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Reactivity of molybdenum nitrosyl alkyl complexes Young, Michelle A. 1995

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REACTiVITY OF MOLYBDENUM NITROSYL ALKYL COMPLEXESbyMICHELLE A. YOUNGB.Sc.(Hons), University of Cape Town, 1988A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIES(Department of Chemistry)We accept this thesis as conformingto the required standards.icTHE UNIVERSITY OF BRITISH COLUMBIAJuly 1995© Michelle A. Young, 1995In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis br scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)______________________Department of_____________________The University of British ColumbiaVancouver, CanadaDate• tkSt V? qsDE.6 (2188)11AbstractHydrogenation of the bis(alkyl) complex, Cp*Mo(NO)(CH2SiMe3)yields thebimetallic oxo bridging-nitrido complex, [Cp*Mo(NO)(CHSi e)](p.N)[Cp*Mo(O)(CH2Si e3].When the alkyl group of the bis(alkyl) precursor is changedto neopentyl (R CH2Me3)or neophyl (R =CH2MePh), intermediate bridgingnitrosyl complexes, (Cp*MoR),.tNO) are isolated. These bridging nitrosyl complexesisomerize to bimetallic oxo bridging-nitrido complexes, [Cp*Mo(NO)R](p.N)[Cp*Mo(O)R]. A kinetic study of the isomerization (R = neophyl) reveals that it is firstorder in (Cp*MoR)2p.NO) with kobs(2O °C) 1.1 ± 0.3 x io s1. Furthermore,kinetic analyses at various temperatures establish that AH = 39 ± 3 kJ mol1 and-188 ± 6 1 mold K1, which is consistent with the isomerizations occurring in anintramolecular manner. Hydrogenation of a mixture of Cp*Mo(NO)(CH2SiMe3)andCp*W(NO)(CH2S1Me3)produces the heterobimetallic bridging nitrosyl complex,[Cp*Mo(CHSi e)](.tNO)W(j,which similarly isomerizes to a 60:40mixture of[Cp*W(NO)(CHSiMe)]Qj,N)[Cp*Mo(O)(CH]and[Cp*Mo(NO)(CHSi e)](.tN)[Cp*W(O)(C].The products obtained, when Cp*Mo(NO)(CH2SiMe3is hydrogenated in thepresence of added substrates, are dependent on the nature of the substrate. A transientlygenerated molybdenum hydride complex, Cp*Mo(NO)(CH2SiMe3)H,is trapped in thepresence of acetone or benzaldehyde to produce alkyl alkoxide complexes,Cp*Mo(NO)(CH2SiMe3)(OR)(R = CHMe2,CH2Ph). In the presence of acyclic dienes,x, f3-unsaturated ketones, PPh3 or PhSSPh, both alkyl groups undergo hydrogenolysis andcomplexes of the type, Cp*Mo(NO)L (L =q4-diene, x, 13-unsaturated ketone, (PPh3)2(SPh)2), are obtained. In the presence of 1,3-COD, however, the cyclic diene undergoes111initial C-H bond activation and couples in a novel way to produce, Cp*Mo(N0)(rl4C16H24).A pyridine trapped alkylidene complex, CpMo(N0)(=CHCMe3)(py), is generatedfrom the thermolysis of the bis(neopentyl) complex, CpMo(N0)(CH2ein thepresence of pyridine. The ci-carbon of this neopentylidene is very nucleophilic andundergoes stereoselective addition reactions with a variety of polar REH complexes(E 0, 5, N; R = alkyl, aryl) to produce the corresponding neopentyl alkoxide, thiolateand amide complexes. Similarly, it reacts with carboxylic acids to yield neopentylcarboxylate complexes, CpMo(NO)(CH2Me3) i-OC{O}R). Dimeric species areobtained when bifunctional REH reagents are used. For example, oxalic acid produces[CpMo(NO)(CH2e3)]-i-(rOC{ 0) C { 0)0) and neopentyl glycol produces[CpMo(N0)(CHe(i-0CHH0 .These addition reactions probablyproceed via the formation of an initial Lewis-acid Lewis-base pair, followed by protontransfer.CpMo(N0)(CH2e3)(ER)complexes readily undergo exchange reactions withRE’H (E’ = 0, S, N) reagents (pKa RE’H <REH). For example, the amide ligand ofCpMo(N0)(CHe)(NH-p-tolyl) readily undergoes exchange with one equivalent ofHOSiPh3,p-cresol, t-butyl thiol and acetic acid to produce CpMo(N0)(CH2e3)(E)(E OSiPh3,0-p-tolyl, SCMe3 and OC{0}Me) and free p-toluidine. An equilibriummixture ofCpMo(N0)(CH2e)(0-p-tolyl) and CpMo(N0)(CHe)(OSiPhisgenerated when one equivalent ofHOSiPh3is reacted with CpMo(N0)(CH2e3)(O-p-tolyl) or one equivalent ofH0-p-tolyl is reacted with CpMo(N0)(CHe)(OSiPh.Labeling studies are consistent with these heteroatom exchange reactions occurring withthe retention of configuration at the CpMo(N0)(CH2e3)fragment.ivTABLE OF CONTENTSAbstract iiTable of Contents ivList of Tables xiiList ofFigures xiiiList of Schemes xviList of Abbreviations xviiAcknowledgments xxiQuotation xxiiCHAPTER 1: Introduction 11.1 General Introduction 21.2 Organotransition-Metal Chemistry 21.3 Organotransition-Metal Nitrosyl Complexes 31.3.1 Nitric Oxide 31.3.2 The Binding ofNitric Oxide to Organotransition-Metals 41.3.3 18-Electron Rule and Oxidation States 61.3.4 Synthesis and Characterization of Organotransition-MetalNitrosylComplexes 61.4 Scope and Format of this Thesis 71.5 References and Notes 10VCHAPTER 2: Reactions of Cp*Mo(NO)R2Complexes with MolecularHydrogen 122.1 Introduction 132.1.1 Nitrosyl N-O Bond Cleavage 142.2 Experimental Procedures 172.2.1 Methods 172.2.2 Reagents 182.2.3 Synthesis Preparation of [Cp*Mo(NO)(CH2Si e3)] lIN)[Cp*Mo(O)(CHSi e](2.1) Preparation of[Cp*MoR]2QiNO) [R = CH2Me3(2.2),CH2MePh(2.3)] Preparation of [Cp*Mo(CH2Si e)](p.NO)[Cp*W(CHSIMe)](2.4) Preparation of[Cp*Mo(NO)R](pN)[Cp*Mo(O)R] [R =CH2Me3(2.5), CH2MePh(2.6)] Preparation of [Cp*W(NO)(CHSiMe3)](tN)[Cp*Mo(o)(cHSiMe)1(2.7a) and [Cp*Mo(NO)(CH2Si e3)]QiN)[Cp*W(O)(CH2SiMe3](2.7b) 212.2.4 Reaction of Cp*Mo(NO)(pto1y1)2with H2 212.2.5 Reaction of [Cp*Mo(CHe3)]QtNO)(2.2) with[Cp*Mo(CHePh)](p,NO)(2.3) 222.2.6 Reactions of [Cp*Mo(CH2ePh)](pNO)(2.3) with L (LPPh3,PMe3,pyridine) 222.2.7 Kinetic Measurements 22vi2.3 Results and Discussion.272.3.1 Hydrogenation ofCp*Mo(NO)(CH2Si3) 272.3.2 Hydrogenation of Cp*Mo(NO)R2(R = neopentyl or neophyl) 302.3.3 Hydrogenation of Cp*Mo(NO)(ptolyl) 362.3.4 Kinetic Study 362.3.5 Hydrogenation of a Mixture of Cp*Mo(NO)R2and Cp*W(NO)R2(R= CH2SiMe3) 392.3.6 Proposed Mechanism of Isomerization 402.3.7 Fluxional Processes in Solution 442.3.8 Reactivity ofBridging-Nitrido and Bridging-Nitrosyl Complexes 452.4 Epilogue 452.5 References and Notes 47CHAPTER 3: Reactions of Cp*Mo(NO)(CII2SiMe3)Complexes with 112 inthe Presence of Trapping Substrates 533.1 Introduction 543.2 Experimental Procedures 553.2.1 Methods 553.2.2 Reagents 553.2.3 Synthesis 563.2.3.1 Preparation ofCp*Mo(NO)(rj4frans1,3butadiene) (3.1) 563.2.3.2 Preparation ofCp*Mo(NO)(ritrans2,3-dimethyl-butadiene)(3.2) 57vu3.2.3.3 Preparation ofCp*Mo(NO)(CH2SiMe3)(OR)[R = CHMe2(3.3), CH2Ph (3.4)] 573.2.3.4 Preparation ofCp*M(NO)(r1i6H24[M = Mo (3.5), W (3.6)] 583.2.3.5 Preparation ofCp*W(NO)(q3811)H (3.7) 583.2.3.6 Preparation ofC16H24 (3.8) 583.2.3.7 Preparation ofCp*Mo(NO)(r13. enten2one) (3.9) 593.2.3.8 Preparation of Cp*Mo(NO)(114-butenone) (3.10) 593.2.3.9 Preparation of Cp*Mo(NO)(PPh3)2(3.11) 603.2.3.10 Preparation of Cp*Mo(NO)(SPh) (3.12) 603.2.4 Labeling Studies Using D2 613.2.5 Reaction of Cp*Mo(NO)(CH2SiMe3)with 1,3 -Cyclohexadiene andH2 613.2.6 Reaction of Cp*W(NO)(CH2SiMe3)with 1,3 -Cyclohexadiene andH2 623.2.7 Reactions ofCp*Mo(NO)(CH2SiMe3)with Other UnsaturatedHydrocarbons and H2 (Other Unsaturated Hydrocarbons = 1,5-COD,cyclooctatetraene, ethylene, or diphenylacetylene) 623.2.81HN]VlRMonitoring 623.3 Results and Discussion 683.3.1 Reactions with Acyclic Dienes 683.3.2 Reactions with Acetone and Benzaldehyde 713.3.3 Reactions with Cyclic Dienes 723.3.4 Reactions with c, 13-Unsaturated Ketones 833.3.5 Reaction with Triphenyiphosphine 86viii3.3.6 Reaction with Diphenyl Disuiphide.903.3.7 Attempted Reactions with Other Substrates 923.4 Epilogue 923.5 References and Notes 94CHAPTER 4 Reactivity of CpMo(NO)(=CHCMe3)(py) 984.1 Introduction 994.2 Experimental Procedures 1024.2.1 Methods 1024.2.2 Reagents 1024.2.3 Synthesis 1034.2.3.1 Preparation of CpMo(NO)(=CDCMe3)(py)(4.1-d1) 1034.2.3.2 Preparation ofCpMo(NO)(CH2Me)(OR)[R =C6H4-p-Me(4.2), SiPh3 (4.3)] 1034.2.3.3 Generation ofCpMo(NO)(CHDCMe)(06H- -Me)(4.2-d1) 1044.2.3.4 Preparation ofCpMo(NO)(CH2Me3)(SCMe (4.4) 1044.2.3.5 Generation of CpMo(NO)(CHDCMe)(SCMe(•-‘i) 1054.2.3.6 Generation ofCpMo(NO)(CH2Me3)(NR)[R = H-o-tolyl (4.5),R = H-p-tolyl (4.6), C(O)CHH{O} (4.7), HCMe3 (4.8)] 1054.2.3.7 Generation of CpMo(NO)(CHDCMe3)(NHR) [R = p-tolyl (4.6-d1), CMe3 (4.8-d1)] 1064.2.3.8 Preparation ofCpMo(NO)(CH2Me3) r-OC{O }Me) (4.9) 1064.2.3.9 Preparation ofCpMo(NO)(CHMe) i-OC{O }Ph) (4.10) 106ix4.2.3. 10 Preparation ofCpMo(NO)(CH2Me3) r-OC{ 0 }R) [R =C6H4-o-N02(4.11),C6H4-p-N02(4.12)] 1074.2.3.11 Preparation ofCpMo(NO)(CHMe)[(OC(M ))- , J(4.13) 1074.2.3.12 Preparation of[CpMo(N0)(CH2C e3)](ji-OCHCMeH)(4.14) 1084.2.3.13 Preparation of[CpMo(NO)(CHC e]-t-(rOC{O)CO{O)) (4.15) 1084.2.3.14 Preparation of [CpMo(NO)(CH2e3)]( .- )(4.16) 1084.2.4 Reaction of CpMo(NO)(=CHCMe)(py)with PPh2H 1094.2.5 Reactions of CpMo(NO)(=CHCMe3)(py)with Other Reagents (OtherReagents = H2, CO, CO2.C2H4,Ph2SiH,PhCO, Me2CO, (MeO)3SiH,PhCCH, MeCN) 1094.3 Results and Discussion 1154.3.1 Alcohols 1154.3.2 Thiol 1174.3.3 Amines 1184.3.4 Carboxylic Acids 1204.3.5 Acetylacetone 1224.3.6 Neopentyl Glycol, Oxalic Acid and Water 1254.3.7 Other Reagents 1304.3.8 Related Research Efforts 1314.3.9 Labeling Studies 1314.3.10 Mechanistic Considerations 1344.4 Epilogue 136x4.5 References and Notes.137CHAPTER 5 Heteroatom Exchange Reactions inCpMo(NO)(CH2Me3)(ER)Systems 1425.1 Introduction 1435.2 Experimental Procedures 1445.2.1 The Reactions of Complexes 4.2, 4.3, 4.6 and 4.6-d1with HOS1Ph3and p-Cresol 1445.2.2 The Reactions ofp-Toluidine with Complexes 4.2, 4.3, 4.4 and 4.9 1465.2.3 The Reactions of Complexes 4.2, 4.3, 4.6, 4.6-d1 and 4.9 withHSCMe3 1465.2.4 The Reactions of Complexes 4.2, 4.3, 4.4, 4.6 and 4.6-d1with AceticAcid 1475.2.5 The Reactions of Complex 4.10 with p-and o-Nitrobenzoic Acid andOxalic Acid 1485.2.6 The Reaction of Complexes 4.13 and 4.15 with Benzoic Acid 1485.2.7 Treatment of CpMo(NO)(CH2Me3) r-OC{ 0)C6H4-p-N02(4.11)with o-Nitrobenzoic Acid 1495.2.8 Treatment ofCpMo(NO)(CH2Me3)-OC{ 0)C6H4-o-N02(4.12)with p-Nitrobenzoic Acid 1495.2.9 The Generation ofCpMo(NO)(CD2Me3)(OM )(5.1-d2) 1505.2.10 The Reaction ofCpMo(NO)(CDMe)(OM )(5.1-d2)withpCresol 1505.3 Results and Discussion 151xi5.3.1 AmideExchange.1515.3.2 AlcoholExchange 1555.3.3 Thiolate Exchange 1585.3.4 Carboxylate Exchange 1595.3.5 Labeling Studies 1645.3.6 Exchange Mechanisms 1655.4 Epilogue 1685.5 References and Notes 170xliList of TablesTable 2.1 Numbering Scheme, Color, Yield and Elemental Analysis Data 25Table 2.2 Selected Mass Spectral and Infrared Data 26Table 2.3 NMR Data (C6D) 27Table 2.4 Rate Constants (kobs) as a Function of Temperature for theIsomerization of 2.3 to 2.6 in Toluene 42Table 3.1 Numbering Scheme, Color, Yield and Elemental Analysis Data 63Table 3.2 Selected Mass Spectral and Infrared Data 64Table 3.3 NMR Data (C6D) 65Table 4.1 Numbering Scheme, Color, Yield, and Elemental Analysis Data 110Table 4.2 Selected Mass Spectral and Infrared Data 111Table 4.3 NIVIR Data (C6D) 112Table 5.1 Selected pKa Values 153xliiList of FiguresFigure 1.1 The molecular-orbital diagram of nitric oxide 4Figure 1.2 The synergic interaction between a nitrosyl ligand and a transitionmetal 5Figure 2.1 View of the solid-state molecular structure of[Cp*Mo(NO)(CHSi e3)](.tN)[Cp*Mo(O)(C](2.1) (50% thermalellipsoids); including selected bond lengths and angles (with esds in parentheses) 28Figure 2.2(a) View of the solid-state molecular structure of[Cp*Mo(CHe3)](.tNO)(2.2) at 185 K (50% thermal ellipsoids); includingselected bond lengths and angles (with esds in parentheses) 32Figure 2.2(b) Second view, at right angles to that shown in Figure 2.2(a), of thesolid-state molecular structure of 2.2 at 185 K (50% thermal ellipsoids) 33Figure 2.3 300 MHz 1H NMR spectrum of [Cp*Mo(CHePh)](I.tNO)(2.3) in C6D. (a) Spectrum run immediately; only complex 2.3 (inset representslow-field region, 7.7 - 6.9 ppm). (b) After 2 h; mixture of complexes 2.3 (marked•) and 2.6 (marked.) (inset represents low-field region, 7.8- 6.9 ppm) 34Figure 2.3(c) 300 MHz 1H NMR spectrum of [Cp*Mo(CHePh)](,.iNO)(2.3) in C6D after 4.5 h (full conversion to complex 2.6). Inset represents low-field region, 7.8 - 6.9 ppm 35Figure 2.4 UV-vis plot showing a decrease in absorption as a function of time forthe isomerization of 2.3 to 2.6 in the region 600 - 800 nm (20 °C in toluene) 37Figure 2.5 Plot of ln(A-A) vs t for the isomerization of 2.3 to 2.6 (50 °C intoluene) 38Figure 2.6 Eyring Plot for the isomerization of 2.3 to 2.6 (toluene) 39xivFigure 2.7 View of the solid-state molecular structure of[Cp*Mi(NO)(CHSi e3)]Q.LN)[Cp*M(O) H](2.7) (50% thermalellipsoids); including selected bond lengths and angles (with esds in parentheses) 41Figure 3.1 View of the solid-state molecular structure of Cp*Mo(NO)(4_C16H24)(3.5), including selected bond lengths and angles (with esds inparentheses) 75Figure 3.2 300 MHz 1H NMR spectrum ofCp*Mo(NO)(q416H24)(3.5) inC6D 76Figure 3.3 75 MF{z APT spectrum of Cp*Mo(NO)(rI4i6H24)(3.5) in C6D 76Figure 3.4 75 MHz APT spectrum ofC16H24 (3.8) in C6D 79Figure 3.5 300 MHz1HNMR spectrum ofC16H24 (3.8) in C6D 79Figure 3.6 300 MHz1HNMR. spectrum ofCp*W(NO)(r138Hii)H(3.7) inDMSO-d6 81Figure 3.7 75 MHz APT spectrum ofCp*W(NO)(q38H11)H(3.7) inDMSO-d6 81Figure 3.8 500 IVIHz HETCOR spectra ofCp*Mo(NO)(rI43 enten2one), 3.9in DMSO-d6 87Figure 3.9 500 MHz COSY spectrum ofCp*Mo(NO)(q3 enten2one), 3.9 inDMSO-d6 88Figure 4.1 300 MHz 1H NMR spectrum ofCpMo(NO)(CH2Me)(0C6H4-p-Me) (4.2) in C6D 116Figure 4.2 300 MHz 1H NMR spectrum of the crude reaction mixture ofCpMo(NO)(=CHCMe3)(py)with succinimide in C6D 116xvFigure 4.3 75 MHz 13C{ ‘H) NMR spectrum ofCpMo(NO)(CH2e)[(OC(M ))H-O,O](4.13) in C6D. The insetrepresents the quaternary carbon peak at 190.0 ppm 124Figure 4.4 300 MHz 1H NMR spectrum of [CpMo(NO)(CH2e3)](j.t-OCH2CMeH)(4.14) in C6D. The inset represents an expanded view of thedoublets 124Figure 4.5 300 MHz 1H NMR spectrum of [CpMo(NO)(CH2e3)]-j.t-(rOC{O)C{O}O) (4.15) in CD21. The inset represents an expanded view of the 4sets of doublets 127Figure 4.6 75 MHz‘3C{1H) NMR spectrum of[CpMo(NO)(CH2C e3)]- .-(92-OC{O}C{O}O) (4.15) in CD21. The inset represents the quaternary carbonpeak at 203.5 ppm 128Figure 5.1 300 MHz 1H NMR spectrum of the reaction ofCpMo(NO)(CHe)(q-OC{O}Ph) (4.10) with o-nitrobenzoic acid in CDCI3 162Figure 5.2 300 MHz 1H NMR spectrum of the reaction ofCpMo(NO)(CHe)[(OC(M ))H-O,O](4.13) with benzoic acid in CDC13 163xviList of SchemesScheme 2.1 Proposed Mechanism ofFormation of Complexes 2.5 and 2.6 42Scheme 2.2 Proposed Mechanism ofFormation of Complex 2.7 43Scheme 2.3 Proposed Manner ofNitrosyl NO Bond Cleavage 44Scheme 3.1 Proposed Mechanism ofFormation of Complexes 3.1 and 3.2 70Scheme 3.2 Proposed Mechanism ofFormation of Complexes 3.5 and 3.6 77Scheme 3.3 Bonding Modes of Unsaturated Ketones 84Scheme 3.4 Proposed Mechanism ofFormation of Complex 3.11 90Scheme 4.1 Proposed Mechanism of Formation of Complex 4.14 126Scheme 4.2 Cis and Trans Addition ofREH with Complex 4.1-d1 134Scheme 5.1 Exchange Reactions of Complex 4.6 152Scheme 5.2 Exchange Equilibria 154Scheme 5.3 Exchange Reactions of Complex 4.10 160Scheme 5.4 Exchange Reactions of Complex 4.6-d1 165Scheme 5.5 Mechanism ofExchange of Complex 5.1-d2with p-Cresol 166Scheme 5.6 Associative Ligand Exchange Mechanism 167Scheme 5.7 Free Ion Ligand Exchange Mechanism 167xvList of AbbreviationsThe following list of abbreviations employed in this thesis are commonly used in thechemical literature.A angstrom, 10-10 macacH acetylacetonate (2,4-pentanedionate)anal, analysisAr arylatm atmospherebr broad (spectral)Bu butyl (superscript n or t refers to normal or tertiary)°C degree celsiuscarbon-13‘3C{’H} proton-decoupled carbon-13cal calories (1 cal=4.148J)calcd calculatedC6D benzene-d6CDC13 chloroform-d1CD21 dichloromethane-d2cm1 wavenumbers1 ,3-CHD 1,3 -cyclohexadiene1,3-COD 1 ,3-cyclooctadieneCOSY correlation spectroscopy (in NMR spectroscopy)Cp ri5-CH,perhydrocyclopentadienylCp* ri5-CMe,pentamethylcyclopentadienylCp’ both Cp and Cp*Cym cymene (4-isopropyl toluene)ö chemical shift in ppm referenced to Me4Si at ö 0xviiid doublet (in an NMR spectrum) or day(s)dmpe 1 ,2-bis(dimethylphosphino)ethanedppe 1 ,2-bis(diphenylphosphino)ethaneE heteroatom (0, S, N)El electron-impact (in mass spectroscopy)eq equation(s)equiv equivalent(s)esd estimated standard deviation (used in X-ray crystallography)Et ethyl (CH3C2)Et20 (CH3C2)0(ether or diethyl ether)g gramh Planck’s constant, or hour(s)proton2fl deuteriumenthalpy of activationHETCOR heteronuclear correlation (in NMR spectroscopy)Hz Hertz (s-i)JR infraredJ coupling constant (in NIVIR spectroscopy)flJ n-bond coupling constant between atoms A and BK degree KelvinU kilojoulekB Boltzmann’s constantkobs observed rate constantLUMO lowest unoccupied molecular orbitalm multiplet (in NMR spectroscopy)M Mo and W, or molar, or megam/z mass-to-charge ratio (in mass spectrometry)Me methyl (CH3)Mes mesityl (C6H2-2, 4, 6-Me)mg milligram(s)mm minute(s)mL millilitremmol millimmolemol moleMS mass spectrumv stretching frequency (in JR spectroscopy)neopentyl CH2Me3neophyl CH2MePhNMR nuclear magnetic resonanceORTEP Oak Ridge Thermal Ellipsoid Plotphosphorus-3 1[p]+ parent molecular ion (in mass spectrometry)Ph phenyl, C6H5ppm parts per million (in NMR spectroscopy)Pr propyl (C3H5)py pyridineq quartet (in an NMR spectrum)quat quaternary carbon atomR alkylRT room temperatures singlet (in an NIvIR spectrum)AS entropy of activationSCE standard calomel electrodet triplet (in an NMR spectrum)tetraglyme tetraethylene glycol dimethyl ether (Me(OCH2CH4OMe)o-tolyl C6H4-2-Me (ortho-tolyl)p-tolyl C6H4-4-Me (para-tolyl)THF C4H8O (tetrahydrofuran)THF-d8 C4D80UV ultraviolet (in electronic spectroscopy)vis visible (in electronic spectroscopy)xxxUAcknowledgementsI owe my thanks and gratitude to a number of people who, over these past four andhalf years, have made nitrosyl chemistry enjoyable (most of the time, at least) and possible.Peter Legzdins, thank you for your supervision, encouragement and support, as well ascreating a research environment that encourages individuality and freedom of expression.My thanks goes to past group members, in particular John Veitheer (for Chapter 4),Mike Shaw (always willing to lend a hand), JeffDebad (getting me started), Kevin Ross(A254 line companion), Eric Brouwer (for outside lunches), as well as former group postdocs, George Richter-Addo and Roser Reina, for insightful suggestions and help. Mypresent A246 lab-mates, Kevin Smith (always ready with ideas and references) and SteveMcNeil (our computer advisor) thanks for good humor, laughter and general silliness.Thanks Steve Sayers (the underwater explorer), Sean Lumb (the energetic adventurer) andElizabeth Tran for practical advice, help and conversation.My thanks goes to the UBC technical staff, especially Leanne Darge, MariettaAustria, Peter Borda and Marshall Lapawa, for providing some of the characterizationdata. Thank you Fred Einstein and Ray Batchelor (SFU) for solving the crystal structuresin this thesis, and Chris Brion and Alan Storr (UBC) for reading portions of this thesis andmaking suggestions for improvement.I am also grateful for financial support from the University of British Columbia in theform of a University Graduate Fellowship.Ursula and Mike Bothma, my parents, thanks for love and opportunities. And to myother parents, Jack and Gerda Young, thank you for making it financially possible to cometo Canada and always being interested in my work.Finally, my thanks goes to my husband, Brett, for love, friendship, and the Brodie.Thanks for supporting me though these years and keeping everything in perspective.xdiGet your facts firstthen you can distort ‘emas much as you please.Rudyard Kipling1CHAPTER 1Introduction1.1 General Introduction 21.2 Organotransition-Metal Chemistry 21.3 Organotransition-Metal Nitrosyl Complexes 31.4 Scope and Format of this Thesis 71.5 References and Notes 1021.1 General IntroductionThis thesis discusses the synthesis and reactivity of organotransition-metal complexes:complexes with organic groups bonded to transition metals through direct metal-carboninteractions. More specifically, the organotransition-metal complexes introduced in thisthesis are bound to a nitrosyl ligand.The introduction addresses the basic question ofwhy organotransition-metalcomplexes are studied, as well as the fundamental concepts related to organotransitionmetal nitrosyl complexes. Finally, a brief synopsis of the whole thesis is provided at theend of this chapter.1.2 Organotransition-Metal ChemistryThe past two decades have witnessed an explosive growth in the field oforganotransition-metal chemistry.1 Few other fields have experienced such significantdevelopment in a relatively short period. Although the first use of an organometallicreagent for organic synthesis was reported in 18482, it was the discovery of ferrocene3approximately 103 years later which triggered the rapid development of the organicchemistry of transition metals. This led to a wealth of new and interesting compounds,mechanistic ideas, and theoretical advances.The applications of organotransition-metal complexes are numerous. Many are usedas catalysts in industrial organic reactions such as Ziegler-Natta polymerizations, theWacker process, and Fischer-Tropsch synthesis, to name but a few.4 They often mediateinteresting organic transformations which do not occur in the absence of thesecomplexes.5 They also stabilize or trap extremely reactive organic fragments therebyallowing these fragments to be studied.6 A number of enzymatic processes involvetransition metals,7 and organotransition-metal complexes are used to mimic or modelthese systems in order to better understand their chemistry.8 Another important area of3research involving organotransition-metal complexes is the activation ofbonds of smallmolecules.9 The goal of activation chemistry is to use simple, abundant, and cheapmolecules (alkanes, for example) and transform them into more valuable molecules. 10Recently, organometallic dendrimers have been recognized as potentially important newmaterials with uses including acting as catalysts for multielectron transfer.11Apart from the above-mentioned practical applications of organotransition-metalcomplexes, the interaction of transition metals with organic ligands continually results innew reactivity patterns and mechanisms which provoke and challenge the interest ofacademic chemists.1.3 Organotransition-Metal Nitrosyl ComplexesThe formation of a transition-metal nitrosyl bond (M-NO) results in interestingchemistry at both the transition metal as well as at the nitrosyl ligand.12 Since anunderstanding of the bonding principles in free NO is essential for the understanding ofbonding in transition-metal nitrosyl complexes, a basic description of free nitric oxide isprovided below.1.3.1 Nitric OxideNitric oxide is a small, paramagnetic molecule which is thermodynamically unstablewith respect to N2 and 02.12 It has received recent fame as an important biochemical,13and in 1992 was named Molecule of the Year by the editors ofScience.14The major resonance forms ofNO, depicted below, may be used to understand thebonding in nitric oxide.+.:N=o4The resonance form ofNO in which the negative charge is localized on the nitrogenatom accounts for NO binding to transition metals through the nitrogen atom. Thebinding preference of nitrogen can also be explained by molecular-orbital theory. Themolecular-orbital diagram ofNO is shown in Figure 1.1. The a orbital (containing theelectrons that interact with the transition metal) is predominately N in character andaccording to this theory, the odd electron is based in a 1t*molecular orbital.2p-2sNatomic orbitalsa2*-12pZs0atomic orbitalsFigure 1.1 The molecular-orbital diagram of nitric oxide.1.3.2 The Binding of Nitric Oxide to Organotransition-MetalsNO binds to transition metals in many different modes.12 In this thesis mainly linear,terminal nitrosyl ligands are observed, although a few examples of nitrosyls bridging twometal centers are presented in Chapter 2. The bonding in linear M-NO linkages can be5described using Lewis-dot or molecular-orbital theories. The resonance forms (using theformer bonding description) extant in linear M-NO linkages are represented below.+ - + .. +M—NO MNO M=N=O 4 MN—o:.. •1..The molecular-orbital approach describes the bonding ofNO to a transition metal as asynergic interaction involving the following components: (a) the transfer of one electronfrom NO to the metal, (b) a-donation of two electrons from NO to the metal and (c) thebackdonation of electron density from the occupied metal d-orbitals into a(antibonding) orbital of the NO ligand. This latter interaction results in NO being a strongt-acid (even stronger than CO). This synergic interaction is represented in Figure 1.2.a-donation M ()N=== oM(a)— N(o)it-backdonationM(di) —, NO(lt*)Figure 1.2 The synergic interaction between a nitrosyl ligand and a transition metal.6Hence, in linear M-NO linkages, NO binds as NO (providing three electrons to themetal), whereas in bent linkages to transition metals, NO binds as NO- (providing oneelectron to the metal).1.3.3 18-Electron Rule and Oxidation StatesOrganometallic chemists often use an electron-counting formalism to describeorganometallic compounds. The 18-Electron Rule (or Effective Atomic Number Rule)arises from the fact that a filled transition metal’s valence shell will contain 18 electrons (9bonding molecular orbitals form from the interaction of the metal and ligand atomicorbitals). lb Thus, in general, 18-valence-electron organotransition-metal compounds arestable. The class of complexes, Cp’M(NO)R2(Cp’ = Cp or Cp*; M = Mo or W and R =alkyl), which form the basis of this thesis, exhibit a 16-valence-electron configuration (5e(Cp) + 6e- (M Mo or W) + 3e (NO) + 2e- (2 x R) = l6ej. Even though they defy the18-Electron Rule, these complexes are stable because the LUMO is non-bonding in natureand is much higher in energy than the HOMO.15Assigning formal oxidation states to organotransition-metal nitrosyl complexes isundesirable. The assignment does not reflect a true representation of the electron densityresiding on the metal center since the bonding ofNO to a metal can be rationalized in anumber of ways (vide supra).161.3.4 Synthesis and Characterization of Organotransition-Metal NitrosylComplexesNitrosyl complexes can be synthesized in a number ofways.12 The parent nitrosylcomplexes used in this thesis are derived from the reaction of the N-nitroso compound,Diazald (N-methyl-N-nitroso-p-toluenesulfonamide), with the tricarbonyl anion,Cp’M(CO)3 (M = Cr, Mo, W) (eq 1.1). Subsequent treatment of the molybdenum and7tungsten dicarbonyl nitrosyl complexes with PC15 results in the formation of the dichiorideprecursors, Cp’M(NO)(Cl)2.17[Cp’M(COhl- Dia1 Cp’M(NO)(CI)2 (1.1)Standard physical techniques, namely NIvIR and JR spectroscopy, mass spectrometry,elemental analysis, and X-ray crystallography are used throughout this thesis for theidentification and characterization of the new organotransition-metal nitrosyl complexessynthesized.JR spectroscopy is especially usefhl for characterizing metal-nitrosyl complexes. Theinfrared absorption of a nitrosyl ligand is a distinctive physical property which aids themonitoring of reactions of nitrosyl complexes. The value of VNO also reflects the degreeof it-backbonding from the transition-metal’s filled d-orbitals to the 7t*acceptor orbitalson the linear nitrosyl ligand. In other words, electron-rich metal centers donate moreelectron density into this 7t*acceptor orbital, thereby resulting in lower vNO values thanthose exhibited by electron-deficient metals.1.4 Scope and Format of this ThesisStudies of the reactivity of Cp’M(NO)X2complexes (Cp’ = Cp, Cp*; M = W, Mo;X = halide, alkyl or aryl) constitute the main focus of research in the Legzdin& group. Areview of this chemistry has appeared in the literature. 15,18 The work in this thesisextends this research and involves an investigation of the reactivity of the molybdenumbis(alkyl) complexes, Cp’Mo(NO)R2(R = CH2SiMe3,CH2Me3,CH2MePh).Chapter 2 deals with the reactivity of the bis(alkyl) complexes with molecularhydrogen. This chemistry results in the formation of bimetallic bridging-nitrosyl8complexes, [Cp*Mo(NO)R](,iNO)2which isomerize in solution to form products of thetype, [Cp*Mo(NO)R}(1.LN)[Cp*Mo(O)R]. These isomerizations are unique and involvethe activation of the NO ligand.The reactivity ofCp*Mo(NO)(CH2SIMe3)with molecular hydrogen in the presenceof a variety of substrates constitutes the focus of Chapter 3. While the transientmolybdenum alkyl hydride, Cp*Mo(NO)(CH2SIMe3)H,can be trapped with polarreagents (acetone and benzaldehyde), in the presence of other non-polar reagents bothalkyl ligands undergo hydrogenolysis and compounds of the type, Cp*Mo(NO)L2(L =PPh3, SPh) or Cp*Mo(NO)L’ (L’ =i4-diene, or cc f3-unsaturated ketone) are obtained.The hydrogenation ofCp*M(NO)(CH2S1Me3(M Mo, W) in the presence of 1,3-cyclooctadiene results in a novel coupling of the cyclooctadiene substrate to formCp*M(NO)(rl4i6H24).The bis(alkyl) complex, CpMo(NO)(CH2Me3)thermolyses at room temperaturein the presence of pyridine, to produce the alkylidene complex,CpMo(NO)(=CHCMe3)(py). The reactivity of this alkylidene forms the basis ofChapter 4. It reacts stereoselectively with a range of polar HER substrates (E = 0, S, N;R = alkyl or aryl) to form complexes of the type, CpMo(NO)(CH2Me3)(ER),with theloss of free pyridine.Finally, Chapter 5 is an extension of Chapter 4 and covers a variety of exchangereactions of the CpMo(NO)(CH2Me3)(ER)complexes generated in Chapter 4. Thesecomplexes undergo stereoselective exchange reactions with HER’ acids to formCpMo(NO)(CH2Me3)(E’R’)complexes and HER.This thesis is formatted with Chapters 2 through 5 having five major sections: X.1Introduction, X.2 Experimental Procedures, X..3 Results and Discussion, X.4 Epilogueand X.5 References and Notes (where X corresponds to the chapter number).9Subsections of these categories are numbered using legal outlining procedures, e.g. X.1.1,X.1.1.1, X.1.2, X.1.2.1 etc. All new compounds prepared are catalogued numerically ineach chapter. For example those in Chapter 3 are referred to as 3.1, 3.2, 3.3 and so on.Schemes, tables, figures and equations are similarly sequenced. The standardmethodologies employed throughout this thesis are described in detail in Chapter 2,section References and Notes(1) There are many books on the topic of transition-metal organometaflic chemistry.For example, (a) Crabtree, R. H. The Organometaiic Chemistry of the TransitionMetals; John Wiley & Sons: New York, NY, 1994. (b) Coilman, J. P.; Hegedus, L.S.; Norton, J. R.; Finke, R. G. Principles andApplications ofOrganotransitionMetal Chemistry; University Science Books: Mill Valley, CA, 1987.(2) Frankland, E. J. Chem. Soc. 1848, 2, 263.(3) Kealy, T. J.; Pauson, P. L. Nature 1951, 168, 1039.(4) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: the Applications andChemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.; WileyInterscience: New York, NY, 1992.(5) For examples of transition-metal mediated ROMP (ring-opening-olefin-metathesis)polymerizations, see: Grubbs, R. H. In Comprehensive Organometallic Chemistry;Wilkenson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: New York, NY,1982; Vol.8, Chapter 54.(6) (a) For example, the high-energy benzyne fragment is stabilized by tantalum in thecomplex, Cp*Ta(Me)2(C6H4).McClean, S. J.; Schrock, R. R.; Scharp, P. R.;Churchill, M. R.; Youngs, W. J. J. Am. Chem. Soc. 1979, 101, 263. (b) Silenecomplexes have also been stabilized by electron-rich transition metals, see:Campion, B. K.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1990, 112, 4079.(7) Bertini, I.; Gray, H. B.; Lippard, S. J.; Valentine, J. S. Bioinorganic Chemistry;University Science Books: Mill Valley, CA, 1994.(8) Researchers hope that, for example, organotransition metal-amido complexes, willbe useful models for nitrogen fixation. See, for example: Glassman, T. E.; Vale, M.G.; Schrock, R. R. J. Am. Chem. Soc. 1992, 114, 8098.11(9) For recent references on C-H bond activation see: (a) Gutiérrez, E.; Monge, A.;Nicasio, M. C.; Poveda, M. P.; Carmona, E. J. Am. Chem. Soc. 1994, 116, 791.(b) Debad, 3. D.; Legzdins, P.; Lumb, S. A.; Batchelor, R. J.; Einstein, F. W. B. J.Am. Chem. Soc. 1995, 117, 3288.(10) For example, a methylene group (CH2)can be abstracted from a methyl group(CH3), using a Rh complex, and inserted into a variety ofbonds, such as Si-H, Si-Siand C-H. Gozin, M.; Aizenberg, M.; Liou, S-Y.; Weisman, A.; Ben-David, Y.;Milstein, D. Nature 1994, 370, 42.(11) For a recent review of dendrimers, see: Issberner, J.; Moors, R.; Vogtle, F. Angew.Chem., mt. Ed. Engi. 1994, 33, 2413.(12) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University Press: NewYork, NY, 1992.(13) (a) Snyder, S. H.; Bredt, D. S. ScientfIc American 1992, 266(5), 68. (b)Feldman, P. L.; Griffith, 0. W.; Stuehr, D. J. Chem. Eng. News 1993, 71(50), 26.(c) Galla, H-J. Angew. Chem., mt. Ed. Engi. 1993, 32, 378. (d) Young, S. NewScientist 1993, 137, 36. (d) Snyder, S. H. Science 1992, 257, 494.(14) Koshland, D. E., Jr. Science 1992, 258, 1861.(15) Legzdins, P.; Veltheer, J. E. Ace. Chem. Res. 1993, 26, 41.(16) Enemark and Feitham have devised a notation to describe metal nitrosyls whichallows for the prediction of reactivity patterns, but is especially usefhl in predictingM(NO) geometries, see: Enemark, J. H.; Feitham, R. D. Coord. Chem. Rev.1974, 13, 339.(17) Dryden, N. H.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B. Organometallics1991, 10, 2077.(18) Legzdins, P.; Richter-Addo, G. B. Ace. Chem. Res. 1988, 88, 989.12CHAPTER 2Reactions of Cp*Mo(NO)R2Complexes with Molecular Hydrogen2.1 Introduction 132.2 Experimental Procedures 172.3 Results and Discussion 272.4 Epilogue 452.5 References and Notes 47132.1 IntroductionThe reactions of molecular hydrogen with organotransition-metal alkyl complexesusually result in the formation of metal hydride complexes. For example, the zirconiumalkyl halide complex, Cp2Zr(Cl)R, reacts with molecular hydrogen to form alkane and thehydride complex, Cp2Zr(Cl)H (eq 2. 1)1 This type of dihydrogen activation is referred toas hydrogenolysis. The hydrogenolysis of metal-alkyl bonds also produces dimericbridging hydride species (eq 2.2).2Cl — ,ClZr 2 Zr” (2 1)R= alkyl_______Th Th (2.2)The reactivity of the dialkyl tungsten nitrosyl complex, Cp*W(NO)(CH2SiMe3)towards molecular hydrogen was studied a few years ago by Legzdins and coworkers.3They found that the hydrogenation of Cp*W(NO)(CH2SiMe3)at low pressures producedno isolable products. Higher pressures of molecular hydrogen (60 atm), however, resultedin a mixture of two bridging dihydride tungsten species, namely, [Cp*W(NO)Hj2(I1H)14and [Cp*W(NO)H](,.LH)2p*W(NO)(CSiMe3)](eq 2.3).60 tm H/W\20min (2.3)ONRRR = CH2SiMe3This chapter deals with the reactions of the dialkyl molybdenum complexes,Cp*Mo(NO)R2(R = CH2S1Me3,CH2Me3,CH2MePh), with molecular hydrogen.Molybdenum hydride complexes are relatively rare, and this investigation was partlyprompted with the hope of isolating an elusive molybdenum nitrosyl hydride complex.Unfortunately, during this investigation no molybdenum hydrides were isolated. However,the reactions of Cp*Mo(NO)R2with molecular hydrogen do yield novel bimetallic oxobridging-nitrido species of the type [Cp*Mo(NO)R](p.N)[Cp*Mo(O)R] which result fromnitrosyl N-O bond cleavage. A closer investigation of this reaction resulted in the isolationof intermediate bimetallic bridging-nitrosyl complexes, [Cp*M0R]2(I.L_NO) Thesebridging-nitrosyl species are thermally unstable and isomerize in solution in anunprecedented manner to form the bimetallic oxo bridging-nitrido species,[Cp*Mo(NO)R](.iN)[Cp*Mo(O)R].2.1.1 Nitrosyl N-O Bond CleavageThe surface chemistry of nitric oxide has always received considerable attention dueto its relevance in environmental chemistry.4 NO activation has been reported to occurboth in the gas-phase ion chemistry of metal clusters5 as well as in the condensed phase.6The degradation of the dinitrosyl complex, CpCr(NO)21, when observed by mass15spectroscopy shows fragmentation patterns consistent with nitrosyl N-O bond cleavage.7Recently, tandem mass spectrometry has shown thatCp2Fe(NO)+ undergoes nitrosylN-O bond cleavage with the resulting fragments being pyridine and CpFe2O.8 Therehave also been reports on the conversion of terminal nitrosyl ligands into terminal oxo andnitrido linkages induced by Lewis acids.9 Although nitrosyl metal cluster complexes havebeen shown to undergo dissociation of nitrosyl N-O bonds to yield mtrido clusters,10 thereis a scarcity of reports in the literature concerning monomeric or dimeric nitrosylorganometallic systems undergoing nitrosyl N-O bond dissociation.The bimetallic oxo bridging-nitrido complexes, [Cp*Mo(NO)R](l.tN)[Cp*Mo(O)R],presented in this chapter, result from nitrosyl N-O bond cleavage. In studying thereactivity of Group 6 metal nitrosyl complexes, the Legzdins’ group has generally foundthat the nitrosyl ligand remains intact during the various reactions involving thesecompounds. Recently, however, more and more examples of nitrosyl N-O bond cleavagereactions (under a variety of unrelated conditions) have been encountered.-1The first example of nitrosyl N-O bond cleavage was recognized in the unusual water-catalyzed isomerization of the diary! complex, CpW(NO)(o-tolyl)2to the oxo aryl imidocomplex, CpW(O)(o-tolyl)(N-o-tolyl).12Nitrosyl N-O bond cleavage products have alsobeen identified from the reaction of Cp*W(NO)(CH2SiMe3)Clwith either KOMe orKOCMe3 (eqs 2.4 and 2.5).132 W 2 KOCMe3,pentane W—N—W (2.4)NCISiMe3R = CJI2SiMe316N” I\R = CH2SIMe3Zinc reduction of the alkyl chloride complexes, Cp*W(NO)(Cl)R (R = CH2SIMe3,CH2Me3,Ph), results in the isolation of bimetallic oxo bridging-nitrido tungstencomplexes, [Cp*W(NO)Rj(p.N)[Cp*W(O)Rj.l4Thermolysis of Cp*W(NO)Ph2produces three products, all ofwhich formally result from nitrosyl N-O bond cleavage (eq2.6). 15An example of nitrosyl N-O bond cleavage in a chromium nitrosyl complex is thereaction of CpCr(NO)(THF)I with the sterically demanding Grignard reagent (mes)MgBrand trace amounts of molecular oxygen to produce CpCr(mes)(N-mes)(O).16Mostrecently, the Legzdins’ group reported that hydrolysis of the bis(acetonitrile) salt,[Cp*W(NO)(CH2SiMe3)(NCM )1BF4with one equivalent of water results in theformation of the isolable hydroxylamido containing salt, [Cp*W(O)(112KOMe, pentaneo’j%o (2.5)NIPhPh55°CC6H+Ph+ (2.6)2.2 Experimental Procedures172.2.1 MethodsThe methodologies described in this chapter apply to the entire thesis. All reactionsand subsequent manipulations involving organometallic reagents were performed underanaerobic and anhydrous conditions in an atmosphere of purified argon or dinitrogen.Purification of inert gases was achieved by passing them first through a column containingMnO and then a column of activated 4A molecular sieves.Solvents were freshly distilled from the appropriate drying agents under a dinitrogenatmosphere and either purged for 10 mm with argon prior to use or were directly vacuumtransferred from the appropriate drying agent. Tetrahydrofliran and diethyl ether weredistilled from sodiumlbenzophenone; hexanes, toluene and pentane were distilled fromsodiumlbenzophenone/tetraglyme; dichloromethane was doubly distilled fromP205;acetone and pyridine were doubly distilled from CaH2.18 C6D and C7D8were dried overactivated 4A molecular sieves, degassed using 3 freeze-thaw-pump cycles and filteredthrough Celite before use. CDC13 was dried onP205 for two days and then filteredthrough a column of neutral alumina 1 and degassed.Conventional glovebox and vacuum line Schienk techniques were utilizedthroughout.19 The gloveboxes used in this work were Vacuum Atmospheres HE-553-2and HE-43-2 models as well as a two-station Braun Labmaster 130 glovebox. IR spectrawere recorded on either an ATI Mattson Genesis Series FT-JR or a Nicolet 5DX FT-JRinstrument. All samples were recorded as Nujol mulls sandwiched between NaCl plates oras KBr pellets. All NMR spectra were recorded in parts per million on a VarianAssociates XL-300 or Bruker AMS-500 spectrometer. 1H NMR spectra are referenced tothe residual proton signal ofC6D (ö 7.15), CD21 (ö 5.34), CDC13 (ö 7.24), DMSO-d6(ö 2.50), or TI-d6 (6 3.58, 1.73). 31P{1H) NMR spectra (121.42 MiJz are referencedto external P(OMe)3 set at 8 141.00 relative to 85%H3P04.‘3C{’H} NMR spectra18(75.43 M1{z) are referenced to the natural abundance carbon signals of the solventemployed: C6D( 128.00), CDCI3 (ö 77.00) or CD2I (8 53.80). Ms. M. T. Austriaand Ms. L. K. Darge assisted in obtaining some of the NIvER data. Mass spectra wererecorded by the staff of the mass spectrometry laboratory. Low-resolution mass spectra(El, 70 eV) were recorded on a Kratos MS5O spectrometer using the direct-insertionmethod. All elemental analyses were performed by Mr. P. Borda. X-ray crystallographicanalyses were performed by Drs. R. J. Batchelor and F. W. B. Einstein of Simon FraserUniversity.2.2.2 ReagentsThe organometallic reagents, namely Cp*Mo(NO)(CH2e3)0Cp*Mo(NO)(CHePh,20Cp*Mo(NO)(CHSiMe21Cp*W(NO)(CH2S1Me32and Cp*Mo(NO)(pto1yl)23were prepared by published procedures. H2 (Linde, extradry) was used as received. PPh3 (Aldrich) was recrystallized from hexanes. Pyridine(Aldrich) was distilled before use. PMe3 was dried over and transferred fromsodium/benzophenone. The column chromatographic material employed during this workwas Florisil (60-100 mesh, Fisher). Filtrations were performed through Celite 545diatomaceous earth (Fisher) that had been oven-dried and cooled in vacuo. All frits usedwere of medium porosity.2.2.3 SynthesisIsolated yields, physical properties, and spectroscopic data for all new complexes arelisted in Tables 2.1- Preparation of ICp*Mo(NO)(CII2Si e3)]O.L_N)[Cp*Mo(O)(CH](2.1)A purple solution of Cp*Mo(NO)(CH2SIMe3)(410 mg, 0.94 mmol) in C6H (20mL) was exposed to an atmosphere ofH2 at 5 °C. The reaction mixture was stirred whilebeing permitted to warm to room temperature over 1 h, during which time its colorchanged to red-brown. Solvent was removed from the final mixture in vacuo to obtain ared-brown residue which was extracted with hexanes (2 x 15 mL). The combined extractswere concentrated under reduced pressure and then chromatographed on a Florisil column(3 x 8 cm) using hexanes/Et20(2:1) as eluant. A green band which developed wascollected, and solvent was removed from the eluate in vacuo to obtain a green residue.This residue was dissolved in a minimum of pentane, and the solution was maintained at-8 °C for 4 weeks to induce the deposition of analytically pure 2.1 (130 mg, 40% yield). Preparation of [Cp*MoR]2(p.NO) [R = CH2Me3(2.2), C}IMePh(2.3)]The preparation of [Cp*Mo(CH2e3)](,.tNO)(2.2) is given as a representativeexample. C6H (20 mL) was vacuum transferred onto a solid sample ofCp*Mo(NO)(CH2e3)(405 mg, 1.00 mmol) and then the solution was reacted withH2 (1 atm) at 5 °C. The stirred reaction mixture was warmed slowly to room temperature(20 mm) during which time its color changed from red to bright green. The solvent wasremoved in vacuo. The resulting green-blue residue was then washed with pentane(5 mL). The washed residue was then dissolved in Et20 (30 mL) and transferred to thetop of a Florisil column (2 x 2 cm) supported on a flit. The column was washed with Et20(2 x 20 mL). The eluate was then taken to dryness and the resulting residue wasredissolved in a minimal amount of toluene. Cooling of the solution overnight induced the20formation of blue crystals of 2.2 (85 mg, 26% yield). Complex 2.3 was prepared in 33%yield in a similar manner from Cp*Mo(NO)(CHePh2.2.3.3 Preparation of [Cp*Mo(CHSi e3)1(Ij,NO)W(](2.4)A C6H (20 mL) mixture of Cp*Mo(NO)(CH2S1M (435 mg, 1.00 mmol) andCp*W(NO)(CH2SiMe3)(523 mg, 1.00 mmol) was reacted with H2 (1 atm) at 5 °C. Thestirred reaction mixture was warmed slowly to room temperature (20 mm) during whichtime its color changed from purple to blue. The solvent was removed in vacuo, and theresulting blue powder was then washed with Et20 (2 x 10 mL). This powder was notrecrystallized since it was found to be analytically pure 2.4 (181 mg, 30% yield). Preparation of [Cp*Mo(NO)R](ij.N)[Cp*Mo(O)R] jR = CH2Me3(2.5),CII2MePh(2.6)1The preparation of [Cp*Mo(NO)(CHe3)]Q.tN)[Cp*Mo(O)(CH2CM3)](2.5) isgiven as a representative example. A toluene or Et20 solution of [Cp*Mo(CHe(j.t-NO)2 (100 mg, 0.14 mmol) was left at room temperature overnight, during which time.a color change from blue to brown occurred. The brown solution was then taken todryness in vacuo. The brown residue was suspended in pentane (20 mL) and filteredthrough Florisil (2 x 2 cm) supported on a fit. Cooling of a concentrated solution of thisfiltrate at -30 °C for 4-6 weeks resulted in the precipitation of 2.5 as a brown powder.Complex 2.6 was prepared similarly. Based on 1H NMR spectroscopy, theconversions of 2.2—* 2.5 and 2.3 —* 2.6 were quantitative. Preparation of [Cp*W(NO)(CIISiMe)](p.N)[Cp*Mo(O)(CII2Si e3](2.7a) and [Cp*Mo(NO)(CRSi e)](I.t_N)[Cp*W(O)(CH1(2.7b)21Complexes 2.7a and 2.7b were prepared in a manner analogous to that of the Modimeric species, complexes 2.5 and 2.6. Isomers 2.7a and 2.7b, however, could be easilycrystallized (although not separated) by cooling the final concentrated pentane solutionovernight. As before, based on 1H NIvIR spectroscopy, the conversions of 2.4 —* 2.7aand 2.7b were quantitative.2.2.4 Reaction of Cp*Mo(NO)(pto1yI)2with H2This reaction was performed in a manner identical to that outlined in section substituting the appropriate diaryl precursor. Workup yielded extremely low yields(<5%) of [Cp*Mo(ptolyl)]2( NO.lB. (Nujol mull): 1366 cm’ (s). 1HNMR (C6D): ö 6.55 (m, 411,C6H4-Me), 2.05(s, 311,C6H4-Me), 1.62 (s, 1 5H, C5Me). Low-resolution mass spectrum (probetemperature 120 °C): m/z 704 [P].A blue C6D solution of [Cp*Mo(ptolyl)]2(,.tNO left in an NMR tube for 2 hchanged to brown. The 1H MvIR spectrum after 12 h was consistent with the quantitativeconversion of [Cp*Mo(ptolyl)j2(,.tNO to [Cp*Mo(NO)(ptolyl)j(N)[Cp*Mo(O)(ptolyl)].1HNMR (C6D): 6 7.82, 7.61, 7.10 (m, 8H,C64-Me), 2.22 (s, 6H,C64-Me),1.67, 1.66 (s, 2 x 15H, C5Me).2.2.5 Reaction of [Cp*Mo(CH2e3)](.tNO) (2.2) with[Cp*Mo(CHePh)]4iNO)(2.3)22An equimolar mixture of [Cp*Mo(CH2e3)](,.LNO) (2.2) (20 mg, 0.03 mmol)and [Cp*Mo(CH2ePh)j(.tNO)(2.3) (24 mg, 0.03 mmol) were dissolved in C6D(0.6 mL) in an NMR tube. Over the course of two hours a color change from blue tobrown was observed. The 1H NMR spectrum taken 12 h later revealed peaks attributableto a mixture of[Cp*Mo(NO)(CHPh)](pN)[Cp*Mo(O)(CHh)](2.6) and[Cp*Mo(NO)(CH2e3)](p.N)[Cp*Mo(O)(Cj(2.5).2.2.6 Reactions of [Cp*Mo(CH2ePti)](I. NO) (2.3) with L (L = PPh3,PMe3, pyridine)The reaction of[Cp*Mo(CHePh)](pNO)(2.3) with PPh3 is given as arepresentative example. The solid mixture of[Cp*Mo(CHePh)]($.iNO)(2.3)(0.30 mg, 0.038 mmol) and PPh3 (20 mg, 0.076 mmol) was dissolved in C6D (0.6 mL) inan NMR tube. Over the course of two hours a color change from blue to brown occurred.The 1H NMR spectrum taken 12 h later revealed peaks attributable to free PPh3 and[Cp*Mo(NO)(CH2ePh)](I.IN)[Cp*Mo(O)(CHh)](2.6).The other reactions were performed similarly, except that an excess ofPMe3 wasvacuum transferred onto a C6D solution of [Cp*Mo(CH2ePh)](pNO)(2.3).2.2.7 Kinetic MeasurementsThe conversion of 2.3 to 2.6 was studied in order to gain some insight into themechanism operative in this conversion. A quantity (3 mg, 0.004 mmol) of[Cp*Mo(CH2ePh)](.tNO)(2.3) was weighed into a 10-mL volumetric flask in adrybox. The flask was filled with toluene (0.0004 M solution) and shaken. An aliquotwas transferred to a 1.00-cm UV-vis spectrophotometer cell equipped with a 4-mm Teflonstopcock. The cell was placed in the cell holder of a Hewlett-Packard 8542A diode arrayspectrometer. The temperature of the cell holder was maintained constant (± 0.1 °C) by a23Haake W19 temperature bath equipped with a Haake D8 temperature controller or aVWR 9501-1156 temperature bath with a digital temperature controller. The solutionwas left to equilibrate (300 s) with the temperature of the water bath before spectra wererecorded. Spectra were then recorded at regular intervals, and data were collected for atleast 3.5 half-lives. The absorbance values at infinity were computer optimized. The rateconstants (k) were then calculated from plots of 1n(At-A) versus time (in seconds).zH1 and AS were determined from the Eyring plot of ln(kOb/T) versus lIT, where= -R(slope) and tS = R[intercept- ln(kB/h)j, and R, kB and h are the gas constant,Boltzmann’s constant and Planck’s constant, respectively.24Table 2.1 Numbering Scheme, Color, Yield, and Elemental Analysis Datacomp coloranal. found (calc)complex no. (yield, %) C H N[Cp*Mo(NO)(CH2Si e3)](j. ) 2.1 brown-red 48.40 (48.25) 7.70 (7.54) 3.99 (4.02)[Cp*Mo(O)(CHSi e)J (40)[Cp*Mo(CHe](.tNO) 2.2 blue (26) 54.42 (54.20) 8.03 (7.90) 4.23 (4.21)[Cp*Mo(CH2ePh)](.tNO) 2.3 blue (33) 60.67 (60.90) 7.10 (7.17) 3.40 (3.55)[Cp*Mo(CHSi e3)j(j.tNO) 2.4 blue (30) 42.85 (42.85) 6.74 (6.69) 3.45 (3.57)[Cp*W(CH2SiMe)][Cp*Mo(NO)(CHe)]Q ) 2.5 brown (lOO)b 54.03 (54.20) 7.83 (7.90) 4.03 (4.21)[Cp*Mo(O)(CHe3][Cp*Mo(NO)(CH2ePh)j(.t.N) 2.6 brown (lO0)’ 61.07 (60.90) 7.23 (7.17) 3.53 (3.55)[Cp*Mo(O)(CHePh)1[Cp*M1(NO)(CHSi e3)](I.LN) 2.7 brown-red 42.87 (42.85) 6.86 (6.69) 3.57 (3.57)[Cp*M2(O)(CHSiMe)]a (100)a A mixture of isomers: 2.7a, M1 = W (60%); 2.7b, M1= Mo (40%).b By 1H NMR. (based on the corresponding bimetallic bridging NO precursor).Table 2.2 Selected Mass Spectral and Infrared Datacomp MS tempo IR (Nujol mull)no. (°C) VNO other strongbands2.1 698 [Pj 120 1581 (br) 824, 829, 8002.2 664 LPj 100 1332 (vs)2.3 788 [Pj 120 1339 (vs)2.4 784 [P] 150 1314 (br) 950, 930, 909,845, 8292.5 664 [Pj 100 1591 (vs) 919, 8412.6 788 [Pj 120 1592 (vs) 919, 842, 8012.7 784 [P] 150 1576, 1555 (br) 997, 978, 956,933, 917a Probe temperatures.b Values for the highest intensity peak of the calculated isotopic cluster(98Mo and 184W).25Table 2.3 NMR Data (C6D)comp 111 NMR (6) ‘3C{1H} NMR (6)no.2.1 1.75 (s, 15H, Cf/vie5) a1.65 (s, 15H, C5Me)0.55 (s, 9H, SiMe3)0.40 (s, 911, S1Me3)0.93 (br s, 2H, CH2)0.00 (br s, 211, CH2)2.2 1.68 (s, 1511, C5Me) 112.1 (C5Me)’1.19 (s, 9H, CH2Me3) 62.5 (CH2)1.13 (br s, 2H, ClEf2) 38.3 (CMe)35.4 (CMe)10.2 (Cf/vIe5)2.3 7.42 (m, 211, Ph) 155.3, 127.6, 126.1, 124.8 (Ph)”7.21 (m, 2H, Ph) 112.5 (CMe)7.08 (m, 1H, Ph) 61.0 (CH2)1.60 (s, 1511, C5Me) 43.6 (CMe2Ph)1.46 (s, 2H, CH2) 33.9 (CMePh)1.34 (s, 6H, CHMePh) 10.1 (C5Me)2.4 1.72, 1.68 (s, 1511, C5Me) a1.52, 0.21 (s, 9H, CH2SiMe30.27, 0.24, 0.01, -0.20 (s, 1H, CH2)2.5 2.23 (d, 2H, CH2 HH 11.6Hz) 115.8, 112.4 (C5Me)1’1.86 (s, 15H, C5Me) 87.4, 59.6 (CH2)1.83 (s, 15H, C5Me) 50.5, 38.2 (CMe3)1.34 (s, 911, CH2Me3) 33.5, 33.5, 33.4 (CMe3)1.16 (s, 9H, CH2A’Ie3) 10.9, 9.9 (C5Me)1.04 (d, 2H, CH2 HH = 11.6 Hz)2.6 7.59 (m, 2H, Ph) 155.2, 155.1, 129.1, 128.3, 127.9, 127.8,7.28 (m, 5H, Ph) 126.0, 125.4, 124.9, 124.7 (Ph)”7.10 (m, 3H, Ph) 115.7, 112.4 (C5Me)2.90 (d, 2H, CH2 HH = 11.3 Hz) 73.1, 50.6 (CH2)1.78 (s, 1511, C5Me) 44.5, 40.3 (CMe2Ph)1.73 (s, 15H,C5Me) 32.5, 31.5, 29.8 (CMe2Ph)1.47 (s, 6H, CH2MePh) 11.1, 10.5 (CMe)1.45 (s, 6H, CHMePh)1.30 (d, 211, CH2 HH = 11.3 Hz)a Not recorded.b Recorded in CDCI3.Ratio of isomers 60:40.262.7 189A’ ‘•85B’‘82A’ ‘80B (s, 1511, C5Me)c 116.1, 115.3, 112.5, 111.4 (C5Me)°41A’°39B’°28B’°26A (s, 9H, CH2SIMe3) 41.0, 28.9, 15.4, 15.0 (CH2)1.27, 0.84, 0.69, 0.10, 0.09, 0.04 (d, 211, CH2 11.2, 11.1, 10.1 (C5Me)HH = 12.6 Hz) 3.7, 3.3, 2.7, 2.5 (CHSiMe3272.3 Results and DiscussionThe reactivity of the molybdenum dialkyl species, Cp*Mo(NO)R2with molecularhydrogen is discussed in this chapter. Since intermediate complexes are obtained when thealkyl group is either neopentyl or neophyl, these hydrogenations are discussed separately.2.3.1 Hydrogenation of Cp*Mo(NO)(CH2SiMe3)Exposure of a purple solution ofCp*Mo(NO)(CH2SiMe3to 1 atm of molecularhydrogen results in an immediate color change to brown. Workup of the reaction mixtureleads to the isolation of large brown crystals in moderate yield. Mass spectral data andelemental analyses for these crystals suggest that the product is a dimeric species ofmolecular formula [Cp*Mo(NO)(CH2Si e3)].The ‘H NMR spectrum of the productcontains peaks attributable to two Cp* signals, two methyl signals (SiMe3)as well as twobroad signals due to the methylene protons of the trimethylsilylmethyl ligand. Thespectroscopic data are thus inconsistent with the complex being a symmetric dimer. Sincethe molecular structure of the product could not be unambiguously determined usingspectroscopic and physical techniques, an X-ray crystallographic analysis was performed24and the resulting ORTEP diagram is shown in Figure 2.1. The complex formed from thehydrogenation of Cp*Mo(NO)(CH2SiMe3)is the bimetallic oxo bridging-nitrido species,[Cp*Mo(NO)(CHSi e)J(p.N)[Cp*Mo(O)(C](2.1) (eq 2.7).HR272’ 6 6—0NIR-2RHR = CH2SIMe3 2.128Figure 2.1 View of the solid-state molecular structure of[Cp*Mo(N0)(CH2Si e3)](j.t_N)[Cp*Mo(O)(CHS1 e3](2.1) (50% thermal ellipsoids); including selected bondlengths and angles (with esds in parentheses).____N(i)N(2)Mo(2)0(2)Si(1)Si(2)Mo(1)-N(2)-Mo(2) 169.6 (2)N(2)Mo(1)Cp*a 122.2N(2)Mo(2)Cp*a 118.6Mo(1)-N(1)-O(1) 166.2 (4)Mo(1)Bond Lengths (A)Mo(1)-N(2) 1.908 (3)Mo(2)-N(2) 1.8 12 (3)Mo(2)-0(2) 1.714 (3)Mo(1)-N(1) 1.769 (4)N(1)-0(1) 1.203 (5)Bond Angles (deg)a Refers to centroid of Cp* ligand29Complex 2.1 was the first bimetallic bridging-nitrido nitrosyl complex to bestructurally characterized. Subsequently, a tungsten analogue,[Cp*W(NO)(CH2SiMe3)](.LN)[Cp*Mo(O)Cl], was also structurally characterized.14There are numerous reports in the literature of structurally-characterized asymmetricbridging-nitrido complexes,25while symmetric isomers are relatively rare.26Complex 2.1 contains a nearly linear nitrido bridge, the Mo(1)-N(2)-Mo(2) anglebeing 169.6 (2)°, which implies that the nitrogen’s lone pair of electrons is also utilized formultiple bonding in the bridge. In the tungsten analogue mentioned above,[Cp*W(NO)(CHMe3)](I.tN)[Cp*W(O)Cl], the W-N-W angle is 177.4 (3)°. TheMo(1)-N(2) bond length is 1.908 (3) A, while that ofMo(2)-N(2) is 1.8 12 (3) A. Thesebond lengths fall in the range between double and single molybdenum-nitrogen bonds.27Since each molybdenum center contains different ligands, the Mo(1)-N(2)-Mo(2) bridge isunsymmetrical, and this is reflected in the different bond lengths. In the molybdenumanion, [Mo(py)2Cl3](.t-N), however, the two Mo-N distances involving the nitridoligand differ by 0.1 A [Mo(1)-N: 1.887 (10) A and Mo(2)-N: 1.793 (10) A] which isunusual for a symmetrically disposed nitrido bridge.28 The two Cp* ligands in 2.1 areorthogonal, with a dihedral angle about the Mo(1)-N(2)-Mo(2) link of 99.4°. Theorthogonal relationship of the Cp* ligands suggests that the Mo-N-Mo bondinginteraction involves orthogonal p orbitals on the central N atom for the formation of the itbonds to the metals in a manner analogous to that found for related oxo-bridged species.21The Cp* ligands are similarly orthogonally disposed in the tungsten analogue with adihedral angle of 101°. The molybdenum-oxo link is of typical double-bond length(1.714 (3) A).29The IR spectrum of 2.1 shows a broad peak (1581 cml) due to the nitrosyl ligand aswell as sharp molybdenum oxo and molybdenum nitrido peaks (824, 829, 800 cm1).Typical VM0,O bands occur in the region 800 - 900 cm1; for example, the asymmetric and30symmetric vM.,rj bands ofCp*Mo(O)2(CHSi e3)exist at 912 and 893 cm’.° Theassignment of the three bands of 2.1, however, to specific vibrational modes is impossiblewithout conducting a labeling study. For example, labeling of [Cp*WMe3]2QiN)with15N results in a shift of the VV to a lower energy (800 —> 789 cm-1).31 The tungstenanalogue, [Cp*W(NO)(CH2SiMe3)](.tN)[Cp*W(O)(C],shows only two peaksin this region (962, 891 cm’) although all the other spectroscopic properties of 2.1 aresimilar to its tungsten analogue.’42.3.2 Hydrogenation of Cp*Mo(NO)R2(R = neopentyl or neophyl)If the bis(neopentyl) or bis(neophyl) complex, Cp*Mo(NO)R2is reacted with 1 atmof molecular hydrogen and the solution is worked up after 20 mm, thermally unstable bluecomplexes (2.2 and 2.3, respectively) are isolable in crystalline form in 26- 33% yield.Elemental analyses indicate that the crystals of 2.2 and 2.3 have the molecular compositionCp*Mo(NO)R, and low-resolution mass spectra display features which suggest that thecomplexes are dimeric. Consistent with the dimers being symmetric, the ‘H and 13CNIvIR spectra of the blue crystals exhibit signals attributable to one equivalent Cp* andone equivalent R group, and their Nujol mull JR spectra display VNO bands at 1332 cm-1(2.2) and 1339 cm4 (2.3) which is in the region expected fori2-NO or bridging NOligands.32 The solid-state molecular structure of 2.2 has been established by a single-crystal X-ray crystallographic analysis and is shown as an ORTEP diagram inFigure 2. The most chemically interesting features of the molecular structure are itscis geometry of the Cp* ligands and its symmetrical Mo(p.-NO)2obridging systemwhose intramolecular dimensions resemble those extant in [CpCr(NO)](ji,-N34(CpFe)2Qt-NO,35and (CpCo)2Qi-NO.36 In particular, the Mo-Mo separation of2.5930 (7) A in 2.2 is indicative of the existence of a relatively short single metal-metal31bond.37 Each metal center in 2.2 may thus be viewed as having a formal 16-valence-electron configuration.Both complexes 2.2 and 2.3 isomerize in solution within 2 h at ambient temperaturesto the bimetallic oxo bridging-nitrido species, [Cp*Mo(NO)RJ(N)[Cp*Mo(O)R](R = CH2Me3(2.5), R = CH2MePh(2.6)), in which the metal centers have attainedthe favored 18-valence-electron configurations (eq 2.8). Figure 2.3 contains1HNMRspectra showing the disappearance of peaks due to complex 2.3 over the course of 4.5 hand the appearance of peaks due to complex 2.6.H2, C6H/R0NIR21WR\(2.8)R = CH2Me (2.2) R CH,CMe3(2.5)R = CHMePh(2.3) R = CH2MePh(2.6)These isomerizations involving the conversion of bridging-nitrosyl ligands to their oxoand nitrido constituents are unique. Wolczanski and coworkers have reported a noveldissociation of a CO ligand during the thermolysis (120 °C for 4 h) of[(silox)2W(Cl)(CO ] which affords the oxo-p.-carbido complex,(silox)O)W=C=W(C1).3832Figure 2.2(a) View of the. solid-state molecular structure of [Cp*Mo(CH2e3)](.tNO)2 (2.2) at 185 K (50% thermal ellipsoids); including selected bond lengths and angles(with esds in parentheses).C(15)C(14))_C(6)f___C(3)C(13)C(9)C(12) C(10) 0Bond Lengths (A) Bond Angles (deg)Mo-Mo’ 2.5930 (7)Mo-N 1.944 (3)Mo-N’ 1.977 (3)N-O 1.259 (3)N-Mo-C(6) 102.06 (11)Mo-N-O 138.52 (21)NMoCp*a 120.92Mo-N-Mo’ 82.80 (10)a Refers to centroid of the Cp* ligand33Figure 2.2(b) Second view, at right angles to that shown in Figure 2.2(a), of the solidstate molecular structure of 2.2 at 185 K (50% thermal ellipsoids).34_________________________________________________________111:1111111 liii 11111119 II IIIIIIIIIIIII II7.874 7.2I I I I I I I I I I I3 2PPMFigure 2.3 300 MHz 1H NMR spectrum of [Cp*Mo(CH2èPh)](I.jNO)(2.3) inC6D. (a) Spectrum run immediately; only complex 2.3 (inset represents low-fieldregion, 7.7 - 6.9 ppm). (b) After 2 h; mixture of complexes 2.3 (marked +) and 2.6(marked.) (inset represents low-field region, 7.8 - 6.9 ppm).(b)..4(a)JULI Ii ililill III III 11111 IIIIIIII7.4 7.3 7 2 7. 1WLI I I2 PPM35tiLj I I I I I I I I I I I I3 2PPMFigure 2.3(c) 300 MHz 1H NMR spectrum of[Cp*Mo(CHePh)J(siNO)(2.3) inC6D after 4.5 h (full conversion to complex 2.6). Inset represents low-field region, 7.8 -6.9 ppm.Furthermore, these isomerizations occur intramolecularly as evidenced by thecrossover experiment depicted in eq 2.9. An equimolar mixture of 2.2 and 2.3 convertscleanly to an equimolar mixture of 2.5 and 2.6 upon warming.(c)I1 j 11111 11111 II I III I I I 11117. 7.4 7.2 7.0 PPM36/ O\R = CH2Me3(2.2)MoN—/-RN0R = CH2Me3(2.5)2.3.3 Hydrogenation of Cp*Mo(NO)(ptoIy1)2The hydrogenation of a solution of the diary! complex, Cp*Mo(NO)(ptolyl)2produces a blue powder in very low (< 5%) yield. Mass spectral, IR and 1H NMR dataare consistent with the product being the bridging-nitrosyl complex, [Cp*Mo(ptolyl)]2(I1NO)2. Over the course of—’ 4 h, the color of a C6D solution of this product changesfrom blue to brown, and the final 1H NMR spectrum displays peaks characteristic of thecorresponding bimetallic oxo bridging-nitrido complex, [Cp*Mo(NO)(ptolyl)](p.N)[Cp*Mo(O)(ptolyl)]. Since the yield is so low, however, no further attempts weremade to fUlly characterize this complex. Difficulties were also experienced in successfullyrepeating the preparation.2.3.4 Kinetic StudyUV-vis spectroscopy was used to monitor the progress of the isomerization of[Cp*Mo(CHePh)](IINO)(2.3) to [(Cp*Mo(NO)(CHePh)](p.+ NR’ = CH2MePh(2.3)—+ Mo=N—MoR’N= CH2MePh(2.6)(2.9)37N)[Cp*Mo(O)(CH2C ePh)j(2.6) since the former complex is blue while the latter isbrown (Figure 2.4.) Complex 2.3 exhibits a band at 684 nm in its UV-vis spectrum whilethe oxo nitrido complex 2.6 shows no features at this wavelength.The isomerization of 2.3 to 2.6 in toluene is first order with k0 (20 °C) = 1.1 ± 0.3 xio- s1 .39 A representative plot of ln(At - A) vs t is shown in Figure 2.5. Kineticanalyses at different temperatures were also conducted and the calculated rate constants(kobs) at 10.0, 20.0, 30.0, 40.0 and 50.0 °C in toluene are contained in Table 2.4; theconcentration of 2.3 was maintained at 0.0004 M (3 mg in 10 mL toluene). These ratesand temperatures were used to construct an Eyring plot (Figure 2.6). zH was calculatedto be 39 ± 3 kJ mold while z\S was found to equal -188 ± 6 J mol1K1(-45 ± 2 calmo!-1K1). The transition state of the isomerization reaction (depicted on page 44,Scheme 2.3) involves bond making (Mo-O and Mo-N-Mo) as well as bond breaking (Mo-Mo and N-O). The relatively low zS.H value and high negative S value imply thatAS, resulting from a symmetric ground state to unsymmetric transition state, is the majorcontributor to the reaction rate. The high negative AS is also consistent with theisomerization occurring in an intramolecular fashion.401.61 .41.2aC€—o 0 0 0 0 0 0 0 0 00 0 C’10 10 10 10 10 r. 1.. 10w.veiength (nm)Figure 2.4 UV-vis plot showing a decrease in absorption as a function of time for theisomerization of 2.3 to 2.6 in the region 600 - 800 nm (20 °C in toluene).38time (s)Figure 2.5 Plot of ln(At-A,0)vs t for the isomerization of 2.3 to 2.6 (50 °C in toluene).Table 2.4 Rate Constants (k€Jb) as a Function of Temperature for the Isomerization of2.3 to 2.6 in tolueneTemperature (°C) Rate Constant (r1)10.0 6.8 (± 1.3) x20.0 1.1 (±0.3)x io30.0 1.7 (± 0.4) x40.0 3.3 (± 0.8) x50.0 4.8 (± 1.4) x io-39-13-13.5-140C-‘ -14.5-15-15.50.0035Figure 2.6 Eyring Plot for the isomerization of 2.3 to 2.6 (toluene).2.3.5 Hydrogenation of a Mixture of Cp*Mo(NO)R2and Cp*W(NO)R2(R =CH2SIMe3)Exposure of an equimolar mixture ofCp*Mo(NO)(CH2SiMe3)andCp*W(NO)(CH2SiMe3)to molecular hydrogen results in the formation of theheterobimetallic species [Cp*Mo(CH2Si e3)](p.NO)W(](2.4). Thephysical properties of 2.4 are consistent with it possessing a molecular structure analogousto that of complexes 2.2 and 2.3. Tn 1984 Bergman reported two heterobimetallicbridging-nitrosyl dimers, CpoCr(NO)3andCp2oMn(NO)P e3.The former isproduced when NaCpCo(NO) is treated with CpCr(NO)21, while the latter results fromthe reaction ofNaCpCo(NO) with [CpMn(CO)(NO)PMe]BF4.Reports of otherheterobimetallic bridging-nitrosyl complexes have appeared in the literature.42.0.0031 0.0032 0.0033 0.0034l/T40Complex 2.4 is also thermally unstable in solution and converts to a mixture of twostructural isomers 2.7a and 2.7b in a 60:40 ratio (eq 2.10). A single-crystal X-raycrystallographic analysis of has established that both structural isomers, namely[Cp*W(NO)(CHSiMe3)](E.tN)[Cp*Mo(O)(CH](2.7a, 60%) and[Cp*Mo(NO)(CHSi e)J(I1.N)[Cp*W(O)(Cj(2.7b, 40%) are present in theunit cell. The ORTEP diagram of 2.7 is shown in Figure 2.7.The most noticeable feature of the solid-state structure of 2.7 is the trans arrangementof the Cp* ligands. This is different from the solid-state structure of 2.1 (Figure 2.1)which shows the orthogonal arrangement of the Cp* ligands. Other structural featuresare, however, similar.H2,Cfl_______— II /R2100NJR 01(JR-2RflR”oW\ (. )R = CH2SIMe3 24 (Z.7a, M, = W; M2 = Mo)(2.7b,M Mo;M W)2.3.6 Proposed Mechanism of IsomerizationPlausible mechanisms for the first steps in the conversions shown in equations 2.8 and2.10 are shown in Schemes 2.1 and 2.2, respectively.The first step in Scheme 2.1 involves the formation of an unstable Mo alkyl hydridecomplex from the hydrogenolysis of one of the alkyl bonds of the starting dialkyl Mocomplex.44 Alkane is observed when this reaction is monitored by 1H NMRspectroscopy. This unstable Mo hydride complex can dimerize and then intramolecularlylose H245 which results in the nitrosyl ligands adopting bridging positions and causes41Figure 2.7 View of the solid-state molecular structure of [Cp*Mj(N0)(CH2Si e3)](t_N)[Cp*M(0)(CHSIMe3)j(2.7) (50% thermal ellipsoids); including selected bondlengths and angles (with esds in parentheses).0(2)N(2)M(2)/NWtO(l)Bond Lengths (A) Bond Angles (deg)WJMo(1)-N(2) 1.913 (7)W/Mo(1)-N(1) 1.770 (7)W/Mo(2)-0(2) 1.7 18 (6)W/Mo(2)-N(2) 1.818 (6)N(1)-0(1) 1.194 (10)W/Mo(1)-N(2)-WfMo(2) 157.3 (4)N(2)Mo/W(1)Cp*a 115.7WIMo(1)-N(1)-0(1) 170.0 (7)a Refers to the centroid of the Cp* ligand42the formation of a molybdenum-molybdenum bond due to the electronic unsaturation atthe Mo centers.The M(ji-NO)2bridging systems in these products could then cleave in the mannerdepicted in Scheme 2.3 to form the final bridging-nitrido complexes, 2.5, 2.6 or 2.7. Thethermodynamic driving force of the isomerizations is the formation of strong Mo=O andMo-N-Mo linkages.46The cleavage of nitrosyls to form nitrido and oxo ligands has been observed in metalclusters containing NO ligands. lOa It has been speculated in these systems that terminalnitrosyl ligands become bridging when an ancillary ligand, for example CO, is lost. Toexplain the observed nitrido ligand that results, bent bridging-nitrosyls similar to thosedepicted in Scheme 2.3 have been postulated.Scheme 2.1IR2’ C611-Ru 2 Moo R112R = CIEI2Me3(2.5)R = CHMePh(2.6)4/°—°NR RR = CIEI2Me3(2.2)R = CHMePh(2.3)/RN=43It is postulated in Scheme 2.2 that the intermediate Mo hydride species forms anadduct with a molecule of the bis(alkyl) tungsten starting complex which then readilyeliminates alkane and rearranges to form complex It has been demonstrated thatCp*W(NO)(CH2SiMe3)does not react with 1 atm of molecular hydrogen in the timescale of the experiment. This is consistent with the proposed adduct formation betweenthe tungsten dialkyl complex and the transient molybdenum hydride with concomitant lossof alkane, but is not consistent with the association of molybdenum and tungsten hydridecomplexes followed by the intramolecular loss of molecular hydrogen. Cleavage of one ofthe bridging-nitrosyls in this dimer (2.4) can result in two isomers, namely, the final oxoligand ending up on either the Mo (2.7a) or W (2.7b) center. Since Mo evidently has agreater propensity to form oxo ligands (as observed in some Legzdins’ group chemistry),21the ratio ofMoO to W=O is slightly greater than one.Scheme 2.2112, C611—sE::-+______R = CH2SiMe3-RH-/R__/—M2 4/o_WNRN R N R(2.7a, M1 = W; 2.7b, M1=Mo) 2.444Scheme 2.3I?N NI 0/\____IIM M M M MN=M\ / NZ00M = Cp*MR (M’ = Mo, W; R = CIEI2SIMe3,CII2Me3,CIE2MePh)2.3.7 Fluxional Processes in SolutionIn solution [CpCr(NO)2jexists as a mixture of cis and trans NO-bridged dimers.48The mechanistic pathway for the interconversion of the isomers involves bridge-terminalNO equilibration and rotation about the chromium-chromium bond. There have also beenreports ofNO groups converting from linear to bent to bridging.49 An attempt was madeto ascertain whether mixtures of isomers exist in solutions of [Cp*Mo(CH2ePh)](i.iNO)2 (2.3) using variable temperature (-50 °C to 20 °C) 1H NIvIR spectroscopy. Thisstudy only revealed broadening of the Cp* signals at lower temperatures and it was alsocomplicated by the spontaneous isomerization of 2.3 to 2.6. Hence, by the completion ofthe study, all of 2.3 had isomerized to 2.6. Solution JR spectroscopy in CD21 revealsone nitrosyl band (1337 cml), consistent with only one isomer being present in solution.The solid-state molecular structure is thus probably similar to that in solution. Thus, thereis little evidence to support the existence of a mixture of isomers in solutions of 2.3(eq 2.11) similar to those found in solutions of [CpCr(NO)2}.45________/R211R” °N / Io0N ‘- / -47o R RN N K R o0 02.3.8 Reactivity of Bridging-Nitrido and Bridging-Nitrosyl ComplexesThe reactivity of the 16-valence-electron bridging-nitrosyl complexes 2.2, 2.3 and 2.4has not been extensively investigated because of their propensity to isomerize to the oxonitrido complexes 2.5, 2.6 and 2.7. Interestingly, they do not form adducts with simpleLewis bases such as phosphines (PPh3 and PMe3)or pyridine. Similarly, they do not reactwith acetone. The resultant products of these reactions are the corresponding bimetallicoxo bridging-nitrido species.As solids, complexes 2.1, 2.5, 2.6 and 2.7 can be handled in air for short periods oftime without any noticeable decomposition. Solutions of these complexes are thermallystable, and no noticeable decomposition occurs when the solutions are heated to 60 °C inC6D (as judged by 1H Nl’vlR spectroscopy). Exposure of solutions of 2.1 to air results inthe formation of the dioxo bridging-oxo species, [Cp*Mo(O)2](I.LO).5O2.4 EpilogueHydrogenation of solutions of Cp*Mo(NO)R2(R = neopentyl, neophyl) producesthermally sensitive intermediate blue complexes, [Cp*M0R](p._NO)2which isomerize tobimetallic oxo bridging-nitrido complexes, [Cp*Mo(NO)R]Q.tN)[Cp*Mo(O)R]. Thebridging-nitrosyl intermediate cannot be isolated when Cp*Mo(NO)(CH2SIMe3ishydrogenated, and only the final bimetallic oxo bridging-nitrido complex is obtained.A heterobimetallic bridging-nitrosyl complex, [Cp*Mo(CH2Si e3)](.LNO)2[Cp*W(CHSiMe3)J,is formed when a mixture of Cp*Mo(NO)(CHSiMeand46Cp*W(NO)(CH2SiMe3)is hydrogenated in benzene. Subsequent isomerization of thiscomplex results in a 60:40 mixture of the bridging-nitrido species,[Cp*W(NO)(CHSiMe)J(I.tN)[Cp*Mo(O)(CH]and[Cp*Mo(NO)(CHi e)](p.N)[Cp*W(O)(Cj,with the molybdenum oxolinkage being slightly preferred.The oxo bridging-nitrido complexes represent a class of products that results fromnitrosyl N-O bond cleavage. i12-NO linkages have been invoked as intermediates in otherexamples of nitrosyl N-O bond cleavage but, in this instance, the nitrosyl ligand adopts abridging position before cleaving to form the final thermodynamically stable bridgingnitrido complex.The fundamental question, however, concerns the reason why these particular M(i.iNO)2Mgroupings are prone to undergo this isomerization. Other such linkages, e.g.,those in [CpCr(NO)]2(p-N,(CpFe)2.t-NO,and (CpCo)2.t-NO,cited earlier, aswell as those in [Cp*Ru(Cl)](NO5l,[Cp*Ru($.tNO)}252,[Cp*Ru(Ph)]2(p._NO,52and [Cp*FeQ.L_NO)]253,are electronically saturated and show no such proclivity. Futurestudies should focus on both a theoretical treatment of the molecular orbitals extant inthese complexes and a subsequent investigation of the reactivity of these complexes.These studies may lead to an answer to this question.An interesting study would involve the synthesis of the mixed heterobimetallicbridging-nitrosyl complex, [CpCoJ(j.tNO)2p*MoR], by reacting Cp*Mo(NO)(R)Clwith NaCpCo(NO). This dimer, although electronically saturated at the Co center,would be electronically unsaturated at the Mo center. It would be interesting to determinewhether this dimer would cleave to its bimetallic oxo bridging-nitrido counterpart,[CpCo(NO)](p.N)[Cp*Mo(O)R].472.5 References and Notes(1) Gell, K. I.; Posin, B.; Schwartz, J.; Williams, G. M. J. Am. Chem. Soc. 1982, 104,1846.(2) Fagan, P. J.; Manriquez, J. M.; Maata, E. A.; Seyam, A. M.; Marks, T. J. J Am.Chem. Soc. 1981, 103, 6650.(3) Legzdins, P.; Martin, J. T.; Einstein, F. W. B.; Jones, R. H. Organometallics 1987,6, 1826.(4) Kummer, J. T. J. Phys. Chem. 1986, 90, 4747.(5) (a) Fredeen, D. J. A.; Russell, D. H. .1 Am. Chem. Soc. 1986, 108, 1860. (b)Jacobson, D. B. J. Am. Chem. Soc. 1987, 109, 6851. (c) Klaassen, J. J.; Jacobson,D. B. I Am. Chem. Soc. 1988, 110, 974. (d) Gord, J. R.; Freiser, B. S. J. Am.Chem. Soc. 1989, 111, 3754.(6) (a) Gland, J. L.; Sexton, B. A. Surf Sd. 1980, 94, 355. (b) Baldwin, E. K.;Friend, C. M. .1 Phys. Chem. 1985, 89, 2576.(7) MUller, J.; Ludemann, F.; Schmitt, S. J. Organomet. Qiem. 1979, 169, 25.(8) Schroder, D.; Muller, J.; Schwarz, H. Organometallics 1993, 12, 1972.(9) Seyferth, K.; Taube, R. .1. Mo!. Catal. 1985, 28, 53.(10) (a) Gladfelter, W. L. Adv. Organomet. Chem. 1985, 24, 41. (b) Gibson, C. P.;Dahi, L. F. Organometallics, 1988, 7, 543. (c) Gibson, C. P.; Bern, D. S.;Falloon, S. B.; Hitchens, T. K.; Cortopassi, J. E. Organometallics 1991, 10, 1742.(d) Feasey, N. D.; Knox, S. A. R. I. Chem. Soc., Chem. Commun. 1982, 1063.(11) Legzdins, P.; Young, M. A. Comments Inorg. Chem. 1995, 17, 239.(12) Legzdins, P.; Rettig, S. J.; Ross, K. J.; Veltheer, J. E. I Am. Chem. Soc. 1991, 113,4361.48(13) Lundmark, P. J. Ph.D. Dissertation, University ofBritish Columbia, 1993.(14) Debad, J. D.; Legzdins, P.; Reina, R.; Young, M. A.; Batchelor, R. J.; Einstein, F.W. B. Organometallics 1994, 13, 4315.(15) Brouwer, E. B.; Legzdins, P.; Rettig, S. J.; Ross, K. J. Organometallics 1994, 13,2088.(16) Shaw, M. J. Ph.D. Dissertation, University of British Columbia, 1993.(17) Legzdins, P.; Sayers, S. F.; Rettig, S. J. J. Am. Chem. Soc. 1995, 116, 12105.(18) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purfication ofLaboratoryChemicals, 3rd ed.; Pergamon Press: Oxford, 1988.(19) Shriver, D. F.; Drezdon, M. A. The Manipulation ofAir-Sensitive Compounds, 2nded.; Wiley-Interscience: New York, NY, 1986.(20) Veitheer, J. E. Ph.D. Dissertation, The University ofBritish Columbia, 1993.(21) Legzdins, P.; Lundmark, P. J.; Phillips, E. C.; Rettig, S. J.; Veitheer, J. E.Organometallics 1992, 11, 2991.(22) Legzdins, P.; Veltheer, J. E. In Handbuch der Praparativen AnorganischenChemie, 4th ed.; Herrmann, W. A., Ed.; in press.(23) Dryden, N. H., Legzdins, P., Rettig, S. J.; Veitheer, J. E. Organometallics 1992,11, 2583.(24) Crystals of 2.1 are triclinic of space group P1; Z = 2; a = 10.284 (4) A; b = 11.043(2) A; c = 16.979 (3) A; a= 85.97 (2)°; fl= 75.29 (2)°; y= 65.70 (2)°; V= 1698.5A. Drs. Ray Batchelor and Fred Einstein solved the structure using the Pattersonmethod and full-matrix least-squares refinement procedures to RF 0.03 7 for 4422reflections with I, 2.5a(10).49(25) Examples include: (a) [(TMEDA)C12VJ(j.i-N)[V3(TIvI DA)]: Sorensen, K. L.;Lerchen, M. E.; Ziller, J. W.; Doherty, N. M. Inorg. Chem. 1992, 31, 2679. (b)[( e3SiO)V] p.-N)[Pt(PEt)Me]: Doherty, N. M.; Critchiow, S. C. J. Am. Chem.Soc. 1987, 109, 7906.(26) Examples include: (a) [Cp*WMe3j2(.tN): Glassman, T. E.; Liu, A. H.; Schrock,R. R. Inorg. Chem. 1991, 30, 4723. (b) [Ru2N(en)5]C1.H0:Griffith, W. P.;McManus, N. T.; Skapski, A. C. J Chem. Soc., Chem. Commun. 1984, 434.(27) The Mo-N bond distances in [{MoN[S2P(OMe)]}4are 1.865 A whichcorresponds to the value expected for double bonds: Dehnicke, K.; Strahle, J.Angew. Chem., mt. Ed. Engi. 1992, 31, 955. In {[(tBuMe2SiNCHCH)3N]Mo}(p.-N)the Mo-N single bond distances average 1.907A: Shih, K.-Y.; Schrock, R. R.; Kempe, R. I Am. Chem. Soc. 1994, 116, 8804.(28) Du, Y.; Rheingold, A. L.; Maatta, E. A. J. Chem. Soc., Chem. Commun. 1994,2163.(29) For example: (a) The Mo=O bond lengths in Mo(O)2e(bpy are 1.707 (2) and1.708 (2) A: Schrauzer, G. N.; Hughes, L. A.; Strampach, N.; Robinson, P. R.;Schlemper, E. 0. Organometallics 1982, 1, 44. (b) The Mo=O bond length in{IIB(Mepz)3}MoO(SCNEtis 1.669 (3) A: Young, G. C.; Roberts, S. A.;Ortega, R. B.; Enemark, J. H. J. Am. Chem. Soc. 1987, 109, 2938.(30) Legzdins, P.; Phillips, E. C.; Sanchez, L. Organometallics 1989, 8, 940.(31) Glassman, T. E.; Liu, A. H.; Schrock, R. R. Inorg. Chem. 1991, 30, 4723.(32) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University Press: NewYork, 1992, Chapter 2.50(33) Crystal data for 2.2 at 185 K: orthorhombic, space group Pbcn, Z = 4, a = 12.570(3) A, b = 15.566 (2) A, c = 15.610 (3) A, V= 3054.3 A3. Drs. Ray Batchelor andFred Einstein solved the structure using the Patterson method and full-matrix least-squares refinement procedures to RF = 0.025 for 1921 reflections with I, 2.5a(I).(34) Calderón, J. L.; Fontana, S.; Frauendorfer, E.; Day, V. W. J. Organomet. Chem.1974, 64, ClO.(35) Calderón, J. L.; Fontana, S.; Frauendorfer, E.; Day, V. W.; Iske, S. D. A. .1Organomet. Chem. 1974, 64, C16.(36) Brunner, H. J Organomet. Chem. 1968, 12, 517.(37) Cotton, F. A.; Walton, R. A.. Multiple Bonds Between Atoms, 2nd ed; OxfordUniversity Press: New York, 1993, Chapter 5.(38) Miller, R. L.; Wolczanski, P. T.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115,10422.(39) Kinetic parameters were derived from the average of 3 runs. The error limits werecalulated using standard statistical methods: Gordon, A. I.; Ford, R. A. TheChemist’s Companion; Wiley-Interscience: New York, NY, 1972.(40) If the isomerization was bimolecular, the high negative entropy of activation wouldbe consistent with the isomerization occurring in an intermolecular fashion.(41) Weiner, W. P.; Hollander, F. J.; Bergman, R. G. J Am. Chem. Soc. 1984, 106,7462.(42) see: (a) Tiripicchio, A.; Camellini, M. T.; Neve, F.; Ghedini, M. J. Chem. Soc.,Dalton Trans. 1990, 1651. (b) Neve, F.; Ghedini, M. Inorg. Chim. Acta 1990,175, 111. (c) Delgado, E.; Jeffery, J. C.; Simmons, N. D.; Stone, F. G. A. J.51Chem. Soc., Dalton Trans. 1986, 869. (d) Tiripicchio, A.; Lanfredi, A. M. M.;Neve, F.; Ghedini, M. J Chem. Soc., Chem. Commun. 1983, 97.(43) Crystal data for 2.7 at 297 K: monoclinic, space group P21/c, Z = 4, a = 13.9440 (3)A, b = 19.953 (3) A, c = 12.112 (3) A, 13 = 97.99 (2)°, V 3336.2 A. Drs. RayBatchelor and Fred Einstein solved the structure using the Patterson method andfull-matrix least-squares refinement procedures to RF = 0.029 for 3096 reflectionswith 10 2.5a(10).(44) This hydride has been trapped with acetone or benzaldehyde to formCp*Mo(NO)(CHSiMe3)(OCHMe and Cp*Mo(NO)(CH2SiMe3)(OCHPh seeChapter 3, section 3.3.2.(45) (a) Dinuclear H2 elimination has been observed in the reaction ofOsH(CO)Me(C with Os(CO)4(H)Me to produce the trinuclear complex,0s3(CO)12Me2:Norton, J. R. Acc. Chem. Res. 1979, 12, 139. (b) Adecomposition pathway of transition metal hydride complexes involves theformation of bimetallic bridging hydride complexes which then lose H2 and result inthe formation of a metal-metal bond; M-H —* M(p.-H)M-H —* M-M + H2; Crabtree,R. H. Comprehensive Coord. Chem. 1987, 2, 689.(46) Mayer and coworkers have estimated that the W=O and W=NR bonds are 577kJ/mol and 420 kJ/mol, respectively, in complexes of the typeL3C12W=X (L =phosphine, X 0, NR). Hall, K. A.; Mayer, J. M. J. Am. Chem. Soc. 1992, 114,10402.(47) Labeling studies have shown that the osmium complex, Os(CO)4(H)Me,decomposes with the reductive dinuclear elimination of methane to yield thebimetallic complex, Os(H)(CO)40 (CO)Me); see reference 45(a).52(48) Kirchner, R. M.; Marks, T. J.; Kristoff, J. S.; Ibers, J. A. J. Am. Chem. Soc. 1973,95, 6602.(49) (a) Neve, F.; Ghedini, M. Inorg. Chim. Acta 1990, 175, 111. (b) KimberlyFjeldsted, D. 0.; Stobart, S. R.; Zaworotko, M. J. J. Am. Chem. Soc. 1985, 107,8258.(50) Faller, J. W.; Ma, Y. J. Organomet. Chem. 1988, 340, 59.(51) Hubbard, J. L.; Morneau, A.; Burns, R. M.; Zoch, C. R. J. Am. Chem. Soc. 1991,113, 9176.(52) Chang, J.; Bergman, R. G. J. Am. Chem. Soc. 1987, 109, 4298.(53) Lichtenberger, D. L.; Copenhaver, A. S.; Hubbard, J. L. Polyhedron 1990, 9, 1783.53CHAPTER 3Reactions of Cp*Mo(NO)(CH2SiMe3)with H2 in the Presence ofTrapping Substrates3.1 Introduction 543.2 Experimental Procedures 553.3 Results and Discussion 683.4 Epilogue 923.5 References and Notes 94543.1 IntroductionThe reactivity of Cp*W(NO)(CH2SiMe3)with molecular H2 in the presence ofvarious substrates has been extensively studied in the Legzdins’ group.1 In most cases the16-valence-electron tungsten alkyl hydride complex, Cp*W(NO)(CH2SiMe3)H,is initiallygenerated and, in the presence of unsaturated bonds such as C=O or C=N, the hydrideadds across these bonds to give complexes with tungsten-oxygen or -nitrogen links. Forexample, the hydrogenation of Cp*W(NO)(CH2S1Me3)in the presence of acetomtrileyields the azomethine complex, Cp*W(NO)(CHSIMe)(N=CHMe) (eq 3.1). Similarly,the reaction ofCp*W(NO)(CH2SIMe3)in the presence ofH2 and acetone yields thetungsten alkyl alkoxide complex, Cp*W(NO)(CH2SiMe3)(OCHMe.l1 112, NCMe W (3.1)/ N-RU N’N R o0 R RR = CH2SiMe3The related reactivity of the analogous molybdenum dialkyl system,Cp*Mo(NO)(CH2SiMe3),with molecular H2 in the presence of unsaturated linksconstitutes the focus of this chapter. Cp*Mo(NO)(CHSiMe3)has been reacted withmolecular H2 in the presence of the following reagents: Lewis bases (PPh3), dienes (1,3-butadiene, 2,3-dimethyl-butadiene, 1,3-cyclooctadiene), acetone and benzaldehyde, as wellas c, f3-unsaturated ketones (3-penten-2-one and butenone) and diphenyl disuiphide.None of these reagents react with the dialkyl molybdenum starting material prior to theaddition of molecular H2.55The products resulting from the reaction of Cp*Mo(NO)(CH2SIMe3)with molecularH2 in the presence of these various reagents suggest that different reaction pathways arefollowed depending on the nature of the reagent. A transient Mo alkyl hydride complexdoes appear to be generated initially and, in certain cases, this hydride complex can betrapped. However, these hydrogenations often result in the loss of both alkyl groups. Thereactivity does appear to parallel that of the tungsten system; however, the tungstenhydrogenations are complete within 12 h, whereas the molybdenum dialkyl system reactsinstantaneously with molecular H2 in the presence of trapping substrates. Comparisonsbetween the two systems are drawn as appropriate.3.2 Experimental Procedures3.2.1 MethodsThe synthetic methodologies employed throughout this thesis are described in detailin section ReagentsAll reagents were purchased from commercial suppliers or were prepared accordingto literature methods. For details of Cp*M(NO)(CH2Si e3)(M = Mo, W), H2 andPPh3 see section 2.2.2. 1,3-butadiene (Matheson), and ethylene (Matheson) were used asreceived. 1,3 -Cyclooctadiene, 1,3 -cyclohexadiene, 1, 5-cyclooctadiene, cyclooctatetraene,diphenylacetylene, 3-penten-2-one, butenone and 2,3-dimethyl-butadiene (Aldrich) weredried over 4A molecular sieves, filtered through alumina I (neutral) and degassed with 3freeze-thaw-pump cycles prior to use. Acetone and benzaldehyde (Fisher) were dried onCaH2 and distilled before use. Diphenyldisuiphide (PhSSPh) was used as received from56Aldrich. The column chromatographic materials used during this work were Florisil(60-100 mesh, Fisher) and alumina I (neutral, Fisher). Celite (Fisher) was used for allfiltrations. All fits used during this work were of medium porosity.The reaction vessels utilized during these hydrogenation reactions were glass bombs(thick-walled glass vessels) equipped with Kontes stopcocks.3.2.3 SynthesisIsolated yields, physical properties, and spectroscopic data for all complexes are listedin Tables 3.1 - Preparation of Cp*Mo(NO)(q4-trans-l ,3-butadiene) (3.1)An excess of 1,3-butadiene was condensed into a purple solution ofCp*Mo(NO)(CH2SiMe3)(360 mg, 0.82 mmol) in Et20 (20 mL) maintained at -78 °C.The reaction vessel was evacuated and filled with H2 (1 atm). The stirred reactionmixture was warmed slowly to room temperature (1 h) whereupon its color changed frompurple to green-yellow. The final reaction mixture was filtered through a column(3 x 3 cm) of Florisil supported on a fit. The column was washed with Et20until thefiltrate was colorless. Solvent was removed from the combined filtrates in vacuo, and theremaining residue was dissolved in a minimum of hexanes (ca 15 mL). Cooling of thissolution to -8 °C for 2 d resulted in the formation of yellow crystals of Cp*Mo(NO)(q4trans-i ,3-butadiene) (87 mg, 34%) which were collected by filtration.573.2.3.2 Preparation of Cp*Mo(NO) i4-trans-2,3-dimethy1-butadiene) (3.2)H2 (1 atm) was added to an evacuated reaction vessel containing a mixture ofCp*Mo(NO)(CHSiMe3)(410 mg, 0.94 mmol), excess 2,3-dimethyl-butadiene (0.2 mL,1.7 mmol), and Et20 (20 mL) at -78°C. The stirred reaction mixture was warmed slowlyto room temperature (1 h) during which time its color changed from purple to brown.Solvent was removed in vacuo to obtain a brown residue which was extracted with Et20(2 x 15 mL). The combined extracts were concentrated under reduced pressure and thenchromatographed on a Florisil column (3 x 8 cm) using hexanesfEt2O(2:1) as eluant. Agreen-yellow band developed and was collected. Solvent was removed from the eluate invacuo, and the remaining green residue was dissolved in a minimum of hexanes andmaintained at -8 °C for 2 d to induce the formation of yellow crystals (94 mg, 29% yield)of analytically pure Cp*Mo(NO)(ri4ans2,3-dimethyl-butadiene). Preparation of Cp*Mo(NO)(CH2SIMe3)(OR) [R = CliMe2 (3.3), CII2Ph(3.4)1The preparation ofCp*Mo(NO)(CHSiMe)(OCHIvle is given as a representativeexample of the method used to synthesize both 3.3 and 3.4. H2 was added (1 atm) to anevacuated reaction vessel containing a mixture of Cp*Mo(NO)(CH2SiMe)(410 mg,0.94 mmol), acetone (0.1 mL, 1.4 mmol), and pentane (20 mL) at -78°C. The stirredreaction mixture was warmed slowly to room temperature (1 h) during which time itscolor changed from purple to red. Solvent was removed in vacuo, and the reaction vesselwas kept under vacuum for two hours to obtain a red residue. This residue wasredissolved in pentane (10 mL) and filtered through a column of Celite (3 x 3 cm)supported on a fit. The eluate was then concentrated under reduced pressure (3 mL).Cooling at -30 °C for 3 weeks did not induce crystallization, and thus the solvent was58removed in vacuo for 12 h. The red residue was then redissolved in C6D (0.6 mL) andtransferred into an NMR tube for spectroscopic characterization. Preparation of Cp*M(NO)(1l4i6H24)[M = Mo (3.5), W (3.6)]The preparation ofCp*Mo(NO)(rIfrans2,3dimethylbutadiene) (3.2) describedabove is a representative example of the method used to synthesize both 3.5 and 3.6, using1,3-cyclooctadiene as the organic reagent. The tungsten conversion required a longertime (18 h) to reach completion. Yellow crystals of analytically pure Cp*M(NO)(r14_C16H24)were isolated in 25% (M— Mo) and 14% (M = W) yields. Preparation of Cp*W(NO)(rl38Hii)H (3.7)The preparation of Cp*Mo(NO)(14trans2,3-dimethyl-butadiene) (3.2) describedabove is a representative example of the method used to synthesizeCp*W(NO)(i13-C8H1)H(3.7), except that the solvent used was pentane rather thanEt20. When Cp*W(NO)(CH2SiMe(523 mg, 1.00 mmol) was reacted with 1,3-CODand H2 (1 atm) in pentane (20 mL), two products crystallized from the final concentratedsolution. The red crystalline material corresponded to analytically pureCp*W(NO)(r14i6H24)(3.6) (58 mg, 10%), while the yellow powder corresponded tothe tungsten hydride complex, Cp*W(NO)(qgH11)H(3.7) (52 mg, 11%). As the twoproducts were differently colored, they were separated with a spatula in the glovebox. Preparation of Ci6H (3.8)02 was bubbled through a solution of Cp*Mo(NO)(rI4i6H24(478 mg, 1.00 mmol)in Et2O (30 mL) for 1 h. Solution JR showed the slow disappearance of VNO at1564 cm1 during this time. Once the nitrosyl band had completely disappeared, the59solution was concentrated and chromatographed on an alumina I column (3 x 8 cm). Thischromatography, as well as the manipulations that follow, was performed in air. Thecolumn was then washed with Et20 (50 mL). During this washing a yellow banddeveloped but was not eluted. Solvent was removed from the clear filtrate and theremaining clear oil ( 3 mL) was redissolved in Et20 (5 mL), filtered on a second columnof alumina 1(3 x 5 cm) supported on a fit and eluted with Et20 (20 mL). The removal ofsolvent from the eluate with a rotary evaporator yielded an analytically pure clear viscousoil (76 mg, 3 5%). Preparation of Cp*Mo(NO)(q4-3-penten-2-one) (3.9)H2 (1 atm) was added to an evacuated reaction vessel containing a mixture ofCp*Mo(NO)(CHSiMe3)(410 mg, 0.94 mmol), excess 3-penten-2-one (0.15 mL,1.50 mmol), and Et20 (20 mL) at -78°C. The stirred reaction mixture was warmed slowlyto room temperature (1 h) during which time its color changed from purple to brown.Solvent was removed in vacuo to obtain a brown residue which was extracted with Et20(2 x 15 mL). The combined extracts were concentrated under reduced pressure and thenchromatographed on a Florisil column (3 x 8 cm) using hexanes/Et20(1:2) as eluant. Ayellow band developed and was collected. Solvent was removed from the eluate in vacuo,and the remaining yellow residue was dissolved in a minimum ofEt20 (5 mL) andmaintained at -8 °C for 2 d to induce the formation of a yellow powder of analytically pureCp*Mo(NO)(fl43 enten2one) (41 mg, 12%). Preparation of Cp*Mo(NO)(q4-butenone) (3.10)C6H (10 mL) was vacuum transferred onto a mixture of Cp*Mo(NO)(CH2SiMe(436 mg, 1.00 mmol) and an excess of butenone (0.16 mL, 2.00 mmol). H2 (1 atm) wasthen introduced into the reaction vessel. Upon warming the reaction mixture changed60color from purple to red. The reaction mixture was stirred for 20 mm after which time thesolvent was removed in vacuo. The residue was washed with pentane (10 mL) and Et20(5 mL). The resulting yellow powder was dissolved in CH21 (15 mL) and filteredthrough Celite (2 x 3 cm) supported on a fit. The filtrate was concentrated, hexanes(5 mL) were added, and the mixture was then maintained at -8 °C for 5 d to induce theformation of a yellow powder of analytically pure Cp*Mo(NO)(14butenone) (73 mg,22%). Preparation of Cp*Mo(NO)(PPh3)2(3.11)H2 (1 atm) was added to an evacuated reaction vessel containing a mixture ofCp*Mo(NO)(CHSIMe3)(440 mg, 1.00 mmol), PPh3 (524 mg, 2.00 mmol), and C6H(20 mL) at -78°C. The stirred reaction mixture was warmed to room temperature(30 mm) during which time its color changed from purple to red. The solvent wasremoved in vacuo to obtain a red powder which was washed with Et20 (1 x 10 mL) andrecrystallized fromCH21/hexanes. Two fractions of product 3.11 were obtained (totalof 326 mg, 44%). Preparation of Cp*Mo(NO)(SPh)2(3.12)H2 (1 atm) was added to an evacuated reaction vessel containing a mixture ofCp*Mo(NO)(CHSi3)(440 mg, 1.00 mmol), PhSSPh (218 mg, 1.00 mmol), andC6H (20 mL) at -78°C. The stirred reaction mixture was warmed to room temperature(40 mm) during which time its color changed from purple to green. C6H was removed invacuo to obtain a green residue which was then extracted with pentane (2 x 25 mL). Thecombined extracts were filtered through a column ofFlorisil (2 x 3 cm) supported on a fitusing pentane/Et20(1:1) as eluant. The solvent was removed from the filtrate, and theconcentrated solution was maintained at -8 °C for 2 d to induce the formation of green61crystals of 3.12 (200 mg, 4 1%). These crystals were recrystallized from a minimumamount of pentane (5 mL).3.2.4 Labeling Studies Using D2The reaction ofCp*Mo(NO)(CH2Si3)with 1,3-COD and D2 was performed inmaimer identical to that described in section except that D2 was substituted for H2.1H NMR and mass spectral data of the yellow product obtained were identical to that ofauthentic Cp*Mo(No)O4.Ci6H24)(3.5).Similarly, the reaction of Cp*W(NO)(CH2S1Mewith 1,3-COD and D2 (inpentane) was performed identically to that described in section, again substitutingD2 for H2. 1H NMR and mass spectral data of the yellow and red products obtained wereidentical to those of authentic Cp*W(NO)(q38H11)H(3.7) andCp*W(NO)(q416H24)(3.6).3.2.5 Reaction of Cp*Mo(NO)(CH2S1Me3)with 1,3-Cyclohexadiene and H2H2 (1 atm) was added to an evacuated reaction vessel containing a mixture ofCp*Mo(NO)(CHSiMe3)(410 mg, 0.94 mmol), excess 1,3-cyclohexadiene (0.2 mL,2.0 mmol), and pentane (20 mL) at -78°C. The stirred reaction mixture was warmedslowly to room temperature (40 mm) during which time its color changed from purple tobrown and a yellow powder precipitated from the reaction solution. The powder wasseparated from the reaction solution via cannulation and was washed with pentane (5 mL)and Et20 (5 mL). All attempts to recrystallize this powder were unsuccessftil as itssolutions in various solvents appeared to decompose over the course of a few days at-8°C.623.2.6 Reaction of Cp*W(NO)(CH2SIMe3)with 1,3-Cyclohexadiene and 112H2 (1 atm) was added to an evacuated reaction vessel containing a mixture ofCp*W(NO)(CHSiMe3)(523 mg, 1.00 mmol), excess 1,3-cyclohexadiene (0.2 mL, 2.0mmol), and pentane (20 mL) at -78°C. The reaction mixture was warmed to roomtemperature and was stirred for 12 h. During this time a color change from purple to redoccurred. Removal of solvent in vacuo yielded a red powder. A 1H NMR spectrum ofthis crude red powder revealed peaks attributable to a mixture of the two known tungstendimers, [Cp*W(NO)H]2(j..LH) and [Cp*W(NO)(CHSiMe3)](I.tH)( )H],previously characterized.23.2.7 Reactions of Cp*Mo(NO)(CH2S1Me3)with Other UnsaturatedHydrocarbons and 112 (Other Unsaturated Hydrocarbons = 1,5-COD,cyclooctatetraene, ethylene, or diphenylacetylene)These reactions were performed in a manner similar to that described in section3.2.3.2. Workup, however, only yielded intractable brown solids.3.2.8 1H NMR MonitoringThe reactions described in sections, and were also performedin NMR tubes in C6D to facilitate monitoring. In each case two equivalents of trappingagent were weighed into a vial in the glovebox with a one molar equivalent ofCp*Mo(NO)(CH2SiMe3).C6D (0.6 mL) was then added to the solids and the solutionwas transferred via pipette into an NN4R tube equipped with a Teflon gas inlet. After theinitial spectrum had been recorded, the solution was frozen and the atmosphere in the tubewas replaced with H2. The NI’vfR tube was then warmed to room temperature and thecontents shaken to dissolve the 112. 1H NIvIR spectra were recorded periodically until thereaction was deemed to be complete.63Table 3.1 Numbering Scheme, Color, Yield, and Elemental Analysis Datacomp coloranal. found (calc)complex no. (yield, %) C H NCp*Mo(NO)(i41,3butadiene) 3.1 yellow (34) 53.47 (53.36) 6.69 (6.66) 4.39 (4.44)Cp*Mo(NO)(12,3dimethyl 3.2 yellow (30) 55.69 (55.97) 7.45 (7.35) 4.22 (4.08))utadlene)Cp*Mo(NO)(R)(OCHMe 3,3 red (90)” bCp*Mo(NO)(R)(OCH2Ph 3,4 red (90)” bCp*Mo(NO)(i14_C6H24) 3,5 yellow (25) 65.52 (65.56) 8.27 (8.33) 2.91 (2.95)Cp*W(NO)(r1624 3.6 yellow (15) 54.99 (55.22) 6.97 (6.97) 2.38 (2.48)Cp*W(NO)(ii38H11)H 37 yellow (11) 47.23 (47.27) 5.97 (5.96) 2.91 (3.06)C16H24 3.8 clear (35) 88.48 (88.40) 11.17 (11.20) 0.00 (0.00)3,9 yellow (12) 51.83 (52.17) 6.80 (6.73) 4.16 (4.06)Cp*Mo(NO)(rbutenone) 3.10 yellow (22) 50.39 (50.75) 6.36 (6.40) 4.25 (4.23)CpMo(NO)(PPh3)2 311 red (44) 69.30 (70.31) 5.68 (5.78) 1.66 (1.78)Cp*Mo(NO)(SPh) 3.12 green (41) 55.13 (55.10) 5.24 (5.27) 2.90 (2.93)R CH2SiMe3a Based on the 1H NMR of the reaction mixture.b An elemental analysis was not obtained due to the oily nature of the material.64Table 3.2 Selected Mass Spectral and Infrared Datacomp MS temp” IR (Nujol mull)no. m/z1’ (°C) VNO other strongbands3.1 317 [P4] 200 1616(vs)3.2 345 [P] 180 1574 (vs)33 C3.4 457 [P1 120 c35 479 [P] 120 1564 (vs)263 LP -C16H24]3.6 565 [P1j 80 1554 (vs)3.7 457 IPI 80 (1605, 15801574, 1568)3.8 216 [P] 120 1639, 1446 (C=C)3.9 347 LPI 120 1514 (vs) 1564 Cs, CO)263 LP -C5H8013.10 662 [2Pl 120 1512 (vs) 1572 (s, CO)333 [P]3.11 786 [P] 80 1530 (vs)3.12 740 [2P - SPh] 100 1615 (vs) 1655 (Ph)a Probe temperatures.b Values for the highest intensity peak of the calculated isotopic cluster(98Mo and 1841,If)•Not obtained due to the oily nature of the product.65Table 3.3 NMR Data (C6D)comp 1 NMR (6) ‘3C{1H} NMR (6)no.3.1 3.57 (m, 1H, CII) 105.5 (CMe)3.47 (m, 1H, CII) 96.7 (CH)3.08 (m, 1H, CII) 91.3 (CII)2.45 (m, 1H, CII) 61.4 (CH2)1.65 (s, 15H, (C5Me) 58.9 (CH2)1.57 (m, 111, CII) 10.5 (CMe)1.21 (m, 111, CR)3.2 3.37 (m, 1H, CH) 109.3 (CH)3.27 (d, 1H, CH, HH = 9.9 Hz) 105.8 (C5Me)2.66 (d, 111, CII, HH = 9.9 Hz) 102.8 (CR)1.76 (br, 111, CII) 60.6 (CR2)1.74 (s, 311, Me) 59.2 (CH2)1.69 (s, 15H, C5Me) 22.4 (Me)1.00 (s, 3H, Me) 21.2 (Me)10.7 (C5Me)33 5.22 (ses, 111, OCHMe2) 113.2 (C5Me)1.58 (s, 15H, C5Me) 82.7 (CII)1.32 (d, 311, OCHMe2) 39.1 (CHS1Me31.29 (d, 3H, OCIIUe2) 28.5 (Me)1.13 (d, 1H,CHSiMe3,JHH= 11.4 Hz) 26.9 (Me)0.85 (d, 1H, CH2SiMe3HH = 11.4 Hz) 10.2 (C5Me)0.34 (s, 9H, CHSiMe) 2.5 (CHSiMe33.4 7.46 (m, 2H, Ph) C1 not observed7.15 (m, 3H, Ph) 142.4 (OCH2)5.91 (m, 2H, OCH2Ph) 129.3 (C.)1.42 (s, 1511, C5Me) 128.6 (Cme)1.31 (d, 1H, CH2SiMe3,JHH = 9.9 Hz) 127.8 (Cpa)1.16 (d, 111, CHSiMe HH = 9.9 Hz) 112.9 (C5Me)0.40 (s, 9H, CII2SiMe3) 42.0 (CR2SiMe39.6 (C5Me)2.1 (CR2SIMe3665.27 (t, 1H, CR)4.09 (brt, 1H, CII)3.55 (m, 211, CH)3.11 (m, 1H, CR)2.71 (m, 1H, CH2)2.56 (m, 2H, CH and CH2)2.41 (m, 2H, CH2)2.15 (m, 2H, CH2)1.80 (m, hR. CR2)1.45 (s, 1511, C5Me)1.22 (m, 1H, CH2)5.32 (t, 1H, CII)4.58 (brt, 1H,CH)3.36 (m, 2H, CR)3.01 (m, 1H, CR)2.88 (m, 1H, CH2)2.63 (m, 1H, CR2)2.44 (m, 211, CH and CR2)2.20 (m, 2H, CH2)1.77 (m, 12H, CH2)2.48 (s, 15H, C5Me)1.25 (m, 111, CH2)6.05 (d, 1H, CHy44.55 (q, 111, CR)4.75 (q, 1H, CR)4.04 (m, 111, CR)2.95 (m, 1H, CR)2.78 (ses, 1H, Cl2)2.08 (m, 1H, CH2)1.97 (s, 15H,C5Me)1.90 (m, 111, CR2)1.84 (m, 1H, CR2)1.15 (m, 111, CH2)0.87 (m, 111, CH2)-0.71 (s, hR. WH,JwH=63liz)151.3 (C4)122.8 (CR)103.3 (C5Me)86.8, 81.6, 70.0, 66.8, 53.2 (CR)32.5, 31.7, 31.5, 30.8, 29.5, 28.6, 28.0, 26.8,23.7 (CR2)9.2 (C5Me)154.0 (Cquat)123.7 (CR)102.1 (C5Me)80.4, 76.1, 60.8, 57.3, 54.3 (CR)32.7, 32.6, 32.4, 30.8, 29.5, 28.4, 28.0, 27.0,23.6 (CR2)9.1 (C5Me)136.4 (CR)121.1 (CR)104.0 (C5Me)92.2 (CR)75.1 (CR)62.2 (CR)31.9 (CH2)25.6 (CR2)24.4 (CH2)10.4 (C5Me) 5.70 (m, 511, CR) 141.7 (Cquat)3.25 (m, 111, CR) 133.5, 132.3, 128.5, 127.1, 125.3 (CR)1.12 (m, 6H, C112) 43.9 (CR)1.43 (m, 12H, CR2) 35.7, 29.9, 29.0, 28.1, 26.9, 26.7,26.3, 24.8, 23.1 (CR2)675.28 (d, 1H, CHb)’4.96 (d, 1H, CHa)2.65 (m, 111, CHa)2.25 (m, 1H, CHb)2.05 (d, 3H, Mea)1.90 (d, 311, Meb)1.66 (d, 3H, Mea)1.62 (s, 15H,C5MCa)1.58 (s, 15H,C5MeSb)1.56 (d, 3H, Meb)185.8 (CO)110.0 (C5Mea)108.8 (C5Meb)88.0, 77.8 (CHa)69.6, 65.5 (CHb)23.4 (Me)21.4 (Me)20.8 (Me)9.6 (C5Me)9.5 (CMè)3.10 4.67 (dd, 111, CH, HH = 18.0, 7.5 HZ)c 187.3 (CO)C2.57 (m, 2H, CH) 110.2 (C5Me)1.79 (s, 1511, C5Me) 72.0 (CH)1.63 (s, 3H, Me) 51.3 (CH2)29.1 (Me)10.0 (C5Me)3.11 7.47 (m, 12H, Phme)d 138.4 (d, = 35.5 HZ, C)e6.92 (m, 18H, Ph010 para) 133.7, 133.6, 133.5, 133.4, 128.8,1.52 (s, 15H, C5Me) 128.6, 128.5, 128.2, 127.6, 127.5,127.5 (Ph)101.8 (C5Me)10.3 (C5Me)3.12 7.81, 7.40, 7.01 (m, 1OH, Ph) f1.57 (s, 1511, C5Me)a Recorded in DMSO-d6.b Ratio ofisomerato isomerbis 1:1.Recorded in CD21.d 31P{’H} in CDC13,71.3 ppm.e Recorded in CDC13.f Not recorded.3.9683.3 Results and DiscussionThe reactivity of the molybdenum dialkyl complex, Cp*Mo(NO)(CH2SiMe3)withmolecular in the presence of a variety of substrates is discussed in this chapter. Eachclass of reagent is reviewed separately since different products and modes of reactivity areobtained for different reagents.3.3.1 Reactions with Acydic DienesFormer members of the Legzdins’ group, in particular Nancy Christensen, devotedtheir efforts towards the synthesis and reactivity of molybdenum diene complexes of thetype, Cp’Mo(NO)(t4-frans-diene).3These diene complexes were initially prepared viasodium amalgam reduction in TRF of Cp’Mo(NO)I2in the presence of acyclic conjugateddienes (eq 3.2).Mo Na/Fig,- 2 Na! Mo (3 2)0N” I ‘ butadieneThis reduction method is not ideal because the reactions need to be monitored closelyby FTIR spectroscopy as the products decompose in the highly reducing environment.3aThe workup procedures for this synthetic method are also difficult as repeatedchromatography is necessary to separate the products. Earlier efforts also producedinconsistent product yields.When solutions of Cp*Mo(NO)(CH2SIMe3)are treated with molecular H2 in thepresence of acylic conjugated dienes such as 1 ,3-butadiene or 2,3-dimethyl-butadiene, the69corresponding Cp*Mo(NO)(t14ft.ansdiene) complexes are obtained (eq 3.3) in moderateyields. The reaction of the dialkyl complex with 2,3-dimethyl-butadiene was followed by1H NMR spectroscopy, and the overall yield was found to be approximately 80%, basedon the integration of the peaks due to the product. The isolated yields are less due to thehigh solubility of the resulting products in pentane which prevents their complete recoveryby crystallization.I H2,-2RHMo R’ (3.3)0 R R’R = CH2S1Me3 R’ = II (3.1), Me (3.2)Complexes 3.1 and 3.2 have been identified by their spectroscopic properties, and bycomparison with analogous compounds synthesized by the reduction method describedabove. An earlier X-ray crystallographic analysis ofCpMo(NO)(r4-1,3-butadiene) showsthe diene ligand bound to the metal center in a very twisted, transoidal fashion.4 Theinteresting feature of these complexes is that the trans isomer is the thermodynamicallymore stable isomer, which is in contrast to theCp’2M(i4-diene) complexes (M = Zr or Hf)in which the cis isomer is the thermodynamically more stable isomer.5 This is apparently amanifestation of the frontier-orbital properties of the Cp’Mo(NO) fragment.6The proposed mechanism for the formation of complexes 3.1 and 3.2 is shown inScheme 3.1.70Scheme 3.1MO2 MoNIH0 R 0 RR = CH2SiMe3The reaction of the starting dialkyl molybdenum complex with molecular hydrogenresults in an initial hydrogenolysis of one of the molybdenum alkyl bonds, producing atransient alkyl hydride complex with the loss of SiMe4. Although no molybdenum hydridecomplexes have been isolated to date, this hydride can be trapped in the presence ofsuitable reagents (section 3.3.2). This 16-valence-electron coordinatively unsaturatedhydride species can then be envisioned as binding one of the double bonds of the diene in ai2-fashion. This is followed by the intermediate complex undergoing the loss of a secondmolar equivalent of SiMe4with the concerted coordination of the dangling second doublebond of the diene to the molybdenum center.The proposed intermediate, the alkyl hydride complex with the coordinated2-diene,is not detectable while monitoring the reaction of Cp*Mo(NO)(CH2SiMe3)with 2,3-71dimethyl-butadiene and H2 by 1H NMR spectroscopy. Tetramethylsilane is eliminatedvery rapid and is the only identifiable species (other than the product) in the monitoredreaction mixture.This hydrogenation methodology has been used to prepare the first nitrosyl r14-frans-diene complex of tungsten.7 These Cp*W(NO)(q4fransdiene) complexes cannot beobtained by Na/Hg amalgam reduction of Cp*W(NO)12in the presence of acyclic dienessince this method simply yields intractable decomposition products.33.3.2 Reactions with Acetone and BenzaldehydePurple solutions of Cp*Mo(NO)(CH2S1Me3)containing either acetone orbenzaldehyde react instantaneously when placed under an atmosphere of molecular H2.The solution colors turn deep red, and workup after 5 mm results in the generation ofalkyl alkoxide species, Cp*Mo(NO)(CH2SiMe3)(OCHMe (3.3) andCp*Mo(NO)(CH2SiMe3)(OCHPh (3.4), respectively (eq 3.4). The unsaturated oxygen-carbon bond of the ketone or aldehyde inserts into the molybdenum hydride bond of thetransient Cp*Mo(NO)(CH2SiMe3)Hspecies to yield these alkyl alkoxide products. Thisreactivity parallels that of the analogous reaction of Cp*W(NO)(CH2SIMe3)withmolecular H2 in the presence of acetone or benzaldehyde. lbMo 2 Mo (34)/ \ R’C(=O)R” / \N R (-RU) OCHR’R”0 R RR = CH2SiMe3 R’ = = Me (3.3)= Ph; R” = H (3.4)72All attempts to isolate pure 3.3 and 3.4 have been futile to date, and these complexeshave only been spectroscopically characterized by comparison with the analogous tungstencomplexes. Unfortunately, Mo alkyl alkoxides cannot be filtered through anychromatographic support (Florisil or alumina) since the products have a propensity tostick to these materials.8 The products are also extremely soluble in pentane, a featurewhich has prevented their isolation as crystalline materials. Thus, the reaction mixturesare merely filtered through Celite, and the pentane is removed in vacuo for 12 h. Thesealkyl alkoxide species are generated in high yields (- 90%) as observed by 1H NMRspectroscopy.Berke and coworkers have observed similar insertions of the C0 double bond ofpropanal and benzaldehyde into the W-H bond ofWH(CO)2(NO)(PMe3)affording thealkoxide complexes, W(OPr)(CO)2(N )(PMe3)andW(OCHPh)(CO)(NO)(PMe,respectively.9 The reaction of the polymetallic hydrido complex, [W2H(O-i-Pr)7]withaldehydes and ketones similarly results in insertion into the tungsten hydride bond formingalkoxide ligands.103.3.3 Reactions with Cyclic DienesThe Na/Hg amalgam reduction of CpMo(NO)12in the presence of cyclic conjugatedclienes simply results in the decomposition of the organometallic reactant.3’7However,when 1,3-cyclooctadiene (1,3-COD) is employed as a trapping agent in the hydrogenationof Cp*M(NO)(CH2SIMe)(M = Mo, W), it undergoes an unprecedented coupling in thecoordination sphere of the metal (eq 3.5).73M 2 M (3.5)/ \ 1,3-COD / \RR (-2 RH) 0NR = CH2SiMe3 M = Mo (3.5), W (3.6)There are many examples in the literature of catalytic and stoichiometric metal-mediated dimerizations of acyclic and cyclic olefins.H For example, when Cp*Ru(14butadiene)Cl is treated with excess butadiene in the presence of AgOTf and CO (1 atm),the 1,5-cyclooctadiene complex, [Cp*Ru(112:rlgHi)(CO)]OTf, is isolated.12 Aclosely related coupling, although catalytic, is the coupling of cyclooctene to 1 -cyclooctylcyclooctene in the presence of a nickel catalyst which is shown in equation 3.6.13Q Ni/Al . (3.6)Another example of a metal-facilitated dimerization involves the reductivedimerization of benzene in[(ri6-CH)Mn(CO)3]to yield [{Mn(CO)3}]2p-(i4-C6Hó:ri4-C6H)12,in which the two manganese centers are bridged by the newly formedtetrahydrobiphenylene ligand.14 Aryl halides have also been shown to couple in thepresence ofNi(COD)2and PPh3 to give diaryl products.15 Recently, a process forforming 4-vinylcyclohexene from butadiene using an iron nitrosyl halide catalyst in thepresence ofNO has been patcnted.1674The spectroscopic properties of 3.5 did not permit an unambiguous assignment of itsmolecular structure. Consequently, Cp*Mo(NO)(q416H24)was subjected to an X-raycrystallographic analysis. The resulting ORTEP drawing of the solid-state molecularstructure of 3.5 is shown in Figure 3.1.17Three double bonds are present within theC16H24 ligand. C(2)-C(3) and C(12)-C(13) (average 1.39 A) are involved in ri2-it bonding to molybdenum while the thirddouble bond is not involved with the metal [C(1 1)-C(18), 1.340 A]. The N-O bonddistance is 1.243 A which is typical of other Group 6 monomeric nitrosyl complexes. 18The intramolecular angle Mo-N-O is 173.3 (2)° which is essentially linear. C(8), C(1),C(2), C(3) and C(4) are roughly coplanar since the double bond at C(2) and C(3) imposeseclipsing of C(1) and C(4). In the same way, the second ring has imposed eclipsing aboutthe double bonds, C(17)-C(18) and C(1 1)-C(12). The potential conjugation between thetwo double bonds in this ring is disrupted by the C(18)-C(1 1)-C(12)-C(13) torsion angleof 53.6 (2)°.In solution, Cp*Mo(NO)(rI4i6H24displays a complicated 1H NMR spectrumwhich contains complex coupling patterns of the CH and CH2 protons of the 2-cyclooct-2-en-1-yl-1,3-cyclooctadiene ligand (Figure 3.2). ‘3C{’H} and APT data (Figure 3.3),however, proved invaluable for spectroscopic assignment. Thus, in addition to the Cp*signals, the APT spectrum exhibits the expected nine peaks assignable to the CH2 carbons,six peaks assignable to the CH carbons and, finally, one peak attributable to the lonequaternary carbon in theC16H24 ligand. A 2D COSY experiment was used to assign thepeaks attributable to the CH and CH2 protons in the 1H NMR spectrum.Monitoring this reaction by 1H NMR spectroscopy did not reveal peaks assignable toany reaction intermediates, except the byproduct, SIMe4 (0.0 ppm inC6D). This isprobably due to the fact that intermediate species do not persist in solution for very long.75Figure. 3.1 View of the solid-state molecular structure ofCp*Mo(NO)(416H24)(3.5),including selected bond lengths and angles (with esds in parentheses).C(35)C(31)E2IIIIIIIIIIC(21) C(25)CC(32) C(22)C(23)::) C(24) C(34)C(14)çc(12)C(4)Q C(2)c(11) C(1)C(15)C(6)C(17) C(18)C(16) cmBond Lengths (A) Bond Angles (deg)C(12)-C(13) 1.383 (3)C(11)-C(18) 1.340 (4)C(2)-C(3) 1.392 (3)C(11)-C(1) 1.5 16 (4)C(1)-C(2) 1.521 (3)C(1)-C(8) 1.498 (3)N-O 1.217 (2)Mo-N-O 172.3 (2)NMOCPa 123.0C(2)-C(3)-C(4) 128.1 (2)C(1)-C(3)-C(3) 130.1 (2)C(14)-C(13)-C(12) 125.7 (2)C(18)-C(11)-C(12) 121.9 (2)a Refers to centroid of the Cp ligand765 5 5 0 4.5 4.0 3.5 3.0 2.5 20 1.5 PPMFigure 3.2 300 MHz 1H NMR spectrum ofCp*Mo(NO)(14i6H24)(3.5) in C6D.10... %, rJt 44J , w rrtdo do 40 20 PPMFigure 3.3 75 MHz APT spectrum ofCp*Mo(NO)(416H24)(3.5) in C6D.L77In order to gain some insight into the mechanism of this unusual reaction, the mixtureof 1,3-COD and Cp*Mo(NO)(CH2S1Me3)was reacted with D2 instead ofH2.Surprisingly, this labeling study revealed that deuterium was not incorporated in the finalcyclooctenyl-cyclooctadiene ligand. Taking this into account, a possible mechanism forthe formation of the triene, is presented in Scheme 3.2.Scheme 3.2M/\R RH M2-RHR H1,3-COD M/I\RjHM = Cp*(Mo or W)(NO)(1) 1,3-COD(2) addition acrosshydride(1)- Rh (2) C-H bondactivation1,2-H shiftC-C bondformationAs presented in Scheme 3.1, an initial alkyl hydride complex,Cp*Mo(NO)(CH2SiMe3)H,could be formed by the hydrogenolysis of one of the alkyl78ligands of the molybdenum dialkyl complex. Earlier (section 3.3.2), it was shown that analkyl hydride can be trapped in the presence of acetone or benzaldehyde. One of thedouble bonds of 1,3-COD then coordinates in an i2 fashion to this 16-valence-electroncomplex. The loss of a second molar equivalent of SiMe4 is accompanied by C-H bondactivation of theq2-bound 1,3-COD. Another equivalent of 1,3-COD then adds acrossthe Mo hydride bond and results in thei3-coordination of this new incoming 1,3-CODligand.’9 1,2-Hydrogen shift on the activated diene followed by nucleophilic attack20 onthei3-al1yl carbon of the ally! COD ligand results in the newly formed C-C bond and thecoordinated, coupled triene ligand.The novel coupled organic ligand, 2-cyclooct-2-en-1-yl-1,3-cyclooctadiene, can easilybe liberated from the molybdenum center by treatment of solutions of complex 3.5 with02 as shown in eq 3721ri/I O2 (3.7)M= Mo (3.5) 3.8The spectroscopic properties of 3.8 are similar to those attributable to thecoordinated species in 3.5. Thus, in the‘3C{’H} NMR and APT spectra of the freeorganic molecule, 2-cyclooct-2-en- 1 -yl- 1,3 -cyclooctadiene, the signals attributable to theCR carbons (133.4, 132.3, 128.5, 127.1, 125.3, 43.9 ppm in C6D)occur downfield tothose of the CH signals of the corresponding molybdenum-coordinated species 3.5 (122.8,86.8, 81.6, 69.7, 66.8, 53.2 ppm in C6D)(the APT spectrum of 3.8 is shown in Figure793.4). This is as expected as coordination results in electron density from the C-C doublebonds contributing to the Mo-diene bond.The 1H NMR spectrum of 3.8 is shown in Figure 3.5. The 1H NMR signals for thefive CH protons occur as a complex multiplet around 5.6 ppm while the signal for the CHproton (at the position of the newly formed C-C bond) occurs at 3.5 ppm. The remainingsignals for the CH2 protons occur as two complex multiplets at around 2.1 and 1.4 ppm.14: 120 100 80Figure 3.4 75 MHz APT spectrum ofC16H24 (3.8) in C6]).I20 15 10 PPMFigure 3.5 300 MHz 1H NMR spectrum ofC16H24 (3.8) in C6]).40 20 PPM80Intermediate complexes in reactions are sometimes isolated and identified by changingthe metal to the heavier third-row congener.22’23 Consequently,Cp*W(NO)(CH2SiMe3)was hydrogenated in the presence of 1,3-COD in an attempt toisolate any possible intermediate complexes. When this reaction is performed in THF orEt2O only the analogous tungsten product, Cp*W(NO)(416H24)(3.6), is obtained.However, when this hydrogenation is performed in pentane or hexanes, a second productis formed in addition to complex 3.6, viz. Cp*W(NO)(ri38Hi1)H(3.7) (eq 3.8).I 1,3-COD (3 8)/ W pentaneRR (-2R1EI)R = CH2S1Me3Unfortunately, suitable crystals of this product could not be grown for an X-raycrystallographic analysis. However, other standard spectroscopic methods were used toidenti1,’ 3.7. Elemental analysis is consistent with a C8H12 fragment being bound toCp*W(NO), and mass spectral data suggest that the product is monomeric in nature. Theuse of 1H and ‘3C{1H} NMR spectroscopy, as well as 2D COSY and HETCORexperiments, once again proved invaluable in the identification of 3.7. Characteristictungsten-hydride satellites (J = 63 Hz) were evident in the 1H NMR spectrum (Figure 3.6)of this complex at -0.71 ppm in DMSO-d6.24 An APT spectrum (Figure 3.7) exhibited, inaddition to the Cp* signals, signals due to three CIT2 carbons and five CH carbons. This isconsistent with the formulation of the C8H11 fragment consisting of one double bond anda it-ally! fi.inctionality. Cp*W(NO)(C8H11)H is thus formally an 18-valence-electroncomplex./ +3.6 3.781Figure 3.6 300 MHz 1H NMR spectrum ofCp*W(NO)(138Hii)H(3.7) in DMSO-d6.e w1Mtnri.II II lIIiJiIlIIIIlIIi If Ii40 i a ibo 80 60 40 0 0 —20 PPMFigure 3.7 75 MHz APT spectrum ofCp*W(NO)(q3gH11)H(3.7) in DMSO-d6.82The use ofD2 instead ofH2 in this reaction did not result in the incorporation ofdeuterium into 3.7. Thus, C-H bond activation of 1,3-COD must again be operative inthis reaction. Unfortunately, this complex does not appear to be a reaction intermediate asit does not react with further 1,3-COD to produce 3.6. In Scheme 3.2, the product (3.5)results from the selective activation of one of the C-H bonds of 1,3-COD. Complex 3.7 isprobably a byproduct resulting from a side reaction which involves the activation of one ofthe other C-H bonds in 1,3-COD. This can also account for the low isolated yields of thetungsten complex (3.6) as compared to the molybdenum analogue (3.5). The formation of3.7 appears to be solvent dependent since it does not appear to be formed when thehydrogenation of the tungsten system is performed in THF or Et2O. The analogousmolybdenum complex has not been identified when the hydrogenation of this system isconducted in THF, Et20, pentane or hexanes.In order to investigate the generality of this coupling reaction,Cp*Mo(NO)(CH2S1Me3)was hydrogenated in the presence of another cyclic diene, i.e.,1,3 -cyclohexadiene (1,3 -CHD). When this reaction is conducted in THF or Et20, noisolable species can be identified. However, when it is conducted in hexanes or pentane,an insoluble yellow powder precipitates from the reaction mixture when the solutionwarms to room temperature. At this point the reaction mixture was cannulated from theyellow powder and the powder washed with pentane and Et2O. Mass spectral data andelemental analysis25 of this powder are consistent with its formulation asCp*Mo(NO)(C12H6).This is encouraging since the reaction appears to proceedsimilarly to that of the 1,3-COD reaction. Unfortunately, problems are encounted whenthis yellow powder is in solution because it does not persist in solution for very long. Forinstance, when a 1H NMR sample is made up, the solution mixture changes from yellow toorange within a couple of minutes. Attempts to run 1H NIVER spectra at cold temperatureswere unsuccessful as only broad signals were observed. A‘3C{1H} NMR and APT83spectrum at -60 °C in toluene-dg revealed signals very similar to those of the coupled 1,3-COD product to molybdenum (3.5)26 If 1,3-CHD had coupled in ananalogous manner, one would have expected signals due to one quaternary C, six CHcarbons and five CH2 carbons. However, eight signals due to CH carbons and four due toCH2 carbons are observed at this temperature. It is thus evident that the coupling isapparently different to that of the 1,3-COD product. 2D NMR experiments (HETCORand COSY) were not attempted due to the reactive nature of the product in solution. Theidentity of this organometallic product is thus impossible to deduce from the limited dataavailable. Attempts to trap this product with PMe3 have also proven fhtileThe reaction of 1,3-CHD with Cp*W(NO)(CH2S1M3)and molecular H2 simplyyields the bridging hydride dimers, [Cp*W(NO)H](J.IH) and[Cp*W(NO)(CH2SiMe3)]QtH)(NO)H] from the final reaction mixture.23.3.4 Reactions with a, 3-Unsaturated KetonesThe reaction of solutions of Cp*Mo(NO)(CH2SiMe)with molecular H2 in thepresence of either 3-penten-2-one or butenone results in the formation ofCp*Mo(NO)(rj43-penten-2-one) (3.9) and Cp*Mo(NO)(4butenone) (3.10) respectively(eq 3.9)N”l\ etieCp*Mo(NO)(ReHC=CH2{O}Me) (39)0 R butenoneR = CH2SiMe3 R’ = Me (3.9), H (3.10)84Mass spectral data suggest that the products are monomeric in nature, whileelemental analyses are consistent with there being one unsaturated ketone ligand perCp*Mo(NO) fragment. Two different bonding modes of the unsaturated ketone towardthe molybdenum center can be envisioned (Scheme 3.3).Scheme 3.3A BMo MoR.Mo = Cp*Mo(NO)R = Me, IIThe first mode (A) involves the unsaturated ketone being bound to the Cp*Mo(NO)fragment in a transoidal q4 fashion. This is not unreasonable since acyclic dienes bind tothe molybdenum fragment in this manner (vide supra). Another binding mode of theunsaturated ketone towards molybdenum can be a a interaction of the lone pair ofelectrons on the carbonyl oxygen with the metal as well as the it interaction with thedouble bond of the unsaturated ketone (B). This interaction would similarly result in a 18-valence-electron molybdenum complex. Gladysz and coworkers have shown that 13CNMR spectroscopy can be very useful in discerning ii2 and i1 binding modes in metalcoordinated aldehyde and ketone complexes.27 While‘q1-aldehyde and ketone ligandsshow CO resonances in the normal downfield range of organic carbonyl groups, those of2 ligands typically occur in the range 45 - 111 ppm. An equilibrium between it and abinding modes has been observed in the cationic complex,CpRe(NO)PPh3(O=CHAr)X.2885The CO resonances for complexes 3.9 and 3.10 occur at 8 185.5 and 187.7respectively, which are out of the range of typicalq2-bound carbonyl complexes. Thus,the bonding of these unsaturated ketones is probably similar to B depicted in Scheme 3.3.No unambiguous assignment of the exact binding mode of 3-penten-2-one or butenone tomolybdenum can be made without a crystal structure of the complex. Regrettably, allattempts to crystallize 3.9 and 3.10 have been unsuccessfbl to date.In solution two isomers of complex 3.9 (1:1 ratio based on the integration of the Cp*signals) are present as observed by 1H and 13C{’H} NME. spectroscopy. No attemptswere made to separate the isomers by crystallization due to the low isolated yield of theproduct. These isomers probably differ with respect to the orientation of the unsaturatedketone towards the metal center as shown below.Mo = Cp*Mo(NO)-?° -oThere is no reason to expect that one isomer would be more preferred than the other,and so the 1:1 isomeric mixture in solution is reasonable.The 1H NMR spectrum of the crude yellow powder of complex 3.10 suggests that insolution two isomers exist for this complex as well.29 In a recrystallization attempt,however, one isomer powdered out of solution (the 1H and 13C(’H} NIVIR data inTable 3.3 are for the one isomer). No attempt was made to isolate the second isomer bycrystallization, again due to the low yield.862D HETCOR and COSY experiments were utilized for complete spectral assignmentof the 1H and 13C{’H} NtvER spectra of complex 3.9. Figures 3.8 and 3.9 show the 2DHETCOR and COSY spectra, respectively.The product yields of these reactions are low. This may be due in part to possibleformation of other species under these reaction conditions, namely alkyl alkoxidecomplexes, Cp*Mo(NO)(CH2SiMe3)(OCHMeHC=CMe ) andCp*Mo(NO)(CHSi e)(OCITMeHC=.Such products could arise from a reactionpath analogous to that proposed for the reaction of acetone and benzaldehyde withCp*Mo(NO)(CH2Si3)and molecular H2 (section Although these productsare not observed or isolated, it is expected that they would not be stable under the reactionconditions employed.8 The same reaction with the tungsten congener yields the alkylalkoxide products exclusively.303.3.5 Reaction with TriphenyiphosphineThe orthometalated-phosphine hydride product, Cp*W(NO)(112PPh5H4)H,isisolated from the reaction ofCp*W(NO)(CH2SiMe3)and molecular H2 in the presenceof triphenyiphosphine (eq 3.lO).hl The product expected from this reaction was the alkylhydride complex, Cp*W(NO)(CHSiMe)(PPhH.However, this hydride complex isunstable and eliminates another equivalent of tetramethylsilane, followed byorthometalation of one of the phenyl ligands of triphenylphosphine to yield the finalproduct.87VVFigure 3.8 500 MHz HETCOR spectra ofCp*Mo(NO)(q43 enten2one), 3.9 inDMSO-d6.1n1.w,rn—.0.i I-00.0I’.ee01aC-F88ILaci, -.0CI I I I I I I I I I IFigure 3.9 500 MHz COSY spectrum ofCp*Mo(NO)(rl43 enten2one), 3.9 inDMSO-d6.89w H2,PPh3 w (3.10)N” I R (-2 N< \7Ph2o R OH(R = CH2S1Me3The analogous reaction with the dialkyl molybdenum species yields the bis(phosphine)complex, Cp*Mo(NO)(PPh3)2(3.11) (eq 3.1 1), in moderate yield (44%).Mo________Mo (3.11)/ \ 2PPh3 / \N R (2Q N PPh3o R 0 PPh3R = CH2SIMe3 3.11The most likely mechanistic pathway for the formation of 3.11 is presented inScheme 3.4. This pathway is similar to that presented in Scheme 3.1 (the formation of theCp*Mo(NO)(4fransdiene) complexes). An initially formed alkyl hydride complex canbe trapped with one equivalent of triphenylphosphine. With the loss of a second molarequivalent of tetramethylsilane, a second molecule of triphenyiphosphine thenbinds to the molybdenum center. This reaction proceeds rapidly at room temperature, andno intermediates have been observed when the reaction is followed by 1H NMRspectroscopy.The photolysis of the dicarbonyl nitrosyl complexes, Cp’M(NO)(CO)2in the presenceof triphenyiphosphine yields the analogous Cp’M(NO)(PPh3)2(M = Cr, Mo) complexes.3190Scheme 3.4Mo 2 Mo/\ -RH /\N Ro R RR = CH2SIMe3 IPPh3Mo MoN’s’ I ‘PPh3 NI \PPh3o PPh3 0 R113.3.6 Reaction with Diphenyl DisuiphideDiphenyl disuiphide, PhSSPh, has been successfully used in the trapping ofCp*W(NO)R fragments generated by radical mechanisms to yield the corresponding alkylthiolate complexes, Cp*W(NO)(R)SPh.32 In order to establish whether any radicalintermediates are produced in these hydrogenation reactions, diphenyl disulphide wasreacted with Cp*Mo(NO)(CH2SiMe3)in the presence of molecular H2. The onlyisolable product formed in this reaction is the dithiolate complex, Cp*Mo(NO)(SPh)(3.12) as shown in eq 3.12.91Mo 112 Mo (3.12)/ \ PhSSPh / \RR (-2 RH) 0NSPhSPhR = CH2S1Me3 3.12Thiolate nitrosyl complexes are not new to the Legzdins’ group. A range of tungstenthiolate complexes of the type, CpW(NO)(SCH2SiMe3)have been isolated via theinsertion of sulfur into the tungsten-alkyl bonds of the dialkyl species,CpW(NO)(CH2SiMe3).3The molecular structure of Cp*Mo(NO)(SPh)2is probably dimeric in nature as is theCp analogue, which is synthesized by the reaction of CpMo(NO)12with a two molarequivalent ofNaSPh.34 This conclusion is consistent with mass spectral data which showa peak at m/z = 740 which corresponds to 2P-SPh.This hydrogenation reaction (eq 3.12) results in the formal dissociation of the S-Sbond of diphenyl disuiphide. It simplistically appears that phenyl disuphide has oxidativelyadded to the Cp*Mo(NO) fragment. Kubas and coworkers have activated diphenyldisuiphide with W(PPr3)2(CO) to yield the radicals, W(PPr3)2(CO)S h and SPh.35Teuben and coworkers have invoked a bis(thiolate) vanadium complex,CpV(SPh)2(PMe3)as the product from the reaction ofCpV(2-CH4)(PMe3withdiphenyl disulphide.36923.3.7 Attempted Reactions with Other SubstratesOnly intractable brown solids are obtained when Cp*Mo(NO)(CH2SiMe3)is reactedwith molecular H2 in the presence of any of the following substrates: 1,5-COD,cyclooctatetraene, ethylene, diphenylacetylene.3.4 EpilogueThe hydrogenation reactions ofCp*Mo(NO)(CH2SiMe3)in the presence of addedsubstrates likely proceed initially with the formation of a very reactive molybdenum alkylhydride intermediate, Cp*Mo(NO)(CH2SiMe3)H. Although this hydride has not beenisolated as such, it can be trapped in the presence of polar substrates such as acetone orbenzaldehyde, yielding alkyl alkoxide complexes.The reactivity of the molybdenum dialkyl complex, Cp*Mo(NO)(CH2SiMe3)withmolecular H2 in the presence of added substrates appears to be dependent on the nature ofthe substrate. For example, in the presence of unsaturated non-polar reagents such asdienes, the transient molybdenum hydride bond does not appear to add across the C=Clinkage. Rather, another molar equivalent of tetramethylsilane is lost and the second CCbond of the conjugated diene binds to the metal, thereby resulting in4-diene complexes.When the trapping agent is a Lewis base, such as triphenyphosphine, both alkylgroups undergo hydrogenolysis and the corresponding bis(phosphine) complex is formed.In the same way, the hydrogenation in the presence of diphenyl disulphide yields thedithiolate complex. The latter result is consistent with the fact that these hydrogenationreactions do not appear to proceed via radical intermediates.The hydrogenation reaction in the presence of a conjugated cyclic diene, 1,3-COD,causes the cyclic diene to couple, resulting in a novel triene product. This interestingreactivity has not yet been extended to other cyclic dienes.93The yields of all the products are moderate to low. This is due to a combination ofreasons. Firstly, as shown in Chapter 2, Cp*Mo(NO)(CHSIMe3)reacts rapidly withmolecular H2 to form oxo nitrido bimetallic complexes. If the reaction of the putativeCp*Mo(NO)(CH2SiMe3) with the added substrate is slower than, or occurs at a ratecomparable to that of, the formation of the oxo nitrido bimetallic species, a number ofproducts are expected in the final reaction mixtures. Although[Cp*Mo(NO)(CHSi e)](pN)[Cp*Mo(O)(C](2.1) is not isolated from anyof the above reactions this does not preclude its formation in low yields during thehydrogenation reactions. As mentioned in, 2.1 does not crystallize very easilyfrom solution. Secondly, the products slowly decompose under the reaction conditions,i.e., the presence of excess H2.Generally the reaction of the molybdenum dialkyl complex with molecular hydrogenin the presence of trapping substrates parallels that of the tungsten systems. However, theanalogous tungsten alkyl hydride is generated at a slower rate.The isolation of a molybdenum nitrosyl hydride should be the focus of future studies,since to my knowledge no such complexes have been isolated to date. Even if amolybdenum nitrosyl hydride could be generated under more controlled conditions, itwould greatly facilitate further investigations of the reactivity of such a complex.943.5 References and Notes(1) (a) Legzdins, P.; Martin, I. T.; Einstein, F. W. B.; Jones, R. H. Organometallics1987, 6, 1826. (b) Debad, J. D.; Legzdins, P.; Lumb, S. A.; Batchelor, R. J.;Einstein, F. W. B. Organometallics 1995, 14, 2543.(2) Martin, I. T. Ph.D. Dissertation, The University ofBritish Columbia, 1987.(3) (a) Christensen, N. J.; Hunter, A. D.; Legzdins, P. Organometallics 1989, 8, 930.(b) Christensen, N. J.; Legzdins, P.; Einstein, F. W. B.; Jones, R. H.Organometallics 1991, 10, 3070. (c) Christensen, N. J.; Legzdins, P.; Trotter, J.;Yee, V. C. Organometallics 1991, 10, 4021. (d) Christensen, N. J. Ph.D.Dissertation, The University of British Columbia, 1990.(4) Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.; Willis, A. C. J. Am. Chem. Soc.1986, 108, 3843.(5) (a) Yasuda, H.; Nakamura, A. Angew. Chem., mt. Ed. Engi. 1987, 26, 723. (b)Erker, G.; Kruger, C.; Muller, G. Adv. Organomet. C’hem. 1985, 24, 1.(6) Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.; Willis, A. C.; Bursten, B. E.; Gatter,M. G. J Am. Chem. Soc. 1986, 108, 3843.(7) Debad, J. D.; Legzdins, P.; Young, M. A.; Batchelor, R. J.; Einstein, F. W. B. J.Am. Chem. Soc. 1993, 115, 2051.(8) Legzdins, P.; Lundmark, P. J.; Rettig, S. J. Organometallics 1993, 12, 3545.(9) Van der Zeijden, A. A. H.; Bosch, H. W.; Berke, H. Organometallics 1992, 12,2051.(10) Barry, J. T.; Chacon, S. T.; Chisholm, M. H.; Huffman, J. C.; Streib, W. E. J. Am.Chem. Soc. 1995, 117, 1974.95(11) (a) For the coupling of cyclooctatetrenes by palladium, see: Siesel, D. A.; Staley,S. W. Tetrahedron Lelt. 1993, 34, 3679. (b) For the use ofq3-allyl nickelalkoxides as homo-and heterogeneous catalysts in the dimerization of olefins, see:Boennemann, H.; Jentsch, J. D. App!. Organomet. Chem. 1993, 7, 553. (c) Forthe selective dimerization of aldehydes to esters catalyzed by zirconocene andhafhocene complexes, see: Morita, K.; Nishiyama, Y.; Ishii, Y. Organometaiics1993, 12, 3748. (d) For the catalytic dimerization of norbornadiene topentacyclotetradecadine by ruthenium catalysts, see: Mitsudo, T.; Zhang, S. W.;Watanabe, Y. J. Chem. Soc., Chem. Commun. 1994, 435.(12) Itoh, K.; Masuda, K.; Fukahon, T.; Nakano, T.; Aoki, K.; Nagashima, H.Organometallics, 1994, 13, 1020.(13) Bogdanovic, B. Adv. Organomet. Chem. 1979, 17, 105.(14) Cooper, N. J.; Geib, S. J.; Thompson, R. L. J Am. Chem. Soc. 1991, 113, 8961.(15) Yamamoto, T.; Wakabayashi, S.; Osakada, K. J. Organomet. Chem. 1992, 428,223.(16) Duisters, H. A. M.; Haenen, J. G. D. PCT Tnt. App!. WO 94 10, 110(Cl.C07C13/20); Chem. Abstr. 1994, 121, 979.(17) Crystals of 3.5 are triclinic of space group P1; a = 8.772 (1) A, b = 8.898 (1) A,c= 16.137(2)A,ct= 103.26(1)°, 3= 103.98(1)°,y=94.58(1)°,Z=2. DrsRayBatchelor and Fred Einstein solved the structure using the Patterson method andfull-matrix least-squares refinement procedures to RF = 0.023 for 3452 reflectionswith J 2.5c(I0).96(18) The N-O bond distance in CpMo(NO)(r4-trans-2,5-dimethyl-2,4-hexadiene) is1.2 13 (3) A: Hunter, A. D.; Legzdins, P.; Nurse, C. R.; Einstein, F. W. B.; Willis,A. C. J. Am. Chem. Soc. 1985, 107, 1791.(19) ‘q3-Mlyl complexes ofPd and Ni have been invoked as intermediates in thedimerization of olefins: Coilman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.Principles andApplications ofOrganotransition Metal Chemistry; UniversityScience Books: Mill Valley, CA, 1987; Chapter 11.(20) Facile nucleophilic attack atr3-allyl ligands bound to transition metals is common,see ref 19, Chapter 7.(21) The organometallic product of this transformation is the well known[Cp*Mo(O)](iO): Faller, J. W.; Ma, Y. .1 Organomet. Chem. 1988, 340, 59.(22) Using third row congeners to investigate possible intermediates in catalytic reactionsis common. For example, in order to gain information regarding an intermediate inthe rhodium catalyzed dimerization of methyl acrylate, Brookhart investigated theiridium analogue: Hauptman, E.; Sabo-Etienne, S.; White, P. S.; Brookhart, M.;Garner, J. M.; Fagan, P. J.; Calabrese, J. C. J. Am. Chem. Soc. 1994, 116, 8038.(23) Using the bulkier Cp4(C5Ph4H) ligand instead of Cp* did not result in the isolationof any reaction intermediate: Reina, R.; Young, M. A. unpublished results.(24) Moore, D. S.; Robinson, S. D. Chem. Soc. Rev. 1983, 12, 415.(25) Anal, found (calc): C 62.68 (62.69), H 7.35 (7.43), N 3.29 (3.32), m/z 423 [P],160 [C12H6].(26) ‘3C{’H} NMR (toluene-d6): ö 140.0, 119.8, 106.0, (3 x CH); 103.1 (C5Me);92.1, 79.1, 57.3, 44.9, 39.2(5 x CH); 29.1, 24.9, 22.0, 20.1 (4 x CH2); 9.9(C5Me).97(27) Huang, Y.-H.; Gladysz, J. A. I Chem. Ed. 1988, 65, 298.(28) Méndez, N. Q.; Seyler, J. W.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. Soc. 1993,115, 2323.(29) 1H NMR (CD2C1): ö 4.67 (2dd, 2H, CH, Jp = 7.5 Hz), 2.57 (dd, 2H, CH), 2.48(dd, 1H, CH), 2.30 (dd, 1H, CH), 1.81 (s, 15H,C5Me),1.79 (s, 15H, C5Me), 1.63(s, 3H, Me), 1.55 (s, 3H, Me).(30) Debad, J. D., Young, M. A. unpublished results.(31) Brunner, H. J. J. Organomet. Chem. 1969, 16, 119.(32) (a) Debad, J. D.; Legzdins, P.; Reina, R.; Young, M. A., Batchelor, R. 3.; Einstein,F. W. B. Organometallics 1994, 13, 4315. (b) Brouwer, E. B.; Legzdins, P.;Rettig, S. J.; Ross, K. J. Organometallics 1994, 13, 2088.(33) Legzdins, P.; Sanchez, L. I Am. Chem. Soc. 1985, 107, 5525.(34) (a) McCleverty, J. A.; Seddon, D. I Chem. Soc., Dalton Trans. 1972, 2588. (b)McCleverty, J. A.; James, T. A. I Chem. Soc. (A) 1970, 1068.(35) Lang, R. F.; Ju, T. D.; Kiss, G.; Hoff, C. D.; Bryan, J. C.; Kubas, G. J. I Am.Chem. Soc. 1994, 116, 7917.(36) Hesson, B.; Meetsma, A.; van Bolhuis, F.; Teuben, J. H.; Helgesson, G.; Jagner, S.Organometallics 1990, 9, 1925.98CHAPTER 4Reactivity of CpMo(NO)(=CHCMe3)(py)4.1 Introduction 994.2 Experimental Procedures 1024.3 Results and Discussion 1154.4 Epilogue 1364.5 References and Notes 137994.1 IntroductionFischer and Maasböl introduced the first carbene complex to the world in 1964.1Since then M=C complexes have been extensively investigated. Fischer carbenes aremostly prepared by alkylation of a metal-bound carbonyl ligand (eq 4.1).(a) PhLI OMeW(CO)6 + (CO)5W=C (4.1)(b)Mc30 PhThe first carbene complex without an x-carbon-heteroatom bond was preparedserendipitously by Schrock ten years later (eq 4.2).22Me3CCHLi 11Ta(CH2CMe3)1 (Me3CCH2)Ta=C (4.2)-2 Lid, - CMe4-CMe3Carbenes of this type are referred to as alkylidenes since they react differently fromFischer carbenes.3 Fischer carbenes usually react as electrophiles while Schrockalkylidenes react as nucleophiles. Typical reactions ofFischer carbenes include Lewisbase adduct formation as well as stoichiometric cyclopropanation of olefins.3 Alkylideneson the other hand typically undergo Wittig-type alkylations as well as olefin metathesisreactions.4’5 The difference in reactivity is attributed to a difference in metal-carbonbonding.6 The bonding in Fischer carbenes is dominated by a single dative bond from adoubly occupied orbital on the carbene ligand (M — C(ER)R). The system is thenstabilized by back bonding from filled metal d orbitals (M (dit) —* C (pi*)). The bondingin alkylidenes typically resembles that extant in covalent olefin double bonds.100Traditionally alkylidene complexes have been synthesized exclusively via x-hydrogenabstraction routes.7 However, many have been synthesized recently by rearrangements oftransition-metal-bound cyclopropene ligands.8Not all M=C complexes strictly qualify as Fischer carbenes or Schrock alkylidenes.Many complexes are amphiphilic, the a-carbon being both electrophilic and nucleophilic.Examples include complexes such as CpRe(CO)2(=CHR (R = (CH2)CMe39as well as(CO)2Ph3P)Ru(=CF)i0There are also many transition-metal methylene complexeswhich undergo unique reactivity.11’12In general, the bis(hydrocarbyl) nitrosyl complexes of the type Cp’M(NO)R2(Cp’ = Cp, Cp*; M Mo, W; R = alkyl, aryl) are thermally stable at room temperatureeither as solids or in solution. John Veitheer, a former member of the Legzdins’ group,however, discovered that the bis(neopentyl) complex, CpMo(NO)(CH2Me3)thermallydecomposes at room temperature. Leaving a solution of this complex at roomtemperature results in the formation of an unsymmetric dimer with a unique bridgingrl:r2 nitrosyl group (eq 4.3).13RT__NN2-2CMC4H><.-\(4.3)Me3C N CMe3 CMe3 e3 oThis thermolysis presumably occurs via intramolecular x-hydrogen elimination ofneopentane yielding a transient neopentylidene fragment, CpMo(NO)(=CHCMe3),whichthen dimerizes to form the resulting product. In the presence of suitable trapping agents101such as phosphines or pyridine, the corresponding 18-valence-electron neopentylidenecomplexes CpMo(NO)(=CHCMe3)L(L = phosphine or pyridine) are isolated (eq 4.4). 14Mo L Mo (44)-CMe4Me3C N CMe3 L CMe30L = Pyridine, PPh3PPh2MeKinetic studies show that the generation of this alkylidene complex is first order indialkyl and independent of the nature of the trapping phosphinei7 Unfortunately, thephosphine-trapped alkylidenes are thermally robust and show limited reactivity.Teuben similarly prepared the first “Schrock”-type alkylidene complex of vanadium bythermal decomposition via a-H abstraction from a neopentyl group (eq 4.5).15CpV(PMe3)(CII2Me dmpe CpV(dmpe)(CHCMe3) (4.5)- Me4, - PMe3The X-ray structure of this vanadium alkylidene shows that the a-H is strongly interactingwith the vanadium center. It is proposed to be an intermediate between an alkylidene andalkylidyne hydride species. The equilibrium between an alkylidene(Ta(CHCMe3)(dmpe)21)and alkylidyne hydride species (Ta(CCMe3)(H)(dmpe)21)hasbeen reported (dmpe = 1,2-bis(dimethylphosphino)ethane).6102The remainder of this chapter deals with an investigation of the reactivity of thepyridine-trapped alkylidene complex, CpMo(NO)(=CHCMe3)(py),which, due to thelability of the pyridine ligand, is more reactive than the phosphine-trapped analogue.4.2 Experimental Procedures4.2.1 MethodsThe synthetic methodologies employed throughout this thesis are described in detailin section ReagentsThe organometallic precursors used in this work, CpMo(NO)(CX2Me3)(X = H,D) and CpMo(NO)(=CHCMe3)(py),were prepared according to published procedures.’7Me3CNH2(Aldrich) was vacuum transferred from CaH2. Acetic acid (glacial, Fisher) wasdried on CuSO4 and filtered prior to use. Acetylacetone (Fisher), Me2CO (BDH) andPhCHO (Aldrich) were distilled from CaR2. All other reagents, i.e., H2 (Linde, extradry), CO (Linde), CO2 (Matheson, 99.99%), C2H4 (Matheson), (p-tolyl)NH2(Aldrich),(o-tolyl)NH2(Aldrich), neopentyl glycol (2,2-dimethyl-1,3-propanediol, Eastman Kodak),benzoic acid (Fisher), o- and p-nitrobenzoic acid (Eastman Kodak), oxalic acid (Fisher),succinimide (Aldrich), p-cresol (Aldrich), t-BuSH (Aldrich), Ph3SiOH (Aldrich), Ph2SiH(Aldrich), (Me3O)SIR (Aldrich), PPh2H(Aldrich) and PhCCH (Aldrich) were used asreceived.Filtrations were performed using Celite 545 diatomaceous earth (Fisher) supported onmedium porosity fits. NMR tubes used were equipped with Kontes gas inlets.1034.2.3 SynthesisIsolated yields, physical properties, and spectroscopic data for all new complexes arelisted in Tables 4.1-4.3. 1H and 2H N1v1R data for the generated monodeutero complexes(4.2-ti1,4.4-d1,4.6-d1 and 4.8-ti1)are contained in paragraphs following experimentaldetails of the non-deutero analogues. Preparation of CpMo(NO)(=CDCMe3)(py)(4.1-d1)Et20(20 mL) was added to CpMo(NO)(CD2Me3(100 mg, 0.294 mmol) andpyridine (45 mg, 0.57 mmol) and the mixture was stirred at 45 °C for 4 h. The final amberreaction mixture was taken to a brown oil in vacuo. Pentane (20 mL) was added to theoil, and the mixture was stirred at ambient temperatures for 30 mm. The solvent wasremoved in vacuo, the remaining yellow-brown powder was dissolved in Et20 (20 mL),and the solution was filtered through Celite (2 x 2 cm). The filtrate was concentratedunder reduced pressure and was then cooled at -30 °C overnight to induce the depositionof yellow-brown crystals of 4.1-ti1 (48 mg, 48% yield). Drying of these crystals in vacuoresulted in their becoming slightly discolored, probably due to their losing some of thecoordinated pyridine. Preparation of CpMo(NO)(CH2Me3)(OR) FR =C6114-p-Me (4.2), SiPh3(4.3)]The preparation ofCpMo(NO)(CHMe)(0C6H4- - e)(4.2)is given as arepresentative example. Benzene (10 mL) was vacuum transferred onto a solid mixture ofCpMo(NO)(CHCMe3)(py)(100 mg, 0.294 mmol) andp-cresol (32 mg, 0.30 mmol).Over the course of 1 h at room temperature a color change of the stirred mixture fromorange to red occurred. The final reaction mixture was taken to dryness in vacuo and wasextracted with Et2O (10 mL). The extract was filtered through Celite (2 x 2 cm),104concentrated in vacuo, and then maintained at -30 °C overnight in a freezer. In total,three fractions of red crystals (96 mg, 88% yield) were isolated by cannulation afterrepeated concentrations and crystallizations.CpMo(NO)(CH2Me3)(OSiPh(4.3) formed well defined blocks in 75% yield. Generation of CpMo(NO) (CHDCMe3(0C6H4-p-Me) (4.2-d1)CpMo(NO)(=CDCMe)(py)(4.1-d1)(30 mg, 0.087 mmol) and p-cresol (9 mg,0.09 mmol) were weighed into an NMR tube. C6D (0.8 mL) was added to the tube, andthe contents were shaken. The color of the NMR solution changed from amber to redwithin 5 mm. The 1H NMR spectrum revealed peaks assignable to complex 4.2-d1( 7.25 (m, 2H, Ph), 7.04 (m, 2H, Ph), 5.21 (s, 5H, C5H), 2.10 (s, 3H, Me), 1.45 (s, 1H,CHD), 1.19 (s, 9H, CMe3)) as well as free pyridine (ö 8.55 (2H), 7.31 (2ff), 6.95 (1ff)).C6D was then removed in vacuo, the contents of the NMR tube redissolved in C6H andthe 2H NMR (40 MHz) spectrum recorded. The spectrum revealed one broad peak(ö 3.43 (br s, 1D, CHD)). Preparation of CpMo(NO)(CH2Me3)(SCMe(4.4)CpMo(NO)(=CHCMe)(py)(273 mg, 0.803 mmol) in benzene (20 mL) was treatedwith an excess of t-butyl thiol introduced by vacuum transfer. The reaction mixture wasstirred for 1 h until it was deep red, and the solvent and excess thiol were then removed invacuo. The red residue was extracted with pentane (2 x 10 mL). The extracts werefiltered through Celite (2 x 2 cm), concentrated in vacuo, and then maintained at -30 °Covernight. Deep red crystals of 4.4 were isolated by filtration, and the remaining motherliquor was concentrated and cooled to obtain a second crop ofCpMo(NO)(CH2Me3)(SCMe(total of 205 mg, 73% yield).1054.2.3.5 Generation of CpMo(NO)(CIIDCMe3(SCMe3)(4.4-cu)Complex 4.4-d1was generated in a manner similar to that of complex 4.2-d1 (section4.2.3.3) except that the thiol was vacuum transferred onto the sample ofCpMo(NO)(=CDCMe)(py)(4.1-d1).1H NMR: 6 5.15 (s, 5H, C5H), 1.77 (s, 9H, CMe3), 1.22 (s, 9H, SCMe3)0.79(s, 1H, CHD). 2HNK4R (40 MFTz, C6H): 3.05 (br s, 1D, CHD). Generation of CpMo(NO)(CH2Me3)(NR) [R = H-o-tolyl (4.5), R = II-ptolyl (4.6), C{O}CII2H{O} (4.7), HCMe3(4.8)]The generation ofCpMo(NO)(CHMe)(NH-o-tolyl) (4.5) is given as arepresentative example. Samples of CpMo(NO)(=CHCMe3)(py)(30 mg, 0.087 mmol)and (o-tolyl)NH2(9 mg, 0.09 mmol) were weighed into an NMR tube. C6D (0.8 mL)was added to the tube, and the contents were shaken. After 1 week at ambienttemperatures, the once amber solution appeared red, and a 1H NMR spectrum of thereaction mixture revealed the quantitative conversion of the pyridine complex to complex4.5. Signals due to free pyridine were also evident in the spectrum.NMR spectra revealed that complex 4.6 was quantitatively generated in 24 h,while complexes 4.7 and 4.8 were generated in 5 mm. Over the course of several days atroom temperature the neopentyl succinimate complex (4.7) decomposed completely to avariety of Cp-containing products. Hence, no effort was made to isolate complex 4.7.Complexes 4.6 and 4.8 have previously been fully characterized.171064.2.3.7 Generation of CpMo(NO)(CRDCMe3)(NH ) [R = p-tolyl (4.6.-d1), CMe3(4.8-c1)]Complexes 4.6-d1 and 4.8-d1were generated in a manner similar to that of complex4.2-d1 (section except that t-butylamine was vacuum transferred onto the sampleof CpMo(NO)(=CDCMe3)(py)(4.1-d1)to generate complex 4.8-d1Complex 4.6-d1:1HNMR: ö 10.48, 9.83 (br s, 2H, 2 xNIi), 7.36, 6.97, 6.81, 6.38(d, 8H,2xHohoand2xHme,J=8.11Hz), 5.14, 5.12(s, 1OH,2xC5) 2.08,2.05 (s, 6H, 2 xp-Me), 1.78, 1.63 (br s, 2H, 2 x CHD), 1.32, 1.28 (s, 18H, 2 x CMe3).2H NMR (40 MHz, C6H): 2.32 (br s, 2D, CUD).Complex 4.8-d1:1HNIvIR: ö 8.92 (br s, 1H, NH), 5.21 (s, 5H, C5H), 1.59 (s, 1H,CHD), 1.30 (s, 18H, 2 x CMe3). 2H NMR (40 MHz, C6H): 2.12 (br s, ID, CUD). Preparation ofCpMo(NO)(CHMe)(q-OC{O}Me) (4.9)CpMo(NO)(=CHCMe)(py)(150 mg, 0.441 mmol) in benzene (20 mL) was treatedwith glacial acetic acid (290 p.L, 0.5 mmol) dropwise via a microsyringe. The color of thereaction mixture faded from dark orange to yellow as the acetic acid was added. Thereaction mixture was stirred for 30 mm to ensure complete reaction. The solvent andexcess acid were then removed in vacuo. The residue was extracted with pentane(2 x 10 mL). The extracts were filtered through Celite (2 x 2 cm), concentrated in vacuo,and then maintained at -30 °C overnight. Pale orange crystals of complex 4.9 (114 mg,81% yield) were isolated. Preparation of CpMo(NO)(CH2CMe3(ii2-OC{O}Ph) (4.10)C6H (20 mL) was vacuum transferred onto a solid mixture ofCpMo(NO)(=CHCMe3)(py)(106 mg, 0.3 09 mmol) and benzoic acid107(38 mg, 0.311 mmol). The reaction mixture was stirred for 1.5 h after which time thesolvent was removed in vacuo. The yellow residue was dissolved in Et20 (20 mL) andfiltered through Celite (2 x 2 cm). The filtrate was concentrated in vacuo and thenmaintained at -30 °C overnight. Two fractions of yellow crystals of complex 4.10 (totalof 103 mg, 86% yield) were isolated. Preparation ofCpMo(NO)(CH2Me3)(q-OC{O}R) IR =C6H4-o-N02(4.11),C61{4-p-N02(4.12)1The preparation ofCpMo(NO)(CHMe3) r-OC{ 0 }C6H4-o-N02)(4.11) is givenas a representative example. Et20 (20 mL) was added to a solid mixture ofCpMo(NO)(CHCMe3)(py)(100 mg, 0.292 mmol) and o-nitrobenzoic acid (49 mg,0.29 mmol) and the reaction mixture was stirred for 12 h. During this time a yellowpowder precipitated from the Et20 solution. The supernatant was removed from theyellow powder via cannulation, and the powder was washed with hexanes (10 mL) andEt20 (5 mL). The powder was then dissolved in CH21 (20 mL) and filtered throughCelite (2 x 2 cm). Complex 4.11 was recrystallized from aCH1/hexanes mixture(81 mg, 65%).Complex 4.12 was similarly prepared and recrystallized from aCH21/hexanesmixture in 75% yield. Preparation of CpMo(NO)(CH2CMe3[(OC(Me))2CH-O,0] (4.13)Complex 4.13 was synthesized in a manner similar to that of complexes 4.11 and4.12, except that the reaction mixture was stirred for 1.5 h and the product wasrecrystallized from aEt20/hexanes mixture in 72% yield.1084.2.3.12 Preparation of [CpMo(NO)(CR2e3)1(ji-OCHHO (4.14)C6H (20 mL) was vacuum transferred onto a solid mixture ofCpMo(NO)(=CHCMe3)(py)(320 mg, 0.94 1 mmol) and neopentyl glycol (49 mg,0.50 mmol). Over the course of 3 d at room temperature a color change from orange tomaroon occurred. The final reaction mixture was taken to dryness in vacuo, and theresidue was extracted with pentane (25 mL). The extract was filtered through Celite(2 x 2 cm), concentrated in vacuo, and then maintained at -30 °C for several hours. Afterthis time a brown precipitate was separated from the solution by cannulation, and the redsolution was returned to the freezer. The desired complex precipitated overnight as a redpowder. The red powder was recrystallized from pentane to obtain complex 4.14(149 mg, 50% yield) as an analytically pure red solid. Preparation of [CpMo(NO)(CH2CMe3]-ii-(riOC{O}CO{O}) (4.15)Et20 (20 mL) was vacuum transferred onto a solid mixture ofCpMo(NO)(=CHCMe3)(py)(100 mg, 0.292 mmol) and oxalic acid (14 mg, 0.16 mmol).The reaction mixture was stirred for 1 h during which time yellow solids precipitated fromthe reaction solution. The solvent was removed in vacuo, and the yellow powder wasdissolved in CH21 (20 mL) and filtered through Celite (2 x 2 cm). The filtrate wasconcentrated (10 mL) and hexanes (5 mL) added. This solution was then maintained at-30 °C overnight to obtain small orange crystals (68 mg, 76%) of complex Preparation of [CpMo(NO)(CH2e3)]( .- )(4.16)CpMo(NO)(=CHCMe)(py)(180 mg, 0.529 mmol) was treated with water (0.5 mL,excess) in benzene (20 mL). Over the course of 2 h at room temperature the stirredreaction mixture became red-brown. The final mixture was then taken to dryness invacuo, and Et20 (20 mL) was used to extract the red-brown residue. The red-brown109extract was filtered through Ceite (2 x 2 cm), concentrated in vacuo, and then maintainedat -30 °C for several days. Black-red, air-stable crystals of 4.16 (100 mg, 70% yield) wereisolated and dried in vacuo for several hours.4.2.4 Reaction of CpMo(NO)(=CHCMe3)(py)with PPh2HCpMo(NO)(=CHCMe3)(py)(320 mg, 0.941 mg) was treated with PPh2H(0.17 mL,0.95 mmol) in THF (20 mL) at -78 °C. The reaction solution was warmed slowly to roomtemperature and stirred for 1 h. Removing the solvent in vacuo after this time yieldedbrown intractable solids.This reaction was also performed in an NMR tube for monitoring purposes. In aglovebox, CpMo(NO)(=CHCMe3)(py)(30 mg. 0.9 mmol) was dissolved in C6D(0.6 mL), the solution transferred to an NIVIR tube and two pipette drops ofPPh2Hwereadded to the solution. The 1H NMR spectrum was recorded immediately.4.2.5 Reactions of CpMo(NO)(=CHCMe3)(py)with Other Reagents (OtherReagents = H2, CO, CO2C2H4,Ph2SiIl,PhCO, Me2CO, (MeO)3SiH, PhCCH,MeCN)These reactions were performed in NMR tubes in a manner similar to that describedabove (section 4.2.4). In the case of the gases, the NMR tube containingCpMo(NO)(=CHCMe3)(py)and C6D was removed from the glovebox and then placedin a Dewar of liquid N2. The N2 atmosphere in the NMR tube was removed and then theappropriate gas introduced (1 atm). The NMR tube was shaken to dissolve the gas.NMR spectra were recorded within 1 h of being made up as well as 2 d later.110Table 4.1 Numbering Scheme, Color, Yield, and Elemental Analysis Datacomp coloranal._found_(caic)complex no. (yield, C H N%)CpMo(NO)(=CDCMe3)(py) 4.1...iJ amber (48) 52.02 (52.95) 5.96 (5.92) 8.09 (8.23)CpMo(NO)R(0C6H4- -Me) 42 red (88) 55.32 (55.28) 6.29 (6.29) 3.85 (3.97)CpMo(NO)R(OS1Ph3) 43 red (75) 62.63 (62.55) 5.76 (5.82) 2.47(2.61)CpMo(NO)R(SCMe 44 red (73) 47.98 (47.85) 7.12 (7.19) 3.96 (3.99)CpMo(NO)R(N1{-o-tolyl) 45 red (100) bCpMo(NO)R(N1{-p-tolyl) 4.6 red (100) bCpMo(NO)R(NC{O}CH2H 47 red (100) bC{O})CpMo(NO)R(NHCMe3 48 red (100) bCpMo(NO)R(i12-OC{O}Me) 4.9 yellow (81) 45.06 (44.86) 6.13 (5.97) 4.37 (4.36)CpMo(NO)R(-OC{O}Ph) 4.10 yellow (86) 53.19 (53.26) 5.42 (5.53) 3.56 (3.65)CpMo(NO)R(-OC{O}C6H4 4.11 yellow (65) 47.35 (47.67) 4.63 (4.72) 6.32 (6.54)o-N02)CpMo(NO)R(1-OC{O}C 4.12 yellow (75) 47.96 (47.67) 4.61 (4.72) 6.69 (6.54)p-NO2)CpMo(NO)R[(OC(Me))H- 4.13 orange- 49.90 (49.86) 6.56 (6.42) 3.90 (3.87)0,01 yellow (72)[CpMo(NO)Rj2(- 4.14 red (50) 47.94 (47.92) 6.86 (6.77) 4.44 (4.47)OCHCMeH)[CpMo(NO)R]2-j.t-(r 4.15 orange 42.94 (43.14) 5.20 (5.28) 4.53 (4.57)OC{O}C{O}O) (76)[CpMo(NO)R](j.t- ) 4.16 red (70) 44.46 (44.46) 5.99 (5.97) 5.20 (5.18)R = CH2Me3a Based on 1H NMR.b Product not isolated (NMR experiment).111Table 4.2 Selected Mass Spectral and Infrared Datacomp MS temp” JR (Nujol mull)no. nilzb (°C) VNO other strong bands4.1 343 [P1 120 1532 (vs) 1545 (br,vs)4.2 317 [P - CMe3] 100 1612 (vs)43 539[Pj 150 1607 (vbr)482 [P-CMe3]4,4 353 [Pj 120 1631 (vs)295 [P-CMe3] 1608 (sh)4.5 C46 C47 C4.8 C49 323 [P9 120 1612 (br) 1530 (s), 1426 (s) (12_ carboxylate)252 [P-CHCMe]4.10 385 [Pj 100 1620 (vs)’1 1498 (s), 1447 (s) (2_ carboxylate)870, 852, 810, 714, 6914.11 430 [P1 150 1616 (br)” 1533 (vs), 1495 (sh), 1435 (br), 1357(2.. caiboxylate)931, 877, 848, 823, 795, 777, 731,711, 6494.12 430 [P9 150 1637 (sh), 1537 (s), 1441 (s), 1342 (s), 13171619 (br)’1 (sh) (i2.. carboxylate)843, 820, 719, 6674.13 363 [P1 100 1602 (sh), 1524 (s), 1423 (m), 1368 (br)1580 (vs br)’ (2.. carboxylate)929, 812, 7764.14 555 LP-CH2CMe3i 100 1598 (br)4.15 612 LP1 200 1676, 1607 (vs 1471 (sh), 1461 (m), 1441 (sh), 1425br)’ (m), 1352 (m) (2_ carboxylate)928, 818, 7944.16 540 [P4] 120 1599, 1575 933, 839, 804(vs) (Mo-O-Mo)a Probe temperatures.b Values for the highest intensity peak of the calculated isotopic cluster(98Mo).C Not recorded (NMR experiment).d KBr pellet.112Table 4.3 NMR Data (C6D)comp 111 NMR (6) 13C NMR (8)no.4.1 8.27 (d, 2H, pyridine protons, HH = 1.5 1.r) b6.65 (t, 1H, pyridine proton, HH =6.23 (t, 2H, pyridine protons, HH = 5.8 Hz)5.50 (s, 511, C5H)1.50 (s, 911, CMe3)4.2 7.25 (m, 2H, Ph00) 131.9 (C10)7.03 (m, 2H, meta) 130.1 (C00)5.21 (s, 5H, C5H) 117.4 (Cme)3.43 (d, 1H, CH2HH = 10.8 Hz) 104.6 (C5H)2.12 (s, 3H, Me) 75.1 (CH2)1.47 (d, 111, Cl2HH = 10.8 Hz) 39.1 (CMe3)1.18 (m, 9H, CMe3) 33.4 (CMe3)20.7 (Me)43 7.78 (m, 6H, Ph) 137.5 (C10)7.15 (m, 911, Ph) 135.7 (C.)5.07 (s, 511, C5H) 130.0 (Cme)3.79 (d, 1H, CH2HH 9.9 HZ) 128.2 (Cpai.a)1.01 (s, 911, CMe3) 104.3 (C5H)0.99 (d, 1H, CH2HH = 9.9 Hz) 83.3 (CH2)39.6 (CMe3)33.1 (CMe3)44 5.15 (s, 5 H, C5H) SCMe3 not observed3.05 (ci, 111, CH2HH 10.2 Hz) 101.8 (C5H)1.78 (s, 9H, CMe3) 70.9 (CH2)1.24 (s, 9H, SMe3) 39.2 (CMe3)0.79 (d, 111, CH2HH = 10.2 Hz) 35.1 (CMe3)33.7 (SCMe3)4•5 10.58, 9.90 (br s, 2H, NH”) b7.20 (m, 2H, Ph)6.91 (m, 4H, Ph)6.72 (m, 2H, Ph)5.21, 5.17 (s, 1OH, C5H)2.41 (dd, 2H, CIT2.HH = 9.9 Hz)1.75 (dci, 2H, CH2HH = 9.9 Hz)1.71, 1.60 (s, 611, 0-Me)1.38, 1.31 (s, 1811, CMe3)1134.6 10.49, 9.84 (br s, 2H, 2 x NH)” b7.37, 6.96 (d, 4H, 2 xH00,HH = 7.4 lIz)6.81, 6.38 (d, 4H, 2 x Hmeta, HH = Hz)5.14, 5.12 (s, 1OH, 2 xC5H)2.26 (2 d, 1H each, CH2HH = 12.9 Hz)2.08, 2.05 (s, 6H, 2 xp-Me)1.82 (d, 1H, CH2HH = 10.8 Hz)1.67 (d, IH, CH2,HH = 11.1 Hz)1.32, 1.28 (s, 1811, 2 x CMe3)47 5.25 (s, 5H, C5H) b4.56 (d, 1H, CH2HH = 9.6 Hz)1.90 (s, 4H, 2 x CH2)1.27 Cs, 9H, CMe3)0.41 (d, 1H, CIT2.HH = 12.0 Hz)4.8 8.92 (br s, 1H, NH) b5.22 (s, 5H, C5H)2.16 (d, 111, CH2HH 11.8 Hz)1.61 (d, 111, CH2HH = 11.8 Hz)1.30 (s, 1811, 2 x CMe3)49 5.14 (s, 5H, C5H) 188.2 (CO)2.74 (d, 1H, CH2HH = 12.0 Hz) 104.2 (C5H)1.80 (d, 1H, CH2HH = 12.0 Hz) 67.0 (CH2)1.56 (s, 311, Me) 38.9 (CMe3)1.33 (s, 9H, CMe3) 33.9 (CMe3)23.0 (Me)4.10 8.02 (m, 211, Ph/ 181.2 (OCOPh/7.59 (m, 2H, Ph) 133.6, 130.2, 129.1, 128.4 (C1)7.42 (t, 1H, Ph) 104.5 (CH)5.89 (s, 5H, C5H) 68.7 (CH2)2.96 (d, 1H, CH2‘HH = 11.4 Hz) 39.1 (CMe3)1.77 (d, 1H, Cl2HH = 11.4 Hz) 33.6 (CMe3)1.14 (s, 9H, CMe3)4.11 7.87 (m, 3H, Ph) OCO not observedf7.65 (m, IH, Ph) 133.2, 132.9, 130.3, 127.7, 124.15.94 (s, 5H, C5H) (C1)3.04 (d, 1H, CU2HH = 11.1 Hz) 105.0 (C5H)1.72 (d, 1H, CU2,JHH = 11.1 Hz) 71.0 (CH2)1.13 (s, 9H, CMe3) 39.4 (CMe3)33.6 (CA’fe3)1144.12 8.28 (m, 4H, Phf OCO not observed!5.91 (s, 5H, C5H) 151.2, 135.8, 130.5, 123.1 (C1)3.06 (d, IH, CH2HH = 11.4 Hz) 105.0 (C5H)1.77 (d, 111, Cl2HH = 11.4 Hz) 69.9 (CH2)1.14 (s, 9H, CMe3) 39.3 (CMe)33.6 (CMe3)4.13 5.72 (s, 5H, C5H/ 190.0 (OCMe2CH O5.44 (s, 1H, CH) 105.4 (C5H)2.27 (d, 1H, CH2HH = 11.4 Hz) 101.4 (OCMe2CHO)2.00 (obs, 1H, Cl2) 66.6 (CH2)1.99 (s, 6H, 2 x CMe2) 38.2 (CMe3)1.06 (s, 9H, CMe3) 33.9 (CMe)27.1 (OCMeCHO)4.14 5.40, 5.37 (s, 1OH, 2 x C5H) 104.9, 104.8 (2 x C5H)5.01, 4.97 (d, 2H, CHMeHH = 5.7 Hz) 90.7, 90.3 (CHCMe)4.76, 4.74 (d, 2H, CHMeHH = 2.7 Hz) 63.8, 63.1 (2 x Cl2)2.87, 2.83 (d, 2H, CH2,HH = 11.4 Hz) 40.9, 40.6 (2 x CMe3)1.67, 1.59 (d, 2H, CH2HH = 11.4 Hz) 38.0 (CH2CMe)1.28, 1.27 (s, 18H, 2 x CMe3) 33.6 (CMe3)0.93 (m, 6H, CH2Me) 21.6, 21.5 (2 xCH?vfe)4.15 5.82 (s, 511, C5H/ 203.5 (02CC0)r2.96 (d, 1H, CH2,JH = 11.4 Hz) 105.1 (C5H)2.91 (d, 1H, CH2HH = 11.4 Hz) 67.5 (CH2)2.14 (d, 1H, CH2,JH = 11.4 Hz) 39.4 (CMe3)2.13 (d, 111, CH2HH = 11.4 Hz) 33.8 (CMe3)1.08 (s, 9H, CMe3)4.16 5.37, 5.30 Cs, 1OH, 2 x C5H) CMe3 not observed3.03, 2.99 (d, 2H, CH2HH = 11.4 Hz) 105.0, 104.8 (2 x C5H)2.74, 2.70 (d, 2H, CH2HH = 11.4 Hz) 65.8, 65.0 (2 x CH2)1.99, 1.96(d, 2H, CH2,JH ’ 11.4 Hz) 34.0 (CMe3)1.67, 1.63 (d, 2H, CH,JH = 11.4 Hz)1.29, 1.25 (s, 18H, 2 x CMe3)a 2H NMR (40 MHz, C6H) 13.61 ppm.b Not Recorded.C Isomer ratio = 1:1.7.d Isomerratio=1:1.e Two superimposed doublets.f Recorded in CDC13.g Recorded in CD21.1154.3 Results and DiscussionSince this chapter deals with the reactivity of the pyridine-trapped neopentylidenecomplex, CpMo(N0)(=CHCMe3)(py),with a range of reagents, each class of reagent isdiscussed separately.4.3.1 AlcoholsAddition of the 0-H bond ofp-MeC6H4OHor Ph3SiOH across the Mo=C doublebond of CpMo(N0)(=CHCMe3)(py)affords the alkyl alkoxo complexesCpMo(N0)(CH2e)(0C6H4- -M )(4.2) and CpMo(N0)(CHe)(OSiPh(4.3)respectively (eq 4.6).oNH\ N”I”O(4.6)PY CMe3 0 R’ RR p-tolyl (4.2), SIPh3 (4.3)R’ = CH2Me3The 1H NIVIR spectrum of complex 4.2 is shown in Figure 4.1. It is a typical1H NIvIR spectrum of an alkyl alkoxide complex with peaks attributable to twodiastereotopic methylene protons, Cp protons, and t-butyl protons, as well as peaks due tothe methyl and phenyl protons of the alkoxide ligand.The two reactions depicted in eq 4.6 are quantitative in C6D as judged by the NMRspectra of the appropriate mixtures and are complete in less than 30 mm at roomtemperature. Although the Cp* analogue complexes, Cp*M(N0)(R)(OR), for tungsten116, .......•,.. .Figure 4.1 300 MHz 1H NMR spectrum ofCpMo(NO)(CH2Me3)(0C6H4- - )(4.2)in C6D.— I I I • I II J I I I I I •:_I:__:•. ,Figure 4.2 300 MHz 1H NMR spectrum of the crude reaction mixture ofCpMo(NO)(CHCMe3)(py)with succinimide in C6D.117and molybdenum have been previously synthesized by metathesis routes, thecorresponding Cp complexes are not accessible via this synthetic method.18 This isprobably due to ease of reduction of the alkyl chloro precursor complexesCpM(NO)(R)(C1) with the alkoxide salts. It may also be noted here that Schrock andcoworkers have shown that Cp*WMe3(=CH2)reacts withC6F5OH to giveCp*WMe4(0C6F5).19 Osborn and coworkers have also demonstrated thatMo(NR)(=CHCMe3) C2CMereacts withC6F5OH to produceMo(NR)(CHCMe(0C.0Similarly,HCHMe)(=CHSiMe(Si=NPh)reacts with t-butyl alcohol to yield64TMe)( HSi(=NPh)(OCMe,but reacts with HOSiPh3to producethe new alkylidene complex,W(C64H2N e)(CHSiMe3(=NPh)(OSiPh.1Thisreplacement of an alkyl group by the weakly it-donating triphenylsiloxy group has alsobeen observed by Osbom et al.20The Mo-CH2CMe3bond ofCpMo(NO)(CHMe3)(0C64- - e)(4.2) is resistantto protonolysis at room temperature. Complex 4.2 does not react with excess p-cresol atroom temperature. At elevated temperatures (60 °C) for 4 days, however, thebis(alkoxide) complex, CpMo(NO)(0C6H4- -Me)2is formed.224.3.2 ThiolTreatment of the pyridine adduct CpMo(NO)(=CHCMe3)(py)with t-butyl thiolresults in the rapid formation of the neopentyl thiolate complex,CpMo(NO)(CH2Me3)(SCMe(4.4), in a manner completely analogous to that of theneopentyl alkoxide complexes considered above (eq 4.7).118Mo RSCM4 Mo (4.7)oNH\PY CMe3 0 R’ RR’ = CH2Me3,R = CMe3 4.4The reaction is also quantitative in C6D as judged by the 1H NMR spectrum.Complex 4.4 probably has a molecular structure similar to that possessed byCpW(NO)(CH2SiMe3)(SCHwhich has been previously prepared by the insertionof elemental sulfur into CpW(NO)(CH2SIMe3.34.3.3 AminesTreatment of a C6D solution of CpMo(NO)(=CHCMe3)(py)with primary orsecondary amines quantitatively affords the neopentyl amido complexes of the typeCpMo(NO)(CH2Me3)(NR)[R = H-o-tolyl (4.5), H-p-tolyl (4.6), C{O}CH2H{O)(4.7), HCMe3 (4.8)] (eq 4.8).119MoN/HN0 R’04.7Complexes 4.5 and 4.6 are similarly produced when the bis(neopentyl) complex,CpMo(NO)(CH2Me3),is thermolysed in the presence ofH2N-o-tolyl orH2N-p-tolyl(eq 4.9). 17 This thermolysis method of generating the transient neopentylidene fragment,CpMo(NO)(CHCMe3in situ, and trapping it in the presence of the appropriate amine, isalso successful in the formation ofCpMo(NO)(CH2Me3)(NHCMe.17Me3C N CMe30Mo0N”N-V...siicinimideP0Nl0HPY CMe3R’ = CII2Me3R = R-o-tolyl (4.5), Hp-toIyI (4.6),HCMe3 (4.8)(4.8)RT- CMe4NHR1110N” J,\NR(4.9)R’ CH2Me3R = H-p-tolyl (4.6), HCMe3(4.8)120The succinimide complex, CpMo(NO)(CH2Me3)(NC(0)CH2(0)) (4.7) has,not, however, been previously characterized. The 1H NMR spectrum of an equimolarmixture of CpMo(NO)(=CHCMe3)(py)and succinimide in C6D is shown in Figure 4.2.The reaction is quantitative as judged by the spectrum with peaks attributable to freepyridine as well as the resulting neopentyl succinimide complex,CpMo(NO)(CH2Me3)(N (0)CHCHC(0)) (4.7). This complex decomposes overthe course of hours at room temperature and converts to numerous products as judged by1H NMR spectroscopy. Consequently, all attempts to isolate this complex have beenunsuccessful.Complexes 4.5 and 4.6 form as 1.7:1 and 1:1 mixtures of rotational isomers,respectively, probably resulting from hindered rotation about the multiple Mo-amidelinkage.24 Crystallographic evidence supports the fact that multiple-bond character existsin the Mo-N linkage ofCpMo(NO)(CH2Me3)(NH-p-tolyl) (4.6). 17 Furthermore, theplanarity of the amide ligand in this complex rules out slow inversion at N as a cause forthe existence of two isomers. A variable temperature NMR study of 4.6 reveals only linebroadening (80 °C in C6D)and then decomposition of the compound before anycoalescence of signals is observed.4.3.4 Carboxylic AcidsThe reaction of CpMo(NO)(=CHCMe3)(py)with acetic acid is instantaneous andquantitative, and the transformation is accompanied by a color change from amber to paleyellow. The acetate complex formed (4.9) is stable to the presence of excess acetic acid, afact which again shows the resistance of the Mo-CH2CMe3bond to protonolysis (videsupra) (eq 4.10). The pyridine-trapped neopentylidene complex reacts similarly withbenzoic and o- and p-nitrobenzoic acids. The reaction time for benzoic acid is 30 mm121while that for o- and p-nitrobenzoic acid is considerably longer (24 h). The longerreaction time of the latter two carboxylic acids is probably a manifestation of theinsolubility of the acids in the reaction solvent.M RCO{O}II M°- py / °c•,Q- (4.10)0 NPY CMe3 0= CH2Me3R = Me (4.9), Ph (4.10), Q114-o-N02(4.11),C.5H4-p-N02(4.12)Complexes 4.9, 4.10, 4.11 and 4.12 are formulated as containing2-OC{O}R groupsbecause of the difference between the symmetric and asymmetric stretching modes of theCO moieties in either their Nujol-mull or KBr 1R spectra. The two-band separation(104 cm-1 for 4.9, 51 cm-’ for 4.10, 98 cm1 for 4.11 and 96 cm for 4.12) are wellwithin the range normally associated with bidentate carboxylate ligands.25 For 11 LOC{O}R groups the band separation is typically> 200 cm. By coordinating thecarboxylate group in a bidentate fashion the metal can also attain the favorable 18-valence-electron configuration.The carboxylate ligand is well established, and numerous examples of bidentatecarboxylate transition-metal complexes are reported in the literature.26 Othermolybdenum and tungsten nitrosyl bidentate carboxylate complexes have been synthesizedby standard metathesis methods. For example, reactions of the benzyl chloride complexes,Cp’M(NO)(CH2Ph)Cl (Cp’ = Cp, Cp*, M = Mo, W) with the silver phenylbutyrate salt,122AgO2CCH(Et)Ph, results in benzyl carboxylate products, Cp’M(NO)(CH2Ph) r-OC{O}CH(Et)Ph).27 Other carboxylate complexes have been formed from the insertionof CO2 into metal-aryl bonds (eq 4.1 1).28w 2 W “9 (4 11)/ I N’ I “OPh0 Ph 0 Ph4.3.5 AcetylacetoneAcetylacetone (2,4-pentanedione) exists mainly in the enol form, shown below, andCpMo(NO)(=CHCMe3)(py)reacts with this enol form of acacH in a similar manner tothat of a carboxylic acid.1*The reaction of acacH with CpMo(NO)(=CHCMe3)(py)is instantaneous as judged bythe color change of the reaction mixture from amber to yellow (eq 4.12). The productCpMo(NO)(CH2Me3)[(OC(M ))H-O,OJ(4.13) is formulated as having the acacligand bidentate in a manner analogous to the carboxylate complexes (4.9 - 4.12) (videsupra).123oNH\ oN”IOX(4.12)PY CMe3 R IR = CH2Me3 4.13‘3C{’H} NMR spectroscopy proved usefi.il in the characterization of complex 4.13.The‘3C{’H} NMR spectrum (Figure 4.3) exhibits peaks due to the two quaternarycarbons, two methyl carbons, and one methine carbon, as well as peaks due to theneopentyl and cyclopentadienyl ligands.Maitlis and coworkers have previously synthesized diketonato complexes of the typeCp*Rh(Cl)[(OCRCHRCO)O,O) from the reaction ofCp*2Rh14withNa(RCOCHCOR’).29Bergman’s group has also reported osmium diketonato complexesof the type, (Cym)Os[(OCR)2CH-O,O](X where Cym =t6-cymene and X = Cl,CH(CO2Me).30 Other nitrosyl diketonate complexes that have been synthesized includeCpMo(NO)(I)[(OC(Me))H-O,O]1as well as the 1 6-valence-electron chromiumcomplex, CpCr(NO)[(OC(Me))H- ,OJ.32Lippard has recently reported a generalroute to heterobimetallic complexes with bidentate bridging acac ligands which is asignificant development since many metalloenzymes contain bridging carboxylate-typeligands.33Erker and coworkers have recently shown that protonation by acetylacetone of acationic(2-pyrazolyl-N, N’)zirconocene complex results in a pyrazole acetylacetonatezirconocene salt (eq 4. 13).34124Figure 4.3 75 MHz‘3C{’H) NMR spectrum ofCpMo(NO)(CH2Me3)[(OC(M ))H-0,01(4.13) in CDCI3.The inset represents the quaternary carbon peak at 190.0 ppm.Figure 4.4 300 MHz 1H NMR spectrum of [CpMo(NO)(CH2e3)J( -OCH2CMeH)(4.14) in C6D. The inset represents an expanded view of thedoublets.I1 III III liii I I I III IjI I liii I I III I I I I I I I I I I IiOO Sb 50 40 PPMJJ&iPPN125NHCp2Zr acac Cp2Zr\. (4.13)THF4.3.6 Neopentyl Glycol, Oxalic Acid and WaterThe above-mentioned reagents are classed together since each have two reactive 0-Hbonds which react with CpMo(N0)(CHCMe3)(py).Neopentyl glycol (2,2-dimethyl-1,3-propanediol), reacts with two equivalents of thepyridine-trapped neopentylidene complex to yield the bimetallic species,[CpMo(N0)(CH2e3)](j.t-OCHHO (4.14) (Scheme 4.1).The 1H NvIR spectrum of complex 4.14 is shown in Figure 4.4 and reveals a mixtureof isomers (1:1 ratio). The four sets of doublets due to the diastereotopic methyleneprotons on the neopentyl groups occur upfield (ö 1.67, 1.59, 2.87, 2.83) of the fourdoublets due to the methylene protons in the bridging neopentyl glycol ligand (ö 4.76,4.74, 5.01, 4.97). The former doublets exhibit coupling constants (11.4 and 11.7 Hz)greater than those of the latter doublets (2.7 and 5.7 Hz).This reaction can be viewed as proceeding via an intermediate monomeric neopentylalkoxide species, CpMo(N0)(CH2e3)(OCHHO )(Scheme 4.1).The 0-H bond of this putative intermediate species then adds across the Mo=C bond ofanother equivalent of alkylidene resulting in the diolate bridging two molecules ofCpMo(N0)(CH2e3).Even in the presence of excess diol, the 0-H bond in thisputative intermediate complex is evidently more reactive than that of the diol.126Scheme 4.1HOROHON-LON/ IPY CMe3 R’= CII2Me3R =CHMeII(4.14)X =CH2MeOUMo— ORO—MOiV\ /RN N0 0As mentioned earlier, the experimental results are consistent with the existence ofMoCH2Me3bonds in the alkyl alkoxide complexes that are stable to excess alcohol. Forinstance, treatment of a CpMo(NO)(CH2Me3)(OR)complex with an excess ofROHdoes not lead to the formation of CpMo(NO)(OR)2under ambient conditions (see section4.3.1).Interestingly, work by Stephan and coworkers35has shown that treatment ofCp2ZrMe with neopentyl glycol does not afford the cyclic bis(alkoxide) complex, butrather a bimetallic complex with two bridging dialkoxy ligands (eq 4.14).127...iiMe Ho OH2 ZrMe -2CH4(4.14)JI..IIIlij Ill 1111111 I IIIFigure 4.5 300 MHz 1H NMR spectrum of[CpMo(NO)(CH2C e3)]-ji-(iOC{O)C{O)O) (4.15) in CD21. The inset represents an expanded view of the 4 sets ofdoublets.Figure 4.6 75 MHz‘3C{’H} NMR spectrum of[CpMo(NO)(CH2C e3)]-.t..(rOC{O}C{O}O) (4.15) in CD2I. The inset represents the quaternary carbon peak at203.5 ppm.128Similarly, the reaction of CpMo(NO)(CHCMe3)(py)with oxalic acid results in theformation of the bridging bis(carboxylate) complex, [CpMo(NO)(CH2e3]-.i-1OC{O)C{O}O) (4.15) (eq 4.15).2 Mo../’ONPY CMe3i20 200 ISD 4b PPMHOC{O}CC{O}OH- py (4.15)R CI{2Me3 4.15129The reaction leading to 4.15 occurs in the presence of 0.5 equivalent or excess oxalic acid,but no monomeric CpMo(NO)(CH2Me3) i-OC(O}C{O}O ) species can be isolatedfrom the final reaction mixture. Experimental results are again consistent with the MoCH2Me3bond in the carboxylate alkyl complex being stable to excess acid at ambienttemperatures.The 1H and‘3C{1H} NMR spectra of complex 4.15 are shown in Figures 4.5 and4.6, respectively. In the 1H NMR spectrum four sets of doublets occur, each withcoupling constants of 11.4 Hz which is typical of that exhibited by the diastereotopicmethylene protons on the neopentyl group. Two other peaks are evident in this spectrum,one peak attributable to the Cp protons and one due to the methyl protons of theneopentyl group. The‘3C{’H} NMR spectrum of this complex exhibits one peak due tothe quaternary carbons of the bridging carboxylate ligand, and one due to the Cp protons,as well three peaks due to the carbons in the neopentyl ligand.Treatment of the pyridine adduct CpMo(NO)(=CHCMe3)(py)with water leads to thebridging oxo complex, [CpMo(NO)(CH2e3)](i- )(4.16) (eq 4.16).2MoçJ1 (4.16)PY CMe34.16Crystals of 4.16 are air-stable, red and diamagnetic. NtvlR spectra of complex 4.16indicate that it exists as a mixture of isomers (1:1 ratio), a feature that has been found for130other complexes of the general type [Cp’Mo(NO)R]2( -O)which are formed by directhydrolysis of Cp’Mo(NO)R2complexes.364.3.7 Other ReagentsThe alkylidene complex is unreactive towards H2 (1 atm), CO (1 atm), CO2 (1 atm),C2H4(1 atm), Ph2SiH PhCHO, Me2CO, (MeO)3SIH, or PhCCH at room temperature.In the presence of these reagents the pyridine adduct merely decomposes to the bridgingnitrosyl dimer (eq 4.17). Ideas on why these reagents are unreactive towards thealkylidene complex are presented in section Mo L Mo Mo (4.17)ON -py HPY CMe CMH CMe3 N0e3L = olefins, Me2CO, (Ph)RCO, PhCC1I,Ph2S1H CO, C022The reaction of CpMo(NO)(=CHCMe3)(py)with MeCN yields a plethora ofproducts, based on the number of peaks assignable to Cp in the 1H NMR spectrum of thefinal reaction mixture. By way of contrast, the reaction of the tantalum alkylideneTa(CH2CMe3)(=CH Me with MeCN yields a mixture ofE and Z isomers ofTa(CHCMe(N(Me)C=CHCMe3).71314.3.8 Related Research EffortsAddition reactions of nucleophilic alkylidene complexes with acids such as HCI havebeen reported.38 The Bergman group has shown that Cp*Ir(PMe3)(=CH2reacts withphenols, primary alcohols, succinimide, and t-butyl thiol to afford alkoxide, hydride, amideand thiolate methyl complexes, respectively.39 Interestingly, CO2 also reacts withCp*Ir(PMe3)(=CH2to form an addition product, Cp*]lr(PMe3)(CH2{O}O), but olefinsand acetylenes fail to give addition products. Thus, both Cp*Ir(PMe)(CH andCpMo(NO)(=CHCMe3)(py)have a very polar M=C bond and are very nucleophilic at C.An interesting reaction that is the reverse of that observed in this chemistry occurswith Fischer carbene complexes. Thus, (CO)5Cr(=C{Ph}OMe) reacts withR3M-H[M = Si, Ge, Sn] to produce (CO)5Cr(C{H} {Ph}OMe)(MR3complexes which upontreatment with Lewis bases, L, convert to (CO)5CrL and3M-C(H)(Ph)OMe.40Themechanistic path for these conversions is believed to be dominated by associativenucleophilic attack of hydride on the carbene carbon.4.3.9 Labeling StudiesSince the bis(neopentyl) complex, CpMo(NO)(CH2Me3)is very thermally sensitiveand cannot be isolated, John Veltheer synthesized the tetradeutero analogue,CpMo(NO)(CD2Me3)which is considerably more stable.’7 He established that thethermolysis of CpMo(NO)(CD2Me3in the presence ofNH2-p-tolyl affords the monodeutero complex CpMo(NO)(CHDCMe)(NH-p-tolyl) (eq 4.18).132______EH (4.18)N’ I R’ -CMeCD3 I0 W RR’ = CD2Me3 E = Op-to1yI (4.2-d1),NH-p-tolyl (4.6-d1),NHCM.3(4.X.4)R = CHDCMeThe resulting monodeutero product confirms that the N-H bond of the amide liganddoes in fact add across the Mo=C bond of the transient CpMo(NO)(=CDCMe3)fragment(Route A below). If mere protonolysis of the alkyl ligand of the bis(deutero) neopentylcomplex by the amine had occurred, the resulting product would beCpMo(NO)(CD2Me3)(NH-p-tolyl) (Route B).Route A:- Me4C-d3 + RNR2Mo(CD2CMe3) Mo(CDCMe3) Mo(CDHCMe3)(NIIR)Route B:Mo(CD2CMe3)+ RNH2 Mo(CD2CMe3)(NRR)- Me4C-d2 Mo(CD2CMe3)(NHR)The pyridine-trapped deutero(neopentylidene), CpMo(NO)(=CDMe3)(py)wassubsequently synthesized in order to ascertain the stereoselectivity of these additionreactions. The reaction of CpMo(NO)(=CDMe3)(py)with p-cresol for example could inprinciple result in the generation of two diastereomeric pairs of enantiomers (R,S/S,R andR,R/S,S) due to the chiral centers at molybdenum and at the alkyl carbon. The additionreactions of the pyridine adduct with p-cresol, amines or t-butyl thiol are in factstereoselective as only one diastereomeric pair of enantiomers is formed (eq 4.19). Since133the reaction products are only spectroscopically characterized (see section 4.2.3), exactlywhich pair is formed cannot be determined.N/IKE(4.19)PY CMe3 0 RR=CIIDCMe3£ = O-p-tolyl (4.2-d1),SCMe3 (4.4-d1),NH-p-tolyl (4.6-d1),NIICMe3(4.8-d1)Gladysz and coworkers have shown that the nitrosyl rhenium complex,CpRe(NO)(PPh3)(OTf), reacts with primary and secondary amines to give the aminecomplexes, [CpRe(NO)(PPh3)(NHRR)j+OTf-, with retention of configuration atrhenium.41Interestingly, the same pair of enantiomers is formed regardless of whetherCpMo(NO)(=CDCMe3)(py)is reacted with the E-H (alcohol, amine or thiol) reagent orCpMo(NO)(CD2Meis thermolysed in the presence of these reagents (eq 4.18).The question of whether the addition of the E-H bond across Mo=C is cis or trans, cannotbe answered without knowing exactly which diastereomer the product is, the identity ofwhich can only be unambiguously determined by X-ray crystallography. If one onlyconsiders the chirality at the carbon center of the alkyl group, trans addition would givethe S isomer while cis addition would afford the R isomer as shown in Scheme 4.2.42134Scheme 4.2Mo_________ER •Mo D- py0N >\tMe3py CMe3cis addition R, RER Mo iiMo- pyj )‘CMe3py CMe3trans addition R, S4.3.10 Mechanistic ConsiderationsThe most plausible mechanistic pathway for the reactions of the alkylidene with E-Hreagents is shown in eq 4.20.Mo cUR’Mo = Mo —CH2R’_______I I I I(4.20)/EH E£ HJR” LR /RMo = CpMo(NO); R’ = CMe3;E =0, S, Nil; R = alkyl, aryl135This mechanism involves initial pyridine exchange for incoming base to form a Lewis-acid Lewis-base adduct. This is rapidly followed by proton transfer, possibly via a fourcentered transition state. Base exchange of pyridine for PMe3 has been established. 14The necessity to form an initial Lewis-base Lewis-acid adduct (on the left in eq 4.20)could explain why reagents without a lone electron pair on E (E = C, Si) fail to react withCpMo(NO)(=CHCMe3)(py). Monitoring of the reaction of CpMo(NO)(=CHCMe3)(py)with Ph3SiOH by low-temperature 1H NMR spectroscopy failed to reveal the presence ofany intermediate species such as the putative alcohol complex,CpMo(NO)(=CHCMe)(O H}SiPh.This may be due to the rapid rate of the secondproposed step, namely, proton transfer to the nucleophic cs-carbon.In an effort to support this mechanism, an ideal reagent to react with the alkylidenewas thought to be diphenyiphosphine (PPh2H). At low temperatures this reagent mightexchange with pyridine to form the phosphine trapped alkylidene,CpMo(NO)(=CHCMe3)(PPh2H)which upon warming could effect intramolecular protontransfer to form the alkyl phosphide complex, CpMo(NO)(CH2Me3)(PPh.Unfortunately, diphenylphosphine reacts very rapidly with CpMo(NO)(=CHCMe3)(py)even at low temperatures (-50 oC)43 The one product that appears to be formed is notvery stable in C6D at room temperature and decomposes to a plethora of Cp containingproducts as observed from the 1H NMR spectrum after 3 h.The PKa of a reagent is not necessarily an indication of its reactivity towards theneopentylidene complex. For example, the pKa of phenylacetylene (‘-29, DMSO)44which does not react with the neopentylidene, is much lower than that of amines which doreact with it (vide supra). This fact supports the mechanistic idea that the formation of aLewis-base Lewis-acid adduct is paramount to the eventual formation of additionproducts.1364.4 EpilogueThe characteristic reactivity of the pyridine-trapped neopentylidene complex,CpMo(NO)(=CHCMe3)(py), has been explored in this chapter.CpMo(NO)(=CHCMe)(py)reacts stereoselectively with a range of heteroatom-hydrogen(E-H) bonds (for example, alcohols, thiols, amines, carboxylic acids) which readily addacross the Mo=C linkage. In the presence of reagents with two heteroatom-hydrogenbonds (for example, neopentyl glycol, oxalic acid) dimeric species with bridging ligandsare obtained. In the presence ofunsaturated reagents (olefins, acetylenes etc.) or nonpolar reagents (112), the neopentylidene complex merely decomposes to form theunsymmetric .t-i’ :r2 nitrosyl dimer, [CpMo(NO)](p.-i’:2-NO)(p.-CHCMe3)[CpMo(=CHCMej. The reactivity pattern is clearly different to that displayedby classic Schrock alkylidenes.Previously, the Legzdins’ group had been unable to prepare perhydrocyclopentadienylmolybdenum amide and alkoxide complexes via metathesis reactions from theCpMo(NO)Cl2precursors because of its propensity to be reduced by the MNHR andM0R18 salts (M = Li, Na, K). The reduction potential of CpMo(NO)Cl2is -100 mV(CH2C1 vs SCE) whereas the analogous potential of the Cp* derivative is -350 mV.46The reactivity of the neopentylidene complex has, thus, afforded a range of previouslyinaccessible products.This work can be extended by investigating the following questions: Does thethermolysis of the Cp* analogue, Cp*Mo(NO)(CH2e3)similarly yield an analogousalkylidene complex? Is a tungsten alkylidene accessible by thermolysingCpW(NO)(CH2Me3)and Cp*W(NO)(CH2Me3)?47Future work may also encompass the synthesis of new nitrosyl alkylidene complexesby other methods; for example using alkylidene transfer reagents such as the phosphorane,Et3P=C(H)Me.481374.5 References and Notes(1) Fischer, E. 0.; Maasbol, A. Angew. Chem., mt. Ed. EngI. 1964, 3, 580.(2) Schrock, R. R. J. Am. Chem. Soc. 1974, 96, 6796.(3) Coliman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles andApplications ofOrganotransition Metal Chemistry; University Science Books: MillValley, CA, 1987.(4) (a) Schrock, R. R. Ace. Chem. Res. 1979, 12, 98. (b) Schrock, R. R. .1.Organomet. Chem. 1986, 300, 249. (c) Fox, H. H.; Schrock, R. R.; O’Deil, R.Organometallics 1994, 13, 635. (d) Toreki, R.; Schrock, R. R.; Davis, W. M. J.Am. Chem. Soc. 1992, 114, 3367. (e) Fu, G. C.; Nguyen, S. T.; Grubbs, R. H. .1.Am. Chem. Soc. 1993, 115, 9856. (f) Feher, F. J.; Tajima, T. L. .1 Am. Chem. Soc.1994, 116, 2145.(5) For a recent example of the stereoselective olefination of ketones by transition metalalkylidenes, see: Fujimura, 0.; Fu, G. C.; Rothemund, P. W. K.; Grubbs, R. H. J.Am. Chem. Soc. 1995, 117, 2355.(6) Cater, E. A.; Goddard, W. A. J Am. Chem. Soc. 1986, 108, 4746.(7) (a) Nugent, W. A.; Mayer, J. M. Metal-LigandMultiple Bonds; Wiley: New York,1988, Chapter 3. (b) LaPointe, A. M.; Schrock, R. R. Organometallics 1993, 12,3379.(8) Johnson, L. K.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993, 115, 8130.(9) Casey, C.; Vosejpka, P. C.; Askham, F. R. J. Am. Chem. Soc. 1990, 112, 3713.(10) Clark, G. R.; Hoskins, S. V.; Jones, T. C.; Roper, W. R. I. Chem. Soc., Chem.Commun. 1983, 719.138(11) For example: (a) Ir=CH2[N(SIMeCHPPh]undergoes oxidative additionreactions; see: Fryzuk, M. D.; Gao, X.; Joshi, K.; MacNeil, P. A. J Am. Chem.Soc. 1993, 115, 10581. (b) CpFe(CO)2=CHPF6reacts with CO to form a (2..C,C) ketene complex, CpFe(CO)(CHO)PF;see: Bodnar, T. W.; Cutler, A.R. J. Am. Chem. Soc. 1983, 105, 5926.(12) For examples of cationic electrophilic methylene complexes of the typeCp’Fe(L)(=CH),see: (a) Guerchais, V.; Astruc, D. J. Chem. Soc., Chem.Commun. 1985, 835. (b) Riley, P. E.; Capshew, C. E.; Pettit, R.; Davis, R. E.Inorg. Chem. 1978, 17, 409. (c) Brookhart, M.; Liu, Y. Advances in MetalCarbene Chemistry; Kiuwer Academic: Dordrecht, 1988, Vol 269, p 251. (d) Foran example of a cationic methylene rhenium complex, CpRe(NO)(PPh3)(=CH;see: Merrifield, J. H.; Strouse, C. E.; Gladysz, J. A. Organometallics 1982, 1,1204.(13) Legzdins, P.; Rettig, S. J.; Veitheer, J. E. I Am. Chem. Soc. 1992, 114, 6922.(14) Legzdins, P.; Veltheer, J. E.; Batchelor, R. B.; Einstein, F. W. B. Organometallics1993, 12, 3575.(15) Hessen, B.; Meetsma, A.; Teuben, J. H. I Am. Chem. Soc. 1989, 111, 5977.(16) Churchill, M. R.; Wasserman, H. J.; Turner, H. W.; Schrock, R. R. I Am. Chem.Soc. 1982, 104, 1710.(17) Legzdins, P.; Veitheer, J. E.; Young, M. A.; Batchelor, R. 3.; Einstein, F. W. B.Organometallics 1995, 14, 407.(18) (a) Legzdins, P.; Lundmark, P. J.; Rettig, S. J. Organometallics 1993, 12, 3545.(b) Lundmark, P. J. Ph.D. Dissertation, The University of British Columbia, 1993.139(19) Liu, A. H.; Murray, R. C.; Dewan, 3. C.; Santarsiero, B. D.; Schrock, R. R. J. Am.Chem. Soc. 1987, 109, 4282.(20) Ehrefeld, D.; Kress, 3.; Moore, B. M.; Osborn, J. A.; Schoettel, G. J. Chem. Soc.,Chem. Commun. 1987, 129.(21) Van der Schaaf, P. A.; Grove, D. M.; Smeets, W. J. J.; Spek, A. L.; van Koten, G.Organometallics 1993, 12, 3955.(22) Veltheer, J. E.; Young, M. A. unpublished observations.(23) Evans, S. V.; Legzdins, P.; Rettig, S. J.; Sanchez, L.; Trotter, 3. Organometallics1987, 6, 7.(24) We have also observed the existence of such geometrical isomers in relatedCpW(NO)(amido)(aryl) complexes: Legzdins, P.; Ross, K. 3. unpublishedobservations.(25) (a) Cutler, A.; Raja, M.; Todaro, A. Inorg. Chem. 1987, 26, 2877. (b) Grove, D.M.; van Koten, G.; Ubbels, H. J. C.; Zoet, R.; Spek, A. L. Organometallics 1984,3, 1003. (c) Decon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. (d)Nakamoto, K. Infrared and Raman Spectra ofInorganic and CoordinationCompounds, 3rd. Ed.; Wiley: New York, 1978, p 232. (e) Coutts, R. S. P.; Wailes,P. C. Aust. J. Chem. 1967, 20, 1579.(26) For example: (a) Pirrung, M. C.; Morehead, A. T. J. Am. Chem. Soc. 1994, 116,8991. (b) Holl, M. H.; Hilihouse, G. L.; Folting, K.; Huffman, J. C.Organometallics 1987, 26, 1522. (c) Bradley, M. G.; Roberts, D. A.; Geoffroy, G.L. J. Am. Chem. Soc. 1981, 103, 379. (d) Smith, S. A.; Blake, D. M.; Kubota, M.Inorg. Chem. 1972, 11, 660.(27) Dryden, N. H. Ph.D. Dissertation, University ofBritish Columbia, 1990.140(28) (a) Brouwer, E. B.; Legzdins, P.; Rettig, S. J.; Ross, K. J. Organometallics 1993,12, 4234. (b) Darensbourg, D. J.; Grotsch, G.; Wiegreffe, P.; Rheingold, A. L.Inorg. Chem. 1987, 26, 3827.(29) Rigby, W.; Lee, H.-B.; Bailey, P. M.; McCleverty, J. A.; Maitlis, P. M. I Chem.Soc., Dalton Trans. 1979, 388.(30) Michelman, R. I.; Ball, G. E.; Bergman, R. G.; Anderson, R. A. Organometallics1993, 13, 869.(31) Hunt, M. M.; Kita, W. G.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 1978,474.(32) Legzdins, P.; Smith, K. M. unpublished results.(33) Tanase, T.; Watton, S. P.; Lippard, S. J. J. Am. Chem. Soc. 1994, 116, 9401.(34) Rottger, D.; Erker, G.; Grehi, M.; FrOhlich, R. Organometallics 1994, 13, 3897.(35) Stephan, D. W. Organometallics 1990, 9, 2718.(36) Legzdins, P.; Lundmark, P. J.; Phillips, E. C.; Rettig, S. J.; Veitheer, 3. E.Organometallics 1992, 11, 2991.(37) Schrock, R. R.; Fellmann, 3. D. J. Am. Chem. Soc. 1978, 100, 3359.(38) Hill, A. F.; Roper, W. R.; Waters, 3. M.; Wright, A. H. .1. Am. Chem. Soc. 1983,105, 5939.(39) Klein, D. P.; Bergman, R. G. I Am. Chem. Soc. 1989, 111, 3079.(40) Dötz, K. H. Angew. Chem., mt. Ed. Engl. 1984, 23, 587.(41) Dewey, M. A.; Bakke, J. M.; Gladysz, J. A. Organometallics 1990, 9, 1349.(42) The R and S designations have been arrived at by assigning the following priorities:Cp > E > NO> CHDCMe3and Mo > CMe3 > D > H; see: Stanley, K.; Baird, M.C. I Am. Chem. Soc. 1975, 97, 6599.141(43) Spectrum run at low and room temperature showed the same features. 1H NMR(300 Mhz, C6D): ö 7.68, 7.12 (m, 10H, 2Ph), 5.22 (s, 5H, C5H), 2.21 (d, 1H, J =11 lIz), 1.25 (s, 15H, CMe3), 1.05 (m, 1H), -1.62 (d, 1H, J=37.2Hz). 31P{’H}:ö 3.62.(44) Lowry, K. S.; Richardson, T. H. Mechanism and Theory in Organic Chemistry;Harper and Row: New York, NY, 1987.(45) (a) Legzdins, P.; Rettig, S. J.; Ross, K. J. Organometallics 1993, 12, 2103. (b)Ross, K. J. Ph.D. Dissertation, The University of British Columbia, 1994.(46) Herring, F. G.; Legzdins, P.; Richter-Addo, G. E. Organometallics 1989, 8, 1485.(47) Preliminary work by K. J. Ross on the thermolysis of Cp*W(NO)(CH2Me3)suggests that a possible Cp*W(NO)(=CHCMe3)fragment is generated; however, itcannot be trapped in an anaologous manner to that of the molybdenum alkylidene.See reference 45 (b).(48) For example the reaction ofWC12(NPh)(PPhMe)3with ethyl phosphorane,Et3P=CITR, produces the alkylidene complex,WC12(=CHR)(PPhMe)NPh)see:Johnson, L. K.; Frey, M.; Ulibarri, T. A.; Virgil, S. C.; Grubbs, R. H.; Ziller, J. W.J Am. Chem. Soc. 1993, 115, 8167.142CHAPTER 5Heteroatom Exchange Reactions in CpMo(NO)(CH2Me3)(ER)Systems5.1 Introduction 1435.2 Experimental Procedures 1445.3 Results and Discussion 1515.4 Epilogue 1685.5 References and Notes 1701435.1 IntroductionMany transition-metal complexes with M-ER bonds (E 0, S, N) are criticalintermediates in catalytic processes such as hydrodesulfl.irization1and hydrodeamination.2Exchange reactions of the type M-ER + ER —> M-E’R + ER have been the focus ofnumerous reports for a number of reasons. The most important reason is that thedetermination of bond-dissociation energies3 are crucial for the understanding of thethermodynamics of organometallic processes such as the individual steps in catalyticcycles. o-Ligand metathesis reactions also offer a route to new heteroatom-bondedspecies which are often inaccessible through standard synthetic methods.Chapter 4 presents a range of molybdenum-heteroatom species which weresynthesized using the pyridine-trapped alkylidene complex, CpMo(NO)(=CHCMe3)(py),as precursor. These molybdenum-heteroatom species are, in turn, convent precursors forstudying a-ligand metathesis reactions of the type depicted in eq 5.1. This preliminaryinvestigation of ligand-exchange reactions of molybdenum nitrosyl complexes constitutesthe focus of this chapter.Mo Mo (5.1)/ \ -Eli / \N’ E N”0 R 0 RR = CIIXCMe3(X Ii, D)E = E’ = aikoxide, amide, suiphide, carboxylatea-Ligand metathesis reactions have been utilized previously to synthesize new aryltungsten nitrosyl complexes of the type, CpW(NO)(o-tolyl)X (X = NR2, OR). Forexample, the reaction of CpW(NO)(o-tolyl)2with t-butyl amine or HOCHMe2results in144the formation of CpW(NO)(o-tolyl)(NHCMe3)and CpW(NO)(o-tolyl)(OCHMe2),respectively.4McCleverty and coworkers have also successfully used exchange reactionsto prepare a variety of[HB(Me2pz)3]Mo(NO)(I)X (Me2pz = 3,5-dimethylpyrazolyl; X =OR, NR2)complexes from the diiodide precursors and corresponding acid, HX.55.2 Experimental ProceduresSynthetic details and characterization data of the organometallic precursors used inthis chapter, namely, CpMo(NO)(CH2Me3)(ER)(ER = O-p-tolyl (4.2), OSiPh3 (4.3),SCMe3 (4.4), NEI-p-tolyl (4.6), O2CMe (4.9), O2CPh (4.10),2CC6H4-o-N0 (4.11),02CC6H4-p-N0 (4.12), (OCMe)CH(4.13)), [CpMo(NO)(CHeJ- .-(rOC { 0) C { 0 } 0) (4.15), CpMo(NO)(=CDCMe3)(py)(4.1-d),CpMo(NO)(CHDCMe3)(O-p-tolyl) (4.2-d1)and CpMo(NO)(CHDCMe)(SCMe(4.4-d1) are contained in Chapter 4, section 4.2.All exchange reactions were performed in 3. J. Young NMR tubes fitted withRotoflow screw tops. Acetylacetone (Fisher) was distilled from CaCl2 and degassed invacuo. MeOD (Aldrich) was dried over activated 4A sieves, degassed using 3 freeze-thaw-pump cycles and filtered through Celite. Acetic acid (glacial, Fisher) was dried overCuSO4 and HO-p-tolyl (Aldrich) was dried over CaH2. All other reagents viz., H2N-p-tolyl (Aldrich); t-BuSH (Aldrich), oxalic acid (Fisher), benzoic acid (Fisher), o- and pnitrobenzoic acid (Eastman Kodak) and Ph3SiOH (Aldrich), were used as received.5.2.1 The Reactions of Complexes 4.2, 4.3, 4.6 and 4.6-d1 with HOS1Ph3and pCresolThe reaction ofCpMo(NO)(CH2Me3)(NH-p-tolyl) (4.6) with HOS1Ph3 is given as arepresentative example. In a glovebox, complex 4.6 (30 mg, 0.08 mmol) was dissolved inC6D (0.8 mL) and transferred to an NMR tube containing HOSiPh3 (43 mg, 0.08 mmol).145The reaction was accompanied by an immediate color change from orange to red. A1H NMR spectrum of the resultant red solution showed that peaks due to complex 4.6were replaced by new resonances at 5 7.78 (m, 611), 7.15 (m, 911), 5.07 (s, 5H), 3.79 (d,1H, Jpj = 9.9 Hz), 1.01 (s, 9H) and 0.99 (d, 1H, Jj = 9.9 Hz). 1 equivalent offreeptoluidine was also detected at 5 6.91 (m, 2H, Ar), 6.33 (m, 2H, Ar), 2.72 (br s, 2H, NH)and 2.18 (s, 3H, Me).A1HNMR spectrum of the solution resulting from the reaction of 4.6 with 1equivalent ofp-cresol showed that peaks due to complex 4.6 were replaced by newresonances at 5 7.25 (m, 2H), 7.03 (m, 211), 5.21 (s, 5H), 3.43 (d, 1H, Jj = 10.8 Hz),2.12 (s, 3H), 1.47 (d, 1H, Jp1= 10.8 Hz) and 1.18 (s, 9H). Peaks due to freep-toluidinewere also evident.A 111 NMR spectrum of the solution resulting from the reaction of 4.6-d1with 1equivalent ofp-cresol showed that peaks due to complex 4.6-d1were replaced by newresonances at 57.25 (m, 2H), 7.04 (m, 2H), 5.21 (s, 5H), 2.10 (s, 3H), 1.45 (s, 1H), and1.19 (s, 911). Peaks due to free p-toluidine were also evident.A 1H NMR spectrum of the solution resulting from the reaction of 4.3 with 1equivalent ofp-cresol showed broad peaks at 5 7.8 (br s), 7.2 (br s), 7.0 - 6.8 (br s), 6.5(br s), 5.1 -5.3 (br s), 3.9 (br s), 3.5 (br s), 2.1 (br s), 1.9 (br s) and 1.1 - 1.3 (br s). Thereverse reaction, namely, the reaction of 4.2 with 1 equiv ofHOSiPh3,showed the sameset of resonances.A 111 NMR spectrum of the solution resulting from the reaction of 4.3 with 4equivalents ofp-cresol showed peaks at 5 7.10 (br m, 4H), 5.21 (s, 5H), 3.43 (br s, 1H),2.12 (s, 3ff), 1.47 (br s, 1H) and 1.18 (s, 9H). Peaks due to free p-cresol and HOSIPh3were also evident.A 1H NMR spectrum of the solution resulting from the reaction of 4.2 with 4equivalents ofHOSiPh3showed peaks at 5 7.78 (br s), 7.15 (br s), 5.10 (s, 5H), 3.79 (br146s, 1H), 1.01 (s, 9H) and 0.99 (br s, 1ff). Peaks due to freep-cresol and HOSiPh3werealso evident.5.2.2 The Reactions of p-Toluidine with Complexes 4.2, 4.3, 4.4 and 4.9The reactions were performed in a manner similar to that described above. 1H NI’vIRspectra recorded after 2 d revealed peaks only due to the starting molybdenum complexand free p-toluidine.5.2.3 The Reactions of Complexes 4.2, 4.3, 4.6, 4.6-d1 and 4.9 with HSCMe3The reaction ofCpMo(NO)(CH2Me3)(OSiPh(4.3) with HSCMe3is given as arepresentative example. In a glovebox, complex 4.3 (30 mg, 0.06 mmol) was dissolved inC6D (0.8 mL) and transferred to an NMP. tube. The tube was then removed from theglovebox and HSCMe3 (-‘ 1 equivalent) was vacuum transferred onto the solution. A1H NIVIR spectrum of the resultant red solution showed that resonances due to complex4.3 were replaced by new resonances at ö 5.15 (s, 5H), 3.05 (d, 1H, = 10.2 Hz), 1.78(s, 9H), 1.24 (s, 911) and 0.79 (d, 1H, = 10.2 Hz). Resonances due to free HOSiPh3were also detected at ö 7.77 (m, 611, Ph) and 7.18 (m, 9H, Ph).The reactions of 4.2 and 4.6 with 1 equivalent ofHSCMe3generated the same set ofresonances in the 1H NMR spectra, except signals due to either free p-cresol or ptoluidine (not HOSiPh3)were also evident.A 1H NMR spectrum of the solution resulting from the reaction of 4.6-d1 with 1equivalent of HSCMe3 showed that peaks due to complex 4.6-d1 were replaced by newresonances at ö 5.15 (s, 5H), 1.77 (s, 911), 1.22 (s, 911), and 0.79 (s, 1H). Peaks due tofree p-toluidine were also evident.147A1HNMR spectrum of the solution resulting from the reaction of 4.9 with an excessofHSCMe3 showed that peaks due to complex 4.9 were replaced by new resonances at ö5.15 (s, 5ff), 3.05 (d, 1H, Jpjj = 10.2 Hz), 1.78 (s, 9H), 1.24 (s, 9H) and 0.79 (s, 1H,= 10.2 Hz). Peaks due to free acetic acid (ö 1.53) and excess HSCMe3( 1.24) were alsoevident.5.2.4 The Reactions of Complexes 4.2, 4.3, 4.4, 4.6 and 4.6-d1 with Acetic AcidThe reaction ofCpMo(NO)(CH2Me3)(SCMe (4.4) with acetic acid is given as arepresentative example. In a glovebox, complex 4.4 (30 mg, 0.08 mmol) was dissolved inC6D (0.8 mL) in an NMR tube and acetic acid (- 1 equivalent) was added. The reactionwas accompanied by an immediate color change from red to yellow. A 1H NMRspectrum of the resultant yellow solution showed that resonances due to complex 4.4 werecompletely replaced by new resonances at 3 5.14 (s, 5H), 2.74 (ci, 1H, Jjjj = 12.0 Hz),1.80 (d, 1H, Jp = 12.0 Hz), 1.56 (s, 3H) and 1.33 (s, 9H). A resonance due to freeHSCMe3was detected at 3 1.24 (s, 9H, SCMe3).The reactions of complexes 4.2, 4.3 and 4.6 with acetic acid (— 1 equivalent)generated the same set of resonances in the 1H NIVIR spectra, except signals due to eitherfree p-cresol, triphenylsilanol or p-toluidine (not HSCMe3)were also evident.A 1H NMR spectrum of the solution resulting from the reaction of 4.6-d1with aceticacid (-- 1 equivalent) showed that peaks due to complex 4.6-d1were replaced by newresonances at 3 5.14 (s, 5ff), 1.78 (s, 1H), 1.54 (s, 3H), and 1.34 (s, 1ff). Peaks due tofree p-toluidine were also evident. A 2H NIVIR (C6H)spectrum showed a peak at 6 2.71.1485.2.5 The Reactions of Complex 4.10 with p-and o-Nitrobenzoic Acid and OxalicAcidThe reaction ofCpMo(NO)(CH2Me3)-OC{ O}Ph) (4.10) with p-nitrobenzoicacid is given as a representative example. In a glovebox, complex 4.10 (30 mg, 0.08mmol) was dissolved in CDC13 (0.8 mL) in an NEvIR tube and p-nitrobenzoic acid (12 mg,0.08 mmol) was added. A 1H NIvIR spectrum of the yellow solution showed thatresonances due to complex 4.10 were completely replaced by new resonances at ö 8.28(m, 4H), 5.91 (s, 5ff), 3.06 (d, 1H, Jp11 = 11.4Hz), 1.77 (d, IH, Jp1= 11.4 Hz) and 1.14(s, 9H). The phenyl resonance due to free benzoic acid (ö 8.30) was also evident in thefinal 1H NMR spectrum.A 1H NMR spectrum of the solution resulting from the reaction of 4.10 witho-nitrobenzoic acid (1 equiv) showed that peaks due to complex 4.10 were replaced bynew resonances at ö 7.78 (m, 3H), 7.65 (m, 1H), 5.94 (s, 5H), 3.04 (d, 1H,J = 11.1Hz), 1.72 (d, 1H, Jpj = 11.1 Hz) and 1.13 (s, 9H). The phenyl resonance due to freebenzoic acid was also evident in the final 1H NMR spectrum.A 1H NIvIR spectrum of the solution resulting from the reaction of 4.10 with oxalicacid (0.5 equiv) showed that peaks due to complex 4.10 were replaced by new resonancesat 5.82 (s, 5H), 2.96 (d, 1H, = 11.4 Hz), 2.91 (d, 1H,J1 = 11.4 Hz), 2.14 (d, 1H,Jj = 11.4Hz), 2.13 (d, 1H, Jj = 11.4Hz) and 1.08 (s, 9H). The phenyl resonance dueto free benzoic acid was also evident in the final 1H NMR spectrum.5.2.6 The Reaction of Complexes 4.13 and 4.15 with Benzoic AcidThe reaction ofCpMo(NO)(CH2Me3)[(OC(M ))H-O,O](4.13) with benzoicacid is given as a representative example. In a glovebox, complex 4.13 (30 mg, 0.08mmol) was dissolved in CDC13 (0.8 niL) in an NMR tube and benzoic acid (10 mg, 0.08mmol) was added. A ‘H NMR spectrum of the yellow solution showed that resonances149due to complex 4.13 were completely replaced by new resonances at ö 8.02 (m, 2H), 7.59(m, 2H), 7.42 (t, 1H), 5.89 (s, 5H), 2.96 (d, 1H, Jp = 11.4 Hz), 1.77 (d, 1R, Jp = 11.4Hz) and 1.14 (s, 9H). A methyl resonance due to free acacH was detected at ö 2.18 (s,6H, Me).A 1H NtvIR spectrum of the solution resulting from the reaction of 4.15 with benzoicacid (2 equiv) showed peaks due to complex 4.15 and free benzoic acid only.5.2.7 Treatment of CpMo(NO)(CH2CMe3(q2-OC{O}C64p-N0)(4.11) with oNitrobenzoic AcidCpMo(NO)(CHMe3)-OC{ }6H4- O(4.12) (30 mg, 0.07 mmol) wasreacted with o-nitrobenzoic acid (12 mg, 0.07 mmol) in a manner similar to that describedabove (section 5.2.6). A 1H NMR spectrum of the yellow solution showed thatresonances due to complex 4.12 were completely replaced by new resonances at ö 7.87(m, 3H), 7.65 (m, 1H), 5.94 (s, 5H), 3.04 (d, 1H, = 11.1 Hz), 1.72 (d, IH, Jp =11.1 Hz) and 1.13 (s, 9H). Phenyl resonances due to free p-nitrobenzoic acid weredetected at ö 8.31 (m, 2H, Ar) and 7.18 (m, 2H, Ar).5.2.8 Treatment of CpMo(NO)(CHMe)(q-OC{O}Cj4o- O(4.12) with pNitrobenzoic AcidThis reaction was performed in a manner similar to that described above. After 2days the 1H NMR spectrum revealed peaks due to complex 4.12 and free p-nitrobenzoicacid only.1505.2.9 The Generation of CpMo(NO)(CD2Me3)(OM ) (5.1-t12)In a glovebox, CpMo(NO)(=CDCMe)(py)(4.1-d1)(30 mg, 0.09 mmol) wasdissolved in C6D (0.8 mL) and transferred to an NMR tube containing MeOD (3 mg,0.08 mmol). The 1H NMR spectrum of the resultant red solution revealed peaks due tothe generated alkoxo complex, CpMo(NO)(CD2Me3)(OM )(5.1-d2), (ö 5.25 (s, 5H,C5H), 3.10 (s, 3H, OMe), 1.22 (s, 9H, CMe3)) and free pyridine.5.2.10 The Reaction of CpMo(NO)(CD2Me3)(OM ) (5.1-d2)with p-CresolC6D and pyridine were removed in vacuo from the NMR solution of complex 5.1-t12generated above (section 5.2.9). Complex 5.1-d2was then redissolved in C6D (0.8 mL)and p-cresol (10 mg, 0.09 mmol) added. A 1H NMR spectrum of the red reactionsolution showed that resonances due to complex 5.1-d2were completely replaced by newresonances at 87.26 (m, 2H,), 7.03 (m, 2H), 5.20 (s, 511), 2.12 (s, 3H) and 1.18 (s, 9H),as well as a peak (6 3.31) due to free MeOH. A 2H NMR (C6H)spectrum revealed twopeaks at 8 3.45 (br s, 1D) and 1.56 (br s, 1D).1515.3 Results and DiscussionExchange reactions of different ligands are discussed separately. Mechanistic ideasare presented at the end of the section.5.3.1 Amide ExchangeTreatment of a C6D solution of the neopentyl amide complex,CpMo(NO)(CH2Me)(NH-p-tolyl) (4.6) with one equivalent ofp-cresol results in thequantitative formation of the neopentyl alkoxide complex, CpMo(NO)(CH2Me3)(O-p-tolyl) (4.2). This reaction occurs rapidly judging by the color change from orange to redin less than 2 mm. Free p-toluidine is also observed in the reaction solution by 1H NMRspectroscopy. Similarly, the reaction of one equivalent of triphenylsilanol with complex4.6 results in the rapid formation ofCpMo(NO)(CH2Me3)(OSiPh (4.3) and freep-toluidine. These amide ligand exchange reactions are summarized in Scheme 5.1.t-Butyl thiol (— one equivalent) rapidly reacts with complex 4.6 resulting in theformation of the neopentyl thiolate complex, CpMo(NO)(CH2Me3)(SCMe(4.4).Once again, this reaction is accompanied by a color change from orange to bright red inless than 2 mm. Free p-toluidine is similarly observed in the reaction solution by 1H NIvIRspectroscopy. Even when this reaction is performed with an excess of t-butyl thiol, theneopentyl ligand does not undergo metathesis to yield the bis thiolate complex,CpMo(NO)(SCMe3)2,at room temperature.The amide ligand of complex 4.6 also readily undergoes exchange with acetic acid inC6D solutions to yield the neopentyl carboxylate complex, CpMo(NO)(CH2Me3)-OC{O}Me) (4.9). This carboxylate complex is unreactive at room temperature in C6D,even in the presence of excess acetic acid.152Scheme 5.17N” NHR’0 RH2NR’MeC{O}O&21’VHOR’ N”T” Ic0 R / \ 0 RN SCMe3R’ =p-tolyl (4.2); SiPh3 (4.3) 0 R 494.4R = CH2Me3The reverse reactions, for example the reactions of complexes 4.2 or 4.4 with oneequivalent ofp-toluidine, do not proceed at room temperature. The reactions depicted inScheme 5.1 proceed in the direction expected based on pKa values of the free acids(Table 1). For example, p-toluidine (pKa> 30.6) is less acidic than p-cresol (PKa = 10.28)and the reaction ofp-cresol with CpMo(NO)(CH2Me)(NH-p-tolyl) (4.2) proceedsreadily at room temperature while the back-reaction does not proceed at all.153Table 5.1 Selected PKa ValuesAcid PKaHO2CC64-o-N0 2. 16 (90)(o-nitrobenzoic_acid)HO2CC64-p-N0 3. 14 (10.8)(p-nitrobenzoic_acid)(C0211) (oxalic acid) 1.23, 4. 19PhCO2H 4.19a (11.1)(benzoic_acid)MeCO2H(acetic acid) 475a (12.3)acacH 8.9b(i3)HOC64-p-Me(p-cresol)HSCMe3 1105b (17.0)(t-butyl thiol)HOMeHOSiPh3 (l657)d(triphenylsilanol)N112-C6H4p-Me > 30.6c(p-toluidine)PKa Values in brackets determined in DMSO.6a Determined inH20.7b Determined inH20.8C Based on the fact that the PKa ofPhNH2 is 30.6 (H20).9d Determined in DMSO.10It is important to note that the direction of exchange reactions is not only dependenton PKa values but also on metal-ligand dissociation energies.9 In Scheme 5.2, theequilibria (a) and (b) give rise to the pKa values. Only if the ionic dissociation energies ofM-NR2 (c) and M-OR (d) are equal will the direction of exchange be controlled merely byPKa values. If the energies of(c) and (d) work in opposition to that of(a) and (b), the154exchange reaction may not proceed in the predicted direction. In other words, if theM-OR ionic dissociation energy is greater than that for M-NR2 and the pKa ofR2N-H <that ofRO-H, the reaction M-OR +R2N-H will probably not proceed.Scheme 5.2RO-il -* R0 + IP (a)1I + NR -+ R-NR2 (b)M-NR2 -* RN + M (c)M + OR M-OR (d)M-NR2+ RO-Il -* M-OR +R2N-llE, E’ =0, S, NFor example, in a 1:1 exchange the less acidic 1 ,2-(HN)C6H4displaces catechol from(Cym)Os(1,2-026H4)(Cym =r6-p-cymene), but the back-reaction is not observed.9McCleverty’s group has similarly shown that the more acidic PhOH does not exchangewith the less acidic amine in the reaction of[HB(Me2pz)3]Mo(NO)(OCOMe)(NHMe)with PhOH. Rather, theq1-OCOMe ligand is displaced to form the alkoxo amidecomplex, [HB(Me2pz)3]Mo(NO)(OPh)(NHMe).1 This i1-OCOMe ligand displacementby less acidic alcohols has also been observed in other chemistry involving molybdenumnitrosyl pyrazolylborate complexes. 12Consistent with the fact that exchange reactions are not merely governed by PKavalues, the neopentyl group of CpMo(NO)(CH2Me3)(ER)does not undergo exchangeeven in the presence of excess added acid (pKa of neopentane> 40). Ideas on why theneopentyl group is not involved in these exchange reactions are presented in section by Bryndza and coworkers has established that an equilibrium exists in thereaction of the platinum(II) amide complex, (dppe)Pt(Me)(NMePh) (dppe = 1,2-bis(diphenylphosphino)ethane) with MeOH (eq 5.2). 13(dppe)Pt(Me)(NMePh) + HOMe -* (dppe)Pt(Me)(OMe) + HNMePh (5.2)Bergman and coworkers have shown that the osmium imido complex, (Cym)Os(N-tBu) undergoes exchange reactions with a variety ofH-X bonds (X = 0, S, C) resulting inthe cleavage of the Os-N multiple bond and the formation of new Os-X bonds.9 Forexample, (Cym)Os(N-t-Bu) reacts with HOCMe2CMeOHto yield the alkoxide complex,(Cym)Os(OCMe2MeO),and free t-butyl amine. (Cym)Os(N-t-Bu) also undergoesamine exchange reactions.14 The substitution of amido groups for alkoxo groups has beenobserved in reactivity studies of chromium nitrosyl complexes.155.3.2 Alcohol ExchangeThe neopentyl alkoxide complexes, CpMo(NO)(CH2Me3)(OR)(R = p-tolyl (4.2),SiPh3 (4.3)) undergo rapid exchange with t-butyl thiol or acetic acid to result in theformation ofCpMo(NO)(CH2Me)(SCMe (4.4) and CpMo(NO)(CH2Me3)(q-OC{O}Me) (4.9) respectively (eq 5.3). Resonances due to the free alcohols, p-cresol andtriphenylsilanol, are also evident in the 1H NMR spectra of the resulting reaction mixtures.156-EHN”fl’E’(5.3)0 R 0 RR = CII2Me3= SCMe3 (4.4), C11302(4.9)E = O-p-tolyl (4.2), OS1Ph3 (4.3)Similar alkoxo for thiolate exchange has been observed by Bergman and coworkerswho have shown that the osmium bis(thiolate) complex, (Cym)Os(SCMe3)2is formedfrom the exchange reaction of the bis(alkoxide) complex, (Cym)Os(OCMe withHSCMe3.9Irreversible exchange of the Re-O bond occurs in complexes of the type(CO)L2Re( R) (R = Me, Et; L phosphines) with HSR, PHR2 and RBH reagents. 16When one equivalent ofp-cresol is reacted with CpMo(NO)(CHMe3)(OSiPh(4.3) in C6D,resonances due to an equilibrium mixture of complexes 4.3 andCpMo(NO)(CH2Me)(O-p-tolyl) (4.2) are evident in the 1H NMR spectrum, as well assignals attributable to the free alcohols, p-cresol and triphenylsilanol (eq 5.4). The sameresonances are generated when one equivalent ofHOSiPh3is reacted with complex 4.2 inC6D.p-cresol (-Ph3SiOB)Mo_________Mo 54/ Ph3S1OII (- p-cresol) / (.)N OSiPh3 N 0-p-toIyJ0 R 0 R4.3 4.2R = CH2Me3157Consistent with Le Chatelier’s Principle, the equilibrium can be shifted in theappropriate direction with an excess of one alcohol (eqs 5.5 and 5.6). For example, thereaction of complex 4.3 with an excess ofp-cresol results in the formation of thecorresponding alkyl alkoxide complex 4.2, and free triphenylsilanol and p-cresol (asobserved by 1H NMR spectroscopy). Even in the presence of the excess added alcohol nometathesis of the neopentyl ligand, to form the bis(alkoxide) complex, is observed at roomtemperature. Consistent with this observation, the reactions of the bis(alkyl) tungstennitrosyl complex, Cp*W(NO)(CH2SiMe3)with excess HOR (R = Me, Ph, p-tolyl) resultin exchange of only one of the alkyl ligands for an alkoxide ligand, forming thecorresponding alkyl alkoxide complexes, Cp*W(NO)(CH2SIMe3)(OR), and free alkane.17Mo excess p-cresol Mo 5 5/ \ -Ph3S1011 / \ (.)o ROSiPh3RO-p-tolyl4.3 4.2R = CH2Me3excessPh3SiOIl(5.6)/\ -p-cresol /\RO-p-tolylROSiPh34.2 43R = CH2Me3158Phenoxide for methoxide ligand exchange has been observed for other tungstennitrosyl complexes (eq 5.7).18PhOH (- MeOR)/W\ 4 1/ /W\ (5.7)OMe MeOH OPhR = CH2SIMe3The Ir-O bond in the iridium hydroxo complex, Cp*Tr(PMe3)(Ph)(OH), alsoundergoes exchange with phenol to form the phenyl aryloxide complex,Cp*Ir(PMe3)(Ph)(OPh).19Other studied exchange reactions of transition metal alkoxidecomplexes with added alcohols include Cp*Ir(PPh3)(OEt)H20and(Cym)Os[OC(Me)2(Me)0] Thiolate ExchangeAs expected, acetic acid (1 equiv) with a PKa = 4.75 reacts rapidly with the thiolatecomplex, CpMo(NO)(CH2Me)(SCMe(4.4), to yield the carboxylate complex,CpMo(NO)(CHe3) r-OC{O}Me) (4.9). The reverse reaction occurs when anexcess ofHSCMe3reacts with complex 4.9 to give complex 4.4 (eq 5.8).159MeC{O}OH (-HSCMe3)Mo_________________________Mo., (5.8)N” ‘SCMe3excess HSCMe3(- MeC{O}OH)N” ‘0’Me0 R 0 R4.4 4.9R= CU2Me35.3.4 Carboxylate ExchangeThe addition of one equivalent of o- or p-nitrobenzoic acid to CDC13 solutions ofCpMo(NO)(CH2Me) r-OC{O}Ph) (4.10) results in carboxylate exchange to formCpMo(NO)(CHMe3)(q-OC{ 0 }C6H4-o-N02)(4.11) and CpMo(NO)(CH2Me3) r-OC{O}C6H4-p-N02(4.12), respectively (Scheme 5.3). Free benzoic acid is alsoobserved in the 1H NMR spectra of these reactions. The 1H NfvIR spectrum resultingfrom the exchange reaction of complex 4.10 with p-nitrobenzoic acid is shown in Figure5.1. These carboxylate exchange reactions occur rapidly at room temperature (< 5 mm)and can be rationalized on the PKa values of the free acids. The PKa of benzoic acid is4.19, while those ofp and o-mtrobenzoic acids are 3.14 and 2.16, respectively.In a similar manner, the treatment of one-half an equivalent of oxalic acid withcomplex 4.10 results in the formation of the dimeric bridging-carboxylate complex,[CpMo(NO)(CH2e3)]- .-(OC{O}C{O}O) (4.15), and free benzoic acid(Scheme 5.3). The reverse reaction, i.e., the reaction of 4.15 with two equivalents ofbenzoic acid, does not proceed at room temperature.The greater acidity of o-nitrobenzoic acid compared to p-nitrobenzoic acid accountsfor the formation of complex 4.11 and free p-nitrobenzoic acid from the reaction ofcomplex 4.12 with one equivalent of o-nitrobenzoic acid. Consistent with the PKa160argument, the reverse reaction i.e., the treatment of complex 4.11 with p-nitrobenzoic aciddoes not proceed (eq 5.9).Scheme 5.3Mo .ORp4.12-PhCOMo 1/2 (C0H) (-PhCO2H) /MOcCCMON” ‘0-’-Ph N0 R2PhCOH 0 R I NR4.10 4.15— PhCORoiIhMoR = CH2Me3N”=64-p-N0 0 R=CH-o-N024.11161o-N02-C6H4CO,cEP7,Mo... // ,Mo ... (5.9)N’p-NO,-CCN”o R 0 R4.12 4.11R = CH2Me3 =C6H4-o-N02R =64-p-N0The same acidity argument accounts for the fact that treatment of the acetylacetonatocomplex, CpMo(NO)(CH2Me3)[(OC(M ))H-O,O] (4.13), in CDC13with oneequivalent of benzoic acid results in the formation ofCpMo(NO)(CH2Me)(q-OC{O}Ph) (4.11) and free acetylacetonate (eq 5.10), judged by the 1H NMR. spectrum ofthe reaction solution (shown in Figure 5.2).ON“(5.10)4.13 4.10R = CH2Me3IIIII1I1IIIIIIII11IIIIIIIIIIIIIIIIII8642OPPMFigure5.1300MI-Iz1HNMRspectrumofthereactionofCpMoNO)(CH2CMe3)(r2-OC(O}Ph)(4.10) withp-nitrobenzoicacidinCDC13.IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII11118642OPPMFigure5.2300N’lI-Iz111NMRspectrumofthereactionofCpMo(NO)(CH2CMe3)[(OC(Me))2CH-O,O](4.13)withbenzoicacidinCDCI3.1645.3.5 Labeling StudiesIn order to establish whether these exchange reactions proceed with retention ofstereochemistry at the CpMo(NO)(CH2Me3)fragment, labeling studies were conductedusing the (mono)deutero-alkyl amide complex, CpMo(NO)(CHDCMe3)(NH-p-tolyl)(4.6-d1). In Chapter 4, the deutero-alkylidene complex, CpMo(NO)(=CDCMe)(py)(4.1-d1),was reacted with various REH reagents resulting in the stereoselective formationof one diastereomeric pairs of enantiomers, CpMo(NO)(CHDCMe3)(ER)as judged by 1HNMR spectroscopy. The exchange reactions of CpMo(NO)(CHDCMe)(NH-p-tolyl)(4.6-d1)with HSCMe3,HO-p-tolyl and Me3CO2H(Scheme 5.4) occur with the retentionof stereochemistry at the CpMo(NO)(CHDCMe3)fragment, as judged by 1H and 2HNMR spectroscopy. Signals attributable to only one diasteromeric pair of enantiomers areobserved in these spectra.A second labeling study was conducted using the generated bis(deutero)-alkylcomplex, CpMo(NO)(CD2Me3)(OM )(5.1-d2). Complex 5.1-d2 is generated in anNMR tube by the reaction of one equivalent of MeOD, with the pyridine trapped deuteroalkylidene complex, CpMo(NO)(=CDCMe3)(py)(4.1-d1). Treatment of complex 5.1-d2with one equivalent ofp-cresol results in the formation ofCpMo(NO)(CD2Me3)(O-p-tolyl) (4.2-d2). The formulation of this product is based on 1H and 2H NN’IR spectra.Two signals are observed in the 2H NMR spectrum, while no methylene signals areevident in the 1H NMR spectrum. The formation of 4.2-d2 is consistent with theexchange reaction proceeding along pathway B depicted in Scheme 5.5. Pathway Binvolves the initial formation of a Lewis acid-Lewis base pair between 4.2-d2 and p-cresol.This is followed by rapid protonolysis of OMe and elimination ofMeOH. An alternatepathway (A) could involve the initial loss ofMeOD, generating a transient alkylidenefragment which then could react with p-cresol. If the reaction had proceeded via thispathway, only one signal each in the 2H NIVIR and 1H NMR spectra would have been165observed. The exchange reaction proceeding via pathway B is also consistent with theretention of stereochemistry at the CpMo(NO)R fragment (vide supra).Scheme 5.4N” I NHR0 Rd4.6-d1ROHZ MI2R7-NH2Me3CSH -NII2R MeC{O}iN%0N0R0N”I”SCMe3/R’ =p-tolyiRd CHDCMe3 4.4-d15.3.6 Exchange MechanismsIn general, ligand exchange reactions proceed via two mechanisms. The firstmechanism (Scheme 5.6) involves an initial hydrogen bond forming between the ligatedER group (E = 0, S, N) and the free acid, HER. This is followed by a four-centeredtransition state which rapidly loses HER with the concomitant formation of the new166Scheme 5.5DMo -.-./ON\py CMe34.l-d1MeODMo0N2CMe35.1-d2A//MeOD Pcres\ Br MODl MoON NJL CMe3] /(OMe..p-tolyl-O-—Hp-creso1 MeOHMo MoN” \HDCMe3 N’ CD2Me30 R 0 R4.2-d1 R = O-p-tolyl 4.1-t12167M-E’R’ bond. This concerted associative exchange mechanism is generally believed to beoperative in alcohol exchange reactions in coordinatively unsaturated transition metals.21Scheme 5.6R ,ER. ,HER/ S.’ I’LM—E LM H LM—E’A BE, E’ =0, S, NThe mechanisms of the exchange reactions of alkoxo and aryloxo rhenium(I)complexes, (CO)3L2Re( R) and (CO)3L2Re(OAr , with phenols and alcohols haverecently been investigated.22 Bergman and Simpson have established that these exchangereactions proceed via an initial hydrogen-bonded adduct between the coordinated alkoxideand incoming phenol.Another potential ligand-exchange mechanism (Scheme 5.7) involves the coordinatedER ligand reversibly dissociating from the metal to give a metal cation and ER anion. 16, 22Exchange could then occur between the free ions (ER and E’R’) in solution.Scheme 5.7RLflM—E - LM ER flE’R LM—E’R’ + HERE, = 0, N, S168It is generally accepted that this type of mechanism is most likely occur when strongacids such as HC1 or CF3SO3H are added to alkoxo or phosphino complexes.’6It is not unreasonable to expect that the exchange reactions of the molybdenumnitrosyl complexes discussed in this chapter proceed in a manner similar to that depicted inScheme 5.6. The initial step in these exchanges, however, probably involves a Lewis acid-Lewis base interaction between the molybdenum complex and the incoming acid (path B,Scheme 5.5). Theoretical studies have confirmed the presence of a metal-based LUMOorbital existing between the two methyl ligands in CpMo(NO)Me2and reactivity studieson 16-valence-electron complexes of this type have supported the fact that thesecomplexes are Lewis acidic in nature.23 A four-centered transition state, similar to thatdepicted in Scheme 5.6, involving hydrogen-bonding between the ligated ER group andthe free acid then forms. In the exchange reactions considered in this chapter, theneopentyl group never undergoes exchange. This is consistent with the view thathydrogen-bonding is important in the transition state.5.4 EpilogueThe amide ligand ofCpMo(NO)(CH2Me3)(NH-p-tolyl) (4.6) is readily exchangedwith alkoxo, thiolate or carboxylate ligands when reacted with HOSiPh3,HO-p-tolyl,HSCMe3and acetic acid. The corresponding Mo-alkoxo, Mo-thiolate or Mo-carboxylatecomplexes in turn do not undergo exchange with p-toluidine. Ionic dissociation energiesas well as pKa values of the free acids predict the direction of the exchange reactions. Thedirection of exchange of molybdenum carboxylate complexes with free carboxylic acids ispredicted purely by pKa values.169An equilibrium exists when an equimolar mixture ofCpMo(NO)(CH2Me3)(O-p-tolyl) (4.2) is reacted with HOS1Ph3or CpMo(NO)(CH2Me3)(OSiPh (4.2) with HO-ptolyl. An excess of free alcohol can shift the reaction in the desired direction.These exchange reactions occur with retention of the stereochemistry at theCpMo(NO)(CH2Me3)fragment. The mechanism of these reactions probably involves aninitial Lewis acid-Lewis base adduct. Interestingly, the neopentyl ligand does not undergoexchange even in the presence of an excess of added acid.Future studies can focus on exchange reactions of analogous complexes with differentalkyl and aryl ligands, as well as changing the metal to tungsten. The exchange reactionspresented in this chapter occur very rapidly, a fact which has precluded any kinetic studies.By changing alkyl group or metal, an exchange system more amenable to a kinetic studymight be found. Competitive exchange between the different alkyl or aryl ligands and theheteroatom ligand could possibly be prompted this way. Furthermore, the influence ofelectronic and steric factors of different ligands on these exchange reactions could beinvestigated. The determination of bond-dissociation energies would also be interesting,especially for comparison with other studied systems.Using exchange reactions to form new metal-carbon bonds is also suggested forfuture work. For example, Teuben and coworkers have taken advantage of this syntheticmethodology to prepare [Zr(CpN)(CCPh)2](CpN =i5:a-CH4(CH2)3NMe)from theexchange reaction of [Zr(CpN)(NMe)with PhCCH.241705.5 References and Notes(1) Angelici, R. J. Ace. Chem. Res. 1988, 21, 387.(2) Djukic, J-P.; Rose-Munch, F.; Rose, E. J Chem. Soc., Chem. Commun. 1991,1634.(3) For example, relative M-E (E = S, 0, N, C) bond strengths have been determinedfrom equibrium studies of Cp*Ru(PMe3)2X(X = hydride, alkoxide, amide, alkyl,alkynyl, hydrosulfide, cyanide) and (dppe)Pt(Me)X with EH reagents: Bryndza, H.E., Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J. Am. Chem. Soc. 1987,109, 1444. (b) For the determination and significance of transition-metal-alkylbond dissociation energies, see: Halpern, J. Ace. Chem. Res. 1982, 15, 238.(4) Ross, K. J. Ph.D. Dissertation, The University of British Columbia, 1994.(5) Reynolds, S. J.; Smith, C. F.; Jones, C. J.; McCleverty, J. A. Inorg Synth. 1985,23, 4.(6) Bordwell, F. G. Ace. Chem. Res. 1988, 21, 456.(7) Lide, D. R. CRC Handbook ofChemistry and Physics, 75th ed.; CRC Press:London, 1994.(8) (a) March, J. Advanced Organic Chemistry, 3rd ed.; John Wiley and Sons: NewYork, 1985. (b) Fessendon, R. J.; Fessendon, J, S. Organic Chemistry, 3rd ed.;Brooks/Cole Publishing Co.: Monterey, CA, 1986.(9) Michelman, R. I.; Ball, G. E.; Bergman, R. G.; Anderson, R. A. Organometallics1994, 13, 869.(10) Bassindale, A. R.; Taylor, P. G. The Chemistry of Organic Silicon Compounds;Patai, S.; Pappoprt, Z., Eds.; Wiley-Interscience: New York, 1989; Vol. 1, Chapter12, p 809.171(11) McCleverty, J. A.; Denti, G.; Reynolds, S. J.; Drane, A. S.; Muff, N. E.; Rae, A. E.;Bailey, N. A.; Adams, H.; Smith, J. M. A. J Chem. Soc., Dalton Trans. 1983,1, 81.(12) (a) Wlodarczyk, A.; Edwards, A. J.; McCleverty, J. A. Polyhedron 1988, 7(2),103. (b) McCleverty, J. A.; Rae, A. E.; Wolochowicz, I.; Bailey, N. A.; Smith, J.M. A. J. Chem. Soc., Dalton Trans. 1982, 5, 951.(13) Bryndza, H. E.; Fultz, W. C.; Tam, W. Organometallics 1985, 4, 939.(14) Michelman, R. I.; Bergman, R. G.; Anderson, R. A. Organometallics 1993, 12,2741.(15) Bradley, D. C.; Newing, C. W. Chem. Comm. 1970, 219.(16) Simpson, R. D.; Bergman, R. G. Organometallics 1992, 11, 3980.(17) Phillips, E. C. Ph.D. Dissertation, The University ofBritish Columbia, 1990.(18) Lundmark, P. J. Ph.D. Dissertation, The University of British Columbia, 1993.(19) Woerpel, K. A.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 7888.(20) Newman, L. J.; Bergman, R. G. J. Am. Chem. Soc. 1985, 107, 5314.(21) (a) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. MetalAlkoxides; Academic Press:New York, 1978. (b) Lunder, D. M.; Lobkovsky, E. B.; Streib, W. E.; Caulton, K.G. J. Am. Chem. Soc. 1991, 113, 1837.(22) Simpson, R. D.; Bergman, R. G. Organometallics 1993, 12, 781.(23) Legzdins, P.; Veltheer, 3. E. Acc. Chem. Res. 1993, 26, 41.(24) Hughes, A. K.; Meetsma, A.; Teuben, J. H. Organometallics 1993, 12, 1936.


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