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Syntheses, kinetic and homogeneous hydrogenation studies of ditertiary phosphine rhodium(I) complexes Fung, Dawning Chui Mun 1988

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SYNTHESES, KINETIC AND HOMOGENEOUS HYDROGENATION STUDIES OF DITERTIARY PHOSPHINE RHODIUM(I) COMPLEXES by DAWNING CHUI MUN FUNG B.Sc, University of London, England, 1982 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY . in THE FACULTY OF GRADUATE STUDIES CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF JUNE ® DAWNING CHUI BRITISH COLUMBIA 1988 MUN FUNG, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of CHEMISTRY The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6(3/81) A B S T R A C T The original purpose of this work was to investigate the catalytic properties of a series of Rh 2(CO) 4(P-P) 2 complexes (where P-P = ditertiary phosphines of the type PR 2(CH 2) nPR 2 > R = alkyl or aryl) for hydroformylation. The preparation of Rh 2(CO) 4(P-P) 2 involves the synthesis of Rh(P-P)2Cl, followed by reaction with NaBH^ to give RhH(P—P)2> which when treated with CO in benzene yields Rh 2(CO) 4(P-P) 2 > as reported in the literature. The dimer, Rh2(CO)4(dpp)2> where dpp = PPh2(CH2) 3PPh 2 > was prepared and examined for its interaction with H 2 > and H 2 /CO, in order to test its capabilities for catalytic homogeneous hydroformylation. The interaction of Rh2(CO)4(dpp)2 (42) with H 2 > and the reaction of CO with RhH(dpp)- (£2) to yield 42, are summarized as follows: P » - Rft—R c6 g /° H H 2 unknown ^ ' _ ^ h y d f i d M *=* oc -RJ-co 49 co ||-co (><> 52 -co CO / \ - P ^ P / V P W P 50 59 -50// 50 H H „ _4h — P ^ P — i J h - C O P* * P P P P ^ 51 P ^ All the species shown, except .52, have been detected by H and Pf. H} NMR spectroscopy. Formation of the monomeric hydride, iQ, from 42, occurs at high [dpp]. ii The reaction of Rh 2(CO) 4(dpp) 2 and 6 equivalents dpp with synthesis gas ( H 2 : CO = 1 : 1) gives initially 5J) and R l ^ C O ^ d p p ^ reforms after 30 minutes of interaction. This is consistent with the previous finding of low turnover rate for hydroformylation of 1-hexene using as catalyst the co-ordinatively saturated Rh 2(CO) 4(dpp) 2. Treatment of 52 in toluene with - 1 atm CO, followed by treatment with -1 atm H 2 > sets up the following equilibria (where dpp* = monodentate dpp): 4 CO, -H. H. 2 RhH(dpp) 2 4 i > Rh 2(CO) 4(dpp) 2 + 2 dpp 4 •1/2 dpp + 2 CO 52 42 1/2 Rh 2H 2(CO) 2(dpp) 3 + RhH(CO)(dpp*)(dpp) The homogeneous hydrogenation of 1-hexene at 31° C, - 1 atm H 2, catalyzed by "the RhH(dpp) 2/CO/H 2 system" in toluene is ascribed to the formation of an unidentified "RhH" from 5D and/or 51. The H2~uptake curve displayed an initial ("inductive") period required for the generation of an active species "RhH", a second period of maximum rate, and a final slowing down period. The mechanism suggested for homogeneous hydrogenation of 1-hexene catalyzed by the "RhH(dpp) 2/CO/H 2 system" is presented as follows: Rhalkyl ' "RhH" + hexane RhH + 1 -hexene k-3 "Rhalkyl" + H s iii The corresponding rate law for the maximum rate, consistent with the kinetic data, is given by: Rate = k 4k 3[H 2][ 1-hexene] ["RhH"] tk_3 + k 4[H 2] +k^[ 1-hexene] where ["RhH']t is total concentration of the active "RhH" catalyst. At high [1-hexene], where k^fl-hexene] >> k_3 + k 4[H 2], the rate law is simplified to: Rate = k 4[H 2]["RhH'] t where ["RhH']^ ~ total rhodium concentration in solution. The values of k^  and k 4 at 31° C were found to be 0.42 M - 1 s - 1 and 20 M - 1 s \ respectively. The Rh(dcpe)2 + X" complexes (X = CI, BF 4, PF &; dcpe PCy 2(CH 2) 2PCy 2) were prepared and found to have no reactions with NaBH 4 or Li A l H 4 > Consequently, the dcpe carbonyl dimer could not be prepared. The Rh(p = p) 2 + C f complex, where p = p = PPh 2C 2H 2PPh 2 > was isolated and characterized; its reaction with NaBH 4 was incomplete, partially generating RhH(p=p) 2. Treatment of the mixture with CO gave partially Rh(CO)(p=p) 2 + C l ~ and another uncharacterized carbonyl complex. A single crystal X-ray structure determination of Rh(dcpe)2 + C l ~ showed that the geometry around Rh is distorted square planar. Also, the extremely air-sensitive species [RhCl(dcpe)- solv] f i (solv = THF or 0.1 C 6H 6) and RhCl(dcpe)(CH2Cl2)« were isolated. The interaction of Rh(dcpe)2 + Cl with small gas molecules was studied in order to test its potential as a catalyst. There is interaction between Rh(dcpe)2 + Cl~" and HCI, C l 2 > and CO, in CH 2C1 2. The reaction with HCI to give cis-RhHCl(dcpe) 2 + Cl is extremely rapid. The use of stopped-flow kinetics and UV-VIS spectrophotometric techniques at 25° C gave an equilibrium constant of iv 4.2 x 10 M for the reaction. The forward reaction was first-order in both [Rh(dcpe)2 + Cl ] and [HCI], indicating a concerted oxidative addition reaction. The RhHCl(dcpe) 2 + species reacts further with HCI to give RhHC^dcpe) and the diphosphonium salt, dcpe(HCl)2. The Rh(dcpe)2 + Cl complex reacts with C l 2 to give RhCl 2(dcpe) 2^Cl , which was also obtained by prolonged treatment of RhHCl(dcpe) 2 + C f with CDC\y The reaction of Rh(dcpe)2 + C f with CO to give Rh(CO)(dcpe)2 + C f yielded k Q n and k ff values of 2.2 x l ( f 2 M _ 1 s" 1 and 5.02 x 10 ^ s \ respectively at 25° C. The Rh(dcpe)2 + Cl complex was inactive as a catalyst for decarbonylation of benzaldehyde, or hydrogenation of 1-hexene. v T A B L E OF CONTENTS Abstract ii Table of Contents vi List of Figures xi List of Tables xviii Abbreviations and Symbols xx Acknowledgements xxiv Chapter 1. INTRODUCTION 1 1.1. General Introduction 1 1.2. Scope of this Thesis 3 1.3. Homogeneous Hydrogenation 6 1.3.1. Hydrogen Activation 8 1.3.2. Mechanisms of Homogeneous Hydrogenation 13 1.3.2.1. Monohydride Catalysts 14 1.3.2.2. Dihydride Catalysts 16 1.3.2.3. Free Radical Pathways 20 1.3.3. Asymmetric Hydrogenation 21 1.4. Homogeneous Hydroformylation 24 1.4.1. Cobalt Catalyzed Hydroformylation Reactions 25 1.4.2. Rhodium Catalyzed Hydroformylation Reactions 25 1.4.3. Asymmetric Hydroformylation 29 1.5. Homogeneous Decarbonylation 33 1.6. Binuclear Rhodium Complexes in Homogeneous Hydrogenation .... 41 1.7. The R h ( P - P ) 2 + Systems in Homogeneous Catalysis 42 Chapter 2. Experimental 46 2.1. Materials 46 2.1.1. Solvents 46 2.1.2. Gases 46 2.1.3. Phosphines 47 2.1.4. Diphosphonium Salt 47 2.1.4.1. Diphosphonium Salt of l,2-Bis(dicyclohexylphosphino)ethane, dcpe(HCl) 2 47 2.1.5. Alkene Substrates 48 2.1.6. Inorganic Silver Salts 48 2.1.7. Rhodium Compounds 49 2.1.7.1. Dichlorotetrakis(cyclo- octene)dirhodium(I), [RhCKCOE)^ 2 49 2.1.7.2. Bis[l,3-bis(diphenylphpsphino)propane]rhodium(I) chloride, Rh(dpp) 2 + C f 49 vi 2.1.7.3. Bis[l,3-bis(diphenylphosphino)propane] rhodium(I) hexafluorophosphate-0.3CH-CL, Rh(dpp) 2 + P F g .0.3CH 2Cl 2 50 2.1.7.4. Carbonylbis[l,3-bis(diphenylposphino)propane] rhodium(I) hexafluorophosphate, Rh(CO)(dpp)2 + P F 6 51 2.1.7.5. Hydridobis[l,3-bis(diphenylphosphino)propane] rhodium(I), RhH(dpp) 2 51 2.1.7.6. Tetra(carbonyl)bis[l,3-bis(diphenylphosphino) propane] dirhodium(O)' C..H,-, Rh 2(CO) 4(dpp) 2.C 6H 6 52 2.1.7.7. Carbonylhydridotris(lriphenylphosphine)rhodium(I), RhH(CO)(PPh 3) 3 53 2.1.7.8. Bis[l,2-bis(dicyclohexylphosphino)ethane] rhodium(I) chloride, Rh(dcpe)2 CI 53 2.1.7.9. Bis[l,2-bis(dicyclohexylphosphino)ethane] rhodium(I) tetrafluoroborate • 1/2CH.CL, Rh(dcpe)2 + BF 4 • 1/2CH2C12 54 2.1.7.10. Bis[l,2-bis(dicyclohexylphosphino)ethane] rhodium(I) hexafluorophosphate, Rh(dcpe)2 + PF 6 56 2.1.7.11. Chlorohydridobis[l,2-bis(dicyclohexylphosphino) ethane]rhodiurn(III2 chloride* 0.3CfiHfi, RhHCl(dcpe) 2 + Cl • HCI- OJCgHg ....?. 57 2.1.7.12. Dichlorobis[l,2-bis(dicyclohexylphosphino) ethane] rhodium(III) chloride, RhCl 2(dcpe) 2 CI 58 2.1.7.13. Reactions between [RhCKCOE)^ 2 and 2 Equivalents of dcpe .^.....^  59 2.1.7.14. Reactions between Rh(dcpe)2 CI and Gases/Reagents, including CO, H ?, HCI, N 2 > 0 2, LiAlH 4, and NaBH 4 62 2.1.7.15. Reactions of Rh 2(CO) 4(dpp) 2 under Vacuum 64 2.1.7.16. Attempted Synthesis and Isolation of RhH(CO)(dpp*)(dpp) 65 2.1.7.17. Bis[l,2-bis(diphenylphosphino)ethylene] rhodium(I) chloride- C H X L , Rh(p=p) 2 + 6".CH2C12 66 2.1.7.18. Hydridobis[l,2-bis(diphenylphosphino)ethylene] rhodium(I), RhH(p=p) 2 67 2.1.7.19. Reaction of a Mixture of Rh(p=p) ? + C f and RhH(p=p), with CO 69 vii 2.1.7.20. Reaction between Rh(p = p)~ C f • CH-CL and CO ........ 69 2.2. Instrumentation 70 2.3. Gas-uptake Apparatus 73 2.3.1. The Apparatus 73 2.3.2. Procedure for Catalytic Hydrogenation Experimental Run 75 2.3.2.1. In Situ Hydrogenation of 1-Hexene using the RhH(dpp)2/CO Catalyst System 75 2.3.2.2. Hydrogenation of 1-Hexene using Rh(dcpe), Cl or Rh 2(CO) 4(dpp) 2 as Catalyst "Precursor 78 2.4. Stoichiometric Gas-Uptake 79 2.5. Measurements of Gas Solubilities 79 . 2.6. The Catalysis Apparatus and Conditions for Attempted Decarbonylation of Benzaldehyde 84 2.7. Isolation and Detection of Hydrogenated Alkene Products or Decarbonylated Aldehyde Products by GC 86 Chapter 3. Syntheses, Characterization, and Reactions of l,2-Bis(dicyclohexylphosphino)ethane Complexes of Rhodium(I) 88 3.1. Introduction 88 3.2. Reactions of [RhCl(COE)J ? with 2 or 4 Equivalents of dcpe T „ 90 3.3. X-Ray Structure Determination of Rh(dcpe)2 Cl Bis[l,2-bis(dicyclohexylphosphino)ethane^rhoaium(I) chloride 102 3.4. Attempted Reactions between Rh(dcpe)-) Cl and NaBH., LiAlH 4, H 2 > 0 2 , and N 2 .. 108 3.5. Reactivity of Rh(dcpe)2 + C f toward CO 109 3.5.1. Formation of Rh(CO)(dcpe)2 + 109 3.5.2. Spectrophotometric Kinetic Studies on the CO Interaction with Rh(dcpe)2 I l l 3.6. Reaction of Rh(dcpe)2 + C f with HCI 120 3.6.1. Characterization of RhHCl(dcpe) 2 + 120 3.6.2. Stopped-flow Kinetic Studies on the Reaction of HCI with Rh(dcpe)2 + C f 124 3.6.3. Further Reaction of RhHCl(dcpe) 9 + C l " with HCI 135 3.7. Attempted Use of Rh(dcpe)2 Cl in Homogeneous Catalysis .... 142 3.8. Conclusions 143 Chapter 4. Formation of RhH(CO)(dpp*)(dpp) from Rh ?(CO) 4(dpp) ?, RhH(dpp) 2, and RhH(CO)(PPh 3) 3 1 .7. 145 4.1. Introduction 145 4.2. Synthesis and Properties of Rh 2(CO) 4(dpp) 2 146 viii 4.2.1. Synthesis of Rh 2(CO) 4(P-P) 2 146 4.2.2. Properties of Rh 2(CO) 4(dpp) 2 (42) 147 4.3. Reactions of Rh 2(CO) 4(dpp) 2, in the Presence of Various Equivalents of dpp, with l-L, and H 2/CO 153 4.3.1. Formation of RhH(CO)(dpp*)(dpp) from Rh 2(CO) 4(dpp) 2 153 4.3.1.1. Characterization of RhH(CO)(dpp*)(dpp) 153 4.3.2. Attempted Isolation of RhH(CO)(dpp*)(dpp) 166 4.3.3. Reactions of Rh 2(CO) 4(dpp)- and n Equivalents of dpp (n = 0, 1, 2, 6, 20) in the Presence of H 2 and H 2/CO 172 4.3.4. Gas-uptake Studies 181 4.4. Formation of RhH(CO)(dpp*)(dpp) from RhH(CO)(PPh 3) 3 181 4.4.1. NMR data 184 4.4.2. FT-IR data 190 4.4.3. Discussion 190 4.5. Generation of RhH(CO)(dpp*)(dpp) from RhH(dpp) 2 194 4.5.1. Titrations with CO 194 4.5.2. Sequential Addition of CO and H 2 to RhH(dpp) 2 196 4.6. Possible Mechanism for the Interaction of Rh 2(CO) 4(dpp) 2 plus Various Equivalents of dpp with H 2 and the Reverse Reactions 199 4.7. Conclusion 204 Chapter 5. Homogeneous Hydrogenation of 1-Hexene via In Situ Generation of RhH(CO)(dpp )(dpp) and Rh 2H 2(CO) 2(dpp) 3 as Catalyst Precursors .... 205 5.1. Introduction 205 5.2. Catalytic Hydrogenation of 1-Hexene 206 5.2.1. Homogeneous Hydrogenation of 1-Hexene Using Rh 2(CO) 4(dpp) 2 as Catalyst 206 5.2.2. Homogeneous Hydrogenation of 1-Hexene Using the In Situ RhH(dpp) 2/CO/H 2 Catalyst System 207. 5.2.2.1. Preparation of Catalyst Solution 207 5.2.2.2. Gas-uptake, GC, and NMR Studies 208 5.3. Kinetic Data Obtained for the Catalytic Hydrogenation of 1-Hexene Using the RhH(dpp) 2/CO/H 2 System 218 5.3.1. Initial Region 218 5.3.2. Maximum Rate Region 222 5.4. Analysis of Kinetic Results, and Discussion 235 5.4.1. Initial period 235 5.4.2. Maximum Rate Period 236 Chapter 6. General Conclusions and Recommendations for Future Work 244 ix References Appendix LIST OF FIGURES Figure 1.1. The uncatalyzed addition of H 2 to an olefin via (a) concerted or (b) stepwise pathways .'. 7 Figure 1.2. (a) Symmetry-forbidden addition of H2 to a double bond, (b) Metal-catalyzed allowed concerted addition of two H atoms to a double bond 8 Figure 1.3. Approach of Ft 2 to the transition metal centre M 10 Figure 1.4. Transition state for the heterolytic cleavage of H 2 13 Figure 1.5. Mechanisms for monohydride hydrogenation catalysts. Two pathways (A) and (B) are indicated 15 Figure 1.6. Mechanisms for dihydride hydrogenation catalysts 17 Figure 1.7. Mechanism for homogeneous hydrogenation of 1-hexene using [Rh(NBD)(diphos)]BF4 as a catalyst 19 Figure 1.8. Free radical mechanism in homogeneous hydrogenation of aryl substituted olefins * 22 Figure 1.9. Chiral ditertiary phosphine ligands 24 Figure 1.10 Equilibria of Rh carbonyls under H 2 and CO 26 Figure 1.11. Associative (A) and dissociative (D) pathways of the hydroformylation of alkenes 27 Figure 1.12. Acyl species resulting from the reaction of RhH(CO)(PPh 3) 2 with 1-octene under CO 28 Figure 1.13. Mechanism of asymmetric hydroformylation catalyzed by the RhH(CO)(PPh 3) 3/(-)diop system 31 Figure 1.14. Structure of [(-)BPPM]PtCl(SnCl 3) 32 Figure 1.15. Suggested mechanism for the stoichiometric decarbonylation of aldehydes with RhCl(PPh 3) 3 35 Figure 1.16 Possible mechanism for the decarbonylation of aldehydes using Rh(P—P )2 + as catalyst, where P—P is dpe or dpp (species are mono-cations throughout) 37 Figure 1.17. Mechanism for intramolecular hydroacylation and decarbonylation 39 xi Figure 1.18. Mechanism for the stoichiometric decarbonylation of acid chlorides with RhCl(PPh 3) 3 40 Figure 1.19. Mechanism for the hydrogenation of olefins using Rh(diop)2 + as catalyst 44 Figure 2.1. Schematic representation of an anaerobic spectral cell 71 Figure 2.2. Schematic representation of the constant pressure gas-uptake apparatus 74 Figure 2.3. Reaction flask used for the measurements of gas solubilities in different solvents 80 Figure 2.4. Hydrogen solubility in toluene at 31, 26, and 18° C, and specific pressures 81 Figure 2.5. The apparatus set-up used for studying decarbonylation of aldehydes 85 Figure 3.1. (a) ^P{^H] NMR spectrum of the complex isolated from the substitution reaction of [RhQ(COE)J- with 2 equivalents of dcpe in CH 2C1 2 .i„ 93 Figure 3.1. (b) "^P^H} NMR spectra of the complexes isolated from the substitution reactions of [RhCl(COE)~L with 2 equivalents of dcpe in C 6 H 6 „.... 95 Figure 3.1. (c) ^ P{^H} NMR spectrum of the complex isolated from the substitution reaction of [RhCl(COE)J ~ with 2 equivalents of dcpe in THF : ...... 97 Figure 3.2. Substitution reactions between [RhCl(COE)2] 2 and dcpe (2 or 4 equivalents) in CH2CI2, C6H5 or THF, based on the characterization of the isolated products 98 Figure 3.3. The 3 1P{ 1H] NMR spectrum of Rh(dcpe)2 + BF 4" • 1/2CH 2C1 2 in Cfl^C^/dg- acetone at 19° C under Ar (the same spectrum for Rh(dcpe)2 + C r was recorded at 19° C and -84°C) 101 Figure 3.4. Stereoscopic view of the Rh(dcpe) 2 + cation, and the numbering scheme used; 50% probability thermal ellipsoids are shown and hydrogen atoms have been omitted for the sake of clarity 104 Figure 3.5. Decrease in v(CO) of the Nujol mull of Rh(CO)(dcpe)2 + C f on subjection of the solid to vacuum 110 Figure 3.6. Changes in UV-VIS absorbance for the reaction of Rh(dcpe)2 + C l " with CO in C H j C l j at 25° C, and 1 atm total pressure 113 xii Figure 3.7. Analysis of data from Figure 3.6 in terms of an equilibrium reaction composed of a forward pseudo first-order carbonylation reaction, opposed by a first-order decarbonylation process. Absorbance measured at 403.6 nm. .114 Figure 3.8. Changes in absorbance for the decarbonylation (via bubbling of Ar) of the in situ formed Rh(CO)(dcpe)2 + C l " (from [Rh] = 0.456 mM, and [CO] = 2.34 mM) in CH 2 C1 2 at 25° C 116 Figure 3.9. Analysis of data from Figure 3.8 in terms of an irreversible loss of CO from Rh(CO)(dcpe)2 + C r 117 Figure 3.10. Changes in absorbance of FT -IR spectrum at 1993 cm ^ after CO was introduced into a CH2CI2 solution of Rh(dcpe)2 + C l ~ at 18° C (absorbance has been corrected for solvent contribution) 119 Figure 3.11. 3 1P{ 1H] NMR spectrum of RhHCl(dcpe) 2 + in CD 2Cl2 (1-5 x 10" 2 M) under vacuum at (a) 22°C, (b) -49 C, (c) -89°C 123 Figure 3.12. A typical curve obtained from the stopped-flow kinetic studies at [Rh] = 4.97 x 10 - 5 M, [HCI] = 4.50 x 10" 2 M, and 18.3°C, due to the disappearance of absorbance of Rh(dcpe)2 + C l " at 403.6 nm, where RhHCl(dcpe)2 + C l ~ has negligible absorbance (see Figure 3.15 later) 126 Figure 3.13. A plot of the dependence of k , on [HCI] at 18.3° C, [Rh] = 4.97 x 10" 5 M ° ! 129 Figure 3.14. 3 1P[ 1H} NMR spectra of RhHCl(dcpe) 2 + in DMA under Ar, at ambient temperatures recorded at various times 132 Figure 3.15. Changes in UV-VIS spectrum due to the reaction of Rh(dcpe)2 + C r (5.80 x 10" 4 M) with ~1 atm HCI in CH-CL at 18° C 133 Figure 3.16. Possible approaches of HCI toward Rh(dcpe)2 + Cl in CH 2 C1 2 136 Figure 3.17. Changes in the high field ^H NMR spectrum with time, when RhHCl(dcpe) 2 + C r in CDC13 is exposed to - 1 atm HCI 138 Figure 3.18. (a) lH NMR and (b) 1H{ 3 1P} NMR spectra of high field hydrides, pertaining to the interaction of Rh(dcpe)2 + C l - with ~1 atm HCI in CD 2C1 2 139 31 1 Figure 3.19. P{ HI NMR spectrum for the in situ reaction of Rh(dcpe)2 + G -with HCI ( ~1 atm) in CH2CI2; the solution was transferred into an NMR tube under Ar after a reaction time of (a) 10 min and (b) 2 h 140 Figure 3.20. Proposed pathways for the interaction of HCI with Rh(dcpe)2 + Cl 142 Figure 4.1. FT-IR spectra (Nujol, Csl plates) of (a) Rh2(CO)4(dpp)2, and (b) Rh2(CO)4(dpp)2 after being subjected to vacuum for 1 h 148 xiii / Figure 4.2. FT-IR spectrum (Nujol, Csl plates) of a yellow compound, Rh2(CO)2(dpp)2, isolated via addition of hexanes to a vacuum-concentrated solution of Rh2(CO)4(dpp)2 in C F ^ C ^ 150 Figure 4.3. FT-IR spectrum of a brown solid isolated by evaporation to dryness of a CH 2C1 2 solution of Rh2(CO)4(dpp)2: a mixture of Rh?(CO)7(dpp)~ and Rh2(M-CO)2(dpp)2 ... 7. 151 Figure 4.4. Proposed structures for RhH(CO)(dpp*)(dpp), 5Q, and Rh2H2(CO)2(dpp)3, 51 154 Figure 4.5. (a) P{ H} NMR spectra (121.4 MHz) for the reaction of Rh2(CO)4(dpp)2 and 2 equivalents of dpp with H 2 at ambient temperatures for 10 min ; the complicated spectrum in the region 25-35 ppm is due to an overlap of the spectral features of Rh2H2(CO)2(dpp)3, RhH(CO)(dpp*)(dpp) and RhH(dpp)2; [RhJ = 20 mM, [dpp] = 42 mM. 158 Figure 4.5. (b) P{ H] NMR spectra (121.4 MHz) for the reaction of Rh2(CO)4(dpp)2 and 6 equivalents of dpp with H 2 at ambient temperatures for 25 min; the spectrum is more simple compared to that in (a), due to a conversion of Rh2H2(CO)2(dpp)3 into RhH(CO)(dpp*)(dpp); [Rhj] = 24 mM, [dpp] = 150 mM 159 Figure 4.6. ^P^H} NMR computer simulated spectra for the mixture of hydrides, RhH(dpp)2, Rh2H2(CO)2(dpp)3, and RhH(CO)(dpp*)(dpp), at 121.4 MHz in CD2Cl2, according to the information from the spectrum in Figure 4.5 (a) 160 Figure 4.7. 1 H NMR spectrum of 5Q (Rh2(CO)4(dpp)2 with 5.9 equivalents of dpp in CD 2C1 2 at ambient temperatures under H 2 for 15 min) (400 MHz, high field region only); [Rhj] = 24 mM, [dpp] = 150 mM 161 Figure 4.8. (a) ~H NMR spectrum of Rh2(CO)4(dpp)2 with 2 equivalents of dpp under H 2 for 15 min in CD 2C1 2 (400 MHz, high field region only); [RI12] = 20 mM, [dpp] = 42 mM, (b) V i ? } NMR spectrum of the system shown in Figure 4.8 (a) 163 Figure 4.9. (a) 1 H NMR spectrum of a 1 : 0.6 mixture of RhH(CO)(dpp*)(dpp) and Rh2H2(CO)2(dpp)3 in CD2C12, obtained from the Rh2(CO)4(dpp)2/2dpp/H2 system (300 MHz, high field region only; RhH(dpp)2, 52, is not included in this spectrum), (b) Computer simulated spectrum of (a) (300 MHz) 164 Figure 4.10. FT-IR spectrum of Rh2(CO)4(dpp)2 under CO in CH 2C1 2 165 Figure 4.11. FT-IR spectrum of Rh2(CO)4(dpp)2 and 6 equivalents of dpp, dissolved in CH 2C1 2, after bubbling with H 2 for 30 min 165 xiv Figure 4.12. FT-IR spectrum of Rh2(CO)4(dpp)2 and 6 equivalents of dpp, after bubbling the solution with D 2 for 1 h in CFL^C^ 166 Figure 4.13. Proposed synthetic route for RhH(CO)(dpp*)(dpp) 166 Figure 4.14. FT-IR spectrum (Nujol mull, Csl plates) of an isolated mixture of RhH(dpp)2, RhH(CO)(dpp*)(dpp), and Rh 2H 2(CO) 2(dpp) 3 169 Figure 4.15. 3 1Pf 1H} NMR spectrum of Rh2(CO)4(dpp)2/6dpp in CH 2Cl2 under CO at ambient temperatures; [Rh^] = 27.3 mM, [dpp] = 167 mM 176 Figure 4.16. NMR spectrum for the reaction between Rh2(CO)4(dpp)2 (with no added dpp) in CyDg at ambient tempratures and H2 for 10 min (high field region only, 400 MHz); [Rh^ = 29.2 mM (Table 4.3, entry i) 176 Figure 4.17. *H NMR spectrum for the reaction of Rh2(CO)4(dpp)2 and 1 equivalent dpp in CD2CI2 at ambient temperatures with H2 for 4 h (high field region, 400 MHz); [RhJ = 23.7 mM, [dpp] = 28.4 mM (Table 4.3, entry ii) .. 177 Figure 4.18. *H NMR spectrum for the reaction of Rh2(CO)4(dpp)2 and 2.4 equivalents dpp in CD 2Cl2 at ambient temperatures with H2 for 12 h (high field region, 400 MHz); [RhJ = 29.2 mM, [dpp] = 65.1 mM (Table 4.3, entry iii) .. 178 Figure 4.19. ^ H NMR spectrum for the reaction of Rh2(CO)4(dpp)2 and 6 or 20 equivalents dpp in CD2CI2 at ambient temperatures with H 2 for 1 h (high field region, 400 MHz); [RhJ = 14.1 mM, [dpp] = 87.5 mM (Table 4.4, entry ii) 179 Figure 4.20. *H NMR spectrum for the reaction of Rh2(CO)4(dpp)2 and 6 equivalents excess dpp in CD 2Cl2 at ambient temperatures with H~ for 14 h (high field region, 400 MHz); [RhJ = 14.1 mM, [dpp] = 87.5 mM (Table 4.4, entry iv) 180 Figure 4.21. Changes in gas stoichiometry for a reaction of Rh2(CO)4(dpp)2 and 6 equivalents of added dpp, with ~1 atm H 2 in toluene (10 mL) at 31° C (using the data in Table 4.5) 183 31 1 Figure 4.22. P{ Hj NMR spectrum for exchange reaction between RhH(CO)(PPh 3)3 and 1.5 equivalents of' added dpp in CD-CL under Ar (Table 4.6, entry 1) 187 31 1 Figure 4.23. P{ HI NMR spectrum for exchange reaction between RhH(CO)(PPh 3) 3 and 5 equivalents of added dpp in C D X L under Ar (Table 4.6, entry 3) .......7. 188 xv Figure 4.24. 3 P{ H} NMR spectrum for exchange reaction of RhH(CO)(PPh3)3 with 5 equivalents of dpp, and 0.2 equivalents of added CO in CD2CI2 under Ar (Table 4.6, entry 5) 189 Figure 4.25. Proposed structures for (a) RhH(CO)(dpp*)(dpp) and (b) Rl^P^CO^dpp)^ by Hughes and Young, and Kastrup et al 191 Figure 4.26. *H NMR spectrum for exchange reaction between RhH(CO)(PPh3)3 and 2 equivalents of added dpe in CD2CI2 under Ar (Table 4.6, entry 7). 193 Figure 4.27. (a) *H NMR spectrum of RhH(dpp), in C D X L ( :H probe : 300 MHz) .. „.....„ 197 Figure 4.27. (b) *H NMR spectrum for the titration of RhH(dpp)- with 0.43 equivalents of CO in CD 2 C1 2 probe : 300 MHz) 7. 197 Figure 4.27. (c) 1H NMR spectrum for the titration of RhH(dpp)- with 0.86 equivalents of CO in CD 2 C1 2 ( :H probe : 300 MHz) 198 Figure 4.27. (d) ^H NMR spectrum for the titration of RhH(dpp)- with 1.3 equivalents of CO in CD 2 C1 2 (lH probe : 300 MHz) 7. 198 Figure 4.27. (e) 1 H NMR spectrum for the titration of RhH(dpp)- with 2.2 equivalents of CO in CD 2 C1 2 (*H probe : 300 MHz) .7. 199 Figure 4.28. Proposed pathways for the interaction of Rh2(CO)4(dpp)2 with H2 in CH2CI2, and its formation from RhH(dpp)2 and CO. 201 Figure 4.29. Reaction of Rh 4(CO) i n[(-)diop] with H 2 203 Figure 5.1. A typical H2- uptake plot for the hydrogenation of 1-hexene in toluene (5 mL), at 31° C using the RhH(dpp)2/CO/H2 catalyst system; [1-hexene] = 0.0478 M, [Rh] = 1.17 mM, [ H j = 3.10 mM 209 Figure 5.2. (a) Dependence of the inhibition time on [Rh] at [1-hexene] = 0.0319 M, [H 2] = 3.10 mM 219 Figure 5.2. (b) Dependence of the inhibition time on [Rh] at [1-hexene] = 0.857 M, [H 2] = 3.10 mM 220 Figure 5.3. Dependence of 1/inhibition time on [1-hexene] (at [Rh] = 1.17 mM, [ H j = 0.0319 M) 221 Figure 5.4. (a) Hydrogen uptake curves at various [Rh] for hydrogenation of 1-hexene at low [1-hexene] catalyzed by the RhH(dpp)2/CO/H2 system in toluene (5 mL) at 31° C ([1-hexene] = 0.0319 M, [ H j = 3.10 mM, [Rh] = 0.219-0.727 mM) ... 223 xvi Figure 5.4. (b) Hydrogen uptake curves at various [Rh] for hydrogenation of 1-hexene at low [1-hexene] catalyzed by the RhH(dpp)2/CO/H2 system in toluene (5 mL) at 31° C ([1-hexene] = 0.0319 M, [ H J = 3.10 mM, [Rh] = 1.17-3.56 mM) 7. 224 Figure 5.4. (c) Hydrogen uptake curves at various [Rh] for hydrogenation of 1-hexene at high [1-hexene] catalyzed by the RhH(dpp)2/CO/H2 system in toluene (5 mL) at 31°C ([1-hexene] = 0.857 M, [ H J = 3.10 mM, [Rh] = 0.273-2.20 mM) 225 Figure 5.5. Hydrogen uptake curves at various [1-hexene] for hydrogenation of 1-hexene catalyzed by the RhH(dpp)2/CO/H2 system in toluene (5 mL) at 31 °C ([Rh] = 1.17 mM, [HJ, = 3.10 mM) 226 Figure 5.6. Hydrogen uptake curves at various [H2] for hydrogenation of 1-hexene at low [1-hexene] catalyzed by the RhH(dpp)2/CO/H2 system in toluene at 31° C ([Rh] = 1.17 mM, [1-hexene] = 0.0319 M) 227 Figure 5.7. Dependence of the second region maximum rate of hydrogenation on [H2] ((a) [Rh] = 1.17 mM, [1-hexene] = 0.0319 M, (b) [Rh] = 0.273 mM, [1-hexene] = 0.857 M) 228 Figure 5.8. (a) Dependence of the maximum hydrogenation rate (second region) on [Rh], at [1-hexene] = 0.0319 M, [ H J = 3.10 mM (injection method) 229 Figure 5.8. (b) Dependence of the maximum rate of hydrogenation on [ R h ] l / 2 (up to [Rh] = 1.17 mM), at [1-hexene] = 0.0319 M, [ H J = 3.10 mM (injection method) 230 Figure 5.8. (c) Dependence of the maximum rate of hydrogenation on [Rh], at [1-hexene] = 0.857 M, [HJ, = 3.10 mM 231 Figure 5.9. Dependence of the maximum rate of hydrogenation (second region) on [1-hexene] ([Rh] = 1.17 mM, [HJ, = 3.10 mM) 232 Figure 5.10. Hydrogen uptake curves for various conditions of hydrogenation employing in situ RhH(dpp) 2/CO/H 2 as catalyst precursor. Curve 3 shows the hydrogenation using RhH(dpp) 2 alone as catalyst (only ~25% reduction to hexane observed in 5 h) 234 Figure 5.11. Proposed reaction pathways for the homogeneous hydrogenation of 1-hexene using the RhH(dpp)2/CO/H2 system; the active catalytic species is labelled as "RhH" 237 Figure 5.12. A plot of 1/rate versus l/[ 1-hexene] (for the maximum rates of hydrogenation at [Rh] = 1.17 mM, [ H J = 3.10 mM) 239 Figure 5.13. A plot of 1/rate versus 1/[H2J (for the maximum rates of hydrogenation at [Rh] = 1.17 mM, [1-hexene] = 0.0319 M) 240 xvii LIST OF TABLES Table 2.1. FAB mass spectrum (high mass portion) of Rh(dcpe)2 Cl 55 Table 2.2. FAB mass spectrum of Rh(p = p) 2 + C l ~ • CH 2 C1 2 68 Table 2.3. Solubility of H 2 in toluene at specific temperatures and pressures 82 Table 3.1. Compounds isolated from the substitution reactions of [RhCl(COE) ?] 2 with 2 equivalents of dcpe per dimer .7. 92 Table 3.2. Bond lengths (A) with estimated standard deviations in parentheses for Rh(dcpe)2 + C f 105 Table 3.3. Bond angles (deg) with estimated standard deviations in parentheses for Rh(dcpe)2 + C r 106 Table 3.4. Kinetic and equilibria data for the binding of CO to Rh(P-P )2 + complexes 121 Table 3.5. 1 H and 3 1P{ 1H1 NMR data for various RhHCl(P-P)~ + complexes at 25° C, in CDC13 .. 125 Table 3.6. Pseudo first-order rate constants (k 0b S) of the reaction Rh(dcpe)2 + Cr + H C l ^ = * - R h H C l ( d c p e ) 2 + C l - , at 18.3°C 128 Table 3.7. Distribution of products via dissolution of RhHCl(dcpe)2 + in CH^CN and DMA, observed by 31p{lH} and/or UV-VIS spectrophotometric techniques 131 Table 3.8. Dependence of k - on [DMA] for the rate of loss of HCI from RhHCl(dcpe) 2 + (5.80 x 10" 4 M) dissolved in CH 2C1 2-DMA at 18°C 134 Table 3.9. Interaction of R h f d c p e ^ C f with HCI (-1 atm) in CD 2 C1 2 - the distribution of high field hydrides (^ H NMR region) with time 137 Table 4.1. Spectroscopic data for the various hydrides observed during the interaction of Rh2(CO)4(dpp)2 with H2, in the presence of dpp ligand. The exact amount of these species present under various conditions is presented in Table 4.3 (P. 173) 155 Table 4.2. Spectroscopic data for the changes in IR when a CH2CI2 solution of Rh2(CO)4(dpp)2 and 6 equivalents of dpp is treated with H2, or D2 167 Table 4.3. Species generated when Rh2(CO)4(dpp)2 is treated with H2 or H2/CO in CD2CI2 in the absence or presence of added dpp 173 xviii Table 4.4. Hydrides generated when Rh2(CO)4(dpp)2/6 dpp interacts with H2 in CD2Q2 at ambient temperatures. The products and their distribution varied with time ([RhJ = 14.1 mM, [dpp] = 87.5 mM) 174 Table 4.5. Gas-uptake (evolution) for a reaction of Rh~(CO)4(dpp)-/6 dpp with H 2 in toluene (10 mL) at 31 °C 7 .7. 182 Table 4.6. Solution compositions of the phosphine exchange reactions of RhH(CO)(PPh3)3 with various equivalents of dpp in CD2CI2 at 19° C (31p probe operated at 121.4 MHz, and *H probe operated at 300 MHz unless stated otherwise) 185 Table 4.7. Products pertinent to the various solutions listed in Table 4.6 186 Table 4.8. 1 H NMR data for the RhH(CO)(dpe*)(dpe), Rh2H2(CO)2(dpe)3, and RhH(dpe) 2 species in CD2CI2 at ambient temperatures (high field region only, 400 MHz); solution composition is presented in Table 4.6, entry 7 . ... 193 Table 4.9. Titrations of RhH(dpp)2 (1.82 x 10 ^ mol) with different amounts of gaseous CO, in 0.6 mL CD 2 C1 2 195 Table 4.10. Product distribution of the hydrides formed by sequential addition of ~ 1 atm CO, and then ~ 1 atm H 2 to RhH(dpp) 2 in CD 2 C1 2 after 15 min at ambient temperatures 200 Table 5.1. Kinetic data for the homogeneous hydrogenation of 1-hexene catalyzed by the RhH(dpp) 2/CO/H 2 system (toluene, at 31° C) 210 Table 5.2. Kinetic data for various conditions of hydrogenation employing the RhH(dpp)2/CO/H2 system, or RhH(dpp)2, a s catalyst at [H2] = 3.10 mM, [1-hexene] = 0.0319 M, 31° C in 5 mL toluene (unless stated otherwise) 214 Table 5.3. Product distribution for 1-hexene catalyzed by the RhH(dpp) 2/CO/H 2 system in toluene (5 mL) at 31° C 216 Table 5.4. Maximum rate of hydrogenation for some common catalytic systems 242 xix ABBREVIATIONS AND SYMBOLS The following list of abbreviations and symbols will be employed in this thesis. Abbreviations for phosphine ligands not frequently used are presented at the end of this section. A angstrOm(s) A absorbance acac acetylacetonate Ar aryl atm atmosphere; 1 atm = 760 mmHg br broad Bu butyl COD 1,5-cyclo-octadiene COE cyclo-octene d day(s); doublet dcpe 1,2- bis(dicyclohexylphosphino)ethane ddt doublet of doublets of triplets diop (2R,3R) or (2S,3S)-o-isopropylidene-2,3— dihydroxy-1,4- bis(diphenylphosphino)butane DMA N,N'-dimethylacetamide DMA* HCI N,N'-dimethylacetamide hydrochloride dmgh2 dimethylglyoxime DMI 1,3- dimethyl- 2- imidazolidinone dpb l,4-bis(diphenylphosphino)butane dpe 1,2- bis(diphenylphosphino)ethane xx dpm bis(diphenylphosphino)methane dpp l,3-bis(diphenylphosphino)propane dq doublet of quintets dt doublet of triplets e.e. enantiomeric excess Et ethyl g gram(s) h hour(s) Hz hertz, cycles per second IR infrared J coupling constant k rate constant K equilibrium constant In natural logarithm log logarithm m multiplet M molarity, moles per liter Me methyl group min minute(s) mL milliliter NBD norbornadiene nm nanometer(s) NMR nuclear magnetic resonance Pa Pascal, the unit of pressure in SI system, 1 MPa = 0.987 atm Ph phenyl xxi P—P chelating dilertiary phosphine p=p 1,2- bis(diphenylphosphino)ethylene PPh^ triphenylphosphine ppm parts per million PR^ trialkylphosphine q quintet RDS rate determining step s second(s), singlet sh sharp solv solvent sq py square pyramidal t time, triplet t- trans-tbp trigonal bipyramidal temp temperature TEMPO 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical THF tetrahydrofuran TMS tetramethylsilane Tol toluene torr unit of pressure, 760 torr = 1 atm vbr very broad v/v volume by volume w weak X anionic ligand 6 chemical shift in ppm from standard xxii X X max A M v [ ] 31 r PI Phosphines chiraphos DiPAMP diphos dippe dmpe dtbpe PCy 3 P(-iPr) 3 Prophos molar extinction coefficient, M cm wavelength, nm wavelength of maximum absorbance, nm -1 2 -1 equivalent molar conductivity, Q, cm mol frequency, cm * concentration broadband proton decoupled broadband phosphorus decoupled the dangling end of a monodentate ditertiary phosphine, or a chiral centre (2S,3S)-bis(diphenylphosphino)butane 1,2- bis(o- anisylphenylphosphino)ethane 1,2- bis(diphenylphosphino)ethane (dpe) l,2-bis(diisoproylphosphino)ethane l,2-bis(dimethylphosphino)ethane l,2-bis(ditertiarybutylphosphino)ethane tricyclohexylphosphine tri - isopropylphosphine l,2-bis(diphenylphosphino)propane xxiii ACKNOWLEDGEMENTS I am grateful to Professor B.R. James for his guidance and support throughout the course of this work. I would like to express my deepest gratitude to him for his help in the completion of this thesis. Many thanks must also go to the members of the group, both past and present, particularly for their support and useful discussion. I would also like to acknowledge the help of Professor A. Storr for reading and commenting on this thesis. Thanks are also extended to D. Thackray, CY. Sue, and L. Xie for reading part of the thesis. I wish to thank Dr. S.J. Rettig for the crystal structure determination. The assistance of microanalyses, NMR, mass spectroscopic, glass-blowing, and mechanical services is also gratefully acknowledged. xxiv CHAPTER 1. INTRODUCTION 1.1. GENERAL INTRODUCTION Significant development has been achieved in the past 25 years or so in the field of homogeneous catalysis. There is an increase in industrial interest in utilizing homogeneously catalyzed processes. Successful applications include the Wacker process, the Oxo process, methanol carbonylation, polymerization, arene oxidation, and the 1-4 synthesis of L-Dopa drug. A homogeneous catalyst possesses many advantages not 5-7 found in the heterogeneous catalyst. A homogeneous catalyst may have high selectivity achieved via catalyst tailoring especially by ligand modification, so as to minimize by-product formation. Such a catalyst is also a discrete molecular species and has higher catalyst activity in terms of turnover frequency than the available surface metal atoms in a heterogeneous system. Milder reaction conditions are generally required for the homogeneous catalyst and this offers economic benefits in energy saving. A homogeneous catalyst gains over a heterogeneous catalyst in being reproducible, and having a definite stoichiometry and structure, which allow foT mechanistic studies. Despite all the above advantages, homogeneous catalysis still lags behind in industrial applications, compared to heterogeneous catalysis. Major drawbacks include the greater sensitivity of homogeneous catalysts to oxygen and moisture, and their lower thermal stability compared to the heterogeneous catalysts. Particularly, there is the problem of separating the usually expensive homogeneous catalyst from the product at the end of the reaction. A remedial approach has been subsequently 1 2 adopted to combine the advantages of both homogeneous and heterogeneous catalysts. The homogeneous catalysts are either attached to inert supports (organic or inorganic 5 8 materials), ' or are modified by the use of polar, water soluble ligands such as 9 sulphonated triarylphosphines in aqueous-organic two-phase solvent systems. Phase transfer catalysis has been employed in various catalytic processes, and tetralkylammonium salts such as benzyl triethylammonium chloride or tetramethylammonium chloride are used as the phase transfer catalyst.^ ^  In these approaches, the separation of products is easy and the molecular character of the catalyst is preserved. Developments in homogeneous catalysis are often guided by the concepts of mechanism. A mechanism, of course, can be disproved but never be fully substantiated. It is a hypothesis that best fits the experimental data. A non-free radical mechanism is composed of a series of organometallic reaction steps, namely, ligand substitution, oxidative-addition, migratory insertion, and reductive elimination linked together in a cycle. Each species in the cycle generally obeys the sixteen- or eighteen- electron rule. Transition metal complexes in which the metals possess a variable oxidation state, co-ordination number, and the ability to form complexes with different types of ligands, are typical examples of precursor catalysts. The majority of homogeneous hydrogenation precursor catalysts are also co-ordinatively unsaturated, or otherwise susceptible to producing vacant co-ordination sites. Weakly co-ordinating ligands such as CH 3CN in Cr(CO) 3(CH 3CN) 3 or MeOH in [ R h f P R^MeOHy +; photolabile ligands such as CO in Cr(CO)^ or Fe(CO)^; and ligands which can be irreversibly destroyed by hydrogenation (e.g. olefins or allyls), are good choices of ligands for precursor 3 catalysts. Bi- or multi-dentate ligands such as r\ -allyls, an orthometallated group, or ditertiary phosphine, may open up a site by reversible ring opening. Complexes having 3 severely reduced co-ordination numbers have a tendency to associate into catalytically inactive forms, which can be prevented by using non-bridging labile ligands such as 3 MeOH or bulky ligands such as PCy^. Amongst the transition metal complexes, the Group 8-10 triads, especially rhodium phosphine species, are commonly found to be efficient precursor catalysts.3'"^ ^  t 1.2. SCOPE OF THIS THESIS Our group has researched the binding and activation of small gas molecules by bis(diteriary phosphine) complexes of Rh(I) of the type Rh(P—P) 2X, where P—P is 18 19 20 21 P P l ^ C ^ ^ P P l ^ (n = 1 - 4), ' diop, or chiraphos, and X is a co-ordinated or associated anionic moiety, including H, CI, BF^, PF^ and SbF^. The Rh(P— P^X type complexes were found to be efficient for (a) hydrogenation of prochiral olefinic 20 22- 23 24 acids, ' (b) decarbonylation of aldehydes, and (c) intramolecular hydroacylation 25— 26 of olefinic aldehydes to give cyclopentanones, ~ depending upon the types of P—P and X employed. Of interest, the RhH(P—P) 2 complexes, where P—P = dpe, dpp, and diop, were found to react with CO to yield Rh 2(CO) 4(P— P ) 2 complexes, which contain 27 one P—P ligand per Rh. Rhodium carbonyl complexes have been used as catalysts for hydroformylation (see Section 1.4), while asymmetric hydroformylation catalyzed by rhodium complexes containing chiral ditertiary phosphines was a subject of intensive investigation by Pino's 28—31 28 group. The mechanism proposed by Pino et al. for asymmetric hydroformylation t Precursor catalyst : A well characterized complex added to the flask to bring about the reaction. The precursor catalyst may react with the reactant gas and/or the substrate to form the actual catalyst, which may be considered as the first identifiable complex that participates directly in the catalytic cycle3. 4 of 1-butene to (S)-2-methylbutanal, using the RhH(CO)(PPh 3) 3/(-)diop catalyst system, involved the "RhH(CO)(diop)" species as reactive intermediate. Nevertheless, the PPh^ dissociated from RhH(CO)(PPh 3) 3 would compete with 1-butene for a co-ordination site on "RhH(CO)(diop)". It seemed that the problem of excess PPh^ in the hydroformylation system might be avoided by employing Rh 2(CO) 4(P— P) 2 complexes as catalyst precursors. The original aim of this work was to study the catalytic properties of a series of Rh 2(CO) 4(P—P) 2 complexes for asymmetric hydroformylation of alkenes. Due to the high cost of chiral ditertiary phosphines, much of the work was performed on nonchiral ditertiary phosphines, including dpp, p = p, and dcpe. 27 The literature method was adopted for the synthesis of Rh 2(CO) 4(P—P) 2. The Rh(P—P) 2 + C1 complex was synthesized, followed by reaction with NaBH 4 to give RhH(P—P) 2. The bubbling of CO into a benzene solution of RhH(P—P) 2 produced Rh 2(CO) 4(P-P) 2. The dcpe ligand, (PCy 2CH 2CH 2PCy 2), which consists of bulky cyclohexyl groups, was chosen for studying steric effects in the Rh 2(CO) 4(P—P) 2 series. The Rh(dcpe)2 + X~ complex (X = C l " , or BF 4") was synthesized and 31 characterized by P NMR and X-ray analysis (Chapter 3). Also isolated and characterized were [RhCl(dcpe)-solvate]n complexes (solvate = THF, O.lC^Hg) and RhCl(dcpe)(CH2Cl2)« CgHg. There was no reaction observed between Rh(dcpe)2 + C l ~ and NaBH 4. Consequently, the preparation of RhH(P—P) 2 was not achieved, and the synthesis leading to the formation of the Rh 2(CO) 4(dcpe) 2 dimer failed. A digression was made to study the activation of small gas molecules by Rh(dcpe)2 + Cl , in an attempt to test its potential as a catalyst, and understand the reactivity pattern of a series of Rh(P—P) 2X complexes (previously studied by our group), particularly with relevance to those consisting of a P—P ligand which forms a 5-membered ring with Rh. The Rh(dcpe)2 + Cl compound was found to be unreactive toward or N^. Reactions of Rh(dcpe)2 + Cl with HCI and CO were observed and the kinetics were studied. Mechanisms are proposed for such reactions. The Rh(dcpe)2 + Cl system was found to be inactive for hydrogenation of 1-hexene (at 31° C) and decarbonylation of benzaldehyde (at 24 - 90° C), under mild conditions (Chapter 3). The Rb^CO^dpe^ complex had previously been found to give RhH(dpe)2 as product, when Rh^CO^dpe^ was treated with excess dpe in dg- toluene under 32 vacuum. The source of hydride was speculated to be from either trace water in dg-toluene or the dpe ligand. The abstraction of two H atoms from dpe (PPh 2CH 2CH 2PPh 2) could give p = p (PPh 2CH = CHPPh 2), and thus the p = p ligand was used in the synthesis of an Rh 2(CO) 4(P—P) 2 analogue, in an attempt to understand the nature of intermediates arising from the possible hydride abstraction of the dpe dimer. The Rh(p=p)2 + X (X = CI , BF^ -) complexes were isolated and characterized. Reaction of Rh(p = p)2 + Cl with NaBH^ was incomplete, generating RhH(p=p)2 in a mixture with Rh(p=p)2 + Cl . The mixture, when treated with CO, -I- — yielded partially Rh(CO)(p = p)2 CI , and another unknown carbonyl compound (Chapter 2). Thus, the preparation of the Rh2(CO) 4(p = p)2 dimer was also unsuccessful. The interaction of RhjtCO^dpp^ with Hj, and ^/CO, was studied in an attempt to test its potential for the homogeneous hydroformylation reaction (Chapter 4). In the presence of ~ 1 atm of Hj, the integrity of Rh^COJ^dpp^ as a dimer was largely maintained. However, in the presence of excess dpp ligand, Rh^CO^dpp^ was converted into monomeric and phosphine bridged dimeric hydrides, and a mechanism is suggested, based on IR, ^ P NMR, ^H NMR, and gas-uptake measurements. The same monomeric and dimeric hydrides were also generated via (i) in situ exchange of 6 RhHtCOXPPh.^ with excess dpp, (ii) in situ reaction of RhH(dpp)2 with ~1 atm CO, followed by degassing the system thrice and exposing it to ~ 1 atm (abbreviated as the RhH^pp^/CO/^ system) (Chapter 4). Though the interaction of R l ^ C O ^ d p p ^ and 6 equivalents of dpp with Hj/CO initially resulted in the monomeric hydrides, the dpp dimer was later reformed and, being very stable under synthesis gas, was considered unlikely to be an efficient catalyst for homogeneous hydroformylation (Chapter 4). The R l ^ C O ^ d p p ^ species was then found to be an inactive hydrogenation catalyst (Chapter 5). The RhH(dpp)2 complex, which initially generates R t ^ C O ^ d p p ^ in the presence of CO, and subsequently forms RhH(CO)(dpp*)(dpp), and R h j r ^ C O ^ d p p ) ^ under (abbreviated as the RhF^dpp^/CO/r^ system), was studied for hydrogenation of 1-hexene, and a mechanism is proposed (Chapter 5). 1.3. H O M O G E N E O U S HYDROGENATION Homogeneous hydrogenation is one of the most widely studied and well understood processes. The first homogeneous hydrogenation was reported by Calvin, who discovered that copper(I) salts, notably the acetate in quinoline solution, catalyzed the reduction of copper(II) and quinone by H2, under homogeneous and relatively mild 33 34 conditions. ' The real breakthrough in this field was the discovery of one of the most practical catalysts available, namely the so-called Wilkinson catalyst, RhCKPPh^)^, in 1965,^ when selective reductions of alkenes at 25° C and 1 atm H2 in benzene became possible. Attention has been focused on using tertiary phosphines as ligands in 15 36 catalyst design ' and, at the same time, the discovery has spurred much interest in mechanistic studies, in order to understand the underlying molecular processes. The uncatalyzed concerted addition of H2 to an alkene is a symmetry 7 37 forbidden process (Figure 1.1 (a), Figure 1.2 (a)). The stepwise process (Figure 1.1 (b)) is thermodynamically favorable but it involves high energy H atoms since the bond dissociation energy of hydrogen is about 435 kJ mol * at least in the gas 38 phase. These restrictions are overcome by the use of transition metals with d-orbitals, which have the correct symmetry to interact with H 7 (Figure 1.2). Figure 1.1. The uncatalyzed addition of to an olefin via (a) concerted or 39 (b) stepwise pathways. Thus, the essential requirement for the transition metal in the homogeneous hydrogenauon of alkene is the formation of a metal hydride bond via the activation process. Other requirements (in non-radical processes) include the activation of the alkene substrate, the subsequent hydride transfer from the central metal atom to the co-ordinated substrate, and the reductive elimination of the product. 8 0 9 Q C — C ( a ) (b) Figure 1.2. (a) Symmetry-forbidden addition of H 2 to a double bond, (b) Metal-catalyzed allowed concerted addition of two H atoms to a double bond. 1.3.1. Hydrogen Activation Three common modes of activation are as follows: (a) Homolvtic splitting or oxidative addition This mode of activation involves H j addition to the complex with a concomittant increase in the oxidation state and co-ordination number of the metal (equation (1.1)). H M' • H 2 MI (1.1) H The reversible reaction is common at d metal centres in a square planar complex, especially Rh' and Ir'; this results in an octahedral d^ configuration, with a change in oxidation state of metal by two. The forward reaction is promoted by a low initial oxidation state, high metal basicity and unsaturation in the co-ordination sphere. Equation 1.2 shows an example where the product RhH 2Cl(P P P) does not undergo phosphine dissociation, and hence there is no vacant site for further alkene 40 activation. Thus, the dihydride is not a hydrogenation catalyst H ' W s • H 2 (1.2) ?S > C I I H P"Y> = C 6H 5P(CH 2CH 2CH 2P(C 6H 5) 2) 2 CI Equation 1.3 demonstrates the uncommon oxidative addition of H 2 to a d ^ complex. The cis form is favored by polar solvents, whereas the trans form is favored 4142 by sterically encumbered phosphines. ' i^Pt — 1 | S^ " L>eH o.3> 43 A study by the Eisenberg's group shows that cis H 2 oxidative-addition is under kinetic control (equation 1.4). This step is vital to the diastereoselectivity in the asymmetric hydrogenation of alkenes, e.g. as in L-Dopa synthesis via the Monsanto 44 process. 10 Pho °C/" O U l Pho P-P = dpe H H 2 H '/, T P 3 — • ^ Ir C l l C O P Ph 2 <> C>„ V ' Ir H^c', Ph2 P, (kinetic) o Ph 2 (thermodynamic) (1.4) The exact nature of the interaction between a transition metal complex and H 2 during the formation of hydrides is not known. Hydrogen may initially approach 45-49 the metal complex in a side-on or an end-on fashion (Figure 1.3). H M - - . H — H < 1 * H (a) end -on (b) s i d e - o n Figure 1.3. Approach of H^ to the transition metal centre M. Recently, the isolation and structural characterization of the complexes o so M ( C O ) 3 ( P R 3 ) 2 ( T T - H 2 ) (M = Mo, W; R = Cy, i-Pr) by Kubas et al. lend weight 2 to the side-on approach. This TJ - H 2 complex at ambient temperature in soluuon undergoes equilibration to give the dihydride (10 - 30%) MH 2(CO) 3(PR 3) 2 > and thus the side-bonded hydrogen complexes are very plausible intermediates en route to 2 oxidative addition of H 2 to metal complexes. The stability of such TJ - H 2 complexes 5152 depends on a fine balance of steric and electronic factors. ' Bulky ligands such as 11 2 PCy^, and P(i-Pr) 3 inhibit dihydride formation and stabilize TJ -H^ complexes. Strong 2 acceptor ligands such as CO also stabilize TJ - H 2 complexes by reducing back donation from Tr(metal) into the a*(H 2) orbitals. The TJ -H^ complexes have been found in n 53-56 T 57 _ 58 D 58 ,.,59 D ( 60 , — 61 Cr, Ir, Fe, Ru, Pd, Pt, and Rh systems. Hydrogen oxidative addition at bimetallic centres (two mononuclear or one binuclear) involves a one-electron change for each metal (equation 1.5). 2M n or (M n-M n) + H 2 < > 2 M n + 1 H (1.5) or ( H M n ^ - M n + 1H) 62 The exact mechanism for the net one-electron oxidation is not known. ^  A binuclear complex could invoke a concerted mechanism with migration of hydrogen atoms^3 64 (equation 1.6), or a free radical process (equation 1.7). Bu l Ph,P S CO Ir I r ^ + H 2 ^ = ^ [ I r H ( M - S B u l X C O X P P h 3 ) ] 2 (i. 6) OC S f PPh, Bu i Co 2(CO) g + „ 2Co(CO) 4* H 2 % 2HCo(CO) 4 (1.7) In the mononuclear case, the d 7 C o ^ C N ) ^ - and Co H(dmgh) 2 systems are amongst the best-studied examples.^'*^ The reversible activation of H 2 by Co ^ ( C N ) ^ -II 3- 2 proceeds with a rate proportional to [ H J and [Co (CN)^ ] , consistent with an activated complex of composition [ C o 2 H 2 ( C N ) ^ ^ ~ (equation 1.8).^ 12 2 C o n ( C N ) 5 3 - + H 2 2Co n iH(CN) 5 3" (1.8) [Co:;'JJ;;;co 0 r o>-- - H - - . H " - c o f H (b) Heteiolvtic Cleavage M-X + H 2 * + M-H + HX (1.9) X = ususally a halide There is no change in the formal oxidation state of the metal (equation 1.9). The overall process can occur via oxidative addition of H 2 followed by loss of HX, which 38 can be assisted by base (equation 1.10). Such an example is given in equation 1.11. M*-X + H 2 ^ = > M I H H 2 X •M-H + Base-H + X" (1.10) RhClP 3 + H 2 < 4 = = = i * h H 2 C l P 3 m s e • RhHP 3 + Base.H + C l " (1.11) P = PPh 3 The acidity of at least one M-H in many M H 2 systems may be the reason for a general occurrence of the oxidative-addition deprotonation mechanism. However, with metals in higher oxidation states, e.g. Rh(lII), an initial oxidative addition is less likely and a genuine heterolytic cleavage via the following 38 transition state may occur (Figure 1.4). 13 — M-- -X • i • i - 1 • • H- - -H Figure 1.4. Transition state for the heterolytic cleavage of 1.3.2. Mechanisms of Homogeneous Hydrogenation Homogeneous, olefin hydrogenation catalysts can be divided into two main classes: 1. Monohydride catalysts, having a single M-H group present at some characterized stage of the catalytic cycle, e.g. RhH(CO)(PPh 3) 3. 2. Dihydride catalysts, having two adjacent hydrides (cis-MH 2) present in one stage of the catalytic cycle. Examples of such precursor complexes are RhCl(PPh 3) 3 > [Rh(NBD)(diphos)]BF4, and [Rh(NBD)(PPh 3) 2]BF 4. The mechanisms for these two types of catalysts will be discussed in the following sections. However, it is worth mentioning that, in order to propose such mechanisms, many studies have been performed, including kinetic experiments on individual stoichiometric reaction steps, product composition studies, overall kinetics of the system, isotope labelling experiments, X-ray crystallography, and low temperature NMR experiments. Since catalysis is a purely kinetic phenomenon, the kinetic experiments are key in elucidating the mechanism because the actual catalytic species is usually too short-lived, unstable or present in too low a concentration to be detected. 14 Species observed during the hydrogenation may not necessarily be directly involved in the catalysis. 1.3.2.1. Monohydride Catalysts The often proposed mechanisms are illustrated in Figure 1.5. There are two pathways that differ in the hydrogenolysis of the metal alkyl intermediate to form the products. This may occur via either intramolecular alkyl-hydride elimination (pathway (A)) or intermolecular elimination with a second metal-hydride (pathway (B)). Pathway (A) requires the presence of vacant co-ordination sites on the metal alkyl intermediate for the oxidative addition of H ^ . Pathway (B) does not require the oxidative addition step, but this process can be inhibited by steric bulk at the metal 67 site in the binuclear reductive step. A typical well studied example via pathway (A) is hydrogenation using RhH(CO)(PPh 3).^ which is highly selective toward terminal olefins but not the more heavily substituted substrates. With monohydride complexes, isomerization of olefins can occur both in the presence and absence of PL,. Isomerization can occur via the B-hydride elimination of the co-ordinated alkyl intermediate, provided that the reductive elimination process is slow relative to the formation of alkyl. This is especially true in the presence of a strong 7r-acid ligand, which reduces the electron density on the metal atom and thus inhibits the oxidative addition of H~ to the metal alkyl complex. Figure 1.5. Mechanisms for monohydride hydrogenation catalysts. Two pathways (A) and (B) are indicated. 16 1.3.2.2. Dihydride Catalysts A generalized scheme for this class of catalysts is shown in Figure 1.6. The two plausible routes are well documented. The distinction rests on the sequence of addition of alkene or H 2 to the metal complex. The hydride route (pathway (C)) involves oxidative addition of to the metal centre, followed by alkene co-ordination. The unsaturate route (pathway (D)) proceeds by the binding of substrate, followed by the activation of F^. Both pathways involve a common dihydride substrate intermediate, which converts to an alkyl hydride which then reductively eliminates to release the alkane product. In general, the hydride route is more common since the addition of alkene, which is a 7r-acid ligand, would reduce electron density at the metal centre, and thus prevent the necessary oxidative addition of H 2 in the unsaturate route. The RhCl(PPh 3) 3 catalyst has been studied exhaustively by many groups, 69 7 3 particularly those of Wilkinson and Halpern. Substrates such as 1-hexene and cyclohexene are reduced by the hydride route. However, stronger binding substrates such as styrene, ethylene or 1,3-butadiene, which show different kinetic behaviour, may follow the unsaturate route. The mechanism for the hydrogenation of cyclohexene is now fully elucidated. It is noteworthy that five species are observed spectroscopically, namely RhCl(PPh 3) 3, RhCl(cyclohexene)(PPh3)2> Rh 2Cl 2(PPh 3) 4 > RhH 2Cl(PPh 3) 3 > and 71 74 Rh 2H 2Cl 2(PPh 3) 4. ' However, none of them is the active species. The active, undetected catalyst is RhCl(PPh3)2(solvent), which is rapidly intercepted by H 2, followed by alkene activation, hydride transfer, and product release. The suggestion of such a species is based purely upon kinetic investigations. Cationic rhodium complexes also fall into this category of dihydride catalysts. The phosphine ligands determine whether the catalysts follow the hydride route or the 17 H, V M-H fi ( D ) « - r - c H / \ H C H, M - H I H A (C) Figure 1.6. Mechanisms for dihydride hydrogenation catalysts. 18 unsaturate route. The [Rh(NBD)(PPh3)J. BF^ system proceeds via the hydride route. The first step is activation ..;{equation. 1.12). HoweveT, for the chelating ditertiary phosphine complex, [RhCNBD.)(diphos)]BF4, where an initial oxidative addition of would force a hydride to- be trans to phosphorus (thus destabilizing a metal hydride), 75 76 H2 activation is not feasible (equation 1.13). ' Rather, the complex yields the square I 77 78 79 planar bis(solvated) Rh complex (equation 1.14), ' ' and follows the unsaturate route (Figure 1.7). Complex 1 is then in...£qiiilibrium with the olefin complex 2- The equilibrium 19 \ -M-CH I H.I | . s fast | H 2 P^ I > H S - CH3CN (solvent) P-P = diphos Rate(25°C) = k<\ K M Total [C=c] CH21 1 + K [C=C] Figure 1.7. Mechanism for homogeneous hydrogenation of 1-hexene using [Rh(NBDXdiphos)]BF4 as a catalyst. 20 constants K have been measured for various olefin substrates; e.g. for unsaturated 3 -1 amides (a-acetamidocinnamic acid, K = 8 x 10 M ) and aromatic rings (toluene, K = 97 M \ Thus the choice of solvent is important to avoid competitive inhibition 76 in the hydrogenation processes. At ambient temperatures, the rate determining step is kj, which involves oxidative addition of Hj. This is then followed by rapid migratory 80 75 insertion and reductive elimination to yield the alkane. Species 1, 2 and 3_ have 80 been detected by NMR measurements, while 3_ and 4 have also been characterized 75 by X-ray diffraction. Species 4 is the first alkyl hydride complex directly observed in hydrogenation processes; this requires low temperatures, which slow down kj relative 75 to kj, and thus allows the concentration of 4 to build up. 1.3.2.3. Free Radical Pathways Homogeneous hydrogenation can also occur via a free radical mechanism, 81 82 which involves H atom transfer. ' There has been relatively little research in this area compared to studies on non free-radical mechanisms, probably because it is less widespread. A free radical mechanism is observed in some mononuclear metal carbonyl hydride systems. The olefin substrates are typically a,fi-substituted arenes such as styrene and polycyclic arenes, which can stabilize radicals via electron derealization, and also alkynes. Examples are given below : 21 PhCH = C H 2 + 2CoH(CO) 4 ••PhCH 2CH 3 + Co2(CO), (1.16)' ,81 '8 C H 2 = C(CH 3)(C 6H 5) + 2MnH(CO) 5 *-(CH 3) 2CH(C 6H 5) + Mn 2(CO) 1 Q (1.17)' 84 The mechanism involves successive H atom transfers from metal to olefin 82 substrate (Figure 1.8). Metal hydride bond dissociation energies are about 60 kcal -1 82 85 mol , and appear to be quite insensitive to the nature of the metal and ligand ' Knowledge of the heat of formation of the alkyl free radical product allows for the estimation of the enthalpies for the H transfer process (step (a)) to be about 15 to 25 kcal mol .^ Thus, the reaction is feasible at mild reaction conditions. After the second H atom transfer, dimerization of the metal radical species occurs along with formation of the alkane product. If the hydride can be regenerated by introducing to the dimer (step (b)), this would give rise to catalytic hydrogenation. This type of mechanism does not require prior co-ordination of the substrate. Several other systems 86—92 exhibit free radical mechanisms. However, it is still not possible at this stage to predict whether a particular catalyst prefers free radical pathways to non-free radical mechanisms or vice versa. 1.3.3. Asymmetric Hydrogenation I 2 3 R iR zC = CHR J H, 2 > R 1R 2CHCH 2R 3 (1.18) * cat Asymmetric hydrogenation involves the use of a chiral catalyst which directs 22 Figure 1.8. Free radical mechanism in homogeneous hydrogenation of aryl substituted olefins. 23 the hydrogenation of the prochiral olefin preferably to give one enantiomeric product (equation 1.18). Significant advancement has been made in this field, partly because of its attractive applicability. Many agricultural and pharmaceutical products are optically active substances. An asymmetric hydrogenation process is much more appealing than resolution of racemic mixtures. Apart from industrial interest, applications can be found in laboratory organic syntheses, or in the study of reaction mechanisms using chirality 44 93-100 as a probe. Numerous reviews have emerged in the past ten years. ' Mechanistic aspects are now known for cauonic Rh complexes containing chelating ditertiary phosphine ligands. However, the choice of prochiral olefins, that give good e.e., is restricted to functionalized olefins, with the best being the enamide precursor of a-amino acids (Z-isomer). The olefin acts as a chelate ligand via its carbonyl group and the olefin double bond. Electron-withdrawing substituents such as carboxylic acid or phenyl groups further enhance such binding. The reaction mechanism is similar to that shown in Figure 1.7. The chiral catalysts used involve a chiral phosphine, with chirality being present in either the phosphorus atom, such as in DiPAMP, or in the carbon backbone, as in diop or chiraphos (Figure 1.9). The chiral ditertiary phosphine ligands generally give higher enantioselectivity than the monotertiary phosphine analogues, probably because of the rigid ring configuration, which renders less chance for substitution or rearrangement of the chiral phosphine. The mechanisms of asymmetric hydrogenation using Rh(I) chiral ditertiary phosphine complexes have been investigated by many groups with major advances coming from the groups of B r o w n ^ and Halpern.^ It is found that the major diastereomer formed in an equilibrium step does not determine the nature of the 24 Ph H Me H R = Me Chiraphos DiPAMP Diop Figure 1.9. Chiral ditertiary phosphine ligands. product. Rather, the minor diastereomer is kinetically moTe active in the subsequent rate determining H 2 oxidative addition step that gives, the preferred enantiomeric product 1.4. H O M O G E N E O U S HYDROFORMYLATION Hydroformylation, also known as the Oxo reaction,- is the reaction between an 103 alkene, H 9 and CO to form an aldehyde (equation 1.19). Commercial practice has been primarily concerned with Co, phosphine-modified Co, and phosphine-modified Rh catalysts. These catalysts, to some extent are complementary. Rhodium ones are suitable for the hydroformylation of low molecular weight alkenes under mild conditions, while Co carbonyls pertain to high molecular weight feedstocks containing significant amounts of internal alkenes.^4.105 ^ e p r m c j p a j industrial application is the hydroformylation of propene to n-butanal, which can be RCH = C H 2 + CO + H 2 +-RCH 2CH 2CHO or RCH(CH 3)CHO (13 25 further hydrogenated to n-butanol, a useful solvent. Alternatively, aldol condensation and subsequent hydrogenation yields 2-ethylhexanol, useful as a plasticizer. Rhodium catalysts account for one third of n-butanol production worldwide, amounting to a billion pounds per year. Reviews in this topic are listed in the references.^ ^ 1.4.1. Cobalt Catalyzed Hydroformylation Reactions Cornils,^ Orchin,"^ and Botteghi^ 3'"'"^ et al. have given detailed surveys of mechanistic studies with Q^CCCOg. The process requires rather severe conditions (100 - 120° C, 100 - 300 atm of CO and H J . Heck and Breslow115 proposed a widely-cited mechanism, in which the key active species is CoH(CO)3. A free radical pathway has also been invoked by Halpern^ and Orchin^ for stoichiometric hydroformylation of aryl-substituted ethylenes with CoH(CO)4. The phosphine-substituted Co catalyst offers additional advantages, namely higher normal to iso product ratios are realized. Internal olefins can be converted into terminal linear alcohols via olefin isomerization and regioselective hydroformylation. However, the phosphine-modified catalyst also promotes some olefin hydrogenation, 118 which is an undesirable reaction. 1.4.2. Rhodium Catalyzed Hydroformylation Reactions The remarkably active Rh non-phosphine containing catalysts are troubled by competing equilibria (Figure 1.10). The tetrameric j> and hexameric & clusters are very inactive, while the active 119 species is thought to be RhH(CO)4> 1. Phosphines are often added to increase the normal to iso product ratio, and to suppress olefin hydrogenation and rearrangement 26 Figure 1.10 Equilibria of Rh carbonyls under H 2 and CO. The phosphine catalyst RhH(CO)(PPh 3) 3 has been studied thoroughly by 120 121 Wilkinson et al. and Pruett et al. The high activity of the complex at ambient temperature and 1 atm pressure facilitates mechanistic and kinetic investigations. Two mechanisms have been proposed, namely, associative and dissociative (Figure 1.11). In both pathways, the key intermediate is RhH(CO) 2(PPh 3) 2, j£, as 122 evidenced by NMR and IR studies. The dissociative mechanism (pathway D) involves the formation of a sixteen-electron, unsaturated hydride RhH(CO) 2(PPh 3), j), via the loss of a phosphine ligand. Olefin co-ordination and insertion afford an alkyl complex, JJ, and then addition of a phosphine, and CO insertion, yield the unsaturated acyl derivative, JJ. Oxidative addition of H 2 is the rate determining step. There is also the possibility of binuclear elimination as the product forming step. The associative mechanism (pathway A) involves the initial attack of alkene to RhH(CO) 2(PPhj) 2 species. Subsequent formation of the alkyl and acyl derivatives follows the same pathways as the dissociative mechanism. However, such mechanism schemes are over-simplified. The equilibria involving 27 H H ^ r L H u>r c o # L - R r c o ^ o c > , h H f C O C O C O § 12 Figure 1.11. Associative (A) and dissociative (D) pathways of the hydroformylation of alkenes. 28 123 other less active dimeric species are not included in the cycle. Considering just the 124-127 dissociative mechanism, several groups studied the initial phosphine dissociation from RhH(CO)(PPh3)3> It was found that RhH(CO)(PPh3)2 was intercepted more rapidly by PPh3 and by CO rather than alkene. Thus, RhH(CO)(PPh3)2 is not very likely an intermediate. Rhodium acyl complexes, 13, of 1-octene, 1-decene, and styrene 31 13 have been observed by P and C NMR techniques. On treatment of a solution of RhH(CO)2(PPh3)2 with 1-octene under a ^CO atmosphere at 0°C, and then cooling o 128 to -95° C, species JLZ was observed (Figure 1.12). This observation indicates that the subsequent hydrogenolysis step is rate determining. There is also a rapid acyl-alkyl 128 equilibration taking place. The authors thus pointed out that stereochemistry of the product can be controlled by subsequent hydrogenolysis of JJ following phosphine or CO loss (kinetic control). \ ^ - n - C 8 H 1 7 o c , l ^JRh— PPh 3 OC PPh 3 17 Figure 1.12. Acyl species resulting from the reaction of RhH(CO)(PPh3)2 with 1-octene under CO. The associative mechanism pathway affords more steric hindrance to the co-ordinating olefin and would be expected to provide preferential formation of the linear alkyl Rh intermediate. This mechanism is thus preferred at higher concentrations 29 of catalyst and PPhj Progress in this field of homogeneous hydroformylation has not been as impressive as in homogeneous hydrogenation. The hydroformylation involves complicated underlying equilibria, and the system is sensitive to catalyst, phosphine, H^, and CO concentrations as well as temperature, whereby different species may be present for different combinations of the above factors. Despite all these mechanistic limitations, hydroformylation has great application value industrially, and the number of patents is increasing steadily every year. 1.4.3. Asymmetric Hydroformylation Asymmetric hydroformylation involves the use of optically active catalyst to 28-31 convert the olefin into an optically active aldehyde (equation 1.20). R « * * * H 2 • co c a t a ln' R v C H 0 ( 1 . 2 0 ) 28 30 Pino's group ' investigated the asymmetric hydroformylau'on catalyzed by Rh complexes containing chiral phosphines. The combination of RhH(CO)(PPh.j)3 and four 28 equivalents of diop catalyzes the formation of 12 with 25 % e.e. (equation 1.21). II H 2 , C O Me CHO P t T ^ H RhH(CO)(pPh3)3 P f f * H (1.21) 18 4 d i o p 19 Some conjugated dienes were recently hydroformylated (e.e. up to 30 %) in 129 the presence of Rh/diop catalyst Enamides have also been used, in an attempt to improve the optical yields by the chelating effect of the substrate as in asymmetric hydrogenation. However, the presence of the amide group gives no major improvement 30 28 29 to the optical yields compared with simple olefins. ' A mechanism for the asymmetric hydroformylation of butene, using the RhH(CO)(PPh 3) 3/(-)diop system was proposed by Pino et a l . 2 8 (Figure 1.13). The asymmetric induction should take place in the step giving the alkyl rhodium intermediate 22. The CO insertion is more rapid than the rhodium-alkyl formation and its decomposition by the reverse reaction. The origin of the asymmetric induction could be due to the free energy difference between the diastereomeric ir-olefinic complexes and their relative reactivities for giving the alkylrhodium intermediates. Other groups researched into the use of RhH(CO)(PPh 3) 3/P—P systems (where P—P = dpe, dpp, diop etc.) in hydroformylation of 1-alkene and concluded that the reactive intermediates responsible for high normal : iso products consist of both PPh 3 130-132 and P—P groups attached to the rhodium(I) metal centre. Thus, the presence of PPh 3 is unfavorable in the asymmetric hydroformylation of 1-alkene, when the iso product formation is preferred. Platinum complexes generally give hydroformylation products with higher asymmetric induction than those of rhodium. The hydroformylation of styrene, 2-ethenyl-6-methoxynaphthalene, and vinyl acetate with [(-)-BPPM]PtCl(SnCl 3), carried out in the presence of triethylorthoformate, yielded enantiomerically pure acetals 133 (equations 1.22-1.24, Figure 1.14). 31 Figure 1.13. Mechanism of asymmetric hydroformylation catalyzed by the RhH(CO)(PPh 3) 3/(-)diop system. 32 Ph 1 HQ/C0=1 P = 2700psi „ "ii [M-BPPMJPt(SnCI-OCI hi triethylorthoforrrtate iv Temp = 60°C.substrate/Pt Ph 400 • — H CH(0Et) 2 > 96 % e.e. (1.22) i s o : n = 0-5 « 1 JOQT-A C 0 i - i v MeO i ,CH(0Et)2 JQJQT > 96 % e.e. iso: n = 07 : 1 i - IV J A c 0— - v t i CHl0Et)2 >98% e.e. i S O : n = 1-5 = 1 (1.23) (1.24) r <x ppn 2 C 1x ' ^ s n c i 3 Figure 1.14. Structure of [(-)BPPM]PtCl(SnCl3). ( )-BPPM _ (2S,4S)-N-(tert-butoxycarbonyl)-4-(diphenylphosphino)-2-[(diphenyl-phosphino)methyl] pyrrolidine. 33 Such a system reported by Parrinello et al. is no doubt the most efficient one in effecting asymmetric hydroformylation of alkenes. However, the nature of the asymmetric induction is still not known. 1.5. HOMOGENEOUS DECARBONYLATION Aldehydes, acid chlorides and other organic carbonyl derivatives can undergo transition- metal catalyzed decarbonylation."^'^ Transition metal complexes, which are efficient in binding CO, may be good stoichiometric decarbonylation reagents and, if CO is not tenaciously retained, then the reagents become catalytic. An example is provided by the Wilkinson compound (equations 1.25, 1.26). Under mild condition, decarbonylation of aldehyde is achieved stoichiometrically (equation 1.25). At a sufficiently high temperature, 23 itself and therefore RhCl(PPh3)3 are catalysts for the decarbonylation of aromatic aldehydes (equation 1.26). The RhCl(PPh3)3 complex is one of the most efficient reagents for decarbonylation of alkyl, aryl and vinyl aldehydes (equations 1.25 - 1.27). RCHO + RhCl(PPh3)3 CH^Cl 2^2 w RhCl(CO)(PPh3)2 + PPh3 (1.25) 136 25° C 23 + RH RhCl(CO)(PPh3)2 ArCHO • ArH + CO 180° C 34 H CHO H H \ / \ / C = C + RhCl(PPh,), • C = C + PPh, X \ 3 / \ 3 Ph R Ph R + RhCl(CO)(PPh 3) 2 (1.27) 1 3 6 Chiral aldehydes are converted stoichiometrically to chiral hydrocarbons with a retention of configuration at the chiral centres (equations 1.28, 1.29). Ph CHO Ph H R = a|kyl 73-94% optical purity Et Et ,137 Phm- C —CHO Me RhCI(PPha)3 i p n ^ e ^ H (1.29)1 PhCN, 110*0 = Me 81% optical purity The mechanistic cycle for stoichiometric decarbonylation of aldehydes using 138 RhCKPPh^)^ is proposed in Figure 1.15. The first step involves the oxidative 139 addition of the aldehyde to the Rh complex. Suggs isolated a species corresponding to 24 by using 8-quinoline carboxyaldehyde as substrate. The acyl hydride Rh complex is stabilized by chelation and more easily isolated (equation 1.30). 35 0 Cl _p RhClP, ^  r RhHP 2 4R C H 0 I 1 y R C Rh P 1 ^ H P RH + RhCl(CO)P2 •* • Cl RhC ocy 1 P H P = PPh 3 Figure 1.15. Suggested mechanism for the stoichiometric decarbonylation of aldehydes with RhCl(PPh,) r (1.30) The next step is the migration of the alkyl group from carbonyl to metal. This 138 proceeds with retention of configuration, and hence gives the retention of stereochemistry in the decarbonylation of chiral aldehydes. The last step is the reductive elimination of alkane (RH) and the formation of RhCl(CO)(PPh 3) 2. The catalytic decarbonylation, using RhCKPPh^)^, requires much more severe conditions (at 200 - 300° C, see above). In this respect, Pignolet et a l . ^ ^ ~ ^ 3 and 22 24 James and Mahajan ' found that the cationic ditertiary phosphine complexes of Rh such as Rh(dpe)2 + Cl and Rh(dpp) 2 + Cl catalytically decarbonylated aldehydes at lower temperatures (120 - 150° C). The major points concerning the catalytic 36 141 decarbonylation of benzaldehyde using ditertiary phosphine Rh complexes are: 1. the activities are significantly larger than with RhCKPPh.^, 2. the activities show a marked dependence on chelate ring size with the order in activity dpp>dpe>dpb>dpm, observed at various temperatures (115 - 180° C), 3. the activities using Rh(P—P)^ + are approximately two times larger than with Rh(P-P) + species, 4. the Rh(dpp)2 + system shows catalytic behaviour at temperatures as low as 100° C (activity = 3 turnovers h \ A possible mechanism was proposed as in Figure 1.16, in which a species containing a dangling monodentate ditertiary phosphine ligand is invoked. Pignolet et al. have suggested that inhibition of the catalytic decarbonylation by added CO resulted from a competition between CO and aldehyde for the undetectable intermediate 24 [Rh(dpp*)(dpp)] +, 2 5 . ^ Kinetic data on the reversible binding of CO by Rh(dpp)0 show that inhibition by CO would result simply from formation of Rh(CO)(dpp)2 +. The intermediate 26 is co-ordinatively saturated and thus B-hydride abstraction leading 138 to olefin, a common side-product in aldehyde decarbonylations, does not occur. In terms of the ditertiary phosphine ligand series, PPb^Cr^) PPhj, + 19143 Rh(dpb) 2 has a high tendency to form bimetallic species with CO, ' while + 144 Rh(dpm)2 presumably does not form a dangling phosphine ligand readily. This rationalizes why Rh(dpe)2 + and Rh(dpp>2+ complexes are better reagents for catalytic 141 + decarbonylation. According to Pignolet et al., the reactivity of Rh(P— P) 2 for aldehyde decarbonylation depends upon Rh-P bond lability, and chelate ring flexibility. 137 Early work of Walborsky and Allen had shown that the stoichiometric decarbonylation of an optically active aldehyde using RhCKPPh^)^ occurs with retention 2125 26 of configuration (equations 1.28, 1.29, see above), and James and Young ' ' 37 r~ p P^| P rn P P RCHO P-O ,R "^iu ~* -£2 f \ / ^ ^ • Rh- 0=< ^ P j h - C 0 ^ V XP P^l ^ ^ ^ R h ^ ^ ? P ^ P^ V P ' ^CO Y v p ^ P 26 * 0 II Figure 1.16 Possible mechanism for the decarbonylation of aldehydes using Rh(P-P) 2 + as catalyst, where P-P is dpe or dpp (species are 141 mono-cations throughout). 38 • + expanded on this idea and employed chiral catalysts of the type Rh(P—P)2 , where (P—P) is a chiral ditertiary phosphine ligand, with racemic aldehyde substrates with the aim of inducing asymmetry in a decarbonylated product (equation 1.31). R 1 R 2 R 3 C C H O Rh(P-P) 2 + ^ R 1 R 2 R 3 C H + CO (1.31) The choice of a racemic alkenic aldehyde as substrate, in order to build in a functional group for resolution of optically active product by use of chiral shift reagents, led, however, to a catalytic synthesis of chiral disubstituted cyclopentanones (equation 1.32). Thus, intramolecular hydroacylation occurred, instead of the expected decarbonylation, and the chiral product results via a kinetic resolution process involving the reactant racemic aldehyde; indeed, the unreacted aldehyde is isolated in high 26 optical purity of chirality opposite to that of the cyclopentanone product. Ph Me P r i N r ^ PhCN ^ y S ^ o (1.32) ^ C H 0 130-180°C H-(s) P-P = (S,s)-chiraphos . . v ' highest e.e. = 69f-The mechanism depicted in Figure 1.17 was suggested. Co-ordination of the aldehyde via the carbonyl and/or more probably the olefinic group, followed by oxidative addition of the CHO group would yield eventually a Rh(III) acyl hydride species 21- Subsequent hydrometallation to the metallocycle .28, followed by reductive elimination of the C - C fragment to give cyclopentanone, completes the catalytic cycle. The kinetic resolution of an initially racemic aldehyde with chiral catalyst could result from binding of the olefinic group and the orientation of the phenyl groups within 44 the catalyst, analogous to the example in asymmetric hydrogenation. Other products 39 including alkenes formed via a decarbonylation process were also observed, but they were not chiral because of accompanying isomerization. Figure 1.17. Mechanism for intramolecular hydroacylation and decarbonylation. The results of attempted decarbonylation reactions of acid chlorides, by-treatment with RhCl(PPh3)3, Rh(dpp)2 + , or Rh(dpp)+, depend upon the substrates used. Aroyl chlorides, such as PhCOCl, form stable oxidative addition chloro(aroyl) adducts with the above complexes and do not undergo decarbonylation even under 145-147 prolonged heating, while alkyl acid chlorides undergo stoichiometric decarbonylation quite readily. The initial oxidative addition step of alkyl acid chloride to the Rh is fast, and the rate determining step is thought to be the reductive-elimination step of the alkyl chloride product (Figure 1.18). Thus, the acyl to Ph Me Rh^CO) 40 - P P h C H 9 C 0 C I § f „ Cl RhCIP 3 + = t » RhCIP2 ^ * » PhCH 2 -G -Rh ' ' fast p ^ C l RhCl(CQ)p s t o w 0 C ^ R V ^ I PhCH-T 1 ^ C l PhCHgCI P = P P h 3 * P Figure 1.18. Mechanism for the stoichiometric decarbonylation of acid chlorides with RhCl(PPh 3) 3. alkyl rearrangement, step (a), prior to the rate determining step, contributes to the racemization within a chiral substrate. For example, the reaction of S-(-)-a-trifluoromethylphenylacctyl chloride with RhCl(PPh 3) 3 produces racemic 148 149 a-trifluoromethylbenzylchloride. a, B- Unsaturated acid chlorides undergo stoichiometric decarbonylation by RhCl(PPh 3) 3 to give vinyl phosphonium salts (equation 1.33).^ O + : R < P P h 3 + R ^ V ^ C ( • R ^ P P h 3 C r + (Ph3P,2RhCHCO) P P h3 41 I. 6. BINUCLEAR RHODIUM COMPLEXES IN HOMOGENEOUS HYDROGENATION Metal clusters are regarded as compounds containing more than one transition metal atom.^ ^ Thus, binuclear complexes can be regarded as falling into this category. The principal problem in the area of cluster catalysis is the tendency of clusters to fragment into mononuclear species, particularly in solvents of good donor 38 155 ability or at high temperatures. Another important question remaining in the area of cluster catalysis is whether the intermediates are mononuclear or polynuclear in character. Recently, binuclear phosphine-containing Rh complexes of two types have been studied for hydrogenation of alkenes and alkynes. The first category involves a bridging phosphine ligand, that brings about two metal atoms close to one another. Notably, the dpm ligand easily forms bridging complexes, with or without a Rh-Rh bond. Examples are Rh 2(CN) 2(CO) 4(dpm) 2, 1 5 6 22, Rh 2Cl(CO) 2(dpm) 2 + , 1 5 7 1Q, Rh 2Cl 2 ( M-CO)(dpm) 2, 1 5 8 159 I I , and Rh 2(dpm) 2(CO) 2, 12. Complexes 1D-12 form complexes with bridging acetylenes, which can be subsequently hydrogenated. No oxidative addition product was observed when 22 was treated with H2- Hence, it is speculated that the hydrogenation occurs by an initial formation of an alkyne binuclear Rh complex, which then reacts with H 2 < ^ In the case of the substrate being alkene, it is not certain whether one metal atom or both are involved in the catalytic cycle, since alkene may either bridge or attach terminally to the metal atoms. The second category involves the use of co-ordinatively unsaturated dimers, containing a bridging hydride, and terminal monotertiary phosphines'^'"'"^ or ditertiary phosphines,^ for example [RhH(PR 3) 2] 2, and [RhH(P—P)] 2_ The hydrogenation mechanisms of alkenes and alkynes by [RhHCPR^)^ where R = O-i-C^H^ have been proposed by the Muetterties 42 g r o u p . ^ ' ^ ' ^ The evidence for the proposed mechanisms is based on low temperature NMR studies and X-ray crystallographic determination on isolated intermediates. Unfortunately, decomposition of the dimer to monomer occurred during the hydrogenation of alkynes and this lowered both the activity and the specificity of the system. 1.7. THE Rh(P-P)2"r SYSTEMS IN HOMOGENEOUS CATALYSIS The R h ( P - P ) 2 + type complexes, where P-P is PPh 2(CH 2) nPPh 2 (n = 1 to 4), ( + )-diop, or S,S-chiraphos, were found to be reactive toward 0 2 > H2, CO and HCI, at ambient conditions, depending upon the types of P—P employed."^ 22,24,165 169 ^ e h y u r 0 g e n a n c j carbon monoxide systems will be discussed because the knowledge of the interactions of R h ( P — P ) 2 + with H 2 and CO can provide information as to whether the systems will possess catalytic properties for hydrogenation (HJ, carbonylation (CO), or hydroformylation (H2/CO). 24 Quantitative data for CO binding reveal rapid on- and off- rates for Rh(dpm) 2 + (k 1 > 10 5 M - 1 s" 1; k_j > 10 s" 1 in CH 2C1 2 at 30° C), no binding by Rh(dpe) 2 + and Rh(chiraphos)2 + , and a rapid on-rate and slow off-rate for Rh(dpp) 2 + (kj = 5 x 103 M" 1 s" 1; k ^ = 4 x 10~ 4 s" 1 in both CH 2C1 2 and DMA at 30° C). The dpb and diop cations give a mixture of products, including 18 19 22 143 167 dinuclear species with bridging ditertiary phospines. ' ' ' ' There are no obvious simple reactivity patterns correlating with the chain lengths of the phosphines. The strong affinity of the dpm and dpp cations for CO, together with reversible loss of the gas, led our group to test P—P systems for catalytic decarbonylation of aldehydes (see Section 1.5). The Rh ( P — P ) 2 + systems were found to effect catalytic decarbonylation of benzaldehyde to benzene under refluxing conditions in toluene. Our group had established the reactivity sequence of dpp > dpb > dpm. 43 24 when Pignolet's group published on the same systems under comparable conditions and 140 reported optimum activity for the dpp cation (dpp > dpe > dpm) (see Section 1.5 for the proposed mechanism for decarbonylation). The Rh(dpe)2 + cation was previously reported to have very low olefin 170 hydrogenation activity, consistent with the idea that bis(ditertiary phosphine) chelate complexes could not furnish a vacant co-ordination site, a requirement in homogeneous catalysis. However, it was later found by our group that Rh(diop)2 + and RhH(diop)2 are efficient catalysts for asymmetric hydrogenation because of the generation of the vacant site by one of the ditertiary phosphine ligand becoming monodentate (i.e. 20 23 "dangling"). ' Preliminary data indicated that the most active hydrogenation catalysts of the Rh[PPh 2(CH 2) PPhJ, + (n = l-4) series were the dpp (n = 3) and dpb (n = 4) 18 complexes, together with the bis(diop) analogue. These three systems were also the 19 + + only ones that reacted reversibly with H 2 > while Rh(dpm)2 , Rh(dpe)2 , and Rh(chiraphos)2 did not react with H 2 at any measurable rate. The Rh(dpp) 2 system reacts rapidly with 1 atm H 2; the forward rate was determined using stopped-flow techniques (k 1 = 1.0 x 10 3 M - 1 s - 1 ; = 0.45 s - 1 ; at 30°C in + of DMA). The Rh(dpb)2 cation reacts rapidly with H 2 but gives a mixture 19 + + products. Of interest, the formation of RhH 2(diop) 2 from Rh(diop)2 a n c * * a t r a H2 proceeds via a two-stage process, rationalized in terms of intermediate solvated species containing monodentate (or "dangling") ditertiary phosphine. Therefore, qualitatively, it was argued that the "reactivity" of R h t P P h ^ C H ^ P P h J + cations toward H2 increases with n.^ The low temperature 3^P NMR data for Rh(dpp) 2 + in acetone reveal an AjB^X pattern consistent with a trigonal bipyramidal (tbp) species 168 containing an equatorial solvent, while the dpm and dpe species show equivalent P 44 i o i n I cn atoms consistent with square planar or fluxional tbp solvated species. ' ' The replacement of the solvent by H 2 in the solvated tbp species would give a geometry close to that expected for the transition state during formation of a cis-RhH 2(P—P) 2 + product; such a rationalization was used to explain the larger on-rate with H 2 for Rh(dpp) 2 + . A mechanism was proposed for the hydrogenation of olefins particularly 2-methylenesuccinic acid using as catalyst Rh(P— P) 2 + , where P—P = diop (Figure 1.19). 1 8" 2 0 K 2. Olefin (A) RhH 2(P-- P>2 + * RhH^AVP-PVP-p'^"1" M \ I ' , H 2 \ k / ^ A H 2 \ / Rh(P-P) 2 + 3J P-P = diop A = 2-methylenesucdnic : acid Figure 1.19. Mechanism for the hydrogenation of olefin using Rh(diop), as catalyst 45 The mechanism, suggested in Figure 1.19, leads to the rate law : - d f t y / d t = kK2[Rh]-p o t aj[A]/(l + K^tA]), consistent with the experimental data. Under H^, [34] >> [3J], and the rate law is independent of [Hj . Though hydrogenation of prochiral olefinic acids using RhCdiop^"1" is much slower than with a comparable + 22 Rh(diop)(solvent)2 system, higher optical yields (e.g. up' to 94 % e.e. of N-acetylphenylalanine from (Z)-N-ace tarn idocinnamic acid) can be obtained for the former system. 24 + The dioxygen systems for the Rh(P— cations have also been studied but will not be discussed in this thesis. CHAPTER 2. EXPERIMENTAL 2.1. MATERIALS 2.1.1. Solvents Spectral or analytical grade solvents were obtained from MCB, BDH, Aldrich, Eastman, Fisher or Mallinckrodt Chemical Co. Benzene, hexanes, THF, and toluene were distilled from sodium/benzophenone under 1 atm of N 0. N,N'-Dimethylacetamide (DMA) was stirred over CaH 2 for 24 h prior to fractional distillation under vacuum, and subsequently stored in the dark. Methanol, ethanol and dichloromethane were distilled after refluxing with the appropriate drying agents (Mg/^ for methanol and ethanol, P 2^5 *° r dichloromethane). Acetonitrile was stored over molecular sieves (Fisher : Type 5A, Grade 522, 8-12 mesh) prior to use. Anhydrous diethyl ether and isopropanol were used without further purification. Prior to use, all solvents were deoxygenated by N^ or Ar bubbling. 2.1.2. Gases Purified argon (H.P.), nitrogen (U.S.P.), oxygen (U.S.P.), carbon monoxide (CP.), hydrogen (U.S.P.), and synthesis gas (50% hydrogen, 50% carbon monoxide) were obtained from Union Carbide Canada Ltd. Hydrogen was passed through an Engelhard Deoxo catalytic purifier to remove trace oxygen. A lecture bottle of anhydrous 46 47 hydrogen chloride was obtained from Matheson Gas Co. All gases, except hydrogen, were used without further purification. 2.1.3. Phosphines Reagent grade triphenylphosphine (PPhJ, l,2-bis(diphenylphosphino)ethane (dpe), l,3-bis(diphenylphosphino)propane (dpp), l,2-bis(dicyclohexylphosphino)ethane (dcpe), and cis-l,2-bis(diphenylphosphino)ethylene (p = p) were supplied from Strem Chemicals, Inc. Before use, dpp and dpe were recrystallized from hot EtOH/hexanes mixture under Ar, while dcpe was recrystallized from THF/EtOH under Ar. Both PPh3 and p = p were used as supplied for syntheses. 2.1.4. Diphosphonium Salt 2.1.4.1. Diphosphonium Salt of 1,2- Bis(dicyclohexylphosphino)ethane, dcpe(HCl)2 This compound was prepared by a method similar to that reported in the literature for the monotertiary phosphine hydrobromide salt1 Anhydrous hydrogen chloride was bubbled into a solution of dcpe (0.23 g, 0.54 mmol) in CHjClj (7 mL) for 10 min. The solution was then reduced to dryness under vacuum. Dichloromethane (1 mL) and hexanes (5 mL) were added to precipitate out a white hygroscopic solid, which was then washed with hexanes and dried under vacuum for 4 h. Yield: 0.2 g (70%); C 2 6 H 50 C 1 2 P 2 r e c * u i r e s C : 6 3 - 0 2 ' H : 1 0 - 1 7 % : found C : 63.59, H : 10.49%. 3 1 P NMR (CDC1J: 5 26.1 (d, J p _ H = 490 Hz). 3 1P{1H] NMR (CDC13, 22 to -51°C): 5 26.1 (s). lH NMR (CDCIJ: 6 1.20-3.30 (m, 48H), 7.20 (br, 2H, J p _ H = 490 Hz). IR (Nujol) : 2288 cm"1 (vbr, y(P-H)). Conductivity (22° C, Ar, CH 3CN), [dcpe^COJ = 4.3 x 10~4 M : A M = 48 -1 2 -1 165 cm mol . The molar conductivity value is somewhat between that of a 1:1 2 electrolyte and a 2:1 electrolyte in CH^CN (Lit : range for a 2:1 electrolyte is 220 -1 2 - 1 - 1 2 -1 - 300 ft cm mol ; range for a 1:1 electrolyte is 120 - 160 Q, cm mol ). This implies an incomplete dissociation of 2 protons from dcpe(HCl)2 dissolved in CH^CN. The amount of co-ordinated HCI in dcpe(HCl)2 was found by titration with an ethanolic NaOH solution (7.3 x 10 M). The dcpe(HCl) 2 compound (3.11 mg, 6.3 x 10 ^  mol) was dissolved in EtOH (~0.5 mL) in a 10 mL Erlenmeyer flask. Three drops of phenolphthalein indicator solution (< 1 mg of phenolphthalein dissolved in 10 mL EtOH) was added. The colorless solution was titrated with the standardized NaOH solution to a pink end-point (1.9 (±0.1) mL NaOH). The amount of HCI in dcpe(HCl)2 was calculated to be 1.4 x 10~^ mol, which was 2.2 equivalents of the dcpe(HCl)2 complex. It was then concluded that the 2 phosphine groups of dcpe had reacted with HCI gas dissolved in CHjC^. 2.1.5. Alkene Substrates 1-Hexene and cycle-octene were obtained from Aldrich and passed through an alumina column (Fisher, A-950, neutral chromatographic grade, 80 - 200 mesh) preceeding their use. The alumina treatment removed peroxides present in alkenes. 2.1.6. Inorganic Silver Salts Silver hexafluorophosphate and silver tetrafluoroborate were supplied by Alpha Inorganics and Cationics Inc., and were stored in a vacuum-desiccator for protection from moisture. 49 2.1.7. Rhodium Compounds The rhodium was obtained as RhCl^ • 3H 20 (containing 39% Rh) which was supplied on loan from Johnson Matthey Ltd. All reactions, unless specified otherwise, were carried out in deoxygenated solvents under an atmosphere of argon by employing Schlenk techniques. 2.1.7.1. Dichlorotetrakis(cyclo-octene)dirhodium(I), [RhClfCOE)^ ^  To a mixture of isopropanol and water (40 mL : 10 mL) previously degassed by N 2 bubbling was added RhCL^ 3H 20 (1.3 g, 4.8 mmol). This was then followed by the addition of a 1:1 mixture of cyclo-octene (6.5 mL, 50 mmol) and isopropanol (6.5 mL). The brownish-red solution was stirred under Ar for 4 h, and then kept in the dark at room temperature for 5 d. The orange-brown crystals that dropped out were filtered off, washed carefully with EtOH (30 mL), dried under vacuum, and stored in a fridge at 0°C. Yield: 1.3 g (76%). The yield was increased to 1.5 g (88%) after further precipitation occurred on leaving the mixture in the dark for 8 d. C 1 6 H 2 g C l R h requires C : 53.57, H : 7.87%; found C : 53.64, H : 7.83%. The 3 synthetic procedure was based on that reported in literature, but the concentration of RhCl-j • 3H 20 in isopropanol/water mixture was lowered by 40%, in order to allow for complete dissolution of RhCl^ • 3H 20. 2.1.7.2. Bis[l,3-bis(diphenylphosphino)propane\rhodium(I) chloride, Rh(dpp)^rCl 4 The preparative procedure followed exactiy a previous, literature method. To dpp (0.45 g, 1.1 mmol) dissolved in benzene (10 mL) was added [RhCKCOE)^ (0.18 g, 0.25 mmol) in benzene (10 mL). A yellow solid immediately precipitated out, and the reaction was completed by stirring the mixture manually with a glass rod 50 under a blanket of Ar. The yellow solid was then filtered and reprecipitated from EtOH-hexanes to give yellow needles. Yield: 0.48 g (80%); C 5 4 H 5 2 C l P 4 R h requires C : 67.33, H : 5.40%; found C : 67.40, H : 5.30%. 3 1 R 1 H} NMR (CF^CIJ: 5 7.3 (d, ^Rh-P = 1 3 2 ^ c n e i ™ c a l s nift being 0.5 ppm upfield compared to the value of 7.79 ppm reported in the literature, where dg-acetone-CH2C12 (1:2, v/v) was used 7 as solvent 2.1.7.3. Bis[l,3-bis(diphenylphosphino)propane~\rhodium(I) hexafluorophosphate0.3C'H^Cl^ Rh(dpp)2+ PF~ • 0.3CH2C12 The complex was prepared from the corresponding chloro complex as 4 + -described previously. To a CH 2 C1 2 solution (5 mL) of Rh(dpp)2 CI (0.13 g, 0.14 mmol) was added AgPF^ (0.035g, 0.14 mmol) in acetone (4 mL). The mixture was stin-ed in the dark for 15 min. The resulting white precipitate of AgCl was then filtered off using Celite, and the filtrate evaporated to dryness. The residue was dissolved in CH 2 C1 2 (2 mL) and precipitated with Et 20 (10 mL) to yield orange-yellow crystals. Yield: 0.12 g (70%). The crystals were found to be contaminated with a small amount of CH 2 C1 2 (0.3 equivalents) as seen by "^ H NMR. C54 3H52 6C10 6 F 6 P 5 R h r e c l u i r e s C : 5 9 3 9 - H : 4 - 8 3 % ; f o u n d C : 59-54> H : 5 - 0 9 % -31P{1H} NMR (CDC13): 5 7.5 (d, J R H _ P = 136 Hz), -145.1 (septet, J p _ F = 710 Hz). J H NMR (CDC13): 5 1.90 (m, 4H), 2.30 (m, 8H), 5.20 (CH2C12, s, 0.6H), 7.20 (m, 40H). IR (Nujol) : 833 cm"1 (r(P-F) of non-co-ordinated PF "). 51 2.1.7'A. Carbonylbis[l,3- bis(diphenylposphino)propane] rhodium(I) hexafluorophosphate, Rh(CO)(dpp)2+PF~ The complex was prepared according to the literature method for the corresponding Rh(CO)(dpp) 2 + B F 4 compound."* Carbon monoxide was bubbled into a CH 2C1 2 (10 mL) solution of Rh(dpp) 2 + ? ¥ ~ • 0.3CH2C12 (0.10 g, 9 x 10" 5 mol) until the volume was reduced to 2 mL. Diethyl ether was added to precipitate out a yellow solid which was filtered off, washed with diethyl ether several times, and then dried under a slow stream of CO for 12 h. Yield: 0.09 g (86%); C 5 5 H 5 2 F 6 O P 5 R h requires C : 60.01, H : 4.76%; found C : 60.30, H : 4.89%. 3 1P{ 1H] NMR (CH 2C1 2 > under CO, 30°C): 5 -12.6 (t of d, J R h _ p = 86 Hz, J p _ p = 27 Hz), 14.5 (t of d, J R h _ p = 122 Hz, J p _ p = 27 Hz), -145 (septet, J p _ p = 710 Hz). IR (Nujol): 1945 cm" 1 (KCO)), 833 cm" 1 (i>(P-F) of non-co-ordinated [Lit 5: Rh(CO)(dpp)2 + BF 4", 3 1P{ 1H] NMR (CH2C12-dfi-acetone, 2:1, v/v): 6 -12.9 (t of d, JRh-P = 8 5 , 1 H z ' JP-P = 3 0 H z ) l 1 5 1 ( t o f d > JRh-P = 1 2 1 5 H z ' JP-P = 3 0 Hz); IR (Nujol) : 1930 cm"1, (u(CO))]. 2.1.7.5. Hydridobis[l,3-bis(diphenylphosphino)propane\rhodium(I), RhH(dpp)2 4 The compound was prepared exactly according to the literature method. A three molar excess of NaBH 4 (0.05 g, 1.3 mmol) in EtOH (10 mL) was added to Rh(dpp) 2 + C l ~ (0.4 g, 0.42 mmol) dissolved in EtOH (10 mL). The mixture was stirred for 30 min, and the orange solid filtered off was reprecipitated from benzene-hexanes (10 mL : 20 mL) and washed with EtOH. The bright orange solid was dried under vacuum for 2 d. Yield: 0.27 g (70%); C ^ H ^ P ^ h requires C : 69.83, H : 5.75%; found C : 69.57, H : 5.70%. 3 1P{ 1H] NMR (CD 2C1 2): 8 18.5 (d, JRh-P = 1 4 3 H z )- l f i N M R ( C D 2 C 1 2 ) : 8 1 4 3 ( m ' 4 H ) ' 2- 0 0 (t' 8 H ) > 7 1 0 ( m ' 52 24H), 7.40 (m, 16H), -10.32 (d of q, 1H, J R h _ H = 8 Hz, J p _ R = 22 Hz). IR (Nujol) : 2000 cm" 1 (y(Rh-H)). IR (CH 2C1 2) : 1979 cm" 1 (p(Rh-H)). The 4 spectroscopic data agree with those reported in the literature. A method based on the single step synthesis adopted by Ahmad et al.^ to synthesize RhH(PPh 3) 4 direcdy from RhCl3« 3H 20 failed to produce RhH(dpp)2. 2.1.7.6. Tetra(carbonyl)bis[ 1,3- bis(diphenylphosphino)propane] dirhodium(0)- C^H^, Rh 2(CO) 4(dpp) 2.C 6H 6 7 The tide compound was prepared exactly according to the literature method, by bubbling CO into a benzene solution (20 mL) of RhH(dpp) 2 (0.20 g, 0.22 mmol) and precipitating the yellow Rh dimer product with hexanes. Reprecipitau'on from benzene and hexanes (5 mL : 12 mL) under CO and drying under a slow stream of CO for 4 d afforded the dimer compound with benzene trapped in the crystal lattice.:}: Yield: 0.10 g (80% based on RhH(dpp)2; 34% based on R h C y 3H 20); C 6 4 H 5 g 0 4 P 4 R h 2 requires C : 62.96, H : 4.79%; found C : 62.50, H : 4.75%. 3 1Pf 1H} NMR (CH 2C1 2): 6 7.8 (d, J R h _ p = 150 Hz) [ L i t 7 : 3 1P{ 1H] NMR (C 7D g): 5 9.7, J R h _ p = 154 Hz]. 1 H NMR (CD 2C1 2): 6 1.84 (m, 4H), 2.45 (t, 8H), 7.16 (m, 24H), 7.24 (s, 6H), 7.30 (m, 16H) [ L i t 7 : J H NMR (C ?D 8): 6 1.66 (m, 4H), 2.36 (m, 8H), 7.08 (m, 24H), 7.52 (m, 16H)]. IR (Nujol) : 1985, 1973 cm" 1 (i»(CO terminal)), 1744, 1721 cm" 1 (v(CO bridging)) [Lit. 7: IR (Nujol) : 1967 cm" 1 (v{CO terminal)); 1718 cm" 1 (v(CO bridging))]. IR (CH 2C1 2) : 1968 cm" 1 (v(CO terminal)) and 1761, 1734, 1719 cm" 1 (*»(CO bridging)). $The Rh dimer, air-sensitive when wet, must be dried under a slow stream of CO for at least 4 d in order that the complex can be handled subsequently in air for a short period of time (~15 min). 53 2.1.7.7. Carbonylhydridotris(triphenylphosphine)rhodium(I), RhH(CO)(PPh^ The yellow compound was prepared according to a literature method,6 but scaled down by a factor of about 2. To a vigorously stirred, boiling solution of PPh^ (1.0 g, 3.9 mmol) in EtOH (100 mL) was added RhCly 3H 20 (0.10 g, 0.38 mmol) in EtOH (20 mL). After a delay of 15 s, aqueous formaldehyde (3.9 mL, ~ 35%) and a solution of KOH (0.31 g, 5.5 mmol) in EtOH (7.7 mL) were added rapidly and successively to the boiling mixture. The yellow mixture was heated under reflux for 15 min and then cooled to room temperature with a water-bath. The bright yellow solid was filtered off, washed with EtOH, water, EtOH, and hexanes, and dried under vacuum for 2 d. Yield: 0.33 g (87%); C 5 5H 4 6OP 3Rh requires C : 71.90, H: 5.05%; found C : 71.60, H : 5.05%. 3 1P{ 1H] NMR (CgHg): 6 39.0 (d, J R h _ p = 159 Hz) [L i t 8 : 3 1P{ 1H] NMR (C 7D g): 5 39.8, J R h _ p = 159 Hz]. 1 H NMR (CD 2C1 2): 6 7.07 (m, 27H), 7.18 (m, 18H), -9.71 (vbr, 1H). IR (Nujol) : 2037 cm - 1 (v(Rh-H)), 1921 cm" 1 (p(CO)) [Lit. 6 IR (Nujol) : v(Rh-H) = 2041 cm"1, v(CO) = 1918 cm"1]. 2.1.7.8. Bis[l ,2-bis(dicyclohexylphosphino)ethane]rhodium(I) chloride, Rh(dcpe)^rCl To a brownish-orange solution of [RhCKCOE)^ 2 (0.17 g, 0.24 mmol) in benzene (7 mL) was added dcpe ligand (0.41 g, 0.95 mmol) in benzene (3 mL). The solution turned bright orange-yellow and yellow solid started to precipitate out in minutes. The suspension was allowed to stir for 1 h and then kept at 4°C for 3 h to allow for complete precipitation. The compound was dissolved in CH 2C1 2 (3 mL) and precipitated from EL,0 (20 mL) and dried under vacuum. Yield: 0.37 g (79%). The Rh(dcpe)2 + Cl complex is soluble in polar solvents such as EtOH, acetone, CH,CN, and CELCL but only slightly soluble in non-polar solvents such as petroleum 54 ether, benzene (< 1 mg/10 mL). C 5 2 H % C l P 4 R h requires C : 63.50, H : 9.84, CI : 3.60%; found C : 63.43, H : 9.68, CI : 3.60%. 3 1Pf 1H] NMR (CDC\y 25° C): 6 65.6 (d, J R h _ p = 131 Hz); (CD 2C1 2, -83°C): 6 65.9 (d, J R h _ p = 130 Hz). lH NMR (CDCL): 6 1-2 (br). UV-VIS spectra measured under Ar at 25° C, X „ nm (log 1 Qe, M" 1 cm - 1): DMA 403.6 (3.70), 315 (3.73), 285 (3.53); CHjCN 403.5 (3.67), 315 (3.69), 285 (3.50); CH 2C1 2 403.6 (3.70). Solid state UV-VIS spectrum (Nujol) : X m „ v at 403.6, 315, and 285 nm. Molar conductivity (21°C, Ar, CHXN), [Rh] = 1.7 mM : A M = 115 B - 1 cm 2 mof 1. Molar conductivity (21° C, Ar, CH 3N0 2), -1 2 -1 [Rh] = 1.7 mM : A ^ = 120 cm' mol . The molar conductivity values are typical of a 1:1 electrolyte (the A M values for 1:1 electrolytes range from 120 to 160 S2"1 cm 2 m o f 1 in C H X N , and 60 to 115 S T 1 cm 2 m o f 1 in CH 3N0 2). 2 FAB (M/Z) - 947 (± 3) amu; Rh(dcpe) 2 + has a mass of 948 amu. Apart from the parent ion peak at 947 amu, fragments corresponding to the stepwise loss of the cyclohexyl groups attached to the phosphine ligand were also observed in the FAB mass spectrum (Table 2.1). The X-ray crystal structure of Rh(dcpe)2 + C f was determined (Section 3.3). 2.1.7.9. Bis[l,2-bis(dicyclohexylphosphino)ethane]rhodium(I) tetrajluoroborale- 1/2CH2C12> Rh(dcpe)? + BF 4" • 1/2CH2C12 Some Rh(dcpe)2 + C f (0.37 g, 0.37 mmol) was dissolved in CH 2C1 2 (5 mL) and added to AgBF 4 (0.07 g, 0.38 mmol) dissolved in MeOH (7 mL). The mixture was stirred for 1 h, reduced to dryness, and the residue extracted with CH 2C1 2 (5 mL); the extract was filtered through Celite and precipitation effected with Et^O (15 mL). The solid obtained was recrystallized from CH 2C1 2-Et 20 (4 mL : 15 mL) to give orange crystals. The presence of C H X L in the crystals was seen in the *H Table 2.1. FAB mass spectrum (high mass portion) of Rh(dcpe)2 CI . b c M/Z (± 3), amu possible assignment ' relative abundance 947 Rh(dcpe)2 v strong 861 Rh[(dcpe) 2-Cy] + medium 779 Rh[(dcpe) 2-2Cy] + weak 616 Rh[(dcpe) 2-4Cy] + weak 521 Rh(dcpe) + medium a. The EI mass spectrum of Rh(dcpe)2 CI displays a parent peak at 1120 amu which is attributed to [RhCl(dcpe)]2. b. Cy = cyclohexyl c. The intensity of Rh[(dcpe)-Cy] + with a mass number of 439 was taken as the intensity reference. 56 NMR spectrum. Yield: 0.25 g (65%); C 5 2 j H ^ B C l F ^ R h requires C : 58.53, H 9.07%; found C : 58.22, H : 9.02%. 3l?{lHi NMR (dg-acetone): 5 65.6 (d, J R h _ p = 131 Hz). : H NMR (CDC13): 6 1.0-2.0 (vbr, 96H), 5.25 (CH 2C1 2, s, 1H). UV-VIS spectrum measured under Ar at 22° C, X , nm (log i r ie, M - 1 cm - 1): 403.6 (3.74), 313 (3.79), 285 (3.65). Molar conductivity (22° C, Ar, CH 3CN), [Rh] = 0.35 mM : - 1 2 - 1 A j ^ = 118 ft cm mol . The molar conductivity value is typical of a 1:1 electrolyte.2 2.1.7.10. Bis[l,2-bis(dicyclohexylphosphino)ethane\rhodium(I) hexafluorophosphate, Rh(dcpe)2+PF~ The complex was prepared (while trying to make a hydrido species) similar to o the literature method for the synthesis of {RhHCl[o-CgH 4(AsMe 2) 2] 2}PF g from NH 4PF 6 and {Rhto-CgH^AsMe^ J^Cl in EtOH. A solution of Rh(dcpe)2 + C f (0.10 g, 0.10 mmol) in EtOH (3 mL) was treated with an equimolar amount of NH 4PF 6 (0.016 g, 0.10 mmol) dissolved in EtOH (1 mL). The mixture was stirred for 15 min and concentrated to dryness under vacuum. The residue was extracted with EtOH (2 mL) and diethyl ether (10 mL) was added to precipitate out a yellow solid, which was dried under vacuum for 10 h. Yield: 0.094 g (86%); C ^ H ^ F ^ R h requires C : 57.14, H : 8.85%; found C : 57.08, H : 8.78%. 3 1P{ 1H] NMR (CH 2C1 2): 6 65.6 (d, J R h _ p = 131 Hz), -146 (septet, J p _ p = 712 Hz). IR (Nujol) : 839 cm 1 (i>(P-F) of non-co-ordinated PF g~). 57 2.1.7.11. Chlorohydridobis[l,2-bis(dicyclohexylphosphino)ethane] rhodium(IH) chloride' 0.3C6H6> RhHCl(dcpe) 2 + C l " • HCI - 0.3C6H6 To a yellowish-orange benzene suspension (5 mL) of Rh(dcpe)2 + C l ~ (0.09 g, 0.092 mmol) was added 10 equivalents of hydrogen chloride (20 mL, 0.90 mmol) by a gas-tight syringe. The suspension turned clear yellow in 10 min, followed by precipitation of a white solid, which was then Filtered off, washed with benzene, and vacuum dried for 5 h. A small amount of benzene ( -0.3 equivalents) was observed in the lH NMR spectrum of the isolated white solid. Yield: 0.09 g (90%); C53 8 H99 8 C 1 3 P 4 R h r e £l u i r e s C : 5 9- 8 4- H : 9-31> C 1 : 9-%5%> f o u n d C : 5 9- 8 0- H : 9.21, Cl : 9.70%. 3 1 R 1 H ] NMR (CD 2C1 2): 6 61.9 (d, J R h _ p = 89.4 Hz). XH NMR (CD 2C1 2): 6 -19 (overlap of a q of d, J p _ H = 13 Hz, J R h _ H = 15 Hz; high field NMR region for the hydride). IR (Nujol) : 2091 cm' 1 (p(Rh-H)). It is known that the oxidative addition of HCI to Rh(P-P) 2 + species5'1^ or the iridium analogues11 often gives products contaminated with lattice-held HCI. Further, the excess - 12 HCI can also be present in the form of HC1 2 , which has IR bands at 1080, 1185 and 1575 cm 1, as in the solid state spectrum of (NMe 4)(HCl 2). Such bands were not observed in the IR (Nujol) spectrum of the title complex. Molar conductivity (22° C, Ar, CH 3CN), [Rh] = 0.565 mM : A M = 126 fl-1 cm 2 mol" 1 (typical of a 1:1 2 electrolyte). The conductivity of HCI (0.25 M) in CH^CN was found to be = — 1 2 -1 o 27.6 fi cm mol at 22° C (see Section 2.5), and thus there is little contribution -1 2 -1 to the conductivity of 126 fi cm mol from the lattice-held HCI in the title complex. The total amount of HCI in RhHCl(dcpe) 2 + Cl"«HCI • 0.3CgH^ was obtained _3 by titration with a standardized NaOH solution (7.3 x 10 M). The HCI adduct (1.4 mg, 1.3 x 10" 6 mol) dissolved in EtOH-CH 2Cl 2 solution (0.5 mL : 0.05 mL) was titrated with NaOH solution. The end-point was observed to be 0.3 (± 0.05) 58 mL. The amount of H + in the HCI adduct was thus 2.2 x 10~ 6 mol, i.e. 1.70 equivalents of HCI were found to be in the RhHCl(dcpe) 2 + C l ~ • HC1» 0.3CgHg species, close to the theoretical value of 2.0. An attempt to remove the trapped HCI in RhHCl(dcpe) 2 + C f • HCI•• OJCgHg, 13a + -according to a literature method employed to produce RhHCl(dmpe) 2 CI from RhHCl(dmpe) 2 + HCL, , by refluxing a benzene suspension of the HCI adduct for 30 min under Ar, was not successful. Rather, the Rh(dcpe)2 + Cl species was generated, indicating a ready loss of both the co-ordinated and lattice held HCI in RhHCl(dcpe) 2 + C f - H C I * 0.3C6H6 at elevated temperature. 2.1.7.12. Dichlorobis[l,2- bis(dicyclohexylphosphino)ethane] rhodium(III) chloride, RhCl2(dcpe)2+ Cl~ The title compound was prepared similar to the literature method, in which the action of C l 2 on Rh(dmpe)2 + C l ~ resulted in the cis-[RhCl 2(dmpe) 2 + C f ] complex. 1 3 a The compound Rh(dcpe)2 + C l ~ (22 mg, 2.3 x 10~ 5 mol) was dissolved in CH 2C1 2 (5 mL), and C l 2 gas bubbled through the solution slowly until the solvent was removed completely. Then CH 2C1 2 (4 mL) was added and Ar used to purge the solution to remove excess C l 2 > and concentrate the solution to a small volume (~0.3 mL). Hexanes (10 mL) were then added to precipitate out a yellow solid. Yield: 22 mg (90%); C 5 2 H % C l 3 P 4 R h requires C : 59.23, H : 9.18%; found C : 59.70, H : 9.50%. 3 1P{ 1H} NMR (CH 2C1 2): 5 40.8 (d, J R h _ p = 81 Hz). 59 2.1.7.13. Reactions between [RhCl(COE)^ 2 and 2 Equivalents of dcpe The reactions between [RhCKCOE)^ 2 and 2 equivalents of dcpe depended upon the solvents employed. (A) Chloro(dichloromethane)- l,2-bis(dicyclohexylphosphino)ethanerhodium(I)' CgHg, RhCl(dcpe)(CH 2Cl 2)- C f iH 6 To a CH 2C1 2 (20 mL) suspension of [RhCl(COE) 2] 2 (0.34g, 0.48 mmol) was added dcpe ligand (0.41 g, 0.96 mmol) dissolved in CH 2C1 2 (5 mL). The mixture turned into a brownish-orange solution and that was evacuated to yield a reddish-brown oil. Hexanes were then added to precipitate out a yellow solid. The 3 1P{^H] NMR spectrum of the isolated compound in CH 2C1 2 showed a mixture of 2 compounds, RhCl(dcpe)(CH 2Cl 2) and Rh(dcpe)2 + C l ~ , in the ratio of 3.4 : 1. A pure sample of RhCl(dcpe)(CH 2Cl 2)'C^H^ was obtained by repeated reprecipitation (3 times) from benzene (5 mL) and hexanes (20 mL) and washing with EtOH to remove the ionic Rh(dcpe)2 + C f complex. Yield: 0.1 g (14%). C ^ H ^ C l ^ R h requires C : 54.75, H : 7.80%; found C : 54.55, H : 7.75%. 3 1P{ 1H} NMR (CH 2C1 2): 6 75.3 ( J R h _ p = 125 Hz, J p _ p = 25 Hz), 99.2 ( J R h _ p = 157 Hz, J p _ p = 25 Hz). lH NMR (CDClj): 5 1.0-3.0 (vbr, m, 48H), 7.3 (C 6H 6 > s, 6H). The elemental analysis and the 31 1 P{ HI NMR data fit well the assigned structure; however, the CH 2C1 2 peak could not be located in the ^H NMR spectrum. UV-V1S spectrum measured under Ar at 25° C, X , nm (log i ne, M _ 1 cm"1): CH-CL 397 (3.22), 356 (3.17), 271 (3.55). IT13.X J-U Z Z (B) Chloro-l,2-bis(dicyclohexylphosphino)ethanerhodium(I)' O.lCgHg, RhCl(dcpe)-O.lCgH^ To [RhCl(COE) 2] 2 (0.17 g, 0.24 mmol) dissolved in benzene (10 mL) was added dcpe (0.20 g, 0.47 mmol) in benzene (3 mL). After stirring for 5 min, the mixture turned yellowish-orange. The solution was then concentrated to 3 mL under 60 vacuum, and hexanes (15 mL) were added to afford a yellow solid which was then filtered off, pumped under vacuum (~ 1 h) and stored under Ar at -70° C. The filtrate was set aside in a fridge at ~ 4°C overnight, during which time, more yellow 31 1 crystals having a P{ H] NMR spectrum in CD2CI2 almost identical to that of the yellow solid described below in C, precipitated out The total yield for both crops of product was 0.08 g. The brown filtrate was then vacuum concentrated to 2 mL and hexanes (15 mL) added to precipitate out a brown solid, with a yield of 0.07 g. The yellow product also had a UV-VIS spectrum very similar to that of the species, RhCl(dcpe)* THF, described in the following section. However, the ^H NMR spectrum revealed contamination by a small amount of benzene (-0.1 equivalent). Thus, the yellow product is tentatively assigned as RhCl(dcpe)-O.lCgHg. Yield: 0.08 g (30%); C26 6 H48 6 C 1 P 2 R h T e c* u i r e s C : 56-17> H : 8- 6 1 %' f o u n d c : 5 6- 7 1- H : 8- 0 4 %-3 1P{ 1H3 NMR (CH 2C1 2, from 20 to -75° C): 5 95.8 (d, J R h _ p = 200 Hz). J H NMR (CD 2C1 2) 5 1.0-2.0 (br, m, 48H), 7.3 (m, 0.6H). UV-VIS spectrum measured under Ar at 25° C, X m a x > nm (log 1 Qe, M _ 1 cm"1): C H X l j 397 (2.87), 335 (3.62), 289 (3.79). The brown solid has very low values for C and H elemental analysis (e.g., C : 43.58, H : 6.81%). The molecular weight of the compound was found to be > 31 1 21,000 amu,* which indicates that the isolated brown compound is a polymer. P{ H} NMR (CH2CI2): 6 47 (br, s). This species (7.22 mg) was slowly converted into Rh(dcpe)2 + C f in CT^CL, (2 mL) by adding dcpe (12 mg), as monitored by 3 1P{ 1H} NMR spectral changes. However, the conversion is incomplete in 9 h. The brown compound was soluble in CEI^Clj and benzene, sparingly soluble in DMA and acetone, and insoluble in C H X N , EtOH, CH 3N0 2 > and Etp. *The Signer method was used in the molecular weight determination. 61 (C) Chloro-l,2-bis(dicydohexylphosphino)emanerhodium(I)« THF, RhCl(dcpe)- THF The dcpe ligand (0.16 g, 0.38 mmol) dissolved in THF (7 mL) was added dropwise to a suspension of [RhCKCOE)^ (0.14 g, 0.19 mmol) in THF (15 mL) until the mixture turned to a homogeneous orange solution. The rest of the dcpe solution (~ 5 mL) was then added rapidly to give finally a yellow-orange solution, which was stirred for 20 min and then concentrated to about 3 mL under vacuum. Hexanes (20 mL) were then added to precipitate out a yellow solid. The mixture was set aside for 30 min in the fridge, filtered, and the solid washed with hexanes and dried under vacuum for 1 h. The solid yellow compound was extremely air-sensitive, and turned greenish-brown on exposure to air for a few min. Yield: 0.12 g (49%); C 3 0 H 5 6 C 1 O P 2 R h r e c l u i r e s C : 5 6 - 9 2 - H : 8 - 9 2 % ; f o u n d C : 5 7- 1 6> H : 9 3 4 % -3 1P{ 1H3 NMR (THF): 5 94.3 (d, J R h _ p = 196 Hz); (CD 2C1 2, 20° C to - 7 5 °C): 5 96.1 (d, J R h _ p = 199 Hz). lH NMR (CD 2C1 2): 1.0-2.5 (m, br), 3.6 (-CHjO. free THF). UV-VIS spectrum measured under Ar at 25° C, ^ m a x> n m ^°&IQ€> M" 1 cm - 1): CH 2C1 2 397 (2.89), 335 (3.59), 289 (3.71). Molar conductivity (22° C, Ar, CHXN), [Rh] = 1.3 x 10"3 M : A M = 45.2 ft-1 cm 2 mof 1. There is a partial dissociation of RhCl(dcpe)* THF dissolved in CH^CN, because the A ^ value is less 2 than that for a 1:1 electrolyte. The isolated tide complex was extremely air-sensitive; _3 even prolonged exposure of the yellow RhCl(dcpe)* THF compound to a 10 torr vacuum (~3 h) caused a darkening of the color to brownish-yellow, possibly due to 31 1 the formation of a polymeric species, the P{ H] NMR spectrum of which was identical to that of the brown compound described in Section 2.1.7.13 (B). The "polymerization" process was induced in the solid state under vacuum, possibly because 62 of the presence of trace C^. The [RhCl(diop)(CgHg)] 2 complex is known to undergo 14 polymerization in the presence of C^. The isolated title complex, especially in the crystalline form, was very stable under 1 atm Ar at low temperature (-60° C). It is still uncertain whether the title compound is a monomer, or dimer formulated as [RhCl(dcpe)* THF] 2. The molecular weight determinations were unsuccessful, due to a decomposition of the complex during the measurements. There are previous examples of complexes of the type [RhCl(P-P)] n > where n = 1, P-P = 2 PCy 3; 1 5 n = 2, P-P = diop, 1 4 dtbpe,1^ dippe;1^ with P—P = dcpe, a polymer formulation has been 17 presented. 2.1.7.14. Reactions between Rh(dcpe)2+ CI and Gases/Reagents, including CO, H^, HCI, N2, 02, LiAlH4, and NaBH^ _i_ A C H X 1 2 solution of Rh(dcpe)2 CI was allowed to react with ~ 1 atm of small gas molecules, namely, C^, N 2 > H 2, CO, and HCI in a UV-VIS anaerobic spectral cell (see Section 2.2). The reaction was monitored by changes in UV-VIS spectra, particularly with respect to a decrease in the absorption band of Rh(dcpe)2 + Cl at 403.6 nm. For those reactions which gave some UV-VIS changes, syntheses were carried out in attempts to characterize products. The Rh(dcpe)2 + C l ~ complex was also treated with NaBH 4 in EtOH, or L i A l H 4 in THF. The Rh(dcpe)2 + Cl compound was unreactive toward ~ 1 atm of H 2 > N 2 > and 0 2. The UV-VIS spectrum of Rh(dcpe)2 + C f (200 to 900 nm) also does not change on exposure to air. However, Rh(dcpe)2 + Cl interacts with CO and HCI in CH 2C1 2. The hydride reagents, NaBH 4 and LiAlH 4, failed to generate any rhodium(I) hydride, and the starting Rh(dcpe)2 + Cl was recovered from these reactions. A typical reaction of the. yellow Rh(dcpe)2 + CI with CO was carried out as 63 follows. The complex (0.05 g, 0.05 mmol) was dissolved in CE^Clj (10 mL), and CO bubbled into the solution contained in a Schlenk tube; the solution was then concentrated to about 0.5 mL by continued bubbling of CO. Carbon monoxide-saturated hexanes were then layered above the solution to precipitate out a yellow solid, which was washed with CO-saturated hexanes, and dried under CO for 2 h. Yield: 0.043 g. A Nujol mull of the CO complex was prepared in a glove-bag Filled with CO. Before the sample was prepared, the Nujol oil was warmed to 60° C, CO bubbled through the viscous liquid for half an hour, and then the Nujol was cooled down to room temperature under a CO atmosphere. An FT-IR spectrum revealed v(CO) at 1979 cm \ Solution FT-IR spectra also displayed P(CO) during the in situ preparation of the carbonyl complex (CH^CN: v(CO, free) = 2137, v(CO) = 1993; DMA: KCO, free) = 2130, v(CO) = 1982 cm - 1). However, the elemental analysis Fitted the Rh(dcpe)2 + Cl formulation, presumably because of the ready loss of CO before and during the analysis procedure under Ar, and the incomplete formation of the Rh(CO)(dcpe)2 + C f complex as judged by UV-VIS spectra (Section 3.5.2). C 5 3H 9 6C10P 4Rh requires C : 62.93, H : 9.56%; while C 5 2 H % C l P 4 R h requires C : 63.50, H : 9.84%; found C : 63.50, H : 10.00%. 3 1P{ 1H} NMR spectrum' (CH 2C1 2, under 1 atm of CO, 19.8°C): 5 65.6 (d, J R h _ p = 131 Hz); at -50°C, 6 59.9 (d, 128 Hz); at -88.6°C, 6 61.0 (d, 128 Hz); (under 4 atm of CO, 25°C): 6 66 (d, 130 Hz). The HCI adduct of Rh(dcpe)2 Cl was prepared according to the method described in Section 2.1.7.11. 64 2.1.7.15. Reactions of Rh nd^O)^(dpp)^ under Vacuum The dpp dimer, both in the solid state and in CT^Clj solution, is sensitive to vacuum. Pumping on the Rh 2(CO) 4(dpp) 2 solid under vacuum for 1 h changed the colour from yellow to light brownish-yellow. The FT-IR solid state spectrum revealed new peaks of very low intensity at 2063, 1817, and 1784 cm .^ However, the major species present after the vacuum treatment is still Rh 2(CO) 4(dpp) 2 as shown by the strong peaks at 1985, 1973, 1744, 1721 cm - 1 pertinent to the dpp dimer (see Section 4.2.2 for discussion). The yellow CH 2C1 2 solution (20 mL) of the dpp dimer (70 mg, 5.7 x 10~ 6 mol) turned red on subjection to vacuum. The FT-IR spectrum of the red solution showed peaks at 1968, 1761, 1736, and 1721 cm - 1, which is little different to that of Rh 2(CO) 4(dpp) 2 (3.0 x 10" 3 M) dissolved in CH 2C1 2 under CO. 31 1 The P{ Hi NMR spectrum of the red solution revealed a broad doublet centred at 9.5 ppm ( J R ^ _ p = 151 Hz), similar to that of Rh 2(CO) 4(dpp) 2 dissolved in CH 2C1 2 (see Section 2.1.7.6). Precipitation from the red solution (5 mL) using hexanes (10 mL) afforded a yellow solid with an IR peak at 1966 cm 1 (Nujol), due to a terminal v(CO) stretch. Yield: 0.02 g. The result of the elemental analysis of the isolated yellow solid is as follows : C : 44.87, H : 4.11%. Evacuation to dryness of the CH 2C1 2 solution of Rh 2(CO) 4(dpp) 2 yielded a brownish-orange solid, with both teminal and bridging CO stretchings at 1968 and 1721 cm \ respectively (see Section 4.2.2 for discussion). 2.1.7.16. Attempted Synthesis and Isolation of RhH(CO)(dpp*)(dpp) 65 (A) Method 1. -4 A benzene (3 mL) suspension of Rh 2(CO) 4(dpp) 2 • CgH g (0.16 g, 1.31 x 10 mol) was degassed twice by freeze-thaw cycles and H 2 (-1 atm) was introduced. The suspension was stirred under H 2 for 16 h. A golden yellow-orange solution resulted. Hexanes were added and the mixture was stored in a fridge at ~4°C for 27 h. A yellow solid was isolated and dried under H 2 for 9 h. Yield: 0.1 g. Elemental analysis does not fit the formulation for RhH(CO)(dpp*)(dpp). C^H^^OP^Rh requires C : 69.04, H : 5.58%; found C : 65.86, H : 5.84%. lH NMR shows a mixture of species, including RhH(CO)(dpp*)(dpp), Rh 2H 2(CO) 2(dpp) 3, and RhH(dpp) 2 in a ratio of 6 : 1 : 1.4 (see Sections 4.3.1 and 4.3.2). IR (Nujol) : 1981 cm - 1 (»>(Rh-H)), 1908 cm" 1 (i>(CO)). IR (CH 2C1 2) : 1981 cm" 1 (j/(Rh-H)), 1952 cm" 1 (v(CO) of an unknown carbonyl species), 1910 cm" 1 (v(CO)). The RhH(dpp) 2 compound has v(Rh-H) at 2000 cm 1 in Nujol and 1979 cm" 1 in CH 2C1 2 > and there is an overlap of the KRh-H) peak of RhH(dpp) 2 with that of RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3 (see Sections 4.3.1 and 4.3.2). (B) Method 2. To Rh(CO)(dpp)2 + PF 6" (0.05 g, 4.5 x 10" 5 mol) dissolved in EtOH (3 mL) was added NaBH^ (0.003 g, 8.0 x 10" 5 mol) dissolved in EtOH (3 mL). The solution was stirred under Ar for 15 min and the volume reduced to 3 mL. Hexanes were added to precipitate out a yellow solid, which was filtered off, washed with hexanes, and dried under vacuum. Yield: 0.037 g (78%). 3 1P{ !H] NMR (CH 2C1 2): 6 7.5 ( J R h _ p = 136 Hz); -145.1 (septet, J p _ p = 710 Hz). IR (Nujol) : 833 cm" 1 66 (i'(P-F) of non-co-ordinated PFg") Thus, the reaction of Rh(CO)(dpp) 2 + PF~ with NaBH^ in EtOH under Ar generates Rh(dpp) 2 + PF^ - (see Section 4.3.2), rather than the hoped for RhH(CO)(dpp*)(dpp) species, possibly due to the labile loss of CO from Rh(CO)(dpp) 2 + PFg~, leading to the formation of Rh(dpp)2 + PFg~. Nevertheless, it is surprising that Rh(dpp) 2 + PF^ did not react with NaBH 4, in contrast to the + - 4 chloride analogue, Rh(dpp) 2 CI , which readily yields RhH(dpp)2-2.1.7.17. Bis[l,2-bis(diphenylphosphino)elhylene\rhodium(I) chloride' CH 2C1 2 > Rh(p = p) 2 + C f - C H 2 C 1 2 Some [RhCl(COE) 2] 2 dimer (0.40 g, 0.56 mmol) was dissolved in benzene (20 mL) to give a brownish-red solution, and a suspension of p = p (0.89 g, 2.2 mmol) in benzene (30 mL) was added to it. The mixture turned to a bright orange-yellow suspension, which was stirred under Ar for 30 min, concentrated to ~ 10 mL, and filtered; the yellow solid was collected, washed with benzene, and reprecipitated from CH 2C1 2 (10 mL) and Et 2 0 (30 mL). Yield: 0.9 g (86%); C ^ H ^ C l ^ R h requires C : 62.65, H : 4.56, CI : 10.47%; found C : 62.60, H : 4.70, CI : 10.19%. 3 1P{ 1H} NMR (CH 2C1 2): 6 71.9 (d, J R h _ p = 134 Hz). The compound crystallized with a mole of CH 2C1 2, which was retained even after prolonged drying under vacuum for 5 d. The CH 2C1 2 solvate was observed in *H NMR spectrum taken in CDCl^. 1 H NMR (CD 2C1 2): 8 7.18 (m, 4H), 7.39 (m, 40H). The CD 2C1 2 was used as solvent in the *H- NMR measurement in order to prevent the overlap of peaks of the complex in the aromatic region with those of the residual protons of the deuterated solvent, such as CDCL. UV-VIS spectra measured under Ar at 25° C, X „ , nm (log, Ae, 3 max v °10 M _ 1 cm - 1): C H X N 403.7 (3.77), 328 (3.83), 313 (3.94); toluene-CH 2C1 2 (2:1) 403.7 (3.71) [ L i t 1 8 : for Rh(p=p) 2 + BPh 4~, UV-VIS spectrum measured under Ar at 20°C, 67 Xmax' n m ( l o g 1 0 e ' M _ 1 c m _ 1 ) : c h l o r o b e n z e n e 4 0 5 (3-?9). 340 (3.82), 313 (4.04)]. Molar conductivity (22° C, Ar, CHXN), [Rh] = 0.30 mM : A M = 125 fi 1 cm 2 -1 2 mol . The conductivity value is typical of a 1:1 electrolyte. FAB (M/Z) : 896 amu; Rh(p = p ) 2 + has a mass of 895.7 amu. Table 2.3 illustrates the high mass fragments of the FAB mass spectrum of Rh(p = p)2 + . 2.1.7.18. Hydridobis[ 1,2- bis(diphenylphosphino)ethylene] rhodium(I), RhH(p=p)2 The preparative method was similar to that described in Section 2.1.7.5 for the corresponding dpp derivative. The complex Rh(p = p)2 + C l ~ • CF^C^ (0.15 g, 0.16 mmol) was dissolved in EtOH (45 mL) and stirred with NaBH 4 (0.02 g, 0.52 mmol) in EtOH (10 mL) for 1 h; the resulting solid was filtered off and washed with EtOH (20 mL). The brownish-orange solid was reprecipitated from benzene-hexanes (5 mL : 15 mL), washed with hexanes and vacuum dried to yield an orange solid. Yield: 0.08 g. The isolated compound was a mixture of RhH(p = p)2 and Rh(p = p) 2 + C f • CH 2C1 2 in the ratio of ~ 3:1, as confirmed by 3 1 P and lH NMR spectra. 3 H 4 5 3C1 Q 7 5 P 4 R h requires C : 67.75, H : 4.92%; found C : 67.05, H : 5.12%. 3 1P{ 1H] NMR (CD 2C1 2): 5 62.5 (d, J R h _ p = 145 Hz, RhH(p=p) 2), 71.9 (d, JRh-P = 1 3 4 H z ' R n ( P = P ) 2 + C f - C H 2 C y ; ^ r a t i o o f ^ n y d r i d e t 0 chloride complex is -3:1. lH NMR (CD 2C1 2): 8 -10.5 (d of q, J p _ H = 18 Hz, J R h _ H = 12 Hz, RhH(p = p)2). Other peaks in the aromatic region between 6.98 and 7.35 ppm cannot be assigned. IR (Nujol) : 1912 c m - 1 (i>(Rh-H)). The RhH(p = p)2 complex is 31 1 readily distinguished from RhHtdpe^; the P{ H3 NMR signal of the latter compound 4 occurs at higher field in C^D^: 5 56.4 (d, J R^_p = 142.5 Hz), and that for 68 Table 2.2. FAB mass spectrum of Rh(p=p), + C f • CH.C1 M/Z, amu a b possible assignment ' relative abundance 896 Rh(p=p) 2 + v strong 819 Rh[(p = p) 2-Ph] + weak 711 Rh[(p = p)2-PPh2] + very weak 684 Rh[(p = p) 2-C 2H 2PPh 2] + very weak a. P=P = PPh 2C 2H 2PPh 2 b. The intensity of Rh(p=p) + with a mass number of 499 was taken as the intensity reference. 69 RhH(p=p) 2tt in CgDg is at 8 61.2 ( J R h _ p = 144 Hz). 2.1.7.19. Reaction of a Mixture of Rh(p-p) * Cf and RhH(p=p)2 with CO The reaction of a mixture of Rh(p = p) 2 + Cr • C H j C ^ and RhH(p=p) 2, in the ratio of 1 : 3, with CO in benzene was carried out according to the literature 7 method used to synthesize Rh 2(CO) 4(dpp) 2 from RhH(dpp) 2 > by bubbling CO into a benzene solution (15 mL) of the mixture (0.1 g) and precipitating an orange-yellow solid with hexanes from the red-orange filtrate. Yield of a mixture: 0.050 g. IR (Nujol) : 2038 (P(CO) of an unknown species), 1973 (same as v(CO) for Rh(CO)(p = p) 2 + C f , see next section), and 1912 cm - 1 (i/(Rh-H) for RhH(p=p) 2). 2.1.7.20. Reaction between Rh(p= p) * Cf • CH 2 C1 2 and CO The title reaction was performed according to the literature method for the + - 5 formation of the analogous Rh(CO)(dpp)2 BF^ complex. Carbon monoxide was bubbled into a yellow suspension of Rh(p=p) 2 + Cl • CH 2 C1 2 (0.1 g, 0.1 mmol) in CH 2 C1 2 > The suspension turned clear in ~ 5 min, and the yellow solution was concentrated to ~ 1 mL under vacuum. Hexanes (10 mL) were added to afford a yellow solid, which was collected and dried under vacuum for 20 min. Yield: 0.09 g. IR (Nujol) : 1973 cm - 1 (f(CO), w). The weak terminal *>(CO) stretch tentatively -I- — suggests the existence of Rh(CO)(p=p) 2 Cl , corresponding to other reported Rh(CO)(P—P) 2 + complexes synthesized via the same routed The characterization is incomplete at this stage. This finding is in contrast to a previous literature report, claiming that the Rh(p=p) 2 + BPh 4 complex did not react measurably with CO in t t A 1 3C-APT NMR spectrum of the isolated mixture of Rh(p=p)2 + C l - • CH2CI2 and RhH(p=p)2 in CD2CI2 shows no resonances pertinent to any CH2 group; this shows that the reaction of Rh(p=p)2 + C l ~ with NaBH4 does not give RhH(dpe)2-70 o 18 chlorobenzene at 25° C. 2.2. INSTRUMENTATION (i) Infrared spectra, in Nujol mulls between Csl plates, in KBr pellets, or in Q-^C^ solution using KBr cells of 0.5 mm pathlength, were recorded on a Perkin Elmer 598 grating spectrophotometer or a Nicolet 5DX FT-IR instrument. (ii) UV-VIS spectra were recorded on a thermostatted Perkin Elmer 552 A spectrophotometer. Specially constructed, anaerobic spectral cells made of quartz (Figure 2.1) with 1.0 and 0.1 cm cell pathlengths were used. The weighed sample was placed in the quartz cell while the solvent was deoxygenated by a freeze and thaw static vacuum technique in the side-arm flask prior to mixing. In order to obtain the solid state UV-VIS spectrum, the Nujol mull sample was applied to a strip of filter paper, which was then attached to the central window of a quartz cell for measurement. (iii) *H NMR spectra were recorded on Bruker WP-80, Varian XL-300 or Bruker WH-400 spectrometers, with tetramethylsilane (TMS) at 5 0.0 ppm as standard. 3 1P{ 1H] NMR spectra were recorded on a Varian XL-300 (121.4 MHz for 3 1P) or a Bruker WP-80 (32.3 MHz for 3 1P). The reference for the 3 1P{ 1H] NMR spectra was the signal for triphenylphosphine at -6 ppm (relative to 85% 19 H^PO^). Chemical shifts are positive in the direction of decreasing field and are reported relative to 85% H 3P0 4. Simulation of the 3 1 P and 1 H NMR spectra was performed using a Bruker ASPECT 200 NMR PANIC program. UC{lKl NMR spectra were recorded on a Varian XL-300 (75 MHz for 1 3C), 71 attached to vacuum line B14 **— Quartz -cell Figure 2.1. Schematic representation of an anaerobic spectral cell. 72 with the internal standard being TMS at 6 0.0 ppm. All spectrometers were operating in the Fourier transform mode and were equipped with variable temperature control. All samples were sealed under Ar by a rubber septum, unless otherwise specified. A high pressure thick-walled NMR tube, sealed by a Teflon stopper, was kindly supplied by Professor Cullen's group. (iv) Conductivity measurements were made at 21° C, under anaerobic conditions, using a Thomas Serfass conductivity bridge and cell. (v) FAB spectra were recorded on a mass spectrometer (AEI MS9), modified and converted by personnel of the electronics shop of this department. A 7-8 kV accelerated xenon beam is employed. EI mass spectra were taken from Kratos MS 50 Mass Spectrometer; the electron energy was maintained at 70 eV. Both the FAB and EI MS spectra were recorded by the Mass Spectrometry Services in this department (vi) Elemental analyses were performed by Mr. P. Borda of this department. (vii) Fast kinetics ( t ^ in the 5 ms - 10 s region) were measured using a thermostatted Durrum 110 stopped-flow apparatus, equipped with a 2-cm light path cuvette. (viii) Gas chromatograph analyses were performed on: (a) a Varian 6000 GC instrument equipped with a 20\ 20% tri-o-cresylphosphate on 60/80 chromosorb-P column, and flame ionization detector (FID) for GLC separation and detection. Nitrogen was used as the carrier gas, and the column was operated at 40° C; 73 (b) a Carle 311 machine fitted with a 6', 8% OV-101 on chromosorb column (He carrier, FID detector). (ix) Gas-uptake for stoichiometric and kinetic studies, as well as for solubility data, was measured on the constant pressure gas-uptake apparatus as described in the following section. 2.3. GAS-UPTAKE APPARATUS 2.3.1. The Apparatus The constant pressure gas-uptake apparatus, used for determination of reaction stoichiometries and for kinetic studies of reactions involving the gas molecules Hj, CO, and 0 2, is represented schematically in Figure 2.2. The apparatus consisted principally of three parts, the reaction vessel, the measurement unit, and the gas-handling part The reaction vessel was a small, pyrex, two-neck flask (A), equipped with a dropping side-arm bucket The flask was connected to a capillary oil manometer (D) containing n-butyl phthalate of negligible vapour pressure, and was clipped to the piston-and-wheel mechanical shaker (I) driven by a Welch variable speed electric motor, whilst held in the thermostatted oil-bath (B). The oil-bath consisted of a four-litre glass beaker with silicone oil, and was placed in a polystyrene-foam lined wooden box for insulation. The capillary manometer (D) was connected through tap K to a gas measuring burette, consisting of a precision bored tube (N) of known diameter and a mercury reservoir (E). This measuring unit was thermostatted at 25° C in a water bath. The gas burette was connected via an Edwards high vacuum needle valve (M) to the gas-handling part of the apparatus, which consisted of a mercury 74 Figure 2 . 2 . Schematic representation of the constant pressure gas-uptake apparatus. 75 manometer (F), a gas inlet (Y) and a Welch Duo-Seal rotatory vacuum pump (G). Thermostatting was controlled by Jumo thermo- regulators and Merc-to-Merc relay control units, stainless steel jacketed, electrically isolated heating coils, along with mechanical stirring. The temperatures of the 2 baths could be maintained within ± 0.1° C of the set temperatures. The gas-uptake was measured with a vertically mounted cathetometer, and times were recorded from a Lab-Chron 1400 timer. 2.3.2. Procedure for Catalytic Hydrogenation Experimental Run 2.3.2.1. In Situ Hydrogenation of 1-Hexene using the RhH(dpp)j/CO Catalyst System (a) Non-injection method Stage 1 : In a typical experiment, 5 mL of toluene and an appropriate amount of 1-hexene (10 uL - 0.6 mL) were transferred by either a pipette or a syringe into the 25 mL reaction flask. The precursor catalyst complex, RhH(dpp)2, was placed in a small sample bucket and suspended in the flask on the side-arm hook. The flask with the connected spiral arm was attached to the gas-handling part of the apparatus at joint O. The solution was degassed three times by a freeze and thaw static vacuum technique. The reaction flask was then filled with CO to a pressure slightly less than 760 torr, and taps C and P were then closed. The flask and spiral arm could be removed and connected to the capillary manometer at H. The flask was then placed in the oil bath, and clipped to the mechanical shaker, which was then started. The rest of the apparatus up to tap C was evacuated with taps H, K, L, J, and M open. After about 10 min shaking to obtain thermal equilibration between the reaction vessel and the oil bath, and to saturate the solution with gas, the shaker was stopped. 76 Meanwhile, CO was admitted to the rest of the apparatus until the gas pressure was somewhat lower than that desired. Tap C was opened and the pressure increased to 760 torr. The needle valve (M) and taps K and L were closed, and the initial reading of the mercury level in the gas measurement burette (N) taken. The gas pressure in manometer F was then raised above 760 torr. The bucket containing the RhH(dpp)2 complex was dropped into the solution and the shaker and timer were both started. The gas-uptake was indicated by the difference in the oil levels of the manometer (D). The manometer was balanced by allowing gas to enter the burette through the needle valve (M) to balance the oil levels and noting the corresponding rise in the mercury level of the burette at appropriate time intervals. Stage 2: The uptake stopped after a net 1.5 equivalents of gas were consumed in less than 10 min [ 2RhH(dpp)2 + a m i > Rh (rn) Mpp) + 2dpp + H 2 (2.1) (see Section 4.3.4)]. Taps H and C were closed,, and taps K. and L open. The spiral coil and flask were then reattached to joint O of the gas-handling part of the apparatus. The yellow solution in the reaction flask was then frozen and degassed three times with the freeze and thaw static vacuum technique. Stage 3 : Taps C and P were then closed, and the spiral arm and flask were then removed and attached to the manometer at H. The flask was placed in the oil bath preheated to the desired temperature, and clipped to the mechanical shaker. The rest of the apparatus up to tap C was evacuated keeping the taps H, K, L, J, and M open. About 5 min were allowed for thermal equilibration between the reaction vessel and oil-bath. Meanwhile, H 2 was admitted into the rest of the apparatus until the gas pressure was somewhat lower than that desired. This procedure was repeated three times. Tap C was opened and the pressure adjusted to the desired value. The needle 77 valve (M) and taps K and L were closed, and the initial reading of the mercury level in the gas measurement burette (N) taken immediately. The shaker and timer were started. The gas pressure in the gas-handling part was then raised over that in the reaction flask. The gas-uptake indicated by the difference in the oil levels of the capillary manometer (D) was measured (as mentioned before) by allowing the gas into the mercury reservoir via the needle valve (M) to balance the oil levels and noting the corresponding rise in the mercury level of the burette (N) at appropriate time intervals. The initial rapid gas absorption (within 50 s) that results from H 2 dissolving in toluene at the specific temperature is then subtracted from the total absorption at a particular time to give the net H 2~ uptake caused by the hydrogenation process, (b) Injection method Alternatively, the same results for such in situ hydrogenation of 1-hexene were obtained by the injection method. The above experimental procedure up to Stage 2 was .adopted for the injection method. After the yellow solution, containing Rh 2(CO) 4(dpp) 2, in the reaction flask was frozen and degassed three times, ~ 770 torr of Ar was introduced. Under a slow flow of Ar, the side-arm was taken out of the flask and replaced rapidly by a silicon rubber septum. The yellow solution in the flask was then frozen immediately by immersing the vessel into liquid N 2 and degassed three times with the freeze and thaw static vacuum technique. Hydrogen was then introduced at slightly less than 760 torr. The taps C and P were closed and the coil and flask were then transferred and attached to the capillary manometer at H. The system up to tap C was evacuated and then filled with H 2 up to 760 torr thrice. Finally, tap C was opened and the whole system was quickly adjusted to 760 torr; taps K, L closed and the shaking and timing started. A net gas evolution (~ 1 equivalent) was observed and this was complete in about 450 s. At this stage, the 78 rhodium hydride containing a dangling phosphine, RhH(CO)(dpp*)(dpp), has been generated (see Sections 4.3.1, and 4.5.2) : H2/2dpp Rh 2(CO) 4(dpp) 2 < _ > 2 RhH(CO)(dpp*)(dpp) + 2 CO (2.2) Then the taps K and L were opened, previously degassed 1-hexene under Ar was syringed in via the septum, and the shaking and timing started. Taps K and L were closed after ~45 s shaking. This procedure was used because in the blank experiments using only the toluene solution with injection of 1-hexene, there is initial "apparent gas absorption" in the oil manometer (-45 s) that results from a change in the temperature of the reaction vessel contents, when 1-hexene (the substrate) at temperature lower than 31°C (reaction temperature) is injected. Though the 1-hexene placed in a Schlenk tube was pre-heated to 31°C in a temperature regulated oil-bath, there was a decrease in temperature of 1-hexene during its transfer from the oil-bath to the gas-uptake apparatus via a syringe. Fortunately, in this catalytic hydrogenation system, the H 2 uptake in this initial 45 s time period is negligible (< 5% of the reaction). The rise in the mercury level in N was then measured at appropriate intervals of time. 2.3.2.2. Hydrogenation of 1-Hexene using Rh(dcpe)^CI or Rh 2(CO)^(dpp)2 as Catalyst Precursor The hydrogenation procedure followed exactly that described in stage 1 in Section 2.3.2.1, except that H 2 is used instead of CO and the appropriate Rh catalyst complex employed instead of RhH(dpp) r 79 2.4. S T O I C H I O M E T R I C G A S - U P T A K E The same procedure as that used for gas solubility experiments was applied (see the following Section). The total gas-uptake observed was corrected for gas solubility under the same conditions. 2.5. M E A S U R E M E N T S O F G A S S O L U B I L I T I E S The gas-uptake apparatus described in Section 2.3.1 was used for measurement of solubility of H 2 in toluene, at specific temperatures and pressures. The following procedure was used to measure the solubility of H 2 in toluene. Typically, toluene (5 mL) was transferred to a flask consisting of a tap Z (Figure 2.3) and degassed using the freeze-thaw procedure; the taps C and P were closed and the flask along with the spiral arm transferred to the capillary manometer (D). The flask was shaken for ~ 5 min to allow for thermal equilibration between the flask and the oil bath. Meanwhile, the system up to tap Z was evacuated, H 2 was introduced and its pressure adjusted approximately to that required. After the 5 min thermal equilibration period, the shaking was stopped and tap Z then opened. The exact pressure was noted; the taps K, L and the needle valve (M) were closed, and the timer and shaker started immediately. The immediate, diffusion controlled gas-uptake was then measured. The solubility of H 2 in toluene was found to obey Henry's Law (Figure 2.4, Table 2.3) to give K values of 3.16 mM atm - 1 at 18° C, 3.23 mM atm 1 at 26°C, and 3.29 mM atm-''' at 31°C, where K = [Hj/partial pressure of 20 H 2 - The literature values for the solubility of [HJ in toluene at different temperatures were as follows : 3.45 mM atm 1 at 18° C, 3.58 mM atm - 1 at 26° C, — 1 o 21 and 3.69 mM atm at 31° C. Another literature source reports lower values for H 2 solubility in toluene: 2.77 mM atm 1 at 15° C, 2.94 mM atm - 1 at 25° C, and 80 CSS connection to spiral coil e Tap 4 V ^ \ Figure 2.3. Reaction flask used for the measurements of gas solubilities in different solvents. Tap Z prevents the evaporation of volatile solvent from the flask into the spiral coil during the thermal equilibration process under vacuum. Figure 2.4. Hydrogen solubility in toluene at 31, 26, and 18° C, and specific pressures. 82 Table 2.3. Solubility of H. in toluene at specific temperatures and pressures. P J J , torr [Hj], mM Temperature, °C 716 3.10 31 526 2.29 31 356 1.48 31 146 0.61 31 727 3.09 26 567 2.42 26 367 1.54 26 167 0.72 26 738 3.07 18 548 2.28 18 358 1.40 18 168 0.68 18 83 3.13 mM atm 1 at 35° C. The sets of data obtained from the gas-uptake studies were used. The solubility of CO in CH 2C1 2 cannot be measured accurately at 18-25°C, using the gas-uptake apparatus, because of the high vapour pressure of CH 2C1 2- The shaking of the flask during the uptake caused the vaporization of CH 2C1 2, which is in competition with the gas absorption process. A net "gas evolution" was observed for 20 the attempted solubility measurement. The same literature source cited for the H 2 solubility provides the solubility of CO in CHCl^ as follows : P^Q = 610 torr, [CO] = 6.29 mM at 18° C; P C Q = 593 torr, [CO] = 6.16 mM at 20° C; P C Q = 539 torr, [CO] = 5.71 mM at 25° C. Assuming that these solubility values also hold for CH 2C1 2 solvent, and after correction for the vapour pressure and density data, the solubility of CO in CH 2C1 2 is 3.86 mM at P C O = 421 torr, 18° C; 3.64 mM at P C Q = 393 torr, 20° C; and 2.34 mM at P C Q = 246 torr, 25° C. These sets of values are taken for use in the kinetic calculations in Chapter 3. The solubility of HCI in CH 2C1 2 or CH^CN was measured by volumetric titration method. A literature value available for CHCl^ solution (0.255 M at P^Q = o 20 593 torr, 18° C) is not considered applicable, because HCI is a polar gas and its interaction with polar solvent is likely to yield a wide range of solubilities in various solvents. The method is a back titration procedure (modified by Mr. C.Y. Sue in this laboratory) where excess NaOH solution is added to neutralize HCI dissolved in CH 2C1 2, and the unreacted NaOH is then titrated with potassium hydrogen phthalate solution. To a temperature regulated CH 2C1 2 (20 mL) solution degassed thrice in a Schlenk tube by freeze and thaw static vacuum tehnique, HCI gas at a specific pressure was introduced via a manometer; the dissolution of HCI was instantaneous as 84 manifested by a sudden drop of pressure in the manometer. Since the HCI gas reacted with a silicone rubber septum, a ground glass stopper was used for sealing the Schlenk tube. After the first withdrawal of HCl/CHjClj solution, the ? H Q in the container dropped. Thus, it was not possible to keep an account of the PJ^Q accurately. However, the HCI content in the C H X l j solution can be analyzed accurately by volumetric titration. Typically, C H j C l j (2 mL) solution, consisting of dissolved HCI, was added to excess NaOH (10 mL, 47.8 mM) dissolved in wet EtOH. This mixture was swirled for 2 min, 3 drops of phenolphthalein added, and titrated with a standardized ethanolic solution of potassium hydrogen phthalate (9.86 mM). The titration end-point was reached when the color of the mixture changed from pink to colorless. The back titration procedure gave a value of 0.18 M under ~360 torr HCI at 18° C for the solubility of HCI in C H X l j . Using the same procedure described above, the solubility of HCI in CH^CN was found to be 0.25 M under ~320 torr HCI at 22° C, and the conductivity of such -1 2 -1 a CHjCN solution was measured to give A ^ as 27.6 £2 cm mol . Thus, the HCI dissolved in C H X N does not dissociate completely to give a 1:1 electrolyte 2 (according to literature , a typical value for a 1:1 electrolyte in CH^CN is in the range of 120 to 160 fi-1 cm 2 mof 1). 2.6. THE CATALYSIS APPARATUS AND CONDITIONS FOR ATTEMPTED DECARBONYLATION OF BENZALDEHYDE A one-piece glass reaction vessel (~ 4 mL volume), having a gas-inlet tube and a 20 cm water condenser with top sampling port, was used (Figure 2.5). A -78° C trap was placed after the condenser to collect any condensate. The vessel was typically charged with a solution of benzaldehyde (0.3 mL, 3.2 mmol) and catalyst 85 r Suba-seal septum H 20 out reflux condenser >-H2Q In • acetone/ dry-ice . -78*C rr - • to vacuum line/Ar IT- Temperature regulated oil-bath o i l -bubbler Figure 2.5. The apparatus set-up used for studying decarbonylation of aldehydes. 86 (30 mg, 0.03 mmol for Rh(dcpe)2 + Cl~) in C H X N (0.6 mL), which was then deoxygenated by three freeze-thaw cycles. The reaction mixture was then stirred at constant temperature in the range of 25 - 96° C (at 25° C, solvent = CH j C l j ; 70° C, solvent = CH^CN; 95° C, solvent = neat benzaldehyde). The course of the reactions was monitored by periodic sampling, using long glass capillary tubing, and subsequent GC analysis. The yellow catalyst was recovered by the addition of diethyl ether 31 (~5 mL) and found to be unchanged after the reaction as observed by P NMR. No Rh(CO)(dcpe)2 + Cl species was observed in the IR spectrum of the recovered solid. 2.7. ISOLATION AND DETECTION OF HYDROGENATED ALKENE PRODUCTS OR DECARBONYLATED ALDEHYDE PRODUCTS BY GC. (i) Decarbonylation of aldehyde An OV-101 column (6' 8% OV-101 on chromosorb) was found to be suitable to separate benzaldehyde from benzene (the decarbonylated product). The results showed that no decarbonylation occurred for [benzaldehyde] = 3.6 - 9.8 M, [Rh(dcpe)2 + Cl ] = 0.036 - 0.12 M, temperature = 25 - 95° C in C H X I j or CHXN. Conditions: Carrier gas: He, 25 mL min ^ ; Column temperature: 95°C; Injected volume: 1 M L ; Heated inlet: 165° C; Attenuation: 128; Retention times: (1) C H X l j : 0.3 min (2) CHjCN : 0.4 min (3) benzene : 0.8 min (4) benzaldehyde : 3.5 min. (ii) Hydrogenation of 1-hexene 87 The organic products of the hydrogenation of 1-hexene were separated from the Rh catalyst by vacuum distillation and then subjected to GC analyses. A 20% tri-o-cresylphosphate on 60/80 chromosorb-P column (2fJ) was used to separate the hexenes-hexane mixture. Conditions: Carrier gas: N^ ; Column temperature: 40°C; Injection volume: 1 ML; Injection port temperature: 220° C; Attenuation: 16. A 0.4% toluene solution of pentane (v/v) was used as internal standard for quantitative analysis of the hydrogenated products. The response factor for a species Y is defined as: Area of Y # of mol of pentane injected x Area of pentane # of mol of Y injected Retention times (response factor): (i) pentane : 5.2 min (1); (ii) hexane : 11.8 min (1.3); (iii) 1-hexene : 13.9 min (1.4); (iv) trans-2-hexene : 15.9 min (1.4); (v) cis-2-hexene : 17.1 min (1.4). One juL of the solution containing the hydrogenated products and 1 ML of the standard pentane solution were co-injected into the GC. The areas for the pentane, and hydrogenated products (hexane, 1-hexene, cis-, and trans-2-hexenes) were measured and divided by the corresponding response factors. Since the amount of pentane injected and its corrected area is known, the quantitative amounts of hydrogenated products can be calculated. CHAPTER 3. SYNTHESES, CHARACTERIZATION, AND REACTIONS OF l,2-BIS(DICYCLOHEXYLPHOSPHINO)ETHANE COMPLEXES OF RHODIUM(I) 3.1. INTRODUCTION A major strength of homogeneous catalysis, especially with systems involving tertiary phosphine ligands, is the opportunity afforded to tailor ligands via varying the substituent groups on the phosphorus atoms.1 Bulky ligand systems frequently give non typical reactions or products. Thus, it has proven possible to isolate 14-electron 2 3 complexes with very bulky ligands, such as Pd[P(t-Bu).j]2 and RhCKPCy.^. Novel complexes containing, for example, arene, C0 2, N 2 and hydride ligands are also reported. The complexes Ni(arene)[PCy 2(CH 2) nPCy 2] (where arene is benzene, naphthalene or anthracene, and n is 2 or 3) and Ni(naphthalene)(PCy3)2 have been 4 prepared. While the reaction of tertiary phosphines (L) with [RhCl(COE) 2] 2 is a common method for preparing RhClL^ complexes, a similar reaction of PCy^ under N 2 yields RhCKN^PCy.^. 5 Some transition metal hydrides can be stabilized through 6 7 steric effects. Thus, while NiHBrL 2 and PdHBrL 2 > with L = PEt^, decompose rapidly at room temperature, the corresponding complexes with L being P(i-Pr) 3 or PCy^ are stable and isolable. A crystal structure of trans- PtH^PCy.^ has been reported, this type of trans-PtH 2L 2 complex being stabilized only with bulky ligands. Also, use of bulky phosphines has led to compounds of unusual oxidation states being isolated. Thus, while the reaction of RhClj- 3H 20 with PEt^ gives the RhCl^L^ complex,^ similar reactions with PEt(t-Bu) 2,^ and P(o-Tol) 3 1 1 give the paramagnetic 89 Rh(II) nionomeric complexes, RhCljI^. Carbon dioxide complexes are rare because of the weakness of bonding in most cases. The crystal structure of a carbon dioxide 12 complex containing bulky phosphine ligand, NKCOjXPCy^, has been reported. The reaction of NiH(CH 3)(PCy 3) 2 with C 0 2 yields NiH(0 2CH)(PCy 3) 2; 1 3 the analogous 14 trans- PtHCOjCHXPCy^ n a s been characterized by X-ray crystallography. Novel chemistry has sometimes been achieved, such as C-H bond activation by Pt(dcpe),^ which was prepared by reductive elimination of neopentane (npH) from cis-Pt^H(npXdcpe). The complex, Pt(dcpe), reacted with C-H bonds in some saturated, and unsaturated hydrocarbons, such as cyclopentane, 1,1,2,2,-tetramethylcyclopropane, tetramethylsilane, benzene, and mesitylene. An excellent review by Garrou 1 6 has described the susceptibility of co-ordinated tertiary phosphine ligands to P-C bond scission reactions at transition metal centres of low oxidation state and co-ordination unsaturation. Thus, the use of alternative ancillary ligands is to be encouraged, especially for prolonging catalyst stability and activity. It appears that the ease of P-C cleavage in some iron subgroup cluster systems follows the order of P-C > P-C 2 > P-C 3. 3 sp sp sp Despite the novelty of the phosphine systems containing bulky aliphatic substituents on the phosphorus atoms, these systems have been less thoroughly studied compared to the phosphine systems containing aromatic substituents. Part of the theme of this thesis was to synthesize the Rl^CCCO^CP— P^ series of complexes containing phosphine ligands with bulky aliphatic substituents, such as the dcpe group. 90 3.2. REACTIONS OF [RhCl(COE)2] 2 WITH 2 OR 4 EQUIVALENTS OF DCPE The synthesis of Rh(dcpe)2 + X followed the route of James and Mahajan 1 7 via a stoichiometric substitution reaction at [RhCl(COE) 2] 2 using 4 equivalents of dcpe. If less than a 4-fold excess of dcpe were used, then a mixture of products in equilibrium with one another results (see below). Common starting materials for syntheses for Rh(I) phosphine complexes are the dimeric [Rh*Cl(diene)]2 or [RI^CKalkene)^ 2 complexes, where diene = 1,5-COD,18 NBD, 1 9 1,5-hexadiene,20 and alkene = COE, 2 1 or ethylene.22 The labile 22 23 precursor complexes, [RhCl(CO) 2] 2 or [RhCl(acac)]2 , are also frequendy used. Substitution reactions proceed via the cleavage of the chloride-bridged dimer. A relatively strongly co-ordinated diene, such as 1,5-COD, is retained in the product. With the monoene precursor (e.g. [RhCl(COE) 2] 2), in which COE is a more weakly co-ordinated species, the chloride bridge is retained in the product, while the phosphine ligand displaces some COE ligand (equations 3.1 and 3.2). In the case of [Rh C K C O ) ^ as precursor, the substitution product contains one carbonyl ligand per rhodium atom. / IK / C K / 1/2 K Rh OK ^L l ' N < L ci" (31)' 24 8 36 = N B D . C O D , I 1,3-butadiene, 1,3- cyclohexadiene PPh 3, MePPh2, PMe2Ph, diop.etc 2L= dpp, * P N M R ( 3 6 , C D 3 0 D ) = 2 5 Mpprr^d, JRh-p = 148H2) = N B D 91 / /^ / C K v>\ . d-acetone p ^ C K .^w Rh Rh-" + d PP • ( Rh Rh^ cr * \ o p « 3 2 p p m V P P JRh-P=187Hz O p =6-4 ppm jRh-P= 132Hz 5 p -30-3 ppm JRh-P=184Hz The substitution reaction of [RhCKCOE)^ using 2 equivalents of dcpe is solvent, temperature, and concentration dependent Different products were isolated when using CH-^C^, benzene or THF as solvents, as shown in Table 3.1. With Qh^Clj as solvent, a mixture of products, namely Rh(dcpe)2 + Cl , 22, and RhCl(dcpe)(CH 2Cl 2), 28, was isolated 02 : 28 = 1 : 3.4). Compound 28 had an 31 AMX P NMR solution structure. The products were separated from one another by reprecipitation thrice from benzene-hexanes mixture, since the cauon Rh(dcpe)2 + Cl 31 1 had lower solubility in benzene. Figure 3.1 (a) shows the Pi HI NMR spectrum of isolated RhCKdcpeXC^Clj)* C^H^; the spectrum shows 2 sets of doublet of doublets, Fitting a proposed structure with one P trans to Cl, and the other one trans to CH2CI2. One mole of benzene solvate was trapped in the isolated 28 during the puriFication procedure, as observed in the ^ H NMR spectrum. The elemental analysis also Fitted the formulation for RhCl(dcpe)(CH 2Cl 2)-C 6H 6 (Section 2.1.7.13 (A)); however, co-ordinated CH 2 C 1 2 could not be identified in the *H NMR spectrum recorded in CDC1,. Table 3.1. Compounds isolated from the substitution reactions of [RhCKCOE)^ with 2 equivalents of dcpe per 3 dimer. Solvent Product Color 31 1 PI H} JRh-P" Jp_ p, NMR : 8, Hz Hz ppm a (i) C H 2 C l 2 b RhCl(dcpe)(CH 2Cl 2). C g H 6 orange-yellow 75.3 125 25 99.2 157 25 (ii) Benzenec RhCl(dcpe)-0.1C 6H 6 yellow 95.8 200 -possibly polymer brown 47 - — (iii) THF d RhCl(dcpe)- THF yellow 96.1 199 -a. In CH 2C1 2 or CD 2C1 2 b. See Section 2.1.7.13 (A) c. See Section 2.1.7.13 (B) d. See Section 2.1.7.13 (C) 93 <5 p = 99-: J R h-p=157 ^ RhCl(dcpe)(CH2Cl2)-C6H6 : ^ 2 <5 p = 75 jJRh-P=125jf •3 1 T— I T - ' I I I 1 130 110 90 70 50 ppm Figure 3.1. (a) P{ H] NMR spectrum of the complex isolated from the substitution reaction of [RhCl(COE)2]2 with 2 equivalents of dcpe in C H 2 C 1 2 . 94 When the reaction was carried out in benzene solution, a yellow solid was initially isolated, followed by the precipitation of a significant amount of brown compound upon concentrating the solution under vacuum. The yellow solid was identified as RhCl(dcpe)* O.IC^H^, 22, which was extremely air-sensitive in the solid state, but stable when stored under Ar at -60°C. The P{ H] NMR spectrum revealed a doublet (Figure 3.1 (b)), which was invariant from 20°C to -75°C, showing the presence of equivalent phosphorus atoms. The elemental analyses for 22 are slightly high in the C and H contents ( ~0.5% error), and a small amount of CgHg presence was detected in the ] H NMR spectrum (Section 2.1.7.13 (B)). The *H NMR, 3 1P{ 1H} NMR, and UV-VIS spectra of 22 are very similar to those of the substitution product isolated from the THF solvent (see later). It is not possible at this stage to identify if compound RhCl(dcpe)« O.lC^Hg exists as a monomer or dimer, because the attempted molecular weight determinations, which required at least 2 to 3 d for measurements, were unsuccessful; decomposition of 22 occurred as seen in the darkening of color in C H j C l j solution. There have been previous reports concerning the formation of [RhCl(P-P)] n complexes (where n = l for P-P=2 PCy 3; 3 n = 2 for 25 26 26 27 28 dpp, dippe, dtbpe, diop; n = n, a polymer formulation for dcpe ). The 3 elemental analysis of RhCKPCy^ showed low carbon and hydrogen contents ( ~ 1 % 25 from the expected values), while [RhCl(dpp)] 2 was only formed in situ: 76 78 characterization details of the dippe, dtbpe and dcpe complexes were not provided. ' 27 The compound [RhCl(diop)(CgHg)] ^ has been reported, but the characterization was based solely on elemental analysis and molecular weight determination. The brown solid, 4Q, another compound isolated from the substitution reaction taking place in the benzene solution, was found to have very low C and H contents. 31 1 The P{ H} NMR in C H j C l j gave a very broad singlet (Figure 3.1 (b)), which was 95 6 p = 95-8 J Rh-P= 200 Hz RhCI (dcpe)-0-1 C g H 6 150 100 50 20° tO -75° C T -50 PPm <50 = P -47 room temp 31 1 Figure 3.1. (b) P( H} NMR spectra of the complexes isolated from the substitution reactions of [RhCKCOE)^ 2 with 2 equivalents of dcpe in C 6H 6. 96 further broadened at -60 C. The species 4Q must be a polymer, since the molecular weight was measured and found to be higher than 21,000 amu. The brown product was soluble in C H j C ^ but sparingly so or insoluble in most polar solvents. Of interest, compound 4Q slowly converts into Rh(dcpe)2 + Ci , in the presence of excess dpp (see Section 2.1.7.13 (B)). With THF as solvent, RhCl(dcpe)-THF, 41, was isolated. The elemental analysis fits the formulation (Section 2.1.7.13 (C)). The 3 1P[ 1H] NMR spectrum (Figure 3.1 (c)) of 41 is nearly identical to that of RhCl(dcpe)* 0.1CgHg, implying the same solution structure (see discussion later). The summary of the substitution reactions is provided in Figure 3.2. The products obtained from the reactions between [RhCKCOE)^ and 2 equivalents of dcpe in CH2CI2, benzene, and THF were in low yield, which indicated that other species are also present in the substitution process but are not precipitated out during the 3 isolation of the reaction products. Van Gaal and Van Den Bekerom proposed the reaction pathways shown in equation 3.3 for the stoichiometric substitution reaction of [RhCKCOE^j with PCy^. Several species were observed on the NMR time scale in C^D, al room temperature. 6^6 1/2 [RhCKCOE)^ * 2 PCy 3 *• RhCl(PCy 3) 2(COE) * -COE *- RhCl(PCy3). 209.9 Hz (3.3) 6 P 41.6, J Rh-P = 193.8 Hz 97 5p\=96-1 JRh-P^199 Hz RhCI (dcpe)-THF —U T I 150 i 50 -50 i -150 PP"i Figure 3.1. (c) P{ H] NMR spectxum of the complex isolated from the substitution reaction of [RhCKCOE)^ with 2 equivalents of dcpe in THF. 98 Rh Rh 2 p ^ p THF fi ID = COE ^ 2 „ Yield =49% \ ~ - ^ * R h C l ( d c p e ) ( C H 2 C l 2 ) - C 6 H 6 \ 38 4 P - P \ C 6 H 6 \ Yield= 14% RhCl(dcpe)0 .1C 6 H 6 39 + Yield = 30% polymer 40 37 Yield = 79% Figure 3.2. Substitution reactions between [RhCKCOE)^ 2 and dcpe (2 or 4 equivalents) in C H 2 C l 2 , C^H^ or THF, based on the characterization of the isolated products. 99 A series of equilibria takes place; in the presence of COE, the equilibria shift to the 3 left; in more concentrated solutions of rhodium, the equilibria shift to the right The large co-ordination chemical shift:}:$ of dcpe in RhCl(dcpe)* THF and RhCl(dcpe)* O.lCgHg, which is about 96 ppm downfield, is consistent with the chelating ligand forming a 5-membered ring with the Rh(I) centre.16 The ring strain expected for a phosphorus atom contained in a 5-membered ring is small, but an unusually large deshielding (versus, e.g. 4-, 6-, and 7- membered rings) 1 6 is noted, irrespective of the metal or other substituents in the ring. As commented by Garrou, the exact 25 nature of this phenomenon is still unknown. Slack et al. observed that as the trans 31 1 influence of the ligand trans to the phosphine decreases, the P{ H] NMR resonances shift to lower field and values of J R n _ p increase (see equations 3.2 and 3.3; trans 31 effects decrease in the order P > Cl). Thus, the P chemical shifts and coupling constants of [RhCl(dcpe)-solvate] n (solvate = THF or O.lCgHg, average: 96 ppm, 199 Hz) and Rh(dcpe)2 Cl (65.6 ppm, 131 Hz, trans phosphine) fit into the trend noted 25 by Slack et al. The RhCl(dcpe)-solvate could exist as one of the following structures: 1. A trigonal planar Y-shaped monomer, analogous to RhX(PCy 3) 2 (X = F, Cl, Br, I), 3 29 2. A T-shaped planar monomer, which has been proposed for RhH(PCy 3) 2 > 3. A four-co-ordinate chloride bridged dimer, suggested for [RhCl(P— P)] 2 > where P-P = dpp,25 dippe,26 dtbpe,26 and d i o p 2 7 The extremely air-sensitive nature of the RhCl(dcpe)-solvate species precludes accurate molecular weight determination. The [RhCl(dcpe)] 0 species with a molecular weight of 1120 amu is always observed as the parent peak in the EI mass spectra of tt Co- ordination chemical shift is defined as the change between the chemical shift of a non-co-ordinated phosphine ligand and the observed chemical shift upon co-ordination of the phosphorus ligand to a metal. 100 Rh(dcpe)2 + C f , RhHCl(dcpe) 2 + C f • HCI- OJCgHg, and RhCl 2(dcpe) 2 + C f . The decomposition of these species during the EI mass spectrum measurement is confirmed, when the FAB method is employed and gives an entirely different spectrum with parent mass at 947 (±3) amu for Rh(dcpe)2 + C l ~ (see Section 2.1.7.8). The formation of [RhCl(dcpe)] 2 as a major stable decomposition product indicates the possibility of the extremely air-sensitive, isolated RhCl(dcpe)-solvate being a monomer. The Y-shaped trigonal planar structure is more likely because the T-shaped structure would require inequivalent phosphorus groups in the RhCl(dcpe)-solvate, which does not fit the 31 1 observed doublet in the PI H] NMR spectrum. The unusual instability of RhCl(dcpe)-solvate in both the solid state and solution lends weight to the idea that such species are in fact intermediates en route to the formation of Rh(dcpe)2 + C l ~ , obtained from the reaction between [RhCl(COE) 2] 2 and 4 equivalents of dcpe in benzene, which is complete in 3 minutes (see below). The RhCl(dcpe)(CH2Cl2)-CgHg species is proposed to have a four-co-ordinate square planar structure, as evidenced in two sets of inequivalent phosphorus groups 31 1 observed by the P{ H] NMR spectrum. The NMR data also suggest that the RhCl(dcpe)-solvate monomer does not have a solvent molecule present as a fourth ligand. The Rh(dcpe)2 + Cl complex (22), a 1:1 electrolyte, can be prepared in high yield (79%) by adding > 4-fold excess of dcpe to [RhCl(COE) 2] 2 in C g H 6 (Section 17 17 2.1.7.8), the same method being used for preparation of the analogous dpm, dpe, 17 17 17 30 dpp, dpb, diop, and chiraphos complexes of type Rh(P—P) 2X (X = CI or BF 4). Treatment of 21 with AgX (X = BF^) or NH 4X (X = PF g) yielded the corresponding X~ salts. According to the 3 1P{ 1H} NMR spectrum down to -84°C (Figure 3.3), all 101 5 p = 6&6 JRh-P = 131 Hz 80 40 0 ppm Figure 3.3. The 3 1P{ 1H1 NMR spectrum of Rh(dcpe)2 + BF 4" • l/ 2 C H 2 C l 2 in CH 2Cl 2/d g-acetone at 19° C under Ar (the same spectrum for Rh(dcpe)2 + C f was recorded at 19° C and -84°C). the 4 P atoms are equivalent, the data being consistent with a square planar configuration. Four-co-ordinate, d complexes tend to have square planar rather than tetrahedral geometry, because of the stabilization achieved by a lower HOMO anti-bonding energy for the square planar geometry. More d electrons occupy the anti-bonding orbital in the T^ case. Steric requirements may sometimes overcome this effect and cause distortion toward T^ geometry. Crystallographic data are now available for the ionic complexes Rh(dpe) 2 + C10 4~, 3 1 Rh(dpp) 2 + B F 4 " 3 2 Rh(dpb) 2 + BF 4", 3 3 R h ( c h i r a p h o s ) 2 + C f 3 0 and Rh(dcpe) 2 +Cf (see Section 3.3). Distortion toward T d 102 geometry is observed in all the complexes described above, and to a much larger extent for Rh(dpb) 2 + (see Section 3.3 for discussion). However, the equivalence of the 4 P atoms on the NMR time scale at room temperature was observed for all the Rh(P—P) 2 + X complexes described above, resulting from a fast intramolecular rearrangement. A solid state visible spectrum of 21 showed absorption maxima at 403.6, 315, and 285 nm, the same as the solution values. The electronic absorption and emission 34-36 spectral data for square planar complexes have been analyzed in some detail. The R h ( P — P ) 2 + complexes, where P—P = dpm, dpe, dpp, dpb, diop, chiraphos, and dcpe, all exhibit a metal to ligand charge transfer band in the region of 400 nm. The equivalence of the solid state and solution UV-VIS spectra indicates that 21 has the same structure (ionic, 4 co-ordination around Rh) in the solid state and in polar solvents. It should be noted that the Rh(P—P) 2C1 complexes with P—P = dpm, dpb, and diop are five-co-ordinate in the solid state and in nonpolar solvents, but dissociate to give ionic chloride in polar medium, while the dpe and dpp analogues 17 contain ionic chloride. The UV-VIS spectrum of 21 obeys Beer's Law for [Rh] in both DMA and C H X N (DMA : [Rh] = 4.3 x 10~ 5 - 6.4 x 1 0 - 4 M; C H X N : [Rh] = 2.7 x 10" 5 - 4.8 x 10~ 4 M). 3.3. X-RAY STRUCTURE DETERMINATION OF Rh(dcpe)2 + C l " , BIS[l ,2-BIS(DICYCLOHEXYLPHOSPHINO)ETHANE] RHODIUM(I) CHLORIDE Yellow crystals of Rh(dcpe)2 + Cl were grown in air by slow diffusion of diethyl ether into a CH 2C1 2 solution of the compound. A single crystal X-ray diffraction study carried out by Dr. S.J. Rettig of this department revealed the 103 complex to be distorted square planar, consisting of a discrete cation and anion, as shown in Figure 3.4. Tables 3.2 and 3.3 show the bond lengths and bond angles for the structure of Rh(dcpe)2 + Cl . The Rh(dcpe)2 + cation, as a whole, possesses approximate T>^ symmetry. The Rh atom lies approximately in the P^ mean plane (displacement = 0.010 X ) , while the P atoms lie alternatively above and below P^ mean plane (displacement : -0.20, 0.20, -0.24, 0.24 X respectively for P(l)-P(4)). Such displacements are also observed in Rh(S,S-chiraphos)2 + ^ (Rh displacement out of plane = -0.008 X ; 4 P atoms lie 0.1 X alternatively above and below plane), and R h ( d p e ) 2 + 3 1 (Rh displacement = 0.03 X ; 4 P atoms displacement = 0.04 X ) . The average Rh-P distances in 3J is 2.33 X , very similar to that of Rh(S,S-chiraphos)2 + 3° (2.3 X mean), and Rh(dpe)2 + 3 1 (2.31 X mean). All the cyclohexyl groups are in chair conformations. The P-C and C-C bond distances are within the normal range for the cyclohexyl groups. The extent of distortion can be observed in the angles at the metal atom between trans ligands (trans P atoms), which would be 180° in a square-planar situation. Such angles were found to be 169 and 170° in Rh(dcpe)2 + C f , 150 and 156° in Rh(dpb) 2 + , 3 3 175 and 178° in Rh(dpe)2 + , and 173 and 175° in Rh(chiraphos)2+. Thus, the distortion toward T^ geometry in Rh(P— P ^ + complexes follows the following trend : Rh(dpb)2 + > Rh(dcpe)2 + > Rh(S,S-chiraphos)2+ > Rh(dpe)2 + . The distortion is determined by both the chain size and the bulkiness of the substituent group on the P atoms. The hydrogen atoms attached to C(6), C(12), C(18), C(30), C(42), and C(48) block any axial approach to the metal. The "blocking" hydrogen atoms probably contribute to the lack of reactivity in solution toward CL, and FL (see next Section). 104 Figure 3.4. Stereoscopic view of the RhCdcpe^ cation, and the numbering scheme used; 50% probability thermal ellipsoids are shown and hydrogen atoms have been omitted for the sake of clarity. 105 Table 3.2. Bond lengths (X.) with estimated standard deviations in parentheses for Rh(dcpe)2 + C f . Bond L e n g t h ( A ) Bond L e n g t h ( A ) Rh -P(1) 2 .3536(4) C f 23 -c 24) 1.536(3 Rh -P (2 ) 2 .3527(4) C 23 1 -c 28) 1.516(3 Rh -P(3) 2 .3156(5) C 24] -CI 25) 1.528(3 Rh -P (4 ) 2 .3304(5) C 25 -c '26) 1.496(4 P ( 1)-C(1 ) 1 .840(2) CI 26 1 f-C 27) 1.524(4 P( 1)-C ( 5 ) 1 .862(2) C f27] -c '28) 1.534(3 P( 1 )-C(11) 1 .870(2) C [29, -c r30) 1.525(3 P ( 2)-C ( 2 ) 1 .832(2) CI 29] -CI i34) 1.531(3 P ( 2)-C(1 7 ) 1 .870(2) c< 30] -c 31 ) 1.526(3 P(2)-C (23) 1 .865(2) CI '31 ; -c '32) 1.524(4 P ( 3)-C ( 3 ) 1 .850(2) c< 32] -c< [33) 1.509(4 P (3)-C (29 ) 1 .862(2) c '33] -c 34) 1.539(4 P (3)-C (35 ) 1 .867(2) c 35] -c 36) 1.535(3 P ( 4)-C ( 4 ) 1 .848(2) c< 35] -CI ,40) 1.500(3 P ( 4 ) - C ( 4 1 ) 1 .863(2) c< 36] -CI 37) 1.528(3 P(4)-C (47 ) 1 .862(2) Ci 37] -c 38) 1 .487(4 C( 1)-C ( 2 ) 1 .529(2) c< 38] -CI 39) 1.522(4 C ( 3)-C ( 4 ) 1 .517(3) CI 39] -CI 40) 1.541(3 C ( 5)-C ( 6 ) 1 .533(3) CI 41 ] -c 42) 1.523(3 C ( 5)-C(1 0 ) 1 .526(3) CI 41 ) -c< 46) 1.518(3 C ( 6)-C ( 7 ) 1 .522(3) CI 42] -c< 43) 1.509(3 C ( 7)-C ( 8 ) 1 .511(4) CI 43] -CI 44) 1.511(4 C ( 8)-C ( 9 ) 1 .524(4) C( 44) -ci 45) 1.502(4 C ( 9)-C(1 0 ) 1 .534(3) CI 45] -CI 46) 1.528(3 C( 1 1 )-C(12) 1 .524(3) CI 47) -c< 48) 1.527(3 C ( 1 1 ) - C ( 1 6 ) 1 .533(3) CI 47) -c< 52) 1.540(3 C( 1 2)-C( 1 3 ) 1 .520(3) CI 48) -c< 49) 1.517(3 C( 1 3)-C(1 4 ) 1 .516(3) CI 49) -ci 50) 1.517(3 C( 1 4)-C(1 5 ) 1 .510(3) c< 50) -C( 51 ) 1.526(4 C( 1 5)-C( 1 6 ) 1 .528(3) c< 51 ) -c< 52) 1.526(3 C( 17)-C(18) 1 .521(3) C( 53) -ci 54) 1.46(3) C( 17)-C(22) 1 .533(3) CI 53) -ci 54*) 1.53(3) C ( 1 8 ) - C ( 1 9 ) 1 .520(3) CI 53) -c< 53) ' 1.11(2) C(1 9)-C ( 2 0 ) 1 .521(3) C( 54) -CI 55) 1.67(4) C (20)-C (21 ) 1 .518(4) c< 55) -ci 54*) 1.29(3) C(21 )-C(22) 1 .527(3) 106 Table 3.3. Bond angles (deg) with estimated standard deviations in parentheses for Rh(dcpe)2 + C f . Bonds A n g l e ( d e g ) Bonds A n g l e ( d e g ) P ( l ) - R h - P ( 2 ) 81 . 9 2 ( 2 ) C( 18) - C ( 1 7 ) - C ( 2 2 ) 108 . 8 ( 2 ) P O ) - R h - P ( 3 ) 1 6 8 . 7 5 ( 2 ) C( 17) - C ( 1 8 ) - C ( 1 9 ) 1 1 1 . 4 ( 2 ) P ( l ) - R h - P ( 4 ) 9 7 . 8 7 ( 2 ) C( 18) - C ( 1 9 ) - C ( 2 0 ) 1 1 1 . 1 ( 2 ) P ( 2 ) - R h - P ( 3 ) 9 8 . 3 0 ( 2 ) C( 19) - C ( 2 0 ) - C ( 2 1 ) 1 10 . 7 ( 2 ) P ( 2 ) - R h - P ( 4 ) 1 6 9 . 7 3 ( 2 ) C( 20) - C ( 2 1 ] - C ( 2 2 ) 1 10 . 6 ( 2 ) P ( 3 ) - R h - P ( 4 ) 8 3 . 9 3 ( 2 ) C( 17 1 - C ( 2 2 ) - C ( 2 1 ) 109 . 6 ( 2 ) Rh - P ( 1 ) - C ( 1 ) 1 0 9 . 9 4 ( 6 ) P( 2 ) - - C ( 2 3 ) - •C(24) 1 1 1 . 3 6 ( 1 3) Rh - P ( 1 ) - C ( 5 ) 1 2 3 . 6 9 ( 6 ) P( 2 ) - - C ( 2 3 ) - •C(28) 117 . 2 8 ( 1 5) Rh - P ( 1 ) - C ( 1 1 ) 1 0 9 . 5 6 ( 6 ) C l 24 - C ( 2 3 ] - C ( 2 8 ) 1 10 . 0 ( 2 C ( 1 ) - P ( 1 ) - C ( 5 ) 1 0 2 . 2 3 ( 8 ) C l 2 3 1 >-C(24) - C ( 2 5 ) 1 1 1 .4(2*-C ( 1 ) - P ( 1 ) - C ( 1 1 ) 1 0 4 . 2 5 ( 8 ) C( 24 >-C(25 1 - C ( 2 6 ) 1 1 1 . 6 ( 2 C ( 5 ) - P ( 1 ) - C ( 1 1 ) 1 0 5 . 4 2 ( 8 ) C l 25 >-C(26 J - C ( 2 7 ) 1 1 1 . 3 ( 2 Rh - P ( 2 ) - C ( 2 ) 1 1 0 . 1 2 ( 6 ) Cl 26 >"C(27; - C ( 2 8 ) 110 . 7 ( 2 Rh - P ( 2 ) - C ( 1 7 ) 1 1 0 . 1 5 ( 6 ) Cl 23 >-C(28 1 - C ( 2 7 ) 1 1 2 . 1 ( 2 Rh - P ( 2 ) - C ( 2 3 ) 1 2 4 . 1 3 ( 6 ) PI 3) - - C ( 2 9 ) - - C O O ) 1 1 0 .27 ( 4) C ( 2 ) - P ( 2 ) - C ( 1 7 ) 1 0 4 . 1 4 ( 8 ) PI 3) - - C ( 2 9 ) - -C (34 ) 1 1 5 . 8 ( 2 C ( 2 ) - P ( 2 ) - C ( 2 3 ) 1 0 2 . 3 8 ( 8 ) Cl 30 1 - C ( 2 9 - C ( 3 4 ) 1 1 0 . 1 (2 C ( 1 7 ) - P ( 2 ) - C ( 2 3 ) 1 0 4 . 0 2 ( 8 ) Cl 29 1 - C ( 3 0 - C ( 3 1 ) 1 1 2 . 6 ( 2 Rh - P ( 3 ) - C ( 3 ) 1 1 0 . 2 4 ( 7 ) Cl 30 | - C ( 3 1 >-C(32) 1 10 . 1(2 Rh - P ( 3 ) - C ( 2 9 ) 1 1 4 . 9 5 ( 6 ) Cl 31 ) -C (32 >.-C(33) 109 . 5 ( 2 Rh - P ( 3 ) - C ( 3 5 ) 1 2 3 . 7 4 ( 7 ) Cl 32 ) - C ( 3 3 • - C ( 3 4 ) 1 1 1 . 8 ( 3 C ( 3 ) - P ( 3 ) - C ( 2 9 ) 1 0 1 . 8 2 ( 1 0 ) Cl 29 ) -C (34 >-C(33) 110 . 3 ( 2 C ( 3 ) - P ( 3 ) - C ( 3 5 ) 9 8 . 8 9 ( 1 0 ) P 3)- - C ( 3 5 ) - •C (36 ) 1 1 1 .33 ( 5) C ( 2 9 ) - P ( 3 ) - C ( 3 5 ) 1 0 4 . 0 6 ( 9 ) PI 3)- - C ( 3 5 ) - -C (40 ) 1 1 4 .99 ( 5) Rh - P ( 4 ) - C ( 4 ) 1 0 9 . 9 8 ( 7 ) C r 36 >-C(35 >-C(40) 1 1 0 . 7 ( 2 Rh - P ( 4 ) - C ( 4 1) 1 2 3 . 4 7 ( 6 ) c ,35 >-C(36 >-C(37) 1 1 1 . 4 ( 2 ' Rh - P ( 4 ) - C ( 4 7 ) 1 1 5 . 3 5 ( 6 ) c 36 ) - C ( 3 7 I - C O B ) 1 1 1 . 5 ( 2 C ( 4 ) - P ( 4 ) - C ( 4 1 ) 9 9 . 2 7 ( 9 ) c r 37 ) - C ( 3 8 ) - C ( 3 9 ) 1 1 1 . 2 ( 2 -C ( 4 ) - P ( 4 ) - C ( 4 7 ) 1 0 1 . 9 2 ( 1 0 ) c [38 ) - C ( 3 9 ) - C ( 4 0 ) 1 1 1 . 5 ( 2 C ( 4 1 ) - P ( 4 ) - C ( 4 7 ) 1 0 3 . 7 6 ( 9 ) c [35 ) - C ( 4 0 >-C(39) 1 1 1 . 9 ( 2 P( 1 ) - G ( 1 ) - C ( 2 ) 109.041* 12) p [4)- -C(41 )-- C ( 4 2 ) 1 1 1 .50 ( 4) P ( 2 ) - C ( 2 ) - C ( 1 ) 1 0 9 . 5 1 ( 1 2 ) p [4)- -C(41 )-- C ( 4 6 ) 1 1 4 .76 ( 4) P ( 3 ) - C ( 3 ) - C ( 4 ) 1 1 1 . 8 1 ( 1 4 ) c [42 ) -C(41 >-C(46) 1 1 0 . 8 ( 2 P ( 4 ) - C ( 4 ) - C ( 3 ) 1 1 1 . 6 2 ( 1 4 ) c [41 ) - C ( 4 2 ) - C ( 4 3 ) 1 1 1 . 7 ( 2 P( 1 ) - C ( 5 ) - C ( 6 ) 1 1 1 . 1 0 ( 1 3 ) c [42 ) - C ( 4 3 >-C(44) 1 12 .1 (2 P( 1 ) - C ( 5 ) - C ( 1 0 ) 1 1 6 . 2 5 ( 1 3 ) c [43 ) - C ( 4 4 ) - C ( 4 5 ) 1 1 1 . 0 ( 2 C ( 6 ) - C ( 5 ) - C ( 1 0 ) 1 0 9 . 8 ( 2 ) c [44 ) - C ( 4 5 ) - C ( 4 6 ) 1 1 1 . 5 ( 2 C ( 5 ) - C ( 6 ) - C ( 7 ) 1 1 2 . 0 ( 2 ) c [41 ) - C ( 4 6 ) - C ( 4 5 ) 1 1 1 . K 2 C ( 6 ) - C ( 7 ) - C ( 8 ) 1 1 0 . 9 ( 2 ) p [4)- - C ( 4 7 ) - - C ( 4 8 ) 1 1 1 .28( 4) C ( 7 ) - C ( 8 ) - C ( 9 ) 1 1 0 . 6 ( 2 ) p [4)- - C ( 4 7 ) - - C ( 5 2 ) 1 1 5 . 14( 5) C ( 8 ) - C ( 9 ) - C ( 1 0 ) 1 1 1 . 6 ( 2 ) c [48 ) - C ( 4 7 ) - C ( 5 2 ) 1 1 0 . 7 ( 2 C ( 5 ) - C ( 1 0 ) - C ( 9 ) 1 1 1 . 6 ( 2 ) c [47 ) - C ( 4 8 ) - C ( 4 9 ) 1 1 2 . 1(2 P( 1 ) - C ( 1 1 ) - C ( 1 2 ) 1 1 0 . 9 5 ( 1 3 ) c [48 >-C(49 ) - C ( 5 0 ) 1 1 1 . 6 ( 2 P( 1 ) - C ( 1 1 ) - C ( 1 6 ) 1 1 7 . 7 5 ( 1 3 ) c [49 ) - C ( 5 0 ) -C(51 ) 1 1 0 . 8 ( 2 c o n t i n u e d /... 107 C ( 1 2 ) - C ( 1 1 ) - C ( 1 6 ) 108.6(2) C ( 5 0 ) - C ( 5 1 ) - C ( 5 2 ) 111.7(2) C ( 1 1 ) - C ( 1 2 ) - C ( 1 3 ) 111.6(2) C ( 4 7 ) - C ( 5 2 ) - C ( 5 1 ) 110.7(2) C ( 1 2 ) - C ( 1 3 ) - C ( 1 4 ) 110.8(2) C ( 5 4 ) - C ( 5 3 ) - C ( 5 4 * ) 62(2) C ( 1 3 ) - C ( 1 4 ) - C ( 1 5 ) 110.5(2) C ( 5 4 ) - C ( 5 3 ) - C ( 5 3 ) ' 100(2) C ( 1 4 ) - C ( 1 5 ) - C ( 1 6 ) 111.6(2) C ( 5 4 * ) - C ( 5 3 ) - C ( 5 3 ) ' 105(2) C ( 1 1 ) - C ( 1 6 ) - C ( 1 5 ) 110.1(2) C ( 5 3 ) - C ( 5 4 ) - C ( 5 5 ) 75(2) P ( 2 ) - C ( 1 7 ) - C ( 1 8 ) 112.06(13) C ( 5 4 ) - C ( 5 5 ) - C ( 5 4 * ) 61(2) P ( 2 ) - C ( 1 7 ) - C ( 2 2 ) 117.18(13) C ( 5 3 ) - C ( 5 4 * ) - C ( 5 5 ) 85.3(15) 108 3.4. ATTEMPTED REACTIONS BETWEEN Rh(dcpe)2 + C f AND NaBHj, LiAlHj, H 2 , 0 2 , AND N 2 The Rh(dcpe)2 + Cl compound in CH 2C1 2 was unreactive toward NaBH^, or LiAlH 4, and the required RhH(P—P) 2 type product (Section 2.1.7.14) could not be made. Consequently, the preparation of Rh 2(CO) 4(P—P) 2 > via the bubbling of CO into a benzene solution of RhH(P—P) 2, cannot be achieved. A digression was made to study the interactions of Rh(dcpe)2 + Cl with small gas molecules, in an attempt to test its potential as a catalyst and for a comparison with the reactivity of other R h ( P - P ) 2 + systems. The Rh(dcpe)2 + Cl complex (21) in CH 2C1 2, when treated with ~1 atm of H 2 > 0 2 > or N 2 > was found to be unreactive toward these gas molecules. The same + 37 non-reactivity pattern was also observed for Rh(chiraphos)2 . However, the Rh(dpe) 2 + compound, 42, also containing 5-membered chelate phosphine rings, reacted reversibly with 0 0 (k = 0.25 M - 1 s" 1, k ,, = 2.6 x 10 - 3 s _ 1 at 30° C in l on ott 42 + + MeOH). Species Rh(chiraphos)2 , and Rh(dpe)2 (like 21) were both unreactive 37 toward H 2. The lack of reactivity of 21 toward H 2, and 0 2 can be rationalized either in terms of a very large k F F relative to k , or an immeasurably small k value. J 6 off on J on Blocking of the axial sites at the metal by the hydrogen atoms of the cyclohexyl rings is apparent in the solid state structure (Section 3.3) and could contribute to the non-reactivity (negligible k Q f i) in solution. 109 3.5. REACTIVITY OF Rh(dcpe)2 + C f TOWARD CO 3.5.1. Formation of Rh(CO)(dcpe)2 + The interaction of 22 with CO leads to the formation of Rh(CO)(dcpe)2 + C l ~ , but the only evidence is the IR data. Qualitative and quantitative data for CO binding have been reported previously by James and Mahajan1^ on Rh(dpm)2 + , and Rh(dpp) 2 +; both of these complexes form terminal CO adducts (Rh(CO)(dpm)2 + : HCO) = 1948 cm"1; Rh(CO)(dpp) 2 + : v(CO) = 1930 cm"1), whereas Rh(diop) 2 + and Rh(dpb) 2 + under a CO atmosphere give a mixture of products including mononuclear species with "dangling" phosphine ligand and binuclear species with 17 38 39 bridging ditertiary phosphine. ' ' An FT-IR spectrum (Nujol mull) of Rh(CO)(dcpe)2 + C l " , 42, reveals v(CO) at 1979 cm" 1 (see Section 2.1.7.14). The solution FT-IR spectra also displayed v(CO) during the in situ preparation of the carbonyl complex. The u(CO) is in the region for a terminal CO (- 1900 cm"1). The Rh(CO)(dcpe)2 + Cl complex was very labile in terms of CO loss. Figure 3.5 shows the CO loss from 43 in the solid state under vacuum. Because of the ready loss of CO from 42, the result of the elemental analysis was not very satisfactory (expected C : 62.93, H : 9.56%; found C : 63.50, H : 10.00%). Surprisingly, the 3 1 R 1 H J NMR spectrum of Rh(dcpe)2 + C l " (2.5 x 10~ 2 M) under CO ( ~1 atm) in CFL X l j showed no sign of new peaks pertinent to Rh(CO)(dcpe)2 + C l ~ , even at -89° C. The spectrum is typical of that noted for Rh(dcpe)2 + Cl . The small changes in chemical shift with 40 41 + -varying temperature (Section 2.1.7.14) is common. ' When Rh(dcpe)2 CI was subjected to 4 atm of CO in a high pressure NMR tube, only peaks corresponding to + - -2 the starting Rh(dcpe)2 CI (3.0 x 10 M) were observed. Variable temperature 31 1 P{ Ffj NMR data in CH 2Cl 2-d 6-acetone (2:1, v/v) have been reported for other 110 Rh(C0)(dcpe)2+ *\ 1 1 1 1 —1 T -3800 3200 2600 2000 1700 1400 1100 cm Figure 3.5. Decrease in v(CO) of the Nujol mull of Rh(CO)(dcpe)2 + C f on subjection of the solid to vacuum. (a) Under CO, (b) Under vacuum (>(CO) labelled as *). I l l Rh(CO)(P-P) 2 + complexes such as Rh(CO)(dpm) 2 +BF 4" and Rh(CO)(dpp) 2 + BF 4". 1 7 The Rh(CO)(dpm)2 + B F 4 species was considered to be present as a fluxional tbp species in solution from 22° C to -50°C. The 3 1P{ 1H] NMR chemical shifts for Rh(CO)(dpm)2 + BF 4" (44) and Rh(dpm)2 + BF 4" (45) were very similar, with a difference of 20 Hz in the Jpji-P coupling constants (44 at 25° C in CH 2Cl 2-d 6-acetone (2:1, v/v) : 5 22.6 ( J R h _ p = 97.5 Hz.); 45 at 25° C in CH 2C1 2-C 6D 6 : 5 23.50 ( J R h _ p = 117 Hz)). An equilibrium mixture of 44 and 45 exists at 25° C when 44 is placed under Ar. A static tbp structure for Rh(CO)(dpp)2 + BF 4" (46) was observed at -50°C under CO, and the 3 1P{ 1H] NMR spectrum showed 2 sets of triplet of doublets (46 in CH 2Cl 2~dg-acetone (2:1, v/v) : 8 5.14 ( J R h _ p = 91.6 Hz, J p _ p = 46 Hz), -9.94 ( J R h _ p = 114.7 Hz, J p _ p = 46 Hz)). It is not immediately clear why the Rh(CO)(dcpe)2 + CI species is not 31 detected by P NMR; equilibrium constant data (see Section 3.5.2) show that under 4 atm CO, there should be ~ 30% conversion to the carbonyl. The only explanation is 31 that the P NMR data (chemical shifts and coupling constants) are the same, within experimental error, for Rh(dcpe)2 + C l ~ and Rh(CO)(dcpe) 2 +Cl~. The Rh(CO)(dcpe)2 + CI complex is the first case of a detected CO adduct for a R h [ P R 2 ( C H 2 ) 2 P R 2 ] + type cation. The Rh(chiraphos)2 + and Rh(dpe) 2 + species are known to be inert to CO addition. 3.5.2. Spectrophotometric Kinetic Studies on the CO Interaction with Rh(dcpe) 2 + Studies on the equilibrium constant for binding CO in CH 2C1 2 solutions (via kinetics of the forward and reverse reactions) were made by UV-VIS spectrophotometric measurements, as described in Section 2.2. Pseudo first-order conditions for the forward reaction were implemented by using low Rh concentrations 112 (0.456 mM and 0.460 mM), with excess and constant CO concentration, by maintaining a total pressure of 1 atm of CO and CF^C^ vapour over the solutions in the cell. The concentration of CO in CH^C^ at a specific temperature was obtained from the solubility data (see Section 2.5): 3.86 mM at 18° C, 3.64 mM at 20 ° C, and 2.34 mM at 25 ° C (PQQ + PQYI Q = 1 A^M< at the specific temperature). The data were analyzed according to the standard first-order rate law, ln(A^ - A ) = ln(Ag -A ) - k , t, where A n, A., and A are the solution absorbances at t=0, time t, eq obs 0 t eq and completion of the reaction, respectively (see Appendix I). The observed pseudo first-order rate constant, k , , was obtained from the ln[(A n - A )/(A - A )] obs l v 0 eq t eq' versus time plot The decrease of the peak intensity at 403.6 nm was monitored during intermittent stirring of the RhCdcpe^"1"Cl solution with an excess amount of CO (Figure 3.6). The interaction of Rh(dcpe)? + Cl with CO to form a monocarbonyl is represented by equation 3.4: Rh(dcpe)9 + C f + CO < » Rh(CO)(dcpe)? + Cl (3.4) k -1 The observed rate constant k Q^ s is a function of k^  and k_j (^011S = ^fCO] + k_j, see Appendix I for proof). The plots of ln[(A 0 - A )/(A - A )] versus time at two initial [Rh] of ~ sobs 0.45 mM and [CO] = 2.34 mM, yield k . values of (5.40 and 5.67) x 1 0 - 4 s"1, -4 -1 averaging to 5.54 (±0.14) x 10 s (see Figure 3.7 for one of the plots). The reverse "decarbonylation" process was studied by slow bubbling of Ar, presaturated with CK^Clj, into the in situ generated solution of Rh(CO)(dcpe)2 + C l ~ 113 Figure 3.6. Changes in UV-VIS absorbance for the reaction of Rh(dcpe)2 + C f with CO in CH 2C1 2 at 25° C, and 1 atm total pressure. 4-3.5-Time, h Figure 3.7. Analysis of data from Figure 3.6 in terms of an equilibrium reaction composed of a forward pseudo first-order carbonylation reaction, opposed by a first-order decarbonylation process. Absorbance measured at 403.6 nm. 115 complex, and monitoring the increase in absorbance (A^) at 403.6 nm (Figure 3.8). The decarbonylation process is represented by equation 3.5: k . Rh(CO)(dcpe)2 + C f I i * . Rh(dcpe)2 + C l ~ + CO (3.5) Ar The data were analyzed according to the equation, MA^ - A^) = k_^ t + constant (see Appendix I). The loss of CO is irreversible, since the flow of Ar is considered to remove CO from the mixture. The slope of a plot of ^ ( A ^ - A T ) versus time -4 -1 gives k_j (Figure 3.9). The k_^ values were (5.15 and 4.89) x 10 s (average = 5.02 (±0.13) x 1 0 " 4 s"1) for the two in situ Rh(CO)(dcpe)0 + C f systems. Thus, kj[CO] = k Q b s - k = (5.54 - 5.02) x 1 0 - 4 = 0.52 x 1 0 - 4 s" 1; and kj = 0.52 x 1 0 " 4 s _ 1 /2 .34 x 1 0 - 3 M = 0.022 M _ 1 s" 1; then K can be calculated as k j / k ^ = 0.022 M _ 1 s _ 1 /5 .02 x 1 0 - 4 s" 1 -44 M - 1. This value of K can be used to obtain the final absorbance data (Appendix 1) at the equilibrium of the interaction of Rh(dcpe)2 + with CO at 25 ° C in CH 0 C 1 ? where [Rh] Q = 4.56 x 1 0 " 4 M [ R h ] £ q = [Rh] Q - [RhCO] e q [RhCO] e q = [Rh] Q - [ R h ] £ q and [Rh] = [Rh(dcpe)2 + ]; [RhCO] = [Rh(CO)(dcpe) 2 +] Let A £ Q = Absorbance of [Rh] at equilibrium at 403.6 nm AQ = Absorbance of [R I JQ at 403.6 nm 1 = pathlength of cell = 1 cm, e = molar extinction coefficient of [Rh] at 403.6 nm at 25 ° C e ^ Q = molar extinction coefficient of [RhCO] at 403.6 nm at 25 ° C 116 Figure 3.8. Changes in absorbance for the decarbonylation (via bubbling of Ar) of the in Situ formed Rh(CO)(dcpe)2 + C f (from [Rh] = 0.456 mM, and [CO] = 2.34 mM) in CH 2C1 2 at 25° C. 117 Figure 3.9. Analysis of data from Figure 3.8 in terms of an irreversible loss of CO from Rh(CO)(dcpe)2 + C f . 118 [RhCO] _ i [Rh]eq[co] [RhCO] g q = 44 x (4.56 x 1 0 - 4 - [RhCO] e q) x 2.34 x 10 - 3 M [RhCO] = 4.26 x 10" 5 M eq [ R h ] £ q = 4.56 x 10" 4 M - 4.26 x 10~ 5 M = 4.13 x 10" 4 M VAeq = ([Rh]0-[R«eq> x <e"eCO> X 1 eq "l \j Jeq 2.307-2.121 = (4.56 x 10_4-4.13 x 10"4) x (5.06 x 10 3-<? c o) x 1 eco = 7 4 4 M _ 1 c m _ 1 •'.Absorbance of Rh(CO)(dcpe)2 + at 100% formation is: A = 4.56 x 10" 4 x 744 x 1 = 0.339 The dashed line in Figure 3.6 was thus assumed to approximate the spectrum for 100% formation Rh(CO)(dcpe)2 + (point A is the isosbestic point, while point B is the absorbance for 100% formation of Rh(CO)(dcpe)2 + ) . Thus, the kinetic data give an equilibrium value for CO binding of~44 M - 1. The data analysis is limited by only partial formation of the carbonyl (- 10%) at a total pressure of 1 atm at 25° C. The achievement of equilibrium is relatively slow at 25° C with k]_ being 2.2 x 10" 2 M _ 1 s" 1 and k ^ = 5.02 x 1 0 - 4 s"1. The interaction between CO and Rh(dcpe)2 + Cl was also followed by the growth of i'(CO) absorbance at 1993 cm - 1 in CH 2C1 2 in the FT-IR spectrum; the absorbance reaches a maximum value after about 2.0 h using a [R1I]Q of 1.31 mM and a constant [CO] of 3.86 mM at 18° C (see Section 2.5 for the CO solubility data, Appendix I for calculation and kinetic data). Figure 3.10 shows the growth in IR absorbance at 1993 cm ,^ assigned to the v(CO) of Rh(CO)(dcpe)2 + , during the reaction of Rh(dcpe)2 + C f with CO in CH 2C1 2 at 18° C. The k Q b s value was found to be 4.15 x 10 4 s \ which is in good agreement with that determined at 25° C 119 Figure 3.10. Changes in absorbance of FT-IR spectrum at 1993 cm after CO was introduced into a C H X l j solution of Rh(dcpe)2 + C f at 18° C (absorbance has been corrected for solvent contribution). 120 using the UV/VIS data (5.54 x 10 4 s - 1 ) . Other Rh(P—P) 2 + complexes, containing 5-membered rings, such as Rh(chiraphos)2 + ,^ or Rh(dpe)2 + , 1 7 are inert toward CO. Table 3.4 summarizes the kinetic and equilibria data for binding of CO to R h ( P — P ) 2 + complexes. The data reveal rapid on- and off- rates for Rh(dpm)2 , no binding by Rh(dpe)2 , and Rh(chiraphos)2 , a rapid on-rate and slow off-rate for Rh(dpp) 2 , and slow on-and off-rates for Rh(dcpe)2 + . The dpb and diop cations give a mixture of products, 17 37 38 including binuclear species with bridging ditertiary phosphines. ' ' There are no obvious simple reactivity patterns for the Rh(P— P ) 2 + complexes. The approach by CO toward Rh(dcpe)2 + C l ~ is presumably end-on, and the structure of the resulting Rh(CO)(dcpe)2 + complex could be distorted square pyramidal or fluxional trigonal bipyramidal. 3.6. REACTION OF Rh(dcpe)2 + C1~ WITH HCI 3.6.1. Characterization of RhHCl(dcpe)2 + The Rh(dcpe)2 + Cl complex reacts with HCI in CH 2C1 2 > and benzene. The _L — product is isolated as the cis-RhHCl(dcpe) 2 Cl • HCI • OJCgH^ (42) species, following stirring a benzene suspension of 22 with 10 equivalents of HCI. Contamination with ~ 1 equivalent of lattice held HCI was confirmed by titration of 42 with NaOH and by Cl elemental analysis (see Section 2.1.7.11). Contamination with lattice-held HCI is + 17 30 43 found with other RhHCl(P—P) 2 complexes ' and iridium analogues. The benzene solvate in 42 is observed by ^H NMR. Complex 42 is a 1:1 electrolyte with a molar conductivity of 126 S T 1 cm 2 m o f 1 in CH 3CN at [Rh] - 5.65 x 10~ 4 M. The value -1 2 for molar conductivity of HCI in CH^CN is very low and found to be 27.6 ft cm 121 Table 3.4. Kinetic and equilibria data for the binding of CO to Rh(P-P) 2 + complexes. P-P k r M _ 1 s" 1 k _ l ' s _ 1 k 1/k_ 1 (K), M" 1 A a,42 dpm > 105 >10 b ( ~io4) A 42 dpe - - -. a,c,42 dpp 5 x 10 3 4 x 10" 4 1.25 x 10 7 chiraphos 4 2 - - -A d,f dcpe 2.2 x 10"2 5.02 x 10" 4' e 44 a. In CH 2C1 2, at 30° C. b. Estimated from kj and K. . c. In dma, at 30° C. d. In CH 2C1 2 > at 25° C. e. Determined spectrophotometrically by flowing Ar into a solution of the in sift} formed Rh(CO)(dcpe)2 + . f. k Q b s (= kj[CO] + k_p was found to be 4.02 x 1 0 - 4 s" 1 at 18° C, using FT-IR to monitor the CO reaction. 122 mol \ at [HCI] = 0.25 M (see Section 2.5). Thus, there is little contribution to the -1 2 -1 conductivity of 126 ft cm mol from the lattice-held HCI in 42. That the hydride is cis to chloride in 42 is indicated by a relatively high field chemical shift of the hydride (overlapping quintet of doublets) at -19 ppm, analogous to that of + - 43 + -cis-IrHCl(dpp)2 PF g ; while the hydride resonance of trans-RhHCKdpm^ Cl 46 adduct occurs at lower field ( — 1 2 ppm). However, the Jp_pj value of 42 is 13 Hz (Table 3.5), which is small in terms of the average of three Jp_j_j terms at 90° o 44 and one J p _ H at 180 (trans Jp_j-i usually > 50 Hz ), even though cis-IrHCl(dpp)2 + PFg manifests a similar value J p _ H (12.5 Hz). The p(Rh-H) value is low in 42 (Nujol mull : 2091 cm - 1), in contrast to that of the trans-RhHCl(chiraphos) 2 + adduct.30 The 3 1P{ 1H] NMR doublet signal in CD 2C1 2 at room temperature implies equivalent P atoms but there is a coalescence of peaks to give a broad signal at -50° C; further cooling down to -89° C gives a doublet (Figure 3.11). The spectral changes were reversible upon temperature changes. The coalescence at -50°C is probably due to the overlap of 2 broadening signals. There is no solubility problem for a 1.5 x 10~ 2 M CD 2C1 2 solution of RhHCl(dcpe) 2 + at -90°C. The broadening of signal is probably due to intramolecular rearrangement, but the temperature is not low enough to freeze out the fluxionality. The intramolecular + - 43 rearrangement of a cis-IrHCKdpp^ PF g has been studied by Miller et al., using variable temperature 3 1 P NMR. At room temperature, the 3 1P{ 1H} NMR spectrum of cis-IrHCl(dpp)2 + PFg in THF displays a singlet; on cooling, no coalescence of peaks is observed but a rather more complicated spectrum consisting of an apparent A^i^ spectrum results at -74° C. A rapid exchange between H and Cl was assumed and the axial and equatorial phosphorus atoms in cis-IrHCl(dpp)2 + PF g become equivalent by intramolecular rearrangement. The ^H NMR spectra (high field region) of 124 cis-RhHCl(dcpe) 2 + Cl at -50° and -89°C show only a slight broadening of signal. Of other isolated RhHCl(P-P) 2 + ( P - P 1 7 - 3 0 = dpm, dpe, dpp, chiraphos), the only crystallographic data available are those for the P—P = dpm system, which has trans geometry 4 6 Table 3.5 lists the *H and 3 1P1 1H] NMR data of known RhHCl(P-P) 2 + complexes. An attempt to prepare RhHCl(dcpe) 2 + P F F from the reaction of Rh(P-P) 2 + C f with N H 4 + PF 6~, according to the method reported by Mague,48 failed. Instead, Rh(dcpe)2 + PFg was isolated, due to the C F exchange with P F F (see Section 2.1.7.10). The kinetics of the reaction between Rh(dcpe)2 + Cl and HCI in CH 2C1 2 were examined in an attempt to understand the mechanism of the formation of cis-RhHCl(dcpe) 2 + (see below). 3.6.2. Stopped-flow Kinetic Studies on the Reaction of HCI with Rh(dcpe)2 + C l ~ The kinetics of the rapid reaction between Rh(dcpe) 2 + and HCI in CH 0C1 2 were studied at 18.3 (±0.1)°C using stopped-flow apparatus. The [Rh] was varied from (2.67 to 0.267) x 10~ 4 M, and the [HCI] from 0.18 to 3.59 x 10" 2 M. The [HCI] was always at least in a 670-fold excess over the [Rh], to implement pseudo first-order conditions. The equation for the reaction is represented as follows: + - k2 + -Rh(dcpe)2 CI + HCI .^-i h> RhHCl(dct>e)2 CI (3.6) k-2 The stopped-flow instrumentation used was described in Section 2.2. The reaction was monitored by following the decay of absorbance at 403.6 nm, corresponding to Rh(dcpe)2 + Cl . A typical curve obtained from the stopped-flow kinetic studies is shown in Figure 3.12. Table 3.5. J H and 3 1P{ 1H] NMR data for various RhHCl(P-P) 2 + complexes at 25° C, in CDC13. Compounds 5P, J p _ p , J R h _ p 6H, J R h _ H > J p _ H , Lit-ppm Hz Hz ppm Hz Hz trans-RhHCl(dpm)2 + C f -15.8 - 83 -11.9 17.5 13 17.46 RhHCl(dpe) 2 +Cf 51.8 - 93.8 -15.9 16 12 17,47 RhHCl(dpp) 2 + C f 2.3 - 91 -15 RhHCl(chiraphos)2 + C f 38.4 94 27 -11.23 15 62.7 94 27 17 13 30 cis-RhHCl(dcpe) 2 + C f 61.9 - 89.4 -19 15 13 this work 126 0 0-19 0-57 0-95 1-33 Time.s Figure 3.12. A typical curve obtained from the stopped-flow kinetic studies at [Rh] = 4.97 x 10" 5 M, [HCI] = 4.50 x 1 0 - 2 M, and 18.3° C, due to the disappearance of absorbance of Rh(dcpe)2 + C l ~ at 403.6 nm, where RhHCl(dcpe) 2 + C f has negligible absorbance (see Figure 3.15 later). 127 Analogous to the CO system, the derived rate-law gives k Q b s = kjtHCl] + k 0 . The k , was obtained from a plot of ln[(A n -A ) / ( A - A )] versus time, where -L obs 0 eq t eq A Q , A t > and A are the absorbances on the ordinate axis (arbitrary units) at time = 0, time t, and completion of reaction respectively. The experimental data are listed in Appendix I. The results of the k ^ values over a range of [Rh], and [HCI] are reported in Table 3.6. At a constant [HCI] = 0.18 M , k b s > over a range of [Rh], is found to be 7.6 (±0.2) s 1 (see Table 3.6), thus showing the rate is first order in [Rh]. At a constant [Rh] = 4.96 x 10~ 5 M , the plot of k Q b g versus [HCI] gives a straight line, with slope (k2) being 42.1 (±2.3) M - 1 s - 1 (Figure 3.13). The intercept is close to the origin, showing that k_ 2 is 'negligible' compared to k^tHCl] i.e. at the conditions used, the forward reaction is complete. In order to obtain data for the reverse (off) reaction, it is necessary to choose a reagent that will remove the free HCI product, so as to prevent the on-rate (k2) from occurring; the k_ 2 rate constant can then be conveniently measured by conventional techniques. 49 N.N'-Dimethylacetamide (DMA) reacts exothermically with HCI, and was thus considered a good choice for removal of free HCI released from RhHCl(dcpe)2 + (equation 3.7). RhHCl(dcpe)- + « Rh(dcpe)-+ + HCI k ^2 D M A D M A - HCI (3.7) 128 Table 3.6. Pseudo first-order rate constants (k 0b s) ° f *-he reaction Rh(dcpe)2 + C f + HCI < > R h H C K r i r p e^n' at 18.3° C. [Rh] x 105, M [HCI] x 102, M k s"1 a obs' s 1. 4.97 3.59 1.39 2. 4.97 4.50 1.95 3. 4.97 5.32 2.21 4. 4.97 6.10 2.85 5. 4.97 8.50 3.11 6. 4.97 9.36 3.65 7. 4.97 18.0 7.70 8. 26.7 18.0 7.84 9. 5.34 18.0 7.41 10. 2.67 18.0 7.69 a. k 2 = 42.1 ( ± 2 . 3 ) M s Figure 3.13. A plot of the dependence of k Q b s on [HCI] at 18.3° C, [Rh] 4.97 x 10" 5 M. 130 31 1 Both P{ H] NMR and UV-VIS spectra were taken at different intervals following dissolution of RhHCl(dcpe) 2 + in solvents such as DMA and CH^CN. The results are presented in Table 3.7. The 3 1P{ 1H} NMR spectra for dissolution of RhHCl(dcpe) 2 + in DMA, recorded at different time intervals, show only the transformation of RhHCl(dcpe) 2 + into Rh(dcpe)2 + , the HCI presumably being removed as DMA-HC1. No other side-products were observed (Figure 3.14). Similarly, the loss of HCI also occurs for RhHCl(dcpe) 2 + dissolved in CH^CN, but this is a much slower process. It is known that DMA reacts with HCI to give a DMA* HCI adduct, whereby the proton is attached to the oxygen of the carbonyl group, and the molecule features a double 49 bond between the carbonyl carbon and nitrogen, as evidenced by the X-ray analysis. Whether there is chemical interaction between CH^CN and HCI is not clear; such interaction might involve N-protonation, but this is probably less feasible than for O-protonation in DMA- HCI. Figure 3.15 shows the UV-VIS changes for the HCI addition to Rh(dcpe) 2 + (5.8 x 10~ 4 M) in CH 2C1 2 to give RhHCl(dcpe) 2 + . There is a decrease in the absorption band at 403.6 nm; an isosbestic point is observed at 310 nm. The reverse process (i.e. a loss of HCI from RhHCl(dcpe) 2 + ) can be monitored by an increase in the absorption band at 403.6 nm, using the UV-VIS spectrophotometric technique with DMA as the base to remove the released HCI. The kinetics for the loss of HCI from RhHCl(dcpe) 2 + were studied in DMA-CH 2C1 2 at 18° C. The [DMA] was varied from 2.15 to 10.8 M (neat DMA), and [Rh] was kept at 5.60 x 10 4 M. The analysis was carried out according to a first order irreversible reaction, as exemplified in the decarbonylation of Rh(CO)(dcpe) 2 + (see Section 3.5.2). A plot of hXA^-A^ versus t gives k b s as the slope. Table 3.8 lists k ^ as a function of [DMA], the values of 131 Table 3.7. Distribution of products via dissolution of RhHCl(dcpe) 2 + in CH 3CN and DMA, observed by 3 1P} 1H] NMR and/or UV-VIS spectrophotometric techniques. [RhHCl(dcpe) 2 +], M Solvents Time, h RhHCl(dcpe) 2 + Rh(dcpe)2 + 1.01 x 10" 2 a DMA 9 67% 33% 1.01 x 10" 2 a DMA 70 - 100% 5.6 x 10" 4 b DMA 21 10% 90% 1.49 x 10" 2 a CH 3CN 75 57% 43% 1.1 x 10" 4 b CH 3CN 50 62% 38% a. Observed by P{ H} NMR b. Observed by UV-VIS spectral changes " i 1 1 1 1 r ~ — i 1 1 100 90 80 70 60 50 40 30 20 ppm P{ H} NMR spectra of RhHCl(dcpe)2 + in D M A under Ar, at ambient temperatures recorded at various times, (a) Time = 0 h, (b) Time = 9 h, (c) Time = 70 h. 133 200 400 600 Wavelength, nm Figure 3.15. Changes in UV-VIS spectrum due to the reaction of Rh(dcpe) 2 +CF (5.80 x 1 0 - 4 M) with ~1 atm HCI in CH 2C1 2 at 18° C. 1.0 (±0.1) x 10~ 6 s _ 1 being essentially independent of [DMA] at [DMA] < 5.40 M. and are thus considered equal to k_2 (see Appendix I for proof). The equilibrium constant for the HCI addition is k 2 / k _ 2 = 4 1 1 M _ 1 s _ 1 ' ]-° x 1 0 ~ 6 s _ 1 = 7 -1 4.2 x 10 M , the large value clearly showing favored formation of the cis-HCI adduct. The k Q b s value is significantly larger in neat DMA than values obtained in the DMA-CH 2C1 2 solvent mixtures, perhaps implying that the dielectric constant of the medium plays a kinetic role in the process. 134 Table 3.8. Dependence of k_ 2 on [DMA] for the rate of loss of HCI from RhHCl(dcpe) 2 + (5.80 x 10~H M) dissolved in C H 2 C 1 2 - D M A at 1 8 ° C . ,-4 [DMA], M k v , s obs' -1 1. 2. 3. 4. 5. 0.00 2.15 5.40 7.56 10.8 (neat DMA) (no apparent reaction after 24 h) 1.0 x 10 -6 9.0 x 10' -7 1.5 x 10' ,-6 7.5 x 10 -5 a. The dielectric constants for C H 2 C 1 2 and D M A are 9 and 39, respectively. The rate of HCI addition to Rh(dcpe)2 + C f in C H 2 C 1 2 is first order with respect to HCI and Rh(dcpe)2 + . This indicates that a concerted oxidative addition of HCI to give a cis-HCI adduct occurs. An ionic mechanism which involves two consecutive steps such as chloride attack and protonation is probably not operative because it would involve a non-linear dependence of k Q b s on [HCl]."^ There are two plausible "geometric" approaches of HCI towards Rh(dcpe)2 + Cl , 17. The first approach involves the replacement of the solvent molecule in a solvated tbp form of 12 by HX (see Figure 3.16). Such replacement would give a geometry close to that expected for a transition state during formation of a cis-RhHCl(P—P) 2 + product, as suggested previously by James and Mahajan~^ for corresponding formation of cis-RhH 2(P—P) 2 + species. The ^Pl^H] NMR solution spectrum of 3JZ is unchanged on cooling down to -83°C. Hence, it is impossible to 135 know if 22 exists as a fluxional solvated tbp species in solution, although the low temperature 3 1 P NMR data for Rh(dpp) 2 + in acetone are consistent with a tbp 17 species. A second approach of HQ to form the product requires the bending of 2 of the Rh-P bonds simultaneously to a pseudo-octahedral transition state (Figure 3.16). Such a three-centre mechanism for cis addition, involving the bending of trans ligands, has been previously suggested in the cis-oxidative addition of alkyl halides to Vaska's 52 compound by Ugo's group, and also in the cis-oxidative addition of H 2 to Ir(dpe)2 + by Eisenberg's group.53 3.6.3. Further Reaction of RhHCl(dcpe)2 + C f with HCI The HCI addition to Rh(dcpe)2 + C f gives initially cis-RhHCl(dcpe) 2 + , 42, but subsequently a further reaction product is observed. Complex 42 is the only adduct formed in the first ten minutes of interaction, and hence the stopped-flow kinetics are quite clean. In CDCl^ or CD 2C1 2 > a new hydride, 48, is detected if Rh(dcpe)2 + C l ~ is exposed to - 1 atm HCI, which represents a large excess over the required stoichiometric amount to form RhHCl(dcpe) 2 + , for over 20 min (Table 3.9, Figure 3.17). A sample of isolated RhHCl(dcpe) 2 + C l " • HCI-OJCgHg, 42, dissolved in CD 2C1 2, and sealed under vacuum does not show the formation of 48 in 2 h; however, when RhHCl(dcpe) 2 + Cf-HCl-0.3C 6H 6 is treated with a large excess of HCI ( -1 atm) in CH 2C1 2, species 48 (~ 3%) appears in 15 min. The new hydride (48) has a high field resonance at 8 -18.3 in the *H NMR spectrum. The 1H{ 3 1Pj NMR spectrum of 48 (Figure 3.18) reveals that J R h _ H is 20 Hz, and the coupling within the "quartet" is also 20 Hz. Thus, the peak structure results from a coincidence of equal coupling constants for Jpjj-H a n c* ^P-H 31 1 for two equivalent P atoms. The P{ H] NMR spectrum of the reaction solution has 136 p", U HCI (a) P • | V- P r P P ^ R h ^ H + s o , v |^CI tbp V V distorted square planar solv • solvent p =.• dcpe Figure 3.16. Possible approaches of HCI toward Rh(dcpe)2 C-F in CH 2C1 2. (a) Replacement of solvent in tbp species of Rh(dcpe)2 + C l ~ by HCI. (b) Bending of 2 trans phosphorus ligands toward one another. 137 Table 3.9. Interaction of Rh(dcpe) 2 +Cf with HCI (~ 1 atm) in CD 2C1 2 - the distribution of high field hydrides (*H NMR region) with time. Time, h RhHCl2(dcpe) RhHCl(dcpe) 2 + Cl at -18.3 ppm (48) at -19 ppm (42) 0.17 - 100% 0.42 4% 96% 2.0 25% 75% a doublet at 5 98 ( J R h _ p = 134 Hz), and a singlet at 26.1 ppm (Figure 3.19). The free dcpe phosphine reacts rapidly with HCI in CH 2C1 2 to give a 31 product, dcpe(HCl)2, that shows a single P NMR peak at 26.1 ppm. The isolated diphosphonium salt, dcpe(HCl)2, was characterized by elemental analysis, spectroscopic measurements, and titration with NaOH in ethanol (see Section 2.1.4.1). About two moles of HCI are present per mole of dcpe(HCl)2. The dcpe(HCl) 2 species accounts for the appearance of one of the P signals during the formation of the new hydride 48- Thus, one of the dcpe ligands in RhHCl(dcpe) 2 + C l " is removed during the reaction with excess HCI to give dcpe(HCl) r —I 1 1 T 1 1 18-2 18-6 19*0 PPm Figure 3.17. Changes in the high field H NMR spectrum with time, when RhHCl(dcpe) 9 + C f in CDCL is exposed to ~ 1 atm HCI. Figure 3.18. (a) H NMR and (b) lH{ ?} NMR spectra of high field hydrides. pertaining to the interaction of Rh(dcpe)2 + C f with ~ 1 atm HCI in CD 2C1 2. 140 P=61-9 J R h - P = 8 9 - 4 Hz KP^\ >p P j (*) H ( > h — C l C l = 98 J Rh -p = 134H^ J l | - -P=1C C I H P ^ PHCI 5 P =26-1 Figure 3.19. PI H] NMR spectrum for the in_situ reaction of Rh^dcpe^ Cl with HCI (- 1 atm) in G ^ C l j ; the solution was transferred into an NMR tube under Ar after a reaction time of (a) 10 min and (b) 2 h. 141 Based on the NMR data, the structure of 48 is proposed to be: H P. I ( " R h — a Cl 48 The 3 1P{ 1H] NMR signal of P-P in 48 is expected to be split by Rh into a doublet, while the *H NMR signal for the hydride should be a quartet if Jpj^pj ~ J P - P i -Prolonged exposure of RhHCl(dcpe)2 + Cl to CDCl-j gives rise to new 3 1 P NMR peaks at 6 40.8 ppm with -"p^.p - 81 Hz, the data corresponding to those for RhCl2(dcpe)2 + Cl , synthesized independendy (Section 2.1.7.12). Decomposition of 54 hydrides in chlorinated solvents is well documented. The reaction scheme proposed for the Rh(dcpe)2 + Cl /HCI chemistry is shown in Figure 3.20. Presumably, removal of the P—P ligand as dcpe(HCl)2 creates vacant co-ordination sites, one of which becomes occupied by the anionic chloride associated with the RhHCl(dcpe)2 + cation. Because the cis-RhHCl(dcpe)2 + compound consists of two bulky phosphines arranged cis- to one another, a fast equilibrium involving the dissociation of dcpe from RhHCKdcpe)^ may take place; the interaction of the free dcpe ligand with excess HCI in solution could give dcpe(HCl)2 and promote the decomposition of RhHCl(dcpe)2 + . 142 C H 2 C I 2 C l " + B • » H C * n V CI 2 H C I P I (>< )^ sr C k C>-« >p DMA v P^|>p p/" | p^ > ci C I H P ^ P H C I Figure 3.20. Proposed pathways for the interaction of HCI with Rh(dcpe)2 CI . 3.7. ATTEMPTED USE OF Rh(dcpe)2 + C l ~ IN HOMOGENEOUS CATALYSIS (i) Decarbonylation Because Rh(dcpe)2 + Cl was found to interact with CO, decarbonylation of benzaldehyde with Rh(dcpe)2 + Cl in CH 2C1 2 or in CH^CN was attempted. The temperature was varied from 25 to 90° C, and an 85- to 300- fold excess of substrate was used. There was no sign of reaction based on GC analysis of the reaction products, after 20 to 27 h, under a slow flow of Ar (see Section 2.6 for experimental details). 143 The kinetics for the reversible CO interaction with Rh(dcpe)2 + C f reveal a small affinity of the complex for CO gas (K - 44 M 1 at 25° C), but this does not necessarily imply poor activity for catalytic decarbonylation of aldehydes.55 (ii) Hydrogenation of 1-hexene. Hydrogenation of 1-hexene (0.0319 M) was attempted at 31° C with Rh(dcpe)2 + Cl (1.32 mM) under ~ 1 atm of H 2 in toluene (5 mL). No hydrogenation occurred over an 8 h period (see Section 2.3.2.2). The inertness of Rh(dcpe)2 + Cl toward H j and the steric hindrances of dcpe around the Rh presumably prevent the simultaneous binding of both alkene substrate and hydride at the Rh atom, usually a key criterion for effective homogeneous hydrogenation. 3.8. C O N C L U S I O N S The Rh(dcpe)2 + Cl complex was synthesized according to the substitution reaction of [RhCl(COE) 2] 2 with four equivalents of dcpe. If two equivalents of dcpe were used in the substitution reaction, [RhCl(dcpe)« solvate] n (solvate = THF, O.lCgH^) type complexes and RhCl(dcpe)(CH2Cl2)« C^Hg were isolated. The X-ray crystal structure of Rh(dcpe)2 + Cl was determined, and found to consist of distorted square planar geometry at the metal. Reaction of Rh(dcpe)2 + Cl with hydride generating agents, in an attempt to produce RhH(P— P ) 2 en route to the preparation of Rh 2(CO) 4(P— P) 2 > was unsuccessful. The PF 6" and BF 4" analogues of Rh(P-P) 2 + C l " were obtained by C f exchange with NH^PFg, and AgBF^. The binding of small gas molecules by Rh(dcpe)~ + Cl was studied in an attempt to discover the potential of using the 144 complex as a catalyst in homogeneous catalysis, and also to understand the reactivity pattern of a series of Rh(P— P) 2 + complexes (previously studied by our group), particularly with relevance to those consisting of P—P ligands which form 5-membered chelate rings with Rh. It was found that Rh(dcpe)2 + C f reacted with CO ( K g q -44 M _ 1 at 25° C) to give Rh(CO)(dcpe)2 + , and with HCI ( K g q ~ 4.2 x 107 M _ 1 at 18° C) to give cis-RhHCl(dcpe) 2 + . The kinetics were studied. The mechanism of reaction between HCI and Rh(dcpe)2 + Cl is proposed to be a concerted oxidative addition. The Rh(dcpe)2 + Cl complex was found to be inactive for homogeneous decarbonylation of benzaldehyde and hydrogenation of 1-hexene. Further investigation into the organometallic reactions of the extremely reactive [RhCl(dcpe)' solvate] species seems worthwhile. CHAPTER 4. FORMATION OF RhH(CO)(dpp*)(dpp) FROM Rh 2(CO) 4(dpp) 2, RhH(dpp)2, AND RhH(CO)(PPh 3) 3 4.1. INTRODUCTION The original purpose of this research work was to synthesize a series of Rh 2(CO) 4(P—P) 2 complexes, and study their catalytic properties in homogeneous hydroformylation, with the aim of achieving asymmetric hydroformylation as the ultimate goal (Section 1.2). Earlier work performed by another member1 of this group showed that both Rh 2(CO) 4(diop) 2 and Rh 2(CO) 4(dpp) 2 catalyzed the hydroformylation of 1-hexene at 55°C in benzene (turnover frequency ~1.25 h 1 for the diop dimer). In order to understand the underlying mechanism for hydrogenation or hydroformylation, the relatively inexpensive dpp dimer was used to study reactivity with H 2. This chapter treats in depth the formation of hydrides from such reactions, and also from other routes. The Rh 2(CO) 4(dpp) 2/dpp system was subsequently investigated for use in homogeneous hydrogenation of 1-hexene (see ChapteT 5). 145 146 4.2. SYNTHESIS AND PROPERTIES OF Rh 2(CO ) 4 (dpp) 2 4.2.1. Synthesis of Rh 2(CO) 4(P-P) 2 The synthesis of Rh 2(CO) 4(dpp) 2 (42) followed the method reported in the 2 literature (Section 2.1.7.6). The procedures are demonstrated in equations 4.1-4.3. C H [ R hCKCOE)^ + 4 P-P 6 6 » 2 R h ( P - P ) 2 + C f + 4 COE (4.1) NaBH 4 Rh(P-P) 0 + C f % - R h H ( P - P ) ? (4.2) 1 EtOH C 6 H f i 2 RhH(P-P) 2 + 4 CO »Rh 2(CO) 4(P-P) 2 + 2 P-P + H 2 (4.3) The synthetic steps are also applicable to P—P = diop, and dpe. Two other ligands, dcpe and p=p, were investigated in the present work. The Rh(dcpe)2 + Cl complex (22) was synthesized (see Chapter 3), but it does not react with NaBH 4 or L i A l H 4 to yield the RhH(P— P) 2 type complex. However, some studies were made regarding die binding of small gas molecules (CO and HCI) by 21. As regards to the p=p ligand, partial success has been achieved by preparing RhH(p = p) 2 in a 3 : 1 mixture with Rh(p=p) 2 + C f , by treating Rh(p = p) 2 + C f • CH 2C1 2 with NaBH 4. The reaction of the mixture of chloride and hydride with CO produced several species, including Rh(CO)(p = p) 2 + Cl , and another unknown carbonyl compound (Section 2.1.7.19). Hence, the attention in this chapter is focused on the study of 147 31 1 Rh 2(CO) 4(dpp) 2 > which has been previously fully characterized by P{ H] NMR, 2 FT-IR, and elemental analysis (see Section 2.1.7.6), as well as X-ray crystallography. The X-ray crystal structure of 42 revealed both terminal and bridging carbon monoxides, while the two dpp ligands were described as being trans to the Rh-Rh bond (2.72 A). Mutin et al. reported the in situ generation of a mixture of cis and trans isomers of Rh 2(CO) 4(diop) 2 in CF^C^, the IR spectrum of which shows four bands at 2005, 1980, 1810, and 1720 cm - 1. The 31P{1FT} NMR spectrum shows two 31 1 sets of doublets, corresponding to the cis and trans isomers. However, the P{ H] NMR spectrum of Rh 2(CO) 4(dpp) 2 consists of only a doublet, supporting the presence of only one isomer. The presence of 4 IR v(CO) bands (Figure 4.1) indicates that there is a lack of symmetry within 42, which is reported to possess an "approximate" C 2 symmetry.2 4.2.2. Properties of Rh 2(CO) 4(dpp) 2 (42) The dpp dimer is sensitive to vacuum (see Section 2.1.7.15). Pumping on the solid under vacuum for 1 h generates new peaks in the FT-IR spectrum (Figure 4.1), with a concomitant darkening of the yellow color. The new peaks appeared at 2063, 1817, and 1784 cm \ and were of very low intensity (Figure 4.1). The changes are tentatively rationalized in terms of loss of CO and subsequent polymerization resulting in cluster formation, so as to minimize the unsaturation in bonding. Rhodium cluster complexes with substituted phosphine ligands also exhibit v(CO) in these regions. Examples are Rh 4(CO) 1 0(PPh 3) 2, i>(CO) (cm - 1, CH 2C1 2) : 2076, 2050, 2023, 1840, 1820; Rh 4(CO) 1 Q(dpe), v(CO) (cm'1, CHjCy : 2087, 2072, 2049, 2015, 1833, 1825.3 However, the major species present after the vacuum treatment is still Rh 2(CO) 4(dpp) 2. The yellow CH 2C1 2 solution of Rh 2(CO) 4(dpp) 2 turned red on being subjected Figure 4.1. FT-IR spectra (Nujol, Csl plates) of (a) Rh2(CO)4(dpp)2> and (b) Rh2(CO)4(dpp)2 after being subjected to vacuum for 1 h. 149 to vacuum. Precipitation with hexanes yielded a yellow solid with an IR peak at 1966 cm 1 (Nujol) due to a terminal v(CO) stretch (see Figure 4.2). This peak occurs at a lower streching frequency than that of the terminal CO ligands in Rh 2(CO) 4(dpp) 2 (1985, 1973 cm - 1), consistent with the loss of an electron withdrawing ligand such as CO. Evacuation to dryness of the CH^Clj solution of Rh 2(CO) 4(dpp) 2 afforded a brownish-orange solid, with both terminal and bridging u(CO) bands at 1968 and 1721 cm - 1, respectively (see Figure 4.3). These results fit into a proposal by 4 5 Evans et al. ' for the corresponding Rh 2(CO) 4(PPh 3) 4 system (equation 4.4). 2 Rh 2(CO) 4P 4 CO Jin — R h ' P ' X C ^ P 2CH 2C1 2 v(COXNujol): 2005 sh, 1985 s, 1790 sh, 1765 s cm"1; y(CO)(CH 2Cl 2): 2017 m, 1992 s, 1800 m, 1770 m cm" 1 P = PPh, v(COXNujol): 1765 w, 1739 s cm' v(CO) (CH 2C1 2): 1740 s cm" 1 + P 2(CO)Rh-Rh(CO)P 2 r(CO)(CH 2Cl 2): 1980 s cm"1. -1 (4.4) In the work from the Wilkinson group, ' an orange benzene solution of Rh 2(CO) 4(PPhj) 4 was slowly concentrated in a stream of N 2 to which small portions of CH 2C1 2 were added at intervals. The concentrated red solution was found to contain two complexes, Rh 0(CO) 0(PPh,) 7, and Rh,(/i-CO)-(PPh,),, and the latter 150 0) o c CO E (0 c I-3200 2000 Wavenumber. cm 1400 -1 Figure 4.2. FT-IR spectrum (Nujol, Csl plates) of a yellow compound, Rh^CO^dpp^, isolated via addition of hexanes to a vacuum-concentrated solution of Rh^COJ^dpp^ in CHjC^. 151 Figure 4.3. FT-IR spectrum of a brown solid isolated by evaporation to dryness of a CH 2C1 2 solution of Rh 2(CO) 4(dpp) 2: a mixture of Rh 2(CO) 2(dpp) 2 and Rh 2(y-CO) 2(dpp) 2. 152 slowly precipitated out as the dark red solvate, R l ^ M - C O ^ P P h ^ * ICYi^ZXj, leaving only Rl^CCO^CPPh^ in the filtrate. The proposed sequence was suggested by IR 4 5 data. ' The X-ray crystal structure of R l ^ u - C O ^ P P h ^ was obtained later by Singh et al., 6 and showed it to be co-ordinatively unsaturated, with no interaction between Rh and CE^C^ molecules (the shortest Rh-Cl approach being 5.662(7) A). The complex is dimeric, with the two Rh(0) atoms linked by two carbonyl bridges, and there is a two-fold rotation axis passing through the centre of the dimer. The Rh-Rh separation is 2.630(1) A, indicative of the presence of a metal-metal bond. By analogy to the R l ^ C O ^ P P h ^ system described above, the addition of hexanes to an evacuated reddish CE^C^ solution of Rh^CCO^dpp^ preferentially precipitates out an unsaturated dimeric complex with terminal CO ligands, R l ^ C O ^ d p p ^ (f(CO) = 1966 cm *), whereas complete evacuation to dryness yields two unsaturated dimeric isomers, i.e. Rh^CO^dpp^, and Rb^/u-CO^dpp^ (*"(CO) = 1721 cm \ The values for C and H elemental analysis for the hexanes precipitated solid were C : 44.87, H : 4.11%; the relatively low values would be consistent with a formulation Rh^CO^dpp^-nCP^Clj, but only if n were 7. The C I ^ C ^ solution containing the unsaturated dpp dimers was found to react with trace Oj to generate green solutions, perhaps converting Rh° into Rh^, 7 which is characteristically green in color. This oxidation is more likely for solutions containing initially Rh^COJ^dpp^ at lower concentrations. 153 4.3. REACTIONS OF Rh 2(CO) 4(dpp) r IN THE PRESENCE OF VARIOUS EQUIVALENTS OF DPP, WITH H 2 AND H 2/CO 4.3.1. Formation of RhH(CO)(dpp*)(dpp) from Rh 2(CO) 4(dpp) 2 The interaction of Rh 2(CO) 4(dpp) 2 (42) with H 2 in CgDg, C ? D g or CD 2C1 2 > in the absence and presence of varying equivalents of added dpp, was studied by performing the reaction in a 5 mL Schlenk tube. The solution was stirred under ~1 atm H 2 for 10 to 16 h, or H 2 was bubbled into the solution by using a 10-inch stainless steel needle fitted into the Schlenk tube via a Suba-seal septum; an outlet needle was connected to an oil bubbler. The solution was then transferred into an NMR tube placed in a wide-mouth Schlenk tube by means of a 1 mL dropper pipette under a blanket of H 2 > The NMR tube (5 mm diameter) was subsequently sealed with a Suba-seal rubber septum and wrapped tightly with paraffin film. If H 2 was bubbled through the solution of 42 for -10 min, or an approximately 10-molar excess of H 2 (-1 atm) was allowed to react with 42, in the presence of dpp ligand (dpp : Rh > 1), the predominant species formed is believed to be RhH(CO)(dpp*)(dpp), iQ. 4.3.1.1. Characterization of RhH(CO)(dpp*)(dpp) The complex iQ could not be isolated pure, and the characterization is based on the spectroscopic information of 5$ produced in situ. There are other routes to generate in situ these routes will be discussed in Sections 4.4 and 4.5. 154 (a) 3 1P{ 1H1 NMR data Table 4.1 shows the various types of hydrides observed during the interaction of 42 with H 2, in the presence of dpp ligand. Both RhH(CO)(dpp*)(dpp), 5Q, and Rh 2H 2(CO) 2(dpp) 3 > i i , were formed at dpp : Rh < 2 : 1; a small amount of RhH(dpp) 2, 52, was also observed. These hydrides have overlapping "^ P!1!!} NMR, and NMR resonances. The proposed structures for JvQ and 51 are shown in Figure 4.4. H 1 0 C _ R ' h _ P 1 ~ P 3 P^- P 2 H 2 H — Rh—CO 50 51 Figure 4.4. Proposed structures for RhH(CO)(dpp*)(dpp), 5Q, and Rh 2H 2(CO) 2(dpp) 3, 51. 1 2 3 There are three types of phosphorus atoms for 5Q (P , P , P ), and two types for 51 4 5 (P , P ). When -6 equivalents molar excess of dpp ligand were added to 42, the 155 Table 4.1. Spectroscopic data for the various hydrides observed during the interaction of Rh 2(CO) 4(dpp) 2 with H.,, in the presence of dpp ligand. The exact amount of these species present under various conditions is presented in Table 4.3 (P. 173). (a) 3 1P} 1Hj NMR data (in CF^Clj, free dpp is seen at 6 P -17.4) Compound P, ppm Jph .P - H Z JP_P> H Z RhH(dpp) 2 (52) 18.5 143 RhH(CO)(dpp*)(dpp) (5Q) 18.1 (P 2) 123 47 26.3 (P 1) 140 47 -17.6 (P 3) Rh 2H 2(CO) 2(dpp) 3 (51) 18.7 (P 4) 122 47.3 26.5 (P 5) 139 47.3 cis-RhH 2(dpp) 2 + C f (58) 19 99 30 9 82 30 156 Table 4.1 (cont) (b) *H NMR data (only the high field region, in CD-CL) Compound H, ppm J Rh-H' H z Jp_j_j, Hz. RhH(CO)2(dpp)a (56) -9.22, dt 11 54 RhH(dpp)2 (52)14 -10.32, dq 8 22 RhH(CO)(dpp*)(dpp) (5Q) -9.53, ddt 10.5 46.7, 17 Rh2H2(CO)2(dpp)3 (51) -9.70, ddt 10.9 44, 16.1 cis-RhH2(dpp)2 + C f (58) -8.60, m - 142 a. See Figure 4.16. The J p _ H value (54 Hz) indicates that the structure of RhH(CO)2(dpp) is similar to that of RhH(CO)(dpp*)(dpp), having the hydride "somewhere between" cis and trans to phosphine. This results in trans carbonyls in RhH(CO)2(dpp). The ^P^H} NMR peaks for ifj, which is present in very low concentration, are not detected. Species RhH(CO)2(dpp) is proposed because it is an appropriate intermediate in some overall reaction pathways (Section 4.6, Figure 4.28). 157 H2 treatment of the mixture in CHjCl^ gave a simplified spectrum, owing to the complete conversion of 51 and 52 to 5Q (Figures 4.5 (a) and (b), see Section 4.6 for discussion). For species iQ (see Figure 4.5 (b)), the P at 26.3 ppm, which is a triplet of doublets, corresponds to the chemical shift for p\ which is coupled to two 2 2 P atoms and the Rh. The P has a chemical shift at 18.1 ppm (a doublet of 3 doublets), while P is a singlet at 6 —17.6 ppm, the chemical shift being very 1 2 similar to that of free dpp (-17.4 ppm). The co-ordination shifts for P and P are about 35.5 to 43.7 ppm downfield, this being characteristic of a 6-membered chelating ditertiary phosphine ligand attached to Rh(I). The difference between J p ^ . p l and 2 ^Rh-P 2 's a t t r'buted to electronic effects because P is approximately trans to a 1 9 hydride, while P is trans to a carbonyl, a strong 7r-acid ligand. Both coupling constants are in the range expected for Rh(I) complexes.^ 31 1 The P{ H} NMR spectra for species similar to 5Q and 51 have been 11 12 previously reported by Hughes and Young and Kastrup et al. However, there were uncertainties in both cases in the assignment of spectra and structure (see section 4.4 for more detailed discussion). 31 With information on the spectrum of iQ, the P chemical shifts and coupling constants for 51 were obtained by subtracting the spectral data of Figure 4.5 (b) and the RhH(dpp) 2 (52) resonances from the spectrum in Figure 4.5 (a). Not surprisingly, the NMR parameters are very similar for the two complexes, 5Q, and 51 (Table 4.1 (a)). Figure 4.6 shows a computer simulated spectrum corresponding to a mixture of 5D, 51 and 52, as found experimentally in Figure 4.5 (a). (b) H NMR data The ^H NMR spectrum of 5Q in the high field region consists of a doublet 158 6 p*-18-7—* JR»i-P4=122 • V- ,*V47-3—P.! .^_°i>2=i8-i ,*-jRh-p=123 (2^=47 6 P = 2 6 6 — J ^ V - JRh-P1=MO M3 H 6 52 O C - f i ' h - p 1 - p 3 P v - P 2 50 H ' H o c _ l l h - - P ^ N P — d h — C O OP4 O* 51 _ J A — —r-15 «t-U6 Ire* dpp-. at -17-4 -17 Figure 4.5. (a) P{ H3 NMR spectra (121.4 MHz) for the reaction of Rl^CCO^dpp^ and 2 equivalents of dpp with H 2 at ambient temperatures for 10 min ; the complicated spectrum in the region 25-35 ppm is due to an overlap of the spectral features of Rh2H2(CO)2(dpp)3> RhH(CO)(dpp*)(dpp) and RhH(dpp)2; [Rh^ = 20 mM, [dpp] = 42 mM. 159 r — — i 1 1 1 1 11—r 25 15 5 -17 ppm Figure 4.5. (b) J iPTH} NMR spectra (121.4 MHz) for the reaction of Rh 2(CO) 4(dpp) 2 and 6 equivalents of dpp with H 2 at ambient temperatures for 25 min; the spectrum is more simple compared to that in (a), due to a conversion of Rh 2H 2(CO) 2(dpp) 3 into RhH(CO)(dpp*)(dpp); [Rh^ = 24 mM, [dpp] = 150 mM. 160 ) p H 1 O C - R h - P ^ P 3 /V, oc-r H ' H _»lh_^~Np—flh—CO 25 15 ~I -17 ppm Figure 4.6. Pi H] NMR computer simulated spectra for the mixture of hydrides, RhH(dpp)2, Rh 2H 2(CO) 2(dpp) 3 > and RhH(CO)(dpp*)(dpp), at 121.4 MHz in CD 2C1 2, according to the information from the spectrum in Figure 4.5 (a). of doublet of triplets (Figure 4.7). The proton H* is split by P* and Rh into a 2 doublet of doublets, and by P into a triplet, The coupling constant J R ^ . ] . } (10.5 Hz), was established by performing a 3 * P decoupling experiment on the "^H NMR spectrum. There is a difference of 29 Hz between Jpl_pj (17 Hz) and Jp2_j^ 1 2 (46.7 Hz), because P is cis to H, while P is "somewhere between" cis and trans (Table 4.1 (b)). Trans Jp_pj (80-150 Hz) values are usually larger than cis Jp_pj 13 (10-40 Hz) values, and structures £fj and £1 are proposed to account for the differences in the observed coupling constants. In the case where a mixture of £Q, i l , and £2 is observed from a reaction Figure 4.7. H NMR spectrum of £Q (Rh 2(CO) 4(dpp) 2 with 5.9 equivalents of dpp in CH 2C1 2 at ambient temperatures under H 2 for 15 min) (400 MHz, high field region only); [Rh£ = 24 mM, [dpp] = 150 mM. 162 of R l ^ C O ^ d p p ^ and 2 equivalents excess dpp under H^, the J R h _ p j values for the 1 31 various hydrides were obtained from the H{ P} spectrum (Figure 4.8). Again, subtraction of the spectral data of Figure 4.7 and the RhH^dpp^ peaks from the spectrum shown in Figure 4.8 (a) yields the *H NMR parameters for 5J. Figure 4.9 shows the simulated spectrum for a 1 : 0.6 mixture of and 51. (c) FT-IR data An in situ solution FT-IR spectrum of Rh 2(CO) 4(dpp) 2 (42) (Figure 4.10) in CH 2Cl2 shows little change in the 1600-2000 cm - 1 region on bubbling H 2 into the solution for 15 minutes (see Table 4.2, Section 4.3.3, Table 4.3). However, in the presence of excess dpp ligand, the ^(CO) bands of 42 disappear, while three new bands appear at 1981 (shoulder), 1952 and 1910 cm - 1 (Figure 4.11). Repeating the experiment using D 2 instead of H 2 gives comparable data (Figure 4.12) but the shoulder peak at 1981 cm 1 is not observed, implying its assignment as a t>(Rh-H). The species, presumably a carbonyl, associated with the band at 1952 cm - 1 is unknown because the peak grows if the solution is exposed to air, at the expense of the 1910 cm 1 peak. The 1952 cm 1 peak almost certainly forms as a result of oxidation during the transfer of solution from the Schlenk tube to the IR cell via a syringe. The peak at 1910 cm 1 is assigned to a terminal v(CO) of a species produced by the H 2 reaction with R l ^ C O ^ d p p ^ ; the peak position does not change upon deuteration, and hence H and CO are considered to be cis to one another 1 1 (Table 4.2). The data are consistent with the formation of RhH(CO)(dpp*)(dpp) species (5_Q) when Rx^CO^dpp^ in the presence of 6 equivalents of dpp is treated with H 2 for 30 min, as observed by 3 1 P and *H NMR (Sections 4.3.1.1 (a), and 4.3.1.1 (b)). 163 Figure 4.8. (a) H NMR spectrum of Rh 2(CO) 4(dpp) 2 with 2 equivalents of dpp under H 2 for 15 min in CT>2C12 (400 MHz, high field region only); [Rh£ = 20 mM, [dpp] = 42 mM, (b) lmn?) NMR spectrum of the system shown in Figure 4.8 (a). 164 Figure 4.9. (a) H NMR spectrum of a 1 : 0.6 mixture of RhH(CO)(dpp*)(dpp) and Rh 2H2(CO) 2(dpp)3 in CD 2C1 2, obtained from the Rh2(CO)4(dpp)2/2dpp/H2 system (300 MHz, high field region only; RhH(dpp>2, 52, is not included in this spectrum), (b) Computer simulated spectrum of (a) (300 MHz). 165 e E CB 2263 1 7 0 3 Wavenutnber. cm"1 1275 Figure 4.10. FT-IR spectrum of Rh 2(CO) 4(dpp) 2 under CO in CH 2C1 2. E m c 2263 Wavenumber. cm -1 Figure 4.11. FT-IR spectrum of Rh 2(CO) 4(dpp) 2 and 6 equivalents of dpp, dissolved in CH 2C1 2, after bubbling with H 2 for 30 min. 0) u c (0 E c 19521 I u(co)|°f a n unknown carbonyl species 1910 v(CO) 166 2263 1703 Wavenumber, cm 1275 -1 Figure 4.12. FT-IR spectrum of Rh 2(CO) 4(dpp) 2 and 6 equivalents of dpp, after bubbling the solution with D 2 for 1 h in CH 2C1 2. 4.3.2. Attempted Isolation of RhH(CO)(dpp*)(dpp) Two routes were investigated for preparing the pure RhH(CO)(dpp*)(dpp) complex. The first route involved the attempted attack of Rh(CO)(dpp)2 + PF^~ by NaBH^ under Ar (Section 2.1.7.16). The chemistry is summarized below: Rh(dpp) 2 CI AgPF, CO + ^ Rh(dpp) 2 + PF 6" • 0 . 3 C H 2 C 1 2 — R h ( C O K d p p ) 2 + P F / NaBH. Rh(dpp) 2^PF 6 NaBH. RhH(CO)(dpp*Xdpp) Figure 4.13. Proposed synthetic route for RhH(CO)(dpp*)(dpp). 167 Table 4.2. Spectroscopic data for the changes in IR when a C H j C l j solution of Rh2(CO)4(dpp), 2 and 6 equivalents of dpp is treated with H 2 > or Compound [Rh^, [dpp], v, cm - 1 (in CE^Clj) mM mM Rh 2(CO) 4(dpp) 2 a 20 j>(CO) : 1968, 1761, 1734, 1719 Rh 2(CO) 4(dpp) 2, with 21.2 i'(CO) : 1968, 1761, 1734, 1720 H 2 for 15 min new peak at 1821 (weak) Rh 2(CO) 4(dpp) 2 + 6 11.6 67.3 j>(Rh-H) : 1981 equivalents dpp, with KCO) : 1910 H 2 for 30 min y(CO) : 1952 (unknown carbonyl species) Rh 2(CO) 4(dpp) 2 + 6 12.3 71.6 i>(CO) : 1910 equivalents dpp, with v(CO) : 1952 (unknown D 2 for 1 h carbonyl species) a. Measured under CO (see Figure 4.10). 168 However, the Rh(CO)(dpp)2 + PFg~ complex yielded only Rh(dpp) 2 + PF^ - upon reaction with NaBH 4. The loss of CO from Rh(CO)(dpp)2 + BF 4" is known to be very 14 + -facile, and correspondingly the Rh(CO)(dpp)2 PFg species also undergoes CO loss under Ar to give Rh(dpp) 2 + PFg~. Of interest, the Rh(dpp) 2 + C l ~ complex is known 14 + -to react with NaBH 4 to give RhH(dpp) 2 > but the Rh(dpp) 2 PF^ species was found to have no reaction with NaBH 4 under corresponding conditions, and thus Cl must play an important mechanistic role in the NaBH 4 reaction. The second route follows from the in situ NMR studies and involves the stirring of a benzene suspension of Rh 2(CO) 4(dpp) 2 with 6 equivalents dpp under H 2 for 14 h. During this time, the suspension changed to a clear yellow-orange solution which yielded a mixture of RhH(dpp) 2 > RhH(CO)(dpp*)(dpp), and Rh 2H 2(CO) 2(dpp) 3. The yield could be increased by setting aside the product solution at a lower temperature (-4 °C). The high field *H' NMR spectrum of the isolated solid under Ar discloses a mixture, with the ratio of RhH(dpp) 2 : RhH(CO)(dpp*)(dpp) : Rh 2H 2(CO) 2(dpp)j being 1.4 : 6 : 1. The elemental analysis (Section 2.1.7.16) does not fit the formulation for a pure sample of RhH(CO)(dpp*)(dpp). The FT-IR spectrum of the isolated solid shows v(Rh-H) at 1981 cm - 1 and v(CO) at 1908 cm 1 (Figure 4.14). The shoulder peak at 1981 cm 1 is thus considered to be an overlap of v(Rh-H) for RhH(dpp) 2 (2000 cm" 1), 1 4 RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3. A solution FT-IR spectrum in CH 2C1 2 exhibits bands at 1981, -1 * 1952, and 1910 cm , the same as for the in situ generation of RhH(CO)(dpp )(dpp) from the Rh 2(CO) 4(dpp) 2/6 equivalents/H2 system (Section 4.3.1.1 (c), Figure 4.11). Complexes containing ditertiary phosphine ligands bonded via one phosphorus atom (i.e. "dangling" phosphine ligands) have been isolated and characterized by several 15 16 17 18 groups (e.g. Sanger, Tolman et al., Isaacs and Graham, Bao et al., 169 H 1 1 1 r— 3800 3200 2600 2000 1700 W a v e n u m b e r , c m - 1 Figure 4.14. FT-IR spectrum (Nujol mull, Csl plates) of an isolated mixture of RhH(dpp)2, RhH(CO)(dpp*)(dpp), and Rh 2H 2(CO) 2(dpp) 3. 170 19. Higgins et al. ). Other such "dangling" species have been suggested by more 20 21 circumstantial evidence, either by implications from kinetic studies ' or in situ 22 23 detection at variable temperatures by spectroscopic techniques. ' In most cases, the complexes involve the smaller nuclei of the first row transition metals, thus suggesting that steric effects may overcome chelate effects. However, there is no useful rule to predict the occurrence of a ditertiary phosphine as monodentate, chelate or bridging chelate. Several groups have proposed species with a "dangling" phosphine ligand as an intermediate in the conversion into the bidentate species, or have isolated such compounds. These studies include: (a) the CO substitution of [V(CO),]" by P-P, followed kinetically. 2 4 P-P = dpm, dpe, dpp, dpb (b) the formation of a bridged phosphine complex via a species containing a monodentate ditertiary phosphine, followed by 3 1 P NMR. 1 6 b [V(CO) 6]~ + P - P ^ = K V ( C O ) 5 P - P ' ] " + CO (fast) [ V ( C O ) 5 P - P * ] V = ^ [ V ( C O ) 4 ( P - P ) ] " + CO (slow) (4.5) (4.6) r f p ~ p r~ P + naphthalene r- P P P •» dmpe F e — P ^ P + naphthalene (4-7) 171 (c) the stabilization of species with a "dangling" phosphine ligand by alkene co-ordination, based on kinetic evidence (54) 2 2 or isolation of a complex CLS).16'"5 P-P = dpe P-P = dmpe 18 A more systematic approach by Bao et al. allowed for the investigation of the stability of a series of chelates with the ditertiary phosphine ligands P P r ^ C H ^ P P l ^ where n = 1-4. The stability of the chelates increased in the order : dpb < dpm < dpp ~ dpe. However, the formation of the "dangling" phosphine species was enhanced by oxidizing one end of the ligand (equation 4.8). ® Co CO P ~ P room temp CH2CI2 3h Co Co Co I 90 h, air I room temp, CH2CI2 Co p (4-8) Higgins et al. have reported on reactions of PtCL(NCBu%, PtCL(l,5-C0D), 172 PtMe 2(l,5-COD), and PdCl 2(NCPh) 2 with P-P = 0 (where P-P = dpm, dpe, dpp, dpb) which give species with a "dangling" phosphine oxide ligand, where the oxygen end is not bound to the metal centre. However, P = 0 co-ordination could be promoted by removal of a co-ordinating halide or methyl group, which creates a vacant site for the chelation. 4.3.3. Reactions of Rh2(CO)^(dpp)2 and n Equivalents of dpp (n = 0, 1, 2, 6, 20) in the Presence of H 2 and H 2/CO Sections 4.3.1.1 (a) - (c) deal with the characterizations of RhH(CO)(dpp*)(dpp) (50) and Rh 2H 2(CO) 2(dpp) 3 (51). Tables 4.3 and 4.4 show the distribution of hydrides formed from the interaction of 42, containing different equivalents of added dpp, with H 2 in CD 2C1 2 for a certain period of time. 1 31 1 Calculations of percentages were based on H and P{ H} NMR spectroscopic data. When a C D 2C1 2 solution of Rh 2(CO) 4(dpp) 2 was treated with H 2 > in the absence of 31 1 1 any added dpp, there were small changes in the P{ H} NMR and H NMR spectra. The major species was unreacted Rh 2(CO) 4(dpp) 2 (- 95 % ) . A small amount of RhH(CO)2(dpp) (1.1%), and other unknown hydrides with broad signals at high field formed during the treatment. The assignment for RhH(CO)2(dpp) is based on ^H NMR data, where the hydride signal at -9.2 ppm is split by dpp into a triplet Cfp-H = 54 Hz) and further by Rh into a doublet (JRJJ-H - H Hz). The range of the coupling constants is within that for Rh(I) species containing dpp chelating ligand, such as iQ, and .51. However, when excess dpp ligand (n = 1, 2, 6, or 20 equivalents) was added to 42 in CD 2C1 2, the major hydride observed was RhH(CO)(dpp*)(dpp), 50. In one of the runs with n = 1 (Table 4.3), Rh(dpp)2 + C l " was observed due to the decomposition of RhH(dpp)~ in CD-CL. The decomposition Table 4.3. Species generated when Rh2(CO)4(dpp)2 is treated with H 2 or H 2 / C O in CD 2C1 2 in the absence or presence of added dpp. [RhJ, [dpp], Equivalents Species detected time, h mM mM of dpp (% of total) added (i) 29.2a 0 0 42 (> 95%); £6. (1.1%) 0.17 unknown hydrides at high field (5 ~-20, -27.7) (ii) 23.7a 28.4 1 £2 (17%); £8 (22.2%); £Q (26.5%); 4 £1 (26.5%); £6 (2.9%); £2 (4.9%) (iii) 29.2a 65.1 2 £0 (69.8%); £1 (21%);£2 (9.2%) 12 (iv) 14.1a 87.5 6 £Q (87.5%); £1 (8.7%); £2 (3.8%) 14 (v) 17.8a 353 20 £Q (80%); 42 (20%) 10 (vi) 16.2b 99.0 6 42 (66%); £Q (14%) 0.17 unknown hydrides at high field (vii) 16.2b 99.0 6 42 (100%) 0.50 a. Under H 2 > b. Under E y C O (1 : 1). 42 = Rh2(CO)4(dpp)2; £0 = RhH(CO)(dpp*)(dpp); £1 = Rh2H2(CO)2(dpp)3; £2 = RhH(dpp)2; £6 = RhH(CO)2(dpp); £2 = Rh(dpp) 2 + Cf; £8 = RhH2(dpp)2 + C f . 174 Table 4.4. Hydrides generated when Rt^CCO^dpp^^ dpp interacts with H 2 in CD 2C1 2 at ambient temperatures. The products and their distribution varied with time ([Rh^ = 14.1 mM, [dpp] = 87.5 mM). Time, h Amount of species3 (i) 0.16 42 (15%); £Q (60%); unknown hydrides at -25, -26, -30.5 to -30.7 ppm (25%) (ii) 1 h £Q (100%) (iii) 6 h £Q (100%) (iv) 14 h £Q (87.5%); £1 (8.7%); £2 (3.8%) 42 = Rh 2(CO) 4(dpp) 2; £D = RhH(CO)(dpp*)(dpp); £1 = Rh 2H 2(CO) 2(dpp) 3; £2 = RhH(dpp)2-a. The assignments were based on the *H NMR spectrum. 175 of metal hydrides in halogenated solvents is well documented. 25a There is no interaction between R l ^ C O ^ d p p ^ and dpp under ~1 atm CO, as shown in Figure 4.15. In the presence of -1 atm F^, the dimer 42 disintegrates into monomers, with the final concentration of RhH(CO)(dpp*)(dpp) (5_Q) increasing (Figure 4.16 - Figure 4.19) as the concentration of dpp increases. There is also a conversion of the bridged dimer R l ^ F ^ C O ^ d p p ) ^ (51) into the monomer (5_Q), on increasing the concentration of added dpp. The *H NMR spectrum for the interaction of 42 and 6 equivalents of dpp ligand with F^ varies with time (Table 4.4, compare Figure 4.19 and Figure 4.20). g Some unknown high field hydrides at H -31, -26, and -25 ppm, and the RhH(CO)(dpp*)(dpp) (5Q) species appear in the first 10 min. After 1 h, the signals of the high field hydrides (-25 ppm to -31 ppm) disappear. The peaks pertinent to Rh^H-XCO^dpp)^ and RhH(dpp)2 grow in after 14 h. There is a slow conversion of 5Q into 51 and 52, indicating that the "dangling" phosphine end of 5Q (a) forms a bridged dimer via another rhodium hydride (equation 4.9), or (b) becomes chelating and displaces CO from 5Q (equation 4.10). Species 52 could be obtained from P—P dissociation from 5Q-free P P Sp = -17.6 —r— 20 - r o -20 ppm 3 1 „ 1 ¥ Figure 4.15. H HI NMR spectrum of Rh 2(CO) 4(dpp) 2/6dpp in CH 2C1 2 under CO at ambient temperatures; [Rh^ = 27.3 mM, [dpp] = 167 mM. H OC — Rh—CO A unknown species 5 H = -9 22 ^ t -84 -200 ppm Figure 4.16. H NMR spectrum for the reaction between Rh 2(CO) 4(dpp) 2 (with no added dpp) in C^Dg at ambient tempratures and H 2 for 10 min (high field region only, 400 MHz); [Rhj] = 29.2 mM (Table 4.3, entry i). 177 Figure 4.17. H NMR spectrum for the reaction of Rh 2(CO) 4(dpp) 2 and 1 equivalent dpp in CX>2C12 at ambient temperatures with H 2 for 4 h (high field region, 400 MHz); [RbJ = 23.7 mM, [dpp] - 28.4 mM (Table 4.3, entry ii). 178 Figure 4.18. H NMR spectrum for the reaction of Rh2(CO)4(dpp)2 and 2.4 equivalents dpp in CD 2 C1 2 at ambient temperatures with H 2 for 12 h (high field region. 400 MHz); [RhJ = 29.2 mM, [dpp] = 65.1 mM (Table 4.3, entry iii). —I — -95 ppm Figure 4.19. H NMR spectrum for the reaction of Rh 2(CO) 4(dpp) 2 and 6 or 20 equivalents dpp in CD 2C1 2 at ambient temperatures with H 2 for 1 h (high field region, 400 MHz); [Rh^ = 14.1 mM, [dpp] = 87.5 mM (Table 4.4, entry ii). 180 T 1 1 1 -9-5 -10-5 Ppm Figure 4.20. H NMR spectrum for the reaction of Rh 2(CO) 4(dpp) 2 and 6 equivalents excess dpp in C D 2 C l 2 at ambient temperatures with H 2 for 14 h (high field region, 400 MHz.); [Rh^ = 14.1 mM, [dpp] = 87.5 mM (Table 4.4, entry iv). 181 In the presence of 760 ton (total pressure) of Hj/CO (1:1), the Rh 2(CO) 4(dpp) 2 was converted initially into RhH(CO)(dpp*)(dpp) (~ 14%) in the first ten min, followed by complete regeneration of Rh 2(CO) 4(dpp) 2 after 30 min, as observed by 3 1 P and lH NMR (see Table 4.3). 4.3.4. Gas-uptake Studies Changes in gas stoichiometry were measured for the reaction of H 2 with Rh ?(CO) 4(dpp) 2 in the presence of 6 equivalents dpp (toluene, 10 mL, 31 °C). The average of three determinations was 1.0 ± 0.1 equivalent of gas evolution per mole of Rh 0 dimer (Table 4.5, Figure 4.21). The gas stoichiometry fits, for example, the following equation (compare Table 4.4): Rh 2(CO) 4(dpp) 2 + 2 dpp + H 2^r—»-2 RhH(CO)(dpp*)(dpp) + 2 CO (4.11) A net 1 mole equivalent of gas evolution per Rh 2 is expected for the formation of RhH(CO)(dpp*)(dpp) from Rh^CO^dpp)^ An excess amount of dpp ligand (> 2 equivalents) is also required. 4.4. FORMATION OF RhH(CO)(dpp*)(dpp) FROM RhH(CO)(PPh 3) 3 12 11 Kastrup et al. and Hughes and Young have both reported on the in situ generation of RhH(CO)(dpp*)(dpp) via a phosphine exchange reaction of RhH(CO)(PPh 3) 3 (6.Q) with 3 or 5 equivalents of dpp in toluene. Both groups had 31 1 conducted P{ H} NMR studies on the exchange of ditertiary phosphines with Table 4.5. Gas-uptake (evolution) for a reaction of ^ 2 ( 0 0 ) 4 ( 0 ^ ) 2 / 6 dpp with H~ in toluene (10 mL) at 31° C. Time, h Equivalents of gas-uptake (i) a 0 0 0.075 -0.30 0.20 -0.80 0.8 to 2 -0.92 0 0 0.040 -0.20 0.1 -0.48 0.2 -0.88 0.3 -0.94 0.8 -0.98 ,....c (in) 0 0 0.15 -1.0 0.8 -1.1 a. [Rh^ = 3.56 mM, [dpp] = 2.16 x 10~ 2 M b. [Rh^ = 3.60 mM, [dpp] = 2.20 x 10~ 2 M c. [Rh^ = 5.44 mM, [dpp] = 3.48 x 10" 2 M Figure 4.21. Changes in gas stoichiometry for a reaction of Rh 2(CO) 4(dpp) 2 and 6 equivalents of added dpp, with -1 atm H 2 in toluene (10 mL) at 31 °C (using the data in Table 4.5). 184 RhH(CO)(PPh,), under N«, implying that the species observed are those participating in some in situ olefin hydroformylation under C O / H 2 mixtures. However, one must note that during hydroformylation, RhH(CO)2(PPh3)2 is the predominant species 4 present, and the interaction of RhH(CO)2(PPh3)2 with ditertiary phosphines may well be different from that of RhH(CO)(PPh.p3 with ditertiary phosphines under Nj. The studies under N 2 are more relevant to the conditions for catalytic hydrogenation. 4.4.1. NMR data Tables 4.6 and 4.7 show the results of in situ phosphine exchange reactions of RhH(CO)(PPh3)3 (£Q) with dpp, under various conditions in the present studies. The assignments were based on the ^H and ^Pi^H] NMR spectra (Figures 4.22 to 4.24). Increasing the amount of dpp ligand added (from 1.5 equivalents to 5 equivalents) and/or injecting 0.2 equivalents of CO to the RhH(CO)(PPh3)3/dpp/Ar system simplify the spectrum because the system preferably forms RhH(CO)(dpp*)(dpp). Increasing the dpp ligand concentration (compare Figure 4.22 and Figure 4.23) also converts Rh2H2(CO)2(dpp)3 to RhH(CO)(dpp*)(dpp). Also, increasing the CO concentration transforms some RhH(dpp)2 into RhH(CO)(dpp*)(dpp) (compare Figure 4.23 and Figure 4.24). The above observations can be summarized as follows: OC (4-12) H CO H O C - R h — P ~ P (413) P v ^ P Table 4.6. Solution compositions of the phosphine exchange reactions of RhH(CO)(PPh 3) 3 with various equivalents of dpp in CD 2C1 2 at 19° C ( 3 1P probe operated at 1.21.4 MHz, and *H probe operated at 300 MHz unless stated otherwise). Solution [RhH(CO)(PPh 3) 3], M [P-P], M P-P:Rh Comments l a 3.10 X 10 1 4.72 x 10" 2 1.5 ^ V m NMR 2 a 3.10 X 10" 2 4.72 x 10" 2 1.5 *H NMR 3 a 2.30 X lO" 2 0.11 5 3l?{lHl NMR 4 a 2.30 X lO" 2 0.11 5 ! H NMR 5 a 2.30 X lO" 2 0.11 5 3 1P{ 1H3 NMR, 0.2 equivalents of CO injected 6 a 2.30 X _2 10 0.11 5 1 H NMR, 0.2 equivalents of CO injected 7 b 2.50 X _2 10 4.95 x 10"2 2 : H NMR a. P-P = dpp b. P-P = dpe, *H probe operated at 400 MHz. Table 4.7. Products pertinent to the various solutions listed in Table 4.6. e f Solution Amount of species present ' 1, 2 5Q (7.7%); 51 (23.2%); 52 (20.6%); PPh 3 (34.3%); dpp (3.9%); A 3 (< 7.0%); J3b (-1.7%); £ c (- 1.7%); E>d (< 1%) 3, 4 5Q (39.6%); 52 (5%); PPh 3 (14.9%); dpp (39.6%); L>d (< 1%) . 5, 6 50 (70.2%); PPh 3 (7.0%); dpp (17.5%); £ b (-1.8%); E) d (-3.5%) 5Q = RhH(CO)(dpp*)(dpp), 51 = Rh 2H 2(CO) 2(dpp) 3 > 52 = RhH(dpp) 2 a. Unknown triplet of doublets at ^ P = 41 ppm (A). b. Unknown doublet at 5 P = 9.3 ppm (B). c. Unknown doublet at P = 8.5 ppm (£). d. Unknown singlet at P = 28.9 ppm (D). e. Reaction time is -15 minutes. f. See Table 4.1 for the NMR data of species 5Q, 51. and 52. JRh-p^13Q J (^1^47 3 V - P 5 -47-iJ J^-ip2=ia-i _JRh-p1=WO / H p'=47 1" K - |"tvr / »1-p2-47 J * pea1fs9ned H 1 • W 3 OC—Rh—P P 40 30 OC H H CO PPh, at - 6 f re* dpp. at -17-4 6p3 •I -17-6 I.) I 10 0 -10 ppm Figure 4.22. 3 1 R 1 F 0 NMR spectrum for exchange reaction between RhH(CO)(PPh 3) 3 and 1.5 equivalents of added dpp in CD 2C1 2 under Ar (Table 4.6, entry 1). O O —i 188 Figure 4.23. P{ H] NMR spectrum for exchange ""reaction between RhH(CO)(PPh 3) 3 and 5 equivalents of added dpp in CT>2C12 under Ar (Table 4.6, entry 3). 189 H 1 1 O C — R h—P ' / V . 6p> at -17-6 6 p1=26 3 JRh-p 1«i40 J . J p1- p^ B 47 rnrfi 4 t - r -30 \ 6 p2s 181 :123 47 b p6=185 ' j R h . | & i 4 3 s / free PPh3-at-6 free dpp" at -17-4 -r-20 1 10 -10 ppm Figure 4.24. J1P{ HI NMR spectrum for with 5 equivalents of dpp, CTLCU under Ar (Table 4.6, exchange reaction of RhH(CO)(PPh 3) 3 and 0.2 equivalent of added CO in entry 5). 190 4.4.2. FT -IR data When a mixture of RhH(CO)(PPh 3) 3 (1.36 x 10~ 2 M) and dpp ligand _2 (6.46 x 10 M) dissolved in C H j C l j was stirred under Ar for 15 min, the color changed from yellow to yellow-orange. The IR spectrum displayed peaks at 1981, 1952, and 1910 cm \ resembling those observed in the R l ^ C O ^ d p p ^ ^ dpp system (Secdon 4.3.1.1 (c)). The peaks, 1981 and 1910 cm"1, are assigned as the y(Rh-H) and i'(CO) respectively, for the RhH(CO)(dpp*)(dpp) species, while the peak at 1952 cm 1 is assigned to an unknown carbonyl species formed from the oxidation of the solution in the FT-IR cell. 4.4.3. Discussion The spectroscopic data presented in Sections 4.4.1 and 4.4.2 fit the following stepwise processes for substitution of PPh, in RhH(CO)(PPh,), by dpp. P H OC h—CO 2 0 C — R h — p P w P 57 51 50 p ^ p = dpp -CO CO P = P P b 3 H (414) 52 191 All the species in equation 4.14 were observed at low concentration of dpp ligand. Increasing the concentration of dpp shifts the equilibrium toward the right, while CO addition enhances the formation of species iQ (see also equations 4.9, 4.10). The same substitution process had been studied by two groups; Kastrup et 12 11 31 1 al., and Hughes and Young via P{ H} NMR experiments. There were discrepancies in the assignments of the NMR data by both groups. Hughes and Young proposed that both 5Q and 51 (or more specifically, isomers of these - see Figure 4.25 below) had exactly the same *H and "^Pi^H} NMR data. The complications in Figure 4.25. Proposed structures for (a) RhH(CO)(dpp*)(dpp) and (b) Rh~H-(CO)0(dpp), by Hughes and Young, and Kastrup et al. their spectra, and the subsequent tenuous interpretation of the solution Pi H} NMR spectra arise because of the use of a low frequency spectrometer ( 3 1P probe : 36.5 31 MHz used by Kastrup et al.; and the 40.5 MHz P probe used by Hughes and Young). Non first-order "^Pl^H} NMR spectrum resulted when a 36.5 MHz probe is 8 5 employed. The separation between P^ and P 2 is 299 Hz, while J p l _ p 2 is 47 Hz (see Figures 4.4 or 4.22 for the numbering of P atoms), and thus A P/AJ is about 6.4 Hz. Besides, our data show that the spectrum of 51 is not quite superimposed on 192 that of iQ ( 5 P 1 - 6 P 5 = 0.2 ppm; 5 P 2 - 6 P 4 = 0.6 ppm). It is indeed erroneous to interpret the spectrum of complex 5_Q as a first order spectrum (for which A^P/AJ > 9)^k a s w a s c i o n e by Kastrup et a l . 1 2 There is also a discrepancy within the values of Jpj,_pl and Jpjj. p2 reported by Kastrup et al., and Hughes and Young (i.e. Kastrup et al. : Jp n_p2 = 144 Hz; Jpjj.pl = 126 Hz; Hughes and Young : J R n _ p 2 = 126 Hz; J R h _ p l = 143 Hz). Our data match those of Hughes and Young. The 3 1P! 1H] NMR spectra obtained by Hughes and Young ( 3 1P NMR probe : 40.5 MHz) were recognized as non first-order (A P/AJ ~ 7.7), and the simulated 16-line multiplet was assigned to species similar to (isomers of) £Q or £1, but with no effort being directed toward distinguishing between the two (and indeed it may be impossible to do so in a low frequency spectrometer). The hydrides in £Q and £1 were proposed as being trans to CO (see Figure 4.25) by both groups because the three phosphines were considered better sterically accommodated in the equatorial positions of a tbp complex; however, there was no direct evidence for such structures. The *H NMR and IR spectra for £Q and £1 were not reported by these groups, and such data in the present studies argue strongly for cis-orientation of the hydride and CO. There is also phosphine exchange between RhH(CO)(PPhj) 3 and dpe (Table 4.6, entry 7), which produces a mixture of RhH(CO)(dpe )(dpe), Rh 2H 2(CO) 2(dpe) 3 > and RhH(dpe) 2 in a ratio of 5.9 : 2.6 : 1 (Figure 4.26, Table 4.8). The assignments are made by comparison with the chemical shifts and coupling constants of the species identified for the corresponding dpp system (Table 4.1). Apparendy, the dpe ligand can 22 also exist in the dangling mode, and there are other examples in the literature. Pw P7 6 H « B-917 n r _ R h _ P ^ P — R h — C O • una P4T"56 Jp?H 4=18 JRh-H 4=io •10 " -,11 . , 9-87 ft. I JfP-f^se W 6 H « . . 1 0 . 7 0 ppm 193 Figure 4.26. H NMR spectrum for exchange reaction between RhH(CO)(PPh 3) 3 and 2 equivalents of added dpe in CD 2C1 2 under Ar (Table 4.6, entry 7). Table 4.8. A H NMR data for the RhH(COXdpe*)dpe, Rh 2H 2(CO) 2(dpe) 3 > and RhF£(dpe)2 species in CT>2C12 at ambient temperatures (high field region only, 400 MHz); solution composition is presented in Table 4.6, entry 7. Complex 6, ppm ^Rh-H' *^ P-H' H z RhH(CO)(dpe')(dpe) -9.17 10 18, 56 Rh 2H 2(CO) 2(dpe) 3 -9.37 11 18, 56 RhH(dpe) 2 -10.70 10 18 194 4.5. GENERATION OF RhH(CO)(dpp*)(dpp) FROM RhH(dpp)2 In order to substantiate further the pathways for the interactions within the Rl^CO^dpp^/excess dpp/F^ system, which will be discussed in the next section, titrations of RhH(dpp) 2 with CO were carried out in CL^C^, and the formation of the RhH(CO)(dpp*)(dpp) (50) and R h ^ C O ^ d p p ^ (51) followed by XH and ^P{^H} NMR techniques. Moreover, these last two mentioned hydrides were also generated via sequential addidon of ~1 atm CO to RhHtdpp^, followed by treatment with ~1 atm H^. 4.5.1. Titrations with CO Titrations of RhH(dpp) 2 (3.03 x 10" 2 M) in C D ^ with different equivalents of CO were carried out under Ar in a 5 mm tube capped with a Suba-seal rubber septum at ambient temperatures. The tube was shaken for 3 min after each injection of CO into the gas phase above the solution, before the commencement of data acquisition of the spectrum. Table 4.9 summarizes the results. Increasing the amount of added CO increases the proportion of RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3 at the expense of RhH(dpp) 2 (Figures 4.27 (a)-(e)). The Rh^CO^dpp^ species also grows in. In experiment 5, where the 31 1 Rh 2(CO) 4(dpp) 2 peak is apparent in the P{ H] NMR, F^ gas in solution is also observed in the *H NMR spectrum at 4.75 ppm. Thus, the addition of CO causes the conversion of RhFf(dpp)2 to RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3, and in the presence of ~ 2 : 1 excess of [CO], two molecules of the hydridocarbonyl RhH(CO)2(dpp) (Section 4.3.3) are considered to dimerize with loss of H 2 to give Rh 2(CO) 4(dpp) 2. The RhHfCO^dpp) species is not observable in the titrations. This implies that the complex RhH(CO)(dpp*)(dpp) is a stable hydride under PL, and the Table 4.9. Titrations of RhH(dpp). ? (1.82 x 10" 5 mol) with different amounts of gaseous CO, in 0.6 mL CD 2C1 '2-Experiments Moles equivalent Product distribution (%) of CO injected 52 5Q 51 42 1 0.0 100 - - -2 0.43 95 (5) -3 0.86 52 31.5 18.5 -4 1.3 27 46 27 « 1) 5 2.2 3.8 55.6 29.2 11.4 a. Average value from } H and 3 1P{ !H} NMR data probe : 300 MHz, 3 1 P probe : 121.4 MHz). 42 = Rh 2(CO) 4(dpp) 2, 5D = RhH(CO)(dpp*)(dpp), 51 = Rh 2H 2(CO) 2(dpp) 3, 52 = RhH(dpp) 2 > 196 equilibrium constant governing the formation of RhH(CO)2(dpp) from RhH(CO)(dpp*)(dpp) is small, while the dimerization of RhH(CO) 2(dpp) with loss of H 2 to give Rh 2(CO) 4(dpp) 2 is rapid (see Figure 4.28). 4.5.2. Sequential Addition of CO and H 2 to RhH(dpp)2 The RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3 mixture can also be generated by saturating a CH 2C1 2 solution of RhH(dpp) 2 with CO ( -1 atm) for 10 min, followed by degassing three times and introducing ~ 1 atm H 2 for 15 min. The process can be represented by the following scheme: 2RhH(dpp) 4 CO, -H 2 2 < Rh 2(CO) 4(dpp) 2 + 2 dpp FL 1/2 dpp + 2 CO + RhH(CO)(dpp*)(dpp) + 1/2 Rh 2H 2(CO) 2(dpp) 3 (4.15) Degassing of a CO-saturated CH 2C1 2 solution of the Rh 2(CO) 4(dpp) 2/2 dpp mixture gave no apparent change in the yellow color of the original solution; a red color, however, was observed on evaporating the CH 2C1 2 solution of Rh 2(CO) 4(dpp) 2 (Section 4.2.2) due to a loss of CO from Rh 2(CO) 4(dpp) 2 to give Rh 2(CO) 2(dpp) 2 and Rh 2(y-CO) 2(dpp) 2. Thus, an increase in [Rh^CO^dpp)^ and treatment time under Ax or vacuum facilitates such conversion. The immediate introduction of H 2 to the degassed yellow solution produced a mixture of .51), 51, and 52 as shown in Table 4.10. Low temperature *H NMR experiments were also performed down to -70° C: Figure 4.27. (a) ! H NMR spectrum of RhH(dpp). in CD.CL probe : 300 t MHz). 1 1 1 Figure 4.27. (b) *H NMR spectrum for the titration of RhH(dpp), with 0.43 equivalents of CO in CD 2C1 2 (lH probe : 300 MHz). H H o c — R h —P^P—Rh—CO OC-RV -P^P fJP O POP \^  T 1 -106 ppm 198 Figure 4.27. (c) H NMR spectrum for the titration of RhH(dpp) 2 with 0. equivalents of CO in CD 2C1 2 (*H probe : 300 MHz). 86 . _ Rh — P^ P — J h — . C O / f ' O C - R h — P P P v P ppm Figure 4.27. (d) H NMR spectrum for the titration of RhH(dpp), with 1.3 equivalents of CO in CD 2C1 2 ( H probe : 300 MHz). H I O C - R h - P P « H n r — R h _ P ^ P — Rh—CO A. /V p. . p T 1 1 1 1 1 1 1 1 1 1 -8-9 -9-5 -10-1 - -107 PPm 199 Figure 4.27. (e) H NMR spectrum for the titration of RhH(dpp) 2 with 2.2 equivalents of CO in C D 2 C l 2 (*H probe : 300 MHz). broadened features were observed possibly due to a slowing down of an intramolecular rearrangement in the tbp 5Q species. When excess dpp ligand was added, the conversion to £Q was preferentially facilitated (Table 4.10). 4.6. POSSIBLE MECHANISM FOR THE LNTERACTION OF Rh7(CO)4(dpp)2 PLUS VARIOUS EQUIVALENTS OF DPP WITH H 2 AND THE REVERSE REACTIONS The following mechanism is suggested for the interaction of Rh 2(CO) 4(dpp) 2/dpp with H 2 > and the formation of Rh 2(CO) 4(dpp) 2 from the reaction of CO with RhH(dpp) 2 (Figure 4.28). 1 31 1 All the species, except 52, have been detected in the H and P{ H] NMR spectra. When Rh 2(CO) 4(dpp) 2 (42) is treated with H 2 (via H 2 bubbling into the solution) in CH 2C1 2, only a very small amount of 5J5 and other unknown hydrides are observed (Section 4.3.3). Upon addition of a large excess of dpp, the equilibria shift 200 Table 4.10. Product distribution of the hydrides formed by sequential addition of -1 atm CO, and then -1 atm H 2 to RhH(dpp)2 in CD2C12 after 15 min at ambient temperatures. [RhHCdpp)^ , M [dpp], M Products 50 51 52 42 2.26 x 10 -2 - 56.4% 33.7% 9.9% (< 1%) 3.15 x 10"2 7.86 x 10"2 86.5% - 13.5% 42 = Rh2(CO)4(dpp)2; 50 = RhH(CO)(dpp*)(dpp); 51 = Rh2H2(CO)2(dpp)3; 52 = RhH(dpp)2; 201 p O (e) H W C CO H 2 unknown I OC—Rh—CO pV\ / - " * P ^ HYDRIDFTS V " A c6 c --pj P ^ P 40 co TI-co ,(d) H H H p& p V ( N / ) ^ 2 o C - R h - P ^ P ^ OC-Rh_so lv V*V ~ £ i \ - P - P g \ 52 P w P P W P 50 59 - 5 0 ^ 50 H H O C - J h - P ^ P - R ' h - C O Figure 4.28. Proposed pathways for the interaction of Rh2(CO)4(dpp)2 with H 2 in CHjClj, and its formation from RhH(dpp)2 and CO. toward the formation of 50 and 51. There is a competition among the various pathways, (b), (c), and (d) for the consumption of 52. Since 50 is the major species observed when 6 equivalents of dpp are added to 42 under ~1 atm H 2 for 10 min to 14 h, the formation of iQ from 52 must be very facile, compared to other pathways (such as (c) and (d)) leading to the consumption of 52- Species 42 is very stable under CO. Because the formation of 52 from 56 requires the loss of CO (step (d)), it is important that this process is facilitated by either bubbling in the FL gas 202 or degassing the Rh dimer/dpp solution before the introduction of (Section 4.5.2). The conversion of £1 to £Q is achieved by adding excess dpp ligand (see equation 4.12), while the loss of CO from £Q to give 52 is slow (step (a), see equation 4.10, Section 4.3.3). Therefore, the conversion of 42 into the : monomeric hydrides £Q, 52, and the ditertiary phosphine-bridged hydride 51 probably occurs via the formation of the unstable Rh(I) species, £2, which reacts rapidly with dpp or £Q, depending upon the concentrations of £Q and dpp. The mode of activation by 42 in step (e), in the presence or absence of dpp, remains uncertain. In either case, some unknown hydrides were detected (^ H NMR) at relatively high fields, but the signals appeared only for the first 10 min of interaction. These hydrides are plausibly bridging hydrides in dimeric Rh complexes; such species are well documented and some e.xamples are provided below. H P. (Ref. 27) (Ref. 28) (Ref. 29) It proved impossible to elucidate the nature of the intermediate hydrides in the present system (Rh^CO^dpp^/dpp/Hy because of their transient existence. There have been previous reports on the. transformations of dimeric rhodium(O) carbonyl complexes into monomeric hydrides "under Hj, 'as exemplified in equations 4.1630 and 4.174 : 203 DMI u Rh(CO)2(acac) + PR 3 M • 1/2 [Rh(CO) 3PR 3] 2 + 1 «> RhH(CO) PR, H ?/CO 1 atm, x 3 (1/1), • 47 MPa, 230° C, 45min + (3-x) CO (4.16) R = c-C 5H 9, i-Pr H. added P, -CO Rh 2(CO) 4P 4 < « _ J _ » 2RhH(CO) 2P 2 4= 2RhH(CO)P, (4.17) CO CO The exact nature of the H 2 interaction with the Rh dimeric complexes in equations 4.16 and 4.17 is not known. During the course of the present work, Bassett et al. also suggested some similar pathways to those shown in Figure 4.28 for the decomposition of the Rh4(CO)jQ[(-)diop] complex, as shown in Figure 4.29.^ Rh 4(CO) 1 0t(-)diop] (-)diop -2 CO Rh 4(CO) g[(-)diop] ; 2 (-)diop 2 Rh 2(CO) 4[(-)(diop)] 2 RhH(CO)[(-)diop*][(-)diop] + Rh,H,,(CO),,[(-)diop]3<* -4 CO (-)diop £2 £2 -CO H 2 (500 torr) 2RhH(CO)2[(-)diop] £1 Figure 4.29. Reaction of Rh 4(CO) 1 ( )[(-)diop] with H. 204 There was no evidence presented for the existence of .61, while the species £2 and £3 31 were identified only by their P NMR spectra, the peaks appearing as a complex multiplet between 20 and 30 ppm. Species .62 and £2 were not distinguished 11 31 spectroscopically. ' 4.7. CONCLUSION The rhodium hydride, RhH(CO)(dpp*)(dpp) (5_Q), containing a "dangling" ditertiary phosphine can be generated in situ by (i) an exchange reaction of RhH(CO)(PPh 3) 3 with 5 equivalents of dpp; (ii) titrating RhH(dpp) 7 with ~ 2 equivalents of CO; (iii) cleaving Rh 2(CO) 4(dpp) 2 with an excess amount of dpp (~6 equivalents) under The distribution of products results from a subtle balance among [F^], [CO], [dpp], and the type of starting complex, i.e. RhH(dpp) 2 > RhH(CO)(PPh 3) 3, or Rh 2(CO) 4(dpp) 2. The structural assignment of the in situ formed RhH(CO)(dpp*)(dpp) is based on IR, NMR, and tensometric (changes in gas volume stoichiometries) data. The pathways for the cleavage of the Rh 2(CO) 4(dpp) 2 complex are proposed in Figure 4.28. Without addition of excess dpp ligand, Rh 2(CO) 4(dpp) 2 does not react with H 2 to give RhH(CO)(dpp*)(dpp). More important still, without the cleavage to such a hydride, the Rh 2(CO) 4(dpp) 2 complex is ineffective for catalytic hydrogenation (see Section 5.1). The next chapter will deal with the homogeneous hydrogenation of 1-hexene, using the system consisting of the in situ formed RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3 species. CHAPTER 5. HOMOGENEOUS HYDROGENATION OF 1-HEXENE VIA IN SITU GENERATION OF RhH(CO)(dpp*)(dpp) AND Rh 2H 2(CO) 2(dpp) 3 AS CATALYST PRECURSORS 5.1. INTRODUCTION The original purpose of the research project was to investigate the possibility of hydroformylation of alkenes using Rh^CO^P— P) 2 complexes (P—P = ditertiary phosphine) as catalysts. In Chapter 4, the interaction of Rh 2(CO) 4(dpp) 2 (42) with H 2 was described. Essentially, the dpp dimer, in the presence of excess dpp ligand, was converted to monomeric hydrides and a phosphine-bridged dimeric hydride. The same hydrides were also generated initially using H 2/CO (1/1) synthesis gas, but the Rh 2(CO) 4(dpp) 2 species was completely regenerated after 30 min (Section 4.3.3). This dpp dimer is considered unlikely to be an efficient catalyst for hydroformylation because it is co-ordinal!vely saturated. The low turnover rate (1.25 h *) observed for the hydroformylation of 1-hexene at 55°C in benzene using Rh 2(CO) 4(diop) 2 as catalyst^ is consistent with this. There have been few reports on the study of homogeneous hydrogenation of alkenes using binuclear Rh complexes containing ditertiary phosphine ligands (see Section 1.6). The field of homogeneous hydrogenation is dominated by reports on monomeric Rh complexes (see Sections 1.3.2, and 1.3.3). Because the kinetics and mechanisms for the hydrogenation of alkenes using 205 206 dimeric Rh(0) complexes or monomeric Rh hydrides containing "dangling" phosphine ligands have not been studied, we decided to undertake such a study, and this chapter describes the homogeneous hydrogenation of 1-hexene using such compounds as catalysts. 5.2. CATALYTIC HYDROGENATION OF 1-HEXENE 5.2.1. Homogeneous Hydrogenation of 1-Hexene Using Rh 2(CO) 4(dpp) 2 as Catalyst A sample of Rh 2(CO) 4(dpp) 2 (42), weighed in a glove bag, was used as a potential catalyst for hydrogenation of 1-hexene in toluene. The solution remained yellow for 16 h, but no H 2 consumption was observed at temperatures up to 40° C, using [Rh] of (1.24-2.05) x 10 - 3 M, and [1-hexene] up to 1.0 M . The co-ordinatively saturated 42 is thus an ineffective catalyst for homogeneous hydrogenation of 1-hexene. Photolysis to remove a CO ligand, or the addition of dpp ligand to cleave the dimer, may be necessary in order to open up a vacant site in 42 for the activation of substrate molecule or H^ The dpp dimer, 42, was cleaved into monomeric and dimeric hydrides in CH 2C1 2 by the addition of excess dpp under H 2 (Section 4.3.3). The reaction of 42 in CH 2C1 2 with ~1 atm H 2, in the presence of various equivalents of dpp (dpp : Rh > 1), yields mixtures of the RhH(CO)(dpp*)(dpp), 50, Rh 2H 2(CO) 2(dpp) 3 > 51, and RhH(dpp) 2 > 52, complexes (Section 4.3), and CH 2C1 2 solutions of RhH(dpp) 2 following treatment with CO and subsequently H 2 also produce the same mixture (Section 4.5.2). Because Rh 2(CO) 4(dpp) 2 is quite air-sensitive in the solid state, it proved to be more convenient to use RhH(dpp) 2 as the catalyst precursor. The catalytic activity of RhH(dpp)- is found to be negligible in the present context (see Table 5.2, entry 5, 207 Section 5.3), and thus the catalytic activity of 1Q and/or 5_1 for homogeneous hydrogenation of 1-hexene can be evaluated using the "RhH(dpp)2/CO/H2" system. 5.2.2. Homogeneous Hydrogenation of 1-Hexene Using the In Situ RhHfdpp^/CO/^ Catalyst System 5.2.2.1. Preparation of Catalyst Solution Solutions of "the RhH(dpp)2/CO/H2 system" were found to be efficient catalysts for the homogeneous hydrogenation of 1-hexene under mild condidons. The system generated an in situ mixture of 49, 1Q, H , and 12; the procedure involved equilibrating RhH(dpp)2> 12, with CO (at a total pressure of 1 atm) to yield 42 and dpp, followed by degassing thrice by freeze-thaw cycles, and then introducing - 760 ton H 2 (equation 5.1). 4 CO, -Hj Hy 2 RhH(dpp)2 ^ fc. Rh2(CO)4(dpp)2 + 2 dpp ^ I » 1/2 dpp + 2 CO 12 42 + 1/2 Rh2H2(CO)2(dpp)3 + RhH(CO)(dpp*)(dpp) (5.1) II ID Two experimental procedures were used for the introduction of the 1-hexene substrate into the RhH(dpp)2/CO/H2 system (see Section 2.3.2 for operation of the gas-uptake apparatus). The first method involved the addition of 1-hexene to RhH(dpp)2 before equilibrating the system with CO, and then H 2 sequentially, while the second method involved injecting 1-hexene after the equilibration procedure with CO and H r Both methods gave the same results and, consistent with this finding, 208 31 1 RhH(dpp)2 did not react with 1-hexene or FL,, as monitored by P and H NMR. 5.2.2.2. Gas-uptake, GC, and NMR Studies Figure 5.1 shows a typical gas-uptake plot at 31°C for [1-hexene] at 0.0478 M, [Rh] at 1.17 mM, and [ H ^ at 3.10 mM. The H2~ uptake was followed up to at least 80% completion for most of the runs, except for those with [1-hexene] = 0. 857 M, where the reaction was followed up to only - 4% completion. The solution remained clear throughout the hydrogenation reactions with no metal deposition being observed. The uptake curve can be divided into three regions: the initial (or "induction-type") period (see "A" in Figure 5.1), the second period of a maximum, essentially linear rate, and a final slowing down period. Tables 5.1 (i), (ii), and (iii) list the results of the uptake experiments, under varying concentrations of F^, 1-hexene, and RhH(dpp) 2; listed are the maximum rate, the "induction time", and the approximate H^-uptake at the end of the induction period (Ah). The kinetic data (i.e. [F^] absorption versus time) for each experiment are provided in Appendix II. Such an S-shaped uptake curve (i.e. initially a slow reaction period followed by a region of more rapid rate) shown in Figure 5.1, in general, could be due to any of the following reasons: 1. slow generation of the active species, such has been found, for example, in the hydroformylation of alkene using trans-RhCl(CO)(PPh 3) 2; the observed induction (i.e. inhibition) period, which could be removed by the addition of base, was attributed to a slow hydrogenolysis reaction that generated the active monohydride 2 species (equation 5.2). 209 Figure 5.1. A typical reuptake plot for the hydrogenation of 1-hexene in toluene (5 mL), at 31° C using the RhH(dpp) 2/CO/H 2 catalyst system; [1-hexene] = 0.0478 M, [Rh] = 1.17 mM, [ H ^ = 3.10 mM. 210 Table 5.1. Kinetic data for the homogeneous hydrogenation of 1-hexene catalyzed by the RhH(dpp) 2/CO/H 2 system (toluene, 31° C). (i) Dependence of maximum rate on [Rh]. [Rh] x 103, M a Maximum rate x 10^, inhibition time, h A h b M s _ 1 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 0.219L 0.328^  0.328° 0.727^  0.727d 1.17c. 1.17d 1.71c 1.71e 2.55c 3.56cf 0.2731 0.273f 0.273 0.650 1.17* 2.20 f 5.66 0.38 3.6 7.53 0.47 3.8 7.52 0.47 3.6 10.0 0.80 2.1 9.68 0.83 3.1 13.3 1.54 2.1 13.5 1.48 2.5 14.2 2.14 3.2 14.4 — — 8.15 3.31 3.0 7.67 4.88 2.3 33.3 0.012 2.9 26.1 0.009 3.3 5.23 0.0007 2.0 42.6 0.044 2.4 81.5 0.088 2.9 139 0.17 2.2 a. [Rh] = Initial [RhFKdpp)^ b. Ah = estimate of equivalents of H2~uptake defined by the intersection at B in Figure 5.1 per Rh at the transition from the initial reaction period to the second period. c. [1-Hexene] = 0.0319 M, [H^) = 3.10 mM; injection of 1-hexene. d. [1-Hexene] = 0.0319 M, [H^ = 3.10 mM; non-injection addition of 1-hexene. e. Reinjection of 1-hexene (20 M L , 0.0317 M) into the reaction mixture near the end of the catalysis (- 84 % completion). f. [1-Hexene] = 0.857 M, [ H ^ = 3.10 mM; non-injection addition of 1-hexene. g. [1-Hexene] = 0.857 M, [H^] = 2.69 mM; non-injection addition of 1-hexene. h. [1-Hexene] = 0.857 M, [ H ^ = 1.11 mM; non-injection addition of 1-hexene. Table 5.1 (cont.) (ii) Dependence of maximum rate on [1-hexene] ([Rh] = 1.17 mM, [ H ^ = 3.10 mM, injection method). 5 a [1-Hexene], M Maximum rate x 10 , 1/inhibition time, Ah KA "I U - l M s h 1. 0.0160 1.16 0.42 3.8 2. 0.0319 1.33 0.65 2.1 3. 0.0478 2.75 1.20 1.7 4. 0.0635 3.53 1.28 3.0 5. 0.127 5.63 3.28 3.4 6. 0.857° 7.46 11.3 2.9 a. As for footnote b in Table 5.1 (i). b. Non-injection addition of 1-hexene. 212 Table 5.1 (conL) (iii) Dependence of maximum rate on [H^l ([Rh] = 1.17 mM; [1-hexene] = 0.0319 M, injection method; total pressure = 760 torr = P^ + + P j 0 i u e n e -P H , torr [ H ^ x 103, M Maximum rate x 106, inhibition time, h A h a M s" 1 1. 76 0.489 7.40 1.57 2.6 2. 116 0.653 8.96 1.63 3.1 3. 156 0.816 12.2 1.31 1.5 4. 206 1.02 12.3 1.45 2.7 5. 256 1.22 13.1 1.71 3.2 6. 556 2.45 13.3 1.70 2.2 7. 716 . 3.10 13.3 1.54 2.1 a. As for footnote b in Table 5.1 (i). 213 RhCl(CO)P 2 + N E l 3 + H 2^=»vRhH(CO)P 2 + E^NHCl (5.2) P = PPh 3 2. autocatalysis, a reaction product itself becoming a catalyst Examples of such kinetics are found in the acid-catalyzed hydrolyses of esters and related compounds; the carboxylic acid product obtained from the ester hydrolysis acts as 3 a catalyst 3. a free radical mechanism, with the induction period being the time required for accumulation of the concentration of free radicals required for propagation of the subsequent chain reactions.3 An autocatalytic reaction involving the hexane product is unlikely because it would be necessary to invoke some process requiring C-H activation at an sp 3 carbon atom; this exciting improbability was ruled out because only hexane. cis- and trans-2-hexenes were found as quantitative products in the initial "induction" period (see Table 5.3 for the results of the analysis of reaction products in the intial and second periods by GC, and Section 2.7 for the conditions employed in the GC analysis). No other product possibly arising from C-H activation was detected. Free radical mechanisms were tested for by injecting TEMPO during both the initial and second periods of the catalysis. The concentration of added TEMPO was the same as [1-hexene], both being ~ 30 equivalents in excess over the [Rh]. The results (see Table 5.2, entries 2,3) showed that there was essentially no change in the induction time or in the rates during either the initial or maximum rate periods. Thus, TEMPO, a free radical trap, has no effect on the hydrogenation system. The organics, at any stage, are tested by GC (Table 5.3) and found to consist of 1-hexene, cis-and trans- 2-hexenes, and hexane, without formation of any coupling or branched 214 Table 5.2. Kinetic data for various conditions of hydrogenation employing the RhH(dpp) 2/CO/H 2 system, or RhH(dpp) 2 > as catalyst at 31° C in 5 mL toluene (unless stated otherwise). [Rh] x 10 3 a 6 M Maximum rate x 10 , Inhibition time, h A h b M s _ 1 1. 1.17c 13.3 1.54 2.1 2. 1.17c 12.4d 1.53 2.3 3. 1.17c l l . l e 1.82 2.0 4. f 1.171 5.25 1.72 4.3 5. 1.17g 0.79 - -6. 1.17h 9.71 1.73 4.1 7. L I T 0 4 (0.091) after 12 h, unable to estimate a., b. See corresponding footnotes of Table 5.1 (i). c. The standard in situ RhH(dpp) 2/CO/H 2 system. d. 0.0319 M TEMPO injected in the initial period (after -30 min). e. 0.0319 M TEMPO injected in the second period (after -1.8 h). f. In 5 mL DMA solution, [H^ = 1.76 mM. g. Using only RhH(dpp) 2 as catalyst h. -2.0 equivalents of CO injected into gas phase above a toluene (5 mL) solution of RhH(dpp) 2 under H 2; the system was then shaken for 10 min prior to the injection of 1-hexene. i. Added [dpp] = 3.51 mM. 215 alkanes (Table 5.3). Thus, free radical pathways are highly unlikely. During the transition from the initial to the second period, there was a color change of the solution from pale to a more intense orange-yellow and this was accompanied by on the average ~ 2.7 equivalents of H 2 consumption per Rh (see Table 5.1). This is suggestive of a process forming an active catalyst. The initial period is thus considered to result from the generation of an active species. In order to test whether the initial H 2~ uptake was used mainly for the hydrogenation of 1-hexene or for the generation of another active species, the following experiment was performed. The uptake by a solution with a [RhH(dpp)2] of 1.17 mM and [1-hexene] of 0.0319 M, under ~ 760 torr P„ at 31° C, was monitored. After ~1 h of reaction (still in the initial period), 1.0 (±0.1) equivalent of was consumed, while 0.8 (+0.2) equivalents of hexane were produced and detected by GC (Table 5.3). This indicates that the generation of the more active catalytic species (on going to the maximum rate region) does not require "stoichiometric" amounts of H 2 for the formation of a Rh hydride; the slow hydrogenation and isomerization of 1-hexene that takes place during the initial period must utilize some hydridorhodium species. Addition of 1-hexene into the reaction mixture near the end of catalysis ( ~84% completion), i.e in the final slowing down period, caused an immediate H 2 uptake, with the rate of H 2 consumption being nearly the same as that observed in the maximum rate period prior to the injection of 1-hexene. The slow period of H 2 consumption was not again observed presumably because the active catalytic species had already been generated before the injection (Table 5.1 (i), entry 9), i.e. the active catalytic species has been fully formed in the maximum rate period. For the gas-uptake plots, the close to linear maximum hydrogenation rates for the second period were readily measured (± 10% error). The slow rates for the initial 216 Table 5.3. Product distribution for 1-hexene catalyzed by the RhH(dpp) 2/CO/H 2 system in toluene (5 mL) at 31 °C. Reaction H 2~ uptake Product distribution (%) time, h (%) hexane 1-hexene t-2-hexene c-2-hexene 1.0b 3.5 2.8 93.4 2.5 1.3 6.0C 94.4 94.7 2.7 2.6 a. Determined by GC (see Section 2.7). b. [Rh] = 1.17 mM, [1-hexene] = 0.0319 M, [ H ^ = 3.10 mM; injection method. c. [Rh] = 0.328 mM, [1-hexene] = 0.0319 M, [Hj\ = 3.10 mM; injection method. 217 period were difficult to obtain accurately because very small changes were involved. The dependences of the maximum rate on concentrations of Rh, 1-hexene, were investigated. The [Rh] was varied from 0.219 to 3.56 mM, the [1-hexene] from 0.0160 to 0.857 M, while P H was changed from 36 to 716 torr. Toluene (5 mL) was used as solvent and the hydrogenations were carried out at 31° C. Addition of three-fold excess dpp, the use of DMA as solvent, and the injection of ~2 equivalents CO into solution of the RhH(dpp)2 complex under H 2 , which generates the same types of hydrides as in the RhH(dpp) 2/CO/H 2 system, were also studied. The distribution of the species in the RhH(dpp)2/CO/H2 system (species 42, 5D, 51, 52) in the initial and second periods was investigated by ^H and ^P^H} NMR studies. A toluene (0.7 mL) solution of 1-hexene (30 ML , 0.329 M) was added to a Schlenk tube ( ~10 mL in volume) constructed with a side-arm NMR tube (5 mm diameter). The solution was degassed three times and RhH(dpp)2 (14.5 mg, 0.0214 M) added under a blanket of Ar. The resulting solution was then frozen immediately in liquid N 2 > and degassed 3 times using freeze-thaw cycles. Then, ~ 1 atm CO was introduced and the solution stirred for 15 min. The yellow solution, now containing Rh2(CO)4(dpp)2 and dpp (see equation 5.1), was then degassed three times, and H 2 ( ~1 atm) added. The whole system was stirred under H 2 for various times, during which hydrogenation occurred as shown by the appearance of the hexane peaks in ^H NMR. There was also a gradual color change of the solution from pale to a more intense yellow-orange during a 4-h period. The solution was then degassed twice and decanted into the side-arm NMR tube, which was then sealed under vacuum. A blank experiment was also performed by excluding 1-hexene from the system. The distribution of species within the RhH(dpp)-/CO/H7 catalyst system, before and during 218 the initial and second periods (67% of 1-hexene being hydrogenated) of the catalytic hydrogenation of 1-hexene in C^Dg, is found to be constant (the percentage ratios of Rh2(CO)4(dpp)2 : RhH(CO)(dpp*)(dpp) : Rh2H2(CO)2(dpp)3 : RhH(dpp)2 ~6 : 42 : 7 : 45, see Section 4.5 for discussion). 5.3. KINETIC DATA OBTAINED FOR THE CATALYTIC HYDROGENATION OF 1-HEXENE USING THE RhH(dpp) 2/CO/H 2 SYSTEM 5.3.1. Initial Region The data of Table 5.1 show that the inhibition time (i.e. the time measured at the abscissa intercept of the line drawn for the second region of the uptake curve, the slope of this line giving the maximum rate - see "A" in Figure 5.1) was found to be approximately directly proportional to the [Rh] at low (Figure 5.2 (a)) and high (Figure 5.2 (b)) [1-hexene], inversely dependent on [1-hexene] (Figure 5.3), and essentially insensitive to [H2] (Table 5.1 (ii)). The maximum rate of hydrogenation was sensitive to [Rh], [1-hexene], and [H2]. At the high [1-hexene] (0.857 M), the inhibition period was short and the H2-uptake in the second period was relatively fast (Table 5.1 (i)). When the [1-hexene] was lower, the maximum hydrogenation rate decreased and the inhibition period became longer; the induction periods increased also with increasing [Rh]. Clearly, 1-hexene plays a very important role in the conversion of the catalyst precursors into the active species, i.e. in the "induction" period, during the transition from the initial slow period to the second faster region of hydrogenation. 219 Figure 5.2. (a) Dependence of the mhibttion time on TRh] at [1-hexene] = 0.0319 M, f i y = 3.10 m M 220 Figure 5.2. (b) Dependence of the inhibition time on [Rh] at [1-hexene] = 0.857 M, [Hj] = 3.10 mM. 12 [1-Hexene], M Figure 5.3. Dependence of 1/inhibition time on [1-hexene] (at [Rh] = 1. mM, [Hj] = 0.0319 M). 222 5.3.2. Maximum Rate Region The maximum hydrogenation rates measured for the second period are listed in Table 5.1. Both the injection and non-injection methods (Section 5.2.2) give essentially the same rates at a fixed [Rh], [1-hexene] and [Hj] (Table 5.1 (i), e.g. entries 2,3 and 4,5). Figures 5.4-5.6 show the variation in uptake plots for various [Rh], [1-hexene], and [ly. The dependence of the second region maxmimum rate on P„ . at [Rh] = 1.17 mM and [1-hexene] = 0.0319 M, was approximately 1st order up to 160 torr (0.816 mM), but this then decreased and approached zero order at pressure > 260 torr (1.22 mM) (Figure 5.7 (a)); the dependence is probably best described as 1st order at low pressure, and becoming less than 1st order and approaching zero order at pressures above -260 torr. However, at higher [1-hexene] (0.857 M) and at [Rh] = 0.273 mM, the dependence of the maximum rate on P H is approximately first-order (Figure 5.7 (b)). The dependence on [Rh] at a lower [1-hexene] (= 0.0319 M) approximates half order up to 1.17 mM in metal (Figure 5.8 (b)) and then levels off to zero and becomes negative at [Rh] > 2 mM. The maximum H j - uptake rate for these conditions centred at ~ 1.4 mM Rh (Figure 5.8 (a)). The unusual negative rate dependence on metal at high [Rh] is tentatively suggested as resulting from the aggregation of Rh nuclei to form polymeric species. At higher [1-hexene] (~ 0.857 M), the dependence on [Rh] remained first order up to 2.5 mM (Figure 5.8 (c)). The dependence on [1-hexene] was approximately first order up to 0.1 M and then decreased and approached zero order at ~1.0 M (Figure 5.9). Such saturation 4 kinetics are common, and the olefin dependence shows that the rate law must contain a [1-hexene] term in both the numerator and the denominator. 223 35 30-S 25 © X 20 o c n 15-1 0 -• TRh] = 0.219 mM O TBh] = 0.326 mM • [Rh] = 0.328 mM <tHnwnjectian) O [Rh] = 0.727 mM V [Rh] = 0.727 mM (non-injection) O TP « V 0 ••»-L«J"u D 0.6 1.5 2.5 Time.li Figure 5.4. (a) Hydrogen uptake curves at various .{RhJ for hydrogenation of 1-hexene at low [1-hexene] catalyzed by the RhH(dpp) 2/CO/H 2 system in toluene (5 mL) at 31°C ([1-hexene] = 0.0319 M, [ H ^ = 3J0 mM, [Rh] = 0.219-0.727 mM). 35-224 30-CO 25-O o 20-o es 15-10-• rRh] = 1.17 mM OrRh] = 1.17 mM (non-injection) Q • [Rh] = 1.71 mM O D [Rh] = 2.55 mM o * V [Rh] = 3.56 mM O C« O I % : o l » • o 6 • • • • o • o • D D • V V V V 7 7 7 V V V 3 4 Time, h Figure 5.4. (b) Hydrogen uptake curves at various [Rh] for hydrogenation of 1-hexene at low [1-hexene] catalyzed by the RhH(dpp) 2/CO/H 2 system in toluene (5 mL) at 31° C ([1-hexene] = 0.0319 M, [ H ^ = 3.10 mM, [Rh] = 1.17-3.56 mM). 225 40 35-30 25-eo © 8 20 • f i © •8 15 10-o • [Rh] = 0.273 mM O [Rh] = 0.650 mM • o • [Rh] = 1.17 mM • • [Rh] = 2.20 mM • o D O • • D • • • • • o • • • D • 0 0.00 0.05 0.10 0.15 0.20 Time, h 0.25 0.30 0.35 0.40 Figure 5.4. (c) Hydrogen uptake curves at various [Rh] for hydrogenation of 1-hexene at high [1-hexene] catalyzed by the RhHCdpp^/CO/Hj system in toluene (5 mL) at 31° C ([1-hexene] = 0.857 M, [ H ^ = 3.10 mM, [Rh] = 0.273-2.20 mM). 80 CO o 7 0 -6 0 -« SO-'S f o •a E 40-30-2 0 -10-V V V V V V V • [1-Hexene] O [1-Hexene] • [1-Hexene] • [1-Hexene] V [1-Hexene] © [1-Hexene] • D • B' • • <9 o o o 8 0.0160 M 0.0319 M 0.0478 M 0.0635 M 0.127 M 0.857 M • o o 0.S T -1.5 —T— 2.5 - r 3 3.5 Time, h Figure 5.5. Hydrogen uptake curves at various [1-hexene] for hydrogenation of 1-hexene catalyzed by the RhH(dpp) 2/CO/H 2 system in toluene (5 mL) at 31° C ([Rh] = 1.17 mM, [ H ^ = 3.10 mM). 227 35 CO O • f i o 30 25 20 15-10-5 -D 7 S • [H2l = 0.489 mM [H2l = 0.653 mM [H2l = 0.816 mM [H2l = 1.02 mM [H2l = 1.22 mM [H2l = 2.45 mM [H2] = 3.10 mM V • © • © • V • • v» o DOOJCL! I*,*© • D ' o _9> 6> 0.5 T -1.5 2.5 3.5 Time, h Figure 5.6. Hydrogen uptake curves at various [ iy for hydrogenadon of 1-hexene at low [1-hexene] catalyzed by the RhH(dpp)2/CO/H2 system in toluene at 31°C ([Rh] = 1.17 mM. [1-hexene] = 0.0319 M). 228 35 [ H 2 ] * l o 3 > M Figure 5.7. Dependence of the second region maximum rate of hydrogenation on [Hj] ((a) [Rh] = 1.17 mM, [1-hexene] = 0.0319 M, (b). [Rh] = 0.273 mM, [1-hexene] = 0.857 M). 229 16 14-Of 1 I I 1 1 1 1 1 ! 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 [Rh] x 10 3, M Figure 5.8. (a) Dependence of the maximum hydrogenation rate (second region) on [Rh], at [1-hexene] = 0.0319 M, [HJ = 3.10 mM (injection method). 230 Figure 5.8. (b) Dependence of the maximum rate of hydrogenation on [Rh] (up to [Rh] = 1.17 mM), at [1-hexene] = 0.0319 M, [ H j = 3.10 mM (injection method). 160 0 0.5 1 1.5 2 2.5 3 [Rh] x 103, M Figure 5.8. (c) Dependence of the maximum rate of hydrogenation on [Rh], [1-hexene] = 0.857 M, [ H j = 3.10 mM. 10 8-o • 1 1 1 1 i 1 1 ! 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 [1-Hexene], M Figure 5.9. Dependence of the maximum rate of hydrogenation (second on [1-hexene] ([Rh] = 1.17 mM, [HJ = 3.10 mM). region) 233 Other kinetic studies included the use of RhHCdpp^ complex alone as the catalyst, the use of excess dpp ligand in the RhF^dpp^/CO/Hj system, the injection of TEMPO into the RhHCdpp^/CO/Hj system during both the initial and second periods, the use of DMA as solvent, and the injection of CO into a solution of the RhH(dpp)2 complex under H j to generate an in situ catalyst. The RhH(dpp)2 catalyst gave hydrogenation rate about 17 times lower than that found for the RhH^dpp^/CO/Hj system under corresponding conditions (Table 5.2, entry 5, Figure 5.10). When - 2.0 equivalents of CO were injected into the gas phase above a toluene solution of RhH(dpp)2 (1.17 mM) under Hj, the same species were generated as in the standard in situ RhFKdpp^/CO/Hj system (Table 5.2, entry 6, Sections 4.5.1, and 4.5.2). Further, similar kinetic data (i.e. maximum rate, and inhibition time) were obtained as in the RhF^dpp^/CO/Hj system under corresponding conditions (Table 5.2, entry 1). Excess added dpp ( ~3 equivalents) caused a very long initial period and only about -15% of the total possible Hj-uptake was observed in 14 h (Table 5.2, entry 7, Figure 5.10). Addition of TEMPO, a free radical trap, during the initial or second periods did not change appreciably the maximum rate (Section 5.2.2.2, Table 5.2, entries 2,3). Thus, a contribution from free radical pathways appears unlikely, and the mechanism more plausibly consists of the conventional organometallic pathways involving "two-electron" processes. The use of DMA as solvent lowered the rate of reaction by a factor of about 2-3, and also increased the inhibition time. The solubility of H j in DMA is -3 -1 somewhat lower than in toluene ( - 1.76 x 10 M atm versus CO © 30 25 20 *0 v •fi o •8 15 10-5 -O o • o •j 1° o o •o • j o .* o •j • • D 0' D 0 • XL T -6 Standard in situ catalyst (Table 5.2, entry 1). RhH(dpp)2/2.0 equivalents CO injected (Table 5.2, entry 6). DMA as solvent (Table 5.2, entry 4). RhH(dpp)2 alone as catalyst (Table 5.2, entry 5). Addition of 3 equivalents of dpp to standard in situ catalyst (Table 5.2, entry 7). T 8 10 12 14 Time, h 16 Figure 5.10. Hydrogen uptake curves for various conditions of hydrogenation employing in situ RhHCdpp^/CO/Hj as catalyst precursor. Curve 3 shows the hydrogenation using RhHCdpp^ alone as catalyst (only -25% reduction to hexane observed in 5 h). 235 — 3 — 1 o 5 3.29 x 10 M atm at 30° C). However, when using DMA as solvent at [Rh] = 1.17 mM, [Hj] = 1.76 mM, and [1-hexene] = 0.0319 M, the oxidative addition of H j was not the rate determining step because the rate of hydrogenation is independent of [Hj] under the reaction condition (see Figure 5.7 (a)). Therefore, the differences in the solubility of H j in various solvents do not contribute to the observed changes in the rate of hydrogenation in employing different solvents. Consequentiy, the use of the polar solvent instead of toluene has some effect on the rate of homogeneous hydrogenation of 1-hexene catalyzed by the in situ Rh system. 5.4. ANALYSIS OF KINETIC RESULTS, AND DISCUSSION The in situ RhH(dpp)j/CO/Hj system produces the same types and distribution of species as the Rhj(CO) 4(dpp)j/2dpp/Hj system, or that formed by the injection of 2 equivalents of CO into a RhH(dpp)j solution under Ar (Sections 4.3, 4.5.1, and 4.5.2), as shown in equation 5.1. Since Rhj(CO) 4(dpp)j and RhH(dpp)j were found to be inefficient catalysts (Sections 5.2.1 and 5.2.2) compared to the RhH(dpp)j/CO/Hj system, the observed hydrogenation catalysis with the in situ system must be attributed to active species generated from the two compounds, RhH(CO)(dpp*)(dpp) and Rh 2Hj(CO)j(dpp) 3. 5.4.1. Initial period The Hj-uptake does not give a smooth S-shaped curve. There is a very sharp increase in rate near the end of the induction period (Figure 5.1). The initial period is tentatively explained as arising from the generation of an active Rh catalytic species. This is manifested in a color change during the transition from the initial period to the second period (Section 5.2.2.2). The NMR experiments, however, show 236 the same distribution of species 42~i2 in the initial and second periods. This may be caused by a transformation of the active catalytic species back into the RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3 complexes when the sample was sealed under vacuum (Section 5.2.2.2), i.e., in the absence of Hj. The fact that there is no induction period observed after the injection of 1-hexene into the reaction mixture in the final period suggests that the active species has been fully formed in the maximum rate period. The exact mechanism for the generation of the active species in the induction period is not known. Induction periods and S-shaped H j - uptake curves were also observed by the Basset group^ during the asymmetric hydrogenation of the methylester of Z-a-acetamido cinnamic acid using Rh4(CO)-^Q[(-)diop]3 as catalyst, in the presence of various equivalents of (-)diop, at 70° C in EtOH. The system was suggested to involve the cluster framework breakdown of Rh 4(CO)^Q[(-)diop] 3 to give RhH(CO)2(diop), Rh 2H 2(CO) 2(diop) 3 > and RhH(CO)(diop )(diop) complexes as catalysts. These species generated are very similar to those observed in the present system. However, no explanation was given for the observation of the induction period in their work. 5.4.2. Maximum Rate Period When the second period of hydrogenation is reached, a mechanism can be proposed to account for the observed dependence of maximum rate of hydrogen-uptake on [Rh], [1-hexene], and [ H j (Figure 5.11). Isomerization of 1-hexene also occurred during the hydrogenation process to give a mixture of cis- and trans- 2-hexenes. The active species generated in the induction period is plausibly a rhodium hydrido species because isomerization of 1-alkenes by monohydride rhodium catalysts is well documented (see Section 1.3.2). Assuming that the active species responsible for 237 RhH" +' 1 -hexene k 3 "Rhalkyl" (5-3) " "-3 ' v / "Rhalkyl" + H 2 • RhH" + hexane (5-4) Figure 5.11. Proposed reaction pathways for the homogeneous hydrogenation of 1-hexene using the RhH(dpp)2/CO/H2 system; the active catalytic species is labelled as "RhFf. 238 hydrogenation in the second period is "RhH" and the rate determining step is the reaction of "RhH" with 1-hexene, the following mathematical treatment is presented (according to the reaction scheme proposed in Figure 5.11). Rate = k 3[l-hexene]["RhH"]-k_ 3[Rhalkyl] (5.5) d[Rhalkyl]/dt = k 3["RhH"][l-hexene]-k_ 3[Rhalkyl] -k^RhalkyfJtHj] = 0 (steady state assumption) (5.6) k,[l-hexene]["RhH"] [Rhalkyl] = J _______ (5.7) k_3 + k 4[H 2] k„k J H j [ 1- hexene] [ "RhH"] Rate = 4 3 2 (5.8) k.3 + k 4 [ H 2 ] Let ["RhH"] = total concentration of the active "RhH" species ["RhH']t = ["RhH"] + k3[l-hexene]["RhH"] k_3 + k 4[H 2] ["RhH1(k - + k,[Hj) ["RhH"] 1 ~ 3 4 ? k ^ + k^Hj] +k 3[l-hexene] (5.9) Substituting equation 5.9 into equation 5.8 gives: k,k,[HJ [1-hexene] ["RhH'] tRate = 4 3 2 t (5.10) k_3 + k4[H2] +k 3[l-hexene] A plot of 1/rate versus l/[l-hexene] gives a straight line (Figure 5.12) with an intercept at 1.25 (±0.77) x 10 4 M _ 1 s, which is l / ^ t H ^ p R l i H " ] ). Similarly, a plot of 1/rate versus 1/[H2] also yields a straight line from [H 2] = 0.653 mM to 4 -1 3.10 mM (Figure 5.13), with an intercept at 5.79 (±0.70) x 10 M s, corresponding to l/(k3["RhH']T 1-hexene]). At high [1-hexene] where k3[l-hexene] > > k_ 3 + k 4[H 2], the rate law is simplified to: Rate = k4[H2]["RhH']t (5.11) The observed first order rate dependence on [H 2] (see Figure 5.7 (b)), the 239 Figure 5.12. A plot of 1/rate versus l/[ 1-hexene] (for the maximum rates of hydrogenation at [Rh] = 1.17 mM, [H2] = 3.10 mM). 240 Figure 5.13. A plot of 1/rate versus l / t H j (for the maximum rates of hydrogenation at [Rh] = 1.17 mM, [1-hexene] = 0.0319 M). . 241 independence of rate on [1-hexene] (Figure 5.9), and first order rate dependence on total rhodium concentration (see Figure 5.8 (c)) support the derived rate law (equation 5.11). This indicates that ["RhFF]t is nearly equal to the total rhodium concentration in solution, i.e., the starting catalyst precursor, RhH(dpp)2, has been fully transformed into the active "RhH" compound in the second period. A plot of rate versus total rhodium -2 -1 concentration (see Figure 5.8 (c)) yields a slope of 6.08 (±0.44) x 10 s , -2 -1 corresponding to k ^ H j . Consequently, k 4 is calculated as 6.08 x 10 s /3.10 x -3 -1 -1 10 M = 20 M s . Also, the value of k^ can be obtained from the intercept of the plot of 1/rate versus l/[ 1-hexene], where l/(k 4[H 2]["RhH'] t) = 1.25 x 10 4 M - 1 s, as 22 M - 1 s _ 1. Thus the average value of k 4 is 21 (±1) M - 1 s _ 1. The value of k^ is found to be 0.42 M'1 s~* from the plot of 1/rate versus 1/[H2]. The use of DMA as the solvent in place of toluene is more likely to inactivate the "RhH" species by co-ordination. Subsequent replacement of DMA by 1-hexene for the substrate activation would be difficult and this could perhaps contribute to a lowering of rate ( ~2.6 times) during the second period. A large reduction in maximum rate when a three-fold excess of dpp was added to the RhH(dpp) 2/CO/H 2 system shows that dpp competes with 1-hexene and/or H 2 for reaction with the active "RhH" species. The maximum rates of hydrogenation of 1-hexene for the RhH(dpp) 2/CO/H 2 system are within the range observed for several Rh catalysts and the well known RuHCKPPh^)^ system (Table 5.4). The maximum turnover frequency for the present system was 440 h" 1 at [Rh] = 0.273 mM, [ H j = 3.10 mM, [1-hexene] = 0.857 M, and 31° C. One of the most efficient systems reported for homogeneous hydrogenation of alkenes is the use of in situ Rh 2Cl 2(CgH^ 4) 4 (or RhCl 2(C 4H 7)) with the 2-aminopyridine ligand, CrH,r>LR , where R is n-propyl, n-butyl, n-amyl or 242 Table 5.4. Maximum rate of hydrogenation for some common catalytic systems. a Ref [Catalyst], mM [Substrate], M Maximum rate, Temp, M s - 1 °C (Turnover rate, h ^) 1. RuHCl(PPh 3) 3, D 2 2. RhCl(PPh 3) 3, c 1.25 3. RhH(CO)(PPh 3) 3, c 1.25 4. RhH(diop) 2, d 1 5. RhH(dpe)2,e 4 6. RhH(dpp)2,f 1.17 7. RhH(dpp) 2/CO/H 2, f 0.27 acrylamide, 0.1 2.60 X 10~5(47) 35 4 1-hexene, 0.6 2.71 X 10~4(781) 25 5 1-hexene, 0.6 1.55 X 10~4(446) 25 2 MSA, 0.1 1.5 X 10"4(540) 606 MSA, 0.1 7.9 X 10"6(7.1) 60 6 1-hexene, 0.0319 8.1 X 10"7(2.5) 3 1 t w 1-hexene, 0.857 3.31 X 10"5(440) 3 1 t w a. P u = 760 torr - P , , b. In DMA. c. In benzene. H 2 solvent vapour d. In n-butanol. e. In n-butanol/toluene = 2/l, v/v. f. In toluene. MSA = 2-Methylene succinic acid, tw = this work. 243 7 cyclohexyl; C g H 1 4 is cyclo-octene, and C 4 H 7 is 2-methylallyl. The turnover rate is high as 5000 h - 1 (mol Hj/mol Rh) at 30° C, with [RhJ = 0.09 mM, [cyclohexene] = 0.45 M in EtOH ([RhJ : [C,H,N,R] = 1 : 4, P u = 760 torr -I J 3 l x i j PEtOH vapour^ T h e m e c n a m s m 's thought to involve several species such as mono-, di-, and tri-nuclear Rh species. CHAPTER 6. GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK The original aim of this work was to study homogeneous hydroformylation of alkenes, using a series of Rh 2(CO) 4(P— P)j type complexes (where P—P = dpe, dpp, diop) as catalysts.1 Other ditertiary phosphines to be investigated included dcpe, and p = p. The synthesis of Rh 2(CO) 4(P— P) 2 followed the routes reported by James et al. 1 The synthesis involved the substitution reaction of [RhCl(COE) 2] 2 with 4 equivalents of P—P to yield the Rh(P—P) 2 + C1~ complex, which was then reacted with NaBH 4 to give RhH(P-P) 2- Bubbling CO into a benzene solution of RhH(P-P) 2 produced Rh 2(CO) 4(P—P) 2 > The dpe, dpp, and diop dimers were reported on previously.1 With use of the same route of preparation described above, the Rh(dcpe)2 + X (where X = C l ~ , BF 4~, PF^~) complexes were synthesized and characterized. A single crystal X-ray structure determination of Rh(dcpe)2 + C l ~ showed that the complex was distorted from square planar geometry. Compounds containing only one ditertiary phosphine per rhodium, such as RhCl(dcpe)* THF, RhCl(dcpe)* O.lCgHg, and RhCl(dcpe)(CH 2Cl 2)-C f iH 6 were isolated by a substitution reaction of [RhCl(COE) 2] 2 with 2 equivalents of dcpe in the various solvents. The yield was generally low (< 30 % ) . Preliminary studies of the reactions of these extremely air-sensitive [RhCl(dcpe)- solvate] compounds with CO showed that the reaction was extremely facile and took place even in the solid state. Further investigation into the activation of small gas molecules by these co-ordinatively unsaturated complexes is recommended. 244 245 The attempts to synthesize the hydride RhH(dcpe)2 from Rh(dcpe)2 + C l ~ by reaction with NaBH^ or LiAlH^ failed. Consequently, the routes leading to the synthesis of R f ^ C O ^ P — P)j failed for P—P = dcpe. A digression was made to study the interaction of Rh(dcpe)2 + Cl , _ , with small gas molecules, in an attempt to test its potential as a catalyst. There was no reaction observed between Rh(dcpe)2 + Cl and Hj, O2, or Nj, but the complex reacted with HCI in C H j C ^ or benzene initially to give the cis-RhHCl(dcpe)2 + Cl adduct A stopped-flow kinetic study was employed to give a k of 42.1 M 1 s 1 at 18.3° C. The k F F value, obtained by treating the on off 0 isolated and characterized RhHCl(dcpe) 2 + C f • HCI • OJCgHg with DMA, which rapidly removed the free HCI released from the adduct, was 1.0 x 10~^ s - 1 ; the equilibrium 7 -1 constant for the HCI addition to _ is thus 4.2 x 10 M . The mechanism of reaction between 21 and HCI is proposed to involve a concerted oxidative addition. The cis-RhHCl(dcpe) 2 + adduct reacted further with HCI ( -1 atm) in CH 2C1 2 to 1 31 1 give a RhHCl 2(dcpe) complex, characterized by H and P{ H} NMR. In the presence of CDCl^, cis-RhHCl(dcpe) 2 + c r was converted into R h C l j C d c p e ^ C f slowly, the latter compound being prepared also by bubbling C^ into a C H j C ^ solution of Rh(dcpe)2 + Cl~. The interaction of 12 with CO was evidenced by FT-IR data in Nujol and in various solvents such as DMA, CHjClj, and CH^CN, v(CO) being in the region for a terminal CO stretch. The compound is tentatively assigned to be Rh(CO)(dcpe)2 + C f by comparison with the analogous Rh(CO)(P-P) 2 + complexes (where P—P = dpm, dpp). The equilibrium constant for CO binding was - 4 4 M - 1 (by UV-VIS spectrophotometric techniques) at 25° C in C ^ C l j . The mechanism for the interaction of CO with Rh(dcpe)2 + Cl is proposed to involve an end-on interaction, with very little steric changes on going from the reactant to the product: surprisingly, no ^P{*H] NMR signals pertinent to Rh(CO)(dcpe)j + could be 246 observed, presumably because of the similarity in and J j ^ - p f°r Rh(dcpe)2 + Cl and Rh(CO)(dcpe)2 + C f . The k Q n (2.2 x 10~ 2 M - 1 s"1) and k ff (5.02 x 10" 4 s - 1) values at 25° C were relatively small, indicating that the doublet observed in the 31 1 P{ H] NMR is not an averaged signal resulting from the exchange between Rh(dcpe)2 + C f and Rh(CO)(dcpe)2 + C f . The Rh(dcpe)2 + Cl complex did not catalyze decarbonylation of benzaldehyde or hydrogenation of 1-hexene under mild conditions, possibly because of the difficulty of activating the substrate and/or gas molecule. The Rh(p = p) 2 + Cl complex was isolated via the substitution reaction of [RhCl(COE) 2] 2 with p=p. Reaction of Rh(p = p) 2 + C f with NaBH^ gave partial conversion to RhH(p = p) 2. Treatment of the product mixture with CO gave Rh(CO)(p = p) 2 + Cl and another unknown carbonyl compound. Attempts to synthesize the p=p dimer of the type Rh 2(CO) 4(P—P) 2 were unsuccessful. Syntheses of the dpp, diop, and dpe dimers were available according to the literature report.1 Because diop is an expensive ligand, and the dpe dimer is extremely air-sensitive in the solid state, the dpp dimer was chosen for examination for reactivity toward H 2 > in order to understand better the mechanism for subsequent hydrogenation or hydroformylation processes. The interaction of Rh 2(CO) 4(dpp) 2 with H 2 in CH 2C1 2 is summarized in the reaction scheme shown in Figure 4.28 (Chapter 4). In the presence of H 2, Rh 2(CO) 4(dpp) 2 was cleaved by excess dpp ligand to give a mixture of species, the distribution of which depended on the amount of dpp added. The formation of bridged dimer, Rh 2H 2(CO) 2(dpp) 3 > which was subsequently transformed into RhH(CO)(dpp*)(dpp) species, was observed at low [dpp]. The J H and 3 1P{ 1H} NMR spectra of these two species were obtained at 300 and 121.4 MHz, respectively. Use of 247 a low frequency 3 1 P probe (32.3 MHz) gave sceond order spectra for the species RhH(CO)(dpp*)(dpp) and Rh 2H 2(CO) 2(dpp) 3. These hydrides could also be obtained by a phosphine exchange reaction of RhH(CO)(PPh 3) 3 with 3 to 5 equivalents of dpp, while the species RhH(CO)(dpp*)(dpp) could be stabilized by small amount of CO ( ~ 0.2 equivalents) injected into the RhH(CO)(PPh 3) 3/dpp system. Reaction of RhH(dpp)j with 2 equivalents of CO also generated both RhH(CO)(dpp*)(dpp) and RhjHjtCO^dpp)^. The breakdown of Rh 2(CO) 4(dpp) 2 is thought to proceed by the formation of undetected "RhH(CO)(dpp)(solv)':. Species RhH(CO)(dpp*)(dpp) was stable toward loss of CO to give RhH(dpp)0, or loss of dpp to give "RhH(CO)(dpp)(solv)". The following reactions took place in an in situ RhH(dpp) 2/CO/H 2 system, which also generated the RhH(CO)(dpp*)(dpp) and Rh 7H 2(CO) 2(dpp) 3 species: 4 CO, - H ? H, 2 RhH(dpp) 2 ^ fc> Rh,(CO) 4(dpp) 2 + 2 dpp + >\/l dpp + 2 CO 52 .42 1/2 Rh 2H 2(CO) 2(dpp) 3 + RhH(CO)(dpp*)(dpp) 51 5D Because 42 and 52 were found to be ineffective or inefficient for homogeneous hydrogenation of 1-hexene at 31°C, under a total pressure of 760 torr H 2 > an observed catalytic hydrogenation of 1-hexene by the RhH(dpp) 2/CO/H 2 system was ascribed to 51) and/or 51- The H 2 uptake curve showed three regions, an initial ("induction") period, a second period of a maximum rate, and a final slowing down period. The initial period is interpreted as the time required to generate an active species from the catalyst precursors 50 and 51- The active, unidentified catalyst is written as "RhH". The addition of a large excess of 1-hexene facilitates the formation of "RhH". At low [1-hexene], reaction of "RhH" with added dpp, and perhaps other 248 rhodium species (at higher [Rh]) in solution causes an increase in inhibition time and a decrease in the maximum hydrogenation Tate. The reaction mechanism is outlined below. RhH" + 1-hexene „ 3 r Rhalkyl k-5 Rhalkyl" + H. k 4 RhH + hexane The corresponding rate law for the maximum rate, consistent with the kinetic data, is given by: Rate = k ^ ^ H j ] [1-hexene] ["RhH"] tk_3 + k 4[Hj] +kj[ 1-hexene] where ["RhH']t is the total concentration of the active "RhH" species. At high [1-hexene], where k^fl-hexene] > > k ^ + k J H j ] , the rate law is simplified to: Rate = k^H^pRhH"] The values of k^ and k 4 at 31° C were 0.42 M~* s-'' and 20 M~* s - 1 , respectively. The reaction of RhH(dpp) 2 to give co-ordinatively saturated Rh 2(CO) 4(dpp) 2 occurs under synthesis gas ( H 2 : CO = 1 : 1). Thus, the use of lower CO partial pressures in H-/CO mixtures will be required to effect any possible hydroformylation. 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Williams, Organometallics 2, 1452 (1983). 9 f) APPENDIX Appends 1 1 (•) Kinetics of CO addition to Rh(drpe)2 + C f Rh(dcpe ) 2 + Cf * CO j 1 » Rn(COXdcpe) 2 +a' 1=0 [Rh] 0 1CO] 0 l=t IRh) t (CO] Q tRWo-IRW, - d l R I ^ / d l = kj |Rh) t [00] - t.jawflj-lRH),) U I k" = k,[CO] (7.2) -dtRh^/dl = [RMjOT + k_j) - k.jIRhJp = fk" + k . ^ l R h ] , - [k.,/(k- + k . j M R l i t y f U ) At equilibrium : -d[Rh] /dt = 0 en SuFsftituring equation 7.4 into equation 7.3 lives : -dtRH^/d! » V * k . jKIRh] , - ( R h y -dtRJO^flRh], - [Rh]^) = (k- + k . , ) dl (7.5) (7.6) Integrating equation 7.5 gives : miaRi]0 - [ R W ^ V O R h l , - IRil^D - (k' + k - i > 1 " kobs * .*. k » V + k and thus 001 "I k ob, - k l t C 0 ) + k - l Assuming that the abscrtMce or RWCOXdc j*> 2 + Cf it 403 6 mn is negligible omrpartd to that or Rn(dcpe) 2 *Cf (analogous to the case ot Rh(dpp) 2 +CI ). Ibra (Rh] 0 • A„. [Rhlj - A,, and I R h ] ^ - A ^ al 403.6 nm. Equation 7.6 can be rewritten as : InUA,, - A e q ) / ( A , - A ^ B = k ^ . O.V Tabulated tpcoral changes for the data presented in Table 3.1. (a) CirbonyUtion or Rh( depend" (j) {Rh] = 4.56 x 10"4 M . (C Tempera cure = 25° C, CHJQJ Time, h A403.6- m D taflA,, - V'<A. " V 0.000 1307 0.000 0.0628 1246 0.397 0.176 2.233 0.507 0.362 2.205 0.795 0J24 2194 0.935 0.634 2.173 1.28 0.840 2.158 1.62 0.996 2.155 1.70 1.12 2.140 2.28 1.28 2.132 183 A eq 2.121 -atope = 5.40 (±0.37) I l O " " a" 1 (ii) [Rh] = 4.60 i 10 ' * M . |CO) = 2.34 > 10" 3 M Temperature = 25° C. C H 2 a 2 . in dark. Time, h A403.6' m lnllA,, - v<\ - V 0.000 2.310 0.000 0.0311 2300 0.0617 0.0S31 2.284 0.169 0.192 2.257 0.382 0.320 2.227 0.667 0.507 1201 1.06 0.600 2.193 1.21 0.799 2.177 1.59 0.946 1169 1.86 1.18 1157 2.48 A eq 2143 -- J O stopt = 5.67 (±0.09) I 10" 4 s" (b) Kinetics or decajtooylariOD of in dm formed RlKCOXdcpe). Rh(COXilcpe),+a" in* W ( l c r x ) , + C f + CO , = 0 <0 "0 1 = 1 c . "< + "0 = <0 " c . + "0 •=- - •<, + c„ = a. -dc,/di = I . , c, *. = c0 " c , • • - « T ) " V/<h = k - i «o - V -«I«0 - •,>'"<> * = k - l d l  l * c 0 ' 'i ' k - l * * c ' - » : \ = "0 ••• talc,, - 1,1 = i _ , 1 + talc,, - aj V V + «0 V - •o + £ V "o + t * o ' •o •'• % ~ *l' A - - A l Subsuntting equation 7.10 into equation 7.9 gives : tajA^ - AJJ - k_j t + coostast (b) dearbonylauoo or in_s_ formed RblCOXdcpe)^4 C f from (i) [Rn] = 4.56 x 10"4 M. (CO] = 2.34 x 10"3 M Tanperauire = 25° C C H - Q , . Time, b A ( 1 nm ln(A_ - A () 0.000 1128 -1.89 0.0239 2-138 -1.96 0.138 2.150 -2.05 0.27! 2.188 -2.40 0.373 1197 -2.50 0.519 1209 -2.66 0.686 1234 -3.10 0.871 1248 -3.47 1.165 1264 -4.20 A_ 1279 -slope = 5.15 (10.85) x 10"4 * (ii) (Rh) = 4.60 I 10~4 M. ICO] = 134 x 10"3 M Temperature = 25° C CHJQJ- in dark. Tune, h mn - A,) 0.000 1143 -1.74 0.100 1172 -1.92 0.175 2.186 -2.04 0.270 1210 -2.23 0.405 2-235 -2 49 0.560 1255 -2.76 0.725 1271 -3.06 0.875 1282 -3.32 1.385 1304 -4.27 \ . 1318 • slope = 4.89 (±0.69) i 10~4 s' (c) Use or FT - IR to follow the kinetics of carbonyialion or RlKdcpeJj O . Assuming that me absorbance at 1993 cm~^. due to the formation of M ( O O X d c p e ) 2 + . obeys Beer's L a * , a plot of I n l A ^ / f A ^ - A,)] versus runt gives 'obs » R M r f c p e l ^ C l " -t C O j ' *• Rh(COXdcpe) 2 + a" (7.111 ,=0 v t=t a, c, 'eq *0 " 'eq drydt = k , a t c O ] - k ^ c , Let i - = kj ICO]. a, = a„ - c, dc,/dt = k ^ - (k- + k_j)c t At equilibrium. " * « , ' * = ° * c eq " k > 0 ' ( k ' * k - l > dc,/dt = - cr + k . , ) [c t - k^ / tk - + k_,)] « -or + k . , x c , - c ^ i IrHc^ - c,) = (V + k_,X + c At tune=0. c t = 0 • ' • m ' c eq / ( c e< l " c t » = ( k " + k - l " • W The A ^ and A ( uunesuuud to the absorbances at 1993 c m - 1 of R h ( C O X d c p e ) j + . at time - • , and L * e q " c e q : A t " S ta'Aeq/<Ae, " N1 = k o b s l w l " k obs = * * k - l = k l ! C 0 ] + k - l (c) Cirbonylaiion or Rh(dcpe) 2 + C f : F T - I R ipecoiJ cfaanfes {Rh] = U l m M . [CO] = 3.86 mM. TempcraQirc = 18° C . C H j Q j . A,)] 0.000 0.000 0.000 0.480 0.0095 0.68S 0.962 0.0145 1.42 1.19 0.0158 1.76 1.37 0.0163 1.92 1.95 0.0181 2.95 A _ 0.0191 slope = k ^ = 4.15 ( ± 0 . 1 1 ) s 10~ 4 s " 1 Kxprrimenul : The RrKdcpclj * C\~ complcs (6.5 nig. 6.6 * I0~ f t mol) was dissolved in CHjC I } (5 ml.) in a small round-bottom flask ( - 15 ml.) capped with a Suba-seal septum placed in a thcrmoslalled ail bath. The solution was degassed three times by freere-lhaw cycles, then -I atm C O was introduced. The reaction mixture (250 ul . ) was syringed out at regular time interval under C O and injected into an anaerobic 1-T-lK cell, previously flushed with C O . The l-T-IR spectrum was taken. —J r-J 2 (a) Kinetics or HO addition to Rh(dcpe)j+a" u 18.3° C in CHjClj Tabulated spectral changes for data presented in Table 3.6. (1) Table 3.6. entry 1. [HO] = 3.59 I 10"2 M. (Rb) = 4.97 i 10"5 M Time, s InKA^ - V" A t - v 0.000 0.306 0.800 1.29 1.78 2.27 0.000 0.357 1.12 1.82 2.44 3.12 Hope = = 1.39 (±0.02) s"1 (2) Table 3.6. entry 2. [HO] = 4.50 i 10"3 M. [Rh] = 4.97 l 10'5 M Time. 1 InKAj, " Aed>'<Al - V 0.000 0.109 0.244 0380 0.570 0.896 1.25 0.000 0.168 0.409 0.675 1.11 1.70 241 atope = k ^ = 1.95 (±0.03) s"1 (3) Table 3.6. entry 3. [HCTJ = 5J2 a lO" 2 M. [Rh] = 4.97 I 10"5 M time, i - V 0.000 0.162 0J24 0.486 0.648 0.810 0.972 1.13 O.OOO 0.236 0.379 0.930 1.34 1.69 2.03 2.39 slope = k = 121 (±0.10) s' (4) Table 3.6. entry 4. [HCT] - 6.10 a 10"J M. [Rh] > 4.97 s lo" 5 M 0.000 0.000 0.0590 0.0810 0.119 0.247 0.249 0.643 0.439 U3 0.629 1.70 0.819 137 1.01 2.77 0.000 0.000 0.0625 0.109 0.163 0.539 0.263 0.875 0.363 1.20 0.463 1.57 0.563 1.79 0.663 1.93 slope = k . = 3.11 (±0.15) s '"ICS) " V / ( A , " "eg" 0.000 0.000 0.100 0.363 0.200 0.651 0JO0 0.998 0.400 1.43 0.500 1.75 0.600 122 slope = k = 3.65 (±0.10) a —) (7) Table 3.6. entry 7. [Hd] = 0.180 M . [Rh] •= 4.97 I I f f 5 M 0.000 0.000 00200 0.147 0.0650 0.502 0.100 0.756 0.120 0.920 0.200 1.54 Hope •= k o b J = 7.70 (±0.04) s" 1 (8) Table 3.6. entry 8. I H O l = 0.180 M , [Rh] e 2-67 X 10" 4 M Time. • ID[(AQ - V < A , " W 0.000 0.000 0.0250 D.134 00600 0.446 0.140 1.07 0.220 1.70 llope = 'obs = 7.84 ( ± a i 4 ) i " 1 (9) table 3.6. entry 9. [HCTJ = 0.180 M . [Rh] = 5.34 i 10" 5 M Time, s lnKAj, - V " A t - v 0.000 0.000 0.0200 0.0895 0.0600 0.377 0.140 1.00 0.220 1.60 slope = k . = 7.41 (±0.17) s' (10) Table 3.6. entn 10. [HCI] = 0.180 M . [Rh] = 2.67 x 10" 5 M Time, s lnI(A 0 - A ^ M A , - A ^ ) ) 0.000 0.000 00450 0.201 0.110 0.790 0.190 1.47 O270 1.99 slope = k 0 | R = 7.69 (±0.03) s"1 2 (b) Kinetics of removal of H a from R h H a ( d c p e ) 2 + . fadhltted by the D M A (Section 3.6.2). - k - 2 RhHCKdcpe), < ^ - M U i ( d c p e ) , + H Q k 3 fast D M A D M A - H Q Rate = -d[RhHCfJ/dt = l . j I R h H O ] - kjIRhHHCl] Applying steady state assumption to [HCI]. -d[HCl] /d l = 0 Thus. k . j I R h H a ] - kjIRiillHCI] - kj[HCI)[DMA] = 0 [ H d ] = k_ 2 [RhHO]/(k 2 [Rh) + kjIDMA)) Substituting equation 7.14 iato equation 7.13 : -dlRhHCTJ/dt = k_ 2[RhHCI] - lk 2tRh]k_ 2[RhHCf)/(k 2[Rh] + kj[DMA])l -d[RbHCl]/dl = k_ 2 k 3 [RhHCT][DMA]/(k 2 [Rli] + kjlDMA]) ir k 3 (DMA] > > kjIRh]. then equation 7.15 can X simplified to : - d l R i H C O ' d l = k_ 2 IRhHO] A plot of lnfA w - A ( ) versus t yields k ^ = k_ 2' 2 (b) Kinetics of HO iddition to Rh(dcpe)2* C f it 18.3° C in CHjQj. (1) Table 3i. entry 5. [DMA] = 10.8 M. |Rh) = 5.8 s 10~4 M. temperature = 18.3° C Time, b A403 6' n m 1°<A» " V o.ooo 0.005 1.02 0.352 0.037 1.01 0.590 0.057 1.00 1.28 0.117 0.979 2.88 0.276 0.917 4.18 0.40) 0.866 5.77 0.532 0.809 6.18 0.564 0.795 7.77 0.668 0.747 U J 0.848 0.658 12.6 0.899 0.631 150 0.996 0.578 21.7 1.187 0.464 A„ 1778 -11 (±037) i 10"5 r1 3.8. entry A, [DMA] = 7.56 M. |Rh] = 5.8 I 10"4 M. temperature = slope -(2) Table 18-3° C WA„ - A) 0 0.040 1.05 0J00 0.052 1.05 1.18 0.066 1.045 1.55 0.074 1.042 112 0.085 1.039 168 0.098 1.034 3.10 0.101 1.033 3.57 0.106 1.031 4.02 0.114 1.026 17.3 0.303 0.958 A_ 2.91 -slope = IS (±0.02) » 10"6 s" (3) Table 3.8. entry 3. [DMA] = 5.40 M. [Rh] = 5.8 x 10 M. temperature = 18 J ° C. Time, b A403.6' n m hXA„ - A,) 0 0.089 1.044 0.500 0.095 1.042 1.18 0.100 1.040 1.53 0.105 1.039 210 0.111 1.036 168 0.118 1.034 3.53 0.121 1.033 4.00 0.127 1.031 A w 193 -(4) Table 3.8. entry 1 [DMA] = 115 M. [Rh] = 5.8 I 10"4 M. temperature 18.3° C WA„ - A,) 0 0.116 1.045 0.53 0.121 1.044 0.93 0.126 1.042 1.43 0.132 1.040 2.00 0.137 1.038 160 0.143 1.036 3.00 0.146 1.035 4.00 0.145 1.031 N. 2.96 -slope = 1.0 (±0.37) t 10"6 s"1 Table 3.8. entry 1. [DMA) = 0 M. [Rh] = 5.80 x 10"4 M. temperature (5) 16.3° C. 0 0.028 0.500 0.028 1.18 0.028 1.52 0.028 108 0.027 168 0.027 3.52 0.027 4.03 0.027 A_ 198 Appendix D 1. Tabulated uptake foi the dau presented in Table 5.1 (i). A regression line is obtained fat the kinetic dau enclosed in brackets in each or the fotlowing experiments. (i) Table 5.1 (i). entry 1. [Rh] = 0.219 mM. [1-hexene] = 0.0319M. [H^ = 3.10 mM. injection method. Tune, b [Hj] absorbed i 10J. M 0.209 0.436 (0484) (1.89) (0.633) (5.48) (0.787) (8.25) (0.858) (9.59) 0.954 11.2 1.15 13.1 1.32 14.4 1.88 17.1 slope = 5.66 (±0.11) I 10"° M s~' (ii) Title 5.1 (i). entry 2. [Rh] = 0J28 mM. [1-beicne] • 0.0319M. [HJ ~ 3.10 mM. injection metbod. Time, b [HJ absorbed I 10 . M 0.0650 0.0996 0.180 0.219 0.256 0.299 0.331 0.49S 0.384 1.06 0.668 4.82 (0.729) (683) (0.756) (7.89) (0.791) (9.18) (0.852) (10.9) (0.894) (12.0) (0.957) (13.7) (1.06) (16.2) (109) (16.5) 1.35 20.5 1.44 22.1 1.50 22.8 1.55 23.4 1.70 24.4 2.00 26.4 slope = 7.53 (±1.71) > 10"° M s" (iii) Tible 5.1 (i). entry 3. [Rh] = 0J28 mM. [1-heiene] = O0319M. [HJ = 3.10 mM. non-injection method Time, h [HJ «bsorbed I 103. M 0 0 0.0236 0.209 0.398 1.04 0.572 2J9 (0.650) (4.82) (0.697) (5.95) (0.739) (7.32) (0.783) (8.81) (0.866) (10.5) 0.991 13.5 1.03 13.8 1.10 15.1 1.26 17.0 1J9 18.1 1.48 19.1 1.70 20.4 1.93 20.8 slope = 7J2 (±0.44) s 10"6 M s"1 (iv) Tible 5.1 (i). entry 4. [Rh) = 0.727 mM. [1-heiene] = O0319M. [HJ = 3.10 mM. injection method. Time, h [HJ ibsorbed i 103. M 0 0 0.313 0.242 0.391 0.423 0.694 1.26 0.801 1.56 0.870 2.38 0.888 3.03 (0.912) (3.88) (0.927) (4.43) (0.947) (5.17) (0.987) (6.60) (1.02) (7.70) (1.06) (9.37) (110) (10.7) (1.12) (113) 1.19 13.5 1.26 15.6 1.40 19.5 1.50 22.4 1.55 23.9 slope = 10.0 (±0.11) i 10 M s" t o —1 (v) Table 5.1 (i). entry 5. [Rh] = 0.727 mM. (1-hexene) = 0.0319M. [Hj] = 3.10 mM. non-injection method Tune, h [Hj) ibsorhed x 10 . M 0 0 0.223 0478 0.474 0.936 0.556 1.35 0.623 1.67 0.841 1.91 0.893 7.45 0.999 5.90 1.02 6.65 1.04 7.36 O.05) (7.71) (108) (877) (111) (9.82) (113) (10.5) (1.15) (11.2) 1.19 11.6 1.21 12.3 1.31 15.8 1J4 16.8 U 5 17.2 U 7 17.9 slope = 9.68 (±0.02) x 10" 6 M s" <ri) Tible 5.1 (i). entry 6. (Rh) = 1.17 mM, |l-bexene] = 0.0319M. [Hj) = 3.10 mM, injection method. time, h [Hj] ibsorbed > 10 . M 0 0 0.153 0.259 0.766 1.22 1.27 2.12 1.41 2.37 1.66 5.% 1.68 6.29 1.76 9.46 (1.80) (12.3) (1.84) (14.7) (1.89) (16.9) (1.92) (16.1) 2.05 22.5 2.20 25.2 2.43 27.3 slope = 13.3 (±0.83) > 10"* M s" (vii) Table 5.1 (i). entry 7. [Rb] = 1.17 mM. [1-hexene] = 0.0319M. [Hj] = 3.10 mM. non-injection method. Time, b [Hj) absorbed x 103. M 0 0 0.423 1.06 0.717 1J3 0.833 1.37 0.871 1.57 0.982 2.01 1.07 119 1.15 2.33 (165) (7.88) (166) (6.41) (1-67) (8.93) (1.70) (10.5) (173) (111) (1.76) (13.7) 1.79 15.2 1.84 17.9 1.90 21.0 1.93 216 1.99 25.8 2.03 26.6 112 28.2 2.19 29.4 126 30.2 133 30.4 slope = 13.5 (±D72) s 10" 6 M s" 1 (viii) Table 5.1 (I), emry !. [Rh] = 1.71 mM. (l-bexene) = 0.0319M. [Hp = 3.10 mM. injection method. Time, h [Hj] absorbed t 10 . M 0 0 0.751 1.28 0.878 1.75 1.19 2.10 1.39 2.46 2.12 3.72 2.27 5.52 2.30 7.31 2.33 8.71 (2.38) (12.1) (2.41) (14.6) (2.44) (15.61 (153) (20.1) 2.58 21.4 2.61 22.2 2.72 24.9 283 26.9 ~J slope = 14.2 (± U) x 10 0 M s (ii) Tible 5.1 (i). entry 9 (Rh) = 1.71 mM, [1-bexeneJ 1-hexene. = 0.0319M. (Hj) = 3.10 mM. re-injection of Turn, b [HjJ ibsorbed 1 10 . M (0) (0) (0.106) (5.50) (0.16!) (S.37) (0.196) (10.2) (0.330) (11.7) (0.270) (14.1) (0.319) (16.5) (0.425) (22.1) slope = 14.4 (±0.09) i 10" 6 M I* 1 (x) Tible J . l (i). entry 10. (Rh] = U S mM. [1-beicoe] = D.0319M. [HJ = 3.10 mM. mjection method. Ttrae. b [Hj] ibsorbed I 10 . M 0 0 0.112 0.499 0.284 0.747 0.446 1.10 104 2.38 1.82 4.06 104 4.54 129 4.76 151 5.50 (3 56) (8.07) (3.72) (11.0) (3.90) (17.6) (3.94) (18.9) 4.01 20.7 414 23.4 413 24.7 4.42 26.9 4.53 28.3 4.63 29.4 4.94 30.4 512 31.0 slope = 8.15 (±0.87) i H f 6 M i " 1 (ii) T«ble 5.1 (i). entry 11. (Rh] = 3.56 mM. (1-heiene) - 0.0319M. [Hj] = 310 mM. injection method. Time, h (Hj] ibsorbed I 10 . M 0 0 0.520 0.674 1.00 1.19 1.33 1.70 2.16 3.25 2.76 4.22 2.80 4.23 2.96 4.73 3.53 5.44 3.72 6.12 4.56 7.44 5.27 9.51 5.34 12.1 5.37 13.3 (5.39) (14.2) (5.4!) (14.9) (5.44) (15.7) (5.48) (16.5) (5.53) (18.1) 5.63 20.5 5.66 21.3 5.88 25.1 5.97 25.9 slope = 7.67 (±0.04) i 10" 4 M «" ' (xii) Tible 5.1 (i). entry 11 [Rh] - 0.273 mM. [1-hexene] = 0.857M. [Hj] = 3.10 mM, non-injeciion method. Time. h [Hj] ibtorbed x 103, M 0 0 0.0381 157 0.047! 4.22 0.0564 5.32 (0.0672) (6.60) (0.0753) (7.57) (0.0881) (9.10) (0.098)6 (10.4) (0.106) (11.3) (0.113) ( I D ) (0.120) (12.9) 0.141 137 0.184 18.8 slope = 33.3 (±0.17) I I 0 " 6 M s" (liiJ) Table S.1 (i). entry 13. [Rb] = <U73 m M . [)-beiene] - 0.857M. [HJ = 2.69 m M . non-injecrjori method. Time, h (HJ absorbed i 10 3. M 0 0 0.058 3.50 (0.066) (4.S4) (0.093) (8.00) (0.122) (11.0) (0.181) (16.3) (0.299) (27.0) dope (xiv) Table 5.1 (i) entry 14. [Rh] 0J73 m M . [1-bexene] - 0.857M. [HJ - 3.10 mM. non-injection method Tunc, h [HJ absorbed s 10 3. M 0 0 0.067 0.97 0.088 1.60 (0.141) (2.62) (0.201) (3.81) (0.264) (4.93) (0.453) (8.54) (0.672) (12.6) (0.850) (16.0) 0.921 17.2 1J0 22.5 L60 30.0 slope = 5.23 ( ± 0 . 0 2 ) > 10" 6 M s " 1 (iv) Table 5.1 (i). ermy 15. [Rh] = 0.650 m M , [1-hexene] = 0.857M. [HJ = 3.10 m M . rjon-unecu'on method Tune, h [HJ absorbed i 10 3. M 0 0 0.0481 0.378 0.0600 1.69 (0.0925) (7.69) (0.111) (11.1) (0.136) (12.5) (0.183) (21.2) (0.208) (25.8) (0.231) (28.5) 0.277 34.0 0.305 37.3 slope = 42J ( ± 2 . 1 8 ) i K f 6 M s " 1 (xvi) Table 5.1 (i). entry 16. [Rh] = 1.17 m M . [i-hexene] = 0.857M. [HJ • 3.10 mM. rxm-injecrion method. Time, h [HJ absorbed x 10 . M 0 0 0.0256 0.737 0.0497 1.06 0.0808 2.09 0.104 5.14 (0.119) (8.76) (0.126) (10.9) (0.148) (18.1) (0.156) (20.3) (0.194) (307) 0.204 33.3 slope = 81.5 ( ± 2 . 3 ) i 10" 6 M • (xvii) Table 5.1 (i). entry 17. [Rh] => 2J0 m M . [1-hexene] = 0.857M. [Hj] - 3.10 m M . Kn^uijcction method Tune, b [HJ absorbed x 10 3. M 0.0590 0.470 0.0830 1.14 0.102 1.66 0.139 3.20 0.149 5.29 (0.207) (19.1) (0.216) (23-6) (0.225) (28.1) (0.237) (34.1) slope = 139 ( ± 0 . 1 8 ) x 10" 6 M s" 2. Tabulated H 2 uptake Tor tbe dau preaeoted in Table 5.1 (ii). (i) Table 5.1 (ii). entry 1. [Rb] = 1.17 m M . [1-beiene] = 0.0160 M. [ H J = 3.10 mM. injection method Tune, h [ H J absorbed i io'. M 0 0 0.790 1.32 1.18 2.10 1.60 2.89 1.91 3.44 2 14 3.71 (2.55) (6.54) (2.58) (7.50) (2.64) (10.8) (2.71) (13.0) 2.8) 14.1 slope = 1.16 ( ± 0 . 1 0 ) > ) 0 " 5 M a' (ii) Table 5.1 (ii). entry 2, same as Table 5.1 (i). entry 6. (iii) Table 5.1 (ii). entry 3. [Rh] = L17 m M . [l-hexene] = 0.0478 M . [Hj) = J.10 m M . injection method Time, b ( H J absorbed > 10 . M 0 0 0.138 0.198 0.502 1.17 0.749 2.20 (0.876) (4.00) (0.889) (5.30) (0899) (6.28) (0.913) (7.64) (0.928) (9.15) (0951) (11.4) (0.970) (13.3) (0.989) (15.3) (1.03) (18.9) (1.05) (21.3) (1.06) (22.4) 1.09 24.4 1.10 25.0 LIS 29.2 1.18 32.8 1J2 40.3 1.44 43.0 1.61 44.0 tlope = 2.75 ( ± 0 . 0 2 ) i H f 5 M s" (iv) Table 5.1 (ii). entry 4. (Rh) = L17 m M . [1-heiene] = 0.0635 M. [ H J = 3.10 m M . injection method T i n t , b [ H J absorbed i 10 3 . M 0.0875 0.744 0.159 1.03 0.284 1.49 0.420 2.48 0.588 3.25 0.824 4.53 (0.918) (15.8) (0.931) (18.4) (0.943) (20.7) (0.951) (21.8) (0.968) (24.6) (0.981) (26.7) (0.989) (27.9) (102) (30.9) 0.03) (32.3) (1.05) (34.2) (1.09) (37.7) 1.13 40.9 1.19 45.2 1.26 48.8 1.34 52.5 1.51 56.2 slope = 3.53 ( ± 0 . 1 8 ) a 10" 5 M s " 1 (v) Table 5.1 (ii). entry 5. [Rh] = 1.17 mM. [1-beaene] = 0.127 M . [ H J = 3.10 m M . injection method. ( H J absorbed i 10 3 . M 0 0 0.0375 0.630 0.179 2.25 0.293 3.70 0.327 6.20 (0.351) (8.94) (0.384) (16!) (0.409) (21.0) (0.473) (34.2) (0.515) (42.8) (0.539) (47.3) 0.565 51.6 0.596 55.2 0703 61.4 0.840 66.2 0.955 70.3 slope = 5.63 ( ± 0 . 0 7 ) > 10" 5 M s" (vi) Tible S.l (ii). entry 6. [Rh] = 1.11 m M . [1-hexene] = 0.857 M. [ H J = 3.10 mM. non-injection method Time, h [ H J ibsorbed x 10 3. M 0 0 0.0256 0.661 0.0497 0.946 0.0808 1.88 (0.104) (4.61) (0.119) (7.86) (0.126) (9.79) (0.148) 06.3) 0.156 18.2 0.194 27.5 0.204 29.8 •lope = 7.46 (±0.01) x 10" 5 M f 3. Tihtutted H j spake Tor the mtti presented in T i M e 5.1 (iii) (i) T iMe S.1 (iiix entry 1 (Rh] = L17 m M . [1-bexene] « O0J19M. ( H J •= 0.489 m M . irjecoco method Time, h ( H J ibsorbed x 10 3. M 0 0 0.0747 0.117 0.174 0.249 0.728 1.27 0.861 L52 134 146 LSI 3.05 \m 6.43 (8.64) (193) (9 JO) (1.94) (9.98) (197) (10.7) (W2) (Lt2) (2-06) (13.0) (2-10) (142) 122 16.6 124 17.0 128 17.9 133 18.9 139 19.8 147 20.9 157 21.7 176 23.7 181 24.4 Hope = 7.40 (±0.17) i 10" 6 M T (U) T ib le 5.1 (iii). entry 2 [Rh] = L17 m M . (1-hexene] = 0.0319M. [ H j = 0.653 m M . injection method. Time, h [ H J ibiorbed x 10 3 . M 0 0 0.391 0.799 0.930 1.94 1.21 2.43 1.23 2.66 1.46 3.08 1.55 3.57 1.64 4.01 1.71 4.48 1.75 5.53 1.83 7.28 (1.92) (9.34) (1.94) (10.1) (1.97) (10.9) (2.01) (113) 2.05 13.7 107 14.6 2.11 15.8 119 18.4 134 21.5 2.40 23.7 2.45 24.4 llope = 8.96 ( ± 0 . 3 6 ) x 10" 6 M s (iii) Tible 5.1 (iii). entry 3 [Rh] = L17 m M . [1-hexene] = 0.0319M. ( H J = 0.816 m M . injection method Time, h [ H J ibiorbed x 10 . M 0 0 0.478 0.647 0.799 1.05 1.15 1.49 1.40 4.52 1.43 5.83 (1.47) (7.63) (1.49) (7.49) (1.52) (9.75) (1.63) (14.5) (168) (16.4) 1.72 17.5 1.76 18.7 1.85 20.5 1.93 22.0 110 23.9 slope = 12.2 ( ± 0 . 7 2 ) x 10" 6 M s " 1 (iv) Tabic 5.1 (iii). entry 4. (RhJ = 1.17 m M . [1-hexeneJ = 0.0319M. ( H J « 1.02 m M . injection method. Time, h [ H J absorbed I 10 3. M 0 0 0.450 0.898 0.960 1.78 1.06 2.00 1.26 267 1.34 2.93 1.37 3.20 1.51 4.67 1.60 5.93 1.62 7.06 (166) (8.71) (1.69) (10.7) (178) (15.0) (183) (16.7) (1.89) (19.0) 1.94 21.3 1.97 22.3 2.01 23.2 slope = 12.3 ( ± 0 . 6 7 ) i 10 M s' (v) Table 5.1 (iii). entry 5. [RhJ = 1.17 m M . [1-bexene] = 0.0319M. ( H J = U 2 mM. injection method. Time, h ( H J absorbed a 10 3. M 0 0 0184 0.349 0.600 1.12 0.889 1.91 1.04 130 1.57 3.63 1.70 4.00 1.74 4.33 1.78 4J8 1.83 6.09 (1.87) (7.51) (1.90) (9.18) (1.93) (10.5) 0.96) (11.8) (2.02) (14.7) 105 16.0 2.14 19.5 120 21.6 124 216 129 23.9 138 25.2 146 264 151 26.8 slope = 13.1 ( ± 1 2 6 ) x 10" ' M i" (vi) Table 5.1 (iii). entry 6. [RhJ = 1.17 m M . [1-bexene] = 0.O319M. [ H J = 245 m M . injection method. Time, h [ H J absorbed x 10 . M 0 0 0.139 0.253 0.387 0 797 0.533 1.12 0962 212 1.53 3.12 1.70 4.07 (185) (7.41) (1.88) (8.63) (1.92) (10.6) (1.98) (13.5) (102) (15.5) 113 19.7 125 23.2 136 25.3 slope = 13.3 ( ± 0 . 2 1 ) i 10" 6 M s" (vii) Table 5.1 (iii). entry 7. ume as Table 5.1 (i). entry 6. 4. Tabulated H j uptake Tar the dtut praented in Tible 5.2. (OTable 5 J , entry 1, lame ai Table S.l (i). entry 6-(ii) Table 5.2, entry 2. [Rh] e L17 mM. [ 1 - h n n r ] = 0.0319M, ( H j = 3.10 m M . [TEMPO] = 0.0319 M injected in first region of reaction, injection method. Time, b [H£ absorbed i 10 . M 0 0 0J22 0.413 0.813 1.46 1.1)8 1.54 1J15 2.41 1.541 315 1.643 4.65 (1.676) (5.81) (1.695) (6.65) (1.734) (8.23) (1.780) (10.4) (1.858) (13.8) (1.916) (15.5) 1.949 16.6 1.981 17.6 2021 18.7 2.071 20.0 2117 21.1 2146 21.6 2.179 22.1 slope = 114 ( ± 0 . 3 9 ) i 10" M »~ after correction Tor a change in volume (8%) due to the iirjectioa of • toluene solution of T E M P O . (iii) Table 5.2, entry 3. [Rh] = 1.17 m M . [1-hexene] = 0.0319M. [Hj] = 3.10 mM. [TEMPO] = 0.0319 M injected in second region of reaction, injection method. Time, h [ H J absorbed i \0 . M 0 0 0.599 0.0371 0.762 0.286 0.981 0.778 1.254 1.40 1 454 1.76 1.557 113 1.642 2.36 1.909 3.44 1.941 4.50 (1.966) (5.42) (2.003) (6.83) (2.042) (8.49) (2.092) (9.90) (2.113) (11.0) (2)49) (12.0) (2)92) (13.6) 2.215 14.1 2304 15.) 2356 16.1 2.407 18.4 2.492 19.6 2557 20.4 2.611 21.1 2725 22.2 2819 23.0 1878 23.4 3.015 24.2 dope = 11.1 ( ± 0 . 3 3 ) i 10 M t after cxxrrectiorj for a change in volume (81) due lo the injection of a toluene toluhon of T E M P O . O O (tv) Tible 5.2, entry 4. (Rh) = L n mM. (1-beitoe) = 0.0319M. (HJ = 3.10 mM. in DMA. injeciion method. Time, b (HJ ibsorbed i 103. M 0 0 0.O992 0.514 0.294 0.836 0.683 1.77 0.965 2.45 1.49 3.78 1.87 4.74 (2.01) (5.65) (109) (6.76) (116) (8.27) (126) (10.2) (132) (114) 138 12.5 2.47 14.1 154 15.5 169 17.8 2.84 19.9 3.02 2)7 3.14 23.1 3.39 25.0 3.59 26.3 slope = 5.25 (±0.19) I 10"6 M t (v) Tible 5.2, entry 5. [Rh] = L17 mM. [1-beiene] = 00319M. [HJ = 3.10 mM. using only RhH(dpp). •s catalyst. iiv}ection method. Time, n [HJ ibsorbed x 10. M 0 0 1.63 0.219 2.17 1.03 3.56 2.99 (3.83) (3.99) (4.34) (5.58) (5.07) (7.36) (5.74) (9 09) (5.89) (9.831 (6.06) (10.7) slope = 0.79 (±0.04) > 10"6 M s " 1 (vi) Tible 5.2, entry 6. (Rb) = 1.17 mM. [1-besene] = 0.0319M. [HJ = 3.10 mM. - 20 equivalents of CO injected into gas phase above the toluene solution, injeciion method. (HJ absorbed i 0 0 0.346 0.784 0.814 125 1.15 2.80 1.46 3.94 1.63 4.01 (194) (7.30) (2.00) (9.18) (2.03) (104) (111) (13.1) (116) (14.9) (2.21) (16.7) 126 17.8 129 18.8 133 19.6 139 20.9 2.45 21.9 2.50 23.0 157 23.9 2.68 25.3 (vii) Table 51, entry 7. [Rh) = 1.17 mM. [1-hcxene] = 0.0319M. [HJ = 3.10 mM. [dpp] = 331 mM. injection method. thrrf. b [HJ absorbed t 103. M (0) (0) (0.600) (0.274) (2.82) (0.700) (4.15) (0.992) (5.05) (1.08) (14.3) ' (4.68) OO 4^ 

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