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Synthesis and characteristic chemistry of sixteen-electron group 6 hydrocarbyl-containing nitrosyl complexes Veltheer, John E. 1993

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SYNTHESIS AND CHARACTERISTIC CHEMISTRY OF SIXTEEN-ELECTRON GROUP 6 HYDROCARBYL-CONTAINING NITROSYL COMPLEXES by JOHN E. VELTHEER B.Sc.H., Queen's University at Kingston, 1988  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  in THE FACULTY OF GRADUATE STUDIES (Department of Chemistry)  We accept this thesis as conforming to the required stand  THE UNIVERSITY OF BRITISH COLUMBIA April 1993 © John E. Veltheer, 1993  In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  (Signature)  Department of  ^Ckervt • 5+10"1  The University of British Columbia Vancouver, Canada  Date  DE-6 (2/88)  ol q Art I -  iii3  ii Abstract  This thesis addresses the non-generality of the previous method used to prepare CpW(NO)(alkyl) 2 complexes from their diiodide precursors. The three most significant problems inherent to the original synthetic method were: (a) the reagents being used (i.e. transition-metal dihalide, alkylating agent and solvent), (b) excessive hydrolysis during workup, and (c) thermal decomposition. This work presents a rationalization for, and utilization of, a new general methodology for the preparation of Cp'M(NO)R 2 [Cp' = Cp (i 5 -0 5 H5), Cp * (i 5 -05Me5); M = Mo, W; R = alkyl, aryl] complexes. Reactions of Cp * M(NO)C12 with (ary1) 2Mg•X(dioxane) [aryl = Ph, p-tolyl, o-tolyl] in THE provide Cp * M(NO)(aryl)2 complexes. These 16-electron diaryl complexes irreversibly form 1:1 metal-centered adducts with PMe 3 . CpW(NO)(o-tolyl) 2 is the only isolable diaryl complex of this type that does not incorporate a Cp * ligand. Electrochemical and IR studies indicate that these diaryl complexes are more potent Lewis acids than their Cp'M(NO)(alkyl) 2 congeners. X-ray crystallographic analyses indicate that these diaryl complexes have more sterically accessible metal centers than related dialkyl complexes. The thermal stability of Cp'M(NO)R2 complexes is Cp * > Cp, W > Mo and alkyl > aryl. The relative Lewis acidities of this family of nitrosyl complexes mirrors the thermal stability trend. Reactions of Cp * Mo(NO)C12 with (alky1) 2Mg•X(dioxane) [alkyl = CH 2 CMe 3 , CH2 CMe2Ph, CH2SiMe3] in THE provide previously inaccessible Cp *Mo(NO)(alkyl)C! and Cp * Mo(N0)(alky1)2 complexes in a stepwise fashion. Cp * Mo(NO)(alkyl)Cl species are comparable in Lewis acidity to related Cp * Mo(N0)(ary1)2 complexes. Cp * Mo(NO)(CH2CMe3)Cl reacts with PMe 3 and pyridine to afford metal-centered 18-electron adducts while CO and CNCMe3 readily insert into its Mo-neopentyl bond. The chloride ligand in Cp * Mo(NO)(CH2CMe3)Cl is easily abstracted by Ag+ in NCMe to form [Cp * Mo(NO)(CH2CMe3)(NCMe)]BF4. The phosphine complex,  iii Cp * Mo(N0)(CH2CMe3)(PMe3)C1, is readily dehydrohalogenated by LDA in THE to afford the novel cyclometallated dialkyl complex (1 5 ,11 1 -0 5 Me4CH2 )Mo(N0)(CH2 CMe3 )(PMe 3 ). Reactions of Cp *Mo(NO)R2 complexes with water afford bimetallic complexes of the type, [Cp *Mo(NO)R]2-(11-0), and free hydrocarbon. A kinetic analysis of the reaction between H 2 O and Cp*Mo(N0)(CH 2 SiMe 3 )2 implicates Cp * Mo(NO)(CH2SiMe3)(OH) as a key intermediate in the formation of [Cp * Mo(N0)(CH2SiMe3)]2-(A-0). Cp'W(NO)(alkyl)2 complexes are hydrolytically stable, whereas Cp'W(NO)(aryl)2 complexes react vigorously with water providing Cp'W(0)2(aryl) complexes. CpMo(NO)(CH2 CMe3) 2 thermally decomposes via a first-order intramolecular extrusion of neopentane yielding the 16-electron alkylidene complex, CpMo(NO)(=CHCMe 3 ). A mechanistic analysis of the above-mentioned thermal reaction shows little solvent dependence and suggests a highly ordered transition state. CpMo(NO)(=CHCMe3) cannot be isolated, but it can be trapped with phosphines or pyridine (L) yielding CpMo(NO)(=CHCMe3)L complexes. Primary amines and alcohols add their heteroatom-hydrogen bonds across the Mo=C double bond of CpMo(NO)(=CHCMe3) stereoselectively to give alkyl amide and alkyl alkoxide complexes. CpMo(NO)(=CHCMe3) reacts with CpMo(NO)(CH2CMe3)2 to give [CpMo(NO)](12-11 1 :11 2 -N0)(A-CHCMe3)[CpMo(=CHCMe3)], which contains the first g-11 1 :1 2 -nitrosyl ligand.  iv  TABLE OF CONTENTS  Abstract ^  ^ii  Table of Contents ^  iv  List of Figures ^  xi  List of Tables ^  xiii  List of Schemes ^  xv  List of Abbreviations ^  xvi  Acknowledgements ^  xx  Quotation ^  xxi  Dedication ^  xxii  CHAPTER 1: General Introduction and Scope of This Thesis ^ 1 1.1 Background ^  1  1.2 Early Work ^  3  1.3 Scope of This Thesis ^  6  1.4 Format of This Thesis ^  8  1.5 References and Notes ^  8  CHAPTER 2: 16-Electron Cp * M(NO)(aryl)2 [M = Mo or W] Complexes: Their Synthesis, Characterization, and Properties ^  10  2.1 Introduction ^  10  2.2 Experimental Procedures ^  12  2.2.1 Methods ^  12  2.2.2 Reagents ^  13  2.2.3 Electrochemical Measurements ^  14  2.2.4 General Preparation of R2Mg•X(dioxane) (X= 1-2) ^  14  2.2.5 Preparation of Cp * M(NO)(aryl) 2 ^  15  V  2.2.6 Attempted Syntheses of CpM(NO)(aryl) 2 ^  16  2.2.7 Synthesis of CpW(NO)(o-tolyl) 2 ^  17  2.2.8 Preparation of Cp * W(N0)(p-toly1)2(PMe3) ^  17  2.2.9 Preparation of Cp * W(N0)(o-toly1)2(PMe 3 ) ^  18  2.2.10 Characterization Data ^  19  2.2.11 Other Characterization Data ^  21  2.3 Results and Discussion ^  22  2.3.1 A New General Synthetic Method ^  22  2.3.2 Physical Properties ^  23  2.3.3 Spectroscopic Characterization ^  23  2.3.4 Attempts to Synthesize CpM(NO)(aryl) 2 Complexes ^  24  2.3.5 X-ray Crystallographic Analyses of Complexes 2.3 and 2.6 ^ 25 2.3.6 Reactivity Patterns of the Diaryl Complexes ^  28  2.3.7 Electrochemical Properties of Complex 2.5 ^  31  2.4 Epilogue and Future Work ^  33  2.5 References and Notes ^  34  CHAPTER 3: Chemistry of Cp * Mo(NO)(R)Cl and CpMo(NO)R2 [R = alkyl] Complexes: Synthesis and Reactivity of Some Neutral Nitrosyl Lewis Acids ^ 37 3.1 Introduction ^  37  3.2 Experimental Procedures ^  38  3.2.1 Methods ^  38  3.2.2 Reagents ^  38  3.2.3 Preparation of CpMo(NO)R2 ^  39  3.2.4 Preparation of Cp *Mo(NO)(R)C1 ^  39  3.2.5 Preparation of Cp * Mo(NO)R2 ^  40  3.2.6 Preparation of Cp * Mo(NO)(CH2SiMe3)2 ^  41  3.2.7 Preparation of Cp *Mo(N0)(CH2 CMe3 )R ^  42  vi 3.2.8 Preparation of Cp * Mo(NO)(CH2CMe3)(Cl)(PMe3) ^  42  3.2.9 Preparation of Cp * Mo(N0)(CH2CMe3)(C1)(py) ^  42  3.2.10 Preparation of Cp *Mo(N0)(1 2 -C {0}CH2 CMe3)C1 ^  43  3.2.11 Preparation of Cp * Mo(NO)(/1 2 -C {NCMe3}CH2CMe3)C1 ^ 43 3.2.12 Preparation of [Cp * Mo(N0)(CH2 CMe3)(NCMe)}13F4 ^ 43 3.2.13 Reaction of 3.11 with LDA: Preparation of (11 5 ,71 1 -05Me4CH2)Mo(N0)(CH2CMe3)(PMe3) ^  44  3.2.14 Reaction of 3.13 with PMe3: Preparation of [Cp *Mo(N0)(C {0} {PMe3}CH2CMe3)]Cl ^  45  3.2.15 Other Characterization Data ^  46  3.3 Results and Discussion ^ 3.3.1 Alkylations of Cp'Mo(NO)C1 2 Complexes ^ 3.3.1.1 Synthesis of Cp'Mo(NO)(alkyl)2 Complexes ^  52 52 52  3.3.1.2 Synthesis of Cp * Mo(N0)(allcyl)C1 Complexes ^ 53 3.3.1.3 Synthesis of Cp * Mo(N0)(CH2CMe3)R Complexes ^ 57 3.3.2 Electrochemical and Infrared Studies: Assessment of Relative Lewis Acidity ^ 59 3.3.2.1 The Cp * M(NO)C12-to-Cp *M(N0)(CH2CMe3)C1-toCp * M(N0)(CH2CMe3)2 Series ^  59  3.3.2.2 The Cp *Mo(NO)(CH2 CMe 3 )2 -to-Cp *Mo(N0)(CH2CMe3)(p-toly1)-toCp * Mo(N0)(p-toly1)2 Series ^  62  3.3.2.3 Further Comparisons ^  63  3.3.3 Reactivity of Cp * Mo(NO)(CH2CMe3)C1 ^  64  3.3.3.1 Formation of 1:1 Adducts: Reactions With Pure Lewis Bases ^ 64 3.3.3.2 Formation of Insertion Products: Reactions With Unsat'd Lewis Bases ^ 68 3.3.3.3 Halide Abstraction: Reaction with AgBF 4 ^  71  3.3.4 Reactions of 18-Electron Derivatives of Cp * Mo(N0)(CH2CMe3)C1 ^ 72 3.3.4.1 Reaction of Cp * Mo(NO)(CH2CMe3)(Cl)(PMe3) with LDA ^ 72 3.3.4.2 Reaction of Cp * Mo(N0)(11 2 -C{O}CH2CMe 3 )C1 with PMe 3 ^ 74  vii 3.4 Epilogue and Future Work ^  75  3.5 References and Notes ^  76  CHAPTER 4: Reactions of 16-Electron Cp'M(NO)R2 Complexes of Molybdenum and Tungsten with Water ^  79  4.1 Introduction ^  79  4.2 Experimental Procedures ^  80  4.2.1 Methods ^  80  4.2.2 Reagents ^  80  4.2.3 Electrochemical Measurements ^  81  4.2.4 Kinetic Measurements ^  81  4.2.5 Preparation of [Cp *Mo(N0)(CH2SiMe3)]2-(11-0) ^  82  4.2.6 Preparation of [Cp *Mo(N0)(CH2CMe3)]2-(1-0) ^  83  4.2.7 Preparation of [Cp * Mo(N0)(CH2CMe2Ph)]2-(1.1-0) ^  84  4.2.8 Preparation of [Cp * Mo(N0)(o-toly1)} 2 -(1.1.-0) ^  85  4.2.9 Preparation of [Cp * Mo(N0)(Ph)] 2 -(4-0) ^  85  4.2.10 Preparation of Cp * W(0)2(aryl) ^  86  4.2.11 Preparation of CpW(0)2(o-tolyl) ^  87  4.2.12 Reaction of Cp * Mo(N0)(CH2SiMe 3 ) 2 with 18 0H2 ^  87  4.2.13 Reaction of Cp *Mo(N0)(CH2 SiMe 3 ) 2 with D 2 0 ^  88  4.2.14 Reaction of 4.1 with 02 ^  88  4.2.15 Reaction of 4.1 with Excess Water ^  89  4.2.16 Characterization Data for Complexes 4.1 - 4.9 ^  89  4.2.17 Other Characterization Data ^  92  4.2.18 Kinetic Data ^  93  4.3 Results and Discussion ^  95  4.3.1 Two Similar Reactions? ^  95  4.3.2 Reactions of Cp * Mo(NO)R 2 Complexes with Water ^  96  viii 4.3.3 Physical Properties of [Cp * Mo(NO)R]2-(1-1-0) Complexes ^ 98 4.3.4 Spectroscopic Properties of [Cp * Mo(NO)R] 2 -(1-0) Complexes ^ 99 4.3.5 Related Bridging Oxo Complexes ^  101  4.3.6 Electrochemical Studies ^  102  4.3.7 Labeling Studies ^  103  4.3.8 Kinetic Studies ^  105  4.3.8.1 Monitoring of the Hydrolysis Reaction ^  105  4.3.8.2 Variation of [3.8], [H 2 0] and [D 2 0] ^  107  4.3.8.3 Variation of Temperature: Measurement of Activation Parameters ^ 109 4.3.8.4 Mechanistic Proposals ^ 4.3.9 Comparisons of Reactivity ^  111 113  4.4 Epilogue and Future Work ^  116  4.5 References and Notes ^  117  CHAPTER 5: Some Characteristic Chemistry of CpMo(NO)(CH2CMe 3 ) 2 and its Transient Thermal Decomposition Product, CpMo(NO)(=CHCMe 3 ) ^ 121 5.1 Introduction ^  121  5.1.1 Historical Development ^  121  5.1.2 Relationship to Cp'M(NO)R2 Complexes ^  122  5.2 Experimental Procedures ^  123  5.2.1 Methods ^  123  5.2.2 Reagents ^  123  5.2.3 Preparation of CpMo(NO)(CH2CMe3)2 ^  123  5.2.4 Preparation of (Me3CCD2)2Mg•dioxane) ^  124  5.2.5 Preparation of CpMo(NO)(CD 2 CMe3 )2  ^  126  5.2.6 Preparation of [CpMo(N0)(CHCMe 3 )] 2  ^  127  5.2.7 Preparation of CpMo(NO)(=CHCMe 3 )L ^  128  5.2.8 Reaction of CpMo(NO)(CD2CMe3) 2 with PPh2Me ^  130  ix 5.2.9 Preparation of [(Me3P) 4Mo(NO)Cl]2 ^  130  5.2.10 Preparation of CpMo(NO)(=CHCMe3)(py) ^  131  5.2.11 Preparation of CpMo(NO)(=CHCMe3)(PMe3): Reaction of 5.7 with PMe3 ^ 131 5.2.12 Attempted Preparations of CpMo(NO)(=CHCMe3)(L) [L = acetone, acetonitrile, thiophene, PhCCH, P(CMe3)3] ^  132  5.2.13 Preparation of CpMo(N0)(11 2 -C (NCMe3)CH2CMe3)(CH2CMe3) ^ 132 5.2.14 Preparation of CpMo(NO)(CH2 CMe3)(NHR) ^  133  5.2.15 Preparation of CpMo(N0)(CHDCMe 3 )(NH-p-toly1) ^ 134 5.2.16 Preparation of CpMo(NO)(CH2 CMe3 )(OPh) ^  134  5.2.17 Kinetic Measurements ^  135  5.3 Results and Discussion ^  136  5.3.1 Synthesis and Characterization of CpMo(NO)(neopentyl)2 ^ 136 5.3.1.1 Solid-State Molecular Structure of CpMo(NO)(CD 2 CMe3) 2 ^ 137 5.3.2 Synthesis and Characterization of [CpMo(N0)(CHCMe3)]2 ^ 139 5.3.2.1 X-ray Crystallographic Analysis of [CpMo(N0)(CHCMe 3 )] 2 ^ 141 5.3.2.2 Known Nitrosyl Bonding Modes ^ 5.3.3 Trapping Reactions ^  143 144  5.3.3.1 Synthesis and Characterization of CpMo(NO)(=CHCMe3)(L) ^ 145 5.3.3.2 X-ray Crystallographic Analysis of CpMo(NO)(=CHCMe3)(PPh2Me) ^ 147 5.3.3.3 Attempted Preparation of CpMo(NO)(=CHCMe 3 )(PMe 3 ) ^ 149 5.3.3.4 Preparation of CpMo(NO)(=CHCMe3)(PMe 3 ) ^ 149 5.3.3.5 Thermal Stability of Adduct Complexes ^  150  5.3.3.6 Attempted Preparation of CpMo(NO)(=CHCMe 3 )(CNCMe3 ): Synthesis of an Insertion Product ^ 5.3.4 Kinetic Studies ^  150 152  5.3.4.1 Variation of Ligand and Ligand Concentration ^ 153 5.3.4.2 Variation of Temperature ^  156  5.3.4.3 Implications on the Formation of [CpMo(N0)(CHCMe3))2 ^ 159  x 5.3.5 Further Chemistry of CpMo(NO)(=CHCMe3): Trapping Reactions with Ligands Containing E-H bonds (E = N, 0) ^  160  5.3.5.1 Reactivity of CpMo(NO)(=CHCMe3) With Amines ^ 160 5.3.5.2 X-ray Crystallographic Analysis of Complex 5.10 ^ 163 5.3.5.3 Reactivity of CpMo(NO)(=CHCMe3) with Phenol ^ 165 5.4 Epilogue and Future Work ^  165  5.5 References and Notes ^  166  xi List of Figures  Figure 1.1 Synergic bonding interactions between a transition metal and a linear  nitrosyl ligand ^  2  Figure 2.1 Views of the solid-state molecular structures of Cp * Mo(N0)(o-toly1)2 and  Cp * W(N0)(o-toly1)2 ^  26  Figure 2.2 Pictorial representations of the open coordination sites at each metal center in  representative Cp'M(NO)R2 complexes ^  27  Figure 2.3 300 MHz 1 H NMR spectrum of Cp * W(N0)(o-toly1)2(PMe3) in C 6D 6 ^ 30 Figure 2.4 Ambient temperature cyclic voltammogram of Cp*W(N0)(p-toly1)2 in THF ^ 31 Figure 3.1 300 MHz 1 H NMR spectra of 3.3 in C6D6 ^  56  Figure 3.2 300 MHz 1 H NMR spectrum of 3.9 in C 6D 6 ^  58  Figure 3.3 Cyclic voltammogram of Cp*Mo(NO)(CH2 CMe3)Cl in THE ^ 61 Figure 3.4 300 MHz 1 H NMR spectrum of 3.11 in C 6D 6 (inset: Newman projection of  Cp * Mo(NO)(CH2CMe3)(Cl)(PMe3) down the metal-Cp * centroid axis) ^ 65 Figure 3.5 300 MHz 1 H NMR spectra of 3.12 at (a) room temperature and  (b) -100 °C in THF-d8 ^  67  Figure 3.6 ORTEP diagram of 3.13 ^  70  Figure 4.1 Designation of isomers for [Cp * Mo(N0)(R)]2-(p-0) complexes ^ 99 Figure 4.2 Overlaid UV-vis spectra of 3.8 and 4.1 as THF solutions ^ 106  Figure 4.3 First-order plots for the hydrolysis reaction ([3.8] = 1.116 x 10 -3 M) ^ 108 Figure 4.4 kob s vs [H2O] for the reaction of Cp * Mo(NO)(CH2SiMe3)2 with H2 O ^ 108 Figure 4.5 Variable temperature first-order rate plots for reactions of Cp*Mo(N0)(CH2SiMe3)2 (3.8) with excess H 2 O ^  110  Figure 4.6 Erying plot for the reaction of Cp * Mo(NO)(CH2SiMe3)2 with H 2 O ^ 111 Figure 5.1 ORTEP diagram of 5.1-d4 ^  138  Figure 5.2 Spectroscopic characterization of 5.2: (a) partial Nujol mull infrared spectrum  (1700 - 700 cm-1 ); (b) 300 MHz 1 H NMR spectrum in C6D6 ^  140  xii Figure 5.3 ORTEP diagram of 5.2 ^  142  Figure 5.4 Valence-bond representation of a 1.1 2 -1 1 :1 2 -NO ligand serving as a 5-electron donor to two Mo centers ^  143  Figure 5.5 Known nitrosyl bonding modes ^  144  Figure 5.6 IR spectral monitoring (1650 to 1540 cm -1 ) of the formation of CpMo(NO)(=CHCMe3)(PPh2Me) (5.3) from 5.1 and excess PPh 2Me in Et20 ^ 146 Figure 5.7 300 MHz 1 H NMR spectrum of CpMo(NO)(=CHCMe3)(PPh3) in C 6D 6 ^ 147 Figure 5.8 ORTEP diagrams of CpMo(NO)(=CHCMe3)(PPh2Me) (5.3): (a) view perpendicular to alkylidene plane; (b) view parallel to alkylidene plane ^ 148 Figure 5.9 1 H NMR spectrum of complex 5.8 in C 6D6 ^  151  Figure 5.10 Absorption spectral changes for the thermolysis of CpMo(NO)(CD2CMe3)2 in CH2 C1 2 in the presence of 6.49 equiv PPh 3 (run 5) ^  154  Figure 5.11 Spectral expansion and first-order log plot of data (run 5) ^ 155 Figure 5.12 Representative Eyring plots for the conversion of CpMo(NO)(CD2CMe3)2 (5.1-d4 ) to CpMo(N0)(=CDCMe3) in hexanes and CH2C12 ^  158  Figure 5.13 1 H NMR spectrum of 5.10 in C 6D6 ^  162  Figure 5.14 ORTEP diagram of CpMo(N0)(CH2CMe3)(NH-p-toly1) (5.10) ^ 164 Figure 5.15 Comparison of the angles about Mo in complexes 5.1 and 5.10 ^ 163  List of Tables  Table 2.1 Numbering Scheme, Color, Yield and Elemental Analysis Data ^ 19 Table 2.2 Mass Spectral and Infrared Data ^  19  Table 2.3 1 11NMR Data ^  20  Table 2.4 Electrochemical Data for the First and Second Reductions of Complex 2.5 ^ 21 Table 3.1 Numbering Scheme, Color, Yield, and Elemental Analysis Data ^ 46 Table 3.2 Mass Spectral and Infrared Data ^  47  Table 3.3 NMR Data (C 6D6) ^  48  Table 3.4 Electrochemical Data for the First Reductions of Cp *M(NO)C1 2 ,  Cp * M(NO)(CH2CMe3)Cl, and Cp * M(NO)(CH2CMe3) 2 ^  51  Table 3.5 Comparative LR and Electrochemical Data (THF) ^  60  Table 3.6 Selected bond lengths and angles for complex 3.13 ^  70  Table 4.1 Numbering Scheme, Color, Yield, and Elemental Analysis Data ^ 89 Table 4.2 Mass Spectral and Infrared Data ^  90  Table 4.3 1 H and 13 C{ 1 H} NMR Spectral Data ^  90  Table 4.4 Electrochemical Data for 3.8 and 4.1 ^  92  Table 4.5 Variation of Water Concentration Data ^  93  Table 4.6 Rate Constants for Various Concentrations of 3.8 using H2 O and D 2 0 ^ 94 Table 4.7 Rate Constants as a Function of Temperature ^ Table 4.8 UV-vis Data (in THF) for Complexes 3.8 and 4.1 ^  94 106  Table 5.1 Representative bond lengths and angles for complex 5.1-d 4 ^ 138 Table 5.2 Representative bond lengths and angles for complex 5.2 ^ 142 Table 5.3 Representative bond lengths and angles about Mo in 5.3 ^ 148 Table 5.4 UV-vis Data for CpMo(NO)(CD 2 CMe3 ) 2 in Various Solvents ^ 153 Table 5.5 Thermal Decomposition Reactions of 5.1-d4 in the Presence of  Phosphines in CH 2 C1 2 ^  154  Table 5.6 Rate Constants as a Function of Temperature in Various Solvents ^ 157  xiv Table 5.7 Activation Parameters Derived from Eyring Plots ^  158  Table 5.8 Representative bond lengths and angles for complex 5.10 ^ 164  XV  List of Schemes  Scheme 1.1 Characteristic reactions of CpW(NO)(CH2SiMe 3 )2 ^  5  Scheme 3.1 IR and electrochemical comparisons of pairs of 16-electron organometallic nitrosyl complexes differing in only one component of their respective structures ^ 63 Scheme 4.1 Labelling experiments ^  104  Scheme 4.2 Mechanistic pathway from Cp *Mo(NO)R2 to [Cp * Mo(NO)R] 2 -(2-0) ^ 111 Scheme 4.3 Elementary reaction sequence for the conversion of Cp *Mo(NO)R2 to [Cp * Mo(NO)R]2-(4-0) ^  112  Scheme 4.4 Interrelationships of oxo derivatives of Cp * Mo(NO)(CH 2 SiMe 3 ) 2 ^ 114 Scheme 5.1 Possible mechanism for the formation of 5.2 from 5.1 ^ 159 Scheme 5.2 Possible paths for the reaction of CpMo(N0)(CD 2 CMe3)2 with RNH2 ^ 161  xvi List of Abbreviations  The following list of abbreviations, most of which are commonly used in the chemical literature, have been employed in this thesis.  A^angstrom, 10 -10 m anal.^analysis atm^atmosphere(s) by^boiling point br^broad (spectral) °C^degree centigrade 13c^carbon-13  '3Com^proton-decoupled carbon-13 (observe carbon while decoupling proton) calcd^calculated C 6 D6^benzene-d6 CDC1 3^chloroform-d1 CD2C12^dichloromethane-d2 cm-1^wavenumbers COSY^correlation spectroscopy (in NMR spectroscopy) Cp^i5-05H5, perhydrocyclopentadienyl Cp *^15-05Me5, pentamethylcyclopentadienyl Cp'^both Cp and Cp * CV^cyclic voltammogram 5^chemical shift in ppm referenced to Me 4 Si at 5 0 d^doublet (in an NMR spectrum); or day(s) A^heat (in thermolysis) DCI^direct chemical ionization (in mass spectrometry) Ea^activation energy  xvii EI^electron-impact (in mass spectrometry) Et2 0^(CH3CH2)20, ether, diethyl ether eV^electron volts FAB^fast-atom bombardment (in mass spectrometry) Fc^ferrocene, Cp2Fe g^gram(s)  free energy of activation h^Planck's constant; or hour(s)  1H^proton 2H^deuterium enthalpy of activation Hz^Hertz (s -1 )  iR^(Current) x (resistance) IR^infrared J^coupling constant (in NMR spectroscopy) n JAB^n-bond coupling constant between atoms A and B  K^degree Kelvin kJ^kilojoule kB^Boltzmann's constant kobs^observed rate constant LDA^lithium diisopropylamide LUMO^lowest unoccupied molecular orbital m^multiplet (in NMR spectroscopy) M^Mo and W; or molar; or mega m/z^mass-to-charge ratio (in mass spectrometry) Me^CH3, methyl mg^milligram(s) min^minute(s)  xviii mL^milliliter mmol^millimole mol^mole MS^mass spectrum v^stretching frequency (in IR spectroscopy); or scan rate (in electrochemistry) neopentyl^CH2CMe3 neophyl^CH2CMe2Ph NMR^nuclear magnetic resonance ORTEP^Oak Ridge Thermal Ellipsoid Plot 31p^phosphorus-3 1  [11 +^parent molecular ion (in mass spectrometry) Ph^C6H5, phenyl ppm^parts per million (in NMR spectroscopy) pz^pyrazolyl q^quartet (in an NMR spectrum) quat^quaternary carbon atom RT^room temperature s^singlet (in an NMR spectrum) AS:^entropy of activation SCE^standard calomel electrode t^triplet (in an NMR spectrum) TBAH^tetrabutylammoniumhexafluorophosphate tetraglyme^tetraethylene glycol dimethyl ether, Me(OCH2CH2)4OMe o-tolyl^C6H4-2-Me, ortho-tolyl p-tolyl^C6H4-4-Me, para-tolyl THE^C41180, tetrahydrofuran THF-d8^C4D80 UV^ultraviolet (in electronic spectroscopy)  xix V^volt vis^visible (in electronic spectroscopy)  XX  Acknowledgements  Many people made the pages that follow an easier accomplishment.  I thank Peter Legzdins for being the bane of mediocrity and for giving me the opportunity to achieve this goal. I also thank him for his support, encouragement, enthusiasm and absolute commitment to excellence. I'm grateful to past and present members of the Legzdins group (P. J. Lundmark, G. B. Richter-Addo, N. H. Dryden, J. D. Debad, K. J. Ross, W. S. McNeil, M. A. Young, E. B. Brouwer, E. C. Phillips, N. J. Christensen, E. G. Vessey, T. C. Chin, S. F. Sayers, M. J. Shaw, R. Reina and K. M. Smith), for their help, humor and friendship. Specific thanks to PJL for continuous moral support and conversation. These four long years weren't so bad. Thanks to GRA for his endless enthusiasm for nitrosyl chemistry. Thanks to NHD for coining "skim the cream". Thanks also to JDD and KJR for numerous mountain biking adventures. Thanks to WSM for proofreading nearly everything I have written. Thanks to Andrew McKerrow, the true definition of a colleague and friend. Thanks to Dave McConville, the eternal pessimist, for his realistic views of science and research. Thanks to Mike Fryzuk and Brian James for the advice they provided me in numerous hours of discussion. Thanks to Professors R. A. Whitney, M. C. Baird and S. Wolfe who were my biggest influences in pursuing a Ph.D. degree. Thanks to Drs. S. J. Rettig, R. J. Batchelor and F. W. B. Einstein for solving ten crystal structures, and attempting what seems like hundreds more, for me. Thanks to S. Rak, P. Borda, M. Austria, M. Lapawa and L. Darge for their countless hours of work on my behalf. Thanks to Irene Rodway for always having an open door and ear. I am indebted to my parents, Peter and Gail, for their love and continuing support. I sincerely acknowledge my in-laws, Jim and Emily Taylor, for their love, understanding and interest in my work. I also thank my grandfather, Ed McMahon, and my aunt and late uncle, Pheme and Bill Brown, for always believing in me. Lastly, I owe everything to my wife, Kim Taylor, for her unwavering love, support and companionship. You have made me what I am.  xxi  Science is indeed a mirror to nature, a process of describing the natural world in increasingly elaborate terms.  Peter Knudtson  To Kim  1  CHAPTER 1 General Introduction and Scope of This Thesis  1.1 Background ^  1  1.2 Early Work ^  3  1.3 Scope of This Thesis ^  6  1.4 Format of This Thesis ^  8  1.5 References and Notes ^  8  1.1 Background A fundamental principle of transition-metal organometallic chemistry is the 18-valenceelectron rule which states that stable organometallic complexes will contain a total of 18 valence electrons. 1 The valence-bond basis of this rule, which is analogous to the octet rule in organic chemistry, is that organometallic complexes generally owe their stability to the filling of the nine valence orbitals of the transition metal with eighteen electrons provided by the metal and the ligands of the complex. In molecular-orbital terms, the rule may be rationalized in terms of the filling of all bonding and nonbonding valence molecular orbitals of the complex by 18 electrons and the existence of a large HOMO-LUMO energy gap. Although the vast majority of known transition-metal organometallic complexes obey this "rule", many compounds have now been isolated that violate it. Particularly interesting among the latter are the complexes that contain 16 valence electrons since such electronically, and often coordinatively, unsaturated organometallic  complexes are known to play an important role in catalytic systems. 2  2 Like 18-electron species, isolable 16-electron complexes also owe their stability to a large HOMO-LUMO gap, but their LUMOs need not be filled with a pair of electrons in order to attain a stable electronic configuration. Classic examples of sixteen-electron complexes that contain ancillary hydrocarbyl ligands are the square-planar d 8 Group 10 compounds of the type L2MR 2 [L = two-electron donor; M = Pt or Pd; R = hydrocarbyl] 3 and the pseudo-tetrahedral d 0 Group 4 metallocene bis(hydrocarbyl) complexes Cp2MR2 [Cp = 1 5 -05H5; M = Ti, Zr or Hf]. 4 More recent examples are the (PNP)IrR2 5 [PNP = N(SiMe 2 CH2PPh 2 )2] and Cp'M(NO)R2 [Cp' = Cp or Cp* (i 5 -C5Me5); M = Mo or W; R = alkyl or aryl] systems, 6 the latter of which form the basis of this thesis. Cp'M(NO)R2 complexes are 16-electron coordinatively and electronically unsaturated pseudo-octahedral molecules whose chemistry our group has been actively investigating since the discovery of CpW(NO)(CH2SiMe3)2 in 1985. 7 These Cp'M(NO)R2 complexes are monomeric and possess 3-legged piano-stool molecular structures in which the M-NO linkages are essentially linear. In this configuration, the nitrosyl group functions as a formal three-electron donor to the metal center and is a strong n-acid ligand, stronger indeed than the well known CO group. 8,9 The M-NO orbital overlaps commonly invoked to account for this synergic bonding are diagrammed in Figure 1.1.  a-donation  ^M C  —  C  M(a)  "  -  11-  N=. =.0 -  N(a)  7C -backdonation  M(dn)  --Pi- NO(7c*)  Figure 1.1 Synergic bonding interactions between a transition metal and a linear nitrosyl ligand  3 The strong n-acidity of the nitrosyl ligand is a key contributor to the molecular-orbital energy levels of the model complex CpMo(NO)Me 2 which has the requisite large HOMO-LUMO gap for a stable 16-electron complex. The LUMO of CpMo(NO)Me 2 is calculated to be a nonbonding, metal-centered orbital. 7a It may be noted at this point that the degree of M(d7c)-NO(Tc * ) backbonding is a direct function of the amount of electron density at the metal center in the compounds studied in this thesis. Transfer of electron density into the NO(7c * ) antibonding orbital decreases the N-0 bond order and thus decreases the nitrosyl stretching frequency (v NO) as measured in the IR spectrum of the complex. As we shall see later, this sensitive indicator is one of the most useful spectroscopic probes in organometallic nitrosyl chemistry.  1.2 Early Work  It was thought that the bis(hydrocarbyl) derivatives, Cp'M(NO)R 2 , should be preparable by metathesis reactions from their known dihalide precursors, Cp'M(NO)X 2 [X = I, Br, or C11. 10 The preparations of these latter complexes described in the literature involved treatment of Cp'M(NO)(CO)2 complexes with elemental halogen (eq 1.1). 12,13  CH2 Cp'M(NO)(CO)2 X2, Cp'M(NO)X2 2 COCA ^ Since solid iodine is the easiest of the halogens to handle stoichiometrically, our work in this area began by using the diiodide complexes as synthetic precursors. In 1985, Luis Sanchez, a Spanish postdoctoral fellow in our group, discovered that treatment of CpW(NO)I 2 with two equivalents of the Grignard reagent, Me 3 SiCH2MgC1, in Et 2 0 at room temperature afforded an unusual 4-coordinate magnesium complex, {CpW(N0)(CH 2 SiMe 3 )2 } 2 MgX2 , in which the nitrosyl oxygen atoms of the organometallic ligands act as donors to the central Mg (eq 1.2). Hydrolysis of this intermediate isonitrosyl complex (eq 1.3) afforded a high yield of the prototypal  4 complex, CpW(N0)(CH2SiMe3)2, as relatively air-stable and thermally-stable violet crystals. 7a An X-ray crystallographic analysis established the monomeric nature of the 16-electron complex in the solid state.  \ .:ZII./ i  4 RMgCI ^Yo. Et2 0, RT  2  ....,--W _ ---..,.. R^_=^R N 0 I X Mg X + 3 MgX2 I 0 N R \ ..-' R W  X=IorC1  p/  R = CH2 SiMe 3  i  W\ = R/ _,^R N 0 I  xs H2 O ^)0..  X 1g X 0 N R =: ,,,-R --...,,, W  (1.2)  Et2 0, RT  2^ ..„...--W_ --...,  R : R N 0  (1.3)  A preliminary survey of the reactivity of this 16-electron dialkyl compound soon showed its chemistry to be dominated by its ability to function as a Lewis acid. In other words, the LUMO of the parent complex is sufficiently low in energy that it can accept electrons from classic Lewis bases. After initial coordination to the metal center, some Lewis bases often underwent subsequent intramolecular insertion reactions involving the alkyl ligands (Scheme 1.1).Th In addition to functioning as a Lewis acid at tungsten, the prototypal dialkyl complex also possesses  5 a good Lewis base site at the nitrosyl oxygen atom (cf. eq 1.2). Hence, the reaction of CpW(NO)(CH 2 SiMe3)2 with AIMe 3 provides a monomeric isonitrosyl AIMe3 adduct (Scheme 1.1).  PMe3 R ' / \ ' PMe3 N R 0  vacuum  R  „...--W 7, ---.........  -  CNR'  -----)p.  E^R  N 0  trans  W ,,,^,- R' R ' / \ '/■1 N C 0^\ R  R = CH2 SiMe 3 R' = p-tolyl W -,' z. ."--. R^1^R N  ,  o. A1Me3  Scheme 1.1 Characteristic reactions of CpW(NO)(CH2SiMe3) 2  Subsequent work showed that neopentyl (CH2 CMe3 ) and neophyl (CH 2 CMe2Ph) analogues of CpW(NO)(CH2SiMe3)2 could be prepared in a similar manner to that described above, albeit in lower isolated yields. 7,12 b On occasion, CpMo(NO)(CH 2 SiMe 3 )2 could also be prepared. Since further research efforts showed that reactions 1.2 and 1.3 could be extended no further, a more reliable synthetic route to presumably more reactive complexes was desired. Clearly, room temperature alkylation in diethyl ether was a useful methodology for only a small proportion of our target molecules. The limited set of only Cp'W(NO)(alkyl) 2 complexes precluded a comprehensive investigation of the chemistry of the Cp'M(NO)R2 family of Lewis acids in general.  6 1.3 Scope of This Thesis  My thesis addresses the non-generality of the previous method (eq 1.2 - 1.3) and puts forth a new, completely general route to a much larger family of Cp'M(NO)R 2 complexes. The three most significant problems inherent to the original synthetic method were: (a) the reagents being used (i.e. transition-metal dihalide, alkylating agent and solvent), (b) excessive hydrolysis during workup (eq 1.3) (c) thermal decomposition The research in this thesis attempts to rationalize why the original synthetic method was not appropriate for synthesizing most of our desired Cp'M(NO)R 2 species. The methodology described in this thesis is now routinely employed in our laboratories and is useful for synthesizing dialkyl, diaryl, alkyl chloro, aryl chloro, mixed dialkyl, mixed diaryl and even mixed alkyl aryl complexes of both Mo and W. Under the original reaction conditions (eq 1.2 - 1.3) most Cp'M(NO)R2 species decompose rapidly, although in many cases to isolable, characterizable and often very interesting materials. Specifically, Chapter 2 of this thesis presents the general synthetic methodology to Cp'M(NO)R2 complexes and applies that method to the preparation of Cp'M(NO)(aryl) 2 species. The major chemical conclusion to emanate from the work in this chapter is that Cp'M(NO)(aryl) 2 complexes have much more accessible metal centers and are more potent Lewis acids than their Cp'M(NO)(alkyl) 2 congeners. 14 Chapter 3 extends the method put forth in Chapter 2 to previously inaccessible Mo dialkyl and alkyl chloro complexes. The chemical reactivity of Cp * Mo(NO)(CH 2 CMe3 )Cl is examined, as it serves as the prototypal monoalkyl complex. Chapter 3 ties together the concept of relative Lewis acidities of the whole family of Cp'M(NO)R 2 complexes using structural, electrochemical, and infrared data. On the basis of such data, we can rank Cp'M(NO)R 2 complexes in terms of their relative Lewis acidities (i.e. electron-deficiencies) as Cp > Cp*, Mo > W, and aryl > alkyl. We thus conclude that CpMo(NO)(aryl) 2 complexes, if isolable, would be the most electrondeficient of the Cp'M(NO)R2 family of complexes. Conversely, the most easily handled  7 complexes, namely the Cp*W(NO)(alkyl) 2 species, are also the weakest Lewis acids. 11,14,15 An interesting manifestation of the structure/reactivity relationship developed in Chapter 3 is that sterically or electronically the same conclusions are reached about the relative Lewis acidities of Cp'M(NO)R2 complexes. Chapter 4 presents the reactions of dialkyl and diaryl complexes of Mo and W with water, a reagent integral to the original synthetic method (eq 1.3). All isolable Mo dialkyl and diaryl compounds are readily hydrolyzed to form bimetallic [Cp'Mo(NO)R]2-(11-0) complexes and free hydrocarbon. 16 The reaction between H 2 O and Cp * Mo(NO)(CH2 SiMe3 )2 has been subjected to a kinetic analysis in order to gain some insight into the above-mentioned hydrolysis reactions. The dialkyl complexes of W are hydrolytically stable, as evidenced by the success of reaction 1.3. However, the diaryl complexes of W are exceedingly moisture-sensitive and are readily converted to dioxo complexes of the type Cp'W(0) 2R upon exposure to water. The final chapter of this thesis reports the investigation of the characteristic chemistry of CpMo(NO)(CH2 CMe 3 ) 2 and its thermal decomposition product, namely the transient 16-electron alkylidene nitrosyl complex, CpMo(N0)(=CHCMe 3 ). A detailed kinetic analysis has been performed on the thermolysis of CpMo(NO)(CD 2 CMe 3 ) 2 , the tetradeutero analogue of CpMo(NO)(CH2CMe3)2. The thermal instability of CpMo(NO)(CH2 CMe3) 2 (which is the least thermally stable, yet isolable Cp'M(NO)R 2 species) emphasizes the problematic role that temperature played in the original alkylation reactions which were conducted at room temperature. In addition to a variety of new chemical species formed when CpMo(N0)(=CHCMe 3 ) is trapped with Lewis bases or reacted with heteroatom-hydrogen bonds, the chemistry of CpMo(NO)(=CHCMe 3 ) led to the discovery of a new bonding mode for coordinated nitric oxide.17  8 1.4 Format of This Thesis  I have organized this thesis in fairly rigorous format in order to facilitate cross-referencing and readability. Allowing X to be the chapter number, Chapters 2 through 5 all have five major sections: X.1 Introduction, X.2 Experimental Procedures, X.3 Results and Discussion, X.4 Epilogue and Future Work and finally X.5 References and Notes. Subsections of these major categories are numbered using standard legal outlining procedures, e.g. X.1.1, X.1.1.1 etc. All compounds prepared in each chapter are catalogued numerically, e.g. X.1, X.2 etc. Thus, the conversion of a compound prepared in Chapter 3 to one prepared in Chapter 5 might read "3.4 was reacted under appropriate conditions to produce 5.11." Schemes, figures and equations are similarily sequenced. The standard methodologies and experimental procedures outlined in Section 2.2.1 apply to the entire thesis.  1.5 References and Notes  (1)  Elschenbroich, C.; Salzer, A. Organometallics - A Concise Introduction, 2nd ed.; VCH Publishers: New York, 1992; pp 186, 412.  (2)  Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987.  (3)  Purcell, K. F.; Kotz, J. C. Inorganic Chemistry; W. B. Saunders: Toronto, 1977; p 804.  (4)  Negishi, E.; Takahashi, T. Aldrichimica Acta 1985, 18, 31 and references therein.  (5)  (a) Fryzuk, M. D.; MacNeil, P. A.; Massey, R. L.; Ball, R. G. J. Organomet. Chem. 1989, 368, 231. (b) Fryzuk, M. D.; MacNeil, P. A.; Ball, R. G. J. Am. Chem. Soc. 1986, 108, 6414. (c) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J. J. Am. Chem. Soc. 1985, 107, 6708.  (6)  Unless explicitly stated otherwise, throughout this thesis Cp' will always denote both IliC 5 H5 (Cp) and 1 5 -05Me5 (Cp * ), M will indicate both Mo and W, and R will indicate both alkyl and aryl.  9 (7)  (a) Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B. E.; Gatter, M. G. J. Am. Chem. Soc. 1985, 107, 1411. (b) Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394.  (8)  Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley-Interscience: New York, 1988; Chapter 22.  (9)  Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University Press: New York, 1992; Chapter 1.  (10) These complexes possess either monomeric or halide-bridged dimeric molecular structures in the solid state, but in solution they are all formulated as solvated monomers. 11 Hence, they are represented exclusively by their monomeric formulae throughout this thesis. (11) Dryden, N. H. Ph.D. Dissertation, The University of British Columbia, 1990. (12) For Cp' = Cp see: (a) Seddon, D.; Kita, W. G.; Bray, J.; McCleverty, J. A. Inorg. Synth. 1976, 16, 24. (b) Hunter, A. D.; Legzdins, P.; Martin, J. T.; Sanchez, L. Organomet.  Synth. 1986, 3, 66. For Cp' = Cp * see: (c) Dryden, N. H.; Legzdins, P.; Einstein, F. W. B.; Jones, R. H. Can. J. Chem. 1988, 66, 2100. (d) Gomez-Sal, P.; de Jesia, E.; Michiels, W.; Royo, P.; de Miguel, A. V.; Martinez-Carrera, S. J. Chem. Soc., Dalton Trans. 1990, 2445. (13) A more efficient preparation of the dichloride complexes involves the use of PC1 5 as a stoichiometric source of C12 . See Dryden, N. H.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B. Organometallics 1991, 10, 2077. (14) Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2583. (15) Legzdins, P.; Veltheer, J. E. In Handbuch der Praparativen Anorganischen Chemie, 4th ed.; Herrmann, W. A., Ed.; Thieme-Verlag: Stuttgart, Germany, in press. (16) Legzdins, P.; Lundmark, P. J.; Phillips, E. C.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2991.  (17) Legzdins, P.; Rettig, S. J.; Veltheer, J. E. J. Am. Chem. Soc. 1992, 114, 6922.  10  CHAPTER 2 16-Electron Cp * M(NO)(aryl) 2 [M = Mo or W] Complexes: Their Synthesis, Characterization, and Properties  2.1 Introduction ^  10  2.2 Experimental Procedures ^  12  2.3 Results and Discussion ^  22  2.4 Epilogue and Future Work ^  33  2.5 References and Notes ^  33  2.1 Introduction  Previous work in our laboratories resulted in the discovery of a family of 16-electron dialkyl complexes having the general formula, CpW(NO)R2 [R = CH 2 SiMe3, CH2CMe3, CH2CMe2Ph]. 1,2 A surprising property of these electronically and coordinatively unsaturated compounds is their thermal and oxidative stability. For example, under inert atmospheres they are indefinitely thermally stable at room temperature. While they may be handled as solids in air for short periods of time at ambient temperatures with no deleterious effects, exposure to 0 2 for several days at 25 °C converts them to the oxo complexes CpW(0) 2 R. 3 The results of FenskeHall MO calculations on the model complexes CpM(NO)Me2 (M = Mol and W 4 ) indicate that their LUMOs are metal-centered and non-bonding in nature. As a consequence, many Cp'M(NO)R2 complexes are stable species, 5 despite their formal electron-deficiency. Originally, the dialkyl nitrosyl complexes were prepared by the sequential treatment of CpW(NO)I2 with RMgC1 in Et 2 0 at room temperature and then hydrolysis of the intermediate  11 isonitrosyl adducts as summarized in the introductory Chapter of this thesis (Section 1.2: eqs 1.2 and 1.3 ).1,2,6 The methodology described in Section 1.2 is successful for sterically demanding alkyl groups (R) without 13-hydrogens, namely CH2SiMe3, CH2CMe3 and CH 2 CMe2Ph. However, if R = aryl or the metal is molybdenum (R = alkyl or aryl) the reaction conditions do not give the desired complexes. Therefore, room temperature alkylation in Et20 was, at the inception of this work, useful for only 1/4 of our target molecules if we desired both dialkyl and diaryl complexes of Mo and W. Logically, we wished to extend this family of tungsten alkyl nitrosyl compounds to encompass related aryl complexes, as well as molybdenum alkyl and aryl complexes.? It was our expectation that the characteristic chemistry exhibited by these new Cp'M(NO)(aryl)2 species would be dependent on the nature of the aryl group. Such ligand dependent behavior has been documented in the related family of 18-electron complexes, Cp * Ru(NO)R2, for which Bergman and coworkers have established differences in the comparative reactivity of the alkyl- versus arylsubstituted complexes, 8 ' 9 e.g. Cp*Ru(NO)Me2 + 2 PMe 3 —LI' —D. Cp*Ru(CN)(PMe 3 )2 + H2 O + CH4^(2.1) Cp*Ru(NO)Ph2 + 2 PMe3 --L. Cp*Ru(NO)(PMe3 )2 + Ph-Ph^(2.2) Furthermore, it was probable that the metal centers in the Cp'M(NO)(aryl)2 complexes would be less sterically hindered than in their dialkyl analogues, thereby enhancing the reactivity of the diaryl complexes toward larger, or less reactive Lewis base substrates. Very early on we discovered that the synthetic methodology employed to prepare the dialkyl complexes (Section 1.2) could not be used to synthesize the analogous diaryl complexes, principally for three reasons. First, the choice of reagents (including the starting material, the alkylating agent and the solvent) was inappropriate. Second, the water needed to hydrolyze the intermediate isonitrosyl adducts was simply disastrous, and third, many of the target complexes  12 appeared to be very sensitive to thermal decomposition. Indeed, all Cp'M(N0)(ary1) 2 complexes synthesized in our laboratories to date react instantaneously with H 2 O at ambient temperatures (Chapter 4) 10, 11 and thermally decompose in solution at room temperature. Thus, given that the previous synthetic methodology could not be extended to include congeneric diaryl complexes, we had good reason to develop a new set of experimental conditions and reagents to synthesize these compounds. Development of a general synthetic approach to CpIM(NO)R2 [M = Mo and W, R = alkyl  and aryl] complexes began with the Ph.D. research of Neil Dryden and is further elaborated in the present work. A general methodology for the preparation of a wide variety of Cp'M(NO)R2 complexes has now been established.  2.2 Experimental Procedures  2.2.1 Methods The methodologies described in this section apply to the entire thesis. All reactions and  subsequent manipulations involving organometallic reagents were performed under anhydrous conditions in an atmosphere of prepurified argon or nitrogen. Purification of inert gases was achieved by passing them through a double-walled glass column (10 x 60 cm) containing MnO and activated 4A molecular sieves. Conventional glovebox and greaseless vacuum line Schlenk techniques were utilized throughout. 12 The specific gloveboxes utilized in this work were Vacuum Atmospheres HE-553-2 and HE-43-2 models. All IR samples were either as THE', Et 2 0, hexanes or CH2 Cl 2 solutions in NaCI cells or as Nujol mulls sandwiched between NaCI or CsI plates. IR spectra were recorded on a Nicolet 5DX FT-IR instrument, internally calibrated with a He/Ne laser. All NMR spectra were obtained on Varian Associates XL-300 spectrometer and are reported in parts per million. 1 H NMR spectra (299.94 MHz) are referenced to the residual proton signal of C 6D 6 (8 7.15), CD 2 C12 (8 5.34), CDC13 (8 7.24) or THF-d 8 (8 3.60 and 1.75). 31p{lm NMR spectra (121.42 MHz) are referenced to external P(OMe)3 set at 8 141.00 ppm  13 relative to 85% H3PO4. 13c{1H} NMR spectra (75.43 MHz) are referenced to the natural abundance carbon signals of the solvent employed: C 6D6 (8 128.00), CDCI 3 (8 77.00) or CD 2 Cl 2 (8 53.80). Mrs. M. T. Austria, Ms. L. K. Darge, and Dr. S. 0. Chan assisted in obtaining the NMR data, as did J. D. Debad. Mass spectra were recorded by Dr. G. K. Eigendorf and the staff of the mass spectroscopy laboratory. Low-resolution mass spectra (EI, 70 eV) were recorded on a Kratos MS50 spectrometer using the direct-insertion method. Fast-atom bombardment (6 kV ion source, 7-8 kV xenon FAB gun) mass spectra were recorded on a AEI MS 9 spectrometer using 3-nitrobenzyl alcohol as matrix. All elemental analyses were performed by Mr. P. Borda of this Department.  2.2.2 Reagents The organometallic reagents, Cp'M(NO)Cl 2 (M = Mo or W), were prepared by established procedures. 13 Cp'Mo(NO)Cl 2 complexes were further purified by Soxhlet extraction with CH2 C1 2 , and the Cp'W(NO)C1 2 complexes were stored at -30 °C and used within one month of preparation. PhMgC1, (p-toly1)MgC1 and (o-toly1)MgC1 (1.0 M in THF) were purchased from Aldrich Chemical Co. and were used as received. Activated magnesium powder was obtained from Baker Scientific. PMe 3 was prepared from P(OMe) 3 and MeMgI 14 and was dried over and transferred from sodium/benzophenone. Solvents were freshly distilled from appropriate drying agents under a dinitrogen atmosphere and were either purged for 10 min with argon prior to use or were directly vacuum transferred from the appropriate drying agent. Dioxane, tetrahydrofuran and diethyl ether were distilled from sodium/benzophenone; hexanes, benzene, toluene and pentane were distilled from sodium/benzophenone/tetraglyme; dichloromethane was doubly distilled from P 2 0 5 . 15 C 6D6 was dried over activated 4A molecular sieves and degassed using 3 freeze-thaw-pump cycles.  14 2.2.3 Electrochemical Measurements  The detailed methodology employed for cyclic voltammetry studies in these laboratories has been outlined elsewhere. 16,17 Potentials were supplied by a BAS CV27 voltammograph, and the resulting cyclic voltammograms were recorded on a Hewlett-Packard Model 7090A Measurement Plotting System in buffered recording mode. All measurements were made at room temperature. The potentials, E°', are defined as the average of the cathodic and anodic peak potentials, (Eps + Ep,a)/2, and are reported versus the aqueous saturated calomel electrode (SCE). In a drybox, solutions were prepared at 0.1 M in the [n-Bu4/4]PF6 support electrolyte and —6 x 10 -4 M in the organometallic complex to be studied. During the course of an electrochemical experiment the solutions were maintained under an atmosphere of argon. The 3electrode cell consisted of a Pt bead working electrode (-1 mm diameter), a Pt wire auxiliary electrode, and a SCE in a separate holder connected to the cell compartment via a Luggin probe separated by a frit. Compensation for iR drop in potential measurements was not employed during this study. Ferrocene (Fc) was used as an internal reference in these studies, with the redox couple Fc/Fc+, occurring at Eci'= 0.54 V versus SCE in THE over the range of scan rates (v) used (0.10 to 0.80 V s -1 ). The anodic and cathodic peak separation (AE) for this couple increases with increasing scan rate (80 - 240 mV between 0.10 - 0.80 Vs -1 ), but since the Fc/Fc+ couple is known to be highly reversible, other redox couples exhibiting similar peak separations and integrations to the internal standard were also considered to be electrochemically reversible, single-electron processes. The ratio of cathodic peak current to anodic peak current, ^for the oxidation of Fc was unity over all scan rates used, as expected for a chemically reversible process. The linearity of a plot of i p,a vs. v 12 was checked for all reduction processes to establish the presence of diffusion control. 17  2.2.4 General Preparation of R 2 M•X(dioxane) (X= 1 - 2)  A general procedure for the preparation of diarylmagnesium reagents is outlined below; this methodology is a modification of a published procedure for the preparation of unsolvated  15 dialkylmagnesium complexes. 18 Since Neil Dryden (Ph.D. 1990) conceived the idea of using diaryl magnesium complexes as synthetic reagents to prepare Cp'M(NO)(aryl)2 complexes, he is thankfully acknowledged. 19 The method presented below was utilized to synthesize (Ph) 2Mg•(dioxane), (p-toly1) 2Mg•(dioxane) and (o-toly1)2Mg•(dioxane). A 0.4 M solution of the Grignard reagent in THF was prepared (either from activated Mg powder and aryl chloride or by dilution of purchased RMgC1 solution). The solution was then treated with 1,4-dioxane (2.2 equivalents) dropwise from an addition funnel. The resulting white slurry was stirred for 6 - 24 h at room temperature. The supernatant solution containing the desired R2Mg•(dioxane) was isolated by centrifuging portions of the slurry for 20 min at 3500 rpm and then removing the clear supernatant solutions by cannulation. The remaining solids were similarily reextracted with fresh THF. The solvent was removed from the combined supernatant solutions under reduced pressure to obtain tarry residues. These residues were triturated exhaustively with hexanes, the hexanes being carefully removed after each trituration by cannulation. The remaining solid was dried in vacuo (24 h at 25 °C) to obtain the organomagnesium products as dusty white pyrophoric powders. Higher purity diaryl magnesium reagents were prepared by crystallization of these powders from THF/Et20. The compositions of the powders or crystals with respect to their active carbanion equivalents were established by hydrolysis of weighed solid samples and titration of the resulting solutions with 0.100 N HC1 using phenolphthalein as the indicator.  2.2.5 Preparation of Cp * M(NO)(aryl)2 [M = Mo, W; aryl = Ph, p-tolyl, o-tolyl] (2.1 - 2.6) Since the synthetic approach to each of these six complexes is similar, their syntheses are presented in a generalized manner. Solid Cp * M(NO)C12 (2.00 mmol) and solid R2Mg•(dioxane) (1.98 mmol, slight deficiency) were intimately mixed in a Schlenk tube contained in a glovebox. The tube was removed from the box and was attached to a vacuum line, whereupon THF (25 mL) was vacuum transferred onto the solids at -196 °C. The reaction mixture was then allowed to warm to -50 °C under an  16 atmosphere of argon. After being stirred for 30 min at -50 °C, the mixture was warmed to 0 °C. The solvent was removed in vacuo without further warming of the reaction mixture, and the residues were washed with pentane. The dried blue-to-purple residues were then extracted with cold Et 2 0 (10 °C) and filtered through Celite (4 x 15 cm) at -20 °C supported on a sintered glass frit. 20 The extracts were concentrated in vacuo until the first signs of crystallization were evident. If brown solids appeared on the sides of the flask during the initial concentration procedure, a second low-temperature filtration was done. Complete crystallization occurred upon storing the concentrated extracts in a freezer overnight. Finally, the desired diaryl complex was isolated by removal of the supernatant solution by cannulation and drying of the remaining crystals in vacuo. The tungsten complexes, 2.4 2.6, are more robust than the molybdenum complexes, 2.1 -  -  2.3, and can therefore be filtered through Florisil (60 -100 mesh) maintained at low temperatures  rather than Celite as specified above. However, transfer of the THF by syringe or the use of Grignard reagents in THF as the arylating agents reduce the isolated yields of products by at least 20 - 30%. Occasionally, complete decomposition of the entire reaction mixtures occurs when using commercial Grignard reagents. This observation is attributed to the fact that commercially available aryl Grignard reagents contain residual aryl halide, as well as Wurtz coupling products, which complicate the purification and isolation of the desired organometallic products. The physical properties of 2.1 2.6 are collected in Tables 2.1 - 2.3 (Section 2.2.10). -  2.2.6 Attempted Syntheses of CpM(NO)(aryl)2 [M = Mo, aryl = Ph, p-tolyl, o-tolyl; M = W, aryl = Ph, p-tolyl]  Efforts to prepare the analogous CpMo(NO)(aryl) 2 (aryl = Ph, o-tolyl, p-tolyl) and CpW(NO)(aryl) 2 (aryl = Ph, p-tolyl) complexes via methodology similar to that outlined above have so far been unsuccessful. Reaction of CpM(NO)C1 2 complexes with the appropriate diaryl magnesium reagents in THE results in distinctive red-purple to blue solutions. The IR spectra of these reaction solutions show strong bands in the region anticipated for the vNo of these diaryl complexes, specifically CpMo(NO)Ph 2 , 1628 cm -1 ; CpMo(NO)(p-tolyl)2, 1624 cm-1;  17 CpMo(NO)(o-tolyl)2, 1624 cm -1 ; CpW(NO)Ph2 , 1601 cm-1 ; and CpW(NO)(p-tolyl)2, 1599 cm -1 . However, any attempts to isolate these aryl complexes that have involved manipulations above -40 °C resulted in a rapid color change to red-brown and loss of their distinctive IR spectral features.  2.2.7 Synthesis of CpW(NO)(o-tolyl)2 (2.7) CpW(NO)(o-tolyl)2 is the only isolable diary' nitrosyl complex that does not incorporate a pentamethylcyclopentadienyl ligand (Cp * ). It is synthesized from CpW(NO)C1 2 and (o-tolyl)2MgX(dioxane) in a manner analogous to the method used to synthesize complexes 2.1 - 2.6; filtration through Florisil during work-up, however, is mandatory. 21 Crystallization from diethyl ether provides 2.7 as violet needles in variable yields (10 - 30%). Anal. calcd for C19H 1 9NOW: C, 49.48; H, 4.15; N, 3.04. Found: C, 49.19; H, 4.21; N, 3.05. IR (THF) v No 1602 cm-1 . IR (Nujol) vNO 1601 cm -1 . 1 H NMR (C 6D6) 5 7.43 - 7.02 (m, 8H, aryl H), 5.22 (s, 5H, C5H5), 2.83 (s, 6H, 2 x o-CH 3 ). Low-resolution mass spectrum (probe temperature 120 °C): m/z 461 (Pl.  2.2.8 Preparation of Cp * W(N0)(p-toly1)2(PMe3) (2.8) A sample of Cp * W(N0)(p-toly1)2 (35 mg, 0.066 mmol) was dissolved in hexanes (10 mL) at room temperature. PMe3 (1 atm) was introduced into the Schlenk tube via vacuum transfer from Na/benzophenone whereupon the deep blue solution instantly became yellow. Concentration and cooling of the reaction solution overnight at -30 °C induced the precipitation of a white powder which was collected and washed with cold pentane (5 mL) to obtain analytically pure Cp * W(N0)(p-toly1)2(PMe3) (32 mg, 80%). Anal. calcd for C 2 7H3 8NOPW: C, 53.39; H, 6.31; N, 2.31. Found: C, 53.43; H, 6.22; N, 2.30. IR (THF) vN ID 1562 cm -1 . IR (Nujol) vNo 1559 cm -1 . 1 H NMR (C6D6) 5 7.86 (dm, 2H, 2 of 4 ortho protons, transoidal to PMe 3 , Jiip not resolved), 7.07 (d, 2H, Jig/ = 7.1 Hz, 2 of 4 meta protons), 6.99 (d, 2H, ./pH = 7.1 Hz, 2 of 4 ortho protons), 6.91 (d, 2H, JHH = 7.1 Hz, 2 of  18 4 meta protons), 2.23 (s, 6H, 2 xp-CH3 ), 1.48 (s, 15H, C5(CH3 )5), 0.59 (d, 9H, JHp = 9.5 Hz, P(CH3 ) 3 ). 31 P NMR (C6D6) 8 -21.11 (s, Jwp = 320 Hz). Low-resolution mass spectrum (probe temperature 180 °C): m/z 531 (P+-PMe3). FAB mass spectrum: m/z 607 (Pl.  2.2.9 Preparation of Cp * W(N0)(o toly1) 2 (PMe3) (2.9) -  A solution of Cp * W(N0)(o-toly1) 2 was generated from Cp * W(NO)C12 (840 mg, 2.00 mmol) and (o-toly1)2Mg•X(dioxane) (640 mg, 4.00 mmol o-tolyl -) in THE (40 mL) at -20 °C. The solvent was removed in vacuo and the residue extracted with CH 2 C12/Et20 (1:1, 50 mL). The extracts were filtered through Florisil (3 x 5 cm). The column was rinsed with CH 2 Cl 2 (3 x 25 mL) and the purple filtrate was taken to a purple powder in vacuo. The powder was dissolved in Et 2 0 (80 mL) and CH2 C1 2 (3 mL). PMe 3 (1 atm) was introduced into the Schlenk tube containing the solution of Cp * W(N0)(o-toly1) 2 via vacuum transfer from Na/benzophenone whereupon the violet solution instantly became yellow. The yellow solution was taken to dryness and dried in vacuo for 5 h. The dried extracts were then extracted with CH 2 C1 2 (40 mL) and filtered through alumina I neutral (3 x 4 cm). The column of alumina was rinsed exhaustively with CH2C1 2 (150 mL). Removal of solvent from the filtrate, redissolution in a minimum of Et 2 0 (-50 mL) and crystallization overnight at -30 °C induced the precipitation of Cp * W(N0)(o-toly1)2(PMe3) (180 mg, 15% based on Cp * W(NO)C12) as an off-white microcrystalline solid. Anal. calcd for C27H38NOPW: C, 53.39; H, 6.31; N, 2.31. Found: C, 53.46; H, 6.29; N, 2.29. IR (Nujol) vNo 1536 cm 1 (br). 1 H NMR (C 6 D6 ) 5 8.10, 7.45 6.91 (m, aryl protons for -  -  both cis and trans isomers), 2.86 (s, o-CH3, cis isomer), 2.82 (s, 2 x o-CH3, trans isomer), 2.23 (s, o-CH3, cis isomer), 1.60 (s, C 5 (CH3 ) 5 , trans isomer), 1.58 (s, C 5 (CH3 ) 5 , cis isomer), 0.87 (d, JHp = 6.3 Hz, P(CH3 ) 3 , trans isomer), 0.81 (d, JHp = 9.0 Hz, P(CH3 ) 3 , cis isomer). 31 P NMR  (C6D6) 8 -17.54 (s, trans isomer, Jwp = 311 Hz), -20.40 (s, cis isomer Jwp = 309 Hz). Lowresolution mass spectrum (probe temperature 100 °C): m/z 531 (P+-PMe 3 ). FAB mass spectrum:  m/z 607 (Pt).  19 2.2.10 Characterization Data for Complexes 2.1 - 2.6  Table 2.1 Numbering Scheme, Color, Yield and Elemental Analysis Data  elemental analysis found (calcd)  compd  color  no.  (yield, %)  Cp*Mo(NO)Ph 2 a  2.1  violet (42)  60.51 (63.61)  5.87 (6.07)  3.19 (3.37)  Cp*Mo(N0)(p-toly1) 2  2.2  purple (65)  64.83 (65.01)  6.70 (6.59)  2.92 (3.16)  Cp*Mo(N0)(o-toly1) 2  2.3  purple (32)  65.06 (65.01)  6.57 (6.59)  3.16 (3.16)  Cp*W(NO)Ph2  2.4  blue (55)  52.30 (52.50)  5.00 (5.01)  2.66 (2.78)  Cp*W(N0)(p-toly1) 2  2.5  blue (81)  53.98 (54.25)  5.52 (5.50)  2.62 (2.63)  Cp*W(N0)(o-toly1) 2  2.6  violet (40)  55.15 (54.25)  5.89 (5.50)  2.59 (2.63)  complex  H  C  N  a The elemental analysis of C, H and N are proportionally low due to a small quantity of residual MgC1 2 (approx. 4.9%) which could not be removed without decomposing the organometallic complex.  Table 2.2 Mass Spectral and Infrared Data  IR  compd  MS  tempb  no.  m/za  (°C)  VNO (THY)  vN0 (Nujol)  2.1  417^[P4]  90  1615  1592  2.2  445 [P1  100  1609  1603  80  1601  1587  415^[P+-NO]  2.3  445 [P1 415 [P + -NO]  2.4  503 [P1  120  1588  1576  2.5  531^[P1  100  1582  1576  2.6  531^[P+]  80  1601  1545  439^[P+- C71181 a m/z values are for the highest intensity peak of the calculated isotopic cluster ( 98 Mo and isaw). b Probe temperatures.  20 Table 2.3 1 H NMR Data  compd no. 2.1  1H NMR (C6D6) 7.81 (dd, 4H, J= 7.2, 1.2 Hz, ortho protons) 7.23 (t, 4H, J= 7.2 Hz, meta protons) 7.12 (t, 211, J= 7.2 Hz, para protons) 1.59 (s, 1511, C 5 (CH3 ) 5 )  2.2  7.77 (d, 4H, J= 6.9 Hz, aryl protons) 6.98 (d, 4H, J= 6.9 Hz, aryl protons) 2.07 (s, 611, 2 xp-CH3 ) 1.59 (s, 15H, C 5 (CH3 ) 5 )  2.3  7.25 - 6.94 (m, 8H, aryl protons) 2.20 (s, 6H, 2 x o-CH3 ) 1.60 (s, 15H, C 5 (CH3 ) 5 )  2.4  7.99 (d, 4H, J= 8.0 Hz, ortho protons) 7.18 (t, 4H, J= 7.4 Hz, meta protons) 7.07 (d, 211, J= 7.2 Hz, para protons) 1.58 (s, 15H, C 5 (CH3 ) 5 )  2.5  8.01 (d, 4H, J= 6.4 Hz, ortho protons) 6.99 (d, 4H, J= 6.4 Hz, meta protons) 2.01 (s, 6H, 2 x p-CH3 ) 1.65 (s, 15H, C 5 (CH3 ) 5 )  2.6  7.88 - 7.00 (m, 8H, aryl protons) 2.92 (s, 611, 2 x o-CH3 ) 1.62 (s, 1511, C5(CH3)5)  21 2.2.11 Other Characterization Data  Table 2.4 Electrochemical Data for the First and Second Reductions of Complex 2.5a  elb (V)  AEc  i^/i^ PA PXd  0.80  -1.21  0.13 (0.14)  1.0  0.40  -1.21  0.10 (0.10)  1.0  0.20  -1.21  0.08 (0.10)  1.0  0.10  -1.21  0.07 (0.08)  1.0  scan rate  .E°'26 (V)  AEc  ip,a/ip,cd  0.80  -2.46  0.12 (0.14)  0.8e  0.40  -2.46  0.10 (0.10)  0.8e  0.20  -2.46  0.07 (0.10)  0.8e  0.10  -2.46  0.06 (0.08)  0.8e  scan rate (v, VS-1 )  (v, Vs -1 )  a In THE containing 0.10 M [n-Bu 4NJPF6 , at a Pt-bead working electrode. Potentials are measured vs SCE.  b Defined as the average of the cathodic and anodic peak potentials (± 0.02 V). c Defined as the separation of the cathodic and anodic peak potentials. Values of AE given in parentheses are for the Cp2 Fe/Cp 2Fe+ couple under the same conditions. d Ratio of anodic peak current to cathodic peak current e Accurate determination of this ratio was not possible since the reduction wave occurred very close to the solvent limit of the THF/TBAH electrolyte system.  22 2.3 Results and Discussion 22  2.3.1 A New General Synthetic Methodology  A reliable, reproducible synthesis of CVM(NO)R 2 complexes began, oddly enough, with electrochemistry. Cyclic voltammetry showed that the metathesis reactions forming dialkyl nitrosyl complexes proceeded through isolable radical anion intermediates of the type [Cp 1M(NO)X2 ]. - . 16 This work empirically showed that [CVM(NO)C1 2 ]• - are the longest lived and most thermally stable dihalo radical anions in this family. Therefore, it was our view that a general synthetic methodology for the preparation of Cp 1M(NO)R2 complexes should be designed to maximize the stability of these radical anion intermediates. This requirement implies the use of (a) dichloro nitrosyl starting materials, (b) highly solvating solvents to stabilize ionic intermediates, and (c) low temperatures to avoid possible decomposition pathways of the newly formed organometallics. Obviously, the use of the diiodide complexes at room temperature in diethyl ether did not meet these requirements (Section 1.2). Cp 1M(NO)C1 2 complexes are now used exclusively as starting materials in our work since we have previously established that the dichloro complexes, Cp'M(NO)C1 2 (M = Mo or W), are superior to their iodo analogues as synthetic precursors for the preparation of Cp'M(NO)(alkyl) 2 complexes. 3,19,22,23,24,25 Our next step was to overcome the inherent need for water in reaction 1.3. The use of R2Mg•X(dioxane) reagents instead of Grignard reagents accomplishes this goal. Equation 2.3 summarizes the synthetic method that was designed to synthesize Cp *M(NO)(aryl)2 complexes. TIE , argon Cp * M(NO)C12 (ary1)2MgX(dioxane)^  -50 to Cc  Cp*M(N0)(ary1) 2 + Cl2M•dioxane) (2.3)  After metathesis, the dioxane from R 2 Mg•(dioxane) ligates the MgC1 2 formed considerably more efficiently than does the oxygen atom of the nitrosyl ligand of the desired Cp*M(NO)R 2 complexes, thereby removing the need for water to liberate the desired organometallics from isonitrosyl adducts of the type [Cp*M(NO)R 2 ] 2 •MgC1 2 (eq 2.1). 26 Additionally, these reagents  23 are solids and, when properly prepared, are of exact stoichiometries. An additional benefit of reaction 2.3 is that Cl2Mg•dioxane is very insoluble in even moderately polar solvents thus allowing uncontaminated Cp * M(NO)(aryl) 2 complexes to be extracted from the final reaction mixtures. The desired products of reactions 2.3 range in color from deep indigo-blue to violet and are obtainable as analytically pure, crystalline solids in yields of 32-81%. 27  2.3.2 Physical Properties Cp'M(NO)(aryl)2 complexes are thermally stable as solids when analytically pure, but in solution at room temperature decompose over the course of several days or weeks. Overall, these complexes are markedly less stable than their dialkyl analogues since they are prone to decompose autocatalytically, even at low temperatures. The diaryl complexes are soluble in most common organic solvents, but to a noticeably lesser extent than their dialkyl analogues. For instance, Cp*W(N0)(p-toly1) 2 is the only diaryl complex that is soluble in hexanes, whereas all Cp'M(NO)(alkyl)2 species are pentane soluble. In general, the p-tolyl derivatives are more soluble than the o-tolyl and phenyl derivatives. Both in solution and in the solid state, Cp * M(NO)(aryl)2 species decompose rapidly when exposed to the atmosphere.  2.3.3 Spectroscopic Characterization The spectroscopic properties of the six Cp * diaryl complexes are consistent with their possessing monomeric, three-legged piano-stool molecular structures having a 16-valenceelectron configuration at the metal center. Thus, just as observed previously for related dialkyl complexes, the 1 H NMR spectra of these diaryl complexes (Table 2.3) are not unusual and are fully consistent with their formulations. The v NO bands in the IR spectra of the molybdenum diaryl complexes are, on average, some 23 cm-1 higher in energy than their tungsten congeners. These vNO bands are also some 30 - 60 cm-1 lower in energy than those exhibited by their parent dichloro complexes, a manifestation of the better electron donation from an aryl ligand than from an electronegative  24 chloride ligand. However, when the IR spectra of the diaryl complexes are compared to those of the dialkyl complexes, it is obvious than aryl ligands are considerably less electron-donating than alkyl ligands. Both the solution and solid-state ER. spectra of the dialkyl derivatives display v NO values which are 10 - 30 cm -1 lower in energy than those of similar diaryl complexes (see Section 1.1 for the conclusions that can be drawn from vNO). In other words, their IR spectroscopic properties imply that in the Cp*M(NO)(aryl) 2 complexes the metal centers are more electrondeficient than in their alkyl analogues, and this in turn is manifested by the molybdenum and tungsten centers in the aryl species being more Lewis acidic than in related Cp * M(N0)(alky1) 2 complexes. The topic of relative Lewis acidities is further addressed in Section 3.3.2.  2.3.4 Attempts to Synthesize CpM(N0)(ary1) 2 Complexes  Efforts to prepare the Cp-containing members of this class of complexes by reaction 2.3 indicate that the desired diaryl products are indeed formed in solution. This conclusion is supported by the observation of a single v NO in the IR spectra of the final reaction mixtures (Section 2.2.6) at values consistent with those expected for diaryl nitrosyl complexes. Furthermore, the reaction solutions are a very distinctive blue to purple color which is characteristic of isolable diary! nitrosyl complexes. However, all attempts to isolate the Cpcontaining products from these solutions have been unsuccessful so far, the decomposition of the desired products being indicated by the loss of the characteristic colors of the reaction mixtures upon attempted workup. The only exception to this generalization is that CpW(N0)(o-toly1) 2 is isolable, albeit in fairly low yields (< 35%). 11,32 Solutions containing in situ generated CpW diaryl complexes can be derivatized (vide infra), but in situ generated CpMo(N0)(ary1) 2 complexes have resisted derivatization. For example, a violet THE solution of CpMo(NO)(p-tolyl) 2 (1 mmol generated in situ) can be treated with PMe 3 (1 atm) at -50 °C. The product after workup, a yellow solid, gives an amber solution in C 6 D6 . A 1 H NMR spectrum of this solution reveals 4,4'-dimethylbiphenyl (8 7.45, d; 7.03, d; 2.16, s) to be  the major product in the mixture. An unidentified Cp peak at 8 5.24 quickly diminishes in  25  intensity as new spectra are recorded. Within 30 minutes, the peak at 8 5.24 is gone and the walls of the NMR tube are coated with an insoluble dark brown precipitate. The only two identifiable compounds left in solution at this time are PMe 3 and 4,4'-dimethylbiphenyl. Clearly, the aryl ligands of CpMo(NO)(p-tolyl)2 are extremely labile. Our inability to isolate CpW(NO)(aryl) 2 complexes other than CpW(NO)(o-tolyl) 2 was unexpected, since three CpW(NO)(alkyl) 2 complexes have been isolated in our laboratories. Since the Cp*-containing analogues are accessible, however, it may be that a planar aromatic ligand in conjunction with the less sterically demanding Cp group does not shield the unsaturated metal center as well as do bulky alkyl and Cp * ligands. Our recent discovery of the thermal instability of CpMo(NO)(CH2 CMe3 ) 2 25 indicates to us that CpMo(NO)(aryl) 2 species are likely to be even more thermally sensitive, particularly since it appears that low activation energy pathways for their decomposition exist (Section 2.2.6). Evidently, the additional steric shielding and greater electron-donating ability of the Cp* ligand versus the Cp ligand provide the necessary factors to render a variety of Cp * M(NO)(aryl) 2 complexes stable enough to be isolated. This general stabilizing ability of the Cp * group has also been documented for other systems. 28  2.3.5 X ray Crystallographic Analyses of Complexes 2.3 and 2.6 -  The solid-state molecular structures of both Cp *M(N0)(o-toly1) 2 complexes have been established, and their monomeric natures have been confirmed. 29 ORTEP diagrams of these two very similar diaryl complexes are presented in Figure 2.1. The intramolecular geometrical parameters of both molecules resemble those found in CpW(NO)(CH 2 SiMe3 )22 and CpMo(NO)(CD2CMe3)2.30  26  Figure 2.1 Views of the solid-state molecular structures of Cp *Mo(N0)(o-toly1) 2 , 2.3 (on the left) and Cp * W(N0)(o-toly1)2 , 2.6 (on the right); 33% probability ellipsoids are shown for the non-hydrogen atoms  The solid-state structures of the four crystallographically characterized Cp'M(NO)R 2 complexes can be compared. The chemically most interesting metrical parameters of these molecules is the apparent size of the open coordination site at their metal centers. Figure 2.2 shows a triangle with the metal-carbon bond lengths and C-M-C bond angle defining the empty coordination site of each complex. The non-bonded separations between the two carbon atoms directly attached to the central metals are also given.  27 CpW(NOXCH2 SMe 3 )2^CpMo(NO)(CD 2CMe 3 )2^CP 8 W(NO)(0-toly1)2^Cp * Mo(N0)(0-tolY02 A^B^ C^D  A  1.09.6  C4^ C  C4  -  3.44A^  0  C^C^C C •^ C • 3.59A^ 3.65A  3.53A^  Figure 2.2 Pictorial representations of the open coordination sites at each metal center in representative Cp'M(NO)R2 complexes  For simplicity, allow A = CpW(NO)(CH2SiMe3)2; B = CpMo(NO)(CH2CMe3)2; C = Cp * W(N0)(o-toly1)2; and D = Cp * Mo(N0)(o-toly1) 2 . Then, from the series of non-bonded C-C separations it is apparent that aryl complexes have more room at their metal centers than alkyl complexes, i.e. Dcc > BCC (i.e. 3.65 A > 3.53 A) and Ccc > Acc (i.e. 3.59 A > 3.44 A). Additionally, molybdenum complexes have increased accessibility to their metal centers compared to analogous tungsten complexes, thus BCC > ACC (i.e. 3.53 A > 3.44 A) and Dcc > CCC (i.e. 3.65 A > 3.59 A). Furthermore, it is also evident from the structures of 2.3 (C) and 2.6 (D) (Figure 2.1) that the a-bonded aryl groups are considerably less sterically demanding than are similarly attached bulky alkyl groups. The upshot of the crystallographic analyses is that the diaryl complexes are more accessible Lewis acids, this in addition to being more potent Lewis acids (Section 2.3.3). Thus, the nonbonded C-C separation, used as a measure of metal accessibility, allows us to conclude that diaryl species are considerably more spatially accessible than their dialkyl analogues. This feature is also manifested by the characteristic chemistry of these complexes which is considered in the next section.  28 2.3.6 Reactivity Patterns of the Diaryl Complexes In all cases investigated to date, diaryl complexes react with substrates orders of magnitude faster than analogous dialkyl complexes under similar conditions, e.g. eqs 2.4 and 2.5.  W .-/- _ )  r-, \ -F  CO (1 atm) ^'P.^ hours  W ..,_  ------  CMe2 Ph  (2.4)  N 0 PhMe 2C NO CM e2 Ph^ PhMe 2 C 0  CO (1 atm)  W  seconds  0  (2.5)  N 0  The transformation shown in eq 2.5 involves CO insertion to form an acyl complex. It is very facile, being complete in a matter of seconds under ambient conditions. In comparison, Cp *W(NO)(CH2CMe2Ph)2 reacts with CO to form the analogous monoacyl complex only after 6 h under the same reaction conditions (eq 2.4). 31 In a similar manner, in situ generated CpW(NO)(p-tolyl)2 reacts instantaneously with elemental sulfur in nonpolar solvents at 25 °C to provide CpW(N0)(S-p-toly1)2. 32 In contrast, CpW(N0)(CH2SiMe3)2 reacts with sulfur in toluene (50 °C) over a period of one day in a sequential manner to form first CpW(NO)(SCH2SiMe3)(CH2SiMe3), then CpW(N0)(ri 2 -SfS)CH2SiMe 3 )(CH2 SiMe 3 ), and then finally the analogous bis(thiolate) complex, CpW(NO)(SCH2SiMe3)2. 2,33 Cp * M(NO)(aryl)2 complexes are also more potent Lewis acids than analogous dialkyl species. Previous work has demonstrated that CpW(NO)(CH2SiMe3)2 and PMe 3 establish an equilibrium in toluene at room temperature. 2 Hence, the monophosphine adduct,  29 CpW(NO)(CH2SiMe3)2(PMe3), can only be isolated in the presence of an excess of PMe3, and it loses PMe3 when exposed to vacuum. In contrast, prolonged exposure of isolated Cp*W(N0)(ary1) 2 (PMe3) species (2.8 and 2.9) to high vacuum for days at room temperature does not regenerate Cp * W(N0)(p-toly1) 2 (2.5) or Cp * W(N0)(o-toly1) 2 (2.6), respectively. From its spectral data (Table 2.3), complex 2.8 [Cp * W(N0)(p-toly1)2(PMe3)] clearly has equivalent aryl ligands. Thus, the structure in solution must be such that the PMe3 ligand is trans to the nitrosyl group of the complex (eq 2.6).  PMe3  (2.6)  In contrast, complex 2.9, the o-tolyl analog of 2.8, exists as a 60/40 mixture of cis and trans isomers in C 6D6. Thus, the 1 H NMR spectrum of 2.9 (Figure 2.3) shows three distinct o-Me environments; two for the cis isomer and one for the trans isomer. We can present no rationale for the ability of complex 2.9 to isomerize while 2.8 is configurationally stable, since both are nominally 7-coordinate molecules. The nitrosyl-stretching frequencies of Cp * W(NO)(aryl) 2 complexes and their 18-electron adduct derivatives can be compared. Adding PMe 3 to Cp * W(N0)(p-toly1) 2 causes a drop in vNo of 20 cm-1 in THE (16 cm -1 in the solid state). Upon going from 2.6 to 2.9, a drop of 9 cm -1 (solid-state) is observed. These observations are consistent with the metal centers in 2.8 and 2.9 being more electron-rich than the parent complexes 2.5 and 2.6, respectively.  30  Figure 2.3 300 MHz 1 H NMR spectrum of Cp * W(N0)(0-toly1)2(PMe3) (2.9) in C6D6  31 2.3.7 Electrochemical Properties of Complex 2.5 Knowledge of the redox properties of molecules is of fundamental importance to understanding their reactivity behavior and their chemical properties. Hence, Cp *W(N0)(p-toly1)2 (2.5), has been investigated by cyclic voltammetry as a representative member of the Cp *M(NO)(aryl)2 family. The cyclic voltammogram of 2.5 in THF (Figure 2.4) exhibits reduction features consistent with the occurrence of two reversible, one-electron reductions at E" values of -1.21 V and -2.46 V. These reductions imply the consecutive formation of [Cp*W(N0)(p-toly1)2• - and [Cp *W(N0)(p-toly1) 2 1 2- in solution. The cyclic voltammogram of 2.5 is devoid of any oxidation features out to the solvent limit of 0.8 V.  -1'-.  +e--.41.  .......-^W^ : --....,  41--.--  e  24.e.11--  -e N 0 17-electrons  16-electrons  0.0^  -'54.......,^.W -..,,  -1.0  N 0 ^  ^  18-electrons  - 2.0^  Votts vs SCE  Figure 2.4 Ambient temperature cyclic voltanunogram of Cp * W(N0)(p-toly1) 2 (scan rate = 400 mV r 1 ) in THF  - 3.0  32 The linearity of plots of i p,a vs. v 112 for these reduction waves shows that the reductions are diffusion controlled. Comparison of the AE values for these reductions with the internal Fc/Fc+ reference indicate that the first reduction is electrochemically reversible. The i p,a/ip , c value for the first reduction is unity over the range of scan speeds used, thereby establishing its chemical reversibility. From Figure 2.4 and the data presented in Table 2.4, it is apparent that an accurate determination of i p,a/ip,c for the second reduction feature is not possible, but the reduction wave is electrochemically reversible and in all likelihood is chemically reversible as well. Some of the  [Cp * W(N0)(p-toly1)21 2- formed in the second reduction may react with Cp * W(N0)(p-toly1) 2 to give two molecules of [Cp * W(N0)(p-toly1)2]• - before reoxidizing to [Cp*W(N0)(p-toly1) 2 ]. electrochemically, a phenomenon that would lower i p,a/ip,c ratios. Cp * W(N0)(p-toly1)2 is easier to reduce by 410 mV than is Cp * W(NO)(CH 2 CMe2Ph) 2 which exhibits a reversible first reduction in THF at -1.62 V. 22 Interestingly, this result is consistent with the diaryl complex containing a more electron-deficient metal center than does the related Cp * W(NO)(alkyl)2 species. Hence, the aryl ligands of Cp * W(NO)(aryl) 2 evidently provide less electron density to the metal center thus allowing such complexes to accept electrons more easily than analogous Cp * W(NO)(alkyl)2 complexes. Additionally, Cp * W(N0)(p-toly1) 2 exhibits a second reduction wave at more negative potential, i.e. -2.46 V. The second reduction of Cp * W(NO)(CH2CMe2Ph)2 in THF apparently occurs beyond the solvent limit of -2.7 V, thereby precluding a comparison of this feature with that exhibited by the Cp * W(N0)(p-toly1)2 complex. Thus the formation of [Cp * W(N0)(CH2 CMe2Ph) 2 ] 2- occurs at more than 250 mV more negative potential than the second reduction of Cp * W(N0)(p-toly1)2. The electrochemical investigation of Cp * W(N0)(p-toly1) 2 has established that its reduction behavior is consistent with the Fenske-Hall MO description of CpMo(NO)Me2 (Section 2.1), i.e. the LUMO of Cp'M(NO)R2 complexes is non-bonding and filling of this orbital with electrons is completely reversible and does not disrupt the bonding in these complexes. Finally, infrared spectral studies of diaryl nitrosyl complexes are also consistent with their being more electron-deficient than comparable dialkyl complexes. Thus, a Nujol mull IR  33 spectrum of Cp*W(N0)(p-toly1) 2 shows a strong nitrosyl band at 1576 cm -1 , where the more electron-rich Cp * W(NO)(CH2CMe2Ph)2 complex exhibits its vNO at 1551 cm -1 under identical conditions.  2.4 Epilogue and Future Work  In addition to developing a reliable synthetic route to various Cp *M(NO)(aryl)2 complexes from their Cp *M(NO)C12 precursors, this work has established some fundamental facts about the diaryl species vis-à-vis their better known alkyl analogues. Specifically, the thermal stability of Cp'M(NO)R2 complexes appears to be Cp * > Cp, W > Mo, and alkyl > aryl. Hence, CpMo(NO)(aryl)2 complexes have yet to be isolated. Furthermore, the investigations summarized in this chapter have established that the diaryl complexes are more reactive than their dialkyl counterparts, principally because their metal centers are more electron deficient and sterically more accessible to incoming nucleophilic reactants. In principle it would of great interest to separate the respective magnitudes of steric and electronic contributions to the Lewis acidity of Cp'M(NO)R2 complexes, but it seems unlikely that this will be possible. Ongoing research in our group includes a study of the reactions of diaryl complexes with organic substrates that do not react with dialkyl complexes. For example, heterocumulenes (CO2 and isoelectronic analogues) do not react with Cp'W(NO)(alkyl)2 species, but they do react with the highly activated diaryl complexes to give insertion products.34  34 2.5 References and Notes (1)  Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B. E.; Gatter, M. G. J. Am. Chem. Soc. 1985, 107, 1411.  (2)  Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394.  (3)  Legzdins, P.; Phillips, E. C.; Sanchez, L. Organometallics 1989, 8, 940.  (4)  Bursten, B. E.; Cayton, R. H. Organometallics 1987, 6, 2004.  (5)  In this context, stability of a complex refers to resistance to decomposition, within a reasonable period of time (hours) under "normal" physical conditions (inert atmosphere and ambient temperature) to any intra- or intermolecular chemical processes.  (6)  Hunter, A. D.; Legzdins, P.; Martin, J. T.; Sanchez, L. Organomet. Synth. 1986, 3, 66.  (7)  While Cp'Mo(NO)(CH 2 SiMe3) 2 were known compounds at the inception of this work neither was synthesizable reproducibly or in greater than 25% yield.  1,3  (8)  Seidler, M. D.; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 6110.  (9)  Chang, J.; Bergman, R. G. J. Am. Chem. Soc. 1987, 109, 4298.  (10) Legzdins, P.; Lundmark, P. J.; Phillips, E. C.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2991.  (11) A particularly interesting example of this reactivity is provided by our discovery of water converting one of these diaryl nitrosyl compounds to its arylimido oxo analogue. Legzdins, P.; Rettig, S. J.; Ross, K. J.; Veltheer, J. E. J. Am. Chem. Soc. 1991, 113, 4361.  (12) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-Sensitive Compounds, 2nd ed.; Wiley-Interscience: New York, NY, 1986. (13) Dryden, N. H.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B. Organometallics 1991, 10, 2077. (14) Our method of preparing PMe3 is adapted from Wolfsberger, W.; Schmidbaur, H. Synth. React. Inorg. Met.-Org. Chem. 1974, 4, 149.  35 (15) Perrin, D. D.; Armarego, W. L. F.; Pen -in, D. R. Purification of Laboratory Chemicals, 3rd ed., Pergamon Press: Oxford, 1988. (16) Herring, F. G.; Legzdins, P.; Richter-Addo, G. B. Organometallics 1989, 8, 1485. (17) Legzdins, P.; Wassink, B. Organometallics 1984, 3, 1811. (18) Anderson, R. A.; Wilkinson, G. Inorg. Synth. 1979, 19, 262. (19) Dryden, N. H., Ph.D. Dissertation, The University of British Columbia, 1990. (20) The requisite low-temperature frit for performing this operation is available from Kimble Science Products under the Kontes trademark (product # 215400; p 157 of the 1988 catalog). (21) If the filtration is conducted using Celite, the product isolated is the isonitrosyl complex, [CpW(NO)(o-tolyl)2] 2 . MgC1 2 (50 - 70 % yield). Anal. Calcd for C38H38N 2 O2MgC1 2 W2: C, 44.85; H, 3.76; N, 2.75. Found: C, 44.52; H, 3.97; N, 2.47. In solution this complex behaves as if it were free CpW(NO)(o-tolyl) 2 . 32 (22) The work presented in this chapter has been published, see: Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2583. (23) Veltheer, J. E.; Legzdins, P. In Handbuch der Preiparativen Anorganischen Chemie, 4th ed.; Herrmann, W. A., Ed.; in press. (24) (a) Dryden, N. H.; Legzdins, P.; Phillips, E. C.; Trotter, J.; Yee, V. C. Organometallics 1990, 9, 882. (b) Legzdins, P.; Jones, R. H.; Phillips, E. C.; Yee, V. C.; Trotter, J.;  Einstein, F. W. B. Organometallics 1991, 10, 986. (c) Dryden, N. H.; Legzdins, P.; Trotter, J.; Yee, V. C. Organometallics 1991, 10, 2857. (25) Legzdins, P.; Rettig, S. J.; Veltheer, J. E. J. Am. Chem. Soc. 1992, 114, 6922. (26) These adducts are recognizable as red-to-brown Et 2 0-insoluble materials. (27) Cp * W(N0)(p-toly1)2 and Cp * W(NO)Ph2 were first prepared by N. H. Dryden, 19 but the methodology employed in this thesis is considerably more reliable and higher yielding.  36 (28) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Inorg. Chem. 1984, 23, 1739 and references cited therein. (29) Crystals of 2.3 are monoclinic of space group P21/a; a = 11.570 (2) A, b = 10.128 (3) A, c = 18.439 (4) A, 13= 92.50 (2)°; Z = 4. Crystals of 2.6 are monoclinic of space group P2 1/a;  a = 11.612 (3) A, b = 10.168 (5) A, c = 18.250 (3) A, 13 = 91.87 (2)°; Z = 4. Dr. Steve Rettig solved both structures using the Patterson method and full-matrix least-squares refinement procedures to R= 0.030, Rw = 0.033 and R= 0.037, Rw = 0.040, for 3387 and 3019 reflections with I 3a(1), respectively. (30) The solid-state structure of CpMo(NO)(CD 2 CMe 3 ) 2 has been determined, see Chapter 5. (31) Dryden, N. H.; Legzdins, P.; Lundmark, P. J.; Einstein, F. W. B.; Reisen, A.  Organometallics, in press. (32) Legzdins, P.; Ross, K. J. unpublished observations. (33) Evans, S. V.; Legzdins, P.; Rettig, S. J.; Sanchez, L.; Trotter, J. Organometallics 1987, 6,  7. (34) Brouwer, E. B., M.Sc. Dissertation, The University of British Columbia, 1992.  37  CHAPTER 3 Chemistry of Cp*Mo(NO)(R)Cl and Cp'Mo(NO)R2 [R = alkyl] Complexes: Synthesis and Reactivity of Some Neutral Nitrosyl Lewis Acids  3.1 Introduction ^  37  3.2 Experimental Procedures ^  38  3.3 Results and Discussion ^  52  3.4 Epilogue and Future Work ^  75  3.5 References and Notes ^  76  3.1 Introduction  Systems containing transition-metal-carbon a bonds, especially those with coordinatively and/or electronically unsaturated metal centers, have received the attention of many workers. 1 This fact is due not only to the synthetic utility of such complexes because of their involvement in a variety of stoichiometric and catalytic transformations, 2 but it is also a reflection of the desire of many researchers to continue elucidating the fundamental chemistry of the transition-metal-carbon bond. Our work has centered on the characteristic chemistry of hydrocarbon ligands attached to the 14-valence electron Cp 1M(NO) fragments. In a recent Account, 3 we correlated our current knowledge of the symmetric 16-electron complexes CpM(NO)R2 [R = alkyl, aryl], much of which is described in Chapters 1 and 2. Chapter 3 focuses mainly on related unsymmetric species; specifically, this chapter reports four areas of my research. Firstly, the methodology from Chapter 2 is utilized to synthesize a variety of previously inaccessible Cp'Mo(NO)(alkyl)2 complexes. Secondly, that same method allows  38 straightforward syntheses of Cp * Mo(NO)(alkyl)C1 complexes which were previously deemed "unsynthesizable" by past members of our group and are the obvious precursors to unsymmetric hydrocarbyl complexes, Cp *Mo(N0)(R)R'. Thirdly, a complete assessment of the relative Lewis acidities (electron deficiencies) of all known Cp'M(NO)(R)Cl and Cp 1M(NO)R2 species is presented. The structure/reactivity relationship amongst all known CpM(NO)-containing complexes bearing hydrocarbyl ligands is substantiated by spectroscopic, electrochemical, crystallographic and chemical reactivity data. Lastly, the characteristic reactivity of Cp * Mo(N0)(CH2CMe3)C1 is described in detail. This molecule reacts with Lewis bases of many types, silver (I) salts and alkylating (or arylating) agents.  3.2 Experimental Procedures  3.2.1 Methods  The synthetic methodologies employed throughout this thesis are described in detail in section 2.2.1.  3.2.2 Reagents  The organometallic dihalide complexes Cp'Mo(NO)C12, 4 were prepared and handled as described in Section 2.2.2. The (alky1)2Mg•Y(dioxane) reagents [R = CH2CMe3, CH2CMe 2Ph, CH2 SiMe3 ] were prepared in manner analogous to that described for (ary1)2Mg•X(dioxane) in Section 2.2.4. PMe 3 was prepared as described in Section 2.2.2 and was vacuum transferred from Na/benzophenone. Me3SiCH 2MgC1 (1.0 M in Et20, Aldrich) was used as received. Other reagents such as CO (Matheson), CNCMe 3 , AgBF4 and LDA (Aldrich) were also used as received. Pyridine and acetonitrile (BDH) were twice distilled from CaH2 . Samples of Cp * W(NO)(CH2CMe3)Cl and Cp * W(NO)(CH2CMe3)2 were provided by Jeff D. Debad. THF-d8 (Aldrich) was used as received. CDCI 3 and CD2C12 (MSD Isotopes) were dried over P205 and stored in the dark.  39 The column chromatography material used during this work was alumina (80 - 200 mesh, Fisher neutral, Brockman activity I). Filtrations were performed through Celite 545 diatomaceous earth (Fisher) that had been oven-dried and cooled in vacuo.  3.2.3 Preparation of CpMo(NO)R 2 [R = CH 2 CMe2 Ph (3.1), CH 2 SiMe3 (3.2)] The preparation of CpMo(NO)(CH2CMe 2Ph)2 is given as a representative example of the method used to synthesize both 3.1 and 3.2. In a glovebox, CpMo(NO)C1 2 (262 mg, 1.00 mmol) and (PhMe2CCH2)2Mg•X(dioxane) (409 mg, 1.00 mmol) were weighed into a Schlenk tube. THF (20 mL) was then vacuum transferred onto the solids at -196 °C. The stirred reaction mixture was warmed to room temperature over the course of 10 min. The deep red solution (v NO 1607 cm -1 ) was then taken to dryness in vacuo. The oily red residue was suspended in Et 2 0 (40 mL), and transferred to the top of an alumina I neutral column (2 x 2 cm) supported on a medium-porosity flit. The column was washed with Et 2 0 (2 x 30 mL) until the filtrate was colorless. The filtrate was then taken to dryness and redissolved in pentane (10 mL). Cooling of the pentane solution of 3.1 for several days at -30 °C induced the formation of red crystals of the product (197 mg, 43% yield). Complex 3.2 (51% yield) was synthesized similarly except Celite was used in place of alumina during the filtration step.  3.2.4 Preparation of Cp * Mo(NO)(R)CI [R = CH2CMe 3 (3.3), CH 2 CMe2 Ph (3.4), CH 2 SiMe3 (3.5)] The preparation of Cp * Mo(N0)(CH2CMe 3 )C1 (3.3) is given as a representative example. Cp * Mo(NO)C12 (332 mg, 1.00 mmol) and (Me3CCH2)2MgX(dioxane) (165 mg, 0.50 mmol) were weighed into a Schlenk tube and the solids were ground to a fine powder with a spatula. THF (40 mL) was then vacuum transferred onto the solids at -196 °C. The stirred reaction mixture was slowly allowed to warm to -45 °C, whereupon the insoluble orange Cp *Mo(NO)Cl 2 was slowly taken into the increasingly purple solution (vNO 1618 cm -1 ). When no solid  40 Cp * Mo(NO)C12 remained in the bottom of the Schlenk tube (2-6 h at -45 °C), the reaction was deemed complete. The reaction mixture was then warmed to -10 °C and the THF was removed in vacuo. The oily purple residue was suspended in pentane (25 mL), stirred for 5 min at room temperature and then taken to dryness to yield a purple powder. The powder was extracted with pentane/Et 2 0 (4:1, 30 mL), filtered through Celite (2 x 5 cm) and concentrated in vacuo. Cooling of the solution at -30 °C for several days induced the formation of purple needles of 3.3 (203 mg, 55% yield). For father reactivity studies, solutions of Cp * Mo(NO)(CH2CMe3)Cl (ca. 90% yield) 5 were used directly after the Celite filtration described in the previous paragraph. Complexes 3.4 and 3.5 were prepared in a similar manner but substituting the appropriate R2 Mg reagent where necessary. Cp * Mo(N0)(CH2 CMe2 Ph)C1 forms maroon crystals from hexanes in 64% yield. Cp * Mo(N0)(CH2 SiMe 3 )C1 is a blue-purple oil and has not yet been crystallized.  3.2.5 Preparation of Cp * Mo(NO)R2 [R = CH 2 CMe3 (3.6), CH2 CMe 2 Ph (3.7)] The Cp * Mo(NO)R2 complexes were prepared in a manner analogous to that described for their diaryl congeners (Section 2.2.5). The preparation of 3.6 is given as a representative example of the synthetic procedure. In a glovebox, Cp *Mo(NO)C1 2 (332 mg, 1.00 mmol) and (Me3CCH2)2Mg•X(dioxane) (330 mg, 1.00 mmol) were weighed into a Schlenk tube. THF (15 mL) was vacuum transferred onto the solids at -196 °C. The stirred reaction mixture was slowly allowed to warm to 0 °C whereupon the mixture first turned purple then red (v Nic, 1588 cm -1 ). The red reaction mixture was taken to dryness in vacuo. The residue was suspended in Et20 (30 mL), treated with 2 drops of water and quickly transferred onto a column of alumina I neutral (2 x 3 cm) supported on a glass frit. The column was washed with Et20 (30 mL) until the eluate was colorless. The ether was then removed from the filtrate to afford a red powder which was dissolved in a minimum of pentane (5 mL). The pentane solution was cooled at -30 °C overnight in a freezer. Large red  41 crystals of Cp * Mo(NO)(CH2CMe3)2 (240 mg, 59% yield) formed and were isolated by removing the mother liquor via cannulation. Similarily, Cp *Mo(N0)(CH2CMe2Ph)2 (3.7) was isolated as red crystals in 70% yield. However, the addition of H2 O was omitted from the procedure leading to 3.7.  3.2.6 Preparation of Cp * Mo(NO)(CH2SiMe3)2 (3.8)  The procedure outlined below is a significant improvement of the methodology that we employed previously to synthesize this compound. 6 This procedure provided the bulk quantities of complex 3.8 that were required for the chemistry described in Chapter 4. To a rapidly stirred, brown suspension of Cp * Mo(NO)C12 (4.58 g, 13.8 mmol) in Et20 (200 mL), maintained at -60 °C, was added Me3SiCH2MgC1 (27.4 mL of a 1.0 M Et 2 0 solution, 27.4 mmol) in a dropwise fashion from an addition funnel under a constant purge of Ar. Immediately, the reaction mixture turned purple, and an off-white precipitate slowly formed. The reaction mixture was stirred while the cold bath was permitted to warm slowly to room temperature over 2 h. The final deep purple reaction mixture (v NO 1593 cm-1 ) was taken to dryness in vacuo. Under rigorously anaerobic conditions, the remaining lavender solid was suspended in hexanes (50 mL), and the resulting slurry was filtered through Celite (3 x 3 cm). The Celite plug was then rinsed with hexanes (2 x 15 mL). The combined, deep purple filtrates were taken to dryness and extracted with pentane (3 x 15 mL). Cooling of the pentane extracts for 2 days at -30 °C in a freezer induced the deposition of very large (e.g. 10 mm x 5 mm x 2 mm) violet crystals. These crystals (2.55 g) were collected by filtration. Further concentration of the filtrate and cooling afforded a second crop of crystals (1.08 g). Total yield of 3.8 thus obtained was 3.63 g (60%). Cp * Mo(N0)(CH2 SiMe 3 )2 can also be synthesized using (Me3SiCH2 )MeX(dioxane) as the alkylating reagent, but there is no significant increase in yield.  42 3.2.7 Preparation of Cp * Mo(N0)(CH2CMe 3 )R [R = CH2 SiMe3 (3.9); R =p-tolyl (3.10)]  The preparation of 3.9 is representative of the method used for both unsymmetric complexes. Cp * Mo(NO)(CH2CMe3)Cl (262 mg, 0.71 mmol) and (Me 3 SiCH2 ) 2 Mg•X(dioxane) (92 mg, 0.35 mmol) were weighed into a Schlenk tube. Et 2 0 (15 mL) was vacuum transferred onto the solids at -196 °C. The stirred reaction mixture was slowly allowed to warm to 0 °C, and the final burgundy mixture was taken to dryness in vacuo. The residue was extracted with pentane (2 x 10 mL), and the combined extracts were filtered through a pad of Celite (2 x 5 cm). The filtrate was taken to dryness in vacuo to obtain 3.9 (123 mg, 41% yield) as an analytically pure red powder. Similarily, complex 3.10 was isolated as lumps of maroon crystals in 30% yield by crystallization from Et 2 0.  3.2.8 Preparation of Cp * Mo(N0)(CH 2 CMe 3 )(CI)(PMe 3 ) (3.11)  A solution of Cp * Mo(NO)(CH2CMe3)Cl (3.3) in Et 2 0 (10 mL) was generated from Cp *Mo(NO)C1 2 (1.00 mmol) as described in Section 3.2.4. The purple solution was treated with PMe3 (excess) which was vacuum transferred from Na/benzophenone. The resulting orange solution was concentrated and cooled for 1 week in a freezer (-30 °C) to obtain 3.11 as large yellow blocks (197 mg, 49% yield).  3.2.9 Preparation of Cp * Mo(N0)(CH2 CMe3 )(CI)(py) (3.12)  A sample of Cp * Mo(N0)(CH2CMe3)CI (184 mg, 0.500 mmol) in Et 2 0 (10 mL) was treated with pyridine (0.50 mL, excess) at room temperature with no noticeable change in color. Upon cooling to -30 °C, the purple reaction solution turned orange-red, and the desired product began to crystallize. The reaction mixture was stored in a freezer (-30 °C) overnight to complete the crystallization. 3.12 (192 mg, 86% yield) was isolated as orange lumps which were dried in vacuo at low temperature (-10 °C). Prolonged drying at room temperature caused the complex to lose pyridine and darken to the red-purple characteristic of its Cp * Mo(N0)(CH2CMe 3 )CI precursor.  43 3.2.10 Preparation of Cp * Mo(N0)(71 2 -C{O}CH2 CMe3)C1 (3.13) A sample of Cp *Mo(NO)(CH2CMe3)Cl (188 mg, 0.511 mmol) contained in a 300 mL -  heavy-walled reaction vessel was dissolved in Et20 (25 mL). The solution was frozen (-196 °C) and thoroughly degassed via several freeze-pump-thaw cycles. The vessel was charged with CO (2 atm), sealed, and then warmed to room temperature. The reaction solution changed from purple to brown over the course of 4 h at which time the CO pressure and solvent were removed in vacuo. The brown residue was extracted with hexanes/Et20 (1:2, 85 mL), and the extracts were filtered through Celite (2 x 2 cm). The filtrate was then concentrated and cooled to -30 °C to induce crystallization. Three days later, amber needles of 3.13 (60 mg, 30%) were isolated by removal of the red mother liquor, which was subsequently discarded, since attempts to obtain more pure product from it were unsuccessful.  3.2.11 Preparation of Cp * Mo(N0)(i 2 C{NCMe3 }CH2 CMe 3 )C1 (3.14) -  In a drybox, a sample of Cp * Mo(NO)(CH2CMe3)Cl (184 mg, 0.500 mmol) contained in a 100-mL reaction vessel was dissolved in Et20 (10 mL). To the solution was added CNCMe 3 (-100 pi.„ —0.9 mmol) via a pipette. The unstirred solution instantly changed color from purple to amber. Within seconds the amber solution began to turn bright yellow and a yellow powder precipitated. The reaction vessel was sealed, removed from the drybox, and reattached to a vacuum line. The solvent and excess isocyanide were removed in vacuo. The yellow residue was then dissolved in a minimum of Et20 (-80 mL) and placed in a freezer to crystallize. Overnight, the desired product crystallized in nearly quantitative yield (212 mg, 95%) as microcrystalline yellow needles. The mother liquor was removed from the product by cannulation, and the remaining needles were dried in vacuo.  3.2.12 Preparation of [Cp * Mo(N0)(CH2 CMe3)(NCMe)P3F4 (3.15)  Cp * Mo(NO)(CH2 CMe3 )Cl (331 mg, 0.900 mmol) and AgBF 4 (175 mg, 0.897 mmol) were combined in a Schlenk tube. The tube was cooled to -60 °C and NCMe (10 mL) was slowly  44 added with a syringe such that it solidified upon entering the tube. The tube and its contents were then warmed to -20 °C, whereupon a color change from purple to red-brown occurred and a white precipitate formed. The precipitate (presumably AgCI) was removed by filtration of the reaction mixture through Celite (2 x 2 cm). The filtrate was then reduced to a brown oil in vacuo. First Et 2 0 (50 mL) and then pentane (2 x 30 mL) were used to triturate the oil into a tractable yellow powder which was dried in vacuo for 24 h. The powder was dissolved in a minimum of CH2C12 (-3 mL), and Et 2 0 (30 mL) was added to the amber solution. Filtration of this solution through Celite (1 x 4 cm) followed by cooling of the filtrate at -60 °C for several hours induced the deposition of 3.15 as irregular orange-yellow crystals (355 mg, 86% yield).  3.2.13 Reaction of Cp * Mo(NO)(CH2CMe3)(C1)(PMe3) (3.11) with LDA: Preparation of (g 5 ,11 1 -05Me4CH2)Mo(NO)(CH2CMe 3 )(PMe3 ) (3.16)  Cp * Mo(N0)(CH2CMe3)(C1)(PMe3) (900 mg, 2.03 mmol) and LDA (220 mg, 2.06 mmol) were combined in a Schlenk tube. THE (25 mL) was vacuum transferred onto the solids at -196 °C. The rapidly stirred reaction mixture was then warmed to room temperature (15 min), and the solvent was removed in vacuo. The oily brown residue was dried in vacuo for 6 h, then suspended in Et 2 0 (50 mL), and filtered through a column of neutral alumina I (2 x 3 cm). The alumina column was washed with Et 2 0 until the amber band had been completely eluted. The ether was removed from the filtrate in vacuo, and the remaining yellow residue was extracted with pentane (50 mL). The pentane extraction was filtered through Celite (1 x 2 cm), concentrated and placed in a freezer to induce crystallization. After 1 week at -30 °C, dark amber crystals of complex 3.16 (530 mg, 64% yield) were isolated by removing the mother liquor via cannulation.  45  3.2.14 Reaction of Cp * Mo(NO)(r1 2-C{O}CH2 CMe3 )C1 (3.13) with PMe 3 : Generation of [Cp * Mo(N0)(C{0}{PMe3}CH2CMe3)(PMe3)1C1 (3.17) An NMR tube was charged with a CDC1 3 (0.8 mL) solution of complex 3.13 (15 mg). An initial 1 H NMR spectrum was recorded to verify the integrity of the sample.? PMe 3 (excess) was then vacuum transferred into the NMR tube. No visible color change occurred. However, a 1 H NMR spectrum of the reaction mixture revealed that all of complex 3.13 had been consumed to form a single product, formulated as [Cp *Mo(N0)(C{O} (PMe3 }CH2CMe3)(PMe3)]Cl (3.17). Spectroscopic data for 3.17: 1 H NMR (CDC13): 5 2.15 (dd, 1H, CH2 , 2J1 1 = 15.0 Hz, 3JHp = 6.9 Hz), 1.84 (s, 15H, Cp*), 1.76 (dd, 9H, PMe 3 , 2JHE) = 12.9 Hz, 3Jpp < 1 Hz), 1.63 (dd,  1H, CH2 , 2JHH = 15.0 Hz, 3JHp = 28.5 Hz), 1.43 (dd, 9H, PMe3, 2.4ip = 9.9 Hz, 3Jpp < 1 Hz), 0.84 (s, 9H, C(CH3)3)•  All attempts to isolate the above-mentioned compound were fruitless. For example, removal of the solvent in vacuo yielded a white powder which did not completely redissolve in CDC13, CD 2 Cl 2 or C 6D6 . A 1 H NMR spectrum of the portion that did dissolve revealed the presence of a multitude of compounds, none of which was 3.13 or 3.17.  46 3.2.15 Characterization Data for Complexes 3.1 - 3.16  Table 3.1 Numbering Scheme, Color, Yield, and Elemental Analysis Data  complex CpMo(NO)(CH2 CMe 2Ph) 2  compd no.  color (yield, %)  anal. found (calcd) H  3.1  red (43)  C 65.89 (65.64)  CpMo(N0)(CH 2 SiMe3 )2  3.2  violet (51)  42.60 (42.73)  7.60 (7.39) 4.01 (3.83)  Cp'Mo(NO)(CH2 CMe3 )Cl  3.3  violet (55)  48.97 (48.99)  7.05 (7.13) 3.88 (3.81)  Cp*Mo(N0)(CH2 CMe2 Ph)C1  3.4  maroon (64)  55.69 (55.89)  6.71 (6.57)  3.30 (3.26)  Cp *Mo(N0)(CH 2 SiMe3 )C1  3.5  blue-violeta  b  Cp`Mo(NO)(CH 2 CMe 3 )2  3.6  red (59)  59.70 (59.54)  9.35 (9.24)  3.50 (3.47)  Cp'Mo(NO)(CH 2 CMe2 Ph) 2  3.7  red (70)  65.89 (65.64)  6.84 (6.83)  3.14 (3.06)  Cp*Mo(N0)(CH 2 SiMe3 ) 2  3.8  violet (60)  49.74 (49.63)  8.52 (8.57) 3.44 (3.21)  Cp *Mo(NO)(CH2 CMe3 )(CH2 SiMe3 )  3.9  red (41)  54.35 (54.40)  8.91 (8.89)  3.25 (3.34)  Cp *Mo(N0)(CH2 CMe 3 )(p-toly1)  3.10  maroon (30)  62.38 (62.40)  7.80 (7.85)  3.50 (3.31)  Cp *Mo(NO)(CH2 CMe3 )(Cl)(PMe 3 )  3.11  yellow (49)  48.89 (48.71)  7.79 (7.95)  3.25 (3.16)  Cp *Mo(N0)(CH 2 CMe 3)(C1)(py)  3.12  orange (86)  53.88 (53.76)  7.06 (6.99) 6.48 (6.27)  Cp *Mo(NO)(r1 2 -C{O}CH2 CMe3 )C1  3.13  amber (30)  48.63 (48.56)  6.56 (6.62) 3.43 (3.54)  Cp *Mo(N0)(11 2 -C{NCMe3 }CH2 CMe3 )C1  3.14  yellow (95)  53.47 (53.28)  7.80 (7.82) 6.27 (6.21)  [Cp* Mo(N0)(CH2 CMe3 )(NCMOIBF4a  3.15  yellow (86)  43.58 (44.37)  6.27 (6.35)  (C 5 Me4 CH2)Mo(N0)(CH2 CMe3 )(PMe3 )  3.16  amber (64)  N 6.84 (6.83) 3.14 (3.06)  5.57 (6.09)  53.04 (53.07) 8.51 (8.41) 3.37 (3.44) a Due to the viscosity of this oil, the yield can only be approximated (-60%). b Due to oily nature of this material, satisfactory elemental analysis could not be obtained. c The difficulty in obtaining satisfactory combustion analysis may be attributed to the formation of CF 4 .  47 Table 3.2 Mass Spectral and Infrared Data  compd  MS  tempb  no.  We  (°C)  IR (Nujol mull) VNO  other bands  1575 (vs)  3.1  c  3.2  367 [P+]  3.3  c  1606 (vs)  3.4  c  1606 (vs)  3.5  c  1628 (vs)  1243 (w, SiMe 3 )  3.6  405 [P+]  150  1593 (vs)  1235 (w, CMe 3 )  3.7  529 [P1  100  1594 (vs)  3.8  437 [P41  100  1595 (vs)  3.9  421 [P+]  120  1599 (vs)  3.10  425 [P+]  150  1611 (vs)  1234 (w, CMe 3 )  3.11  387 [P+ - CH2 CMe3 J 369 [P+- PMe3 ]  DCId  1575 (vs)  1233 (w, CMe 3 ) 958 (m, PMe 3 )  3.12  c  1600 (sh)e 1580 (s) 1561 (s)  1223 (w, CMe 3 )  3.13  397 [P1  180  1601 (vs)  1246 (w, CMe3 ) 1624 (s, CO)  3.14  452 [P1 422 [P+- NO]  180  1574 (vs)  1715 (m, CN) 1231 (w, CMe 3 )  3.15  462 [P+]  + FABf  1606 (vs)  2316, 2290 (NCMe) 1230 (w, CMe 3 ) 1063 (br, BF4)  3.16  409 [P1  120  1595 (m) 1544 (br)  952 (m, PMe 3 )  100  1587 (vs)  a m/z values are for the highest intensity peak of the calculated isotopic cluster ( 98Mo and 184w). b Probe temperatures. c Decomposes upon warming. d NH3 carrier. e Which of these three bands is v N0 could not be determined unambiguously. f FAB-MS (matrix: 3-nitrobenzyl alcohol).  48 Table 3.3 NMR Data (C6D6)  compd  13com NMR (8, ppm)  1H NMR (8, ppm)  no. 3.1  7.39 (m, 414, Ph) 7.30 (m, 4H, Ph) 7.17 (m, 2H, Ph) 5.26 (s, 5H, Cp) 4.18 (d, 2H, CH2 , JHH = 9.6 Hz) 1.53 (s, 6H, C(CH3 )2 ) 1.33 (s, 6H, C(CH3 )2 ) -0.52 (d, 2H, CH2 , JHH = 9.6 Hz)  152.26 (Cipso) 137.76 (C0) 126.00 (C^) 125.66 (C) 101.32 (C5H5 ) 96.28 (CH2) 46.13 (C(CH3 )2 ) 32.36 (C(CH3 ) 2 ) 32.23 (C(CH3 ) 2 )  3.2  5.18 (s, 5H, Cp) 3.11 (d, 2H, CH2 , JH H = 8.7 Hz) 0.35 (s, Si(CH3 ) 3 ) -0.24 (d, 2H, CH2 , JHH = 9.6 Hz)  101.36 (C5 H5 ) 65.96 (C112 ) 2.37 (Si(CH3 ) 3 )  33  3.73 (d, 1H, CH2 , JH H = 10.8 Hz) 1.44 (s, 15H, Cp*) 1.20 (s, 9H, C(CH3 )3 ) -0.06 (d, 1H, CH2 , JHH = 10.8 Hz)  113.69 (CH 2 ) 110.41 (C5 (CH3 ) 5 ) 40.12 (C(CH3 )3 ) 32.89 (C(CH3 ) 3 ) 9.86 (C 5 (CH3 ) 5 )  3.3a  3.68 (d, 1H, CH2 , JH H = 11.1 Hz) 1.87 (s, 15H, Cp*) 1.15 (s, 9H, C(CH3 )3 ) 0.11(d, 1H, CH2 , JHH = 11.1 Hz)  b  3.4  7.19 (d, 2H, Ph) 7.09 (d, 2H, Ph) 7.17 (t, 1H, Ph) 4.01 (d, 1H, CH2 , JHH = 9.0 Hz) 1.71 (s, 3H, C(CH3 ) 2 ) 1.64 (s, 3H, C(CH3 ) 2 ) 1.41 (s, 15H, Cp*) 0.38 (d, 1H, CH2 , JH H = 9.0 Hz)  151.72 (g ra ) 127.51 (Com() 126.19 (Cme) 125.90 (Cp...) 114.09 (C112 ) 110.50 (C5 (CH3 ) 5 ) 46.74 (C(CH3 )2 ) 31.79 (C(CH 3 ) 2 ) 30.63 (C(CH 3 ) 2) 9.74 (C 5 (CH3 ) 5  3.5  2.61 (d, 1H, CH2 , JHH = 10.5 Hz) 1.94 (d, 1H, CH2 , JHH = 10.5 Hz) 1.51 (s, 15H, Cp*) 0.32 (s, 9H, Si(CH3 ) 3 )  113.73 (C5 (CH3 ) 5 ) 70.65 (CH2) 10.01 (C5 (CH3 ) 5 ) 1.65 (Si(CH3 ) 3 )  3.6  3.08 (d, 2H, CH2 , JHH = 12.1 Hz) 1.61 (s, 18H, C(CH3 )3 ) 1.45 (s, 15H, Cp*) -0.95 (d, 2H, CH2 , JHH = 12.1 Hz)  109.78 (C5 (CH3 ) 5 ) 97.97 (CH2) 39.05 (C(CH3) 3 ) 33.85 (C(0.13 )3 ) 9.92 (C5(CH3)5)  49 3.7  7.43 (m, 4H, Ph) 7.20 (m, 4H, Ph) 7.09 (m, 2H, Ph) 3.21 (d, 2H, CH2 , Jim = 10.2 Hz) 1.55 (s, 6H, C(CH3 )2 ) 1.37 (s, 6H, C(CH3 )2 ) 1.21 (s, 15H, Cp s ) -0.85 (d, 2H, CH2 , JH H = 10.2 Hz)  152.86 (Cipsd 135.20 (Cmg ) 126.05 (Cm ) 117.62 (C 110.01 ( 5 (CH3 ) 5 ) 101.24 (CH2 ) 33.53 (C(CH3 )2 ) 31.62 (C(CH3 )2 ) 31.40 (C(CH3 )2 ) 9.84 (C 5 (CH3 ) 5 )  3.8  2.21 (d, 2H, CH2 , Jmi = 10.8 Hz) 1.49 (s, 1511, Cp*) 0.37 (s, 18H, Si(CH3 ) 3) -1.17 (d, 2H, CH2 , ^HH = 10.8 Hz)  110.52 (C5(CH3)5) 66.54 (CH2) 9.92 (C 5 (CH3 ) 5 ) 2.56 (Si(CH3 ) 3 )  3.9  3.96 (d, 1H, CH2 CMe3 , 2JHH = 10.8 Hz) 1.51 (s, 15H, Cp*) 1.33 (s, 9H, CH2 C(CH3 )3 ) 1.19 (d, 1H, CH2 SiMe3 , 2JHH = 12.0 Hz, 2Jsm = 19.8 Hz) 0.37 (s, 9H, CH 2 Si(CH3 )3 ) -0.67 (d, 1H, CH2 SiMe3 , 2JHH = 12.0 Hz, 4^HH = 1.8 Hz) -2.15 (d, 1H, CH2 CMe3 , 2JHH = 10.8 Hz, 4JHH = 1.8 Hz)  118.78 (CH2 CMe3 ) 110.06 (C5 (CH3 ) 5 ) 46.21 (CH2 SiMe3 ) 40.86 (C(CH3 ) 3 ) 33.43 (C(CH3 )3 ) 9.96 (C 5 (CH3 ) 5 ) 2.57 (Si(CH3 )3 )  3.10  7.71 (d, 2H, Nary 1 , JHH = 7.8 Hz) 7.14 (d, 2H, .ii,1, jHH = 7 . 8 Hz) 4.67 (d, 1H, CH2' 2JHH = 9 . 0 Hz) 2.22 (s, 3H, p-CH3) 1.57 (s, 15H, Cp*) 1.28 (s, 9H, C(CH3 )3 ) -1.55 (d, 1H, CH2 , 2 d'HH =. 9 . 0 Hz)  b  3.11c  2.30 (dd, 1H, CH transoidal to P, Jim = 9.6 Hz, ^ip = 3.3 Hz) 1.79 (s, 9H, C(CH3 )3 ) 1.69 (s, 15H, Cp*) 1.53 (d, 1H, CH cisoidal to P, JHH = 9.6 Hz) 1.32 (d, 9H, P(CH3 )3 ,./HP = 9.6 Hz)  109.79 (C5 (CH3 ) 5 ) 64.70 (d, CH 2 , .16 = 7.3 Hz) 38.50 (C(CH3 )3 ) 35.16 (C(CH3 )3 ) 12.42 (d, P(CH3 ) 3 , ./cp = 26.6 Hz) 9.75 (C 5 (CH3 ) 5 )  3.12d  8.98 (br s, 2H, NC 5H5 ) 8.04 (br s, 1H, NC 5 H5 ) 7.61 (br s, 2H, NC 5H5 ) 2.08 (br s, 1H, CH2 ) 1.53 (br s, 15H, Cp*) 1.10 (br s, 9H, C(CH3 )3 ) 0.49 (br s, 1H, CH2 )  b  3.13  3.08 (d, 1H, CH2 , Jim = 15.5 Hz) 2.62 (d, 1H, CH2 , JHH = 15.5 Hz) 1.61 (s, 15H, Cp*) 0.91 (s, 9H, C(CH3 )3 )  280.66 (CO) 112.97 (C5 (CH3 ) 5 ) 54.34 (CH2) 33.08 (C(CH3 ) 3) 29.64 (C(CH3 ) 3 ) 10.10 (C5(CH3)5)  50 3.14  3.00 (d, 2H, CH2 , Jim = 18.0 Hz) 2.57 (d, 2H, CH2 , Jim = 18.0 Hz) 1.64 (s, 15H, Cp*) 1.33 (s, 9H, NC(CH3 ) 3 ) 1.00 (s, 9H, CH 2 C(CH3 )3 )  b  3.151  2.50 (s, 3H, CH3 CN) 1.80 (s, 15H, Cp*) 1.30 (d, 1H, CH2, Jim = 10.5 Hz) 1.08 (s, 9H, C(CH3 ) 3) 0.70 (d, 1H, CH2 , Jmi = 10.5 Hz)  CH3CN not observed 113.67 (C5 (CH3 ) 5 ) 66.52 (CH2) 38.80 (C(CH3 ) 3) 34.25 (C(CH3 )3 ) 9.31 (C 5 (CH3 ) 5 ) 4.29 (CH3 CN)  3.16c e  2.88 (vt, 1H, C 5Me 4CH2 ,J= 3.0 Hz) 2.72 (dd, 1H, C 5Me 4CH2 , J = 3.0, 13.2 Hz) 1.93 (d, 3H, Me, J= 2.4 Hz) 1.73 (d, 3H, Me, J= 3.3 Hz) 1.69 (s, 3H, Me) 1.40 (d, 9H, P(CH3 )3 , Jim = 7.8 Hz) 1.17 (d, 3H, Me, Jim = 3.6 Hz) 0.99 (s, 9H, C(CH3 )3 ) 0.87 (m, 1H, CH2 CMe3 ) 0.34 (d, 1H, CH2 CMe3 , J = 12.9 Hz)  131.00 (C 5Me4CH2) 110.16 (C5Me 4CH2) 108.36 (C5 Me4 CH2) 107.29 (d, C5 Me4 CH2 , Jcp ^8.0 Hz) 104.36 (C5 Me4 CH2 ) 64.85 (d, CH2 CMe3 , Jc p = 6.5 Hz) 53.94 (C 5 Me4 CH2 , Jc p = 10.4 Hz) 39.26 (d, CH2 CMe3 , Jc p = 14.2 Hz) 35.19 (C(CH3 )3 ) 16.76 (d, P(CH3 ) 3 , Jcp = 25.0 Hz) 12.82 (d, C5(CH3 ) 4CH2 , Jc p = 26.8 Hz) 11.20 (C 5 (CH3 ) 4 CH2, Jul = 19.5 Hz) 9.92 (C 5 (CH3 ) 4 CH2 ) 9.02 (C 5 (CH3 ) 4 CH2 , Jc p = 30.7 Hz)  3.16  2.83 (vt, 1H, C 5 Me4CH2 , J = 2.7 Hz) 2.49 (dd, 1H, C 5 Me4 CH2 , J = 2.7, 13.2 Hz) 1.82 (s, 3H, Me) 1.81 (s, 3H, Me) 1.49 (s, 311, Me) 1.40 (s, 9H, C(CH3 )3 ) 1.07 (d, 1H, CH2 CMe3 , J= 12.9 Hz) 0.98 (d, 91I, P(CH3 )3 , Jma = 7.8 Hz) 0.95 (d, 311, Me, Jiii, = 3.6 Hz) 0.48 (dd, 11-1, CH2 CMe3 , J = 12.9, 1.2 Hz)  b  ,  a In THF-dg (provided for comparison to low-temperature spectrum of complex 3.12). b Not recorded. c 31P NMR (6, ppm): 3.11 = 6.38 (s); 3.16 = -2.89 (s). d In THF-d8 at -100 °C. e In CDC1 3 . I In CD 2 C1 2 .  51 Table 3.4 Electrochemical Data for the First Reductions of Cp * M(NO)C1 2 , Cp * M(NO)(CH2CMe3)C1, and Cp *M(N0)(CH2CMe3)2•a  complex Cp *Mo(NO)C1 2  Cp * Mo(N0)(CH 2 CMe3 )C1  Cp * Mo(N0)(CH 2 CMe3 )2  Cp * W(NO)C1 2  Cp * W(N0)(CH2 CMe3 )CI  Cp * W(NO)(CH2 CMe3 )2  scan rate (v, Vs -1 ) 1.0  AEC  i  a  0.6  L' i b (mV) -350 -350  0.2  -350  113 (94)  1.0  1.0  -970  207 (230)  1.0  0.6  -960  172 (188)  1.0  0.2  -970  125 (132)  0.9  1.0  - -1590  510 (186)  0.7  1.0  -750  234 (167)  1.0  0.6  -750  178 (136)  1.0  0.2  -740  116 (109)  1.0  8.0  - -1220  550  0.7  4.0  - -1220  360  0.6  1.0  - -1210  170  0.4  1.0  -1720  310 (167)  0.9  0.6  -1700  250 (136)  0.8  0.2  -1680  180 (109)  0.8  p,a p,c  194 (171)  1.0  162 (136)  1.0  d  a In THE containing 0.10 M [n-Bu 4N]PF 6 , at a Pt-bead working electrode. Potentials are measured vs SCE. b Defined as the average of the cathodic and anodic peak potentials. c Defined as the separation of the cathodic and anodic peak potentials. Values in brackets are for the Cp2Fe/Cp2 Fe+ couple under identical experimental conditions. d Ratio of anodic peak current to cathodic peak current.  52 3.3 Results and Discussion  3.3.1 Alkylations of Cp'Mo(NO)C12 Complexes  Since the methodology described in Chapter 2 was successful for the preparation of Cp'M(N0)(ary1) 2 complexes, I decided to extend the scope of the methodology to include three hitherto unknown Cp'Mo(NO)(alkyl) 2 complexes. Additionally, the preparations of the known complexes CpMo(NO)(CH 2 SiMe 3 ) 2 and Cp*Mo(N0)(CH 2 SiMe 3 ) 2 have been markedly improved, both in terms of yields and reproducibility, by employing this methodology. Also as a direct result of this study, three Cp * Mo(NO)(alkyl)Cl complexes, which had previously been thought to be inaccessible, were prepared in a single synthetic procedure. These complexes led in turn to the synthesis and isolation of mixed alkyl and alkyl aryl complexes of Mo.  3.3.1.1 Synthesis of Cp'Mo(NO)(alkyl) 2 Complexes  Five complexes of this type have been prepared in a straightforward manner. The Cp'Mo(NO)(CH2SiMe3)2 complexes were previously known, but could not be reliably prepared. 6 ' 8 As discussed in Chapter 1, several factors were responsible for this lack of reproducibility. Of these, the major reason was the use of water in the original preparation of these complexes. The use of water is unnecessary with the alkylating agents containing dioxane ligands, and thus the preparation of the moisture-sensitive Mo dialkyl complexes is not problematic. Since Cp * Mo(NO)(CH2 CMe3 )2 reacts slowly with water, small quantities can be used during its synthesis to optimize the yield of this complex. As a group, the Mo dialkyl complexes are extremely air-sensitive, converting to a variety of oxo-containing species when exposed to the atmosphere (Chapter 4). 9 All five complexes are thermally sensitive. As solids they decompose within months of storage under argon at -30 °C. The neopentyl and neophyl complexes (3.1, 3.6, 3.7) can be chromatographed on neutral alumina I. The trimethylsilylmethyl complexes (3.2, 3.8) are not very stable to alumina. Accordingly, they are best purified by filtration through Celite.  ^  53 The spectroscopic properties of the dialkyl complexes prepared in this chapter are very similar to those exhibited by their W congeners. 6,8,10,11  3.3.1.2 Synthesis of Cp*Mo(NO)(alkyl)C1 Complexes For many years our group had been unable to synthesize the 16-electron Cp'W(NO)(R)X [R = alkyl, aryl; X = I, Cl] complexes from their dihalide precursors. The reason that all previous attempts to prepare Cp'W(NO)(R)X compounds failed is because, under the various reaction conditions employed, 12 any such complexes formed react with the alkylating agent present in solution faster that Cp'W(NO)X 2 (vide infra). Indeed, only recently did we communicate the preparation of Cp * W(NO)(CH2 SiMe 3 )Cl from Cp * W(NO)(CH2 SiMe3 )2 (eq 3.1), albeit via a somewhat indirect route. 13  H2, NCMe^ ^)  / 1N ^V  _N I R u R  2 HCI  *^W ^/ J 1 CI N N 1^ 0 R C^0 R / T  ^  I  (3.1)  Me  R = CH2SiMe3 However, this route is not particularly appealing synthetically, since the overall yield of Cp * W(NO)(CH2 SiMe 3 )Cl from Cp*W(NO)C1 2 suffers from the number of transformations involved (30% overall for 3 steps). Knowing that Cp*W(N0)(CH 2 SiMe 3 )C1 is inherently stable, I was convinced that it was our previous methodology rather than any intrinsic instability that had thwarted attempts to prepare mono(hydrocarbyl) species. Application of the method used to synthesize Cp * M(NO)(aryl) 2 complexes (Chapter 2) to their preparation is a much more efficient route to these compounds. The reaction leading to Cp *Mo(NO)(alkyl)Cl is portrayed in eq 3.2.  54  1/2 R2 Mg'X(dioxane) Ii.  1  Cl^THF, -45 °C  ^ N^ 0  Cl  (3.2)  N 0  R = CH2 CMe3 (3.3); CH2 CMe2Ph (3.4); CH2SiMe 3 (3.5) Control of reaction temperature is extremely critical for the success of reaction 3.2. For example, above -45 °C, Cp * Mo(NO)(CH2CMe3)Cl (3.3) is reactive with (Me3CCH2)2Mg-X(dioxane), giving Cp * Mo(NO)(CH2CMe3)2 and unused starting material. Since Cp *Mo(NO)C12 is nearly insoluble in cold THF, extended reaction times in dilute solution (40 mL/mmol) are required to achieve solutions of Cp * Mo(N0)(CH2CMe 3 )C1 uncontaminated by Cp * Mo(NO)(CH2 CMe3) 2 . It should be noted that it is nearly impossible to isolate pure monoalkylated products from mixtures of the mono- and dialkyl complexes. However, it is easy to perform the reverse separation since the monoalkyl complex is unstable to alumina and the dialkyl complex is chromatographable. Finally, it is also important to note that the Cp * Mo(NO)C1 2 starting material must be highly purified in order to obtain clean, stoichiometric metathesis. 14 Since the reactivity of Cp * Mo(NO)(CH2CMe3)Cl (3.3) forms a large portion of this chapter, a solution of the complex was often generated in situ, filtered to remove MgC12•dioxane, and the next solvent or reagent was added directly. Typical yields of solutions of 3.3 generated in this manner are about 90%. 5 The solid-state structure of Cp*W(NO)(CH2SiMe3)Cl is that of a three-legged, monomeric piano-stool, 13 and there is no reason to believe that any of the Mo monoalkyl complexes (3.3 3.5) are anything but monomeric. Thus, in solution and the solid-state, they are deep purple, maroon or blue and exhibit strong vNO bands in their Nujol mull IR spectra (1628 - 1606 cm -1 ) in the same energy range (1650 - 1550 cm -1 ) as their monomeric dialkyl and diaryl analogues. The 1 H NMR spectra of 3.3 -  3.5 (Table 3.3) are also consistent with their formulations. The spectra  display two doublets attributable to the diastereotopic methylene protons of the alkyl groups at  55 markedly different chemical shifts indicative of their substantially different magnetic environments in the three-legged piano-stool, i.e. one proton is close to the Cl ligand and the other is close to the nitrosyl ligand. For example, the methylene protons of the 3.3 resonate at 8 3.73 and -0.06 ppm in C6D6 ( 2J = 10.8 Hz) (Figure 3.1(a)). Complex 3.3 is moderately thermally sensitive (more so than its analogues 3.4 or 3.5). In the solid state it decomposes uncleanly to a brown oil over the course of several weeks, as evidenced by 1 H NMR spectroscopy. However, in benzene, the thermal decomposition (8 h at 100 °C or 6 - 10 weeks at room temperature) is much cleaner (Figure 3.1(b)). From the spectrum in Figure 3.1(b), it is clear that 3.3 has undergone some sort of isomerization since all resonances from Figure 3.1(a) remain, but are found at different chemical shifts. 15 The structure of the thermolysis product remains a mystery, since the complex can only be described as an oily film that is completely resistant to crystallization from Et 2 0, CH2 C1 2 , benzene, toluene or THE or typical combinations of these solvents. Interestingly, the IR spectrum of the complex exhibits no band in the region 2000-1500 cm -1 that would be expected for a terminal nitrosyl ligand.  56  ^iiiirillIIIIIIIII^1^1^1^1^ifilliffilliiiiiitli  4.0^3.5^3.0^2.5^2.0^1.5^1.0^0.5 PPM 0.0  (b)  ^h....k.14...■•■••■•................ ^1111111t1iii^i^i  4.0^3.5^3.0  '^1 ^' 1^1^i^1^i^1^t 2.5^2.0  111'111'1ml  1.0^0.5 PPM 0.0  Figure 3.1 300 MHz 1 H NMR spectra (C 6D6 ) of (a) complex 3.3 at room temperature; (b) complex 3.3 after 8 h at 100 °C  57 3.3.1.3 Synthesis of Cp * Mo(NO)(CH2CMe3)R Complexes [R = alkyl (3.9), p-tolyl (3.10)]  Using the methodology described in this Chapter, Jeff Debad of our group recently synthesized a family of asymmetric bis(hydrocarbyl) complexes of tungsten. 16 Not unexpectedly, analogous Mo complexes can be prepared as well. Using similar methodology to that which provided us with diaryl complexes of Mo and W (Chapter 2), Cp *Mo(NO)(CH2 CMe3)(alkyl) and Cp * Mo(N0)(CH2CMe3)(aryl) complexes are preparable (eq 3.3). Cp * Mo(NOXCH 2CMe3 )Cl + 1/2 R2Mg÷ 10I- Cp * Mo(NO)(CH2 CMe 3 )R^(3.3)  R = CH2 SNIe 3 (3.9), p-tolyl (3.10) As one might expect, the nitrosyl-stretching frequencies of the mixed complexes are essentially the average of their symmetric congeners (vide infra, Section 3.3.2.2). The 1 H NMR spectrum of Cp * Mo(NO)(CH2CMe3)(CH2SiMe3) (Figure 3.2) shows some interesting features. First, all four methylene protons of 3.9 are magnetically inequivalent and thus appear as four separate resonances. Two of the resonances (8 3.96 and -2.15 ppm) show 2.4111 couplings of 10.8 Hz and are assigned to CH2 CMe3. The other two resonances (8 1.19 and -0.67 ppm) show 2JHH couplings of 12.0 Hz and are assigned to CH2SiMe3. Second, one of each of these sets of resonances (8 -0.67 and -2.15 ppm) shows additional 4.4.11/ couplings of 1.8 Hz. The observation of these long-range couplings is attributed to a phenomenon called "W" coupling in organic chemistry. 17 In organic compounds when 4 bonds lie in a plane and align themselves in the conformation of a W, 4./H1. couplings can be observed. The arrangement of the two alkyl groups in complex 3.9 must therefore be similar to that shown below.  R = CMe3 ; Iti = SNIe3  )1 IIIIIIIIIIII IF -Ili -rim iiiiiirliii n iiiirriltrilliiiiiiiiiiiiiii mil 2^ 0^-1^-2 PPM Figure 3.2 300 MHz 1 H NMR spectrum of  _...*_. ..._,_.___..  Cn M in 1( (We 2 ____3 __ _2SiMe ___ __3, in r_6_156 nq0r14 VCR 13_._ 9 in  59 3.3.2 Electrochemical and Infrared Studies: Assessment of Relative Lewis Acidity  The existence of vacant, metal-centered orbitals on 16-electron CpM(NO)-containing compounds imparts Lewis-acid characteristics to these materials. In previous worlc,3,18,20 i t h as been found that virtually all Cp'M(NO)X2 [X = I, Br, Cl] and Cp'M(NO)R2 [R = alkyl, aryl] complexes exhibit IR nitrosyl-stretching frequencies which correlate with the relative electron density at the metal centers in these 16-electron species (Section 1.1). Additionally, these complexes exhibit reversible electrochemical reductions in THE or CH 2 C12 , features attributable to the non-bonding natures of the LUMOs in these species, that are also directly proportional to the electron density at their metal centers. In this section, a correlation of a wide variety of experimental observations is used to establish a structure/reactivity relationship amongst the various Cp'M(NO)-containing compounds prepared in this thesis. Under any criterion used (apparent size of coordination site (Section 2.3.5), nitrosyl-stretching frequency or reduction potential), Cp, Mo and aryl complexes are respectively better Lewis acids than related Cp*, W and alkyl complexes, without exception.  3.3.2.1 The Cp * M(NO)C12 - to - Cp * M(N0)(CH2 CMe3 )C1 - to - Cp * M(N0)(CH 2 CMe 3 )2 Series The Mo and W series of compounds Cp * M(NO)C1 2 , Cp * M(N0)(CH2 CMe3 )CI and  Cp * M(NO)(CH2CMe 3 ) 2 are compared using both IR spectroscopy and electrochemistry in Table 3.5. More detailed electrochemical results are located in Table 3.4 (Section 3.2.15).  60 Table 3.5 Comparative IR and Electrochemical Data (THF) vp,10 (cm - 1 )  E°' 1 (mV vs SCE at 1.0 V/s)  Cp*Mo(NO)C12  1655  -350a  Cp*W(NO)C12  1630  -750a  Cp*Mo(NO)(CH2CMe3)C1 (1)  1618  -960a  Cp*W(N0)(CH2CMe3)CI (1')  1595  -1210b  Cp*Mo(NO)(CH2CMe3)2 (2)  1588  - 1590b  compd  1568 Cp*W(NO)(CH2CMe3)2 (2') a Reversible. b Quasi-reversible. c Reversible at high scan rates, otherwise quasi-reversible.  - 1700a  As evidenced by the data in Table 3.5, replacement of electronegative chloride ligands with neopentyl groups causes significant shifts in vN ID to lower energy and increases in reduction potential (E°') to more negative potentials. Consequently, it can be concluded that the metal centers in Cp * M(NO)(CH2CMe3)2 are more electron-rich than their mononeopentyl and dichloride precursors. Additionally, in comparing each of the three pairs of compounds, the W complex always exhibits the lower vNo (average of 23 cm -1 ) and the more negative reduction potential (by 400, 260 and 110 mV) than its corresponding Mo analogue. Of the six complexes studied, Cp *Mo(NO)(CH2CMe3)2 and Cp * W(N0)(CH2 CMe3)CI exhibit quasi-reversible reductions in THF. For example, at a scan rate of 1.0 V/s Cp * Mo(NO)(CH2CMe3)2 exhibits a reduction wave (ip,a/ip,c = 0.7), having AE almost three times that of ferrocene; in contrast, a scan rate of 8.0 V/s is required to achieve a reduction wave with ip,a/ipx = 0.7 for Cp * W(NO)(CH2CMe3)Cl. Additionally, Cp * W(NO)(CH2 CMe3)2 exhibits a reduction wave that approaches reversibility at high scan rates, but is quasi-reversible at low scan rates (Table 3.4). 19 The other three complexes studied exhibit completely reversible reductions as would be expected on the basis of previous studies" and calculations. 21 A representative cyclic voltammogram of Cp * Mo(N0)(CH2CMe3)C1 is shown in Figure 3.3.  61  0.5 -  0.4 -  0.3 -  0.2 .7lik  E t E ,..  0.1  1^I^I^I^I^I^I^I  0  -0.7  -0.8  ^  -0.9  ^ ^ ^ ^ ^ -1 -1.2 -1.3 -1.4 -1.5  -0.1 -  -0.2 -  -0.3 -  potential (V)  Figure 3.3 Ambient temperature cyclic voltammogram of Cp*Mo(N0)(CH 2 CMe 3 )C1  in THE (scan rate = 1 Vs -1 )  62 The general conclusion to emanate from the IR. data and the electrochemical reduction potentials is that the Mo complexes are more electron-deficient than their W partners. This increased electron-deficiency at the metal is manifested in the higher reactivity of Mo nitrosyl complexes of this type. We have previously determined that Cp *M(NO)(aryl)2 complexes are better Lewis acids than their Cp * M(NO)(alkyl)2 congeners due to the increased electronwithdrawing ability of aryl groups (Chapter 2). Cp * M(NO)(aryl)2 complexes also show a similar trend in reactivity with the more electron-deficient complexes being more reactive. 20 This study indicates that Cp * M(NO)(CH2CMe3)Cl complexes are comparable in electron-deficiency to related Cp * M(NO)(aryl) 2 complexes for both the Mo and W systems. For example, the nitrosylstretching frequency of Cp*Mo(N0)(Ph) 220 in THF is 1618 cm -1 , only 3 wavenumbers higher than that for Cp * Mo(NO)(CH2CMe3)Cl. From an electrochemical standpoint, the reduction potential of Cp * W(N0)(p-toly1 )220 in THF is -1210 mV and that of Cp * W(NO)(CH2CMe3)Cl is a comparable -1220 mV.  3.3.2.2 The Cp * Mo(N0)(CH2CMe3)2-to-Cp * Mo(N0)(CH2CMe3)(p-toly1)-toCp * Mo(N0)(p-toly1)2 Series  Cp * Mo(N0)(CH2CMe3)(p-toly1) (3.10) exhibits a vNo band at 1600 cm -1 in its THF IR spectrum. Similar spectra of 3.10's symmetric analogues, Cp *Mo(NO)(CH2CMe3)2 (3.6) and Cp * Mo(N0)(p-toly1)2 (2.2) exhibit v No bands at 1609 and 1588 cm -1 , respectively, the average of which is 1598.5 cm -1 (1.5 cm -1 less than that observed for 3.10). Two other series of nitrosylstretching frequencies add to this argument. The mean vi,To value (THF) of Cp * Mo(NO)C1 2 (1655 cm -1 ) and Cp * Mo(NO)(CH2CMe3)2 (1588 cm -1 ) is 1621.5 cm -1 . The measured value of vNID for Cp *Mo(NO)(CH2 CMe3)Cl is 1618 cm -1 . Similarily, the mean v No value of Cp * W(NO)C12 (1630 cm -1 ) and Cp *Mo(NO)(CH2CMe3)2 (1568 cm -1 ) is 1599 cm -1 , 4 cm -1 more than the value of 1595 cm -1 observed for Cp*W(NO)(CH2CMe3)Cl.  63 3.3.2.3 Further Comparisons Scheme 3.1 shows a typical set of related complexes compared to Cp*W(NO)(CH2CMe3)2. Changing the Cp * ring to Cp causes an increase in vNo (19 cm -1 ) and a drop in E° (230 mV). Similarly, changing the metal from W to Mo causes an increase in v NO (20 cm -1 ) and a drop in E° (110 mV). Lastly, changing both alkyl groups for aryl ligands also causes an increase in vNo (14 cm-1 ) and a drop in E° (490 mV). Clearly, all three complexes in these comparisons are more electron-deficient than Cp*W(NO)(CH2CMe3)2.  Cp vs Cp  w  r= 3 o  *  Me C N CMe 3  Mo vs W  -- , /W\  -4(---------)"  •011-----)11,  Me3 C N o CMe3  vN0 = 1587 cm -1^vN0 = 1568 cm -1^vNo = 1588 cm -1 E ° = -1470 mV  ^  E ° = -1700 mV  ^  E ° = -1590 mV  aryl vs alkyl  W N 0  vN0 = 1582 cm -1  °  E = -1210 mV Scheme 3.1 IR and electrochemical comparisons (THF) of pairs of 16-electron organometallic nitrosyl complexes differing only in one component of their respective structures (i.e. Cp vs Cp * , Mo vs W, aryl vs alkyl)  64 3.3.3 Reactivity of Cp * Mo(N0)(CH2CMe3)CI  The reactivity of Cp * Mo(N0)(CH2CMe 3 )CI (3.3), described in this section is part of a companion study of the characteristic chemistry of both the Mo and W chloro neopentyl complexes. 22 Compounds (3.11 3.14, 3.16) have isostructural tungsten analogues that were -  prepared by Jeff Debad of our group. As expected, the chloro neopentyl complex 3.3 is more reactive than its W congener.  3.3.3.1 Formation of 1:1 Adducts: Reactions With Pure Lewis Bases  Like related 16-electron bis(hydrocarbyl) nitrosyl complexes, the mononeopentyl complex prepared in this work is very reactive towards Lewis bases. 8,20 The coordinative and electronic unsaturation of the metal center in Cp*Mo(N0)(CH 2 CMe3 )C1 (3.3) is satisfied by coordination of a Lewis base. For example, the addition of PMe 3 to a solution of Cp * Mo(NO)(CH 2 CMe3)C1 causes an immediate color change from purple to yellow and formation of the metal-centered 1:1 adduct (3.11) (eq 3.4).  PMe3  CI  R  N 0  Mo  te ; 1^C1  (3.4)  N PMe3  0  R = CH2CMe3 The 1 H NMR spectrum (Figure 3.4) of the 18-electron phosphine adduct is diagnostic of a 4-legged piano-stool molecular structure having the PMe 3 ligand trans to the nitrosyl group. A Newman projection of this molecule depicts the stereochemical scenario (Figure 3.4 inset). The methylene protons of the alkyl group are positioned such that HA is transoidal to the P nucleus ( 34p = 3.3 Hz) and HB is cisoidal to the P nucleus ( 3JHp = 0 Hz). Since MO calculations and X ray crystallographic analyses have indicated that the vacant coordination site in all 16-electron Cp'M(NO)(Y)(Z) [Y, Z = 1-electron-donor ligand] complexes is trans to the NO ligand it is not  65  Cl PMe3  ON HA  HB CMe3  iiiiiitiIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIiiiiiiriIIIIIIIIII  2.2^2.0^1.8^1.6^1.4 PPM^1.2  Figure 3.4 300 MHz 1 H NMR spectrum of 3.11 in C 6D6 (inset: Newman projection of  Cp * Mo(N0)(CH2 CMe3)(C1)(PMe3) down the metal-Cp* centroid axis)  66 surprising that the PMe3 enters the metal's coordination sphere at this position. 3 The isolated adduct isomer appears to be the thermodynamically favoured isomer given that no isomerization is observed for this complex. Thermolysis in C 6D6 leads only to decomposition. The addition of PMe 3 to 3.3 causes a drop in v NO of 31 cm -1 in the IR spectrum of 3.11, a feature consistent with the metal center in the adduct becoming markedly more electron-rich. Prolonged exposure of complex 3.11 to high vacuum does not effect the dissociation of the phosphine ligand, as occurs for CpW(NO)(CH2SiMe3)2(PMe3). 8 Again, this substantiates the conclusion that 3.3 is about as electron-deficient as related diary! complexes (Section 3.3.2.1) which also bind PMe3 irreversibly (Section 2.3.6)." Interestingly, a bulkier base such as pyridine also coordinates to 3.3, but in a reversible fashion (eq 3.5). Cp*Mo(NOXCH2 CMe3 )Cl  py - PY  3.3^  Cp*Mo(NO)(CH2CMe3XCIXPY)  (3.5)  3.12  At low temperatures, the adduct is stabilized and by virtue of its diminished solubility (compared to 3.3) will crystallize from Et2 0 solutions containing an excess of pyridine. As a solid, the adduct shows a tendency to lose pyridine slowly under vacuum. Dissolution of 3.12 in a coordinating solvent such as THE-d8 or even a non-coordinating solvent such as C 6 D6 quantitatively liberates pyridine and the starting material as evidenced by 1 H NMR spectroscopy (Figure 3.5a). Consequently, in order to spectroscopically characterize the pyridine adduct, lowtemperature (-100 °C) NMR studies in THF-d 8 are required (Figure 3.5b). 23 As a solution of 3.3 and 1 equiv of pyridine is cooled, signals due to the methylene protons of the neopentyl group are the first to coalesce (-30 °C), presumably due to the slowed rotation of the alkyl ligand about its Mo-C bond axis. At lower temperature (-75 °C) signals due to the methyl protons of the Cp * rings coalesce. At -90 °C, the pyridine in the sample begins to coordinate to the metal, and all signals begin to sharpen. Finally, at -100 °C, only signals attributable to the pyridine complex 3.12 are observable. At this temperature, the sample solution is orange (characteristic of 3.12) and not purple (characteristic of 3.3).  67  11111111I^III^ti^tir^till^ii^111111'111^1111^ill 1^li^III!^li^i^I 10^8^6^4^g PPM^0  ir !Ili li ii t iiiimit  II iiiiiii 10^  fliiiiiii r t it 1 I 1111 aPPM^0  Figure 3.5 300 MHz 1 H NMR spectra of 3.12 at (a) room temperature; (b) -100 °C in MF-d8 (5 3.60 and 1.75)  68 3.3.3.2 Formation of Insertion Products: Reactions With Unsaturated Lewis Bases  Lewis-base substrates containing unsaturated linkages also form adducts with Cp*Mo(NO)(CH2 CMe3)Cl, but once coordinated they undergo facile insertion into the molybdenum-neopentyl bond. Thus, the reactions of Cp*Mo(NO)(CH2CMe3)C1 with CO or CNCMe 3 provide the dihapto acyl (3.13) and iminoacyl (3.14) complexes, respectively (eq 3.6).  CE  N  Cl  0  Mo R—C' / 'CI N 0  (3.6)  E = 0, NCMe 3 R = CH2 CMe3 The yield of 3.13 (30%) is substantially lower than the yield of 3.14 (95%) primarily due to the complexity of the CO insertion reaction. After isolation of amber Cp*Mo(NO)(rl 2 -C{O}CH2 CMe3 )C1 from diethyl ether, the red supernatant solution can be taken to dryness, and the residue can be redissolved in pentane. Crystallization of the pentane solution yields an impure red solid which, by 1 H NMR spectroscopy, exhibits a multitude of signals attributable to Cp * , CMe3 and CH2 protons. Attempts to purify this mixture have not as yet been successful. On the other hand, the tert-butyl isocyanide insertion reaction is facile and very clean. Thus, addition of CNCMe 3 to a purple solution of 3.3 causes an immediate color change to amber, presumably due to the adduct Cp * Mo(N0)(CH2 CMe3 )(CI)(CNCMe 3 ), followed by a color change to yellow and precipitation of the inserted species Cp*Mo(NO)(rl 2 -C {NCMe 3 } CH2 CMe3)C1 (3.14). In accord with other acyl complexes that we have isolated, 8 ' 24 the neopentyl-acyl derivatives prepared in this work exhibit classic i 2 -acyl ligands. 25 The IR spectrum of 3.13 as a Nujol mull exhibits two strong bands at 1624 and 1601 cm -1 attributable to NO- and CO-stretching frequencies, but unambigous assignment of these bands is not possible without labeling studies.  69 However, by analogy to related complexes, the 1624 cm -1 band can most likely be attributed to vNo. To confirm the nature of the acyl group in complex 3.13, it was subjected to an X-ray crystallographic analysis. 26 An ORTEP diagram of the molecular structure of 3.13 is shown in Figure 3.6. The ti 2 -acyl ligand is clearly evident in the structure of 3.13. The two independent molecules of 3.13 that were crystallographically determined exhibit short Mo-O bond lengths of 2.217(2) and 2.226(2) A and very acute C11-Mo-02 bond angles of 32.9(1) and 33.3(1)°, respectively (Table 3.6). The differences between the Mo-02 and Mo-Cll bond lengths of 0.153 and 0.182  A  are characteristic of an 71 2 -acyl group. 25 A nearly isostructural analogue of complex 3.13 has been reported by Hersh; however, the route to this latter complex does not resemble that in which 3.13 was prepared. 27 This complex, CpMo(N0)(11 2 -C{O)p-toly1)(I), exhibits a A(Mo-0)-(Mo-C) of 0.163 A. As of 1988, 25 the known range of 61(Mo-0)-(Mo-C)] for structurally characterized 1 2 -acyl complexes was 0.17 - 0.30  A. Both the Hersh complex and  complex 3.13 are slightly outside of this range. The low values of 0[(Mo-0)-(Mo-C)] observed for these two complexes are more similar to electron-deficient Group 4 / 2 -acyl complexes and are likely a function of the Lewis-acidic metal centers in these compounds. The IR spectrum of 3.14 is diagnostic for an i 2-iminoac yl moiety. Specifically, a vCN at 1715 cm -1 is characteristic of this ligand type. 25 It is of interest to note that the nitrosylstretching frequency for the iminoacyl complex is 50 cm -1 lower than that of the acyl complex 3.13. It can therefore be concluded that an NCMe 3 group is a better source of electron density for the Cp * Mo(NO) fragment than is an oxygen atom from an i 2 -acyl group.  70  Figure 3.6 ORTEP diagram of 3.13 (only one of the two crystallographically independent molecules is shown). 33% probability ellipsoids are shown for the non-hydrogen atoms.  Table 3.6 Selected bond lengths and angles for complex 3.13  bond lengths (A) (esd) molecule A molecule B 2.217(2) 2.226(2) Mo-02 2.044(3) 2.064(3) Mo-C11 2.4204(8) 2.4258(8) Mo-Cl 1.763(3) 1.785(3) Mo-N  bond angles (°) (esd) molecule A molecule B Mo-C11-02 80.5(2) 81.5(2) Mo-02-C11 66.6(2) 65.3(2) Mo-N-01 167.8(2) 168.1(2) 32.9(1) Cl 1-Mo-02 33.3(1) C11-Mo-N 92.8(1) 91.3(1) N-Mo-C1 98.46(9) 98.52(8) 85.79(6) C I -Mo-02 85.45(6)  71 3.3.3.3 Halide Abstraction: Reaction with AgBF4  The chloro ligand in 3.3 is very susceptible to abstraction by Ag (I) salts. Treatment of Cp * Mo(NO)(CH2 CMe3)Cl with AgBF 4 in acetonitrile leads to a good yield of the 16-electron acetonitrile cation [Cp * Mo(NO)(CH2CMe3)(NCMe)]BF4 (3.15) (eq 3.7).  Mo E  BF4  AgBF4, NCMe -AgCI  N 0  NCMe  (3.7)  N O  R CH2 CMe3 Complex 3.15 is related to other Mo and W cations that our group has previously described. 28 However, 3.15 is the first nitrosyl cation of this type to bear a hydrocarbyl ligand. At room temperature, in CD2C12, 3.15 exhibits only 1 peak for the coordinated nitrile ligand whereas the analogous W cation exhibits two broad singlets attributable to two coordinated acetonitrile groups. 22 Evidently, the Mo complex would rather not coordinate a second molecule of acetonitrile for steric or electronic reasons. Why this is so is not clear, since the analogous molybdenum halo bis(acetonitrile) cations, [Cp * Mo(NO)(X)(NCMe)2]PF6 [X = I, Br, Cl], are known. 28 Possibly the BF 4 anion is weakly coordinated to Mo in 3.15. Increased energy vcN (vs free NCMe at 2230 cm -1 ) at 2316 and 2290 cm -1 in the IR spectrum of 3.15 are diagnostic for the presence of an N-bound nitrile ligand; 29 therefore, it is unlikely that the nitrile ligand is it-bound as occurs in the 16-electron acetonitrile cation, [W(bpy)(PMe3)2(CI)(1 2 -NCMe)]+. 30 Being an ionic species, 3.15 is best crystallized from CH2C12/Et2 0 mixtures, from which it forms irregular orange-yellow crystals. However, upon standing in CD 2 C12 for several hours at room temperature, the complex spontaneously decomposes to a mixture of products and free acetonitrile. Clearly, the nitrile ligand in the 16-electron Mo cation is quite labile. Interestingly, a related complex, [Cp2Zr(Ph)(NCMe)]BPh4, does not persist in the presence of excess acetonitrile, but rather undergoes insertion of NCMe into the Zr-Ph bond. The thermally stable complex thus formed, [Cp2Zr(NC{Me}Ph)(NCMe)]BPh 4 , contains one NCMe ligand.31  72 Halide abstraction from compound 3.3 in a non-coordinating solvent such as dichloromethane does not lead to tractable products, presumably because the unsolvated 14-electron [Cp*Mo(NO)(CH2CMe3)] + , if formed, would be exceedingly reactive. The scope of ligand types that can be coordinated to the [Cp * Mo(N0)(CH2CMe3)] + fragment remains to be ascertained. It should be noted that a family of cations isoelectronic with [Cp * Mo(NO)(CH2CMe3)] + have been extensively studied by Jordan and coworkers. 32 The [Cp 1 2Zr(R)(L)]BX 4 [R = H, Me, CH2Ph, Ph; L = NCMe, THE; X = F, Ph] salts mediate a variety of productive insertion processes. Similar reactivity can be envisaged for complexes derived from 3.15.  3.3.4 Reactions of 18 Electron Derivatives of Cp * Mo(N0)(CH2CMe3)C1 -  As mentioned in the text, many derivatives of Cp*Mo(N0)(CH 2 CMe3 )C1 (3.3) are very reactive and/or decompose readily in solution. This section outlines two reactions that should be a foundation for much future research.  3.3.4.1 Reaction of Cp * Mo(N0)(CH2CMe3)(CI)(PMe3) with LDA  Dehydrohalogenation of Cp*Mo(NO)(CH 2 CMe3 )(PMe3)Cl (3.11) with LDA in THE gives moderate yields of the unusual dialkyl complex 3.16 in which the metallated Cp* ring serves as the second alkyl ligand (eq 3.8).  LDA  MO M^ O -(i-P r)2 NH, - LiCI 2'.^/- - ■ R R^ :.^ 'CI^  (3.8)  -  N PMe3^N O  PMe 3  R = CH2 CMe3 Conversion 3.8 probably proceeds via direct deprotonation by LDA of a Me substituent of the Cp * ligand followed by intramolecular metathesis. Alternatively, deprotonation of the  73  hydrocarbyl ligand would afford a transient alkylidene species that could rearrange to the final product. Bercaw and coworkers have established a precedent for such a rearrangement (eq 3.9). 33  Hf  ,,.CH2Ph ' CH2Ph  CH2 Ph (3.9)  The latter pathway is mechanistically unlikely in the Mo system for two reasons. First, the methylene protons of 3.11 are more sterically protected than the methyl groups of the Cp* ligands and LDA is a hindered base. Secondly, alkylidene complexes of the type, CpMo(NO)(=CHCMe 3 )L [L = Lewis base], 34 have been shown to be isolable and thermally stable to at least 150 °C in C 6D 6 (Chapter 5). The NMR spectra (CDC1 3 ) of 3.16 show four different methylene-proton resonances ( 1 H NMR) and four inequivalent Cp * methyl-group signals ( 1 H and 13 C{ 1 1-1}-APT NMR) in addition to the CMe3 resonance for the neopentyl ligand. The two sets of methylene protons show markedly different 2411.1 couplings characteristic of their chemical environments. Thus, the neopentyl methylene protons are bound to a purely spa methylene carbon and exhibit coupling constants of 12.9 Hz, whereas the methylene protons of the tucked-in Cp* ring are bound to a carbon with considerable sp 2 character and thus are weakly (or geminally) coupled (3.0 Hz). 35 Fortuitously, a single diastereomer is isolated from reaction 3.8.  36 This feature is most clearly  evidenced by a single resonance in the 31 P NMR spectrum (5 -2.89 ppm) of this material in C 6D 6 (Table 3.3). The proposed structure of complex 3.16 is based on the solid-state structural analysis of its W congener. 22 Since both Mo and W complexes display nearly identical spectroscopic features, they are very likely isostructural. While other i 5 ;n 1 -Cp * -containing complexes such as  74 [Cp *Ru(i 5 ,71 1 -05Me 4 CH2)][BF 4 ], 37 Cp * Hf(11 5 ,71 1 -0 5Me 4CH2)(CH2Ph), 33 and Cp *Zr(r1 5 ,i 1 -05Me4CH2)(Ph) 38 are known, none was prepared by direct deprotonation of a Cp * ligand.  3.3.4.2 Reaction of Cp * Mo(N0)(t1 2 C{O)CH2 CMe3)C1 with PMe 3 -  In order to determine if the 11 2 -acyl ligand in 3.13 could be displaced from its metal center to .11 1 coordination, the complex was reacted with PMe3. Treatment of 3.13 with excess PMe3 in  CDC13 causes the formation of a single organometallic product that has two different PMe3 groups. Since the 1 H NMR spectrum of the product reveals that the signals attributable to the methylene protons of the acyl ligand show 31 P coupling ( 3./Hp = 28.5 and 6.9 Hz), in addition to a mutual 2JRH of 15.0 Hz, it seems most likely that one PMe 3 ligand is attached to the acyl carbon and one is attached to the metal, thereby causing the chloro ligand to become a counter anion (eq 3.10).  xs PMe3  Mo  Of N  0  Me P +--—I c:F.4 3^Mo„ ;4 'PMe3  O  Cl-  (3.10)  0  R = CH2 CMe3 The complex, [Cp *Mo(N0)(C{0}{PMe3}CH2CMe3)(PMe3)]Cl (3.17) is stable only in solution (in the presence of excess PMe3) and decomposes readily upon removal of solvent. The insolubility of 3.17 in C 6D 6 is consistent with the ionic formulation of the complex. Nevertheless, without a crystallographic analysis of this salt, the true nature of complex 3.17 remains ambiguous. Anion metathesis may well lead to a more stable derivative of this species.  75  3.4 Epilogue and Future Work  Two major conclusions about the entire family of Mo alkyl nitrosyl complexes can be made. First, they are synthesizable and are not as unstable as previously speculated. In fact, the alkylations can be conducted in a stepwise fashion, thereby enabling us to prepare asymmetric complexes of the type Cp * Mo(N0)(R)(R'). Future attempts to exploit the chiral nature of these molecules in various asymmetric syntheses should be made. Second, structural, electrochemical and IR studies all lead to the same conclusions about the relative Lewis acidities of the entire Cp'M(NO)(Y)(Z) [Cp' = Cp, Cp * ; M = Mo, W; Y, Z = halide, alkyl, aryl] family of complexes. Namely, Cp-containing complexes have more electron-deficient and more accessible, and thus more reactive, metal centers than their Cp * congeners. The same statement is true for Mo vs W and aryl vs alkyl. Thus, the most Lewis acidic complex in this series that we can prepare is CpMo(NO)(aryl)2. It is a shame that such species are so thermally sensitive as to de& isolation. The last part of this chapter describes reactions of Cp * Mo(NO)(CH2 CMe3 )Cl and some of its derivatives. Clearly, Cp*Mo(N0)(CH 2 CMe3 )CI is a potent Lewis acid. Its neopentyl group migrates readily to unsaturated substrates and its chloride ligand can be metathesized or abstracted. The reactions of Cp *Mo(NO)(CH2 CMe3)Cl (3.3) should be extended to other alkyl or aryl complexes since the comparative chemistry of those species would be of interest. Sitnilarily, the chemistry of several reaction products would be of considerable practical interest. For instance, what organic substrates are activated by coordination to [Cp*Mo(N0)(CH2 CMe3 )]+-containing complexes and what types of bonds can be activated by the strained ring in (r15,111-05Me4CH2)Mo(N0)(CH2CMe3)(PMe3)?  76 3.5 References and Notes (1)  The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Ed.; Wiley: New York, 1987; Vol. 4, Part 2.  (2)  Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; Chapters 3 and 14.  (3)  Legzdins, P.; Veltheer, J. E. Acc. Chem. Res. 1993, 26, 41.  (4)  Dryden, N. H.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B. Organometallics 1991, 10, 2077.  (5)  Several reactions mixtures of this type were taken to dryness in tared flasks without crystallization, and the mass of the crude product established by difference. The yields of four separate experiments ranged from 83 - 93%.  (6)  Legzdins, P.; Phillips, E. C.; Sanchez, L. Organometallics 1989, 8, 940.  (7)  1 H NMR data for 3.13 in CDC1 3 : 5 3.45 (d, 1H, CH2 , Jp  = 15.9 Hz), 3.06 (d, 1H, CH2,  JHH = 15.9 Hz), 1.95 (s, 15H, C5(CH3 ) 5 ), 1.23 (s, 9H, C(CH3 ) 3 )• (8)  Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394.  (9)  Legzdins, P.; Lundmark, P. J.; Phillips, E. C. Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2991.  (10) Hunter, A. D.; Legzdins, P.; Martin, J. T.; Sanchez, L. Organomet. Synth. 1986, 3, 66. (11) Legzdins, P.; Veltheer, J. E. In Handbuch der Preiparativen Anorganischen Chemie, 4th ed.; Herrmann, W. A., Ed.; Thieme-Verlag: Stuttgart, Germany, in press. (12) A commentary of our attempts to synthesize such complexes is provided in a paper describing the chemistry of CpM(NO)(rl 2 -benzyl)C1 complexes (see: Dryden, N. H.; Legzdins, P.; Trotter, J.; Yee, V. C. Organometallics 1991, 10, 2857). These benzyl complexes are 18-electron species by virtue of their containing 3-electron-donor benzyl groups.  77 (13) Debad, J. D.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B. Organometallics 1992, 11, 6. (14) Crude Cp*Mo(NO)C12 prepared from Cp*Mo(NO)(CO)2 and PC1 5 is best purified by Soxhlet extraction and crystallization from CH 2 C12. 4 (15)  1 H NMR spectral data for this complex (C6D6): 8 3.83 (d, 111, CH2, Jig{ = 16.2 Hz), 3.62  (d, 1H, CH2, fu/ = 16.2 Hz), 1.75 (s, 15H, Cp * ), 1.04 (s, 9H, CMe3). (16) Debad, J. D.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B. Organometallics, in press. (17) Jackman, L. M.; Sternhell, S. Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry; Pergamon Press: Toronto, 1969; 2nd ed., p 334. (18) Chapter 2 of this thesis. (19) For moderate charge-transfer rates, quasi-reversible electrochemical behaviour is often observed. The slowness of electron transfer is often ascribed to subtle structural changes that occur upon reduction (or oxidation). For a general description of this phenomenon, see: Geiger, W. E. Prog. Inorg. Chem. 1984, 275. For more detailed descriptions of electrochemical quasi-reversibility, see: (a) Matsuda, H.; Ayabe Y. Z Elekirochem. 1955, 59, 494. (b) Bard, A. J.; Faulkner, L. F. Electrochemical Methods; John Wiley and Sons: New York, NY, 1980; Chapter 6. (20) Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2583. (21) Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B. E.; Gatter, M. G. J. Am. Chem. Soc. 1985, 107, 1411.  (22) Debad, J. D.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics, submitted for publication. (23) Initial attempts to obtain a limiting spectrum for this adduct, Cp * Mo(N0)(CH2CMe3)(C1)(py), in CHC13 were unsuccessful since the solvent freezes at a temperature higher than that required to form the adduct.  78 (24) Dryden, N. H.; Legzdins, P.; Lundmark, P. J.; Reisen, A.; Einstein, F. W. B. Organometallics, in press. (25) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059. (26) Crystals of 3.13 are monoclinic of space group P21In; a = 15.226 (2) A, b = 12.376 (2) A, c = 19.641 (2) A, 0 = 95.921 (8)°, Z = 8. Dr. Steve Rettig solved the structure using the Patterson method and full-matrix least-squares refinement procedures to RF = 0.032, RWF = 0.032 for 7215 reflections with I 3a(1). (27) Bonnesen, P. V.; Yau, P. K. L.; Hersh, W. H. Organometallics 1987, 6, 1587. (28) Chin, T. T.; Legzdins. P.; Trotter, J.; Yee, V. C. Organometallics 1992, 11, 913. (29) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley-Interscience: New York, 1986; pp 280-281. (30) Barrera, J.; Sabat, M.; Harman, W. D. J Am. Chem. Soc. 1991, 113, 8178. (31) Borkowsky, S. L.; Jordan, R. F.; Hinch, G. D. Organometallics 1991, 10, 1268. (32) Jordan, R. F.; Bradley, P. K.; LaPointe, R. E.; Taylor, D. F. New J Chem. 1990, 14, 505 and references therein. (33) Bulls, A. R.; Schaefer, W. P; Serfas, M.; Bercaw, J. E. Organometallics 1987, 6, 1219. (34) Legzdins, P.; Rettig, S. J.; Veltheer, J. E. J. Am. Chem. Soc. 1992, 114, 6922. (35) The exact assignments of the methylene protons were eludicated from a two-dimensional 1 H: 1 H-COSY spectrum of 3.16.  (36) Based on the X-ray crystallographic analysis of the W analogue of 3.16, the isolated product has a configuration in which the PMe 3 ligand is trans to the NO ligand, just as in the starting material, Cp * Mo(NO)(CH2CMe3)(Cl)(PMe 3 ) (3.11). (37) Kreindlin, A. Z.; Petrovskii, P. V.; Rybinskaya, M. I.; Yanovskii, A. I.; Struchkov, Yu. T. J. Organomet. Chem. 1987, 319, 229. (38) Schock, L. E.; Brock, C. P.; Marks, T. J. Organometallics 1987, 6, 232.  79  CHAPTER 4 Reactions of 16-Electron Cp'M(NO)R2 Complexes of Molybdenum and Tungsten with Water  4.1 Introduction^  79  4.2 Experimental Procedures ^  80  4.3 Results and Discussion ^  95  4.4 Epilogue and Future Work^  116  4.5 References and Notes ^  117  4.1 Introduction  At present, little is known about the reactions of transition-metal-carbon a bonds with 112 0. 1 This seems rather anomalous given water's ubiquity and the ease at which most organometallic complexes are hydrolyzed. As outlined in Chapters 2 and 3, the impetus for developing a new synthetic methodology to a range of Cp'M(NO)R 2 complexes was necessitated by the propensity of all but the tungsten dialkyl complexes to react rather vigorously with water which was a key reagent in our original preparations of this family of organometallic complexes. This chapter will focus on the products obtained when isolable Cp 1M(NO)R2 complexes are treated with water. 2 A fundamental goal of our research group has been the determination of the distinctive physical and chemical properties that complexes containing 14-valence-electron Cp'M(NO) fragments exhibit. Typically, these properties are dependent upon all three  80 variables: the metal, the cyclopentadienyl ring, and the ancillary hydrocarbyl ligands. The work described in this chapter provides the clearest illustration of the dependence upon M so far investigated in our laboratories. In general, the W-C a-bonds in Cp'W(NO)(alkyl)2 compounds are stable and are not easily hydrolyzed (Chapters 1 and 2). The four isolable Cp'W(NO)(aryl) 2 complexes, on the other hand, are converted to their aryl dioxo analogues, Cp'W(0)2(aryl) when exposed to water. In contrast, the Mo-C a bonds in all (both alkyl and aryl) isolable Cp*Mo(NO)R2 complexes are readily hydrolyzed to form bimetallic complexes of the type [Cp*Mo(NO)R] 2 -(.1-0) and free hydrocarbon. This chapter will focus primarily on these molybdenum systems and present the synthesis, characterization and properties of five such complexes. In light of a preliminary kinetic analysis, a probable mechanism for the formation of these complexes is presented. The chemical relationships between Cp * Mo(NO)R2 and its oxo derivatives, [Cp * Mo(NO)12.] 2 -(.1-0), Cp * Mo(0) 2 R, and [Cp * M0(0)2]2-(1-0) are presented. Lastly, the chemistry of the molybdenum systems is contrasted with that of the congeneric tungsten systems.  4.2 Experimental Procedures  4.2.1 Methods The synthetic methodologies employed throughout this thesis are described in detail in section 2.2.1.  4.2.2 Reagents The organometallic dihalide complexes, Cp 1M(NO)C1 2 , were prepared and handled as described in Section 2.2.2. Preparations of (ary1) 2Mg•dioxane) reagents are outlined in  81 Section 2.2.4. Other (alky1) 2Mg•X(dioxane) reagents were prepared similarily by substituting the appropriate Grignard reagent. Commercial Grignard reagents were purchased from Aldrich as 1.0 M Et20 or THF solutions. The Cp'W(NO)(aryl)2 complexes [Cp' = Cp, aryl = o-tolyl; Cp' = Cp*, aryl = Ph, p-tolyl, o-tolyl], were prepared using the methodology outlined in Sections 2.2.5 and 2.2.7. Bulk quantities (1-5 g) of Cp * Mo(NO)(CH2SiMe3)2 were prepared as described in Section 3.2.6. D 2 0 (MSD Isotopes, 99.8% D) and 18 0H2 (Aldrich, 97% 180) were used as received. H202 (30% aqueous, Fisher) was used as received. The column chromatography materials used during this work were alumina (80 - 200 mesh, Fisher neutral, Brockman activity I), alumina I deactivated to alumina III with 6% w/w deaerated water, and Florisil (60 - 100 mesh, Fisher). Filtrations were performed through Celite 545 diatomaceous earth (Fisher) that had been oven-dried and cooled in vacuo.  4.2.3 Electrochemical Measurements The methodology used for electrochemistry was very similar to that described in Section 2.2.3 except that Ag wire was used as the reference electrode. Ferrocene was used as an internal reference during this work, with the redox couple, Cp2Fe/Cp2Fe+, occurring at E°'= 530 mV versus Ag wire in THE over the range of scan rates used (0.10 to 0.80 V s -1 ). 3  4.2.4 Kinetic Measurements The conversion of Cp * Mo(N0)(CH2SiMe3)2 (3.8) to [Cp * Mo(N0)(CH2SiMe3)]2-(11-0) (4.1) was studied in order to gain some insight about the mechanism of the hydrolysis reaction. In general, a standard solution of Cp * Mo(NO)(CH2SiMe3)2 was prepared in a drybox in a volumetric flask using THF as  82 solvent. Aliquots of this standard solution were diluted as required, and 2 mL of the diluted solution was then quickly mixed with 2 mL of a standard solution of water in THE in a 1.00 cm UV-vis spectrophotometric cell equipped with a 4-mm Teflon stopcock. The concentration of the water was sufficiently high such that all runs were effectively conducted under pseudo-first-order conditions. The cell and its contents were shaken, quickly removed from the drybox, and placed in a Hewlett-Packard 8542A diode array spectrometer, which, in conjunction with a Haake W19 temperature bath equipped with a Haake D8 temperature controller, allowed the cell to be maintained at constant temperature (±0.1 °C). The solution was left unstirred and allowed to equilibrate with the spectrometer temperature for 300 s. Spectra were then recorded at regular intervals (usually 150 s), and data were collected for approximately one half-life or until significant deviations from linearity of the monitored parameters occurred. The absorbance values at infinity were determined by calculation, i.e. the correlation coefficients of pseudo-firstorder log plots were optimized by empirically varying the values of Am . 4 The observed rate constants (kth s) were then calculated from plots of ln(k o-At) versus time (s). Values of AH: and AS: were determined from Erying plots of ln(kth s/T) versus 1/T where AH: = -R(slope) and AS = R[intercept - In(k B/h)] (R, kB and h are the gas constant, Boltzmann's constant and Planck's constant, respectively).  4.2.5 Preparation of [Cp * Mo(N0)(CH 2 SiMe3)1 2 4 0) (4.1) -  A solution of Cp *Mo(NO)(CH2SiMe3)2 (3.8) generated from Cp *Mo(NO)C12 (1180 mg, 3.55 mmol) and Me3SiCH2MgC1 (8.0 mL of a 1.0 M Et20 solution, 8.0 mmol) in Et20 (150 mL) was prepared at 0 °C. Treatment of the resultant purple solution with deaerated H 2 O (0.20 mL) destroyed any excess Grignard reagent and caused the decomposition of Cp * Mo(N0)(CH2SiMe3) 2 (vNO 1593 cm -1 ) to [Cp * Mo(N0)(CH2SiMe3)]2-(1-0) (v Nic, 1584 cm -1 ). The solvent volume was then  83 reduced to approximately 15 mL by slow evaporation of the Et20 under reduced pressure. At this point the mixture contained mostly 4.1, but small quantities of Cp * Mo(NO)(CH2SiMe3)2 persisted in solution. The mixture was then very slowly chromatographed on an alumina III column (3 x 8 cm) made up in Et20 using Et20 as eluant. As the purple band indicative of the dialkyl starting material passed through the column, it changed color, becoming the red-brown characteristic of 4.1. The rest of 4.1 eluted from the column next. The combined red-brown eluates were collected, and their volume was reduced in vacuo until the first signs of crystallization became evident. Maintaining the concentrated solution at -20 °C overnight resulted in the deposition of large red-black crystals (650 mg) of [Cp *Mo(N0)(CH2SiMe3)]2-(11-0) which were collected by filtration. The filtrate was taken to dryness, and the residue was redissolved in hexanes. Crystallization of this second fraction afforded an additional 302 mg of product (75% total yield based on Cp *Mo(NO)C12). The conversion of Cp *Mo(NO)(CH2SiMe3)2 to [Cp * Mo(N0)(CH2SiMe3)}2-(.1-0) could be conveniently monitored by 1 H NMR spectroscopy which indicated that the yield of the reaction was > 95%. For example, the reaction of Cp *Mo(NO)(CH2SiMe3)2 (38 mg) in C 6D6 (1.2 mL) with H 2 O (10 ;AL, excess) required approximately 2 min at room temperature to go to completion. The signal due to the 2 equiv of Meg Si formed in the reaction could be easily detected at 5 0.00 ppm.  4.2.6 Preparation of [Cp * Mo(N0)(CH 2 CMe3)12 (1 0) (4.2) -  -  Solid Cp * Mo(NO)C12 (498 mg, 1.50 mmol) and solid (Me3CCH2)2Mg•(dioxane) (510 mg, 3.00 mmol of Me3CCH2 -) were intimately mixed in a Schlenk tube. The tube was cooled to -80 °C, and THE (20 mL) was added. When the temperature was raised to -20 °C, the solution became orange-red (vNO 1588 cm -1 ), a feature indicative of the formation of Cp * Mo(NO)(CH2CMe3)2. The reaction mixture was maintained at  84 temperatures below 0 °C as the solvent was removed in vacuo. The remaining red solid was then suspended in Et 2 0 (20 mL). While this suspension was being rapidly stirred, deaerated water (-0.7 mL) was added, thereby producing a red solution and a brown precipitate. After 15 min, the mixture was filtered through a column of alumina I (3 x 3 cm), and the column was rinsed with Et20 (75 mL). The combined filtrates were taken to dryness. The red solid was dissolved in THE (5 mL), and water (0.5 mL) was added. The mixture was stirred for 6 h, whereupon it was filtered through a column of alumina I (2 x 6 cm) supported on a fiit. The filtrate was taken to dryness, and the residue was crystallized from hexanes/Et 2 0 (5:1) to obtain black crystals of 4.2 (451 mg, 88% yield).  4.2.7 Preparation of [Cp * Mo(N0)(CH2CMe 2 Ph)1 2 (1 0) (4.3) -  -  Solid Cp *Mo(NO)C1 2 (664 mg, 2.00 mmol) and solid (PhMe2CCH 2 )2Mg•(dioxane) (398 mg, 3.90 mmol of PhMe2CCH2 -) were intimately mixed in a Schlenk tube. The tube  was cooled to -80 °C, and THE (20 mL) was added. Upon being warmed to -20 °C, the solids dissolved and the solution became red (vNO 1586 cm -1 ), thereby indicating the formation of Cp * Mo(NO)(CH2CMe2Ph)2. The solution was warmed to room temperature and stirred for 1 h, and then the solvent was removed in vacuo. The residues were suspended in Et 2 0/hexanes (50 mL, 4:1 mixture), the suspension was stirred vigorously, and deaerated water (0.4 mL) was added. The solution quickly became deep red, and an oily beige precipitate formed on the sides of the Schlenk tube. After 10 min, the reaction mixture was filtered through alumina I (2 x 6 cm) supported on a fiit. Removal of the solvent from the filtrate left a red oil. This oil was dissolved in pentane and transferred to an alumina I column (2 x 4 cm) supported on a fiit. The column was washed with pentane (100 mL), and the washes were discarded. Complex 4.3 was then removed from the column with Et20 (100 mL). Solvent was removed from the red Et20  85 filtrate, and the residue was dried in vacuo for 48 h at 35 - 40 °C. The red solid thus obtained tended to be somewhat oily, presumably reflecting its contamination with Me 3 CPh. Crystallization of this residue from pentane yielded 390 mg (50% yield) of analytically pure 4.3 as dark red needles. Subsequent crops of crystals obtained from the supernatant liquid were too oily to be isolated in analytically pure form.  4.2.8 Preparation of [Cp*Mo(N0)(0 - toly1)12 - (11- 0) (4.4) Cp * Mo(NO)C12 (664 mg, 2.00 mmol) was partially dissolved in THF (10 mL) and cooled to -50 °C. A single aliquot of (o-toly1)MgC1 (4.2 mL of a 1.0 M THF solution, 4.2 mmol) was added via syringe to the dichloride slurry. The resulting purple solution of Cp * Mo(N0)(o-toly1)2 (vNO 1601 cm -1 ) was immediately treated with deaerated water (0.1 mL) at -50 °C. The purple solution changed instantly to red-brown. The volume of the solution was reduced to 3 mL in vacuo, and it was transferred to the top of an alumina III column (2 x 5 cm). A brown band was eluted from the column with acetone and collected in a flask. Slow evaporation of the acetone solvent from the eluate in air afforded 512 mg (71% yield) of 4.4 as medium-brown microcrystals which were washed with pentane (10 mL) and dried in vacuo.  4.2.9 Preparation of [Cp * Mo(N0)(Ph)1 2 - (2- 0) (4.5) Cp * Mo(NO)Cl2 (332 mg, 1.00 mmol) and Ph2Mg•X(dioxane) (356 mg, 2.02 mmol of Ph -) were intimately mixed in a Schlenk tube. The tube and contents were then cooled to -80 °C, and THF (10 mL) was added to the solids by vacuum transfer. The Schlenk tube and its contents were then quickly warmed to room temperature by immersing the tube for 1 min in a warm water bath. The violet solution of Cp *Mo(NO)Ph2 (vNO 1615 cm -1 ) thus produced was taken to dryness, and the residue was redissolved in Et 2 0 (50 mL) and the solution filtered on Celite (1 x 4 cm). The violet filtrate was cooled to 0 °C and treated  86 with water (50 gL), whereupon the color of the solution changed to red-brown within 1 min. The ether was removed from the final mixture in vacuo, and the residue was crystallized from CH2 C12/hexanes (1:1) to obtain 269 mg (78% yield) of 4.5 as a dark brown microcrystalline solid.  4.2.10 Preparation of Cp * W(0)2(aryl) [aryl = Ph (4.6), p-tolyl (4.7), and o-tolyl ( 4 . 8 )]  The three pentamethylcyclopentadienyl aryl dioxo complexes were prepared in a similar manner, the synthesis of Cp * W(0)2(p-toly1) (4.7) being presented below as a representative example. A sample of Cp * W(N0)(p-toly1)2 (200 mg, 0.377 mmol) was dissolved in Et 2 0 (20 mL). The resulting blue solution was stirred at room temperature, and a single drop of deaerated H 2 O was added, whereupon the solution became green immediately. The solution was taken to dryness in vacuo, and the brown residue was extracted with Et20 (5 x 10 mL). The combined extracts were filtered through Florisil (2 x 3 cm), concentrated under reduced pressure, and placed in a freezer overnight. These operations induced the crystallization of 70 mg (42% yield) of colorless, microcrystalline Cp * W(0)2 (p-toly1) 4.7, which was collected by filtration. Similarly, Cp * W(0)2Ph (4.6) and Cp * W(0) 2 (o-toly1) (4.8) were obtained in 45 and 80% yield from Cp * W(NO)Ph 2 and Cp * W(N0)(o-toly1) 2 , respectively. The three aryl dioxo complexes could also be obtained in comparable yields by treatment of their diaryl precursors in the solid state with 02.  87  4.2.11 Preparation of CpW(0)2(o tolyl) (4.9) -  CpW(NO)C12 (1050 mg, 3.00 mmol) and (o-toly1) 2 Mg•(dioxane) (960 mg, 6.00 mmol of o-tolyl -) were intimately mixed in a Schlenk tube. The tube and contents were then cooled to -196 °C, and THE (50 mL) was added by vacuum transfer. The Schlenk tube and its contents were warmed to 10 °C over a period of 2 h. The violet solution of CpW(NO)(o-tolyl)2 (vNO 1601 cm -1 ) thus produced was taken to dryness, and the residue was extracted with Et 2 0 (5 x 50 mL) and the combined extracts filtered through Celite (3 x 4 cm). The violet filtrate was concentrated to one-half its original volume and treated with 30% aqueous hydrogen peroxide (-0.2 mL), whereupon the color of the solution changed to red-amber within seconds. 5 The reaction mixture was taken to dryness in vacuo and the resulting brown powder extracted with acetone (10 mL). The acetone solution was applied to the top of an alumina I column (2 x 5 cm), and the column was eluted with acetone (100 mL). The eluate was taken to dryness and the residue was extracted with Et 2 0 (5 x 20 mL). The ether extracts were filtered through Celite (1 x 5 cm), concentrated, and maintained at -30 °C for two weeks. These operations induced the deposition of 583 mg (52% yield) of complex 4.9 as a pale amber, microcrystalline solid.  4.2.12 Reaction of Cp * Mo(N0)(CH 2 SiMe3)2 with 18 0112 Cp * Mo(N0)(CH2SiMe3)2 (24 mg, 0.055 mmol) was dissolved in C 6D 6 (1.2 mL) and was treated with excess 18 0112 (10 1AL). After being shaken vigorously for 2 min, the sample was filtered through a column of Celite (0.3 x 0.5 cm) into an NMR tube to remove residual 18 0112. Approximately 95 % of the signal intensity in the 1 H NMR spectrum of the filtrate was attributable to [Cp *Mo(N0)(CH2SiMe3)] 2 -(1- 18 0). A signal at 8 0.00 ppm confirmed the presence of Me4Si, the byproduct of the hydrolysis reaction.  88 4.2.13 Reaction of Cp * Mo(NO)(CH2SiMe3)2 with D20 Cp *Mo(N0)(CH2SiMe3)2 (20 mg, 0.046 mmol) was dissolved in C 6 D 6 (1.2 mL) and treated with excess D 2 0 (10 121). After being shaken vigorously for 2 min, the sample was filtered through Celite (0.3 x 0.5 cm) into an NMR tube to remove unused D20. The 1 H NMR spectrum of the filtrate revealed the presence of 2 equiv of DH2 CSiMe3 by its  characteristic singlet at 8 0.00 and its 1:1:1 triplet (JHD = 2.1 Hz) at a slightly higher field.  4.2.14 Reaction of 4.1 with 02  [Cp * Mo(N0)(CH2SiMe3)]2-(.1-0) (100 mg, 0.14 mmol) was dissolved in hexanes (5 mL). The atmosphere in the flask containing the deep red solution was evacuated, and the vessel was then pressurized to 1 atm with 02. After being stirred at room temperature for 24 h, the resulting amber solution was taken to dryness in vacuo. Extraction of the residue with Et20 and subsequent crystallization afforded yellow-brown crystals of [Cp * Mo( 0)2]2 - (4 -0) (43 mg, 56% yield). 6 There was no evidence for any Cp * Mo(0)2(CH2SiMe3) having been formed. Data determined for [Cp*M0(0)2]2-(11-0) in this work are as follows: Anal. calcd for C20H30 0 5MO2: C, 44.29; H, 5.58. Found: C, 44.31; H, 5.67. IR (Nujol mull): vm 0_, 910 (s), 878 (s) cm -1 ; vmo_o_m0 762 (s, br) cm -1 . 1 H NMR (C6D6): 6 1.78 (s). 1 H NMR (THF-d8): 6 1.94 (s). Low-resolution mass spectrum (probe temperature 120 °C): m/z 526 (P+-0).  In related experiments, various solutions of complexes 4.2 and 4.5 were allowed to evaporate to dryness in a beaker in the atmosphere. The only organometallic complex detectable in each dried residue by 1 H NMR spectroscopy was [Cp*M0(0)2]2-(.1.-0).  89 4.2.15 Reaction of 4.1 with Excess Water  Treatment of [Cp *Mo(N0)(CH2 SiMe3)J2-(.1.-0) (28 mg) with an excess of water (25 giL) in THF-d8 (0.8 mL) for 48 h afforded an amber solution whose 1 H NMR spectrum showed [Cp *Mo(0)0 2 -(2-0) (8 1.94) to constitute > 95 % of the signal intensity. The other principal signals were due to H2O and Me 4 Si.  4.2.16 Characterization Data for Complexes 4.1 - 4.9 Table 4.1 Numbering Scheme, Color, Yield and Elemental Analysis Data  Color no.  (yield, %)  [Cp *Mo(N0)(C 112SiMe3)12 - (1,1-0 )  4.1  [Cp *Mo(N0)(CH2CMe3)12-(1- 1.-0)  Complex  Anal. Found (Calcd) C  H  N  red-black (75)a  47.07(47.16)  7.64 (7.35)  3.85 (3.93)  4.2  black (88)a  53.00 (52.94)  7.74 (7.70)  3.95 (4.12)  [Cp*Mo(N0)(CH2CMe2PM2-(1-0)  4.3  dark red (50)a  59.33 (59.69)  7.15 (7.01)  3.23 (3.48)  [Cp*Mo(N0)(o-toly1)J 2 -(11-0)  4.4  brown (71)a  56.68 (56.67)  6.28 (6.15)  3.77 (3.89)  [Cp*Mo(N0)(Ph)1 2 -(.J.-0)  4,5  dark brown (78)a  54.73 (55.50)  5.81 (5.82)  4.00 (4.04)  Cp*W(0) 2Ph  4.6  colorless (45)b  44.93 (44.88)  4.68 (4.71)  0.00 (0.00)  Cp*W(0) 2 (p-toly1)  4.7  colorless (42) 6  46.03 (46.17)  5.05 (5.01)  0.00 (0.00)  Cp*W(0) 2 (o-toly1)  4.8  colorless (80)b  46.65 (46.17)  5.11 (5.01)  0.00 (0.00)  CpW(0) 2 (o-tolyl)  4.9  colorless (52)c  38.80 (38.74)  3.30 (3.25)  0.00 (0.00)  a Yield based on Cp * Mo(NO)C1 2 . b Yield based on Cp * W(N0)(ary1) 2 (aryl = Ph, p-tolyl, o-tolyl). c Yield based on CpW(NO)C1 2 .  90  Table 4.2 Mass Spectral and Infrared Data  no.  MS  tempb  mka  (°C)  IR (Nujol mull) vm0.0  VNO  vw.0  4.1  716 [P+], 686 [P +-NO]  120  1576, 1561  774, 750  4.2  680 [P1, 609 [P+-CH2 CMe3 ]  200  1582, 1566  768  4.3  805 [P+]  100  1578, 1564  774  4.4  720 [12+ ], 602 [P+-NO-tolyl]  225  1584, 1576, 1568  771, 743  4.5  692 [Pi, 662 [P + -NO]  100  1589, 1575, 1554  803, 785  4.6  428 [P1  120  938,900  4.7  442 [VI  100  939,901  4.8  442 [P+]  4.9  372 [P4 ], 354 [P+-0]  940, 899 951,910  120  a m/z values are for the highest intensity peak of the calculated isotopic cluster. b Probe temperatures.  Table 4.3 1 H and 13 C{ 1 }1} NMR Spectral Data  no. 4.1  1H NMR (8, C6D6) Isomer A (90 °M 1.59 (s, 30H, C5 (CH3 ) 5 ) 0.84 (d, 2H, J HH ''.- 12.3 Hz, CHAHB SiMe3 ) 0.75 (d, 2H, JHH = 12.3 Hz, CHAHB SiMe3 ) 0.45 (s, 18H, Si(CH3 )3 ) Isomer B (10%1 1.71 (s, 30H, C 5 (CH3 ) 5 ) methylene protons not observed 0.42 (s, 18H, Si(CH3 )3 )  13C{111} NMR (8, C6D6) Isomer A 112.60 (C 5 (CH3 ) 5 ) 36.50 (CH2 SiMe3 ) 9.96 (C 5 (CH3 ) 5 ) 2.83 (CH2 Si(CH3 )3 ) Isomer B 113.50 (C 5 (CH3 ) 5 ) 36.20 (CH2 SiMe3 ) 10.26 (C 5 (CH3 ) 5 ) 3.43 (CH2Si(CH3)3)  91 4.2  Isomer A (100%) 1.80 (d, 211, J HH = 13.2 Hz, CHAHBCMe3 ) 1.74 (d, 2H, J HH = 13.2 Hz, CH AHBCMe3 ) 1.63 (s, 3011, C 5 (CH3 ) 5 ) 1.45 (s, 1811, C(CH3 )3 )  Isomer A 112.44 (C5 (CH3 ) 5 ) 68.06 (C(CH3 ) 3) 33.89 (CH2 CMe3 ) 28.05 (C(CH3 ) 3 ) 9.81 (C 5 (CH3 ) 5 )  4.3  Isomer A (100 % 7.71 (d, 4H, ortho protons) 7.32 (t, 411, meta protons) 7.10 (t, 2H, para protons) 2.18 (d, 211, JHH = 12.4 Hz, CHAHBCMe2Ph) 2.04 (d, 2H, JHH = 12.4 Hz, CHAHB CMe2 Ph) 1.84 (s, 6H, CH2 C(CH3 ) A(CH3 ) BPh) 1.78 (s, 611, CH2 C(CH3 ) A(CH3 ) BPh) 1.56 (s, 30H, C 5 (CH3 ) 5 )  Isomer A 155.42 (Cipso) 128.40 (Cpheyi ) 125.91 (Cphenyi) 125.34 (Cphenyi ) 112.83 (C5 (CH3 ) 5 ) 69.91 (CH2C(CH3 ) 2 Ph) 43.78 (CH2 C(CH3 ) 2Ph) 32.02 (CH3 ) A 30.77 (CH3 )B 9.76 (C 5 (CH3 ) 5 )  4.4  Isomer A (65 %) 7.80 - 7.05 (m, 8H, aryl protons) 2.40 (s, 6H, aryl CH3 ) 1.59 (s, 30H, C 5 (CH3 ) 5 ) Isomer B (35 %) 7.80 - 7.05 (m, 8H, aryl protons) 2.29 (s, 611, aryl CH3 ) 1.62 (s, 3011, C 5 (CH3 ) 5 )  c  4.5  Isomer A (97%) 7.77 (d, 411, ortho protons) 7.31 (t, 4H, meta protons) 7.21 (m, 2H, para protons) 1.56 (s, 30H, C 5 (CH3 ) 5)  Isomer A 178.10 (Cir..) 135.30 (Corth ) 127.50 (Cmeta) 127.21 (Cpera) 114.45 (C5 (CH3 ) 5 ) 9.69 (C5(CH3)5) Isomer B signals not observed  Isomer B (3%) 1.62 (s, 30H, C 5 (CH3 ) 5 ) other signals not observed  4.6  7.52 - 7.46 (m, 2H, ortho protons) 7.12 - 6.98 (rn, 311, other aryl protons) 1.66 (s, 1511, C 5 (CH3 ) 5 )  c  4.7a  7.21 (d, 2H, J= 10.1 Hz, aryl protons) 7.11 (d, 2H, J = 10.1 Hz, aryl protons) 2.30 (s, 3H, p-CH3) 2.01 (s, 15H, C 5 (CH3 ) 5 )  C.^ Ipso not observed  141.15 (Com.) 141.01 (C^) 129.69 (C a) 119.40 (C5 (CH3 ) 5 ) 21.52 (p-CH3) 11.03 (C3(CH3)5)  92 4.8  7.58 (dd, 1H, J= 7.5, 1.5 Hz, ortho proton) 7.20 (d, 1H, J = 7.8 Hz, aryl proton) 7.04 (m, 1H, aryl proton) 6.95 (m, 1H, aryl proton) 2.26 (s, 3H, o-C!13 ) 1.67 (s, 15H, C 5 (CH3 ) 5)  c  4.9b  7.80 (m, 1H, aryl proton) 7.12 - 6.98 (m, 31I, aryl protons) 5.68 (s, 5H, C 5115 ) 2.27 (s, 3H, o-CH3 )  c  -  a Both spectra recorded in CD2 C1 2 .  b Relaxation delays of 20 s were used to ensure full integration of Cp proton signals. c Not recorded.  4.2.17 Other Characterization Data Table 4.4 Electrochemical Data for 3.8 and 4.1a  scan rate (v, Vs -1 )  complex 3.8 Ac E•• 1b  ip,anp,cd  (V)  complex 4.1 Ac •'ib  scan rate (v, Vs -1 )  (V)  ip,a/ipl  0.80  -1.52  0.69 (0.20)  1.0  0.80  -1.37  0.19 (0.18)  1.0  0.60  -1.55  0.47 (0.18)  1.0  0.60  -1.38  0.17 (0.16)  1.0  0.40  -1.54  0.49 (0.15)  1.0  0.40  -1.38  0.15 (0.14)  1.0  0.20  -1.56  0.36 (0.12)  1.0  0.20  -1.39  0.12 (0.11)  1.0  0.10  -1.59  0.28 (0.10)  0.9  0.10  -1.37  0.09 (0.10)  1.0  a In THE containing 0.10 M [n-Bu 4N]PF6 , at Pt-bead working electrode. Potentials measured vs Ag wire. b Defined as the average of the cathodic and anodic peak potentials (± 0.02 V). c Defined as the separation of the cathodic and anodic peak potentials. Values of AE given in brackets are for the Cp2Fe/Cp2 Fe+ couple under the same conditions. d Ratio of anodic peak current to cathodic peak current.  93 4.2.18 Kinetic Data for the Conversion of 3.8 to 4.1 Table 4.5 Variation of Water Concentration Data (Absorbance at 404 run vs time)  time(s)  0.624  0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500  0.0675 0.0781 0.0880 0.0982 0.1080 0.1177 0.1273 0.1369 0.1464 0.1557 0.1650 0.1743 0.1831 0.1920 0.2007 0.2094 0.90 1.245  Aco  kobs (x 10 4 r 1)  water concentration (x 10 2 M) 1.09 1.56 3.12 4.59 0.0560 0.0738 0.0905 0.1068 0.1226 0.1381 0.1532 0.1683 0.1827 0.1970 0.2113 0.2250 0.2384 0.2517 0.2644 0.2770 0.86 2.138  0.0342 0.0571 0.0786 0.0999 0.1207 0.1411 0.1606 0.1799 0.1986 0.2163 0.2338 0.2508 0.2670 0.2830 0.2979 0.3126 0.80 3.020  0.0667 0.1012 0.1386 0.1745 0.2083 0.2404 0.2704 0.2991 0.3256 0.3505 0.3739 0.3954 0.4157 0.4347 0.4519 0.4682 0.73 6.273  0.0529 0.1049 0.1526 0.1966 0.2370 0.2735 0.3069 0.3368 0.3637 0.3881 0.4099 0.4295 0.4468 0.4627 0.4768 0.4894 0.61 10.27  6.24 0.0815 0.1483 0.2038 0.2526 0.2955 0.3330 0.3657 0.3942 0.4191 0.4403 0.4589 0.4749 0.4883 0.5003 0.5107 0.5193 0.58 14.10  94  Table 4.6 Rate Constants (25 °C) for Various Concentrations of 3.8 using H2 O and D 2 0 [1120] = 5.55 x 10 3 M I [D201= 5.55 x 10 3 M kobs (0) -  13.151 (M) 2.56 x 10 -4 3.84 x 10-4 5.12 x 104 6.40 x 10 4 1.15 x 10 -3 1.58 x 10 -3 2.54 x 10 -3  -  1.668 x 10 4 1.844 x 104 1.821 x 10 4 1.798 x 10 4 1.446 x 10 4 1.436 x 10 -4 1.577 x 10 4  3.170 x 10 -5 3.690 x 10 -5 3.591 x 10 -5  Table 4.7 Rate Constants as a Function of Temperature Data (Absorbance at 404 nm vs time)  time(s)  15.0  0 300 600  0.029 0.031 0.055 0.083 0.113 0.145 0.174 0.206 0.237 0.266 0.295 1.17 0.959  900 1200 1500 1800 2100 2400 2700 3000 ko koh, (x 104 s-1 )  Temperature (°C) 20.0 25.0 0.055 0.058 0.097 0.139 0.182 0.225 0.267 0.308 0.347 0.385 0.421 1.17 1.438  0.070 0.073 0.127 0.181 0.236 0.287 0.337 0.383 0.427 0.468 0.506 1.17 1.839  30.0 0.070 0.112 0.186 0.260 0.328 0.391 0.447 0.502 0.549 0.593 0.631 1.17 2.509  95  4.3 Results and Discussion  4.3.1 Two Similar Reactions?  This section begins with the comparison of two reactions which, at first glance, would be expected to be quite similar. Previous work in these laboratories 7 ' 8 has established that the preparation of the tungsten dialkyl complexes Cp'W(NO)(CH2SiMe3)2 is straightforward via reaction of Cp'W(NO)I 2 with 2 equiv of a (trimethylsilyl)methyl Grignard reagent followed by hydrolysis with water, i.e. Cp'W(NO)I2  I) 2 Me3SiCH2MgC1' Et20 2) excess H2 0  >  Cp'W(NOXCH2SMe3)2  (4.1)  Conversion 4.1 consistently affords the Cp'W(NO)(CH 2 SiMe 3 ) 2 complexes in 80% isolated yields. 9 A similar reaction involving one of the congeneric molybdenum diiodide starting materials is not as straightforward. Thus, treating Cp * Mo(NO)I2 with 2 equiv of an ethereal solution of Me3 SiCH2MgC1 followed by quenching with water leads to the formation of three isolable organometallic products, 2,10 the minor one being the expected Cp *Mo(N0)(CH2 SiMe3 )2 (eq 4.2). Mo R/ R 0  (4.2) N 0 0 N R  0  96 Hence, the desired dialkyl complex is indeed among the final isolated products but in abysmal yield. [Cp *Mo(NO)I]2 results via the loss of iodide from the [Cp * Mo(NO)I2]. radical anion, which is the first intermediate formed in metathesis reactions of this type. 11,12 However, the key point to emerge from reaction 4.2 is that, unlike its tungsten  congener, Cp * Mo(NO)(CH2 SiMe3)2 is unstable with respect to hydrolysis. During efforts to synthesize tungsten diaryl and molybdenum dialkyl and diaryl nitrosyl complexes it was very obvious that these desired compounds were extremely sensitive to moisture, thus prompting me to develop a new synthetic strategy to these complexes (Chapters 2 and 3). Having a set of Cp'M(NO)R2 complexes available, I was then able to conduct a systematic investigation of their reactions with water.  4.3.2 Reactions of Cp * Mo(NO)R2 Complexes with Water  Five dialkyl and diaryl complexes of molybdenum (2.1, 2.3, 3.6, 3.7 and 3.8) have been reacted with water, and it has been found that all five react in a similar manner. Thus, the transformations of Cp * Mo(NO)C1 2 to [Cp * Mo(NO)R]2-(.1-0) (R = alkyl or aryl) via Cp * Mo(NO)R2 , as portrayed in eq 4.3, appear to be a general phenomenon. 13  2 Mg  H0  —s Cp*Mo(NO)R2 ---a- 1/2 [Cp*Mo(NOXRA2-(1-0) (4.3) Cp*Mo(NO)C12^ or R2Mg  R = alkyl or aryl  As indicated in eq 4.3, both RMgX and R2Mg reagents may be used to generate the requisite Cp *Mo(NO)R2 species in situ. Subsequent hydrolysis then affords a variety of bimetallic bridging oxo complexes (4.1 - 4.5) whose physical properties are collected in Tables 4.1 - 4.3. The yields of the bimetallic complexes are higher (based on Cp *Mo(NO)C12) if their precursor Cp * Mo(NO)R2 complexes are generated in situ and not isolated before their reactions with water. For example,  97 [Cp * Mo(N0)(CH2 SiMe3)}2-(A-0) is best synthesized by generating Cp * Mo(NO)(CH2SiMe3)2 in Et 2 0, treating the solution with water, and then chromatographing the final mixture on alumina III with Et20 as eluant (eq 4.4). Nevertheless, it can also be obtained by treatment of isolated Cp *Mo(NO)(CH2SiMe3)2 with water in benzene or THF (eq 4.5). 1) 2 Me 3 SiCH2MgC1, Et 2 0 Cp * MO(NO)C 12 excess H2O^[Cp*Mo(NOXCH2SNIe3A2-(PrO)  (4.4)  (75% based on Cp*Mo(NO)C1 2 )  Cp*Mo(NO)(CH2SiMe3)2  H2O  ^  [Cp*Mo(N0)(CH2SiMe3)]2(.1-0)^(4.5) (95% based on Cp*Mo(NOXCH 2 SiMe 3 )2 ) (57% based on Cp*Mo(NO)C1 2)  The transformation from Cp * Mo(NO)(CH2SiMe3)2 to 4.1 is clean and chemically straightforward (eq 4.5); however, the highest yield of complex 4.1 is obtained if one does not isolate the dialkyl complex before conducting the hydrolysis reaction. Several features of these otherwise straightforward conversions (4.3) merit some comment. The relative reactivities of Cp * Mo(NO)R2 complexes towards water are R = aryl >> CH 2 SiMe 3 > CH2 CMe2Ph >> CH 2 CMe3 . Thus, treatment of Cp * Mo(NO)(aryl)2 complexes with water causes their instantaneous consumption. 14 The (trimethylsilyOmethyl and neophyl derivatives (4.1 and 4.3) are formed in about 5-10 min in Et 2 0 (with which water is immiscible) at ambient temperatures. However, Cp * Mo(NO)(CH2CMe3)2 takes several hours at 20 °C to decompose to 4.2 in THF (with which water is miscible) even in the presence of 30 equiv of water. The reason other CpMo(NO)R2 complexes prepared in Chapter 3 were not reacted with water is because competitive thermal decomposition pathways complicate these  98 hydrolysis reactions. This feature is best exemplified by CpMo(NO)(CH 2 CMe3)2 which at room temperature decomposes to [CpMo(NO)(CHCMe3)]2 (Chapter 5) markedly faster than it reacts with water. 15 Additionally, attempts to synthesize the Cp analogues of 4.4 and 4.5 have been thwarted by the thermal instability of the precursor CpMo(NO)(aryl)2 complexes (Chapter 2). 14  4.3.3 Physical Properties of [Cp * Mo(NO)R]2-(A-0) (R = alkyl, aryl) Complexes These bimetallic complexes are moderately air-stable as solids, but in solution they are generally air-sensitive and decompose eventually to [Cp * Mo(0)2] 2 -(12-0). This latter result has been established specifically for complexes 4.1, 4.2 and 4.5 (vide infra). In contrast, complex 4.4 is sufficiently air-stable to be crystallized in air by slow evaporation of solvent. Perhaps the steric bulk of the ortho-methyl groups in [Cp * Mo(N0)(o-toly1)} 2 -(11-0) provides enough protection for the metal centers to shield them from attack by further water or oxygen. The alkyl derivatives (4.1 - 4.3) are dark red-brown to red-black crystalline solids which exhibit high thermal stability. For instance, thermolysis of a sample of 4.1 in benzene in an NMR tube at 80 °C for 2 days results in no detectable change in the NMR spectrum of the sample. The aryl derivatives (4.4 and 4.5) are chocolate brown and tend to be less crystalline than their alkyl congeners. The elemental analysis, IR, mass spectral, and NMR data for complexes 4.1 - 4.5 are collected in Tables 4.1 - 4.3.  4.3.4 Spectroscopic Properties of [Cp * Mo(NO)R)1 2 -(1-0) Complexes Since 4.1 is the prototypal complex in this family, its properties will be considered in some detail. The 300 MHz 1 H NMR spectrum of [Cp *Mo(N0)(CH2 SiMe 3 )] 2 -(2-0) in C6D6 exhibits an AB pattern at 0.84 and 0.75 ppm ( 2Jp = 12.3 Hz) for the two pairs of diastereotopic methylene protons and singlet resonances at 1.59 and 0.45 ppm due to the  99 methyl protons of the Cp* ligands and the methyl groups attached to the silicon atoms, respectively. In comparison, the two pairs of diastereotopic methylene protons in Cp * Mo(N0)(CH2SiMe3)2 give rise to an AX doublet of doublets at 2.21 and -1.17 ppm (2J = 10.8 Hz) while the Cp * and SiMe3 protons resonate at 1.49 and 0.37 ppm, respectively. The principal feature that differentiates the two spectra is the existence of two small singlets at 1.71 and 0.42 ppm in the proton NMR spectrum of [Cp *Mo(N0)(CH2SiMe3)]2-(.1-0). These latter resonances can be assigned to the Cp * and SiMe3 protons, respectively, of a minor isomer of 4.1. However, signals due to the methylene protons of the minor isomer are not observable. The isomers are probably diastereomers, since they do not interconvert thermally (vide supra), and have thus been assigned the molecular structures shown in Figure 4.1.  C  NO  ONNO Isomer A (SR)  ^  Isomer B (SS)  Figure 4.1 Designation of isomers for [Cp * Mo(N0)(R)]2-(1-0) complexes. Newman projections are drawn down the Mo-O-Mo axes and are approximate representations.  The solid-state molecular structure of [Cp *Mo(NO)(CH2SiMe3)]2-(.1-0) reveals it to be the SR isomer with the two chiral [Cp * Mo(NO)(CH2SiMe3)] units being twisted by 90 ° with respect to each other about the Mo-O-Mo bond. 16 The other diastereomer probably has the same relative configurations at the metal centers, i.e. RR or SS, the latter  100  isomer being shown in Figure 4.1. The SR isomer is probably the dominant form in solution, as molecular models show that either the RR or SS isomers possess severe steric interactions between the methyl groups of the Cp s ligand on one molybdenum with the bulky CH2 SiMe 3 group on the other Mo center. As summarized in Table 4.3, where the major and minor isomers are designated as A and B, respectively, the ratios of [Cp sMo(NO)R] 2 -(1-0) isomers are generally dependent on the steric bulk of the ancillary ligands. Consistent with the view that steric factors control the ratio of the two isomers is the fact that for [Cp sMo(N0)(CH2 SiMe 3 )] 2 -(11-0) the ratio major:minor is 9:1 while the less sterically encumbered [CpMo(N0)(CH 2 SiMe3 )] 2 -(4-0) exhibits an isomer ratio of major:minor = 3:1. 10 The most sterically crowded complex [Cp s Mo(N0)(CH2 CMe 3 )] 2 -(1-0) exhibits an isomer ratio of at least 50:1. The IR spectral features exhibited by these bimetallic 12-oxo complexes (Table 4.2) are different from those displayed by their parent bis(hydrocarbyl) complexes, Cp s Mo(NO)R2. The IR spectra of [Cp sMo(NO)R] 2 -(.1.-0) exhibit strong bands in the region 803 - 743 cm -1 which are diagnostic of Mo-O-Mo vibrations.  17  The nitrosyl-  stretching frequencies of these bimetallic complexes are some 5-55 cm -1 (average = 23 cm-1 ) lower in energy than the v NO bands exhibited by their parent bis(hydrocarbyl) complexes. This spectral feature is indicative of there being more electron density available on the molybdenum centers for backbonding into the antibonding orbitals of the nitrosyl ligands in these bridging oxo complexes than in their parent dialkyl and diaryl complexes. Therefore, this would suggest that the bridging oxo ligand is providing each molybdenum center with one electron in the a Mo-O link and some electron density associated with the lone pairs of electrons in its p orbitals. This view is in accord with the X-ray crystallographic analysis of [Cp sMo(N0)(CH2 SiMe3 )] 2 -(1-0). 16 The oxo bridge between the two molybdenum centers is essentially linear and the two {Cp *Mo(NO)(CH2 SiMe3)} units are twisted by  101 approximately 90° with respect to one another. This, taken together with the short Mo-O bond lengths (ca. 1.90 A) which are intermediate between typical Mo-0 and Mo=O bond lengths, 18 suggests that there is a considerable degree of multiple bonding in the central Mo-O-Mo linkage of the bimetallic complex. If the Mo-O-Mo axis is defined as the z axis, the Mo=O=Mo n interaction involves overlap between an empty d orbital on each of the {Cp*Mo(NO)(CH2 SiMe3 )} groups with the appropriate filled p orbital (i.e. px for one group and py for the one orthogonal to it) on the bridging oxygen atom. This localized view of the 7C bonding in the Mo=O=Mo bridge suggests that each Mo center thus achieves the favored 18-valence-electron configuration. Therefore, in these complexes the oxo ligand may be regarded as a formal 6-electron donor, three electrons being donated to each metal center so that the metal can reach electronic saturation. These complexes are very similar to the monomeric 18-electron alkoxyalkyl complexes Cp*Mo(NO)(R)(OR), in which the alkoxide group also functions as a formal 3-electron donor to the 15-electron Cp * Mo(N0)(R) fragments. 19  4.3.5 Related Bridging Oxo Complexes The bridging oxo complexes presented in this chapter have valence isoelectronic analogues in Group 4. Specifically, [Cp 2Zr(Me)] 2 -(1-0), 20 [Cp 2Hf(Me)J 2 -(g-0), 21 and [Cp* 2Zr(H)] 2 -(11-0) 22 have been prepared from their Cp' 2MR2 precursors by reaction with water although systematic study of their formation or chemistry has not been reported in the literature. Non-nitrosyl containing complexes having Mo-O-Mo linkages have been characterized as well. For example, [(11Bpz 3 )Mo(0)C1] 2 -(11-0) 23 and [Cp * Mo(0)C1] 2 -(.1-0) 24 have been structurally characterized, and both were found to contain nearly linear Mo-O-Mo linkages. In contrast, [Ir(PPh 3 )(NO)1 2 -(11-0) (the only other oxo-bridged nitrosyl complex that has been structurally characterized) exhibits an acute Ir-O-Ir angle of 82.3 (3)°, long Ir-O  102 bond lengths of about 1.94 A and an Ir-Ir separation of 2.55 A. 25 The Ir(PPh 3 )(NO) fragment seemingly would rather engage in metal-metal bonding to satiate its electronic requirements rather than accepting extra electron density from the oxo ligand. This is probably a function of iridium being a softer Lewis acid than molybdenum.  4.3.6 Electrochemical Studies During previous investigations, we have found the knowledge of fundamental redox properties of organometallic complexes to be a useful guide in directing our studies of their characteristic chemical properties. 11 It was therefore of interest to determine the electrochemical behavior of Cp * Mo(NO)(CH2 SiMe 3 )2 and [Cp*Mo(N0)(CH 2 SiMe3 )} 2 -(.1.-0) as representative examples of their particular classes of compounds. The cyclic voltammogram of Cp *Mo(NO)(CH2 SiMe 3 )2 in THF reveals that it undergoes a quasi-reversible, one-electron reduction at approximately E°' = 1.55 V vs -  Ag wire. While the ip,a/ip,c ratio for this feature indicates reversibility, it is clear from the AE values (Table 4.4) that electron transfer in this system occurs at a slower rate than for the ferrocene standard. For example, at 0.8 Vs -1 the peak separation is about 700 mV and not the 200 mV observed for Cp2Fe. Compared to the dialkyl and diaryl complexes in Chapters 2 and 3 this phenomenon of quasi-reversibility, while rare in these systems, is observed. 26 Regardless of the kinetics of the electron-transfer step, the reduction potential of Cp * Mo(N0)(CH2SiMe3) 2 is comparable to that exhibited by related dialkyl complexes such as Cp *Mo(NO)(CH2 CMe3 ) 2 (E°' = -1.45 V) 27 and Cp * W(NO)(CH2CMe2Ph)2 (E°' = -1.62 V). 14 Interestingly, no second reduction of Cp * Mo(NO)(CH 2 SiMe 3 )2 occurs out to the solvent limit even though the dianion [Cp * Mo(N0)(CH2 SiMe3)2} 2- should exist. 14,28 The cyclic voltammogram of complex 4.1 in THF indicates that the compound undergoes a reversible, one-electron reduction and a second, irreversible, one-electron  103 reduction. The first reduction potential for 4.1 occurs at E°'= -1.38 V; the linearity of the plot of ip,c vs v 1/2 establishes that this reduction is diffusion controlled. Furthermore, comparison of the i p,a/ip , c and AE values for this reduction with the internal Cp2Fe/Cp2Fe+ reference indicates that it is reversible (Table 4.4). The second reduction of complex 4.1 occurs at -2.18 V but is irreversible, thus implying that the compound is not retaining its structural integrity upon addition of the second electron. Neither molybdenum complex undergoes any oxidation processes up to the solvent limit of 0.80 V vs Ag wire. Thus, with the information gleaned from this study, it seems that the preparation and isolation of [Cp * Mo(NO)(CH2SiMe3)2T - and 4.1' radical anions should be possible. It is equally clear that Cp *Mo(N0)(CH2SiMe3)2 does not undergo any bond-cleavage processes upon initial electron addition. Hence, it seems unlikely that its reactions with water involve any redox processes (vide infra).  4.3.7 Labeling Studies  A series of experiments was conducted in order to gain some mechanistic insights into the transformation of Cp * Mo(N0)(CH2SiMe3)2 to [Cp * Mo(N0)(CH2SiMe3)] 2 -(µ-0) by H2O. The results of these experiments are summarized pictorially in Scheme 4.1. Treatment of Cp *Mo(NO)(CH2SiMe3)2 (3.8) with excess water results in its clean conversion to 4.1 (-95% by 1 H NMR spectroscopy) with concomitant formation of 2 equiv of Me 4 Si. The 1 H NMR spectrum also contains a small singlet at 5 1.78 ppm attributable to — 2% of [Cp * Mo(0) 2 ] 2 -(.1-0), which results from further hydrolysis of 4.1 (vide infra). Reaction of Cp * Mo(NO)(CH2SiMe3)2 with D20 results in the synthesis of 4.1, but with DH2CSiMe3 being produced as the byproduct of the reaction. DH2CSiMe3  is readily identified by the appearance of a singlet in the 1 H NMR spectrum at 5 0.00 ppm with a smaller 1:1:1 triplet (JHE, = 2.1 Hz) at slightly higher field, 29 the latter signal being due to the mono-deuterated methyl group of DH 2 CSiMe3 . Finally, treatment of the  104 bis(trimethylsilyl)methyl complex with 18 0H2 cleanly converts it to 4.1', the 18 0 labelled analogue of 4.1. This product may be conveniently characterized by mass spectrometry. The mass spectrum of 4.1' shows a parent-ion peak at m/z 714 (as compared to 712 for unlabeled 4.1 recorded under identical experimental conditions).  2  - 2 RH  H2 O V  0  0  O  R = CH2 SiMe3  Scheme 4.1 Labelling experiments  Taken together, these experiments clearly demonstrate that the water used to convert 2 moles of Cp *Mo(NO)(CH2SiMe3)2 to 1 mole of complex 4.1 ends up as 2 moles of Me4 Si (RH) and the bridging oxo ligand of complex 4.1. In other words, the balanced chemical equation for this transformation is as shown in eq 4.6. 2 Cp*Mo(NOXCH2 Silv1e3 )2 + H2 O --).- [Cp*Mo(N0)(CH2 Sa4e3)]-(A-0) + 2 Me 4 Si^(4.6)  105  One could propose a number of plausible mechanistic pathways for reaction 4.6. In order to gain some further insight into the mechanism of this reaction, a kinetic analysis of the hydrolysis of Cp * Mo(NO)(CH2SiMe3)2 was undertaken.  4.3.8 Kinetic Studies  4.3.8.1 Monitoring of the Hydrolysis Reaction  The kinetic analysis of reaction 4.6 was hampered by several factors which merit explanation. The first is that the product of reaction 4.6, complex 4.1, undergoes further reaction with water to produce [Cp *Mo(0) 2 ] 2 -(11-0) (vide infra). The relative effect of this competitive and undesirable reaction is heightened at low concentrations of the organometallic starting material and under pseudo-first-order conditions where water is in excess. The second factor is the difficulty of monitoring the concentration of Cp *Mo(NO)(CH2SiMe3)2 as conversion 4.6 proceeds. For instance, attempts to probe the kinetics of reaction 4.6 by 1 H NMR spectroscopy were unsuccessful. Since water is immiscible with benzene, it could not be used as a pseudo-first-order reagent in that solvent. In THE (a solvent with which water is miscible), the Cp * resonances for Cp *Mo(NO)(CH2SiMe 3 )2 (5 1.84 ppm) and 4.1 (8 1.86 ppm) are not sufficiently resolved to be integrated accurately. Furthermore, the signals in the spectral region around 5 0.00 ppm are not sufficiently resolved to permit monitoring of the Me4Si byproduct. Consequently, I decided to effect the kinetic analysis of reaction 4.6 in THF using UV-vis spectroscopy, since starting material and product exhibit different spectral properties (Table 4.8). The UV-vis spectra of Cp *Mo(NO)(CH2 SiMe3 ) 2 and 4.1 in THF are similar except that 4.1 exhibits a strong band at X^= 404 nm (a —14000 M-1 cm -1 ) (Figure 4.2).  106 Table 4.8 UV-vis Data (in THF) for Complexes 3.8 and 4.1  Complex (conc.)  X (nm)  A  a (Mlcm-t)  3.8  223 300 526 236 276 404  1.613 0.909 0.086 3.210 3.102 0.744  14038 7909 752 60112 58089 13932  (1.149 x 10 -4 M) 4.1  (5.340 x 10 -5 Mr  a Concentration is 1/2 of [3.s] since it requires 2 moles of 3.8 to create 1 mole of 4.1.  3.5  -  32.5 414 4.• a -2O  1.5 -  Cp*Mo(N0)(CH2SiMe3)2 (3.8) [Cp * Mo(N0)(CH2SiMe3)]2-(1-0) (4.1)  10.5 0 200  300^400^500^600  700  800  wavelength (nm) Figure 4.2 Overlaid UV-vis spectra of 3.8 and 4.1 as THE solutions  From the spectral overlay (Figure 4.2) it is clear that monitoring of the disappearance of starting material would be very difficult, since no absorption band of Cp * Mo(N0)(CH2SiMe3)2 is sufficiently resolved from bands of its hydrolysis product 4.1. However, a Beer's law plot showed the absorptivity of 4.1 at 404 nm to be linear (r 2 > 0.99) up to A — 1.8, and so this spectral feature was monitored as the hydrolysis reactions  107 progressed. Reactions were performed under pseudo-first order conditions with an excess of water present. Because complex 4.1 does react further with water to form [Cp *Mo(0)2]2-(A-0), the occurrence of this latter process was minimized by keeping the reactants as concentrated as possible within the linear absorption limits and monitoring only the first half-life or so of each run. For these reasons, the data extracted from the kinetic measurements applies to the initial part of the reaction between water and Cp * Mo(N0)(CH2SiMe3)2.  4.3.8.2 Variation of [3.8], [11 2 0] and [D 2 0]  Under the experimental conditions employed, the progress of reaction 4.6 in THE is initially first-order in both water and Cp * Mo(NO)(CH2SiMe3)2 since all plots of In (A00 -At) versus time are linear. First, reaction 4.6 was studied by varying the water concentration and keeping the initial concentration of Cp *Mo(N0)(CH 2 SiMe3 ) 2 constant at 1.116 x 10 -3 M. Thus, [H2O] was varied, but still kept in excess, from 0.62 x 10 -2 to 6.24 x 10 -2 M in six experiments . The experimental data for these runs are compiled in Table 4.5. Figure 4.3 shows the lowest and highest water-concentration runs. Both lines are nearly linear (r2 > 0.999). As expected, the rates of the hydrolysis runs increase with increasing [H2O]. The plot of k ob s vs [H2O], confirming that reaction 4.6 is first-order in water, is shown in Figure 4.4 (data taken from Table 4.5).  108  0  ■ ■ a ■ ■ ■ ■ ■ •  a • Emma  [water] = 6.24 E-3 M  -1  [water] = 6.24 E-2 M -2  /  -3 300  0  900  600  1200  1500  time (s)  Figure 4.3 Selected first-order plots for the hydrolysis reaction ([3.8] = 1.116 x 10 -3 M)  0.0016 0.0012 ......  Q 0.0008 0.0004 0  0^0.01^0.02^0.03^0.04^0.05^0.06^0.07  [water] (M) Figure 4.4 Plot of k ob s vs [H 2 O] for the reaction of Cp * Mo(NO)(CH2SiMe3)2 with H2O  109 Additionally, over an order of magnitude change in the concentration of the dialkyl complex ([3.8] 0 = 2.56 x 10 4 - 25.39 x 10 4 M), reaction 4.6 is also first-order in the metal complex. The concentration of water for the seven experiments defining the concentration range of Cp *Mo(N0)(CH2SiMe3)2 was held constant at 5.55 x 10 -3 M (pseudo-first-order excess). Values of k obs for these runs are located in Table 4.6. From the plots of In (A00 -A) versus time, the average value of kobs is 1.66 x 10 4 ri at 25 °C. At very low concentrations of Cp * Mo(NO)(CH2SiMe3)2 (i.e. [Cp *Mo(N0)(CH2 SiMe3 ) 2 J 0 < 1.0 x 10 4 M), plots of In (A 00 -At) versus time are not linear, a feature which probably reflects that the conversion of 4.1 to [Cp*Mo(0) 2 J 2 -(1-0) by water becomes much more significant at very low concentrations of the organometallics. The hydrolysis reaction was also studied using D 2 0 instead of H2 O in three selected experiments. Values of k ob s for these runs are located in Table 4.6. Again, the reaction appears to be first-order with respect to D 2 0, the average value of kob s being 3.48 x 10 -5 s -1 . The primary kinetic isotope effect (kH/kD) for the hydrolysis reaction is thus 0.000166/0.0000348 = 4.77, thus indicating that the rate of the hydrolysis reaction is slowed substantially by using the heavier isotope. This observation is significant in that it suggests that there is a considerable degree of 0-H bond breaking in the transition state, since the maximum kinetic isotope effect, k H/kD , expected for breaking an 0-H bond is 11. 30  4.3.8.3 Variation of Temperature: Measurement of Activation Parameters The kinetics of the hydrolysis reaction were also investigated at five-degree intervals between 15 and 30 °C. The data for these experiments are tabulated in Table 4.7 and graphically depicted in Figure 4.5.  110  0.2 -  -0.8 0^500^1000^1500^2000^2500^3000  time (s) Figure 4.5 Variable temperature first-order rate plots for reactions of  Cp *Mo(NO)(CH2SiMe3)2 (3.8) with excess H2 O  The data from the four variable-temperature runs afford a straight-line Eyring plot (Figure 4.6) of In (k obs/T) versus 1/T from which the following activation parameters may be calculated: AH: = 43 kJ mol -1 , AS: = -41 eu, AG: 2 98 = 94 kJ mol -1 . 31 The sign and magnitude of AS: clearly indicates that the rate-determining step of conversion 4.6 is highly associative in nature. Additionally, an Arrhenius plot of In kobs vs 1/T yields a straight line from which the activation energy, E a, is calculated to be 49.0 Id mo1-1.  111 -13.5  • -15 0.00328^0.00332^0.00336^0.0034^0.00344^0.00348  1/T (1/K) Figure 4.6 Erying plot (15 - 30 °C) for the reaction of Cp * Mo(NO)(CH2SiMe3)2 with H2 O  4.3.8.4 Mechanistic Proposals The most likely scenario for the mechanistic transformation of Cp * Mo(NO)R2 to [Cp * Mo(NO)R] 2 -(1-0) is presented in Scheme 4.2. 0 NR  + H2O, +3.8  Mo „......--^---,..... R E^R N 0  =-- 0 --- Mo  - 2 RH  0  3.8  4.1  H2011 -H20 RH  -574---  0 RN  . .  Mo // --- .N„N ^'OH, ki R^R^‘-  A  -R11 --).  Mo \ OH ;.N 0  \ fMo^Mo  +3.8  R /' '0c HI 0  B^  C  Scheme 4.2 Mechanistic pathway from Cp *Mo(NO)R2 to [Cp * Mo(NO)R]2-(1-0)  R  ^  112 Water, a good Lewis base, could form an initial 1:1 adduct (A) with the 16-electron dialkyl complex. Intramolecular expulsion of RH would afford the hydroxo alkyl intermediate (B) 32 which could function as a Lewis base towards a second molecule of the 16-electron dialkyl Lewis acid to form a bimetallic (1-0H) complex (C). In the final step this latter compound could eliminate RH to form the final bridging oxo product. An alternative mechanism in which two molecules of B couple and eliminate water is very remote since the concentration of B is never appreciable in the reaction mixture. This claim is substantiated by monitoring of the reaction in THF-d 8 by variable temperature 1 H NMR spectroscopy (-80 to +20 °C). Variable temperature NMR spectroscopy failed to detect any signals that could be attributed to anything other than Cp *Mo(NO)(CH2SiMe3)2, either isomer of 4.1, or Me4 Si. While the mechanism of the hydrolysis reaction is not positively determined, the rate-determining step is highly associative in nature (AS: = -41 eu) and therefore likely involves 0-H bond breaking and Mo-0 bond formation. The mechanism put forth in Scheme 4.2 is rewritten in Scheme 4.3 to simplify the rate law determination. ^3.8  ^B  +  H20^A  A  +  ^k2^ - RH  B  k3 3.8^C  ka ^C ^4.1 - RH Scheme 4.3 Elementary reaction sequence for the conversion of Cp * Mo(NO)R 2 to  [Cp * Mo(NO)R] 2 -(.1-0)  113 A simple analysis of the proposed mechanism for reaction 4.6 must assume a steadystate approximation for the intermediate complexes A, B and C:  • rate = d[4.1]/dt = k 4 [C] • assume d[C]/dt = 0 = k3 [B][3.8] - k 4 [C] .-. 14[C] = k 3 [B][3.8] and rate = k 3 [B][3.8] • assume d[B]/dt = 0 = k2 [A] - k3 [B][3.8] k3 [B][3.8] = k 2 [A] and rate = k 2 [A] • assume d[A]/dt = 0 = k 1 [3.8][H2 0] - k- 1 [A] k2 [A] .-. [A] = (k 1 /(k-1 + k2 ))[3.8][H20] rate = k2 [A] = (k i k2/(k- 1 + k2 ))[3.8][H2 0] = kobs[3.8][H2O]  Thus, this mechanism predicts a second-order rate law which exhibits first-order dependence on both reagents. This is indeed the case.  4.3.9 Comparisons of Reactivity Scheme 4.4 summarizes the reactions of Cp *Mo(NO)(CH2 SiMe3) 2 and its oxo derivatives with water and oxygen and illustrates the variety of products, both mono- and bimetallic, that may be obtained. To date, three compounds resulting from oxidation or hydrolysis of Cp*Mo(NO)(CH 2 SiMe3 )2 have been isolated. As previously reported, the dialkyl complex reacts with dry 0 2 to give the dioxo complex Cp * Mo(0)2(CH2 SiMe 3 ) in moderate yield. 33 This work has shown that Cp *Mo(NO)(CH2 SiMe3)2 reacts with H 2 O to provide high yields of [Cp*Mo(N0)(CH 2 SiMe 3 )} 2 -(1-0). While Cp *Mo(0)2(CH2SiMe3) and [Cp * Mo(N0)(CH2 SiMe3)] 2 -(µ-0) cannot be interconverted, both species do react with excess water or excess oxygen to form the known  ^  114 [Cp * Mo(0)2]2-(4-0) complex as the only isolable organometallic product. 34 Exposure of Cp*Mo(N0)(CH2SiMe3)2 to the atmosphere leads to the formation of a mixture of all three oxo derivatives in variable yields depending on the length of exposure and the relative humidity. Thus, sagacious choices of both reagents and reaction conditions are necessary to obtain a particular oxo derivative of Cp * Mo(N0)(CH2 SiMe 3 )2. The reactions of Cp*Mo(NO)(CH 2 SiMe3 ) 2 outlined in Scheme 4.4 appear to be quite general for the family of Cp'Mo(NO)R 2 (R = alkyl, aryl) complexes (vide supra).  .......--.Mo _^---..,  02 (1 atm) _______),„  ,, Mo 0^I^Cs %^'''''  R^i R N 0  R  1 xs 02  i H2 O  xs H 2 O (or 0 2 )  ---10.  O  01  !  Scheme 4.4 Interrelationships of known oxo derivatives of Cp * Mo(N0)(CH 2 SiMe 3 ) 2  In marked contrast, the reactions of Cp'W(NO)(alkyl)2 species with water and oxygen differ dramatically from those of their Cp'W(NO)(aryl) 2 analogues. The Cp'W(NO)(alkyl) 2 complexes resist reaction with water, persisting even in the presence of a vast excess of water for prolonged periods of time under ambient conditions. However, all isolable Cp'W(NO)(aryl) 2 complexes react instantaneouly with water to form  115 monometallic aryl dioxo complexes 2 (eq 4.7) which are also the products formed when these diaryl complexes are reacted with molecular oxygen.  H2O to  w  O I 0  (4.7) 0  R  R = aryl  Interestingly, CpW(NO)(o-tolyl) 2 reacts with water to form two products in an essentially equimolar ratio (eq 4.8).  ri..  <C]::>1  H2 O  -)o.^.,‘;/.-,., 0^N  The synthesis of the dioxo complex is more easily accomplished by treatment of CpW(NO)(o-tolyl) 2 with H2 02 (aq). The imido oxo complex, 5 while clearly the more interesting of the two complexes formed by reaction 4.8, is not formed in solutions containing H2 0 2 . The characterization data for these Cp'W(0) 2 (aryl) complexes are collected in Tables 4.1 - 4.3. The spectroscopic features of these latter complexes are similar to those exhibited by their alkyl dioxo analogues. 33 Most notably, they exhibit two strong v Ar z) stretches in their IR spectra. Aryl dioxo complexes tend to be less crystalline and less soluble in common organic solvents than their alkyl congeners, but they are equally air- and moisture-stable.  116 The fact that the molybdenum and tungsten dialkyl and diaryl species exhibit such different hydrolysis chemistry is unique. Typically, related compounds of the second- and third-row transition metals react in a similar manner, with the third-row compound being the slower to react. 35 Such is indeed the case for the hydrolyses of Cp 2MMe2 (M = Zr, Hf). Thus, reaction of Cp2MMe 2 with water in air leads to the formation of isoelectronic analogues of 4.1 4.5, namely [Cp 2 M(Me)] 2 -(u-0). 20,21 Clearly, the differences created -  by changing the metal in related, valence-isoelectronic Cp 1M(NO)R2 (M = Mo, W) complexes have a much more pronounced effect on the characteristic chemical behavior of these species. 36  4.4 Epilogue and Future Work  The work presented in this chapter clarifies the chemical reactivity of the family of Cp'M(NO)R2 complexes with water. Originally, it was thought that these complexes were hydrolytically stable, but this false assumption was based on the reactivity of complexes available at that time, namely the tungsten dialkyl complexes which are very hydrolytically stable. Now that both dialkyl and diaryl complexes of Mo and W are known, such a sweeping generalization no longer holds. The hydrolytic decomposition of the Mo species is general and leads to [Cp*Mo(NO)R] 2 -(1-0) complexes, whereas W diaryl complexes react to form Cp'W(0) 2 (aryl) species. Presumably, this dependence on metal is reflective of the different mechanisms by which these Mo and W complexes react with water. A preliminary look at the mechanism of the hydrolysis reactions of the Mo species has determined that Cp * Mo(NO)R2 complexes in all likelihood react with water to form hydroxy complexes {Cp *Mo(NO)(R)(OH)}, which then react further with starting material to yield the [Cp * Mo(NO)12] 2 -(p-0) bridging oxo complexes. In the case of the W systems, such an intermediate does not seem likely, given the outcome of their  117 hydrolysis reactions. The mechanism of the formation of Cp'W(0)2(aryl) complexes is yet to be determined. For future studies in this area, I would suggest an examination of the reactivity of [Cp *Mo(N0)(CH2CMe3)]2-(g-0) (4.2), since it is the easiest to prepare, highest yielding and the most stable of the bridging oxo complexes presented in this chapter. Additionally, complex 4.2 exists as only one diastereomer in solution, thus simplifying spectral interpretations.  4.5 References and Notes (1)  See for example: The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patai, S., Eds.; Wiley: New York, 1985; Vol 2.  (2)  The work described in this chapter has been published. See Legzdins, P.; Lundmark, P. J.; Phillips, E. C. Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2991.  (3)  Nicholson, R. S. Anal. Chem. 1966, 38, 1406.  (4)  Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms; Brooks/Cole Publishing Co.: Monterey, CA, 1985.  (5)  The reason hydrogen peroxide is used, instead of water, is to maximize the yield of the dioxo complex. If water is reacted with CpW(NO)(o-tolyl) 2 , two organometallic products are isolable: the dioxo complex CpW(0)2(o-tolyl) and, more interestingly, CpW(N-o-tolyl)(0)(o-tolyl) which is a structural isomer of the starting material. See Legzdins, P.; Rettig, S. J.; Ross, K. J.; Veltheer, J. E. J. Am. Chem. Soc. 1991, 113, 4361.  (6)  [Cp*M0(0)2]2-(11-0) was first synthesized by Faller's group. See Faller, J. W.; Ma,  Y. J. Organomet. Chem. 1988, 340, 59. Later, Rheingold's group published a  118 detailed crystallographic analysis of all three polymorphs of [Cp * Mo(0)2]2-(.1-0), see Rheingold, A. L.; Harper, J. R. J. Organomet. Chem. 1991, 403, 335. (7)  Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394.  (8)  Hunter, A. D.; Legzdins, P.; Martin, J. T.; Sanchez, L. Organomet. Synth. 1986, 3, 66.  (9)  Even higher yields of Cp*W(NO)(CH2SiMe3)2 are obtained using Cp*W(NO)C1 2 , see Legzdins, P.; Veltheer, J. E. In Handbuch der Preiparativen Anorganischen  Chemie, 4th ed.; Herrmann, W. A., Ed.; Thieme-Verlag: Stuttgart, Germany, in press. (10) Phillips, E. C. Ph.D. Dissertation, The University of British Columbia, 1989. (11) Herring, F. G.; Legzdins, P.; Richter-Addo, G. B. Organometallics 1989, 8, 1485. (12) Christensen, N. J.; Debad, J. D.; Legzdins, P.; Lundmark, P. J.; Reina, R.; Ross, K. J.; Trotter, J.; Veltheer, J. E.; Yee, V. C., manuscript in preparation. (13) The only isolable CpMo(NO)R2 complex which reacts faster with water than it thermally decomposes is CpMo(NO)(CH2SiMe3)2. Reaction of CpMo(N0)(CH2SiMe3) 2 with water provides [CpMo(N0)(CH2SiMe3]2-(A-0) in about 60% isolated yield. 2 This reaction was first performed by E. C. Phillips. 10 (14) Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992,  11, 2583. (15) Legzdins, P.; Rettig, S. J.; Veltheer, J. E. J. Am. Chem. Soc. 1992, 114, 6922. (16) Dr. Steve Rettig solved the structure of [Cp * Mo(NO)(CH2SiMe3)} 2 -(A-0). 2,10 (17) Barraclough, C. G.; Lewis, J.; Nyholm, R. S. J. Chem. Soc. 1959, 3552. (18) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, 0.; Watson, D. G.; Taylor, R. J.  Chem. Soc., Dalton Trans. 1989, 51.  119 (19) Lundmark, P. J. Ph.D. Dissertation, The University of British Columbia, in preparation. (20) Hunter, W. E.; Hrncir, D. C.; Bynum, R. V.; Pentilla, R. A.; Atwood, J. L. Organometallics 1983, 2, 750. (21) Fronzek, F. R.; Baker, E. C.; Sharp, P. R.; Raymond, K. N.; Alt, H. G.; Rausch, M. D. Inorg. Chem. 1976, 15, 2284. (22) Hillhouse, G. L.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5472. (23) Lincoln, S.; Koch, S. A. Inorg. Chem. 1986, 25, 1594. (24) Umakoshi, K.; Isobe, K. J. Organomet. Chem. 1990, 395, 47. (25) Carty, P.; Walker, A.; Mathew, M.; Palenik, G. J. J. Chem. Soc. D 1969, 1374. (26) See Section 3.3.2. (27) Chapter 3 of this thesis. (28) Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B. E.; Gatter, M. G. J. Am. Chem. Soc. 1985, 107, 1411. (29) JHD couplings are typically small (J)a{ — 6.5 JxD). (30) Moore, J. W.; Pearson, R. G. Kinetics and Mechanism, 3rd ed.; Wiley-Interscience: Toronto, 1981, p 369. (31) Reference 4, p 16. (32) It may be noted that Bercaw has invoked an analogous hydroxo complex, namely Cp * 2Zr(H)(OH), as an intermediate species formed when either Cp * 2Zr(H) 2 or Cp * 2Zr(N2) reacts with water. The ultimate product in these reactions is [Cp * 2Zr(H)J2-(.1-0), a valence-isoelectronic analogue of complexes 4.1 - 4.5. 22 (33) Legzdins, P.; Phillips, E. C.; Sinchez, L. Organometallics 1989, 8, 940.  120 (34) This ubiquitous complex appears to be the ultimate oxidative decomposition product of a wide variety of Cp * Mo(NO)-containing species. Legzdins, P.; Lundmark, P. J.; Phillips, E. C.; Veltheer, J. E.; Young, M. A., unpublished observations. (35) "In general, the second and third transition series elements of a given group have similar chemical properties, but show pronounced differences from their light congeners." Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: Toronto, 1988; p 776. (36) It has been noted that Zr and Hf complexes are more nearly identical than for any other two congeneric elements, mostly due to the lanthanide contraction (see ref 35, chapter 19).  121  CHAPTER 5 Some Characteristic Chemistry of CpMo(NO)(CH2CMe3)2 and its Transient Thermal Decomposition Product, CpMo(NO)(=CHCMe3)  5.1 Introduction ^  121  5.2 Experimental Procedures ^  123  5.3 Results and Discussion ^  136  5.4 Epilogue and Future Work ^  165  5.5 References and Notes ^  166  5.1 Introduction  5.1.1 Historical Development The first compound containing a metal-carbon double bond was prepared in 1964 by E. 0. Fischer (eq 5.1). 1 Due to the large volume of work to emanate from the Fischer group, complexes containing M=C(ER)Regroups became inexorably tagged with Fischer's name. i) Phil^/ OMe W(CO)6 2)H+ ^(C 0 )5 W=C,^ (5.1) 'Ph In general, Fischer carbene complexes contain alkoxy or amido substituents (ER) directly attached to the carbene carbon. Consequently, the thermal stability of Fischer carbenes is often attributed to their having a variety of stabilizing resonance structures. Fischer carbene ligands are almost always derived from metal-bound carbonyl moieties.2  122 The first unstabilized carbene complex (i.e. no a-carbon-heteroatom bond) was prepared nearly a decade later by Schrock during an attempt to prepare Ta(CH2CMe3)5 (eq 5.2). 3  Ta(cH2 cme 3 )3 c12  2 Me3 CCH2 11  ^ (Me3CCH2)3Ta=C  -2 lie, -CMe4^  /  H (5.2)  \CMe3  This neopentylidene complex had completely different chemical properties than previously studied carbene complexes, and thus constituted a new class of compounds that have since been named Schrock alkylidenes. Typically, Schrock alkylidene complexes are derived via alkane loss from precursors containing cis-dialkyl ligands. 4 In terms of reactivity, Fischer carbenes are typically electrophilic at C a whereas Schrock alkylidenes are nucleophilic at C a . 5 The bonding of Fischer carbenes is typically described as M4--C(ER)R' dative interactions with substantial M(d7r)—>C(pe) backbonding. 6 Schrock-type complexes are typically described as containing M=C covalent bonds similar to, but certainly more polar than ethylene.?  5.1.2 Relationship to Cp'M(NO)R 2 Complexes  As alluded to in the previous chapters of this thesis, many Cp'M(NO)R 2 complexes are thermally sensitive. Indeed it is the extreme thermal instability of CpMo(NO)(aryl)2 complexes that has precluded their isolation (Chapter 2). The next most sensitive complexes are the CpMo(NO)(alkyl) 2 family (Chapter 3) of which CpMo(NO)(CH2CMe3) 2 is by far the most thermally sensitive. This chapter describes the characteristic chemistry of this complex and presents the results of a detailed mechanistic study of its thermal decomposition. CpMo(NO)(CH2CMe3) 2 decomposes via an intramolecular a-hydrogen elimination of neopentane to yield CpMo(NO)(=CHCMe 3 ). While this 16-electron neopentylidene complex exists only transiently, its reactivity can be, and has been, investigated.  8  123 5.2 Experimental Procedures  5.2.1 Methods The synthetic methodologies employed throughout this thesis are described in detail in section 2.2.1. The flow system for in situ IR spectral monitoring was constructed from a Teflon diaphragm pump (Cole-Parmer 07090-62) and a 0.2 mm NaCl cell (Wilmad 105A10-5).  5.2.2 Reagents CpMo(NO)C12 was prepared and handled as described in Section 2.2.2. The preparation of (Me 3 CCH2 )2Mg•X(dioxane) is described in Section 3.2.3. LiAID 4 (Aldrich, 98% D) and P(n-Bu) 3 , P(p-tolyl) 3 , PPh2Me, CNCMe3, (p-tolyl)NH2 (Aldrich) and PPh 3 (Strem) were used as received. Me3CC{O)C1 (Aldrich) was degassed with several freeze-pump-thaw cycles. DMF (BDH) was freshly distilled in vacuo from activated 4A molecular sieves. 9 Mesitylene (MCB) was dried over CaC1 2 and degassed in vacuo. PMe3 was prepared as described in Section 2.2.2 and was vacuum transferred from Na/benzophenone. Me 3 CNH2 (Aldrich) was vacuum transferred from CaH2. The column chromatography material used during this work was alumina (80 - 200 mesh, Fisher neutral, Brockman activity I). Filtrations were performed through Celite 545 diatomaceous earth (Fisher) that had been oven-dried and cooled in vacuo.  5.2.3 Preparation of CpMo(NO)(CH 2 CMe3) 2 (5.1) This complex was prepared in a manner analogous to that described for related complexes found in Chapters 2 and 3, but with a few added precautions. These precautions are required because samples of CpMo(NO)(CH2CMe3)2 (5.1), when taken to dryness, decompose in a matter of minutes at ambient temperatures. Therefore, in order to isolate spectroscopically pure 5.1, the following synthetic procedure was used: THE (3 mL) was vacuum transferred onto  124 CpMo(NO)C12 (100 mg, 0.38 mmol) and (Me 3 CCH2 ) 2Mg•(dioxane) (105 mg, 0.75 mmol Me3 CCH2-) at -196 °C. The reaction mixture was stirred for 1 h as it was warmed to -40 °C, the temperature at which all subsequent manipulations were performed. The final red solution (vNO 1608 cm -1 ) was taken almost to dryness in vacuo. The oily red residue was suspended in Et 2 0 (10 mL), and the mixture was filtered through alumina I (1 x 2 cm) supported on a mediumporosity glass frit. Solvent was removed from the filtrate to obtain 5.1 as a red solid (-70 mg, —55% yield). Five different estimates of the yield of similar-scale reactions leading to 5.1 indicated it varied only slightly from 55% (50-59%). IR (THF) vNo 1608 cm -1 . 1 H NMR (C 6 D6 ) 5 5.04 (s, 5H, C5H5), 4.01 (d, 2H, CH2, JHEI = 12.1 Hz), 1.31 (s, 18H, C(CH 3 ) 3 ), 0.27 (d, 2H, CH2 , 411.1 = 12.1 Hz). Due to the thermal instability of CpMo(NO)(CH2CMe3) 2 as a solid, no elemental analysis or mass spectral data could be obtained for the complex.  5.2.4 Preparation of (Me 3 CCD 2 ) 2 Mg•X(dioxane) 1 ° LiAID 4 (3.50 g, 79.1 mmol) was transferred to a 500-mL round-bottom flask in a drybox. The flask, equipped with a large magnetic stirring bar, was removed from the drybox and attached to a vacuum line. A 250-mL pressure-equalized addition funnel was attached to the flask and cooled under a flow of argon. Once cool, the addition funnel was charged with Me3CC{O}C1 (19.1 mL, 155 mmol, 0.98 equiv) diluted with Et 2 O (100 mL). Et2O (300 mL) was added to the solid LiA1D4. The flask was cooled to -78 °C and dropwise addition of the acetyl chloride solution to the LiAID 4 suspension was initiated. After the addition was complete, the reaction mixture was warmed to room temperature and stirred for 4 h. No visible change in the reaction mixture was observed. At that time the flask was cooled to -10 °C, and H 2 O (2 mL) was added via a disposable syringe. The contents of the flask were stirred for 5 min, and then NaOH (0.6 g dissolved in 3 mL of water) was added to the flask, thereby causing the contents of the flask to become increasingly viscous. The volatiles were then vacuum transferred from the white solids into a new flask using an oil bath to supply the needed thermal energy. [Attempts to filter  125 the above reaction mixtures proved to be very difficult and decreased yields substantially.] After thawing, the vacuum-transferred solution was dried overnight on MgSO4. MgSO 4 was removed by filtration into a tared 500-mL flask. Removal of the solvent at -25 °C yielded Me3CCD2OH (-10 g, 72% yield) as a waxy, white solid. This solid was used directly in the next step. Me3CCD 2 OH (-10 g, —111 mmol) was dissolved in DMF (150 mL) in a flask equipped with two addition funnels and a Liebig condenser connected to a 250-mL, one-necked flask. One addition funnel was charged with P(n-Bu) 3 (42.4 mL, 170 mmol), and the other with Br2 (8.2 mL, 160 mmol). The P(n-Bu) 3 was added to the DMF solution over a 5-min period at room temperature. The flask was then immersed in an ice bath, and the bromine addition commenced. After the bromine had been completely added (30 min), the addition funnels were removed from the reaction flask and the orange reaction mixture was stirred at room temperature for 30 min. Distillation of the crude reaction mixture under argon (1 atm) at 140 - 155 °C provided a colorless mixture of DMF and Me 3 CCD 2 Br. The distillation pot contained a dark red tar which solidified upon cooling. The following manipulations were executed in air. Addition of H 2 O (80 mL) to the DMF/Me 3 CCD 2 Br mixture caused the solution to separate into two layers. The lower layer of Me 3 CCD 2Br was removed using a separatory funnel and was washed with water (20 mL). At this point, the organic product appeared cloudy. Both fractions of water were combined and extracted with Et20 (50 mL). The ether wash was combined with the crude Me3CCD 2Br. The resulting Et20 solution was dried over CaC12 for 1 h, and was filtered into a Schlenk tube, equipped with a 4-mm Kontes Teflon stopcock, containing P 2 O5 (2 g). The contents of the vessel were subjected to 3 freeze-pump-thaw cycles and left to dry in the dark for 3 days. The Me 3 CCD 2Br solution was then cannulated onto fresh P 2 O5 (2 g). The residual drying agent was washed with Et 2 0 (5 mL). After 1 day over P2O5, the Me3CCD2Br and ether were filtered through Celite (2 x 2 cm) supported on a fit into a new flask. To this were added dibromoethane (1.0 mL) and Et 2 0 (50 mL). Meanwhile, in a glovebox, a 500-mL flask was charged with Mg powder (5 g, excess). The flask was removed from the glovebox and attached to a vacuum line.  126 Et2 0 (250 mL) was added by syringe to the flask. An addition funnel and a reflux condenser were attached to the flask. The contents of the flask were brought to reflux using an oil bath. The Me 3 CCD 2Br/BrCH2 CH2Br/Et20 solution was transferred to the addition funnel and dropwise addition of the solution to the refluxing Mg/Et20 mixture was effected over the course of several hours. The reaction mixture was then refluxed overnight. The resulting yellow mixture was filtered through Celite (3 x 10 cm). The clear yellow filtrate was treated dropwise with 1,4-dioxane (13 mL, 153 mmol, excess). Subsequent workup as described in Section 2.2.4 afforded a pale orange solution of (Me 3 CCD 2)2MgX(dioxane). This solution was used directly in further reactions rather than isolating the organomagnesium species by crystallization, thus minimizing the losses normally associated with crystallization. Hydrolysis of a sample, then titration against standard HC1 (0.1000 M) using phenolphthalein as indicator established the carbanion concentration of the stock solution to be 0.12 M.  5.2.5 Preparation of CpMo(NO)(CD2 CMe3) 2 (5.1-d4 ) CpMo(NO)(CD2CMe3)2 is markedly more thermally stable than its perhydro analogue. Thus, while its synthesis is similar to that described in Section 5.2.3, the scale of the reaction can be greatly increased with relative ease. THE (50 mL) was vacuum transferred onto CpMo(NO)C12 (1048 mg, 4.00 mmol), and (Me3CCD2) 2MgX(dioxane) (33.6 mL of a 0.12 M stock solution, 8.1 mmol Me3CCD21 was added dropwise to the stirred dichloride slurry at -78 °C. The reaction temperature was slowly raised to 0 °C, and approximately one-half of the solvent was removed in vacuo. An IR spectrum of the final red solution exhibited a v i,,To of 1609 cm -1 . Without warming, the reaction mixture was then taken to dryness under reduced pressure. The red residue was suspended in Et20 (100 mL), and the mixture was filtered through alumina I (2 x 5 cm) supported on a medium-porosity glass frit. The alumina column was washed with ether until the extracts were colorless (-50 mL). The ether was removed in vacuum to leave a red powder which was dissolved in pentane and refiltered through a column of Celite  127 (2 x 2 cm). Cooling of the pentane solution at -30 °C for several days gave red needles of the desired complex in 39% yield (526 mg). Anal. Calcd for C15H 2 3D4NOMo: C, 53.41; H, 8.07; N, 4.15. Found: C, 53.60; H, 8.00; N, 4.07. IR (THF) vNO 1609 cm-1 . IR (Nujol) v NO 1623 cm -1 . 1 H NMR (CDCI 3 ) 8 5.67 (s, 5H, C5H5), 1.17 (s, 18H, C(CH3) 3 ). 1 H NMR spectroscopy revealed no detectable methylene proton signals, thus indicating at least 99.5% D incorporation at C a . 2H NMR (C 6H6, 40 MHz) 8 4.00 (s, br, no resolvable couplings), -1.09 (s, br, no resolvable couplings).  5.2.6 Preparation of [CpMo(NO)(CHCMe3)]2 (5.2) In a typical experiment, THF (30 mL) was vacuum transferred onto a mixture of CpMo(NO)C1 2 (467 mg, 1.78 mmol) and (Me 3 CCH2) 2Mg•X(dioxane) (512 mg, 3.66 mmol Me3CCH2 -) at -78° C. The stirred reaction mixture was allowed to warm to -40° C over 1 h to ensure complete reaction as judged by IR spectroscopy. The final red solution of 5.1 (vNO 1608 cm -1 ) was taken almost to dryness in vacuo. The oily red residue was redissolved in CH2C12/hexanes (1:2, 50 mL), and the red reaction mixture was stirred in the dark at room temperature for 2-3 h during which time it became pale orange. Solvent was removed under reduced pressure, and the brown residue was extracted with Et 2 0 (2 x 50 mL). Filtration of the combined extracts through neutral alumina I (2 x 3 cm) followed by exhaustive washing with Et2 0 (200 mL), concentration in vacuo to —25 mL, and slow cooling of the filtrate to -30° C induced the deposition of pale orange crystals of 5.2 (220 mg, 47% yield) which were collected by filtration. Anal. Calcd for C20H30N2O2Mo2: C, 45.99; H, 5.79; N, 5.36. Found: C, 45.96; H, 5.84; N, 5.34. IR (Nujol) vblo(tenninal) 1595 (vs) cm-1, vN0(bridging) 1315 (s) cm -1 . IR (CH2Cl2) vNO 1594 (vs), 1328 (s) cm -1 . 1 H NMR (C 6D6) 8 12.42 (s, 1H, CHrerrni nni), 9.30 (s, 1H, CHbridgi ng), 5.20 (s, C 5H5 ), 5.16 (s, C5H5), 1.28 (s, C(CH3 ) 3 ), 1.23 (s, C(CH3 ) 3 ). 13 C( 1 H) NMR (C 6D6) 8 301.12 (s, =CHrermi nal), 243.05 (s, =CHbridgi ng), 101.64 (s, C5H5), 100.83 (s, C 5 H5 ), 34.99 (s,  128 C(CH3)3), 32.23 (s, C(CH3)3), no resonances for C(CH 3 )3 were unambiguously assignable. Low-resolution mass spectrum (probe temperature 120 °C): m/z 522 (P+).  5.2.7 Preparation of CpMo(NO)(=CHCMe3)L [5.3, L = PPh 2 Me; 5.4, L = PPh 3 ; 5.5, L = P(p-toly1) 3 )]  All three complexes were synthesized in a similar manner. A solution of CpMo(NO)(CH2CMe3)2 was generated from CpMo(NO)C12 (2.00 mmol) and (Me3CCH2)2Mg.X(dioxane) (4.05 mmol Me3CCH2) in THE (30 mL). The solution was taken to dryness in vacuo at -20 °C, extracted with Et 2 0 (50 mL) and quickly filtered through alumina I (2 x 4 cm) into a Schlenk tube containing the appropriate tertiary phosphine (4.0 mmol, —4 equiv assuming 50% conversion of CpMo(NO)C1 2 to CpMo(NO)(CH2CMe3)2). The reaction mixture was then stirred at room temperature for 3 h in the dark. [Dichloromethane or hexanes were also suitable solvents for these reactions although diethyl ether was the most convenient since the filtration of the dialkyl starting material on alumina must be effected using Et 2 0 as the solvent.] The transformations of CpMo(NO)(CH2 CMe3) 2 to CpMo(NO)(=CHCMe3)L could be conveniently monitored by IR spectroscopy. For example, an IR spectrum of a solution of CpMo(NO)(CH2CMe 3 )2 and PPh 2Me (4.0 equiv) exhibited a vNo of 1616 cm -1 in Et 2 0. Monitoring of this solution by in situ flow-cell IR spectroscopy showed loss of the band at 1616 cm -1 and appearance of a new band at 1577 cm - I. After 3 h only the 1577 cm -1 band remained, and the reaction was deemed to be complete. After the reaction was complete, the solvent was removed in vacuo leaving a brown powder. The residue was extracted with Et 2 0 (5.3 and 5.5) or CH2 C12 (5.4) and filtered through alumina I (2 x 2 cm). The alumina was washed with the appropriate solvent until the washings were colorless. The filtrate (yellow to amber in color) was taken to dryness and washed with pentane (15 mL) to remove excess phosphine. The remaining residue was crystallized from a minimum of Et 2 0. Complex 5.3 and 5.4 were isolated as amber crystals in 57 and 42% yields, respectively.  129 Complex 5.5 was isolated as a yellow powder in 50% yield. All isolated yields are based on CpMo(NO)C12 as the limiting reagent. Complex 5.3: Anal. Calcd for C23H28NOPMo: C, 59.87; H, 6.12; N, 3.04. Found: C,  59.80; H, 5.99; N, 2.94. IR (Nujol) vm, 1540 (vs) cm -1 . 1 H NMR (C 6D6) 6 13.59 (d, 1H, CH, JpH = 3.7 Hz), 7.41 (m, 4H, PPh 2Me), 7.01 (m, 6H, PPh 2Me), 5.26 (s, 5H, C5H5), 1.82 (d, 3H, PPh2 (CH3 ), JpH = 11.2 Hz), 1.34 (s, 9H, C(CH3)3). 13 C{ 1 H} NMR (C6D6) 5 316.1 (s, =CH), 138.9 (d, Clips° of PPh2Me, Jpc = 42.4 Hz), 137.1 (d, Ci pso of PPh 2Me, ./p c = 42.1 Hz), 132.5 (d, Cmthe of PPh2Me, Jpc = 14.1 Hz), 131.3 (d, C o mo of PPh 2Me, Jpc = 14.0 Hz), 130.2 (s, C^of PPh 2Me), 139.7 (s, C^of PPh2Me), 128.6 (d, C meta of PPh 2Me, Jpc = 9.9 Hz), 128.4 (d, Cmeta of PPh2Me, Jpc = 9.9 Hz), 98.6 (s, C5 H5 ), 52.6 (s, C(CH 3 ) 3 ), 31.3 (s, C(CH3) 3 ), 17.6 (d, PPh2 (CH3 ), Jpc = 21.3 Hz). 31 P{ 1 H} NMR (C6D6) 6 36.9 (s). Low-resolution mass spectrum (probe temperature 120 °C): m/z 393 (P+-CHCMe3). Complex 5.4: Anal. Calcd for C28H30NOPMo: C, 64.25; H, 5.78; N, 2.68. Found: C,  64.15; H, 5.74; N, 2.88. IR. (Nujol) vNo 1540 (vs) cm -1 . 1 H NMR (C 6 D6) 6 13.80 (d, 1H, CH, JPH = 3.6 Hz), 7.77 (m, 6H, PPh3), 7.15 (m, 9H, PPh 3 ), 5.40 (s, 5H, C 5H5), 1.39 (s, 9H,C(CH3)3). ' 3 COM NMR (C 6 D 6 ) 6 319.62 (s, =CH), 136.29 (d, Ci pse of PPh3, Jpc = 43.2 Hz), 133.70 (d, Como of PPh 3 , Jpc = 12.1 Hz), 130.17 (s, Cpa of PPh 3 ), 128.66 (d, Cmeta of PPh3, Jpc = 9.8 Hz), 98.86 (s, C5H5), 53.34 (s, C(CH 3 )3), 30.87 (s, C(CH3 )3). 31 P { 1 H) NMR (C6D6) 5 41.2 (s). Low-resolution mass spectrum (probe temperature 120 °C): m/z 525 (Pi. Complex 5.5: Anal. Calcd for C31H36NOPMo: C, 65.84; H, 6.42; N, 2.48. Found: C,  66.11; H, 6.61; N, 2.33. IR (Nujol) vNo 1553 (vs) cm-1 . 1 H NMR (C 6D6) 5 13.70 (d, 1H, CH, JFE = 3.6 Hz), 7.61 (m, 6H, P(p-tolyl)3), 6.90 (m, 6H, P(p-tolyl) 3 ), 5.34 (s, 5H, C 5H5 ), 1.98 (s, 9H, p-CH3), 1.33 (s, 9H, C(CH3)3). 13 C{ 1 H} NMR (C 6D6) 5 316.60 (d, =CH, Jpc = 13.9 Hz), 176.65 (s, Cps of P(p-tolyl) 3 ), 133.71 (d, Corm° of P(p-tolyl) 3 , Jpc = 12.3 Hz), 129.45 (d, Cmeta of P(p-tolyl)3, Jpc = 10.3 Hz), 98.85 (d, C5H5, Jpc = 7.0 Hz), 53.15 (d, C(CH3)3, Jpc = 6.0 Hz), 30.92 (d, C(CH3)3, Jpc = 5.5 Hz), 21.07 (d,p-CH3, Jpc = 6.4 Hz). 31 P{ 1 H} NMR  130 (C 6 D 6 ) 6 56.6 (s). Low-resolution mass spectrum (probe temperature 120 °C): m/z 567 (Pt), 497 (P+-CHCMe3).  5.2.8 Reaction of CpMo(NO)(CD2CMe3)2 with PPh 2 Me  The conversion of CpMo(NO)(CD2CMe3)2 to CpMo(NO)(=CDCMe 3 )(PPh 2 Me) (5.3 d1 ) -  could be conveniently monitored by NMR spectroscopy. For example, CpMo(NO)(CD2CMe 3 ) (10 mg) and PPh2Me (10 mg, excess) were dissolved in CD 2 C12 (0.6 ml) in an NMR tube and allowed to react overnight. A 1 H NMR spectrum of the reaction mixture revealed CpMo(NO)(=CDCMe 3 )(PPh2Me) to be the only organometallic product. Other resonances were attributable to neopentane-d3 and residual PPh 2 Me.  5.2.9 Preparation of [(Me3P) 4 Mo(NO)C11 2 (5.6)  A solution of CpMo(NO)(CH2CMe 3 ) 2 was generated from CpMo(NO)C12 (2.00 mmol) and (Me3CCH2)2Mg•X(dioxane) (4.05 mmol Me3CCH 2 -) in THE (30 mL). The volume of the red solution was reduced to 5 mL, and CH 2 C12 (50 mL) was added. The solution was cooled to -50 °C and PMe3 (1 atm) was vacuum transferred onto the reaction mixture. The reaction mixture was allowed to warm to room temperature was stirred for an additional 5 h. The resulting orange slurry was taken to dryness in vacuo and washed with pentane (20 mL). The residue was extracted with Et20/THF (10:1, 4 x 50 mL), and the combined extracts were filtered through alumina I (3 x 3 cm). The orange filtrate was taken to dryness and the reamining solid recrystallized from Et20. Over time the Et 2 0 solution darkenedll and deposited gold flakes of 5.6 (204 mg, 22% yield).  Anal. Calcd for C12H36NOP4C1Mo: C, 30.95; H, 7.79; N, 3.01. Found: C, 31.22; H, 7.77; N, 3.14. IR (Nujol) v No 1528 (vs), vpc 939 (vs) cm -1 . 1 H NMR (C6D6) 5 1.30 (br t, P(CH3)3,  J= 2.6 Hz). 13 C{ 1 H} NMR (C6D 6 ) 5 19.29 (br s, P(CH3)3). 31 P ( 1 1-1) NMR (C6D 6 ) 5 -3.03 (s).  131 5.2.10 Preparation of CpMo(NO)(=CHCMe3)(py) (5.7) A solution of CpMo(NO)(CH2CMe3)2 was generated from CpMo(NO)C12 (1.80 mmol) and (Me3 CCH2 )2Mg•X(dioxane) (3.62 mmol Me 3 CCH2-) in THE (30 mL). The solution was taken to dryness in vacuo at -20 °C, extracted with Et20 (40 mL), and quickly filtered through alumina I (2 x 2 cm) into a reaction vessel containing pyridine (1.0 mL, excess). The contents of the bomb were sealed under vacuum and were stirred overnight in the dark. The amber reaction mixture was then taken to a brown oil in vacuo. Pentane (50 mL) was added to the oil, the resulting solution was stirred rapidly for 1 h, and the pentane was removed in vacuo. Drying under high vacuum for 4 h provided a brown powder which was dissolved in Et 2 0 (20 mL), filtered through Celite (2 x 2 cm). The filtrate was concentrated in vacuo and placed in a freezer at -30 °C to induce crystallization. Brown crystals of the pyridine adduct were isolated after several days (230 mg, 38% yield). When crushed, the brown crystals became a bright yellow powder. Anal. Calcd for C1 5 H2oN2 0Mo: C, 52.95; H, 5.92; N, 8.23. Found: C, 52.56; H, 6.02; N, 8.08. IR (Nujol) v NO 1532 (vs), 1545 (sh) cm -1 . 1 H NMR (C 6 D 6 ) 5 13.56 (s, 1H, CH), 8.28 (d, 2H, pyridine protons, Jim = 1.5 Hz), 6.62 (t, 1H, pyridine proton, JHH = 7.8 Hz), 6.20 (t, 21I, pyridine protons, JHH = 5.7 Hz), 5.52 (s, 51I, C5H5), 1.56 (s, 91I, C(CH3)3).  13 C { 1 H}  NMR  (C 6 D6) 5 303.51 (s, =CH), 155.47, 136.82, 123.93 (s, pyridine carbons), 100.10 (s, C5H 5 ), 50.80 (s, C(CH3)3), 32.33 (s, C(CH 3 )3). Low-resolution mass spectrum (probe temperature 150 °C):  m/z 540 [(P+-CHCMe3)]2, 272 (P+-CHCMe3).  5.2.11 Preparation of CpMo(N0)(=CHCMe 3 )(PMe3) by Reaction of 5.7 with PMe3 An NMR tube was charged with CpMo(NO)(=CHCMe3)(py) (5.7) and C 6D 6 (0.7 mL). The tube and its contents were frozen to -196 °C, then PMe3 (1.1 equiv) was vacuumed transferred into the tube. The reaction mixture was then warmed to room temperature. Over the course of 30 min the solution darkened to red-brown from amber. A 1 H NMR spectrum of the sample revealed that the pyridine adduct had been converted to the PMe3 adduct and 1 equiv of pyridine had been liberated. The complex, CpMo(NO)(=CHCMe3)(PMe3), was not isolated.  132 1 H NMR (C 6D6 ) 8 13.21 (d, 1H, CH, JpH = 3.9 Hz), 5.32 (d, 5H, C 5H5, JpH = 1.2 Hz), 1.43  (s, 9H, C(CH3 ) 3 ), 0.92 (d, 9H, PMe 3 , JpH = 9.0 Hz).  5.2.12 Attempted Preparations of CpMo(NO)(=CHCMe 3 )(L) [L = acetone, acetonitrile, thiophene, PhCCH, P(CMe3)3]  All five of these experiments led to the same result. Typically, a solution of CpMo(NO)(CH2CMe3)2 was generated in THE then worked-up into CH 2 C12 or Et 2 0. An excess of the appropriate ligand (5 - 10 eq) was then introduced into the red solution. Stirring of the mixture overnight in the dark at room temperature resulted in a brown solution over copious quantities of a intractable brown powder. Filtration of the reaction mixture through Celite followed by attempts at crystallization provided only more brown powders which were found to consist of many products as judged by 1 H NMR spectroscopy. In some cases resonances attributable to small quantities of 5.2 were apparent in the NMR spectra.  5.2.13 Preparation of CpMo(N0)(11 2 - C{NCMe3}CH2CMe3)(CH2 CMe3) (5.8)  A solution of CpMo(NO)(CH 2 CMe 3 ) 2 (from 1 mmol of CpMo(NO)C1 2 ) in Et 2 0 (30 mL) was prepared in the usual manner (Section 5.2.7). This solution was cannulated into a reaction vessel containing CNCMe3 (339 IAL, 3.00 mmol). As the red solution encountered the isocyanide, it immediately turned yellow. The yellow solution was taken to dryness in vacuo to remove excess CNCMe3. The resulting yellow powder was crystallized from Et20 in 3 fractions to obtain 200 mg (48% yield) of 5.8. Anal. Calcd for C20H36N2OMo: C, 57.68; H, 8.71; N, 6.73. Found: C, 57.89; H, 8.71; N, 6.70. IR (Nujol) vm, 1549 (vs), vw 1680 (m) cm -1 . 1 H NMR (C6D6) 8 5.21 (s, 5H, C5H5), 3.17 (d, 1H, CH2 , JHH = 13.5 Hz), 2.68 (d, 1H, CH2, JHH = 12.0 Hz), 2.25 (d, 1H, CH 2 , JHH = 12.0 Hz), 2.20 (d, 1H, CH2, iHR = 13.5 Hz), 1.51 (s, 9H, NC(CH3)3), 1.13 (s, 9H, C(CH3)3), 1.12 (s, 9H, C(CH3)3). nctim NMR (C6D 6 ) 8 204.86 (s, CN), 100.49 (s, C 5 H5 ), 61.25 (s, NC(CH3 ) 3 ), 48.30 (s, CH2 ), 37.86 (s, C(CH 3 ) 3 ), 35.20 (s, NC(CH 3 ) 3 ), 32.80 (s, C(CH3)3),  133 31.87 (s, CH2), 30.30 (s, C(CH 3 ) 3 ), 30.25 (s, C(CH3)3). Low-resolution mass spectrum (probe temperature 180 °C): m/z 418 (P+).  5.2.14 Preparation of CpMo(NO)(CH2CMe3)(NHR) [R = CMe3 (5.9); p-tolyl (5.10)]  The reactions leading to 5.9 and 5.10 were conducted in a similar manner. A sample of CpMo(NO)(CH2 CMe3)2 (from 2 mmol of CpMo(NO)C12 ) in Et 2 0 (50 mL) was prepared in the usual manner (Section 5.2.7). The red solution was treated with an excess of amine either as a solid ((p-tolyl)NH 2 , 600 mg) or by vacuum transfer (Me3CNH2, —1 mL) and stirred for 8 h in the dark at 15 °C. The final reaction mixture (brown for 5.9, orange for 5.10) was taken to dryness and the excess amine removed in vacuo. Several hours exposure to dynamic vacuum were necessary to sublime all excess (p-tolyl)NH2 from 5.10. CpMo(NO)(CH2CMe 3 )(NHCMe3) (5.9) was recrystallized from pentane as amber needles in 56% yield. Complex 5.10, CpMo(N0)(CH2 CMe3 )(NH-p-toly1), was recrystallized from Et20 as orange blocks (33% yield). Complex 5.9: Anal. Calcd for C 14 H2 6N20Mo: C, 50.30; H, 7.84; N, 8.38. Found: C,  50.60; H, 7.79; N, 8.10. IR (Nujol) v No 1554 (br, vs) cm -1 . 1 H NMR (C 6D6) 8 8.92 (br s, 1H, NH), 5.22 (s, 5H, C 5H5 ), 2.16 (d, 1H, CH2, JHH = 11.8 Hz), 1.61 (d, 1H, CH2 , Jp1 = 11.8 Hz), 1.30 (s, 18H, 2 x C(CH3 ) 3 ). 13 C( 1 H) NMR (C6D6) 8 102.55 (s, C5H5), 68.00 (s, NHC(CH 3 )3), 46.16 (s, CH2), 36.20 (s, CH 2 C(CH3) 3 ), 34.24, 32.75 (s, 2 x C(CH3) 3 ). Low-resolution mass spectrum (probe temperature 180 °C): m/z 335 (P+-H). Complex 5.10: Anal. Calcd for C 17H24N2 OMo: C, 55.44; H, 6.57; N, 7.61. Found: C,  55.25; H, 6.49; N, 7.70. IR (Nujol) vNo 1548 (s), 1563 (sh) cm -1 . 1 H NMR (C 6 D6) 8 10.49, 9.84 (br s, 2H, 2 x NH), 7.37, 6.96, 6.81, 6.38 (d, 8H, 2 x Holm° and 2 x Hmeta , J = 7.4 or 9.3 Hz), 5.14, 5.12 (s, 10H, 2 x C5H5), 2.26 (2 superimposed doublets, 111 each, CH ,u, J = 12.9 Hz), 2.08, 2.05 (s, 6H, 2 xp-CH3 ), 1.82 (d, 1H, CHI, Jp = 10.8 Hz), 1.67 (d, 1H, CH5, = 11.1 Hz), 1.32, 1.28 (s, 18H, 2 x C(CH 3 ) 3 ). 13 C NMR (C 6D 6 ) 5 153.56, 153.15 (2 x Ci spe), 133.59, 133.29 (2 x C pl, 130.08, 129.42 (2 x C e am ), 119.80, 119.65 (2 x C meta), 103.25, 103.07 (2 x C5H 5 ), 57.42, 55.18 (2 x CH2), 37.52, 36.89 (2 x CMe 3 ), 34.05, 33.99 (2 x  134 C(CH3)3), 20.85 (2 xp-CH3 ). Low-resolution mass spectrum (probe temperature 200 °C): m/z 370 (Pl.  5.2.15 Preparation of CpMo(N0)(CHDCMe 3 )(NH-p- toly1) (5.10-d1 ) In a drybox, solid CpMo(NO)(CD2CMe3)2 (5.1 -d4) 100 mg, 0.296 mmol) and solid (  (p-tolyl)NH2 (100 mg, 0.935 mmol, 3.16 eq) were weighed into a Schlenk tube. Et20 (10 mL) was vacuum transferred onto the solids to yield a red solution which over the course of 24 h turned dark orange while being stirred. The orange solution was taken to dryness, and excess amine was removed by sublimation at room temperature. The remaining orange solid was extracted with Et 2 0 (15 mL), filtered through Celite (1 x 2 cm), concentrated, and crystallized over 1 week at -30 °C. Orange crystals of 5.10-d 1 (74 mg, 68% yield) were isolated by removing the mother liquor with a pipette. 1H  NMR (C 6 D6 ) 5 10.48, 9.83 (br s, 2H, 2 x NH), 7.36, 6.97, 6.81, 6.38 (d, 8H, 2 x Hortho  and 2 x Hmeta, JHH = 8.1 Hz), 5.14, 5.12 (s, 10H, 2 x C5H5), 2.08, 2.05 (s, 6H, 2 xp-CH3 ), 1.78, 1.63 (br s, 2H, 2 x CHD), 1.32, 1.28 (s, 18H, 2 x C(CH3 ) 3 ). 2 H NMR (C 6H6 , 40 MHz) 5 2.32 (br s, CHDCMe3). Low-resolution mass spectrum (probe temperature 200 °C): m/z 371 (P±).  5.2.16 Preparation of CpMo(NO)(CH2CMe 3 )(OPh) (5.11) A solution of CpMo(NO)(CH2CMe3)2 (from 1 mmol of CpMo(NO)C12) in Et 2 0 (25 mL) was prepared in the usual manner (Section 5.2.7). The red solution was cannulated into a bomb containing PhOH (250 mg, 2.5 equiv). The solution was stirred in the dark for 12 h whereupon the solvent and excess phenol were removed in vacuo (8 h at room temperature). The residual red oil was dissolved in hexanes (20 mL) and filtered through Celite (1 x 2 cm). The filtrate was concentrated slightly, and then maintained at -30 °C for 1 week in a freezer. The red oil which settled was separated from the hexanes mother liquor via cannulation. The oil was dried overnight in vacuo (-90 mg, —25% yield).  135 Anal. Calcd for C16H2 1 NO 2 Mo: C, 54.09; H, 5.96; N, 3.94. Found: C, 54.32; H, 6.08; N, 4.00. IR (Nujol) vNo 1608 - 1588 (v br, vs) cm -1 . 1 H NMR (C 6D 6 ) 5 7.65 - 6.59 (3 br m, 5H, Ph), 5.19 (s, 5H, C5H5 ), 3.51 (d, 1H, CH2 , .41i = 10.2 Hz), 1.41 (d, 1H, CH2 , Ji.ili = 10.2 Hz), 1.15 (s, 9H, C(CH3)3). 13 C{ 1 H) NMR (C6D6) 5 129.69, 122.39, 117.74, 108.93 (C aryl), 104.58 (s, C5H5), 77.08 (s, CH2 ), 39.16 (s, C(CH3)3), 33.28 (s, C(CH3)3)•  5.2.17 Kinetic Measurements  The conversion of CpMo(NO)(CH2CMe 3 )2 (5.1) to CpMo(NO)(=CHCMe3) was studied in order to gain some insight into the mechanism of thermal decomposition operative in this system. All mechanistic studies were performed using the tetradeutero analogue of 5.1, CpMo(NO)(CD2CMe3) 2 (5.1-d4 ). Solid CpMo(NO)(CD2CMe3)2 and the appropriate trapping ligand were weighed into a volumetric flask in a drybox. Dissolution with the appropriate solvent was quickly executed. The volumetric flask was shaken and an aliquot was transferred to a 1.00-cm UV-vis spectrophotometer cell equipped with a 4-mm Teflon stopcock. The concentration of the trapping ligand was sufficiently high such that all runs were effectively conducted under pseudo-first-order conditions. The cell was quickly removed from the drybox, and placed in a Hewlett-Packard 8542A diode array spectrometer, which in conjunction with a Haake W19 temperature bath equipped with a Haake D8 temperature controller allowed the cell to be maintained at constant temperature (±0.1 °C). The solution was left unstirred and allowed to equilibrate with the spectrometer temperature for 300 s. Spectra were then recorded at regular intervals and data were collected for at least 3 half-lives. The absorbance values at infinity were determined experimentally by allowing certain runs to proceed for at least 10 half-lives. The measured infinity values were always within 0.1 of computer-optimized infinity values. For example, the infinity value for run 4 was measured and found to be 0.130, while the calculated value for Aco was optimized to 0.1299. The observed rate constants (kob s) were then calculated from plots of ln (A,-Ar) versus time (in seconds). Values of OH: and AS: were determined from  136 Erying plots of ln(kth s/T) versus 1/T where AH: = -R(slope) and AS = R[intercept - In(1 3/h)] (R, kB and h are the gas constant, Boltzmann's constant and Planck's constant, respectively).  5.3 Results and Discussion In accord with the theme of this thesis it seemed appropriate to investigate the thermal behaviour of Cp 1M(NO)R2 complexes. Since the original method to bis(hydrocarbyl) complexes of this type involved their preparation at room temperature, it was of interest to determine if the reason that many such complexes could not be prepared was due to their thermal instability. In general, 16-electron Cp 1M(NO)R2 complexes are thermally stable at 20 °C when isolated in analytically pure form. 12 This stability reflects both the non-bonding nature of the metal-centered LUMO of these complexes and their kinetic inertness toward decomposition.  13  However, as the  electron deficiency of these complexes increases, so too does their thermal sensitivity (Chapter 2). Since we were unable to isolate the most thermally sensitive compounds in this class, namely the CpMo(NO)(aryl) 2 species, we were not able to effect a systematic study of their thermal decomposition. The next most thermally sensitive class of these nitrosyl complexes involves the related Mo dialkyl complexes, CpMo(NO)(alkyl) 2 . A representative member of this class, namely CpMo(NO)(CH 2 CMe3) 2 , was purposely left out of Chapter 3 since the chemistry of this very thermally sensitive compound is sufficient to warrant an entire chapter in this thesis. As it turns out, the most thermally sensitive, yet isolable, Cp'M(NO)R2 complex is the one that decomposes the most cleanly. It has been observed in our group that most Cp 1M(NO)R2 complexes do not decompose along one pathway, rather a multitude of thermal decomposition products are formed.14  5.3.1 Synthesis and Characterization of CpMo(NO)(neopentyl) 2 (5.1 and 5.1 d4) -  CpMo(NO)(CH 2 CMe3 )2 and its deuterated analogue CpMo(NO)(CD 2 CMe3)2 are prepared by the methodology developed in Chapters 2 and 3 (eq 5.3).  137 (Me 3 C0C2)Mtdioxane  CpMo(NO)C12 ^ ►^CpMo(N0)(C X2CMe3)2^(5.3) THE, - 40 °C  X = H (5.1), D (5.1-d4 ) Complex 5.1 must be prepared at low temperatures (yield ca. 55%) and used immediately in further derivatization reactions. On the other hand, 5.144 can be crystallized from pentane in about 40% yield, but must be stored at -30 °C. Both complexes are exceptionally thermally and air-sensitive. They are, however, not particularily sensitive to deaerated water (Chapter 4). The spectral properties of both complexes are consistent with their formulation as monomeric species. Interestingly, the 1 H NMR. spectrum of 5.144 shows no signals attributable to methylene protons, thus implying nearly 100% deuteration at C. A possible explanation for this observation is that any a-protio-containing complexes decompose, and are removed, under the conditions in which 5.144 is handled during workup. The 2H NMR spectrum of 5.1-d4 exhibits two broad singlets at approximately the same chemical shifts as the two doublets ( 2.4114 = 12.1 Hz) in the 1 H NMR spectrum of 5.1 attributed to the methylene protons of the complex;  however, no D-D coupling constants could be extracted from the 2 H NMR spectrum.  5.3.1.1 Solid-State Molecular Structure of CpMo(NO)(CD2CMe 3 ) 2 When carefully prepared, complex 5.144 forms high quality red crystals suitable for an X-ray crystallographic analysis. 15 The molecular structure is shown in Figure 5.1. The analysis was performed at low temperature (170 K) since the complex, even in highly crystalline form, decomposes completely in under 12 h in the solid state at room temperature.  16  No anomalously  close contacts were found to exist between the alkyl residues or between the metal and the CD 2 groups. The refined positions of the methylene deuterons do not suggest any agostic-type C-DMo interactions which might be expected for a coordinatively unsaturated 16-electron complex. We found this somewhat surprising given the abnormally high thermal sensitivity of this complex. The intramolecular dimensions about Mo (Table 5.1) are also completely as expected and consistent with this species being a potent Lewis acid (for a full discussion of the structure/Lewis  138  C( 1)  Figure 5.1 ORTEP diagram of 5.1-d4 . 50% probability ellipsoids are shown for the non-hydrogen atoms  Table 5.1 Representative bond lengths and angles for complex 5.144  bond lengths (A) (esd) Mo-N 1.766(3) N-0 1.213(3) Mo-C6 2.143(3) Mo-C16 2.129(3)  bond angles (°) (esd) C6-Mo-C16 111.52(11) C16-Mo-N 98.27(11) N-Mo-C6 97.51(11) Mo-N-O 168.5(3)  139 acidity relationship, see Chapter 3). The only intermolecular separations significantly less than the sums of the appropriate pairs of van der Waals radii are O--C(1)' (3.148(4) A) and O--C(5)' (3.083(4) A) where the primes indicate the operation -1/2+x, 1/2-y, 1/2+z (space group: P2 1 /n).  5.3.2 Synthesis and Characterization of [CpMo(NO)(CHCMe3)12 To demonstrate the thermal instability of CpMo(NO)(CH2CMe3)2, a deep red solution of the complex in dichloromethane (v NO 1609 cm -1 ) is stirred at room temperature for several hours. The color of the solution fades with time such that the final solution is pale orange and exhibits a broad band at 1594 cm -1 in its ER. spectrum. Removal of the solvent, followed by exhaustive extraction with Et20 affords an amber solution. Attempts to crystallize product from this solution yield an impure brown powder. Additionally, the color of the solution continues to darken with time, thereby implying that reactions are still occurring. Chromatography, however, simplfies the isolation of a pure product. Filtration of the above-mentioned ether solution through alumina I neutral and crystallization of the eluted complex from Et20 yields pale orange crystals of 5.2. To date, the highest isolated yield of the complex is 47%. 1 H NMR spectroscopic monitoring of the above reaction shows that the yield is variable (30 - 60%). Complex 5.2 exhibits very unusual spectral properties which are not easily interpretable. For example, its ER. spectra (solution or Nujol mull (Figure 5.2a)) exhibit two strong bands at 1595 and 1315 cm -1 , and its 1 H (Figure 5.2b) and 13 C NMR spectra exhibit two sets of equal intensity C 5H5 , CH and CMe3 resonances, and no resonances characteristic of methylene-type protons.  By elemental analysis, the formula of the isolated compound is CpMo(NO)(CHCMe3), but a mass-spectral parent ion having m/z 526 implies that the complex is dimeric. 2 CpMo(NOXCH2 CMe3 )2 ----■ [CpMo(NOXCHCMe 3 )]2^(5.4) 5.1^  5.2  140 • 0  1  1558^1438^1308^1178^1048  818^789  wa venumbers (cm -I )  lb Figure 5.2 Spectroscopic characterization of 5.2: (a) partial Nujol (marked by X) mull infrared spectrum (1700 - 700 cm -1 ), (b) 300 MHz 1 H NMR spectrum in C6D6  141 But, somehow, the two halves of the dimer are inequivalent. Thus, to resolve any ambiguity as to the molecular structure of complex 5.2, an X-ray crystallographic analysis was performed on a suitable crystal of the complex.  5.3.2.1 X-ray Crystallographic Analysis of [CpMo(NO)(CHCMe3)12 An X-ray analysis of 5.2 17 establishes that the molecular unit is a dimer of CpMo(NO)(CHCMe 3 ) whose structure is unique for two significant reasons; (1) the CpMo(NO)(CHCMe3) units are associated very asymmetrically through bridging nitrosyl and alkylidene groups even though there are no apparent factors prohibiting symmetric association; 18 (2) a 11-1 1 :1 2 -NO group has never before been observed. 21 An ORTEP diagram of the solidstate molecular structure of 5.2 is shown in Figure 5.3.  The bonding of the bridging nitrosyl group in 5.2 (Figure 5.3) is clearly ill to Mo(1) and 1  2  to Mo(2). The bonding is best described as the bridging NO ligand acting as a 3-electron donor to Mo(1) and a 2-electron donor to Mo(2) (Figure 5.4). Thus, the intramolecular dimensions (Table 5.2) of the Mo(g-ril:1 2 -NO)Mo entity involve an elongated N-0 linkage (1.296 (3) A) 19 which is essentially linear with respect to Mo(1) [Mo(1)-N(1)-O(1) = 165.5 (2)1 and is symmetrically disposed with respect to Mo(2) [Mo(2)-N(1) = 2.187 (3) and Mo(2)-O(1) = 2.149 (2) Al  142  C4  C18  ^  C13  Figure 5.3 ORTEP diagram of [CpMo(NO)](g-r1 1 :71 2 -N0)(11-CHCMe3)[CpMo(=CHCMe3)) (5.2). 33% probability ellipsoids are shown for the non-hydrogen atoms  Table 5.2 Representative bond lengths and angles for complex 5.2  bond lengths (A) (esd) 2.9607 (5) Mol-Mo2 Mol-N1 1.784 (3) Mo2-N2 1.763 (3) 1.296 (3) N1-01 N2-02 1.216 (4) Mo2-N1 2.187 (3) Mo2-01 2.149 (2) 1.940 (3) Mol-C16 Mol-Cll 2.186 (3) Mo2-C11 2.192 (3)  bond angles (°) (esd) Mol-N1-01 165.5 (2) 171.0 (3) Mo2-N2-02 Mo2-N1-01 71.0 (2) Mol-C11-Mo2 85.1 (1) 137.1 (2) Mol-C16-C17 Mol-N1-Mo2 95.9 (1) Mol-C11-C12 127.4 (2) Mo2-C11-C12 125.7 (2)  143  N  0  Mo(1) ^Mo(2) Figure 5.4 Valence-bond representation of a 1.1 2 -11 1 :i 2 -NO ligand serving as a 5-electron  donor to two Mo centers. The Mo-Mo separation of 2.9607 (5) A in complex 5.2 is consistent with the existence of a single metal-metal bond which may be invoked to account for the compound's diamagnetism and to provide each metal center with an 18-valence-electron configuration.  20  The spectroscopic  properties of 5.2 confirm that the solid-state molecular structure persists in solution (vide supra).  5.3.2.2 Known Nitrosyl Bonding Modes  At the inception of this work there were eight known bonding modes for coordinated nitric oxide reported in the literature (Figure 5.5). 21 Monomeric complexes can have linear (A) or bent (B) terminal nitrosyl ligands, although structurally characterized bent nitrosyl groups are quite rare. 22 Three types of nitrosyl ligands can be found for bimetallic complexes. Two of these are il bound through nitrogen and can be symmetrically (C) or asymmetrically (D) bound to the two  metal centers. Isonitrosyl complexes (E), on the other hand are bound through nitrogen to one metal and oxygen to another metal. Three cluster complexes fill out the eight known bonding modes of NO. These complexes contain NO ligands that are g 3 -1 1 bound (F) and two variants on A4-712 coordination (G, H).  144  M —N  MAN = 0  A  ^  M MNO  (I(^1 / N \^z N,  i,  0^M ^ -M^M ^ -M  B^C^D M^M \ / N .- 0  d M E^F^G^H Figure 5.5 Known nitrosyl bonding modes (dashed lines indicate that a bonding interaction may or may not be present).  Clearly, the p-1 1 :1 2 -NO ligand in complex 5.2 constitutes the ninth type of nitrosyl ligand to be structurally characterized. It might be noted that a few examples of wil:1 2 -CO ligands can be found in the literature. 23 A theoretical study of 11-11 1 11 2 -CO bonding has been presented by Hall and Simpson. 24 Their conclusion was that such CO ligands are a consequence of steric crowding.  5.3.3 Trapping Reactions While the transformation of 5.1 into 5.2 is interesting and unusual, it also appears to be quite complex. Compound 5.2 is the major product of numerous ones resulting from reaction 5.4, as evidenced by NMR spectroscopic monitoring. In order to gain some chemical evidence for the transient formation of CpMo(NO)(=CHCMe 3 ) from 5.1, the dialkyl complex (5.1) was allowed to decompose in the presence of potential trapping ligands such as phosphines.  145 5.3.3.1 Synthesis and Characterization of CpMo(N0)(=CHCMe3)(L) Complexes [L = Lewis base] Stirring a solution of 5.1 in the presence of excess tertiary phosphines or pyridine, L, produces the adducts CpMo(NO)(=CHCMe3)(L) [5.3, L = PPh2Me; 5.4, L = PPh3; 5.5, L = P(p-tolyl) 3 ; 5.7, L = py] in good yields in a matter of hours at room temperature (eq 5.5).  A,L -CMe4  H O  N  (5.5)  L CMe3  The above-mentioned alkylidene complexes are thermally stable, 18-electron species that can be handled as solids in air for short periods of time. These complexes (5.3 - 5.5, 5.7) are the first monomeric Group 6 mononitrosyl Schrock-type alkylidene compounds to have been isolated. However, a few Fischer-type complexes are known. These are the chromium dipheny1 25 carbene complex, CpCr(NO)(CO)(=CPh2) 26 prepared by Herrmann's group and the three CpM(NO)(CO)(=C(OMe)(Ph)) [M = Cr, Mo, W] complexes prepared by Fischer. 27 As expected, all four of these complexes originate from their dicarbonyl nitrosyl precursors. While the yields of the phosphine adducts are about 50% based on CpMo(NO)C12, this is more a function of the CpMo(NO)C12 to CpMo(NO)(CH2CMe3)2 step (-55%) rather than the trapping step (Section 5.2.3). The cleanness of the trapping step can be demonstrated by NMR spectroscopy. For example, an NMR-tube experiment in CD2C12 of the reaction of 5.1 d4 and -  excess PPh 2Me gives CpMo(NO)(=CDCMe 3 )(PPh2Me) (5.341 ) in quantitative yield (Section 5.2.8). Since the nitrosyl-stretching frequencies (in Et20) of the starting material (1616 cm -1 ) and product (1577 cm -1 ) are so different, the progress of this trapping reaction can be easily monitored by IR spectroscopy (Figure 5.6). The isosbestic point at 1602 cm -1 is not as clear as in the UV-vis spectral monitoring (vide infra) of this same reaction, primarily due to the fact that our  146 IR flow cell system is not perfectly sealed and thus some evaporation of solvent occurs over the course of the reaction. 12 — CpMo(NO)(=CHCMe 3 )(PPh2Me) (5.3, v N0 = 1577 cm -1 )  10  2—  t  CpMo(N0)(CH2 CMe3 ) 2 (5.1, v N0 = 1616 cm -1 ) 0^ 1650 1640 1630 1620 1610 1600 1590 1580 1570 1560 1550 1540  wavenumbers Figure 5.6 IR spectral monitoring (1650 to 1540 cm -1 ) of the formation of  CpMo(NO)(=CHCMe3)(PPh 2Me) (5.3) from 5.1 and excess PPh2Me in Et2 0. Scans are taken at room temperature in 30-min intervals  CpMo(NO)(=CHCMe3)(L) complexes exhibit classic neopentylidene resonances in their NMR spectra. 28 For example, complex 5.4 exhibits a very low-field doublet (5 13.80 ppm, 3JpH = 3.6 Hz) in its 1 H NMR spectrum (Figure 5.7 inset). The doublet arises from the coupling of the alkylidene proton to the 31 P nucleus of the phosphine ligand.  147  l i milimilliiiiiiiiiiiiimilt 13.9^13.8^13.7^13.8  11111111111111111111111111111111truillu irinimplit4111111,11111111111 14^1^10^  d PPM  Figure 5.7 300 MHz 1 H NMR spectrum of CpMo(N0)(=CHCMe 3 )(PPh 3 ) (5.4) in C 6D6  The 1 H NMR spectrum of the pyridine adduct (5.7) shows 3 sets of clearly resolved resonances for the pyridine ring protons implying that the pyridine ring rotates quickly about the Mo-N axis, on the NMR timescale, at 25 °C.  5.3.3.2 X-ray Crystallographic Analysis of CpMo(NO)(=CHCMe 3 )(PPh2 Me) (5.3) The solid-state structure of CpMo(NO)(=CHCMe3)(PPh2Me) 29 (Figure 5.8) additionally confirms the monomeric nature of this and related neopentylidene nitrosyl complexes prepared in this work. The intramolecular dimensions of 5.3, as established by X-ray diffraction, resemble those exhibited by the related [CpRe(NO)(=CHPh)(PPh 3 )]+ cation. 30 Typically, high-valent alkylidene complexes exhibit M-C a {H)-C bond angle distortions and complex 5.3 is no exception. 31 However, the distortion from idealized sp 2 geometry for complex  148  Figure 5.8 ORTEP diagrams of CpMo(NO)(=CHCMe3)(PPh2Me) (5.3): (a) view perpendicular to alkylidene plane; (b) view parallel to alkylidene plane. 50% probability ellipsoids are shown for the non-hydrogen atoms. Labels for most carbons are omitted for clarity  Table 5.3 Representative bond lengths and angles about Mo in 5.3  bond lengths (A) (esd) Mo-P 2.451 (1) Mo-N 1.757 (3) Mo-C6 1.950 (3) 0-N 1.233 (3) C6-C7 1.521 (5) C6-H6 1.01 (4)  bond angles (°) (esd) P-Mo-N 92.94 (8) P-Mo-C6 94.3 (1) 100.8 (1) N-Mo-C6 Mo-N-0 172.7 (2) Mo-C6-C7 140.8 (3) 113 (2) Mo-C6-H6 105 (2) C7-C6-H6  149 5.3 is not severe. Typically, an M=C a-C bond angle of 150° implies a significant M=C-H agostic interaction. 4,32 CpMo(NO)(=CHCMe3)(PPh2Me), formally a Mo(II) species, exhibits a smaller angle at C a of 140.8(3) A. M=C a bond lengths vary from 1.83 to 2.14 A, with the shorter bond lengths being found in increasingly electron-deficient complexes. Structurally characterized Mo alkylidene complexes show very little variance of Mo=C a bond length even though many "oxidation states" 33 have been characterized. The Mo=C a bond length of 1.950(3)  A in 5.3 is in  the middle of the known range (1.917 to 1.989 A). 34  5.3.3.3 Attempted Preparation of CpMo(NO)(=CHCMe3)(PMe3) When a dichloromethane solution of 5.1 is treated with PMe3, the expected CpMo(NO)(=CHCMe3)(PMe3) complex is not obtained. Instead, [(Me3P)4Mo(NO)C1] 2 (5.6) is the only isolable product. Evidently the Cp ligand can be removed from the coordination sphere of 5.1 by the potent base PMe3, whereas weaker and bulkier nucleophiles cannot access the Mo center until the thermally induced loss of neopentane has occurred. The chloride ligand in the ultimate product must arise from the solvent. The dimeric structure of [(Me3P) 4Mo(NO)Cl] 2 has been established by a partial X-ray crystallographic analysis of the complex, but the structure did not refine due to severe disorder amongst the ligands. 1 H, 13 C and 31 P NMR spectra of complex 5.7 exhibit only broad singlets at room temperature thereby implying that fast chemical exchange of the PMe3 ligands occurs in solution. The related iodide complexes, [(Me3P) 4 M(NO)I] x [M = Mo, W; x is unknown], can be prepared by treating CpM(NO)I 2 with excess PMe 3 . 35  5.3.3.4 Preparation of CpMo(NO)(=CHCMe3)(PMe3) While the reaction of CpMo(NO)(CH2CMe 3 )2 with PMe 3 does not afford a PMe3 adduct of CpMo(NO)(=CHCMe3), the complex can be prepared by a different route. Treatment of the pyridine complex (5.7) with 1 equiv of PMe 3 in benzene cleanly forms the PMe 3 complex and free pyridine (eq 5.6).  150  N10^H O  N  PMC3  Mo  - PY  O  N  PY CMe3  y  (5.6)  Me3 P CMe3  With the success of reaction 5.6 it is clear that the pyridine ligand in complex 5.7 is very labile. It is anticipated that CpMo(N0)(HCMe3)(py) can be used as a stable (storable) source of CpMo(NO)(=CHCMe3) in further reactivity studies.  5.3.3.5 Thermal Stability of Adduct Complexes The three phosphine adducts (5.3 -5.5) are thermally robust species that are unreactive to other Lewis bases. Thermolysis of any of these complexes results only in equilibration of the two possible alkylidene rotamers. The pyridine adduct 5.7, however, is quite thermally sensitive due to the lability of the pyridine ligand in solution. Over the course of several months at room temperature in C6D6 the pyridine complex decomposes to yield (-90%) the asymmetric dimer 5.2 (eq 5.7).  Mo  0 \„,  1/2  PY CMe 3  o ^  Me3 C  (5.7)  Mo  CMe3  N  O  5.3.3.6 Attempted Preparation of CpMo(N0)(=CHCMe3)(CNCMe 3 ): Synthesis of an Insertion Product CNCMe3 is very reactive towards CpMo(NO)(CH2CMe3)2. Unlike other trapping ligands which cannot access the Mo center of this complex, t-butyl isocyanide reacts instantly at room temperature to give an air-stable iminoacyl complex, CpMo(N0)(71 2 -C{NCMe3)CH2CMe3)(CH2CMe3) (5.8) (eq 5.8).  151  CNCMe3  mo z  (5.8)  11 Me3C^ O CMe3 )  CpMo(NO)(CH2CMe3)2 is a potent 16-electron Lewis acid (v NO 1623 cm -1 ) and insertion of CNCMe3 into one of the Mo-neopentyl bonds causes a dramatic drop in vNO by 74 cm -1 to that observed for 5.8 (v NO 1549 cm -1 ). The r1 2 nature of the iminoacyl group is best characterized by a vC N band at 1680 cm -1 in the ER spectrum of the product. This low-energy stretch is typical of other rig-iminoacyl ligands. 36 As shown in Figure 5.9, the 1 H NMR spectrum of complex 5.8 displays 4 resonances due to the diastereotopic methylene protons of the complex. Signals due to the three inequivalent butyl groups are also apparent at 8 1.51, 1.13 and 1.12 ppm.  3.2^3.0^2.9^2.6^2.4^2.2 PPM  5.0^4.5^4.0^3.5^3.0^2.5^2.0^i.5 PPM 1.0  Figure 5.9 1 H NMR spectrum of complex 5.8 in C6D6  152 Since CNCMe 3 is sufficiently small to access the Mo atom in CpMo(NO)(CH 2 CMe 3 )2 , no attempts to trap CpMo(N0)(=CHCMe 3 ) with small Lewis bases such as CO or H2 were effected since it was obvious that they would preferentially react with the starting material, CpMo(NO)(CH2 CMe3 )2 . Many trapping reactions are not successful (Section 5.2.11). For example, acetone, acetonitrile, thiophene, PhCCH and P(CMe3) 3 are not appropriate ligands for the formation of CpMo(NO)(=CHCMe3)L species. Thus, when complex 5.1 is allowed to thermally decompose in the presence of these ligands, total decomposition of the reaction mixture is observed.  5.3.4 Kinetic Studies  With substantial chemical evidence for the transient existence of CpMo(NO)(=CHCMe 3 ), a kinetic investigation of the trapping reactions (Section 5.3.3.1) seemed in order. Since the transformation leading to the asymmetric alkylidene dimer was not clean, and the reactions leading to the phosphine-trapped alkylidene monomers were very clean as evidenced by both NMR and lit spectroscopies, a detailed analysis of the reactions of CpMo(NO)(CH 2 CMe3 ) 2 with phosphines was conducted. However, the thermal instability of CpMo(NO)(CH 2 CMe 3 )2 makes a kinetic analysis of its thermal decomposition rather difficult. For example, the complex decomposes in air within seconds of exposure. Also, in the time it takes to get a sample into the drybox, it has thermally decomposed quite substantially. 37 Thus, it seemed that the logical next step was to use the tetra-a-deutero analogue, namely CpMo(NO)(CD 2 CMe 3 ) 2 , since it was our view that if a-H abstraction was a mechanistic path to the transient alkylidene, it would show a substantial kinetic isotope effect. As it turns out, 5.1-d4 is markedly more thermally stable than its perhydro congener and thus was used in all kinetic investigations (vide infra). The increased thermal stability of 5.1 d4 is also consistent with the view that its primary pathway for thermal -  decomposition involves a-H(D) elimination as the rate-determining step. Since the dialkyl complex, CpMo(NO)(CD2CMe3)2, is red and most of its derivatives are markedly less colored, this study was conducted using UV-vis spectroscopy to monitor reaction progress.  153 CpMo(NO)(CD2CMe3)2 exhibits a band at —470-490 nm in its UV-vis spectra, (Table 5.4) whereas the phosphine adducts (5.3-5.5) show no features in their UV-vis spectra above 400 nm.  Table 5.4 Monitored Peak UV-vis Data for CpMo(NO)(CD2CMe3)2 in Various Solvents  solvent CH2C12 Et20 hexanes THE mesitylene  X (nm) 472 484 490 482 486  c (M- lcm - 1 ) 390 391 391 393 380  I quickly discovered that the generation of the transient alkylidene complex, CpMo(NO)(=CDCMe3), was first-order in its dialkyl precursor and independent of the nature of the trapping ligand or its concentration.  5.3.4.1 Variation of Ligand and Ligand Concentration The rate-determining formation of CpMo(NO)(=CHCMe3) from 5.1 is supported by kinetic data. Thermolysis of CpMo(NO)(CD2CMe3)2 (5.1-d4) in the presence of trapping ligands, L, is cleanly first-order in the complex and zero-order in L (eq 5.5). The decomposition of 1.684 x 10 -3 M solutions of 5.1-d4 in CH2C1 2 at 40 °C in the presence of 3.56, 5.24, 10.17 or 13.64 equiv of PPh 2Me, 6.49 equiv of PPh3 or 9.00 equiv of P(p-tolyl)3 affords kobs values of 2.83 ± 0.16 x 10 4 s -1 (all 6 experiments are tabulated in Table 5.5). Typical plots are shown for run 5 in Figures 5.10 and 5.11.  154 Table 5.5 Thermal Decomposition Reactions of 5.1-d4 in the Presence of Phosphines in CH 2 C12 [CpMo(N0)(CD 2 CMe3 )2 ] = 1.684 x 10 -3 M run  trapping ligand  [ligand] (M)  equiv of ligand  1 2 3  PPh2Me  5.994 x 10 -3  3.56  PPh2Me  8.831 x 10 -3  PPh2Me  1.712 x 10 -2  5.24 10.17  2.891 x 10 4  4  PPh2Me  2.298 x 10 -2  13.64  2.898 x 10 -4  5  PPh3  1.093 x 10 -2  6.49  2.930 x 10 4  6  P(p-tolyl)3  1.456 x 10 -2  9.00  2.935 x 10-4  250^300^350^400^450^500  ^  550  kobs (s -1 ) 2.785 x 104 2.514 x 10 -4  ^  600  ^  650  wavelength (nm)  Figure 5.10 Absorption spectral changes for the thermolysis of CpMo(NO)(CD2CMe 3 ) 2 in CH2 C1 2 in the presence of 6.49 equiv PPh 3 (run 5)  155 r2 = 0.997  0.7  P_^ •=4:^  0.6  c tr.' C  0.5 0.4  reaction time = 5.1 T slope = -2.930 x 10 4 r 1  Q 1^-  0.3  ......  40  0.2 0.1 0 ^ ^ ^ ^ ^ 3000 6000^9000 430^465^500^535 570 0 12000  wavelength (nm)  ^  time (s)  Figure 5.11 Spectral expansion of region around 472 nm and first-order log plot of data (run 5)  Since the reaction to form adducts of CpMo(NO)(=CHCMe3) is independent of the nature or concentration of the trapping ligand an intramolecular elimination of neopentane via an a-H abstraction reaction as the rate-determining step is strongly inferred as the mechanistic path to the key neopentylidene nitrosyl intermediate (eq 5.9).  T .... C. o ---, /Mom Me3C 18 CMe3  A -CMe  4  O  N  /  H  (5.9)  CMe3  The transient neopentylidene complex has a very short lifetime and in solution is quickly trapped by phosphine. This conclusion is substantiated by the observation of an isosbestic point in both lR and UV-vis spectral monitoring of these thermolysis reactiolis.38  156 5.3.4.2 Variation of Temperature  To gain some insight into the nature of the transition state during the generation of CpMo(NO)(=CDCMe3) a kinetic analysis at various temperatures was conducted. Measurements were carried out in five solvents under identical chemical conditions. In all 27 runs (Table 5.6) the concentration of 5.1-d4 was maintained at 3.558 x 10 -3 M (30.0 mg in 25 mL). The trapping ligand used was PPh 2Me (concentration = 2.490 x 10 -2 M: 124.7 mg in 25 mL, 7.000 equiv). Comparing rates at one temperature (25 °C), the reaction is fastest in dichloromethane. The other four solvents shows very similar rate constants. Thus, if we set k ob s (THF) = 100 then the rate increases THF (100) < mesitylene (111) < Et 2 0 (117) < hexanes (121) < CH2C12 (235) at 25 °C. At 40 °C, the order changes slightly to THF (100) < Et 2 0 (108) < hexanes (114) < mesitylene (115) < CH2 C12 (207). Thus, within the errors of the measurements, dichloromethane seems to double reaction rates for reaction 5.9 but the rates are nearly independent of all other solvents used in this study. These results stand in marked constrast to those observed by Schrock in related studies of some Ta systems where he found rate constants at room temperature to increase as Et 2 0 (100) < pentane (200) < benzene (400) < CHCI 3 and CH2 C1 2 (1500). 39 In both systems halogenated solvents seem to be promoters of a-H abstraction reactions, but the role the solvent plays is still ambiguous. Kinetic measurements were also attempted in CHCI 3 , benzene and toluene, but it appears that these solvents are reactive towards CpMo(N0)(=CDCMe 3 ). It might be speculated that C-H activation of solvent molecules is competitive with phosphine trapping. In the case of benzene activation, the product would be CpMo(NO)(CH2CMe3)Ph which is expected to be far more thermally unstable than complex 5.1 and would quickly decompose under the conditions in which it would be formed. Conversion of selected data in Table 5.6 to Eyring plots is shown in Figure 5.12, and the respective activation parameters for all solvents are compiled in Table 5.7.  157 Table 5.6 Rate Constants as a Function of Temperature in Various Solvents  run  solvent  7  15.0  kobs ( 0 ) 1.964 x 10 -5  8  20.0  3.453 x 10 -5  -15.95  9  25.0  5.856 x 10 -5  -15.44  30.0  9.523 x 10 -5  -14.97  11  35.0  1.824 x 10 4  -14.34  12  40.0  2.912 x 10 -1  -13.89  13  45.0  4.834 x 10 -4  -13.40  14  25.0  2.920 x 10 -5  -16.14  15  30.0  4.779 x 10 -5  -15.66  32.5  6.632 x 10 -5  -15.34  17  37.5  1.052 x 10 4  -14.90  18  40.0  1.515 x 10 4  -14.54  19  25.0  3.010 x 10 -5  -16.11  20  32.5  6.387 x 10 -5  -15.38  40.0  1.610 x 10 -4  -14.48  22  47.5  2.819 x 10 -4  -13.94  23  55.0  6.014 x 10 .4  -13.21  24  25.0  2.491 x 10 -5  -16.30  25  32.5  5.674 x 10 -5  -15.50  40.0  1.409 x 10 4  -14.61  27  45.0  2.016 x 10 -4  -14.27  28  60.0  7.893 x 10 -1  -12.95  29  25.0  2.757 x 10 -5  -16.20  30  32.5  6.956 x 10 -5  -15.30  40.0  1.619 x 10 4  -14.48  32  45.0  2.268 x 10 4  -14.15  33  65.0  1.103 x 10 -3  -12.63  10  16  21  26  31  CH2Cl2  Et20  hexanes  THE  mesitylene  temp. (°C)  In (kobsiT) -16.50  158 -12 -  dichloromethane i hexanes  -16 -17 0.0030  ^  0.0031  ^  0.0032^0.0033  ^  0.0034  ^  0.0035  1/T Figure 5.12 Representative Eyring plots for the conversion of CpMo(NO)(CD2CMe3)2 (5.1-d4 ) to CpMo(NO)(=CDCMe3) in hexanes and CH2C12  Table 5.7 Activation Parameters Derived from Eyring Plots  runs 7 - 13 14 - 18 19 - 23 24 - 28 29 - 33  solvent CH2C12 Et20 hexanes THE mesitylene  All* (kJ/mol) + 79.2 + 81.4 + 78.5 + 78.9 + 73.7  AS (J/mol•K) - 60.1 (-14.4 eu) - 58.9 (-14.1 eu) - 68.1 (-16.3 eu) - 68.1 (-16.3 eu) - 84.0 (-20.1 eu)  AG1298  (kJ/mol) + 97.1 + 99.0 + 98.8 + 99.2 + 98.7  Interpretation of the thermolysis data leads to several conclusions. Fisrt, in all cases the entropy of activation is negative, thereby implying a highly ordered transition state. 39 Second, since both AH: and AS: do not really vary with solvent, it is proposed that the intramolecular expulsion of neopentane in this reaction is not particularly affected by the solvent and that the transition state is not highly solvated.  159  5.3.4.3 Mechanistic Implications on the Formation of [CpMo(N0)(CHCMe3)1 2  Since the formation of CpMo(NO)(=CHCMe 3 ) is rate-determining, simple coupling of two molecules of CpMo(NO)(=CHCMe3) is not a mechanistic path to the asymmetric dimer 5.2. Instead, the transient neopentylidene monomer in all likelihood forms an adduct with a second molecule of 5.1 which then eliminates neopentane and rearranges to produce 5.2. Just why the dimer adopts such an asymmetric structure remains to be ascertained. A proposal for the mechanistic sequence of events from 5.1 to 5.2 is outlined in Scheme 5.1.  -CMe4  +5.1^„.” NO  ^ 0),,A1 ----)." ^,,.,M^JML, Ce „,,, No .TisAT  ON^  IK  --  CMe3^Me 3 C^e^H^CMe3^  4  —)  "  Mo ...t. NO ---.. mo  II--j  CMe3  14° H^CMe3  Scheme 5.1 Possible mechanism for the formation of [CpMo(NO)](1-71 1 :1 2 -NO)  (p-CHCMe3)[CpMo(=CHCMe3)] (5.2) from CpMo(NO)(CH 2 CMe3 ) 2 (5.1)  It is interesting that CpMo(NO)(=CHCMe 3 )(py) (5.6) thermally decomposes to complex 5.2, albeit at a much slower rate than does complex 5.1. It is likely that the mechanistic pathways to the dimer 5.2 from 5.1 and 5.6 are similar, but they clearly cannot be the same.  160 5.3.5 Further Chemistry of CpMo(N0)(=CHCMe3): Trapping Reactions with Ligands Containing E-H bonds (E = N, 0)  In order to learn more about the nature of the alkylidene ligand in the transient CpMo(NO)(=CHCMe 3 ) complex, two types of reactions were carried out. The N-H and 0-H bonds of amines and alcohols can be added stereoselectively across the Mo=C double bond of CpMo(NO)(=CHCMe3) yielding complexes of the general type CpMo(NO)(CH2CMe3)(ER').  5.3.5.1 Reactivity of CpMo(N0)(=CHCMe3) With Amines  The reaction of CpMo(N0)(CH 2 CMe3)2 with primary amines gives alkyl amide 40 complexes CpMo(N0)(CH2CMe3)(NHR) [R = CMe3 (5.9), p-tolyl (5.10)] (eq 5.10).  F^c...T.7  Cc  i... ..  Mo^  e, RNH2  ,„.^,.,,, Mo \ -CMe4^ f. ^NHR  Me3 C N o CMe 3^Me3C  (5.10)  o N  In order to determine whether these products arise from protonolysis of the dialkyl complex or NH addition across the Mo=C double bond in CpMo(N0)(=CHCMe3), a control experiment using the deuterated starting material was conducted. The reaction of CpMo(NO)(CD2CMe3)2 with (p-tolyl)NH2 affords CpMo(N0)(CHDCMe3)(NH-p-toly1) (5.10-d 1 ), thus confirming that the mechanism of the reaction proceeds via the alkylidene complex (Route B, Scheme 5.2) and not via direct protonolysis of an alkyl ligand (Route A, Scheme 5.2).  161  Route A D^D^D^D^D = R-y./ :^M°^: R - Me4C-d2, R !^M° \ ..N ..*. R Mo^: R RNH2 ♦ R:^ y' ..^• ■H I^D^D D^D^D RNH 2  Mo-d2 Route B R ??  ?RM°'y  D^D  Mo 4 C-d3^RN Me H2 _______, R--...,(^___,  D  N  .R  H  Mo-d1 Scheme 5.2 Possible pathways for the reaction of CpMo(NO)(CD2CMe3)2 with RNH2  Additionally, this reaction of amine with CpMo(NO)(=CDCMe 3 ) is stereoselective. There is only one 2H NMR resonance and one 1 H NMR resonance attributable to the methylene protons of the newly created CHDCMe3 group when CpMo(NO)(CD2CMe3)2 is allowed to react with Me3CNH2. An N-H addition across the Mo=C linkage is the only possible explanation for this experimental observation. This rules out prior coordination of the amine to the Mo center followed by proton transfer to the alkylidene carbon. Since only a single product is formed, we can assume that the transient alkylidene complex does not have C s symmetry. Curiously, when (p-tolyl)NH2 is used in reaction 5.10, two diastereomeric products are isolated in a 1:1 ratio as evidenced by two sets of all resonances in the 1 H NMR spectra of 5.10 (Figure 5.13). It is left to future work to determine the nature of the two products, although it is most likely that the complexes are related by rotation about the Mo-amide bond (vide infra).  Figure 5.13 1 H NMR spectrum of CpMo(N0)(CH2CMe 3 )(NH-p-toly1) (5.10) in C6D6  163 5.3.5.2 X-ray Crystallographic Analysis of Complex 5.10 Due to the complicated nature of the NMR spectra of complex 5.10, an X-ray  crystallographic analysis of the compound seemed in order. 41 The molecular structure is shown in Figure 5.14, and the complex is indeed monomeric as suggested by its mass spectrum. The internal metrical parameters (Table 5.8) for the CpMo(NO)(CH2CMe3) fragment resemble those found in CpMo(NO)(CD2CMe3)2 (Section 5.3.1.1). The anilide hydrogen atom, H(20), lies in the plane defined by Mo-N(2)-C(11) (deviation from the plane = 0.003 (4) A). The Mo-N(2) bond length of 1.955 (3) A appears to be typical for a Mo-NHR linkage. 42 The large Mo-N(2)-C(11) bond angle of 137.1 (3)° indicates some degree of Mo-N multiple bonding is present in this molecule. However, the most obvious difference between this structure and its parent dineopentyl complex (5.1) is within the angles in the legs of the piano-stool. A comparison of projections down the Cp-centroid-Mo axis is shown in Figure 5.15.  CH2CMe3  Me3CCH 2  Me3 CCH  N 0  5.1  5.10  Figure 5.15 Comparison of the angles about Mo in complexes 5.1 and 5.10  As discussed in Chapter 3 the bond angles about the metal centers in 16-electron alkyl and aryl complexes, Cp'M(NO)R2 are substantially different (for complex 5.1 these angles are 97.5, 98.3 and 111.5°). In the structure of complex 5.10 the analogous angles are very similar (95.3, 100.6, 96.0°). Clearly, the ability of the amide ligand to donate extra electron density to the Mo center satiates some of the electron deficiency inherent in CpMo(NO)(CH2CMe3)2 and effectively negates any open coordination position between the alkyl and amide linkages. Thus, the angle between the two neopentyl groups in 5.1 is 111.5°, and the angle between the neopentyl and  164 C(4)  Figure 5.14 ORTEP diagram of CpMo(N0)(CH2CMe3)(NH-p-toly1) (5.10). 50% probability ellipsoids are shown for the non-hydrogen atoms. Hydrogen atoms other than H(20) have been omitted for clarity  Table 5.8 Representative bond lengths and angles for complex 5.10  bond lengths (A) (esd) N2-H20 0.76 (4) Mo-C6 2.1904 Mo-N1 1.751 (3) Mo-N2 1.955 (3) N1-0 1.221 (3)  a  Parameter restrained during refinement.  bond angles (°) (esd) C6-Mo-N1 95.3 (5) N1-Mo-N2 100.6 (1) N2-Mo-C6 96.0 (5) Mo-C6-C7 130.4a Mo-N2-H20 113.3 (32) Mo-N2-C11 137.1 (3) Mo-N1-O 170.2 (3)  165 anilide groups in 5.10 is only 96.0°, a significant drop of 15.5°. Consistent with these structural observations are the IR spectra of both 5.1 and 5.10. The observed values of vNo for these materials are 1623 and 1548 cm -1 , respectively, i.e. the spatially constricted complex (5.10) is the weaker Lewis acid by a substantial margin. The above evidence would indicate that the amide ligand can donate significant amounts of it-electron density to the Mo atom. Multiple-bond character in the Mo-N linkage likely accounts for the two rotational isomers of the complex observed in the room temperature NMR spectra of complex 5.10 (vide supra).  5.3.5.3 Reactivity of CpMo(NO)(=CHCMe3) with Phenol Addition of the 0-H bond of PhOH across the Mo=C double bond of CpMo(N0)(=CHCMe3) gives the alkyl alkoxide complex CpMo(NO)(CH2CMe3)(OPh) (5.11) (eq 5.11). The complex is a red oil at room temperature, but may be purified by precipitation from hexanes at low temperatures.  A -CMe4  H O  PhOH  Mo  N CMe3  Me3 C  OPh  (5.11)  N  0  The fact that 5.11 is thermally stable implies that previously unsuccessful attempts 43 to synthesize CpMo(N0)(R)(OR') complexes were more a function of the metathesis methodology employed and not the inherent instability of the product complexes.  5.4 Epilogue and Future Work The original metathesis method (Chapter 1) leading to Cp 1M(NO)R2 complexes was conducted at room temperature. Clearly, this is the reason previous workers were unable to synthesize CpMo(N0)(CH2CMe 3 )2 . It is interesting that this complex, the most thermally  166 sensitive, yet isolable Cp'M(NO)R2 complex, displays the cleanest thermolysis chemistry. The work described in this chapter has shown that CpMo(NO)(CH2CMe3)2 decomposes via a firstorder process to the transient 16-electron neopentylidene complex, CpMo(N0)(=CHCMe3). The characteristic chemistry of CpMo(N0)(=CHCMe3) investigated to date has shown that it can be trapped either with 2-electron donor ligands or with heteroatom-hydrogen bonds which readily add across the Mo=C bond. Since the pyridine complex, CpMo(NO)(=CHCMe3)(py), serves as a stable source of the transient CpMo(N0)(=CHCMe3) complex, all future research should begin with this synthetic precursor. The characteristic reactivity of the transient CpMo(N0)(=CHCMe3) fragment with other E-H bonds than those described at the end of this chapter, such as primary phosphines and thiols, should reveal the entire manifold of CpMo(N0)(R)(ER') complexes that are synthesizable. Additionally, unsaturated organic entities capable of coupling with the alkylidene ligand should also be investigated." Preliminary investigations into the thermal behaviour of other CpMo(NO)R2 complexes and CpW(N0)(CH2 CMe3)2 have shown some promise, but more work is required to fine tune the conditions under which alkylidene complexes are best generated.  5.5 References and Notes  (1)  Fischer E. 0.; Maasb61, A Angew. Chem., Int. Ed Engl. 1964, 3, 580.  (2)  Gallop, M. A.; Roper, W. R. Adv. Organomet. Chem. 1986, 25, 121.  (3)  Schrock, R. R. J Am. Chem. Soc. 1974, 96, 6796.  (4)  Schrock, R. R. Acc. Chem. Res. 1979, 12, 98.  (5) This is only a very general comparison. Many complexes of both types have been found to be amphiphilic. For example, see the chemistry of (a) CpRe(CO)2(=CHR) in Casey, C.; Vosejpka, P. C.; Askham, F. R. J. Am. Chem. Soc. 1990, 112, 3713 or (b) (C0)2(Ph3P)2Ru(=CF2) in Clark, G. R.; Hoskins, S. V.; Jones, T. C.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1983, 719.  167 (6)  Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987.  (7)  Carter, E. A.; Goddard, W. A. J. Am. Chem. Soc. 1986, 108, 4746.  (8)  Legzdins. P.; Rettig, S. J.; Veltheer, J. E. J Am. Chem. Soc. 1992, 114, 6922.  (9)  Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 3rd ed.; Pergamon Press: Oxford, U.K., 1988.  (10) This preparation is derived from that found in the literature for Me3CCD2MgBr, see Caulton, K. G.; Chisholm, M. H.; Streib, W. E.; Xue, Z. J. Am. Chem. Soc. 1991, 113, 6082. (11) The darkening of this solution is consistent with the dimerization of CpH to (CpH)2. Additionally, the crystallizing solution had the characteristic odor of dicyclopentadiene. (12) Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B. E.; Gatter, M. G. Organometallics 1988, 7, 2394. Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992, 11, 2583.  (13) (a) For Mo see: Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B. E.; Gatter, M. G. J.  Am. Chem. Soc. 1985, 107, 1411. (b) For W see: Bursten, B. E.; Cayton, R. H. Organometallics 1987, 6, 2004. (14) Brouwer, E. B.; Debad, J. D.; Legzdins, P.; Ross, K. J.; Veltheer, J. E. unpublished observations. (15) Crystals of 5.1 -d4 are monoclinic of space group P21/n; a = 5.9781 (6) A, b = 20.065 (4)  A, c = 13.6355 (17) Á; 13 = 95.520 (10)°; Z= 4. Drs. Ray Batchelor and Fred Einstein solved the structure using the Patterson method and full-matrix least-squares refinement procedures to R= 0.025, Rw = 0.032 for 2494 reflections with /0 ?. 2.5(7(4). (16) The red solid turns brown and exhibits two nitrosyl-stretching frequencies at 1597 and 1578 cm-1 in its Nujol mull IR spectrum. A 1 H NMR spectrum of the solid in C6D6 displays two major Cp resonances at 5 5.43 and 5.34 ppm in a 1:2 ratio as well as two CMe3 peaks at 5  168 1.35 and 1.29 ppm in a 2:1 ratio. Numerous other smaller resonances are also observed. As more spectra are recorded signals due to numerous products appear and/or disappear. There is no evidence for the existence of complex 5.2 evident in these spectra. (17) Crystals of 5.2 are orthorhombic of space group Pcab; a = 17.836 (2) A, b = 18.265 (2) A, c = 13.198 (2) Z = 8. Dr. Steve Rettig solved the structure using the Patterson method and full-matrix least-squares refinement procedures to R = 0.027, Rai, = 0.026 for 3406 reflections with I 3a(I). (18) Examples of complexes being associated symmetrically via two bridging alkylidene ligands, 16a two conventionally bridging nitrosyl ligands,  16 b or multiple metal-metal  bonds, 16c are well known. For instance, see (a) Holton, J.; Lappert, M. F.; Pearce, R.; Yarrow, P. I. W. Chem. Rev. 1983, 83, 135. (b) CalderOn, J. L.; Fontana, S.; Frauendorfer, E.; Day, V. W.; Iske, S. D. J. Organomet. Chem. 1974, 64, C10 and C16. (c) Toreki, R.; Schrock, R. R.; Vale, M. G. J. Am. Chem. Soc. 1991, 113, 3610 and references therein. (19) For comparison, other long N-0 bonds are: (a) 1.247 (5)  A for the µ3-NO of  Cp3 Mn3(NO) 4 : Elder, R. C. Inorg. Chem. 1974, 13, 1037; (b) 1.271 (7) A for the µ4-T12NO of a mixed Co/Mo cluster: Kyba, E. P.; Kerby, M. C.; Kashyap, R. P.; Mountzouris, J. A.; Davis, R. E. J. Am. Chem. Soc. 1990, 112, 905; and (c) 1.47 (1) A for CpRe(PPh3)(SiMe2C1)(NO-BC13): Lee, K. E.; Arif, A. M.; Gladysz, J. A. Inorg. Chem. 1990, 29, 2885.  (20) Other long Mo-Mo single bonds have been reported: (a) 3.056(1)  A for [CpMo(C0) 2 ] 2 -(g  -NCNMe2): Chisholm, M. H.; Cotton, F. A.; Extine, M. W.; Rankel, L. A. J. Am. Chem. Soc. 1978, 100, 807. (b) 3.117(1) A for [CpMo(C0)2]2-(.1-allene): Bailey, W. I.; Chisholm, M. H.; Cotton, F. A.; Murillo, C. A.; Rankel, L. A. J. Am. Chem. Soc. 1978, 100, 802. (21) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University Press: New York, 1992; Chapter 2 and references therein.  169 (22) Reference 21, p 64. (23) Examples of g2 -1 1 :1 2 -co ligands have been documented; see, (a) Horwitz, C. P.; Shriver, D. F. Adv. Organomet. Chem. 1984, 23, 219 and references therein. (b) Marsella, J. A.; Huffman, J. C.; Caulton, K. G.; Longato, B.; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 6360. (24) Simpson, C. Q.; Hall, M. B. J. Am. Chem. Soc. 1992, 114, 1641. (25) Reference 6, p 126 states that CPh 2 groups are electrophilic and thus best considered as Fischer carbenes. (26) Herrmann, W. A.; Hubbard, J. L.; Bernal, I.; Korp, J. D.; Haymore, B. L.; Hillhouse, G. L. Inorg. Chem. 1984, 23, 2978. (27) Fischer, E. 0. Pure Appl. Chem. 1970, 24, 407. (28) Nugent, W. A.; Mayer, J. M. Metal Ligand Multiple Bonds, John-Wiley and Sons Ltd., -  NY, 1988, Chapter 4. (29) Crystals of 5.3 are monoclinic of space group P2 1/n; a = 9.328 (2) A, b = 15.516 (3) A, c = 15.188 (2) A, B = 93.72 (1)°; Z= 4. Dr. Steve Rettig solved the structure using the Patterson method and full-matrix least-squares refinement procedures to R = 0.042, R w = 0.037 for 4081 reflections with 4 ?_ 3a(4). (30) Kiel, W. A.; Lin, G-Y.; Constable, A. G.; McCormick, F. B.; Strouse, C. E.; Eisenstein, 0.; Gladysz, J. A. J. Am. Chem. Soc. 1982, 104, 4865. (31) Reference 28, p 132 (32) Schultz, A. J.; Williams, J. M.; Schrock, R. R.; Rupprecht, G. A.; Fellmann, J. D. J. Am. Chem. Soc. 1979, 101, 1593. (33) For the purposes of this comparison formal oxidation state is determined assuming alkylidene ligands to be CHR 2- groups and not neutral dative donor ligands. (34) Reference 28, p 187. (35) Christensen, N. J.; Hunter, A. D.; Legzdins, P.; Sanchez, L. Inorg. Chem. 1987, 26, 3344.  170 (36) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059. (37) Three attempts were made to determine the kinetic isotope effect for this system, but difficulty in handling samples of the non-deuterated dialkyl complex gave unreliable and non-reproducible measurements. The 3 values of kH/kD that were determined were 4, 7 and 11. (38) Ebsworth, E. A. V.; Rankin, D. W. H.; Cradock, S. Structural Methods in Inorganic Chemistry, Blackwell Scientific, Oxford, U.K., 1987, p 276. (39) Wood, C. D.; McLain, S. J.; Schrock, R. R. J. Am. Chem. Soc. 1979, 101, 3210 (40) Related Cp * -containing alkyl amide complexes have been prepared by methathesis reactions of Li(amide) and Cp * M(NO)(R)Cl. Legzdins, P; Rettig, S. J.; Ross, K. Organometallics, in press. (41) Crystals of 5.10 are monoclinic of space group P2 1/c; a = 11.243 (2) A, b = 9.632 (2) A, c = 16.845 (3) A, 13 = 99.74 (1)°; Z = 4. Drs. Ray Batchelor and Fred Einstein solved the structure using the Patterson method and full-matrix least-squares refinement procedures to RF= 0.031 for 2213 data with 10 2.5a(10 ). (42) For example, the average Mo-amide bond length in (a) Mo2(OCMe 3)4 (P hNH) 2(PlINH2)2 is 1.97 A, see: Chisholm, M. H.; Parkin, I. P.; Streib, W. E.; Folting, K. S. Polyhedron 1991, 10, 2309 and (b) [HB(Me2pz)3]Mo(NH-n-Bu) 2 is 1.94 A, see: Obaidi, N. A.; Hamor, T. A.; Jones, C. J., McCleverty, J. A.; Paxton, K. J. Chem. Soc., Dalton Trans. 1986, 1525. (43) Lundmark, P. J., Ph.D. Dissertation, The University of British Columbia, 1993. (44) For recent examples of this type of chemistry, see: Garrett, K. E.; Sheridan, J. B.; Pourreau, D. B.; Feng, W. C.; Geoffroy, G. L.; Staley, D. L.; Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 8383 and references therein.  

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