn'" requires very strong oxyanionic ligands, nearly all such MnJ" complexes have at least one bridging 0x0 or hydroxo ligand, and they may contain additional bridges such as carboxylate ions, all of which contribute to a generally strong coupling in such systems. Consequently, there are few examples of weakly coupled MnF systems to make generalizations about the presence or lack of EPR spectroscopy in these systems. However, we might expect that because ground-state spin-orbit contributions to zero-field splitting are absent in MnN ions, higher spin triplet and quintet states might be EPR active here just as they sometimes are in the MnH complexes. A recent example of an EPR signal arising from a weakly coupled ( J = -2.3 cm-') Mnk" dimer was observed for the complex [Mn'"(di-t-butylsalpn)lzDCBI (Fig. 70) (312). 2. Polynuclear Complexes Several factors unique to polynuclear systems make the interpretation of their spectra more complicated than for the binuclear cases. One such factor is that a given manganese ion may couple to two or more other ions with different exchange magnitudes, J , and J z . The interpretation of such spectra is often simplified by assuming that Jz

388


A 5.5K B 50K

I

0

I

I

I

I

I

I

1000 2000 3000 4000 5000 6000 7C Field (gauss)

FIG.70. X-band EPR spectra of the weakly coupled [Mn1"(3,5-di-t-busa1pn)l2DCBI complex. [Reproduced with permission from (312).Copyright 1997 the American Chemical Society.]

is much smaller than J 1 and can either be neglected or treated as a perturbation on the spin splitting by J,. On the other hand, J , may be exactly equal to Jz (in a highly symmetric trinuclear species for example), thus reducing the spin exchange problem to a simple manifold of states treatment as for binuclear species. However, when J1is of the same order of magnitude as J2,then the spin splitting is complex and cannot be treated by a simple manifold of states. A second factor complicating the EPR spectroscopy of polynuclear species is the fact that the possible number of Mn hyperfine splittings increases as 6" where n is the number of manganese ions contributing to the hyperfine interaction. Furthermore, the hyperfine splitting constant A decreases as lln. Therefore, EPR spectra of polynuclear complexes may exhibit broadened signals due t o a large number of unresolved hyperfine splittings, which may not be distinguishable from broadening from other sources such as slow tumbling or anisotropy. 3. EPR of Biological Manganese Complexes

EPR is one of the most useful techniques brought to bear in the study of polynuclear manganese centers in biology. For example, it was the hyperfine splitting in the CW EPR spectrum of spinach thylakoid membranes that first showed conclusively that the oxygen evolving complex in PS I1 consisted of a multinuclear cluster of coupled manganese ions (160, 161). The EPR from the S, oxidation state of active PS I1 centers has also been the primary signal for tracking the redox cycling of the OEC during oxygen evolution by PS I1 (33, 42, 46). There have also been numerous attempts to establish the nuclear


389

and oxidation levels of the manganese cluster. Brudvig, for example, used variable-temperature EPR data to support a tetranuclear cluster (342, 343). Several other groups have variously assigned these EPR spectra to two dimers (3441, dimers of dimers, and tetramers (345). More recently, Dismukes has argued that the Sz spectra result from a tetranuclear cluster composed of 3 Mn"' and 1 MnWions (346).To obtain a good fit of the data, the Mn"' ions are predicted to be five coordinate with an inverted order for the &orbital energies. The Sz signal has also yielded information on the ligand environment (187, 347) through 15N labeling and observation of the 15N hyperfine splitting with ENDOR spectroscopy, and on the binding of substrate and inhibitors (348) to the OEC and model complexes (203, 349). Very recently, two new EPR signals have been observed. The Sosignal was shown to be multiline in the presence of methanol (170-172). Using parallel-mode EPR spectroscopy, Britt has shown a beautiful multiline signal for the S, oxidation level (174). Interpretations of these new signals hold great promise for understanding this enigmatic cluster's structure. Enzymes containing binuclear Mn sites are especially amenable to structural elucidation using EPR spectroscopy. The EPR spectra of various oxidation levels in the binuclear manganese catalase were used to assign the low-valent active form as a weakly coupled MnL) center and the "superoxidized inactive form as a strongly coupled MnlIIMnWcenters (7, 23, 136, 138-140, 350, 351). These assignments were based in part on a comparison with the EPR spectra of crystallographically characterized model compounds. The arginase active site was shown to be a binuclear Mn" center by the shape of the spectra and the number and spacing of Mn hyperfine couplings (7). The structures proposed for both the manganese-containing catalase and arginase from these studies were essentially confirmed by subsequent X-ray crystal structure analysis (8). D. X-RAYA~SORPTION SPECTROSCOPY Although it is an unconventional spectroscopic technique, X-ray absorption spectroscopy (XAS) has been the primary tool in the elucidation of the structural features of biological multinuclear Mn centers for which high-resolution crystallographic data are not yet available (23, 24, 134). This is typically done by comparing features observed in the XAS of the biological complex with similar features found in the spectra of crystallographically characterized Mn model complexes. We defer an extended discussion of the theory and principles of XAS

390


to several reviews on the subject of XAS in biological and model manganese complexes (24, 134). However, a few basics will suffice to understand the information that is contained in XAS of polynuclear complexes. Unlike UV and visible electronic spectroscopy, which involve promotion of outer-shell valence electrons, X-ray absorption involves the promotion of core electrons to higher energy using X-ray frequencies (24,134).As such, X-ray absorption is a high-energy technique. There are really two techniques that come under the heading of X-ray absorption. X-ray Absorption Near Edge Structure, XANES, involves promotion of a 1s electron to the 3d level in the case of manganese. (This is for the K shell; L edges with core electrons from the n = 2 shell are now being examined for models and enzyme.) This gives rise to an absorption “edge” whose energy is very characteristic of the oxidation state of the manganese. As a general rule, manganese in higher oxidation states exhibits absorption edges at higher energies, as might be expected from the tighter binding of core electrons in higher oxidation states. In fact, the edge energy seems to correlate best to the average number and bond lengths of ligands bound to the metal. In polynuclear complexes, shifts in the edge energy are correlated with changes in the average oxidation state of the manganese ions. This complicates the use of XANES in higher nuclearity complexes because one-electron oxidation results in a smaller change in the average oxidation state as complex nuclearity increases. Even so, this technique has been used extensively in tracking the oxidation states of the manganese cluster in PS I1 upon S-state advancements. The progressive edge shift upon increasing oxidation state for polynuclear manganese complexes is nicely illustrated by the XANES spectra of the isostructural series Mn2(2-OHsalpn)i(n = -2 to +1)in four successive oxidation states (Fig. 71) (467). As the X-ray energy is increased beyond that required for promotion of the core electron, ejection of core electrons into the continuum occurs. This ejected electron propagates from the Mn center until it encounters another atom from which it can be back-scattered. The interference of back-scattered waves with propagating waves leads to an interference pattern that is manifested as an oscillation in the X-ray absorption pattern. Fourier transformation of this oscillating spectrum from the frequency domain to the distance domain gives a new spectrum whose abscissa contains information on the distance between the target atom (i.e., the Mn center) and the back-scattering atoms. This second technique is called Extended X-ray Absorption Fine Structure, E M S , and has been the only spectroscopic tech-


6531

6538

6545

6552

391

6559

Energy (ev)

FIG.71. XANES spectra of the family of complexes IMn(20Hsalpn)l" (n = -2, - 1, 0, +l), and the oxidation states on the manganese go from Mn"Mn" to Mnl'lMnl". Noticeable changes occur in the edge energy with each increase in the oxidation state of the manganese. [Reproduced with permission of the author, from (4671.1

nique to yield direct direct structural information about the manganese cluster in PS 11. The XANES of the Mn catalase provided the first definitive proof that this enzyme cycles between the Mnil and MnkI' oxidation levels (137, 352) The extent of catalase activity correlated with the proporenzyme; however, samples with Mn*" quantitation of Mnf or tively showed reduced activity. The EXAFS of the Mn catalases have been less informative because of the Mn-Mn separations in the reduced, active enzymes (135).Nevertheless, EXAF'S of the "superoxidized" enzyme demonstrated that the MnlIIMnl" enzyme has a Mn-Mn separation of ~ 2 . 7 A, which is consistent with a di-p,-oxo core (135).Subsequent spectroscopic analysis confirmed that a "diamond core" with a bridging syn,syn acetate formed the enzyme active site ( 9 ) . While interpretation of the XAS for the Mn catalase has been relatively straightforward, the application of XANES and EXAFS to the OEC has led to several significant disagreements among workers in this field (24,28,33,46, 48, 134, 154). Klein and Sauer first collected XAS data on the OEC and provided the first detailed structural and oxidation-state information about the Mn cluster (353, 354). The XANES indicated that the S , and S , enzyme forms contained significant amounts of Mn"' and Mn'". At the same time, both S states could be examined by EXAFS and exhibited short Mn-Mn vectors of 2.7 A (154-159), which is consistent with a high-valent di-pa-oxocore, for example the structure in Figs. 49 and 50. Because the S , state contains a Kramer's spin ladder (i.e., nonintegral spin system) as shown by EPR spectroscopy, the S , state was assigned as MniI'MnY and the

392


S2 state was assigned as Mn"'Mn?. All of the groups using XAS have agreed upon these oxidation levels and general structural details (24, 33,154, 156, 158, 159). However, there are members of the EPR community that suggest that the oxidation levels should be adjusted downward by two oxidation states (346).The justification for this is that low-coordination-number Mn"' ions may have aberrantly high edge energies. If this is true, fits of the EPR spectra would be consistent with XAS data. However, investigations of many model compounds have not supported the idea that XANES edges are this sensitive to coordination number changes (24,33,134). One area of significant controversy is whether the Mn center is oxidized upon each S-state transition (24, 33) Klein and Sauer argued that upon forming S3,the edge energy did not increase sufficiently to support manganese oxidation (355).However, the &-state EPR spectrum disappears upon additional oxidation to S3,which indicates that the oxidizing equivalent must be in close proximity to the manganese cluster (46).These observations have led some investigators to suggest that the substrate (water) is partially oxidized or that a protein residue near the active center is oxidized. Significant excitement was generated when Boussac and Rutherford formed a new EPR signal in EGTA (Ca-depleted) samples (this signal was later shown to occur with acetate addition) (176, 356). The broad radical signal was assigned to histidine radical. However, Britt has shown recently that this signal originates from tyrosine Y;, which is the intermediate oxidized charge carrier of the enzyme (177, 178) Subsequent investigation has placed this tyrosine at

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