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Towards transition metal-catalyzed hydration of olefins, aquo ions, and pyridylphosphine-platinum and… Xie, Yun 1990

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Towards Transition Metal-Catalyzed Hydration of Olefins; Aquo Ions, and Pyridylphosphine-Platinurn and Palladium Complexes By YUNXIE B.Sc. Peking University, 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept this thesis as confirming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA July, 1990 © YUN XIE, 1990 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 The University of British Columbia Vancouver, Canada DE-6 (2/88) ABSTRACT This thesis work resulted from an on-going project in this laboratory focusing on the hydration of olefins, using transition metal complexes as catalysts, with the ultimate aim of achieving catalytic asymmetric hydration, for example: (H02C)CH=CH(C02H) (H02C)CH2-CH(OH)(C02H) * (C = chiral carbon atom). Initially, the hydration of maleic to malic acid, catalyzed by Cr(H 20)6 3 + at 100°C in aqueous solution was studied, including the kinetic dependences on Cr 3 + , maleic acid and pH. A proposed mechanism involving 1:1 complexes of C r 3 + with the maleato and malato monoanions is consistent qualitatively with the kinetic data. This Cr system was, however, ineffective for hydration of prochiral olefins, and the work became a minor component of the thesis and is described in the last chapter. Emphasis was switched to the study of water-soluble phosphine systems based on Pd and Pt. The major part of this thesis describes the synthesis and characterization, principally by lK, 3 1P{1H} and l 9 5Pt{1H} NMR spectroscopies, of: square-planar complexes of the type MX 2(PPh.3.npyn) 2 (M = Pd, Pt; X = halides; n = 1, 2, 3); the binuclear species M 2 X 2 (u-PPh3.npyn)2 (head-to-tail, HT) and Pt2I2(u-PPh3_„pyn)2 (head-to-head, HH; n = 1,10a, n = 2, 10b and n = 3, 10c) ; and the Pt(PPh2py>3,27a, and Pt(Ppy3)3, 26c , complexes. The reactivities of the binuclear complexes toward acetylenes, and the Pt(0) species toward 0 2 , olefins, HC1 and Mel, are also described. With use of PPhpy2 within the binuclear phosphine-bridged species, the P atom incidentally becomes chiral. The diastereomers of 10b were isolated and characterized by 3lp{ lH} NMR spectral data. All the isolated binuclear complexes react in CH 2 C1 2 with dimethylacetylene-dicarboxylate, DMAD, to form an A-frame insertion product. The HH or HT configuration of the precursor is maintained in every case except for 10b and 10c which form initially an HH-DMAD adduct that slowly isomerizes to the corresponding HT-DMAD adduct. Detailed ii 3 IP{ 1H} NMR spectroscopic studies show that the presence of a properly positioned pyridyl group promotes the isomerization by forming a detectable chelated P-N intermediate, and that insertion of DMAD precedes chelation. The reactions of Pt2l2(u-PPh3_npyn)2 (HH) (n = 1, 2, 3) with DMAD in CH2CI2 are kinetically first-order in both [Pt.2] and [DMAD] for the insertion step, and first-order in [Pt2] and zero-order in [DMAD] for the isomerization step. The activation parameters for the insertion step are consistent with oxidative addition to a binuclear system. A proposed mechanism is fully supported by 3 1P{ lU) and 1 9 5 P t { lH] NMR spectral data. Complex 26c, reacts in CH2CI2 or CDCI3 with limited oxygen to give Pt(Ppy3)3(C>2), which may contain an end-on superoxo structure as judged by an IR band at 1114 c m - 1 . Complex 26c, under 1 arm O2, forms the 'expected' peroxo species Pt(Ppy3>202. Complexes 26c and 2 7 a , react with the olefins (maleic anhydride, acrylonitrile, methacrylonitrile and crotonitrile) to give the square-planar species Pt(PPh3-npyn)2Cn2-olefin). The square-planar geometry infers strong rc-back donation from metal to olefin, a state which is probably undesirable for the purpose of olefin activation toward hydration. Indeed, complex Pt(PPh2py)2Cn2-maleic anhydride), 47a , shows no olefin hydration product when heated at 80°C in aqueous NaOH solution. Trans-Pt(H)Cl(PPh2py)2, 50a, was prepared from 27a and gaseous HCl in THF; 50a in acetone-d6, reacts with acrylonitrile to give cis-PtCl(CH2CH2CN)(PPh2py)2» but in the presence of aqueous NaOH at 80°C, 50a was inactive for hydration of acrylonitrile to either (3-cyanoethanol or acrylamide. ui Table of Contents Page Abstract ii Table of contents iv List of tables x List of figures . xii List of schemes xvi Abbreviations and symbols xvii Acknowledgements xxii CHAPTER ONE: INTRODUCTION 1.1 Overview of homogeneous catalysis 1 1.2 Objective of this thesis 2 1.3 Alcohol manufacture and synthesis of asymmetric alcohols 3 1.4 Feasibility of catalytic hydration by transition metal complexes 9 1.4.1 Activation of water 9 1.4.2 Activation of olefins toward nucleophilic attack by hydroxide, and migratory insertion of olefin into an M-OH bond 12 1.4.3 Reductive elimination of alkyl-hydride and alkyl hydroxy moieties 15 1.4.4 Olefin hydration catalyzed by metal phosphine complexes 15 1.4.5 Olefin hydration catalyzed by metal non-phosphine complexes.... 18 1.4.6 A comparison of catalytic hydrocyanation and hydration of olefins; olefin oxidation via hydration 19 1.5 Chemistry of pyridylphosphines 21 1.5.1 Developments in syntheses of pyridylphosphines 21 1.5.2 Coordination chemistry of pyridylphosphines 22 1.5.3 Reactivity of the coordinated pyridylphosphines and catalytic activities of their metal complexes 25 iv 1.6 The scope of this thesis 26 References 28 CHAPTER TWO: GENERAL EXPERIMENTAL PROCEDURES 2.1 Materials 37 2.2 General instrumentation 40 2.3 Preparation of Pd and Pt starring materials 40 2.3.1 Dichlorobis(benzonitrile)-palladium(II) and -platinumCJf) 41 2.3.2 Cis-dichloro(l,5-cyclooctadiene)platinum(II) 41 2.3.3 Tris(dibenzyMeneacetone)dipalladium(0) 41 2.3.4 Bis(dibenzylideneacetone)platinum(0) 42 2.4 Preparation of 2-pyridylphosphines 42 2.4.1 2-(Diphenylphosphino)pyridine 43 2.4.2 Bis(2-pyridyl)phenylphosphine 43 2.4.3 Tris(2-pyridyl)phosphine 44 2.5 Preparation of Pt(D) and Pd(H) complexes of 2-pyridylphosphines 44 2.5.1 Cis-dihalobis(2-pyridylphosphine)platinum(II) 44 2.5.2 Dmalobis(2-pyridylphosphine)palladium(II) 45 2.6 Preparation of homo- and hetero- dinuclear palladium(I) and platinum© complexes of pyridylphosphines 47 2.6.1 Dihalobis(2-pyridylphosphine)diplatinum(T) (head-to-tail, HT)... 47 2.6.2 Duodobis(2-pyridylphospnine)diplatinum(I) (head-to-head, HH) 48 2.6.3 Dihalobis(2-pyridylphosphine)dlpalladium(I) (HT) 50 2.6.4 Diahalobis[tri(2-pvridyl)phospr^ (HT) 52 2.7 Preparation of DMAD A-frame adducts 52 2.8 Preparation of Pt(0) complexes of pyridylphosphines and their derivatives from reactions with olefin, hydrogen chloride and methyl iodide 53 2.8.1 Terrakis[tri(2-pyridyl)phosphine]plarinum(0) 54 2.8.2 Tris[2-(diphenylphosphino)pyridine]platinum(0) 55 v 2.8.3 Olefin derivatives from Pt(0) complexes of pyrdylphosphines.... 55 2.8.3.1 Acrylonitrile, methacrylonitrile and crotonitrile complexes 55 2.8.3.2 Maleic anhydride and diethyl maleate complexes 56 2.8.4 Hydridochloro derivatives from Pt(0) complexes of pyridylphosphines 57 2.8.4.1 Tr^s-cWorohyd^dobis[(2-aUphenylphosphmo)pyridine]-platinum(II) 57 2.8.4.2 Attempted preparation of hydridochlorobis[rri(2-pyridyl)-phosphine]platinum(n) 58 2.8.5 Iodo(methyl) derivatives from Pt(0) complexes of pyridylphosphines 59 2.9 Preparation of bis(maleato) and bis(malato) chromium(JJI) species 59 2.9.1 Cis-potassium (haquobis(maleato)chromate(in) trihydrate 60 2.9.2 Cis-potassium (h^uobis(n^ato)chrormte(IJJ) monohydrate 60 References 61 CHAPTER THREE: STRUCTURES OF M(TJ) AND M(I) (M = Pd, Pt) PYRIDYL-PHOSPHJNE COMPLEXES AND THEIR SOLUTION CHEMISTRY 3.1 Introduction 63 3.2 Structural characterization of pyridylphosphine complexes 66 3.2.1 Pt(II) pyridylphosphine complexes 66 3.2.2 Pd(II) pyridylphosphine complexes 69 3.2.3 Pt(I) dinuclear pyridylphosphine complexes 81 3.2.4 Pd(I) dinuclear pyridylphosphine complexes 83 3.3 Solution equilibrium of the cis-Ptl2(PN2)2 complex in CDCI3 96 3.4 Aqueous solution studies 102 3.4.1 Dichlorobis[tri(2-pyridyl)phospWne]dipalladium(I) 102 3.4.2 Cis-dichlorobis[tri(2-pyridyl)phosphine]paUadium(n) 105 References 112 vi CHAPTER FOUR: REACTIVITIES OF THE HEAD-TO-HEAD ISOMERS OF Pt2l2(H-PN3)2 COMPLEXES TOWARD DIMETHYLACETYLENEDICARBOXYLATE AND TOWARD PHOSPHINE 4.1 Introduction , 116 4.2 Experimental 118 4.3 Results and discussion 120 4.3.1 Reactions of Pt2l2(H-PN3)2 (HT) with acetylenes 120 4.3.2 Reaction of Pt2l2(H-PN3>2 (HH) with DMAD 122 4.3.3 Reaction of Pt2l2(^-PNi)2 (HH) with DMAD 132 4.3.4 Reactions of Pt2l2(u-PN2)2 (HH) with DMAD 137 4.3.4.1 Reaction of Pt2l2(u-PN2)2 (HH) (IS, 2R) with DMAD 137 4.3.4.2 Reaction of Pt2l2(^ -PN2)2 (HH) (IS, 2S)/(1R, 2R) mixture with DMAD 140 4.3.4.3 Kinetic studies on the reactions of the diastereomers of Pt2l2(^-PN2)2 (HH) with DMAD 141 4.3.5 Mechanism of the reaction of Pt2l2(u-pNn)2 (HH) with DMAD. 146 4.4 Reaction of Pt2l2(H-PN3)2 (HH) with P N 3 under air in organic solvents 150 References 159 CHAPTER FIVE: PREPARATIONS AND REACTIVITIES OF PYRIDYLPHOSPHINE PLATINUM(O) COMPLEXES 5.1 Platinum(O) complexes of pyridylphosphines 161 5.1.1 Tetrakis[tris(2-pyridyl)phosphine]platinum(0) 161 5.1.2 Tris[2-(diphenylphospruno)pyridme]platinum(0) 171 5.2 Characterization of olefin complexes of 2-((Hphenylphosphino)pyridine and tris(2-pyridyl)phosphine 174 5.2.1 Acrylonitrile, methacrylonitrile and crotonitrile complexes 174 5.2.2 Maleic anhydride and diethyl maleate complexes 178 vii 5.2.3 An ethylene complex 181 5.3 Reactions of Pt(PNi)3 and Pt(PN3)4 with HC1 183 5.3.1 Reaction of Pt(PNi)3 with HC1 183 5.3.2 Reaction of Pt(PN3)4 with HC1 187 5.4 Reaction of Pt(PN 1)3 and Pt(PN3)4 with methyl iodide 188 5.5 Attempted hydration of olefinic compounds using platinum and palladium pyridylphosphine complexes 188 5.5.1 Hydration of acrylonitrile 188 5.5.2 Hydration of maleic acid 190 References 192 CHAPTER SIX: CATALYTIC HYDRATION OF MALEIC ACID BY CHROMTUM(in) SPECIES 6.1 Kinetic experiments 194 6.2 Results and discussion 197 6.2.1 Spectroscopic properties of Cr(III) and complexation with maleic and malic acid 197 6.2.2 Kinetic results 201 6.2.2.1 Maleic acid dependence 201 6.2.2.2 Catalyst concentration dependence 204 6.2.2.3 pH dependence 206 6.2.3 Mechanisms 207 6.2.4 Other metal ion catalysts and the hydration of other olefinic carboxylic acids 211 6.3 Conclusion 212 References 214 APPENDICIES AI Crystallographic data for Pt2Cl2(^-PN2)2 (HT) (IS, 2S) 216 viii AH Acid-base titration data for Pd2Cl2(u-PN3)2 (HT) and PdCl2(PN3)2 in aqueous solution 219 Am Kinetics of oxidative addition of DMAD to Pt2I2(u-PN3)2 (HT) in CH2C12 221 ATV Kinetics of oxidative addition of DMAD to Pt2I2(u.-PN3)2 (HH) in CH 2C1 2 226 AV Kinetics of isomerization of Pt2l2(u-DMAD)(u-PN3)2(HH) in CH2C12 229 AVI Kinetics of oxidative addition of DMAD to Pt2I2(u-PNi)2 (HH) in CH 2C1 2 232 AVH Kinetics of oxidative addition of DMAD to Pt2I2(u-PN2)2 (HH) (1R, 2R)/(1S, 2S) in C H 2 C 1 2 234 AVUI Kinetics of oxidative addition of DMAD to Pt2l2(n-PN2)2 (HH) (1R, 2S) in C H 2 C 1 2 237 ATX 1.Calculation of the AG 0 and AH 0 values for Eq. 6.1 and the AG values at different temperatures 239 2. Visible absorption spectral parameters of CrCl3-6H20 solutions at various concentrations 239 3. Determination of composition and formation constant of maleato and malato C r 3 + aquo complexes in aqueous solution at 100°C 240 ix List of Tables Page 2.1 Elemental analyses of cis-PtX2(PNn)2 complexes 44 2.2 Elemental analyses of PdX2(PNn>2 complexes 46 2.3 Visible spectral data of PdX2(PNn)2 complexes 46 2.4 Elemental analyses of Pt2X2(p>PNn)2 complexes 48 2.5 Elemental analyses of Pd2X2(p.-PNn)2 (HT) complexes 51 2.6 Visible spectral data of Pd2X2(u-PNn>2 (HT) complexes 51 2.7 Elemental analysis of PtPdX2(u-PN3)2 (HT) complexes 52 2.8 Elemental analyses of DMAD insertion products 53 3.1 3 !p {*H} NMR parameters of PtX2(PNn)2 complexes 67 3.2 3!P{*H} NMR parameters of PdX2(PNn)2 complexes 70 3.3 1 3 C {JH) NMR chemical shifts of PdX2(PNn)2 (n = 1 and 2, X = I; n = 3, X = CI) in CD2CI2 at -20°C 77 3.4 Solid state 31P{ !H} NMR data of PdX2(PN„)2 at ambient temperature 79 3.5 31P{ lH) and 195Pt{ lH} NMR data of Pt2X2(H-PNn)2 (HT) complexes 85 3.6 31P{ *H} and 195pt{ lU} NMR data of Pt2l2(u-PNn)2 (HH) complexes 90 3.7 3 1P{ !H} NMR Data of PtPdX2(u-PN3)2 (HT) complexes 93 3.8 31P{lH) chemical shifts of Pd2X2(n-PNn)2 (HT) complexes 95 3.9 31P{ iH} NMR data of cis-PtX2(PNn)2 species in CDCI3 102 3.10 Chemical analyses of [Pd2(u-PN3)2(OH2)2JX2 (X = BF4, PF6 and BPI14) species.. 103 3.11 JR., 3 1P{ lU} NMR and conductivity data of bis(aquo)dipalladium salts 103 3.12 Visible spectral data of Pd2Cl2(H-PN3)2 (HT) in different solvents 104 4.1 31P{ lH) NMR parameters of DMAD insertion adducts 119 4.2 Kezdy-Swinbourne treatment of the data in Table JV 7 for estimating A«> 125 4.3 First-order analysis of the data in Table ATV 7 by the Guggenheim method. 128 4.4 The observed pseduo first-order rate constants at different [DMAD], x at 25°C in CH2CI2 128 4.5 Temperature dependence of rate constants ki and k 2 in CH 2C1 2 129 4.6 Dependence of kobs on [DMAD] for reaction of Pt2I2(Li-PN3)2 (HT) with DMAD, at l8°CinCH 2 Cl 2 132 4.7 Rate constants of reaction Pt2l2(H-PN3)2 (HT) with DMAD at various temperatures in CH2C12, and the activation parameters 132 4.8 Rate constants of reaction 4.3 at various temperatures and the activation parameters.. 135 5.1 3 1P{ JH} NMR parameters of compounds Pt(PNn)2(T|2-ol) (n = 1, ol = acrylonitrile, methacrylonitrile and crotonitrile; n = 3, ol = acrylonitrile) in CDC13 175 5.2 31p{ in} and *H NMR parameters of Pt(PNn)2(Ti2-ol) (n = 1, ol = MA, DEMA andDEFM;n=3,ol=MA) 179 5.3 Hydration of acrylonitrile catalyzed by Pt and Pd complexes at 80°C 189 6.1 *H NMR spectroscopic analysis of control samples 197 6.2 Visible spectral data of some hydration isomers of CrCl3-6H20 198 6.3 Visible spectral parameters of chromium(IH) oligomers 199 6.4 Initial rate of the catalytic hydration of maleic acid at various [H2MA] 202 6.5 Catalytic conversion of maleic acid to malic acid and fumaric acid 203 6.6 Data for Cr 3 + concentration dependence 205 6.7 pH dependence data 206 6.8 Results of aquation of maleato complexes of Cr 3 + 209 6.9 Hydration products of maleic acid using diaquo-bis(maleato) and -bis(malato) Cr 3 + complexes as catalysts 212 xi List of Figures Page 2.1 Schematic representation of a spectral cell 39 3.1 Relative energy levels of d-orbitals in different geometries 64 3.2 31p{ IH} NMR spectrum of cis-Ptl2(PN2)2 in CD2CI2 at -50°C 68 3.3 Infrared spectrum of the phosphine ligand PN3 in Nujol 72 3.4 Infrared spectra of cis-PtCl2(PN3)2 and cis-PdCl2(PN3)2 on Nujol 73 3.5 1 3C{ *H} NMR spectra of cis-PdCl2(PN3)2, trans-PdI2(PNi)2 and trans-Pdl2(PN2)2 in CD2CI2 at -20°C 76 3.6 Solid-state 3lp{iH} NMR spectrum of PdCl2(PNi>2 at r.t 78 3.7 Solid-state 31P{ *H} NMR spectra of PtCl2(PN3)2 and PdX2(PN3)2 (X = CI, Br) at r.t 80 3.8 The isotopomers of a Pt2X2 (|i-PNn)2(HT) type compound 81 3.9 3 1P{ lH\ and 195Pt{ *H} NMR spectra of Pt2Cl2(n-PN3)2(HT) in CDC13 at r.t 83 3.10 31P{ J H J NMR spectrum of the diastereomers of Pt2Cl2(u-PN2)2 in CDC13 at r.t 84 3.11 3D structure of Pt2Cl2(n-PN2)2 (HT) (IS, 2S) 87 3.12 The isotopomers of a Pt2l2(H-PNn)2 (HH) type compound 88 3.13 31P{ JH} and 195Pt{ ^H) NMR spectra of Pt2I-2(H-PN3)2 (HH) in CDCI3 at r.t 89 3.14 31P{ iH) NMR spectrum of the mixture of diastereomers of Pt2l2(u-PN2)2 (HH) in CD2CI2 at r.t 91 3.15 31P{ iH} NMR spectra of the separated diastereomers of Pt2l2(u.-PN2)2 (HH) in CD2Q2 at different temperatures 92 3.16 3lP{lH} NMR spectrum of PtPdCl2(|i-PN3)2 (HT) in CDC13 at r.t 94 3.17 Variable temperature 31P{!H} NMR spectra of Pd2Cl2(n-PN2)2 (HT) in CD2CI2 97 3.18 Variable temperature 31P{!H} spectra of Pd2I-2(u-PN2)2 (HT) in CD 2C1 2 98 3.19 Low temperature 31P{1H} NMR spectra of cis-Ptl2(PN2)2 in CDC13 99 3.20 3 1P{ JH} NMR spectrum of the reaction product Pt(PN3)2(S)2+ (S = solvent), formed in situ from cis-PtCl2(PN3)2 and AgN0 3 in CH 3CN 101 3.21 The pH-utration curves of Pd2Ch(H-PN3)2 (HT) in 25.0 mL HN0 3 solution 105 3.22 The pH-utration curves of PdCl2(PN3)2 in 10.0 mL H20 106 xii 3.23 3lp{lH} NMR spectrum of PdCl2(PN3)2 in D 2 0 at r.t 107 3.24 The pH-titration curves of PdCl2(PN3)2 in 25.0 mL HNO3 solution 110 3.25 Infrared spectrum of [Pd(PN3)2(H20)(OH)](PF6) in Nujol at r.t I l l 4.1 ORTEP drawing of Pd2Cl2(u-DMAD)(u-PN3)2 117 4.2 31P{ 1H} NMR spectrum of Pt2l2(u-DMAD)(u-PN3)2 (HT) in CDC13 at r.t 121 4.3 31P{ lR) NMR spectrum of Pt2I2(n-MPP)(ii-PN3)2 (HT) in CDC13 at r.t 121 4.4 (a) 31P{ *H} NMR spectrum of the Pt2l2(H-DMAD)(u-PN3)2 (HH) intermediate in CD 2C1 2 at r.t.; (b) 195Pt{ lH] NMR of the intermediate in CD 2C1 2 at -20°C; (c) 31P{ lH} NMR spectrum of the same sample taken 20 h after spectrum (b) was taken 123 4.5 The observed changes in visible spectra for reaction 4.1 in CH2CI2 at 25°C 124 4.6 Pseudo first-order rate plot for the DMAD binding step of reaction 4.1 at 25°C 126 4.7 Estimation of the Aoo value by Kezdy-Swinbourne method 127 4.8 The Guggenheim treatment to give kobs 127 4.9 Pseudo first-order rate plot for the isomerization step of reaction 4.1 inCH2Cl 2 at25°C 130 4.10 Eyring plots for the rate constants of step 1 and step 2 of reaction 4.1 130 4.11 The spectral changes for reaction 4.2 in CH2CI2 at 21°C 131 4.12 Pseudo first-order rate plot for reaction 4.2 in CH2CI2 at 18°C 133 4.13 The Eyring plot for reaction 4.2 133 4.14 3lp{lH} and ^ Pt^H} NMR spectra of Pt2l2(u-DMAD)(n-PNi)2 (HH) in CDC13 at r.t 134 4.15 Pseudo first-order rate plots for reaction 4.3 in CH2CI2 at 25°C 146 4.16 The Eyring plot of the rate constants of reaction 4.3 146 4.17 3lp{ iH} NMR spectra for the products of reactions 4.4 and 4.5 in CDC13 at r.t, 139 4.18 31p{lH} NMR spectra for the products of reactions 4.6 and 4.7 in CDCI3 at r.t 142 4.19 The spectral changes for reactions 4.4 and 4.5 in CH2CI2 at 25°C 144 4.20 Pseudo first-order rate plot for reactions 4.4 and 4.5 in CH2CI2 at 25°C 144 4.21 3lp NMR spectra of the diastereomeric mixture of Pd2l2Qj.-DMAD)(^-PN2)2 and Pt2l2(H-DMAD)(n-PN2)2 in CDCI3 at r.t 145 4.22 Pseudo first-order rate plot for reactions 4.6 and 4.7 in CH2CI2 at 20°C 147 xiii 4.23 The Eyring plot for the rate constants of reaction 4.6 and 4.7 147 4.24 Reaction of Pt2l2(H-PN3)2 in air with one equivalent of PN3, as followed by 31p{ 1H} NMR spectroscopy in CDC13 at r.t 152 4.25 31p{ 1H} and 195Pt{ !H} NMR spectra of in situ formed Pt2l2(H-PN3)2(PN3)2 in CDC13 at r.t 155 4.26 The gradual dissociation and isomerization of Pt2l2(u-PN3)2(PN3)2 in CDCI3 in air to the final product Pt2I2(u.-PN3)2 (HT), as followed by 31P{lU] NMR spectroscopy. 156 4.27 31P{ JH} NMR spectrum of [Pt2(u-PN3)2(PN3)2](BPh4)2 in acetone-d6 at r.t 158 5.1 3 *P {!H} NMR spectra of isolated Pt(PN3)4 in CD2CI2 at various temperatures 163 5.2 195pt{ lH} NMR spectrum of Pt(PN3)4 in CD2CI2 at -45°C 163 5.3 31P{ !H} NMR spectrum, recorded in CDC13 at -45°C, of a freshly isolated but unpurified "Pt(PN3)4" sample by N2H4 reduction of cis-PtCl2(PN3)2 in the absence of extra PN3 ligand 165 5.4 31P{ lH) spectrum, recorded at -10°C, of the same sample of Fig. 5.3, after 48 h in CDCl 3 at -20°C 165 5.5 Infrared spectrum of Pt(PN3)302 in Nujol at r.t 168 5.6 31P{ lH) NMR spectrum in CDC13 at r.t. of an in situ reaction product formed from Pt(PN3)4 with pure oxygen gas 168 5.7 (a) Infrared spectrum of solvent CDCI3; (b) infrared spectrum of Pt(PN3)202 in CDCI3 at r.t 169 5.8 3*P{*H} NMR spectrum, recorded at -50°C, of Pt(PN3)4 in toluene-dg after a week at -20°C 170 5.9 ^P^H} NMR spectrum of Pt(PNi)3 in CD 2C1 2 at -50°C 172 5.10 195Pt{lH} NMR spectrum of Pt(PNi)3 in CDCI3 at -45°C 172 5.11 3 1P{ lH] NMR spectrum of the in situ formed Pt(PNi)4 in CDC13 at -45°C 172 5.12 Decomposition of Pt(PNi)3 in CDCI3 after 48 h at -20°C, as shown by the 31P{ ln) NMR spectrum (CDC1 3 , -20°C) 173 5.13 31P{ lH) NMR spectrum of Pt(PNi)2(Ti2-CH2CHCN) in CDCI3 at 25°C 175 5 14 31P{!H} NMR spectra of Pt(PNi)2(i12-CH2C(CH3)CN) in CDC13 at various temperatures 176 5.15 31p{lH} NMR spectrum of Pt(PNi)2(T|2-(CH3)CHCHCN) in CDC13 at 25°C 177 5.16 *H and 31P{ lK] NMR spectra of Pt(PNi)2(MA) in CDCI3 at r.t 179 xiv 5.17 31P{ lH) NMR spectra of complexes Pt(PNi)2(DEMA) and Pt(PN!)2(DEFM) in CDCI3 • 180 5.18 31P{ lH) NMR spectrum, recorded at -85°C in toluene-ds, of the Pt(0) ethylene adduct Pt(PNi)2(rt2-CH2CH2) formed in situ at r.t 182 5.19 ! H NMR spectrum of trans-Pt(H)Cl(PNi)2 in CDCI3 at r.t 184 5.20 3 1P{ lH} NMR spectrum of trans-Pt(H)Cl(PNi)2 in CDC13 at r.t 184 5.21 *H NMR spectrum of the in situ mixture of trans-Pt(H)Cl(PNi)2 and cis-PtCl(CH2CH2CN)(PNi)2 after lh in CDC1 3 at r.t 190 5.22 3*P{ lH} NMR spectra of in situ, fully formed cis-PtCl(CH2CH2CN)(PNi)2 in CDCI3 190 5.23 Infrared spectrum of "HPtCl(PN3)2" in Nujol at r.t 191 6.1 A typical lH NMR spectrum of the hydration product mixture containing maleic, malic and fumaric acid in acetone-d6 at r.L 196 6.2 The UV/vis absorption spectrum, recorded at 25°C, of Cr(HMA)(H20)5 2 + in H 2 0 200 6.3 The UV/vis absorption spectrum, recorded at 25°C, of Cr(Hmal)(H20)52+ in H20 200 6.4 Plot of A[H2MA]/At vs. [H2MA]i, at the initial stage of the reaction 202 6.5 Plot of In [H 2MA] vs. t 204 6.6 Initial rate of loss of maleic acid as a function of [Cr] 205 6.7 Conversion of maleic acid after 10 h as a function of pH 207 xv List of Schemes Page 1.1 Major processes in industrial alcohol production 4 1.2 Regioselective synthesis of alcohol via hydroboration of alkene 5 1.3 Asymmetric synthesis of an alcohol via a chiral borane 5 1.4 Acohol formation via mercuration of alkene 5 1.5 Asymmetric synthesis of alcohol via a carboxylate complex of Hg(H) 6 1.6 Hydrosilylation of a prochiral ketone catalyzed by a Rh complex 7 1.7 Mechanism of hydrosilylation of a ketone 8 1.8 Examples of enzyme-catalyzed hydration of biologically important olefins 8 1.9 Possible mechanistic pathways in catalytic hydration using a platinum metal complex.... 10 1.10 Interaction between metal and olefin 13 1.11 Possible reactions for a coordinated olefin with nucleophile 13 1.12 The proposed mechanism for catalytic hydration of fluoroethylene by Ru 2 + 19 1.13 Synthetic route for 2-pyridylphosphine via lithiopyridine 21 1.14 Possible coordination modes of pyridylphosphines to transition metals 23 3.1 The equilibria between the species possibly present in CDCI3 for cis-Ptl2(PN2)2 100 3.2 Solution behaviour of the palladium(II) bisaquo species 108 4.1 Expected isomerization products based on a concerted pathway involving phosphorus migration and nitrogen coordination 147 4.2 The proposed mechanism for the oxidative addition of DMAD to Pt2l2(H-PNn)2 (HH) and the subsequent isomerization 149 4.3 Schematic presentation of the isomerization reaction promoted by one equivalent of PN3 153 4.4 Schematic diagram of possible reaction pathway with excess PN 3 157 6.1 The plausible mechanism of maleic acid hydration catalyzed by the Cr 3 + aquo ion 209 xvi ABBREVIATIONS AND SYMBOLS The following list of abbreviations and symbols will be employed in this thesis. A Angstrom (s) (10"10 metre) A absorbance Ar aryl aq. aqueous anal, calcd. analysis calculated atm atmosphere (1 arm = 760 mmHg) br broad, in NMR spectroscopy nBu normal butyl BIN AP 2,2'-bis(diphenylphosphino)-1, l'-dinaphthyl BPPM (2S,4S)-N-(tert-butoxycarbonyl)-4-(diphenylphosphino)-2-[(diphenylphospMno)-memyl]pyrroUdine BPPFOH 1 -[(s)- 17-(diphenylphosphino)ferrocenyl]ethanol BMPP benzylmethylphenylphosphine Ch. chapter COD 1,5-cyclooctadiene cm centimetre cone. concentration °C degree Celsius 1 3 C carbon-13 isotope 1 3C{ lH} proton broad band decoupled carbon-13 NMR spectroscopy CAMP cyclohexylanisolylmemylphosphine Cy cyclohexyl CP cross polarization in solid state NMR spectroscopy d doublet, in NMR spectroscopy xvii dba dibenzylideneacetone DMAD dimeuiylacetylenedicarboxylate, (CH3CX3COCCDOCH3) DEFM diethylfumarate, (trans-C2H500CCH=CHCOOC2H5) DEMA diethylmaleate, (cis-C2Hs<X)CCH=CHCOOC2H5) dppm bis(diphenylphosphino)methane dmpm (dimethylphosphino)methane DMA-HQ N, N'-dimethylacetarnide hydrochloride DIOP 4,5-bis[(diphenylphosphmo)memyl]-2,2-dimethyl-l,3-dioxolane e.e enantiomeric excess e.u. entropy unit, cal moHK-1 en emylenediamine equiv. equivalent Eq. equation Et ethyl (C2H5-) Et20 diethyl ether FT Fourier transform FA fumarate dianion Fig. figure g gram (s) GC gas chromatography HH head-to-head HT head-to-tail H 2MA maleic acid HMA maleate monoanion H2FA fumaric acid HFA fumarate monoanion H2mal malic acid Hmal malate monoanion xviii h hour (s) HEDTA trianion of ethylenecUaminetetraacetic acid Hz Hertz, cycles per second AH* activation enthalpy IR infrared spectroscopy J coupling constant k rate constant K equilibrium constant kobs observed rate constant Kcal kilocalories °K degree Kelvin lit. literature L ligand, litre(s) In natural logarithm MA maleate dianion, or maleic anhydride mal malate dianion M metal, or molarity (moles per liter) m. p. melting point m multiplet, in NMR spectroscopy mg milligram(s) mL inilliliter(s) mmol millimole(s) Me methyl (CH3-) MCPBA m-chloroperbenzoic acid min minute(s) MAS magic angle spinning in solid state NMR spectroscopy MPP methyl propiolate, MeOOCCsCH nm nanometre(s) xix NMR nuclear magnetic resonance spectroscopy ol olefin 3 1 P phosphorus-31 isotope 3 *P {*H} proton broad band decoupled phosphorus NMR spectroscopy 1 9 5Pt platinum-195 isotope 195pt { 1H } proton broad band decoupled platinum NMR spectroscopy Ph phenyl (C6H5-) i-Pr iso-propyl ppm parts per million PPh3 triphenylphosphine PR3 trialkylphosphine py pyridine PNi 2-(cUphenylphosphino)pyridine, PPh2py, P(C6H5)2(2-C5H4N) PN 2 bis(2-pyridyl)phenylphosphine, PPhpy2, P(C6H5)(2-C5H4N)2 PN 3 tris(2-pyridyl)phosphine, Ppy3, P(2-CsH4N)3 PN n (2-pyridyl)phosphines q quartet, in NMR spectroscopy ref. reference RLi organolithium RMgX Grignard reagent \ r.t. room temperature r. d. s rate determining step sh shoulder in UV/visible spectroscopy Sect. section AS* activation entropy s second t time, triplet in NMR spectroscopy temp. temperature xx THF tetrahydrofuran TLC thin layer chromatography TMS tetramethylsilane TCNE tetracyanoethylene UV/vis ultra-violet/visible spectroscopy X wavelength, in nm bridging coordination mode V wavenumber in infrared (cm'1) e extinction coefficient, M^cnr1; or dielectric constant A M equivalent molar conductivity (Q^mor 1cm2) hapticity [ ] concentration {»H} proton broad band decoupled xxi A C K N O W L E D G E M E N T S I wish to thank Professor B. R. James for his guidance and encouragement throughout the preparation of this thesis. I would also l ike to thank the members of this group for their helpful comments and discussions. Sincere gratitude is due to my husband, Gang, whose encouragement and understanding were a constant support throughout the work. The C G P Award (1984 -1985) from the State Education Commission of the People's Republic of China is also acknowledged. xxi i Chapter 1 Introduction 1.1. Overview of homogeneous catalysis Advances in the field of homogeneous catalysis have been particularly impressive since the discovery of the potent alkene hydrogenation catalyst, RhCl(PPh3)3, by Wilkinson's group in 1965. In spite of the problems in separating the product from the catalyst, homogeneous catalysts are utilized in many important industrial processes, including the Wacker process (oxidation of olefin to aldehyde or ketone), the Oxo process (hydroformylation of olefin), the Monsanto process (carbonylation of methanol), and the Ziegler process (olefin polymerization).^  - 5 Growing interests in industrial application of homogeneously catalyzed processes have stimulated research in the area called heterogenization of homogeneous catalysts. The approaches toward heterogenization, which are currently under active study, include direct immobilization of homogeneous catalysts on inert supports and application of phase transfer catalysts. In the former approach, the catalyst is attached to an insoluble support (including inorganic supports, such as silica and zeolites, and organic supports, such as organic polymers). In the second approach, a tetraalkylammonium salt, such as benzyltriethylammonium chloride, is used to transport the catalyst to the organic phase and to keep the catalyst in the aqueous phase when the reaction is complete. By either of these approaches, the major disadvantage of product separation in homogeneous catalysis is compensated by the advantages of heterogeneous catalysis. A new generation of catalysts — heterogenized homogeneous catalysts — could have a much wider industrial application and represent a revolutionary progress in the field of catalysis.*5 - 1 0 The field of homogeneous catalysis ranges from simple acid-base catalysis to more complicated enzyme catalysis. In this thesis, transition metal catalysts are of interest. The principal reasons why transition metals contribute the essential ingredient in a wide range of catalyst systems are: the bonding ability of transition metal to a catholic choice of ligands, the variability of oxidation states accessible by the transition metals, and the variability of 1 coordination sites accessible by the ligands. Delicate combinations of ligand steric and electronic effects influence strongly the structure and reactivity of the catalytically active species. Most reactions in homogeneous catalysis using transition metal complexes can be described by virtue of fundamental reactions of coordination and organometallic chemistry. These reactions, including ligand substitution, oxidative addition, reductive elimination and migratory insertion, occur in a logical sequence bringing about the necessary transformation in a catalytic cycle. Each species in the catalytic cycle obeys, in general, the sixteen- or eighteen-electron rule. These species may not all be detected by spectroscopic methods; however, characterization of the detected species often provides valuable information on the mechanism involved. Knowledge of catalytic mechanism is very essential because it will serve as a guide for tailoring a catalyst intentionally to meet the special need of more sophisticated organic synthesis in the research laboratory as well as in the fine chemical industry 1.2. Ob jec t ive of this thesis As one interest of our group is in bleaching and delignification chemistry of pulp, we became aware of the need for developing new chemistry for the bleaching of high-yield pulp (i.e. containing a large amount of lignin fragment* from chemical pulping) because current CI2-bleaching processes raise concern in the environment as a result of the formation of toxic chloro-organic materials, and because the sulfite-bleached pulp is prone to re-oxidation by air which leads to yellowingt.^ -15 \i j s certainly of interest to study what olefin hydration would do in regard to reduction of the chromophoric group. If the hydration of the C=C bond could be *Basic units of lignin (ref. 15). CH2OH CH2OH CH CH •< II CH CH OH OH 3 p-coumatyl coniferyl alcohol alcohol tOrigin of color: (1) formation of carbonyl-olefinic double bond conjugation; (2) formation of alkali metal complex with chromophoric and auxochromic groups which are the products of oxidation of the phenolic group (ref. 11). The principal objective of pulp bleaching is the reduction or removal of the color constituents (ref. 16). 2 CHjOH CH 11 CH sinapyl alcohol done catalytically, the chemistry could be useful for the utilization of lignin rich pulp in the paper making industry. When this work began in 1985, there had been examples in the literature of olefin hydration to give alcohols catalyzed in solution by both non-phosphine or phosphine transition metal complexes. A few examples are presented here, while a more comprehensive coverage is given in Sect. 1.4. Chromium trichloride hexahydrate (CrCl3-6H20) activates hydration of maleic acid at 170°C.^7 Several a.P-unsaturated nitriles were hydrated using platinum phosphine catalysts in a side-reaction during nitrile hydration to the amide.^"20 Addition of water to non-activated C=C bonds in 1,3-dienes was catalyzed by a palladium(O) triphenylphosphine complex in the presence of CO2.21' 2 2 Jensen and Trogler later claimed successful direct hydration of the C=C bond in both simple olefins and in a,p"-unsaturated nitriles catalyzed by trans-Pt(H)Cl(PMe3)2,2-? although the reproducibility remains controversial.24 The initial objective of this thesis work was to realize catalytic hydration of olefins, particularly activated ones such as fumaric acid, by either non-phosphine or phosphine transition metal complexes. Limited success with some non-phosphine aquo metal ions (Ch. 6) led to the exploration of several pyridylphosphine palladium and platinum complexes (Ch. 3, 4, 5), and indeed the syntheses and characterizations of these pyridylphosphine complexes form the major part of this thesis. Their potential as hydration catalysts of olefins was investigated. 1.3. Alcohol manufacture and synthesis of asymmetric alcohols Current industrial alcohol manufacturing is largely based on the Oxo process and the aldol process, using ethylene or propene as feedstocks; these processes are catalyzed homogeneously by organometallic complexes.2-5 The Oxo process produces straight chain alcohol, while the aldol process gives branched alcohol. 3 Oxo process: RCH=CH 2 + CO + H 2 -^ -R(CH2)2CHO R(CH2)3OH cat = Co, Rh metal complexes Aldol process: 2CH3(CH2)2CHO forc , CH 3CH 2CH 2CH=CCHO + H 2 0 p o t CH 3CH 2CH 2CH=CCHO + 2H 2 — — C H ^ C H ^ C H O ^ O H CH 2 CH 3 CH^CH, cat. = Co, Rh, Ru metal complexes Scheme 1.1. Major processes in industrial alcohol production (ref.25). Another industrial synthesis of an aliphatic alcohol directly from olefin is via hydration catalyzed by solid phosphoric acid (H3PO4) in the vapour phase, the process requiring severe reaction conditions;2*5- 2 7 the corrosion of the reactor, and deactivation of catalyst due to elimination of phosphoric acid during operation, have limited the application of the methods. Research developments in catalytic hydration of olefins on ion-exchanged zeolites (including exchange with metal ions and proton) under mild operating conditions are currently under active investigation by many researchers worldwide and provide a promising alternative to the direct hydration.25" 37 In laboratory organic syntheses, the hydroxyl group can be introduced by either hydroboration or mercuration of alkenes.^ *- & The anti-Markovnikov alcohol is formed from hydroboration of alkene followed by oxidation of the alkylborane intermediate, as illustrated below: 4 3 OIT H OH 1-methyl-cyclopentene trans-2-methyl-cyclopentanol racemic mixture racemic mixture Scheme 1.2. Regioselective synthesis of alcohol via hydroboration of alkene. The addition of diborane to alkene is rapid and quantitative, while oxidation of alkylborane gives highly regioselective alcohol with the OH situated on the less hindered carbon atom. Use of a modified chiral borane, such as a pinene derivative, gives for example 67-70% e.e of exo-norborneol from norbornene after the oxidation:-39'40 Scheme 1.3. Asymmetric synthesis of an alcohol via a chiral borane (ref.40). The example for asymmetric synthesis of 2-butanol from cis-butene given in ref. 40 appears to be erroneous. As an alternative to acid-catalyzed hydration of olefins in solution, mercuration of the alkene, followed by sodium borohydride reduction of the intermediate hydroxyalkyl mercuric salt, produces the Markovnikov alcohol with the OH group on the more sterically hindered carbon atom (Scheme 1A).3S Asymmetric synthesis of an alcohol using modified mercuric salts is less successful, a tartaric acid complex of mercury(U) giving about 32% e.e in hydration of styrene (Scheme 1.5).^  K , R 3 , / — \ +H20 2^ H r2+ R Scheme 1.4. Alcohol formation via mercuration of alkene. 5 H I H H NaBH, C6HsC*HCH3 OH Scheme 1.5. Asymmetric synthesis of alcohol via a carboxylate complex of Hg(U) (ref. 41). Unfortunately, both the hydroboration and mercuration reactions are stoichiometric and have limited uses in laboratory synthesis, and they are not suitable for industrial scale synthesis. Because of the importance of chiral alcohols in the fine chemical industry, as well as in laboratory synthesis of chiral alcohol ligands for asymmetric induction,'*2 asymmetric synthesis of alcohols has been pursued by many research groups using catalytic hydrogenation of ketones4-? - 5 1 or catalytic hydrosilylation of ketones followed by hydrolysis.52"58 Chiral alcohol synthesis with 50% e.e from a prochiral olefin has also been realized via catalytic hydrosilylation of the olefin, followed by stoichiometric oxidation by MCPBA, using dichloro[(R)-N,N-dimethyl-l-[(S)-2-(aUphenylphosphmo)ferrocenyl]emylamme]palladium(II) as the hydrosilylation catalyst^ Homogeneous asymmetric hydrogenation of ketones has generally been less fruitful than for the olefinic substrates in terms of enantiomeric excess and the predictability of product configuration. The optical yields of the alcohol products normally range from 5-40% normally; a few ketones that are reduced with over 80% e.e. have a second functionality, such as keto or hydroxyl, at the a or p* position capable of interacting with the metal centre, although the chelation has not yet been substantiated during the course of hydrogenation. The best homogeneous catalysts60 are complexes of rhodium with l-[(S)-17-(diphenylphosphino)ferro-cenyl]ethanol (BPPFOH)/*9 4,5-bis[(diphenylphosphmo)memyl]-2,2-dimethyl-1,3-dioxolane (DIOP),50 and (2S,4S)-N-(tert-butoxycarbonyl)-4-(diphenylphosphino)-2-[(diphenylphos-phino)methyl]pyrolidine (BPPM),5J and of ruthenium with 2,2'-bis(diphenylphosphino)-l,r-dinaphthyl (BINAP).4-* The understanding of specific catalyst-substrate interactions which give rise to the high enantioselectivity is far from clear. On the other hand, asymmetric reduction of ketone via initial hydrosilylation is better understood. Although PtL*Cl2 [L*= benzylmethyl-6 phenylphosphine, (+)BMPP] was the first catalyst reported to catalyze hydrosilylation of ketonic substrates,52 rhodium is again found to be the most effective metal, giving higher conversion and asymmetric induction. The optical yield for hydrosilylation of simple ketones is, in general, higher than for the corresponding hydrogenation reaction. The results from many studies show that the steric requirements for a match of the chiral ligand, hydrosilane and ketone are of critical importance for success of the asymmetric induction (Scheme 1.6). [(+)BMPPjRh(S)Cl Off PnCOtBu Ph-*CH-tBu HSiMe2Ph OH (S, 54% e.e) P K r n t P K+)BMPP]Rh(S)Cl OH" PhCOtBu Ph-*CH-tBu HSiMe2Et OH (R, 56% e.e) Scheme 1.6. Hydrosilylation of a prochiral ketone catalyzed by a Rh complex, S = solvent (ref.58); the importance of matching the ligand, hydrosilane and ketone is demonstrated by the fact that a change of phenyl to ethyl group on the hydrosilane results in a change of the absolute configuration of the major optical isomer. The catalytic cycle for the asymmetric hydrosilylation of prochiral ketones using Rh complexes involves four basic steps (Scheme 1.7): (a) oxidative addition of hydrosilane to the metal centre, (b) coordination of ketone, (c) insertion of carbonyl into the Si-M bond to form an a-siloxyalkylrhodium hydride, and (d) reductive elimination of the optically active silyl ether.-5*'61 As the stereorelationship of ligand, silane and ketone has become clearer, the enantiomeric excesses have improved; the highest e.e of 97.6% was achieved in the hydrosilylation of acetophenone using a rhodium diamine complex.-55 The rapid developments in catalytic reduction of ketones via hydrogenation or hydrosilylation have perhaps led to an impediment in research on olefin hydration. Further, despite the great amount of work done in the hydrogenation/hydrosilylation areas, the results are still lacking in certain aspects — the rates for ketone reduction are much slower than those for 7 — S i - R h ^ C L 2 ) C l Scheme 1.7. Mechanism of hydrosilylation of a ketone; S = solvent, * L 2 = a chiral, bidentate ligand (modified from ref. 61). olefins, and the number of ketones which give a high optical yield is far smaller than that of olefins.42- 43> 47< 48> 60> 6 1 Direct catalytic hydration of olefins, using transition metal complexes, leading to asymmetric synthesis of alcohols is still well worth pursuing. The only systems, which effect asymmetric hydration of olefins directly, are those of a biologically active enzyme, such as fumarase, aconitase and enoyl-CoA-hydratase (Scheme 1.8).65 Specific substrate-enzyme interactions, are believed to play key role in these systems, although the detailed mechanisms of these hydration processes are not well understood.65 The aconitase system is iron-dependent65 XC O O H fumarase C O O H + H 2 0 = r C H O H H O O C H C H 2 C O O H fumaric acid 1-malic acid H Y C 0 ° H aconitase £ H 2 C O O H C H 2 C O O H + H 2 0 = C H C O O H + H O C C O O H H O O C H 2 C C O O H H O C H C C H 2 C O O H C O O H cis-aconitic acid iso-citric acid citric acid Scheme 1.8. Examples of enzyme-catalyzed hydration of biologically important olefins. 8 1.4. Feasibility of catalytic hydration by transition metal complexes Hydration of olefins is a thermodynamically favourable process64 and, in principle, transition metal complexes capable of activating water and/or olefin can catalyze the conversion of olefin to alcohol.65 Three possible catalytic cycles will be dealt with: the first one (Scheme 1.9a) involves oxidative addition of water by a low valent metal complex, followed by coordination of olefin and then its insertion into the M-OH bond, and finally reductive elimination of the hydroxyalkyl-hydride as alcohol; the second one (Scheme 1.9b) involves coordination of an olefin followed by nucleophilic attack at the olefin by external OH" and then protonation of the metal-carbon bond to give alcohol; alternatively the protonation could go via oxidative addition to give a Pt(II) hydride with subsequent reductive elimination of the hydroxyalkyl-hydride; the third one (Scheme 1.9c) involves oxidative addition of water, insertion of olefin into the M-H bond followed by reductive elimination of alkyl-hydroxyl to form an alcohol. All three possible catalytic cycles will be commented upon in later sections (Sect. 1.4.1-1.4.5), and also comparisons between catalytic hydration, hydrocyanation, and the Wacker process, will be discussed. The following sections (Sect. 1.4.1-1.4.4) are divided according to the four major steps in the catalytic cycles, separate accounts being given on the possibility of the individual step. Following a review of the bond strengths of the M-C and M-0 bonds, and thermodynamic consideration with respect to reductive elimination, a summary of the feasibility of catalytic hydration will be presented at the end of Sect. 1.4.4. 1.4.1 Activation of water Transition metals interact with water molecules in the following fashions: coordination of a water molecule to a metal centre of higher oxidation state promotes the deprotonation of the water (Eq. 1.1), and oxidative addition of a water molecule to a low valent transition metal M(H 2 0) n + 1 • M(OH)"+ + H+ (1.1) M° + H 2 0 • Mn(H)OH (1.2) 9 Scheme 1.9. Possible mechanistic pathways for catalytic hydration using, for example, a platinum metal complex;25 oxidative addition of H2O is written as giving trans species (Sect. 1.4.1). 10 complex gives a hydridohydroxo species (e.g., Eq. 1.2). The chemistry exemplified in Eq. 1.1 has been recognized since the early days of aqueous solution chemistry while that of the latter is relatively new, within last twenty years. In aqueous solution, the proton dissociation constant or hydrolysis constant of a transition metal aquo complex increases several orders of magnitude with respect to that of free water, the higher the charge of the central ion, the more acidic the coordinated water becomes.66 As a result of a strong M-OH interaction, the electron density is largely drawn to the metal and the basicity of hydroxide drops accordingly. On the other hand, oxidative addition of water to low-valent metal complexes is generally believed to promote the basicity of the OH group. Oxidative addition of protic compounds to low-valent transition metal complexes has been known for many years to give a metal hydride species,67 and water has been shown in several cases to add oxidatively to give M(H)OH species, in which the hydroxo group can be coordinated or present as an associated anion. Within the coordinated OH species, both cis- and trans- geometry have been revealed. The cis-hydridohydroxo complexes are more stable mermodynamically;68 however, trans addition products are more evident. Very few cases of cis addition have been reported. The early transition metal complexes of M(H)OH type tend to dimerize and form a bridging oxo species, and are thus less useful for the purpose of hydration. The only example of an early transition metal monomeric M(H)OH species is a hafnium permethylated cyclopentadienyl (Cp*) complex, which is not formed directly from an oxidative addition of H2O but from an exchange reaction of Hf (H)(NH3)Cp* with H 2 O . 6 9 Complexes of the cis-M(H)OH type, formed directly from oxidative addition of H2O, are cis-Os(H)(OH)(PMe3)470 and cis-Ir(H)(OH)(PMe3)4(PF6).6« The iridium compound, which has been characterized crystallographically, is both air and thermally stable and does not eliminate water even at 200°C. 6 8 The more reactive oxidative adduct is of the type trans-M(H)OH. As a result of the strong trans influence of hydride 7 i the nucleophilicity of OH is greatly enhanced, and the basicity and nucleophilicity of the OH group have been recognized in several catalytic processes. Trans oxidative addition of water has been seen for several group Vm transition metal complexes. The Os 3 (CO) i2 complex reacts with H2O at 200°C giving Os3(H)(OH)(CO)io;72 [Rh(en)2]+, electrochemically produced from [RhCl2(en)2]+ in aqueous 11 solution, adds water to give [Rh(H)(OH)(en)2] + , which has been isolated as the tetraphenylborate salt.7-* The rhodium phosphine complexes, Rh(H)[P(i-Pr)3]3 and Rh2(H)2(n-N 2)(PCy3)4 > are capable of activating H2O/D2O via oxidative addition for an H-D exchange process in aromatic compounds via orthometallated species.74 The key intermediate in the Water Gas Shift ( W G S ) reaction catalyzed by Rh(H)(PEt3>3 or Rh(H)(PEt3>4 was found to be [Rh(H)(PEt 3) 3]OH 7 5 The non-phosphine complex, Ru(HEDTA)(CO)- (HEDTA = trianion of ethylenediaminetetraacetic acid), was recently discovered by Khan et al. to catalyze the W G S reaction efficiently under mild conditions (20 ~ 80°C, 1 ~ 35 arm CO) ; kinetic investigations indicate that oxidative addition of water to form trans-Ru(H)(OH)(CO)(HEDTA)- is the rate limiting step. 7 6 The complex trans-[Ru(H)(OH)(S)(PPh3)2] (S = THF, H2O), reported by Wilkinson's group, is however formed by substituting a CI - with O H - ; 7 7 this hydridohydroxo species readily undergoes reductive elimination of H2O under vacuum in dry solvents. The possibility that Ru(CO)2(PPh2py)3 (where py represents a 2-pyridyl substituent) might form a Ru(H)(OH) species was examined by Prystay, but a dihydride formed instead.7 8 The platinum tertiary phosphine derivatives, PtLn [L = P(i-Pr)3, n = 2, 3], catalyze the conversion of CO and H2O to CO2 and H2, and D - H exchange in water via a Pt(H)OH intermediate; hydration of nitriles to amides, and olefins to alcohols, has also been investigated using the same systems. 7 9 _ & ? The IrCl(PCy3)2 complex was found by James et al. to react reversibly with H2O in CH3CN form the unstable Ir(H)(OH)Cl(CH3CN)(PCy3) 2 species.8 4 It is interesting to point out that oxidative addition of water has so far been limited to the second and third row group VILI metal complexes. / 1.4.2. Activation of olefins toward nucleophilic attack by hydroxide, and migratory insertion of olefin into an M - O H bond A n olefin is electron-rich and generally prone to attack by an electrophile, such as H + ' Br+, etc. However, upon complexation to an appropriate electron-deficient metal centre, the olefin ligand becomes subject to nucleophilic attack, because the electron-density of the olefin is now shifted towards the metal centre via a a- M-olefin bond (Scheme 1.10). 12 a-donation from olefin to M d-7t back donation from M to olefin Scheme 1.10. Interactions between metal and olefin. Transition metal complexes capable of activating an olefin must be in a relatively high oxidation state and have a number of associated electron withdrawing ligands, such as carbon monoxide, which will stabilize the negative charge resulting from an external nucleophilic attack.*5 The most characteristic reaction of a coordinated olefin complex is the attack of nucleophile to produce a o-alkyl metal complex. Two basic modes of attack are possible and are distinguishable by both their stereo- and regiochemistry. Trans attack, from the face opposite the metal by a nucleophile (Nuc-) without prior coordination, is commonly observed with the Nuc" attack occurring predominantly at the more substituted olefin terminus:55 Cis attack, by a previously coordinated nucleophile, from the same side as the metal, appears as the net insertion of olefin into the M-Nuc bond. The cis attack normally occurs at the less substituted olefin terminus.*5 In both processes, a major competing reaction is the displacement of olefin by the nucleophile, particularly if the nucleophile is a good ligand for the metal, as illustrated below (Scheme 1.11 J.* 5 MLn MLn R R R T N u c ' M Nuc" displacement 13 Hydroxide has long been perceived as a weak ligand for a late-transition-metal because of the mismatch of hard ligand base with soft metal ac id . 5 6 - 8 7 Before 1986, the success of the Wacker process had been attributed to the weak coordination of hydroxide to Pd, so that displacement of olefin did not compete with OH* attack at the olefin.5* Bryndza and his coworkers have since investigated the relative bond strength of M-H, M-O, M-N and M-C bonds through equilibrium studies and found them to be M-C^) > M-O > M-H > M-C(Sp3) > M-N, 6 4 indicating that the M-O bond is not intrinsically weak Studies on the thermochemistry of the late transition metal hydroxide, alkoxide, and amide complexes further revealed that the bond strength of M-OH was 15 Kcal/mol greater than that of M-OMe.8 9 The stability of the M-OAr bond (M = Pd n , Ni11) with respect to the M-Me bond has been demonstrated by Yamamoto et al. by the preferential insertion of OO into the M-Me bond in some chelating diphosphine complexes.90-91 On the other hand, insertion of CO and/or olefin into the M-OMe rather than the M-Me bond is more common in platinum(U) phosphine complexes.65'92 • 9 4 The higher reactivity of the M-OMe bond compared to the M-Me bond toward CO and/or olefin in these examples has been attributed to kinetic lability.89 In fact, Yamamoto's group reported recently that reaction of CO with mixed alkyl alkoxo (not aryloxo) Pd 1 1 and N i n complexes results in the formation of an alkoxycarbonyl, instead of an acetyl, before reductive eh'mination of an ester takes place.95 These low valent metal alkoxide complexes display high reactivity, quite different from that of the high valent earlier group metal alkoxides.96 Migratory insertion of olefin into a Pt-OH bond was proposed by Jensen and Trogler as one option in a scheme for catalytic 1-hexene hydration, although there was no proof of a formation of a hydroxyalkyl intermediate.25 Tetrafluroethylene has been shown to insert into a Pt-OMe bond,65 but there are no data on reactivity with a Pt-OH species. The thermodynamics of olefin insertion into an M-OH bond are as favourable as those of the uncatalyzed olefin hydration, as inferred by the relative bond strengths of M-OH and M-CH2CH2OH 8 9 The olefin insertion step may not be actually observed in catalysis by spectroscopic methods and the 14 hydroxyalkyl insertion product may not be isolable but, nevertheless, the insertion product is energetically accessible, and therefore a viable catalytic intermediate. 1.4.3. Reductive elimination of alkyl-hydride and alkyl-hydroxy moieties Reductive elimination, the reverse of oxidative addition, is accompanied by a decrease in coordination number and lowering of metal oxidation state, and is of interest because the process leads to the formation of bonds between two ligands; this is particularly important in most homogeneously catalyzed reactions for it frees the organic product at the end of cycle. For reductive elimination to occur, the cis geometry is required; usually the reductive elimination of trans disposed ligands requires a preceding isomerization to a cis disposition. A trans hydrido(cyanomethyl)platinum(II) species, for instance, undergoes isomerization by photolysis before acetonitrile is produced by reductive elimination of the cis hydrido/cyanomethyl groups:97 PPh3 PPh3 I hv I fast H - P t - CH 2 CN H - P t - PPh3 - CH 3 CN + "Pt(PPh3)2" PPh3 CH 2 CN Reductive elimination from hydrido(alkyl) metal complexes has been well documented, whereas that from a hydroxo(alkyl) is very rare. The cis-Pt(H)MeL2 and the cis-Pt(H)(CH2CF3)(PPh3)2 complexes have been shown to ehminate C H 4 and 1,1,1-trifluoroethane at -25°C, respectively.98'99 The relative rate of reductive elimination of hydrido-R is related to the metal-ligand (R) bond strength, and decreases in the order R = C H 3 C O > C H 3 > PI1CH2 > 100,101 it is not surprising that M(H)(R'OH) species should eliminate alcohol more readily than does M(OH)R'. Indeed, as noted in the literature, many hydroxo(alkyl) complexes appear to be very stable^ 9- 1 0 2 the corresponding alkoxo(alkyl) complex is, in certain cases, also resistant to reductive elimination.^05 1.4.4. Olefin hydration catalyzed by metal phosphine complexes The only such transition metal hydration catalysts reported thus far are trans-hydridochlorobis(trimethylphosphine)platinum(II), trans-Pt(H)Cl(PMe3)2 and the tris(iso-15 propyl)phosphmeplatinvjm(0) species, Pt[P(i-Pr)3]3. The former catalyst was claimed to catalyze the hydration of an unsaturated terminal olefin such as 1-hexene to the primary alcohol with a reasonable turnover rate at moderate temperatures.2-3'' It was also found by the same research group, in a study of the catalytic hydration of acrylonitrile to amide, that in a side-reaction the a,P-unsaturated olefinic bond was hydrated at a competitive rate.2-*0 In the presence of base, trans-Pt(H)Cl(PMe3)2 was converted to a hydridoaquobisphosphineplatinum(n) species. The reported turnover rates were 6.9 Ir1 at 60°C for n-hexanol, and 8.3 h' 1 at 100°C for n-dodecanol: Pt(H)Cl(PMe3)2/NaOH (PhCH2)Et3NCl RCH=CH2 RCH2CH2OH R=CH3(CH2)3.60°C R=CH3(CH2)9, 100°C The proposed mechanism is similar to cycle (a) given on page 10, and basically consists of oxidative addition of H2O, coordination of olefin followed by OH - attack at the coordinated olefin to form a hydroxyalkyl complex, and reductive elimination of alcohol; catalyst is regenerated by oxidative addition of H2O to PtL2 (L = PMe3). A kinetic study showed that the rate of olefin hydration was independent of [OH"] when present in excess, and first-order in the alkene concentration. Water replacement by olefin was said to be the rate hmiting step. Deuterium labelling using D2O showed exclusive P-deuterium incorporation, indicative of reductive elimination occurring at the end of cycle; deuterium scrambling or olefin isomerization was not observed, ruling out olefin insertion into the Pt-H bond which is often reversible.-*0-? The complex did not catalyze the hydration of an internal olefin such as 2-hexene, or 3-hexene. The P-hydrogen elimination from hydroxyalkyl, which occurs very readily in oxidation of olefins catalyzed by Pd1 1 to form ketone or aldehyde, does not compete with a trans to cis isomerization step which leads to the formation of alcohol. The primary alcohol formed in the reaction was better accounted for by olefin insertion into the Pt-OH bond, rather than OH external attack which would predominantly produce a secondary alcohol (Sect. 1.4.2). A five-coordinate, square 16 pyramid intermediate may be involved before insertion. A controversy has arisen, however, concerning the olefin insertion step and the question of alcohol formation. The original authors claimed that slow insertion into Pt-H by the olefin did not compete with the insertion into Pt-OH which eventually led to the alcohol formation. On the contrary, some J H and 3 1 P NMR spectroscopic studies conducted by Ramprasad et al. showed that trans-[Pt(H)(PMe3)2(l-hexene)]+ was formed at -54°C and was only stable below -30°C 2 4 Significant decomposition of the hydride/olefin complex occurred at -12°C and resulted in total isomerization of the 1-hexene, with no alcohol being detected. Formation of trans-Pt(H)OH(PMe3)2, a key intermediate in Trogler's cycle, by adding NMe4+OH*-5H20 to trans-[Pt(H)(PMe3)2(l-hexene)]+ at -59°C, was confirmed by *H NMR spectroscopy, but no primary alcohol was formed on gradually warming the mixture to room temperature.24 Yoshida et al. have reported olefin hydration using Pt[P(i-Pr)3]n/H20 (n = 2, 3) systems-^ 8 via a pathway similar to cycle (a) in Scheme 1.9. Although Pt(OH)RL2 has also been reported as a catalyst precursor for the hydration of the olefinic bond in acrylonitrile/9 the poor reproducibility suggested the involvement of a hydride impurity J04 Platinum is so far the only metal with coordinated phosphines showing promise for catalytic hydration. The chemistry related to mechanistic aspects still remains to be explored. Within the three cycles shown in Scheme 1.9, the metal-hydride in cycle (a) seems to be essential for product liberation at the end of catalytic cycle; meanwhile, the presence of both metal hydride and hydroxide initiates a major competition for olefin insertion; in cycle (b), there is no precedence for an PtRL2 complex to undergo protonation of the alkyl by a H2O molecule; in order to complete the catalytic cycle, an extra step may be required to cleave the Pt-C bond;58 in cycle (c), the major concern is that the Pt(OH)RL2 complex is probably stable to reductive eUmination/9'64>101 The challenge of finding an effective catalyst remains. 17 1.4.5. Olefin hydration catalyzed by metal non-phosphine complexes Catalytic activation of olefinic substrates by non-phosphine transition metal complexes was pioneered by Halpem and coworkers in the early 1960s, when maleic, fumaric and acrylic acids were catalytically hydrogenated in aqueous acid solutions containing chlororuthenate(II) species.-*05 The formation of a Run-olefin complex, which later reacted with hydrogen, was demonstrated; in the case of maleic acid, a 1:1 olefin complex was confirmed spectrophotometrically and the stability constant was measured to be 5x103 M* 1 (aq. 3M HCl, 20°C). Further studies^0*5 using D2 and D2O demonstrated that the hydrogen atoms which added to the double bond originated from the aqueous solvent rather than hydrogen gas. However, no hydration product was revealed. The same group reported the catalytic hydration of acetylenic compounds by Rum chlorides in aqueous acid solution.^07 Acetylene, methylacetylene and ethylacetylene were converted under mild conditions to acetylaldehyde, acetone and methyl ethyl ketone, respectively. In the mechanism proposed for hydration of acetylene, a coordinated hydroxo ligand underwent migratory insertion into the coordinated acetylene, and the resulting hydroxyalkene group was cleaved off Rufflby a proton to give the corresponding product, 'vinyl alcohol', which rearranged to acetaldehyde.^07 Aqueous acid solutions of some rhodium(IU) chloro complexes, [Rh(H20)6-nCln]^n"3^", were also found later to be active for acetylene hydration.^08 Hydration of fluoroolefins was also observed in the aqueous Ru^3M HCl system, under conditions similar to those for hydrogenation of the olefinic acids; hydration was thus competitive with hydrogenation of the C=C bond in fluoro- or 1,1-difluoroethylene even under 1 atm H2, so that only acetaldehyde and acetic acid, respectively, were detected following hydrolysis of the hydroxyfluorides.^09 Thus, chlororuthenate(II) species can function as a hydration catalyst for fluoroethylene as well as a hydrogenation catalyst for olefinic acids. Scheme 1.12 was the suggested mechanism of the olefin hydration process/*09 18 OH F H(F) H slow OHR H(F) H fast at high [olefin] H Q , , F OH + H3C- C- F H(F) Scheme 1.12. The proposed mechanism for catalytic hydration of fluoroethylene by Ru2-*.109 The unique reactivity of fluoroethylenes in this Ru(U) system is not fully understood, but the suggestions were that fluoroolefins stabilize Ru n sufficiently to prevent the reduction (e.g. under H2) to metal, while the high electronegativity of fluorine promotes the nucleophilicity of the coordinated OH which attacks the olefin bond leading to a P-hydroxyalkyl intermediate. This reaction is a true olefin hydration catalyzed by a non-phosphine transition metal complex. Another example found in the literature in the late 1960s is that reported by Bzhasso and Pyatnitskii who found that CrCl3-6H 20 catalyzes the hydration of maleic acid at 170°C.^7 Formation of a [C-Cr]3 + cation analogous to a carbonium ion was suggested to play a crucial role in the catalysis. Matching a metal complex catalyst with a substrate under suitable conditions was the initial major aim of this thesis project, although the goal has not yet been achieved. In Chapter 6, a kinetic study of the Cr3+/maleic acid system is described. 1.4.6. A comparison of catalytic hydrocyanation and hydration of olefins, and olefin oxidation via hydration Hydrogen cyanide has a bond dissociation energy of 123 Kcal/mol, which is very close to that of water, 119 Kcal/mol,^0 and it is beneficial to compare activation of hydrogen cyanide with activation of water. Hydrocyanation is one of the most successful homogeneously catalyzed reactions in chemical industry, the catalysis utilizing Ni(0) phosphite and Pd(0) phosphine 19 complexes. Asymmetric induction has been achieved in a Pd(DIOP)2 system with up to 40% e.e.111 The mechanism of catalytic hydrocyanation using nickel complexes has been reported recently in detail/ 7 2 - 1 1 4 whereas studies on the Pd-catalyzed reaction are still at a preliminary stage. Some similarities between systems using Ni and Pd catalysts have been revealed/72 Nickel-catalyzed hydrocyanation has been shown to involve oxidative addition of hydrogen cyanide, coordination of olefin followed by olefin insertion into the Ni-H bond, and reductive elimination of alkyl cyanide.772 - 1 1 6 The relative order of oxidative addition and olefin coordination may be reversed depending on the alkene; isomerization of olefin often takes place in the olefin insertion step because of its reversibility which may lead to the formation of linear and branched nitriles. Addition of a Lewis acid has been demonstrated to promote the formation of the linear nitrile.775 Oxidative addition of HCN is shown to be cis, and olefin is shown to insert into the M-H bond exclusively/75 The reductive elimination of alkyl cyanide is facile. Oxidative addition of H2O can give either cis or trans hydridohydroxo species. There is little information on the reactivity of the cis type toward olefin insertion, while in the trans type olefin appears to insert readily into an M-OH bond.25 Although insertion of olefin into the Pt-H bond of Pt(H)OH species was observed by Ramprasad et al., no reductive elirnination of alkyl-hydroxide to give alcohol formation was detected.24 Oxidation of olefins to aldehyde or ketones has been catalyzed successfully by a Pd n/02/CuCl 2 system in the Wacker process. Olefin is hydroxylated but, because of the (3-hydride elimination, no alcohol is formed. The order of olefin reactivity is CH2=CH2 > RCH=CH2 > RiCH=CHR2, gerninal di-substituted, tri-substituted and tetra-substituted olefins, and electrophilic olefins generally do not coordinate sufficiently well to permit further chemistry/77 The olefin, once coordinated to palladium, is generally subject to nucleophilic attack by OH- or H 2 0 , which occurs rapidly at the more substituted position. The a-alkylpalladium species then formed is unstable and undergoes rapid, spontaneous P-hydride elirnination to form an OH-substituted olefin which isomerizes to ketone. The Pd(0) formed after P-hydride elimination is oxidized to Pd n by O2/CUCI2 and is continually used as catalyst. The 20 success of the Wacker process for formation of acetaldehyde from ethylene indicates that alcohol formation in this system is not attainable. 1.5. Chemistry of pyridylphosphines 1.5.1. Developments in synthesis of pyridylphosphines Phosphines with substituted benzene rings and heteroaromatic groups, such as pyridine, were first reported by Davis and Mann,118 and the synthetic strategy was further developed by M a n n and W a t s o n . ^ 9 The overall yield of pyridylphosphine production, PPh3. n py n (abbreviated as PN„, n = 1,2,3; py represents the 2-py moiety throughout this thesis), was very poor because the use of a Grignard reagent created problems in product separation, so that extensive purification procedures including vacuum distillation were required. The introduction of 2-Uthiopyridine by Wibaut et a l . ^ 2 0 provided an alternative route for pyridylphosphine synthesis. A revised experimental procedure using hthiopyridine was laid out by Plazek and T y W 2 - * and has been adopted with minor modifications in more recent years by several research groups.-* 2 2" -*2-* Scheme 1.13. Synthetic route for 2-pyridylphosphines via hthiopyridine. Because of the instability of Hthiopyridine above -60°C, the yield of this one pot synthesis is normally around 40 ~ 50%. PPh2py (PNi) and PPhpy2 (PN2) can also be made via a Uthiophenylphosphine route, which is sometimes complicated by the coupling reaction of metallated phosphine; nevertheless, the yields are often comparable with those using lithiopyridine. UtMophenylphosphine can be prepared by reacting lithium metal with P P h 3 , ^ 2 6 -PPI12CI or PPhCl2 J 2 7 , or from reaction of methyllithium with diphenylphosphine, i 2 S as in equations (1.3) to (1.6) PPh2py, PPhpy2, Ppya. n= 1 n = 2 n = 3 21 PPh3 + 2Li -PPh 2 Li + PhLi (1.3) PPh2Cl +2Li ~PPh 2Li + LiCl (1.4) PPhCl2 + 4Li -PPhLi 2 + 2LiCl (1.5) PPh2H + MeLi -PPh 2 Li + CH4 (1.6) PPh3 + Na/NH3 «• PPh2Na + PhNH2 (1.7) Tolmachev et al. reported that triphenylphosphine could be metallated using sodium in liquid ammonia, Eq. (1.7), with the yield of PPh2py being 50~60%.729 A minor modification of the experimental conditions of Plazek and Tyka's method was used in the present work, giving an average yield of 40~56% for the three phosphines (Ch. 2). 1.5.2. Coordination chemistry of pyridylphosphines Since a 1965 breakthrough in the field of homogeneous catalysis made by Wilkinson's group, who used RhCl(PPh3)3 as a catalyst to hydrogenate alkenes at 25°C and latm H 2 in organic solvents, attention has focused on tertiary phosphines as ligands in catalyst design. Phosphines are, in fact, found to be the most popular ligands in many catalytic systems, and it is not surprising that phosphine chemistry has flourished ever since. Triphenylphosphine is the most exhaustively studied phosphine; 2-pyridylphosphines, PPh3.npyn> appear to be a good choice for systematic studies on the electronic effect of pyridine on the phosphine, because the geometry of a pyridylphosphine remains relatively constant with respect to triphenyl-phosphine.^0 Equally important and significant are the heteropolydentate characteristics of these pyridylphosphines, which could be highly desirable in areas such as catalysis, asymmetric synthesis and organometallic stereochemistry.^ -?^ Pyridylphosphine, as a heteropolydentate ligand, is more attractive perhaps than the chelating diphosphines and polyphosphines because the binding characteristic of phosphorus and nitrogen allow for a more rationalized design of a catalyst. Pyridylphosphines, however, received little attention until the late 1970s and, in fact, the X-ray crystallographic structure of Ppy3 was only done in 1988,7-?2 forty-four years after it was 22 first made. The binding options offered by these 2-pyridylphosphines are shown in the following diagram (Scheme 1.14). I m —[:p—M ; p - ^ N 0 i ffV II rv v vi Scheme 1.14. Possible coordination modes of pyridylphosphines to transition metals. Transition metals (Group VTJB -VTJI, IB) form complexes with 2-pyridylphosphines of the various structures I - V shown in Scheme 1.14. Although there is no example of an isolated type VI compound reported in the literature, this coordination mode is nevertheless possible from electronic and steric points of v iew/ 2 2 - 1 3 3 and it has been proposed to coexist in solution with type I (square planar) and type II (square pyramid) complexes in the case of the trans-RhCl2(CO)(PN2) and trans-RhCl2(CO)(PN3).^ The most common coordination mode among the second and the third row transition metal 2-pyridylphosphine complexes is type I; i.e. only the phosphorus donor coordinates, for example, Mo(CO)4(PNi)2,^ PdX2(PNn)2 (n = \ \ 1 2 8 n = 2 and 3, Ch. 3), and PtX2(PN„)2 (n = 1 ; 1 3 5 n =2 and 3, Ch. 3). Cu, Ag and Au form M(X)PN n species (X = halides) which have unusually short M-P distances; under suitable conditions M(X)P is believed to aggregate to form a cluster.7-36 The chelating coordination type II is found in some isolated Pt1 1 ([PtI(PNi) 2]PF 6),7 5 5 RuCl2(CO) 2(PNi)^ 7 and U 1 1 1 (U(BH4)3(PNi)2) complexes,^8 and also is present in solution for several type I complexes as indicated by 3 1 P NMR data (Ch. 3). The U 1 1 1 complex is shown by X-ray crystallography to be 13-coordinate with three BH4" ligands being tridentate and two chelating phosphines being 23 coplanar with one of the boron atoms/5 5 Tetrahedral CoCl2(PN2) is a type Ul compound, the phosphorus lone-pair not interacting with the central metal atom as shown by crystallographic data/ 5 9 The unsymmetrical bridging mode in type IV is another common coordination mode for both hetero and homo binuclear complexes; examples include Pd2X2(PNn)2 (Ch. 3), PdMoQi-CO)(CO)2(H-PNi) 2 / 5 4 Re2Cl4(u-PNi)3^0, RhPtCl3(CO)(PNi) (head-to-tail, HT)141, Rh2Cl2(u-CO)(u-PNi)2 (HT)142, and RhPdCl3(CO)(u-PNi)2 (HT)/ 4 5 Because of the different softness of phosphorus and nitrogen donors, Mi and M2 can be linked together even if they are of different hardness. The tridentate mode via three nitrogen atoms in type V is common among the first row transition divalent metals, as in [M(T|3-PN3)2](C104)2/22,144 and a Ru n complex can also be formed of the same structure/24 The interesting coordination chemistry of these phosphine ligands has been extended to their oxides of both the phosphine and pyridine. Phosphine oxides have found practical applications in selective extraction processes for the platinum metals/4-5 Formation of (OPNi)PtBr4, containing a five-member chelating structure as in (a)/ 4 6 demonstrates the affinity of pyridylphosphine oxides for platinum in a high oxidation state. Pyridylphosphine P, N-oxide is a 1,3-bifunctionalized ligand as in (b), structurally similar to ligands containing monophosphoryl and carbonyl groups, or carbonyl and phosphine oxide groups which are good extractant ligands for lanthanide and actinide ions/ 4 7 > 1 4 8 The ligand OP(Ph)2(Opy) was made in an attempt to replace these extractants so that selective extraction of uranium as UC>2(N03)2[OP(Ph)2(Opy)] could be carried out. The structure of the uranium complex contains a six-member ring arranged in a distorted boat form/ 4 9 0 5-member ring (a) 24 1.5.3. Reactivity of the coordinated pyridylphosphines and catalytic activities of their metal complexes The pyridylphosphine complexes are also interesting in their distinctive reactivity. The triply bonded dirhenium(U) complex Re2Cl4(PNi)3 readily undergoes HC1 elirnination in the presence of base to give the ortho-metallated complex Re2Cl3(PNi)2[PPh(o-C0il4)py];7-?9'140 the base can be the 2-(diphenylphosphino) pyridine ligand itself. This is reported to be the first example of ortho-metallation occurring at a metal-metal multiple bond. When Ru3(CO)n(PNi) is heated under reflux in methanol, the spontaneous cleavage of a phosphorus-carbon bond of 2-(diphenylphosphino)pyridine takes place to give a doubly bridged phosphido ligand with the pyridyl nitrogen coordinated to the third ruthenium, while a bridging benzoyl group is formed by migratory insertion of a phenyl group onto a terminal carbonyl group/ 5 0 A similar phosphorus-carbon bond cleavage reaction for triphenylphosphine occurs, but at a much higher temperature, at the boiling point of xylene for some Mn, Rh and Os clusters/-57 A few catalytic aspects of pyridylphosphine complexes have also been investigated, but mechanistic studies of the catalysis are non-existent. The catalyst precursor in most cases is formed in situ; for instance, when a mixture of Pd(OAc)2, PNi, p-Me-C6H4-S03H, and propyne is heated at 45°C in methanol under 60 bar CO pressure, methylmethacrylate was obtained with 99% selectivity at a very high turnover rate (20,000 h ' 1 ) / 5 2 Catalytic homologation from methanol to ethanol can also be achieved at 155~180°C under 200~300 bar syn-gas (CO:H2 = 1:1) pressure using a ruthenium pyridylphosphine catalyst, the selectivity being over 30% with 60 ~ 74% conversion/55 A rhodium complex containing tripyridylphosphine formed in situ from Rh2(CH3COO)4 catalyzes the conversion of a water and carbon monoxide mixture to hydrogen and carbon dioxide, the water gas shift (WGS) reaction/54 The only example of a catalytic process using a well-defined catalyst precursor, RhH(CO)(PPh3)(PN3)2, is the hydroformylation of 1-hexene at low CO and H2 pressures in the presence of excess PN 3; no mechanistic detail was given by the authors/25 25 Pyridylphosphine complexes thus exhibit many differences in their coordination chemistry, reactivity and catalytic activity from those of the triphenylphosphine analogs. In this thesis, efforts in revealing these aspects of pyridylphosphine complexes are presented. 1.6. Scope of th is thesis The water solubility of the tris-2-pyridylphosphine palladium complex, Pd2Cl2(H-PN3)2 (HT) 11c, appeared to be an asset for the complex as a candidate for a hydration catalyst, and the complex attracted our attention since it was discovered earlier in our group by Lee and Yang.7-5-5 To our knowledge, few neutral tertiary phosphine complexes, excluding those with highly polar substituents (-SO3H, -CO2H, -OH, and -NH2) in the phosphine molecules,75*5 are soluble in water.74- 7 5 • 7 9 - 8 4 • 1 5 7 Cis-PdCl2(PN3)2 was later found in the present work to have even better solubility in water than the binuclear complex. The solution behaviours are discussed in Chapter 3. Some monomeric and binuclear metal pyridylphosphine complexes of palladium and platinum were synthesized via methods based on literature procedures for 2-(diphenylphosphino)pyridine derivatives described in Chapter 2, and structurally characterized by 3 1P{ 1H), 3lC{1H} and *H NMR spectroscopic methods (Ch. 3). The molecule structure of Pt 2 Cl2(u-PN 2 )2 (HT) (IS, 2S) was determined crystallographically (Ch. 3). The diastereoisomers Pt2l2(H-PN2)2 (HH) (IS, 2S) and Pt2l2(u.-PN2)2 ( H H ) < 1 S» 2 R ) w e r e successfully separated by preparative TLC (Ch. 2) but, unfortunately, the high symmetry of the single crystals grown precluded crystallographic characterization. None of the M(I) or M(II) pyridylphosphine halide complexes binds olefin in organic solvents or in aqueous solution. The binuclear head-to-head (HH) or head-to-tail (HT) complexes MiM2X2(H-PNn)2 (Mi = M2 = Pd, or Pt; Mi = Pd, M2 = Pt; X = CI, Br, I; n = 1, 2, 3) form a bridging adduct with dimethylacetylenedicarboxylate (DMAD). The isolated products bear the HT-configuration in most cases, except in Pt2l2(M-"PNi)2 (HH) where the HH-configuration is maintained (Ch. 4). Kinetic and spectroscopic studies on the reactions of Pt2l2(H-PNn)2 (HH) (n = 1, 2, 3) with DMAD are presented in Chapter 4: a five-coordinate Pt(H) intermediate with chelating pyridine is 26 proposed based on 3 1 P { lK] N M R data, while a destabilization effect of a non-bridging pyridine substituent plays a crucial role in isomerization of the H H D M A D adduct to the H T adduct. Isomerization of the H H to H T form of the binuclear complex Pt2l2(M-- p N3)2 in the presence of excess phosphine PN3 was also investigated by 3 1 P { !H} N M R in some detail, and the findings are included in Chapter 4. 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Catal., 1980,8, 369. 157. (a) KuUberg, M.L.; Kubiak, CP. Organometallics, 1984,3, 632. (b) Kullberg, M.L.; Kukiak, CP. C; Mol. Chem., 1984,1, 171. (c) Kullerg, M.L.; Lemke, F.R.; Powell, D.R., Kubiak, CP. Inorg. Chem., 1985,4, 3589. 36 Chapter 2 General Experimental Procedures 2.1. Materials Spectral or reagent grade solvents, such as dichloromethane, benzonitrile, and diethyl ether, etc., were obtained from the commercial suppliers, Aldrich, Eastman, Fisher, or BDH. Where necessary, dichloromethane was dried over phosphorus pentoxide and distilled under nitrogen prior to use. Hexanes, THF, benzene and toluene were dried with sodium/ benzophenone ketyl and distilled just before use. Chloroform was purified by passing through an Alumina (neutral, Fisher Scientific) column. Methanol and ethanol were refluxed over alkoxides formed by reaction of Mg ftrrrrings with alcohol initiated by trace amount of I2, and were distilled under nitrogen. Deuterated solvents used in the present study, acetone-d6, chloroform-di, toluene-ds, dichloromethane-d2, and benzene-d6, were obtained from Merck Frosst Canada Inc., and degassed by three to six "freeze-pump-thaw" cycles for anaerobic NMR studies. Anhydrous HCl(g) was supplied by BDH Chemical Co. Ethylene (CP Grade) was used as supplied from Matheson Gas Co. Argon, nitrogen, and oxygen (99.99%) were supplied by Union Carbide of Canada Ltd. All gases were used without purification. Platinum and palladium were supplied on loan from Johnson Matthey Ltd. as (PtCl2)n, K2PtCl4 and (PdCl2)n- Chromium trichloride hexahydrate, CrCl3-6H20, was purchased from Aldrich Chemical Co. Potassium dichromate, K2CT207 (Aldrich), was used as supplied. Diphenylchlorophosphine (PPh2Cl) and dichlorophenylphosphine (PPhCl2) were purchased from Aldrich chemical Co. and distilled before use. Phosphorus trichloride, PCI3, was supplied by Mallinckrodt chemical Co., and was purified by distillation at 70 - 72°C after 30 min of vigorous reflux under N 2 . 2-Bromopyridine, purchased from Aldrich, was purified by 37 stirring with NaOH pellets overnight at room temperature and then distilling from CaO under vacuum prior to use. 2-Chloropyridine (Aldrich Chemical Co.) was used without purification. Dimethylacetylenedicarboxylate, DMAD, supplied by Aldrich, was distilled under vacuum for kinetic studies only. Acrylonitrile, methacrylonitrile, crotonitrile and diethyl maleate (Aldrich) were distilled over CaO under reduced pressure and kept in the fridge. n-Butyllithium (1.60 M in hexane) and methyllithium (1.56 M in ether) were used as supplied from Aldrich. Dibenzylideneacetone, dba, was synthesized by a literature method.7 Anhydrous hydrazine and the hydrate (reagent grades) were supplied by MCB Co. and used without further purification. Maleic acid and fumaric acid, purchased from Fisher Scientific Co., were recrystallized twice from water. Aconitic acid, mesaconic acid, R, S-malic acid and maleic anhydride were supplied by Aldrich and used without further purification. 2.2. G e n e r a l ins t rumenta t ion Visible absorption spectra were recorded on a thermostated (± 0.1°C) Perkin-Elmer 552A spectrophotometer, using spectral quartz cells of 1 or 10 mm path length. Absorbance data for kinetic experiments were obtained either from the digital display on the instrument at the wavelength of interest, or taken from the repetitive scanning spectra. Times were recorded either directly from a Lab-chron 1400 timer for faster reactions (10 - 20 s between readings), or by a built-in timer for slower reaction (3 to 90 min between readings). For recording visible spectra under anaerobic condition, the one type of cell shown in Fig. 2.1 was employed. The details of sample preparation and time recording for the kinetic studies will be described in Chapter 4, Sect. 4.2. Infrared spectra were recorded on a Nicolet 5DX-FT or a Perkin-Elmer 598 spectrophotometer calibrated with the 1601 cnr 1 peak of polystyrene film. Nujol mulls between KBr or Csl plates were normally used for solid state IR, while KRS-5 plates (42% TlBr + 58% HI, Harshaw Chem. Co.) were used for the cis-PdCl2(PN3)2 and cis-PtCl2(PN3)2 species. A KBr solution cell with path length 0.1 mm was used for solution IR samples. 38 High vacuum Teflon stop-cock / \ Fig. 2.1. Schematic representation of a spectral cell. Nuclear magnetic resonance spectra were obtained on a Varian XL-300 MHz spectrometer using 5 mm NMR tubes. JH chemical shifts were recorded in ppm relative to external TMS, and 1 3 C chemical shifts were relative to external TMS or the CDCI3 solvent peak at 77.1 ppm. All 3 1 P chemical shifts, both in solution and in solid state, were referenced to external 85% H3PO4. The 1 9 5Pt chemical shifts were referenced to the absolute resonance frequency of 195Pt which itself is relative to the lU resonance of TMS; at 100 MHz, HPt = 21.4 MHz, 2' 3 or 64.2 MHz on the Varian XL-300 spectrometer. This Pt reference is -4535 ppm upfield from the PtCl62" standard4 which is frequendy cited in the literature. Solid state 31P{ lH) NMR spectra were recorded on a Braker CXL-100 MHz instrument using a cross polarization and magic angle spinning FT programme. The high-frequency-positive convention, recommended by RJPAC, has been used in reporting all chemical shifts. Simulation of 3 1P and 39 1 9 5 Pt NMR spectra were performed using the Bruker software PANIC85 on BDS-1000, or UBCPANIC on ASPECT-2000. The fit of the simulation was based on matching the individual peak positions of the experimental and calculated spectra. Melting points of phosphine ligands were determined using a 6548-J17 microscope equipped with a Thomas model 40 micro, hot stage, melting point apparatus. GC analyses for organic compounds (in Ch. 5) were performed with an Hewlett-Packard 5890A instrument using a thermal conductivity detector and a 6-ft Porapak-Q column. Conductivity measurements were recorded in a cell (Fisher Scientific Co., cell constant 1.0) connected to a Beckman Serfass conductance bridge. pH-Titrations were carried out in aqueous media using a Corning 12 pH-meter and a combination glass electrode; care was taken to ensure that each reading recorded represented equilibrium conditions. The atmosphere was kept C02-free by bubbling presaturated N 2 through the solution. The temperature was maintained constant within 0.2°C by a water-bath equipped with a thermocouple. Elemental analyses were performed by Mr. P. Borda of this department. The X-ray , crystal structure determination of Pt2Cl2(|X-PN2)2 (HT) was carried out by Dr. S.J. Rettig of this department. 2.3. P repara t ion of P t and P d star t ing mater ia ls 2.3.1. Dichlorobis(benzonitrUe)-paUadium(n) and -platinum(U) The precursors PdCl2(PhCN)2 and PtCl2(PhCN)2 were prepared by standard methods described in the literature 5 using (PdCl2)n and (PtCl2)n, respectively. The PtCl2(PhCN)2 compound was also synthesized from K2PtCl4 by reaction with benzonitrile in a biphasic system. Some K2PtCU (2.0 g, 4.8 mmol) was dissolved in a minimum amount of water to which 30 mL of benzonitrile was added, and the mixture was then heated on a steam bath until the aqueous layer turned colourless. The yellow benzonitrile solution of PtCl2(PhCN)2 was separated from the aqueous layer while still hot and then left at 40 -20°C overnight. The yellow crystals that separated were collected by filtration, washed carefully with Et20, and dried in vacuo at 80°C; yield 1.2 g (55%). Anal, calcd. for Ci4HioN2Cl2Pt: C 35.61, H 2.13, N 5.93; found: C 35.77, H 2.11, N 5.96. 2.3.2. Cis-dichloro(l, 5-cyclc«x:tadiene)platinum(ir) The cis-PtCl2(COD) complex was prepared by a published procedure,6 the purity being ascertained by elemental analysis; yield 76%. Anal, calcd, for C8Hi 2 Cl 2 Pt: C 25.68, H 3.23; found: C 26.02, H 3.45. 2.3.3. Tris(dibenzyUdeneacetone)dipalladium(0), Pd2(dba)3 The title compound, Pd2(dba)3, was prepared by a modified literature procedure;7 the benzonitrile precursor was used instead of [PdCl2]n. To a reaction flask containing 150 mL MeOH was added PdCl2(PhCN)2 (2.3 g, 6 mmol), dba (4.6 g, 19.0 mmol) and sodium acetate (3.9 g, 47.5 mmol). The mixture was refluxed for 1 h and then cooled to r.t. The precipitated product was filtered, washed with H 2 0 and acetone successively, and dried in vacuo to give a reddish purple solid; yield 4.0 g (73%). This compound was redissolved in hot CHCI3 (130 mL) and reprecipitated with diethyl ether (100 mL) to yield the solvated, dark purple, microcrystalline compound Pd2(dba)3-CHCl3. Anal, calcd. for C 5 i H 4 2 0 3 P d 2 CHCI3 : C 60.34, H 4.19; found: C 59.87, H 4.01. 2.3.4. Bis(dibenzyhdeneacetone)platmum(0), Pt(dba)2 The Pt(dba)2 complex was prepared according to a literature procedure.8 A hot aqueous solution (5 mL) of K2PtCU (1.0 g, 2.4 mmol) was added to a well stirred, refluxing solution of dba (1.6 g, 6.7 mmol) and sodium acetate (2.6 g, 32 mmol) in EtOH (50 mL) under N 2 . The solution rapidly turned dark green and was left refluxing for 5 h (10 min in the original procedure). The mixture was cooled and the resulting black precipitate was filtered off. The crude product was run through a silica gel column using CH 2C1 2 . The purple eluent was 41 collected, and evaporated to dryness, and the resulting solid dried in vacuo; yield 0.97 g (61%). Anal , calcd. for C34H280 2 Pt: C 61.53, H 4.25; found: C 60.97, H 4.41. 2.4. P r e p a r a t i o n o f 2 -Py r i dy lphosph ines Syntheses o f P P h 3 . n p y n , or P N n (n = 1, 2, 3), were carried out under N 2 using conventional bench-top techniques for manipulation of air-sensitive compounds. 9 The phosphines, for convenience, are designated as P N i , P N 2 , and PN3, respectively, according to the number of incorporated pyr idyl substituents. These abbreviations w i l l be used throughout this thesis. 2.4.1. 2-(Diphenylphosphmo)pyridine The l igand P P h 2 p y , or P N i , was previously prepared in this group fo l lowing the reported procedure 7 0 v ia diphenylphosphine, P P h 2 H , which was itself made according to a literature method. 7 7 Diphenylphosphine is a bad smelling, toxic compound and is very air-sensitive. A l l the manipulations were carried out in an efficient fumehood. The yield of the P N i production step is 80%, while the overall yield from PPh3 is 56%. The reaction procedure is outlined in the following equations: THF, r.t. 2Hp P P h 3 . P P h 2 L i + P h L i • P P h 2 H + P h H +2Li -2LiOH 2 THF, r.t. 2K;Woropyridine PPh2H -zsr P P h 2 L i — s r - p p h 2 P y A n alternative route was v ia the procedure used to make PN3, but using P P h 2 C l (80 mmol) in place of PCI3 (see below); white crystals of P N i were obtained after recrystallization, yield 45 - 50%. The latter preparation is preferred because it requires a shorter reaction time and simpler work-up procedure. Ana l , calcd. for C17H14NP: C 77.54, H 5.36, N 5.32; found: C 42 76.92, H 5.37, N 5.14. M.p. 84.5 - 86°C; lit. 82° 7 0 and 85°C. 7 2 3lp{lH} NMR data (CDCI3, r.t.): -3.28 (s). 2.4.2. Bis(2-pyridyl)phenylphosphine The ligand PPhpy2 (PN2) was prepared by the method described in the next section for Ppy3, but using PPhCl2 (40 mmol) instead of PC13. White crystals were obtained after recrystaUization from acetone-petroleum ether (b.p. 30 - 60°C), yield 53 - 60%. Anal, calcd. for Ci6H 1 3 N 2 P: C 72.71, H 4.96, N 10.60; found: C 72.90, H 4.99, N 10.43. 3lp{lH} NMR data ( CDCl3,r.t.): -1.98 (s). M.p. 96.7 - 98.3°C; lit. 96°C. 7 2 2.4.3. Tris(2-pyridyl)phosphine The ligand Ppy3 (PN3) was prepared by a modified version of a procedure described in the literature.75 A 50 mL of n-butyllithium (1.6 M in hexane, 80 mmol) was transferred to a flask containing 50 mL of diethyl ether at -100°C. 2-Bromopyridine (8 mL, 80 mmol) was introduced and then the mixture was stirred vigorously at -100°C for 4 h until the colour turned to deep red. Liquid PCI3 (2.3 mL, 27 mmol) in Et 2 0 (15 mL) was then added dropwise via a syringe with vigorous stirring of the reaction solution. A light brown slurry was obtained. Before being warmed to r.t. gradually, the reaction mixture was stirred at -90°C for another 2 h. The mixture was extracted with degassed H2SC>4 (2M, 2xl00mL), and the aqueous layer collected. The aqueous solution was then made alkaline by adding saturated NaOH at 0°C and stirred, whereupon yellow crystals were deposited. The collected yellow crystals were washed thoroughly with H 2 0 and then acetone:H20 (1:1), and recrystallized from acetone-petroleum ether (1:1) and dried in vacuo; yield 30 - 46%. Anal, calcd for C i 5 H i 2 N 3 P : C 67.91, H 4.56, N 15.84; found: C 68.10, H 4.45, N 15.87. 3lp{lR} data ( CDCI3, r.t.): -0.05 (s). M.p. 114.5 - 116<>C; lit. 115° 7 2 and 111 -113°C. 7 4. 7 5 43 2.5. Preparation of Pt(II) and Pd(II) complexes of 2-pyridylphosphines 2.5.1. Qs-dmalobis(2-pyridyl)phosphmeplatinum(n), la - lc The complexes cis-PtCl2(PNn)2, n = 1 (la* ), 2 (lb) and 3 (lc), were obtained by reacting a yellow CH2CI2 solution (50 mL) of cis-PtCl2 (PhCN)2 (~ 1.0 g, 3.76 mmol) with a colourless solution of 2.0 equivalent of PN n (n = 1 - 3) in CH2CI2 (15 mL). The reaction niixture was stirred for 1 h at r.t, and the resulting colourless solution was concentrated to ~ 15 mL before Et20 (20 mL) was added to complete the precipitation. The white solid (la - lc) were collected by filtration and washed by Et20; yield ~90%. These compounds can also be made using cis-PtCl2(COD) in place of cis-PtCl2(PhCN)2. The elemental analyses of compounds la - lc are listed in Table 2.1, while the 31P{ lH) NMR data are listed in Table 3.1. Table 2.1. Elemental Analyses of Cis-PtX2(PNn)2 Complexes Complexes 1 calcd. C found H calcd. found N calcd. found C 34H 2 8N 2Cl2P2Pt, la 51.52 51.32 3.56 3.67 3.54 3.29 C34H28N2l2P2Pt, 3a 41.86 42.03 2.89 3.05 2.87 2.60 C32H26N4Cl2P2Pt, lb 48.36 46.44 3.30 3.32 7.05 6.66 C32H26N4l2P2Pt, 3b 39.32 38.94 2.68 2.74 5.73 5.57 C3oH24N6Cl 2P2Pt, lc 45.23 45.46 3.04 2.94 10.55 10.51 C3oH 24N6Br 2 P 2 Pt, 2c 40.23 40.55 2.73 2.83 9.49 9.28 C30H24N6l2P2Pt, 3c 36.77 36.53 2.47 2.46 8.58 8.41 The complexes cis-PtX2(PNn)2 (X = Br, n = 3 (2c); X = I, n = 1 (3a), 2 (3b) and 3 (3c)) were made by metathesis of cis-PtCl2(PNn)2 with the appropriate NaX. Complex la (0.20 g, 0.250 mmol), lb (0.20 g, 0.250 mmol) or lc (0.20 g, 0.251 mmol) was suspended in * a, b and c will be used throughout this thesis to label complexes containing P N i , PN2 and PN3 phosphine, respectively. 44 CH2CI2 (20 mL), and about a five-fold excess of NaX dissolved in a minimum amount of water was added to the suspension. A small amount of MeOH (5 mL) was introduced to homogenize the mixture, which was then stirred at r.t. for about 1 h. The solution turned to pale or bright yellow for bromide or iodide, respectively, and was then concentrated to about 8 mL; the product precipitated was collected by filtration, washed extensively with H2O and MeOH, and dried in vacuo; yields ~85%. The elemental analyses of compounds 2c and 3a - 3c are also listed in Table 2.1; the 31P{1H} NMR data are given in Table 3.1. Compounds PtCl2(PNi)2, la, and PtI2(PNi)2,3a, have been reported previously by Farr et al.16 2.5.2. Dihalobis(2-pyridylphosphme)paUadium(n), 4-6 Most of the complexes under this heading have been made before, except 5b and 6b. PdCl2(PNi)2, 4a, and PdCl2(PN2)2,4b, were reported by Balch's group77 and Newkome's group78 respectively. The others were made in our own group via a simple substitution of the coordinated benzonitrile groups by the corresponding phosphine;79 however, these complexes were not characterized in terms of geometry and the elemental analyses were not satisfactory. Therefore, complexes 4c, 5a - 5c, and 6a - 6c were prepared by the following procedures and characterized; 4a and 4b were also prepared. The 31P{1H} NMR data of the complete series are listed in Table 3.2. Compounds PdCl2(PNn)2 (4a - 4c) were made by dissolving PdCl2(PhCN)2 (300 mg, 0.781 mmol) in CH2CI2 (20 mL), and adding to this reddish-brown solution two equivalents of PN n (n = 1 - 3) dissolved in a rninimum amount of CH2CI2 (7 mL). After the solution mixture was stirred at r.t. for 30 min, a yellow solid slowly precipitated; Et20 (40 mL) was added to complete the precipitation. Species 4a - 4c were purified after dissolution and reprecipitation from CH2Cl2/Et20 twice and dried in vacuo; yields ~95%. The compounds PdX2(PNn)2 (X = Br, I; n = 1 - 3, 5a - 5c and 6a - 6c) were prepared through the same method of metathesis as used for 2c and 3b, 3c; yields 85 - 90%. The 45 elemental analyses of 4c, 5a - 5c, and 6a - 6c, and the visible spectral data of 4a - 4c, 5a -5c, and 6a - 6c, are summarized in Tables 2.2 and 2.3, respectively. Table 2.2. Elemental Analyses of PdX2(PNn)2 Complexes Complexes C H N calcd. found calcd. found calcd. found C34H28N2Cl2P2Pd, 4a 58.01 57.89 4.01 4.27 3.98 4.20 C34H28N2Br2P2Pd, 5a 51.51 51.42 3.56 3.53 3.53 3.42 C34H28N2l2P2Pd, 6a 46.05 45.94 3.18 3.09 3.16 2.99 C32H26N4Cl2P2Pd, 4b 54.45 54.67 3.71 3.99 7.94 7.88 C32H26N4Br2P2Pd, 5 b 48.36 46.84 3.30 3.52 7.05 6.71 C32H26N4l2P2Pd, 6b 43.24 41.85 2.95 2.82 6.31 6.59 C 3oH24N 6Cl 2P 2Pd, 4c 50.90 50.62 3.42 3.34 11.87 11.90 C 3 oH24N6Br 2 P2Pd, 5c 45.22 44.55 3.04 3.00 10.55 9.94 C3oH2 4N 6l2P2Pd, 6c 40.45 39.90 2.72 2.65 9.44 9.51 Table 2.3. Visible Spectral Data of PdX2(PNn)2 Complexesa Complexes Xmax(exl0-3) 4a 340(13.2) 5a 363(9.73) 6a 425(3.11) 4b 337(6.21) 5b 362(6.63) 6b 421(3.62) 4c 338(3.42) 5c 362(4.13) 6c 428(3.03) (a) Measured in CH2CI2, at r.t.; X, in nm and e in M^cnr1. 46 2.6. Preparation of homo- and hetero-binuclear platinum(I) and palladium(I) complexes of pyridylphosphines 2.6.1. Dihalobis(2-pyridylphosphme)diplatinum(I) (head-to-tail, HT), 7-9 The complexes Pt2X2(u-PNn)2(HT) (X = CI, n = 1 (7a), 2 (7b), 3 (7c); X = Br, n = 3 (8c)) were made using a modified literature procedure.76 A purple CHCI3 solution (60 mL) of Pt(dba)2 (0.25 g, 0.38 mmol) was added dropwise to a CHCI3 solution (40 mL) of the corresponding cis-PtX2(PNn) complex, la - lc, or 2c (0.38 mmol), and the mixture was then refluxed under N2 for 5 h. TLC was performed every 30 min until the Pt(dba)2 had all been consumed. The CHC13 solvent was completely removed on the rotary evaporator, and the crude product mixture was redissolved in CH2CI2. The solution was then charged on to a silica gel column, and 2% MeOH in CH2C12 was used as eluent. The reddish-orange eluate following the yellow dba band was collected. An orange (7a - 7c) or brownish-orange solid (8c) was obtained after solvent evaporation, respectively. The products were recrystallized from CH2Cl2/Et20 (1:1), and dried in vacuo; yield 50 - 60%. The complex Pt2Cl2(u.-PNi)2 (HT), 7a, was previously made by Balch's group.76 The Pt2Cl2(M-PN2)2(HT) (7b) sample separated by column chromatography (~3 mg) was dissolved in CH2CI2 (1.0 mL), on top of which MeOH (1.0 mL) was carefully placed. The red, hexagonal flakey single crystals of Pt2Cl2(M>N2)2(HT) (IS, 2S) (see Sect. 3.2.3) for X-ray diffraction analysis were thus grown by the slow diffusion of MeOH into the CH2CI2 solution at 5°C. Complexes Pt2l2(M--PNn)2 (HT), n = 2 (9b) or 3 (9c), were prepared by metathesis of Pt2Cl2(M.-PNn)2 (7b or 7c) with Nal, using the method described for 3c (Sect. 2.5.1). The products were obtained as pure HT form in nearly quantitative yield. Analytical data of the HT isomers of Pt2X2(H-PNn)2 are given in Table 2.4, and 31P{ NMR data in Table 3.5. 47 2.6.2. Duodobis(2-pyridylphosphme)pktmum(I) (head-to-head, HH), 10 Complexes Pt2l2(H-PNn)2(HH), n = 1 (10a), 2 (10b) and 3 (10c), were synthesized by the procedure described for Pt2X2(u-PNn)2 (HT), X = CI (7c) and Br (8c); see Sect. 2.6.1. The reactions were completed within 30 min to 1 h. The crude product was separated from free dba by column chromatography as described in Sect 2.6.1. Pure HH isomer of Pt2l2(H-PNi)2, 10a, was isolated (silica gel, CH2C12) from the first chromatography separation, while a mixture of HH and HT isomers was obtained for Pt2l2(M--PN2)2» 10b, and Pt2l2(H-PN3)2,10c, with the HH isomer being a major product (HH/HT ~ 80%) (silica gel, 2% MeOH in CH2CI2); the yield of 10a was 60%, and the yields of 10b (+ 9b) and 10c (+ 9c) were ~ 65%. Table 2.4. Elemental Analyses of Pt2X2(u,-PNn)2 Complexes Complexes C H N calcd. found calcd. found calcd. found C34H28N2Cl2P2Pt2,7a(HT) 41.35 41.60 2.86 2.96 2.84 3.15 C34H28N2l2P2Pt2,10a (HH) 34.89 34.77 2.41 2.30 2.39 2.13 C32H26N4Cl2P2Pt2, 7b (HT> * 38.83 37.37 2.65 2.93 5.66 5.23 C32H26N4l2P2Pt2, 9b (HT)* 32.78 32.42 2.23 2.25 4.78 4.60 C32H26N4I2P2Pt2,10b (HH)* 32.78 33.16 2.23 2.54 4.78 4.45 C3oH24N6Cl2P2Pt2,7c (HT) 36.34 36.46 2.44 2.50 8.48 8.39 C3oH24N6Br2P2Pt2,8c (HT) 33.34 33.20 2.24 2.34 7.78 7.72 C30H24N6l2P2Pt2,9c(HT) 30.67 30.69 2.06 2.14 7.15 6.96 C30H24N6l2P2Pt2,10c (HH) 30.67 30.65 2.06 2.01 7.15 7.08 (a) This compound is hygroscopic. The X-ray crystal structure is done for the (IS, 2S) diastereomer (Chapter 3, Fig. 3.11). (b) Mixture of diastereomers is analyzed. The 10b and 9b mixture (0.28 g, 0.24 mmol), obtained after initial column chromatography (silica gel, 2% MeOH in CH2CI2), was separated as three bands using a preparatory TLC plate (2 mm silica gel, 2% MeOH in CH2CI2,5 runs). The first band contained 48 10b.2/10b.3 (83 mg, 30%), the second band contained lOb.l (92 mg, 33%), and the third band contained 9b (36 mg, 13%). Complexes lOb.l, 10b.2 and 10b.3 are optical isomers (see Sect. 3.2.3). The red, single crystals of 10b. 1 and 10b.2/10b.3 were grown by the same diffusion method as described for 7b (Sect. 2.6.1) in CH2CI2 (1.0 mL) and MeOH (1.0 mL) at 5°C. Compound lOb.l grew as twin crystals on three occasions, and the 10b.2/10b.3 mixture grew as highly disordered single crystals. Complex Pt2l2( i^-PN3)2(HH), 10c, was obtained pure by fractional crystallization from CH 2Cl2 and MeOH. The HT isomer 9c was presumably more soluble and was left in the mother solution. The orange solid that came out from the solution after overnight standing in a freezer was the desired isomer. Analytical data of the HH isomers 10a to 10c are listed in Table 2.4, and the 3 1P{ 1H} NMR data are given in Chapter 3 (Table 3.6). Compound Pt2l2(H-PNi)2(HH), 10a, has been made previously by Balch's group.76 The spectroscopic data of 10a, prepared in the present study, are in good agreement with the literature values. Reaction of 10c with greater than a two-fold excess of phosphine PN3 yields a new tetrakisphosphine binuclear platinum species. The procedure for the isolation of the tetraphenylborate salt of this binuclear species, [Pt2(H-PN3)2(PN3)2](BPh4)2, 41.2, is described here. To an orange solution of 10c (30 mg, 0.03 mmol) in CH2CI2 (20 mL) was added solid PN3 (30 mg, 0.11 mmol), and the mixture was stirred at r.t. for 15 rnin under air until the orange colour faded to nearly colourless. Water (15 mL) was added to extract the ionic species that had formed; to the extract was added excess NaBPh4 (0.1 g, 0.3 mmol) in MeOH (5 mL). The bright yellow precipitate that formed was collected by filtration, purified by reprecipitation from acetone using Et20, and dried in vacuo; yield 55 mg (88%). Anal, calcd. for Ci08H88Ni2B2P4Pt2: C 62.07, H 4.24, N 8.04; found: C 62.43, H 4.35, N 7.94. UV/Vis. 49 (acetone, r.t.): ^ max = 336, 424 nm; emax = 1.6X104, l.lxlO 4 M^cnr 1. Molar conductivity (CH3CN, r.t.): 204.3 Q-imoHcm2 at 1.9xl0"3 M. 3 1 P and 1 9 5Pt NMR: see Sect. 4.4. 2.6.3. Dmalobis(2-pyridylphosphine)dipaUadium(r) (HT) (X = CI, Br, I), 11 -13 Most of the binuclear palladium© HT isomers have been made before, except Pd2X2(H-PN 2 ) 2 , X = CI (lib), Br (12b), or I (13b). Complex Pd2Cl2(u.-PNi)2,11a, was made by Maisonnat et al. 2 0 and complexes Pd2X2(H-PNn)2 (n = 1, X = Br (12a) and I (13a); n = 3, X = CI, (11c), Br (12c), and I (13c)) were made previously in this laboratory.79 The X-ray crystal structure of Pd2l2(H-PNn)2> 13c, revealed the HT phosphine arrangement.76 However, it was uncertain whether the rest of the binuclear palladium complexes were also adopting the same arrangement. In an effort to clear up the ambiguity regarding the relative orientation of these pyridylphosphine complexes, the complete series 11 to 13, a to c inclusive (Table 2.5), were made according to the following procedure. A dark violet, CH2CI2 solution (20 mL) of Pd2(dba)3CHCl3 (0.22 g, 0.21 mmol) was added slowly into a suspension of PdX2(PNn)2 (0.42 mmol) in CH2CI2 (20 mL). The resulting solution was stirred at r.t. for about 1 h until the suspended solid was all dissolved. The solution was concentrated to 10 mL, and Et 20 (10 mL) was added slowly to yield the desired products, which were reprecipitated from CH2CI2 (15 mL) and Et20 (10 mL) two or three times, and dried under vacuum; yield ~90%. The elemental analyses and the visible spectral data are summarized in Tables 2.5 and 2.6. The 31P{ iH} NMR data of the complete series are listed in Chapter 3 (Table 3.8). The structural characterization of these binuclear compounds, with emphasis on the diastereomers present, is discussed in Chapter 3. 50 Table 2.5. Elemental Analyses of Pd2X2(M>PNn)2 (HT) Complexes Complexes C H N calcd. found calcd. found calcd. found C34H2gN2Cl2P2Pd2, Ha 51.40 51.17 3.48 3.25 3.46 3.73 C34H28N2Br2P2Pd2,12a 45.42 45.31 3.14 3.15 3.11 2.95 C34H28N2l2P2Pd2,13a 41.11 41.47 2.84 3.07 2.82 2.76 C32H26N4Cl2P2Pd2-2CH 2Cl 2, lib* 44.17 44.06 3.37 3.23 6.24 6.27 C32H26N4Br2P2Pd2,12b 42.65 42.30 2.91 2.80 6.22 6.34 C32H26N4I2P2Pd2,13b 38.62 37.60 2.63 2.78 5.63 5.29 C 3oH2 4N 6Cl2P2Pd 2 ,11c 44.25 44.28 2.97 3.01 10.32 10.26 C30H24N6Br2P2Pd2,12c 39.90 39.63 2.68 2.70 9.31 9.00 C3oH24N6l2P2Pd2,13c 36.14 36.08 2.43 2.40 8.43 8.20 (a) The chlorine analysis was done for this particular compound: calcd. 15.80, found 16.00. Table 2.6. Visible Spectral Data of Pd2X2(u,-PNn)2(HT) Complexes a Complexes Xmax. (exlO-3) 12a 348(13.7) 483(7.92) 13a 379(13.4) 540(10.8) l ib 333(10.8) 462(9.47) 12b 345(15.0) 476(9.90) 13b 376(15.3) 530(13.7) 11c 335(12.0) 460((8.43) 12c 348(11.6) 475(10.3) 13c 376(9.7) 525(11.7) (a) Measured in CH2CI2; the wavelength X is in nm and the molar absorptivity e is in M^cnr1. 51 2.6.4. Dmalobis[tri(2-pyridyl)phosprune]paJladimn (HT), 14c - 16c Complexes PtPdX2(n-PN3)2 (HT) (X = CI (14c), Br (15c) and I (16c)) were prepared by reaction of the appropriate cis-PtX2(PN3)2 with Pd2(dba)3-CHC13 using the same conditions described for the syntheses of Pt2X2(^ -PNn)2 (HT) (Sect 2.6.1). Head-to-tail isomers were formed exclusively. Analytical data are given in Table 2.7, while the 31P{ NMR data are given in Chapter 3, Table 3.7. Table 2.7. Elemental Analyses of PtPdX2(u,-PN3)2 (HT) Complexes Complexes (? H N calcd. found calcd. found calcd. found C30H24N6Cl2P2PdPt, 14c 39.90 39.96 2.68 2.75 9.31 9.03 C3oH24N6Br2P2PdPt, 15c 36.63 35.59 2.44 2.54 8.47 8.16 C3oH24N6I2P2PdPt, 16c 33.17 33.37 2.23 2.46 7.74 7.40 2.7. Preparation of the DMAD A-frame adducts An excess amount of DMAD (0.2 mL, 1.6 mmol) was added to a 20 mL CH2CI2 solution containing (0.1 g, 0.1 mmol) of the appropriate Pt2l2(^ -PNn)2 (HT) or (HH) (n = 1 - 3) complexes at r.t. The reaction mixture was stirred for about 15 min until the colour of solution turned from orange to yellow. The volume of solution was then reduced on a rotary evaporator to about 3 mL, when the orange solid that deposited was collected, and washed with Et20 (3x10 mL) to get rid of excess DMAD. The products Pt2l2(H-DMAD)(n-PNi)2 (HH) (17), Pt2l2(H-DMAD)(n-PN2)2 (HT) (18), and Pt2l2(^ -DMAD)(u,-PN3)2 (HT) (19),were dried at 80°C in vacuo; yield 95%. The complexes Pt2Cl2(n-DMAD)(u,-PN3)2,20, and PtPdCl2(H-DMAD)(n-PN3)2, 21, were also isolated in this way. A methylpropiolate (MPP) A-frame adduct, 22, was formed from 9c in situ in an NMR tube by adding the acetylene to a CDCI3 solution (0.8 mL) of 9c. 31P{1H} NMR data of 22, together with those of the isolated complexes, 17 - 21, inclusive are listed in Table 4.1. 52 The palladium DMAD adducts were also prepared from the Pd2X2(p.-PNn)2 precursors, in the same manner as the platinum adducts (Sect 2.6.3). The compound Pd2Cl2(p.-PNi)2, which was found not to react with CO, 2 0 reacts with DMAD, yielding an orange solid Pd2Cl2(n-DMAD)(n-PNi)2,23. The corresponding PN2 complexes Pd2Cl2(^-DMAD)(|x-PN2)2 (HT), 24, and Pd2l2(M--DMAD)(^.-PN2)2 (HT), 25, are also among those isolated and characterized. The elemental analyses of 17 - 21 and 23 - 25 are presented in Table 2.8. Table 2.8. Elemental Analyses of DMAD Insertion Products complex C H N calcd. found calcd found calcd. found C40H34N2l2O4P2Pt2,17 36.60 36.45 2.61 2.65 2.13 2.01 C38H32N4l204P2Pt2,18 34.72 34.88 2.45 2.41 4.26 4.50 C36H30N6l2O4P2Pt2,19 32.84 32.80 2.30 2.14 6.38 6.16 C36H30N6Cl2O4P2Pt2, 20 38.14 38.11 2.67 2.86 7.42 7.17 C36H30N6Cl2O4P2PdPt, 21 41.37 41.27 2.89 2.99 8.04 7.91 C40H34N2Cl2O4P2Pd2, 23 50.45 50.68 3.60 3.51 2.94 3.09 C38H32N4Cl204P2Pd2, 24 47.72 47.59 3.38 3.50 5.87 5.69 C38H32N4l204P2Pd2, 25 40.13 39.87 2.84 2.68 4.93 5.04 2.8. Preparation of Pt(0) complexes of pyridylphosphines and their derivatives from reactions with olefin, hydrogen chloride and methyl iodide Experimental procedures similar to those described for the preparation of triphenylphosphine Pt(0) complexes5 were adopted for syntheses of the pyridylphosphine Pt(0) derivatives. Some modifications in the experimental conditions were made. All reactions and subsequent manipulations were performed under anaerobic conditions, using conventional techniques for handling air-sensitive compounds.9 53 2.8.1. Tetrakis(tripyridylphosphme)platmvmi(0), 26c (1) A saturated aqueous solution of K2PtCl4 (0.50 g, 1.2 mmol; in 5 mL) was added to a refluxing THF solution (20 mL) containing KOH (0.13 g, 2.4 mmol) and five equivalents of PN3 (2.92 g, 11 mmol). The mixture was re fluxed for about 10 min, and the resulting orange solution was cooled to r.t., when the water and THF layers separated. The THF was pumped off, and the yellow precipitate that had formed in the aqueous layer upon cooling was extracted with CH2CI2 (10 mL); this CH2C12 solution was subsequently transferred via a cannula tube to another flask where the extract was dried over MgSC«4 for 2 h. The drying agent was filtered off, and hexane was introduced to the CH2CI2 filtrate until the precipitate reformed again; the solid was collected, washed thoroughly with hexane, and recrystallized from CH2Cl2/hexane (v/v 1:1) to give a yellow solid, which was dried in vacuo at r.t.; yield 0.65 g, 43%. (2) A benzene suspension of cis-PtCl2(PN3)2 (0.30 g, 0.38 mmol; in 30 mL) with PN3 ligand (0.30 g, 1.14 mmol) was refluxed at 80°C. Hydrazine hydrate in benzene (10% by vol) was added dropwise to the mixture with vigorous stirring until all the white solid had dissolved. The yellow solution thus formed was refluxed for another 10 min and then cooled to 5°C. The yellow solid that formed upon concentration of the solution to 10 mL was collected by filtration and redissolved in CH2CI2 (15 mL). Compound 26c, obtained by a reprecipitation procedure using hexane (~ 20 mL), was then collected and dried in vacuo; yield 0.19 g, 40%.* Compound 26c obtained from either method (1) or (2) has the same 3 1P and 195Pt NMR solution spectra which prove the identity of the complex but satisfactory elemental analysis was not obtained. Anal, (optimum) calcd for C6oH48Ni2P4Pt: C 57.37, H 3.85, N 13.38; found: C 59.38, H 3.80, N 13.49. 31p{lH} NMR (CDC13, -40<>C): 8P= 30.1 (st), lJ P t P= 3829 Hz. 195pt{lH} NMR (CDCI3, -40°C): 8P = -538.3 (quintet), UR-P = 3840 Hz. * An identical procedure but using no added PN3 gave a smaller yield (18%) of 26c. t The 3 1P{ 1H} NMR pattern containing a major singlet, doublet or triplet, as well as the corresponding platinum satellites, is noted in this thesis simply as a singlet, a doublet or a triplet. 54 Use of ethanol, the solvent used in the original preparation of Pt(PPh3)4,5 resulted in formation of orange solutions on reaction of PtCL;2' with PN3 or cis-PtCl2(PN3)2 in the presence of N2H4; upon concentration, the solutions yielded only a red oil. An oxygen complex of 26c, tentatively formulated as "Pt(PN3)302", was isolated as an orange solid by leaving crude 26c, obtained from method (2) (~80 mg, 0.064 mmol), in CH2C12 (15 mL) and hexanes (15 mL) at 0°C under N2 for four days (-30 mg, yield 38%). Anal, calcd. for C45H36N902Pt: C 52.83, H 3.55, N 12.32; found: C 52.23, H 3.51, N 11.85. 31p{lH} NMR (CDCI3, r.t.): 6> = 23.3 (d), 22.5 (t); Jpff = 2700, 4050 Hz; J P P = 14 Hz. IR data are given in Fig. 5.5. 2.8.2. Tris[2-(diphenylphosphmo)pyridine]platinum(0), 27a The exact experimental procedure described for the synthesis of Pt(PPh3)3 was followed.-5 A saturated aqueous solution of K P^tCU (1.0 g, 2.4 mmol; in 10 mL) was added to a refluxing EtOH solution (20 mL) containing KOH (0.27g, 4.8 mmol) and 3.5 equivalents of PNi ligand (2.3 g, 8.5 mmol). The mixture was refluxed for 20 min, and the resulting yellowish-orange solution was cooled to r.L and then concentrated to 15 mL. A yellow solid that formed was collected by filtration and then washed with cold EtOH (5 mL) thrice. This product was then redissolved in CH2CI2 (12 mL), reprecipitated by the addition of hexane (20 mL), and then dried in vacuo; yield of Pt(PNi)3 1.7 g (72%). Anal, calcd. for CsiH42N3P3Pt: C 62.19, H 4.30, N 4.27; found: C 62.63, H 4.49, N 3.90. 31p{lH} NMR (toluene-dg, -70°C): 8P = 57.02 (s), ^ P I P = 4444 Hz. l95Pt{lH} NMR (toluene-dg, -70°C): 8pt = -300.3 (quartet), UptP = 4433 Hz. 1 . . 55 2.8.3. Olefin derivatives from Pt(0) complexes of pyridylphosphines 2.8.3.1. Acrylonitrile, methacrylonitrile and crotonitrile complexes (44a, 44c, 45a, 46a) A solution of excess acrylonitrile (distilled, ~lml) in Et20 (5 mL) was degassed by the freeze-and-thaw method three times. This solution was added dropwise to a suspension of Pt(PNi)3 (93 mg, 0.094 mmol) in Et20 (20 mL) until the yellow solid dissolved and the solution turned colourless. This reaction mixture was then left at -20°C overnight. A white solid which deposited on the side-wall of the flask was collected. The supernatant was then transferred to a beaker where a second crop of solid was obtained by scratching the side of the beaker. The product Pt(PNi)2(T|2-CH2CHCN), 44a, is air-stable; yield 54 mg (air dried), ~75%. Pumping on the wet reaction product resulted in reformation of yellow Pt(PNi)3. The complex Pt(PN3)2(T|2-CH2CHCN), 44c, was formed in situ in an NMR tube when an excess of liquid acrylonitrile was added to a yellow CDCI3 solution of Pt(PN3)4 at r.t. The colour of the resulting solution became pale yellow. The exact procedure outlined for isolation of the acrylonitrile complex 44a was also used for synthesis of the methacrylonitrile and crotonitrile species. The complexes Pt(PN 1)2(112-CH2C(CH3)CN), 45a, and Pt(PNi)2(Ti2-CH3CHCHCN), 46a, are also white solids. Because a mixture of cis and trans crotonitrile is used, two isomers of 46a which differ in the geometry of the olefin are obtained (Sect. 5.2.1). Yields of 45a, 46a are ~60%. Unfortunately, because of the loss of coordinated olefin on pumping, these olefin complexes (44a, 45a, 46a) could not be dried properly and therefore did not analyze to a satisfactory standard for confirmation of their chemical formulation. 56 2.8.3.2. Male ic anhydride ( M A ) and diethyl maleate ( D E M A ) complexes (47a, 47c 48a) A saturated solution of maleic anhydride, M A , in E t 2 0 (3 mL) was degassed by three freeze-and-thaw cycles and added slowly to a solution of Pt (PNi )3 (98 mg, 0.1 mmol) in benzene (20 mL) . After the solution rapidly became pale yellow, n-hexane (20 mL) was added. The pale yellow precipitate that gradually farmed upon cooling the solution to -20°C was filtered, washed with E t20 , and dried in vacuo; yield of Pt (PNi )2 [ r i 2 - (CHCO)20] , 47a, is 89% (72 mg). Ana l , calcd. for C3 8 H28N203Pt: C 55.68, H 3.69, N 3.42; found: C 55.61, H 4.00, N 3.19. The Pt(PN3)4 complex also reacts with M A , under the same experimental conditions used for the synthesis of 47a, to form a pale yellow compound Pt (PN 3 )2 [ r | 2 - (CHCO)20] , 47c; y ield 80%. Ana l , calcd. for C 3 4 H 2 6 N 6 0 3 P t : C 49.58, H 3.18, N 10.21; found: C 49.25, H 3.19, N 10.42. The preparation of a mixture of Pt(PNi)2(DEMA), 48a.l, and Pt (PNi )2(DEFM) ( D E F M = diethyl fumarate), 48a.2, was carried out in E t 2 0 using Pt (PNi )3 as described for the preparation of 47a. The addition of D E M A in E t 2 0 was controlled very carefully in order to obtain a solid product, because excess D E M A resulted in the formation of a white, col loidal product. White microcrystals of 48a.1 and 48a.2 were obtained upon leaving the solution at -20°C for two days; y ield 30%. Ana l , ca lcd. for C42H4()N2C>4Pt: C 56.43, H 4.51, N 3.13; found: C 55.95, H 4.72, N 3.37. The 3 1 P { lH) N M R data of 47a, 47c, 48a.l and 48a.2 are presented in Table 5.2. 2.8.4. Hydridochloro derivatives from Pt(0) complexes of pyridylphosphines 2.8.4.1. Trans-cMorohydridobis[(2-(h^henylphosphmo)pyrid^ 50a The complex trans-Pt(H)Cl(PNi)2,50a, was obtained by reaction of a T H F solution (40 mL) of P t ( P N i ) 3 27a (200 mg, 0.2 mmol), initially under N 2 , with anhydrous H C l gas at 1 atm. 57 The solution was stirred vigorously for 20 min until the yellow colour dissipated. Excess HC1 was pumped off completely and an N 2 atmosphere was re-established. Hexane (40 mL) was then laid on top of the colourless THF solution, the mixture then being left in a freezer at -20°C for 72 h. White crystals formed and these were collected by filtration and washed with hexanes/THF (v/v 1:1) and then THF alone; yield 67 mg (45%). The complex, 50a, is air-sensitive and turns orange when exposed to air. Anal, calcd. for C34H29N2ClP2Pt: C 53.87, H 3.86, N 3.70; found: C 54.01, H 4.31, N 3.34. IR (Nujol): vptH = 2212 cm-1; *H NMR of the hydride (CD2C12, r.t.): 8 = -16.32 (t), J P H = 12.7 Hz, J P t H = 1213 Hz; 3 1 P NMR (CD2C12, r.t.): 8 = 30.00 (s), JptP = 3031 Hz. Excess HC1 in solution, after the reactant solution turned colourless, must be removed immediately because otherwise the reaction is more complex. A new, white solid was isolated under similar experimental conditions in the presence of excess HC1. The IR and *H and 31P{1H} NMR data of this unknown platinum hydride species are different from those of 50a: IR (CDCI3): = 2219 cm"1; *H NMR of the hydride (CDCI3, r.t.): 8 = -15.5 (br. s), Ji>tH = 1173 Hz; 3 1P{1H} NMR (CDCI3, r.t.): 8 = 35.2 (s), = 3136 Hz. 2.8.4.2. Attempted preparation of hydridocMorobis[tri(2-pyridyl)phosphine]platinum(II) The following procedures were used for the preparation of the title complex: (1) Tetrakis[tris(2-pyridyl)phosphine]platinum(0), 26c, (31.4 mg, 0.024 mmol) was dissolved in CH2CI2 (10 mL). Anhydrous H Q (g) was bubbled through deoxygenated CH2CI2 for about 5 min, and this HCl-saturated solution (0.2 mL) was added slowly, using a syringe, to the CH2CI2 solution of 26c, the colour of the reaction mixture turning from bright yellow to greyish yellow. The solid that gradually deposited (30 rnin) was filtered, washed three times with CH2Cl2/hexanes (5 mL, v/v 1:1) and dried in vacuo. (2) Compound 26c (15.4 mg, 0.012 mmol) was suspended in benzene (10 mL) under a positive pressure of N2 in a septum-capped Schlenk tube. The HC1 (g) (5 mL at 1 atm) was 58 injected slowly, and the suspension was then stirred for about 10 min until the yellow solid dissolved. A canary yellow solid formed after 30 min, and was collected by filtration, washed twice with benzene (5 mL), and dried under vacuum. (3) Some 26c (10.2 mg, 0.008 mmol) was suspended in THF (10 mL) at r.t.; and 0.5 mL THF solution of DMA-HQ* (0.016M, 0.008 mmol; 1.0 equiv) was then added dropwise. The colour of the suspension turned from bright yellow to pale yellow upon stirring (-10 min), and the suspension was allowed to settle at 0°C. No further colour change was observed. The resulting canary yellow solid was collected and washed with THF (3x5 mL) to remove DMA. 2.8.5. Iodo(methyl) derivatives from Pt(0) complexes of pyridylphosphines, trans-PtI(Me)(PNi)2,52a,* and trans-PtI(Me)(PN3)2,52c Some Pt(PNi)3 (100 mg, 0.10 mmol) was dissolved in benzene (20 mL) and four equivalents of Mel (0.035 mL, 0.40 mmol) were added. The reaction mixture was refluxed at 80°C for 20 min and then cooled to ~5°C. A pale yellow precipitate formed when the solution was concentrated by evacuation to 5 mL at this temperature. The solid was collected by filtration, redissolved in water (5 mL), and extracted with CH2C12 (20 mL). The colourless CH2C12 portion was then transferred via a cannula tube to another Schlenk flask where the solution was concentrated to 5 mL. The white solid of trans-PtI(Me)(PNi)2, 52a, was precipitated by addition of hexanes (10 mL); yield 58 mg (67%). Anal, calcd. for C3sH3iIN2P2Pt: C 48.68 , H 3.62 , N 3.24; found: C 47.71, H 3.61, N 2.92. 31P{lH} NMR (CDCI3, r.t.): 8p = 26.82 (s), iJptP = 3078 Hz; *H NMR (CDCI3, r.t.): 5M. = 0.05 (t) 3j P H = 5.7,2JptH = 75 Hz. ([MePNiJI was made in situ in the present study by adding excess Mel to the CDCI3 solution of PNi at r.t.: 5P = 17.5 (s): 8Me = 3.21 (d), 2 J P H = 12.1 Hz). * DMA-HCL N,N<umethylacetamide hydrogen chloride was provided by A. Joshi, its synthesis is described in ref. 21. * A recent paper (Jain, V.K. et al. J. Organomet. Chem., 1990,389, 417.) has noted the synthesis of trans-PtI(Me)0,Ni)2 from the reaction of PtI(Me)(COD) with 2 equivalents of PNi in benzene; the reported spectral properties are essentially the same as those presented here. 59 Trans-PtI(Me)(PN3)2, 52c, was obtained in the same manner. The final product was isolated as a pale yellow solid; yield 57%. Anal, calcd. for C3iH27lN6P2Pt: C 42.92, H 3.14, N 9.69; found: C 43.45, H 3.37, N 9.51. 31p{lH} NMR (CDCI3, r.t.): 8P = 25.64 (s), IJptP = 3133 Hz. ([MePN3]I was made in situ in the same manner as described for [MePNi]!: 8p = 12.46 (s)). 2.9. Preparation of bis(maleato) and bis(malato) chromium (III) species Preparations of K[Cr(maleato)2(H20)2], 53, and K[Cr(malato)2(H20)2], 54, complexes have not been reported in the literature. The procedure adopted here follows the reported synthesis for the cis-K[Cr(malonato)2(H20)2] complex.22 2.9.1. Cis-potassium diaquobis(maleato)chromate(III) trihydrate, 53 Solutions of K2Cr2C»7 (2.95 g, 0.01 mol; 6 mL) and maleic acid (5.80 g, 0.05 mol; 6 mL) in boiling water were mixed, and allowed to react until no more carbon dioxide evolved. About 45 mL water was then added to the mixture. After being cooled, the solution was added very slowly with vigorous stirring to 100 mL of 95% ethanol; the product thus precipitated was collected by filtration, washed with 95% ethanol and ether, and dried in air, yield of the greyish blue solid was 5.4 g (78%). Anal. Calcd. for CgHwOnCrK: C 23.47, H 3.45; found: C 23.59, H 3.75. Visible spectral data in H 2 0 (kmax nm, e M^cnr 1): 420 (72.9), 572 (74.4). 2.9.2. Cis-potassium diaquobis(malato)chromate(III) monohydrate, 54 The procedure described for the preparation of the maleato complex was adopted, except that 3.42 g R, S-malic acid and 1.5 g K2Cr2C>7 were used instead; yield 4.1 g (90%). Anal. Calcd. for CgHuOoCrK: C 23.47, H 3.45; found: C 23.46, H 3.35. Visible spectral data in H2O ( W nm, e M-lcnr1): 420 (76.4), 572 (77.0) 60 References 1. Conard, C.R.; Dolliver, M.A.; Organic Synthesis; Blatt, A.H., Ed.; John Wiley & Sons: New York, 1943; Coll. Vol.2, p. 167. 2. Brevard, C; Granger, P. Handbook of High Resolution Multinuclear NMR; Wiley: New York, 1981; p.200. 3. Georgii, I.; Mann, B.E.; Taylor, B.F. Inorg. Chim. Acta , 1984, 86, L81. 4. Kerrison, S.T.S.; Sadler, P.J. J. Mag. Res., 1978, 31, 321. 5. Hartley, F.R. Organometallic Rev. A., 1976, 6,119. 6. McDermott, J.X.; White, J.F.; Whitesides, G.M. / . Am. Chem. Soc, 1976, 98, 652. 7. Ukai, T.; Kawazura, H.; Ishii, Y.; Bonnett, J.J.; Ibers, J.A. / . Organomet. Chem., 1974, 65, 253. 8. Moseley, K.; Maitlis, P.M. / . Chem. Soc. Dalton Trans., 191 A, 169 . 9. Shriver, D.F.; Drezdon, M.A. The Manipulation of Air-Sensitive Compounds, 2nd ed.; Wiley: New York, 1986. 10. Maisonnat, A.; Farr, J.P.; Olmstead, M.M.; Hunt, C.T.; Balch, A.L. Inorg. Chem., 1982, 21, 3961. 11. Bianco, V.P.; Boronzo, S. Inorg. Synth, 1976, 16, 161. 12. Mann, F.G.; Watson, J. J. Org. Chem., 1948, 13, 502. 13. Kurtev, K.; Ribola, D.; Jones, R.A.; Cole-Hamilton, D.J.; Wilkinson, G. / . Chem. Soc Dalton , 1980, 55. 14. Keene, F.R.; Snow, M.R.; Stephenson, P.J.; Tiekink, E.R.T. Inorg. Chem., 1987, 27, 2040. 15. Boggess, R.K.; Zatko, D.A. J. Coord. Chem., 1975,4, 111. 16. Farr, J.P.; Wood, F.E.; Balch, A.L. Inorg. Chem., 1983, 22, 3387. 17. Farr, J.P.; Olmstead, M.M.; Balch, A.L. Inorg. Chem., 1983, 22, 1229. 18. Newkome, G.R.; Evans, D.W.; Fronczek, F.R. Inorg. Chem. 1987, 26, 3500. 19. Lee, C-L.; James, B.R. unpublished results. 61 20. Maisonnat, A.; Fair, J.P; Balch, A.L. Inorg. Chim. Acta, 1981,53, L217. 21. Thorburn, I.S. PhD Dissertation, University of British Columbia, 1985; Ch.2. 22. Chong, J.C. Inorg. Synth., 1976,16, 81. 62 Chapter 3 Structures of M(II) and M(I) Pyridylphosphine Complexes and Their Solution Chemistry 3.1. Introduction The oxidation state M(U) is the most common for both palladium and platinum. Due largely to the crystal field splitting effects (Fig. 3.1),7 these d 8 metal ions prefer square-planar geometry because the greatest stabilization energy is achieved compared to any other coordination number and geometry.2 All the M(U) bisphosphine complexes are of square-planar geometry and are generally formulated as MX2P2, and consequently, two possible geometric isomers— cis and trans* — exist (see below). Because of the potential catalytic properties of these divalent complexes, isomerization of one geometric form to another has been a research topic for several decades and studies in this area still remain active.-? * 9 These mononuclear complexes of Pt11 have also been used as models for catalytic intermediates in mechanistic investigations.70 * 7 2 \ / \ / M = Pd, Pt; P = tertiary phosphines; p / \ x X ' ^ P ^ = hahdes, or other monoanions cis trans Another now well established oxidation state for Pt and Pd is +1, which was previously considered to be rare.7-? However, this oxidation state is readily found among binuclear complexes, both homonuclear and heteronuclear, that contain a formal single metal-metal bond.74- 7 5 The metal-metal bond formed by the overlap of single electrons on each metal stabilizes the binuclear framework against the disproportionation reaction (M2 1 —> M n + M°) — the reverse of the M 2 * synthesis, the conproportionation reaction. Two metal centres, acting cooperatively, offer unique reactivity patterns which differ from the well-established reactivity of * The term cis and trans are used throughout with reference to the position of two phosphine ligands in a complex. 63 the mononuclear metal complexes: examples include the insertion of small molecules such as U2S,16 C O , 1 4 ' 1 7 • 1 9 and S0 2 7 7 - 1 8 into the metal-metal bond. In the field of catalysis particularly, the binuclear complexes have been used as simplified models for understanding the mechanisms of catalysis taking place on metal surfaces, because the bridging coordination mode available with binuclear systems may resemble a chemisorption mode of small molecules on such surfaces.70 d * V d*J-yJ,.<izJ \ d x V dz* dxy, dyz, dxz dxV. d / dxy dz* / \ dxy \ dxV.dxy ' dxz, dyz dxy, dyz/dxz\ / \ dz2 / dxz, dyf \ dxz, dyz .. .— tetrahedron free ion octahedron square-planar square-pyramid trigonal-bipyramid Fig. 3.1. Relative energy levels of d-orbitals in different geometries. The M(I) binuclear complexes (M = Pt, Pd) to be described here are made from a conproportionation reaction of M(0) and M(H) (see below).27 - 2 2 The pyridylphosphine ligands in these binuclear complexes adopt a P-N bridging mode, forming a five-member ring with the two metal atoms. The Pt(I) or Pd(I) centres in the molecule resemble Pt(U) or Pd(U) centres, and adopt a four-coordinated, square-planar configuration. Stereoisomerism thus arises from two different orientations of the bridging pyridyl ligand, that is, the head-to-head (HH) and head-to-tail (HT) configurations. The complexes prepared in the present work are shown below: 64 X — M i M2—X X — M x — M 2 — X N. P P. .N. Head-to-Tail, HT M 1 = M 2 = Pd,Pt M^Pd, M 2 = Pt X = CI, Br, I Head-to-Head, HH Mi = M 2 = Pt X = I P= Ph2P, PhPpy, Ppy2 P = Ph2P, PhPpy, Ppy2 The compound Pd2Cl2(H-PNi)2,11a, was prepared in 1981 by Maisonnat et al.27 Two years later, the syntheses of Pt2X2(|i-PNi)2(HT) (X = CI (7a), I (9a)) and Pt2l2(H-PNi)2(HH) (10a) were reported by Farr et al.22 Independently, the tris(2-pyridyl)phosphine palladium binuclear complex, Pd2Cl2(H.-PN3)2 (11c), was made in our group 2 0 The Pd2Cl2(M-PNi)2 complex was reported not to give the insertion product with CO;2 7 also Pd2Cl2(M--PN3)2 was shown in the present work not to react with 1 arm CO in CH2CI2 at room temperature. The reactivity of the M-M bond toward insertion of small molecules is common for the systems with the bridging diphosphine ligand bis(diphenylphosphino)methane, dppm, which was being used for gas separation study in our group originally.77 The lack of flexibility of the bridging pyridylphosphines was thought responsible for the loss of reactivity.27 However, the bridging pyridylphosphines are perhaps not as rigid as suggested because Lee and Yang in our group discovered that the reaction of dimethylacetylenedicarboxylate, DMAD, with Pd2Cl2(p.-PN3)2 led to the formation of the bridged acetylene compound, Pd2Cl2(^-DMAD)(p.-PN3)2.20 The interest in this thesis work was then directed more toward activation of unsaturated hydrocarbons, such as olefins and acetylenes. This study was also extended here to the platinum system. Use of bis-and tris(2-pyridyl)phosphine increases gradually the hydrophilicity of the complexes, which was considered to be an asset for hydration catalysis. The aqueous solution chemistry of the tris(2-65 pyridylphosphine complexes Pd2Cl2(^ -PN3)2(HT) and Pda2(PN3)2 will be considered in Sect. 3.4. The incidental phosphine chirality induced by the bridging of one pyridyl group within the PN2 systems leads to many interesting features in the 3 iPpH} NMR spectroscopic data, which will be dealt with later on. The present study was originally directed to the potential catalytic hydration of activated olefins by the pyridylphosphine complexes. However, preliminary results showed that none of the M(I) and M(U) complexes made activated olefins (acrylonitrile and maleic acid) either in organic solvents or in water, although, acetylenes (dimethylacetylenedicarboxylate, DMAD, and methylpropiolate, MPP) inserted into M2* metal-metal bond, forrning olefinic complexes (Sect. 2.7), further chemistry on the bridging molecules was not observed. In this Chapter, the structural characterizations of M(II) mononuclear and M(I) binuclear complexes of pyridylphosphines by spectroscopic methods are presented. 3.2. Structural characterization of pyridylphosphine complexes 3.2.1. Pt(U) pyridylphosphine complexes The complexes synthesized are shown below: PtX2(PNi)2 PtX2(PN2)2 PtX2(PN3)2 X = C1, la X = C1, lb X = C1, lc X = I, 3a X = I, 3b X = Br, 2c X = I, 3c The synthetic route to dichlorobisphosphineplatinum(II) complexes is basically via reaction of phosphine with potassium tetrachloroplatinate(II) in ethanol solution, or phosphine reaction with a platinum(II) precursor containing a readily displaceable ligand such as cyclooctadiene and benzonitrile. The latter was chosen in the present study, and PtCl2(PhCN)2 and PtCl2(COD) were used as precursor complexes. Dichlorobis(2-pyridylphosphine) derivatives of platinum(U) were obtained by simple replacement of PhCN or COD with the appropriate phosphine ligand. The corresponding bromo and iodo complexes were readily 66 obtained by metathesis of PtCl2(PhCN)2 with appropriate sodium halides. All these isolated mononuclear compounds show satisfactory elemental analysis (see Table 2.1). The PtX2(PNi)2 (X = CI, I) species have been made previously.22 The geometry of the dmalobis(2-pyridylphosphine)platinum(Il) species 1-3 in solution can be easily identified by means of 31P{ lH) NMR spectroscopy. The magnitude of the 195Pt-3 1P coupling constant diagnoses the geometry of the complex. It was first revealed by Pidcock and coworkers in 1962 that the cis iJpt-p value was significantly larger than the corresponding trans one in PtX2(PR3)2 systems.2-3 This observation was later supported by Grim et al. using a different series of Pt(U) bisphosphine complexes.24 Since these studies, the one bond Upt-p coupling constants have been well documented.2-5 - 2 9 The ratio of1 Jpt-p cis to trans is normally about 1.5.24 The assignments for all the PtX2(PNn)2 listed in Table 3.1 are made according to the above coupling constant criteria. A typical 31P{1H} NMR spectrum of a cis-PtX2(PNn)2 species is shown in Fig. 3.2. Table 3.1. 31P{ lH) NMR Parameters for PtX2(PNn)2 Complexes Complexes Solvent 5(ppm) lJPtP(Hz) Ref a cis-PtCl2(PNi)2, la CD2CI2 11.6(s) 3676 tw cis-Ptl2(PNi)2. 3a CD2CI2 6.7(s) 3514 tw trans-PtI2(PNi)2 CDCI3 9.8(s) 2503 22 cis-PtCl2(PN2)2. lb 00202 13.3(s) 3765 tw cis-Ptl2(PN2)2. 3bb CD2CI2 7.4(s) 3630 tw trans-Ptl2(PN2)2b CDCI3 10.4(s) 2500 tw cis-PtCl2(PN3)2. lc° CD2CI2 20.0(s) 3910 tw cis-PtBr2(PN3)2. 2c° CD2CI2 17.3(s) 3833 tw cis-Pt2l2(PN3). 3cc CD2Q2 11.24(s) 3660 tw (a) tw = this work; (b) Recorded at -50°C; (c) Recorded at -70°C. 67 F ig . 3.2. 121.4 M H z 31p{ lH} N M R spectrum of cis-Ptl2(PN 2)2,3b, in CD2CI2 at -50<>C. Based on the assignments in Table 3.1, general trends in the 3 *P N M R chemical shifts and coupling constants of these pyridylphosphine P t n complexes become obvious. A shift to lower resonance frequency is seen on going from la (11.6), through lb (13.3), to lc (20.0 ppm), when P N i is replaced by PN2 and then PN3 for the chloro complexes; and an upfield shift of the phosphorus resonance is observed on going, for example, from lc (20.0), through 2c (17.3), to 3c (11.2 ppm), when chlorine is replaced by bromine and then iodine for the complexes of the same phosphine. The coupling constant Jptp o f the complex increases, for example, on going from la (3676), through lb (3765), to lc (3910 Hz ) , and decreases on going from lc (3910), through 2c (3833), to 3c (3660 Hz) . The downfield shift in the phosphorus resonance signals of the series compared to those of the free ligands, and the trend within la to lc, can be understood in a simplif ied manner in terms of the ligand basicity and the metal deshielding ability. The 3 1 P { J H} N M R chemical shifts of the free ligands move downfield from -3.28 for P N i through -1.7 for PN2 to -0.05 ppm for PN3, indicating that the phosphorus nucleus in P N i is more shielded than that in P N 3 by the lone pair electron density and implying that as a free phosphine, P N i is more basic than PN3. Upon coordination, the phosphorus nucleus is deshielded by the metal. The M - P bond is formed by the a donation of the phosphine lone pair of electrons to the empty dsp 2 hybrid orbital and n back donation from the fi l led d ^ , d y z of the metal to the 3d orbital of the phosphorus. A reduction of 68 electron density on the phosphorus nucleus occurs, and the phosphorus resonance on coordination is expected to shift downfield with respect to the free ligand resonance. The difference between the chemical shift of phosphorus in a coordination compound and in the free ligand is referred to as the coordination shift The larger the coordination shift is , the greater the degree of electron transfer from the ligand to the metal. Apparently, PN3 with a coordination shift of 20.05 ppm in lc perhaps forms a stronger interaction with Pt(U) than PNi which shows a coordination shift of 14.9 ppm in la. This interaction between metal and ligand is also reflected in another NMR parameter, the Jptp coupling constant, where the trend is JptP(ic) > JPtP(lb) > JPtP(la)' consistent with the conclusion made based on the coordination shifts. For a particular phosphine ligand, the observed upfield chemical shift trend 8ci > Sfir > §1, as the halogen changes from chlorine through bromine to iodine, possibly reflects the successive weakening of the platinum-phosphorus a-bond which is consistent with the decrease in the directly bonded spin-spin coupling constant 1 Jp ,^ JptP(Cl) > JptP(Br) > JptP(l). as observed previously.290 3.2.2. Pd(II) pyridylphosphine complexes The complexes synthesized in this category are shown below: As described in Chapter 2 (Sect. 2.5.2), PdCl2(PNn)2 complexes were synthesized by the reaction of PdCl2(PhCN)2 with PNn ligand in dichloromethane solvent The bromo and iodo complexes were again obtained from the metathesis reaction of the PNn dichloro species with sodium halides. In contrast to the Pt(U) phosphine complexes, the corresponding Pd(II) complexes presented problems in assignment of cis or trans geometry. The 31P{1H} NMR spectroscopy reveals no information regarding metal-phosphorus (JMP) coupling — palladium is PdX2(PNi)2 PdX2(PN2)2 PdX2(PN3)2 X = C1, 4a X = I, 6a X = C1, 4b X = I, 6b X = C1, 4c X = Br, 5c X = I, 6c 69 NMR inactive. The 31P{ iH} NMR data of these palladium complexes (Table 3.2) do not serve independently as a structural probe. Apart from the NMR spectroscopy including 31P{ iH}, iH and 13C{ iH} which will be discussed later on in more detail, two other techniques — dipole moment measurement and infrared spectroscopy — used for identification of cis and trans isomers of PdX2p2 type compounds in the solution and in the solid state fail to provide a concrete assignment for these complexes. The basic principles and limitations of these techniques are discussed in the following paragraphs. Table 3.2. 31p{lH} NMR Parameters of PdX2(PNn)2 Complexes3 Complexes 8 Chemical Shifts (ppm) cis trans Ref.b PdCl2(PNi)2. 4a 29.5(s) 23.4(s) 21 &tw PdBr2(PNi)2, 5ac 26.5(s) 20.5(s) tw PdI2(PNi)2. 6ac — 9.6(s) tw PdCl2(PN2)2. 4b 30.9(s)d 17.7(s)d 30&tw PdBr2(PN2)2. 5bc 27.9(s) — tw PdI2(PN2)2. 6bc — 8.9(s) tw Pd2Cl2(PN3)2. 4c 34.6(s) — tw PdBr2(PN3)2. 5^ 32.1(s) — tw PdI2(PN3)2. 6cc — 7.0(s) tw (a) Recorded in CD2O2 at r.L unless otherwise noted; (b) tw = this work; (c) Recorded at -70°C; (d) Cis and trans were made separately in ref. 30. Dipole moment measurement: The geometric isomers of the square-planar type complexes MX2L2 are distinguishable by their dipole moments.57 - 3 2 In principle, trans-PdX2P2 with D2h symmetry has zero dipole moment, as opposed to the cis-complex having a non-zero dipole moment, and measurement of the dielectric constant of a non-polar solvent containing either a cis or a trans isomer of PCIX2P2 relates to the dipole moment of the solute. A zero dipole moment indicates that the solute is of trans structure; however, a non-zero dipole moment does not 70 exclude the presence of trans isomer. To distinguish whether the solute is pure cis or a mixture of cis and trans, a standard cis-compound of similar composition with known dipole moment is required. It is important for the test compound to have good solubility in a non-polar solvent. Unfortunately, the pyridylphosphine Pd(U) series do not dissolve in benzene or carbon tetrachloride (two common solvents used for measurement of dielectric constant) and are only slightly soluble in dichloromethane; the low solubilities made the measurement impractical. Infrared spectroscopy (IR): The position and band shape of palladium halogen (vpd-x) and palladium phosphorus (vpd-p) stretches are often cited in the literature in reporting the geometry of PdX2P2 compounds. In principle, for the cis-isomer with C 2 v symmetry, Pd-X and Pd-P stretches appear as two bands; for the trans isomer with D 2 n symmetry, the symmetric stretch is IR inactive, therefore the Pd-X and Pd-P stretches appear as single bands.26'33 The palladium-halogen stretching frequencies in cis isomers are lower than those in trans isomers [vpd-Cl(cis) = 280 - 310 cm*1, vpd-ci(trans)= 350 - 357 car1],34'40 because the trans influence of phosphine in a cis isomer weakens the palladium halogen bond.-37* On the other hand, the palladium-phosphorus stretching frequencies in cis isomers are higher than those in trans ones [vPd-P(cis) = 400 - 450 cm"1, vpd-P(trans) = 340 - 400 car1]34 ~36<37a because the greater trans influence of the second trans phosphine reduces the palladium phosphorus bond strength more efficiently than does the halogen. As the halogen changes from chlorine through bromine to iodine, the reduced mass \i [|i = mim2/(mi + m2)] increases, which leads to the lowering of vibration frequency according to the following equation: The stretching frequencies of Pd-X bands (X = Br, I) should apparently be below 280 cm*1. Unfortunately, the free pyridylphosphines absorb strongly in the stretching frequency range of the palladium-halogen and palladium-phosphorus atoms; the absorption bands of the coordinated pyridylphosphines also cover up the bands of interest and make the identification of the Pd-X and Pd-P vibration modes and geometry assignment impossible (Figs. 3.3 and 3.4). 71 i i 1 r 800.0 700.0 600.0 500.0 400.0 300.0 200.0 Wavcnumbcrs (cm"1) Fig. 3.3. Infrared spectrum of the phosphine ligand PN3 in Nujol on KBr plate. Nuclear magnetic resonance: Apart from the one bond metal-phosphorus coupling constant criterion which has been used very successfully in P1X2P2 systems, the magnitude of a two bond phosphorus-phosphorus coupling constant is also known to be dependent on whether the atoms are mutually cis or trans, and is thereby characteristic of the geometry of the complex.2 5-4 1 • 4 2 This is particularly important for the palladium systems where the one bond metal-phosphorus coupling does not exist It is well established that, in bis (phosphine) complexes of paUadium(IT), phosphorus-phosphorus coupling in trans geometry is much greater than in cis; for example, the values of 2Jpp in cis and trans PdCl2(PMe3)2 are -8.0 and +610 Hz respectively.42 Direct observation of a 3 1 P NMR spectrum does not show this 2Jpp because the two phosphorus atoms are always chemically and magnetically equivalent Indirect observation 72 600.0 500.0 400.0 300.0 200.0 600.0 500.0 400.0 300.0 200.0 Wavenumber (cm-1) Wavenumbcr (cm"1) Fig. 3.4. Infrared spectra of the complexes cis-PtCl2(PN3)2, lc, and cis-PdCl2(PN3)2,4c, in Nujol on KRS-5 plate (42% TlBr + 58% Til). of 2Jppis possible, however, by * H or 1 3C{lH} NMR spectroscopies in which the magnetic inequivalence of the phosphorus nuclei gives rise to a complicated second order spectrum.26-42 • 4 6 These techniques work particularly well for deterrriining the gross geometry of bis(phosphine) palladium(U) complexes with phosphines containing cc-methyl or methylene groups, as the phosphines when trans are virtually coupled. The spin systems for these complexes are AnXX'An' (n = 3 or 2) for the * H and AXX' for the 1 3 C { !H} NMR spectra, respectively, of the phosphine methyl and methylene groups (A = the measured nucleus, lH, or 1 3 C ; X = 3 1 P ) . Mann et al. extended this method to an aromatic 1 3 C atom and obtained similar results.44 The multiplicity of the ! H or 1 3 C signals in the * H or ^ C p H } NMR spectra is largely dependent on the magnitude of 2Jpp. Unfortunately, the limited resolving power of the NMR spectrometer in many cases only allows for an estimate of 2Jpp after lengthy mathematical calculations and spectrum simulation. In practice, the splitting pattern — a doublet or triplet — is found to be characteristic of the geometry. This is possible because in trans complexes the oc-methyls or -methylenes are virtually coupled, and 2Jpp is usually large (approximately 400 Hz) such that the phosphine * H and ^ C ^ H } resonances appear as a 1:2:1 triplet.26 When the phosphines are mutually cis, these resonances normally appear as a 1:1 doublet since 2Jpp is usually small. A lH NMR 1:2:1 triplet for the methyl or methylene protons will be observed when |JPH - Jp'H I2 < 2JppAvi/2, where Avi/2 is the resolving power of the spectrometer; and a 1:1 doublet will be observed when [JPH - Jpn I2 > 4JppAvi/2. The condition for the occurrence of a 1:2:1 triplet in the 1 3 C { lH} NMR spectrum is that |jpc - Jp'C I2 < 8Jpp-Avi/2- The intensity ratio of 1:2:1 is essential in judging the authenticity of trans geometry because cis complexes sometimes give a pseudo triplet with variable intensity ratio, the so-called "filled-in" doublet. This pseudo triplet can be misleading, especially when the signal-to-noise ratio is low. 4 c « 2 5 • 4 5 As mentioned above, the 3 1 P { 1 H } NMR spectrum of either a cis or a trans PCIX2P2 species in solution shows as a singlet because in both circumstances the phosphorus nuclei are chemically and magnetically equivalent. When only one isomer is present, the 3 1 P { lB.} NMR spectroscopy does not tell whether that species is cis or trans. If both isomers are present, the 74 resonance at lower field is often assigned to the cis-PdX2P2 species,-30' 40< 4 6 despite the opposite order being seen for the platinum analogues.24- - 3 0 The 31P{ lK) NMR spectral data in Table 3.2 show that all except 4a and 5a of the PdX2(PNn)2 complexes made give one 31P{ lH} singlet in CD2CI2, which presents difficulty in making assignments for these phosphine complexes. However, if the singlet at 29.5 ppm of 4a is assigned to the cis isomer and the upfield singlet at 23.4 ppm to the trans one (the assignments to cis and trans were not made by the authors who first reported 4a) 2 7 a downfield shift, 54a(cis) < S4b(cis) < 84c(cis)» becomes evident. Newkome and coworkers-30 also found that the 3 1P{ lH) NMR chemical shifts of cis-PdCl2(PPh3)2, cis-PdCl2(PNi)2, and the crystallographically characterized cis-PdCl2(PN2)2, moved downfield with the sequential replacement of the phenyl groups by the pyridyl groups on the phosphorus. Apart from this, an upfield shift trend is established for the trans iodo complexes, 86a(trans) > S6b(trans) > 86c(trans)- The halide substitution induced shifts within both series can be understood in terms of the trans influence order I > Br > CI. The *H NMR spectra of the above phosphine complexes showed overlapping multiplets in the pyridyl and phenyl resonance regions, and the lack of methyl or methylene protons makes a clear-cut judgement of the signal pattern impossible. Fortunately, the 1 3C{ *H} NMR spectra of the complexes, whose 31P{!H} NMR spectra show one singlet at -40°C in CDCI3, showed more definitive results. The basic patterns described previously — doublet for a cis isomer and triplet for a trans isomer — are observed. The complexes PdJ.2(PNi )2,6a, and PdJ.2(PN2)2,6b, showed 1:2:1 triplets for the ortho- and meta-carbons on both pyridyl and phenyl rings, indicative of a trans geometry; on the other hand, PdCl2(PN3)2,4c, gave 1:1 doublets for the ortho- and meta-carbons on the pyridyl rings, indicative of a cis geometry (Fig. 3.5). These results are consistent with the previous assignments based on the 3 1 P NMR chemical shift trends. The positions of the 1 3 C signals of the selected compounds are listed in Table 3.3. Because the assignments are based on the solution 31P{1H} and 13C{VH} NMR data, the assigned structures are then the preferred geometries ia solution under the conditions of study. Isomerization may have taken place when the solid complex dissolves in solution. An attempt was made using 75 (a) KC6 \ C2 J J.i u i / l . id C3 C4 C9 |c7 UUU T T H - 1 — ! — I — I — r -C8 ~i—i—t j *i—i—i—i—i—i—i i T — j i i i—r—r—i—i 1 i—ff 155 ISO 14S 140 !35 330 125 120 1(5 PP*i U O (b) C6 C8 C2 • 1 ' '" T ~' I 1 • 1 1 I ' ' 1 ' I ' 1 ' ' l • 1 ! • ' 1 • t ' • • HQ l i S 150 140 1(0 ;3S U C 125 C9 C7/C4 C3 C8 C6 C9 k C7 11 1111 1. 11 I4f I4» 13' 136 I3J I 3 » 133 I )J 131 • / / . . , l i i . | I ' . . , i n t| 126 1J5 124 ffff Fig. 3.5. 13C{1H} NMR spectra (75.4 MHz) of palladium pyridylphosphine complexes: (a) trans-PdI2(PNi)2, 6a, (b) trans-Pdl2(PN2)2, 6b and (c) cis-PdCl2(PN3)2, 4c; in CD2C12 at -20°C (see Table 3.3 for C atom numbering system). 76 Table 3.3. 13C{ lH) NMR Chemical Shifts of 4c, 6a and 6b, in CD2CI2 at -20P& C(l) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) PNi 136.2(d,11.6) 134.2(d,20.0) 128.7(d,7.4) 129.1(s) 164.0 150.4(d,12.7) 127.9(d,15.2) 122.2(s) 135.8(d,2.1) PN2 — 135.1(d,20.6) 128.9(d,7.8) 129.7(s) 162.9(d) 150.5(d,11.9) 128.5(d,17.9) 122.6(s) 135.9(d,3.0) PN3 — — — — 161.9(d) 150.5(d,11.5) 129.3(d,19.5) 122.9(s) 136.0(d,3.4) 6a — 131.6(1,11.8) 123.6(t,10.2) 126.6(s) — 145.4(t,15.2) 126.3(t,24.6) 119.9(s) 131.0(br.s) 6b — 136.8(t,14.0) 127.6(t,12.2) 131.0(s) — 149.1(t,17.0) 131.5(t,24.0) 124.0(s) 135.1(t,10.0) 4c — — — — — 148.7(d,20.0) 131.5(d,21.7) 124.3(s) 135.5(d,17.4) (a) The 13C{ lU) NMR spectra of the free ligands were measured in CDCI3 solvent at r.t. and those of the complexes were recorded in CD2CI2 at -20°C; the chemical shifts are referenced to TMS in ppm, and the number in the brackets is the I nJpc + n + 2Jpd v a l u e m s, d and t indicate singlet, doublet and triplet, respectively. The carbon atoms C(l) to C(9) correspond to the individual carbon atoms on the phosphine ligand as depicted below. 0 ; C » 3-n 6 7 n solid state 31P{ *H} NMR (cross-polarization and magic angle spinning, CP/MAS) to correlate structures in solution with those in the solid state, but with little success (Table 3.4). The selected samples analyzed by CP/MAS 31P{1H) NMR show a pronounced chemical shift anisotropy for the phosphorus nuclei. The 3 1 P peaks are normally broad, up to 10 ppm; for instance, PdCl2(PNi)2,4a, gives a singlet at 25.2 ppm with linewidth at half height of 8 ppm (Fig. 3.6). It is uncertain, therefore, whether one isomer or both are present in the solid state, knowing that the solution chemical shift of the cis is seen at 29.5 ppm and that of the trans at 23.4 ppm. Solid state site symmetry47 also presents a problem in interpreting the spectra. For example, the 31P{1H} NMR spectrum of cis-Ptl2(PN3)2,3c, (Fig. 3.7a) shows two sets of signals with Jptp values of 3599 and 3294 Hz. Both of these are assigned to cis isomers according to a literature report48 in which a similar splitting pattern is seen for cis-PtCl2(PPh3)2-The splitting is attributed to the site symmetry in the solid state. It is relatively easy to identify such splitting for Pt but is difficult for Pd. For example, even when two peaks appear at 46.4 Fig. 3.6. Solid-state ^P^H) NMR spectrum (40.4 MHz) of PdCi2(PNi)2 obtained at r.t. using cross-polarization and magic angle spinning; 8 = 25.5 (s). 78 and 40.3 ppm in the case of PdCl2(PN3>2 (Fig. 3.7b), it is still uncertain whether they are due to the resonances of cis and trans isomers or arise from the local site symmetry. In addition, chemical shifts measured in the solid state are somewhat different from those measured in solution. Nevertheless, for the iodo complexes 6a - 6c, the solid state 3 1P{1H} NMR data agree well with the solution spectra. The trans-Pdl2(PNn)2 species is corifirmed to be the only isomer present both in solution and in the solid state. In the case of PdBr2(PN3)2,5c, it seems reasonable to conclude that both cis and trans isomers are present in the solid state with their chemical shifts about 20 ppm apart But it is impossible to correlate the observed solution singlet at 31.0 ppm with either of the two singlets observed in the solid state (Fig. 3.7c). The phosphorus resonances at 42.1 and 23.8 ppm are assigned to the cis and trans isomers, respectively. Table 3.4. Solid State 31P{ ^ H} NMR Data of PdX2(PNn)2 Complexes at Ambient Temperature Complexes 8 Chemical Shifts (ppm) PdCl 2PN 1) 2,4a 25.2 PdI2(PNi)2,6a 8.0 PdCl2(PN3)2,4c 46.4 (50%), 40.3 (50%) PdBr2(PN3)2,5c 42.1 (50%), 23.8 (50%) PdI2(PN3)2,6c -2.5 Thus despite the problems with the mononuclear Pd pyridylphosphine complexes, assignments are made. We also notice that the chloro and bromo complexes prefer cis geometry in solution at low temperature. A weak JC interaction of the pyridyl group on one phosphine with the pyridyl group on the adjacent phosphine in the PdCl2(PN2)2 crystal structure was noticed by Newkome et al.;-30 this weak interaction might provide extra stabilization energy for the cis geometry, especially at low temperature when molecular motion is restricted. 79 (a) i r 160.0 ~~i r 80.0 ~i—i 1 r 0.0 -80.0 -160.0 ppm (b) I 1 1 1 240.0 160.0 80.0 0.0 ~i r -80.0 -160 ppm (c) -1 r 160.0 80.0 0.0 ~i r -80.0 •160.0 ppm Fig. 3.7. Solid-state 31P{1H} NMR spectrum (40.4 MHz) of (a) 3c, (5 = 13.1 (s) and 1.9 (s), Uptp = 3599 and 3294 Hz, respectively), (b) 4c, and (c) 5c, obtained at r.L using cross-polarization and magic angle spinning. 80 3.2.3. Pt(I) binuclear pyridylphosphine complexes The Pt1 binuclear bridging complexes discussed here were synthesized via the conproportionation reaction of Pt(0) and Pt(H) precursors (Sect. 2.6).22 Two stereoisomers: head-to-head, HH, and head-to-tail, HT, are possible (see Sect. 3.1). However, the only isolable HH isomers were limited to the three iodo complexes Pt2l2(H-PNn)2- The head-to-tail isomers 7 - 9 discussed here are shown below: X = CI, 7a X = CI, 7b x = CI, 7c X = I, 9a X = I, 9b X = Br, 8c X = I, 9c The 31P{1H} NMR spectroscopy was commonly used as the structural tool in assigning configurations. The isotopomers drawn below (Fig. 3.8) can be applied to any of the Pt-PNn systems with an HT arrangement. The natural abundances indicated for each isotopomer arise from the presence of the 195Pt spin labelled isotope. P N P N P N N ^ P N ^ P N ^ P A B C 43.8% 44.8% 11.4% Fig. 3.8. The isotopomers of a Pt2(y>PNn)2X2 (HT) type compound, n = 1,2, 3, X = CI, Br, I; Pt* = 195pt> natural abundance of 1 9 5Pt = 33.8%, I = 1/2; P = PPh2, PPhpy or Ppy2; N = pyridyl moiety; halides are not shown. 81 The 31P{1H} NMR spectrum shown in Fig. 3.9 is that of Pt2l2(ji-PN3)2(HT), 9c, which is typical of an HT-isomer. The major central peak A in Fig. 3.9 arises from the unlabelled isotopomer A , because in the absence of 195Pt, the two phosphorus atoms are chemically as well as magnetically equivalent. The doublet of doublets labelled B comes from the singly labelled isotopomer B, in which the two phosphorus atoms are chemically equivalent but magnetically inequivalent, this giving rise to the fine satellite splitting centred at the A position. This is an A A X spin system with J A X - J A ' X (*JptP -2JptP) being » Jpp, and hence the spectrum is pseudo-first-order with the outermost doublets given by the phosphorus directly bonded to 195Pt centre and the innermost doublets arising from the phosphorus bonded to the non-spin labelled Pt. The signal labelled C arises from isotopomer C , but such signals are not always observable due to the low intensity. The only situation where the 31P{1H} NMR spectrum is ambiguous for an HT/HH assignment is when the 3Jpp coupling constant becomes very small or unresolvable (see Ch. 4, Sect. 4.3.1). When the ambiguity arises, ^ PtpH} NMR can be used to distinguish the two possible phosphine orientations. The typical 195Pt{ *H} spectrum of an HT isomer, e.g. 9c, is shown in Fig. 3.9(b) as a "doublet of doublets" arising from the overlap of the signals due to the isotopomer B and C. The 1 Jptp and 2Jptp can be obtained by spectrum simulation with good agreement with the 3 *P {*H} data. The basic spectral pattern does not change as the number of pyridyl substituents on the phosphorus atom changes. A downfield 8p chemical shift is seen on going from 7a through 7b to 7c (Table 3.5). Fig. 3.10 shows the 31P{1H} NMR spectrum of 7b as two distinct sets of the basic spectral pattern discussed above, and can be understood in terms of stereoisomers drawn in Fig. 3.10. 82 A (a) —I—i : — i 1 — | — ! 1—i—i—]— i r—i—i 1—i 1 1 1 — | i i 1—I 1—i—r - ' . G O -150 -200 -250 -300 -350 PPM Fig. 3.9. * lP[lH) and 195pt{lH} NMR spectra of Pt2l2(H-PN3)2 (HT), 9c, in CDCI3 at r.t.; (a) 31p{lH} NMR (121.4 MHz) and (b) ^Pt^H} NMR (64.2 MHz) (data given in Table 3.5). 83 Jf) CI-Pi Pi-CI (IS, 2S)* 7b.1 P N py- po) y Cl-Pt Pt-Cl I i N (2h>...n ph (IR, 2R) 7b.2 Cl-Pt Pi-CI I I N <z)p...» py U -(IR, 2S) 7b.3 I—I—i—I—i—|—i—I—i—r—|—I—I—I—I—|—I—l—I—i—|—I—r->.—i—|—i—i—i—I—|—i—n—i—|—I—I—:—i—i—i—:—>—i j—i—i—n— 25 20 15 10 5 0 -5 -10 -15 -2C RPM Fig. 3.10. 31P{ 1H} NMR spectrum of the diastereomers of 7b in CDCI3 at r.L (data given in Table 3.5). Peaks are not assigned to specific diastereomers. * Only the chiral phosphorus atoms are numbered; regardless of the configuration of the molecule drawn, in the paper plane, the P atom above Pt is labelled P(l), and the P atom below is labelled P(2). 84 Table 3.5. 31p{lH.} and 195pt{lH} NMR Data of Pt2X2(H-PNn)2 (HT) a Complexes Solvent 6 P 8pt ijpt-p 2Jpt-P 3 j p p Ref.b Pt2a2(u-PNi)2(HT) t 7a CDC13 -1.5 4124 215 17.8 22 Pt2I2(^-PNi)2 (HT), 9a C D a 3 5.7 3905 198 17.8 22 Pt2Cl2(p.-PN2)2 (HT), 7b CDCI3 0.3 4090 215 20.0 tw -0.2 4090 215 20.0 tw Pt2I2(^-PN2)2(HT), 9b CD2G2 -4.67 3945 196 ~16c tw -5.04 3945 196 ~16c tw Pt2Cl2(u-PN3)2(HT), 7c C D Q 3 0.66 4074 214 18.2 tw Pt2Br2(H-PN3)2 (HT), 8c CDC13 -0.27 4006 214 17.6 tw Pt2I2(n-PN3)2(HT), 9c CDC13 -3.09 3936 205 18.9 tw -197.3 3901 207 — tw (a) Recorded at r.t.; op and 8pt measured in ppm, coupling constants in Hz. (b) tw = this work, (c) Satellites difficult to resolve. When one of the two pyridyl groups attached to a phosphorus atom forms a bridging bond across Pt-Pt, that phosphorus atom becomes asymmetric. Assuming no or slow exchange between the bridging and non-bridging pyridyl groups, three compounds, 7b.1, 7b.2 and 7b.3, are expected. The stereochemical relationships between these isomers are described by symmetry operations. 7b.l and 7b.2, related by a mirror plane, are enantiomeric; 7b.3 is not related by any symmetry to 7b.l/7b.2 and therefore, is a diastereomer of these.49- 5 0 In an achiral solvent, diastereomers are chemically different, but enantiomers are ^ distinguishable.49 Consequently, 7b.l and 7b.2 give one set of signals and 7b.3 gives another set (Fig. 3.10). The well-resolved resonances in the 31P{ NMR spectrum of Pt2Cl2(p.-PN2)2 (HT) at room temperature imply that either no exchange of the bridging pyridyl with nonbridging pyridyl takes place or the exchange is too slow relative to the NMR time scale. 85 The crystal structure of 7b.l, Pt2Cl2(iU-PN2)2(HT) (IS, 2S) was done by SJ. Rettig in this department (Fig. 3.11). In the solid state, this molecule is, in fact, not coplanar as drawn above. The dihedral angle between two coordination square planes of platinum is 36.19°. The Pt, P, and N atoms are arranged rather like a boat conformation. There is a C2 axis through the centre of the Pti-Pt2 bond for the main frame, and the noncoordinated phenyl and pyridyl groups are severely twisted (the dihedral angles between two phenyl rings and two pyridyl rings are 44.10 and 41.25°, respectively). On the whole, the C2 symmetry of 7b.l, present in solution, no longer exists; the phosphorus atoms become diastereotopic in the crystal lattice. The other P-N phosphine arrangement is the head-to-head, as described in Sect. 3.1. The iodo complexes of HH configuration discussed here are shown as follows: The change of ligand orientation from HT to HH brings about significant changes in the appearance of the 31P{1H} spectrum. In general, four isotopomers with appropriate abundances need to be considered (see Fig. 3.12). The representative 31P{1H} spectrum of Pt2J-2(H-PN3)2 (HH), 10c, is shown in Fig. 3.13. The major singlet D is due to the unlabelled isotopomer D. The outermost satellites labelled E arise from isotopomer E, where the splitting, Upt-p, is determined by the 3260 Hz difference between the two lines, which correspond to the one bond Pt-P coupling. This value is significantly smaller than that of the HT isomer, 3936 Hz, due to the trans effect of the phosphine ligands. The inner satellites labelled F are considerably broader and are due to the isotopomer F. The broadening is accounted for by the fast relaxation 10a 10b 10c 86 Fig. 3.11. 3D structure of Pt2Cl2(p:-PN2)2 (HT), 7b.l; the ORTEP drawing and atomic numbering scheme. Representative bond lengths and angli Pt(l)-Pt(2) = 2.574 (1) Pt(l)-P(l) = 2.169(3) Pt(l)-N(3) = 2.10(1) Pt(2)-P(2) = 2.169(1) Pt(2)-N(l) = 2.08 (1) i, in A and ° , respectively: N(3)-Pt(l)-Pt(2) = 91.4 N(3)-Pt(l)-Cl(l) = 88.0 P(l)-Pt(l)-Cl(2) = 99.1 P(l)-Pt(l)-Pt(2) = 81.5 Cl(l)-Pt(l)-Pt(2) = 173.5 Cl(2)-Pt(2)-Pt(l) = 176.6 87 transmitted from the quadrupolar nitrogen atoms through 1 9 5 Pt to P. Isotopomer G is supposed to give a doublet of doublets, but the signal-to-noise ratio is insufficient to allow identification of the peaks. P N P N P N P N jt — Pt J 1 " * ! 1 Pt —Pt* Pt*-Pt P ^ N D E F G 43.8% 22.4% 22.4% 11.4% Fig. 3.12. The isotopomers of a Pt2l2(H-PNn)2 (HH), n = 1, 2, 3; natural abundance of 1 9 5 Pt = 33.8%, I = 1/2. Pt* = 1 9 5Pt; N = pyridyl moiety; P = PPI12, PhPpy, or Ppy2; halides are not shown. In all the isotopomers shown, the phosphorus nuclei within each isotopomer are always equivalent. The splitting of satellites seen in the HT isomers is no longer visible in the HH system. The differences in the 3 1 P NMR spectral appearance, as well as the magnitude of ijptp coupling constant, provide a good probe for elucidating the structure of these bridging Pt complexes. The typical ^Pt^H} NMR spectrum of an HH isomer, for example 10c, is shown in Fig. 3.13(b) as an authentic triplet arising from the isotopomer E. The 31P{1H} and 195Pt{ lH) NMR data for the Pt2l20>PNn)2(HH) complexes are listed in Table 3.6. Examining the stereochemical relationship between the stereoisomers of Pt2l2(H-PN2)2 (HH), 10b, generated in situ, one expects for planar structures two similar sets of signals for lOb.l and 10b.2/10b.3 because 10b.2 and 10b.3 are enantiomers and both of them are diastereomers of lOb.l by external comparison.42-43 The two phosphorus nuclei in lOb.l, related by a mirror plane perpendicular to the P-Pt-P axis, are enantiotopic and, therefore, a 3 1 P 88 D (a) 'jpip 10 i i • i i i > i i .1 • i i i i • • • ! • • • • i • • 1 1 i 1 1 1 1 i 1 1 '• 1 I ' 1 1 1 I 1 ' ' ' I 3L 3 b 25 so i's 10 5 0 - 5 PPM -10 (b) Fig. 3.13. 3 1P{ JH} and 1 9 5Pt{ 41} NMR spectra of Pt2l2(u.-PN3)2 (HH), 10c, in CDC13 at r.t.; (a) 31P{ lH} NMR (121.4 MHz) and (b) 195Pt{ 41} NMR (64.2 MHz) (data given in Table 3.6). 89 Table 3.6. 31P{ lH) and 19SPt{ *H} NMR Data of Pt2(M>PNn)2l2 (HH)a Complexes Solvent 8P 8pt Uptp 2JptP 3 j p p Ref.b Pt2l2(^-PNi)2,10a CDCI3 9.6 3260 153 — 22&tw Pt2I2(H-PN2)2, lOb.lc CD2CI2 12.55 3276 122 441 tw 11.0 3276 122 441 tw -739.4(t) 3261 — — tw 10b.2/10b.3c CD2CI2 11.30 3249 182 — tw Pt2l2(^PN3)2,10c CDCI3 14.11 3260 136 — tw -759.5(t) 3259 — — tw (a) Recorded at r.t.; chemical shifts 8p and 8pt measured in ppm, coupling constants in Hz. (b) tw = this work, (c) Recorded at -20°C. (IS, 2R) (IS, 2S) (IR, 2R) 10b.1 10b.2 10b.3 singlet with Pt satellites is expected. The two phosphorus atoms in 10b.2/10b.3, related by the C2 axis in the molecular plane perpendicular to the P-Pt-P axis, are equivalent and should give rise to a singlet and Pt satellites. However, the actual experimental spectrum of the diastereomeric mixture of 10b (Fig. 3.14) is different from that expected for planar structures. It appears to be composed of a singlet and an AB quartet with their respective Pt satellites. This was ascertained by measuring the 3 1P NMR spectrum of both the separated diastereomer lOb.l and the 10b.2/10b.3 mixture — enantiomers are not separable by TLC (see Sect. 2.6.2 for detailed separation procedure) (Fig. 3.15). The tight AB quartet reflects e^quivalent P atoms in the molecule. The coupling constants of this compound were resolved by spectrum simulation: 90 Jpp^Jptp and 2Jptp were found to be 441, 3276 and 122 Hz, respectively. The magnitude of the P-P coupling confirms that the two P atoms are trans to each other, and this is reinforced by the value of 1 Jptp (3276 Hz). The inequivalence of the P atoms can only arise from lack of symmetry element in the molecule: the P atoms are no longer enantiotopic as in lOb.l, or equivalent as in 10b.2/10b.3, and must be diastereotopic. I — I — I — i — r — l — I — r — I — | — i — I — I — I — | — l — i — i — i — | — i — i — i — i — ] — i — i — i — i — | — i — i — i — i — i — i — : — : — : — : 10 25 20 15 10 5 0 -5 P P « -:o' Fig. 3.14. 121.4 MHz 31P{tH} NMR spectrum of the mixture of diastereomers of 10b in CD2CI2 at r.L The correlation between the diastereomeric structures depicted above and the 3 1P{ 1H) NMR spectra in Fig. 3.15 was not obvious. Some single crystals of lOb.l obtained were highly disordered, and species 10b.2/10b.3 grew as twin crystals in three occasions (Sect. 2.6.2). The problem in assigning the structures was finally solved later by the 31P{ *H} NMR spectral analyses of the DMAD derivatives of 10b. 1 and 10b.2/10b.3 which will be discussed in full detail in Chapter 4, Sect 4.3.4. Species 10b. 1, the second fraction obtained from a TLC plate, turns out to be the one giving the "abnormal" 3 1 P spectrum shown in Fig. 3.15(b). The 10b.2/10b.3 mixture, the first fraction from the TLC, gives the typical 3 1 P NMR spectrum as in Fig. 3.15(d) expected for coplanar geometry. The inequivalence of phosphorus atoms in lOb.l arises because the mirror plane relating these atoms must be disrupted. However, the C2 91 Fig. 3.15. 31P{1H} NMR spectra (121.4 MHz) of the separated diastereomers of 10b in CD2C12; (a) lOb.l at r.t, (b) lOb.l at -10°C, (c) 10b.2/10b.3 at r.t, and (d) 10b.2/10b.3 at -20°C (data given in Table 3.6). 92 symmetry of the molecule does not seem to be affacted. Recall that in the X-ray structure of the HT isomer 7b. 1 (Fig. 3.11), the noncoordinated pyridyl and phenyl groups on one phosphorus atom are twisted with respect to those on the another, despite these groups being relatively far away from one another spacewise. In the HH isomer, two phosphorus atoms are situated on one platinum, the interaction between the groups on the phosphorus atoms being so severe that, even in solution, the expected mirror plane for lOb.l molecules symmetry cannot be maintained. 3.2.4. Pd(I) pyridylphosphine complexes Dichlorobis[2-(diphenylphosphino)pyridine]dipalladium(I), Pd2Cl2(|i-PNi)2, was reported by Maisonnat et al.27 and was assigned the head-to-tail structure based on analogous structures for heterodinuclear complexes of the same phosphine ligand, for example, with PdW 22,51 ^ PdJRh11.57 • 5 2 In these cases, the P-N phosphine arrangement on the metal centre can be easily distinguished by the spectroscopic analysis. The PdW/PNj heterodinuclear complexes made in the present work, 14c-16c, were shown to have exclusively the HT configuration by their 3 1 P NMR spectra (Table 3.7, Fig. 3.16). The doublet must arise from the chemical inequivalence of the two P atoms attached to two different metal centres. The P atom attached to Pt is split by 195Pt and gives rise to the larger satellite (3900 Hz), while the P atom attached to Pd is split (by 195Pt) by only about 100 Hz (Table 3.7). The splitting between the two phosphorus atoms, 14.3 -16.8 Hz, is characteristic of an HT disposed phosphine (see the corresponding Pt2 analog, Sect. 3.2.3). No HH-isomer with both P atoms on palladium has ever been found. Table 3.7. 31P{ iH} NMR Data for PtPdX2(u-PN3)2(HT) Complexes3 Complexes Sp(Pt) 8p(Pd) 2Jptp 3j p p PtPdCl20i-PN3)2. 14c -3.92 (d) 9.4 (d) 3988 110.5 14.3 PtPdBr20i-PN3)2. 15c -5.62 (d) 7.78 (d) 3951 100.0 15.6 PtPdl2(H-PN3)2. 16c -8.36 (d) 4.46 (d) 3863 75.4 16.8 (a) The spectra were recorded at r.L in CDC13; 8p's are measured in ppm and the coupling constants are in Hz. 93 I i j i i i i i i i i i I i i i i j i i i i i i i i i j i i i i i i : i i | i i i i I i i i i • r i 0 15 10 5 0 - 5 -10 -15 -20 PPM -25 F ig . 3.16. 3lp{ lH} N M R spectrum of 14c in CDCI3 at r.t, see Table 3.7. The singlet at 6.3 labelled X is the Pd2Cl2(M--PN3>2 impurity. The molecular structure of Pd2l2(p>PNi)2 was determined crystal lographically, 2 0 where the H T orientation was shown explicit ly. The downfield shift of 3 1 P N M R resonances through the series 13a, 13b and 13c (Table 3.8) is normal, as is the downfield shift induced by the halogen substitution f rom 11a, 12a, to 13a (Sect 3.2.2). In the case of Pd2Cl2(u,-PN3)2, support for the H T configuration comes indirectly from the H T configuration (ktermined for the D M A D insertion adduct, assuming no rearrangement occurs during the course of reaction (see X -ray structure in F i g . 4.1). Hence, the H T configuration is assigned to al l the dipalladium complexes, based on the relationships between their 3 l p { l H } N M R data (Table 3.8). The structures of the H T Pd2X2(j i -PNn) 2 species discussed here are shown below: X = Q , 11a X = CI , lib X = CI, 11c X = B r , 12a X = B r , 12b X = Br , 12c X = 1,13a X = 1,13b X = 1,13c 94 Table 3.8. 3 1 P { l H ) Chemical Shifts for P d ^ t J i - P N n h (HT) Complexes 3 Complexes Solvent 5p(ppm) Ref.b Pd 2 Cl20i-PNi)2.11a CD2CI2 4.4 21 & t w Pd 2 Br 2 ( ^ -PNi ) 2 .1 2 a CDCI3 2.92 tw Pd 2 I 2 (n-PNi) 2 .13a C D a 3 -0.44 tw P d 2 C l 2 ( p . - P N 2 ) 2 . l ib CD2CI2 5.2 tw Pd 2 Br 2 (u -PN 2 ) 2 .12b CD2CI2 3.28 tw Pd 2 I 2 0a -PN 2 ) 2 .13b CD2Q2 -0.16 tw Pd 2 Cl 2 (n -PN 3 ) 2 .11c CDCI3 6.3 tw Pd2Br 2(n-PN 3) 2.12c CDCI3 4.35 tw Pd 2 0 i -PN 3 ) 2 I 2 .13c CDCI3 0.55 tw (a) A l l these chemical shifts are measured at r.t (b) tw = this work. A sharp 3 1 P N M R singlet signal is seen for 11a and 11c, whi le a broad singlet is obtained for l ib where the diastereomeric mixture is present as l lb . l , llb.2 and llb.3, as depicted below: l l b . l l lb.2 l lb .3 The P nuclei in llb.l/llb.2 are equivalent and should give one singlet. The P atoms in llb.3, related by a centre o f inversion located in the centre of the P d - P d bond, are enantiotopic and should also give rise to a singlet Therefore, two singlets reflecting the presence of two sets of diastereomers are anticipated. The broad singlet in experimental spectrum in C D 2 C 1 2 at room temperature indicates the presence of an exchange process responsible for an 95 averaging of the two singlets. Two sharp singlets do appear at -20°C and remain unchanged down to -85°C (Fig. 3.17). This broadening phenomenon is not seen in the Pt2 case (see 7b, Sect. 3.2.3). It is well-known that Pd systems are substitutionally more labile than Pt systems,-5-*-54 therefore, it is not surprising that l lb. l , llb.2 and llb.3 are interconvertible. Direct inversion of the phosphorus centre is not feasible.-5-5'56 Dissociation of the pyridyl moiety from the Pd seems to be involved: subsequent rotation of the Pd-P bond would allow for the two pyridyl groups to be indistinguishable in reforrning the bridging binuclear compound. The inversion of absolute configuration of the P atom is realized via the dissociation-rotation-recoordination. This cycle is prevented when the sample is subjected to sufficiently low temperature. This mechanism also explains the resolution of the diastereomers at low temperature. The iodo complex 13b exhibits much more complex 3 1 P NMR behaviour in a variable temperature experiment (Fig. 3.18). These spectra are reproducible in the temperature range from room temperature down to -85°C, and are unaffected by addition of an acetone solution of sodium iodide. The broad peak seen at room temperature separates into two broad peaks at -20°C; however, further cooling allows the downfield signal to coalesce at -40°C and to reappear at -60°C as a doublets of doublets (or an AB quartet) centred at the original position*. The upfield singlet stays at the same position with enhanced intensity. We have no definite explanation at this stage for the observed 3 1 P NMR spectra; perhaps, a non-planar structure (cf. 7b and lOb.l) is responsible for the inequivalence of the two phosphorus atoms, especially when the molecular motion is restricted at low temperature. 3.3. So lu t ion e q u i l i b r i u m of the c i s - P t I 2 ( P N 2 ) 2 c o m p l e x i n CDCI3 In previous discussion, the geometry of the PtX2(PNn)2 compounds was assumed to be cis, according to the magnitude of their Jptp coupling constants in the 3*P NMR spectra (Sect. 3.2.1). No signals of any other species are detected from room temperature down to -70°C from 100 to -200 ppm in CD 2C1 2 solvent for such species. However, the 3 1 P NMR spectrum of 3b 96 (a) (c) - I — i — I — i — i — i — i — i — i — i — i — i — i — I — i — I — i — r - ! i i i r | — i — i — i — r 25 20 - i — i — | — i — i — i — i — | — i — i — i — i — | r 15 10 5 - 5 10 -15 PPM Fig. 3.17. Variable temperature 3ip{lH} NMR spectra (121.4 MHz) of l ib in CD2CI2: (a) at r.t., (b) at -20° and (c) -85°C. Peaks are not assigned. 97 (a) (b) 1 (c) ^ ^ ^ ^ I (d) (e) (0 20 15 10 T -10 -15 P=>v. Fig. 3.18. Variable temperature 31p{lH} NMR spectra (121.4 MHz) of 13b in CD2CI2: (a) at r.t., (b) 0°. (c) -20°, (d) -45°, (e) -60° and (f) -85°C. Peaks are not assigned. in GDCI3 at -55°C exhibits a complex pattern (Fig. 3.19); the analysis is based on the presence of three species, cis and trans isomers, and another unknown compound which is believed to be responsible for the broad peaks at 24.0 and -70.3 ppm. The relative intensities of all these signals vary with temperature. As the solution is warmed from -55°C to -30°C, the intensity of the unknown species grows in and the resonance of the cis isomer broadens (5 = 7.4 ppm, based on the data in CD2O2). This unusual behaviour of cis-Ptl2(PN2)2 resembles that of the 98 (a) (b) trans-3b cis-chelate cis-3b cis-chelate trans-3b cis-chelate « cis-3b cis-chelate 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 ' ' ' ' | 1 1 1 1 1 1 1 1 1 l'1 ' 1 ' I 1 ' ' ' ! ' ' ' 1 • 40 20 0 -20 -40 "60 -80 PPM Fig. 3.19. Low temperature 31P{1H} NMR (121.4 MHz) spectra of cis-Ptl2(PN2)2,3b, in CDCI3; (a) at -30°C and (b) at -55°C. The singlet at 7.4 with Pt-satellites are the signals of cis-3b, and the singlet at 10.4 with corresrxmding satellites are the resonances of trans-isomer of 3b; the broad singlets at 24.0 and -70.3 with Pt-satellites are assigned to the signals of the cis-chelate. 99 analogous cis-Ptl2(PNi)2 system in CDCI3 reported in the literature.22 The peaks at 24.0 and -70.3 ppm in Fig. 3.19 are assigned to a cis-chelated ionic compound (Scheme 3.1). This assignment is supported by the reported 3 1 P NMR data of the corresponding isolated ionic compound cis-[PtI(PNi)2]PF6,22 and those of cis-Pt(PN3)2(S)+ (S= solvent) generated in situ in the present work by reacting cis-Pt(PN3)2Cl2 with two equivalents of AgPFg in CH3CN (Fig. 3.20 and Table 3.9). The colourless solution of cis-Pt(PN3)2(S)+ gradually changes on standing to a bright yellow, a colour characteristic of a trans-isomer.-57 The following scheme outlines the solution equilibria, which account for the three species observed; that the trans-chelated ionic species is not seen implies instability of this intermediate with respect to the trans neutral isomer. N ^ I v y PPhPy2 I \ I /PPhpy XP t/ • Pt I / \ PPhpy2 I ^  ^ PPhpy2 cis- 3 b Pt PPhpy2"l + N PPhpy cis-chelate N = chelating pyridyl py2PhP. A 1 + \ / Pt N\ )PPhpy r trans-chelate Pt pyzPhP^ ^ 1 PPhpy2 trans- 3 b Scheme 3.1. The equilibria between the species possibly present in CDCI3 solvent for cis-PtI2(PN2)2,3b. 100 - r r p r --«5 -rp-r -to r r p m - 1 4 0 r t f , - I t O - i to P P U Fig. 3.20. ^PpHJ NMR spectrum (121.4 MHz) of the reaction product Pt(PN3)2(S)2+, formed in situ from cis-PtCl2(PN3)2 and AgN03 in CH3CN; the heptet centred at -143.6 is the resonance of PF6-; X = PF2O2" impurity from PF6- hydrolysis (ref.58). Despite the solution behaviour observed in CDCI3, only the cis-PtX2(PNn)2 species are observed in C D 2 O 2 . The solvent dependent nature of the equilibria cannot be rationalized in terms of solvent polarity as CD2CI2 (dielectric constant e = 9.0) is more polar than CDCI3 (e = 6.0). The ionic dissociation was originally thought to be caused by H2O impurity present in CDCI3; however, this is ruled out by a 3 1P experiment using 3b in CD2CI2 with added D 2 O — the cis-isomer is still the only one present This ionic dissociation phenomenon was observed by 3 1P NMR for several cis-PtX2(PNn)2 complexes in CDCI3 (Table 3.9). 3.4. Aqueous solution studies The water solubility of the neutral PdX2(PNn)2 and Pd2X2(ji-PNn)2 complexes increases with n (the number of pyridyl groups on phosphorus) and decreases in the order of CI > Br > I. The cis-PdCl2(PN3)2 complex is the most water soluble species, 100 mg in 40 mL at ambient conditions (Cone. = 3.7xl0"3 M). The chemistry of Pd2Cl2(u.-PN3)2 (HT), 11c, and cis-PdCl2(PN3)2,4c, in aqueous solution is discussed in the following sections. 101 Table 3.9. 3*P{ lH} NMR Data for as-PtX2(PNn)2 Species in CDCl3a Complex 5P 8PC ^PtPc 2 J P P c Ref.b la 18.5 -51.5 3695 3443 c 22 3a 19.2 -61.0 3645 3240 c 22 lb 22.4 -60.9 c c c tw 3b 24.0 -70.3 3654 3280 c tw 3c 24.4 -73.7 3693 3457 16.5 tw [Pd(PN!)2]PF6 19.3 -61.2 3637 3241 c 22 Pt(PN3)2(S)2+ 26.34 -53.74 3642 3478 12.0 tw (a) Only the data for the cis-chelated species are included in this Table; all the spectra except that of the Pt(PN3)2(S)2+ done in the present work were taken at -55°C. The chemical shifts are recorded in ppm and the coupling constants are in Hz. P represents the P atom of the non-chelated phosphine and the Pc represents the P atom of the chelated phosphine. (b) tw = this work, (c) The Pt-satellites are buried in the background noise, or the coupling constants are not resolved. 3.4.1. DicWorobis[Ms-(2-pyridyl)phosphme]dipaUadium(I) Pd2Cl2(u,-PN3)2(HT), 11c p y 2 P ^ N' py2P' a — P d Pd—a+ 2H20 N ^ / P p y 2 (X reddish orange N' -H20—Pd Pd— OH2 + 2C1" (3.1) ,NL^Ppy 2 green Dissolution of 11c in water occurs according to equation (3.1); the liberation of chlorides and the formation of bisaquo dipalladium species are demonstrated by the isolation of bisaquo dipalladium salts using NaBF4, KPF6 and NaBPh4. These green products can be redissolved in 102 acetone and acetonitrile for further purification by reprecipitation using diethyl ether, and do not dissolve in dichloromethane or chloroform. The elemental chemical analyses for these salts agree well with the proposed formulation [Pd20>PN3)2(OH2)2]X2 (X = BF4, PF6 or BPh*) (Table 3.10). In addition, the conductivity data listed in Table 3.11 also support that the green products are 1:2 electrolytes (for the ideal 1:2 electrolyte, A = 210 - 240 fl^moHcm2),59 although the molar conductivity of [Pd20i-PN3)2(OH2)2](BPh4)2 is somewhat low. The 3 1P NMR spectra show a singlet at much higher field than that of the corresponding dichloro complex 11c (A8 = 30 ppm); a blue shift of 460 nm band of 11c is observed in the visible spectra (Table 3.12). The infrared spectra of the B F 4 - and PFg- salts show a band assignable to the coordinated water and reveal no band assignable to a palladium-hydroxide stretch. The IR band of coordinated water in the BPI14- salt is obscured by stretching bands of the phenyl groups (Table 3.11). Although the configuration (HT or HH) of the ionic green species is not obvious from the above data, the reversibility of reaction (3.1) implies that the head-to-tail configuration is perhaps preserved: when a 10-fold excess of lithium chloride is introduced into the green aqueous solution, an orange solid, the original dichloro dipalladium complex, is precipitated. Table 3.10. Chemical Analyses of [Pd2(H-PN3)2(OH2)2]X2 Species C H N calcd. found calcd. found calcd. found BF4 37.81 37.37 2.96 2.90 8.82 8.47 33.70 33.71 2.64 2.55 7.86 7.72 BPh4 66.08 66.45 4.83 4.78 5.93 5.92 Table 3.11. IR, 31P{ lH) NMR and Conductivity Data for Bis(aquo)dipalladium Salts X JRa 31P{1H) conductivity15 VH^cnr1) 8(ppm) A(Q-1mol-1cm2) BF4 1627 -21.81(s, CH3CN) 269 PF6 1627 . -22.19(s, CH3CN) 256 BPh4 c -21.63(s, acetone) 188 (a) In Nujol mull, (b) Solvent = CH3CN. (c) Obscured by the IR bands of B P I H - . 103 Table 3.12. Visible Spectral Data for Pd2Cl2(p>PN3)2 Complexes in Different Solvents* Complex Solvent Xi(exlf>3) \2(exl0-3) X3(exlO-3) Pd2Cl2(u.-PN3)2,11c CH2C12 335 (12) 460 (8.4) — H 2 0 — 409 (9.3) 600 (1.7) [Pd2(iI-PN3)2(H20)2](BF4)2 CH3CN — 420 (6.1) 610 (0.41) [Pd2(^PN3)2(H20)2](PF6)2 CH3CN — 420 (6.1) 610 (0.41) [Pd2(^ PN3)2(H20)2](BPh4)2 CH3CN — 420 (5.7) 610 (0.41) (a) X is a wavelength maximum in nm; e is the molar absorptivity in iVHcm"1. The other reported cases of a neutral phosphine binuclear bridged complex being soluble in water is the dihalobis(dimethylphosphino)methane bridged dipalladium complex Pd2X2(p> dmpm)2 (X = CI and Br), which liberates hydrochloric acid and forms a neutral dihydroxo derivative.74 However, the coordinated H2O on the ionic bisaquo PN3 species is not as acidic. Attempts were made to measure the pKa value of the coordinated water by standard pH-titration experiments60 in aqueous solution under vigorous bubbling of N2. No appreciable amount of proton is titrated up to pH 11.0 (Fig. 3.21), implying that dissociation of proton from coordinated H2O is negligible. An increase in the initial pH of the solution containing the complex with respect to the initial pH of the dilute acid without complex is attributed to perhaps protonation of nitrogen atoms on the noncoordinated pyridine rings by the added acid at pH < 5.6. The difference between two curves above pH 5.6 is perhaps due to some residual CO2 in solution. Unfortunately, neither the ionic compounds in aqueous solution nor the salts in organic solvents (acetone, acetonitrile) react with olefins (acrylonitrile, maleic acid) or acetylenes (DMAD, methylpropiolate) at room temperature. 104 Fig. 3.21. The pH-titration curves of 11c in 25.0 mL HNO3 solution. [HNO3] = 9.8X10-4 M, [11c] = 4.98x10^  M, [NaOH] = 4.71x10-2 M; I = 0.3 M using NaNQ3, temp. = 22QC. 3.4.2. Cis-mcMorobis[tris(2-pyridyl)phosphme]paUadium(II), cis-PdCl2(PN3)2,4c On dissolution in water, 4c instantly gives a molar conductivity (267 Q^moHcm2 at 1.08xlO"3 M) corresponding to a 1:2 electrolyte which results from loss of two chloride ligands as estimated by AgN03 titration. The relative high value infers the possible involvement of conduction by proton arising from the dissociation of coordinated water. In fact, one proton is titratable by NaOH (Fig. 3.22). The protonation/deprotonation equilibration between bisaquo and aquohydroxo is fast on the NMR time scale, as a result, only one singlet peak should be observed at the concentration-weighted average chemical shifts of bisaquo and aquohydroxo species.62-63 However, the 31P{1H) NMR specrum of 4c in D 20 consists of two distinct singlets at 27.3 and 21.6 ppm with intensity ratio of approximately 2:1 (Fig. 3.23), and these peaks did not vary noticeably when the solution was titrated with NaOH. The observed 3 1P NMR spectrum can be rationalized, based on a literature report that cis-palladium(II) bisaquo 105 i c B 2.0 0.0 10.0 20.0 30.0 40.0 volNaOH (mL) Fig. 3.22. The pH-titration curve of 4c in 10 mL of H 2 O in the absence of H N O 3 . [4c] = 3.34x10-3 M, [NaOH] = 1.0x10-3 M; I = 0.3 M using NaN03, temp. = 25°C, under N 2. The method for graphical determination of pKa is described in ref. 61. A: equivalence point, B: volume at equivalence point, C: volume at half equivalence point, D: half equivalence point, and E: pH at half equivalence point. species dimerize rapidly in aqueous solution, equation (3.2);6^ it is perhaps reasonable to assume that an analogous binuclear phosphine complex of this sort is formed in aqueous solution. Indeed, a purple solid, precipitated at the end of pH titration (pH = 11.0) as a BPI14-H O 2(en)Pd(OH2)22+ (en)Pd Pd(en) + 2H30+ (3.2) O H 106 salt and recrystallized from CH3CN and EtOH at -15°C as dark red crystals, analyzed well for [Pd(^ -OH)(PN3)CH3CN]2(BPh4)2 (calcd.: C 65.77, H 4.85, N 7.48, O 2.14; found: C 65.50, H. 4.64, N 7.34,0 2.24). The 31P{ NMR spectrum of this product consists of a singlet at 29.1 ppm in CD3CN, indirectly suggesting that the phosphine is monocoordinated and that CH3CN must occupy the fourth available coordination site. The presence of CH3CN in this molecule is confirmed by a *H resonance at 2.35 ppm in the NMR spectrum in GD3CN. The IR stretching band for OH is observed at 3680 cnr1 in CH2Cl2 after solvent subtraction, but no band is observed assignable to the coordinated CH3CN. -1 ! I 1 1 ! 30 25 - i — r ~i—1—f—j—1—'—'—'—l 15 PP. - -: :o - 1 — r 40 1 1 r 35 20 Fig. 3.23. 121.4 MHz 31p{ lH) NMR spectrum of PdCl2(PN3)2,4c, in D20 at r.t. Efforts were made to isolate the bisaquo species cis-Pd(PN3)2(H20)22+ by adding methanol solutions of NaBF4, KPF6 or NaBPh4 to the aqueous solution of PdCl2(PN3)2. Because of the high solubility of the BF4- and PFty product salts in water, isolation of these salts was more difficult; even when the solids were sometimes collected by filtration, further purification of the solids by a reprecipitation procedure proved to be difficult because of their poor solubilities in organic solvents (acetone or acetonitrile). Therefore, the attempted characterization is largely based on the BPI14- salt An orange tetraphenylborate salt, obtained by reacting the aqueous solution of cis-PdCl2(PN3)2 with a methanol solution of NaBPh4, is readily dissolved in acetone and can be reprecipitated from the acetone solution by the addition of diethyl 107 ether. This isolated orange solid is believed to be the cis bisaquo exclusively, with a 3 1P NMR singlet at 33.4 ppm in CD3CN. Possibly because of the solubility difference between the mononuclear and binuclear species, only the mononuclear species appears to be precipitated. The equilibria are outlined in Scheme 3.2. H 2 + -2H30+ n2+ 2(PN3)2Pd(H20)22+ = r ( P N ^ d ^ ^ P d C P N ^ + * ^ V / 0 H 1 V ^ 2 H 2 0 2(PN3)2P(r ' OH2 Scheme 3.2. Solution behaviour of the palladium(U) bisaquo species. Any bisaquo and aquohydroxo species would equilibrate rapidly on the NMR time scale, as mentioned previously, perhaps giving the one singlet at 33.4 ppm in CH3CN. This singlet may correlate with the 27.3 ppm singlet in I>20» and thus the downfield singlet in the 31P{ lH) NMR spectrum in D2O is assigned to the bisaquo/aquohydroxo species, while the upfield one is assigned to the binuclear hydroxo bridged species. The elemental analysis required for [Pd(PN3)2(H20)2](BPh4)2 (calcd: C 71.43, H 5.23, N 6.41; found: C 69.43, H 4.73, N 6.87), however, was low in carbon and hydrogen contents and somewhat high in nitrogen content. Further reprecipitation procedures gave species with gradually decreasing carbon and increasing nitrogen contents (for example, C 67.08, H 4.85, N 7.18). The molar conductivity of the 'best' salt sample in acetonitrile (Cone. = 8.4X10-4 M) was 170 Q^moHcm2 which is too high for a 1:1 electrolyte and somewhat low for a 1:2 electrolyte (for 1:1 electrolyte A M = 110 - 120 ft-1 mol-1 cm2,1:2 electrolyte A M = 210 - 240 Q^moHcm2).58 The analytical and conductivity results are more sensible, however, when the above equilibria are taken into consideration (Scheme 3.2). In fact, the pH of an aqueous solution of 4c at 3.7xl0"3 M is 2.9, corresponding to 34% proton dissociation. The chemical analysis data, recalculated based on a 34% : 66% rnixture of [Pd(PN3)2(H20)(OH)](BPh4) and [Pd(PN3)2(H20)2](BPh4)2, assuming that this ratio is unaltered by precipitation ( C 69.39, H 5.08, N 7.11), fit much better the experimental 108 result This treatment is also applicable for the BF4- salt The isolated solid, assumed to be a mixture of 66% [Pd(PN3)2(H20)2](BF4)2 (calcd: C 42.56, H 3.33, N 9.93) and 34% [Pd(PN3)2(H20)(OH)](BF4) (calcd: C 47.48, H 3.59, N 11.08) gives experimentally found values for C, H and N of 43.84, 3.20 and 10.63, respectively, which match well the calculated values, 44.23, 3.42 and 10.32 . The pKa value of bisaquo species was estimated to be 3.8 ± 0.2 by a pH-titration shown in Fig. 3.22 and 4.8 ± 0.1 by the previously mentioned standard titration60 (Fig. 3.24). These pKa values are not true representations of the acid dissociation constants of a bisaquo species, but are mixed constants of proton dissociation and dimerization (Scheme 3.2). In the absence of acid, the measured constant reflects more of the dimerization; in the presence of acid, dimerization being inhibited, the measured constant reflects more proton dissociation. There are no comparable acid dissociation constant data available in the literature for this type of aquo/phosphine Pd 2 + species; there are a few for the bisaquo/amino acid Pd 2 + compounds Pd(L)(H20)2 2 + (L = methionine, pKa = 4.96; L = alanine, pKa = 5.50; L = tyrosine, pKa =4.77).°^ The reason for this is perhaps attributable to the complexity of reactions taking place in solution, dissociation of proton being usually accompanied by dimerization.65'65 The same problem was encountered for Pd(en)(OH2)22+, where only the endpoint of a titration, pH 7.5, was reported.65 In contrast with the rapid dimer formation from Pd(en)(H20)22+ (Eq. 3.2), the formation of the corresponding Pt2+ analog is slow with respect to the pKa (^ termination so that the pKa's of Pt(en)(H20)22+ were found to be 5.8 and 7.6 6 5 This complexity in pKa determination for bisaquo/phosphine Pd2 + species is best shown in the n vs pH plot (Fig. 3.24, inset) based on the data from the two titrations. A simple, one proton pH titration should give a smooth S-shaped curve. Despite the uncertainties, the pKa values measured (3.8 and 4.8) seem to fall into the same range as the data for the bisaquo/amino acid species. 109 0.00 0.50 1.00 1.50 2.00 volN.OH (mL) Fig. 3.24. The pH-titration curves of (1) H N O 3 and (2) 4c in H N O 3 . [ H N O 3 ] = 2.0x10-3 M, [4c] = 2.0x10-3 M, [NaOH] = 9.42xl0"2 M; I = 0.3 using NaNC*3; temp. = 25.0 ± 0.2<>C. Inset: n vs. pH plot, yielding pKa = 4.8. n is defined as the molar fraction of the protonated species present, the value of which can be calculated from the volume difference between curves (2) and (1) divided by the volume used to titrate 2 mM H N O 3 . To avoid formation of a possible bisaquo and aquo-hydroxo mixture, the pH values of two aqueous 4c solutions were adjusted to 1.5 by adding dilute H N O 3 and to 5.6 by adding solid NaHC03, respectively. The colour of the former solution (pH = 1.5) is orange-yellow and the latter (pH = 5.6) is brownish orange. A tetraphenylborate salt precipitates out from the acidified solution as a yellow powder with an improved chemical analysis result (found: C 70.12, H 5.10, N 6.60), and a CD3CN solution of the sample still showed the same 3lp NMR shift, 33.4 ppm. The conductivity of this product in acetonitrile solution (Cone. = 7.5X10-4 M) is 217.6 Q^moHcm 2 . The orange solid precipitated from the pH 5.6 solution analyzes for [Pd(PN3)2(H20)(OH)](BPh4) with relatively satisfactory results except for nitrogen content 110 which is about 0.8% off (calcd: C 65.42, H 4.78, N 8.48; found: C 65.39, H 4.60, N 7.72); the conductivity is 183.6 Q^moHcm2 (Cone. = 6.3x10^  M). The VOH and VH 2 O stretches are detected in a sample assumed to be [Pd(PN3)2(H20)(OH)](PF6) (Fig- 3.26) but not in [Pd(PN3)2(H20)(OH)](BPh4). On addition of lithium chloride to the aqueous solution of 4c, the precursor 4c complex is not precipitated; instead, an unknown pale yellow, water soluble species, possibly Pda3(PN3>2-, is formed, giving a 3lP{ lH) resonance at 54.3 ppm in D2O with no formation of an intermediate species between the assumed anionic species and the cationic species. Even if PdCl2(PN3)2 is formed, it is only a transient species. The labile character of the Pd 2 + centre would promote such product formation. 3200.0 2600.0 2000.0 1700.0 1400.0 1100.0 800.0 500.0 Wavenumbers (cm1) Fig. 3.25. hifrared spectrum of pd(PN3)2(H20)(OH)](PF6) in Nujol on KBr at r.t. I l l References 1. Basolo, F.; Pearson, R.G. Mechanisms of Inorganic Reactions — A Study of Metal Complexes in Solution, 2nd Ed.; John Wiley & Sons: New York, 1969; p.69. 2. Hartley, F.R. The Chemistry of Platinum and Palladium; John Wiley & Sons: New York, 1973; p.19. 3. Ref. 1, pp.423-427. 4. (a) Redfield, D.A.; Nelson, J.H. Inorg. Chem., 1973, 12, 15. (b) Redfield, D.A.; Nelson, J.H.; Henry, R.A.; Moore, D.W.; Jonassen, H.B. /. Am. Chem. Soc, 1974, 96, 6298. (c) Verstuyft, A.W.; Redfield, D.A.; Cary. L.W.; Nelson, J.H. Inorg. Chem., 1916,15, 1128 5. (a) MacDougall, J.J.; Mathey, F.; Nelson, J.H. Inorg. Chem., 1980,79, 1400 (b) MacDougall, J.J.; Nelson, J.H.; Mathey, F. Inorg. Chem., 1982, 21, 2145. 6. (a) Haake, P.; Pfeiffer, R.M. / . Chem. Soc. D , 1969, 1330. (b) Haake, P.; Pfeiffer, R.M. /. Am. Chem. Soc, 1970, 92, 5243. (c) Haake, P.; Pfeiffer, R.M. /. Am. Chem. Soc, 1970, 92, 4996. 7. (a) Cooper, D.G.; Powell, J. Can. J. Chem., 1973, 51, 1634. (b) Cooper, D.G.; Powell, J. / . Am. Chem. Soc, 1973, 95, 1102. 8. Romeo, R. Inorg. Chem., 1978, 17, 2040 . 9. Favez, R.; Roulet, R.; Pinkerton, A.A.; Schwarzenbach, D. Inorg. Chem., 1980, 19, 1356. 10. 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U.S. Patent 4,693,875, 1987; CA. 1988,107, 239618x. (c) Barnabas, A.F.; Sallin, D.; James, B.R. Can. J. Chem., 1989, 67, 2009. 17. (a) Lee, C-L.; James, B.R.; Nelson, D.A.; Hallen, R.T. Organometallics, 1984,3, 1360. (b) Lyke, S.F.; Lilga, M.A.; Nelson, D.A.; James, B.R.; Lee, C-L. Ind. Eng. Chem. Prod. Res. Dev., 1986,25, 517. 18. Balch, A.L.; Benner, L.S.; Olmstead, M.M. Inorg. Chem., 1979,18, 2996. 19. (a) Olmstead, M.M.; Hope, H.; Benner, L.S.; Balch, A.L. / . Am. Chem. Soc, 1977, 99, 5502. (b)Holloway, R.G.; Penfold, B.R.; Colton, R.; McCormick, M.M. /. Chem. Soc. Chem. Comm., 1976,485. 20. Lee, C-L.; Yang, Y-P.; James, B.R. to be submitted to Organometallics. 21. Maisonnat, A.; Farr, J.P.; Balch, A.L. Inorg. Chim. Acta , 1981,53, L217. 22. Farr, J.P.; Wood, F.E.; Balch, A.L. Inorg. Chem., 1983, 22, 3387. 23. Pidcock, A.; Richards, R.W.; Venanzi, L.M. Proc. Chem. Soc, 1962,184. 24. Grim, S.O.; Keiter, R.L.; McFarlane, W. Inorg. Chem., 1967, 6, 1133. 25. 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Ibid., 1956, 525. 33. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed; Wiley: New York, 1986; p.324, 339. 34. Coates, G.E.; Parkin, C. / . Chem. Soc, 1963, 421. 35. Park, P.J.D.; Hendra, P.J. Spectrochimica Acta A, 1969,25, 227. 36. Rosevear, D.T.; Stone, F.G.A. / . Chem. Soc, 1965, 5275. 37. (a)Adams, D.M.; Chandler, PJ. / . Chem. Soc. Chem. Commun., 1966, 69. (b)Adams, D.M.; Chatt, J.; Gerratt, J.; Westland, A.D. / . Chem. Soc, 1964, 734. 38. Goggin, P.L.; Goodfellow, R.J. / . Chem. Soc. A, 1966, 1402. 39. (a)Cheney, A.J.; Mann, B.E.; Shaw, B.L.; Slade, R.M. /. Chem. Soc. A, 1971, 3833. (b)Cheney, A.L.; Shaw, B.L. / . Chem. Soc. Dalton Trans., 1972, 860. 40. Grim, S.O.; Keiter, R.L. Inorg. Chim. Acta , 1970, 4, 56. 41. Cheney, A.L.; Shaw, B.L. /. Chem. Soc. Dalton Trans., 1972, 754. 42. Goodfellow, R.J.; Taylor, B.F. / . Chem. Soc Dalton Trans., 191 A, 1676. 43. Jenkins, J.M.; Shaw, B.L. J. Chem. Soc. A , 1966, 770. 44. Mann, B.E.; Shaw, B.L.; Stainbank, R.E. J. Chem. Soc. Chem. Comm., 1972, 151. 45. (a)Verstuyft, A.W.; Nelson, J.H.; Cary, L.W. Inorg. Chem., 1976,15, 732. (b) Redfield, D.A.; Cary, L.W.; Nelson, J.H. Inorg. Chem., 1975,14, 50. 46. Goggin, D.L.; Goodfellow, R.J.; Haddock, S.R.; Knight, J.R.; Reed, F.J.S.; Taylor, B.F. / . Chem. Soc. Dalton Trans., 1974, 523. 47. Fyfe, CA. Solid State NMR for Chemists; CF.CPress: Guelph, Ontario, 1983; pp.363-365, 375-377. 48. Bermi, L.; Clark, H.C; Davies, J.A.; Fyfe, C.A.; Wasylishen, R.E. /. Am. Chem. Soc, 1982, 104, 438. 49. Mislow, K.; Raban, M. Topics. Stereochem., 1974,1, 1. 50. Mislow, K.; Bickart, P. Isr. J. Chem., 1976/1977, 15, 1. 114 51. Maisonnat, A.; Farr, J.P.; Olmstead, M.M.; Hunt, C.T.; Balch, A.L. Inorg. Chem., 1982, 21, 3961. 52. Farr, J.P.; Olmstead, M.M.; Balch, A.L. Inorg. Chem., 1983, 22, 1229. 53. Ref. 11, p.214. 54. Ref. 1, pp. 414-417. 55. McEwen, W.E. In Topics in Phosphorus Chemistry, Vol. 2; Grayson, M., Griffith, E.L., Eds.; Interscience: New York, 1965; pp.17-19. 56. Gallagher, M.L.; Jenkins, I.D. In Topics in Stereochemistry, Vol. 3; Eliel, E.L., Alliger, N.L., Eds.; Interscience: New York, 1968; p. 11. 57. Hartley, F.R. Organomet. Chem. Rev. A , 1970, 6, 119. 58. Sue, C-Y. PhD. dissertation , University of British Columbia, 1989; p. 189. 59. (a) Dodd, R.E.; Robinson, P.L. Experimental Inorganic Chemistry —A Guide to Laboratory Practice, Elsevier: Amsterdam, 1954; p.378. (b) Geary, W.J. Coord. Chem. Rev., 1971, 7, 81. 60. James, B.R. D. Phil, dissertation , Oxford, 1960. 61. Reynolds, C.A. Principles of Analytical Chemistry, Allyn & Bacon: Boston, 1966; p.79. 62. Appleton, T.G.; Hall, J.R.; Ralph, S.F.; Thompson, C.S.M. Inorg. Chem., 1989, 28, 1989. 63. Lippard, S.J. Science, (Washington, D.C.) 1982,218, 1075. 64. Chernova, N.N.; Kurskii, I.G. Zh. Neorg. Khim., 1978,23, 1014 through C.A., 1978, 89, 31695x. 65. Lim, M.C.; Martin, R.B. / . Inorg. Nucl. Chem., 1976,38, 1911. 115 Chapter 4 Reactivities of the Head-to-Head Isomers of Pt2l2(^-PNn)2 Complexes (n = 1, 2 or 3) toward Dimethylacetylenedicarboxylate and toward Phosphine 4.1. Introduction Many metal acetylene complexes have been characterized since the mid 1970s in a search for hydrogenation and cyclotrimerization catalysts for acetylenes. There is no ambiguity in the bonding mode of mononuclear metal acetylene complexes in which the acetylene molecule acts as a a donor and acceptor. In binuclear complexes, two acetylene bonding modes are most commonly observed: the tetrahedral \12-Tp geometry (A) in which the acetylene sits perpendicular to the metal-metal a x i s / • 9 and the cis-bimetallated olefinic geometry (B) in which the acetylene lies parallel to the metal-metal a x i s / 0 * ^  Most of type A complexes are carbonyl complexes of the first row transition metals, whereas the B type complexes usually involve the phosphine complexes of the second or third row transition metals. A B It is anticipated^ that the binuclear acetylene complexes wi l l show significantly enhanced activity over that of free acetylene because the 'coordinated' acetylenic bond is significantly lengthened with respect to that in free acety lene/ 7 or even with respect to acetylene bound to one single m e t a l / 8 One aspect of the present study was originally aimed at activation toward hydration of olefins and acetylenes via coordination to the previously described pyridylphosphine binuclear metal complexes. 116 As previously discussed, binuclear complexes containing the bridging PNn ligands do not form A-frame insertion adducts with CO J9 However, the dimethylacetylenedicarboxylate, DMAD, insertion complex with Pd2d2(ji-PN3)2,11c, was prepared and crystallographically characterized by our group to have the cis-bimetallated olefinic structure (type B) (Fig. 4.1)20 The reactivity seen in a chelating diphosphine to accommodate DMAD75 is also observed in pyridylphosphine complexes, and the noted absence of a Pd-Pd bond is seen also in the similar insertion product Pd2Q2(^^30<!CF3)(dppm)2.72 The head-to-tail, HT, configuration of the product allows us to suggest tentatively the HT configuration for the precursor 11c, assuming no isomerization during the insertion reaction; the HT geometry is not demonstrated from other physical characterization data (Sect 3.2.4). This study was extended in the present work to platinum complexes, and the insertion reaction seems to be a quite general one (Sect 2.7). Fig. 4.1. ORTEP drawing of Pd2Cl2(u,-DMAD)(|i-PN3)2 and the atom numbering scheme.20 Some representative bond lengths (A) and bond angles (°). Pd(l)-C(l) = 2.010 (3) Pd(l)-P(2) = 2.2434 (9) Pd(l)-N(l) = 2.128 (3) Pd(l)-CIQ) = 2.3929(10) Pd(2)-C(2) = 2.003 (3) Pd(2)-P(l) = 2.2260 (9) Pd(2)-N(2) = 2.127 (4) Pd(2)-Cl(2) = 2.3841 (10) C(l)-C(2) = 1.330 (5) Pd(l)-C(l)-C(2) = 112.1 (3) Pd(l)-C(l)-C(3) = 121.0 (2) P(2)-Pd(l)-N(l) = 174.8 (8) Cl(l)-Pd(l)-C(l) = 175.0 (1) Cl(l)-Pd(l)-N(l) = 89.04 (8) Cl(l)-Pd(l)-P(2) = 95.71 (4) 117 In this Chapter, general reactivities of the Pt2l2(M>Nn)2 head-to-tail (HT) and head-to-head (HH) species, 9 and 10, toward DMAD will be described. The kinetic and spectroscopic studies are focused on the simple oxidative addition of DMAD to the HT isomer, and the same oxidative addition to the HH isomer followed by isomerization to the HT form (see below). To our knowledge, there has been no similar study reported in the literature on such isomerizations involving a binuclear metal framework. It was considered to be of interest from an organometallic chemistry point of view to investigate the detailed mechanism and the origin of such isomerization. Also in this Chapter, a related isomerization reaction of Pt2l2(H-PN3)2 (HH), 10c, in the presence of PN3 to give Pt2J-2(p>PN3)2 (HT), 9c, will be discussed. 4.2. Experimental Syntheses of the HH isomers of Pt2l2(M--PNn)2 (n = 1 - 3), 10a - 10c, and separation of the HH from the HT isomers have been described in Chapter 2, Sect. 2.6.1 - 2.6.2, and the structures of complexes Pt2l2(u.-pN2)2» 10b, were discussed in detail in Sect. 3.2.3. Reactions of 10a - 10c with DMAD have also been described in Chapter 2, Sect. 2.7. The isolated reaction products were characterized as 1:1 insertion complexes, and the configuration of adducts were determined unambiguously using 3 1P and 195Pt NMR spectroscopies. Table 4.1 lists the 31P{ *H} parameters of the DMAD adducts made. Kinetic experiments for reactions of the binuclear platinum complexes with DMAD were carried out using a thermostated (± 0.1°C) Perkin-Elmer 502 A UV/vis spectrophotometer. Pure DMAD liquid was added by a microsyringe so that the concentration of DMAD could be calculated from the known volume and density data, and was placed in the side-flask of the anaerobic optical cell (Fig. 2.1); a solution of the Pt complex of known concentration was transferred from a volumetric flask and placed in the cell (3 mL solution for 10 mm path length 118 Table 4.1. 3 1P NMR Parameters of DMAD Insertion Adducts3 Compound 5 (ppm) p^tp 3Jptp Pt2l20i-DMAD)Oa-PNi)2 (HH), 17 21.5 (s) 3496 242 Pt2I2(n-DMAD)0a-PN2)2 (HT), 18b 6.85 (s) (32%) 4480 268 7.35 (s) & 8.47 (s)c (59%) 4517 257 9.32 (s) (9%) 4540 272 R2l2(^ -DMAD)(ji-PN3)2 (HT), 19 8.37 (s) 4455 272 Pt2Cl20i-DMAD)(n-PN3)2 (HT), 20 9.88 (s) 4605 272 PtPdCl2(^ -DMAD)(^ PN3)2 (HT), 21<* 12.84 (d) 29.58 (d) 4486 478 Pt2I2(^-MPP)(n-PN3)2 (HT), 22e 12.24 (s) 4620 279 10.12 (s) 4584 242 Pd2Cl2(^-DMAD)(n-PNi)2 (HT), 23 35.77 (s) Pd2Cl2(^-DMAD)(n-PN2)2 (HT), 24b 35.46 (s) (44%) 35.25 (s) & 33.83 (s)c 33.33 (s) (12%) (44%) Pd2l2(^ -DMAD)(n-PN2)2 (HT), 25b 35.02 (s) (34%) 34.61 (s) & 33.46 (s)c 32.98 (s) (18%) (48%) (a) The spectra were recorded in CDC13 solvent at r.t (b) Diastereomers of the DMAD adducts were detected, the relative intensities of the individual peaks being given as percentages in the brackets, (c) Diastereotopic P atoms give rise to two singlets with equal intensities, (d) 4Jpp was resolved as 5.0 Hz. (e) This adduct was not isolated (Sect 2.7). quartz cell) shown in Fig. 2.1. No degassing procedure was required. Reaction times were recorded, following mixing of the reactants, on a Chron-Lab 1400 timer for faster reactions (10 -20 seconds between readings), or recorded with the repetitive scanning spectra for the slower reactions (3 - 90 minutes between readings) by a built-in timer. For the binding of DMAD, a decrease of absorbance was monitored at one specific wavelength (e.g. 500 nm for 10c, and 520 nm for 9c, see Sect 4.3.2) for most reactions, except for the reaction of 10a and 10b with DMAD in which the spectra from 400 to 600 nm were recorded (see Sect. 4.3.3); for the isomerization step (from 28.2 to 19), a subsequent increase of absorbance was monitored from 119 450 to 600 nm (see below). The reactions were run under pseudo-first order conditions by using a 10 to 100-fold excess of DMAD. The concentrations of platinum complexes were usually in the range of 0.20 - 1.30X10-4 M. The absorbance data versus time, and the corresponding -ln(Ar Aoo) or -ln(Aoo-At) vs. time data, are listed Tables AJJI -AVIE in the Appendix. The observed first-order rate constants were obtained from the slopes of the plots of ln(At-Aoo) vs. t by a least-square analysis; the plots were generally linear with correlation coefficients higher than 0.996 (A t is the absorbance at time t, and Aoo normally is the absorbance at completion of the reaction for a one-step process). For consecutive DMAD binding and isomerization reactions, the lowest absorbances (see Fig. 4.5) were taken as A«, for the binding step. This method is justified by application of the Kezdy-Swinbourne treatment.2^•22 The estimated Aoo values from a K-S plots were usually very close (within two percent or less) to those obtained directly from the spectra. The Guggenheim treatment2-* was applied to confirm the accuracy of the rate constants in some cases. Detailed descriptions are given in Sect. 4.3.2. The activation enthalpies and entropies were obtained from Eyring plots of In kfl vs. 1/T. 4.3. Results and discussion 4.3.1. Reactions of Pt2l2(M--PN3)2 (HT), 9c, with acetylenes Reaction of 9c with DMAD yielded the A-frame product shown as 19 in Fig. 4.2, the expected analogue of the Pd2Cl2(H-DMAD)(|i-PN3)2 (HT) complex (Fig. 4.1). Initially surprising, however, was the 31P{1H) NMR spectrum (Fig. 4.2), which showed no splitting of the Pt satellites. The 3 1 P spectrum was expected to be similar to that of 9c (Fig. 3.9) because of magnetic inequivalence of the P atoms in the 1 9 5Pt spin labelled isotopomer. The apparent anomaly was clarified when 9c was treated in situ in an NMR tube with methylpropiolate (MPP), and the 3 1 P NMR spectrum of the product Pt2l2(H-MPP)(p>PN3)2 (HT), 22, was measured (Fig. 4.3); the now chemically ^ equivalent P atoms are seen as two singlet (with Pt satellites) peaks, and there is no PP coupling (i.e. Jpp » 0 Hz or is unresolvable). Thus in 19, the magnetic inequivalence of the two P atoms is not detected. The HT geometry is also supported by the 120 9c + DMAD 19 m?^ N i Or"* 19 \ — / - / > = c \ MeOOC COOMe i j 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 ' | 20 15 10 5 0 -5 -10 PPM-15 Fig. 4.2. 31p{ 1H} N M R spectrum (121.4 M H z ) of 19 in CDCI3 at r.t.: 5 = 8.37; iJptP = 4455 H z , 3 jp tP = 272 H z . 9c + MeOOCCsCH - 22 • Fig. 4.3. 3lp{ 1H) NMR spectrum (121.4 MHz) of 22 in CDCI3 at r.t: 8 = 12.24 (s), 10.12 (s); P^tp = 4620, 4584 Hz; 3Jptp = 279, 242 Hz, respectively. Peaks are not assigned to individual P atoms. 121 195pt NMR spectrum of 19 (8 = 293.2 (d), 1Jw = 4453 Hz; in CDCI3, r.t) which is similar to the pattern observed for 9c in Fig. 3.9b and apparently dissimilar to that of 10c in Fig. 3.13b. 4.3.2. Reaction of Pt2l2(H-PN3)2 (HH), 10c, with DMAD Extension of the concept of the acetylene reaction to reaction of 10c with DMAD would lead to the expectation of a product with structure 28.1. The 3 1 P NMR spectrum of 28.1 would basically look similar to that of 10c (Fig. 3.13). However, the reaction of 10c with DMAD finally yielded an orange compound which was clearly identical with 19. 10c + DMAD py 2p-^ N L Pt Pt p y 2 P ^ j N . MeOOC COOMe 19 (4.1) The difference in 28.1 and 19 lies only in the relative P-N orientation, one is being HH, and the other is HT. Obviously, an isomerization has taken place during the reaction. This reaction was monitored in situ by 3 1P NMR spectroscopy and the results are shown in Fig. 4.4. In Fig. 4.4(a), the immediately formed species, 28.2, has in fact a 3 1 P spectrum very different from either that of 19 (Fig. 4.2) or the expected pattern for 28.1 (cf. 10c in Fig. 3.13a and 17 in Fig. 4.14). As indicated by the doublet of doublets, two phosphorus atoms in 28.2 are chemically inequivalent. The value of Jpp, 13 Hz, is typical of either a cis phosphorus, or diagonally oriented-HT, phosphorus coupling. The ^ PtpH} NMR spectrum of intermediate 28.2, shown in Fig. 4.4(b), consists of a pseudo-triplet which is in fact a doublet of doublets, ruling out the possibility of an HT configuration because of the apparent dissimilarity between (b) 122 19 28.2 (c) •cO 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 1 I I [ 1 ' ' ' I ' 30 20 10 0 -10 Fig. 4.4. (a) 31P{1H} NMR spectrum (121.4 MHz) of 28.2 the initially formed DMAD adduct of 10c, in CD2CI2 at r.t: 5Pl = 21.2 (d), 8p2 = 26.5 (d); = 3719, ijpff^ 3625 Hz; 2 J P P = 13.5 Hz, respectively, (b) 195Pt{ lH} NMR spectrum (64.2 MHz) of 28.2 obtained at -20°C in CD2CI2: 5 = -128.1(dd); Uptp = 3712, 3613 Hz. The broad peaks at 230.0 and 300.0 are assignable to the HT DMAD adduct, 19. (c) 31P{ lR] NMR spectrum (121.4 MHz) of the same sample taken 20 h after spectrum (b) was taken (sample was stored at -20°C); 3JptP, of 28.2 was resolved here as 190 Hz. X = impurity. 123 and the 195Pt spectrum of 9c, Pt2l2G>PN3)2 (HT), (Fig. 3.9b). The basic "HH" configuration must therefore be maintained in 28.2 while distortion from 'perfect HH1 must result in chemical inequivalence of the P nuclei. The 1 JptPi and 1 Jptp2 values measured in (b) are consistent with the values measured in (a). In Fig. 4.4(c), the upfield peaks, growing in with time, are the peaks due to the HT-A-frame adduct 19, the result of a geometrical rearrangement. Monitoring reaction 4.1 by visible spectroscopy consistently reveals that this overall reaction involves two consecutive steps; typical visible spectra recorded in kinetic experiments are displayed in Fig. 4.5. 0.00 H 1 - r 1 1 450.0 500.0 550.0 600.0 650.0 X (nm) Fig. 4.5. The observed changes in visible spectra for reaction. 4.1 in CH2CI2 at 25°C. is the spectrum of the precursor 10c (Xrnax = 498 nm, e = 456 M^cnr1); is the spectrum of the intermediate 28.2 (see Fig. 4.4a); — is the spectrum of the HT DMAD adduct 19. The solid lines between 19 and 28.2 are the spectra recorded every 30 min for the formation of 19 from 28.2; changes for the conversion of 10c to 28.2 are not shown. 124 The first-order plot, ln(At-A„) vs. t, for the spectral change from spectrum of 10c to that of 28.2 in Fig. 4.5 is linear to about four half-lives (ti/2 = 40.1 s) (Fig. 4.6). The direct reading of the lowest absorbance 0.170 at 500 nm was assumed to be A«>, and the observed first-order rate constant obtained thereby is 1.73xl0*2 s_1 (Appendix Tables ATV 1-11). The problem of using (Aoo)exp directly for the kobs calculation is the degree of reliability of A w because of the slow secondary reaction. Kezdy et al.27 and Swinbourne22 developed a method to estimate Aoo, based on the following equation, when the final instrument reading is unreliable or unavailable: At = (At+x)ekT - Aoo(ekx-l), where % is a suitably chosen time interval; and at the theoretical end point A t = A t + T = Aoo. The intersection of the line (At vs. A t+X) for the experimental data and the 45° line is the estimated Aoo. In this method, the x value is chosen between ti/2 and 1.5 ti/2 to give the best accuracy. The absorbances were thus processed also according to the Kezdy-Swinbourne method, and the analysis is listed in Table 4.2. Fig. 4.7 shows the A t vs. At+x plot for the data, and the Aoo thus found is 0.175. Several other experimental data analyzed in the same way show that the difference between the estimated Aoo and the experimental value is less than two percent. The assumption that the lowest absorbance value is A«, is thus validated; hereafter, (Aoo)exp- is used as the real Aoo, without further verification. Table 4.2. Kezdy-Swinbourne Treatment of the Data in Table A T V 7 for Estimating A * , t(s) A t (t + t)a(s) At+x 0 0.488 40 0.325 30 0.352 70 0.263 40 0.325 80 0.250 50 0.299 90 0.237b 60 0.280 100 0.228 70 0.263 110 0.220b 80 0.250 120 0.212 90 0.237b 130 0.205b 100 0.228 140 0.200 110 0.220b 150 0.195b 120 0.212 160 0.191 (a) T = 40 second was used based on the ti/2 result from data in Table ATV 7. (b)These values are interpolated from the At vs. t plot. 125 -1.00 -4.00+--— • i i 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 t(s) Fig. 4.6. Pseudo first-order rate plot, In (At-Aoo) vs. t, for the DMAD binding step of reaction 4.1 (10c to 28.2), at 25°C: [10c] = LlOxlO"3 M, [DMAD] = 6.50xl0-2 M (Table AIV 7). Inset: the observed pseudo first-order rate constants vs. [DMAD] (see Table 4.4). At this particular temp., only two [DMAD] values were used; at 34°C, three [DMAD] values were used (Table ATV 1-3). The Guggenheim treatment25 in which ln(At-At+x) is plotted against time, was also applied to avoid the complication of estimating A«, in order to obtain the kobs value. The observed first-order rate constant is obtained from the slope without knowing the real A»; for the best accuracy, x is chosen between two to three half-lives (Table 4.3, Fig. 4.8). The rate constant obtained in this way is 1.80xl0-2 s-1. Changing the DMAD concentration resulted in a change of the observed psuedo first-order rate constant, and a first-order dependence on [DMAD] was demonstrated by the linear relationship of the kobs vs. [DMAD] plot including the origin (Fig. 4.6 inset). A summary of DMAD dependence data is given in Table 4.4. Direct involvement of DMAD in the conversion of 10c to 28.2 is clearly indicated by the DMAD dependence result, and this step in later sections is called the DMAD binding step. The measured second-order rate constant ki at 25°C is 0.270 M ' V 1 (Fig. 4.6 inset). 126 Fig. 4.7. Estimation of the Aoo value by Kezdy-Swinbourne method; plot of A t vs. A t + X (see Table 4.2; x = 40.0 s - ti/2). Fig. 4.8. The Guggenheim treatment to give kobs; plot of -ln(At - A t + T) vs. t (see Table 4.3; x = 80 s - 2ti/2). 127 The first-order plot for the subsequent step involving conversion of 28.2 to 19, ln(Aoo-At) vs. t (Fig. 4.9) is also essentially linear. The two observed rate constants, 4.77xl0"5 and 4.70xl0-5 s-1, are essentially independent of [DMAD]. The zero-order kinetics with respect to DMAD are characteristic of an intramolecular rearrangement process; this step (28.2 to 19) is called the isomerization step. Table 4.3. First-Order Analysis of the Data in Table ATV 7 by the Guggenheim Method t(s) A t t +1 (s) At+T At-At-K -ln(At-At+x) 0 0.488 80 0.250 0.238 1.435 10 0.427 90 0.236 0.191 1.655 20 0.381 100 0.228 0.153 1.877 30 0.352 110 0.220 0.132 2.025 40 0.325 120 0.212 0.113 2.180 50 0.299 130 0.205 0.094 2.364 kobs = 1.80xl0-2 s"1 Table 4.4. The Observed Pseudo First-Order Rate Constants at Different [DMAD], in CH2CI2 at 25°C PMADJxlO2 (M) 0.00 4.06 6.50 kobs (s"1) 0.00 1.08 1.73 ki= 0.270 M - V 1 The binding of DMAD to 10c (4.1) is an oxidative addition reaction, as is the net DMAD insertion reaction into the metal-metal bond with 9c (Sect 4.3.1). However, the expected insertion product 28.1 (p. 106) is simply not observed throughout the reaction although, from the spectroscopic data, we know that the initially observed product 28.2 is structurally similar to 28.1. 128 Based on the results from studies on reaction 4.1, a plausible reaction pathway is outlined as: 10c + DMAD — [28.2] — 19 (k,>k2) The second-order rate constants for the binding of DMAD (ki), and the first-order rate constants for the isomerization (k2) at various temperatures in CH2CI2 are summarized in Table 4.5. The plot of lnki/T vs. 1/T (Fig. 4.10) yields the activation enthalpy, AH" = 8.5 ± 0.3 kcal/mol, and the activation entropy, AS* = -32.4 ± 0.9 e.u; and the plot of - Ink2/T vs. 1/T gives AH" (24.0 ± 1.0 kcal/mol) and AS* (6 ± 4 e.u) for the isomerization reaction step. Table 4.5. Temperature Dependence of Rate Constants ki and k2 in CH2CI2 T(K) l/TxlO3 ki (M-V1) -lnki/T k2xl05(s-l) -Ink2/T 290.15 3.446 0.176 7.385 1.39 16.85 294.15 3.400 0.229 7.158 2.74 16.19 298.15 3.354 0.270 7.007 4.74 15.64 302.15 3.310 0.339 6.793 9.10 15.02 307.15 3.256 0.436 6.557 16.7 14.42 AH* = 8.5 ± 0.3 kcal/mol AS* = -32.4 ± 0.9 e.u. AH* = 24.0 ±1.0 kcal/mol AS* = 6 ± 4 e.u. The activation parameters found for the oxidative addition step are typical of data for oxidative addition at a single metal centre, which has been reported upon extensively in the literature.24 The parameters are also comparable to the data for oxidative addition of H2S at Pd2Br2(Mppm)2 (m* = 1 3 kcal/mol, AS* = -27 e.u.),25 and DMAD at Pt2i2(^-PN3)2 (HT), 9c, (AH* = 9.0 ± 0.2 kcal/mol, AS* =-33.0± 0.8 e.u.) (see below), which are among the very few known for oxidative addition systems at binuclear metal centres.26 The relatively high AH* for the isomerization step is considered to result from a required Pt-P bond cleavage, while the AS* value close to zero implies 'minimal' geometric changes in the process (see below for further discussion). 129 .5 -1.0 -12 -1.4 -1.6 -1.8 -2.0 -22 -2.4 -2.6 -2.8 slope = -2.82x10 • [DMAD] = 0JD4O6U • [DMAD] = OMSM I I I 0.0 100.0 I I I I 200.0 300.0 t(min) 400.0 Fig. 4.9. Pseudo first-order rate plot, In (A r A«,) vs. t, for the isomerization step (28.2 to 19) in CH2C12 at 25°C; [10c] = 1.10x10-3 M (Table AV 6-7). -6.0 -10.0 • -12.0--14.0 • -16.0 -18.0 0.0032 0.0033 0.0034 1/T (K"1) 0.0035 Fig. 4.10. The Eyring plots, In k/T vs. 1/T, for the rate constants of step 1 (binding of DMAD) and step 2 (isomerization) (see Table 4.S). 130 By a similar approach, the rate of the reaction of Pt2l2(M--PN3)2.9c, with DMAD to give the expected HT insertion product 19 was investigated: 9c + DMAD > 19 (4.2) The reaction was monitored by the absorbance decrease at the absorption maximum of 9c at 520 nm or from spectral changes over 360 - 600 nm region (Fig. 4.11). The first-order plot, ln(At-A«,) vs. t, is linear with a correlation coefficient 0.999 (Fig. 4.12, Table ADJ.). The observed first-order rate constant is also DMAD dependent, and the straight line plot of kobsvs- [DMAD] plot demonstrates the first-order relationship (Fig. 4.12, inset). The DMAD dependence data are summarized in Table 4.6. The temperature dependence of the second order rate constants for this reaction in CH2CI2 (Table 4.7) is shown in Fig. 4.13 by the Eyring plot In k/T against 1/T. AH" and AS* are found to be 9.0 ± 0.2 kcal/mol, and -33.0 ± 0.8 e.u. respectively. 360 400 560 X (nm) Fig. 4.11. The spectral changes for reaction 4.2 (9c to 19) at 21°C in CH2CI2: the spectrum labelled 9c was taken before the mixing (Xmax = 320,403 and 518 nm; e = 1.55X104, 5.04xl03 and 725 M^cnr1); the spectrum labelled 19 was taken at the experimental infinite time; the solid lines between 9c and 19 were recorded every 3 min (an isosbestic point at 474 nm is seen). [9c] = 1.45x10^  M , [ D M A D ] = 8.13x10-3 M (Table A J J J 10). 131 Table 4.6. Dependence of kobs °n [DMAD] for Reaction 9c with DMAD, at 18°C in CH2CI2 [DMAD]xl02 (M) 0.00 1.19 2.39 2.71 4.54 kobsxl03 (s-1) 0.00 0.764 1.49 2.00 2.75 k = 6.2xlO-2M-1s-1 Table 4.7. Rate Constants for Reaction of 9c with DMAD, at Various Temperatures in CH2CI2, and the Activation Parameters \ T(K) 1/TxlO3 (K-1) kxlO2 (M-V 1) -lnk/T 286.15 3.495 4.54 8.749 291.15 3.435 6.19 8.456 294.15 3.400 8.11 8.196 298.15 3.354 9.00 8.106 302.15 3.310 11.7 7.840 307.15 3.256 14.4 7.645 AH* = 9.0 ± 0.2 kcal/mol AS* = -33.0 ± 0.8 e.u. 4.3.3. Reaction of Pt2l20>PNi)2 (HH), 10a, with DMAD In order to assist in identifying the structure of 28.2 in reaction (4.1), the structurally related compounds, 10a and 10b (the corresponding PN2 complex), were reacted with DMAD. Reactions of the latter, as the various diastereomers lOb.l and 10b.2/10b.3, with DMAD will be dealt with in the next section. When the title compound 10a was reacted with DMAD, an orange solid, 17, was isolated and characterized by 3 1 P and 1 9 5 Pt NMR spectroscopies (Fig. 4.14), as well as elemental analysis (Table 2.8). Both spectra strongly suggest the HH configuration. This particular product, 17, is stable in solution for more than a week, and no isomerization process is apparent. 132 Fig. 4.12. Pseudo first-order rate plot, In (At-Aoo) vs. t, for reaction 4.2 at 18°C in CH2CI2: [9c] = 1.18x10-3 M, [DMAD] = 2.39xl(r2 M (Table ADJ. 12). Inset: the observed first-order rate constants vs. [DMAD] (see Table 4.6). .8.80 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.00320 0.00330 0.00340 0.00350 l / roc 1 ) Fig. 4.13. The Eyring plot of In k/T vs. 1/T for reaction 4.2 (Table 4.7). 133 Fig. 4.14. (a) 31p{lH} NMR spectrum (121.4 MHz) of 17 in CDCI3 at r.t.: 8 = 21.2 (s), ^PtP = 3496 Hz, 3jptp = 242 Hz. (b) 195Pt{ lH) NMR spectrum (64.2 MHz) of 17 in CDCI3 at r.t.: 8pt = -43.3 (t),1 JptP = 3496 Hz, JptPt = 670 Hz. 134 The rate of reaction (4.3) was studied by monitoring absorbance decrease in the 400 to 600 nm region spectrophotometrically (the spectral changes are similar to those in Fig. 4.11, except only one absorbance maximum at 496 nm (405 M^cnr1) is seen; an isosbestic point occurs at 476 nm). Analyzing data for the spectral changes at selected wavelengths (Tables AVI) gives a first-order dependence in [Pt2], i.e. the ln(ArA«.) vs. t plot is a straight line (Fig. 4.15). Doubling the concentration of DMAD doubles the pseudo first-order rate constant, implying a first-order dependence on [DMAD]. The simple first-order dependences in both metal and DMAD are the same as those found for the first steps of reaction 4.1. The temperature dependence of the rate constants was also investigated, and the results are summarized in Table 4.8 and Fig. 4.16. The AH* and AS* values are again in the same range as for the oxidative addition reactions at binuclear metal centres described in the previous section. Table 4.8. Rate Constants for Reaction 4.3 at Various Temperatures, and the Activation Parameters T(K) lAxlO3 (K-1) kxlO2 (M-V1) k/TxlO4 -lnk/T 293.15 3.411 2.60 0.887 9.330 298.15 3.354 3.21 1.077 9.136 303.15 3.299 4.04 1.323 8.923 308.15 3.245 4.90 1.590 8.747 AH* = 7.1 ±0.2 kcal/mol AS* = -41.0 ± 0.7 e.u. The key point that the studies on this reaction demonstrate (in conjuction with findings on reaction 4.1) is that the instability of an HH DMAD adduct is not governed by simple steric factors: the phenyl group is little different in size to pyridine (cf. 17 vs. 28.1 (p. 106)). The nonbridging pyridyl groups are clearly playing an important role in destabilizing the HH insertion product and in promoting isomerization to the HT product. The role of the nonbridging pyridyl group will be illustrated further in the following section. 135 0.0 -3.0 I 1 1 1 i i • • • i | 0.0 1000.0 2000.0 3000.0 4000.0 5000.0 6000.0 t(s) F ig . 4.15. Pseudo first-order rate plot, In ( A t -Aoo) vs. t, for reaction 4.3 (10a to 17) at 25°C in CH2CI2: [10a] = 2 .89x l (H M ; [ D M A D ] = 1.63xl0- 2 M . Inset: the observed rate constants at 25°C vs. [ D M A D ] (data in Table A V I 6). -9.70 • -9.80 I • 0.0032 0.0033 0.0034 0.0035 l/rox:1) F ig . 4.16. The Eyring plot for the rate constants of reaction 4.3 (Table 4.8). 136 4.3.4. Reactions of Pt2l2(H-PN2)2 (HH), 10b, with D M A D Reactions of the diastereomers of 10b with D M A D are shown to undergo initially the same oxidative addition with D M A D to form an A-frame insertion adduct; the diastereomers, in which the binuclear P - N bridging complex has one non-chelated pyridyl group and one phenyl group on the phosphorus atom, are found to combine reaction patterns of both 10a and 10c, leading to H H , H T or a mixture of H H and H T adducts (see below). Knowledge of the fates of the diastereomeric insertion adducts is essential in elucidating the origin of the isomerization. The use of the isolated diastereomers 10b. 1 and 10b.2/10b.3 (Sect. 3.2.4) allows for a thorough understanding of the relationship between the structures of the starting complexes and the end-products. In this section, the results of 3 1 P { 41} N M R and kinetic studies are presented. 4.3.4.1. Reaction of lOb.l with D M A D (4.4) (4.5) (1R.2S) 30 The reaction of lOb.l with D M A D is expected to yield two diastereomers 29.1 and 30 (Eqs. 4.4 and 4.5). Each diastereomer is expected to give one set of 3 1 P N M R signals. The 3 l P 137 NMR spectrum of the reaction mixture after 20 min is shown in Fig. 4.17(a), and consists Of one upfield singlet and two downfield doublets with corresponding Pt satellites. The striking similarity in the spectral pattern of the downfield signals to that of 28.2 (Fig. 4.4a), and in the pattern of the upfield signals to that of 17 (Fig. 4.14a) suggests that 29.1 gives rise to the downfield doublet of doublets and 30 to the upfield singlet More convincing evidence for these assignments is obtained from the 3 1 P NMR spectrum of the same sample taken after one week (Fig. 4.17b). Signals corresponding to 30 stay unchanged while those of 29.1 disappear completely. The upfield four singlets with different intensities are those of Pt2l2(^-DMAD)(|J.-PN2)2 (HT), 18, present as a diastereomeric mixture. In other words, adduct 30 niimics the reactivity of 17 as 30 and 17 are structurally similar, and adduct 29.1 mimics the reactivity of 28.2. The stability of 30 against isomerization is thought to be due to steric reasons. Formation of the A-frame insertion product pushs the iodo group backwards in the same plane as the pyridyl groups; as a result, the pyridyl groups are blocked from possible contact with central Pt atom. On the other hand, the pyridyl groups in compound 29.1 are not blocked. The presence of properly positioned pyridyl group(s) is essential for the isomerization, which is thus predictable based on the structure of the initial A-frame adduct. The equivalence of the phosphorus nuclei seen in compound 30 is due to the a symmetry in the molecule. Obviously, the inequivalence of the P nuclei seen in 29.1 must result from an interaction similar to that observed in 28.2; this is thought to be the chelating form of 29.1 is labelled as 29.2. 28.2 29.2 138 29.2 30 « (a) (b) 30 18 ' I i I | I I I i 1 l l l l | l l 1 I 1 l l l l | l l l l | l i l l | i l i i i i i i i i i T i i | i i i i ; i i : i i 40 30 20 10 6 -10 PPM Fig. 4.17. 31P{1H} NMR spectra (121.4 MHz) for the products of reactions 4.4 and 4.5, in CDCI3 at r.t.: (a) initially formed adducts 29.2 and 30 after 20 min, (b) products 18, after isomerization (see also Fig. 4.18b), and 30 after one week; unlabelled peaks are unassigned. (29.2: 8 = 28.3 (d), UptP = 3705 Hz;8 = 26.4 (d), lJPiP = 3763, 3JptP = 215 Hz; 2 J P P = 15.0 Hz. 30: 8 = 21.15 (s), UptP = 3459,3JptP = 238 Hz). The difference between a phenyl group and a pyridyl group is that the latter contains a nitrogen atom capable of chelation such that a five-coordinate square-pyramidal phosphorus-nitrogen chelatied intermediate can be formed. There are several examples in the literature of chelating pyridylphosphine complexes containing 4-membered rings.27- 2 8 With this assumption, the splitting of the phosphorus signals in the 3 1P{ 1H) NMR spectra of 28.2 and 29.2 can be accounted for. This type of chemical inequivalence of the P nuclei has never been 139 observed for the HH precursors in the absence of DMAD. The isomerization of Pt2l2(H-PN3)2 (HH), 10c, to the HT form, 9c, does not proceed even at 60°C in CHCI3 for 24 h, at least in the absence of PN3 (see Sect. 4.4). The isomerization of 28.2 and 29.2 to their corresponding HT isomers is therefore promoted by the chelation which is itself promoted by the DMAD oxidative addition. Whether the chelation of the pyridyl group takes place simultaneously upon, or is preceded by, the oxidative addition of DMAD is clarified in the following section. 4.3.4.2. Reaction of the 10b.2/10b.3 mixture with DMAD The reactions of the mixture of 10b.2/10b.3 with DMAD are assumed to proceed initially as follows: (4.6) (4.7) (IR, 2R) (IR, 2R) 10b.3 3 2 - ! Externally related by a mirror plane, 31.1 and 32.1 are enantiomers, as are the starting complexes 10b.2 and 10b.3. Enantiomers are not distinguishable from each other in an achiral environment by NMR spectroscopy. The P atoms are diastereotopic within one enantiomer by internal comparison.29 These inequivalent P atoms show strong mutual trans coupling, and 140 therefore a strongly coupled AB quartet with corresponding Pt satellites is expected in the 3 *P NMR spectrum. The peaks labelled with stars in Fig. 4.18a are the expected signals for 31.1/32.1. The inequivalence arising from the diastereotopic phosphorus nuclei is reflected in the Jpp value of these enantiomers, 454 Hz, which is comparable to the J Pp value in Pt2l2(H-PN2)2 (HH), lOb.l (Fig. 3.15). The downfield closely spaced doublet of doublets is assigned to the subsequent chelating intermediate 31.2/32.2 (2 enantiomers) because of the similarity in spectral pattern to that of 28.2. The prominent feature of this spectrum is that the direct insertion products in the HH forms, 31.1 and 32.1, are detected; the corresponding products were never seen in reactions (4.1) and (4.4). There is no obvious explanation for the difference, except that statistically 31.1 and 32.1 have fifty percent less chance for chelation than the precursors (28.1 and 29.1) to 28.2 and 29.2 in which chelation is apparent. Also during the NMR experiment, the signals of 31.1/32.1 disappeared much faster than the other set. This evidence favours pathways involving initial oxidative addition of the acetylene, followed by chelation. With the availability of one pyridyl group for chelation, 31.1/32.1 have favourable geometry to isomerize. The 3 1 P NMR spectrum of the reaction products after one week is displayed in Fig. 4.18b. The final reaction mixture contains only the HT-insertion adduct 18, the NMR signals being similar to the upfield signals in Fig. 4.17b. These experiments demonstrate again that if the Pt centre is accessible by the pyridyl group, the isomerization is inevitable. 4.3.4.3. Kinetic studies on the reaction of the diastereomers of 10b with DMAD The rate measurements for the reactions (4.4) and (4.5) were done by using the previously described methods, that is by monitoring the spectral changes from 400 to 600 nm, Fig. 4.19. Reaction of lOb.l with DMAD produces the two diastereomers 29.2 and 30, presumably by two, direct parallel reactions. The rate law, in terms of optical density data, is derived easily, assuming that 29.2 and 30 have the same absorptivities: lOb.l + DMAD 141 31.2/32.2 (a) 31.1/32.1 18 (b) .1 40 30 i r i I i i i i I i i i 20 T T 10 1 •! | I I I I | I I ) I [ I I M [ I'l I f | 0 -10 P P M Fig. 4.18. 31P{1H} NMR spectra (121.4 MHz) for the products of reactions 4.6 and 4.7, in CDCI3 at r.t.: (a) the initially formed (20 min) adducts 31.2/32.1 are labelled with stars, and the chelated intermediates 31.2/32.2 via which isomerization occurs, (b) the end products 18, Pt2l2G/i-DMAD)(u,-PN2)2 (HT), after one week (Pt satellites are not visible, see also Fig. 4.21b). (31.1/32.1: AB quartet centred at 18.5, Uptp = 3459, 3 J P t P = 249, 2 J P P = 454 Hz; 31.2/32.2: 6 = 25.5 (d), 25.9 (d); ^  = 3763, 3654 Hz; 3 ^ = 169 Hz; 2JP P= 14.7 Hz. 142 -d[10b.l] /dt = ki[10b.l][DMAD] + k2[10b.l][DMAD] =(ki+k2)[10b.l][DMAD] =k'obs[10b.l] where k'obs = 0 q + k2)[DMAD] By monitoring the decrease of lOb.l concentration in conditions using excess DMAD, the observed pseudo first-order rate constant k'0bs is obtained from the slope of the In (A t - Aoo) vs. t plot. The true second-order rate constant for the loss of lOb.l is the sum of rate constants for each of the parallel reactions. The difference in these latter rate constants is reflected by the difference in the final concentrations of 29.2 and 30: that is, ki/k 2 = [29.2]/[30]. The pseudo first-order plot, ln(At-Aoo) vs. t, gives a very good straight line and the second-order rate constant, the sum of ki and k2, obtained from the slope of kobs vs. [DMAD] plot, is 8.10xl0"3 M - V 1 at 25°C (Fig. 4.20). The ratio of [29.2] to [30] was deterrnined through the 3 1 P signal integration to be 3.5:1 (Fig. 4.17a); these data give ki ~ 6.3xl0"3, and k 2 « 1.8xl0-3 M-V 1 . The difference reflected in the rates must be attributed to the preferential binding of DMAD to the 'pyridyl face' over the 'phenyl face'. This preference seems to be applicable also in the reactions of the diastereomeric mixture within the Pd2X2(M--PN2)2 (HT) (X = CI, I) and Pt2I2(n-PN2)2 (HT) complexes with DMAD (Fig. 4.21). The ln(At-Aoo) vs. t data of reactions (4.4) and (4.5) are listed in Tables AVUI. Monitoring of the subsequent isomerization step (29.2 to 18) by visible spectroscopy was not possible because the acccompanying largest total absorbance change was too small (-0.03). However, a rough estimate of tip. for this step at 25°C is 6 h (cf. for 10c, ti/2 = 4.1 h, Table 4.5). The rate measurements for reactions (4.6) and (4.7) were also monitored by visible spectroscopy; spectral changes similar to those in Fig. 4.19 were observed. Compounds 10b.2 and 10b.3 are enantiomers, and so are the insertion adducts 31.1 and 32.1, and the subsequently formed chelated species 31.2 and 32.2. Reactions (4.6) and (4.7) are independent, occurring with the same rate constants, 143 400 500 600 X(nm) Fig. 4.19. The spectral changes for reactions 4.4 and 4.5 at 25°C in CH2CI2: the spectrum labelled lOb.l ( ) was taken at t = 0 (Xmax = 496 nm, e = 608 M_1cm_1); the spectrum labelled 29.2 + 30 ( ) gave the lowest absorption among the repetitive scan spectra; the solid lines in between were recorded every 5 min. [lOb.l] = 1 . 9 9 X 1 0 " 4 , [DMAD] = 1.22xl0"2 M(TableAVin4). 0.0 -2.0-.S -3.0" -4.0 0.0 slope = -7.23x10 1000.0 2000.0 t(s) 3000.0 4000.0 Fig. 4.20. Pseudo first-order plot, In (A t -Aoo) vs. t, for reactions 4.4 and 4.5, at 25°C in CH2C12: [lOb.l] = 1.99xl(H M, [DMAD] = 8.13xl0"3 M . Inset: the observed rate constants (25°Q vs. [DMAD] (data in Tables A W ) . 144 Ph i 1 1 .J '•Pd Ph py XT p" 'Ph n i p vj CM py.py O-p-py i' Li py-jp. PIT N. py. PtjVjp N CT 11 i J '•PdPtf py Ph,py py.py (a) py, _ Pn*s .p-^ N '-Po^Pd* py. Ph,py «»py py Ph Ph1 •v Ph.Ph N ^ P ' '•Pd P<f .Ph Ph,Ph 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 | i n i 11111111) 11111111111,11111111 ] 111111111 [ 1111;! I i i ! ; 1111111: i 38 36 34 32 30 2B 26 24 22 20 IB 16 PPM Ph.py (b) py.py Ph,Ph | I I I I | I I I I | ! ! I I | ! I I 1 J I I I 1 | I I I I ] I ! I ! I I I I I | I I I : | I I I I : I I I 30 20 10 6 -10 PPM Fig. 4.21. 3 1 P NMR spectra (121.4 MHz) of the diastereomeric mixture of (a): Pd2l2(^-DMAD)(n-PN2)2 (HT), 25, and (b): Pt2l2(H-DMAD)(n-PN2)2 (HT), 18, in CDC13 at r.t. (data see Table 4.1); the structures of the diastereomeric mixture of 18 are identical to the Pd analogues except that the central metal atoms are Pt (Scheme 4.1). 145 k3 10b.2 +DMAD - 31.1 ^  ^31.2 k3 = k4 = k 10b.3 + DMAD — 32.1 ^ 32.2 The rate law is simply -d{[10b.2]+[10b.3]}/dt = k[DMAD]{[10b.2]+[10b.3]} = kobs{[10b.2]+[10b.3]} Therefore, the rate of the individual reaction is first-order with respect to the concentration of the corresponding enantiomer, and the overall reaction rate is first-order with respect to the total [Pt2]. With the assumption that the non-chelated and the chelated species are formed in a fixed ratio (by the fast equilibrium shown in Scheme 4.2), the plot of ln(At-Aoo) vs. t should appear linear (see derivation in Appendix VJJ), and kobs (= k[DMAD]) should be directly proportional to [DMAD]. Such analyses are obtained for this system (Fig. 4.22). At 20°C, the second-order rate constant is found to be 8.05xl0*3 M_1s_1. The activation enthalpy and activation entropy are found to be 8.2 ± 0.5 Kcal/mol and -35.0 ± 0.8 e.u., respectively (Fig. 4.23) (the raw data are listed in Tables A VII). Accurate monitoring of the subsequent isomerization step was again very difficult because the largest total absorbance change was only about 0.06. A rough estimate of ti/2 at 25°C is 5 h (cf. for 10c, ti/2 = 4.1 h, Table 4.5; for lOb.l, ti/2 = 6 h). 4.3.5. Mechanism of the reaction of Pt2l20>-PNn)2 (HH) with DMAD The ratios of relative intensities of the diastereomeric isomers of the final product Pt2l2(H-DMAD)(u.-PN2)2 (HT), 18, in Fig. 4.17 and 4.18 are similar. This observation cannot be explained in terms of a concerted migration of a phosphorus atom from one Pt to the other, because this would lead to the stereospecific (HT) isomers, as shown in Scheme 4.1. According to Scheme 4.1, compound 292 would undergo isomerization to give the enantiomers 33 and 34. The expected 3 1P NMR spectrum for 33 and 34 should contain only one singlet with the corresponding Pt-satellites, because the phosphorus atoms are chemically equivalent due to the presence of a C2 axis. Compounds 31.2 and 32.2 would generate 35 and 36, respectively, which are also enantiomers. The expected 3 1P spectrum for 35 and 36 should 146 -4.5 I i • • i i • • i • • • i • • • i i i • 0.0 1000.0 2000.0 3000.0 4000.0 5000.0 t(s) Fig. 4.22. Pseudo first-order plot, In (At -A») vs. t, for reactions 4.6 and 4.7, at 20°C in CH2CI2: [10b.2/10b.3] = 2.18xl(H M, [DMAD] = 8.13x10-3 M. Inset: the dependence of the pseudo first-order rate on [DMAD] (data in Tables AVE). -8.80 I 1 1 1 1 • t 1 1 1 1 1 0.0032 0.0033 0.0034 0.0035 i / r Fig. 4.23. The Eyring plot, In k/T vs. 1/T, for the rate constants of reactions 4.6. and 4.7 (see Tables AVE 8). 147 enantiomers (1R.2R) (IS, 25) 3 7 » , ' 3 8 enantiomers Scheme 4.1. The expected isomerization products, based on a concerted pathway involving phosphorus migration and nitrogen coordination, and the actual isomerization products observed. contain two singlets with equal intensity due to the diastereotopic P atoms in the molecule (Jpp = 0, see Sect. 4.3.1). Instead, four singlets are seen (Figs. 4.17,4.18). These are assigned to the three possible sets of enantiomers drawn above, and thus the concerted migration pathway of Scheme 4.1 does not account for the observed products. The mechanism is best explained as involving complete dissociation of a P atom from Pt, followed by recoordination to the other Pt, during which process the chiral identity of the P atom is lost. The activation enthalpy of 24 kcal/mol for reaction (4.1) seems consistent with the involvement of a P ligand dissociation in the isomerization step.30 Thus, the general reaction pathway for the reaction of DMAD with 10b and 10c (HH) species leading to the isomerized insertion adduct is presented in Scheme 4.2: C D E Scheme 4.2. The proposed mechanism for the oxidative addition of DMAD to Pt2l2(|^ -PNn)2 (HH) and the subsequent isomerization; s and = represent the free and bridged acetylene, respectively; P* represents the chiral P atom; r.cLs = rate determining step. In this scheme, the HH starting complexes A, comprising lOb.l, 10b.2/10b.3 and 10c, and the corresponding isomerized A-frame products, E (18 and 19), have been well 149 characterized. Among the three suggested intermediate species (B, C and D), only D has not been detected, and this is consistent with its subsequent rearrangement in a fast step to give the product E. Nevertheless formation of D is suggested by the relatively high activation enthalpy (AH* = 24 kcal/mol) for the slow step in the isomerization (28.2 to 19) and also by the formation of the nonstereospecific HT products 18 from species 29.2, 31.2 and 32.2 (Scheme 4.1). B is detected in situ by 3 1P NMR spectroscopy in reaction (4.5) as 30, and in reactions (4.6) and (4.7) as 31.1 and 32.1. When the phosphine is PNi, the HH-DMAD adduct was isolated as 17. C, a key intermediate species, has been seen in every isomerization reaction as 28.2, 29.2, 31.2 and 32.2. The suggested reaction pathway is fully supported by the 3 1 P and 1 9 5Pt NMR spectral data and is consistent with the results of the kinetic experiments. 4.4. Reaction of Pt2l2(|A-PN3)2 (HH), 10c, with PN3 under air in organic solvents The HH isomer PtPdl2(M--PNi)2, with two P atoms on the Pt, has been shown to isomerize in solution under reflux in air to the HT isomer,-?7 with the P coordinated to both Pd and Pt. This HT isomer was more stable thennodynamically, and the HH isomer was believed to be the kinetic product of the synthesis procedure used.57 The Pt2l2(|^-PN3)2 (HH) complex, 10c, shows no tendency to isomerize in solution after refluxing in CHCI3 or CH2Cl2 under air for 48 h. When tris(2-pyridyl)phosphine (PN3) was introduced to the CH2CI2 solution of 10c at room temperature, the isomerization of 10c and the oxidation of phosphine ligand were observed. If one equivalent of phosphine was added, the HT complex, 9c, was produced; if two or more equivalents of phosphine were added, an ionic complex containing two HT bridging phosphines and two terminal phosphines could be isolated as the BPI14- salt (see below). Both of these reactions were investigated by 3 1P NMR spectroscopy and the results are discussed here. Reaction of 10c with one equivalent of P N 3 in air, leading to the formation of 9c, was followed by 3 1P NMR spectroscopy; although a reaction of phosphine with the HH isomer is 150 observed almost instantaneously, as indicated by the colour change from orange to yellow, the isomerization proceeds relatively slowly. The 3 1 P NMR spectrum of the immediately formed yellow species 39 is shown in Fig. 4.24a; the splitting pattern and the coupling constants strongly suggest that compound 39 still contains an HH skeleton, while the third phosphine could be on either one of the Pt centres. Fig. 4.24b, taken one week after the mixing, suggests that the terminal phosphine coordinates to the Pt with two nitrogen atoms because the intermediate 40 has an HT isomerized skeleton. This conclusion is drawn based on the matching of the 3 1 P NMR spectrum of the reaction product from 9c and PN3 in situ, and the understanding of the spectral patterns; the 3 1 P NMR pattern of 40 indicates three ^equivalent P atoms, these giving three sets of doublets of doublets. The only reasonable structure for 40 is the HT binuclear complex with an extra terminal phosphine on one of the Pt centres. As the isomerization proceeds via 39, the terminal phosphine must become bridging while one of the bridged phosphines becomes non-bridging. The overall picture of the isomerization process is given in Scheme 4.3. The equilibrium between 10c and 39 is established instantaneously and very much favours the formation of 39, while the formation of 40 from 9c is not as favourable, perhaps because of steric reasons. This infers that the mcorning phosphine adds to the Pt with the two coordinated N atoms; the phosphine perhaps replaces the iodide so that the Pt still retains a square-planar geometry (39). The Scheme 4.3 illustrates the chemical processes which are monitored spectroscopically (Fig. 4.24). The formation of a cationic species from a P121 phosphine dihalide on reaction with phosphine is precedented in the literature.52 In the final stage, the phosphine is oxidized to the phosphine oxide; this process does not occur normally in air in the absence of metal complex. 151 (a) 39 Pl,2 P3 (b) 9c P2 Pi I (C) 9c OPN3 I 1 1 i 1 1 1 1 1 1 1 'i 1 1 1 1 1 1 1 1 I 1 1 1 1 ! 1 1 1 1 I 1 1 1 3 0 2 0 1 0 0 I I I ! 1 ! I I I 1 -10 1 1 : -20 PPM Fig. 4.24. Reaction of 10c in air with one equivalent of PN3, as followed by 31P{ lH) NMR (121.4 MHz) spectroscopy at r.L in CDC13. (a) immediately after addition of PN3, (b) one week after addition of PN3, (c) twenty days after. (39: 8Pl 2 = 6.0 (d), ^ P I P = 3305,2Jptp = 126 Hz; 8p3 = 16.2 (t), the Pt satellites of P3 not being visible; 2JpP = 17.6 Hz. 40: see Fig. 4.26). 152 + W—I + Ppy3 10c 39 I — P t — I t - e ^ y a i -f r N ^ ) P p y 2 air Py2P^ I—Pt t—I + OPpy3 40 9c Scheme 4.3. Schematic presentation of the isomerization reaction promoted by one equivalent PN3. As the ratio of added phosphine to [Pt2l increases (£ 2equiv.), the immediately formed species 41.1 has a 3 1P NMR spectrum completely different to that of 39; the spectrum of 41.1 in CDCI3 consists of two parts — an upfield "singlet" with pseudo-first order Pt satellites and a downfield "singlet" with non-first-order Pt satellites — with equal integrals (Fig. 4.25). The upfield "singlet" is actually a triplet with some fine splitting of 5 Hz, and so is the downfield one. The upfield half of the spectrum indicates that 41.1 has at least two HT-arranged bridging phosphines, while the downfield half resembles the coupling pattern of the 3 1P NMR spectrum of Pt2Cl2(PPh3)2(CO)2 3 3 (JJptP = 2189,2Jptp = 475, 3JPP = 227 Hz) in which two phosphines are colinear with the binuclear Pt2 core. Because of the magnetic inequivalence of four phosphorus nuclei and the quadruple broadening effect of two nitrogen nuclei, the fine splittings between cis-phosphorus atoms and between diagonally situated phosphorus atoms are not resolvable and the spectra are thus broadened. The broad peaks are not caused by fluxional behaviour of the molecule, as the low temperature 3 1P NMR spectrum shows no improvement of 153 resolution. The 1 9 5Pt NMR spectrum of 41.1 (Fig. 4.25b) indicates chemical equivalence of the Pt nuclei in the molecule. A tetrakisphosphine diplatinum complex is proposed with the structure shown in Scheme 4.4. The major coupling constants: lJp\?i> 2JptP2» *JptP3t 2JptP4 and 3 Jpp are resolved by spectrum simulation to be 3936,126,2240,750 and 220 Hz, respectively. Compound 41.1 is not stable in CH2CI2 or CHCI3 in air, and gradually the phosphine is 'removed' as the oxide, and the HT complex, 9c, is formed. The 3 1 P NMR spectrum of the species present in solution after one week consists of about 25% of 40, while only the HT isomer 9c is present after two weeks (Fig. 4.26). Scheme 4.4 shows a possible route for the formation of 41.1 and 9c in solution. The ionic nature of 41.1 is demonstrated by its large solubility in water. The tetraphenylborate salt of 41.1 was isolated and characterized as 41.2 (Sect. 2.6); the molecular conductivity measurement agrees with the presence of a 1:2 electrolyte (204.3 Q^moHcm 2 at 1.9xl0"3 M).34 Complex 41.2 readily dissolves in acetone-d6 and acetonitrile-d3, giving a 3 1 P NMR spectrum which is the same as that of the 41.1 in CDCI3; and this spectrum does not vary over a period of at least three weeks (Fig. 4.27). No decomposition was seen during an extensive purification procedure.* Complexes 41.1 and 41.2 do not react with olefins, such as styrene, acrylonitrile and maleic acid, and no hydration of these olefins was found when 41.1 or 41.2 (~ 0.5 mM, in 1:1 acetone/H20 mixture) was mixed with olefin substrate (~ 50 equivalents) under N2 at 80°C for a period of 2 h. * A recrystallization procedure from acetone generated yellow crystals of 41.2, but unfortunately they were unsuitable for X-ray analysis. 154 Fig. 4.25. (a) 31P{1H} NMR spectrum (121.4 MHz) of in situ formed Pt2l2(H-PN3)2(PN3)2, 41.1, in CDCI3 at r.L: 8Pl>2 = -12.3 (t); I j p ^ = 3926, Sjptp, = 126 Hz; 8P34= 27.9 (t), Ijpu^  = 2240, 2JPu>3 = 750,3Ji>3P4 = 220 Hz; the two major peaks are split into a triplet with 5 Hz coupling, (b) ^Pt^H} NMR spectrum (64.2 MHz) of 41.1 in CDCI3 at r.t.: multiplets centred at 250.3, natural line width = 80 Hz. 155 (c) 0PN3 9c (b) -jA... i J 40 (a) P3 OPN3 OPN3 L^\_A , , . , . . A _ A _ ^ J L 40 Pi 9c A I PN3 4/0 P2 A • 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -i 1 1 1 40 30 20 I I I I I I I I I I I I I I I I I M I I I I I I I I I I I 10 - 1 0 -A. •I I I I I I M I I I I I 'I M I I -^0 PPM - 3 0 Fig. 4.26. The gradual dissociation and isomerization of 41.1 in CDC13 in air to the final product 9c, as followed by 31P{ *H) NMR spectroscopy (121.4 MHz) at r.t.: (a) immediately after addition of excess PN3, (b) one week after addition, (c) three weeks after addition. (40: o>, = -2.5 (dd), 3 J P , P 2 = 18.3,3JPlp3= 3.6; i jptp, = 3897, = 120 Hz; 5?2 = -10.8 (dd), 2jpjp3 = 9.7 ; lJp&2 = 3933, 2jptp 2 = 194 Hz; 5p3 = 16.8 (dd), 1 J p t p 3 = 2430, 2 J p t p 3 = 550 Hz). Ppy3 = (excess) 2+ py2P^  N' p y 3 P - P t Pt -Ppy 3 2T OPpy3 + I—Pt Pt-Ppy 3 r - N ^ P p y z air 2+ : p y 3 p (3 )_Pt_Pt_(4 )p p y 3 2r 40 41.1 air py2P-^  1 i ^ N ^ - P p y a u 9c NaBPh4 H20/MeOH 2+ OPpy3 py3P—] py2P^0 ^ Pt Pt—Ppy3 2BPh4" C r 41.2 Scheme 4.4. Schematic diagram of possible reaction pathway with excess PN3. In conclusion, the HH configuration appears to be very stable toward frame rearrangement in the absence of added PN3 or DMAD. The strong tendency to form the HT configuration in the presence of such reagents in air, however, suggests that the HH isomer is formed in a metastable state, and that a large kinetic barrier normally stops the isomerization in the absence of a nucleophile. Any factor which lowers this kinetic barrier will accelerate the isomerization. The fact that the HH configuration was not obtained in any Pd(I) binuclear 157 species is likely attributed to a low kinetic barrier under the experimental conditions for this metal which is a much more labile system. With Pd systems, even if an HH isomer is formed as an intermediate, it will soon isomerize to the HT form. On the other hand, the HT form plus PN3 even in the Pt system, will never go to the HH configuration, because the process is thermodynamically unfavourable. Fig. 4.27. 3lp{lH} NMR spectrum of [Pt2(^ -PN3)2(PN3)2](BPh4)2, 41.2, in acetone-de at r.t (see also Fig. 4.25). P3.4 Pl,2 I : 1 1 1 j 1 1 1 1 _3£ p3 / j 158 References 1. Muetterties, EX ; Pretzer, W.R.; Thomas, M.G.; Beier, B.F.; Thorn, D.L.; Day, V.W.; Anderson, A.B. / . Am. Chem. Soc., 1978, 100, 2090. 2. Day, V.W.; Abdel-Meguid, S. S.; Dabestani, S.; Thomas, M.G.; Pretzer, W.R.; Muetterties, E.L. / . Am. Chem. Soc, 1976, 98, 8289. 3. Bailey, W.I. Jr.; Cotton, F.A.; Jamerson, J.D.; Kolb. J.R. / . Organomet. Chem., 1976, 121, C23. 4. Cotton, F.A.; Jamerson, J.D.; Stults, B.R. / . Am. Chem. Soc., 1976, 98, 1774. 5. Sly, W.G. / . Am. Chem. Soc, 1959, 81, 18. 6. Bennett, M.A.; Johnson, R.N.; Robertson, G.B.; Turney, T.W.; Whimp, P.O. Inorg. Chem., 1976, 15, 97. 7. Wang, Y.; Coppens, P. Inorg. Chem., 1976,15, 1122 . 8. Millis, O.S.; Shaw, B.W. / . Organomet. Chem., 1968,11, 595. 9. Dickson, R.S.; Pain, G.N.; Mackay, M.F. Acta Crystallogr. Sect B , 1979, 35, 2321. 10. Davidson, J.L.; Harrison, W.; Sharp, D.W.A.; Sim, G.A. / . Organomet. Chem., 1972, 46, C47. 11. Dickson, R.S.; Johnson, S.H.; Kirsch, H.P.; Lloyd, D.J. Acta Crystallogr. Sect. B , 1977, 33, 2057. 12. Balch, A.L.; Lee, C-L.; Lindsay, C.H.; Olmstead, M.M.; J. Organomet. Chem., 1979, 177, C22. 13. Southern, T.G.; Cowie, M.A. Inorg. Chem., 1982, 21, 246. 14. Mague, J.T. Inorg. Chem., 1989,28, 2215. 15. (a) Dickson, R.S.; Hames, B.W.; Cowie, M.A. Organometallics , 1984, 3, 1879. (b)/dem. 1985, 4, 852. 16. Koie, Y.; Shinoda, S.; Saito, Y.; Fitzgerald, B.; Pierpont, C.G. Inorg. Chem., 1980, 19, 770. 17. International Tables for X-ray Crystallography; Ibers, J. A., Ed.; Kynoch Press: Birmingham, England, 1973 ; Vol.3, Table 4.2.2. 159 18. Ittel, S.D.; Ibers, J.A. Adv. Organomet. Chem., 1976,14, 33. 19. Maisonnat, A.; Farr, J.P.; Balch, A.L. Inorg. Chim. Acta, 1981, 53, L217, and Sect. 3.1. of this thesis. 20. Lee, C.-L.; Yang, Y-P.; James, B.R. unpublished results. 21. Kezdy, F.J.; Kaz, J.; Bruylants, A. Bull. Soc. Chim. Belg., 1958,67, 687. 22. (a)Swinbourne, E.S. / . Chem. Soc, 1960, 2371. (b)Manglesdorf, P.C. / . Appl. Phys., 1958,30, 443. 23. (a) Guggenheim, E.A. Phil. Mag., 1926,2, 538. (b) King, E.L. / . Am. Chem. Soc, 1952, 74, 563. 24. Atwood, J.D. Inorganic and Organometallic Reaction Mechanisms; Brooks/Cole: New York, 1985; p. 166. 25. Barnabas, A.F.; Sallin, D.; James, B.R. Can. J. Chem., 1989,67, 2009. 26. Brost, R.D.; Kimberly Freldsted, D.O.; Stobart, S.R. Chem. Commun., 1989, 488. 27. Farr, J.P.; Olmstead, M.M.; Wood, F.; Balch, A.L. / . Am. Chem. Soc, 1983, 105, 792. 28. Farr, J.P.; Olmstead, MM.; Balch, A.L. Inorg. Chem., 1983,22, 1229. 29. Mislow, K.; Roban, M. Topics in Stereochemistry 1977,1, 1. 30. (a) Joshi, A.M.; James, B.R. Organometallics, 1990, 9, 199. (b) Krassowski, D.W.; Nelson, J.H.; Brower, K.R.; Nauenstein, D.; Jacobson, R.A. Inorg. Chem. 1988, 27, 4294. 31. Farr, J.P.; Wood, F.E.; Balch, A.L. Inorg. Chem., 1983, 22, 3387. 32. Blau, R.J. Energ Res. Abstr., 1985,10, No.21297 through CA. 1985,103, 129835a. 33. Koie, Y.; Schinoda, S.; Saito, Y. Inorg. Chem., 1976, 20, 4408. 34. (a) Dodd, R.E.; Robinson, P.L. Experimental Inorganic Chemistry — A Guide to Laborotory Practice; Elsevier: Amsterdam, 1954; p.378. (b) Geary, W.J. Coord. Chem. Rev., 1971, 7, 81. 160 Chapter 5 Preparation and Reactivity of Pyridylphosphine Platinum(O) Complexes The chemistry of the Pt(0) species, Pt(PR.3)m (m = 2,3,4), has been investigated in great detail since the synthesis of Pt(PPh3)4 was reported in 1958;7 yet interest in this subject is not (hminishing. Among the Pt(PR3)m species (R = Et,2 \-¥T?>4 aryl-5-6), Pt(PPh3>3 and Pt(PPh3)4 are the most studied because they are easily handled and readily available. Even though the pyridylphosphine ligands were made forty years ago,7 the chemistry of their platinum(O) compounds has not been investigated prior to this work. In this Chapter, the preparation of the platinum precursors Pt(PN3)4,26c, and Pt(PNi)3,27a, and their reactivities toward oxygen, olefin, HCl and methyl iodide will be discussed. By and large, the chemical behaviour of the Pt(PNn)m species (n = 1, 3; m = 3,4) resembles that of Pt(PPh3)m, but the differences in their reactivities with respect to this complex will be emphasized. The catalysis attempted using these platinum pyridylphosphine derivatives will be discussed at the end of this Chapter. 5.1. Platinum(O) complexes of pyridylphosphines 5.1.1. Tetraltis[tris(2-pyridyl)phosphme]platinum(0), 26c The synthesis of Pt(PN3>4, 26c, has been described previously in Chapter 2 (Sect. 2.8.1); 26c is made by refluxing R^PtCL; in aqueous solution with a THF solution of KOH in the presence of PN3 ligand at temperatures higher than 60°C, or by reducing cis-PtCl2(PN3)2, lc, with hydrazine in the presence of PN 3 in benzene solution. The yield in the former preparation varies from 18 to 43% with reaction time, and in the latter is about 40%. The solvents used in these reactions are critical, THF and benzene being found through trial-and-error to be the best solvents for the particular reactions, respectively. When ethanol, the solvent used for the successful preparation of Pt(PPh3)4 by either method,8 is used in the present work, a deep red oil, which is not air-sensitive, results upon concentration and gives a complicated 31P{ VH} NMR 161 spectrum. The role of ethanol played in these reduction reactions is unknown. The hydrazine reduction route was chosen for the larger scale preparation of 26c because of its better reproducibility. Sodium or potassium amalgam reduction of trans-PtCl2(PR3)2 in THF is also known in the literature4 for the preparation of bisphosphine and trisphosphine Pt(0) complexes. This method, however, does not yield any Pt(PN3)m (m = 2, 3) species from a cis-PtCl2(PN3)2 precursor. Preparation of trans-PtCl2(PN3)2 for the starting material for the amalgam reduction (via Hg lamp photolysis of cis-PtCl2(PN3)2 in benzene, as in the procedure for making trans-PtCl^Phs)^ leads to decomposition of the starting cis complex. The 31P{ lH} NMR spectrum of 26c shows a singlet at 8 30.1 ppm, and two singlet satellites, at -20°C in CD2CI2 (Fig. 5.1). The signals have relative intensities of 1:4:1, and the typtp coupling constant is 3824 Hz, a typical value for Pt(PR3)4 tetrahedral complexes.5 The 195Pt{ !H} NMR spectrum of 26c (Fig. 5.2) consists of a quintet centred at 8 -538.3 ppm, having a Uptp coupling constant of 3839 Hz, close to the value measured from the 31P{ lH} NMR spectrum. Complex 26c certainly remains undissociated from -85° to -20°C in CD2CI2, the signals beginning to broaden only around 0°C, behaviour unlike that of Pt(PPh3)4. The variable temperature 31P{ lH] NMR experiment for Pt(PPh3)4, investigated by Sue in this group, shows that the 1:4:1 signal pattern of Pt(PPh3)4 can only be seen at -90°C, and the equihbrium constants for Pt(PPh3)4 dissociation into Pt(PPh3)3 and free phosphine in toluene-dg are 0.24 at 20°C and 0.013 M at -90°C, corresponding to 40 and 10% dissociation at 10"2 M, respectively/0 Consequently, compound Pt(PN3)3, 27c, tris[tri(2-pyridyl)phosphine]-platinum(O), cannot be made by re fluxing 26c in benzene, which is the corresponding method used for obtaining pure Pt(PPh3)3.i0 The unusual stability of 26c was also noticed during the course of its synthesis via the hydrazine reduction method; 26c was still the major product obtained even when the extra three equivalents of PN3 were omitted (Sect. 2.8.1). Compound 27c is only formed as a minor product (Fig. 5.3). The 31P chemical shift, 52.9 ppm, and the 162 (a) (b) (c) x JL 1 J11:ijiII111. II 111 70 50 50 ; I:' • • 11:1111111| i; 11; 111111111|1111[n m11n111n;i;;i);11111111n 111 ]! i AO 30 20 10 -10 ~io PPM-30 Fig. 5.1. 31p{lH) NMR spectra (121.4 MHz) of isolated Pt(PN3)4, 26c, at 3.2xlfJ-2 M in CD2CI2 at various temperatures; (a) 0°C, (b) -20°C, (c) -85°C: 8 = 30.1 (s), JjptP = 3824 Hz. Fig. 5.2. 195pt{lH) NMR spectrum (64.2 MHz) of 26c, at 3.2xl0"2 M in CD 2C1 2 at -45°C: 8 = -538.3 (quintet, the low intensity peaks being indicated by arrows), Uptp = 3839 Hz (the natural line width = 60 Hz). 163 coupling constant, 4435 Hz, for 27c are comparable with those of Pt(PPh3)3 and Pt(PNi)3: (Pt(PPh3)3: 50.07 (s), 4449 Hz;^Pt(PNi)3: 52.0 (s), 4417 Hz, see below). The 31P{ lH} NMR spectra of 26c mentioned above were recorded either immediately or within a few hours after sample preparation. Samples for 31P{ lH) NMR studies were prepared under a vigorous flow of nitrogen using degassed deuterated solvents, and the sample tubes were capped with a rubber septum and then wrapped with parafilm. The original bright yellow sample solution gradually changes to finally deep red (after one week) on standing. The 31P{ ^H) NMR spectrum of a mixture of 26c and 27c after 48 h in CDC13 solution (still orange) displays a major doublet at 81 23.3 ppm and a major triplet at 82 22.5 ppm with an integration ratio of 2:1 (Fig. 5.4). The coupling constants ^JptPi, 1 J p t P 2 and 2Jpp) for the new complex are 2700, 4050 and 14 Hz, respectively. Another noticeable change in the 31P{ ^H) NMR spectrum is the significant enhancement of signal intensities of the free phosphine and the phosphine oxide. Tentative structures for the newly formed complex, 42, are postulated to have a T-shaped phosphine arrangement, based on the coupling pattern and the magnitude of the coupling constants (see below for possible structures). There is no precedence in the literature for a Pt(0) (d10) complex possessing a T-shaped phosphine geometry within a 3- or 4-coordinate complex. Logically, the oxidation state of compound 42 should be either I or JJ. The increase in the amount of phosphine oxide implies that oxygen coordination might be involved. A similar "doublet and triplet" 31P{1H} NMR pattern was reported in a spectroscopic investigation of phosphine oxidation catalyzed by Pt(0).^  A five-coordinate, square pyramidal, trisphosphine peroxo intermediate (see (iii) below, where P = PMePh2) was proposed based entirely on the 31P{ lH) NMR parameters of this in situ formed compound, the data being indicative of a cis phosphine arrangement (8 = 13.2 (d), ^PtPi = 2740 Hz; 8 = -12.7 (t), 1J^P2 = 2930 Hz; 2 J P P = 22 Hz). The oxidation of the phosphine in the presence of KOH was demonstrated by both kinetic and spectroscopic data to go via hydrogen peroxide formed in situ. The Pt-P coupling constants in 42 are very different from those in Pt(PMePh2)3C»2, and also no formation of such peroxo species is detected (see below). 164 26c OPN 3 PN3 27c T 42 I I I I ! GC T 1 -70 ; 1 ! 1 , 11 I 1 I 1 ) ' I I 1 1 u: 1 1 : 1 1 I 1 1 1 1 I 1 1 ; 60 50 40 30  1 1 ; 1 : 1 1 | 1 1 1 1 I 1 1 1 1 | M 1 1 J 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l I 1 1 20 10 0 PPM Fig. 5.3. 31p{lR} NMR spectrum (121.4 MHz), recorded in CDCI3 at -45°C, of a freshly isolated but unpurified "Pt(PN3)4" sample formed by N H 2 N H 2 reduction of cis-PtCl2(PN3)2, lc, in the absence of extra PN3 phosphine ligand: 8 = 52.9 (s), 1 Jptp = 4435 Hz for 27c. O P N 3 Pi P2 * P N 3 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 j 1 1 i 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 : 1 1 I 1 1 1 1 45 40 35 30 25 20 15 10 0 PPM Fig. 5.4. 3lp{lH} spectrum (121.4 MHz), recorded at -10°C, of the same sample of Fig. 5.3, after 48 h in CDCI3 at -20°C; compounds Pt(PN3)4 and Pt(PN3)3 have disappeared completely. 42: 8 P l = 23.3 (d), 8p2 = 22.5 (t); JptP,, J P t P 2 and J P P are 2700, 4050 and 14 Hz, respectively (see text). 165 p p J ^ P t C ? ; P a -rt / \ OP p Pi (ii) (iii) (possible structures for 42) OP, (i) (iv) Compound 42 was isolated as an orange solid from the reprecipitation of the product from hydrazine reduction of cis-lc (see Sect. 2.8.1). The 31P{1H} NMR spectrum of the isolated orange crystals in CDCI3 shows only signals of 42 when taken immediately following sample preparation. When the solution of 42 in CDCI3 was left for ten days, a dark red oil formed in the NMR tube; this sample showed only a singlet for OPN3 in the 31P{1H} NMR spectrum. Similar chemical behaviour has been reported for triphenylphosphine platinum(O) species in the literature/2'13 An analogous red material was formed slowly in the presence of trace oxygen (impurity in commercial nitrogen gas75) and was characterized as the trinuclear [Pt(PPh3)2]3 cluster/2 The red oil in the present study is assumed to be a similar oligomeric platinum(O) phosphine species. The lack of a 31P signal for this species is perhaps due to its low concentration because of the poor solubility of the oil. The source of oxygen in the in situ NMR experiment may be attributed to the slow permeation of air through the rubber septum, and in the preparatory scale, oxygen may come as the impurity in the nitrogen gas used. Elemental analysis of the orange solid isolated fits the formulation of "Pt(PN3)302" reasonably well (Sect. 2.8.1). The lack of a peroxo stretching band in the 740 - 930 cm-1 region74 (Fig. 5.5) and the presence of new bands between 1093 - 1135 cm"1 suggest that complex 42 possibly contains a superoxo ligand( (i) or (ii) ). The classical superoxo metal complexes invariably bind the O2 molecule in a "bent end-on" fashion, exhibiting an IR band between 1070 - 1200 cm-1/-5 The only side-on, symmetrical bound superoxo species, characterized recently is Tp'Co(02) (Tp' = hydridotris(3-tert-butyl-5-methylpyrazolyl)borate which has an IR band at 961 cm"1/6 Pt(0) is not a transition metal moiety favouring superoxo complex formation. Nonetheless, Pt(PN3)3 partially satisfies the general requirements; the metal must have one coordination site available and possibly a favourable one-electron oxidation 166 potential.77 Compound 42 is unstable both in solution and in the solid state (a single crystal of the isolated material decomposed after a week in a grease-sealed capillary tube under N2 atmosphere). In order to elucidate the likely role of oxygen in this reaction, a parallel reaction using pure oxygen gas (1 atm) with Pt(PN3)4 (~ 15 mg) in CH2CI2 (8 mL) was performed at room temperature. There was no noticeable colour change during this reaction. The reaction was stopped after 15 man. The 31P{1H} NMR spectrum of the orange-yellow solid obtained by pumping off the solvent shows a major singlet at 18.2 ppm with the Pt-satellites (Fig. 5.6). The 31P{ lH) data for this compound are comparable with those for the Pt(PPh3)202 peroxo species (8 = 14.5 (s), 1 JptP = 4045 Hr,10 8 = 16.4 (s), Ijptp = 4059 Hz77). The typical IR band for peroxide is not obvious in CDCI3, although a shoulder is evident at 812 cm-1 (Fig. 5.7); the corresponding band for the Pt(PPh3)202 peroxo species is at 821 cm*1. Based on these limited data, the formation of the Pt(PN3)2C«2 peroxide, 43c, seems likely. A superoxo structure (the more usual end-on78>19, or novel side-on arrangement76) for the unknown, supposed oxygen complex 42 is very speculative at this stage, corifirmation by an X-ray crystal structure being critical. The possibility of 42 being a dinitrogen complex (a non air-sensitive, mononuclear dinitrogen complex of Pt(AsPh3)3 has been characterized recently20) is ruled out by the lack of a V N N stretching vibration around 2200 cm-1, and also the elemental analysis which corresponds closely to that for a Pt(PN3)3(02) formulation. The possibility of H2O participation to give -OOH or -OH ligands is also ruled out by the lack of vrjOH and V O H stretching vibrations around 3500-3600 cm"1. As the phosphine oxide OPN3 is formed gradually in a relatively large quantity, the possibility of a phosphine oxide complex was examined. Balch's group has made a Pt(IV) phosphine oxide complex of OPNi (PtBr4(OPNi), see below), this showing a vpo band at 1115 cm'1 (the band of the free phosphine oxide is seen at 1180 cnr 1) 2 7 The 3 1P NMR spectrum of this Pt(TV) complex shows a singlet at 25.4 ppm with no Pt satellites being seen, this being attributed to the weak interaction between the Pt and P atoms separated by an O atom (the 3 1P signal for the free OPNi is seen at 21.4 ppm)27 The 3 1P resonance of free OPN3 is at 15.7 ppm, and the resonances at 23.3 and 22.5 ppm could be those of coordinated phosphine oxide, 167 1700.0 1400.0 1100.0 800.0 500.0 200.0 Wavcnumbers (cm ) Fig. 5.5. Infrared spectrum of 42c in Nujol. 43c # OPN 3 I 1 « < > — j — 1 1 1 1 1 1 » — J O ^ . PN 3 40 30 "T—r- 20 "I—1—'—I—I—'—I—I—I—1—r—1—1—i — 1 — 1 — r -10 0 P P K Fig. 5.6. 3 1P{ 1H) NMR spectrum (121.4 MHz) in CDCI3 at r.t of an in situ reaction product formed from Pt(PN3)4 with pure oxygen gas: 8 = 18.2 (s), UptP = 3848 Hz for Pt(PN3)202, 43c. 168 Fig. 5.7. (a) Infrared spectrum of solvent CDCI3; (b) infrared spectrum of Pt(PN3)202, 43c, inCDCl3 at r.t. . P = O N Pt — Br Br Br particularly in view of the IR band at 1114 cm"1 (Fig. 5.5). A possible structure of the Pt(0) phosphine oxide complex is depicted as (iv) (see above). The doublet in the 31p{lH) NMR spectrum could then be assigned to the two equivalent phosphorus atoms of coordinated OPN3. The relatively small Jptp value of 2700 Hz (compared to values of ~3800 Hz in Pt(PN3)4 and ~4400 Hz in Pt(PN3)3) would then be attributed to a two-bond Pt-P coupling, and the Jpp value of 14 Hz to a three-bond coupling. However, the evidence against this assignment is the value of the 2Jptp coupling, because the corresponding value in the PtBr4(OPNi) species was reported to be too small to be resolved.27 Further, the hard nature of the oxygen of the phosphine oxide ligand makes such oxygen coordination at a soft centre such as Pt(0) unlikely. The different products formed from the oxygen reactions of 26c may be caused by two different reaction pathways. Complex 26c reacts quickly with oxygen molecules under O2 atmosphere to form the peroxide 43c. On the other hand, the formation of 42 occurs much more slowly, and is also solvent dependent, being promoted by chlorinated solvents, CH2CI2 (or CD2CI2), CDCI3. Toluene-d8 does not promote the formation of 42, as demonstrated by the 31P{ lH) NMR spectrum of 26c taken one week after the sample preparation (Fig. 5.8). 26c * OPN3 27c T .,^ iL.w.,-.«>A,.... . u > - , . , w s Fig. 5.8. 3ip{lR} NMR spectrum (121.4 MHz), recorded at -50°C, of 26c in toluene-dg after a week being stored in a freezer at r.20°C. 170 5.1.2. Tris[2-(cUphenylphosphmo)pyTidme]platm 27a The synthesis of Pt(PNi>3,27a, has been described in Sect. 2.8 2, the purity of the isolated product being dependent on the used phosphine concentration. A mixture of tetrakis-and tris-phosphine platinum(O) species resulted if the ratio of phosphine to platinum used was between 4 and 6.5. The 3 !P{ iH} NMR spectrum in CD2CI2 at -50°C of 27a contains a singlet at 52.0 ppm and two singlet satellites (Fig. 5.9); the coupling constant Jptp is 4417 Hz which is typical of a Pt(PR3)3 trigonal planar complex.5 The ^Ptf1!!} NMR spectrum of 27a reveals a quartet centred at 8 -300.3 ppm at -45°C in CDCI3 with a coupling constant ^ptp of 4433 Hz (Fig. 5.10). An attempt was made to isolate the tetrakis PNi compound, Pt(PNi)4 26a, by adding five equivalents of PNi ligand (~ 60 mg, 0.23 mmol) of the tetrakis and tris species to a mixture at room temperature, with limited success: although in a solution of 27a containing excess free ligand PNi, 26a is detected as a major singlet at 25.6 ppm ^JptP 3910 Hz) (Fig. 5.11), a precipitation procedure again produces the mixture of the tetrakis and tris Pt(0) species. The coupling constant and the chemical shift of 26a are similar to those of Pt(PN3)4,26c. The reason for the unusually high coordination shifts for both tetrakis phosphine platinum(O) species, 26a (A8 = 28.1 ppm) and 26c (A8 = 29.8 ppm), compared to those of other four-coordinate complexes Pt(PR3)4, AS ~ 4.6 to 13.8 ppm,5-10-22 is not understood. The 31P{ !H} NMR spectrum of 27a after 48 h at -20°C indicates that more than two thirds of the Pt(PNi)3 complex is decomposed in CDCI3 (Fig. 5.12). Three major, new, platinum containing complexes in addition to 27a are seen: one has a singlet at 20.5 ppm with Pt satellites at 31.2 and 9.65 ppm (*JptP = 2616 Hz) and could be trans-PtCl2(PNi)2 [cf. trans-PtCl2(PPh3)2, 20.6 (s), ijptp = 2634 Hz25]; the second one has a singlet at 13.0 ppm with Pt satellites at 29.2 ppm and -3.1 ppm (*Jptp = 3921 Hz) and is possibly a peroxo species, Pt(PN 1)202,43a, (cf. Pt(PN3)202 43c: 18.2 (s), iJptP = 3848 Hz, Fig. 5.6; Pt(PPh3)202, 14.5 (s), Uptp = 4045 Hz10-11); the third compound whose 3 1P NMR signals are centred at 30.1 171 Fig. 5.9. 31p{lH) NMR spectrum (121.4 MHz) of Pt(PNi)3,27a, at 4.5x10-2 M in CD2CI2 at -50°C: 8 = 52.0 (s), Uptp = 4417 Hz. I 1 111 1 1 1 1 1 [ 11 1' 1} 111 1 1 111 1 1 1 1 1 1 1 11 1 1 I \ 1 1 1 J 11 iH ) 1 1 1 r j 1 '1 11 1 111 11 1 1 1 1 1 11 1 1 1 1 1 1 1 I 1 ' 1 1 I -100 -200 -300 -400 -500 -600 -700 -600 PPM -900 Fig. 5.10. 64.2 MHz 195pt{lH) NMR spectrum of 27a at 4.5xl0"2 M in CDCI3 at -45°C: 8 = -300.3 (quartet), Uptp = 4433 Hz (the natural line width is 40 Hz). 26a OPNi PNi 111 1 11 1 1 r 1 J1 i 1 : 1 1 1 i i j \ 1 1 1 [ 1 11 1 1 1 11 ; ; 1 1 1 i 11 i 1 1 j 1 r r 1 1 1 1 1 i ; 1 i 1 1 1 1 : • 1 : . 1 i J1 . 1 1 T . ; 1 1 j 1 1 T T ) Fig. 5.11. 31P{!H} NMR spectrum (121.4 MHz) of the in situ formed Pt(PNi)4, 26a, in CDCI3 at -45°C: 8 = 25.6 (s), Ijptp = 3910 Hz. 172 OPNi 27a Xi • | . I I I | I I 70 60 i i I i i n [ i i i i | i i { X 2 X 3 50 1 | 1 1 1 ' I 40 I i | I i I I | I 30 ~i I i i i i | i r i i | i i i i | i i i i | i i i , | i i i i | i i i i | i i 20 10 0 -10 I I -20 PPM Fig. 5.12. Decomposition of Pt(PNi)3 in CDCI3 after 48 h at -20°C shown by the 31P{ !H} NMR spectrum (CDCI3, -20°C). Xj: "HPtCl(PNi)2", see Fig. 5.20a; X 2: trans-PtCl2(PNi)2, X 3: Pt(PNi)202,43a: 8 = 13.0 (s), ^PIP = 3921 Hz. ppm as a doublet with satellites at 42.2 and 18.0 pm (Jj>tp = 2938 Hz) has parameters very similar to those of the trans-Pt(H)Cl(PNi)2 complex which will be discussed in Sect. 5.3.1 (30.2 (d), p^tp = 3023 Hz, Fig. 5.20a). The decomposition of Pt(PNi)3 is clearly complicated; no analog of Pt(PN3)302,42, is detected. The formation of trans-Pt(H)Cl(PNi)2 and trans-PtCl2(PNi)2 may be rationalized by the presence of HCl, CI2 and COCI2 impurities produced by the light-promoted chloroform oxygen reaction.24 5.2. Characterization of olefin complexes of 2-(diphenylphosphino)-pyridine and tris(2-pyridyl)phosphine 5.2.1. Acrylonitrile, methacrylonitrile and crotonitrile complexes (44a, 44c, 45a, 46a) Syntheses of these olefin complexes have been described in detail in Chapter 2, Sect. 2.8.3.1. Although satisfactory chemical analyses were not obtained for complexes 44a - 46a, because of the loss of olefin in drying procedures, the *H and 31P{1H} NMR spectroscopic evidence (Table 5.1) unambiguously reveals that these complexes are essentially square-planar with a cis-phosphine arrangement (see below). For instance, the 31P{!H} NMR spectrum of Pt(PNi)2(rt2-CH2CHCN), 44a, contains a major AB quartet and the corresponding Pt satellites with a phosphorus-to-phosphorus coupling of 34.7 Hz, typical of cis-phosphorus coupling (Fig. 5.13). Carbon C 1 probably has a stronger trans influence than does the C 2 carbon atom. Therefore, the downfield half of the AB quartet is assigned to P 2 which has the platinum-phosphorus coupling of 3969 Hz; the upfield half of the AB quartet is then due to the resonance of P1 which has the smaller platinum-phosphorus coupling, 3445 Hz. The 31P{1H} NMR spectrum at 25°C shows that 44a is non-fluxional, implying that the bonding between Pt and the hydrocarbon is more like the metallocyclic structure shown, than a 7t-bonded olefin structure which more readily allows for rotation of olefin around the Pt-olefin bond. Strong Pt d-7t-back donation is also indicated by the multiplets at 2.0 and 3.8 ppm in the *H NMR spectrum of the coordinated acrylonitrile (free acrylonitrile appears as a multiplet between 5-6 ppm); strong olefin-to-metal o-donation via the olefin n molecular orbital would result in a downfield shift of the olefinic protons to around 7 ppm.2-5'26 174 P 1 ^ ^ C 2 H C N p ^ ^ ^ C 1 ^ < 44 P t - » 44 C^HCN P 2 ^ C 1 ^ 5C ~i—r j ' I I I | 1 I ! I I I I r~T |—I—!—i—i—i—I—I—I—I—!—!—1—I—1—j—1—I—!—I—I ^5 £0 35 30 2 5 20 15 PPM 10' Fig. 5.13. 31p{lH} NMR spectrum (121.4 MHz) of Pt(PNi) 2(Tl 2-CH2CHCN), 44a, in CDCI3 at 25°C (data an given in Table 5.1). Table 5.1. 3 1P{ lH) NMR Parameters of Compounds 44a - 46a in CDCl3a Complexes Temp. (°C) Spi 8p2 *JptPi *JptP2 2JPP Pt(PNi)2(T|2-CH2CHCN), 44a 25 30.7 31.7 3445 3969 34.7 Pt(PN3)2(ri2-CH2CHCN), 44c -45 34.6 35.4 3406 3934 31.9 Pt(PNi)2(ri2-CH2C(CH3)CN), 45a 25 31.3 31.3 3538 3718 35.2 Pt(PNi)2(rt2-CH(CH3)CHCN), 46a.l 25 31.8 32.6 3265 4095 38.5 Pt(PNi)2(Ti2-CH(CH3)CHCN), 46a.2 25 31.6 32.1 3240 4086 39.4 (a) The chemical shifts and coupling constants are in ppm and Hz, respectively. The platinum(O) precursor Pt(PN3)4, 26c, forms a complex with acrylonitrile, 44c, whose 31P{1H} NMR spectrum (Table 5.1) contains also an AB quartet (with satellites) similar to that seen in Fig. 5.13, and the complex is considered to possess the same geometry as 44a. 175 The Pt(0)-methacrylonitrile complex Pt(PNi)2(r|2-CH2C(CH3)CN), 45a, displays a distinct 31P{!H} NMR spectrum containing a very intense singlet at 31.3 ppm and symmetric AB quartets as Pt-satellites (Fig. 5.14a). The "accidental equivalence" of the two chemically different phosphines in 45a is disrupted upon cooling the system to -20°C when the central singlet splits into a very close AB quartet (Fig. 5.14b). The P-P coupling constant in the AB quartet, arising from inequivalent phosphorus atoms, is 35.2 Hz which is similar to that of compound 44a. The UptPj and 1Jptp2 values are found to be 3718 and 3538 Hz, respectively. The difference (180 Hz) is small compared to the difference in 44a (524 Hz), this presumably reflecting the similar electron densities at the two olefinic carbon atoms: the electron density on the carbon with the electron withdrawing CN group and electron donating CH3 group appears comparable with that on the methylene carbon, which consequently leads to similar trans influences imposed by the two carbon fragments. By inference, the difference in the two Pt-P coupling constants within the crotonitrile complexes 46a. 1 or 46a.2 should be the largest among 44a, 44c, 45a and 46a. 1/ 46a.2 (see below), (a) JUL (b) j L i u (c) J U I I I I I I 50 1 I 1 < 1 ' I 1 40 Fig. 5.14. 31p{lH} NMR spectra (121.4 MHz) of Pt(PNi)2(Ti2-CH2C(CH3)CN), 45a, in CDCI3 at various temperatures, (a>r.t., (b) -20°C, (c) -45°C; see Table 5.1. 176 When Pt(PNi)3, 27a, is treated with a mixture of cis and trans crotonitrile, two product isomers are obtained (Sect. 2.8.3). The 3 1P{1H} NMR spectrum of this mixture basically consists of two sets of signals, each set composed of an AB quartet and the corresponding Pt satellites (Fig. 5.15). These two complexes 46a. 1 and46a.2 do not interconvert at room temperature. The peaks labelled with an asterisk (*) belong to one complex which has a major AB quartet centred at 32.2 ppm and the corresponding Pt-satellites (}Jptpv 1JptP2 and 2Jpp are 3265, 4095 and 38.5 Hz, respectively); the unlabelled resonances belong to a second complex with an AB quartet centred at 31.9 ppm and Pt-satellites (/JptPt, 1JptP2 and 2Jpp are 3240,4086 and 39.4 Hz, respectively). The average difference of 840 Hz between iJptPt and 1JptP2» the largest among 44a - 46a, is attributed to the large difference in electron densities of the olefinic carbon atoms (see above) because one has the electron-withdrawing CN substituent and the other has the electron-donating methyl group. The 31P{1H} NMR spectra of Pt(0) complexes with prochiral olefins have not been reported before. The splitting pattern in the 3 lP NMR signals in these complexes (44a - 46a) strongly supports the assigned square-planar geometry. I l i l i | i i i i | i l i i | I i i l | l l l l [ l i i i | i i i i • i i i i ; i i i i j 1 , i i S O 4 0 3 0 2 0 10 P f V . Fig. 5.15. 31P{1H} NMR spectrum (121.4 MHz) of Pt(PNi)2(ri2-(CH3)CHCHCN) in CDCI3 at 25°C; peaks labelled with * belong to 46a. 1, unlabelled peaks belong to 46a.2, see Table 5.1. No assignment regarding the geometry of the coordinated olefin for 46a. 1 and 46a.2 is made. 177 5.2.2. Maleic anhydride (MA) and diethyl maleate (DEMA) complexes 47a, 47c, 48a Syntheses of Pt(PNi)2(MA), 47a, Pt(PN3)2(MA), 47c, and Pt(PNi)2(DEMA), 48a, have been described in Sect 2.8.3.2. The reaction of diethyl maleate with Pt(PNi)3,27a, was carried out in diethyl ether, the addition of DEMA was done carefully to obtain a solid product, because excess DEMA results in the formation of a white, colloidal product Reactions of MA or DEMA with the Pt(0) complexes were noticably more rapid than the nitrile reactions possibly because activation by the two carboxylate groups is greater than that by the one CN group. The geometries of these olefinic complexes are characterized using NMR techniques. The 31P{ *H} NMR spectra of these complexes are not as informative as those of 44a - 46a because MA and DEMA are symmetric. For instance, the 31P NMR spectrum of 47a, shown in Fig. 5.16b, contains a major singlet at 27.0 ppm, indicating that the two P nuclei are equivalent. Although the coupling constant value of 3841 Hz implies a cis-phosphine geometry (cf. trans-Ptl2(PN2)2, ^PtP = 2500 Hz, Table 3.1), whether the geometry of this molecule is square-planar or tetrahedral cannot be assertained from the 31P data. Fig. 5.16a shows the *H NMR spectrum of 47a: a pseudo triplet of doublets centred at 3.45 ppm is assigned to the olefinic protons of the coordinated maleic anhydride, which have shifted upfield by 3.65 ppm upon coordination.^5 The shift to high field is similar to that observed for complex 44a. This infers that the olefin bonding in 47a and 44a is perhaps similar. This shift is caused by the increase of electron density on the carbon atom to which the olefinic protons are attached. These protons resonances, split by the adjacent 31P into a doublet (3JPH = 8.0 Hz) and by the 195Pt into a larger doublet (2jptp = 60.0 Hz), are similar in position to those of the protons on saturated sp3 carbon atoms, perhaps reflecting a lowering of the C-C bond order. The maleic anhydride complex Pt(PN3)2(MA), 47c, formed from 26c and MA (Sect. 2.8.3.3), is assumed to have a geometry similar to that of 47a. The 31P NMR data of 47c are given in Table 5.2. 178 Table 5.2. 31P{1H} and lH NMR Parameters of 47a • 48aa Complexes 5 P 5 H JptP JptH JPH Pt(PNi)2(MA), 47a 27.0 3.45 3841 60.0 8.0 Pt(PN3)2(MA), 47c 32.3 — 3836 — — Pt(PNi)2(DEMA), 48a.l 30.0 — 3750 — — Pt(PNi)2(DEFM), 48a.2 28.1 — 3815 — — a) The spectra are recorded in CDCI3 at r.t; the chemical shifts are in ppm and the coupling constants are in Hz. j t 1 ; 1 ; 1 1 1 1 i I I I I ; ! I i 1 1 I I I 1 1 1 i 1 1 ; 1 1 J I ; I ! 1 ; 1 I I ' 1 : • • • • ~ 10 a 6 i 2 b ??v. (b) 1 A i 1 1 i 1 , 1 ,; 1 1 1 1 1 1 1 1 1 1 1 M I 1 1 : 1 1 1 1 1 1 1 1 1 1 1 1 ; 1 : 1 1 1 1 1 1 1 1 1 1 1 M | • 1 1 ! 1 1 1 60 50 <0 30 20 10 0 PPM -10 Fig. 5.16. ! H (300 MHz) and 31P{1H} (121.4 MHz) NMR spectra of Pt(PNi)2(MA), 47a, in CDCI3 at r.t. (a) l H NMR: 8 = 3.45 (d), 2JptP = 60.0, 3 J P H = 8.0 Hz; X i are signals due to Et 20 and X 2 is the H 2 0 peak in CDCI3. (b) 3 1P{ lH] NMR: 8 = 27.0 (s), Upt? = 3841 Hz. The 3 1P{ 1H) NMR spectrum of the isolated Pt(PNi)2(DEMA) (DEMA = diethyl maleate), 48a. 1, at room temperature in CDC13 unexpectedly consists of two sets of signals with relative integration ratio 1:1.2 — the singlet at 30.0 ppm with coupling constant ^ptp of about 3750 Hz and another singlet at 28.1 ppm with a UptP of 3815 Hz (Fig. 5.17b). The mother solution, as indicated by the 3 1 P NMR spectrum, contains the same two compounds, but at remarkably different concentrations. The reaction of Pt(PNi)3 with DEMA initially (~ 20 min) gives one major compound (Fig. 5.17a), which on standing slowly converts to another species 179 (a) 48a.l JL (b) 48a. 1 J L i 48a.2 (c) 48a.2 i i i i | • • i i | i i i i i i i i i | r i i I | | i ' ' | i i i i | i i ' i | ' • i i | i T • ' | i i i : , ' ' 50 40 30 20 10 PPM Fig. 5.17. 3lp{ IR) NMR spectra (121.4 MHz) of complexes 48a.l and 48a.2 in CDCI3. (a) the initial (20 min) in situ reaction product from Pt(PNi)3 27a + DEMA at -20°C. (b) the isolated white crystals of the Pt(PNi)2(DEMA) 48a.l and Pt(PNi)2(DEFM) 48a.2 mixture at r.t. (c) the initial (10 min) in situ reaction product from 27a + DEFM at -20°C. Data are given in Table 5.2. 180 whose 3 1P signal is centred at 28.1 ppm. Isomerization of the coordinated olefin is involved. This is confirmed by the formation of Pt(PNi)2(DEFM) (DEFM = diethyl fumarate), 48a.2, in situ by reacting 27a with DEFM: the single species formed immediately (~ 10 min) has the same 3 1P NMR chemical shift and Uptp coupling constant as the isomerization product from 48a. 1; 48a.2 does not isomerize for 72 h in solution. A similar cis-trans isomerization of maleate to fumarate ester catalyzed by Co2(CO)8 complex has been reported recently by Ungvary.27 5.2.3. An ethylene complex 49a The ethylene complex, Pt(PNi)2(Tj2-CH2CH2)» 49a, was formed in situ by introducing via a syringe ethylene gas at 1 arm (2 mL) into a toluene-ds solution (lmL, ~7.0xl0'3 M) of Pt(PNi)3,27a, in an NMR tube filled with N2. The solution turned from yellow to colourless for reactions at room temperature. The 31P{ lH) NMR spectrum of this colourless solution at -85°C (Fig. 5.18) showed the formation of new species whose 3 1P parameters are very similar to those of Pt(PPh3)2(r(2-CH2CH2) reported in the literature (8 = 32.5 (s), ^ P I P = 3694 Hz at -80°C in toluene-d8).22 On warming up to room temperature, the same sample gave a very broad peak centred at 26 ppm because species 27a and 49a exchange very rapidly according to (Eq. 5.1) in the presence of free PNi ligand. This exchange behaviour has also been observed in Pt(PPh3)2(ri2-CH2CH2) system. Pt(PNi)3 + CH2=CH2 • - Pt(PNi)2(ri2-CH2CH2) + PNi (5.1) toluene-dg 27a 49a The Pt(PN3)4 species, 26c, does not react with ethylene under the same conditions, as indicated by the 31P{1H} NMR spectrum of 26c which remains unaltered after an attempted reaction. 181 49a T T 27a U U J J r m - r p i ' M I i : :. 1111 : I 11; I f 11; 111111 11! 111111 11 i i j 11 r) 11 111 | 1111111 11 ] 1111 111111111 111111 [ 60 SO 40 30 20 10 0 -10 PPM -20 Fig. 5.18. 31p{ lH) NMR spectrum (121.4 MHz) recorded at -85°C in toluene-dg of the Pt(0) ethylene adduct Pt(PNi)2(Tl2-CH2CH2), 49a, formed in situ at r.t.: 8 = 35.5 (s), Uptp = 3654 Hz. In conclusion, Pt(PN3)4,26c, and Pt(PNi)3,27a, react with activated olefins in ways similar to the triphenylphosphine Pt(0) complexes Pt(PPh3)2 or Pt(PPh3)3; in the case of ethylene and DEMA, Pt(PN3)4 appears to be less reactive. The geometries of all the olefinic complexes made here are unambiguously square-planar, based on the 31P{ lH} and *H NMR spectroscopic data. The square-planar geometry in these platinum olefin complexes infers that there is strong 7t-back-donation from the Pt to olefin which lessens the electron density on the platinum, of the formal oxidation state zero. As a result, the coordinated olefin is reduced to a nearly saturated state. This is consistent with the molecular orbital interpretation of the bonding between platinum and olefin in such complexes; that is, a negligible o olefin to metal bond and a strong TC metal to olefin bond,25 The C-C distance in the tetracyanoethylene complex Pt(PPh3)2[C2(CN)4] (1.52 k28a, \A9k28b) is stretched 0.21 or 0.18 A compared to that in free TCNE, and is closer to a single C-C bond distance. By analogy, the C-C bond distances in the olefin complexes made here are likely to approximate those of a single bond. 182 5.3. Reactions of Pt(PNi)3 and Pt(PN3)4 with HQ 5.3.1. Reaction of Pt(PN 1)3 with HQ Oxidative addition of HC1 to Pt(0) phosphine complexes generally yields trans-hydridocMorobisphosphmeplatinum(II) species.29-30 A white crystal isolated using Pt(PNi>3 as reactant (Sect. 2.8.4.1) was analyzed to be trans-Pt(H)Cl(PNi)2, 50a. The complication described in Sect 2.8.4.1, regarding the necessity of removing excess HC1 because of protonation of the pyridyl group, is not apparent in the triphenylphosphine system70-30 The hydride resonance in the *H NMR spectrum of 50a appears at -16.32 ppm as a resolved pseudo triplet of triplets at room temperature, due to coupling to the 195Pt nucleus and two equivalent 3 1P nuclei (Fig. 5.19). The signals at 1.76 ppm and 3.73 ppm can be assigned to the presence of solvated THF in the crystal lattice, the integration of these signals corresponding to one half mole of THF per platinum complex. The microanalysis results for the hydrogen and nitrogen improve significantly based on the formula trans-Pt(H)Cl(PNi)2l/2C4H80 (calcd. for C36H3 ClN2Oo.5P2Pt: C 54.44, H 4.19, N 3.53; found: C 54.01, H 4.31, N 3.34). The Pt-H IR band at 2212 cm"1 (Nujol) is similar to those found for trans-Pt(H)Cl(PPh3)2 in the literature (2209, 2217 and 2210 cm"1 in refs. 10, 29 and 30, respectively). The original 3lp{iH} NMR spectrum of 50a showed a major doublet with corresponding doublet platinum satellites; the separation between the major doublet is -12.0 Hz (Fig. 5.20a) which is indicative of phosphorus-hydride coupling (see Fig. 5.19). The appearance of the doublet in the 31P{1H} NMR spectrum was indeed due to the insufficient decoupling power applied to suppress the interference of the hydride; this coupling was suppressed by applying a stronger decoupling power (Fig. 5.20b). 183 T ! ! i "T i—i—I . ' - ] ! ! I — I — r — i — ; — i — i — i — i — i — : — i — j — i — i — ; — ! — -5 0 -5 -10 - i 5 PPM -20 Fig. 5.19. !H NMR spectrum (300 MHz) of trans-Pt(H)Cl(PNi)2, 50a, in CD2C12 at r.t.: 5(hydride) = -16.32 (t), 2 J P t H = 1213, 3 J P H = 12.7 Hz. Free THF is seen as multiplets at 1.76 and 3.73. S = solvent. • l ' 40 I i i i i ' i i i i i i i i i ; i i 30 20 10 0 PPM •'.0 Fig. 5.20. 31p{lH} NMR spectra (121.4 MHz) of 50a in CD2C12 at r.t; (a) obtained with the decoupling power DLP = 6: 8 = 30.2 (d), UptP = 3023 Hz, 2 JPH = 12.1 Hz. (b) obtained with the decoupling power increased by 26 (DLP = 0): 8 = 30.2 (s), p^tp = 3026 Hz. 184 Transfer of a single hydrogen atom to an olefin is a common reaction for transition metal hydride complexes.57 Compound 50a reacts with acrylonitrile to form an alkyl complex, as indicated by the loss of intensity of the high field hydride signal and the appearance of multiplets in the methylene proton region in the *H NMR spectrum (Fig. 5.21). It is clear from the lH NMR data that the linear -CH2CH2CN rather than the branched -CH(CH3)CN group is formed. Olefin insertion into an M-H bond in this anti-Markovrdkov fashion is more commonly seen in the literature.52 Accompanying this olefin insertion into the Pt-H bond is the isomerization of a trans phosphorus complex to a cis one (Eq. 5.2). (5.2) ^ \ / acetone-d \ „ / Pt' +CH2=CH-CN -5— Pt P2 CH 2CH 2CN 50a 51 The above reaction proceeds with no colour change, the solution remaining colourless. Fifty percent of the hydride is converted to the alkyl species in about an hour at room temperature. Compound 51 is not isolated. However, the 3 1P{ 1H} spectrum at -45°C in acetone-d6 of in situ, fully formed 51 contains two doublets centred at 20.22 and 17.15 ppm with a Jpp coupling of 16.5 Hz, indicative of cis-phosphine geometry (Fig. 5.22b). The ^ptP coupling for the phosphorus trans to the alkyl group is 1954 Hz, and that for the phosphorus trans to the chloride is 4254 Hz. The doublet assigned to the phosphorus trans to alkyl group collapses to a broad singlet when the sample is warmed to room temperature (Fig. 5.22a). The collapse of the phosphorus resonance at room temperature seemed not being caused by P-hydride elimination55 of the cyanoethyl group because the J H NMR of the reaction mixture at room temperature showed no exchange between the methylene proton and the hydride resonances (Fig. 5.21). Interaction between the CN group and the central Pt atom in the following fashion seems likely. c#lHt<-P X C H ^ 185 r — i — r Fig. 5.21. h reaction methylene solvent. (a) — i — r 5 i—r i—r ± JL -10 1—i—|— i— i—r -15 PPM 4T *H NMR spectrum (300 MHz) of the in situ mixture of 50a and 51 formed after 1 (Eq. 5.2) in acetone-d^ at r.t. The signals at 2.53 and 1.19 (expanded) are the resonances of 51; the upfield triplet is the hydride resonance of 50a; X = THF; S= P2 Pi I (b) | t i i i I i i i i | i i i i | i M i ] i i i ! | i ! • : |' : : i i ; i i i i [ : 50 -10 30 20 , 10 I : ! I I I I i I ; : : I : | ! ! 1 : ! ! I 1 ! ' 10 PPM -20 Fig. 5.22. 31P{1H} NMR spectra (121.4 MHz) of in situ, fully formed 51 in acetone-d6: (a) at r.t., (b) at -45°C; 8 P l = 20.22 (d), 8 P z = 17.15 (d); ljptp,= 1954, l j P t P 2 = 4254, J P P = 16.5 Hz. l g 6 5.3.2. Reaction of Pt(PN3)4 with HC1 Attempted preparations of trans-Pt(H)Cl(PN3)2,50c, were unsuccessful using either HCl(g) or DMA-HQ (dirnethylacetamide hydrogen chloride) as a source of HQ (Sect 2.8.4.2). A canary yellow solid, isolated from the reaction of Pt(PN3)4 with HC1, is extremely air-sensitive in the solid state (turning to orange-red) and is sparsely soluble in chlorinated solvents, moderately soluble in acetone and in CH3CN, and very soluble in EtOH and in H2O giving deep red solutions under an inert atmosphere. Of interest, these red solutions appear to be less air-sensitive (see below for 3 1 P NMR data). The IR spectrum of this yellow solid displays three bands in the Pt-H region (2056, 1983 and 1947 cm"1) and two bands in the N-H stretching region (3516, 3443 cm*1) that presumably arise from protonation of the pyridine groups (Fig. 5.23). Samples for IR were prepared using degassed Nujol under a N2 atmosphere in a glove-bag immediately before the measurement 3 8 0 0 . 0 3 2 0 0 . 0 2 6 0 0 . 0 2 0 0 0 . 0 1 7 0 0 . 0 1 4 0 0 . 0 1 1 0 0 . 0 8 0 0 . 0 5 0 0 . 0 2 0 0 . 0 Wavenumbers (cm"1) Fig. 5.23. Infrared spectrum of "Pt(H)Cl(PN3)2", 50c, in Nujol at r.t. under N2. The yellow solid isolated from method 1 (Sect. 2.8.4.2) has a ^P^H} NMR spectrum in C D 3 C N containing a major singlet at 26.7 ppm and two singlet satellites ^ JptP = 3869 Hz). 187 The lH NMR spectrum in CD3CN showed a very weak multiplet around -5.5 ppm with no satellites; this signal was not seen when the spectrum was measured in D2O. The 31P{1H) NMR spectrum of this unknown compound in either D2O (8 = 27.1 (s), Uptp = 3875 Hz) or CD3OD (8 = 26.8 (s), Uptp = 3879 Hz) showed no change under air. The reason for the different reactivities of Pt(PN3)4,26c, and Pt(PNi)3, 27a, toward HQ are not fully understood. One possible reason is the difference in the number of pyridyl groups on phosphine. The twelve pyridyl groups of 26c appear to stabilize the tetrakis compound against dissociation (Sect 5.1.1) and make bisphosphine Pt(0) species less readily available for reaction with HC1; furthermore, the pyridyl groups can compete with the Pt centre for HQ (see Fig. 5.23 for evidence of protonation of pyridine). 5.4. Reactions of Pt(PNi) 3 and Pt(PN 3) 4 with methyl iodide Both Pt(PNi)3 and Pt(PN3)4 react with Mel to give trans-PtI(Me)(PN„)2, in ways perhaps analogous to the triphenylphosphine Pt(0) systems.54 The isolation of these complexes has been described in Sect 2.8.5. 5.5. Attempted hydration of olefinic compounds using platinum and palladium pyridylphosphine complexes 5.5.1. Hydration of acrylonitrile Compounds Pt(PNi)3,27a, Pt(PN3)4,26c, Pt(PNi)2(T|2-CH2CHCN), 44a, and trans-Pt(H)Cl(PNi)2,50a, synthesized in the present work, were used as catalyst precursors in the hydration of acrylonitrile. The reaction mixture typically consisted of 0.5 mL of acrylonitrile, 0.5 mL of H20,0.02 mmol of the complex and 0.02 mmol of NaOH. The reactant solutions were contained in ampules that were charged under N 2 and sealed under vacuum. After being loaded, the reaction vessels were placed in a flask containing refluxing benzene. At the end of the prescribed time (1 h), the vessels were opened and the metal complexes present were separated from organic products via vacuum distillation at ~40°C. The solid residues were extracted with Et20 and the extracts combined with the organic distillates. The organic mixtures, in the 188 distillates and extracts were analyzed by gas chromatography (Sect. 2.9). An initial column pressure of 40 psi and an initial temperature of 150°C were used. After 14 min, the temperature was raised 15°C/min to 220°C. Table 5.3 summarizes the results obtained by GC. Sodium hydroxide was shown to be the likely catalyst in the hydration of acrylonitrile to the alcohol and the amide, because the platinum complexes were found to be inactive in the absence of NaOH. The turnover numbers of around 40 disagree with the number of 1.16 presented in reference 35. The acrylonitrile in the present study, distilled over CaH2 (b.p. 54 - 56°C, 40 mmHg), has perhaps higher purity than that used in reference 35, because purification of acrylonitrile has been reported to increase the P-cyanoethanol production.56 Another possible difference could result from the procedures used in separating organic compounds from the metal species prior to sample injection into the GC. Platinum metal, if contaminating the column, could be the active species performing the catalysis. It is interesting to point out that trans-Pt(H)Cl(PNi)2, cis-PdCl2(PN3)2 and cis-PtCl2(PN3)2 inhibit the base catalysis. In aqueous solution, the aquo complexes formed by substitution of chloride(s) show acidic behaviour (at least with lc, see Sect. 3.4.2); the added base likely generates M(OH) species that are inactive under these experimental conditions. Table 5.3. Hydration of Acrylonitrile Catalyzed by Pt and Pd Complexes at 80°C Catalyst3 Turnoversb P-cyanoethanol acrylamide Pt(PNi)3,27a 40.2 — Pt(PN3)4, 26c 37.8 3.9 Pt(PNi)2(t|2-CH2CHCN), 44a 42.5 — trans-Pt(H)Cl(PNi)2,50a — — cis-PtCl2(PN3)2, lc — — cis-PdCl2(PN3)2,4c — — NaOH 42.4 15.2 (a) Catalyst = 0.02 mmol complex + 0.02 mmol NaOH. (b) Turnover numbers = mole per mole of catalyst per hour, (—) implies non-detection. 189 5.5.2. Hydration of maleic acid A strong acid, such as HC1 (see below), and metal ions such as Cr 3 + (Sect. 6.2.4), were found to catalyze the hydration of maleic acid at 100°C, a thermodynamically favourable but kinetically slow process. The water solubilities of the pyridylphosphine complexes cis-PdCl2(PN3)2,4c, and Pd2Cl2(H-PN3)2,11c (Sect 3.4), led to the investigation of these species as potential catalyst precursors for the hydration of maleic acid, during at the earlier stages of this thesis work. The reaction mixture typically consisted of a solution containing 2.0xl(H to l.OxlO"3 M complex and 0.1 M maleic acid in 20 mL of H2O (pH = 1.8). The pH of the solutions were adjusted by adding solid NaOH. Initially, the reaction rnixtures at 80°, 100° or 120°C were contained in a 80 mL autoclave that was charged either under air or N2, and later the autoclave was replaced by a regular Schlenk flask (see below). After being loaded, the autoclave or flask was placed in a thermostated oil-bath. At the end of 16 h, the autoclave was opened and the solution was concentrated to dryness. The solid obtained thereby was redissolved in acetone-d6 for *H NMR analysis. A typical *H NMR spectrum that contains maleic acid, fumaric acid and malic acid is shown in Fig. 6.1 (Ch. 6). Initially, complex 4c in the presence of NaOH (pH » 7.0) showed positive results (3-40 turnovers in 16 h) but with poor reproducibility; Fe3+ impurity from the autoclave is almost certainly responsible for the catalysis (see also Sect. 6.2.4), because no catalysis was detected in a wide range of pH (3.0 - 12.5) when the autoclave was replaced by a glass flask and experiments were carried out at ~100°C. At pH > 10.0,4c started to decompose gradually to metal at 100°C under either an air or N 2 atmosphere. Because strong acid HC1 (3.0 M) completely converted maleic acid (0.86 M) to fumaric acid (50%) and malic acid (50%) in 3 h at 100°C, the addition of acid was avoided in the catalysis experiments. Complex 11c is green in H 20 and the solution is stable for at least 5 h at 120°C; however, the solution decomposed to give metal after 3 h at 80°C when maleic acid was introduced. Neither malic acid nor fumaric acid was detected after the reaction. 190 The ionic complexes [Pd(H20)(OH)(PN3)2](BPh4), [Pd2(H20)2(PN3)2](BPh4)2, [Pd2(H20)2(PN3)2](BF4)2, and [Pd2(H20)2(PN3)2](PF6)2, derived from 4c and 11c (Sect. 3.4) are generally less water-soluble than 4c and 11c, and were shown in the present study to be inactive toward hydration of maleic acid. The catalytic activity of the Pt(0) complex of maleic anhydride Pt(PNi)2(MA), 47a, was also tested; 47a was reacted with NaOH (5-fold excess) in aqueous solution at 80°C for 1 h. No hydration product was found by *H NMR spectroscopy. 191 References 1. Malatesta, L.; Cariello, C. J. Chem. Soc, 1958, 2323. 2. Pearson, R.G.; Louw, W.; Rajaran, J. Inorg. Chim. Acta , 1974,9, 251. 3. Mann, B.E.; Musco, A. / Chem. Soc. Dalton Trans., 1980, 776. 4. Otsuka, S.; Yoshida, T.; Matsumoto, M; Nakatsu, K. / . Am. Chem. Soc, 1976, 98, 5850. 5. Tolman, C.A.; Seidel, W.C; Gerlock, D.M. / . Am. Chem. Soc, 1972,4, 2669. 6. Halpern, J.; Weil, T.A. / . Chem. Soc. Chem. Commun., 1976, 631. 7. Mann, F.G.; Watson, J. / . Org. Chem., 1948,13, 502. 8. Hartley, F.R. Organomet. Chem. Rev. A , 1976,6,119. 9. Hartley, F.R. The Chemistry of Platinum and Palladium; John Wiley & Sons: New York, 1973; p.458. 10. Sue, C-Y. PhD dessertation, University of British Columbia , 1989; Chs. 2, 5. 11. Sweany, R.L.; Halpern, J. / . Am. Chem. Soc, 1977, 99, 8337. 12. Gillard, R.D.; Ugo, R.; Cariati, F.; Cenini, S.; Bonati, F. Chem. Commun., 1966, 869. 13. Ugo, R.; La Monica, G.; Cariati, F.; Cenini, S.; Conti, F. Inorg. Chim. Acta, 1970, 4, 390. 14. Jones, R.D.; Summerville, D.A.; Basolo, F. Chem. Rev., 1979, 79, 139. 15. Suzuki, M.; Ishiguro, T.; Kozuka, M.; Nakamoto, K. Inorg. Chem., 1981, 20, 1993. 16. Egan, J.W. Jr.; Haggerty, B.S.; Rheingold, A.L.; Sendlinger, S.C; Theopold, K.H. / . Am. Chem. Soc, 1990,112, 2445. 17. Collman, J.P.; Hegedus, L.S.; Norton, J.R.; Finke, R.G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; University Science Books: Mill Valley, CA., 1987; p.400. 18. Neiderhoffer, E.C.; Timmons, J.H.; Martell, A.E. Chem. Rev., 1984, 84, 137. 19. Ref. 17, pp.200-203. 192 20. Frem, R. Unpublished data, communicated to B.R. James. 21. Wood, F.E.; Olmstead, M.M.; Farr, J.P.; Balch, A.L. Inorg. Chim. Acta, 1985, 97, 77. 22. Sen, A. ; Halpern, J. Inorg. Chem., 1976,19, 1073. 23. Farr, J.P.; Wood, F.E.; Balch, A.L. Inorg. Chem., 1983, 22, 3387. 24. Perrin, D.D.; Armarego, W.C.F.; Perrin, D.R. Purification of Laboratory Chemicals, 2nd ed.; Pergaman Press: Oxford, 1980; p.167. 25. (a)Cenini, S.; Ugo, R.; La Monica, G. /. Chem. Soc. A, 1971,409. (b)Cenini, S.; Ugo, R.; Bonati, F.; La Monica, G. Inorg. Nucl. Chem. Letters , 1976,3, 191. 26. Maddox, M.L.; Stafford, S.L.; Kaesz, H.D. Adv. Organometallic Chem. 1965,3, 1. 27. Ungvary, F. Fifth IUPAC Symposium on Organometallic Chemistry Directed towards Organic Synthesis; Florence, Italy, 1989; PSI-60. 28. (a) Panattoni, C; Bombienri, G.; Belluco, U.; Baddley, W. H. /. Am. Chem. Soc, 1968, 90, 798. (b) Bombieri, G.; Forsellini, E.; Parattoni, C; Graziani, R.; Bandoli, G. / . Chem. Soc. A, 1970, 1313. 29. Collamati, I.; Furlani, A.; AttioU, G. / . Chem. Soc. A, 1970, 1694. 30. Carriati, F.; Ugo, R.; Bonati, F. Inorg. Chem., 1966, J, 1128. 31. Ref. 17, pp. 384, 698-700. 32. Docherty, N.M.; Bercaw, J.E. /. Am. Chem. Soc, 1985,107, 2670. 33. Clark, H.C.; Kurosawa, H. Inorg. Chem., 1972,11, 1275. 34. Hall, T.L.; Lappert, M.F.; Lednor, P.W. / . Chem. Soc, Dalton Trans., 1980, 1448. 35. Jensen, CM.; Trogler, W.C. / . Am. Chem. Soc, 1986,108, 723. 36. Arnold, D.P.; Bennett, M.A. / . Organomet. Chem., 1980,199,119. 193 Chapter 6 Catalytic Hydration of Maleic Acid by Chromium(III) Species Olefins, such as maleic and fumaric acids, are activated toward nucleophilic attack by the presence of the electron-withdrawing carboxylic groups and, if coordinated to a metal via the C=C bond, the double bond is seen more susceptible to the nucleophilic attack (see Sect. 1.4.2). The hydration product of maleic or fumaric acid, malic acid, is a good synthetic substitute for citric acid used in food acidification.7 Malic acid, containing a chiral carbon atom, is often obtained as a mixture of enantiomers. Asymmetric synthesis of S-malic acid is achieved using the biologically active enzyme, fumarase, via the hydration of fumaric acid under physiological conditions (Sect. 1.3). The salt CrCl3-6H20 has been reported to convert maleic acid in aqueous solution to malic and fumaric acid catalytically at 170°C.2 In the absence of a strong acid or a metal ion, kinetic and thermodynamic data for the maleic acid hydration reaction shows that lowering the temperature favours the formation of malic acid.7 The temperature employed in the Cr3+/maleic acid system studied in the present work is 100°C, at which the catalysis operates at a measurable rate. Attempts to develop new catalysts and to extend the catalysis to other olefinic substrates were unsuccessful. In this chapter, preliminary kinetic results of this non-phosphine, transition metal complex catalyzed hydration of maleic acid are presented; preliminary data on coordination of the various carboxylic acids are also presented. 6.1. Kinetic experiments The total chromium content of the catalyst precursor, CrCl3-6H20, was determined by the spectrophotometric measurement of the chromate (Cr042-) concentration, formed quantitatively by oxidation of CrQS) species by the action of peroxide in alkaline solution.5 The characteristic absorption band at 371.9 nm of the formed yellow species (Cr042-) has a molar extinction coefficient of 4.8xl03 M^cnr 1. The average Cr 1 1 content cletermined for standards prepared from CrCl3-6H20 was 1.5% higher than the calculated value. 194 The kinetic experiments were carried out in a series of reaction vessels for various periods of time, the reaction conditions being maintained as close as possible; each separate reaction yielded one kinetic data point The typical initial [Cr3+] in kinetic runs was 3.60xl0-2 M, with the maleic acid present in a 50-fold excess. The reactions were carried out at 100 ± 1°C on a circulating steam-bath. The reaction mixture, 10 mL of an aqueous solution, whose pH was measured, was kept in a 50 mL Schlenk flask under air or N 2 which held more than 1 atm pressure when the quickfit stoppers and the stopcocks were tightened by rubber bands. The reaction was stopped immediately at any specified time by cooling the flask in an ice-water bath; no measurable product formation resulting from isomerization or hydration is detected at 40°C for 8 h. The volume of the resulting aqueous solution was then reduced to about 1 mL, and the organic compounds were extracted thoroughly by Et20 (10 xlO mL portions) until the ethereal extract contained no more acids as shown by TLC. The blue, aqueous solution containing Cr 3 + was discarded. The Et20 solution was dried over anhydrous MgSC>4 for 5 h before evaporation to dryness on a rotary evaporator. The white solid obtained thereby was dried under vacuum overnight. Analysis for product composition by gas chromatography was impractical because of the low vapour pressures and high boiling points of maleic, fumaric and malic acids. The molar composition of the reaction mixture was thus determined from the integration ratios of each component in the *H NMR spectrum; the isolated white solid product was dissolved in acetone-d6 for the NMR analysis and, as noted above, each analysis provided one point for the rate plot A typical *H NMR spectrum is shown in Fig. 6.1. Table 6.1 summarizes the results of !H NMR analysis of samples with known composition, which had been subjected to the same work-up procedure as described for the unknown samples from the kinetic runs. Some errors in the kinetic experiments are derived from the work-up procedure. The extraction recovery is about 98%, with some organic substrate possibly being lost in the coordination sphere of Cr 3 +. Another error is from the measurement of the integrals in the *H NMR spectrum: the relative error is normally 1 - 2% for maleic acid, 2 - 5% for malic acid and 3 -10% for fumaric acid. 195 Fig.6.1. A typical *H NMR (300 MHz) spectrum of the hydration product mixture containing maleic, malic and fumaric acids in acetone-d6 at r.t.; H 2MA: 8=CH- = 6.4 (s); H 2FA: 6.8 (s); H2mal: 5.CH- = 4.52 , 8.CH2- = 2.75; 2JH aHb = 15.9, 3JHaHc = 4.5, 3JHbHc = 7.2 Hz. S = solvent peak. Table 6.1. *H NMR Analysis of Control Samples0 Mixture I H 2 MA H2mal H 2 FA Mixture JJ H 2MA H2mal H 2 FA calcd. (%) 49.5 42.2 8.3 calcd.(%) 47.2 43.6 9.2 found(%) 48.6 44.4 7.0 found(%) 50.0 42.4 7.6 (a) Maleic, malic and fumaric acids are abbreviated as H 2 MA, H2mal and H 2 FA, respectively, throughout this Chapter. 6.2. Results and discussion Hydration of maleic acid to R, S-malic acid (Eq. 6.1) at room temperature is, in fact, a thermodynamically favourable process with a A G 0 value of -5.3 kcal/mol* (see Appendix AIX). However, in the present studies, the hydration was found not to proceed in the absence of a catalyst even at 100°C. Chromium(m), added as CrCl3-6H20, was found to be active in favour cis-HOOCCH=CHCOOH + H20 — HOOCCH2CH(OH)COOH maleic acid R, S-malic acid of malic acid formation at 100°C, while Fe(IU), added as FeCl3-6H20, gave a much higher percentage of the isomerization product fumaric acid, which was not hydrated at this temperature (see below). 6.2.1. Spectroscopic properties of Cr(JH) and complexation with maleic and malic acids Chromium(III) has a d 3 electronic configuration and is substitutionally inert.5 , 6 The weak electronic absorption of chromium(III) is the result of symmetry (Laporte) forbidden d-d transitions.6 Two absorption maxima are often observed in the visible region, and their positions are influenced by the ligands coordinated according to the ligand field strength(s). When trans-[CrCl2(H20)4]Cl-2H20, the catalyst precursor (commercially available as CrCl3-6H207-8), dissolves in water, H 2 0 slowly replaces one chloride, resulting in a blue shift of both original absorption maxima. This process has a half life of 2.5 h at room temperature,9 but the rate is * A AG0 value of -7 kcal/mol in acidic solution was given in ref. 4. 197 greatly accelerated at 100°C. The chloropentaaquo species is relatively stable (ti/2 = 70 h, r.t.).10 The following table lists the room temperature visible spectral data of the hydration isomers, and these data serve as a guide for identification of the chemical species present in the catalytic reactions. Table 6.2. Visible Spectral Data of Some Hydration Isomers of CrCl3-6H20a complex Xi.ei X2,e2 Ref. trans-[CrCl2(H20)4]+ 650, 24.1 455, 22.0 8,9 cis-[CrCl2(H20)4]+ 650.0, 18.0 455, 25.7 8,9 [CrCl(H20)5]2+ 609, 16.4 425, 20.8 10 [Cr(H20)6]3+ 572, 13.4 409, 15.4 11, 12 (a) Wavelengths in nm and extinction coefficients in M^cm-1. The oligomerization of Cr(H20)63+ at elevated temperature to form hydroxo bridged dimeric, trimeric and tetrameric species often becomes more important in basic, concentrated solutions.75'14 A study by Laswick and Plane74 has shown that in the absence of the added base, some 70.4% of the Cr 3 + species remained as hexaaquo species in a chromic perchlorate solution (0.05 - 0.10 M) after 24 h boiling, while only 38.8% remained as Cr(H20)63+ after 3 h if NaOH (0.05 - 0.10 M) was added prior to reflux. The rest of the Cr(TII) species were present as oligomerized hydroxo bridged forms. Oligomerization, however, could be controlled to a certain extent by lowering the pH of the solution. More detailed studies of chromic oligomers by Stunzi and Marty led these authors to state that the absorbance ratio of the shorter wavelength band over the longer one was 1.17 ± 0.01 for the monomer, 1.18 ± 0.01 for the dimer, 1.60 ± 0.01 for the trimer, 1.95 ± 0.04 for the tetramer, and 1.5 - 1.56 for the oligomers higher than four Cr units respectively.7-5 A red shift of both maxima is also indicative of the oligomer formation.75 198 Table 6.3. Visible Spectral Parameters of ChromiumCuT) Oligomers75 monomer dimer trimer tetramer hexamer Xi (ei) 575 (13.2) 582 (17.4) 584 (19.2) 580 (15.6) 585 (18.6) Xi (e2) 409 (15.5) 417 (20.4) 425 (30.5) 426 (30.3) 426 (29.0) e2/eia 1.17 1.17 1.59 1.94 1.56 (a) Calculated from data given in the ref. 15. In the present study, both maleic and malic acid are found to form complexes with Cr 3 + in solutions containing 0.20 M KNO3 and 0.02 M HNO3 at 100°C. These complexes are different from those formed by reduction of K2Cr207 with the same acids (Sect. 2.9). The absorbances of CrCl3-6H20 solutions at different concentrations (in solutions containing 0.20 M KNO3 and 0.02 M HNO3 at pH = 2.0) which had been heated at 100°C for 30 min were recorded as 'blanks'; plots of e at 410 and 572 nm against [Cr3 +] is presented in Appendix (Fig. A.2). The ratios of the two e values are between 1.16 -1.17 (Table ADC 1), indicating the formation of Cr(H20)63+ under these conditions. The acid nitrate solutions containing Cr(H20)63+ and the maleic (or malic) acid at various ratios were heated at 100°C for 30 min and the absorbances were recorded. Through Job's Continuous Variation method76 (the data and treatment are shown in Appendix ATX 3), the composition of both complexes was determined to be 1:1, and the formation constants are 8-lxlO2 and 9.1xl03 M"1 for the maleate and malate complexes, respectively. The in situ maleate complex 55, considered to be [Cr(HMA)(H20)5]2+, has absorption band maxima at 418 and 572 nm (Fig. 6.2), while the corresponding malate complex (56) has maxima at 410 and 562 nm (Fig. 6.3). The positions of the absorption maxima indicate that these complexes are free of coordinated chloride ligands (cf. Table 6.2) because a carboxylate ligand is comparable to H2O within the spectrochemical series. The malate complex formed under similar conditions from malic acid (0.25 M) and QC13-6H20 (0.25 M) was separated on a cation-exchange column, yielding largely a charge 2+ species with the same absorption maxima (410, 562 nm) as those of the in situ formed species. 199 I 1 1 1 1 315 400 500 600 700 nm Fig. 6.2. The UV/v is absorption spectrum recorded at 25°C of [C r (HMA) (H 2 0)5]2 +, 55, in H 2 0 ; X(e, M ' W 1 ) data are noted, on the spectrum. F ig . 6.3. The U V / v i s absorption spectrum recorded at 25°C of [ C r ( H m a l ) ( H 2 0 ) 5 ] 2 + , 56, in H 2 0 ; X (e, M ' W 1 ) data are noted on the spectrum. The maleate and malate complexes are thus written as containing the mono-protonated, monodentate, mono-anion. It was also shown in Olson and Taube's work that some 92% of the maleate C r 3 + complex was monodentate containing the HMA anion.4 However, this monodentate and the chelate species [Cr(MA)(H20)4]+ containing the maleate dianion were found to be present in equilibrium with an equilibrium constant of 15.4 M _ 1 at 25°C, in favour of the formation of the monodentate form (Eq. 6.2)/ 7 Mixing fumaric acid and CrCl3-6H20 solutions under the same conditions gave green solutions whose absorbances varied with the concentration of the added chromic species, perhaps indicating no formation of a complex. [Cr(MA)(H20)4]+ + H+ . [Cr(HMA)(H20)5]2+ (6.2) 6.2.2. Kinetic results The experimental conditions of the kinetic runs have been described in the experimental section (Sect. 6.1). The reaction solution is homogeneous throughout. In this section, the maleic acid concentration dependence, the catalyst concentration dependence, and the pH dependence will be discussed. An observed competition between the malic acid formed in situ and the maleic acid for the Cr(H20)63 + ion complicates the reaction kinetics. Another complication arises from a slow formation of fumaric acid from the coordinated maleato complex. Nevertheless, with certain simplifications and assumptions, the results of the kinetic study can be interpreted to some satisfaction. 6.2.2.1. Maleic acid dependence Maleic acid dependence experiments were done through two different methods — measuring the initial rate of reaction at different starting maleic acid concentration, and monitoring the H2MA concentration change with time. The initial rate measured at t = 2.0 h with <16% conversion gives a zero-order dependence on maleic acid concentration (Table 6.4), the slope of A[H2MA]/At vs. [H2MA] plot being essentially zero (Fig. 6.4). The second method shows that 201 even up to first 8 h the rate is independent of maleic acid concentration (Fig. 6.5 inset), although after about 8 h a first-order dependence seemed apparent (Table 6.5, Fig. 6.5). Table 6.4. Initial Rate of the Catalytic Hydration of Maleic Acid at Various [H2MA]a [H2MA]i(M) 0.344 0.688 1.03 [Cr]xl03 (M) 3.6 3.6 3.6 [H2MA]t(M) 0.289 0.638 0.972 [H2mal]t (M) 0.039 0.020 0.025 [H2FA]t(M) 0.015 0.029 0.033 A[H2MA]/At (M/h) 0.0273 0.0248 0.0288 (a) Data measured after 2 h; subscripts i and t refer to initial and time of measurements, respectively. The pH levels of these reactions are between 1.6 to 1.8. 0.05 0.04-^ 0.03 -\ < 2 < 0.02 0.01 0.20 0.40 0.60 0.80 [H2MA]i(M) 1.00 1.20 Fig. 6.4. Plot of A[H2MA]/At vs. [H2MA]i at the initial stage of the reaction; [Cr] = 3.60xl0"3 M, temp. = 100 ± 1°C, t = 2.0 h; pH = 1.6 - 1.8. 202 Table 6.5. Catalytic Conversion of Maleic Acid to Malic Acid and Fumaric Acid3 t(h) H2MA H2mal H2FA (%, M) (%,M) (%, M) 0.0 100.0, 1.72 0.0, 0.0 0.0, 0.0 0.5 94.5, 1.63 3.9,0.067 1.5, 0.026 1.0 90.4, 1.55 7.3, 0.126 2.4, 0.041 1.0 90.0, 1.56 6.1, 0.105 3.0, 0.052 2.0 87.7, 1.51 7.8, 0.134 4.5, 0.077 2.0 86.1, 1.48 12.6, 0.217 1.4, 0.024 3.0 84.4, 1.45 11.2, 0.193 4.4, 0.076 4.0 78.6, 1.35 17.8, 0.306 3.7, 0.064 4.0 73.8, 1.27 23.7, 0.408 2.5, 0.043 5.0 80.8, 1.39 12.0, 0.206 7.1, 0.122 5.0 78.5, 1.35 15.9, 0.273 5.6, 0.096 6.0 68.6, 1.18 27.5, 0.473 4.0, 0.069 7.7 66.0, 1.14 30.4, 0.523 3.7, 0.064 8.0 66.2, 1.14 30.8, 0.530 3.1, 0.053 16.0 55.1, 0.948 37.9, 0.652 6.9, 0.119 20.0 47.2, 0.812 46.8, 0.805 6.0, 0.103 24.0 34.3, 0.590 53.8, 0.925 11.9, 0.205 24.0 31.9, 0.549 53.9, 0.927 14.2, 0.244 24.0 38.0, 0.654 52.6, 0.905 9.5, 0.163 32.0 27.4, 0.471 63.1, 1.09 9.4, 0.162 32.0 29.3, 0.504 62.6, 1.08 8.2, 0.141 32.0 29.9, 0.514 55.1, 0.948 15.6, 0.268 40.0 23.6, 0.406 67.7, 1.15 7.9, 0.136 40.0 23.4, 0.402 62.8, 1.08 12.4, 0.213 203 (Table 6.5 continued)  40.0 20.9, 0.359 69.3, 1.19 9.8, 0.169 48.0 14.4,0.248 71.5, 1.23 14.1, 0.243 48.0 17.4, 0,299 72.8, 1.25 9.8, 0.169 (a) [H2MA], = 1.72 M, [Cr3+] = 3.6x10-2 M; temp. = 100 ± 1°C; pH = 1.3. L7» -2.00 I ' ' ' ' i ' 1 • • — i i • • • • | 0.0 10.0 20.0 30.0 40.0 50.0 t(h) Fig. 6.5. Plot of ln[H2MA] vs. t (0.0 <, t <. 48.0 h), ti/2 = 19.3 h; inset: plot of [H2MA] vs. t (0.0 £ t <, 8.0 h). 6.2.2.2. Catalyst concentration dependence The [Cr3+] dependence was determined by varying the concentration of Cr 3 + species, while keeping the concentration of maleic acid relatively constant (Table 6.6, Fig. 6.6). At low Cr 3 + concentration, the order approximates to one (Fig. 6.6), while a gradual decrease from the first-order dependence is seen at higher Cr 3 + concentration and eventually the dependence is approaching zero-order. The decrease in order on Cr is tentatively attributed to the interference of the malic acid product, by coordination of the malate mono-anion to the Cr 3 + (Sect 6.2.1); reduction of the effective Cr 3 + concentration because of the oligomerization is also a possibility. 204 Table 6.6. Data for Cr3* Concentration Dependence8 [CrS+JxlO3 (M) H2MA H2mal H2FA A[MA]/At (%, M) (%,M) (%,M) (M/hxlO2) 3.0 96.3, 0.830 1.7, 0.015 1.9, 0.016 1.60 7.4 92.2, 0.795 6.0, 0.052 1.7, 0.015 3.35 9.0 91.3, 0.787 6.4, 0.055 2.2, 0.019 3.75 14.4 89.2, 0.769 8.7, 0.075 2.1, 0.018 4.65 18.0 86.6, 0.746 11.5, 0.099 2.1, 0.018 5.80 22.0 84.3, 0.727 13.8, 0.119 1.9, 0.016 6.75 27.0 81.8, 0.705 15.9, 0.137 2.2, 0.019 7.85 36.0 79.2, 0.683 18.0, 0.155 2.8, 0.024 8.95 45.0 78.1, 0.673 18.3, 0.158 3.6, 0.031 9.45 60.8 77.3, 0.666 19.7, 0.170 3.0, 0.026 9.80 65.0 76.1, 0.660 19.9, 0.172 3.9, 0.034 10.1 (a) [H2MA]i = 0.862 M, temp.= 100 ± 1°C, t = 2.0 h, pH = 1.5. 12.0 10.0 •£ ^ 8 - 0 0J0 V i ' ' • I i ' • « I • • i • I • " ' • 0.0 20.0 40.0 60.0 80.0 [CrKMxlO3) Fig. 6.6. Initial rate of loss of maleic acid as a function of [Cr]. 205 6.2.2.3. pH dependence The adjustable pH range for kinetic measurements is very narrow in this system because of the formation of a grey, colloidal precipitate, possibly Cr(OH)3, around pH 2.6. For those solutions required with pH > 1.6, the pH levels were initially brought to selected values by addition of solid NaOH; the solutions were then self-buffered by the substrate (maleic acid/maleate conjugate pair action). For those solutions with pH < 1.6 , the pH levels were adjusted using HC1-KC1 buffer solution. A ApH change of 0.2 between the initial and final solutions was observed (Table 6.7), the unbuffered solutions normally have a ApH change of around 0.2 - 0.3. Spectra of some of the 'final' catalysis solutions after 10 h reaction were measured at room temperature; broad maxima, noted at ~415 and ~570 nm, are between the maxima of the maleate and malate complexes. The experimental conditions and work-up procedures were similar to those of the unbuffered experiments, except that aqueous HC1 was added to neutralize the added base before the Et20 extraction. The data are summarized in Table 6.7 and Fig. 6.7. Table 6.7. pH Dependence Data3 pH pH H 2MA H2mal H 2 FA (initial) (final) (%, M) (%, M) (%, M) 0.87 0.75 65.6, 0.565 25.5, 0.220 8.9, 0.077 1.00 1.07 44.7, 0.385 43.4, 0.374 11.8, 0.102 1.30 1.46 37.9, 0.327 52.7, 0.454 9.4, 0.081 1.77b 1.96 54.0, 0.465 36.0, 0.310 10.0, 0.086 1.92b 2.12 61.9, 0.533 31.6, 0.272 6.5, 0.056 2.0 2.15 51.7, 0.446 32.5, 0.280 15.9, 0.137 2.35 2.44 84.4, 0.728 9.1, 0.078 6.5, 0.056 2.64 2.60 91.5, 0.789 3.9, 0.034 4.6, 0.040 3.0 2.86 92.3, 0.796 3.5, 0.030 4.2, 0.036 (a) [H2MA]i = 0.862 M, [Cr3+] = 3.6xl0"2 M; temp. = 100 ± 1°C, t = 10.0 h. (b) Buffered by potassium hydrogen phthalate and NaOH solutions. 206 7 0 . 0 Fig. 6.7. Conversion of maleic acid after 10 h as a function of pH; pH values are averaged between the initial and final values (Table 6.7) From the data in Fig. 6.7, the rate of catalytic hydration is obviously pH dependent; however, no quantitative information can be obtained from these results. As mentioned previously, in the presence of base the oligomerization becomes more important, and this may explain the drastic reduction of the conversion at higher pH. 6.2.3. Mechanism There are several possible mechanisms for the formation of malic and fumaric acids. Three of the most plausible ones are discussed below. The Cr(H20)63+ ion may form a Cr(T|2-olefin)(H20)43+ complex analogous to the known Ru(rj2-H2MA) complex;75 then free or coordinated H2O attacks the coordinated C=C, forming a hydroxyalkyl ligand (Scheme 1.12). The alkyl group, as in the case of a fluoroolefin substrate,79 is cleaved off in a subsequent protonation step. This mechanism is considered unlikely because of the fact that the maleato ligand is coordinated through the oxygen donor of the carboxylic group instead of the olefinic bond, as indicated by the small shift of absorption maxima on coordination of the maleate mono-anion to the Cr(H20)63+species. Other O-bonded maleato complexes have also been isolated (see below). 207 A second possible mechanism involves the direct formation of a [C-Cr] 3 + cation, analogous to a protonated C=C bond. Cleavage of the C-Cr bond yields a zwiterion, and addition of H2O to this unstable intermediate gives the hydrated product (see Eq. 6.3). This mechanism was proposed by Bzhasso and Pyatnitskii,2 and was used to explain the formation of fumaric acid and malic acid. This mechanism also accounts for a kinetic dependence on the catalyst concentration that was noted in the present study, but it would predict that C r 3 + should catalyze the hydration of fumaric acid; also a fumarate complex would be expected to form during the reaction. Cr"13+ H» , H + r jc=c4 + cr* " H-p-C-H HOOC COOH HOOC COOH H + _ H H °H H HOOC' SCOOH HOOC' 'NCOOH H. (6.3) The coordination of carboxylate oxygen to the Cr centre has been demonstrated by Olson and Taube in the stoichiometric aquation of Cr(H20)5(HMA)2+ at temperatures of 40 - 60°C,4 and more recently a similar monomethyl maleate CoOQT) complex, Co(CH3-MA)(H20)53+, was characterized by Sargeson and coworkers.20 The experimental conditions in the present study differ from those in the reported stoichiometric reaction in the temperature, pH and the concentration of maleic acid.4 However, the reactions occurring in these two systems appear to be very much related. The chromium(III) maleato complexes, formed in situ by mixing perchloric acid, hexaaquochromium(in) and maleic acid in a 1:1:2 ratio at 40°C for 115 h, were separated by cation exchange chromatography into three species:4 a chelate Cr(H20)4(MA)+, a 'malato intermediate' (see below), and a monodentate maleate species Cr(H20)5(HMA)2+. On aquation at 60°C for 20 h in 0.94 M perchloric acid, each species gave maleic, fumaric and malic acids with the compositions shown in Table 6.8.4 This intermediate was characterized by an absorption maxima at 417 nm (e 36.3 M^cnr 1) and, from its elution behaviour, was more like a +1 than a +2 complex.4 This chromic species, giving predominantly malic acid on aquation, was suggested to be the 5-member ring chelate (57, see structure in Scheme 6.1) formed by the 208 hydroxyl and carboxylate groups of malic acid. With knowledge of this structure for 57, the elution behaviour can be explained by dissociation of the proton on the coordinated hydroxo group (the pKa of this proton in the Co(IU) complex was found to be 2.75 20). The scheme presented in the reference 4, showing a conversion of a monodentate maleate species to monodentate fumarate and chelate malate species 57, seems applicable to the catalytic hydration system. Although the possibility of a "C-Cr3+ cation" mechanism cannot be excluded, the similarity between the stoichiometric and catalytic reactions is very evident. Based on the spectroscopic and kinetic results, the catalytic cycle for the hydration of maleic acid is outlined below (Scheme 6.1). In this mechanism, formation of fumaric acid is very slow and irreversible. Table 6.8. Results of Aquation of Maleato Complexes of Cr 3 + 4 Complex (% yield3) H2MA (%) H2mal (%) H2FA (%) chelate (6.4) intermediate (1.6) monodentate (~92) 76 28 87 22 69 10 1 3 3 (a) The yields are calculated from the data given in ref. 4. Hmal Cr(H20)6 3+ HP + Hmal K-(H20)5Cr(Hmal)' malato 56 2+ HFA (H20)5Cr(HFA) fumarato 2+ (H20)5Cr(HMA) maleato 55 2+ (H20)4Cr. 57 CH2COOH OH CH+ I CHjCOOH Scheme 6.1. A plausible mechanism for maleic acid hydration catalyzed by the Cr 3 + aquo ion; the HMA, HFA and Hmal species are monoanions, but the charges are omitted for convenience both in the Scheme and in the derivation of the rate-law. 209 The rate-law for the disappearance of maleic acid can be derived, assuming that the H + transfer from the coordinated water to the double bond of the coordinated maleate is the rate determining step. rate = k2[55] = k2Ki[Cr][HMA], where [Cr] = [Cr(H20)63+] and as [Cr]tot= [Cr] + [55] + [56] = [Cr] (1 + KifHMA] + K2[Hmal]), [Cr]toITHMA] m t e - k 2 K l l + K1rHMA]+K2[Hmal] <6-4> Assuming dissociation of the second proton is negligible for both maleic acid and malic acid under the catalysis condition (pK^ values are 6.07 and S.10, respectively), we have _ [HMA][H+] _ .1.8 ' _ [Hmal][H+] _ -3.4 j u  K ai [H2MA] 1 0 m d a ' [H^aT] 1 0 .and thus • + At the beginning of the reaction, the concentrations of malic acid (and fumaric acid) are very small, and at the pH values used, with knowledge of the Ki and K 2 values (~103 and 104 M"1, respectively) and the pKa, values (see above), it is simple to show that the KifHMA] term in the denominator of Eq. 6.4 dominates (i.e. 'all' the Cr is present as 55). Thus rate = k2[Cr]tot The rate is first-order in [Cr]t0t concentration, and independent of the maleic acid concentration as found experimentally. When malic acid has accumulated to a significant amount, it will compete with maleic acid for a coordination site on Cr 3 +; i.e. K2[Hmal] and Ki[HMA] become comparable. Under these conditions (assuming that the concentration of free Cr3* is negligible), Eq. 6.4 gives [Cr]t0JHMA]  rate " K 2 K l K 1 [HMA] + K2[Hmal] 210 The rate expression at the later stages of the hydration is thus k ^ i KJCr l^ jVlAl^+rHl )  ~ Kj KapCa+|H+])[H2MA]tot+ K 2 K4K a +[Hl)rH2mal] t o t Approximately, [H2mal]tot = [H2MA]i - [H2MA]tot. where [H2MA]i is the initial concentration of H2MA. Thus, _ A[Cr]tJH2MA] tot " B + C[H2MA]tot where A = k2KiKai([H+] + K'ai), B = K2K,ai[H2MA]i(Kai + [H+]), C = KiKa i(K'a i + [H+]) - K 2K' a i(K a i + [H+]) The rate is still first-order in chromium, and is between first- and zero-order in maleic acid concentration. At the stage where K2[Hmal] > Ki[HMA] (Eq. 6.4), the rate then becomes first-order in maleic acid concentration. The above analysis based on the plausible mechanism shown in Scheme 6.1 is in qualitative agreement with the kinetic results. The rate of the isomerization reaction is very slow compared to the hydration, and no kinetic information is available on the formation of fumaric acid because of the difficulty in measuring the fumaric acid concentration accurately. 6.2.4. Other metal ion catalysts, and the hydration of other olefinic carboxylic acids Besides Cr 3 +, Fe3 + ion (added as FeCl3-6H20) was found in the present study and A l 3 + (added as Al2(S04)3-18H20) is known from the literature,2 to catalyze the hydration of maleic acid in a similar fashion. The last two catalyst systems, however, give more isomerization product, fumaric acid. The A l 3 + catalyst is reported to yield 73.9% malic acid and 26.1% fumaric acid while the Cr 3 + catalyst yields 88.8 and 11.2% of malic and fumaric acid, respectively, at 170°C.2 In the present study, the Fe3 + catalyst gave a system that analyzed for a mixture of 38.6, 44.0 and 17.4% of H2MA, I^ mal and H2FA, respectively, while Cr 3 + under the same conditions (Table 6.5, 8 h entry) gave a mixture of 66.2, 30.8 and 3.1% of H2MA, 211 H2mal and H2FA, respectively. Other metal ions, such as, Rh 3 + (added as RhCl3-3H20), Rh3+/Rh+ (prepared in situ from RhCl3-3H20 using C2H4) 2 7 , Ru3+ (RuCl3-3H20), Ru2+ ( via TiCl3 reduction of (NH4)2RuCl6 in 3 M HCl)22 and Pt2+ ^PtCL;), were shown in the present studies to be ineffective for either hydration or isomerization of maleic acid under the same conditions (100°C, closed system). Among the olefinic carboxylic acids tested with the Cr3* system, maleic acid was the only substrate catalyticaUy hydra ted. Fumaric acid, mesaconic acid (HC^CC(CH3)=CHC02H) and aconitic acid (H02CC(CH2C00H)=CHC02H) were shown in the present work to be unreactive. When the mixture of R, S -malic acid and Cr 3 + was heated at 100°C for 8 h, no dehydration product, either as maleic acid or fumaric acid, was detected; furthermore, under the same conditions, S-malic acid was not racemized. The replacement of malato ligand by maleate must be facile when maleic acid is present in large excess, however, because under similar experimental conditions, both cis-K[Cr(MA)2(H20)2] and cis-K[Cr(mal)2(H20)2] as catalysts gave comparable hydration mixtures after ~ 2 h (Table 6.9). Table 6.9. Hydration Products of Maleic Acid Using Diaquo- bis(MA) and -bis(mal) Cr 3 + Complexes as Catalysts cis-K[Cr(MA)2(H20)2] cis-K[Cr(mal)2(H20)2] [H2MA]i = 0.862 M, [Cr] = l.OlxlO"2 M t = 2.17 h, temp. = 100.0°C [H2MA]i = 0.862 M, [Cr] = 9.52xl0"3 M t = 2.17 h, temp. = 100.0°C [H2MA] [H2mal] [H2FA] 86.7% 10.0% 3.3% [H2MA] [H2mal] [H2FA] 84.6% 12.0%a 3.4% (a) Malic acid from the catalyst contributes 9.52xl0"3x2/[0.862 + (2x9.52x10"3)] = 2.2%. 6.3. Conclusion The catalytic hydration of maleic acid by a non-phosphine metal complex was realized at 100°C using Cr 3 + ion, and the kinetics were studied in some detail in the present work. 212 Previous workers had reported on such catalytic activity at 170°C.2 Because of the practical problems of this system (Sect. 6.1), large errors in the kinetic data became inevitable. A temperature variation study was impossible because of the concentration change of the key catalytic species resulting from oligomerization. The maleic acid consumption is first-order in Crtotal concentration below approximately 2.7X10-2 M, but the initial rate dependence gradually drops off at higher Cr concentrations. This decrease is attributable perhaps partially to the oligomerization and partially to the interference of malic acid (see Scheme 6.1). A pH dependence of the maleic acid conversion is evident. Addition of base causes extensive oligomerization and formation of possibly a hydroxide; the reduction of catalyst concentration at higher pH (2.0 - 3.0) is believed to be responsible for the reduction in conversion. The specific interaction between maleate and Cr involved in the catalysis is not yet clear. Several other metal ions tested showed negative results with maleic acid, and the hydration of fumaric acid and other prochiral olefins using Cr 3 + was also unsuccessful. Asymmetric synthesis of malic acid from prochiral fumaric acid was thus not pursued further. Olson and Taube's results4 provide very useful information in attempts to elucidate the reaction mechanism. The malato ligand of Cr(malato)(H20)52+ can be replaced readily by maleate at the initial stage of reaction. As malic acid accumulates, it becomes more difficult for the product to be released; the rate thus becomes substrate concentration dependent because of the competition of maleic and malic acid for the active Cr(H20)63+ catalyst. 213 References 1. Kisa, Z.; D6ak, Gy. Hung. J. Ind. Chem., 1983,11, 313. 2. (a) Bzhasso, N.A.; Pyatnitskii, M.P. Zh. Prikl. Khim., 1969,42, 1610. (b) Bzhasso, N.A.; Pyatnitskii, M.P. Izv. Vyssh. Ucheb. Zaved., Pishch. Tekhnol., 1967,5, 207; through CA. 1968, 68, 39013k. 3. Haupt, G.W. /. Res. Natl. Bur. Stand. (U.S.), 1952,48, 414. 4. Olson, M.V.; Taube, H. / . Am. Chem. Soc., 1970, 92, 3236. 5. Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Wiley and Sons: New York, 1980; p.727. 6. (a) Wood, D.L.; Ferguson, J.; Knox, K.; Dillon, J.F. Jr. / . Chem. Phys., 1963, 39, 890. (b) Figgis, B.N. Introduction to Ligand Fields; Wiley: New York, 1966; p.209, 222. 7. O'Brien, T.O. In Chemistry of Coordination Compounds, Ed. Bailar, J.C. Jr.; Reinhold: New York, 1956; pp.261 - 262. 8. Gates, H.S.; King, E.L. / . Am. Chem. Soc, 1958, 80, 5011. 9. Johnson, H.B.; Reynolds, W.L. Inorg. Chem., 1963,2, 468. 10. Swaddle, T.W.; King, E.L. Inorg. Chem., 1965,4, 532. 11. Halpern, J.; Harkness, A.C. / . Chem. Phys., 1959,37, 1147. 12. Cathers, R.E.; Wendlandt, W.W. /. Inorg. Nucl. Chem., 1965,27, 1015. 13. (a) Thompson, M.; Connik, R.E. Inorg. Chem., 1981,20, 2279. (b) Finholt, J.E.; Thompson, M.; Connik, R.E. Inorg. Chem., 1981, 20, 4151. 14. Laswick, J.A.; Plane, R.A. /. Am. Chem. Soc, 1959,81, 3564. 15. Stunzi, H.; Marty, W. Inorg. Chem., 1983,22, 2145. 16. Bauer, H.H.; Christian, G.D.; O'Reilly, J.E. Instrumental Analysis; Allyn and Bacon: Boston, 1978; pp.178 - 179. 17. Olson, M.V.; Taube, H. Inorg. Chem., 1970, 9, 2072. 18. Halpern, J.; Harrod, J.F.; James, B.R. / . Am. Chem. Soc, 1961, 83, 753. 19. James, B.R.; Louie, J. Inorg. Chim. Acta, 1969,3, 568. 214 20. Gahan, L.R.; Harrowfield, J.M.; Herlt, A.J.; Lindoy, L.F.; Vftiimp, P.O.; Sargeson, A.M. J. Am. Chem. Soc, 1985,107, 6231. 21. (a) James, B.R.; Kastner, M.; Rempel, G.L. Can. J. Chem., 1969,47, 349. (b) James, B.R.; Rempel, G.L. Can. J. Chem., 1968,46, 571. 22. Halpern, J.; James, B.R. Can. J. Chem., 1966,44, 495. 215 Appendix Al. Structural parameters of Pt2Cl2("-PN2)2 (HT). 7b.l Empirical formula C32ll26CI2N4P2Pt2 Formula weight 989.62 Crystal system Monoclinic Lattice parameters: a = 15.142(5) A b = 9.853(3) A c = 23.74(1) A p = 107.98(3)° V = 3369(2) A3 Space group P21/n (#14) Z value 4 Density calculated 1.95 g/cm' Residuals: R; Rw 0.056; 0.069 Goodness of Tit indicator 2.09 Maximum shift in final cycle 0.08 Largest peak in final diff. map 4.11 e/A' Intramolecular distances involving the nonhydrogen atoms atom atom distance atom atom distance Ptd) N(3) 2.10(1) C(6) C(7) 1.36(2) PtO) P(D 2.169(3) C(7> C(8) 1.36(2) Pl(l) Cl(l) 2.405(4) C(8) C(9) 1.36(3) Pt(D Pt(2) 2.574(1) C(9) CtIO) 1.33(3) Pt(2) N(l) 2.08(1) C(ll) C(12) 1.35(2) Pt(2) P(2) 2.169(4) C(ll) C(16) 1.36(2) Pt(2) a(2) 2.383(4) C(12) C(13) 1.46(3) Pd) C(6) 1.83(2) C(13) C(14) 1.35(3) P(D C(l) 1.83(2) C(14) C(15) 1.26(3) P(D C(ll) 1.86(2) C(15) C(16) 1.37(3) P(2) C(I7) 1.80(2) C(17) C(18) -1.41(2) P(2) C(22) 1.80(2) C(18) C(19) 1.35(2) P(2) C(27) 1.84(2) C(19) C(20) 1.36(3) N(l) C(l) 1.36(2) C(20) C(2I) 1.40(2) N(l) C(5) 1.36(2) C(22) C(23) 1.36(2) N(2) C(6) 1.33(2) C(23) Q24) 1.39(2) N(2) C(10) 1.37(2) C(24) C(25) 1.37(3) N(3) C(21) 131(2) C(25) C(26) ••34(3) N(3) C(17) 1.38(2) C(27) C(32) 1.35(2) N(4) C(22) 1.38(2) C(27) C(28) 1.37(2) N(4) C(26) 1.39(2) C(28) C(29) 1.31(2) C(l) C(2) 1.36(2) C(29) C(30) 1.31(3) C(2) C(3) 1.38(3) C(30) C(31) 1.40(3) C(3) C(4) 1.37(3) C(31) C(32) 1.39(2) <M C(5) 1.30(3) Distances are in A. Estimated standard deviations in the least significant figure are given in parentheses. Torsion and conformation angles Pt(l) N(3) C(2I) C(20) 178(2) Pt(2) Pt(l) PO) C(l) 39.2(6) Pt(l) N(3) CO 7) C(l 8) 176(1) Pt(2) Pt(l) PO) C(ll) 159.8(6) PtO) N(3) C(I7) P(2) -20) Cl(l) PlO) N(3) C(21) 39(1) P((l) P(D C(6) N(2) 118(1) CIO) PtO) N(3) C(I7) -143(1) Pt(1) P(D C(6) C(7) -60(1) Cl(l) Pt(D PO) C(6) 91.2(5) PtO) P(D C(l) NO) -29(1) Cl(l) PtO) PO) CO) -147.4(6) Pt(l) P(l) C(l) C(2) 149(2) Cl(l) Pt(D PO) C(ll) -26.7(6) Pl(1) P(l) C(ll) C(12) -26(2) O(l) PtO) Pt(2) N(l) -130(1) PtO) P<1) C(ll) C(16) 159(1) ao) PtO) Pt(2) P(2) 48(1) Pt(l) Pt(2) N(I) CO) 32(1) ao) PtO) Pt(2) a(2) 48(3) Pt(l) Pt(2) N(l) C(5) -150(1) a<2) Pt(2) N(l) C(l) -148(1) PtO) Pt(2) P(2) C(17) 44.7(5) Cl(2) Pt(2) N(l) C(5) 31(1) Pl(l) Pt(2) P(2) C(22) -76.7(5) a<2) Pt(2) P(2) CO 7) -135.3(5) Pt(l) N(l) CO) C(2) 174(1) a(2) Pt(2) P(2) C(27) -17.1(6) Pl(1) Pt(2) P(2) C(27) 162.9(6) a(2) Pt(2) P(2) C(22) 103.3(6) Pt(2) N(l) C(5) PO) -8(2) a(2) Pt(2) PtO) N(3) -37(2) Pt(2) N(l) C(5) C(4) -176(2) Cl(2) Pt(2) Pt(l) PO) 143(2) Pt(2) P(2) C(17) N(3) -37(1) PO) C(6) N(2) C(10) -178(1) Pt(2) P(2) C07) C(18) 145(2) p(D C(6) C(7) C(8) 179(1) Pt(2) P(2) C(22) C(23) -25(2) PO) CO) NO) C(5) 174(1) Pt(2) P(2) C(22) N(4) 159(1) PO) CO) C(2) C(3) -173(2) Pt(2) P(2) C(27) C(32) 110(2) PO) C(ll) C(12) C(13) -179(1) Pt(2) P(2) C(27) C(28) -64(1) PO) C(ll) C(16) C(15) 180(2) Pt(2) PtO) N(3) C(21) -147(1) PO) Pt(l) N(3) C(21) -146(2) Pt(2) PtO) N(3) C(17) 30(1) PO) PtO) N(3) C(17) 32(3) Pt(2) PtO) PO) C(6) -82.2(5) PO) Pt(D Pt(2) N(l) -34.2(4) P(l) PtO) Pi(2) P(2) 143.4(1) N(3) Pt(l) PO) C(6) -84(3) P(2) C(17) N(3) C(2I) 176(1) N(3) PtO) PO) CO) 38(3) P(2) C(I7) CO 8) C(19) -174(2) N(3) PtO) PO) C(ll) 158(3) P(2) C(22) C(23) C(24) -177(1) N(4) C(22) C(23) C(24) -1(3) P<2) C(22) N(4) C(26) 177(1) N(4) C(22) P(2) CO 7) 35(1) (to be continued) Tortion and conformation angles W (?) W angle (D W Q) w angle P(2) C(27) C(32) C(3I) -168(2) N(4) C(22) P(2) C(27) -73(1) P(2) C(27) C(28) C(29) 176(2) N(4) C(26) C(25) C(24) 0(3) P(2) Pt(2) N(l) CO) 10(4) C(l) NO) C(5) C(4) 2(3) P(2) Pt(2) PtO) N(3) -36.8(3) CO) PO) C(6) C(7) 175(1) NO) C(l) C(2) C(3) 5(3) C(l) PO) C(ll) C(l 2) 100(1) NO) CO) P(D C(6) 96(D C(l) P(l) COD C(l 6) -75(1) NO) C(l) P(D COD -158(1) C(2) C(D N(l) C(5) -4(2) NO) C(5) C(4) C(3) 1(4) C(2) CO) PO) C(6) -87(2) NO) Pt(2) P(2) C(17) 67(3) cm C(l) PO) C(ID 20(2) NO) Pt(2) P(2) C(22) -54(3) C(2) Ct3) C(4) C(5) 2(4) NO) Pt(2) P(2) C(27) -175(3) C(6) N(2) C(IO) C(9) 0(3) NO) Pt(2) PtO) N(3) 145.6(5) C(6) C(7) C(8) C(9) 1(3) N(2) C(6) C(7) C(8) 1(2) C(6) PO) COD C(12) -149(1) N(2) C(6) PO) C(l) -7(1) C(6) PO) con C(16) 36(1) N(2) C(6) PO) C(ll) -115(1) C(7) C(8) a.9) C(10) 0(3) N(2) C(10) C(9) C(8) 0(4) W) C(6) N(2) C(10) 0(2) N(3) C(21) C(20) C(19) 4(3) C(7) C(6) P(l) COD 67(1) N(3) C(l 7) C(18) C(19) 8(3) C(ll) C(12) C(13) C(14) 3(3) N(3) C(17) P(2) C(22) 89(1) COD C(16) C(15) C(14) -3(4) N(3) C(17) P(2) C(27) -163(1) C(I2) C(ll) C(16) C(15) 5(3) C(12) CO 3) C(14) C(15) -1(4) CO 3) C(14) C(15) C(16) 1(4) CO 3) C(12) C(ll) C(16) -5(3) C(17) N(3) C(2D C(20) 0(3) C(I7) CO 8) C(19) C(20) -5(4) C(17) P(2) Q27) C(32) -128(2) CO 7) P(2) C(27) C(28) 58(1) C(18) C(I9) C(20) C(21) -1(4) C(18) C(17) N(3) C(21) -6(2) C(18) C(17) P(2) C(22) -89(2) CO 8) C(17) P(2) C(27) 19(2) C(22) C(23) C(24) C(25) , K3) C(22) N(4) C(26) C(25) -1(3) C(22) P(2) C(27) C(32) -17(2) C(22) P(2) C(27) C(28) 169(1) C(23) C(22) N(4) C(26) 1(3) C(23) C(22) P(2) C(27) 103(1) C(23) C(24) Ct25) C(26) 0(3) C(27) C(32) C(31) C(30) -9(4) C(27) C(28) C(29) C(30) -6(4) C(28) C(29) C(30) C(31) 3(4) C(28) C(27) C(32) C(31) 6(3) C(29) C(28) C(27) C(,32) 1(3) C(29) C(,30) q3l) C(,32) 5(4) The sign is positive if when looking fron atom 2 to atom 3 a clockwise motion of atom 1 would superimpose it on atom 4. Intramolecular bond angles involving the nonhydrogen atoms atom atom atom angle atom atom atom angle N(3) pid) P(D 172.9(3) C(5) N(l) Pt(2) 120(1) N(3) pt(D Cl(l) 88.0(3) C(6) N(2) QI0) 116(1) N(3) ptd) Pt(2) 91.4(3) C(2I) N(3) C(17) 118(1) P(D pt(D a(l) 99.1(1) C(2I) N(3) Pt(D 122(1) P(D ptd) Pt(2) 81.5(1) C(17) N(3) Pt(l) 119.0(9) OO) ptd) Pt(2) 173.5(1) C(22) N(4) C(26) 119(2) N(l) Pl(2) P(2) 173.7(3) N(l) C(l) C(2) 121(2) N(l) Pt(2) 0(2) 90.6(4) N(l) C(l) P(D 113(1) N(l) Pt(2) Pt(D 92.8(3) C(2) C(l) P(l) 126(1) P(2) Pt(2) Cl(2) 95.3(2) C(l) C(2) C(3) 117(2) P(2) Pt(2) Pt(l) 81.4(1) C(4) C(3) C(2) 122(2) Cl(2) Pt(2) Pl(» 176.6(1) C(5) C(4) C(3) 120(2) C(6) P(D C(l) 107.0(7) C(4) C(5) N(l) 121(2) C(6) P(D C(ll) 101.7(7) N(2) C(6) C(7) 122(1) C(6) P(D Pt(l) 112.3(5) N(2) C(6) P(D 120(1) C(l) P(l) C(ll) 103.0(7) C(7) C(6) Pd) 118(1) C(l) P(D Pt(l) 113.4(5) C(8) C(7) C(6) 120(1) C(ll) P(D Pi(l) 118.2(6) ca> C(8) C(9) 119(2) C(I7) P(2) C(22) 106.9(7) C(10) C(9) C(8) 119(2) C(I7) P(2) C(27) 103.3(7) C(9) C(10) N(2) 124(2) Q17) P(2) Pt(2) 111.0(5) C(12) C(ll) C(16) 122(2) C(22) P(2) C(27) 102.6(8) C(12) C(ll) P(D 120(1) C(22) P(2) Pt(2) 114.9(5) C(16) C(ll) P(l) 118(1) C(27) P(2) Pt(2) 117.0(6) C(ll) C(12) C(13) 116(2) C(l) N(l) C(5) 120(1) C(14) C(I3) C(12) 120(2) C(l) N(I) Pt(2) 120(1) C(15) C(14) C(13) 120(2) C(14) C(15) C(16) 124(2) C(ll) C(16) C(15) 118(2) N(3) C(I7) C(18) 120(1) N(3) C(I7) C(I8) 120(1) N(3) C(17) P(2) 113(1) C(18) C(17) P(2) 127(1) qi9) qi8) C(17) 120(2) C(18) am C(20) 120(2) Intramolecular bond angles involving the nonhydrogen atoms (continued) atom atom atom angle atom atom atom angle C(I9) C(20) C(21) 119(2) N(3) C(21) C(20) 123(2) C(23) C(22) N(4) 120(2) C(23) C(22) P(2) 121(1) N(4) C(22) P(2) U9(l) C(22) C(23) C(24) 119(2) C(25) C(24) C(23) 121(2) C(26) C(25) C(24) 119(2) C(25) C(26) N(4) 122(2) C(32) C(27) C(28) 121(1) C(32) C(27) P(2) 124(1) C(28) C(27) P(2) 115(1) C(29) C(28) C(27) 117(2) C(28) C(29) C(30) 125(2) C(29) C(30) C(31) 119(2) C(32) C(31) C(30) 116(2) C(27) C(32) C(31) 120(2) Angles are in degrees. Estimated standard deviations in the least significant figure are "given in parentheses. A l l . The acid-base titration data for Pd2Cl2(|i-PN3h(HT), l i e , and PdCl2(PN3)2, 4c, in aqueous solution. Table A H 1. NaOH titration against HN03a vol (mL) PH vol (ml.) PH 0.000 2.905 0.618 9.650 0.126 3.030 0.649 9.820 0.262 3.228 0.681 9.950 0.390 3.570 0.778 10.195 0.427 3.725 0.905 10.395 0.455 3.962 1.134 10.615 0.488 4.460 1.382 10.770 0.520 6.795 1.610 10.910 0.552 8.970 1.962 10.990 (a) I H N O 3 I = 9.80x10-* M, |NaOHl = 4.7 U10"2 M; ionic strength 1 = 0.3 M using NaN03. temp. = 25.0 ± 0.2°C, under N 2 (see Fig. 3.21). Table All 2. NaOH titration against H N O 3 and l ie" >—* VO vol (mL) PH vol (mL) PH 0.000 2.970 0.682 9.610 0.102 3.075 0.712 9.780 0.166 3.166 0.745 9.910 0.259 3.330 0.807 10.100 0.324 3.490 0.870 10.240 0.394 3.765 0.934 10.345 0.427 3.995 0.996 10.430 0.460 4.442 1.061 10.500 0.490 4.920 1.157 10.590 0.524 5.845 1.316 10.705 0.554 7.075 1.506 10.815 0.584 8.200 1.665 10.885 0.618 8.960 1.859 10.955 0.650 9.360 1.986 10.995 (a) [ I I N O 3 ) = 9.80xl(H M, |NaOH| = 4.7UI0"2M. | l l c | = 4.98xl0"4 M; ionic strength I = 0.3 M using NaN03, temp. = 25.0 ± 0.2°C, under N2(see Fig. 3.21). Table All 3 NaOH titration against HNfo' vol (mL) PH vol (mL) pH 0.000 2.890 0.613 9.575 0.129 3.016 0.646 9.760 0.228 3.150 0.709 10.005 0.291 3.266 0.805 10.228 0.387 3.530 0.969 10.454 0.420 3.680 1.166 10.626 0.452 3.895 1.427 10.780 0.483 4.300 1.631 10.870 0.516 6.070 1.828 10.940 0.548 8.770 1.993 10.990 0.580 9.285 (a) The conditions are exactly the same as described in fable All 1 (a). Table All 4:.NaOH titration against H N O 3 and lie" vol (mL) PH vol (mL) PH 0.000 2.830 0.871 8.680 0.101 2.904 0.904 9.055 0.165 2.960 0.936 9.330 0.229 3.022 0.967 9.520 0.293 3.097 1.000 9.700 0.358 3.186 1.033 9.840 0.422 3.295 1.065 9.952 0.486 3.435 1.096 10.040 0.550 3.624 1.162 10.182 0.582 3.750 1.226 10.290 0.614 3.906 1.322 10.420 0.647 4.114 1.423 10.520 0.678 4.422 1.523 10.595 0.710 4.975 1.650 10.685 0.743 6.080 1.810 10.775 0.774 6.560 2.004 10.865 0.808 7.185 0.840 8.005 (a) The conditions are exactly the same as described in Table All 2 (a). Table All 5. NaOH titration against UNO)" vol (mL) PH vol (mL) Pll 0.000 2.610 0.611 10.080 0.100 2.715 0.644 10.225 0.150 2.791 0.675 10.333 0.200 2.865 0.707 10.421 0.264 2.980 0.803 10.618 0.329 3.130 0.997 10.860 0.394 3.365 1.250 10.990 0.426 3.555 1.347 11.100 0.450 3.790 1.564 11.202 0.482 4.705 1.724 11.260 0.498 7.145 1.855 11.302 0.516 8.780 1.959 11.335 0.546 9.520 1 999 11.402 0.578 9.875 (a) I H N O 3 ] = l.98xl0"3 M, (NaOH) = 9.42xlO"2 M; ionic strength I = 0.3 M using NaN03. temp. = 25.0 ± 0.2°C, under nitrogen (see Fig. 3.24). Table All 6. NaOH titration against UNO} and 4c* vol (mL) Pll vol (111L) PH 0.000 2.635 0.909 9.27 0.099 2.708 0.941 9.65 0.163 2.765 0.972 9.90 0.260 2.865 1.005 10.07 0.356 2.990 1.038 10.20 0.423 3.100 1.069 10.30 0.579 3.335 1.136 10.47 0.583 3.585 1.200 10.585 0.616 3.770 1.264 10.675 0.650 4.040 1.330 10.752 0.681 4.460 1.417 10.850 0.714 4.940 1.555 10.957 0.740 5.350 1.654 11.020 0.811 6.610 1.749 11.072 0.843 7.38 1.880 11.135 0.875 8.43 1.986 111X0 (a) IHNO3] = 1.98x10-3 M. |4c] = 2'00x10"3 M. [NaOH] = 9.42x10-2 M; ionic strength 1 = 0.3 M using NaN03, temp. = 25.0 ±0.2°C, under N 2 . Table All 7. NaUH titration against HNO3 and 4c8 vol (111L) PH vol (mL) PH 0.000 2.500 0.888 6.29 0.132 2.607 0.923 7.09 0.230 2.712 0.954 8.15 0.381 2.925 0.991 9.06 0.478 3.120 1.025 9.25 0.575 3.430 1.089 9.590 0.640 3.825 1.155 9.960 0.682 4.320 1.221 10.215 0.716 4.660 . 1.319 10.440 0.748 4.960 1.448 10.643 0.781 5.270 1.583 10.780 0.824 5.855 1.844 10.965 0.856 5.99 1.972 11.030 (a) Hie conditions are exactly die same as described in Table All 6 (see Fig. 3.24) Table All 8. NaOH titration against 4c in the absence of HNO3" vol (ml.) PH vol (mL) pH 0.000 2.900 28.83 5.80 2.28 3.03 29.44 6.00 5.09 3.12 29.81 6.20 7.90 3.24 30.35 6.50 11.76 3.50 31.09 6.80 15.73 3.75 31.61 7.03 19.04 4.00 32.31 7.40 21.70 4.30 32.95 7.88 23.45 4.51 33.70 8.45 25.03 4.75 34.70 8.92 25.37 4.85 35.75 9.40 26.09 5.00 36.98 9.60 26.80 5.15 38.21 9.80 27.35 5.30 39.36 9.92 27.85 5.45 40.95 10.10 28.18 5.60 45.45 10.32 (a) lNaOH| = 1.0x10-' M. |4c] = 3.34xIO"3 M. 10 mL; ionic strength I = 0.3 M using NaN03; temp. = 25.0 ± 0.2 °C, under N2 (see Fig. 3.22). AMI. Kinetic data of the oxidative addition of DMAD to i^hOi-PNjh (1 IT), 9c, in CH2CI2. Table All! 1. (9c] - 1.18x10-3 M. IDAMD] - 3.58xlO"2 M, temp. -34.0°C. t(s) A,(A. = 518nm) -ln(At- A«.> 0.0 0.852 0.681 20.0 0.810 0.768 40.0 0.760 0.882 60.0 0.715 0.997 80.0 0.676 1.109 100.0 0.643 1.214 120.0 0.612 1.324 140.0 0.587 1.423 160.0 0.563 1.528 180.0 0.544 1.619 200.0 0.526 1.715 220.0 0.509 1.814 260.0 0.483 1.988 300.0 0.462 2.154 340.0 0.455 2.313 0.346 slope - 4.77x10-3 s>, R2 - 0.996. Table AMI 2. |9c] 34.n°c. - 1.18x10-3 M. (DAMD| •= 2.39x10-* M. temp. Ks) A, (X = 5l8nm) -ln(A,-A.) 0.0 0.857 0.671 40.0 0.793 0.805 60.0 0.758 0.887 80.0 0.726 0.968 100.0 0.697 1.047 120.0 0.671 1.124 140.0 0.649 1.194 180.0 0.609 1.336 200.0 0.592 1.402 250.0 0.554 1.570 280.0 0.535 1.666 320.0 0.513 1.790 380.0 0.486 1.960 450.0 0.462 2.154 0.346 slope = 3.29x10-3 j >, R2 = 0.996. Table All! 3 |9c| - 1 18)» 10 3 M. |DAMD| = 34 0°C. 1.19x10-2 M, temp. = t(s) A,(X = 5l8nm) -ln(A,-A.) 0.0 0.852 0.681 40.0 0.822 0.742 60.0 0.803 0.783 80.0 0.784 0.826 100.0 0.766 0.868 120.0 0.748 0.911 140.0 0.732 0.952 160.0 0.717 0.992 180.0 0.702 1.033 200.0 0.689 1.070 240.0 0.663 1.149 280.0 0.640 1.224 320.0 0.619 1.298 390.0 0.587 1.423 450.0 0.564 1.523 480.0 0.553 1.575 0.347 slope = 1.90x10-3 ,1, R2 = fj.999. Table AI1I 4. [9c] - 1.18x10-3 M, [DAMDJ - 3.58xl0"2 M. lemp. » 29.0°C. t(») A,(X = 518nm) -ln(A, - A_) 0.0 0.858 0.717 20.0 0.817 0.805 40.0 0.776 0.901 60.0 0.738 1.000 80.0 0.705 1.090 100.0 0.674 1.190 120.0 0.648 1.280 140.0 0.624 1.370 160.0 0.603 1.457 180.0 0.584 1.542 200.0 0.565 1.635 220.0 0.549 1.720 250.0 0.527 1.852 280.0 0.509 1.973 300.0 0.497 2.064 340.0 0.476 2.244 400.0 0.455 2.465 510.0 0.423 2.937 600.0 0.407 3.297 0.370 slpoe - 4.28xl0-3 «•, R2 - 0.999. Table AMI 5. [9cJ » 1.18x10-3 M, [DAMD] = 2.39xlfr2 M, lemp. = 29.0°C. t(s) A, (X = 518nm) -ln(A,-A«,) 0.0 0.858 0.687 20.0 0.837 0.730 40.0 0.807 0.794 60.0 0.780 0.856 80.0 0.753 0.921 100.0 0.730 0.981 120.0 0.708 1.041 140.0 0.688 1.100 160.0 0.669 1.158 200.0 0.636 1.269 -240.0 0.607 1.378 300.0 0.571 1.532 360.0 0.540 1.687 400.0 0.522 1.790 450.0 0.503 1.911 oo 0.355 slope = 2.74x10-3 i-l, R2 - 0.998. Table AIII 6. (9c] » 1.18x10-3 M. (DAMD] = 1.19xl0"2 M, temp. -29.0°C. t(s) A, (X = 518 nm) -ln(A, - A..) 0.0 0.858 0.687 20.0 0.854 0.695 40.0 0.840 0.724 60.0 0.825 0.755 80.0 0.810 0.787 100.0 0.796 0.819 120.0 0.783 0.849 140.0 0.770 0.879 160.0 0.758 0.909 180.0 0.747 0.936 200.0 0.736 0.965 240.0 0.714 1.024 280.0 0.696 1.076 320.0 0.677 1.133 380.0 0.653 1.211 450.0 0.627 1.302 510.0 0.607 1.378 570.0 0.589 1.452 0.355 slope - 1.38x10-5 ,1, R2 _ 0.999. Table A m 7. (9c] - 1.18x10-3 M , [ D A M D ] » 3.25x10-2 M , temp. -25.0°C. Ks) A, (X = 518 run) -ln(A,-A.) 0.0 0.885 0.679 40.0 0.829 0.796 60.0 0.802 0.858 80.0 0.775 0.924 100.0 0.751 0.986 120.0 0.729 1.047 150.0 0.701 1.130 180.0 0.675 1.214 200.0 0.660 1.266 240.0 0.632 1.370 280.0 0.606 1.478 360.0 0.564 1.682 400.0 0.547 1.778 0.378 slope = 2.74x10-3 s ->,R2 = 0.998. Table A m 8. (9c] = • 1.18x10-3 M, (DAMD] = 1.63x10-2 M, temp. -25.0°C. Ks) A| (X '518 nm) -ln(A,-A») 0.0 0.884 0.681 40.0 0.857 0.736 60.0 0.839 0.774 80.0 0.824 0.807 100.0 0.807 0.846 140.0 0.784 0.901 180.0 0.755 0.976 220.0 0.733 1.036 260.0 0.711 1.100 300.0 0.693 1.155 360.0 0.666 1.245 450.0 0.632 1.370 0.378 slope = 1.55x10-3 s-l l .R 2 = 0.998. Tible AID 9. [9c] - 1.45x1a4 M. [DAMD] - 5.70x10-3 M, temp. -21.0°C. t(min) At(X-413nm) -ln(A,-A>) 0.0 0.645 1.019 4.0 0.611 1.146 8.0 0.576 1.262 12.0 0.549 1.363 16.0 0.523 1.470 20.0 0.498 1.585 24.0 0.478 1.687 0.293 slope = 2.76x10-2 min'. R* - 0.999. Tible AIII 11. [9cJ - 1.18x10-3 M, [DAMD] - 2.7U10-* M, temp. -I8.0°C. t(«) A,(X = 518nra) -ln(A,-A-) 0.0 0.881 0.687 40.0 0.844 0.764 60.0 0.819 0.819 80.0 0.798 0.868 100.0 0.779 0.914 120.0 0.761 0.960 160.0 0.729 1.047 200.0 0.700 1.133 240.0 0.674 1.217 280.0 0.651 1.298 330.0 0.625 1.398 390.0 0.597 1.519 450.0 0.573 1.635 510.0 0.552 1.749 570.0 0.534 1.858 660.0 0.510 2.025 780.0 0.485 2.235 OO 0.378 slope » 2.00x10-3 ,1, R2 . 0.998. Table AID 10. [9c] - 1.4SxHH M. [DAMD] • 8.13xMr3 M, m y . -21.0°C. t(min) A,(X-428nm) -ln(A,-A«) 0.0 0.631 1.073 3.0 0.596 1.189 6.0 0.561 1.302 9.0 0.532 1.415 12.0 0.505 1.532 15.0 0.478 1.666 18.0 0.457 1.784 21.0 0.438 1.904 24.0 0.420 2.033 OO 0.289 •lope-4.01x10-2 min-l,R2. |.000. Table AID 12. [9c] - 1.18x10-3 M, [DAMD] * 2.39x10-2 M, temp. -18.0°C. t(i) A,fX-5I8nm) -ln(VA-) 0.0 0.883 0.683 40.0 0.839 0.774 60.0 0.821 0.814 80.0 0.805 0.851 110.0 0.780 0.911 150.0 0.754 0.978 180.0 0.736 1.027 240.0 0.705 1.118 300.0 0.678 1.204 360.0 0.652 1.295 420.0 0.630 1.378 480.0 0.609 1.465 540.0 0.390 1.551 660.0 0.556 1.726 780.0 0.528 1.897 OO 0.378 slope - 1.49x10-31- '. R 2 - 0.991. Table AIII 13. (9c| - 1.18x10"3 M. |DAMD| - I.l9xl0"2 M. lemp. = 18.0°C. t(s) A,(X = 5l8nm) -ln(A, - A J 0.0 0.876 0.697 40.0 0.864 0.722 80.0 0.844 0.764 120.0 0.827 0.801 180.0 0.805 0.851 220.0 0.792 0.882 260.0 0.780 0.911 300.0 0.767 0.944 380.0 0.745 1.002 460.0 0.734 1.033 540.0 0.703 1.124 620.0 0.683 1.187 720.0 0.662 1.259 840.0 0.639 1.343 960.0 0.618 1.427 0.378 slope » 7 . 6 4 X 1 0 " 4 s •'. R2 - 0.999. Table AIII IS. [9c] = 1.18xlO-3 M, [DAMD] - 3.58xIO-2 M, temp. -1?.0°C. Ks) A, (X. = 518nm) -ln(A,-A„) 0.0 0.892 0.666 40.0 0.857 0.736 60.0 0.837 0.779 80.0 0.818 0.821 100.0 0.800 0.863 120.0 0.785 0.899 160.0 0.758 0.968 200.0 0.735 1.030 240.0 0.712 1.100 280.0 0.693 1.155 330.0 0.669 1.234 390.0 0.646 1.317 480.0 0.613 1.448 600.0 0.576 1.619 720.0 0.545 1.780 OO 0.378 slope = 1.48x10-3 s>, R2 = 0.999. Table AIII 14. [9c| - 1.18x10-3 M. [DAMD] - 4.54xl0"2 M. lemp. -18.0°C. I(S) A, (X = 518 nm) -ln(A,-A.) 0.0 0.886 0.677 40.0 0.812 0.835 60.0 0.782 0.906 80.0 0.757 0.970 100.0 0.734 1.033 120.0 0.714 1.091 140.0 0.694 1.156 160.0 0.677 1.207 180.0 0.661 1.262 200.0 0.646 1.317 240.0 0.618 1.427 ' 270.0 0.599 1.510 330.0 0.566 1.671 390.0 0.538 1.833 480.0 0.505 2.064 550.0 0.483 2.254 780.0 0.434 2.882 oo 0.378 slope =2.75x10-3 s-l, R2 - 0.999. Table Affl 16. [9c] - 1.18xlO-3 M, [DAMDI - 2.39xl0"2 M. temp. -13.0°C. t (» A, (X = 5l8nm) -ln(A, - A„) 0.0 0.890 0.641 40.0 0.873 0.673 60.0 0.858 0.703 800 0.845 0.730 100.0 0.833 0.755 120.0 0.821 0.781 140.0 0.811 0.803 2000 0.782 0.870 280.0 0.750 0.949 330.0 0.731 1.000 390 0 0.710 1.058 480.0 0.682 1.143 6000 0.648 1.255 720.0 0.618 1.366 0.378 M slope = 1.03x10-3«', R2 « 0.998. as Table ATJ117. (9cJ - 1.18x10-3 M, [DAMD] - 1.19xl0"2 M, temp. -13.0°C. t(s) A, (X = 518nm) -ln(A,-A_) 0.0 0.885 0.650 40.0 0.867 0.685 80.0 0.852 0.715 120.0 0.838 0.744 160.0 0.824 0.774 210.0 0.810 0.855 270.0 0.793 0.844 360.0 0.774 0.889 420.0 0.760 0.924 540.0 0.735 0.989 600.0 0.722 1.024 720.0 0.700 1.088 0.378 slope = 6 . 0 8 X 1 0 - 4 s >, R2 = 0.998. AIV. Kinetic data of the oxidative addition of DMAD to Pt2h(u-PN3)2 (HH) 10c in CH2Cl2 (binding of DMAD. k|). Table AIV 1. |10c] = 1.10x10-3 M. [DAMD) - 4.88xlO"2 M. temp. = 34.0°C. t(s) A, (X = 500 nm) -ln(A,- A_*) 0.0 0.499 1.162 40.0 0.332 1.924 50.0 0.305 2.129 60.0 0.285 2.313 70.0 0.268 2.501 80.0 0.254 2.688 90.0 0.240 2.919 100.0 0.229 3.147 120.0 0.214 3.576 140.0 0.204 4.017 0.186 *The lowest absorbance recorded before the subsequent increase of absorbance reading, similarly hereinafter. slope = 2.00x10 2 s- 1, R2 = 0.998. Table AIV 2. [10c] = 34.0°C. - 1.10x10-3 M, [DAMD] = 2.28xl0"2 M, temp. t(s) A, (X = 500 nm) -ln(A,-A-*) 0.0 0.508 1.146 40.0 0.415 1.492 60.0 0.379 1.666 80.0 0.349 1.839 100.0 0.324 2.010 120.0 0.304 2.172 140.0 0.288 2.323 160.0 0.275 2.465 180.0 0.263 2.617 200.0 0.253 2.765 o o 0.190 slope = 9.85x10-3 s', R2 = 1.000. TAble A1V 3. [10c] - 1.10x10-' M, [DAMD] - 3.23x10-2 M. temp. - 34.0°C. t(s) A,(X = 500nm) -ln(A,-A«*) 0 0.501 1.168 30 0.399 1.565 40 0.377 1.677 60 0.336 1.924 80 0.305 2.163 100 0.278 2.430 120 0.258 2.688 140 0.242 2.957 160 0.230 3.219 ' 180 0.219 3.540 200 0.212 3.817 0.190 slope » -1.31x10-2 i-l. R2- 0.997 Table AIV 4. [10c] = UOxlO"3 M. [DAMD] » 3.25x10-2 M. temp. -29.0°C. Ms) A, (X = 500 nm) -ln(A,-A«*) 0.0 0.498 1.158 40.0 0.382 1.619 60.0 0.346 1.820 80.0 0.318 2.010 100.0 0.295 2.198 120.0 0.275 2.397 140.0 0.259 2.590 160.0 0.245 2.797 180.0 0.234 2.996 200.0 0.225 3.194 ~ 0.184 slope - 1.00x10-21->.R2 - 0.999. TableAIV5. [10c] 29.0°C. - 1.10x10-3 M, [DAMD] - 6.50x10-2 M, temp. . !(•) A, (X-500 nm) -JnCAt-A.*) 0.0 0.509 1.136 40.0 0.320 2.025 50.0 0.298 2.207 60.0 0.276 2.430 70.0 0.260 2.631 80.0 0.247 2.830 90.0 0.236 3.037 100.0 0.227 3.244 120.0 0.213 3.689 OO 0.190 slope = 2.20x10-2 ,1, R2 „ Q.999. Table AIV 6. 110c] = 25.0°C. 1.10xl0-s M. [DAMDI = 4.06xl0"2 M, temp.= C(S) A, (X = 500 nm) -ln(A, • A„*) 0.0 0.488 1.130 30.0 0.384 1.519 40.0 0.359 1.640 50.0 0.340 1.743 60.0 0.323 1.843 70.0 0.308 1.945 80.0 0.294 2.048 100.0 0.270 2.254 120.0 0.250 2.465 140.0 0.235 2.659 160.0 0.222 2.865 OO 0.168 $lope = 9.08x10-3 »- R2 - 1.000. Tab!eATV7. [10c] - 1.10x10-3 M, [DAMD] » 6.50xl0-2 M, temp. -25.0°C. t(s) A, (X = 500 nm) -ln(A,-A-*) 0.0 0.488 1.106 30.0 0.352 1.704 40.0 0.325 1.864 50.0 0.299 2.048 60.0 0.280 2.207 70.0 0.263 2.375 80.0 0.250 2.526 100.0 0.228 2.847 120.0 0.212 3.170 0.170 slope = 1.73xlO"2 s'.R 2= 1.000. Table ATV8. [10c) = l.lOxlO"3 M, [DAMD] = 3.25xlO-2 M, temp. =• 2I.0°C. l(s) A, (X = 500 nm) -ln(A, - A-*) 0.0 0.490 1.152 40.0 0.398 1.500 50.0 0.375 1.604 60.0 0.359 1.687 70.0 0.344 1.772 80.0 0.331 1.852 90.0 0.319 1.931 100.0 0.308 2.010 120.0 0.290 2.154 140.0 0.275 2.293 160.0 0.260 2.453 - -200.0 0.238 2.749 240.0 0.223 3.016 280.0 0.211 3.297 oo 0.174 slope = 7.58x10-3 , >.R2 = 0.999 Table AIV 9. [10c] - 1.10x10-3 M. [DAMD] -6.50xl0"2 M, temp. -21.0°C. t(s) A, (X = 500 nm) 0.0 0.499 1.124 40.0 0.340 1.833 60.0 0.299 2.129 70.0 0.286 2.244 80.0 0.270 2.408 90.0 0.259 2.538 100.0 0.249 2.674 120.0 0.232 2.957 160.0 0.210 3.502 OO 0.177 slope = 1.33xlO-2 s', R2 = 0.999. Table AIV 10. [10c) >= 1.10x10-3 M, (DAMD) = 6.50xlO"2 M. temp-17.0°C. Ks) A, (X = 500 nm) -ln(A,-A_*) 0.0 0.493 1.121 30.0 0.356 1.614 40.0 0.328 1.766 50.0 0.307 1.897 60.0 0.290 2.017 70.0 0.274 2.146 100.0 0.239 2.501 140.0 0.213 2.882 180.0 0.192 3.352 oo 0.164 slope = l.WxlO^ »». R 2 = 0.997. Table AIV 11. (10c) - 1.10x10-3 M, (DAMD) - 4.07xl0-2 M. temp.-17.0°C. t(J) A t (X - 500 nm) -ln(A( - A *^) 0.0 0.492 1.109 30.0 0.396 1.431 40.0 0.376 1.519 50.0 0.357 1.609 60.0 0.340 1.698 80.0 0.313 1.858 100.0 0.293 1.995 120 0 0.273 2.154 160.0 0.245 2.430 220.0 0.217 2.813 « , 0.159 slope - 7.14x10-3 s1, R 2 = 0.997. AV. Kinetic data of the isomerization of PtzhOl-DMADX^-PNjh (HH). 28.2 to Pt2l2(H-DMAD)(n-PN3)2(HT). 19. in CH2CI2 (k2). Table AV 1. |10c) = 1.10x10-3 M, (DAMD) = 2.28x|f>2 M. temp. -34.0°C. t (min) A, (X = 476 nm) -ln(A_-A0 0.0 0.363 1.174 5.0 0.373 1.207 35.0 0.460 1.551 65.0 0.517 1.864 95.0 0.558 2.172 125.0 0.589 2.489 155.0 0.609 2.763 185.0 0.625 3.058 215.0 0.636 3.324 0.672 slope - l.OlxlO"2 min'. R 2 - 0.997. Table AV 2. (10c) - 1.10x10-3 M. (DAMD] - 3.25xl0-2 M. temp. -34.0°C. t(h) A,(X = 460nm) -ln(A_-At) 0.0 0.478 0.872 0.04 0.503 0.929 1.04 0.698 1.609 2.04 0.787 2.198 3.04 0.830 2.688 4.04 0.860 3.270 OO 0.898 slope = 9.76x10-3 min-l, R2 = Q.997. Table AV3. [10c] - l.lOxlO"3 M, [DAMD] - 4.88xl0"2 M, lemp. -34.0OC. t (min) A, (X = 460 nm) -ln(A--Ai) 0.0 0.451 0.920 3.0 0.469 0.965 28.0 0.568 1.266 53.0 0.634 1.532 78.0 0.685 1.802 103.0 0.715 2.002 128.0 0.745 2.250 153.0 0.765 2.470 178.0 0.782 2.690 203.0 0.795 2.900 - 0.850 slope = 9.72x10-3 min'. R2 » 0.997. UJ O Table AV 4. [10c] = l.lOxlO"3 M. [DAMD] - 3.25xl02 M, lemp. -29.0°C. 1 (min) A, (X = 480 nm) -ln(A_AJ 0.0 0.286 1.115 9.0 0.296 1 146 39.0 0.356 1.355 69.0 0.399 1.537 99.0 0.435 1.720 129.0 0.463 1.890 159.0 0.486 2.056 189.0 0.507 2.235 219.0 0.522 2.386 249.0 0.539 2.590 279.0 0.548 2.718 OO 0.614 slope = 5.72xl0-3 min1, R2 = 0.999. Table AV 5. [10c] - 1.10x10-3 M, [DAMD] - 6.50xl0"2 M. temp. » 29.0°C. I (min) A,(X = 470nm) -ln(A.-A0 0.0 0.386 1.013 3.0 0.393 1.033 33.0 0.457 1.231 63.0 0.502 1.398 93.0 0.538 1.556 123.0 0.570 1.720 153.0 0.595 1.871 183.0 0.620 2.048 213.0 0.638 2.198 243.0 0.652 2.333 273.0 0.668 2.477 303.0 0.686 2.740 0.749 slope = 5.35x10-3 mini, R2 = 0.999. r a S P* 6 M < o s 6 I 2 CN r"i m O O — »-l vO ? 5 Or d d d o o © 6 S S o o •ri d o o q «n d « <N I— - « N M f l _ . o 1 . " N 3 o " 2 S - S - 2 - 3 S — <N — f o n o oo w> m vv d d d d d d d d * 5 « « s ^ r» oo r- O » O - N 1 ^ r- r» r- o> d d d d d d o o o o o o o o © © o © © © o II a 6 n 8, o •3 < N - « r o o o » r i r - o r ~ ll <f © © © d o d o o © o o q o p o o p o *^ *^ V v n * 12 2 2 N o a 6 s ao > 4 «n r» t*\ * N <n . <s m m m © © © © t>> ao >G t*i O oo ft « m ^ w Q tn ^ ^ •* « © d © d d © o o o o o o o o o © © © © ' © o d d © tr> « * <s «n eo — • 231 Table AV 10. [10c] » l.lOxlO"3 M, [DAMD] = 6.50xlO"2 M. lemp. -17.0»C. I (min) A, (X = 470 nm) -ln(A».-A0 0.0 0.342 1.016 60.0 0.360 1.070 120.0 0.375 1.112 180.0 0.392 1.165 240.0 0.405 1.207 300.0 0.419 1.255 360.0 0.432 1.302 420.0 0.446 1.355 480.0 0.457 1.398 ee 0.704 slope - 7.95X10-4 min1, R2 - 1.000. Table AV 11. [10c] -17.0°C. l.lOxlO"3 M, [DAMD] = 4.07xl0"2 M, lemp. = t(min) A,(X = 478nm) -ln(A»-A0 0.0 0.275 1.162 80.0 0.301 1.248 160.0 0.320 1.317 240.0 0.337 1.382 320.0 0.351 1.440 400.0 0.366 1.505 480.0 0.379 1.565 560.0 0.392 1.630 OO 0.588 slope - 7.68x10-* min', R2 - 0.999. AVI. Kinetic data of the oxidative addition of DMAD to PtzhOl-PNita (HH), 10a, in CH2CI2. Table AVI 1. [10a] = 2.89x10-* M. [DAMD] = l.OSxlO"2 M, temp. = 35.0°C. t(min) A, (X - 425 nm) -ln(ArA«) 0.0 0.709 0.904 3.0 0.673 0.997 13.0 0.578 1.295 23.0 0.506 1.599 33.0 0.449 1.931 43.0 0.411 2.235 53.0 0.381 2.564 63.0 0.361 2.865 73.0 0.346 3.170 OO 0.304 slope = 3.16X10"2 min1, R2 = 0.997. Table AVI 2. [10a] 35.0°C. - 2.89x10-4 M, [DAMD] - 8.13x10-3 M, temp. » t(min) A,(X = 425 nm) -In(ArA-) 0.0 0.709 0.880 20.0 0.546 1.366 30.0 0.489 1.619 40.0 0.449 1.845 50.0 0.414 2.096 60.0 0.390 2.313 70.0 0.366 2.590 80.0 0.350 2.830 90.0 0.337 3.079 OO 0.291 slope = 2.39xl0-2 min', R2 = 0.999. Table AVI 3. |IOa| = 30.0°C. = 2.89x10^  M, |DAMD| = I.OSxlO"2 M, temp. ((min) A, (X = 425 nm) -ln(A,-A„) 0.0 0.723 0.931 2.5 0.703 0.983 17.5 0.586 1.359 32.5 0.502 1.754 47.5 0.447 2.137 62.5 0.408 2.538 77.5 0.384 2.900 OO 0.329 slope = 2.64xl0"2 min1, R2 = 0.999. Table AVI 4. [10a] = 30.0°C. = 2.89X104 M, [DAMD] = 1.63xlO-2 M, temp. t(min) A, (X = 425 nm) -ln(At-A„) 0.0 0.732 0.919 3.0 0.684 1.047 13.0 0.574 1.423 23.0 0.494 1.826 33.0 0.439 2.244 43.0 0.401 2.688 53.0 0.375 0.333 3.170 slope = 4.20xl0"2min-l, R2 = 0.998. Table AVI 5. [10a] = 25.0°C = 2.89x10-* M, [DAMD] = 1.63xl0"2 M, temp. t(min) A t (X = 425 nm) -ln(At-A«,) 3.0 0.681 0.965 11.0 0.601 1.201 19.0 0.536 1.444 27.0 0.483 1.698 35.0 0.444 1.938 43.0 0.413 2.180 51.0 0.390 2.408 59.0 0.369 2.674 OO 0.300 slope = 3.05xl0-2 min-', R2 = 1.000. Table AVI 6. |IOa] = 2.89xl04 M, |DAMD| = 8.l3xl03 M, temp. = 25.0°C. t (min) A, (X = 425 nm) -ln(A,-A«) 3.0 0.683 0.965 15.0 0.619 1.149 27.0 0.563 1.343 39.0 0.519 1.528 51.0 0.476 1.749 63.0 0.446 1.938 75.0 0.420 2.137 87.0 0.399 2.333 OO 0.302 slope = 1.65x10-2 min-1, R2 = 1.000. Table AVI 7. [10a] = 2.89x10-* M, [DAMD] = 1.63xl0-2 M, temp. « 20.0°C. t (min) A t (X - 425 nm) -ln(Ar-A<») 3.0 0.693 0.949 18.0 0.570 1.332 33.0 0.482 1.737 48.0 0.425 2.129 63.0 0.386 2.526 78.0 0.357 2.976 OO 0.306 slope = 2.69x10-2 min-1, R 2 = 0.999. Table AVI 8. [10a] = 2.89x10-* M. [DAMD] = 1.08x10-2 M, temp. = 20.0°C. t(min) A t (X = 425 nm) -ln(At-A») 0.0 0.714 0.856 5.0 0.684 0.929 25.0 0.571 1.266 45.0 0.488 1.614 65.0 0.432 1.945 85.0 0.393 2.263 105.0 0.365 2.577 125.0 0.346 2.865 OO 0.289 slope - 1.63x10-2 min-1, R2 = 0.999. Table AVI 9. |IOa]= - 30.0°C. = 6.63xl04 M, [DAMD) = 1.33x10-2 M. temp. 1 (min) A, (X = 425 nm) -ln(A, A.) 0.0 1.614 0.091 3.5 1.551 0.163 13.5 1.304 0.506 23.5 1.138 0.828 33.5 1.021 1.139 43.5 0.930 1.474 53.5 0.869 1.784 63.5 0.833 2.025 O O 0.701 slope = 3.26x10-2 ^ -l^ R2 = i .000. Table AVI 10. [10a] = 30.0°C. = 5.31xlO-4M,[DAMD] = 1.33x10-2 M. temp. t(min) A, (X = 430 nm) -In(ArA>) 0.0 1.178 0.392 3.5 1.123 0.476 13.5 0.946 0.812 23.5 0.820 1.146 33.5 0.721 1.519 43.5 0.669 1.790 53.5 0.621 2.129 O O 0.502 slope = 3.25xl0-2 min1, R2 = 1.000. Table AVI 11. [10a] = 30.0°C. = 4.09x10-* M. [DAMD] = 1.33x10-2 M, temp. t(min) A,(X = 430nm) -ln(Ar-A„) 0.0 0.908 3.5 0.857 13.5 0.726 23.5 0.631 33.5 0.564 43.5 0.517 53.5 0.482 O O 0.390 slope = 3.23x10-2 min1, R2 = 1.000. Table AVI 12. |10a| = 2.83x10-* M. |DAMD) = 1.33x10 2 M. temp. = 30.0°C. t (min) A| (X = 430 nm) -ln(ArA,) 0.0 0.629 3.5 0.601 13.5 0.513 23.5 0.448 33.5 0.405 43.5 0.373 53.5 0.348 O O 0.285 slope = 3.17x10-2 mini, R 2 = 1.000. AVH Kinetics of the oxidative addition of DMAD to Pt2h0t-PN2)2 (HH), 10b.2/10b.3. in CH2CI2 Derivation of the rate-law for reactions 4.6 and 4.7: Because 10b.2 and 10b.3, 31.1 and 32.1, as well as 31.2 and 32.2 are three enantiomeric sets, e(10b.2) = e(10b.3) etc. A 0 = Ao(10b.2) + Ao(10b.3) = e(10b.2)([10b.2] + [10b.3])l = e(10b.2)[10b.2/10b.311 - eQl Assuming 31.1/32.1 and 31.2/32.2 are formed in a fixed ratio, at the end of reaction: [31.2/32.2]:[31.1/32.1] = K, and [31.2/32.2] + [31.1/32.1] = Ci; x is the concentration of 10b.2/10bJ at time t. A - = e 3 l I 1 + e 3 l 2 1 3 i . i 1 + K " - 2 i + K . C i ~ x . K(Ci-x) A t = x £ i o b . J + Y ^ « 3 i . i l + 1 + R 1 A A , e S ! . l + R E 3 1 . 2 . , A , - A_= x(el0bl —-—=)1 e 3 1 . 1 + K c 3 M O b . 2 " 1 + K L*elob.2 i~Tv » '10b.2 1 + K  A , - A , A 0 - A . = Cj Thus, for a first-order reaction, a plot of ln(At - A») vs. t should be a straight line. Table AVD 1. I10b.2/10b.3j = 2.18x10-* M. [DAMD) = 8.13x10-3 M. temp. - 25.0°C. t (min) A, (X = 425 nm) -ln(A,-A_) 0.0 0.798 0.732 3.3 0.703 0.952 5.0 0.669 1.044 8.0 0.630 1.162 13.0 0.559 1.419 18.0 0.514 1.625 23.0 0.471 1.870 28.0 0.441 2.087 33.0 0.415 2.323 38.0 0.396 2.538 OO 0.317 »lope«4.55xl0-2 min-', R2 » 0.999. N Table AVHZ {10b.2/10bJl = 2.18x10-* M, [DAMD] = 8.13x10-3 M. V\ temp. » 35.0°C. l(min) A, (X = 425 nm) -ln(A,-A») 0.0 0.819 0.757 2.5 0.708 1.027 6.5 0.600 1.386 10.5 0.532 1.704 14.5 0.479 2.048 18.5 0.445 2.354 22.5 0.422 2.631 ' OO 0.350 slope = 8.08X10"2 min1, R2 = 0.998. TaMeAVm. [10b.2/10b.3] = 2.18x10-* M. |DAMDJ » 8.13x10-3 M, temp. = 15.0°C. t (min) Ai(X = 425nm) -ln(A,-A») 0.0 0.788 0.633 3.0 0.704 0.805 13.0 0.559 1.197 23.0 0.484 1.483 33.0 0.427 1.772 43.0 0.385 2.056 53.0 0.352 2.354 63.0 0.329 2.631 oo 0.257 slope = 2.99xl0"2 min', R2 - 0.996. Table AVH4. [10b.2/10bJJ - 2.18x10-* M, (DAMD] - 8.13x10-3 M. temp. - 30.0°C. t (min) A,(X = 425nm) -ln(ArA») 0.0 0.794 0.699 2.5 0.696 0.919 7.5 0.581 1.259 12.5 0.506 1.565 17.5 0.450 1.877 22.5 0.413 2.154 27.5 0.386 2.419 32.5 0.366 2.674 37.5 0.352 2.900 OO 0.297 slope = 6.02xl0"2 min1, R2 = 0.996. Table AVfl 5. [10b.2/10b.3J - 2.18x1a4 M. [DAMD] = 8.13xlO-3 M, lemp. - 20.0°C. to t (min) A, (X = 425 nm) -ln(A,-A_) 0.0 0.815 0.656 3.0 0.723 0.851 13.0 0.567 1.306 23.0 0.475 1.720 33.0 0.420 2.087 43.0 0.380 2.477 53.0 0.354 2.847 OO 0.296 tlope - 3.93xlO-2 min1, R2-0.997. TableAVD.6. [10b.2/10b.3) = 2.18x1a4 M. [DAMD] = 4.07x10-3 M. lemp. - 20.0°C. 1 (min) A, (X = 425 nm) -ln(ArA..) 0.0 0.806 0.669 3.0 0.743 0.801 15.0 0.626 1.103 27.0 0.549 1.366 39.0 0.488 1.640 51.0 0.447 1.877 63.0 0.412 2.137 75.0 0.387 2.375 87.0 0.369 2.590 OO 0.294 slope = 2.07x10-2 mi,,-!. R2 . Q.996. Table AVTJ 7. [10b.2/10b.3] = 2.18xl04 M. [DAMD] = 1.62x10-2 M, temp. = 20.0"C. I (min) A, (X = 425 nm) -ln(A,-A„) 0.0 0.791 0.650 2.5 0.669 0.916 7.5 0.527 1.355 12.5 0.440 1.766 17.5 0.338 2.674 22.5 0.350 2.513 27.5 0.324 2.900 32.5 0.309 3.219 OO 0.269 slope * 7.88x10-2 mi,,-!, R2 „ 0.998. Table AVU 8. Activation parameters of reaction 4.6 and 4.7. T(K) k (xlO2 M's 1) 1/T(xl03 K') lnk/T 288.15 5.80 3.47 -8.511 293.15 8.06 3.41 -8.199 298.15 9.33 3.35 -8.070 303.15 12.3 3.30 -7.810 308.15 16.6 3.25 -7.526 AVU1 Kinetic data of the oxidative addition of DMAD to Pt2l2(u.-PN2)2 (HH), lOb.l, in C H 2 C I 2 . TableAVmi. [lOb.l] - 1.99xl(H M, [DAMD] = 8.13x10-3 M, temp. - 25.0°C. t (min) A, (X»425nm) -ln(A,-A„) 0.0 0.712 0.764 2.5 0.643 0.924 6.5 0.572 1.121 10.5 0.521 1.291 14.5 0.473 1.483 18.5 0.438 1.650 22.5 0.407 1.826 26.5 0.383 1.988 30.5 0.361 2.163 OO 0.246 slope - 4.37xl0"2 min', R 2 « 0.999. Table AVfflZ [10b. 1] = 1.99x10-* M, [DAMD] = 4.07x10-3 M, lemp. - 25.0°C. t(tnin) A, (X = 425 nm) -ln(A,-A.) 0.0 0.713 0.819 3.7 0.665 0.934 13.7 0.576 1.191 23.7 0.512 1.427 33.7 0.461 1.666 43.7 0.424 1.884 53.7 0.394 2.104 63.7 0.370 2.323 OO 0.272 slope = 2.29x10-2 min', R2 = 0.999. Table AVDJ3. llOb.l] = 1.99x10-* M, [DAMD] = 1.62xl0-2 M, temp. = 25.0°C. t (min) A, (X = 425 nm) -ln(ArA») 0.0 0.706 0.730 2.5 0.597 0.986 6.5 0.489 1.328 10.5 0.415 1.655 14.5 0.363 1.973 18.5 0.327 2.273 22.5 0.300 2.577 26.5 0.280 2.882 OO 0.224 slope = 7.85x10-2 min1. R2 = 0.999. Table AVffl4. • [lOb.l] = 1.99x10* M, [DAMD] = 1.22xl02 M. lemp. = 25.0°C. t (min) A, (X = 425 nm) -ln(A1-A») 0.0 0.727 0.753 2.5 0.641 0.955 7.5 0.528 1.302 12.5 0.449 1.645 17.5 0.401 1.931 22.5 0.361 2.254 27.5 0.332 2.577 OO 0.256 slope = 6.37xl0-2 min1. R2 = 0.999. TableAVmS. [lOb.l] = 1.99x10-* M, [DAMD| = 2.04xl02 M. lemp. » 25.0°C. t(min) A, (X = 425 nm) -ln(A,-A«.) 0.0 0.707 0.730 2.5 0.589 1.011 7.5 0.419 1.640 J2.5 0.337 2.189 17.5 0.288 2.765 22.5 0.261 3.324 27.5 0.245 3.912 0.225 slope » 1.15x10-' min'. R2 = 1.000. Table AVDI6. [lOb.l] •= 1 99x10 4 M. (DAMD] = 2.44xl0"2 M. temp. - 25.0°C. t(min) A, (X = 425 nm) -ln(A,-A») 0.00 0.725 0.724 2.12 0.598 1.027 5.12 0.482 1.419 8.12 0.408 1.784 11.12 0.350 2.207 14.12 0.323 2.489 17.12 0.298 2.847 OO 0.240 slope = 1.21x10-' min', R2 = 0.998. TableAVffl7. |10b.l] = 1.99xl04 M. (DAMD| = 2.71xl02 M. temp. = 25.0°C. t (min) A, (X = 425 nm) -ln(A,-A.,) 0.00 0.718 0.709 2.12 0.571 1.064 5.12 0.453 1.483 8.12 0.380 1.871 11.12 0.331 2.254 14.12 0.296 2.659 17.12 0.276 2.996 0.226 slope = 1.31x10-' min'. R2 = 0.998. TablcAVU18. [10b.1] = 1.99xl0« M. [DAMD] = 3.26xlO'2 M. temp. = 25.0°C. t (min) A, (X = 425 nm) -ln(A,-A_) 0.00 0.707 0.717 2.12 0.551 1.103 5.12 0.412 1.645 8.12 0.338 2.129 11.12 0.294 2.590 14.12 0.266 3.058 17.12 0.250 3.474 OO 0.219 slope = 1.61x10-' min'. R2 = 0.998. A1X 1.Calculation of the AG° and AH° values for Eq. 6.1,* and the AO values at different temperatures. maleic acid + H2O » R, S-malic add crystalline liquid crystalline -149.40 -56.75 -2U.45b -188.94 -68.37 -264.27 AC-o = -5.3 kcal/mol AH° = -6.96 kcal/mol According to the Gibbs-Helmholtz equation: 3(AG/T) AH° AG values at 100 and 170°C are calculated to be -4.9 and -4.5 kcal/mol, (sj respectively. (a) The AG(° and AHf° values are taken from: 1) Dean, LA. Handbook of Organic Chemistry, McGraw-Hill: New York, 1987; S-22, S-26. 2) Kaye, G.W.C.; Laby, T.H. Tables of Physical and Chemical Constants, 15th ed.; Longman: London, 1986; p.268. (b) The AOf0 value for R, S-malic acid is unavailable; the value used in calculation is that for S-malic acid. Standard states: AGr3 (kcal/mol) AHfO (kcal/mol) 2. Visible absorption spectral parameters of CTCI36H2O solutions at various concentrations" Table ADC 1. Visible absorption spectral data for C1CI3-6H2O solutions at various concentrations lCrl(M) 410 nm 572 nm 6410*572 0.007 0.148 0.128 1.16 0.014 0.277 0.240 1.16 0.021 0.372 0.319 1.17 0.028 0.485 0.418 1.16 0.035 0.605 0.523 1.16 (a) Solutions in this series of experiments had been heated for 30 min at 100°C prior to the measurements; the ionic strength and acidity of the solutions were held relatively constant using 0.02 M HNO3 and 0.20 M KNO3; pH = 2.0. [Cr](M) Fig. A 1. Plot of absorbance against [Or] for CrCl3-6H20 solutions. 3. Determination of compositions and formation constants of maleato and malato Cr3* complexes in aqueous solution at 1U0°C. Table AIX 2. Absorbances at various Cr/1H2MA| ratios" Vcr (mL) V H 2 M A (mL) iaii/(|Cr)i • |H 2 MA)i) A 5 7 2 5.0 0.0 1.0 0.47 4.0 1.0 0.8 0.58 3.5 1.5 0.7 0.60 3.0 2.0 0.6 0.61 2.5 2.5 0.5 0.54 2.0 3.0 0.4 0.52 1.5 3.5 0.3 0.47 1.0 4.0 0.2 0.30 0.0 5.0 0.0 0.0 (a) (Cr^ +Jj = [H2MA]; = 0.035 M. Absorbances (A) were recorded at r.t, after (he solutions were heated at I00°C for 30 min. The solution pH (-2.0) and ionic strength were held relatively constant using 0.02 M HNO3 and 0.20 M KNO3. 0.8 " 0.0 0.2 0.4 0.6 0.8 1.0 laii/UCrJi + lrhMAJi) Fig. A.2. Job's plot; absorbance al 572 nm vs. lCr|i/((Cr)i + (H2MA|i); data from Table AIX 2. Table AIX 3. Formation constants calculated based on the data from Fig. A.2 at selected molar ratios, pH = 2.0. [Cr], ICrQlMA)!^ (CrJ. + lHjMAiJj [Cr]^ + ICKHMA)).,, ( % ) 0.50 71.6 8.3X102 0.45 76.9 7.6x102 0.40 81.7 7.6x102 0.35 87.0 9.0x102 K, =8.1 xlO2 M ' Table AIX 4. Absorbances at various |Crl/|H2mal) ratios* Ver(mL) Venial (mL) lCHi/([Q|i + |H2malli) A562 5.0 0.0 1.0 0.44 4.0 1.0 0.8 0.56 3.5 1.5 0.7 0.56 3.0 2.0 0.6 0.57 2.5 2.5 0.5 0.57 2.0 3.0 0.4 0.52 1.5 3.5 0.3 0.42 1.0 4.0 0.2 0.30 0.5 4.5 0.1 0.17 0.0 5.0 0.0 0.0 (a) |Cr 3 +li = |H2mal|i » 0.035 M. Absorbances (A) were recorded at r.i, after the solutions were heated at 100"C for 30 min. The solution pH (-2.0) and ionic strength were held relatively constant using 0.02 M HNO3 and 0.20 M KNO3. Tabic AIX 5. Formation constants calculated based on the data from Fig. A.3 at selected molar ratios, pH = 2.0. tali .cvtffa^u ~ " i&]i+[H2mal)]i ICrl., + ICKHmal)]^  w K l 0.50 67.9 9.8xl03 0.45 72.6 8.8x103 0.40 76.9 8.5x103 0.35 82.4 9.6x103 K 2 = 9.2x105 M 1 Table AIX 6. Absorbances at various lCr|/|H2mal| ratios* Vcr(mL) Vn2mnl ('nL) |Cr|i/(|Crli+|H2mall,) A562 4.0 1.0 0.8 0.58 3.5 1.5 0.7 0.63 3.0 20 0.6 0.64 2.5 2.5 0.5 0.62 2.0 3.0 0.4 0.53 1.5 3.5 0.3 0.48 1.0 4.0 0.2 0.31 0.5 4.5 0.1 0.19 (a) |Cr3+|, = |H2mal]j = 0.035 M. Absorbances (A) were recorded at r.t, after the solutions were healed at 100°C for 30 min. The solution pH (-2.4) and ionic strength were held relatively constant by 0.01 M IINO3 and 0.20 M KNO3. / 00 0.2 o.4 0.6 0.8 1.0 laii/aaii + IHamalli) Fig. A.4. Job's plot; absorbance at 562 nm vs. lCry([Cr|, + [l^ mal),); data from Table AIX 6. Table AIX 7. Formation constants calculated based on the data from Fig. A.3 at selected molar ratios, pH = 2.4. Z M Z ICWUmaft^. ~ " ICrlj+U^maDIi [ & [ „ . + ICrQlmal)]^ ™ * 2 0.50 76.5 8.7x10s 0.45 81.0 7.2x103 0.40 87.7 9.0X103  0.35 93J) 1.1x10* K 2 = 9.0x103 NI' Sample calculation for the formation constant: The equilibrium concentration for the maiaic monoanion |Hmal| is calculated using the K',, expression, i.e. K.JHjtnal],,,, to ^i. Thus, to |Hmal] = -[Cr(Hmal)| lCrfHmal)](KJ + IH*]) 2 [Cr][Hmal) • .„ ..„ .. K.jCrJIHjmall.o, |Cr] = [H2mal|u,(= Cj - |Cr(Hmal)J, where C, is the initial concentration for both |Cr| and [H2mal|ioi as they are mixed in a 1:1 ratio. From the Job's plot, the percentage of the undissociated complex at the equilibrium is determined to be 76.5%, i.e. another 23.5% Cr3* is present as the aquated ion. Therefore, based on the data in Table AIX 6, „ _ 0.765x0.0175(10 3 4+ 10 J 4) 2 7 T 4 (0.235x0.0175) xlO = 8.7x103 (M-l) Similarly, at other molar ratios, K2 values are calculated and given in Table AIX 7, the averaged K2 value is 9.0x I03 M 1 . 

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