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A study of catalytic autoxidation of organic substrates using H2/O2 mixtures in the presence of rhodium… Gamage, Sujatha Nandani 1985

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A STUDY OF CATALYTIC AUTOXIDATION OF ORGANIC SUBSTRATES USING H 2 / 0 2 MIXTURES IN THE PRESENCE OF RHODIUM COMPLEXES CONTAINING DIMETHYLSULFOXIDE LIGANDS BY SUJATHA NANDANI GAMAGE B.Sc, U n i v e r s i t y of S r i Lanka, Peradeniya, 1976 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CHEMISTRY WE ACCEPT THIS THESIS AS CONFORMING TO THE REQUIRED STANDARD THE UNIVERSlty^OF^BRITISH COLUMBIA SEPTEMBER, 1985 © Sujatha Nandani Gamage, 1985 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 CMB M J S T I ? / The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date Oah i q g > ^ DE-6(3/81) i i ABSTRACT Dimethylacetamide (DMA) solvent i s oxidized c a t a l y t i c a l l y to CH3CON(CH3)CH2OOH and CH3CON(CH3)CHO under H 2/0 2 mixtures at 50°C i n the presence of the dimethylsulfoxide complex RhCl3(DMSO)3 (I) at a rate which i s much f a s t e r than p e r o x i d e - i n i t i a t e d autoxidation of DMA under n2 alone. The hydroperoxide i s thought to be the i n i t i a l product, and the N-formyl d e r i v a t i v e i t s decomposition product. An accompanying metal-catalyzed hydrogenolysis of 0 2 leads to H 20 2 and H 20. Hydrogen peroxide and CH3CON(CH3)CH2OOH are the only products formed i n the early stages of the c a t a l y t i c reaction. The maximum rate of gas uptake i n t h i s i n i t i a l region i s independent of the p a r t i a l pressure of 0 2, but shows l i n e a r dependences on Rh and H 2. Stoichiometry, rate and sp e c t r a l data are consistent with an i n i t i a t i o n r e a c t i o n between complex I and H 2, and then 0 2 to give a c a t a l y t i c a l l y a ctive RhU^ ( 0 2 = ) (DMA) species (II) (eq. 1). -DMSO H 2 Rh I I l :(DMSO) + DMA v Rh I I ] :(DMA) > [RhI(DMA)] DMA _ 2 H+ R h I l : E ( 0 2 = ) (DMA) II (1) The autoxidation of DMA and the hydrogenolysis of 0 2 are postulat-ed to occur v i a independent pathways inv o l v i n g II (eqs. 2 and 3). i i i II •CH3CON(CH3)CH2OOH slow H 2, -H 20 2 slow Rh , 0 2, D M A 1 > f a s t Rh1(DMA) II f a s t II (2) (3) In the absence of H 2, II degenerates to c a t a l y t i c a l l y i n a c t i v e species. The r o l e of H 2 i n the DMA autoxidation i s thought to be the regeneration of Rh* species and hence II, from deactivated forms of II. Eventual slow, i r r e v e r s i b l e deactivation of the c a t a l y s t and the probable p a r t i c i p a t i o n of the H 20 2 product i n p e r o x i d e — i n i t i a t e d f r e e - r a d i c a l autoxidations complicate the i n t e r p r e t a t i o n of l a t e r stages of reaction. Diphenylsulfide (DPS) i s c a t a l y t i c a l l y oxidized to the sulfoxide by complex I under H2/G*2 i n DMA at 50°C, but accompanying oxidation of the solvent p e r s i s t s even i n the presence of a 100-fold excess of DPS over Rh. Oxidation of the s u l f i d e i s thought to involve H 20 2 l i b e r a t e d i n the c a t a l y t i c hydrogenolysis of 0 2. Complex I i n CH 2C1 2 or C2H.4C12 reacts with CO to give the dimethylsulfide complex RhCl3(DMS)3 v i a a f a c i l e reduction of DMSO ligands. Dimethylsulfoxide i s reduced also by Rh* species i n CH 2C1 2 i n the presence of two equivalents of acid to y i e l d DMS, Rh*** and H 20. However, Rh*/2H+/DMS0 systems are r e l a t i v e l y stable i n DMA, because of the proton a f f i n i t y of the solvent. i v Complex I reacts also with the strongly basic t e r t i a r y amine NEt3 v i a a redox process i n which the Rh*** i s reduced to Rh* with an accompanying dehydrogenation of the amine (eq. 4). RhCl 3 + 3NEt 3 •> RhCl + 2NEt 3-HCl + CH2=CHNEt2 (4) The r e s u l t i n g ethenamine then reacts with I to give the 7] 1 - y l i d i c complex, RhCl 3(DMSO) 2("CH 2CH= +NEt 2). Data from an e a r l i e r t h e s i s , on a rea c t i o n between complex I and 1,8-bis(dimethylamino)naphthalene (or Proton Sponge), are rein t e r p r e t e d i n terms of a s i m i l a r redox re a c t i o n that gives an N-carbene fragment (eq. 5),which i s s t a b i l i z e d within the R h 1 1 1 complex, RhCl 3(DMSO) 2(=CH-N(Me)-C 1 0H 6NMe 2•HCl). RhCl 3 + 2 P.S. > RhCl + P.S.HCl + :CHN (5) V TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS v LIST OF FIGURES i x LIST OF TABLES x i i ABBREVIATIONS AND SYMBOLS xv ACKNOWLEDGEMENTS x v i i i CHAPTER I Introduction 1.1 Metal - c a t a l y s i s i n the autoxidation of organic substrates 2 1.2 Metal-dioxygen complexes 9 1.3 Metal-catalyzed a c t i v a t i o n and transf e r of 0 2 to organic substrates 13 1.4 C a t a l y t i c autoxidations i n the presence of H 2 co-substrate 19 1.5 Contents of t h i s thesis 24 CHAPTER II Experimental 11.1 General instrumentation 26 11.2 Gas-uptake apparatus and measurement 29 11.2.1 The apparatus 29 11.2.2 A general experiment 32 11.2.3 S o l u b i l i t y of gases 32 11.2.4 Gas-stoichiometries 33 11.2.5 Gas evolution 33 v i II.2.6 Introduction of gas mixtures 34 11.3 Spectral measurements 36 11.4 Materials 38 11.4.1 Solvents 38 11.4.2 Gases 38 11.4.3 Rhodium complexes 39 11.4.3.1 General 39 11.4.3.2 Mer-cis-trichloroaquo-bis(S-dimethylsulfoxide)-rhodium(III), mer-cis-RhCl 3(H 20) (DMSO)2 39 11.4.3.3 Trichloro(diethylethenamine)bis(S-dimethyl-sulfoxide)rhodium(III), RhCl 3(-CH 2CH= +NEt 2)(DMSO) 2 40 11.4.3.4 Mer- c i s - t r i c h l o r o ( d i m e t h y l s u l f i d e ) b i s ( S - d i m e t h y l sulfoxide)rhodium(III), mer-cis-RhCl 3(DMS)(DMS0) 2 41 11.4.3.5 Mer - t r i c h l o r o t r i s ( d i m e t h y l s u l f i d e ) r h o d i u m ( I I I ) , mer-RhCl 3(DMS) 3 42 11.4.3.6 Product from the reaction between RhCl 3(DMSO) 3 and CO i n DMA i n the presence of an added equivalent of H 20 42 11.4.4 Derivatives of dimethylacetamide N-methyl-N-formylacetamide, CH3CON(CH3)CHO N-methylacetamido-methylhydroperoxide, CH3CON(CH3)CH2OOH 43 11.4.5 Other materials 45 11.5 Analysis of oxidation products i n dimethyl-acetamide solvent 46 I I . 5.1 H 20 46 11.5.2 Peroxides 50 11.5.3 HCHO, HC02H and C0 2 51 11.5.4 Other (a) Diphenylsulfide, diphenylsulfoxide and diphenylsulfone 52 (b) N-methyl-N-formylacetamide and N-methyl-acetamido-methylhydroperoxide 53 V X 1 (c) N-methylacetamide, N-methyl-N-methylol-acetamide, dimethylsulfone and other 53 CHAPTER I I I Synthesis and c h a r a c t e r i z a t i o n of DMSO complexes of R h 1 1 1 and t h e i r reactions with H 2 and CO 111.1 Some general properties of metal sulfoxides 55 111.2 Synthesis and c h a r a c t e r i z a t i o n of some Rh I I i :(DMSO) complexes 57 111.2.1 RhCl 3(DMSO) 3 57 111.2.2 Mer-cis-RhCl 3(DMSO) 2(H 20) 60 111.2.3 Mer-cis-RhCl 3(DMS0) 2(DMS) 63 I I I . 3 Reactions of Rh m(DMS0) complexes with H 2 63 111.3.1 RhCl 3(DMS0) 3 i n DMA 64 111.3.2 Reaction of RhCl 3(DMSO) 3 i n DMA with H 2 67 (a) In s i t u c h a r a c t e r i z a t i o n of products 72 (b) Redox decomposition of i n s i t u [Rh*/2H +/ 3C1"/3DMS0] species 81 111.3.3 Hydrogen reactions of other complexes 86 111.4 Reactions of RhCl 3(DMS0) 3 with CO 86 (a) In CH 2C1 2 or C 2H 4C1 2 88 (b) With added H 20 i n DMA 92 111.5 Summary 100 CHAPTER IV Reaction of RhCl 3(DMSO) 3 with t e r t i a r y amines IV.1 Introduction 103 IV.2 Characterization of RhCl 3(DMSO) 2CCH 2CH= +NEt 2) 104 IV.3 Reaction stoichiometry 120 IV.4 Mechanistic aspects 128 IV.5 Other routes to dehydrogenation of amines 133 IV.6 Hydrolysis of the enamine ligand within the RhCl 3(DMS0) 2("CH 2CH= +NEt 2) complex 135 v i i i IV.7 An i n t e r p r e t a t i o n of the data obtained by Morris f o r the reaction between RhCl3(DMSO>3 and 1,8-bis(dimethylamino)naphthalene 143 IV. 8 Concluding remarks 146 CHAPTER V A study of the c a t a l y t i c uptake of H 2 / O 2 mixtures i n the presence of RhCl 3(DMS0) 3 complex i n DMA and the accompanying oxidation of the solvent V. l Introduction 149 V.2 Product analysis 151 V.3 Mechanistic studies. 162 V.3.1 Results 163 V.3.2 Discussion 174 (a) I n i t i a t i o n r eaction 174 (b) C a t a l y s i s 184 (c) The ro l e of H 2 i n the autoxidation of DMA 194 (d) Later stages of the rea c t i o n 197 V. 4 Implications f o r further work 198 (a) Hydrogenolysis of O2 198 (b) Metal-centred transfer of 0 2 to C-H bonds 202 (c) Role of reducing co-cubstrates i n c a t a l y t i c autoxidations 203 CHAPTER VI Other c a t a l y s t systems u t i l i z i n g H 2/0 2 mixtures VI. 1 Introduction 206 VI.2 Results and discussion 206 CHAPTER VII Conclusions and suggestions f o r further work 215 CHAPTER VIII References 218 Appendix I 228 Appendix I I 229 1 X LIST OF FIGURES Figure Page 1.1 (a) Molecular o r b i t a l s of 0 2 (b) Molecular o r b i t a l diagram f o r an M02 complex 11.1 Schematic representation of an anaerobic s p e c t r a l c e l l 27 11.2 Schematic representation of a combination anaerobic s p e c t r a l c e l l and gas uptake f l a s k 28 11.3 Schematic representation of a constant pressure gas uptake apparatus 30 11.4 A s i m p l i f i e d version of a gas uptake apparatus with d e t a i l s of the modified gas i n l e t 35 11.5 Absorbance (A) of DMA i n a 1 cm c e l l against a reference containing the same: A Q, no added H 20; A-^ , 0.057 M; A 2, 0.117 M; A3, 0.170 M; A 4, 0.222 M; A 5, 0.281 M H 20 48 11.6 Plot of A-A 0 at 1930 nm vs. the concentration of added HoO i n DMA i n 1 cm c e l l 49 111.1 400 MHz % nmr spectrum of RhCl 3(DMS0) 3 i n CDCI3 59 111.2 400 MHz multiple solvent suppression *H nmr of RhCl 3(DMS0) 3 i n DMA/toluene-dg (-1:1) 66 111.3 H 2 uptake p l o t f o r 1.0 x 10" 2 M s o l u t i o n of RhCl 3(DMS0) 3 i n DMA at 50°C under 1 atm of H 2 68 111.4 V i s i b l e spectra of i n s i t u Rh* i n DMA obtained from the r e a c t i o n between 3.0xl0" 3 M RhCl3(DMS0) 2 i n DMA and 0.8 equivalents of H 2 at 50°C under various conditions 70,71 111.5 80 MHz *H nmr spectrum of: (a) the product i s o l a t e d from s o l u t i o n B i n F i g . III.4, i n CDCl 3 under Ar; (b) residue from s o l u t i o n E i n F i g . III.4, i n CDCI3 73 III.5.1 FT i r spectra of: (a) 4x10"^ M RhCl 3(DMS0) 3 i n DMA, (b) species D (see F i g . III.4) within a few minutes a f t e r i t s formation 75 X 111.6 0 2 u p t a k e p l o t s f o r [ R h C l ( C O E ) 2 ] 2 dimer i n DMA c o n t a i n i n g 2 e q u i v a l e n t s o f HC1 and 3 e q u i v a l e n t s o f DMSO p e r rhodium a t 50°C 78 111.7 V i s i b l e s p e c t r a o f 4.5x10"^ M s o l u t i o n s o f [ R h C l ( C 0 E ) 2 ] 2 dimer i n DMA c o n t a i n i n g 2 e q u i v a l e n t s o f HC1 and 3 e q u i v a l e n t s o f DMSO p e r rhodium a t r . t . under v a r i o u s c o n d i t i o n s 79 111.8 80 MHz lH nmr sp e c t r u m o f RhCl 3•2DMS0•DMS i n CDC1 3 82 111.9 An u p t a k e p l o t f o r the r e a c t i o n o f 2 . 4 x l 0 " 2 M R h C l 3 ( D M S 0 ) 3 i n C 2 H 4 C 1 2 , w i t h CO (1 atm) a t 30°C 89 111.10 V i s i b l e s p e c t r a l changes f o r a s o l u t i o n o f 2 . 2 5 x l 0 " 2 M R h C l 3 ( D M S 0 ) 3 i n CH 2 C 1 2 , under CO (1 atm) a t ambient c o n d i t i o n s 89 111.11 Uptake p l o t s f o r r e a c t i o n s o f Rh complexes i n DMA w i t h CO (1 atm) 94 111.12 V i s i b l e s p e c t r a l changes under CO (1 atm) a t 30°C f o r a s o l u t i o n o f 2 . 0 x l 0 " 2 M R h C l 3 ( D M S 0 ) 3 i n DMA c o n t a i n i n g added H 20 ( 2 x l 0 " 2 M) 94 111.13 An FT i r s p e c t r u m o f a s o l u t i o n from the r e a c t i o n o f R h C l 3 ( D M S 0 ) 3 i n DMA w i t h CO (1 atm) a t r . t . f o r 24 h 95 I I I . 14 (a) A 80 MHz ^H nmr s p e c t r u m i n CDCI3 and (b) an i r s p e c t r u m i n KBr o f [ ( D M A ) 2 H ] [ R h C l 2 ( C O ) 2 ] 97 IV.1 400 MHz XH nmr sp e c t r u m o f R h C l 3 ( D M S O ) 2 ( " C H 2 C H = + N E t 2 ) i n CDC1 3: (a) -5 min a f t e r d i s s o l u t i o n , (b) 3 h a f t e r (a) 105 IV. 2 400 MHz XH nmr sp e c t r u m o f R h C l 3 ( D M S O ) 2 ( " C H 2 C H = + N E t 2 ) -4 h a f t e r d i s s o l u t i o n i n CDC1 3 106 IV.3 D e t a i l s o f the m u l t i p l e t a t -58.3 o f F i g . IV.2 r e l e v a n t t o -CH= a s s i g n m e n t s 107 IV.4 D e t a i l s o f the 63.5-4.5 r e g i o n o f F i g . IV.2 r e l e v a n t t o -CH 2- a s s i g n m e n t s 107 IV.5 D e t a i l s o f t h e §3.5-4.5 r e g i o n o f F i g . IV.2 r e l e v a n t t o = +NCH 2- a s s i g n m e n t s 109 IV.6 A 1H nmr s p e c t r u m o f R h C l 3 ( D M S O ) 2 ( " C H 2 C H = + N E t 2 ) i n acet o n e - d g 113 IV.7 An i r s p e c t r u m o f R h C l 3 ( D M S 0 ) 2 ( " C H 2 C H = + N E t 2 ) i n KBr 116 x i A stereo-view ORTEP diagram of mer-cis-RhCl3(DMS0) 2 (•CH2CH=+NEt2) An i l l u s t r a t i o n of the bond lengths of the ethenamine liga n d on mer-cis-RhCl3(DMS0) 2-CCH 2CH= +NEt 2) 400 MHz 1H nmr spectra of RhCl 3(DMSO)3: (a) i n CDCI3 under Ar, (b) -5 min a f t e r adding 1.3-1.5 equivalent of NEt 3 under Ar, (c) 24 h a f t e r (b) (a) 80 MHz XH nmr spectrum of RhCl 3(DMS0) 3 + 0.8 NEt 3 i n CDCI3 under Ar, and (b) An i r spectrum of the above s o l u t i o n i n a i r An i r spectrum [Et 2NH 2] [trans-RI1CI4(DMSO) 2] i n KBr A stereo-view ORTEP diagram of [Et 2NH 2][trans-RhCl4(DMS0)2] ( c r y s t a l type A) A stereo-view ORTEP diagram of [Et 2NH 2[trans -RhCl 4(DMS0) 2] ( c r y s t a l type B) 400 MHz XH nmr spectra of RhCl 3(DMS0) 2("CH 2CH= +NEt 2) (a) i n acetone-dg, (b) a f t e r adding one equivalent of DMA-Hcl to (a), (c) 24 h a f t e r (b) 80 MHz -"-H nmr spectrum i n CDCI3 of a re a c t i o n residue from the c a t a l y t i c uptake of H 2/0 2 (—2:1) mixture by RhCl 3(DMS0) 3 i n DMA (5.0xl0" 3 M) a f t e r -4 h of rea c t i o n 80 MHz ltt nmr spectrum of CH3C0N(CH3)CH200H i n CDCI3 The p l o t s of concentrations of (a) t o t a l H 2 + 0 2 uptake, (b) H20, (c) 16 + 17, and (d) H 20 2, as functions of time f o r reaction mixtures containing 5.0x10"3 M RhCl3(DMSO)3 i n DMA under a mixture of H 2/0 2 (500/260 i n tor r ) at 50°C (data i n Table V.3) The rate of t o t a l H 2/0 2 (500/260 i n tor r ) uptake by solutions of RhCl3(DMS0)3 i n DMA at 50°C at various concentrations of Rh The rate of t o t a l H 2/0 2 uptake by 5.0x10"3 M solutions of RhCl3(DMS0)3 i n DMA 50°C at various p a r t i a l pressures (p.p.) of H 2 x i i V.6 P l o t of maximum rate vs. p a r t i a l pressure of H 2 f o r 5.0x10"3 M RhCl 3(DMSO) 3 i n DMA at 50°C under H 2/0 2 at a t o t a l pressure of 760 t o r r 172 V.7 P l o t of maximum rate vs. the concentration of Rh for RhCl 3(DMS0) 3 i n DMA at 50°C under a H 2/0 2 (500/260 i n to r r ) mixture 173 V.8 P l o t of maximum r a t e " 1 vs. [DMSO] for RhCl 3(DMS0) 3 i n DMA at 50°C under a H 2/0 2 (500/260 i n tor r ) mixture 173 V.9 V i s i b l e s p e c t r a l changes for a 3.0xl0" 3M s o l u t i o n of RhCl 3(DMS0) 3 i n DMA at 50°C under a H 2/0 2 (500/260 i n t o r r ) mixture within 0-2000 s of reac t i o n 176 V.10 V i s i b l e s p e c t r a l changes over 4 h f o r a 3.0x10" 3 M s o l u t i o n of RhCl 3(DMS0) 3 i n DMA at 50°C under a H 2/0 2 (500/260 i n torr) mixture 177 V.10.1 FT i r spectra of: (a) 4 x l 0 " 2 M s o l u t i o n of RhCl 3(DMS0) 3 i n DMA; (b) a s o l u t i o n r e s u l t i n g from the H 2/0 2 (500/260 i n torr) r e a c t i o n of 4 x l 0 " 2 M 1 i n DMA at 50°C for -2000 s 180 V . l l H 2 uptake plo t s f o r : (a) 1.0x10"2 M RhCl 3(DMSO) 3 i n DMA; (b) s o l u t i o n (a) a f t e r r e a c t i o n at 50°C under H 2/0 2 (500/260 i n tor r ) f o r -2000 s; (c) s o l u t i o n (a) a f t e r r e a c t i o n at 50°C under H 2/0 2 (500/260 i n to r r ) f o r - 4 h 181 x i i i LIST OF TABLES St r u c t u r a l c l a s s i f i c a t i o n and nomenclature of dioxygen complexes 400 MHz iH nmr data f or RhCl 3(DMS0) 3 i n CDC1 3 Summary of 400 MHz iH nmr data f o r an e q u i l i b r a t e d s o l u t i o n of RhCl 3(DMS0) 2("CH 2CH= +NEt 2) i n CDC1 3 Summary of 1 3 C {*H} nmr data f o r an e q u i l i b r a t e d s o l u t i o n of RhCl 3(DMS0) 2CCH 2CH= +NEt 2) i n CDC13 100 MHz !H nmr data f o r NEt 3, NEt 3-HCl and mixtures of both i n CDC13 100 MHz 1H nmr data f o r the -NCH2CH3 protons of NEt 3, 1-2 h a f t e r adding the amine to RhCl 3(DMS0) 3 under Ar Interpre t a t i o n of 100 MHz *H nmr data obtained by Morris f o r the orange c r y s t a l s i s o l a t e d i n the react i o n between RhCl 3(DMS0) 3 and P.S. i n CH 2C1 2 80 MHz 1H nmr sp e c t r a l data i n CDC1 3 for DMA and some de r i v a t i v e s Summary of a n a l y t i c a l data f o r the re a c t i o n mixtures from the c a t a l y t i c uptake of a H 2/0 2 (2:1) mixture at 1.0 atm i n the presence of 5.0x10"3 M RhCl 3(DMS0) 3 c a t a l y s t i n DMA at 50°C Maximum rate of uptake of a H 2/0 2 (500/260 i n tor r ) mixture at 1.0 atm by RhCl 3(DMS0) 3 i n DMA at 50°C, with or without various additives Maximum rate of H 2/0 2 uptake by 5.0x10"3 M RhCl 3(DMS0) 3 i n DMA at 50°C under various p a r t i a l pressures (p.p.) of 0 2, f o r a given p.p. of H 2 Maximum rate of H 2/0 2 uptake by 5.0xl0" 3 M RhCl 3(DMSO) 3 i n DMA at 50°C under various p a r t i a l pressures of H 2 and a t o t a l pressure of 760 t o r r xiv V.6 Maximum rate of uptake of a H2/02 (500/260 i n torr) mixture by RhCl3(DMS0)3 i n DMA at 50°C at various concentrations of Rh, with or without added DMSO 168 V.7 Maximum rate of H 2/0 2 uptake of a H 2/0 2 (500/260 t o r r ) mixture by 2.5xl0" 3 M RhCl3(DMSO) 3 i n DMA at 50°C f o r various concentrations of DMSO 169 V.8 Rate of oxidation of DMA solvent by 1.0 atm 0 2 at 50°C with or without added RhCl3(DMS0)3 complex (5.0xl0" 3 M) under various conditions 175 VI.1 Maximum rate of gas uptake from an approximately 2:1 mixture of H 2 and 0 2 at 1 atm by 1.0x10"2 M RhCl3(DMSO) 3, with or without thioether substrates, at 50°C i n DMA 207 VI.2 Product analysis a f t e r a c a t a l y t i c uptake of H 2/0 2 (2:1) mixture at 1 atm at 50°C i n the presence of l.OxlO" 2 M RhCl 3(DMS0) 3 or RhCl 3(DES) 3 c a t a l y s t i n DMA with added DPS substrate 208 VI.3 Maximum rate of gas uptake from an approximately 2:1 mixture of H 2 and 0 2 at 1 atm by Rh c a t a l y s t systems (1.0x10" 2 M i n Rh) at 50°C i n various solvents 212 X V ABBREVIATIONS AND SYMBOLS The following l i s t of abbreviations and symbols w i l l be employed i n t h i s t h e s i s . A angstrom(s) A absorbance atm atmosphere; 1 atm — 760 mm Hg COE cyclooctene d day(s); doublet DES d i e t h y l s u l f i d e DESO di e t h y l s u l f o x i d e DMA N.N-dimethylacetamide, CH3C0N(CH 3) 2 DMA-HCl N,N-dimethylacetamidehydrochloride DMA-HBF4 N,N-dimethylacetamidehydrofluoroboride DMF dimethylformamide DMS dimethylsulfide DMSO dimethylsulfoxide DMS02 dimethylsulfone DPS diphenylsulfide DPSO diphenylsulfoxide DPS02 diphenylsulfone g gram(s) h hour(s) Hz hertz, cycles per second i r i n f r a r e d J coupling constant, i n Hz xv i k rate constant K equilibrium constant log logarithm m medium M molar, moles per l i t e r mL m i l l i l i t e r nm nanometers nmr nuclear magnetic resonance Ph phenyl PPI13 triphenylphosphine ppm parts per m i l l i o n P.S. Proton Sponge, 1,8-bis(dimethylamino)-naphthalene P.S. HC1 Proton Sponge hydrochloride py pyridine r c o r r e l a t i o n c o e f f i c i e n t r . t . room temperature s second(s); s i n g l e t ; strong t time; t r i p l e t TMS tetramethylsilane TMSO2 tetramethylene sulfone w weak 5 chemical s h i f t i n ppm downfield from TMS 6 molar e x t i n c t i o n c o e f f i c i e n t , M"^ - cm"-'-X wavelength, nm X m a x wavelength of maximum absorbance P rocking modes i n i r v i b r a t i o n s V s t r e t c h i n g modes i n i r v i b r a t i o n s x v i i [ ] concentration [x/y/z] uncharacterized species p o s s i b l y made from components x, y, and z. {*H} proton decoupled x v i i i Acknowledgements I am g r a t e f u l to Professor B.R. James f o r h i s guidance, support and encouragement through the course of t h i s work. Discussions with him on the content and form of th i s thesis were p a r t i c u l a r l y rewarding. I thank him for h i s patience and time. I also thank members of the group, both past and present, f o r t h e i r support, p a r t i c u l a r l y during some lean times. Dr. S. R e t t i g d i d the c r y s t a l l o g r a p h i c work. I thank him for h i s cooperation. The assistance of micro analyses, nmr, mass spectroscopic, glass-blowing, e l e c t r i c a l and mechanical services are also g r a t e f u l l y acknowledged. I am indebted to David Thackray f or wading through some early d r a f t s and f o r us e f u l discussions i n general, and to Dr. Ian Thorburn fo r c r i t i c a l comments and proof-reading. Rani Theeparajah typed t h i s t h e s i s ; I thank her for a s k i l f u l work c a r r i e d out with utmost patience and cooperation. F i n a l l y , I dedicate t h i s thesis to Rohan and J i t f o r g i v i n g me the strength to keep my head above the murky waters of autoxidation c a t a l y s i s . - 1 -CHAPTER I INTRODUCTION - 2 -I INTRODUCTION 1.1 Metal-catalysis in the autoxidation of organic substrates The slow deterioration of organic materials such as rubber, natural o i l and fats has long been understood to be caused by aerial oxidation. The oxidative decay processes have hi s t o r i c a l l y been known as autoxidations, probably because of their apparent spontaneous nature. Now the term autoxidation is generally applied to distinguish oxidations by O2, from other types of oxidations.^• 2 The control of autoxidation is desirable not only from -the standpoint of inhibiting the oxidative deterioration of industrially important raw materials and products, but also for promoting selective autoxidations of industrial importance.3 Catalysis by metals and/or metal derivatives plays an important role in the control of selective, partial oxidation of alkanes, olefins and aromatic hydrocarbons to useful products. 3"^ The use of metals or metal-derivatives for the heterogeneous catalysis of the autoxidation of organic substrates has been, and is of, widespread use in the petro-chemical industry.** Homogeneous catalysis in the liq u i d phase offers some advantages over heterogeneous catalysis because of the lower energy requirements and better level of process control attainable in the liqu i d phase; not only can temperature and mixing be better controlled than in the heterogeneous phase, but also the nature of the active catalytic species is regulated more effectively. Homogeneous processes are also more amenable to studies by spectroscopic and kinetic methods, and therefore offer more potential for the fine tuning of known catalysts, or the design of new catalysts.^ - 3 -Homogeneous c a t a l y s i s of autoxidations by soluble metal s a l t s or complexes can be complicated, because of the v a r i e t y of mechanistic pathways a v a i l a b l e . A major pathway i s v i a the i n t e r a c t i o n between metal c a t a l y s t and trace hydroperoxide impurities i n a system, while d i r e c t i n t e r a c t i o n s between metal and substrate, or metal and dioxygen, can also lead to autoxidations. Metal-hydroperoxide in t e r a c t i o n s T r a n s i t i o n -metal ions can c a t a l y t i c a l l y decompose hydroperoxides to give the free r a d i c a l s RO- and R0 2• (eqs. 1.1-1.3). M n + + ROOH > M n + 1 + RO- + HO" 1.1 Mn+1 + R 0 0 H > Mn+ + RO 2- + H + 1.2 Mn+ 2 ROOH > RO- + R0 2- + H 20 1.3 Trace f r e e - r a d i c a l s may i n i t i a t e f r e e - r a d i c a l chain autoxidations of organic substrates. Mechanisms of such autoxidations are well characterized;'' a simple form of such an oxidation i s given i n Scheme 1.1. Since the immediate products of hydrocarbon autoxidations are themselves hydroperoxides, the rate of production of f r e e - r a d i c a l s (eq. 1.3) and therefore the rate of i n i t i a t i o n (eq. 1.4) increase c o n t i n u a l l y , to give an autoacceleration of the oxidation at the beginning. A f t e r some time the hydroperoxide concentration reaches a - 4 -steady state, and a maximum rate of reaction i s attained. In the maximum rate region the rate i s independent of the rate of i n i t i a t i o n (eq. 1.4) and i s dependent only on the rate of propagation (eq. 1.6). In such cases, the c a t a l y s i s of f r e e - r a d i c a l chain oxidations by t r a n s i t i o n metal ions i s only a c a t a l y s i s of i n i t i a t i o n . Scheme 1.1 I n i t i a t i o n : In. RH -> R- + InH 1.4 (In.; i n i t i a t i n g f r e e - r a d i c a l s , usually RO- or RO2') Propagation: R- R0 2- 1.5 fa s t 1.6 R0 2H RH Termination: 2 R0 2- > non f r e e - r a d i c a l products 1.7 - 5 -Metal-substrate Interactions In some s i t u a t i o n s the metal ions or complexes may catalyze autoxidations v i a d i r e c t i n t e r a c t i o n s with the substrate.** The i n d u s t r i a l l y important oxidation of a l k y l benzenes to carboxylic acids with a cobalt c a t a l y s t i s a relevant example (eq. I . 8 ) . ^ a Co(OAc) n, (n=2,3), 0 2 AcOH(solvent),100°C C0 o H 1.8 The mechanism suggested involves an i n i t i a l i n t e r a c t i o n between Co*** and the substrate, to give a benzyl r a d i c a l (eqs. 1.9 and I.10). Under autoxidation conditions the benzyl r a d i c a l i s trapped by 0 2 (eq. I.11). The peroxy r a d i c a l s formed are good o x i d i z i n g agents which catalyze the regeneration of Co*** species from Co** (eq. 1.12). ArCH-j + Co III -> [ArCH 3] + . + Co II 1.9 [ArCH 3] +- -> ArCH 2- + H + 1.10 ArCH 2• + 0 2 -> ArCH 20 2• 1.11 ArCH 20 2• + Co II -> ArCH 20 2" + Co III 1.12 The e f f e c t i v e t o t a l r e a c t i o n i s a metal-catalyzed f r e e - r a d i c a l autoxidation (Scheme 1.2). - 6 -Scheme I.2 ArCH 3 ArCH 20 2H ArCH 20 2 • ^^== > ArCH 2 • * C o 1 1 1 °2 A net "autoxidation" r e a c t i o n r e s u l t s also from Wacker-type oxidations^ which are widely used i n industry to f u n c t i o n a l i z e o l e f i n s , f o r example, to produce acetaldehyde and vinylacetates from ethylene; these involve the a c t i v a t i o n of the o l e f i n by coordination to the metal centre (reaction A, Scheme 1.3). The oxygen atom(s) incorporated i n the oxidation are derived from H 20. A raetal-02 i n t e r a c t i o n may or may not occur with the Cu cocatalyst, whose role i s e s s e n t i a l l y the regeneration of the active P d 2 + species from the i n a c t i v e Pd° produced i n the oxidation of the o l e f i n (reactions B and C, Scheme 1.3). Metal-dioxygen in t e r a c t i o n s In the presence of sui t a b l e ligands, transition-metal ions react with 0 2 to give metal-dioxygen complexes.* u A large number of such complexes, p a r t i c u l a r l y those of Pt°, Pd°, Co**, Rh* and I r * , have been i s o l a t e d and characterized. Metal-dioxygen complexes are widely used i n nature not only f o r binding and r e v e r s i b l y carrying 0 2 (e.g. myoglobin, - 7 -Scheme I.3 CH2=CH2 CH3CHO, 2H + hemoglobin), but also f o r the oxidation of organic s u b s t r a t e s . 1 The oxidation processes occur e i t h e r v i a enzyroic oxygenases, which incorporate one or two atoms of 0 2 to a substrate, or v i a oxidases that convert both atoms of 0 2 to water or hydrogen peroxide. The heme u n i t which contains an Fe-porphyrin active centre, i s p a r t i c u l a r l y prevalent, and, f o r example, i s found i n myoglobin and hemoglobin, the monoxygenase cytochrome P-450, tryptophan dioxygenases, and i n cytochrome C oxidase- the terminal enzyme i n the r e s p i r a t o r y redox chain that reduces 0 2 to w a t e r . T h e monoxygenase enzyme, cytochrome P-450, i s of p a r t i c u l a r i n t e r e s t because of i t s a b i l i t y to hydroxylate hydrocarbons with regio- and s t e r e o s e l e c t i v i t y (eq. 1.13). RH + 0 2 + NADH + H + > ROH + H 20 + NAD+ 1.13 - 8 -The suggested mechanism for the hydroxylation r e a c t i o n '(Scheme I.4) l l a involves a s e l e c t i v e oxygen atom transfer within the constraints of an enzyme-C>2 substrate complex, where the dioxygen i s ac t i v a t e d by coordination to the i r o n centre. Achieving s e l e c t i v e autoxidations i n v i t r o , v i a a s i m i l a r a c t i v a t i o n of 02 by a d i r e c t i n t e r a c t i o n between the metal c a t a l y s t s and O2, has received much attention i n the l a s t two decades. 1 2 Although, a c a t a l y s t system of any i n d u s t r i a l s i g n i f i c a n c e has not yet emerged from such s t u d i e s , 1 3 there i s a continuing i n t e r e s t i n t h i s area of study. Scheme 1 . 4 Fe z +0 2-RH - 9 -Some background information on metal-dioxygen complexes and a b r i e f summary of the l i t e r a t u r e i n the area are given i n the next two sections. 1.2 Metal-dioxygen complexes Dioxygen i n i t s ground state contains two unpaired electrons i n the 7T o r b i t a l s ( F i g . I . l . a ) . The bonding i n metal-dioxygen complexes ar i s e s e s s e n t i a l l y from the i n t e r a c t i o n of the dioxygen 7T o r b i t a l s (Fig. 1.1) with the d - o r b i t a l s of the metal ( F i g . I . l . b ) , the 0 2 l i g a n d a c t i n g e s s e n t i a l l y as an el e c t r o n acceptor.* 0 F i g . 1.1 (a) The molecular o r b i t a l s of 0 2 (b) Molecular o r b i t a l diagram f o r an M02 complex - 10 -Table 1.1 S t r u c t u r a l c l a s s i f i c a t i o n and nomenclature of dioxygen complexes* Structure II Nomenclature (Vaska) III Nomenclature (Gubelmann and Williams) IV Example 0 I M Type Ia (superoxo) rjL dioxygen [Co(CN) 50 2] 3-M Type H a (peroxo) T7Z dioxygen P t ( 0 2 ) ( P P h 3 ) 2 M-0 0-M Type lb (superoxo) Type l i b (peroxo) T/1:^1 dioxygen [ (NH 3) 5Co0 2Co(NH 3) 5 ] 5 + TT1:!?1 dioxygen [ (NH 3 ) 5 C o 0 2 C o ( N H 3 ) 5 ] 4 + M / \ \ M rj2:7]2 dioxygen [ ( U 0 2 C 1 3 ) 0 2 ] 4 " M 0 — M 7J2:t]1 dioxygen [RhCl(0 2) (PPh 3) 2] 2 (a) Ref 10 - 11 -Table 1.2 Properties of some dioxygen species Species Bond Order Compound 0-0, A ^0-0 > cm 0 2 +- 2.5 0 2AsF 6 1.123 1858 0 2 2 0 2 1.207 1555 02"- 1.5 K0 2 1.32-1.35 1146 0 2 = 1 Na 20 2 1.45-1.50 738-880 In h i s review on metal-dioxygen complexes, V a s k a 1 3 i d e n t i f i e d four s t r u c t u r a l types (column II , Table 1.1); superoxo compounds (types Ia and lb) where the 0-0 distance i s roughly constant (-1.3A) and close to the value reported for the superoxide anion 02"* (Table 1.2), and peroxo compounds (type I l a and l i b ) where the 0-0 distance (-1.5 A) i s close to the values reported f o r H 20 2 and 0 2 2"- (Table 1.2). The notations a or b d i s t i n g u i s h complexes where the 0 2 i s bound to one metal atom (type a) or bridges two metal atoms (type b) . Gubelmann and Williams 1*^ have recently introduced an alternate nomenclature using "hapto" notations (column I I I , Table 1.1). They argue that the metal-dioxygen bond, although p o l a r i z e d with the dioxygen carrying at l e a s t a p a r t i a l - 12 -p a r t i a l negative charge, generally has an appreciable covalent character, and therefore, the assignment of formal oxidation states may not be too accurate.* 0 In t h i s thesis most of the discu s s i o n i s based on Vaska's c l a s s i f i c a t i o n s since i t i s a convenient nomenclature which i s s t i l l widely used i n the l i t e r a t u r e , * 2 though the exact e l e c t r o n i c d i s t r i b u t i o n i n the c i t e d cases may or may not correspond to the meanings implied by the terms superoxo and peroxo. Superperoxo- or jH.-peroxo- metal complexes are generally found f o r t r a n s i t i o n metal ions such as Fe**, Co**, Mn** and Cu*, which show one ele c t r o n oxidations, while a majority of peroxo complexes are found f o r Rh*, I r 1 , Pd° and Pt° metals; some examples are found i n Table 1.1 . Measurement of the dioxygen s t r e t c h i n g frequency i s the most us e f u l method a v a i l a b l e f o r d i s t i n g u i s h i n g between superoxo- and peroxo-metal complexes. In h i s review, Vaska showed how the VQ.Q values then known corresponded to the two p r i n c i p a l types of complexes, Type I (or superoxo) and Type II (or peroxo), i n h i s c l a s s i f i c a t i o n . For superoxo complexes the frequencies lay i n the range 1075-1195 cm"* and for peroxo complexes i n the range 790-932 cm"*. The frequencies of i o n i c super-oxides and peroxides, re s p e c t i v e l y , are close to the middle of these ranges (Table 1.2). Although some dioxygen s t r e t c h i n g frequencies outside these l i m i t s are noted i n some recent s t u d i e s , * 0 Vaska's observations s t i l l remain v a l i d for most metal-dioxygen complexes. The e l e c t r o n i c spectra of metal-dioxygen complexes are not well characterized. Systematic studies i n the area have been l i m i t e d by the i n s t a b i l i t y of many of these complexes i n s o l u t i o n . * 0 Some chemical properties of metal-dioxygen complexes are discussed i n the next section, i n the context of t h e i r c a t a l y t i c a c t i v i t y . - 13 -1.3 Metal-catalyzed a c t i v a t i o n and transf e r of 0 2 to organic substrates The reactions are c l a s s i f i e d broadly i n the l i t e r a t u r e 1 2 3 as those i n v o l v i n g superoxo and/or jUrperoxo, and peroxo metal complexes (see column I I , Table 1.1). Ca t a l y s i s v i a superoxo or jU.-peroxo complexes Oxidation of organic substrates catalyzed by superoxo or jUrperoxo complexes are often interpreted i n terms of i n i t i a l production of r a d i c a l s v i a H-atom abstraction by the complexes (eq. 1.14) perhaps by analogy with the R-0-0- r a d i c a l . 1 2 * 3 M-0-0- + RH > M-O-O-H + R- 1.14 Such mechanisms were e a r l i e r thought to be c e n t r a l to the observed c a t a l y t i c e f f e c t of superoxo-metal complexes i n the autoxidation of hydrocarbons, but i t was l a t e r shown that the e f f e c t arose from the metal-catalyzed decomposition of hydroperoxide i m p u r i t i e s , 1 2 a as i n eq 1.3. The c a t a l y t i c autoxidation of phenolic substrates i s interpreted also i n terms of i n i t i a l production of r a d i c a l s by H-atom abstrac t i o n by Co I I I(02"') or C o I 1 1 ( 0 2 = ) C o 1 1 1 moieties (eq. 1.15) and subsequent metal-catalyzed steps to produce quinones (eq. I.16). 1 2* 3 - 14 -1.15 1.16 Sheldon and Kochi suggest that perhaps the superoxo-metal complexes react f i r s t as bases or nucleophiles to abstract protons from phenols (e.g. eq I . 1 7 ) 1 2 a ; because of the net e l e c t r o n t r a n s f e r involved i n a metal-dioxygen bond, the 0 2 l i g a n d may act as a base or a n u c l e o p h i l e . l u Subsequent metal-catalyzed steps involve alkylperoxocobalt species s i m i l a r to eq. 1.16 above (e.g. eq. 1.17.1). 1.17 1.17.1 - 15 -C a t a l y s i s v i a peroxo-metal complexes The dioxygen ligands of peroxo-metal complexes also e x h i b i t basic or n u c l e o p h i l i c b e h aviour. 1 0 The r e a c t i v i t y of the Pt(0 2)(PPh3>2 complex towards e l e c t r o p h i l e s i s an i l l u s t r a t i o n of t h i s property (eq 1.18- I . 2 1 ) . 1 6 1.18 1.19 1.20 1.21 Simple o l e f i n s do not react with peroxo complexes but they may be rendered e l e c t r o p h i l i c by coordination to m e t a l . 1 2 a The i s o l a t i o n of the - 16 -complex IrCl(PPh3)2(C2H4)(O2), perhaps, res u l t e d from such an attempt at oxidation of ethylene.* 2* 3 Since from about 1967 many e f f o r t s have been made to use metal-dioxygen complexes or t h e i r precursor complexes such as RhCl(PPh 3)3, RhCl(CO)(PPh 3) 3 and Pd(PPh 3)4, for the c a t a l y t i c autoxidation of organic substrates. The majority of the reports involve the oxidation of c y c l i c o l e f i n s , t y p i c a l l y at temperatures >60°C. Some i n i t i a l r e s u l t s on these sytems were thought to be due to metal-centred transf e r of 0 2, but more d e t a i l e d studies have i n v a r i a b l y revealed that the r o l e of the metal i n such reactions i s to catalyze the i n i t i a t i o n of free r a d i c a l reactions as i n eq. I.3.* 2 a'^ The f i r s t authentic example of a non f r e e - r a d i c a l c a t a l y t i c r e a c t i o n was the co-oxidation of terminal o l e f i n s and triphenylphosphine to methyl ketones and triphenylphosphine oxide, r e s p e c t i v e l y , by the RhCl(PPh3)3 complex i n benzene, at r . t . , reported by Read and Walker*^ (eq. 1.22). Radical chain processes were not detected and triphenylphosphine was found to be e s s e n t i a l f o r the c a t a l y t i c a c t i v i t y . RCH=CH2 + PPh 3 + 0 2 > RCOCH3 + PPh 30 1.22 Later, Mimoun and coworkers demonstrated a c a t a l y s t system for the oxidation of terminal o l e f i n s without the need for a coreducing agent such as PPh 3. Using mixtures of Rh and Cu s a l t s i n a l c o h o l i c solvents, sometimes with added acid, two molecules of terminal o l e f i n s were converted to two molecules of methyl ketone (eq. 1.23). 2 RCH=CH2 + 0 2 > 2RCOCH3 1.23 - 17 -In both of the above systems, the suggested mechanism involved the i n i t i a l formation of a peroxometallacycle by rea c t i o n of a Rh* c a t a l y t i c species with the o l e f i n and 0 2 (eq 1.24). 0—0 H / \ / Rh 1 + RCH=CHo + Oo > Rh C v 1.24 \ / • \ / \ H H R In the d e t a i l e d mechanistic scheme suggested by Mimoun and coworkers, the peroxometallocycle i s thought to break down into the ketone and a Rh***~0 intermediate (path A, Scheme 1.5); such oxo intermediates are invoked i n P-450 catalyzed systems.** The second oxygen atom transfer and the regeneration of the c a t a l y s t occurs v i a a Wacker process (path B; c f . r e a c t i o n A i n Scheme 1.3). In the presence of excess PPh 3 the Rh* species may be regenerated by the r e a c t i o n between Rh***-0 and PPh 3 (path B'). Scheme 1.5 I - 18 -Autoxidation of no n - o l e f i n i c substrates such as PPh 3 and thioethers by peroxometal complexes has received some att e n t i o n also. I n i t i a l l y , there had been high expectations for the f a c i l e transfer of coordinated dioxygen to coordinated substrates (eq 1.24.1).*^ Ph 3P 0 \ / M / \ Ph 3P -> : o M -> M + 2PPhoO 1.24.1 Later, d e t a i l e d studies on the c a t a l y t i c oxidation of PPh 3 by the Pt(0 2)(PPh 3)2 complex 2 0 showed that the phosphine i s oxidized by free H02", which i s c a t a l y t i c a l l y generated i n the presence of p r o t i c impurities; a second PPh 3 molecule regenerates the active oxidant (Scheme 1.6). Free peroxide i s thought to be the active oxidant also i n the Ru**~ catalyzed autoxidation of thioethers i n primary or secondary alcohol solvents (eq I. 25-1.27) 21 R u 1 1 + 0 2 R u I V + 0 2 2- 1.25 R u I V + R1R2CHOH Ru II + R 1R 2CO + 2H + 1.26 R 2S + H 20 2 > R2SO + H 20 1.27 19 -Scheme I.6 P t ( 0 2 ) ( P P h 3 ) 2 •2PPh 3 +0 2 PPh 3 -> P t ( P P h 3 ) 3 ( 0 2 ) PPh3,H+ [ P t ( P P h 3 ) 4 ] 2 + + H0 2" ^ P P h 3 S>PPh30 - 0H"< ^->PPh 30,H + P t ( P P h 3 ) 4 2+ 1.4 C a t a l y t i c autoxidations i n the presence of H 2 cosubstrate Studies on metal-catalyzed autoxidations i n t h i s laboratory i n the past few years have concerned some oxidations i n the presence of H 2, where the 0 2 appears to be activated as a MOOH species which forms v i a an e f f e c t i v e i n s e r t i o n of 0 2 to an M-H bond (eq. 1.28). M-H + 0 2 > M-02H 1.28 The work i n i t i a t e d from an accidental f i n d i n g about 15 years ago: while solvent e f f e c t s on c a t a l y t i c hydrogenation were being studied, a - 20 -R h 1 1 1 - c a t a l y z e d H 2 reduction of (CH 3) 2SO to the s u l f i d e and H 20 was d i s c o v e r e d . 2 3 The suggested mechanism involved a hydridorhodium(III) intermediate containing S-bonded sulfoxides. (Reactions of Rh*** complexes with H 2 generally lead to Rh* species, most l i k e l y v i a h e t e r o l y t i c a c t i v a t i o n of H 2 to give i n i t i a l Rh***(H) species which then d i s s o c i a t e to Rh* and H + (eq I . 2 9 ) . 2 2 ) . - H + - H + R h * * * + Ho R h * * * H v s R h * 1.29 + H + + H + The r e a c t i o n rates of the DMSO reduction decreased eventually and t h i s was a t t r i b u t e d to the build-up of i n a c t i v e Rh* species. In an attempt to maintain c a t a l y t i c a c t i v i t y by reoxidation of the Rh*, a H 2/0 2 mixture was used. This l e d to a s u r p r i s i n g c a t a l y t i c oxidation of the sulfoxide to the sulfone and H 20 (reaction I . 3 0 ) 2 3 which i s s t o i c h i o -m e t r i c a l l y s i m i l a r to the oxidations c a r r i e d out by the P-450 enzyme systems (eq. 1.13). (CH 3) 2SO + H 2 + 0 2 > ( C H 3 ) 2 S 0 2 + H 20 1.30 Neither Rh*** nor Rh* species under 0 2 alone performed t h i s c a t a l y s i s , which was interpreted as a strong i n d i c a t i o n that a hydridorhodium(III) species was the e f f e c t i v e 0 2 - c a r r i e r . Trocha-Grimshaw and Henbest also had found that isopropanol solutions of Rh*** and I r * * * s a l t s c a r r i e d out the same oxidations using j u s t 0 2, 2 Z f and such a medium i s very e f f e c t i v e f o r forming hydride species. 2-* The suggested c a t a l y t i c cycle (Scheme 1.7) 2^ included an M(OOH) - 21 -intermediate (M-Rh 1 1 1), which i s formed by a r e a c t i o n between MH and 0 2. The oxidation of the substrate (S=DMSO) was postulated to occur v i a the metal hydroperoxide. Scheme I.7 The e f f e c t i v e i n s e r t i o n of 0 2 to Rh i i J-(H) to give Rh i i i(OOH) species i s well documented for RhH(NH 3) 5 and RhH(CN)4(H 20) 2", 2 7 while oxygen atom-tra n s f e r from a Pd-OOH species i s reported i n the c a t a l y t i c oxidation of terminal o l e f i n s by H 20 2 i n the presence of Pd**-carboxylate c a t a l y s t s (Scheme 1 . 8 ) . 2 9 In other studies attempting to detect the r o l e of M-OOH i n t e r -mediates, a CH 2C1 2 s o l u t i o n of the dimer [ I r C l 2 ( H ) (CgH]_2) ] 2 was reported - 22 to y i e l d with 0 2, a single cyclooctene-one product and an I r species, the i n f r a r e d spectrum of which indicated the conversion of coordinated -00H to -OH; the data were interpreted i n terms of eq 1.31 (written f o r a monomer).26,28 °2 I r C l 2 ( H ) ( C 8 H 1 2 ) > I r C l 2 ( 0 0 H ) ( C 8 H 1 2 ) > 'Ir(OH)Cl 2 + C 8H 1 20 1.31 The suggested mechanism f o r oxygen atom transf e r i s s i m i l a r to that suggested by Roussel and Mimoun for the oxidation of terminal o l e f i n s by H 20 2 v i a a Pd-OOH intermediate (Scheme I . 8 ) .2 9 Scheme I.8 (RC0 2) 2Pd H 20 2 -RC02H RC02-Pd-00H H 2 0 ^ H 20 2 RC0 2 -Pd-OH <-RCOCH-; RC02-Pd-OOH R C O o - P d Addition of excess CgH]_4 to the [ I r C l 2 (H) (C 8H^ 2) ] 2 system and the use of a H 2/0 2 mixture to regenerate the Ir***(H) from the Ir***(OH), as i n - 23 -Scheme 1.7, gave low but c a t a l y t i c y i e l d s of the ketone and a large excess of H 20 from an independent, metal-catalyzed hydrogenolysis of O2. The low c a t a l y t i c a c t i v i t y towards the o l e f i n oxidation was a t t r i b u t e d to the low a c t i v i t y of the 'lr(OH)' intermediate towards H 2 (Scheme I.7).26 A c a t a l y t i c cycle analogous to that i n Scheme I.7 was proposed by Tabushi and Y a z a k i 3 0 f o r the epoxidation of o l e f i n s using a Mn*** porphyrin under 0 2 with H 2 / c o l l o i d a l platinum as the coreductant. The suggested mechanism (Scheme 1.9) i s analogous to that postulated f o r the Ir system i n Scheme 1.7 i n that both involve M-OOH intermediates, form-ed by the protonation of coordinated dioxygen species and the i n s e r t i o n of 0 2 into a Ir***-H bond, r e s p e c t i v e l y . The subsequent oxygen trans f e r i n the Mn system i s thought to occur v i a a Mn*V_0: species, s i m i l a r to the Fe***=0 species implicated i n the P-450 systems (Scheme 1.4). Scheme 1.9 24 1.5 Contents of t h i s thesis The general aim behind the work described i n t h i s thesis was to use H2/O2 mixtures i n the presence of rhodium c a t a l y s t s f o r s e l e c t i v e autoxidation of organic substrates as i n Scheme 1 . 7 . As a prelude to such a study i t was attempted to generate Rh***(00H) species by reaction of s u i t a b l e Rh*** complexes with H2 followed by 0 2 . Sulfoxide complexes were selected as suitable precursors for the hydride complexes, since such chemistry had already been studied with some success f o r these complexes. 2 3 Attempts at generating Rh***(H) species containing DMSO ligands are summarized i n Chapter I I I . An i n t e r e s t i n g redox r e a c t i o n between RhCl3(DMSO)3 and N E t 3 , which was observed i n the course of the work given i n Chapter I I I , i s d e t a i l e d i n Chapter IV. A rhodium- and In-dependent oxidation of DMA, which was discovered a c c i d e n t a l l y i n some preliminary studies on the use of Rh*** c a t a l y s t f or autoxidations under H 2 / 0 2 mixtures, i s described i n Chapter V. - 25 -CHAPTER II EXPERIMENTAL - 26 -II EXPERIMENTAL II.1 General instrumentation V i s i b l e spectra were recorded on a Perkin-Elmer 552A spectro-photometer, f i t t e d with thermostatted c e l l compartments. Near i n f r a - r e d spectra were obtained using a Cary 17D spectrophotometer. Spectral c e l l s used were anaerobic type with quartz c e l l s of 1 cm or 0.1 cm pathlength ( F i g II.1 and F i g II.2). Infra-red spectra were recorded on a Perkin-Elmer 457 or 598 grating spectrometer c a l i b r a t e d with the 1601.4 cm"* or 906.7 peaks of polystyrene, or a Ni c o l e t 5DX FT-IR instrument. S o l i d samples were run as Nujol mulls between Csl plates or as KBr di s c s . Solution samples were run i n 0.1 cm pathlength c a v i t y c e l l s with KBr windows. Proton and carbon-13 nuclear magnetic resonance {*H nmr} spectra were recorded on Bruker WP80 (80 MHz), Varian XL100 (100 MHz) or Bruker WH400 (400 MHz) spectrometers operating i n the Fourier transform mode, generally with tetramethylsilane (TMS) at 5o.O as an i n t e r n a l standard. For gas chromatographic analysis a Carle A n a l y t i c a l Gas Chromato-graph (Model 311), or a temperature programmable Hewlett-Packard 5750, both i n the thermal conductivity detector mode, was used. The columns used were of s t a i n l e s s - s t e e l with 1/8" i n t e r n a l diameter. A hand-packed 6' foot column containing PPQ (Water Associates, Inc., 80-100 mesh) was used f o r H 20 determination while a 12' foot column was used to determine H 2, 0 2 and C0 2. Carbowax 20 M on Chromosorb WHP (80-100 mesh) was the packing material used f o r estimating organic oxidation products. A 6' - 27 -TEFLON STOP-COCK B U • B U \ / \ V I II.1 Schematic representation of an anaerobic spectral c e l l - 28 -F i g . II.2 Schematic representation of a combination anaerobic s p e c t r a l c e l l and gas uptake f l a s k - 29 -column containing 5% Carbowax purchased from Chromatographic S p e c i a l i -t i e s Ltd., and a hand-packed 6' column containing home-made packing material with -1% Carbowax 20 M, were both used. Conductivity measurements were made at 25°C using a Thomas Serfass conductivity bridge and c e l l . II.2 Gas-uptake apparatus and measurement II.2.1 The apparatus The constant pressure gas-uptake apparatus used f o r determining gas stoichiometrics and k i n e t i c studies i s shown schematically i n F i g . II.3. The pyrex two-neck rea c t i o n f l a s k (A), equipped with a dropping side-arm bucket, was attached to a f l e x i b l e glass s p i r a l tube, which connected f l a s k A to a c a p i l l a r y manometer (D) at tap C. The re a c t i o n f l a s k was c l i p p e d to a Welch v a r i a b l e speed e l e c t r i c motor so that the f l a s k could be shaken w h i l s t held i n the thermostatted o i l - b a t h (B). The o i l - b a t h consisted of a four l i t e r glass beaker with s i l i c o n e o i l and was held i n a polystyrene-foam l i n e d wooden box f o r i n s u l a t i o n . The c a p i l l a r y manometer contained n-butyl phthalate, a l i q u i d of n e g l i g i b l e vapor pressure, and was connected to the gas measuring burette which had a p r e c i s i o n bored tube (N) of known diameter and a mercury r e s e r v o i r (E). The c a p i l l a r y manometer and gas measuring burette were thermo-s t a t t e d at 25°C i n a perspex water bath. By means of an Edwards high vacuum needle valve (M), the burette was connected to the gas-handling Fig. II.3 Schematic representation of a constant pressure gas uptake apparatus - 31 -part of the apparatus. The l a t t e r consisted of a mercury manometer (F), the gas i n l e t (Y), and connections to a Welch Duo-Seal rotary vacuum pump (G). The thermostatting of the two baths was c o n t r o l l e d by Jumo thermo-regulators and Merc-to-Mere r e l a y c o n t r o l c i r c u i t s , with 40 Watt elongated l i g h t bulbs used for heating. This, with mechanical s t i r r i n g , meant that the temperature could be maintained within — 0.5°C. The gas-uptake was measured with a v e r t i c a l l y mounted cathetometer. In each experiment 5.0 mL of solvent was p i p e t t e d into the 25 mL r e a c t i o n f l a s k and any other cocatalysts or substrates required were weighed or pipe t t e d i n . The weighed complex was suspended on the hook of the side arm of the r e a c t i o n f l a s k . The f l a s k with the connected s p i r a l arm was attached to the gas-handling part of the apparatus at j o i n t 0. The substrate s o l u t i o n was degassed by a freeze and thaw s t a t i c vacuum technique which was c a r r i e d out three times. The r e a c t i o n f l a s k was then f i l l e d with gas to a pressure s l i g h t l y l e s s than that required, and taps C and P were closed. The f l a s k and s p i r a l arm could then be removed and connected to the c a p i l l a r y manometer at H. The f l a s k was placed i n the o i l - b a t h , and attached to the motor driven shaker (I) which was then started. The whole system up to tap C was evacuated with taps, H, K, L, J , and M open. A f t e r a 10 minute shaking to a t t a i n thermal e q u i l i b r a t i o n of the r e a c t i o n f l a s k and to saturate the s o l u t i o n with gas, the shaking was stopped, and the required gas was admitted to the r e s t of the apparatus at pressure s l i g h t l y l e s s than required. Tap C was opened and the pressure increased to that desired. The needle valve and taps K and L were closed; shaking was continued for another 5 min to complete thermal e q u i l i b r a t i o n and the i n i t i a l reading of the mercury l e v e l i n N taken. - 32 -11.2.2 A general experiment An experimental run was st a r t e d by dropping a bucket containing the complex, and s t a r t i n g the shaker and timer. The gas-uptake was indica t e d by the difference i n the o i l l e v e l s of the manometer (D). The manometer was balanced by allowing gas to the burette through the needle valve and thereby maintaining a constant pressure i n the rea c t i o n f l a s k . The corresponding r i s e i n the mercury l e v e l i n N was measured at appropriate i n t e r v a l s of time. Since the diameter of the manometer (N) was known, the volume of gas consumed could be c a l c u l a t e d and expressed as mmols of uptake per mg (mmol) of complex. 11.2.3 S o l u b i l i t y of gases The s o l u b i l i t y of H 2 and 0 2 i n DMA at s p e c i f i c temperatures and pressures was determined using the gas-uptake apparatus discussed previously. The DMA was degassed but was l e f t under vacuum when taps C and P were closed, and the f l a s k with the s p i r a l arm transferred to the c a p i l l a r y manometer. The system was then evacuated to tap C and f i l l e d with the required gas to the approximate pressure required. Tap C was opened, and the pressure immediately adjusted to that required. With taps K and L, and the needle closed, the timer and shaker could be started, and the immediate uptake of gas could be measured. The s o l u b i l i t y of H 2 i n DMA at 50°C was measured to be (1.90 ± 0.05)10"3M and (1.36 ± 0.05)10"3M at 700 and 500 t o r r , r e s p e c t i v e l y . At 60°C the s o l u b i l i t y of H 2 i n DMA obeys Henry's Law to give a K value of - 33 -2.82 x 10"6M t o r r " 1 where K - [ H 2 ] / p a r t i a l pressure of H 2 . 3 1 A K value of (2.70 x 0.05) x 10'6M t o r r " 1 was deduced for the s o l u b i l i t y at 50°C. The s o l u b i l i t y of 0 2 at 50°C at 760 t o r r was deduced to be 4.0 x 10" 3 M. 11.2.4 Gas-stoichiometries The procedure described i n Section II.2.3 was used to determine gas stoichiometries f or reaction solutions. For accurate measurements i t was necessary to keep the contact time between the r e a c t i o n s o l u t i o n and the gas phase to a minimum to avoid r e a c t i o n between the gas phase and the s o l u t i o n surface, between the time the gas i s introduced and the i n i t i a l monitoring of the uptake. The t o t a l gas-uptake observed was corrected f o r gas s o l u b i l i t y under the same conditions. 11.2.5 Gas evolution The same uptake apparatus was used to measure gas evolutions corresponding to about 4 x 10" 2 mmol of gas at S.T.P. The upper l i m i t given was determined by the height of the mercury column i n the gas buret (N,E; F i g II.3) of the p a r t i c u l a r apparatus used. The procedure was the same as that f or an uptake experiment except that the manometer was balanced i n the present case by rel e a s i n g gas into the part r i g h t of the needle-valve which was under a pressure less than that used i n the rea c t i o n conditions. - 34 -II.2.6 Introduction of gas mixtures General procedure A pre-mixed gas mixture, contained i n a mixing f l a s k T ( F i g II.4) at a pressure P', was allowed to d i f f u s e i n s t a n t l y into the evacuated r e a c t i o n f l a s k and the re s t of the apparatus to the desired pressure P. For ease of explanation the part of the apparatus from the reaction f l a s k A to the needle valve M w i l l be designated V3, that enclosed by M, P, Y and E taps designated V 2 , and the YTQ + QX + QZ part, V-± (Fig I I . 4 ) . To pre-mix the gases, f o r example i n a 1:1 r a t i o i n H 2 and 0 2, the two gases were introduced separately and consecutively to the evacuated v l + V 2 at a pressure of P/2 each, v i a gas l i n e s (1) and (2), res p e c t i v e l y , and was i s o l a t e d by c l o s i n g taps Y, X and Z. The r e s t of the system upto the greaseless tap A' was evacuated and tap R was closed. The greaseless tap A' was opened to connect the already evacuated r e a c t i o n system to the r e s t of the apparatus. On opening Y slowly, the gas at a pressure of P' i n V^, flowed i n s t a n t l y into V 2 + V3 to give the f i n a l desired pressure of P i n + V 2 + V3. The system was allowed to e q u i l i b r a t e f o r 1/2 h before c l o s i n g Y to proceed as required f o r a general uptake experiment (Section II.2.2). The f i n a l t o t a l pressure P was estimated using the r e l a t i o n P=P'V^/(V^+V2+V3>. The r a t i o VT_/V^+V2+V3 was experimentally derived f o r the apparatus used. In the p a r t i c u l a r apparatus a P' of 920 t o r r i n was required to obtain a f i n a l pressure of 760 t o r r i n V]_+V2+V3. - 35 -gases Fig. I I . 4 A s i m p l i f i e d version of a gas uptake apparatus with d e t a i l s of the modified gas i n l e t - 36 -The part V 2 consists of several narrow tubes making up the manometer and the connections to i t . To ensure that the problem of incomplete mixing of gases d i d not s i g n i f i c a n t l y a f f e c t the r a t i o of gases i n V]_, a volume of V]_ > 10V 2 was used. Hydrogen d i f f u s e s 3.6 times f a s t e r than 0 2 i n a i r under comparable c o n d i t i o n s . 3 2 I t i s not too c l e a r what rate d i f f e r e n t i a l s are operative when a mixture of H 2 and 0 2 at, for example, 920 t o r r , i s opened to a vacuum composed of several interconnected narrow tubes. I f the rates are s i g n i f i c a n t l y d i f f e r e n t , the gas mixtures i n V3, p a r t i c u l a r l y i n the rea c t i o n f l a s k A and the immediate surroundings, w i l l be r i c h e r i n H 2 than cal c u l a t e d . To check the homogeneity of the gas mixture i n V^+V2+V3, 0.5 mL aliquots of a 1:1 gas mixture i n V^+V2+V3 e q u i l i b r a t e d f o r 30 min were sampled at A and near T i n F i g II.4 by gas chromatography using 12' PPQ packed column at 30°C with the c a r r i e r gas He de l i v e r e d at 30 p s i ; Retention times: H 2 2 min, 0 2 2.5 min; thermal conductivity detector s e n s i t i v i t y : 0 2/H 2, 27. The ali q u o t s from A gave a r a t i o of 28 — 2 for the peak area of 0 2/H 2 while those from T gave 30 i 2. Therefore the gas mixture i n V^+V2+V3 can be considered homogeneous within the l i m i t s of the a n a l y t i c a l method used. I I . 3 Spectral measurements Anaerobic c e l l s as shown i n Figs II.1 and II.2 were used to monitor reactions spectrophotometrically. - 37 -In a t y p i c a l experiment, a weighed amount of the complex was placed i n the c e l l ( F i g II.1) and 1.0-5.0 mL of solvent p i p e t t e d into the 25 mL f l a s k . The solvent was degassed three times by employing a freeze and thaw s t a t i c vacuum technique and the gas(es) of choice was introduced v i a attachments to the B7 socket. The s o l i d and solvent were then mixed and shaken to obtain a homogeneous s o l u t i o n . For reactions at temperatures close to ambient, the c e l l was placed i n the thermostatted c e l l compartment and the s p e c t r a l changes recorded immediately. For reactions at higher temperatures the solvent i n the f l a s k was brought to that temperatures before mixing with the s o l i d and p l a c i n g i n the thermostatted c e l l . To obtain the v i s i b l e spectrum of a s o l u t i o n used i n an uptake experiment a re a c t i o n f l a s k with a quartz c e l l attachment was used (Fig II.2). A f t e r the uptake was monitored f o r the desired time period, the greaseless tap was closed and the f l a s k detached from the apparatus. The s o l u t i o n was transferred into the c e l l by t i l t i n g the f l a s k i n order to record the v i s i b l e spectrum. Further experiments were c a r r i e d out, i f necessary, by reconnecting the f l a s k to the uptake apparatus. This method was p a r t i c u l a r l y u s e f u l f or obtaining the v i s i b l e spectra of extremely a i r - s e n s i t i v e species. The same rea c t i o n f l a s k was used to obtain the nmr spectra of h i g h l y a i r - s e n s i t i v e reaction solutions by attaching a s e a l i n g nmr tube v i a the B7 socket (2) ( F i g II.2). 38 -II.4 Materials 11.4.1 Solvents Spectral or reagent grade solvents were obtained from A l d r i c h , Eastman, Fisher, Mallinckdrodt or MCB Chemical Co. Dimethylacetamide (DMA) was s t i r r e d over CaH 2 under a N 2 atmosphere over 24 h and vacuum d i s t i l l e d . When s p e c i f i e d as dry, other solvents were d i s t i l l e d from the following drying agents under N 2:CH 2C1 2 from P 205; acetone from anhydrous K 2C n3; alcohols with the corresponding magnesium alkoxide. A l l dr i e d solvents were stored under N 2 or Ar and dispensed under the same. 11.4.2 Gases P u r i f i e d oxygen, argon, nitrogen and carbon monoxide were obtained from Canada L i q u i d A i r Ltd., Union Carbide of Canada Ltd., or Matheson Gas Co., and were used without further p u r i f i c a t i o n . P u r i f i e d hydrogen fo r hydrogenation reactions was obtained from Union Carbide and was passed through an Engelhard Deoxo c a t a l y t i c p u r i f i e r to remove traces of 0 2. The gases were d r i e d where necessary by passing them through C a C l 2 drying tubes. - 39 -II.4.3 Rhodium Complexes II.4.3.1 General Rhodium(III) t r i c h l o r i d e was obtained as the trihydrate from Johnson Matthey Ltd. and was kept i n a desiccator. L i t e r a t u r e methods were used to prepare RhCl 3(DMSO)3, 3 3 [ N E t 4 ] [ R h C l 4 ( D M S 0 ) 2 ] 3 3 , [ R h C l ( D M S 0 ) 5 ] [ B F 4 ] 2 3 4 , R h C l 3 ( E t 2 S ) 3 3 5 , R h C l 3 ( M e 2 S ) 3 3 5 and [ R h C l ( C 0 E ) 2 ] 2 3 6 . The complex [RhCl(CO) 2] 2 was a g i f t from Dr. D. Mahajan. Some experiments are described i n Chapter III i n cases where departures from the l i t e r a t u r e reports were observed or modifications of the reported methods were used. II.4.3.2 Mer-cis-trichloroaquobis(S-dimethylsulfoxide)rhodium(III), mer-cis-RhCl 3(H 20)(DMSO) 2 To RhCl 3-3H 20 (0.2 g, 0.8 mmol) dissolved i n 2-propanol solvent (3 mL) containing 20% H 20 (0.6 mL), about 3 equivalents of DMSO (0.16 mL, 2.3 mmol) were added. The yellow suspension which formed a f t e r s t i r r i n g the s o l u t i o n f o r approximately 1 h, was f i l t e r e d , washed with 2-propanol and ether, and dr i e d at r . t . under vacuum. Anal. calcd. f o r C 4 H 1 4 C l 3 0 3 S 2 R h : C 12.86, H 3.72; found: C 12.81, H 3.74. 1H nmrfijjMS ((CD 3) 2CO): 3.67 (s, 6H, S-CH 3), 3.50 (s, 6H, -SCH 3). I r (nujol) 1140 cm"1 (s, VSQ); 1060 cm"1 s and 980 cm"1 w (P C H ); 3200 cm"1 (br PQ-H-3 H 20) ; 1600 cm"1 (w, br, §HOH' h2°-) • T ^ e complex was soluble i n H 20, acetone and CH 3N0 2 and insoluble i n CH 2C1 2. - 40 -II.4.3.3 Trichloro(diethylethenamine)bis(S-dimethylsulfoxide)-rhodium(III), RhCl 3("CH 2CH=%Et 2) (DMS0) 2 To a suspension of RhCl 3(DMS0)3 (1.0 g, 2.25 mmol) i n acetone (25 mL), NEt 3 (0.63 mL, 4.5 mmol) was added with s t i r r i n g , a l l under Ar. Most of the suspension disso l v e d immediately to give a red-brown solu-t i o n and a t u r b i d i t y appeared a f t e r 10-20 min. The mixture was s t i r r e d overnight to y i e l d an off-white p r e c i p i t a t e and a red-brown so l u t i o n . The off-white s o l i d mixture was f i l t e r e d o f f under Ar, r e c r y s t a l l i z e d from EtOH and i d e n t i f i e d as NEt 3*HCl (156 mg, 1.1 mmol). Anal. calcd. for C 6H 1 6C1N: C 52.13, H 11.70, N 10.17; found: C 51.13, H 11.54, N 9.68. LH nmr 5 T M s ( C D C 1 3 ) : (fc. 9 H> N-CH 2CH 3), 3.10 (dq, 6H, NCH 2CH 3) and 12.2 (br, -1H, N +-H). Ir (Nuiol): 2400-2600 cm - 1, br, s (J>N_H) • Ir(CHoClo): 2400 cm"1, br, s and 2560 cm"1, s (^N.H) . The red-brown s o l u t i o n was concentrated by pumping to about 5 mL and warmed to dissolve any p r e c i p i t a t e s formed. The r e s u l t i n g s o l u t i o n was allowed to cool slowly when a c r y s t a l l i n e yellow p r e c i p i t a t e deposited. The s o l u t i o n was f i l t e r e d under Ar to y i e l d an a i r - s t a b l e yellow s o l i d and an a i r - s e n s i t i v e mother l i q u o r . Repeating the sequence of concentrating, warming and slow cooling gave two other crops of the yellow product. A l l three crops were combined and r e c r y s t a l l i z e d from warm acetone to y i e l d 250 mg (0.5 mmol) of product i n 22% y i e l d based on rhodium. Further p r e c i p i t a t i o n y i e l d e d material contaminated by an a i r -s e n s i t i v e species. Anal. calcd. f o r C 1 0 H 2 5 N O 2 s 2 c l 3 R h : c 25.85, H 5.42, N 3.02; found: C 25.86, H 5.34, N 2.99. LE nmr data ( F i g IV.2 and Table IV.1). 1 3 C ^H) nmr data (Table IV.2). IrCKBr): 1645 cm"1, s (J> C = N+ ); 1110 cm"1, s and 1125 cm"1, sh ( V $Q) ; other d e t a i l s i n Fig. - 41 -IV.8). The complex dissolves r e a d i l y i n C H C I 3 , CH 2C1 2, acetone, CH3OH and H 20 and s l i g h t l y i n EtOH, and i s non-conducting i n CH3N02. The compound e x i s t s i n CDCI3 s o l u t i o n as two isomers --60% mer-trans and 40% mer-cis a f t e r e q u i l i b r a t i o n for <4 h (see F i g . IV.l.b and Section IV.2.3), and e s s e n t i a l l y as the mer-trans i n acetone (see F i g . IV.10a and Section IV.2) or immediately a f t e r d i s s o l u t i o n i n CDCI3 ( F i g . IV.1.a). Slow d i f f u s i o n of ether into an acetone s o l u t i o n or a CDCI3 s o l u t i o n y i e l d s a compound containing about 70% of the mer-cis isomer as judged by a *H nmr of the sample. I I . 4.3.4 Mer-cIs-trichloro(dimethylsulfide)bis(S-dimethylsulfoxide)-rhodium(III), mer-cis-RhCl 3(DMS)(DMS0) 2 Approximately one equivalent of DMS (20 /XL, 0.25 mmol) was added to a s o l u t i o n of RhCl 3(DMS0)3 (110 mg, 0.25 mmol) i n dry CH 2C1 2 (10 mL). The s o l u t i o n was s t i r r e d overnight and ether was added slowly to obtain a yellow p r e c i p i t a t e . Anal, c a l c d for C g H ^ C ^ O ^ R h : C 16.85, H 4.24; found: C 17.00, H 4.25. 1H nmr 6 T M S ( C D C I 3 ) : 2.44 (d, J = 2 Hz, 6H); 3.64 (s, 6H); 3.56 (s, 6H). I r (Nujol): 1130 cm"1, s (J>SOiDMSO); 1030 and 985 cm"*, m (PQJ asymm. and symm., r e s p e c t i v e l y ) ; also see Section I I I . 3.2. - 42 -II.4.3.5 Me r - t r i c h l o r o t r i s ( d i m e t h y l s u l f i d e ) r h o d i u m ( I I I ) , mer-RhCl 3(DMS) 3 A degassed s o l u t i o n of RhCl 3(DMSO) 3 (50 mg, 0.11 mmol) i n C2H4CI2 (15 mL) was s t i r r e d at 50°C under a CO atmosphere f o r 3 d . A yellow p r e c i p i t a t e was obtained from the concentrated s o l u t i o n by adding ether. R e c r y s t a l l i z a t i o n from C ^ C ^ / e t h e r gave a deep orange c r y s t a l l i n e product i n 65% y i e l d . Anal, calcd. f o r C 6 H 1 8 C l 3 0 3 S 3 R h : C 18.21, H 4.59; found: C 18.31, H 4.59. XH nmr <5TMS (CDC1 3): 2.50 (d, J •= 2 Hz, 12H) and 2.30 (d, J = 2 Hz, 6H). This complex was prepared also by a l i t e r a t u r e procedure using RhCl 3-3H 20 and DMS.35 II.4.3.6 Product from the reac t i o n between RhCl3<DMS0)3 and CO i n DMA i n the presence of an equivalent of added H2O A degassed s o l u t i o n of RhCl 3(DMS0) 3 (0.2 g, 0.45 mmol) i n DMA (15 mL) containing added H2O (9 flL, 0.5 mmol) was s t i r r e d under 1 atm CO for 24 h. A s o l u t i o n c o l o r change from deep to l i g h t orange indi c a t e d the completion of the reaction. The solvent was d i s t i l l e d o f f at 50°C under vacuum to y i e l d a yellow residue. Further pumping turned the p r e c i -p i t a t e into a yellow-orange o i l , while white c r y s t a l s were c o l l e c t e d on the walls of the t a l l Schlenk tube used. The hi g h l y hygroscopic c r y s t a l s were c a r e f u l l y scraped o f f under Ar and were shown to be DMA-HCl by comparing the *H nmr and i r (Nujol) data with those of the authentic compound (Section II.4.5). The amount of DMA-HC1 recovered - 43 -was approximately 20 mg (-0.2 mmol). The o i l y residue was dissol v e d i n CO-saturated CH2CI2 and CO-saturated ether was added slowly to make the s o l u t i o n j u s t t u r b i d . Leaving the mixture i n an i c e / s a l t bath at ~-10°C for a few hours y i e l d e d pale-yellow, needle-like c r y s t a l s . These were f i l t e r e d under Ar, washed with ether and d r i e d by leaving under a dynamic vacuum. F i l t e r i n g under CO changed the color of the f i l t e r e d c r y s t a l s from pale-yellow to light-green.; t h i s c o l o r change was reversed on resubjecting the c r y s t a l s to Ar. Y i e l d 60 mg. found: C 29.46, H 4.74, N 6.92. I r (KBr): 2025 m and 1985, s (VCO); 1850, m (*v c o); 600-1700, br (absorptions due to the (DMA) 2H + c a t i o n ) ; see also F i g III.14.a. 1H nmr 5 T M S ( C D C I 3 ): 2.38 (s, 6H, C H 3 C O - ) , 3.14 (s, 12H, CH3CON(CH3),.6.56 (br, IH, D 20 s e n s i t i v e , (DMA) 2H +). In some other t r i a l s o i l y products were obtained. A discu s s i o n on the c r y s t a l l i n e product i s given l a t e r i n Section III.4, p. 97. II.4.4 Derivatives of dimethylacetamide The N-methyl-N-methylolacetamide compound CH3CON(CH3)CH20H, was prepared by a l i t e r a t u r e method 3 7 as an i n s i t u mixture containing about 30% s t a r t i n g materials N-methylacetamide, CH3C0N(CH3)H (Eastman), and paraformaldehyde, H(CH0H)n0H. The preparation and some properties of CH3C0N(CH3)CH200H and CH3CON(CH3)CHO are described below. Proton nmr data f o r a l l of the derivatives are summarized also i n Table V . l . - 44 -N-Methyl-N-formylacetamide; CH3CON(CH3)CHO Dimethylacetamide (2.0 mL) was heated at 100°C under 0 2 ( 1 atm) f o r 2.5 d. A f t e r pumping o f f most of the DMA, the r e s i d u a l l i q u i d (-0.5 mL) contained about 10% CH3C0N(CH3)CH0 i n DMA, as judged by XH nmr. Further enrichment was not possible because of the loss of the compound with DMA on prolonged pumping. A small quantity of CH 3 C0N(CH 3 )CH0 containing <10% DMA was i s o l a t e d using 4 x 50 (ll portions of the enriched r e a c t i o n mixture on an 0V-17 column i n a Varian Aerograph model 90-P preparative gas chromatograph at 95°C. 1H nmr 6TMs ( C D C I 3 ): 2.40 (s, 3 H , CH 3 C 0 - ) , 3 . 11 (s, 3 H , N-CH 3), 9 . 2 1 (s, IH, N-CHO), see also Table V . l . Mass spec: m/e 101 ( 0 . 1 ) , CH 3 C 0 N(CH 3 ) C H 0 + ; 73 (36), CH 3 C 0 N(CH 3) +; 58 ( 2 1 ) , CH 3 C 0 N H + ; 43 ( 1 0 0 ) , CH 3C0 +. The percentage i n t e n s i t i e s f o r a l l fragments except the parent ion were obtained from a gc/ms of the 90% pure sample, i n order to minimize interference from impurities. The parent ion was not d i s c e r n i b l e i n a pc/ms on a column containing 1% carbowax on Chromasorb WHP at 90°C. N-Methylacetamido-methylhydroperoxide, CH 3 C 0 N(CH 3)CH 2 0 0 H Dimethylacetamide (5.0 mL, 60 mmol) containing a trace of -^BuOOH (10 (ll of 70% aqueous solu t i o n , 0.08 mmol) at 80°C was shaken under 1 atm of 0 2 f o r 24 h. Most of the unreacted DMA was then vacuum trans-f e r r e d i n about 45 min to leave a syrupy l i q u i d (-0.5 mL) . Further pumping f o r about 2 days at room temperature y i e l d e d a small amount of CH 3 C 0 N(CH 3)CH 2 0 0 H (-60 (ll, 80 mg, 0.8 mmol) containing <5% of DMA as - 45 -impurity, as judged by *H nmr data i n CDCI3 (F i g V . l . a ) . Anal. calcd. f o r C 4H 9N0 3: C 40.33, H 7.62, N 11.75; found: C 40.05, H 7.60, H 11.47. I r : 3399, 3231 cm - 1 (s, J>00-H)• 1 6 3 4  cm~ l <s- ^C0>• 7 7 5 a n d 8 2 2 <m' "0-0H>- Ir ( C D C l Q : 3526 (m, J> 0 0_ H) . 1H_jimr 5 T M S (CDCI3) : F i g V . 2 , Table V . l ) . Longer reaction times gave lower y i e l d s of the hydro-peroxide and increasing amounts of CH3C0N(CH3)CH0, as judged by *H nmr. Attempts at p u r i f y i n g an approximately 70% mixture of CH3CON(CH3>-CH2OOH i n DMA by separatory gas chromatography gave a decomposition product i d e n t i f i e d as CH3CON(CH3)CHO by -^H nmr: 5 T M S ( C D C 1 3 ) : 2.4 (s, 3H, CH3CO), 3.1 (s, 3H, N-CH3) and 9.2 (s, IH, N-CHO). Almost quantitative decomposition of the hydroperoxide to the formyl d e r i v a t i v e i s observed under a n a l y t i c a l gas-chromatographic conditions (Section II.5.4). II.4.5 Other materials Proton sponge, (1,8-bis(dimethylamino)naphthalene), supplied by A l d r i c h , was sublimed before use. Triethylamine, 2,6-di-t-butylpyridine and dimethylaniline (MCB Chemical Co.) were used as supplied. Dimethyl-, d i e t h y l - and diphenylsulfides ( A l d r i c h ) , s p e c t r a l grade dimethylsulfoxide (Section II.4.1), diphenylsulfoxide (Eastman), diphenylsulfone and dimethylsulfone ( A l d r i c h ) , were used without further p u r i f i c a t i o n . Care should be taken i n handling dimethyl-s u l f oxide since the compound i s r e a d i l y absorbed by the skin. Cyclooctanone ( A l d r i c h ) , catalase from bovine l i v e r (Sigma Chemicals), chromotropic a c i d (Fisher S c i e n t i f i c Co.), 2 - t h i o b a r b i t u r i c a c i d ( A l d r i c h ) , and 53% HBF 4 i n ether ( A l f a Chemicals) were used as - 46 -supplied. Dimethylacetamide hydrochloride (DMA*HCl) was prepared by passing gaseous HCl through neat DMA and r e c r y s t a l l i z e d from EtOH. *H nmr C"TMS (CDCI3): 2.32, 3H, CH3CO; 3.10, 6H, N(CH 3) 2; ~8 br, -IH (DMA)2H. I r (Nujol): 1900-2700 cm - 1, br, V0H; 1770 cm"1, br <5QH; 1 6 6 0 cm - 1, s, */ c = N. The *H nmr and i r data gave good correspondence with l i t e r a t u r e data. 3** Dimethylacetamide hydrofluoroborate (DMA"HBF4) was prepared by adding 53% HBF 4 i n ether to neat DMA. The p r e c i p i t a t e formed was r e c r y s t a l l i z e d from EtOH. Anal. calcd. f o r C 4H 1 0N0BF 4: C 27.47, H 5.76, N 8.00; found: C 27.36, H 5.50, N 7.89. II.5 Analysis of oxidation products i n DMA solvent This s e c t i o n describes the a n a l y t i c a l methods used to estimate oxidation products of DMA and diphenylsulfide i n DMA solvent. II.5.1 H 2 0 Major l i t e r a t u r e methods used f o r analysis of water include the K a r l Fischer method, 3 9 ,^ u gas chromatography,^* and -.near-infrared spectroscopic analysis .40,42 - j ^ e n e a r i n f r a - r e d method was found to be a very convenient, non-destructive method r e q u i r i n g no i n t e r n a l standards and was convenient f o r the purposes of the present work. Water shows an absorption maximum at around 1900 nm which has been used p r e v i o u s l y ^ 2 f o r analysis of H 20 i n a v a r i e t y of solvents, including - 47 -polar hygroscopic solvents such as methanol or isopropanol. In the present work the same method was used s u c c e s s f u l l y f o r i n DMA solvent. Though not emphasized i n the l i t e r a t u r e reports, handling of the sample under anaerobic conditions was found to be e s s e n t i a l f or the success of the method for hygroscopic solvents l i k e DMA. Dimethylacetamide (5.0 mL) was transferred under Ar to a reaction f l a s k equipped with a quartz c e l l of 1 cm pathlength. The absorbance of the solvent A D (eq. II.1) was measured at 1930 nm against a 1 cm reference c e l l containing DMA under Ar. Quartz glass and DMA solvent (€^930. 0.2 mol"*Lcm"*) absorb i n the n e a r - i r region. Since the two c e l l s used were not p e r f e c t l y matched a net absorbance, p o s i t i v e or negative, can be expected f o r A Q. For the p a r t i c u l a r two c e l l s used, a net p o s i t i v e absorbance was noted ( F i g II. 5 . ) . Known amounts of H2O were syringed into the sealed r e a c t i o n f l a s k v i a the septum cap to obtain the absorbance A (eq II.2) f o r d i f f e r e n t amounts of added H2O (Curves 2-6, F i g II.5.a). The difference (A-Ao) obeyed Beer's law for concentrations upto 0.10 M H2O i n DMA ( F i g II.6). For concentrations l e s s than -0.01M the method i s not too u s e f u l because of low s e n s i t i -v i t y . The e x t i n c t i o n c o e f f i c i e n t of H2O was determined to be 2 mol~*Lcm~*. A Q - A s - A r II.1 (A s and A r are the absorbances of the sample and reference c e l l s , r e s p e c t i v e l y ) . ( Aadded H 20 + A s) - A r II.2 - 48 -1900 nm 2000 Fig. II.5 Absorbance (A) of DMA in a 1 cm c e l l against a reference containing the same: A Q, no added H20; A l f 0.057 M; A o , 0.117 M; A 3, 0.170 M; A 4, 0.222 M; A 5, 0.281 M H20 - 49 -lH20]added. M Fig. II.6 Plot of A-AQ at 1930 nm vs. the concentration of added H2O in DMA in 1 cm c e l l (see Fig. II.5 for details and some spectra) - 50 -A - A o " Aadded H 20 - k [ H2°]added I T-- 3 In an experimental run, the i n i t i a l absorbance of the solvent, with or without added substrate ( A 0 ) , was obtained as described above. The r e a c t i o n f l a s k was opened under Ar to introduce the bucket contain-ing the c a t a l y s t into the side arm (Fi g II.2), and the f l a s k was re-stoppered. A f t e r a gas-uptake experiment (see Section II.2.2), the new absorbance (A t) of the reaction s o l u t i o n was obtained. The amount of H2O formed i n the reaction was estimated by using the c a l i b r a t i o n data (eq I I . 3) f o r the same set of sample and reference c e l l s . II.5.2 Peroxides Organic peroxides (ROOR), hydroperoxides (ROOH) and H2O2 were analyzed together as ' t o t a l peroxides' by a s l i g h t l y modified l i t e r a t u r e method. 4 3 An approximately 2 mM s o l u t i o n of Na2S203 was standardized using a n a l y t i c a l grade KBr03 j u s t p r i o r to the peroxide determination. To 1.0 mL of the reac t i o n s o l u t i o n d i l u t e d with H 20 (20 mL), KI (1.0 g) 4 M H2SO4 (6 mL) and 10% aqueous ammonium molybdate (2-3 drops) were added i n quick succession and the s o l u t i o n l e f t i n the dark, f o r 20 minutes. Blank solutions of DMA (1.0 mL) containing c a t a l y t i c amounts of RhCl3(DMS0)3 were used to make appropriate corrections f o r a e r i a l oxidation, and usually accounted for less than 0.5 mL of 2 mM Na2S203 i n a t y p i c a l t i t r a t i o n r e q u i r i n g 10-20 mL. A l i t e r a t u r e method 4 3 with s l i g h t modifications was used f o r determining organic peroxides i n the presence of H2O2. An a l i q u o t of - 51 -the r e a c t i o n mixture (1.0 mL) was d i l u t e d with H2O (20 mL). A saturated NaHC03 s o l u t i o n (3 mL) was then added, followed by 0.1% aq. catalase (-5 drops) and the s o l u t i o n was allowed to stand f o r about 10 min to remove any ^2^2 (e(l- II-4). Peroxidic analysis on the r e s u l t i n g s o l u t i o n was then c a r r i e d out as described above. The d i f f e r e n c e between the above two peroxide analyses was taken to be the concentration of H2O2 i n a sample. II.5.3 Formaldehyde, formic a c i d and carbon dioxide A c o l o r i m e t r i c method was used for determining HCHO.^ The method employed the development of a chromophore (Xmax, 580 nm) by the reac t i o n between HCHO or (HCH0) n with chronotropic a c i d under a c i d i c conditions. As a minor modification f o r analysis i n DMA solvent, a 0.10 mL al i q u o t of the reac t i o n s o l u t i o n was d i l u t e d to 4.0 mL with H2O and the suggested procedure for 4.0 mL aliquo t of an aquous s o l u t i o n was followed. A reference s o l u t i o n of RhCl3(DMSO)3 c a t a l y s t i n DMA was used as a blank. For the analysis of HCO2H an al i q u o t of the re a c t i o n s o l u t i o n (2.0 mL) was d i l u t e d with H2O (20 mL) and t i t r a t e d with approximately 0.01M NaOH using phenolphthalein i n d i c a t o r . A DMA s o l u t i o n containing RhCl3(DMSO)3 c a t a l y s t was used as a blank. The NaOH s o l u t i o n was standardized using potassium hydrogen phthalate. Other more s e n s i t i v e methods^ given i n the l i t e r a t u r e f o r HCO2H were not su i t a b l e f o r the conditions used i n the present work. For example, i t was not possible to employ a l i t e r a t u r e c o l o r i m e t r i c method - 52 -using t h i o b a r b i t u r i c a c i d reagent 4- 3 0 because of strong interference from the CH3CON(CH3)CHO product formed i n oxidation reactions of i n t e r e s t . A gas chromatographic method using a glass column containing a porapak Q support coated with 3% phosphoric acid 4- 3 1* was thought to be unsuitable f o r analysis of basic DMA solutions. Carbon dioxide was detected q u a l i t a t i v e l y by taking 0.5 mL aliquots from the gas phase of a r e a c t i o n mixture and analyzing by gas chromatography using a 12' column packed with Porapak Q support at 30°C with the He c a r r i e r gas d e l i v e r e d at 30 p s i (Retention time, 10.5 min). II.5.4 Other organic products (a) Diphenylsulfide. diphenylsulfoxide and diphenvlsulfone The r e a c t i o n solvent DMA was pumped o f f under vacuum and the r e s i d u e J with added DMSO2 as the i n t e r n a l standard, was d i s s o l v e d i n CDCI3, and the products were i d e n t i f i e d and q u a n t i f i e d by t h e i r c h a r a c t e r i s t i c chemical s h i f t s and integrations of the para protons i n a 270 MHz or 400 MHz -^H nmr spectrum. 5 T M S ( C D C 1 3 ) f o r DPS: 7.33, m, 10 H; 6 T M S(CDCl3) for DPS0: 7.45, m, 6H, ortho and meta to -SO; 7.66, m, 2H, para to -SO. 5 X M S ( c d c 1 3 ) f o r DPS0 2: 7.50, m, 4H, ortho; 7.56, m, 4H, meta; 7.90, m, 2H, para. - 53 -(b) N-methyl-N-formylacetamide and N-methvl-acetamido-methyl- hydroperoxide A known amount of mesitylene was added as the i n t e r n a l standard to a rea c t i o n s o l u t i o n to obtain a gas chromatogram (1% carbowax on chromosorb WHP at 80°C f o r 1 min, increased to 200°C at 10°C/min). Flow rate of He, 25 mL min"*; retention times, min: mesitylene 1.4, DMA 3.2, CH3CON(CH3)CHO and CH3CON(CH3)CH2OOH 4.8, CH 3CON(CH 3)H 5.1). The quantitative decomposition of the hydroperoxide to the formyl under the above gas chromatographic conditions was determined independently using an authentic sample of the former. (c) N-methvlacetamide. N-methyl-N-methvlolacetamide. dimethyl- sulfone and other organics A column containing 5% carbowax on chromosorb WHP at 100°C with He c a r r i e r gas deli v e r e d at 20 p s i was used f o r detection by gas chromato-graphy. Retention times: H 20; -2 min, DMA, -4 min; cyclooctanone, -8 min; DMSO, 10 min; CH3CON(CH3)CHO, 11 min; CH 3CON(CH 3)H and CH 3C0N(CH 3)CH 20H: 15 min; DMS02, 45 min. Cyclooctanone was used as an i n t e r n a l standard. Dimethylsulfone can be also detected by *H nmr; 5 T M S ( C D C 1 3 ) : 3.0. - 54 -CHAPTER I I I SYNTHESIS AND CHARACTERIZATION OF DMSO COMPLEXES OF Rh AND THEIR REACTIONS WITH H 2 AND CO 5 5 I I I Synthesis and ch a r a c t e r i z a t i o n of DMSO complexes of R h 1 1 1 and t h e i r reactions with H2 and CO. I I I . l Some general properties of metal sulfoxides The chemistry of s u l f o x i d e s 4 " and t h e i r m e t a l - c o m p l e x e s 4 ' i s well reviewed. A free sulfoxide, f o r example, DMSO, has a pseudo-pyramidal structure with the s u l f u r atom at the apex, i n the gas phase or i n the s o l i d state. The molecular structure of DMSO i s represented by two canonical forms I and I I , based on b a s i c i t y , dipole-moment, and the evidence f o r a net p o s i t i v e charge on the s u l f u r atom. 4**' 4 8 The bonding between the s u l f u r and the oxygen i s represented as invo l v i n g a <J i n t e r a c t i o n and a d7T-p7r i n t e r a c t i o n . \ \ + -S = 0 < > s — 0 / / I I I Because of the p o l a r i t y of the bond i n the d i r e c t i o n of s u l f u r to oxygen, coordination to metal i s expected to occur v i a the oxygen atom, and t h i s mode of bonding i s i n f a c t the most commonly observed. Coordination through the s u l f u r atom, generally found i n Pt metal sulfoxides, i s harder to envisage. Some workers have included a t h i r d cannonical form, I I I , to explain bonding through the s u l f u r atom. 4 9 - 56 -III In h i s d e t a i l e d review on metal sulfoxides, Davies argues against such a formulation, based l a r g e l y on evidence f o r an increase i n the net posi-t i v e charge on the s u l f u r atom on S-coordination. Instead, a general explanation based on 'soft' centres p r e f e r r i n g 'soft' donor atoms^ 1 i s offered. The occurrence of both modes of bonding i n Pt metal complexes i s explained on the basis of the usual balance of s t e r i c and e l e c t r o n i c e f f e c t s . Coordination through the s u l f u r atom (DMSO) or the oxygen atom (DMSO) can be r e a d i l y distinguished by i r and nmr spectroscopy. 3 3 *34,48 Coordination through s u l f u r i s expected to increase the oxygen to s u l f u r bond order. For example, i n the i r spectra of rhodium complexes of DMSO, t h i s i s r e f l e c t e d by an increase i n V$Q by about 30-100 cm"* from the 1055 cm"1 value found f o r free DMSO. The PcH3 m o d e s o f f r e e DMSO, found at 918(w), 945 (m), and 1008 (s) cm"1; also s h i f t to higher frequency on coordination and may overlap with the J>gQ region. Because of t h i s problem, the assignments f o r V§Q a r e often checked by comparison of t h e i r i r with those of the DMSO-dg analogues of the complexes. The Pc\l m o a e s show is o t o p i e s h i f t s on coordination, while the V$Q peaks remain unchanged. Coordination through oxygen decreases the p7T-dX bonding and r e s u l t s i n a decrease i n Vso f r o m t n e free value; f o r example, the VC^Q peaks of rhodium complexes of DMSO generally occur i n the range 920-1000 cm"1. - 57 -Proton nmr spectra of metal sulfoxides are e s p e c i a l l y u s e f u l i n s t r u c t u r a l studies of these complexes. The methyl protons of both DMS.0 and DMSO are found at a lower f i e l d than the free molecule chemical s h i f t at 5 2.6. Because of t h e i r proximity to the metal centre, the methyls on S-bonded sulfoxides are more deshielded than t h e i r 0-bonded analogues; f o r example the methyl protons of DMS.0 and DMSO on Rh*** generally appear at 5 3.1-3.7 and 5 2.7-3.0, r e s p e c t i v e l y . 3 3 , 3 4 I I I . 2 Synthesis and ch a r a c t e r i z a t i o n of some RhXJ-L(DMS0) complexes The synthesis of a v a r i e t y of neutral, c a t i o n i c or anionic Rh***(DMS0) i s documented. 3 3' 3 4 A major part of the work i n t h i s thesis involved the RhCl3(DMS0)3 (1) complex. Some relevant d e t a i l s of i t s synthesis and ch a r a c t e r i z a t i o n and the preparation of two r e l a t e d complexes, mer-cis-RhCl 3(DMS0)2(H20) and mer-cis-RhCl3(DMSO)2(DMS) are described i n t h i s section. Two other complexes RhCl3(DMS0)2("CH2CH= + N E t 2 ) and [NEt 2H 2] [RhCl^DMSO) 2] are described i n Sections IV.2 and IV. 6, r e s p e c t i v e l y . III.2.1 RhCl 3(DMS0) 3 (1) The t i t l e complex was i s o l a t e d i n about 70% y i e l d by react i n g RhCl 3-3H 20 (1.0 g, 3.8 mmol) i n DMSO (5 mL) at 70°C f o r ~4 h. The product was p r e c i p i t a t e d by adding dry EtOH, and r e c r y s t a l l i z e d from CH2Cl2/ether as a yellow-range c r y s t a l l i n e product. The i r data i n - 58 -Nujol (1145 cm"1, s, V^Q; 935, s, VSQ\ 1032 and 980, s, p C H ) and the  LU nmr data i n CDCI3 (§ 3.625, 6H, DMSO; 3.443, 6H, DMSO; 2.871, 6H, DMSO) correspond w e l l with l i t e r a t u r e values f o r the mer-cis - R h C l 3 ( D M S 0 ) 2 -(DMSO) isomer (IA).33,34 ^ e ^'rimr spectrum ( F i g . I I I . l ) shows several other DMSO and DMSO peaks corresponding to - 2 0 % of the t o t a l DMSO and DMSO. Attempts to obtain a purer sample of the mer-cis isomer by repeated r e c r y s t a l l i z a t i o n s with CF^C^/ether gave a highest p u r i t y of -90% f o r a t o t a l y i e l d of <50%. Barnes et a l . had obtained the mer-cis isomer i n about 95% p u r i t y f o r an o v e r a l l y i e l d of 35%, using d i f f e r e n t r e a c t i o n c o n d i t i o n s . 3 4 For the purposes of the present work where R h C l 3 ( D M S 0 ) 3 was expected to be used mainly as a synthetic precursor, a higher o v e r a l l y i e l d was considered more important than a r e l a t i v e l y smaller increase i n present p u r i t y , and the product containing -80% of the mer-cis isomer was employed. I d e n t i f i c a t i o n of the minor isomers was c a r r i e d out by s o l u t i o n *H nmr spectroscopy. Barnes et a l . have done a d e t a i l e d study on the isomeric d i s t r i b u t i o n of a se r i e s of [ R h C l n ( D M S 0 ) g . n ] 3 " n complexes. 3 4 The chemical s h i f t s of overlapping peaks due to minor isomers of R h C l 3 ( D M S 0 ) 3 were deduced i n d i r e c t l y by 1H-{ 1 0 3Rh} INDOR measurements and were assigned to various geometrical and valence isomers. The higher r e s o l u t i o n of the 400 MHz *H nmr spectrometer used i n the present work enabled us to obtain the proton features d i r e c t l y . Of the extreme-l y small peaks i n a *H nmr spectrum of 1 i n CDCI3 ( F i g . I I I . l ) , the apparent doublet at § 2.439 and two other peaks at § 3.645 and 3.560 (marked with a s t e r i s k s i n the Figure) are assigned to mer-cis - R h C l 3 -(DMS0)2(DMS) by comparison with data f o r the authentic compound (Section I I I . 2 . 3 ) . Four r e a d i l y d i s c e r n i b l e minor peaks are found at §3.525, - 59 -Fig. I I I . l 400 MHz XH nmr spectrum of RhCl3(DMSO)3 in CDC1 - 60 -3.501, 3.496 and 2.780. In the presence of excess DMSO the peaks at <5 3.525 and 2.780 disappear, while the other two appear unchanged i n in t e n s i t y . The two peaks at 5 3.501 and 3.491 which occur i n an approximate 2:1 r a t i o , are assigned to mer-RhCl3(DMSO3) (IB) to account fo r three DMSO ligands and the inequivalence of two of them. The two peaks at 5 3.525 and 2.780, which occur also i n an approximately 2:1 r a t i o are assigned to mer-trans-RhCl3(DMS.0)2(DMSO) (IC) to account for two equivalent DMSO ligands and a singl e DMSO. The alternate fac geometry i s discounted on the grounds that the p a r t i c u l a r geometry i s not found with RhCl3L.3 complexes (L = DMS35, and DMSO34) reported i n the l i t e r a t u r e . The assignments are summarized i n Table I I I . l along with those deduced by Barnes et a l . 3 4 using a sample of RhCl 3(DMSO)3 i n CH2CI2. Discrepancies i n the r e l a t i v e chemical s h i f t s of the DMSO ligands of the minor isomers could be due to the differences i n the solvent e f f e c t s of the two systems. III.2.2 Mer-cis-RhCl 3(DMSO) 2(H 20) The t i t l e complex was i s o l a t e d i n an attempt to repeat another l i t e r a t u r e method for the synthesis of RhCl3(DMSO)2(DMSO) under ambient conditions. According to the l i t e r a t u r e , a s o l u t i o n containing RhCl3 -3H20 and three equivalents of DMSO i n isopropanol solvent contain-ing 20% H 20 changed color from red to yellow within an hour of reaction, and deposited orange c r y s t a l s on leaving overnight. Some departures from these general observations were noted i n that i n i t i a l brown or yellow suspensions were sometimes obtained. In the present work, a - 61 -Table I I I . l 400 MHz XH nmr data f o r RhCl 3(DMSO) 3 (1) i n CDC13 RhCl 3(DMSO) 3 i n CDC1 3 RhCl 3(DMSO) 3 i n C H 2 C l 2 ( a * Assignment 3.625 3.551 3.443 (~80%)<b) 3.363 (~95%)(b> 2.871 2.811 Cl S . C l C l IA, mer-cis-RhCl 3(DMSO) 2(DMSO) 3.501 3.496 } (-13%)<b> 3.452 3.445 C l S . C l s > r - s C l IB, mer-RhCl 3(DMS0) 3 3.525 2.780 } (~6%)<b> 3.450 2.722 C l C l 1C, mer-trans-RhCl 3(DMSO) 2(DMSO) Other (-1%) - RhCl 3(DMS0) 2(DMS) and other species ( a) L i t e r a t u r e values f o r a sample prepared by a d i f f e r e n t method ( b) As a percent of t o t a l coordinated DMSO. - 62 -yellow suspension r e s u l t e d from the i n i t i a l l y c l e a r s o l u t i o n , w i t h i n one to s i x hours of reaction, i n a l l three experiments attempted. However, i n two of these, the f i n a l product obtained a f t e r s t i r r i n g the i n i t i a l yellow suspension overnight, was contaminated with metal. In the t h i r d experiment the i n i t i a l yellow product was f i l t e r e d , washed with i s o -propanol and c o l d acetone, and dried. A *H nmr spectrum of the sample i n acetone-dg showed two peaks of equal i n t e n s i t y f o r sulfur-coordinated DMSO (Section II.4.3), while the i r spectrum showed a c h a r a c t e r i s t i c V^Q peak at 1140 cm"*. The i r peaks at 1060 cm"*, s, and 980 cm"*, w, were assigned to PQE^ a s v m m e t r * c a n d symmetric modes, r e s p e c t i v e l y , by comparison with the data f o r the RhCl3(DMSO)2(DMSO) complex (Section III.2.1) which have been confirmed by deuteration s t u d i e s . 3 3 Evidence for. the coordinated H 20 was seen i n the i r spectrum (Section II.4.3). Isopropanol or ethanol i s widely used as a solvent f o r substitu-t i o n reactions of Rh*** complexes, 5 3 which are generally rather slow ( p a r t i c u l a r l y those of c a t i o n i c and neutral complexes), because of t h e i r d*> e l e c t r o n i c c o n f i g u r a t i o n . 5 4 The a b i l i t y of reducing alcohols to catalyze these reactions has been known f o r a long t i m e . 5 5 The r o l e of the solvent i s to generate a c a t a l y t i c amount of Rh(I) (eq. I I I . l ) , which then catalyzes the s u b s t i t u t i o n process. 5^ The formation of metal by-product i s almost c e r t a i n l y due to the decomposition of the intermediate Rh(I) species (eq. I I I . 2 ) . 5 7 a Rh*** + RjR^HOH > Rh* + 2H + + RjR^CO I I I . l 2Rh* > Rh** + Rh° I I I . 2 III.2.3 Mer-cis-RhCl 3(DMSO) 2(DMS) Mer-cis-RhCl3(DMSO)2(DMS) was i s o l a t e d i n -70% y i e l d by adding an equivalent of DMS to a CH 2C1 2 s o l u t i o n of RhCl3(DMSO)2(DMSO) (Section II.4.3). The doublet at d 2.44 (J - 1 Hz) i s assigned to DMS coordina-ted to Rh***; the occurrence of doublets i n the same region, with * 0 3 R h ( I — 1/2) coupling constants of -1 Hz, are reported f o r the mer-RhCl3(DMS)3 complex.3-* An i r spectrum of the mixed DMSO/DMS complex shows two symmetric peaks of medium i n t e n s i t y at 980 and 1035 cm"*, assigned to rfjjj^ modes of coordinated DMS.3-* Dimethylsulfoxide ligands also show two c h a r a c t e r i s t i c peaks centred at —1000 cm"* assigned to PQ^ modes, J but these appear as two non equivalent peaks f o r complexes containing only s u l f u r coordinated DMSO (e.g. i r spectrum of RI1CI3-(DMS0) 2("CH 2CH=N +Et 2) i n F i g . IV.7). In the presence of oxygen-coordinated DMSO, e.g. i n RI1CI3(DMSO)2(DMSO) , the PQR^ modes appear as two approximately equivalent peaks centred at -1000 cm"*. 3 3 Therefore i n mixed DMSO/DMS complexes of Rh***, two approximately symmetric modes centred -1000 cm"* may be taken as evidence f o r the presence of DMS ligands, provided that no DMSO i s present i n the complex. III.3 Reactions of Rh 1 1 1(DMS0) complexes with H 2 The reactions of Rh***(DMS0) complexes with H 2 were studied f o r the purpose of generating Rh***(H) species (Section 1.4). The ad d i t i o n of strong bases i s us u a l l y required f o r the a c t i v a t i o n of H 2 by Rh(III) complexes, -*7b but the discovery of a reac t i o n between RhCl3(DMS0)3 and - 64 -strongly basic t e r t i a r y amines l i k e P.S. and NEt 3 (Section IV), pre-cluded the use of such bases. Weaker bases such as 2,6-di-t-butyl-pyridine or 2,6-dimethylpyridine d i d not f a c i l i t a t e a re a c t i o n between RhCl3(DMSO)3 and H 2 i n C2H4CI2. Dimethylacetamide (DMA) has been used previously with success, as a basic medium f o r the c a t a l y t i c hydrogena-t i o n of o l e f i n s by RhCl3(DES)3, where the c a t a l y t i c Rh* species was derived from the i n i t i a l H2 reac t i o n of the Rh*** precursor (eq. I I I . 3 ) . 5 7 b -H+ -H+ Rh*** + H 2 v Rh***(H) v % Rh* I I I . 3 +H+ +H+ Hydridoruthenium (II) complexes have been i s o l a t e d by reactions of Ru(II)/PPh3 complexes with H 2 i n DMA.58 The reac t i o n between Rh***(DMSO) complexes and H2 i n DMA was thus investigated i n the present work to see i f Rh***(H) species were formed. Although i t was not possible to i s o l a t e or detect Rh***(H) species using Rh***(DMSO) pre-cursors, the reac t i o n between RhCl3(DMSO)3 (1) and H2 i n DMA was investigated i n d e t a i l , because of the discovery of an i n t e r e s t i n g c a t a l y t i c autoxidation of DMA by that complex i n the presence of H2 (Chapter V). This s e c t i o n summarizes mainly the work on complex (1). III.3.1 RhCl 3(DMSO) 3 (1) i n DMA James and Morris reported the f a c i l e s u b s t i t u t i o n of the DMSO - 65 -ligand i n the major mer-cis isomer i n CDCI3 by various ligands (eq. I I I . 4 ) ; 3 3 mer-cis-RhCl 3(DMSO) 2(DMSO) + L v * RhCl 3(DMSO) 2L + DMSO III.4 (e.g., L - DMF, 0PPMe2, pyridine-N-oxide, DMA) The s u b s t i t u t i o n reactions were monitored by *H nmr. Dimethylacetamide was found to be a weaker donor (K eq = 0.01^0.005) than, f o r example, DMF (K eq = 0.8±0.4). In the present work, because of the u n a v a i l a b i l i t y of DMA-do, i t was not possible to observe the equilibrium i n neat DMA by *H nmr. A multiple solvent suppression *H nmr of a -0.02 M s o l u t i o n of (1) i n DMA/toluene-dg (-1:1) shows two sets of major peaks at 53.42 and 3.62, and 6 3.46 and 3.67 and other smaller peaks ( F i g . I I I . 2 ) . The pos i t i o n s of the major peaks correspond with those assigned f o r unreact-ed IA and RhCl 3(DMSO) 2(DMA) i n CDC13 (5 3.44 and 3.62, and 3.49 and 3.67, r e s p e c t i v e l y ) by James and M o r r i s . 3 3 However, the absence or the presence of peaks due to coordinated DMA or free DMSO could not be ascertained because of the t o t a l suppression of peaks l y i n g near or between the i r r a d i a t e d solvent peaks ( 5 2.1, CH3C0; 2.9 and 3.1, N(CH 3) 2; f o r example, a multiple solvent suppression *H nmr spectrum of a 0.06 M s o l u t i o n of DMSO i n DMA/toluene-d g (-1:1) d i d not show the expected peak at 5-2.6 for DMSO. A s o l u t i o n of complex 1 i n DMA does not show any conductance. The v i s i b l e spectrum of 1 i n DMA i s inva r i a n t with time f o r at l e a s t 1 h and gives an e x t i n c t i o n c o e f f i c i e n t of 360 mol L"* cm"* at X m a x 435 nm. Species containing coordinated DMA could not be i s o l a t e d from the 66 -- 67 -so l u t i o n . A d d i t i o n of ether to a s o l u t i o n of 1 i n DMA or pumping o f f the solvent y i e l d e d RhCl3<DMSO)3 species with or without adsorbed DMA ( I r : 1630 cm - 1, Vco; *H nmr 5(CDCI3): 2.1, s, 3H, and 3.0, d, 6H). The amount of the mer-cis isomer i n the i s o l a t e d product was -50%, compared to the 80% i n the s t a r t i n g complex. By using the data i n the l i t e r a t u r e 3 3 f o r K eq i n CDCI3 (eq. III. 4 ) , and the assumption that the solvent e f f e c t s on K eq are minimal, a 1 x 10" 2 M s o l u t i o n of mer-cis-RhCl 3(DMS0)2(DMS0) i n DMA i s c a l c u l a t e d to contain about 90% of mer-cis-RhCl 3(DMSO)2(DMA). The i n a b i l i t y to i s o l a t e any DMA-substituted Rh***(DMSO) from solutions of complex 1 i n DMA presumably r e s u l t s from a s h i f t of eq. III.4 (L = DMA) to the l e f t on removing or d i l u t i n g the DMA solvent. The absence of any detectable v i s i b l e s p e c t r a l changes suggests that the l i g a n d s u b s t i t u t i o n i s a ra p i d reaction. III.3.2 Reaction of RhCl 3(DMS0) 3 (1) i n DMA with H 2 A 1.0 x 10"^ M s o l u t i o n of 1 i n DMA takes up H 2 under ambient conditions. Increasing the temperature to 50°C gave conveniently measurable rates, and at 50°C about 0.8 equivalents of H2 per rhodium were taken up i n the f i r s t 25 min (Fig. I I I . 3). The uptake data beyond t h i s point were not reproducible; a sudden increase i n the rate was observed i n some t r i a l s . Leaving the s o l u t i o n under H2 f o r longer periods of time l e d to the formation of Rh metal. The i r r e p r o d u c i b i l i t y of uptake data i n the l a t t e r part of the re a c t i o n r e s u l t s from metal p r e c i p i t a t i o n . V i s i b l e s p e c t r a l changes for the r e a c t i o n under s i m i l a r Time, s F i g . I I I . 3 H 2 uptake pl o t f o r 1.0 x 10" z M s o l u t i o n of RhCl 3(DMS0) 3 i n DMA at 50°C under 1 atm H 2 - 69 -conditions gave non-isosbestic behaviour ( Fig. III.4, inset I ) . A sudden increase i n the i n t e n s i t y of the v i s i b l e absorption was a d e f i n i t e i n d i c a t o r of metal formation i n these reactions. Because of problems of metal p r e c i p i t a t i o n on prolonged exposure of the s o l u t i o n to H 2, the f i n a l r e a c t i o n stoichiometry could not be ascertained d i r e c t l y . However, metal p r e c i p i t a t i o n was avoided by removing H 2 from the system a f t e r 25 min of reaction, or a f t e r about 0.8 equivalents of uptake, by freeze-thaw degassing the s o l u t i o n three times. The v i s i b l e absorption spectrum a f t e r degassing (Curve C, F i g . III.4) was e s s e n t i a l l y unchanged from that under H 2 (Curve B, F i g . III. 4 ) . The i r r e v e r s i b i l i t y of the H 2 r e a c t i o n was shown by the absence of any further H 2 uptake by the degassed product s o l u t i o n i n about the f i r s t 500 seconds of exposure (c f . F i g . III.3); leaving the s o l u t i o n under H 2 any longer l e d to metal p r e c i p i t a t i o n . (In further discussion, the various r e a c t i o n solutions w i l l be r e f e r r e d to as A or B ....or E, depending on the designation given to the v i s i b l e spectrum of each i n F i g . I I I . 4 ) . Solution C was stable under Ar for at l e a s t 1-2 h, but decomposed slowly over a period of several days to give small amounts of p r e c i p i -tated metal and an a i r - s t a b l e s o l u t i o n F (spectrum F, F i g . III. 4 ) . A *H nmr spectrum of the residue from F, dissolved i n CDCI3, showed some Rh***(DMS) (52.2-2.5, Section III.2.3) which accounted f or about 15-20% of a l l the DMSO and DMS i n the residue. Attempts to i s o l a t e pure Rh species from s o l u t i o n C gave a i r -stable products containing Rh***(DMS). In a t y p i c a l workup procedure, the DMA solvent was pumped o f f to y i e l d a red o i l y r e a c t i o n residue which was r e c r y s t a l l i z e d from CH 2Cl 2/ether or acetone/ether, a l l under Ar. The r e s u l t i n g yellow-orange product appeared a i r - s t a b l e , and showed o W a v e l e n g t h , n m n m F i g . III.4 V i s i b l e spectra of i n s i t u Rh 1 i n DMA obtained from the r e a c t i o n between 3.0xl0 - 3 M RhCl 3(DMS0) 3 i n DMA and 0.8 equivalents of H 2 at 50°C under various conditions (A gives the spectrum " of RhCl 3(DMS0) 3 i n DMA): B, under H 2 ( i n s e t I, changes from A to B taken at int e r v a l s of 100 s ) ; C, under Ar or vacuum (remains unchanged for at l e a s t 1 h) (cont'd, on next page) Fig. I I I . 4 (cont'd) D, within 50-100 s a f t e r introducing 0 2 to C at 1 atm; E, a f t e r leaving D at 50°C under 0 2 f o r ~25 min (inset I I , changes from D to E at r . t . ) ; F, s o l u t i o n C a f t e r several days - 72 -peaks c h a r a c t e r i s t i c of RhII:[/DMSO ( §3.4-3.6), RhIi:i/DMSO (52.8), RhII]:/DMS (52.2-2.4), and free DMA (5 2.1, 2.95, 3.05) i n CDC13 (Fig. II I . 5a); the RhI'II/DMS accounted f or -15% of the t o t a l DMSO and DMS. An i r spectrum of the yellow p r e c i p i t a t e i n Nujol showed bands due to DMSO (1130 cm"1) and DMSO (930 cm" 1); the i r data were not useful f o r ch a r a c t e r i z i n g coordinated DMS because of interference from the oxygen-coordinated DMSO (Section I I I . 2.3). (a) In s i t u c h a r a c t e r i z a t i o n of products The product solutions d i d not show any high f i e l d signals i n the 5 0 to -50 region i n a 1H nmr analysis, using e i t h e r a 60 MHz CW or a 80 MHz FT spectrometer. For the l a t t e r , the reactions with H 2 were c a r r i e d out i n a 4:1 mixture of DMA and toluene-dg mixture, and a multiple solvent suppression technique was used to reduce the i n t e n s i t y of DMA peaks; a s i m i l a r technique was used by Dekleva^ 9 to observe Ru 1 1H(Cl) species i n DMA solvent. No information about the fate of the DMSO ligands a f t e r the H 2 r e a c t i o n could be obtained from the •'-H nmr, because of the i n t e n s i t y of solvent peaks i n the.60 MHz CW experiment, and the suppression of the peaks neighbouring the DMA peaks i n the multiple solvent suppression experiment (Section III.3.1). The DMA solvent a v a i l s two windows i n the regions -1800-2900 cm"1 and -750-900 cm"1 for i r a n a l y s i s . No s i g n i f i c a n t peaks were found i n eit h e r region. The v i s i b l e spectrum of the H 2-product s o l u t i o n under vacuum (curve C, F i g . III.4) changed r a p i d l y on exposure to 1 atm of 0 2 at r . t . or 50°C (curve D). To obtain more quantitative data, the amount of 0 2 73 -Fi g . III.5 80 MHz 1H nmr spectrum of: (a) the product i s o l a t e d from s o l u t i o n B i n F i g . III.4, i n CDC13 under Ar; (b) residue from s o l u t i o n E i n F i g . III.4, i n CDCI3 (x, solvent impurity) - 74 -taken up by degassed product solutions was measured as described i n Section II.2.4; i n a l l the experiments, the i n i t i a l H 2 r e a c t i o n was stopped a f t e r a H2/Rh uptake r a t i o of -0.8 i n order to avoid complica-tions due to metal p r e c i p i t a t i o n on prolonged exposure to H 2. The amount of 0 2 taken up per equivalent of H 2 used gave values of 0.70, 0.75, 0.75 for three d i f f e r e n t experiments. A l l 0 2 uptakes were complete within 100-200 s of exposure, and there was no further uptake f o r at le a s t another hour at 50°C. The absorbance of the i n i t i a l oxygenated species D decreased continuously under Ar or under 0 2 to give a stable spectrum (curve E, F i g . III.4) within - 20 min. At room temperature, the change from D to E took a few hours but there was a rapid decrease i n about the f i r s t 0.5 h. The i r spectrum of so l u t i o n D, taken within a few minutes a f t e r the exposure to 0 2, showed a weak, broad hump -850 cm"*, which was p a r t l y masked by another broad band at -930 cm"* (F i g . III.5.1). S t r i p p i n g o f f solvent DMA from s o l u t i o n E y i e l d e d a yellow, o i l y residue. A gas chromatogram of the stripped solvent (Section II.5.4.c) showed the presence of -0.2 equivalents of CH3C0N(CH3)CH0 per rhodium and some unquantified amount of H 20. A *H nmr of a CDCI3 s o l u t i o n of the r e a c t i o n residue ( Fig. III.5.b) shows free DMA ( 5 2.1, 2.95 and 3.03), DMSO (53.4-3.6), DMSO (§2.85 and 2.75) and DMS (52.2-2.5). The D 2 0 - s e n s i t i v e broad peak around 5 6.7 roughly accounts f o r -0.5-1.0 equivalents of protons for 3 equivalents of t o t a l DMSO, DMSO and DMS i n sol u t i o n . The broad peak was not observable i n some other t r i a l s . An i r spectrum of the residue showed peaks due to free DMA (1630 cm"*, s, *> c o), DMSO (1140 cm"1, s, V^0) and DMSO (930 cm"*, s, PSO) . Rhodium(III) complexes generally take up one equivalent of H 2 to give Rh* species, most l i k e l y v i a Rh***(H) intermediates (eq. I I I . 3 ) . 5 7 b - 75 -900 800 700 Wavenumbers , cm"4 600 76o Wavenumbers, cm"1 600 Fig. III.5.1 F . T . i r spectra of: (a) telO' 2 M RhCl3(DMSO)3 in DMA, (b) species D (see Fig. III.4) within a few minutes after i t s formation (solution in (a) was used to generate D; path length of KBr windows used, 0.5 mm) - 76 -Both Rh* and Rh***(H) species may be O2-sensitive; the s e n s i t i v i t y of Rh(I) complexes to 0 2 i s a noted feature i n t h e i r chemistry (see l a t e r ) , while the reactions of Rh***(H) with 0 2 to given Rh***(OOH) are documented a l s o . 2 ^ The 0 2 - s e n s i t i v i t y of the product s o l u t i o n and the absence of any evidence for Rh***(H) species -- i . e , h i g h - f i e l d *H nmr s i g n a l s , or i r bands at -2000 cm"* 60 indicate the product to be a Rh* species. Rhodium(I) complexes are known to form a v a r i e t y of Rh*-0 2 adducts mostly of the peroxo type (Section 1.2). The i r band due to the 0-0 s t r e t c h , occurring at 750-900 cm"*, i s the most us e f u l method av a i l a b l e f o r i n s i t u analysis of these adducts.* 0 Reaction of the [RhCl(COE) 2] 2 dimer i n DMA, containing an excess of L i C l , with 0 2 at 25°C i s reported to form a Rh*-02 adduct i n s i t u , which shows a band at 895 cm"* assignable to an 0-0 s t r e t c h . 6 * The broad band at -850 cm"* observed i n the present case l i k e l y a r ises also from an 0-0 s t r e t c h of a Rh*-0 2 adduct. The i r spectrum of the s t a r t i n g material 1 i n DMA shows a band at 930 cm"* (a, F i g . III.5.1) assigned to V^Q of DMSO (Section III.2.1).In the spectrum of s o l u t i o n D (b, F i g . III.5.1) the 930 cm"* band i s broader. T n e peak at 698 cm"* i n spectrum (a), a t t r i b u t a b l e to V Q - S ^ < * s reduced i n i n t e n s i t y and i s accompanied by the appearance of two new bands at 678 and 668 cm"* (spectrum b). The reasons f o r these changes are not c l e a r ; further discussion on the postulated Rh*-02 species i s given i n Section V.3.2. The amount of 0 2 taken up by the H 2 product s o l u t i o n of complex 1 ( s o l u t i o n B, F i g . III.4) i s less than that expected f o r a p r i o r t o t a l reduction of 1 to a Rh* species. I f a l l of complex 1 reduces to a Rh(I) species, and the Rh* reacts with 0 2 i n a 1:1 r a t i o (eq. III . 5 ) , the - 77 -O2/H2 r a t i o should be equal to 1; but however, the observed O2/H2 r a t i o averages only -0.75. H 2 0 2 RhCl 3(DMSO) 3 > [Rh I/2H +/3Cl"/3 DMSO] > [Rh I-0 2/2H +/3Cl"/3 DMSO] III.5 The low value very l i k e l y r e s u l t s from a si d e - r e a c t i o n where H2 i s consumed i n a reduction of DMSO to DMS. A Rh***-catalyzed reduction of DMSO to DMS by H 2 i s documented (eq. I I I . 6 ) . 2 3 Rh*** DMSO + H 2 > DMS + H 20 I I I . 6 A *H nmr of the residue from s o l u t i o n E (F i g . III.5b) indeed shows some Rh***(DMS) ( 5(CDC1 3): 2.2-2.5), accounting f o r about 5-10% of the t o t a l DMSO and DMS i n the residue. The Rh***(DMS) observed almost c e r t a i n l y r e s u l t s from the H2 reaction of A to B, because the 0 2 r e a c t i o n of B to give D and then E cannot p o s s i b l y lead to a reduction of DMSO. A t o t a l r e a c t i o n stoichiometry as i n eq. III.7 accommodates both the O2/H2 r a t i o of 0.75, and the reduction of some DMSO to DMS. RhCl 3(DMSO) 3 + 0.8 H 2 > 0.6 [Rh*/2H +/3Cl"/3 DMSO] + 0.2 [Rh***/3C1V2DMS0/DMS] + 0.2 [unreacted RhCl 3(DMS0) 3] +0.2 H 20 III.7 - 78 -According to eq. III.7, 0.6 equivalents of t o t a l H 2 used i s spent on the reduction of Rh*** to Rh* (eq. III.5), and 0.2 equivalents on the reduc-t i o n of DMSO (eq. III. 6 ) . In the 0 2 r e a c t i o n only the Rh* w i l l form a Rh*-0 2 adduct. Therefore, the corresponding 0 2/H 2 r a t i o w i l l be only 0.75. S i m i l a r l y , the expected y i e l d of Rh***(DMS) i s c a l c u l a t e d as -7% of the t o t a l DMSO/DMS (eq. III.7). These assumptions allow f o r good correspondence with the experimental data. Further evidence f o r an i n s i t u Rh* product that reacts with 0 2 i n a 1:1 stoichiometry i s provided by some comparative studies with a model system using the l a b i l e [RhCl(C0E) 2] 2 dimer^ 2 as the Rh* precursor. Addi t i o n of the dimer to DMA solutions containing 2 equivalents of DMA'HCl and 3 equivalents of DMSO per equivalent of Rh gives a system s t o i c h i o m e t r i c a l l y equivalent to (Rh*/2H+/3C1"/3DMS0] (cf. eq. III.5). Under 0 2, the model system r a p i d l y absorbs one equivalent of gas per rhodium at 50°C (Fig. III.6); the longer r e a c t i o n time of -2000-3000s taken by the more concentrated s o l u t i o n corresponds with the time taken by the complex to dissolve. Both reaction solutions show a slower c a t a l y t i c uptake, following the i n i t i a l r a p i d reaction; the c a t a l y t i c uptake was not monitored f o r s o l u t i o n (a). E s s e n t i a l l y the same r e a c t i v i t y pattern was observed when 2 equivalents of L i C l were used i n place of DMA'HCl (see Section V.3.2 for further discussion). The v i s i b l e spectrum of the model system under Ar (curve C D, Fig. III.7) changes slowly with time over 24 h to give a stable spectrum (curve C). The e q u i l i b r a t e d s o l u t i o n r a p i d l y absorbs approximately one equivalent of 0 2 per Rh to give a sp e c t r a l change (C — > D i n Fi g . I I I . 7) s i m i l a r to that of the H 2 product s o l u t i o n of 1 (C — > D i n Fig. III.4) . The spectrum of the oxygenated model system changes at r . t . , 5000 ~ 10,000 Time, s F i g . III.6 0 2 uptake plots f o r [RhCl(C0E) 2] 2 dimer i n DMA containing 2 equivalents of HCl and 3 equivalents of DMSO per rhodium at 50°C at Rh concentration of: (a) 0.60 x 10" 2, (b) 2.1 x 1 0 - 2 M Fig. III.7 V i s i b l e spectra of 4.5xlO _ J M solutions of [RhCl(COE)] 2 dimer i n DMA containing 2 equivalents of HC1 and 3 equivalents of DMSO per rhodium at r . t . under various conditions: C 0, immediately a f t e r d i s s o l v i n g under Ar; C, a f t e r leaving C Q under Ar over a few hours; D, 50-100 s a f t e r f lushing C with 0 2; E, several hours a f t e r leaving C under 0 2 at 1 atm - 81 -ra p i d l y i n approximately the f i r s t 0.5 h and then more slowly to give a f i n a l stable s o l u t i o n (curve E, Fi g . III.7). Attempts to i s o l a t e characterizable products from e i t h e r 0 2 reac-t i o n of the i n s i t u Rh* species generated from complex 1 i n DMA, or the Rh* model system, were not successful. Further discussions of the oxygen rea c t i o n of i n s i t u Rh* are found on p. 178 and p. 179. (b) Redox decomposition of i n s i t u [Rh*/2H+/3C1"/3DMS0] s p e c i e s The r e l a t i v e s t a b i l i t y of the i n s i t u Rh* product i n DMA solvent, and i t s decomposition to Rh***(DMS) species i n the workup procedure, suggest that a redox reaction as i n eq. III.8 i s f a c i l i t a t e d i n the absence of DMA. Rh*/2H+/3C1"/3DMS0 > RhI**/3Cl'/2DMSO/DMS + H 20 III.8 Similar redox reactions are documented f or several other t r a n s i t i o n -metal complexes. Reduction of sulfoxides i s an important r e a c t i o n i n organic synthesis where intermediate sulfoxides need to be reduced s e l e c t i v e l y to the respective t h i o e t h e r s . 4 8 Salts or complexes of T i * * * , V** or Mo*v which act as oxygen acceptors are used f o r such reductions (e.g. eq. III.9). 63 MoO(S 2CNEt 2) + DMSO > Mo(0) 2(S 2CNEt 2) + DMS III.9 For Pt group metal complexes, the presence of 2 equivalents of strong a c i d i s generally required f o r the deoxygenation of the sulfoxide; the - 82 -concommitant oxidation of the metal centre i s noted i n some cases.^ 4 The suggested decomposition re a c t i o n i n eq. III.8 may be accom-p l i s h e d by using the dimer [RhCl(COE)2]2 a s the Rh 1 precursor. The dimer (110 mg, 0.025 mmol i n Rh) was added to a degassed s o l u t i o n of CH 2C1 2 (10 mL) containing 2 equivalents of DMA-HC1 (55 mg, 0.05 mmol) and 3 equivalents of DMSO (50 fil, 0.09 mmol) under Ar, and the s o l u t i o n was s t i r r e d overnight. The f i n a l s o l u t i o n remained q u a l i t a t i v e l y unchanged from the o r i g i n a l yellow-orange, and was a i r - s t a b l e . Ether was added slowly to obtain, a yellow p r e c i p i t a t e which was r e c r y s t a l l i z -ed as [Rh-3C1-2DMS0-DMS] i n -60% Y i e l d . ' Anal. calcd. f o r C 6H 1 8Cl 302S 3Rh: C 16.85, H 4.24; found: C 17.09, H 4.26. The *H nmr spectrum of the product i n CDC13 (Fig. III.8) shows an approximate DMSO, DMSO/DMS r a t i o of 2:1. A s o l u t i o n of the product i n DMA i s non-conduct-ing, showing the product to be non-ionic. The multitude of peaks observed f o r Rh***/DMS i n the *H nmr spectrum suggests the presence of several Rh*** species. Dimethyl-s u l f i d e ligands are known to form dimeric Rh*** complexes containing b r i d g i n g DMS l i g a n d s . ^ 5 Since the compound i s non-conducting, only monomeric structures with a 6:1 ligand to metal r a t i o are possible; a dimeric formulation would require an i o n i c nature with at l e a s t one non-associated C l " ion. Further c h a r a c t e r i z a t i o n was not possible because of the d i f f i c u l t y of analyzing the *H nmr spectrum. The complexity of *H nmr spectrum suggests the product to be composed of several isomeric forms. The redox reaction between the Rh* precursor dimer and DMSO i n the presence of added HC1 was e s s e n t i a l l y complete within 15 min of reaction as judged by *H nmr. The spectrum of a reac t i o n s o l u t i o n , a f t e r about - 83 -F i g III.8 80 MHz AH nmr spectrum of RhCl 3 2DMS0-DMS i n CDC1 3 - 84 -15 min of mixing Rh 1 precursor with 2 equivalents of HCl and 3 equiva-lents of DMSO i n CDCI3 under Ar, d i d not show peaks of DMA*HCl (6 3.1, s, 3H; 2.3, s, 6H; -9, br, IH), but only of DMA (5 3.0, d, 6H; 2.1, s, 3H). Peaks due to coordinated DMS and DMSO were i n an approximate r a t i o of 1:2, as f o r the i s o l a t e d complex. The r a p i d i t y of the redox decomposition of the [Rh I/2H +/3Cl"/ 3DMS0] model system i n CH2CI2 contrasts with the r e l a t i v e s t a b i l i t y of the same system i n DMA; the so l u t i o n changes under Ar over 24 h (CQ — > C i n F i g . III.7) but the e q u i l i b r a t e d s o l u t i o n remains s e n s i t i v e to O2, according to s p e c t r a l (C — > D, F i g - III.7) data, the observations on the model system p a r a l l e l those on the H2-product s o l u t i o n of 1 i n DMA. The r e l a t i v e s t a b i l i t y of the RhI/2H+/3Cl"/3DMSO systems i n DMA re s u l t s perhaps from the u n a v a i l a b i l i t y of protons i n t h i s medium com-pared to CH2CI2. James et al.^3 suggested that the i n i t i a l step i n the Rh^^ - c a t a l y z e d reduction of DMSO by H2 was the protonation of a coordinated DMSO; the c a t a l y t i c a l l y a ctive rhodium species was postulated to be Rh I ] : i(H) (eq. III.10). The mechanism proposed for the Rh 1 system i n the present work involves a s i m i l a r i n i t i a l protonation step given as an equilibrium r e a c t i o n (eq. III.11), and an el e c t r o n transfer from the Rh 1 to the ele c t r o n d e f i c i e n t s u l f u r atom (eq. III.12). \ \ CH3 CH3 III.10 - 8 5 -0 0H + . // + ^ j // R h ^ — S — C H 3 + H + ^ R h i < — S —C H 3 1 1 1 . 1 1 \ \ CH 3 CH 3 r*OH + Rh 1 :S—CH 3 > R h 1 1 1 + S(CH 3) 2 + H 2 0 I I I . 1 2 \ CH 3 The r e a c t i v i t y difference i n DMA and CH2CI2 solvent may be a t t r i -buted to the proton scavenging a c t i o n of the DMA solvent. Dimethyl-acetamide reacts instantaneously with gaseous HC1 to give s o l i d DMA1 H C 1 . In DMA solvent, competition between complex and solvent f o r protons w i l l s h i f t eq. I I I . 1 1 to the l e f t and thereby slow the redox re a c t i o n I I I . 1 2 . The suggested presence of about 0 . 2 equivalents of Rh***(DMS) i n the H 2 product of complex 1 (eq. I I I . 7 ) requires comment. Since the Rh*/ DMSO product i s stable i n DMA s o l u t i o n f o r a few hours, the Rh i : [ I(DMS) most l i k e l y r e s u l t s from a R h 1 1 1 - c a t a l y z e d reduction of DMSO by H 2 (eq. I I I . 1 0 ) which occurs v i a a Rh 1 1 1(H)(DMSO) intermediate formed i n the H 2 r e a c t i o n of R h C l 3 ( D M S 0 ) 3 2 3 (eq. III.13). In keeping with the arguments developed i n t h i s section, a protonation of the DMSO, within the coordination sphere (eq. III.14) i s preferred over protonation by a 'free' H + as suggested i n eq. I I I . 1 0 . H 2 R h I I L : ( D M S 0 ) > [ R h m ( H ) (DMSO) ] + H + — > Rh I (DMS0) + 2 H + III.13 - 86 -^ I l I I ("*0H -> Rh 1 +S—CHi \ CH3 •> R h 1 1 1 + S(CH 3) 2 + H 20 III.14 III.3.3 Hydrogen reactions of other complexes The c a t i o n i c species [RhCl(DMSO) 5](BF 4) 2 i n DMA under H 2 at 50°C gave metal i n s t a n t l y , while [NEt 4][RhCl 4(DMS0) 2] took up H 2 e s s e n t i a l l y at the same rate as complex 1. The Rh^^/DMSC- complex containing an ethenamine ligand, RhCl 3(DMSO) 2(~CH 2CH= +NEt 2), was i n a c t i v e towards H 2 i n C 2H4C1 2. Thioether complexes of R h 1 1 1 , e.g. RhCl 3(DES) 3 and RhCl 3(DMS) 3, i n DMA were les s stable under H 2 than the analogous DMSO complex, 1, and gave metal within 200-500 s at 50°C. III.4 Reactions of RhCl 3(DMS0) 3 (1) with CO The reactions with CO were studied with the aim of using CO as an a l t e r n a t i v e reagent to H 2. In the presence of stoichiometric amounts of H 20, CO can act as a two equivalent reducing agent to generate Rh 1 species (eq. III.15), or perhaps, as a H" donor reagent to generate Rh 1 1 1(H) species (eq. III.16). The former re a c t i o n i s well documented for R h 1 1 1 complexes, 66-68 w n j . ] . e the l a t t e r i s based on the success with - 87 P t 2 + systems (eq. III.17). 69 R h 1 1 1 + CO + H 20 > Rh 1 + C0 2 + 2H + I I I . 15 R h 1 1 1 + CO + H 20 > Rh I I ] :(H) + C0 2 + H + I I I . 16 P t 1 1 + CO + H 20 > P t n ( H ) + C0 2 + H + I I I . 17 In a d e t a i l e d mechanistic study of the reac t i o n of RhCl3'3H 20 with CO i n DMA, James and Rempel found the i n i t i a l step to be the formation of a R h H I ( C O ) species (eq. III.18). Hydrolysis of the Rh I ] C I(CO) to y i e l d a Rh*(C0) 2 species (eq. III.19) was a much slower r e a c t i o n . ^ 8 R h 1 1 1 + CO > R h i n ( C O ) I I I . 18 -C0 2,-2H + +2C0 Rh I I i :(CO) + H 20 -> [Rh 1] > Rh I(CO) 2 I I I . 19 In the present work i t was of i n t e r e s t to t r y and i s o l a t e a Rh***(CO) intermediate from a reaction between RhCl3(DMSO)3 and CO under anhydrous conditions, i n order to be able to con t r o l the hydro l y s i s step to obtain Rh I ] : I(H) species (eq. III.16). The RhCl 5(CO) 2' complex has been made v i a the carbonylation of chlororhodate(III) s p e c i e s , 7 0 while several other Rh***(CO) species have been made by the oxidative a d d i t i o n of, f o r example, a halogen X 2 to Rh*(CO) complexes. 7 1 Some reactions were done with the deliberate a d d i t i o n of 1-2 equi-valents of H 20 per rhodium, i n order to check whether t h i s could be a convenient route to a Rh 1 complex containing CO and DMSO ligands. Such a - 88 -complex, RhCl(CO)(DMSO)2> has been made by the re a c t i o n between [ R h ( C 0 ) 2 C l ] 2 and DMSO.72 The reactions r e q u i r i n g anhydrous conditions were conducted i n CH2CI2 or C2H4CI2, and those re q u i r i n g H2O conducted i n DMA. (a) In CH 2Cl2 or G^H^C^ The reactions with CO were monitored using the rate of net gas uptake with time i n C2H4CI2 solvent. A 2.4 x 10" 2 M C 2H4Cl2 s o l u t i o n of RhCl3(DMSO)3 (1) takes up approximately one net equivalent of gas per equivalent of rhodium i n less than 2 h (Fig. III . 9 ) . A slow gas evolu-t i o n was noticeable a f t e r about 5500 s, and the presence of CO2 i n the gaseous phase at t h i s stage was v e r i f i e d by gas chromatography (Section II.5.3). V i s i b l e s p e c t r a l changes f o r a 2.25 mM CH2CI2 s o l u t i o n of RhCl 3(DMS0)3 under CO are given i n F i g . III.10. Within the f i r s t 30 min of reaction, the absorbance at 435 nm ( € = 350 M"1 cm"1) decreases s t e a d i l y . A f t e r 30 min, an increase i n the o v e r a l l absorbance at 435 nm i s noted and t h i s continues for about 24 h to give a f i n a l spectrum with a shoulder at 405 nm ( 6= 520 M"1 cm" 1). The f i n a l v i s i b l e spectrum corresponds well with the absorption spectrum of RhCl3(DMS)3 i n CH2CI2 ( € =• 542 M"1 cm"1 at 405 nm). The i n i t i a l v i s i b l e s p e c t r a l changes correspond with the time scale for the uptake of one equivalent of gas (Fig. I I I . 9 ) , suggesting the i n i t i a l formation of a Rh 1 1 1(C0) i n t e r -mediate. An a i r - s e n s i t i v e o i l y - r e s i d u e i s o l a t e d from a re a c t i o n mixture a f t e r a net uptake of one equivalent of CO per Rh shows two peaks at - 89 -ICOl/RhJ Time, s F i g . III.9 An uptake p l o t f o r the rea c t i o n of 2 . 4 x l 0 - 2 M RhClo(DMSO)o i n C 2H 4C1 2, with CO (1 atm) at 30°C € , M cnr 1 500-450 Wavelength, nm F i g . III.10 V i s i b l e s p e c t r a l changes f o r a s o l u t i o n of 2.25xl0~ 2 M RhCl 3(DMSO) 3 i n CH 2C1 2, under CO (1 atm) at ambient conditions (between 10-70 min of rea c t i o n time, the spectra were recorded every 5 min) ; A, i n i t i a l spectrum - 90 -1980 and 2100 cm"1. Attempts to i s o l a t e a characterizable product from the residue were not successful. The [Rh-^CO^C^]" anion shows two absorptions at 1975 cm"1 and 2060 cm" 1, 7 1 but the. formation of a dicarbonylrhodium(I) product i s not consistent with the approximately 1:1 uptake of CO by R h 1 1 1 , and a Rh 1 1 1(CO) intermediate seems l i k e l y . James et a l . noted s i m i l a r VQQ absorptions (1970 and 2070 cm"1, re s p e c t i v e l y ) i n an impure compound i s o l a t e d a f t e r an i n i t i a l , r e l a t -i v e l y r a p i d uptake of one equivalent of CO by R h 1 1 1 i n DMA;6** attempts to i s o l a t e an a n a l y t i c a l l y pure compound from the r e a c t i o n residue were also unsuccessful. A complex analyzing as mer-RhCl3(DMS)3 was i s o l a t e d i n -65% y i e l d as the f i n a l product from a r e a c t i o n of an approximately 0.01 M s o l u t i o n of RhCl 3(DMS0) 3 i n C2H4CI2 with 1 atm of CO at 50°C, a f t e r a reaction time of over 24 h (Section I I . 4.3.5). The 1H nmr spectrum of a sample i n CDCI3 showed two doublets (J = 1 Hz) i n a 2:1 r a t i o at 5 2.50 and 2.30, r e s p e c t i v e l y ; the data correspond with those f o r an authentic sample of mer-RhCl3(DMS)3 prepared by a l i t e r a t u r e method3-* ( 5 (CDCI3): 2.53, d, J - 1 Hz, 12H and 2.30, d, J = 1 Hz, 6H). An i r spectrum of the complex shows two almost equivalent peaks at 980 and 1035 cm"1 which are assigned to the PQ^^ modes of the DMS ligands.35 Larger scale preparations using concentrations of RhCl3(DMSO)3 greater than 0.05 M i n CH2CI2 or C2H4C12 y i e l d e d mixtures of products, the main components of which were i d e n t i f i e d as mer-RhCl3(DMS)3 and mer-cis-RhCl3(DMS)2(DMS0) , res p e c t i v e l y , by a -^H nmr spectrum of the product i n CDCI3. I d e n t i f i c a t i o n of the former was by reference to the peak pos i t i o n s of the authentic compound given above. The remaining peaks occurring at an -1:1:1 r a t i o at 53.55 (s, 6H, (CH 3) 2SO), 2.48 (d, J - 1 - 91 -Hz, 6H, (CH 3) 2S), 2.38 (d, J = 1 Hz, 6H, (CH 3) 2S) were assigned to mer-cis-RhCl 3(DMS) 2(DMSO) to account f o r two non-equivalent DMS ligands and an S-coordinated DMSO ligand. Longer rea c t i o n times or r e f l u x i n g conditions improved the y i e l d of mer-RhCl 3(DMS) 3, but s t i l l gave at le a s t 20% of the bis(di m e t h y l s u l f i d e ) product. Attempts to obtain pure RhCl 3(DMS) 2(DMSO) by a d i f f e r e n t route using RhCl 3(DMS0) 3 and 2 equivalents of DMS i n CDC13 gave an approximately 1:1 mixture of mer-c i s -RhCl3(DMSO)2(DMS) (Section II.4.3.4) and mer-cis-RhCl 3(DMSO)-(DMS)2, as judged by s o l u t i o n *H nmr. Longer rea c t i o n times showed a d d i t i o n a l peaks due to some mer-RhCl 3(DMS) 3. In the CO reduction of complex 1 to RhCl 3(DMS) 3 i n CH 2C1 2 or C 2H 4C1 2 under ambient conditions (eq. III.20) the observed s p e c t r a l data are consistent with an i n i t i a l 1:1 reac t i o n between 1 and CO to give a Rh I l : [(C0) species (eq. III.21). RhCl 3(DMS0) 3 + 3C0 > RhCl 3(DMS) 3 + C0 2 III.20 RhCl 3(DMSO) 3 + CO > RhCl 3(DMS0) 2(C0) + DMSO III.21 The reduction of DMSO then can occur v i a a n u c l e o p h i l i c attack on the coordinated CO by DMSO to y i e l d a Rh I i : t(DMS) species and C0 2 (eq. I I I . 22); f o r e l e c t r o n i c book-keeping purposes the R h ( C O ) moiety i s represented as Rh^-CSQ"4". 92 -0 0 Rh I ] :-C£0 + + S-CH3 > Rh I I^-Q rO^S +(CH3) 2 CH-: -> Rh I i : r(S(CH3)2) + C0 2 III.22 A c l o s e l y analogous reaction i s the reduction of t e r t i a r y amines oxides to t e r t i a r y amines by metal-bound carbon monoxide (e.g. eq. I I I . 2 3 ) . 7 3 a 0 (C0)4Fe=C=0 + ( C H 3 ) 3 Nt-O" > (CO) 4Fe"—0—0—1^(CH3)3 -> (C0) 4Fe(N(CH 3) 3) + C0 2 III.23 In the present case, the r e s u l t i n g RI1CI3(DMSO)2(DMS) complex can react further, i n a s i m i l a r manner, to give RI1CI3(DMSO)(DMS)2 and then RhCl 3(DMS) 3. (b) With added H 20 i n DMA A gas uptake p l o t f o r the CO reaction of a 0.020 M s o l u t i o n of complex 1 i n DMA i n the presence of an added equivalent of H2O at 30°C - 93 -(Fig . III.11) shows a net uptake of approximately 2 equivalents of CO per equivalent of Rh within the f i r s t 6000-7000 s. Uptake data f o r the RhCl3 -3H20 complex i n DMA, reproduced from a l i t e r a t u r e r e p o r t , ^ 8 are given i n the same figu r e ; a b r i e f discussion on these data i s given l a t e r i n t h i s Section. With 1, the s o l u t i o n color changes from a deep orange to a l i g h t orange during the uptake. There i s no s i g n i f i c a n t increase i n the t o t a l uptake over the next 20 h. Carbon dioxide i s detected i n the gas phase of the rea c t i o n mixture wi t h i n 2 h of r e a c t i o n time. V i s i b l e s p e c t r a l changes f o r an analogous system under ambient conditions are given i n F i g . III.12. The changes are non-isosbestic i n behaviour. A f t e r some r e l a t i v e l y r a p i d changes i n about the f i r s t 1.75 h (or -6000 s ) , slower changes continue to occur over the next few hours to give a f i n a l stable spectrum E i n < 24 h ( F i g . III.12). The r e l a t i v e l y r a p i d changes i n the f i r s t 6000 s, correspond well with the net uptake region of the uptake p l o t (Fig. III.11). An i r spectrum of a s o l u t i o n a f t e r 24 h of rea c t i o n shows two strong bands at 1982 and 2060 cm"-'-, and two other much weaker bands at 1950 and 2087 cm"* (Fig. III.13). The two strong bands correspond to the two c h a r a c t e r i s t i c bands at 1975 and 2060 cm"* reported f o r the cis-[Rh(C0)2Cl2]" anion; the l i t e r a t u r e data are for the s a l t s of e i t h e r ( C ^ Q ^ N 4 " or (Cg^^As" 1", i n N u j o l . 7 * The source of the two smaller bands i s not c l e a r . A 60 MHz cw *H nmr spectrum of the rea c t i o n s o l u t i o n i n the So to -40 region does not show any peaks due to rhodium-hydride protons. 94 -(NetCOl/lRhJ 2000 4000 Time, s 6000 1 80,000 Fi g . III.11 Uptake p l o t s f o r reactions of Rh complexes i n DMA ( 2 . 0 x l 0 - 2 M) with CO (1 atm): (a) RhCl 3(DMSO) 3 with one equivalent of added H 20 per Rh at 30°C; (b) RhCl 3-3H 20 at 40°C 800 C . M ' c m - 1 400 B ; 600 S C ; 3600 s D ;6000 s E ;~24h 400 450 Wavelength, nm F i g . III.12 V i s i b l e s p e c t r a l changes under CO (1 atm) at 30°C f o r a . s o l u t i o n of 2 . 0 x l 0 - 2 M RhCl 3(DMSO) 3 i n DMA containing added H 20 ( 2 x l 0 " 2 M); A, i n i t i a l spectrum (between B and C, the spectra were recorded every 60s) - 95 -2100 Wavenumbers. cm" 1900 F i g . III.13 An FT i r spectrum of a s o l u t i o n from the r e a c t i o n of RhCl 3(DMSO) 3 i n DMA with CO (1 atm) at r . t . f o r 24 h In the workup procedure to i s o l a t e the r e a c t i o n products, DMA-HCl was i s o l a t e d i n about 50% y i e l d based on rhodium (Section II.4.3.6). A rhodium species, postulated to contain the novel [(DMA) 2H] + cation, was also obtained from the r e a c t i o n residue. A discussion on the i s o l a t e d rhodium product i s given l a t e r i n t h i s section. The experimental data are consistent with a net stoichiometry i n v o l v i n g a reduction of R h 1 1 1 to Rh 1: H 20 R h 1 1 1 + 3C0 > Rh I(CO) 2 + C0 2 + 2H + I I I . 24 Whether or not a CO reduction of DMSO to DMS and C0 2 occurs, s i m i l a r to - 96 -the CH2CT2 system discussed previously (eq. III.20), cannot be ascer-tained from the uptake data, since such a sid e - r e a c t i o n w i l l release an equivalent of CO2 fo r each equivalent of CO taken up. The net uptake of 2 equivalents of gas, the detection of the [Rh(CO)2CI2]" anion i n the rea c t i o n s o l u t i o n and CO2 i n the gas phase, and i s o l a t i o n of DMA-HC1 as by-product, support the proposed stoichiometry. The major re a c t i o n product detected and the r e a c t i o n stoichiometry are i d e n t i c a l to those reported f o r the r e a c t i o n between RhCl3-3H20 and CO i n DMA;68 thus, the DMSO ligands have no e f f e c t on the CO/H2O reac t i o n of Rh*** i n terms of the f i n a l product. However, the o v e r a l l rate of re a c t i o n i n the RhCl 3(DMSO)3 system at 30°C i s f a s t e r than that r e p o r t e d 6 8 f o r the RhCl 3-3H 20 system at 40°C (Fig. III.11). In the l a t t e r case, an i n i t i a l f a s t e r gas uptake was a t t r i b u t e d to formation of a Rh***(C0) species (eq. III.18). The a d d i t i o n a l net uptake of one equivalent of gas i n a slower second phase of the re a c t i o n was assigned to the combination of a slow hydrolysis step (that releases an equivalent of CO2) and a ra p i d uptake of 2 equivalents of CO by the Rh* species subsequently formed (eq. III.19). In the present case a s i m i l a r mechanism i s probable, but the absence of a d i s t i n c t i n i t i a l phase corresponding to a CO/Rh r a t i o of 1 shows the h y d r o l y s i s step i s fast e r than i n RhCl 3 • 3^0 system even at 30°C. Presumably, the DMSO ligands exert an e f f e c t on the rate of the hydrolysis reaction. I s o l a t i o n of a rhodium product James et a l . i s o l a t e d the cis[RhCl2(CO)2]" anion as the [Ph^As]-- 97 -[RhCl2(CO)2] s a l t , by adding a methanol s o l u t i o n of the Ph^AsCl s a l t to the RhCl3-3H 20 and CO reaction product i n DMA.68 In the present work, a product was i s o l a t e d by working up the re a c t i o n mixture without an added cat i o n (Section II.4.3.6). The proton nmr spectrum of the i s o l a t e d complex i n CDCI3 ( F i g . III.14.a) showed three peaks at 5 2.38, 3.14 and 6.6 (br) i n the r a t i o 3:6:~0.5, a t t r i b u t e d to CH3CO-, -N(CH.3)2 and the a c i d i c proton, re s p e c t i v e l y , of the [(DMA^H] 4" cation. The peak and the a c i d i c proton, respectively, of the [(DMA^H]"1" cation. The peak positi o n s assigned to the methyl protons of the (DMA)2H+ cation are close to those of the DMA"HCl adduct (Section II.4.5), but the i r spectrum of the complex (Fig. III.14.b) i s quite d i s t i n c t from that of the DMA"HCl s a l t (Section II.4.5). In f a c t , the broad band i n the i r spectrum which extends from -600 cm"1 to -1700 cm"1 confirms the presence of a (DMA) 2H + cation. Salts containing symmetrically H-bonded cations are known to cause such broad bands i n t h i s r e g i o n . 7 4 A c l o s e l y r e l a t e d l i t e r a t u r e example i s (DMF^H"1" cat i o n i n the [(DMF) 2H][Pd 2Cl 6] s a l t . 7 4 a In the case of (DMS0) 2H + c a t i o n i n [(DMSO) 2H][RhCl 4(DMS0) 2], the symmetric nature of the H-bond has been confirmed by a c r y s t a l structure (2, Ortep diagram of (DMS0) 2H + c a t i o n ) . 7 4 c (2) - 98 -Wavenumber, cm F i g . III.14 (a) A 80MHz iH nmr spectrum i n CDCI3 and (b) an i r spectrum i n KBr of the product i s o l a t e d from the rea c t i o n between RhCl 3(DMSO)3 and CO i n DMA i n the presence of an equivalent of added H2O - 99 -By analogy with the (DMF) 2H + and (DMSO) 2H + cations a s i m i l a r symmetrically H-bonded structure (3) i s proposed f o r the (DMA) 2H + cation. A s i m i l a r protonation at the carbonyl oxygen i s observed with the DMAH+ c a t i o n i n the DMA-HCl adduct. 7 5 CH3 CH^ ( C H 3 I 2 N — ^ N(CH3)2 0 H*- 0 (3) The product analyzes well f o r a complex s a l t of the molecular formula [(DMA) 2H][RhCl 2(CO) 2] (Anal, c a l c d . f o r C 1 0 H 1 9 c l 2 N 2 ° 4 R h : c 29.65, H 4.73, N 6.91; found: C 29.46, H 4.74, N 6.92), but the 1>CQ region of i t s i r spectrum (Fig. III.14b) shows three well-defined absorptions (1875 m, 1985 s and 2025 m, cm"1) i n the s o l i d state. The pattern i s quite d i f f e r e n t from that of the i n s i t u product i n DMA solvent ( F i g . III.13) assigned to c i s - R h C l 2 ( C O ) 2 , but resembles that of the [RhCl(CO) 2] 2 dimer (2035 s, 2090 s and 2105 m, cm"1) i n terms of the number of bands. The appearance of three bands i n the i r spectrum of [ R h C l ( C 0 ) 2 ] 2 i s explained i n terms of a bent molecule (4). 7 3° (4) - 100 -In the s o l i d state the DMA^H cat i o n may well i n t e r a c t with the RhCl2(CO)2~ anion v i a the halide ligands, e.g. as i n structure 5; an in t e r a c t i o n between the c a t i o n and C l " ligands i n the counter anion i s observed also with the [EtNH 2] [RhCl^DMSO^] s a l t ( Fig. IV.14). However, such an i n t e r a c t i o n does not explain the occurrence of 3 bands assignable to VQQ and further studies ( s o l u t i o n i r experiments) are needed to resolve t h i s problem. I l l . 5 Summary Reaction between RhCl3(DMSO)3 and H2 i n DMA leads to a Rh 1 species as the major product. Slow decomposition of the Rh* product i s postulated to occur v i a a redox decomposition reaction between Rh* and DMSO i n the presence of 2 equivalents of acid; the proton a f f i n i t y of the DMA solvent i s thought to slow down the decomposition i n this solvent at l e a s t for 1-2 h at r . t . The clean reduction of RhCl 3(DMSO) 3 to Rh***(DMS) species by CO i n CH2CI2 or C2H4CI2 i s a novel reaction. In a l i t e r a t u r e report on the carbonylation of Pd**(DMS0) complexes, formation of a mixed C0/DMS0 complex was observed. 4 8 No other reports on carbonylation of metal-DMSO OC (5) - 101 -complexes or any other reports on the reduction of DMSO by CO has come to our attention. The r e a c t i o n between RhCl3(DMSO)3 and CO i n the presence of H2O, i n DMA gave the [RhCl2(CO)2l" anion which was i s o l a t e d i n low y i e l d as the [(DMA)2H][RhCl2(CO)2] complex. The rhodium anion product i s the same as that generated by the corresponding r e a c t i o n with RhC^-SH^O. - 102 -CHAPTER IV REACTION OF RhCl 3(DMSO) 3 WITH TERTIARY AMINES - 103 -IV Reaction of RhCl 3(DMSO)3 with T e r t i a r y Amines IV.1 Introduction The a d d i t i o n of strong bases i s sometimes required f o r a c t i v a t i o n of H 2 by higher-valent metal complexes. 5 7' 3 The strong base, 1,8-bis-(dimethylamino)naphthalene, commercially known as Proton Sponge (P.S.), has been s u c c e s s f u l l y used, f o r example, to f a c i l i t a t e the r e a c t i o n between Ru(III) and H 2 . 7 6 During attempts to b r i n g about a base-promoted rea c t i o n between RhCl3(DMSO) 3 (1) and H 2 i n C 2H4Cl 2, M o r r i s 5 0 discovered a r e a c t i o n between 1 and P.S. A protonated P.S. (P.S.H +) species was detected by *H nmr and i r spectroscopy, and a complex containing a P.S. d e r i v a t i v e was i s o l a t e d but not characterized. As part of the work i n t h i s t h e s i s , several other bases were test-ed as possible proton scavengers. The same r e a c t i v i t y of 1 was found towards Et3N (pK^ 3.4), which i s comparable i n base strength to P.S. (pK^ 2.7), but no reactions occurred with weaker bases such as dimethylphenylamine (pK^ 8.8) and 2-6-di-t-butyl pyridine (pK^ 8.2). The r e a c t i o n between RhCl3(DMSO)3 and NEt3 appeared p a r t i c u l a r l y i n t e r e s t i n g because of the widespread use of the base as a cocatalyst i n reactions catalyzed by Rh complexes. 7 7" 8 1 A Rh-ethenamine complex was i s o l a t e d as a product of the r e a c t i o n between RhCl3(DMSO)3 and NEt3. This chapter summarizes the c h a r a c t e r i z a t i o n of the species and d e t a i l s of the o v e r a l l reaction. - 104 -IV.2 Characterization of RhCl 3(DMSO) 2(~CH2CH=» +NEt2) » 6 Complex 6 (see Section II.4.3.2 f o r d e t a i l s on i s o l a t i o n ) e x i s t s i n CDCI3 i n two isomeric forms. The nmr spectrum of 6 within -5 min of d i s s o l u t i o n i n CDCI3 shows mainly the peaks due to one isomer, designated A ( F i g . IV.1.a). The r e l a t i v e i n t e n s i t y of peaks due to another isomer (B) increased over 3 h to give an equ i l i b r i u m mixture estimated to be 60% i n A and 40% i n B (Fig. I V . l . b ) ; the t r i p l e t and quartet at §1.42 and 3.10, respectively, are due to NEt3*HCl impurity i n the sample of 6 used. A purer sample of 6 was e q u i l i b r a t e d i n CDCI3 s o l u t i o n f o r -4 h to obtain a nmr spectrum s u i t a b l e f o r more d e t a i l e d analysis ( F i g. IV.2). The multitude of peaks was f i r s t divided into two sets A and B by reference to Figs. IV.1.a and IV.l.b; assignments w i t h i n each isomer, summarized i n Table IV.1, were made by s e l e c t i v e decoupling experiments. The two overlapping t r i p l e t s ( 5-8.3) are assigned to the "CH2-CH=-+NEt2 protons of each isomer. There are several reports i n the l i t e r a t u r e on metal-ethenamine complexes. 8 2" 8 6 A. d e f i n i t e assignment of an r / ^ - y l i d i c coordination mode i s made i n two of the r e p o r t s , 8 2 ' 8 3 where hi g h l y deshielded -CH=+NEt2 protons with chemical s h i f t s i n the range 5 7-8 have been noted. The doublet of doublets at 5 4.049 and another at 3.626 are assigned to the "CH.2-CH=+NEt2 protons of isomers B and A, re s p e c t i v e l y . I r r a d i a t i o n of each doublet of doublets collapsed the corresponding t r i p l e t s ( F ig. IV.3). The simultaneous i r r a d i a t i o n of the overlapping t r i p l e t s at 5-8.3 showed the collapse of each doublet of doublets into a doublet with a coupling constant of 2.5 Hz ( F i g . IV.4). The smaller coupling i s assigned to the expected i n t e r a c t i o n between the J DMS 1 0 DMSO A (ak M J s \ V i - j 1 1 * , , , ,— 3 2 K / f J \ w " * \ t A * ppm i F i g . IV.1 400 MHz 1H nmr spectrum of RhCl 3(DMS0)2(~CH 2CH= +NEt2) In CDCI3: (a) ~5 min a f t e r d i s s o l u t i o n , shows peaks mainly due to isomer A ; (b) 3 h a f t e r (a) (x, NEt 3-HCl impurity; *, isomer B ' other unlabelled peaks due to the ethenamine ligand) DMSO A Fig. IV.2 400 MHz XH nmr spectrum of RhCl 3(DMSO) 2CCH2CH= +NEt 2) -4 h a f t e r d i s s o l u t i o n i n CDC13 (a,b ....,h r e f e r to centres of quartets or t r i p l e t s ; see also Table IV.1) - 107 IV.3 (a) D e t a i l s of the m u l t i p l e t at ~68.3 of F i g . IV.2 relevant to -CH= assignments; (b) and ( c ) , a f t e r i r r a d i a t i o n at 4.049 and 3.626, r e s p e c t i v e l y IV. 4 (a) D e t a i l s of the 53.5-4.5 region of F i g . IV.2 relevant to -CH2- assignments; (b), a f t e r i r r a d i a t i o n at 6-8.3 - 108 -Rh(III) (I = 1/2) and the methylene protons of the "CH2-CH=+NEt2, and corresponds to l i t e r a t u r e values; e.g. the Rh(III)/"CH3 couplings i n Rh(C 5H 5)(CH 3)2(S(CH3)2) and a r e l a t e d complex have been found to be 2.7 and 2.6 Hz, r e s p e c t i v e l y . 8 7 There i s no apparent coupling between the Rh nucleus and the "CH2-CH=+NEt2 protons. The r e l a t i v e magnitudes of coupling between the metal centre and the o l e f i n i c protons of the ethenol ligand, CH2=CH-uH, have been used s u c c e s s f u l l y to d i s t i n g u i s h between the 7 j * - y l i d i c and 7 7 2 - o l e f i n i c modes of t h i s ligand. For example, i n Pt(acac)Cl(7/ z-CH 2=CH-0H) the Pt-Htl coupling (J=76 Hz) i s of the same order of magnitude as the Pt-H coupling (J=71 H z ) , 8 8 while i n Pt(acac)Cl( T/1--CH2CH0) the Pt-H^ coupling (J = 113 Hz) i s much greater than the Pt-H 0* coupling (J=20 H z ) . 8 9 Further evidence f o r an r / ^ - y l i d i c coordination mode for 6 i n solu-t i o n i s the observed non-equivalence of N-Et groups i n the ethenamine ligand i n both isomeric forms 6A and 6B. The d e t a i l e d assignments for the N-Et protons were made by i r r a d i a t i o n experiments. Each of the l e t t e r s a, b, .. g,h i n F i g . IV.2 re f e r s to a centre of a mul t i p l e t . The c i r c l e s round a, c, e and g denote that they a l l belong to the ethenamine liga n d on isomer 6B. The t r i p l e t s e, f, g and h, due to -NCH2CH3 protons of both isomers, are well separated from the other peaks, but, because of overlap or the proximity of the peaks, clean i r r a d i a t i o n of each was not possible. Simultaneous i r r a d i a t i o n of the two overlapping t r i p l e t s e and f caused the collapse of the c, d quartet p a i r ( F i g. IV.5). Since e and c are already known to belong to isomer B, they are assigned to an -NEt group on the B isomer, and d and f mul t i p l e t s to an -NEt group on the A isomer. The remaining multiplets are assigned s i m i l a r l y and a l l assignments are summarized i n Table IV.1. 109 F i g . IV.5 ( a ) D e t a i l s of the 63.5-4.5 region of F i g . IV.2 relevant to -+NCH2- assignments; (b) and ( c ) , a f t e r i r r a d i a t i o n at e and f, and g and h, t r i p l e t s i n F i g . IV.2, r e s p e c t i v e l y / 110 -Table IV.1 Summary of 400 MHz ^H nmr data f o r an e q u i l i b r a t e d s o l u t i o n of RhCl 3(DMSO)2("CH 2CH= +NEt2) (6) i n CDC13 (Fig. IV.2)<a> Peaks Assignments Isomer A 3.626 (dd, J = 8.5 2.5 Hz, 2H) 8.258 ( t , J = 8.5 Hz, IH) b: 3.768 (q); h: 1.367 (t)< b) d: 3.587 (q); f: 1.478 (t) 3.517 ( s ) , 12H CH 2CH= +NEt 2 CH 2CH= +NEt 2  /CH 2 bCH3 h \CH 2 dCH 3 f (CH3) 2S0(S-coordinated) =+N' Isomer B 4.049 (dd, J = 9, 2.5 Hz, 2H) 8.299 ( t , J - 8.5 Hz, IH) a: 3.845 (q); g: 1.396 (t)< b> c: 3.653 (q); e: 1.497 (t) 3.466 ( s ) , 6H 3.263 ( s ) , 6H "CH 2CH= +NEt 2 •CH 2CH= +NEt 2 /CH2 a CH 3 S \ c H 2 c C H 3 e (CH.3)2S0 (S-coordinated) (CH3) 2S0 (S-coordinated) Consists of two isomers A and B i n an approximately 60:40 r a t i o . Integrations f o r the N-Et protons are not given. Chemical s h i f t given i n 5-a, b, g and h re f e r also to the centres of the correspond-ing peaks i n F i g . IV.2. Coupling constants f o r the quartets and t r i p l e t s average to -7 Hz. - I l l -The summarized assignments show that the ethenamine ligand, whether i n isomer A or B, contains non-equivalent -NEt groups (see below f o r a d d i t i o n a l features). Coordination of the ethenamine as an o l e f i n , as i n 7 , would lead to equivalent -NEt groups because of free r o t a t i o n about the C-N bond; the non-equivalence d e f i n i t i v e l y eliminates the p o s s i b i l i t y of 7/2-o l e f i n i c coordination. " \ XNEt 2 Rh +/ CH2—CH=NX Rh Et Et IV. 1 (7) The non-equivalence of the -NEt groups further shows that there i s no s o l u t i o n equilibrium such as eq. IV.1 for complex 6, since t h i s w i l l lead to scrambling of -NEt groups on a ligand. 8^ H a H NEt, Rh H Et (8) (9) Other atom, 8, or possible coordination modes such as bonding through the N an ^^-coordination, 9, have not been reported i n the - 112 -l i t e r a t u r e . In addition, the experimental data i n the present case as 8 i s expected to give an ABX pattern f o r the H , HP 1 and HP^ o l e f i n i c protons, while an a l l y l i c form such as 9 w i l l require the complex to be i n an i o n i c form, e.g. [RhCl2(DMSO)2(r/3-CH2=CHNEt2)]C1, to maintain an 18 e l e c t r o n configuration i n the valence s h e l l (the evidence f o r the coordination of both DMSO molecules to Rh(III) i s given below). The experimental data show an A2X pattern f or the o l e f i n i c protons (Table IV.1) and the complex i s non-conducting i n s o l u t i o n (Section II.4.3.1). Sulfur-bonded DMSO ligands of R h 1 1 1 are r e a d i l y assigned by nmr spectroscopy (see Section I I I . l ) . The s i n g l e t s at 6 3.517, 3.466 and 3.263 ( F i g . IV.2 and Table IV.1) are i n the range expected for DMSO. Changes i n the nmr spectrum of 6 with time (Fig. IV.1) show that the peak at 8 3.517 belongs to isomer A and the other two peaks to B. The small peak due to free DMSO at 6 2.635 most l i k e l y r e s u l t s from some ligand d i s s o c i a t i o n i n so l u t i o n . An nmr spectrum of 6 i n acetone shows only the one DMSO peak a t t r i b u t a b l e to isomer A i n s i g n i f i c a n t amounts ( F i g . IV.6); smaller peaks due to isomer B may or may not be concealed by other peaks. Since the compound was i s o l a t e d on a preparative scale from acetone, i t i s highl y l i k e l y that i t e x i s t s , mostly or t o t a l l y , as the A isomer i n the s o l i d s tate. On r e d i s s o l u t i o n of 6 i n CDCI3, the A isomer i s found i n about 85% abundance, as judged from a spectrum taken a f t e r about 5 min (Fig. IV.1.a). The decrease i n A with time again suggests that 6 probably e x i s t e d s o l e l y as isomer A at zero time. The presence of two DMSO peaks of equal i n t e n s i t y f o r the isomer B (Fig . IV.2) indicates the non-equivalence of the two DMSO ligands. exclude such p o s s i b i l i t i e s ; f o r example^ an 7 7 -N-ethenamine complex such acetone-<% 8.0 6.0 4.0 2.0 ppm F i g . IV.6 A 1H nmr spectrum of RhCl 3(DMSO)2(~CH 2CH="^Et 2) i n acetone-d 6 (x, NEt 3-HCl impurity) - 114 -Therefore a mer-cis geometry, 10, i s assigned to B. For isomer 6A ei t h e r a mer-trans, 11, or a f a c - c i s , 12, geometry would account f o r the observed equivalence of DMSO ligands ( F i g . IV.2). Generally DMSO ligands trans to one another occur at a lower f i e l d i n t h e i r *H nmr than those to trans to C l " . 3 4 The DMSO i n isomer A i s more deshielded than e i t h e r DMS.0 i n isomer B. Therefore isomer A i s most l i k e l y to show the mer-trans geometry, 11. In fa c t , a mer-trans geometry should be the more favorable s t e r i c arrangement f o r the three l a r g e r ligands. C l SO CH 2-OS OS ci CH2-C l C l C l ' C l ' C H o -SO OS OS C l C l C l 10, mer-cis 11, mer-trans 12, f a c - c i s •L->C{-"-H)-nmr data obtained f o r an e q u i l i b r a t e d s o l u t i o n of 6 i n CDCI3 (Table IV.2) are also consistent with the given formulation for 6. For example, the assignments for the C^ N"1" and the "CH2- carbons, and other N-Et carbons correspond well with those r e p o r t e d 8 2 for the Pd I I["CH 2CH=N +(CH(CH 3) 2) 2] complex (<5(CDC13): 17.45, CH 2; 164.92, C=N; 50.74, C-N; 52.45, C-N; 19.16, 2 x CH 3; 23.04, 2 x CH 3); the assignments f o r A and B isomers i n the present case were made on the assumption that the trend i n 1 3 C chemical s h i f t s p a r a l l e l s that of *H chemical s h i f t s i n Table IV.1. The observed 1 0 3 R h - 1 3 C coupling constant i s 10 Hz. The 1 9 5 P t - 1 3 C coupling i n the Pt I I("CH 2-C +(NMe 2) 2) complex i s reported to be 584 Hz. 8 5 The value of 10 Hz obtained for the Rh-ethenamine complex (6) - 115 -i n the present case appears reasonable since the ---^Pt-^H coupling constant f o r the "CH2- protons i n the above Pt complex (J = 104 H z ) 8 5 i s also about 50 times greater than the corresponding  LE coupling constant i n complex 6 (Table- IV.2). The three peaks at § 42.6, 42.8 and 43.3 (Table IV.2), which are deshielded from the free DMSO value at 39.4, are assigned to coordinated DMSO. No data are a v a i l a b l e i n the l i t e r a t u r e f o r comparison. A s o l i d state i r spectrum of 6 i s given i n F i g . IV.7. The strong absorption at 1643 cm"1 agrees well with the values of 1613 and 1615 cm"1 reported f o r c^=N ofr^-ethenamine complexes of P d ( I I ) 8 2 and o o P t ( I I ) , J r e s p e c t i v e l y . The ^c=N absorptions of c y c l i c imminium perchlorates, e.g. 12.1, were reported to l i e i n the range 1660-1700 cm"1.90 The strong band at 1110 cm"1 i n F i g . IV.7 i s assigned to V$Q of equivalent DMSO ligands of 6A. The other strong band at 1020 cm"1 and the weak band at 985 cm"1 are assigned to PQ^ modes of s u l f u r -coordinated DMSO by reference to other Rh(III)/DMSO complexes (Section I I I . 2), though a comparison with a DMSO-dg analogue of the complex i s necessary f o r a d e f i n i t i v e assignment (Section I I I . l ) . The small shoulder at 1125 cm"1 i s perhaps due to a small amount of isomer B i n the s o l i d state. 116 -Table IV.2 Summary of 400 MHz 1 3 C {*H} nmr data f o r an e q u i l i b r a t e d s o l u t i o n of RhCl 3(DMSO) 2CCH 2CH= +NEt 2) (<) i n C D C l 3 ( a ) Peaks Assignments Isomer A 12.6 ( d , J •» 10 Hz) " C H 2 — 187.5 "C= +NEt 2 45.8 / C H o — -V 53.3 \ C H 2 — 19.2 / C H 2 - C H 3 =+N 23.2 \ C H 2 - C H 3 42.8 (CH 3) 2SO Isomer B 14.0 ( d , J = 10 Hz) _ C H 2 — 188.3 ( d , J = 10 Hz) _ C= +NEt 2 46.2 / C H o — 53.4 \ C H 2 — 19.4 /CH 2 - C H 3 =+N 23.3 \ C H 2 - C H 3 42.6 ( C H 3 ) 2 S 0 42.4 ( a) Shown by •'•H nmr to contain -60% 6A and -40% 6B; chemical s h i f t s given i n 6. F i g . IV.7 An i r spectrum of RhCl 3(DMSO) 2(-CH 2CH= +NEt 2) i n KBr - 118 -F i g . IV.8 A stereo-view ORTEP diagram of mer-cis-RhCl3(DMSO)2("CH 2CH= +NEt 2) (Some s t r u c t u r a l parameters given i n Appendix I) - 119 -(6) (4) CH3-(1.511) ( U 8 1k / 1.291 1?CH/ (1.503) F i g . IV.9 An i l l u s t r a t i o n of some bond lengths of the ethenamine ligand on mer-cis-RhCl 3(DMSO) 2 ("CH 2CH= +NEt 2), 6B; d e t a i l s i n Appendix I. An X-ray c r y s t a l structure of a si n g l e c r y s t a l i s o l a t e d from a CDCI3 s o l u t i o n of 6 by slow p r e c i p i t a t i o n with ether confirms the structure assigned to 6 by spectroscopic data. The c r y s t a l i s o l a t e d shows the mer-cis geometry assigned to the minor 6B isomer (Fig. IV.8). The sulfoxide ligands on 6B are both s u l f u r coordinated as predicted. The S=0 bond lengths at 1.472(2) and 1.476(2) A (Appendix I) correspond well with the S=0 bond lengths i n RhCl 3(py) 2(DMSO) 2 (1.47 A)48 or [(DMS0) 2H][RhCl 4(DMSO) 2] (1.48 A)74c. The ethenamine liga n d shows an T i ^ - y l i d i c coordination mode as predicted. The bond angles C ^ - N - C ^ 5 ) , c ( 2 ) - N - c ( 3 > and N ^ 2 ) - ^ 1 ) (see Fi g . IV.9 f o r an i l l u s t r a t i o n ) at 120.3°(2), 122.9°(2) and 126.7°(2), - 120 -re s p e c t i v e l y (Appendix I ) , correspond well with the 120° angle expected i f there i s a double bond between N and C ^ 2 ) . T h e length of the N-C^2) bond (1.291(3) A), as compared to the N-C<3) (1.475(3) A) and N-C<5) (1.481(3) A) bond lengths, further confirm a higher bond order between rj(2) and N. McCrindle et a l . 8 2 found the N-C^-C^ 1) bond angle f o r an analogous Pd 1 1(ethenamine) complex to be r e l a t i v e l y large at a value of 136°(10), but the reported N-C^2) bond length at 1.249(12) A compares well with the corresponding bond length i n the present Rh 1 1 1(ethenamine) complex 6B. IV.3 Reaction Stoichiometry The complex RhCl 3(DMSO) 2("CH 2CH= +NEt 2), 6, and NEt 3-HCl were i s o l a t e d as pure products from the re a c t i o n between RhCl 3(DMSO) 3 and NEt 3 (Section II.4.3.2). Addition of NEt 3 to a s o l u t i o n of RhCl 3(DMSO-d 6) 3 i n CDC13 under Ar showed peaks due to NEt 3H + ( (CDC1 3): 1.38, t; 3.20, q, -11, br) consistent with the abs t r a c t i o n of protons from NEt 3. Equation IV.2 shows the dehydrogenation of one NEt 3 molecule by two others. I f an accompanying rea c t i o n such as eq. IV.3 i s invoked to maintain the redox equivalence of the system the t o t a l redox r e a c t i o n would be as i n eq. IV.4. However, the i s o l a t i o n of RhCl 3(DMS0) 2("CH 2-CH= +NEt 2) requires at l e a s t another equivalent of Rh(III) i n an o v e r a l l reaction such as eq. IV.5. 3NEt 3 •> 2NEt 3H + + CH2=CHNEt2 + 2e IV. 2 - 121 -Rh(III) + 2e > Rh(I) IV.3 Rh(III) + 3NEt 3 > Rh(I) + 2NEt 3H + + CH2=CHNEt2 IV.4 2RhCl 3(DMSO) 3 + 3NEt 3 > RhCl 3("CH 2CH= +NEt 2)(DMSO) 2 + 2NEt 3HCl + Rh(I)/Cl"/4DMS0 IV.5 Early i n the in v e s t i g a t i o n , r e a c t i o n mixtures containing various r a t i o s of NEt 3:Rh were tested to f i n d the optimum conditions for i s o l a t i o n of pure products. I t was f i r s t noted that the ^ H nmr peak positi o n s f o r N-CH2-CH3 and N-CH2CH3 protons of a re a c t i o n mixture were dependent on the amount of excess NEt 3 i n the system. A •'-H nmr study with several mixtures of pure NEt 3 and NEt 3 - H C l gave a single t r i p l e t and a sing l e quartet with averaged chemical s h i f t s dependent on the composition of each mixture (Table IV.3). For the analysis of i n - s i t u r e a c t i o n mixtures by 100 MHz ^H nmr, the set of t r i p l e t s due to -N-CH2-CH3 protons was selected as a sui t a b l e probe, since the interferences by other peaks were l e a s t i n that region. Data f o r various mixtures are summarized i n Table IV.4. Reaction mixtures containing NEt 3/Rh(III) i n a r a t i o < 1.5 show a t r i p l e t at 5l.38. The use of higher r a t i o s of NEt 3/Rh(III) leads to an u p f i e l d s h i f t of the averaged t r i p l e t which, according to the studies with NEt 3/NEt 3H + mixtures, suggests the presence of excess NEt 3 i n the system. The observations are reasonably consistent with the NEt 3/Rh r a t i o of 1.5 proposed i n eq. IV.5. - 122 -Table IV.3 100 MHz *H nmr data f o r NEt 3, NEt 3-HCl, and mixtures of both i n CDC13 Molar r a t i o of NEt 3/NEt 3•HCT t, -NCH2CH3 q, -NCH2CH3 -N+H Pure NEt 3 0.95 2.42 (a) 10 0.94 2.48 8.92 (br) 1.09 2.65 .(a) 1.25 2.85 7.7 (br) 0.5 1.32 2.97 11.7 (br) 0.1 1.38 3.10 (a) Pure NEt 3-HCl 1.42 3.10 12.2 (br) ( a) Peaks not observed. - 123 -Table IV.4 100 MHz 1 H nmr data f o r the -NCH2CH3 protons of N E t 3 > 1-2 h a f t e r adding the amine to RhCl 3(DMSO) 3 In CDC13 under Ar Molar r a t i o of NEt 3:RhCl 3(DMSO) 3 t, -NCH2CH3 0.5 1.38 1 1.38 1.5 1.39 2 1.32 1.20 1.10 8.5 1.05 50 0.90 - 124 -By inspection of a *H nmr spectrum ( F i g . IV.lO.c), a "CH2CH= + N E t 2 / ( t o t a l NEt3 added) r a t i o of 1:3.5 was estimated f o r a f i n a l r e a c t i o n mixture that i n i t i a l l y contained NEt3/Rh i n a r a t i o of 1.5. In other words, about 30% of the i n i t i a l NEt3 had been converted to the dehydrogenated form. This value corresponds to the 33% conversion expected from the stoichiometry of eq. IV.5. Dire c t evidence f o r the presence of Rh(I) species was harder to obtain. Several unsuccessful attempts were made to i s o l a t e a Rh(I) species such as [NEt3H][RhCl2" (DMS0) 2], Two high l y a i r - s e n s i t i v e Rh(I)/DMS0 species, [RhCl(DMSO)2l2 and [PSH][RhCl2(DMS0)2], i s o l a t e d by M o r r i s , 5 0 gave methyl proton nmr peaks at 8(CDC13/(CD3)2C0), 3.32 (br), and 5(CDC1 3), 2.8 (br), res p e c t i v e l y . No such s i g n i f i c a n t peaks were found i n the re a c t i o n between RhCl3(DMS0)3 and NEt3 (1:1.5) i n CDCI3 under Ar, and indeed free DMSO i s present ( F i g . IV.lO.c). I t may be that, i n so l u t i o n , Rh(I) exi s t s i n as s o c i a t i o n with other ligands such as C l " and/or unreacted NEt3. Strong i n d i r e c t evidence f o r the presence of Rh(I) i n concentrations approximating the expected 50% of the t o t a l Rh (eq. IV.5) i s given by data f o r the uptake of O2 by the re a c t i o n s o l u t i o n . A s o l u t i o n of RhCl3(DMS0)3 and NEt3 (1:2) i n C2H4CI2 was contained i n a re a c t i o n f l a s k connected to an uptake apparatus (see Section II.2.1) and shaken under Ar f o r over 2 hours. The s o l u t i o n was then degassed and the subsequent r a p i d uptake of O2 monitored as described i n Section II.2.4. The instantaneous uptake of O2 corresponded to 0.32 and 0.45 mol of O2 per mol of RhCl3(DMSO)3 i n two t r i a l s , suggesting the presence of about 0.3-0.5 equivalents of Rh(I) per i n i t i a l RhCl 3(DMSO)3 i n the system. Further d e t a i l s on the reactions of such Rh(I) species with O2 are given i n Section III.3.2.a. - 125 -P a F i g . IV.10 (a) 400 MHz iH nmr spectra of RhCl 3(DMSO) 3 i n CDC1 3 under Ar, (b) ~5 min a f t e r adding 1.3-1.5 equivalents of NEt 3 to (a) under Ar, (c) 24 h a f t e r (b) (i n s e t , expanded 2.3- 2.5 region of ( c ) ) ; o , DMSO of RhCl 3(DMSO) 3; x, impurity; o, DMSO of RhCl 3(DMSO) 2("CH 2CH= +NEt 2); •, new DMSO and DMS peaks - 126 -The recent communication reporting the r e a c t i o n between PdCl2(PhCN)2 and several t e r t i a r y amines 8 2 described the i s o l a t i o n of several Pd(II)/ethenamine complexes. A metal-centred dehydrogenation of the amines was proposed to explain the i s o l a t i o n and the detection of, f o r example, Pd(II)/ethenamines (35%), Pd(0) (28%) and the amine-HCl (7%), using e t h y l di-isopropylamine. The o v e r a l l reaction, represented as eq. IV.6, p a r a l l e l s that established i n the present work (eq. IV.5). In a s e r i e s of a r t i c l e s published i n the e a r l y 1950's on the organic chemistry of c y c l i c amines, Leonard et a l . used mercuric acetate 2 Pd(II) + 3( iPr) 2NCH 2CH3 — > Pd(0) + [( iPr) 2N +=CH-CH 2")Pd(II) + 2 amine.HCl IV.6 as the dehydrogenating agent i n a convenient synthesis of c y c l i c ethenamines.91 i n a t y p i c a l experiment, a f o u r f o l d excess of Hg(0Ac)2 over the amine was used to obtain near quantitative dehydrogenation. The acetate ligands were found to act as proton acceptors, and the metal centre as the e l e c t r o n acceptor (eq. IV.7), i n a r e a c t i o n analogous to those of eqs. IV.5 and IV.6. IV. 7 - 127 -A s i m i l a r redox process (eq. IV.8), coupled with other reactions (eqs. IV.9 and IV.10), was suggested to be intermediate i n a c a t a l y t i c generation of H 2 from H2O, where NEt 3 i s the e f f e c t i v e reductant (eq. IV.11 i n Scheme I V . l ) . 9 2 Scheme I V . l hV 2 R u L 3 2 + + 2NEt 3 > 2RuL 3 + + Et 3NH + + CH 3CH= +NEt 2 IV.8 Pt/0 2 2RuL 3 + + 2H + > 2 R u L 3 2 + + H 2 IV. 9 CH 3CH= +NEt 2 + H 20 > CH3CHO + E t 2 N H 2 + IV. 10 The t o t a l reaction: hp, R u L 3 z + Et 3N + H 20 > Et 2NH + CH3CH0 + H 2 IV. 11 Pt/0 2, L = 2,2'-dipyridyl - 128 -IV.4 Mechanistic Aspects The mechanism of the dehydrogenation re a c t i o n (eq. IV.5) could t h e o r e t i c a l l y involve e i t h e r an intermediate Rh(III)/NEt3 complex, or an outer-sphere e l e c t r o n transfer from NEt3 to the Rh(III) metal centre (see below). Of the three l i t e r a t u r e examples of t e r t i a r y amine dehydrogenations 82,91,92 d i s c u s s e d above, p r i o r coordination of the amine i s postulated f o r the P d ( I I ) 8 2 a n d H g ( I I ) 9 1 systems. For example, the dehydrogenation of the c y c l i c amine was thought to be f a c i l i t a t e d by the coordination of the amine to Hg(II) as i n Scheme IV.2. In the r e a c t i o n between PdCl2(PhCN)2 and several a c y c l i c t e r t i a r y amines (eq. IV.6), McCrindle et a l . postulated an i n i t i a l coordination of the amine to the Pd(II), followed by a metal i n s e r t i o n into an adjacent C-H bond and a /3-hydride e l i m i n a t i o n to give the enamine which complexes to P d C l 2 (Scheme I V . 3 ) . 8 2 - 129 -Scheme IV.3 -N-CH-CH-/ PdCI2 -HCl > -(-N»C-CH-)PdCl2«-^ l2_ I I N—C-PdCt I I I -N—C=CH -> HPdCl Pd° + HCl In the absence of a vacant coordination s i t e on the metal complex, an outer-sphere e l e c t r o n tr a n s f e r mechanism i s a p o s s i b i l i t y . 9 3 The R u L 3 2 + (L = 2,2'-dipyridyl)-mediated dehydrogenation of NEt 3 (eq. IV.8, Scheme IV.l) 9 2 probably occurs v i a such a mechanism because of the absence of a possible coordination s i t e on the strongly chelated complex. In the present work, attempts were made to i s o l a t e or at l e a s t detect a Rh(III)/NEt 3 intermediate and follow i t s decomposition path. In a 1H-nmr monitored re a c t i o n between RhCl 3(DMSO) 3, and -1.5 equivalents of NEt 3 i n CDC1 3 , the spectrum obtained - 5 min a f t e r the addi t i o n of NEt 3 to the RhCl 3(DMSO) 3 s o l u t i o n ( F ig. IV.lO.b) shows an immediate decrease i n the peaks due to the major isomer mer-cis-RhCl 3-(DMS0)2-(DMSO), accompanied by a newly developed peak due to free DMSO. The added NEt 3 shows two broad humps at 5 1.1 and 2.6, which are s i g n i -f i c a n t l y higher than the free NEt 3 values (Table IV.3). These observa-tions strongly suggest the formation of a Rh(III)/NEt 3 intermediate i n the s o l u t i o n : - 130 -RhCl3(DMSO)2(DMSO) + NEt 3 v % RhCl 3(DMSO) 2(NEt 3) + DMSO IV.12 2NEt 3 PvhCl 3(DMSO) 2(NEt 3) > RhCl + 2NEt 3HCl + C H 2=CHNEt 2 IV. 13 + 2DMSO The 1 H nmr spectrum of a (0.8 NEt 3 + RhCl 3(DMSO) 3) mixture, which has a NEt 3/Rh r a t i o less than the s t o i c h i o m e t r i c a l l y required 1.5 (eq. IV.5), i s given i n F i g . IV.11.a. The deshielding of the t r i p l e t and the quartet set due to -NCH2CH3 and N-CH2CH3, res p e c t i v e l y , with respect to free NEt 3, and the sharpness of the peaks .suggest that here NEt 3 e x i s t s as N E t 3H + or some deshielded form. The minute peaks at 5-8.3, and between 3.6-4.0 are reasonably s i m i l a r to the "CH 2-CH= +NEt 2 protons of the Rh(III)/ethenamine complex, 6, but the upper l i m i t of the amount of ethenamine formed i s estimated to be -10% of the t o t a l NEt 3 added i n i t i a l l y (the estimate i s made by comparing the integrations due to "CH 2-CH- +N(CH 2CH 3) 2 protons i n the 53.5-4.0 region and the peaks due to t o t a l NCH 2CH 3 protons i n the 5l.0-1.5 region). An i r spectrum of the same s o l u t i o n ( F i g I V . l l . b ) shows a well defined but r e l a t i v e l y weak peak at 1630 cm"1, a t t r i b u t a b l e to ^c=N °^ a coordinated ethenamine (see Section IV.2). The smaller peak at 1710 cm"1 could be due to an enamine s a l t of type CH 3CH- +NEt 2•Cl". The i r absorptions f o r such s a l t s of pyridine-enamines are known to occur i n the range 1640-1700 cm" 1. 9 0 Peaks due to ^N +-H o f NEt 3H +, expected at 2400 (br) and 2560 cm"1 (see Section II.4.3.3), are absent. The occurrence of Rh(III)/ethanamine unaccompanied by NEt 3H + i s a curious observation i n terms of the chemistry discussed thus f a r i n t h i s chapter, and w i l l be further discussed i n the next section. The absence of NEt 3H + i s of i n t e r e s t - 131 -F i g . IV.11 (a) 80 MHz iH nmr spectrum of RhCl 3(DMSO) 3 +0.8 NEt 3 i n CDC13 under Ar, and (b) an i r spectrum of the above so l u t i o n i n a i r - 132 -since i t shows that the deshielded -NCH2CH3 protons i n F i g . IV.11a are most l i k e l y to be due to a Rh(III)/NEt3 species. In f a c t , the large peak of free DMSO at 6 2.64 i n the same figure,corresponding to about 30% of the t o t a l free and coordinated DMSO i n so l u t i o n , further supports a s u b s t i t u t i o n r e a c t i o n as i n eq. IV.12 to give a Rh(III)/NEt 3 complex. On leaving the above rea c t i o n s o l u t i o n under Ar over 24 h, broad peaks a t t r i b u t a b l e to PJ\J+_H of NEt3H + appeared at -2400-2600 cm"1, and peaks due to ^rj=j\j, at -1630 cm"1, increased i n i n t e n s i t y . These data are consistent with a redox re a c t i o n as i n eq. IV.13, but without the presence of excess NEt3. Such a reac t i o n proceeds most l i k e l y v i a a slow proton abstra c t i o n from the coordinated NEt3 (eq. IV.13) by the small amount of free NEt3 e x i s t i n g i n s o l u t i o n at any time due to the equilibrium IV.12; the slowness of reaction IV.13 i s a t t r i b u t a b l e to the low concentration free NEt3 a v a i l a b l e . Repeated attempts to i s o l a t e the postulated Rh(III)/NEt3 were not successful. No examples of Rh(III) complexes with coordinated triethylamine could be found i n the l i t e r a t u r e . An unsuccessful attempt to i s o l a t e a Rh(III)/NEt3 complex was mentioned i n a report by Mori et a l . on the ro l e of t e r t i a r y amines i n a RhCl 3 -3H20 catalyzed a r y l a t i o n of ethylene with iodobenzene; the f a i l u r e was a t t r i b u t e d to s t e r i c f a c t o r s . 8 0 A redox reaction, s i m i l a r to that of eq. IV.5 discussed i n t h i s chapter, may have been an unnoticed complication i n the Japanese work. A l i k e l y mechanism for the redox reaction between complex 1 and NEt3 i s given i n Scheme IV.4. - 133 -Scheme IV.4 Rhm( DMSO) N E t -DMSO v N E t3 s+OMSO H — C H , Rh l UrCH 2 CH=NEt 2 ) « • RhSlDMSO) -DMSO CH3— C H = N E t 2 NEt 3 - N E t 3 H * C H 2 = C H - N E t 2 R h 1 + N E t 3 H * The suggested mechanistic pathway i s analogous to that suggested by Leonard et a l . (Scheme I V . 2 ) . A mechanism in v o l v i n g a hydride abstra c t i o n by R h 1 1 1 to give an intermediate R h 1 1 1 ( H ) , as i n the mechanism suggested f o r P d 1 1 systems (Scheme IV.3), i s discounted i n the present case on the grounds that a Rh^ 1 1(NEt 3) intermediate i s u n l i k e l y to provide a vacant coordination s i t e f o r such a H" abstraction. In addition, no peaks a t t r i b u t a b l e to Rh^^-H species were detected i n the 5 0 to 40 region of the nmr spectrum of a re a c t i o n s o l u t i o n . IV.5 Other Routes to Dehydrogenation of NEt 3 A dehydrogenation of about 10% of added NEt 3 without an accompany-ing formation of NEt3H + was mentioned i n the previous section. In order - 134 -to check whether t h i s dehydrogenation was due to trace 0 2 i n the system, a re a c t i o n was c a r r i e d out i n a i r . Here i t was thought that 0 2 might act as an alternate acceptor of hydrogen, as i n eq. IV.14, to give 100% conversion of the NEt 3 to the ethenamine. RhCl 3(DMSO) 3 + NEt 3 + l / 2 0 2 > RhCl 3(DMS0) 2(•CH 2CH= +NEt 2) + H 20 + DMSO IV. 14 Similar dehydrogenation reactions of amines by 0 2 are well-document-e ( j 94-96 However, i n a l l such cases, the amines are i n v a r i a b l y primary or secondary amines that remain coordinated to the metal both before and a f t e r the oxidation (eqs. IV.15 9 5 and I V . 1 6 9 6 ) . A r e a c t i o n between RhCl 3(DMSO) 3 and 0.8 NEt 3 i n CDC13 i n a i r s t i l l gave <10% conversion of NEt 3 to ethenamine (as judged by a *H nmr IV. 15 [Ru(NH 3) 5(NH 2CH 2R)] z + + 0 2 > [Ru(NH 3) 5(N=CR)] z + + 2H 20 IV.16 spectrum of the s o l u t i o n ) , showing that trace 0 2 was not responsible f o r the observed dehydrogenation. - 135 -Another p l a u s i b l e hydrogen acceptor i n the system i s DMSO. Under su i t a b l e conditions DMSO can act as a hydrogen acceptor to form DMS and H 2 0 (eq. IV.17, see Section III.3.2.b for d e t a i l s ) . DMSO + Rh(I) + 2H + > DMS + H 2 0 + Rh(III) IV. 17 In f a c t , the *H nmr spectrum of a so l u t i o n of R h C l 3 ( D M S 0 ) 3 with 0.8 NEt 3 under Ar showed several small peaks i n the 8 2.1-2.5 region (Fig. IV.11.a). The protons of Rh(III)/DMS species generally occur i n t h i s region as doublets (see Section III.4.1). The features of the peaks due to a Rh(III)/DMS by-product are more d i s c e r n i b l e i n the 400 MHz ^H nmr-monitored re a c t i o n between R h C l 3 ( D M S 0 ) 3 and 1.5 N E t 3 i n CDCI3 (Fig. IV.lO.d). Therefore, a slow reaction involving the reduction of DMSO (eq. IV.18) i s probably the reason for the unusual dehydrogenation noted i n Section IV.4. Rh(III)/DMSO + NEt 3 > Rh(III)/DMS + H 2 0 + CH2=CHNEt2 IV.18 IV.6 Hydrolysis of the enamine ligand within the RhCl 3(DMSO) 2-(CH 2CH=NEt 2) complex, 6 The t i t l e r e a c t i o n was discovered during attempts to i s o l a t e a pure species from the reaction between RhCl3(DMSO)3 and NEt3- I t was f i r s t thought that NEt3 reduced a l l the Rh(III) to Rh(I) as i n eq. IV.4, - 136 -and that the formation of the Rh(III)/ethenamine complex was due to the oxidation of the Rh(I) by excess DMSO (eq. IV.19, see also Section III.3.2.b). Rh(I) + 2H + + DMSO > Rh(III) + H 20 + DMS IV.19 To check the p o s s i b i l i t y of such chemistry, the RhCl3(DMSO)3 complex (200 mg, 0.45 mmol) i n acetone (25 mL) was s t i r r e d under Ar i n the presence of excess NEt3 (0.2 mL, 1.4 mmol) overnight and the s o l u t i o n concentrated to give an off-white p r e c i p i t a t e (NEt3 -HCl) and a red-brown s o l u t i o n . Excess DMA'HCl (180 mg, 1.5 mmol) was added under Ar to the red-brown f i l t r a t e , and the s o l u t i o n was s t i r r e d overnight to obtain an a i r - s t a b l e , orange s o l u t i o n and a yellow p r e c i p i t a t e . An i r spectrum of the crude product i n Nujol showed the presence of Et2NH2 + ( ^N IH' 3140 cm"1, s) NEt 3H + (^N+_H; 2400-2600 cm"1, s) and only a weak peak due to "CH 2-CH= +NEt 2 ( ^ c=N^ 1 6 4 0 cm"1, w). A f t e r r e c r y s t a l l i z a t i o n from CH 2Cl 2/ether i n a i r , a complex analyzing f or [NEt 3H] [RhCL/^DMSO^] was i s o l a t e d i n about 20% y i e l d (-40 mg, 0.08 mmol). Anal. calcd. for c 1 0 H 2 8 c l 4 N O 2 s 2 R h : c 23.86; H 5.61; N 2.78; found: C 23.35; H 5.29; N 2.71.  lU nmr. 5(CDC1 3): 1.38, t, 9H, (CH 3CH 2)3NH +; 3.13, q, 6H, (CH 3CH 2) 3NH* (see Table IV.3); 3.48, s, -11H, trans (CH 3) 2S0; 3.53, s, -IH, c i s (CH.3)2S0.34 The mother-liquor from the above r e c r y s t a l l i z a t i o n step was concentrated to about 1-2 mL and l e f t i n a Schlenk tube closed with a rubber septum. A f t e r a few days, several red c r y s t a l s , estimated to be -20 mg (0.04 mmol), appeared. An i r spectrum of the c r y s t a l s showed strong peaks at 3140 cm"1 (V^n c f R 2NH 2 +) and 1128 and 1140 cm'1 ( Vgo °f DMSO) (F i g . IV.12). X-ray analysis of the red c r y s t a l s showed F i g . IV.12 An Ir spectrum [Et 2NH 2][trans-RhCl 4(DMSO) 2] i n KBr - 138 -there to be two d i s t i n c t c r y s t a l forms, both with the molecular formula [Et 2NH2] [trans-RhCl 4(DMS0)2] , 13. In one type, the ca t i o n shows H-bonding with a C l " lig a n d each from two adjacent complex anions (13A, Fi g . IV.13), while i n the other, the i n t e r a c t i o n i s with the DMSO ligands on the complex anion (13A, F i g . IV.14). On an average, the Rh-Cl bond lengths (2.331-2.346 A) and the S=0 bond lengths (1.422-1.472 A) i n 13A and 13B are s l i g h t l y shorter than those reported f o r the complex anion i n [H(DMS0)2] [trans-RhCl 4(DMSO) 2] (2.33-2.37 and 1.479-1.482 A, r e s p e c t i v e l y ) 5 0 . Within the two c r y s t a l types, the e f f e c t of d i f f e r e n t H-bonding modes are r e f l e c t e d i n t h e i r Rh-Cl and S=0 bond lengths (Appendix I I ) . The R h ( l ) - C l ( l ) bond length i n 13A i s longer than the corresponding bond length i n 13B, while the reverse i s true f o r the S(2)-0(2) bond lengths. The mother-liquor s t i l l showed J/JVJ+.H peaks due to Et2NH2 + and Et3NH + cations and DMSO but no further pure compounds could be i s o l a t e d from the mixture. The absence of the ethenamine i r peaks i n the re a c t i o n mixture and the appearance of peaks due to Et2NH 2 + suggested the p o s s i b i l i t y of reac t i o n of HCl with the Rh(III)/ethanamine complex (e.g. as eq. IV.20). RhCl 3(DMS0)2("CH2CH= +NEt 2) + HCl > [CH 3CH= +NEt 2][RhCl 4(DMS0) 2"] H2O impurity CH3CH0 + [Et 2NH 2][RhCl 4(DMS0) 2] IV. 20 - 139 -F i g . IV.13 A stereo-view ORTEP diagram of [Et 2NH 2][trans-RhCl 4(DMSO) 2] ( c r y s t a l type A). Diagram shows an a d d i t i o n a l anion per molecule (Some s t r u c t u r a l parameters given i n Appendix II) - 140 -F i g . IV.14 A stereo-view ORTEP diagram of [Et 2NH 2][trans-RhCl4(DMS0) 2] ( c r y s t a l type B). Diagram shows an a d d i t i o n a l anion per molecule.(Some s t r u c t u r a l parameters given i n Appendix II) - 141 -The r e a c t i o n between pure RhCl 3(DMSO) 2(CH2CH=NEt2), 6, (6 mg, -0.015 mmol) and DMA'HCl (4 mg, -0.03 mmol) i n acetone-dg (-0.6 mL) was thus monitored by 80 MHz *H nmr. No H2O was added to the system since the acetone-dg already contained -0.04 mmol H2O as an impurity ( §2.93, F i g . IV.15.a). The o r i g i n a l nmr peaks due to Rh(III)/"CH 2CH= +NEt2 and DMA'HCl ( F i g . IV.15.b) were replaced over 2 days by a new set of peaks (F i g . IV.15.C) assignable to E t 2 N H 2 + (5 1.4, t, 6H; 3.2, m, 4H), DMA ( 8 3.0 br, N(CH 3) 2) and -CH3CH0 (§9.8, IH, D 20 s t a b l e ) . The peak p o s i t i o n of DMSO remained e s s e n t i a l l y unchanged. The i r spectrum of the products i n the form of a t h i n f i l m , made by slow drying of a drop of the s o l u t i o n on a NaCl window, showed the absence of the V Q ^ $ peak at -1640 cm"1 and the presence of a sharp absorption at 3140 cm"1, c h a r a c t e r i s t i c of ^N-H °^ Et2NH2 +. The D 2 0 - i n s e n s i t i v e peak at §9.8 was assigned to CH3CH0, though the expected s p l i t t i n g into a quartet (J-2Hz) was not d i s c e r n i b l e . The expected doublet due to CH3-CH0, 5-2.2, was obscured by the acetone-d5 peak and the CH3C0-peak of DMA. The same reactants (6 + DMA'HCl) i n CDC13 s o l u t i o n showed a s i m i l a r but very slow reaction; a f t e r 10 days the quartet (J-2 Hz) at §9.8 was observed. McCrindle et a l . noted a s i m i l a r slow decomposition of Pd 1 1(ethenamine) i n CH2C12 containing traces of H2O to give the corresponding aldehyde or ketone product. 8^ - 142 -IV. 15 (a) 400 MHz LH nmr spectra of RhCl 3(DMSO)2(~CH 2CH= +NEt2) In acetone-dg; (b) a f t e r adding one equivalent of DMA'HCl to (a); (c) 24 h a f t e r (b); x, NEt3'HCl impurity - 143 -IV.7 An i n t e r p r e t a t i o n of the data obtained by M o r r i s 5 0 f o r the re a c t i o n between RhCl3(DMSO)3 and l , 8-bis(dimethylamino)-naphthalene As discussed i n the introduction to t h i s chapter, a reaction s i m i l a r to that between RhCl3(DMSO)3 and NEt3 was discovered by Morris for another strongly basic (pK a 11.3) t e r t i a r y amine, 1,8-bis(dimethyl-amino)naphthalene (or P.S.). Morris noted that a r e a c t i o n between RhCl3(DMSO-dg)3 and P.S. i n CDCI3 yi e l d e d PS.HC1, suggesting proton abstra c t i o n from one P.S. molecule by another. An a i r - s t a b l e species, containing a P.S. d e r i v a t i v e , was i s o l a t e d i n an impure form from a r e a c t i o n s o l u t i o n containing other a i r - s e n s i t i v e species, but was not c h a r a c t e r i z e d . 5 0 The a b s t r a c t i o n of protons from one NEt3 molecule by another was concluded i n the present work to be part of a 2-electron redox re a c t i o n i n v o l v i n g a Rh(III) metal centre (eqs. IV.2-IV.5). The r e a c t i o n between RhCl3(DMSO)3 and P.S., discovered by Morris, may now be explained based on the same p r i n c i p l e s . The suggested t o t a l r e a c t i o n i s given i n eq. IV.21. The formation of an N-carbene d e r i v a t i v e , appearing as a ligand on the complex, 14, i s invoked to accommodate a 2-electron oxidation of an N-methyl amine. Proton Sponge scavenges protons to form mono-protonated P.S.H+ species, where the proton i s strongly hydrogen-bonded to both nitrogen atoms on the molecule. In the r e a c t i o n stoichiometry suggested (eq. IV.21), one proton i s held as P.S.H+, while the other proton i s held by the Me2N- group on the P.S./carbene d e r i v a t i v e as i n 14. The l a t t e r type of protonation i s hypothesized to accommodate the approximately 1:1 P.S./Rh stoichiometry noted by M o r r i s . 5 0 - 144 -2RhCl 3(DMSO) 3 + 2P.S. CH 3 I -> RhCl 3(DMSO) 2(:CHN- ) HC1-(CH 3) 2N 14 + P.S. HC1 + RhI/2Cl"/ADMS0 IV. 21 L i t e r a t u r e reports show that Rh(III) can form stable complexes with carbenes containing a hetero-atom OL to the c a r b o n . 9 7 , 9 8 The N-carbene complexes are characterized by a strong vc=N absorption at -1600 cm"1, since the C-N bond i n a carbene i s expected to show considerable double bond character, e.g. R h 1 1 1 (:C—N—) < > R h 1 1 1 (:"C=+N—) I I I I H W According to Morris's i r data, the crude complex containing the P.S. d e r i v a t i v e shows absorbances at 1605 and 1590 cm"1, e i t h e r of which correspond w e l l with the l i t e r a t u r e values f o r the R h 1 1 1 carbene c o m p l e x e s . 9 7 , 9 8 The -^H nmr data obtained by Morris (Table IV. 5) are also compatible with the proposed structure, 14, although the peak assigned f o r the carbene proton i s at a somewhat lower 5 than those reported f o r Rh(III)/carbene complexes. For example, the carbene protons of RhCl 3(PPh 3) 2(CHNEt 2)-CHC1 3 and RhCl 3(PEt 3) 2(CHNEt 2) appear at §8.53 and 11.26, r e s p e c t i v e l y . 9 7 A change i n the chemical s h i f t s of the carbene ligands by 2.7 ppm f o r a change i n the li g a n d environment from PPh 3 to P E t 3 suggests the chemical s h i f t of a carbene proton to be - 145 -Table IV.5 I n t e r p r e t a t i o n ^ of the 100 MHz *H nmr data obtained by Morris< b) f o r the orange c r y s t a l s i s o l a t e d i n the reaction between RhCl3(DMSO)3 (1.0 mmol) and P.S. (0.8 mmol) i n CH 2C1 2 Peaks Assignment Rh(III)/carbene complex, 14 7.5-7.7 (5H, m) 7.0 (IH, dd, J = 2, 7 Hz) 5.15 (IH, s, br) 3.75 (6H, s, br) 3.44 (3H, s) 3.54 (12H, s) H 2-H 6 «1 Rh=CH-N HC1. (CH 3) 2N-CH-N(CH3)-(CH) 2S0 (S-bonded) [P.S.H.][RhCl 4(DMS0) 2], 15 7.5-8.0 (0.2 x 6H, m) 3.29 (0.2 x 12H, d, J = 2 Hz) 3.50 (0.2 x 6H, s) aromatic protons of P.S.H+ ( c) -N(CH 3) 2H + of P.S.<C> (CH 3) 2S0 (S-bonded) Based on the s t r u c t u r a l formula 14. Ref. 50; chemical s h i f t s given i n p.p.m. Protons due to N+H not observed; ascribed to the low concentration. Compares well with the values f o r P.S.H. +. 5 u - 146 -h i g h l y s e n s i t i v e to i t s environment. Therefore the value of 5.15 assigned i n the present case i s considered reasonable. The impurity i n the P.S. d e r i v a t i v e appears to be [P. S .H] [RhCl^DMSO) 2 ] , 15, which i s present i n amounts up to 20%. The elemental analysis c a l c u l a t e d for 14 (C 35.20, H 4.75 and N 4.56) agrees well with the experimental values (C 35.35, H 4.83 and N 4.61), despite the presence of the P.S.H.+ impurity, presumably because the general formulae of 14 (C]_4H3QN202S2Rh) and 15 (C^ 4H3 2N 20 2S2Rb) d i f f e r only by two hydrogens. Morris reported the f a i l u r e to obtain 14 i n pure form. 5 0 Attempts i n the present work, using CH2Cl 2/EtOH solvent, f a i l e d also to give pure products. IV.8 Concluding Remarks The dehydrogenation of t e r t i a r y amines by RhCl 3(DMSO)3 could be a more general r e a c t i o n for most t r a n s i t i o n metal complexes, i f suitable conditions p r e v a i l , since s i m i l a r dehydrogenations with Hg(II) and Q O Q1 Pd(II) are already known.'-' 7 1 P r i o r coordination of the amine could be a general feature, though t h i s has not been established i n the l i t e r a t u r e examples. A r e l a t i o n between the b a s i c i t y of the t e r t i a r y amine and i t s redox r e a c t i v i t y was noted at the outset of t h i s chapter (see Section I V . l ) . This r e l a t i o n cannot be a t t r i b u t e d simply to the proton a f f i n i t y of the base since a strong base i s not e s s e n t i a l f or the proton abstrac-t i o n steps i n the three suggested pathways (Schemes IV.1-IV.3). For example, i n Scheme IV.2, the weak base AcO" i s used to abstract the proton from a C-H bond already weakened by coordination of the amine to - 147 -the metal. In Scheme IV.3, the metal i n s e r t i o n s p l i t s the C - H bond. A co n t r i b u t i n g f a c t o r to the b a s i c i t y dependence i s almost c e r t a i n l y the differences i n the bond strengths wi t h i n R h ( I I I ) / t e r t i a r y amine intermediates. For example, an i n t e r a c t i o n between a Rh(III) and a strongly basic amine should have a greater weakening e f f e c t on the N-C-H protons than an i n t e r a c t i o n with a weaker base. Therefore, proton abstrac t i o n by any base B, e i t h e r d i r e c t l y (Schemes IV.2 and IV.4) or v i a a j3-hydride el i m i n a t i o n (Scheme IV.3), should be easier from a stronger base amine. For larger amines such as P.S., the concentration of the Rh I ] : i( amine) intermediate may be low but, i f the weakening of the C-H bond wit h i n such an intermediate i s s u f f i c i e n t l y large, the re a c t i o n should proceed. In terms of the arguments based on b a s i c i t y of amines developed above, primary or secondary amines should show s i m i l a r r e a c t i v i t y since they too have high pK a values (e.g. (C 2H 5)NH 2 11.0; (C 2H 5) 2NH 10.6). Primary and secondary amines form stable complexes with platinum metals such as R h ( I I I ) 8 0 ' 9 9 ' 1 0 0 and R u ( I I ) . 9 4 " 9 6 The amine ligands on some of these metal complexes are known to dehydrogenate i n the presence of external o x i d i z i n g agents (e.g. eqs. IV.15 and IV.16 i n Section I V . 5 ) . 9 4 - 9 6 To our knowledge there are no reports on the dehydrogenation of primary and secondary amines i n the absence of an external o x i d i z i n g agent. whether the observed dehydrogenation of t e r t i a r y amines, v i a coordination to a t r a n s i t i o n metal (Section IV.4) with the metal centre acting as an e f f e c t i v e i n t e r n a l o x i d i z i n g agent (eq. IV.4), i s s p e c i f i c to t e r t i a r y amines merits further i n v e s t i g a t i o n . - 148 -CHAPTER V A STUDY OF THE CATALYZED UPTAKE OF H 2/0 2 MIXTURES IN THE PRESENCE OF RhCl 3(DMSO) 3 COMPLEX IN DMA AND THE ACCOMPANYING OXIDATION OF THE SOLVENT - 149 -V A study of the catalyzed uptake of H2/O2 mixtures i n the presence of RhCl3(DMSO)3 complex i n DMA and the accompanying oxidation of the solvent V . l Introduction The p o t e n t i a l of using H2/02 mixtures i n the presence of rhodium c a t a l y s t s f o r the autoxidation of organic substrates was discussed i n Section 1.4. During some preliminary studies, an i n t e r e s t i n g rhodium-and H2-dependent autoxidation of solvent DMA i n the presence of RhCT3(DMSO)3 (1) complex was discovered. There was no autoxidation of DMA i n the absence of e i t h e r complex 1 or H2, showing that both were e s s e n t i a l f o r the observed c a t a l y t i c a c t i v i t y . Autoxidations of N-alkyl or N,N-dialkylamides are well docu-m e n t e d . 1 0 1 " 1 0 6 T h e studies appear to have been motivated mainly by the importance of preventing the autoxidation of i n d u s t r i a l l y important amide polymers. The l i t e r a t u r e on oxidation of amides by other oxidants involves p e r o x y s u l f a t e , 1 0 7 RuO^, 1 0 8 and a l k y l hydroperoxides or peracids with or without added metal c a t a l y s t . 1 0 9 , 1 1 0 Autoxidation of N-alkyl-amides generally occurs v i a f r e e - r a d i c a l chain mechanisms, which are i n i t i a t e d e i t h e r photochemically at low temperatures, or thermally at ~100-130°C. The oxidations occur at C-H bonds (X to the nitrogen to y i e l d -N-C=0 products (e.g. CH3CON(CH3)CHO (16) i n DMA autoxidation) or other decomposition products. The mechanistic d e t a i l s of the reactions are also well documented. 1 0 1' 1 0 5 Major mechanistic pathways f o r DMA autoxidation are given by eqs. V.1-V.7 i n Scheme V . l . At temperatures - 150 ->100°C the N-alkyl hydroperoxide (17) i s the i n i t i a l product, while at temperatures -0°C the termination re a c t i o n products (eqs. V.4-V.7) predominate. Scheme V . l I n i t i a t i o n : CH 3CON(CH 3) 2 Propagation: In. InH CH 3CON(CH 3)CH 2-(In. = i n i t i a t o r ) V . l CH 3CON(CH 3)CH 2-CH3CON(CH3)CH2OOH (17) CH 3CON(CH 3)CH 20 2' CH 3CON(CH 3) 2 V.2 V.3 Termination: 2CH3CON(CH3)CHO + H 20 2 16 V.4 2CH 3CON(CH 3)CH 20 2' CH3CON(CH3)CH2OH 19 ->[CH 3CON(CH 3)CH 2] 20 2 + 0 2 18 V.5 -> CH3CON(CH3)CHO + [CH3CON(CH3)CH2OH] + 0 2 19 CH 3C0N(CH 3)H + HCHO 20 V.6 V.7 Met a l - c a t a l y s i s of N-alkylamide autoxidations does not appear to have received much attention. Work i n our laboratory i n the 1960's showed that the dimer [RhCl(COE) 2] 2 i n the presence of excess L i C l i n DMA at 80°C c a t a l y t i c a l l y oxidized both the COE li g a n d and the s o l v e n t . 6 1 - 151 -V.2 Product analysis Small amounts of solvent oxidation products were detectable by gas chromatography (Section II.5.4.c) within 0.5 h of shaking a 5.0 x 10" 3 M s o l u t i o n of 1 i n DMA at 50°C under a H2/02 (2:1) mixture. For ' the purposes of product characterization, the reactions were allowed to pro-ceed for at l e a s t 4-5 h to allow the accumulation of s u f f i c i e n t products f o r a n a l y s i s . A gas chromatogram of the r e a c t i o n mixture a f t e r 4-5 h of r e a c t i o n showed a major peak with a retention time of 11.5 min and a smaller peak at 15 min accounting f o r <5% of the major peak. The major peak was i d e n t i f i e d as that of CH3CON(CH3)CHO (16) and the minor peak as that of CH3CON(CH3)H (20) by c o i n j e c t i o n with authentic samples. A gc/ms of the major peak d i d not show the expected parent ion (m/e = 101), but three other d i s t i n c t m/e values observed at 73 (40), [CH 3C0N(CH 3)H] +; 58 (30), [CH 3C0NH] +; and 43 (100), [CH 3C0] +; matched those of authentic CH3C0N(CH3)CH0 (Section II.4.4). Since the parent ion i n a mass spectrum of the authentic compound occurred at le s s than 0.1% r e l a t i v e i n t e n s i t y , the absence of the parent ion i n a gc/ms of the r e a c t i o n mixture was considered acceptable. Lock and Sagar noted the decomposition of some N-alkyl hydro-peroxides under high temperature (75-135°C) gas chromatographic condi-t i o n s . 1 0 3 Authentic CH3C0N(CH3)CH200H (17), prepared i n the present work, was also found to decompose, almost q u a n t i t a t i v e l y , to CH3C0N(CH3)CH0 (16) under the gc conditions used for analysis (Section II.4.4). Thus, the major peak i n a gas chromatogram from the product s o l u t i o n can be due to e i t h e r 16 or 17, or both. A 1H nmr spectrum i n CDCI3 of a re a c t i o n residue from a product s o l u t i o n a f t e r -0.2 M t o t a l - 152 -uptake i s given i n F i g . V . l . The large peak at §2.1 and the corres-ponding two peaks centred at §3.0 i n F i g . V . l are assigned to DMA. The peaks at §9.2, 3.1, and 2.4, which occur i n an approximate 1:3:3 r a t i o , are assigned to CH3CON(CH3)CHO. The set of peaks at §5.15, 3.0 (hidden under -NCH3 peak of DMA) and 2.15, and §5.2, 3.15 and 2.12 ' are assigned to c i s and trans isomers of CH3C0N(CH3)CH200H, re s p e c t i v e l y . The assignments were made by comparison with *H nmr data f o r authentic samples (Table V . l ) . Table V . l 80 MHz *H nmr sp e c t r a l data i n CDCI3 f o r DMA and some deri v a t i v e s (measured In ppm with TMS i n t e r n a l standard) Compound NCH3 NCH202/NCH20H NCHO OH/NH CH3CON CH 3C0N(CH 3)H( a) 2.87(d) - - -8(br) 1.98 CH 3CON(CH 3) 2( a) 3.00(d) - - 2.10 trans-CH 3C0N(CH 3)CH 200H( b) 3.19 5.18 - - l l ( b r ) 2.15 cis-CH 3C0N(CH 3)CH 200H( b) 3.02 5.14 - ~ l l ( b r ) 2.22 CH 3C0N(CH 3)CH0 3.13 - 9.2 2.46 Commercial Samples Cis:trans =1:3; r a t i o estimated by *H nmr (Fig. V.2); disc u s s i o n given i n the text. F i g . V . l 80 MHz nmr spectrum i n CDCI3 of a r e a c t i o n residue from the c a t a l y t i c uptake of H2/O2 (~2:1) mixture by RhCl 3(DMS0) 3 i n DMA ( 5 . 0 x l 0 - 3 M) a f t e r ~4 h of r e a c t i o n -OOH 12.0 H o " NCH 2 0 O H o ppm NCH3 CH3CO-F i g . V.2 80 MHz ^ nmr spectrum of CH3CON(CH3)CH2OOH i n CDCI3 (x, DMA impurity) - 155 -The occurrence of c i s and trans isomers i n c e r t a i n N,N-alkylamides or t h e i r d e r i v a t i v e s i s well documented. 1 0 4• 1 1 1 The hydroperoxide de r i v a t i v e s generally show a greater abundance i n the c i s isomer (17A) than the trans isomer (17B), and t h i s i s explained i n terms of a greater s t a b i l i t y imparted to the c i s isomer by i n t e r n a l H-bonding. H x ° — 0 0<v /CH2 % / CrC X CH 2 OOH CH 3 C H 3 J 17A 17B A iH nmr spectrum of authentic 17 prepared i n the present work (Section II.4.4) showed two sets of peaks occurring i n a —3:1 r a t i o ( Fig. V.2). The set of peaks found i n a greater abundance i s assigned to the c i s isomer based on the r a t i o n a l e given above. The peaks assigned to 17A and 17B, and the i n t e n s i t y r a t i o of the two isomers i n CDCI3 (Fig. V.l) i n the *H nmr of r e a c t i o n residues, show good correspondence with those of the authentic samples. The *H nmr spectrum of the residue showed much smaller amounts of the N-formyl (16) and H 20 products since they are l a r g e l y removed with the stripped solvent; a gas chromatogram of the stri p p e d solvent shows the presence of both. The smaller peaks at 6 2.15, 3.0 and 5.25 could be due to another peroxo d e r i v a t i v e , perhaps some 18 shown i n Scheme V . l , but t h i s was not ascertained. The set of peaks at 3.4-3.6 are assigned to methyl protons of DMSO (Section - 156 -I I I . l ) . The D2O-sensitive broad peak at - 8 2.3 could be due to H2O2, since H20 2 i s found as a s i g n i f i c a n t product i n a r e a c t i o n s o l u t i o n a f t e r -0.2 M H2/O2 uptake (see l a t e r ) . Analysis f o r the t o t a l organic peroxide content i n a product s o l u t i o n (Section II.5.2) a f t e r -0.14 M t o t a l gas uptake gave a value of 8 x 10" 3 M. The amount of 17 i n the same product s o l u t i o n was estimated to be about 1 x 10" 2 M, using the r a t i o of integrations between the N-CH2 of 17 and the phenyl protons of added i n t e r n a l standard DPSO i n a nmr spectrum of the reaction residue i n CDCI3. The correspondence between the amount of organic peroxide and the amount of 17 i n a product s o l u t i o n suggests the organic peroxide to c o n s i s t e s s e n t i a l l y of the hydroperoxide 17. The thermally unstable methylol product CK^CON^^CI^OH 1 1 1 (19) (Scheme V . l ) gives the same retention time as CH3CON(CH3)H (20) i n a gas chromatogram (Section II.5.4.b), perhaps because of decomposition to 20 under gc conditions. The methylol 19 has been detected previously as an unstable intermediate i n the autoxidation of DMA by p e r a c i d s . 1 1 2 In the present case, the absence of s i g n i f i c a n t amounts of CH3CON(CH3)H i n a gas chromatogram was taken as evidence for the absence of s i g n i f i c a n t amounts of both 19 and 20 i n a product s o l u t i o n . Gas chromatograms of the product solutions f a i l e d to show other possible h i g h - v o l a t i l e oxidation products of DMA such as (CI^^NH. The absence of s i g n i f i c a n t amounts of HCHO, HCO2H or CH3CO2H, or CO2 was v e r i f i e d by colorimetry, t i t r i m e t r y , and gas chromatography, r e s p e c t i v e l y (Section II.5.3). Analysis f or H2O (Section II.5.1) and H2O2 (Section I I . 5.2) showed su b s t a n t i a l amounts of both i n a product s o l u t i o n (see l a t e r ) . - 157 -To obtain the product d i s t r i b u t i o n as a function of time, product solutions were analyzed at various points up to -4.5 h of reac t i o n time. Because of the destructive nature of some a n a l y t i c a l methods (e.g. iodometric assay f o r peroxides), several experiments were needed to get the time dependence of the product d i s t r i b u t i o n . The r e s u l t s are summarized i n Table V.2. The extent of the H2/O2 re a c t i o n i s given by both the time of reaction and the t o t a l uptake of gas. Plots of time vs. concentrations of the t o t a l gas uptake, H2O, 16 + 17, and H2O2 products are given, by a-d i n Fi g . V.3. The rate of t o t a l gas uptake remains e s s e n t i a l l y l i n e a r for approximately the f i r s t 1 h of reac t i o n time and then f a l l s o f f slowly. The product d i s t r i b u t i o n up to about 0.75 h of re a c t i o n time i s also markedly d i f f e r e n t from that found between 0.75 h to -4.5 h of monitored r e a c t i o n time. Oxidation products of DMA (16 + 17) can be detected as ea r l y as 0.25 h of reaction, though i t was d i f f i c u l t to determine accurately the concentration. Although the major oxidation product detected i s the N-formyl 16, the i n i t i a l product of autoxidation i s almost c e r t a i n l y the hydroperoxide 17. The i n i t i a l hydroperoxide products of N-alkyl amides (eq. V.3, Scheme V.l) decompose r e a d i l y under re a c t i o n conditions to give mixtures of p r o d u c t s . * 0 3 " * 0 5 The decomposition of 17 to 16 (eq. V.8) i s noted also i n the present work (Section II.4.4); although the rea c t i o n stoichiometry i s not determined, e i t h e r i n the l i t e r a t u r e examples or i n the present work, H2O i s included i n the equation as the obvious by-product. CH3C0N(CH3)CH200H > CH3C0N(CH3)CH0 + H 20 V.8 - 158 -Table V.2 Summary of a n a l y t i c a l data( a) f o r the rea c t i o n mixtures from the c a t a l y t i c uptake of a H2/O2 (2:1) mixture at 1.0 atm i n the presence of 5.0 x 10" 3 M RhCl 3(DMSO) 3 c a t a l y s t i n DMA at 50°C Experiment Time of Tota l 16+17 Total 17(b) H 20 2 H 20 No reaction, h uptake peroxides 1 0. 55 2. .8 0.52 1.0 0.2 0.8 -2-7 0. 75 4. .2 - - - 1.0±0 8 1. 1 6. .0 - - - -9 1. 1 6. .0 0.96 1.2 0.2 1.0 -10 1. 1 6. .0 1.4 0.2 1.2 2.2 11 2. 0 10. .8 1.8 - - -12 2. 6 14. .0 3.0 0.8 2.2 5.4 13 4. 2 20. .0 3.8 1.6 2.2 -14 4. 2 20. .0 4.1 - - -15 4. .2 20. .0 4.0 - - -16-19 4. .2 20. .0 3.8±0.4< d) - - - 8.2±0 20 4. .6 21. .4 4.1 3.8 1.2 2.6 9.4 De t a i l s of a n a l y t i c a l methods are given i n Section II.5. A l l concentrations are reported i n mol L" 1 x 10 2. A blank indicates that the p a r t i c u l a r analysis was not c a r r i e d out. Determined as organic peroxides (Section II.5.4); see text f o r ra t i o n a l e f o r assuming e s s e n t i a l l y a l l organic peroxides e x i s t as the hydroperoxide 17. Average of 6 sets of data: 0.7, 0.8, 0.8, 1.0, 1.0 and 1.4. Average of 4 sets of data: 3.6, 3.6, 4.0, and 4.2. e Average of 4 sets of data: 7.8, 8.1, 8.2 and 8.5. - 159 -T i m e , h F i g . V.3 The p l o t s of concentrations of (a) t o t a l H 2 + 0 2 uptake, (b) H 20, (c) 16 + 17, and (d) H 20 2, as functions of time f o r r e a c t i o n mixtures containing 5.0xl0" 3 M RhCl3(DMS0) 3 i n DMA under a mixture of H 2/0 2 (500/260 i n t o r r ) at 50°C (data i n Table V.3). - 160 -Hydrogen peroxide occurs at a greater concentration than e i t h e r H2O or (16 + 17) i n about the f i r s t 0.75 h of re a c t i o n time. Photo-i n i t i a t e d f r e e - r a d i c a l autoxidation of a simple N,N-alkyl amide such as DMA could produce H2O2 v i a a termination pathway (eq. V.4, Scheme V . l ) . I f the H2O2 detected i n the present case i s due to an autoxidation of DMA, then f o r each equivalent of H2O2 detected, at l e a s t two equivalents of 16 should be produced. However, i f there i s metal-catalyzed decomposition of H20 2, the observed 16: H2O2 r a t i o w i l l be greater s t i l l . Since the concentration of H2O2 remains c o n s i s t e n t l y higher than that of 16 + 17, at l e a s t upto about 2.5 h, i t i s almost c e r t a i n that some or a l l of the detected H2O2 r e s u l t s from a r e a c t i o n other than the autoxidation of DMA. Homogeneously catalyzed hydrogenolysis of O2 to y i e l d H2O2 (eq. V.9) i s documented i n some patent l i t e r a t u r e . H 2 + 0 2 > H 20 2 V.9 In the few other reports available 2 6,114-116 a n e t hydrogenolysis of O2 to H 20 (eq. V.10) i s ei t h e r v e r i f i e d 2 6 ' 1 1 5 or a s s u m e d 1 1 4 ' 1 1 6 . H 2 + 0.50 2 > H 20 V.10 Hydrogen peroxide decomposes r e a d i l y i n the presence of c a t a l y s t s , p a r t i c u l a r l y t r a n s i t i o n metal ions (eqs. V.11-V.14). 1 1 7 M n + + H 20 2 > M n + 1 + HO- + HO- V . l l - 161 -Mn+1 + H 2 Q 2 > Mn+ + H+ + HO2" V.12 HO- + H02- > H 20 + 0 2 V.13 Adding eqs. V.11-V.13 gives 2H 20 2 > 2H 20 + 0 2 V.14 In the patented p r o c e s s e s , 1 1 3 the decomposition of H 20 2 was avoided by containing the transition-metal c a t a l y s t s used, f o r example, I r C l ( C O ) ( P P h 3 ) 2 , 1 1 3 c * i n water immiscible solvents which are continuously extracted with water to remove promptly any H 20 2 formed. The H 20 2 detected i n the present case almost c e r t a i n l y r e s u l t s from a metal-catalyzed hydrogenolysis of 0 2 to H 20 2, s i m i l a r to those noted i n the patent l i t e r a t u r e . I t was d i f f i c u l t to estimate the amount of H 20 formed i n the early stages of the reac t i o n ( i . e . <0.5 h of reac t i o n time), because of the l o w - s e n s i t i v i t y of the ne a r - i r method for concentrations <0.01 M i n DMA (Section II.5.1). A f t e r 0.75 h of reaction time the concentration of H 20 increases s t e a d i l y (curve b, F i g . V.3). The data obtained at > 0.75 h (Table V.2) f i t a l i n e a r p l o t with a c o r r e l a t i o n c o e f f i c i e n t of 0.996 and an intercept e q u a l l i n g 0.25 h on the x-axis, suggesting the amount of H 20 i n the 0-0.25 h period to be close to zero. Some or a l l of the H 20 observed i n -0.25-4.5 h of reaction time c e r t a i n l y r e s u l t s from the decomposition of H 20 2 (eq. V.14) and/or the decomposition of the hydroperoxide 17 (eq. V.8). In p r i n c i p l e , the homogeneously catalyzed hydrogenolysis of 0 2 could lead to H 20 (eq. V.10), without the - 162 -intermediacy of H2O2 (eq. V.9) and i t s subsequent metal-catalyzed decomposition (eq. V.14). In the present case the i n i t i a l product of O2 hydrogenolysis i s most c e r t a i n l y H2O2, since i t i s found i n la r g e r amounts than H2O i n the 0-0.75 h rea c t i o n period, and the amount of H2O from 0-0.25 h i s deduced to be n e g l i g i b l e or zero. Occurrence of larger amounts of H2O, and a steady state amount of H2O2 at longer r e a c t i o n times, are consistent with a metal-catalyzed decomposition of H2O2 to give H2O (e.g. eq. V.14; see l a t e r f o r other possible modes). I f the assumption that CH3CON(CH3)CH2OOH i s the i n i t i a l product of DMA autoxidation i s correct, the net gas uptake should correspond to the amounts of underlined atom equivalents i n H 20 2 and H.2Q and CH3CON(CH3)CH2OOH products found at any time. The amount of H 20 used i n a c a l c u l a t i o n should be corrected, of course, f o r any H2O r e s u l t i n g from the decomposition of hydroperoxide 17 to 16 (eq. V.8). The above r e l a t i o n between t o t a l gas-uptake and the corresponding H and 0 atom equivalents i n the products should hold true i r r e s p e c t i v e of mechanism(s) of H2O production involved, since the r e l a t i o n considers only the net stoichiometry of reaction. The product d i s t r i b u t i o n at -4.6 h of reaction, given i n experi-ment 20, Table V.2, i s used to tes t the correspondence between the t o t a l gas uptake and the product d i s t r i b u t i o n , because a f u l l a nalysis of a product s o l u t i o n i s given by that set of data. The concentration of H 20 2 (2.6 x 10" 2 M) corresponds to 5.2 x 10" 2 M uptake of H 2+0 2 (eq. V.9) and the concentration of 16 + 17 (4.1 x 10" 2 M) corresponds to an O2 uptake of 4.1 x 10" 2 M (eq. V.15). The difference between the t o t a l of 16 + 17, and 17 amounts to 2.9 x 10" 2 M. Since the production of 16 from 17 should lead to an equivalent amount of H2O (eq. V.8), 2.9 x - 163 -10" 2 M H2O i n the product s o l u t i o n r e s u l t s from such a decomposition. The remainder of the H2O (6.5 x 10" 2 M) should o r i g i n a t e from H 2 and 02, and therefore, corresponds to 9.8 x 10" 2 M uptake, according to the net stoichiometry i n eq. V.10. The t o t a l uptake corresponding to the production of H 202, 1 6 + 17, and H2O, adds up to 19.1 x 10" 2 M, while the t o t a l experimental uptake i s 21.4 x 10" 2 M . The rough corres-pondence shows that the postulated net stoichiometries (eqs. V.9, V.10 and V.15) are consistent with the t o t a l uptake of gas. CH3CON(CH3)2 + 0 2 > CH3CON(CH3)CH2OOH V.15 V.3 Mechanistic studies V.3.1 Results The rate of t o t a l gas uptake by a 5.0 x 10" 3 M s o l u t i o n of RhCl 3(DMSO) 3 ( 1 ) i n DMA under 1 atm H 2/0 2 (2:1) at 50°C remains e s s e n t i a l l y l i n e a r i n about the f i r s t 1 h of re a c t i o n and f a l l s o f f slowly i n the next 3.5 h of monitored reaction time (Fig. V.3.a). The maximum rate of rea c t i o n was used for k i n e t i c analysis of the i n i t i a l stage. The isomeric d i s t r i b u t i o n of complex 1 does not appear to a f f e c t the maximum rate of the H2/O2 reaction. A t y p i c a l sample of complex 1 used contains about 86% of mer-RhCl3(DMSO)2(DMSO) (1A+ 1C), -13% of mer-RhCl 3(DMSO) 3 (IB) and -1% of other minor isomers. A further sample which contained -70% of mer-cis-RhCl 3(DMSO) 2(DMSO) (IA) and -30% of - 164 -mer-trans-RhCl 3(DMSO) 2(DMSO) (IC), or almost 100% of mer-RhCl 3-(DMSO)2(DMSO), gave e s s e n t i a l l y the same rate as a t y p i c a l sample (cf. experiments 1 and 2, Table V.3). The rate was unaffected also by a 20-f o l d excess of h*20 or 16-fold excess of hydroquinone, but was reduced by -30% i n the presence of a two-fold excess of DMSO (experiments 3-5). The rate of re a c t i o n at 2.5 x 10" 3 M Rh i s v i r t u a l l y unchanged by eith e r 60-fold or 125-fold excess of HBF4 added as DMA-HBF^ (experiments 6-8). The rate p l o t s at various Rh (a-c, F i g . V.4), and H 2 and 0 2 (a-c, Fig . V.5) concentrations/partial pressures remain l i n e a r f o r at l e a s t upto -2000 s. The maximum rate of uptake i s independent of the p a r t i a l pressure of 0 2 at high (540 torr) or low p a r t i a l pressure of H 2 (Table V.4), but shows a l i n e a r dependence on H 2 at a t o t a l of 1 atm pressure (Table V.5, F i g . V.6). The rhodium dependence of the maximum rate i s f i r s t order at lower concentrations but becomes les s than f i r s t order at higher concentrations (Experiments 1-12, Table V.6; Plot a, Fi g . V.7). The rate becomes f i r s t - o r d e r on rhodium i n the presence of about a 50-fold excess of added DMSO (Experiments 13-15; Table V.6; p l o t b, F i g . V.7). The rate data at various DMSO concentrations are given i n Table V.7 and the corresponding p l o t of (max. r a t e ) " 1 vs [DMSO] i n F i g . V.8. The v i s i b l e s p e c t r a l changes f or a 3.0 x 10" 3 M s o l u t i o n of 1 i n DMA under a H 2/0 2 (-2:1) mixture at 50°C are given i n Figs. V.9 and V.10. The spectrum of complex 1 (curve A, F i g . V.9) changes r a p i d l y under H 2/0 2 i n the f i r s t 600 s (curves B - G) and then the spectrum remains unchanged f o r a further 1500 s, a f t e r which slow changes are observed. The spectra a f t e r a t o t a l of 1 h and 4 h reac t i o n are given by curves H and J (Fig. V.10), r e s p e c t i v e l y . Under 0 2 alone at 50°C spectrum G changed to K within -20 min (Fig. V.10). An i r spectrum of - 165 -Table V.3 Maximum rate of uptake of a H2/O2 (500/260) mixture at 1 atm by RhCl 3(DMSO) 3 (l)< a> i n DMA at 50°C with or without various additives Experiment [Rh] x 10 3 M Additive Max. rate x 10 5, M s" 1 5.0 - 1.49 2 5.0<b) - 1.48 3 5.0 0.1 M H 20 1.48 4 5.0 0.08 M hydroquinone 1.46 5 5.0 0.008 M DMSO 1.19 6 2.5 - 0.82 7 2.5 0.16 M DMA-HBF4 0.79 8 2.5 0.31 M DMA-HBF4 0.79 ( a) See Table I I I . l f o r the isomeric d i s t r i b u t i o n i n a t y p i c a l sample. (b) A sample RhCl3(DMSO)2(DMSO) containing about 70% of the mer-cis isomer (IA) and 30% of the mer-trans isomer (1C) was used. - 166 -Table V.4 Maximum rate of H2/O2 uptake by 5.0 x 10" 3 M RhCl 3(DMSO) 3 i n DMA at 50°C under various p a r t i a l pressures (p.p.) of O2, f o r a given p.p. of H2. p.p. of H2, t o r r P-P- of H2, t o r r Max. rate x 10 , M s 540 225 1.67 125 1.60 70 1.60 160 600 0.53 300 0.54 100 0.53 - 167 -Table V.5 Maximum rate of H2/O2 uptake by 5.0 x 10" 3 M RhCl 3(DMSO) 3 i n DMA at 50°C under various p a r t i a l pressures of H2 and a t o t a l pressure of 760 t o r r ^ a ) p.p. H2, t o r r Max. rate x IO 5, M s " 1 700 1. 89 645 1. .79 600 1. ,79 540 1. .67 500 1. .49 480 1. .41 400 1. .17 360 1. .16 300 0. .94 270 0. .90, 0.86 230 0. .72 160 0. .53 120 0. .50 Since the rate i s independent of the p a r t i a l pressure of O2 (Table V.4), various p a r t i a l pressures of O2 were used to maintain a t o t a l pressure of 760 t o r r f o r reasons of convenience i n the preparation of gas mixtures (Section II.2.6). - 168 -Table V.6 Maximum rate of uptake of a H2/O2 (500/260 t o r r ) mixture by RhCl 3(DMSO)3 i n DMA at 50°C at various concentrations of Rh, with or without added DMSO Experiment [Rh] x 10 J, M Max. rate x 10 3, M 1 7. ,0 1. ,83 2 5. ,9 1. ,68 3 5. .4 1. ,62 4 5. .0 1. .49 5 4. .4 1. .39 6 4. .0 1. .34 7 3. .1 0. .96 8 2. .5 0. .82 9 2. .2 0. .82 10 1. .65 0. .56 11 1. .10 0. .42 12 0. .57 0. ,23 13 2. .5 <a) 0. ,225 14 4. .9 <a) 0. ,435 15 7. .2 (a> 0. 64 In the presence of 0.115 M added DMSO. - 169 -Table V.7 Maximum rate of uptake of a H 2/0 2 (500/260 t o r r ) mixture by 2.5 x 10" 3 M RhCl 3(DMSO) 3 i n DMA at 50°C fo r various concentrations of DMSO Experiment [DMS0]( a), Max. rate x 10 6, Max. r a t e " 1 x 10" 5, M M s " 1 M"1 s 1 <0.0025 8.20 1.22 2 0.027 4.70 2.13 3 0.082 2.90 3.45 4 0.092 2.85 3.51 5 , 0 . 1 0 5 2.46 4.06 6 0.115 2.25 4.44 7 0.145 2.07 4.83 8 0.165 1.73 5.78 In experiment 1 there was no added DMSO, but a value less than the concentration of RhCl 3(DMSO) 3 i s given to account f o r ligand d i s s o c i a t i o n i n DMA (Section III.3.1.a). The [DMSO] given i n experiments 2-8 r e f e r to [DMSO] added only, since the e f f e c t of lig a n d d i s s o c i a t i o n i s minimal at high concentrations 33 DMSO. 1000 2000 3000 Time, s F i g . V.4 The rate of t o t a l H 2/0 2 (500/260 In t o r r ) uptake by solutions of RhCl3(DMSO)3 i n DMA at 50°C at various concentrations of Rh: (a) 4.0xl0~ 2, (b) 3.1xl0" 3, (c) 1.7xl0 - 3 M Time, s F i g . V.5 The rate of t o t a l H2/O2 uptake by 5.0x10" 3 M solutions of RhCl3(DMSO)3 i n DMA at 50°C at various p a r t i a l pressures of H 2; (a) 645, (b) 300, (c) 160 t o r r t o t a l pressure 760 t o r r - 172 -F i g . V.6 Plot of maximum rate vs. p a r t i a l pressure (p.p.) of H 2 f o r 5.0x10"3 M RhCl 3(DMSO) 3 i n DMA at 50°C under H 2/0 2 at a t o t a l pressure of 760 t o r r (data i n Table V.5 ) - 173 -V.7 (a) Plot of maximum rate vs. the concentration of Rh f o r RhCl 3(DMSO) 3 i n DMA at 50°C under a H 2/o 2 (500/260 i n t o r r ) mixture; (b) same conditions as (a), but with added 0.115 M DMSO .050 .10 IDMS0], M .15 V.8 Plot of maximum r a t e " 1 vs. [DMSO] f o r RhCl 3(DMSO) 3 i n DMA at 50°C under a H 2/0 2 (500/260 i n t o r r ) mixture (data i n Table V.7) - 174 -s o l u t i o n G (meaning the s o l u t i o n corresponding to spectrum G i n Figs. V.9 or V.10) does not show any s i g n i f i c a n t new bands i n the narrow 650-950 cm"1 window a v a i l a b l e f o r DMA solvent i n the 250-1700 cm"1 region ( F i g. V.10.1); see below f o r other d e t a i l s . Hydrogen uptake p l o t s f o r freeze-thaw degassed solutions G and J are given by curves b and c i n F i g . V . l l , r e s p e c t i v e l y . The uptake p l o t f o r s o l u t i o n G (curve b) i s e s s e n t i a l l y the same as that of complex 1 under analogous conditions (curve a; see also Section III.3.2). Solution J takes up H 2 much more slowly than e i t h e r G or complex 1 i n DMA. I t was not possible to obtain the t o t a l r e a c t i o n stoichiometry f o r e i t h e r s o l u t i o n G or J , because of metal p r e c i p i t a t i o n on prolonged exposure (>2500 s) to H 2. Some comparative studies on the rate of autoxidation of DMA under various conditions are summarized i n Table V.8 V.3.2 Discussion (a) I n i t i a t i o n r e a c t i o n The r a p i d v i s i b l e s p e c t r a l changes i n about the f i r s t 600 s of rea c t i o n and the r e l a t i v e s t a b i l i t y of the spectrum f o r at l e a s t a further 1500 s (F i g . V.9) suggest that the c a t a l y t i c a l l y a ctive species i s generated i n the f i r s t 600 s. An uptake p l o t obtained under condi-tions analogous to those i n F i g . V.9 (plot b, F i g . V.4) does not show an induction period, but the t o t a l gas uptake at -600 s corresponds to -2 equivalents of t o t a l gas per an equivalent of Rh. The uptake data are consistent with an i n i t i a t i o n r eaction as i n eq. IV.16 to give an - 175 -Table V.8 Rate of oxidation of DMA so l v e n t ^ 3 ) by 1.0 atm 0 2 at 50°C with or without added RhCl 3(DMSO) 3 complex (5.0 x 10"3M) under various conditions Experiment Additive Average rate of oxidation x 10 6, M s " 1 1 - 0 2 H 20 2 (~3 x 10" 2 M) and CH3CON(CH3)CH2OOH (17) 0.3 (-1 x 10' 2 M)( b) 3 H 2 0 2 ( ° ) (4 x 10" 2 M) 0.4±0.1< d) 4 H 20 2( c> (4 x 10" 2 M) + 1 0.4 5 t-BuOOH (4 x 10" 2 M) 0.6±0.1 6 t-BuOOH (4 x 10" 2 M) + 1 0.5 7 H 2 (500 torr) with l< e) , 2.5±0.2< f) ( a) Defined as the amount of (CH3CON(CH3)CHO, 16 + CH 3C0N(CH 3)CH 200H, 17), detected by gas chromatography (Section II.5.4.c) a f t e r a re a c t i o n period of -4 h. (k) A sample of a product s o l u t i o n from a H 2/0 2 r e a c t i o n corresponding to experiment 20 i n Table V.2 was used. The given concentrations for H 20 2 and 17 i n the product solutions were estimated by analogy with the a n a l y t i c a l data f o r experiment 20. ( c) H 20 2 added as a 34% aqueous so l u t i o n . (d) (e) (f) Some CH 3C0N(CH 3)H (20) approximating to about 15% of t o t a l products was detected; the given rates r e f e r to the formation of 16 + 17 + 20 The p a r t i a l pressure of 0 2 i n experiment 7 was 260 t o r r . Deduced from p l o t C i n F i g . V.3. F i g V 9 V i s i b l e spectral changes f o r a 3.0x10"J M s o l u t i o n of RhCl 3(DMSO) 3 i n DMA at 50°C under a H 2/0 2 (500/260 i n t o r r ) mixture within 0-2000 s of reaction (A, spectrum of 1 i n DMA) Zoo 500 Wavelength, nm F i g . V.10 V i s i b l e spectral changes (A -> G) over 4 h f o r a 3.0xl0" J M solu t i o n of RhCl 3(DMSO) 3 i n DMA at 50°C under a H2/02 (500/260 i n torr) mixture (A; 1 i n DMA; K and L: ~20 min and ~ 6 h, respectively, a f t e r leaving G under 0 2 (1 atm) at 50°C 178 i n i t i a l Rh 1 species by rea c t i o n between complex 1 and H2, and a subsequent r a p i d r e a c t i o n between the Rh 1 species and O2. -2H+ 0 2 R h 1 1 1 + H 2 > Rh 1 > Rh I-0 2 IV. 16 slow f a s t The H2 r e a c t i o n of complex 1 i s postulated to y i e l d a Rh 1 species which r a p i d l y reacts with O2 to give a Rh^-G^ adduct (Section III.3.2.a); a f u l l c h a r a c t e r i z a t i o n of the species was not possible because of i t s i n s t a b i l i t y . The v i s i b l e spectrum of the c a t a l y t i c a l l y a c t i v e species i n DMA (curve G, F i g . V.9) i s s i m i l a r to the spectrum of the postulated Rh^C^ species i n Section III.3.2.a (curve D, F i g . III.4) i n that both show increased £ ( e x t i n c t i o n c o e f f i c i e n t s ) over the 400-600 nm range with respect to the spectrum A of complex 1 i n DMA. Species D shows a higher £ than G i n the range 400-435 nm. The discrepancy between the v i s i b l e spectra of species D and G i s not s u r p r i s i n g considering that species D i n Fi g . III.4 i s postulated to be a mixture -60% of Rh I0 2, -20% of unreacted 1, and -20% of a Rh I I ] :(DMS) species (Section II.3.2.a). Because of the n u c l e o p h i l i c i t y of the dioxygen ligand, the Rh 1 -02 species may e x i s t i n a s s o c i a t i o n with 1 and/or Rh I ] : i(DMS) species.. The dimeric [RhCl(0 2) (PPh 3) 2] 2 complex ( 2 1 ) 1 1 8 a i s thought to form v i a a n u c l e o p h i l i c attack by the dioxygen lig a n d of a RhCl(02)(PPh 3)2 species on the metal centre of another (eq. V.17, E = R h C l ( 0 2 ) ( P P h 3 ) 2 ) . 1 1 9 By analogy, a Rh I-0 2 species could form (Rh-0 2)2 or (Rh)202 species v i a n u c l e o p h i l i c attacks by coordinated O2 of Rh*-02 on another R h 1 ^ or on -40% of unreacted 1 or R h 1 1 1 (DMS) species, - 179 -resp e c t i v e l y ; a dioxygen bridged (Rh^C^ i s postulated to be the intermediate i n a rea c t i o n between K0 2 and the formally R h 1 1 1 complex [Rh(C 3H 5)2Cl]2 (eq. V . 1 7 . 1 ) . 1 1 8 b PPh Cl PPh / 21 Rh / \ Rh / Rh(OOH) V.17 An i r spectrum of the species G (b, F i g . V.10.1) i s e s s e n t i a l l y the same as the spectrum of complex 1 i n DMA (a, F i g . V.10.1), except that the peak at 930 cm"1 appears broader and there i s a new absorption at -950 cm"1. Subtracting spectrum (a) from spectrum (b) gives a weak - 180 -900 800 700 600 Wavenumbers , cm* 100 Wavelength, cm F i g . V.10.1 FT i r spectra of: (a) 4x10"^ M s o l u t i o n of RhCl 3(DMS0) 3 i n DMA; (b) a s o l u t i o n r e s u l t i n g from the H 2/0 2 (500/260 i n t o r r ) r e a c t i o n of (a) at 50°C f o r -2000 s (path length of KBr windows, 0.5 mm) 1.0-Time, s F i g . V . l l H 2 uptake plo t s f o r : (a) 1.0xl0 _ z M RhCl 3(DMSO) 3 i n DMA; (b) solution (a) a f t e r r e a c t i o n at 50°C under H2/O2 (500/260 i n tor r ) f o r ~2000 s; (c) s o l u t i o n (a) a f t e r reaction at 50°C under H 2/0 2 (500/260 i n t o r r ) f o r ~ 4 h - 182 -broad band at -895 cm"1 which may be due to a 0—0 str e t c h , but the evidence i s not compelling. Although the sp e c t r a l data a v a i l a b l e are i n s u f f i c i e n t to cha-r a c t e r i z e the c a t a l y t i c a l l y a ctive species G, the observed 2:1 gas:rhodium r a t i o required to generate the c a t a l y t i c a l l y a ctive species G i s best i n t e r p r e t e d i n terms of a stepwise r e a c t i o n between 1 and H 2 and then 0 2 (eq. IV.16). The s i m i l a r i t y between the H 2 r e a c t i o n of complex 1 i n DMA and species G i n DMA, i n t h e i r r e a c t i o n rates and stoichiometry (curves a and b, respectively, i n F i g . V . l l ) suggests the R h 1 - 0 2 species resembles complex 1 i n DMA i n e l e c t r o n i c and s t r u c t u r a l properties. The inverse dependence of the rate on the concentration of DMSO (Fig. V.8) suggests that the rate determining step(s) involve(s) a DMSO d i s s o c i a t i o n from G. A R h 1 1 1 peroxo complex containing DMSO ligands such as Rh I I ICl(0 2)(DMS0)3 i s consistent with the observed properties of species G i n DMA. In p r i n c i p l e , the peroxide ligand i s replaci n g two C l " ligands, and thus the s i m i l a r i t y with complex 1 i n DMA. Although the v i s i b l e s p e c t r a l data c l e a r l y i n d i c a t e a change from complex 1 to species G under the c a t a l y t i c conditions ( F i g. V.9), the rate of H 2/0 2 uptake (Figs. V.4 and V.5) remains e s s e n t i a l l y l i n e a r during the corresponding periods of time, consistent with the above proposed s i m i l a r i t y between species G and complex 1 i n DMA. The net stoichiometry of the H 2/0 2 r e a c t i o n of 1 (eq. V.16) requires that two protons are generated for each equivalent of species G formed. Dioxygen adducts of M°(M=Pd,Pt) are known to react with acids (HX) to y i e l d MX2 species and H 20 2 v i a a postulated M I I(00H) intermediate (e.g. eq. V.18). 1 20a A n analogous protonation of a Rh-0 2 complex has also been reported (eq. V . 1 8 . 1 ) . 1 2 0 d - 183 -Ph 3P 0 \ / Pt / \ Ph 3P 0 HX Ph 3P OOH Ph 3P X \ / HX \ / > Pt > Pt + H2O2 CH 2C1 2 / \ / \ Ph 3P X Ph 3P X V.18 acacH R h C l ( 0 2 ) ( P P h 3 ) 3 > RhCl(0 2H)(acac)(PPh 3) 2 + PPh 3 V.18.1 -benzene The absence of a s i g n i f i c a n t e f f e c t on the c a t a l y t i c H2/O2 reac-t i o n by e i t h e r a 60- or 125-fold excess of HBF4 over rhodium (experi-ments 7 and 8, Table V.3) suggests that such a protonation step(s) i s not involved i n the rate determining step(s) of the c a t a l y t i c c y c l e ( s ) . I t i s l i k e l y that any such protonation i s i n h i b i t e d by DMA solvent. I f E i n eq. V.17 i s equivalent to a proton, an analogous mechanism can be proposed f o r protonation of R h 1 , 0 2 i n DMA (eq. V.19). (OOH) V.19 Solvent DMA i s l i k e l y to compete with the O2 ligand and thus i n h i b i t the protonation of the dioxygen lig a n d (cf. eq. III.11, p. 85). (However, rapid protonation of dioxygen ligands under the re a c t i o n conditions cannot be t o t a l l y r uled out. For example, i f the protons generated i n the H2 re a c t i o n of Rh**-'- (eq. IV.16) remain i n the coordination sphere of the metal during a ra p i d r e a c t i o n between Rh* and O2, solvent effects, - 184 -as discussed above, would be minimal. The p o s s i b i l i t y of r a p i d protona-t i o n under rea c t i o n conditions i s considered l a t e r i n t h i s s e c t i o n ) . (b) C a t a l y s i s The rate data and the product d i s t r i b u t i o n i n the i n i t i a l phase of the c a t a l y t i c r e a c t i o n are t e n t a t i v e l y , but best, in t e r p r e t e d i n terms of a mechanistic scheme invo l v i n g a Rh^-^-peroxo intermediate (Scheme V.2). The requirement of a p r e d i s s o c i a t i o n of a DMSO ligand (eq. V.20) i s invoked to explain the DMSO dependence of the rate. The d i s s o c i a t i o n Scheme V.2 K Rh I I i :(02 =) (DMSO) + DMA ^ R h 1 1 1 ( 0 2 = ) (DMA) + DMSO V.20 Rh I i : [(02 =) (DMA) - 185 -i s given as a s u b s t i t u t i o n of DMSO by solvent DMA (Section III.3.2). Subsequent H 2 r e a c t i o n of the Rh***(02)(DMA) species (path A) may or may not require a p r e d i s s o c i a t i o n of a DMA ligand: the p o s s i b i l i t y of a DMA lig a n d d i s s o c i a t i o n i s discussed l a t e r . I f the t o t a l concentration of rhodium i s [Rhp] and rate of react i o n i s defined as the rate of t o t a l uptake, the mechanism i n Scheme V.2 y i e l d s the rate law, rate = 2k x [Rh***(0 2)(DMA)][H 2] + k 2 [Rh***(0 2)(DMA)] 2k xK'[Rh T][H 2] k 2K'[Rh T] = + V.21 K' + [DMSO] K' + [DMSO] where, K' = K[DMA] V.22 At s u f f i c i e n t l y low rhodium concentrations, [DMSO] < K' and the rate law i n eq. V.21 leads to a f i r s t - o r d e r dependence on [Rhp]; at the higher rhodium concentrations, [DMSO] > K' and i n the absence of added DMSO i s equated to [Rh***(0 2 =)(DMA)] (which i s given by (K'[Rh T])*/ 2. Thus the rate law then becomes: rate = 2k x (K'[Rh T])*/ 2[H 2] + k 2 ( K ' [ R h x ] ) * / 2 The complex dependence on Rh (plot a, F i g . V.7) i s thus accounted f o r by the K equilibrium, i . e . i n terms of the r e l a t i v e magnitudes of K and [DMSO] i n the denominator of eq. V.21 as [Rhp] increases. In the presence of s u f f i c i e n t added DMSO ([DMSO] » K i n eq. V.21), a f i r s t - o r d e r dependence on c a t a l y s t i s obtained (plot b, F i g . V.5). - 186 -The rate dependence on added DMSO can be analyzed by taking the re c i p r o c a l form of eq. V.21 (eq. V.23) and p l o t t i n g ( r a t e ) " 1 vs. [DMSO] (Fig. V.8). [DMSO] 1 ( r a t e ) " 1 = : + V.23 K'(2k 1[H 2] + k 2 ) [ R h T ] (2k 1[H 2] + k 2 ) [ R h T ] Analysis of various rate dependences using eqs. V.21 and V.23 lead to i n t e r n a l l y consistent values f o r k^, k 2 and K'. The slope of rate vs. [Rh T] i n the presence of 0.115 M added DMSO (Fig. V.7.b) i s given by (2k x[H 2] + k 2)K' -• = 8.9 x 10" 4 s " 1 V.24 K' + 0.115 and the slope of ( r a t e ) " 1 vs [DMSO] (Fig. V.8) y i e l d s a value of (2k 1[H 2] + k 2) K' - 1.4 x 10" 4 M s " 1 for r = 0.998 V.25 Dividi n g equation V.24 by V.25 leads to a K' = 0.042 M. A p l o t of rate vs [H 2] gives a l i n e a r p l o t ( Fig. V.6), the slope of which leads to 2k xK'[Rh T] ^ = 9.6 x 10" 3 s " 1 for r=0.994 V.26 K' + [DMSO] Using the value of K' derived above, [DMSO] i s c a l c u l a t e d to be 4.5 x 10" 3 M at 5.0 x 10" 3 M [Rh T] , and thus k]^ = 1.05 M" 1s" 1 i s obtained from eq. V.26. Substituting values f o r K' and k^ i n eqs. V.25 or V.24 y i e l d s - 187 -k 2 = 5.0 x 10" 4 s" 1. The intercept i n F i g . V.6 equals 0.17 x 10" 5 and y i e l d s k 2 = 3.7 x 10" 4 s"*, while the intercept i n F i g . V.8 equals 1.1 x 10~ 5 M s"* and y i e l d s K' = 0.039 v i a the intercept:slope r a t i o (eq. V.23). The values of k 2 and K' derived using the intercepts w i l l be le s s accurate than those derived using the slopes. The value of K' = 0.042 M derived above y i e l d s K = 0.004 v i a eq. V.22; the density of DMA expressed i n mol L"* i s used f o r [DMA]. The K value derived corresponds well with the K = 0.01^0.005 value reported for the analogous equilibrium of complex 1 i n CDCI3 2 3 (see also Section III.3.1). According to the rate expression i n eq. V.21, the intercept i n the H 2-dependence p l o t ( Fig. V.5) should give the rate of autoxidation of DMA at [Rh T] = 5.0 x 10" 3 M. The value of -0.2 x 10" 5 M s"* derived from F i g . V.5 corresponds well with the rate of production of oxidized DMA (16 + 17) equalling 0.25 x 10" 5 M s"*, derived from p l o t c i n F i g . V.3. The i n t e r n a l consistency of the derived k^,k 2 and K values and the correspondence of derived k 2 and K' with independent measurements lend strong support to the postulated mechanism i n Scheme V.2. Hydrogenolysis of 0 2 The r e a c t i o n pathways A and B i n Scheme V.2 require elaboration. In path A, the H 2 reaction of a Rh***-peroxo intermediate i s postulated to lead to the production of H 20 2 and a Rh*. Mechanistic d e t a i l s of the hydrogenolysis of 0 2 to water or hydrogen peroxide, whether homo-geneously or heterogeneously catalyzed, remain l a r g e l y unknown.**5 In - 188 -the C u 2 + - c a t a l y z e d hydrogenolysis of 0 2 to H 20 i n aqueous media at high temperatures and pressures, f i r s t - o r d e r dependences on the c a t a l y s t and on H 2, and zero-order dependence on 0 2, were reported, but the suggested mechanism (eqs. V.27-V.29) d i d not consider the d e t a i l s of the 0 2 reduction (eq. V . 2 9 ) . 1 1 4 k l C u 2 + + H 2 CuH + + H + V.27 k-1 k 2 CuH + + C u 2 + > 2Cu + + H + V.28 f a s t Cu + + l / 2 0 2 + 2H + > 2C u 2 + + H 20 V.29 In the report by Vaska et a l . on the hydrogenolysis of 0 2 to H 20 i n toluene solvent under ambient conditions by several Pt metal c a t a l y s t s containing phosphine ligands, complex rate dependences on metal, and f i r s t - o r d e r dependences on H 2 and 0 2 were noted and the c a t a l y t i c a l l y a ctive species were postulated to be metal hydrides such as IrH(CO)-( P P h 3 ) 3 and I r H 3 ( C O ) ( P P h 3 ) 2 . 1 1 5 The mechanism suggested i n the present work postulates H 20 2 to be the i n i t i a l product of a Rh catalyzed hydrogenolysis of 0 2. The occurrence of r e l a t i v e l y larger amounts of H 20 2 over H 20 i n the i n i t i a l phase of the H 2/0 2 r e a c t i o n suggests the i n i t i a l product to be H 20 2, and the smaller amount of H 20 detected to r e s u l t from the decomposition of hydroperoxide (17) product (eq. V.8). The good rate dependences obtained w i t h i n about the f i r s t 2000 s suggest that production of H 20 - 189 -and 0 2 v i a a metal catalyzed decomposition of H2O2 (eqs. V.11-V.14) i s not s i g n i f i c a n t within t h i s time. To our knowledge there are no precedents i n the l i t e r a t u r e f o r H2 reactions with metal-peroxides. Hydrogen reactions of Rh*** complexes containing h a l i d e ligands (eqs. V.30 and V.32) are well docu-m e n ted. 1 2* 1* 2 2 R h * * * C l 6 3 _ + H 2 > R h * C l 4 3 _ + 2HC1 V.30 RhCl 3(DES) 3 + H 2 > Rh*Cl(DES) 3 + 2HC1 V.31 A s i m i l a r r e a c t i o n where a peroxide ligand acts e s s e n t i a l l y as two C l " ligands could presumably produce H 202 and Rh* i n a H2 re a c t i o n with Rh***-peroxide. Hydrogen reduction of Rh***Cl n species are thought to occur v i a Rh***(H) i n t e r m e d i a t e s , 5 7 but the mechanism of the H2 a c t i v a t i o n i s not c l e a r . 1 2 3 The requirement of a vacant coordination s i t e i s invoked i n one s t u d y * 2 2 but not i n another.* 2* A mechanism in v o l v i n g a p r e d i s s o c i a t i o n of a ligan d i s given i n Scheme V.3. I f equilibrium K2 i s important, the k2 (measured) i n a c a t a l y t i c H2/02 react i o n (Scheme V.3) i s a c t u a l l y the value of k 2 ^ . Autoxidation of DMA According to path B i n Scheme V.2 autoxidation of DMA occurs v i a a metal-centred t r a n s f e r of peroxidic dioxygen to a coordinated DMA. The - 190 -proposed mechanism i s based on k i n e t i c evidence. Solvent DMA i s not e f f e c t i v e l y autoxidized i n the absence of complex 1 or H 2 Scheme V.3 \ .0 0 K 2 -DMA ,Rh III DMA +DMA / Rh / H2.-H4 .0 Rh III reductive e l i m i n a t i o n Rh 1 <- R h x ( 0 2 = ) L - DMSO or DMA (experiment 1, Table V.8). The rate of p e r o x i d e - i n i t i a t e d autoxidations with or without complex 1 amounts only to about (0.3-0.6) x 10" 6 Ms" 1 (experiment 2-6), while i n the presence of H 2 the rate of autoxidation of DMA remains e s s e n t i a l l y l i n e a r i n the 4.5 h of reac t i o n at 2.5 x 10" 6 Ms" 1 (curve c, F i g . V.3). A 15-fold excess of the f r e e - r a d i c a l scavenger hydroquinone has no e f f e c t on the rate of H 2/0 2 uptake f o r about 2000 s of monitored re a c t i o n time, showing that there are no s i g n i f i c a n t f r e e - r a d i c a l chain autoxidations i n t h i s period (experiment 4, Table V.3); hydroquinone has been used previously with success to - 191 -i n h i b i t p h o t o - i n i t i a t e d autoxidation of DMA. 1 0 6 The rate data (Tables V.4-V.7) are also consistent with a Rh-dependent t o t a l r e a c t i o n (eq. V.19). Ng suggested a metal-centred transfer of 0 2 to a coordinated DMA to explain an observed slow autoxidation of solvent DMA i n the presence of [RhCl(COE) 2]2 complex and excess L i C l at 80°C (Scheme V . 4 ) 1 2 4 a . A homolytic pathway v i a a r a d i c a l abstraction from coordinated DMA was postulated to occur v i a a R h 1 1 ( 0 2 " - ) form of a Rh^0 2 adduct; about 3% of the Rh^0 2 adduct was postulated to e x i s t as Rh 1 10 2"- based on an e.s.r. s t u d y . 1 2 4 b Scheme V.4 Rhl:[(Oo-• ) (DMA) _ _ _ _ _ _ _ RhIII(Oo=) (DMA) o 2 Since the above work, more information on the coordination mode of DMA has become a v a i l a b l e . Dimethylacetamide protonates at the carbonyl oxygen; f or example, within the DMA"HCl adduct protonation at the oxygen gives a p o l a r i z e d molecule ( 2 2 ) . 7 5 - 192 -CH 3 0---H C l \+ / N=C / \ CH 3 CH 3 22 Coordination through the oxygen atom was postulated f o r Rh2Clg(DMA)2, the only DMA complex of Rh*** reported i n the l i t e r a t u r e , but further s t r u c t u r a l data are not a v a i l a b l e . * 2 5 However, DMA i s now considered to bind i n v a r i a b l y through the oxygen. 7 5 A p l a u s i b l e mechanism f o r the autoxidation of DMA by Rh***(C>2=)-(DMA) (23) species, where the DMA i s coordinated v i a the oxygen atom, i s given i n Scheme V.5. Scheme V.5 - 193 -A p o l a r i z a t i o n of the molecule s i m i l a r to structure 22 w i l l make the N-CH3 protons s u f f i c i e n t l y a c i d i c f o r a proton abstra c t i o n by the peroxidic 0 2 ligand to give an intermediate -N-CH2" species: the N-methyl groups i n amides are known to react v i a N-CH2~ moieties under su i t a b l e c o n d i t i o n s . 1 2 6 The proposed mechanism assumes a slow proton ab s t r a c t i o n by the n u c l e o p h i l i c peroxide ligand to give an intermediate R h 1 1 1 complex containing an N-methylene ligand, p o s s i b l y v i a a 5-membered metallocycle (24). Reductive e l i m i n a t i o n of the NCH2" and H0 2" ligands would y i e l d CH3CON(CH3)CH2OOH, with concomitant generation of an 0 2 - s e n s i t i v e Rh 1 species; rapid r e a c t i o n of Rh 1 with 0 2 regenerates the c a t a l y t i c a l l y active R h 1 1 1 ( 0 2 = ) complex (23). Precedents f o r metal-centred proton abstraction from N-CH3 groups are found i n Chapter IV, where the coordination of t e r t i a r y amines to R h 1 1 1 metal centres are postulated to render the N-CH2- proton s u f f i c i e n t l y a c i d i c f o r proton abstraction. In p r i n c i p l e , i f a rapid protonation(s) of the peroxide ligand occurs under the reac t i o n conditions (see p. 183), the reac t i v e i n t e r -mediate could be a Rh 1 I I(OOH) (eq. V.32), or a R h 1 1 1 species which does not contain a peroxo liga n d (eq. V.33). H + R h I i : i ( 0 2 = ) > R h I I I ( O O H ) 2 + V.32 fa s t H + H + R h I ] : i ( 0 2 = ) > [Rh I I i :(OOH)] 2 + > R h 1 1 1 + H 20 2 V.33 fa s t f a s t In e i t h e r case,subsequent reaction with H 2 could lead to a c a t a l y t i c hydrogenolysis of 0 2 v i a a mechanism analogous to Scheme V.3, but a - 194 -mechanism for DMA oxidation v i a a Rh 1 1 1(00H) or R h 1 1 1 species i s not r e a d i l y conceivable. Some Mn 2 + or Mn 3 + complexes are reported to catalyze the oxidation of N-alkyl amides to the respective keto d e r i v a t i v e s by a l k y l hydroperoxides (eq. V.34) but mechanistic d e t a i l s are not a v a i l a b l e . 1 1 0 0 0 II ll / C \ Mn 1 1 or Mn 1 1 1 / \ ( C H 2 ) n NR' + 2R00H > ( C H 2 ) n N R ' + H 2 ° + R 0 H \ / \ / ^CHo C II 0 V.34 Results from 'blank' reactions i n the present study (experiments 2-6, Table V.8) show that R h ^ C ^ D M S O ^ i n the presence of H 20 2 cannot oxidize DMA at the same rate as the H 2/0 2 reaction. (c) The r o l e of H 2 i n the autoxidation of DMA According to path B, Scheme V.2, or Scheme V.5, i t should be possi b l e to catalyze the autoxidation of DMA by the R h 1 1 1 ( 0 2 = ) intermediate even i n the absence of H 2, but such c a t a l y s i s i s not r e a l i z e d . When a reaction s o l u t i o n containing the postulated R h 1 1 1(0 2=^ intermediate ( f o r example, a s o l u t i o n corresponding to spectrum G i n Fi g . V.9 or V.10) i s put under 1 atm of 0 2 at 50°C, rapid, v i s i b l e s p e c t r a l changes (G — > K, F i g . V.10) are observed, but there i s no - 195 -d i s c e r n i b l e uptake of gas for at l e a s t 1 h. There were further slower s p e c t r a l changes with time to give a stable spectrum L (Fig. V.10) a f t e r a few hours. Attempts to i s o l a t e characterizable species from r e a c t i o n solutions corresponding to spectrum L (for [Rh] = 3 x 10" J or 1 x 10"*^  M) were not successful. A study on immobilized, solid-supported organosulfide complexes of Rh 1 for autoxidation of o l e f i n s , reported by Nyberg and D r a g o , 1 2 7 has been based on the assumption that the r e l a t i v e l y short l i f e times of rhodium c a t a l y s t s i n autoxidations 1''' 1 8 r e s u l t from processes that are multiordered i n rhodium. Read and coworkers r e p o r t e d 1 2 8 that the decomposition of RhCl(02)(PPI13)3 species i n benzene s o l u t i o n under anaerobic conditions leads to dimeric Rh species and PPI13O (eq. V.35). 4R h C l ( 0 2 ) ( P P h 3 ) 3 > [RhCl(PPh 3) 2]2 + [ R h C l ( 0 2 ) ( P P h 3 ) 2 ] 2 + 4PPh 30 V.35 The changes G — > K i n the v i s i b l e spectrum of the R h 1 1 1 ( 0 2 = ) species i n s o l u t i o n (Fig. V.10), i n the absence of H 2, could involve s i m i l a r dimerization processes. The proposed mechanism f o r the autoxidation of DMA (Scheme V.5, p. 184) requires that the 0 2 l i g a n d be n u c l e o p h i l i c . Since dimeric [Rh0 2] 2 species (e.g. 21, p. 179) are known to be unreactive towards e l e c t r o p h i l e s , 1 2 9 dimerization of R h 1 1 1 ( 0 2 = ) species i n s o l u t i o n w i l l render the r e a c t i o n s o l u t i o n c a t a l y t i c a l l y i n a c t i v e towards autoxidation of DMA. At any given time a reaction s o l u t i o n containing R h 1 1 1 ( 0 2 = ) species w i l l have 2 equivalents of protons per equivalent of R h 1 1 1 ( 0 2 = ) (p. 182). Dioxygen ligands on monomeric dioxygen complexes of Rh and Pt - 196 -complexes are known to protonate r e a d i l y , 1 2 0 but such a protonation does not appear to be a factor i n the c a t a l y t i c i n a c t i v i t y of postulated R h 1 1 1 ( 0 2 ™ ) species i n s o l u t i o n . For example, a Rh 1 1 1(02~) model system, made of l a b i l e [RhCl(COE)2]2 dimer, and 2 equivalents of L i C l and 3 equivalents of DMSO per equivalent of Rh i n DMA, i s also c a t a l y t i c a l l y i nactive towards the uptake of O2 (p. 80) although the system does not contain protons. Under H2/O2 mixtures the postulated Rh 1 1 1(02~) intermediate remains stable for -1400 s (Fig. V.10), a f t e r which i t changes slowly. The apparent s t a b i l i t y of Rh 1 1 1(02~) species under H2 f o r -1400 s i s most l i k e l y a r e s u l t of constant regeneration of R h 1 1 1 ( 0 2 ~ ) species from i t s i n a c t i v e forms i n the presence of H2. I f the i n a c t i v e form i s a [Rh 1 1 1(02*")]2 dimeric species as postulated above, H2 reaction of such species could give Rh 1 species (eq. V.36), which w i l l then react r a p i d l y with O2 to regenerate the c a t a l y t i c a l l y a ctive Rh 1 1 1(02~) species. -2H 20 2 20 2 [Rh I ] : I(02~)]2 + 2H 2 •« > ZRh 1 -* > 2 [ R h i n ( 0 2 ~ ) ] V.36 The low n u c l e o p h i l i c r e a c t i v i t y of the bridging dioxygen ligands i n a dimeric s p e c i e s , 1 2 9 i n p r i n c i p l e , should not have a d i r e c t e f f e c t on the H2 a c t i v a t i o n by the formally R h 1 1 1 metal centres (also see p. 189). - 197 -(d) Later stages of the rea c t i o n A f t e r -1400 s of reac t i o n under H2/O2 mixtures, the v i s i b l e spectrum of the c a t a l y t i c a l l y active Rh11-'-(02==) species (spectrum G, Fi g . V.10) changes slowly (G — > J ) , suggesting a slow decomposition of the c a t a l y t i c a l l y active species even under H2. A s o l u t i o n a f t e r from -4 h of re a c t i o n (or the s o l u t i o n corresponding to spectrum J) takes up H2 much more slowly than the i n i t i a l Rh11-'-(02=') species (cf. curves (a) and (b) i n F i g . V . l l ) , consistent with the decomposition of R h 1 1 1 ( 0 2 = ) species to other ^ - i n a c t i v e or low-active form(s). The decomposition could occur v i a several paths. Some data for an analogous Rh-dioxygen species, which i s formed v i a the stoichiometric H2/O2 rea c t i o n of complex 1 i n -60% pu r i t y , are given i n Chapter I I I (p. 74). The rea c t i o n mixture obtained from the above s o l u t i o n on leaving i t under Ar or O2 showed evidence for an -OH species (broad D2O-sensitive band i n the nmr spectrum of rea c t i o n residue i n CDCI3, F i g . III.5.b). Although protonation reactions were postulated to be slow f o r the R h 1 1 1 ( 0 2 = ) species i n the presence case (see p. 183), such protonations may become s i g n i g i c a n t at longer r e a c t i o n times. Another p l a u s i b l e explanation for the deactivation o f Rh I i : i(02 =) species i s the accumulation of H2O2 and/or CH3C0N(CH3)CH200H products i n the course of the c a t a l y t i c H2/0 2 uptake. A f t e r about 0.75 h of rea c t i o n time (Fig. V.3), the concentration of H2O2 i n the system approaches a steady state and the concentration of H2O increases s t e a d i l y , most c e r t a i n l y as a r e s u l t of metal-catalyzed decomposition of H2O2 (eqs. V.11-V.14). As the concentration of H2O2 increases, reaction between H2O2 and Rh species w i l l compete with the H2 and/or O2 reactions - 198 -of the metal (Scheme V.2). For example, the steady state concentration of H2O2 i n the system (curve c, F i g . V.3) i s about 20 times greater than the concentration of O2 (<1 x 10" 3 M) at any time. Therefore at l a t e r stages, r e a c t i o n between Rh 1 and H2O2 may compete with the f a s t r e a c t i o n between Rh 1 and O2 i n Scheme V.2 to lead to c a t a l y t i c a l l y i n a c t i v e Rh species (eq. V . 3 7 ) . 1 3 0 Rh 1 + H 20 2 > R h 1 1 + HO" + HO- V.37 As shown i n the above equation, such reactions also generate HO* f r e e - r a d i c a l s . Therefore, at l a t e r stages of the c a t a l y t i c H2/O2 reaction, f r e e - r a d i c a l autoxidations of DMA w i l l play a s i g n i f i c a n t r o l e . V i s i b l e s p e c t r a l changes (Fig. V.10) suggest the deactivation of ca t a l y s t , but the autoxidation rate of DMA (curve c, F i g . V.3) remains e s s e n t i a l l y l i n e a r , consistent with increased contributions from free-r a d i c a l autoxidations at l a t e r stages of the reaction. V .4 Implications f o r further work (a) Hydrogenolysis of O2 Homogeneously catalyzed hydrogenolysis of 0 2 to give H2O or H2O2 i s documented, but the mechanistic information on such systems i s li m i t e d , 114,115 or i s not reported at a n . 26 ,113 ,116 T h e o r e t i c a l l y , the hydrogenolysis of O2 could lead e i t h e r to H2O2, where i t may be further decomposed to H2O (eq. V.38), or d i r e c t l y to H2O without the release of - 199 -H2O2 (eq. V.39). In the present study, hydrogenolysis of O2 within Rh systems i s proposed to occur v i a the d i r e c t H2 rea c t i o n of a Rh I ] : i(p eroxo) complex to give H2O2; production of H2O i s thought to r e s u l t from a metal- catalyzed decomposition of the i n i t i a l H2O2 product (eq. V.37). In a l l the Journal l i t e r a t u r e reports ( i . e . with the exception of the patent l i t e r a t u r e ) , H2O i s the only observed product and the reactions are represented as concerted hydrogenolysis reactions of 0 2 to give H2O (eq. V.39). In b i o l o g i c a l systems, the cytochrome oxidase enzyme c a r r i e s out a concerted 4-electron reduction of O2 to H2O without the release of intermediary H2C,2;1''"a the 4e + 4H + requirement of the enzyme system (eq. V.40) i s s t o i c h i o m e t r i c a l l y equivalent to two hydrogens required i n the hydrogenolysis of O2 to give H2O (eq. V.39). Cytochrome oxidase i s a multicomponent enzyme system containing two heme porphyrins and two copper centres. Although the mechanism of the rea c t i o n i s poorly understood, the four redox centres are thought, perhaps, to be involved i n the storage and transfe r of the required four electrons. 1-*-a M M H 2 + 0 2 > H 20 2 > H 20 + l / 2 0 2 V.38 M 2H 2 + 0 2 > 2H 20 V.39 Cyto. oxidase 4H + + 4e + 0 2 > 2H20 V.40 - 200 -The i n v i t r o c a t a l y s t systems, which y i e l d from H 2 and 02, are r e l a t i v e l y simple and are comprised of monomeric Pt-metal complexes containing p h o s p h i n e , 1 1 5 p o r p h y r i n , 1 1 6 o r cyclooctadiene l i g a n d s 2 6 i n non-aqueous solvents, or of copper s a l t s i n w a t e r . 1 1 4 In such systems, 2-electron reduction of O2 to i n i t i a l H2O2 product appears more l i k e l y . The evidence f o r H2O2 production v i a metal-catalyzed hydrogenolysis, suggested i n the patent l i t e r a t u r e and i n the present study, supports such a hypothesis, but further studies are required before any generalizations can be made. Further mechanistic studies to understand the d e t a i l s of hydro-genolysis of O2 are important i n the context of H2O2 production and f u e l - c e l l technology. Hydrogen peroxide i s used widely i n industry as a non-polluting o x i d i z i n g agent. I t i s manufactured commercially, mainly by the autoxidation of anthroquinol (eq. V.41); the anthroquinone co-product i s then hydrogenated back to the quinol using supported Pd or Ni c a t a l y s t (eq. V . 4 2 ) . 1 3 1 V.41 V.42 OH - 201 -A second process for H2O2 production developed by Shell- L J- L involves the oxidation of isopropanol to acetone and H2O2 i n ei t h e r vapor or l i q u i d phase with 15 to 20 atm of 0 2 at ~100°C (eq. V.43). (CH 3) 2CH(0H) ( C H 3 ) 2 C ( 0 0 H ) 0 H (CH 3) 2C0 H o O 2 U2 V.43 Hydrogen peroxide can also be formed d i r e c t l y by the thermal, T O T e l e c t r i c discharge, or metal-activated reaction of H 2 and O2. S i l e n t e l e c t r i c discharge processes have been patented and a process p i l o t e d i n Germany, but the power requirements are too high for commercial u s e . 1 3 1 The recent patent a c t i v i t y i n the area of metal-catalyzed homogeneous hydrogenolysis of O2 indicates an increasing i n t e r e s t i n such reactions 1 0 - 1 as v i a b l e a l t e r n a t i v e s to methods presently used. J J- An understanding of the mechanism of hydrogenolysis should enable the design of cheaper and/or improved c a t a l y s t s . On the other hand, the p r a c t i c a l need to reduce dioxygen to H 20 without the release of H2O2 i s important i n f u e l - c e l l t e c h n o l o g y , 1 1 3 while the p h o t o - i n i t i a t e d s p l i t t i n g of H2O (the reverse of the hydrogenolysis reaction i n eq. V.39) demands much current i n t e r e s t . 1 3 2 Increased understanding of the metal-catalyzed hydrogenolysis w i l l provide some insigh t s into the 4-electron reduction of O2. - 202 -(b) Metal-centred t r a n s f e r of 0 2 to C-H bonds Metal-centred oxygen transfers are postulated i n metal-catalyzed 1 9 17 1ft autoxidations of terminal o l e f i n s using Rh c a t a l y s t s . L > » M e t a l -mediated autoxidations are reported also f o r phenolic substrates, where Cu, Mn and Co c a t a l y s t s are used to obtain quinone products 1^ 3,12b ( s e e also Section 1.3). To our knowledge, metal-centred a c t i v a t i o n and tra n s f e r of 0 2 to C-H bonds to give C-00H species i s not documented except f o r the study by Ng on Rh-catalyzed autoxidation of DMA. 1 2 4 a Enzymic oxidations are known to lead to hydroperoxide intermediates v i a pathways which may or may not involve active metal-centres (e.g. eq. V . 4 4 ) . l l c lipoxygenase R1CH=CHCH2CH=CHR' + 0 2 > R1CH=CHCH=CHCH(02H)R' V.44 Sheldon and Kochi suggest that the hydroxylation of phenolic substrates mediated by nonheme i r o n and copper monoxygenases proceeds also v i a the i n i t i a l incorporation of a dioxygen molecule across the aromatic C-H bond (eq. V . 4 5 ) . l l c - 203 -The findings i n the present study suggest that a metal-centred incorporation of a dioxygen molecule across a C-H bond i s a f e a s i b l e reaction, though the hydroperoxide products formed i n such reactions could i n i t i a t e f r e e - r a d i c a l chain autoxidations (see Section 1.1). (c) Role of reducing co-substrates i n autoxidations The requirements of added co-substrates (e.g. PPh3) 1 7 or alcohol s o l v e n t 1 8 ' 2 1 , 1 3 3 are noted i n almost a l l the successful autoxidation c a t a l y s t systems reported, but the ro l e of the co-substrate or the solvents remains generally u n c l e a r . 1 7 ' 1 8 , 1 3 3 ' 1 3 4 In the report by Read and Walker the rhodium-catalyzed autoxidation of oct-l-ene to octan-2-one i n the presence of excess PPh 3 i s accompanied by oxidation of the phosphine to PPI13O, and the reac t i o n mixture a f t e r -3 h contains about a 6-fold excess of PPI13O over the ketone product of i n t e r e s t . 1 7 S i m i l a r l y , i n a recent report on the autoxidation of terminal o l e f i n s to ketones by c a t i o n i c Rh 1 complexes i n EtOH or MeOH solvent, the aldehyde product of solvent autoxidation i s present i n about a 5-fold excess over the o l e f i n oxidation product a f t e r -7 h of r e a c t i o n . 1 3 3 In the present Rh/DMA work, the rates of production of CH 3C0N(CH3)CH 200H and H 20 2 are about 2 x 10" 6 M s " 1 and 6 x 10" 6 Ms" 1, resp e c t i v e l y , i n the i n i t i a l 0.5 h of reac t i o n for a 5 x 10" 3 M c a t a l y s t i n DMA at 50°C under a 1 atm of H 2/0 2 (2:1) (Fig. V.2). According to the mechanistic i n t e r p r e t a t i o n , the requirement of H 2 co-substrate i n the autoxidation of DMA could be avoided i f measures were taken to prevent the deactivation of the c a t a l y s t . I t i s possible - 204 -that the co-substrates or the reducing solvents in other related systems act in a similar manner, i.e. by regenerating species active for oxygen transfer. Systematic studies on the role of reducing agents, and the reasons for catalyst deactivation in such systems, would be useful in the design of autoxidation cataylsts that do not require the employment of co-substrates, since these consume a large proportion of the oxidant in wasteful side-reactions. - 205 -CHAPTER VI OTHER AUTOXIDATION CATALYST SYSTEMS UTILIZING H 2 / 0 2 MIXTURES - 206 -VI. Other autoxidation c a t a l y s t systems u t i l i z i n g H 2 / O 2 mixtures VI.1 Introduction Hydrogenolysis of O2 by RhCl 3(DMSO) 3 c a t a l y s t i n DMA leads to the production of H2O2 and an accompanying autoxidation of the solvent (Chapter V). In the presence of suitable substrate(s), i t should be possible to u t i l i z e the H2O2 formed i n such c a t a l y s i s towards the i n s i t u autoxidation of a substrate S to SO (eq. VI.1). metal S H 2 + 0 2 > [H 20 2] > H 20 + SO VI. 1 Thioethers (R-S-R') are oxidized r e a d i l y to the corresponding sulfoxides (R-SO-R') by H2O2 or alky H y d r o p e r o x i d e s 1 3 5 . This chapter describes some attempts at c a t a l y t i c autoxidation of thioether substrates u t i l i z i n g H2/O2 mixtures. Several other systems which do not contain added substrates are also described. VI.2 Results and Discussion The rates of H2/O2 uptake by RhCl 3(DMSO) 3 i n DMA systems, with or without added thioether substrates, are summarized i n Table VI.1. The rate of c a t a l y t i c H2/O2 uptake by RhCl 3(DMSO) 3 complex i n DMA - 207 -Table VI.1 Maximum rate of gas uptake from an approximately 2:1 mixture of H 2 and 0 2 at 1 atm by RhCl 3(DMSO) 3 (1.0 x 10" 2 M ) , with or without thioether substrates, at 50°C i n DMA Experiment Added substrate maximum rate x 10 5, M s " 1 1 - 2.0 2 DPS (4 x 10" 2 M) 2.0 3 DPS (-1 M) 1.0 4 DES (~1 M) 0.2 5 DMS (-1 M) no uptake (experiment 1) i s unaffected by a 4-fol d excess of DPS over rhodium (experiment 2) but was decreased by -50% i n the presence of a 100-fold excess (experiment 3). Similar suppression i n rate was observed with excess DES or DMS (experiments 4 and 5, res p e c t i v e l y , Table VI.1). The e f f e c t of added thioethers on the reaction rates probably r e s u l t s from changes i n the coordination sphere of the rhodium. Dimethylsulfide r e a d i l y displaces DMSO ligands from complex 1 to give Rh^-^DMS) species (Section II.2.3), and R h 1 1 1 complexes of DES are known. 3 5 The observed suppression i n rates could r e s u l t from a suppression of DMA autoxidation rate (path B, Scheme V.2, p. 184) due to a lower e f f e c t i v e concentration of the c a t a l y t i c a l l y active R h 1 1 1 ( 0 2 = ) species i n the presence of the - 208 thioether (eq. VI.2), and/or from a lower r e a c t i v i t y of a thioether-substituted R h 1 1 1 complex towards H 2 (eq. VI.3, c f . path A, Scheme V.2). R h 1 1 1 ^ " ) (DMA) + R 2S v R h 1 1 1 ( 0 2 = ) (R 2S) + DMA VI. 2 2 3 R h I I : [ ( 0 2 = ) (R 2S) + H 2 > Rh I(R 2S) + H 2 0 2 VI. 3 In the DPS systems, diphenylsulfoxide (DPSO), H 20, and CH 3C0N(CH 3)CH0 (16) and/or CH 3C0N(CH 3)CH 200H (17) are detected as r e a c t i o n products at DPS concentrations of 0.2-1 M, a f t e r reaction times of 3.5-5.5 h (Table VI.2). S i g n i f i c a n t l y , no H2O2 was detected i n a r e a c t i o n mixture a f t e r 5.5 h of r e a c t i o n (experiment 4, c f . with corresponding systems i n the absence of DPS where H 202 i s detected, Table V.2, p. 158). In addition, solutions containing 0.2 M DPS, and 4 x 1 0 M H2O2 added as a 35% aqueous solu t i o n , under Ar, also showed ~3 x 10" 2 M DPSO a f t e r ~ 4 h reaction, with or without added complex 1 (at 5 x 10" 3 M); 4 x 10" 2 M H 202 was used i n the l a t t e r r e a c t i o n since such a concentration i s of the same order as that found i n a c a t a l y s t system i n the absence of added thioether substrates (Table V.2). Therefore i t i s highly probable that the observed DPSO product r e s u l t s from the oxidation of DPS by the H 2 0 2 generated by the c a t a l y t i c hydrogenolysis of 0 2. An analogous autoxidation of thioethers by i n s i t u generated H 20 2 was reported r e c e n t l y . 2 1 The H20 2 was postulated to r e s u l t from an e f f e c t i v e reduction of 0 2 by alcohol solvent, catalyzed by Ru 1 1(DMS0) complexes (eq. VI.4). - 209 -Table VI.2 Product analysis^ 3) after a catalytic uptake of a H2/O2 (2:1) mixture at 1 atm, at 50°C In the presence of 0.010 M RhCl3(DMS0)3 or RhCl3(DES)3 catalyst In DMA with added DPS substrate Experi- Catalyst [DPS], Time of To t a l gas DPSO, H 20, 16+17<b) H 20 2 ment M reaction, h uptake, M M M 1 RhCl 3(DMSO) 3 1.1 5.5 2 RhCl 3(DMSO) 3 0.24 3.5 0.20 -0.04 >1<C) -<d) -<c) 0.12 -0.04 >1<C) -<d) -<c) 3 RhCl 3(DMS0) 3 1.1 5.5 4 RhCl 3(DMSO) 3 1.1 5.5 5 RhCl 3(DES) 3 0.24 4.6 0.18 0.04 0.074 0.02 -<c) 0.20 -<d) -<e) -<d) 0 0.13 0.06 0.04 -<d> -( c> (a) See Section II.5.4 f or d e t a i l s of a n a l y t i c a l methods. (°) Detected and/or q u a n t i f i e d by gas chromatography as CH 3C0N(CH 3)CH0 (16) (see Section V.2 for d e t a i l s ) . ( c) Analyses were done before the s a t i s f a c t o r y method described i n Section II.5.1 was f u l l y developed. ( d) Detected but not quan t i f i e d . ( e) Not analyzed for. - 210 -Ru 1 1 R1R2C.HOH + 0 2 > R^R2C0 + H 20 2 VI. 4 The mechanism proposed includes a d i r e c t e l e c t r o n t r a n s f e r from R u 1 1 to 0 2 to y i e l d R u I V and 0 2 = (eq. VI.5), and a reduction of the Ru(IV) species by the alcohol to regenerate R u 1 1 (eq. VI.6). The mode of ele c t r o n t r a n s f e r i n eq. VI.5, i . e . whether outer-sphere or inner-sphere, i s not s p e c i f i e d . R u 1 1 + 0 2 + 2H + > R u I V + 0 2~ VI. 5 R u I V + R 1R 2CH0H > R u 1 1 + R 1R 2C0 + 2H + VI. 6 The above mechanism i s s i m i l a r i n p r i n c i p l e to that proposed i n the present work f o r the Rh-catalyzed hydrogenolysis of 0 2 by H 2 to y i e l d H 20 2 (Scheme V.3, p. 190), although the l a t t e r mechanism involves an inner-sphere e l e c t r o n transfer from H 2 to 0 2^to y i e l d 2H + and 0 2 = . In the Ru system, turnover numbers (t.n.) from 1 to 19 mol product (mol c a t a l y s t ) " 1 h " 1 have been obtained f o r thioether autoxidations. For example, trans -RuBr2(DMSO)4 i n methanol at 100°C under 0 2 (100 ps i ) gives a t.n. of 19 f o r the s e l e c t i v e autoxidation of decyl methyl s u l f i d e to the corresponding s u l f o x i d e . 2 1 The c a t a l y t i c oxidation of DPS to DPSO under H 2/0 2 (2:1, 1 atm) at 50°C i n the present work gives a t.n. of -1. A major problem i n the present system i s the accompanying autoxidation of the DMA solvent. For example, i n the presence of 1.1 M - 211 -DPS, autoxidation products of DMA (16 + 17) accounts for about 33% of t o t a l organic products (experiment 3, Table VI.2). In an e f f o r t to f i n d an a l t e r n a t i v e to DMA solvent, c a t a l y s i s i n C2H4CI2 was investigated. Because of i t s r e l a t i v e l y high b o i l i n g point (83.5°C), C 2H 4Cl2 i s a suitable solvent for gas-uptake studies. There was no H 2/0 2 (2:1) uptake by RhCl 3(DMSO) 3 (1), RhCl 3(DES) 3, or RhCl 3(DMSO) 2(~CH2CH= +NEt2) complexes i n C2H4CI2 solvent, and added base d i d not i n i t i a t e any r e a c t i v i t y with complex 1 (experiments 1-4, Table VI.3). In contrast, complexes 1, RhCl 3•3H 20 and RhCl 3(DES) 3 i n DMA showed c a t a l y t i c gas uptake from H2/O2 (2:1) mixtures (experiments 5-7, Table VI.3). Another Rh system containing [RhCl(COE) 2] 2 as a l a b i l e Rh 1 p r e c u r s o r , 6 2 together with 3 equivalents of DMA'HCl and 2 equivalents of DMSO i n C2H4CI2, gave an i n i t i a l r a p i d uptake which corresponded roughly to one equivalent of gas per rhodium, but there was no uptake over the next 0.5 h (experiment 8). E s s e n t i a l l y the same r e a c t i v i t y was observed when L i C l was used i n place of DMA'HCl (experiment 10). However, analogous systems i n DMA solvent gave c a t a l y t i c uptake of H2/0 2 at rates (experiments 9 and 11) close to that found f o r complex 1 i n DMA. The necessity of DMA solvent f or c a t a l y t i c a c t i v a t i o n suggests, perhaps, that the reactions proceed v i a the a c t i v a t i o n of H2 by Rh11-'-(02=) i n a h e t e r o l y t i c mode which requires a basic solvent (see p.p. 64 and 189). The ' c a t a l y t i c i n a c t i v i t y ' of Rh 1 species i n C2H4CI2 towards H2/02 contrasts with the reported c a t a l y t i c a c t i v i t y of other low-valent Pt metal complexes such as Pt(PPh 3)4, RhCl(CO)(PPh 3) 2 and Ir C l ( C O ) ( P P h 3 ) 2 , to give H 2 0 . 1 1 5 The l a t t e r systems at 2 x 10"3M are reported to produce H2O from H2/O2 mixtures (2:1) at rates of 13.2 x - 212 -Table VI.3 Maximum rate of gas uptake( a) from an approximately 2:1 mixture of H 2 and 0 2 at 1 atm by Rh c a t a l y s t systems (1.0 x 10" 2 M i n Rh), at 50°C i n various solvents Experi-ment Catalyst solvent added base/ligand max. rate x 10 5, M s -1 1 RhCl 3(DMS0) 3 (1) C 2H 4C1 2 no uptake 2 RhCl 3(DMS0) 3 (1) C 2H 4C1 2 2,6-di-t-butyl-p y r i d i n e ( b ) (0.1 M) no uptake 3 RhCl 3(DES) C 2H 4C1 2 no uptake RhCl 3(DMS0)-(-CH 2CH- +NEt 2) C 2H 4C1 2 no uptake DMA 2.0 6 RhCl 3(DES) 3 DMA 1.0 RhCl 3•3H 20 DMA 0.5 8 [RhCl(C0E) 2] 2 C 2H 4C1 2 DMA-HCl (3x10*2 M) DMSO(3x10 _ 2 M) no c a t a l y t i c uptake 9 [RhCl(COE) 2] 2 DMA 1.8 10 [RhCl(COE) 2] 2 C 2H 4C1 2 L i C l ( 2 x l 0 " 2 M) DMSO (3x10" 2 M) no c a t a l y t i c uptake 11 [RhCl(C0E) 2] 2 DMA 1.7 (a) (b) (c) A l l uptakes, unless otherwise stated, were c a t a l y t i c with respect to rhodium. Stronger bases such as P.S. and NEt 3 were not used because they gave complicating side reactions with 1 (Ch. 3). A r a p i d uptake corresponding to one equivalent of gas per rhodium was observed i n i t i a l l y . - 213 -10" 6, 9.6 x 10" 6 and 6.9 x 10" 6 M sec" 1, r e s p e c t i v e l y , at 25°C i n toluene. Phosphine and/or CO ligands i n such systems markedly increase the r e a c t i v i t y of Rh 1 species towards H 2 and/or 0 2. I f H 20 2 i s r e a l l y the i n i t i a l product of hydrogenolysis i n those systems and the observed H 20 product i s a r e s u l t of the decomposition of H 20 2, i t should be possible to u t i l i z e the intermediate H 20 2 to oxidize added thioether substrates. - 214 -CHAPTER VII CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK - 215 -VII. Conclusions and Suggestions f o r Further Work The general aim of the work described i n t h i s thesis was to obtain a s u i t a b l e Rh c a t a l y s t f or the autoxidation of organic substrates by H2/O2 mixtures. Dimethylacetamide (DMA) was chosen as a solvent, because of i t s known a b i l i t y to promote formation of monohydrides from reaction of platinum metal complexes with H2. However, i t soon became c l e a r that the amide solvent i t s e l f underwent oxidation under H2/O2 mixtures, and t h i s system was then investigated. This autoxidation catalyzed by RhCl3(DMSO)3 y i e l d e d u s e f u l information on the hydrogenolysis of O2 to H2O2, the f e a s i b i l i t y of metal-centred dioxygen t r a n s f e r to substrates, and a p o t e n t i a l r o l e of H2 i n such c a t a l y s i s . Both the Rh complex and H2 are e s s e n t i a l for. the observed e f f e c t i v e autoxidation of DMA. Rate data obtained are consistent with a metal-centred autoxidation of DMA i n a Rh 1 1 1(C-2 =)(DMA) (23) intermediate and an independent hydrogenolysis of O2 v i a the same intermediate. Although the proposed structure f o r the c a t a l y t i c a l l y active intermediate, and the postulated d e t a i l s of the mechanistic pathways, (Schemes V.2, V.3 and V.5; p. 184-192) are tent a t i v e , the o v e r a l l i n t e r n a l consistency of the data lends strong support to the proposed mechanism. The c a t a l y t i c i n a c t i v i t y of 23 towards the autoxidation of DMA under O2 alone i s a t t r i b u t e d to the i n s t a b i l i t y of such species i n sol u t i o n . Species 23 i s postulated to give i n a c t i v e [Rh 1 1 1(C>2 =)]2 t v P e products v i a dimerizations and the H2 cosubstrate i s thought to regenerate Rh 1 species, hence 23 (eq. V.36, p. 196), from such products. Slow deactivation of the c a t a l y s t , and the p a r t i c i p a t i o n of H2O2 product - 216 -i n other autoxidation pathways of DMA, l i m i t the mechanistic i n t e r p r e t a -t i o n to the k i n e t i c a l l y analyzed i n i t i a l phase of the r e a c t i o n that l a s t s about 2000 s. The findings i n the present study r a i s e several possible areas of further work. For example, systematic studies on the metal-catalyzed hydrogenolysis of 0 2 w i l l be important i n the development of c a t a l y s t s fo r commercial production of H 20 2 from H 2 and 0 2 (see p. 43). Secondly, H 20 2 generated from such autoxidations could be used for i n s i t u autoxidation or organic substrates (Chapter VI). Screening various known hydrogenolysis c a t a l y s t s y s t e m s 1 1 3 " 1 1 6 f o r such c a t a l y t i c a c t i v i t y w i l l be a f i r s t step i n such an i n v e s t i g a t i o n . T h i r d l y , the proposed r o l e of H 2 i n the autoxidation of DMA suggests that the use of H 2 or any other co-substrates i n autoxidations w i l l be unnecessary, i f measures are taken to prevent the deactivation of c a t a l y s t s . In t h i s respect, studies on deactivation pathways of metal-dioxygen adducts should lead to the development of robust c a t a l y s t s f o r autoxidations. The complex RhCl3(DMS0)3 (1) was found to undergo f a c i l e reduction to Rh 1 1 1(DMS) species by CO, but y i e l d e d a Rh 1 species as the major reduction product under H 2 i n DMA (Chapter I I I ) . Dimethylsulfoxide was r e a d i l y reduced by Rh 1 i n the presence of added acids i n CH 2C1 2, but the r e a c t i o n was slow i n DMA. The s t a b i l i t y of the Rh I/2H +, DMSO redox couple i n DMA was a t t r i b u t e d to the proton a f f i n i t y of solvent DMA. The i n v e s t i g a t i o n s into the reactions of t e r t i a r y amines and RhCl3(DMS0)3 (1) (Chapter IV) revealed i n t e r e s t i n g redox processes between 1 and the t e r t i a r y amines, NEt3 or P.S. The proposed redox reactions are represented i n eqs. VII.1 and VII.2, although the observed f i n a l r e a c t i o n stoichiometries (see eqs. IV.5 (p. 121) and IV.21 (p. - 217 -144), r e s p e c t i v e l y ) are more complicated, because of secondary reactions between the dehyrogenated amines and complex 1. RhCl 3 + 3NEt 3 > RhCl + 2NEt 3 + + CH2=CHNEt2 VII. 1 CH 3 I RhCl 3 + 2 P.S. > RhCl +• P • S .HCl +:CHN ^ 7 V I 1-2 HCl-(CH 3) 2N The above redox reactions are, i n p r i n c i p l e , analogous to the more f a m i l i a r reactions between R h 1 1 1 and alcohols to y i e l d aldehydes (e.g. eq. V I I . 3 ) . 5 6 RhCl 3 + HOCH2R > RhCl + 2HC1 + 0=CHR VII. 3 T e r t i a r y amines are widely used as cocatalysts i n Rh-catalyzed reactions and are generally thought to act as proton scavengers i n such s y s t e m s . 7 8 - 8 1 The f a c i l e redox reactions discovered i n the present work suggest that t e r t i a r y amines should be considered also as p o t e n t i a l 2-electron reducing agents, and that any discussions on the ro l e of amines i n c a t a l y s i s should consider t h i s feature. To our knowledge, the i s o l a t e d Rh^^-ethenamine complex (6) i s the f i r s t recorded f o r t h i s metal, although analogous P t 1 1 and P d 1 1 complexes are known. 8 2" 8 6 The proposed redox re a c t i o n between R h 1 1 1 and an N-CH3 group of P.S. (eq. 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I, 1734 (1976). - 227 -APPENDIXES - 228 -Appendix I Some structural parameters for mer-cis-RhCl3(DMSO)("CH2CH=+NEt2) Table I Bond lengths (X) with estimated standard deviations in parentheses Bond Length(X) Bond Length(A) Rh -CIO) 2.3342(6) S(2) -0(2) 1 .476(2) Rh -Cl(2) 2.3480(5) S(2) -C(9) 1 .770(3) Rh -CK3) 2.3578(5) S(2) -COO) 1 .785(4) Rh -SO ) 2.2800(5) N -C(2) 1 .291(3) Rh -S(2) 2.3906(5) N -C(3) 1 .475(3) Rh -CO) 2.125(2) N -C(5) 1 .481(3) SO) -OO) 1.472(2) c o ) -C(2) 1 .439(3) SO) -C(7) 1.771(3) C(3) -C(4) 1 .511(4) SO) -C(8) 1.780(3) C(5) -C(6) 1 .503(4) Table II Bond angles (deg) with estimated standard deviations in parentheses Bonds Angle(deg) Bonds Angle(deg) CIO )-Rh -CK2) 177.25(2) Rh -SO )-C(8) 112.21(11) CIO )-Rh -CK3) 90.44(2) OO) -S(1)-C(7) 107.3(2) CIO )-Rh -SO ) 89.96(2) OO) -SO )-C(8) 106.80(14) Cl(1)-Rh -S(2) 92.23(2) C(7) -SO )-C(8) 99.8(2) CIO )-Rh -CO) 88.06(7) Rh -S(2)-0(2) 117.17(8) Cl(2)-Rh -CK3) 89.33(2) Rh -S(2)-C(9) 111.90(15) Cl(2)-Rh -SO) 90.22(2) Rh -S(2)-C(10) 110.78(11) Cl(2)-Rh -S(2) 90.48(2) 0(2) -S(2)-C(9) 107.4(2) Cl(2)-Rh -CO) 89.22(6) 0(2) -S(2)-CO0) 108.8(2) Cl(3)-Rh -SO) 179.30(2) C(9) -S(2)-C(10) 99.3(2) Cl(3)-Rh -S(2) 85.52(2) C(2) -N -C(3) 122.9(2) Cl(3)-Rh -CO) 92.19(6) C(2) -N -C(5) 120.3(2) SO )-Rh -S(2) 95.03(2) C(3) -N -C(5) 116.8(2) SO )-Rh -CO) 87.26(6) Rh -CO )-C(2) 110.4503) S(2)-Rh -CO) 177.69(6) N -C(2)-C(1) 126.7(2) Rh -SO) -OO) 116.84(7) N -C(3)-C(4) 111.8(2) Rh -SO) -C(7) 112.42(12) N -C(5)-C(6) 112.8(2) - 229 -Appendix II Some s t r u c t u r a l parameters f o r [Et 2NH 2][trans-RhCl4(DMSO) 2] Table I Bond lengths (A) with estimated standard deviations in parentheses Bond A B Bond A B R h ( i ) - C K l ) 2 . 3 4 5 9 ( 4 ) 2 . 3 3 0 9 ( 1 2 ) S ( 1 ) - C ( 2 ) 1 . 7 7 4 ( 3 ) 1 . 7 7 8 ( 6 ) R h d ) - C l ( 2 ) 2 . 3 4 5 2 ( 5 ) 2 . 3 3 7 1 ( 1 3 ) S ( 2 ) - 0 ( 2 ) 1 . 4 2 2 ( 2 ) 1 . 4 7 2 ( 4 ) R h O ) - S ( l ) 2 . 3 1 7 3 ( 5 ) 2 . 3 1 2 4 ( 1 0 ) S ( 2 ) - C ( 3 ) 1 . 7 7 3 ( 3 ) 1 . 7 7 9 ( 6 ) R h ( 2 ) - C l ( 3 ) 2 . 3 4 0 0 ( 5 ) 2 . 3 3 3 0 ( 1 1 ) S ( 2 ) - C ( 4 ) 1 . 7 8 6 ( 4 ) 1 . 7 7 7 ( 6 ) R h ( 2 ) - C l ( 4 ) 2 . 3 4 2 9 ( 5 ) 2 . 3 3 3 9 ( 1 1 ) N - C ( 5 ) 1 . 4 8 2 ( 3 ) 1 . 4 9 3 ( 8 ) R h ( 2 ) - S ( 2 ) 2 . 3 2 9 4 ( 5 ) 2 . 3 3 1 8 ( 1 1 ) N - C ( 7 ) 1 . 4 9 9 ( 3 ) 1 . 4 9 1 ( 7 ) S ( 1 ) - 0 ( 1 ) 1 . 4 6 3 ( 2 ) 1 . 4 6 3 ( 2 ) C ( 5 ) - C ( 6 ) 1 . 4 9 5 ( 4 ) 1 . 4 9 2 ( 1 1 ) S ( l ) - C ( l ) 1 . 7 8 3 ( 2 ) 1 . 7 8 3 ( 2 ) C ( 7 ) - C ( 8 ) 1 . 4 8 6 ( 4 ) 1 . 4 8 5 ( 1 1 ) Table II Bond angles (deg) with estimated standard deviations in parentheses Bonds A B Bonds A e C l ( l ) - R h ( l ) - C l ( 2 ) 8 9 . 6 1 ( 2 ) 9 0 . 2 4 ( 6 ) C(D-S ( 1 ) - C ( 2 ) 9 9 . 7 4 ( 1 4 ) 101 . 6 ( 4 ) C I O ) - R h ( l ) - S O ) 8 7 . 7 5 ( 2 ) 9 4 . 4 2 ( 4 ) R h ( 2 ) - S ( 2 ) - 0 ( 2 ) 1 1 6 . 2 7 ( 1 1 ) 117 . 0 ( 2 ) C l ( 2 ) - R h ( l ) - S ( 1 ) 9 2 . 5 1 ( 2 ) 9 0 . 11 ( 5 ) R h ( 2 ) - S ( 2 ) - C ( 3 ) 111 . 3 1 ( 1 2 ) 112 . 3 ( 2 ) C l ( 3 ) - R h ( 2 ) - C l ( 4 ) 8 9 . 7 6 ( 2 ) 9 0 . 0 9 ( 5 ) R h ( 2 ) - S ( 2 ) - C ( 4 ) 111 . 6 7 ( 1 3 ) 1 11 . 7 ( 2 ) C l ( 3 ) - R h ( 2 ) - S ( 2 ) 9 0 . 5 9 ( 2 ) 8 9 . 8 9 ( 4 ) 0 ( 2 ) - S ( 2 ) - C ( 3 ) 1 0 5 . 9 ( 2 ) 1 0 6 . 9 ( 3 ) C l ( 4 ) - R h ( 2 ) - S ( 2 ) 9 1 . 6 1 ( 2 ) 8 6 . 1 4 ( 4 ) 0 ( 2 ) - S ( 2 ) - C ( 4 ) 1 1 0 . 8 ( 2 ) 107 . 5 ( 3 ) R h ( l ) - S ( 1 ) - 0 ( 1 ) 1 1 5 . 2 3 ( 8 ) 1 1 5 . 8 5 ( 1 5 ) C ( 3 ) - S ( 2 ) - C ( 4 ) 9 9 . 5 ( 2 ) 100 .1 ( 4 ) R h d ) - s ( D - c d ) 1 1 1 . 7 B ( 9 ) 1 1 1 . 3 ( 2 ) C ( 5 ) - N - C ( 7 ) 1 1 5 . 0 ( 2 ) 113 . 9 ( 5 ) R h ( 1) - S ( 1 ) - C ( 2 ) 1 1 0 . 9 2 ( 1 0 ) 1 1 2 . 5 ( 2 ) N C ( 5 ) - C ( 6 ) 1 11 . 8 ( 2 ) 11 1 . 5 ( 6 ) 0 ( 1 ) - S ( 1 ) - C ( 1 ) 1 0 8 . 1 6 ( 1 2 ) 1 0 7 . 0 ( 3 ) N C ( 7 ) - C ( 8 ) 1 1 1 . 8 ( 2 ) 110 . 5 ( 6 ) 0 ( 1 ) - S ( 1 ) - C ( 2 ) 1 0 9 . 6 8 ( 1 4 ) 1 0 7 . 4 ( 3 ) <a) Found i n two c r y s t a l forms, termed type A and B (Figs. IV.13 and IV.14 r e s p e c t i v e l y ) , both t r i c l i n i c . See also p. 1 3 8 . 

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