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Metal basicity : coordination behaviour of some non-conventional Lewis-acids Clentsmith, Guy Kenneth Bruce 1996

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M E T A L BASICITY : COORDINAT ION B E H A V I O U R OF S O M E N O N -C O N V E N T I O N A L LEWIS -AC IDS . by Guy Kenneth Bruce Clentsmith B.Sc. (Hons.), University of New South Wales, 1988. M . Sc., University of British Columbia, 1991. A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in THE F A C U L T Y OF G R A D U A T E STUDIES (Department of Chemistry) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1996 © Guy Kenneth Bruce Clentsmith, 1996 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. Department of C M E ^ t S T ^ The University of British Columbia Vancouver, Canada Date 3US .. <+ . °>G DE-6 (2/88) ABSTRACT The potential of a series of transition metal hydride and alkyl complexes to act as metal-bases is examined, and their interaction with some conventional and non-conventional Lewis-acids is explored. Stoichiometric amounts of LiBEt2H2, LiBEu, and Pd{PPri2(CH2)3PPri2}Cl2 generated [{PPri2(CH2)3PPr^}Pd]2(p>H)2»LiBEt4 , a neutral adduct of the metal-base [{PPri2(CH2)3PPri2}Pd]20>-H)2 and Lewis-acidic LiBEta. The parent metal hydride, [{PPri2(CH2)3PPri2}Pd]2u>-H)2, can be prepared independently from K B E t 3 H and Pd{PPri2(CH2)3PPri2}l2, and quantitatively forms the adduct with LiBEt4. Similarly, the NaBEt4 and LiAlEu adducts of [{PPri2(CH2)3PPri2)Pd]2(u.-H)2 are prepared. Solution spectroscopy of the adducts indicates lability of the coordinated Lewis-acid, but adduct formation is thermodynamically favoured. Adduct formation with LiBEt4 is also observed for Cp2ReH, CP2WH2, and Cp2TaH3, with Cp2TaH3«LiBEt4 being the most stable. Treatment of Pd{PPri2(CH2>2PPri2 } C12 with KBEt3H gives zerovalent [{PPri2(CH2)2PPri2}Pd]2(p-{PPri2(CH2)2PPri2 }). [ {PPr* 2(CH 2) 2PPr i2 } P d ] 2 a n d [{PPri2(CH2)3PPri2}Pd]2 have also been prepared and characterized, and found to undergo reaction with the known complexes PdtPPr^CCFJ^hPPr^h and PdfPPr^CE^bPPr^h to give [{PPri2(CH2)2PPri2}Pd]2u>-{PPri2(CH2)2PPri2}) and [ { P P ^ C ^ P P r ^ P d h O i -{PPri2(CH2)3PPri2}) respectively. Pd{PPri2(CH2)2PPri2}Me2, when treated with one equiv of [H(OEt2)2]BArf (where BArf = B[3,5-(F3C)2C6H3J4)-), irreversibly gives the methyl cation [Pd{PPr12(CH2)2PPri2}Me]BArf. This complex is highly Lewis-acidic, and activates H2 to give [{PPri2(CH2)2PPri2}Pd]20>-H)2{BArf}2 and methane. Carbonylation of [Pd{PPr12(CH2)2PPri2}]vle]BArf gives an unprecedented mixed valence complex of palladium, {[Pd{PPri2(CH2)2PPri2}Me][Pd{PPri2(CH2)2PPri2}CO]} {BAr f }, a l o n g w i t h ii [Pd{PPri2(CH2)2PPri2}(CO)2]{BArf}2 and acetone. Similar chemistry was also performed with palladium methyl complexes bearing the new ligand Pr^PCCMehCHNBu 1 . iii TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iv LIST OF FIGURES , xi LIST OF TABLES xiv GLOSSARY OF TERMS xvi ACKNOWLEDGMENTS xx Chapter 1: INTRODUCTION 1 1.1 Preamble 1 1.2 Metal Basicity 1 1.2.1 Background and history 1 1.2.2 Nature of Lewis-acid/metal-base interactions 3 1.2.3 Influence upon reactivity 4 1.3 Summary of previous work in this laboratory 8 1.4 Outline and scope of this work 11 1.5 References 12 Chapter 2: BINUCLEAR PALLADIUM COMPLEXES WITH BRIDGING HYDRIDES. UNUSUAL COORDINATION BEHAVIOUR OF LiBEt4 AND NaBEt* 16 2.1 Introduction to Chapter 2 16 2.2 Isolation and structure of [(dippp)Pd]20J.-H)2,LiBEt4 (1) 16 2.3 Isolation and structure of [(dippp)Pd]2(p.-H)2 (2) 22 2.3.1 Comparison of 2 to analogous Pt systems 27 2.4 Metal-basicity of [(dippp)Pd]2(u.-H)2 (2) 34 2.5 Structure of [(dippp)Pd]2(p.-H)2,NaBEt4 (3) and comparison to 1 36 iv 2.6 Thermodynamic parameters associated with the formation of 1 38 2.7 Lewis-acid exchange between 1, 2,3, and 4 40 2.8 Bonding Considerations 42 2.9 Reactivity Studies 43 2.10 Conclusions 44 2.11 Experimental 45 2.11.1 Procedures 45 2.11.2 Materials 46 2.11.3 Syntheses 47 2.11.3.1 Pd(dippp)Cl2 • 47 2.11.3.2 Pd(dippp)I2 47 2.11.3.3 [(dippp)Pd]2(p-H)2 (2) 47 2.11.3.4 [(dippp)Pd]2(p-D)2 (2-d2) 48 2.11.3.5 [(cbppp)Pd]2(p-H)2'LiBEt4 (1) • 48 2.11.3.6 [(dippp)Pd]2(p-H)2*NaBEu (3) 49 2.11.3.7 [(dippp)Pd]2(p-H)2«LiAlEt4 (4) 50 2.11.3.8 Pd(dippp)(Ti2-H2C=CH2) 50 2.11.3.9 Pd(dippp)PPh3 50 2.11.3.10 Pd(dippp)(DMAD) .' 51 2.11.3.11 Pt(dippp)I2 51 2.11.3.12 Pt(dippp)H2 (5) ...52 2.11.3.13 [Pt(dippp)H]2 (6) 52 2.11.3.14 [(dippp)Ni]2(M.-H)2 53 2.11.3.15 [(dippp)Ni]20>-H)2«LiBEt4 53 2.11.3.16 [(dippp)Rh]2(u:-H)2«LiBEt4 53 2.12 References : 54 v Chapter 3: COORDINATION BEHAVIOUR OF L i B E t 4 TOWARDS (n*-C 5 H 5 ) 2 ReH, (Tl5-C 5H5) 2WH2, AND (Tt5-C 5Hs) 2TaH3. SOLID STATE STRUCTURE OF [(n5-C5H5)2Ta(uJl)2AlH]2(u.-OBu)2 57 3.1 Intrcxiuction 57 3.2 General 58 3.2.1 Metal basicity of C f o M H n 5 8 3.2.2 Syntheses of Cp2MH n 59 3.3 Coordination behaviour of L iBEU to the metallocene hydrides 60 3.4 Competition behaviour by 1, 2, and 3 for LiBEt4 and considerations of bonding 64 3.5 Routes to Cp2TaH3; isolation of [Cp2Ta(u-H)2AlH]2(u-OBu) (4) 68 3.6 Solid-state and solution structure of [Cp2Ta(p-H)2AlH]2(p-OBu)2 (4) 70 3.7 Bonding considerations 75 3.8 Mechanism of formation of 4 75 3.9 Reactivity of [Cp2Ta(p.-H)2AlH]2(p:-OBu)2 (4) 80 3.9.1 Hydrolysis of 4 to give Cp2TaH3 80 3.9.2 Reactivity of 4 towards CO and olefins 81 3.10 Conclusions 82 3.11 Experimental 83 3.11.1 Procedures 83 3.11.2 Materials . . 83 3.11.3 Syntheses . 83 3.11.3.1 Cp2ReH»LiBEt 4 (1) 83 3.11.3.2 Cp2WH2»LiBEt4 (2) 84 3.11.3.3 Cp2TaH3»LiBEt4 (3) 84 3.11.3.4 [Cp2Ta(u,-H)2AlH]2(p.-OBu)2 (4) 84 3.11.3.5 Cp2TaH3»LiAlEt4 (6) 85 vi 3.11.3.6 Cp 2TaAlH4»THF (5) 85 3.12 References.. 85 Chapter 4: Z E R O V A L E N T 16- A N D 1 4 - E L E C T R O N P A L L A D I U M C O M P L E X E S 88 4.1 Introduction to Chapter 4 88 4.1.1 General 88 4.1.2 Zerovalent palladium phosphine complexes 89 4.2 Discussion 92 4.2.1 Isolation of [(dippe)Pd](p-dippe) 92 4.2.2 Single crystal X-ray structure of [(dippe)Pdj(p-dippe) (4) 94 4.2.3 Solution structure of [(dippe)Pd](p-dippe) (4) 96 4.3 Routes to zerovalent, 1:1 complexes of phosphines and palladium 97 4.4 Catalytic studies 99 4.5 Analysis of the 3 lP[ lH) N M R spectrum of [Pd(dippe)]2(u>dippe) 102 4.6 Conclusions 106 4.7 Experimental 106 4.7.1 Procedures 106 4.7.2 Materials 107 4.7.3 Syntheses 107 4.7.3.1 [(dippe)Pd]2(p-dippe) (4) 107 4.7.3.2 [(dippp)Pd]2(dippe) (5) 108 4.7.3.3 [(dippe)Pd]2(dippp) (6) 108 4.7.3.4 Pd(dippe){Ti2-H2C=C(Me)CH2OMe} (7) and [Pd(dippe)]n (8) 109 4.7.3.5 Pd(dippp) {Ti2-H2C=C(Me)CH2OMe} (9) and [Pd(dippp)]n(10) 109 4.7.4 Catalyses 110 vii 4.8 References 110 Chapter 5: C A T I O N I C P A L L A D I U M A L K Y L S P E C I E S S T A B I L I Z E D B Y E L E C T R O N - R I C H PHOSPHINES 112 5.1 Introduction to Chapter 5 , 112 5.2 General... 113 5.3 Routes to [Pd(dippe)(r|3-CH2C6H5)]+ • -.114 5.4 Preparation of [Pd(dippe)Me]+ 119 5.5 Reactivity of [Pd(dippe)Me(OEt2)](BArf) (2) 122 5.5.1 Reaction with dihydrogen 122 5.5.2 Lewis-acidity 123 5.5.3 Reaction with ethylene 125 5.5.4 Reaction with carbon monoxide 127 5.6 Solid-state and solution structure of {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6) 130 5.7 Formation of {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6).. 134 5.8 Copolymerization experiments 142 5.9 Palladium cations stabilized by P N type donors 142 5.9.1 Ligand and metal precursor syntheses 142 5.9.2 Solid-state structure of [Pd(dippim)Me(OEt2)](BArf) (13) 146 5.9.3 Solution structure of [Pd(dippim)Me(OEt2)](BArf) (13) 149 5.9.4 Solid-state structure of Pd(dippim)Me2 (12) and comparison to that of [Pd(dippim)Me](BAr f) (13) 151 5.10 Conclusions 153 5.11 Experimental 155 5.11.1 Procedures 155 5.11.2 Materials .155 5.11.3 Syntheses.... 155 vii i 5.11.3.1 Pd(dippe)Me2 155 5.11.3.2 Pd(dippp)Me2 • 156 5.11.3.3 l-(diisopropyl)isobutylidene tert-butylamine (dippim) 157 5.11.3.4 Pd(dippim)Cl2 157 5.11.3.5 Pd(dippe)(CH2Ph)2 158 5.11.3.6 [Pd(dippe)CH2Ph]{B(C6F5)3CH2Ph} 158 5.11.3.7 Pd(dippe)(CH2Ph)Cl 159 5.11.3.8 rPd(dippe)(CH2Ph)I 159 5.11.3.9 [Pd(dippe)(r|3-CH2Ph](BArf) (1) 160 5.11.3.10 [Pd(dippe)Me(OEt2)](BArf) (2) 160 5.11.3.11 [Pd(dippp)Me(OEt2)](BArf) (3) 161 5.11.3.12 {[Pd(dippe)]2(p-H)2}(BArf)2(4) 161 5.11.3.13 Methyl transfer between 2 and Pd(dippe)(13CH3)2 162 5.11.3.14 Pd(dippe)(Me)CH2Ph (5) .....162 5.11.3.15 {[Pd(dippe)CO] [Pd(dippe)Me]} (BArf) (6) and {[Pd(dippe)(13CO)2][Pd(dippe)Me]}(BArf) (6a) 162 5.11.3.16 [Pd(dippe)(C01 3CH3)CO](BAr f) (7) 163 5.11.3.17 [Pd(dippe)(1 3CH3)CO](BAr f) (8) 164 5.11.3.18 [Pd(dippe)(CO)2](BArf)2 (9), [Pd(dippe)(N=CCH3)](BArf)2 (10a), and [Pd(dippe)(TFIF)2](BArf)2 (10b) 164 5.11.3.19 {[Pd(dippp)CO][Pd(dippe)Me]} (BAr f) (11) • 165 5.11.3.20 Pd(dippim)Me2 (12) 165 5.11.3.21 rPd(dippim)(Me)Et20](BArf) a3) 166 5.12 References 166 Chapter 6: CONCLUSIONS AND SECOND THOUGHTS 171 6.1 Retrospective 171 ix 6.2 Prospective 172 6.2.1 Adducts of LiBEu 172 6.2.2 Development of [Pd(dippe)Me]+ chemistry 172 6.2.3 Development of P-N chemistry 175 6.2.4 Development of [Pd(dippe)]2+ chemistry 177 6.3 References 178 APPENDICES .180 x List of Figures Figure Caption Page Figure 1.1 Chem 3D® view of [(cuppp)Pd]2(p-H)2»LiBEt4. 10 Figure 2.1 (a) Molecular structure of [(dippp)Pd]2(l|.-H)2,LiBEt4 (1); and 19 (b), core view of 1 showing the interaction of the L i cation with the three a - C - H bonds in the ethyl groups, the bridging hydrides and the palladium centres (isopropyl groups have been removed for clarity). Figure 2.2 (a) Molecular structure and of [(dippp)Pd]2(p-H)2 (2); and (b), 25 core view of 2 (isopropyl groups have been removed for clarity). Figure 2.3 A view looking down the Pd-Pd axis of [(dippp)Pd]2(p-H)2 (2), 26 [(dippp)Pd]2(p-H)2»LiBEt4 (1), and [(dippp)Pd]2(u,-H)2«NaBEt4 (3) to show the non-planar nature of the hydride and phosphorus cores. Figure 2.4 Chem 3D® view of [Pt(dippp)H]2 (6). 32 Figure 2.5 (a) Molecular structure of [(dippp)Pd]2(p-H)2,NaBEt4 (3); and 37 (b), core view of 3 showing the interaction of the Na cation with the four a - C - H bonds in the ethyl groups, the bridging hydrides and the palladium centres (isopropyl groups have been removed for clarity). Figure 2.6 Experimental (left) and simulated (right) variable temperature *H 39 N M R spectra of an equimolar mixture of [(dippp)Pd]2(p.-H)2*LiBEt4 (1) and [(dippp)Pd]2(p-H)2 (2) in the hydride region (0 Hz = -3.00 ppm; [Pd] = 4.20 x 10"2 mol L _ 1 ) ; the very sharp quintet at low temperatures is due to 2 while the broader resonance is due to the LiBEt4 adduct 1. xi Figure 3.1 300.1 M H z *H N M R spectrum of the hydride region of (a) 62 Cp2TaH3 and (b) Cp2TaH3»LiBEt4 (3), in ^-benzene. Figure 3.2 Proposed structures for Cp2ReH (1), Cp2ReH (1), CP2WH2 (2), 68 and Cp2TaH3 (3). Figure 3.3 Chem 3D® view of molecular structure of [Cp2TaAlH]2(p- 70 H)2(p-OBu)2 (4) (Cp ring centroids have been added for clarity). Figure 3.4 Chem 3D® view of one half of the [Cp2TaAlH]2(p-H)2(p-OBu)2 72 (4) molecule normal to the TaAlH2 plane. Figure 4.1 Experimental 202.47 MHz 3 1 P{ *H} spectrum of [(dippe)Pdh(p- 93 dippe) (4) in ds-toluene: (a) total spectrum; (b) downfield region (A-resonance, bridging phosphorus nuclei); (c) upfield region (B2-resonance, chelating phosphorus nuclei). The absorption at 50.9 ppm results from a folded-in peak due to an impurity. Molecular structure of [Pd(dippe)]2(p-dippe) (4) The 3 1 P{ lU} N M R [AB2k spin system of 4 and spin labels used (R = Pr1). Simulated (top trace) and experimental (bottom trace) 202.47 MHz 3 1 P{ 1 H} N M R spectra of [(dippe)Pd]2(p--dippe) (4). For data see Table 4.2. Figure5.1 500.13 M H z *H (b), and ! H { 3 1 P } (a) N M R spectra of [Pd(dippe)(ri3-CH2C6H5)](BArf) (1) in ^ -methylene chloride. Figure 5.2 (a) 50 .32 M H z ^ C ^ H ) N M R spectrum of [Pd(dippe)(l3CH3)(OEt2)](BArf) (2-1 3 C 1 ) ; and (b), corresponding 81.015 M H z ^ P p H } N M R spectrum of 2- 1 3 Ci. Figure 5.3 Chem 3D® view of the cat ionic port ion of 130 {[Pd(dippe)CO][Pd(dippe)Me]} (BAr f) (6). Figure 4.2 Figure 4.3 Figure 4.4 94 102 105 116 121 xii Figure 5.4 O R T E P v iew of the ca t ion ic por t ion of 148 [Pd(cUppim)Me(OEt2)](BArf) (13). Figure 5.5 Chem 3D® view of Pd(dippim)Me2 (12). 153 Figure A l ORTEP view of, and numbering scheme for [Pt(dippp)]2(u.-H)2 182 (6, Chapter 1). Figure A2 ORTEP view of, and numbering scheme for [Cp2Ta(p- 185 H) 2AlH] 2(u-OBu)2 (4, Chapter 3). Figure A3 ORTEP view of, and numbering scheme for cationic portion of 187 {[Pd(dippe)CO][Pd(dippe)Me])(BArf) (6', Chapter 5). Figure A4 ORTEP view of, and numbering scheme for [Pd(dippim)Me2 (12, 192 Chapter 5). xii i List of Tables Table Title Page Table 2.1 Selected intramolecular distances (A) observed in 20 [(dippp)Pd]2(u-H)2'LiBEt4 (1), [(dippp)Pd]2(u-H)2 (2), and [(dippp)Pd]2(p-H)2»NaBEt4 (3). Table 2.2 Selected intramolecular angles (deg) observed in [(dippp)Pd]2(p- 21 H) 2 -LiBEt4 (1), [(dippp)Pd]2(u-H)2 (2), and [(dippp)Pd]2(p-H) 2«NaBEt4 (3). Table 2.3 Selected intramolecular bond lengths (A) and bond angles (deg) 33 observed in [Pt(dippp)Ff]2 (6). Table 3.1 300.1 MHz *H NMR data (ppm) for Cp 2 ReH»LiBEt 4 (1), 63 Cp2WH2«LiBEt4 (2), and Cp2TaH3»LiBEt4 (3); the values for the parent hydrides Cp2ReH, CP2WH2, and Cp2TaH3 are reported in parentheses. Table 3.2 Selected intramolecular distances (A) observed in 71 [Cp2TaAlH]2(ji-H)2(p-OBu)2 (4). Table 3.3 Selected intramolecular angles (deg) observed in 71 [Cp2TaAlH]2(jl-H)2(p-OBu)2 (4). Table 4.1 Selected intramolecular distances (A) and angles (deg) observed 96 in [(dippe)Pd]2(p-dippe) (4). Table 4.2 Starting parameters using the [AX2]2 approximation, and final 104 iterated parameters for the [AB2]2 system. Table 5.1 Selected intramolecular distances (A) observed in 130 {[Pd(dippe)Me][Pd(dippe)CO] }(BArf) (6). Table 5.2 Selected intramolecular angles (deg) observed in 130 {[Pd(dippe)Me][Pd(dippe)CO]} (BAr f) (6) xiv Table 5.3 Selected intramolecular bond lengths (A) and bond angles (deg) 149 observed in [Pd(dippim)Me(OEt2)](BArf) (13) and Pd(dippim)Me2 (12). Table A l Crystallographic data for [Pt(dippp)]2(u,-H)2 (6 , Chapter 1), 180 iCp2Ta(p-H)2AlH]2(u,-OBu)2 (4, C h a p t e r 3 ) , {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6', Chapter 5), and Pd(dippim)Me2 (12, Chapter 5). a Final atomic coordinates (fractional) and 2? e q (A2) for [Pt(dippp)]2(|i-H)2 (6, Chapter 1). Final atomic coordinates (fractional) and Z?eq (A 2) for [Cp2Ta(p> H)2AlH]2(u,-OBu)2 (4, Chapter 3). Final atomic coordinates (fractional) and 2? e q (A 2 ) for {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6', Chapter 5). Atoms C(63)-C(69), the toluene solvent, are disordered about a centre of symmetry and have variable occupancy. Table A2.4 Final atomic coordinates (fractional) and 5 e q (A2) for 193 [Pd(dippim)Me2 (12, Chapter 5). Table A2.1 Table A2.2 Table A2.3 183 186 188 xv GLOSSARY OF TERMS The following abbreviations, most of which are commonly found in the literature, are used in this thesis. observe proton while decoupling heteronucleus nnX 2 H or D deuterium 11B{ JH} observe boron while decoupling proton observe carbon while decoupling proton 31P{1H} observe phosphorus while decoupling proton A angstrom (10"10 m) Anal. analysis atm atmosphere avg average BAr f {B[3,5-(F3C)2C6H3]4}-br broad BuJ isobutyl group, -CH2CH(CH3>2 Bun n-butyl group, -CH2CH2CH2CH3 Bul tertiary butyl group, -C(CH3)3 BulO tertiary butoxy group, -OC(CH3)3 Calcd calculated Chem 3D® molecular modelling program for the Macintosh computer cm -1 wave number COD 1,5-cyclooctadiene Cp cyclopentadienyl group, [C5H5]" Cp'"' substituted Cp ligand, e.g. [CsKUMe] -Cp* pentamethylcyclopentadienyl group, {05(013)5}-d doublet xvi d sept doublet of septets dcype l,2-bis(dicyclohexylphosphino)ethane dd doublet of doublets deg (or °) degrees dippb l,4-bis(diisopropylphosphino)butane dippe 1,2-bis(dn^propylphosphino)ethane dippirh 1 -(dusopropylphosphmo)isobutylidene-tert-butylarxiine dippp 1,3-bis(diisopropylphosphino)propane dmpe l,2-bis(dimethylphosphino)ethane dppe l,2-bis(diphenylphosphino)ethane dppm bis(diphenylphosphino)methane dtbpm bis(ditert-butylphosphino)methane EI electron ionization equiv equivalent(s) eV electron Volt GCMS Gas Chromatography/Mass spectrometry h Hour H B A r f Brookhart's acid, [H(OEt2)2]{B[3,5-(F3C)2C6H3]4} Hz Hertz, seconds -1 IR infrared K degrees Kelvin kcal kilocalories Keq equilibrium constant L n M H m Generic metal hydride complex, " H m " refers to terminal metal hydrides, e.g. M - H . Non-classical dihydrogen ligands are denoted (TJ2-H2>. L U M O lowest unoccupied molecular orbital xvii M central metal atom (or "molar", when referring to concentration) m multiplet (or "medium", for infrared data) M + parent ion m- meta Me methyl group, -CH3 mg milligram(s) MHz megaHertz mL millilitre mm millimetre mmol millimole M O molecular orbital mol mole MS mass spectrometry n / A B n-bond scalar coupling constant between nuclei A and B N M R nuclear magnetic resonance o- ortho ORTEP Oakridge Thermal Ellipsoid Plotting Program OTf triflate group, -OSO2CF3 p para Ph phenyl group, -C6H5 ppm parts per million Pr1 isopropyl group, -CH(CH3)2 quin quintet R hydrocarbyl substituent RT room temperature s singlet (or "strong", for infrared data) sept septet xviii T temperature t triplet THF ether tetrahydrofuran tmeda 1 J,2,2-7V^^^-tetramemylemylenediamine TMS tetramethylsilane VT variable temperature x . y - 1 3 C n Complex x.y has n number of 1 2 C atoms replaced by 1 3 C atoms x.y-dn Complex x.y has n number of *H atoms replaced by 2 H atoms A heat AG^ Gibbs free energy of activation AH0 enthalpy of reaction °C degrees Celsius xix ACKNOWLEDGMENTS I would like to express my thanks to my supervisor, Professor Michael Fryzuk, for the guidance, patience, and humour he has shown me throughout the course of this work. I would also like to thank past and present members of his research group, including Drs. Jesse Ng, Tim Haddad, Patrick Paglia, David M c Convil le, Murugesapillai Mylvaganam, Xiaoliang Gao, Jim Kickham, Jason Love, Chris Haar, and Messrs. Garth Giesbrecht, Paul Duval, Danny Leznoff, Mike Bowdridge, Samuel Johnson, Ms. Laleh Jafarpour, and M . Frederic Naud, for their friendship and advice. The assistance of the technical staff, in particular Mr. Steve Rak and Mr. Peter Borda, was always cheerfully given and was greatly appreciated. The assistance of the staff of the N M R facility, Dr. Orson Chan, Mrs. Liane Darge, and Mrs. Marietta Austria, was also invaluable. A special thanks goes to Dr. Steve Rettig, who painstakingly solved all the crystal structures in this thesis. G. K. B. C. xx Chapter 1: INTRODUCTION 1.1 Preamble TO THE INORGANIC CHEMIST the notion of a metal acting as an acceptor of electron density is very familiar. A picture such as the following (a) underlies the most fundamental ideas of coordination chemistry. L n M ^ — : L LnM: •-E" a. b. That a metal should act in the reverse sense, that is as a source of electrons (cf. b), is not appreciated as readily, yet there are many examples of this sort of behaviour in the chemical literature. Such a phenomenon is systematically known as "Metal Basicity" in which the metal nucleus acts in a non-traditional role as a net donor of electron density.1 There are essentially two manifestations of metal-base behaviour: (i), oxidative-addition type chemistry in which a lone pair of electrons formally associated with the metal adds to an electrophilic species; and (ii), Lewis-acid/metal-base adducts in which the transition metal donates electron density to some main-group metal compound, typically of aluminum or boron. The chemistry described in this work is broadly relevant to the second category, and examines the potential of certain transition metal hydride and alkyl complexes as metal-bases. In order to set this work in context we shall first briefly examine some of the ideas and concepts of metal basicity; in particular we wish to emphasize how Lewis-acid/metal-base interactions can have profound effects upon inorganic structure and reactivity. 1.2 Metal Basicity 1.2.1 Background and history Probably the most widely accepted definition of acid-base behaviour was proposed by G. N. Lewis, 2 who treated an acid as an electron-pair acceptor, and a base as an electron-pair donor. Traditionally, metal cations, both from the main-group and the transition series, are 1 References begin on page 12 Chapter 1: Introduction regarded as acids. However, many complexes whose metal centres are in low oxidation states can serve as donors to electrophilic species, and thus may be classified as bases. The first examples of metal-basicity are attributed to Hieber who demonstrated the basicity of some anionic metal carbonyls as shown in eq1-1. 1 HM(CO)n ^ ^ [M(CO)m- + H + ^ (M = Mn, Re, n = 5; M = Co, n = 4) Acid dissociation constants measured for these reactions ranged from 1 0 ' 1 0 to 1, with basicity increasing down a Group. Also, upon protonation a shift to higher wave number of the carbonyl stretching frequency occurs, which should reasonably be a reflection of the excess electron density lost by the metal nucleus. But the most interesting feature of these acid-base reactions was the change in structure of the acid versus its conjugate metal-base, with quite striking changes observed in geometry (eq 1-2). OC OC CO M n — C O CO © CO OC + H + O C , £ 0 [1-2] Mri N CO H The dramatic change in symmetry upon protonation (i.e. £>3h to C 4 v ) is not observed for bases of the main-group elements (cf. NH3/NH4+ or OH7H2O) whose lone-pair electrons determine molecular geometries. As we shall see later, coordinative unsaturation, in the sense of a formal lone-pair of electrons associated with the metal centre, is not a necessary condition for metal-basicity. While H + is the archetypal Lewis-acid, low-valent electron-rich transition metal complexes have also been found to interact with a range of neutral, molecular Lewis-acids such as BX3 and AIX3 (X = halide), AIR3 (typically R = Me), and the mercuric halides. The metallocene hydrides of general formula Cp2MH n were an early example and were recognized 2 References begin on page 12 Chapter 1: Introduction as metal-bases par excellence. Species such as Cp2ReH and CP2WH2 could not only be protonated by mineral acids,3 but could also form stable Lewis-acid/metal-base adducts with a variety of molecular Lewis-acids such as AlMe3 and BF3. Cp2TaH3, without ^ -electrons, can also function in this second capacity as can a legion of other electron-rich transition metal complexes.4 Some of the typical neutral Lewis-acids that undergo adduct formation with transition metal species are the following: A^Brg in Co2(CO)8»AlBr3;5 HgCl2 in a variety of complexes of the type (rt5-C5H5)Co(CO)2»HgCl2 and Fe(CO)3(PPh3>2'HgCl2;6,7 Al2Me6 in (Ti5-C5H5)2WH2»AlMe3,1>8-11 (Ti5-C 5H 5)3ZrH«AlMe3, 1 2 and (ri5-C5H5)2ReH»AlMe3; B F 3 in (ri5-C5H5)2WH2*BF313 and (rj 5. C 5H 5)2ReH»BF3; 8 and dimeric A1 2 M e 4 C l 2 in (y\5-C5H5)Rh(PMe3)2*Al2Me4Cl2.14 While this list is not comprehensive, it does focus on the main types of interaction possible; these are illustrated by the structures A to D which follow. C D 1.2.2 Nature of Lewis-acid/metal-base interactions As just illustrated, the Lewis-acid/metal-base interaction can be mediated in two ways: (i), an interaction between a coordinated ligand and Lewis-acid (cf. C and D); and (ii), an 3 References begin on page 12 Chapter 1: Introduction interaction directly with the transition metal centre by way of a formal dative bond (cf. A and B). The presence of dative or covalent bonding may be verfied by reference to high quality X -ray crystallography data which shows that the separation between the transition metal nucleus and the Lewis-acidic site is shorter than (or reasonably close to) the sum of the metallic radii (Zmet) of the metal nuclei. For example, in structure A this distance is 2.58 A,7 whereas £met(Co, Hg) = 2.75 A, 1 5 and the bonding interaction between transition metal and main-group metal may be formally represented by a dative bond. This contrasts to C , whose structural data were obtained by a low-temperature single-crystal X-ray diffraction experiment,11 in which the separation between tungsten and aluminum is 3.110 A (cf. SmetCW, Al) = 2.80 A).1 5 This length is too far for a covalent interaction and therefore a five- or six-coordinate aluminum participating in multicentre electron-deficient bonding is inferred. The presence of lone-pairs on the transition metal nucleus or on its ancillary ligands, or, in the case of coordinative saturation, energetically high-lying occupied molecular orbitals with non-bonding character with respect to the parent metal-base,16 may thus be qualitatively used to rationalize adduct formation with Lewis-acids. Metal complexes with formally no d-electrons can also engage in interactions with Lewis-acids as evidenced by recent work in which Zr(IV) methyl and hydride derivatives generate cationic, base-free derivatives by reaction with B(C6F5)3. 1 7 - 1 9 1.2.3 Influence upon reactivity At best, many of the preceding examples serve as structural curiosities. However, other such species exhibit moieties that have only a transient existence as separate entities: for example Cp 2 T i in Cp2TiBH3 2 0 or the methylene unit in Cp2TiCH2AlMe2Cl. 2 1 In these and other cases, the Lewis-acid/metal-base interaction has served to stabilize a very reactive unit, the net effect of which has been to modulate the reactivity of the parent transition metal complex. Such interactions are often implied in catalysis, which habitually relies on the addition of quantities of molecular Lewis-acids as necessary cofactors and cocatalysts in the 4 References begin on page 12 Chapter 1: Introduction catalytic cycle (e.g. AIR3 in Ziegler-Natta polymerization of olefins, BPI13 in hydrocyanation of olefins, 2 2 dimerization of ethylene,2 3 AIX3 and Snl2 in migratory insertion reactions)2 4"2 6. Until quite recently, however, no real attempt had been made to understand the enhanced reactivity that implicitly occurs when a Lewis-acid undergoes adduct formation with a transition metal complex. This is somewhat surprising given the ubiquity of molecular Lewis-acids in catalysis and the presumption that the Lewis-acid/metal-base interaction generates the reactive species in the cycle. No doubt the problems associated with the identification and study of reactive intermediates undermine comprehensive research, but this nevertheless represents a serious gap in chemical knowledge. For example addition of AlEt3 to a slurry of T i C l 3 or a solution of T iCU in hexanes produces the infamous Ziegler catalyst whose use is widespread industrially; likewise addition of methyl alumoxane to zirconium metallocenes generates the "single-site" olefin polymerization catalyst which now enjoys great popularity.2 7 However, only for the second example has a convincing model system and associated evidence been advanced, and that only recently.1 9 While it is ambitious to study the reactive species in such catalytic systems, die study of characterizable Lewis-acid/metal base systems can potentially yield much information on the reactivity patterns observed in catalysis. The first such study by Shriver, who had originally introduced the concept of Metal Basicity in the 60's, involved the Lewis-acid "promotion" of carbonyl insertion into the metal-carbon bond of Mn(CO)5CH3. 2 4 Migratory insertion is of course a fundamental organometallic process and is illustrated in eq 1-3. 2 8 L n M + L' • L nM C O \ > = 0 R This complex, Mn(CO)5CH3, undergoes this process at negligible rates, even under many atmospheres of added CO ligand which might be expected to favour migration. The addition of 5 References begin on page 12 Chapter 1: Introduction stoichiometric AlBr3, however, greatly enhances the rate constant (up to a factor of 108) and the process operates even without added CO. This phenomenon is summarized in Scheme 1.1. Scheme 1.1 Path 1 H 3CMn(CO)« "unpromoted" AlBro Path 2 "promoted" ° C O C :Mri ,*Br- -AlBr, C-y Me •O CO O ( C O ) 4 M n — C C H 3 CO O II (CO) 5 Mn—Q O C Mn' 8 CO C-y Me •O AIBr3 The addition of Lewis-acid thus induces a very rapid carbonyl insertion into the M n - M e bond by both activating the carbonyl towards migration through interaction between Lewis-acidic A l and the carbonyl oxygen, and by providing an electron-rich nucleus (Br) which acts as a ligand to fi l l the coordination site left vacant when the acyl group is formed. This demonstration of the effects of Lewis-acids upon fundamental patterns of reactivity was very influential and inspired much work directed at the possible role of Lewis-acids in catalysis and how chemists 6 References begin on page 12 Chapter 1: Introduction could exploit such Lewis-acid interactions to modify or accelerate reactivity. Structures E to H which follow are prime representatives of this sort of approach. In E the Lewis-acid, AlMe2Cl, has stabilized the very reactive methylene unit;29 in F a Lewis-acid/metal-base interaction has allowed direct observation of the insertion process of a Ziegler-Natta polymerization cycle.30 In G, addition of Lewis-acidic B(C6Fs)3 to the parent metal-base Cp2ZrMe(OCMe2CH2CH2CHCH2) gives an analogue of the elusive [Cp2ZrR(olefin)]+ fragment;31 likewise in H addition of B(C6Fs)3 to Cp'2ZrMe2 generates an olefin polymerization catalyst of definite structure and mode of action.17 It is anticipated that such interactions will become increasingly important as specific Lewis-acid/metal-base adducts are used in place of (or at the very least to model) established transition metal catalysts which operate in the presence of added molecular Lewis-acids. This is especially true as the design of sophisticated Lewis-acids, such as H(OEt2)2(B[3,5-(F3C)2C6H3]4}32 (Brookhart's acid or HBArf), [HNBU3] {Bu tCH2CH[B(C6F5)2]2H},33 or more recently [(Bu^AKL^-Oyjn,34 proceeds apace. As shown above the Lewis-acid may be used to generate an open coordination site, or 7 References begin on page 12 Chapter 1: Introduction more ambitiously the Lewis-acid nucleus itself may serve as a site of coordinative unsaturation.11 It is clear that some interesting chemistry awaits further research in this area. 1.3 Summary of previous work in this laboratory Previously in this laboratory much research has been devoted to the synthesis and reactivity of transition metal dimers bearing bridging hydride ligands. 3 5^ 3 The ancillary ligand of choice was a bidentate phosphine with alkyl substituents attached to the donor phosphorus nuclei and a general formula of R.2P(CH2)nPR2 ( « = 1 to 4; R = alkyl). Research by this group has found the use of the isopropyl group and chain-lengths of two to three to be exceptionally versatile. This substitution guarantees a very electron-rich environment for the metal atom to which the ligand binds, and thus enhanced reactivity towards small molecules such as H2, CO, and olefinic substrates. Another consequence of the use of these very basic ligands which was not originally anticipated is that their metal complexes are so electron-rich that they are likely good candidates for metal-base behaviour. The rhodium hydride dimer series ([{Pr i2P(CH2)nPPr i2}2Rh]2(p-H)2; n = 2, 1,2-bis(diisopropylphosphino)ethane "dippe"; n = 3, l,3-bis(diisopropylphosphino)propane "dippp"; n = 4, l,4-bis(diisopropylphosphino)butane "dippb"), as 28 electron complexes, shows a remarkable array of reactivity both in a stoichiometric and catalytic role which is summarized in Scheme 1.2. While insertion chemistry predominates, the Rh2H2 system will oxidatively add dihydrogen and in this sense it is a metal-base. Similar complexes have also been isolated for cobalt and these have been implicated in the hydrogenation of arenes. The logical extension of the chemistry of the rhodium system was to undertake an examination of the chemistry of the analogous platinum group complexes. Nickel and platinum complexes of this type are precedented, and for both metals there exist well characterized examples of phosphine stabilized metal hydrides having both mononuclear and binuclear structures: for 8 References begin on page 12 Chapter 1: Introduction nickel, [(dcypp)Ni]2(p.-H)2;4 4 for platinum, Pt(dcypp)H2 and [(dcypp)Pt]2(p-H)2,' [(dppe)Pt]2(p-H)2 4 6 and Pt(dtbpm)H 2. 4 7 ' 4 8 45 Scheme 1.2 Y A Rh Rh . -R IC i«> H n = 2, 3, 4 / ° \ Rh Rh^ V allene 1 -alkene n = 2 ( H 2 ^ l p / R h \ / R \ p J / H 2 > n Ph Rh'' N R h PhgSiHa y N H n = 2 X (-Hz) Z n Z n Zn(CH 2Ph) 2 J / ^ , — = — * - Rh. Rh n = 3 SjH^ H H (H)R"VC( Rh H(R) Rh 1,3-butadiene n = 2,3 (-H2) REH n=2 (-Ha) I E = O, S. NR R h ' N R h ( H 2 C ) „ Rh In contrast, the corresponding palladium complexes are conspicuously absent. A few hydrides of general formula rrans-Pd(PR.3)2H2 ex is t 4 9 but no examples of higher complexity, for instance complexes bearing P2-hydrides,50 have been isolated and unequivocally characterized before the appearance of this work . 5 1 , 5 2 This is somewhat surprising given the fact that homogeneous catalysis by zerovalent palladium complexes and simple palladium salts is widespread in organic synthesis, and in many cases a palladium hydride intermediate is inferred. The initial attempt to synthesize [(dippp)Pd]2(p-H)2 was thus simply motivated by the desire to complete the triad of Group 10 metal dimers of general formula [{R2P(CH2)nPR2}2M]2(p-H)2. It was also hoped that a rich Group 10 metal chemistry could be developed to complement that of Group 9 (vide supra). The unexpected result in fulfilling 9 References begin on page 12 Clwpter 1: Introduction this aim was the demonstration of metal-base behaviour and the identification of a new class of Lewis-acids, the alkali tetraethylborates. Early investigations conducted by Dr Brian Lloyd, a former post-doctoral fellow of this group, involved the addition of two equiv of LiBEt3H to a slurry of Pd(dippp)Cl2 in THF at low temperature. A deep red solution resulted from which maroon crystals could be isolated in approximately 6 0 % yield. The crystals were structurally characterized by X-ray crystallography to reveal the structure of [(dippp)Pd]2(p-H)2*LiBEt4 as shown in Figure 1.1. Pd-Pd 2.8248(5) A; Pd-H ( a v ) 1.70(3) A; L i - H ( a v ) 2.30(6) A; Pd(1)-Li 2.733(8) A; Pd(2)-Li 2.635(8) A F i g u r e 1.1. Chem 3D® view of [(dippp)Pd] 2(p-H) 2»LiBEt 4 . 10 References begin on page 12 Chapter 1: Introduction Detailed discussion of this complex, including the question of the origin of LiBEu from a stock solution that was nominally LiBEt3H, will be delayed until the next chapter. The outstanding feature of this complex is that it represents a Lewis-acid/metal-base adduct between a neutral LiBEt4 moiety, the Lewis-acid, and the metal-base, [(dippp)Pd]2(p>H)2. The Lewis-acidity of L i + , as its tetraethylborate salt, had not been previously demonstrated. Predictably the Lewis-acid is associated with the hydrides of the metal core, the most basic site of the palladium complex.2 5 1.4 Outline and scope of this work The isolation of [(dippp)Pd]2(H-H)2*LiBEt4 signalled the starting point for a detailed examination of the coordination behaviour of LiBEt4, a non-conventional Lewis-acid, towards a range of metal-hydrides. Several important questions were raised as to the nature of the interaction: what was its generality; what were its thermodynamic parameters; how did coordination by LiBEt4 affect the reactivity of the parent hydride? Clearly, these questions demanded answers and our attempts to address these gave rise to some unexpected chemistry. The enduring result of this work was the discovery that tetraethylborate salts of lithium and sodium could form stable 1:1 adducts with both hydride-bridged dimers of the later transition metals and terminal metal hydrides of the earlier metals of the transition series. Chapter 2 describes this chemistry in relation to [(dippp)Pd]2((i-H)2 (dippp = 1,3-bis(diisopropylphosphino)propane), and Chapter 3 deals with the coordination behaviour of LiBEu towards the more classic metal-bases, Cp2ReH, CP2WH2, and Cp2TaH3. Our attempts to extend the palladium chemistry described in Chapter 1 to the two-carbon backbone ligand, l,2-bis(diisopropylphosphino)propane (dippe), led to the isolation of some zerovalent palladium complexes which, while not exhibiting metal-base behaviour, nevertheless displayed some interesting chemistry, in particular the fundamental reactivity patterns of Pd(0) complexes of the type [Pd(dippe)]n and [Pd(dippp)]n; this is the subject of Chapter 4. Chapter 5 returns to metal-base orthodoxy and describes the chemistry of palladium complexes of the type 11 References begin on page 12 Chapter 1: Introduction Pd(dippe)R.2 and Pd(dippp)R-2, which serve as the parent metal-bases for some cationic Pd(II) hydrocarbyl complexes stabilized by the poorly coordinating anion BArf (BArf = {B[3,5-(F3C)2C6H3]4}_). These possess profound electrophilic character and react with a range of small molecules to give results that are relevant to our earlier discussion on Lewis-acid promoted reactivity. Finally, Chapter 6 advances some second thoughts and conclusions, and includes some proposals for future work. 1.5 References (1) Shriver, D. F. Acc. Chem. Res. 1970,3, 231. (2) Huheey, J. E.; Keiter, E. A. ; Keiter, R. L. Inorganic Chemistry; 4th ed.; Harper Collins: New York, 1993. (3) Green, M . L. H.; McCleverty, J. A.; Pratt, L.; Wilkinson, G. /. Chem. Soc. 1961,4854. (4) Tebbe, F. N. J. Am. Chem. Soc. 1973, 95, 5413. (5) Kristoff.J. S.; Shriver, D.F. Inorg. Chem. 1974,73,499. (6) Cook, D. J.; Daws, J. L.; Kemmitt, R. D. W. /. Chem. Soc. (A) 1967, 1547. (7) Nowell, I. M. ; Russell, D. R. 1967, (8) Brunner, H.; Wailes, P. C ; Kaesz, H. D. J. Inorg. Nucl. Chem. Lett. 1965,1, 125. (9) Johnson, M . P.; Shriver, D. F. /. Am. Chem. Soc. 1966,88, 301. (10) Storr, A.; Thomas, B. S. Can. J. Chem. 1971,49,2504. (11) Bruno, J. W.; Huffman, J. C ; Caulton, K. G. J. Am. Chem. Soc. 1984,106, AAA. (12) Kopf, J.; Vollmer, H.-J.; Kaminsky, W. Cry**. Struct. Commun. 1980, 9,985. (13) Shriver, D. F. /. Am. Chem. Soc. 1963,85, 3509. (14) Mayer, J. M. ; Calabrese, J. C. Organometallics 1984,3,1292. (15) Aylward, G. H.; Findlay, T. J. V. SI Chemical Data; 2nd ed.; John Wiley and Sons: Auckland, 1971. (16) Werner, H. Angew. Chem. Int. Ed. Engl. 1983,22,927. (17) Yang, X. ; Stem, C. L.; Marks, T. J. /. Am. Chem. Soc. 1991,113, 3623. 12 References begin on page 12 Chapter 1: Introduction (18) Yang, X. ; Stern, C. L.; Marks, T. J. Angew. Chem. Int. Ed. Engl. 1992,31,1375. (19) Yang, X. ; Stern, C. L.; Marks, T. J. 7. Am. Chem. Soc. 1994,116,10015. (20) Melmed, K. M. ; Coucouvanis, D.; Lippard, S. J. Inorg. Chem. 1972,12, 232. (21) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. /. Am. Chem. Soc. 1978,100, 3611. (22) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; 2nd ed.; John Wiley and Sons, Inc.: New York, 1992. (23) Fischer, K.; Jonas, K.; Misbach, P.; Stabba, R.; Wilke, G. Angew. Chem., Int. Ed. Engl. 1973,12, 943. (24) Butts, S. B.; Holt, E. M. ; Strauss, S. H ; Alcock, N. W.; Stimson, R. E.; Shriver, D. F. J. Am. Chem. Soc. 1979,101, 5864. (25) Butts, S. B.; Strauss, S. H ; Holt, E. M. ; Stimson, R. E.; Alcock, N. W.; Shriver, D. F. J. Am. Chem. Soc. 1980,102, 5093. (26) Pearson, J. M. ; Haynes, A.; Morris, G. E.; Sunley, G. J.; Maitlis, P. M. J. Chem. Soc, Chem. Commun. 1995, 1045. (27) For recent reviews, consult: (a) Jordan, R. F. Adv. Organomet. Chem. 1991,32, 325. (b) Marks, T. J. Acc. Chem. Res. 1992,25, 57. For recent leading references, also see ref. 19. (28) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: M i l l Valley, California, 1987. (29) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc. 1978,100, 3611. (30) Eisch, J. J.; Piotrowski, A. M. ; Brownstein, S. K.; Gabe, E. J.; Lee, F. L. /. Am. Chem. Soc. 1985,107,7219. (3.1) Wu, Z.; Jordan, R. F.; Petersen, J. L. 7. Am. Chem. Soc. 1995,117,5867. (32) Brookhart, M. ; Grant, M. ; Volpe, J., A. F. Organometallics 1992,11, 3920. (33) Jia, L.; Yang, X. ; Stern, C ; Marks, T. J. Organometallics 1994,13, 3755. (34) Harlan, C. J.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1995,117, 6465. 13 References begin on page 12 Chapter 1: Introduction (35) Fryzuk, M. D. Can. J. Chem. 1983, 61, 1347. (36) Fryzuk, M. D.; Jones, T.; Einstein; F. W. B. Organometallics 1984,3,1556. (37) Fryzuk, M. D.; Piers, W. E.; Rettig, S. J.; Jones, T.; Einstein, F. W. B.; Albright, T. S. /. Am. Chem. Soc. 1989, 111, 5709. (38) Fryzuk, M . D.; Piers, W. E.; Einstein, F. W. B.; Jones, T. Can. J. Chem. 1989,67, 883. (39) Fryzuk, M. D.; Piers, W. E. Organometallics 1990,9,986. (40) Fryzuk, M . D.; McConville, D. H.; Rettig, S. J. Organometallics 1990, 9, 1359. (41) Fryzuk, M . D.; Rosenberg, L.; Rettig, S. J. Organometallics 1991,10, 2537. (42) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J. J. Organomet. Chem. 1993,445, 245. (43) Fryzuk, M . D.; Rosenberg, L.; Rettig, S. J. Inorg. Chim. Acta 1994,222, 345. (44) Jonas, K.; Wilke, G. Angew. Chem. Int. Ed. Engl. 1970, 9, 312. (45) Clark, H. C ; Hampden-Smith, M . J. Am. Chem. Soc. 1986,108, 3829. (46) Carmichael, D.; Hitchcock, P. B.; Nixon, J. F.; Pidcock, A . /. Chem. Soc. Chem. Commun. 1988, 1554. (47) Hofmann, P.; Heiss, H.; Neiteler, P.; Miiller, G.; Lachmann, J. Angew. Chem., Int. Ed. Engl. 1990,29, 880. (48) For a recent, definitive work featuring the related complex [Pt(dippe)H]2 see: Schwartz, D. J.; Andersen, R. A. /. Am. Chem. Soc. 1995,117,4014. (49) See for example: (a) Brooks, E. H.; Glockling, F. J. Chem. Soc. A 1967, 1030. (b) Munakata, H.; Green, M . L. H. J. Chem. Soc, Chem. Commun. 1970, 881. (c) D i Bugno, C ; Pasquali, M. ; Leoni, P.; Sabatino, P.; Braga, D. Inorg. Chem. 1989,28, 1390. (50) Palladium complexes having bridging hydrides have been postulated on the basis of spectroscopic data, see: (a) Schunn, R. A. Inorg. Chem. 1976,15, 208. (b) Goel, R. G.; Ogini, W. O. Inorg. Chim. Acta 1980,44, 2165. (c) Young, S. J.; Kellenberger, B.; Reibenspies, J. H.; Himmel, S. E.; Manning, M. ; Anderson, O. P.; Stille, J. K. /. Am. Chem. Soc. 1988,110, 5744. (d) Zudin, V. N.; Chinakov, V. D.; Nekipelov, V. M. ; 14 References begin on page 12 Chapter 1: Introduction Likhobolov, V. A.; Yermakov, Y . U. /. Organomet. Chem. 1989,255, 425. (e) Zudin, V. N.; Chinakov, V . D.; Nekipelov, V . M . ; Rogov, V . A . ; Likholobov, V. A. ; Yermakov, Y . I. /. Mol. Catal. 1989,52, 27. (f) Siedle, A. R.; Newmark, R. A. ; Gleason, W. B. Inorg. Chem. 1991, 30, 2005. Structural data for the complex {[(dippp)Pd]2(p-H)(p-CO)}+ has recently appeared, see: Portnoy, M . ; Frolow, F.; Milstein, D. Organometallics 1991,10, 3960. [(dcype)Pd]2(p-H)2 has also lately been isolated but not structurally characterized, see: Pan, Y . ; Mague, J . T.; Fink, M . J. Organometallics 1991,11, 3495. (51) Fryzuk, M . D.; Lloyd, B. R ; Clentsmith, G. K. B.; Rettig, S. J. /. Am. Chem. Soc. 1991, 113, 4332. (52) Fryzuk, M . D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. /. Am. Chem. Soc. 1994, 116, 3804. 15 References begin on page 12 Chapter 2 : B I N U C L E A R P A L L A D I U M C O M P L E X E S W I T H B R I D G I N G H Y D R I D E S . U N U S U A L C O O R D I N A T I O N B E H A V I O U R O F L i B E t 4 A N D N a B E t 4 2 .1 Introduction to Chapter 2 THE L O N G - S T A N D I N G INTEREST of this research group in the synthesis and reactivity of transition-metal dimers bridged by hydride ligands and stabilized by electron-rich bidentate phosphines was sufficient incentive to attempt the synthesis of [(dippp)Pd]2(p-H)2 (dippp = l,3-bis(diisopropylphosphino)propane). Synthesis of this complex would complete the triad of Group 10 complexes of general formula [P2M]2(p-H)2 (P2 = bidentate phosphine) and it was also hoped that this Group 10 metal chemistry would parallel that of the Group 9 which was discussed in Chapter 1. In the event we were indeed successful in isolating such a complex of palladium with p2-hydride ligands. The unexpected bonus of the work was the inclusion of a LiBEty unit in the coordination sphere of the palladium nucleus which gave the first evidence for a Lewis-acid/metal-base interaction between a late transition-metal hydride complex and the non-traditional Lewis-acid, LiBEu. In this chapter we relate the solid-state and solution structures of adducts of general formula [(dippp)Pd]2(p-H)2 ,ME'Et4 (M = L i or Na; E' = B or Al) as well as their relative stabilities. The solid-state structure of the parent palladium hydride dimer [(dippp)Pd]2(p-H)2 is also included for purposes of comparison. 2 . 2 Isolation and structure of [(dippp)Pd] 2(p-H) 2»LiBEt 4 (1) The addition of a THF solution of LiBEt3H (Aldrich Super Hydride) to a slurry of Pd(dippp)Cl2 in THF at low temperature (-78 °C) led to the formation of a deep red solution from which maroon crystals having the formula [(dippp)Pd]2(p-H)2#LiBEt4 (1)+ could be isolated in 68% yield (eq 2-1). The ! H N M R spectrum of 1 exhibited a broad quintet at -3.66 ppm (2/p-H = 35 Hz), due to coupling to four equivalent phosphorus-31 nuclei, which clearly f In each individual chapter, the first compound discussed is numbered 1, the second 2, etc.; the numbering scheme does not continue to the next chapter and the sequence begins again. 16 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... indicates the presence of bridging hydrides; in addition, resonances due to a LiBEu residue were observed at 1.30 and 0 . 4 2 ppm for the methyl and methylene resonances, respectively (cf. 1.29 and 0 .21 ppm for crystalline LiBEu in ^8-toluene). The question of the origin of LiBEu from a proprietary solution that was nominally LiBEt3H was easily resolved by proton-coupled n B N M R spectroscopy; the spectrum of our particular bottle displayed two resonances: a singlet at -17.5 ppm, overlayed by a triplet at -17.8 (7B-H = 67.4 Hz), attributed to LiBEu and LiBEt2H2, respectively. Other bottles examined contained only authentic L iBEtsH. 1 [2-1] 1H NMR: - 3.66 ppm (quintet) v • -* The X-ray crystal structure of 1 is shown in Figure 2.1. The binuclear structure is clearly evident as are the bridging hydrides which were located and refined. The palladium-hydride bond lengths range from 1.68(3) to 1,79(4) A (cf., Pd( l ) -H( l ) , 1.68(3) A; Pd(l)-H(2), 1.70(3) A; Pd(2)-H(l), 1.79(4) A; Pd(2)-H(2), 1.76(3) A); the P d - H - P d angle is 109(2)°. Other selected bond lengths and bond angles appear in Tables 2.1 and 2.2. However, the most interesting feature of this structure is the presence of a coordinated LiBEu unit with the lithium cation perched atop the Pd2(p>H)2 core beneath the B E u anion. The palladium-lithium distances are somewhat different with the lithium being closer to Pd(2): Pd( l ) -L i , 2.733(8) A and Pd(2)-Li, 2.635(8) A. Also noteworthy is the interaction of the lithium with the B E u unit as the lithium binds to one C -H bond on each of three ct-carbon ethyl substituents ( L i - H = 17 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... 1.69, 1.72, and 1.86 A; L i - a - C a v . = 2.32 A). The neutron diffraction study of LiBMe4 shows similar L i - H - C interactions.2 18 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. C38 Figure 2.1. (a) Molecular structure of [(dippp)Pd]2(p>H)2»LiBEt4 (1); and (b), core view of 1 showing the interaction of the L i cation with the three a - C - H bonds in the ethyl groups, the bridging hydrides and the palladium centres (isopropyl groups have been removed for clarity). 19 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. Table 2.1. Selected intramolecular distances (A) observed in [(dippp)Pd]2(p--H)2#LiBEt4 (l),[(dippp)Pd]2(p-H)2(2),and[(dippp)Pd]2(p-H)2'NaBEt4 (3). [(dippp)Pd]2(U-H)2-L.iBEt4 (1) Pd(l)-Pd(2) 2.8248(5) PdQ)-H( l ) 1.68(3) Pd(l)-H(2) 1.70(3) PdQ)-P(l ) 2.344(1) Pd(l)-P(2) 2.337(1) Pd(l>-Li 2.733(8) Pd(2)-H(l) 1.79(4) Pd(2)-H(2) 1.76(3) Pd(2)-P(3) 2.336(1) Pd(2)-P(4) 2.336(1) Pd(2)-Li 2.635(8) L i - B 2.275(9) L i - H ( l ) 2.54(4) Li -H(2) 2.25(3) Li-C(31) 2.371(10) Li-C(33) 2.339(9) Li-C(35) 2.316(10) [(dippp)Pd]2(p-H)2 (2) Pd(l)-Pd(2) 2.8245(8) Pd(l ) -H( l ) 1.67(5) Pd(l)-H(2) 2.13(4) PdU) -P( l ) 2.304(2) Pd(l)-P(2) 2.299(2) Pd(2)-H(l) 2.11(5) Pd(2)-H(2) 1.73(4) Pd(2)-P(3)/P(4) 2.306(2) [(dippp)Pd]2(p-H)2«NaBEu (3) Pd(l)-Pd(2) 2.8169(6) Pd(l ) -H( l ) 1.82(5) Pd(l)-H(2) 1.79(4) Pd(l ) -P( l ) 2.326(1) Pd(l)-P(2) 2.325(1) Pd(l ) -Na 2.874(2) Pd(2)-H(l) 1.74(5) Pd(2)-H(2) 1.77(4) Pd(2>-P(3) 2.324(3) Pd(2)-P(4) 2.320(2) Pd(2)-Na 2.897(2) Na -B 3.329(2) Na-H( l ) 2.60(5) Na-H(2) 2.56(5) Na-C(31) 2.67(2) Na-C(33) 2.78(2) 20 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. Table 2.2. Selected intramolecular angles (deg) observed in [(dippp)Pd]2(p>H)2»LiBEt4 (1), [(dippp)Pd]2(p.-H)2 (2), and [(dipPP)Pd]2(p.-H)2»NaBEt4 (3). [(dippp)Pd]2(^-H)2«LiBEt4 ( 1 ) _ H(l) -Pd(l) -H(2) P(l) -Pd(l) -P(2) Pd(l) -Li -Pd(2) Pd(l)-H(2)-Pd(2) 72(2) 99.91(4) 63.5(2) 109(2) H(l)-Pd(2)-H(2) P(3)-Pd(2)-P(4) Pd(l)-H(l) -Pd(2) H( l ) -L i -H(2) 68.5(1.5) 100.54(5) 109(2) 49(1) [(dippp)Pd]2(li-H)2 (2) H(l) -Pd(l) -H(2) P(l) -Pd(l) -P(2) Pd(l)-H(l) -Pd(2) 85.9(2) 99.81(8) 95.8(3) H(l)-Pd(2)-H(2) P(3)-Pd(2)-P(4) Pd(l)-H(2)-Pd(2) 84.8(2) 101.93(8) 93.5(3) [(dippp)Pd]2(u:-H)2«NaBEt4 (3) H(l) -Pd(l) -H(2) P(l) -Pd(l) -P(2) Pd(l)-Na-Pd(2) Pd(l)-H(2)-Pd(2) 73(2) 100.49(5) 58.43(4) 105(3) H(l)-Pd(2)-H(2) P(3)-Pd(2)-P(4) Pd(l)-H(l)-Pd(2) H(l ) -Na-H(2) 76(2) 100.95(6) 104(2) 49(3) The unsymmetrical solid-state structure is not maintained in solution as evidenced by N M R spectroscopy. A singlet in the 3 1 P{ 1 H} N M R spectrum is observed even at low temperatures (-90 °C) which indicates that all the phosphines are equivalent. A symmetrical structure in solution is therefore required since the two ends of the binuclear unit are spectroscopically equivalent. In addition, the LiBEu remains associated with the transition-metal core since the resonances due to the tetraethylborate unit remain shifted in the lH N M R spectrum from those measured for pure LiBEu 1° d^-tolnene. Variable-temperature lU NMR studies show that, in solution, all four ethyl groups at boron are equivalent, indicating that the B E u unit is rather mobile and maintains a rapid tumbling motion even at low temperatures 21 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... interchanging the methylene protons. Moreover, from the variable temperature N M R data, it is also evident that the top and the bottom of the binuclear palladium hydride are apparently equivalent, thus suggesting that the whole.LiBEu unit is labile and does not differentiate each side of the complex (vide infra). 2.3 Isolation and structure of [(dippp)Pd]2(u,-H)2 (2) Clearly, any attempt to rationalize the nature of the bonding by the LiBEu unit in the palladium-lithium adduct 1 could not be addressed without the parent hydride dimer, [(dippp)Pd]2(p-H)2. Unfortunately, attempts to prepare this complex by hydride metathesis of Pd(dippp)Cl2 with pure LiBEt3H were unsuccessful; however, treatment of the more soluble diiodide derivative Pd(dippp)l2 with 2 equiv of crystalline KBEt3H in toluene does give [(dippp)Pd]2(p-H)2 (2) as shown in eq 2-2. Particularly diagnostic of 2 is the hydride resonance in the *H N M R spectrum at -2.52 ppm which appears as a binomial quintet (/p-H = 34.9 Hz). [2-2] 1H NMR: - 2.52 ppm (quintet) Typical formal oxidation states for palladium are +2 and 0. However, both of the binuclear complexes, 1 and 2, have formal oxidation states of +1, assuming that the hydrogens are bridging hydrides, Pd2(p>H)2, and not a bridging dihydrogen unit; Pd2(p.-T|2-H2). 22 References begin on page 54 Chapter 2:Binuclear Palladium Complexes withBridgingHydrides.. A T P d ' Pd Pd — P d H Pd2(jl-H)2 Pd2(n-T|2-H2) From the X-ray structure of 1, the presence of bridging hydrides is reasonable since the H - H separation is 2.00(5) A, longer than the 1.6 A distance normally ascribed as the cutoff for a rt 2-H2 ligand. 4 Evidence that the parent binuclear complex 2 is also best described as a Pd(I) dimer comes from the formation of [(dippp)Pd]2(p-D)2 by reaction of KBEt3D with the diiodide as before. The *H N M R spectrum of 2-^ 2 shows a small quintet of 1:1:1 triplets at -2.53 ppm due to residual protons coupled to deuterium and four equivalent phosphorus-31 nuclei indicative of [(dippp)Pd]2(p-D)(p-H) (2-diy, the observed H - D coupling constant (/H-D) of 2.4 Hz is much too small to be due to a T|2-H2 moiety 4 The solution structure of bare palladium dimer 2 indicates that the phosphine donors are equivalent at all temperatures as are the hydrides. While any of the following geometries can account for the equivalence of the phosphines in the 3 1 P{ 1 H} N M R spectrum, a fluxional process interchanging the bridging hydrides must be invoked to account for the observed magnetic equivalence of the hydrides in the *H N M R spectrum if the preferred planar D2h geometry is accepted. Such fluxional processes are well precedented.5,6 23 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... The X-ray crystal structure of this bare palladium hydride dimer 2 is shown in Figure 2.2. The Pd-Pd bond distance in 2 is 2.8245(8) A, virtually identical to that found in the palladium-lithium adduct 1 (i.e. 2.8233(6) A). What is striking, however, is the unsymmetrical nature of the hydride bridges between the two palladium atoms: the Pd( l ) -H( l ) distance is 1.67(3) A while Pd(l)-H(2) is 2.13(4) A; similarly, Pd(2)-H(2) is 1.73(3) A and Pd(2)-H(l) is 2.11(3) A. It has lately been suggested that this asymmetric bridge manifests a relativistic preference for terminal hydride bonds (vide infra).6 However, given the symmetrically bridged Pd 2 H2 cores of 1 and 3, and the fact that X-ray crystallography is not the best way to determine hydride geometries, it seems more likely that the asymmetric bridge of 2 is an artefact, or represents some packing effect. Infrared spectroscopy, which might offer a more reliable probe for transition-metal hydrides, was unable to identify the P d - H stretching frequency. 24 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. 25 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... The shorter metal-hydride distances are similar to that found in the L iBEu adduct 1 but longer than the 1.531(11) A found in the recently reported cation {[(dippp)Pd]2(u,-H)(p-CO)}+.7 The solid state structure is not completely planar as evidenced by the end-on view in Figure 2.3. Looking down the Pd(l)-Pd(2) axis, the plane defined by the two palladium atoms and the two hydrides is canted by 19.5' to the P(l)-Pd(2)-P(2) plane and 15.1* to the P(3)-Pd(2)-P(4) plane. Essentially the same geometry is observed for the coordination planes of the LiBEu and N a B E u adducts, 1 and 3 (vide infra), respectively. P2 2 1 3 Figure 2.3. A view looking down the Pd-Pd axis of [(dippp)Pd]2(p.-H)2 (2), [(dippp)Pd]2(p-H)2*LiBEt4 (1), and [(dippp)Pd]2(p>H)2»NaBEu (3) to show the non-planar nature of the hydride and phosphorus cores. The analogous d 9 nickel dimer, [(dcypp)Ni]2(p.-H)2 (dcypp = l,3-bis(dicyclohexylphosphino)-propane), has a solid-state structure more resembling a £>2 symmetric species with the two Ni(dcypp) ends of the molecule staggered by 63.3 0 . 8 , 9 However, a molecular orbital calculation performed upon the model complex [(H3P)2Ni]2(p>H)2 concluded that the energy minimum 26 References begin on page 54 Chapter 2:Binuclear Palladium Complexes with Bridging Hydrides.... occurred for the completely planar £>2h structure, and thus the observed departure from planarity in [(dcypp)Ni]2(u>H)2 was proposed to be due to steric repulsion between the cyclohexyl groups on alternate ligands.9 For the structure of the lithium adduct 1, this angle is 34.6°, while for the hydride dimer 2, this angle is 24.0°. Presumably, because of the larger size of Pd as compared to N i , steric repulsion between isopropyl groups across the ends of the binuclear unit is minimal and the structure of the parent palladium hydride dimer 2 is nearly planar. In solution, the structure of the bare palladium dimer 2 is completely symmetrical as evidenced by equivalent phosphorus nuclei by 3 1P{^H} N M R spectroscopy at all temperatures. For this reason, a symmetrical £>2h geometry is assumed for all discussions of the solution structure of this dimer 2. 2.3.1 Comparison of 2 to analogous Pt systems Curiously, in every attempt we have made to monitor the reaction'leading to 2, we have been unable to observe a signal attributable to a cfx-dihydride Pd(II) species, that is Pd(dippp)H2, which would be a likely intermediate following hydride metathesis of the metal-halide bonds. Variable temperature 3 1 P{ 1H} N M R spectroscopy of a mixture of Pd(dippp)l2 and two equiv of KBEt3H in toluene shows only one signal at 27.9 ppm which grows in intensity as the temperature increases. This resonance is precisely the value of that of the end product 2. Presumably the Pd(dippp)H2 species undergoes loss of dihydrogen and dimerization too quickly to be observed by N M R spectroscopy. We further attempted to access the cis-dihydride species by placing a solution of 2 under 50 arm of dihydrogen in a sapphire N M R tube, and even under these forcing conditions only signals relating to 2 were observed in the lH and 3 1 P{ 1 H} N M R spectra. Conversely, for the series of allied platinum compounds bearing bidentate phosphine ligands and bridging hydrides the cw-dihydride Pt(U) species is indeed accessible, with an 27 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... observable equilibrium operating between the Pt(I) dimer and the Pt(II) dihydride in the presence or absence of dihydrogen. In fact very recent work formulates the dippe compound as [(dippe)PtH]2,6 that is as a dimeric system of Pt(I) with terminal hydrides and a metal-metal bond as shown below. The authors attribute this phenomenon to a relativistic preference for terminal metal-hydrides which operates strongly for the sixth-row platinum nucleus, marginally for the fifth-row palladium nucleus (hence one short and one long bridging hydride per palladium centre, vide supra), and not at all for the fourth-row nickel nucleus.1 0 We were reluctant to accept this conclusion because of the restrictive chelate bite angle imposed by each of the ligands, dippe and dcyppe, used in this work, which seemed to us to be an important determinant of molecular structure. With reference to the following diagram theoretical studies have shown that if the L -M - L bond angle is changed the H - M - H angle changes to compensate.11"13 k "*VK X H r f \ / \ / > rVI M V v V \1 H v L M M-H Thus if a bidentate ligand of large chelate bite (i. e. dippp) is replaced by a ligand of small chelate bite (i. e. dippe), thereby closing the L - M - L bond angle, the H - M - H angle opens out. For the Pt systems under discussion such an effect may render the hydrides incapable of bridging the gap between two metal centres and terminal-hydride ligation is preferred, and this consideration is especially important given the shortness of Pt-Pt bonds relative to Pd-Pd ones. 28 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... The unusual structure of [Pt(dippe)H]2 may thus simply be an expression of the different chelate bite angle of the dippe and dippp ligands, and had the three-carbon backbone ligand been used the hydride would have had a regular geometry. For the Pd system a piece of negative evidence for this argument may be advanced: when Pd(dippe)i2 is treated with two equiv of KBEt3H a hydride-bridged species analogous to 2 is not obtained and a zerovalent species is isolated (see Chapter 4). Conceivably, the Pd(0) product results from an intermediate Pd hydride species whose H - P d - H angle was too obtuse to allow a P-2-H interaction and underwent reduction to Pd(0) with the elimination of dihydrogen.1 4 In order to obtain more compelling evidence for our hypothesis the obvious course was to attempt to prepare the Pt hydride dimer based on the three-carbon backbone ligand dippp. To this end Pt(COD)l2 was placed under dihydrogen in the presence of dippp, and in a second experiment Pt(dippp)l2 was treated with two equiv of KBEt3H as for the synthesis of 2. For both experiments crystalline, material was obtained whose microanalyses corresponded to a formula of CisH35P2Pt (i.e. "Pt(dippp)H"), but each had completely distinct structural and spectroscopic parameters. In the first instance reduction of Pt(COD)l2 yielded colourless crystals whose IR spectrum displayed a strong M - H stretch at 1976 and 1961 c m - 1 , values which indicate terminal hydride ligation. This was not a dippp analogue of [Pt(dippe)H]2 with two terminal hydrides and a metal-metal bond but in fact the Pt(II) monomer cw-Pt(dippp)H2 (5), as witnessed by an AX2 pattern in the 3 1 P{ 1 H} N M R spectrum (i.e. the 1 9 5Pt(dippp)H 2 isotopomer), and an [AX]2 pattern for the hydrides in the lH N M R spectrum at -0.92 ppm (7p.H (trans) = 149.1 Hz; /P_H (cis) = -16.9 Hz; J?i H = 1040 Hz) (eq 2-3). 1 5 29 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... RCODI 2 + dippp [2-3] IR:v-Pt-H 1976,1961 cm'1 1H NMR: hydrides -0.92 ppm, [AX]2 multiplet In the second instance a new platinum hydride species was isolated along with traces of 5 (eq 2-4). Its 3 1 P{ lH) N M R spectrum displayed the [A 2]2X multiplets characteristic of a binuclear structure (i.e. a P t - 1 9 5 P t isotopomer coupled to four chemically equivalent phosphorus nuclei); the hydride region of its *H N M R spectrum displayed binomial quintets with platinum satellites centred at -1.10 ppm (/p-H = 37.7 Hz; /pt-H = 486.4 Hz). This Pt hydride species displayed an uncharacteristically weak absorption at 1939 c m - 1 attributable to terminal M - H stretches in the IR spectrum.16 [2-4] 30 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... Under 4 atm of H2, 6 cleanly converts to monomeric 5 within minutes. This is a reversible chemical equilibrium because when 5 is placed under 4 atm of N2 in a sealed N M R tube signals attributable to its reduction product 6 are observed when the sample is heated (eq 2-5). It seems likely therefore that c«-Pt(dippp)H2 was the initial product in the metathesis reaction but after loss of dihydrogen 6 was isolated. Crystals suitable for X-ray diffraction experiments were obtained for 6, and its solid state structure is shown in Figure 2.3. Relevant bond lengths and bond angles appear in Table 2.3. [2-5] 6 5 Given the shortness of the Pt-Pt bond at 2.6002(5) A, and the orthogonality of the two P - P t - P coordination planes, it is readily apparent that the structure of 6 is not a variant of the structure of 2 or of [(dcypp)Ni]2(p-H)2, and that a formulation of [Pt(dippp)H]2 is the most correct representation of the molecular geometry. However, a closer examination of the structure of 6 leads to a partial vindication of the original thesis. The P - P t - P angles seem to depart markedly from the 90° or 180" angles that square planar geometry at the Pt nucleus would enforce. The bond angles involving the hydrides, which in this structure were located and refined, also suggest a distortion towards a bridging hydride structure. In addition, the lH and 3 1 P { 1 H ) N M R spectra in d%-tohicne at -90 *C show equivalent phosphorus and hydrogen nuclei, not the A B type 3 1 P{ , 1 H} N M R spectrum observed for [Pt(dippe)H]2 at low temperatures.6 While the unusual geometry of 6 and [Pt(dippe)HJ2 indicates that a relativistic effect favouring terminal hydrides does operate, other factors such as chelate bite angle can 31 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... also influence the preferred binding mode of the hydrides. A ligand of even greater chelate bite than dippp, such as the four-carbon backbone dippb, when bound to platinum, might undermine the a priori preference of the metal for terminal hydrides. Figure 2.4. Chem 3D® view of [Pt(dippp)H]2 (6). 32 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... Table 2.3. Selected intramolecular bond lengths (A) and bond angles (deg) observed in [Pt(dippp)H]2 (6). Pt(l)-Pt(2) 2.6002(5) Ptd) -H( l ) 1.65(6) Pt(l)-H(2) 2.14(6) Pt(2)-H(l) 2.53(6) Pt(2)-H(2) 1.73(6) Pt( l ) -P(l ) , P(2) 2.241(2), 2.263(2) Pt(2)-P(3),P(4) 2.248(3), 2.261(2) H(l) -Pt(l) -H(2) 80(2) H(l)-Pt(2)-H(2) 68(2) P(l) -Pt(l) -P(2) 99.48(9) P(3)-Pt(2)-P(4) 99.51(10) Pt(2)-Pt(l)-P(l) 157.23(6) Pt(2)-Pt(l)-P(2) 100.64(6) Pt(l)-Pt(2)-P(3) 154.08(7) Pt(l)-Pt(2)-P(4) 105.19(6) P(2)-Pt(l) -H(l) 169(2) P(D-Pt ( l ) -H( l ) 90(1) P(3)-Pt(2)-H(2) 100(1) P(4)-Pt(2)-H(2) 160(1) 33 References begin on page 54 -Chapter^-BinueleanPaUadium Complexes with Bridging Hydrides.. [2-5] As an aside it is worthwhile adding that the Pt system is, like the Pd system, a potent metal base, as witness recent work in which the-coordination behaviour of certain cw-P2'PtR2 (R = H, Me; P2 = bidentate phosphine) towards Lewis-acidic Cp*2Yb is documented.17 2.4 Metal-basicity of [(dippp)Pd]2(p.-H)2 (2) Examination of the lH N M R spectrum of the original reaction mixture of Pd(dippp)Cl2 and the THF solution of LiBEt 2H2/LiBEt4 that ultimately produces 1 revealed a quintet at -2.52 ppm which is identical to that observed for the parent palladium hydride dimer 2. This is good evidence that the bare hydride dimer 2 is the initial product and only upon workup and removal of the THF, which was undoubtedly coordinated to the LiBEt4, does coordination of free LiBEt4 to the palladium-hydride dimer occur. The proposal that the bare hydride dimer 2 is an intermediate in the formation of the palladium-lithium adduct 1 can be confirmed by the preparation of adduct 1 from dimer 2 by addition of LiBEt4 in an aromatic solvent. Isolated yields of 1 by this procedure are in excess of 90%. This appears to be a general procedure since addition of the corresponding sodium salt, NaBEt4, to the parent palladium dimer 2 generates the corresponding adduct [(dippp)Pd]2(p.-H)2iNaBEt4 (3) while the addition of LiAlEt* also generates the related adduct [(cUppp)Pd]2(p>H)2»LiAlEt4 (4). A l l of these transformations are summarized in Scheme 2.1. 34 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... The Lewis-basicity of 2 may also be demonstrated in another respect, by its interaction with the archetypal Lewis-acid, H + . Solutions of 2 in inadequately dried volumes of ^ -benzene displayed a small quintet upfield at -5.52 ppm which indicated formation of anew complex. Since this peak may be intensified by the addition of a few drops of D 2 0 it seems clear that the new product may be formulated as cationic {[(dippp)Pd]2(p-H)3)+ with an hydroxide counterion. The ability of the generic [P2Pt]2(p-H)2 complex to deprotonate water has also been observed.5 , 6 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... 2.5 Structure of [(dippp)Pd] 2(p--H) 2»NaBEt4 (3) and comparison to 1 The solid-state X-ray structure of the NaBEty adduct 3 is shown in Figure 2.5. As can be seen from the ORTEP diagram, it displays similar features to that of the LiBEt4 adduct 1. The binuclear structure is apparent as are the bridging hydrides; the Pd-Pd distance of 2.8169(6)A is very slightly shorter than either of the comparable Pd-Pd bond distances in 1 or 2 and the P d - H bond lengths of 1.80(3) A are long but within the range of 1.67(3)-2.13(4) A observed for the bare dimer 2. The disposition of the tetraethylborate unit around the alkali metal constitutes the most important difference in the structures of adducts 1 and 3. The lithium cation in 1 (Figure 2.1) is proximate to three of the a-carbons of the tetraethylborate unit, and a bridged interaction with one C - H bond of each of the three methylene units is inferred. As a caveat it should be noted that the methylene hydrogens are in calculated positions based on the observed C - C angle of the ethyl group, nonetheless, only one C - H unit points at the L i cation. The L i - C separation (2.316(10)-2.371(10) A) is comparable to the corresponding distances in the structures of LiBMe4 and LiAlEt4 (2.36(2) and 2.37(2) A respectively).2 , 1 8 The sodium nucleus in the solid state structure of 3 (Figure 2.5) is contiguous to only two a-carbons, but now positioned to interact with four a - C - H bonds; again, the hydrogens are placed in calculated positions based on the observed C - C bond angle of the ethyl group. The difference in the interaction of N a + and L i + with the tetraethylborate unit may be attributed to some peculiarity of the solid state, however, it may also reflect that sodium ion is larger than lithium ion and thus prefers a higher coordination number. The average N a - C distance is 2.67 A. 36 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. C 3 7 Figure 2.5. (a) Molecular structure of [(dippp)Pd]2(p-H)2 ,NaBEt4 (3); and (b), core view of 3 showing the interaction of the Na cation with the four a - C - H bonds in the ethyl groups, the bridging hydrides and the palladium centres (isopropyl groups have been removed for clarity). 37 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... Other distances of note are the lithium-hydride bond lengths of 2.25(3) and 2.54(4) A and the lithium-palladium separations of 2.733(8) and 2.635(8) A, the differences again indicative of the unsyrnmetrical structure in the solid-state structure of 1. The observed sodium-hydride distances are 2.56(5) and 2.60(4) A and the sodiumrpalladium lengths are 2.90(5) and 2.87(3) A. Variable-temperature, *H N M R experiments on the NaBEty adduct 3 show no inequivalence among the ethyl groups of the tetraethylborate groups, even at -90 °C, similar to that observed for the LiBEu adduct 1; this is again consistent with the tetraethylborate anion undergoing a rapid tumbling motion in solution. Also comparable to 1 is the fact that, in the sodium adduct 3, the top and bottom of the binuclear hydride unit are equivalent by J H N M R spectroscopy even at low temperature which is a feature consistent with a labile NaBEu unit. 2.6 Thermodynamic parameters associated with the formation of 1 As outlined in Scheme 2.1, upon addition of stoichiometric amounts of solid LiBEu to a benzene or toluene solution of the parent hydride dimer 2, the LiBEu adduct 1 is produced as determined by 3 1 P{ 1 H} N M R spectroscopy. How strong, thermodynamically, is the interaction of the perethylborate and aluminate salts with the palladium hydride dimer 2? Attempts to measure the equilibrium constant for adduct 1 in the presence of 2 by N M R spectroscopy were unsuccessful presumably because the adduct forms completely even at low concentrations of L i B E u - hi addition, if a mixture of adduct 1 and the bare hydride dimer 2 are mixed together,, exchange of the LiBEu unit occurs as evidenced by broad resonances in the hydride region of the lH N M R spectrum (Figure 2.6); as the temperature is lowered, this particular exchange process becomes slow and the separate signals due to the hydride resonances of 1 and 2 are observed in the exact ratio that they were originally mixed. As the temperature is raised, the peaks coalesce to eventually generate a single resonance (at 100 °C) due to averaged intermolecular exchange. 38 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... I I | I I I I | I I I I | I I I I | I I I I | I I I I | I I I I | I I I I | I I I I | I I I I | I I I I | N U | I I I I | I I I I | I I I I | I I I I | I I I I | I I l l | l l l l | M I I | l l l l | l l l l | l l l l | l l l l | i l l l | l l l l [ M I I | l l l l | l l l l | l l l l | l l l l | l l l l | l l l l | l l l l | M 400 200 0 -200 Hz 400 200 0 -200 Hz Figure 2.6. Experimental (left) and simulated (right) variable temperature *H N M R spectra of an equimolar mixture of [(dippp)Pd]2(p.-H)2 ,LiBEt4 (1) and [(dippp)Pd]2(p>H)2 (2) i n the hydride region (0 Hz = -3.00 ppm; [Pd] = 4.20 x 10" 2 mol L"1); the very sharp quintet at low temperatures is due to 2 while the broader resonance is due to the LiBEu adduct 1. 39 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... The kinetics of the exchange reaction have.been estimated by variable temperature *H N M R spectroscopy in fife-toluene, followed by simulation of the experimental spectra. A graph of the interpolated rate constant, k&x, versus inverse temperature gave an exponential dependence over a temperature range of 243-373 K. The rate of exchange, /? e x , was found to vary as the concentration of adduct 1 and was insensitive to the concentration of the bare hydride dimer 2. A simple first order dissociative mechanism is proposed and the rate law is formulated as Rex = fceX[l]- The Eyring activation parameters may be extracted from a plot of InCitexT"1) versus T 1 : zW* e x = 7.9 ± 1.0 kcal mol ' 1 and A S * e x = -21.3 ± 4.0 cal K ' 1 mo l ' 1 . 1 9 The negative value of zlS^ex is troublesome for what is suggested to be a dissociative mechanism; however, if solvent molecules participate in the activated complex by solvating free LiBEt4, then this negative AS$ex can be rationalized.2 0 A more comprehensive kinetic analysis, embracing solvents other than ds-toluene, could not be performed due to the insolubility of 1 in aliphatic solvents, and its complete decomposition in chlorinated solvents. As expected, in the presence of donor solvents such as THF or Et20, adduct formation is disrupted and the bare palladium dimer 2 is produced as evinced by 3 1 P{ 1 H} N M R spectroscopy. These experiments provide further evidence that, thermodynamically, the adduct 1 is favored yet, kinetically, the LiBEt4 is rather labile; this latter point corroborates the *H N M R spectroscopic data concerning the equivalence of both the ethyl groups at boron and the isopropyl substituents at phosphorus at low temperatures mentioned earlier. 2.7 Lewis-acid exchange between 1,2,3, and 4 The addition of L1BEL4 to the sodium adduct 3 results in virtually quantitative formation of the lithium adduct 1 (eq 2-4). Although one equiv of LiBEt4 added to the sodium adduct 3 is enough to generate the lithium adduct 1 as the only detectable product, this is an equilibrium process since addition of approx. five equiv of NaBEty to the lithium adduct 1 does result in 40 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... the formation of a mixture of 1 and 3 in a 2:1 ratio; in other words, the reaction shown in eq 2-6 can be reversed by the addition of an excess of NaBEu. [2-6] Interestingly, if L i A l E u is added to the sodium tetraethylftorafe adduct 3, an equal mixture of the lithium adducts 1 and 4 is obtained indicating that there is little preference for the anion portion of the adduct (eq 2-7). [2-7] 41 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... 2.8 Bonding Considerations With regard to the nature of the bonding in complexes 1, 3, and 4, we have been unable to find another derivative with a similar type of interaction. Complexes that contain lithium are quite well known. 2 1 - 2 3 Examples range from ionic derivatives such as the tetrameric {[(TJ 5 -C5Ff5)2MoH]Li}4 2 4 , 2 5 to organolithium adducts of Ni(0) similar to the following which are considered to be ion pairs. 2 6" 2 8 H 2 It should be stressed that the interaction of the lithium ion with the metal complex shown here is fundamentally different than in the Pd2H2-LiBEt4 adduct 1 or the corresponding NaBEt4 derivative 3 since, in the nickel complex, the methyl anion is directly attached to the metal center, whereas in the latter molecules, the anionic BEt4 moiety coordinates only to the L i + or N a + ions. With regard to the electronic structure of 2,1, and 3, the Pd-Pd separation should be a fairly sensitive indicator of electronic distribution around the Pd2H2 core. By the 3a criterion this separation is identical for each species and therefore it is reasonable to preclude a covalent interaction between metal-base and Lewis-acid. If a covalent interaction operated in the Pd2-Li and Pd2-Na systems the Pd-Pd bond distance should be significantly longer than in the parent hydride 2. On the other hand if the interaction of the LiBEu with the palladium hydride dimer is not covalent then it must be an ion-dipole type bond (i.e., the lithium ion is electrostatically attracted to electron density) since it is fairly well established that the bonding in organolithium derivatives is largely ion ic . 2 1 , 2 9 42 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... 2.9 Reactivity Studies Reactions of ethylene (or propylene) with the lithium adduct 1 result in the loss of H2 (detected by N M R spectroscopy) and the release of the LiBEu unit to generate the three coordinate Pd(0) adduct Pd(rj2-C2H4)(dippp); a similar reaction ensues when PPh 3 is allowed to react with 1 since Pd(PPh3)(dippp) is obtained, and the reaction with dimethyl acetylenedicarboxylate, MeO^CC^CCO^Me, also generates the corresponding Pd(0) adduct Pd(ri 2 -Me02CCsCC02Me)(dippp). The same sequence of reactions carried out with the bare palladium dimer 2 produces the exact same palladium(O) adducts in virtually identical yields; all of this is summarized in Scheme 2.2. Thus, these formally palladium(I) hydride derivatives would appear to be sources of Pd(0) since H2 elimination is observed rather than any insertion type chemistry. In the case of the L i adduct 1, the presence of the Lewis acidic LiBEu moiety does not apparently influence any of the simple reaction types shown here since it is liberated along with the H2. There is precedent for the release of H2 from formally Ni(I) hydride dimers upon addition of olefins as well as liberation of alkylaluminum fragments from adducts of Ni(0) derivatives.3 0 43 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... In retrospect, it is difficult to imagine how the Lewis acidic portion of 1, the LiBEu unit, could have any influence on these types of reactions since the particular substrates used would be expected to be relatively innocuous to group 13 "ate" species. In addition, the similarity in the reaction outcomes shown in Scheme 2.2 is consistent with the lability of the LiBEu unit in that it is the palladium hydride dimer portion of adduct 1 that is undergoing the H.2 displacement and ligand addition reactions. 2.10 Conclusions In the work reported in this chapter we document a whole new kind of Lewis-acid/metal-base interaction that involves a neutral, binuclear metal complex bearing bridging hydride ligands and the alkali metal of Group 13 tetraalkyl "ate" derivatives. By variable 44 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... temperature N M R studies these adducts were shown to be quite labile and this aspect is reflected in the reactivity of the hydride dimers with a variety of simple substrates. In solution, however, the N M R experiments show that the interaction is qualitatively quite strong since no evidence for adduct dissociation was observed, and under no circumstances was the parent dimer 2 observed in the presence of stoichiometric amounts of LiBEty even at the low temperature limit (approx. -90 °C) imposed by the N M R solvent c?8-toluene. The kinetic parameters we obtained (AH$CX = 7.9 ± 1.0 kcal mol" 1 and AS* e x = -21.3 ± 4.0 cal K"1 mol"1) measured the activation energy for the exchange of LiBEt4 between adduct 1 and parent 2 rather than a free energy difference between the two species, nevertheless the value of AG$QX SO obtained (14.3 kcal mol"1 at 300 K) may be loosely interpreted as an estimate of the difference in stabilities of 1 and 2. Such a high value would preclude observation of free [(dippp)Pd]2(p-Ff)2 across the temperature range used here. In trying to gauge what promotes the interaction, a number of features deserve mention: (i) the solution structures of the lithium and sodium tetraalkyl borates (and aluminates) are unknown; however it may be inferred that the cation is labile and is strongly involved in oligomeric aggregates, and the suggestion that LiBEt4 in combination with [(dippp)Pd](p-Ff)2 is en tropically more favourable than solvated LiBEt4 is advanced;2 0 and (ii), the palladium hydride dimer itself has very electron-rich metal nuclei {formally d9 for each Pd(I)) and bridging hydrides, both sources of electron density for an ion-dipole type of interaction with soluble alkali metal cations. Taken together, it is not unreasonable to predict that other electron-rich metal complexes with and maybe even without hydrides should undergo adduct formation with species such as LiBEt4 and NaBEt4. 2.11 Experimental 2.11.1 Procedures Unless otherwise stated all manipulations were performed under an atmosphere of dry, oxygen-free dinitrogen or argon by means of standard Schlenk or glovebox techniques.31 The glovebox used was a Vacuum Atmospheres HE-553-2 device equipped with a MO-40-2H purification system and a -40 °C freezer. and 3 1 P{ ^K) N M R spectroscopy were performed 45 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... on a Varian XL-300 instrument operating at 300 MHz and 121.4 MHz , respectively. 2 D N M R spectroscopy was also performed on the Varian instrument operating at 46 MHz . J H{ 3 1 P} N M R spectroscopy was performed upon a Bruker AMX-500 instrument operating at 500.1 MHz . *H N M R spectra were referenced to internal C 6 D 5 H (7.15 ppm), C 6 D 5 C D 2 H (2.09 ppm), or CHCI3 (7.24 ppm). ^ P p H ) N M R spectra were referenced to external P(OMe)3 (141.0 ppm with respect to 85% H3PO4 at 0.0 ppm). Longitudinal relaxation times for the hydrides were measured by a standard inversion-recovery sequence supplied with Version 6.0c of the Varian V X R Series software. Kinetic data were simulated by the D N M R - S I M program.3 2 Microanalyses (C, H, halogen) were performed by Mr. P. Borda of this department. 2.11.2 Materials Dippp was prepared by a published procedure.3 3 EtLi was prepared by a published procedure,34 and recrystallized from toluene. LiAlEty and L i B E u were prepared by adding AlEt3 (Aldrich) or BEt3 (Aldrich) respectively to EtLi and recrystallized from toluene.3 5 NaBEt4 (Strem) was used as supplied. KBEt3H was prepared by addition of BEt3 to a slurry of K H (BDH) in toluene and was recrystallized from toluene; KBEt3D was prepared likewise from K D . 3 6 PdCk* was obtained on loan from Johnson-Matthey and used to prepare P d ( C 6 H 5 O N ) 2 C l 2 ; 3 7 H2PtCLi and PtC l 2 were also obtained on loan from Johnson-Matthey and used to prepare Pt(COD)l2. [(dippp)Rh]2(p>H)2 was prepared by a literature method.3 8 Hexanes, toluene, pentane, Et20, and THF were heated to reflux over CaH2 prior to a final distillation from either sodium metal or sodium benzophenone ketyl under an Ar atmosphere. Prior to drying, olefins were removed from the aliphatic solvents by treatment with cone. H2SO4 and acidic KMn04(aq). Acetone was dried over 4-A sieves and sparged with dinitogen. A l l deuterated solvents were purchased from Cambridge Isotope Laboratories; dy-chloroform was use as supplied, d^-benzene and ds-toluene were dried by distillation from sodium-benzophenone ketyl; oxygen was removed by 4 fteeze-pump-thaw cycles. 46 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. 2.11.3 Syn theses 2.11.3.1 Pd(dippp)CI2 To a solution of P d C Q H s O N h C ^ (4.37 g; 11.4 mmol) in acetone (100 mL) was added dippp (3.15 g; 11.4 nimol) dropwise. With each drop of phosphine a yellow precipitate formed and the deep amber colour of the solution discharged completely to give a clear supernatant. After 30 min of stirring the reaction mixture was exposed to the air and the fine yellow precipitate was collected upon a frit, washed with copious acetone (100 mL), and air-dried (4.34 g; 84% yield). Anal. Calcd for C i 5 H34Cl 2 P 2 Pd: C, 39.71; H, 7.55; CI, 15.63. Found: C, 40.00; H, 7.65; CI, 15.63. ^ P p H } and *H N M R assignments are reported elsewhere.7 2.11.3.2 Pd(dippp)I2 To a slurry of Pd(dippp)Cl2 (1.70 g; 3.75 mmol) in acetone (50 mL) was added Nal (1.50 g; 10.0 mmol). The colour of the slurry deepened to ochre and stirring was continued for 2 h. Workup was as for Pd(dippp)Cl2 giving Pd(dippp)I2 as a fine ochre powder (2.12 g; 89% yield). Anal. Calcd for C i s H ^ I ^ P d : C, 28.30; H, 5.38; I, 39.87. Found: C, 27.98; H, 5.12; I, 40.05. *H N M R (299.99 MHz, CDC13): 8 2.90 (d sept, 4H, C#Me 2 , / P . H = 12.0 Hz, ^H-Me = 7.2 Hz), 2.25 (m, 2H, PCH 2 C// 2 CH 2 P), 1.61 (m, 4H, PC// 2CH 2C// 2P), 1.41 (dd, 12H, CHMe, /p.Me = 19.2 Hz, / M e - H = 7.2 Hz), 1.24 (dd, 12H, CHMe', / p . M e ' = 13.8 Hz, /Me'-H = 7.2 Hz). 3ip{ lR} N M R (121.42 MHz, CDCI3): 8 30.7 (s). Pd(dippp)I2 may also be prepared directly without isolation of Pd(dippp)Cl2 (76% yield). 2.11.3.3 [(dippp)Pd]2(p-H)2 (2) Pd(dippp)I2 (1.15 g; 1.80 mmol) was slurried in toluene (100 mL) and copied to -60 'C in a dry-ice/acetone bath. A solution of KBEt3H (0.499 g; 3.81 mmol) in toluene (10 mL) was added dropwise with stirring and the temperature was raised to -40 °C. A slight effervescence was noted and the red colour of the dimer developed as the Pd(dippp)I2 reacted. The temperature was maintained at -40 "C for 2 h after which time gas evolution had ceased 47 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. and the solution was deep red in colour. The temperature was further raised to -20 °C for 30 min after which the reaction mixture was passed through a frit lined with Cetite. The solvent was removed in vacuo and the residue dissolved in pentane (9 mL). The filtered solution was cooled to -30 °C and deep-red crystals appeared after 24 h (0.300 g, 44% yield). 'lH{ilP] N M R (500.13 MHz, rf6-benzene): 8 1.93 (sept, 8H, C//Me2, /H-Me = 8.4 Hz), 1.90 (quin, 4H, P C H 2 C # 2 C H 2 P , / H - H = 4.5 Hz), 1.32 (t, 8H, PC7/ 2 CH 2 C// 2 P, /H-IT = .4.5 Hz), 1.30 and 1.10 (d, 48H, C H M e 2 , 7 M e . H = 8.4 Hz), -2.50 (s, 2H, p-H). IH N M R (500.13 MHz, ^-benzene): 5 -2.52 (quin, p>H, 7p_H = 34.9 Hz), other resonances appeared as multiplets, TiQi-H) = 0.62 s. 31p'{lH} N M R (121.42 MHz, d6-benzene): 8 27.0. Anal. Calcd for C 3 oH 7 oP4Pd 2 : C, 46.94; H, 9.19. Found: C, 47.22; H, 9.25. 2.11.3.4 [(dippp)Pd]2(p-D)2 (2-d2) The deuterium analogue was prepared by a procedure identical to that for 2 using KBEt3D (0.518 g; 3.72 mmol) and Pd(dippp)I2 (1.16 g; 1.82 mmol). Recrystallization from pentane (10 mL) gave deep-red crystals (0.356 g, 51% yield). *H N M R (500.13 MHz, d&-benzene) 8 1.93 (m, 12H, P C H 2 C # 2 C H 2 P , C//Me2), 1.30 (m, 8H, PC# 2 CH 2 C# 2 P) , 1.30 and I. 10 (m, 48H, CHMe 2 ) , -2.55 (quin of t, residual p-(H-D), 7 P . H - 35.0 Hz, 7 H -D = 2.4 Hz). 2 D N M R (46.0 MHz, toluene): 8 -2.52 (quin, p>D, 7 P . D = 5.5 Hz). 3 1 P N M R (121.42 M H z ; de-benzene) 8 27.0 (t, 7 p . D = 5.5 Hz). Anal. Calcd for C3oH 7 0 P4Pd 2 : C, 46.94; H, 9.19. Found: C, 46.75; H, 9.10. 2.11.3.5 [(dippp)Pd]2(p.-H)2»LiBEt4 (1) To a slurry of Pd(dippp)Cl2 (0.398 g; 0.877 rnmol) in THF (60 mL) at -60 °C was added a mixture of L i B E t 2 H 2 and LiBEt4 in THF (1.80 mL; 1.80 mmol Li+). The temperature was raised to -40 °C and the red colour of the product developed as the Pd(dippp)Cl2 reacted. A faint effervescence was also noted. The mixture was stirred for 2 h at the end of which time a deep red solution was observed and gas evolution had ceased. The temperature was further raised to -20 °C and stirring was continued for 1 h, after which the 48 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. THF was removed under vacuum. The deep-red residue was extracted with toluene (30 mL) and the solution was passed through a frit lined with Celite to remove the precipitated L i CI. The toluene was removed by suction and the residue was dissolved in an extra volume of toluene (5 mL). This deep-red solution was layered with hexanes (10 mL) and yielded maroon crystals after 6 h at -40 °C (0.300 g; 68% yield). lH{31p} N M R (500.13 MHz, ^-benzene): 8 1.85 (sept, 8H, C//Me2, /H-Me = 8.4 Hz), 1.78 (quin, 4H, PCH2C//2CH2P, 7H-H' = 4.4 Hz), 1.34 (br t, 12H, BCH2C//3, /H-H1 = 7.5 Hz), 1.24 (t, 8H, PC//2CH2C//2P, JH-W = 4.4 Hz), 1.19 and 1.07 (d, 48H, CHMe 2 , Jut-K = 8-4 Hz), 0.42 (br q, 8H, BCZ/2CH3, JH-W = 7.5 Hz), -3.63 (br s, 2H, p-H). lH N M R (500.13 MHz, ^-benzene): 8-3.66 (br quin, p-H, / P . H = 35 Hz), other resonances appeared as multiplets, TKp-H) = 0.49 s. 31p{lH) N M R (121.42 MHz, ds-benzene): 8 22.4. Anal. Calcd for C38H9oBLiP4Pd2: C, 50.62; H, 10.06. Found: C, 50.57; H, 10.06. Alternatively LiBEt4 may be added to a solution of [(dippp)Pd]2(p-H)2 in toluene as for 3. Recrystallization from toluene/pentane gives the product in yields over 90%. 2.11.3.6 [(dippp)Pd] 2 (p-H) 2 »NaBEt4 (3) 2 (0.100 g; 0.130 mmol) and NaBEt4 (0.020 g; 0.133 mmol) were treated as for the preparation of 4. Upon addition of NaBEt4 to the solution of 2 the colour darkened to a bottle-green. Removal of the solvent gave a blue, crystalline residue. The residue was taken up in toluene (1.2 mL) and the green solution was layered with pentane (5.0 mL). Blue crystals appeared after 6 h (0.050 g; 41% yield). IR N M R (299.99 MHz, ^-benzene): 8 1.74 (m, 12H, PCH2C//2CH2P, C//Me 2), 1.39 (t, 12H, BCH2C//3, /H-H* = 7.0 Hz), 1.12 (m, 8H, PC// 2CH 2C// 2P), 1.10 and 0.99 (m, 48H, CHMe 2 ) , 0.19 (q, 8H, BC// 2 CH 3 , /ir-H = 7.0 Hz), -3.43 (quin, 2H, p-H, 7 P . H = 35.4 Hz), T;(u.-H) = 0.18 s. 31p N M R (121.42 MHz, ^-benzene): 8 25.4. Anal. Calcd for C38H9oBNaP4Pd2: C, 49.74; H, 9.89. Found: C, 50.00; H, 10.07. 49 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. 2.11.3.7 [(dippp)Pd]2(u.-H)2.LiAlEt4 (4) 2 (0.100 g; 0.130 mmol) and L i A l E u (0.020 g; 0.133 mmol) were dissolved in toluene (10 mL) and stirred for 1 h. The red colour of the solution becomes slightly less intense during this time. The solvent was removed by suction and recrystallization from pentane/toluene (3:1; 10 mL) gave purple crystals after 48 h (0.109 g; 89% yield). *H N M R (299.99 MHz , d6-benzene): 8 1.80 (m, 8H, C//Me 2 ) , 1.75 .(m, 4H, PCH2C//2CH2P), 1.60 (t, 12H, AICH2C//3,7H '-H = 8.1 Hz), 1.16 (m, 8H, PC//2CH2C//2P), 1.20 and 1.07 (m, 48H, CHMe 2 ) , 0.05 (q, 8H, A1C// 2 CH 3 , 7 H ' -H = 8.1 Hz), -3.87 (quin, 2H, f i -H , 7p.H = 35.0 Hz). 31p{lH} N M R (121.42 MHz , d6-benzene): 8 23.3. Anal. Calcd for C38H 9 oAlLiP 4 Pd2: C, 49.73; H, 9.88. Found: C, 49.48; H, 9.85. 2.11.3.8 Pd(diPPp)(Ti2.H2C=CH2) To a solution of 2 (0.081 g; 0.106 mmol) in toluene (10 mL), subjected to several freeze-pump-thaw cycles, was added ethylene (0.500 mmol) from a constant volume bomb. The initial red colour of the solution rapidly discharged to give a colourless solution. The solvent was stripped off, the residue was dissolved in a minimum of pentane (2.5 mL), and the clear solution was cooled to -30 °C. Colourless crystals appeared after 48 h (0.070 g; 80% yield). *H N M R (299.99 MHz , d6-benzene): 8 2.86 (d, 4H, H 2 C = C H 2 , 7 P - H ='1.5 Hz), 1.69 (sept, 4H , C//Me2, /H-MC = 7.0 Hz), 1.68 (m, 2H, PCH2C//2CH2P), 1,25 (m, 4H, PC//2CH2C//2P), 1.08 and 0.96 (dd, 24H, CHMe2,7p-H = 15.0 Hz, / M e - H = 7.0 Hz). 3lp NMR (121.42 MHz, ^-benzene): 8 29.5. Anal. Calcd for C i 7 H 3 8 P 2 P d : C, 49.70; H, 9.32. Found: C, 49.90; H, 9.41. 2.11.3.9 Pd(dippp)(PPh3) To a solution of 2 (0.090 g; 0.117 mmol) in toluene (15 mL), was added PPh 3 (0.061 g; 0.233 mmol). The red colour discharged rapidly and a vigorous evolution of gas was observed. The solvent was stripped off and the yellow residue taken up in Et2Q (15 mL). A yellow, micro crystalline powder deposited on standing (0.132 g; 87% yield). *H N M R 50 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. (200.13 MHz , <i6 benzene): § 7 88 (m, 6H, o-Ph), 7.15 (m, 9H, m-, p-Ph) 1.75 (m, 6H, PCH 2 C// 2 CH 2 P, C//Me2), 1.23 (m, 4H, PC# 2 CH 2 C// 2 P), 1.08 and 0.96 (dd, 24H, CHMe 2 , / P . H = 9.6 Hz, /H-H' = 3.0 Hz). 31p N M R (81.015 MHz, ^-benzene): 8 33.8 (t, IP, 7 P . F = 88.5 Hz), 23.7 (d, 2P, 7 P . P - = 88.5 Hz). Anal. Calcd for C 3 3 H 4 9 P 3 P d : C, 61.44; H, 7.66. Found: C, 61.96; H, 7.90. ; 2.11.3.10 Pd(dipppXDMAD) To a solution of 2 (0.075 g; 0,098 mmol) in toluene (10 mL) was added D M A D in toluene (0.029 g; 0.200 mmol). The initial red colour of the solution slowly discharged to give a colourless solution within 1 h. The solvent was stripped off, and the residue dissolved in minimum pentane (5 mL). The Colourless solution was cooled to -40 *C and colourless crystals appeared after 48 h (0.087 g; 85% yield). lH N M R (299.99 MHz , d6-benzene): 8 3.54 (s, 6H, C 0 2 C H 3 ) , 1.82 (sept, 4H, C//Me2, / H -Me = 7.0 Hz), 1.51 (m, 2H, PCH 2 C// 2 CH 2 P) , 1.22 (m, 4H, PC// 2CH 2C// 2P), 1.11 and 0.87 (dd, 24H, C H M e 2 , / P . H = 16.0 Hz, / M e - H = 7.0 Hz). 3lp N M R (121.42 MHz, d6-benzene): 8 34.5. Anal. Calcd for C 2 iH4oP 2 04Pd: C, 48.05; H, 7.68. Found: C, 48.27; H, 7.90. , ^ 2.11.3.11 Pt(dippp)I 2 A slurry of P t C l 2 (1.00 g; 3.76 mmol) in PhC=N (40 mL) was heated at 100 °C for 20 minutes at the end of which time the PtC l 2 had dissolved to give a light amber solution. The solution was passed hot, through Celite, and the filtrate treated with hexanes (250 mL). A chalky, light yellow powder precipitated which was collected upon a frit and washed with hexanes (100 mL), and then E t 2 0 (lOOmL), to give pure P t ( P h O N ) 2 C l 2 (1.69 g; 95.2% yield). Pt (PhCsN) 2 Cl 2 (0.85 g; 1.79 mmol) was then slurried in acetone (60 mL), and Nal (0.85 g; 6.40 mmol) was added. The slurry was stirred for 24 h, during which time the colour of the solid became progressively darker. A solution of dippp (0.497 g; 1.79 mmol) in toluene (5 mL) was added dropwise with stirring, and a flocculent yellow powder precipitated. The precipitate was collected upon a frit, washed successively with water (50 mL) and E t 2 0 (2 x 50 mL 51 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. volumes), and air dried to give Pt(dippp)l2 as a fine yellow powder (1.15 g; 89.5% yield based on Pt (PhON) 2 Ci 2 ) . Anal. Calcd for Ci 5 H34l 2 P2Pt: C, 24.84; H, 4.72. Found: C, 24.69; H, 4.60. 2.11.3.12 Pt(dippp)H2 (5) Pt(COD)l2 (0.500 g; 8.98 x 10"4 mol) was dissolved in THF to give a pale yellow solution and dippp (0.235 g; 8.98 x 10"4 mol) was added dropwise. A yellow solid precipitated. Amalgam (5.0 g; 0.8%) was added and the reaction vessel was charged with 4 atm H2 after several freeze-pump-thaw cycles. After 24 h of stirring the supernatant solution was decanted and passed through a frit lined with Celite. The solvent was removed by suction, and the colourless material remaining was recrystallized from toluene (5 mL), layered with pentane (10 mL). Colourless crystals of 5 appeared after 12 h at -40 °C (0.200 g; 47% yield). *H N M R (500.13 MHz, ^-benzene): 6 1.83 (sept, 4H, C//Me2, /H-Me = 7.2 Hz), 1.63 (m, 2H, PCH2C//2CH2P), 0.91 (m, 4H, PC//2CH2C//2P), 1.15 and 0.91 (dd, 24H, CUMeMe', 7 M e - P = 15.5 Hz, 7 M e - H = 7.2 Hz), -0.96 (m, 2H, p-H, /pt-H = 1064.4 Hz, 7 H - P (trans) = 149.1 Hz, 7 H - P (cis) = -19.7 Hz, /H-H 1 = 5.1 Hz). 31p.{ lH} (81.015 MHz, ^-benzene): 8 31.2 (s, / P .pt = 1904 Hz). IR 1976 cm-l v(Pt-H). Anal. Calcd for Ci 5 H 3 6 P2Pt: C, 38.05; H, 7.66. Found: C, 38.25; H, 7.80. 2.11.3.13 [Pt(dippp)H]2 (6) Pt(dippp)l2 (0.500 g; 6.89 x 10"4 mol) was slurried in toluene (50 mL) and cooled to -78 "C in a dry-ice ethanol bath. A solution of K B E t 3 H (0.192 g; 2.82 mmol) in toluene (10 mL) was added dropwise with stirring, and the vessel allowed to warm up to room temperature. The mixture was stirred for 24 h, during which time the Pt(dippp)l2 went up to give a pale yellow solution and a precipitate of KI was observed. The solution was passed through Celite and the toluene removed in vacuo. The pale yellow, solid was dissolved in toluene (2 mL), filtered, layered with hexane (5 mL), and allowed to stand at -40 °C. The first crop of colourless crystals were the more insoluble Pt(dippp)H2 (5) (0.030 g; 9% yield). The 52 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. supernatant solution was sucked to dryness, and the yellow residue was dissolved in hexane (10 mL). Pale yellow crystals appeared after 12 h at -40 °C (0.130 g; 40% yield). *H N M R (200.13 MHz, d6-benzene): 8 1.88 and 1.85 (m, 12H, C//Me2, PCH 2 C// 2 CH 2 P), 1.20 (m, 8H, PC// 2CH 2C// 2P), 1.31 and 1.08 (dd, 48H, CUMeMe', 7Me-P = 12.6 H z^ M E-H = 6.0 Hz), -1.09 (quin of quin, 2H, p-H, 7 P t . H = 486.4 Hz, 7 P . H = 37.7 Hz). 3 1 P{ lH} N M R (81.015 MHz, d^-benzene): 8 46.6 (s, lJpi-p = 2515.9 Hz, 2/pt- P = 401.3 Hz, 3 / P . P = 38:9 Hz; coupling constants refer to the 1 9 5 Pt -P t isotopomer). Anal. Calcd for C3oH7oP4Pt2: C, 38.13; H, 7.13. Found: C, 38.20; H, 7.15. 2.11.3.14 [(dippp)Ni]2(u.-H)2 Ni(dippp)Cl 2 (1.058 g; 2.60 mmol) and KBEt3H (0.737 g; 5.34 mmol) were treated as for the preparation of 2. Recrystallization from toluene (5 mL) layered with pentane (15 mL) gave black-red platelets (0.345 g, 42% yield). *H N M R (299.99 MHz, ^-benzene): 8 1.61 (m, 12H, PCH 2 C// 2 CH 2 P, CHM&2), 1.27 (m, 8H, PC// 2CH 2C// 2P), 1.35 and 1.16 (d, 48H, CHMeMe\), -10.79 (quin, 2H, p-H, 7 H-P = 18.5 Hz). 3 lP { lH } N M R (121.42 MHz, d%-toluene): 8 36.3. Anal. Calcd for C3oH7oNi2P4: C, 53.61; H, 10.50. Found: C, 53.49; H, 10.44. 2.11.3.15 [(dippp)Ni]2(p-H)rLiBEt4 As for 1 with [(dippp)Ni]2(jj.-H)2 (0.130 g; 1.93 x l O 4 mol) and LiBEu (0.026 g; 1.93 x 1(H mol). The complex was characterized in solution. ! H N M R (299.99 MHz, d(,-benzene): 8 1.80 (m, 12H, P C H 2 C # 2 C H 2 P , C#Me 2 ) , 1.29 (m, 8H, PC// 2 CH 2 C# 2 P), 1.29 and 1.13 (d, 48H, CHMe Me'), 1.25 (br t, 12H, B C H 2 C t f 3,/H-H' = 8.0 Hz), 0.40 (br q, 8H, BC// 2 CH 3 ,7 H-H' = 8.0 Hz), -10.9 (br quin, 2H, p-H, 7 H-P = 18.5 Hz). 3*P{lH} N M R (121.42 MHz, dfrbcnzene): 8 31.9. 2.11.3.16 [(dippp)Rh]2(p.-H)2»LiBEt4 As for 1 with [(dippp)Rh]2(p-H)2 (0.100 g; 1.30 x 10"4 mol) and LiBEu (0.020 g; 1.30 x lO- 4 mol). The complex was characterized in solution. J H N M R (299.99 MHz, d§-53 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. benzene): 8 1.78 (m, 12H, PCH2C//2CH2P, CHMe2), 1.29 (m, 8H, PC//2CH2C//2P), 1.29 and 1.16 (d, 48H, CHMeMe'), 1.25 (br t, 12H, B C H 2 C / 7 3 , 7 H - H ' = 8.0 Hz), 0.35 (br q, 8H, B O / 2 C H 3 , 7 H - H ' = 8.0 Hz), -6.65 (m, 2H, n-H). 31p{lH) N M R (121.42 MHz, ^-benzene): 8 47.5 (m, 1/p.Rh = 155 Hz). 2.12 References (1) For another example of this see ref. 11 in the following: Thaler, E. F., K.; Huffman, J. C ; Caulton, K. G. Inorg. Chem. 1987,26, 374. (2) Rhine, W. E.; Stucky, G. D.; Peterson, S. W. J. Am. Chem. Soc. 1975,97, 6401. (3) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. J. Principles and Applications of Organotransition Metal Chemistry; University Science Books: M i l l Valley, CA , 1987, pp412. (4) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992,121, 155. (5) Clark, H. C ; Hampden-Smith, M . J. Am. Chem. Soc. 1986,108, 3829. (6) Schwartz, D. J.; Andersen, R. A. J. Am. Chem. Soc. 1995,117,4014. (7) Portnoy, M. ; Frolow, F.; Milstein, D. Organometallics 1991,10, 3960. (8) Jonas, K.; Wilke, G. Angew. Chem. Int. Ed. Engl. 1970,9,312. (9) Barnett, B. L.; Kruger, C ; Tsay, Y . -H . ; Summerville, R. H.; Hoffmann, R. Chem. Ber. 1977,770,3900. (10) It is argued that a relativistic stabilization of the 6s orbital over the 5s operates, citing the shortness of the A u - H bond (1.52 A) compared to the A g - H (1.62 A). Since terminal M - H bonds are shorter than bridging M - H bonds, the terminal M - H is capable of incorporating more metal-based 6s orbital character. (11) Otsuka, S. J. Organomet. Chem. 1980,200,191. (12) Hofmann, P.; Heiss, H.; Muller, G. Z. Naturforsch. 1987, B42, 395. (13) Hofmann, P.; Heiss, H.; Neiteler, P.; Muller, G.; Lachmann, J. Angew. Chem., Int. Ed. Engl. 1990,29, 880. 54 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.. (14) Admittedly a complex formulated as [(dcype)Pd]2(H-H)2» where dcype. is the two-carbon backbone ligand l,2-bis(dicyclohexylphosphino)propane, has recently been : isolated and characterized in solution. However, the reported value for the Pd-H stretching frequency in the IR spectrum at 1620 c m - 1 seems suspiciously high for a bridging hydride ligand. We suggest therefore that its structure may be a variant of the platinum complex, [Pt(dippe)H]2< See: Pan, Y . ; Mague, J . T.; Fink, M . J. Organometallics 1991,11, 3495. (15) The large magnitude of Vpt-H value as compared to that for 6 is also strongly indicative of a Pt(II) species. See ref. 6, and references cited therein. (16) The corresponding deuterides, 5-d2 and 6 - ^ 2 . whose value for Vp^-D would allow definitive assignment, have not yet been characterized. (17) Schwartz, D. J.; Ball , G. E.; Andersen, R. A . /. Am. Chem. Soc. 1995,117, 6021. (18) Gerteis, R. L.; Dickerson, R. E.; Brown, T. L: Inorg. Chem. 1964,3, 872. (19) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms; McGraw-Hi l l Book Company: New York, 1981, pp 50. (20) While the example is drawn from the solid state the crystal structure of NaBEt3H is relevant. The stucture comprises a tetrameric unit of N a B E t 3 H stabilized by two mesitylene. molecules. See: Koster, R.; SchtiBler, W.; Boese, R.; Blaser,D. Chem. Ber. 1991,724,2259. (21) Setzer, w! N.; Schleyer, P. v. R.Adv. Organomet. Chem. 1985,24,353: (22) Darensbourg, M . Y. ; Ash, C. E.Adv. Organomet. Chem. 1987,27,1. (23) Jonas, K.; Kriiger, C. Angew Chem. Int. Ed. Engl. 1980, 79,520. (24) Benfield, F. W. S.; Green, M . L. H.; Moser, G. A.; Prout, C. K . 7 . Chem. Soc, Chem. Commun. 191 A, 759. (25) Forder, R. A.; Prout, K. Acta Cryst. 1974, B30,2318. (26) Wilke, G. Angew. Chem. Int. Ed. Engl. 1988,27,185. (27) Poerschke, K. R; Wilke, G.J. Organomet. Chem. 1988,358, 519. 55 References begin on page 54 Chapter 2: Binuclear Palladium Complexes with Bridging Hydrides.... (28) Poerschke, K. R.; Wilke, G ,/ . Organomet. Chem. 1988,349,257. (29) Schade, C ; Schleyer, P. v. R. Adv. Organomet. Chem. 1988,27, 169. (30) Fischer, K.; Jonas, K.; Misbach, P.; Stabba, R.; Wilke, G. Angew. Chem. Int. Ed. Engl. 1973,72,943. (31) Brown, T. L.; Dickerhoof, D. W.; Bafus, D. A.; Morgan, G. L. Rev. Sci. Inst. 1962, 33, 491. (32) Version 1.0 was obtained from Drs. G. Hagele and R. Fuhler of the Heinrich-Heine Univsersity, Dusseldorf, Germany. It is now available as part of the W I N - N M R software package for the D3M PC. (33) Tani, K.; Tanigawa, E.; Yatsuno, Y. ; Otsuka, S. J. Organomet. Chem. 1985,279, 87. (34) Bryce-Smith, D.; Turner, E. E. J. Chem. Soc. 1953, 861. (35) Baker, E. B.; Sisler, H. H./ . C/iem. Soc. 1953,5193. (36) Klusener, P. A. A. ; Brandsma, L.; Verloruijsse, H. D.; Schleyer, P. v. R.; Friedl, T.; Pi , R. Angew. Chem., Int. Ed. Engl. 1986,25,465. (37) Doyle, J. R.; Slade, P. E.; Jonassen, H. B. Inorg. Synth. 1960, 6, 216. (38) Fryzuk, M . D.; Piers, W. E.; Einstein, F. W. B.; Jones, T. Can. J. Chem. 1989, 67, 883. 56 References begin on page 54 Chapter 3: COORDINATION BEHAVIOUR OF LiBEt 4 TOWARDS (r|5-C5H5)2ReH, ( r i 5 - C 5 H 5 ) 2 W H 2 , AND (rjS-CsHskTaHa. SOLID STATE STRUCTURE OF[(Ti5.C5H5)2Ta(p-H)2AlH] 2(p-OBu)2 3.1 Introduction W E H A V E SEEN HOW LiBEu acts as an excellent Lewis-acid towards the late transition metal hydride [(dippp)Pd]2(p-H)2. It remained for us to establish the coordination behaviour of the alkali tetraethylborates to metal hydrides across the periodic table. As illustrated in Scheme 3.1, such a task may be systematically approached by varying each component of the interaction, comprising the Lewis-acid, composed of anion and cation, and the transition metal-base. Scheme 3.1 anion, • Lewis-acid ^cationJ Metal-base For [(dippp)Pd]2(p>H)2 we had already substituted each element of the Lewis-acid, the cation and the anion (cf. N a + replaced L i + , and AlEt4" replaced BE14"), and had observed adduct formation. As a preliminary variation of the metal-base, we also performed the trivial 57 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4„ replacement of [(dippp)Pd]2(p>H)2, by its congeners [(dippp)Ni]2(p--H)2 and [(dippp)Rh]2(p-H ) 2 and in both instances adduct formation was likewise observed in solution; this is represented in eq 3-1. toluene [(dippp)M]2(|i-H)2 + LiBEu ^ - — - » [(dippp)M]2ai-H)2«LiBEt4 ^ (M = Pd, Ni, Rh) A potentially more interesting line of enquiry was to depart from transition metal phosphine complexes entirely and examine the behaviour of the alkali tetraethylborates towards the classic metal-bases, the metallocene hydrides of general formula Cp2MH n . These species had of course provided some of the earliest examples of metal-base behaviour, and with Cp2ReH, CP2WH2, and Cp2TaH3 accessible we had the opportunity to assess both the effect of formal lone-pairs at the metal centre, and of varying the number of hydride ligands available for coordination to the Lewis-acid. It turns out that L i B E u does indeed undergo adduct formation with each of these terminal metal hydride complexes. In this chapter we relate this chemistry in terms of synthesis, solution behaviour, and competition of the metallocene hydrides for L iBEu- As a bonus a new synthetic route to Cp2TaH3 was developed from the new, unequivocally characterized Ta(IH) complex, [Cp2Ta(p-H)2AlH]2(p-OBu)2- This species may be formulated as a Lewis-acid/metal-base adduct of the unknown Cp2TaH and the conventional Lewis-acid, AIH2OBU; its single-crystal X-ray structure is also included as well as details of its hydrolysis reaction to give Cp2TaH3. 3.2 General 3.2.1 Metal basicity of Cp2MH n As we have seen, the metallocene hydrides have long been known to give isolable Lewis-acid/metal-base adducts with a variety of molecular and non-molecular Lewis-acids. The electronic structure of the metallocene hydrides is not far removed from that of ferrocene, and the molecular orbitals of the Cp2M fragment have been dealt with in several studies. The 58 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. metal-based molecular orbitals can function as donors or acceptors and thus the structure of C p 2 M H n and its protonated salt, [ C p 2 M H n + i ] + , can be rationalized. 1 - 4 However, for Lewis-acids other than H + , for example, B or A l trialkyl derivatives, recent evidence suggests that the Lewis-acidic centre is bound to the metal-base by reason of a multi-centre interaction with the hydrides.5 The formal lone-pairs of electrons residing on the tungsten or rhenium nucleus in CP2WH2 or Cp2ReH do not apparently participate in bonding to molecular Lewis-acids. As will be seen, for Lewis-acidic LiBEt4 etc. we likewise find a similar situation. 3.2.2 Syntheses of C p 2 M H n Synthetic methods used to access the metallocene hydrides are quite indirect and the most general route involves thermolysis of the metal chloride precursor (TaCls, WC16, and ReCIs) in THF in the presence of excess NaCp, with NaBFL; to act as the reductant (eq 3-2). 6 , 7 R e — H MCI n [3-2] 59 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. Once the solvent is removed, sublimation of the residue yields crystalline Cp2MH n in generally acceptable yields. However, synthesis of Cp2TaH3 by these means tends to be quite erratic and a variety of methods have been employed to increase synthetic yields; 7 ' 8 for instance, Cp2TaCl2, which may be prepared from TaCls and CpSnBu3 as shown in eq 3-3, may be treated with hydride transfer reagents to yield Cp2TaH3 after hydrolysis 8 The identity of the key intermediate, before hydrolysis, had not been unequivocally characterized prior to this work. + 3SnBu3CI P-3] 3.3 Coordination behaviour of LiBEt* to the metallocene hydrides Given the ability of LiBEt4 to coordinate to late transition metal complexes bearing bridging hydrides it was a logical extension to examine its coordination behaviour to terminal hydride complexes of the early transition metals. The addition of one equiv of crystalline LiBEt4 to Cp2ReH, CP2WH2 or Cp2TaH3 in toluene or benzene, results in the quantitative formation of the 1:1 adducts Cp2ReH»LiBEu (1), Cp2WH2»LiBEu (2), and Cp2TaH3»LiBEu (3) respectively, as determined by *H N M R spectroscopy. For each adduct the cyclopentadienyl resonances shift slightly in the *H N M R spectrum and the hydride resonances shift markedly upfield by approx. 1-3 ppm with respect to the parent metal-bases. Also indicative of adduct formation is the appearance of signals due to the ethyl groups of the borate anion at different chemical shifts compared to authentic LiBEu in c/8-toluene (cf. 1.29 and 0.21 ppm for the methyl and methylene resonances respectively): the methylene resonances move downfield slightly by 0.20 ppm and the methyl resonances remain relatively unperturbed. This behaviour is entirely consistent with what we have observed for [(dippp)Pd]2(p:-H)2*LiBEt4. 60 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. Complete ! H N M R data for each of the adducts are tabulated in Table 3.1. The hydride region of the *H N M R spectrum of 3 and Cp2TaH3 are shown in Figure 3.1. Adduct formation is complete within the time of mixing in ^-toluene at -78 °C (approx. 10 minutes) and as before variable temperature *H N M R spectroscopy indicates the presence of only one species, the adduct, across the temperature range 183-330K. 61 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. a. - A 1 b . I I I I I I I T T I 1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 I I I 0.0 -0 .5 -1 .0 -1 .5 -2 .0 -2 .5 -3 .0 -3 .5 -4 .0 -4 .5 PPM F i g u r e 3.1. 300.1 M H z *H N M R spectrum of the hydride region of (a) Cp2TaH3 and (b) Cp2TaH3»LiBEt4 (3), in ^-benzene. 62 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. Table 3.1. 300.1 MHz !H NMR data (ppm) for Cp2ReH»LiBEt 4 (1), Cp 2WHrLiBEt 4 (2), and Cp2TaH3»LiBEt4 (3); the values for the parent hydrides Cp2ReH, Cp 2 WH 2 , and Cp 2TaH3 are reported in parentheses. Cp2ReH«LiBEt4 (1) Cp 2 WH 2 «LiBEt4 (2) Cp 2 TaH 3 «LiBEt4 (3) C5H5 3.97 (4.20) 4.03 (4.23) 4.47 (4.75) BCH 2 C// 3 1.28 1.27 1.43 BC// 2 CH 3 0.44 0.48 0.40 M-H -14.02 (-12.92) -14.90 (-12.23) -3.80 (d) (-3.00) -4.71 (t) (-1.62) Cp2ReH«LiBEt4 (1) is more soluble than the parent hydride and thus can only be characterized in solution, nevertheless both 2 and 3 can be isolated as crystalline solids in 80-90% yields. Only for 3 could crystals suitable for X-ray structure determination be obtained, however the data set was subject to a very high degree of thermal motion at room temperature and while the formulation of Cp 2 TaH 3 «L iBEt4 could be confirmed only the position of the heavy tantalum nucleus could be meaningfully fixed.9 Judged by the change in chemical shift, the unique central hydride of Cp 2 TaH 3 »L iBEt4 is the one most strongly perturbed by the interaction with L i + , and its resonance occurs strongly upfield with respect to the corresponding signal of the parent hydride. Early work had also observed that the central hydride of Cp 2 TaH 3 is the one most influenced by adduct formation with aluminum reagents, and had suggested therefore that the central hydride was the most basic. 1 0 Also consistent with our results was the observation that the non-hydride resonances of comparable adducts, e.g. those of the cyclopentadienyl and the ethyl groups of Cp 2 TaH 3 »AlEt3 and C p 2 W H 2 » A l E t 3 , 1 0 , 1 1 were only slightly affected by adduct formation. 63 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. These results strongly suggest that it is the interaction between the hydride ligands and the L i nucleus that is responsible for adduct formation between C p 2 M H n and LiBEt4 (vide infra). 3.4 Competition behaviour by 1,2, and 3 for L iBEt4 and considerations of bonding The addition of one equiv of C p 2 M H m to Cp2M 'H n , LiBEt4 would allow us to establish the relative importance of lone pairs on the metal centre and also the effect of varying the number of hydride ligands available for coordination to L i + . Accordingly, solutions of Cp2TaH3/Cp2ReH«LiBEt4, Cp2TaH3/Cp2WH2»LiBEt4, and Cp2WH2/Cp2ReH»LiBEt4 were prepared in ^-benzene and their lH N M R spectra were acquired. As a check, the converse experiments were performed with solutions of Cp2TaH3»LiBEt4/Cp2ReH, etc., and these were found to give equivalent results to the former series. As indicated by the shifts in the hydride resonances in the *H N M R spectra, the LiBEt4 moiety was coordinated by the metal base with the most number of hydride ligands: i.e. Cp2TaH3 abstracts LiBEt4 from both Cp2ReH*LiBEt4 (1) and Cp 2 WH 2 »LiBEt 4 (2) to form Cp2TaH3»LiBEt4 (3) and Cp2ReFf and C p 2 W H 2 respectively; CP2WH2 likewise abstracts LiBEt4 from 1 to form 2. The exchange can also be reversed by the addition of 1-2 extra equiv of the metal-base which bears fewer hydride ligands. This exchange process is shown in Scheme 3.2 and is complete within the time of mixing at 20 °C. 64 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4... Scheme 3.2 MH n»LiBEt 4 + I m M = Re, n = 1 i M = W, n = 2 1 M = Ta, n = 3 | cfe-benzene 20 °C MH n M ,Hm»LiBEt 4 The facile exchange of L i + between the metal-bases demonstrates that it is kinetically quite labile though its interaction with the metal-base is thermodynamically strong. The kinetic lability of LiBEu can also be demonstrated by the addition of one equiv of C p 2 M H n to its adduct Cp2MH n »LiBEt4 (i.e., m = n in the reaction shown in Scheme 3.2); in the lH NMR spectrum at 20 °C average values of the hydride chemical shift are obtained with respect to those of the adduct and the parent hydride, which indicates rapid intermolecular transfer of L i B E u between C p 2 M H n . Such fluxionality was also observed in the L iBEu a °d NaBEu adducts of [(dippp)Pd]2(p-H)2, and seems to be a feature of the solution behaviour of the alkali tetraethylborates. Significantly, the presence of 6?-electrons on the metal-base does not 65 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4„ apparently influence the strength of the interaction with the Lewis acid as witnessed by the behaviour of Cp2ReH. The Re complex has formally two lone-pairs on the metal centre (d4-electron configuration) and yet it is a fairly lacklustre competitor for LiBEt4. Conversely, Cp2TaH3, with a ^-configuration, competes for the Lewis-acid quite tenaciously. These results point to the idea that adduct formation is solely due to the interaction between L i + and the hydride ligands, and that no dative interaction operates between L i + and the transition metal centre. Again, since L i + can only be negligibly involved in covalent bonding,1 2 , 1 3 an ion-dipole type of interaction must operate between it and the hydrides. Such simple electrostatic interactions are not governed by octet considerations or the like1 2 and thus Cp2TaH3, with three polar Ta-H bonds, is a better donor towards L i + than CP2WH2 or Cp2ReH with two or one hydride ligands respectively. This explanation will be expanded below and is fully consistent with what we have observed for [(dippp)Pd]2(p-H)2; the late metal hydride also manifested Li+-hydride interaction but did not display marked structural variation in its adducts with LiBEu or NaBEU- 1 4 The ion-dipole interaction is also probably responsible for the marked upfield shift of the hydrides in the *H NMR spectrum upon adduct formation: as electron density is polarized towards the site of the interaction, the hydrides experience a shielding effect leading to the observed shift.15 Given the outcome of the competition experiments, it is possible to infer likely solution for the adducts 1, 2, and 3 with reasonable confidence. As a starting point, we take the structure of the parent molecules, which, on the basis of the structure of [(dippp)Pd]2(p-H)2*LiBEt4, should vary very little upon adduct formation with LiBEu. Our formulation of the structure of the adducts is presented in Figure 3.2. In 1, the lone hydride is interposed linearly between the Re centre, to which it is covalently bound, and the Li nucleus, with which it interacts by ion-dipole attraction. Such a geometry would maximize the attraction between the R e - H dipole and ionic L i + . The tetraethylborate anion, which accompanies L i + , must also undergo a rapid tumbling motion interchanging the ethyl protons, as witnessed by the 66 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4„ equivalence of the borate group in the *H N M R spectrum. Conversely, in 2, L i + is equidistant between the terminal hydrides and interacts with both. The structure of 3 is harder to induce in that here a symmetrical structure in which L i + interacts equally with all of the hydrides cannot be drawn and two alternative structures are depicted. In 3a L i + interacts directly with the central hydride of Cp2TaH3 as in 1, whereas in 3b/3b' L i + interacts with two hydrides which is similar to the interaction in 2. The structures 3a and 3b may rapidly equilibrate by a process in which not only the borate anion is conformationally mobile (vide supra), but the LiBEt4 group as a whole. Of the two possibilities, we opt for 3a as the most likely as 3b/3b' should give rise to an A B C pattern for the hydrides in the *H N M R spectrum of 3, something we have never seen even at low temperature. Also, we cannot ignore the shift in the resonance of the central hydride in the *H N M R spectrum of 3; the dramatic upfield shift of the central triplet from -1.80 for Cp2TaH3 to -4.71 ppm for the adduct almost certainly indicates that the central hydride is the greatest contributor to L i + binding. Of course, the given structure of 3a is qualitatively no different from that of the Cp2ReH adduct 1, likewise 3b/3b' is no different from 2, and hence we must explain why Cp2TaH3 wil l abstract LiBEt4 from both 2 and 1. No dative interaction takes place, as we have seen, because in that case the order of stabilities would be reversed; that is Cp2ReH would form the most stable adduct if lone pair electrons were binding L i + . We propose that the origin of the selectivity for L i + lies in the relative strengths of the metal-hydride dipoles. As we move from Cp2TaH3 to CP2WH2 to Cp2ReH we go from a <i°- to a d 2 - to a ^-electron configuration. This trend to higher electron density at the metal nucleus is reflected by the respective formal oxidation states, i.e. Ta(V), W(IV), and Re(III). As dipoles depend on the degree of charge separation between the elements comprising them, the strength of the dipole formed by the transition metal nucleus and the hydride ligand should follow the opposite trend. 1 6 Thus the electropositive Ta(V) nucleus, which is denuded of electron density, forms the most effective M - H dipole, and Re(IU), with two lone pairs, the weakest. L i + is electrostatically attracted most strongly to the excess charge density resident on the central hydride of Cp2TaH3 and a stronger interaction results. Thus we 67 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4.... attribute the superior activity of Cp2TaH3 as a metal-base to the enhanced ion-dipole interaction between L i + and the central hydride. Figure 3.2. Proposed structures for Cp2ReH«LiBEt4 (1), Cp2WH2»LiBEt4 (2), and Cp2TaH3«LiBEt4 (3). 3.5 Routes to Cp 2 TaH 3 ; isolation of [Cp2Ta(p.-H)2AlH]2(u-OBu)2 (4) . While Cp2TaH3 can be accessed by way of the reaction depicted in eq 3-2, Cp2TaCl2 (eq 3-3) offers a more dependable route as it is stable to hydride transfer reagents. The original preparation called for "Vi t r ide" , a commercially available benzene solution of NaAlH2(OCH2CH 2OCH3)2, to effect metathesis of the Ta-Cl bonds, and subsequent 68 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. hydrolysis produced the required Cp2TaH3. The identity of the intermediate product, before hydrolysis, was unknown.17 For the purposes of our investigation we had initially required a high yield route to Cp2TaFf3 but pursuant to this goal we were able to isolate and structurally characterize a Ta(IJJ) intermediate whose hydrolysis gave the desired Cp2TaH3. Cp2TaCl2 can be prepared with ease according to eq 3-3, however, since Vitride is a something of an exotic reagent, we elected to do the reduction with L1AIH4 in THF. The addition of THF solvent to a mixture of Cp2TaCl2 and two equiv of LiAlFLt at -78 °C gives an effervescent rose-pink solution whose colour changes to amber upon being warmed to room temperature. Upon removal of the solvent an orange powder was obtained which may be recrystallized from toluene/hexane to give highly pyrophoric, amber crystals. Microanalysis and the presence of butoxy and hydride resonances in the *H NMR spectrum led to a formulation of Cp2TaH3A!OBu (87% yield based on Cp2TaCl2) as shown in eq 3-4. This formulation was confirmed by a single-crystal X-ray diffraction experiment and the structure of [Cp2Ta(p-H)2AlH]2(p-OBu)2 (4) is shown in Figure 3.3. [3-4] H NMR: hydrides -10.6 ppm 69 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4„.. C(14) Figure 3.3. Chem 3D® view of molecular structure and numbering scheme for [Cp2Ta(u.-H)2AlH]2(u.-OBu)2 (4); (Cp ring centroids have been added for clarity.) 3.6 Solid-state and solution structure of [Cp2Ta(u,-H)2AlH]2(p-OBu)2 (4) As can be seen in Figure 3.3 the structure of 4 may be described as a Lewis-acid/metal-base adduct of Cp2TaH and AlH2(OBu). Of particular note is the dimeric structure of the molecule which only became evident during the last stages of refinement.9 The propensity of the aluminum nucleus to fill its valence shell by participating in multi-centre bonding is again manifested by the AI2O2 metallacycle which is completely planar and symmetric and is centred on a crystallographic point of inversion. The molecule can thus attain Q symmetry. The hydride which is terminally bound to the Al centre was located and refined in the structure; the hydrides associated with the Ta nucleus were placed in idealized positions.9 Selected bond lengths and bond angles appear in Tables 3.2 and 3.3. 70 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4.... Table 3.2. Selected intramolecular distances (A) observed in [Cp2TaOJ.-H)2AlH]2(p-OBu)2 (4). Ta( l ) -Al ( l ) 2.671(4) Al ( l ) -0(1) 1.849(7) Ta( l ) -C( l ) 2.32(1) • Ta(l)-C(6) 2.34(1) Ta(l)-<:(2) 2.37(1) Ta(l)-C(7) 2.31(1) Ta(l)-C(3) 2.36(1) Ta(l)-C(8) 2.32(1) Ta( lH : (4) 2.37(1) Ta(l)-C(9) 2.37(2) Ta(l)-C(5) 2.34(1) Ta(l)-C(10) 2.38(1) Ta(l) -Cp(l) 2.03(4) Ta(l)-Cp(2) 2.04(3) Ta( l ) -H( l ) 1.95 Ta(l)-H(2) 1.81 A l ( l ) -H ( l ) 1.93 Al( l) -H(2) 1.82 Al( l ) -H(3) 1.39(5) Al( l ) -0(1)* 1.839(7) 0(1)-C(11) 1.43(1) Cp(l-2) refer to the unweighted centroids of the cyclopentadienyl ring Table 3.3. Selected intramolecular angles (deg) observed in [Cp2Ta(p>H)2AlH]2(p>OBu)2 (4). : Cp(l)-Ta-Cp(2) 141.6(5) 0(1)-A1(1)-0(1)* 78.0(3) H(l) -Ta(l) -H(2) 88.8(5) A l ( l )* -Q( l ) -C ( l l ) 130.2(7) A1(1)-0(1)-A1(1)* 102.0(3) H(1)-A1(1)-H(2) 89.5(5) A l ( l ) -0 (1 ) -C ( l l ) 125.7(6) Cp(l-2) refer to the unweighted centroids of the cyclopentadienyl ring As the tantalum hydrides lie in idealized positions, the Cp(l)-Ta-Cp(2) angle is perhaps the most important structural parameter of the molecule and has a value of 141.6°. This value is greater than the corresponding angle observed in Cp2TaH3 of 139.9° but is less than that observed in [Cp2WH3]+of 148.2°.3 Without reliable hydrogen coordinates, however, meaningful comparison cannot be made with respect to the H - T a - H bond angle which should 71 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. be substantially reduced from the 125.8" value observed in Cp2TaH3 (i.e. between the equatorial hydrides related by the C2 axis)3 but possibly greater than the corresponding angle for Cp2MoH2of 75.5°:1 8 if a dative interaction operates between the Ta and A l nuclei then the H - T a - H bond angle should be strongly influenced and become greater accordingly. The carbon nuclei of the butoxy group were subject to a large degree of thermal motion resulting in some unusual C - C bond lengths in the refined structure (i.e. between C(12) and C(13)). Nevertheless, it is clear from the lH N M R and ^ C p H } N M R spectra that the hydrocarbyl chain contains saturated carbon atoms only and so the possibility of a butenyl type fragment may be ruled out. Figure 3.4. Chem 3D® view of one half of the [Cp2Ta(p-H)2AlH]2(p-OBu)2 (4) molecule normal to the TaAlH.2 plane. As can be seen in Figure 3.4, which presents a view of the asymmetric unit of 4 normal to the plane defined by the Ta, A l , and hydride atoms, the position of the Ta nucleus is 'slipped' relative to the planes of the cyclopentadienyl rings. Ring slippage is also observed in the structure of Cp2TaH3 and Cp2NbH3 but in contrast to the structures of these species the Cp rings of 4 are slightly offset from an eclipsed geometry.3 72 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. Solution spectroscopy of 4 reveals a symmetric structure with all the cyclopentadienyl protons equivalent in the *H NMR spectrum; the hydrides appear as a broad singlet upfield at -10.6 ppm. Insofar as *H. NMR spectra may be accurately integrated, the ratio of CTpH-.OCH2CH2CH2CHy.TaH was measured at 9:9:2 (i.e. the cyclopentadienyl protons could not be accurately integrated). The terminal hydride bound to the aluminum nucleus apparendy remains unobserved in the *H NMR spectrum, either due to quadrupolar broadening of the signal due to interaction with the 2 7 A l nucleus, or the signal remains buried under the peak due to the cyclopentadienyl protons at 4.76 ppm. The broadness of the other hydride signals is also proposed to be due to quadrupolar interactions, as a dynamic process interchanging the hydrides bound to the metal centres may be ruled out because of other evidence (vide infra). It is also possible that the observed hydrides could exchange with the protons of the Cp ring in a process mediated by the aluminum nucleus. Consistent with this is the observation that for sealed NMR samples of 4 in d(,-benzene that had stood for several weeks, it was found that the integrated ratio of the Cp protons diminished in intensity relative to the butoxy protons, and that signals for the hydridic protons had disappeared entirely. When the solvent was removed from these samples and the residue redissolved in toluene, the 2D{ !H} NMR spectrum showed signals at -10.8 and 4.70 ppm, that is 4 had activated the original deuterated solvent and had incorporated deuterium nuclei in the Cp ring and the hydride positions. That Al could thus act as such an intramolecular site of coordinative unsaturation has been demonstrated before in the solution behaviour of Cp2WE>2#AlMe3.5 It also been shown that both Cp2TaH3 and Cp2NbH3 can activate C-H bonds through postulated Ta(III)/Nb(III) intermediates but in 4 the intermediacy of the aluminum nucleus in a H/D exchange must be invoked to account for deuterium labelling of the Cp rings. As regards the molecularity of 4 in solution, our attempts to determine the molecular weight by the isopiestic method were unsuccessful. However, the molecularity of 4 in d(,-benzene or G?8-toluene solution may be inferred from the *H NMR spectrum in which the resonances of the methylene protons of the butoxy group are well resolved and symmetric. As 73 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4„ well as singlets for the Cp protons at 4.76 ppm and the hydridic protons at -10.6 ppm, the 500.13 MHz *H N M R spectrum of 4 features the following absorptions: a pair of multiplets at 4.09 and 3.99 ppm attributed to OC//2CH2CH2CH3 with an integrated ratio of 1:1; a pair of multiplets at 2.03 and 1.81 ppm attributed to OCH2C//2CH2CH3 again with an integrated ratio of 1:1; a multiplet at 1.44 ppm attributed to OCH2CH2C//2CH3 with an integrated ratio of 2; and a simple triplet at 0.93 ppm attributed to OCH2CH2CH2C//3 with the required integral ratio of 3. Since the separation in Hz between each pair of the downfield multiplets becomes smaller when the spectrum is acquired at 299.99 or 200.12 M H z it is clear that the geminal protons of each methylene group are diastereotopic resulting in an A B coupling pattern. Had the appearance of the methylene protons been due to some coupling phenomenon, Av would have been the same on each spectrometer and the spectrum would not have simplified at high field strength. In the solid-state structure of 4 these methylene protons cannot be interchanged by any symmetry element (i.e. one of the protons is proximate to the terminal aluniinum hydride) which is therefore consistent with the observed *H N M R spectrum. (The cyclopentadienyl rings are also diastereotopic by this reasoning, and the chemical shift of each ring must be assumed to be coincident.) This factor also precludes interchange of the hydrides bridging Ta and A l with the hydride terminal on A l , as mentioned above. The hypothetical monomer, Cp2Ta(p-H)2AlH(OBu), can achieve C s symmetry, and therefore only four multiplets would be observed in the J H N M R spectrum arising from the geminal protons of the methylene groups, each of which would share an enantiotopic relationship. Solution spectroscopy therefore indicates that 4 retains the dimeric structure of the solid-state and the AI2O2 metallacycle remains intact. Admittedly, the methylene resonance at 1.44 ppm occurs as a multiplet corresponding to two protons. However, as this methylene position (i.e. OCH2CH2C//2CH3) on the butoxy tail is furthest removed from the metal centres, the chemical difference should become negligible and while formally diastereotopic the resonances for each proton may coincide. 74 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4.... 3.7 Bonding considerations The structure of 4 in relation to the allied complex Cp2WH2,AlMe3 forms a useful basis for discussion. The interaction between the transition metal and the main-group metal in Cp2WH2»AlMe3 has been described as 'non-classical' on the basis of a low-temperature X-ray diffraction experiment which located all the hydrogen atoms of the complex: the two hydride ligands were shown to interact with both the tungsten and aluminum nuclei to give a five-coordinate aluminum centre.5 An aluminum nucleus engaging in electron-deficient, multicentre bonding accounts for the large separation between the A l and the W centre, and its value of 3.110(3) A effectively precludes any dative interaction. Cp3ZrH»AlMe3, as expected for a <i°-hydride, shares this 'non-classical' structure.19 In contrast to Cp2WH2*AlMe3, examination of the structure of 4 leads to an interpretation in which the metal-metal interaction is much stronger. The distance of 2.671(4) A separating the tantalum and aluminum nuclei lies well inside the sum of the metallic radii of the metal nuclei (Z m e t Ta, A l = 2.86 A), and perhaps represents a dative interaction from Ta to Al . 3.8 Mechanism of formation of 4 The mechanism of formation of 4, involving a formal one-electron reduction of Ta(IV) to Ta(III), is clearly open to question although, undoubtedly, the butoxy group derives from the well known cleavage of THF by AIH3 in the presence of a transition metal (vide infra)?0 For the reaction between Cp2NbCl2 and Vitride or LiAlH4 a mechanistic study reported the following findings: with Vitride as the hydride-transfer reagent two products were observed in benzene solution in variable proportions, Cp2NbH3 and a species formulated as Cp2NbH 2Al(OCH2CH 2OCH3)2; with L1AIH4 in THF an equimolar mixture of Cp2NbH 3 and Cp2NbH2AlH2 was obtained.21 These results were loosely interpreted on the basis of the formation of an intermediate Nb(IV) species, Cp 2 NbH2, which undergoes rapid disproportionation to the Nb(IU) and Nb(V) species as shown in eq 3-5. 75 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. [3-5] Following the formation of C P 2 N D H 3 , and extraction into benzene, the Nb(V) species undergoes reduction to a presumed Nb(III) species, Cp2NbFf2AlH2, by elimination of H2. Subsequent hydrolysis of Cp2NbFf2AlH2 to give Cp2NbFf3 proceeds quantitatively. Whether a similar disproportionation reaction operates for the Ta system is highly questionable. When the *H N M R spectrum of the crude reaction mixture obtained from Cp2TaCl2 and IJAIH4 was periodically sampled by *H N M R spectroscopy, the only species present was the final product 4. (d%-THF was used as the N M R solvent because in d^-bcnzene a reduction reaction might occur for Cp2TaFf3 as it does for Cp2NbFf3, and might therefore lead to the conclusion that no Ta(V) species were present.) Moreover, i f a disproportionation reaction did occur during the reaction, the oxidation product, a Ta(V) species, Cp2TaH-3 or the adduct it may form with A1H 3 , must surely undergo subsequent reduction to give the final Ta(III) product 4 to account for the mass balance (> 80% yield). However, in separate 76 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. experiments we found that neither Cp2TaH3 nor Cp2TaH3»LiBEt4 underwent reaction with excess L i A l F L ; in THF or de-benzenc, and even at reflux temperature, more aggressive conditions than those which generated 4, both Ta(V) species were stable. These data suggest that a disproportionation mechanism does not in fact operate and the presumed initial product, Cp 2TaH2, rapidly undergoes reduction to a Ta(DT) species. It is speculated that the Cp2TaH2 intermediate is responsible for the pink-red colour momentarily observed at low temperature when the solvent is introduced at -78 °C. We presume that this Ta(IV) species undergoes homolysis with the loss of dihydrogen to give the required Ta(IJJ) (eq 3-6). We further believe that the Ta(UI) intermediate mediates the addition of AIH3 to a bound THF solvent molecule to give the final product 4. This point will be discussed further below. [3-6] -1/2H2 77 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4„ We repeated the addition of THF to a mixture of L1AIH4 and Cp2TaCl2 under carefully controlled conditions of temperature, i.e. the solvent was transferred by trap-to-trap distillation and THF was introduced at -196 "C. The reaction mixture was warmed to -78 °C and an effervescent rose-pink solution resulted. When the solution was warmed to ice-bath temperature the rose colour dissipated to give an amber colour but the solid product we subsequently isolated was not 4 but a species we formulate as Cp2TaAlH4»THF (5). Yields of 5 are in excess of 80%, and once again the Ta centre has undergone a formal one-electron reduction from Ta(IV) to Ta(III). As before, a Cp2TaH2 species is proposed as the intermediate. To account for the stoichiometry in this reaction we propose that two equiv of LiCl, one equiv of (AlH3)n, and one half an equiv of dihydrogen are produced as shown in eq 3-7. [3-7] 1 H NMR: hydrides-9.6 ppm It will be seen that the empirical formula of 5 is identical to that of 4, and while solid structural details for 5 are lacking, in ^ 6-benzene or THF solution 5 slowly converts into 4 and the conversion may be driven to completion simply by heating the solution to reflux temperature (eq3-8). 78 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. Ta >AIH4»THF dR-benzene Ta(p-H)2AIH(0(CH2)3CH3) [3-8] J2 This reaction may be monitored by *H N M R spectroscopy which indicates disappearance of the second-order multiplets due to the bound THF molecules of 5 and the appearance of the butoxy resonances of 4. Also the hydride signal at -9.6 ppm is replaced by a signal at -10.6 ppm. Given this result it is reasonable to infer that 5 was an intermediate in the original synthesis of 4, but could not be isolated due to the local heating caused by the dissolution of Cp2TaCl2 in THF. Under stricter temperature control, 5 can be intercepted and converts to 4 upon heating. Importantly, the fact that a sample of Cp2TaAlH4»THF (5) in ^-benzene can undergo this conversion shows that the activation of the THF solvent molecule was indeed transition metal assisted (i.e., the possibility that excess LiAlFLj alone had activated the solvent in the original synthesis of 4 may be discounted). As yet, we have no definitive picture of the solid state structure of 5, however, in solution, an idea of molecular geometry may be developed. The *H N M R spectrum of 5 in d§-benzene solution exhibits multiplets at 3.55 (OCH2C//2) and 1.27 ppm (OC//2CH2) due to THF. The question we wish to ask is whether the THF moiety is associated with the Ta or the A l centre, and two possibilities are shown in the following. For simplicity we have assumed that 5 is monomeric (perhaps an unjustified assumption) and that the aluminate binds in a bidentate fashion. Of the two possibilities we might expect that structure i is the most likely, as THF coordination to A l would allow easy delivery of an aluminum hydride to the a-carbon of the THF and thus ready conversion of 5 to 4. 79 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. H i. ii. In order to test this hypothesis a solution of 5 was placed under an atmosphere of CO in the hope of displacing the bound THF molecule, and foraiing a Ta -CO species. This reaction only went part way to completion as judged by the appearance of some new hydride and new Cp resonances in the *H NMR. The resonance due to the a and (5-protons of the THF remained unchanged. Likewise when excess PMe3 was added, new hydride and Cp resonances were noted but the THF resonance was again unperturbed. These results are consistent with an interpretation in which the added donor molecule binds to the Ta(III) centre, and the THF remains bound to the A l centre. The reverse scenario in which the tantulum nucleus was bound to the THF (cf. ii) would lead to the appearance of THF resonances at their conventional values of 3.55 and 1.73 ppm. 3.9 Reactivity of [Cp 2Ta(u,-H) 2AlH]2(p-OBu)2 (4) 3.9.1 Hydrolysis of 4 to give C p 2 T a H 3 The addition of an excess of H 2 0 to a solution of 4 in toluene results in a exothermic reaction with a white inorganic salt precipitating, presumably hydrated AI2O3. After extraction of the mixture with Et20, and drying of the ethereal layer with Na2S04, Cp2TaH3 may be obtained in yields of 90-100% based on the dry weight of the residue. Recrystallization from toluene/hexanes gives yellow crystals of Cp2TaH3 in 70-80% yields. When the reaction is repeated with D2O and the deuterolysis product sublimed from the aluminum salts, the following results are obtained. The 2 D{ 1 H} N M R spectrum of the sublimate in toluene 80 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. solution features a pair of broad singlets at -1.77 and -3.13 ppm respectively; in the proton-coupled 2 D N M R spectrum the up-field deuteride signal is split into a triplet indicative of coupling to a pair of *H nuclei ( /H-D =1.5 Hz), and the lower field signal is observed as a broad doublet. It would appear that while the deuterolysis of 4 results in indiscriminate placement of the deuteride nuclei (i.e. both hydride positions are isotopically labelled), only one deuteron of the D 2 0 reactant is incorporated in the final product. This interpretation is supported by the *H N M R spectrum of the deuterolysis product in d6-benzene. Three hydride signals are observed in different intensities (1:1:2): a multiplet at -1.58 ppm and a pair of broad doublets at -2.92 and -2.97 ppm. The multiplet at lower field is coupled to one of the upfield doublets, but the doublets are not mutually coupled. The identity of the deuterolysis product may therefore be inferred as isotopomeric Cp 2 TaH 2 D, 4-d\, as shown below. Evidently the deuteride nucleus has a sufficient magnetic influence to cause a chemical shift difference in the *H N M R spectra of i and i i . Isotopomer i, whose equatorial hydride occurs at -2.92 ppm, is magnetically different to isotopomer i i , which has twice as many equatorial hydrides. Significantly, the low and high field signals of the hydride region may be integrated at a ratio of 3:1; isotopomeric Cp 2 TaH 2 D has a total of three flanking hydrides but only one central hydride ligand. Likewise the high-field signals at -2.92 and -2.97 ppm occur in a 1:2 ratio. 3.9.2 Reactivity of 4 towards CO and olefins A surprising feature of the chemistry of 4 is that the complex is quite inert to small molecules such as CO or ethylene, and while it reacts pyrophorically with air the only chemical D II 81 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. transformation we have observed is its hydrolysis reaction. This result is unexpected for a coordinatively unsaturated Ta(UI) nucleus with a formal electron count of 16. However, neither of the components of 4, Cp 2 TaH and AlH2(OBu), have an independent existence, yet are quite stable in combination. One way to formalize this stability is by electron counting. If the Ta(p-H) 2 A1 core is considered as a four centre, four electron interaction (i.e. the AlH 2 (OBu) unit donates two electrons to Ta), then the Ta nucleus can achieve an 18-electron count, and should thus be fairly unreactive. 3.10 Conclusions In this chapter we have shown that an interaction with LiBEty occurs across the board for the early-metal metallocene hydrides, Cp 2 ReH, C p 2 W H 2 , and Cp 2TaH3. Although both the Re and W nuclei possess rf-electrons these are non-decisive factors in adduct formation, and the interaction depends solely on the extent of ion-dipole attractions between L i + and the polar M -H bonds. The absence of concrete structural data for the adducts is disappointing, although, as we do not expect gross variation between the structures of the adducts and the known structures of the parent molecules, this does not represent a serious failure. In any case, X-ray crystallography would be unlikely to locate the hydride atoms of interest in these early transition metal complexes, and recourse would have to be made to a low-temperature experiment or even a neutron-&ffraction study. The isolation and structure of the Ta(IIi) species 4 was a fortuitous result which provides important information in relation to metallocene hydride synthesis as well as evidence for a metal-metal interaction between the Ta and A l nuclei from the X-ray crystal structure. Cp 2 TaCl 2 was found to undergo reduction with L i A l H t to give the new tantalum tetrahydroaluminate species 5, which then undergoes conversion to 4. The difference in the chemistry of the third row transition metal versus that of its second row congener is manifested in this reduction reaction. Whereas C p 2 N b C l 2 is believed to undergo disproportionation with L i A l H t , Cp 2 TaC l 2 only undergoes reduction with the probable intermediacy of a Ta(IV) 82 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. hydride. The use of substituted cyclopentadienyl ligands, such as Cp*, may well serve to stabilize such a species. The well-known kinetic stability of the third row transition metal over the second row metal offers the hope that paramagnetic tantalum hydrides may be isolated. 3.11 Experimental 3.11.1 Procedures Unless otherwise stated, general procedures were performed according to Section 2.12.1. lH N M R details for the adducts 1, 2, and 3 appear in Table 3.1. 3.11.2 Materials TaCIs (Strem), WC16 (Strem), and ReCls (Sandrich Rhenium Alloys) were purchased from the usual suppliers and used as received (sublimation of the per-halides did not markedly improve yields). Solutions of NaCp were prepared from freshly cracked dicyclopentadiene by addition of the CpH to stoichiometric Na metal in THF; after the initial reaction the reaction vessel was heated at reflux for 3 h, before filtration and use (titration gave concentrations of 1.5-2.0 mol L ' 1 ; storage of the solution at -40 °C did not diminish the titre over 2-3 weeks). CpSnBu3 was prepared by addition of NaCp to a solution of SnBu3Br (Aldrich) in THF, followed by a water workup, and distillation under vacuum; 2 2 the reagent was used to prepare Cp2TaCl2 by literature procedures.2 3 , 2 4 Methylene chloride was dried by prolonged heating at reflux over CaH2 and then distilled. L iA lH t (Aldrich) was extracted with Et20 before use; L iA lD4 (Aldrich) was used as supplied. The metallocene hydrides, Cp2ReH, CP2WH2, and Cp2TaH3 were prepared according to published procedures.6"8 , 2 4 D2O (99.7% d-atom) was purchased from Cambridge Isotopes Laboratories and subjected to several freeze-pump-thaw cycles before use. 3.11.3 Syntheses 3.11.3.1 Cp 2ReH*LiBEt4 (1) To a solution of Cp2ReH (0.045 g; 2.36 x 10"4 mol) in ^-benzene (0.5 mL) was added LiBEt4 (0.037 g; 2.36 x 10"4 mol). The adduct was characterized in solution. 83 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4. 3.11.3.2 Cp 2 WH 2 »L iBEt4 (2) To a yellow solution of C p 2 W H 2 (0.145 g; 4.59 x 10"4 mol) in toluene (10 mL) was added LiBEt.4 (0.61 g; 4.59 x 1 0 - 4 mol). The solution was stirred for 1 h and then the solvent was removed in vacuo. The yellow residue was taken up in toluene (2 mL) and this solution was layered with pentane (10 mL). After 12 h at -40 °C pale yellow needles appeared (0.171 g; 83%). IR 1917 cm" 1 v(W-H). Anal. Calcd for C i 8 H 3 2 B L i W : C, 48.04; H, 7.17. Found: C, 47.70; H, 7.17. 3.11.3.3 Cp 2 TaH 3 »L iBEt4 (3) As for 2 with Cp 2 TaH 3 (0.106 g; 3.36 x 10- 4 mol) and LiBEt4 (0.045 g; 3.36 x 10 - 4 mol). Recrystallization from toluene (1 mL) layered with hexanes (4 mL) afforded yellow crystals (0.139 g; 92%). ^ C p H } N M R (d6-benzene, 75.3 MHz): 8 87.0 (s, C5H5), 16.0 (q, B C H 2 C H 3 , 7 B -C = 24.9 Hz), 11.4 (s, B C H 2 C H 3 ) . IR 1743 cm" 1 v(Ta-H). Anal. Calcd for C i 8 H 3 3 B L i T a : C, 48.24; H, 7.42. Found: C, 47.84; H, 7.40. 3.11.3.4 [Cp 2 Ta(p-H) 2 AlH] 2 (p-OBu) 2 (4) A volume of THF (approx. 50 mL) was transferred at -78 °C to a solid mixture of C p 2 T a C l 2 (1.00 g; 2.62 mmol) and LiAlFLj (0.199 g; 5.24 mmol). Upon dissolution of the solids a pink-red solution was obtained which slowly effervesced. Upon warming to room temperature the colour of the solution slowly changed to orange-amber. The solution was stirred at room temperature for 1 h after which time the solvent was stripped off in vacuo. The orange-red residue remaining was extracted with toluene (40 mL), this solution passed through Celite, and then concentrated to a volume of 10 mL. The solution was layered with hexanes (20 mL) and chilled to -40 °C. Amber crystals appeared after 24 h (0.943 g; 87%). *H N M R (^-benzene, 500.1 MHz): 5 4.75 (s, 20H, CpH), 4.09 and 3.99 (m, 4H, OCH2), 2.03 and 1.81 (m, 4H, OCH 2C// 2), 1.44 (m, 4H, OCH 2 CH 2 C// 2 ) , 0.93 (t, 6H, /H-IT = 7.0 Hz), -10.6 (s, 4H, Ta-//). 13c{lH} N M R (d6-benzene, 75.3 MHz): 8 81.5 (s ,C 5 H 5 ) , 61.8 (s, OCH 2 ) , 34.9 (s, 84 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEt4. OCH2CH2), 19.4 (s, C H 2 C H 3 ) , 14.2 (s, CH 3 ) . IR 1657 cm"' v(Ta-H). Anal. Calcd for Ci4H 22A10Ta: C, 40.59; H, 5.35. Found: C, 40.57; H, 5.34. 3.11.3.5 C p 2 T a H 3 » L i A I E t 4 (6) As for 2 with Cp 2 TaH 3 (0.134 g; 4.27 x 10^ mol) and LiAlEu (0.064 g; 4.27 x 10"4 mol). Recrystallization from toluene (1 mL) layered with hexanes (4 mL) afforded yellow crystals (0.180 g; 91%). ! H N M R (d6-benzene, 299.99 MHz): 5 4.50 (s, 9H, CpH), 1.58 (t, 3H, AICH2C//3, 7 H - H ' = 8.1 Hz), 0.26 (q, 2H, A1C// 2CH 3, / H - I T ='8.1 Hz), -3.85 (d, 2H, Ta-tfflanking., JR-H = 13.0 Hz), -5.05 (t, IH, T a - / / c e n t r a i , 7 H -H- = 13.0 Hz). Anal. Calcd for C i 8 H 3 3 A l L i T a : C, 46.56; H, 7.16. Found: C, 46.84; H, 7.40. 3.11.3.6 C p 2 T a A l H 4 ' T H F (5) A volume of THF (approx. 60 mL) at 0 °C was transferred to a solid mixture of Cp 2 TaCl 2 (0.979 g; 2.56 mmol) and L1AIH4 (0.194 g; 5.12 mmol) at -196 °C by trap-to trap distillation. The mixture was warmed to -78 °C, and as the solvent thawed the reactants went up to give an effervescent, rose-pink solution. The solution was allowed to warm to 0 "C, and the rose colour discharged to give a light amber solution. The THF was removed in vacuo and the yellow residue was extracted with toluene and passed through Celite to give an orange solution. A grey material was retained by the frit and discarded. The toluene was removed in vacuo and the residue was recrystallized from THF (5 mL) layered with hexanes (25 mL). A fine yellow powder deposited after the solution had stood for 12 h at -40 °C (0.860 g; 81%). Found: C, 40.67; H, 5.29. *H N M R (^-benzene, 200.13 MHz): 54.65 (s, 10H, CpH), 3.55 (m, 4H, OCH 2C// 2), 1.27 (m, 4H, OC// 2CH 2), -9.36 (s, IH, Ta-H). IR 1784 cm"' v(Ta-H), 1687 cm- 1 v(Al-H). Anal. Calcd for Ci4H22A10Ta: C, 40.59; H, 5.35. 3.12 References (1) Ballhausen, C. J.; Dahl, J. P. Acta Chem. Scand. 1961,15,1333. (2) Lauher, J. W.; Hoffmann, R. /. Am. Chem. Soc. 1976, 98,1729. 85 References begin on page 85 Chapter 3: Coordination Behaviour of LiBEtj. (3) Wilson, R. D.; Koetzle, T. F.; Hart, D. W.; Kvick, A.; Tipton, D. L.; Bau, R. J. Am. Chem. Soc. 1977,99,1775. (4) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interactions in Chemistry; John Wiley & Sons: New York, 1985. (5) Bruno, J. W.; Huffman, J. C ; Caulton, K. G. /. Am. Chem. Soc. 1984,106,444. (6) Green, M . L. H ; Pratt, L.; Wilkinson, G. J. Chem. Soc. 1958, 3916. (7) Green, M . L. H.; McCleverty, J. A.; Pratt, L.; Wilkinson, G. J. Chem. Soc. 1961, 4854. (8) Bunker, M. J.; Cian, A . D.; Green, M . L. H.; Moreau, J. J. E.; Siganporia, N. /. Chem. Soc, Dalton Trans. 1980, 2155. (9) Rettig, S. J., personal communication, 1995. (10) Tebbe, F. N. J. Am. Chem. Soc. 1973, 95, 5412. (11) Storr, A. ; Thomas, B. S. Can. J. Chem. 1971,49, 2540. (12) Setzer, W. N.; Schleyer, P. v. R. Adv. Organomet. Chem. 1985,24, 353. (13) Schade, C ; Schleyer, P. v. R. Adv. Organomet. Chem. 1988,27,169. (14) Fryzuk, M . D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. /. Am. Chem. Soc. 1994, 116, 3804. (15) Kao, S. C ; Darensbourg, M. Y. ; Schenk, W. Organometallics 1984,3, 871. (16) The energy of an ion-dipole interaction is given as \z±\ye 47tr2£o where is the charge of the ion, fx is the dipole moment, e the electronic charge, eo permittivity of free space, and r is the distance between the ion and the molecular dipole. The dipole moment, fx, is defined by H = qr' where q represents the charge and r' is the distance separating the charges. Measurement of the charges associated with the transition metal nucleus and the hydride ligands and hence determination of the magnitude of the dipole is clearly a non-56 References begin on page 85 Chapter 3: Coordination Behaviour ofLiBEt4.. trivial matter. See: Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry; 4th ed.; Harper Collins: New York, 1993. (17) [(ri5-C5H4Me)2Ta(p-H)2AlH(p>H)2]2, the hydrolysis of which gives the corresponding Ta(V) trihydride has been reported but not structurally characterized, see ref. 24 in: Barron, A. R.; Salt, J. E.; Wilkinson, G. J. Chem. Soc, Dalton Trans. 1986, 1329. (18) Schultz, A. J.; Stearley, K. L.; Williams, J. M. ; Mink, R.; Stucky, G. D. Inorg. Chem. 1977,16, 3303. (19) Kopf, J.; Vollmer, H. J.; Kaminsky, W. Cryst. Struct. Commun. 1980, 9, 985. (20) Wiberg, E.; Amberger, E. Hydrides of the Main Groups I-TV; Elsevier: New York, 1971. (21) Labinger, J. A. ; Wong, K. S. J. Organomet. Chem. 1979,170, 373. (22) Fritz, H. P.; Kreiter, C. G. J. Organomet. Chem. 1964,1, 323. (23) Bunker, M . J.; Cian, A. D.; Green, M. L. H. J. Chem. Soc, Chem. Commun. 1977, 59. (24) Curtis, M . D.; Bell, L. G.; Butler, W. G. Organometallics 1985,4, 701. 87 References begin on page 85 Chapter 4: ZEROVALENT 16- AND 14-ELECTRON PALLADIUM COMPLEXES 4.1 Introduction to Chapter 4 4.1.1 General THE L A C K OF REACTIVITY of the palladium complexes so far presented was a bit disappointing in comparison to the wide range of transformations available with the analogous rhodium species. In contrast to [(CH2)n(PR2)2Rh]2(p.-H)2, [(dippp)Pd]2(P--H)2 underwent no insertion type chemistry in the presence of olefins or imines, and even in combination with L iBEt4 only reductive elimination took place to give tricoordinate, zerovalent species of general formula Pd(dippp)L (L = donor).1 , 2 Strategies were therefore sought to make the P2Pd unit more reactive (P2 = bidentate phosphine) and two courses suggested themselves as shown in Scheme 4.1. Scheme 4.1 88 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes Both strategies rely on reducing the degree of. coofdinative saturation at the metal nucleus, i.e. by reducing the formal electron count at the Pd nucleus from 16 to 14 electrons. A 14 electron Pd species should show similar reactivity to Rh(I) or Ir(I) with respect to oxidative-addition or insertion chemistry. In the second instance a hydrocarbyl group bound to the Pd(U) precursor is removed by protonolysis as the corresponding hydrocarbon to give a cationic Pd(II) hydrocarbyl; this is the subject of later chapters. In the first instance halides bound to the metal centre are removed reductively to give a neutral, 1:1 complexes of zerovalent Pd and bidentate phosphines. While this approach departs from the Lewis-acid/metal-base formalism advanced in the preceding chapters some interesting chemistry nevertheless results. In this chapter we detail synthetic routes to, and the reactivity of neutral, 16 and potentially 14 electron Pd complexes stabilized by the very electron-rich, bidentate phosphines, dippe (1,2-bis(diisopropylphosphino)ethane) and dippp (l,3-bis(diisopropylphosphino)propane). 4.1.2 Zerovalent palladium phosphine complexes Mononuclear zerovalent palladium phosphine complexes of the type Pd(PR.3)n ( « = 2 to 4) provide one of the classic examples of ligand dissociation equilibria in coordination chemistry,3'4 and such equilibria are paramount to numerous catalytic processes. Normally, the use of chelating, bidentate phosphines might be expected to reduce the tendency for dissociation. However, the recent study by Portnoy and Milstein clearly shows that complexes of Pd(0) incorporating bulky bidentate phosphines undergo more complicated equilibria involving mononuclear and binuclear species, with the nature of the equilibria dependent on the chelate ring size.5 For example, the solution behaviour of a complex with the formula Pd(dippp)2 was shown to involve a mononuclear trigonal planar derivative in equilibrium with a binuclear complex having a bridging dippp unit and free dippp (eq 4-1) 89 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes [4-1] In contrast, the related derivative Pd(dippe)2 (3) (viz. dippe is the two-carbon backbone ligand) exists exclusively as the four-coordinate mononuclear complex. This difference in solution structures for these bis(ligand) complexes 1 and 3 is further manifested in the enhanced reactivity of the former in the carbonylation of chloroarenes.5 It is interesting to note that while the binuclear species 2 in eq 4-1 can be isolated, no such analogous complex with dippe, i.e. [(dippe)Pd]2(|i-dippe), has been reported.6 During our investigations on the preparation of binuclear palladium hydride complexes as described in Chapter 2, we attempted to prepare the analogous hydride dimer using the two-90 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes carbon backbone ligand, dippe. However, instead of forming [(dippe)Pd]2(p>H)2, the binuclear zerovalent complex [(dippe)Pd]2(u,-dippe) (4), the dippe analogue of 2, was isolated in moderate yield (eq 4-2). [4-2] This result gave rise to allied attempts to produce 1:1 complexes of palladium and bidentate phosphines, and in fact these rare species were directly observed and characterized.7 Equilibria similar to eq 4-1 were found to operate which has possible implications in catalysis. In addition, the single crystal X-ray structure of 4, which exhibits a trigonal planar coordination geometry around the Pd nuclei, was determined; furthermore its solution structure was detailed by analysis of its second order 3 1 P{ 1 H} N M R spectrum, which presents an interesting case of magnetic inequivalence. 91 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes 4.2 Discussion 4.2.1 Isolation of [(dippe)Pd](p-dippe) In Chapter 2 we showed that the binuclear palladium hydride complex [(dippp)Pd]2(p-H)2 could be prepared by reaction of the corresponding mononuclear Pd(II) precursor Pd(dippp)l2 with two equiv of KBEt3H in toluene.1 , 2 In an effort to extend this to the two-carbon backbone diphosphine ligand, we treated Pd(dippe)X2 (X = CI, I) with two equiv of KBEt3H in toluene. From the brown solution that results, amber crystals of empirical formula Pd(dippe)i.5 could be isolated in 35% yield based on Pd (53% yield based on dippe). Signals attributable to palladium hydrides Were conspicuously absent from the *H N M R spectrum, and the 3 1 P { 1 H ] N M R spectrum at different field strengths showed magnetically inequivalent phosphorus nuclei; in Figure 4.1, the second order 202.47 MHz 3 1 P{ *H} N M R spectrum of 4 is shown. This same material could also be isolated by means of nucleophilic cleavage of the 2-methylallyl derivative, [(r|3-C4H7)Pd]2(p-Cl)2, in the presence of 1.5 equiv of dippe per Pd (eq 4-3). A binuclear structure was suggested on the basis of the 3 1 P{ 1 H} N M R spectrum and confirmed by the solid state X-ray crystal structure analysis (Figure 4.2). 92 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes 56 55 54 53 52 51 50 49 (ppm) I 1 1 • i < 1 ' 1 1 1 > 1 r—i 1 1 1 1 1 I 1 r— i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 55.6 55.2 54.8 54.4 50.0 49.6 49.2 48.8 (ppm) (ppm) Figure 4 . 1 . Experimental 202.47 MHz 3lp{ iH} spectrum of [(dippe)Pd]2(ii-dippe) (4) in d%-toluene: (a) total spectrum ; (b) downfield region (A-resonance, bridging phosphorus nuclei); (c) upfield region (B2-resonance, chelating phosphorus nuclei). The absorption at 50.9 ppm results from a folded-in peak due to an impurity. 93 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes Figure 4.2. Chem 3D® view of [(dippe)Pd]2(p-dippe) (4). 4.2.2 Single crystal X-ray structure of [(dippe)Pd](p,-dippe) (4) As seen in Figure 4 .2, the most notable feature of the solid state structure of 4 is the flexible coordination behaviour of the bidentate ligand dippe. Two dippe ligands adopt a chelating mode, whereas one dippe adopts an uncharacteristic bridging mode, linking the two palladium nuclei. The centre of the molecule lies on a crystallographic point of inversion which equates each half of the binuclear metal complex. The coordination geometry around each palladium nucleus is trigonal planar with each Pd-P separation being approximately 2.30 A, a typical value for palladium phosphine complexes in zerovalent or +1 oxidation states. 94 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes Unlike the homologous species Pd2(dppm)3, (dppm = bis(diphenylphosphino)methane) there is no suggestion of a metal-metal interaction, since each metal nucleus in 4 resides in an isolated coordination environment. The chelating P(2)-Pd(l)-P(3) bond angle is 90.08(2)% whereas the P( l ) -Pd( l ) -P(3) and P(2)-Pd(l)-P(3) angles are 132.40(2) and 137.46(2)° respectively. Other bond distances and bond angles appear in Table 4.1. The three donor nuclei and the palladium nucleus are all co-planar, with a mean deviation from the plane of 0.02 A. This plane is canted to that of the bridging backbone (the plane comprising P(3), C(l), C(l*), and P(3*)) by an angle of 18°. 2 MeONa / MeOH (- 2 NaCl) [4-3] 95 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes Table 4 .1 . Selected intramolecular distances (A) and angles (deg) observed in [(dippe)Pd]2(p--dippe) (4). Pd(l ) -P( l ) 2.2966(6) Pd(l)-P(2) 2.2970(6) Pd(l)-P(3) 2.3019(6) C(2)-C(3) 1.486(4) C( l ) -C( l )* 1.529(4) P(l) -Pd(l) -P(2) 132.40(2) P(l) -Pd(l) -P(3) 137.46(2) P(2)-Pd(l)-P(3) 90.08(2) 4.2.3 Solution structure of [(dippe)Pd](p-dippe) (4) If the solid state structure of 4 persisted in solution, both phosphorus donors of the chelating dippe would be inequivalent and so an [ABC] 2 type spectrum would be observed in the '^P^H} N M R spectrum. What is in fact observed is the highly symmetric [ A B 2 ] 2 type system with each coordination plane providing an A B 2 component to the overall 3 1 P{ 1 H} spectrum (vide infra). Evidently, rotation of the Pd(dippe) unit around the remaining P-Pd bond must be quite facile in order to account for the equivalence of the chelating phosphorus nuclei. The P C H 2 C H 2 P backbone linking the palladium nuclei is not sufficiently long enough to magnetically isolate each component and a highly second order spectrum results. The three-carbon backbone analogue, [(dippp)Pd]2(p>dippp), has been reported and characterized in solution, and this exhibits a simpler A X 2 spectrum with only marginal second order complication.5 Since the origin of the second order pattern observed in the 3 1 P{ ! H} N M R spectrum of 4 is due to the presence of the two-carbon backbone of the bridging dippe, we attempted to 96 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes prepare the mixed ligand system [(dippp)Pd]2(p--dippe) in which the chelating diphosphihe is the three-carbon backbone dippp ligand and the bridging unit is the dippe ligand. We had previously shown that addition of ligands to the hydride dimer [(dippp)Pd]2(p>H)2 results in loss of H2 and formation of zerovalent palladium complexes;2 thus, addition of dippe to [(dippp)Pd]2(p-H)2 resulted in the isolation of a material that reproducibly analyzed as [(dippp)Pd]2(dippe) (5). The complementary mixed ligand complex [(dippe)Pd]2(dippp) (6) was prepared as ochre crystals by the reaction of a 2:1 mixture of dippe and dippp with [(T|3-C4H7)Pd]2(p--Cl)2 in the presence of MeONa. 9 In solution both 5 and 6 rearrange to give mixtures of the known complexes,5 Pd(dippp)2 (1), [(dippp)Pd]2(p-dippp) (2) and [(dippe)PdJ2(p-dippe) (4) and other unidentified complexes (approx. 10%); no evidence for a second order pattern in the 3 1 P{ 1 H} N M R spectrum of 5 was detected even when the N M R sample was prepared at low temperatures. The preference of dippe to chelate rather than bridge is presumably a significant determining factor.1 0 4.3 Routes to zerovalent, 1:1 complexes of phosphines and palladium Given the versatility of the reaction represented in eq 4-3, we also ventured the addition of two equiv of the bidentate phosphines to a single equiv of [(n,3-C4H7)Pd]2(u.-Cl)2 in the hope of isolating 14 electron complexes of the empirical formula Pd(dippe) or Pd(dippp); this latter species is the putative intermediate in the reactions catalyzed by Pd(dippp)2 (1) but it has not been directly observed.6 When dippe is used, vinylic and methoxide protons are observed in the *H N M R spectrum of the crude reaction mixture, and the 3 1 P{ 1 H} N M R spectrum exhibits three sets of resonances in an approximate 7:2:1 ratio: an A B quartet at 61.2 and 55.0 ppm (/p.p- = 66.2 Hz), the familiar second order pattern of 4, and a singlet at 32.7 ppm (eq 4-4) 97 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes + 2 X n A J P p ^ Y Y 2 MeONa/MeOH (- 2 NaCl) [4-4] [Pd(dippe)]n 8 7 : 2 : 1 When authentic Pd(dippe)2 ( 3 ) is added to this product mixture, the A B quartet and the high-field singlet disappear and the concentration of 4 is enhanced. This result suggests that the A B signal arises from the olefin adduct of Pd(dippe), Pd(dippe)(ri2-H2C=C(Me)CH20Me) (7), 1 1 and the singlet from actual [Pd(dippe)]n (8), and thus the reaction between 3 and 8 may be formulated as shown in eq 4 - 5 . The stoichiometry of eq 4 -4 requires another Pd nucleus but the mass balance may be satisfied by precipitation of palladium metal. The identity of the olefin may be further verified by an nOe experiment: the diastereotopic vinyl protons were observed to be in close proximity by the nOe criterion, as were the diastereotopic methylene protons (diastereotopic because the olefin has an endo and exo face when bound to the metal centre). Also, when the N M R sample was exposed to an atmosphere of CO, presumably to form the new complex Pd(dippe)CO (11) by displacement, the displaced olefin gives rise to 5 98 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes signals in the J H N M R spectrum: due to the two diastereotopic vinyl protons, the methoxy, the now equivalent methylene protons, and the methyl group. Neither 7 nor 8 could be isolated in a pure form from the product mixture but when the experiment was performed with dippp in place of dippe the analogous complexes were observed in the 3 1 P{ N M R spectrum: thus, the formation of Pd(dippp)(ri2-H2C=C(Me)CH20Me) (9), and [Pd(dippp)]n (10) is proposed along with a trace of binuclear [(dippp)Pd]2(p-dippp) (2). Slow crystallization from pentane afforded an amber material which was a 9:1 mixture of 9 and 10; drying under vacuum removed the olefin to give crystals of empirical formula Pd(dippp). As has been recently suggested with the chelating diphosphine l,2-bis(dicyclohexylphosphino)ethane (dcype), a monomer-dimer equilibrium may be operative.1 2 Molecular weight determination by the Signer method1 3 gave a value of 750 ± 80 g mol" 1 , consistent with a formulation for 10 as [Pd(dippp)]2- [(dippp)Pd]2(p-dippp) (2) is also accessible from 10 by simple addition of mononuclear Pd(dippp)2 (1). 4.4 Catalytic studies Since 4 may be assembled from its sub-units Pd(dippe)2 and Pd(dippe) (eq 4-5) it is an obvious choice for inclusion in some catalytic cycle. In this sense 4 is a protected Pd(dippe) complex: the 14 electron fragment may bind to the saturated 18 electron complex Pd(dippe)2 to give the stabilized complex 4 with 16 electrons per Pd nucleus. Our model reaction was the reductive dechlorination of PhCl by MeONa in MeOH, catalyzed by Pd(0), a system developed by Milstein et al.14 Substrate and catalyst were mixed in a 100:1 ratio, and heating to 90 °C gave a homogeneous solution whose deep brown colour and 3 1 P { 1 H ) NMR spectrum indicated formation of Pd(dippe)PhCl. After 2 hours, G C analysis indicated 65-70% conversion of PhCl to benzene whereas, under identical conditions, Pd(dippp)2 (1) gave 80-85% conversion.1 4 Further heating of the former resulted in precipitation of metallic Pd and no improvement in conversion. As a catalyst, 4 is therefore superior to the coordinatively 99 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes saturated bis-chelato complex 3 (40-45% conversion at 100 °C) 1 4 and comparable to 1 but it is not as thermally stable. These results support the idea that coordinative unsaturation is a prerequisite for reactivity. r <>1 1/n < > P d n A [4-5] Benzene, room temperature It is tempting to speculate that the reaction represented in eq 4-5 does indeed operate in the reductive dechlorination catalysis as shown in Scheme 4 ,2 . 1 4 However the question of whether Pd(dippe)2 is kinetically competent to participate in the catalytic cycle is moot and likely to be answered in the negative given the lifetime of the catalytic system as compared to 100 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes that based on Pd(dippp)2. The catalytic cycle may be completed with 4 acting as an equilibrium source of monomeric, 14 electron Pd(dippe), with this nucleophilic species performing the oxidative addition of PhCl to give Pd(dippe)PhCl. Subsequent methanolysis of the P d - C l bond and precipitation of NaCl , followed successively by [3-hydride elimination upon the coordinated methoxy unit to give formaldehyde, and then reductive elimination to give benzene, finally regenerates the 14 electron Pd(dippe) unit. Scheme 4.2 Reductive elimination of PhH [(dippe) Pdktji-dippe) -Pd(dippe)2 Pd(dippe) Oxidative addition of PhCl -CH 2 0 by p-hydride elimination +MeONa -NaCl It seems unlikely that binuclear 4 could act as a resting state of the catalytic cycle, i.e. that the mononuclear, 14 electron Pd(dippe) complex could be protected by adding to Pd(dippe)2 to give 4 before further oxidative addition occurs. This view is reinforced by theoretical studies, 101 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes which have consistently indicated thatfor a P 2 M fragment, its nucleophilicity (i.e. its ability to perform oxidative-addition reactions) is strongly related to the P - M - P bond angle: acute P - M -P angles correspond to strong nucleophiles. 7 , 1 5 Thus coordinatively unsaturated metal complexes with bidentate ligands of small chelate bite angles are inherently more reactive than those with ligands of larger bite angles. Since the two-carbon backbone ligand, dippe, has a smaller chelate bite angle than the three-carbon backbone dippp, the proposed Pd(dippe) unit is likely to be more reactive than Pd(dippp), and undergo oxidative addition with C - H bonds as well as the C - G l bonds of the PhCl substrate. This is a probable cause of the thermal instability of [(dippe)Pd]2(ji-dippe) as compared to Pd(dippp)2 in the catalytic reaction. 4.5 Analysis of the 3 1 P{ 1 H} N M R spectrum of [Pd(dippe)]2(p-diPpe) The 202.47 MHz 3 1 P{ lH) N M R spectrum of a 2.5% solution of 4 in ds-toluene has been presented in Figure 4.1 which reveals the corresponding [AB2J2 spin system consisting of 6 phosphorus nuclei labelled according to Figure 4.3. ' . Figure 4.3. The 3 1 P{ !H} N M R [AB2]2 spin system of 4 and spin labels used (R = Pr*). 102 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes While complete details of sub-spectral analysis of this system wil l appear elsewhere,16 some little discussion of its parameters and appearance wil l be presented. The bridging phosphorus nuclei (Pi , Pr) give rise to the A , A' multiplet centred at 5p = 54.88 ppm. Each pair of chelating phosphorus nuclei (P 2 , P3 and P 2 ' , P3') is magnetically equivalent and forms the composite particles B 2 and B' 2 respectively, corresponding to a multiplet located at 5p = 49.62 ppm. Integration confirms the intensity relation of 1:2 for the A - and the B 2 -regions respectively. The spectral appearance of this [ A B 2 ] 2 spin system is therefore determined by the following parameters, the resonance frequencies, VA (= VA') and VR (= VBO, and three non-negligible coupling constants, /AB (=,/A'B')» JAB' (= JA"B), and JAA- The other coupling constant operating between the pairs of chemically equivalent nuclei, P2/P3 and P2/P3', corresponds to the long range interaction 7/BB'> the value of which approaches zero. Therefore a special situation results which allows treatment of the [ A B 2 ] 2 spin system of 4 as a simpler [ A X 2 ] 2 system. 1 7" 1 9 The value of the main coupling constant /AB is estimated at 96.8 Hz, a value that is consistent with other phosphorus nuclei that possess a trigonal planar relationship.5 The value of the coupling constant between the chemically equivalent A A ' phosphorus nuclei, /AA'> is estimated at 39.4 Hz. For the value of JAB' (= JA"B), 0.3 Hz was assumed. Use of these starting parameters followed by iteration and analysis by WIN -DAISY 1 6 , 2 0 leads to the extraction of all N M R parameters from the experimental spectrum, as shown in Table 4.2. The long range interaction, 7/BB', is vanishingly small, and is well beneath resolution or zero. Agreement between the calculated and experimental spectra is better than the limits of resolution (< 0.1 Hz). The experimental and simulated spectra appear in Figure 4.4. 103 References begin on page 110 Chapter 4: Zerovalent 14 and16 Electron Palladium Complexes Table 4.2. Starting parameters (Hz) using the [AX 2 ] 2 approximation and final iterated parameters for the [AB 2 ] 2 system. General System Phosphorus system Start Iteration Error3 v A = VA v p i = v p r 11111.8 11111.52 0.078 VB = VB- VP2/P3 = Vp2'/3' 10041.7 10045.52 0.088 JAB =JAB 2/pi.p2/3 96.8 96.99 0.134. JAB' = JA'B 5^Pl-P2'/3' 0.3 0.27 0.0029 JAA 3Jp\-Pl' 39.4 39.57 0.147 JBB' 1Jp213-P2'iy 0.0 0.03 0.087 Half-width (A) 2.25 . 2.50 Half-width(B) 2.75 2.50 rms 0.496 0.395 a. Quoted as a standard deviation of the iterated frequency. 104 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes Hz 11200 .11000 . 10800' 10600 10400 10200 10000 Figure 4.4. Simulated (top trace) and experimental (bottom trace) 202.47 MHz 3 1 P{ lH} NMR spectra of [(dippe)Pd]2(p.-dippe) (4). For data see Table 4.2. 105 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes 4.6 Conclusions The isolation of the zerovalent palladium complex [(dippe)Pd]2(M'-dippe), 4, was achieved through our attempts to prepare the binuclear palladium hydride complex [(dippe)Pd]2(u,-H)2. This hydride dimer is still unknown and in Chapter 2 we had advanced some reasons to account for our failure to isolate it. The solid state structure of 4 shows that one of the dippe units acts as a bridging ligand between the two trigonally coordinated palladium centres. The preference of the palladium nucleus to have a 16 d-electron configuration is satisfied both in the solid and the solution state. Although the two palladium centres are electronically isolated from each other, there is magnetic exchange through the bridging dippe unit that results in the observation of a second order 31p{lH} N M R spectrum. This spectrum has been fully analyzed by means of iterative procedures. The direct observation and in one case isolation of 1:1 complexes of palladium and bidentate phosphines, and the reaction that these species undergo with saturated 18 electron complexes, have obvious implications for catalysis, and this is an area where [(dippe)Pd]2(p>dippe) exhibits potential. 4.7 Experimental 4.7.1 Procedures Unless otherwise stated general procedures are detailed in Chapter 2. Product mixtures of the catalytic experiments were analyzed by a Hewlett Packard 5390A gas chromatograph. Solution molecular weight determinations were carried out in d6-benzene by means of the isopiestic method in a Signer molecular weight apparatus;13 Pd(dippe)2 was used as reference. 3 1 P { 1 H } and ^ { ^ P } N M R spectroscopy were performed upon a Bruker A M X - 5 0 0 instrument operating at 202.47 MHz and 500.1 MHz, respectively; 3 1 P{ *H} N M R spectra were referenced to external P(OMe)3 (141.00 ppm with respect to 85% H3PO4 at 0.00 ppm); *H N M R spectra were referenced to internal C 6 D 5 C D 2 H (2.09 ppm) or internal C6D5H (7.15 ppm). 1 3 C { 1 H } N M R spectroscopy was performed upon a Bruker AC-200 instrument 106 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes operating at 50.32 MHz; spectra were referenced to internal ^-toluene ( C i p s o , 137.5 ppm). Spectral analysis was performed upon a 3 1 P { 1 H ) N M R spectrum (32 Kwords) ranging from 9800 to 11450 Hz. Line-broadening by exponential multiplication with L B = 1 Hz was applied to the FID prior to FT. Data were transferred to the ID W I N - N M R 2 1 program system for numerical evaluation. After sub-spectral analysis, starting data and the experimental spectrum were used for iteration and simulation with WIN -DAISY 2 0 running under ID WIN-NMR on a PC 486/66 MHz. Graphical representations were produced by ID WIN-NMR. 4.7.2 Materials PdCl2(dippe) was prepared by adding one equiv of the chelating phosphine to a solution of PdCl2(C6H5C=N)2 in acetone and recovered on a frit. MeOH was distilled from Mg(OMe)2 or CaH2 under Ar and used to prepare MeONa solutions by addition of sodium metal. 4.7.3 Syntheses 4.7.3.1 [(dippe)Pd]2(u-dippe) (4) PdCl2(dippe) (0.510 g; 1.16 mmol) was slurried in toluene (60 mL) and cooled to -40 °C in a dry-ice/acetone bath. A solution of two equiv of KBEt3H (0.320 g; 2.32 mmol) in toluene (10 mL) was added with stirring. The temperature was maintained for 2 hours during which time the PdCl2(dippe) went up and a deep brown colour developed. The reaction mixture was warmed to room temperature and passed through a frit lined with Celite. The solvent was stripped off from the filtrate, the brown residue was dissolved in toluene (2 mL) and layered with pentane (10 mL). After 24 h at -30 °C, amber crystals of 4 deposited from the solution (0.203 g, 35% yield based on Pd). Anal. Calcd for C42H96P6Pd2: C, 50.91; H, 9.68. Found: C, 50.58; H, 9.49. i H p l p ) N M R (dg-toluene): 8 1.99 (s, 4H, bridging-C//2), 1.95 (sept, 4H, bridging-C//Me2, /H-Me = 7.5 Hz), 1.92 (sept, 8H, chelating C//Me2, ^H-Me = 7.5 Hz), 1.42 (s, 8H, chelating CH2), 1.33 (m, 24H, bridging-CHMeMe'), 1.18 and 1.05 (d, 48H, 107 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes chelating-CHMeMe', /Me-H = 7.5 Hz). Details of the 3 1 P{ lK} NMR spectrum are discussed in the text. Alternatively 4 may be prepared by treating [(T|3-C4H7)Pd]2(p-Cl)2 (0.320 g; 0.810 mmol) with MeONa (0.100 g; 1.85 mmol) in MeOH (50 mL), in the presence of stoichiometric dippe (0.640 g; 2.44 mmol, 1.5 equiv per Pd). The phosphine is added directly to the yellow solution after addition of the MeONa solution. The clear solution was stirred for 12 h, and then evaporated to dryness. The ochre precipitate remaining was washed repeatedly with cold hexanes (15 mL) to give pure 4 (0.500 g; 62% yield). 4.7.3.2 [(dippp)Pd]2(dippe) (5) To a deep red solution of [(dippp)Pd]2(p-H)2 (0.100 g; 0.13 mmol) in toluene (5 mL) was added a solution of dippe (0.034 g; 0.130 mmol) in toluene (10 mL). The red colour rapidly discharged to give an amber solution. The solvent was removed in vacuo and the residue recrystallized from pentane (5 mL) to give amber crystals of 5 (0.079 g; 65% yield). Anal. Calcd for C44HiooP6Pd2: C, 51.41; H, 9.81. Found: C, 51.44; H, 9.83. The complex is unstable in solution. 4.7.3.3 [(dippe)Pdk(dippp) (6) The alternative procedure for 4 was employed with [(ri3-C4H7)Pd]2(p-Cl)2 (0.312 g; 0.79 mmol), dippe (0.415 g; 1.58 mmol) and dippp (0.218 g; 0.79 mmol) in toluene (10 mL), and MeONa (0.104 g; 1.90 mmol) in MeOH (4.3 mL). Workup and recrystallization from hexanes (10 mL) gave amber crystals (0.425 g; 53 % yield). Anal. Calcd for C43H9gP6Pd2: C, 50.94; H, 9.74. Found: C, 51.10; H, 9.90. The complex is unstable in solution. 108 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes 4.7.3.4 Pd(dippe){Ti2.H2C=C(Me)CH20Me} (7) and [Pd(dippe)]n (8) As for6 with [(Ti3-C4H7)Pd]2(u,-Cl)2 (0.201 g; 0.51 mmol), dippe (0.268 g; 1.02 mmol), and MeONa (0.055 g; 1.02 mmol) in MeOH (30 mL). Removal of the solvent gave a brown powder (0.379 g) which by 3 1 P{ *H} N M R was a mixture of 7,8, and 4 in a 7:1:2 ratio. Pd(dippe){ri2-H2C=C(Me)CH2OMe} (7): 3lp{iH} N M R (d6-benzene) 8 61.2 (d, IP, 7 P . F = 66.2Hz), 55.0 (d, 1 P , / P ' - P = 6 6 . 2 H Z ) ; lH{31p} N M R 84.83 and 3.57 (d, 2H, CHH'O,7H-H' = 10.0 Hz), 3.46 (s, 3H, OMe), 3.18 and 3.06 (s, 2H, vinylic-C/7/7'), 2.17 (s, 3H, vinylic-Me), isopropyl resonances were obscured by those due to 4 and 8. [Pd(dippe)]n (8): 3lp{lH} NMR (fi?6-benzene) 8 32.7 (s); ^ p l p } N M R resonances obscured by those due to 4 and 7. Recrystallization of the crude product from hexanes or pentane enriched the percentage composition of 4 and further characterization could not be achieved. 4.7.3.5 Pd(dippp){ri2-H2C=C(Me)CH20Me} (9) and [Pd(dippp)]n (10) As for 6 with [(Ti3-C4H7)Pd]2(p-Cl)2 (0.215 g; 0.55 mmol), dippp (0.304 g; 1.10 mmol) in toluene (7 mL), and MeONa (0.061 g; 1.13 mmol) in MeOH (30 mL). The reaction mixture was recrystallized from pentane (0.5 mL) to give amber crystals (0.230 g) which ^ p i p ) N M R spectroscopy revealed to be a 9:1 mixture of 9 and 10. A successive recrystallization and prolonged drying under vacuum gave low-melting amber crystals of 10 (0.055 g; 13% yield). Pd(dippp){Ti2-H2C=C(Me)CH2OMe} (9): 31p{lH} N M R (^-benzene) 8 26.4 (d, IP, 7p.p' = 24.9 Hz), 25.4 (d, IP, 7p.p = 24.9 Hz); lH{31p} (d6-benzene) 8 4.77 and 3.52 (d, 2H, CHff'O, TR-H" = 10.0 Hz), 3.46 (s, 3H, OMe), 2.95 and 2.85 (s, 2H, vinylic-C////'), 2.12 (s, 3H, vinylic-Me), 1.75 and 1.65 (sept, 4H, C7/MeMe', 7 H -Me = 7.0 Hz), 1.67 and 1.22 (m, 4H, CH2CH'2P), 1.09 and 1.05 (d, 12H, CHMeMe', /Me-H = 7.0 Hz), 0.96 and 0.92 (d, 12H, CH'MeMe', / M e - H = 7.0 Hz); 1 3 C{ 1 H) N M R 8 81,7 (m, C H 2 0 ) , 66.6 (d, CCH 2,7p-c = 23 Hz)^56.7 (s, OMe), 49.6 (d, vinylic-CH 2,7p-c =17 Hz), 24.3 (d, vinylic-Me, / P . M e = 2 Hz), ligand resonances were ambiguous. Anal, was not obtained. [Pd(dippp)]n (10): Anal. Calcd for Ci5H34P 2Pd: C, 47.07; H, 8.95. Found: C, 47.35; H, 9.05. 3lp{lH} N M R (^-benzene) 8 109 References begin on page 110 Chapter 4: Zerovalent 14 and 16 Electron Palladium Complexes 34.6 (s); !H{3lp} N M R (^-benzene) 82.44 (m, 4H, C//2CH2P), 1.72 (sept, 8H, C//MeMe\ JH-Mt = 7.0 Hz), 1.46 (m, 8H, C//2P), 1-30 and 1.20 (d, 48H, CHMeMe', / M e - H = 7.0 Hz). Solution molecular weight. Calcd for [Pd(dippp)]2: 766 g mol" 1 . Found: 750 ± 80 g m o H . 4.7.4 Catalyses A bomb reactor was charged with 1 or 4 (0.1 mmol), PhCl (10 mmol), powdered NaOH (10 mmol), and MeOH (4 mL). The bomb was evacuated and heated to 90 "C in an oil bath. Periodically, a drop of solution was diluted with Et20 and its composition analyzed by GC. 4.8 References (1) Fryzuk, M . D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. J. Am. Chem. Soc. 1991, 113, 4332. (2) Fryzuk, M . D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. /. Am. Chem. Soc. 1994, 115, 3804. (3) Tolman, C. A.; Seidel, W. C ; Gerlach, D. H. J. Am. Chem. Soc. 1972, 94, 2669. (4) Mann, B. E.; Musco, A. J . Chem. Soc, Dalton Trans. 1975, 1673. (5) Portnoy, M. ; Milstein, D. Organometallics 1993,12,1655. (6) Portnoy, M. ; Milstein, D. Organometallics 1993, 72, 1665. (7) Otsuka, S. /. Organomet. Chem. 1980,200,191. (8) Kirss, R. U.; Eisenberg, R. Inorg. Chem. 1989,28, 3372. (9) Kuran, W.; Musco, A. Inorg. Chim. Acta 1975, 72,187. (10) Piers, W. E. Ph. D. Thesis, University of British Columbia, 1988. (11) Jolly, P. W. Angew. Chem., Int. Ed. Engl. 1985,24,283. (12) Pan, Y , ; Mague, J. T.; Fink, M . J. J. Am. Chem. Soc. 1993, 775, 3842. (13) (a) Signer, R. Liebigs Ann. Chem. 1930,478,246. (b) An apparatus, similar to the one used here, is described in: Zoellner, R. W. /. Chem. Ed. 1990,67,714. (14) Ben-David, Y. ; Gozin, M. ; Portnoy, M. ; Milstein, D. /. Mol. Catal. 1992, 73,173. 110 References begin on page 110 Chapter 4: Zerovalent.14 and 16 Electron Palladium Complexes (15) Hofmann, P.; Heiss, H.; Muller, G. Z. Naturforsch. 1987, B42, 395. (16) Hagele, G.; Fryzuk, M . D.; Rettig, S. J.; Clentsmith, G. K. B. Organometallics in press. (17) Anet, F. A. L. /. Am. Chem. Soc. 1962,84,747. (18) Anet, F. A. L.J. Mol. Spectrosc. 1968,28,191. (19) Anet, F. A. L. Spectrochimica Acta 1968,24A, 1939. (20) WIN-DAISY program system: Weber, U.; Spiske, R ; Hoffken, H. W.; Hagele, G.; Thiele, H. B R U K E R F R A N Z E N ANALYT IC , Manual 1993. (21) ID W I N - N M R program system: Thiele, H.; Germanus, A . ; Pape, R. B R U K E R F R A N Z E N ANALYT IK , Manual 1993. Ill References begin on page 110 Chapter 5: CATIONIC P A L L A D I U M A L K Y L SPECIES STABILIZED BY ELECTRON-RICH PHOSPHINES 5.1 Introduction to Chapter 5 TO REITERATE THE A R G U M E N T proposed at the beginning of Chapter 4, one way to greatly augment the reactivity of a Pd(dippe)R2 unit (R = hydrocarbyl) is to remove one of the R groups as a hydrocarbon by protonolysis (eq 5-1). [5-1] The cationic [Pd(dippe)R(OEt2)]+ species that results is a potent electrophile, and if the weak coordination offered by the labile ether molecule is ignored, it is potentially a 14-electron species. In terms of the metal-base formalism advanced in Chapter 1 this represents an irreversible interaction between metal-base, the parent P2MR2 unit (P2 = bidentate phosphine), and Lewis-acidic H + , to generate a new reactive species, [P2MR]+. A similar strategy is employed in the synthesis of the single-site olefin polymerization catalysts which rely on methyl abstraction from Cp2Zr(CH3)2 by Lewis-acidic methyl alumoxane (MAO) or B(C6F5)3. In practice, the palladium cation is not three coordinate as shown in Scheme 4.1, but wil l conscript a solvent molecule to occupy the vacant coordination site (cf. Et20 in eq 5-1). Depending on the donor ability of the solvent, it may be displaced by electron-rich species such as CO or olefins. In this chapter we relate the synthesis, structure, and reactivity of several cationic palladium hydrocarbyl complexes that are matched with the poorly coordinating anion, 112 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines BArf (BArf = { B P ^ - ^ C h C ^ F J ^ } - ) . 1 Compared to the neutral Pd(II) and Pd(0) systems, this cation/anion combination results in superior reactivity, indeed akin to the reactivity displayed by such species as [Cp2ZiR]+. One obvious practical application of this chemistry is the development of a cationic Pd system capable of polymerizing functionalized olefins.2 Although this goal was not pursued vigorously, the [Pd(dippe)R(OEt2)]+ system did give intriguing results with respect to copolymerization of CO and olefins and an unprecedented result with respect to migratory insertion reactions of C O . 3 , 4 As a complement to the chemistry of the Pd cations with bidentate phosphorus ligands, some new ligands were devised with both phosphorus and nitrogen donors whose palladium hydrocarbyl complexes can potentially give two isomers upon protonolysis. One isomer was found to form stereoselectively and its solid-state and solution structures are also presented here. 5.2 General The use of cationic Group 10 metals in catalysis has recently attracted considerable attention.5"12 There are similarities between the metallocene cations of the Group 4 metals and the square planar, cationic eft metal complexes, these being, (i), formal positive charge at the metal centre, (ii), a hydrocarbyl group or the equivalent, and (iii), a potentially vacant coordination site cis to that group.1 3 The cfi metal systems might therefore be expected to undergo equivalent chemistry with respect to insertion. While the tendency of late-metal nuclei to |3-eliminate and thus terminate a catalytic cycle must be negotiated,1 4 the reduced oxophilicity of the d 8 electronic configuration as compared to the cf often means that a catalyst based on such a system will tolerate functionality on the substrate,2 something which the traditional Ziegler type catalysts were never able to perform. As softer electrophilic sites they also open up the opportunity of copolymerization of different substrates and this feature has already been exploited in the catalytic copolymerization of CO and ethylene in the synthesis of 113 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Carolan® polymers.1 5 More recently, this methodology has been used in the first stereoregular copolymerization of GO and propylene to produce optically active, isotactic polyketones.1 6 These late metal cations, which are usually stabilized by bidentate ligands with P -P , P - N , or N - N type donors, are therefore a highly active area of current research, and as catalysts they are very much complementary to the metallocene cations and Kaminsky type systems (cf. Section 1.2.3). 5.3 Routes to [Pd(dippe)(Ti3.CH 2G 6H5)] + We therefore wished to develop routes to similar cationic palladium systems and examine their chemistry. As a starting point, we examined the synthesis of a cationic palladium benzyl derivative. Given our experience in phosphine chemistry, we opted first for the electron-rich bidentate phosphines, dippe and dippp, as the ancillary ligands for our palladium hydrocarbyl complexes. First, the protonolysis reactions of Pd(dippe)(CH2C6H5)2 by such acids as Er^N'HJ and CgHsN'HCl were examined. In THF, protonolysis proceeds smoothly and the reaction may be conveniently monitored by 3 1 P{ 1 H} N M R spectroscopy which shows the appearance of an A X system: i.e. the 3 1 P nuclei are rendered non-equivalent upon removal of a benzyl group. The lH and ^ { ^ P } N M R spectra again showed the non-equivalence of each side of the ancillary diphosphine ligand in the palladium complex by exhibiting four sets of isopropyl methyl resonances (each a doublet of doublets due to coupling to a single 3 1 P nucleus and a methine proton), and two sets of isopropyl methine signals (each a septet in the ^ f 3 1 ? } spectrum due to coupling to the six protons of the isopropyl methyl groups). The benzyhc protons too, appeared as a doublet of doublets by coupling to each non-equivalent phosphorus nucleus. However, the chemical shift of the ortho protons of the remaining benzyl group was measured at 7.45 ppm, a value too far downfield to be consistent with T)3-benzyl coordination and conventional rj1-coordination is indicated. 114 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines [5-2] Evidently, upon protonolysis the halide portion of the acid coordinates, as shown in eq 5-2. Clearly, a Lewis-acid whose anion has a poorer coordinating ability than a halide was necessary to ensure rj3-coordination. HBArf (HBArp = [H(OEt2)2]{B[3,5-(F3C)2Q5H3]4}) or 'Brookhart's acid' seemed to fit our requirement. This easily handled, crystalline solid may be deployed in ethereal solvents, and a molecular weight of 1012.47 g m o H allows for accurate weighing of small molar amounts.1 The use of alternative acids such as HBF4 or CF3SO3H leads to decomposition of the palladium starting material. The addition of one equiv of FIB Arf to Pd(dippe)(CH2C6H5)2 does indeed result in the formation of [Pd(dippe)(r|3-CH2Q5H5)](BArf) (1) which may be crystallized out of Et20 in 85% yield (eq 5-3). Crystals suitable for X-ray diffraction could not be obtained, but the structure of the molecule may be deduced by solution spectroscopy. 115 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines b. 1 1 r 1 1 1 1 1 — — i j 1 ! 1 i—:—i 1 1 1 1 1 1 1 1 1 j 1 1 1 1 3.0 2.5 2.0 1.5 1.0 (ppm) Figure 5.1. 500.13 M H z l H (b),'-and i H p l p } (a) N M R spectra of [Pd(dippe)(r|3-CH2C6H5)](BArf) (1) in ^ -methylene chloride. 116 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines The 3 1 P { 1 H ) N M R spectrum of 1 shows the required A X pattern, viz. a doublet of doublets at 88.7 and 75.2 ppm (/p.p = 32.1 Hz); and the lH and ^ t 3 1 ? } N M R spectra, shown in Figure 5.1, are representative of a complex based on a bidentate ligand system whose donor nuclei are non-equivalent. Of particular note are the four isopropyl methyl signals between 1.16 and 0.82 ppm and the two isopropyl methine resonances at 2.37 and 1.89 ppm; the i H p i p } N M R spectrum again shows the inequivalence of the methylene backbone of the ligand with the two triplets required for the A2X2 spin-system of the CH2CH'2 linkage. Significantly, signals due to the ortho protons of the benzyl group are observed at 6.50 ppm. 1 7 This upfield value indicates ri3-coordination (vide supra) and the assignment is further evidenced by an nOe experiment: irradiation of the benzylic protons results in signal enhancement of the peak at 6.5 ppm, due to the ortho protons, and a negative nOe with the meta protons at 7.4 ppm. The ortho and benzylic protons each appear as single multiplets in the *H N M R spectrum and their equivalence is maintained even at -90 "C, as is the inequivalence of the phosphorus nuclei in the 3 1 P{ *H} N M R spectrum. This dynamic behaviour is proposed to be due to a fast r|1-ri3 suprafacial shift of the palladium nucleus, which interchanges both syn and anti benzyl protons, and orfnophenyl protons but maintains the non-equivalence of the phosphine donors, as shown in the following. 1 8 117 References begin on page 166 ChapterS: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines A poorly coordinating anion was thus necessary to ensure T|3-benzyl coordination. This result could also be demonstrated by the preparation of [Pd(dippe)(T|3-CH2C6H5)]+ by benzyl abstraction upon Pd(dippe)(CH2Ph)2 by 6(0^5)3, one of the Lewis-acids that has been used in the synthesis of the single-site olefin polymerization catalysts. While a 1:1 mixture of the two reagents generated an oil, the solution spectra of the product was equivalent to that of 1, except for the benzylic signals due to [B(G6Fs)3CH2Ph]- observed in the J H N M R spectrum, l-dq was also prepared by protonolysis by HBArf upon Pd(dippe)(CD2C6Ds)2 (prepared in turn from Pd(dippe)Cl2 and two equiv of KCD2C6D5), and in the 2 H N M R spectrum of the reaction mixture a doublet was observed at 2 . 3 0 ppm (/H-D = 2.5 Hz) due to extrusion of HD 2 CC6D5. 118 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 5.4 Preparation of [Pd(dippe)Me]+ While 1 could indeed be shown to be a cationic, Ti3-benzyl system, its reactivity is not extraordinary. 1 is inert to both CO and H2, and is in fact air-stable. This is not too surprising as examples of stable T| 3 -allyl palladium cations are legion, and their chemistry is fairly mature. 1 9 We therefore chose to examine protonolysis reactions upon Pd(dippe)Me2 and Pd(dippp)Me2, and synthesis of methyl cations based upon these precursors proved quite straightforward. To an ethereal solution of either palladium dimethyl complex, the addition of one equiv of HBArf causes a rapid effervescence, undoubtedly of methane, and the methyl cations [Pd(dippe)Me(OEt2)]BArf (2) and [Pd(dippp)Me(OEt2)]BArf (3) are obtained in 60-80% yields as shown for 2 in eq 5-4. <> Crv Me \ © / Pd [5-4] / \ ' P* OEt 2 <y 2 Because the remaining methyl group bound to the Pd centre cannot offer additional n-interaction as can an allyl or benzyl group, the metal nucleus is now highly electrophilic and is loosely stabilized by whatever solvent molecules that are available. In different N M R solvents, or indeed an ethereal solution of the cation to which different solvents are successively added, this gives rise to a series of solution spectra which indicates the progressive filling of the vacant coordination site by the stronger donor. The 3 1 P{ *H} N M R spectrum of 2 thus consists of an A X pattern and in different N M R solvents the chemical shift of the downfield doublet changes noticeably, whereas the chemical shift of the upfield doublet remains relatively unperturbed, compared to the resonance of Pd(dippe)Me2 at 61.0 ppm. From a starting solution of Pd(dippe)Me2 plus HBArf in Et^O/cfe-benzene the following 3 1 P{ 1 H] N M R parameters can be 119 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines generated: an A X pattern at 84.2 and 63.1 ppm ( / A X = 17.4 Hz) due to [Pd(dippe)Me(0Et2)]+ (2a); following addition of a few drops of THF, an A X pattern at 86.0 and 69.0 ppm ( / A X = 17.0 Hz) due to [Pd(dippe)Me(THF)]+ (2b); and following addition of excess PPh3, an A M X pattern at 78.3, 70.0, and 29.8 ppm ( / A X = 368.0 Hz, / A M = 21.9 Hz, / M X = 29.5 Hz) due to [Pd(dippe)Me(PPh3)]+ (2c). These data are in accord with a square planar palladium cation whose the fourth coordination site is filled by a labile solvent molecule. The phosphorus donor that is trans to the remaining methyl group is likely to remain uninfluenced magnetically by the occupation of the coordination site cis to it, and thus is assumed to give rise to the upfield multiplet. Meanwhile the downfield signal wil l vary considerably as does the chemical environment of this phosphorus donor, depending on the occupation of the coordination site trans to it. (Such a conclusion may also be obtained by selective irradiation of each 3 1 P resonance while observing 3 / H - P of the methyl signal in the *H N M R spectrum.) The equilibrium between 2a, 2b, and 2c may be simply explained by the relative coordinating abilities of the added donor; the o-donating abilities are expected to increase in the order Et20<THF<PPh3. The solution behaviour of 2 may also be characterized by examination of the ^ C p H ) and 3 1 P{ 1 H} N M R spectra of 2 - 1 3 Ci which are shown in Figure 5.2. The 3 1 P{ 1 H} N M R spectrum of 2 - 1 3 Ci may be described as an A B X pattern with two non-equivalent phosphorus nuclei (/p.p1 = 16.8 Hz) coupled to a single 1 3 C nucleus (/fj-P = 86.1 H, /c-P' - 2.8 Hz). The two values of 2/c-p are again consistent with a trans and cis coupling respectively across a square planar centre. The corresponding 1 3C{ lH] N M R spectrum exhibits an apparent doublet of doublets due to the strong trans coupling and the cis coupling of much smaller magnitude (this splitting is not well resolved in the 3 1 P{ 1 H} N M R spectrum). The non-ambiguous coupling constants and chemical shifts so obtained for the [Pd(dippe)Me]+ system prove useful in monitoring the reactions of this complex with small molecules. 120 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 300 250 200 150 (Hz) Figure 5.2. (a) 50.32 MHz ^ C ^ H } N M R spectrum of [Pd(dippe)(13CH3)(OEt2)](BAr f) (2-1 3 C i ) ; and (b), corresponding 81.015 MHz 3 1 P{ *H} N M R spectrum of 2 - 1 3 C i . 121 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 5.5 Reactivity of [Pd(dippe)Me(OEt2)](BArf) (2) 5.5.1 Reaction with dihydrogen Given that the methyl group in [Pd(dippe)Me]+ cannot offer the Pd nucleus any secondary ligand interaction as can the benzyl portion of 1, the cationic portion of 2 should be highly electrophilic and reactive, and so it proves to be. Both Pd(dippe)Me2, the parent complex of 2, and the benzyl cation 1 are completely inert to dihydrogen. In contrast, a THF solution of 2, placed under 1-4 atm of H2, rapidly develops a red colour, and 3 1 P{ 1 H} NMR spectroscopy reveals a singlet at 92.0 ppm, which indicates equivalence of the phosphorus donors. The corresponding *H N M R spectrum reveals a binomial quintet in the hydride region at -5.75 ppm (7p-H = 55.0 Hz). Solution spectroscopy therefore indicates that a binuclear Pd species, {[(dippe)Pd]2(p.-H)2}(BArf)2 (4), results from the activation of dihydrogen by [Pd(dippe)Me(OEt2)]BArf (2). As shown by the corresponding reaction between H2 and [Pd(dippe)13CH3]+ (2 - 1 3 Ci ) and the observation of 1 3 CH4 in solution, the methyl residue of 2 is lost as methane as shown in eq 5-5. [5-5] v. As we shall see, binuclearity, as is evident for 4, seems to be a fairly common feature of the reaction products of [Pd(dippp)Me(OEt2)]BArf with other substrates. 122 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 5.5.2 Lewis-acidity The electrophilicity of the palladium nucleus in 2 can also be demonstrated by the reaction of 2 with Pd(dippe)(CH2Ph)2. A stoichiometric solution of 2 and Pd(dippe)(CH2Ph)2 in Et20/d6-benzene leads to the formation of two products, of different solubility, both of which give rise to A X patterns in the 3 1 P{ 1 H} N M R spectrum. The identity of the ether-soluble product is the T|3-benzyl cation 1, whereas the other product, which is soluble in benzene and displays both methyl and benzylic resonances in its 1 H N M R spectrum, was identified as Pd(dippe)(Me)CH2Ph (5). 5 was later unequivocally characterized by independent synthesis from Pd(dippe)MeCl and benzyl potassium. Evidently, the acidic palladium centre of 2 abstracts a benzyl group from Pd(dippe)(CH2Ph)2 to give quantitative yields of the neutral, mixed hydrocarbyl complex, 5, and the stabilized T|3-benzyl cation 1 (eq 5-6). [5-6] Et^O/oVbenzene Room temperature 123 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines A potentially more interesting result occurs when an ethereal solution of 2 is mixed with one equiv of its labelled parent molecule, Pd(dippe)(1 3CH3)2. Hydrocarbyl transfer is again observed with the disappearance of the second order [AX]2 multiplet due to Pd(dippe)(13CH3)2 in the 3 1 P{ 1 H} or 1 3 C{ 1 H} N M R spectra and the observation of simpler A B X absorptions due to isotopomeric [Pd(dippe)Me] +/Pd(dippe)Me*] + and Pd(dippe)Me2/Pd(dippe)MeMe* (eq 5-7). [5-7] It is clear that there is no thermodynamic driving force in this last reaction in that a more stable species such as the T) 3-benzyl cation is not generated. The Lewis-acidity of the palladium nucleus of 2 renders the complex exceptionally kinetically labile. Hydrocarbyl transfer was also observed in the reaction between [Cp*2ZrCH 3 ] + and Cp*2Zr( 1 3CH3)2, where the Zr 124 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines nucleus was similarly Lewis-acidic, and the anion [B(QjF5)3Me]% similarly poorly coordinating.1 3 In both reactions an intermediate cannot be observed in the 3 1 P { 1 H ) N M R spectrum, and yet a hydrocarbyl residue has been demonstrably transferred from one Pd(U) nucleus to another. In order to account for this process a binuclear species of Pd, as an intermediate or transition-state, must be invoked. One possibility is shown in the following diagram, in which the hydrocarbyl residues bridge both metal centres. Thus in both of the reactions represented in eq 5-6 and eq 5-7, both Pd(dippe)(CH2Ph)2 and Pd(dippe)*Me2 have acted as metal bases, and have donated electron density to Lewis-acidic [Pd(dippe)Me(OEt2)]+. 5 . 5 . 3 Reaction with ethylene Given the similarity of the methyl cation 2 with the single-site olefin polymerization catalysts, how 2 would interact with olefins was an obvious matter to investigate. As might be expected for the late-metal palladium nucleus,2 0"2 2 (3-hydride elimination plays a dominant role and prevents 2 from being an efficient olefin polymerization catalyst. 3 1 P { 1 H } N M R spectroscopy performed on a sample of 2 1 3 C i in dfe-THF solution under 1-2 atm of ethylene reveals both the A B X pattern of the starting material and a new A X signal centred at 73.5 ppm (/p.P' = 21.3 Hz) which grows in over 24 hours. On standing for a longer period, the 3 1 P{ *H} N M R spectrum indicates a mixture of unidentified products, with 1 3 C-label led propylene 125 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines identified in the ^ C O H ) and *H N M R spectra along with a mixture of unidentified oligomers. It would appear that while the ethylene only slowly displaces the THF solvent molecule from the Pd nucleus, once the ethylene is bound to the metal centre, migration of the methyl residue is facile and elaboration of a propyl group occurs rapidly as shown in Scheme 5.1. Scheme 5.1 Had the intermediate complex been observable, viz. the ethylene adduct of 2- 1 3Ci, the 3 1 P{ ^ H] N M R spectrum would have revealed an A B X pattern rather than the observed doublet of doublets, as presumably, after insertion, the 1 3 C nucleus is too far removed from the phosphorus donors to effect scalar coupling of a Vp-c type, fi-hydride elimination then occurs before further ethylene coordination and insertion occurs, and propylene is ejected. The 126 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines palladium hydride species that results is itself capable of further olefin oligomerization, thus a mixture of oligomers occur in the reaction residue. Other workers have managed to forfend the effects of p-hydride elimination in similar cationic nickel and palladium methyl systems by use of the exceptionally bulky diimine ligand, ArN=C(R)C(R)=NAr (R = H or Me; Ar = 2,6-C6H3(Pri2)2 or 2,6-C6H 3 Me 2 ) . 1 4 5.5.4 Reaction with carbon monoxide We have seen that [Pd(dippe)Me(OEt2)](BArf) (2) is unable to polymerize ethylene in T H F because of a competing P-hydride elimination process. The prospects of copolymerization of ethylene and carbon monoxide mediated by 2 were considerably brighter as it has been established that CO/ethylene coupling is governed by mermodynamic and not kinetic factors.1 5 The obvious preliminary was to examine the behaviour of carbon monoxide towards 2 and then proceed with some copolymerization experiments of CO with ethylene and other a-olefins. In the event the simple reaction of 2 with CO follows an unconventional path. Many model studies have been performed upon the carbonylation of the metal-alkyl bond of palladium phosphine complexes.2 3"2 7 On this basis, we might reasonably expect that CO would first coordinate at the available coordination site of 2, displacing the solvent molecule, and then migratory insertion would occur to finally yield an Pd-acyl species, as is shown in Scheme 5.2. 127 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Scheme 5.2 These acyl species are regarded as important intermediates in the copolymerization of CO and ethylene. 1 5 , 2 8 This process is simply an example of migratory insertion, which has long been recognized as a fundamental organometallic reaction pathway, and would likely proceed by migration of the methyl group to CO rather than insertion of the CO into the palladium-methyl bond as it is generally accepted that formation of metal-acyls proceeds by way of migration 2 7 However, when a solution of 2 in rfs-THF is placed under 1-2 atm of CO, instead of observing a new doublets of doublets in the 3 1 P{ *H} N M R spectrum, as would be generated by the A X spin system of a [Pd(dippe)(COMe)S]+ species (S = solvent molecule or CO), a singlet at 58.7 ppm was observed, indicating equivalence of the phosphorus donors of the dippe ligand, accompanied by the appearance of a deep orange colour in the solution. That the species that 128 References begin on page 166 Chapter 5: Cationic Palladium Alley I Complexes Stabilized by Electron-rich Phosphines gave rise to the singlet resonance occurred as the major product was quite perplexing. Even more perplexing was the observation that this product remained stable under added pressure of CO (i.e. approx. 5 atm), and signals attributable to acyl groups could not be identified in the lH N M R spectrum. Under an atmosphere of 1 3 C O the 3 1 P{ 1 H} N M R signal at 58.7 ppm splits into a doublet of doublets (Jp-c = 28.1 Hz; /p - c = 25.5 Hz) and quintet absorptions with the corresponding coupling constants were observed at low field in the ^ C p H } N M R spectrum (236.4 and 228.1 ppm respectively). When the corresponding experiment was performed upon 2 - 1 3 Ci under CO, the 3 1 P{ 1 H} N M R spectrum revealed a doublet at 58.7 ppm (/p_c = 14.4 Hz), and in the ^ C ^ H } N M R spectrum a binomial quintet was observed at 47.3 ppm (7c-P = 14.4 Hz). These results indicate the formation of some binuclear palladium species that is bridged by two chemically different carbonyl groups and in which the four phosphorus donors are equivalent. However, such an interpretation was not supported by the eventual X-ray crystal structure of crystals grown from Et20/toluene. 129 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Figure 5 .3 . Chem 3D® view of the cationic portion of {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6). 5.6 Solid-state and solution structure of {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6) For the moment we will ignore discussion of the mechanism of formation of 6, and also accept, temporarily at least, that, barring a labile CO molecule, the solid-state structure does represent what is observed in solution. Instead we will concentrate on the highly unusual structure of the crystals obtained from the CO reaction which is shown in Figure 5.3. As could be inferred from the solution spectra, the structure is indeed binuclear as was that of [(dippp)Pd]2(u,-H)2 (Chapter 2). But unlike the palladium hydride each palladium nucleus of 6 resides in a distinct coordination environment. Moreover, the palladium core of 6 is cationic, and the structure depicted is associated with a BArf anion that is clearly well isolated from both palladium nuclei though closer in space to Pd(l). As regards the cationic portion, Pd(2) more 130 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines or less directly interacts with the carbonyl group with a P d - C separation of 1.87(1) A , meanwhile its interaction with C(30) of the methyl group is more long range; here the separation is 2.94(1) A . On the other hand, Pd(l) is separated from C(29) by 2.32(1) A , yet its distance of 2.17(1) A from C(30) clearly indicates that the methyl group is covalently bound to this centre.2 9 Given this non-symmetric bridging of the carbonyl group across the palladium centres, and the disposition of the methyl group, it is reasonable to formulate 6 as a mixed valence species of Pd, that is as an adduct of a basic Pd(0) complex, Pd(dippe)CO, and a Lewis-acidic Pd(II) species, [Pd(dippp)Me]+. Indeed, 6 could be reformulated as a conventional square planar palladium coordination complex, with Pd(l) as the central metal atom and the Pd(dippe)CO moiety as the fourth ligand, if the relevant bond angles are considered. The angles P(2)-Pd(l)-C(30) and P(l) -Pd(l) -Pd(2) are tolerably close to 180° (177.7(2)° and 170.32(8)° respectively), and the angles Pd(2)-Pd(l)-C(30) and P ( l ) -Pd ( l ) -C(30) (78.5(8)° and 91.9(8)°) are likewise close to 90°. In this respect, of possible significance is the separation between the palladium nuclei; its exceptionally short value of 2.6886(8) A (cf. approx. 2.81 A for the [(dippp)Pd]2(p-H)2 series) may represent an electrostatic interaction between the electron-rich Pd(0) centre and the cationic Pd(II), but we may also speak of a coordinate bond from Pd(2) to cationic Pd(l). This bond is bridged by the CO group, which may be designated as semi-bridging 3 0 on the basis of its unequal span across the two non-equivalent palladium centres and the bent Pd(2)-C(29)-0 angle of 160.1(2)°. The P(2)-Pd(l) -C(30) angle of 177.7(2)° also seems to preclude an agostic interaction between Pd(2) and the methyl group; if such an agostic interaction were operating a more acute angle might be expected. Also of interest is the unusual disposition of the coordination planes: the plane defined by P(l) -Pd(l) -P(2) is tilted to that defined by P(3)-Pd(2)-P(4) by an angle of 92.10°, i.e. the coordination planes of the palladium nuclei are normal to each other. Other selected intramolecular distances and angles appear in Tables 5.1 and 5.2. 131 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Table 5.1. Selected intramolecular distances (A) observed in {[Pd(dippe)Me][Pd(dippe)CO]}(BArf) (6). Pd(l)-Pd(2) 2.6886(8) Pd(l)-C(30) 2.172(7) Pd(l ) -P( l ) 2.302(2) Pd(2)-C(30) 2.942(10) Pd(l)-P(2) 2.332(2) Pd(2)-C(29) 1.873(10) Pd(2)-P(3) 2.325(2) Pd(l)-C(29) 2.32(1) 2.33(2) C(29)-0 1.130(9) Table 5 .2 . Selected intramolecular angles (deg) observed in {[Pd(dippe)Me][Pd(dippe)CO]}(BArf)(6). " P(3)-Pd(2)-P(4) 87.66(8) Pd(l)-C(30)-Pd(2) 78.5(8) P(4)-Pd(2)-C(29) 167.2(3) P(3)-Pd(2)-Pd(l) 159.62(6) P(l) -Pd(l) -P(2) 86.74(7) P(l)-Pd(l)-Pd(2) 170.32(8) P(2)-Pd(l)-C(30) 177.7(2) P(2)-Pd(l)-Pd(2) 102.95(2) P(l) -Pd(l) -C(30) 91.9(8) P(3)-Pd(2)-Pd(3) 159.62(3) Pd(l)-C(29)-Pd(2) 79.0(4) P(4)-Pd(2)-Pd(3) 109.67(5) We have yet to reconcile the solid-state structure of 6 with what we observe in solution, and in fact under ^ C O the species in solution is manifestly not 6 but an allied binuclear palladium complex to whose core two distinct carbonyl units are coordinated (viz. the 3 1 P{ 1H} N M R spectrum exhibits a doublet of doublets with two values of/p.c observed). Nevertheless, an analytical sample of 6 dissolved in d%-THF does give a singlet at 58.6 ppm in its 3 1 P{ 1 H} N M R spectrum, and if 1 3 C O is introduced the doublet of doublet pattern is observed centred at 58.8 ppm. It appears that the 3 1 P{ ^ H} N M R chemical shifts of the two species are more or less 132 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines coincident, and that the species giving rise to the A B X pattern is the carbonyl adduct of 6, [Pd(dippe)(13CO)2]{[Pd(dippe)Me](BArf)J (6a).31 Therefore 6a is simply more soluble than 6, and the extra CO group is lost upon crystallization. It is worthwhile adding in passing that zerovalent palladium species such as Pd(dippp)CO (vide infra) and Pd(dippp)(CO)2 have been characterized in solution, and the CO ligand is quite labi le. 3 2 Admittedly, the low-field carbonyl absorptions in the ^ C p H } N M R spectrum observed for 6a, at 8 = 236.5 and 228.2 ppm, could be consistent with acyl carbons, but since both carbonyls bridge two palladium centres (as witnessed by their appearance as binomial quintets), the low-field chemical shifts are quite appropriate for \i2 l i gands . 3 3 , 3 4 As regards the apparent equivalence of the phosphorus nuclei in the 3 1 P{ 1 H} N M R spectrum, clearly an exchange process must be operating which serves to equate the phosphorus nuclei. Such a process must not only interchange the phosphorus nuclei on each one of the dippe ligands, it must also equate the donor nuclei on alternate ligands. Lability of the CO group would partially satisfy the first requirement, however since 6 is a 30-electron binuclear species, it is likely that the CO remains tightly bound to the metal core. Alternatively, the equivalence of the phosphorus donors may be explained by an intermediate with a both a bridging methyl and a bridging carbonyl group, and with the coordination planes coplanar. Transfer of the methyl or carbonyl group may occur in either a stepwise or concerted fashion, and rapid equilibration results in the observation of a singlet in the 3 1 P{ 1 H) N M R spectrum. These processes are depicted in Scheme 5.3. 133 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines At low temperature (< -70 °C), the singlet at 58.6 ppm splits into four broad signals, i.e. four phosphorus environments, and is therefore consistent with the solid state structure. Such fluxionality was also observed in the related cation, {[(dippp)Pd]2(p.-H)(p-CO)}+,33'34 but in this case the limiting spectrum not reached. 5.7 Formation of {[Pd(dippe)CO][Pd(dippe)Me]}(BAr f) (6) While the solid-state and solution structure of 6 poses some problems in interpretation, its mechanism of formation from 2 and CO is even more of a challenge. As shown in eq 5-8, the reactants were [Pd(dippe)Me(OEt2)]+ (2), formally a Pd(JJ) species, and carbon monoxide. 134 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines The product isolated was a Pd(II)/Pd(0) dimer. Somewhere along the reaction pathway a divalent palladium nucleus underwent formal two-electron reduction to Pd(0). It was also observed that the formation of 6 was specific to THF. When the reaction was performed in Et20/d6-benzene with 2 - 1 3 Ci , the 3 1 P{ 1 H} N M R spectrum revealed two sets of doublet of doublets at 73.7 and 70.7 ppm (/p.p1 = 40.6 Hz), and in the ^ C ^ H ] N M R spectrum a doublet of doublets at 46.5 ppm was observed (JQ-P = 38.7 Hz, JQ-P = 17.3 Hz). This species we formulate as the square planar, acyl-carbonyl complex, [Pd(dippe)(C0 1 3CH3)CO] + (7) with the two phosphorus-carbon coupling constants of the 3 /c -P type corresponding to a trans and cis Coupling respectively; and when the reaction was performed in the poorly coordinating solvent o-dichlorobenzene with 2 - 1 3 C i under C O , what was apparently [Pd(dippe)(1 3CH3)CO]+ (8) was observed (eq 5-9), as indicated by a new A B X pattern in the 3 1 P{ 1H} N M R at 84.2 and 83.8 ppm (/P.P' = 21.2 Hz), and the corresponding spectrum in the 1 3C{ iH} N M R centred at -4.9 ppm (7c-P (trans) = 105 Hz, 7c-P' (cis) = -8.5 Hz), whose upfield value indicates a Pd-Me group rather than a Pd-acyl. 3 5 [5-8] ^ P f H } N M R s 58.6 ppm IR v C o (KBr disc) 1830 cm ,-1 135 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines ^PfH} NMR: dd 73.3, dd 70.7 ppm 13C{1H} NMR: dd 46.2 ppm o-dichlorobenzene 8 3 1 P{1H} NMR: m 84.2, m 83.7 ppm 13C{1H} NMR: m-4.90 ppm [5-9] For these latter experiments we may rationalize formation of 7 and 8 based upon the coordinating ability of Et20 versus o-dichlorobenzene. Previous studies appear to indicate that the more strongly coordinating solvent will favour formation of the acyl group in that it serves to stabilize the metal centre upon migration of the methyl group. 1 5 Thus in weakly coordinating o-dichlorobenzene, the barrier to migratory insertion of CO in 8 is apparently too great. However, up to this point, questions regarding the importance of THF in the formation of 6 in THF, remain unanswered. As we have seen, at higher pressures of CO (1-4 atm), signals due to a putative Pd-acyl species were conspicuously absent from a solution of 2 in ds-THF under carbon monoxide. However, when an N M R sample was prepared under 300 mm Hg pressure of CO, signals 136 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines attributable to both [Pd(dippe)(C0 1 3CH 3)CO] + (7) and [Pd(dippe)( 1 3CH 3)CO] + (8) could be observed in the 3 1 P{ N M R spectrum, which diminished in intensity relative to the peak due to 6 on standing. Also a peak at 81.0 ppm was observed which grows in with the peak at 58.7 ppm but diminishes in intensity on standing with several new absorptions above 100 ppm observed. The observation of 7 and 8 seems to indicate that these species are intermediates in the formation of 6/6a, and not the converse. When the volatiles were removed from an N M R sample of 6 under 1 3 C O , a peak at 204.0 ppm was observed in the ^ C ^ H ) N M R spectrum of the distillate, and a doublet peak was observed at 2.03 ppm ( / H -C = 6.0 Hz) in the lH N M R spectrum, i.e. chemical shifts consistent with acetone. 3 6 , 3 7 The identity of this species was confirmed by G C M S spectroscopy, which, along with peaks for Et20 and d$-THF, gave a molecular ion at 59 amu, i.e. corresponding to H 3 C ( 1 3 C O ) C H 3 . In the same experiment performed with 2 - 1 3 C i under 1 2 C O , H 3 1 3 C ( C O ) 1 3 C H 3 was identified by G C M S . The provenance of acetone is thus undoubtedly due to the methyl group of the starting material 2. In order to account for the coupling of two methyl units and the one carbonyl we must invoke hydrocarbyl transfer, presumably between 7 and 8 or 2, of a methyl group to a palladium acyl species, (or conversely, an acyl group to a palladium methyl species), of the kind represented in eq 5-7. Under CO, such a process would produce two new compounds: (i), a neutral Pd(II) complex, Pd(dippe)(COMe)Me, which reductively eliminates to give acetone and a zerovalent palladium species, presumably Pd(dippe)CO; and (ii), a Pd(iJ) species, [Pd(dippe)]2 + or one of its carbonyl adducts. We believe that this dicationic species gives rise to the peak at 81.9 ppm in the 3 1 P { ! H } N M R spectrum. The identity of this species is probably [Pd(dippe)(CO)2](BArf)2 (9) since under 1 3 C O , the peak at 81.0 ppm appears as a second order multiplet, i.e. as would be generated by the [AXfc spin system of the square planar [Pd(dippe)(1 3CO)2]2 + dication. In a separate experiment we tried to access these P d 2 + cornplexes by the addition of two equiv of HBArf to a solution of Pd(dippe)Me2 in THF. The evolution of a gas was indeed observed, but unfortunately the species so produced, presumably [Pd(dippe)(THF)2](BArf)2 (10a), rapidly polymerizes the THF solvent which makes 137 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines characterization impossible. In ^3-acetonitrile spectroscopic parameters for the dication [[Pd(dippe)(N=CD3)2](BArf)2 (10b) can be obtained and its 3 1 P{ ! H} N M R spectrum exhibits a singlet at 118.9 ppm. However, CO is too poorly coordinating to displace the strongly bound acetonitrile molecules from the palladium centre, and 9 was therefore inaccessible by this route. As a last resort, Pd(dippe)Me2 was again treated with two equiv of HBArf in THF, but under stringent temperature control (i.e. a temperature of -78 °C was not exceeded). After the reaction mixture was warmed to -20 °C and the volatiles were removed, a solution of the residue in ^ -methylene chloride under 1 2 C O exhibited a singlet at 83.8 ppm in the 3 1 P{ 1 H} N M R spectrum. When the solution was warmed to room temperature, the singlet at 83.8 ppm diminished in intensity and a new singlet was observed at 115.6 ppm. From the original mixture, crystals of 10a were indeed isolated, after recrystallization from methylene chloride at low temperature (< -20 °C). In ^-methylene chloride 10a briefly gives a singlet at 118.9 ppm, to be replaced a singlet due to the decomposition product at 115.6 ppm. On the basis of these results it therefore seems reasonable to conclude that the singlet at 81.9 ppm in the original reaction between 2 and CO was in fact [Pd(dippe)(CO)2]2 +. The slight discrepancy in chemical shift may be explained by the different N M R solvent used in each case. To recapitulate, 7 combines with a Pd -Me complex, whose methyl group is exchanged for the carbonyl of 7. Pd(dippe)(COMe)Me is thereby formed, which, in the presence of CO, reductively eliminates acetone to generate the zerovalent palladium complex, Pd(dippe)CO. Meanwhile, the other product arising from the reaction of 7 and 8, [Pd(dippe)(CO)2]2+, eventually decomposes by reacting with the solvent. These processes are depicted in Schemes 5.4 and 5.5. 138 References begin on page 166 ChapterS: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Scheme 5.4 139 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines The extrusion of acetone thus produces one half of the elements required for 6/6a, a zerovalent Pd(dippe)CO unit. It is proposed that this electron-rich moiety acts as a donor towards [Pd(dippe)Me]+ (2) that is present in the equilibrium, in a type of Lewis-acid/metal-base interaction. Therefore we can finally formulate the reaction of CO and 2 to give 6 as shown in eq 5-10. [5-10] CO 2© CO We can test this hypothesis by gauging the interaction between independently prepared Pd(dippp)CO, and authentic [Pd(dippe)Me(OEt2)]+ (2). The use of the three-carbon backbone ligand represents a negligible chemical change, but a dramatic change in magnetic resonance results, which gives excellent handles for 3 1 P{ ^ H] N M R spectroscopy. As shown in eq 5-11, if Pd(dippp)CO is generated in situ (i.e. by placing a THF solution of [Pd(dippp)]2 under CO), and one equiv of [Pd(dippe)Me(OEt2)]+ (2) is added, a pair of triplets are observed in the 3 1 P{ ! H ] N M R spectrum of the solution at 53.8 and 23.8 ppm (/p.p = 28 Hz). This result is 140 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines precisely what is expected if adduct formation between Pd(dippp)CO and 2 occurs, and the formation of {[Pd(dippp)CO)][Pd(dippe)Me]}(BArf) (11), a dippp analogue of 6, is indicated. It should be noted that only one of the two limiting, instantaneous structures of 11 is shown in eq 5-11; as before, an exchange process serves to equilibrate the phosphorus nuclei of each ligand (cf. Scheme 5.3). Why then, is the formation of 6 specific to THF? The answer perhaps lies in the fact that THF is the most strongly coordinating medium of all the solvents we have used for the carbonylation reactions of 2. Despite the isolation of 6 from the reaction in THF, our later results indicate that migratory insertion of GO does occur, as it did in Et20. However, the more weakly coordinating Et20 molecule is unlikely to stabilize dicationic species such as 9 and 10, whereas in THF both 9 and 10 have a transient existence. Since the hypothetical Pd(dippe)(COMe)Me complex is formed by intermolecular methyl transfer along with 10, and 141 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines then it presumably reductively eliminates acetone to give the zerovalent palladium moiety, for convenience Pd(dippe)CO. Furthermore, using the stoichiometry of eq 5-10 as a guide, which requires one equiv of unreacted [Pd(dippe)Me] + to undergo adduct formation with Pd(dippe)CO, THF may act as a carrier for [Pd(dippe)Me]+, to provide the other half of 6. In other words, if THF can compete with the added CO for coordination to [Pd(dippe)Me]+, and both the migration reaction of the methyl group onto CO and the elimination of acetone are relatively fast processes, then the required [Pd(dippe)Me]+ unit is available for later interaction with Pd(dippe)CO. Such then, we submit, is the role played by THF in the formation of 6. In the absence of insertion, and the stabilization of the dication 9, formation of 6 becomes a thermodynamic sink. Further work to elucidate the mechanism, in particular an independent synthesis of Pd(dippe)(COMe)Me, is underway. 5.8 Copolymerization experiments While in THF solution carbonylation of 2 gives rise to the unusual binuclear system 6, in Et20 carbonylation generates the mononuclear Pd-acetyl species 7. Et20 was therefore the obvious solvent in which to conduct some copolymerization experiments of ethylene and CO. The addition of equal volumes of ethylene and CO to 1 mol% of 2 in Et20 produced a clear yellow solution from which a white, microcrystalline powder precipitated (total gas pressure, approx. 1 atm). The white powder was insoluble in all solvents tested, and its melting point of 159 °C indicates that this polymer is different from those reported in the literature.38 No further effort was made to characterize this material. 5.9 Palladium cations stabilized by P N type donors 5.9.1 Ligand and metal precursor syntheses For the bidentate phosphine systems so far presented, protonolysis of the dimethyl complex can have only one stereochemical outcome. However, if one of the phosphorus atoms is replaced by a different donor nucleus then the cation can exist in diastereomic forms as shown in eq 5-12. 742 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines [5-12] Isolation of such a complex might therefore provide information on the relative strength of the donor nuclei in this cationic environment; it might also provide a stereochemical probe for fundamental reactions occurring at the metal nucleus. This sort of chemistry was attempted upon a dimethyl palladium complex whose ancillary ligand incorporated both a phosphine and an imine moiety, that is l-(diisopropylphosphino)isobutylidene tert-butylamine ("dippim"), whose synthesis is outlined in Scheme 5.6. 143 References begin on page 166 ChapterS: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Scheme 5.6 ii. As shown, the condensation of isobutyraldehyde with tert-butylamine produces the imine, isobutylidene tert-butylamine, in high yield. 3 9 Upon treatment with lithium diisopropylamide, the imine anion is generated in situ, to which is then added Pr^PC l . Substitution of the phosphorus electrophile occurs regioselectively at carbon to give the ligand dippim in 75-80% yield based on Pr^PCl . The ligand may be bound to Pd in a manner precisely analogous to the synthesis of late-metal phosphine complexes: dropwise addition of dippim to a homogeneous solution of Pd(N=CC6H5)2d2 in acetone gives a bright yellow precipitate of air stable Pd(dippim)Cl2. Metathesis of Pd(dippim)Cl2 with two equiv of M e L i or M e M g X and recrystallization from toluene/pentane gives Pd(dippim)Me2 (12) in high yield. 144 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Dimethyl complexes of palladium that incorporate the diphosphine ligands dippe or dippp are robust; however, the dippim complex 12 was found to be somewhat thermally labile. A nitrogen donor should be intrinsically less potent than a phosphine towards a late transition-metal, and this is purely an electronic effect because, even substituted by a bulky B u l group, the nitrogen is less encumbered than the phosphorus of the diisopropylphosphino group. Solutions of Pd(dippim)Me2 in toluene or benzene at room temperature deposit palladium metal on standing, and *H N M R spectroscopy indicates detachment of the imine donor from the metal nucleus to give three coordinate Pd (viz. additional signals due to the imine arm of the ligand appear which are close to those found for the unbound ligand; meanwhile the phosphorus nucleus remains strongly ligated to the metal nucleus as indicated by a new singlet in the 3 1 P{ lH} N M R spectrum different from that of both the free and the chelating ligand). For this hypothetical three-coordinate Pd complex as shown, the formation of metal-metal bonds, and subsequent precipitation of Pd metal can undoubtedly occur. Me. Me« However, when a chilled solution of 12 in Et2P is treated with one equiv of HBArf the solution effervesces and its clear colour persists over days. Evidentiy, upon protonation and loss of methane, the imine moiety binds tightly to the metal centre. Crystals of [Pd(dippim)Me(OEt2)](BArf) (13) may be isolated from Et20 in approx. 75% yield. These results may be interpreted on the basis of the established Hard Soft Acid Base (HSAB) principle. 4 0 Before protonation, the Pd nucleus of 12 had a choice of two donors (N and P 145 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines based) and as a late metal it would inevitably bind to the phosphorus donor more strongly. In 13 the Pd nucleus is now cationic, and prefers coordination by a nitrogen donor because of its increased Lewis-acidity. The strong binding between the imine donor and the Pd centre in 13 as compared to 12 and their relative stabilities may therefore be rationalized. 5.9.2 Solid-state structure of [Pd(dippim)Me(OEt2)](BArf) (13) A single crystal X-Ray diffraction experiment was performed upon a suitable crystal of 13 to reveal the structure shown in Figure 5.4. The structure was marked by gross disorder of the per-fluoromethyl groups of the anion; also two independent cations appear in the unit cell which made data collection prolonged and the data set unwieldy.4 1 Nevertheless the structure was refined to respectable parameters and both independent cations of the unit cell, the [Pd(dippim)Me(OEt2)]+ fragments of interest, possess essentially the same coordination geometry. As seen in Figure 5.4, the dippim ligand participates in a five-membered chelate ring with the Pd nucleus, and the separation between the metal atom and the donor phosphorous and nitrogen atoms are respectively 2.18 A and 2.17 A. The N-Pd separation is to be compared with values of 2.276(2) A, for N -Pd (nitrogen trans to methyl) in Pd(PAN)CH3Cl (PAN = l-(dimethylamino)-8-(diphenylphosphino)naphthalene),4 2 a neutral complex of palladium and a P - N ligand, 2.200(2) A, reported for Pd(tmeda)Me2,4 2 2.153(5) (trans to carbon) and 2.032(5) (trans to oxygen), reported for [Pd(2,9-dimethyl-l,10-phenanthroline)(K2-C6H-6-{C(0)Me)-2,3,4-(OMe)3)] + 4 3 and 2.00(2)-2.04(3) A, reported for a series of dicationic complexes of palladium and 1,10-phenanthroline.44 The difference in bond lengths may be rationalized by the comparative Lewis-acidities of the metal as part of the neutral, cationic, and dicationic complex, and also by the relative trans influence offered by a carbon or oxygen donor. The cationic complex 13 gives an intermediate P d - N bond length as does [Pd(2,9-cnmethyl-l,10-phenantJTroUne)(K2-(^-6-{C(O)Me}-2,3,4-(OMe)3)]+; the neutral complex, Pd(PAN)CH3Cl, gives a much longer P d - N bond length (cf. the corresponding distance for 12 below, where the nitrogen donor is also trans to a methyl); whereas the dicationic complexes give very short P d - N bond lengths. This effect is also manifested by the Pd -P bond distance; a 146 References begin on page 166 Chapter 5.Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines value of 2.18 A is one of the shortest known, 1 5 and reflects the increased bond strength between the phosphine donor and the cationic palladium nucleus. The strong a-donation provided by the phosphine is also probably responsible for the particular geometry of 13, with Et20 occupying the coordination site trans to the phosphine. The more basic phosphine donor can offer more electron density to the cationic metal centre, and is therefore the better choice, in terms of stability, as the donor trans to a weakly donating ether molecule. The trans directing influence of a phosphine compared to a nitrogen donor is a rough indication of this ; (vide infra).40 The oxygen donor of the diethyl ether molecule is separated from the palladium nucleus by a distance of 2.23 A. Other selected bond lengths and bond angles for both 13 and 12 appear in Table 5.3. 147 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines C15 C17 C16 C19 Figure 5.4. ORTEP view of the cationic portion of [Pd(dippim)Me(OEt2)](BArf) (13). 148 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 5.9.3 Solution structure of [Pd(dippim)Me(0Et2)](BArf) (13) As we have seen, for either independent cation in the unit cell of 13, the remaining methyl group is tranj-coordinated to the nitrogen donor. An ether solvent molecule is trans-coordinated to the phosphorus donor. The obvious question is whether this geometry persists in the solution-state. The *H and i H f 3 1 ? } N M R spectra of a d%-THF solution of 13 are not very informative as the methyl resonance, tentatively assigned at 0.42 ppm (vide infra), appears as a singlet and is not strongly coupled to the 3 1 P nucleus as are the methyl protons of the backbone and the methine and methyl protons of the isopropyl group directly attached to phosphorus. Hence, when the 3 1 P nucleus is irradiated in a ^ t 3 1 ? } N M R experiment, the appearance of the methyl signal is unchanged relative to the 1 H N M R spectrum and assignment is problematic (cf. the multiplet resonances of the strongly phosphorus-coupled isopropyl methyls and methines collapse to doublets and septets respectively when the 3 1 P nucleus is irradiated and the *H N M R spectrum is observed). The alternative was to examine the 3 1 P { l H } and ^ C p H } N M R spectra of [Pd(dippim)(13CH3)(OEt2)](BArf) (13 - l 3 Ci) and compare it to the corresponding spectra of Pd(dippim)(1 3CH 3)2 (12-13C2), prepared as for Pd(dippe)(13CH3)2 (2-13C2) by the action of one equiv of Mg( 1 3CH 3)2»(l,4-dioxane) upon Pd(dippim)Cl2. For 12- 1 3C2 the spin system may be designated as A B X , and the 3 1 P{ 1 H} N M R spectrum in ^-benzene exhibits a doublet of doublets centred at 62.9 ppm with one strong coupling of 118.9 Hz, indicative of coupling to the trans 1 3 C nucleus, and one small coupling of 6.9 Hz to the cis 1 3 C nucleus. This is precisely analogous to the A B X spin system of [Pd(dippe)(13CH3)]+ (2 - 1 3 Ci) , except that for 12-13C2 there are two 1 3 C nuclei and only one 3 1 P nucleus. Similarly the ^ C p H ) N M R spectrum of 12-1 3C2 exhibits two doublets at 8.9 ppm (trans to 3 1 P ) and -7.4 ppm (cis to 3 1 P) with the appropriate coupling values (no coupling was observed between the non-equivalent 1 3 C nuclei which share a cw-relationship). A d%-THF or d6-benzene/Et20 solution of 1 3 - 1 3 C i exhibits a broad singlet at 87.0 ppm in its 3 1 P{ 1 H} N M R spectrum and a singlet at -3.9 ppm in the ^ C p H ) N M R spectrum. In the absence of any doublets with the large coupling values characteristic of frarw-coordination, it 149 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines may be safely inferred that the structure in solution of 13- 1 3Ci and, by extension, 13 proper, parallels that observed in the solid state and that the remaining methyl group is tr.ans-coordinated to the imine donor. If the *H and ^ f 3 1 ? } N M R spectrum of 13 is acquired in a more poorly coordinating solvent than d%-THF, such as d4-c>-dichlorobenzene or ^-methylene chloride, both bound and free Et20 can be observed. In ds-THF the vacant coordination site is undoubtedly filled by C4D8O which of course cannot be observed in the *H N M R spectrum. Conversely, in a more poorly coordinating solvent, the Pd nucleus binds to the residual solvent of crystallization (Et20). Signals due to bound Et20 occur at 3.67 (OC//2CH3) and 1.11 ppm (OCH2C//3) as a quartet and a triplet respectively (cf. 3.34 and 1.07 ppm for free Et20). nOe spectroscopy in this solvent also allowed for definitive assignment of the methyl group bound to the Pd nucleus: when the isopropyl methine or methyl protons were irradiated an enhancement was observed for the peak at 0.21 ppm (this resonance appears at 0.42 ppm in ds-THF), and vice versa. These protons on the phosphine arm of the ligand bear a cis relationship to the Pd-Me group and should thus fulfill the nOe criterion, as indeed they do. The methylene protons of the bound Et20 molecule also bear a cis relationship to the Me group and enhancement is once again observed when this position is irradiated. On the other hand no signal enhancement was observed when the B u l group on the nitrogen was irradiated. A l l these results are entirely consistent with the earlier conclusion: that the same coordination geometry with methyl trans to the imine and 'cis to the phosphine applies to both the solid and solution-state. 150 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Table 5.3. Selected intramolecular bond lengths (A) and bond angles (deg) observed in [Pd(dippim)Me(OEt2)](BArf) (13) and Pd(dippim)Me2 (12). [Pd(dippim)Me(OEt2)](BAr f) (13) Pd(dippim)Me2 (12) Pd-P 2.18 Pd -P 2.2955(8) Pd-N 2.19 Pd-N 2.266(2) Pd-O 2.22 Pd-C(15) 2.098(3) Pd-C( l ) 2.03 Pd-C(16) 2.035(3) N-C(3) 1.23 N - C ( l l ) 1.268(4) C ( 2 X X 3 ) 1.51 C(l)-C(2) 1.503(4) P-Pd-N 82.4 P-Pd-N 71.51(7) C ( l ) -Pd -Q 85.9 C(15)-Pd-C(16) 84.0(1) P -Pd -O 170.9 P-Pd-C(15) 173.0(1) P-Pd-C ( l ) 92.8 P-Pd-C(16) 95.1(1) N -Pd -O 99.7 N-Pd-C(15) 101.8(1) N -Pd -C ( l ) 172.3 N-Pd-C(16) 173.6(1) P-C(2)-C(3) 103.9 P-C(l ) -C(2) 105.1(2) N-C(3)-C(2) 126.3 N -C(2)-C(l) 124.9(1) 5.9.4 Solid-state structure of Pd(dippim)Me2 (12) and comparison to that of [Pd(dippim)Me](BArf) (13) In order to compare the structure of the cation with that of the parent molecule the crystal structure of 12 was commissioned. (The structures were in fact solved in this order as the-structure of 12 only became relevant when me structure of 13 was safely in hand.) The X -ray crystal structure of Pd(dippim)Me2 is shown in Figure 5.5. Relevant bond lengths and angles appear in Table 5.3. In comparing the structure of 12 to its daughter molecule 13 it is 757 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines readily apparent that the gross geometry of 13 is not radically different to that of 12. In both instances the P - N ligand participates in a five-membered chelate ring with the palladium nucleus and the P - P d - N bond angles are quite similar. Et20 substitutes for the methyl donor of 12. What is noteworthy, however, is the shortness of the P d - L bond distances in 13 in comparison to those of 12 (L = donor nucleus). In 12 the P d - P and P d - N distances are 2.2955(8) and 2.266(2) A respectively; whereas in cationic 13 the corresponding distances are 2.18 and 2.17 A., Again, this difference is readily rationalized on the basis of the enhanced Lewis-acidity of the Pd nucleus in 13. The cationic Pd centre now binds tenaciously to its donor nuclei and smaller bond distances result. The P d - C bond distances of 12 are also worthy of comment in that they reflect the respective trans influence exerted by the phosphine and nitrogen donor. Pd-C(16) has a value of 2.035(3) A, whereas the Pd-C(15), trans to the phosphine, is markedly longer at 2.098(3) A (vide supra). As we have argued, the sole Pd -C bond distance of 13, 2.04 A, is not significantly different from the corresponding distance of 12, and indicates that the imine donor, which is much closer to the palladium centre of 13, does not possess a significant trans influence. 152 References begin on page 166 Cliapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Figure 5.5. Chem 3D® view of Pd(dippim)Me 2 (12). 5 .10 Conclusions In this chapter we have described how cationic palladium systems may be developed from neutral palladium hydrocarbyl complexes and some elaborate Lewis-acids, with a view to enhancing the reactivity of the metal centre. [Pd(dippe)(r | 3-CH2C6H5)](BArf) (1) is unremarkable, but [Pd(dippe)Me(OEt2)](BArf) (2) was found to possess exceptionally high reactivity. Nowhere is this more apparent than in its reaction with dihydrogen as compared to the inertness of its parent complex, Pd(dippe)Me 2 , towards small molecules. Late metals can activate small molecules by oxidative addition pathways, and the reaction of 2 to give {[Pd(dippe)] 2(u-H)2}(BArf)2 (4) could be accounted for by an oxidative addition of 2 to give a Pd(IV) species, followed by a reductive elimination of methane to give 4 . Such a mechanism is in marked contrast to the ability of both neutral Pd(II) and Pd(0) to add dihydrogen. 4 5 The rapid reaction of 2 with F b could even be said to resemble hydrogenolysis (i.e. a-bond 153 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines metathesis), a process that is normally associated with the alkyls of the early transition-metals. This apparent reversal of the normal trends of reactivity of the periodic table has been noted in a similar system, viz. {Cp*IrPMe3Me(CH2Cl2)}BArf,46,47 a positively charged complex of Ir(ffl) which wil l induce selective C - H activation in alkanes. For our palladium system, the interaction of Lewis-acidic H + with a neutral Pd(U) complex has likewise greatly magnified the reactivity of the metal centre. Binuclearity seems to be a key feature of its reaction chemistry, as is demonstrated by the isolation of {[Pd(dippe)CO][Pd(dippe)Me]}BArf (6). This is undoubtedly an unusual complex, but, in view of the other Lewis-acid/metal-base adducts we have encountered in this work, its occurrence can be interpreted, if not rationalized. By breaking 6 up into its formal components, Pd(dippe)CO, the metal base, and [Pd(dippe)Me]+, the Lewis-acid, the importance of metal-base/Lewisracid interactions can be readily appreciated. Not only was Pd(dippe)CO a better donor towards [Pd(dippe)Me]+ than THF, it was also a better donor than a CO or an acetyl group, as witnessed by its isolation when the alternative donors were present. We are unable at this stage to determine why THF is crucial to the C O reaction, but we have speculated that in T H F the dicationic species, [Pd(dippe)(CO)2]2 +, is stabilized and that 6 becomes a thermodynamic sink. The formal reduction that one palladium nucleus undergoes during the reaction, with the extrusion of acetone,36 is a new phenomenon associated with migratory insertion reactions, and again emphasizes the importance of Lewis-acid/metal base chemistry. Previously, the existence of binuclear species in carbonylation reactions was unsuspected, although dimeric palladium complexes have been isolated from copolymerization experiments, and have been found to be less active catalysts than their mononuclear precursors.48 We have also endeavoured here to develop a complementary cationic system based on a P - N type ligand and were successful in isolating and structurally characterizing a palladium cation bound to the new dippim ligand. While its solid-state and solution structure have been fully elucidated, much work remains to be done in exploring its reaction chemistry. Preliminary results show that [Pd(dippim)Me(OEt2)](BArf) (13) is a weaker Lewis-acid than 154 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines [Pd(dippe)Me(0Et2)](BArf) (2). Carbonylation reactions upon 13 and assessment of its qualities as a copolymerization catalyst are underway. 5.11 Experimental 5.11.1 Procedures G C M S analysis was performed by the Mass Spectrometry Service of the University of British Columbia upon a KRATOS MS 80 RFA instrument. Other procedures are outlined in Section 2.12.1. 5.11.2 Materials HBArf was prepared by a literature method,1 as was BCCfcFs^.4 9 MeLi (Aldrich, halide content < 0.05 mol L"1) was used as supplied as a 1.4 mol L"1 solution in Et20. PhCH2MgCl (Aldrich) was used as supplied as a 1.0 mol L"1 solution in Et20. Mg( 1 3CH3)2 was prepared by a literature method and used as either a 0.05 mol L"1 solution in Et20, or recrystallized from benzene/pentane as Mg( 1 3CH3)2»(l,4-dioxane).?° The metal precursor Pd(COD)MeCl was prepared according to the literature.5 1 CH2CI2 was distilled from CaH2 under N2. Isopropylamine was dried by heating to reflux over CaH2 and distilled under N2; lithium diisopropyl amide was prepared from isopropylamine and B u n L i (Aldrich) of measured titre in THF and hexanes.52 Isobutylidene tert-butylamine was prepared by a published procedure.39 Acetonitrile was dried over activated 4 A molecular sieves, distilled from trap to trap, and stored over molecular sieves, ^-methylene chloride and ^-acetonitrile was distilled from CaH2 after a prolonged period at reflux under N2; d&-THF was distilled from sodium benzophenone ketyl under N2; d4-o-dichlorobenzene was distilled from CaH2 under N2. A l l deuterated solvents were subjected to four freeze-pump-thaw cycles. 5.11.3 Syntheses 5.11.3.1 Pd(dippe)Me2 To a slurry of Pd(dippe)Cl2 (1.54 g; 3.50 mmol) in THF at -78 °C was added a solution of MeL i (5.0 mL; 7.0 mmol) dropwise over 5 minutes. The cooling bath was then 755 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines removed, and as the mixture attained room temperature a reaction ensued evidenced by the disappearance of the Pd(dippe)Cl2 and the formation of a clear, dark-brown solution. The solution was stirred for 1 h after which time the solvent was removed in vacuo. The brown residue was extracted with toluene (25 mL), passed through a frit lined with Celite to remove L i C l , and concentrated to a small volume (approx. 3 mL). The brown solution was layered with pentane (10 mL) and cooled to -40 "C to give colourless crystals of Pd(dippe)Me2 after 12 h (1.25 g; 89% yield). *H N M R (rfg-toluene, 500.13 MHz): 8 1.88 (sept, 2H, C//MeMe', 7 H - M e = 7.1 Hz), 1.14 (m, 4H, PC//2C//2P), 1.06 (dd, 12H, CHAfeMe', , / H - M e = 7.1 Hz, / M e - P = 15.4 Hz), 0.84 (dd, 12H, CHMeMe', 7 H -Me = 7.1 Hz, / M e - P = 12.4 Hz), 0.72 (dd, 6H, PdMe 2 , / M e - P (trans) = 6.6 Hz, / M e - P (cis) = 1.0 Hz). 3lp{lH) N M R (dg-toluene): 8 61.0. Anal. Calcd for C i 6 H 3 g P 2 P d : C, 48.19; H, 9.60. Found: C, 47.69; H, 9.42. Following the preparation of Pd(dippe)Me2, Pd(dippe)(1 3CH3)2 was similarly prepared, with Pd(dippe)Cl2 (0.509 g; 1.16 x 10-3 mol) and Mg(l 3 CH 3 ) 2 (C4HgO) 2 (0.165 g; 1.16 x 10' 3 mol). Recrystallization from toluene (1 mL) layered with hexanes gave colourless crystals (0.371 g; 80% yield). *H N M R (d6-benzene, 500.13 MHz): 8 1.88 (sept, 2H, CT/MeMe', /n-Me = 7.1 Hz), 1.14 (m, 4H, PC//2C//2P), 1.09 (dd, 12H, CHAfeMe", , / H - M e = 7.1 Hz, / M e - P = 15.4 Hz), 0.83 (dd, 12H, CHMeAfe', 7 H - M e = 7.1 Hz, / M e . p = 12.4 Hz), 0.87 (dq, 6H, Pd*Me2, Ju-C - 124.0 Hz, /Me-P (trans) = 6.6 Hz, / M e - P (cis) = 2.0 Hz). 3 lP{ lH} N M R (d6-benzene, 81.015 MHz): 8 65.4 (m, / P . c (trans) = 108.5 Hz, / P<: (cis) = -10.5 Hz, / P . F = 8.1 Hz), ^ C ^ H } (d6-benzene, 50.32 MHz): 8 0.0 (m, / c .p (trans) = 108.5 Hz, / c .p (cis) = -10.5 Hz, JC-C = 0.5 Hz). Microanalysis was not obtained. 5.11.3.2 Pd(dippp)Me 2 Pd(dippp)Me2 was synthesized as for Pd(dippe)Me2, with Pd(dippp)I2 (0.565.g; 8.88 x 10*4 mol) and M e L i (1.3 mL; 1.80 x 10"3 mol). Successive recrystallizations from toluene (3 mL) layered with hexanes (6 mL) gave Pd(dippp)Me2 as colourless crystals (0.179 g; 49% yield). lH N M R (^-benzene, 299.99 MHz): 8 2.45 (m, 2H, PCH 2 Cr/ 2 CH 2 P) , 1.92 (m, 156 References begin on page 166 CnapterS: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 4H, C//MeMe', / H -Me = 7.2 Hz), 1.58 (m, 4H, PC// 2CH 2C// 2P), 1-14 and 0.91 (dd, 12H, CHMeMe', 7Me^H = 7.2 Hz, 7 M e - P = 15.0 Hz), 0.59 (dd, 6H, PdMe 2 ,7\ie-P (trans) = 5.0 Hz, 7\ie-P (cis) = 1.0 Hz). 3lp{ IR) N M R ( d6-benzene, 121.41 MHz): 6 16.4. Anal. Calcd for Ci 7 H4oP 2 Pd: C, 49.46; H, 9.77. Found: C, 49.15; H, 9.95. 5.11.3.3 l-(diisopropyl)isobutylidene tert-butylamine (dippim) A solution of isobutylidene tert-butylamine (6.29 g; 0.0494 mol) in THF (10 mL) was added dropwise to a solution of lithium dnsopropylamine in THF (40 mL) at -15 "C prepared from diisopropylamine (7.0 mL; 0.050 mol) and B u n L i solution (31.0 mL; 0.050 mol). After the addition the faint yellow solution was allowed to warm to RT, and then stirred for 2 h. The solution was again cooled to -20 °C and a white solid precipitated. To this stirred, chilled slurry, a solution of Pr^PCl (7.54 g; 0.0494 mol) in E t 2 0 (10 mL) was added over 10 min. The white solid went up into solution during this time. The solution was warmed to RT and stirred overnight. The solvent was removed in vacuo to yield a yellow oil which was distilled under vacuum to give l-(diisopropyl)isobutylidene tert-butylamine (dippim) as a water clear distillate (57-65 °C at 0.015 mm Hg, 9.00 g, 75% yield). *H N M R (^-benzene, 500.13 MHz): 8 7.61 (d, IH, C//NBut, / H .p = 2.0 Hz), 1.90 (d sept, 2H, C//MeMe', /H-Me = 7.0 Hz, 7 H - P = 3.8 Hz), 1.32 (d, 6H, C M e 2 C H , / P : M e = H - 0 Hz), 1.18 (dd, 12H, CHMeMe', 7 M e - H = 7.0 Hz, 7 M e - P = 13.5 Hz), 1.16 (s, 9H, NBM*) , 1.07 (dd, 12H, CHMeMe', / M e - H = 7.0 Hz, / M e - P = 11.0 Hz). 31p{lH} N M R (4-benzene, 202.47 MHz): 8 33.3. ^ C O H } (^-benzene, 50.32 MHz): 8 162.1 (d, CHNBu 1 , /c -P = 7.7 Hz), 56.5 (s, NC(CH 3)3), 39.7 (d, P C M e 2 , 7 C - P = 23.0), 30.0 (s, NC(CH 3)3), 25.1 (d, PCH(CH3)(CH*3), 7 C - P = 12.9 Hz), 22.9 (d, PCH(CH 3 )(CH' 3 ) , 7 G - P = 31.5 Hz), 22.3 (d, PC(CH 3 ) 2 CHN, 7 G - P = 21.5 Hz), 21.1 (d, PCH(CH 3)(CH' 3) , / C ' - P = 6.4 Hz). 5.11.3.4 Pd(dippim)CI2 A deep amber solution of Pd(C6H5CsN) 2 Cl 2 (2.43 g; 6.32 mmol) in acetone (50 mL) was treated with a solution of dippim (1.54 g; 6.32 mmol) in toluene (10 mL). With each drop of the ligand solution a yellow precipitate formed, and by the end of the the addition the 157 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines amber colour had discharged completely by the end of the addition. The yellow slurry was left to stir for 2 h, after which the all but a small volume of acetone (approx. 10 mL) was stripped off in vacuo. The reaction mixture was exposed to the air, slurried in Et20 (50 mL), and the yellow precipitate was recovered on a frit. The product was washed with a further two volumes of E t 2 0 (50 mL), and air-dried (2.27 g; 85% yield). *H N M R (d\-chloroform, 500.13 MHz): 8 7.25 (d, IH, O/NBu 1 , / H -P = 30 Hz), 2.52 (d sept, 2H, C/iMeMe", /H-Me = 7.0 Hz, / H .p = 2.0 Hz), 1.63 (d, 6H, C M e 2 C H , / P . M e = 11.0 Hz), 1.60 (s, 9H, NBu*), 1.57 (dd, 12H, CHMeMe', JMZ-YL = 7.0 Hz, / M E - P = 9.5 Hz), 1.29 (dd, 12H, CHMeMe',/ M E'-H = 7.0 Hz, / M E - P = 16.0 Hz). 3 1 P{ 1 H} N M R (^-chloroform, 202.47 MHz): 8 93.9. Anal. Calcd for C i 4 H 3 0 C l 2 N P P d : C, 39.97; H, 7.19; N, 3.33. Found: C, 39.98; H, 7.09; N, 3.21. 5.11.3.5 Pd(dippe)(CH2Ph)2 As for Pd(dippe)Me 2 with Pd(dippe)Cl 2 (1.09 g; 2.48 x 10"3 mol) and P h C H 2 M g C l (5 mL; 5.0 x 10"3 mol). Recrystallization from toluene (2 mL) layered with hexanes (10 mL) afforded Pd(dippe)(CH2Ph)2 as clear brown needles (1.00 g; 73% yield). *H N M R (^-benzene, 500.13 MHz): 8 7.43 (m, 4H, o-Ph), 7.21 (m, 4H, m-Ph), 6.96 (m, 2H, p-Ph), 3.11 (dd, 4H, C//2Ph, / H .p = 9.0 Hz, / H -p- = 7.5 Hz), 1.75 (sept, 4H, C//MeMe', /H-Me = 7.0 Hz), 1.00 (m, 4H, PC# 2C// 2P), 0.91 and 0.82 (dd, 24H, CHMeMe', / M E - P = 16.0 H z , / M E - H = 7.0 Hz). 3 1 P{ !H} N M R (d6-benzene, 202.47 MHz): 8 62.0. Anal. Calcd for C 2 8H4 6 P 2 Pd: C, 61.03; H, 8.01. Found: C, 60.75; H, 7.94. 5.11.3.6 rPd(dippe)CH2Ph]{B(C6F5)3CH2Ph} To a solution of Pd(dippe)(CH2Ph2) (0.100 g; 1.82 x 1CH mol) in E t 2 0 (2 mL) was added solid B((^5)3 (0.093 g; 1.82 x 10"4 mol) with stirring. The initial brown colour discharged to give a colourless solution. The solvent was removed in vacuo to give a slimy brown oil which resisted crystallization. The product was characterized in solution. !H N M R (d3-acetonitrile, 200.12 MHz): 8 7.75 (m, 2H, T^-CH^m-P/z), 7.70 (br s, 2H, m, o-PACH 2 B), 7.58 (m, IH, p-PhCH2n), 7.10 (m, 3H, r i 3 -CH 2 -m , p-Ph), 6.59 (m, 2H, rp-CH2-o-Ph), 3.09 (d, 158 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 2H, T|3-C//2Ph, / H - P = 9.4 .Hz), 2.54 (s, 2H, PhC7/2B), 2^40 and 2.10 (m, 4H, CtfMeMe'), 2.04 and 1.81 (m, 4H, PC# 2C7/ 2 'P), 1.17, 1.06, 0.95 and 0.92 (m, 24H, CHMeMe'). 31p{lH) N M R (d2-methylene chloride, 121.42 MHz): 6 89.3 (d, IP, / P . P - = 30.6 Hz), 76.9 (d, IP, / P .p = 30.6 Hz). 5.11.3.7 Pd(dippe)(CH2Ph)Cl To a solution of Pd(dippe)(CH2Ph)2 (0.117 g; 2.12 x 10' 4 mol) in THF (5 mL) was added solid CeHsN'HCl (0.025 g; 2.12 x lO"4 mol). The mixture was stirred overnight to yield a colourless solution. The volatiles were removed in vacuo and the grey-white residue was recrystallized from E t 2 0 (10 mL) to give colourless crystals of Pd(dippe)(CH 2Ph)Cl (0.068 g; 65% yield). lH N M R (d2-methylene chloride, 299.99 MHz): 5 7.42 (m, 2H, m-Ph), 7.04 (m, 2H, o-Ph), 6.85 (m, IH, p-Ph), 3.00 (dd, 2H, Ct f 2 Ph, / P . H (trans) = 10.8 Hz, 7 P . H (cis) = 3.2 Hz), 2.32 (sept, 4H, CT/MeMe', 7 H - M e = 6.6 Hz), 1.85 and 1.54 (dt, 4H, PC7/2C7/2'P, 7 H-P = 13.5 Hz, / H - i r = 8.0 Hz), 1.28, 1.18, 1.10, and 1.02 (dd, 24H, CHMeMe', /Me-P = 16.2 H z A i e - H = 7.2 Hz). 31p{lH} N M R (d2-methylene chloride, 121.42 MHz): 6 76.4 (d, IP, /p.p = 24.3 Hz), 72.9 (d, IP, /p.p = 24.3 Hz). Anal. Calcd for C 2 i H 3 9 C l P 2 P d : C, 50.92; H, 7.94. Found: C, 50.75; H, 7.94. 5.11.3.8 [Pd(dippe)(CH2Ph)I To a solution of Pd(dippe)(CH2Ph)2 (0.113 g; 2.05 x 10"4 mol) in THF (5 mL) was added solid C6HsN»HCl (0.047 g; 2.12 x 10"4 mol). The mixture was stirred overnight to yield a colourless solution. The solvent was removed in vacuo and the yellow residue was recrystallized from E t 2 0 (10 mL) to give yellow crystals of Pd(dippe)(CH2Ph)I (0.084 g; 70% yield). lH N M R (rf2-methylene chloride, 299.99 MHz): 87.43 (m, 2H, m-Ph), 7.06 (m, 2H, o-Ph), 6.87 (m, IH, p-Ph), 3.37 (dd, 2H, C//2Ph, / P . H (trans) = 10.3 Hz, / P . H (cis). = 3.9 Hz), 2.45 (sept, 4H, O/MeMe', / H - M e = 6.6 Hz), 1.85 and 1.60 (dt, 4H, PCH2CH2T, / H . p = 13.5 Hz, / H - i r = 8.0 Hz), 1.27, 1.18, 1.10, and 1.02 (dd, 24H, CHMeMe', /Me-P = 15.9 Hz^Me -H = 159 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 7.2 Hz). 3lp{lH) N M R (^-methylene chloride, 121.42 MHz): 8 75.3 (d , . lP ,/ P .F = 22,2 Hz), 71.2 (d, IP, >P.p = 22.2 Hz). 5.11.3.9 [Pd(dippe)(T|3.CH2Ph](BArf) (1) To a solution of Pd(dippe)(CH2Ph2)2 (0.072 g; 1.28 x 10' 4 mol) in THF (10 mL) at -10 *C was added a solution of HBArf (0.130 g; 1.28 x 10"* mol) in THF (1 mL). The original dark colour became clear. After the solvent was removed, the residue was dissolved in E t 2 0 (2.5 mL), and the solution placed in the freezer. Colourless crystals appeared after 24 h (0.127 g; 76%). *H N M R (J2-methylene chloride, 500.13 MHz): 8 7.75 (br s, 8H, o-Art), 7.68 (m, 2H, r\l-CH2m-Ph), 7.55 (br s, 4H, p-An), 7.30 (m, 2H, r\l-CH2m-Ph), 6.41 (m, 2H, n 3 -CH2o-Ph), 3.06 (d, 2H, ri3-C# 2Ph, 7 H - P = 7.8 Hz) 2.37 and 1.98 (dsept, 4H, O/MeMe', 7 H -P = 1.8 Hz, 7 H -Me = 7.2 Hz), 2.04 and 1.76 (dt, 4H, PC//2C//2'P, 7 H - P = 19.7 Hz, Ju-W = 7.0 Hz), 1.07, 1.10, 0.92 and 0.75 (dd, 24H, CHMeMe', JMe-P = 14.7 Hz,/ M e _H = 7.2 Hz). 31p{lH} N M R (d2-methylene chloride, 121.42 MHz): 8 88.7 (d, lP ./p .p = 32.1 Hz), 75.2 (d, IP, 7p.p = 32.1 Hz). Anal. Calcd for C 5 3 H 9 i B F 2 4 P 2 P d : C, 48.11; H, 3.89. Found: C, 48.16; H, 3.96. 5.11.3.10 [Pd(dippe)Me(OEt2)](BArf) (2) To a solution of Pd(dippe)Me2 (0.126 g; 3.16 x 1 0 4 mol) in E t 2 0 (5 mL) was added a solution of HBArf (0.320 g; 3.16 x 10"4 mol) in E t 2 0 (5 mL) at -10 °C. The solution vigorously effervesced and lightened in intensity. The volatiles were removed in vacuo and the residue was recrystallized from E t 2 0 (3 mL) and placed in the freezer. Colourless crystals appeared after 24 h (0.323 g; 82% yield). *H N M R (d%-THF, 500.13 MHz): 8 7.79 (br s, 8H, o-Ar{), 7.58 (br s, 4H,p-Ar{), 2.43 and 2.31 (sept, 4H, C//MeMe\ 7 H -Me = 7.2 Hz), 2.15 and 2.11 (dt, 4H, P C / / 2 0 / , 2 P , / H - p = 13.3 Hz, Ju-W = 6.1 Hz), 1.31, 1.26, 1.24, and 1.21 (dd, 24H, CHMeMe', 7 M e - P = 16.0 Hz, 7 M e - H = 7.2 Hz), 0.63 (dd, 3H, Pd-Me, Jue-P (trans) = 6.3 Hz, /Me-P (cis) = 0.9 Hz). 3lp{ IR} N M R (d%-THF, 202.42 MHz): 8 86.0 (d, IP, / P .p = 17.0 Hz), 69.0 (d, IP, 7 P . p = 17.0 Hz). Anal. Calcd for G r z H a v B F ^ P d : C, 45.27; H, 3.80. Found: C, 45.37; H, 4.00. 160 References begin on page 166 ChdpterS: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 2 - 1 3 C i was prepared likewise from Pd(dippe)( 1 3CH3) 2 and HBArf. 3 1 P{ !H} N M R (d6-benzene/Et2d, 81.015 MHz): 5 87.9 (dd, IP, 7 P . F =16.7 Hz, / P . C (cis) = 2.8 Hz), 67.3 (dd, IP, /p-.p = 16.7 H z , / P . c (trans) = 86.1 Hz). ^C{lH) N M R (d6-benzene/Et20, 50.32 MHz): 8 4.49 (dd, 7c-P (trans) = 86.1 Hz, / c .p (cis) = 2.8 Hz). 5.11.3.11 [Pd(dippp)Me (OEt 2 ) ] (BArf ) (3) As for 2 with Pd(dippp)Me2 (0.086 g; 2.08 x 10"4 mol) and HBArf (0.211 g; 2.08 x 10"4 mol). Recrystallization from THF (2 mL) gave clear crystals which darkened on standing (0.100 g; 38% yield). ] H N M R (d2-mettiylene chloride, 200.13 MHz): 8 7.63 (br s, 8H, m-Arf), 7.50 (br s, 4H, prArt), 2.39 (m, 2H, PCH 2 C// 2 CH' 2 P), 2.10 (m, 4H, O/MeMe'), 1.59 and 1.43 (m, 4H, PC//2CH2C//*2P), 1.10 (m, 24H, CHMeMe), 0.60 (dd, 3H, Pd-Me,/Me-P (trans) = 5.0 Hz, 7 M e - P (cis) = 1.0 Hz). 3 1 P{ lH) N M R (d2-methylene chloride, 81.015 MHz): 8 49.6 (d, IP, /p.p- = 39.2 Hz), 16.2 (d, IP, Jp p- = 39,2 Hz). Anal. Calcd for Q g F L ^ B F ^ P d : C, 45.72; H, 3.92. Found: C, 46.01; H, 3.59. 5.11.3.12 {[Pd(dippe)] 2 (p-H) 2 }(BAr f ) 2 (4) 2 (0.250 g; 1.89 x 10' 4 mol) was dissolved in THF (10 mL), and the solution was subjected to four freeze-pump-thaw cycles. The solution was cooled to -196 ° C and placed under 4 atm of dihydrogen., Upon warming to.^78 °C a the solution developed a deep red colour. The solution was warmed with stiring to 0 "C and the THF was removed in vacuo. The red residue remaining was recrystallized from THF (2 mL) and cooled to -40 °C. Deep red needles deposited after 12 h. (0.120 g; 51%). J H N M R (dg-THF, 500.13 MHz): 8 7.79 (br s, 16H, o-Arf), 7.56 (br s, 8H, p-Arr), 2.20 (sept, 8H, Ci/MeMe', /n-Me = 7.0 Hz), 2.08 (m, 8H, PCH 2 C/f 2 P) , 1.97 (m, 8H, OC// 2CH 2), 1.20 and 1.14 (dd, 48H, CHMeMe', /Me-P = 16.5 Hz, / M e - H = 7.0 Hz), -5.75 (quin, 2H, (p-J%, / P - H = 55.0 Hz). 3 1 P{ 1 H} N M R (dg-THF, 202.42 MHz): 8 92.0. Anal. Calcd for C9 2 H9 2 B 2 F48P 4 Pd2 : C, 44.8.1; H, 3.68. Found: C, 45.01; H, 3.71. ' ••-•v. 161 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 5.11.3.13 Methyl transfer between 2 and Pd(dippe)(13GH3)2 To a solution of 2 (0.035 g; 2.81 x 10"5 mol) in Et20 (0.20 mL) was added a solution of Pd(dippe)(1 3CH3)2 (0.12 g; 2.82 x 10"5 mol) in d6-benzene (0.20 mL). The products were characterized in solution. 3 1 P{ 1 H} N M R (d6-benzene/Et20, 81.015 MHz): 8 71.0 (brs). 1 3C{ 1H} NMR: (^6-benzene/Et20, 50.32 MHz): 8 -3.8 (br s). 5.11.3.14 Pd(dippe)(Me)CH2Ph (5) To a solution of Pd(COD)MeCl (0.344 g; 1.-26 x 10"3 mol) in THF (20 mL) was added a solution of dippe (0.330 g; 1.26 x 10 - 3 mol) in toluene (2 mL). A white precipitate was noted and some was evolved. The reaction vessel was then cooled to -78 °C in a dry-ice/acetone bath and a solution of KCH2Ph (0.166 g; 1.26 x 10" 3 mol) in THF (5 mL) was added by cannulation. The reaction mixture was allowed to warm to room temperature with stirring, and the solvent was removed in vacuo to give an orange residue. The residue was taken up in toluene (20 mL), and this solution was passed through Celite. The filtrate was concentrated to 2 mL and then layered with hexanes (5 mL). After 24 h at -40 °C orange crystals deposited (0.453 g; 76% yield). *H N M R (dg-benzene, 500.13 MHz): 8 7.51 (m, 2H, m-Ph), 7.26 (m, 2H, o-Ph), 6.96 (m, IH, p-Ph), 3.18 (dd, 2H, CH 2 Ph, / H - p (trans) = 10.0 Hz, / H -p (cis) = 8.0 Hz), 1.81 and 1.83 (sept, 4H, C//MeMe*. /H-Me = 7.5 Hz), 1.06 and 1.04 (m, 4H, PCH2CH2?), 1.00,0.96,0.82, and 0.75 (dd, 24H, CUMeMe', / H -p = 12.0 Hz, / M e -H= 7.5 Hz), 0.67 (dd, 3H, Pd-Me, / M e - P (trans) = 7.0 Hz, / M e - P ( c w ) = 6.0 Hz). 3 1 P{!H} N M R (d6-benzene, 202.42 MHz): 5 68.6 (d, IP, / P . p = 10.0 Hz), 60.4 (d, IP, / P . P - = 10.0 Hz). Anal. Calcd for C2 H42P2Pd: C, 55.64; H, 8.91. Found: C, 55.62; H, 8.88. 5.11.3.15 {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6) and {[Pd(dippe)(13CO)2][Pd(dippe)Me]}(BArf)(6a) In a thick-walled reactor vessel a solution of 2 (0.480 g; 3.62 x 10 - 4 mol) in THF (10 mL) was subjected to several freeze-pump-thaw cycles. The reactor was charged with 4 atm CO gas and stirred for 1 h. The solution developed a red/amber during this time. The 762 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines volatiles were removed in vacuo and the orange residue was recrystallized from Et20 (2 mL) under CO and a few drops of toluene to give deep amber plates of 6 (0.200 g; 33% yield). *H N M R (rf8-THF, 500.13 MHz): 8 7.79 (br s, 8H, o-Ar{), 7.51 (br s, 4H,p-Ar{), 2.38-2.25 (m, 8H, CHMeMe'), 1.97 (m, 8H, PCH2CH2P), 1.49, 1.39, 1.25, and 1.19 (m, 48H, CHMeMe'), 0.55 (m, 3H, Pd-Me). 31p{lH.} N M R (d^-THF, 202.42 MHz): 8 58.6; at -70 °C, 68.5, 64.5, 59.0, 58.0. Anal. Calcd for C69H87BF240P4Pd2 (i.e. {[Pd(dippe)CO][Pd(dippe)Me]}(BArf>C7H8):. C, 47.74; H, 5.05: Found: C, 48.05; H, 4.94. Similarly was prepared 6 - 1 3 C i with 2 - 1 3 C i (0.050 g; 4.01 x 10" 5 mol) in d 8 -THF (0.4 mL). The product was characterized in solution. 3 1 P { 1 H ) N M R (d$-THF, 81.015 MHz): 8 58.6 (d, 7 P . C = 14.6 Hz). 1 3C{ !H} N M R (rfg-THF, 50.32 MHz): 8 47.3 (quin, Pd-Me, JC-p = 14.6 Hz); 1 3 C NMR, (q of quin, Pd-Me, / H - c = 127.6 Hz, 7 C -P = 14.6 Hz). Similarly was prepared 6a with 2 (0.050 g; 4 .0Lx lO ' 5 mol) in d 8 - THF (0.4 mL). The product was characterized in solution under l 3 C O . *H N M R (d^-THF, 500.13 MHz): 8 7.79 (br s, 8H, o-Ar{), 7.57 (br s, 4H, p-Ar{), 2.62 (m, 3H, Pd-Me), 2.27 (sept, 8H, CHMeMe', 7 H -Me = 7.0 Hz), 1.97 (m, 8H, PCH2CH2P), 1.27 and 1.17 (dd, 48H, CHMeMe', 7 M e - P = 14.0 Hz, 7 M e-H = 7.0 Hz). 3 1 P{!H} N M R (dg-THF, 202.42 MHz): 8 58.7 (dd, 7 P - c = 28.2 Hz, 7 P . c = 25.6 Hz). 3 1 P{ 1 H} N M R (ds-THF, 50.32 MHz): 8 236.4 (quin, CO, /r j - P = 28.2 Hz), 228.1 (quin, CO, 7 C - P = 25.6 Hz). 5.11.3.16 [Pd(dippe)(C0 13CH 3)CO](BAr f) (7) In a 5 mm N M R tube, 2 - 1 3 C i (0.030 g; 2.27 x 10"5 mol) was dissolved in Et20 (0.4 mL) and a few drops of ^ 6 - b e n z e n e were added. The solution was freeze-pump-thawed and placed under 300 mm Hg gas pressure of carbon monoxide, and its 3 1 P{ lH} and 1 3 C{ lH) N M R spectra were recorded. 3 1 P{ 1 H} N M R (<26-benzene/Et20, 81.015 MHz): 8 73.7 (dd, IP, 7 P . P - = 40.6 Hz, 7 C -P = 38.7 Hz) 70.7 (dd, IP, 7 P . P - = 40.6 Hz, 7 C -P = 17.3 Hz). 1 3C( *H} N M R (d6-benzene/Et20, 50.32 MHz): 8 47.3 (dd, C 0 1 3 C H 3 , 7 C - P (trans) = 38.7 Hz, 7C-P' (cis) = 17.3 Hz). 163 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 5.11.3.17 [Pd(dippe)(13CH3)CO](BArf) (8) In a 5 mm N M R tube, 2 (0.030 g; 2.27 x 10' 5 mol) was dissolved in dA-o-dichlorobenzene (0.35 mL). The solution was freeze-pump-thawed and placed under 300 mm Hg gas pressure of carbon monoxide, and its 3 1 P{ 1 H} and 1 3 C{ ! H} N M R spectra were recorded.. 3 1?! 1!!} N M R (^-a-mchlorobenzene, 81.015 MHz): 8 84.2 (dd, lP ,/p .F = 21.2 Hz, / C -p = 105 Hz) 83.8 (dd, IP, /p_ P ' = 21.2 Hz, 7 C - P = -8.5 Hz). 1 3 C ^H} N M R (a\-o-dichlorobenzene, 50.32 MHz): 8 -4.9 (dd, P d - 1 3 C H 3 , / C - P (trans) = 105 Hz, / C - P ' (cis) = -8.5 Hz). 5.11.3.18 [Pd(dippe)(C0)2](BArf)2 (9), rPd(dippe)(NsCCH3)](BArf)2 (10a), and [Pd(dippe)(THF)2](BArf)2 (10b) To a solid mixture of HBArf (0.416 g; 4.11 x 10"4 mol) and Pd(dippe)Me2 (0.133 g; 2.06 x 10"4 mol) under vacuum at -196 °C, was added THF (2.0 mL) by trap to trap distillation. The reaction vessel was backfilled with CO (1 atm) and allowed to warm to -78 °C, at which temperature a clear yellow solution was observed. The solvent was removed in vacuo at -20 °C and [Pd(dippe)(CO)2](BArf)2 (9) was characterized in solution. 3 1 P{ lH} N M R (<i2-methylene chloride, 121.42 MHz) : 8 83.8. The residue of the reaction mixture was recrystallized from methylene chloride (2 mL) and cooled to -40 °C. Yellow crystals of [Pd(dippe)(THF)2](BArf)2 appeared after 12 h (0.220 g; 25%). *H N M R (d2-methylene chloride, 500.13 MHz) : 8 7.64 (br s, 16H, o-Arf), 7.49 (br s, 8H, p-Ar{), 3.86 (m, 8H, OC// 2CH 2), 2.32 (sept, 4H, C//MeMe', 7 H -Me = 7.3 Hz), 2.17 (m, 4H, PC//2C//'2P), 1.97 (m, 8H, OC// 2CH 2) , 1.36 and 1.28 (dd, 24H, CHMeMe', / M e - P = 18.5 Hz, 7 M e - H = 7.3 Hz). 3 1 P { ! H } N M R (d2-methylene chloride, 202.42 MHz) : 8 118.9. Anal . Calcd for C 8 2 H64B2F480P 2 Pd (i.e. [Pd(dippe)(THF)](BArf)2): C, 45.44; H, 2.98. Found: C, 45.48; H, 2.97. [Pd(dippe)(N=CCH3)](BArf)2 (10a) was prepared by adding acetonitrile (1 mL) to a mixture of HBArf (0.102 g; 5.01 x 10"5 mol) and Pd(dippe)Me2 (0.020 g; 2.50 x 10"5 mol), and characterized in solution. J H N M R (^-acetonitrile, 200.13 MHz): 8 7.71 (br s, 16H, o-Ar{), 164 References begin on page 166 ChapterS: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines 7.66 (br s, 8H, p-Arfi, 2.57 (sept, 4H, CHMeMe', /H -Me = 7.3 Hz), 2.31 (m, 4H, PC/^O/^P), 1.38 and 1.30 (dd, 24H, CHMeMe', / M e P =15.0 H z , / M e - H = 7.3 Hz). 31p{lH} N M R , ( d 3 -acetonitrile, 81.015): 8 118.9. 5.11.3.19 {[Pd(dippp)CO][Pd(dippe)Me]}(BArf) (11) To a solution of Pd(dippp)CO (prepared from [Pd(dippp)]2 (0.015 g; 2.00 x l ( r 5 mol) under CO in 5 mL E t 2 0 ) , was added 2 (0.050 g; 4.01 x 10" 5 mol). The product was characterized in solution. 31p{lH} N M R (^-benzene; 81.015 MHz): 8 53.8 (t, IP, / P . F = 28.5 Hz), 23.8 (t, IP, /p-.p = 28.5 Hz). 5.11.3.20 Pd(dippim)Me2 (12) As for Pd(dippe)Me2, with Pd(dippim)Cl2 (1.01 g; 2.41 mmol), MeL i (3;5 mL; 4.9 mmol), and THF (50 mL). Within 2 min of adding the M e L i to the slurry of Pd(dippim)Me2 at -78 °C the starting material went up to give a clear solution with a faint yellow tinge. Upon warming to room temperature the solution darkened somewhat due to precipitation of Pd metal. After workup and recrystallization from toluene (1 mL) layered with pentane (5 mL) grey crystals were isolated (0.72 g; 76%). ! H N M R (dg-benzene, 500.13 MHz): 8 6.97 (d, IH, CT/NBut, / H . p = 16.5 Hz), 1.78 (d sept, 2H, CtfMeMe', 7 H - P = 2.0 Hz, /H -Me = 7.0 Hz), 1.26 (s, 9H, NBu 1), 1.14 (dd, 12H, CHMeMe', 7 M e - P = 14.5 Hz, / M e - H = 7.0 Hz), 0.98 (d, 6H, C M e 2 C H , 7 P . M e = 7.5 Hz), 0.94 and 0.89 (d, 6H, Pd -Me, / M e -P = 7.00 Hz), 0.79 (dd, 12H, CHMeMe',/Me'-P = 11.5 HzJue'-R = 7.0 Hz). 3lp{lH} N M R (d6-benzene, 202.47 MHz): 8 62.9. Anal. Calcd for C i 6 H 3 6NPPd : C, 50.59; H, 9.55; N, 3.69. Found: C, 50.66; H, 9.66; N, 3.49. 1 2 - 1 3 C 2 was prepared likewise from Pd(dippim)Cl2 (0.420 g; 9.98 x 1 0 4 mol) and, Mg( 1 3CH 3 ) 2 »dioxane (0.142 g; 9.98 x 10"4 mol). Recrystallization from toluene (1 mL) layered with pentane (10 mL) gave colourless crystals (0.320 g; 81%). lH N M R (^-benzene, 200.13 MHz): 8 6.97 (d, IH, CZ/NBu1, / H - P = 16.5 Hz), 1.78 (d sept, 2H, C//MeMe', 7 H -P = 2.0 765 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Hz, / H - M e = 7.0 Hz), 1.26 (s, 9H, NBu 1), 1.17 (dd, 12H, CHMeMe*, / M e - P = 14.5 Hz, 7 M e - H = 7.0 Hz), 0.96 (d, 6H, C M e 2 C H , 7 P . M e = 7.5 Hz), 0.93 (dd, 6H, Pd-Me, / M e - C = 130 Hz, 7 M E - P = 7.00 Hz), 0.82 (dd, 12H, CHMeMe', / M e '-P = H-5 Hz/ M e ' -H = 7.0 Hz). 3 1 P{ lH} N M R (de-benzene, 81.015 MHz) : S 62.9 (dd, 7 P - C (trans) = 118.9 Hz, 7 P . C (cis) = 8.9 Hz). "Cf^H;} N M R (^-benzene, 50.32 MHz): 8 8.92 (d, 1C, Pd-Me, 7 C -P (trans) = 118.9 Hz), -7.45 (d, 1C, Pd-Me' , / C -p (cis) = 8.9 Hz). 5.11.3.21 [Pd(dippim)(Me)OEt2](BArf) (13) As for 2 with 12 (0.133 g; 3.50 x 10"4 mol) and HBArf (0.266 g; 3.50 x 10"4 mol). Workup and recrystallization from E t 2 0 (2 mL) gave colourless crystals (0.320 g; 74%). *H N M R (dg-THF, 500.13 MHz): 8 7.79 (br s, 8H, o-Art), 7.62 (d, IH, CHNBu 1 , 7H-P = 20.9 Hz), 7.58 (br s, 4H,p-Ar f ) , 2.47 (dsept, 2H, CHMeMe', / H - M e = 7.1 Hz, 7 H - P = 3.3 Hz), 1.58 (d, 6H, C M e 2 C H , / P . M e = 11.1 Hz), 1.33 and 1.28 (dd, 12H, CHMeMe', 7 M e - P = 16.5 Hz, / M e -H = 7.1 Hz), 0.43 (s, 3H, Pd-Me). 31p{lH} N M R (dg-THF, 202.42 MHz): 8 87.0. Anal. Calcd for C 4 7 H 4 5 B F 2 4 N P P d : C, 45.97; H, 3.69; N, 1.14. Found: C, 46.25; H, 3.88; N, 1.07. 13- 1 3C 2 was prepared likewise from 12- 1 3C 2 (0.030 g; 6.79 x 10' 5 mol) and H B A r f (0.068 g; 6.79 x 10"5 mol). Recrystallization from E t 2 0 (0.5 mL) gave colourless crystals (0.045 g; 50%). 1 3C{ lK} N M R (d6-benzene/Et20, 50.32 MHz): 8 -3.9. 5.12 References (1) Brookhart, M. ; Grant, M. ; Volpe, J., A . F. Organometallics 1992,11, 3920. (2) Johnson, L. K.; Mecking, S.; Brookhart, M . /. Am. Chem. Soc. 1996,118,267. (3) For general treatises on intramolecular insertion reactions including CO migratory insertion, see: (a) Tkatchenko, I. In Comprehensive Organometallic Chemistry; G. . ., - • Wilkinson, F. G. A. Stone and E. W. Abel, Eds.; Pergamon Press: Oxford, 1982; Vol . 8; pp 101-223. (b) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: M i l l 166 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines Valley, CA , 1987, pp 355-399. (c) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; 2nd ed.; John Wiley & Sons: New York, 1994, pp 161-182. (4) For specific accounts of carbonylation reactions of metal alkyls, see: (a) Calderazzo, F. Angew. Chem., Int. Ed. Engl. 1977, 76,299. (b) Kuhlmann, E . J . ; Alexander, J. J. Coord. Chem. Rev. 1980,33, 195. (c) Anderson, G. K.; Cross, R. J. Acc. Chem. Res. 1984, 77, 67. (5) Barsacchi, M. ; Consiglio, G,; Medici, L.; Petrucci, G.; Suter, U. W. Angew. Chem., Int. Ed. Engl. 1991,30,989. (6) Batistini, A.; Consiglio, G.; Suter, U. W. Angew. Chem., Int. Ed. Engl. 1992,31, 303. (7) Batistini, A.; Consiglio, G. Organometallics 1992, 77,1766. (8) . Pisano, C ; Nefkens, S. C. A.; Consiglio, G. Organometallics 1992, 77,1975. (9) van Asselt, R. ; Gielens, E E . C. G.; Riilke, R. E.; Vrieze, K.; Elsevier, C. J. J. Am. • C h e m . Soc. 1994, 776,977. (10) Brookhart, M. ; Rix, F. C ; DeSimone, J. M. ; Barbdrak, J. C.J.Am. Chem. Soc. 1992, 774,5894. (11) Brookhart, M. ; Wagner, M . I.; Balavoine, G. G.; Haddou, H. A. J. Am. Chem. Soc. 1994,114, 3641. (12) NozakLK.; Sato, N.; Takaya, H.7. Am. Chem. Soc. 1995, 777,9911. (13) Yang, X. ; Stern, C. L.; Marks, T. J. J.Am. Chem. Soc. 1994, 776,10015. (14) Johnson, L. K.; Kill ian, C. M. ; Brookhart, M. J. Am. Chem. Soc. 1995, 777, 6414. (15) For reviews see: (a) Sen, A . Chemtech 1986, 48. (b) Sen, A . Adv. Polym. Sci. 1986, 73174,125. (c) Drent, E. Pure Appl. Chem.. 1990,62,661; (d) Sen, A . Acc, Chem. Res. 1993,26, 303. (16) Jiang, Z.; Sen, A . J. Am. Chem. Soc. 1995, 777,4455. (17) Fryzuk, M . D.; McConville, D. H.; Rettig, S. J. J: Organomet. Chem. 1993,445,245. (18) Cotton, F. A. ; Marks, T . J ./ . Am. Chem. Soc.1969,97,1339, (19) Jolly, P. W. Angew. Chem., Int. Ed. Engl. 1985,24,283. 767 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines (20) Peuckert, M. ; Keim, W. Organometallics 1983,2, 594. (21) Wilke, G. Angew. Chem., Int. Ed. Engl. 1988,27, 185. (22) Rix, F. C ; Brookhart, M . J.Am. Chem. Soc. 1995,117, 1137. (23) Ozawa, F.; Hayashi, T.; Koide, H.; Yamamoto, A . /. Chem. Soc, Chem. Commun. 1991,1469. (24) Dekker, G. P. C. M . ; Elsevier, C. J . ; Vrieze, K.; van Leeuwen, P. W. N. M . Organometallics 1992,11,1598. (25) Dekker, G. P. C. M. ; Elsevier, C. J.; Vrieze, K.; van Leeuwen, P. W. N. M . ; Roobeek, C. F. /. Organomet. Chem. 1992,430, 357. (26) T6th, I.; Elsevier, C. J. /. Chem. Soc, Chem. Commun. 1993, 529. (27) van Leeuwen, P. W. N. M. ; Roobeek, C. F.; van der Heijden, H. J. Am. Chem. Soc. 1994,776,12117. (28) Drent, E.; van Broekhoven, J. A. M. ; Doyle, M . J.J. Organomet. Chem. 1991,417, 235, and references to patents cited therein. (29) Seligson, A . L.; Trogler, W. C. J. Am. Chem. Soc 1991, 775, 2520. (30) Cotton, F. A. Prog. Inorg. Chem. 1976,27, 1. (31) An alternative formulation of 6a, as {[Pd(dippe)1 3CO][Pd(dippe)(1 3COMe)]}(BAr f), i.e. a binuclear system bridged by CO and an acetyl group, is untenable due to the observed ^ C p H ) N M R spectrum of 2 - 1 3 C i under 1 3 C O . JQ-C values of 4-10 Hz are measured. Therefore the scalar coupling is not of the lJc-C tyPe (°f- lJc-C= 40.6 Hz for the A X 2 pattern of 1 3 CH3( 1 3 CO)3CH3, which is observed in the same spectrum) and a formulation of {[Pd(dippe)(13CO)2][Pd(dippe)Me]}(BArf) is justified. (32) Portnoy, M. ; Milstein, D. Organometallics 1993, 72, 1655. Also see Chapter 4 of this thesis. (33) Portnoy, M. ; Frolow, F.; Milstein, D. Organometallics 1991,10, 3960. (34) Portnoy, M. ; Milstein, D. Organometallics 1994,13,600. (35) T6th, I.; Elsevier, C. J. /. Am. Chem. Soc. 1993, 775,10388. 768 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines (36) The catalytic formation of ketones, diketones, and aldehydes from CO and Grignard reagents by L 2 N i R 2 ( L 2 = l,2-bis(diphenylphosphino)ethane or 2,2'-bipyridine; R = alkyl) has been reported, see: Yamamoto, T.; Kohara, T.; Yamamoto, A . Chem. Lett. 1976,1217. (37) The isolation of acetic anhydride and palladium metal has recently been reported during the carbonylation of a cationic palladium complex bearing a tridentate nitrogen donor, see: Markies, B. A. ; Wijkens, P.; Dedieu, A. ; Boersma, J.; Spek, A. L.; van Koten, G. Organometallics 1995,14, 5628. (38) Vall i , V. L. K.; Alper, H. /. Polym. Sci., Part A: Polym. Chem. 1995,33, 1715. (39) Stork, G.; O'Dowd, S. R. Org. Synth. 1974,54,46. (40) Huheey, J. E.; Keiter, E. A. ; Keiter, R. L. Inorganic Chemistry; 4th ed.; Harper Collins: New York, 1993. (41) The structure of 13 is preliminary and awaits final refinement; Rettig, S. J., personal communication, 1995. (42) Dekker, G. P. C. M. ; Buijis, A. ; Elsevier, C. J.; Vrieze, K ; van Leeuwen, P. W. N. M.; Smeets, W. J. J.; Spek, A. L.; Wang, Y . F.; Stam, C. H. Organometallics 1992,11, 1937. (43) Vicente, J. ; Abad, J. A. ; Gil-Rubio, J. ; Jones, P. G.; Bembenek, E. Organometallics 1993, 72,4151. (44) Milani, B.; Alessio, A. ; Mestroni, G.; Sommazzi, A. ; Garbussi, F.; Zarigrando, E.; Bresciani-Pahor, E.; Randaccio, L. /. Chem. Soc, Dalton Trans. 1994,1903. (45) See ref. 3b, pp 286-287. (46) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993,115,10462. (47) Arndtsen, B. A.; Bergman, R. G. Science 1995,270,1970. (48) Pisano, C ; Consiglio, G.; Sironi, A.; Moret, M . /. Chem. Soc, Chem. Commun. 1991, 421. (49) Massey, A. G.; Park, A. /. Organomet. Chem. 1966,5, 218. 169 References begin on page 166 Chapter 5: Cationic Palladium Alkyl Complexes Stabilized by Electron-rich Phosphines (50) Dryden, N. H.; Legzdins, P.; Trotter, J.; Yee, V. C. Organometallics 1991,10,2857. (51) Riilke, R. E.; Ernsting, J. M. ; Spek, A . L.; Elsevier, C. J.; van Leeuwen, P. W. N. M. ; Vrieze, K, Inorg. Chem. 1993,32,5769. (52) Brandsma, L.; Verkruijsse, H. Preparative Polar Organometallic Chemistry; Springer-Verlag: Berlin, 1987; Vol. 1. 770 References begin on page 166 Chapter 6 : CONCLUSIONS A N D SECOND T H O U G H T S 6 .1 Retrospective THROUGHOUT THIS THESIS W E have examined the potential of a series of transition metal hydride and alkyl complexes to act as metal bases, and have explored then-interaction with some non-conventional Lewis acids. L i + , as its tetraethylborate salt, was the most unusual of these Lewis acids, and its ability to form adducts with transition metal hydride complexes was both unexpected and unprecedented. The metal-basicity of the hydrides of the early transition metals was never in doubt, but the basicity of the late-metal hydrides had largely been unrecognized. In terms of lithium coordination chemistry it is significant that L i + was associated with the hydrides, i.e. with the most rich source of electron density. Bound to transition metals, the hydride ligand forms a dipole which can interact with ionic species such as L i + and N a + . It is unreasonable and unnecessary to invoke any covalent character for the chemical forces between L i + and the core of the metal complex.1'2^ The bond between L i + and M - H can be explained in terms of ion-dipole attraction, and is conceptually similar to the so-called agostic bonding invoked for the C - H - L i bonds of regular organolithium compounds.2 While adduct formation between LiBEt4 and L n M H n to form stable 1:1 complexes was an unusual finding, the reactivity displayed by these species is pedestrian, and indeed not far removed from that of the parent hydrides themselves. Since one of the stated aims of our research was to magnify the activity of the transition metal centre by exploiting Lewis acid/metal base interactions, we therefore examined how a well established Lewis acid, H + , would interact with some neutral, and fairly inert, transition metal alkyl complexes. The action of HBArf upon Pd(dippe)Me2 irreversibly forms a highly reactive, cationic palladium alkyl species that, as well as showing promise as a copolymerization catalyst, also will activate small molecules such as H 2 and CO. The transition metal centre in [Pd(dippe)Me]+ possesses high Lewis acidity, and its reaction chemistry is dominated by the formation of binuclear complexes 171 References begin on page 178 Chapter 6: Conclusions and Second Thoughts which demonstrate new modes of reactivity. It is this enhanced reactivity compared to the parent complex that is most exciting, and which would form the basis of further work based on this thesis. During the course of our research, inevitably some miscellaneous results occurred, only loosely related to the theme of Lewis acid/metal base interaction. The isolation and structure of [Cp2Ta(p-H)2AlH]2(p-OBu)2 was one of these, even though the complex may be formulated as an adduct of a C p 2 T a H metal base and an AlH2 (OBu) Lewis acid. Also isolated and structurally characterized was a zerovalent complex of palladium and bidentate phosphines, [Pd(dippe)]2(p.-dippe), in which the metal centre displays an unusual trigonal planar mode of coordination. [Pd(dippe)J2(p-dippe) was also found to undergo reactions typical of a coordinatively unsaturated metal centre, i.e. typical of a metal base in a broader sense. 6.2 Prospective 6.2.1 Adducts of L i B E t 4 At this point it is appropriate to consider how the work described herein could be extended, and several possibilities suggest themselves. The Lewis acidity of LiBEt4 and NaBEt4, directed at other transition metal hydrides, would no doubt produce some more unusual structures, but since the adducts so formed are likely to be unreactive, this does not promise to be a very fruitful line of enquiry. One such reaction so far not attempted would involve the addition of LiBEt4 to a metal complex which bears an r j 2 - H 2 ligand. Whether or not the L i + - H interaction would persist with these less polar hydrogen ligands would be an interesting question to answer. 6.2.2 Development of [Pd(dippe)Me]+ chemistry Considerably more scope for development is offered by the cationic palladium methyl complexes. We have so far only touched the surface of the reaction chemistry of the [Pd(dippe)Me]+ system and it is expected that the complex wil l undergo many more chemical transformations. The result obtained with CO is incentive to examine the interaction of 172 References begin on page 178 Chapter 6: Conclusions and Second Thoughts [Pd(dippe)Me(0Et2)]+ with other polar, organic substrates. Isonitriles are an obvious candidate, but given that they are more prone to insertion than CO, 3 multiple insertion processes (i.e. homopolymerization) might dominate this chemistry as shown in Scheme 6.1. Scheme 6.1 y o -Me X © / -s Pd + : C = N R <y S = solvent molecule \ © / Pd Me R N = C 1 <> \ © / C Pd / \ / / R . N -R N = C \ © / Me Pd Me -<y N \ R <v An alternative might be to examine the reactivity of the metal cation towards both primary and secondary silanes, in the hope of developing some sort of Pd-silyl chemistry. Pd-silyl complexes are largely unknown,4"8 and an added incentive is the possible design of a late-metal catalyst for silane coupling to give polysilanes. 9" 1 1 The state of the present art is very underdeveloped, and there are few late metal systems which will couple silanes effectively.1 2'1 3 In fact, a preliminary reaction between [Pd(dippe)Me(OEt2)]+ and Bu n SiH3 showed loss of methane, and quantitative formation of a new complex whose identity is unknown but which 173 References begin on page 178 Chapter 6: Conclusions and Second Thoughts displays a doublet of doublets in its 3 1 P{ 1 H} N M R spectrum, and upfield signals in its *H N M R spectrum. A possible reaction scheme is shown in eq 6-1. [6-1] Obviously this compound will have to be fully characterized, and the reaction wil l have to be repeated with less active silanes such as PhSiH3 and Ph2SiH2, but it is possible that a dehydrogenative coupling mechanism could lead to the synthesis of polymers and oligomers containing S i -S i bonds as shown in Scheme 6.2. 1 4 Scheme 6.2 A reaction of the type envisaged would have significant practical importance as catalysts promoting polysilane formation are very rare. Given the propensity of the 774 References begin on page 178 Chapter 6: Conclusions and Second Thoughts [Pd(dippe)Me(0Et2)]+ complex to form binuclear complexes, a silylene complex might also be accessible.12 Further work in this area might therefore be well rewarded. 6.2.3 Development of P - N chemistry During the above work a new ligand was developed, dippim, a bidentate system which incorporates both an imine and a phosphine type donor. This ligand, which is fundamentally different from the range of existing P - N donors, might serve to bridge the gap between the bidentate phosphines which have traditionally been used to stabilize late metal complexes, and the current crop of diimine ligands which have recently given some spectacular results when bound to nickel and palladium metal centres. 1 5 , 1 6 Both dentates of dippim are amenable to variation and clearly a host of ligands based upon it are accessible (Scheme 6.3). Scheme 6.3 In I the phosphorus substituents have simply been varied by replacing the isopropyl groups of dippim with phenyl groups. Likewise in n, replacement of the isopropyl groups by a chiral substituent leads to a hgand whose metal complexes would have diastereotopic faces. 1 7 Alternatively, variation of the imine arm of the ligand, by replacing tert-butylamine for some 175 References begin on page 178 Chapter 6: Conclusions and Second Thoughts suitably substituted arylarnine, leads to a ligand of type III, which would be invaluable for comparison with the diimine analogues (vide supra). A l l these ligands, and more, would be available by simply varying the starting imine and phosphinous chloride. As an aside, it is worth mentioning a (P-N)" system can easily be generated by treating dippim with a suitable reducing agent. As shown in eq 6-2, when dippim is treated with one equiv of LiAlH4 in THF at reflux temperature, an alane complex is generated. [6-2] Subsequent protonation of the alane, as shown in Scheme 6.4, could produce a new ligand, Pr i2P(CMe2)CHNH(Bu t), that would be suitable for binding to early metals. 176 References begin on page 178 Chapter 6: Conclusions and Second Thoughts Scheme 6.4 n + • MR 4 H + -RH M = Zr, Hf MR3 6.2.4 Development of [Pd(dippe)] 2 +chemistry The final issue raised by this work involves the chemistry of the dicationic palladium complexes, [Pd(dippe)]2 + and [Pd(dippe)(CO)2]2+, whose intermediacy was established in the reaction between CO and [Pd(dippe)Me]+. The [Pd(dippe)(CO)2]2+ species would be of interest in its own right as an unusual Pd(JJ) carbonyl complex. 1 8 Meanwhile, the dication, [Pd(dippe)]2 +, can be isolated by fairly standard means, and this is an extremely reactive entity. A preliminary reaction showed that [Pd(dippe)] 2 + was even more Lewis-acidic than [Pd(dippe)Me]+, as indicated by the formation of two equiv of the methyl cation from a stoichiometric mixture of [Pd(dippe)]2 + and the parent dimethyl complex, Pd(dippe)Me2, as shown in eq 6-3. 777 References begin on page 178 Chapter 6: Conclusions and Second Thoughts [6-3] S = Solvent molecule Although [Pd(dippe)]2+ is unstable in both THF and methylene chloride and a suitable reaction medium would have to be found (perhaps 0-CI2C6H4 or 0-F2C6H4), the exceptionally high Lewis-acidity of this species could result in C - H activation, or C - S i activation.19 Such then are our suggestions: for future work. Many of the proposed reactions, in particular the last, are highly speculative, but others could quite legitimately be studied now that the groundwork is laid. Some interesting chemistry, planned and unplanned, awaits. 6.3 References (1) Schade, C ; Schleyer, P. v. R. Adv. Organomet. Chem. 1988,27, 169. (2) Schlosser, M. In Organometallics in Synthesis; M . Schlosser, Ed.; John Wiley & Sons: Chichester, 1994; pp 13-18. (3) Collman, J. P.; Hegedus, L. S.; Norton, J. R ; Finke, R. G. Principles and Applications ' of Organotransition Metal Chemistry; University Science Books: M i l l Valley, California, 1987, p 377-378. (4) Curtis, M . D.; Greene, J. J. Am. Chem. Soc. 1978,100,6362. (5) Eaborn, C ; Griffiths, R. W.; Pidcock, A . /. Organomet. Chem. 1982,225, 331. (6) Seyferth, D.; Goldman, E. W.; Escudie, J. /. Organomet. Chem. 1984,271,331. (7) Schubert, U.; Muller, C. /. Organomet. Chem. 1989,373,165. (8) Pan, Y. ; Mague, J. T.; Fink, M. J. Organometallics 1992,11, 3495. 178 References begin on page 178 Chapter 6: Conclusions and Second Thoughts (9) Tilley, T. D. Comments Inorg. Chem. 1990,10, 37. (10) Corey, J. Y . Advances in Silicon Chemistry; JA1: Greenwich, Connecticut, 1991; Vol. 1. (11) Tilley, T. D. Acc. Chem. Res. 1993,26, 22. (12) Fryzuk, M . D.; Rosenberg, L.; Rettig, S. J. Inorg. Chim. Acta 1994,222, 345. (13) For an early metal polysilane catalyst, see: Woo, H. G.; Walzer, J. F.; Tilley, T. D. 7. Am. Chem. Soc. 1992,114, 7046. (14) Curtis, M . D.; Epstein, P. S. Adv. Organomet. Chem. 1981,19,213. (15) Johnson, L. K.; KiUian, C. M. ; Brookhart, M.'/. Am. Chem. Soc. 1995,117, 6414. (16) Johnson, L. K.; Mecking, S.; Brookhart, M . /. Am. Chem. Soc. 1996,118, 267. (17) Burk, M . J.; Feaster, J. E.; Harlow, R. L. Organometallics 1990, 9, 2653. (18) Wang, C ; Willner, H ; Bodenbinder, M. ; Batchelor, R. J.; Einstein, F. W. B.; Aubke, F. Inorg. Chem. 1994,33, 3521. (19) Arndtsen, B. A.; Bergman, R. G. Science 1995,270, 1970. 179 References begin on page 178 Appendices Table A l . Crystallographic data for [Pt(dippp)]2(u.-H)2 (6 , Chapter 1), [Cp2Ta(p-H) 2 AlH] 2 (p-OBu)2 (4, Chapter 3), {[Pd(dippe)CO][Pd(dippe)Me]}(BAr f) (6', Chapter 5), and Pd(dippim)Me2 (12, Chapter 5). a Compound 6 4 6' 12 Formula C3oH7oP4Pt2 C 2 8 H 4 4 A l 2 0 2Ta2 C65.5H83BF240P4Pd2 Ci6H 3 6NPPd fw 944.66 828.51 1689.84 379.84 Color, habit yellow, plate orange, prism yellow, plate colourless, prism Crystal system triclinic monoclinic triclinic monoclinic Space group PI P2i/c P\ Plllc a, A 11.529(1) 7.534(2) 17.134(2) 9.397(2) b.k 16.581(1) 11.433(1) 18.454(2) 13.804(2) c,k 11.018(2) 17.733(2) 12.932(2) 15.691(2) a, deg 91.11(1) 90 93.22 90 P, deg 107.10(1) 100.15(1) 94.98(1) 106.77(1) Y.deg 106.166(8) 90 70.366(7) 90 V, A3 1921.7(1) 1503.5(4) 3835.1(8) 1948.6(5) Z 2 2 2 4 Pcalc g / c m 3 1.146 1.830 1.463 1.295 F(000) 932.00 800 1714 800 Radiation M o M o M o M o p, cm" 1 74.25 73.42 6.49 10.26 Crystal size, mm 0.12x0.45x0.50 0.20 x 0.20 x 0.30 0.15x0.35x0.40 0.20 x 0.30 x 0.45 Transmission factors 0.19-1.00 0.84-1.00 0.82-1.00 0.90-1.00 Scan type co-29 co-29 co-29 co-29 Scan range, co° 1.00+0.35 tan 9 1.15+0.35 tan 9 1.26+0.35 tan 9 0.89+0.20 tan 9 Scan speed, °min _ 1 32 (up to 9 scans) 32 (8 rescans) 16 (up to 9 scans) 32 (up to 9 scans) 29max> d e § 60 55 50 65 180 Appendices Crystal, decay, % 8.4 -0.97 22.5 -Total reflections 11727 3905 13984 7682 Unique reflections 11208 3637 13489 7312 n merge 0.034 0.059 0.054 0.035 Number with / > 3CT(/) 6189 1500 5769 3423 Variables 334 154 927 173 R 0.042 0.036 0.044 0.031 Rw 0.038 0.032 0.039 0.028 gof 1.97 1.75 2.05 1.53 Max A/a (final cycle) 0.05 0.003 0.14 0.0005 Residual density e/A3 -3.18 to 2.94 -0.44 to 0.44 -0.51 to 0.48 -0.41 to 0.37 a Temperature 294 K, Rigaku AFC6S diffractometer, Mo Ka (k = 0.71069 A) radiation, graphite monochromator, takeoff angle 6.0°, aperture 6.0 x 6.0 mm at a distance of 285 mm from the crystal, stationary background counts at each end of the scan (scan/background time ratio 2:1), a 2 (F 2 ) = [52(C + 45)]/Lp2 (S = scan rate, C = scan count, B = normalized background count), function minimized Zw(LF0l-LFcl)2 where w = 4 F 0 2 / a 2 ( F 0 2 ) , R = ZIIF0I-IFCII/EIF0I, Rw = (Zw(IF 0l-IF cl) 2yEwF 0l 2) 1 / 2, and gof = [Zw(IF0l-IFcl)2/(/n-n)]1/2 (where m = number of observations, n = number of variables). Values given for R, Rw, and gof are based on those reflections with I > 3CT(7). 181 Appendices Appendices Table A2.1 Final atomic coordinates (fractional) and 5 e q (A 2) f ° r [ p t(dippp)]2(|t-H)2 (6, Chapter 1). atom X y z Bta Pt(D 0.49134(3) 0.26615(2) 0.25401(3) 2.644(8) Pt(2) 0.30918(3) 0.26689(2) 0.04838(3) 2.719(8) P(l) 0.6784(2) 0.3161(1) 0.4085(2) 3.37(5) P(2) 0.3811(2) 0.1780(1) 0.3628(2) 3.08(5) P(3) 0.1650(2) 0.3219(1) -0.0773(3) 4.44(6) P(4) 0.3167(2) 0.1728(1) -0.0983(2) 3.27(5) C(l) 0.6896(9) 0.2773(6) 0.5668(9) 4.2(2) C(2) 0.6113(10) 0.1879(6) 0.5643(10) 5.0(3) C(3) 0.4685(9) 0.1759(5) 0.5319(9) 4.2(2) C(4) 0.7228(9) 0.4313(5) 0.447(1) 4.4(2) C(5) 0.8236(9) 0.3013(6) 0.381(1) 5.0(3) C(6) 0.3218(9) 0.0643(5) 0.2976(9) 3.9(2) C(7) 0.2384(9) 0.1989(5) 0.3829(10) 4.2(2) C(8) 0.614(1) 0.4569(6) 0.469(1) 7.8(4) C(9) 0.843(1) 0.4704(7) 0.563(1) 7.0(3) C(10) 0.815(1) 0.2082(7) 0.370(1) 7.6(4) C ( l l ) 0.847(1) 0.3413(8) 0.264(1) 7.1(4) C(12) 0.429(1) 0.0326(5) 0.281(1) 5.5(3) C(13) 0.247(1) 0.0045(6) 0.369(1) 5.8(3) C(14) 0.268(1) 0.2920(6) 0.432(1) 6.0(3) C(15) 0.1265(9) 0.1740(6) 0.264(1) 5.1(3) C(16) 0.099(1) 0.2840(7) -0.250(1) 6.9(3) C(17) 0.099(1) 0.1941(9) -0.284(1) 7.1(4) C(18) 0.224(1) 0.1780(6) -0.262(1) 5.2(3) 183 Appendices C(19) 0.226(1) 0.4366(7) -0.085(2) 8.1(4) C(20) 0.0215(9) 0.3115(6) -0.025(1) 5.7(3) C(21) 0.2610(8) 0.0596(5) -0.0759(10) 4.1(2) C(22) 0.4737(9) 0.1826(6) -0.1205(9) 4.4(2) C(23) 0.342(1) 0.4526(8) -0.133(2) 9.5(5) C(24) 0.261(1) 0.4870(7) 0.045(2) 10.9(5) C(25) -0.059(1) 0.2202(7) -0.047(1) 7.2(4) C(26) -0.060(1) 0.3678(8) -0.080(2) 9.3(5) C(27) 0.1292(9) 0.0377(5) -0.057(1) 4.9(3) C(28) 0.259(1) -0.0042(6) -0.183(1) 6.3(3) C(29) 0.5388(10) 0.2748(7) -0.132(1) 6.1(3) C(30) 0.5581(9) 0.1498(7) -0.012(1) 5.4(3) H(l ) 0.546(6) 0.328(3) 0.155(6) K D H(2) 0.344(5) 0.325(3) 0.193(6) K D 5 e q = 8/37C2(£/ii(aa*)2 + U 22(bb*)2 + U zzibb*)1 + 2 U i2aa*bb*cosy + 2U i3aa*cc*cosJ3 + 2U23bb*cc*cosa) 184 Appendices Figure A2. ORTEP view of, and numbering scheme for [Cp2Ta(|i-H)2AlH]2(u.-OBu)2 (4, Chapter 3). 185 Appendices Table A2.2 Final atomic coordinates (fractional) and 5 e q (A2) for [Cp2Ta(|i-H)2AlH]2(|i-OBu)2 (4, Chapter 3). atom X y z Ta(l) 0.29644(7) 0.57304(2) 0.29468(3) 4.90(1) Al ( l ) 0.4996(5) 0.4830(3) 0.4199(2) 2.76(8) O(l) 0.6063(9) 0.5738(6) 0.5017(3) 4.9(2) C(l) 0.173(2) 0.650(1) 0.3951(8) 7.2(4) C(2) 0.220(2) 0.749(1) 0.352(1) 8.0(5) C(3) 0.115(2) 0.743(2) 0.2768(10) 8.5(5) C(4) 0.001(2) 0.652(2) 0.2788(10) 8.1(5) C(5) 0.033(2) 0.593(1) 0.346(1) 8.0(5) C(6) 0.520(3) 0.488(3) 0.2360(9) / 8.8(6) C(7) 0.477(4) 0.595(2) 0.203(1) 9.5(7) C(8) 0.314(5) 0.591(2) 0.1659(9) 10.3(8) C(9) 0.235(3) 0.489(3) 0.171(1) 10.6(7) C(10) 0.365(5) 0.419(1) 0.214(1) 10.2(7) C ( l l ) 0.764(2) 0.644(1) 0.5041(7) 8.1(4) C(12) 0.706(4) 0.775(2) 0.516(1) 17.1(9) C(13) 0.798(5) 0.854(2) 0.526(1) 18(1) C(14) 0.760(2) 0.972(1) 0.5399(8) 9.8(5) H(l ) 0.2909 0.4265 0.3506 8.5 H(2) 0.4881 0.6184 0.3655 8.5 H(3) 0.6281 0.4013 0.4081 8.5 ^eq = %/3n2(Un(aa*)2 + U22(bb*)2 + U 33(W>*)2 + 2 U i2aa*bb*cosy + 2£/i3fla*cc*cosp + 2C/23^*cc*cosa) 186 Appendices 187 Appendices ble 2.3 F i n a l atomic coordinates (fractional) and B e q (A 2 ) for {[Pd(dippe)CO][Pd(dippe)Me]}(BArf) (6', Chapter 5). Atoms C(63)-C(69), the toluene solvent, are disordered about a centre of symmetry and have variable occupancy. atom X y z Pd(l) 0.17150(3) 0.14667(4) 0.15305(5) 4.55(2) Pd(2) 0.00695(3) 0.21226(4) 0.1682(5) 4.66(2) P(l) 0.3079(1) 0.0712(1) 0.1384(2) 4.79(6) P(2) 0.2184(1) 0.2520(1) 0.1648(2) 4.27(5) P(3) -0.1244(1) 0.2364(1) 0.2261(2) 5.94(7) P(4) -0.0584(1) 0.2521(1) 0.0047(2) 5.15(6) F(l) 0.7145(3) 0.2875(5) 0.7656(6) 13.4(3) F(2) 0.7656(4) 0.2392(5) 0.6378(6) 14.3(3) F(3) 0.7514(4) 0.1721(4) 0.7473(8) 18.8(4) F(4) 0.6132(4) 0.1067(5) 0.3868(5) 15.0(3) F(5) 0.5015(5) 0.1881(4) 0.3456(4) 12.2(1) F(6) 0.5019(4) 0.1027(6) 0.4330(5) 12.2(3) F(7) 0.3437(5) 0.0849(3) 0.6736(8) 15.2(3) F(8) 0.2508(6) 0.1096(4) 0.5595(6) 17.3(1) F(9) 0.2243(5) 0.1074(4) 0.7068(6) 14.2(3) F(10) 0.0640(5) 0.4385(6) 0.7494(7) 16.8(3) F ( l l ) 0.0241(4) 0.3748(5) 0.6407(9) 19.0(4) F(12) 0.0715(4) 0.4536(6) 0.6020(4) 17.9(4) F(13) 0.189(1) 0.526(3) 0.393(2) 13(1) F(14) 0.277(2) 0.566(2) 0.316(2) 10.9(9) F(15) 0.292(3) 0.452(2) 0.320(3) 8.1(7) F(16) 0.4569(7) 0.6421(5) 0.5712(6) 19.9(4) 188 Appendices F(17) 0.4064(4) 0.6492(5) 0.7074(8) 17.6(4) F(18) 0.5175(4) 0.5752(3) 0.6871(5) 10.7(2) F(19) 0.2972(4) 0.5341(3) 1.0629(6) 12.5(2) F(20) 0.3652(4) 0.5748(3) 0.9740(4) 9.8(2) F(21) 0.4175(4) 0.5273(3) 1.1126(5) 11.6(2) F(22) 0.5170(3) 0.1758(3) 1.0249(4) 9.8(2) F(23) 0.6113(3) 0.2226(4) 1.0301(6) 13.6(3) F(24) 0.5299(4) 0.2459(3) 1.1461(5) 12.1(2) 0(1) 0.1045(4) 0.1630(5) 0.3676(6) 10.7(3) C( l ) 0.3712(4) 0.1312(5) 0.1821(6) 5.4(2) C(2) 0.3319(4) 0.2143(4) 0.1528(6) 5.3(2) C(3) 0.3454(5) -0.0098(5) 0.2251(7) 6.6(3) C(4) 0.3429(6) 0.0281(7) 0.0115(8) 9.0(4) C(5) 0.1807(4) 0.3216(4) 0.0613(6) 5.3(2) C(6) 0.2080(5) 0.3109(4) 0.2869(6) 5.2(2) C(7) 0.3161(6) 0.0160(6) 0.3345(8) 9.2(3) C(8) 0.4379(5) -0.0536(5) 0.2333(8) 9.1(3) C(9) 0.2931(7) -0.0144(7) -0.0420(9) 11.7(4) C(10) 0.368(1) 0.072(1) -0.055(1) 21.9(9) C ( l l ) 0.1931(5) 0.2813(6) -0.0448(7) 7.7(3) C(12) 0.2181(6) 0.3861(5) 0.0684(8) 8.9(3) C(13) 0.1166(6) 0.3584(5) 0.3000(7) 8.1(3) C(14) 0.2483(6) 0.2615(5) 0.3807(7) 7.6(3) C(15) -0.1980(6) 0.2618(7) 0.1104(9) 11.8(4) C(16) -0.1705(5) 0.2815(6) 0.0200(8) 7.7(3) C(17) -0.1652(6) 0.3127(6) 0.3219(9) 8.8(4) C(18) -0.1415(5) 0.1520(6) 0.2770(8) 7.7(3) 189 Appendices C(19) -0.0419(5) 0.1841(5) -0.1068(7) 7.0(3) C(20) -0.0474(5) 0.3382(5) -0.0488(7) 6.2(3) C(21) -0.1656(9) 0.3903(7) 0.285(1) 17.5(6) C(22) -0.1142(8) 0.2948(8) 0.425(1) 14.3(5) C(23) -0.2319(6) 0.1652(7) 0.3027(10) 12.3(4) C(24) -0.1071(7) 0.0809(6) 0.210(1) 12.0(5) C(25) 0.0453(6) 0.1640(6) -0.1420(8) 9.4(3) C(26) -0.0605(6) 0.1125(6) -0.0802(8) 10.0(4) C(27) -0.0607(6) 0.4035(5) 0.0338(8) 8.2(5) C(28) -0.1012(5) 0.3680(5) -0.1476(7) 8.2(3) C(29) 0.0809(6) 0.1778(6) 0.2845(9) 7.7(3) C(30) 0.1304(4) 0.0471(4) 0.1365(7) 6.5(2) C(31) 0.4805(4) 0.2887(4) 0.6496(6) 3.7(2) C(32) 0.5596(5) 0.2839(4) 0.6917(6) 4.4(2) C(33) 0.6312(5) 0.2371(5) 0.6498(6) 4.5(2) C(34) 0.6285(4) 0.1922(4) 0.5621(7) 4.5(2) C(35) 0.5517(5) 0.1954(4) 0.5171(6) 4.2(2) C(36) 0.4800(4) 0.2427(4) 0.5601(6) 3.9(2) C(37) 0.7135(6) 0.2317(8) 0.7023(10) 7.7(4) C(38) 0.5468(6) 0.1468(7) 0.4245(8) 6.9(3) C(39) 0.3175(4) 0.3162(4) 0.6811(5) 3.8(2) C(40) 0.3255(4) 0.2383(5) 0.6762(5) 4.4(2) C(41) 0.2576(6) 0.2135(5) 0.6637(6) 5.3(3) C(42) 0.1776(6) 0.2661(7) 0.6591(6) 6.0(3) C(43) 0.1677(5) 0.3430(6) 0.6681(6) 5.4(3) C(44) 0.2361(5) 0.3678(4) 0.6789(6) 4.7(2) C(45) 0.2718(8) 0.1289(7) 0.6513(10) 7.9(4) 190 Appendices C(46) 0.0825(7) 0.3987(8) 0.666(1) 9.8(5) C(47) 0.3784(4) 0.4270(4) 0.6323(5) 3.8(2) C(48) 0.3286(4) 0.4407(4) 0.5394(6) 4.1(2) C(49) 0.3176(5) 0.5046(5) 0.4799(6) 4.6(2) C(50) 0.3555(5) 0.5579(4) 0.5110(6) 4.8(2) C(51) 0.4059(4) 0.5453(4) 0.6032(6) 4.2(2) C(52) 0.4180(4) 0.4810(4) 0.6607(5) 4.1(2) C(53) 0.259(1) 0.5177(10) 0.382(1) 7.0(4) C(54) 0.4463(7) 0.6018(5) 0.6404(9) 6.9(3) C(55) 0.4129(4) 0.3612(4) 0.8226(5) 3.7(2) C(56) 0.3827(4) 0.4309(4) 0.8763(6) 4.1(2) C(57) 0.4014(4) 0.4398(4) 0.9818(6) 4.0(2) C(58) 0.4504(4) 0.3777(5) 1.0390(5) 3.9(2) C(59) 0.4779(4) 0.3070(4) 0.9892(6) 4.1(6) C(60) 0.4594(4) 0.2983(4) 0.8837(6) 4.2(2) C(61) 0.3700(6) 0.5167(6) 1.0330(8) 5.8(3) C(62) 0.5357(6) 0.2386(5) 1.0449(8) 6.0(3) C(63) -0.063(6) -0.042(5) 0.486(8) 15(1) (0.45 occ.) C(64) 0.03(1) -0.114(5) 0.403(5) 25(3) (0.40 occ.) C(65) -0.094(6) 0.002(3) 0.524(3) 19(2) (0.70 occ.) C(66) -0.112(6) 0.058(6) 0.572(6) 25(3) (0.45 occ.) C(67) -0.054(7) 0.026(5) 0.533(9) 25(3) (0.45 occ.) C(68) 0.028(3) 0.053(6) 0.538(5) 19(3) (0.55 occ.) C(69) 0.026(5) -0.063(5) 0.440(5) 13(1) (0.50 occ.) J3(l) 0.3975(5) 0.3478(5) 0.6964(7) 3.8(2) fieq = mK2(Un(aa*)2 + U 12(bb*)2 + U3i(bb*)2 + 2 U i2aa*bb*cosy + 2Ui2aa*cc*cos$ + 2U2^bb*cc*cosa) 191 Appendices Appendices T a b l e A2.4 Final atomic coordinates (fractional) and £ e q (A2) for Pd(dippim)Me2 (12, Chapter 5). atom X y z Pd(l) 0.24912(3) 0.58016(2) 0.18183(2) 3.152(5) P(l) 0.34177(9) 0.43249(6) 0.15656(5) 3.00(2) N(l ) 0.2085(3) 0.4924(2) 0.2946(2) 3.29(6) C(l) 0.2286(3) 0.3491(2) 0.2058(2) 3.27(7) C(2) 0.2128(3) 0.4011(2) 0.2867(2) 3.59(8) C(3) 0.5374(3) 0.4030(2) 0.2158(2) 3.84(8) C(4) 0.5738(4) 0.4314(3) 0.3140(2) 5.52(9) C(5) 0.6416(4) 0.4553(3) 0.1727(3) 5.7(1) C(6) 0.3253(4) 0.3959(2) 0.0410(2) 3.82(8) C(7) 0.1763(4) 0.4236(3) -0.0253(2) 5.25(9) C(8) 0.3657(5) 0.2915(3) 0.0265(3) 6.0(1) C(9) 0.2934(4) 0.2472(2) 0.2329(2) 4.62(9) C(10) 0.0693(4) 0.3395(2) 0.1427(2) 4.26(8) C ( l l ) 0.1853(4) 0.5333(2) 0.3784(2) 4.15(8) C(12) 0.2990(5) 0.6142(3) 0.4084(2) 6.2(1) C(13) 0.0273(4) 0.5710(3) 0.3539(3) 6.1(1) C(14) 0.2076(5) 0.4595(3) 0.4544(2) 6.4(1) C(15) 0.1411(4) 0.7108(2) 0.1922(2) 4.86(9) C(16) 0.3066(4) 0.6499(2) 0.0823(2) 4.92(9) #eq = 8/37C2(t/n(aa*)2 + U22Qjb*)2 + U 2i(bb*)2 + 2 U \2aa*bb*cosy + 2U i3a<2*cc*cosj3 + 2U23bb* cc* cosa) 193 VITA Guy Kenneth Bruce C L E N T S M I T H Date of Birth: Nationality: 18th October 1967 Australian Education: 1991-1996 University of British Columbia Ph. D. in Chemistry under the supervision of Prof. M . D. Fryzuk, "Metal Basicity: Coordination Behaviour of some Non-Conventional Lewis Acids". Studies include fundamental reactivity patterns of early- and late-metal hydrides; adduct formation with alkali metal salts; zerovalent 14 and 16 electron palladium complexes; cationic palladium hydrocarbyl complexes stabilized by electron-rich phosphines; ligand design and synthesis. 1989-1991 University of British Columbia M . Sc. in Chemistry under the supervision of Prof. M. D. Fryzuk, "Binuclear Palladium Hydrides". The first palladium complexes bearing bridging hydride were isolated and unequivocally characterized. These species were found to undergo reaction with Lewis-acidic alkali metal salts. 1985-1988 University of New South Wales, B. Sc. (Hons) in Chemistry. Fourth year project supervised by Prof. N. K. Roberts, "Rigid Chiral Phosphines of Small Chelate Bite". 1984 Sydney Technical College, N. S. W. Higher School Certificate. Publications: Fryzuk, M . D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. "Binuclear Palladium Complexes with Bridging Hydrides. Unusual Coordination Behavior of LiBEt4 and NaBEu"J- Am. Chem.Soc. 1994,116, 3804-3812. 194 2. Fryzuk, M . D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. "Binuclear Palladium Complexes with Bridging Hydrides. Unusual Coordination Behavior of LiBEt4." /. Am. Chem. Soc. 1991,113, 4332-4334. 3. Fryzuk, M . D.; Hagele, G.; Clentsmith, G. K. B.; Rettig, S. J. "Solution and Solid State Structures of the Binuclear Zerovalent Palladium Complex [Pd(dippe)]2(p-dippe) (dippe = 1, 2-bis(diisopropylphosphino)ethane)." Accepted for publication mOrganometallics . Manuscripts in preparation: Fryzuk, M . D.; Clentsmith, G. K. B.; Rettig, S. J. "Unusual Coordination Behaviour of LiBEt4 to (Ti5-C5H5)2ReH, Cn5-C5H5)2WH2, and (Ti5-C5H 5)2TaH 3. Solid State Structure of (ri5-C5H 5)2TaH«AlH 2(OCH2CH2CH2CH3)." Fryzuk, M . D.; Clentsmith, G. K. B.; Rettig, S. J . "Reactivity of [(dippe)PdMe]{B{3,5-(F3C)2C6H3}4} ( d i p p e = 1 , 2 -bis(di i sopropylphosphino)ethane) . Unusua l Structure of {[Pd(dippe)Me][Pd(dippe)CO]}{B(3,5-(F3C)2C6H3)4}, a Binuclear Complex of Pd(II) and Pd(0)." Fryzuk, M . D.; Clentsmith, G. K. B.; Rettig, S. J. "Cationic Palladium(II) Alkyl Complexes Stabilized by Electron-Rich PN Ligands. Solution and Solid State Structures of Pd(dippim)Me2 (dippim = l-(diisopropylphosphino)isobutylidene tertiary butylamine) and [Pd(dippim)Me]+(B{3,5-(F3C)2C6H3}4)\ Conference papers: Fryzuk, M . D.; Clentsmith. (J. K. B.: Rettig, S. J. "Cationic Palladium(II) Alkyl Complexes Stabilized by Electron-rich Phosphines." Oral presentation. The Xlth F E C H E M Conference on Organometallic Chemistry, Universita Degli Studi di Parma, Parma, Italia (10-15.9.95). 195 

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