Organometallic Chemistry, Volume 32

Contents Chapter 1

Theoretical Organometallic Chemistry By A.J. Bridgeman 1 2 3

Introduction s- and p-Block Metals 2.1 Structural and Spectroscopic Studies 2.2 Mechanistic Studies d- and f-Block Metals 3.1 Structural and Spectroscopic Studies 3.2 Mechanistic Studies References

Chapter 2

Groups 1 and 11: The Alkali and Coinage Metals By S.R. Boss and A.E.H. Wheatley 1

2

Alkali Metals 1.1 Introduction 1.2 Alkyl Derivatives 1.3 Alkenyl, Allyl, Vinyl, Alkynyl and Related Derivatives 1.4 Aryl Derivatives 1.5 Cyclopentadienyl and Related Derivatives Copper, Silver and Gold 2.1 Introduction 2.2 Copper Compounds 2.3 Silver Compounds 2.4 Gold Compounds References

Chapter 3

Group 2 (Be–Ba) and Group 12 (Zn–Hg) By Christopher M. Pask and Dominic S. Wright 1 2 3

Scope and Organisation of the Review Group 2 Group 12 References

Chapter 4

Scandium, Yttrium and the Lanthanides By John G. Brennan and Andrea Sella 1 2

Introduction New Compounds – Structure and Reactivity

2.1

3 4 5

6

Cyclopentadienyl (Cp) and Lightly Substituted Cp Compounds 2.2 Cp* and Related Per-substituted Ancillaries 2.3 Functionalized Cp Ligands 2.4 Linked and Ansa-Cp Ligands 2.5 Indenyl and Fluorenyl Ancillaries 2.6 Hetrocyclic Ancillaries 2.7 Amido and other Nitrogen-based Supports 2.8 Carborane and other Boron-containing Ligands 2.9 Aluminates 2.10 Other Ancillaries Theoretical Studies Gas Phase Chemistry Polymerization Catalysis 5.1 Olefin Polymerization 5.2 Butadiene 5.3 Acrylates 5.4 Polar Monomers Applications in Organic Synthesis References

Chapter 5

Carboranes, Including Their Metal Complexes By Paul A. Jelliss 1 2 3

Introduction Theoretical and Computational Studies Carboranes 3.1 {CB8} and {CB9} 3.2 {CB10} 3.3 {CB11} 3.4 {C2B3} 3.5 {C2B7} 3.6 {C2B8} 3.7 {C2B9} 3.8 {C2B10} 3.9 {C3B3} and {C4B2} 4 Metallacarbaboranes 4.1 {MCB10} 4.2 {exo-MCB11} 4.3 {MC2B4} 4.4 {MC2B8} 4.5 {MC2B9} 4.6 {MC2B10} 4.7 {exo-MC2B10} 5 Biological Carborane Chemistry and BNCT 6 Crystal Engineered Supramolecular and Polymeric (Metalla)carborane Materials References

Chapter 6

Group III – B, Al, Ga, In and Tl By Simon Aldridge 1 2

General Boron 2.1 B(C6F5)3 and Related Boranes 2.2 Borate Anions 2.3 Boron Hydrides 2.4 Boratabenzenes and Related Systems 2.5 Boron-containing Materials 2.6 Boron-based Sensors 2.7 Boron-based Ligand Systems 2.8 Boronic Acids and Suzuki Type Coupling Reactions 2.9 Diboron(4) Reagents 2.10 Borane Functionalized Cyclopentadienyl Ligands 2.11 Miscellaneous 3 Aluminium 3.1 Sub-valent Aluminium and Aluminium Clusters 3.2 Aluminoxanes, MAO Models and Aluminium in Olefin Polymerization 3.3 Aluminium Derivatives Containing Bonds to Group 15 Elements 3.4 Aluminium Derivatives Containing Bonds to Group 16 Elements 3.5 Aluminium Organometallics in Organic Synthesis 3.6 Miscellaneous Examples 4 Gallium 4.1 Sub-valent Gallium Species and Clusters 4.2 Complexes Containing Subvalent Gallium Species as Ligands 4.3 Gallium Derivatives Containing Bonds to Group 15 Elements 4.4 Gallium Derivatives Containing Bonds to Group 16 Elements 4.5 Gallium Hydrides 4.6 Miscellaneous 5 Indium 5.1 Subvalent Indium and Indium Clusters 5.2 Indium Derivatives Containing Bonds to Groups 15 or 16 5.3 Indium Organometallics in Organic Synthesis 5.4 Miscellaneous Examples 6 Thallium References Chapter 7

Group 14: Silicon, Germanium, Tin and Lead By Richard A. Layfield

1 Overview 2 Silylenes 3 Germylenes, Stannylenes and Plumbylenes 4 Multiple Bonds to Main Group Elements 5 Homoaromatics and Related Copmpounds 6 π-Bonded Compounds 7 Transition and Main Group Metal Complexes References Chapter 8

Group 15: Phosphorus, Arsenic, Antimony and Bismuth By Matthew D. Francis 1 2 3 4

Phosphorus Arsenic Antimony Bismuth References

Chapter 9

Organo-Transition Metal Cluster Complexes By Mark G. Humphrey and Marie P. Cifuentes 1 2 3 4 5 6 7 8 9

10

11

12

Introduction General Reviews Spectroscopic Studies Theory Structural Studies Large Clusters 6.1 Homonuclear High-nuclearity Clusters 6.2 Heteronuclear High-nuclearity Clusters Group 6 Group 7 8.1 Rhenium Group 8 9.1 Iron 9.2 Ruthenium 9.3 Osmium 9.4 Mixed-metal Clusters Containing Only Group 8 Metals Group 9 10.1 Cobalt 10.2 Rhodium 10.3 Iridium 10.4 Group 9 Mixed Metal Clusters 10.5 Group 9 Clusters as Catalysts Group 10 11.1 Nickel 11.2 Palladium 11.3 Platinum Group 11

12.1 Copper 12.2 Silver 12.3 Gold 13 Group 12 13.1 Mercury 14 Mixed Metal Clusters 14.1 Group 5 14.2 Group 6 14.3 Group 7 14.4 Group 8 14.5 Group 9 14.6 Group 10 14.7 Group 11 14.8 Clusters Containing Three Different Metals References Chapter 10

Complexes Containing Metal–Carbon σ-Bonds of the Groups Iron, Cobalt and Nickel, Including Carbenes and Carbynes By Philip J. King 1 2 3

Introduction Reviews and Articles of General Interest Metal–Carbon σ-Bonds Involving Group 8, 9 and 10 Metals 3.1 The Iron Triad 3.2 The Cobalt Triad 3.3 The Nickel Triad 4 Carbene and Carbyne Complexes of Groups 8, 9 and 10 References Chapter 11

Reactions of Transition Metal π-Complexes of Alkenes, Alkynes and Dienes By Gregory A. Solan 1 2 3 4 5

6

Introduction Reviews Complexes Containing π-Coordinated Alkenes 3.1 Well-defined π-Coordinated Alkenes 3.2 π-Coordinated Alkenes Generated In-situ Complexes Containing π-Coordinated Alkynes 4.2 π-Coordinated Alkynes Generated In-situ Complexes Containing π-Coordinated Dienes (Chain and Cyclic) 5.1 Well-defined π-Coordinated Conjugated Dienes 5.2 Well-defined π-Coordinated Non-conjugated Dienes 5.3 π-Coordinated Dienes Generated In-situ Bimetallic Complexes Containing π-Coordinated Alkenes, Alkynes or Dienes 6.1 Homobimetallic 6.2 Heterobimetallic

Chapter 12

References Transition Metal Complexes of Cyclopentadienyl Ligands By Ian R. Butler 1 2 3 4 5 6 7

General Introduction Main Group, Lanthanides and Actinides Titanium, Zirconium and Hafnium Vanadium, Niobium and Tantalum Chromium, Molybdenum and Tungsten Manganese, Rhenium and Technetium Iron, Ruthenium and Osmium 7.1 General Ferrocene and Synthetic Chemistry 7.2 Electrochemistry 7.3 Polymer Chemistry 7.4 Schiff Bases Ligand/Complexes 7.5 NMR 7.6 Materials Chemistry 7.7 Liquid Crystals 7.8 Complexes/Coordination 7.9 Carbon-based Structures 7.10 Dppf as a Ligand 7.11 Theoretical 7.12 Miscellaneous Ferrocene Chemistry 8 Cobalt 9 Nickel References

Chapter 13

Organic Aspects of Organometallic Chemistry By Steven D. R. Christie and Jon J. Shotton 1 Introduction 2 Methods for C–C Bond Formation 3 Methods for C–X Bond Formation 4 Metathesis Reactions 5 Metal Mediated Cyclisation and Cycloisomerisation Reactions 6 Metal Mediated Cycloadditions 7 Pauson–Khand Type Reactions 8 Nicholas Reactions References

Abbreviations

Ac acac acacen Ad AIBN ampy Ar Ar* Ar
f arphos ATP Azb 9-BBN BHT Biim BINAP bipy Bis bma BNCT Bp bpcd bpk Bpz4 But2bpy t-bupy Bz Bzac cbd 1,5,9-cdt chd chpt CIDNP [Co] (Co) cod coe cot CP/MAS Cp CpR

acetate acetylacetonate N,N
-ethylenebis(acetylacetone iminate) adamantyl azoisobutyronitrile 2-amino-6-methylpyridine aryl 2,4,6-tri(tert-butyl)phenyl 3,5-bis(trifluoromethyl)phenyl 1-(diphenylphosphino)-2-(diphenylarsino)ethane adenosine triphosphate azobenzene 9-borabicyclo[3.3.1]nonane 2,6-dibutyl-4-methylphenyl biimidazole 2,2
-bis(diphenylphosphino)-1,1
-binaphthyl 2,2
-bipyridyl bis(trimethylsilyl)methyl 2,3-bis(diphenylphosphino)maleic anhydride boron neutron capture therapy biphenyl 4,5-bis(diphenylphosphino)cyclopent-4-ene-1,3-dione benzophenone ketyl (diphenylketyl) tetra(1-pyrazolyl)borate 4,4
-di-tert-butyl-2,2
-bipyridine tert-butylpyridine benzyl benzoylacetonate cyclobutadiene cyclododeca-1,5,9-triene cyclohexadiene cycloheptatriene chemically induced dynamic nuclear polarisation cobalamin cobaloxime [Co(dmg)2 derivative] cycloocta-1,5-diene cyclooctene cyclooctatriene cross polarisation/magnetic angle spinning 5-cyclopentadienyl 5-alkylcyclopentadienyl xv

xvi Cp* Cp
Cp CV CVD Cy Cyclam Cym Cyttp dab dabco dba dbpe DBU DCA depe depm DFT diars diarsop dien diop DIPAMP diphos dipp dipyam DMAD DMAP dmbpy DME DMF dmg dmgH dmgH2 DMP dmpe dmpm dmpz DMSO dpae dpam dppa dppb dppbz dppe dppf dppm dppp

Abbreviations 5-pentamethylcyclopentadienyl trimethylsilylcyclopentadienyl tetramethylethylcyclopentadienyl cyclic voltammetry(ogram) chemical vapour deposition cyclohexyl 1,4,8,11-tetraazacyclotetradecane p-cymene PhP(CH2CH2CH2PCy2)2 1,4-diazabutadiene 1,4-diazabicyclo[2.2.2]octane dibenzylideneacetone 1,2-bis(dibutylphosphino)ethane 1,8-diazabicyclo[5.4.0]undec-7-ene 9,10-dicyanoanthracene 1,2-bis(diethylphosphino)ethane 1,2-bis(diethylphosphino)methane density functional theory o-phenylenebis(dimethyl)arsine [(2,2-dimethyl-1,3-dioxolan-4,5-diyl)bis(methylene)] bis[diphenylarsine] diethylenetriamine [(2,2-dimethyl-1,3-dioxolan-4,5-diyl)bis(methylene)]bis-1[diphenylphosphine] 1,2-bis(phenyl-o-anisoylphosphino)ethane 1,2-bis(diphenylphosphino)ethane 2,6-diisopropylphenyl di-(2-pyridyl)amine dimethyl acetylenedicarboxylate 2-dimethylaminopyridine dimethylbipyridine 1,2-dimethoxyethane N,N-dimethylformamide dimethylglyoximate monoanion of dimethylglyoxime dimethylglyoxime dimethylpiperazine 1,2-bis(dimethylphosphino)ethane bis(dimethylphosphino)methane 1,3-dimethylpyrazolyl dimethyl sulfoxide 1,2-bis(diphenylarsino)ethane bis(diphenylarsino)methane 1,2-bis(diphenylphosphino)ethyne 1,4-bis(diphenylphosphino)butane 1,2-bis(diphenylphosphino)benzene 1,2-bis(diphenylphosphino)ethane 1,1
-bis(diphenylphosphino)ferrocene bis(diphenylphosphino)methane 1,3-bis(diphenylphosphino)propane

Abbreviations DSD edt EDTA ee EELS EH MO ELF en ES EXAFS F6acac Fc Fe* Fp Fp
FTIR FVP glyme GVB HBpz3 HBpz*3 H4cyclen HEDTA hfa hfacac hfb HMPA HNCC HOMO IGLO im Is* ISEELS KTp LDA LiDBB LMCT LNCC MAO Me2bpy Me6[14]dieneN4 Me6[14]N4 4,7-Me2phen 3,4,7,8-Me4phen Mes Mes* MeTHF mcpba MLCT

xvii diamond—square—diamond ethane-1,2-dithiolate ethylenediaminetetraacetate enantiomeric excess electron energy loss spectroscopy extended Hu¨ckel molecular orbital electron localisation function ethylene-1,2-diamine MS electrospray mass spectrometry extended X-ray absorption fine structure hexafluoroacetylacetonate ferrocenyl Fe(CO)2Cp* Fe(CO)2Cp Fe(CO)25-(C5H4Me) fourier transform infrared flash vacuum pyrolysis ethyleneglycol dimethyl ether generalised valence bond tris(pyrazolyl)borate tris(3,5-dimethylpyrazolyl)borate tetraaza-1,4,7,10-cyclododecane N-hydroxyethylethylenediaminetetraacetate hexafluoroacetone hexafluoroacetylacetonato hexafluorobutyne hexamethyl phosphoric triamide high nuclearity carbonyl cluster highest occupied molecular orbital individual gauge for localised orbitals imidazole 2,4,6-triisopropylphenyl inner shell electron energy loss spectroscopy potassium hydrotris(1-pyrazolyl)borate lithium diisopropylamide lithium di-tert-butylbiphenyl ligand to metal charge transfer low nuclearity carbonyl cluster methyl alumoxane 4,4
-dimethyl-2,2
-bypyridyl 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11diene 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane 4,7-dimethyl-1,10-phenanthroline 3,4,7,8,-tetramethyl-1,10-phenanthroline mesityl 2,4,6-tributylphenyl methyltetrahydrofuran metachloroperbenzoic acid metal—ligand charge transfer

xviii MTO nap nb nbd NBS NCS NCT Neo Np np3 nta OEP OTf OTs Pc PES PMDT pd phen pic Pin Pmedta pp3 [PPN]; py pydz pz R-PROPHOS R,R-SKEWPHOS RDF ROMP sal salen saloph SCF TCNE TCNQ terpy tetraphos TFA tfbb tfacac THF thsa tht TMBD TMEDA tmp TMS tol

Abbreviations methylrhenium trioxide 1-naphthyl norbornene norbornadiene N-bromosuccinimide N-chlorosuccinimide neutron capture theory neopentyl 1-naphthyl N(CH2CH2PPh2)3 nitrilotriacetate octaethylporphyrin trifluoromethanesulfonate (triflate) p-toluenesulfonate (tosylate) phthalocyanin photoelectron spectroscopy pentamethylenediethylenetetramine pentane-2,4-dionate 1,10-phenanthroline pyridine-2-carboxylic acid (;)-pinanyl pentamethyldiethylenetriamine P(CH2CH2PPh2)3 [(Ph3P)2N]; pyridine pyridazine pyrazolyl (R)-(;)-1,2-bis(diphenylphosphino)propane (2R,4R)-bis(diphenylphosphino)pentane radial distribution function ring opening metathesis polymerisation salicylaldehyde N,N
-bis(salicylaldehydo)ethylenediamine N,N-bisalicylidene-o-phenylenediamine self consistent field tetracyanoethylene 7,7,8,8-tetracyanoquinodimethane 2,2
,2-terpyridyl 1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane trifluoroacetic acid tetrafluorobenzobarrelene trifluoroacetylacetonato tetrahydrofuran thiosalicylate (2-thiobenzoate) tetrahydrothiophen NNN
N-tetramethyl-2-butene-1,4-diamine (tmena) tetramethylethylenediamine 2,2,6-6-tetramethylpiperidino tetramethylsilane tolyl

Abbreviations TP TP* TPP Trip Triph triphos TRIR Tsi TTF vi WGSR XPS Xyl

xix hydrotris(1-pyrazolyl)borate hydrotris(2,5-dimethylpyrazolyl)borate meso-tetraphenylporphyrin 2,4,6-triisopropylphenyl 2,4,6-(triphenyl)phenyl 1,1,1-tris(diphenylphosphinomethyl)ethane time resolved infrared (spectroscopy) tris(trimethylsilyl)methyl (Me3Si)3C tetrathiafulvalene vinyl water gas shift reaction X-ray photoelectron spectroscopy xylyl

1 Theoretical Organometallic Chemistry BY A.J. BRIDGEMAN

1

Introduction

This chapter aims to cover theoretical and computational studies on organometallic molecules. Section 1 covers the s and p-block elements and Section 2 covers the d- and f-block metals. Clusters, carbonyls and metal-metal bonded systems containing M-C bonds are included. Extended systems and organic species on metal surfaces are excluded except where calculations have been performed on model complexes designed to mimic solid state and surface chemistry. A wide variety of computational methods is employed in the computational chemistry community. As in previous recent years, density functional theory (DFT) continues to grow in prominence. The vast majority of the work described in this chapter has been performed at the DFT level with the hybrid functional B3LYP being the most popular for studies of organometallic molecules and reactions. ‘Traditional’ ab initio approaches including Hartree-Fock (HF) and post-HF methods (including MP2 and MP4) continue to be used, often for comparison with DFT based methods. Semi-empirical methods now appear to have only limited use except in the molecular mechanics (MM) calculations. A relatively new use of molecular mechanics for large systems is in hybrid calculations where the transition metal and its coordination environment or the reactive centre of a molecule are treated at a higher level leaving the remainder to be treated at the MM level. These QM/MM or ONIOM calculations enable the steric bulk of organometallic molecules to be adequately but computationally efficiently treated and are becoming particularly prevalent in studies of reaction mechanisms and profiles. Only a brief mention of the computational method is given. Standard abbreviations for computational methods are employed throughout. Given the plethora of basis sets available in modern computational chemistry programs and the variety of basis set designations employed by authors in this field, no description of basis sets is given. The reader should consult the original work for further details of the computational method and the basis set.

Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 1

Organometallic Chemistry

2

2

s- and p-Block Metals

2.1 Structural and Spectroscopic Studies. — 2.1.1 Metal Alkyls and Analogues. A B3LYP and AIM study1 of the electron density on the ethyl ligand and related organic groups in alkyllithium complexes reveals charge concentrations in the valence shell at the a and b-atoms suggesting delocalization of the lone pair at the a-C atom or of the Li-C bonding electrons. As a consequence, it is suggested that Li . . . H contacts are due to this delocalization, with Li . . . H agostic interactions playing only a minor role. The equilibrium between Al atoms, trimethylaluminum and an Al-TMA complex is predicted by B3LYP and MP2 calculations2 to lead to a complex which contains an Al-Al bond semibridged by a methyl group. The structure of the GaMe3 dimer, optimized at the MP2 level3, is very similar that found in the tetragonal solid. CIS and TDDFT calculations4 on dimethlystannylene, SnMe2, have been used to assign absorption spectrum of the products of the flash photolysis of SnMe4. The S-bound methyl groups in (Me2MCH2SMe)2 complexes (M : Al, Ga, In) are placed in equatorial positions of the chair-like six-membered ring systems with the conformation with both S-Me groups in axial positions being higher in energy by ca. 25 kJ mol91, according to MP2 and B3YLP calculations5. The HeI photoelectron spectra of trimethylaluminum, triethylaluminum, dimethylaluminum, and diethylaluminum hydrides have been interpreted with the aid of ab initio quantum chemical calculations, including Hartree-Fock/Koopmans, outer valence Green’s function, and equation of motion coupled-cluster ionization energy calculations6. Luminescence from the novel blue phosphorescent Group 15 compounds MR3 (M : P, Sb, Bi; R : p-(N-7-azaindolyl)phenyl) are ligandbased emissions with contributions from the lone-pair electrons of the central atom, a Hartree-Fock study has revealed7. The structures, vibrational frequencies and barriers to internal rotation of the MX3 group in CH2:CH-MX3 (M : C, Si, and Ge; X : F and Cl) compounds have been investigated at the B3LYP and MP2 levels8. A decrease in the MX3 rotational barrier in the order C > Si > Ge is predicted and is attributed to the decrease in the C-M covalent bond character. Dissociation energies and structures of the donor-acceptor adducts Et3Al-E(SiMe3)3 and tBu3Al-E(iPr)3 (E : P, As, Sb, Bi) have been studied at the B3LYP level9. The thermodynamic stability of these depends both on the electronic strength of the Lewis acid and base, which is influenced by the central Group 13 and 15 elements, their substituents, and on steric interactions between the Lewis acid and base. B3LYP calculations10 on carbochalcogenoates CH3COEM(CH3)3 complexes (E : Se, Te; M : Ge, Sn, Pb) show a shortening of the C:O . . . Sn distances compared to the C:O . . . Ge bonds due to greater oxygen lone pair ; M-E s* donation. MP2 calculations11 on Cl3Te[CH2CH(Cl)CH2O(H) . . . ] and Cl3Te[CH2CH (CH2Cl)OC(CH3):O . . . ] reveal strong oxygen lone pair ; s*(Te-Cl) and Coulombic interactions for the Te . . . O bonds, leading to significantly longer bonds in the isolated molecules than in the solid state. The structures of the C, Si and Ge analogues of the perfume ingredient majantol [2,2-dimethyl-3-(3-

1: Theoretical Organometallic Chemistry

3

methylphenyl)propan-1-ol ] have been determined at the RI-MP2 level12. The 119 Sn isotropic chemical shifts for the grouping C-N-Sn(CH3)3-N-C, characteristic of a series of solid organometallic compounds have been calculated at the B3LYP level13 revealing a strong influence on the Sn-N-C bond angle. The molecular structures of methylaluminoxanes with three-coordinated aluminium centres, calculated at HF, MP2 and B3LYP levels14, are determined by the strain due to ring formation and/or by the stabilizing p-bonding interactions between the oxygen lone pairs and vacant p-orbitals of the metals. The silanes Me2Si(CRR2)(CRR2) (R : H, R : Me and R : R : Me, CR2 : fluorenyl) and Me2Si(CHR2)2 exhibit extensive intramolecular and intermolecular C-H . . . p interactions leading to supramolecular associations, according to AMI semiempirical molecular orbital calculations15. The isotropic shielding of the tin nucleus in the reference compound tetramethyltin and a series of tetra-organotin compounds have been calculated at the B3LYP level16 to examine the effects of varying ligand chain-length, substitution at the a and b positions and bond order for compounds of the type Me3SnR. The trimeric diphenyltin chalcogenides (Ph2SnS)3 have twisted boat conformations as the global minimum at the B3LYP level17. B3LYP calculations18 suggest that the barrier to inversion of the monomer of 2-lithio-N-formylpyrrolidine is increased by use of non-coordinating solvents but lowered by aggregation into a trimer. 2.1.2 Clusters. The geometries and thermodynamic properties of twenty-four Group 13-Group 16 chalcogen heterocubanes [RM(m3-E)]4 (R : H, CH3; M : Al, Ga, In; E : 0, S, Se, Te) and twelve Group 13-Group 13 pure cubanes [RM(m3-M)]4 (R : F, Cl, CH3, NO2; M : Al, Ga, In) have been studied at the B3LYP level19. The chalcogen heterocubanes are predicted to be thermodynamically resistant to fragmentation. B3LYP calculations20 on the Na2[CAl4]2 dimer predict that structures with a C-C bond are higher in energy than the structures with two isolated structural [CAl4]9 units. Formation of a [C2Al8]49 cluster without a C-C bond are only slightly higher in energy. The structures and bonding of the closo-[1-M(CO)3(m4-E9)]49 (E : Sn, Pb; M : Mo, W) cluster anions have been studied at the DFT level21. The three-dimensional aromaticity of [(AlH)6(AlNMe3)2(CCH2R)6] (R : Ph, CH2SiMe3) has been investigated using ab initio and Hu¨ckel calculations22 suggesting that each Al4C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. B3LYP calculations23 have been used to study the bonding in Al metalloid clusters including [Al7
N(SiMe3)26]9, [Al12
N(SiMe3)28]9, [Al22Br20 12THF], Al14
N(SiMe3)26I6]29, Al14(m8-Si)Cp*6, [(AlEt)8(m4-CCH2Ph)5(m4-H)], [(AlMe)8(m4-CCH2Ph)5(m4-CCPh)] and [(AlMe)7(m4-CCH2Me)4(m2-H)2] and show the relationship between the observed geometry and electron counts. Increasing the proton donor ability of acids leads to the formation of bifurcate H-bonds in the complexes [B10H10]29 HOCH3, [B10H10]29 HOCF3, [B10H10]29 HCN, and [B12H12]29 HOCH3 complexes, a HF study24 reveals, in agreement with experimental observations. The colour of the pyridine-containing azanonaboranes [(C5H5N)B8H11NHR] (R : methyl, ethyl, isopropyl or tertiary









4

Organometallic Chemistry

butyl) is strongly dependent on the energy of the LUMO which is itself strongly influenced by the p-donor ability of the substituent, an AM1 study reveals25. A B3LYP study26 suggests that the macropolyhedral thiaboranes [S2B17H17-SMe2] and [S2B18H19]9 do not form for kinetic reasons only. The hexacoordinated tin atoms in the nanocluster [(CH3Sn)12O14(OH)6]2; are harder than the pentacoordinated tin atoms according to a B3YLP and HF study27. Molecular electrostatic potential calculations indicate that nucleophiles will approach preferentially the macrocation around the poles rather than at the equator of the cage. 2.1.3 Cyclopentadienyl Complexes and Analogues. Recent developments in the chemistry of cyclopentadienyl derivatives of the alkaline-earth metals, including computational studies, have been reviewed28,29. The geometries, metal-ligand bond dissociation energies, and heats of formation of twenty sandwich and half-sandwich complexes of the main-group elements of Groups 1, 2, 13, and 14, and Zn have been calculated at the BP86 and CCSD(T) levels30 and the nature of the metal-ligand bonding analysed with an energy-partitioning method. The structure and anisotropic NMR interaction tensors in the bis (pentamethyl cyclopentadienyl) aluminum cation, [(Cp2Al)]; have been calculated using RHF and B3LYP computations and the GIAO method31. There is a low barrier to rotation of the rings. The orientations of the NMR tensors, the large chemical shielding span, and the very small value of CQ(27Al) can all be rationalized in terms of the high molecular symmetry. The orthorhombic zigzag phase of plumbocene, Pb(C5H5)2 is lower in energy than a lattice comprising uniformly staggered rings by 2.8 kJ mol91 due to the difference in the strength of intermolecular interactions between the chains for the two different lattices, according to plane-wave DFT calculations32. 2.1.4 Carbonyls. B3LYP calculations33 of the structure and IR spectrum of the boron carbonyl (CF3)3BCO confirm its experimental formation and indicate that it has a bond strength of ca. 110 kJ mol91. B3LYP calculations34 confirm experimental observations of the neutral OCBBCO molecule and suggest that it has a linear singlet ground state with a very short boron-boron bond length, indicative of a unusual boron-boron triple bond. Calculation of the vibrational spectrum at the B3LYP level35 confirms that reaction of PhSiMe with CO yields the carbonyl complex PhSi(CO)Me. MR-CI calculations36 on Sr;CO reveal the existence of a bistable species with strontium either bonded to the carbon-end or to the oxygen-end of CO in collinear geometries. 3.1.5 Low Valent and Multiply Bonded Systems. Computational and experimental studies37 of Group 13 complexes, containing M(0), M(I) and M(II) including MCO, MCH3 and HMCH3, and complexes of M2 with substrates such as H2, CO, N2, CH4, NH3 and PH3, have been reviewed. Computational studies of homonuclear triple bonding between heavier Group 14 elements have been reviewed38. Acetylenes may behave either as acidic or basic probes, by virtue of the ∫C-H group, which is acidic, and of the C∫C triple bond, which can interact

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5

with positively charged centres, such as acidic hydroxyls or metal cations suggests a B3LYP study39 of alkali metal cation - acetylene and methylacetylene interactions. The oxidation states and stabilities of group 13 carbene analogues [
HC(CRNR)2E] (E : B, Al, Ga and In) have been investigated at the BP86 and B3LYP levels40 suggesting that the In system should have similar stability to the experimentally known compounds of Al and Ga but that the B homologue is expected to be highly reactive. B3LYP calculations41 on the donor-acceptor complexes R3E-ER and the isomeric R2E-ERR complexes (E, E : B-TI and R, R : H, Cl, CH3) suggest that the latter are stabilized by p- donor R and s- or p-bonded R groups. Calculations on the experimentally known complex Cl3BBCp* suggest it is the strongest bonded donor-acceptor complex of main-group elements that has been synthesized until now. The electropositively substituted Ge-AsX species are thermodynamically and kinetically more stable than their isomeric XGe:As molecules according to B3LYP and CCSD(T) calculations42 for X : H, Li, Na, BeH, MgH, BH2, AlH2, CH3, SiH3, NH2, PH2 OH, SH, F and Cl. HF, MP2, CCSD(T) and B3LYP calculations43 have been reported on the structures and bonding in mono (CH)5XH and diskeletally (CH)4(XH)2 substituted benzenes (X : B9, N;, Al9, Si, P;, Ga9, Ge and As;). For the disubstituted isomers, the ortho isomer is more stable for X : Ga9, Ge and As;, the meta isomer is most stable for X : B9, N;, Al9 and Si whereas the para isomer is most stable for X : P;. B3LYP calculations44 on the first stable germabenzene are in good agreement with its experimentally determined structure and properties whilst nucleus-independent chemical shift and aromatic stabilization energy calculations strongly indicate its aromaticity. B3LYP calculations45 have been used to study the effect of substituents on the structure and bonding of polarized phosphaalkenes revealing that the P:C bond length is dependent on the p-back-donation of the substituent on P. Phosphole oligomers have interesting, tunable electronic properties due to competition between the cyclic- and the carbon backbone p-conjugation, according to HF and B3LYP calculations46. Five and six membered cyclic carbenes containing phosphorus can be stabilized against dimerization by introduction of bridgehead tricoordinate phosphorus in the a-position from the carbene and/or tetracoordinate phosphorus in the b-position from the divalent carbon, according to B3LYP calculations47. The five-membered heterocyclic anion (deloc-1,3,4)-1-sila-3,4-diboracyclopentane-1-ide is strongly distorted at the MP2 level48 and contains a three-centretwo-electron bond between silicon and two boron atoms. The difference in the C-C bond lengths in 1,2,4,5-tetrahydro-1,1,4,4-tetramethyl-1,4-disiladicyclobuta[a,d]benzene, the first silacyclobutene diannelated [a,d]benzene, results primarily from substituent effects and is not caused by ring strain effects, according to MP4 and B3LYP calculations49. The ionization potentials of the silenes tBuMe2Si(Me3Si)Si:Ad and (Me3Si)2Si:Ad calculated50 at the ROVGF, MP4/6, and B3LYP levels reproduce the experimental values and show that the effect of substituents on the HOMO energy of silenes is slightly smaller than in the corresponding alkenes.

6

Organometallic Chemistry

The relative stabilities of the stereoisomers of the four-centre p systems (H2X)HE:EH(XH2), (H2X)HE:EH(XH2)2;, and HX:EHHE:XH (E : Si, C; X : Al, B, P, N) correlate with by the orbital-phase continuity of the terminal atoms. Isodesmic reaction energies calculated at the MP2 and B3LYP levels51 show that p-conjugation is effective for stabilization of compounds with AlH2 and BH2 substituents on the unsaturated silicon, and therefore, aluminum- and boron-substituted disilenes can be potential synthetic targets. The structures and vibrational frequencies of Ga(m-H)2Ga and In (m-H)2In, calculated at the BP86 level52, have enabled these matrix isolated species to be identified. MP2 and B3LYP studies53 of Lewis base ; H2Si coordination shows that the strength of the interaction depends mainly on the nucleophilicity of the base and the extent of p-delocalization of the lone pair on the silylene onto the p-frame of the base and that the base coordination triggers the nucleophilicity of the silylenes. In contrast to earlier studies, the only silylene calculated to have a triplet ground state by a BLYP study54 is bis(tri-tert-butylsilyl)silylene, (t-Bu3Si)2Si. 23 minima on the potential energy surface of silabenzene have been identified using MP2, CCSD(T) and B3LYP calculations55 including two new valence isomeric forms of silabenzene. 61 minima on the potential energy surface of disilabenzene, C4Si2H4 have been identified using B3LYP and CCSD(T) calculations56 with the planar forms corresponding to the most stable isomers but with several H-bridged isomers of high stability. 1-silanaphthalene is aromatic according to B3LYP calculations57 of its structure, vibrational frequencies and NMR chemical shifts. The first stable 9-silaanthracene is similarly aromatic according to analysis of B3LYP calculations58. B3LYP calculations59 on the structure of the overcrowded diaryldilithiogermane, Tbt(Dip)GeLi2 (Tbt : 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Dip : 2,6-diisopropylphenyl) confirm the structure determined experimentally. The monolithiated (organosulfonyl)acetonitriles [PhSO2CHCNLi.TMEDA] and [tBuSO2-CHCNLi.THF] form novel chain and sheet polymeric structures in preference to the more common (SO2Li)2 or Li2N2 ring dimers associated with lithiated sulfones or nitriles due to the inflexibility of the ligand backbone which precludes chelation and consequently destabilizing these structures, according to a B3LYP study60. B3LYP calculations61 on the different isomers of 2,3-dihydro1,2,4-thia-, selena- and tellura-diphospholes, and 2,3-dihydro-1H-1,2,4-triphosphole indicate similar stability with the lone pair on the heteroatom able to conjugate with the adjacent double bond. B3LYP calculations62 on 8-X-1-(pYC6H4Se)C10H6, 8-G-C10H6SeH-1 and HG-SeH2 (X : Cl, Br, and F) indicate that the conformation with the p-YC6H4Se group perpendicular to the naphthyl group is most stable. Orbital-phase theory63 has been used to propose pentagon stability. Cyclic delocalization of the lone pair electrons on the five-membered ring atoms through the vicinal s bonds was shown to be favoured by the orbital-phase properties. The pentagon stability was found to be outstanding in saturated phosphorus five-membered rings in the puckered conformation, and was substantiated by the negative strain energy of cyclopentaphosphane, P5H5. The aromaticity of the all-metal molecules Al429, XAl3, Ga429, In429, Hg469, Al39 and

1: Theoretical Organometallic Chemistry

7

Ga39 aromatic units has been evaluated at the B3LYP level64. All are eletron deficient species compared to the corresponding aromatic hydrocarbons and all should be considered as having both p and s-aromaticity. Cyclopropaannulated benzenes, pyridines and phosphabenzenes are aromatic, cyclobutanaannulated analogues possess some aromaticity but cyclobutena-annulated analagues are not aromatic according to B3LYP calculations65. The most stable isomer of Ge-containing cyanogens at the MP2, B3LYP and QCISD levels66 is GeNCN resulting in the unpaired electron being localized on nitrogen. 2.2 Mechanistic Studies. — B3LYP and CAS- MCQDPT2 calculations67 on the mechanism of the addition of nonenolizable aldehydes and ketones to Ge:C, Si:Si, Si:Ge, and Ge:Ge bonds can be grouped as either involving the formation of singlet diradical or zwitterionic intermediates or as concerted processes. Reactions leading to Si-O bonds can proceed via diradical and zwitterionic intermediates but formation of a Ge-O bond does not proceed via diradical intermediates. The energy barriers for addition of water to nine silenes, studied at the MP4 and B3LYP levels68, depend strongly on the substituents and show a good correlation with the difference in the total NBO charge between Si and C. B3LYP and QCISD calculations69 on the abstraction reactions of silylenes with oxirane and thiirane show that the interactions involve the initial formation of a donor-acceptor ylide-like complex followed by a heteroatom shift via a two-center transition state. B3YLP and G2 studies70,71 of the MC2H6 (M : Si and Ge) potential energy surface shows that MH2 and C2H4 initially form a p-complex, which can either collapse to silirane (germirane) or can isomerise by a 1,2 H-shift to ethylsilylene (ethylgermylene) with a relatively low barrier. The photoabsorption positions and symmetries for the discrete hole states in the dominant dissociation channel from photoionization of Ge(CH3)4 have been studied at the HF level72. The photodecomposition of (CH3)3SnI has been investigated through CASSCF/MS-CASPT2 calculations73 of the low-lying excited states and associated potential energy curves revealing that the a1A and b1A states are dissociative with respect to Sn-I bond elongation but that the upper c1A state is quasi-bound with respect to Sn-I bond elongation. The boron-zinc alkyl exchange reaction exchange reaction, studied at the B3PW91 level74, is predicted to proceed with a low activation barrier, involving two intermediates with unusual bonding structures. The metal-carbon bonding in these intermediates ensures a highly stereoselective exchange process, which can be employed in asymmetric organometallic synthesis. 1,3-silyl migration on acylpolysilane proceeds via a retention pathway and an inversion pathway with respect to the stereochemistry of the migrating silyl group with the former energetically preferred, according to a B3LYP study75. Silene undergoes [2 ; 2] cycloaddition with acetylene in a two-step manner resulting in the formation of silacyclobutene. The migration of silyl groups in silylpyrazolones proceeds via migration of the organosilyl group from the nitrogen atom to the oxygen atom (1,3-Si shift), followed by two consecutive 1,2-H shifts from the carbon to the nitrogen atom, according to a B3LYP and MP2 study76. The repulsive Coulomb barrier for several geometrically stable isomers of the

8

Organometallic Chemistry

[BeC4]2- dianion has been calculated at the HF level77 with the electron-loss lifetime for the most stable linear isomer, C29BeC29, estimated to be about 5 ms. The potential energy surfaces for the reactions of atomic hydrogen with the methylgermanes, GeH3CH3 and GeH2(CH3)2 have been studied at the PMP4SDTQ level78 are in good agreement with available experimental values. The potential energy surface for the excitation of the Ba . . . FCH3 complex to its electronic A state, studied at the CASSCF and MRCI;Q levels79, shows that the complex is repulsive at the ground state geometry and decays to Ba and CH3F fragments. A MP2 study80 of the reaction of atomic hydrogen with GeH(CH3)3 is in reasonable agreement with available experimental values within 298-510 K. As an alternative to the standard Cossee mechanism, the dinuclear alternative of the olefin reactions at aluminium where the chain switches between two different metal centres at each insertion has been investigated at the B3LYP level81 which indicates that the barriers for both insertion and b-hydride transfer at two different metal centers are rather similar to those for the more usual mononuclear mechanisms. The formation of aromatic carboxylic acids by carboxylation of aromatics with a carbon dioxide-Al2Cl6/Al system has been investigated at the B3LYP level82. A possible pathway involving an initial complex between benzene and Al2Cl6, with subsequent formation of organoaluminum intermediates (PhAlCl2 and PhAl2Cl5) is endothermic whereas the formation of
AlCl3n complexes of CO2 is exothermic and presumably preferred. The effect of geminal substitution at silicon on 1-sila- and 1,3-disilacyclobutane strain energies, 2;2 cycloreversion enthalpies and Si:C p-bond energies have been studied using MP2 and MP4 calculations83 on R2Si:CH2, 1-silacyclobutanes cyclo-R2Si(CH2)3 and 1,3-disilacyclobutanes cyclo(R2SiCH2)2. Electronegative substituents increase reaction enthalpies and strain energies and shorten and weaken the Si:C p-bond. The transformation of 2,3-dimethyl-2-siloxy-1,1-disilyl-1-silacyclobut-3-ene to 1,2-dimethyl-3(siloxydisilylsilyl) cycloprop-2-ene, studied at the B3LYP level84, involves is a typical 1,2-siloxyl shift with a triangular geometry in the transition state, resulting in the formation of a cyclopropene ring. A B3LYP study85 of the unimolecular rearrangement XSi∫P ; Si:PX (X : H, Li, BeH, BH2, CH3, NH2, OH and F) suggests that highly electronegative substitution at silicon can greatly stabilize the triple-bonded XSi∫P species relative to the double-bonded Si:PX isomer, thermodynamically and kinetically. A similar study86 of the unimolecular rearrangement of arsasilyne, XSi:As (X : H, Li, Na, BeH, MgH, BH2, AlH2, CH3, SiH3, NH2, PH2, OH, SH, F and Cl), to arsasilene, Si:AsX, also suggests that highly electronegative substitution occurs preferentially at silicon and, thus, strongly stabilizes triply bonded XSi:As, with respect to doubly bonded Si:AsX, again thermodynamically and kinetically. The delocalization energy in silanols, XH2SiOH, and silanediols, XHSi(OH)2 (X : H, OH, OSiH3 and Cl) increases on ionization with the Si-O9 bond gaining a partial double bond character according to HF and B3LYP calculations87. A B3LYP study88 reveals that cis,trans-Os(H)2(OTf)(NO)(PiPr3)2 and related systems react with (Me3Si)2NLi, (Me3Si)2CHLi, lithium 2,2,6,6-tetramethyl-

1: Theoretical Organometallic Chemistry

9

piperidide, Me3SiCH2Li, and B(CH2SiMe3)49 by a highly unusual and facile b-Me9 transfer with Me-C and Me-Si bond cleavage a direct SE2 substitution at carbon with inversion of the Me group.

3

d- and f-Block Metals

3.1 Structural and Spectroscopic Studies. — 3.1.1 Species Containing a Transition Metal-Carbon s-Bond Including Metal-Carbonyls. The electron density distribution in [FeCo(CO)8]9, which contains a semi-bridging carbonyl has been determined through B3LYP / AIM calculations89 to show the changes occurring during the evolution from terminal to bridging coordination. The smooth continuum of conformations observed in the solid state is explained in terms of the mutual interplay of direct M-M and M-CO and indirect M . . . M and M . . . C interactions. The vibrational frequencies of solvated [Ir(CO)6]3; calculated at the DFT level90 have been used to assigned the experimental infrared spectrum and are consistent with the absence of IR ; CO p-back- bonding. BPW91 calculations91 on the vibrational frequencies of [CpFe]2(m-CO)2(m-CH2) predict a 3 B2 ground state. The geometries, n(CO) frequencies, relative enthalpies and Gibbs energies of the alkene carbonyl complexes [W(CO)4(h2-C2H4)2]0/; and [W(CO)5(h2-C2H4)] have been studied at the B3LYP level92 and confirm experimental assignments. The geometries, vibrational frequencies and bonding in OCAgX (X : F, Cl, Br) have been studied by ab initio calculations93 revealing that the C-O distances are short, and the M-C distances are significantly longer than those in other molecules containing a metal-carbonyl bond. B3PW91 calculations94 on n(CO) frequencies of the CO adducts of the bivalent lanthanides, Cp2M(CO)x (M : Eu or Yb; x : 1, 2, M : Ca; x :1 ) are in good agreement with experiment for Cp*2M(CO) with M : Ca, Eu, but in poor agreement for M : Yb unless the CO is bound to the metal through the oxygen atom. CASMCSCF and MRSDCI calculations95 have been used to study the bonding and structure of OUCO, O2U(CO)2, and UO2CO3 and to assign the species present in matrices. A new conformer of Fe(CO)4 has been predicted using MP2 calculations96 with a square planar configuration and a singlet ground state. Computational and experimental studies97 of the spectroscopy, photochemistry and electrochemistry of [M(CO)4(alpha-diimine)] complexes (M : Cr, Mo and W) have been reviewed. B3LYP calculations98 on the ground and excited states of fac-[Re(bpy)(CO)3(4-Etpy)]; provide assignments for the n(CO) modes in the MLCT excited state and show the importance of p*(4,4-X2)bpy.9)-p*(CO) mixing, providing an explanation for the relative intensities of the A2 and A excited-state bands, and an explanation for the large excited-to-ground-state n(CO) shift for the A2 mode and its relative insensitivity to variations in X. The halide ligands in [(2,4-bpy)RuCl2(CO)3] and [(4,4-bpy)(RuCl2(CO)3)2] possess a significant role in the HOMO-LUMO energy gap and can be used to tune the electronic properties of ruthenium bipyridines, a B3PW91 study99 suggests. A similar study100 also suggests that the position of the bipyridine substituents in

10

Organometallic Chemistry

mono(bipyridine) complexes of ruthenium (n,n-L2-2,2-bpy)Ru(CO)2Cl2 (n : 3, 4, 5, or 6; L : CO(O)Me or CO(O)Et) causes changes in their electrochemical properties of the compounds due to variations in the HOMO-LUMO energy gap. Photolysis of Rh(CO)(PMe3)2Cl and Rh(CO)(PBu3)2Cl is predicted by B3LYP calculations101 to yield non-planar, spin-triplet, excited states whilst high energy photolysis of the former is predicted to yield cis-Rh(CO)(PMe3)2Cl. BPW91 calculations102 have been used to predict the products of the photolysis of (CpCo)2FeSiH2CH3 and (CpCo)2FeCH2SiH3. B3LYP calculations103 of the linear and nonlinear ground state absorption in platinum-organic compounds show several ways to tailor the linear absorption to a desired wave length region with nonlinear absorption cross-sections strongly enhanced by the introduction of charge-transfer units and linear oscillator strengths essentially unaffected by the same ligand substitutions. A DFT study104 on Co(CO)3(PPh3)2BEt3 reveals that it contains a weak, novel B-O interaction. The EPR g-values for dinuclear radical anion complexes
(hL)[Re(CO)3Cl]2.9 (L : 2,2-azobispyridine and 2,2-azobis(5-chloropyrimidine) are reproduced reasonably well by DFT calculations105 which also provide insight into the relationship between spin density distribution and g-anisotropy. Molecular models for the reactivity of coordination vacancies of Ni-II ions grafted onto tridentate silica support with CO, studied at the B3LYP level106, suggest that the Ni-CO interaction is stabilized by a magnetic transition from a Ni2; triplet to the Ni2; singlet state occurring upon adsorption. The photoelectron spectra of iron carbonyls have been modelled107 showing that it is possible to track the temporal variation of a metal-carbon bond. The structure and bonding in ruthenium porphyrin and carbonyl diyl complexes (CO)4Ru-EHeq, (CO)4Ru-EHax, (Por)Ru-EH and (Por)Ru-E(trip) (trip — 2,4,6-triisopropylphenyl, E : B, Al, Ga, In, Tl) have been calculated at the B3LYP level108. NBO analyses reveal high Ru-E bond dissociation energies, with B-Ru p-back-bonding similar to that in a M-CO bond. There is no correlation between the M-P bond lengths and bond strengths in the phosphines M(CO)5PX3 (M : Cr, Mo, W; X : H, Me, F, Cl), reveals a BP86 study109, and PMe3 forms the strongest and longest M-P bond whilst PCl3 is the poorest s-donor and a strongest p(P) acceptor. The p effects associated with Rh-P bond lengths, n(CO), and 9DHreaction for Rh(CO)(Cl)(PX3)2 and n(CO) and 9DHreaction for Rh(acac)(CO)(PZ3) have been studied using a QALE analysis110 showing the importance of the synergistic interactions between the phosphine and other ancillary ligands. The thermodynamics of H2, arene, alkane, and CO addition to pincer-ligated three-coordinate d8, four-coordinate d8, and five-coordinate d6 iridium complexes, studied at the B3LYP level111, suggest that addition of one ligand is favoured by p-donor ligands but addition of a second is disfavoured by such ligands. Protonation of [Re2H(CO)9]9 yields the neutral complex [Re2H2(CO)9] which contains classically bonded, containing one m1 and one m2 H-atom but the non-classical [Re2(h2-H2)(CO)9] tautomers is an intermediate in the fast hydride exchange process, according to B3LYP calculations112. [W(N2Npy)(NPh)Me];

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11

reacts with CO2 or isocyanates via cycloaddition reactions at the W:NPh bond and not insertion into the W-Me bond, despite the latter product being the most thermodynamically favourable according to DFT calculations113. The d4 imido complex [TpRu(CO)(PPh3)(NPh)][OTf ] is thermally unstable due to the pconflict arising from the p-antibonding role of the singly occupied molecular orbital, a B3LYP and BLYP study114 reveals. BP86 calculations115 have been used to model the structures and NMR spectra of (L)PtMen (n : 2 or 4 and L : 1-methyl-(2-methylthiomethyl)-1H-benzimidazole and 1-methyl-(2-tert-butylthiomethyl)-1H-benzimidazole). The lowlying electronic transitions of a series of [(iPr-DAB)PtR2] (R : CH3, CD3, adme, neop, neoSi, tBu, CPh, Ph, Mes) compounds have been studied116 at the DFT level revealing the significant R contribution to the high-lying orbitals and the MLCT with partial M-C s-bond-to-ligand charge transfer character of the transitions. First row transition metals prefer N-binding while 2nd and 3rd row metals can prefer C-binding to imidazole according to DFT studies117.B3LYP calculations118 suggest that [Pd(COD)ClL] (L : 2,6-bis(diphenylphosphine sulfide)-3,5-diphenylphosphinine) contains a l4-1-P-chlorophosphinine ligand, bound to the palladium atom through the phosphorus atom and is formed from the internal attack of the chloride counteranion onto the highly electrophilic P atom of the transient l3-phosphinine complex. The structures of [Pd(m-OAc)2]3, Pd(OAc)2.2NHEt2, [Pd(OAc)(m-OAc)(CH3)2SO]2, [Pd(m-OAc)(p-SEt)]4 and [Pd(m-SEt)2]6, calculated at the PBE level119, are in good agreement with X-ray diffraction analyses. The dicyanodicarbonyliron(II) thiolate complexes trans, cis-[(CN)2(CO)2Fe(S, S-C-R)]9 (R : OEt, NEt2) are five-coordinate with a vacant site trans to the CO ligand and two CN- ligands occupying cis positions, according to B3LYP calculations120. The coordination around the metal in [M(h2(C,C)-C3O2)(PPh3)2] (M : Ni, Pd, Pt) complexes is square planar according to BP86 calculations121. HF and MP2 calculations122 predict that relative energy of the isomers of trans-[PtCl4
(E)-NH:C(Me)OEt2], trans-[PtCl4
(E)NH:C(Et)OEt2], trans-[PtCl4
(E)-NH:C(Et)OiPr2], trans-[PtCl4
(E)NH:C(Et)OnPr2], and cis-[PtCl4
(E)-NH:C(Et)OMe2] decrease in the order EE > EZ > ZZ. The structures of (h2-fumarodinitrile) palladium(0) complexes containing dihydro(phosphanylphenyl)oxazole ligands are accurately reproduced by B3LYP calculations123 but require extended basis sets on the metal. The performance of DFT in studying124 the structures of lanthanides organometallic complexes has been reviewed. The 4f electrons are shown to be uninvolved in bonding and can be included in the core but careful modelling of ligands is necessary as oversimplification can lead to structural artefacts. The peculiar pyramidal coordination observed experimentally for Ln[CH(SiR2R)(SiR3)]3 (Ln : La, Sm, R : R : Me, R : H, R : Me, R : R : H) is reproduced by B3LYP calculations125. The Si-C bond lengths are lengthened due to b-Si-C agostic interactions, whereas g-C-H agostic interactions are repulsive, 1.

Organometallic Chemistry

12

1

HF calculations126 confirm the experimental observation that the two N-N distances in [Rh(tpy)(bpy)(N3)](PF6)2 differ. B3LYP calculations127 on [Pt(COMe)Cl
MeN(H)CH:CHN(Me)C(O)Me] and the free ligand MeN(H)CH:CHN(Me)C(O)Me show that the internal H-bonding in the ligand is lost upon coordination. The W-W bond order in [W2(k2-O2CH)4(m-HCCH)2], at the B3LYP level128, is two. The intramolecular XH . . . HM interaction between the imine proton donor and the terminal hydride in H(mH)Os3(CO)10(HN:CPh2) is essentially electrostatic, a B3LYP study reveals129. The structures and frontier orbitals of [M(tmtaa)] and [M(acacen)] fragments employed in the organometallic chemistry of early transition metals have been investigated at the BP86 level130. The Ir-I bond enthalpy in [Ir(m-SBu)I2(CO)2]2 is significant smaller than in trans -[Ir(X)I2(CO)(PPh3)2] (X : F, Cl and Br) and is therefore, not transferable between both types of molecules, according to B3LYP calculations131, despite very similar bond lengths and bond orders. BP86 calculations132 on the structures and vibrational spectra of Re(CO)(PMe3)3(h2BH4), Re(CO)(PMe3)3(h2-BBNH2), Re(CO)2(PMe3)2(h2-BH4) and 2 Re(CO)2(PMe3)2(h -BBNH2) have been used to confirm and assign the synthesis of these systems. BP86 calculations133 have also been used to calculate the structure, relative energy, IR and 11B NMR spectra of various isomers of Tp3R,5R rhodium(I) dicarbonyl complexes (Tp:hydridotris(pyrazol-1-yl)borate; R : H, Me) revealing that the the lowest energy structure of Tp3R,5RRh(CO)2 is a nonclassical square pyramidal structure with a long metal apical ligand distance. B3LYP calculations134 indicate a marked preference for homochiral dimeric fencholate methylzinc complexes over heterochiral dimers with H and Me ortho-substituents in the anisyl moieties but similar stabilities are predicted for SiMe3 and tBu substituents. B3LYP calculations135 on [Os3(m-H)(CO)10(m-NC5H3C5H4N)], [Os3(mH)(CO)10(m-NC5H3C5H3N)Os3(m-H)(CO)10] and [Os3(m-H)(CO)10(mNC5H3C5H4N)W(CO)5] reveal that they all have similar orbitals in the LUMO region and that the first two have the Os---Os s-bonding orbitals as their HOMO orbitals whilst the latter has the t2g set orbitals of the W(CO)5 fragment as the HOMO orbitals. B3LYP calculations136 suggest that the electronic coupling between the M2 units in dimetal tetracarboxylates, M2(O2CR)4 ( R : alkyl

1: Theoretical Organometallic Chemistry

13

and M : Mo or W) occurs by M2 d ; ligand p-conjugation. B3LYP and BLYP calculations137 on Os3H(m-H)(CO)10(benzylamine) show the presence of an unconventional hydrogen-bond M---H · · · H---N. The structure of the first known triosmium carbonyl cluster with a coordinated redox-active ligand 4,4,5,5-tetramethyl-2,2-biphosphinine has been confirmed by B3LYP calculations138 which also show an that each P atom bridges two metal atoms. A review of computational and crystallographic studies of the utility of Tolman cone for P-donor and other ligands in accounting quantitatively for steric effects has been published139. Theoretical advances in the elucidation of the nature, the structure, the spectral and energetic characteristics of the new types of hydrogen bonds (HB) specifically for organometallic compounds — cationic hydride as a proton donor ([MH](;) . . . OP(X-)) and a metal atom (M . . . HX) or hydride ligand (MH . . . HX) as a proton acceptor — have been reviewed140. The coordination and organometallic chemistry of metal-NO complexes141 and the interactions of organic nitroso compounds with metals142, including key computational and bonding studies, have been reviewed. 3.1.2 Cyclopentadienyl Derivatives and Related p-Bonded Species. The Et2C2B4H429 carborane ligand in the ferrocenyl derivative (h6C6H6)Fe(Et2C2B4H3-5-Fc) is strongly electron donating so that there is a substantial flow of electron density from the ferracarborane cluster to the ferrocenyl moiety, according to a PM3 and B3LYP study143. A B3LYP and BP86 study144 of the bonding in ferrocene derivatives with Group-15 heteroligands Fe(h5)-E5)2 and FeCp(h5-E5) (E : N, P, As, Sb) reveals that the strongest bonded homoleptic complex is Fe(h5-P5)2 and that the bonding in the mixed FeCp(h5-E5) is much stronger than in the homoleptic molecules. The bonding in transition-metal boryl complexes of the type [(C5R5)M(CO)2BX2] is dominated by the covalent contribution with s-donation from the boryl ligand overwhelmingly predominating over p-back-donation, according to a B3LYP and BLYP study145. h1W(CO)5 and h5-M(CO)3 complexes (M : Mo, W) of 1,2,4- and 1,3,4-thiadiphospholes, P2SC2But2, have similar stabilities, a B3LYP study reveals146.Diphosphaferrocene bonds to [GaCl2]; using the lone pairs on the phosphorus atoms with a contribution from the P-Fe bond, according to a BP86 study147. The structures and energetics of Au(C5H5N); species have been studied at the MP2 and CCSD(T) levels148 revealing C2v and C1 symmetry isomers. The Jahn-Teller distortion in CrCp*Tp has been studied using DFT calculations149 and is due to the metal-based HOMO which is significantly antibonding with respect to a single pyrazolyl ring only. Both the singlet and triplet lowenergy states of [(h6-benzene)Nb(CO)3]; are subject to Jahn-Teller effects, according to a B3LYP study150. The h6, h6-inter-ring haptotropic rearrangement of (h6-biphenylene)chromium tricarbonyl has been studied at the PBE level151. B3LYP calculations152 have been reported on (h5-C5H5)Fe(CO)2Cl and its fragments in anionic, neutral and cationic states and have been used to assign mass spectrometry experiments on anions. A review of crystallographic and computational results and new BP86 and B3LYP calculations153 show that the rotation angles in MInd2 complexes show a

14

Organometallic Chemistry

wide range of values, depending both on the electron count and on the steric effects of the ring substituents with hapticity change toward h3 induced by extra electrons. The hapticity and ring slippage in IndMo(h3-Ind)(CO)2 has been investigated at the B3LYP level154. Exocyclic h3 coordination for the fluorenyl, cyclopenta[def ]phenanthrenyl and dihydrocyclopenta[def ]phenanthrenyl ligands in the complexes IndMo(L)(CO)2 according to B3LYP calculations155. The two-electron reduction driven h5 to h3 coordination shift of cyclopentadienyl and indenyl ligands in molybdenocene complexes, [X(h5-Cp)Mo(CO)2)] (2;) ( X : Cp or Ind) has been studied at the B3LYP level156. The results verify the indenyl effect, that the rearrangement is much more facile for the indenyl ligand than for the cyclopentadienyl ligand, but suggest that this is due to the Mo-X bond strength rather than the traditionally accepted aromaticity gain in the benzene ring formed in h3-Ind complexes. The energy profiles for the rotamers of mixed cobaltacarbonanes have been calculated at the ZINDO level157. The photochemistry of the CpNiNO complex has been investigated158 using DFT and time dependent DFT. The whole potential energy curve along the NiNO angle coordinate reveals both ground and metastable states, and transition states connecting the minima. The electronic influence of unbridged and ansa-bridged ring substituents on a zirconocene centre, studied at the BP86 level159, suggest that the effects of substituents on the cyclopentadienyl rings are due to a simple inductive effects but that the electronwithdrawing effect of [Me2C] and [Me2Si] ansa-bridges is due to stabilization of the cyclopentadienyl ligand acceptor orbital, which enhances back-donation from the metal. BP86 calculations160 of the structures and bonding in the complexes Cp(CO)2FeX predict a widened angle at the carbon atom for X : CH2SiH2(OH) compared to X : CH3, in agreement with the expectations of Bent’s rule, and that X is weakly bound. The bonding and structure of benzene complexes of V;, Cr2;, Mn;, Fe2;, Co;, Ni2; and Cu; have been studied at the B3LYP level161 revealing large interaction energies. A B3LYP study162 predicts that benzene binds via its p electrons whereas pyridine forms s complexes in the [Fe(L)(L)]n; (L, L : benzene or pyridine; n : 1 or 2). There is a strong perturbation of the metal centres by the hydrazone bridge in [CpFe(h6)-p-HC6H4)-NHN:CMe-(h5)C5H4)FeCp];PF69) as well as some metal-metal interaction through the bridge, a B3YLP study163 reveals. A B3LYP study164 confirms crystallographic results that the heterodimetallic complex [Zn
(h5-C5H5)Fe[(h 5-C5H4)-CH:N-(CH2)3--NMe2]Cl2] exists in two isomeric forms which differ in the conformation of the ligand E(anti-) or Z(syn-). HF calculations165 on [Nb(h5-C5H5)2PDT] (PDT:propane-1,3-dithiolate), [NbCp2EDT] and [NbCp2EDT]; (EDT:ethane-1,2-dithiolate) indicate a tetrahedral configuration around Nb, in agreement with crystallographic studies. LDA and BP86 calculations166 on the vibrational spectra of model complexes have been used to identify Zr-Cl, Zr-CH3 and Zr-Cp bonds present in 2,2-bis(indene-2-yl)-biphenyl-Zirconium dichloride supported on silica in the presence of methylaluminoxane. B3LYP, B3P86 and mPW1PW91 TDDFT calculations167 have been used to study the UV/visible absorption spectra of rac-

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Et(Ind)2ZrCl2, rac-Et(Ind)2ZrMeCl, rac-Et(Ind)2ZrMe2, and the [racEt(Ind)2ZrMe-C2H4]; complex and reproduce fairly well the essential features of the experimental spectra which originate from ligand to metal charge transfer transitions. DFT calculations168 confirm the X-ray structure of
[Cp-C(:CH2)O]-Zr(NEt2)22 with the two Zr centres connected by two symmetry-equivalent h5:k[Cp-C(:CH2)-O] ligands. B3PW91 calculations169 have been used, in conjunction with an electron diffraction study, to study the structure of Ru(C5Me5)(C5F5), Ru(C5H5)2, Ru(C5F5)(C5H5) and Ru(C5F5)2, revealing that F substituents, while s-electron withdrawing in nature, donate to the ring p-systems resulting in shorter ruthenium-to-ring distances. B3LYP calculations170 on ferrocenophanes yield structures in good agreement with those determined experimentally. BP86 calculations171 have been used to rationalize the bonding in [Rh2(m-O2CR)4L] complexes with strong and very weak axial donor-acceptor ligands L such as ‘Arduengo’ carbenes and aromatic hydrocarbons. The structures and vibrational frequencies of Tp(3,5-Me)RhH2(H2) in its ground and various transition states have been studied at the B3LYP and BP86 levels172 revealing canted h2-dihydrogen dihydride local minima. The diabatic couplings between the metal centre in [MCp2]0/; (M : Fe, Co; Cp : C5H5) have been evaluated173 using ab initio methods at the HF level leading to excellent agreement with the experimental estimate for M : Fe. For M : Co, the calculated numbers are substantially higher than the experimental estimates, suggesting that the latter may represent a considerable underestimate. The dissociation of the (C6H6)Cr(CO)3; ion proceeds by the sequential loss of three CO and benzene ligands, according to BP86 calculations174. DFT calculations175 of the electronic structure and normal vibrational modes of CpRe(NO)(CO)H support the localized two-electron valence bond description of the Re-H interaction suggested by the vibrational progressions seen in the valence ionization spectrum. Calculated H-D and H-T coupling176 in [Cp/Cp*Ru(P-P)H2]; (where P-P is a chelating diphosphine ligand) are consistent with experimental studies of their dependence on temperature and magnetic field. The molecular structures and excited states of CpM(CO)2 (M : Rh, Ir) and [Cl2Rh(CO)2]9 have been investigated at the B3LYP and SAC-Cl levels177 suggesting that they have singlet ground electronic states with large singlettriplet separations. The strong transitions in [Cl2Rh(CO)2]9 are identified to be metal to ligand CO MLCT excitations but in the CpM(CO)2 systems both MLCT excitation and ligand Cp to metal and CO charge transfer excitation are strongly mixed. The unsaturated metal centre in (phenylbicycloheptenyl)Pd(PPh3) and related substituted derivatives receives p-electron density of the arene mainly through its C-ipso atom and this effect may be slightly improved if the C-ortho atom also gets closer to the metal but the slipped h2 coordination does not appear seem to be crucial for the stability of the system according to B3LYP calculations178. B3LYP calculations179 suggest that the six membered metallacycle in osmabenzyne Os[∫C-C(SiMe3):C(CH3)C(SiMe3):CH]Cl2(PPh3)2 2 exhibits somewhat aromatic properties with the filled metal dx2-y2-orbital interacting with the equatorial p-orbital of the high

Organometallic Chemistry

16

reactive carbyne carbon in a back-bonding fashion. BP86 calculated180 dipole moments of (h5-C5H5)(PH3)2M(C∫C-p-C6H4X)n; (M : Fe, Ru; X : NO2, CN, H, OMe, NH2; n : 0, 1) have been used to calculate second-order molecular polarizabilities with electron-rich iron sigma-aryl acetylides.

2

A molecular mechanics force field has been developed181 for the conformational analysis of amido- and alpha-aminoferrocenes and used to show that the selectivity of the diastereoselective lithiation of N,N- dimethylferrocenylethylamine and sparteine-mediated enantioselective lithiation of (diisopropylamido)ferrocene using MeLi from the adduct conformer with the shortest C-H-ring---H3C-Li interaction. The 16e two-legged piano-stool complexes [CpRu(PP)]; (PP : (PH3)2, H2PCH2CH2PH2) adopt typically pyramidal structures whereas [CpRu(NN)]; (NN : (NH3)2, H2NCH2CH2NH2) prefer planar structures, according to B3LYP calculations182. A combined molecular mechanics, PM3 and BOP study183 of binding of CH2:CHR to chiral [(h5-C5H5)Re(NO)(PPh3)]; (R : Me, n-Pr, CH2Ph, Ph, i Pr, tBu and SiMe3) has been used to study the stereoselectivity allowing computationally derived diastereoselective excess to be derived and compared with experiment. The Cr-C2X4 bond strengths in Cr(CO)5(C2X4) (X : H, F, Cl) has been investigated using a DFT and bond energy decomposition analysis184 in terms of the the Dewar-Chatt-Duncanson model suggesting that the energy necessary to deform the olefin and the metal-centered fragment is the determining factor in the trends in bond enthalpies. A B3LYP study185 has shown that styrene oxidation by chromylchloride yields an intermediate with a 2:1 stoichiometry with two Cr(V) centres on opposite sides of the former double bond. A B3LYP study186 suggests that back-donation is important for olefins but electron donation is more important for phosphanes in [Pt(PH3)2L] (L : H2C:CHR or PH2R, R : H, CN, F, OH, NH2) complexes. Complexation of azaphosphirane with W(CO)5 is predicted by B3LYP calculations187 to lead to a tighter ring. BP86 and B3LYP calculations188 on [Nb(h5-C5H5)R2(HCCH)] (R : Cl, Me) have been used to study the parallel and the perpendicular conformations of the alkyne ligand with respect to the Cp plane and suggest that the latter is preferred. The Os-C(alkyne) distance shortens between the complex Os(h5-C5H5)Cl
h2-HC∫CC(OH)Ph2(PiPr3) [Os(h5-C5H5)
h2-HC∫CC(OH)Ph2 (PiPr3)]PF6 due the participation of the acetylenic second p-orbital in the metalalkyne bonding so that the 2-electron-donor alkyne ligand transforms to a 4-electron-donor ligand, a B3LYP study reveals189. The structures of [1,2-

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(CH2)2C5Me3MC5Me4CH2]3; trications (M : Ru and Os) have been calculated at the BLYP level190. The Pt-Ga bond length in
(dcpe)Pt[GaC(SiH3)3]2is very short, a BP86 study reveals191, with significant Pt;Ga p-back-donation. The trans and cis isomers of [PdCl2(L-Allyl)2] (L-Allyl : 1,3-diallylimidazolidin-2-ylidene) have comparable stability, according to a B3LYP study192. The barrier to alkyne rotation in [H3(CO)(I)W
HC∫CH]29 and [H3(CO)(I)W
HC∫CCH2]3- is calculated, at the extended Hu¨ckel level193, to be too high for rotation to be important on the NMR timescale. A BP86 study194 has shown that the 1H NMR spectrum of paramagnetic (2-Me-allyl)3Cr can be explained by the presence of two conformers which convert into each other by rotation of one allyl ligand around the allyl-chromium axis. The unusual trigonal prismatic structure of tris(butadiene)molybdenum 3 has been confirmed by calculations at the MP2, BP86 and B3LYP levels195. NBO and NRT analyses of the bonding suggest coordination of diolefins to a d6 Mo(0) atom is a more accurate description that M-C s-bonding of a olefin to d0 Mo(VI). The bonding in the unusual Cu3
m2-(CCHCo2(CO)63 cluster 4 has been studied using a DFT calculations196 to provide a rationalization of its peculiar structure, suggesting that the triangular Cu(I) core interacts strongly with the organometallic fragment with an important p component. The bonding in the 46-electron cluster [(CO)3Fe(m3-Se)
Pt(CO)P(2-C5H4N)Ph22] consists of an open Pt---Fe---Pt triangle capped by a m3-Se atom and can be thought of as a tetrahedral (CO)3FeSe unit stabilized by sidewise interactions of the triple bond with two d10-L2M fragments, a B3LYP study197 reveals. The chemical shift and electric field gradient tensors in the piano-stool compound mesitylenetricarbonylmolybdenum(0) have been calculated at the BP86-ZORA level198 and are found to reproduce the experimental results.

3

The structures and bonding of the closo-[1-M(CO)3(m4-E9)]49 (E : Sn, Pb; M : Mo, W) cluster anions have been studied at the DFT level21. The bis(ferrocenyl)butadiyne ligand in Os3(CO)10(m3-h2-FcC4Fc) has triene character that facilitates electrocommunication through the unsaturated carbon chain, a molecular orbital analysis based on extended Hu¨ckel calculation has revealed199. A B3LYP study200 reveals that the phosphaalkyne ligand in [CpMo
P(OH)32
h2(4e)-PC∫Me]; is a poorer donor and better acceptor than the alkyne ligand in [CpMo
P(OMe)32
h2(4e)-alkyne];. Donor-acceptor interactions, studied at the B3LYP level201, of Lewis bases, such as amine, phosphine, cyanic acid and phosphaalkine with various phosphinidene complexes

Organometallic Chemistry

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4

carrying a W(CO)5 fragment can be divided into two categories. Amines bind stronger than a phosphine to a phosphinidene. p-donors yield shorter N-P bond, but the resulting donor-adduct is even less stable with respect to decomposition. A B3LYP study202 reveals that the greatest bonding contribution in the vinyl coordination to the M(CO)5 fragment in [M(CO)5-
h2-Ph3PAuC(OR):CH2] [M : Cr, R : Me, Et ; M : Mo, R : Me; M : W, R : Me, Et] complexes comes from the terminal, partially negatively charged, CH2 carbon atom via partial end-on h1-bonding rather than the usual h2-bonding of olefins. The Arduengo carbine ligand in a methyltitanocene cation complex serves as a pure s-donor ligand to the titanocene moiety and the observed favoured ‘in-plane’ orientation of the ligand is steric in origin, according to a BP86 and bond energy analysis203. The metal-cumulene bond in the series of metallacumulene complexes [(CO)5Cr(:C)nX2)] (X : F, SiH3, CHCH2, NH2, NO2; n : 2-8) 5 are significantly affected by the substituents, particular by the resonance effect in the NH2 and NO2 systems. A BP86 and bond energy analysis204 reveals that p-donor amino substituents cause a decrease in the bond strength while the p-acceptor nitro substituents cause an increase of the bonding energies, particularly for cumulenes with an odd or an even number of carbon atoms, respectively. The conformations of complexes such as [M2(m-C∫CR)2L4], in which each metal atom is in a square-planar environment depend on the coordination mode of the alkynyl ligands and the angle between the coordination planes around the two metal atoms, a MP2 study reveals205. Each metal centre in [Sm(NHAr)3]2 engages in an h6-arene interaction with the aryl ring of an amide ligand attached to an adjacent samarium with a significant role for the 5d metal acceptor orbitals in stabilizing p-donation from the imido groups to the Sm centres within the Sm2N2 core, according to a B3LYP study206.

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5

The relative energies of the isomers of C5H4-cyclocumulene, ethynylcyclopropene, diethynylmethane, and their organometallic analogues obtained by replacing the CH2 groups by Cp2Ti and Cp2Zr are dramatically altered in the transition-metal analogues according to a B3LYP study207. The metallacyclocumulenes 6a are comparable in energy to 6c and 6d and substituents on the carbon skeleton help to fine-tune the energetics. The influence of the h1- and h5-pyrrolyl coordination modes on the stabilities of [pyrAr2]Zr(NMe2)3 and [pyrAr2]Zr(NMe2)I2 (Ar : Ph, Xyl) has been examined at the B3LYP level208 suggesting that h5-coordination is favoured to a greater degree for the smaller [pyrPh2] ligand than for the [pyrXyl2] ligand, largely due to the increased bulk in the ortho positions destabilizing h5-coordination for the [pyrXyl2] ligand.

6a

6b

6c

A BP86 study209 suggests that the bonding of the ene-diamido ligand is different with respect to the conventional s2,p -description of the bonding of the cis-1,3-butadiene ligand to early transition metals. The experimentally observed folded envelope geometry in high valent early transition complexes containing a substituted 1,4-diaza-1,3-butadiene ligand is due to the reorientation of the N hybrid orbitals in order to saturate the metal centre. The valence structure for the [M---Cx---M] unit in a variety of homonuclear [
Cp(CO)2M2(m-Cx)] (M : Cr, Fe;, x : 3; M : Mn, x : 3, 5, 7) and heteronuclear [
Cp(CO)2M(mCx)
Cp(CO)2M] (M : Fe;, M : Mn, x : 3; M : Fe, M : Cr, x : 3, 5, 7) model compounds is diagnostic of the metal-to-metal communication and a BP86 study210 reveals a simple electron-counting scheme to predict the valence structure, based on the dn configuration of the MLm (MLm) fragments and the number of pp electrons of the ‘linear’ Cx unit. trans-[RhX(L)(PMe3)2] (L:C:CH2, C:CHC6H5, CO, 2,6-Me2C6H3NC, C2H4) complexes have been used as model compounds for the analysis of the vibrational spectra of related complexes at the BPW91 level211 allowing the trans influence of a series of ligands on the metal-carbon bond in vinylidene, carbonyl and isocyanide complexes to be studied. A molecular mechanics study212 of optically active ferrocene containing macrocyclic peptides reveals that the low energy conformations are nonsymmetric. C-H . . . p interactions where the C-H group interacts with the psystem of a six-membered chelate ring take place in quite a number of metal complexes, including organometallic compounds and have bond energies of ca. 1 kcal mol91 at the B3LYP level213.

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3.2 Mechanistic Studies. — Chiral catalysts that are efficient at inducing asymmetry have their region of maximum stereoinduction spatially congruent with the site of chemistry but inefficient catalysts do not according to study214 of a range of catalysts for three Diels-Alder reactions at the PM3 and MM level. Theoretical and experimental studies of photochemical Pt-C bond homolysis reactions of metal-carbon bonded platinum compounds have been reviewed215. The use of computational quantum chemistry in catalysis research has been assessed216 suggesting that single reference theories are not sufficiently accurate and that a multi-configurational self-consistent field (MCSCF) theory must be utilized. 3.2.1 Polymerization Reactions. According to BP86 calculations217 for a model system with L : HC(O)CHC(O)H, cationic titanium complexes of the type L2TiR; (R : growing alkyl chain) can be viable intermediates in the homogeneous olefin polymerisation catalysed by titanium beta-diketonato complexes. The interaction between the catalyst systems (NPR3)2TiMe;, (Cp)(NCR2)TiMe;, (CpSiR2NR)TiMe;, (Cp)OSiR3TiMe;, and (Cp)NPR3TiMe; and the counterions B(C6F5)49, MeB(C6F5)39, TMA-MAOMe9 and MAOMe9 (TMA : trimethylaluminum and MAO : methylalumoxane) have been studied at the BPW91 level218. The ion-pair separation energies increase in the order B(C6F5)49 < MeB(C6F5)39 < TMA-MAOMe9 < MAOMe9. For each counterion, the (NPR3)2TiMe; system has the lowest separation energy. QM/MM models satisfactorily reproduce the behaviour of the ion-pair system in the insertion process. Possible structural candidates for the active(III) and dormant (IV) species in dimethylzirconocene-catalysed, MAO (methylaluminoxane)-activated olefin polymerization have been identified at the BP86 level219 as [Cp2ZrMeAlMe3] ; [MeMAO]9 (III) and [Cp2ZrMe];[MeMAO]9 (IV) respectively. A change in the olefin pressure does not affect the global number of branches in propylene polymerization catalysed by Pd(II) complexes with methyl backbone- and -Ph-iPr2 imine substituents but strongly affects the polymer microstructure, leading to hyperbranched structures at low pressures, according to stochastic simulations using DFT energies220. The performance of DFT and ab initio approaches221 in studying the propagation and termination reactions of olefins with a prototype homogeneous Group 4 ansa-metallocene catalyst has been compared leading to substantially similar geometries at all levels. Pure functionals underestimate the difference between termination and propagation but hybrid functionals lead to very similar energetics to CCSD(T). A reasonably good correlation has been observed at the B3LYP level222 between the turnover frequency of Ni(II) salicylaldiminato catalysis for ethylene polymerization in the presence of additives and the Ni-O (additive) bond dissociation energy. The unusual inverse kinetic isotope effect observed for the catalyst [C5Me5(P(OCH3)3)CoCH2CH2-m-H]; in the polymerization of ethylene is reproduced by a B3LYP study223 of the catalytic cycle. A B3LYP study reveals224 that ethylene is easily inserted into the Ni(II)-CH3 bond of Ni(CH3)L]; with a moderate activation barrier with L : HNCH:CHNH, PhNCH:CHNPh,

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H2PCH2CH2PH2, and Me2PCH2CH2PMe2 but with a larger activation barrier with L : 2,2-bipyridine as the coordinate bond of the latter is less flexible. Coordination of ethylene with the nickel(II) propyl complex [Ni(CH2CH2CH3)L]; involves a b-H agostic interaction which is weakest for L : Me2PCH2CH2PMe2 so allowing this system to propagate the reaction through coordination of further ethylene. A B3LYP study225 of the reaction of alkynes with [RuCp(PR3)(CH3CN)2]PF6 (R : Me, Ph, Cy) leading to allyl carbene or butadienyl carbene complexes suggests that metallacyclopentatriene and vinylidene complexes, respectively, are crucial intermediates. The effects of Me, tBu, OMe, and F substitutions on the insertion aptitude and regioselectivity of propene with a cationic phenylpalladium(II)diimine catalyst have been studied at the B3LYP level226. There is a correlation for symmetrical substitutions between the stability of the catalyst HOMO and the insertion barrier whilst for unsymmetrical substitutions of F and OMe, trans-influences led to relatively large differences in insertion aptitudes. Steric effects are notable when tBu replaced hydrogen at the nitrogen positions, primarily through the interaction between t-Bu and alkene methyl groups. The asymmetric induction observed in palladium-catalysed allylic alkylation reactions with typical substrates has been investigated at the BP86 level227 using three structurally related chiral PN ligands. The stereochemical outcome is determined by a delicate balance of steric repulsions and not by electronic effects. The polymerization of ethene and propene by a dicyclopentadienyl yttrium hydride catalyst involves the overall reactions Cp2YH ; 2C2H4 ; Cp2Y-C4H9 and Cp2YH ; 2C3H6 ; Cp2Y-C6H13 and proceeds via formation of a weakly bound encounter complex between the monomer-catalyst and the olefin, followed by the insertion of the ethene or propene into a Y-C bond and finally the formation of a Cp2Y-C4H9 or Cp2Y-C6H13 species, according to a B3LYP study228. The semi-empirical PM3 method229 has been used to study the transition state of the olefin insertion process of metallocene catalysts leads to results which are in good agreement with those obtained at higher levels of theory. The mechanism of the enantioselective alkylation of aldehydes with diethylzinc using has been studied at the B3LYP level230. The mechanism of copolymerization of CO2 with cyclohexene oxide catalysed by the Zn(II) organometallic compound (BDI)ZnOCH3 has been studied using the ONIOM approach231. Insertion of CO2 into either a zinc-alkoxyl or zinc-carbonate bond is thermodynamically less favourable but kinetically favoured over the insertion of epoxide, due to a high barrier for the latter. The high barrier is associated with a rather asynchronous transition state where the ring opening has taken place and yet the C-O bond is not formed. The nature of the monomer insertion step in the allylnickel(II)-catalysed 1,4polymerization of 1,3-butadiene has been investigated in a BP86 study232. Polymerization by the trans-1,4 regulating cationic allylnickel(II) [RC3H4Ni(C4H6)L]; catalyst proceeds via cis-butadiene insertion into either the h1-s-butenyl-Ni bond or the h3-p-butenyl-Ni bond and anti-syn isomerization. B3LYP calculations233 on [Cr(NR)2C3H7(C2H4)]; (R : H, tBu) reveal a facile reductive elimination reaction involving b-hydrogen transfer from the alkyl chain, suggesting

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that the active species in ethylene polymerisation with bis(imido)chromium(VI) precursors contains a reduced chromium atom. 3.2.2 C-X Activation Reactions. The reaction pathways for the reactions of Ni and CH4 leading to NiCH2 ; H2, NiCH3 ; H and NiH ; CH3 have been studied at the B3LYP level234. Reaction of platinum oxide with methane at elevated temperatures leads to direct abstraction of a hydrogen atom leading to PtOH and free methyl radical, reveals a B3LYP study235. B3LYP studies236 on palladium and platinum alkoxide complexes containing bidentate ligands, L2MX(OCY2H) (L2 : CH2NCHCHNCH2 and PH2CH2CH2PH; M : Pd and Pt; Y : H and F) show that Pd complexes with X : OCH3, NH2, OH and HCOO favour s-bond metathesis followed by a reductive elimination leading to the metal-hydride products while complexes with X : CH3 favour b-hydrogen elimination. Both reaction pathways are possible for complexes with ligands X : Cl and Br when M : Pd and Y : H. Pt complexes have higher reaction barriers. B3LYP studies237 of palladium insertion into alkyne and aryl carbonhalogen bonds suggest that it can proceed via a concerted oxidative addition across the carbon-halogen bond. A stepwise mechanism via a s-complex is favoured when a nitro group is introduced onto the alkyne. The C-H bond dissociation reactions of methane and ethane by the bare FeO; complex and diiron and dicopper models of methane monooxygenase, studied at the B3YLP level238, can proceed via pathways: oxene insertion in which a C-H bond is dissociated and C-O and O-H bonds are formed in a concerted manner via a three-centred transition state (CHO)-H- . . . -O- . . . -Fe, direct abstraction mechanism in which a linear transition state (CHO)-H- . . . -O- . . . -Fe leads to the dissociation into an Fe-OH intermediate and an alkyl radical species and a four-centred transition state (CHO)-H- . . . -O- . . . -Fe in its initial stages which leads to a reaction intermediate involving OH and CH3 ligands. The parallel and perpendicular approach of the incoming substrate in the activation mechanism of the C-X (X : Sn, Ge, Si, C, H) s-bonds of HC∫CR (R : SnH3, GeH3, SiH3, CH3, H) has been examined at the B3LYP level239 using the model complexes (PH3)2M (M : Ni, Pd, Pt). For X : Ge, Si, C, H, the C-X s-bonds approach the Pd parallel to the P-Pd-P plane and are activated in the P-Pd-P plane. In contrast, the highly polarized C-Sn s-bond, the C-Sn s-bond approaches the Pd perpendicularly to the P-Pd-P plane. The reactions of Ti;(4F, 2F), V;(5D, 3F), and Cr;(6S, 4D) with NH3 and CH4, studied using DFT240, proceed via formation of the high-spin ion-dipole complex, followed by a hydrogen shift process leading to the formation of the insertion products, which are more stable in a low-spin state. B3PW91 calculations241 of the potential energy and free energy of the C-F cleavage reaction of OsH3ClL2 (L : PPr3) with vinyl fluoride to produce OsHFCl(:CCH3)L2 and H2 proceeds via OsHCl(H2)(H2C:CHF)L2. The unimolecular C-F cleavage of the coordinated C2H3F has a high activation energy. The dehydrogenation reaction of [m3-2,6-(R2PCH2)2C6H3]IrH2 ‘pincer’ complexes may proceed via classical Ir(V) and nonclassical Ir(III)(h2)-H2) intermediates, according to B3LYP calculations242, with a a dissociative pathway involving initial loss of H2, followed by C-H addition kinetically favoured. ONIOM

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23

calculations243 on RuHCl[(Bu2PCH2CH2)-tBu((E)-CH:CH)CH2PtBu2], RuCl(CO)[CHRuHCl(Py)[(Bu2PCH2-CH2)-tBu-((E)-CH:CH)CH2PBu2], (C2H4PtBu2)2] and RuH(CO)[CH(C2H4PBu2)2] have been used to show that the bulky PtBu2 groups are responsible for the electronically unfavorable cis arrangement of the CO and Cl and to locate the transition state structures for the intramolecular olefin insertion into the Ru-H bond. and Hydrogen scrambling in [CpOs(PH2CH2PH2)(CH3)H*] [Cp*Os(PMe2CH2PMe2)(CH3)H]; takes place via the H-exchange mechanism, reveals a B3YLP and B3LYP:HF ONIOM study244. The first step of this reaction is the C-H* bond formation that takes place via a three-centered transition state. After formation of the methane complex, it isomerises and the reverse process occurs, via methane C-H bond activation. Methane loss from TpPtMe(H)2 (Tp : hydrido-tris(pyrazolyl)borate), studied at the mPW1k level245, cannot occur due to the rigidity of the Tp ligand, which does not allow the required trans geometry. H/D scrambling of the methyl ligand is, however, relatively facile and proceeds through a h2-CH-CH4 complex. Hydrogenation of the double bond in [CsCl(H2)(PPh3)
Ph2P(CH2)2CH:CH(CH2)2PPh2]OTf is predicted to be thermodynamically feasible but kinetically unfavourable by B3LYP calculations246. mer,trans-[(PMe3)3Rh(-C∫C-R)2H] from [(PMe3)4Rh(Me)] and terminal alkyne involves initial elimination of methane and the formation of the trigonal bipyramidal complex [(PMe3)4Rh(-C∫C-R)], a BP86 study reveals247. This intermediate undergoes an oxidative addition reaction with a second equivalent of alkyne to give fac-[(PMe3)3Rh(-C∫C-R)2H] as the kinetic product. The oxidative addition of a Pt(0) fragment, Pt(PH3)2, to one of the rim C-C bonds of semibuckminsterfullerene (C30H12), to give an h2-s-bonded Pt(II)-buckybowl complex has been compared at the BP86 level248 to oxidative addition to the C-C bond in five-membered rings of less strained hydrocarbons to study the effect of ring strain and curvature . The bowl-like curvature of semi-buckminsterfullerene plays an essential role in the C-C bond-breaking reaction, while no attainment of aromaticity is involved in the process. The dirhodium tetracarboxylate-catalysed C-H bond activation/C-C bond formation reaction of a diazo compound with an alkane, studied at the B3LYP level249, is initiated by complexation between of the diazo compound to the rhodium catalyst with N2 extrusion to give a carbine complex driven by back-donation from the Rh 4dxz orbital to the C-N s*-orbital. The C-H activation/C-C formation proceeds in a single step through a threecentered hydride transfer-like transition state with a small activation energy. The dehydrogenation of cyclohexadiene and 9,10-dihydroanthracene by trans(DMPE)2Ru(H)(NH2) to yield benzene (or anthracene), (DMPE)(2)Ru(H)(2), and ammonia has been studied using B3LYP calculations250 suggesting that the NH2 group us exceptionally basic and prefers to extract a proton from C-H rather than a H atom. Oxidation of the hydrocarbons methane, ethane, methyl fluoride, and ethylene catalysed by cis-(H2O)(NH2)Fe(m-O)2(h2-HCOO)2Fe(NH2)(H2O) and cis-(HCOO)(Imd)Fe(m-O)2(h2-HCOO)2Fe(Imd)(HCOO) start by coordination of the substrate molecule to the bridging oxygen atom followed by the H-atom abstraction, according to B3LYP calculations251. The selectivity of C-H activation of R-H by Ti(OR)2(:NR) is determined by the structure and ener-

24

Organometallic Chemistry

getics of the substrate/Ti-imido interaction, according to B3LYP calculations252. The hydride/h2-Si-H) hydrogen exchange observed in the bis(silane) complexes RuH2[(h2-HSiMe2)2(CH2)2](PCY3)2 and RuH2[h2- HSiPh2)2O](PCY3)2 proceed through formation of isomers with dihydrogen ligands, according to NMR and B3LYP studies253. A B3LYP study254 of methyl-Pd heterocyclic silylene and germylene complexes reveals a very low activation barrier for methyl migration to the silylene or germylene ligand, but indicates that, in the absence of solvent or counterion effects, reductive elimination of silicenium or germacenium cations is less likely. A B3LYP, B3PW91 and MP2 study255 reveals that Rh-catalysed alkane borylation proceeds via formation of a reactive boryl intermediate followed by C-H bond activation and B-C bond formation. Rh-catalysed hydrosilylation of ethylene, studied at the B3LYP, MP4-(SDQ) and CCSD(T) levels256, proceeds with the rate-determining step in the Chalk-Harrod mechanism being Si-C reductive elimination. The apical site of the palladium complex on elementary reactions acts to significantly lower the energy barrier for the oxidative addition of the C-X (X : Sn, Ge, Si, C) bonds of heteroles to the Pd of (H2PC2H4PH2)Pd and the insertion of XH2 into the Pd-C bond of (H2PC2H4PH2)Pd(h2-HC:CH), according to a B3LYP study257. The catalysis by bis(alkynyl)(1,5-cyclooctadiene)platinum complexes of the cross-linking of polyorganosiloxanes containing Si-H and vinyl groups could proceed via two possible mechanisms, according to a BP86 study258. The first one involves a sequence of four oxidative additions and reductive eliminations, while the second one requires a reductive coupling that is induced by olefin coordination. In both cases, the initial step is rate-determining. A B3LYP study259 of the decomposition of a novel palladiummethyl complex of the rigid CNC ligand 2,6-bis(1-alkylimidazolin-2-yliden-3yl)pyridine predicts that reductive elimination to give 2-methylimidazolium species is a facile reaction. The mechanism for the activation of the O-H bond in H2O, the C-H of CH4, and the H-H of H2, and the p bonds in C2H2, C2H4, and H is significantly affected by the electronic nature of the Pd:X bond, according to B3LYP calculations260. Activation of O-H is heterolytic when X : Sn or Si but homolytic for X : C whereas activation of C-H and H-H bonds is heterolytic for all X. Activation of CC double and triple bonds is homolytic for X : Sn whereas activation of the C:O p can proceed via homolytic or heterolytic pathways. The rate determining step in the Rh(I)-catalysed intramolecular coupling of an alkenyl group to a C-H bond of a substituted benzimidazole involves insertion of the alkenyl double bond into the rhodium-carbene bond, according to B3LYP calculations261. The conversion of methane to methyl bisulfate using Pt(NH3)2Cl2 and Pt(bpym)Cl2 involves a series of steps beginning with C-H activation to form an intermediate ion-pair Pt(II)-CH4 methane complex prior to forming a Pt(II)-CH3 complex, a B3LYP study reveals262. The oxidation step is rate determining and is more favourable for the ammine catalyst suggesting higher activity than the bipyrimidine catalyst. The three step zirconium-catalysed oxidation of amines in the presence of hydroperoxides to give the corresponding nitro compounds proceeds via N-

1: Theoretical Organometallic Chemistry

25

oxides, hydroxylamines and nitroso intermediates, according to a B3LYP study263. BP86 and Car-Parrinello simulations264 have been used to study the stepwise migratory insertion of methyl isocyanide into the zirconium-carbon bonds in [calix[4](OMe)2(O)2-ZrMe2] showing that methyl isocyanide insertion takes place via the initial formation of an h1-iminoacyl species that is suddenly converted into the more stable h2-isomer. Insertion of the residual alkyl group into the iminoacyl moiety leading to an h2-bound imine is then kinetically favoured. A similar approach has also been used265 to study the acetylene to vinylidene isomerization in (Cp)(CO)2Mn(HC∫CH). The direct 1,2 hydrogen shift, proceeding via an agostic intermediate, is predicted to be the energetically most favourable path. The same methodology266 has been applied to migratory insertion using the cationic Ni(II) [(dppp)Ni(CH3)(CO)]; complex which takes place via a five-coordinate complex is thermodynamically and kinetically favoured and methyl attack on the resting carbonyl group. The alkyl group migration reaction CH3Co(CO)4 ; CH3(CO)Co(CO)3, studied at the B3LYP level267, involves two stable 16-electron acyl intermediates with the carbons of the acyl groups in the axial position. One of the intermediates is stabilized by the formation of an agostic interaction to the formally vacant site of the trigonal bipyramid, and the other is stabilized by the acyl oxygen adopting an h2 coordination geometry. The carbonyl association reaction CH3(CO)Co(CO)3 ; CO ; CH3(CO)Co(CO)4 proceeds via methyl migration. Insertion into the C(aryl)-S and not the C(vinyl)-S bond of (h6-benzothiophene)Mn(CO)3; is favoured and h1-S coordination of benzothiophene to Mn(CO)49 is viable in an intermediate in the C-S insertion reactions, according to B3LYP calculations268. Insertion reactions of alkynes (RC∫CR: R : H, R: H, Me, CF3, Ph; R : Me, R: Ph; R : CO2H, R: H, Me, CF3, Ph) with the model phosphanickelacycle [NiBr(CH:CHCH2PH2-KCP)(PH3)] proceed via associative processes with 5-coordinate intermediates preferred, a BP86 study has revealed269. B3LYP calculations270 on the C-S cleavage reaction in the thermal degradation of tris(3,5-dimethylpyrazolyl)hydridoboratorhenium(V)(oxo)-(1,2-dithiolate) and -(1,2-monothiodiolate) reveals that substitution of each sulfur raises the predicted activation energy significantly. An MP2 and B3LYP study271 on the tautomers [Ru
:C:C:C(H)CH3(h5C5H5)(PH3)2]; and [Ru
:C:C(H)CH:CH2(h5-C5H5)(PH3)2]; predicts that the latter is only slightly more stable than the former and that the spontaneous tautomerization process between both complexes involves a [1,3]-hydrogen sigmatropic rearrangement. A B3LYP study272 of the reaction of a 1,3-diphosphacyclobutane -2,4-diyl-2-ide with chromium or tungsten hexacarbonyl suggests that formation of the anionic complexes [cyclo[P(Mes*)-C(SiMe3)P(Mes*)-C(O)- C[M(CO)5]]9 proceeds by the formal insertion of CO into the four membered ring via two intermediates that can be formulated as a cyclic metal acyl and an acyclic ketenyl complex. 3.2.3 Addition Reactions. The oxidative addition of SiH4 to a platinum-diphosphine complex, Pt(PH3)2 has been studied at the BPW91 and dynamic CarParrinello levels273 indicating that the oxidative addition is both thermodynami-

26

Organometallic Chemistry

cally and kinetically favoured and takes place via a reactant-like transition state leading directly to the more thermodynamically stable cis-[PtH(PH3)2-SiH3] square-planar product. The displacement of the MeB(C6F5)39 anion from seven different zirconocene methyl cations by neutral Lewis bases, such as dimethylaniline, benzyldimethylamine, and dinbutyl ether, studied at the BP86 level274, proceeds by way of an associative mechanism involving a five-coordinated intermediates with the Lewis base coordinated to the central coordination site. SiH4 activation by Cp2LnH complexes has been studied at the B3PW91 level of theory275 and compared to the reactivity of CH4. Formation of Cp2Ln(SiH3) can occur via H/H exchange and silylation and both occur via a single-step s-bond metathesis mechanism. Both pathways are therefore thermally accessible with the H/H exchange path more kinetically favourable and the silylation reaction thermodynamically preferred. The considerably lower activation barrier for silylation relative to methylation is attributed to the ability of Si to become hypervalent. The silyl migration reaction of ORe[N(SiMe2CH2PCy2)2] to give NRe[O(SiMe2CH2PCy2)2] is driven by greater thermodynamic stability of the latter according to DFT calculations276. The bis-silylation reaction of alkynes by palladium complexes, modelled using Pd(PH3)2, studied at the B3LYP level277, proceed by oxidative addition involving H3Si-SiH3 to the catalyst leading to cis-(SiH3)2Pd(PH3)2 and transfer of the two silyl groups to the alkyne C-C triple bond in a many step process involving the formation of two C-Si bonds. The diastereoselectivity of the oxyfunctionalization in the epoxidation of stereolabeled methyl-substituted chiral allylic alcohols, studied at the B3LYP level278, is due to the competition between allylic strain and the electronic advantage for the spiro transition state. The most favourable pathway for the hydroboration reaction of olefins catalysed by Cp2Ti(HBcat) involves coordination of an olefin to an intermediate with a five-membered ring with a Ti-H-B bridging unit, according to a B3LYP study279. B-H bond cleavage then occurs to allow the reductive elimination to form alkylboronate ester as the main product. An isolated Au10 cluster should be able to catalyse the CO oxidation reaction even below room temperature according to DFT calculations280 due to special reaction geometries available for small particles in combination with an enhanced ability of low coordinated gold atoms to interact with molecules from the surroundings. The rhodium(III)-catalysed hydrogenation of carbon dioxide into formic acid has been studied at the B3LYP level281 using cis-[RhH2(PH3)3]; and cis[RhH2(PH3)2(H2O)]; model catalysts. The reaction proceeds by initial CO2 insertion into the Rh(III)-H bond followed by either by isomerization of [RhH(h1-OCOH)(PH3)2(H2O)2];, five-centered H-OCOH reductive elimination and oxidative addition of H2 to [Rh(PH3)2(H2O)2]; or through isomerization of [RhH(h1)-OCOH)(PH3)2(H2O)(H2)]; and six-centered s-bond metathesis of [RhH(h1-OCOH)(PH3)2(H2O)]; with H2. A B3LYP study282 reveals that hydrogen transfer from the dihydrogen to the norbornadiene ligand of [Ru(H2)(norbornadiene)(2,6-(Ph2PCH2)2C6H3)]; proceeds through a stepwise mechanism. A B3PW91 study283 of ether dehydrogenation by [RuHCl(PiPr3)2]2 suggests that

1: Theoretical Organometallic Chemistry

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coordination of the removed H2 is essential for reaction thermodynamics. The regioselectivity of nucleophilic attack in silyl-substituted (diphosphino)(h3-allyl)palladium cations has been studied at the B3LYP level284. For unsymmetrical allyl ligands the shortest Pd-C(terminal) bond is the one corresponding to the carbon atom directly bonded to silicon, electronic factors being determinant. When the silicon group is the bulkiest one, both steric and electronic factors favour the attack at the carbon atom remote from silicon (gcarbon). When the silicon substituent is the least sterically demanding, the attack at carbon bonded to silicon (a-carbon) is sterically more favorable, but electronic effects favor the attack at the g-carbon. The Pt(PPh3)2-catalysed reaction between disilacyclobutene and acetylene proceeds via oxidative addition of the Si-Si bond of disilacyclobutene to Pt, release of one phosphine ligand, coordination of acetylene to form a p-complex, migratory insertion of acetylene into a Pt-Si bond leading to an Si-C bond, coordination of acetylene, and elimination of product disilacyclohexadiene, a B3LYP study reveals285. The rate-determining step is the insertion of acetylene into a Pt-Si bond. B3LYP calculations286 suggest that reaction of Mo(PMe3)6 with 2,6Ph2C6H3OH to give the h6-arene complex [h6-C6H5C6H3(Ph)OH]Mo(PMe3)3 proceeds via oxidative addition of the O-H bond to give a non-classical Mo . . . H-OAr hydrogen bond. A B3LYP study287 of the reductive elimination of methane from cis, trans-Os(H)2(CH3)(NO)L2 (L : PiPr3) with and without LiNMe2 show the experimentally observed rate acceleration is due to preferential stabilization of the oxidative addition transition state, due to the greater back donation to NO possible because of the relief of the filled-filled repulsion between the occupied Os d and C-H s-orbitals by the Li electrophile on NO oxygen. 3.2.4 Cyclization and Metathesis Reactions. BP86 calculations288 have been used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH3)(L)Cl2Ru:CH2] (L : PH3 and imidazol-2-ylidene). Initial addition of ethene is rate-determining on the associative pathway but ring closure to yield a ruthenacyclobutane, or its reverse, is rate determining on the dissociative pathway. Pathways for the reaction of ethene with diazomethane to produce cyclopropane and dinitrogen catalysed by Pd(0) complexes have been investigated at the B3LYP level289 leading to an activation energy of 71.7 kJ mol91 for the most favorable catalytic cycle, far lower than previously reported computed barriers for Pd(II)-catalysed pathways of this reaction. Pd(h2-C2H4)2 is predicted to be the resting state of the catalyst, in equilibrium with Pd(h2C2H4)(k-C-CH2N2), from which N2 is eliminated in the rate-determining step 7. The cyclopropanation reactions with ethylene of several mono zinc carbenoids proceed via an asynthchronous attack on one CH2 whilst the corresponding gem-dizinc carbenoids proceed via synchronous attack on both CH2 groups, according to B3LYP calculations290. The charges on the carbon atoms and protons in Hg(C6H6)2(AlCl4)2 and [Hg(C6H6)2(AlCl4)]; at the DFT level291 are significantly higher than in uncomplexed benzene or HgCl2(C6H6)2 so that proto-

Organometallic Chemistry

28

nation of benzene is thermodynamically favoured in comparison to protonation of benzene by HO2CCF3, a known catalyst for arene H/D exchange. The reaction of allene with carbon dioxide using [RhCl(C2H4)(PiPr3)]2 as catalyst has been studied at the B3LYP level292 and leads to a four membered lactone.

7

A comprehensive investigation293 using BP86 calculations and hybrid BP86 / MM calculations on the influence of the ligand L on the regulation of the product selectivity for the [Ni0L]-catalysed cyclodimerization of 1,3-butadiene show that allylic isomerization, allylic enantioface conversion and oxidative coupling are influenced to a minor extent by electronic and steric effects. The steric bulk on the ligand as well as its p-acceptor ability act to facilitate the reductive elimination, while s-donor abilities serve to retard this process. A similarly comprehensive investigation of the mechanism for cyclodimerization of butadiene by the generic [bis(butadiene)(Ni0PH3)] at the BP86 level294. Starting from a pre-established equilibrium between several configurations of the [(octadienediyl)(NiIIL)] complex, the major cyclodimer products are formed along competing reaction paths via reductive elimination, as the overall rate-determining step. BP86 calculations295 have been used to computational screen latetransition-metal catalysts and nitrogen-containing polar monomers toward an incorporation of amines and nitriles in the polymer chain of polyolefins. The calculations reveal the general trend that the activation energies for the ethylene, propylene, and acrylonitrile insertion in the Brookhart systems are similar, whereas the activation energies for the vinylamine insertion are much higher. A similar approach296 has also been used for an initial screening of late-transitionmetal catalysts and nitrogen-containing polar monomers toward an incorporation of amines or nitriles in the polymer chain of polyolefins. Substrates of the type CH2:CH(CH2)nX (X : polar group) can bind either with the N-containing polar group or with the p moiety to the metal centre of the catalyst, with the latter leading to polymer growth. The activity of the catalyst in vinyl-vinyl reductive elimination reaction from bis-s-vinyl complexes [M(CH:CH2)2Xn], studied at the B3LYP level297, decreases in order PdIV, PdII > PtIV, PtII, RhIII > IrIII, RuII, OsII. The activation mechanism of the p bonds, the nonpolar C∫C of C2H2 and the polar C : O of HCHO and the C:N of HCN at the Pd:Sn bond of the model complex (H2PC2H4PH2)-Pd:SnH2 has been studied at the B3LYP level298. For the nonpolar ethyne C∫C and C:N p bond, the reaction proceeds by the homolytic mechanism supported by the rotation of the (H2PC2H4PH2)Pd group around the Pd-Sn axis. For the strongly polarized formaldehyde C:O p bond, the donation of lone pair electron on the C:O oxygen to the Sn p orbital is so strong that the C:O bond is broken by the heterolytic mechanism with the electrophilic attack

1: Theoretical Organometallic Chemistry

29

of the C:O carbon to the Pd atom and rotation is not necessary. The reaction of nickel dithiolene complexes and ethylene, studied at the B3LYP level299, is a two-step process, in which a trans-product forms first which then rearranges to the more thermodynamically stable cis-product. The syntheses of helicenes based on the modular assembly of key cis,cis- dienetriynes and their nickel(0)-catalysed [2;2;2] cycloisomerization strongly favour the intramolecular simultaneous construction of three aromatic rings, according to B3LYP calculations300. BP86 calculations301 on the ruthenium-catalysed olefin metathesis reactions involving a Grubbs-type (PCy3)2-Cl2Ru:CHPh catalyst and a heteroleptic (pre)catalytic system, in which a N-heterocyclic carbene, NHC, ligand substitutes a single phosphine, predict that both the PCy3-Ru binding and the insertion barrier are lower for heteroleptic system. Endo- and exo-cycloisomerizations of 4-pentyn-1-ol using a tungsten pentacarbonyl catalyst, proceeds with a rate-determining Ca-Cb hydride migration step to form a vinylidene intermediate, according to a B3LYP study302, with the primary role of the tungsten catalyst being to the stabilize the vinylidene intermediate. The thermodynamic products of the reaction between ethyne and the model compounds (HE)3M:M(EH)3 (M : Mo and W; E : O and S) are the alkyne adducts M2(m-C2H2)(EH)6 rather than metathesis products HC:M(EH)3 except when M : W and E : O according to a B3LYP study303. The role of the zinc catalyst in the Diels-Alder reaction between cyclopentadiene and 3-phenyl-1-(2-pyridyl)-2-propen-1-one has been studied at the B3LYP level304. The uncatalysed process proceeds via an asynchronous concerted [4;2] reaction but the presence of a Lewis acid catalyst changes the mechanism drastically into a polar stepwise process. The mechanism of cyclopropanation, catalysed by a 3-oxobutylideneaminatocobalt(II) complex, has been analysed at the B3LYP level305. The axial donor ligand on the catalyst reduces the activation energy for formation of the cobalt carbene complex but increases the activation energy for the cyclopropanation step.

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Organometallic Chemistry

N.T. Nguyen and C. Jones, Organometallics, 2002, 21, 3076. 201. S. Grigoleit, A. Alijah, A.B. Rozhenko, R. Streubel and W.W. Schoeller, J. Organomet. Chem., 2002, 643, 223. 202. H.G. Raubenheimer, M.W. Esterhuysen, A. Timoshkin, Y. Chen and G. Frenking, Organometallics, 2002, 21, 3173. 203. M. Niehues, G. Erker, G. Kehr, P. Schwab, R. Frohlich, O. Blacque and H. Berke, Organometallics, 2002, 21, 2905. 204. A. Marrone and N. Re, Organometallics, 2002, 21, 3562. 205. G. Aullon and S. Alvarez, Organometallics, 2002, 21, 2627. 206. J.C. Gordon, G.R. Giesbrecht, D.L. Clark, P.J. Hay, D.W. Keogh, R. Poli, B.L. Scott and J.G. Watkin, Organometallics, 2002, 21, 4726. 207. E.D. Jemmis, A.K. Phukan and K.T. Giju, Organometallics, 2002, 21, 2254. 208. J.M. Tanski and G. Parkin, Organometallics, 2002, 21, 587. 209. D. del Rio and A. Galindo, J. Organomet. Chem., 655, 16. 210. P. Belanzoni, N. Re and A. Sgamellotti, J. Organomet. Chem., 2002, 656, 156. 211. D. Moigno, B. Callejas-Gaspar, J. Gil-Rubio, H. Werner and W. Kiefer, J. Organomet. Chem., 2002, 661, 181. 212. S. Maricic and T. Frejd, J. Org. Chem., 2002, 67, 7600. 213. G.A. Bogdanovic, A.S.D. Bire and S.D. Zaric, Eur. J. Inorg. Chem, 2002, 7, 1599. 214. K.B. Lipkowitz, C.A. DHue, T. Sakamoto and J.N. Stack, J. Am. Chem. Soc., 2002, 124, 14255. 215. J. van Slageren, A. Klein and S. Zalis, Coord. Chem. Rev., 2002, 230, 193. 216. B.H. Kim and C.R.F. Lund, J. Mol. Catal. A — Chem., 2002, 188, 173. 217. M. Buhl, F.T. Mauschick, J. Organomet. Chem., 2002, 646, 126. 218. Z.T. Xu, K. Vanka, T. Firman, A. Michalak, E. Zurek, C.B. Zhu and T. Ziegler, Organometallics, 2002, 21, 2444. 219. E. Zurek and T. Ziegler, Organometallics, 2002, 21, 83. 220. A. Michalak and T. Ziegler, J. Am. Chem. Soc., 2002, 124, 7519. 221. G. Talarico, A.N.J. Blok, T.K. Wo and L. Cavallo, Organometallics, 2002, 21, 4939. 222. S. Bhaduri, S. Mukhopadhyay and S.A. Kulkarni, J. Organomet. Chem., 2002, 654, 132. 223. A. Zeller and T. Strassner, Organometallics, 2002, 21, 4950. 224. T. Tomita, T. Takahama, M. Sugimoto and S. Sakaki, Organometallics, 2002, 21, 4138. 225. E. Ruba, K. Mereiter, R. Schmid, V.N. Sapunov, K. Kirchner, H. Schottenberger, M.J. Calhorda and L.F. Veiros, Chem. Eur. J., 2002, 8, 3948. 226. H. von Schenck, B. Akermark and M. Svensson, Organometallics, 2002, 21, 2248. 227. M. Widhalm, U. Nettekoven, H. Kalchhauser, K. Mereiter, M.J. Calhorda and V. Felix, Organometallics, 2002, 21, 315. 228. N. Sandig and W. Koch, Organometallics, 2002, 21, 1861. 229. M.J. Young, C.C.M. Ma and C. Ting, Russ. J. Coord. Chem., 2002, 28, 25. 230. I. Shiina, K. Konishi and Y. Kuramoto, Chem. Lett., 2002, 31, 164. 231. Z.W. Liu, M. Torrent and K. Morokuma, Organometallics, 2002, 21, 1056. 232. S. Tobisch, Chem. - Eur. J., 2002, 8, 4756. 233. V.R. Jensen and K.J. Borve, Chem. Commun., 2002, 542. 234. H.Q. Yang, Y.Q. Chen, C.W. Hu, M.C. Gong, H.R. Hu, A.M. Tian and N.B. Wong, Chem. Phys. Lett., 2002, 355, 233. 235. D.Y. Hwang and A.M. Mebel, Chem. Phys. Lett., 2002, 365, 140. 236. S.M. Ng, C. Zhao and Z.Y. Lin, J. Organomet. Chem., 2002, 662, 120. 237. M. Jakt, L. Johannissen, H.S. Rzepa, D.A. Widdowson and R. Wilhelm, J. Chem. Soc.-Perkin Trans. 2, 2002, 576.

1: Theoretical Organometallic Chemistry 238. 239. 240. 241. 242. 243. 244. 245. 246. 247. 248. 249. 250. 251. 252. 253.

254. 255. 256. 257. 258. 259. 260. 261. 262. 263. 264. 265. 266. 267. 268.

269. 270. 271. 272.

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K. Yoshizawa, Coord. Chem. Rev., 2002, 226, 251. T. Matsubara and K. Hirao, Organometallics, 2002, 21, 4482. E. Sicilia and N. Russo, J. Am. Chem. Soc., 2002, 124, 1471. G. Ferrando-Miguel, H. Gerard, O. Eisenstein and K.G. Caulton, Inorg. Chem., 2002, 41, 6440. K. Krogh-Jespersen, M. Czerw, N. Summa, K.B. Renkema, P.D. Achord and A.S. Goldman, J. Am. Chem. Soc., 2002, 124, 11404. D.G. Gusev and A.J. Lough, Organometallics, 2002, 21, 5091. R.K. Szilagyi, D.G. Musaev and K. Morokuma, Organometallics, 2002, 21, 555. M.A. Iron, H.C. Lo, J.M.L. Martin and E. Keinan, J. Am. Chem. Soc., 2002, 124, 7041. S.H. Liu, X. Huang, Z.Y. Lin, C.P. Lau and G.C. Jia, Eur. J. Inorg. Chem., 2002, 7, 1697. J.P. Rourke, G. Stringer, P. Chow, R.J. Deeth, D.S. Yufit, J.A.K. Howard and T.B. Marder, Organometallics, 2002, 21, 429. F. Nunzi, A. Sgamellotti and N. Re, Organometallics, 2002, 21, 2219. E. Nakamura, N. Yoshikai and M. Yamanaka, J. Am. Chem. Soc., 2002, 124, 7181. J.R. Fulton, S. Sklenak, M.W. Bouwkamp and R.G. Bergman, J. Am. Chem. Soc., 2002, 124, 4722. D.G. Musaev, H. Basch and K. Morokuma, J. Am. Chem. Soc., 2002, 124, 4135. T.R. Cundari, T.R. Klinckman and P.T. Wolczanski, J. Am. Chem. Soc., 2002, 124, 1481. I. Atheaux, F. Delpech, B. Donnadieu, S. Sabo-Etienne, B. Chaudret, K. Hussein, J.C. Barthelat, T. Braun, S.B. Duckett and R.N. Perutz, Organometallics, 2002, 21, 5347. D.S. McGuinness, B.F. Yates and K.L.J. Cavell, Organometallics, 2002, 21, 5408. X.H. Wan, X.J. Wang, Y. Luo, S. Takami, M. Kubo and A. Miyamoto, Organometallics, 2002, 21, 3703. S. Sakaki, M. Sumimoto, M. Fukuhara, M. Sugimoto, H. Fujimoto and S. Matsuzaki, Organometallics, 2002, 21, 3788. T. Matsubara and K. Hirao, Organometallics, 2002, 21, 2662. M.N. Jagadeesh, W. Thiel, J. Kohler and A. Fehn, Organometallics, 2002, 21, 2076. D.J. Nielsen, A.M. Magill, B.F. Yates, K.J. Cavell, B.W. Skelton and A.H. White, Chem. Commun., 2002, 2500. T. Matsubara and K. Hirao, J. Am. Chem. Soc., 2002, 124, 679. K.L. Tan, R.G. Bergman and J.A. Ellman, J. Am. Chem. Soc., 2002, 124, 3202. J. Kua, X. Xu, R.A. Periana and W.A. Goddard, Organometallics, 2002, 21, 511. W.R. Thiel and K. Krohn, Chem. — Eur. J., 2002, 8, 1049. S. Fantacci, F. De Angelis, A. Sgamellotti and N. Re, Organometallics, 2002, 21, 4090. F. De Angelis, A. Sgamellotti and N. Re, Organometallics, 2002, 21, 2715. F. De Angelis, A. Sgamellotti and N. Re, Organometallics, 2002, 21, 2036. S.K. Goh and D.S. Marynick, Organometallics, 2002, 21, 2262. H.Z. Li, K.Q. Yu, E.J. Watson, K.L. Virkaitis, J.S. DAcchioli, G.B. Carpenter, D.A. Sweigart, P.T. Czech, K.R. Overly and F. Coughlin, Organometallics, 2002, 21, 1262. S.A. Macgregor and E. Wenger, Organometallics, 2002, 21, 1278. K.P. Gable, P. Chuawong and A.F.T. Yokochi, Organometallics, 2002, 21, 929. V. Cadierno, S. Conejero, M.P. Gamasa, J. Gimeno and M.A. Rodriguez, Organometallics, 2002, 21, 203. A. Fuchs, D. Gudat, M. Nieger, O. Schmidt, M. Sebastian, L. Nyulaszi and E.

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273. 274. 275. 276. 277. 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291. 292. 293. 294. 295. 296. 297. 298. 299. 300. 301. 302. 303. 304. 305.

Organometallic Chemistry Niecke, Chem. — Eur. J., 2002, 8, 2188. G. Giorgi, F. De Angelis, N. Re and A. Sgamellotti, Chem. Phys. Lett., 2002, 364, 87. F. Schaper, A. Geyer and H.H. Brintzinger, Organometallics, 2002, 21, 473. L. Perrin, L. Maron and O. Eisenstein, Inorg. Chem., 2002, 41, 4355. O.V. Ozerov, H.F. Gerard, L.A. Watson, J.C. Huffman and K.G. Caulton, Inorg. Chem., 2002, 41, 5615. A. Bottoni, A.P. Higueruelo and G.P. Miscione, J. Am. Chem. Soc., 2002, 124, 5506. M. Cui, W. Adam, J.H. Shen, X.M. Luo, X.J. Tan, K.X. Chen, R.Y. Ji and H.L. Jiang, J. Org. Chem., 2002, 67, 1427. D. Liu and Z.Y. Lin, Organometallics, 2002, 21, 4750. N. Lopez and J.K. Norskov, J. Am. Chem. Soc., 2002, 124, 11262. Y. Musashi and S. Sakaki, J. Am. Chem. Soc., 2002, 124, 7588. S.H. Liu, S.M. Ng, T.B. Wen, Z.Y. Zhou, Z.Y. Lin, C.P. Lau and G. Jia, Organometallics, 2002, 21, 4281. G. Ferrando-Miguel, J.N. Coalter, H. Gerard, J.C. Huffman, O. Eisenstein and K.G. Caulton, New. J. Chem, 2002, 26, 687. V. Branchadell, M. Moreno-Manas and R. Pleixats, Organometallics, 2002, 21, 2407. S.Y. Kang, T. Yamabe, A. Naka, M. Ishikawa and K. Yoshizawa, Organometallics, 2002, 21, 150. T. Hascall, M.H. Baik, B.A. Bridgewater, J.H. Shin, D.G. Churchill, R.A. Friesner and G. Parkin, Chem. Commun., 2002, 2644. D.V. Yandulov and K.G. Caulton, New. J. Chem., 2002, 26, 498. S.E. Vyboishchikov, M. Buhl and W. Thiel, Chem. - Eur. J., 2002, 8, 3962. B.F. Straub, J. Am. Chem. Soc., 2002, 124, 14195. C.Y. Zhao, D.Q. Wang and D.L. Phillips, J. Am. Chem. Soc., 2002, 124, 12903. A.S. Borovik and A.R. Barron, J. Am. Chem. Soc., 2002, 124, 3743. M. Aresta, A. Dibenedetto, I. Papai and G. Schubert, Inorg. Chim. Acta., 2002, 334, 294. S. Tobisch and T. Ziegler, J. Am. Chem. Soc., 2002, 124, 13290. S. Tobisch and T. Ziegler, J. Am. Chem. Soc., 2002, 124, 4881. D.V. Deubel and T. Ziegler, Organometallics, 2002, 21, 4432. D.V. Deubel and T. Ziegler, Organometallics, 2002, 21, 1603. V.P. Ananikov, D.G. Musaev and K. Morokuma, J. Am. Chem. Soc., 2002, 124, 2839. T. Matsubara and K. Hirao, Organometallics, 2002, 21, 1697. Y.B. Fan and M.B. Hall, J. Am. Chem. Soc., 2002, 124, 12076. F. Teply, I.G. Stara, I. Stary, A. Kollarovic, D. Saman, L. Rulisek and P. Fiedler, J. Am. Chem. Soc.(184) 2002, 124, 9175. L. Cavallo, J. Am. Chem. Soc., 2002, 124, 8965. Y.H. Sheng, D.G. Musaev, K.S. Reddy, F.E. McDonald and K. Morokuma, J. Am. Chem. Soc., 2002, 124, 4149. M.H. Chisholm, E.R. Davidson, M. Pink and K.B. Quinlan, Chem. Commun., 2002, 2770. L.R. Domingo, J. Andres and C.N. Alves, Eur. J. Org. Chem., 2002, 15, 2557. T. Ikeno, I . Iwakura, S. Yabushita and T. Yamada, Org. Lett., 2002, 4, 517.

2 Groups 1 and 11: The Alkali and Coinage Metals BY S.R. BOSS AND A.E.H. WHEATLEY

1

Alkali Metals

1.1 Introduction. — As in recent years, Part 1 of this review deals with alkali metals (M;) and is thereafter categorized primarily according to the organic anion component (R9) of organometallics of the type Rn9(M;)n. Except in cases of extreme interest, emphasis will be placed on compounds that contain at least one carbon-alkali metal interaction. An overview of synthetic and mechanistic studies of alkali metallated organics is to be found at the start of each section. Structural probes are then arranged according to the analytical method used. Solid-state investigations (for the most part by single-crystal X-ray diffraction) precede solution and gas phase studies. 1.2 Alkyl Derivatives. — The employment of organolithium substrates in general, and alkyllithium reagents in particular, has been the subject of extensive review in 2002. Principally, this has taken the form of an overview of the role of organolithiums in synthesis by Clayden.1 The conversion of prochiral methylene units into stereogenic centres utilizing lithiation/substitution has also been reviewed of late.2 A survey of the samp-/ramp-hydrazone methodology [samp : (S)-1-amino-2-(methoxymethyl)pyrrolidine] for generating new carbon-carbon or carbon-heteroatom bonds in a regio-, diastereo- and enantioselective way has been published.3 The use of alkyllithium reagents in the preparation of new and synthetically versatile lithium zincates has been considered,4 as has their role in solid-phase synthesis.5 The employment of butyllithium in the preparation of new ferrocenoylsilanes has been reported.6 Similarly, its use has enabled the intercalation of lithium into MoS2, as has been discussed within a wider review on the intercalation chemistry of the latter species.7 Alkyl- (but also aryl-)lithium substrates have been used to effect the synthesis of 6-substituted phenanthridines via an anionic ring closure process.8 Both alkyllithium and amidolithium species have been tested in attempts to utilise Group 1 cations to accelerate C—H reductive elimination and oxidative addition reactions of Os(II)/Os(0) nitrosyl complexes.9 Similarly, a Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 39

Organometallic Chemistry

40

variety of precursors to [(Pri3P)2(H)2Os(NO)]; have been reacted with methylcontaining organolithium substrates. Results suggest cleavage of these organometallic species to generate methyl anions in the presence of an electrophilic osmium dihydride.10 Butyllithium and potassium menthoxide have been sequentially employed in forming the chelating diamide dianion [ArN(CH2)3NAr]29 (Ar : 2,6-Pri2C6H3).11 The more complicated organolithium substrate (Me2NMe2Si)(Me3Si)2CLi (thf)2 has been used in conjunction with Cl2Mn to effect the formation of new manganese complexes incorporating bulky tris(organosilyl)methyl ligands.12 Styrene retarded anionic polymerisation in hydrocarbon media has been studied using bimetallic initiators rather than straightforward alkyllithium ones. Accordingly, lithium alkoxide-dialkylmagnesium adducts have been found to undergo ligand exchange reactions to afford a reactive alkyllithium in situ.13 The solid-state structures of [Ph2(NC5H10CH2)SiCH2Li]2 (NC5H10 : 1piperidinyl) and
Me2(smpCH2)Si[(R)-CH(Li)smp]2
smp : 1-[(S)-2(methoxymethyl)pyrrolidinyl] have both been reported during an investigation into the stereochemically selective transformations of metal alkyls.14 As part of a study into the reactivities of lithiated phosphoraneimine complexes, the pseudocubic tetramer [Me3Si(Et2P)NCHMeLi]4 has been prepared.15 The 2:1 reaction of 2-biphenylamine with (dimethylamino)trimethylstannane has given a distannylamine that undergoes stannazane cleavage with BunLi in MeOBut to yield methanide-stabilized (2-biphenylNLi2)5 (MeLi)2 (OEt2)5 — the alkali metal centres thereafter having been replaced by boryl or silyl groups.16 The cocomplex
(Me2HSi)[(Me2HC)2HSi]NLi2 (BunLi)4 has resulted from the straightforward treatment of (Me2HSi)[(Me2HC)2HSi]NLi with n-butyllithium.17 New lithiated diphenyl(triorganosilyl)methanes have also been reported recently.18 The lithiation of N,N,N-trimethyl-1,4,7-triazacyclononane has yielded a simple dimer while the same study has reported a monomeric complex of benzyllithium.19 The solid-state structure of the lithiated [(benzylsilyl)methyl]amine Me2(smpCH2)Si[(R)-CH(Li)Ph] has recently been obtained. The absolute configuration at the metallated carbon centre of this alkyllithium compound, which is highly diastereomerically enriched in solution, has thus been determined.20 The pmdeta-solvated sodium derivative of diphenyl(trimethylsilyl)methane has been published recently.21 Meanwhile, the simple, polymeric complex [(Me3Si)2CHK thf ]- has been reported.22 A variety of alkyllithium species have been probed using DFT methods in conjunction with high-resolution X-ray and neutron diffraction techniques to facilitate a topological analysis of charge density and electronic structure of the organic anion. Recent work has established that the Ca lone pair is delocalized over Ca and Cb. This allows the reinterpretation of agostic stabilization of lithium, with the close approach of Cb to the metal being explained in terms of this delocalization.23 The solution structures adopted by adducts between 6Li-enriched n-butyllithium and various chelating diamines (L) have been probed by NMR spectroscopy. Results suggest dimers of the type (RLi L)2 form, with the employment of two different diamines (L and L) yielding both homosolvates (RLi L)2 and













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41

(RLi L)2 and also the heterosolvated dimer (RLi L)(RLi L). The use of multiple solvating agents has also allowed the elucidation of relative binding constants and free energies for sequential solvent substitution.24 The ability of lithium alkoxide-dialkylmagnesium adducts to undergo ligand exchange reactions to afford an alkyllithium has already been discussed in a synthetic context.13 This phenomenon has been recently probed using 6Li,1H HOESY.25 Data reinforce the view that the organometallic derivative of the more electropositive metal will form in situ.





1.3 Alkenyl, Allyl, Vinyl, Alkynyl and Related Derivatives. — The preparation and configurational stability of enantioenriched a-thioallyllithium species has been investigated, with emphasis being placed on understanding the stereochemistry of their electrophilic substitution chemistry.26 Meanwhile, alkynyllithium substrates have been employed in the preparation of new cationic rhenium tricarbonyl bipyridine complexes.27 They have also been used in conjunction with the [1]ferrocenophanes Cl(Me)Si(h-C5H4)2Fe and Cl2Si(h-C5H4)2Fe to yield RC∫C(Me)Si(h-C5H4)2Fe and (RC∫C)2Si(h-C5H4)2Fe (R : Bun, Ph), respectively. These last two species have been shown to undergo transition metal catalysed ring-opening polymer formation to yield polyferrocenylsilanes that bear acetylenic groups.28 The (dimethylamino)ethynylcarbene complexes (OC)5M:C(NMe2)C∫CLi (M : W, Mo, Cr) have been treated with (OC)6M and thence with F3CSO3Me to give homobinuclear complexes that incorporate an aminocarbene-cyclopropenylidene bridging ligand.29 Lithium salts derived from (1R,2R)-1,2-diaminocyclohexane have been prepared. The treatment of this diamine with BunLi has given both (1R,2R)-(9)-1(Me3SiNLi)-2-(Me3SiNH)C6H10 and [(1R,2R)-(9)-1,2-(Me3SiN)2C6H10]Li2. These have each been reacted with PhCN to give (1R,2R)-(9)-1[Me3SiNC(Ph)N]Li-2-(Me3SiNH)C6H10wherein the amidinate moiety acts as an h3-donor.30 In the context of allylic systems, meso-1,3-diphenylallyllithium-[(9)sparteine] has been characterized.31 Two dilithium salts of (S)-N-(a-methylbenzyl)allylamine have been reported lately. Whereas dilithiation of the chiral amine in the absence of a coordinating Lewis base has been shown to give the cyclic hexamer [(S)-a-(PhCHMe)(CH2CH:CHLi)NLi]6, the addition of tmeda has led to deaggregation and the formation of dimeric [(S)-a(PhCHMe)(CH2CH:CHLi)NLi (tmeda)2]2.32 The ability of the a,adiiminopyridine ligand system to form stable trianions has been demonstrated by the recent structural elucidation of ([a-(2,6-Pri2C6H3N:CMe)]
a-[2,6Pri2C6H3N-C(:CH2)]C5H3N)(Li thf)2Li (thf )2.33 Moreover, sequential reaction of the amine substrate with BunLi and BunNa in thf has given [(S)-a(PhCHMe)(CH2CH:CHNa)NLi thf ]4.32 In each of these structures the alkali metals are supported by h2-allyl bonding. Similar stabilization of the metal centres has been noted in the b-diketiminatolithium species obtained lately by the reaction of bis(trimethylsilyl)methyllithium and 1-azaallyllithium reagents with cyanoamines.34 The phosphaalkenyl-substituted h3-azaallyllithium complex Me3Si(But)C: PC(SiMe3)(6-MeC5H3N-2)Li tmeda has been reported.35 Meanwhile, 2-lithio-1









Organometallic Chemistry

42

methyl-1,3-benzaphosphole has lately been shown to adopt dimeric aggregation in the solid state and the reactivity of the species has been probed in an attempt to take advantage of its lack of steric congestion at the carbanion centre.36 Reaction of the mixed halogenated bis(methylene)phosphorane (Me3Si)2C:P(Mes*):C(F)Cl (Mes* : 2,4,6-But3C6H2) with BunLi in thf has yielded the Z-configured carbenoid (Me3Si)2C:P(Mes*):C(F)Li (thf)3.37 The N-delocalized phosphazenolithium dimer [O· · ·C(OMe)· · ·N· · ·P(Ph)2CHMeLi thf ]2 has been prepared and fully characterized.38 Triboracyclopropanates have been shown to incorporate very short B-B distances in, for example, the contact ion triple (Me3SiCH)5CHB4SiMe3(Li thf)Li thp (thp : tetrahydropyrane).39 Meanwhile, the sodium salt of nido-2,3-(Me3Si)-2,3-C2B4H6 has been alkylated to yield nido-5-Bui-2,3(Me3Si)-2,3-C2B4H5, with subsequent deprotonation affording the crystalline material 5-Bui-2,3-(Me3Si)-2,3-C2B4H4-1-Na thf.40 In the context of alkali metal aluminate chemistry, the interaction of lithium, sodium and potassium with ethynyl fragments has been studied. Reaction of the tacn-derivative 1,4-diisopropyl-1,4,7-triazacyclononane with H4AlLi and terminal acetylenes in the presence of thf has afforded monomeric trialkynylaluminates in which one acetylide ligand uses its Ca-centre to bridge between metals.41 Meanwhile, treatment of [2,6-Pri2C6H3N(SiMe3)AlCl2]2 with the corresponding phenylethynyl alkali metal substrate in either dioxane or thf has afforded dioxane-bridged [2,6-Pri2C6H3N(SiMe3)Al(C∫CPh)3Li dioxane]2(m-dioxane) and also [2,6-Pri2C6H3N(SiMe3)Al(C∫CPh)3M thf ]2 (M : Na, K).42 Reaction of the complex Cp*Hf (C6H10)Cl thf (C6H10 : 2,3-dimethyl-1,3-butadiene) with excess (Me3Si)2CHLi in thf has resulted in deprotonation of an SiMe group to yield Cp*Hf (C6H10)CH(SiMe3)SiMe2CH2Li thf. This product contains a 1-hafna-3silacyclobutane ring.43 The alkali metal ion-binding properties of (Me3tacn)Cr(C∫CH)3 have been probed lately with both
[(Me3tacn)Cr(C∫CH)3]2K;F3CSO39 and
[(Me3tacn)Cr(C∫CH)3]2Cs;Br9 having been structurally elucidated.44 The reduction of (2,6-Pri2C6H3N:CH)GaI2 by elemental potassium in either OEt2 or tmeda (: L) has yielded dimers of (2,6-Pri2C6H3N:CH)GaK L.45



















1.4 Aryl Derivatives. — Directed aromatic lithiation has been employed in order to generate a variety of fluoro- and trifluoromethyl-substituted pyrazoles46 and benzyl and phenethyl alcohols.47 1-Aminoboratabenzene complexes of Group 3 metals have been reported — lithium boratabenzenes representing the best source of boratabenzene ligands.48 Moreover, PhLi has been employed in conjunction with Cl5Mo or Cl6W to achieve the anionic polymerisation of methyl methacrylate.49 As in previous years, the solid-state structures of various lithiated terphenyls have been reported50,51 while the chemistry of the cyclobutadienyl dianion has also been expanded. In the context of the latter field, both 1,2-diphenyl-3,4bis(trimethylsilyl)cyclobutadiene dilithium and 1,3-diphenyl-2,4-bis(trimethylsilyl)cyclobutadiene dilithium have been prepared by reacting the corresponding cyclobutadiene cobalt complexes with elemental lithium. Their crystal structures

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have been obtained as dme-solvates and reveal that in each case the metal ions sit above and below the cyclobutadiene plane.52 A study into the reactivities of new hydrogen-bonded borane-ammonia complexes has led to the preparation of [2-MeOCH2OC6H4Li thf ]2.53 Arene-lithium p-interactions have been noted in triarylborane adducts of 2-lithio-N-methylimidazole.54 In the same way as last year, lithium, sodium and potassium salts containing carbons-adjacent nido- and arachno-C2B10 carborane anions have been prepared.55 The p-stabilization of both sodium56,57 and potassium58 by the aryl-terminated side arms of so-called lariat ethers has been reported. Internal aryl-support of potassium has been noted in the amidinate [MesNC(H)NMes] K MesN(H)C(H)NMes.59 Similar p-support of the metal has been noted in polymeric [(Ph2P)2NK (thf)n]- (n : 1.25, 1.5). These species, obtained by reacting (Ph2P)2NH with HK in thf, represent convenient precursors to new homoleptic lanthanides with chelating bis(phosphanyl)amide ligands. These are of interest vis-a`-vis the ring-opening polymerisation of lactones.60 Lastly, new caesium ion complexes of unsubstituted calix[4]arenes have revealed internal arene stabilization of the metals.61 Concerning heterobimetallic species, the debate about gallium-gallium triple bonds continued in 2002, with the isolation and full characterization of both a neutral ‘digallene’ and its reduced sodiated derivative failing to support the triple bond argument.62 Toluene molecules h6-stabilise potassium in Ga[Co2(CO)6(mCO)]2K (PhMe)2 — the heterometallic product of reaction between Cp*GaCl2 and Co(CO)4K.63 Moreover, reaction of I2Yb (thf)2 with diphenyldipyrrolylmethanide has yielded the macrocyclic octamer of diphenyldipyrrolylmethanediylytterbium. This reaction has also afforded both monomeric Yb(diphenyldipyrrolylmethanediyl)3(K thf)3 and oxide-encapsulation cluster [Yb(diphenyldipyrrolylmethanediyl)]4[K (thf)3]2(m4-O).64 Potassiated dipyrrolide dianions have been generated by fragmentation of an (Et8-calix[4]tetrapyrrole)(K dme)4 ligand in the presence of I2Tm (dme)3.65 Mixed-metal K-U systems have also been studied. Hence, [(Et8-calix[4]tetrapyrrole)U dme]K dme has been reacted with potassium naphthalenide under Ar and N2 with contrasting results. Whereas the employment of an argon atmosphere incurs the formation of a mixed-valence, dinuclear U(III)/U(IV) potassiate along with a silicon greasecontaminated by-product, the use of a nitrogen environment results in N2cleavage and deposition of a dinuclear m-nitrido U(IV)/U(V) mixed-valence potassiate.66 Diuranium inverted sandwich compounds incorporating naphthalene and cyclooctatetraene have been prepared and fully characterized. I3U (dme)2 has been treated with Mes(But)CNK in dme to give [Mes(But)CN] IU dme which has, in turn, been reacted with C8K and naphthalene to afford
[Mes(But)CN]3U2(m-h6,h6-C10H8)K2.67 Reaction of [(Me3Si)2N]2Sn with (Me3Si)3SiCs has yielded
PhCH2[(Me3Si)3Si]2SnCs- wherein the benzyl groups facilitate polymer formation by acting as h6:h6 bridges between alkali metal ions.68 Structures and dissociation energies of the sandwich complexes (C6H6)n;1Lin (n : 1-6) have been studied using the G3(MP2) and B3LYP/6-31G(d) levels of theory. In either instance, large dissociation energies have been noted along with

























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short inter-benzene distances.69 1.5 Cyclopentadienyl and Related Derivatives. — The introduction of excess cyclopentadienyllithium to I2Yb has been shown to facilitate formation of a new ytterbium(II) substituted indenyl complex that reveals high activity with respect to ring-opening lactone polymerisation.70 In a similar vein, the potassium salt of tert-butyl(dimethyl)silylcyclopentadienyl has been reacted with lanthanide trichlorides to give the corresponding monosubstituted dichlorides.71 The w-alkenyl functionalized species 1-(but-3-enyl)-2,3,4,5-tetramethylcyclopentadienyllithium, -sodium and -potassium have all been used as precursors to organometallic compounds of the lanthanides.72 As part of an investigation into lithium and potassium complexes of phosphanyl-substituted bis[1,3-bis(trimethylsilyl)cyclopentadienyl]yttriates,ion-separated
[(Me3Si)2C5H3]2Li9[Li (thf)4]; has been fully characterized.73 Just as a simple thf-solvated fluorenyllithium complex has lately been elucidated,74 so a 9,9-dilithiofluorene has been reported.75 Ion-bonded CpNa (10crown-5), C9H7Na (10-crown-5) and C13H9Na (10-crown-5) (C9H7 : indenyl, C13H9 : fluorenyl) have all been shown to incorporate h5-stabilization of the metal.76 Meanwhile, ion-separated Cp2Na9[Na (12-crown-6)]; has been crystallized.77 Simple, monomeric 12-crown-6 adducts of CpNa and MeC5H4Na have been reported,78 as has the polymer of solvate m:h5-CpNa dme.78 As part of a study into the formation of cyclodimeric Group 4 Ziegler-Natta catalysts, the polymeric structure of [MeC(O)C5H4Na thf ]- has been presented.79 In the same way as their Na-based analogues have been published, ionseparated Cp9[K (12-crown-6)2]; and polymeric [(m4:h5:h1:h1-Cp)(m:h5Cp)3K4(m:h2:h1-dme)2]- have also been noted.78 Similarly, the structure of BPh49[m:h5-CpK2 (12-crown-6)2]; has been obtained.80 As part of a study into the formation of new lanthanocenes containing silylated cyclopentadienyl ligands, the polymer [m:h5-(Et3SiC5H4)K C5H4N]- has been reported.81 Several potassiated polyalkylcyclopentadienyl complexes have also been fully characterized. Hence, polymeric structures are revealed by (m:h5-Cp*K thf)-, [m:h5Cp*K (thf )2]and the tethered olefin species [(m:h582 C5Me4SiMe2CH2CH:CH2)K thf ]-. The dimethylindole anion (dmi) is noted to adopt h1-, h3- and h5-bonding modes in the potassium salt (dmiK thf)-.83 Moving to fluorenyl systems, the use of diethyleneglycol-dimethyl ether has facilitated the isolation and characterization of a host of alkali metal complexes including a trimeric solvated potassiate.84 A series of dipotassiated 1-silafluorenyl dianions have been crystallized. The X-ray structure of bis(12-crown6)-solvated 1,1-dipotassio-1-silafluorene chains indicates aromatic delocalization in the silole ring, with polymer chain formation being allowed by the ability of the ether to bridge between dimetallated silafluorene units. Prolonged oxidation of this species in the presence of thf yields the Si-Si bonded dimeric dianionic salt (C12H8Si)2K2 (thf)4 whilst its thermal reaction in the presence of dme, 12crown-6 and elemental potassium gives a diradical dianion salt whose EPR spectrum is reported; in [C12H8Si(Me)CH2]2[K (12-crown-6) dme]2 each metal centre is h5-bonded.85 Analogous fluorenyl and 9-germafluorenyl dianion chem

































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istry has also been investigated.86 A series of crown ether solvated mixed-metal K-Rb, K-Cs and Cs-Rb cyclopentadienyl derivatives have been characterized recently.87 The rubidium cocomplex
m:h5-Cp*[Rb (18-crown-6)]2(m:h5-Cp
Rb(Cp)(m:h5-Cp*)[Rb (18crown-6)]2) has lately been observed.88 Moving to heterobimetallic species, tantalum complexes incorporating diphenyldipyrrolide dianions have been probed. In this context, several lithium and sodium-containing complexes have been achieved. Diphenyldipyrromethane reacts with MeLi in OEt2 to yield [Ph2C(C4H3N)2]2Li4 (OEt2)3. Treatment of this complex with Cl5Ta has given [Ph2C(C4H3N)2]2TaCl2 and from this both [Ph2C(C4H3N)2]Ta[(1,4-h1:h1-2,3-h2-C6H7)PhC(C4H3N)2](Na OEt2) and [Ph2C(C4H3N)2]3Ta(Na OEt2)2 have been prepared. In the first of these, partial hydrogenation of an aryl ring is noted.89 New high-valent zirconacarboranes incorporating h7-carboranyl ligands have been synthesized, as with the Zr(IV) metallacarborane
[(m-h5):h7Me2Si(C5H4)(C2B10H11)]Zr(NEt2)2[Na3 (thf)4]-.90 The investigation of 2,5dimethylpyrrole bonding in divalent complexes of Sm and Yb has led to the synthesis and full characterization of (m-h1:h5-Me2C4H2N)2[Sm(m-h1:h5Me2C4H2N)2Na (thf )2]2. This complex is a dimer in which two samarium atoms are bridged by two s/p-bonded pyrrolide rings, each samarium also bonding to four 2,5-dimethylpyrrolyl units (two s-bonded and two p-bonded). Two sodium ions, both s- and p-bonded to 2,5-dimethylpyrrole ligands, are located at either extrema of the dimer.91 In a similar vein, h5-stabilization of the higher Group 1 metal is noted in the polymeric ytterbium complex [Yb(m-h1:h5Me2C4H2N)4(K thf )2]-.91 New dinuclear catalysts for effecting aldol condensations and the pinacolic coupling of acetylpyrrole have been reported. In this manner, Cl3V (thf )3 and [Cl3V2 (thf)6]2(Zn2Cl6) have been reacted with excess potassiated acetylpyrrole, [a-MeC(O)C4H3N]K, to give
[MeC(O)C4H3N] [(C4H3N)C(O)CH2C(C4H3N)(Me)OV]K thf2 and [MeC(O)C4H3N]4[(C4H3N)C(Me)(O)]2V2(K thf )2, respectively. In the first of these products, four acetylpyrrolide anions from two vanadium moieties have undergone an aldol condensation, whilst in the second, two acetylpyrrolide ligands have been reductively coupled to give a pinacol.92 A report of h1:h5:h5-bonding by a pyrazolate ligand has been noted lately in the potassium manganate
3,5-[(OC)2MnC5H4CH2] 93 2HC3N2K. Reaction of elemental rubidium or caesium, respectively, with 2,3,4,5,2,3,4,5octamethylbi-1,1-phospholyl [(Me4C4P)2] in thf has afforded rubidium 2,3,4,5tetramethylphospholide and caesium 2,3,4,5-tetramethylphospholide, the solidstate structure of the latter having been obtained.94 The polymers of (MeC5H4)Cs and (MeC5H4)2Cs2 (18-crown-6) have lately been presented as part of a study into the preparation and characterization of new caesium tris(cyclopentadienyl)manganates.95 Accordingly, chains of [(MeC5H4)3Mn]Cs and (Cp3Mn)Cs have been reported along with corresponding measurements of magnetic susceptibilities.95

























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2

Copper, Silver and Gold

2.1 Introduction. — Section 2 of this review is divided into three parts, in each of which a different coinage metal is considered. In the same way as for Section 1, compounds that contain at least one carbon-metal interaction dominate. For each of the coinage metals, synthetic and mechanistic studies are reported first and are followed sequentially by solid-state, solution and gas phase investigations. 2.2 Copper Compounds. — Several reviews of copper-based synthetic transformations have appeared recently. Catalytic carbon-carbon bond formation reactions using ambient conditions and in the presence of air and water have been studied,96 as have enantioselective copper-catalysed conjugate addition processes.97 The methylene homologation of organocopper substrates has been reviewed as part of a wider article on carbenoid homologation in organic synthesis.98 The preparation of soluble di- and polynuclear copper alkynyl complexes with a view to the subsequent design of luminescent organometallic oligomers and metal-based functional materials has also been discussed.99 Just as copper-catalysed conjugate addition reactions have been the subject of review,97 so too copper catalysts have been used in tandem with mixed thioetherphosphite and phosphine-phosphite ligands in order to effect asymmetric addition to cyclohexanone.100 The 1,4-migration of a silyl group from sp2-carbon to oxygen in (Z)-g-trimethylsilyl allylic alcohols under the influence of copper(I) tert-butoxide is thought to yield vinylcopper intermediates that can be stereospecifically cross-coupled with allyl, aryl and vinyl halides.101 It has been shown that Cl2Cu catalyses Cl2Sn-mediated coupling between aldehydes and allyl bromide with Cl2Sn reduction of the cuprate yielding Cu; ions that, in turn, participate in a cation-p complex with the allyl bromide.102 Reaction of 2-C5H4NSiMe2C(SiMe3)2Li thf with copper(I) iodide has allowed [2-C5H4NSiMe2C(SiMe3)2Cu]2 to be isolated and fully characterized.103 New organoaryl-Cu(III) complexes based on triazamacrocyclic ligands have been reported as part of a study into the activation of aromatic C-H bonds.104 A stable phosphaarene h2-(P:C) p-interaction with copper is noted in the product of reaction between zwitterionic benzo[c]phospholides and copper(I) iodide.105 The solid-state structure of 2-allyl-6-methylphenoxocopper(I) reveals p-tethered ligands,106 as do adducts of copper halides and (2-allylthio)benzimidazole.107 Similarly, p-interactions stabilize the metals in the products obtained by treating triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione with copper(I) bromide.108 Reaction of copper(I) chloride with 4-vinylpyridine has recently afforded the twodimensional Cu(I)-olefin coordination polymer [ClCu (C5H4NCH:CH2-4)]-, sheets of which are based on rectangular grids.109 Along with various copper(I) ethylene adducts based on fluorinated tris(pyrazolyl)borates,110 the complex carbonyl[hydrotris(3,5-dimethylpyrazol-1yl)borato]copper(I)111 has been characterized lately. While a review of coinage metal alkynyl complexes has recently appeared,112 research has been very much ongoing in the field. Hence, the pseudo-cubic tetramer [Ph3PCuC∫CC∫CPh]4



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has been prepared and had its luminescent behaviour probed.113 Along with those of the simple complex (hfac)Cu(h2-C∫CCH2OMe) (hfac : hexafluoroacetylacetonate), the structural and luminescent properties of the highnuclearity Cu(I)-alkynyl species (hfac)8(3,3-dimethyl-1-butynyl)8Cu16 and (hfac)8(3-phenyl-1-propynyl)12Cu20 have been elucidated.114 A method for the preparation of homovalent Cu(I) clusters of the type (hfac)m(RC∫C)nCum]n has been presented. Hence, the treatment of hfacH with copper(I) oxide in excess n-pentyne is presumed to yield mononuclear hfac(C3H7C∫C)Cu, with heating, evaporation and oxidation affording the convoluted high-nuclearity species (hfac)11(C3H7C∫C)15Cu26 in which two disc-shaped copper arrays are bridged by a (C3H7C∫C)2Cu unit.115 The structure of (hfac)10(C4H9C∫C)8Cu18 has also been reported.116 A variety of geometries have been noted for the polymeric cyanobridged, mixed-valence copper complexes isolated from (and found to incorporate) different diamines.117 A new decacopper(II) array has recently been achieved by expanding the metallasiloxane cage [(PhSiO2)6Cu6]2 (OH2)m (BunOH)6-m using [(tmpa)CuCN]9 [tmpa : tris(2-pyridylmethyl)amide]. The solid-state structure of the
[(PhSiO2)6Cu6]2[(tmpa)CuCN]49 ion reveals a hexagonal and near planar Cu6 arrangement, to which four peripheral (tmpa)Cu units are linked via cyanide bridges.118 The solid-state structures of new lithium cyanocuprates have been reported lately. As part of a study into b-diketoiminate complexes of zinc and copper, [(dipp)nacnac]Li OEt2
(dipp)nacnac : [(2,6-diisopropylphenyl)amino]-4[(2,6-diisopropylphenyl)imino]pent-2-enyl has been reacted with copper(I) iodide to yield
[(dipp)nacnac]CuILi OEt2.119 Meanwhile,
[RCuCN]Li (thf )22 [R : (Me3Si)3C, Me2(Me2N)Si(SiMe3)2C] reveals a 4-membered metallocyclic core. Less straightforwardly, a dimer of
[Me2(MeO)Si]2(Me3Si)CCuCNLi thf has been characterized.120 Various coordination modes have been recently noted for the cyano ligands in the coordination polymer
[(en)2Cu OH2]2;[(en)2CuNi2Cu2(CN)10]29-.121 Meanwhile, the use of 1,10-dithia-18-crown-6 has allowed the isolation and characterization of coordination polymers containing NCCu moieties both with and without alkali metal ions.122 The mixed-metal cation [NCCu(m-S)2Mo(mS)2CuCN]2; has also been prepared.123 The same study that incorporated a report of the characterization of [Me3Si(Et2P)NCHMeLi]4 has also presented the serendipitously obtained solid-state structure of the lithiocuprate, [Me3Si(Me2P)NCH2][Me3Si(Me2P)NCH(SiMe2OLi)]CuLi. This is formed if the organolithium substrate is treated with copper(I) iodide in the presence of trace silicon grease.15 Reaction of the appropriate o-diamine chelated aryllithium substrate with copper(I) bromide has yielded the neutral 2:1 lithiocuprates
Ar[CH2N(Me)CH2CH2NMe2-2]Li2 CuBr.124 In each case the alkali metal centres are stabilized by intramolecular N,N-chelation of the pendant diamine, the donor centers revealing stable configurations such that RN,RN and SN,SN diastereoisomers are noted for Ar : C6H4 and C10H6, respectively. A homologous series of aryl cyanocuprates, -argentates and -aurates of the type Ar2Li2 MX [Ar : C6H4CH2N(Et)CH2CH2NEt2-2; M : Cu, Ag, Au] have been prepared and the first of these has been fully characterized.125 Further concerning

















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heterometallic systems, reaction of (MeO)2Cu with (OC)6Co2(HC∫CCO2H) has resulted in formation of the trigonal cluster [(OC)6Co2(HC∫C)Cu]3.126 The polynuclear Cu(I) and Ag(I) complexes
[(m3:h1-FcC∫C)(dppm)3Cu]PF6- and
[(m3:h1-FcC∫C)(dppm)3Ag]CF3SO3- [dppm : bis(diphenylphosphino)methane] have been prepared and characterized.127 Heterobimetallic Ti(IV)-Cu(I) complexes of the type [(h5-Me3SiC5H4)2Ti(C∫CSiMe3)2]CuR have been prepared wherein R represents a variety of alkyl or aryl groups.128 For R : n-butyl, the crystal structure reveals bis(h2-acetylide) coordination of the copper centre.129 A novel inclusion complex in which [Gd (dmf)8]3; ions reside in the pockets of a three-dimensional
[(NC)9Cu6]39- host lattice has been structurally characterized.130 In a similar vein, the extended and layer-structured complex
[Gd2 (dmf)8](NC)10Cu4 has also been reported.130 Just as various Cu(I) borates have been authenticated,110,111 so too has Cp(OC)2Mn(CNCH2NC)Cu(pz*)3BH (pz* : 3,5-dimethylpyrazolyl).131 The same study that incorporated solid-state structural data for the lithiocuprates
Ar[CH2N(Me)CH2CH2NMe2-2]Li2 CuBr (Ar : C6H4 and C10H6) also reported on the enantioselective self-assembly of aryllithiums and -cuprates containing pendant o-diamines.124 Multinuclear NMR spectra of (R)-Ar, (R)diamine and rac-Ar, rac-diamine
C6H4[CH2N(Me)CH2CH2NMe2-2]Li2 CuBr reveal that chirality manifest in the chelating groups incurs selectivity in the thermodynamic formation of halocuprates of the type Ar2Li2 CuX. As discussed above, Ti(IV)-Cu(I) complexes of the type [(h5Me3SiC5H4)2Ti(C∫CSiMe3)2]CuR reveal bis(h2-acetylide) coordination of the copper centre in the solid state.129 Moreover, intramolecular intermetallic interactions have been studied in systems of this type (and their silver analogues) using cyclic voltammetry.132 The precise structure of monomeric copper(I) cyanide has recently been established in the gas phase using millimeter/submillimeter-wave spectroscopy.133 Metallocarbohedrenes are binary molecular clusters that contain metal centres linked by C229 ions and, while many have been prepared, pure and bulk syntheses remain elusive. Recently, DFT methods have been applied to systems such as these that contain copper or silver with a view to rationalizing new synthetic routes to them.134









2.3 Silver Compounds. — While no alkylsilver systems have been fully characterized in 2002, limited examples of arylsilver systems have been prepared. In this field, a silver carbaporphyrin has been reported.135 A further arylsilver species was revealed by the use of C6F5Ag, with C6F5Ag NCEt forming a polymer in the solid state.136 Meanwhile, a series of NS3-open chain and cage ligands have been used to generate new Cu(I) and Ag(I) species, with the silver salt of 1-aza-4,10,15trithia-7(1,3,5)-benzenabicyclo[5.5.5]heptadecaphane showing a polymeric structure by virtue of intermolecular S-Ag interactions.137 A coordination polymer based on N-Ag and C-Ag bonding, and utilizing the [NCC6H4C(OH)] [NCC6H4C(O)]C5H2CH(C5H10)C6H5 linker, has been prepared by employing a fulvene-type bidentate ligand. This was obtained by effecting the aroylation of cyclohexyl-substituted cyclopentadienyl anions.138

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A paper detailing the direct coupling of oxazolines and N-heterocyclic carbenes has incorporated a discussion of the preparation, through reaction of 1-mesityl imidazole with 2-bromo-4,4-dimethyloxazoline, of a 2-(4,4dimethyl)oxazolinyl imidazolium salt. This has in turn been converted into the corresponding silver N-heterocyclic carbene complex by treatment with silver(I) oxide.139 The straightforward complex (1,3-dimethyl-4,5-dimethylimidazol-2ylidene)silver(I) has been prepared,140 as has the monocarbene dinuclear complex m-C6H4(CH2ImMeAgCl)2 (ImMe : N-methylimidazole).141 A series of imidazol2-ylidene carbene complexes of silver have been characterized lately with bridging Ag-Ag, Ag-halide, Ag-dicarbene and ring p-p interactions all being noted to incur polymerisation.142 Reaction of the carbene 1,3-dimesitylimidazol-2-ylidene (: L) with (closo-CB11H12)Ag has led to the isolation of (L2Ag)9[(closoCB11H12)4Ag2];, the anion of which contains two silver(I) centres bridged by two carbenes, with a terminal carbene on each metal also.143 The lamella structures of di- and tetranuclear carbene complexes of silver containing cyclic (BrAg)2 cores have also been elucidated.144 The porous silver(I) organometallic coordination polymer of triptycene (tpty) has been prepared and structurally investigated. Desorption and re-absorption of toluene guest molecules has been noted to incur changes in the 3D architecture of [(O4Cl)3(tpty)3Ag3] (PhMe)2 such that the tpty ligands adopt different bonding modes depending on the steric requirements of the network.145 The complex that results from treatment of a bicyclo[2.2.2]octa-2,5-diene ligand with silver nitrate reveals p-coordination of the metals by the ligand ene units.146 Several Ag(I) complexes of N-allyl and N-butenyl lariat ethers have been reported, with data suggesting that polymerisation is favoured by the presence of shorter allyl side-arms while longer ones incur monomer formation.147 The study of silver(I) clusters containing acetylide dianions has continued, with 2002 seeing the report of the mixed-valent complex [(tmc)Ag][(tmc)Ag OH2][(F3CCO2)12Ag11 (OH2)4]2(C2)2 (tmc : 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)148 and also, using a quaternary ammonium anion as a mediating agent, (Et4N)6
[(F3CCO2)11Ag10 (OH2)2]2(C2)22 and (Et4N)3
[(NCAg)(F3CCO2)14Ag13 (OH2)6](C2)22.149 A benzyltrimethylammonium ion acts as an h3 p-donor to an endohedral silver(I) centre in the columnar polymer
[(F3CCO2)6Ag7 (PhCH2NMe3)](C2)-.150 The directing effects of different anions have been viewed as important in the reaction of silver(I) salts with tbutylacetylene. This has been undertaken with a variety of anions present (F9, Cl9, Br9, BF49, Tos9, NO39)—the dodecahedral fluoride-encapsulating cage (ButC∫C)12Ag14F having been presented.151 However, it has also been established that TosAg reacts to give polymeric [(ButC∫C)2Ag3Tos]- instead.151 Members of the silver(I) acetylide series [m-4-RC6H4(C∫C)mAg(PCy3)n]4 (R : H, Me, MeO, PhC∫C, m : 1, n : 2; R : H, m : 2, n : 4) have been characterized in the solid state and also had their emission spectra analysed, with intense vibronically structured bands being attributed to 3(pp*) excited states of the arylacetylide ligands.152 The structure of
(4-MeC5H4N)2Cu[(NC)2Ag]2- has been reported.153 Similarly, the complex [(dien)2Cu2(NC)9Ag5]- (dien : diethylenetriamine) has been











Organometallic Chemistry

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fully characterized and found to incorporate a polymeric cation and both [(NC)2Ag]9 and [(NC)3Ag2]9 counter-ions.154 New triorganotelluronium pseudohalides R3Te;[(NC)2Ag]9 (R : Me, Ph) have been prepared.155 The ability of coordination polymers to reveal differing structural dimensionality via M-M and M-cyano interactions has been probed for a group of silver-containing heterobimetallic systems based on [(NC)2M]9 (M : Cu, Ag) and [(NC)3Ag2]9 moieties. Accordingly, differing architectures have been noted for fully characterized en and tren [: tris(2-aminoethyl)amine] complexes.156 Cyanide groups have been noted to bridge between silver and manganese centres in the polymeric complex [(salen)MnNCAg]- [H2salen : N,N-bis(salicylidene)-1,2diaminoethane].157 A series of interpenetrating, three-dimensional cyanidebridged Fe(II)-Ag(I) networks based on the formulation LmFe[(NC)2Ag]2 [L : pyrazine, 4,4-bipyridine, bis(pyridyl)ethylene] have been characterized.158 The P,N,P-tridentate ligand 2,6-bis(diphenylphosphino)pyridine has been used to generate the carbonyl-bridged species trans-Fe(CO)(m-CO)
m-[2,6(Ph2P)2C5H3N]Ag2 OEt2.159 As part of an investigation into the behaviour of the potentially tridentate pyridine-bridged dicarbene ligand 2,6-bis[(3-methylimidazolium-1-yl)methyl]pyridine, its Pd(II) and Ag(I) complexes have been prepared. X-ray crystallography on the latter reveals a dimer in which the silver(I) ions are bridged by two ligands bonding through their carbene donors.160 A bis(calixarene) bearing tert-butylated and de-tert-butylated calix[4]arene units linked by four ethylenoxy bridges has been prepared. It is suitably preorganised for the complexation of Ag; ions and, as such, X-ray crystallography shows that it sandwiches silver(I) ions using the two distal, coplanar phenyl rings on the de-tert-butylated part of the molecule.161 The treatment of (Cp*IrCl2)2 with F6PAg and then with 2,2,5,6,6-pentamethyl4-hepten-3-one has led to the co-syntheses of a homometallic iridium species and the mixed Ir-Ag polymer
Cp*IrAg(m2-OPF2O)2Ag[h1:1-CH:C(But)CH: C(But)O]-.162 The mixed-metal Pt(II)-Ag(I) cation [(PhC∫C)4(mdppm)2Pt2(Ag NCMe)2]; has been prepared and its luminescent behaviour has been probed.163 Meanwhile, neutral pyrazolate species have been investigated, with the solid-state structure of Pt-Ag bonded
[kC,P-(o-toloyl)2PC6H4CH2]2(m4-mepz)2Pt2Ag; (4-mepzH : 4-methylpyrazole) revealing coordination of the silver(I) centre by one o-toloyl component on each of the two phosphine ligands.164 Polymeric Cd-Ag systems based on tren Cd(m-NC)Ag units have also been prepared.165 The silver salt of 1-aza-4,10,15-trithia-7(1,3,5)-benzenabicyclo[5.5.5]heptadecaphane has already been discussed. In the solid state it exists as a polymer by virtue of intermolecular S-Ag coordination. However, NMR spectroscopy suggests that in solution the species is fluxional, with intramolecular NS3coordination of the metal being possible as well.137





2.4 Gold Compounds. — Just as [2-C5H4NSiMe2C(SiMe3)2Cu]2 has been fully characterized, so the solid-state structure of the gold(I) analogue has been reported.103 The anion [(NC)2Au]9 has been employed to counter the charge on the

2: Groups 1 and 11: The Alkali and Coinage Metals

51

diiodo(ethylenedithio)diselenadithiafulvalene cation in a superconductor with Tc : 8.6 K166 and also that on
2-[4,5-(1,2-cyclohexylenedithio)-1,3-dithiol-2ylidene]-5-[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene; in a new organic conductor.167 The structure and temperature-dependent photoluminescent behaviour of [(NC)2Au]9[(tpa)2Au]; (tpa : 1,3,5-triaza-7phosphaadamantane) has been described.168 Similarly, [(RNC)2Au]; (R : But, c-hexyl) has acted as a counter-ion in some recent gold dithiolate complexes.169 A series of ylide- and mixed alkynyl/ylide-gold complexes have been prepared by reaction of ClAu tht with the corresponding ylides. Accordingly, ClAuCH(SO2C6H4Me-4)PPh3 and (ClAu)2[m-CH(PPh3)2] have been obtained using a 2:1 molar ratio. Meanwhile, the use of (Ph3P)2CAu acac yields (Ph3P)2CAuC∫CC6H4NO2-4 upon treatment with excess alkyne and the 1:2 reaction of (ppn)Au (acac)2 with Ph3P(4-MeC6H4SO2)CH2 gives the cation
[Ph3P(4-MeC6H4SO2)CH]2Au;.170 The gold-gold bonded ion (
[(Me2NH)2C]2Au2); has been fully characterized as an ion-separated chloride and bromide salt and its luminescent properties noted. Data reveal that luminescence occurs at higher energy than in chains of the cation with PF69 or BF49 counter-ions.171,172 A recent study into tetragold(I) bis(phenylene-1,2-dithiolate) complexes with isonitrile ligands has led, through the reaction of (c-C6H11)ClAu with 1,2-(SNa)2C6H4, to the low-yield preparation of co-crystalline [(1,2-S2C6H4)Au2CN(c-C6H11)]
[(c-C6H11)NH]2CAuCN(cC6H11)2.173 An isonitrile complex based on a dpp dye (dpp : diketopyrrolopyrrole) has also been reported. In the solid state it reveals stacking by virtue of the formation of gold-gold interactions.174 Acetylenic 3(pp*) emissions have been studied in Cy3PAu(C∫C)nAuPCy3 (n : 3, 4), and for n : 3 the crystal structure has been obtained.175 A study into gold(I) complexes has been conducted, with both HC∫CC∫CAuPPh3 and [(m-dppm)2Cu](m-I)(m3-C∫CC∫CAuC∫CC∫CH) having been prepared and fully authenticated.176 The digold acetylide Me2C(C6H4OCH2C∫CAu)2 has been treated with diphosphane substrates of the type Ph2P(CH2)mPPh2 in order to achieve the self-assembley of various macrocycles or [2]catenanes, the significant effects of changing m having been probed.177 In a like vein, 4-BrC6H4CH(C6H4OCH2C∫CAu)2 has been employed in conjunction with various diphosphines.178 Interpenetrating digold macrocycles have also been prepared by the reaction of 1,2-C2H4(O-3C6H4OCH2C∫CH)2 with ClAu SMe2 and the diphosphines Ph2P(CH2)mPPh2 (m : 1-6), trans-Ph2PCH:CHPPh2 and Ph2PC∫CPPh2.179 Series of tris(cyclohexyl)phosphine supported gold mono- and bis(acetylides)180 and of thiophene-containing gold polyacetylides181 have been crystallized recently. The first of these groups have had their photoluminescent properties studied.180 The analogous silver and gold compounds [C6F5(Ph2C:NH)M]2 have been prepared and characterized. Both species are metal-metal bonded and arranged in antiparallel fashion with only the gold centres being supported by extra NH · · · Fortho interactions.182 The simple gold(I) complexes (2,6Me2NCH2)2C6H3AuPPh3,183 C6F5AuP(Ph2)C∫CH184 and (2-C5H3N)Au185 have been crystallographically characterized. New 2,4,6-tris(trifluoromethyl)phenyl (FMes) complexes of gold have been prepared with ion-separated [(FMes)2







Organometallic Chemistry

52

Au]9[(dppm)2Au]; and diphosphine-bridged (FMesAu)2 Ph2P(CH2)mPPh2 (m : 1, 2) having been authenticated.186 The reaction of 2-amino-4,5-dihydrothiazole (2-amt) with the labile gold(III) species (C6F5)3Au tht has yielded monomeric (C6F5)3Au 2-amt.187 A similar monomer has been noted for (C6F5)3Au Ph2PC6H4SH-2.188 Meanwhile, mixed-valent C6F5(6-MeC6H3PPh22)Au(m-6-MeC6H3PPh2-2)AuC6F5 has been prepared.189 Reaction of (pz)3BHNa (pz : pyrazol-1-yl) with cyclometallated Cl2RAu in the presence of sodium perchlorate has yielded the tetrakis(pyrazol-1-yl)borate gold(III) ion
[k2-N,NB(pz)4](k2-C,N-C6H4CH2NMe2-2)Au;.190 Just as several silver carbenoids have been reported recently so has the gold species
[1,3-bis(5H-dibenzo[a,d]cycloheptenyl)imidazol-2-ylidene](triphenylphosphine)gold(I) chloride.191 Concerning heterobimetallic systems, a two-dimensional Co(II)-Au(I) coordination polymer,
(dmf)2 Co[(NC)2Au]2-, has been created using [(NC)2Au]K in tandem with Co(Ac)2 (OH2)4 in dmf.192 An ongoing study into the properties of m2-alkynyl and vinylidene complexes of transition metals has lately afforded the tungsten acetylide (h5-Cp)(ON)W(m-CO)(m-C∫CBut)AuPPh3.193 The straightforward vinyl ether complexes (OC)5M[CH2:C(OMe)AuPPh3] have been obtained by reacting Ph3PAu; with the deprotonated Fischer-type alkoxy(methyl)carbene complexes (OC)5M:C(OMe)CH29 (M : Cr, W).194 The heterobimetallic unit Ag(m-SPPh2CH2)2Au has been noted to afford a solid-state polymer containing a short intramolecular Ag-Au distance.195 The mixed Ag(I)/Au(III) complex [Ag(m-PPh2)2Au(C6F5)2]2 has been reacted with (PPh3)2ClAg to yield the T-shaped pentanuclear insertion product (m3-Cl)(mAg)[(m-Ph2P)(m-PPh3AgPPh2)]2Au(C6F5)2.196 The luminescent gold(I) butterfly Tl2[(C6Cl5)2Au]2 O:CMe2 reveals a Tl(I)-Tl(I) interaction in the solid state.197 Meanwhile, Tl-Au interactions are noted in the strongly luminescent polymers
[Tl(bipy)]2[(C6F5)2Au]2- and
[Tl(bipy)][Tl(bipy)0.5 thf ][(C6F5)2Au]2-.198 Both gold and silver complexes incorporating (PhX)2Fc [X : S, Se; Fc : (h5-C5H4)2Fe] ligands have been prepared. Of these, the ferrocenyl sulfide- and selenide-bridged gold(III) complexes (PhX)2Fc[(C6F5)3Au]2 have been fully characterized.199 Cleavage of the carbon-sulfur bond in [(OC)3Fe]3(m3-SC∫CBut)Li under the influence of Ph3PClAu has afforded [(OC)3Fe]3(m4,h2C∫CBut)AuPPh3, wherein the gold unit is neither face- nor edge-bridging with respect to the Fe3-cluster.200 In the same way that polymers based on tren Cd(mNC)Ag moieties have been reported lately so too have the corresponding gold compounds been prepared.165 Catalytically active cationic and reduced gold clusters have been observed (by EXAFS and XANES) to be present in powdered MgO-supported CO oxidation catalysts; the initial source of these clusters being adsorbed Me2Au(acac).201 The same organometallic substrate has been grafted onto TiO2(110) and Si(111), with STM observations being made before and after annealing in an ultra-high vacuum.202 The nonlinear optical (NLO) properties of a series of 4-O2NC6H4C∫CAuL (L : PCy3, PMe3) and LAu(m-4-C∫CRC∫C)AuL (L : PCy3, PMe3; R : C6H4, C6H4-4-C6H4) have been studied. Quadratic NLO merit has been found to increase on replacing triarylphosphine with trialkylphosphine, with cubic NLO

















2: Groups 1 and 11: The Alkali and Coinage Metals

53

values being small for all the gold complexes tested.203 The solution (by 1H NMR spectroscopy), cytotoxic and DNA binding properties (by spectrophotometry, circular dichroism, ultradialysis, and monitoring of the thermal denaturation profiles of DNA) of some gold(III) complexes incorporating bipyridyl ligands have been studied because of their stability under physiological conditions and their potential applications as anti-tumour agents.204 The octapropargyl resorcinarene [PhCH2CH2CHC6H2(OCH2C∫CH)2]4 has been used to give the octaalkynylgold(I) derivatives [PhCH2CH2CHC6H2(OCH2C∫CAuL)2]4 (L : PPh3, CNBut) and insoluble, : polymeric
[PhCH2CH2CHC6H2(OCH2C∫CAu)2(m-LL)2]4-(LL Ph2P(CH2)mPPh2, m : 1, 2, 4). These complexes have been extensively probed using NMR spectroscopy.205 Low temperature NMR spectroscopic studies on the above-mentioned acetylide (h5-Cp)(ON)W(m-CO)(m-C∫CBut)AuPPh3 have revealed the presence of three interconverting isomers in CD2Cl2 solution, of which one is (h5-Cp)(ON)(OC)W(h2-ButC∫C)AuPPh3.193 A low kinetic barrier is noted to the fast and reversible intramolecular cleavage of a gold-carbon bond in [(OC)3Fe]3(m4,h2-C∫CBut)AuPPh3, such that the gold unit migrates from its terminal position (as noted in the solid state) to bridge an edge of the Fe3-cluster core.200 Neutral trinuclear complexes of Au(I) and Hg(II) have been probed using 19 1 F, H HOESY and PGSE NMR techniques. Whereas [(m-C2,N3-bzim)Au]3 (bzim : 1-benzylimidazolate) and
[m-C,N-C(OEt):NC6H4Me-4]Au3 are known to stack with [(m-C,C-C6F4)Hg]3 in the solid state, the first evidence for the retention of acid-base stacking in solution has only now been presented.206 Electrospray mass spectrometry has been employed to probe reactions between the metalloligand [(Ph3P)2(m-Se)Pt]2 and various metal-based substrates. Amongst the latter type of species were several gold(III) chlorides bearing anilino- or arylpyridine units. It was noted that manipulating both the electrondonating and chelating ability of these moieties achieved control of chloride lability and affinity for the metalloligand.207 Formation of the vinyl ether complexes (OC)5M[CH2:CH(OMe)AuPPh3] (M : Cr, W) has already been discussed. DFT calculations suggest that the major bonding contribution to vinyl coordination of the Group 6 metal in these systems comes from partial end-on h1-bonding and not more normal h2-interaction.194

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106. B. Gustafsson, M. Ha kansson, G. Westman and S. Jagner, J. Organomet. Chem., 2002, 649, 204. 107. E. A. Goreshnik, D. Schollmeyer and M. G. Myskiv, Z. Anorg. Allg. Chem., 2002, 628, 2118. 108. X. Xue, X.-S. Eang, R.-G. Xiong, X.-Z. You, B. F. Abrahams, C.-M. Che and H.-X. Ju, Angew. Chem. Int. Ed., 2002, 41, 2944. 109. J. Zhang, R.-G. Xiong, X.-T. Chen, Z. Xue, S.-M. Pengand X.-Z. You, Organometallics, 2002, 21, 235. 110. H. V. R. Dias, H.-L. Lu, H.-J. Kim, S. A. Polach, T. K. H. H. Goh, R. G. Browning and C. J. Lovely, Organometallics, 2002, 21, 1466. 111. A. J. Blake, D. A. Carling, M. W. George, P. Hubberstey, R. L. Garcia and C. Wilson, Acta Crystallogr., Sect. E, 2002, 58, m41. 112. D. M. P. Mingos, R. Vilar and D. Rais, J. Organomet. Chem., 2002, 641, 126. 113. V. W.-W. Yam, C.-H. Lam and N. Zhu, Inorg. Chim. Acta, 2002, 331, 239. 114. C. W. Baxter, T. C. Higgs, A. C. Jones, S. Parsons, P. J. Bailey and P. A. Tasker, J. Chem. Soc., Dalton Trans. , 2002, 4395. 115. T. C. Higgs, P. J. Bailey, S. Parsons and P. A. Tasker, Angew. Chem. Int. Ed., 2002, 41, 3038. 116. T. C. Higgs, S. Parsons, A. C. Jones, P. J. Bailey and P. A. Tasker, J. Chem. Soc., Dalton Trans., 2002, 3427. 117. E. Colacio, R. Kiveka¨s, F. Lloret, M. Sunberg, J. Suarez-Varela, M. Bardajı´ and A. Laguna, Inorg. Chem., 2002, 41, 5141. 118. G. L. Abbati, A. Caneschi, A. Cornia, A. C. Fabretti, Y. A. Pozdniakova and O. I. Shchegolikhina, Angew. Chem. Int. Ed., 2002, 41, 4517. 119. J. Prust, H. Hohmeister, A. Stasch, H. W. Roesky, J. Magull, E. Alexopoulos, I. Us´on, H.-G. Schmidt and M. Noltemeyer, Eur. J. Inorg. Chem., 2002, 2156. 120. C. Eaborn, S. M. El-Hamruni, M. S. Hill, P. B. Hitchcock and J. D. Smith, J. Chem. Soc., Dalton Trans. , 2002, 3975. ˇ erna´k and K. A. Abboud, Acta Crystallogr., Sect. C, 2002, 58, m167. 121. J. C 122. T. Ro¨ttgers and W. S. Sheldrick, Z. Anorg. Allg. Chem., 2002, 628, 1305. 123. W. Clegg, A. Beheshti and M. H. Sadr, Acta Crystallogr., Sect. E, 2002, 58, m349. 124. C. M. P. Kronenburg, C. H. M. Amijs, J. T. B. H. Jastrzebski, M. Lutz, A. L. Spek and G. van Koten, Organometallics, 2002, 21, 4662. 125. C. M. P. Kronenburg, J. T. B. H. Jastrzebski, J. Boersma, M. Lutz, A. L. Spek and G. van Koten, J. Am. Chem. Soc., 2002, 124, 11675. 126. A. Vega, V. Calvo, E. Spodine, A. Za´rate, V. Fuenzalida and J.-Y. Saillard, Inorg. Chem., 2002, 41, 3389. 127. J. H. K. Yip, J. Wu, K.-Y. Wong, K.-W. Yeung and J. J. Vittal, Organometallics, 2002, 21, 1612. 128. H. Lang and T. Stein, J. Organomet. Chem., 2002, 641, 41. 129. T. Stein and H. Lang, J. Organomet. Chem., 2002, 664, 142. 130. S. Liu, E. A. Meyers and S. G. Shore, Angew. Chem. Int. Ed., 2002, 41, 3609. 131. T. Bartoloma¨s, D. Lentz, I. Neubert and M. Ro¨ttger, Z. Anorg. Allg. Chem., 2002, 628, 863. 132. T. Stein, H. Lang and R. Holze, J. Electroanal. Chem., 2002, 520, 163. 133. D. B. Grotjahn, M. A. Brewster and L. M. Ziurys, J. Am. Chem. Soc., 2002, 124, 5895. 134. I. Dance, E. Wenger and H. Harris, Chem. Eur. J., 2002, 8, 3497. 135. M. A. Muckey, L. F. Szczepura, G. M. Ferrence and T. D. Lash, Inorg. Chem., 2002, 41, 4840.

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136. W. Tyrra and M. S. Wickleder, Z. Anorg. Allg. Chem., 2002, 628, 1841. 137. J. M. Baumeister, R. Alberto, K. Ortner, B. Spingler, P. A. Schubiger and T. A. Kaden, J. Chem. Soc., Dalton Trans. , 2002, 4143. 138. Y.-B. Dong, G.-X. Jin, M. D. Smith, R.-Q. Huang, B. Tang and H.-C. zur Loye, Inorg. Chem. , 2002, 41, 4909. 139. V. Ce´sar, S. Bellemin-Laponnaz and L. H. Gade, Organometallics, 2002, 21, 5204. 140. M. Chung, Bull. Korean Chem. Soc., 2002, 23, 1160. 141. W. Chen, B. Wu and K. Matsumoto, J. Organomet. Chem., 2002, 654, 233. 142. K. M. Lee, H. M. J. Wang and I. J. B. Lin, J. Chem. Soc., Dalton Trans., 2002, 2852. 143. M. A. Fox, M. F. Mahon, N. J. Patmore and A. S. Weller, Inorg. Chem., 2002, 41, 4567. 144. C. K. Lee, K. M. Lee and I. J. B. Lin, Organometallics, 2002, 21, 10. 145. M. Wen, M. Munakata, Y. Suenaga, T. Kuroda-Sowa and M. Maekawa, Inorg. Chim. Acta, 2002, 340, 8. 146. P. J. Steel and N. C. Webb, Eur. J. Inorg. Chem., 2002, 2257. 147. P. Arya, A. Channa, P. J. Cragg, P. D. Prince and J. W. Steed, New J. Chem., 2002, 26, 440. 148. Q.-M. Wang, H. K. Lee and T. C. W. Mak, New J. Chem., 2002, 26, 513. 149. Q.-M. Wang and T. C. W. Mak, Angew. Chem. Int. Ed., 2002, 41, 4135. 150. Q.-M. Wang and T. C. W. Mak, Chem. Commun., 2002, 2682. 151. D. Rais, D. M. P. Mingos, R. Vilar, A. J. P. White and D. J. Williams, J. Organomet. Chem. , 2002, 652, 87. 152. Y.-Y. Lin, S.-W. Lai, C.-M. Che, K.-K. Cheung and Z.-Y. Zhou, Organometallics, 2002, 21, 2275. 153. L. Trisˇcˇ ı´ kova´, I. Potocˇ n´a´k and C. Wagner, Acta Crystallogr., Sect. C, 2002, 58, m246. ˇ erna´k, J. Chomicˇ and W. Massa, Acta Crystallogr., Sect. C, 2002, 58, m490. 154. J. C 155. T. M. Klapo¨tke, B. Krumm, P. Mayer, H. Piotrowski, I. Schwab and M. Vogt, Eur. J. Inorg. Chem. , 2002, 2701. 156. C. J. Shorrock, B.-Y. Xue, P. B. Kim, R. J. Batchelor, B. O. Patrick and D. B. Leznoff, Inorg. Chem., 2002, 41, 6743. 157. A. Panja, N. Shaikh, P. Vojtı´ sˇek, S. Gao and P. Banerjee, New. J. Chem. , 2002, 26, 1025. 158. V. Niel, M. C. Mun oz, A. B. Gaspar, A. Galet, G. Levchenko and J. A. Real, Chem. Eur. J., 2002, 8, 2446. 159. H.-B. Song, Z.-Z. Zhang, Z. Hui, C.-M. Che and T. C. W. Mak, Inorg. Chem., 2002, 41, 3146. 160. D. J. Nielson, K. J. Cavell, B. W. Skelton and A. H. White, Inorg. Chim. Acta, 2002, 327, 116. 161. J. Budka, P. Lhota´k, I. Stibor, V. Michlova´, J. Sykora and I. Cisarova´, Tetrahedron Lett. , 2002, 43, 2857. 162. I. I. R. Salas, M. A. Paz-Sandoval and H. No¨th, Organometallics, 2002, 21, 4696. 163. V. W.-W. Yam, C.-K. Hui, K. M.-C. Wong, N. Zhu and K.-K. Cheung, Organometallics, 2002, 21, 4326. 164. L. R. Falvello, J. Fornie´s, A. Martı´ n, V. Sicilia and P. Villarroya, Organometallics, 2002, 21, 4604. 165. H. Zhang, J. Cai, X.-L. Feng, T. Li, X.-Y. Li and L.-N. Ji, Inorg. Chem. Commun., 2002, 5, 637. 166. T. Imakubo, N. Tajima, M. Tamura, R. Kato, Y. Nishio and K. Kajita, J. Mater. Chem., 2002, 12, 159.

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167. T. Kawamoto, M. Ashizawa, T. Mori, J.- I. Yamaura, R. Kato, Y. Misaki and K. Tanaka, Bull. Chem. Soc. Jpn., 2002, 75, 435. 168. Z. Assefa, M. A. Omary, B. G. McBurnett, A. A. Mohamed, H. H. Patterson, R. J. Staples and J. P. Fackler Jr., Inorg. Chem., 2002, 41, 6274. 169. H. Ehlich, A. Schier and H. Schmidbaur, Z. Naturforsch., Teil B, 2002, 57, 890. 170. J. Vicente, A. R. Singhal and P. G. Jones, Organometallics, 2002, 21, 5887. 171. R. L. White-Morris, M. M. Olmstead, F. Jiang and A. L. Balch, Inorg. Chem., 2002, 41, 2313. 172. R. L. White-Morris, M. M. Olmstead, F. Jiang, D. S. Tinti and A. L. Balch, J. Am. Chem. Soc., 2002, 124, 2327. 173. H. Ehlich, A. Schier and H. Schmidbaur, Organometallics, 2002, 21, 2400. 174. I.-P. Lorenz, M. Limmert, P. Mayer, H. Piotrowski, H. Langhals, M. Poppe and K. Polborn, Chem. Eur. J., 2002, 8, 4047. 175. W. Lu, H.-F. Xiang, N. Zhu and C.-M. Che, Organometallics, 2002, 21, 2343. 176. M. I. Bruce, B. C. Hall, B. W. Skelton, M. E. Smith and A. H. White, J. Chem. Soc., Dalton Trans., 2002, 995. 177. C. P. McArdle, M. J. Irwin, M. C. Jennings, J. J. Vittal and R. J. Puddephatt, Chem. Eur. J. , 2002, 8, 723. 178. C. P. McArdle, S. Van, M. C. Jennings and R. J. Puddephatt, J. Am. Chem. Soc. , 2002, 124, 3959. 179. W. J. Hunks, J. Lapierre, H. A. Jenkins and R. J. Puddephatt, J. Chem. Soc., Dalton Trans., 2002, 2885. 180. H.-Y. Chao, W. Lu, Y. Li, M. C. W. Chan, C.-M. Che, K.-K. Cheung and N. Zhu, J. Am. Chem. Soc. , 2002, 124, 14696. 181. P. Li, B. Ahrens, K.-H. Choi, M. S. Khan, P. R. Raithby, P. J. Wilson and W.-Y. Wong, Cryst. Eng. Commun., 2002, 4, 405. 182. A. Codina, E. J. Ferna´ndez, P. G. Jones, A. Laguna, J. M. Lo´pez-de-Luzuriaga, M. Monge, M. E. Olmos, J. Pe´rez and M. A. Rodrı´ guez, J. Am. Chem. Soc., 2002, 124, 6781. 183. M. Contel, M. Stol, M. A. Casado, G. P. M. van Klink, D. D. Ellis, A. L. Spek and G. van Koten, Organometallics, 2002, 21, 4556. 184. M. Bardajı´ , P. G. Jones and A. Laguna, J. Chem. Soc., Dalton Trans., 2002, 3624. 185. A. Hayashi, M. M. Olmstead, S. Attar and A. L. Balch, J. Am. Chem. Soc., 2002, 124, 5791. 186. M. Bardajı´ , P. G. Jones, A. Laguna, A. Moracho and A. K. Fischer, J. Organomet. Chem. , 2002, 648, 1. 187. M. Bardajı´ , A. Laguna, M. R. Pe´rez and P. G. Jones, Organometallics, 2002, 21, 1877. 188. P. G. Jones, R. Terroba, E. Ferna´ndez and M. Laguna, Acta Crystallogr., Sect. E, 2002, 58, m90. 189. M. A. Bennett, S. K. Bhargava, F. Mohr, L. L. Welling and A. C. Willis, Aust. J. Chem., 2002, 55, 267. 190. J. Vicente, M. T. Chicote, R. Guerrero and U. Herber, Inorg. Chem., 2002, 41, 1870. 191. C. Bo¨hler, D. Stein, N. Donati and H. Gru¨tzmacher, New J. Chem., 2002, 26, 1291. 192. E. Colacio, F. Lloret, R. Kiveka¨s, J. Ruiz, J. Sua´rez-Varela and M. R. Sundberg, Chem. Commun., 2002, 592. 193. J. Ipaktschi and F. Munz, Organometallics, 2002, 21, 977. 194. H. G. Raubenheimer, M. W. Esterhuysen, A. Timoshkin, Y. Chen and G. Frenking, Organometallics, 2002, 21, 3173. 195. M. A. Rawashdeh-Omary, M. A. Omary and J. P. Fackler Jr., Inorg. Chim. Acta,

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2002, 334, 376. 196. M. C. Blanco, E. J. Ferna´ndez, M. E. Olmos, O. Crespo, A. Laguna and P. G. Jones, Organometallics, 2002, 21, 2426. 197. E. J. Ferna´ndez, J. M. Lo´pez-de-Luzuriaga, M. Monge, M. E. Olmos, J. Pe´rez and A. Laguna, J. Am. Chem. Soc., 2002, 124, 5942. 198. E. J. Ferna´ndez, P. G. Jones, A. Laguna, J. M. Lo´pez-de-Luzuriaga, M. Monge, J. Pe´rez and M. E. Olmos, Inorg. Chem., 2002, 41, 1056. 199. S. Canales, O. Crespo, A. Fortea, M. C. Gimeno, P. G. Jones and A. Laguna, J. Chem. Soc., Dalton Trans. , 2002, 2250. 200. E. Delgado, B. Donnadieu, M. E. Garcı´ a, S. Garcı´ a, M. A. Ruiz and F. Zamora, Organometallics, 2002, 21, 780. 201. J. Guzman and B. C. Gates, J. Phys. Chem. B, 2002, 106, 7659. 202. Y. Maeda, M. Okumura, M. Date´, S. Tsubota and M. Haruta, Surf. Sci., 2002, 514, 267. 203. S. K. Hurst, M. P. Cifuentes, A. M. McDonagh, M. G. Humphrey, M. Samoc, B. Luther-Davies, I. Asselberghs and A. Persoons, J. Organomet. Chem., 2002, 642, 259. 204. G. Marcon, S. Carotti, M. Coronnello, L. Messori, E. Mini, P. Orioli, T. Mazzei, M. A. Cinellu and G. Minghetti, J. Med. Chem., 2002, 45, 1672. 205. D. Eisler, W. Hong, M. C. Jennings and R. J. Puddephatt, Organometallics, 2002, 21, 3955. 206. A. Burini, J. P. Fackler Jr., R. Galassi, A. Macchioni, M. A. Omary, M. A. Rawashdeh-Omary, B. R. Pietroni, S. Sabatini and C. Zuccaccia, J. Am. Chem. Soc., 2002, 124, 4570. 207. J. S. L. Yeo, J. J. Vittal, W. Henderson and T. S. A. Hor, J. Chem. Soc., Dalton Trans., 2002, 328.

3 Group 2 (Be-Ba) and Group 12 (Zn-Hg) CHRISTOPHER M. PASK AND DOMINIC S. WRIGHT

1

Scope and Organisation of the Review

This review presents a perspective of the important structural and synthetic studies reported in the year 2002. As with the previous few years’ reports, the strict definition of an organometallic compound as one containing at least one C-metal interaction or bond has been used in this survey (with some exceptions, where appropriate). Although based on an extensive literature search, the text is not intended to be fully comprehensive but to highlight major areas of current and potential future interest. In order to facilitate rapid access to a particular subject heading, individual topic headings have been placed in bold script in the text.

2

Group 2

Very few structural reports of s-bonded organomagnesium compounds (simple Grignards and diorganomagnesium complexes) appeared in 2002,1—5 although additional complexes were observed in which stabilising C-H · · · Mg interactions were found.6 Major focuses of studies in this area have remained the applications of s-bonded Mg complexes in polymerisation1,5 and in enantioselective transformations.4 A case in point is the synthesis of the unsolvated Mg enolate dimer [
ArN · · · C(Me) · · · CH · · · C(Me) · · · NArMg CH2 · · · C(Mes) · · · O]2 (Ar: 2,6i Pr2C6H3, Mes: 2,4,6-Me3C6H2) (1) via the deprotonation reaction of MesC(:O)Me with [
ArN · · · C(Me) · · · CH · · · C(Me) · · · NArMgiPr] (2).1 1 contains a central, eight-membered MgOCCMgOCC ring which is unique in the area of Mg enolates. The complex has been shown to be an excellent initiator for the living, syndioselective polymersiation of methylmethacrylate under relatively mild conditions (ca. 930°C), with an rr:rm:mm ratio of 92:8:0 (sr: 0.96) and a high softening temperature (Tg) of 135°C. This is the first well-defined magnesium initiator for the living, syndioselective polymerisation of methyl methacrylate under mild conditions. Reports of the first Mg hydrazide monomer [
(Me3Si)2N—NPh2Mg] (3)6 and the first trinuclear organomagnesium complex [
(tBuC∫C)(thf)Mg(m-NiPr2)(m-C∫CtBu)2Mg] (4)3 are also worthy of mention from a purely structural point of view. A side-on (N,N-bonded) coordination

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mode is found for the two hydrazide ligands in 3, resulting (nominally) in a four-coordinate, square-based pyramidal arrangement for the Mg centre which is further stabilised by agostic C-H · · · Mg interactions. The structural arrangement in 4 can be regarded as resulting from the coordination of a central Mg2; ion by two [(tBuC∫C)2(NiPr2)Mg(thf)]9 magnesate anions. Progress in the development of ‘inverse-crown’ arrangements containing Mg has also been made, a recent example being the novel Mg hydride complex [(iPr2N)4Mg2Na2(mH)2 2(h6-C6H5Me)] (5) which exhibits encapsulation of hydride within its cyclic Mg2Na2N4 arrangement (Figure 1).7 The complex is obtained from the reaction of BuNa and Bu2Mg with iPr2NH (1:1:3 equivalents, respectively) in toluene, with the hydrides presumably being generated by b-elimination from an iPr2N9 anion. This is the first example of hydride encapsulation within such an inversecrown structure.

Figure 1

Structural studies of p-complexes of Be8a and Mg9,10 have also been relatively rare in 2002. The first structurally authenticated heteroleptic berrylocene [(h5Cp*)(h1-Cp)Be] (Cp: C5Me4H) (6) was obtained by the high-temperature reaction of [Cp*BeCl] (7) with CpK (8).8a Previous studies of the insertion reactions of the symmetrically-substituted berrylocenes [Cp*2Be] (9) and [Cp2Be] (10) with 2,6-Me2C6H3N∫C (XylN∫C)8b had provided important evidence for 1,5-sigmatropic shift of the Cp* and Cp rings in the observed fluxionality of these berrylocenes in solution. The product of the reaction between 6 and XylN∫C, [(h5-Cp)
h1-(Cp*)XylN:CBe] (11), provides the first evidence that ring inversion of the h5- and h1-bonded ligands (molecular inversion of the h5-/h1- bonded ligands via an h3-/h3-bonded transition state) may also play a role in the fluxionality of berrylocenes.8a The reactions of Cp2Mg with various amines have been investigated with a view to understanding the nature of intermediates in the vapour-phase reaction of Cp2Mg with NH3 which is important in the fabrication of Mg-doped Group 13 semiconductors.9,10 The structures of the

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primary amine adducts [(h5-Cp)(h2-Cp)MgNH2R] (12) (R: CHiPr2, tBu) are of particular interest in providing models for the primary deprotonation step. In both complexes one of the N—H protons H-bonds intramolecularly in an h3fashion to the three non-bonded C atoms of the h2-Cp ligand while the other N—H proton forms an intermolecular H-bond to the h2-Cp carbons of a neighbouring molecule (range 2.62-2.79Å). It is suggested that the stabilisation that this H-bonding imparts results in a longer lifetime of the NH3 adduct, giving more time for slow elimination of CpH in a CVD reactor. The major current interest in the structural chemistry and synthetic potential of the s- and p-organometallics of the heavier Group 2 elements (Sr, Ca and Ba) has been highlighted by two reviews of the area.11 Reports in this field in 2002 concerned a variety of topics,4,12-18 from studies of classical metallocenes13 and ansa-metallocenes,14 to complexes containing unusual s-4,15 and p-ligands,16,17 and the applications of benzyl complexes as polymerisation initiators.18 One of the highlights in the structural chemistry in this area was the report of the first triple-decker sandwich compound of Ba [(CpBu)Ba(cot)Ba(CpBu)] (CpBu: 1,2,3,4t Bu4C5H; cot: cyclooctatetraene) (13), obtained from the slow reaction of [(CpBu)BaI(thf)2]2 (14) with cotNa2 (15) at ambient temperature.16 The geometries of the two, symmetry-related Ba centres are bent [CpBu(centroid) · · · Ba · · · cot(centroid) 169.5°], with the Ba · · · cot(centroid) distance of 2.40Å apparently being the shortest distance observed between a Ba centre and any conjugated aromatic ligand. A full report of the coordination chemistry of calix[4]arenes with alkali and alkaline earth metals is also of interest.17 The Ca complexes [p-tBu-calix[4] (OC5H11)2CaL] [L: Me2NCH2CH2NMe (16); L: MeOCH2CH2OMe (17)] exhibit the fascinating ability to coordinate a further Ca2] ion within the cavity of the calixarene, the reactions with [CaI2(thf)4] in MeCN giving the complex [p-tBu-calix[4](OC5H11)2Ca(MeCN)2CaI2] (18) in which the encapsulated Ca2; ion is coordinated to two h6-bonded benzene rings (Scheme 1).

Scheme 1

The application of the organometallics of the heavier Group 2 metals as polymer initiators continues to be an active area of research. The two unsolvated heteroleptic Ca complexes [[(Me3SiFl)(2-Me2N-C6H4CH2]Ca]2 (Fl: fluorenyl) (19) and [(EtMe4Cp)(2-Me2N-C6H4CH2]Ca]2 (20) have similar dimeric struc-

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tures in the solid state in which the Ca2; ions are bridged symmetrically by the functionalised benzyl ligand [2-Me2N-C6H4CH2].18 The Fl and EtMe4Cp ligands in each are terminal, with the Me2N group of each bridging benzyl ligand coordinating separate Ca centres. The structures of 19 and 20 are retained in non-donor solvents. However, cleavage of the dimers initially into the monosolvated monomers [[(L)(2-Me2N-C6H4CH2)Ca thf ][L: Me3SiFl (21), EtMe4Cp (22)] occurs upon addition of thf and, with further thf, dissociation into [(2-Me2N-C6H4CH2)2Ca] and the solvent-separated ion-pairs [[(L)2]29 [Ca(thf)6]2;[L: Me3SiFl (23), EtMe4Cp (24)]. The structurally characterised benzene solvate [[(Me3SiFl)2]29[Ca(thf)6]2; (C6H6) (23·C6H6) is the first organometallic complex of a Group 2 element that crystallises as a solventseparated ion-pair.19 The structure contains [Ca(thf)6]2;ions that are encapsulated in a ‘box’ of [Me3SiFl]9 ions and benzene molecules, being stabilised by (thf)C—H · · · p-interactions. 19 and 20 initiate the anionic living polymerisation of styrene, with a high degree of syndiotactic enrichment (ca. 85% r-diads) being obtained in the case of 19. The results are consistent with two competing processes being present within the polymerisation mechanism, a selective insertion step and an inversion step which racemises the chiral chain ends. The presence of excess thf increases the rates of inversion of the chiral carbanionic chains ends, thus destroying the stereoselectivity. The applications of bdiketiminato complexes of the heavier Group 2 elements in polymerisation is likely to be a future area of interest.20 The development of new Group 2 reagents in organic synthesis,21-26 and the observation of novel reactions and synthetic methodologies27-33 continues to be a large area of interest.As has been the case in previous years, this review will not seek to provide full details of developments made in 2002 but will aim to highlight new chemistry in this area which is fundmentally reliant on the character of the reagents themselves. Development of Grignard reagents containing functional groups continues to be a highly active field of study.21-24 Previously, it had been thought that NO2 functionality was incompatible with organometallic reagents such as Grignards. One highlight in the synthesis of new Grignard reagents was the discovery that Mg-I exchange of o-nitro-aryliodides [2-NO2-1-I-Ar] (25) with PhMgCl gives o-nitro-aryl Grignard reagents [2-NO2-1-(MgCl)Ar] (26) (Scheme 2).21 The exchange reactions are normally complete in ca. 5 mins. at 940°C and the addition of an extensive range of electrophiles (E;) results in good to excellent yields of the products 27 (64-94%). The choice of Grignard reagent in the exhange reaction is important as other Grignards (such as iPrMgCl or MeMgX) react with the NO2 group, and only the o-substituted iodides exchange cleanly. The formation of the new Grignards 26 can be regioselective and highly tolerant to functional groups. For example, in the case of 2,5-diiodo-nitrobenzene only the o-substituted I exchanges with Mg as a result of the chelate and inductive effect of the NO2 group, and in the case of 2,5-dinitro-iodo-benzene Mg-I exchange occurs despite the presence of two NO2 groups in the product Grignard reagent. The formation of new, functionalised cyclopropylmagnesium reagents has also been achieved using Mg-I exchange of iPrMgCl with 2-iodo-cyclopropanecarboxylate (Scheme



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65

3).22 The new Grignards 28 show good conformational stability (at ca. 94050°C) owing to the inductive and chelate effect of the ester group.A range of electrophiles exchange stereoselectively with Mg (61-90%). Reactions of i PrMgCl with the related geminal diiodo- or dibromo-cyclopropanecarboxylates allow access to the conformationally rigid carbenoids 29 (Scheme 4), with high stereoselectivity being observed in proceeding reactions with electrophiles. By comparison, the Mg-Br exchange reactions of trialkylmagnesate [nBu3MgLi] (30)25 or [EtMgCl]31 with geminal dibromo-cyclopropanes in which there is no directing and/or stabilising donor functionality apparently occur with far lower selectivity under similar conditions. At higher temperatures allenes can be obtained in high yields.31 A further highlight in the development of new organomagnesium reagents was the application of [BuMgNiPr2] (31) as a selective base.26 31 is readily prepared by the reaction of iPr2NH with commercially available s,nBu2Mg and exhibits remarkable activity and selectivity in deprotonation reactions of cyclopropanes and cyclobutanes. The deprotonation of the cyclopropanecarboxamide 32 with s,nBu2Mg occurs slowly and in low yield. However, if a 1:1 mixture of s,nBu2Mg:31 is employed a-deprotonation occurs and if 30 alone is used then b-depronation results (Scheme 5). Addition of electrophiles to the resulting a- and b-Grignards (33 and 34, respectively) is stereoselective. Perhaps most remarkable of all bearing in mind the low acidity of the C-H protons in cyclobutanes (ca. 2500 less acidic than those in cyclopropanes), analogous b-deprotonation of cyclopropanes also occurs using 31.

Scheme 2

Scheme 3

Scheme 4

Organometallic Chemistry

66

Scheme 5

Significant advances have been made in the transition metal catalysed crosscoupling reactions of Grignard reagents with aryl and alkyl chlorides, tosylates and triflates.27-29 High yields (43-99%) of coupled products (ArR) are obtained in the reactions of aryl Grignard reagents (ArMgBr) with a range of primary alkyl chlorides (RCl) using [Pd(AcO)2]/Cy3P at room temperature in N-methylpyrrolidinone solvent. The reaction system is also capable of coupling organozinc reagents. This is the first report of a Pd-catalysed cross-coupling reaction of alkyl chlorides.27 Simple Fe salts such as FeCln and Fe(acac)n (n: 2,3) have also been shown to be efficient, cheap and environmentally-friendly catalysts for a host of cross-coupling reactions of alkyl or aryl Grignards, zincates and organomanganese species with aryl and heteroaryl chlorides, triflates and tosylates (RX).28,29 An ‘inorganic Grignard’ of formal composition [Fe(MgX)2] is thought to be responsible in a catalytic cycle in which the substrates are brought together by a combination of oxidative addition of RX and nucleophilic addition of RMgX to the Fe centre. The reactions occur rapidly at or below room temperature and show a high degree of functional group tolerance. Of a more inorganic flavour, key evidence of the intermediacy of carbanions in the formation of Grignard reagents has been obtained in a full study of the reactions of Mg with aryl bromides containing Group 14 functional groups (E).30 The reactions are accompanied by an unusual intra- or intermolecular migration reaction in which the E group partially exchanges with the anionic centre. This study supports the view that in addition to the formation of radicals (R·), a further single-electron transfer process [converting R· to R:9] may also be a key step in the formation of other Grignards.

3

Group 12

Structural studies of s-bonded organozinc and cadmium compounds continued to be an active area in 2002. The motivations for these studies were widespread,

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

67

with important contributions in this field reflecting fundamental structural interests,4,34-37 the investigation of organic mechanisms and intermediates,38-41 and the applications of organozinc compounds in polymerisation catalysis.42,43 The structural characterisation of the first octanuclear Zn siloxane [Zn4(thf )4(MeZn)4(O3SiR)4] (R: 2,6-iPr2C6H3N(SiMe3)] (35) represents a significant advance in attempts to prepare larger Zn siloxane aggregates.34 35 is formed from the reaction of excess ZnMe2 with RSi(OH)3 in thf at room temperature. The complex can be regarded as being composed of a central Zn4Si4 cubane arrangement that is capped on four of the faces by ZnMe fragments, and contains Zn in trigonal- planar, trigonal-bipyramidal and octahedral coordination geometries. The presence of hydrolysable functionalities in 35 make it a potential precursor to synthetic Zn-containing zeolites. Another large Zn aggregate, the heptanuclear, iodomethylzinc alkoxide [(MeO)8Zn7(CH2I)6] (36), has also been reported (Figure 2).38 Previously, evidence of the precise nature of intermediates involved in directed Simmons-Smith cyclopropanation reactions had been scarce. The structural characterisation of 36 provides that first unequivocal proof of the nature of these species. The complex is obtained most efficiently by a novel, photoinduced alkyl exchange reaction of [MeOZnEt] (37) with CH2I2. 36 has a centrosymmetric cage structure in the solid state formed by the fusion of two Zn4O4 cubane units into a ‘double-cubane’ arrangement. Indeed, the formation of this arrangement is actually thought to involve equilibriation of two [MeOZn(CH2I)]4 cubanes into 36 and [Zn(CH2I)2]. Mechanistic and structural studies of novel C-C coupling reactions mediated by Zn (and Sn) have continued in 2002, and have a great deal of potential in providing direct approaches to an exciting range of organic species.39,40 This future potential is illustrated in particular by the isolation of a 1,4-diazacyclohexane from the thermolysis of [
2NHCH2-C5H4NZnMe]3 (38), in which two C-C bonds are formed.39

Figure 2

Developments in the structural chemistry of s-bonded organomercury compounds in 2002 reflected the unique (predominantly linear) bonding arrangements and Lewis acid character of this metal in Group 12. A broad range of compounds was investigated, including those of the type [RHgX] (X: inorganic or organic anion),35,44,45 [R2Hg],4,46-49 and cyclic arrangements commonly containing more than one Hg centre.50-55 Major themes were the coordination of anionic and neutral guests by Hg hosts and supramolecular chemistry resulting from intermolecular Hg · · · Hg and Hg · · · ligand interactions.

Organometallic Chemistry

68

The presence of weak intramolecular Hg · · · N or O bonding (combined with steric effects) largely precludes intermolecular association in [(2-SiMe2C5H4N)HgCl] (39),35 [
2-RnC6H5-nN:C(H)-C6H42Hg] [R: 4-Me2N (40), 4-Me (41), 2,6-iPr2 (42), 2,6-Me2 (43)],46 [(2,6-(Me2N)2C6H3)2Hg] (44),47 and [
2Ph2P(:O)-C6H42Hg] (45).48 The relatively short intramolecular Hg · · · H-C interactions in 43 [3.184(2)Å] have been used to argue for a larger van der Waals radius for Hg(II) of 2.1-2.2Å.46 In contrast to these previous examples, extensive intermolecular Hg · · · S interactions occur in the series of compounds [MeHg
S(S)COR] [R: Et (46), iPr (47), CH2Ph (48)].44 The supramolecular architectures of these species reveal a subtle dependence on the steric demands of the R group, with a double-chain (tape-like) arrangement being found in 46 and 47 while in 48 monomers associate into dimeric units which interact only weakly into a double chain arrangement. Extensive intermolecular interactions also occur in the series of novel bis(alkynyl) Hg(II) compounds [MeHgC∫C-RC∫CHgMe] (R: thiophene-2,5-diyl (49), bithiophene-5,5-diyl (50)], now by a combination of closed-shell Hg · · · Hg (3.777-3.935Å) and Hg · · · S bonding.45 The compound [
2-Ph2P-C6H52Hg] (51) combines the facets of donor and acceptor character found in all the examples mentioned above in the coordination of a range of metals in [
2-Ph2P-C6H42Hg] HgBr2 (52), [
2-Ph2P-C6H42Hg] PtCl2 (53) and [
2-Ph2P-C6H42Hg] NiCl2 (54).48 51 acts as an intact donor through the coordination of both of the pendent Ph2P groups and as an acceptor through the Hg centre.46 However, some rearrangement of the framework of 51 is observed on reaction with [PdCl2(PnBu3)2] or [PtCl2(cod)]. The Hg · · · metal interactions in 52 [Hg · · · Hg 3.1515(6)Å], 53 [Hg · · · Pt 2.8339(7)Å] and 54 [Hg · · · Ni 2.756(8)Å] suggest only weak bonding. The structurally characterised, trinuclear compound [
2-Ph2P-C6H42Hg]Pd[
2-Ph2P-C6H4Hg
2-Ph2P(:O)-C6H4] (55) exhibiting the coordination of intact and partially oxidised molecules of 51, is of particular interest.48 Compounds containing several Hg(II) centres have also been of considerable interest in recent years as novel acceptor-type ligands. The trimercury derivative [(C3N3S3)(HgMe)3] (56), obtained by reaction of MeHgCl with 2,4,6-trimercaptotriazine [C3N3(SH)3], represents such a potential ligand arrangement.50 56 is almost completely planar in the solid state, having a pseudo C3 (so-called) ‘manxane’ shape which results from the presence of weak, intramolecular Hg · · · N(ring) interactions. One of the highlights in the area of s-bonded organo Hg(II) chemistry reported in 2002 has been the applications of the macrocyclic compound [(o-C6F4Hg)3] (57) as an acceptor ligand (Figure 3).51-54 A series of complexes is formed with neutral O-donor ligands (L) of the types [57·m3-L] [L: Me2C:O,51 CH3CH2CH(:O),52 Ph2C:O52], [57·m3-L2] [L: Me2NCH(:O),53 (Me2N)3P:O,53 Ph(Me)C:O52] and [57·m3-L2·L] [L: Me2S:O,53 MeC(:O)OEt,53 Me2C:O52]. In the solid-state structures of 57·m3L the O-donor bonds almost symmetrically to the three Hg centres of the Hg(II) host above one side of the macrocycle. For 57 m3-L2, a further O-donor engages the other side of the macrocycle in a similar m3-bonding mode, while in [57 m3L2 L] a further O-donor bonds to one of the Hg(II) centres of 57 in a terminal mode. The lattice structure of [57 m3-Me2C:O] is particularly interesting.51













3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

69

Two molecules approach each other at their uncoordinated faces and are associated into dimer pairs by Hg · · · Hg (3.51Å) and (possibly) by Hg · · · p-arene interactions (3.40-3.48Å). This association is related to that observed in the structure of 57 itself, in which cofacial association into dimer pairs occurs largely through Hg · · · p-arene interactions (3.44-3.65Å) with the ipso-carbon centres of the C6F4 rings [the Hg · · · Hg contacts (3.81-4.09Å) being greater than expected for van der Waals interactions].54 The proclivity for the formation of Hg · · · parene interactions in this system is illustrated dramatically by the ability of 57 to coordinate neutral (electron-rich) arene molecules.54 The solid state structures of a series of complexes 57 L (L: 1-biphenyl, 1-naphthalene, 1-triphenylene) are similar, being composed of infinite stacks of alternating 57 and L in which the Hg · · · p-arene interactions are in the range 3.25-3.55Å.

Figure 3

Other examples of p-bonded organo-Group 12 compounds (in addition to 57 and its complexes, mentioned above) are also worthy of note.56-59 Both ZnCl2 and HgCl2 are coordinated by the neutral, quinoline-functionalised cyclopentadienyl ligand [1-quin-3-SiMe3-C5Me4] (quin: 8-quinoline) (58), giving [1-quin-3SiMe3-C5Me4]ZnCl2 (59) and [1-quin-3-SiMe3-C5Me4 HgCl2]n (60), respectively.60 The coordination of the Zn and Hg centres by the quinoline-N atom results in close proximity of the cyclopentadienyl ring to the metal centres and allows weak p-interactions to be predestined. The resulting formation of a relatively short Zn-h1-C interaction [2.316(2)Å] with the cyclopentadienyl ring in 59 is of interest in being one of the first examples of such an interaction between Zn2; and a neutral p-system. The structure of 60 exhibits Hg(II) · · · h2-p-arene interactions within both the monomeric (n: 1; 2.96-3.00Å) and dimeric (n: 2; 2.812.88Å) isomers that are found in the crystal. In addition to the unusual Zn p-bonding noted in 59, Zn and Cd p-bonding to formally neutral segments of porphyrins [Zn · · · h1-C 2.49(1)]57 and carbaporphyrins [Cd · · · h2-C(phenyl) 2.748(2)-2.762(2)Å]58 has also been observed recently. In the latter case, the structural data is also supported by the observation of 111/113Cd · · · H-C coupling (4.4Hz).58 The presence of Cd · · · h1-phenyl interactions in the dimer [
2,6-Ph2C6H3OCd
m-O(2-Ph2-C6H3)]2 (61) [2.622(5)-2.680(5)Å] may also account for

70

Organometallic Chemistry

its relative stability in solution compared to [
2,6-tBu2-C6H3OCd
m-O(2-tBu2C6H3)]2 (62).59 As expected, the applications of Group 12 reagents in organic synthesis was dominated by organozinc compounds in 2002. These applications are very extensive and, like the discussion of Group 2 reagents in organic synthesis (section 2, above), full treatment of this area is outside the scope of this review. Major areas of interest in 2002 concerned new methodologies to organozinc reagents,61-63 fundamental developmens of new organic reactions,64-67 mechanistic insights into organic reactions,68-71 and stereoselective reactions involving organozinc reagents.72-74 A very useful development was the room-temperature preparation of [F2C:CFZnCl] (63) (73%) from the reaction of iPr2NLi wth ZnCl2 and CF3CFH2 at 15-20°C in thf. Subsequent addition of RC6H4I (25-65°C) gives the a,b,b-trifluorstyrenes R-C6H4CF:CF2 (64) (61-86%).61 This procedure, which avoids the direct use of thermally unstable [F2C:CFLi] (65), is important in the industrial-scale synthesis of 64 which are important monomers in the formation of co-polymers used in fuel cell separators and dialysis membranes. A recent report of the electrochemical conversion of functionalised aryl bromides [YC6H4Br] [Y: CN, CF3, halogen, ROC(:O), RC(:O)] (66) with ZnBr2 into the corresponding arylzinc reagents [Y-C6H4ZnX] (67) is also worthy of note.62 The new procedure involves the use of an electrochemical cell fitted with a sacrificial anode in the presence of CoBr2, and avoids the use of pyridine which resulted previously in complications in subsequent organic reactions (particularly with acid chlorides). The merits of electrochemical generation of 67 are that the use of Rieke Zn is avoided and that a broad range of functional groups are tolerated. A detailed study of the kinetics of this reaction has suggested that the mechanism involves reduction of CoIIBr2 to CoIBr, followed by oxidative addition of 66 to give [Y-C6H4CoIIIBr].69 Reduction of the latter to [Y-C6H4CoIIBr] followed by exchange with ZnBr2 regenerates CoIIBr2 and gives 67. The first evidence for the formation of a geminal dizinc carbenoid [(EtZn)2CHI] (68) introduces this new reagent in the highly stereoselective synthesis of 1,2,3-substituted cyclopropanes.63 68 is generated most efficiently (ca. 69%) by the 2:1 reaction of Et2Zn with CHI3 in CH2Cl2. In situ reactions with the protected 2-butene-1,4-diols cis-[PhCH2OCH2CH:CHCH2OCH2Ph] (69) give 1,2,3-substituted cyclopropanes in which the exclusive formation of syn-diastereoisomers occurs as a result of stabilisation of one of the Zn centres by an O centre of 69 in the transition state. A DFT calculational study of the addition of the geminal dizinc carbenoids [(RZn)2CHI] [R: Et (68), I (70), I and Et (71)] to ethene shows that these reactions occur with significantly lower activation energies (ca. 15 kcal mol91) compared to reactions involving mono-zinc carbenoids such as [IZnCH2I] (ca. 20-25 kcal mol91).69 The lower activation barriers are due to the substatially larger positive charge distributions in 68, 70 and 71 and their consequent greater electrophilic character. The mechanism of the reaction of the dizinc carbenoids is also significantly different to mono-zinc reagents, involving a synchronous attack of the CH2 groups of ethene onto the CHI fragment of 68, 70 and 71 [a more asymmetric (asynchronous) attack of one of the CH2 groups of

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

71

ethene being found with the mono-zinc carbenoids]. Of the new organic reactions involving organozinc reagents introduced in 2002 the formation of C:C double bonds in the reactions of a range of benzylic or alkyl organozinc reagents [RCH2ZnX] (72) with aromatic or vinyl aldehydes RCH:O (73) in the presence of [NiCl2(PPh3)2] and Me3SiCl is of particular interest in providing a stereoselective alternative to the Wittig reaction.64 The procedure uses mild conditions (918-25°C), is highly tolerant to functional groups and gives the alkenes RCH:CHR (74) as only the E-isomers in generally good to excellent yields (54-92%). The mechanism of this reaction is proposed to occur according to Scheme 6. Finally, a report of the dramatic improvement of the enantiomeric excess in asymmetric 1,4-conjugate additions of diorganozinc reagents to a,b-unsaturated ketones is also of general interest.72 It was shown that the use of cheaper Cu(AcO)n (n: 1,2) or Cu naphthenate salts in these reactions leads to improved enantioselectivities than were obtained previously using Cu(OCOCF3)2. The latter had been generally accepted as the most active reagent in these reactions, as a result of its highly Lewis acidic character. Furthermore, using the new reaction conditions the use of donor solvents (Et2O or thf ) gave improved enantioselectivities over the existing methodology (which required non-donor solvents). This study illustrates that the Lewis acid character of the Cu salt employed is not decisive in reactions of this type. Indeed, it appears more likely that the bridging ability of the anion present is the most important factor, presumably in the formation of heterometallic Zn/Cu intermediates. In addition, less soluble organic precursors can now be employed using the new reaction conditions.

Scheme 6

References 1. 2. 3.

A. P. Dave, V. C. Gibson, E. L. Marshall, A. J. P. White, D. J. Williams, J. Chem. Soc., Chem. Commun., 2002, 1208. W. Vargas, U. Englich, K. Ruhlandte-Senge. Inorg. Chem., 2002, 41, 5602. K.-C. Yang, C.-C. Chang, J.-Y. Huang, C.-C. Lin, G.-H. Lee, Y. Wang, M. Y.

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4 Scandium, Yttrium and the Lanthanides BY JOHN G. BRENNAN AND ANDREA SELLA

1

Introduction

This review covers the synthesis, characterization, and reaction chemistry of organometallic complexes of Sc, Y and the lanthanides reported in the year 2002.* As usual we have excluded endohedral fullerene compounds and nonmolecular carbides and related species. Among the highlights this year are reports of the first organothulium(II) complexes,1 a dysprosium dinitrogen complex prepared from a transient divalent organodysprosium species,2 a samarium imido complex,3 further intriguing experimental4 and computational5 suggestions of surprising O-bound carbonyls, three unusual peroxo-bridged complexes,6-8, a stunning octanuclear Sm cluster based on pyrrolylmethane ligands,9 and a very detailed mechanistic study of insertions into Y-H bonds.10 In addition, it is increasingly clear from the rapidly rising number of papers in this area that DFT calculations are now proving rather more reliable than previously and computational chemistry is now starting to make a significant contribution to f-element chemistry. Finally this year’s BrenSell award goes to the groups of Maron and Andersen for their near-heretical suggestions of an oxygen-bound carbonyl.4,5

2

New Compounds – Structure and Reactivity

2.1 Cyclopentadienyl (Cp) and Lightly Substituted Cp Compounds. — Investigations of Ln chemistry with the Cp ligand focused almost entirely on straightforward preparative endeavours, although two reports detailed the subsequent use of these molecules in organic transformations. The synthesis, structures, and reactivity of new lanthanocene complexes incorporating amidinate ligand Z :
tBuNC(Bu)NtBu were described. Treatment of Cp2LnBu (Ln : Er, Y, Gd) with N,N-di-t-butylcarbodiimide results in insertion of carbodiimide into the * Abbreviations: Ln : lanthanide; Cp : C5H5; MeCp : C5H4Me; Cp* : C5Me5; Cp : C5H4(SiMe3); Cp : 1,3-C5H3(SiMe3)2; CpBu : C5H4But; CpBu2 : 1,3-C5H3But2; CpBu3 :1,2,4-But3C5H2; Ind : h-C9H7, indenyl; Flu : h-C13H8, fluorenyl; COT : C8H8; COT : 1,4-C8H6(SiMe3)2; TMEDA : tetramethylethylenediamine (1,2bis(dimethylamino)ethane); TMS : SiMe3; HMPA : hexamethylphosphorustriamide; DME : 1,2-(dimethoxy)ethane; MMA : methylmethacrylate; MAO : methylaluminoxane; DFT : density functional theory. Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 75

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Ln-C s-bond to yield Cp2LnZ (Ln : Er , Y, Gd). The Y and Gd compounds were characterized structurally. Within this series the Y compound is a catalyst for cyclotrimerization of PhNCO.11 Organolanthanide complexes, LnCl2Cp(PzA)2, (Ln : Nd, Sm, Eu, Tb; PzA : pyrazinamide) were prepared from NaCp and LnCl3(PzA)4 in THF. Thermal analysis and IR spectra indicate that coordination of pyrazinamide to the lanthanide ions is through the O of the carbonyl group and through one or both N of the pyrazinamide ring. The complexes gave low crystallinity polyethylene using a MAO co-catalyst.12, 13 Five new bis-Cp complexes were prepared by the reaction of Cp2LnCl (Ln : Yb, Dy, Pr, La, Sm) with LiC10H11O4 (piperonal dimethyl acetal) in THF at low temperature.14 The single pot reaction between SmX2 (X : Cl, I) and tBuLi in THF at 940°, followed by the addition of NaCp results in a dark red solution from which crystals of Cp2Sm(THF)2 were isolated.15 Finally, CpCuPPh3 acts as a Cp transfer reagent reacting with Pr, Er, Yb, Cp2Yb, and SmI2(THF)4, with deposition of Cu metal, to give lanthanide cyclopentadienyl derivatives Cp3Ln (Ln : Pr, Er, Yb), and CpSmI2.16 Lastly, in a contribution to an area of growing interest, the organolanthanide hydroxide [CpPr(acacen)2(m3-OH)]2 was prepared by the reaction of Cp3Pr with equimolar bis(acetylacetone)ethylenediamine (H2acacen) in THF, and its crystal structure was determined. The molecular structure contains two inequivalent Pr ions connected via two hydroxide groups and two O atoms from the Schiff base ligands.17 Reactions of LnCl3 (Ln : Nd, Gd, Dy, Er, Yb) with LiCpBu and LiCpBu2 in THF afford the lanthanocene ate complexes of the type [Cp2Ln(m-Cl)2Li(THF)2] (Ln : Nd, Gd, Dy, Er, Yb).18 An account detailed reactions of LnCl3 with one or two equivalents of KC5H4SiMe2But in THF that gave t (C5H4SiMe2Bu )LnCl2(THF)3 [Ln : Y, Er] and [(C5H4SiMe2But)2Ln(m-Cl)]2 [Ln : Gd, Er, Yb], respectively. The Er compound reacts with LiNPri2 and subsequently with PhNCO to form the PhNCO insertion product (C5H4SiMe2But)2Er[OC(NPri2)NPh](THF). The Er compound also reacts with Na benzothiazole-2-thiolate (NaSBT) to yield [(C5H4SiMe2But)2Er(SBT)(THF)].19 Finally, there were a number of reports that focused primarily on structural descriptions of new compounds. The heterometallic cyclopentyl-substituted complex (C5H9C5H4)3NdBrLi(THF)3 was obtained from the reaction of NdCl3 with NaC5H9C5H4 and LiBr in THF. [(C5H9C5H4)3SmTHF] was also prepared and structurally characterized.20 The analogous structure of [(MeCp)3SmTHF] was also determined.21 Anhydrous LnCl3 (Ln : Y, Sm, Lu) react with two K(C5H4SiEt3) to form the dimeric compounds [(h5-C5H4SiEt3)2LnCl]2 (Ln : Y, Sm, Lu), and these halides react further with MeLi to give the corresponding [(h5-C5H4SiEt3)2LnMe]2. All of these products were characterized by conventional and X-ray diffraction methods. Similarly, the reactions of LnCl3 with two KCp followed by one MeLi gave monomeric Me complexes [(h5Cp)2LnMe(THF)] (Ln : Sm, Lu).22 Finally, insertion reactions of CS2 into the Ln-N bond of [(MeCp)2LnNPh2] gave [(MeCp)2Y(h2-S2CNPh2)]2, and [(MeCp)2Yb(h2-S2CNPh2)], with the

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dimeric structure of the former presumably due to the slightly larger ionic radius of Y. Similarly, PhNCS reacts with the Ln-N bond to give dimeric [(MeCp)2Y
h2-SC(NPh2)NPh]2 (1) which also has an interlinked tricycle formed via two bridging S atoms.23 The corresponding insertion of PhNCS into a thiolate bond gave a thioformamide complex, [(MeCp)2Sm
SC(SPh)NPh(THF)] which was structurally characterized.24 2.2 Cp* and Related Per-substituted Ancillaries. — Divalent organolanthanide chemistry was extended further to the first organothulium complex and the reduction of N2 by an organodysprosium compound. In this work, KCp and TmI2(THF)3 in THF under Ar gave [(h5-Cp)2Tm(THF)] (2) and reaction of DyI2 with KCp in Et2O under N2 gave [(m-N2)(Dy(h5-Cp)2)2], (3). Both products were characterized thoroughly.2 Related chemistry with phospholes and arsoles is described in the heterocycles section 2.6.1

In continuing exciting studies of carbon monoxide coordination, the reaction of CO with the decamethylmetallocenes Cp*2M, (M : Mg, Ca, Sr, Ba, Sm, Eu, or Yb), have been studied in toluene or methylcyclohexane solution in highpressure IR cells. In all cases, except for Mg and Ba, the monocarbonyl complex Cp*2MCO is observed to form under CO pressure. The CO stretching frequencies M : Ca (2158 cm91), M : Sr (2159 cm91), M : Sm (2153 cm91), and M : Eu (2150 cm91), (4), are greater than for free CO (2134 cm91). By contrast, for Cp*2YbCO, (5), the 1:1 complex formed at low CO pressure (< 2 bar), nCO : 2114 cm91, below that of free CO. At higher CO pressures the 1:2 adduct Cp*2Yb(CO)2 with an even lower nCO : 2072 cm91 predominates. Equilibrium constants were determined as a function of pressure and temperature for the 1:1 and 1:2 complex formation (M : Ca, Eu, Yb). Models to account for the bonding in these metallocene CO adducts are discussed.4 Related computational studies are reported in the theoretical section 3.5 No evidence for CO homologation, as reported many years ago by Evans, has been found.25 In related studies, the base-free metallocenes [(CpBu3)2M], (M : Ca, Sr, Ba, Sm, Yb) have been prepared by reaction of MI2 and the sodium or potassium salt of the substituted cyclopentadiene anion in THF, while [(CpBu3)2Mg] was prepared from CpBu3H and MgBu2. The structures of the magnesocene and calcocene are slightly bent in the solid state with centroid-metal-centroid angles ranging from 163° to 173°. Adducts of the strontocene and samarocene with THF and of the barocene, samarocene, and ytterbocene with 2,6-Me2C6H3NC have also been prepared and characterized. The massive bulk of the CpBu3 ligand leads to behavior quite different from that previously observed for base-free metallocenes of the alkali

Organometallic Chemistry

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earths and divalent lanthanides.26 A pair of studies have reported magnetic coupling between Cp*2Yb and radical anion nitrogen-donor ligands. These adducts fall into different classes depending on the reduction potential of the base relative to that of Cp*2Yb. 1:1 adducts of the type Cp*2Yb(L) are formed with pyrazine, quinoxaline, 1,5- and 1,8-naphthyridine, and 4,4-bipyridine. All of the adducts in this class are paramagnetic, and their effective magnetic moments are consistent with the formulation of Cp*2Yb(L 9), in which spins on the individual units are uncoupled to 5 K. Adducts between Cp*2Yb and phenazine, 2,2-azopyridine, 2,2-bipyrimidine, 2,2-azobenzene, and 2,3-bis(2-pyridino)quinoxaline are of 2:1 stoicheiometry: (Cp*2Yb)2(m-L). The crystal structure of (Cp*2Yb)2(m-bipyrimidine) shows that the two metallocenes are bridged by the planar bipyrimidine ligand, and the other 2:1 adducts are assumed to have a similar structure. The effective magnetic moment of these 2:1 adducts shows that the ytterbocene fragments behave as isolated paramagnets. At low temperature the last three adducts undergo antiferromagnetic coupling with a Ne´el temperature of about 20 K. A spin polarization model was advanced to account for the electronic exchange coupling.27 A more detailed study of the magnetic behaviour of [CpR2Yb(L)] (L: bipy (6), phenanthroline, CpR : Cp*, Cp, C5H3But2) which includes several crystal structures, indicates coupling between Yb(III) and the single unpaired electron in the bipyridyl radical anion. The rate of intermolecular exchange of the heterocyclic base ligands in solution, the magnetism and the exchange coupling are all affected by the nature of the substituents on the cyclopentadienide rings.28 With a view to making a detailed comparison of neutral donor ligands at yttrium, the structures of Cp*2YCl(L) (L : THF, benzophenone, MMA, e-caprolactone, HMPA, e-caprolactam, 1-methyl-2-pyrrolidinone, N,N-dimethylpropyleneurea)were determined. The X-ray data show that the Y-O bond lengths decrease in the order above, although these bond lengths span only a short range and some are the same within experimental error. 1H NMR experiments indicate that ligand displacement reactions in solution follow a trend consistent with the bond lengths. The e-caprolactam complex is the exception, since there is hydrogen bonding between the ligand and the chloride. In addition, for e-caprolactam a bis adduct, Cp*2YCl(e-caprolactam)2, can be isolated.29

Casey has continued his elegant studies of olefin insertion into Y-H bonds. The rate of dissociation of [Cp*2YH]2 to the corresponding monomer was measured by labeling and by NMR line-broadening techniques. A full range of dissociative and associative mechanisms for reaction of alkenes with [Cp*2YH]2 was found depending on the reactivity of the alkene, which in turn depends on its degree of substitution. It was shown that the reactions of terminal alkenes with

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[Cp*2YH]2 initially gave mixtures of single- and double-alkene-insertion products but no triple-insertion products. The initially formed n-alkyl Y complex reacts with terminal alkenes at a rate similar to the reaction of [Cp*2YH]2 with terminal alkenes. Not surprisingly the more crowded b-alkyl Y double-insertion product was found to be much less reactive toward terminal alkenes.10 DFT calculations have been carried out to understand why Cp2YH polymerizes ethylene but not propene. A surprisingly large electronic, rather than steric, barrier to insertion is found for propene.30 Lanthanidocene silylamide complexes were obtained in high yields by protonolysis reactions with [Ln
N(SiHMe2)23(THF)2] (Ln : Y, La) and highly substituted cyclopentadiene derivatives C5Me5H, C5Me4H2 and C5Ph4H2, although deprotonation of C5Ph4H2 was accomplished only for La suggesting steric constraints play a role in these reactions. The silylamide ligands feature strong Si-H-Ln agostic interactions — nSiH1827 cm91 — that are indicative of a distinct weakening of the SiH bond. X-ray structure analyses of [
C5Me4H2YN(SiHMe2)2], [Cp*2YN(SiHMe2)2] and [
C5Ph4H2YN(SiHMe2)2] show that the structural features of the agostically bonded bis(dimethylsilyl)amide moiety depend on the steric crowding of the ancillary cyclopentadienyl ligand.31 Permethylyttrocene 2-furyl ate-complexes [Cp*2Y(m-C4H3O)(m-Cl)Li(THF)2 and Cp*2Y(m-C4H3O)2Li(TMEDA) were obtained by reaction of Cp*2Y(mCl)2Li(OEt2)2 with 1 and 2 equivalents of 2-lithiofuran respectively. Furan is metalated by [Cp*2YH]2 in hydrocarbon solvents to give the lithium-free 2-furyl complex [Cp*2Y(h2-C4H3O)] which forms Lewis base adducts [Cp*2Y(h1C4H3O)(L)] (L : THF, pyridine). The h2-furyl complex decomposes by deprotonation and ring-opening of the 2-furyl group to give the bimetallic ynylenolate complex [(Cp*2Y)2(m-OCH:CHC∫C)], (7). With excess H2, the h2-furyl complex is transformed into the oxo-complex [(Cp*2Y)2(m-O)], free furan, and n-butane while in the presence of a large excess of furan, this reaction stops at the enolate species [Cp*2YOCH:CHEt].32 LnCl3 (Ln : Y, Sm, Lu) and YbI2 react with the Li, Na, and K salts of 1-(but-3-enyl)-2,3,4,5-tetramethylcyclopentadiene, yielding [(C5Me4R)2Ln(mCl)]2 (Ln : Y, Sm, Lu; R : CH2CH2CH:CH2) and monomeric Yb(II) complex, respectively. The monomeric THF adducts [(C5Me4R)2LnCl(THF)] (Ln : Y, Sm) crystallize from hexane. Mixed sandwich complexes can be made by sequential reaction of trivalent halide with M(C5Me4R) and MCp* (M : Na, K), isolated as [(C5Me4R)Cp*LnCl]2 (Ln : Y , Sm, Lu). The metallocene halides react with LiMe, LiCH(SiMe3)2, and LiN(SiMe3)2, forming the corresponding unstable alkyl derivatives [(C5Me4R)2LnR] (Ln : Y, R : Me, CH(SiMe3)2; Ln : Sm, R : CH(SiMe3)2, and [(C5Me4R)Cp*YCH(SiMe3)2], as well as

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[(C5Me4R)2SmN(SiMe3)2]. Corresponding 1-(4-methylpent-3-enyl)-2,3,4,5-tetramethylcyclopentadiene complexes have also been prepared.33 2.3 Functionalized Cp Ligands. — The oxirane-ring opening of Bu glycidyl ether with NaCp or NaInd afforded cyclopentadienyl- and indenyl-substituted alcohols RHCH2CH(OH)CH2OBu (R : Cp or Ind) which were used as tridentate ligands. Proton transfer reactions of these alcohols with Ln[N(SiMe3)2]3 afforded the corresponding lanthanide complexes
[(h5-R)CH2CH(m2:h1O)CH2OBu]LnN(SiMe3)22 (R : Cp, Ln : La, Pr, Er , Lu; or R : Ind, Ln : La).34 Metalation reactions, particularly with furan, were noted. The alkyl complex [Y(h5:h1-C5Me4SiMe2NCMe3)(CH2SiMe3)(THF)] metalates furan and thiophene to give [Y(h5:h1-C5Me4SiMe2NCMe3)(m-2-C4H3X)]2 (X : O, S). These dimers have a six-membered core with bridging S/O atoms and trans-amido/Cp ligands. Replacement of the THF with DME gives the thermally stable derivatives [Y(h5:h1-C5Me4SiMe2NCMe3)(2-C4H3X)(DME)], that adopt a four-legged piano stool configuration. In the presence of excess DME, [Y(h5:h1C5Me4SiMe2NCMe3)(CH2SiMe3)(THF)] gives the isolable, five-coordinate complex [Y(h5:h1-C5Me4SiMe2NCMe3)(CH2SiMe3)(DME)] that does not react with furan or thiophene, suggesting that the metalation process had an associative mechanism. The alkyl compound reacts with pyrrole to give [Y(h5:h1C5Me4SiMe2NCMe3)(h1-kN-C4H4N)(DME)]. The reaction of [Y(h5:h1C5Me4SiMe2NCMe3)(CH2SiMe3)(THF)] (containing the longer CH2SiMe2bridge) reacts with thiophene to give [Y(h5:h1-C5Me4SiMe2NCMe3)(2C4H3S)(THF)].35 The reaction of YCl3 with 2 or 3 equivalents of methyl-piperidyl substituted Cp, NaC5H4CH([CH2]2)2NMe, in toluene affords mixtures of [
Y[hC5H4CH([CH2]2)2NMe]2(m-Cl)2] and [Y
h-C5H4CH([CH2]2)2NMe3]. The three amino-functionalized cyclopentadienyl units of the chloride dimer are fluxional at room temperature.36 The reaction product of Cp3Y and 2,4-cyclopentadiene-1-ethanol, when recrystallized from DME, gives the bridging pendant arm complex [m-h5:h1-2-(Cp)ethoxy]bis[(h5-Cp)(DME)Y]2 which was structurally characterized.37 A number of new, well-characterized compounds were shown to possess catalytic activity. The synthesis and crystal structure of the tetranuclear Yb(II) half-sandwich complex
[(h5-C5H4)CH2CH(m3:h1-O)CH2OBu]Yb4 was described. The compound forms a cubane-like arrangement of Yb(II) ions connected by O donors. A subsequent redox transmetallation with HgMe2 yields the trivalent methyl derivative [(h5-C5H4)CH2CH(m3:h1-O)CH2OBu]YbMe(THF), a compound that possesses a low catalytic activity in propylene polymerization.38 Cyano-functionalized [(C5H4CH2CH2CN)CpSmCl] was prepared and found to polymerize ethylene less effectively than did a related Ti(IV) compound.39 The metallocene complexes (CH2:CHCH2CH2C5H4)2LnCl(THF)2 (Ln : Sm, Y, Dy, Er) were synthesized as free-flowing oils and were fully characterized by MS, elemental analysis, and IR methods. The analysis and IR measurements indicate the presence of coordinated THF, with no evidence for intramolecular coordination of the butenyl side chain.40

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A ligand combining Cp* with triazacyclononane has been developed. Addition of LnI2(THF)2 (Ln : Sm, Yb) to K[C5Me4SiMe2(iPr2-tacn)] (tacn : 1,4diisopropyl-1,4,7-triazacyclononane) in THF yields the monomeric complexes [C5Me4SiMe2(iPr2-tacn)]LnI. Both the Sm and Yb derivatives have been structurally characterized.41 Heterobimetallic complexes have been obtained by the reaction of the divalent phosphine-substituted complex [Yb(C5H4PPh2)2 (THF)] with W(CO)6. The reaction is solvent dependent — THF yields [Yb(THF)3(C5H4PPh2)2W(CO)4] while toluene gives [Yb(THF)(C5H4PPh2)2(m-OC)W(CO)3]. The latter contains an isocarbonyl link resulting in infinite chains.42 2.4 Linked and Ansa-Cp Ligands. — Increasingly sophisticated organolanthanide compounds continue to evolve with anionic ring systems bridged by a covalent spacer. New chiral C1-symmetric organolanthanide ansa-metallocene catalysts [Me2Si(OHF)(CpR*)LnN(TMS)2] (OHF : h5-octahydrofluorenyl; Cp : h5-C5H3; R* : (—)-menthyl; Ln : Sm (8), Y, Lu; TMS : SiMe3) have been prepared. Lithium-ate complex precursors can be prepared in up to 90% diastereomeric purity and then converted into the corresponding amides, which are isolated in approximately 100% diastereomeric purity after recrystallization. The complex (S)-[Me2Si(OHF)(CpR*)YN(TMS)2] was crystallographically characterized.43 Catalysis by these complexes is described in section 5.6. Amine elimination was employed to incorporate the isopropylidene-bridged C5H5CMe2-C13H9 (CpH-CMe2-FluH) onto group III-metal centers (Y, La, Nd) to give the neutral, ate-free ansa-lanthanidocenes (h5,h5-Cp-CMe2-Flu)Ln(h5-Cp-CMe2FluH) (Ln : Y, La , Nd). These reactions proceed via the rapidly formed bis(amido)lanthanide intermediates (h5-Cp-CMe2-FluH)Ln[N(SiMe3)2]2 (Ln : Y, La), which readily undergo disproportionation/ligand redistribution reactions to give either a mono(amido)lanthanide complex (h5-Cp-CMe2FluH)2Y[N(SiMe3)2] (9) or what is assumed to be the binuclear complex (h5-CpCMe2-FluH)2La[m-N(SiMe3)2]2Ln[N(SiMe3)2]2, respectively. The reversibility of this amine elimination process was investigated in the yttrium case, in which the Y compound, in the presence of (SiMe3)2NH in toluene at 90° undergoes 50% conversion after 12 h. The effect of a non-coordinating apolar solvent on the reaction outcome of tris(amido) complexes was also studied using toluene, in which the low solubility presumably shifts the disproportionation equilibrium and leads to the isolation of Ln(h5-Cp-CMe2-FluH)3 (Ln : Y, La).44 The reactions of rac-[Me2Si(2-MeC9H5)2]YN(SiHMe2)2 with AlR3 are discussed in the aluminates section 2.9. Beautiful ansa-metallocenes (h5:h5-C24H16)M(THF)2 (M : Sm, Yb, Ca) (10) and (h5:h5-C24H16)MI(THF) (M : Dy, Er, Tm, Lu) were prepared by in situ reactions of two equivalents of potassium acenaphthylenide KC12H8 with MI2 or MI3, respectively. The Yb and Ca complexes were also obtained by direct reduction of acenaphthylene with the corrsponding naphthalenides. An ESR signal of the acenaphthylene radical anion for the Yb compound in THF indicates that the [C24H16]2- ansa-ligand dissociates into two [C12H8]·9 radical anions. Hydrolysis of this complex in benzene afforded 1,1,3,3-tetrahydro-3,3-

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biacenaphthylene and 3,3,4,4-tetrahydro-3,3-biacenaphthylene. The reactions of the Yb compound with ZrCl4 and silylation of the Ca compound proceeded with the cleavage of the C-C bond between two acenaphthylene fragments of the [C24H16]2- ansa-ligand to produce (h2-C12H8)ZrCl2(THF)3 and 1,2-bis(trimethylsilyl)acenaphthene, respectively. NMR analysis of the Lu derivative in THF revealed a dynamic exchange of the solvent molecules in the coordination sphere of the Lu atom. The dynamic process is terminated at 210 K, as evidenced by the nonequivalence of the two acenaphthylene fragments. According to the X-ray diffraction data for this Lu compound, dimerization of two acenaphthylene radical anions at the Lu atom produces a rac-ansa-metallocene structure, in which the Lu atom has distorted tetrahedral geometry with two h5-coordinated five-membered rings of the acenaphthylene ligands, the I atom and the THF molecule. The average Lu-C distance is 2.623Å.45

The one-carbon bridged lanthanocene amide complex [
Ph2C(Flu)(Cp)LuN(TMS)2] was synthesized by reaction of the chloride precursor with LiN(TMS)2 in toluene. The X-ray structure of the amide complex exhibits an intramolecular bSi-C agostic interaction with Lu. The complex was used as a stereospecific catalyst producing richly syndiotactic PMMA, and in ring opening polymerization of e-caprolactone and d-valerolactone producing polymers with moderate polydispersity.46 Five homologs of the new triflate series [LnL(m-OTf)]2 with Ln : Pr, Nd, Sm, Y, Yb have been prepared from [Ln(OTf)3] and Na2L, where L designates two cyclopentadienyl rings tethered by a 2,6-bis(methylene)pyridyl unit. According to crystal structure analyses, each triflate ligand bridges two Ln ions (Ln-O-S(O)O-Ln). Variable-temperature 1H NMR spectra of all complexes reveal notable fluxionality. According to a detailed 19F NMR study of the Y derivative, two dinuclear isomers, differing in the trans and cis orientations of their two L fragments, interconvert with a negligibly small DS‡.47 2.5 Indenyl and Fluorenyl Ancillaries. — Indenyl chemistry was relatively scarce this year, with the high point in the field coming from adventitious incorporation of oxygen. The reaction of YbI2 with two cyclopentylindenyl lithium (C5H9C9H6Li) gives divalent [(C5H9C9H6)2Yb(THF)2], a molecule that shows high activity for ring-opening polymerization of lactones. The reaction between YbI2 and C5H9C5H4Na in the presence of trace O2 gave the peroxo dimer [(C5H9C5H4)2Yb(THF)]2O2, an extremely unusual molecule with an asymmetric O2 unit.8 The divalent cyclopentyl-indenyl complexes (1-C5H9C9H6)2Ln-(THF)n (Ln : Sm, Yb) and K(1- C5H9C9H6)3Sm(THF)3 have been prepared and found to

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be active for the polymerization of acrylonitrile.48, 49 The cyclopentane-substituted indenyl complex [Li(THF)4][(C5H9C9H6)3SmCl] was synthesized by treating SmCl3 with two C5H9C9H6Li in THF and shown to adopt a salt-like structure by X-ray diffraction.50 Pendant arm tetrahydrofurfuryl complexes (C4H7OCH2C9H6)2LnCl (Ln : Nd, Sm, Dy, Ho, Er, Yb) have also been prepared. All six compounds are unsolvated nine-coordinate monomers with a trans arrangement of both the sidearm and indenyl rings. They are more stable to air and moisture than the corresponding unsubstituted indenyl lanthanide complexes.51 Finally, a series of silylene- and methylene-bridged fluorenyl cyclopentadienyl ansa-lanthanocene chlorides, hydrocarbyls, amides and tetrahydroborates were prepared and characterized comprehensively. The hydrocarbyls and amides were used to polymerize MMA and lactones.52 In other investigations, metathesis reactions of bis(9-lithiofluorenyl)(methyl)(phenyl)silane with LnCl3 (Ln : Yb, Dy, Pr, La, Sm, Nd) gave six new lanthanide indenyl complexes53 while five new silyl-bridged organolanthanide complexes Me2SiFlu2LnCl (Ln : Yb, Sm, La, Pr, Nd) were synthesized by the reaction of LnCl3 with Li2Me2SiFlu2.54 2.6 Heterocyclic Ancillaries. — The dimethylindolide salt Me2C8H4NK reacts with SmI2(THF)4 in THF to form the distorted trans-octahedral complex (Me2C8H4N)2Sm(THF)4, (11) in which the dimethyindolide anions are bound in the h1 mode to Sm. Reaction of 2,3-dimethylindole with Y(CH2SiMe3)3(THF)2 afforded the amide complex (Me2C8H4N)3Y(THF)2, in which the dimethylindolide anions are also h1 bound. With Cp*2LnCl2K(THF)2 (Ln : Sm, Y), KMe2C8H4N forms the unsolvated amide complexes Cp*2Ln(Me2C8H4N) (Ln : Sm; Y), in which indolide attaches primarily through nitrogen.55 Addition of BH3 THF to 1-alkylimidazoles (alkyl : Me, butyl) and 1-methylbenzimidazole leads to BH3 adducts that can be deprotonated with BuLi to yield organolithium compounds that metathetically displace chloride from ScCl3 to make 16-electron ate complexes. One structurally characterized example contained two larger (140.2 and 142.9°) and four smaller C-Sc-C angles (93.9-98.7°) with four bridging 3c-2e B-H-Sc bonds to give an 8-coordinate structure.56

In pyrrole chemistry, the reaction of diphenylbis(2-pyrrolyl)methane with KH and subsequent treatment with YbI2(THF)2 leads to a complex reaction from which an octameric divalent Yb macrocyclic complex, [diphenyldipyrrolyl-

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methanediyl-Yb]8, (13) was obtained as a major product. A tetrameric cyclic Yb(II)-oxo complex, [(diphenyldipyrrolylmethanediyl)Yb]4[K(THF)3]2(m-O), (12), arising from solvent deoxygenation and a monomeric Yb(III) complex, Yb(diphenyldipyrrolylmethanediyl)3[K(THF)]3, were also isolated as byproducts of this complex reaction.9 Dipyrrolide dianions were formed by a transient Tm(II) complex via fragmentation of the (Et8-calix[4]tetrapyrrole)[K(DME)]4 ligand during the reaction with TmI2(DME)3. The crystal structure of [(Et8calix[4]tetrapyrrole)Tm][K(DME)],
[Et2C(C4H3N)2]3Tm[K(toluene)]3, and
[(Et8-calix[4]tetrapyrrole)TmK(DME)] were established.57 Also, reactions of SmI2(THF)2 and YbI2(THF)2 with the alkali-metal salts of 2,5-dimethylpyrrole, or the reaction of SmCl3(THF)3 and YbCl3(THF)3 with the same ligands followed by reduction with the appropriate alkali metals, led to the formation of divalent mono- and polynuclear complexes. Structural characterization of these complexes indicated that the bonding mode adopted by the ligand depends on the nature of the alkali-metal cation retained in the structure.58

Finally, hydrolysis of lanthanide pyrazolylborate complexes Ln(TpMe2)2X [TpMe2 : (HB(dmpz)3), where X is a basic anionic ligand] in various solvents gave the insoluble dimers [Ln(TpMe2)(m-BOpMe2)]2 [Ln : La, Ce, Sm; BOpMe2 : (HBO(dmpz)2)29; dmpzH : 3,5-dimethylpyrazole], (14). In the several structures noted, one metal-N bond length, that from Ln to the N across from the two negatively charged bridging O atoms, is significantly longer than the others, suggesting an unusual trans influence at a lanthanide center. The formation of these products is proposed to involve the intermediacy of Ln(TpMe2)2OH produced by protonolysis with adventitious water.59 Bonds to phosphorus were most prevalent in the organometallic chemistry of lanthanide ions attached to more electropositive ligands. Most surprisingly, the reactions of TmI2 with substituted phospholide and arsolide salts afforded stable bis(phospholyl)- and bis(arsolyl)thulium(II) complexes, respectively, that were characterized by multinuclear NMR and X-ray diffraction methods. Thus, reaction of KQ [Q : 2,5-bis(trimethylsilyl)-3,4-dimethylphospholide, -arsolide, or 2,5-di-tert-butyl-3,4-dimethylphospholide] with TmI2(THF)3 in Et2O afforded

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the corresponding [Q2Tm(THF)] (15) complexes. The relatively ‘facile’ isolation of these highly reactive products illustrates utility of these sterically demanding phospholyl and arsolyl ligands for the stabilization of divalent Tm.1 In trivalent chemistry, new mixed phospholyl/COT lanthanide complexes were prepared by metathesis of phospholylpotassium salts with (COT)LnCl precursors: [(COT)Sm(Tmp)(THF)], [(COT)Sm(Dsp)], and [(COT)Nd(Dsp)(THF)], (16) have been thus prepared (Tmp : 2,3,4,5-tetramethylphospholyl, Dsp : 3,4dimethyl-2,5-bis(trimethylsilyl)-phospholyl). The Sm compound can also be approached by a redox reaction of [(Tmp)2Sm] with COT.60 A report detailing the crystal structure of [K(C12H24O6)(C4H8O)2]
[C4(CH3)4P]2Nd(BH4)2 also appeared.61 These and other organometallic phosphole compounds were reviewed.62 The metathesis reaction of [(THF)4Li][Cp2Li] with YCl3 yields (THF)2LiCp2YCl2. Metathesis with LiP(SiPri3)2 in toluene occurs stepwise giving [(THF)2Li][Cp2YCl(P(SiPri3)2)] and [Cp2Y(m-P(SiPri3)2)2Li(THF)], a colorless crystalline material with a YP2Li ring and Y-P bond lengths of 2.840Å. In solution a dissociation is observed into Li (triisopropylsilyl)phosphide and [Cp2Y(m-P(SiPri3)2)(THF)]. When K (triisopropylsilyl)phosphide is used in benzene, the product is ([Cp2Y(m-P(SiPri3)2)2K(h6-C6H6)], (17) with a YP2K ring. Similar chemistry is observed using (DME)LiPH2.63

Metalation of HP(SiMe3)2 with [Y
CH(SiMe3)23] in toluene gives the dimeric tris phosphido compound [Y
P(SiMe3)23]2. The metathesis reaction of [1,3(Me3Si)2C5H3]2YCl with K(PSitBu3) yields (tetrahydrofuran-O)-1,1,3,3-tetrakis(trimethylsilyl)yttrocene-tri(tert-butyl)silylphosphide, which was characterized both in solution and in the solid state. The coupling constants 1J(YP) and 1 J(PH) show values of 144.0 Hz and 201.0 Hz respectively.64

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2.7 Amido and other Nitrogen-based Supports. — A range of amide, aryloxide, and b-diketonate complexes of the type [(salen)Y(X)(THF)]2, (salen : N,Nbis(3,5-di-tert-butylsalicylidene)ethylenediamine; X : N(SiMe3)2, 2,6-di-tertbutylphenoxide, hexafluoroacetylacetonate, Cp*) have been obtained either by metathesis or by protonolysis, and their crystal structures were determined.65 Organometallic derivatives of scandium and yttrium supported by a bulky salicylaldiminato ligand were prepared from [Ln(CH2SiMe2R)3(THF)2] (Ln : Sc, Ln : Y; R : CH, Ph) via alkane elimination. The new precursors are convenient alternatives to the methyl derivatives, due to their higher thermal stability and crystallinity. Reaction with protio ligands give isolable mono(ligand) bis(alkyl) derivatives only for Y while for Sc mixtures were obtained. The Y compounds coordinate either one or two THF ligands, and there is a fluxional exchange process that equates the two geometric isomers. Reactions of LnR3 tris(alkyls) with one or two protio ligands gives the five-coordinate, THF-free bis(ligand) mono(alkyl) complexes that decompose via a pathway involving metalation of one of the N-aryl iso-Pr Me groups. These bis-ligand compounds react slowly but cleanly with H2 either by transfer of the in situ formed scandium hydride to the aldimine carbon of one of the ligands or to form a D2-symmetric dimeric m-hydride complex. A survey of hydride reactivity toward deuterated solvents, ethylene, (trimethylsilyl)acetylene, [HB(C6F5)2]2, benzophenone, pyridine, and THF suggests that it reacts as a dimer. Nonetheless, products from the reactions of (trimethylsilyl)acetylene, [HB(C6F5)2]2, benzophenone, pyridine, and THF were isolated and characterized. Finally, reactions of mono(alkyls) with a final equivalent of protio ligand gave nonorganometallic tris(ligand) coordination complexes. The scandium derivative contained an O-bound k1-salicylaldiminato ligand, while for the larger Y nucleus, all three ligands were chelating in the normal k2-bonding mode.66 Chelating diamido compounds were described in abundance. Of the structural reports, reactions of the bis(phosphinimino)methanides [
CH(PPh2NSiMe3)2-k2N,N,k-CLnCl2]2 (Ln : Y, Sm, Er) with NaCp in a 1:4 molar ratio in THF afforded the corresponding metallocenes [Cp2Ln
CH(PPh2NSiMe3)2-k2N,N,kC]. The structures of these new compounds were investigated in solution and in the solid state — both imine groups and the methine carbon coordinate the Ln.67 Five- and six-coordinate Sc and Y complexes of the diamide-diamine donor ligand N2NN (H2N2NN : (2-C5H4N)CH2N
CH2CH2N(H)SiMe32) were described. Reactions of ScCl3 with Li2N2NN gave a five-coordinate scandium monochloride product, while the analogous reaction with YCl3 in THF or py solution afforded ate complexes [YCl(N2NN)(L)]·1.5(LiCl) (L : THF, py). The Sc and Y monochlorides can be derivatized in classic organometallic fashion, i.e.metathetical replacement with LiN(SiMe3)2, LiNMe2 or LiNHR (R : tBu or 2,6-C6H3iPr2) gave the corresponding five-coordinate amides [M(NRR)(N2NN)]. The s-bond metathesis reaction of [Sc(NMe2)(N2NN)] with C6F5NH2 gave the expected fluorophenylamide product. These chlorides also react with Li[PhC(NSiMe3)2] to form fluxional, six-coordinate benzamidinate derivatives [M
PhC(NSiMe3)2(N2NN)] and with LiCH2SiMe3 to give [Sc(CH2SiMe3)(N2NN)].68

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Deprotonation of the bridged diamine ArHN(CH2)3NHAr (Ar : 2,6Pr2C6H3) with BuLi and KH, respectively, gives the corresponding alkali metal derivatives M2
ArN(CH2)3NAr (M : Li, K). The Li salt reacts with YCl3 to form the octahedral Y metalate complex [Y
ArN(CH2)3NAr(THF)2(mCl)2Li(THF)2]. Reaction of this heterometallic product with NaCp in THF affords the complex [Y
ArN(CH2)3NAr(h5-Cp)(THF)] which is best described as a distorted tetrahedron. Switching to LuCl3, however, leads to the unexpected ionic product [LuCl2(THF)5][Lu
ArN(CH2)3NAr2]. As seen from the structures of these products, the steric demands of the
ArN(CH2)3NAr229 ligand are less than that of two Cp*9 moieties.69 Similarly, YI3 reacts with the potassium salt K2[ArN(CH2)3NAr] (Ar : 2,6-iPr2C6H3) to yield the mono-iodide complex Y[ArN(CH2)3NAr]I(THF)2, and subsequent metathetical replacement of the chloride with K[CH(SiMe3)2] gives the corresponding alkyl complex [
ArN(CH2)3NArYCH(SiMe3)2(THF)] (18).70 The synthesis and characterization of three new classes of Sc compounds with mono- or di-anionic supporting ligands possessing N4- (H2L1; 2-((Me3SiNHCH2CH2)2NCH2)pyridine), N3O(HL2; 2,4-tBu2-6-(4,7-Me2-1,4,7-triazacyclonon-1-ylCH2)C6H2OH) or N2O2(H2L3; 2-((3,5-tBu2-2-HOC6H2CH2CH2)2NCH2)pyridine) donor atom sets (i.e. Cl(py)ScL3, Me3SiCH2ScL1, and (4-MeC6H4O)2ScL2) were described.71 Ketimido compounds were described for both Sc sand Yb. A reduction of the b-diketiminato compound (Et2NCH2CH2NC(Me)CHC(Me)NCH2CH2NEt2)ScBr2, with (C3H5)MgBr gave the unexpected scandium complex (Et2NCH2CH2NC(Me)CHC(Me)NCH2CH2NEt2)MgBr)2ScBr, (19). Correlation of all spectroscopic characterization results leads the authors to the conclusion that this is a diamagnetic compound with a formal oxidation state of ;1. However it is worth noting that the electronic structure has not been investigated and the diamagnetism probably arises from strong antiferromagnetic coupling between the metal and the clearly ‘non-innocent’ ligand.72 The reactions of the base-free di-Me Sc complex (L)ScMe2 [L : ArNC(tBu)CHC(tBu)NAr, Ar : 2,6-iPr2C6H3)] with various equivalents of B(C6F5)3 gave a series of cationic species including a spectroscopically characterized monocationic m-Me dimer
(L)ScMe(m-Me)[B(Me)(C6F5)3], a monomeric, fluxional Me cation
(L)ScMe[MeB(C6F5)3], and the unique dication
(L)Sc[B(Me)(C6F5)3] which has a static structure on the NMR time scale. The di-methyl compound.is a catalyst precursor for ethylene polymerization under borane or MAO-type activation, with activity that approaches Group 4 metallocene systems.73 With a different focus, reaction of [(DIPPh)2nacnac]Li ((DIPPh)2nacnac : N,N-diisopropylphenyl-2,4-pentanediimine anion) with YbCl3 in THF, followed by crystallization from toluene, afforded monomeric [(DIPPh)2nacnac]YbCl2(THF)2 in high yield. Recrystallization from toluene causes loss of some THF to give dimeric
[(DIPPh)2nacnac]YbCl(m-Cl)3Yb[(DIPPh)2nacnac](THF), which has a rare triple chloride bridge. The monomer reacts with NaMeCp to form (MeCp)[(DIPPh)2nacnac]YbCl, and this compound reacts further with LiNR2 (R : Ph, Pri) in THF to form (MeCp)[(DIPPh)2nacnac]YbNR2.74 Although not strictly speaking organometallic, the reaction of YbCl3 with 2 equivalents of NaN(SiMe3)2 gave a mixture of various bis(trimethylsilyl)amides i

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with the known complexes [Yb
N(SiMe3)22(m-Cl)(THF)]2 and [Yb
N(SiMe3)23] as the main products and the cluster compound [Yb3Cl4O
N(SiMe3)23(THF)3] as a minor product. Treatment of the mixed halide complexes with hot n-heptane gave the base-free complex [Yb
N(SiMe3)22(m-Cl)]2 in low yield. In addition to reporting the structures of all of these compounds, the related peroxo complex [Yb2
N(SiMe3)24(mO2)(THF)2] was also structurally reported. DFT calculations support the assignment of the O-O stretching vibration (775 cm91) observed in the Raman spectrum and were used to explore the nature of the agostic-type interactions in the THF-free complexes.6 Another unusual non-organometallic peroxo complex, [Ln4(O2)2Cl8(py)10], (Ln : Sm, Eu, Gd) was also reported.7 Finally, phosphorus-supported carbene complexes with a wide variety of ancillary ligands were outlined in a patent.75 An alternative resonance description of these molecules as amido compounds is also reasonable. 2.8 Carborane and other Boron-containing Ligands. — In carborane chemistry, the reaction of closo-exo-5,6-Na(THF)2-1-Na(THF)2-2,4-(SiMe3)2-2,4-C2B4H4 and LnCl3 in a molar ratio of 3:1 gave [Na3][1,1-[5,6-(m-H)2-nido-2,4-(SiMe3)22,4-C2B4H4]-2,2,4,4-(SiMe3)4-1,1-commo-Ln(2,4-C2B4H4)2] (Ln : Er, Dy) as yellow crystalline solids. Use of a 2:1 ratio gave the lutetiacarborane complex, 2,2,4,4-(SiMe3)4-3,5,6-[(m-H)3Na(THF)2]-1,1-commo-Lu(h5-2,4-C2B4H4)2, as an off-white crystalline solid.76 Alkali salts of ‘carbons-adjacent’ carborane anions of the C2B10 system were readily derivatized. Treatment of [
m-1,2-[o-C6H4(CH2)2]-1,2-C2B10H102Na4(THF)6]n with two LnCl3 gave the novel half-sandwiches [h6-
m-1,2-[o-C6H4(CH2)2]-1,2C2B10H10Yb(NC5H5)3
Na(NC5H5)20.5]2(m-Cl) and [h6-
m-1,2-[o-C6H4(CH2)2]1,2-C2B10H10]LnCl(THF)3 (Ln : Nd, Er, Y), which represent the first lanthanacarboranes containing a terminal Ln-Cl bond in the C2B10 systems. The Cl can be substituted, leading to the isolation of [h6-
m-1,2-[o-C6H4(CH2)2]-1,2C2B10H10]Nd[(m-H)3BH](THF)3, (20) and [h6-
m-1,2-[o-C6H4(CH2)2]-1,2C2B10H10]Y(h5-Cp)(THF), respectively. Reduction of the chlorides with excess Li metal afforded the full-sandwich compounds
h6-[m-1,2-[o-C6H4(CH2)2]1,2-C2B10H10]2Ln
Li5(THF)10 (Ln : Er, Y), while reduction with excess Na metal generated the cluster [
h6-[m-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10]2 Er
Na5(THF)6]4. These full-sandwich complexes were also prepared from the reaction of [
m-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10Li4(THF)6]2 with LnCl3. Their Gd analog
h6-[m-1,2-[-C6H4(CH2)2]-1,2-C2B10H10]2Gd
Li5(THF)9 was pre-

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pared via a ‘one-pot’ interaction between m-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10, excess Li metal, and GdCl3.77 Treatment of LnI2 with [iPr2NB(C9H6)(C2B10H10)] Li2(OEt2)2 generated the trivalent organolanthanide complexes meso-[
h5:si Pr2NB(C9H6)(C2B10H10)2Ln][Li(S)n] (Ln : Sm, Yb; S : THF, n : 4; S : DME, n : 3). Both inter- and intramolecular electron-transfer pathways are proposed for these reactions. Reaction of LnCl3 with [iPr2NB(C9H6)(C2B10H10)] Li2(OEt2)2 gave the same ionic complexes (Ln : Nd, Y, Yb; S : THF, n : 4; S : DME, n : 3). Silylamine elimination reactions of iPr2NB(C9H7)(C2B10H11) and Ln[N(SiHMe2)2]3(THF)2 resulted in clear formation of [h5:si Pr2NB(C9H6)(C2B10H10)]LnN(SiHMe2)2(THF)2 (Ln : Nd, Er, Y). The Nd compound is an active catalyst for the polymerization of Me methacrylate (MMA) in toluene, affording syn-rich poly(MMA).78 Finally, the reaction between 1,2(SiMe3)2-closo-1,2-C2B4H4, ErCl3, and K in a 2:1:4 molar ratio, in the absence of an outside electron-transfer agent, produced 2,2,4,4-(SiMe3)4-3,6-[(mH)2K(THF)2]-1,1-commo-Er(h5-2,4-C2B4H4)2. 79 The insertion of elemental chalcogen E (E : S, Se) into Li-C bonds of dilithium o-carborane in THF solution afforded dimers of dilithium dichalcogenolate carboranes, [(THF)3LiE2C2B10H10Li(THF)]2 (E : S, Se). Reactions of the chloride products with these thiol or selenol compounds gave dinuclear complexes of the formula [Li(THF)4]2[Cp2LnE2C2B10H10]2. According to the X-ray structure analyses of a pair of Nd compounds, the dianions contain two o-carborane dichalcogenolate bridges, and each Cp2Ln fragment is attached to one terminal and two bridging chalcogen ligands. The central Ln2E2 four-membered ring is not planar, and the direct metal-metal interaction is, not surprisingly, absent.80 In other boron work, lithium 1-aminoboratabenzenes react with LnCl3 in toluene to give the stepped ladder dimers [(3,5-Me2C5H3BNiPr2)2Y(m-Cl)2Li]2, [(3,5-Me2C5H3BNiPr2)2Lu(m-Cl)2Li]2, and the mononuclear complexes YCl[3,5Me2C5H3BN(SiMe3)2]2 (21), LuCl[3,5-Me2C5H3BN(SiMe3)2]2, and ScCl(3,5Me2C5H3BNiPr2)2. When donor solvents such as THF or THP are used, analogous reactions take place at ambient temperature, giving solvates such as (3,5Me2C5H3BNMe2)2Sm(m-Cl)2Li(THF), (3,5-Me2C5H3BNMe2)Y(m-Cl)2Li(THP), (3,5-Me2C5H3BNMe2)Sm(m(3,5-Me2C5H3BNMe2)Y(m-Cl)2Li(THF)2, Cl)2Li(THF)2, and the ansa complexes [(3,5-Me2C5H3BNMeCH2)2]Lu(mCl)2Li(THF)2 and [(3,5-Me2C5H3BNMeCH2)2]Lu(m-Cl)2Li(THP)2 (22). These bridges are cleaved by donor ligands in toluene. In all polynuclear complexes, the ring-ring rotational positions are close to 180° in order to minimize transannular ligand-ligand repulsions. Site exchange between terminal and internally coordinated B-NMe2 sidearms is observed.81

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2.9 Aluminates. — The year produced an extensive array of heterometallic Ln/Al compounds with useful chemical properties. Silylamide elimination reactions of rac-[Me2Si(2-MeC9H5)2]YN(SiHMe2)2 with AlR3 in hexane gave the tetraalkylaluminate complexes rac-[Me2Si(2-MeC9H5)2]Y(m-R)2AlR2 (R : Me; Et; iso-Bu). Further reactions with THF gave the terminal alkyl complexes rac-[Me2Si(2-MeC9H5)2]YR(THF) (R : Me; Et; iso-Bu) in a reversible process; treatment with AlR3 regenerated the aluminates. Mixed-alkyl aluminate lanthanidocene complexes were prepared by treating the THF complexes with AlR3 (R : Et, Me, iso-Bu), and the structure of the ethyl derivative was confirmed. These mixed alkyl compounds were discussed in terms of models for polymer chain transfer in organoaluminum-dependent Ziegler-Natta catalysts.82 Alkylated Ln form when
Ln(O2CC6H2iPr3-2,4,6)3n (Ln : Y, La, Nd, Lu) react with AlMe3 to yield hexane-soluble monomers [
AlMe2(O2CC6H2iPr32,4,6)22LnAlMe4]. These products contain an h2-coordinating tetraalkylaluminate ligand and a novel ancillary AlMe2-bridged bis(carboxylate). In the presence of AlEt2Cl, the alkylated Lu compound transformed isoprene to a high-cis polymer (> 99%).83 The reaction of [Ln(OAr)3]2 with four equivalents of AlR3 leads to the formation of the bis-AlR3 adducts (ArO)Ln[(m-OAr)(m-R)AlR2]2 [Ln : La, R : Me; Ln : La, R : Et; Ln : Sm, R : Et] (23). The X-ray crystal structure of the Ln-Me compound reveals short La-C(bridging) distances of 2.800(5) and 2.759(5)Å. A reduced 1J(C-H) coupling constant of 110 Hz and a low energy v(C-H) stretch in the solution and solid state IR spectra are consistent with a strong agostic La···H-C interaction in solution. The solid-state structure of the samarium Et compound contains close contacts of 2.627(4) and 2.649(4) Å between the samarium center and the methylene carbons of the triethylaluminum groups. The room temperature 13C NMR spectrum of the Sm compound exhibits a 1J(C-H) coupling constant of 102 Hz; in addition, the fluxional process that exchanges Me groups in the La compounds is slow enough on the NMR time scale to allow distinct methylene groups in the Sm compound to be observed. Thermolysis of these products results in ligand redistribution to form [R2Al(OAr)]2 [R : Me, Et] and other unidentified species. A similar interaction was noted again in the reaction of [Sm(OAr)3]2 with excess trimethylaluminum that gave the bis-trimethylaluminum adduct (ArO)Sm[(mOAr)(m-Me)AlMe2]2, which exhibits very short Sm-C(bridging) distances of 2.620(5) and 2.632(5)Å. A reduced C-H coupling constant of 106 Hz and a low n(C-H) stretch in the solution and solid-state IR spectrum all suggest the presence of a significant Sm···H-C interaction in solution.84 A third weak interaction was noted in the proton transfer reactions of 2,6-diisopropylaniline with Sm[N(SiMe3)2]3, that afford [Sm(NHAr)3]2, a dimer with a clear h6-arene interaction between the aryl ring of an amide ligand attached to an adjacent samarium; this interaction is maintained in solution. This dimer reacts with AlMe3 to give the complex [(m-ArN)Sm(m-NHAr)(m-Me)AlMe2]2, (24) which contains a unique central Sm2N2 core having short bridging Sm-N distances of 2.152(8) and 2.271(7) Å, which the authors claim to be indicative of Sm-N multiple bonding and therefore suggesting an almost unprecedented lanthanide imido complex.

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The authors’ DFT calculations suggest that the Sm 5d metal acceptor orbitals play a significant role in accepting p-electron density from the imido groups.3

2.10 Other Ancillaries. — Reaction of [Yb
N(SiMe3)22(thf)2] with [Me3NH] [BPh4] (1:1) in toluene gives a mixture of products including [Yb
N(SiMe3)2(thf )BPh4], which exists as two distinct linkage isomers (25, 26), and [Yb
N(SiMe3)SiMe2CH2BPh3(thf)2] (27) with an unusual silylamidoborate ligand formed by CH activation.85 Two novel complexes of Sm and Sc containing silsesquioxane silanolate ligands were synthesized and structurally characterized by X-ray diffraction. The first organolanthanide silsesquioxane complex, [Cp*2Sm(m-Cy7Si8O12O)2Li(THF)] was obtained by metathesis. A related complex, [Sc(acac)2(m-Cy7Si8O12O)]2 was also obtained.86

The molecular structures of a number of 2,6-dimesitylphenyl (Dmp)-based complexes of Sc, Y and Yb were reported. Reactions of DmpLi with MCl3 (M : Sc, Yb, Y) in THF at room temperature followed by crystallization from toluene/hexanes produces DmpMCl2(THF)2 (M : Sc, Yb) and DmpMCl2(THF)3 (M : Y), respectively. The one-pot reaction of DmpLi with YbCl3 in THF followed by addition of KOtBu produces the heterobimetallic monoalkoxide complex DmpYb(THF)(OtBu)(m-Cl)2Li(THF)2. The structures of these molecular products feature monomeric complexes with distorted trigonal-bipyramidal or octahedral coordination geometry about the metal atom, with the two Cl atoms occupying the axial positions.87 The structures of Sm(CH2SiMe3)3(THF)3 and Ln(CH2SiMe3)3(THF)2 (Ln : Er, Yb, Lu) show the Sm atom in a fac-

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octahedral coordination and heavier lanthanides Er, Yb, and Lu (29) trigonal bipyramidally coordinated with three alkyl ligands in equatorial and two THF molecules in axial positions.88 Reaction of the rare earth metal-alkyl complexes [Ln(CH2SiMe3)3(THF)2] (Ln : Y, Lu) with B(C6X5)3 (X : H, F) in the presence of crown ethers (CE) gave crystallographically characterized ion pairs [Ln(CH2SiMe3)2(CE)(THF)n]; [B(CH2SiMe3)(C6X5)3]9 (30). Surprisingly, these are the first examples of cationic alkyl complexes of a lanthanide.89

In redox chemistry, the reduced lanthanide iodides LnI2 (Ln : Sc, Y, La, Ce, Pr, Gd, Ho, and Er) were obtained, and the Nd(II), Dy(II), and Tm(II) derivatives form stable divalent compounds that are insoluble THF, DME, or NH3. All of these subhalides reduce alcohols in THF to give the phenoxy- or alkoxylanthanide diiodides ROLnI2(THF)x (R : Ph, i-Pr, t-Bu). They will reduce cyclopentadiene in THF to give CpLnI2(THF)3 and the La compound reduced 2,2bipyridine (bipy), to give LaI2(bipy)2(THF)2. Triphenylcarbinol, stilbene, naphthalene, and anthracene were not reduced by these subhalides.90

3

Theoretical Studies

There has been a sudden growth in the number of computational studies of lanthanide complexes, often accompanying structural and spectroscopic studies, suggesting that the DFT, particularly with relativistic corrections, has now given several groups the confidence to start making structural predictions. Eisenstein and coworkers have taken a look at the remarkable reports of CO binding to lanthanide centres we discussed last year and in Section 2.2. DFT calculations of the bivalent lanthanide metallocene carbonlyls, Cp2M(CO)x , (M : Ca, Eu, Yb; x : 1, 2), and the MeNC adducts of Yb were calculated. The nCO values are in agreement with experiment for Cp*2M(CO) when M is Ca or Eu. However, for M : Yb the calculations are much less successful. The calculated nCO values for M : Yb are brought into agreement with experiment when the CO is allowed to bond to Cp2Yb by way of the oxygen atom.5 These rather astonishing predictions await structural confirmation. Metallocenes were also the focus of studies to understand the nature of lanthanide chalcogenolate interactions. DFT calculations on LnCp2APh (Ln : La; A : O, S) suggest that the linearity of the Ln-O-C vectors arise largely as a result of electrostatic repulsion between the a-C and the trivalent metal center. For the heavier chalcogen analogues which are less electronegative than oxygen,

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the ligand remains bent.91 Again, conclusive experimental/structural tests are lacking. Studies of bond activation remain prominent and have been used to test the DFT method. Models for C5H5 (Cp) in lanthanides to reduce the computational cost are being sought. The H exchange reaction, Cp2LnH* ; H2 ; Cp2LnH ; HH*, previously studied with C5H5 is used as a benchmark. The Cp ligand is replaced by H, by Cl, and also by an effective group potential (EGP). The three models give results close to that with Cp for the entire lanthanide series (La to Lu). As expected, the EGP gives the best agreement; surprisingly, H gives better results than Cl indicating that the electron donor ability of the ligand (Cp being closer to H than to Cl) is more important than isolobality (Cp and Cl).92 Calculations of the activation of methane by models (Cl2LnZ) of Cp*2LnZ (Z : H, Me) have been carried out for the entire lanthanide series. Remarkably, Cl appears to be a good model for Cp* in this system, successfully reproducing the coordination around the metal. The energetics of the transformation X2LnH ; CH4 ; X2LnCH3 ; H2 are fairly close for X : Cl and Cp, the difference in behavior being attributed to the stronger electron donating ability of Cp. Formation of the lanthanide hydride complex is calculated to be exothermic in agreement with experiment. The energy profiles of the reactions: Cl2LnH ; CH4 ; Cl2LnCH3 ; H2 Cl2LnH* ; CH4 ; Cl2LnH ; H*CH3 Cl2LnCH*3 ; CH4 ; Cl2LnCH3 ; H-CH*3 have been calculated. The transition states for the first and third transformations are energetically accessible, in good agreement with the known experimental data, with transition state structures suggestive of a proton transfer between two anionic species. The second reaction on the other hand is found to have a transition state of very high energy and is therefore effectively forbidden.93 The corresponding study of silane activation by Cp2LnH complexes has also been carried out. The reaction paths corresponding to H/H exchange and formation of Cp2Ln(SiH3), have been computed. They both occur via a single-step s-bond metathesis mechanism. For the thermal H/H exchange reaction, the calculated activation barrier averages 1.8 kcal·mol91 relative to the precursor adduct Cp2LnH(k2-SiH4) for all lanthanide elements. The silylation path is slightly exergonic (DE ª 96.5 kcal mol91) with an activation barrier averaging 5.2 kcal mol91 relative to the precursor adduct. Both pathways are therefore thermally accessible. The H/H exchange path is calculated to be kinetically more favorable whereas the silylation reaction is thermodynamically preferred. The reactivity of this family of lanthanide complexes with SiH4 contrasts strongly with that towards CH4. The considerably lower activation barrier for silylation relative to methylation is attributed to the ability of Si to become hypervalent.47 Relativistic DFT calculations have also been used to explore the bending of YbCp*2 and the actinide congener NoCp*2. The calculations, which reproduce the experimental geometries suggest very well, indicate small amounts of d- and f-orbital covalency.94 The homoleptic hydrocarbyls Ln[CH(SiR2R)(SiR3)]3 (Ln : La, Sm ; R : R



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Organometallic Chemistry

: Me, R : H, R : Me, R : R : H) have been modeled by DFT and compared with the X-ray structures in the literature. The calculations reproduce the peculiar pyramidal coordination geometry of these compounds. The results show significant increases in the Si-C bond lengths associated with b-Si-C agostic interactions, whereas few structural changes are found for g-C-H agostic interactions which are, in fact, repulsive. The simplified model with only SiH3 groups, while still adopting a pyramidal coordination geometry, does not describes the real molecule accurately, since the b-Si-C agostic interactions are replaced by b-Si-H interactions. This does not strike us as a surprise as the complexes have little alternative. More interestingly, Mulliken analysis of the electronic structure shows a relatively covalent Ln-C interaction with significant 5d orbital participation in the bonding. A number of calculations with different basis sets on the model system Sm[CH(SiH2Me)(SiH3)]3 shows that the addition of polarization functions (d functions on Si and C, p functions on H, or f functions on Sm) has little or no beneficial effect on the quality of the results.95 Amberger has continued his in-depth studies of the electronic structure of lanthanide amides by reporting absorption and MCD spectra of the trigonalbipyramidal complexes Pr[N(SiMe3)2]3(CNR)2 (R : But, C6H11) measured at ambient and low temps. The interpretation was complicated by the violation of the selection rules for D3h symmetry by both compounds. Nevertheless the authors felt that their results should be seen as a ‘master set of CF parameters’ for future analyses of the electronic structures of trigonal-bipyramidal lanthanide(III) systems.96 A molecular modeling study of the steric and electronic effects in the regioselectivity of propene insertion in metallocene catalysts of general formula Me2Si(Cp)2M-R (M : Sc, Y, Ti and Zr, R : CH3, CH2CH3, -CH(CH3)2) has been reported. The Group 4 metallocenes are more regioselective, and first-row systems are most regioselective.97 Unusual artificial neural network methods were used to predict the decomposition temperatures of the lanthanide complexes Cp2Ln(C6H5CONHO) and compared with traditional regression methods. The authors argue their method to be superior.98 A number of fundamental studies on gas-phase species have also been reported. The potential energy surface and reaction mechanism corresponding to the reaction of ytterbium monocation with fluoromethane, which represents a prototype of the activation of C-F bond in fluorohydrocarbons by bare lanthanide cations, have been investigated for the first time by using DFT calculations. A direct fluorine abstraction mechanism was revealed, and the related thermochemical data were determined.99 DFT (B3LYP) calculations have been used to study the reaction between MO (M : Sc, Ni, Pd) and methane. ScO is calculated to form a molecular complex with methane which at higher temperature gives the CH3ScOH molecule.100 DFT and coupled cluster calculations have been used to probe pathways for Na-, Al- and Y-induced acetylene (HCCH)vinylidene (CCH2) rearrangements in the gas phase. The metal-free and Nainduced rearrangements were shown to proceed via 1,2-hydride shifts, whereas the Al- and Y-induced reactions proceed via 1,2-hydrogen shifts. The latter

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reactions involve homolytic bond cleavage and formation, with the a and b electron densities undergoing redistribution in opposing directions.101 Following up on some crossed molecular beam experiments, theoretical calculations have been reported for the reaction of yttrium atoms with formaldehyde. The reaction is shown to proceed through the exothermic formation of a side-bound pcomplex followed by C-H insertions which branch out to competing pathways and products. Differing pathways involving both hydrides and carbonyls have been identified.102 4

Gas Phase Chemistry

The reaction of ground-state Y atoms with ketene (H2C:C:O) was studied at two collision energies, 22.7 and 10.4 kcal/mol. Four competing processes were observed, corresponding to formation of (YCH2 ; CO), (YCCO ; H2), (YCHCO ; H) and nonreactive scattering. The endoergicity of the YCHCO ; H product channel was 10.5 < 2.0 kcal/mol, leading to D0(Y-CHCO) : 93 < 2.0 kcal/mol. Formation of (YCH2 ; CO) was found to be the dominant process. The trend in branching ratios as a function of collision energy, combined with center-of-mass distributions obtained through fits to the experimental data and analogies to the Y ; H2CO system, allowed for a qualitative description of relevant features of the Y ; H2CCO potential energy surface.103 The study of the kinetics of gas phase polymerization of butadiene with a heterogeneous catalyst based on NdEt3 and AlEt3 was carried out. The effects of reaction temperature, reaction pressure, dispersing medium (SiO2 or Al2O3), and AlMe3 cocatalyst on the polymerization kinetics were investigated. Two active sites were distinguished. The results show that a combined model of first and second order decay of active site of catalyst can be used to describe the phenomena.104 Halogen-ligated transition metal ions ScXn; (X : Cl, Br, I; n : 1-3) were generated and their reactions with benzene explored. Kinetics were analysed to estimate binding energies, modeling the attachment reaction as a radiative associative process. DFT calculations were also performed to give computed binding energies for some of the complexes. Binding of the first benzene molecule to the Sc is strongly enhanced by specific chemical interactions, while binding of the second benzene molecule is more nearly electrostatic. The binding energies are nearly independent of the identity of the halogen ligands.105 5

Polymerization Catalysis

Polymerization catalysis continues to be a major focus of interest for both academics and industrialists, often working in tandem. Yasuda has published a review of organo-rare-earth-metal initiated living polymerizations of polar and nonpolar monomers.106 5.1

Olefin Polymerization. — Cationic TACN-based catalysts, of the type,

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[(Me2-TACN-SiMe2NBut)Y(CH2SiMe3)2], activated by [HNMe2Ph][B(C6F5)4] for the polymerization of ethylene have been patented by Hessen and coworkers.107 A closely related system has also been patented by others.108 Oligoethylenes with narrow molecular weight distributions have been prepared by combining [Cp*2NdCl2Li(OEt2)2] with MgR2 (R : n-Bu, n-hexyl).109 A catalyst system containing Er(CH(SiMe3)2)3, 2,3-dihydrido-2,2-dimethyl-7-benzofuranol, AlEt3, and triphenylcarbenium B(C6F5)4 for the preparation of ethylene-1-octene copolymer has been patented.110 Various patents have been filed for the preparation of copolymers suitable for a variety of applications, using samarocene catalysts.111,112 Ethylene polymerization by (h55 C5H4CH2CH2CN)(h -Cp)SmCl with MAO, has been reported.39 i [(SiMe3)2NC(N Pr)2]2Ln(m-Me)2Li(TMEDA) (Ln : Nd, Yb) were found to have high activity for the styrene polymerization at 70-100°C. The microstructure was examined in detail.113 Finally, in an intriguingly novel approach to the preparation of weakly coordinating anions, reaction of M[CH(SiMe3)2]3 (M : Y, La) with four 2-HOC12F9 results in the clean formation of M(OC12F9)3(HOC12F9) complexes (M : Y; M : La) in high yield. When these are paired with metallocene di-Me precursors, even sterically open precatalysts such as Me2Si(C5Me4)tBuN)TiMe2 or Cp2ZrMe2 yield highly active ethylene polymerization systems.114 5.2 Butadiene. — Taube has continued his studies of allylneodymium complexcatalysed 1,4-cis polymerization of butadiene. A detailed kinetic study suggests the formation of [Nd(h3-C3H4R)(h4- C4H6)2(ClAlR2C3H4R)2], (R : Me, Et). The reaction proceeds according to the p-allyl insertion mechanism, and the high cis-selectivity in accordance with the anti-cis and syn-trans correlation is a consequence of the preferential h4-cis coordination of butadiene. This was the 60th paper in the series.115 A patent for the polymerization of butadiene using [Cp*2Sm(THF)2] in the presence of aluminoxanes has been filed.116 The same group has patented a related aluminoxane-free polymerization system for both conjugated dienes or the copolymerization of a conjugated diene and aromatic vinyl compounds.117 A catalyst system resulting from the reaction of ansa metallocene -ate complex [(Me2CC5H4)2SmCl.MgCl2(THF)3]2 with allyllithium(dioxane), was used to prepare polymers of 1,3-dienes, copolymers of dienes and long chain a-olefins or a,w-dienes. More interestingly, the catalyst was used to prepare block copolymers of isoprene and e-caprolactone and isoprene/a-olefin and e-caprolactone blocks in controlled fashion. The copolymerization of isoprene with the a,w-dienes hexa-1,5-diene and octa-1,7-diene by ansa-samarocenes has been studied. The alternating (block-isoprene-comonomer-block-isoprene) microstructure is related to the constrained geometry of the ansa-samarocene catalyst. Isoprene/a-olefin/a,w-diene terpolymers were also obtained.118 In further work on this system, the exact nature of the active species and of the allyl precursors was investigated.119 5.3

Acrylates. — A set of allyl-functionalized cyclopentadienyl (Cp) lan-

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thanocene chlorides have been synthesized and for which, not very surprisingly, no evidence of intramolecular coordination of the alkenyl C:C to the central metal was observed. Single-component bulk polymerization of Me methacrylate (MMA) was catalysed by the Y derivative. The influence of substituents and additives was studied.40, 120 The complex [CpBu2NdMe)]2 shows high catalytic activity for MMA polymerization at low temperature. Above room temperature, however, catalyst deactivation occurs by reaction with the substrate to give [CpBu2NdOMe)]2.121 A heterodinuclear catalyst, consisting of ethylene-bridged samarocene and titanocene chlorides, acted as a single component catalyst to give ultrahigh-molecular-weight PMMA (1.5;106).122 Studies of MMA polymerization and cyclohexyl, stearyl, butyl, and 2-ethylhexyl analogues were carried out to examine the maximization of their molecular weights using a minute amounts of a single initiator, [SmCp*2Me(THF)], under various conditions. The molecular weights reported were unimpressive but in an extension of this study, tri-block copolymerizations of various MMAs were explored to obtain rubber-like elastic polymers.123 A patent has been taken out for the living polymerization of olefin/MMA copolymers. The polyolefin is generated by means of a hafnium catalyst, a process which is terminated by adding iodine. The resulting polymer is then reacted with Cp*2Sm(THF)2 followed by MMA to give the copolymer with narrow polydispersity.124 Polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) by metallocene amide complexes MeCp2LnNR2(L) (Ln : Y, Er, Yb; R : Ph, IPr, piperidine; L : THF, piperidine) was studied and significant differences were shown to relate to the crowding at the metal centre.16 Polymerization of acrylonitrile by a variety of divalent indenyl species have been reported.48, 49 5.4 Polar Monomers. — The living ring opening polymerization of cyclic esters (e-caprolactone and L-lactide) and cyclic carbonate (1,3-dioxan-2-one (TMC)) initiated with LnCp3 complexes (Ln: Sm, Er, Pr, Gd and Ce) has been reported. The activity decreases as ionic radius increases.125 Finally, a combinatorial approach to discovery and screening of potential polymerization catalysts has been patented.126

6

Applications in Organic Synthesis

The fluorinated ‘ponytail’ methide, tri[tris(nonafluorobutanesulfonyl)methide] [Yb(C(C4F9SO2)3)3] the anhydrous structure of which is unknown but presumed not to involve a Yb-C bond,was found to be a highly effective catalyst for the Friedel-Crafts acylation of anisole with acetic anhydride.127 In combination with a cyclodextrin, similar systems have been used to catalyse Mukaiyama aldol reactions.128 The compounds [Me2Si(OHF)(CpR*)LnN(TMS)2] (OHF : h5-octahydrofluorenyl; Cp : h5-C5H3; R* : (9)-menthyl; Ln : Sm, Y, Lu; TMS : SiMe3)

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described in section 2.4 have been used for the enantio- and diastereoselective cyclizations of aminoalkenes and phosphinoalkenes. Enantioselectivities 67% were obtained in hydroamination, and diastereoselectivities of 96% were obtained in hydrophosphination. Lithium-ate complex precursors can be prepared in up to 90% diastereomeric purity and then converted into the corresponding amido catalysts, which are isolated in approximately 100% diastereomeric purity after recrystallization.43 THF is ring-opened by in situ generated (acyloxy)phosphonium bromide using allylsamarium bromide as catalyst to afford 4-bromobutyl esters under mild conditions in good to excellent yields.129 Addition of allylsamarium bromide to aromatic aldoximes R1CH:NOH gave di(homoallylic) secondary amines in high yields. Oximes derived from acetophenones gave lower yields of the analogous products.130 Allylsamarium bromide was also used for the direct geminal diallylation of lactones, lactams and acyclic amides. Quaternary carbons were constructed and 2,2-diallyl cyclic ethers were prepared in moderate to good yields under mild conditions.131 In addition, a series of homoallylic a,b-unsaturated amines was synthesized via 1,2-addition of the corresponding imines with allylsamarium bromide in excellent yields under mild and neutral conditions.130 N-(1-Allyl-3-butenyl)-N-arylamines were prepared in good yield via the direct diallylation reaction of formanilides with an organosamarium reagent under mild conditions.132 The organolanthanide- and Group 3 metallocene-catalysed hydrosilylation of alkynylsilanes has been found to provide (Z)-1,1-bis(silyl)alkenes. In particular, Cp*2YMe THF, [Cp2YMe]2, and [Cp2LuMe]2 were regioselective for the hydrosilylation of various alkynylsilanes. The process was evaluated for diverse substitution patterns and functional groups on the pendant alkyl chain. Silyl ethers and halogens are stable to the catalytic process, affording excellent chemoand regioselectivities.133 In the presence of Cp2LnX-HgCl2, treatment of RC∫CCH2Br with Mg leads to the formation of benzene derivatives C6H4R2-1,2 (R : H, Ph) in moderate yield, which provides a new method for the construction of the benzene ring skeleton.134 And finally, the amidinate complexes [Cp2Ln
tBuNC(Bu)NtBu] (Ln : Gd, Y) are reported to be ‘a fine catalyst for cyclotrimerization of PhNCO’.11

Acknowledgement JGB was supported by the National Science Foundation under Grant No. CHE0303075.

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34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59.

60. 61.

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5 Carboranes, Including Their Metal Complexes BY PAUL A. JELLISS

1

Introduction

This review covers the 2002 literature of carboranes and metallacarboranes,1 and is formatted in the same manner as for the previous year. Section 2 addresses exclusively theoretical and computational articles, while Sections 3 and 4 deal with the bulk of carborane and metallacarborane papers, respectively. In Section 3 carboranes are ordered in terms of their CxBy formula (increasing x, y) with metal complexes, MnCxBy, similarly ordered in Section 4. Articles concerning exo-metal complexes are listed separately from those for endo-metal species, where the metal forms an integral part of the polyhedral cage structure and bonding. The nomenclature and formulae adopted are generally those used in the cited articles, though readers should be aware of inconsistencies, e.g. M(hnnido-CxBy) vs. closo-MCxBy. Articles directly related to biological applications, particularly boron neutron capture therapy (BNCT), are covered in Section 5, with any crystal engineering and materials-related work in Section 6. The chemical literature has been surveyed using the on-line search engines Chemical Abstracts SciFinder Scholar and ISI Web of Science. Six specific reviews have appeared in 2002 on carboranes and related compounds: Putting Small Metallacarboranes to Work;2 Phosphorus Insertion into Borane Clusters;3 Transition Metal-Catalysed Double Silylation and Germylation with 1,2-Bis(dimethylsilyl)carborane and 1,2-Bis(dimethylgermyl)carborane;4 The Medicinal Chemistry of Carboranes;5 Recent Developments in the Chemistry of Main Group Metallacarboranes of the C2B4-Carborane Ligands;6 Imide- and Amide-Supported Group 5 and 6 Metallacarboranes.7 Throughout this review, the following key is used to describe the cage vertex atoms in the figures used:

Unless otherwise indicated, terminal hydrogen atoms are bound to boron or carbon vertices but not shown.

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Theoretical and Computational Studies

Molecular orbital Hartree-Fock geometry calculations using 3-21G(*) and 631G* basis sets have shown that tropylium derivatives of closo-C2B10H12 and closo-CB11H129 are so-called Type II species with moderate polarization of the cage electrons and little if any charge-transfer HOMO-LUMO interaction.8 A similar closo-CB9H109 derivative, on the other hand, appears to be a Type III species with significant cage-to-ring charge-transfer in the HOMO-LUMO transition. The closo-C2B9H129 cage, despite results suggesting tentative Type III behavior, appears to be in a class of its own. Calculations at the B3LYP/6-31G* level of a series of closo-C2BnHn (n : 4, 5, 8, 10) carborynes have been presented and compared to their silaboryne analogues.9 Results suggested that dehydrogenation of the parent carboranes 2,3-C2BnHn;2 (n : 5, 8) to yield the 2,3C2B5H5 and 2,3-C2B8H8 carborynes should be more thermodynamically favorable than the formation of benzyne and these products should make suitable dienophiles in pericyclic reactions. Density functional calculations at the HF/STO-3G and B3LYP levels have been employed in the study of the reactions of closo-C2B4H6 with oxygen-containing radicals OH, HO2, O, and O2, allowing a kinetic model to be proposed for the initial stage of oxidation of this carborane.10 A quantitative and qualitative computational investigation into the formation of a series of endohedral boranes, carboranes, and silaboranes has been carried out at the B3LYP/6-31G* level.11 3

Carboranes

3.1
CB8 and
CB9 . — Reaction between arachno-4-CB8H14 and PCl3 in the presence of Proton Sponge has yielded the neutral phosphacarborane closo-2,1PCB8H9, while the isomer closo-6,1-PCB8H9 has been isolated from a similar reaction involving nido-1-CB8H12.12 The borane B10H14 has been treated with the aldehydes HCHO and MeCHO under aqueous alkaline conditions to yield 6-Rarachno-6-CB9H119 (R : H, Me), which can be thermolysed to give 4-R-closo-4CB8H89.13 A similar series of reactions starting with B10H14 and PhCHO has given the related species 4-Ph-closo-4-CB8H89, but via nido-6-Ph-6-CB9H119. Iodination of closo-4-CB8H99 has given equimolar amounts of n-I-closo-4CB8H89 (n : 3, 5) and smaller amounts of 5,6-I2-closo-4-CB8H79. Positional isomerism in the monocarboranes n-R-closo-2-CB9H99 (n : 1, 2; R : H, Ph) has been investigated, the higher carbon-connectivity 2-isomer having not been previously isolated.14 3.2
CB10 . — The alkylation of 7-NH2But-nido-7-CB10H12 has been reported and the pH dependence of the 11B NMR spectrum of 7-NHMeBut-nido-7CB10H12 discussed.15 This work has led to the preparation of 7-NMe3-nido-7CB10H12 and 7-NMe2Cy-nido-7-CB10H12, both precursors to nido-7-CB10H139 and closo-1-CB11H129, without recourse to employing NaCN in the synthesis. 3.3


CB11 . — In a ‘least-nucleophilic anion’ optimization study, the penta-

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106

methylhexahalo carborane monoanions 1-H-2,3,4,5,6-Me5-7,8,9,10,11,12-X6closo-CB119 (X : Cl, Br) have been studied with regard to the impact on the 29Si chemical shift of an PriSi; counterion.16 Steric shielding of the silylium cation from the cage negative charge more than offsets the higher charge density input arising from the inductive effects of the methyl groups. This work has also produced the first crystallographically characterized free silylium cation, [Mes3Si][closo-CB11HMe5Br6] C6H6.17 These and related carboranes have been studied by the same research group as to the structural characterization of discrete salts of the solvated proton, [H(solvent)x][closo-CB11HR5X6].18 The work is supported by the observation of diagnostically broadened bands in the IR spectroscopy of these compounds, where linear short, strong, low barrier (SSLB) O · · · H · · · O hydrogen bonds have been identified. The carborane-based superacids H[1-R-2,3,4,5,6-Y5-7,8,9,10,11,12-X6-closo-CB11] (R : H, Me; X, Y : Br or Cl; Y : Br and X : Cl; Y : Cl and X : Br) have also been reported, formed by heating the solvated [H(solvent)n]; species under high temperature and high vacuum conditions.19 The loss of solvent was confirmed by TGA. The CB9 system [H(H2O)4][closo-CB9HBr9] was also included in the report. The anionic species closo-CB11HBr119 has been employed as an effective ion-exchanger in cation selective chemical sensor electrodes and bulk optodes, proving to be a more robust alternative to the commonly used lipophilic 3,5-[bis(trifluoromethyl)phenyl]borate anion and offering improved lifetime and signal stability.20

3.4
C2B3 . — The dicarbapentaborane(5) 1,5-CMe3-2,3,4-Cl3-closo-1,5-C2B3 has been synthesized by treatment of 1,1,1-tris(dicloroboryl)-3,3-dimethylbutane with AlMe3 and loss of BMe3 with heating.21 Substitution of all three chlorides on boron was achieved using Me3SiNMe2 or lithium acetylides (Figure 1).

Figure 1

3.5
C2B7 . — The carborane nido-7,8,9,11-P2C2B7H9 has been halogenated using the AlCl3/CCl4 and I2/AlCl3/C6H6 systems, resulting in the formation of a mixture of mono- and disubstituted derivatives, 10-X-nido-7,8,9,11-P2C2B7H8 and 5,10-X2-nido-7,8,9,11-P2C2B7H7 (X : Cl, I), reaction taking place at vertices most distant from the phosphorus atoms, but adjacent to the carbons.22 Bromination with N-bromosuccinamide, however, produced a mixture of 3-Br-nido7,8,9,11-P2C2B7H8 and 2,3-Br2-nido-7,8,9,11-P2C2B7H7, presumably by a radicalmediated mechanism. 3.6


C2B8 . — The phosphacarboranes nido-7,8,9-PC2B8H11 and nido-7,8,9-

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PC2B8H109 and their 10-Cl derivatives have been isolated, structurally characterized, and their structures optimized up to the RMP2(fc)/6-31G* level in order to facilitate the calculation of 13C and 11B NMR chemical shifts.23 A new multigramscale synthesis of closo-1,10-C2B8H10 has been reported, achieved by thermal dehydrogenation of nido-5,6-C2B8H12 followed by skeletal rearrangement.24 3.7
C2B9 . — The half-sandwich constrained-geometry sila- and germacarboranes 1-M(Me)-2-(1-h1(s)-OCH2)-3-Me-closo-h5-2,3-C2B9H9 (M : Si, Ge) (Figure 2) have been synthesized by treatment of the 7-(OCH2)-8-Me-nido-7,8C2B9H939 trianion with anhydrous MeMCl3 in a 1:1 molar ratio in dry toluene.25

Figure 2

A fundamental question has been addressed concerning the nido-7,8C2B9H1129 anion with regard to its structure with respect to alkali metal (Li;, Na;, K;) counterions.26 The consensus, as a result of detailed measured and calculated (GIAO-B3LYP/6-311G* and GIAO-B3LYP/6-311G*//MP2/631G*) 11B, 11B-11B COSY and 1H
11B NMR spectra, is that it is not discrete, and there is indeed some level of covalency or strong ion-pairing between the alkali metal cations and the carborane dianion, particularly between one of the cations and the open pentagonal face of the cage. The same group has reported the crystal and molecular structures of the nido-carborane anions, 7,9- and 2,9C2B9H129, isolated as their Proton Sponge salts by treatment of the parent ortho or meta closo-C2B10 carboranes with refluxing piperidine/THF or KOH/tetraglyme/toluene, respectively, and metathesis with Proton Sponge hydrochloride.27 The endo-hydrogens have been located and found to bridge B—B connectivities in the open faces. Cage closures of nido-7,9-C2B9 systems (with and without substituents on carbon), triggered by acidification and generally leading to derivatives of closo-2,3-C2B9H11 have been investigated.28 If electron-withdrawing exopolyhedral substituents are present in the open face (F, OH, OEt), they are displaced as their conjugate acid in the formation of the closo-carborane, while 10-Me-nido-7,9-C2B9H11 specifically undergoes nido-nido isomerism to give 7Me-nido-2,8-C2B9H11. The reported nido-carboranes will be useful precursors to ‘carbons-apart’ closo-metallacarboranes. Reaction of 3-X-1-R-closo-1,2C2B10H10 carboranes with KOH/EtOH to yield 3-X-7-R-nido-7,8-C2B9H109 (X : Br, I; R : H, Me, Ph) has been described, deboronation taking place exclusively at a non-halogenated vertex.29 Diastereomeric nido-thioether o-carborane anions have been produced by the deboronation of closo-1-[(2S)-(2-methylbutyl)thio]- and closo-1-[(2S)-(3,7-dimethyl-6-octenyl)thio]-2-methyl-1,2-dicar-

108

Organometallic Chemistry

badodecaborane with (1R,2S)-(—)-2-amino-1,2-diphenylethanol or its enantiomer, with modest diastereomeric excesses.30 The electron donor power of the nido-carboranyl unit has been found to increase the basicity and nucleophilicity of an exopolyhedral phosphine, at variance with the influence of closo-carboranes.31 The nido-carborane-phosphine species [NMe4][7-PR2-8-R-nido-7,8C2B9H10] (R : Ph, Et, Pri; R : Me, Ph) have been studied, with particular reference to the impact on coordination chemical shift in the 31P and 11B NMR spectra. Curiously only weak deshielding occurs with the 31P signal when the free phosphine lone pair is engaged with a BH3 unit, despite the formation of a stable adduct with this Lewis acid. In the area of thiocarborane chemistry, the same researchers have disclosed the syntheses of a series 7,8-R2-10-L-nido-7,8-C2B9H9 (R : H, Me; L : SMe2, SEt2, SEtPh, S(CH2)4), which have yielded isomeric 9-Lcloso-7,8-C2B9H11 or substituent-eliminated 2,3-closo-C2B9H11 upon thermolysis depending upon exact reaction conditions.32 The alkylation of nido-7,8C2B9H1129 with propargyl bromide in liquid ammonia has yielded 9-CH2— C∫CH-nido-7,8-C2B9H119 or the skeletally rearranged 8-CH2—C∫CH-nido-7,9C2B9H119 depending on reaction conditions.33 3.8
C2B10 . — This is typically the dominant class of carborane studied and remains so in this analysis. Ortho-carborane is arguably the most important precursor in carborane and metallacarborane chemistry. A new convenient synthetic procedure has been reported using similar conditions to the original decaborane/acetylene procedure (B10H14, reflux in MeCN/toluene) but employing propiolic acid tert-butyl ester as the source of the carbon vertices.34 Isobutene and carbon dioxide are eliminated via a tentative 6-membered cyclic intermediate to yield o-C2B10H12 directly in 52% yield. Exposure of o-carborane in a C-atom reactor has lead to the formation of 9-carboranylcarbene, which subsequently reacts with alkenes, coated in the inside wall of the reactor at 9196°C, by hydrogen abstraction to give 9-Me-closo-1,2-C2B10H11.35 No addition is observed, as is typically expected for most triplet state carbenes, and is observed to some extent with the 1- and 2-carboranylcarbene congeners. In the deboronation of o-carborane by n-butylamine, it has been found that reaction rate increases with solvent polarity, protic solvents in particular regenerating the amine from the HB(NHBun)2 intermediate formed during the reaction.36 A structural analysis of 1,2-(SPh)2-closo-1,2-C2B10H10 has been carried out and incorporated into a computational study of C—C bond lengths in C-substituted o-carboranes.37 Substituents with donor lone pairs have been found to decrease the C—C bond order, thus increasing the length of the C—C bond. A similar conclusion has been reached with the structure determination of 1-SPh-2-Mecloso-1,2-C2B10H10.38 The X-ray structure of 1-CH:C(H)Me-2-Me-closo-1,2C2B10H10 has been reported, with the expected elongation of the cage C—C bond revealed.39 The group of isomeric o-, m-, and p-carboranes have been subjected to electron transmission spectroscopy in order to assess the energies required to place electrons into the LUMOs.40 The anion states so produced have been studied at the 6-31G* level of computation and potential energy surfaces generated. Unexpected formation of the first 2,3-bis(p-carboran-1-ylimino) azetidine

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109

has been described, resulting from the dehydration of N-(1,12-dicarba-closododecaboran-1-yl)formamide (Figure 3).41

Figure 3

Hydrolysis of 1,2-bis(cyanoethyl)-o-carborane to give (o-carborane-1,2diyl)dipropionic acid, followed by formation of the corresponding acid dichlorides, diamides, dianilides, and diols has been reported,42 in addition to work to synthesize the o-carborane-based ketonitriles 1-C(OH):C(CN)2-2-R-closoC2B10H10.43 The latter have been found to exist in solution mostly in the enol form. Long chain esters of carborane-carboxylic and -biphenylcarboxylic acids have been synthesized by the reaction of the acid chlorides with 1-pentadecanol and 1-hexadecanol in the presence of pyridine.44 A comprehensive article describing the derivative chemistry of o-, m-, and p-alkyl thioether dodecaboranes has been published and includes a discussion of the reaction of pendant alkene moieties with nido-B10H14 or arachno-B10H12(SEt2)2 in refluxing MeCN/toluene, yielding o-carborane-dodecaborane oligomers (Figure 4).45

Figure 4

Treatment of the closo-carboranyl anion in Li[1-SiMe2But-closo-1,2C2B10H10] with excess a.a-dihaloxylenes has yielded 1-
o-(XCH2C6H4CH2)-2SiMe2But-closo-1,2-C2B10H10 (X : Cl, Br), which react further to give substituted ethane or ethane derivatives, depending on the base used.46 Reaction of the same species with Bun4NF has given a cyclized dihydronaphthacarborane. The crystal structures of two silatrane derivatives with carborane-containing substituents have been described.47 In the structures of 1-methyl-2-(1-silatranyl)-1,2- and 9-(2silatranyl)ethyl-1,7-dicarba-closo-dodecaborane significantly shortened transannular Si · · · N bond distances have been attributed to strong s-acceptor-like

Organometallic Chemistry

110

behavior of the o-carborane fragment. Suzuki-Miyaura coupling reactions between 2-iodo-p-carborane and various arylboronic acids have been investigated and show promise for the synthesis of functionalized carboranes through B—C bond coupling.48 The most productive and efficient catalytic conditions employed Pd2(dba)3 in combination with bis(diphenylphosphino)butane and CsF in refluxing dimethoxyethane. However, the activation of 3-iodo-o-carborane toward copper, nickel, and palladium reagents has yielded only the hydrodehalogenation product, o-carborane.49 Phase transfer catalysis has been employed in the cyanoethylation of o-carboranes with acrylonitrile, reaction taking place at a CH vertex.50 The asymmetric synthesis of methyl 2-phenyl-2-(2phenyl-o-carboran-1-yl)pent-4-oate by [PdCl(h3-C3H5)]2-catalysed allylation of 1-C(H)PhCO2Me-2-Ph-closo-1,2-C2B10H10 with CH2:C(H)CH2OCO2Me in the presence of chiral P,N-bidentate phosphates has been reported.51 The first direct C—C coupling of a porphyrin and a carborane has been disclosed, albeit in poor yield, by reaction of iodophenylporphyrin with o-carborane, catalysed by copper (I) chloride.52 Fischer-Tropsch synthesis employing a 10 wt% Co/TiO2 catalyst modified with o-carborane has been reported as part of an investigation of the effect of boron loading on catalytic activity.53 While the carborane was more effective in the reduction of the catalyst compared with the other boron reagents, boric acid and ammonium borate, catalyst activity was markedly decreased leading ultimately to an invariant turnover frequency. There have been two reports of plasma boronization with o-carborane, one in a helium glow discharge TJ-11 stellerator,54 and the other in a HT-7 super-conducting tokamak in a bid to enhance plasma performance.55 3.9
C3B3 and
C4B2 . — 1,3-dimethyl-1,3-diiodo-1,3-diborapentafulvalene has been shown to undergo rearrangement to a nido-C4B2 cage, while treatment of the same fulvalene with the hydroboration agent, HBEt2, has resulted in the formation of a nido-C3B3 moiety (Figure 5).56

Figure 5

4

Metallacarboranes

4.1
MCB10 . — Treatment of the rhodium monocarbollide complex 1NH:CMe2-2-Cl-2-PPh3-closo-2,1-RhCB10H10 with methanol has yielded 1NH3-2-Cl-2-PPh3-closo-2,1-RhCB10H10 which can be further functionalized at the exopolyhedral nitrogen, reacting with a variety of electrophiles in the pres-

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ence of sodium carbonate.57 Stone et al have investigated the same carborane cage incorporated into a rhenium complex, 1,2-m-NHBut-2,2,2-(CO)3-closo-2,1ReCB10H10, which has proven to be a vehicle for the coupling of terminal alkynes, yielding 1,2-m-NHBut-2,2-(CO)2-3,2-s:h2-
C(:CHBut)—CH:CHBut -closo2,1-ReCB10H9 upon treatment with ButC∫CH.58 4.2
exo-MCB11 . — The alumenium ion, Et2Al;, has been structurally characterized by X-ray crystallography with a 7,8,9,10,11-Br6-closo-CB11H69 cage counterion, although tight cage · · · Al ion-pairing was identified.59 Nevertheless, the compound proved a moderately effective Lewis acid catalyst in the oligomerization of ethylene. Intermolecular CH3 . . . Ag interactions have been identified in the structural characterization of [(PPh3)Ag(closo-CHB11Me11)],60 while in other argentacarborane chemistry by the same group, highly active Lewis acid heteroDiels-Alder catalysts, [Ag(PPh3)n(closo-CB11H6X6)] (n : 1, 2; X : H, Br) have been studied.61 Also disclosed by this group is the structural analysis of the ionic compound [Ag(IMes)2][Ag2(closo-CB11H12)4] (IMes : 1,3-dimesitylimidazol-2ylidene). The solid state structure comprises two silver (I) centers bridged by two carborane cages, in addition to one terminal cage on each silver (Figure 6).62

Figure 6

The complex [(PPh3)2Rh(nbd)][closo-CB11H6Br6] has been tested as a source for a cationic hydrogenation catalyst for hindered alkenes, where the non-coordinating carborane anion has proven particularly effective at enhancing the turnover frequency and overall yield of hydrogenated product.63 4.3
MC2B4 . — The synthesis and X-ray structure of a ‘carbons-apart’ erbacarborane, 2,2,4,4-(SiMe3)4-3,6-[(m-H)2K(THF)2]-1,1-commo-Er(h5-2,4C2B4H4)2, has been reported, by treatment of 1,2-(SiMe3)2-closo-1,2-C2B4H4 with erbium (III) chloride activated by metallic potassium, and thus avoiding the use of naphthalene as an electron mediator in cage opening.64 Electronic interactions

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Organometallic Chemistry

in mono-, di-, and trinuclear (h6-arene)ferracarboranes with p-hyperconjugated links have been investigated, primarily by cyclic voltammetry, and compared with computational investigations using semi-empirical and ab initio methods.65 Electronic communication between iron centers appears to be more limited than comparable cobalt systems. 4.4
MC2B8 . — Treatment of [RuCl2(PPh3)3] with arene solutions of nido-5,6C2B8H12 has led to the formation of the 11-vertex arene-ruthenium species n-Cl-1-(h6-arene)-isonido-1,2,4-RuC2B8H9 (n : 3, 6 depending on the nature of the arene).66 4.5
MC2B9 . — The dianionic dimeric molybdenacarborane [N(PPh3)2]2 [Mo(CO)3(h5-7,8-C2B9H11)]2 has been reported to undergo reaction with elemental sulfur to give a completely decarbonylated product, [N(PPh3)2]2[Mo(mS)(S)(h5-7,8-C2B9H11)]2, which has undergone subsequent terminal sulfur substitution by reaction with PhIO to yield [N(PPh3)2]2[Mo(m-S)(O)(h5-7,8C2B9H11)]2.67 From the same vicinity in the d-block, the first tantalasilesquioxanes have been reported.68 One of these incorporates a tantalum atom in a hybrid sandwich complex, CySi7O12Ta(C2B9H11), where the Ta can be considered as both a vertex in an icosahedral closo-3,1,2-TaC2B9H11 unit and in a TaSi7 silesquioxane structure. The ligand 9-SMe2-nido-7,8-C2B9H109 has been employed in two studies, one with rhodium69 and the other with ruthenium.70 The former work has involved the syntheses and structures of a number of 7-SMe2closo-3,1,2-RhC2B9H11 rhodacarborane structures, where rhodium is also ligated by a variety of donor combinations, including halides together with phosphines, as well as cyclopentadienide anions. The latter study has reported the synthesis and structure of the arene-ruthenium complex [(h-arene)Ru(h-9-SMe2-7,8C2B9H10)][BPh4] (arene : benzene, mesitylene). Cylcopropanation with diazoesters has been shown to be efficiently catalysed by 3-H-3,3-(PPh3)2-7-SR1R2closo-3,1,2-RuC2B9H11 (R1 : Me, R2 : Me; R1R2 : (CH2)4) with a moderate stereoselectivity for the trans isomer.71 The structure and stereochemical configuration of a single enantiomer of [NEt4][3-Co-(1,2-C2B9H10)2-8,8-m-S2] has been established by an X-ray diffraction study, resolved crystals spontaneously forming from a racemic mixture in solution.72 Mixed sandwich pyrrole-dicarbollide cobalt complexes have been synthesized and studied electrochemically.73 In particular the complexes 3-Co(h5-NC4H2Me2)-1,2-R2-closo-1,2-C2B9H9 (R : H, Me, Ph, SMe, SPh) have been synthesized, structurally characterized, and their cyclic voltammetry reported, with increasing overall donor strength of the substituents leading to more negative reduction potentials. The same group has identified the freezing out of rotamers in related cobaltacarborane complexes, 3-Co(h5-NC4H2R2)-1-Ph-closo-1,2-C2B9H10 (R : H, Me) by observation of electronic ring effects upon 1H NMR chemical shifts, particularly for the dimethyl pyrrole ligand.74 The salt [Mn(1,10-C12H8N2)3][Co(C2B9H11)2] has been synthesized in a study to assess the impact of the metallacarborane moiety on the temperature-dependent magnetic moment of the cationic manganese (II) complex, with ferromagnetic exchange interactions having been identified at 20 K.75

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Direct electrophilic insertion of a [Ru(h5-C5R5)]; (R : H, Me) fragment into the ruthenacarborane complex [(h5-C5Me5)Ru(C2B9H12)] has been reported, yielding a 13-vertex diruthencarborane [(h5-C5Me5)Ru(C2B9H12)Ru(h5-C5R5)] (Figure 7).76

Figure 7

Among a series of aromatic layered molecules incorporating icosahedral carboranes and aromatic N,N-dimethylurea groups, the commo-cobaltacarborane shown (Figure 8) has been isolated and characterized.77

Figure 8

4.6
MC2B10 . — Thirteen-vertex metallacarboranes are somewhat less common than their icosahedral congeners. The synthesis, structural characterization and reactivity of several lanthanacarboranes have been described, including those of gadolinium, erbium, ytterbium and neodymium.78 A common feature of the cages include the presence of a bridging m-1,2-[o-C6H4(CH2)2] unit, which forces the cage carbon vertices to maintain their connectivity. Similar cages, with bridging naphthyl and biphenyl units have been employed in the synthesis and characterization of ‘carbons-adjacent’ alkali metal nido- and closoMC2B10 complexes.79 Of note is the complex [
m-1,2-[o-C6H4(CH2)2]-1,2C2B10H10Li4(THF)4(DME)]2, which comprises concomitant open C2B4 and C2B3 faces h6- and h5-coordinating Li atoms, respectively. Both h6- and h7-coordinat-

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Organometallic Chemistry

ing carboranes have been identified in the structural determinations of [h5:h6:sand [
(m-h5):h7-SiMe2(C5H4)SiMe2(C9H6)(C2B10H10CH2NMe)]Zr(NC5H5) (C2B10H11)Zr(NEt2)2
Na3(THF)4]n, respectively, the latter being part of an infinite chain.80 The mechanism of formation of the former complex includes a putative intramolecular cage C—C oxidative addition across the Zr atom. The cobalt complexes 4-C5Me5-closo-x,y,z-CoC2B10H12 (x,y,z : 4,1,6; 4,1,8; 4,1,12) have been crystallographically characterized, a response to the failure to overcome disorder problems in the similar characterization of the cylclopentadienide analogs.81 4.7
exo-MC2B10 . — The chemistry of silyl- and germyl-carboranes, 1,2(EHMe2)2-closo-1,2-C2B10H10 (E : Si, Ge) has been exhaustively investigated.82 The chief mode of reactivity probed is that involving Ni-catalysed insertion of alkynes to give five- and six-membered rings comprising the carborane carbons, the group IV elements and one or both carbons of the alkyne. Both the complexes [1,2-(EMe2)2-closo-1,2-C2B10H10]Ni(PEt3)2 (E : Si, Ge) have been isolated, characterized, and implicated as the active catalysts in the alkyne double germylations or silylations. The same authors have reported the synthesis and reactivity of the halodimethylgermyl and -stannyl carboranes, 1,2-(EXMe2)2closo-1,2-C2B10H10 (E : Ge, X : Cl; E : Sn, X : Br).83 The 16-electron complexes [M(h5-C5Me5)(E2C2B10H10)] (M : Rh, Ir; E : S, Se) and [M(h6-pcymene)(S2C2B10H10)] (M : Ru, Os) have also been shown to undergo alkyne insertion reactions.84 Many of the products contain direct boron-metal bonds, as a consequence of activation of the B—H bond adjacent to both CH vertices. Following transfer of the carborane hydride via the metal center to the alkyne, the latter fragment may adopt a kinetic cisoid or thermodynamic transoid configuration with respect to the metal-boron bond. The half-sandwich binuclear ferracarborane complexes [Fe2(h5-L)2(CO)3(Se2C2B10H10)] (L : C5H4Me, C5H3But3) have been synthesized and structurally characterized.85,86 Treatment of [Ln(m-Cl)(h5-C5H5)2]2 (Ln : Nd, Yb, Dy, Gd, Er) with THF solutions of [(THF)3Li(1,2-E2C2B10H10)Li(THF)]2 (E : S, Se) has afforded the dinuclear sandwich complexes [Li(THF)4][
Ln(h5-C5H5)22
1-(m-E)-2-E-C2B10H1087 kE,kE2]. Structural analysis has shown that the central Ln2E2 four-membered rings in these complexes are not planar. Closely related structures formed from reactions between [(h5-L)2Ln(m-Cl)2Li(THF)2] (L : C5H4But, C5H3But2) and the same chalcogenocarborane precursors have also been reported.88 Bis(diphenylphosphino)-o-carborane has been synthesized and complexed with ‘PdClMe’ and ‘PdCl2’ moieties, the latter used as a 31P NMR probe to assess the impact of the cage on the donor/acceptor properties of the phosphorus atoms.89 The syntheses of boron-bridged constrained-geometry Group 4 metal complexes [h5:s-Pri2NB(C9H6)(closo-1,2-C2B10H10)]ZrX2 (C9H6 : indenyl; X : Cl, Me, NMe2) and [h5:s-Pri2NB(C9H6)(closo-1,2-C2B10H10)]MCl2 (M : Ti, Hf ) have been described and their catalytic functionality explored in the polymerization of ethylene.90 The ZrCl2-based complex has been demonstrated to have approximately three times the activity of the catalytic standard [ZrCl2(h5-C5H5)2]. Metallacarborane and metallofullerene chemistry have come together in the report

5: Carboranes, Including Their Metal Complexes

115

of the first X-ray structural characterization of such a transition metal complex with these ligands combined in the same coordination sphere: [Ir(h2-C60)(1CH2PPh2-B,P-1,2-C2B10H9)(ButNC)2].91 Two mercuracarborane crystal structures have been carried out, those of bis[1,2-dicarba-closo-dodecaboran(12)-1yl]mercury (II) and bis[1,12-dicarba-closo-dodecaboran(12)-1-yl]mercury (II).92 These compounds have been synthesized in good yield by treatment of HgCl2 with the appropriate monolithiocarborane. Both structures comprise linear or near-linear mercury coordination, although significant Hg-solvent interactions have been invoked from interatomic separations. Finally, in a bid to assess the contribution of carborane cages as electron-withdrawing terminii in complex molecules with ‘push-pull’ architectures and their viability as non-linear optical materials, specifically in terms of second-order hyperpolarisability, a collection of carborane-ferrocene dyads have been synthesized and studied.93 Specifically, the compounds 1-C6H4—X:CH-Fc-closo-1,n-C2B10H11 (X : CH, N; Fc : ferrocenyl, n : 2, 7, 12) have been tested, with larger responses observed for the p-carboranes, although electrochemical data suggest that the o-carboranes are more strongly electron-withdrawing.

5

Biological Carborane Chemistry and BNCT

This field of carborane chemistry continues to be reenergized with a wide array of studies incorporating carboranes and metallacarboranes into biologically active molecules. A promising tamoxifen analog, which incorporates an ocarborane group in place of a phenyl group, Boroxifen, has been synthesized, with a view to coupling antiestrogenic activity with BNCT capability.94 The activity will be reported in due course. A twelve-step synthesis of a new porphyrin-labeled carboranyl phosphate diester (Figure 9) has been reported.95 The amide-bridged molecule has been designed to accumulate at lipid-aqueous membrane interfaces and taken up preferentially by tumor cells.

Figure 9

Four new meso-tetra[(nido-carboranylmethyl)phenyl]porphyrins have been synthesized and studied for cytotoxicity and cellular uptake.96 In vitro, there appears to be little dependence on the regioisomerism (para vs. meta) of the meso-aryl fragments carrying the o-carboranyl groups. The amphiphilic nature

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Organometallic Chemistry

of these molecules allows for substantial aqueous solubility while still permitting membrane penetration and good cell uptake. The cytotoxicity has been found to be favorably low (IC50 > 150 mM in all cell lines), a key facet of potential BNCT agents, while thanks to the nature of C—C bonding between the carborane and porphyrin, in vivo cleavage is considered unlikely. Furthermore, retention of fluorescence and photosensitizing properties of the porphyrin rings suggests possible application in photodynamic therapy. Further in vivo testing is underway. A series of o-carboranyl-substituted 1,2,3,4-tetrahydroisoquinolines have been synthesized in moderate-to-good yield, as possible mimics of the dopamine antagonists 1-arylmethyl-1,2,3,4-tetrahydroisoquinolines.97 In particular, 1-(ocarborane)-2-aminosulfonyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline has shown low cytotoxicity (IC50 : 22 mM) and significant accumulation in B-16 melanoma cells. These new molecules may be candidates for BNCT specific to brain tumors. New C-hydrazino-C-carboxycarboranes, including 1-hydrazino12-hydroxycarbonyl-1,12-closo-dodecaborane, have been synthesized as possible binding agents for 99mTc and other biomolecules for targeted radiopharmaceuticals and new BNCT/BNCS agents.98 Such materials have the advantage of having their biodistribution evaluated by radioimaging. A new p-carborane analog of tyrosine, 3-[1-hydroxy-1,12-dicarba-closo-dodecaboran(12)-12-yl] alanine has been prepared in a multi-step procedure.99 It has been shown that its enhanced lipophilicity allows it to function as a hydrophobic tyrosine mimic in insect and mammalian neuropeptides. Thymidine analogs with 3-o- and -pcloso-carboranyl and 3-o-nido-carboranyl groups have been synthesized, with carboranes linked to the ribose ring through C—C bonds being more stable to hydrolytic cleavage than their 3-ether-linked analogs, as shown by phosphoryl transfer assays using recombinant thymidine kinases, TK1 and TK2.100 Similar studies have been carried out with a library of 3-(o-carboranylalkyl)thymidines with alkyl spacers of between two and seven methylene units linking the carborane to the thymidine.101 It has been found that spacers of two and five methylene groups between the carborane and thymidine are optimal for the binding of N-3-substituted carboranyl thymidines to the active site of TK1. Potential BNCT reagents with o-carborane bound to C-6 of L-fucose have been synthesized either by reaction of an L-fucose-derived alkyne with decaborane or by coupling of a 6-amino-L-galactopyranose derivative with carborane carboxylic acid chloride.102 A novel cholesterol-carborane conjugate, cholesteryl 1,12dicarba-closo-dodecaborane-1-carboxylate (Figure 10), has been developed for the purpose of replacing cholesteryl esters in low-density lipoprotein for targeted boron delivery in BNCT.103 The compound is stable within a wide pH range when solubilized in liposomes and has a low cytotoxicity. A very low density lipoprotein (VLDL)-resembling phospholipid-submicron emulsion (PMSE) carrier system has been developed for the purpose of the targeted importation of cholesteryl 1,12-dicarba-closo-dodecaborane-1-carboxylate into cancer cells through interaction with native low density lipoprotein (LDL).104 A stable 2:1 b-cyclodextrin-1-phenyl-1,2-dicarba-closododecaborane inclusion complex has been synthesized and characterized by NOE and NOESY spectroscopy, which among other correlations, has shown

5: Carboranes, Including Their Metal Complexes

117

Figure 10

enhancement of the carborane C—H and the phenyl 2,6-H protons.105 Such an encapsulated carborane system may show promise for BNCT drug delivery. In another approach to facilitate BNCT treatment, new functionalized watersoluble carboranyl anions, [1,2-(BH2X)-closo-1,2-C2B10H10]29 (X : H, CN, CO2Me, CO2H) and [1-R-2-BH2X-closo-1,2-C2B10H10]9 (R : Ph, X : H, CN, CO2H, CO2Me, L-CON(H)C(H)(CH2OH)CO2Me, L-CON(H)C(H)(CHMe2)CO2Me, L-CON(H)C(H)(4-CH2C6H4OH)CO2Me) have been synthesized and characterized.106 Further water-soluble closo- and nido-o-carborane derivatives of anthraquinones, flourenones, and sulfones have been reported.107 The synthetic procedures and characterization by IR and NMR spectroscopies have been described. These compounds may function as BNCT drugs through their DNA binding capability, and will be subjected to further screening and toxicological analysis. DNA binding has also been of interest with regard to the recently synthesized platinum complexes, [Pt(1-S(CH2)n-1,x-C2B10H11)(2,2:6,2terpyridine)]; (x : 2, n : 0, 1; x : 7, n : 1).108 Preliminary studies with calf-thymus DNA have revealed intercalation of the complexes, when the molar ratio of Pt added per DNA base pair is high. The cytotoxicity and boron uptake by B-16 melanoma cells has been established for several o-carborane derivatives of 5,6,7-trimethoxyindole, which is known for its DNA binding role in the potent antitumor agent duocarmycin A.109 The compounds discussed show superior in vitro uptake compared to p-boronophenylalanine, currently employed in BNCT, requiring shorter incubation times and lower concentrations. Finally, a facile procedure for the rapid synthesis of carborane complexes of Re and 99Tc has been developed, in an attempt to avoid the difficulties of synthesizing the corresponding cyclopentadienide complexes for application as 99mTc and 186/188Re radiopharmaceutical imaging agents.110 In particular the complexes [M(CO)3(h5-2,3C2B9H11)]9 and rac-[M(CO)3(h5-2-R-2,3-C2B9H10)]9 (M : Re, 99Tc; R : CH2CH2CO2H) have been reported, and studies to achieve labeling at the tracer level are in progress.

6

Crystal Engineered Supramolecular and Polymeric (Metalla)carborane Materials

The self-assembly of 3-I-closo-1,2-C2B10H11 molecules has been described, with a double zig-zag chain of C—H · · · I hydrogen bonding having been identified in the solid state crystal structure, leading to a head-to-tail packing of molecules.111 A

118

Organometallic Chemistry

polymeric caesacarborane system, has been structurally characterized, the monomer consisting of a pentacaesium carborane cage cluster, [Cs5(C2B9H12)4Cl]n.112 Curiously, the formation of this solid state structure is permitted only from solutions containing 2-chloromethyl-4-nitrophenol, which is believed to somehow synergistically facilitate the cluster formation. Three carborane salts of [ZnCl(HpztBu)3]; (HpztBu : 5-tert-butylpyrazole) have been structurally characterized by X-ray crystallography.113 The cations in the structures with [Co(C2B9H11)2]9 and [1-Ph-closo-1-CB9H9]9 counterions have been found to associate into dimers, resulting in a bowl-shaped face, which can accommodate the cobaltacarborane cage anions, while in [ZnCl(HpztBu)3] [6,7,8,9,10-Br5-closo-1-CB9H5], the cations assemble into a one-dimensional hydrogen-bonded polymer. The isolation of o-carborane with cucurbit[n]urils (n : 5, 6, 7, 8 or 10) from trifluoroacetic acid/sulfuric acid mixtures has been described.114 The resulting structures arise from the host-guest self-assembly of molecular ball bearings, with the encapsulated carborane cage able to freely rotate within the cucurbit[7]uril. The insertion of o-carborane molecules into single-walled carbon nanotubes (SWNTs) has been reported.115 High resolution transmission electron microscopy has revealed the packing of both discrete carborane molecules, as well as zig-zag one-dimensional carborane chains in the SWNT capillaries. The synthesis has been reported of linear carborane-siloxaneacetylenic polymers, for the purpose of serving as precursors for thermosets or ceramics for composite applications up to 500 °C or 1500 °C, respectively.116 The poly(carborane-siloxane-acetylene)-coated carbon fibers so-formed, result in a protective barrier against oxidation at these high temperatures, with stability being attributed to the presence of boron, in particular. With a view toward the inclusion of carboranes into cost-effective high performance polymers, a series of derivatives of 1,2-C2B10H12 with PhOC6H4- or HO2CC6H4-functionalized aryl groups bound to one or both boron atoms in the 9 and 12 positions, have been synthesized for potential transformation to phenylene ether carboranylene ketones (PECK) polymers.117 A study of radiation aging of polymethyl methacrylate (PMMA) has been reported.118 Irradiation of PMMA with g radiation in the presence of dibenzyl-o-carborane resulted in improved thermal stability and a higher softening point for the polymer due to chemical binding of the PMMA oxygen-containing groups with the carborane in the polymer matrix. A boronrich boron-carbon alloy has been fabricated for use as a solid-state neutron detector.119 The alloy was formed by subjecting a boron-rich source gas of o-carborane to plasma-enhanced chemical vapor deposition. The detection of single neutrons was reported, with little interference from signals induced by g radiation. It has been demonstrated that m-carborane crystallizes in three distinct phases depending on temperature: orientationally disordered orthorhombic (< 170 K), ordered monoclinic (< 280 K), and quasi-isotropic cubic (> 280 K).120 In addition to solid-echo NMR and spin-lattice relaxation time measurements, it has been shown that in the orthorhombic phase, the m-carborane molecules undergo strongly anisotropic motion with temperature dependent amplitude. There have been two reports on studies of carborane-containing plastic crystals. Firstly, relaxation dynamics of m-carborane have been assessed

5: Carboranes, Including Their Metal Complexes

119

through dielectric measurement,121 while in other work, broadband dielectric spectroscopy of disordered o-carborane has been measured to show that the boson-peak in glass-forming materials is related to the acoustic phonon density of states, coupled with contributions from a librational mode distribution.122

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27.

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Chem., 2002, 651, 34. S. Du, J. A. Kautz, T. D. McGrath and F. G. A. Stone, J. Chem. Soc., Chem. Commun., 2002, 1004. K.-C. Kim, C. A. Reed, G. S. Long and A. Sen, J. Am. Chem. Soc., 2002, 124, 7662. M. J. Ingleson, M. F. Mahon, N. J. Patmore, G. D. Ruggiero and A. S. Weller, Angew. Chem., Int. Ed. Engl., 2002, 41, 3694. N. J. Patmore, C. Hague, J. H. Cotgreave, M. F. Mahon, C. G. Frost and A. S. Weller, Chem. Eur. J., 2002, 8, 2088. M. A. Fox, M. F. Mahon, N. J. Patmore and A. S. Weller, Inorg. Chem., 2002, 41, 4567. A. Rifat, N. J. Patmore, M. F. Mahon and A. S. Weller, Organometallics, 2002, 21, 2856. J. Wang, S. Li, C. Zheng, J. A. Maguire and N. S. Hosmane, Organometallics, 2002, 21, 5149. F. Fabrizi de Biani, M. Fontani, E. Ruiz, P. Zanello, J. M. Russell and R. N. Grimes, Organometallics, 2002, 21, 4129. V. E. Konoplev, I. V. Pisareva, D. A. Lemenovskii, P. V. Petrovskii, O. L. Tok, F. M. Dolgushin and I. T. Chizhevsky, Collecn. Czech. Chem. Commun., 2002, 67, 936. J.-W. Hwang, J.-H. Kim, J. Kim, J.-K. Uhm and Y. Do, Bull. Kor. Chem. Soc., 2002, 23, 1257. Z. Fei, S. Busse and F. T. Edelmann, J. Chem. Soc., Dalton Trans., 2002, 2587. A. R. Kudinov, D. S. Perekalin, P. V. Petrovskii, K. A. Lyssenko, G. V. GrintselevKnyazev and Z. A. Starikova, J. Organomet. Chem., 2002, 657, 115. A. R. Kudinov, D. S. Perekalin, P. V. Petrovskii and G. V. Grintselev-Knyazev, Russ. Chem. Bull., 2002, 51, 1928. O. Tutusaus, S. Delfosse, A. Demonceau, A. F. Noels, R. Nu´n ez, C. Vin as and F. Teixidor, Tetrahedron Lett., 2002, 43, 983. J. Plesˇek, C. Ivana and J. Backovsky, Collecn. Czech. Chem. Commun., 2002, 67, 569. J. Llop, C. Vin as, F. Teixidor, L. Victori, R. Kiveka¨s and R. Sillanpa¨a¨, Inorg. Chem., 2002, 41, 3347. J. Llop, C. Vin as, F. Teixidor, L. Victori, R. Kiveka¨s and R. Sillanpa¨a¨, Organometallics, 2002, 21, 355. T. M. Polyanskaya, V. N. Ikorskii, V. K. Aksenov and E. A. Il’inchik, Russ. J. Coord. Chem, 2002, 28, 407. A. R. Kudinov, D. S. Perekalin, S. S. Rynin, K. A. Lyssenko, G. V. GrintselevKnyazev and P. V. Petrovskii, Angew. Chem., Int. Ed. Engl., 2002, 41, 4112. Y. Endo, C. Songkram, R. Yamasaki, A. Tanatani, H. Kagechika, K. Takaishi and K. Yamaguchi, J. Organomet. Chem., 2002, 657, 48. G. Zi, H.-W. Li and Z. Xie, Organometallics, 2002, 21, 3464. G. Zi, H.-W. Li and Z. Xie, Organometallics, 2002, 21, 5415. Y. Wang, H. Wang, H.-W. Li and Z. Xie, Organometallics, 2002, 21, 3311. A. Burke, R. McIntosh, D. Ellis, G. M. Rosair and A. J. Welch, Collecn. Czech. Chem. Commun., 2002, 67, 991. J. Lee, T. Lee, S. S. Lee, K.-M. Park, S. O. Kang and J. Ko, Organometallics, 2002, 21, 3922. C. Lee, J. Lee, S. W. Lee, S. O. Kang and J. Ko, Inorg. Chem., 2002, 41, 3084. M. Heberhold, H. Yan, W. Milius and B. Wrackmeyer, Chem. Eur. J., 2002, 8, 388. S.-X. Lu, G.-X. Jin and N.-H. Hu, Gao. Xue. Hua. Xue., 2002, 23, 1077. S.-X. Lu, G.-X. Jin, N.-H. Hu and H.-Q. Jia, Chem. Res. Chin. Univ., 2002, 18, 405. X.-Y. Yu, G.-X. Jin and L.-H. Weng, Chin. J. Chem., 2002, 20, 1256.

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6 Group III: B, Al, Ga, In, Tl BY SIMON ALDRIDGE

1

General

This account covers highlights from the chemical literature in the year 2002, and has been compiled from a review of papers published in the primary and review literature. The scope of organometallic chemistry of group 13 is immense with vast uses in organic synthesis stemming for example from the use of hydroboration in functionalization chemistry, of Suzuki-Miyaura and Barbier couplings in C-C bond forming reactions and of tri-coordinate group 13 species in Lewis acid catalysis. Similarly, the use of boron containing Lewis acids in the activation of olefin polymerisation catalysts continues to attract much attention. Given the scope for exploitation of group13 organometallics, a comprehensive review is beyond the scope of this report, and some topics (e.g. the use of boranes in hydroboration chemistry, group 13 hydrides as reducing agents and arylboronic acids in coupling reactions) have been touched on only where significant new developments have been reported. A number of reviews covering topics within the remit of this report have appeared during the year 2002. These include reports on bonding modes and structures of alkynyl aluminium compounds,1 b-diketiminatometal complexes,2 matrix reactions of ground state and electronically excited atoms of main group elements,3,4 boranes as ligands,5 main group 1,8-disubstituted naphthalene derivatives,6 azides of the heavier group 13 elements,7 organoelement chemistry of main-group porphyrin complexes,8 optical properties of molecular and polymeric boron-containing systems,9 metalloidal aluminium and gallium clusters,10,11 recent developments in low-valent organoaluminium chemistry,12 applications of boron-nitrogen and boron-phosphorus adducts in organometallic chemistry,13 cross-coupling reactions of organoboron compounds,14-16 and boron-containing acids as protective agents and catalysts in synthesis.17 In addition a whole issue of Polyhedron was devoted to low valent and multiply bonded compounds of the main group elements. 2

Boron

2.1 B(C6F5)3 and Related Boranes. — The Lewis acidic behaviour of B(C6F5)3 has long been exploited in the synthesis of donor/acceptor adducts, some of Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 124

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which feature as intermediates in Lewis acid catalysed transformations. Thus, B(C6F5)3 reacts with water to form the aqua complex H2OB(C6F5)3 2H2O, which can be deprotonated by 1,8-bis(dimethylamino)naphthalene (proton sponge) to form the salt [C10H6(NMe2)2H][(C6F5)3B(H3O2)B(C6F5)3] (1). The complex anion has been characterized crystallographically suggesting that this is best thought of as a complex of the theoretically predicted asymmetrically hydrogen bonded [H3O2]9 ion.18 Furthermore, B(C6F5)3 has been found to react with n-octadecanol (n-C18H37OH) and n-octadecanethiol (n-C18H37SH) to form equilibrium mixtures of reactants and the 1:1 adducts (n-C18H37EH)B(C6F5)3 (E : O 2, S 3); equilibrium constants for these reactions have been determined. The adducts are further deprotonated by 1,8-bis (dimethylamino) naphthalene to form the salts [C10H6(NMe2)2H][(n-C18H37E)B(C6F5)3], and by Cp2ZrMe2 to give methane and, apparently, the unstable zirconium complexes [Cp2ZrMe][(nC18H37E)B(C6F5)3].19

Adducts of the N-benzyl imines Ph(R)C:NCH2Ph (R : Ph, CH3, H) and Bu(CH3)C:NCH2Ph and of the N-phenyl imine Ph(H)C:NPh with B(C6F5)3, have been also prepared and characterized in solution and in the solid state. For each imine, the Lewis acid is N-bound, with the exception of the sterically demanding base tBu(CH3)C:NCH2Ph, which reacts with B(C6F5)3 through its enamine tautomer to form an a-C bound adduct. In the N-bound imine-borane adducts steric crowding and p-stacking between C6F5 and C6H5 rings results in restricted rotation about the B-N and B-C bonds. The dynamic behavior which results has been studied using variable-temperature 19F and 1H NMR spectroscopy.20 The stoichiometric reaction of B(C6F5)3 and NEt2Ph has also been investigated by 1D and 2D NMR spectroscopy (1H, 11B,13C, 15N and 19F). No Et2PhN B(C6F5)3 adduct was observed, rather an equilibrium between free B(C6F5)3, NEt2Ph, [HB(C6F5)3]9[HNEt2Ph]; and two zwitterionic stereoisomers (E)- and(Z)-[EtPhN;:CH-CH2-B9(C6F5)3] (30%) in an E/Z ratio of 3:2 was found. Addition of a protic reagent ROH [R : H, SiPh3, (cC5H9)7O12Si8, or silanol group of silica], caused the equilibria involved to be quantitatively displaced towards the ionic form [ROB(C6F5)3]9[HNEt2Ph]; (4).21 In related work the reaction of B(C6F5)3 with the silanol groups of silica surfaces, dehydroxylated at different temperatures (300, 500, 700, and 800°C), has been investigated in presence of the Brønsted base NEt2Ph. The structure of the resulting modified silica supports [∫Si-OB(C6F5)3]9[HNEt2Ph]; (5) has been carefully identified by IR and multinuclear solid-state NMR spectroscopies, isotopic 2H and 18O labeling, elemental analysis, molecular modeling, and comt



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parison with synthesized molecular models.22 Addition of N-Methylimidazole to B(C6F5)3 leads to coordination via the donor nitrogen atom to form the adduct 6 which has been characterized by X-ray diffraction. Deprotonation at the imidazole carbon atom C2 could then be achieved by treatment with methyllithium to generate the reactive ‘Arduengo carbene anion’ intermediate 7, which is reported to undergo a rapid intramolecular nucleophilic aromatic substitution reaction at one of the adjacent C6F5 groups to form a tricyclic betaine-type product 8.23 Vinyl C-H activation reactions of vinyl esters mediated by B(C6F5)3 have been reported by Dash and Jordan. Electrophilic addition of B(C6F5)3 to the C:C bond, followed by proton transfer and elimination of C6F5H is reported to yield the chelated vinylborane product (C6F5)2B
k2-CH:CHOC(:O)R (9, R : Me, Ph, scheme 1).24

Scheme 1

Bifunctional analogues of B(C6F5)3 and related boranes have also been synthesised, and in some cases their anion coordination properties evaluated. Thus, the synthesis and olefin polymerization characteristics of catalyst systems utilizing the binuclear organo-Lewis acid cocatalyst 9,10-bis(pentafluorophenyl)9,10-diboraoctafluoroanthracene, C12F8B2(C6F5)2 (10), have been reported by Marks and co-workers. X-ray diffraction analysis reveals a nearly planar C12F8B2 core with -C6F5 rings rotated 75o out of the plane, hindering p communication between the pendant -C6F5 substituents and the C12F8B2 core, and thereby enhancing Lewis acidity at the boron centers. Competition equilibration experiments between 10, B(C6F5)3, and acetonitrile over a wide temperature range demonstrated that 10 is a stronger Lewis acid than B(C6F5)3 by DH : ;1.4(2) kcal/mol and DS : -5.3(1) eu.25

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A related, although more flexible, chelating framework is represented by C6F41,2-[B(C6F5)2]2 (11). The reaction of the MeO9 complex of 11, [Ph3C];[C6F41,2-
B(C6F5)22(m-OCH3)]9 (12), with Bu3SnH has been shown to yield the solvated stannylium ion [Bu3Sn(arene)];[C6F4-1,2-
B(C6F5)22(m-OCH3)]9 (13). This material is thermodynamically stable toward transfer of the chelated OMe9 anion to ‘Bu3Sn;’ as evidenced by the reaction of free diborane 11 with 2 equiv of Bu3SnOMe.26

Bifunctional Lewis acid systems based on napthyl or single atom bridges have also been reported. The reaction of 1,8-dilithionaphthalene with dimesitylboron fluoride affords a rare boron-bridged naphthalene derivative which undergoes ring-opening reactions to afford unsymmetrically substituted 1,8-diborylnaphthalene species (Ar : Ph 14, Me 15).27 Similar ring-opening chemistry with

trimethyltin chloride to has been shown to afford high yields of 1-(dimesitylboron)-8-(trimethyltin)naphthalenediyl (16), a compound in which a boryl and a stannyl moiety coexist at the peri positions of a naphthalene core. 16 is a useful starting material for the synthesis of other heteronuclear naphthalene derivatives. Thus, transmetalation with GaCl3 or InCl3 in THF results in high yields of 1-(dichlorogallium)-8-(dimesitylboron)naphthalenediyl (17) and 1-(dichloroindium)-8-(dimesitylboron)naphthalenediyl (18), respectively.28 Finally, tetraphenyldiboroxane (19) has been isolated as the main by-product in the synthesis of BPh3 and characterized by means of 11B NMR spectroscopy and X-ray structure analysis.29 Taking this approach one step further, a number of polyfunctional Lewis acids have also been reported in 2002. The use of silsesquioxanes modified by the organoboranes (C6F5)2BX (X : H or Cl) for olefin polymerization in combination with Cp2ZrMe2 has been investigated as soluble models for metallocene

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catalyst deactivation.30 In addition, the synthesis and characterization of (perfluoroaryl)borane-functionalized dendrimers and their use as Lewis acid

catalysts for the hydrosilation of acetophenone have been reported by Piers and co-workers. Carbosilane dendrimers capped with 4, 8, and 12 perfluoroarylborane Lewis acids were prepared by self-catalysed aryl ether silation with the appropriate Si-H terminated dendrimer scaffold. The dendrimers were fully characterized by spectroscopic methods and MALDI-TOF mass spectrometry and tested as catalysts for the hydrosilation of acetophenone using triethylsilane, exhibiting only slightly inferior activities in comparison to B(C6F5)3.31 A number of related boranes synthesised during 2002 are also worthy of comment. Tris(trifluoromethyl)borane carbonyl, (CF3)3BCO (20), has been obtained in high yield by the solvolysis of K[B(CF3)4] in concentrated sulfuric acid. The in situ hydrolysis of a single bonded CF3 group has therefore been shown to be a simple, unprecedented route to a new borane carbonyl. The colourless liquid and vapour of (CF3)3BCO decomposes slowly at room temperature (t1/2 : 45 min in the gas phase). (CF3)3BCO reacts in two different ways towards nucleophilic reagents: substitution of CO or addition to the C atom of the carbonyl group are both observed depending on the nucleophile. The molecular structure of (CF3)3BCO in the gas phase was obtained by a combined microwave and electron diffraction analysis and in the solid state by single-crystal X-ray diffraction. The molecule possesses C3 symmetry, since the three CF3 groups are found to be rotated off the two possible positions required for C3v symmetry.32,33

The synthesis and properties of tris(3,3-dimethyl-1-butynyl)borane (21), the first donor-free tris(alkynyl)borane, and of its adducts with pyridine, triphenylphosphine and tetrahydrofuran have been reported. X-ray structure analyses

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129

reveal that the B-C bond lengths in 21 are shorter than those in the adducts, but that the C∫C triple bond lengths are similar.34 The mechanism of the alkyl group exchange between alkylboranes and alkylzinc compounds has been probed by DFT. It was found that the exchange reaction proceeds with a low activation barrier, involving two intermediates with unusual bonding structures. The metal-carbon bonding in these intermediates ensures a highly stereoselective exchange process, which could be employed in asymmetric organometallic synthesis.35 Compounds of the type (C5H4R)2Ta[CH2B(C6F5)2](m-H)CH3 (R : H 22, Me 23) have been generated by treatment of (C5H4R)2Ta(:CH2)CH3 with HB(C6F5)2. When allowed to undergo irreversible methane loss in the presence of an excess of the sterically modest alkynes 2-butyne or phenylacetylene, the putative intermediates (C5H4R)2Ta[h2-CH2B(C6F5)2] (R : H 24, Me 25) have been trapped as the corresponding tantala-3-boratacyclopentene compounds. In these complexes, the alkyne and borataalkene ligands have reductively coupled at the d2 tantalum centre. These studies represent the first studies concerning the reactivity of a borataalkene ligand at a transition metal centre showing that it can behave in an ‘olefin-like’ manner, despite having a more flexible array of bonding modes available to it than an olefin.36 By contrast, trapping of the tantalocene borataalkene complex Cp2Ta[CH2B(C6F5)2] with isocyanide reagents gives a Ta(III) product in which the metallocene fragment is ligated by an h3-1-azaallyl ligand incorporating an iminoacyl boryl fragment on the central carbon atom. This unique ligand assembly arises from reaction of the isocyanide reagent with the h1 form of the borataalkene ligand, thereby demonstrating another new mode of reactivity for this relatively unexplored class of boratahydrocarbon ligand.37 2.2 Borate Anions. — Borate-based anions continue to attract considerable attention, both as counter-ion partners for cationic metallocene systems in olefin polymerisation, and as negatively charged analogues of well known carbonbackbone chelating and tripodal ligand systems. The reaction between NaNH2 and B(C6F5)3 has been shown to afford the remarkably stable amidodiborate anion [H2N
B(C6F5)32]9 (26), the extended structure of which shows multiple intramolecular NH · · · F hydrogen bonds. Reaction with HCl affords [H(OEt2)2][H2N
B(C6F5)32] (27), while treatment of zirconocene dimethyls with [CPh3][H2N
B(C6F5)32] (28) gives highly active alkene polymerization catalysts.38

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Treatment of lithium imidazolide with two equivalents of B(C6F5)3 has been reported to yield the salt Li;[(C3H2N2)B(C6F5)3]9 (29) which has subsequently been used to generate group 4 metallocene cation systems by means of halide abstraction. Treatment of 29 with HCl in ether afforded [H(OEt2)2] ; [(C3H2N2)B(C6F5)3]9 (30) which can similarly be used to generate Group 4 metallocene cations by protonation, e.g. of dimethylzirconocene.39 A number of transition metal complexes have been reported in 2002 incorporating the anionic boron-centred ligands [PhB(CH2PPh2)3]9 (31) and [Ph2B(CH2PPh2)2]9 (32). In part, the use of a borate centred chelating ligand has facilitated the synthesis of overall charge neutral, zwitterionic analogues of catalytically relevant phosphine/metal cations. These include examples featuring cobalt,40,41 iridium,42,43 and palladium (33).44 Of related interest is the bis(amino)borate ligand, [Ph2B(CH2NMe2)2]9 (34), an anionic equivalent of tertiary diamines, which has been used to prepare a series of N-chelated, zwitterionic rhodium(I) complexes (35).45

2.3 Boron Hydrides. — Boron hydrides continue to attract attention, not only as widely used reducing agents, but also as ligands to transition metals through coordinated B-H bonds. Laser-ablated boron has been shown to react with hydrogen on condensation in excess neon to give the BH4 radical (36), BH49 anion, and B2H6 as the major products. Infrared spectra of 36 support the C2v structure deduced from previous ESR spectra and theoretical calculations with two normal B-H bonds and two long B-H bonds for this novel electron-deficient radical. NBO analysis suggests that the two long B-H bonds and the H-H bond are one-electron bonds.46 Metal complexes containing the BH49 anion also continue to attract attention; the molecular structures of Zr[(m-H)3BH]4 (37) and U[(m-H)3BH]4 (38) have been re-investigated by DFT calculations and by gasphase electron diffraction (GED). The triply bridged bonding mode of the tetrahydroborate groups is confirmed, but both DFT calculations and GED structure refinements indicate that the BH4 groups are rotated some 12o away from the orientation in which the three bridging B-H bonds are staggered with respect to the opposing ZrB3 fragment. As a result the symmetry of the equilibrium conformation is reduced from Td to T.47 Metal complexes featuring sigma BH-bound neutral boranes have also been reported, in part as models for the hitherto unknown isoelectronic alkane complexes. Thus, room temperature reaction of the bis(dihydrogen) complex

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131

RuH2(H2)2(PCy3)2 (39) with excess pinacolborane (HBpin) has been shown to generate the novel complex RuH[(m-H)2Bpin](s-HBpin)(PCy3)2 (40) by loss of dihydrogen. Complex 40 contains two pinacolborane moieties coordinated in different fashions, one as a dihydroborate and the other as a s-borane. In addition, reaction of 39 with one equivalent of HBpin yields total conversion to a new complex tentatively formulated as RuH[(m-H)2Bpin](H2)(PCy3)2 (41) on the basis of NMR data. In the presence of excess HBpin, 41 is converted to 40.48 The fluxional behavior of group 6 metal complexes of the monodentate bis(trimethylphosphine)diborane(4) ligand, M(CO)5(h1-B2H4 2PMe3) (M : Cr 42, W 43), have been studied by NMR spectroscopy. These experiments have revealed two fluxional processes in solution: one is the very rapid exchange of the coordinated BH atom with the geminal hydrogen atom, and the other one is the rather slow exchange with the vicinal hydrogens. This behavior embodies the fluxionality predicted for the ethane complex, W(CO)5(h1-C2H6).49

2.4 Boratabenzenes and Related Systems. — p-Bound boron-containing ligand systems (e.g. boratabenzenes and azaborolides) and their complexes with transition metals continue to attract comparison with related cyclopentadienyl species. The structures of a number of borabenzene adducts of ylidic Lewis bases have been reported. Thus, methylenephosphorane Ph3PCH2 reacts with 1chloro-3,5-dimethyl-2-(trimethylsilyl)-1,2-dihydroborinine(in a 2:1 ratio) to produce (triphenylphosphonio)methanide-3,5-dimethylborabenzene, 3,5Me2C5H3BCH2PPh3 (44).A similar reaction with iminophosphorane Ph3PNPh produces N-(triphenylphosphonio)anilide-borabenzene C5H5BN(Ph)PPh3 (45). Neither compound shows conjugation between the two remote charge centres.50

The syntheses of the new lithium boratabenzenes Li(3,5-Me2C5H3BNiPr2), (46), and Li2[(3,5-Me2C5H3BNMe)2(CH2)2], (47), have been described and their reactions with metal trihalides to give new 1-aminoboratabenzene complexes of the scandium group metals have been examined in detail.51 In related work, Cb*Co(3,5-Me2C5H3BNMe2) (48) (Cb* : C4Me4) was obtained from [Cb*Co(NCMe)3]PF6 and Li(TMEDA)(3,5-Me2C5H3BNMe2). Methanolysis of 48 was then shown to afford Cb*Co(3,5-Me2C5H3BOMe) (49), which itself could undergo further reaction with with MeLi or with BCl3 to give the B-methyl analogue 50 or the B-chloro compound 51, respectively. Reaction of chloro compound 51 with iBu2AlH, TlF, and LiSnMe3 was then shown to afford the B-hydrido derivative 52, the B-fluoro complex 53, and the B-(trimethylstannyl) compound 54, respectively.52 In addition, dicationic triple-decker complexes

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containing a bridging boratabenzene ligand [Cp*Fe(m-h:h-C5H5BMe)ML]X2 [ML : CoCp* 55, RhCp 56, IrCp 57, Ru(h6-C6H6) 58, Ru(h6-C6H3Me3-1,3,5) 59, Ru(h6-C6Me6) 60] have been synthesized by stacking reactions of Cp*Fe(h6C5H5BMe) (61) with the corresponding half-sandwich fragments [ML]2;.53 A number of boratabenzene and related complexes of zirconium(IV) have been examined for olefin polymerisation activity. Thus, complexes of the type (C5H5BR)2ZrCl2 (62) have been activated by methylaluminoxane (MAO) to form catalyst mixtures that polymerize (R : dialkylamino) or oligomerize (R : alkyl, alkoxy) ethylene. The selectivity for specific olefin products is reported to be influenced by the exocyclic substituent on the boratabenzene ring. When (C5H5BOEt)2ZrCl2/MAO was exposed to 1 atm of ethylene, a-olefins were produced in greater than 99% purity, while (C5H5BMe)2ZrCl2/MAO/C2H4 gave a mixture of a-olefins, internal olefins, and vinylidenes. Other catalysts, such as (C5H5BOCH2Ph)2ZrCl2/MAO and (C5H5BOCy)2ZrCl2/MAO, were reported to oligomerize ethylene with selectivities for a-olefin production that were between those of their OEt and Me substituted analogues.54 The syntheses of (h6-boratastilbene)(h5-pentamethylcyclopentadienyl)zirconium dichloride (63), (h6-4dichloride boratastyrylstilbene)(h5-pentamethylcyclopentadienyl)zirconium (64), [1,4-bis(boratastyryl)benzene]bis[(dichloro)(pentamethylcyclopentadienyl)zirconium] (65), and bis(h6-boratastilbene)zirconium dichloride (66) have been reported by Lee and Bazan, and the activities of 63 — 66 for ethylene polymerisation in the presence of methylaluminoxane have been investigated. The activity of 63 is reported to be three times of that of Cp2ZrCl2.55

A general method for the syntheses of new families of transition-metal-free 1,2-azaborolides has been developed. These azaborolides react with a variety of transition-metal electrophiles to produce an array of h5-azaborolyl adducts (of type 67) with ERx substituents featuring nitrogen, oxygen, carbon or phosphorus donors.56 In addition, the electronic properties of these isoelectronic analogues of the cyclopentadienyl ligand have been investigated in depth.57

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2.5 Boron-containing Materials. — A series of new donor/p-acceptor type compounds (of generic structure 68) with trivalent boron as the acceptor function and which show strong two-photon excited up-conversion fluorescence have been synthesized.58

A novel procedure for the dehydrocoupling of cyanamide and tris(hydridosilylethyl)boranes has been developed which allows for the first time the synthesis of boron-modified polysilylcarbodiimides with adjustable nitrogen content, which are promising molecular precursors for ultrahigh temperature Si/B/C/N ceramic materials.59 2.6 Boron-based Sensors. — The use of boron-based reagents in the sensing of anionic and charge neutral molecules continues to attract enormous attention. Only a brief discussion of selected developments is therefore possible here. A number of dibenzoborole derivatives have been reported which show significant solvatochromism in their fluorescence spectra. Thus, 100-140 nm blue shifts in the emission maxima and 20-30 fold increments in the quantum yields are observed upon changing the solvent from THF to DMF. Similar fluorescence changes are observed upon the addition of nBu4NF to solutions in THF, thereby demonstrating sensing capabilities towards fluoride. These fluorescence changes result from the on/off control of the pp -p* conjugation in their LUMO by the coordination of donor solvents (or fluoride ion) to the boron atom in the dibenzoborole skeleton (scheme 2).60

Scheme 2

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A novel approach for the potentiometric detection of saccharides using poly(aniline boronic acid) has been devised by Shoji and Freund. A model has been proposed in which the electrochemical potential is sensitive to the change in the pKa of a conducting polymer as a result of boronic acid-diol complexation.61 2.7 Boron-based Ligand Systems. — Boryl and borylene complexes of transition metals continue to attract much attention both from a structure and bonding viewpoint and on account of their implication in extraordinary organic transformations, such as the catalytic, selective functionalization of hydrocarbons. A number of theoretical studies have addressed the nature of bonding in boryl complexes (LnMBX2), in borylene (boranediyl) ligands (BX) and in their complexes (LnMBX). Thus, analysis of the bonding in transition metal boryl complexes of the type (C5R5)M(CO)2BX2 (69) has been carried out by density functional methods, to quantify the relative contributions to the metal boryl linkage from ionic and covalent interactions. Covalent (orbital) terms are reported to account for 60-70% of the overall attractive interaction between metal and boryl fragments, with s donation from the boryl ligand overwhelmingly predominating over p back-donation even in the most favorable cases (e.g. 84.1:15.8 and 81.9:18.0 for CpFe(CO)2BH2 and CpFe(CO)2B(C6F5)2, respectively).62 Similar DFT calculations have been reported for the model tris- and bis(boryl) complexes Rh(PH3)3(BX2)3 (X : H 70; OH 71) and Rh(PH3)4(BX2)2; (X : H 72; OH 73). Analysis of the energetic ordering in the mer isomers of 70 and 71 shows that back-bonding in these complexes involves a Rh-B bonding orbital (and not a d-block orbital as in most previously reported examples). This surprising behaviour has been rationalized through MO and NBO analyses, and backed up by the corresponding calculations on the bis(boryl) complexes 72 and 73.63 DFT calculations have also been carried out on ruthenium porphyrin and carbonyl diyl complexes (CO)4Ru-EH (both axial and equatorial isomers), (Por)Ru-EH, and for (Por)Ru-E(C6H2iPr3-2,4,6) (E : B — Tl). Subsequent natural bond orbital (NBO) analyses have been applied to examine in detail the Ru-E bonding situation and the influence of the porphyrin ligand. The calculations reveal high Ru-E (E : B-Tl) bond dissociation energies, especially for Ru-B bonds and that E-Ru p-back-bonding is most significant in the case of boron. The porphyrin ligand results in similar p-back-bonding to that demonstrated for carbonyl ligand frameworks; however, s-donation from E to Ru is stronger in the case of the porphyrin ligand.64 The structures, bond energies, heats of formation, and quantitative bonding analyses of main-group metallocenes ECp2 (E:Be-Ba, Zn, Si-Pb) and ECp (E:Li-Cs, B-Tl) have been examined by Rayo´n and Frenking using DFT. The nature of the metal-ligand bonding has been analysed by an energy-partitioning method which has given quantitative information about the strength of the covalent and electrostatic interactions between En; and (Cp9)n (n : 1, 2). The contributions of the orbitals with different symmetry to the covalent bonding have also been reported.65 In related work, the complexes of ECp and related Lewis bases, with group 13 centred Lewis acids have been examined. Thus, the

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results of DFT calculations on the donor-acceptor complexes R3E-ER and their isomers R2E-ERR, (E, E : B-Tl; R, R : H, Cl, CH3) have been reported by Timoshkin and Frenking. The theoretically predicted energy differences between the donor-acceptor complex R3E-ER and the classical isomer R2E-ERR have been derived, although there appears to be no simple correlation of the nature of the group-13 elements E, E and the substituents R, R with the relative stability of the complexes. Calculations on Cl3B-BR (R : Cl, Cp or Cp*) indicate that the Cp* group has a particularly strong effect on the complex form. The calculations show that the experimentally known complex Cl3B-BCp* (74) is the strongest bonded donor-acceptor complex of main-group elements that has been synthesized to date. The theoretically predicted B-B bond dissociation energy is Do : 50.6 kcal/mol.66

Synthetic studies on metal boryl complexes have also been reported in 2002, both with a view to elucidating their fundamental reaction chemistry, and to better understand their role in catalytic hydrocarbon functionalization processes. Hence, mild iridium-catalysed borylation of arenes has been reported, together with the isolation of a potential tris(boryl) metal complex intermediate. Ir(I) precursors in conjunction with bipyridine ligands have been shown to catalyse the borylation of arenes bearing either electron-withdrawing or electron-donating substituents at temperatures much lower than those previously reported for direct arene borylation. The combination of [Ir(COE)2Cl]2 and (4,4-di-t-butyl)bipyridine even allows for reaction at room temperature. The same catalyst system at 100oC has been shown to provide remarkably high turnover numbers for a hydrocarbon functionalization process.67Tethered osmium boryl complexes have been reported to be formed from the reaction of Os(BCl2)Cl(CO)(PPh3)2 (75) with 2-hydroxypyridine. Thus, replacement of one chloride on the dichloroboryl ligand and coordination of the pyridine nitrogen atom to osmium, leads to the six-coordinate complex Os[BCl(OC5H4N)] Cl(CO)(PPh3)2 (76). The five-membered chelate ring so formed is stable and persists through reactions involving replacement of chloride both at osmium and at boron. When 76 is treated with anhydrous HI, replacement of chloride at osmium occurs exclusively, leaving the B-Cl bond intact and giving Os[BCl(OC5H4N)]I(CO)(PPh3)2 (77). By contrast, nucleophilic substitution reactions at boron occur readily with EtOH, n-BuNH2 and Et3N 3HF to give the corresponding alkoxy-, amido- and fluoro-boryl complexes, respectively.68 Similar chemistry has been reported in the reaction between Os(BCl2)Cl(CO)(PPh3)2 and 2-aminopyridine, although two products are isolated in this case. The first

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(78) has a structure analogous to that of 76, whereas the second, corresponds to addition of the aminopyridine in the reverse direction, leading to the tethered, pyridine-stabilized borylene complex Os[B(OEt)(NC5H4NH)]Cl(CO)(PPh3)2 (79). Further B- and Os-centred substitution chemistry has been reported for both 78 and 79.69

The silyl(boryl)platinum(II) species cis-Pt(SiMe2Ph)(BX2)L2 [X2 : -OCMe2CMe2O-, L : PMe3 (80), PMe2Ph (81), PEt3 (82); X2 : -NMeCH2CH2NMe-, L : PMe3 (83)] have been prepared by oxidative addition of silylboranes to platinum(0) complexes. Complexes 80 - 82 undergo selective insertion of phenylacetylene into the Pt-B bond at room temperature, giving cis-Pt
C(Ph):CH(Bpin)(SiMe2Ph)L2 [L : PMe3 (84), PMe2Ph (85), PEt3 (86)], respectively, while 83 is inactive toward insertion. The insertion complexes 84 86 undergo subsequent C-Si reductive elimination to give (Z)-a-silyl-b-borylstyrene with reactivity falling off in the order 86 >> 85 > 84.70Salt elimination methodologies have also been used to give access to new boryl systems. Thus, dinuclear species containing iron centres linked via various bridging boryl ligands (e.g. 87) have been synthesised and a combination of crystallographic, computational and spectroscopic (Mo¨ssbauer, IR, Raman) techniques have been used to probe the bonding in these complexes.71The synthesis of the first silyland germylboryl complexes have been reported by Braunschweig and coworkers, utilizing salt elimination reactions starting from anionic transition metal complexes of the type Na[(h5-C5R5)Fe(CO)2] and K[(h5C5R5)Mn(H)(CO)2]. The novel iron and manganese boryl complexes (h5C5R4R)(OC)2FeB(Cl)Si(SiMe3)3 (R : R : H 88; R : H, R : Me 89; R : R : Me 90), (h5-C5H5)(OC)2FeB(Cl)Ge(SiMe3)3 (91), (h5-C5H5)(OC)2FeB(Cl)Si(SiMe3)2Si(SiMe3)3 (92), (h5-C5H4Me)(OC)2(H)MnB(Cl)Si(SiMe3)3 (93) and [(h5C5H4Me)(OC)2(H)MnB(Cl)Ge(SiMe3)3] (94) were obtained in yields between 35 and 70%. All are reported to be characterised by metal-boron s-bonds without indication of metal-to-boron p-backdonation. There is, however, spectroscopic and structural evidence for the presence of a Mn-H-B bridge in the case of the manganese complexes 93 and 94.72 The synthesis and structural characterization of iron and manganese complexes containing the tetrachlorocatecholboryl (BO2C6Cl4) ligand have also been reported. The structure of the methylcyclopentadienyl iron dicarbonyl derivative (h5-C5H4Me)Fe(CO)2BO2C6Cl4 (95) reveals that the relative orientation of (h5-C5H4Me)Fe(CO)2 and BO2C6Cl4 moieties is influenced by intramolecular C--H · · · O hydrogen bonding.73

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A number of interesting reactions involving bridging borylene complexes, LnM(m-BX)MLn, have also been reported. The syntheses of the bromo-boryl complexes (h5-C5R4R)Fe(CO)2B(2,4,6-Me3C6H2)Br (R : R : H 96; R : H, R : Me 97; R : R : Me 98) have been reported. These are shown to be versatile substrates for the synthesis of both asymmetric boryl complexes [e.g. (h5C5H5)Fe(CO)2B(2,4,6-Me3C6H2)OC6H4tBu-4 99], and bridging borylene complexes
e.g. [(h5-C5H4R)Fe(CO)2]2B(2,4,6-Me3C6H2), R : H 100; R : Me 101via substitution chemistry with retention of the metal-boron bond. Complexes 100 and 101 are reported to be the first examples of structurally characterized bridging borylene complexes without a supporting M-M bond and undergo ready photolytic loss of CO according to Scheme 3.73,74

Scheme 3

Irradiation of the bridged chloroborylene complex
(h5C5H4CH3)Mn(CO)22(m-BCl) (103) under an atmosphere of CO has been shown to afford the Mn2B2 butterfly cluster
(h5-C5H4CH3)Mn(CO)22(B2Cl2) (104) and (h5-C5H4CH3)Mn(CO)3. Although the detailed mechanism for the formation of 104 has yet to be elucidated, the terminal chloroborylene complex (h5C5H4CH3)Mn(CO)2(BCl) (105) appears to be a likely transient intermediate.75 2.8 Boronic Acids and Suzuki Type Coupling Reactions. — Although a complete discussion of Suzuki coupling reactions is beyond the scope of this report (and indeed a number of reviews have appeared in this year14-16), several important advances in this field are worthy of note. 1,4- and 1,3-Bis(trimethylstannyl)benzenes and 2,5- and 2,6-bis(trimethylstannyl)pyridines react with BH3.THF to give intermediates which on hydrolysis lead to benzene- and pyridinediboronic acids in 79-83% yield. While oxidation of the

Organometallic Chemistry

138

benzenediboronic acids with alkaline hydrogen peroxide gives the corresponding 1,3- and 1,4-dihydroxybenzenes, the pyridinediboronic acids react via a double Suzuki reaction with 4-iodoanisole to give 2,5- and 2,6-bis(4methoxyphenyl)pyridines and react with pinacol to give the corresponding pyridine-2,5-pinacol and pyridine-2,6-pinacol diboronic esters.76 The syntheses of 2-mercapto- and 2-piperazino-(methyl-phenyl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolanes (106 and 107, respectively) has been described and their inhibitory activity against serine proteases including thrombin was measured.77 A novel preparation of aryl and heteroarylboronic acids by an electrochemical reductive coupling between aromatic or heteroaromatic halides and a trialkyl borate has been reported. The reactions were carried out in DMF or THF with the use of sacrificial aluminium or magnesium anodes in a single-compartment cell, and the arylboronic acids were obtained in moderate to good selectivities and isolated yields.78

Scheme 4

The asymmetric hydrogenation of 1-phenylethenylboronic acid and esters for the synthesis of chiral organoboron compounds has been reported by Miyaura and co-workers. Thus, the hydrogenation of 108 at 920oC in the presence of [Rh(cod)2]BF4/(R)-BINAP (3 mol%) gave 1-phenylethanol (80% e.e.) after oxidative work up.79

Scheme 5

Alkyl-, alkenyl-, and arylboronic acids undergo Ag2O/CrCl2 mediated homocoupling in moderate to good yields under mild conditions (scheme 6). The general utility of this methodology has been illustrated by intramolecular annulation between sp and sp3 centers.80 A versatile method has been developed by Fu and co-workers for the Suzuki cross-coupling of boronic acids with unactivated alkyl (as opposed to aryl or vinyl) electrophiles at room temperature. On

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the mechanistic side, the study reported that Pd(PtBu2Me)2 undergoes oxidative addition under surprisingly mild conditions (i.e. at 0oC), and that the resulting adduct is a chemically competent intermediate in the cross-coupling process.81 A convenient and one-step synthesis of 9,10-diarylanthracenes has been described via a bis Suzuki-Miyaura cross-coupling reaction. Thus, 9,10-dibromoanthracene was reacted with various aryl boronic acids in presence of palladium(0) catalyst to give 9,10-diarylanthracenes in good yield.82

Scheme 6

Finally, asymmetric aryl transfer reactions to aldehydes using boronic acids as the aryl source, and planar chiral ferrocene species as catalysts has been reported to be applicable to a broad range of products with excellent enantioselectivities (up to 98% ee).83 2.9 Diboron(4) Reagents. — Diboration of unsaturated substrates has become an increasingly popular methodology for introducing functionality in organic molecules and a number of noteworthy new approaches have been reported in 2002. Importantly, a new synthetic route to bis(pinacolato)diborane(4) from the corresponding tetra(pyrrolidino) derivative, [(C4H8N)2B]2 (109), has been reported by Srebnik and co-workers, together with the crystal structures of 109 and its precursor tris(pyrrolidino)borane, 110.84 Furthermore, insertion reactions of bis(pinacolato)diborane(4) [(Me4C2O2)BB(O2C2Me4)] with various diazoalkanes has been shown to provide access to a class of one carbon bridged bis(pinacolato)diborane(4) derivatives(111, scheme 7). The reaction was catalysed by Pt(PPh3)4, and single-crystal X-ray diffraction was used to confirm the structures of the new compounds.85

Scheme 7

Platinum and rhodium catalysed addition reactions of diboron(4) reagents to unsaturated organic substrates continue to represent novel routes to valuable synthetic intermediates. Hence, the platinum(0)-catalysed diboration reaction of bis(pinacolato)diborane(4) [(Me4C2O2)BB(O2C2Me4)] with various 1-alkynylphosphonates and 1-alkynylboronates is reported to give the desired cis-1,2diboronated vinylphosphonate and trisboronated alkene products in high yields.86 Diboration of the styrylboronate esters (E)-p-R-C6H4-CH:CH-Bcat (R

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: H, MeO; cat : 1,2-O2C6H4), with B2cat2 in the presence of either Rh(PPh3)3Cl or [Rh(coe)2(m-Cl)]2 and two equivalents of P(o-tol)3 has been shown to yield the unusual tris(boronate) esters p-R-C6H4-CH2C(Bcat)3 or their isomers p-R-C6H4CH(Bcat)CH(Bcat)2 respectively.87 Additionally, the rhodium catalysed 1,4-addition of bis(pinacolato)diboron and bis(neopentyl glycolato)diboron to a,b-unsaturated ketones, esters, nitriles, and aldehydes (scheme 8) has been developed by Kabalka and co-workers.88

Scheme 8

Diboron(4) reagents also feature in a number of interesting new cross-coupling methodologies. The synthesis of 1-alkenylboronic acid pinacol esters via the palladium-catalysed cross-coupling of bis(pinacolato)diboron with 1-alkenyl halides or triflates has been reported. The borylation of acyclic and cyclic 1-alkenyl bromides and triflates was achieved in high yields with complete retention of configuration of the double bonds (scheme 9). Furthermore, the method has been applied to the one-pot synthesis of unsymmetrical 1,3-dienes via a borylation-coupling sequence.89 w-(4-Bromophenyl)alkanoic acids have been obtained from 1-bromo-4-alkenylbenzenes by a combined hydroborationthermal isomerization-oxidation process and their esters transformed into the corresponding boronates by a cross-coupling reaction with bis(pinacolato)diboron.90

Scheme 9

2.10 Borane Functionalized Cyclopentadienyl Ligands. — A number of metal complexes featuring borane- or borate-functionalized cyclopentadienyl (or related) ligands have appeared in 2002. These include group 4 complexes of relevance to olefin polymerisation and ferrocene based systems with novel electrochemical properties. New boron-bridged ligands incorporating both indenyl and carboranyl moieties have been prepared. Reaction of iPr2NB(C9H7)Cl with one equivalent of Li2C2B10H10 gave, after treatment with 1 equiv of n-BuLi, the dilithium salt [iPr2NB(C9H6)(C2B10H10)]Li2(OEt2)2 (112), which was conveniently converted

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into its neutral counterpart iPr2NB(C9H7)(C2B10H11) (113) via reaction with excess C5H6. Mixing 112 and 113 in a 1:1 molar ratio quantitatively afforded the monolithium salt [iPr2NB(C9H6)(C2B10H11)]Li(THF)2 (114). Treatment of LnI2 with one equivalent of 112 generated the trivalent organolanthanide complexes meso-[
h5:s-iPr2NB(C9H6)(C2B10H10)2Ln][Li(S)n] (Ln : Sm, Yb; S : THF, n : 4; S : DME, n : 3), which were also prepared in a much higher yield by reaction of LnI2 with one equivalent of 112 followed by treatment with one equivalent of 114.91 Group 4 complexes have also been examined: reaction of i Pr2NB(C9H7)(C2B10H11) with Zr(NMe2)4 has been shown to yield the constrained-geometry zirconium amide [h5:s-iPr2NB(C9H6)(C2B10H10)]Zr(NMe2)2 (115), which could further be converted to [h5:s -iPr2NB(C9H6)(C2B10H10)]ZrMe2 (116) on reaction with AlMe3. By contrast, the analogous reaction with Ti(NMe2)4, however, led to the isolation of the deborylated product (h5C2B9H11)Ti(NMe2)2(HNMe2) (117). Salt metathesis reaction between MCl4(THF)2 and [iPr2NB(C9H6)(C2B10H10)]Li2(Et2O)2 was shown to afford the dihalide species [h5:s-iPr2NB(C9H6)(C2B10H10)]MCl2 (M : Ti 118, Zr 119, Hf 120). Examination of the ethylene polymerization activity of these compounds revealed moderate to very high activity (104-106 g mol91 atm91 h91).92 Ring borylation of [Me4C2(h5-C5H4)2CrCO] by B(C6F5)3 has been shown by Shapiro and co-workers to afford the zwitterionic complex
Me4(h5-C5H4)(h5C4H3B(C6F5)3)CrH(CO) (121), the first structurally characterized bent-metallocene complex of Cr(4]). This species has been further shown to decompose thermally to the zwitterionic species
Me4(h5-C5H4)(h5-C4H3B(C6F5)3)Cr (122) and the ionic species [Me4C2(5-C5H4)2CrCO][HB(C6F5)3] (123).93 Both 1,1 and 1,2-diborylated ferrocenes have received significant attention. The dinuclear ferrocene complexes
[FcBbipy]2O(PF6)2, (124), and
[FcBbipy] 2C(H)CN(PF6)2, (125), and the ansa-ferrocenes [1,1-fc(Bbipy)2NC6H4OMe] (PF6)2, (126), and [1,1-fc(Bbipy)2NSiMe3](CO2CF3)2, (127), have been synthesized and characterized by X-ray crystallography and cyclic voltammetry. Electronic communication between the two 2,2-bipyridylboronium substituents of each compound was found to be observed, suggesting the reduced radical species to be partially delocalized redox intermediates. Furthermore, the degree of electronic interaction was found to be largely independent of the degree of conformational flexibility of the individual molecule.94 In related work, the reactions of FcB(Me)Br [128, Fc : (C5H5)Fe(C5H4)] and 1,1-fc[B(Me)Br]2 [129, fc : (C5H4)2Fe] with 2,5-bis(pyridyl)pyrazine and 2,2:4,4:2,2-quaterpyridine have been reported to generate a range of charge-transfer complexes and redoxactive macrocycles which have been examined electrochemically.95 The bidentate bis(boronate) Lewis acid 130 has been shown to act as a higly selective colorimetric fluoride ion sensor. Its mode of action has been shown to involve selective anion binding together with an electrochemical response triggered by fluoride binding and effected chemically by atmospheric oxygen.96 A new type of ferrocene-based heteronuclear bidentate Lewis acid with both Lewis acidic centers attached to the same cyclopentadienyl ring has been reported. Borylation of 1,1-bis(trimethylstannyl)ferrocene with BCl3 occurs with high selectivity at the a-position to give 1-(Me3Sn)-2-(Cl2B)fc (131). Only minor

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amounts of the two other isomers, i.e. the 1-stannyl-3-boryl- and the 1-stannyl1-borylferrocene, were observed. The reaction rate and selectivity were found to strongly depend on the steric and electronic properties of the electrophile. With the bulkier electrophile C6F5BCl2 larger amounts of the 1,3-product, form in addition to the 1,2-isomer. Use of the weaker electrophile PhBCl2 resulted in a significantly lower reaction rate.97

2.11 Miscellaneous. — The four coordinate molybdenum complexes Mo(NR)2[OB(Mes)2]2 (R : tBu 132; R : 2,6-iPr2C6H3 133) have been obtained from the reaction of [(Mes)2BOLi(Et2O)n]x with the corresponding Mo(NR)2Cl2(dme) starting material. The related five-coordinate complex Ti(NtBu)[OB(Mes)2]2(py)2 (134) has also been made using analogous procedures. Structural investigations were performed to assess the effect that the boron atom has on the metal-oxygen and metal-imido nitrogen bond lengths and angles. On average, both classes of compound displayed longer M-O bonds and shorter M-N bonds for the boroxide (OBMes2) derivatives compared to analogous species containing the C(H)OMes2 unit.98 In a related study, bimetallic zirconium heterocycles supported by boron/oxygen ligands have been synthesized. Thus, the reaction between Cp2Zr(CH3)2, and phenylboronic anhydride, (PhBO)3, results in the formation of the heterocyclic dimer [ZrCp2(m-O2BPh)]2 (135), although no reaction was observed with the corresponding mesityl derivative, (MesBO)3. 135 was also synthesized by the protonolysis reaction between Cp2Zr(CH3)2 and phenylboronic acid, and this approach was extended to afford the analogous complexes [ZrCp2(m-O2BAr)]2 (Ar : Mes 136, C6F5 137). The molecular structures of 135 -

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137 reveal a common, dimeric motif consisting of a central ‘Zr2B2O4’ metallacycle.99 A number of highly unusual unsaturated boron-containing compounds have been reported in 2002. Experimental and theoretical evidence of the neutral OCBBCO molecule (138) with boron-boron triple bond character has been reported. The molecule was characterized by matrix isolation infrared spectroscopy; quantum chemical calculations indicated that the molecule has a linear singlet ground state with a very short boron-boron bond length.100

A related s-p diradical, B4(CO)2, isolated in a low temperature matrix, has been characterized by infrared spectroscopy and by theoretical calculations. Both open-shell singlet and triplet states have three p electrons but are aromatic; quantum chemical calculations at various levels indicate that the open-shell singlet is slightly more stable than the triplet state. However, the singlet and triplet are computed to have very similar IR features which do not allow experimental differentiation.101 The lithium salt of tris-tert-butylphosphinimide, [tBu3PN]Li, reacts with BCl3 in refluxing toluene to give a white solid [(tBu3PN)2B]Cl in 72 % yield. This salt contains the extended borinium cation, [(tBu3PN)2B]; (139), containing a rare example of a cationic two coordinate boron centre and featuring linear N-B-N and P-N-B moieties.102

Aromatic boranes with planar four coordinate boron atoms and very short B-B distances have been reported by Berndt and co-workers.103 The same group has also reported triboracyclopropanates with similarly short B-B distances, which can be regarded as two-electron double aromatic compounds.104 The

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reaction of a mixture of Me2NBCl2 and (Me2N)2B2Cl2 with NaK2.8 alloy in hexane has been shown to produce a colourless compound of the composition B6(NMe2)6, a constitutional isomer of the known chair-shaped species cyclo(BNMe2)6, which has previously been obtained by the dehalogenation of (Me2N)2BCl. The colourless B6(NMe2)6 (140) displays three 11B NMR signals, a pattern corresponding to a four-membered ring with two different substituents. Subsequent crystal-structure analysis of 140 confirmed a structure based around a planar, diamond-shaped B4 ring.105 Using a novel class of isomerically pure tetrasubstituted 2-alkoxycarbonyl allylboronates, Kennedy and Hall have shown that Sc(OTf)3 and Cu(OTf)2 allow these reagents to add to aldehydes to yield g-lactone products in good yields at temperatures almost 100oC lower than the corresponding uncatalysed reactions. The large rate enhancement over the uncatalysed reaction provides a highly improved practical approach to access aldol-like adducts with a stereogenic quaternary carbon centre.106 The acceleration induced by catalytic amounts of Lewis acids [such as AlCl3 or Sc(OTf)3] in the allylboration of aldehydes has also been examined by Ishiyama, Ahiko and Miyaura, who reported regiospecific, diastereospecific, and enantioselective synthesis of homoallyl alcohols using this protocol.107A highly diastereoselective one-pot double allylboration reaction sequence has also been reported by Flamme and Rousch for theenantioselective synthesis of 1,5-anti- and 1,5-syn-diols.108 Dialkylboron chlorides have been shown react with aryl aldehydes to produce arylalkylmethanols in good to excellent yields.109 New axially chiral borates and boronates of type 141 have been synthesized and shown to be efficient catalysts or reagents in a range of asymmetric syntheses including Diels-Alder reactions, reductions, allylborations and homologization reactions.110

Triallylborane has been found to react with many di-1-alkynylsilanes (or stannanes) to give novel 2-alkylidene-1,3-silaborolenes via intermolecular 1,1allylboration followed by intramolecular 1,2-allylboration. Upon heating, a second intramolecular 1,2-allylboration takes place to give 1,7borasilabicyclo[4.3.0]nona-5,8-diene derivatives.111

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Aluminium

3.1 Sub-valent Aluminium and Aluminium Clusters. — The study of low valent alumininium organometallics, either at low temperatures in an inert gas matrix, or by making use of the stabilisation conferred by sterically demanding substituents, continues to attract attention. Within this sphere, the reactivity of subvalent group 13 species with dihydrogen in low temperature matrices has been the topic of significant investigation. Thus, the photoinduced reaction of AlCp* monomers with H2 in an argon matrix at 12 K has been followed by IR spectroscopy. The experimental results allied to detailed quantum chemical calculations showed that the product of this reaction is the new aluminium hydride species H2AlCp* (142).112 In a similar vane, the matrix reactivity of AlF and AlCl in the presence of HCl and HBr has also been investigated. The generation and characterization of the new Al(III) hydrides HAlFCl, HAlFBr, and HAlClBr and of the monomeric mixed Al(III) Halides AlX2Y (X, Y : F, Cl, or Br) have thus been reported.113 The silane-aluminium complex Al.SiH4 has also been characterized by infrared spectroscopy in an argon matrix and the course of its subsequent photochemistry to give HAlSiH3 and AlSiH3 has been determined.114

A range of ditrielanes R2E-ER2 (R : SitBu3; E : Al, In, Tl), R2E-ER2 (R : Si Bu2Ph; E : In, Tl) and R2Ga-GaR(SiMe3) have been prepared by a number of routes: (i) from ECl3 (E : Al, In, Tl) and NaR via isolable R2ECl derivatives; (ii) from EBr (E : In, Tl) and NaR or NaR; or (iii) from NaGa2R3 and Me3SiCl. Exceptions to these methodologies include the reaction of GaCl3 with NaR, which is reported to lead to R2GaGaR rather than R2Ga-GaR2, and the reaction of AlBr or GaBr with NaR which leads to R4Al4 or R8Ga18 and R8Ga22, respectively. According to X-ray structure analyses, the deeply coloured ditrielanes contain two planar groups R2EE which are orthogonal to each other (R : SitBu3; angle REER ca. 90°) or nearly orthogonal (R : SitBu2Ph; angle REER ca. 80°).115 A combination of photoelectron spectroscopy and ab initio calculations has been used to elucidate the structure and bonding of a series of bimetallic clusters MAl69 (M : Li, Na, K, Cu, and Au). Ab initio calculations showed that all of the MAl69 clusters can be viewed as an M; cation interacting with an Al629 dianion. Al629 was found to possess an Oh ground-state structure, and all of the MAl69 clusters possess a C3v ground-state structure derived from the Oh Al629. Careful comparison between the photoelectron spectral features and the ab initio onet

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electron detachment energies allowed the authors to establish experimentally the C3v ground-state structures for the MAl69 clusters.116 The structures and bonding of the aluminium clusters [Al7
N(SiMe3)26]9 (143), [Al12
N(SiMe3)28]9 (144), [Al22Br20 12THF] (145), [Al14
N(SiMe3)26I6]29 (146), Al14(m8-Si)Cp*6 (147), [(AlEt)8(m4-CCH2Ph)5(m4-H)] (148), [(AlMe)8(m4-CCH2Ph)5(m4-CCPh)] (149) and [(AlMe)7(m4-CCH2Me)4(m2-H)2] (150) have been analysed by DFT. The authors, Lam and Lin, have attempted to elucidate the relationship between observed geometry and electron counts for these metalloid clusters.117

3.2 Aluminoxanes, MAO Models and Aluminium in Olefin Polymerization. — The role of aluminium complexes in olefin polymerisation catalysis continues to be the subject of significant research effort, in part as the metal site of polymerisation activity. Thus, the highly electrophilic diethylaluminium cation Et2Al; has been synthesized in combination with the weakly coordinating carborane anions, CB11H6X69 (X : Cl 151, Br 152). X-ray crystallography reveals ion-like structural features reminiscent of the corresponding trialkylsilylium species. Furthermore, 151 and 152 has been shown to be potent catalysts for the electrophilic ethenation of benzene, the polymerization of cyclohexene oxide, and the oligomerization of ethene to a low molecular weight, highly branched product.118 As an alternative to the standard Cossee mechanism, in which olefin insertion involves two sites at a single metal centre, Talarico and Budzelaar have investigated a true dinuclear alternative where the chain switches between two different aluminium centres at each insertion. The corresponding dinuclear variations of b-hydrogen elimination (BHE) and b-hydrogen transfer to monomer (BHT) were also investigated. Surprisingly, calculations indicated that the barriers for both insertion and BHT at two different metal centres are rather similar to those for the more usual mononuclear mechanisms.119 The involvement of organoaluminium fragments in the anionic counter-ion partner to cationic metallocene catalysts also continues to be assessed. The structural characterization of a cationic zirconocene olefin polymerization catalyst with its methylated boralumoxane counterion has been reported by Hessen and co-workers.120 In addition, a study of the olefin polymerization system MAO/(C5H5)2ZrMe2 has determined the size of the ion pair [(C5H5)2Zr(mMe)2AlMe2];[Me-MAO]9 by pulsed field-gradient NMR of its cationic moiety. A mean effective hydrodynamic radius of 12.2-12.5 Å, determined from diffusion rates in benzene solution at different zirconocene and MAO concentrations, indicated that the ion pair remains associated even at the lowest concentrations studied. At elevated concentrations, aggregation to ion quadruples or higher aggregates has been proporsed on the basis of an apparent size increase and by shifts of the C5H5 and Me 1H NMR signals.121 Possible structural candidates for the active and dormant species in dimethylzirconocene-catalysed, MAO (methylaluminoxane)-activated olefin polymerization have also been studied by DFT. These species were [Cp2ZrMeAlMe3];[MeMAO]9 and [Cp2ZrMe]; [MeMAO]9, and relative energies in the gas phase and in toluene solution as well as 1H and 13C NMR chemical shifts were calculated and compared with

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spectroscopic data from new experimental investigations.122 In a similar vane, the molecular structures and relative stabilities of three coordinate methylaluminoxanes (MAO, e.g.153 and 154) with variable degrees of oligomerisation have been investigated by Hartree-Fock, MP2, and B3LYP methods. The structures and stabilities of various structures were determined on the basis of ring strain, and p-conjugation between the oxygen lone pairs and the vacant porbitals of the aluminium centres.123

3.3 Aluminium Derivatives Containing Bonds to Group 15 Elements. — 3.3.1 Amido and Phosphido Stabilised Species. The structure and bonding in the aluminium radical species Al NH3, HAlNH2, HAlNH2 NH3, and Al(NH2)2, formed by the reaction of Al atoms with NH3 in an argon matrix, have been studied experimentally by infrared spectroscopy and by quantum chemical calculations.124 Reaction of the bulky chelating diamine ligand ArNH(CH2)3NHAr (LH2, Ar : 2,6-iPr2C6H3) with AlR3 cleanly affords the monomeric, three-coordinate, aluminium complexes LAlR (R : Me 155, iBu 156). The Al/B alkyl/aryl ligand exchange between 155 and B(C6F5)3 in refluxing toluene cleanly produces the pentafluorophenyl-substituted derivative LAlC6F5 (157). Studies of model reactions between 155 and e-caprolactone as well as analyses of low-molecularweight oligomers have demonstrated the ability of 155 to produce telechelic oligomers.125 A chelating dianion Me2Si(NDipp)2 has also been used to synthesize [
Me2Si(NDipp)2ZrCl2(THF)2] (158, Dipp : 2,6-diisopropylphenyl). Subsequent reaction of AlMe3 and 158 has been shown to afford the unusual ionic complex [Me2Al(THF)2];[
Me2Si(NDipp)22Zr2Cl5]9 (159).126 The reactions between amine-stabilized H3Al.L (L : NMe3, NMe2Et) and DippNH2 have been studied at different reaction temperatures and in different molar ratios. Lewis base stabilized heterocyclic iminoalanes [L HAlNDipp]2 (L : NMe3 160, NMe2Et 161), monomeric aminoalanes Me3N (H3-x)Al(N(H)Dipp)x (x : 2 162, 3 163) and base-free, heterocyclic aminoalanes [Dipp(H)NAl(H)-m-N(H)Dipp]2 (164) were obtained in high yields.127 In related work, the reaction of AlH3 NMe3 with nitriles, RCN, has been shown to proceed with the evolution of trimethylamine and to afford the hexameric aluminium amides (HAlNCH2R)6 (R : Ph 165, p-MeC6H4 166, p-CF3C6H4 167). 165 has been shown to react further with Me3SiBr and with PhCCH to give compounds of the type (XAlNCH2Ph)6 (X : Br 168, PhCC 169) in which all of the hydridic hydrogen atoms in 165 have been replaced by bromine atoms or PhCC groups,









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respectively.128 Factors influencing the coordination mode of an amido group to aluminium have been investigated via the reactions of amides with organoaluminium compounds. Thus species of varying nuclearity and containing ring systems of varying sizes are produces by the reaction of AlMe3 with a range of functionalised secondary amines.129 Dimeric [Me2AlN(i-Pr)CH2Ph]2 (170) and the related series of orthometallated compounds [RAlN(i-Pr)-m-(CH2C6H4)]2, R : Me 171, n Pr 172, nBu 173, and iBu 174, have been synthesized by thermolysis of 1:1 mixtures of R3Al and HN(i-Pr)CH2Ph.130 Similarly, the aminoalane dimers, [R2AlN(CH2Ph)2]2 (R : Prn 175, Bun 176, Bui 177) have been synthesized by the thermolysis of 1:1 mixtures of the trialkylaluminium and dibenzylamine at 120oC; the dimeric structure of 176 has been determined from single-crystal X-ray data.131 The reaction of OP(NHtBu)3 with AlMe3 has been reported to yield the complexes Me2Al(m-NtBu)2P(m-O)(NHtBu)AlMe2[OP(NHtBu)3] (177) and MeAl[(m-NtBu)( m-O)P(NHtBu)2]2 (178), whereas the analogous reaction with LiAlH4 yieds
LiAl[OP(NtBu)2(NHtBu)]22 (179). The structures of aluminophosphates imido analogues 177 - 179 reveal the anions [OP(NtBu)(NHtBu)2]9 and [OP(NtBu)2(NHtBu)]9 adopting a variety of bonding modes towards aluminium.132

Bimetallic, pentel-bridged complexes of the type (dmap)Me2M-E(SiMe3)2M(CO)n [M : Al, Ga; E : P, As, Sb; M : Cr, Fe, Ni; dmap : 4(dimethylamino)pyridine] have been synthesized by the reactions of dmapcoordinated Group 13/15 compounds, (dmap)Me2M-E(SiMe3)2 with the transition metal complexes (Me3N)Cr(CO)5, Fe3(CO)12 and Ni(CO)4. This reaction is reported to offer a general pathway to compounds containing a Group 13 metal and a transition metal bridged by a pentel atom.133 3.3.2 Adducts Containing ER3 Donors (E : N, P, As, Sb, Bi). Donor-acceptor interactions within the series of Lewis acid-base adducts Et3Al.E(SiMe3)3 (180) and tBu3Al.E(i-Pr)3 (E : P 181, As 182, Sb 183, Bi 184) have been investigated crystallographically in the solid state and in solution by variable temperature NMR spectroscopy. Comparative DFT calculations were also performed to analyse molecular structures and to derive dissociation energies. The thermodynamic stability of these complexes was found to depend both on the electronic strength of the Lewis acid and base, and on steric interactions. Importantly, repulsive steric interactions were found to be decisive for the stability of

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adducts containing small central elements such as P and large substituents such as iPr and tBu.134In a similar fashion, the Lewis acid/base adducts X3M N(SnMe3)3 (M : Al, X : Cl 185; M : Ga, X : Cl 186; M : Ga, X : Br 187; M : In, X : Cl 188; M : In, X : Br 189) and Cl2MeM N(SnMe3)3 (M : Al 190, M : Ga 191) have been prepared by the 1:1 reaction of MX3 or MCl2Me with N(SnMe3)3 at room temperature in diethyl ether. Unusually short M-N bond lengths of 1.87(1) Å for Al-N (185), 1.950(7) Å for Ga-N (186) and 2.148(6) Å for In-N (188) have been attributed to very strong donor-acceptor bonds.135 Reaction of the monoanionic (tacn)9 ligand [(tacn)H : 1,4-diisopropyl-1,4,7triazacyclononane] with LiAlH4 has been shown to afford a dimeric, unsolvated lithium trihydroaluminate, which upon treatment with terminal acetylenes HCCR (R : Ph, SiMe3) yields the corresponding monomeric trialkynylaluminates in which one acetylide ligand bridges the Li and Al atom at the Ca atom while the two terminal acetylide ligands are coordinated to the Al atom.136 Organoaluminium complexes stabilised by the coordination of a tethered amine base have also been examined in 2002. The reaction of AlCl3 with lithiumN,N-dimethylaminoethanolate in a 1:1 ratio has been shown to yield the dimeric compound [Cl2Al(m-OCH2CH2NMe2)]2 (192) from which the diallyl derivative [(CH2:CHCH2)2Al(m-OCH2CH2NMe2)]2 (193) could be synthesized by treatment with two equivalents of allyl magnesium bromide. Complex 193 was further found to be a useful reagent for the transfer of one allyl group to aldehydes, imines and enones, forming the 1,2-addition products in high yields.137 The related complex [(CH3)2AlOCH2CH2N(CH3)2]2 (194) has been shown to react readily with 4-bromophenol to give methane and [(4BrC6H4O)2AlOCH2CH2N(CH3)2]2 (195). The demethylation of the analogous indium complex [(CH3)2InOCH2CH2N(CH3)2]2 (196) is very slow, and enables it to be used in the cross-methylation of bromophenols and other bromoarenes with active hydrogen atoms in the presence of soluble palladium phosphine catalysts.138



3.3.3 Adducts Featuring Imine, Amidinate or b-diketinimate Donors. A number of complexes containing mixed imine/alkoxide donor sets have been reported, and in some cases their activity with respect to olefin polymerisation evaluated. In this respect, reaction of the Schiff base ligands 3,5-But2-2-(OH)C6H2CHNL with Me3Al at room temperature has been shown to afford the complexes (3,5-But2-2(O)C6H2CHNL)AlMe2 [L : CH2CH2NMe2 (197), 2-(PhO)C6H4 (198), 2CH2C5H3N (199), 8-C9H6N (quinoline) (200) and 2-(PPh2)C6H4 (201)]. In the case of L : 8-quinoline, the same reaction conducted in refluxing toluene affords binuclear
[3,5-But2-2-(O)C6H2CHMeN-8-C9H6N]AlMe2 (202) by methyl migration from metal to ligand. Further reaction of the dimethyl compounds with B(C6F5)3 afforded the corresponding cationic systems [(3,5-But2-2(O)C6H2CHNL)AlMe]; (scheme 10).139 A similar series of aluminium complexes featuring the ketiminate ligand, OCMeCHCMeNH(C6H3iPr2-2,6) has also been prepared. Reactions of this ligand with trialkylaluminiums in 1:1 or 1:2 molar ratio has been shown to generate four- and five-coordinate aluminium complexes (OCMeCH-

Organometallic Chemistry

150

Scheme 10

CMeNAr)AlR2 (R : Me 203, R : Et 204) and (OCMeCHCMeNAr)2AlR (R : Me 205; R : Et 206). Similarly, reaction of AlCl3 with 1 or 2 equivalents of the monolithiated ligand afforded the bis(ketiminate) aluminium chloride complexes (OCMeCHCMeNAr)2AlCl (207) or (OCMeCHCMeNAr)AlCl2 (208). The mono- and bis-ketiminate aluminium complexes 203 – 208 have shown moderate activity toward the ring-opening polymerization of e-caprolactone.140

The synthesis and reactivity of bi-, tri-, and hexametallic and zwitterionic methylaluminium complexes containing a mixed phenoxide/imine ligand have been reported. Thus, for example, addition of three equivalents of trimethylaluminium to tris(3,5-di-tert-butyl-2-hydroxyphenyl)methane, L1H3, affords the simple, C3-symmetric, trimetallic species [L1(AlMe2)3].141 The syntheses of new aluminium complexes carrying bidentate monoanionic iminoamide or imino-phenoxide ligands have also been described. Thus, reaction of 1(O,O-diisopropylphenylamino)-6-(O,O-diisopropylphenylimino)cyclohexene with AlMe3 produces dimethylaluminium N-(O,O-diisopropylphenyl)-6-[N(O,O-diisopropylphenyl)imido]-1-cyclohexen-1-amide (209). By contrast, reaction of the same ligand with AlEt3 under the same experimental conditions involved the 1,4-addition of AlEt3 to the a,b-unsaturated imine, generating diethylaluminium N(O,O-diisopropylphenyl)-2-[(O,O-diisopropylphenyl)amino] 3-ethyl-1-cyclohexen-1-amide (210). Similarly, treatment of the salicylaldimine ligands 3-tBu-2-(OH)C6H3CH:N-R with AlMe3 yielded the dimethylaluminium compounds
3-tBu-2-(O)C6H3CH:N-RAlMe2 (R : C6H5 211, 2,6-iPr2C6H3 212, and C6F5 213). Compounds 210, 211, and 213 were subjected to methyl abstraction with B(C6F5)3 and preliminary polymerization

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tests were carried out; polymerisation of ethylene with low activity was observed.142 Chelating amidinate and b-ketinimate ligand systems have also been used to stabilize new aluminium organometallics. Complexes supported by sterically bulky amidinate ligands containing terphenyl substituents on the backbone carbon atoms have been described by Schmidt and Arnold. The ligands featured differing substituents at the ortho positions of the 2,6-phenyl rings: [2,6-(2,4,6Me3Ph)2Ph]C(NH-i-Pr)N-i-Pr (L2H), [2,6-(2,4,6-i-Pr3Ph)2Ph]C(NH-i-Pr)N-i-Pr (L3H), and [2,6-(4-t-BuPh)2Ph]C(NH-i-Pr)N-i-Pr (L4H). Monoamidinate dialkyl complexes L2AlMe2 (214),L3AlMe2 (215)andL4AlMe2 (216) were subsequently generated, and their reactions with methide abstraction reagents to yield cationic aluminium alkyl species studied.143 Furthermore, 2,4,6-triphenylbromobenzene has been shown to react cleanly with two equivalents of n-butyllithium to produce a dilithiated species, from which treatment with dialkylcarbodiimides (RNCNR, R : isopropyl or cyclohexyl) followed by aqueous work-up results in the formation of sterically hindered bifunctional amidines. Further reaction with trimethylaluminium results in the formation of novel dialkylaluminium bisamidinate complexes containing two AlMe2 units.144 The stabilities and oxidation states of compounds of the type
HC(CRNR)2E (217, E : B, Al, Ga, In) have been analysed by Reiher and Sundermann; population analyses and electron localization functions have been used as analytical tools for the elucidation of the electronic density distribution. Singlet-triplet splitting energies, and model reactions probing donor and acceptor properties of the atom E, yield criteria for the reactivity of the cyclic molecules. It has been demonstrated that a unique class of Lewis structures can be selected from the large number of chemically reasonable structures in order to achieve a sufficiently good mapping of the quantum chemical results on simple chemical concepts. The analysis yields that 217 (with E : In) is expected to have similar properties to the experimentally known aluminium and gallium compounds, while the boron homologue is expected to be highly reactive, such that its synthesis is expected to be difficult.145 3.4 Aluminium Derivatives Containing Bonds to Group 16 Elements. — 3.4.1 Alkoxoaluminium Complexes. Alkoxoaluminium species of varying nuclearities, and featuring bridging ligands to both aluminium and other metals, continue to be the subject of research work. The polymeric aluminium 1,4-dioxobenzene complex, [
(tBu)2Al2(m-OC6H4O)]n (218) has been synthesized by the addition of AltBu3 to hydroquinone in a non-coordinating solvent. 218 reacts with Lewis bases, both via solution and solid/vapour phase reactions, to yield adducts of the type [(tBu)2Al(L)]2(m-OC6H4O) (L : py 219, 3,5-Me2py 220 and THF 221) via cleavage of the Al2O2 dimeric core.146 Similarly, R3Al (R : iBu, tBu) has been shown to react with 2,4-dimethylpentane-2,4-diol, butane-1,4-diol and propane-1,3-diol to afford the alkylaluminium i (222), Bu5Al3[(OC(CH3)2diolates [tBuAl(OC(CH3)2CH2C(CH3)2O)]2 t i CH2C(CH3)2O)]2 (223) [ Bu2Al(O(CH2)4OH)]2 (224), Bu5Al3(O(CH2)4O)2 (225) and tBu5Al3(O(CH2)3O)2 (226). The influence on complex structure of the steric

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bulk of the alkyl substituents and of the diol moiety have been examined.147 The aluminium complex [AlEt2(quin)]2, (quin : 8-quinolinolato) (227) has been obtained from the reaction of AlEt3 with 8-quinolinol and its dimeric structure confirmed by X-ray crystallography. Reaction of AlEt3 with 2-methyl8-quinolinol on the other hand gave the mononuclear complex AlEt2(quin) (228, q : 2-methyl-8-qinolinolato).148 The reaction of 2,4,6-triisopropylphenyl methyl ketone with Ph3Al has been shown to form dimeric [Ph2AlO(2,4,6-tri-iPrC6H2)C:CH2]2 (229) as the only product. 229 has been characterised by NMR spectroscopy and its crystal structure determined.149 The synthesis and reactivity of multi-centre aluminium complexes incorporating ligands with four phenoxide arms have also been investigated, with the intention of fixing two Lewis acidic metal centres in close proximity and thereby fostering cooperative binding of external Lewis bases. The ligands used each incorporated a pair of 2,2-methylenebis(4-tert-butyl-6-alkylphenol) groups linked by aromatic spacers (anthracene, dibenzofuran or and xanthene), and depending on reaction conditions, bi-, tri-, or tetrametallic complexes were obtained.150 Reaction of AlH3(OEt2), generated in situ from AlCl3 and 3 equivalents of LiAlH4, with one equivalent of iPrOH has been shown to result in the formation of the pentanuclear complex Al5(m4-O)(m-OiPr)7H6, 230. On the other hand, incomplete reaction of AlCl3 and 3 equivalents of LiAlH4, followed by the addition of iPrOH, afforded the novel species Al5(m5-O)(m-OiPr)8(Cl)H4, 231.151 Mixed aluminium/lithium, aluminium/boron and aluminium/samarium species have also been reported. Thus, the reaction of [Li(thf)2AlH3tsi]2 [232, tsi : C(SiMe3)3] with four equivalents of the bulky ArOH (Ar : C6H3Pri2-2,6) has been shown to yield Li(thf)(m-OAr)2AlH(tsi) (233), which has a structure containing a four-membered LiO2Al ring both in the solid state and in toluene solution. The corresponding reaction with ArOH (Ar : C6H3But2-2,6) gave a mixture that could not be separated by fractional crystallisation, while the reaction of 233 with Ph3SiOH gave Al(OSiPh3)2
C(SiMe3)3(thf ) (234), which was shown by an X-ray structure determination to be monomeric.152 The reaction of Me2MCl with two equivalents of Li(OCH2CH2OMe) has been shown to result in the formation of bimetallic complexes, [Li(OCH2CH2OMe)2MMe2]2 (M : Al 235, Ga 236. The solid state structures of 235 and 236 are shown to dimeric and to contain eight-membered rings. Furthermore, thermal analysis of 236 indicated that it is volatile enough to be suitable as a single precursor for metal organic chemical vapor deposition

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(MOCVD) of LiMO2 films.153 The reaction of LAlMe2 (where L is a monoanionic b-diketiminato ligand) and H2OB(C6F5)3 has been shown to give LAlOB(C6F5)3 (237), which was crystallized from dichloromethane and is the first reported example of a monoalumoxane. By contrast, when the same reagents were allowed to react in THF an oily product was formed which crystallized as an isomer of 237, formulated as LAl(C6F5)OB(C6F5)2 (238).154 Reaction of [Sm(OAr)3]2 with four equivalents of trimethylaluminium leads to formation of the bistrimethylaluminium adduct (ArO)Sm[(m-OAr)(m-Me)AlMe2]2 (239), which exhibits very short Sm-C(bridging) distances of 2.620(5) and 2.632(5) Å. A reduced 1 JC-H coupling constant of 106 Hz and a low n(C-H) stretch in the solution and solid-state IR spectrum were said to be indicative of a strong agostic Sm · · · H-C interaction in solution.155

3.4.2 Other Derivatives. The structures and energetics of possible complexes between THF and AlMe3 have been calculated, along with their 1H, 13C, 17O, and 27 Al NMR shifts and (for O and Al) their quadrupole coupling constants.156 Geometries and thermodynamic properties of twenty four group 13-group 16 chalcogen heterocubanes [RM(m3-E)]4 (R : H, CH3; M : Al, Ga, In; E : O, S, Se, Te) and twelve group 13-group 13 pure cubanes [RM(m 3-M)]4 (R : F, Cl, CH3, NO2; M : Al, Ga, In) have been studied using DFT. The chalcogen heterocubanes are reported to be thermodynamically resistant to fragmentation

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and the M4E4 core proved to be insensitive to ligand choice. The authors hypothesized on this basis that the entropically disfavored nature of the synthesis may hold the key to the as yet unsynthesized complex [RAl(m3-O)]4.157 The dimethyl(methylthiomethyl)metal compounds (Me2MCH2SMe)2 (M : Al 241, Ga 242, In 243) have been prepared from LiCH2SMe and the respective dimethylmetal chlorides. Unlike the corresponding lithium compounds, the thiomethyl compounds with AlMe2 and GaMe2 groups are reported to be sublimable and soluble in nonpolar solvents. The Al and Ga compounds are dimeric in the solid state and in non-donor solvents, but are cleaved by stronger donors such as ethers and amines. They have been shown to adopt conformations in which the S-bound methyl groups are placed in equatorial positions of the chair-like six-membered ring systems.158 3.5 Aluminium Organometallics in Organic Synthesis. — A complete account of the involvement of organo- and hydrido-aluminium complexes in organic synthesis is clearly beyond the scope of this report. Nevertheless a couple of interesting reactions of dialkylaluminiun derivatives are worthy of note. Treatment of (E)-alk-1-enyldicyclohexylboranes with diisobutylaluminium hydride in the presence of hex-1-ene has been found to result in transfer of the alk-1-enyl group from boron to aluminium to give (E)-alk-1-enyldiisobutylalanes with retention of configuration at the double bond. Such chemistry thereby represents a novel way to prepare (E)-alk-1-enyldiisobutylalanes.159A new methodology for the preparation of a-diimines and b-aminoenones has been devised using dialkyl aluminium amides. This methodology represents an alternative route to C:NR functionalities and related nitrogenous ligands bearing highly electronegative substituents.160 3.6 Miscellaneous Examples. — Reaction of Al(C6F5)3 with 0.34 equivalents of CH2Cl2 has been shown to cleanly produce chlorobis(pentafluorophenyl)alane (C6F5)2AlCl (243), the dimeric structure of which has been confirmed in the solid-state by X-ray diffraction studies. 243 has been further shown to exhibit remarkable activity in polymerizations of styrene, e-caprolactone, and cyclohexene oxide.161 The syntheses of organoaluminates featuring three terminal phenylethynyl groups and their interactions with alkali metal cations have been reported. Thus, the crystal structures of [M; THF(2,6-iPr2C6H3N(SiMe3)Al(CCPh)3)9]2 (M : K 244,Na 245) show that the alkali metal cation interacts with the CC bonds of four phenylethynyl groups, with a further coordination site being occupied by a THF molecule. Thus, the cation therein acts as a bridging moiety to form a dimeric unit.162 Hydroalumination of CC triple bonds of a tetraalkynylbenzene derivative has been shown by Uhl and Matar to afford chelating Lewis acids (exemplified by 246) possessing two or four coordinatively unsaturated aluminium atoms.163

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4

155

Gallium

4.1 Sub-valent Gallium Species and Clusters. — The organometallic chemistry of low-valent gallium systems has again seen remarkable advances in 2002, with a variety of interesting systems incorporating cluster aggregation, binuclear GaGa entities and even discrete Ga centres. The reactions of GaCl and InCl with H2 have been studied experimentally and by quantum chemical methods. Experimental results show that reaction with H2 in a solid argon matrix does not occur spontaneously, but requires photoactivation, the trivalent derivatives H2MCl (M : Ga 247, or In 248) being the detectable products. Furthermore, these results indicate that the photoactivated reactions proceed in a concerted fashion. Quantum chemical calculations have been employed to explain the observed reactivities and to provide quantitative estimates for the barriers to reaction. On this basis, the reactions of group 13 subhalogenides with other molecules have been predicted to follow the same pattern.164 The reaction chemistry of ‘GaI’ continues to offer a powerful synthetic approach to low valent gallium organometallic species. Thus, the reaction of ‘GaI’ with potassium cyclopentadienides has been shown to offer a simple route to cyclopentadienylgallium(I) complexes. The compounds Me5C5Ga (249) and Me4EtC5Ga (250) have been prepared in high yields.165 The reactions of two diazabutadiene ligands, Ar-DAB [(ArNCH)2, Ar : 2,6-Pri2C6H3] and But-DAB [(ButNCH)2] with either GaI or an AlI3/Al mixture have been reported to afford the paramagnetic compounds,
IGaII(But-DAB)2 (251), I2GaIII(But-DAB) (252) and I2AlIII(Ar-DAB) (253) which have been characterised by X-ray crystallography and EPR spectroscopy. The reduction of I2GaIII(Ar-DAB) (254) with potassium metal in the presence of various Lewis bases has led to three structurally characterised compounds, which contain in [:GaI(Ar-DAB)]9 (255), the second example of an anionic gallium(I) carbene analogue.166 Finally, the reaction of ‘GaI’ with LiAr has been shown by Power and co-workers to generate both ArGaGaAr (256) and Ar(I)GaGa(I)Ar (257), each in isolated yields of about 25 % (Ar : 2,6-Dipp2C6H3, Dipp:2,6-iPr2C6H3). The structure of 256 has been shown to consists of centrosymmetric Ga-Ga bonded dimers with a planar, trans-bent C-Ga-Ga-C core. Furthermore the authors were able to demonstrate reduction of the neutral ‘digallene’ 256 to Na2ArGaGaAr, containing the [ArGaGaAr]29 dianion.167

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Macrocycles possessing up to thirty six atoms have been formed in high yields by treatment of the tetraalkyldigallium compound R2Ga-GaR2 [R:CH(SiMe3)2] with the bifunctional protic reagents muconic acid and squaric acid. The products have been shown to contain four Ga-Ga bonds bridged by four acid residues.168 The tetragallanediide Na2Ga4R4.2THF (258, R : SitBu3) has been prepared prepared by Schno¨ckel and co-workers by the reduction of RGaCl2.THF (259) or of the tetrahedro-tetragallane R4Ga4 (260) with sodium or potassium. Furthermore, the orange digallane R2Ga2Cl2 (261) and tetragallane 260 were formed as intermediates in the dehalogenation of 259. The extreme air- and moisturesensitive 258 is reported to be stable against Me3SiCl, but to react with oxygen resulting in the formation of 260, and with GaBr to form several products including RGaBr2.THF. According to X-ray structure analyses 258 contains a nonplanar Ga4 ring with short Ga-Ga bonds, whereby three Ga atoms on opposite sides of the ring are each connected with one of the two Na(THF) groups.169 Additionally, thermolysis of [R4Ga3] (261) has been shown to generate octagallane R6Ga8 (262, R : SitBu3), together with digallanyl radical [R3Ga2] (263) and the tetrahedro-tetragallane R4Ga4 (260); 262 can be reduced to the dark-red octagallanediide Na2Ga8R6 2THF (264) by sodium napthalide. The structures of 262 and 263 feature the Ga atoms of four RGa moieties, together with two ‘naked’ Ga atoms, occupying the corners of a distorted octahedron. The ‘naked’ Ga atoms themselves are located, along with Ga atoms of two further silylgallium moieties, at the corners of a distorted square. Furthermore, the reduction of 262 to 264 leads only to a negligible shortening of the Ga-Ga distances from 2.64 to 2.61 Å (mean values). Both the octagallanes possess Ga8 frameworks previously unknown for group 13 clusters.170 Treatment of the alkynes RCCH (R : Me, n-Pr) with dialkylgallium hydrides, R2GaH (R : Me, Et), has been shown to yield symmetrical carbongallium cage compounds with release of dihydrogen and subsequent addition of Ga-H to the CC triple bonds. Heteroadamantane-type compounds (GaR)6(CR)4 (265) were formed in which the positions of the methylene bridges of the carbon skeleton in adamantane are formally replaced by tricoordinate gallium atoms.171





Novel, larger gallium-containing clusters are represented by [Ga22
N(SiMe3)210]29[Li4Br210 THF]2] (266); this compound containing a [Ga22R10]29 unit has been described by Schno¨ckel and co-workers. The average oxidation state of the Ga atoms in 266 (;0.36) mirrors that of previously

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reported Ga22 clusters, although remarkably the structure of 266 diverges markedly from those reported previously.172 The Wade-Mingos (2n;2) and Jemmis (2n;2m) electron-counting rules appear to be applicable to most of organometallic cluster compounds of gallium and indium. The deltahedra found in the supraicosahedral Group 13 metal clusters can be derived from the 12-vertex cuboctahedron and related 13-vertex polyhedra by capping all of the nontriangular faces.173 4.2 Complexes Containing Subvalent Gallium Species as Ligands. — The strongly s-donating properties of gallium(I) organometallic and related species has been exploited in the synthesis of a number of complexes, both with transition metal and main group Lewis acids. A coordination chemistry umpolung in the form of a gallane ligand on a phosphine Lewis acceptor has been reported by Power, Burford and co-workers. Pentaphenyl-phosphinophosphonium trifluoromethanesulfonate reacts with amino-iminogall(I)ane to effect quantitative substitution of Ph3P and form the first complex of a gallium Lewis base on a phosphine Lewis acid (267).174

Reactions of the hexanuclear rhodium cluster Rh6(CO)16 with GaCp* have been shown to result in an unusual substitution reaction of the face bridging carbonyl ligands to give a series of substituted derivatives of the type Rh6(m3CO)4 -x(m3-GaCp*)x(CO)12 (x : 1-4).175 Furthermore, reactions of Cp*GaCl2 and Cp*2GaCl with different carbonylmetalate anions have been reported to lead to the transition metal gallium complexes (h5-Cp*Ga)W(CO)5 (268), Fe2(CO)6
m(h5-Cp*Ga)2
m-(h3-Cp*Ga) (269), Ga
Mn(CO)53 (270), Cp*2Ga
Mn(CO)5 (271), Co2(CO)6
m-(h5-Cp*Ga)2Co(CO)3 (272) and
(h5-Cp)Fe(CO)2(m-CO)
m2(h5-Cp*Ga) (273). The results showed that both starting materials offer an alternative synthetic route to subvalent Cp*Ga(I) compounds in the synthesis of transition metal gallium complexes.176 In related work Cp*GaCl2 has been shown to react with K[Co(CO)4] via elimination of KCp* and KCl to form the novel ionic complexes [K(toluene)2][Ga
Co2(CO)6(-CO)2] (274) and K[GaCl
Co2(CO)6(-CO)
Co(CO)4] (275), where the gallium atoms possess the coordination number four. By contrast, the reaction of [
Co(CO)42GaCl(THF)] with LiCp* leads to [Li(THF)][Ga
Co2(CO)6(-CO)2] (276), and the reaction of GaCl3 with Na2[Fe(CO)4] generates [Na(OEt2)2][Fe(CO)4(GaCl3)2] (277). These ionic complexes have been shown to form polymeric networks in the solid state through coordination of the alkali metal cations.177

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4.3 Gallium Derivatives Containing Bonds to Group 15 Elements. — Cluster species containing Ga-group 15 element bonds continue to attract considerable attention, not least owing to their relevance as single source 13/15 semiconductor precursors. 2002 has also seen the synthesis of the first Ga/Bi cyclic trimer, thereby completing the series of familiar Ga/group 15 six-membered heterocycles. The alkyl- and phenyl-amido diphenylgallium compounds, [Ph2GaN(H)R]2 (R : Me 278, Et 279, nPr 280, iBu 281, Ph 282), have been prepared from the reactions of Ph3Ga with the corresponding amines at elevated temperatures. These dimeric compounds have been shown to contain bridging amido groups and to display both trans and cis isomers in solution. Thermolysis of compounds 278 and 282 has been carried out and two clusters, (PhGaNMe)7 (283) and (PhGaNPh)4 (284), obtained. The structure of 283 consists of a heptameric Ga7N7 core constructed with Ga2N2 and Ga3N3 rings, while that of 284 possesses a Ga4N4 cubane core.178 Related work on the structures of gallium amido/imido clusters has also been reported by Gladfelter and co-workers. Thus, the phenylgallium clusters incorporating bridging imido and amido ligands, (PhGa)4(NHiBu)4(NiBu)2 (285) and (PhGa)7(NHMe)4(NMe)5 (286) were synthesized by the reactions of [PhGa(NMe2)2]2, with isobutylamine and methylamine, respectively. 285 possesses an admantane-like Ga4N6 core, whereas 286 has a novel Ga7N9 core constructed with both chair- and boat-shaped Ga3N3 rings.179 Trimeric [Me2GaBi(SiMe3)2]3 (287) has been synthesized by the equimolar reaction of Me2GaH and Bi(SiMe3)3 and structurally characterized. 287 represents the first Ga-Bi heterocycle, and thereby completes the range of Ga-pnicogen compounds.180

The syntheses and structures of a number of hydrazino derivatives of gallanes and indanes have been reported. Thus, reaction of Me2GaCl with tBuHNNH2

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generates MeCl2Ga H2NNHtBu (288), and only one methyl group of GaMe3 reacts in refluxing toluene with anhydrous hydrazine in a 4:3 ratio to produce the bicyclic dimethylgallium hydrazide 289. Interestingly, a 1:1 GaMe3 adduct of Li(Me3Si)NNHtBu (290) is the unexpected product from the reaction of Me2GaCl with Li(Me3Si)NNHtBu, rather than the substitution product Me2Ga(Me3Si)NNHtBu. The products obtained from reactions of Li(Me3Si)NNHtBu with Me2InCl are found to depend on the solvent. In hexane the dimeric trimethylindium adduct of Li(Me3Si)NNHtBu (291), is produced as the main species, whereas in diethyl ether a cage compound 292 is isolated which can be regarded as an (Me3SiN:InMe)2 adduct with [Li(Me3Si)NNHtBu]2.181 The monomeric complexes GaRCl(acac) (R : Me 293, Et 294, Mes 295), GaCl[N(SiMe3)2](acac) (296), and GaMe[N(SiMe3)2](acac) (297) each containing three different gallium-bound ligands, have been prepared by metathetical and/or ligand redistribution reactions. X-ray structural studies of 293 and 296 confirmed the presence of the three different ligands bonded to gallium in these monomeric molecules. The related derivatives GaMeCl(hfac) (298) and GaMesCl(hfac) (299; hfac : 1,1,1,5,5,5-hexafluoropentanedionato) were also prepared and characterized, but these compounds slowly decomposed at room temperature.182

4.4 Gallium Derivatives Containing Bonds to Group 16 Elements. — The reaction of mesitylboronic acid with GaMe3 has been shown to afford the dimer [m-(MesB(OH)O)GaMe2]2 (300), a simple example of a O-metalated boronic acid with no hydrogen bonding in the crystal lattice. In addition, the reaction of dimesitylborinic acid, Mes2BOH, with ZnEt2 yields the low-valent zinc compound [(m-Mes2BO)ZnEt]2 (301), which was also characterized by X-ray diffraction.183

The mixed gallium transition-metal complexes [FeCl
Ga2(tBu)4(neol)2] (302) and [M
Ga2(tBu)4(neol)2], (M : Co 303, Ni 304, Cu 305), have been prepared by the reaction of [Ga2(tBu)4(neol-H)2] (neol-H2 : 2,2-dimethyl-propane-1,3diol) with the appropriate metal halide and ‘proton sponge’. The molecular structure of compounds 302 - 305 each consists of a tetracyclic core formed from two four-membered and two six-membered rings. On the basis of spectroscopic and structural data, the [Ga2(tBu)4(neol)2]29 ligand appears to be more flexible

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than other chelating ligands; this is proposed to be due to the flexibility in the O-Ga-O bond angle.184 4.5 Gallium Hydrides. — Gallium hydrides continue to attract attention, both with a view to further exploration of their fundamental reactivity, and of their coordination properties toward transition metal centres. The first gallane-coordinated metal complex, (OC)5W(h1-GaH3 quinuclidine) (306), has been synthesized by photolysis of a ca. 1:1 mixture of W(CO)6 and H3Ga (quinuclidine) in toluene and by the reaction of (OC)5W(THF) with one equiv of H3Ga (quinuclidine) in THF. X-ray crystal structure analysis of 306 revealed that the gallane ligand is bound to the tungsten fragment via a W-H-Ga three-center-twoelectron bond.185 Several new reactions of dichlorogallane (HGaCl2)2 (307) have been reported by Nogai and Schmidbaur. Thus the reaction with two equivalents of triethylphosphine results in the formation of the mononuclear complex (Et3P)GaHCl2 (308). Thermal decomposition of 307 affords hydrogen gas and mixed-valent Ga[GaCl4] (309). Treatment of this product with triethylphosphine gives the





symmetrical, Ga-Ga-bonded gallium(II) complex [GaCl2(PEt3)]2 (310) with an ethane-type structure. The corresponding [GaBr2(PEt3)]2 complex (311) is prepared from Ga[GaBr4] and has an analogous structure.186

The formation and characterization of the gallium and indium subhydride molecules Ga2H2 and In2H2 in low temperature matrices has been reported. Thus, Ga2 reacts spontaneously with H2 in solid argon matrices at 12 K to form the cyclic molecule Ga(m-H)2Ga (312). In2 does not react with H2 under similar conditions, but irradiation at wavelengths near 365 nm has been shown to induce the formation of the corresponding indium hydride, In(m-H)2In (313). Both 312 and 313 have planar, dihydrido-bridged structures with D2h symmetry, as endorsed by comparison of the measured spectra (i) with the results of quantum

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161

chemical calculations and (ii) with the spectra of known gallium and indium hydrides.187,188 The use of alkoxo and amido frameworks to stabilise gallium hydrides has also been exploited in 2002. Thus, the addition of four equivalents of tBuLi to [Cl2Ga(m-NEt2)]2 leads to the synthesis of [tBu(H)Ga(m-NEt2)]2 (314), a gallium hydride which is remarkably stable toward air, oxygen, photolysis, and moderate heating. Dissolution in CHCl3 on the other hand is reported to lead to hydride/chloride exchange, thereby producing [tBu(Cl)Ga(m-NEt2)]2 (315).189 tertButoxygallane, [tBuOGaH2]2, reacts with tetraphenyl-1,3-disiloxanediol, HO(Ph2)SiOSi(Ph2)OH, with hydrogen elimination to form the bicyclic compound [(OSiPh2OSiPh2O)[Ga(H)]2(OtBu)2] (316). The skeleton of molecule 316 consists of a four-membered Ga2O2 ring to which a five-membered O-Si-O-Si-O bridge is connected through Ga-O bonds. When the reactant ratio is changed to 1:2 [(tBuOGaH2)2:1,3-disiloxanediol], two new products are formed in almost equal amounts: [(OSiPh2OSiPh2OSiPh2O)GaH]2 (317) and [(OSiPh2OSiPh2OSiPh2O)GaOtBu]2 (318).190 4.6 Miscellaneous. — The reaction of 1,1-bis(trichlorostannyl)ferrocene with trimethylgallium has been reported to yield Fe(C5H4GaMe2)2 (319), the first digallylferrocene. 319 has a polymeric structure in the solid state, but in donor solvents NMR data confirm a monomeric donor-stabilised structure. The reaction of 319 with phenazine results in the formation of the supramolecular polyferrocene (319 phenazine)n the structure of which was confirmed by X-ray crystallography.191 In related work, a redox reaction has been shown to take place between GaCl3 and an excess of ferrocene, leading to the multidecker sandwich complex 320, [9Ga(C5H5)Fe(C5H5)Ga(C5H5)Fe(C5H5)-]n[GaCl4]2n, which features an array of alternating Ga(I) and Fe(II) ions bridged by cyclopentadienyl moieties.192 Tris(pentamethylphenyl)gallium, (Me5C6)3Ga (321), has been prepared by Robinson and co-workers using the reaction of pentamethylphenylmagnesium bromide with gallium(III) chloride in diethyl ether. The pentamethylphenyl ligands are configured about the gallium center in a propeller fashion defining torsion angles of 60.9°, 62.3°, and 67.3°.193 Additionally, the reaction of Ga(C5Me5)3 with the nucleophilic carbene tetramethylimidazol-2-ylidene has been shown to result unexpectedly in the formation of the carbene complex Ga(C5Me5)2H carbene (322).194



5

Indium

5.1 Subvalent Indium and Indium Clusters. — As with gallium, the chemistry of low valent indium organometallics continues to generate remarkable new molecular architectures. Metal-metal bonded systems and indium(I) organometallics as ligands feature among the highlights from this year. The diindene ArInInAr, 323, has been synthesized by Power and co-workers, making use of extremely sterically bulky aryl substituents [Ar : C6H3-2,6-(C6H3-2,6-Pri2)2]. This dimeric

162

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In(I) aryl features an In-In distance of 2.9786(5) Å, an In-In-C angle of 121.23(6)°, and an In-In bond order that is probably less than unity. Nevertheless the authors conclude that 323 features significant metal-metal bonding.195 The reactions of IMes [CN(Mes)C2H2N(Mes), Mes : mesityl] and DAB [(ArNCH)2, Ar : C6H3Pri2-2,6] with indium(I) halides have been shown to afford the first carbene and diazabutadiene indium(II) complexes, [In2Br4(IMes)2] (324) and [In2Cl2(DAB)2] (325), both of which have been crystallographically characterised.196

Treatment of (tmeda)Pd(CH3)2 with an excess of InCp* has been reported to lead to the trinuclear complex Pd3(InCp*)4(m2-InCp*)4 (326) in quantitative yield, rather than the expected monomeric complex Pd(InCp*)4. The molecular structure of 326 is based around three edge-bridging PdIn4 tetrahedra, with the three central palladium atoms are arranged with only a small deviation from linearity of 10o.197 In related work, the complexes (dcpe)Pt(InCp*)2 (327) and (dcpe)Pt[GaC(SiMe3)3]2 (328) have been synthesized by thermal activation of (dcpe)Pt(H)(CH2t-Bu) followed by reaction with InCp* or GaC(SiMe3)3. The short Pt-Ga distances in 328 are in agreement with an NBO analysis which reveals significantly larger Pt;Ga p back-donation for the model compound (dhpe)Pt[GaC(SiH3)3]2 in comparison to the related model compound (dhpe)Pt(GaCp)2 which bears strongly a p-donating organic substituent at the Ga centre.198

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163

5.2 Indium Derivatives Containing Bonds to Groups 15 or 16. — The synthesis and structures of indium complexes incorporating the bidentate monoanionic ligand N,N-diisopropylaminotroponiminate (iPr2-ATI) have been described. The reaction of InCl3 with Li[iPr2-ATI] yields (iPr2-ATI)InCl2 (329), which can be converted to (iPr2-ATI)InMe2 (330) by reaction with MeLi. 330 can also be formed by the reaction of InMe3 with (iPr2-ATI)H. The reaction of 330 with [Ph3C][B(C6F5)4] yields the diimine complex [
1,2-(NiPr)2-5-CPh3-cyclohepta3,6-dieneInMe2][B(C6F5)4] (331) via addition of Ph3C; to the C5 carbon of 330. Subsequent thermolysis of 331 is then reported to yield [(iPr2-ATI)InMe] [B(C6F5)4] (332) and Ph3CMe.199 The attempted synthesis of
HC(CMeNAr)2In: (Ar : C6H3-2,6-i-Pr2) by the reaction of
HC(CMeNAr)2Li with InCl in THF has been shown by Power and Stender to afford the dimeric species [
HC(CMeNAr)2InCl]2 (333) and [
HC(CMeNAr)2InCl(m-OH)]2 (334).200 5.3 Indium Organometallics in Organic Synthesis. — The use of organoindium reagents in C-C bond forming reactions continues to generate much interest, and a comprehensive treatment of such reactivity is beyond the scope of this report. A number of gallium-,201 and indium-mediated allylation,202-206 and alkynylation,207 reactions have been reported and a stereoselective synthesis of halocyclopropanes has been achieved by halogenation of the cyclopropylindium reagents prepared from allylindation of cyclopropenes.208 5.4 Miscellaneous Examples. — A high yielding synthesis of In(C5H5)3 (335) from InCl3 and Li(C5H5) in THF solution has been reoprted. Furthermore, neat In(C5H5)3 has been shown to readily decompose at 150 oC to form In(C5H5) and cyclopentadiene as the primary products, whereas a benzene suspension decomposes under refluxing conditions. In(C5H5)3 forms an isolable 1:1 adduct with PPh3 but NMe3, THF, and Et2O do not form stable adducts. Ligand redistribution reactions with InR3 (R : Me and CH2CMe3) in THF solution have been shown to provide R2In(C5H5) and RIn(C5H5)2.209

6

Thallium

As in previous years, the organometallic chemistry of thallium is scarce in comparison with the lighter elements of group 13. The syntheses of organometallic complexes of gold and thallium containing the sterically bulky 2,4,6tris(trifluoromethyl)phenyl ligand have been described by Jones and coworkers.210 In addition, treatment of TlCl with [Li(tmeda)2][P3C2Bu2t) has been shown to lead to the monomeric triphospholyl thallium(I) complex Tl(h5P3C2Bu2t) (336); the analogous gallium(I) complex was prepared in a similar fashion from metastable GaBr. Both complexes consist of weakly interacting monomeric half sandwich units in the solid state.211

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121. D. E. Babushkin and H.-H. Brintzinger, J. Am. Chem. Soc., 2002, 124, 12869—12873. 122. E. Zurek and T. Ziegler, Organometallics, 2002, 21, 83—92. 123. T. N. P. Luhtanen, M. Linnolahti and T. A. Pakkanen, J. Organomet. Chem., 2002, 648, 49—54. 124. B. Gaertner and H.-J. Himmel, Inorg. Chem., 2002, 41, 2496—2504. 125. D. Chakraborty and E. Y.-X. Chen, Organometallics, 2002, 21, 1438—1442. 126. M. S. Hill and P. B. Hitchcock, Organometallics, 2002, 21, 3258—3262. 127. T. Bauer, S. Schulz, H. Hupfer and M. Nieger, Organometallics, 2002, 21, 2931—2939. 128. N. D. Reddy, H. W. Roesky, M. Noltemeyer and H.-G. Schmidt, Inorg. Chem., 2002, 41, 2374—2378. 129. B.-H. Huang, T.-L. Yu, Y.-L. Huang, B.-T. Ko, and C.-C. Lin, Inorg. Chem., 2002, 41, 2987—2994. 130. O. M. Kekia, L. K. Krannich, C. L. Watkins, C. D. Incarvito and A. L. Rheingold, Organometallics, 2002, 21, 5987—5990. 131. E. K. Styron, C. H. Lake, D. H. Powell, C. L. Watkins and L. K. Krannich, Polyhedron, 2002, 21, 1747—1753. 132. P. Blais, T. Chivers, G. Schatte and M. Krahn, J. Organomet. Chem., 2002, 646, 107—112. 133. F. Thomas, S. Schulz, M. Nieger and K. Na¨ttinen, Chem. Eur. J., 2002, 8, 1915—1924. 134. A. Kuczkowski, S. Schulz, M. Nieger and P. R. Schreiner, Organometallics, 2002, 21, 1408—1419. 135. Q. M. Cheng, O. Stark, K. Merz, M. Winter and R. A. Fischer, J. Chem. Soc., Dalton Trans., 2002, 2933—2936. 136. C. Cui, J. A. R. Schmidt and J. Arnold, J. Chem. Soc., Dalton Trans., 2002, 2992—2994. 137. H. Schumann, J. Kaufmann, S. Dechert and H.-G. Schmalz, Tetrahedron Lett., 2002, 43, 3507—3511. 138. N. Jaber, D. Gelman, H. Schumann, S. Dechert, J. Blum, Eur. J. Org. Chem., 2002, 1628—1632. 139. P. A. Cameron, V. C. Gibson, C. Redshaw, J. A. Segal, A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans., 2002, 415—422. 140. R.-C. Yu, C.-H. Hung, J.-H. Huang, H.-Y. Lee, and J.-T. Chen, Inorg. Chem., 2002, 41, 6450—6455. 141. A. Cottone III, D. Morales, J. L. Lecuivre and M. J. Scott, Organometallics, 2002, 21, 418—428. 142. D. Pappalardo, C. Tedesco and C. Pellecchia, Eur. J. Inorg. Chem., 2002, 621—628. 143. J. A. R. Schmidt and J. Arnold, Organometallics, 2002, 21, 2306—2313. 144. H. A. Jenkins, D. Abeysekera, D. A. Dickie and J. A. C. Clyburne, J. Chem. Soc., Dalton Trans., 2002, 3919—3922. 145. M. Reiher and A. Sundermann, Eur. J. Inorg. Chem., 2002, 1854—1863. 146. L. H. van Poppel, S. G. Bott and A. R. Barron, J. Chem. Soc., Dalton Trans., 2002, 3327—3332. 147. W. Ziemkowska, S. Kwaniewska, R. Wro´blewski and R. Anulewicz-Ostrowska, J. Organomet. Chem., 2002, 651, 72—79. 148. I. Yamaguchi, T. Iijima and T. Yamamoto, J. Organomet. Chem., 2002,654, 229—232. 149. A. R. Kunicki, A. Chojecki, J. Zachara and M. Gliski, J. Organomet. Chem., 2002, 664, 136—141.

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150. A. Cottone, III and M. J. Scott, Organometallics, 2002, 21, 3610 -3627. 151. C. J. Carmalt, J. D. Mileham, A. J. P. White and D. J. Williams, New J. Chem., 2002, 902—905. 152. A. G. Avent, C. Eaborn, I. B. Gorrell, P. B. Hitchcock and J. D. Smith, J. Chem. Soc., Dalton Trans., 2002, 3971—3974. 153. E. Jung, S. H. Yoo, T.-M. Chung, C. G. Kim, Y. Kim and D. Y. Jung, Inorg. Chem. Commun., 2002, 5, 439—441. 154. D. Neculai, H. W. Roesky, A. M. Neculai, J. Magull, B. Walfort and D. Stalke, Angew. Chem., Int. Ed., 2002, 41, 4294—4296. 155. J. C. Gordon, G. R. Giesbrecht, J. T. Brady, D. L. Clark, D. W. Keogh, B. L. Scott and J. G. Watkin, Organometallics, 2002, 21, 127—131. 156. J. A. Tossell, Organometallics, 2002, 21, 4523—4527. 157. C. J. Barden, P. Charbonneau and H. F. Schaefer III, Organometallics, 2002, 21, 3605—3609. 158. C. Lustig and N. W. Mitzel, Organometallics, 2002, 21, 3471—3476. 159. M. Hoshi and K. Shirakawa, Chem. Commun., 2002, 2146—2147. 160. J. C. Gordon, P. Shukla, A. H. Cowley, J. N. Jones, D. W. Keogh and B. L. Scott, Chem. Commun., 2002, 2710—2711. 161. D. Chakraborty and E. Y. -X. Chen, Inorg. Chem. Commun., 2002, 5, 698—701. 162. M. Schiefer, H. Hatop, H. W. Roesky, H.-G. Schmidt and M. Noltemeyer, Organometallics, 2002, 21, 1300—1303. 163. W. Uhl and M. Matar, J. Organomet. Chem., 2002, 664, 110—115. 164. H.-J. Himmel, J. Chem. Soc., Dalton Trans., 2002, 2678—2682. 165. P. Jutzi and L. O. Schebaum, J. Organomet. Chem., 2002, 654, 176—179. 166. R. J. Baker, Robert D. Farley, C. Jones, M. Kloth and D. M. Murphy, J. Chem. Soc., Dalton Trans., 2002, 3844—3850. 167. N. J. Hardman, R. J. Wright, A. D. Phillips and P. P. Power, Angew. Chem., Int. Ed., 2002, 41, 2842—2844. 168. W. Uhl, L. Cuypers, M. Pro¨tt and K. Harms, Polyhedron, 2002, 21, 511—518. 169. N. Wiberg, T. Blank, M. Westerhausen, S. Schneiderbauer, H. Schno¨ckel, I. Krossing and A. Schnepf, Eur. J. Inorg. Chem., 2002, 351—356. 170. N. Wiberg, T. Blank, H. No¨th, M. Suter and M. Warchhold, Eur. J. Inorg. Chem., 2002, 929—934. 171. W. Uhl, L. Cuypers, B. Neumu¨ller and F. Weller, Organometallics, 2002, 21, 2365—2368. 172. A. Schnepf, G. Sto¨ßer and H. Schno¨ckel, Angew. Chem., Int. Ed., 2002, 41, 1882—1884. 173. R. B. King, J. Organomet. Chem., 2002, 646, 146—152. 174. N. Burford, P. J. Ragogna, K. N. Robertson, T. S. Cameron, N. J. Hardman and P. P. Power, J. Am. Chem. Soc., 2002, 124, 382 -383. 175. E. V. Grachova, P. Jutzi, B. Neumann, L. O. Schebaum, H.-G. Stammler and S. P. Tunik, J. Chem. Soc., Dalton Trans., 2002, 302—304. 176. E. Leiner and M. Scheer, J. Organomet. Chem., 2002, 646, 247—254. 177. E. Leiner, O. Hampe and M. Scheer, Eur. J. Inorg. Chem., 2002, 584—590. 178. B. Luo and W. L. Gladfelter, Inorg. Chem., 2002, 41, 590—597. 179. B. Luo and W. L. Gladfelter, Inorg. Chem., 2002, 41, 6249—6257. 180. F. Thomas, S. Schulz and M. Nieger, Organometallics, 2002, 21, 2793—2795. 181. H. No¨th and T. Seifert, Eur. J. Inorg. Chem., 2002, 602—612. 182. O. T. Beachley, Jr., J. R. Gardinier, M. R. Churchill, D. G. Churchill and K. M. Keil, Organometallics, 2002, 21, 946—951.

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183. R. Anulewicz-Ostrowska, S. Luliski, E. Pindelska, and J. Serwatowski, Inorg. Chem., 2002, 41, 2525—2528. 184. Q. M. Cheng, O. Stark, K. Merz, M. Winter and R. A. Fischer, J. Chem. Soc., Dalton Trans., 2002, 2933—2936. 185. K. Ueno, T. Yamaguchi, K. Uchiyama and Hiroshi Ogino, Organometallics, 2002, 21, 2347—2349. 186. (a) S. Nogai and H. Schmidbaur, Inorg. Chem., 2002, 41, 4770—4774. (b) S. Aldridge and A.J. Downs, Chem. Rev., 2001, 101, 3233-3265. 187. H.-J. Himmel, L. Manceron, A. J. Downs and P. Pullumbi, J. Am. Chem. Soc., 2002, 124, 4448—4457. 188. 189. L. Grocholl, S. A. Cullison, J. Wang, D. C. Swenson and E. G. Gillan, Inorg. Chem., 2002, 41, 2920—2926. 190. M. Veith, H. Vogelgesang and V. Huch, Organometallics, 2002, 21, 380—388. 191. A. Althoff, P. Jutzi, N. Lenze, B. Neumann, A. Stammler and H.-G. Stammler, Organometallics, 2002, 21, 3018—3022. 192. S. Scholz, J. C. Green, H.-W. Lerner, M. Bolte and M. Wagner, Chem. Commun., 2002, 36—37. 193. J. K. Vohs, L. E. Downs, M. E. Barfield, S. D. Goodwin and G. H. Robinson, Polyhedron, 2002, 21, 531—534. 194. J. D. Gorden, C. L. B. Macdonald and A. H. Cowley, J. Organomet. Chem., 2002, 643–644, 487—489. 195. R. J. Wright, A. D. Phillips, N. J. Hardman, and P. P. Power,J. Am. Chem. Soc., 2002, 124, 8538—8539. 196. R. J. Baker, R. D. Farley, C. Jones, M. Kloth and D. M. Murphy, Chem. Commun., 2002, 1196—1197. 197. T. Steinke, C. Gemel, M. Winter and R. A. Fischer, Angew. Chem., Int. Ed., 2002, 41, 4761—4763. 198. D. Weiß, M. Winter, K. Merz, A. Knu¨fer, R. A. Fischer, N. Fro¨hlich and G. Frenking, Polyhedron, 2002, 21, 535—542. 199. F. Delpech, I. A. Guzei and R. F. Jordan, Organometallics, 2002, 21, 1167—1176. 200. M. Stender and P. P. Power, Polyhedron, 2002, 21, 525—529. 201. Z. Wang, S. Yuan and C.-J. Li, Tetrahedron Lett., 2002, 43, 5097—5099. 202. D. Pan, S. K. Mal, G. K. Kar and J. K. Ray, Tetrahedron, 2002, 58, 2847—2852. 203. S. Kumar, V. Kumar and S. S. Chimni, Tetrahedron Lett., 2002, 43, 8029—8031. 204. G. Hilt, K. I. Smolko and C. Waloch, Tetrahedron Lett., 2002, 43, 1437—1439. 205. P. H. Lee, S. Sung, K. Lee and S. Chang, Synlett., 2002, 146—148. 206. K. Lee, J. Lee and P. H. Lee, J. Org. Chem., 2002, 67, 8265—8268. 207. Jacques Auge´, N. Lubin-Germain and L. Seghrouchni, Tetrahedron Lett., 2002, 43, 5255—5256. 208. S. Araki, O. Kenji, F. Shiraki and T. Hirashita, Tetrahedron Lett., 2002, 43, 8033—8035. 209. O. T. Beachley, Jr., D. J. MacRae, A. Y. Kovalevsky, Y. Zhang and X. Li, Organometallics, 2002, 21, 4632—4640. 210. M. Bardajı´ , P. G. Jones, A. Lagun, A. Moracho and A. K. Fischer, J. Organomet. Chem., 2002, 648, 1—7. 211. M.D. Francis, P.B. Hitchcock, J.F. Nixon, H. Schno¨ckel and J. Steiner, J. Organomet. Chem., 2002, 646, 191—195.

7 Group 14: Silicon, Germanium, Tin and Lead BY RICHARD A. LAYFIELD

1

Overview

In order to reflect current research trends, and as in previous years, this report will concentrate on low-coordinate and multiply bonded organometallics of the group 14 elements (the ‘tetrels’), compounds with Group 14 elements acting as ligands to transition and main group metals, and any pertinent technological applications. Although the report concentrates on the Group 14 element-carbon bond some important compounds containing bonds between the heavier Group 14 elements and elements other than carbon are also discussed.

2

Silylenes

The quest to find the first stable triplet silylene continued in earnest in 2002 and a review of recent literature was published.1 A Density Functional Theory (DFT) study on a series of bis-(silyl)silylenes has predicted that the singlet-triplet energy separations are essentially zero in every case except one: at various levels of theory only (tBu3Si)2Si was calculated to have a sufficiently stable triplet ground state relative to the singlet electronic isomer.2 Despite the absence of a stable triplet silylene evidence for the transient existence of such species in the photochemical reactions of stable singlet silylenes continued to emerge: the first observation of bimolecular reactions of the excited state of silyenes is illustrated in Scheme 1.

Scheme 1

The outcomes of these photochemical reactions are thought to be consistent with the biradical nature of the triplet silylene.3 Attempts at photochemical synthesis of triplet silylenes from the hexakis-(trialkylsilyl)cyclotrisilanes [(R3Si)2Si]3 (R3Si : tBuMe2Si or iPr2MeSi) generated the corresponding singlet Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 171

Organometallic Chemistry

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silylenes (R3Si)2Si, which undergo stereospecific cycloaddition with cis- and trans-2-butene to afford silacyclopropanes with retention of stereochemistry.4 The fleeting singlet silylene tBu2Si, formed alongside the disilene tBu2Si:Si tBu2 from photochemical cleavage of cyclo-(tBu2Si)3, reacts with a range of alkene- or phenyl-bridged oligoacetylenes selectively at the acetylenic bonds. No reaction of tBu2Si:Si tBu2 with the acetylene substrate was observed, rather, [2 ; 2]dimerisation of the disilene occurred.5 Similarly, photochemically generated 1,6-diphenyl-1,6-disilahexane-1,1,6,6-tetrayl(1,6-bis-silylene) undergoes intramolecular dimerisation to afford 1,2-diphenyl-1,2-disilacyclo-1-hexene.6 The reactions of the stable diaminosilylene [1,2-(tBuN)2C2H2]Si with a range of halocarbons do not simply yield the monosilane products of oxidative addition (or, alternatively, insertion into the C-Hal bond). Rather surprisingly, a series of disilanes is produced even if a 100-fold excess of the halocarbon is employed (Scheme 2).

Scheme 2

Mechanistic investigations have suggested that Scheme 2 is likely to proceed through a silylene-halocarbon acid-base adduct that subsequently adds a second silylene to form an Si-Si bond, with a subsequent 1,3-[CR1R2R3] shift leading to the disilane product.7 Generation of diarylsilylenes from the thermally-induced decomposition of cyclotrisilanes (Ar1Ar2Si)3 (Ar1, Ar2 : 2-(Me2NCH2)C6H4 or mesityl) in the presence of halosilanes R3Si-Hal provides an alternative route to disilanes, via the more traditional formal silylene insertion into the Si-Hal bond. This work was extended to the reactions of the transient diarylsilylenes with halodisilanes to afford a series of trisilanes.8 Some new insertion reactions of diaminosilylenes into C-Li and Si-Li bonds, affording the corresponding silyllithium compounds, were also reported.9 A significant development in the chemistry of silyl and germyl anions was reported whereby controlled one-electron oxidation of (tBu2MeSi)ENa (E : Si, Ge) by GeCl2 afforded 1 and 2, the first examples of stable, heavy group 14 radicals that lack conjugative stability.10

The general nature of the reaction of stable diaminosilylenes such as 3 with 1,3,5-triphosphabenzene, affording the products of [1 ; 4]-cycloaddition 4, was demonstrated (Scheme 3). By careful control of the reaction conditions, it was also possible to employ 1,3,5-triphosphabenzene as a reagent to trap the transient aminosilylsilylene 5 and structurally authenticate the product 6. The thermal reversibility of the [1 ; 4]-cycloaddition reaction was demon-

7: Group 14: Silicon, Germanium, Tin and Lead

173

Scheme 3

strated by treating 4 with [Mo(CO)4(norbornadiene)] to afford [Mo(CO)3(h61,3,5-triphosphabenzene)] and the complexes cis/trans-[Mo(CO)4(silylene)2].11 Matrix isolation spectroscopy and DFT calculations were employed to elucidate the nature of the products arising from the reactions of Me(Ph)Si with CO and PH3. Thus the 1:1 Lewis acid-base adduct Me(Ph)Si PPh3 > AsPPh3. The molecular structures of the starting complex [PtI3(AsPh3)][Bu4N] and the product trans-[PtI2(AsPh3)(py)] were determined by X-ray and based on the Pt-I distances for these complexes and those published for the related P and Sb complexes, the following trans influence series was deduced: PPh3  AsPh3 > SbPh3. This study was, incidentally, the first systematic comparison of all three heavier pnictogen atoms as donors using strictly analogous complexes with identical cis-ligands. Another study also examined Pt and Pd complexes of arsines but the motivation for this work was the possibility of using heavier arsines in CVD processes instead of AsH3 which is highly toxic and problematic to use. A range of Pd and Pt complexes were synthesised from AsPr3 and AsPri3 including [MCl2(AsR3)2], [Pd2Me2(m-Cl)2(AsR3)2] and [MCl(SCNEt2)(AsR3)2].55 The complexes were characterised spectroscopically and [Pd2Me2(m-X)2(AsPri3)2] (X : Cl, pyrazolate) were both structurally characterised by X-ray. Interestingly, [PdCl2(AsR3)2] was found to liberate free arsine below 300°C which raised the question as to whether this may be a suitable way of purifying AsR3 for use in MOCVD applications. Main group adducts of arsines have also been considered this year. Of particular interest are the reports by Schulz and co-workers examining the chemistry of bimetallic group 13 / 15 adducts. In the first of these reports they discussed experimental and theoretical aspects of alane-group 15 adducts, R3Al-ER3 (E : P, As, Sb, Bi ; R : Et, But ; R : SiMe3, Pri).56 The stability of the adducts was found to decrease with increasing atomic number of the group 15 element. Furthermore, the decreasing stability was accompanied by a shortening of the Al-C bond distances and an increase in the C-Al-C angles (also H-Al and H-Al-H parameters in calculated species such as H3Al-PH3. The B3LYP functional was used and, in the case of the As, Sb and Bi compounds, SDD relativistic effective core potentials were utilised). These patterns were also reflected in the solid-state structures of Et3Al-P(SiMe3)3 and But3Al-E(Pri)3. In the case of But3Al-E(Pri)3, the Al-C distances decrease by 0.05 Å from P to Bi whilst the C-Al-C angles show an increase of around 13°. A second report, also from the group of Schulz considered transition metal complexes of dmap coordinated monomers [(dmap)Me2ME(SiMe3)2] (dmap : 4-dimethylaminopyridine; M: Al, E : P, As ; M : Ga, E : P, As, Sb).57 A variety of transition metal complexes were prepared using [(Me3N)Cr(CO)5], [Fe3(CO)12] and [Ni(CO)4]. Treatment of [(dmap)Me2AlE(SiMe3)2] (E : P, As) with [(Me3N)Cr(CO)5] led to the chromium complexes [(dmap)Me2Al-E(SiMe3)2Cr(CO)5] (E : P, 15a 89% ; As, 15b 76%) which were crystallised from dichloromethane. The molecular structures of both complexes were determined by X-ray and the N-Al-E-Cr torsion angles were found to be 36.9(1)° in 15a and 37.6(1)° in 15b. Interestingly, the Cr-P and Cr-As distances of 2.528(1) Å and 2.600(1) Å, respectively, were the longest P-Cr(CO)5 and AsCr(CO)5 distances seen to date and were related to the steric distortion caused by the dmap ligand. The complex [(dmap)Me2Al-P(SiMe3)2Fe(CO)4] was also pre-

202

Organometallic Chemistry

pared in 19% yield and its infrared spectrum showed three CO absorptions at 1911, 1956 and 2008 cm-1 consistent with an Fe(CO)4 moiety of C3v symmetry. The molecular structure showed the presence of a distorted trigonal bipyramidal Fe-centre with the P-centre located in an axial position, consistent with it being a stronger s donor and a weaker p acceptor than CO. Incidentally, the P-Fe(CO)4 distance of 2.377(1) Å was also the longest reported of this type. [(dmap)Me2GaE(SiMe3)2Ni(CO)4] (E : P, As, Sb) complexes were also prepared and the As and Sb species were found to be gauche along the Ga-E bond with torsion angles of 66.2(1)° and 64.3(1)°, respectively. General conclusions drawn from this work are that the s donor/p acceptor ratio increases in the order P benzvalene > dewar benzene > prismane > bicyclopropenyl > trans dewar.

3

Antimony

In two successive papers, Leong and co-workers described the properties of the antimony-osmium cluster compound [Os3(m-SbPh2)(m-H)(m3,h2-C6H4)(CO)9] 18. In the first of these papers it was found that 18 was an effective catalyst for the cyclotrimerisation of diphenylacetylene to hexaphenylbenzene. Conversely, with terminal alkynes (PhC∫CH, ButC∫CH), the formation of cluster compounds

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

205

with C-C bond formation was observed.66 In the second of these reports, the reactivity of 18 with PPh3 was explored. It was found that the cluster is very reactive and interacts with PPh3 even under ambient conditions to initially form a nucleophilic addition product which could isomerise, decarbonylate, deorthometalate or react further with PPh3 to give the ortho-metalated species [Os3(m-SbPh2)(CO)8(PPh2C6H4)(PPh3)2].67 As part of their continued interest in cyclic stibanes, Breunig et al. presented a study of the reactivity of cyclo-(But4Sb4) with sodium or potassium in the presence of pentamethyldiethylenetriamine (pmdeta).68 The results were found to depend on both the reaction times and the concentration of pmdeta. Heating cyclo-(But4Sb4) in thf to reflux for 1 hour (with K) or 4 hours (with Na) afforded the anion [But4Sb3]- which could be crystallised from benzene as [M(pmdeta)(But4Sb3)] (M : Na 19, K) or as [M(pmdeta)2(But4Sb3)] (M : K). Interestingly, prolonging the reaction led to decomposition of the [But4Sb3]- anion and formation of the diantimonide [But3Sb2]- and the X-ray structure of [K(pmdeta)(But3Sb2)] was determined. It exists as centrosymmetric dimers with rhombohedral K2Sb2 cores containing five coordinate potassium centres. The antimony centres are trigonal pyramidal which is an unusual geometry for four coordinate antimony. The Sb-K distances were found to be essentially equivalent at 3.5655 and 3.5926 Å. In another study, the same group investigated the oxidation of tetraaryldistibanes (R2Sb)2 R : (Ph, o-tolyl, p-tolyl) by controlled air oxidation.69 The starting distibanes, which are yellow solids, were prepared by treatment of dibromoethane with the corresponding diaryl antimonide R2SbNa. Under controlled oxidation, the corresponding oxides (R2Sb)2O were formed in very high yields (>90 %) and the solid state structures showed conformational differences depending on the nature of the aryl substituent. (Ph2Sb)2O was found to adopt a syn,syn conformation whilst (o-tolyl2Sb)2O and (p-tolyl2Sb)2O were found both to have syn,anti conformations. Total exposure to air led to further oxidation and formation of the tetrakis-(diarylantimony)hexoxides (R2Sb)4O6 which are insoluble in common organic solvents and water. Treatment of these hexoxides with H2O2 led to a series of antimony peroxo clusters. The phenyl analogue, 20, was crystallographically characterised. It consists of a puckered (SbO)4 ring with O2 units in capping positions above and below the plane of the Sb centres. Two interesting reports appeared concerning the formation and properties of ruthenium complexes based on stibines (and arsines). In particular, one study appeared concerning the characterisation of ruthenium hydride complexes [RuCp*(H2)2(EPh3)]; (E : As, Sb) and concluded that SbPh3 and AsPh3 are good ligands for stabilising the ruthenium-dihydrogen bond.70 Synthesis was achieved by protonation of [RuCp*H3(EPh3)] with HBF4.OEt2 in dichloromethane at 980°C. Conversely, the corresponding reaction with [RuCp*H3(PPh3)] was unsuccessful and led to spontaneous elimination of hydrogen. Isotopomers, [RuCp*(HD)(D2)(EPh3)]; could also be formed by protonation of [RuCp*D3(EPh3)] and measurements of the temperature dependence of T1 as well as the 1JHD coupling led to the confirmation of a bis-(dihydrogen) species in each case (21 As, a ; Sb, b). The second related report discussed the synthesis and reactivity of Ru(II) and

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Organometallic Chemistry

Ru(IV) half sandwich complexes [RuCp(EPh3)(CH3CN)2]; and 3 ; 71 [RuCp(EPh3)(h -C3H5)Br] (E : P, As, Sb). Previous studies on the phosphine complexes had shown them to be active for the catalysis of redox isomerisation reactions of allyl alcohols as well as able to promote the C-C coupling of acetylenes to give allyl, allenyl and butadienyl carbene complexes. The complexes [RuCp(EPh3)(CH3CN)2]; could be quantitatively prepared by addition of EPh3 (E : P, As, Sb) to [RuCp(CH3CN)3];. Subsequent treatment with allyl bromide led to the high yielding formation of [RuCp(EPh3)(h3-C3H5)Br];. The latter species were found to be air stable both in solution and in the solid state. Interestingly, the outcome was completely different with BiPh3. Treatment of [RuCp(CH3CN)3]; with BiPh3 led to a mixture of the arene complexes [RuCp(h6-C6H5BiPh2)]; and [(RuCp)2(m-h6,h6-Ph2BiPh)]2; 22 (X-ray). Of all the complexes prepared, only the P-based ones were found to have any significant catalytic activity. Main group organometallic compounds of antimony include a series of cyclic and polycyclic Ga-Sb compounds reported by Ha¨nisch et al. The first part of this report included the synthesis and spectroscopic characterisation of Pri3SiE(SiMe3) (E : As, Sb) from the treatment of Pri3SiCl with [
(dme)LiE(SiMe3)22] in dme at 0°C. Whereas the arsenic analogue failed to react, treatment of Pri3SiSb(SiMe3) with GaCl3.PPhPrn2 afforded 23 in 42 % yield. The molecular structures consists of a Ga2Sb2 four membered ring with trigonal pyramidal Sb centres and distorted tetrahedral gallium atoms. The Ga-Sb bond distance was found to be 2.641 Å. Compound 23 was found to decompose gradually in benzene solution to afford the tricyclic compound 24 in 54% yield.

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

207

The synthesis and structure of the novel 24-membered mixed-metal macrocycle cyclo-[
(3-Me-1,2-C6H3O2)2SbNa(thf)26] was described this year. This was an unexpected outcome during attempts to prepare a tetraaanionic species of the type [(RO)2Sb(CH2)3Sb(OR)2]4- from the reaction between 3-Me-1,2C6H3(OH)(ONa) and (Me2N)2Sb(CH3)2Sb(NMe2)2. The macrocycle contains an Sb6Na6O24 core with a cyclic array of SbO2Na units. The Na; centres were found to have distorted tetrahedral geometries and the Sb centres were found to be between 4.16 and 4.51 Å apart.

4

Bismuth

Organo-bismuth compounds have featured quite extensively in the literature this year in the context of main group chemistry, transition metal chemistry and catalysis. As in previous years, Breunig and his group have continued to focus on the chemistry of bismuth hydrides. Using the bulky bis-(trimethylsilyl)methyl substituent (Me3Si)2CH- (hereafter known as R) they reported they synthesis of R2SbH, R2BiH and R2Bi-BiR2. The former two compounds could be synthesised by treating the corresponding chloride R2SbCl or R2BiCl with LiAlH4.72 Whereas R2SbH is stable to around 100°C, the bismuth analogue decomposes quickly at room temperature to the corresponding dibismuthane with concomitant elimination of hydrogen. Characterisation of the hydride was possible, however, and a broad signal for the Bi-H proton was seen in the 1H NMR spectrum and a characteristic IR stretch was recorded at 1690 cm-1. R2Bi-BiR2 is a red crystalline solid which decomposes gradually in solution to a mixture of cyclic bismuthanes. Variable temperature studies suggested that at high temperature and concentration, the dibismuthane undergoes a dynamic process leading to equivalent SiMe3 groups. The suggested mechanism involved exchange of R2Bi units between associated molecules resulting in pyramidal inversion of the Bi centres with a calculated DG‡ value of 14.66 kcal mol-1 (cf. 22.5

208

Organometallic Chemistry

— 24.0 kcal mol-1 for tetraorganodiphosphines). In the solid-state, R2Bi-BiR2 was found to have a trans antiperiplanar configuration with a typical Bi-Bi distance of 3.0534(13) Å. A second report by the same group described the preparation of the bis(dialkylbismuth)sulfide and telluride (R2Bi)2E (E : S, Te, R is again (Me3Si)2CH-).73 These compounds were prepared by treatment of R2BiCl with Na2S in water or Na2Te in liquid ammonia. The sulfide is air sensitive and decomposes gradually in solution with formation of the corresponding bismuthine R3Bi and Bi2S3. The telluride is even less thermally robust and was found to decompose in solution or as a solid above 928°C. Despite being unstable, both compounds were successfully characterised by X-ray crystallography. Both were found to be bent molecules with bend angles of 92.24(4)° (Bi-S-Bi) and 88.00(7)° (Bi-Te-Bi). The Bi-S distances were found to be 2.5574(12) Å and 2.5719(12) Å while the Bi-Te distances were measured at 2.872(3) Å and 2.889(2) Å. In both compounds, the organic substituents were in a gauche conformation relative to the Bi · · · Bi axis. In the same report was a discussion of the chemistry of cyclo-(RSbSe)n which exists as a dimer-trimer equilibrium in solution. The dimer could be trapped by treatment with [W(CO)5(thf)] affording a 42 % yield of the selenium bridged complex 25 which was found to decompose slowly in solution. Interestingly, a reversible photochemical transformation of this species was observed but it was uncertain as to whether the product of this transformation was the monomer RSb[W(CO)5]:Se or the trans isomer. The molecular structure was found to consist of a four membered Sb2Se2 ring in which the W(CO)5 groups are cis relative to the plane of the four membered ring. Sb-Se distances of 2.5574(6) Å and 2.5586(6) Å were measured and the Sb-W distances were found to be 2.7491(5) Å. Related to this work, although perhaps not strictly organometallic in the sense of containing a metal-carbon bond, was a report on the synthesis and structure of the first bismuth(III) telluroether complex.74 The compound in question, [BiBr3(PhTeMe)], was prepared in 22% yield by slow evaporation of a solution of BiBr3 in MeCN which had been layered with an equimolar solution of PhTeMe in dichloromethane. The structure contains a central dimeric Br2Bi(m-Br)2BiBr2 motif with apically coordinated PhTeMe units. The units are linked into infinite chains by long contacts of 3.16 Å between the non-bridging Br and Bi centres. The Bi-Te distances of 3.0533(7) Å are only slightly longer than the sum of the covalent radii of the two elements implying a strong bond. Other reports featuring organo-bismuth chemistry in a main group context include the preparation of a nine germanium atom cluster with two terminal diphenylbismuth moieties. The ionic cluster, [Ph2Bi-(Ge)9-BiPh2]2- 26 was prepared in approximately 22% yield from the reaction between K4Ge9 and BiPh3 in ethylenediamine.75 The structure consists of a central Ge9 cluster with two exo-bonded BiPh2 units which are attached to the opposite vertices of the open face of the cluster. The Ge-Bi bond distances were found to be 2.7327(8) Å and 2.7332(8) Å, consistent with them being single bonds. The Ge-Bi-C angles of between 94.7(1) and 98.0(1)° are almost orthogonal as a consequence of the inert pair effect of the bismuth s-pair of electrons. Bismuthanides and bismuthanediides containing bulky silyl groups were the

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209

subject of a report by Linti and co-workers this year. They found that treatment of BiBr3 with [(thf)3LiSi(SiMe3)3] led to the compounds [
BiSi(SiMe3)34] and [Li(thf)4][Bi
Si(SiMe3)32], the latter in 51% yield.76 In the solid-state, [Li(thf)4] [Bi
Si(SiMe3)32] was found to consist of a bent two coordinate bismuth anion and a separate lithium cation coordinated by 4 thf molecules. The Bi-Si distance was found to be 2.67 Å and the Si-Bi-Si bend angle was 114.4°. A similar reaction between BiBr3 and [Na(thf )2Si(CMe3)3] in thf afforded [
BiSi(CMe3)34] 27 as the main product in 32% yield as well as [Na(thf)3][Bi
Si(CMe3)32] and [Na3(thf )14[Na21Bi12
Si(CMe3)312] (dodecameric cluster, X-ray). The structure of [Na(thf)3][Bi
Si(CMe3)32] was found to be very similar to that of [Li(thf)4] [Bi
Si(SiMe3)32] except that there was found to be a contact between the Na and Bi centres of 3.11(1) Å. The Bi-Si distances were found to be 2.717 Å and the Si-Bi-Si angle was 122.3°. The structure of 27 consists of a folded four membered Bi4 cycle with alternating 9Si(CMe3)3 substituents. The fold angle was found to be 150° with Bi-Bi-Bi angles of approximately 90° and Bi-Bi distances between 3.013(1) and 3.088(2) Å. An unusual heterocycle [
Me2GaBi(SiMe3)23] synthesised by the group of Schulz was found to be the first Ga-Bi heterocycle and was prepared in high yield (74 %) from the reaction between Me2GaH and Bi(SiMe3)3 at 925°C.77 The compound is stable for several hours in an inert atmosphere but decomposes in solution, the products of the decomposition dependent upon the nature of the solvent. In non-coordinating solvents, the heterocycle was found to decompose to Bi2(SiMe3)4, Me3Ga and elemental gallium whereas in coordinating solvents the products were Bi(SiMe3)3, Me3Ga and a dark unidentified precipitate. The molecular structure was shown to be isostructural with the related aluminium compound [
Me2AlBi(SiMe3)23] with distorted tetrahedral Ga and Bi environments. The Ga-Bi distances were found to range between 2.744(1) and 2.783(1) Å. Transition metal compounds incorporating organo-bismuth systems have appeared a number of times this year. Motivated by the utility of bismuthmolybdate phases in heterogeneous catalysis for the oxidation of olefins, Limberg et al. described the synthesis of molybdenum complexes incorporating bismuth alkoxides.78 Treatment of [MoCp2(H)2] with [
Bi[OCH(CF3)2]3(thf )2] afforded the complex [MoCp2
Bi[OCH(CF3)2]22] which was characterised by X-ray crystallography. Interestingly, there was a short distance of 2.693(3) Å observed between the bismuth centre of one ligand and the oxygen of an adjoining ligand. It was suggested that this could be responsible for the small Bi-Mo-Bi angle of only 76.72°. [MoCp2
Bi[OCH(CF3)2]22] was found to be an active catalyst for the oxidation of cis-stilbene at 65°C for 63 hours. A study by Shimada and Tanaka and explored the cross coupling reactions of organo-bismuth compounds with aryl bromides and iodides catalysed by Pd(PPh3)4. In this particular work they focussed on bismuth compounds bearing 2,6-pyridinedimethoxide ligands. It was found that the yield of the cross coupled product could be significantly improved by adding a caesium salt such as Cs2CO3 or CsF.79 Some novel chromium complexes of bismuth have been reported this year from the treatment of NaBiO3 with [Cr(CO)6] in highly concentrated

Organometallic Chemistry

210

KOH/MeOH. The outcome of the reaction was found to be dependent on the

reaction conditions.80 Under ambient conditions, treatment of NaBiO3 with [Cr(CO)6] followed by metathesis with [Et4N]Br afforded [Et4N] 33[Bi
Cr(CO)54] in 76% yield. The anion [Bi
Cr(CO)54] was found to consist of a tetrahedral bismuth centre with Bi-Cr distances of between 2.905 and 2.935 Å. When the reaction was carried out at 80°C, the outcome was different and surprising. In this case the product formed was not [Et4N]3[Bi
Cr(CO)54] but [Et4N]2[MeBi
Cr(CO)53] containing the tetrahedral [MeBi
Cr(CO)53]2- anion. This compound, resulting from a rare C-O activation of methanol, was formed in 62% yield. The molecular structure, determined by crystallography, showed Bi-Cr distances between 2.8123 and 2.8196 Å. The mono-O-phenylation of diols using BiPh3(OAc)2 has been discussed in some detail by Costello et al. In this study they chose (-)pinanediol as the substrate (systematic name: [1R(1a,2a,3a,5a)]-2,6,6-trimethylbicyclo[3.1.1]heptane-2,3-diol).81 It was proposed that the mechanism involves light promoted reductive elimination of the trigonal pyramidal Bi(V) centre and transfer of a phenyl substituent to the least stereochemically hindered OH group to afford the monophenyl ether in a yield of 79% as a clear oil.

References 1. 2. 3. 4. 5. 6. 7.

O. Mo´, M. Ya´n ez, J.-C. Guillemin, E. H. Riague, J.-F. Gal, P.-C. Maria, and C. D. Poliart, Chem. Eur. J., 2002, 8, 4919. C. Jones and A. F. Richards, J. Organomet. Chem., 2002, 645, 256. S. Ito, H. Sugiyama, and M. Yoshifuji, Chem. Commun., 2002, 1744. M. D. Francis and P. B. Hitchcock, Chem. Commun., 2002, 86. M. D. Francis and P. B. Hitchcock, Organometallics, 2003, 22, 2891. G. Anderson, J. C. Green, and M. D. Francis, Organometallics, 2003, 22, 2897. P. N. Condick, M. A. Fox, R. Greatrex, C. Jones, and D. L. Ormsby, Chem. Commun., 2002, 1448.

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

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A. D. Burrows, N. Carr, M. Green, J. M. Lynam, M. F. Mahon, M. Murray, B. Kiran, M. T. Nguyen, and C. Jones, Organometallics, 2002, 21, 3076. T. Sasamori, N. Takeda, M. Fujio, M. Kimura, S. Nagase, and N. Tokitoh, Angew. Chem., Int. Ed., 2002, 41, 139. R. C. Smith, T. Ren, and J. D. Protasiewicz, Eur. J. Inorg. Chem., 2002, 2779. C. W. Tsang, C. A. Rohrick, T. S. Saini, B. O. Patrick, and D. P. Gates, Organometallics, 2002, 21, 1008. V. A. Wright and D. P. Gates, Angew. Chem., Int. Ed., 2002, 41, 2389. L. Weber, S. Kleinebekel, and P. Lo¨necke, Z. Anorg. Allg. Chem., 2002, 628, 803. L. Weber, S. Kleinebekel, H.-G. Stammler, and A. Stammler, Z. Anorg. Allg. Chem., 2002, 628, 810. I. Krossing, U. Englert, E. Matern, J. Olkowska-Oetzel, J. Pikies, and G. Fritz, Z. Anorg. Allg. Chem., 2002, 628, 446. L. Weber, U. Lassahn, and H.-G. Stammler, Eur. J. Inorg. Chem., 2002, 3272. C. Jones, P. C. Junk, A. F. Richards, and M. Waugh, New J. Chem., 2002, 2002, 1209. M. Brym, C. Jones, and A. F. Richards, Dalton Trans., 2002, 2800. S. E. d’Arbeloff-Wilson, P. B. Hitchcock, J. F. Nixon, and L. Nyula´szi, J. Organomet. Chem., 2002, 655, 7. M. L. Helm, P. B. Hitchcock, J. F. Nixon, L. Nyula´szi, and D. Szieberth, J. Organomet. Chem., 2002, 659, 84. F. Mathey, J. Organomet. Chem., 2002, 646, 15. L. Weber, Angew. Chem., Int. Ed., 2002, 41, 563. E. Urnezˇ ius, W. W. Brennessel, C. J. Cramer, J. E. Ellis, and P. v. R. Schleyer, Science, 2002, 295, 832. H. Sitzmann, Angew. Chem., Int. Ed., 2002, 41, 2723. A. R. Kudinov, D. A. Loginov, Z. A. Starikova, P. V. Petrovskii, M. Corsini, and P. Zanello, Eur. J. Inorg. Chem., 2002, 3018. J. Bai, A. V. Virovets, and M. Scheer, Angew. Chem., Int. Ed., 2002, 41, 1737. S. O. Agustsson, C. Hu, U. Englert, T. Marx, L. Weserman, and C. Ganter, Organometallics, 2002, 21, 2993. P. Toullec, L. Ricard, and F. Mathey, Organometallics, 2002, 21, 2635. X. Sava, A. Marinetti, L. Ricard, and F. Mathey, Eur. J. Inorg. Chem., 2002, 1657. X. Sava, M. Melaimi, N. Me´zailles, L. Ricard, F. Mathey, and P. L. Floch, New J. Chem., 2002, 26, 1378. D. Carmichael, F. Mathey, L. Ricard, and N. Seeboth, Chem. Commun., 2002, 2976. M. Ogasawara, T. Nagano, K. Yoshida, and T. Hayashi, Organometallics, 2002, 21, 3062. T. K. Hollis, Y. J. Ahn, and F. S. Tham, Chem. Commun., 2002, 2996. F.-X. Buzin, F. Nief, L. Ricard, and F. Mathey, Organometallics, 2002, 21, 259. F. Nief and L. Ricard, J. Organomet. Chem., 2002, 642, 208. F. Nief, D. Turcitu, and L. Ricard, Chem. Commun., 2002, 1646. M. D. Francis, P. B. Hitchcock, J. F. Nixon, H. Schno¨ckel, and J. Steiner, J. Organomet. Chem., 2002, 646, 191. M. Doux, C. Bouet, N. Me´zailles, L. Ricard, and P. L. Floch, Organometallics, 2002, 21, 2785. E. F. DiMauro and M. C. Kozlowski, J. Chem. Soc., Perkin Trans, 1, 2002, 439. N. H. T. Huy, H. Vong, and F. Mathey, Organometallics, 2002, 21, 336. V. L. Rudzevich, H. Gornitzka, K. Miqueu, J. M. Sotiropoulos, G. PfisterGuillouzo, V. D. Romanenko, and G. Bertrand, Angew. Chem., Int. Ed., 2002, 41,

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42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76.

Organometallic Chemistry 1193. N. Huy, C. Compain, L. Ricard, and F. Mathey, J. Organomet. Chem., 2002, 650, 57. J. M. Galbraith, P. P. Gaspar, and W. T. Borden, J. Am. Chem. Soc., 2002, 124, 11669. M. E. Garcia, V. Riera, M. A. Ruiz, D. Sa´ez, J. Vaissermann, and J. C. Jeffery, J. Am. Chem. Soc., 2002, 124, 14304. R. C. Smith, S. Shah, and J. D. Protasiewicz, J. Organomet. Chem., 2002, 646, 255. A. W. Ehlers, E. J. Baerends, and K. Lammertsma, J. Am. Chem. Soc., 2002, 124, 2831. R. Melenkevitz, D. J. Mindiola, and G. L. Hillhouse, J. Am. Chem. Soc., 2002, 124, 3846. A. T. Termaten, T. Nijbacker, M. Schakel, M. Lutz, A. L. Spek, and K. Lammertsma, Organometallics, 2002, 21, 3196. C.-H. Lai, M.-D. Su, and S.-Y. Chu, Organometallics, 2002, 21, 397. J. Frunzke, M. Lein, and G. Frenking, Organometallics, 2002, 21, 3351. M. Bouslikhane, H. Gornitza, H. Ranaivonjatovo, and J. Escudie´, Organometallics, 2002, 21, 1531. C. Jones, P. C. Junk, and T. C. Williams, Dalton Trans., 2002, 2417. S. Sasaki, K. Sutoh, F. Murakami, and M. Yoshifuji, J. Am. Chem. Soc., 2002, 124, 14830. N. Kuznik and O. F. Wendt, Dalton Trans., 2002, 3074. P. P. Phadnis, V. K. Jain, A. Kno¨dler, and W. Kaim, Z. Anorg. Allg. Chem., 2002, 628, 1332. A. Kuczkowski, S. Schulz, M. Nieger, and P. R. Schreiner, Organometallics, 2002, 21, 1408. F. Thomas, S. Schulz, M. Nieger, and K. Na¨ttinen, Chem. Eur. J., 2002, 1915. J. Besinger, J. Treptow, and D. Fenske, Z. Anorg. Allg. Chem., 2002, 628, 512. K. Naka, T. Umeyama, and Y. Chujo, J. Am. Chem. Soc., 2002, 124, 6600. A. L. Rieger and P. H. Rieger, Organometallics, 2002, 21, 5868. M. Driess, H. Ackermann, J. Aust, K. Merz, and C. v. Wu¨llen, Angew. Chem., Int. Ed., 2002, 41, 450. B. Neumu¨ller, S. Chitsaz, and K. Dehnicke, Z. Anorg. Allg. Chem., 2002, 628, 523. A. D. Bond, F. Garcia, K. Jantos, G. T. Lawson, M. Partlin, and D. S. Wright, Chem. Commun., 2002, 1276. A. F. Shihada and F. Weller, Z. Anorg. Allg. Chem., 2002, 628, 1007. U. D. Priyakumar, T. C. Dinadayalane, and G. N. Sastry, New J. Chem., 2002, 26, 347. M. Deng and W. K. Leong, Organometallics, 2002, 21, 1221. G. Chen, M. Deng, C. K. Lee, and W. K. Leong, Organometallics, 2002, 21, 1227. H. J. Breunig, M. E. Ghesner, and E. Lork, J. Organomet. Chem., 2002, 660, 167. H. J. Breunig, T. Kru¨ger, and E. Lork, J. Organomet. Chem., 2002, 648, 209. H. Aneetha, M. J. Tenorio, M. C. Puerta, and P. Valerga, J. Organomet. Chem., 2002, 663, 151. E. Becker, C. Slugovc, E. Ru¨ba, C. Standfest-Hauser, K. Mereiter, R. Schmid, and K. Kirchner, J. Organomet. Chem., 2002, 649, 55. G. Bala´zs, H. J. Breunig, and E. Lork, Organometallics, 2002, 21, 2584. H. J. Breunig, I. Ghesner, and E. Lork, J. Organomet. Chem., 2002, 664, 130. W. Levason, N. J. Hill, and G. Reid, Dalton Trans., 2002, 4316. A. Ugrinov and S. C. Sevov, J. Am. Chem. Soc., 2002, 124, 2442. G. Linti, W. Ko¨stler, and H. Pritzkow, Eur. J. Inorg. Chem., 2002, 2643.

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9 Organo-transition Metal Cluster Complexes BY MARK G. HUMPHREY AND MARIE P. CIFUENTES

1

Introduction

This chapter covers the chemistry of metal carbonyl and organometallic clusters containing three or more metal atoms. The treatment is in Periodic Group order, homometallic compounds being followed by heterometallic clusters. The numbered compounds are illustrated. Ligands are not shown for high-nuclearity clusters, emphasis being placed on core geometry.

2

General Reviews

The chemistry of organometallic clusters has been reviewed for the year 2000.1 A review summarising the synthesis, properties and chemistry of transition metal clusters with phosphorus monoxide and related ligands,2 and an extensive review on the coordination and organometallic chemistry of metal-NO complexes, including cluster complexes, have been published.3A review of complexes containing Pd-Pd bonds includes some limited examples of trinuclear and higher nuclearity cluster complexes.4 The preparation of transition metal chain compounds, relevant to the preparation of polymeric chains, has been highlighted.5

3

Spectroscopic Studies

The spherical harmonic model has been applied to the analysis of bridging n(CO) vibrations in transition metal carbonyl clusters; the spectra of these species were found to be more sensitive to the molecular arrangement of the metal atoms than in the terminal CO ligand clusters.6 Picosecond time-resolved IR spectroscopy has provided the first direct evidence for the existence of a CO-bridged reactive intermediate in the photolysis of Ru3(CO)12.7 Subpicosecond time-resolved transient absorption spectroscopy has been used to elucidate the nature of photoprocesses involved in the formation of biradicals and zwitterions from Os3(CO)10(a-diimine) clusters.8, 9 The FTIR spectra of Rh4(CO)12/Rh6(CO)16 mixtures have been analysed using a new algorithm. The band-target entropy minimisation (BTEM) method successfully recovered pure component spectra Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 214

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for trace species, with enhanced signal-to-noise ratio.10 Analysis of 31P NMR chemical shifts and 2-D NMR data for a number of W6S8L6-n(PR3)n octahedral clusters (n : 0 — 6; L : nonphosphine Lewis base ligand) has revealed a linear correlation analogous to the Dean-Evans relation for 19F NMR shifts in octahedral tin complexes. The technique demonstrates the trans- and cis- influence of the ligands on the clusters, predicts chemical shifts and aids in spectroscopic identification.11 Para-hydrogen has been used as a mechanistic probe in a study of the hydrogenation of alkynes by Os3(m-H)2(CO)1012 and in the reactivity of the triosmium quinolyl clusters Os3(m-H)
m3-h3-C9H7(4Me)N(CO)9, Os3(m-H)
m3-h2-C9H5-(n-NH2)N(CO)9 (n : 2, 3), Os3(m-H)
m3-h2C9H6(4-CMe2CN)(5-NH2)N(CO)9 and Os3(m-H)
m3-h2-C9H6(4-CMe2CN)(5F)N(CO)9.13 An NMR spectroscopic study of the ligand dynamics of Rh4(mPPh2)4(CO)6 has shown that the strongly bonded m-PPh2 groups undergo site exchange with the CO ligands, effectively moving over the metal surface.14 A solid-state NMR study of the dianionic cluster catalyst [NEt4]2[Ru6Pd6(CO)24] supported on MCM-41 has provided the first evidence for a direct linkage between the cluster counterion and the walls of the mesopore through an ethyl group.15

4

Theory

A bonding analysis of electron-rich octahedrally condensed transition metal chalcogenide clusters using a local metal frontier orbital approach complemented by extended Hu¨ckel theory and DFT calculations has been developed. The molecular orbital model, based on metal-metal bonding arising from interaction of the metal frontier orbitals derived from local metal-ligand coordination, allows metal d orbitals which are mainly involved in M-L bonding to be neglected, affording a greatly simplified bonding model.16 DFT calculations using Kohn-Sham orbital energy patterns have been used to rationalise the valence electron counts for stable face-capped octahedral transition metal clusters M6E8L6 (E : S, Se, Te, Cl) for both p-acceptor (L : CO, PMe3) and p-donor (L : Cl9) coligands.17 A general symmetry-based algorithm has been developed for the determination of possible stereoisomers of heteronuclear clusters and applied to the case of octahedral and icosahedral complexes.18 The structures of some high-nuclearity iridium carbonyl clusters comprised of fused octahedra (e.g. [Ir9(CO)20]9)or combinations of tetrahedral and octahedral polyhedra (as in [Ir10(CO)21]29) has been rationalised in terms of skeletal bonding models.19 Structural comparison of some mixed-metal Rh-Ni cluster anions with 9 — 11 vertices with the ‘flattened’ deltahedra of isoelectronic group 13 cluster anions shows that, in all cases, the mixed-metal clusters contain flattened Rh, rather than Ni, vertices. By considering vertex flattening as providing a mechanism for drawing an otherwise external electron pair into the skeletal cluster bonding, the apparently electron deficient mixed-metal clusters can be considered to follow

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the Wade-Mingos rules.20

5

Structural Studies

A comparison of the structure of [NEt4][Fe3Co(m-CO)4(CO)9] to the previously structured [ppn][Fe3Co(m-CO)3(CO)10] has shown a different arrangement of the CO ligands about the tetrahedral core; the two structures are related using the Ligand Polyhedron Model by an inversion of the Fe3Co core within the icosahedral ligand polyhedron.21 The X-ray crystal structures of Ru3(m-H)(m3-C2SiPri3)(CO)9 and Ru3(m-H)(mN2C3H3)(CO)10 have been published.22, 23 The structures of the series of mixedmetal clusters MoIr3(m-CO)3(h5-C5H5-nMen)(CO)8 (n : 1, 4, 5)24 and Mo2Ir2(m3CO)3(CO)7(h5-C5H5-nMen)2 (n : 1, 4),25 as well as Ru3Co(m-H)3(mCO)(CO)10(PCy3)26 have also been reported.

6

Large Clusters

This section covers transition metal clusters containing nine or more metal atoms. Diagrams of some complexes in this section show core interactions only, to enhance clarity. 6.1 Homonuclear High-nuclearity Clusters. — A high yielding route to the decaruthenium dicarbide dianion [Ru10(m6-C)2(CO)24]29 (60 %) has been developed from reaction of Ru3(CO)12 and CaC2 in thf; by-products include 1 and 2. Addition of complexing agent tetramethyldiaminoethane (tmeda) afforded a slightly lower yield of the decaruthenium dianion, along with the tmeda adduct 3, and reaction of CaC2 with Ru3(m-dppm)(CO)10 gave the known vinylidene raft cluster Ru6(m4-CCH2)2(m-dppm)2(CO)12.27

The unstable enneanuclear cluster dianion [Ni9(CO)16]29 (4) was formed from the reaction of [Ni6(CO)12]29 with CuCl(PPh3)x (x : 1, 2), or from the degradation of [H2Ni12(CO)12]29 with PPh3. The complex is structurally related to the three-layer dodecanuclear close-packed cluster anion [Ni12(CO)21]49 by replacement of one of the outer [Ni3(m-CO)3(CO)3]29 layers with a face-capping CO ligand.28

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The first Pd-Tl carbonyl phosphine cluster dication [Pd12Tl2(CO)9(PEt3)9]2; (5) was prepared from the reaction of Pd10(CO)12(PEt3)6 and AuCl(PPh3)/TlPF6 in 40 % yield. A structural study showed that the cation consists of four fused trigonal bipyramidal units, and suggested a better route to the cluster via reaction of Pd4(CO)5(PEt3)4 and AuCl(SMe2)/TlPF6 (90%). DFT calculations on model complexes suggest that the similar electrophilic nature of the [Au(PEt3)]; and Tl; leads to the formation of the less sterically demanding Tl adduct.29

High-nuclearity Cu(I)-alkynyl clusters of the type Cux;y(C∫CR)y(hfac)x have been isolated by using 1,1,1,5,5,5-hexafluoroacetylacetonate (hfac) as the capping ligand in reactions between Cu2O and linear alkynes 1-pentyne, 1-hexyne, 1heptyne and 1-octyne. Complexes contain either 18 or 26 copper atoms, and show weak Cu-Cu interactions.30-32 Reaction of stoichiometric amounts of [Ag(NCMe)4]; and NH4Se2P(OR)2 (R : Et, Pri) has afforded the decanuclear silver complex (6), containing a m10selenium atom encapsulated in a distorted cis-bicapped trapezoid-prismatic metal core, with the diselenophosphate ligands in three different coordination modes. The pseudohexagonal complex (7) was obtained as a minor product.33 Reaction between Ag(BF4) and t-butylacetylene in the presence of F9, Cl9 or Br9 afforded [Ag14(C∫CBut)12X];, containing an encapsulated halide atom within a rhombic dodecahedral Ag metal core. In contrast, reaction of AgTs (Ts : p-toluenesulfonate) and AgNO3 with t-butylacetylene in the absence of halide ions affords polymeric
Ag3(C∫CBut)2Xn (X : Ts, NO3) (8).34

Reaction of silver benzoate with S(Ph)SiMe3 and S(SiMe3)2 (2:1) in the presence of 1,1,1-tris
(diphenylphosphino)methylethane (triphos) afforded

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Ag70S16(SPh)34(O2CPh)4(triphos)4 (27 %). Similarly, silver trifluoroacetate reacted with S(SiMe3)2 and PR3 (R : Prn, Bun) to give Ag188S94(PR3)30, with the structurally-characterised PPrn3-containing cluster showing disorder of the central Ag and S atoms.35 A series of silver-selenide clusters, Ag4(SePri)4(dppm)2,
Ag8(SeEt)8(dppp)•, Ag28Se6(SeBun)16(dppp)4 and t t Ag124Se57(SePBu 2)4Cl6
Bu 2P(CH2)3PBut212 [dppp : 1,3-bis(diphenylphosphino)propane], has been prepared from reaction of silylated selenide reagents RSeSiMe3 (R : Pri, Et, Bun, But) with silver salts in the presence of dppm or dppp.36 AuCl(PPh3) was reacted with two equivalents of ButP:C(NMe2)2 to give a mixture of [Au
P(But):C(NMe2)22]; and [
(Me2N)2C:(But)PAu2(m-Cl)]; which, in dichloromethane, was slowly converted to the tetracation 9, with a novel ‘puckered ladder’ structure.37

6.2 Heteronuclear High-nuclearity Clusters. — A series of isostructural tetradecanuclear bimetallic Rh-Ni clusters, [Rh13Ni(CO)25]59, [Rh12Ni2(CO)25]49 and [Rh9Ni5(CO)25]3- (10), has been prepared by the reaction of
RhCl(cod)2 with [Ni6(CO)12]29. All complexes contain an interstitial Ni atom, with contraction of the central Rh8 cube, and progressive stabilisation of two MOs, on increasing the number of Ni atoms. Oxidation of 10 afforded isostructural [Rh9Ni5(CO)25]29.38 Isomeric 11 was prepared from the redox condensation reaction between [Rh(CO)4]9 and
PtCl(dppm)2 or from reaction of Rh4(CO)12 with Pt2(CO)3(dppm)2. Reaction of 11 with the labile hexanuclear Rh6(CO)15(NCMe) gave the decanuclear cluster 12, consisting of an octahedral Rh6 unit linked to a tetrahedral Rh2Pt2 fragment by a dative Rh-Rh bond. Cluster 12 was also obtained in lower yield from [Rh7(CO)16]39 or [Rh6(CO)15]29 with
PtCl(dppm)2. The related decanuclear tris(dppm) cluster 13 was formed from Rh6(CO)15(NCMe) with two equivalents of Pt2(CO)3(dppm)2; in this case the fragments are linked via a heterometallic Rh-Pt bond.39 The first high-nuclearity Ni-Ag carbonyl cluster, the tetraanionic [Ni24Ag16(m3CO)4(m-CO)24(CO)12]49, has been prepared in low yields from the reaction of silver acetate with [Ni6(CO)12]29. The 40 metal atom core corresponds to a novel 36-atom Td polyhedron encapsulating four interior atoms.40

9: Organo-transition Metal Cluster Complexes

7

219

Group 6

A high-yield route to the tetranuclear divalent cluster Cr4(m-CH2SiMe3)8 (14) from reaction of CrCl3(thf)3 and three equivalents of Me3SiCH2Li has been reported. Reaction of 14 with ethylene afforded polyethylene in modest yields in the absence of a cocatalyst.41



Desulfurisation of Cr(CO)2(SPMe2)Cp with the radical Cr(CO)3Cp afforded triangular 15 amongst a mixture of compounds, the first example of a homotrimetallic group 6 cluster bearing a bridging phosphido ligand.42 Radicalinitiated bond cleavage by
Cr(CO)3Cp2 has also been explored in the preparation of a number of novel Cr cubane complexes. Thermolysis of Cr(CO)2
SCSN(C6H4)Cp with
Cr(CO)3Cp2 gave a mixture of products including the new cubane clusters 16 and 17. Complex 16 contains a unique corner where the Cr-bound Cp ligand has been replaced by a capping dichromiumtrisulfur moiety, whereas complex 17 consists of a novel benzothiazole bridged dicubane. The reaction occurs via bond cleavage of the metal chelate and thiazole rings.43 The reaction between
Cr(CO)3Cp2 and the tetraalkylthiuram

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Organometallic Chemistry

disulfides
R2NC(S)S2 (R : Me, Et, Pri) at 90 ° afforded double cubanes 18 and 19, linked by a m-h2:h4-dithiooxamide/Cr-Cr bond or by m-h1:h2-dithiocarbamate ligands, respectively.44

Thermolysis of
M(CO)3Cp2 (M : Mo, W) in the presence of grey antimony in a sealed Carius tube afforded 20, the first tetrahedral cluster bearing a single ‘naked’ Sb atom. The bridging fulvalene group was formed from C-H activation of the Cp ligand by the antimony.45 Reaction of W(CO)6 and NaSbO3 gave 21 (R : Me), a hybrid metal carbonyl-oxide cluster and the first carbonyl cluster incorporating a metal trioxide group. Protonation with acetic acid gave 21 (R : H).46

9: Organo-transition Metal Cluster Complexes

8

221

Group 7

8.1 Rhenium. — Reaction of the unsaturated cluster anion [Re3(m3-H)(mH)3(CO)9]9 (44 valence electrons) with pyrazole afforded anionic [Re3(mH)4(CO)9(Hpz)]9 (22), containing a terminally bonded pyrazole ligand showing weak hydrogen bonding to one of the hydrides bridging the opposite face of the cluster. The complex disproportionated at room temperature to give the starting cluster and the known [Re3(m-H)3(m-h2-pz)(CO)9(Hpz)]9, and ultimately 23, containing a m3-h2-pyrazolato ligand.47

The lability of the nitrile ligand in octahedral hexarhenium cluster anions has been exploited in the preparation of a number of novel cluster-containing macromolecules and metallodendrimers. Reaction of [Re6(m3Se)8(PEt3)5(NCMe)]2; with one equivalent of 2,4,6-tri-4-pyridyl-1,3,5-triazine and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine afforded star-shaped tri- (24) and tetra- (25) cluster-containing complexes, respectively, containing the metal clusters coordinated to a central organic core.48

The first generation metallodendrimers 26 were prepared by reaction of [Re6(m3-Se)8(NCMe)6]2; with an excess of [Re6(m3-Se)8(PEt3)5(L)]2; [L : 1,2-bis(4-pyridyl)ethane, (E)-1,2-bis(4-pyridyl)ethene], to give a central

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[Re6(m3-Se)8]2; core surrounded by six octahedrally disposed [Re6(m3Se)8(PEt3)5(L)]2; cluster units.49

Addition of a methanol solution of Cd(NO3)2 to cis-[Re6(m3-Se)8(PPh3)4(4,4dipyridyl)2]2; in CH2Cl2, followed by vapour diffusion of diethyl ether into the mixture, gave 27, a one-dimensional chain of squares composed of an octahedrally coordinated Cd2; atom at the shared corner, and a [Re6(m3-Se)8]2; cluster unit on the unshared corner. A similar reaction with excess Cd(NO3)2 afforded 28, consisting of a zigzag chain coordination with the Cd2; atom in a distorted trigonal bipyramidal geometry. Complex 28 is related to 27 by replacement of a rhenium cluster by a nitrato ligand.50

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Reaction of Re4Te4(TeCl2)4Cl8, KSeCN and NH4Cl in acetonitrile at 200 °C over 3 days afforded [Re6(m3-Se)8(CN)6]49, demonstrating an unusual conversion from a tetrahedrally coordinated Re in the cubane structure of the starting cluster to the octahedral core of the product.51 Regioselective substitution of the iodo ligands in cis- and trans-[Re6(m3-Se)8I2(PEt3)4] and in [Re6(m3-Se)8I(PEt3)5]; with AgCN has afforded cis- and trans-[Re6(m3-Se)8(CN)2(PEt3)4] and [Re6(m3Se)8(CN)(PEt3)5];, respectively.52 The isomorphous bimetallic linked complexes [Re6(m3-Se)8(CN)5(CNX)]29 [X : Ni(H2O)5, Mn(H2O)4] were prepared from the reaction of [Re6(m3-Se)8(CN)6]49 with Ni(OAc)2 and MnSO4, respectively, in aqueous solution.53

9

Group 8

9.1 Iron. — The application of the ligand polyhedral model to describe ligand fluxionality on Fe3(CO)12 has been elaborated by Johnson, with the model involving a rotational symmetry operation of the CO ligand icosahedron being rejected in favour of a libration of the iron triangle.54 9.1.1 Trinuclear Clusters. C-ligands. Oligomerisation of 2-methyl-1-buten-3-yne with Fe3(CO)12 gave a mixture of products including two open structured linkedtriiron complexes 29 and 30, containing isomeric carbon chains, and Fe2(CO)5
C15H18(CO) where the organic ligand is formed from three alkyne molecules.55 Reaction of Fe3(m3-CF)2(CO)9 with H2C:C:CF2 proceeded via C-C coupling and fluorine migration to give 31 and 32.56

Cyclic imides have been produced in good yield from the reaction of a series of alkynes with iron carbonyl complexes prepared from Fe3(CO)12-amine reagent systems.57

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Group 15 ligands. [Fe3(m3-Y)(CO)9]29 (Y : Se, Te) reacted with MesSbBr2 (Mes : mesityl) to give trinuclear clusters 33 (Y : Se, Te); the linked triiron complex 34 was obtained by a modified reaction procedure, and contains an unusual Se-Sb bond.58

9.1.2 Tetranuclear Clusters. The tetrairon butterfly clusters Fe4(m4-h2HC2H)2Cp4 (Cp : Cp, C5H4Me) underwent one- or two-electron oxidation in air or chemical oxidation with silver salts (for C5H4Me), respectively; the butterfly geometry was retained in each case.59 A kinetic study of the reaction between [Fe4S4Cl4]29 and ButNC to form Fe4S4Cl2(CNBut)6 in the presence of acid has shown that binding of the ButNC is slower than initial proton transfer to the cluster, but faster than the diprotonation step.60 9.2 Ruthenium. — 9.2.1 Trinuclear Clusters. C-ligands. The reactivity of the triruthenium cluster anion [Ru3(m-Cl)(CO)10]9 towards butyne derivatives MeC∫CCH2OR (R : tosyl) and ROCH2C∫CCH2OR (R : H, acetyl, tosyl) has been shown to be highly dependent on the nature of the leaving group. Reaction of 1-(tosyloxy)but-2-yne was expected to give anionic [Ru3(m-h2MeC2CH2OTs)(m-Cl)(CO)9]9, by analogy with the known product from reaction with propargyl alcohol; in this case, however, the neutral allenyl complex 35 was obtained via substitution of the tosyl group of the acetylene by the nucleophilic [Ru3(m-Cl)(CO)10]9. Reaction of 35 with two equivalents of K-selectride formed anionic [Ru3(m-H)(m-MeC2Me)(CO)9]9,which was protonated to the neutral dihydride Ru3(m-H)2(m-MeC2Me)(CO)9. Reaction with the bisoxypropargylic acetylenes (R : H, acetyl) afforded anionic complexes 36, the related neutral complexes 37 being obtained by protonation of 36 with HBF4, and 38 from the protonation of the hydroxyallenyl complex 37 (R : H). In contrast, the bistosyloxy acetylene afforded a putative butatriene complex, detected by NMR spectroscopy.61 Reaction of M3(CO)10(NCMe)2 (M : Ru, Os) with the fluorescent-labelled acetylenes RCH2C∫CH [R : 5-(dimethylamino)naphthalene-1-sulfonyl, acridone] afforded M3(m3-h2-HC2CH2R)(m-CO)(CO)9; a strong quenching of the fluorescence intensity was observed on coordination.62 Ru3(CO)10(NCMe)2 and Ru3(m-dppm)(CO)10 have been reacted with a series of ferrocenylalkynes HC∫CFc, HC∫CC∫CFc and FcC∫CC∫CFc, to give a series of triruthenium

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225

complexes containing m3-h2-alkyne, and a previously described head-to-tail dimerised 1,3-diyne, ligands.63 A series of tri-ruthenium and osmium complexes containing parallel m3-h2-( )substituted alkynes has been prepared from reactions with HC∫CR (R : SiMe3, SiPh3, But); the structures of M3(m-H)(m3-h2-C2R)(CO)9 (M : Ru, R : SiMe3, SiPh3; M : Os, R : SiPh3, But) and Ru3(m3-h2-HC2But)(CO)10 were reported.64 Reaction of Ru3(m3-H)2(m-H)3Cp*3 with acetylene at 930 °C afforded the bis(mvinyl) complex 39, which forms the m-ethylidene complex 40 on warming to 0 °C. Thermolysis afforded a series of rearrangements of the C2 unit from m-ethylidene (40) to m3-ethylidyne (41) and m3-vinylidene (42). Reaction of 41 with acetylene resulted in C-C bond formation to give 43.65

Thermolysis of Ru3(m3-C:C:CR2)(m-dppm)(m-CO)(CO)7 (R : Ph, pC6H4Me) in toluene gave 44 and 45 (R : H, Me); conversion of 44 to the more symmetric 45 was achieved by further thermolysis. Reaction of Ru3(m-H)(m3C:C:CPh2)(m-OH)(CO)9 with Co2(CO)8 under similar conditions afforded the tetrahedral mixed-metal cluster 46, containing a face-capping indenyl ligand, and four bridging CO ligands bound to the Co atom.66 Facile C-C bond formation is characteristic of reactions between Ru3(m-H)(m3C:C:CPh2)(m-OH)(CO)9 and a series of alkynes and 1,3-diynes. Insertion of the first alkyne into the Ru-C a bond may be followed by isomerisation in the

226

Organometallic Chemistry

case of bulky alkynes, and, in some cases, bis-insertion products were formed with concomitant elimination of water. Thus, reaction with HC∫CSiMe3 afforded both mono- and bis-insertion products 47 (R : SiMe3) and 48 (R : R : SiMe3, R : H), whereas reaction with HC∫CPh gave solely isomers of 48 (R : H; R : R : Ph). Treatment with HC∫CR (R : ferrocenyl : Fc) also resulted in bis-insertion, in this case to give the metallabenzene cluster 49, whereas reaction with PhC∫CPh and the 1,3-diynes RC∫CC∫CR (R : SiMe3, Fc) resulted in mono-insertion to give the allenyl clusters 50; isomeric 47 was also isolated as the first-formed product in the reaction with Me3SiC∫CC∫CSiMe3.67

The preparation and thermolysis products of the diphosphinoalkyne clusters Ru3(CO)10(Ph2PC∫CR)(Ph2PC∫CR) (R : R : Ph, But; R : Ph, R : But) have

9: Organo-transition Metal Cluster Complexes

227

been reported. Thermolysis of Ru3(CO)10(Ph2PC∫CBut)2 in hexane afforded the 48 electron cluster 51 which reversibly added CO or PPh3 to form 50-electron 52 (L : CO, PPh3), whereas heating Ru3(CO)10(Ph2PC∫CPh)(Ph2PC∫CR) (R : Ph, But) in thf afforded a mixture of products including 53 and, for R : Ph, complex 54, resulting from cleavage of a single phosphinoacetylene P-C bond. Complexes 53 were found to readily convert to the known diynes Ru3(m-h2PhC2C∫CR)(m-PPh2)2(CO)7 on thermolysis.68

The 46-electron silylruthenium cluster 55 has been prepared from the oxidative addition of PhMe2SiH to Ru3(m3-h5,h5-4,6,8-trimethylazulene)(CO)7. A comparison of the hydrosilylation activity of 55 and some di-, tri- and tetraruthenium clusters with coordinated 4,6,8-trimethylazulene has shown that 55 has the highest activity and that cluster integrity is maintained during catalysis.69

Group 15 ligands. A review of the chemistry of triruthenium clusters with 2aminopyridines has been published, including a comparison of reactions of

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Organometallic Chemistry

triruthenium clusters with 2-amino, 2-mercapto and 2-hydroxypyridines.194 The reaction of di(2-pyridyl)amine (Hdpa) with Ru3(CO)12 and M3(CO)10(NCMe)2 (M : Ru, Os) afforded a series of products containing the dpa ligand in a number of different coordination modes (56 - 58), including a novel face-capped cluster.71

Reaction of 2-amino-7,8-benzoquinoline (H2abq) with M3(CO)10(NCMe)2 (M : Ru, Os) resulted in the stepwise formation of cyclometalated products in the ruthenium case (59 — 60), and an amido-bridged complex (61) for the osmium cluster.72

The triruthenium ynenyl complex 62 has been successfully prepared from the reaction of Ru3(m-H)(m3-h2-apyr)(CO)9 (Hapyr : 2-aminopyrimidine) and 2,4hexadiyne. Reaction of 62 with further 2,4-hexadiyne gave 63, containing a 5-electron diynedienyl ligand, whereas reaction with diphenylbutadiyne resulted in coupling of two reagent molecules to form a novel diynetetraentriyl ligand (64).73 The phosphine substitution reactions (PPh3 and dppm) of neutral and cationic dimethylhydrazine derivatives Ru3(m-H)(m3-HNNMe2)(CO)9 and [Ru3(m-H)2(m3-HNNMe2)(CO)9]; suggest that reactivity in these clusters is determined predominantly by the electronic, rather than steric, properties.74 The series Ru3(m-dppm)(CO)9(PR3) (R : Et, Ph, Cy, Pri) has been prepared from Ru3(m-dppm)(CO)10 and the corresponding tertiary phosphines; a lengthening of the Ru-Ru bond adjacent to the introduced phosphine ligand was observed.75 The reactivity of Ru3(CO)10(NCMe)2 and M3(CO)12 (M : Fe, Ru) towards cyclic arsines and phosphines has been examined. While reaction with the labile complex Ru3(CO)10(NCMe)2 afforded products containing the intact ring,

9: Organo-transition Metal Cluster Complexes

229

reaction with carbonyl clusters M3(CO)12 (M : Fe, Ru) gave a series of complexes containing ligands formed from ring fragmention, including 65 — 68 and the mixed arsenidene-phosphinidene cluster Fe3(m3-AsPh)(m3-PPh)(CO)9.76

Diselenido clusters Ru3(m3-Se)2(CO)7(PRR2)2 (R : Ph, R : Ph, CH2Ph; R : R : p-C6H4OMe) have been prepared by stepwise reaction of Ru3(CO)10(NCMe)2 or Ru3(CO)12 with phosphine selenides R2RPSe; the crystallographically-characterised Ru3(m3-Se)(m3-CO)(CO)7(PPh3)2 represents the first triruthenium complex capped by selenium and a carbonyl ligand.77 Group 16 Ligands. Facile C-S bond activation, under ambient conditions, has been observed in the reaction between Ru3(CO)12 and levamisole hydrochloride (levamisole : (-)-2,3,5,6-tetrahydro-6-phenylimidazo[2,1-b]thiazole) to give 69.78 Reaction of Ru3(m-H)3(m-PCy2)3(m-dppm)(CO)4 with CS2 at room tempera-

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Organometallic Chemistry

ture gave the 50-electron open cluster 70, containing an unusual m3-h2-C2S3 mercaptocarbyne ligand.79 Ruthenium sulfide clusters 71 — 73 were prepared from the reaction of RuCl2(PPh3)3 with NaSH; 72 represents a rare PPh3-stabilised Chevrel cluster.80

9.2.2 Tetranuclear Clusters. A route into the rational synthesis of tetraruthenium polyhydride clusters,
Ru(h5-C5R5)4H6 (R5 : H5, H4Me, 1,3H3Me2), has been developed from the reaction of RuIV precursors, RuCl2(h3C3H5)(h5-C5R5), with LiAlH4 followed by workup in ethanol. The method was also applied to the preparation of analogues containing mixed Cp and h5-C5Me5 (Cp*) ligands, Ru4H6Cp39nCp*1;n (n : 0-2).81 H4Ru4(CO)12 reacted with 1-penten-3-yne to give 74 — 76, produced via dimerisation and trimerisation at the enynyl triple bond.82

The open-framework complexes Ru4(m4-C2)(m-CO)2(CO)8(h-C5H4R)2 (R : H, Me) have been reacted with C2(CO2Me)2 to give 77 along with spectroscopicallyidentified Ru4
C2C(CO2Me)C(CO2Me)(CO)9(h-C5H4R)2.83 The tetranuclear benzyne cluster 78 was produced from thermolysis of Ru3(m-dppe)(CO)10 in toluene and contains an unusual m4-h2-PCH2CH2PPh2 unit; Ru3(m-dfppe)(CO)10 [dfppe : 1,2-bis
di(pentafluorophenyl)phosphinoethane] proved stable under similar conditions.193 9.2.3 Penta- and Hexanuclear Clusters. The electrochemical properties of pentaruthenium-C60 clusters, Ru5(m5-C)(m3-h2:h2:h2-C60)(CO)10L2 [L2 : (CO)(PPh3),

9: Organo-transition Metal Cluster Complexes

231

(m-h:h-dppf)] [dppf : 1,1-bis(diphenylphosphino)ferrocene] and the mixedmetal Ru5Pt(m5-C)(m3-h2:h2:h2-C60)(CO)11(h2-dppe) have been examined, including chemical reduction with cobaltocene to afford spectroscopically-characterised dianionic species.84 Addition of SnHPh3 to Ru5(m5-C)(CO)12(h6-C6H6) afforded bimetallic 79 and 80.85

Treatment of Ru5(m4-PNPri2)(CO)15 with [ppn][NO2] afforded anionic 81, which reacted with acids via N-O bond cleavage to form the nitrene cluster 82 when treated with HOSO2CF3, or nitride complexes 83 and 84 with MeOSO2CF3 or HBF4, respectively. Complexes 81 and 82 are the first mixed nitrosyl-phosphinidene and nitrene-phosphinidene clusters.86, 87 9.2.4 Ruthenium Clusters in Catalysis. The triruthenium cluster cation [Ru3(mH)2(m3-O)(m-OH)(h6-C6H6)(h6-C6Me6)]; has been found to have an unusually high catalytic activity for the hydrogenation of benzene under biphasic conditions. NMR studies of the hydrogenation of ethylbenzene have shown no substitution of cluster-coordinated benzene or hexamethylbenzene by substrate, suggesting that the substrate does not coordinate to the catalyst but is instead held in a host-guest association in the hydrophobic ‘pocket’ formed by the three h6-coordinated arene ligands. The possibility of supramolecular catalysis is supported by the observed lack of catalytic activity towards bulky olefins which would be too large to fit into the ‘pocket’.88 9.2.5 Surface-bound Ruthenium Carbonyl Clusters. Ru3(CO)12 has been supported on a variety of inorganic oxides and reacted with a series of alkynes;

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Organometallic Chemistry

comparison of reaction products with those obtained from a similar reaction in methanol/KOH mixtures suggests that, in this case, the intermediates are not anionic species.89 TiO2-supported [Ru6(m6-C)(CO)16]2- has been used as a catalyst for the reduction of SO2 with H2; the active form has been shown to consist of amorphous ruthenium sulfide particles.90 9.3 Osmium. — A structural and spectroscopic study of the triosmium complex Os3H(m-H)(CO)10(NH:CPh2) (85) suggested the presence of intramolecular hydrogen bonding between the imine proton and terminal hydride ligand. DFT calculations indicated a bonding energy of 2.6 kcal/mol and charge distributions indicative of a significant electrostatic component in the hydrogen bond.91 In related work, the effect of ligand basicity on the dihydrogen bonding in the series Os3H(m-H)(CO)10(amine) was determined by correlating pKb values with H . . . H distances using the T1 values of the hydride NMR resonances. As expected, the more basic the amine ligand, the stronger the dihydrogen interaction, confirming the electrostatic nature of the bond as predicted by DFT.92

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233

9.3.1 Trinuclear Clusters. C-ligands. Reactions between Os3(CO)11(NCMe) and 1,4-bis(ferrocenyl)butadiyne have yielded the new clusters 86 and 87. Complex 86 contains a unique linear arrangement of the diyne parallel to the Os3 chain; the C-C bond lengths are equal within experimental error. Electrochemical analysis indicated some electronic communication between the ferrocenyl groups. Thermolysis of 87 at 97 °C afforded 88, formed via scission of the diyne to give two bridging ferrocenylacetylide ligands. Reaction of Os3(CO)10(NCMe)2 with 1,4bis(ferrocenyl)butadiyne resulted in cluster fragmentation to give the binuclear 89, resulting from coupling of two diyne molecules to form a ferrole-type structure.93

The longer chain ferrocenylpolyyne, 1,12-bis(ferrocenyl)-1, 3, 5, 7, 9, 11dodecahexayne was prepared by in situ coupling of the triyne FcC∫CC∫CC∫CH, and reacted with Os3(CO)11(NCMe) to give the hexaosmium linked cluster 90; voltammetric studies showed no electronic communication between the ferrocenyl groups.94

Os3(m-H)2(CO)10 reacted with 1,8-bis(ferrocenyl)octatetrayne to give 91 and a minor amount of 92, consisting of a heterobicyclic ring formed from cyclization of the tetrayne chain with a CO ligand. Complex 91 was reacted with a further equivalent of the precursor cluster to give 93, where two osmium triangles are linked via the two inner alkyne groups of the starting cluster.95 Self assembled monolayers of the triosmium-fullerene complex Os3(m3h2:h2:h2-C60)(CO)8
CN(CH2)3Si(OEt)3 on indium tin oxide and gold electrodes have been shown to exhibit similar electrochemical behaviour to the cluster in solution. The layered species proved remarkably stable, with the first four-

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electron reduction of C60 in fullerene-based SAMs demonstrated for each example.96 The conversion of the p-type C60 ligand in Os3(m3-h2:h2:h2-C60)(CO)9 to a s-type m3-h1:h2:h1 ligand by insertion of isocyanide ligands has been elaborated. Photolysis in the presence of benzyl isocyanide gave the s-bound insertion product Os3(m3-h1:h2:h1-C60)(m3-CNCH2Ph)(CO)9, whereas chemical reaction gave the substituted complex Os3(m3-h2:h2:h2-C60)(CNCH2Ph)(CO)8 which could be reacted further to give s-bound isomers of Os3(m3-h1:h2:h1-C60)(m3CNCH2Ph)(CNCH2Ph)(CO)8.97 The reactivity of the s-bound complex Os3(m3h1:h2:h1-C60)(m3-CNCH2Ph)(CNCH2Ph)(CO)8 has been examined; decarbonylation followed by reaction with a variety of 2-electron donor ligands afforded the series Os3(m3-h1:h2:h1-C60)(m3-CNCH2Ph)(CNCH2Ph)(CO)7(L) [L : (m-H)2, CNR, PMe3, PPh3]. X-ray crystallographic studies of these clusters showed that the bulky PPh3 is unique in coordinating to the least hindered osmium atom, resulting in a reorganisation of the C60 bonding from m3-h1:h2:h1-C60 (1,4-s-type) to m3-h1:h1:h2-C60 (1,2-s-type).98, 99 The first-formed product in the reaction between the unsaturated clusters Os3(m-H)(m3-h2-C7H3-2-R-NS)(CO)9 (R : H, Me) and diazomethane was found to be dependent on the nature of the heterocyclic ring substituent. For R : Me, the reaction afforded 94 via insertion of a methylene group into a M-M bond and formation of a methyl ligand from the cluster bound hydride ligand. Complex 94 is the first triosmium cluster bearing both a bridging methylene and a s-bonded methyl group. Thermolysis of 94 gave 96 (R : Me) via the intermediate 95 (R : Me). In contrast, the diazomethane reaction of Os3(m-H)(m3-h2-C7H4NS)(CO)9 afforded 95 (R : H) as the initial adduct, where the methylene has inserted into a M-C s-bond of the heterocyclic ligand; thermolysis of 95 (R : H) gave 96 (R : H).100 The triosmium carbene complex Os3(m-H)(m-h1:h1-OC4H2CMe)(CO)10 has been reacted with a series of aromatic aldehydes under mild conditions to give condensation products 97 via C-H bond activation of the methyl group a to the carbene carbon atom. In the case of benzaldehyde substrate (X : H, Y : H), reaction occured in the absence of catalyst at room temperature, with the substituted substrates requiring only the presence of a weak base, suggesting a unique ‘enhanced carbene’ activation of the organic ligand of the starting cluster.101

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Group 15 ligands. The bipyridyl clusters 98 and 99 have been prepared from the lightly stabilised Os3(CO)10(NCMe)2. Complex 98 was reacted with metalloligands W(CO)5(thf ) and ReCl(CO)5 to give 100 and 101; conversely, 101 could be formed directly from the acetonitrile-containing starting cluster.102

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An extensive study of the chemistry of the electron deficient triosmium clusters Os3(m-H)(m3-h2-L-H)(CO)9 (L : various benzoheterocycles) towards some twoelectron donors and protic acids has revealed differences in reactivity that are dependent on the nature of the coordinated heterocycle, despite similarities in their bonding modes.103 Reactions of Os3(CO)11(NCMe) or Os3(m-H)2(CO)10 with 1,2,3,4-tetraphenyl1,2,3,4-tetraphospholane has led to a series of trinuclear and linked-trinuclear complexes with the phospholane ligands in a number of different coordination modes (102 — 107).104

The biphosphinyl complex 108 was prepared by phosphine substitution on Os3(m2-h2-tmbp)(CO)10 (tmbp : 4,4,5,5-tetramethyl-2,2-biphosphinine); DFT calculations and redox studies were reported for both complexes.105

The reactions of the bimetallic complex Os3(m-H)(m3-h2-C6H4)(m-SbPh2)(CO)9 with alkenes and phosphines have been investigated. Treatment with a variety of alkenes and dienes [RHC:CHR (R : H, Me); PhHC:CHCH:CHPh,

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H2C(Me)CH:CH2] proceeds with loss of an Os(CO)nL fragment and cluster condensation to give the novel pentaosmium complexes 109 (L : CO) and 110. Complex 109 (L : CO) reacted with EPh3 (E : P, As, Sb) to give monosubstituted derivatives, with 109 (L : PPh3) the structurally characterised example.106

Os3(m-H)(m3-h2-C6H4)(m-SbPh2)(CO)9 was also treated with PPh3 and PMe2Ph; reaction occured readily at room temperature to give a mixture of products including two isomers of the addition product 111, complex 112 formed via decarbonylation of 111, the triangular cluster 113, and the orthometalated product 114 (in the PPh3 case only). Complex 114 could be derived from 111 (R : Ph) via reaction with PPh3. Formation of 115 from reaction of 111 (R : p-tolyl) with PPh3 showed that orthometalation occurs via loss of the o-phenylene group rather than loss of C6H6 from the coordinated PPh3.107

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Group 16 and 17 ligands. A study into the geometry of triosmium amine complexes Os3(m-H)2(m3-S)(CO)8L (L : amine) has shown the presence of isomers resulting from a slow turnstile rotation of the Os(CO)2L group. The chiral complex Os3(m-H)2(m3-S)(CO)8
(S)-PhCHMeNH2) was thus prepared as two stereochemically rigid diastereomers which could be separated by HPLC; the complexes showed very slow epimerisation at 90 °C.108 Triosmium sulfido complexs 116, and isomeric 117 - 118 were prepared from reaction of Os3(m-dppm)(CO)10 with H2S and tetramethylthiourea, respectively. Complex 116 was shown to be an intermediate to 117 – 118 by reaction with H2S.109

2-Mercapto-1-methylimidazole has been reacted with Os3(CO)10(NCMe)2 and Ru3(CO)12 to give 119 and 120, respectively; 1H NMR data for 120 indicated hydride ligand migration. Similar reactions with Os3(m-dppm)(CO)10 or the orthometalated complex Os3(m-H)
Ph2PCH2P(Ph)C6H4(CO)8 gave 121.110

9.3.2 Higher-nuclearity Osmium Clusters. The linear clusters Os4(X)2(CNBut)(CO)15 (X : Cl, Br, I) and Os5(X)2(CNBut)2(CO)18 [X : Cl, Br, I (122)] have been prepared by addition of Os(CNBut)(CO)4 to Os3X2(CO)12 at 60 °C. The complexes contain unbridged dative Os-Os bonds, and are remarkably stable, with Os-Os bond distances in 122 shorter than those observed in Os3 chains.111

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9.4 Mixed-metal Clusters Containing Only Group 8 Metals. — CO ligand substitution on Fe2Ru(CO)12 and FeRu2(CO)12 with CNCy and CNBut have afforded the series Fe2Ru(CO)12-n(CNBut)n (n : 1, 2), FeRu2(CO)12-n(CNBut)n (n : 1, 2) (all of which were structurally characterised), and FeRu2(CO)11(CNCy).112

10

Group 9

10.1 Cobalt. — The first tetracobalt acetylide cluster with a spiked triangular geometry (123) was prepared from reaction of Co4(CO)12 with methyldipropargylamine [(HC2CMe2)2NMe] via partial cyclisation of the amine and hydrogen transfer.113 Reaction of propiolic acid with Co4(CO)12 gave the butterfly cluster 124. Comparison with the smaller organometallic carboxylic acid Co2(m3-h2HC2CO2H)(CO)6 showed a decrease in acidity as the complex nuclearity increased.114

The reactions between Co4(CO)12 and a variety of arylsilanes were carried out at 50°C for between four days and two months to give the series of tetracobalt silyl clusters 125; yields varied from 86 % in the case of the diphenylsilyl cluster (R : R : Ph) to 8 — 30 % for the non-symmetric examples. The linked complexes 126 were prepared from a similar reaction using a 2:1 ratio of capping silane : difunctional linking silane. Electrochemical studies showed the metal centres were acting independently.115

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A new boat-shaped hexacobalt carbonyl cluster containing a semi-interstitial phosphorus atom, 127 (L : CO), has been prepared from Co2(CO)8 and ClPSCH2CH2CH2S. Addition of PPh3 or P(OMe3) to 127 (L : CO), or to the related Co6(m6-P)
m-PS(CH2)2S
m-S(CH2)2S(CO)12, afforded mono- or bisligand substituted products, including 127 (L : PPh3).116

10.2 Rhodium. — Carbonyl ligand substitution on the pentarhodium cluster anion [Rh5(CO)15]9 has been reported for a number of primary and secondary amines. The mono- and bis- substituted products represent rare examples of carbonyl substitution by alkylamines on homoleptic clusters. Structured examples include the bis-monosubstituted linked cluster dianion [
Rh5(CO)142H2N(CH2)4NH2]29, and the disubstituted chelate complex 128.117 The lightly stabilised hexarhodium cluster Rh6(CO)15(NCMe) reacted with tripyrrolylphosphine to form 129, via the spectroscopically-characterized intermediate Rh6(CO)15
h1-P(NC4H4)3. The formation of complex 129 is the first example of a coordinated pyrrolyl group displacing a CO ligand to form an orthometalated pyrrole ring; reaction kinetics show that the Rh-C bond is comparable in strength to the Rh-CO bond.118 The first fullerene-cluster sandwich compound, Rh6(m3-h2:h2:h2-C60)2(m-

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dppm)2(CNCH2Ph)(CO)5 was formed in two steps from the reaction of Rh6(m3h2:h2:h2-C60)(m-dppm)2(CO)9 with fullerene, and addition of CNCH2Ph to the resulting, marginally soluble, product. The structural study showed that the two C60 ligands are in different coordination environments, with one C60-coordinated metal face bonded to a dppm-P atom and an isocyanide ligand, and the other linked to two phosphine-P atoms. Interestingly, the apical Rh atom is bound to both of the C60 ligands, linking the two cages via a single metal atom. Electrochemical studies showed pairwise addition of 6 electrons to the two C60 centres, suggesting electronic communication via the cluster unit.119 GaCp* was reacted with Rh6(CO)15(NCMe) to give a mixture of mono- and bis- substituted complexes Rh6(m3-CO)4 — n(m3-GaCp*)n(CO)12 (n : 1, 2), and with the parent cluster Rh6(CO)12 to give the tri- and tetra- substituted derivatives Rh6(m3-CO)4 — n(m3-GaCp*)n(CO)12 (n : 3, 4). Structural characterisation of the n : 1 — 3 derivatives show that the GaCp* have displaced the face-bridging CO ligands of the starting clusters, with spectral characterisation of the tetra-substituted product suggesting complete substitution to give the unique Rh6(m3GaCp*)4(CO)12 complex.120 10.3 Iridium. — Protonation of the triangular cluster Ir3(m-CO)3(h5-C9H7)3 with HBF4 gave [Ir3(m-H)(CO)3(h5-C9H7)3];; spectroscopic characterisation suggested that the complex cation contains terminal CO ligands and Cs-symmetry. Deprotonation with NEt3 afforded the spectroscopically-characterised CsIr3(CO)3(h5-C9H7)3, with conversion to an equilibrium mixture, containing predominantly the bridging-CO C3v complex as the main component, occuring quickly at room temperature.121 A series of phosphine and di-phosphine tetrairidium clusters derived from Ir4(m- H)(m-PPh2)(CO)10 have been prepared. Spectroscopic data were used to determine structural geometry, and assignments confirmed by the structurallycharacterised examples 130 - 132.122

The reaction of Ir6(CO)16 with cycloheptatriene afforded Ir6(m3-h2:h2:h2C7H8)(m-CO)4(CO)9; the anionic and dianionic derivatives [Ir6(m3-h2:h3:h2C7H7)(m-CO)3(CO)9]— and [Ir6(m3-h2:h2:h2-C7H8)(m-CO)3(CO)9]29 were prepared by subsequent reaction with Na2CO3 and NaOH, respectively. The interconversion of the three complexes was studied electrochemically.123 10.4 Group 9 Mixed Metal Clusters. — Co-Rh mixed-metal clusters,

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Co2Rh2(CO)12 and Co3Rh(CO)12, reacted with 1-alkynes HC∫CR [R : Fe(h5C5H4)Cp, CH2OH, (MeO)C10H6CH(Me)CO2CH2] via insertion into the Co-Co bond to give the series of butterfly complexes 133.124

10.5 Group 9 Clusters as Catalysts. — The catalysis of the cyclotrimerisation of oct-4-yne to hexa(n-propyl)benzene by series of tricobalt alkylidyne clusters, Co3(m3-CR)(CO)9 (R : H, Me, Ph, CO2Et, Cl, Br) and
Co3(CO)92(m3-C)2, was found to be dependent on the alkylidyne substitutent, with the thermally unstable clusters (R : H, Br) and
Co3(CO)92(m3-C)2 giving the best yields. Cyclisation of PhC∫CH using Co3(m3-CH)(CO)9 at lower temperatures allowed recovery of the intact cluster catalyst, showing that degradation products were not involved in the catalytic process. Cyclisation of a number of diynes and triynes gave good yields of regioselective products, suggesting that intramolecular cyclotrimerisation of alkynes is faster than the intramolecular Pauson-Khand reaction or intermolecular cyclotrimerisation.125 Ir4(CO)12 was found to catalyse the coupling reaction of imidazoles with aldehydes in the presence of a hydrosilane as a co-reactant, to give 2-alkyl imidazoles. Several other metal clusters were found to have no effect [M3(CO)12, M : Fe, Ru, Os; Rh4(CO)12]. Addition of C2(CO2Me)2 (DMAD) as a hydrogen acceptor was found to increase yields of the desired product from 34 to 94 %, although no DMAD hydrogenation products were found.126

11

Group 10

11.1 Nickel. — Addition of halide abstractor TlOTf to a solution of Ni3(m3-I)2(mdppm)3 resulted in thallium insertion into a Ni-I bond to give 134. The 46 electron cluster is unstable in solvents other than acetonitrile, and quickly formed the paramagnetic [Ni3(m3-I)2(m-dppm)3]; via loss of Tl.127 The first tincapped trinickel cluster, 135, was formed from the reaction of Ni3(m3-I)2(m-dppm)3 with NaSnCl3; subsequent reaction of 135 with TlPF6 afforded the dichlorostannylene derivative 136.128 11.2 Palladium. — Reduction of [Pd4H(m-dppm)4]2; with BPh49 afforded the 56 electron 137; crystallographic studies showed that the complex has a novel symmetric square geometry.129 Thermolysis of B8H12Pd(PPh3)2 in toluene gave the twenty-vertex B16H17Pd4(m-Cl)(PPh3)5 (138). The core can be considered as a triple-cluster structure composed of a nido-five vertex Pd2B3 unit that has oppos-

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ing triangular faces in common with an arachno ten-vertex Pd2B8 unit and a nido eleven-vertex Pd3B8 unit.130 The two-electron reduction of cationic [Pd3(m-dppm)3(I)(CO)]; has been found to proceed via two simultaneous pathways (EEC and ECE) involving loss of iodide to give the neutral Pd3(m-dppm)3(CO). Manipulation of temperature, solvent polarity, sweep rate and iodide concentration allowed control over the specific reduction process observed.131 11.3 Platinum. — Mononuclear Pt(h2-C2H4)(PPh3)2 was treated with silafluorene (H2SiC12H8) to give the trinuclear complex 139, the first triplatinum complex containing bridging silylene units formed from mononuclear precursors.132

Oxidation of Pt3H(m-PBut2)3(CO)2 with [Cp2Fe]; resulted in loss of the hydride ligand, providing a vacant coordination site and a route into new derivatives. Thus, oxidation in the presence of CO, acetonitrile or ethylene gave cationic 140. A series of neutral derivatives, 141, was prepared by substituting the CO ligand on Pt3H(m-PBut2)3(CO)2 with a series of isocyanides. Cluster 141 (R : CNBut) was treated with CHCl3 to form Pt3(m-PBut2)3(Cl)(CNBut)2, which reacted with excess CNBut to give [Pt3(m-PBut2)3(CNBut)3];.133 A rational route into 42 valence electron platinum-containing triangular clusters has been developed; condensation of the phosphine-linked dinuclear complex (C6F5)2Pt(m-PPh2)2Pt(PPh2R)2 (R : Ph, Me, Et) with the platinum(0) compound Pt(C7H10)3 gave 142, containing three Pt atoms in the mixed oxidation states (II, I, I), and an unusual bridging pentafluorophenyl ligand. Reaction of 142 (R : Ph) with silver(I) reagents AgClO4 and Ag(OClO3)(PPh3) gave heteronuclear 143 and 144, respectively.134

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The triplatinum clusters Pt3(m-CO)3(PR3)3 reacted with CF3C∫CCF3 to give diplatinum(0) compounds Pt2(m-h2,h2-CF3C∫CCF3)(CO)2(PR3)2 (PR3 : PPh3, PBzPh2, PCy3, PPri3).135

12

Group 11

12.1 Copper. — The tricopper diferrocenylacetylide complex 145 was prepared from [Cu2(NCMe)2(dppm)2]2; and ethynylferrocene in the presence of KOH. In contrast, a similar reaction with Ag(dppm)2(CF3SO3)2 gave only the monoacetylide product 146. Electrochemical studies of 145 showed two oxidation processes, attributable to ferrocenyl oxidation.136

The tricopper gold acetylide complex 147 was isolated from a copper(I)catalysed reaction between Au(C∫CC∫CH)2(m-dppm) and (AuCl)2(m-dppm); attempts at a rational synthesis by reacting [Cu3(m3-I)2(m-dppm)3]; with [Au(C∫CC∫CH)2]; gave the linked cationic cluster 148.137

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The luminescent tetracopper cuboidal diynyl complexes Cu4(m3-h1C∫CC∫CR)4(PAr3)4 (R : Ph, Ar : Ph, p-C6H4Me, p-C6H4F; R : p-C6H4Me, p-C6H4OMe, Ar : Ph) were prepared from the reaction of [Cu(NCMe)4]; and the appropriate diyne and phosphine in the presence of KOH.138 The preparation and optical properties of trinuclear cluster iodides M3(m3-I)2(m-dppm)3.I (M : Cu, Ag) have been reported.139 12.2 Silver. — Tetranuclear silver(I) acetylides Ag4
m-(C∫C)xC6H4R-p)4(PCy3)y (x : 1, y : 2, R : H, CH2, OMe; x : 2, y : 4, R : H) have been prepared from [Ag
(C∫C)xC6H4R-p]•, and their photophysical properties assesssed.140 Novel luminescent hexanuclear silver(I) chalcogenolate clusters 149 (EAr : SPh, SC6H4Me-p, SePh, SeC6H4Cl-p) were prepared from [Ag2(m-dppm)2(NCMe)2]2;.141

12.3 Gold. — A crystallographic study of the luminescent pyridyl trimer Au3(mNC5H4)3 has revealed both extended chains and discrete dimers formed through self-association via Au . . . Au interactions. An hourglass shape formed within the crystals due to gold deposition is unusual, in that it resulted from a chemical reaction within the crystal.142 The hexanuclear cluster cation [Au6(PPh3)6]2; was deposited onto a TiO2 support and used as a catalyst for CO oxidation to give CO2.143 Reaction of [
(Pri3P)Au3O]; with 1,2-C6H3R(SH)2 (R : H, 4-Me) gave the trinuclear complex 150 (R : Pri) in 83 % yield when pentane was added at 920 °C to the reaction mixture, or tetranuclear 151 (L : PPri3, R : H, Me) in 63 % if the reaction mixture was concentrated at 20 °C. [(Pri3P)2Au]; was identified as a by-product of the ligand redistribution reaction. In contrast, a similar reaction between [
(Ph3P)Au3O]; and 1,2-C6H4(SH)2 gave solely 150 (R : Ph, R : H).

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Complex 151 (L : CNCy, R : H) was also produced in low yield from the analogous reaction with CyNCAuCl and was isolated as a cocrystallate with the gold(I) salt
(RNC)Au[C(NHR)2]Cl, forming infinite chains via Au . . . Au interactions.144

13

Group 12

13.1 Mercury. — The m3-acetone complex Hg3(m3-OCMe2)(m-C6F4-1,2)3 has been found to form novel cofacial dimers (152) via short Hg(II) . . . Hg(II) mercurophilic interactions.195 The preparation of triangular clusters [Hg3(m-mdppm)3] 4; and [Hg3(m-dppa)3]4; [mdppm : Ph2PCH(Me)PPh2; dppa : Ph2PNHPPh2], from [Hg(dmso)6]2;, elemental mercury and the relevant phosphine, has been reported; unambiguous characterisation was achieved using multinuclear NMR spectroscopy.146

14

Mixed Metal Clusters

14.1 Group 5. — The cationic sulfur-bridged early-late heterobimetallic complex 153 was prepared from the reaction of M(SH)2(PMe3)Cp* (M : Rh, Ir) with the

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electron-deficient VCl3(thf )3. The reaction resulted in oxidation of the V(III) to V(V) with concomitant evolution of H2.147 14.2 Group 6. — Mo, W—Fe. The first mixed-metal clusters bearing a h1-acetylide (154) were prepared from the reaction of M(h1-C∫CPh)(CO)3Cp (M : Mo, W) with Fe3(m3-E)2(CO)9 (E : Se, Te) in the presence of trimethylamine-N-oxide. Reaction of 154 (M : Mo, E : Se, Te) with Co2(CO)8 resulted in addition of the Co2 unit across the C∫C to give pentanuclear 155.148

Mo, W—Co. Cluster expansion of the heterometallic alkyne complexes MCo(mRC2R)(CO)5Cp (M : Mo, W; R : R : CO2Me, CO2Et; R : H, R : CO2Me) with Co2(CO)8 afforded the MoCo3 butterfly clusters 156; carbonyl ligands bridge each Mo-Co bond.149 Similarly, Mo2(m-RC2R)(CO)4Cp2 (R : CO2Me, H) reacted with Co2(CO)8 to give a mixture of products including the novel clusters 157 and 158, formed by addition of 4 and 5 Co atoms, and Mo2Co2(m4C2R)(CO)8Cp2, the Mo2Co2 analogoue of 156. Complex 157 contains an unusual edge-sharing bitetrahedral geometry where each Mo atom is bound to four Co atoms; 158 contains a similar core with an additional Co atom. In each case, the starting dimolybdenum complex has undergone scission of the alkyne ligand to give two alkylidyne ligands and one alkylidyne and a carbide ligand, respectively. A minor product, the ethylidyne complex MoCo2(m3-CMe)(CO)8Cp, was the only product involving cleavage of the Mo-Mo bond, and the heptanuclear cluster 158 (R : CO2Me) reacted with CO to give the analogous alkylidyne cluster MoCo2(m3-CCH2CO2Me)(CO)8Cp, formed by coupling of the carbide and alkylidyne ligands with addition of hydrogen.150 Mo, W—Ir. The systematically varied tetrahedral cluster series MIr3(CO)11-x(PMe3)xCp [M : Mo, x : 0, Cp : C5H4Me, C5HMe4, C5Me5 (159); M : W, Cp : C5H4Me, x : 1 (160), 2] and M2Ir2(CO)10-x(PMe3)xCp [M : Mo, x : 0, Cp : C5H4Me, C5HMe4, C5Me5 (161); M : W, Cp : C5H4Me, x : 1, 2] has been prepared and examined with spectroscopic (including Raman), electrochemical, spectroelectrochemical and theoretical techniques.151 The related clusters with diphosphine ligands, W2Ir2(m-L)(CO)8(h5-C5H4Me)2 (L : dppe, dppf) (162) were also prepared; structural characterisation showed that the electron precise complexes contain electronic asymmetry in the Ir atoms (17 and 19 electrons).152

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Mo, W—Pd, Cu. Chromatography of bimetallic MPd(C∫CPh)(CO)3(PPh3)
h5C5H2Ph2(PPh2) (M : Mo, W) afforded the diastereomeric trinuclear complexes 163, with an unusual linear arrangement of metal atoms, with highly distorted octahedral and square-planar M and Pd environments. A rational route to these complexes was developed from reaction of the linked cluster [Pd(mI)M(CO)3
h5-C5H2Ph2(PPh2)]2 with dppe, giving PdI2(dppe) as a coproduct. As with the parent compounds, 163 was found to be an intermediate in the coupling reaction of metal halides with trialkyltin acetylides to form metallacetylides.153 Treatment of the heterothiometallate anionic complex [MoS4Cu6Br8]49 with dppf gave 164, containing two Cu(dppf) units linked through a MoS429 unit and weak Mo . . . Cu bonds. A similar reaction with Ph2Ppy gave the related 165. Complex 164 was found to have superior optical limiting properties to 165.154 Mo—Fe, Cu cubane clusters. The first uncharged bridged [MoFe3S4]3; double cubane clusters have been prepared from the reaction of anionic [(h2-Cl4cat)(MeCN)MoFe3S4Cl3]29 (Cl4-cat : O2C6Cl4) with FeCl2(P2) [P2 : dppe or dmpe (1,2-bis(dimethylphosphino)ethane)].155 Anionic [(Cl4cat)(MeCN)MoFe3S4Cl3]29 was also reacted with CO in the presence of FeCl2(PEt3)2 or PEt3-NaBPh4 to give the triangular cluster 166 in minor yield, along with the cubane 167.156 14.3 Group 7. — Mn, Re—Fe, Ru, Os. The cationic carbyne complexes [M(∫CPh)(CO)2Cp]; (M : Mn, Re) were reacted with [Fe4(CO)13]29 and

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[M3(CO)11]2- (M : Ru, Os) to give a range of new bimetallic m3-carbyne complexes 168 (M : Mn, Re; M : Fe, Ru, Os; all combinations). Interestingly, whereas ReFe2(m-H)(m3-CPh)(m-CO)2Cp (168, M : Re, M : Fe) reacted with PPh3 at low temperature to give the PPh3-monosubstituted product, the corresponding MnFe2 cluster was unreactive at room temperature.157 The linear complexes 169 — 170 were isolated from the reaction of Os(CNBut)(CO)4 with MnX(CO)5 (X : Cl, Br, I) in the appropriate ratios. The complexes are related to 122, and contain nondative M-M bonds formed by initial CO substitution on the manganese compound by Os(CO)4(CNBut), followed by a CO-halide exchange between the Os and Mn atoms. Interestingly, Os(CO)5 did not produce similar results, possibly due to a lower basicity, and reaction with the Re analogue ReX(CO)5 formed the intermediate (OC)4(ButNC)OsReX(CO)4, but failed to isomerise to the product.158 Mn—Co, Rh. The manganese disulfide complex Mn2(m-S2)(CO)7 reacted with Co(CO)2Cp, Co(CO)(PPh3)Cp and Rh(CO)2Cp* via insertion into the S-S bond to give 171, along with the phosphine derivative of the parent compound, Mn2(m-S2)(CO)6(PPh3).159 Re—Cu, Ag, Au. The unsaturated anion [Re3(m-H)4(CO)9(PPh3)]9 reacted with Lewis acids [M(PPh3)]; (M : Cu, Ag, Au) to give the addition products 172, containing a hydride ligand capping the Re2M face, isolobally analogous to a novel bridging H2 unit. All products were found to dissociate in acetone to give

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Organometallic Chemistry

the starting complexes; the extent of the dissolution was found to be dependent on the basicity, rather than the dielectric constant, of the solvent.160

14.4 Group 8. — Fe, Os—Co, Rh. Reaction of Co3(m3-CCl)(CO)9 with [Fe3(CO)11]9 in the presence of thallium salts afforded the hexanuclear dicarbide cluster anion 173, consisting of distorted Co3C and FeCo2C tetrahedra linked via C-C and Co-Co bonds. A similar reaction with [Fe3(m3-CCO)(CO)9]29, followed by addition of ethanol and then treatment with AuCl(PPh3) gave the Fe3Au3 hexanuclear cluster 174 along with the starting complex Co3(m3-CCl)(CO)9, providing an unusual example of iron cluster expansion using gold reagents.161 A study into the substitution reactions of the triangular cluster FeCo2(m3CS)(m3-S)(CO)2(PPh3)Cp2 has shown that the PPh3 ligand is labile. Replacement with tertiary phosphine was possible only under very specific conditions (3 — 4 fold excess of ligand and 1 minute reflux in benzene); replacement with CNR was less demanding, the resulting FeCo2(m3-CS)(m3-S)(CO)2(CNR)Cp2 providing a route into subsequent CO ligand substitution by PPh3 to give FeCo2(m3-CS)(m3S)(CO)(CNR)(PPh3)Cp2.162 The reaction of Fe2(m-S2)(CO)6 with Co(CNS)2 in the presence of LiBHEt3 and PPh3 afforded the structurally-characterised triangular complex [Fe2Co(m3-S)(CO)8(PPh3)]9 with a Co-bound phosphine ligand, and Co(COBEt3)(CO)2(PPh3), containing a novel B-O bond formed from BEt3 bonding to an oxygen atom of a carbonyl ligand.163 Tetranuclear Os3Rh(m-H)3(CO)12 was reacted with a number of vinyl-containing reagents to give the new clusters 175 and 176 with vinylacetic acid, and 175

9: Organo-transition Metal Cluster Complexes

251

and 177 with vinyl acetate. A number of isomeric alkenes were isolated from the reaction with 1-octene.164

Fe, Ru, Os—Pd, Pt. The reaction of Pd(PBut3)2 with a number of ruthenium compounds resulted in a remarkable Lewis acid-base interaction between the palladium and ruthenium reagents to give a number of new complexes. Thus, reaction with Ru3(CO)12 gave 178, formed by addition of three strongly acidic Pd(PBut3) fragments, a reaction similar to the protonation of M-M bonds with protic reagents. Reaction with Ru(CO)5 afforded 179, consisting of a unique Pd(PBut3)-stabilised Ru2(CO)9 fragment, and reaction with Ru6(m6-C)(CO)17 gave two isomers of the known dipalladium complex 180, again without loss of CO.165 The first ruthenium-palladium sulfido cluster was prepared by reaction of the disulfido-bridged diruthenium complex Ru2(m-S2)(m-SPri)2Cp*2 with Pd(PPh3)4 to give 181 (L : L : SPri); the Pd atoms have been oxidized from Pd0 to PdI. A similar reaction with Ru2(m-SH)2Cl2Cp*2 gave the cationic complex 182, which reacted with CO with cleavage of the Pd-Pd and Ru-Ru bonds to give the linked cluster 183. Treatment of 181 (L : L : SPri) with benzyl bromide afforded stepwise substitution of the thiolato ligands to give 181 (L : Br, L : SPri and L : L : Br).166 Hexanuclear Ru5Pt(m6-C)(CO)15(PBut3), formed from the reaction between Ru5(m5-C)(CO)15 and Pt(PBut3)2, was found to undergo a dynamic rearrangement between two core geometries that could each be crystallised depending on the solvent used: crystallisation from a benzene/octane mixture afforded monoclinic and triclinic forms of 184, with an octahedral arrangement of metal atoms, whereas diethyl ether solutions afforded 185, a bridged square pyramidal cluster.

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The reversible rearrangement in solution serves as a model for the hopping mechanism of adatom diffusion on metal surfaces.167 A number of mixed-metal Ru-Pt and Ru-Sn clusters have been deposited into carbon nanotubes and observed by transmission electron microscopy to be homogeneously deposited with a tendency to align at the tips of the tubules.168 Fe—Ag, Au. FeH2(CO)4 was sequentially reacted with the tridentate ligand 2,6-bis(diphenylphosphino)pyridine (L) and AgClO4 to form the dication 186, containing the two trans-L groups coordinated to a linear FeAg2 chain. Mixedmetal Pt-Ag and Pt-Cu complexes containing L in a cis-orientation about the Pt atom were also prepared; reaction of PtMe2(cod) with two equivalents of L formed the 12-membered metallomacrocycle [Me2Pt(m-L)]2, which could be treated with one equivalent of AgBF4 to form the PtAg2 complex 187, or with [Cu(MeCN)4]; to give a Pt2Cu analogue 188.169

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253

The first Fe3Au alkynyl complex was prepared by treatment of the trinuclear alkynylthiolate cluster anion [Fe3(m3-SC∫CBut)(CO)9]9 with AuClPR3 (R : Ph, Pri) to give 189 via C-S bond cleavage; the spiked triangular geometry of 189 is unique for group 8 — Au mixed-metal clusters. In solution, the complex exists in an equilibrium mixture with the m-AuPPh3 bridging the opposite Fe-Fe bond. This low energy process corresponds to a novel reversible process of oxidative addition/reductive elimination of an Au-C bond across a M-M bond.170 [Os4(m-H)3(CO)12]9 was reacted with AuClPPh3 to give 190, shown to be a better catalyst than the homometallic Os4(m4-H)4(CO)12 for the carbonylation of aniline to give methyl phenylcarbamate.171

Os—Hg. Treatment of trinuclear [Os3(m-PR2)(CO)10]9 (R : Ph, Bui) with HgCl2 afforded a mixture of Os-Hg clusters including 191 — 193. Heptanuclear 191 contains a spirocyclic m4-Hg atom. Heating 191 in the absence of light resulted in a rearrangement of the m-phosphido ligand to the Hg-bridged Os-Os bond to give 192. Photochemical rearrangement of complex 191 occured at room temperature to give the wheel-cluster 193, containing a twisted ring of osmium atoms and a central m6-Hg.172

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14.5 Group 9. — Co—Ni, Pt. The reaction between tetranuclear Co3Ni(CO)9Cp and 1,3,5,7-cyclooctatetraene (cot) or 1,4-(SiMe3)2C8H6 has afforded the first trinuclear h8-cot-capped Co-Ni clusters, 194.173 Addition of [Co(CO)4]9 to preformed doubly-bridged Pt2(m-PPh2)Cl(PPh3)3 gave the trinuclear cluster 195. The complex can be considered to have a d9-d9 Pt(I)-Pt(I) bond with a bridging Co(CO)4 group acting as a four-electron donor anionic metalloligand, giving each Pt centre the required 16 electron configuration.174

Co—Cu. The dicobalt carboxylic acid complex Co2(m-HC2CO2H)(CO)6 was added to copper(II) methoxide to give 196, the unusual bonding being rationalised by complementary DFT calculations.175 14.6 Group 10. — Pt—Cu, Au. The diplatinum complex Pt2(m-PPh2)(m-oC6H4PPh2)(PPh3)2 was shown to be sufficiently electron-rich to form stable adducts with electrophilic Cu and Ag reagents [Cu(PPh3)]; and AgOC(O)CF3 to give 197 and 198, respectively.176 The mixed-metal cluster acetylide Pt2Au4(C∫CBut)8 has been adsorbed onto silica and examined for catalytic activity in the reduction of NO by propylene, the oxidation of propylene in the absence of NO, and the 16O/18O homoexchange reaction. In contrast to catalyst systems formed from deposition of individual Pt and Au precursors, where the presence of the Au had no effect on the Pt activity, the mixed-metal cluster catalyst was found to have different behaviour to

9: Organo-transition Metal Cluster Complexes

255

monometallic Pt catalysts, probably a result of the stability of the mixed-metal cluster.177 14.7 Group 11. — 19F, 1H-HOESY (heteronuclear Overhauser effect spectroscopy) and pulsed-field gradient spin- echo NMR spectroscopy have been used to provide the first direct evidence for the presence of stacked adducts of cyclic [Au(m-C2,N3-bzim)]3 (bzim : 1-benzylimidazolate) or [Au(m-C,NC(OEt):NC6H4Me)]3 with [Hg(m-C,C-C6F4)]3, to form supramolecular assemblies such as 199, coordinated via acid-base interactions.178

14.8 Clusters Containing Three Different Metals. — The thermolysis reaction between M(C∫CPh)(CO)3Cp* (M : Mo, W) and MoFe2(m3-E)2(CO)10 (E : S, Se) in benzene proved highly sensitive to oxygen concentration and reaction conditions. The mixed-metal oxo clusters 200 and 201 were prepared by thermolysis under 2.5 — 5 % oxygen, whereas the same reaction under argon resulted in reaction between W(C∫CPh)(CO)3Cp* and MoFe2(m3-S)2(CO)10 to give 202, with no reaction occurring with the other possible combinations.179 Thermolysis of M(C∫CPh)(CO)3Cp (M : W, Mo) and Fe2Ru(m3-E)2(CO)9 (E : Se, S) in toluene gave 203 and 204, with the hexanuclear cluster 204 formed only in the tungsten acetylide reaction.180 The linked dinuclear complex Na2[(M(CO)3
h5-C5H4C(O)CH2)2] (M : Mo, W) was reacted with the linked cluster complex [MFeCo(m3-S)(CO)8
h5C5H4C(O)CH2]2 (M : Mo, W) to give diketo macrocyclic complexes containing single and double cluster cores, 205 and 206, respectively. Reduction of the macrocyclic clusters with NaBH4 afforded the related dihydroxy complexes.181 Ru3(CO)12 reacted with the tetrahedral cluster MoFeNi(m3-S)(CO)5Cp(h5-

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RC5H4) (R : Me, MeCO, MeO2C) to give 207 and 208. The hydride ligand in 207 is probably bridging the Ru-Ru bond or capping the MoRu2 face.182 Chloride abstraction from PdCl2(h2-dppm) with AgNO3 followed by reaction with anionic [Mn2(m-PPh2)(CO)8]9 gave the trimetallic cluster 209 in 15 % yield, along with binuclear MnPd(m-PPh2)(CO)4(h2-dppm). Similar reaction using PdCl2(h2-PP) (PP : dppe, dppf) failed to produce an analogous cluster product, and attempts to produce 209 from reaction of the bimetallic co-product and AgCl were unsuccessful.183

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Inorg. Chem., 2002, 41, 76. 118. C. Babij, C. S. Browning, D. H. Farrar, I. O. Koshevoy, I. S. Podkorytov, A. J. Poe¨ and S. P. Tunik, J. Am. Chem. Soc., 2002, 124, 8922. 119. K. Lee, H. Song, B. Kim, J. T. Park, S. Park and M.-G. Choi, J. Am. Chem. Soc., 2002, 124, 2872. 120. E. V. Grachova, P. Jutzi, B. Neumann, L. O. Schebaum, H.-G. Stammler and S. P. Tunik, J. Chem. Soc., Dalton Trans., 2002, 302. 121. M. C. Comstock and J. R. Shapley, Organometallics, 2002, 21, 5983. 122. C. M. Ziglio, M. D. Vargas, D. Braga, F. Grepioni and J. F. Nixon, J. Organomet. Chem., 2002, 656, 188. 123. R. Della Pergola, A. Bianchi, F. F. de Biani, L. Garlaschelli, M. Manassero, M. Sansoni, D. Strumolo and P. Zanello, Organometallics, 2002, 21, 5642. 124. B.-H. Zhu, W.-Q. Zhang, Q.-Y. Zhao, Z.-G. Bian, B. Hu, Y.-H. Zhang, Y.-Q. Yin and J. Sun, J. Organomet. Chem., 2002, 650, 181. 125. T. Sugihara, A. Wakabayashi, Y. Nagai, H. Takao, H. Imagawa and M. Nishizawa, Chem. Commun., 2002, 576. 126. Y. Fukumoto, K. Sawada, M. Hagihara, N. Chatani and S. Murai, Angew. Chem. Int. Ed., 2002, 41, 2779. 127. M. J. A. Johnson, P. K. Gantzel and C. P. Kubiak, Organometallics, 2002, 21, 3831. 128. B. K. Breedlove, P. E. Fanwick and C. P. Kubiak, Inorg. Chem., 2002, 41, 4306. 129. D. Evrard, D. Meilleur, M. Drouin, Y. Mugnier and P. D. Harvey, Z. Anorg. Allg. Chem.. 2002, 628, 2286. 130. M. G. S. Londesborough, C. A. Kilner, M. Thornton-Pett and J. D. Kennedy, J. Organomet. Chem., 2002, 657, 262. 131. F. Lemaıˆ tre, D. Brevet, D. Lucas, A. Vallet, Y. Mugnier and P. D. Harvey, Inorg. Chem., 2002, 41, 2368. 132. J. Braddock-Wilking, J. Y. Corey, K. Dill and N. P. Rath, Organometallics, 2002, 21, 5467. 133. P. Leoni, F. Marchetti, M. Pasquali, L. Marchetti and A. Albinati, Organometallics, 2002, 21, 2176. 134. L. R. Falvello, J. Fornie´s, C. Fortun o, F. Dura´n and A. Martin, Organometallics, 2002, 21, 2226. 135. R. Ros, A. Tassan, R. Roulet, G. Laurenczy, D. Duprez and K. Schenk, J. Chem. Soc., Dalton Trans., 2002, 3565. 136. J. H. K. Yip, J. Wu, K.-Y. Wong, K.-W. Yeung and J. J. Vittal, Organometallics, 2002, 21, 1612. 137. M. I. Bruce, B. C. Hall, B. W. Skelton, M. E. Smith and A. H. White, J. Chem. Soc., Dalton Trans., 2002, 995. 138. V. W.-W. Yam, C.-H. Lam and N. Zhu, Inorg. Chim. Acta, 2002, 331, 239. 139. W.-B. Zhou, Z.-C. Dong, J.-L. Song, H.-Y. Zeng, R. Cao, G.-C. Guo, J.-S. Huang and J. Li, J. Cluster Sci., 2002, 13, 119. 140. Y.-Y. Lin, S.-W. Lai, C.-M. Che, K.-K. Cheung and Z.-Y. Zhou, Organometallics, 2002, 21, 2275. 141. V. W.-W. Yam, E. C.-C. Cheng and N. Zhu, New J. Chem., 2002, 26, 279. 142. A. Hayashi, M. M. Olmstead, S. Attar and A. L. Balch, J. Am. Chem. Soc., 2002, 124, 5791. 143. T. V. Choudhary, C. Sivadinarayana, C. C. Chusuei, A. K. Datye, J. P. Fackler Jr. and D. W. Goodman, J. Catal., 2002, 207, 247. 144. H. Ehlich, A. Schier and H. Schmidbaur, Organometallics, 2002, 21, 2400. 145. J. B. King, M. R. Haneline, M. Tsunoda and F. P. Gabbai, J. Am. Chem. Soc., 2002,

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124, 9350. 146. W. Schuh, G. Ha¨gele, R. Olschner, A. Lindner, P. Dvorsak, H. Kopacka, K. Wurst and P. Peringer, J. Chem. Soc., Dalton Trans., 2002, 19. 147. S. Kuwata, T. Nagano, A. Matsubayashi, Y. Ishii and M. Hidai, Inorg. Chem., 2002, 41, 4324. 148. P. Mathur, A. K. Bhunia, A. Kumar, S. Chatterjee and S. M. Mobin, Organometallics, 2002, 21, 2215. 149. H. Adams, L. V. Y. Guio, M. J. Morris and F. A. Wildgoose, J. Organomet. Chem., 2002, 659, 142. 150. H. Adams, L. V. Y. Guio, M. J. Morris and S. E. Spey, J. Chem. Soc., Dalton Trans., 2002, 2907. 151. N. T. Lucas, J. P. Blitz, S. Petrie, R. Stranger, M. G. Humphrey, G. A. Heath and V. Otieno-Alego, J. Am. Chem. Soc., 2002, 124, 5139. 152. J. P. Blitz, N. T. Lucas and M. G. Humphrey, J. Organomet. Chem., 2002, 650, 133. 153. F. Angelucci, A. Ricci, C. Lo Sterzo, D. Masi, C. Bianchini and G. Bocelli, Organometallics, 2002, 21, 3001. 154. Y.-Y. Niu, T.-N. Chen, S.-X. Liu, Y.-L. Song, Y.-X. Wang, Z.-L. Xue and X.-Q. Xin, J. Chem. Soc., Dalton Trans., 2002, 1980. 155. J. Han and D. Coucouvanis, Inorg. Chem., 2002, 41, 2738. 156. J. Han, M. Huang and D. Coucouvanis, Polyhedron, 2002, 21, 2523. 157. N. Xiao, Q. Xu, S. Tsubota, J. Sun and J. Chen, Organometallics, 2002, 21, 2764. 158. F. Jiang, H. A. Jenkins, D. F. Green, G. P. A. Yap and R. K. Pomeroy, Can. J. Chem., 2002, 80, 281. 159. R. D. Adams, O.-S. Kwon and M. D. Smith, Organometallics, 2002, 21, 1960. 160. T. Beringhelli, G. D’Alfonso, M. G. Garavaglia, M. Panigati, P. Mercandelli and A. Sironi, Organometallics, 2002, 21, 2705. 161. R. Reina, O. Riba, O. Rossell, M. Seco, M. Font-Bardia and X. Solans, Organometallics, 2002, 21, 5307. 162. A. R. Manning and A. J. Palmer, J. Organomet. Chem., 2002, 651, 60. 163. B. Zhuang, H. Sun, L. He, Z. Zhou, C. Lin, K. Wu and Z. Huang, J. Organomet. Chem., 2002, 655, 233. 164. J. P.-K. Lau and W.-T. Wong, J. Organomet. Chem., 2002, 659, 151. 165. R. D. Adams, B. Captain, W. Fu and M. D. Smith, J. Am. Chem. Soc., 2002, 124, 5628. 166. S. Kuwata, K. Hashizume, Y. Mizobe and M. Hidai, Organometallics, 2002, 21, 5401. 167. R. D. Adams, B. Captain, W. Fu, P. J. Pellechia and M. D. Smith, Angew. Chem. Int. Ed., 2002, 41, 1951. 168. S. Hermans, J. Sloan, D. S. Shepard, B. F. G. Johnson and M. L. H. Green, Chem. Commun., 2002, 276. 169. H.-B. Song, Z.-Z. Zhang, Z. Hui, C.-M. Che and T. C. W. Mak, Inorg. Chem., 2002, 41, 3146. 170. E. Delgado, B. Donnadieu, M. E. Garcı´ a, S. Garcı´ a, M. A. Ruiz and F. Zamora, Organometallics, 2002, 21, 780. 171. Y. Li, W.-X. Pan and W.-T. Wong, J. Cluster Sci., 2002, 13, 223. 172. H. Egold, M. Schraa, U. Flo¨rke and J. Partyka, Organometallics, 2002, 21, 1925. 173. H. Wadepohl, S. Gebert, R. Merkel and H. Pritzlow, J. Organomet. Chem., 2002, 641, 142. 174. R. Bender, P. Braunstein, S.-E. Bouaoud, D. Rouag, P. D. Harvey, S. Golhen and L. Ouahab, Inorg. Chem., 2002, 41, 1739.

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175. A. Vega, V. Calvo, E. Spodine, A. Za´rate, V. Fuenzalida and J.-Y. Saillard, Inorg. Chem., 2002, 41, 3389. 176. C. Archambault, R. Bender, P. Braunstein and Y. Dusausoy, J. Chem. Soc., Dalton Trans., 2002, 4084. 177. C. Mihut, C. Descorme, D. Duprez and M. D. Amiridis, J. Catal., 2002, 212, 125. 178. A. Burini, J. P. Fackler, R. Galassi, A. Macchioni, M. A. Omary, M. A. RawashdehOmary, B. R. Pietroni, S. Sabatini and C. Zuccaccia, J. Am. Chem. Soc., 2002, 124, 4570. 179. P. Mathur, S. Mukhopadhyay, G. K. Lahiri, S. Chakraborty and C. Tho¨ne, Organometallics, 2002, 21, 5209. 180. P. Mathur, C. Srinivasu, M. O. Ahmed, V. G. Puranik and S. B. Umbarkar, J. Organomet. Chem., 2002, 659, 196. 181. L.-C. Song, W.-F. Zhu and Q.-M. Hu, Organometallics, 2002, 21, 5066. 182. L.-C. Song, G.-H. Zeng, Q.-M. Hu, H.-W. Cheng and H.-T. Fan, J. Organomet. Chem., 2002, 656, 228. 183. Y.-C. Liu, K. H. Lee, J. J. Vittal and T. S. A. Hor, J. Chem. Soc., Dalton Trans., 2002, 2747. 193. G. Sa´nchez-Cabrera, F. J. Zuno-Cruz, M. J. Rosales-Hoz, V. I. Bakhmutov, J. Organomet. Chem., 2002, 660, 153. 194. J. A. Cabeza, Eur. J. Inorg. Chem., 2002, 1559. 195. J. B. King, M. R. Haneline, M. Tsunoda and F. P. Gubbai; J. Am. Chem. Soc., 2002, 124, 9350.

10 Complexes Containing Metal-Carbon s -Bonds of the Groups Iron, Cobalt and Nickel, Including Carbenes and Carbynes By PHILIP J. KING

1

Introduction

The format of this chapter will follow that used in previous volumes. Section 2 will deal with any review articles that are of relevance to this chapter. Section 3 will focus on articles relating to metal-carbon s-bonds involving Group 8, 9 and 10 metals, whilst Section 4 will be concerned with the carbene and carbyne complexes of those metals. Over 800 journal articles of relevance to this Chapter were published during the year 2002 and limitations of space prevent the inclusion of them all; that this involves the omission of some interesting pieces of research is of necessity rather than by desire. Concerning the articles contained within this review, descriptions are intentionally brief and where there are several references concerning similar structure types (e.g. Pt-Me bonds), these have been afforded a more general overview. In all cases, the reader is referred to the original reports for more information.

2

Reviews and Articles of General Interest

Several general review articles have appeared including annual surveys on the topics of transition metals in organic synthesis,1 the chemistry of the carbontransition metal double and triple bond,2 the application of transition metals in hydroformylation,3 heterodinuclear transition metal complexes with multiple metal-metal bonds,4 metal-mediated reductive hydrogenation of organic halides5 and asymmetric hydrogenation6. The uses of transition metal complexes in the aqueous catalytic polymerization of olefins,7 stereoselective polymerization,8 copolymerization of carbon monoxide and olefins9 and the production of low molecular weight oxygenates from carbon monoxide and ethene10 have all been reviewed. The topics of time resolved spectroscopic studies of catalytic intermediates,11 homogeneously catalysed isomerisation of allylic alcohols,12 aryl-aryl bond formation,13 1,n-enynes in the presence of transition metals,14 organotransition metal [3;2] cycloaddition reactions,15 polymer16 and polystyrene17 supOrganometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 264

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ported metal complexes for use in catalysis, ROMPgel reagents in parallel synthesis,19 and N-heterocyclic carbene complexes20 have all been reviewed. Accounts of the multi-electron reduction of CO2 via ruthenium species containing Ru-C bonds,21 synthesis and applications of electrophilic, half-sandwich metal-carbene complexes,22 ruthenium and palladium catalysed C-C bond formation,23 kinetic isotope effects in the C-H bond activation of alkanes by iron-oxo complexes24 and reactions of cationic carbyne complexes of di-iron with carbonyl metal anions25 have appeared. Finally, reviews concerning organonickel,26 organoplatinum27-29 and organopalladium30 complexes have been published as have reviews on the molecular mechanics and dynamics of porphyrins and cobalt corrinoids,31 palladium catalysed coupling of aryl chlorides,32 palladium catalysed C-N, C-C33 and M-C34 cross couplings and palladium catalysed multiple arylation of carbonyl compounds.35

3

Metal-Carbon s -Bonds Involving Group 8, 9 and 10 Metals

3.1 The Iron Triad. — The synthesis, structure and reaction chemistry of phosphine-substituted iron carbonyl complexes (1) containing s-alkyl-p-allyl ligands derived from ring-opened pinene systems have been described.36 Treatment of the p-allyltricarbonyliron lactone complexes (2) with lithium naphthalenide has

been shown to cause decomplexation to acyclic dienols without any scrambling of the allylic centre.37 Photolysis of the diiron complexes [FpCH2SiMe2SiMe2CH2Fp] and [FpCH2SiR2CH2SiR2Fp]
Fp : [(hC5H5)Fe(CO)2]; R : Me or Ph, leads to stereospecific formation of the corresponding 1,3-disilacyclobutanes.38 Similarly, photolysis of [FpCH2SiR2SiR2SiR2CH2Fp] (R : Me or Ph) affords the corresponding 1,2,4trisilacyclopentanes in excellent yield.39 Condensation of the ferriomethyl-silanol complex [FpCH2SiMe2OH] with titanocene and zirconacene dichloride, respectively, leads to the heterodisiloxanes [FpCH2SiMe2OM(Cl)(h-C5H5)2] (M : Ti, Zr) in good yields.40 The theoretical pathways of the irradiation of

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[FpCH2SiH3] have been studied by means of DFT calculations.41 The electron deficient iron(II) benzyl complex [(nacnac)Fe(CH2Ph)] (nacnac : ArNCMeCHCMeNAr; Ar : 2,6-diisopropylphenyl) reacts with B(C6F5)3 via abstraction of the benzyl anion to yield the paramagnetic ion contact pair [(nacnac)Fe] [PhCH2B(C6F5)3].42 The tertiary to iso-butyl isomerisation of the three coordinate iron(II) complexes [Fe(ArNCRCHCRNAr)(CMe3)] (R : Me, tBu; Ar : 2,6-diisopropylphenyl) has been reported and shown to proceed via a hydride intermediate.43 The synthesis and reactivity of the analogous iron(II)-alkyl complexes [Fe
ArNC(tBu)CHC(tBu)NAr(R)] (R : Me, Et, CH2tBu, iPr) has also been reported.44,45 The reaction of [FeR2(bipy)2] (R : Me, Et) with CO has been found to proceed with reductive elimination of acetone or diethylketone and formation of carbonyl derivatives of iron(0).46 Reaction of [Fe(CO)5] with Me2NC∫CNMe2 affords the ferribicyclobutenone species (3), the decarbonylation and C-C coupling/cleavage reactions of which have been investigated.47 The

acylferrate anion [Fe
C(O)Me(CO)4]9 reacts with diphenylacetylene, in the presence of methyl iodide, to afford mono and dinuclear products containing alkenyl ketone ligands.48 Thermolysis of the cis-bis(alkoxycarbonyl) monomers [Fe(CO2R)2(CO)4] (R : Me. tBu, allyl, 1,1-dimethylallyl) has been shown to afford the novel trimetallic species [
Fe(m,h2-CO2R)3(CO)32Fe].49 The cyclisation of the pyruvyl(methoxycarbonyl)tetracarbonyl iron complex cis[Fe(CO)4
C(O)C(O)Me(CO2Me)] can be induced by the addition of pronucleophile reagents to afford the series of substituted metallalactones [Fe(CO)4
C(O)C(R)(Me)OC(O)] (R : MeO, EtO, EtS, PPh2, PCy2).50 A series of sandwich and half-sandwich complexes containing the azaferracyclopentadienyl unit (4) have been prepared from the thermal reactions of [Fe
h4PhC:CC:NPh(CO)3].51 The synthesis of a family of bicycic ferra[2.2.1] complexes (5) has been reported.52 Treatment of [MnFe(m-h7-C7H7)(CO)6] with two equivalents of ArLi (Ar : Ph, o-, m-, p-MeC6H4) followed by Et3OBF4 has been shown to afford the corresponding complexes (6).53 The acetylido methyl iron(II) complexes [Fe(dmpe)2(C∫CR)(Me)] (dmpe : 1,2-diemthylphosphinomethane; R : Ph, C6H4OMe) have been synthesised and used in the photochemical synthesis of both bisacetylide and acetylide bridged oligomeric species.54 The synthesis and nonlinear optical properties of the iron(II) acetylide complexes [(h-C5H5)Fe(P-P)(C∫CC6H4R)]
P-P : 1,2-bis(diphenylphosphino)ethane; (R)(;)-1,2-bis(diphenylphosphino)propane; R : NO2, C6H4NO2, (Z)-

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267

C(H):C(H)C6H4NO2, (E)-C(H):C(H)C6H4NO2 have been reported.55 The second-order NLO properties of several iron(II) and iron(III) s-aryl acetylide complexes have been investigated experimentally. The Fe(III) complexes show a strongly depressed second-order polarizability relative to the Fe(II) congoners.56 Vibrational spectroscopic investigations into the substituent effect on the C∫C triple bond of electron rich Fe(II) and Fe(III) s-aryl acetylide complexes have also been carried out.57 A theoretical investigation into carbon bridged metal complexes, such as [(h-C5H5)Fe(CO)2(m-C3)], has allowed the development of a simple electron-counting scheme to predict the valence structure, based on the dn configuration of the metal fragments and the number of pp electrons of the linear carbon bridge.58 The propadienylidene [Fe
P(OEt3)5(:C:C:CPh2)][BPh4]2 and [FeCl
P(OEt3)4(:C:C:CPh2)][BPh4], and acetylide [Fe
P(OEt3)5
C∫CC(R)(R1)OH][BPh4] (R : R1 : Ph; R : Me, R1 : Ph) complexes have been synthesised through reaction of phosphite-containing FeCl2 solutions with propargylic alcohols.59 Reaction of the m-acetylide complex [Fe2(CO)6(m-C∫CPh)(m-PPh2)] with bimetallic carbonyl species has been shown to afford trinuclear, heterometallic complexes containing either m3-acetylide or m3-alkyne units. Further, carbonylation of [(h-C5H5)MoFe2(CO)6(mHC∫CPh)(m-PPh2)] results in the alkyne reorientation from a perpendicular to a parallel position.60 A fast and reversible intramolecular cleavage of the Au-C bond in the m-acetylide complex [Fe3Au(CO)9(PR3)(m4-C∫CtBu)] (R : Ph, iPr) has been observed in solution.61 An investigation into the organometallic chemistry of fluorinated allenes has led to the isolation of the cationic, vinyl iron complexes [(h-C5H5)Fe(CO)2
C(CF3):CX2][BF4] (X : H, F) and the dinuclear allene complex [Fe2(CO)7(m-h1,h3-F2CCCH2)].62 The vinylic C-F bonds of the complex [Fe2(CO)6
m-h2,h3-MeSC(CF3)C:CF2)] can be cleaved upon reaction with nucleophiles to afford a range of diiron derivatives containing new C-S, C-N, C-H and C-O bonds.63 The reactivity of the new carbonyl anion (7), derived from deprotonation of [Fe(H)(CO)2
P(OPh)3
(PhO)2POC6H4], towards

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Organometallic Chemistry

triorganostannyl salts and halogens has been reported.64 Treatment of 4,6dimethyl-2-mercaptopyrimidine with diiron nonocarbonyl affords the two isomeric cyclometallated complexes (8) and (9), which interconvert slowly at room temperaure.65 Exchange of a the monoanionic, NCN-pincer ligand
NCN : C6H6(CH2NMe2)2-2,6 between a stable arylgold(II) phosphine [Au(h1NCN)(PPh3)] and FeCl3 affords the iron species [Fe(h1-NCN)(Cl)2].66 The oneand two-electron oxidised forms of the tetrairon bisacetylene clusters [(hC5H5)4Fe4(m4-HC∫CH)2] and [(h-C5H4Me)4Fe4(m4-HC∫CH)2] have been synthesised and structurally characterised.67 The synthesis, structure and mechanism of formation of a range of mixed-metal, iron-containing clusters featuring acetylide bridging and acetylide coupling have been reported.68 The oligomerisation of HC∫CC(:CH2)Me in the presence of [Fe3(CO)12] yields, amongst other products, the open cluster complex (10).69 The paramagnetic iron-phenyl complex (11) has been synthesised via reaction of the a dibromide precursor with phenylmagnesium bromide.70 The dimeric N-confused porphyrin complex (12) has been obtained from the anaerobic reaction of a monomeric, bromide

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269

precursor with NaSePh. Repeating the reaction under aerobic conditions affords the hydroxy bridged dimeric species (13).71 Complexes containing a Ru-C s-bond have been used as catalysts, or proposed as intermediates, in processes such as oxidative Heck reactions,72 ringopening polymerization,73 ring-closing metathesis,74 conversion of alkynes to nitriles,75 trans additions of silanes to 1-alkynes,76 cycloadditions of 1,6-diynes to ketones,77 and the cycloisomerisation of enynes.78 The stereochemical course of the cleavage of the metal-methyl bond in [
h-C5H4(neomenthyl)RuMe(CO)(PPh3)] by electrophiles has been investigated.79 An investigation into the irreversible transformation of [(h-C5H5)Ru
CH(CN)SO2Ph(PPh3)2] to [(h-C5H5)Ru(NCCHSO2Ph)(PPh3)2] has shown the C to N isomerisation proceeds via an intermolecular mechansim.80 A general synthesis of the trications (14) has been reported.81 The novel ruthenium(0) aqua complex [Ru(OH2)(h2MeO2CC:CCO2Me)2(dppe)]
dppe : bis(diphenylphosphino)ethane isomerises to give the known complex [Ru(OH2)(h1,h182 MeO2CCCCO2Me)2(dppe)] upon heating. The complex [(h-C5Me5)Ru(h1C8H13)(NPN)]
NPN : 1-phenyl-2,5-di(2-pyridyl)phosphole has been synthesised as part of a study into the co-ordinating ability of 2,5-di(pyridyl)phospholes on Ru centres.83 Several diruthenium complexes containing s-bound hydrocarbyl units have been utilised as part of a study into the phosphorus chemical shift tensors of phosphido ligands in ruthenium carbonyl compounds.84 A one-pot synthesis for a series of complexes [(h6-arene)RuMe(PPh3)2]

270

Organometallic Chemistry

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271

[AlCl2Me2] (arene : C6H6, 1,3,5-C6H3Me3, C6H5F) and the h5-thiophene analogue, has been reported.85 The internally and externally cyclometalated complexes [RuH
MeSi(CH2PMe2)2CHPMe2(PMe3)] and [RuH
MeSi(CH2PMe2)3(PMe2CH2)], respectively, have been synthesised and found to be remarkably resistant to interconversion.86 The cyclometalated complexes [Ru(phpy)(trpy)(X)][PF6] (phpy : 2-phenylpyridine, trpy : 2,2:6,2-terpyridine, X : Cl, NO) have been synthesised and structurally chaacterised.87 The cycloruthenated 2-(phenylimino)phenyls (15) have been synthesised for which the position of tautomerization is influenced by the R-substituents.88 The

synthesis of complex (16), the first triruthenium carbonyl cluster containing a ligand derived from di(2-pyridyl)amine, has been reported.89 The cluster complexes [Ru3(CO)9(m3-h3-abq-C,N,N)] and [Ru3(CO)6(m-H)2(m3-h3-abq-C,N,N)2] (H2abq : 2-amino-7,8-benzoquinoline) have been synthesised in a step-wise manner from [Ru3(CO)10(NCMe)2].90 Thermolysis of [Ru3(CO)9(m-H)(m3HNNMe2)] in the presence of triphenylphosphine affords the cycloruthenated species [Ru3(CO)6(m-H)2(m3-HNNMe2)(PPh3)(m-C6H4PPh2)] as the final product.91 The synthesis and protonation reactions of the norbornadiene (NBD) complex [RuH(NBD)(PCP)]
PCP : 2,6-(Ph2PCH2)2C6H3 have been reported.92 A report concerning the synthesis, reactivity and silicon extrusion chemistry of the ruthenium-silylene complex [RuCl(h3-PCP)(SiLN2)]
PCP : bis(dicyclohexyl)-1,4-phoshinobutane, SiLN2 : 1,3-di-tert-butyl-1,3,2-diazasilol2-ylidene has appeared.93 The mononuclear species [RuH(CO)
CH(C2H4PtBu2)2], [RuCl(CO)
CH(C2H4PtBu2)2] and [RuH(N2)t
CH(C2H4P Bu2)2] have been synthesised as part of an experimental and computational study of pincer complexes of ruthenium.94 The methodology of transcyclometalation has been succesfully applied to the synthesis of the hexanuclear complexes (17) and (18), containing ruthenated PCP sites.95 The metallacyclic species (19) have been found to support insertion of tert-butyl isonotrile into the Ru-O bond to afford complexes (20). Complexes (20) spontaneously react in solution to afford the acylruthenium products (21), via C-C bond formation and

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Organometallic Chemistry

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273

aryl migration.96 Reaction of propionic anhydride with [fac-Ru
C(O)Et(CO)2(H2O)3][CF3SO3] affords the novel complex [fac-Ru
OC(Et)OC(Et)OC(Et)O(CO)2(CF3SO3)], containing a propylidin dipropionato group acting as a tridentate OCO-ligand.97 Reaction of [Ru3(CO)12] with levamisole hydrochloride leads to formation of [Ru3(m-Cl)(CO)9(m-h2-C11H13N2S-C,S)] via an unprecedented oxidative addition of the C-S bond of protonated levamisole to a metal atom.98 The synthesis and structure of the first ruthenated benzodiazepines (22) and (23) has been reported.99 A series of new heterobimetallic

complexes [Ru(SCR1:CHCR2:CH)(depe)2]
R1 : (h6-arene)Cr(CO)3, R2 : H; R1 : H, R2 : (h6-arene)Cr(CO)3; depe : 1,2-bis(diethylphosphino)ethane has been synthesised through regioselective insertion of ruthenium into the C-S bond of tricarbonyl-h6-[(thiophenyl)arene]chromium complexes.100 A range of

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Organometallic Chemistry

substitution, polymerisation, cyclometalation and elimination reactions have been reported from the reaction of the bridging 2-furyl complex [Ru2(CO)6
mP(C4H3O)2(m-h1,h2-C4H3O)] with diphosphanes.101 Reaction of [(h5C9H7)RuH(dppm)] with excess 1,4-diphenyl-1,3-butadiyne affords the vinylacetylide species [(h5-C9H7)Ru
C(C∫CPh):C(H)Ph(dppm)]
dppm : bis(diphenylphosphino)methane, an intermediate in the catalytic dimerization of phenylacetylene.102 Treatment of [(h5-C5H5)Ru
C:C(Ph)C(:S)N(Ph)C(: NPh)S(dppe)], containing a heterocyclic thiazine-thione ligand, with organic halides results in alkylation at the thione sulfur terminus of the ligand.103 Reaction of [RuH(Cl)(CO)(PPh3)3] with diynes HC∫CCH2CH(OR)CH(OR)CH2C∫CH (R : H, MeSO2) affords the corresponding linear (CH)8-bridged complexes [
Ru(Cl)(CO)(PPh3)22
m-C(H):C(H)CH2CH(OR)CH(OR)CH2C(H):C(H)]. These 16-electron species can be converted to the 18-electron complexes [
Ru(Cl)(CO)(PMe3)32
m-C(H):C(H)CH2CH(OR)CH(OR)CH2C(H):C(H)] via reaction with trimethylphosphine.104 The alkyne to vinylidene isomerization in [(h5-C5H5)Ru(HC∫CH)(PMe3)2]; and [(h5; C5H5)Ru(HC∫CMe)(PMe3)2] has been investigated by DFT calculations.105 Kinetic studies into the formation of the vinylidene ruthenium complexes [Ru(Cl)2
:C:C(Z)R(dcpmp)] (Z : H, R : p-MeOC6H4, p-MeC6H4, Ph, p-BrC6H4, tBu, ferrocenyl; Z : SiMe3, R : Ph, SiMe3; dcpmp : 2,6bis(dicyclophosphinomethyl)pyridine) have shown that the process occurs faster for electron rich and sterically undemanding alkynes.106 A study into the reactivity of alkynes with the dimers [RuCl(dcypb)(m-Cl)2(m-H)Ru(dcypb)(H2)] and [RuCl(dcypb)(m-Cl)3Ru(dcypb)(N2)]
dcypb : 1,4-bis(dicyclohexylphosphino)butane has led to the isolation of a range of dinuclear monoalkylidene, monovinylidene and mono(hydroxy)vinylidene derivatives.107 The ruthenium allenylidene complex [(h6-p-cymene)- RuCl(:C:C:CPh2)(PCy3)][OTf ] has been prepared and found to be an excellent catalyst for the ring-opening metathesis polymerization of norbornene and cyclooctene.108 A polymer supported analogue of the above allenylidene complex has been prepared and found to catalyse ring-closing olefin metathesis.109 A report describing the ring-closing metathesis of dienes, in ionic liquids, using ruthenium allenylidene salts has also appeared.110 A series of ruthenium(II) allenylidene, vinylidene and acetylide complexes containing bulky, functionalised trialkylphosphines, t Bu2PCH2XC6H5, as ligands has been prepared.111 Insertions of one, two or three ynamines (MeC∫CCNEt2) into the allenylidene ligands of [(h5C9H7)Ru
:C:C:C(Ph)R(PPh3)2][PF6] (R : Ph, H) has been shown to afford the products [(h5-C9H7)Ru
:C:C: C(NEt2)C(Me):C(R)Ph(PPh3)2] [PF6], [(h5-C9H7)Ru
: C:C:C (NEt2)C( Me):C(H)C(Me):CPh2(PPh3)2] [PF6] and [(h5-C9H7)Ru
: C:C:C(NEt2)C(Me):C(H)C(Me): C(H)C(Me):CPh2(PPh3)2][PF6], containing polyunsaturated cumulene chains.112 The allenylidene complexes [(h5-C9H7)Ru
:C:C:C(R1)R2(PPh3)2] [PF6]
R1 : R2 : Ph; R1 : Ph, R2 : H; R1 : H, R2 : C(Me:CPh2) react with grignard reagents, CH2:CH(CH2)nMgBr (n : 1,2), to form the corresponding s-acetylide species [(h5-C9H7)Ru
C∫CC(R1)(R2)(CH2)nCH: CH2(PPh3)2]. Subsequent protonation of the acetylide species followed by

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demetalation affords terminal 1,5 and 1,6-enynes in excellent yield.113 The addition of dienes to the allenylidene ligand in [(h5-C5H5)Ru(:C:C: CPh2)(CO)(PiPr3)][BF4] has been found to occur regioselectively at the Cb-Cg double bond.114 The bimetallic ruthenium species (24), containing a C7 con-

jugated bridge, have been synthesised via an unprecedented coupling of mononuclear allenylidene and diynyl complexes.115 The reactivity of the allenylidene complex [Ru3(CO)9(m-H)(m-OH)(m3-C:C:CPh2)] towards alkynes has been investigated, leading to the isolation of a range of triruthenium species containing either C5 or C7-hydrocarbyl ligands.116 The synthesis of a series of highly polarized ruthenium acetylide complexes with nitrothienyl substituents [(Cp)Ru
C∫CC(S)CH:CHC:NO2(L)(L)]
Cp : (h5-C5H5) or (h5-C5Me5); L, L : CO, PMe2Ph or PPh3 has been reported.117 Formation of the metallacyclic species [
h5,h1-C5H4C(Me):C(Ph)C(O)Ru(PPh3)2], from the reaction of [
h5-C5H4C(O)MeRuCl(PPh3)2] with PhC∫CH, has been found to proceed via a novel acetylide migration and oxygen transfer mechanism.118 Reaction of (CN)2C:C(CN)2 with [(h5-C5Me5)Ru(C∫CPh)(dppm)] affords the tetracyanobuta-1,3-dien-2-yl complex [(h5-C5Me5)Ru
C[:C(CN)2]C(Ph): C(CN)2(dppm)] when carried out in THF, and the cyano-enyl species [(h5C5Me5)Ru
h3-C[:C(CN)2]C(Ph):C(CN)2(dppm)] in benzene.119 The synthesis of the first s-acetylide complexes of ruthenium(IV) and osmium(IV) thiolates [M(Sxylyl)3(C∫CPh)(Cl)]9 (M : Ru, Os; xylyl : 2,6-dimethylphenyl) has been reported.120 Reaction of the acetylide complexes trans[Ru(C∫CCHR2)(Cl)(dppe)2] (R : H, Me, Ph) with either a base or an oxidising agent affords the corresponding allenylidene complexes trans[Ru(:C:C:CR2)(Cl)(dppe)2][PF6].121 The analogous acetylide [Ru(C∫CC6H4N)(H)(dppe)2] and bis(acetylide) [Ru(C∫CC6H4N)2(dppe)2] complexes have been employed as linkers in the formation of heterobimetallic units and oligomeric species such as 25 and 26.122 Three articles concerning the syntheses, structures and electrochemical properties of the diruthenium (27) and tetraruthenium (28) acetylide complexes have appeared.123-125 The related bis(acetylide) complexes [Ru2(DMBA)4(C∫CX)2] (DMBA : N,N-dimethylbenzamidinate; X : C2SiMe3, C2H, C82Ph, C4H, C4SiMe3),126 [Ru2(form)4(C∫CC5H4N)2] (form : N,N-di-p-tolylformamidinate) and [Ru2(dpf)4(C∫CC5H4N)2] (dpf : N,N-diphenylformamidinate)127 have also been

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synthesised. The latter two complexes are able to act as building blocks for heteromolecular rods via co-ordination of pyridylacetylide nitrogen atom to other metal centres. Thermolysis of [Ru3(CO)10
P(C∫CtBu)Ph22] affords the 48-electron bis(acetylide) cluster [Ru3(CO)6(m-PPh2)2(m-h1,h2-C∫CtBu)2], which readily adds CO or PPh3 to give the corresponding 50-electron species [Ru3(CO)6(m-PPh2)2(L)(m-h1,h2-C∫CtBu)2] (L : CO, PPh3).128 Thermolysis of [Ru3(CO)10
P(C∫CPh)Ph2
P(C∫CR)Ph2] (R : Ph, tBu) eventually affords the corresponding m-diyne cluster [Ru3(CO)7(m-PPh2)2(m3-h1,h1,h2-PhCCC∫CR)] via coupling of the two acetylide units. An article detailing a 1H, 13C and 29Si NMR

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study of acetylide and parallel alkyne substituted Ru3 and Os3 clusters derived from HC∫CR (R : SiMe3, SiPh3 and tBu) has appeared.129 The addition of alkynes to cis-[Ru(Cl)2(dppe)2] offers an easy, sterically controlled, synthetic route to alkynyl-ruthenium dendrons (e.g. 29) and nanometer sized p-delocalized species (such as 30).130 Articles concerning the synthesis of the C2-containing complex [(h5-C5H4R)3Ru3(CO)6(m3-C2)] (R : H, Me)131 and the reactivity of the related C2-species [(h5-C5H4R)2Ru4(CO)8(m-CO)2(m4-C2)] (R : H, Me)132 towards alkynes, have appeared. The synthesis of the m-alkyne complexes [Ru2(:X)2(mCO)(dppm)2(m-MeO2CC∫CCO2Me)] (X : Br, I) has been reported as part of a study into the synthesis and structures of co-ordinatively unsaturated Ru2 carbonyl complexes.133 A study into alkyne coupling at diruthenium fulvalene metal centres has led to the isolation of a range of (fulvalene)dimetallacyclopentadiene complexes [(Fv)Ru2(L)(m-h2,h4-C4R4)] (Fv : h5,h5-bicyclopentadienyl; L : CO, THF, OSMe2, SC4H4, PPh3; R : CO2Me).134 The allenyl [Ru3(CO)9(m-Cl)(m3RCCCH2)] (R : Me, CH2OH) and butatriene [Ru3(CO)9(m-Cl)(m3H2C:C:C:CH2)][BF4] complexes have been prepared and characterised from the reaction of propargylic species with [PPN][Ru3(CO)10(m-Cl)]
PPN : bis(triphenylphosphine)nitrogen.135 Reaction of FcC∫CH, FcC∫CC∫CH and FcC∫CC∫CFc (Fc : ferrocenyl) with [Ru3(CO)10(NCMe)2] and [Ru3(CO)10(mdppm)] has afforded a range of triruthenium complexes containing ferrocenyl derived m-acetylide or m-alkyne ligands.136 Thermolysis of 1-penten-3-yne, in the presence of [Ru4H4(CO)12], affords cluster complexes containing ligands derived from dimerization and trimerization of the enyne through the triple bonds.137 The triruthenium yneyl cluster [Ru3(CO)6(m-CO)2(m3-apyr)(mMeCH:CC:CMe)] has been synthesised and found to undergo carbon-carbon bond-forming reactions with substituted butadiynes.138 Two articles concerning the synthesis of a range of ruthenium clusters containing an interstitial carbide ligand have appeared.139,140 Ruthenium clusters containing a carbide moiety have been used as TiO2-supported catalysts for the reduction of SO2 with H2,141 and have been shown to support multiple additions of triphenylstannane.142 Density functional and ONIOM methods have been used to study hydrogen scrambling in the osmium-methyl complex [(C5Me5)Os(PMe2CH2PMe2)(Me)H];.143 The processes of reductive elimination of methane from [Os(H)2(Me)(NO)(PiPr3)2] and oxidative addition of benzene to the transient [Os(H)(NO)(PiPr3)2] are accelerated by LiN(SiMe3)2.144 The reactions of 1,4bis(ferrocenyl)butadiyne145 and 1,8-bis(ferrocenyl)octatetrayne146 towards triosmium carbonyl clusters [Os(CO)11(NCMe)] and [Os(CO)10(m-H)2] have been investigated, and found to afford a range of products derived from addition, cyclization or fragmentation of the ferrocenyl moieties. A synthetic route to the cationic allenylideneosmium complexes [(h6-mes)Os(:C:C:CR2)(Cl)(L)] [PF6] (mes : 1,3,5-trimethylbenzene; R : Ph, p-tol; L : PMe3, PCy3, PPh3, AsiPr3, SbiPr3) and summary of their reactivity has been published.147 The electronic reasons for the stabilization of osmabenzyne 31 have been studied using orbital interaction analysis and density functional theory.148 The reactivity of the m3-quinolyl 32149 and the related m3-benzoheterocycle 33150 clusters towards

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279

para-hydrogen and diazomethane, respectively, has been reported. The reaction with diazomethane affords the first example of a trimetallic species containing a m-methylidene and a s-methyl ligand.150 An article describing the ligand dependent structural changes in the acid-base chemistry of electron deficient m3-benzoheterocycle Os3-clusters has apeared.151 Reaction of 4,4-bipyridine with [Os3(CO)10(NCMe)2] affords the oxidative addition products [Os3(CO)10(mH)(m-NC5H3C5H4N)] and [
Os3(CO)10(m-H)2(m-NC5H3C5H4N)]. The pendant nitrogen atom of [Os3(CO)10(m-H)(m-NC5H3C5H4N)] allows the complex to be used as a metalloligand in the synthesis of homo- and heterometallic carbonyl clusters.152 Thermolysis of [Os3(CO)10(m-H)(m-h2-ON:CPh2)] affords osmium cluster complexes containing ortho-metallated oxime ligands.153 Articles describing the reactivity of the osmium-antimony m3-phenylene cluster [Os3(mSbPh2)(CO)9(m-H)(m-h2-C6H4)] towards alkenes,154 dienes,154 alkynes155 and nucleophiles156 have appeared. The complex [Os(h2-H2)(Cl)(PPh3)(PCP)]
PCP : 2,6-(Ph2PCH2)2C6H3, containing an elongated h2-H2 ligand, has been synthesised from addition of hydrogen to [Os(Cl)(PPh3)(PCP)].157 The osmiumelongated dihydrogen complex 34 has been synthesised and found to react with alkynes to form the bifunctional species 35, which are amphoteric in nature.158 Substitution of CO with PPh3 in [Os3(CO)8(CNR)(m3-CNR)(m3-h1,h2,h1-C60)] affords the 1,2-s-type complex [Os3(CO)7(CNR)(m3-CNR)(PPh3)(m3-h1,h1,h2C60)] (R : CH2Ph). Further reaction with PPh3 affords [Os3(CO)6(CNR)(m3CNCH2C6H4)(PPh3)(m-PPh2)(m3-h2,h2-C60)] via ortho-metallation of one of the CH2Ph units and cleavage of an Os-Os bond.159 3.2 The Cobalt Triad. — An improved synthesis of the pentafluorophenylcobalt complex [Co(CO)3(PPh3)(C6F5)] has been published.160 An article describing the molecular recognition of anions in water by the organocobalt receptor [(h5-C5H5)(H2O)
2,6-(Me2NCH2)2C6H3][PF6] has appeared.161 A series of cobaloxime h1-allyl complexes have been prepared and their reactivity towards electron deficient alkenes studied.162 Reaction of CoBr2 with the organolithium compound Li(THF)2
C(SiMe3)2(SiMe2NMe2) affords the dimeric species [Co(m-Br)
C(SiMe3)2(SiMe2NMe2)]2.163 The inverse kinetic isotope effect observed for the catalyst [(h5-C5Me5)Co(CH2CH2-H)
P(OMe)3] in the

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Organometallic Chemistry

polymerization of ethene has been investigated using density functional theory.164 The bis(imino)pyridine cobalt complexes [Co(CH2CH2R)
NC5H3(CMe: NAr)2] have been synthesised and used as model systems for b-hydrogen chain transfer.165 A series of mononuclear166 and dinuclear167 alkycobalt complexes containing tridentate imino-oximic ligands have been synthesised, and their structural and spectroscopic properties investigated. Equilibrium, kinetic and leaving group effect studies on ligand substitution reactions of [Co(en)2(Me)(H2O)]2; and [Co(en)2(Me)(NH3)]2;, model complexes for coenzyme B12, have been reported.168 The kinetics of formation of cobalt-carbon s-bonds in the reaction between [Co(NH3)5(H2O)]2; or [Co(NH3)6]2; with aliphatic radicals has been studied using pulse radiolysis techniques and found to obey first order rate laws.169 Photoacoustic calorimetry has been used to determine the Co-C bond dissociation energies of the complexes [Co(salen)(R)(L)]2;
L: H2O, pyridine, imidazole or benzylimidazole; R : Me, Et, nBu or iBu; salen : bis(salicylidene)ethylenediamine.170 Articles containing detailed kinetic and thermodynamic studies on the cyanation of alkylcobalamins,171 spectroscopic studies of the photolysis mechanism in alkylcobalamins172 and a density functional study into the electronic properties of the axial Co-C bond of alkylcobalamins173 have appeared. The synthesis and improved purification of the methylcobinamide complex 36 has allowed a thermodynamic study of its ability to bind exogenous imidazole and pyridine bases.174 A series of chlorinated alkenylcobaloximes have been synthesised175 and used to probe the mechanism of vitamin B12-catalysed dechlorination of perchloro- and trichloroethylene.176 A novel artificial model of coenzyme B12, containing a cobalt(II) porphyrin functionality encapsulated within a large poly(aryl ester) dendrimer cage, has been prepared and found to exhibit steric control in the alkyation of the cobalt centres with propargyl alcohols.177 Reaction of [(h5-C5H5)Co(COD)] with cyclic thiadiynes leads to the formation of either 2,5[2.2]thiophenophanes or [(h5C5H5)Co] stabilized cyclobutadienes, depending on the ring size.178 The enol ether ligand of complex 37 undergoes a tandem cobalt mediated rearrangement and Pauson-Khand reaction to afford a functionalised polycyclic system.179 The synthesis of the modified nucleosides 38, incorporating Co(CO)6 units as electrochemical ‘reporter’ groups, has been reported.180 The bonding capacities of the bifunctional ligand Ph2PC∫CPPh2 towards dicobalt carbonyl complexes have been explored.181 The acid catalysed double dehydration of the dynediol complexes [Co2(CO)6(m-XCCC2X)] (X : C5H8OH, C6H10OH) proceeds via an unprecedented type of intermediate in which a carbocationic centre is stabilised by a non-adjacent dicobalt hexacarbonyl moiety.182 Reaction HC∫CC6H4NH2 with

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281

cobalt octacarbonyl affords the m-azobenzene species [
Co2(CO)62(mHC∫CC6H4N:NC6H4C∫CH)], via a cobalt-mediated coupling of the alkynes.183 The addition of Co(CO)6 fragments to one or more of the free acetylenic groups of 2,6-ethynlpyridyls,184 cyclic diynes,185 and octapropargyl resorcinarene186 has been reported. The synthesis of cyclophanetetrayne complexes from Nicholas reactions of bis(propargyl)dicobalt species with electron rich arenes has been reported.187 The diyne-diol complex [
Co2(CO)62(m-XC2C2X)] (X : CH2OH) has been used in the acid catalysed synthesis of thio and mixed donor atom macrocycles containing co-ordinated diyne units.188 Reaction of [Co4(CO)12] with (HC∫CCMe2)2NMe affords the ‘spiked’ triangular cluster [Co4(CO)10(mCO)
H2CCC(Me)2N(Me)C(Me2)C(m4-C)] containing a partially cyclised di-

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Organometallic Chemistry

propargylamine moiety.189 A rational synthesis of tricobalt-molybdenum and -tungsten butterfly clusters containing alkyne ligands has been published.190 Alkyne scission in the cobalt-molybdenum clusters allows the isolation of malkylidyne clusters.191 The sensitivity of the n(MC) vibrational modes to the electronic modifications occurring in rhodium vinylidene complexes has been studied using density functional theory and Raman and IR spectroscopy.192 The syntheses and properties of a new family of organometallic rhodium complexes containing [Rh(PiPr2Ph)2]or [Rh(PiPrPh2)2] molecular units have been reported.193 The mechanism of formation of mer,trans-[Rh(C∫CR)2(H)(PMe3)3] from [Rh(Me)(PMe3)4] and terminal alkynes has been studied and found to involve C-H oxidative addition, isomerization, alkyne exchange and hydride replacement.194 The conversion of hexafluoropropene into the partially fluorinated alkane 1,1,1-trfluoropropane at a rhodium centre proceeds via formation of the vinyl complexes [Rh
CF:CF(CF2)(PEt3)3] and [Rh(H)2
CF: CF(CF2)(PEt3)3].195 A one step synthesis of the vinyl ketone complexes 39 and 40 from [(h5-C5Me5)Rh(Cl)2(PPh3)], alkyne and H2O in the presence of KPF6 has been reported.196 The rhodabenvalene species 41, containing rare h2-cyclopropene and s-vinylic linkages to Rh, has been prepared.197 A study into the C-C and C-P coupling reactions of the allenylidenerhodium complexes

[RhCl
:C:C:C(Ph)R(PiPr3)2] (R : Ph, p-tolyl) has led to the formation of products containing butatriene or unsaturated phosphorus ylide units.198 The novel metallacyclic complexes 42 have been synthesised and found to be catalytically active for the cycloaddition of diynes and several alkynes.199 The synthesis and reactivity of Rh-Co mixed-metal clusters, containing a [Rh2Co2C2] core, towards 1-alkynes has been described.200 An article describing the facile oxidative addition of C-Cl bonds to neutral and cationic rhodium(I)-bipyridine complexes has appeared.201 The rhodium-methyl complex [Rh(I)2(Me)(CO)(SbPh3)2] has

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been synthesised and its solid state structure determined.202 Dioxygen reacts with ethene at a rhodium centre to afford the corresponding 3-rhoda-1,2-dioxolanes 43, which, upon exposure to light, rearrange to the rhodium formylmethyl hydroxy species 44.203 The synthesis of an unprecedented dinuclear alkylrhodium(III) complex, built up by two 14-electron [Rh(Cl)2(Et)(PR3)]units, has

been described.204 A nonradical trapping pathway for reactions of nitroxides with rhodium porphyrin alkyls bearing b-hydrogens has been identified.205 Several organorhodium(I) complexes of the general formula [Rh(R)(CO)(PPh3)2] (R : p-tolyl, o-tolyl, Me) have been isolated and shown to insert aryl aldehydes into the aryl-rhodium bond.206 The chiral dirhodium(II) compounds [Rh2(O2CR2)(pc)2] (pc : orthometalated arylphosphine) have been used in the cyclopropanation of a-diazo ketones.207 The synthesis, molecular structures and solution dynamics of the cyclometalated complexes [Rh(Cl)(H)(RSCH2)2C6H32,6]2 have been reported.208 The synthesis of a mononuclear rhodium complex containing a ferrocene-derived PCP-pincer ligand has been carried out.209 A detailed mechanistic study into electronic and medium effects on the rate of arene C-H bond activation by cationic iridium(III) complexes has been reported.210 The heterolytic activation of hydrogen has been proposed as a trigger

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Organometallic Chemistry

for iridium complex promoted activation of carbon-fluorine bonds.211 The mechanism of silicon-hydrogen activation by [(h5-C5Me5)Ir(Me)(OSO2CF3)] and rearrangements of the resulting silyliridium complexes have been investigated.212 The activation of carbon-fluorine bonds at cationic iridium centres has been reported.213-216 Rhodium and iridium complexes containing a s-bound methyl group have been prepared as part of an investigation into the ability of such complexes to activate Si-H bonds.217 Similar complexes containing two s-bound methyl groups have been found to undergo oxidatively induced M-C bond cleavage reactions.218 The deprotonation of organic compounds bearing acid protons promoted by iridium amido complexes with chiral diamine ligands has led to the formation new organoiridium species.219 Diiridum complexes containing bridging vinyl and bridging alkyl groups have been synthesised from C-H activation of ethene at a diiridium centre.220 The carbon-carbon bond-forming reactions of Ir(III) vinyl complexes have led to the generation of reactive hydride and alkyl-alkylidene compounds and the observation of a reversible 1,2-H shift in stable hydride-Ir(III) alkylidene species.221 The promotion of alkene C-H activations at a diiridium centre by oxidation has been reported.222 The syntheses and reactivities of a range of vinyl-iridium complexes have been reported.223-226 Articles concerning the synthesis and reactivity of h6-complexes of iridathiabenzene227 and an unexpected rearrangement of two regioisomeric iridabenzenes to an h5-cyclopentadienyl iridium(I) complex228 have appeared. The synthesis of iridacyclopentadiene complexes and their reactivity towards regio- and stereoselective C-C bond formation between alkynes has been reported.229,230 An article describing novel single or double insertion reactions of alkynes into rhodium- and iridium-oxygen or -phosphorus atom bonds has appeared.231 Two articles detailing the preparation and structural characterization of iridium containing cumulenes have been written.232-233 A range of (PCP)Ir(III)-pincer complexes (such as complex 45) have been reported and used in C-H oxidative addition reactions,234 the dehydrogenation of alkanes235 and a study into substituent effects on the thermodynamics of H2, CO, arene and alkane addition to iridium.236 Several articles describing the synthesis and reactivity of iridium complexes containing s-aryl237-238 and fluorinated s-aryl239 ligands (such as complex 46) have appeared. The solid state structure and NMR data of 2-substituted benzoquinoline Ir(III) complexes (47) have shown that the pendant R-group does not always become agostic despite the presence of an available co-ordination site at the IR center.240 Carbonyl containing complexes analogous to 47, containing bulky alkyl groups, have been used in an investigation into the effect of increasing steric bulk at the 2-position of the benzoquinoline unit.241 It has been shown that irdium complexes with fluorene-modified phenylpyridine ligands are resistant to crystallization and can be used in the fabrication of single layer light emitting diodes.242 3.3 The Nickel Triad. — A theoretical study into ligand effects in the insertion of ethene into the Ni(II)-Me bond of [Ni(Me)L];
L : diphenyldiimine, diimine, 2,2-bipyridine, 1,2-bis(dimethylphosphino)ethane or 1,2-diphosphinoethane has been published.243 Complexes containing Ni-Me bonds have

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been used as catalysts for the dehydrogenative oligomerization of PhSiH3244 and in cyclometallation and carbonylation reactions.245 An article concerning the synthesis and reactivity of the fluorovinyl nickel complexes trans[Ni(I)(CF:CF2)(PEt3)2] and trans-[Ni(Br)(CBr:CF2)(PEt3)2] has appeared.246 The reactivity of a family of thienyl nickel complexes 48 has been investigated.247 The imido complex

[Ni(dtbpe)
N(2,6-(CHMe2)2C6H3)] [dtbpe : 1,2-bis(di-tert-butylphosphino)ethane] reacts with CO and CNCH2Ph with addition to the Ni-N bond to give [Ni(dtbpe)
(C,N)-h2-C(O)N(2,6-(CHMe2)2C6H3)] and [Ni(dtbpe)
(C,N)-h2-

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C(NCH2Ph)N(2,6-(CHMe2)2C6H3)], respectively.248 The oxidative-addition and reductive-elimination reactions of aziridines with nickel complexes have been investigated.249 A density functional study into the mechanism of oxidativeaddition of the C-X s-bond of the alkynes HC∫CX (X : SnH3, GeH3, SiH3, CH3, H) has been carried out.250 The synthesis, characterisation and some reactions of the acetylide derivatives [Ni(h5-C5H5)(PPh3)C∫CX] has been studied.251 A series of complexes containing propargylic cations stabilised on nickel-molybdenum and nickel-tungsten bonds has been prepared.252 An article concerning the activation of C-H bonds in internal alkenes and dienes by ‘Ni(h5-C5H5)’ species formed in the reaction of nickelocene with alkali metals has appeared.253 Density functional calculations have been employed to study the insertion reactions of alkynes into the metal-carbon bond of the model phosphinanickelacycle [Ni(Br)(CH:CHCH2PH2-kC,P)(PH3)].254 Several articles concerning the synthesis of nickel-aryl complexes and their catalytic activity in ethene oligomerization have appeared.255-257 Several articles concerning the synthesis and reactivity of square planar nickel complexes containing one or more s-bound fluorinated aryl groups (e.g. complex 49) have appeared.258-260 Treatment of azuliporphyrins

with nickel(II) or palladium(II) affords the corresponding N-confused porphyrin species with cross-conjugated borderline aromatic structures.261 An article detailing the first example of a covalently bound dimeric N-confused porphyrin complex (50) has appeared.262 Nickel(II) N-confused porphyrins have been found to act as dienophiles in Diels-Alder reactions with o-benzoquinodimethane to yield N-confused isoquinoporphyrins.263 An experimental and theoretical investigation into the rearrangement between h1 and h2-coordination modes of arylbicycloheptyl ligands at a palladium

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287

centre.264 The syntheses of complexes derived from the cyclopalladation of hydroxyferrocene265 and the ferrocenyl Schiff base [(h5-C5H5)Fe(h5C5H4CH:NCH2CH2CH2NMe2)]266 have been reported. Interestingly, the ferrocenyl Schiff base [(h5-C5H5)Fe
h5-C5H4CH:NCH(CO2Me)CH2CH2SMe] does not undergo cyclopalladation but forms a palladium-carbon bond with one of the sp3-hybridised CH carbons.267 Complexes containing mono-, di- and triene moieties capped by Pd(II) units have been synthesised from reaction of [Pd2(MeCN)6][BF4]2 with dimethyl acetylenedicarboxylate in the presence of nitrogen chelating ligands.268 The crystal and molecular structure of the polymeric species [Pd
CH2C(O)MeCl]n has been found to possess a different stereochemistry from the model proposed on the basis of the IR spectral data.269 The h1-allyl complex (51), containing a chiral bidentate ligand, has been synthesised and characterised by X-ray crystallographic and spectroscopic data.270

Several reports on the synthesis, structure and reactivity of Pd(II)-alkyl and Pd(II)-aryl complexes containing bidentate N-donor ligands have appeared.271-279 The methylpalladium(II) complexes of the Schiff base ligands 2-[(pyridin-2-ylmethylene)amino]phenol and 2-[(1-pyridin-2-ylethylideneamino)benzenethiol display an h2-HNN coordination mode for the neutral ligands and an h3-XNN (X : O or S) coordination mode for the deprotonated ligands.272 5-Methylquinoxaline has been found to act as a versatile mono-, bi- and tridentate ligand in the synthesis of palladium(II) alkyl complexes.274 The dimeric complex 52 has been synthesised and found to react with donor molecules (acetonitrile, phosphines) to afford the monometallic C-bound enolate complexes.275 A study into the reactivity of palladium(II) aryl

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complexes, containing 2,2-bipyridyl units, towards isocyanides has led to the formation of products derived from mono- and triinsertion of isocyanide into the Pd-C bond.276 Palladium(II) alkyl and aryl complexes containing bidentate imine based ligands have been studied as catalysts for the Heck reaction277,278 and the polymerisation of vinyl ethers.279 Articles concerning methyl-palldium(II) and aryl-palldium(II) complexes containing bidentate phosphine ligands or two monodentate phosphine ligands have also appeared.280-293 Thus, complexes containing a bidentate phosphine ligand and a cyclometalated dimesitylphosphine unit have been synthesised and found to have a low barrier to phosphorus inversion.280 The coordination behaviour of large natural bite angle diphosphine ligands towards methyl and 4-cyanophenylpalladium(II) complexes has been investigated using spectroscopic and crystallographic methods.281 The insertion of isocyanides into the palladium-methyl bonds in [(P-P)Pd(Me)Cl] complexes has been found to be highly dependent on the nature of the chelating phosphine.282 The reaction of trans-[Pd(Me)Cl(PEt3)2] with s-alkenylphenyl isocyanide has been shown to result in the successive insertion of the alkenyl and isocyano groups to afford a (h3-indolylmethyl)palladium complex.283 The zwitterionic palldium(II) complex 53 has been found to be an active catalyst for the

copolymerization of CO and ethene.284 A rare example of P-C bond cleavage in the fluoroaryl phosphine ligand (C6F5)2PCH2CH2P(C6F5)2 has been reported at a palladium centre.288 Several artricles concerning the addition of aryl halides to palladium centres have appeared.289-292 New palladacyclopentadiene complexes containing an N,P-donor, such as [Pd
C4(CO2Me)4(o-Ph2PC6H4-CH:NiPr)], have been synthesised and structurally characterised.294 Similar complexes containing N,P-donors and a palladium-methyl bond have been prepared and used as catalysts for the polymerization of vinyl ethers295 and olefin/CO copolymerization.296,297 Chiral, linear quadridentate ligands with a P2N2 donor set have been used in the synthesis of bis[palladium(II)] and bis[platinum(II)] complexes containing metal-aryl s-bonds.298 Throughout 2002 there have been numerous articles concerning the synthesis and reactivity of palladium complexes containing bidentate N,C-donor ligands derived from the cyclometallation of aryl groups bound to a nitrogen atom (e.g. complex 54299).299-333 A variety of Schiff base ligands, such as the 2,3,4-(MeO)3C6H2C(H):NCH2CH2OH ligand present in complex 54, have been used in the synthesis of a range of mono- and dinuclear cyclopalladated species.299-304,306-309,312,321,322,324 The complexes have all been structurally characterised and their reactivity investigated. Palladium complexes containing N,C-donor ligands have been used in the activation of chelated 1,1-bis(diphenylphosphino)ethene,305 a study into C-H versus C-Br bond activa-

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289

tion308 and a study into second and inverse order pathways in the mechanism of orthopalladation of primary amines.316 An unexpected orthometallation of acetophenonephenylhydrazone on the phenyl group has led to the first X-ray characterized example of an orthopalladated E-arylhydrazone complex.317 A series of cyclopalladated complexes of 6-phenyl-2,2-bipyridines with hydrogenbonding functionality have been prepared and found to exhibit intense luminescence at 77 K, with lifetimes in the 10-200 ms range.319 The double cyclopalladation of mesogenic pyridazines and subsequent reaction with b-ketones has been reported, leading to the formation of chiral liquid crystals.320 The tetranuclear palladium(II) complex (55) has been prepared and its reactivity towards neutral

and anionic ligands investigated.321 Nucleophilic addition of 1,3-dicarbonyl compounds to cyclopalladated compounds containing 1,1-bis(diphenylphosphino)ethene has afforded a range of cyclometallated complexes containing

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Organometallic Chemistry

functionalized diphoshine ligands with pendant 1,3-dicarbonylic arms.322 Cyclopalladated complexes have been used as catalysts in Heck,325,326 Suzuki327 and Stille328 coupling reactions of aryl halides. Articles concerning the insertion reactions of isocyanides330 and isonitriles331 into the Pd-C bond of orthopalladated primary amines have appeared. The reactivity of cyclopalladated complexes towards electron defficient alkenes and alkynes has also been studied, leading to insertion and cycloaddition reactions.333 A facile route to unsymmetrical palladium YCY (Y,Y : NR2, PPh2, OPPh2 and SR) ‘pincer’ complexes, involving trans-chlorometalation of hetero-substituted alkynes, has been reported.334 Synthetic strategies for new para-functionalised NCN-pincer palladium(II) and platinum(II) complexes (e.g. complex 56) have been published.335,336 Organopalladium and organoplatinum pincer complexes derived from the bifunctional ligand [IC6H2(CH2NMe2)2-3,5-Br-4] have been prepared, and found to be substrates for organic transformations and novel building blocks for polymetallic materials.337 The syntheses of new mono and tricyclopalladated dendritic systems based on macrocyclic carbodiazasilane molecules functionalised with the monoanionic [2,6-(CH2NMe2)2C6H3]9 pincer ligand (e.g. complex 57) have been published.338 Palladium complexes containing PCP-pincer ligands

derived from [C6H3(OPiPr2)2-2,6-Cl-1]339 and [C6H10(CH2PtBu2)2-2,6]340 have been synthesised and found to be active catalysts for the cross coupling of phenylacetylene with aryl chlorides339 and the vinylation of aryl halides.340 An

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291

article concerning the synthesis, structures and solution behaviour of bis(sulfoxide)-pincer complexes of palladium(II) has appeared.341 An article describing the disproportionation of [PtPh(CH2COMe)(cod)] (cod : 1,5-cyclooctadiene) and conproportionation of [PtPh2(cod)] and [Pt(CH2OMe)2(cod)] via intramolecular phenyl ligand transfer, has appeared.342 Five coordinate palladium(II)- and platinum(II)-methyl complexes with adiimine and cod ligands have been obtained from the reaction of [PtMe(cod)Cl] with the appropriate diimine in the presence of NaBF4.343 Several articles describing the synthesis and reactivity of organoplatinum(IV) complexes conatining either hydridotris(pyrazolyl)borate(Tp) or hydridotris(3,5-dimethylpyrazolyl)borate (Tp) ligands have been written.344-346 In particular, [(Tp)PtMe(H)2] has been proven to form a s-CH4 complex that is kinetically resistant to methane liberation.346 The Pt(IV) complex [LPtMe(H)2]; (L : [2.2.2]-(2,6)-pyridinophane) has been synthesised and its alkyl exchange reactions studied.347 Reaction of [
PtMe3I4] with AgOAc in acetone results in formation of [PtMe3(OAc)] which reacts with 9-methyladenine (9-MeAH) to yield the trimeric Pt(IV) complex [
PtMe3(9-MeA)3].348 The synthesis of the first family of platinum(IV) complexes containing phosphorus ylide ligands (e.g.

complex 58) has been reported.349 The complexes [Pt2(CO)2(PR3)2(m-h2:h2CF3C∫CCF3)] (PR3 : PPh3 or PCy3) and [Pt2(CO)2(PBzPh2)(m-h1:h1CF3C∫CCF3)2] have been synthesised and structurally characterised from reaction of [Pt3(m-CO)3(PR3)3] with hexafluorobutyne.350 Square planar platinum(II) complexes containing an diimine-based bidentate nitrogen donor and at least one methyl group have been synthesised351-355 (e.g. complex 59351) and found to

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Organometallic Chemistry

undergo a number of reactions including, protonation with loss of methane,351 C-H activation of benzene352 and ligand metathesis.353 Related platiunum(II) methyl complexes containing substituted bipyridine-based nitrogen donor ligands have also been prepared356-359 (e.g. complex 60359) and found to undergo ring expansion reactions,356 self-assembly of an organometallic double helix,357 insertion of CO and allene into the Pt-C bond,358 and metallation of unactivated methyl groups.359 Analogous Pt(II)-CH3 complexes containing bidentate pyridine-imidazoline,360 bis(8-quinolinyl)amine,361 phenanthroline362 or N,N,N,Ntetramethyethylenediamine363 ligands have also been reported and their reactivity investigated. Treatment of the alkenyl complex [PtMe(PEt3)2(CH:CHcyclo-C3HPh2)] with a Bronsted acid results in selective splitting of the Pt-Me bond to give the chloro derivative [PtCl(PEt3)2(CH:CH-cyclo-C3HPh2)] or the rearranged h3-diphenylcyclopentadienyl cation [Pt(PEt3)2(h3-C5H3Ph2)];.364 The reactivity of platinum(0) complexes containing bidentate phosphine ligands and an alkynylphosphine365 or diphenylacetylene366 ligand coordinated in a h2-fashion has been investigated resulting in the formation of several complexes containing Pt-C s-bonds. Reductive elimination of neopentane from [Pt(Np)(H)(dtbpm)] [dtbpm : bis(di-tert-butylphosphino)methane; Np : neopentyl] affords the highly reactive intermediate [Pt(dtbpm)], the reactivity of which towards a variety of electron-defficient benzenes has been investigated.367 Articles concerning insertion reactions supported by the Pt(II) complexes trans[Pt(H)(SAr)(PPh3)2] (Ar : para-C6H4Cl),368 cis-[Pt(SiHPh2)2(PMe3)2]369 and cis-[Pt(C6F5)2(PPh2C∫CR)2] (R : Ph, tolyl)370 have appeared. The P,N,P-tridentate ligand 2,6-bis(diphenylphosphino)pyridine has been used to generate a twelve-membered macrocyclic host species 61, able to encapsulate Tl(I) and

Cu(I) guest ions.371 The reaction in toluene between [(C6F5)2Pt(mPPh2)2Pt(PPh2R)2] and [Pt(C7H10)3] yields the triangulo complexes [Pt3(mPPh2)(m-C6F5)(C6F5)(PPh2R)2] (R : Ph, Me, Et).372 The utility of metalla-supramolecular rectangles containing orthometallated Pt(II) centres as electron reservoirs for multielectron reduction and oxidation has been investigated.373 There have been several reports concerning the synthesis and reactivity of platinum complexes containing bidentate N,C-donor ligands derived from the cyclometalation of aryl groups bound to a nitrogen atom374-379 (e.g. complex 62374 which represents the first seven-membered cyclometalated platinum compound to be structurally characterised). The reported complexes have been used for multiple C-H bond activation,375 construction of supported organometallics,376

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an investigation of photophysical properties377 and substitution reactions.378 Three articles concerning the synthesis of platinum NCN-pincer complexes have appeared380-382 and the complexes described therein utilised in catalysis of carbon-carbon bond formation380 and a study of emissive properties of Pt(II) complexes.381 Analogous Pt(II) PCP-pincer complexes have also been reported and their reactivity towards nucleophiles383 and protonation384,385 investigated. There have been several reports concerning the synthesis and chemistry of platinum complexes containing one and two acetylide ligands. Thus, a series of mono acetylide species have been synthesised through the thermal and photochemical activation of silicon-carbon bonds of the h2-bound alkyne complexes [Pt(h2-Me3SiC∫CPh)(R2PCH2CH2ER2)] (E : N, R : iPr, R : Me; E : P, R : R : Cy).386 Two articles concerning the photoluminescence properties of monoacetylide complexes containing tridentate ligands have appeared.387,388 A number of diplatinum complexes containing h1-acetylide and m,h1:h2-acetylide ligands have been prepared from the stepwise reaction of terminal alkynes with [Pt2(CO)(H)(PHtBu2)(m-PtBu2)2];.389 Far more common than their monoacetylide counterparts are the bis(acetylide) platinum complexes. There have been two reports detailing the synthesis and photophysical investigation of face-to-face diplatinum(II) complexes (e.g. complex 63).390,391 Other reports have detailed the synthesis and use of bis(acetylide)species in the formation of organometallic triangles,392 squares,393 oligomers394-396 (e.g. complex 64394) and dendrimers.397 An ab initio theoretical study into the experimentally observed structures of doubly bridged dinuclear acetylide complexes has been carried out.398 An investigation into electronic communication through the C∫C-Pt-Pt-C∫C linkage of [Pt2(C∫CFc)2(dppm)2] (Fc : ferrocenyl) has been carried out.399 The bis(acetylide) species [Pt
C∫CC6H4SC(O)Me2(PPh3)2] has been prepared and found to act as a single molecule insulator.400

4

Carbene and Carbyne Complexes of Group 8, 9 and 10

Numerous articles have appeared in the literature regarding carbene complexes in the context of catalysis. Thus, there are reports on the synthesis and use of carbene complexes in olefin metathesis,401,402 cross-metathesis,403 ring-closing metathesis,404,405 selective olefination of aldehydes406 and synthesis of an enantiospecific cis-cyclopropane.407 Species containing N-heterocyclic carbene ligands (imidazol-2-ylidene moieties), such as complex 65,408 represent a growing class of

294

Organometallic Chemistry

carbene complexes. Throughout 2002 there have been many reports regarding their ability to catalyse a number of processes including ring-opening metathesis,408-412 ring-closing metathesis,413-424 cross-metathesis,425-428 polymerization of vinyl monomers,429 Heck430-432 and Suzuki433 coupling reactions, carbonylation of aryl halides,434 transfer hydrogenation of carbonyl compounds,435-437 imines437 and olefins,438 C-H activation,439-440 olefin dimerization,441 hydroboration of olefins442 and telomerization of 1,3-dienes with alcohols.443 The reaction of [(C5H5)Co(PPh3)Me2] with the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPR) leads to an equlibrium mixture of [(C5H5)Co(PPh3)Me2], IPR, [(C5H5)Co(IPR)Me2] and PPh3.444 Density functional calculations have been used to rationalise the bonding in [Rh2(mO2CR)4L] complexes with ‘Arduengo’ carbenes.445 The first example of intramolecular, room temperature activation of a C-H bond in N-pyridine functionalised N-heterocyclic carbene complexes of iridium has been reported.446 Changing the counter ion in 2-pyridylmethyl imidazolium salts causes their kinetic reaction products with IrH5(PPh3)2 to be switched from the normal C-2 (complex 66) to abnormal C-5 (complex 67) binding.447,448 Two new synthetic routes to second-

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295

generation ruthenium olefin metathesis catalysts (complexes containing both a Schrock-type and a N-heterocyclic-type carbene fragment such as 65) have appeared. Both methods involve deprotonation of an imidazolium salt ligand precursor and reaction with an appropriate Schrock-type carbene complex.449,450 Studies into the effect of chelation on the structure and catalytic activity of ruthenium carbene complexes451 and C-C and C-H bond activation reactions in N-heterocyclic carbene complexes of ruthenium452 have been reported. An article detailing the synthesis and characterisation of the saddle-shaped nickel-carbene complex 68, derived from an imidazolium linked meta-cyclophane, has appeared.453 Articles describing the synthesis of a variety of palladium complexes

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Organometallic Chemistry

containing N-heterocyclic carbene moieties derived from bis(azido),454 1,1alkylene-3-3-di-tert-butyldiimidazol-2,2-dylidene,455 imidazolium,456 oxazolinylimidazolium (e.g. complex 69)457,458 and dinitroxide (e.g. complex 70)459 species. Articles concerning an experimental and computational study of a reductive elimination in a methyl-Pd(II)-carbene complex460 and the first example of simple oxidative addition of an aryl chloride to a discrete Pd N-heterocyclic carbene amination precatalyst461 have appeared. A series of Fischer carbene complexes (e.g. complex 71) have been synthesised via an asymmetric aldol-type condensation of an isocyanide component on bis(oxazolinyl)phenylrhodium(III) complexes with aldehydes.462 Three articles describing the syntheses and reactivity of dinuclear rhodium complexes containing m-carbene units have appeared.463-465 A series of iridium(I) and iridium(III) carbene complexes containing the metal

centre in different stereochemical environments have been synthesised and structurally characterised.466 The synthesis and characterisation of the novel iridaphosphirene complex 72 has been published.467 A series of remarkably stable iron porphyrin complexes containing nonheteroatom stabilised carbene or (alkoxycarbonyl)carbenes have been prepared and structurally characterised.468 An article concerning the synthesis and structural characterisation of carbeneruthenium(II) complexes binding nucleobases has appeared.469 A series of carbene and vinylidene ‘pincer’ complexes has been synthesised from reactions of alkynes with ruthenium and osmium hydrides.470 Protonation of the vinylalkenyl complexes 73 affords the corresponding alkenylcarbene species 74 in excellent yield.471 Articles describing the synthesis and reactivity of ruthenium allyl, butadienyl and allenyl carbene complexes have appeared.472,473 A novel ruthenium mediated conversion of aminals and aldimines to aminocarbene complexes has been published.474 The ruthenium carbido complex 76 has been synthesised from addition of excess PCy3 to the carbene species 75, and found to be an excellent s-donor ligand.475 Three articles concerning the synthesis and reactivity of ruthenium476,477 and osmium478 cluster bound carbene units have appeared. The three coordinate nickel carbene complex [Ni:CPh2(dtbpe)] [dtbpe : 1,2bis(di-tert-butylphosphino)ethane] has been synthesised and its reactivity investigated.479 The m3-carbyne complex [Co3(CO)9(m3-CH)] has been found to be an active catalyst for inter- and intramolecular cyclotrimerization of

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297

alkynes, producing substituted benezene derivatives.480 The reactivity of the related cluster complex [Co3(CO)9(m3-CCl)] towards metal iron carbonyl clusters has been investigated.481 The addition of N-nucleophiles and ring-breaking of coordinated cyclooctatetraene in diiron bridging carbyne complexes has been investigated.482 The reactivity of the electron-rich allenylidene-ruthenium complexes [(h5-C5Me5)Ru
:C:C:C(R)Ph(dippe)][BPh4] [R : H, Ph; dippe : 1,2-bis(diisopropylphosphine)ethane] has been studied, leading to the formation of novel dicationic ruthenium carbyne species.483 The osmium complex [Os(H)3Cl(PiPr3)2] reacts with vinyl fluoride to produce the corresponding carbyne complex [Os(H)(F)Cl(∫CMe)(PiPr3)2] and H2.484

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2002, 41, 4732. 417. K. Grela, S. Harutyunyan and A. Michrowska, Angew. Chem., Int. Ed. Engl., 2002, 41, 4038. 418. S. J. Connon, A. M. Dunne and S. Biechert, Angew. Chem., Int. Ed. Engl., 2002, 41, 3835. 419. A. K. Chatterjee and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 2002, 41, 3172. 420. H. Wakamatsu and S. Biechert, Angew. Chem., Int. Ed. Engl., 2002, 41, 2403. 421. J. Sun and S. C. Sinha, Angew. Chem., Int. Ed. Engl., 2002, 41, 1381. 422. H. Wakamatsu and S. Biechert, Angew. Chem., Int. Ed. Engl., 2002, 41, 794. 423. J. R. Rogriguez, L. Castedo and J. L. Mascarenas, Chem. Eur. J., 2002, 8, 2923. 424. A. E. Sutton, B. A. Segal, D. F. Finnegan and M. L. Snapper, J. Am. Chem. Soc., 2002, 124, 13390. 425. L. C. Usher, M. Estrella-Jimenez, I. Ghiviriga and D. L. Wright, Angew. Chem., Int. Ed. Engl., 2002, 41, 4560. 426. J. A. Love, J. P. Morgan, T. M. Trnka and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 2002, 41, 4035. 427. S. D. Goldberg and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 2002, 41, 807. 428. T. Choi, I. M. Rutenberg and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 2002, 41, 3839. 429. F. Simal, S. Delfosse, A. Demonceau, A. F. Noels, K. Denk, F. J. Kohl, T. Weskamp and W. A. Herrmann, Chem. Eur. J., 2002, 8, 3047. 430. D. J. Nielsen, K. J. Cavell, B. W. Skelton and A. H. White, Inorg. Chim. Acta., 2002, 327, 116. 431. E. Diez-Barra, J. Guerra, R. I. Rodriguez-Curiel, S. Merino and J. Tejeda, J. Organomet. Chem., 2002, 660, 50. 432. J. A. Loch, M. Albrecht, E. Peris, J. Mata, J. W. Faller and R. H. Crabtree, Organometallics, 2002, 21, 700. 433. C. W. K. Gstottmayr, V. P. W. Bohm, E. Herdtweck, M. Grosche and W. A. Herrmann, Angew. Chem., Int. Ed. Engl., 2002, 41, 1363. 434. V. Calo, P. Giannoccaro, A. Nacci and A. Monopoli, J. Organomet. Chem., 2002, 645, 152. 435. M. Albrecht, J. R. Miencznikowski, A. Samuel, J. W. Faller and R. H. Crabtree, Organometallics, 2002, 21, 3596. 436. A. A. Danapoulos, S. Winston and W. B. Motherwell, J. Chem. Soc., Chem. Commun., 2002, 1376. 437. M. Albrecht, R. H. Crabtree, J. Maria and E. Peris, J. Chem. Soc., Chem. Commun., 2002, 32. 438. L. D. Vasquez-Serrano, B. T. Owens and J. M. Buriak, J. Chem. Soc., Chem. Commun., 2002, 2518. 439. K. L. Tan, R. G. Bergman and J. A. Ellman, J. Am. Chem. Soc., 2002, 124, 3202. 440. M. Muehlhofer, T. Strassner and W. A. Herrmann, Angew. Chem., Int. Ed. Engl., 2002, 41, 1745. 441. D. S. McGuinness, W. Mueller, P. Wasserscheid, K. J. Cavell, B. W. Skelton, A. H. White and U. Englert, Organometallics, 2002, 21, 175. 442. G. A. Grasa, Z. Moore, K. L. Martin, E. D. Stevens, S. P. Nolan, V. Paquet and H. Lebel, J. Organomet. Chem., 2002, 658, 126. 443. R. Jackstell, M. G. Andreu, A. Frisch, K. Selvakumar, A. Zapf, H. Klein, A. Spannenberg, D. Rottger, O. Briel, R. Karch and M. Beller, Angew. Chem., Int. Ed. Engl., 2002, 41, 986. 444. R. W. Simms, M. J. Drewitt and M. C. Baird, Organometallics, 2002, 21, 2958.

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445. D. V. Deubel, Organometallics, 2002, 21, 4303. 446. A. A. Danopoulos, S. Winston and M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 2002, 3090. 447. A. Kovacevic, S. Grundemann, J. R. Miecznikowski, E. Clot, O. Eisenstein and R. H. Crabtree, J. Chem. Soc., Chem. Commun., 2002, 2580. 448. S. Grundemann, A. Kovacevic, M. Albrecht, J. W. Faller and R. H. Crabtree, J. Am. Chem. Soc., 2002, 124, 10473. 449. L. Jafarpour, A. C. Hillier and S. P. Nolan, Organometallics, 2002, 21, 442. 450. W. Baratta, E. Herdtweck, W. A. Herrmann, P. Rigo and J. Schwarz, Organometallics, 2002, 21, 2101. 451. A. Furstner, O. R. Thiel and C. W. Lehmann, Organometallics, 2002, 21, 331. 452. R. F. R. Jazzar, S. A. Macgregor, M. F. Mahon, S. P. Richards and M. K. Whittlesey, J. Am. Chem. Soc., 2002, 124, 4944. 453. M. V. Baker, B. W. Skelton, A. H. White and C. C. Williams, Organometallics, 2002, 21, 2674. 454. Y. Kim, Y. Kwak, Y. Joo and S. Lee, J. Chem. Soc., Dalton Trans., 2002, 144. 455. R. E. Douthwaite, M. L. H. Green, P. J. Silcock and P. T. Gomes, J. Chem. Soc., Dalton Trans., 2002, 1386. 456. S. Grundemann, M. A. Albrecht, A. Kovacevic, J. W. Faller and R. H. Crabtree, J. Chem. Soc., Dalton Trans., 2002, 2163. 457. F. Glorius, G. Altenhoff, R. Goddard and C. Lehmann, J. Chem. Soc., Chem. Commun., 2002, 2704. 458. V. Cesar, S. Bellemin-Laponnaz and L. H. Gade, Organometallics, 2002, 21, 5204. 459. R. Weiss and N. Kraut, Angew. Chem., Int. Ed. Engl., 2002, 41, 311. 460. D. J. Nielsen, A. M. Magill, B. F. Yates, K. J. Cavell, B. W. Skelton and A. H. White, J. Chem. Soc., Chem. Commun., 2002, 2500. 461. S. Caddick, F. G. N. Cloke, P. B. Hitchcock, J. Leonard, A. Lewis, D. McKerrecher and L. R. Titcomb, Organometallics, 2002, 21, 4318. 462. Y. Motoyama, K. Shimozono, K. Aoki and H. Nishiyama, Organometallics, 2002, 21, 1684. 463. U. Herber, T. Pechmann, B. Weberndorfer, K. Ilg and H. Werner, Chem. Eur. J., 2002, 8, 309. 464. T. Pechmann, C. D. Brandt, C. Roger and H. Werner, Angew. Chem., Int. Ed. Engl., 2002, 41, 2301. 465. K. Shiomi, B. K. Breedlove, H. Kitayama, T. Nishioka, I. Kinoshita, N. Koga and K. Isobec, J. Chem. Soc., Chem. Commun., 2002, 1756. 466. D. A. Ortmann, B. Weberndorfer, K. Ilg, M. Laubender and H. Werner, Organometallics, 2002, 21, 2369. 467. M. Brym, C. Jones and A. F. Richards, J. Chem. Soc., Dalton Trans., 2002, 2800. 468. Y. Li, J. Huang, Z. Zhou, C. Che and X. You, J. Am. Chem. Soc., 2002, 124, 13185. 469. J. Fillaut, I. de los Rios, D. Masi, A. Romerosa, F. Zanobini and M. Peruzzini, Eur. J. Inorg. Chem., 2002, 935. 470. D. G. Gusev, T. Maxwell, F. M. Dolgushin, M. Lyssenko and A. J. Lough, Organometallics, 2002, 21, 1095. 471. K. Bieger, J. Diez, M. P. Gamasa, J. Gimeno, M. Pavlista, Y. Rodriguez-Alvarez, S. Garcia-Grande and R. Santiago-Garcia, Eur. J. Inorg. Chem., 2002, 1647. 472. E. Ruba, K. Mereiter, R. Schmid, V. N. Sapunov, K. Kirchner, H. Schottenberger, M. J. Calhorda and L. F. Veiros, Chem. Eur. J., 2002, 8, 3948. 473. E. Ruba, K. Mereiter, R. Schmid, K. Kirchner, E. Bustelo, M. C. Puerta and P. Valerga, Organometallics, 2002, 21, 2912.

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474. C. M. Standfest-Hauser, K. Mereiter, R. Schmid and K. Kirchner, Organometallics, 2002, 21, 4891. 475. A. Hejl, T. M. Trnka, M. W. Day and R. H. Crabtree, J. Chem. Soc., Chem. Commun., 2002, 2524. 476. T. Takao, T. Takemori, M. Moriya and H. Suzuki, Organometallics, 2002, 21, 5190. 477. H. Yan, A. M. Beatty and T. P. Fehlner, J. Am. Chem. Soc., 2002, 124, 10280. 478. S. P. Tunik, I. A. Balova, M. E. Borovitov, E. Nordlander, M. Haukka and T. A. Pakkanen, J. Chem. Soc., Dalton Trans., 2002, 827. 479. D. J. Mindiola and G. L. Hillhouse, J. Am. Chem. Soc., 2002, 124, 9976. 480. T. Sugihara, A. Wakabayashi, Y. Nagai, H. Takao, H. Imagawa and M. Nishizawa, J. Chem. Soc., Chem. Commun., 2002, 576. 481. R. Reina, O. Riba, O. Rossell, M. Seco, M. Font-Bardia and X. Solans, Organometallics, 2002, 21, 5307. 482. S. Zhang, Q. Xu, J. Sun and J. Chen, Organometallics, 2002, 21, 4572. 483. E. Bustelo, M. Jimenez-Tenorio, K. Mereiter, M. C. Puerta and P. Valerga, Organometallics, 2002, 21, 1903. 484. G. Ferrando-Miguel, H. Gerard, O. Eisenstein and K. G. Caulton, Inorg. Chem., 2002, 41, 6440.

11 Reactions of Transition Metal p -Complexes of Alkenes, Alkynes and Dienes GREGORY A. SOLAN

1

Introduction

This chapter covers highlights from the chemical literature in the years 2001 and 2002 and is compiled from a review of papers published in the primary and review literature. The scope of the review is to focus on the reactivity of transition metal complexes containing p-bonded carbon-based ligands and, specifically, alkenes (R2C:CR2), alkynes (RC∫CR) and cyclic (but not cyclopentadienyl) and non-cyclic dienes [R2C:CR-CR:CR2 (conjugated) or R2C:CR-(CR2)nCR:CR2 (non-conjugated)] where R : H or hydrocarbyl. In the main, the reactivity will be illustrated by the use of well-defined p-complexes; transition states or intermediates that can be considered as involving these types of pinteractions will be included where deemed important. Computational work in the field will be also included where supporting the reactivity of these p-complexes. Both mono- and bi-metallic organotransition metal complexes will be discussed; cluster based complexes with three or more metal centres will not. As is clear from the immense nature of the field, this review is not and cannot be comprehensive. As a consequence the author has selected a number of areas in which there has been, in his opinion, outstanding developments or sustained interest. Catalytic and stoichiometric organometallic species are considered with equal preference and the coverage is divided into the following sections dealing with monoalkenes, alkynes, dienes and bimetallic complexes.

2

Reviews

A number of reviews covering topics within the remit of this report have appeared during the years 2001 and 2002. A dedicated issue of Journal of Organometallic Chemistry celebrating the 50th anniversary of Dewar’s landmark paper on the development of a bonding model for metal-alkene complexes contained many relevant papers as did a special issue in the same journal on the subject of interactions of p-systems with metals.1,2 An issue dedicated to metals in organic synthesis also includes a number of related papers.3 A series of Dalton Transaction ‘Perspectives’ has appeared on Ziegler-Natta heterogeneous cataOrganometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 314

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315

lyst compounds,4 on transition metal-carbon multiple bonds5 and on metalcatalysed polymerisation reactions.6 Angewandte Chemie ‘Highlights’ have been published on late transition metal olefin polymerisation catalysis,7 the use of Ruand Rh-catalysed asymmetric hydrogenations8 and on asymmetric hydrovinylation of olefins.9 A number of reviews in Angewandte Chemie have appeared on the subject of transition metal-mediated catalytic reactions and contain a variety of examples of p hydrocarbon-containing complexes.10-14 Also presented in the same journal are relevant reviews on olefin polymerisation catalysts in aqueous media15 and catalysts for the living polymerisation of alkenes.16 A Polyhedron ‘Report’ on the subject of homogeneous catalysis using platinum complexes17 has appeared as has a Angewandte Chemie review focusing on the use of pincer ligands in platinum group chemistry.10 The use of homogeneous vanadiumbased catalysts for the polymerisation of a-olefins has been the subject of a Chemical Society review.18 An account of the general aspects of mononuclear s-trans 1,3-diene complexes is reported in Journal of Organometallic Chemistry.19 A series of European Journal of Inorganic Chemistry ‘Microreviews’ have appeared on the subjects of developments in half-sandwich (carbene)metal (metal : Fe, Ru, Os) complexes,20 on catalysts for the hydroxycarbonylation of styrene21 and on the chemistry of boron-bridged group IV metallocene complexes.22 An award article in Inorganic Chemistry by Professor F.A. Cotton reviews developments in ‘nonclassical’ organometallic chemistry over a halfcentury covers related material.23A review covering ruthenium-catalysed carboncarbon bond formation has also been published.24

3

Complexes Containing p-Coordinated Alkenes

3.1 Well-defined p-Coordinated Alkenes. — 3.1.1 Group 4 (Ti, Zr, Hf ). Two groups have independently reported zirconocene cations, [
Me2Si(h5C5H4)2Zr
CH2Si(Me)2CH2CH:CH2]; (1) and [(h5-Cp5)2Zr
CH2Si(Me)2CH2CH:CH2]; (2), containing coordinated tethered olefins. 1H NMR studies on the ansa-metallocene cation 1 reveals that dissociation and re-coordination is apparent in solution of the pendant olefin (Scheme 1).25,26 A related dissociation of the olefin has been proposed to explain the interconversion of the diastereoisomers in [(h5:h1-C5Me4SiMe2NButTi(OCMe2CH2CH2CH:CH2)] [MeB(C6F5)3] (3).27 Treatment of [(h5-Cp)Zr(h2-H2C:CHCH2CH3)(PMe)3] (4) with methylenecyclopropane gave, via a [2;2] cycloaddition, the zirconacyclopentanecontaining species [(h5-Cp)Zr
-C(-CH2CH2-)CH2C(-CH2CH2-)CH2-] (5); the size of the metallcycle formed is found to be dependent on the nature of the h5-ancillary ligand (Scheme 2).28 A related zirconacyclopentane formation has also been reported on reaction of the 2-vinyl-pyridine-coordinated ansa-zirconocene complex [
(C2H4)(h5-C9H10)2Zr(C5H4N-2-CH:CH2)] (6) with excess norbornadiene (NBD).29 Reaction of [(h5-Cp)Zr(h2-H2CCH2)] (7) with diethyl chlorophosphate results in an usual metal-phosphorylation of the alkene to give [(h5-

Organometallic Chemistry

316

Me2Si

SiMe2

Zr

Me2Si

Zr

Si Me2

Me2Si

Zr

SiMe2

1

Zr

Me2Si

SiMe2

Scheme 1

Zr

Zr PMe3

5

4 Scheme 2

Cp)Zr
CH2CH2P(O)(OEt)2] (8) (Scheme 3).30 In contrast, reaction of 7 with ButCH:NCH2Ph in the presence of excess grignard reagent RMgBr (R : Me, Et) affords, following hydrolysis, the organic coupling product ButCH(CH2CH3):NHCH2Ph.31

Zr

ClP(O)(OEt)2 P(O)(OEt)2

Zr Cl

7

8 Scheme 3

3.1.2 Group 6 (Cr, Mo, W). The substitution kinetics of the reaction of h2-Zcyclooctene (h2-co) complex [Cr(CO)5(h2-co)] (9) with tetracyanoethylene (tcne) to give [Cr(CO)5(h2-tcne)] (10) were found to obey a pseudo-first order kinetics with respect to the concentration of the starting complex and the rate determining step is the cleavage of the metal olefin bond in 9.32 The Mo(IV) h2-olefin complexes, [Mo(NPh)(h2-olefin)
o-(Me3SiN)2C6H4]

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

317

(11: olefin : propene, isobutene) react with molecular hydrogen in the presence of excess arene affording Mo(IV) h6-arene complexes of the type [Mo(NPh)(h6arene)
o-(Me3SiN)2C6H4] (12: arene : benzene, toluene, m-xylene) (Scheme 4); the complexes have been used as catalysts for the hydrogenation of olefins.33,34 Ph N MeSi N MeSi

Ph R1

Mo N

H

N R2

H2 (15 psi)

MeSi N MeSi

3

R

Mo N

R3

H

12

11

R3 = benzene, toluene, m-xylene

R1 = H, R2 = Me R1 = R2 = Me

Scheme 4

Reaction of the calix[4]arene-containing complex [
p-But-calix[4](O)4W(h2C6H10)] (13) with azides and diazoalkanes has been examined. With organoazides (RN3), following alkene displacement, two possible reaction pathways are evident (see products 14 vs. 15), which are found to be dependent on the nature of the R group (Scheme 5).35 Density functional theory calculations (DFT) have been performed on the ethylene rearrangements that are assisted by tungsten calix[4]arenes using [
p-But-calix[4](O)4W(h2-C2H4)] (16) as the starting reactant in the rearrangement process.36 Ph N R

N

N O

W O

O O

RN3

O

O

W O

O O

PhN3

N

O

W O

(R = CPh3, SiMe3)

14

O

13

O W O

N

O O

N N Ph

15

Scheme 5

3.1.3 Group 8 (Fe, Ru, Os). Hybrid HF-DFT calculations are used to clarify the energetics of the competing disrotatory and conrotatory ring-opening pathways for Fe(CO)3-templated rearrangement of methylenecyclopropane (17) to trimethylenemethanes (18).37 It is shown that the barrier for conrotatory ringopening is significantly higher than the barrier for disrotatory ring-opening away from the metal. On the basis of a range of exchange experiments, the h2alkene/hydrido iron(II) species 20 has been proposed as an intermediate to account for the tert- to iso-butyl isomerisation that occurs between [
CH(CMeNAr)2Fe
C(CH3)3] (19) and [
CH(CMeNAr)2Fe
CH2CH(CH3)2] (21) (Scheme 6).38

Organometallic Chemistry

318

NAr

NAr

Fe

NAr

H

Fe

NAr

Fe

NAr

19

NAr

21

20 i

Ar = 2,6-Pr 2C6H3

Scheme 6

A stable Ru(0) aqua complex, [Ru
h2-(MeO2C)HC:CH(CO2Me)2(dppe)(OH2)] (23) was prepared from the reaction of the h2-alkene species [Ru(h6-1,3,5C8H10)
h2-(MeO2C)HC:CH(CO2Me)2] (22) with dppe (diphenylphosphinoethane) in a mixture of dichloroethane and water (Scheme 7).39 CO2Me CO2Me MeO2C

Ph2P Me2OC

dppe Ru

CH2Cl2/H2O

CO2Me

PPh2

Ru MeO

O

OMe

MeO2C

H

O

22

O

H

23

Scheme 7

3.1.4 Group 9 (Co, Rh, Ir). Reaction of the h2-ethylene Co(I) complex [(h5C5H4CH2CH2PBut)Co(h2-CH2CH2)] (24) with an Arduengo-type carbene results in displacement of the phosphino-arm and coordination of the carbene to give 25 (Scheme 8).40 Complex 24 has also been used as a catalyst for the [2;2;2] cyclotrimerisation of terminal alkynes in an aqueous medium.41 t

Me i

Pr N

PBu 2

Me

..

NPr

i

H 2C

P t Bu 2

CH2

Pr N

Co

Co

i

H2C

Me CH 2

i

Pr N

25

24

Me

Scheme 8

The spectroscopically characterised Co(I) complex [
2,6-(C(Me):N(2,6Pri2C6H3))2C5H3NCo(h2-C2H4)]
MeB(C6F5)3 (26) acts as catalyst for ethylene polymerisation (Scheme 9); the mechanism of the polymerisation remains uncer-

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

319

tain.42 Notably the Co(I) alkyl complex [
2,6-(C(Me):N(2,6PriC6H3))2C5H3NCoR] (27: R : n-propyl, n-butyl) is not active for ethylene polymerisation but reacts with ethylene to give [
2,6-(C(Me):N(2,6PriC6H3))2C5H3NCoEt] (28) and an a-olefin; a mechanism involving b-hydrogen transfer to metal, a-olefin displacement and insertion of ethylene into the metal-hydride has been proposed.43 [MeB(C6F5)3]

cat. N Ar

N Co

N Ar

28 (Ar = 2,6-Pri2C6H3)

n

Scheme 9

The reaction of the 14 electron complex [
ArNC(Me)CHC(Me)NArRh(h2COE)] (29: Ar : 2,6-Me2C6H3) with Ph-X (X : Cl, Br), in the presence of dihydrogen, leads to the displacement of COE (cyclooctene) and oxidative addition to give the dinuclear complex [
ArNC(Me)CHC(Me)NArRhPh (m-X)]2 (30: X : Br, Cl) (Scheme 10).44 In contrast, the reaction of 29 with Ph-F gives the dinuclear inverse sandwich complex [
(ArNC(Me)CHC(Me)NAr)Rh2(anti-m-h4:h4-PhF)]2 (31).

F

N Rh

N Rh

N

N

31

Ph-F

N

H2

H2

N

29

X

N

Ph-X

Rh

Ph

Rh N

N Rh

X

Ph

N

30: X = Br, Cl

Scheme 10

The bis(alkene) Rh(I) complex [(h5-Cp*)Rh
h2-H2C:CH(SiMe3)2] (32) catalyses the dehydrocoupling of secondary phosphines (PR2H) giving the diphosphane (P2R4) along with dihydrogen.45 In the presence of an olefin, transfer hydrogenation occurs giving the corresponding alkane and diphosphane. The Rh(I)-TPA system [(TPA)Rh(h2-H2C:CH2)](PF6) (33: TPA : tris(2-pyridyl)amine)) undergoes a dioxygenation reaction with air to give the stable geometric rhod-1,2-dioxolanes isomers [(TPA)Rh(-O-OCH2CH2)](PF6) (34 and 35) of which complex 35 has been structurally characterised (Scheme 11).46 The more sterically bulky complex, [(Me3TPA)Rh(h2-H2C:CH2)](PF6) (36) also undergoes reaction with dioxygen, in this case resulting in displacement of the ethylene molecule and formation of the Rh(III) peroxo complex [(Me3TPA)Rh(h2-O2)](PF6) (37) (Scheme 12).47 Remarkably, the reaction of the

Organometallic Chemistry

320

N N

N

N Rh

N

N

N

Air

N

N

+

N Rh

N O

O O

O

33

N Rh

35

34

Scheme 11

N N N

O2

N Rh

O

-C2H4

N

N N N

N M

O2

O N N

O N

Ir

O

37

36 M = Rh 38 M = Ir

39

Scheme 12

Ir analogue of 36, [(Me3TPA)Ir(h2-H2C:CH2)](PF6) (38), with O2 results in displacement of one of the pyridyl arms and formation of the unprecedented Ir(III) ethene-peroxo complex [(Me3TPA)Ir(h2-H2C:CH2)(h2-O2)](PF6) (39). The rare mononuclear Ir(II) species [(MenTPA)Ir(h2-H2C:CH2)]2; (40a n : 2; 40b n : 3) convert in acetonitrile to the C2H4-bridged dinuclear Ir(III) species [(MenTPA)(MeCN)Ir(m-H2CCH2)Ir(NCMe)(MenTPA)] (41a n : 2; 41b n : 3) via coupling of Ir(II)ethene with Ir(II) (Scheme 13).48 In contrast, treatment of 40b with dioxygen results in C-O bond formation and gives the Ir(III)-formylmethyl complex [(Me3TPA)Ir
CH2C(O)H(NCMe)]2; (42).49

Scheme 13

Heating of a solution of [(Tp)Ir(h2-C2H4)2] (43: Tp : hydridotris(pyrazolyl)borate) in THF, in the presence of dimethyl acetylenedicarboxylate, gives the metallacyclopent-2-ene species [(Tp)Ir
CH2CH2C(CO2Me): C(CO2Me)(THF)] (44) (Scheme 14).50 Interestingly, 44 undergoes further reaction with dimethyl acetylenedicarboxylate to afford the metallacyclopentadiene-

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

321

alkene complex [(Tp)Ir
C4(CO2Me)4(h2-CH2:CH2)] (45). Reaction of 43 with diphenylphosphinomethane (dppm) leads to displacement of ethylene and intramolecular proton transfer from the methylene bridge of the dppm ligand to the iridium centre to give the neutral hydride complex [(Tp)IrH(Ph2PCHPPh2)] (46).51 Ph2 P

H

MeO2C H

TpIr

Ph2P

PPh2

-2 C2H4

C2(CO2Me)2

TpIr

MeO2C C2(CO2Me)2

TpIr

THF

P Ph2

CO2Me

C6H6

45

44

43

CO2Me CO2Me

THF

46

CO2Me

TpIr

Scheme 14

Thermolysis of the Me2-substituted Tp derivative of 43, [(TpMe2)Ir(h2-C2H4)2] (47:TpMe2 : tris(3,5-dimethylpyrazolyl)borate) in wet acetonitrile at 100 oC gives the iradapyrrole complex [(TpMe2)Ir
CHCHCMeNH(Et)] (49); the intermediate [(TpMe2)Ir(h2-CH:CH2)(Et)] (48) could also be isolated on treating 47 with acetonitrile at lower temperature (Scheme 15).52

MeCN

TpMe2Ir

60 oC

MeCN (H2O)

TpMe2Ir

NCMe

100 oC

Et H N Ir H

Me H

49

48

47

Tp

Me2

Scheme 15

A study has been carried out into the structural and chemical properties of cationic [(TpmMe2)Ir(h2-C2H4)2](PF6) (50: TpmMe2 : tris(3,5-dimethylpyrazolyl)methane) and compared with its neutral analogue 47.53 It has been concluded that, while 50 displays structural properties closely reminiscent of those in 47, it does display some differences in reactivity. For example, the hydride-a,w-butenyl species 51 can be obtained on thermal solid-state activation of 50 (Scheme 16), a reaction pathway not observed on treating 47 under similar reaction conditions.

Tpm

Me2

Heat

Ir

Tpm

Me2

Ir

51

50

Scheme 16

3.1.5 Group 10 (Ni, Pd, Pt). Warming a solution of the palladium(II) 5-hexenyl chelate complex [(phen)Pd
h1:h2-CH(CH2SiEt3)CH2C(CO2Me)2CH2CH:

Organometallic Chemistry

322

CH2][B
3,5-(CF3)2C6H34] (52: phen : 1,10-phenanthroline) in ArCN leads to a intramolecular b-migratory insertion to form the palladium cyclopentylmethyl complex 53 (Scheme 17).54 The transformation represents the first direct evidence of b-migratory insertion of a coordinated olefin into a M-C bond of a transition metal alkyl-olefin chelate complex. SiEt3 Et3Si N

CO2Me CO2Me

warming

Pd

ArCN

N

Pd

N MeO2C

NCAr

CO2Me N

52

53

Scheme 17

Displacement of the coordinated ethylene molecule in the Pt(0) complex [(dcpe)Pt(h2-C2H4)] (54) (dcpe : dicyclohexylphosphinoethane) by Ph2PC∫CMe, results in coordination of the alkyne moiety of the phosphinoalkyne to give complex [(dcpe)Pt(h2-Ph2PCCMe)] (55) (Scheme 18). Treatment of 55 with HCl results in oxidation and the formation of the 4-coordinate h1-vinyl complex [(dcpe)PtCl
C:CHMe)PPh2] (56). In dichloromethane, reversible dissociation of the chloride ion in 56 gives the cation [(dcpe)Pt
C(:CHMe)PPh2]; (57), in which the phosphorus atom is constrained into an unusual 3-membered methylenephosphaplatinacycle. Treatment of 57 with methyl iodide results in the formation of a salt, [(dcpe)Pt
h2C(:CH2):CHPPh2Me];I9 (58), in which the alkynylphosphine has rearranged to the corresponding allene.55 Ph2P Cy2 P C

PPh2 Cy2 P

CH2

Pt CH2

P Cy2

54

Ph2PCCMe -C2H4

Cy2 P

C

HCl

Pt

CH(Me)

Pt C

P Cy2

Cl

P Cy2

Me

56

55

CH(Me) Cy2 P

Cl

CH2Cl2

C

Pt P Cy2

PPh2

57

Scheme 18

Ethylene displacement also occurs during the reaction of [(Ph3P)2Pt(h2CH2CH2)] (59) with the P-coordinated phosphinoalkyne complex cis[Pt(CCR)2(PPh2CCR)2] (60: R : Ph, Tol; R : Ph, But) affording the mixedvalence Pt(II)-Pt(0) bimetallic complexes [(Ph3P)2Pt(m:h2:h1-PPh2CCR) Pt(CCR)2(PPh2CCR)] (61) (Scheme 19).56 The enhanced Brønsted acidity of the allylic protons in propylene when co-

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes R'CC

PPh3

+

Pt

PPh2CCR' R

R'CC

PPh2CCR

Pt

Pt -C2H4

PPh3

PPh3

60

59

Pt

Ph2P

R'CC

PPh2CCR

R'CC

PPh3

323

61

Scheme 19

ordinated to Pt(II) in cationic [(tmeda)Pt(h2-CH2CHCH3)Cl](ClO4) (62: tmeda : N,N,NN-tetramethylethylenediamine) has been exploited to furnish, via 63, bimetallic 64, in which the metal centres are linked by a bridging h1,h3-allyl moiety (Scheme 20).57 A further study has also been reported with a range of terminal alkenes giving both mono- and di-nuclear allyl-containing Pt(II) complexes, resulting from a series of deprotonation reactions.58 Me

base

Cl

Cl

H C N

base

Pt

N

Cl

N

N Pt

Pt

Pt N

H2 C

N

N

N

63

62

H2C

2+

Cl

N Pt

C H

N

64

Scheme 20

Platinum(II) complexes of the type, [
(C5H3N)-2,6-(CH2PPh2)2Pt(h2CHR:CHR)](BF4)2 (65) are highly reactive to weakly basic nucleophiles such as MeOH, H2O and aromatic amines giving substituted alkyls of the type [
(C5H3N)-2,6-(CH2PPh2)2Pt(CHRCHRNu)]; (66) (Scheme 21). Nucleophilic addition was found to be kinetically and thermodynamically favoured over alkene substitution.59 A related study has also been performed on the olefins coordinated to the highly electrophilic site in the dicationic palladium(II) complexes [
(C5H3N)-2,6-(CH2PPh2)2Pd(h2-olefin)](BF4)2 (67).60 2+

PPh2 CHR N

PPh2 MeOH

Pt

N

Pt

CH(R)(MeO) C HR

CHR PPh2

PPh2

66

65 Scheme 21

3.1.6 Other Metal Centres. Group 11 (Cu)- The copper(I) h2-ethylene complex, [
(SiMe3)NP(But)2N(SiMe3)Cu(h2-H2CCH2)] (68), was used to prepare the highly reactive copper(I) carbene complex [
(SiMe3)NP(But)2N(SiMe3)Cu
: C(CO2Me)Ph] (69) by reacting 68 with N2:C(CO2Me)Ph (Scheme 22). Complex 69 has proved active in olefin cyclopropanation.61 The copper(I) h2-ethylene complex, [
HB(3-(CF3),R-5-Pz)3Cu(h2-H2CCH2)] (70: R : CF3, Ph H; Pz : pyrazolyl), containing fluorinated substituents and is found to catalyse olefin aziridinations.62

Organometallic Chemistry

324

SiMe3 N

SiMe3 N CH2

But

P

But

Cu

CO2Me

+ N2

N SiMe3

-C2H4/-N2

Ph

CH2

But But

CO2Me P

Cu Ph

N SiMe3

69

68

Scheme 22

The neutral b-diketiminato copper(I) complex, [(CH
C(Me)N(2,6Me2C6H3)2)Cu(h2-C2H4)] (71), is found to activate dioxygen to give the crystallographically characterised dicopper complex [(CH
C(Me)N(2,6Me2C6H3)2)Cu(m-OH)]2 (73); the Cu(III)2(m-O)2 dimer (72) has been proposed as an intermediate (Scheme 23).63

H

N

CH2 Cu CH2

N

71

O2

N

N

O

N

Cu O

N

O

2 'H'

Cu

-C2H4

N

Cu

Cu N

72

N

O

N

H

73

Scheme 23

3.2 p-Coordinated Alkenes Generated In-situ. — 3.2.1 Alkene Polymerisation. 2001 and 2002 have witnessed a large number of reports in which ethylene or an a-olefin can be oligomerised or polymerised in the presence of a transition metal catalyst. While group 4 cyclopentadienyl-based systems (viz. metallocene and constrained geometry catalysts) have continued to lead the way64-126 the emergence of non-cyclopentadienyl systems127-153 and, in particular, late transition metal catalysts (see Table 1) has seen a steady increase in the number of reports.154-187 In general, both types of catalytic systems operate by coordinationinsertion mechanism whereby the alkene coordinates then is inserted into a M-H or M-C bond. Hessen et al. have modelled the first insertion of an alkene by an aluminoxaneactivated zirconocene by preparing [(h5-Cp*)2Zr
-CH2C(Me)CH2S(Me)-] [Al4B4(R)4(Ar)4Me(m-O)8] (74) from the reaction of [(h5-Cp*)2ZrMe2] (73) with the boraaluminoxane and thioalkene (Scheme 24).188 While the complexation of an alkene monomer to a metal-alkyl complex has been proposed as a key intermediate for olefin polymerisation catalysts, their characterisation has proved extremely challenging. Several groups have used

Fe

Si

O

Si

:xs MAO

X

O

ArN

ArN

R

:xs MAO

X

:xs MAO

H

O

X

Fe

PR3

N

N

PR3

X

Fe

NAr

ArN

Cl

X

N

X

R

Cl

NAr H

:xs MAO

NAr

NAr

Fe

O

N

Fe

N

Fe

MAO : methylaluminoxane

X

X

ArN

N

:xs MAO

ArN

ArN

H Cl

O

Cl

Co

N

Co

:xs MAO

:xs MAO

Cl

NAr

NAr

Co catalysts163-165

Cl

ArN

(C6F5)3B

Pd

Ni

Ni

Cl

Ph

PPh3

:xs MAO

P(But)2

N Ar

O

N Ar

N

Ni, Pd catalysts166-186

EA

Ph

MeN P Ar2

Ar2 P

O

Ni

Ph2 P

Ni Br

PPh3

Ph

:xs MAO

Br

Selected examples of late transition metal alkene polymerisation catalysts reported in 2001-2002

Fe catalysts154-162

Table 1

Ph

Ph

Cu Cl

N

:xs MAO

Cl

N

Cu catalysts187

Ph

Ph

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes 325

Organometallic Chemistry

326 Ar O B Ar Al O B O O O Al Ar B Al R O O R

R R

Al O B

Ar O B Ar Al Me O B O Me O O Al Al Ar B R O O R

Ar

Cp*2ZrMe2

Cp*2Zr S Me

S Me

73

R R

Al

O

B Ar

74

Scheme 24

variable temperature 1H NMR spectroscopy to monitor the coordination of the alkene with the catalyst.189,190 For example Brookhart et al. have generated the h2-propylene complex [(a-diimine)PdMe(h2-CH2:CH-CH3)](BAr4) (75: Ar : 3,5-(CF3)2C6H3) at low temperature and have monitored the insertion reaction of the alkene on warming (Scheme 25). Both 1,2- and 2,1-insertions followed by isomerisation reactions are apparent and large number of kinetic parameters have been established about the reaction mechanism.190 Ar 1,2-insertion 80%

Ar

N

N Pd

Pd H

N

N

Ar

H Ar

Ar N Pd

Ar = 2,6-PriC6H3

N Ar

75 Ar 2,1-insertion

N

N Pd

20%

Ar

Ar

N

Pd

Pd H

N

H

N

Ar

Ar

H

N Ar

Scheme 25

3.2.2 Alkene Metathesis. The strong impact of olefin metathesis, on organic synthesis [in ring closing metathesis (RCM),191-201 cross metathesis (CM)202-208 and enyne metathesis209-211] and polymer chemistry [ring-opening metatheis polymerisation (ROMP)212-221] has seen a large number of reports of transition metal-based systems during the years 2001 and 2002.191-239 Considerable emphasis has been placed on catalysts that combine high activity, good durability, tolerance towards functional groups and adaptability to alternative reaction media. Table 2 illustrates some of the key catalysts that have been employed. 3.2.3 Other Catalytic Transformations Involving p-Coordinated Alkenes. Catalysts for the alternating copolymerisation of alkene and carbon monoxide240-264 have continued to be the source of a number of reports as has the related oxidative carbonylation265-272 and hydroformylation273-291 of olefins. In addition, hydrosilylation292-294 and hydrovinylation9,295 of alkenes mediated by transition metal-based catalysts has generated a number of publications.

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

Table 2

Selected examples of ruthenium and molybdenum metathesis catalysts employed in 2001-2002

Ru catalysts

Mo catalysts

Mes N

Mes N

N Mes Cl

N Mes N

Cl

H Ru

R

H

N

R

PCy3

Br

Mes N

N Mes

R

Cl

N Mes

R

H

N

Ru Cl

H Ru

Ph Me

O

Ph

Cl

Me

Mo

O

Ph

Cl

R N

R

Ru

Br

Cl

Mes N

327

O i

Pr OPr'

Mo

O O

Me Ph Me

Cl O i

Pr

4

Complexes Containing p-Coordinated Alkynes

4.1 Well-defined p-Coordinated Alkynes. — 4.1.1 Group 4 (Ti, Zr, Hf ). The titanocene-h2-alkyne complexes [(h5-Cp)2Ti(h2-Me3SiC∫CR)] (76a R : Ph, 76b R : SiMe3) act as efficient catalysts for the hydroamination of terminal alkynes, RC∫CH (R : alkyl), to give imines [RCH2C(:NBut)H], with good to excellent regioselectivities for the anti-Markownikov products.296 Complexes 76a and 76b have been reacted with diisobutylaluminium hydride at room temperature to give the heterobimetallic complexes [(h5-Cp)2Ti(m-h1:h2-RCCSiMe3)(mH)
Al(Bui)2] (77a R : Ph, 77b R : SiMe3) (Scheme 26);297 both complexes act as initiators for the ring-opening polymerisation of lactones. Thermolysis of titanocene-bis(trimethysilyl)ethyne complexes [Ti
h5-C5Me4(CH2R)2(h2Me3SiC∫CSiMe3)] (78) affords complex 80, in which displacement of the alkyne has occurred and activation of one the cyclopentadienyl groups has taken place.298 The ansa-metallocene [
h5:h5-O(Me2SiC5Me4)2Ti(h2-Me3SiC2SiMe3)] (79) decomposes at 170 oC in high vacuum with liberation of 1,2-bis(trimethylsilyl)ethene to give the allylidene complex [Ti
h5-C5Me4)SiMe2OSiMe2
h3:h4C5Me2(CH2)2] (81).299 The titanium h2-dialkyne complex [(OPri)2Ti(h2-Me3SiCC-C∫CSiMe3)] (82) undergoes coupling with 1-octyne to form a metallacyclopentadiene species 83 which undergoes demetallation on treatment with acid to give the corresponding dienyne (Scheme 27).300 The reactions of the zirconocene-alkyne complex [(h5-Cp)2Zr(h2-

Organometallic Chemistry

328

Scheme 26 PriO Me3Si

Me3Si Ti(OPr i)2

Me3Si

82

OPri

C6H13

Me3Si

Me3Si

Ti C6H13

H+

C6H13

Me3Si 83

Scheme 27

Me3SiC∫CSiMe3)(Py)] (84) with the a,b-unsaturated carbonyl compound, CH2:C(O)Me, gives three different products, the 1,2-inserted complex [(h5Cp)2Zr
C(SiMe3):C(SiMe3)C(CH:CH2)(Me)O] (85), the alkyne eliminated species [(h5-Cp)2Zr
CH2CH:C(Me)HO] (86) and the seven-membered zirconacyclic complex [(h5-Cp)2Zr
C(SiMe3):C(SiMe3)CH2CH:CH(Me)-O] (87) (Scheme 28).301 Complex 84 can also be used for the macrocyclisation of bipyridine-containing diynes to give 88 in which the zirconocene reagent acts as both coupling and templating agent.302 4.1.2 Group 6 (Cr, Mo, W). Single crystal X-ray studies have been carried out to study the effect of a one electron oxidation on complex [Cr(CO)2(h2-RCCR)(h6C6Me6)] (89a R : Ph, 89b R : C6H4OMe-p) and reveal a shortening of the Cr-C(alkyne) bond and a lengthening of the Cr-CO bonds in [Cr(CO)2(h2PhCCPh)(h6-C6Me6)](PF6) (90a).303 Substitution of a CO group in 90 with P(OCH2)3CEt gives [Cr(CO)
P(OCH2)3CEt(h2-RCCR)(h6-C6Me6)]; (91a R :

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

329

SiMe3 Me3Si Me3Si

Cp2Zr

Cp2Zr

SiMe3

N

O N

85 SiMe3

N

N N

SiMe3

SiMe3

O Cp2Zr O

Cp2Zr

86

N

Py

SiMe3

84

Me3Si

SiMe3

Cp2Zr Me3Si

SiMe3

O

Cp2Zr

87

88

Scheme 28

Ph, 91b R : C6H4OMe-p) which, in the case of 91b, readily undergoes oxidation with AgPF6 to afford the dication [Cr(CO)
P(OCH2)3Cet(h2-RCCR)(h6C6Me6)](PF6)2 (92 R : C6H4OMe-p) (Scheme 29). These series of experiments demonstrate that the alkyne ligand has the capacity to act as two-, three- or four-electron donor.

Scheme 29 5

The complex [(h -Cp*)Mo(CO)(h2-PhCCPh)2](BF4) (93) reacts with four equivalents of 2,6-dimethylphenyl isocyanide, CN(xyl) to give, following alkyne-isocyanide coupling, the diiminometallacyclopentene [(h5Cp*)Mo
C(:Nxyl)C(Ph):C(Ph)C:Nxyl(CNxyl)2](BF4) (94) (Scheme 30).304 Alkyne-isocyanide coupling also results when [(h5t Cp*)Mo(CNBu )3(PhC∫CPh)2](BF4) (95) is sequentially reduced and protonated to give the Fischer-type metallacyclopentatriene [(h5-Cp*)Mo
: C(NHBut)C(Ph):C(Ph)C(NHBut):(CNBut)](BF4) (96). In contrast, treatment of 93 with CNBut gives the vinyl-carbene system [(h5t t 305 Cp*)Mo
:C(NHBu )C(Ph):C(Ph)CN(CNBu )2](BF4) (97). In the reaction of [(h5-Cp)Mo(h2-PhCCPh)
P(OMe)32](BF4) (98) with an excess of CN(2,5Me2C6H3) a coupling reaction between the alkyne and the isocyanide again occurs but in this case involving three molecules of the isocyanide to give the substituted indole complex 99 in good yield (Scheme 31).306 Carbon-carbon

Organometallic Chemistry

330

coupling between alkynes occurs during the trimerisation of diphenylacetylene with [W(NMe3)(h2-PhCCPh)3] (100) to give the metallacyloheptatrienyl complex [WCl(h2-PhCCPh)(h6-C6Ph6H)] (101).307

BF4

BF4 Ph

Ph CNxyl

Mo

Mo

NxylC

CNxyl

Nxyl CO

Ph

Nxyl

Ph Ph

Ph

93

94

Scheme 30 R Ph

BF4

BF4

N

H N

L

Ph

R Ph

Mo

Mo

xs. CNR

L

L

L R = 2,5-Me2C6H3 L = P(OMe)3

98

NR

99

Scheme 31

Alkyne-phosphaalkyne coupling is the outcome when [(h5-Cp)MoCl(CO)(h2PhCCPh)] (102) is treated with two equivalents of PCBut in the presence of TlPF6 to give the 16 electron product [(h5Cp)Mo
:C(But)PC(But):PC(Ph):C(Ph)(CO)](PF6) (103) (Scheme 32).308 Phosphorus-carbon bond formation also results when [Mo(h2-HCCH)(dppe)2] (104) is treated with two equivalents of [(h5-Cp)2Fe](BF4) in THF-MeCN followed by anion metathesis with NaOTf (OTf : OSO2CF3) to give the metallacyclopropene-containing complex [Mo(h3-CHCHPPh2CH2CH2PPh2309 C,C,P)(MeCN)2(dppe)](OTf)2 (105).

Cl

Ph C

Mo

But

C CO Ph

But

Mo

Tl+, PCBut

CO

P

P

Ph

102

Ph

103

Scheme 32

Metallacyclopropene formation is also apparent during the addition of a hydrido ligand to the alkyne-coordinated Mo(IV) calixarene-supported com-

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

331

plex, [p-But-calix[4]-(O)4Mo(h2-PhCCPh)] (106) to afford [p-But-calix[4](O)4Mo
:C(Ph)CH(Ph)]9(LiSn); (107) (Scheme 33).310

Ph

Ph Mo

O

LiHBEt3

O

O

Ph

Ph

O

H

Mo O

-BEt3

O

LiSn

O O

107

106

Scheme 33

The reaction of [WI2(CO)(NCMe)(h2-HC2But)2] (108) with nitriles and phosphites results in substitution of the coordinated acetonitrile ligand to gives the crystallographically characterised [WI2(CO)(L)(h2-HC2But)2] [109a L : NCPh, 109b L : P(OMe)3] respectively (Scheme 34).311 In a similar fashion acetonitrile exchange products result when [WI(CO)(NCMe)
P(OPri)32(h2-MeC2Me)] (BPh4) (110) is treated with RCN to give [WI(CO)(NCR)
P(OPri)32(h2MeC2Me)](BPh4) (111: R : Et, Pri, But, Ph and PhCH2).312 CO H

CO I

I

But

H

L

W

108

But

W L

NCMe But

I

I

But

H

H 109a L = PhCN 109b L = P(OMe)3

Scheme 34

Alkylation of the propargyl carbon atom of the coordinated alkyne in the tris(pyrazoyl)borate-supported chiral W(II) complex [(TpMe2)WI(CO)(h2-EtCCEt)] (111) to give 112 is regio- and stereo-selective (Scheme 35); the chiral alkyne in 112 can be liberated from the metal centre by photolysis.313 By contrast, reduction of the W(IV) complex [(TpMe2)WF2(h2-PhCCPh)](BF4) (113) with [(h5Cp)2Co] gave the neutral d3 complex [(TpMe2)WF2(h2-PhCCPh)] (114); the d2/d3 redox pair has been regarded as the missing link in a ‘redox family tree’ relating the d6 tricarbonyls [M(CO)3L]9 to the d2 trihalides [MX3L].314 4.1.3 Group 8 (Fe, Ru, Os). Insertion of CO into each of the Fe-C(alkyne) bonds in the h2-alkyne complex [Fe(CO)2
P(OPh)32(h2-PhCCPh)] (115) gives the six-coordinate ferracylopent-3-ene-2,5-dione complexes [Fe(CO)2
P(OPh)32
h1:h1-C(O)C(Ph)C(Ph)C(O)] (116) (Scheme 36). On the other hand, alkyne-CNR coupling occurs on treatment of 115 with CNR to give [Fe(CO)2
P(OPh)3
h1:h1:h1:h1-C(:NR):C(Ph)C(Ph):C(:NR)] (117).315 The nucleophilic addition of Nu to the cationic h2-alkyne complex, [(h5-

Organometallic Chemistry

332

TpMe2

Me

Me

TpMe2

n

(i) Bu Li (ii) PhCH2Br

*

W

*

W I

I

* CH2Ph

CO

Me

CO

C

H

Me

111

112 Scheme 35

CO

NR

P(OPh)3 Ph

NR OC

Fe

CNR

CO OC

OC

Ph

Ph

O P(OPh)3

P(OPh)3 Ph

115

117

Ph

Fe

Fe

OC (PhO)3P

P(OPh)3 O

OC

Ph

116

Scheme 36

Cp)Fe(CO)2(h2-RCCR)](BF4) (118) gives the alkenyl products, [(h5Cp)Fe(CO)2(h1-RC:CR(Nu))] (119), the stereochemistry of which depends on basicity of the nucleophiles employed (Scheme 37).316 Nucleophilic addition also occurs on reaction of the equilibrium mixture of [(h5-C12H12)Ru(h1(120a) and [(h5-C12H12)Ru(h2CH:CH2)(CO)(PPh3)](BF4) PhCCH)(CO)(PPh3)](BF4) (120b) with triphenylphosphine to give the alkenylphosphonio derivative [(h5-C12H12)Ru(h1-CH:C(PPh3)Ph)(CO)(PPh3)](BF4) (121).317

R

Fe

Nu-

Fe

R

OC

OC

OC

OC R

R

Nu

119

118 Scheme 37

4.1.4 Group 9 (Co, Rh, Ir). The CpCo-stabilised cyclobutadiene complexes [(h5Cp)Co
h4-C(SO2But)C(SO2But)CR1CR1] (123) are obtainable from the reaction of [(h5-Cp)Co(h2-ButO2SC∫CSO2But)(CO)] (122) with dithiaalkynes at room

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

333

Scheme 38

temperature, while the corresponding products from dialkylalkynes and diarylalkynes need more elevated temperatures (Scheme 38).318 The square planar Rh(I) complex [Rh(MeCO2)(PPriPh2)2(h2MeO2CC∫CCO2Me)] (124) undergoes an oxidative reaction with diphenylacetylene to give the six-coordinate alkynyl(vinyl)rhodium(III) complex [Rh(MeCO2)(PPriPh2)2
C(CO2Me):CH(CO2Me)(C∫CPh)] (125) (Scheme 39).319 i

PPr Ph2

O

Me

PriPh2P CO2Me O

O HCCPh

Rh

i

Ph2Pr P

Ph

Me

Rh

CO2Me

O PriPh2P

CO2Me

H

124

CO2Me

125

Scheme 39

4.1.5 Group 10 (Ni, Pd, Pt). The h2-alkyne complex [(bipy)Ni(h2-PhCCPh)] (126: bipy : 2,2-bipyridine) has been crystallographically characterised and shown to act as a catalyst for the cyclotrimerisation of diphenylacetylene to give hexaphenylbenzene and dimethyl acetylenedicarboxylate to give hexamethyloxycarbonylbenzene (Scheme 40)320 and supports the proposed hypothesis that h2-alkyne complexes are crucial intermediates in the cyclotrimerisation of alkynes by Ni(0) complexes. As with 126, complex [
(Pri)2PCH2CH2NMe2Pt(h2-PhCCPh)] (127a) also catalyses the cyclotrimerisation of diphenylacetylene to give hexaphenylbenzene. When the trimerisation is performed in the presence of biphenylene, the 9,10disubstituted phenyl phenanthrene results.321 Complex 127a also reacts quantitatively with diphenylacetylene to give the platinacyclopenta-2,4-diene compound [
(Pri)2PCH2CH2NMe2Pt(CPh)4] (128).322 In contrast, [
(Pri)2PCH2CH2NMe2Pt(h2-PhCCSiMe3)] (127b) reacts at room temperature to give the Si-C(sp) cleavage product [
(Pri)2PCH2CH2NMe2Pt(SiMe3)CCPh] (129) (Scheme 41).323 Notably, no reaction was observed when the diphosphine com-

Organometallic Chemistry

334 Ph N

R

Ni

cat.

R

N Ph

126

R = Ph or CO2Me

R

R

R

R

R R

Scheme 40

plex [
(Pri)2PCH2CH2P(Pri)2Pt(h2-PhCCPh)] (130a) was treated with diphenylacetylene nor when [
(Pri)2PCH2CH2P(Pri)2Pt(h2-PhCCSiMe3)] (130b) is heated. Silicon-carbon bond cleavage can occur, however, in 130b when photochemical conditions are employed. Related C-C bond cleavage products are observed on photolysing [LPt(h2-PhCCPh)] [131: L : bis(diisopropylphosphino)ethane, bis(dicyclohexylphosphino)ethane].324 Pri2 P

R

Pri2 P

SiMe3

Pt

C

PhCCPh

C

(R = Ph)

Pri2 Ph P

Pt (R = SiMe3)

N Me 2 Ph

129

N Me2

Ph

Pt N Me2

Ph

127a R = Ph 127b R = SiMe3

Ph Ph

128

Scheme 41

4.1.6 Other Metals. The dialkyl alkyne niobium complex [(h5C5H4SiMe3)Nb(h2-Me3SiC∫CSiMe3)(CH2SiMe3)2] (132a) undergoes an insertion reaction with the isocyanide, ArNC (Ar : 2,6-Me2C6H3), leading to h2iminoacycl compounds 133 (Scheme 42).325 Conversely, displacement of the h2-alkyne occurs on treatment of the dichloride alkyne niobium complex [(h5C5H4SiMe3)Nb(h2-Me3SiC∫CSiMe3)Cl2] (132b) with PhN:C(Me)C(Me):NPh or O:C(Me)C(Me):O to give the ene-diamido complex [(h5C5H4SiMe3)Nb
PhNC(Me):C(Me)NPhCl2] (134a) and the ene-diolato complex [(h5-C5H4SiMe3)Nb
OC(Me):C(Me)OCl2] (134b).326 SiMe3

SiMe3 SiMe3 Me3Si E

Nb

E X

E

Me3Si

Nb

E

ArNC

X X

(Ar = 2,6-Me2Ph)

X SiMe3

134a E = NPh 134b E = O

CH2SiMe3

Nb

132a X = CH2SiMe3 132b X = Cl

Scheme 42

N

Ar

CH2SiMe3 SiMe3

133

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

335

On heating under vacuum the [(hfac)Cu
(h2-CH3CH2C∫CC(:CH2)H] (135) (hfac : hexafluoroacetylacetonate) transforms into the structurally characterised bimetallic complex [(hfac)Cu
(m-h2:h2’-H3CH2C∫CC(:CH2)HCu(hfac)] (136) (Scheme 43); the reaction was shown to be reversible.327

F3 C O

2

O

+ O

O F 3C

CF3

Cu

F3 C

Cu

O

Cu

O

CF3

CF3

135

136

Scheme 43

4.2 p-Coordinated Alkynes Generated In-situ. — 4.2.1 Cyclotrimerisation reactions. Complexes that mediate the trimerisation of alkynes to arenes have continued to be the source of interest in 2001/2001 (see also section 4.1.5). The h2-alkyne complex [(CO)5W(h2-RC∫CH)] (137), generated in-situ from the photochemical reaction of W(CO)6 with RC∫CH, has been shown to be involved in the cyclotrimerisation of alkynes to arenes.328 By a variation of the classic Reppe reaction known as the ‘Metalative Reppe reaction’, three different, unsymmetrical acetylenes can be successfully cyclotrimerised with [(h2propene)Ti(O-i-Pr)2] (138) to afford an arylmetal complex 139 as a single isomer (Scheme 44).329 CO2But

i

Ti(O-Pr )2 138

+ C6H13

C6H13

CO2But

CO2But

H13C6 i

Ti(O-Pr )2 C6H13

C6H13

TiX3

TolO2S

C6H13 139 X3 = (O-Pri)2(O2STol)

Scheme 44

The tris(acetonitrile) iron complex [(h5-Cp*)Fe(NCMe)3]; (140) also promotes the cycloaddition of RC∫CH (R : Ph, Hex) to give substituted benzene complexes [(h5-Cp*)Fe(h6-C6H3R3)]; (141) (Scheme 45).330 However, on reaction of 140 with the acetylenes, RC∫CH (R : CO2Et, CH2NMe2), cycloaddition of C∫C and C∫N triple bonds occurs to give the substituted pyridine containing complexes, [(h5-Cp*)Fe(h6-C5H2MeR2)]; (142: R : CO2Et, CH2NMe2). 4.2.2 Insertion and Coupling Reactions. 4.2.2.1 Group 8 (Fe, Ru, Os). The reaction of Collman’s reagent (NEt4)[
CH3C(O)Fe(CO)4] (143) with a half equivalent of methyl iodide and diphenylacetylene in acetone gave (NEt4)[Fe2(CO)6(m2-CPh:CPhCOCH3)] (145). The reaction pathway is believed to involve an h2-alkyne intermediate (144) which then reacts with the acylferrate anion 143 to give 145 (Scheme 46).331

Organometallic Chemistry

336

Fe

Fe

R NCMe

MeCN

R

R

3 RCCH

3 RCCH

Fe

Me

MeCN

R

140

N

R

142 R = CO2Et, CH2NMe2

141 R = Ph, hex

Scheme 45

(OC)4Fe

143

C(O)CH3

- CH3I -I

PhCCPh (OC)4Fe

Ph

Ph

CH3 O CH3

Scheme 46

Ph

143 (OC)3Fe

(OC)4Fe

144

Ph

Fe(CO)3 CH3

O

145

The cyclopentadienyl cationic complex [(h5-Cp)Ru(NCMe)2(PPh3)]; (146) undergoes reaction with two equivalents of a terminal alkyne to form firstly the phosphine-coupled allyl carbene complex [(h5-Cp)Ru(CRCHCRCH(PPh3)]; (147) and then further reacts to give the h4-butadienyl complex [(h5-Cp)Ru(C6H42-P(Ph)2CHCRCHCR(H)]; (148) (Scheme 47).332,333 On the other hand the neutral pentamethylcyclopentadienyl complex [(h5-Cp*)RuCl(h4-COD)] (149) promotes the head-to-head dimerisation of propargylic alcohols to give the [(h5Cp*)RuCl
h4-H2C4(CR2OH)2] (150).334

Scheme 47

The reaction of [Ru(C6H2OH-2-CHNR-3-Me-5)(PPh3)2(CO)(NO2)] (151) with excess HC∫CX (X : Ph, CH2OH) is found to result in both insertion of a molecule of alkyne regiospecifically into a Ru-C bond and linkage isomerisation of the nitrite ligand to give [Ru(h2-C6H2CXCH-1-O-2-CHNHR-3-Me5)(PPh3)2(CO)(NO2)] (152: X : Ph, CH2OH) (Scheme 48).335 Disubstituted alkynes react with the ruthenium-based methathesis catalyst [(H2IMes)(PCy3)(Cl)2Ru:CHPh)] (153a) to form a phosphine-free h3-vinylcarbene complex, [(H2IMes)(Cl)2Ru
h3-(CHPh)(CPh)(CPh)] (154) (Scheme 49), a step which has relevance to the mechanism of olefin metathesis and alkyne polymerisation.336 Thermal treatment of [(PCy3)2(Cl)2Ru(:CHPh)] (153b) with two equivalents of phenylacetylene gives a vinylidene complex tentatively ascribed as ‘[Ru](PCy3):C:CHPh’ (155). Significantly, 155 catalyses the selective product formation for trans-tail-to-tail enynes.337

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

337

Me Me

PPh3 O N

Ru O

N R

OC

H

X HCCX N

O

PPh3

O

R

H

PPh3

O

H

Ru CO

N

PPh3

O

151

152 X = Ph, CH2OH

Scheme 48

L

'[Ru](PCy3)=C=CHPh' 155

Heat/PhCCH (153b)

Ru Cl

Cl

PhCCPh Ph

PCy3

H

(153a) -PCy3 Ph

153a L = H2IMes 153b L = PCy3

Ru Cl

Cl Ph Ph

154

Scheme 49

Treatment of [OsCl2(PPh3)3] (156) with an excess of HC∫CSiMe3 in wet benzene affords the structurally characterised metallabenzyne complex [(PPh3)2Os
-CHC(SiMe3)C(Me)C(SiMe3)C∫Cl2] (157) (Scheme 50).338 Fragment orbital interaction analysis and DFT calculations have been used to determine the electronic reason for the stabilisation of the osmabenzyne.339 PPh3

PPh3 PPh3

Cl

Os

HCCSiMe3

Cl

SiMe3

Cl Os

Me

Cl

PPh3

SiMe3

PPh3

156

157 Scheme 50

4.2.2.2 Group 9 (Co, Rh, Ir). The metal-mediated [2;2] dimerisation of two alkyne molecules to give cyclobutadienyl ligands is apparent during the stepwise oligomerisation of 1,6-cyclodecadiyne units. For example, the reaction of [(h5Cp)Co
h4-C4(-CH2CH2CH2C∫CCH2CH2CH2-)2] (158) with [(h5-Cp)Co(h4COD)] (159) affords the tetrameric belt-like cyclophane 160 (Scheme 51); octameric species are also obtainable.340 Related products are also accessible from the reaction of cyclic thiadiynes and disiladiynes with 159.341,342 Cobalt-mediated cyclodimerisation of alkynes are also employed during the synthesis of various

Organometallic Chemistry

338

oligo-alkyne-bridged [(h5-C5H4R)Co(h4-C4R4)] fragments and during the synthesis of various nanoscale and barrel-type structures.343,344 CoCp

CpCo CpCo(COD) (159)

CpCo

CoCp

158

CoCp

160

Scheme 51

The reactions of the bis-alkenyl cationic complex [Ir(CH:CH2)2(NCCH3)2(PPh3)2]; (161) with the alkynes, RCCH (R : Ph, p-MeC6H5), have been found to give cross-conjugated hexatrienes and and octatraenes (Scheme 52). By reacting 161 with firstly CO and then RCCH, the alkynyl cis-bis(ethenyl) complex [Ir(CH:CH2)2(CCR)(CO)(PPh3)2] (162) can be isolated which on treatment with H;, gives the h4-hexatriene complex [Ir
h4RCH:C(CH:CH2)2(CO)(PPh3)2]; (163); addition of base to 163 affords the free hexatriene.345 Interaction of [(h5-Cp*)RhCl2(PPh3)] (164) with phenylacetylene and water in the presence of KPF6, affords the alkenyl-ketone complex [(h5R

PPh3

PPh3

+

R

RCCH

OC

(i) CO

H3CCN Ir

(ii) RCCH

H3CCN

Ir

R

PPh3

PPh3 R

162

161

PPh3

H+

OC

Ir

PPh3

R

163

Scheme 52

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

339

Cp*)Rh(PPh3)
CPh:CHC(O)CH2Ph](PF6) (165) (Scheme 53).346 The mechansim for this transformation remains uncertain. 4.2.2.3 Group 10 (Ni, Pd, Pt). Seven-membered nickelacycles (167) have been prepared by insertions of unsymmetrical alkynes into the Ni-C bond of the nickelacycles in [NiBr(C6H4CH2PPh2-2)(PEt3)] (166) (Scheme 54).347 DFT studies have been used to successfully model the transformation; the calculations show that for alkynes bearing electron-withdrawing substituents, the favoured insertion pathway depends on the nature of the ligand trans to the Ni-C bond.348 Insertion of an alkyne into a metal-aryl bond is also apparent during the reaction PF6

Ph3P

Rh

HCCPh / KPF6

Rh

H2O

Cl Cl

Ph3P

O

C C

Ph

CH2Ph

H

164

165

Scheme 53

PPh2 Ni

PPh2

R1CCR2

Br

Ni

Br R1

Et3P

166

R2

PEt3

167

Scheme 54

of [Pd(py)2
-C6H4-2-C(Me)2CH2-] (167) with dimethyl acetylenedicarboxylate giving, via reductive elimination of the inserted product, the 2,3-dihydronaphthalene C6H2-1,2-C(CO2Me):C(CO2Me)CH2C(Me)2.349 The number of insertions of alkyne molecules into the metal-carbon bonds in the PN-supported palladium species, [
C6H4-1-(PPh2)-2-(N: CHPh)PdR(NCMe)](BF4) (168a R : COMe, 168b R : Ph), is found to be dependent on the R substituent in 168. When R : COMe, a single insertion occurs to give 169, while a double insertion occurs when R : Ph to afford 170 (Scheme 55)·.350 The catalytic silylboration of phenylacetylene has been examined by reacting [(PMe3)2Pt(SiMe2Ph)(Bpin)] (171: Bpin :B(-OCMe2CMe2O-)) with PhC∫CH to give the Pt-B inserted product [(PMe3)2Pt(SiMe2Ph)
C(Ph):CH(Bpin)] (172) (Scheme 56), which then undergoes reductive elimination to yield the silylborated compound, (SiMe2Ph)(Ph)C:CH(Bpin).351

Organometallic Chemistry

340 Ph2 Ph P

Ph

Ph2 P

Ph

Pd

PhCCPh

Ph

N

R

PhCCEt

Pd

(R = Ph)

NCMe

N

CHPh

(R = COMe)

CHPh

170

168a R = COMe 168b R = Ph Ph2 P

Ph Et

Pd N

O

CHPh

169

Scheme 55 Me3P

SiMe2Ph Pt

Me3P

PhCCH

B(pin)

Me3P

B(pin)

Me3P

SiMe2Ph Pt

Ph

171

Scheme 56

172

4.2.2.4 Other metals. The zirconacyclopentadiene complex [(h5-Cp)2Zr(h4C4R4)] (173), generated from the zirconium-mediated coupling of two alkyne molecules has been shown, following transmetallation with copper chloride and quenching with chlorodiphenylphosphine, to give the bidentate phosphine ligand NUPHOS (Scheme 57).352 R1 R1 R2

R1

+ R3

Cp2ZrCl2/BunLi

R2

Cp2Zr

(ii) Ph2PCl

R4

R3

R4

173

PPh2

R2 (i) CuCl

R3

Ph2P R4

NUPHOS

Scheme 57

Thermolysis of the symmetrically substituted alkynylcarbene complex [(h5Cp*)(CO)2Re:C(Ph)CCPh] (174) is believed to proceed via an h2-alkyne intermediate (175) followed by an insertion of the carbene fragment into a C-H bond of the pentamethylcyclopentadienyl group to give [h5:h2353
C5(CH3)4CH2CH(Ph)CCPhRe(CO)2] (176) (Scheme 58). Insertion of an electron rich alkyne into a metal-nitrogen bond has been

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

Ph OC OC

341

H 120 oC

Re

Ph H

Ph Re

OC OC Ph

OC OC

Ph

Ph

174

Re

176

175

Scheme 58

observed during the reaction of [(dmpa)Mo(NMe2)2(NMe2H)] (177: dmpa : diN,N-(pyrrolyl-a-methyl)-N-methylamine) with 3-hexyne to give 179; an h2-alkyne complex (178) has been proposed as an intermediate (Scheme 59).354

Mo

Me2N N

NMe2 N

Mo

Me2N

EtCCEt

N

-HNMe 2

N

NMe2

Et

Et

Et

Et

NHMe 2

1,2-insertion

Mo

Me2N

N N

N Me

NMe2

N

N

Me

Me

177

178

179

Scheme 59

4.3.2 Carbene Generation. Kinetic studies have been carried out on the formation of the vinylideneruthenium(II) complexes [
C5H4N-2,6(CH2PCy2)RuCl2(:C:C(Z)R] (181: Z : H, SiMe3; R : aryl, alkyl) generated from the reaction of [
C5H4N-2,6-(CH2PCy2)RuCl2(NCMe)] (180) with RC∫CZ (Scheme 60); higher reaction rates are achieved with more electron rich and less sterically crowded alkynes.355 The propensity for formation of vinylidene ligands on reaction of some transition metal complexes with alkynes has been the subject of two theoretical papers.356,357 For example, DFT calculations have been performed on the alkyne to vinylidene isomerisation in [(h5-Cp)(PMe3)2Ru (HCCR)]; (182: R : H, Me) and it has been determined that the vinylidene complexes are more stable than the alkyne complexes. In addition, a 1,2hydrogen shift via an h2-(C-H)-coordinated alkyne intermediate is the energetically favourable step for the isomerisation.356 A related paper concerning acetylene to vinylidene isomerisation in [(h5-Cp)Mn(CO)2(h2-HCCH)] (183) has also been published.357 PCy2

PCy2 Cl Ru

N Cl

NCMe

RCCZ -MeCN

Cl N

Ru

C Z

Cl PCy2

PCy2

181 R= aryl, alkyl Z = H, SiMe3

180 Scheme 60

R

Organometallic Chemistry

342

The reaction of [(bpy)2Ru(napy)](PF6)2 (184: bpy : 2,2-bipyridine; napy : 1,8-naphthyridine) with propiolic acid in ROH (R : Me, Et) gives the Rucarbene complex [(bpy)2Ru
C5H3N-2,3-(CHCHC(OR)HN(CMe)](PF6)2 (185) (Scheme 61).358 Carbene generation also results when the triphos-containing complex [
PhP(CH2CH2PPh2)2Ir(CR:CRCR:CR)Cl] (186) is treated with AgBF4 and 3-butyn-1-ol giving [
PhP(CH2CH2PPh2)2Ir(CR:CRCR: CR)(:COCH2CH2CH2)](BF4) (187).359

Scheme 61

5

Complexes Containing p-Coordinated Dienes (Chain and Cyclic)

5.1 Well-defined p-Coordinated Conjugated Dienes. — 5.1.1 Group 4 (Ti, Zr, Hf). Reaction of the s-trans/cis equilibrium mixture of [(h5-Cp)2Zr(h4-C4H6)] (188) with the imine MeN:CHPh results in C-C coupling to form cis-[(h5Cp)2Zr
N(Me)C(Ph)HCH2-h3-(CHCHCH2)] (189), in which only one of the two possible diastereoisomers forms (Scheme 62).360 Use of the more sterically bulky imine, PhCH2N:CHPh, results in exclusively trans-[(h5Cp)2Zr
N(CH2Ph)C(Ph)HCH2-h3-(CHCHCH2)] (190).

Scheme 62

The reaction of [(C5H4Me)Zr(h3-crotyl)(h4-C4H6)] (191) with B(C6F5)3 gives the triple-decker complex 192, in which all three C6F5 substituents of one B(C6F5)3 group have been transferred to zirconium (Scheme 63);361 interestingly, complex 192 shows some activity for ethylene polymerisation on activation with MAO. The trimethylsilylmethyl hafnium complex, [(h5-Cp*)Hf (h4C6H10)CH(SiMe)2] (193), reacts with one equivalent of LiCH(SiMe3)2 to give, following deprotonation of one of the alkyl SiMe groups, [(h5-Cp*)Hf (h4C6H10)
CH2SiMe2CHSiMe3][Li(THF)2] (194) (Scheme 64).362 The unusual h2-cyclobutadiene complex [(h5-Cp)2Zr(h2-C2C6H4)(PMe3)] (195) cycloadds a range of unsaturated organic molecules including ButNC to give the

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

343

diamido-Dewar naphthalene complex [(h5-Cp)2Zr(h2-C(NBut)C(NBut)C2C6H4)] (196) (Scheme 65).363 CpMe

CpMeF

C6F5 B(C6F5)3

Zr

Cp

Me

Zr

Zr

191

F F C6F5 C6F5

B

C6F5

F

F B

192 Scheme 63

Scheme 64

But N

ButNC Cp2Zr

Cp2Zr N But

PMe3

196

195 Scheme 65

5.1.2 Group 6 (Cr, Mo, W). The Mo(IV) cis-butadiene complex 197 reacts with one equivalent of 2-butyne to afford the h4-2,3-dimethyl-1,3-cyclohexadiene complex 198, in which carbon-carbon coupling of the coordinated butadiene and alkyne has occurred (Scheme 66). In contrast, addition of two equivalents of 2-butyne to 197 gives free 1,2-dimethyl-1,4-cyclohexadiene and the molybdenum 2-butyne complex 199.364 The bis(diene)tungsten complex [W(O2-1,2-C6H4)(h4-C4H4Me2)2] (200), with supine-prone structure, undergoes reaction with CF3SO3H to give [W(O2C6H4)(h4-C4H4Me2)(OSO2CF3)2] (201), in which selective removal of the prone-diene has occurred (Scheme 67).365 An article dedicated to the reactivity of half-sandwich complexes containing dienes exhibiting both supine and prone coordination modes has also been published.366

Organometallic Chemistry

344 NPh Me3Si

Me3Si

Mo

N

NPh

Me MeCCMe Me

N

Mo

N

-Me2C6H6

MeCCMe

N SiMe3

SiMe3

199

197 NPh Me Me3Si

Mo

N

Me

N SiMe3

198 Scheme 66

O

O

O

O W

CF3SO3H

W OSO2CF 3 OSO2CF3

201

200 Scheme 67

OMe

OH OMe

BF3.Et2O

Fe(CO)3

Fe(CO)3 202

203

Scheme 68

5.1.3 Group 8 (Fe, Ru, Os). The use of the tricarbonyl iron fragment to mediate organic transformations involving diene units has seen a number of reports.367,368 For example, it has been used to stabilise dienyl cations that are generated in situ from 202 which then undergo stereospecific intramolecular addition of aromatic nucleophiles (Scheme 68) to give 203.367 The isomeric h4-Fe complexes, 204a and 204b, synthesisable from the reaction of [(h5-Cp)Fe(CO)2I] with NaCp (in the presence of PPh3), give on deprotonation with BunLi and treatment with FeCl3 the ferrocene-bridged trimetallic species [Fe
[m,h5:h4-5-exo-(1-C5H4)C5H5]Fe(CO)2(PPh3)2] (205) (Scheme 69).369

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

CH2

H2C

Ph3P OC

Fe

OC

Fe

OC OC

204a

CO PPh3

Fe

(ii) FeCl3

Fe PPh3

CO Fe

(i) BunLi

+ OC OC

345

PPh3

204b

205

Scheme 69

Interaction of [(h5-Cp*)RuCl(h4-2,3-Me2C4H6)] (206) with three equivalents of 2-bromo-4,5-dimethylphosphinine in the presence of AgBF4 furnishes cationic 207, in which three molecules of the phosphinine are bound to the metal centre in a h1-fashion (Scheme 70).370 Conversely, use of the more sterically bulky 2,6bis(trimethylsilyl)phosphinine gives complex 208 in which the phosphinine adopts an h6-coordination mode. BF4

Me3Si

Ru Me3Si

P

SiMe3

Ru Cl

AgBF4

P

P AgBF4

Br

Br

Ru

Br

P

P P

Br

SiMe3

208

206

207

Scheme 70

5.1.4 Group 8 (Co, Rh, Ir). Photolysis of the cyclobutadiene-containing complex [(h6-C6H6)Co(h4-C4Me4)](PF6) (209) in acetonitrile results in displacement of benzene to give [Co(h4-C4Me4)(NCMe)3](PF6) (210), which can further react with a wide variety of Lewis bases (L) to produce substitution products [Co(h4C4Me4)(L)3](PF6) [211: L : P(OMe)3, Py, CNBut] (Scheme 71).371 Complex 210 also reacts with Li(TMEDA)(3,5-Me2C5H3BNMe2) to give the boratabenzene complex [Co(h4-C4Me4)(3,5-Me2C5H3BNMe2)] (212), which can undergo further reaction with methanol to give [Co(h4-C4Me4)(3,5-Me2C5H3BOMe)] (213).372 Lithiation of [(h5-Cp)Co
h4-C4(SiMe3)4] (214a) allows access to the cyclobutadiene dianion Li2[(h5-Cp)Co
h4-C4(SiMe3)4] (215) which, by 7Li NMR spectroscopy, indicates that the equivalent lithium ions are shielded by a diatropic ring current.373 On the other hand the tetraethynyl-substituted species [(h5-Cp)Co
h4-C4
(C∫C-C6H4-2-C∫CH)4] (214b) undergoes ring closure to give the unusual butterfly-shaped cyclobutadiene complex [(h5-Cp)Co
h4-C4
(-C∫CC6H4-2-C∫C-C∫C-2-C6H4-C∫C-)2] (216) (Scheme 72).374 A similar methodology has been employed to make half-wheel type cyclynes with the cobalt-cyclobutadiene moiety filling the centre of the wheel.375 5.1.5 Group 10 (Ni, Pd, Pt). Addition of B(C6F5)3 to the h4-butadiene complex [
(Ar)N:C(Me)C(Me):N(Ar)Ni(h4-CH2:CHCH:CH2)] (217: Ar : 2,6-

Organometallic Chemistry

346

Me

Me

Me

PF6

Me Co

Me

L

Me PF6

Me

Me

MeCN hυ

Co

Me PF6

Me

L

L

L

211 L = P(OMe)3, Py, CNBut

Me

Me Co

MeCN

Me NCMe

MeCN

210 209

Me

Me Me

BNMe2

Me

Me

Co Me BNMe2 Me

212

Scheme 71

Cu(OAc)2

CoCp

CoCp

214b

216

Scheme 72 i

Pr C6H3) leads to the zwitterionic complex [
(Ar)N:C(Me)C(Me):N(Ar)Ni
h3-CH2CHCHCH2B(C6F5)3] (218) (Scheme 73); 218 has been employed as a catalyst for the polymerisation of ethylene.376 Methyl activation of the h4-cyclobutadiene palladium(II) complex [
h4C4Me2(But)2PdCl2]2 (219), promoted by triethylamine, gives the bimetallic exomethylene-h3-cyclobutenyl complex [
h3-C4(:CH2)(Me(But)2Pd(m-Cl)]2 (220), while in the presence of 2,2-bipyridine and followed by addition of AgBF4, the cationic mononuclear complex [
h3-C4(:CH2)(Me(But)2Pd(bipy)](BF4) (221) can be isolated (Scheme 74).377 5.1.6 Other Metals. Several theoretical papers have appeared on the subject of the bonding and reactivity of butadiene-containing complexes.378-380 For example, the redetermined molecular structure of [Mn(h4CH2:CHCH:CH2)2(CO)] (222) has been investigated by DFT-quantum chemical calculations and it is found that the typical understanding of bonding of

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes Ar N

347

Ar N B(C6F5)3

Ni N Ar

Ni N Ar

Ar = 2,6-PriC6H3

217

B(C6F5)3

218

Scheme 73 Me CH2 BF4

Me

But Pd

But

But

(i) NEt3, bipy

Pd N

Cl

N

NEt 3

Cl But

But

Me

Me

But Pd

Pd

Cl But

221

But

But Cl Cl

(ii) AgBF4

CH2

Me

Me

Cl Pd

H2C

But

Me

220

219

Scheme 74

butadienes to transition metal complexes and the geometric parameters to be in good agreement with the quantum chemical calculations.378 A DFT study of the polymerisation propagation step for butadiene-containing complex [(h5Cp)Nb(butadiene)R]; (223) for ethylene polymerisation has also been reported.379 5.2 Well-defined p-Coordinated Non-conjugated Dienes. — 5.2.1 Group 8 (Fe, Ru, Os). The cationic h2-diene complexes [(h5-Cp)Fe(CO)2(h2-diene)](PF6) (224: diene : 1,4-pentadiene, 1,5-hexadiene and 1,7-octadiene) have been reacted with a range of nucleophiles giving products the result of nucleophilic addition (225) to the coordinated diene or displacement of the diene (226) (Scheme 75).381 PF6 Fe OC

CO

226

PF6

PPh3

(CH2)n

CH(COOCH2CH3)2

Li[CH(COOCH2CH3)2]

PPh3

Fe OC

Fe

CO

(CH2)n

224 n = 1,2,4

OC

(CH2)n CO

225

Scheme 75

The reactivity of ruthenium complexes supported by the h4-COD (COD : 1,5-cyclooctadiene) ligands has been the subject of a large number of reports. In many cases the h4-COD acts as a labile ligand allowing the introducion of an alternative ligand frame382 or allowing a coupling reaction to occur at the metal

Organometallic Chemistry

348

centre.383,384 For example, treatment of [(Tp)Ru(h4-COD)Cl] (227) with 2aminopyridine in the presence of a terminal alkynes, RC∫CH (R : Ph, Bun, C6H9), affords the cyclic aminocarbene complexes [(Tp)Ru
:C(CH2R)N(R)-2C5H4N)Cl] (228), in which carbon-nitrogen coupling has occurred (Scheme 76).384 Conversely, protonation of [(h4-COD)Ru(h6-C8H10)] (229) with HBF4.Et2O followed by reaction with chiral bisphosphine ligands results in hydride formation and displacement of the COD ligand to give the corresponding cationic complex [(P-P)Ru(H)(h6-C8H10)]; (230: P-P : Me-DUPHOS, BINAP, Tol-BINAP, JOSIPHOS); complexes 230 are catalysts for the enantioselective hydrogenation of alkenes.382 Alternatively the COD ligand can behave as a spectator ligand.385-387 For example, optically active complexes [Ru(h6-(S)-1,3,5-C6H3
CH(Me)R3(h4COD)] (232) have been prepared by the stoichiometric cyclotrimerisation of (S)-MeCH(R)CCH with [Ru(h6-naphthalene)(h4-COD)] (231) (Scheme 77).385 On the other hand, dealkylation of the chelating amine N,N,NN-tetraethylethylenediamine occurs on reaction with [(h4-COD)RuCl2]n (233) to give [(h4-COD)Ru(EtHNCH2CH2NHEt)] (234); an intramolecular pathway has been proposed.386

N TpRu

NHR'

RCCH

N

NR'

TpRu

Cl

C H2

Cl

27

R

228

Scheme 76

R Me R H Me

H R

(S)-MeCH(R)CCH

Ru

RT

231

Ru

H Me

232

Scheme 77

Protonation of the Ru(II) complex [(h4-NBD)RuH(Cl)(PPh3)2] (235) with HOTf in dichloromethane affords [(h4-NBD)RuH(OTf)(PPh3)2] (236) and trimetallic [Ru3Cl5(PPh3)6](OTf) (237), the result of displacement of an NBD ligand and cluster formation (Scheme 78); the product type and ratio is found to be dependent on the reaction solvent employed.388

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

H

H

PPh3

Ru PPh3

HOTf

Cl PPh3

-NBD

PPh3

H

OTf

235

Ph3P +

Ru

236

Ru

PPh3

Cl

Ph3P

Ru

Cl

349 OTf

Ru Cl PPh3

Cl 237

Scheme 78

5.2.2 Group 9 (Co, Rh, Ir). Bis(COD) cationic Rh(I) complexes [(h4-COD)2Rh] (X) (238) have been used for a variety of applications. For example, treatment of [(h4-COD)2Rh](BF4) (238a) with the dinuclear gold complex [C6H4-1,2S2(Au(PPh3))2] results in loss of a molecule of COD and addition of the digold complex to give the trimetallic Au2Rh complex [
C6H4-1,2-S2(Au(PPh3))2Rh(h4COD)](BF4) (239) (Scheme 79).389 On the other hand, [(h4-COD)2Rh](X) (238b X : OTf, 238c X : PF6) have been employed as catalysts for ring-opening polymerisation (ROP) of silicon-bridged [1]ferrocenaphanes and tetramethyldisilacyclobutane at ambient temperature.390 Notably, introduction of a diphosphine in place of a COD ligands in 238b and 238c results in a reduction in the activity. Reaction of [(h4-COD)Ir(Py)2](PF6) (240) with SIMes results in displacement of a molecule of pyridine and formation of the carbene complex [(h4COD)Ir(Py)(SIMes)](PF6) (241). Complex 241 has proved an active hydrogenation catalyst capable of hydrogenating simple olefins.391 The chiral rhodium complex, [C6H4-1-2-
CH2P(Me)(But)2Rh(h4-NBD)] (242), is also an active hydrogenation catalyst but gives only modest enantioselection. Interestingly, the AuPPh 3 S

[Rh(η4-COD)2](BF4)

238a

AuPPh 3

BF4

S Rh

AuPPh 3

S

S

AuPPh 3 239

Scheme 79

reaction of 242 with dihydrogen at -70 oC results in NBD displacement and the formation of two diastereomers of the solvate dihydride [C6H4-1-2
CH2P(Me)(But)2Rh(H)2(S)2] (243) (Scheme 80).392 The substitional lability of [(h4-COD)Rh
C(O)C(C(O)H)C(R)NN(Ph)] (244: R : 2-thienyl, 2-furyl) towards nitrogen and phosphorus donor ligands has been studied and found to give preferentially the ionic species [(h4-COD)Rh (phen)];
C(O)C(C(O)R)C(Me)NN(Ph)9 (245) when 1,10-phenanthroline is employed.393 In contrast, reaction of 244 with triphenylphosphine results in displacement of COD and the formation of [(PPh3)2Rh
C(O)C(C(O)R)C(Me)NN(Ph)] (246) which then reacts rapidly with dioxygen

Organometallic Chemistry

350 But

Me P

P Me

242

Me H2, 2 atm

Rh

But

BF4

-70 oC

P

But H

BF4

P Me

243a

Scheme 80

S

P

+

Rh

But

Me

H

But H

BF4 S

Rh

But P

S Me

S

H

243b S = CD3OD

to give the Rh(III) complex [(PPh3)2Rh
C(O)C(C(O)R)C(Me)NN(Ph)(h2-O2)] (247) (Scheme 81). 5.2.3 Group 10 (Ni, Pd, Pt). The synthesis394 and application of [Ni(h4-COD)2] (248), along with its derivatives, as sources of Ni(0) has been the subject of a series of articles.394-398 Oxidative addition of range of substrates, via displacement

Scheme 81

of the COD ligands, has allowed access to the Ni(II) complexes 249,394 250395 and 251 (Scheme 82).396 Conversely, treatment of 248 with cyclotetra(vinylmethylsilazane) in the presence of a triphenylphosphine was effective for the preparation of the structurally characterised bimetallic nickel(0)-vinylcyclosilazane complex, [(PPh3)Ni
m-(h2-H2C:CH(Me)Si(m-NH))4Ni(PPh3)2] (252).397 Like complex 248, the reactivity of divalent palladium and platinum complexes of the type [M(h4-COD)Cl2] (253a M : Pd, 253b M : Pt) have been thoroughly studied.399-405 Examples of COD displacement reactions range from the interaction of 253 with simple pyrazole-based ligands through to b-cyclodextrin- (CDNNP) and calix[4]arene-functionalised phosphine ligands, giving 254,399 255400 and 256401 respectively (Scheme 83). Displacement of the COD ligand also occurs during the reaction of 253b with a nucleophilic 3-vinylpropene.402 However, in this case unexpectedly cyclopentadienyl and metallacyclic ring formation occurs to give the highly unusual platinabenzene complex [(h5-C5H3-Ph2)Pt
:C(Ph)-C(Ph):CHCH:CH-] (257). Preservation of an h4COD ligand occurs during the reaction of 253b with [Pt2(m-Se)2(PPh3)4];403 the PF6 salts of the reaction products can be isolated on treatment with NH4PF6 to

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes Me N O

Ni

Si N

Me NH

Si

NH

(PPh3)Ni

Ph

Si

351

NH

Ni(PPh3)

Si

NH

Me

Me

252 251 O

Me Me Si NH Si

Ph H

NH

NH

Si NH Si Me Me

tmeda

PPh3

F F

F Et3P

Ni

PEt3

R F

F

F

Br

Br

Br

Ni(η4-COD)2 248

I

R Ni

R = H, Me

Ni Br

249

R

250

Scheme 82

give [Pt2(m3-Se)2(PPh3)4
Pt(h4-COD)](PF6)2 (258a) and [Pt(m3Se)2(PPh3)2
Pt(h4-COD)2](PF6)2 (258b). Transformations can also occur at the h4-COD ligand itself. For example, reaction of o-dimethylaminophenyl(8-quinolinyl)amine [o-(NMe2)Ph-QAH] with 253b in the presence of triethylamine results in the formation of [
o(NMe2)Ph-QAPt
1,2-h2-6-s-COD)] (259) (Scheme 84), in which the neutral h4-COD ligand has been converted into a monoanionic ligand and the amine ligand has been deprotonated.405 In contrast, treatment of the amino-functionalised cyclopentadienyl anion, (C5H4CH2CH2NMe2)9, with [(h4-COD)PtMeCl] (260) results in partial displacement of the COD ligand (h4 to h2) to give [(h2-COD)
h5-C5Me4(CH2CH2NMe2)PtMe] (261), which then slowly dimerises to give the COD-bridged species [PtMe
h5-C5Me4(CH2CH2NMe2)(m-COD)] (262).406 5.3 p-Coordinated Dienes Generated In-situ. — 5.3.1 Butadiene polymerisation. Both theoretical and synthetic studies have been published on the design and application of transition metal catalysts for the polymerisation of butadiene.407-410

6

Bimetallic Complexes Containing p-Coordinated Alkenes, Alkynes or Dienes

6.1 Homobimetallic. — 6.1.1 (Alkynes). The use of alkyne-bridged dicobalt carbonyl complexes (263) for metal-mediated organic synthesis (e.g., Pauson-

Organometallic Chemistry

352 Ph2 P

Ph2 P

Cl

Pt N

N

N

N M

O

O O

O OH OH

P Ph2

Cl Ph2P

255

HN O

256

HN

PPh2

CDNNP

O O O OHOH

(M = Pd, Pt)

(PF6)2

(M = Pt)

Pt Li

Ph

Ph

Pt

(i) [Pt2(µ-Se)2(PPh3)4]

253a M = Pd 253b M = Pt

Ph (M = Pt)

Ph

(η4-COD)MCl2

(ii) NH4PF6

R

R

(M = Pd)

(PPh3)2 Pt

N

Cl

Pt

Pt

Pd Cl

N

258b

NH

257

(PF6)2

Se

Se

NH

Ph

Pt(PPh3)2

(Ph 3P)2Pt

258a N NH

Ph

Se

Se

254 R = Me, But

Scheme 83 R

N H N

Pt(η4 -COD)ClX

Pt N

N

253b X = Cl 260 X = Me

Li(C5Me4R) (R = CH2CH2NMe2)

259

R

Pt CH3

Pt CH3 H3C

261

R

Pt

262

Scheme 84 411-413

Khand reaction) or as starting materials for reactivity studies on the organometallic framework has continued to be a subject of numerous publications.414-418 For example, reaction of [Co2(CO)4(m:h2:h2-FcCCH)] (263a: Fc : ferrocenyl) with a diphosphine such as dppm or dppf (dppf : diphenylphosphinoferrocene) can result in bimetallic [Co2(CO)4(m:h1:h1-dppm)(m:h2:h2FcCCR)] (264a) or linked bimetallic species [(Co2(CO)4)2(m:h1:h1-dppf)2(h1-m-h1dppf) (m:h2:h2-FcCCR)] (265) (Scheme 85).414 Tri- and tetra-metallic clusters are accessible by treatment of [Co2(CO)6(m-CF3-CC-CF3)] (263b) with [(h5Cp)2Co2(m-SMe)2] affording [(h5-Cp)Co3(CO)4(m-SMe)2(m-CF3C2CF3)] (265) [Co2(CO)4(m-CF3C2CF3)
m-(h5-Cp)2Co2(m-SMe)2] (266) respectively.415 Reaction of [Co2(CO)6(m-(Me2Si)C2
C(OH)(C4Ph4)] (263c) with Fe(CO)5 gives the mixedmetal trimetallic complex [Co2(CO)6
m-(Me2Si)C2
h5-CC4Ph4)Fe(CO)2H] (267).416 The reactivity of dppm-substituted alkyne-bridged dicobalt carbonyl complexes (264) has also been the subject of some research activity. For example,

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

353

desilylation of the non-metallated alkynes in [Co2(CO)4(m:h1:h1-dppm)(m:h2:h2Me3SiCC
3,5-(SiMe3C∫C)2C6H3] (264b) occurred on treatment with KOH417 while the mixed-metal trimetallic RuCo2 complex [Co2(CO)4(m:h1:h1-dppm)(mh2-Me3SiC2CC
(h5-Cp)Ru(PPh3)2] (268) is accessible on reaction of [Co2(CO)4(m:h1:h1-dppm)(m-h2-Me3SiC2CCH)] (264c) with [(h5Cp)RuCl(PPh3)2] (in the presence of NH4PF6) in methanol followed by deprotonation with sodium methoxide.418 The dimolybdenum alkyne-bridged framework has also been the source of a number of reports. Treatment of the perpendicularly-bridged complex [(h5Cp)2Mo2(m-C2Ph2)(CO)4] (269) reacts with PriSH to afford two separable isomers of [(h5-Cp)2Mo2(m-h1,h1-C2Ph2)(m-S)(m-SPri)2] (270a and 270b), both of which have the bridging alkyne located parallel to the Mo-Mo bond (Scheme 86).419 Further reaction of both isomers of 270 with [Ru3(CO)12] has been examined and found to give both the tetrahedral [(h5-Cp)2Mo2Ru2(m3-C2Ph2)(m3-S)(mSPri)2(CO)4] (271) and trinuclear [(h5-Cp)2Mo2Ru(m-C2Ph2)(m3-S)2(CO)3] (272) clusters. In a related study, the dinuclear alkyne cation [(h5-Cp)2Mo2(mPh Ph Ph

SiMe3

Fe Ph

OC

H

OC

268 F3C

Co (CO)3

Co(CO)3

CF3

(OC)2Co

Fe(CO)5

Co(CO)2

MeS

(263c: R = SiMe3, R' = C(OH)C4Ph4)

SMe Co Cp

R'

Fc

R

H

266 dppm

Cp2Co2(µ-SMe)2

F 3C

CF3

(OC)3Co

(263b: R = R' = CF3)

Co(CO)3

(263a: R = H, R' = Fc)

263

(OC)2Co

Co(CO)2

Ph2P

PPh2

264a (OC)2Co MeS

Cp Co

Co(CO)2 dppf

SMe

(263a: R = H, R' = Fc)

Co Cp

267

Fc

Fc

H

(OC)2Co

Co(CO)Ph2P

Ph2P

Fc

PPh2

H

PPh2(OC)Co

Co(CO)2

Ph2P

Fc

265

Scheme 85

PPh2 Fc

Organometallic Chemistry

354

PhCCH)(m-SMe)3](BF4) (273) either underwent an 1,2-H shift to give the vinylidene species 274 or yielded, on addition of unsaturated molecules (e.g., RCCH, RNC, SCS), the vinyl-thioether complex 275.421 The reactivity of bimetallic complexes towards free alkynes has been the basis of a number of reports and in several cases products resulting from alkynealkyne coupling result. For example, the binuclear complex [Ru2(CO)4(m-h5:h5Fv)] (276: Fv : fulvalene) promotes, on photochemical activation, alkyne coupling to give, following initial coordination of the demethyl acetylendicarboxylate (277), the metallacyclopentadienyl complex [Ru2(mCR:CRCR:CR)4(m-h5:h5-Fv)(CO)] (278) (Scheme 87).421 Similarly, the silyllinked bis-cyclopentadienyl species [
(h5-C5H3)2(SiMe3)2Ru2(CO)4] (279) reacts photolytically with diphenylacetylene to give 280 and 281 and, in addition, 282 the result of the coupling of two alkyne molecules and a CO.422 In contrast, the phosphido-bridged cluster, [Ru2(CO)6
m-PC(CH)3O)2(m-h1,h2C(CH)3O)] (283), Ph

Ph

Ph

Ph

PriSH Cp(OC)2Mo

S

269

L

PhC

CpMo MeS

Ph

BF4

SMe MoCp

S Me

275 L = RCCH, RNC, SCS

MoCp

S

Pri

270a

i

MoCp Pr

CpMo

+

S

S

Pri

H

Ph

S CpMo

Mo(CO)2Cp

C

Ph

Pri

S

270b

BF4

H SMe

1,2-insertion L

CpMo MeS

MoCp S Me

273

BF4

Ph C 1,2-H shift

C CpMo MeS

H

SMe MoCp

S Me

274

Scheme 86

mediates the coupling of two HC∫CR molecules and a furan group to form [Ru2(CO)4
m-P(C(CH)3O)2
m-h1,h1,h2,h3-RCC(H)C(R)C(H)C(CH)3O] (284).423 6.1.2 (alkenes). The pentaalkyl 1,3-di-t-butyl-2,3-dihydro-1,3-diborole derivative reacts with [(h2-C2H4)2RhCl]2 (285a) to yield, via displacement of the ethylene ligands, the Cl-bridged dimer 286 (Scheme 88).424 Dimer cleavage also occurs during the reaction of iridium analogue of 285a, [(h2-C2H4)2IrCl]2 (285b), with the enantiopure PN ligand, (4S)-2-[2-(diphenylphosphanyl)phenyl]-4-isopropyl1,3-oxazoline to give [IrCl(h2-C2H4)(PN)] (287); introduction of ethylene to 287 generates the five-coordinate bis-alkene complex [IrCl(h2-C2H4)2(PN)] (288), which in the absence of ethylene converts back to 287.425 The borabenzene complex [(C5H5B-Ph)Rh(h2-C2H4)2] (289), prepared by treating 285a with Li[C5H5B-Ph], is an active catalyst for the boration of octane with 4,4,4,4,5,5,5,5-octamethyl-2,2-bi-1,3,2-dioxaboralane (pinBBpin).426 C-H activation occurs on treatment of 285a with But2PCH2CH2C6H5 at room temperature to give the Rh(III) complex [(But2PCH2CH2C6H5)

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

(OC)2Ru

Ru(CO)2

R

O C

hυ, RCCR (R = CO2Me)

(OC)Ru

+

Ru(CO)

Ru

OC

Ru R

R R

276

R

R

277

Si Si

Si (R = Ph)

(OC)2Ru

278

(OC)Ru

Ru(CO)

O

281

Ru(CO)3

283

CO

Ru R

O

CO

282

O HCCR

O

R

O O

(OC)3Ru

R

R

Ru R

280

Ru

R

R

R

+

Ru

R R

Si

Si

+

Ru(CO)2

279

Si

Si

Si O C

hυ, RCCR

355

(R = Ph, p-MePh)

(OC)2Ru R

Ru(CO)2

O H

R

284

Scheme 87

(But2PCH2CH2C6H4)Rh(H)Cl] (290); reversal of the reaction occurs on treatment of 290 with PhCCH or H2.427 Reaction of [Rh(h2-COE)2Cl]2 (291a) with the highly electron donating tridentate ligand, 2,6-bis-(di-tert-butylphosphinomethyl)pyridine (PNP), results in dimer cleavage and displacement of COE to give 292 (Scheme 89). In contrast, treatment of [Ir(h2-COE)2Cl]2 (291b) with PNP allows access to the h2-vinyl hydrido Ir(III) complex 293, the result of vinylic C-H activation.428 C-H bond activatation can be promoted when the bis(alkene) complex [(Pri3P)2(h2-CH3CCCH3)2Ir2(m-HNC10H6NH)] (294) is oxidised with [(h5giving complex [(Pri3P)(h3-MeCHCHCH2)Ir(m-H)(mCp)2Fe](CF3SO3) i HNC10H6NH)Ir(Pr 3P)(NCMe)2](CF3SO3)2 (295) (Scheme 90).429 Conversely, carbon-carbon coupling occurs during the thermally induced reaction of the bis(alkene) complex [(Pri3P)2(h2-CH2CH2)2Ir2
m-NNC3H2-2-NC5H102] (296) to give [(Pri3P)2(m-CH2CH2CHCH2)Ir2
m-NNC3H2-2-NC5H102] (297).430 Reaction of the cationic h2-propene hydrido complex [
(h5C5H3)2(SiMe3)2Ru2(CO)3(h2-CH2:CH-CH3)(m-H)](BF4) (298) with amines gives the neutral amine complexes [
(h5-C5H3)2(SiMe3)2Ru2(CO)3(NHR1R2)] (299: NHR1R2 : NH3, MeNH2, Me2NH) and the corresponding alkylated amines in a 1:1 ratio (Scheme 91).431 6.1.3 (Dienes). The chloride-bridged group 9 complexes [(h4-COD)MCl]2 (300a M : Rh, 300b M : Ir) have served as starting materials for a variety of stoichiometric432-435 and catalytic transformations.436 Scheme 92 illustrates some examples of the reactivity of 300a and 300b including towards the platinum acetylide [Pt(CCSiMe3)4]29, the tethered imadazole salt [CH2
NCHCHN(R)CH2]I2, the alkene-substituted alkene iron complex

Organometallic Chemistry

356 H

Me

t-Bu

Cl

B

Rh

Rh

B

B B

Cl

But

But

Me

Me

Me

Me

t-Bu

Me

286

H

But Me

B

(M = Rh)

H Me

B

Me

But

t

PBu 2 H Rh

B ButPCH2CH2C6H5

Cl Cl

M

M

Rh

(M = Rh)

Cl But2P

Ph

Li[C5H5B-Ph] (M = Rh)

285a M = Rh 285b M = Ir

C6H5

290

289

O

H H

N

Pri

PPh2

H

O

H H

H

PPh2

N Pri

PPh2

N Pri

-C2H4

Ir

H

O

H

C2H4

Ir

H Cl

Cl

288

287

Scheme 88 PBut2

PBut2 H PNP

Ir

N Cl

M

Cl Cl

PNP M

N

-COE

Rh

Cl t

PBu 2

t

PBu 2

291a (M = Rh) 291b (M = Ir)

293

292

Scheme 89

(CF3SO3)2

i Pr 3P

HN

NH

Ir

Ir

Me Me Me

294

2 [Cp2Fe](CF3SO3) i PPr 3

MeCN

HN Me

Ir i

Pr 3P

Me

Scheme 90

NH Ir H

NCMe NCMe i

PPr 3

295

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes

357

BF4 Si

Si

Si

Si

+

1 2

NHR R (OC) 2Ru

Ru(CO) H

BF4

H 1

(OC) 2Ru

Ru(CO)

NHR R2

1 2 NHR R

298

NHR1R2 = NH3, MeNH2, Me2NH

299

Scheme 91

[Fe(h -C5H5)
h -C6H5C(CH:CHCH2)3](PF6) and the phosphonothioate [(h5-Cp)Fe(C5H4PS2OCH2C6H4N3)]9, affording the unsymmetrical heterobinuclear complexes (NBu4)[(h4-COD)Ir(m-CCSiM3)2Pt(CCSiMe3)2] (301),432 [Fe(h5Cp)
m-h6-C6H5C(CH:CHCH2)3IrCl(h4-COD)](PF6) (302),433 the mononuclear Ir(III) complex [Ir(bis-carbene)(OAc)I2](303)434 and (h4-COD)Rh[(h5C5H5)Fe(C5H4PS2OCH2C6H4N3)] (304).435 5

6

Scheme 92

Treatment of the bimetallic butadiene-bridged palladium complex [Pd2(PPh3)2)(m-C4H6)2](PF6) (305) with sodium hydroxide in THF gives the first report of a Pd-Pd bonded hydroxo-bridged organopalladium complex, [Pd2(PPh3)2)(m-C4H6)(m-OH)](PF6).THF (306) (Scheme 93).437

Scheme 93

Interaction of the hydroxide-bridged dirhodium complex [(h4-COD)Rh(mOH)]2 (307a) with [(h5-Cp*)TiMe(m-O2SiPh2)]2 forms the tetranuclear species 308 (Scheme 94); 308 which has provided a useful model for the catalytically active sites for rhodium supported on a titania-silica systems.438 Mixed-bridged

Organometallic Chemistry

358

Scheme 94

methoxo-amido complexes [
(h -COD)Rh2(m-OMe)(m-NHAr)] (309: Ar : ptolyl, p-NO2C6H4, a-napthyl) are accessible on treatment of [(h4-COD)Rh(mOMe)]2 (307b) with primary arylamines (ArNH2).439 4

6.2 Heterobimetallic. — Mixed-metal systems featuring combinations of earlylate, mid-late and late-late tansition metals have been the subject of a number of papers during 2001-2002. The Zr-Rh heterobimetallic complexes, meso-[(h5C9H7)Rh
(h2-CH2:CH)2Si(h5-C5H2Me2)2ZrCl2] (310) and [(h5-C9H7)Rh
(h2CH2:CH)2Si(h5-C5Me4)2ZrCl2] (311) represent examples of early-late p-containing bimetallic complexes that have interest as precatalysts for hexene polymerisation.440 Notably, on treatment with MAO the activity of 310 and 311 was found to be significantly higher than that for the corresponding monometallic complexes and they formed higher moleculear weight polymers. Reactions of the alkyne-bridged M-Co carbonyl complexes [(h5Cp)(OC)2M(m-RCCR’)Co(CO)3] (312 M : Mo, 313 M : W) have the been the subject of a series of papers. Selective substitution of a cobalt carbonyl occurs on reaction of [(h5-Cp)(OC)2Mo(m-HCCFc)Co(CO)3] (312a: Fc : ferrocenyl) with the secondary phosphine Ph2PH to give 314 (Scheme 95),414 while reaction of the phosphinoalkyne, Ph2PC∫CPh, with 313 results in two different reaction pathways depending on the alkyne R substituents. With [(h5-Cp)(OC)2W(mHCCBut)Co(CO)3] (313a) phosphinoalkyne-alkyne coupling occurs to give 315 while with [(h5-Cp)(OC)2W
m-C2(COMe)2Co(CO)3] (313b) insertion of the bridging alkyne into a phosphorus-carbon bond occurs to afford 316.441 Treatment of 312 and 313 with [Co2(CO)8] gives the Co3Mo and Co3W butterfly clusters [(h5-Cp)MCo3(CO)9(m4-RCCR’)] (317 M : Mo, 318 M : W) respectively.442

11: Reactions of Transition Metal p-Complexes of Alkenes, Alkynes and Dienes R1C

CR2

M

OC

Co

OC

OC

CO2Me

MeO2C

Ph

CO

359

Co

Co

OC CO

CO

PPh 2

CO CO

Cp(OC)2W

316

317 M = Mo 318 M = W

Co2(CO)8

R

(M = W, R = R' = CO2Me)

R'

Ph2PCCPh Cp(OC)2M H

PPh2H

Fc

Co(CO)3

(M = W, R = H, R' = But)

312 M = Mo 313 M = W

Ph

H

But

Ph2P

(M = Mo, R =H, R' = Fc) Cp(OC)2Mo

Co (CO)2

W Co(CO)3 (CO)2Cp

Co(CO)2PPh 2H

315

314

Scheme 95

The propargylic Ni-Mo cationic complex [(h5-Cp)Ni(m5 HC2CCMe2)Mo(CO)2(h -C5H4Me)](BF4) (320) can be obtained by protonation of 319 with HBF4.Et2O (Scheme 96); 320 has been found to exhibit dynamic behaviour on the 1H NMR time scale.443 Treatment of [(h5-Cp*)Ni(mCO)Mo(CO)2(h5-Cp)] (321) with methyl acrylate (MA) affords [(h5Cp)Mo(CO)2
h3-(MeO2C)CHCHCHCH2(CO2Me)] (322), the result of the unusual tail-to-tail dimerisation and C-H activation of MA.444 The spectroscopically characterised Pd-Pt complex [(C6F5)2Pd(m(324), formed during the reaction of PPh2CCPh)2Pt(C6F5)2] [(C6F5)2Pd(PPh2C∫CTol)2] with cis-[Pt(C6F5)2(THF)2], reacts further to give [(C6F5)2Pd
m-CTol:C(PPh2)C(PPh2):CTol(C6F5)Pt(C6F5)(S)] (325: S : THF/H2O), in which insertion of two P-coordinated PPh2C∫CTol ligands into a Pt-C6F5 bond has occurred (Scheme 97).445 H

CMe2(OMe)

Mo(CO)2(η5-C5H4Me)

CpNi

H

HBF4.Et2O

Mo(CO)2(η5-C5H4Me)

CpNi

319

320

OC OC Cp*Ni

CMe2

MoCp

O OC xs. CH2=CHC(O)OMe

Mo

CO OMe

MeO O

C O

321

323 Scheme 96

Organometallic Chemistry

360

Tol

Tol C6F5

Ph2P Pd

C6F5

C6F5 Pt

Pd C6F5

Ph2P

Ph2 P

C6F5

C6F5

Tol

C6F5 S Pt

P Ph2

C6F5 Tol

325

324 Scheme 97

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309. H. Ishino, S. Kuwata, Y. Ishii and M. Hidai, Organometallics, 2001, 20, 13. 310. G. Guillemot, E. Solari, R. Scopelliti and C. Floriani, Organometallics, 2001, 20, 2446. 311. P. K. Baker, M.G.B. Drew and D.S. Moore, J. Organomet. Chem., 2002, 658, 77. 312. P.K. Baker, M.G.B. Drew and D.S. Moore, J. Organomet. Chem., 2002, 664, 37. 313. M.B. Wells, J.E. McConathy, P.S. White and J.L. Templeton, Organometallics, 2002, 21, 5007. 314. C.J. Adams, K.M. Anderson, N.G. Connelly, D.J. Harding, A.G. Orpen, E. Patron and P.H. Rieger, Chem. Commun., 2002, 130. 315. M. Barrow, N.L. Cromhout, A.R. Manning and J.F. Gallagher, J. Chem. Soc., Dalton Trans., 2001, 1352-1358. 316. M. Akita, S. Kakuta, S. Sugimoto, M. Terada, M. Tanaka and Y. Moro-oka, Organometallics, 2001, 20, 2736. 317. V. Cadierno, M.P. Gamasa, J. Gimeno, C. Gonzalez-Bernardo, E. Perez-Carreno, S. Garcia-Granda, Organometallics, 2001, 20, 5177. 318. C. Benisch, R. Gleitner, T.H. Staeb, B. Nuber, T. Oeser, H. Pritzkow and F. Rominger, J. Organomet. Chem., 2002, 641, 102. 319. H. Werner, F. Kukla and P. Steinert, Eur. J. Inorg. Chem., 2002, 1377. 320. J.J. Eisch, X. Ma, K.I. Han, J.N. Gitua and C. Kruger, Eur. J. Inorg. Chem., 2001, 77. 321. C. Muller, R.J. Lachiotte and W.D. Jones, Organometallics, 2002, 21, 1975. 322. C. Muller, R.J. Lachicotte and W.D. Jones, Organometallics, 2002, 21, 1118. 323. C. Muller, R.J.Lachicotte and W.D. Jones, Organometallics, 2002, 21, 1190. 324. C. Muller, C.N. Iverson, R.J. Lachiotte and W.D. Jones, J. Am. Chem. Soc., 2001, 123, 9718. 325. A. Galindo, M. Gomez, P.Gomez-Sal, A. Martin, D. del Rio and F. Sanchez, Organometallics, 2002, 21, 293. 326. A. Galindo, M. Gomez, D. del Rio and F. Sanchez, Eur. J. Inorg. Chem., 2002, 1326. 327. T.-Y. Chen, J. Vaissermann and P. Doppelt, Inorg. Chem., 2001, 40, 6167. 328. T. Szymanska-Buzar and K. Kern, J. Organomet. Chem., 2001, 622, 74. 329. D. Suzuki, H. Urabe and F. Sato, J. Am. Chem. Soc., 2001, 123, 7925. 330. K. Ferre, L. Toupet and V. Guerchais, Organanometallics, 2002, 21, 2578. 331. A. Elarraoui, J. Ros, R. Yanez, X. Solans and M. Font-Bardia, J. Organomet. Chem., 2002, 642, 107. 332. E. Ruba, K. Mereiter, R. Schmid, K. Kirchner, E. Bustelo, M.C. Puerta and P. Valerga, Organometallics, 2002, 21, 2912. 333. E. Ruba, K. Mereiter, R. Schmid, V.N. Sapunov, K. Kirchner, H. Schottenberger, M. J. Calhorda and L.F. Veiros, Chem. Eur. J., 2002, 8, 3848. 334. J. Le Paih, S. Derien, C. Bruneau, B. Demerseman, L. Toupet and P.H. Dixneuf, Angew. Chem., Int. Ed. Engl., 2001, 40, 2912. 335. S. Chattopadhyay, K. Ghosh, S. Pattanayak and A. Chakravorty, J. Chem. Soc., Dalton Trans., 2001, 1259. 336. T.M. Trnka, M.W. Day and R.H. Grubbs, Organometallics, 2001, 20, 3845. 337. K. Melis, D. De Vos, P. Jacobs and F. Verpoort, J. Organomet. Chem., 2002, 659, 159. 338. T.B. Wen, Z.Y. Zhou and G. Jia, Angew. Chem., Int. Ed. Engl., 2001, 113, 1951. 339. S.-Y. Yang, X.-Y. Li and Y.-Z. Huang, J. Organomet. Chem., 2002, 658, 9. 340. R.J. Schaller, R. Gleiter, J. Hofmann and F. Rominger, Angew. Chem., Int. Ed. Engl., 2002, 114, 1181. 341. R. Gleiter, J. Classen, B.J. Rausch, T. Oeser and F. Rominger, J. Organonmet. Chem., 2002, 641, 3.

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342. R.S. Schaller, G. Haberhauer, R. Gleiter and F. Rominger, Eur. J. Inorg. Chem., 2002, 2296. 343. J. Classen, R. Gleiter and F. Rominger, Eur. J. Inorg. Chem., 2002, 2040. 344. S. C.Johannessen, R.G. Brisbois, J.P. Fischer, P.A. Grieco, A.E. Counterman and D.E. Clemmer, J. Am. Chem. Soc., 2001, 123, 3818. 345. C.S. Chin, H. Lee, H. Park and M. Kim, Organometallics, 2002, 21, 3889. 346. K. Ogata, K. Kuge, K. Tatsumi and Y. Yamamoto, Chem. Commun., 2002, 128. 347. A.J. Edwards, S.A. Macgregor, A.D. Rae, E. Wenger and A.C. Willis, Organometallics, 2001, 20, 2864. 348. S.A. Macgregor and E. Wenger, Organometallics, 2002, 21, 1278. 349. J. Campora, J.A. Lopez, P. Palma, D. del Rio, E. Carmona, P. Valerga, C. Graiff and A. Tiripicchio, Inorg. Chem., 2001, 40, 4116. 350. K.R. Reddy, K. Surekha, G.-H. Lee, S.-M. Peng and S.-T. Liu, Organometallics, 2001, 20, 5557. 351. T. Sagawa, Y. Asano and F. Ozawa, Organometallics, 2002, 21, 5879. 352. S. Doherty, E.G. Robins, M. Nieuwenhuysen, J.G. Knight, P.A. Champkin and W. Clegg, Organometallics, 2002, 21, 1383. 353. C.P. Casey, S. Kraft and M. Kavana, Organometallics, 2001, 20, 3795. 354. E. Katayev, Y. Li and A.L. Odom, Chem. Commun., 2002, 838. 355. H. Katayama, C.Wada, K. Taniguchi and F. Ozawa, Organometallics, 2002, 21, 3285. 356. F. De Angelis, A. Sgamellotti and N. Re, Organometallics, 2002, 21, 5944. 357. F. De Angelis, A. Sgamellotti and N. Re, Organometallics, 2002, 21, 2715. 358. T. Tomon, D. Ooyama, T. Wada, K. Shiren and K. Tanaka, Chem. Commun., 2001, 1100. 359. J.M. O’Connor, K. Hiibner, A. Closson and P. Gantzel, Organometallics, 2001, 20, 1482. 360. C. Holtke, G. Erker, G. Kehr, R. Frohlich and O. Kataeva, Eur. J. Inorg. Chem., 2002, 2789. 361. (a) T.J. Woodman, M. Thornton-Pett, D.L. Hughes and M. Bochmann, Organometallics, 2001, 20, 4080; (b) T.J. Woodman, M. Thornton-Pett and M. Bochmann, Chem. Commun., 2001, 329. 362. C. Visser, A. Meetsma and B. Hessen, Organometallics, 2002, 21, 1912. 363. T.V.V. Ramakrishna, S. Lushnikova and P.R. Sharp, Organometallics, 2002, 21, 5685. 364. T.M. Cameron, I.Ghiviriga, K.A. Abboud and J.M. Boncella, Organometallics, 2001, 20, 4378. 365. Y. Nakayama, H. Saito, N. Ueyama and A. Nakamura, J. Organomet. Chem., 2001, 627, 221. 366. A. Nakamura and K. Mashima, J. Organomet. Chem., 2001, 621, 224. 367. A.J. Pearson, A. R. Allimardanov and W.D. Kerber, J. Organomet. Chem., 2001, 630, 23. 368. R.D.A. Hudson, C.E. Anson, M.F. Mahaon and G.R. Stephenson, J. Organomet. Chem., 2001, 630, 88. 369. L.-S. Luh and L.-K. Liu, J. Organomet. Chem., 2001, 637-639, 549. 370. N. Mezailles, L. Ricard, F. Matthey and P. Le Floch, Organometallics, 2001, 20, 3304. 371. M. V. Butovskii, U. Englert, A.A. Fil’chikov, G.E. Herberich, U. Koelle and A.R. Kudinov, Eur. J. Inorg. Chem., 2002, 2656. 372. G.E. Herberich, T. S.Basu Baul and U. Englert, Eur. J. Inorg. Chem., 2002, 43.

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12 Transition Metal Complexes of Cyclopentadienyl Ligands BY IAN R. BUTLER

1

General Introduction

The review follows the same general format of previous years1 with the exception that, in view of the explosion in the number of literature reports in the general area of metal- cyclopentadienyl ligand chemistry, the focus will be very much on metallocene chemistry. Inevitably this means that the major emphasis will be on ferrocene chemistry. The standard abbreviations: Cp : h5-C5H5, Cp : h5C5H4Me, Cp* : h5-C5Me5, fc : 1,1-disubstituted ferrocene, Fc : monosubstituted ferrocene, Fp : -Fe(CO)2Cp will be used as in previous volumes, unless otherwise indicated.

2

Main Group, Lanthanides and Actinides

The full characterisation of the bis(pentamethylcyclopentadienyl)aluminum cation, [(Cp*2Al)]; has been achieved. Characterisation methods include solidstate NMR with analytical and numerical simulations of solid NMR powder patterns, ab initio self-consistent field and hybrid density functional theory calculations, and single-crystal X-ray diffraction.2 Magnesocene adducts of alkylamines which are stable and sublimable have been prepared and characterised for use in chemical vapour deposition studies.3 An example of a typical adduct is shown as 1. Again new linked di-germanocenes and di-stannocenes have been obtained in the reaction of [Cp*MCl], M : Ge, Sn, with the dilithium salts of [Cp-ArCpH] ligands, [Ar : phenylene, biphenylene or thiophene bridging group]. The product complexes are observed to lose the linked cyclopentadienyl ligand on treatment with iodine.4 The isopropylidene-bridged unsymmetrical ligand C5H5CMe2-C13H9,(Cp-C(Me)2-FluH), Flu : fluorenyl have been bound to Y, La, and Nd(III) metal centres to give a range of new complexes. The synthetic route used is the amine elimination reactions between the metal amides, Ln(N(SiMe3)2)3, and the free ligand in THF. An example of one of the new yttrium complexes obtained is shown as 2.5

Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 375

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1

2

The simple metathesis reaction of [
CH(PPh2NSiMe3)2-T 2-N,N-T -CLnCl2]2 ; Ln : Y, Sc, Er, with NaCp in a 1:4 molar ratio in THF gave the metallocenes with the structure [Cp2Ln
CH(PPh2NSiMe3)2-T 2-N,N-T -C]2 which were crystallographically characterised.6 Triazacyclononane-functionalised tetramethylcyclopentadienyl ligands have been bound to samarium and yterrbium in the direct reactions of [Ln I2(THF2] with the potassium salt of [C5Me4SiMe2(iPr2-tacn], where iPr2-tacn : 1,4-diispropyl-1,4,7-triazacyclononane. The crystal structure of one of the products thus obtained, the samarium complex, is shown as 3.7 A general review has also highlighted the use of a broad range of cyclopentadienyl- lanthanide complexes as polymerisation catalysts.8 Using similar methodology to that used in reference 5 the silylamine elimination

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3

reaction has been used in the preparation of a series of complexes such as [(Me5C5)2Y(NSiNMe2)2] and [(Me4C5H)2Y
N(SiNMe2)2], [(Ph4C5H)2La(NSiHMe2)2].9 A new series of b-diketimato ligand complexes of Sc(III) have been prepared in which involve the formation of multicylic aggregates.10 Following on with the general theme a series of five new complexes of the general type [ML(m-OTf)]2 have been prepared where M : Nd, Sm, Y, Yb in the reaction of [M(OTf)3] with the disodium salt of a tethered cyclopentadienyl ligand with the spacer —CH2-pyr-CH2 unit, where pyr :2,6-disustituted pyridyl group. Interestingly there is significant bonding between the N-pyridyl donor in the bridge and the metal centre, with the consequence that the complexes are very rigid, at least at ambient temperature. Despite this rigidity an in depth study of the fluxonality was also carried out.11 The lutetium complex of bis-(diphenylphosphinocyclopentadienyl)lutetium chloride was prepared and structurally characterised and its dynamic behaviour in solution was studied by NMR. Interestingly, it was observed that the phosphorus donors remain coordinated to the metal centre throughout the NMR study.12 The reaction of [Cp3Y] with 2,4-cylopentadiene-1-ethanol resulted in the formation of both the complex [CpY(m:h5:h1-C5H4CH2O)(dme)]2 and the complex [Cp2 Y2(C7H8))2(C4H10O2)2].13 A thorough investigation has been carried out on group 3 and 4 hydrido- and alkyl- complexes containing functionalised cyclopentadienyl ligands. The lanthanide tris alkyl complexes [Ln(CH2SiMe3)3(THF)2], where Ln : Y, Er, Yb, Er and Tb, have been used as the precursors to give complexes of the general formula [Ln(h5:h1-C5H4SiMe2NCMe3)(CH2SiMe3)(THF)]. A hydride bridged dimer, [Ln(h5:h1- C5H4SiMe2NCMe3)(THF)(m-H)]2 results on hydrogenolysis of the amido-linked cyclopentadienyl complex.14 In a related report by the same research group the reaction of the half-sandwich alkyl complexes [Y(h5:h1C5Me4SiMe2NCMe3)(THF)] with furan and thiophene has been shown to give

Organometallic Chemistry

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the metallated product complex [Y(h5:h1-C5Me4SiMe2NCMe3)(m2-C4H3E)]2, where E : O, S, which have been shown to be sparingly soluble in hydrocarbon solvents. A crystal structure on the thienyl complex indicates that the complex contains a six-membered ring with bridging sulfur atoms and trans-diposed tetramethylcyclopentadienyl ligands.15 In a paper numbered 156 in the series it has been shown that YCl3, LuCl3 and YbI2 react with the lithium and potassium salts of 1-(but-3-enyl)-2,3,4,5-tetramethylcyclopentadiene to give the complexes [(CH2:CHCH2CH2C5Me4)2LnCl(THF)] for the trivalent metals and [(CH2:CHCH2CH2C5Me4)2Yb] respectively for Yb(II). Further reactions of the salts with SmCl3 were then carried out in addition to the preparation of mixed cyclopentadienyl complexes using the pentamethylcyclopentadienyl ligand as the second cyclopentadienyl group.16 In the previous paper in this series, (number 155), Y, Sm and Lu halide and methyl complexes were prepared using the the (h5-C5H4SiEt3)-ligand.17 The oxirane ring-opening reaction of butyl glycidyl ether using sodium cyclopentadienide or sodium indenide results in the formation of cyclopentadienyl- and indenyl-substituted alcohols which have been used as tridentate ligands with a number of lanthanide metals. A crystal structure of one of the lanthanum complexes obtained is shown as 4.18

4

The metathesis reaction of [(THF)4Li][Cp"2Li], where Cp" : h5(Me3Si)2C5H3, with yttrium trichloride also results in the formation of a bis THF complex of the appropriate substituted cyclopentadienyldichloroytteriate. The halides were subsequently exchanged for phosphides in this paper.19 A number of new synthetic routes have been devised to the sterically crowded [Cp*3U] complex. These are reaction of the dihydride [Cp*2UH2] with tetramethylful-

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vene, reaction of [Cp*2Pb] with [Cp*2UH(DMPE)], reaction of [Cp*2U(DMPE or THF)]; with the K(18-crown-6)(Cp*) and finally the reaction of [Cp*2U] [BPh4] with KCp*.20 Finally it has been shown that the ligand 1,2,3,4-tetramethyl-5-(2-pyridyl)cyclopentadiene (Cp*py) forms a dinuclear complex with U (IV), i.e. the complex di-m-oxo-bis
chloro(Et2O-T -O)[(h5-T -N)-1,2,3,4-tetramethyl-5-(2-pyridyl)cyclopentadienyl]uranium(IV) is formed.21

3

Titanium, Zirconium and Hafnium

Once again the main feature of group 4 chemistry is the use of the metallocenes in alkene polymerisation reactions; in fact the number of references in this area runs into several hundred, therefore the review will focus on the more general synthetic chemistry of these metallocenes rather than their use in catalysis, although inevitably catalysis will feature in some of the synthetic reports in cases where new synthetic methods are used or there are new supports, materials etc. The reactions of [(Cp2TiCH3)(THF)]; BPh49 with a series of tripeptides have been investigated — 16 protected tripeptides were reacted to give methyl(peptide) titanocene cationic complexes in which the organometallic fragment is coordinated to a carboxamide carbonyl oxygen. Generally it was observed that the carbonyl oxygen of the central amino acid was bound to the titanium, however there were a few exceptions to this. The scheme as represented in the original paper is shown as 5. 22

5

It has been observed that methylalumoxane cocatalyst activated complexes of the type [(h5-C5H3R)-(bridge)-Ar]TiCl3 are very active catalysts in the trimerisation of ethene to yield 1-hexane as the main product. Interestingly, the trimerisa-

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tion selectivity depends on the presence of a pendant arene group on the cyclopentadienyl ligand and the nature of the bridge between the Cp- and the arene and indeed in the absence of a pendant group polyethene is observed as the main product.23 The reductive opening of epoxyimonolactams with [Cp2TiCl] gives rise to radicals, which may be trapped by intramolecular p-containing molecules such as lactone and amide carbonyls and conjugated alkenes. A number of new carbocycles such as the trilactam 6 are obtained.24 The crystal structure of the decamethyltitanocene cation, shown as 7, has been described. In this the molecule shows two agostic C-H bonds are evident (dotted lines). In this work its interaction with fluorocarbons is further described.25

6

7

The treatment of (butadiene)zirconocene with benzylmercaptan gives [Cp2Zr(SCH2Ph)2], which on treatment with [Cp2ZrCH3];[CH3B(C6F5)3]9 yields the dinuclear complex [(m:h2-thiobenzaldehyde)(m-SCH2Ph)ZrCp2]2; which is accompanied by the elimination of methane.26 Also, the butadiene zirconocene reacts with the imine MeN:CHPh to give a 1:1, C-C coupled

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8

product shown as 8, which is [kN-MeN-CHPh-CH2-h3-(CHCHCH2)ZrCp2]. This product contains a distorted p-allyl group and a trigonal planar bound nitrogen atom. When more bulky amines are used the 1:1 addition products are observed.27 A comprehensive review of early transition metal hydride complexes has also been published by the research group of Stephan. While containing many articles on Ti, Zr, Hf cyclopentadienyls Sc, Y, V, Nb and Ta compounds are also described.28 Cyclic voltammetry and UV-visible spectroscopy have been used as the key instrumental techniques to examine the substituent effects in the titanium(IV) half sandwich complexes [h5-CpTiCl3] and [h5-(C9H7)TiCl3]. The ring substituents were principally methyl and trimethylsilyl groups. The paper concludes with a tabulation of the data which gives an additive incremental system for the effects of addition of trimethylsilyl and alkyl groups on the spectra of these molecules.29 The reactions of [h5-Cp2ZrCl2] with [K(THF)n][N(PPh2)2], n : 1.25-1.5 and K[CH(PPh2NSiMe3)2] have been reported. In the first reaction studied the monoamino [h5-Cp2Zr(Cl)N(PPh2)2] is obtained by simple metathesis in which there is chelating P and N bonded [(Ph2P)2N] group. In the second case a monocyclopentadienyl complex [h5-CpZr(Cl)
C(PPh2NSiMe3)2], which is described as ‘carbene like’ is obtained as the initial metathesis product which is followed by Cp ligand extrusion.30 It has been observed that [Me2Sn(NCl2)2] and [(Me2N)4Sn] react with alkyl-substituted bis-cyclopentadienyldimethylsilanes to give tetrahydrostannocenes, which on treatment with ZrCl4 give meso and rac configured ansa-zirconocenes as shown in the scheme in 9.31 The reaction of the titanocene complex [Cp2Ti(PhC∫CPh)2 ] and its zirconocene analogue with vanadocene leads to the formation of a series of heterobimetallic products with the alkyne ligand bridging the metal centres.32 An interesting zirconocene-mediated synthesis of (;)-epiafricanol from D-glucose has appeared in which the zirconocene is used in the key ring contraction step.33 A series of microgel-based catalysts have been prepared and tested in ethylene polymerisations. In particular metallocene dichlorides were complexed and activated by aluminium and the results obtained from the polymerisation studies were compared with those obtained from similar silica gel supported catalysts.34

Organometallic Chemistry

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9

The direct addition of alkylzirconocenes to activated alkenes such as benzylidenemalonitrile has been achieved and similarly zirconacyclopentanes reacted with alkanes containing electron withdrawing groups. Treatment of the products of zirconocyclopentanes with ylidene malononitriles with iodine results in the formation of six-membered cyclic products such as 12 (product of 10 with 11).35

10

11

12

The thermally robust complex ansa-[Ti
h5:h5-(C5Me4SiMe2)2O(h2Me3SiC∫CSiMe3)] 13 was obtained by reduction of dichloro-
1,3-bis(h5C5Me4H)-1,1,3,3-tetramethyldisiloxane titanium(IV), [TiCl2(h5:h5(C5Me4SiMe2)2O] with Mg in the presence of bis(trimethylsilyl)ethyne. At 170oC, however, 1,2-bis(trimethylsilyl)ethene is liberated to give the complex ansa[Ti
h5-C5Me4)SiMe2OSiMe2
h3:h4-C5Me2(CH2)2] 14. Both complexes have been structurally characterised and they are shown below.36 The synthesis of a range of 1,2,3-tri-substituted zirconocene dichlorides has been carried out using internal alkyne precursors all of which are commercially available. The crystal structures of six of these have been determined. The cyclic voltammetry of these compounds has also been carried out to determine the effect of the substituents on the Lewis acidity of the metal centre.37 A onepot synthetic procedure for the preparation of substituted cyclopentadiene indenes and tetrahydroindenes has been devised. Essentially this method relies on the use of zirconacyclopentadiene oxophilicity in their reactions with

12: Transition Metal Complexes of Cyclopentadienyl Ligands

13

383

14

Molecular structure of 13 and 14 (30% probability elipsoids)

aldehydes.38 A general review of the ansa-bridge and its effects of the range a metallocene chemistry has appeared which highlights the importance of the bridge, in particular in titanocenes, and zirconocenes.39 In addition to dibenzyl, two complexes [
Zr(C6F4N(CH2CH2O)CH2CH2NC6F5][CH2Ph]2] and [
Zr(C6F5N(CH2CH2O)2CH2CH2NC6F5)(F)2(m-F)2]2 have been obtained in the photolysis of [Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2](C7H8)2 at 435 nm. Interestingly when the methyl complexes (replacing benzyl with methyl) were prepared, there were found not to be photochemically active at this particular wavelength.40 Dendrimer-substituted titanocenes and zirconocenes have been used in ethene polymerisation — the actual dendritic structures were based on silane wedges, prepared by hydrosilylation of chloro-tri-vinyl silane with SiHR1R2. These were reacted with KCp, then with KH to give the salts of the type K
C5H4Si(R2R1SiCH2CH2)3], R : Ph, R1 : Me, which were then used to prepare the metallocenes, 15. (compound numbering is as shown in the paper)41 Asymmetric ligand precursors which contain phosphines have been used to make a range of ansa-titanocenes and zirconocenes and halfnocenes. In this case the ligand precursors have the form [(C5Me4H)Si(Me2)(C5H4R)] when R : (CH2)2PPh or PPh2. A typical complex which was prepared was [Ti
Me2Si(h5C5H3R)Cl2], R : CH2CH2PPh2. Other complexes obtained included the dimethyl derivatives in which methyllithium was used to exchange the metalchlorides. Preliminary results were presented for the catalytic activity of these complexes in ethene polymerisation.42 A theoretical investigation has been carried out on the electronic structure of ‘[Cp2Ti]’ — the calculations produce a triplet ground state with finely rotating (parallel) cyclopentadienyl rings. In the discussion the experimental results are compared with those theoretical ones to support the well known dimer formulation which is observed experimentally.43 The 1,2-addition of diphosphine oxide (HR2PO) to a titanocene carbene complex has been reported in which the titanocene acts as an oxyphile and the phosphine produces (R2P)2CH2.44 A variable temperature 1H NMR study has been carried out on the complex [a,a-ortho-xylene-bis(5,6-dimethyl-1-indenyl)zirconocene

Organometallic Chemistry

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15: Dendrimer substituted titanocenes and zirconocenes

dichloride. When activated with methylaluminoxane, ethene/hexene copolymers could be obtained with broad molecular weight distributions.45 The complexes [exo, exo bis(isocyclopentadienyl)titanocene dichloride], and its trimethylsilyl substituted analogue have been reduced to afford the paramagnetic complexes dimeric exo,exo-bis(h5-isocyclopentadienyl)titanium(III) chloride (plus the monomeric trimethyl silyl analogue) and the diamagnetic exo,exo-bis(isocyclopentadienyl)(h2-bis(trimethylsilyl)ethyne)titanium(II) complexes. The crystal structure of one of the titanium(II) complexes, exo,exo-[Ti(h5-TMS-isodiCp)2(h2-btmse)], btmse : bis-trimethylsilylethene. is shown as 1646

16

The reactions of methylenecyclopropane with [Cp2Ti(PMe3)2] (A) and [Cp2Zr-h2-(1-butene)(PMe3)] (B) by a [2;2] cyclo-addition process to give metallacyclopentanes whereas the indenyl analogue of the zirconocene complex reacts by a [3;2] cyclo-addition to give a 1-zircona-4-methylenecyclohexane

12: Transition Metal Complexes of Cyclopentadienyl Ligands

385

complex. The precursor complexes A and B were also reacted with benzylidene cyclopropane to give olefin complexes, while the indenyl analogue of complex B gave a 1-zircona-2-benzylidene cyclobutane which was structurally characterised.47 It has been observed that thioacetals are transformed into terminal olefins in the presence of a titanocene(II) complex under an ethene atmosphere, with the mode of action controlled by the ligands bound to the titanocene (as shown in 17).48

17 Scheme from reference 48 with original numbering.

The same research group have also reported the preparation of alkynylcyclopropanes by titanocene(II), [(Cp2Ti(P(OEt)3)2], promoted reaction of 1,1bis(phenylthio)-2-alkynes with 1-alkenes.49 An interesting review has appeared on the effect of titanium-containing complexes in cancer treatment. Discussed are the differences observed with the titanocenes and the conventional platinum based anti-cancer drugs.50 Related to this is the effect of [Cp2Ti(NCS)2] on the haematopoietic response of Ehrlich tumour-bearing mice in which it is concluded that the compounds with (NCS) ligands are poorer than the dichloride analogue compounds.51 The zirconadihydrofuran [Cp2ZrOCMe:CHCH2] is obtained in the reaction of [Cp2Zr(py)(h2-Me3SiC2SiMe3)] with methylvinylketone, whereas rac-[(L)Zr(h2-Si(Me3C2SiMe3)], L : (1,2-ethylene-1,1-bis(h5indenyl) on treatment with methylvinylketone gives a 1,4-concerted addition product [rac-(L)-ZrOC((OBu)9tBu):CH2CH2] which has been crystallographically characterised. The reaction of the zirconocenes with mesityl oxide has also been studied in this paper, which has been shown to give a different zirconadihydrofuran complex of the type [rac-(L)ZrOCMe(CH2:CH2)-C(SiMe3):C(SiMe3)].52 Anti-Markovnikov hydroamination of terminal alkynes has been achieved using complexes of the form [Cp2Ti(h2-Me3SiCCR], R : Ph, SiMe3.53 The effect of metallocene dichlorides on the crystal growth of cadmium carbonate has been examined — it is observed that these reduce the crystal growth by 27-66% through possible absorption onto the active sites. Examination of the kinetics of the process suggests a surface diffusion controlled spiral growth mechanism.54 The compound [Cp*Ti(h6-C5H4C(H)(tBu)]Cl reacts with aldehydes, ketones and esters to give s, p-chelated complexes containing the Cp-O ligand by insertion of the carbonyl group into the TiC(H)(tBu) bond. Interestingly, beginning with diasteromerically pure starting material it was observed that

Organometallic Chemistry

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the ratios of the diastereomeric products varied and were dependent on the nature of the R group in the carbonyl containing organic compounds. Four crystal structures were reported in this paper.55 A range of half-open zircanocenes of the type [CpZr(2,4-C7H11)(dmpe)] C7H11 : dimethylpentadienyl, dmpe : Me2P(CH2)2PMe2 have been obtained in the reaction of CpZrCl2Br with three equivalents of K[2,4-C7H17] in the presence of one equivalent of a phosphine. An example of such a complex is shown as 18.

18

As a consequence of a structural investigation of the products it was observed that the product complexes exhibited short Zr-C bonds and this was explained. The reaction of pentadienylmagnesium reagent (3 equivalents) gave [CpZr(C14H21)], in absence of the phosphate, in which the new C14H21 ligand was formed as a consequence of coupling and elimination of one proton from two 2,4-C7H11 ligands. The reaction of only 2 equivalents of the same reagent, this time in presence of the phosphine, leads to the formation of [CpZr(Br)(dmp)2].56 The reaction of [Cp*2ZrMe2] with [Ph3C][B(C6F5)4] leads to the formation of [Cp*2ZrMe)2(m-CH2)][B(C6F5)4] and [Cp*2ZrMe-B(C6F5)4] and not the arene complex previously reported in the literature. 57,58 The reaction of [Cp2ZrMe2] with the phosphate gold thiolates [Au
S(CH2)nCOOH(PMe2Ph)], n : 1, 2, result in the formation of trinuclear complexes which are best represented in structural form: an example is shown as 19, where n : 2 in which the mono and bidentate coordination of the two metalloid ligands to one zircanocene fragment is observed.59 A review on the literature of titanocenes containing a Ti-C bond has also appeared.60 In a synthetic paper titanocenes as catalysts for the intermolecular

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387

19

hydroamination of alkynes have been explored. These catalysts are compared with those which contain no cyclopentadienyl ligands in hydroamination/reduction processes. 61

4

Vanadium, Niobium and Tantalum

The reactions of titananocenes and zirconocenes with vanadocene have been used to afford a range of heterobimetallic complexes of the general type
[Cp2V(h2:h4-PhC4Ph.MCp2], M : Ti, Zr, Cp : Cp, C5H4SiMe3. The crystal structure of one of these indicates that the vanadocene and zirconocene units are built into a butadiene (or butadienyl) scaffold.62 Following on from the resurgence of amino-substituted ferrocenes, the synthesis and characterisation of aminofunctionalised Cp-vanadium complexes has been achieved, and a series of amino-functionalized cyclopentadienyl vanadium phosphine chloride complexes were also synthesized, and their catalytic behavior toward the polymerization of butadiene was examined. These examples, one of which is shown as 20 represent those first structurally characterised.63 The effect of metallocene chlorides on the growth of calcite has been examined which includes the use of a vanadocene.64 This work has also been reported in a related article which the synthesis is reported and the crystallography discussed.65 The same research group also report the reactions of vanadocene with a number of active nitriles RCN·L (L : Lewis Acid) where the Lewis acid is a borane adduct. The products are the borane adducts [Cp2V(h2-RC:N·L)].66 Yet further work by the same research group has resulted in the publication of the vanadocene alkyl complexes [Cp2VMe(CH3CN)]; [BPh4]9.67 A cyclopentadienylvanadioxy complex was obtained in the reaction of vanadocene with 1-hydroxybenzotriazole.68 Interestingly it is observed that vanadocene is a rapid and potent inhibitor of human sperm mobility which exhibits potential for a new type of contraceptive reagent. In a recent paper in vivo studies on mice have been carried out and the data tabulated.69 A general review has documented the synthesis of early transition metal hydrides which have been published over the previous ten years.70 In the general

Organometallic Chemistry

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20

reaction of the ligand precursors [Me2S2(C5Me4H)(C5H4R)], (R : CH2CH2PPh2, PPh2), and their lithium salts a range of niobocene imide complexes of the type [ButN:Nb(h5-C5H4PPh2)2Cl] have been obtained using [ButN:NbCl3(PO)2] as a precursor complex.71 Again using the same precursors further ansa-niobocene imido complexes have been reported by the same research group. A typical product complex obtained is structurally shown as 21.72

21

These researchers have also documented the preparations of the ansa-niobocene dichloride complexes of the type [Nb
Me2Si(h5-C5Me4)(h5-C5H3R)Cl2], R : H, Me, SiMe3, iPr, PPh2 and have gone on to examine the electrochemical behaviour of these complexes.73 The reactivity of a range of electrophilic organic substrates with the tantalum-lithium bridging alkylidine [(Me3SiCH2)(ArN:)Ta(m-CHSiMe3)(m-h1:h3-Pri2-tacn)-Li has been probed. These complexes have been observed to undergo a series of insertion reactions with CH3CN, CO, CS2 etc.74 It has been observed that heating [Cp2Nb.H3] in the presence of silanes leads to the formation of complexes such as [Cp2Nb(SiR3)H2], SiR3 : SiMe2Cl, Si(OEt)3, SiMe2OEt. The complex [Cp2Nb(S:Me2I)2H] was also obtained in the reaction of the previously reported complex [Cp2Nb(SiMe2H)2H] with iodine.75 The paper goes on to describe the preparation of a large number of derivatives obtained by reactions with organolithium, LiPPh2, fluorinating reagents and reducing reagents. Silica immobilised catalysts

12: Transition Metal Complexes of Cyclopentadienyl Ligands

389

have been obtained in the reactions of Cp2NbCl2 with silica, however, these species were found to be inactive in ethylene polymerisation, however with further modification to include zirconocenes highly active catalysts were indeed obtained.76 The reaction of [Cp2NbH3], Cp : tBuC5H4, with tellurium powder in THF resulted in the formation of [CpNb2(Te2)H] and [Cp6Nb4Te4O] which were further reacted with [Cr(CO)5THF] to give [Cp2NbTe2HCr(CO)5] and [Cp6Nb4Te4O.2Cr(CO)5], the structures of which have been crystallographically described.77 In related reactions the formation of triniobocene cationic and neutral complexes such as the example shown as 22 have been obtained from [Cp2Nb
(m-H)2BR2], R-2 : C4H8, C5H10, C8H10) on treatment with B(C6F5)3. 78

22

A number of new phosphorus complexes of tantalum hydrides of the type [CpCpTaH2(PMe2H)]PF6, Cp : C5H2But(Me)2 and [Cp2Ta(H)(m-PPh2, H)Fe(CO)3] have been prepared in a study which examines the reactions of phosphorus with tantalum trihydrides of the type [Cp2TaH3]. The crystal structure of one of the products is shown as 23.79 The reaction of dichloro bis cyclopentadienyl vanadium with dialkyl and alkylene dithiophosphoric acids proceed in a 1:2 molar ratio to yield di cyclopentadienyl vanadium bis-dialkyl and -alkylene dithiophosphates of the type, [Cp2V(S2P(OR)2)2], where R : Et, iPr, Pr, iBu, Ph and [Cp2V(S2POEO)2] where E : CMe2CMe2, CH2CEt2CH2, CH2CMe2CH2. These complexes are semi solids or are solids that are soluble in common organic solvents.80

5

Chromium, Molybdenum and Tungsten

It has been observed that the cis and trans [h5-(C5H4Me)Mo(CO)2(P((OPr)iPr)3]I complexes undergo a two way thermal isomerisation reaction at 90°C to yield a 30:70 cis/trans ratio of isomers in the solid state. It was calculated that the activation energy for this process was approximately 68 kJ mol-1. Interestingly, in

Organometallic Chemistry

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23

benzene the opposite isomer ratio was observed i.e. 70:30 cis/trans.81 In a short communication it has been stated that the cyclopentadienyl complexes of Cr (II) and (III) are stabilized by bis(trimethylsilyl)benzamidinato ligands to allow the resultant complexes to be model compounds to study alkyl halide activation reactions.82 The co-thermolysis of the complex [CpCr(CO)2(S2CNEt2)] with [CpCr(CO)3]2 in refluxing toluene gave a mixture of products comprising the following compounds in order of yield: [Cp4Cr4S4], [CpCr(CO)2(SCNEt2)], [Cr(S2CNEt2)3], [CpCr(CO)2(CNEt2)] and [Cp6Cr8S8(C(S)NEt2)2], [CpCr(CO)2(h2-(C,O)-C(O)C(NEt2)CH(NEt2)]. Three new complexes were crystallographically characterised in this study.83 Zwitterionic ring borylated chromocene complexes have been reported in an article which describes the first structurally characterised Cr(IV) bent metallocene. This complex shown here as compound, 24 was obtained following the ring borylation of [Me4C2 (h5—C5H4)2Cr(CO)] with B(C6F5)3. 84 The crystal structure of the compound [bis-(h5-Cp)-bis(4-methylbenzenesulfonato-O)-molybdenum (IV) has been reported.85 The complexes [CpM(CO)3Cl], M : Mo, W have been included into a b-cyclodextrin host.86 Half-sandwich 17-electron Cr(III) complexes have been isolated following isocyanide stabilisation following the reaction of chromocene with CF3SO3H and acetonitrile yielding [CpCr(CNCH3)4]2; (OTf)92. The reaction of Tl triflate with [Cp*CrCl2], Cp : Cp*, Cp etc. similarly gave the complexes [Cp*Cr(CNCH3)]2;.87 The chromium indenyl complexes [(1-R-ind)2Cr], where R : t-Bu, SiMe3, Ind : h5-C9H6, which were obtained from CrCl2 and the appropriate lithium indenate salts, are high spin, four unpaired electron complexes. The analogous 1,3-disubstituted indenyl complexes are however low spin complexes with two unpaired electrons. It has been shown that the reason for

12: Transition Metal Complexes of Cyclopentadienyl Ligands

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24

this phenomenon is that the bulky substituents on the indenyl ligands are responsible for a conformational change.88 The complexes [(h5C5Me4)(CH2NMe2)CrCl] and unmethylated cyclopentadienyl and indenyl analogues have been shown to be alkene polymerisation catalysts in the presence of methylaluminoxane.89 In an interesting synthetic paper the reductive generation and dimerisation of the complexes [CpM(NR)], M : Mo, W; R : alkyl have been described. One of these complexes, R : Me, which was characterised by X-ray diffraction is shown as 25.90

25

Sulfur dioxide inserts into the S-S bond in the complex [Cp2MoS2] in a dichloromethane/ethanol solvent mix to give a dithiosulfate ligand complex.91 The complexes [h-(C5HiPr4)W(CO)3CH3] and [h-(C5Et5)WCl4] have been prepared in a general synthetic study. It has been observed in this paper that [Cp"Mo(CO)3]9 on treatment with an aqueous ferric solution, yielded the hydride [Cp"Mo(CO)3H], which then had to be converted to the neutral dim-

392

Organometallic Chemistry

mer by oxidation with PhICl2.92 Continuing their now well established work on nitro-ligand complexes, the Legzdins’ group have crystallographically characterised the complex [Cp2Mo2(CH3O)2(C4H11Si)2(NO)2], which is formed in a high yielding synthesis by treating [Cp2Mo(NO)(CH2SiMe3)2] with methanol.93 (see also ref. 107) The complex [M2(CO)6(h5:h5-C5H4CR2C5H4)], M : Mo, W, R : H, Me when protonated with CF3SO3H at the metal-metal bond yields [M2(CO)6(m-H)(h5:h5-C5H4CR2C5H4)]. These complexes also give [M(CO)3H] 5 1 2(h :h -C5H2CR2C5H4) on reduction with sodium/potassium alloy but revert to the starting compounds on standing in daylight. Further photochemical reactions on the initial compounds were subsequently carried out where it was observed both compounds undergo reaction to form ‘twist compounds’ [M(CO)3][M(CO)3H](h5:h5:h1-C5H4CHRC5H3)] which also revert to the starting materials slowly (the W- compounds last much longer).94 A simple and effective synthesis of highly substituted cyclopentadienyl-molybdenum complexes has been reported. Specifically the compounds [Cp"2Mo2O5] have been described in which Cp" : Cp, Cp*, C5H(iPr)4, C5H2tBu3-1,2,4. The compound [Cp2Mo2O5] was obtained in good yield by PhIO oxidation of [CpMoO2]4 in dichloromethane. The [Cp*2Mo2O5] compounds were produced by acidification of the anions [Cp*MoO3]9, which were in turn obtained from [Cp*MoCl4] on treatment with sodium hydroxide under aerobic conditions.95 A series of 60electron clusters of the general type [W2Ir2(m-phos)(CO)8(h5-C5H4Me)] where phos : dppf or dppe have been obtained in the reaction of the analogous decacarbonyl complex with the appropriate phosphine. These clusters exhibit a reversible a one electron oxidation process and two irreversible two-electron reductions.96 The synthesis of the substituted cyclopentadienyl ligand [CpHSiMe2Ph], LH has been reported and it has been treated with CrCl3 to yield the chloro-bridged complex [(h5-L)Cr(m-Cl)Cl]2 which was crystallography characterised.97 Again on the theme of polymerisation catalysts [CpMoCl4]based catalysts have been used in the polymerisation of [O-trifluoromethylphenylacetylene] and O-trimethylsilylphenylacetylene.98 Following this trend the complexes [(h5-C5R4CH2NMe2)CrX2], R : H, Me, X : Cl have been used in ethylene polymerisation studies.99 The complex [Cp*CrTp] was prepared from [Cp*CrCl]2 and KTp and magnetic measurements were obtained together with its crystal structure. A pronounced Jahn-Teller distortion was observed for this high spin complex and thus DFT calculations were used to probe the nature of this distortion.100 In another theoretical study, the mechanisms for generating tungsten carbene complexes from [Cp"W(NO)(CH3)2], Cp" : Cp, Cp, Cp* by methane elimination have been examined, again using DFT calculations.101 Again the co-polymerisation of a range of dipropargyl derivatives has been carried out using the ‘Cp[2MoCl2-EtAlCl2]’ catalyst system.102 The crystal structure of [
Me4C2(C5H4)2Cr(CO)];[BR4]9 R : 3,5-C4H3(CF3)2 was determined following its isolation from the product mixture obtained following the metalprotonation of [
Me4C2(C5H4)2Cr(CO)] in THF.103 The reaction chemistry of the complex [
h5-C5H3(SiMe3)2W2(CO)6(m-H)] which was obtained from [(h5C5H3(SiMe3)2W2(CO)6(H)2] on treatment with Ph3C; has been investigated. The metal-metal bond is cleaved upon treatment with I9. Two crystal structures, the

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protonated and unprotonated compounds are shown as 26 and 27.104

26

27

Further theoretical work has been carried out on the h5- to h3- coordination shift observed in the molybdenocene complexes [(h5-Cp;)(h5-Cp)Mo(CO)2]2;, where Cp; : Cp, Ind which is driven by a two electron reduction. The results concur with the well known fact that ring slippage occurs more readily in the indenyl system, however it is stated that this is not simply an effect of the aromaticity gain in the benzene ring of the h3-indenyl ligand.105 The synthesis of bicyclic cyclopentadienes have been reported in which an N,S-chelate is attached to the Cp-ring. This has been achieved in the reaction of the [2-(1-cycloalkenyl)ethynyl]carbene complexes with 1 equiv. of pyridine-2(1H)-thione or Nphenylthioacetamide at 20 °C gave metal complexes. It was observed that the reaction of a tungsten [2-(1-cyclopentenyl)ethynyl]carbene complex with pyridine-2(1H)-thione led to an intractable mixture of products, due to the thermal instability of the resulting tetrahydropentalene.106 The crystal structure of the complex [CpMo(C6H4FO)(Me3Si)CH2)(NO)], which was obtained from the dimer [CpMo(NO)((CH3)3SiCH2)2]2 on treatment with the phenol 4-FC6H4OH has appeared.107 The carbonyl abstraction reaction chemistry of [Cp*Mo(PMe3)3H] with CO2, formaldehyde, formic acid and methanol to yield [Cp*Mo(PMe3)2(CO)H] have been investigated and in the case of the reaction with formic acid it has been shown that initially the metal centre is protonated to give [Cp*Mo(PMe3)3(H)2][HCO2], which eliminates the secondary phosphine on heating. In the presence of excess formic acid further intermediates containing the h1-O2CH unit are observed.108 The complexes [CpCr(CO)2]2(m-Se)2 and [CpCr(CO)2]2(m-Se) have been prepared in the direct reaction of the tris-carbonyl chromium cyclopentadienyl complex with elemental selenium. Irradiation of the m-Se derivative in the presence of diphenylacetylene gave [(CpCrSe)4(2.C2Ph2)] which was crystalographically characterised.109 A general review article has highlighted the results obtained on the reactions of the metal-

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metal bonded complexes [(h5-C5H4R)2M2(CO)6] and [(h5-C5H4R)2M2(CO)2(MEPh)], E : S, Se, Te complexes.110 The reaction of [CpMoCl4] with [Bi(OEt)3]x was attempted but it was not possible to obtain a tractable product, however on reaction with [Bi
OCH(CF3)23(THF)]2 the complex [CpMoCl
OCH(CF3)23] was isolated, which was formed by a halide/alkoxide exchange. The crystal structure of this complex was determined and discussed.111 A series of cyclopentadiene and indenyl complexes which were functionalised with dimethylamine or quinolyl groups were used as ligands for metal carbonyls. It was observed in some cases that these particular ligands added as neutral ligands without the loss of hydrogen, whereas in other cases the expected cyclopentadienyl complex was indeed obtained. The rare example of h2-C5R3H complex was obtained on reaction of Mo(CO)6 with the ligands.112 The polymerization of ethene has also been carried out by a series of cyclopentadienyl, indenyl and fluorenyl chromium complexes.113 A study of the reactions of the half sandwich complexes [CpMo(h4C4H4R2)CH2]2, R : H, CH3 with B(C6F5)3 has shown that an addition product forms and the crystal structure of a decomposition product which was obtained indicates that the attack by boron was at the methyl site.114 The complexes [CpCr(CO)2(SPR2)], where R: Me or Et are isolated as dark red solids in moderate yield from the reaction of [CpCo(CO)3]2 with one quivalent of the appropriate tetraalkyldiphosphine disulfide. Subsequent desulfurisation R : Me) with [CpCr(CO))3] gave the hydride -bridged complex [Cp2Cr2 (CO)4(mH)(m-PMe2)] together with the [Cp2Cr2(CO)2(-PMe2)2] and [Cp3Cr3(CO)3S(PMe2)]. All of the new complexes were characterised by single crystal X-Ray diffraction.115 Finally biscyclopentadienylchromium composites of mesoporous niobium oxide have been obtained in which the metallocene sits in the pores intact.116

6

Manganese, Rhenium and Technetium

The insertion reaction of ‘CpM’, M : metal groups into the sulfur-sulfur bond of [Mn2(CO)7(m-S2)] has been reported: for example the reaction of this compound with [CpCo(CO)2] or [Cp*Rh(CO)2] gave the new complexes [CpCoMn2(CO)6(m3-S)2] and [Cp*RhMn2(CO)6(m3-S)] respectively. The subsequent reactions of the initial manganese precursor complex with [CpCo(CO)(PPh3)] also yielded a phosphine-substituted analogue.117 In a general paper on metallocene chemistry (metal complexes of Zr, Hf, Fe, Sn, Pb, Rh, Ir) the use of the ligands [h5-C5R4CR-2PMe2] where R,R: H, Me has been described in a general synthesis In the case of manganese the new metallocene [Mn(h5-C5Me4CH2PMe2)2] has been obtained.118 Both high and low oxidation state complexes of the type CpML3 , whee M : Mn, Re and L : O (high oxidation state) and L : CO (low oxidation state) have been investigated using vibrational spectroscopy- in this investigation it is reported tat the vibrational spectra of the complexes [CpM(CO)3], M : Re, Mn have been reinterpretated and in the case of the oxo-complexes a complete spectral assignment has been made. In addition, characteristic force constants have been determined for the

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Cp, and Cp* ligands.119 The same rhenium carbonyl compounds may be prepared by double ligand transfer from ferrocenes with perrhenate, it is proposed that the corresponding technecium compounds when radio-labelled would be useful as imaging reagents.120 Another synthetic paper towards the preparation of cyclopentadienyl-manganese tricarbonyls describes the use of substituted furylcarbinols, which are transformed into hydroxycyclopentadienones. These, in turn, are subjected to conjugate Grignard addition reactions and dehydration to obtain multiply-substituted cyclopentadienyl complexes. In this case 1,2,- and 1,2,4-trisubstituted manganese tricarbonyl complexes.121 The reaction of [(triphos)Re(CO)2(OTf)], where triphos : MeC(CH2PPh2)3, with P4S3 and P4Se3 respectively resulted in the formation and isolation of the coordination isomers, [(triphos) Re(CO)2 (h1-P-apical-P4X3)] and [(triphos)Re (CO)2(h1-P-basal-P4X3)] X : S,Se. Further reaction with higher stoichiometric ratios of the P4X3 unit led to the production of dinulear complexes in which the P4X3 unit coodinates as an h1- and h2- and m-bridging ligand.122 The synthesis of a range of anionic manganese-cyclopentadienyl complexes where Cs-bound crown ethers are used as the corresponding cations has been achieved. The following complexes have been obtained :- [C2Cs];[Cp3Mn], [C2Cs];[Cp3Mn], [CsCp], [(CsCp)2][C]4, and Cs[MnCp3] (N.B. C signifies the crown).123 Finally the photophysics of the complex [CpRe(CO)2(NO)]; has been studied and it has been observed that two metal to ligand charge transfer bands at 330 and 380 nm( Re(I) to NO;) exist. In solution the complex reacts with PPh3 to displace a carbonyl ligand under photolysis, in line with most transition-metal carbonyl chemistry.124 Continuing the theme of using radiolabelled pharmaceuticals, a number of hydroxyphenylsubstituted complexes of the type [CpM(CO)3], where M : Re, Tc have been prepared and used as ligands towards estrogen. The synthesis used was standard in that the lithium salt of the arene substituted cyclopentadienyl with rhenium carbonyl precursors.125 The cyclopentadienyltricarbonylmanganese group has also been used in solid phase metallo-immunoassays in which the organometallic fragment was attached to a protein.126

7

Iron, Ruthenium and Osmium

In a general and very personal account, the structural assignment of ferrocene has been reviewed from an eye-witness perspective by Cotton. This work, combined with the historical accounts published in the Journal of Organometalic Chemistry, volume 638, should provide useful information for future historians on this important milestone in organometallc chemistry.127 In addition a review on the evolution of the use of the ansa-bridge and its effect on the scope of metallocene chemistry has been published.39 7.1 General Ferrocene and Synthetic Chemistry. — Following on from the work in the previous years, further variable temperature Mossbauer studies have been carried out on ferrocene to probe the anisotropy of iron’s atom motion.128 The kinetics of substitution of ferrocenyl-containing b-diketonato ligands by

Organometallic Chemistry

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phenanthroline from b-diketonato-1,5-cyclooctadienerhodium(I) complexes have been examined and the second-order rate constants, k2, for the substitution have been reported.129 The reaction of ferrocene with beta-ketoesters, shown as 28, in the presence of BF3.Et2O or CF3SO3H gives the b-ferrocenyl-a,b-unsaturated esters in 56-83% yield and in the case of the a-unsubstituted-bketoesters the reaction is totally E-stereoselective.130

28

Interestingly, it has been observed that the crystallisation of a ferrocenyl bis(nitrosyl nitroxide) with manganese(II) bis(hexafluoroacetylacetonate) gives rise to spontaneous resolution of a coordination polymer formed from the two units which creates seven sources of chirality.131 The low temperature borylation of 1,1-bis(trimethylstannyl)ferrocene with BCl3 in hexanes occurs with high selectivity at the a-position to give 1-(Me3Sn)-2-(Cl2B)fc as the major product in ca. 87% spectroscopic yield with only minor amounts of the two other isomers, the 1-stannyl-3-boryl-ferrocene and the 1-stannyl-1-borylferrocene, observed. It has been shown that the reaction rate and selectivity strongly depend on the steric and electronic properties of the electrophiles because with bulkier electrophiles such as C6F5BCl2 larger amounts of the 1,3-product, [1-(Me3Sn)-3(C6F5ClB)fc] (ca. 40%), are formed in addition to the 1,2-isomer [1-(Me3Sn)-2(C6F5ClB)fc] (ca. 60%). Interestingly with PhBCl2 significantly lower reaction rates are observed. These data will be useful in synthetic methodologies as both goups can be readily substituted.132 The dilithium 1,1-ferrocene dichalcogenolates, [Fe(h5-C5H4ELi)2], previously used in the preparation of ferrocene dithiols etc react with [Cp*RhCl2], in THF solution, to give the dirhodium compounds, Cp*Rh-[m-(h5-EC5H4)2Fe](E : S, Se, and Te) which contain a metal-metal bond (similar to complexes of platinum previously reported). On the other hand, dimeric complexes
Cp*Ir[m-(h5-EC5H4)2Fe]2 (E : S, Se) with no Ir . . . Ir interaction were obtained in low yields in the corresponding reaction of [Cp*IrCl2]2, with the lithio-salts. The latter products were prepared more efficiently by phosphane elimination from [Cp* Ir(PPh3)(h5-EC5H4)2Fe] (E : S, Se) using elemental sulfur.133 A series of N-para-(ferrocenyl)benzoyl amino-acid esters, para-ferrocenesubstituted compounds of the type Fc(C6H4)CONHCH(R)CO2CH3
Fc : (h 5 -C5H5)Fe(h 5-C5H4); R : H, CH3, CH2CH(CH3), CH2C6H5, have been prepared by coupling p-(ferrocenyl)benzoic acid with the amino-acid esters (esters of gly, L-Ala, L-Leu, L-Phe) using the standard 1,3-dicyclohexylcarbodiimide (DCC)/ 1-hydroxybenzotriazole (HOBt) method. The products obtained were fully characterised by standard methods including the structural characterisation of parent para-(ferrocenyl)benzoyl

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methyl ester, Fc-C6H4CO2Me, and a chiral derivative N-
para-(ferrocenyl)benzoyl-L-alanine methyl ester, namely FC-C6H4CONHCH(CH3)CO2Me.134 In a paper detailing the synthesis and characterization of the heterodimetallic complex [Zn
(h5-C5H5)Fe[(h5-C5H4)-CH:N-(CH2)3-NMe2]Cl2] it has been shown that in solution (NMR study in CDCl3 or acetone-d6) this complex may exist as two species. The crystal structure of this compound shows that the ligand has an anti-conformation and acts as a neutral bidentate forming a six-membered chelate ring.135 A number of new rhodium and iridium complexes containing ferrocene ligands have been obtained in a synthetic study in which the reaction of the ferrocene-containing bridging ligands 1,1-bis(4-pyridylthio)ferrocene, 1,1bis(4-pyridylthio)biferrocene, and 1,1-bis(diphenylphosphanyl)biferrocene, with the chloro-bridged dinuclear complexes [(p-cymene)RuCl(m-Cl)]2 and [(CpMCl)-M-*(m-Cl)]2 (M:Rh, Ir). Two of the product complexes have been structurally characterised. These complexes show quasi-reversible redox processes of the bridging ligands as well as irreversible redox processes of the terminal metal ions.136 A series of new planar chiral amino-alcohol ferrocene ligands based on (S)-2-(methoxymethyl) pyrrolidine have been prepared and these ligands have been used in the standard enantioselective addition of diethylzinc to benzaldehyde with enantioselectivities up to 82%.137 A useful synthetic method, based on the Staudinger reaction of a bis-azide with an appropriately designed diphosphine has been developed for the preparation of macrocycles in which a ferrocene unit is either appended to an O-N-P-macrocycle or is incorporated within the macrocyclic unit.138 2,5-Bis(ferrocenyl)thiazole has been prepared from a-aminoacetylferrocene and chlorocarbonylferrocene in two steps. Interestingly this compound exhibits two reversible redox processes, indicating a significant electronic interaction between the iron centres. A further study of the protonated ligand revealed a reversible electrochemically-driven deprotonation/reprotonation process.139 Again the same research group has carried out the four step synthesis of new strained 2-aza[3]ferrocenophanes, which comprise a benzoquinuclidine framework incorporating a 1,1-disubstituted ferrocene.140 A new class of coordination compounds of organotin(IV) and organosilicon(IV) with a sulfur-containing ligand moiety, derived by the condensation of 1-acetylferrocene and thiosemicarbazide has been described and the product compounds have been used in biological studies.141 Scandium triflate catalysed cycloaddition of a bis[1,2]dithiolo[1,4]thiazine ketothione with either mono- or bis(ferrocenecarbonyl)acetylenes yields mono- and bis-ferrocenecarbonyl-1,3dithiolylidene[1,2]dithiolo[1,4]thiazines, while the cycloaddition of a bisdithiolothiazine dithione with bis-ferrocenylbutynedione gave 3,5-di(bis-ferrocenecarbonyl-1,3-dithiolylidene)[1,4]thiazine, see sheme in 29, the latter structure has been related to extended tetrathiafulvalenes.142 The reactions of two doubly silyl-bridged cyclopentadienyl anions with the metal halides MX2 (M : Fe, Ni, and Cr) results in the formation of a series of ring compound containing between six and seventeen ferrocene (M : Fe) units in the backbone. These products were separated by medium pressure liquid

398

Organometallic Chemistry

29 note – compound numbering as in orginal paper

chromatography. Characterisation was made using MALDI time of flight mass spectrometry. The temperature depedendance (between 25oC and 920oC) of the product ratios with reaction temperature was examined and it was observed that the average number of ferrocene rings in the products increased. It was also observed that terminal groups were cyclopentadienes.143 Two series of new ferrocene-bearing cyclo-pseudo-peptides, including three 1;1 type and three 2;2 type compounds, were designed and synthesized by liquid method. The interactions of the 1;1 type cyclo-pseudo-peptides as the host compounds with some anions were investigated by cyclic voltammetry (CV) and H-1 NMR methods. The results show that the 1 ; 1 type hosts can selectively interact with the guest anions by intermolecular hydrogen bond. A reasonable interaction model of the 1 ; 1 type cyclo-pseudo-peptides with anions was proposed in this publication.144 The first homoleptic complexes of isocyanoferrocene have been prepared. An example is shown in 30.145 During the lactamization of 1,1-ferrocenylbis(alanine) two macrocyclic peptides, shown as 31 and 32 were obtained. The complete characterisation of these compounds was carried out which includes electrochemical and structural methods.146 Hydrolysis reactions of 1,1-bis[W-(methoxycarbonyl)alkanoyl]ferrocenes have been carried out and it has been observed that in the compounds which have the same 1- and 1-substituents, have different k1 and k2 values. It is postulated that this may be due to the electrostatic interaction between the two carbonyl groups.147 Four novel, rigid, and conjugated 1,1-ferrocenediyl-bridged, bis(pyridine), bis(bipyridine), and bis(phenanthroline) ligands have been prepared the palladium catalysed C-C coupling reactions and the products have been fully characterised using a range of spectroscopic methods. Two general coupling methods

12: Transition Metal Complexes of Cyclopentadienyl Ligands

399

30

31

32

were used which were either reacting ferrocenylacetylene with halopyridine or the reverse method using di-iodo ferrocene and a pyridylacetylene. Examples of the compounds prepared are shown in 33, and 34.148 N-Protected and unprotected 2-azetidinones, protolytically, which have been activated by trifluoromethanesulfonic acid, react with ferrocene to give beta-amino ferrocenyl ketones in good yield.149 Eight new cyclobutadiene or ferrocene-fused organometallic dehydroannu-

33

34

400

Organometallic Chemistry

lenes have been prepared in a Cadiot-type coupling reaction of a 1-bromoethynyl-2-silylethynylbenzene derivative to an organometallic diyne (1,2diethynyl-3,4-bis(trimethylsilyl)cyclobutadiene(cyclopentadienyl)cobalt or 1,2diethynylferrocene). This was followed by deprotection and Cu(OAc)2-promoted ring closure. Five of these have been structurally characterized. Interestingly three of the novel cycles explode at temperatures from 196 to 293oC and form insoluble carbon materials which show a high abundance of onion-like carbon nanostructures.150 Ferrocene-containing N-BOC-protected 1-allyl-2-(allylamino)imidazolones have been reacted to give mono- and bis(2,5,8,9-tetrahydro-3H-imidazo[1,2-a][1,3]diazepin-3-ones) by ring-closing metathesis (RCM) reaction using Grubbs’ catalyst. The new compounds were all fully spectroscopically identified.151 The syntheses of a range of novel quinoline-containing ferrocenes of the type shown as 35 has been achieved starting from ferrocene and ruthenocene via the well known dimethylaminomethylmetallocenes. Also obtained were the compounds where the aminoquinoline is directly attached to the a-carbon of the ferrocene or ruthenocene. These compounds have been prepared for their anti-malarial activity. The crucial step in the reaction scheme (ruthenocene example) is the regioselective synthesis of 2dimethylaminomethylruthenocene carboxaldehyde by deprotonation of dimethylaminomethylruthenocene with t-BuLi in diethyl ether, followed by the addition of DME. The single crystal structures of two derivatives have been reported one of which, a ruthenocene, is shown as 36. The 4-aminoquinoline complexes show high efficacy against the chloroquine sensitive and resistant strains of the Plasmodium falciparum parasite in vitro.152Further work on this ferroquin complex has been reported which focuses on the activity of this anti-malarial agent,153 and the reports of in vitro studies of ferrochloroquine.154 In order to address the long-standing questions regarding the now industrially important stereoselective lithiation of ferrocenylmethylamines (shown in figure 37) a molecular mechanics force field has been developed for the conformational analysis of amido- and a-aminoferrocenes. The modelled structures of (diisopropylamido)- and (dimethylamido)ferrocene and N,N-dimethyl-a-ferrocenylethylamine are in agreement with those determined through single-crystal X-ray diffraction. Interestingly the diastereoselective lithiation of N,N-dimethylferrocenylethylamine and sparteine-mediated enantioselective lithiation of (diisopropylamido)ferrocene using MeLi have also been modelled. The results indicate that selectivity results from ring lithiation via the adduct conformer with the shortest C-H-ring- - - H3C-Li interaction.155 A broad range of metallatricarbadecaboranyl analogues of ferrocene, ruthenocene, and osmocene, for example compounds such as 38 shown, have been prepared and structurally characterised. Processes analogous to the h5-h3cyclopentadienyl ring-slippage process in the carboranes have also been examined.156 In an interesting application of biological use of ferrocenes a series of water soluble surfactant substituted ferrocenes have been prepared and used as oneequivalent reductants for the active-R2 subunit of Escherichia coli ribonucleotide reductase.157 A new combinatorial approach has been taken to investigate

12: Transition Metal Complexes of Cyclopentadienyl Ligands

35

401

36

37

the exchange between solution adsorbates and nanoscale features. Among the compounds studied were ferrocene (11-mercaptoundecyl) and ferrocene (11-mercapto-1-oxoundecyl) on amorphous and single-crystal gold substrates. This allowed the comparison of the exchange properties of patterns of nano-scale features as a function of composition, feature size, and nature of the underlying substrate.158 Continuing their work on the oxidation chemistry of biferrocenes the Dong group have prepared another series of mixed-valence 1,1-dinaphthylmethylbiferrocenium cations which have been studied and the effects of changing the counter ion are discussed. Several crystallographic structure determinations are included, a representative example of which is shown 39.159 A further series of asymmetric mixed-valence biferrocenium cations have been structurally characterised and their Mossbauer spectra discussd in detail.160 The dehydrohalogenation of isomeric 2-chloro- and 2-bromo-1,1-diferrocenylclopropanes (Z- and E-isomers with respect to the ‘bisecting’ ferrocenyl substituent) under the action of BuOK in DMSO gave the 3,3-diferrocenylcyclopropene,

Organometallic Chemistry

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38

39

which subsequently underwent facile opening of the small ring to give 3-ferrocenyl-1H-cyclopentaferrocene and 1,1-diferrocenylpropene.161

12: Transition Metal Complexes of Cyclopentadienyl Ligands

403

It has been observed that the trans-to-cis isomerization of azoferrocene (useful in molecular switches) proceeds not only by the photoexcitation of the p-p* transition band but also by the photoexcitation of the MLCT(d-p* transition) band, by a protonation-deprotonation cycle.162 Ferrocene-containing polyaspartamides have been prepared as water-soluble polymeric drug carrier/drug conjugates and their electrochemistry and cytotoxicity have been investigated. This further explores the use of ferrocenes in anti-tumour work.163A series of novel dinuclear and tetranuclear ortho-, meta-, and para- osmacycloferrocenophanes of the general type [h5-C5H4(CH2)n-1,m- C6H4(CH2)nOs(CO)4(CH2)n-1,MC6H4(CH2)nC5H4-h5]xFe-x (n : 2-4, m : 2-4; x : 1: x : 2: 4) have been prepared from the reaction of the corresponding bis(triflates) [h5-C5H4(CH2)n1,m-C6H4(CH2)nOTf ]2Fe with Na2[Os(CO)4] in refluxing dimethyl ether.164 The ferrocenyl hydroxymethylphosphines FcP(CH2OH)2 [Fc : (h5-C5H5)Fe(h5C5H4)] and 1,1-Fc’[P(CH2OH)2]2 [Fc’ : Fe(h5- C5H4)2] have been prepared in the reactions of the corresponding primary phosphines FcPH2 and 1,1-Fc’(PH2)2 with excess aqueous formaldehyde. Some of these phosphines have been crystallographically characerised and that of the mono substituted derivatives is shown as 40. The platinum complex cis-[PtCl2
FcP(CH2OH)22] was subsequently prepared in the reaction of 2 molar equivalents of the ligand FcP(CH2OH)2 with [PtCl2(1,5-cyclooctadiene)].165

40

It has been observed that decamethylferrocene, decamethylcobaltocene, bisbenzenechromium and cobaltocene react with oxalic acid in the presence of oxygen, by a multi-step process involving oxidation, deprotonation and precipitation, to give crystalline superstructures containing columns of the organometallic sandwich cations and hydrogen oxalate anions.166 The spontaneous ambient temperature ring opening polymersation of tin-bridged ferrocenophanes [Fe(h5--C5H4)2SnR2], R : tBu, Me, has also been examined in detail in which the effect of added reagents such as radical traps radicals nucleophiles, lewis acids etc. The mechanistic conclusion reached was that there are two mechanisms in operation, the frst a nucleophile promoted process and

Organometallic Chemistry

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the second an electrophile promoted process where added electrophile such as H;, SnBu3; produced a cationic ring-opening polymerization. The reactivity of stanna-ferrocenophanes towards transition metal carbonyls has also been examined.167 An alternate synthesis of ferrocene derivatives with pendant arms containing a terminal 4-pyridyl binding has been described. The method uses the reaction of acylferrocenes with 4-methylpyridine which is followed by dehydration in dry state using p-toluensulfonic acids/SiO2 systems.168 The reaction of lithium acetylides LiC∫CR with the silicon-bridged ferrocenophane [Fe(h5-C5H4)2Si(Me)Cl] at low temperature resulted in the selective substitution of the chloride to produce [Fe(h5-C5H4)2SiMe(C∫CR)], R : Ph, n Bu. A similar result was obtained in the reaction of the dichloride monomer [Fe(h5-C5H4)2SiCl2]. Subsequent metal-mediated polymerization of the products resulted in the formation of high moleclar weight polymers. The cyclic dendrimer [Fe(h5-C5H4)2SiMe(C∫CPh)]2 was isolated in one of the polymerizations and it was crystalographically characterized.169 It has been shown that coupling of ferrocenyl chromium-carbene complexes with cyclobutenediones leads to ferrocenyl-substituted 5-alkylidenefuranones together with 4-cyclopentene-1,3diones, methyl ferrocenoate and acetylferrocene in varying ratios.170 It has been shown that octamethylferrocenecarbaldehyde reacts with excess nitromethane in the presence of potassium tert-butoxide to give, after silylation and hydrolysis, 2-nitro-3-(octamethylferrocenyl) acrylonitrile, which is solvatochromatic.171 The synthesis and characterisation of a series of ferrocenylaminoalcohols of the type shown as 41 have been reported. These were prepared from the respective parent ferrocenyl aldehydes by reaction with trimethylsilylcyanide followed by hydride reduction. The 1,1-dimethyl-3-(2-

41 R = H, Me

amino-1-hydroxyethyl) ferrocene is used as a redox mediator to glucose oxidase in a commercial biosensor for determining blood glucose levels in diabetics. The new compounds were evaluated in structure-activity relationships.172 The reaction of aminomethylferrocene with ethoxyethyl 2-cyano-3-methylthioacrylate (or 2-cyano-3-alkoxyacrylates) has been used to obtain a series of novel ferrocene-containing cyanoacrylates. Some of the product compounds were observed to have excellent herbicidal activities.173 Organometallic multi-

12: Transition Metal Complexes of Cyclopentadienyl Ligands

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layer thin films, composed of poly(ferrocenylsilane) polyanions and polycations have been made and were deposited electrostatically onto a variety of substrates including quartz, silicon, gold, and hydrophilic/hydrophobically patterned substrates, using layer by layer self-assembly. UV/visible absorption spectroscopy and ellipsometry was the technique of choice to monitor the growth of the films, which were subsequently characterised using X-ray photoelectron spectroscopy (XPS) and by cyclic voltammetry.174 A range of ferrocenylcarbaldehyde thiosemicarbazones have been complexed by reaction with [ReBr(CO)5] and their reaction chemisty further explored. In the course of this investigation several new complexes were structurally characterised, one of which is shown as 42.175

42

Bis[1-(2,4-dichlorobenzoylhydrazonomethyl)-ferrocene(1-)]nickel(II), [Ni(C18H13Cl2FeN2O)2], has been prepared in the reaction of hydrated Ni(OAc)2 with ferrocenyl-2,4-dichlorobenzoylhydrazine in anhydrous ethanol.176 Ferrocene-1,1 -dicarboxylic acid has been used as a building block in supramolecular chemistry, in which it is reacted with a range of organic diamines. The structure of these superstructures is discussed and these vary, dependent on the nature of the amine used.177 Continuing this theme the synthesis and crystal structure of binuclear copper(II) complex containing dimethylaminomethyl)ferrocene has appeared.178 A number of ferrocenylmethanols which have a phosphorus substituent in position two of the ferrocene unit, rac-2-(diphenylphosphino)ferrocenylmethanol, rac-2-diphenylphosphinoyl)ferrocenylmethanol, and rac-2-(diphenylthiophosphoryl)ferrocenylmethanol), have been synthesized and structurally characterized.179 Ferrocene has been included into some alternate cyclodextrins, the latest being a new b-cyclodextrin derivative with the anion side-arm, mono[6-deoxy-6-(2-sodium thio-1,2-dicyane ethylenylthio)]-b-cyclodextrin.180 In related work the inclusion of (11-ferrocenylundecyl)trimethylam-

Organometallic Chemistry

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monium bromide by b-cyclodextrin has been studied.181 The preparation of tetrahydro-1,4-thiazepines containing the ferrocene groups has been made possible by the synthesis of enantiomerically pure b-hydroxyalkyl and b-aminoalkyl ferrocenyl sulfides. Some of compounds obtained were subsequently complexed with palladium and the complexes used as precatalysts in asymmetric allyic substitution reactions.182 In a paper on the synthesis of 1,1-ferrocenylenebridged analogue of BIPNOR it has been shown that 1,1-bis(3,4-dimethylphospholyl)ferrocene isomerises at 170°C through the corresponding 2H-phospholes that are trapped by diphenylacetylene to give a 1,1-ferrocenylene-bridged bis-1phosphanorbornadiene. Following the resolution of the ligands an optically pure palladium complex the ligands have been used in the asymmetric hydrogenation of a dehydroamino acid, hydroformylation of styrene, and allylation of sodium malonate.183 Ferrocene derivatives containing the 8-quinoline-tetramethyl-Cp ligand have been obtained and used to probe the reactivity with Lewis donor species.184 A number of dinuclear ferrocene complexes of the type
[FcBbipy]2O(PF6)2, and
[FcBbipy]2C(H)-CN(PF6)2, together with ansa-ferrocenes [1,1fc(Bbipy)2NC6H4OMe](PF6)2, and [1;1-fc(Bbipy)2NSiMe3](CO2CF3)2, have been synthesized and characterized by crystallography. On examination of the electrochemistry of these complexes it has been observed that there is electronic communication between the two 2,2-bipyridylboronium substituents in these species, with the degree of electronic interaction largely independent of the degree of conformational flexibility.185 In a separate paper the synthesis and characterisation of ruthenium(II) complexes containing ferrocenyl-pyridyl derived ligands has been examined, an example of which is shown as 43. 186

43

A number of sodium amino-functionalised cyclopentadienide salts with a number of Group 14 derivatives have been synthesised and characterised and these have been utilised as amino-functionalised cyclopentadienyl transfer reagents. Also a new type of 1,3-bisfunctionalised ferrocene compound , which is shown as 44 has been obtained.187

12: Transition Metal Complexes of Cyclopentadienyl Ligands

407

44

Continuing on their now well established ferrocene research program, the Herberhold group have investigated the reactions of chlorosilyl-substituted ferrocenes with indenyl lithium and fluorenyl lithium. The products obtained from this synthetic methodology have 1-indenyl (ind) or 9-fluorenyl (flu) substituents attached to ferrocene with a silicon-linker. Examples of the compounds obtained are shown in 45.188

45

A synthetic equivalent of ferrocene 1,2-dithiol which derives from of the corresponding a ferrocenyldithiastannole has been used in the preparation of a stable ferrocenyl-pentathiepin. Deprotection of the dithiostannole gave a novel tetrathiocin comprised of two ferrocene units, which is shown as 46.189 Mixed cobaltocenium-ferrocene compounds, an example of which is shown as 47, have been prepared an electrochemically characterised and it has been included into b-cyclodextrin, (see also previous references on b-cyclodextrin inclusion).190 The preparation of fullerene complexes with metal tetraarylporphyrins in the

Organometallic Chemistry

408

46

presence of excess ferrocene has resulted in the formation of new solvent-free and multicomponent molecular crystals. Examples are shown as 48.191 Palladium-catalysed cross coupling has been used in the preparation of ferrocenylthymidines. Ethynyl-ferrocene or vinylferrocene were coupled with 5-iodo2-deoxyuridine to give products, which in turn were subsequently hydrogenated to the corresponding ethylferrocene derivatves. The crystal structures of ferrocenyl-2-furano-pyrimidone and the uridine-based compound were examined and it was shown that the nucleobase is essentially co-planar with the substituted Cp ring of the metallocene, which is a commonly observed feature of ferrocenylaryl compounds. Incorporation of one of the bases into DNA oligonucleotides was also achieved using automated solid phase synthesis.192 In related work using two ferrocene-labelled analogues of dTTP, 5-(3-ferrocenecarboxamidopropenyl-1)-2-deoxyuridine-5-triphosphate (Fc1-dUTP) and 5-(3-ferroceneacetamidopropenyl-1)-2-deoxyuridine-5-triphosphate (Fc2-dUTP) it has also been demonstrated that facile incorporation of redox labels into DNA by polymerases may be achieved.193 The structure of 11-ferrocenyl-6,14-bis(ferrocenylmethylidene)-2,3-ferroceno1,2-tetracyclo[8.8.0.0(5.10).0(13.18)]octadeca-2,13(18)-diene, 49, has been established by crystallography in a study on the transformations of spirocyclodimers of 1,3-bis(ferrocenylmethylidene)-2-methylidenecycloalkanes in acid medium.194 The reaction of 1,1-bis(trichlorostannyl)ferrocene with trimethylgallium results in the formation of Fe(C5H4GaMe2)2, which is the first digallylferrocene. An analysis of the crystal structure of is compound reveals it to be polymeric in nature. The futher reaction of the product compound, shown as 50, with phenazine results in the formation of the supramolecular polyferrocene, which was crystallographically characterised and is shown as 51.195 The synthesis, structure and reactivity of a new 1,1-bis(N-tert-butyl-N-hydroxyamino)ferrocene has been reported. This compound shows two eclipsed positions of the two tert-butylhydroxyamino groups.196 In a synthetic study which re-examined the well known reactions of ferrocene dithiol with titanocene dichloride and related reactions the novel tetrasulfane fcS4 was obtained. 1,3Dithia[3]ferrocenophane, fcS2CH2 was obtained from fc(SH)2 and CH2Cl2 in the

12: Transition Metal Complexes of Cyclopentadienyl Ligands

409

47

presence of Cp2TiCl2 and KOH and its molecular structure was obtained and is shown as 52.197 A ferrocene/quinone linked dyad with a rigid amine spacer, shown as 53, has also been prepared.198 Ferrocene has been included into the supercages of NaY zeolite; its position in these cages was located by comparison of the neutron and X-ray powder patterns of undoped and doped samples of the zeolite. Thus it has been shown that the ferrocene is located just above a line joining two neighbouring sodium ions.199 It is astonishing that the crystal structure of the heterodinuclear complex [W(CO)5(Me2NCH2)Fc)] (Fc : ferrocenyl) has only just been reported, considering that the ligand is one of the oldest and most widely used ferrocenyl ligands and synthetic precursor compounds.200 Again in related work a reassessment of the surface activity of ferrocenyldimethylammonium surfactants has been made .201

Organometallic Chemistry

410

48

49

A range of 1,3-di (metallocenyl)allylium salts of the type [Mc(CH)3Mc];[X]9 (Mc, Mc : ferrocenyl (Fc), 2,3,4,5,1,2,3,4-octamethylferrocen-1-yl (Fc), ruthenocenyl (Rc); X : BF4, PF6 have been prepared by reacting (2- lithiovinyl) metallocenes with formylmetallocenes, followed by treatment of the resulting alcohols with HX. Differences in the solvatochromism of [RcCH3Rc]; and

12: Transition Metal Complexes of Cyclopentadienyl Ligands

50

411

51

[FcCH3Fc]; suggested a localized structure for the former cation in solution.202 The synthesis and characterization of 4,6-benzyliden-2,3-(ferrocene-1,1-dicarbonyl)-O-methyl-a-D-glucopyranoside, 2,3-(ferrocene-1,1-dicarbonyl)-Omethyl-a-D-glucopyranoside and 4,6:2,3-bis(ferrocene-1,1-dicarbonyl)-Omethyl-a-D-glucopyranoside have reported, and in addition the cytostatic activity of 2,3-(ferrocene-1,1-dicarbonyl)-methyl-a-D-glucopyranoside has been determined.203 The coordination of ferrocenyl ligands bearing bipyridine subunits have been further investgated: in particular the complexes of copper (I and II), nickel, cobalt, iron, lead and mercury (II) and silver (I).204 Following similar trends from previous years, yet another paper has described the Sonogashira-Heck-Cassar reaction of ferrocenylacetylene with aryl- and heteroaryl halides to be a facile and convenient route for the synthesis of 1-ferrocenyl-2-aryl- and 1-ferrocenyl-2heteroarylacetylenes.205 An efficient synthesis of 1-aminoferrocene-1-carboxylic acid derivatives has at last been reported- the synthess is based on the reaction of a protected ferrocenylcarboxylic acid.206 Although this has been previously demonstrated, the synthesis of symmetric and unsymmetric 1,1-dialkenylferrocenes via samarium diiodide promoted reactions of 1,1-diacetylferrocene with halides has been carried out to give a further range of symmetric and unsymmetric dialkenylferrocenes an example of which is shown in 54.207 It has been observed that the reactions of N-(ferrocenylmethyl)-N-methyl-

Organometallic Chemistry

412

52

53

54

amine, [(h5-C5H5)Fe(h5-C5H4CH2(Me)NH)], with hexachlorocyclotriphosphazene, N3P3Cl6, leads to the formation of the N-(ferrocenylmethyl)-N-methylaminocyclotriphosphazene derivatives, [N39P3Cl69n[N(Me)CH2(h5-C5H4Fe (h5C5H5)] n where n : 1-3 with only traces of higher substituted products. The effect of solvent polarity on the reaction has been studied, in particular the effect on the ratio of product isomers formed in the case of the products where n : 2, 55.208

55

A number of new ferrocenophanes have been prepared in a Gomberg reaction of the ferroceneophane 56 with a diazotized p-nitroaniline to give a mixture of

12: Transition Metal Complexes of Cyclopentadienyl Ligands

413

mono- and di- substituted products. The product isomers shown as 57 were isolated following column chromatographic separation.209

56

57

Further substitution reactions have been carried out on ruthenium carbonyl complexes using ferrocenyl alkynes as precursor ligands. An example of one the complex products obtained which was crystallographically characterised is shown as 58.210

58

Further examples of the incorporation of ferrocene into peptides include the synthesis constrained Leu(5)-enkephalin amide analogue 59, in which the cyclic ferrocenyl containing subunit was introduced as a mimetic of the tetrapeptide Tyr-Gly-Gly-Phe unit.211

Organometallic Chemistry

414

59

Modification of cyclopentadienene to incorporate pendant perfluoroalkyl ethyl groups has been carried out and thus bis-[2-(perfluoroalkyl)ethyl] containing ferrocene (as 3 regioisomers) has been obtained.212 It has been observed that Mn(hfacac)3, hfacac : (tris(hexafluoroacetylacetonate)), rapidly oxidizes ferrocene, 1-acetylferrocene, and 1,1-diacetylferrocene, which may prove to be useful in synthesis.213 A stable epoxyferrocene, trans-2-phenyl-3-ferrocenyloxirane, 60, has been prepared in the sulfur ylide-mediated synthesis of epoxides from aldehydes.214

60

The reaction of ferocenyllithium with diphenylsufide or diphenyl selanide gives the derivatives Fc(XPh)2 (X : S, Se) which have been ligated to gold.215 A series of novel hetero-bimetallic metalla-macrocycles based on the bis(1-pyridyl)ferrocene for example the zinc and copper complexes shown in 61 have also been prepared.216

61

The synthesis and structures of three bis(trialkyltin)-1,1-ferrocene dicarboxylates have been achieved; as an example the crystal structure of one of these, namely, MeCy2SnO2CC5H4FeC5H4CO2SnCy2Me has been determined in which the Sn atom is in slighly distorted trigonal-bipyrimidal environment.217

12: Transition Metal Complexes of Cyclopentadienyl Ligands

415

The synthesis of ‘three-petal’ carbocyclic systems based on s-cis-diferrocenyltrienes has been achieved during the acid-catalysed intramolecular, homoannular alkylation of one of the ferrocenyl substituents in a series of spirocyclodimers. The latter compounds were derived from 1,3-bis(ferrocenylmethylidene)-2methylidene-cyclohexane, -cycloheptane, and 3,5-bis(ferrocenylmethylidene)-4methylidene-N-methylpiperidine. The structure of one of these, shown as 62, 1ferrocenyl-3,11-bis(ferrocenylmethylidene)-2,3,4,5,6,6b,10,10a,11,12,13,14dodecahydro-1H-benzo[d](ferroceno[a])phenanthrene has been determined.218 In related work the Long group have also reported the preparation of platinum ethynyl dimers and polymers with pendant ferrocenyl groups, which derive

62

from the square-planar platinum(II) monomer complex trans — [(Ph2FcP)2PtCl2] (Fc : ferrocenyl). These new products are more easily shown in drawings such as the crystallographic structures shown as 63, 64, and 65.219 Another large molecule more easily visualised in graphic form is the novel ferrocene-containing helical triangular macrocycle, which is shown as 66. This was obtained in a Co(II)-templated synthesis.220 A series of charge-transfer complexes and redox-active macrocycles have been obtained in an investigation of the reactions of FcB(Me)Br and 1,1- fc[B(Me)Br] with 2,5-bis(pyridyl) pyrazine (bppz) and 2,2: 4,4: 2,2-quaterpyridine. It has been shown that the bppz adducts possess an intense green colour, while the 2,2: 4, 4: 2,2-quaterpyridine complexes are deep blue. These colours are indicative of charge-transfer interactions between the ferrocene and their electron-poor aromatic substituents.221 A new redox-active labelling reagent, N-ferro-

Organometallic Chemistry

416

63

64

65

cenyliodoacetamide has been prepared and characterised and it has been used to label a number of various biomolecules including a sulfhydryl-modified oligonucleotide, cysteine, glutathione and sulfhydryl-modified bovine serum albumin.222 6,6-Dimethylfulvene has been treated with LiPHMes (Mes : 2,4,6Me3C6H12) to give Li[(C5H4)CMe2PHMes], which was then used to prepare the ferrocenes rac-[Fe
( h5-C5H4)CMe2PHR2] (R : Ph, Mes). The phenyl derivative was then used as a ligand in the reaction with [Cp*TaCl4] to give the heterotrinuclear complex rac-[Fe
(h5-C5H4)CMe2PHPh(Cp*TaCl4))2].223 A series of eleven novel aminophosphine ligands have been synthesized, all of which contain a chiral 2,2-bridged biferrocenyl unit as part of a biferrocenoazepine substructure: these have been used as chiral auxiliaries in the palladium-mediated allylic substitution reactions. An example of one of the palladium complexes is crystallographically shown as 67.224 7.2 Electrochemistry. — The electrochemical acylation of ferrocene, in which a catalyst is generated from a sacrificial aluminium anode, in the presence of a mixture of phosphorus trichloride and a carboxylic acid, has been carried out. Monoacylated ferrocenes were obtained in high yield when the acid was present in a 1.2 stoichiometric excess, however no conditions for the corresponding diacylation were obtained. Diacylation, of course, was however observed when commercial anhydrous aluminum trichloride was added.225 In-situ Fourier trans-

12: Transition Metal Complexes of Cyclopentadienyl Ligands

66

67

417

Organometallic Chemistry

418

form infrared (FTIR) spectroelectrochemistry has been used to probe the mechanism of electron transfer in a bridged biferrocene complex, Ni(LSB)2, where HLSB : S-benzyl-N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate. Possible pathways have been postulated to explain the two one-electron oxidation process which occurs. This thus indicates moderately strong electronic communication between the iron centres.226 It has been observed that octamethyl1,1-di(2-pyridyl)ferrocene may act as molecular electrochemical sensor for magnesium, calcium, zinc, and cadmium ions in acetonitrile solution. The method uses the tradional shift in the ferrocene oxidation peak by ca 0.40 V, in the cyclic voltammogram in the presence of even small amounts (10 mol %) of these ions. In addition the metal complexation is accompanied by a batho- and hyperchromic shift of the band in the visible region of the UV-Vis spectrum. In this paper a detailed study of the behaviour of this ligand with zinc chloride in acetonitrile solution has also shown the ligand is able to accommodate a maximum of two zinc ions. In addition octamethyl-1,1-di(2-thiophenyl)ferrocene has been prepared and structurally characterized.227 Cyclic voltammetry has been used to probe the potential-controlled electron-transfer reaction between cytochrome c and 1,1-dimethylferrocene at a liquid-liquid interface. It is concluded that the electron transfer process is apparently transport controlled, rather than adsorption limited resembling the electron transfer of the protein in vivo.228 A ferrocene-based modular electrochemical saccharide sensor has been prepared which contains two boronic acid receptor groups together with a hexamethylene linker.229 The electrocatalytic oxidation of sulfite with some ferrocene derivatives such as ferroceneacetic acid, ferrocenemonocarboxylic acid and ferrocenemethanol as mediator have been studied using cyclic voltammetry in aqueous solution with various pH at glassy carbon electrode.230 A new ferrocene receptor, shown as 68, has been obtained and used to bind calcium in a

68

complex process involving the whole unsaturated core of the ligand. The complexation leads to significant changes in the ligand material properties as witnessed by the unprecedented cation sensing observed by electrochemistry and fluorescence spectroscopy.231 Electrochemically driven clathration/declathration of ferrocene has been ob-

12: Transition Metal Complexes of Cyclopentadienyl Ligands

419

served and its derivatives by a nanometer-sized coordination cage.232 The electro-oxidation of azaferrocene (AF) and 2,5-dimethylazaferrocene (DMAF) has been investigated in several solvents and under various conditions of basicity and nucleophilicity and it is concluded that the observed decomposition of AF(;) cations is more probably associated with a deprotonation reaction than with a nucleophilic attack.233 A number of p-extended conjugated tetrathiafulvalenes with quinonoidal structures bearing a ferrocene have been made as new p-donors, and their structures and physical properties have been determined. In these compounds three-electron redox behaviour has been observed.234 The mechanisms of the electron transfer of S-benzyl-N-(ferrocenyl-1-methyl-methylidene)-dithiocarbazate palladium(II)/zinc(II) complexes [Pd(lsb)2)/[Zn(lsb)2] have been investigated using a combination of cyclic voltammetry, differential pulse voltammetry, digital simulation and in-situ subtractively normalized interfacial Fourier transform infrared (SNIFTIR) spectroelectrochemistry. The conconclusion reached in this study is that in the square-planar complex [Pd(lsb)2], two consecutive one-electron steps in the redox process occur, while the reaction in the tetrahedral configuration of the Zn complex involved one two-electron step.235 Polyethylleneglygol-ferrocene linear polymeric chains have been attached to gold electrodes and the surface concentration of grafted polymeric chains has been measured electrochemically under good solvent (water) conditions. The results indicate that the electrode is covered with terminally attached PEG chains in a mushroom configuration.236 The voltammetric and impedance study of the binding of ferrocene derivatives to a sulfonated calixarene host has been carried out in aqueous solutions.237 The solubility of ferrocene has been determined using normalized chronoamperometry in an unusual maner.238 Meanwhile the electron transfer mechanism during redox process of a platinum complex of of S-benzyl-N-(ferrocenyl-1-methyl- metylidene)dithiocarbazate has been studied by cyclic voltammetry, differential pulse voltammetry, digital simulation and in situ FTIR spectroelectrochemistry.239 Finally the synthesis and electrochemistry of 5-ferrocene-glucosamide, 5-ferrocene-glucosamide phosphate and 5-ferrocene-amido-5-adenosine has been studied in aqueous solution240 and a series of electrochemical studies on ferrocene-containing surfactants have been carried out.241 7.3 Polymer Chemistry. — Recent developments on the synthesis of high molecular weight polyferrocenophanes has been reviewed, which include the mechanistic aspects of thermal, anionic, and transition metal catalysed ring-opening polymerization (ROP).242 In a TGA study on the degradation processes which occur in ferrocene, ferrocenecarboxylic and ferrocenedicarboxylic acids together with a series of ferrocene-containing polyamides it has been concluded that the decomposition of these materials occurs by a multistage process. From the activation energy values obtained together with the results from FTIR spectroscopy it is suggested that in the case of the polymers the degradation mechanism occurs by either scission of weak links or by random scission of the chains with the primary site of oxygen attack on the polymer chains being at the N-vicinal methylene group.243 The X-ray structure of 3-(4-ferrocenylbutyl)-N-(triisopropyl-

420

Organometallic Chemistry

silyl)pyrrole, a the synthetic intermediate in the multi-step synthesis of the novel ferrocene-substituted pyrrole monomers, N-(3-ferrocenylpropyl)pyrrole, and 3(4-ferrocenylbutyl)pyrrole has been obtained. The product ferrocenes were electropolymerized to form the homopolymers and copolymers of the type, pyrroleco-ferrocenylpyrrole and these were characterized using cyclic voltammetry, UV-visible spectroscopy, scanning electron microscopy (SEM) and four-point probe conductivity measurements. Finally the use of one of these polymer coatings in the determination of the redox-active enzyme cytochrome C in solution has been demonstrated.244 Further ferrocene terminated dendrimers have been prepared on polystyrene (PS) combs and dendrigrafts. The attachment of the ferrocene was achieved by reaction of di(hydroxymethyl) groups on the starting dendrimers: quantitative acetalisation of the 1,3-propanol ends using ferrocenecarboxaldehyde yields polymers with one ferrocenyl moiety per branch terminus. The electrochemical properties of the dendrimers have been studied by cyclic voltammetry in organic solution and compared with the corresponding alpha-functional linear polystyrene. The results obtained indicate that all the ferrocenyl units in the dendrigrafts are available and participate to the oxidationreduction cycles while only a small fraction of the ferrocene groups is active in combs and linear polystyrene.245 A series of polymers in which both neutral (e.g. ferrocene) and cationic groups are within and pendent to their backbones have been synthesised and these materials have been photolylitically decomposed which leads to decoordination of the cationic cyclopentadienyliron moieties from the polymer backbones. Glass transition temperatures of the resulting ferrocene-based polymers were lower than those of the mixed neutral/cationic polymers.246 A new series of liquid crystalline main chain copolyesters were prepared, having ferrocene containing liquid crystals consisting of a ferrocenecontaining mesogenic group with a methyl phosphate group along with a methylene spacer (varied between 2, 4, 6, 8 and 10 carbons) have been prepared. The liquid crystalline behaviour has been studied on a hot stage microscope and the physical properties have been documented using thermal gravimetric and differential scanning calorimetry. It has been observed that the glass transition (T-g) temperatures of these polymers are low as would be anticipated.247 Dehalogenation polycondensation with Ni(0) has been used as a synthetic method to obtain a new p-conjugated polymer containing a ferrocene group: the synthesis used 3,4-bis(4-bromophenyl)-N-methylferrocene-pyrrole-2,5-dione as a precursor. The average molecular weight of the polymer was 2620 and it was found that the polymer had good solubility in common organic solvents. Interestingly the polymer was fluorescent in its ferrocene-oxidised form.248 X-ray absorption spectroscopy (XAS) has been used to investigate the solution phase electrochemical incorporation of perrhenate anion into polyvinylferrocene.249 A novel 2D ferrocene-containing sandwich sheet coordination polymer, consisting of NaZnO inorganic inner and ferrocene-packing outer layers shown as 69, has been achieved by diffusion of a solution of Zn(BF4)2 into a solution of 1,1ferrocenedicarboxylic acid in the presence of NaOH.250 Charge-transfer (CT) effects between the Fc groups and the conjugated poly-

12: Transition Metal Complexes of Cyclopentadienyl Ligands

421

mer backbone has been described in a series of poly(thiophene)s with pendant vinylene ferrocene (Fc) or cyanovinylene ferrocene.251 The physical properties and the glass transition temperatures of polymethyl methacrylates obtained with ferrocene-based diimine Pd(II) catalyst have been studied in relation to the relative stoichiometries of the catalyst and scavenging agent, triisobutylaluminium.252 The polymerisation of phenylacetylene catalysed by palladium(II) dppf has also been carried out.253A series of moderate molecular weight substituted

69

polyferrocenylenes [(h )-C5H3X)Fe(h -C5H3X)]n (where X : H, SiMe3, Me) have been obtained by either Ullmann coupling reactions of the diiodoferrocenes, (h5-C5H3XI)2Fe, with an excess of copper metal or alternatively CuCl-mediated coupling of the bis(stannyl)ferrocene, (h5-C5H3SiMe3SnBu3)2Fe. The new monomers were characterized by 1H- and 13C-NMR spectroscopy, elemental analysis and mass spectrometry. A single-crystal X-ray diffraction study of (h5C5H3SiMe3I)2Fe indicated that the disubstituted cyclopentadienyl rings are eclipsed with the substituents slightly staggered (20°).254 A one-step synthesis of 2ferrocenylethyl alcohol has been developed, which led to the high yield of the monomers 2-ferrocenylethyl acrylate, 2-ferrocenylethyl methacrylate, 2-ferrocenylethylacrylamide, and 2-ferrocenylethylmethacrylamide. These monomers were subsequently copolymerised using AIBN as initiator to either homopolymers or copolymers of these monomers with N-isopropylacrylamide in toluene or THF.255 5

5

7.4 Schiff Bases Ligand/Complexes. — The condensation of 1-acetylferrocene with thiosemicarbazides and semicarbazide hydrochlorides allow the synthesis of the Schiff bases 1-acetylferrocenethiosemicarbazone and 1-acetylferrocenesemicarbazone, which have been further reacted with diorganosilicon (IV) chlorides to give complexes with Si-S or Si-O and Si 105) polyferrocenylsilanes (e.g., [fcSiMe2]n. However, it was also observed that prolonged exposure of the resulting polyferrocene solution to the catalyst resulted in a

432

Organometallic Chemistry

dramatic molecular weight decline. A series of related metal catalysts were also used in this study although some e.g. [Rh(1,5-cod)(dmpe)]PF6 (dmpe : bis(dimethylphosphinoethane)) were less efficient.324 Once again Manners has continued his research on ferrocene-containing polymers. The thermal ringopening polymerization of spirocyclic [1]silaferrocenophanes confined within the interstitial void spaces of silica crystal colloidal templates has led to the preparation of crosslinked polyferrocenylsilane inverse opals, which subsequently were pyrolsed to form magnetic ceramics.325 The reactivity of the stanna[1] ferrocenophane Fe(h5—C5H4)2Sn(tBu)2 toward transition-metal carbonyls have also been investigated. Insertion of a Fe(CO)4 fragment into an Sn-Cp bond of this compound was observed on treatment with Fe2(CO)9, forming the ferrostanna[2]ferrocenophane, whereas in the case of Co2(CO)8, a highly unusual rearrangement/transfer of CO occurred, forming the ketone-bridged dimer.326 Polyferrocenyl microspheres have been prepared under very mild conditions in the Pt(0)-catalysed ring opening polymeristion of the [1]-silaferrocenophane fcSiMe2 together with the cross linking agent linker fcSi(CH2)3. Following chemical oxidation the polymeric microspheres were treated with negatively charged silca microspheres to yield core-corona composite particles. Pyrolysis of the orginal microspheres led to the formation of magnetic ceramics with tunable properties.327 Two papers have been reported by the same authors on the preparation of ‘tubelike’ polymers of poly-(ferrocenyldimethylsilane-b-dimethylsiloxane) when the polymers were dissolved in non-polar solvents such as hexane and decane. This is unusual because the formation of spherical starlike miscelles had previously been observed when these polymers had been dissolved in more polar solvents.328,329 The a-ferrocenylalkylation of some biologically active compounds has been achieved starting from hydroxymethylferrocene.330 A series of 1,5-disubstitued acetals have been prepared by lithiation of a chiral dioxane, followed by electrophilic trapping of the lithiated intermediate. The acetals can be easily hydrolysed to the 1,5-disubstitued ferrocenecarboxaldehydes (in an enantiomerically pure form if the two substituents are different), which can be furthermore substituted on the other Cp ring to yield unprecedented 2,5,1ferrocenecarboxaldehydes. Interestingly the three substituents on the product ferrocenecarboxaldehyde may be different, which is the first example of an enantiomerically pure 1,2,3,1-tetrasubstituted ferrocene with only planar chirality, an example of which is shown as 83. 331 Polypyrolles featuring amidoferrocenes have been used as materials which are useful electrochemical H2PO49 and ATP2- sensors.332 A range of bis(ferrocenyl) derivatives shown as 84, which contain secondary chelating substituents, have been used as voltammetric sensors of Mg2; and Zn2; ions in acetonitrile solution.333 Other related anion receptors which have ferrocenyl reporter groups, shown as 85, have been structurally characterised.334 Anthracene-based fluorescent sensors which contain a ferrocene have been prepared, an example of which is shown as 86. These have been used to detect metal ions elcrochemicaly under a range of p.H. conditions.335 A ferrocene-oligonucleotide conjugate has been used as a DNA probe, the

12: Transition Metal Complexes of Cyclopentadienyl Ligands

433

novelty being that this is an electrochemical gene sensor, which is sensitive, convenient, and does not use radioisotopes.336 The separation of a number of planar chiral ferrocene derivatives, rac-ferroceno[2,3a]inden-1-one, rac-6-(3-

83

84

85

hydroxy-3-methylbut-1-yn-1-yl)ferroceno[2,3a]inden-1-one, rac-ferrocene[2,3a]indene, rac-endo-1-methoxy-1-allylferroceno[2,3a]indene, and rac-1,4-

Organometallic Chemistry

434

86

dihydroxybutylferrocene has been attempted using a series of b-cyclo-dextrin based chiral stationary phases which were prepared by the ring-opening metathesis graft polymerization of various norborn-2-ene (NBE) substituted CDs.337 Ferrocene has also been bound to a silica stationary phase in an effort to use the medium to separate alkylbenzenes, halobenzenes, polycyclic aromatic hydrocarbons, and even basic compounds under reversed-phase conditions.338 A chemionic switch has been prepared in which reversible photoinsertion of a ferrocene into a hydrophobic semiconductor surface occurs. This bi-stable supramolecular device can be switched on by light and off by chemical oxidation.339 Again a range of new chiral N, S-ligands with oxazoline and thiophenyl substituents at benzene ring mid benzylic position have been prepared and applied in palladium-catalysed asymmetric allylic alkylation which exhibit high entantioselectivity (82%-93% e.e.).340 1-(naphthylphenylphosphino) ferrocene has been used as a chiral ligand in the ruthenium-catalysed asymmetric hydrogenation of C:C and C:O double bonds.341 Ferrocenyloxazolines continue to be of general interest: these have now been used as ligands in the vanadiumcatalysed asymmetric epoxidation of allylic alcohols .342 The same research group have also examined the use of polymer-supported ferrocenyl oxazolines for the catalysed highly enantioselective phenyl transfer to aldehydes.343 A range of new aminophosphine ligands, which are based on different ferrocene, cis- and trans-decalin backbones have been used for the preparation of enantioselective transition metal catalysts.344 An efficient procedure for the enantioselective synthesis of b-ferrocenyl-b-amino alcohols has been developed.345 Further synthetic work has been carried out on ferrocenyloxazolines as ligands in palladiumcatalysed coupling reactons. In this paper the focus is on the role of planar chirality of the ferrocene ligands in allylic substitution ractions. Extremely high e.e.’s have been achieved in allylic amination although in allylic alkylations lower e.e.’s were possible when ligands with only planar chirality were used.346

12: Transition Metal Complexes of Cyclopentadienyl Ligands

435

The preparation of
2-(S-p)-[(trans-(2R,5R)-2,5-dialkylpyrrolidinyl)methyl]] ferrocenyldiphenyl phosphines, a range of new ferrocenylphosphinamine ligands which possess one planar site and two stereogenic centres, has been described. In this synthetic methodology trans-(2R,5R)-2,5-dialkyl-1-(ferrocenylmethyl)pyrrolidines were diastereoselectively lithiated and quenched with chlorodiphenylphosphine. The new ligands were structurally characterised, an example of which is shown as 87.347

87

The hydrolytic cleavage of the dihydrooxazole ring in (S-p)-2-
2- (diphenylphosphino)ferrocenyl-4-(1-methylethyl)-4,5-dihydrooxazole has been used to give the planar chiral functionalized phosphine, (S-p)-2-(diphenylphosphino)ferrocenecarboxylic acid, (S-p)-Hpfc, in two steps and 65% yield.348 (-)-Sparteinemediated metalation of ferrocenesulfonates has been used to obtain the first case of double asymmetric induction of ferrocene planar chirality: in this reaction s-BuLi metalates (-)-menthylferrocenesulfonate with (-)-sparteine to give a the 2-formyl product shown as 88.349

88

Enantiomerically pure 1,1-bis
(1R)-1,7,7-trimethylbicyclo[2.2.1]hept-2-en-2ylferrocene has been prepared starting from (1R)-(;)-camphor which, when treated with cyclopentadienylmagnesium chloride gave the corresponding tertiary alcohol, which was condensed by acid catalysed elimination to the optically active pentafulvene 5-(1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene)cyclopentadiene. This, on deprotonation with an organolithium followed by reaction with an iron salt gave the product.350 The enantioselective addition of aryllithiums (Ar : Ph(51%), 2-MeC6H4 (91%), 2-MeOC6H4, (90%), 1-naphthyl (83%) to 6-

Organometallic Chemistry

436

(dimethylamino)fulvene in the presence of (-)-sparteine in toluene at low temperature generated chiral cyclopentadienyllithiums substituted with an N,Ndimethylamino(aryl)methyl group, where the enantioselectivities are shown in brackets. This route serves as a useful basis for the preparation of new chiral ferocenes.351 The chiral ferrocene-based ligands (S,S)-iPr-Phosferrox, (S,R)-iPrPhosferrox, (R,S)-BCyPFA have been used in the enantioselective Pd-catalysed allylic substitution of rac-(E)-1,3-diphenyl-3-acetoxyprop-1-ene with dimethyl malonate has been studied in the ionic liquid 3-butyl-1-methylimidazolium hexafluorophosphate ([bmim][PF6]).352 The asymmetric synthesis of new chiral ferrocenyl-thiazoline ligands: (R)-2-Azido-1-ferrocenylethanol and (R,R)-1,1bis(2-azido-1-hydroxyethyl)ferrocene, which were obtained in the enantioselective borane reduction of alpha-azidoacetyl ferrocene 1- and 1,1-bis(2-azidoacetyl)ferrocene has been achieved.353 The molecular structure of 4,6-bis-O(diphenylphosphino)-2,3-(ferrocene-1,1-dicarbonyl)-O-methyl-alpha-Dglucopyranoside has been determined in a study which involved the synthesis and characterization of a range of new phosphorus containing ligands. An example of these ligands is shown as 89. Furthermoe the corresponding Pd(Il) and Pt(Il) complexes were generated and it was shown that the derived complexes showed good activity (90%) and moderate stereoselectivity (39% e.e.) in the Pd-catalysed enantioselective allylic alkylations.354

89

A range of new chiral ferrocene-based ligands have also been used in ruthenium asymmetric hydrogenation and 355 the synthesis and structure of an enantiomerically pure C-2 symmetric ferrocenyl carbenes has appeared.356 Further ferrocene end capped dendrimers have also been prepared,357 together with ferrocene terminated dendrimers, in this case they contain phophorus and the ferrocene is part of each layer structure.358 The reaction of a nickelocene bound to a cyclopentadienyl (Cp) anion by two SiMe2 groups was prepared and then reacted with solvated iron dichloride to give a trimetallic metallocene, shown as 90. Following structural characteristion, the efficiency of the spin transfer was

12: Transition Metal Complexes of Cyclopentadienyl Ligands

437

then investigated which was found to depend on the bending angle of the ligand bridge.359

90

A 4-ferrocenylpyridine organometallic Lewis base adduct of methyltrioxorhenium(VII) (MTO), has been fully characterized. Furthermore, both the binuclear complex and also the ferrocenylpyridine have been immobilized in b-cyclodextrin (CD) to give inclusion complexes with a 1:1 stoichiometry, which have been investigated using powder X-ray diffraction (XRD) which verifies that the microcrystalline powders which were obtained are true homogeneous inclusion complexes.360 Other papers of interest in this general area are listed as follows: the use of polyvinylferrocenium (PVF) film in enzyme electrodes;361 redox-conjugated reversible isomerization of ferrocenylazobenzene with a single green light;362 optimising the synthesis of ferrocene bridged bis(2-indenyl) ligands by palladiumcatalysed coupling;363 an examination of the behaviour of ferrocene-derivitised thiol guests in b-cyclodextrins;364 the use of Fe-57 Mossbauer spectroscopy in a study of the difference in interactions between guest and host molecules among (R), (S) and racemic isomers for (2-methylbutyl)-, (2-phenylpropyl)- and (2phenylbutyl)ferrocenes enclathrated in deoxycholic acid;365 the formation and electrochemical properties of multilayers of Au nanoclusters covered by selfassembled monolayers containing ferrocene and carboxylate groups on Au and ITO;366 a study on the femtosecond dynamics of relaxation of photoexcited meso-tetraferrocenylporphyrin in the nonprotonated and diprotonated forms;367 novel ferrocene derivatives with PH-functionalized phosphanylalkylcyclopentadienyl ligands: the syntheses and molecular structures of rac-[Fe
(h5(R : Ph, Mes) and rac-[Fe
(h5C5H4)CMe2PHR2] 368 C5H4)CMe2PHPh(Cp*TaCl4)]; the preparation, characterisation and biosensor application of conducting polymers based on ferrocene substituted thiophenes;369 further use of ferrocenes in nanotube production: ferrocene as a precursor reagent for metal-catalysed carbon nanotubes;370 ferrocene inclusion ino b-cyclodextrin;371 unusual electrochemical properties of unsymmetric viologen dendrimers;372 the interaction of diaminoanthraquinones with ferrocenecarboxylic acids;373 the preparation and characterization and prpoperties of dipolar 1,2-N,N-dimethylaminomethylferrocenylsilanes;374 the preparation of multifunc-

Organometallic Chemistry

438

tional nickel-bisdithiolate complexes with trans-4-2-(1-ferrocenyl)vinyl-1methylpyridinium cation for NLO studies;375 the synthesis, structure and magnetic properties of a charge transfer salt of a ferrocene-based diradical, (1,3diferrocenylbenzene)(F-4-TCNQ)2(chlorobenzene);376 the asymmetric synthesis of chiral ferrocene derivatives;377 ferrocene-based heteronuclear bifunctional Lewis acids: synthesis and properties of 1-stannyl-2-borylferrocenes;378 synthesis and electrochemistry of 1,1-bis(diphenylphosphino) ferrocene derivatives of ruthenium carbonyl;.379 the synthesis of ferrocene-quinone conjugated oligomers and polymers and their protonation-induced intramolecular electron-transfer reaction380 and the synthesis and characterization of perfluoroaryl ferrocene oligomers.381 Finally in an intriguing dendrimer study a group of three types of phosphoruscontaining dendrimers with chiral ferrocenyl subunits precisely placed at one individual shell within their skeleton have been synthesized and the influence of the progressive ‘burying’ of the chiral ferrocene derivatives on their electrochemical and chiroptical properties has been reported.382

8

Cobalt

In similarity with ferrocene, cobaltocene has also been used as a precursor in the production of carbon fibres. A dry carrier was used as a carrier gas, thiophene was used to promote growth in this system with the decomposition temperature at 110oC.383 The electrochemistry of cobaltocene has also been studied in an ionic liquid (1-n-butyl-3-methylimidozoliumhexafluorophosphate) using gold, platinum and glassy carbon electrodes in an effort to produce values for standard reference potentials in these media.384 The synthesis and characterization of a cobaltocenium-based redox-active cryptand containing a phenanthroline unit has been described. Complex formation (with Ca2;) was studied by H1/C13-NMR and the effects of complexation on the redox properties were monitored by cyclic voltammetry.385 In a paper refered to earlier where the electrochemistry of dppf ruthenium-capped C60 compounds were investigated cobaltocene was used as a reducing agent.309 The synthesis of mixed cyclopentadienyl/cyclobutadienyl cobalt complexes which are bridged by buta-1,3-diyne, octa-1,3,5,7-tetrayne and dodeca-1,3,5,7,9,11-hexayne units are described using a Hay coupling synthetic procedure.386 In a general metallocene study on the spacial distribution of their valence electrons using Penning ionization electron spectroscopy it has been observed that cobaltocene C1 orbitals exhibit quite diffuse electron distributions due to strong antibonding d-interactions.387 Cobaltocene has also been used as a reducing agent in the reaction of [(h6-benzothiophene)Mn(CO)3]; under carbon monoxide — this leads to insertion of [Mn(CO)4]- into the C(aryl)-S bond to afford a neutral bimetallic complex.388 In synthetic work cobaltocene has been used as a precursor to a series of organophosphorus cage compounds — the chemistry simply involves the reaction of the triphosphate [(P3C2B2CH)-C(SiMe3)2] with cobaltocene. The nature of the products is best represented by a structural example, one of which is shown as 91.389

12: Transition Metal Complexes of Cyclopentadienyl Ligands

439

A general method for rapid construction of metallocene-based hosts is described in which polyether bridged bis(diphenylacetylenes) react with a source of, for example, cyclopentadienyl cobalt at high temperature which leads, via

91

macrocyclization and capture of the intermediate cyclobutadiene, to macrocyclic systems tethered to an integral metallocene platform.390 Oxalic acid has also been reacted with decamethylcobaltocene in the presence of oxygen to give [h5Cp*Co]2[HC2O4][H2C2O4]0.5 and related hydrates, one of which is shown as 92.391

92

Cobaltocene has also been used as a reductant to leave it as a cation in the preparation of a series of 5,10,15-tris(pentafluorophenyl)corrole chromium com-

Organometallic Chemistry

440

plexes.392 Cobaltocene has also been used as a reductant in the synthesis of a range of molybdenum indenyl complexes.393

9

Nickel

A review entitled ‘From nickelocene to novel organonickel compounds’ explores the reactions of nickelocene with organolithium reagents.394 Two papers have apapeared on the inclusion of nickelocene into organometallic macromolecules. Doubly silyl-bridged cyclopentadienyl anions have been reacted with Ni, Fe and Cr salts to give a range of macromolecules which were separated by medium pressure liquid chromatography. A nickelocene-silyl linked ferrocene has been obtained and structurally characterised in the second of these papers. The product complex is shown as 93.369, 395

93

Two papers have also described the synthesis of nickel-containing metalloborane cluster compounds.396, 397 These deal with the reaction of nickelocene with anti-B18H22. Nickelocene has been used as the source of cyclopentadiene in the production of polymer supported cyclopentadienes — in this work nickelocene is reacted with polymer-supported butylchloride to give the Cpfunctionalised resin.398

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286. K. W. Zhao, P. Hu, H. B. Xu and L. F. Zhang, Chin. J. Chem., 2002, 20, 479. 287. R. Pastorek, J. Kamenicek, M. Pavlicek, J. Husarek, Z. Sindelar, and Z. Zak, J. Coord. Chem. 2002, 55, 1301. 288. C. M. Thomas, B. Therrien, A. Neels, H. Stoeckli-Evans and G. Suss-Fink, J. Organomet. Chem., 2002, 658, 251. 289. K. B. Ma, M. Scheibitz, S. Scholz and M. Wagner, J. Organomet. Chem., 2002, 652, 11. 290. R. Horikoshi, T. Mochida and H. Moriyama, Inorg. Chem., 2002, 41, 3017. 291. G. Diehl, U. Karst, J. Chromatography, A, 2002, 974, 103. 292. P. Heszler, K. Elihn, L. Landstrom and M. Boman, Smart Materials Struct., 2002, 11, 631. 293. W. S. Kim, H. S. Song, B. O. Lee, K. W. Kwon, Y. S. Lim, and M. S. Kim, Macromol. Res., 2002, 10, 253. 294. H. C. Shin, M. L. Liu, B. Sadanadan and A. M. Rao, J. Power Sources, 2002, 112, 216. 295. M. Sawamura, Y. Kuninobu, M. Toganoh, Y. Matsuo, M. Yamanaka and E. Nakamura, J. Am. Chem. Soc., 2002, 124, 9354. 296. A. L. Chistyakov and I. V. Stankevich, Russ. Chem. Bull., 2002, 51, 770. 297. S. Barazzouk, S. Hotchandani and P. V. Kamat, J. Mater. Chem., 2002, 12, 2021. 298. V. T. Hoang, L. M. Rogers and F. D’Souza, Electrochem. Commun., 2002, 4, 50. 299. Z. J. Zhang, B. Q. Wei and P. M. Ajayan, J. Phys.-Condes. Matter, 2002, 14, L511. 300. P. Heszler, Appl. Surf. Sci., 2002, 186, 538. 301. J. B. Yang, L. C. Ling, L. Liu, F. Y. Kang, Z. H. Huang and H. Wu, Carbon, 2002, 40, 911. 302. C. Prados, P. Crespo, J. M. Gonzalez, A. Hernando, J. F. Marco, R. Gancedo, N. Grobert, M. Terrones, R.M. Walton and H. W. Kroto, Phys. Rev. B, 2002, 65, 113405. 303. X. F. Zhang, A. Y. Cao, B. Q. Wei, Y. H. Li, J. Q. Wei, C. L. Xu and D. H. Wu, Chem. Phys. Lett., 2002, 362, 285. 304. N. Chopra, P. D. Kichambare, R. Andrews and B. J. Hinds, Nano letters, 2002, 2, 1177. 305. H. Q. Hou, A. K. Schaper, F. Weller and A. Greiner, Chem. Mat., 2002, 14, 3990. 306. W. Y. Wong, G. L. Lu and K. H. Choi, J. Organomet. Chem., 2002, 659, 107. 307. C. M. Dee and E. R. T. Tekink, Acta. Crystlalogr. E., 2002, 58, 4, M136. 308. Y. C. Neo, J. S. L. Yeo, P. M. N. Low, S. W. Chien, T. C. W. Mak, J. J. Vittal and T. S. A. Hor, J. Organomet. Chem., 2002, 658, 159. 309. A. J. Babcock, J. H. Li, K. Lee and J. R. Shapley, Organometallics, 2002, 21, 3940. 310. V. C. Gibson, N. J. Long, A. J. P. White, C. K. Williams, D. J. Williams, M. Fontani and P. Zanello, J. Chem. Soc.-Dalton Trans., 2002, 3280. 311. O. V. Gusev, A. M. Kalsin, M. G. Peterleitner, P. V. Petrovskii, K. A. Lyssenko, N. G. Akhmedov, C. Bianchini, A. Meli and W. Oberhauser, Organometallics, 2002, 21, 3637. 312. Y. Kato, K. Niiyama, H. Jona, S. Okada, A. Akao, S. Hiraga, Y. Tsuchiya, K. Tomimoto and T. Mase, Chem. Pharm. Bull., 2002, 50, 1066. 313. E. Andre-Bentabet, R. Broussier, R. Amardeil, J. C. Hierso, P. Richard, D. Fasseur, B. Gautheron and P. Meunier, J. Chem. Soc.-Dalton Trans., 2002, 2322. 314. M. A. Zuideveld, B. H. G. Swennenhuis, M. D. K. Boele, Y. Guari, G. P .F. van Strijdonck, J. N. H. Reek, P. C. J. Kamer, K. Goubitz, J. Fraanje, M. Lutz, A. L. Spek and P. van Leeuwen, J. Chem. Soc.-Dalton Trans., 2002, 2308. 315. Y. Cai, Y. L. Song, H. Zheng, Y. Y. Niu, C. X. Du and X. Q. Xin, Chem. Lett., 2002,

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508. 316. T. Koizumi, J. Sakamoto, Y. Gondo and T. Endo, Macromolecules, 2002, 35, 2898. 317. C. M. Dee and E. R. T. Tiekink, Acta Crystallogr. Sect. E.-Struct Rep. Online, 2002, 58, M136. 318. I. A. Razak, A. Usman, H. K. Fun, B. M. Yamin and N. A. M. Kasim, Acta Crystallogr. Sect. C-Cryst. Struct. Commun., 2002, 58, M225. 319. D. R. Smyth, J. Hester, V. G. Young and E. R. T. Tiekink, Crystengcomm, 2002, 517. 320. W. S. Kim, Y. J. Kim and S. W. Lee, Bull. Korean Chem. Soc., 2002, 23, 1177. 321. H. Zhang, Q. L. Suo, Y. B. Wang, L. Wang, L. H. Weng and X. B. Leng, Acta Phys.-Chim. Sin., 2002, 18, 746. 322. B. F. Johnston, D. W. R. Rankin, H. E. Robertson, R. P. Hughes and J. R. Lomprey, Organometallics, 2002, 21, 4840. 323. J. Frunzke, M. Lein and G. Frenking, Organometallics, 2002, 21, 3351. 324. K. Temple, S. Dziadek and I. Manners, Organometallics, 2002, 21, 4377. 325. J. Galloro, M. Ginzburg, H. Miguez, S. M. Yang, N. Coombs, A. Safa-Sefat, J. E. Greedan, I. Manners, G. A. Ozin, Adv. Functional Mater., 2002, 12, 382. 326. A. Berenbaum, F. Jakle, A. J. Lough and I. Manners, Organometallics, 2002, 21, 2359. 327. K. Kulbaba, A. Cheng, A. Bartole, S. Greenberg, R. Resendes, N. Coombs, A. Safa-Sefat, J. E. Greedan, H. D. H. Stover, G. A. Ozin and I. Manners, J. Am. Chem. Soc., 2002, 124, 12522. 328. J. Raez, I. Manners, M. A. Winnik, Langmuir, 2002, 18, 7229. 329. J. Raez, I. Manners, M. A. Winnik, J. Am. Chem. Soc., 2002, 124, 10381. 330. L. V. Snegur, V. I. Boev, V. N. Babin, A. I. Moskalenko and Yu. S. Nekrasov, Rus. J. Org. Chem., 2002, 7, 1076. 331. J. Chiffre, Y. Coppel, G. G. A. Balavoine, J. C. Daran and E. Manoury, Organometallics, 2002, 21, 4552. 332. O. Reynes, T. Gulon, J. C. Moutet, G. Royal and E. Saint-Aman, J. Organomet. Chem., 2002, 656, 116. 333. O. B. Sutcliffe, M. R. Bryce and A. S. Batsanov, J. Organomet. Chem., 2002, 656, 211. 334. G. Denuault, P. A. Gale, M. B. Hursthouse, M. E. Light and C. N. Warriner, New J. Chem., 2002, 26, 811. 335. F. Sancenon, A. Benito, F. J. Hernandez, J. M. Lloris, R. Martinez-Manez, T. Pardo and J. Soto, Eur. J. Inorg. Chem., 2002, 866. 336. M. Nakayama, T. Ihara, K. Nakano and M. Maeda, Talanta, 2002, 56, 857. 337. B. Mayr, H. Schottenberger, O. Elsner, and M. R. Buchmeiser, J. Chromatography A., 2002, 973, 115. 338. Q. Y. Huai, X. L. Wang and Y. M. Zuo, Chromatographia, 2002, 55, 549. 339. M. Muraoka, S. L. Gillett and T. W. Bell, Angew. Chem. Int. Ed. Engl., 2002, 41, 3653. 340. H. Wu, X. W. Wu, X. L. Hou, L. X. Dai and Q. R. Wang, Chin. J. Chem., 2002, 20, 816. 341. F. Maienza, F. Santoro, F. Spindler, C. Malan and A. Mezzetti, Tetrahedron: Asymmetry, 2002, 13, 1817. 342. C. Bolm and T. Kuhn, Isr. J. Chem., 2002, 41, 263. 343. C. Bolm, N. Hermanns, A. Classen and K. Muniz, Bioorg. Med. Chem. Lett., 2002, 12, 1795. 344. M. Weissenbacher, T. Sturm, H. Kalchhauser, C. Kratky and W. Weissensteiner, Mon. Chem., 2002, 133, 991. 345. M. Catasus, A. Bueno, A. Moyano, M. A. Maestro and J. Mahia, J. Organomet.

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13 Organic Aspects of Organometallic Chemistry BY STEVEN D. R. CHRISTIE AND JON J. SHOTTON

1

Introduction

This chapter highlights some of the major advances in the application of organometallic reagents to the synthesis of organic compounds which have been reported in the literature during 2002. As is evident from the vast nature of the field, this review is not and cannot be comprehensive. Consequently, the authors have selected a number of areas and placed an emphasis on those reactions in which there is an issue of selectivity (chemo-, diastereo-, regio-, or enantioselectivity) or unusual reactivity and those in which mechanistic understanding is developing rapidly. Catalytic and stoichiometric organometallic species are considered with equal preference and the coverage is divided into sections. Each section contains examples which are related either by reaction type or by element (individually or in closely related transition or main-block groups).

2

Methods for C–C Bond Formation

One of the main uses of organometallics as applied to organic synthesis continues to be new and improved methods for the preparation of carbon-carbon bonds. This continues to be a theme, with a number of diverse papers published during the review period. Kabalka and co-workers have shown that titanium tetrachloride can be used to react two terminal acetylenes with an aldehyde.1 The reaction proceeds with stereocontrol of the new 1,4-diene unit; one alkene is Z and the other E. On the basis of this, it was proposed that the initial reaction proceed via a six-membered transition state between the aldehyde, terminal alkyne and titanium halide, providing an allylic titanium alkoxide where the aldehyde and a halogen have formally added across the alkyne. Further reaction with another equivalent of alkyne allows a vinyl titanium species to be formed by addition of Ti-Cl across the triple bond. The product is then produced by migration of this vinyl group, and formation of a titanium oxide species. The reaction works for both aliphatic and aromatic aldehydes, and both titanium chloride and bromide. Given the utility of 1,4-dienes and the presence of the halogens, this would appear to be a useful reaction for more elaborate synthesis (Scheme 1). Organometallic Chemistry, Volume 32 ” The Royal Society of Chemistry, 2005 454

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Scheme 1

Mascaren as and co-workers have used a cyclopentadienyl ruthenium cation as a means of adding an allylic ether to a triple bond.2 By using the Trost procedure for coupling an alkyne and an alkene,3 the requisite diene was produced in a simple reaction sequence. Subsequent reaction with acid allows the anomeric centre to form, which is then subjected to a Prins type cyclisation with a suitable nucleophile which was carried through the reaction sequence. This method provides a very atom economical method for the preparation of medium sized oxabicycles in a mild, efficient manner (Scheme 2).

Scheme 2

Li and Wei have introduced a method for the addition of terminal alkynes to imines using a copper-ruthenium C-H bond activation.4 The reaction, similar overall to a Grignard attack of a terminal acetylide on to an imine, proceeds in water, or under solvent free conditions. The reaction uses catalytic quantities of both ruthenium(III) chloride and copper(I) bromide, with the tentative mechanism of the formation of a ruthenium acetylide which attacks the copper complexed imine. Release of the metal species allows catalytic turnover (Scheme 3). Yang and Nolan have reported the use of a palladium-carbene catalytic system for the preparation of enynes by the dimerisation of terminal acetylenes.5 Different carbene ligands produced different ratios of products, the most selective was the IMes carbene (1,3-bis[2,4,6-trimethylphenyl]-imidazolium carbene). Also of interest is that differing the base in the reaction produced a variation in the product distribution. In order to obtain the E-alkene, caesium carbonate was found to be the base of choice (Scheme 4). The cross coupling of terminal alkynes with aromatic halides has been known for some time, but normally requires the use of a stoichiometric quantity of

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Scheme 3

Scheme 4

copper salt. Molander and co-workers have used sodium tetraalkynylaluminates for this purpose.6 The requisite tetraalkynylaluminates are produced from the terminal alkyne under study and sodium aluminium hydride. They are moderately stable to air and moisture, but in this case they are used in situ after formation in THF solvent. When coupled with an organic bromide partner, clean reactions were obtained with little or no homocoupling products being detected. A 1:4 ratio of aluminate to bromide could be employed, however, the reaction is sluggish towards the end, so often a 1:3 ratio was used. Presumably this relates to the lack of reactivity of the last aluminium-carbon bond, similar to LiAlH4 reactions. Polybrominated aromatics can also be used directly to provide a direct route into poly-alkynyl aromatics in a single step (Scheme 5).

Scheme 5

Cross coupling reactions using palladium as the catalytic reagent are of course now commonplace in the literature. Many examples exist, and reactions of this type are now of primary importance to many organic chemists. Nonetheless, there still exists scope to extend the range of reactions, and we will consider some recent developments here. Miura and co-workers have published a method for the multiple arylation of thiophene compounds in a single step.7 These are especially noteworthy given the use of polysubstituted thiophenes in electronic devices. The reaction uses a carbomyl substituted thiophene as the substrate. Initial coupling proceeds with the carbamoyl group intact, with the latter stages

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involving Pd(II)-catalysed and base-promoted hydrolytic C-N fission, followed by decarboxylative C-C cleavage, allowing coupling at the third site (Scheme 6).

Scheme 6

Rathore et al have introduced an astute method for the preparation of cissubstituted stilbenes with complete control of stereochemistry.8 The starting materials are the transdibromoalkenes, and the reagent must be an arylmagnesium halide with ortho substituents. The coupling then proceeds with very high selectivity for the cis substituted alkene where both bromides have been replaced with aryl groups. This intriguing reaction seems to require the use of sterically demanding Grignard reagents, otherwise the corresponding alkyne is produced. In addition, the reaction does not produce any of the mono-substituted product; only the product where both halogens are replaced has been detected (Scheme 7).

Scheme 7

The copper mediated coupling of alkyl, alkenyl and aryl nucleophiles with cyclopropenes has been highlighted by Liao and Fox.9 Using a hydroxymethyl substituted cyclopropene, the Grignard reagent is added to the alkene using copper catlaysis. Work-up can provide a means for further functionalisation through judicious choice of quench. The directing effect of the hydroxymethyl serves to position the incoming nucleophile trans to itself. It is also interesting to note that the example show provides access to a cyclopropane with two quaternary centres, with the third position functionalised as an iodide (Scheme 8).

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Scheme 8

Furstner has introduced iron as a catalyst for cross-coupling reactions of Grignard reagents with aryl chlorides.10 Using an iron source such as FeCln, Fe(acac)n, or certain salen complexes act as catalysts, and the range of substrates comprises Grignard and zinc reagents as well as organomanganese complexes. Both aryl and heteroaryl chlorides can be employed: bromides and iodides are normally reduced under the reaction conditions. Functional group tolerance is impressive with esters, nitriles, sulfonamides, acetals and alkynes all being compatible with the reaction conditions. Given the cost and benign environmental effects of inorganic iron complexes, this process is a highly valuable addition to cross-coupling methodology (Scheme 9).

Scheme 9

Trost has prepared the anti-HIV compound Tipranavir through a palladium and molybdenum asymmetric allylic alkylation reactions.11 The two key intermediates were prepared by transition metal catalysis. Firstly, in Scheme 10, the racemic epoxide was transformed into the allylic ether via a palladium and boron co-catalysed process. The reaction is a dynamic kinetic resolution, providing a good yield and high stereoselectivity from racemic starting material. The other major component was prepared from a related reaction, this time mediated by molybdenum. Here (Scheme 11), a malonate nucleophile is added to the internal terminus of a Mo-allyl system, with stereocontrol provided by a chiral ligand. Again the process is a dynamic kinetic resolution, and a very high yield and level of stereocontrol were achieved. These two key components were later coupled on route to the target compound. Organometallic methods for the formation of small rings are very important, and two publications for the generation of cyclopropanes deserve mention. Takeda and co-workers have employed titanocene to effect a desulfurisation of a thioacetal to reveal a titanium carbene.12 This can then be added across alkenes to produce three membered rings. Although a stoichiometric quantity of titanium is used, this is a relatively cheap reagent and is non-toxic. The products are produced in good yield, with high stereoselectivity (Scheme 12).

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Scheme 10

Scheme 11

Scheme 12

Cyclopropanes have also been formed using zinc carbenoids on reaction with dienes.13 The zinc carbenoid is generated from the carbonyl group and then addition to the less hindered double bond is aided by coordination to the acetate group of the diene (Scheme 13). One other developing area which deserves mention is the use of palladium catalysis to effect C-C coupling reaction between aromatic and alkenic systems and enols and enolates. This chemistry has arisen from the more established cross- coupling reactions such as Stille and Suzuki reactions. Hartwig has shown that palladium plus either a N-heterocyclic carbene or a bulky phosphine can be used to produce a-aryl esters in good yield at room temperature.14 The reaction is tolerant of both functionality and substitution on the substrates. Application to important products such as Naproxen were also highlighted (Scheme 14).

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Scheme 13

Scheme 14

Buchwald has taken this one step further by establishing which chiral ligands can effect the coupling in a stereoselective manner.15 The axially chiral phophinobinaphthyl ligand was used to achieve formation of a chiral carbon centre on reaction of enolate with an aryl bromide. High reactivity, good yields and high enantioselectivity were noted in the reaction (Scheme 15).

Scheme 15

In a further modification, the ligand was further developed to contain both an axially chiral backbone, and a chirogenic phosphorus centre.16 This combination provides very good levels of stereoselectivity in the reaction (Scheme 16),

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Scheme 16

3

Methods for C–X Bond Formation

One of the major developments this year has been the advancement in technology for carbon-nitrogen bond formation. This has primarily been as a result of two groups, those of Buchwald and Hartwig, applying their results from carboncarbon cross coupling reactions to those of carbon-heretoatom reactions. Specialised reaction conditions and catalysts have been employed that allow formation of nitrogen containing compounds in simple reactions that were impossible only a few years ago. The synthesis of anilines via the formation of the aromatic-nitrogen bond has been an area of intense study. Hartwig has published on the preparation of anilines by reaction of aryl chlorides with amines, Scheme 17.17 Optimisation of the reaction conditions found that a palladium source with bulky phophine ligands was required, as well as a source of base, and a phase transfer catalyst, cetyltrimethylammonium bromide. When using sodium tertbutoxide as the base, some functionalities such as esters, enolizable ketones, nitriles, and nitro groups were not tolerated. However, switching to sodium hydroxide with toluene as the solvent did allow these reactions to give the desired products. Yields on the whole were excellent with most reactions proceeding in greater than 90% (Scheme 17).

Scheme 17

Buchwald has also reported on the amination of aryl chloride that contain sensitive functionalities, Scheme 18.18 Although the specific conditions are differ-

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462

ent to those of Hartwig, some generalities are evident: a palladium source with bulky ligands are required, as is a strong base. Buchwald has focussed specifically on substrates with functional groups that previously failed in the reaction. Under the new conditions, free hydroxyls, amides and enolisible ketones now undergo clean reaction to give the desired products.

Scheme 18

The formation of aryl-nitrogen bonds can also be carried out with amides.19 Buchwald has illustrated the use of a palldium mediated coupling that proceeds in the presence of a Xantphos catalyst. Yields and functional group tolerance are both good, however, the most intriguing feature is the x-ray crystal structure of an intermediate in the reaction. On insertion of the palladium into the carbonbromide bond of the aryl bromide starting material, the x-ray structure was determined to have the Xantphos occupying trans positions on a distorted square planar palladium, with the bromide and aryl group occupying the other positions, again trans to each other. The authors offer suggestions for how a reductive elimination could proceed to give the product, a three coordinate palladium or isomerisation to the cis complex, but mechanistic evidence is not available as yet (Scheme 19).

Scheme 19

A related procedure, but this time mediated by copper, also provides a means to couple aryl halides with amides.20 The procedure is simple with only the source of copper, and diamine ligand and a base being required. This may account for the greater functional group tolerance of this reaction over the palladium system. Aryl iodide and bromides are both active substrates, and some chlorides do participate. Although higher temperatures are required than those of the palladium system, these are still lower than those of the traditional Ullman type couplings in which copper has been used previously (Scheme 20).

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Scheme 20

The coupling can also be extended to other nitrogen nucleophiles, as Buchwald has shown by through the use of indoles.21 Again this is a copper mediated process that requires the use of a diamine ligand. Yields are good with a range of aryl iodide and bromide substrates. Funstional group tolerance is also good with amine (alkyl or aryl), amide, cyano-, nitro-, ester, allyl, and hydroxyl groups all shown to be compatible with the reaction conditions. The use of amine and hydroxyl groups is particularly noteworthy for a chemoselectivity point of view (Scheme 21).

Scheme 21

Extension of this type of reaction to other heteroatom bond formations has been achieved with thiols.22 The reaction conditions are similar to the amine coupling reaction mentioned above, with the diamine ligand being replaced with a diol. The most notable improvement on previous methods is the use of a cheap inexpensive base, potassium carbonate: previously the expensive phosphazene type base had to be employed for a similar reaction (Scheme 22).

Scheme 22

One of the generalities of the carbon-heteroatom couplings appears to be the necessity for a bulky ligand to coordinate to the palladium during the catalytic cycle. Hartwig has addressed this, and has introduced a large ferrocene based

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ligand for this purpose.23 The ligand appears to generate a stable palladium catalytic species which can be used for carbon-nitrogen bond formation, as well as carbon-carbon bond generation through a Suzuki reaction, and carbonoxygen bond formation. Turnover numbers in the thousands were noted. A range of aryl halides, both electron rich and electron poor were shown to react under the conditions. Furthermore, the catalytic species appears to be stable both in solution and in the solid s tate (Scheme 23).

Scheme 23

Another method for the formation of carbon-nitrogen bonds has been illustrated, again by Hartwig.24 The reaction of a 1,3 diene with a amine in the presence of an amine was shown to produce an allylic amine. Mechanistic understanding of this process led to the discovery that nickel could promote the exchange of amines from several allylic amine species. The exchange process seems to be dominated by the relative thermodynamic stability of the products. In addition, the process proceeds by racemisation of the original allylic amine, a process which may be useful when used in tandem with the amination of dienes (Scheme 24).

Scheme 24

4

Metathesis Reactions

Alkene metathesis is now used routinely in the preparation of organic com-

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pounds. The ready availability of catalysts which can be handled without stringent reaction conditions has made the technique much more accessible to a wide range of both academic and industrial chemists. The number of papers that utilise the technique are still increasing every year, and it is only possible to highlight a few here. One of the difficulties that still arises is the sensitivity of the catalysts, especially the Grubbs catalysts, to heteroatoms that can coordinate to the metal centre. This can slow and stop the reaction by removing the metal from the catalytic cycle. Mioskowski has published a paper that achieves metathesis with sulfur based substrates.25 It was found that the N-heterocyclic carbene ruthenium catalyst gave similar results to Schrock molybdenum based catalyst. All of the catalysts struggled to cyclise sterically encumbered alkene with various forms of sulfur present (sulfide, disulfide, dithioacetal) (Scheme 25).

Scheme 25

The preparation of nitrogen containing rings is also appealing, but again has caused problems in terms of compatibility with the catalyst. However, a simple solution here is to employ an amide in preference to an amine. Blechert has exploited this to great effect in a number of papers. This year, he has used a tandem ring-closing, ring-opening, ring-closing metathesis as part of the route towards the natural products (;)-Dihydrocuscohygrine and Cuscohygrine.26 This ring rearrangement metathesis is achieved through standard methodology to build up the precursor seven membered ring illustrated in Scheme 26. Exposure of this to the metathesis catalyst allows the initial reaction to take place on one of the terminal allyl groups. The metal carbene then performs a ring-opening metathesis of the seven membered ring, concomitantly closing the first of the five membered rings. The rearrangement scheme is completed when the metal carbene closes on to the second terminal allyl group. The stereochemistry is fully transferred during the course of the reaction (Scheme 26). Another notable double metathesis scheme has been performed by Ma and Ni.27 Here, the tetraene show in Scheme 27 was exposed to the Grubbs catalyst, facilitating a tandem metathesis reaction. Two points are worthy of note; firstly, the a,b-unsaturated alkene is the least reactive double bond. This was illustrated if the reaction was stopped prior to completion; the mixture of products contained a monocycle where the acrylamide alkene had not taken part in a cyclisation. However, leaving the reaction for longer allowed the second cyclisation to take place. Secondly, it is possible for an alternative mode of metathesis to

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occur to provide a two five membered rings connected by a single bond. This was

Scheme 26

only achieved to any great extent with specific substitution in the starting material.

Scheme 27

The use of metathesis has also allowed the production of new types of molecules that may be difficult to access by any other routes. An example of this is shown in Scheme 28, where the tetrahydropyridine has been access from the diene shown. This product was then taken on to produce the first sterochemically controlled synthesis of 4-substituted 3-amino piperidines.28 (Scheme 28)

Scheme 28

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Alkyne metatheis is nowhere near as extensively examined as its alkene relation. Nonetheless, there is still work in the area which provides scope for use in organic chemistry. The alkyne metathesis catalysts are often easier to access, and Grela has shown how a mixture of molybdenum hexacarbonyl and 2fluorophenol can provide an ‘instant‘ catalyst.29 Simply mixing the two reagent in the presence of the substrate gives a highly active catalyst which does not require the exclusion of air or moisture, but facilitates the reaction in good yield in a reasonable reaction time (Scheme 29).

Scheme 29

5

Metal Mediated Cyclisation and Cycloisomerisation Reactions

Metal mediated cyclisations continue to be a highly useful and efficient means of performing ring forming reactions. The metal often imparts a rigidity to the transition state of the reaction, and often confers reactivity onto otherwise unreactive species, such as alkene and alkynes. The indolizidine skeleton is often a target of organomentallic mediated reactions; it contains a bicyclic ring structure with nitrogen at the bridgehead, suggesting various disconnections strategies for its preparation. Venecko and West have employed an ammonium ylide on route to this ring structure.30 The initial organometallic reaction uses a copper catalysed decomposition of the diazo compound. Attack of the pyrrolidine nitrogen gives the spirocyclic ylide, which then undergoes a stereoselective Stevens rearrangement to allow access to the bicyclic system. The silyl group is essential to control the stereochemistry, and can be used later to furnish a hydroxyl group via a Tamao oxidation (Scheme 30).

Scheme 30

A related ring system has also been access by Ojima and co-workers.31 Here, a

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rhodium hydrocarbonylation of a terminal alkene furnishes and aldehyde in situ. This is immediately condensed with the amine and amide functionalities to furnish the bicyclic ring structure. The amine can be replaced with oxygen or sulfur to increase the range of products that can be accessed by this chemistry. The products are of interest as conformationally restricted dipeptide surrogates (Scheme 31).

Scheme 31

Mongomery has introduced a interesting nickel mediated cyclisation which allows an allene to attack an aldehyde, producing a five membered carbocycle.32 The nickel(0) catalyst coordinates to both the aldehyde and the allene. This facilitates the carbon-carbon bond forming reaction with concomitant generation of a formal nickel-carbon sigma bond. Reaction of this with a carbon nucleophile (a zinc or lithium reagent) allows the reductive elimination of the nickel(0) to regenerate the catalyst, and formation of a second carbon-carbon bond (Scheme 32).

Scheme 32

A related nickel(0) coupling has been published by Takimoto and Mori. 33 This novel reaction involves the incorporation of carbon dioxide into bis-1,3-dienes while at the same time accomplishing a stereoselective ring closing reaction. The reaction proceeds via coordination of the diene units, then ring formation to provide a bis-allyl nickel system. Reaction of this with carbon dioxide at one allyl group allows a nucleophile to attack the other allyl unit. If diethyl zinc is employed, then a elimination of ethylene produces a nickel hydride which then undergoes a reductive elimination to give the terminal alkene. However, if a

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nucleophile without a b-hydrogen is employed, this group is directly incorporated into the substrate by reductive elimination, thus providing more diversity for the reaction. Also of note is that when unsymmetrical bis-dienes are employed, regioselective introduction of carbon dioxide and nucleophile are noted (Scheme 33).

Scheme 33

Reductive cyclisation of unsaturated systems is possible with a number of low valent transition metals, and Brummond has continued her work in this area.34 Whilst working on the allenyl Pauson-Khand reaction, Brummond postulated that lack of carbon monoxide would not allow formation of cyclopentenone to proceed. Instead, formation of a cross-conjugated triene would be postulated through breakdown of the proposed metallocyclic intermediate. Using a rhodium catalysed process, this was indeed found to be the case. Alkynyl allenes were cyclised through a putative rhodocycle, which can then regenerate the catalyst through reductive elimination to provide the organic triene in good to excellent yield, with good control of the alkene geometry (Scheme 34).

Scheme 34

Continuing their work on N-acyl iminium ions, Allin and co-workers have reported a route to the tetracyclic core of the Erythrinane skeleton.35 Two points of note relating to organometallic chemistry are of interest here: firstly, the titanium mediated rearrangment which furnishes the tetracyclic framework with a high level of diastereoselectivity; secondly, the hydroxymethyl group generated in this reaction is surplus to requirement in the final product, so this is removed in a rhodium mediated decarboxylation after oxidation of the hydroxyl to an aldehyde (Scheme 35).

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Scheme 35

The formation of simple heterocycles remains an area of prime importance, with new and milder techniques required. Kel’in and Gevorgyan have employed a copper (I) catalysed process to effect the synthesis of furans.36 The method is quite general for a range of alkyl and aryl substituted alkynyl ketone starting materials, as well us proceeding under mild reaction conditions. This later fact allows a both acid and base sensitive groups to be tolerated during the reaction. The proposed mechanism proceeds via the allene, then a cycloisomerisation reaction. Some evidence for this was provided since an isolated allenyl ketone was shown to give the same product, albeit in a lower yield (Scheme 36).

Scheme 36

The palladium mediated allylation reactions has already been highlighted above. Poli has extended the scope of the reaction and used it in a tandem palladium reaction which gives rise to a double cyclisation.37 As shown in Scheme 37, the starting material contains a malonate type centre which can react

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with the allyl palladium species formed from the allylic acetate; this forms the 5-membered lactam. However, the vinyl group thus formed can then undergo a Heck reaction with the aryl bromide to furnish the second ring, producing the core of an aza analogue of Epiisopicropodophyllin. The order of the cyclisation was confirmed when the allylic substitution product was detected after 20 mins at 85°C.

Scheme 37

The fenestrane skeleton has been the target of cobalt mediated reactions previously, and Chung and co-workers have revisited this area of chemistry to produce a simple and effective means of accessing the tetracyclic core in a one pot process from an acylic precursor.38 As shown in Scheme 38, the diene-diyne is the requisite starting point for this procedure. Cobalt ocatacarbonyl performs a [2;2;1] Pauson-Khand like cycloaddition of the two alkynes to give a cyclopenatdienone. This is then trapped by the diene in a Diels-Alder reaction to give the third and fourth rings in the structure. This remarkable sequence is only 4 steps in total from readily available starting materials, and provides an excellent example of the use of organometallics in accessing complex structures in short, efficient syntheses (Scheme 38).

6

Metal Mediated Cycloadditions

Cycloaddition reactions remain the mainstay of organometallic mediated processes, with a number of new and improved developments reported during the period of this review. Wender has been looking into the use of cyclopropyl groups as components in cycloaddition reactions. A [5;2] cycloaddition employing a cycopropyl imine and an alkyne required rhodium catalysis to provide a substituted seven membered ring.39 This hetero [5;2] process is the first to

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Scheme 38

produce a heterocycle through such a protocol. The proposed mechanism allows the metal to insert into a carbon-carbon- bond of the cyclopropane, followed by addition of the alkyne. Yields were increased when the alkyne was added slowly to the reaction mixture as this minimised by-products. Reductive elimination regenerated the catalyst, and produced the product. Also of note is that the imine can be produced in situ by condensation of the corresponding aldehyde, thus eliminating a step in the overall procedure (Scheme 39).

Scheme 39

A new development in this area is a [5;2;1] cycloaddition, outlined in Scheme 40.40 Here, the initial two component cycloaddition is enhanced by the addition of a third constituent: carbon monoxide. Thus, a vinyl cyclopropane, an alkyne and carbon monoxide add together to produce, initially, an eight membered ring. Again, the metal catalyst activates and opens the cyclopropane unit. Addition of the alkyne is followed by insertion of CO, before reductive elimination gives eight membered ring, and regenerates the catalyst. However, the product isolated from the reaction is the bicycle shown in Scheme 40. This arises from an transannular aldol reaction of the intermediate enol ether onto the carbonyl group which arises from the CO ligand. 1,3-Dipolar cycloadditions have also seen some usage during the review period, by a number of different groups. The formation of susbstituted triazoles through a three component coupling reaction has been reported.41 Allyl azide is

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Scheme 40

formed in situ through the reaction of trimethylsilylazide with an allyl electrophile in the presence of a palladium catalyst. This can then undergo a dipolar cycloaddition with an alkyne to give the product in reasonable to good yield. In this way, the allyl group is introduced regioselectivly at the 2-position of the triazole ring (Scheme 41).

Scheme 41

Ku¨ndig has published a transition metal Lewis acid mediated dipolar cycloaddition between nitrones and a,b-unsaturated aldehydes.42 The iron or ruthenium promoted process proceeds by coordinating the enal as opposed to the nitrone group in the one point Lewis acid binding site. This accelerates the reaction, with the chiral backbone of the Lewis acid providing the stereoselectivity of the reaction. The background reaction, which proceeds without coordina-

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tion to the catalyst, depletes the enantioselectivity, and this was suppressed by slow addition of the nitrone to the reaction mixture. Brief molecular modelling studies explained the formation of the endo product through minimisation of steric interactions of the nitrone, enal and the ligands of the catalyst (Scheme 42). Fu has also employed nitrones in a transition metal Lewis acid coupling reaction, this time with alkynes.43 Using the innovative bis-azaferrocene catalyst with a source of copper, stereoselectivity is good to excellent, and yields reasonable to good, although there is a trade-off between these two. The cis isomer predominates over the trans in all cases (Scheme 43).

Scheme 42

[2;2;2] Cycloaddition reactions of alkynes and alkenes are among the most popular type of metal mediated annelation processes, and this continues to be the case. Cobalt is one of the metals known to catalyse this process, and Cheung and co-workers have extended this protocol to incorporate allenes into the cyclisation with diynes.44 The allene is the synthetic equivalent of an alkyne since the initially formed product has an exocyclic double bond that is isomerized to the aromatic species. Also, the product is formed with a greater degree of regioselectivity than the corresponding alkyne reaction (Scheme 44). Two hetero [2;2;2] cycloaddition have also been reported. This first uses carbonyl compounds as the third component.45 Itoh and co-workers have successfully utilised a ruthenium catalyst to promote a cycloaddition between a diyne and an electron-deficient ketone. The initially formed pyran then undergoes an eletrocyclic ring-opening to afford the substituted cyclopentene product (Scheme 45). A related system employs carbon dioxide as the coupling partner, providing a route to pyrones.46 This nickel catalysed reaction proceeds with only an atmospheric pressure of carbon dioxide. Mechanistic insight is garnered using differentially protected diynes; the product pyrones were found to only one of the

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Scheme 43

Scheme 44

Scheme 45

475

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possible regioisomers. This suggests that the initial reaction occurs between the carbon dioxide and the least hindered alkyne unit, and the last bond to form is that between the more hindered alkyne and the oxygen of the CO2 (Scheme 46).

Scheme 46

7

Pauson-Khand Type Reactions

The Pauson-Khand reaction remains one of the most effective ways for producing five membered rings with control of regio and stereochemistry. Advances on the original technique continue to be made, which is another factor in its continued popularity. Building on the traditional method of a stoichiometric bimetallic system, Moyano and co-workers have reported a heterobimetallic variant.47 The cobalt-tungsten system is produced from the readily available bis-cobalt complex by displacement of one of the cobalt vertices by a tungsten nucleophile. This has a similar reactivity to the cobalt-molybdenum system48 which has also been reported. However, the tungsten analogue provides more of the endo product when electron deficient alkynes are employed (Scheme 47). The recent interest in the Pauson-Khand reaction has been to attain a catalytic scheme. Again, with the traditional bis-cobalt system, one way of achieving this is to attach ligands to the metals which can stabilise the intermediate low-valent metal complex which is prone to degradation during the recycling stage of the catalytic cycle. Gibson has shown that a triphenylphosphine substituted complex can act as a catalyst during the reaction.49 The complex is prepared from the commercial cobalt octacarbonyl, and is isolated as a stable solid and purified by chromatography. The complex is active at a 5mol% loading and has none of the sensitivities and handing problems of the parent metal carbonyl (Scheme 48). A related system has used Bu3PS as the ligand in a catalytic Pauson-Khand reaction.50 The enyne precursor is prepared from a three component coupling between a propargyl amine, a vinyl boronic acid and glyoxylic acid. The annelation step proceeds with control of stereochemistry from the chiral auxiliary in the chiral amine starting material (Scheme 49). Use of a chiral ligand to prepare enantioenriched products is another logical step forward, and Buchwald has achieved this through use of an optically pure

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Scheme 47

phophite.51 This is the first chiral phosphite which has induced a reasonable level of stereocontrol on the reaction. The drawback is that it is limited to aryl substituted 1,6-enynes (Scheme 50). Alternatives to the bimetallic system have also seen some interest this year. Cobalt nanoparticles on charcoal have been used as a catalyst for the PausonKhand reaction.52 The nanoparticles are prepared by thermal reduction of cobalt octacarbonyl, and stabilsed by surfactants. These are then deposited on to a charcoal surface. Both inter and intramolecular reaction proceed under an atmosphere is carbon monoxide (Scheme 51). In an even more impressive reaction, the cobalt nanoparticle mediated Pauson-Khand cyclisation can be combined with a palladium mediated allylic

Scheme 48

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Scheme 49

Scheme 50

substitution reaction.53 This then sets up a second Pauson-Khand reaction to produce the tetracycle shown in Scheme 52. Here, seven carbon-carbon bonds have been synthesised in a single synthetic sequence in a remarkable 84% yield. One other modification of the Pauson-Khand type reaction deserves mention. The use of carbon monoxide in the catalytic protocol can be unwelcome since it

Scheme 51

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479

Scheme 52

requires high pressure and use of a toxic gas. However, using a rhodium based system, aldehydes can be employed as a CO surrogate.54 The rhodium complex facilitates a decarbonylation of the aldehyde to the hydrocarbon. This CO is then utilised in the enyne cyclisation which is promoted by the same catalyst. When naphthalene carboxaldehyde was employed as the CO source, recovery of the naphthalene was possible. This showed that almost all of the available CO was utilised during the course of the reaction (Scheme 53).

Scheme 53

8

Nicholas Reactions

The bimetallic complexes of use in the Pauson-Khand reaction can also be employed to different ends. The Nicholas reaction refers to the stabilisation of a cation in the propargylic position on a metal-alkyne complex. Recent discoveries extended the methodology to solid phase synthesis.55 The propargylic alcohol was attached to the sold phase through an ether linkage. Complexation of this to cobalt carbonyl was straightforward, and use of the Nicholas reaction allowed cleavage of the organometallic fragment and generation of new alkyne compounds, after oxidation of the metals (Scheme 54). Extension of the protocol to hetermobimetallic systems with two different metals has also been reported.56 The inherent chirality of the metal-alkyne core induces asymmetry on formation of the new carbon-nucleophile bond at the propargylic position. In addition, reaction of a base with the air-stable propargylic cation salts provided a route to enynes in good yield (Scheme 55).

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Scheme 54

Scheme 55

A related reaction has also been reported which achieves the same overall reaction, but with only a catalytic quantity of transition metal.57 Using a cationic diruthenium species, propargylic alcohols were reacted to form the corresponding sulfides in good to excellent yields using only 5mol% of the catalyst (Scheme 56).

Scheme 56

Matsuda and co-workers have introduced the iridium mediated scheme allowing the coupling of a propargylic acetate with a silyl enol ether.58 The advantage over the traditional Nicholas reaction is again the use of only catalytic quantities of transition metal (Scheme 57).

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Scheme 57

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