Science, Faculty of
Chemistry, Department of
DSpace
UBCV
Rempel, Garry Llewellyn
Doctor of Philosophy - PhD
University of British Columbia
Kinetic studies of a number of interesting and significant reactions involving activation of molecular hydrogen, simple olefins and carbon monoxide by solutions of rhodium trichloride trihydrate are described. RhCl₃∙3H₂O in N,N-dimethylacetamide solution is an effective catalyst for the homogeneous hydrogenation of a variety of substituted ethylenes as well as for ethylene itself. The kinetics of the RhCl₃∙3H₂O catalyzed hydrogenation of maleic acid in dimethylacetamide media suggest a mechanism involving an initial hydrogen reduction of rhodium (III) to rhodium (I), which is stabilized in solution by rapid complexing with maleic acid. The rhodium (i) maleic acid complex undergoes a subsequent reaction with hydrogen to produce succinic acid and rhodium (l) again, via an intermediate containing both coordinated maleic acid and hydrogen. In aqueous acid-chloride solution, rhodium (I) olefin complexes also form, but no subsequent homogeneous hydrogenation is apparent. Solutions of RhCl₃∙3H₂O in dimethylacetamide react with ethylene to form a rhodium (I) ethylene complex. Examination of the kinetics of the reaction suggest that the reaction proceeds through an initial dissociation of a chloro rhodium (III) species. A resulting intermediate rhodium (III) ethylene complex is decomposed by water to rhodium (I) which is stabilized by rapid reaction with further ethylene as an olefin complex. The resulting rhodium (I) ethylene complex subsequently acts as a dimerization catalyst for the production of butenes. Ethylene catalyzes at ambient temperatures the production of the [Rh(H₂O)₄Cl₂ ]⁺ cation from aqueous solutions of RhCl₃∙3H₂O; at higher temperatures metallic rhodium is formed. Carbon monoxide reacts with chlororhodate (III) complexes in aqueous HCl solution to form the anionic species [Rh(I)(CO)₂Cl₂]⁻. The observed kinetics indicate that [Rh(I)(CO)₂Cl₂]⁻ is autocatalytically produced; the mechanism postulated involves initial production of some [Rh(I)(CO)₂Cl₂]⁻ through a CO "insertion" reaction with a chlororhodate (III) complex. The [Rh(I)(CO)₂Cl₂]⁻ species Is then involved in a two electron transfer process via a chloride bridged [Rh(I)...Cl…Rh(III) ] intermediate to produce further Rh(I) more efficiently than by the initial direct insertion-reduction process. Carbon monoxide is catalytically activated through coordination to rhodium for subsequent reduction of rhodium (III) resulting from the electron transfer process. An inorganic substrate, ferric chloride, is catalytically reduced by [Rh(I)(CO)₂Cl₂]⁻ in this way. Kinetic studies of direct carbonylation of RhCl₃∙3H₂O in dimethylacetamide solutions have shown the importance of the presence of a water molecule for the production of the [Rh(I)(CO)₂Cl₂]⁻ species from an initially formed Rh(III)(CO) complex. The introduction of carbonyl groups into chlororhodate complexes is found to inhibit catalytic activity for the hydrogenation of olefinic substrates.
KINETIC STUDIES ON CATALYTIC PROPERTIES OF RHODIUM COMPLEXES IK SOLUTION by GARRY LLEWELLYN REMPEL B.Sc.(Hons.) The University of British Columbia, 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY ' in the Department of CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA Ap r i l , 1968 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n -t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t n f Chemistry The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, C a n a d a D a t e 23rd A p r i l , 1968 ABSTRACT K inet ic studies of a number of i n te res t ing and s i g n i f i c a n t reac -t ions invo lv ing c . c t i vat ion of molecular hydrogen, simple o l e f i n s and carbon monoxide by solut ions of rhodium t r i c h l o r i d e t r ihydrate are descr ibed. RhCl^'SH^O i n N,N-dimethylacetamide so lut ion i s an e f fec t i ve cata lys t fo r the homogeneous hydrogenation of a var ie ty of subst i tuted ethylenes as w e l l as for ethylene i t s e l f . The k i n e t i c s of the RhCl^^H^O catalyzed hydrogenation of maleic ac id in dimethylacetamide media suggest a mechanism invo lv ing an i n i t i a l hydrogen reduction of rhodium (I I I ) to rhodium ( i ) , which i s s t a b i l i z e d i n so lut ion by rapid complexing with maleic a c i d . The rhodium ( i ) maleic ac id complex undergoes a subsequent reaction, with hydrogen to produce succinic ac id and rhodium ( l ) aga in , v i a an i n t e r -mediate containing both" coordinated maleic ac id and hydrogen. In aqueous ac id - ch lo r ide s o l u t i o n , rhodium (I) o l e f i n complexes also form, but no subsequent homogeneous hydrogenation i s apparent. Solutions of RhCl^*31^,0 i n dimethylacetamide react with ethylene to form a rhodium ( i ) ethylene complex. Examination of the k i n e t i c s of the react ion suggest that the react ion proceeds through an i n i t i a l d i ssoc ia t ion of a chloro rhodium ( i l l ) species . A r e s u l t i n g intermediate rhodium ( i l l ) ethylene complex i s decomposed by water to rhodium (I) which i s s t a b i l i z e d by rap id react ion with further ethylene as an o l e f i n complex. The r e s u l t i n g rhodium (I) ethylene complex subsequently acts as a dimerizat ion cata lys t for the production of butenes. Ethylene catalyzes at ambient temperatures the production of the [Rh(H 0),C1 ] + cat ion from aqueous solut ions of RhCl *3H 0; at higher temperatures metallic rhodium is formed. Carbon monoxide reacts with chlororhodate (III) complexes in aqueous HC1 solution to form the anionic species [Rh^CO^Cl^] -. The observed kinetics indicate that [Rh*(COJ^Cl^] is autocatalytically pro^ duced; the mechanism postulated involves i n i t i a l production of some [Rh I(C0) 2Cl 2]~ through a CO "insertion" reaction with a chlororhodate (III) complex. The [Rh^CO^Clg] species Is then involved in a two electron transfer process via a chloride bridged R^h"*".. .Cl.. .Rh"*"^ ] intermediate to produce further Rh* more ef f i c i e n t l y than by the i n i t i a l direct insertion-reduction process. Carbon monoxide is catalytically activated through coordination to rhodium for subsequent reduction of rhodium (III) resulting from the electron transfer process. An inorganic substrate, f e r r i c chloride, is catalytically reduced by [Rh^COjgCl ]~ in this way. Kinetic studies of direct carbonylation of RhCl^'SH^O in dimethyl-acetamide solutions have shown the importance of the presence of a water molecule for the production of the [Rh^COjgCl ]~ species from an i n i t i a l l y formed Rh"''**(CO) complex. The introduction of carbonyl groups into chlororhodate complexes is found to inhibit catalytic activity for the' hydrogenation of olefinic substrates. TABLE OF CONTENTS ABSTRACT LIST OF TABLES LIST OF FIGURES ABBREVIATIONS ACKNOWLEDGMENTS CHAPTER I. INTRODUCTION 1 1:01 General introduction 1 1:02 Aim of the vork 1 1:03 Catalytic activation of molecular hydrogen 2 1:0'+ Inorganic substrates 2 1:05 Organic substrates k 1:06 Literature reports on the catalytic properties of 6 rhodium complexes 1:07 Rhodium complexes as hydrogenation catalysts 6 1:08 Catalytic isomerization of olefins 8 1:09 Polymerization of olefins and acetylenes 11 1:10 Carbonyl complexes of rhodium 15 CHAPTER II. APPARATUS AND EXPERIMENTAL PROCEDURE 18 2:01 Apparatus for constant pressure gas-uptake or evolution 18 measurements 2:02 General procedure for a typical gas-uptake experiment 20 Page i i i x x i xv x v i i V Page 2:03 Gas solubility measurements 21 2:0U iTipectrophotometric kinetic measurements 22 2:05 Reaction product analysis , 23 2:06 Instrumentation 2k CHAPTER III. THE RHODIUM CHLORIDE CATALYZED HYDROGENATION OF 26 OLEFINIC COMPOUNDS 3:01 Aqueous systems 26 3:02 Non-aqueous systems 30 3:03 The rhodium chloride-maleic acid system in 30 dimethylacetamide 3:0U Kinetics of the RhCl •3H20/maleic acid/H 2 system in DMA 37 3:05 The i n i t i a l region 37' 3:06 Effect of chloride on the i n i t i a l region k6 3:07 The hydrogenation of maleic acid 53 3:08 Hydrogenation of other substituted ethylenes in 59 dimethylacetamide solution 3:09 The rhodium chloride catalyzed hydrogenation of 63 ethylene in dimethylacetamide Discussion 3:10 The reaction in aqueous solution 6k 3:11 The reaction in non-aqueous solution 65 3:12 I n i t i a l reaction .65 3:13 Effect of chloride on the i n i t i a l reaction 68 3:1^ Solvent effects 69 3:*15 Effect of different substrates on the i n i t i a l 72 reaction V I 3 : l 6 The catalytic hydrogenaticn 3 : 1 7 Hydrogenaticn in the absence of added chloride 3 : 1 8 Hydrogenation at high cl.loride concentration Page 72 73 CHAPTER IV. REACTIONS BETWEEN RHODIUM (III) CHLORIDE AND ETHYLENE 80 U :01 Introduction 80 h:02 The reaction in aqueous media 80 k:03 The reaction in dimethylacetamide solution 8 l UiOk Kinetics of the reaction in DMA 8 l U :05 Discussion of the reaction in DMA 8 3 CHAPTER V. DIRECT CARBONYLATION OF RHODIUM ( i l l ) CHLORIDE 9 6 COMPLEXES IN AQUEOUS SOLUTION 5 : 0 1 Introduction 9 6 5 : 0 2 The reaction of RhCl 3*3H 2 0 in aqueous solution with CO 9 6 5 : 0 3 Kinetics of the reaction in HC1 media 99 5 : 0 U The i n i t i a l reaction 1 0 1 5 : 0 5 The reaction involving the autocatalytic species 1 0 1 5 : 0 6 The effect of chloride concentration on reaction 1 2 0 rate 5 : 0 7 Discussion 120 CHAPTER VI. THE FORMATION OF SOME RHODIUM CARB0NYL COMPLEXES IN DIMETHYLACETAMIDE SOLUTION 6:01 Stoichiometrv 130. 1 3 0 v i i Page 6:02 K i n e t i c s of the f i r s t stage 133 6:03 K i n e t i c s of the second stage 138 6:0U D i s c u s s i o n 138 CHAPTER V I I . CATALYTIC PROPERTIES OF SOME RHODIUM CARBONYL ihh COMPLEXES 7:01 I n t r o d u c t i o n ikk 7:02 A c t i v i t y of c h l o r o carbonyl complexes i n aqueous ihh s o l u t i o n 7:03 A c t i v i t y of c h l o r o carbonyl complexes i n DMA lh9 7: Oil A c t i v i t y o f Rh C l ( C 0 ) ( P P h ^ i n DMA s o l u t i o n 151 CHAPTER V I I I . GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR 153 FUTURE WORK REFERENCES 157 APPENDIX I . MATERIALS l6k Al:01 Rhodium s a l t s _ l6k AI:02 Gases l6U Al:03 Substrates l6k A l ^ l i Solvents 165 Al:05 Other m a t e r i a l s 165 APPENDIX I I . VAPOR PRESSURE AND SOLUBILITY DATA FOR 167 ,N,N-DIMETHYLACETAMIDE A2:01 Vapor pressure o f DMA 167 V l l l Page A 2 : 0 2 Solubility of gases in DMA 167. A 2 : 0 3 Hydrogen solubility in DMA 167 A2:0k Ethylene solubility in DMA - 167 A2:05 Carbon monoxide solubility in DMA 170 LIST OF TABLES Page Rhodium trichloride catalyzed hydrogenation of maleic acid in DMA solutions. I The effect of [Rh 1 1 1], [ l l j and [Maleic acid] on the 38 i n i t i a l reaction rate in DMA. II The effect of additives on the i n i t i a l rate in DMA. h2 III Temperature dependence of in DMA. k3 IV The effect of [Rh 1 1 1] , [ H J and [Maleic acid] on the 1*8 i n i t i a l reaction rate in 1.2 M LiCl/DMA. V Temperature dependence of k in 1.2 M LiCl/DMA. 51 VI The effect of [Rh] , [ H ] and [Maleic acid] on the second 3k (linear) reaction rate in DMA. VII The effect of [Rh], (Maleic acid] and additives on the 56 ~ second (linear) reaction rate in DMA. VIII The effect of [Rh], [ H 2 ] and [Maleic acid] on the second 57 (linear) reaction rate in 1.2 M LiCl/DMA. IX Temperature dependence of k 2 in 1.2 M LiCl/DMA. 60 Complex formation and hydrogenation of various substituted ethylenes. X Summary of kinetic data at 80°. 62 Rh^ - maleic acid complex formation. .XI Rate constarts and activation parameters. 70 Page Ethylene reduction of rhodium chloride in DMA. XII oummary of kinetic data at 80 . - 8U XIII Temperature, dependence of k'. 9 3 Carbonylation of ^ hodium chloride in 3 M HC1. XIV Summary of kinetic data at 8 0 ° . 1 0 2 XV Temperature dependence of k^. 1 0 5 XVI Maximum rates at 8 0 ° . 1 0 8 XVII Calculation of k^ from ratio of maximum rate to 113 i n i t i a l rate. . XVIII Calculation of by use of modified integrated 116 expression. XIX Calculation of k from R h 1 1 1 addition to [Rh I(C0) 2Cl 2]~ 1 1 6 ' XX Temperature dependence of k„. 1 1 8 XXI Effect of [ci~] on the rates of reaction. 1 2 1 Formation of the Rh**1 carbonyl species in DMA. XXII Summary of kinetic data at 1+0°. 1 3 5 XXIII Temperature dependence of k n. 1 3 6 Formation of the Rh* carbonyl species in DMA XXIV . Summary of kinetic data. 1**0 XXV Temperature dependence of k^. 1**0 Solubility of gases in DMA. XXVI Effect of temperature and additives. ~~ • 170 LIST OF FIGURES Page 1. Apparatus for constant pressure gas-uptake or evolution 19 measurements. 2. Rate plot for Rh* - maleic acid complex formation in 3 M HC1 27 at 80°. 3. Absorption spectrum of Rh* - maleic acid complex in 3 M HC1. 29 Rhodium chloride catalyzed hydrogenation of maleic acid in DMA solutions. k. Rate plots at 80° for various [Rh] in DMA. 31 5. - Rate plots at 80° for various [Hg] in DMA. 32 6. Absorption spectra of RhCl^'BHgO in DMA and in 1.2 M 3h LiCl/DMA. • • 7. Absorption spectra of Rh* - maleic acid complex in DMA 35 and in 1.2 M LiCl/DMA. 8. Rate plot for oxidation of the Rh* - maleic acid complex 36 at 63° in DMA. 9. Dependence of the rate of Rh* - maleic acid complex forma- 39 tion on [Rh] in DMA. 10. Dependence of the rate of Rh* - maleic acid complex forma- ho tion on [llg] in DMA. 11. Arrhenius plot for the Rh* - maleic acid complex formation kh in DMA. . ' . I l l 12. F i r s t order plot for loss of Rh from optical density k$ measurements at 530 nut. 13. Kate plots at 8 0 ° for various [Rh] in 1 . 2 M LiCl/DMA. 1 ^ . Dependence, of the rate of Rh"'' - maleic acid complex formation on [Rh] in 1 . 2 M LiCl/DMA. 1 5 . Dependence of the rate of Rh"*" - maleic acid complex. • formation on [H 2] in 1 . 2 M LiCl/DMA. 1 6 . Arrhenius plot for the Rh^ " - maleic acid complex formation in 1 . 2 M LiCl/DMA. i f . Dependence of the rate of hydrogenation of maleic acid on [Rh] in DMA. 1 8 . Dependence of the rate of hydrogenation of maleic acid on . [H 2 ] in DMA. 1 9 . Dependence of the rate of hydrogenation of maleic acid on [Rh] in 1 . 2 M LiCl/DMA. 2 0 . Dependence of the rate of hydrogenation of maleic acid on [H 2] in 1 . 2 M LiCl/DMA. 2 1 . Arrhenius plot for the rhodium chloride catalyzed hydrogens tion of maleic acid in 1 . 2 M LiCl/DMA. Ethylene reduction of rhodium chloride in DMA. 2 2 . Rate plots at 8 0 ° for various [Rh] . 23. Dependence of i n i t i a l rate of reaction on [Rh"*'"'""'"]. 2k. Dependence of i n i t i a l rate of reaction on [CgHjJ . 2 5 . Optical density plot at 550 m/t for loss of original chloro rhodium ( i l l ) species. 2 6 . F i r s t order plot for i n i t i a l reaction. 2 7 . Dependence of i n i t i a l rate of reaction on [CgHjJ . x i i i Page 28. Dependence of i n i t i a l rate of reaction on [ci ]. 90 29. Arrhenius plot for i n i t i a l reaction. 9 ^ Carbonylation of rhodium chloride in 3 M FC1. 30. Rate plots at 8 0 ° for various [Rh 1 1 1.]. 9 7 31. Typical plots of -d [co]/dt vs. [ R h 1 1 1 ] . 100 32. Dependence of i n i t i a l rate of reaction on [Rh*"'"^ ]. 103 3 3 . Dependence of i n i t i a l rate of reaction on [co]. 10k 3k. Arrhenius plot for i n i t i a l reaction. 1 0 6 3 5 . Dependence of maximum rate on and on 109 [ R h m ] x [Rh 1]. 3 6 . Effect of CO pressure on maximum rate. 110 3 7 . Plots of log [Rh I(C0) 2Cl 2]~ - log [Rh 1 1 1] vs. time 115 for various CO pressures. 3 8 . Rate plot for R h 1 1 1 reduction when [Rh I(C0) 2Cl 2 ]~ present 117 i n i t i a l l y . 3 9 - Arrhenius plot for reduction of Rh 1 1 1 by [RhI(C0) Cl ] ~ 119 in the presence of CO. k0. Dependence of maximum rate on [ci ]. 122 Formation of -Rh carbonyl species in DMA. hi. Rate plot for formation of Rh I i : E(C0) and Rh I(C0) 2 species 131 at k0°. h2. Kinetic plot for formation of Rh I i : E(C0) at k0°. 13k 1»3. Arrhenius plot for reaction of Rh'^ "'' with CO. 137 hh. Kinetic plot for the loss of Rh I I I(C0) in the presence 139 of CO at 1+0°. X I V Page l » 5 . Arrhenius plot for reaction of Rh***(CO) with CO. l U l U 6 . CO uptake plot for the reduction of Rh*** in the presence of ikQ Fe*** in 3 M HC1 at 8 0 ° . Vapor pressure and solubility data for DMA.' i+T. Vapor pressure of DMA from 0 - 1 0 0 ° . 1 6 8 U 8 . Solubility of H 2 in DMA at 8 0 ° . 1 6 9 I49. Solubility of C ^ in DMA at 8 0 ° . 1 7 1 5 0 . Solubility of CO in DMA at k0°. 172 ABBREVIATIONS The following l i s t of abbreviations, most of which are commonly adopted in chemical research literature, w i l l be employed in this thesis. A l l temperatures are in °G unless specifically denoted °K. acac acetylac'etonate anion aq aquated, water Ar aryl b.pt. boiling point decomp. decomposition DMA N,N-dimethylacetamide, CH CON(CH3) DMF N,N-dimethylformamide, HCON(CH3)2 DMSO dimethylsulfoxide, (CH^SO en ethylenediamine, KHgCH CHgNH esr electron spin resonance Et ethyl, C 2H 5 IR - infrared L ligand M metal atom MA maleic acid Me methyl, CH 3 nmr nuclear magnetic .resonance P* p a r t i a l pressure Ph phenyl, C^ H py pyridine 2 alkyl or aryl group succinic acid triethylenetetramihe halogen unless stated othervi mclar extinction coefficient frequency, cm ^ ACKNOWLEDGMENTS The author wishes to express his gratitude for the advice, help and encouragement given by Dr. B. R. James throughout the course of the research and preparation of this thesis. The author also wishes to thank the National Research Council of Canada for financial support of the research and for the award of a Bursary (1965-6) and Studentships (1966-8). CHAPTER I INTRODUCTION •1:01 General introduction In recent years there has been a considerable increase in the amount of interest and progress in the field of homogeneous catalysis. Important developments, particularly in the elucidation of many new and unusual catalytic reactions of transition metal ions and coordination compounds have resulted. The field of coordination chemistry and homo-geneous catalysis involving organic substrates is concerned with a variety of reactions such as homogeneous hydrogenation, hydroformylation, oxidation of olefins, hydration of acetylene compounds and polymerization. Complexes' of the Group VIII metals, (particularly Platinum metal complexes), have often been found to be especially active for these types of reactions and have received much attention within the last few years. 1:02 Aim of the work The aim of the work undertaken for this thesis was primarly to investigate the use of certain rhodium complexes, in both aqueous and non-aqueous solution, for the activation of molecular hydrogen. In instances where activation of molecular hydrogen was apparent, the detailed kinetics of the reactions were investigated and reaction mechanisms have been postulated. In order to learn more about the detailed mechanism of some of these catalytic reactions, a variety of rhodium complexes and different substrates were used.- Both aqueous and non aqueous solvent media were employed in attempts to study the effect of solvent on the catalytic activity of the complexes involved. The work presented in this thesis describes these general studies. The study of the catalytic activity of some rhodium carbonyl complexes produced some interesting observations regarding the activation of molecular carbon monoxide and thus sections of the thesis are concerned with the f i e l d of metal carbonyl chemistry. At the commencement of this work in early 1965 there was relatively l i t t l e reported regarding the catalytic aspects of rhodium complexes in solution. However, since that time there has been a remarkable amount published on this particular topic. Some of the more pertinent data, with regards to this thesis, w i l l be br i e f l y reviewed in the following sections. 1:03 Catalytic activation of molecular hydrogen The homogeneous catalytic activation of molecular hydrogen by 1-3 transition metal ions or complexes in solution is well known . The hydrogen activating process usually results in production of a reactive intermediate hydrido complex of the transition metal catalyst. The catalytic activity of these hydrido intermediates i s dependent on factors affecting their s t a b i l i t y and l a b i l i t y . It i s necessary to consider the reactivity of metal complexes toward hydrogen since such reactions are capable of forming the intermediate hydride by various mechanisms. 1:01* Inorganic substrates A common feature of the mechanism of reactions involving inorganic substrates appears to be a rate-determining heterolytic or homolytic splitting of molecular hydrogen by the catalytic species. This step results in the formation of a metal-hydride complex which subsequently reduces tho sub-strate in a rapid step. An example of the heterolytic s p l i t t i n g of molecular hydrogen is the rhodium (III) catalyzed reduction of iron (III) in 3 M hydrochloric li 5 acid ' which can be represented by the following reaction scheme: k l R h 1 1 1 + H ? — R h m i r + H + k - l Rh 1 1 V + 2 F e m -V R h 1 1 1 + 2Fe 1 1 + H + fast (the ligand species Cl , H^ O attached to the Rh*^ ion have been omitted) Direct evidence of an equilibrium such as shown in the f i r s t reaction has been obtained for the corresponding ruthenium (III) system ' by isotopic exchange experiments using deuterium. The heterolytic splitting involves a substitutional process, (eg. replacement of a chloride ligand by a hydride ion), without change in the formal oxidation number of the metal. Hence, the reactivity of a particular metal complex is governed by i t s substitution l a b i l i t y and the s t a b i l i t y of the hydride formed. The presence of a suitable base to stabilize the released proton can also promote activity. A system involving the homolytic splitting mechanism is the copper 8 (I) catalyzed reduction of copper ( l l ) in quinoline solution , which has a rate determining step: 2CU1 + H 2 " 2(CuH)+ The formal oxidation number of the metal is changed on hydride formation and a further factor affecting reactivity of the complex is the ease of metal oxidation. Many other transition metal ions and complexes have been found to activate hydrogen for the reduction of inorganic substrates via such hetero-1 2 l y t i c or homolytic processes ' . 1:05 Organic substrates Although a considerable number of transition metal complexes are known to activate molecular hydrogen, relatively few have been found to be effective catalysts for the homogeneous hydrogenation of organic substrates. Three mechanisms for activation of molecular hydrogen, (involving heterolytic s p l i t t i n g , homolytic s p l i t t i n g , and dihydride formation), have 1-3 been postulated for catalytic hydrogenation of olefinic substances , and i t i s convenient to consider each of these in turn. Heterolytic splitting of molecular hydrogen is exemplified by the chlororuthenate (II) catalyzed hydrogenation of olefinic carboxylic acids Q such as fumaric and maleic acids . An i n i t i a l rapid formation of a 1:1 Ru(ll)-olefin tf-complex is followed by reaction with hydrogen in a rate-determining step as shown in the following scheme: H~-\ / - Ru • C + H 5> A 2 slow / j H I / Ru C A --C - Ru<-'' A +H (fast) \ 1/ \ / H* - R u - + H - C - C - H ^ - R u - C - C - H In the case of the pentacyanocobaltate (II) catalyzed reduction of 3 10 conjugated olefins ' such as butadiene, a homolytic splitting of molecular hydrogen occurs. Detailed studies have suggested that this reaction takes place by the following mechanism: 2Co(CN)5"3 + H 2 2HCo(CN)53 HCo(CN).3 + CH = CHCH = CH0 '• > CH0CH = CHCH0Co(CN)3 5 2 d 3 d ? - HCo(CN) 2 CH0CH= CHCHnCo(CN).-J 2—> CH_CH0CH=CH0 + 2Co(CH)c-3 2 5 . 5 d d 5 CH CH -w CH / CH„ CN + CN - HCo(CN) J -CoCCN)^ —> CH3CH=CKCH3 + 2Co(CN) J The homogeneous catalytic hydrogenation of a variety of olefinic 11 12 substances by chloro, t r i s - (triphenylphosphine) rhodium (I) complex ' involves the formation of an intermediate dihydride for the hydrogen activa-tion. A mechanism suggested for the overall catalytic hydrogenation is given by the following reaction scheme: solvent RhCl(PPh3)3 PPh, RhCl(PPh3)2 + PPh3 K, RhCl(PPh3)2 + H2 2 olefin H2RhCl(PPh3)2 Is.' olefin RhCl(PPh-)-(olefin) „ id rt 11 k" RhCl(PPh3)2 + paraffin Wilkinson et al have shown that attack of hydrogen on the olefin complex does not take place, and paraffin formation results entirely from attack of the olefin on a dihydride complex formed in an i n i t i a l rapid equilibrium. An interesting feature of these olefin hydrogenation reactions is whether both of the reactants, (hydrogen, olefin), or just the hydrogen need to be activated by the metal atom. A l l three mechanisms at some stage involve addition of a metal hydride to the olefin i.e. \ / \ / M - H + C = C M - C - C - H . and such hydi'ometallation reactions have been observed directly; two i n -volving rhodiun: complexes are with the specie s [RhH(CW)i+H20]2 1 3 and •RhHClgfPPh^g^** Hydrometallation is an example of a much wider class of "insertion'-type reactions whose general widespread role in catalytic 2 reactions has also been considered , (see also section 1:09). The existence of the olefin complex as an intermediate in catalytic reactions is clearly demonstrated in such cases where olefin complex forma-tion is followed by subsequent reaction with hydrogen (usually in the presence of excess olefin) resulting in production of reduced olefin. As mentioned previously for intermediate hydrido-complexes, (section 1 : 0 3 , 1 . 0 U ) , the intermediate olefin complex must also show a favorable balance of properties affecting i t s s t a b i l i t y and l a b i l i t y in order that successful homogeneous hydrogenation may occur. 1 : 0 6 Literature reports on the catalytic properties of rhodium complexes A very comprehensive review, by James*'', on the reactions and catalytic properties of rhodium complexes in solution surveys the literature up to the middle of 1 9 6 6 . A number of these reports which are relevant to the present work w i l l be b r i e f l y mentioned in the following sections. 1 : 0 7 Rhodium complexes as hydrogenation catalysts In 1939> Iguti*^ f i r s t reported that certain rhodium (III) com-plexes in aqueous solution activated molecular hydrogen for the reduction of some organic and inorganic substrates such as qiiinone, fumaric acid and sodium n i t r i t e . Rhodium trichloride and unsaturated amine complexes such as CRh(NH3) H 2 o ] c i 3 and Qtti(NH 3)^Cl 2]ci were found to be active while sat-urated amine complexes were inactive. However under the conditions used (aqueous acetate solutions at 2 5 ° ) traces of metallic rhodium, a powerful heterogeneous catalyst'', are formed, so the validity of these early results k may be questionable. Harrod and Halpern in 1 9 5 9 studied the hydrogen reduction of fe r r i c ions catalyzed homogeneously by chlororhodate (III) species in aqueous acid solutions. Extension of this work by James and Rempel^ in 1 9 6 5 revealed that only the anionic, labile chlororhodate (III) species were effective catalysts, the activity of the species increasing with increasing number of coordinated chloride ligands. It is thought that the increased strength of the metal-chloride bonding in the lower chloro species causes the removal of chloride or i t s replacement by hydride in the formation of an intermediate hydridic species to be more d i f f i c u l t . Hydrogenation (and hydroformylation) of olefinic and acetylenic bonds catalyzed by both rhodium ( i l l ) and rhodium (i) complexes has recently been reported for homogeneous conditions, but in non aqueous solvents. Wilkinson and coworkers have reported that effective catalysts for reduction of substrates such as hex-l-ene include RhCl •SRgO ', 1 , 2 , 6 -(py) 3RhCl 3 '» , 1 8 -I O O f-DTDU \ O V , m l 8 f l 9 B v p W B O l , \ H , 1 2 , 2 0 , 2 1 Rh 2Cl 2(SnCl 2 , E t 0 H ) l t , 1 , 2 , 3 -(PPh^RhC^ ' ^, RhCltPPh^ ' ' * -and [RhCN(PPh 3) 2] 2* 3 in ethanol, benzene or ethanol-benzene mixtures. The de-t a i l e d kinetics of these systems have not generally been given. However, some of the systems are probably complex, eg. hex-l-ene is catalytically 2 2 isomerized by ethanolic RhCl3*3.H20 , (section 1:08). It is also known that reaction of triphenylphosphine and arsine type complexes of the platinum metal ions with ethanol can result in the formation of hydrido- and hydrido-2 3 2h carbonyl-type complexes with the metal in a reduced valence state ' . Thus, these hydrogenation systems ara in general l i k e l y to he complex kinetically. The catalytic activity of RhC^PPh^)^ in benzene solution is a more well-defined system in which a molecular hydride intermediate has been detected (section 1:05). The rhodium complexes RhCl(C0)(PPh } ^ , 2 6 and RhH(C0)(PPh 3)^' 2 8 also catalyze the hydrogenation of olefins and acetylenes in benzene solu-tion but these complexes, unlike RhC^PPh^)^, show no reaction with hydrogen 11 20 29 30 or olefin ' * ' RhH(CO)(PPh^)^ does, however, catalyze hydrogen-deuterium exchange and like the RhC^PPh^)^ complex i t does dissociate in 29 solution . The catalytic activity of RhCl(CO)(PPh^)^ is much less than that of the RhCltPPh^)^ species'*""'". Substitution of CO into chlororuthenate ( l l ) 31 complexes similarly results in decreased catalytic activity 1:08 Catalytic isomerization of olefins Of related interest to catalytic hydrogenation reactions is the catalyzed isomerization of'olefins, (i.e. double bond migration). 32 Cramer has carried out an important study on the isomerization of linear butenes using soluble rhodium catalysts. The active catalyst is ob-tained as an equilibrium mixture containing rhodium (III) by a fast anaerobic reaction of inactive [(CgH^ JgRhCl] 2 or (acac) Rh.(C with HC1. [(C 2H u) 2RhCl] 0 — Rh ICl s (a) +s (a) + HC1•~ -s R h m c i _ s 0 I 3 (b) (s i s a solvent molecule, the olefin in (a) is ethylene or a linear butene and the alkyl in (c) is ethyl or butyl). The overall mechanism for the isomerization is formulated as: Rh j L L 4 RCH2CH=CH2 CH_R I CH Rh CH„ H + + L RCH=CHCH, CHR L = Cl or solvent H CHR -(H + + L) Rh /CHCH, fRh 1 1 1 /CHCH, -L L | L L +L CH2R H CH /Rh 1 1 1 / CH, L | L L +L -L T Rh I ] • ,CH •CH, Some general points on this type of mechanism are considered in the following section 1:09. It has been suggested that the formation of a 7 r - a l l y l i c derivative, C^ 2H^RhCl 2, may involve the i n i t i a l isomerization of the starting olefin (trans, trans, trans-l,5,9-cyclododecatriene), by the reversible nucleophllic attack on the olefin bond by a hydride HRhClg (as in the above figure), and the isomerization of olefins in the hydroformylation reaction is thought to 2k 35 proceed via a similar mechanism ' . 10 22 Chalk and Karrod have reported that double bond migration of olefins if catalyzed by rhodium trichloride and other platinum-metal com-plexes. They interpreted this in terms of a reversible hydrometallation of the double bond via a metal-olefin complex. Such a process is illustrated in the following equation where M is a transition metal ion: CH3 CH_ CH HC d I •MH Z "~ HC - M MH CH CH. HC I \ \ CH„ On the basis of varying degrees of stereospecifity shown, and a wide variety of conditions under which isomerization was observed, i t was concluded that the reaction is sensitive to the metal, coordinated ligands and the source of hydride, which originates from the olefin itself or added co-catalysts such as alcohols. Rinehart and Lasky studied the isomerization of 1,3-cyclooctadiene to 1,5-cyclooctadiene, in the presence of RhCl^, and postulated a somewhat different mechanism involving rearrangement through a if-aHyl hydride inter-mediate. RCH^-CH = CH0 RCif' \. N'CH0 RCH = CH - CH_ 2 1 2 M 2 . 3 M i M n A recent similar study involving isomerization of cis, trans-l,5-cyclodecadiene 37 has however been explained in terms of a hydrido-alkyl intermediate . The isomerization of olefins has been reviewed in some detail 38 -recently by Orchin 1:09 Polymerization of olefins and acetylenes Rhodium chloride or -nitrate has been found to catalyze stereo-specifically the polymerization of butadJene in water or alcohol to crystalline t r a n s - l , ^ - p o l y b u t a d i e n e 3 ^ . The fact that butadiene poly-merization is greatly accelerated on the addition of reducing agents suggests the participation of a lower oxidation state, possibly Rh"*" or alternatively a 1+1 k2 hydride complex . Natta and coworkers report that the RhCl^-catalyzed polymerization of cyclobutene in aqueous emulsion to a crystalline poly-cyclobutylenamer -2, occurs through a mechanism involving coordinated anions. Bawn and coworkers have carried out a mechanistic study for the solution polymerization of butadiene to long-chain polymers in the presence of rhodium salts; they conclude that the active species is not a rhodium (I)- TT-allylic complex but instead a tf-allylic rhodium ( I I I ) hydride. The polymerization of butadiene catalyzed by rhodium nitrate in ethanol has also been studied by E.S.R. spectroscopy . It was found that a radical is formed during the polymerization and that the steady-state concentration of the radical was in proportion to the concentration of nitrate added. However, a radical i n -hibitor such as hydroquinone did not inhibit the polymerization and also no random polymer products were obtained. This inconsistency has been rational-ized by postulation of a mechanism involving a coordinated radical (i.e. an 2+ a l l y l radical coordinated to Rh ). This a l l y l radical can be in the stable syn form as shown in (i) and ( l l ) of the following scheme and may probably be detected by an electron spin resonance method. 12 CH2=:CH - CH=CH + H20 + Rh(NO ) CH. CH CHgOH 'CH Rh(N03)2 HOH C CH (I) + HNO, + CH2== CH - CH=CH, ^CH .CHo0H CH' 1 ^CH H 2 C ! *ll Rh(NO_)_ HC J 3 2 .CH CH"2 I "CH Rh(NO ) g CH2 .CH CH2 CH ( I I ) CH20H 1*5 Alderson and coworkers have reported the rhodium chloride catalyzed addition of ethylene and propylene to such dienes as butadiene, isoprene, and 1,3 pentadiene. Rhodium derivatives were also found to catalyze the dimeriza-tion of ethylene to butenes, butadiene to 2 ,k ,6-octatriene, and methyl acry-late to dimethyl 2-hexenedioate. The exclusively straight chain structures of the latter two dimers are in marked contrast to the cyclic and branched chain U6 dimeric products formed by other methods. Cramer has investigated in con-siderable detail the mechanism of the rhodium chloride-catalyzed ethylene dimerization in ethanolic HC1 solution. During a short induction period, when ethylene is absorbed only slowly,, the catalytically active species [(CgH^). Rh^Cl,,] is thought to be formed. This anion i s obtainable directly by dissolution of [(C^H^^Rh^Cl] ^ in ethanolic HC1, and no induction period is observed. Cramer has postulated the' following four step mechanism: (a) Protonation of the bis (ethylene) rhodium (I) complex to an ethyl-rhodium (III) ethylene complex. (b) The rate determining rearrangement of the ethyl-rhodium ( i l l ) ethylene complex to a butylrhodium ( i l l ) complex. (c) The collapse of the butylrhodium ( i l l ) complex to rhodium (I) and butene. (d) Coordination of this resulting rhodium (I) in step (c) with two moles of ethylene to provide the i n i t i a l bis (ethylene) rhodium (I) complex. Thus the mechanism can be formulated as: CH, Cl. C1L + 2CH2=CH2 CL CH, Rh 1 7 C H 2 H + + Cl" /Rh 1 1 1 C H2 L CH, - CH3CH2CH=CH2 - L CHCH2CH3 CH, Cl C H 2 CHCKgCH Rh 1 / C H2 ^-(H + + Cl~) /Rh III /CH 2; <_zk Cl, Rh III CH2CH3 c r Cl C H2 ,CH2CH2CH2CH3 III Cl Cl Cl | L Cl (L = solvent) The actual dimerization process occvrs in step (b) above where the co-ordinated ethylene in the Rh(lll) derivative inserts itself between the C^Hj. group and the rhodium atom giving v.he ion [c^H^Ri^^Cl^Lg] ; which subsequently loses HC1 giving (j^HgRh^Cl^i] where L may be water or ethanol. It is of interest to note that dimerization occurs only when halide ligands are present. Rhodium (I) square planar complexes have also been found efficient for the allene polymerization in polar media 1*8 Teyssie and coworkers have reported that RhCl^ 'SH^ O catalyzes the polymerization of phenylacetylene in benzene-ethanol solutions. The mechanism is thought not to be of the radical type but may be similar to that of the emulsion polymerization of butadiene which involves formation of a rhodium-diene complex. Dickson and Wilkinson^ have shown that hexafluoro-2-butyne 51 can be oligomerized by rhodium (i) complexes. Maitlis and McVey have studied the oligomerization of diphenylacetylene by dicarbonyl complexes such as n'-C^K^Rh(C0)2 and [ciRh(C0) 2]^. Recently, Collman and Kang^2 have prepared a series of mononuclear acetylene complexes which are possible intermediates in metal catalyzed cyclooligomerizations and polymerizations of acetylenes. James"'"'', in his review on rhodium complexes, pointed out a number of interesting points which are very important, in general, to reactions of coordinated olefins including hydrogenation, isomerization and polymerization. These main points are: (i) The reversible oxidation of Rh* by a proton acid to give a Rh*** hydride. (ii) The insertion of coordinated olefin between an alkyl group and the metal ion to which i t is attached. ( i i i ) The lability of hydrogen in rhodium (III) alkyl and olefin complexes. (iv) The importance cC appropriate auxiliary ligands to the catalytic effectiveness of rhodium.-Point ( i i ) i s an example of an addition reaction which involves the forma-tion of an intermediate tf-complex, subsequently undergoing rearrangement to add the elements of the original complex across the unsaturated bond of the olefin: * \ i. 9. L ^ c i / C — M — + C = C > - M - - M - M I ' N I c I c I c R R ' R' I R ( R = H, alkyl group, growing chain). This type of mechanism is an example of a wide class of "insertion"-type reactions and has been suggested for a number of catalytic reactions i n -cluding hydrogenation, hydration, hydroformylation, isomerization and poly-1-3 53 merization ' ' . Insertion reactions of metal complexes have been 55 discussed in detail by Heck and their importance in catalytic reactions 1-3 has been discussed by Halpern 1:10 Carbonyl Complexes of Rhodium Hieber and L a g a l l y ^ found that hydrated RhCl^ when heated in a stream of CO produced [Rh(C0)2Cl] ; the reaction was promoted by saturating the CO stream with methanol. When hydrated RhCl^ was heated with CO for 2h hours at 200 atmospheres and 200°, HRh(C0)|+ was obtained. A large number of both mono and dicarbonyl derivatives of rhodium (i) have been prepared from the (RhCl(C0) 2 J 2 dimer 1^. These include the series of complexes RhL2(C0)X, where L = A^P, A r ^ s , Ar^b, (PhO^P and X = CI, I, SCN ' \ the anions [Rh(C0) 2X 2]", (X = CI, Br, I) and [Rh^CO^xJ 2 , (X = Br, I) 5 T> 5 8; the compounds [Rh(C0) 2X] 2 where X i s carboxylate, nitrate, thiocyanate, sulfate or cyanide ' ; [Rh(COigPRg]n vhere R = C^K^ ; the amine complexes RhCl(amine,(CO)2 5 8 ; and the complexes (/9-ketone)Rh(CO)g 6 ° . ir-^H Rh(C0)'2 ^ 1 and i t s mono and disubstituted derivatives containing phosphines, phosphites and is o n i t r i l e s have been prepared^ 2. The complex RhCl(CO)(PPh 3) 2 has been synthesized by the decarbonyla-4-- — *i u i 2 6 , 6 3 . , 11,lU , . , 6 3 , 6 » + . . . . . 65 ' -, tion of alcohols , aldehydes , acylhalides , aroyl halides , and dimethylformamide, dioxan and certain ketones^ using RhCl^ in the presence of triphenylphosphine or by using RhCl^Ph^)^ i t s e l f . 67 Recently, Chatt and Shaw have reported the formation of rhodium carbonyl complexes by a prolonged treatment of rhodium chloride solutions with carbon monoxide in boiling ethanol followed by addition of the phosphine or arsine to provide complexes of type trans- RhCl(C0)L 2 . g Complexes such as RhCl(CO)(PPh^)2 of d configuration undergo oxi-dation-addition reactions by adding a wide variety of covalent molecules to form hexacocrdinated complexes of d^ c o n f i g u r a t i o n ^ 8 T h e rhodium com-plexes are considerably less reactive than their iridium analogs, but deriva-tives with S0 2 , HC1 , organic halides , halogens ' , tetracyano-7 3 5 2 ethylene and acetylenes , have been prepared. Equilibria such as: Rh ICl(CO)(PPh 3) 2 + H 2 - R h m c i ( C 0 ) ( P P h 3 ) 2 H 2 and the corresponding reactions with molecular 0 2 and CO probably do exist, but the equilibria l i e far to the l e f t . Although rhodium (I; carbonyls are quite stable thermodynamically they are highly l a b i l e , eg. the Cl and CO in RhCl(CO)(PPh 3) 2 undergo rapid e x c h a n g e ^ 5 S u b s t i t u t i o n of CO in rhodium carbonyl complexes has been studied by various workers and both associative 7 5 - 7 9 and disso iative mechanisms have been reported17 Of the rhodium carbonyl derivatives prepared only RhCl(CO)(PPh^^ 27 and RhH(CO)(PPh^)^ have been reported as active catalysts for homo-geneous hydrogenation, (section .1:07)• CHAPTEP II APPARATUS AND EXPERIMENTAL PROCEDURE 2:01 Apparatus for constant pressure gas-uptake or evolution measurements The experimental technique used for most of the kinetic studies involved the measurement of gas-uptake or evolution at constant pressure. The apparatus which was used for this purpose is shown diagramatically in Figure 1. The pyrex-glass reaction flask A, which for shaking purposes could be clipped to a metal piston rod and wheel driven by a Welch variable speed electric motor, was immersed in a thermostated bath B containing Dow Corning (550 Fluid) silicone o i l at the desired reaction temperature. The reaction flask was connected by a length of pyrex glass tubing containing two spring coil arrangements arid a tap C, to a capillary manometer D. The capillary manometer D contained butyl phthalate (a liquid of negligible vapor pressure at 25°) and was in turn connected to the gas burette E, containing mercury and a precision bore tubing N of known diameter. Both the capillary manometer and the gas burette were immersed in a thermostated water bath maintained at 25°C. The gas burette was connected, by means of an Edward's high vacuum needle valve M, to a simple gas-handling system con-structed of pyrex glass. This gas-handling system consisted of a mercury manometer F and connections leading to a Welch Duo Seal rotary vacuum pump G and the gas inlet Y. The thermostat units operated using a Jumo thermo regulator with a Merc to Merc relay control circuit. A perspex rectangular tank was used for Figure 1. Apparatus for constant pressure gas-uptake or evolut ion measurements. the water thermostat. A four li t r e pyrex beaker housed the silicone o i l thermostat unit; this was well lagged with asbestos medium fibre and syn-thetic sponge and enclosed by a 3A inch thick plywood box constructed with a circular hole suitable for viewing the reaction vessel. Sections of . polystyrene foam together with thin asbestos sheeting were used to f i t around the variable speed stirrer, thermometer, etc. and form an insulating cover c-i' the top of the o i l bath. Use of efficient stirring and 25 watt elongated light bulbs as heaters, enabled the thermostat temperatures to be controlled to within ± 0.05°. ' A Lab-chron lhOO timer was used for recording the reaction time during the kinetic experiments. 2:02 General procedure for a typical gas-uptake experiment The following describes the technique employed for a typical gas-uptake measurement experiment. Known amounts of the reactant solutions, (eg. rhodium complex and substrate, i f used), were pipetted into the reaction flask A which was sub-sequently fitted with the spiral tubing containing tap C, (Figure l ) . The solution was degassed by repeated cooling and warming under vacuum by attach-ing the flask and tubing at position 0 on the vacuum and gas handling side of the apparatus. Reactant gas, at a pressure somewhat greater than the vapor pressure of the solvent at the reaction temperature, was then admitted through tap Y to the reaction vessel and solution, and then tap C was closed. The flask and spiral tubing were then disconnected from 0 and attached to the capillary manometer at H; the flask being placed in the thermostated bath B. The reaction flask was also attached to the motor driven shaker I. The remainder of the system beyond tap C was then evacuated and reactant gas was • • 21 introduced through tap Y to a pressure slightly less than the desired pressure for reaction. Tap C was then opened and the pressure adjusted to the desired reaction pressure by introduction of gas through tap Y. A run was started by closing taps K and L and simultaneously start-ing the shaker and timer. As the reaction proceeded and gas was consumed, the o i l level in the l e f t limb of the capillary manometer D began to rise.. ; The manometer D was balanced (to the i n i t i a l reaction pressure) by admitting gas into the buret through the needle valve M. The mercury level in the tube N was thus raised and the level was measured at suitable intervals by means of a vertically mounted Pye cathetometer. Since the diameter of the tube N was known, a direct measure of the volume of gas taken up was obtained and a plot of gas uptake, as moles per l i t r e of solution at S.T.P., against time could be drawn. For gas evolution experiments tap L was closed prior to the f i n a l pressure adjustment which resulted in a rise in the mercury level in N. Gas evolution was followed by withdrawing gas through the needle valve M and following the f a l l in the mercury level. A rapid shaking rate combined with the use of small volumes of solution (3-6 ml), in a relatively large indented reaction vessel (25 ml), ensured absence of physical control (i.e. due to slow dissolution of the reactant gas). 2:03 Gas solubility measurements Some experiments were carried out to determine approximate gas solu b i l i t i e s in dimethylacetamide at the conditions of temperature and pressure used in the kinetic experiments. These so l u b i l i t i e s were determined by f i t t i n g the previously described gas-uptake apparatus (Figure l) with a reaction flask containing a stop cock in i t s neck. The entire system i n -cluding the reaction flask with a measured amount of dimethylacetamide was pumped dowr. at room temperature. The tap on the flask was now closed and the flask pltced in the o i l bath B at the desired temperature. The system was now evacuated to the flask tap and then f i l l e d with the gas to the approximate pres.-ure desired. The flask tap was now opened and the pressure ' adjusted immediately to that required. Taps K and L were closed and the shaker started. The uptake was measured as described in section 2:02 and the solubility of the gas in dimethylacetamide at the particular temperature and pressure could be evaluated. The determined solubility data are recorded in Appendix II. 2:0h Spectrophotometric kinetic measurements The kinetics of some reactions were studied using spectrophotometric techniques in the visible range. A kinetic experiment, involving gas uptake, could be carried out .directly in a 1 cm optical s i l i c a c e l l f i t t e d with a tight teflon cover. The c e l l cover was f i t t e d with capillary tubes for the introduction and exhaust of a gaseous stream through the solution. The rapid gas bubbling rate through the solution ensured the absence of physical control. The reaction was followed by the decrease in optical density of the absorption maxima of the reacting species as a function of time, in a spectrophotometer'fitted with a thermostated c e l l compartment. The gaseous stream was b r i e f l y stopped during optical density measurements. An alternative sampling method of studying the kinetics of this type of reaction was also used in some cases. A reaction was carried out with the constant pressure gas-uptake apparatus but using a short double necked reaction flask, one neck f i t t e d with a serum cap. Small samples of solution ^ould be removed at suitable intervals by means of a micro-syringe injected through the serum cap. The samples were then immediately trans-ferred to c e l l s , i f necessary in the absence of a i r , for subsequent optical density measurements. 2:05 Reaction product analysis Gaseous products Where gaseous products were indicated or questionable, in a given reaction, a gas sample was collected at the end of the reaction and was analyzed. The technique involved use of a double-necked reaction flask, one neck being connected to an evacuated sample bulb with a stop cock. At the completion of the reaction the stop cock on the bulb was opened and a gas • sample collected. The sample was then analyzed by either gas-chromatography or mass spectrometry. Carbon dioxide was also identified by absorbtion on soda lime, which could be loosely packed between glass-wool in one neck of a double necked reaction flask. Liquid products Liquid organic products were separated from the inorganic components of the f i n a l reaction mixture by d i s t i l l a t i o n . The d i s t i l l a t e was analyzed by use of a vapor phase chromatograph with a suitable column, the retention time and peak area of each product being compared with standard solutions. Solid organic products For isolation of some hydrogenated products present in dimethyl-acetamic'e solution, a f a i r l y simple procedure, involving evaporation and sublimation was used. The solvent was removed by pumping through a liquid nitrogen co.Vd trap. The sticky residue which resulted was gently heated, while pumping was maintained, and the hydrogenated product sublimed onto, the clean glass surface of the flask neck. The sublimate was collected, recrystallized fr~>m d i s t i l l e d water, and dried in a vacuum desiccator. The 1 product was identified from i t s melting point and infra-red spectrum (KBr disc method). A l l other organic products, which were isolated by means of solvent extraction, were characterized by their melting point and infra-red spectrum. Solid inorganic products Inorganic products were isolated by reducing the f i n a l reaction solutions to low volumes by pumping through a li q u i d nitrogen cold trap followed by the addition of a complexing ligand in a small volume of an appropriate solvent. The solids, which formed immediately or on slow evap-oration, were collected and recrystallized from an appropriate solvent or solvent mixture. The solids were characterized by means of some or a l l of the following: melting point determination, infra-red spectrum (KBr pellet method), mass spectrum, micro-analysis. Micro-analyses were performed by Schwarzkopf Micro-analytical Laboratory, Woodside, N.Y., and Mr. Borda of this department. 2:06 Instrumentation . Visible and ultraviolet absorption spectra were recorded using a Cary lit recording spectrophotometer or a Perkin Elmer 202 spectrophotometer; these could be f i t t e d when necessary with thermostated c e l l compartments. Matched s i l i c a cells of 1 mm or 1 cm path length were used. Infra-red spectra were recorded on a Perkin Elmer Model 21 using KBr discs whenever possible. Proton nmr spectra were obtained on a Varian HR-100 spectrometer. Mat's spectra were recorded on an Associated E l e c t r i c a l Industries MS 9 mass spectrometer. An Aerograph Model ,A'«90*P gas chromatograph with a molecular sieve column was used for analysis of gas samples ; liq u i d samples were analyzed by use of a Beckman GC-2A chromatography unit with a dinonyl phthalate column. Conductivity measurements were carried out using a Thomas Serfass conductance bridge and dip type conductivity cells with c e l l constants of about 0.1 or 1.0 cm *. pH measurements were obtained on a Corning Model 12 pH meter using glass and calomel electrodes calibrated in standard buffer solutions. Melting points were measured on a Superior Electric melting point apparatus. CHAPTER III THE RHODIUM CHLORIDE CATALYZED HYDROGENATION OF OLEFINIC COMPOUNDS 3 : 0 1 Aqueous systems Since chloro ruthenate complexes, in aqueous solution, were known g to be active for the hydrogenation of olefins , the u t i l i t y of rhodium chloride complexes in aqueous acid solution for homogeneous hydrogenation was investigated. Excess maleic acid in 3 M HC1 was added to a solution of RhCl^'SH^O in 3 M HC1, which contained principally the 5 : 1 chloro species^, and hydro-genation was attempted at 8 0 ° and a total pressure of one atmosphere; H 2 was absorbed as shown in Figure 2. The rate of uptake f e l l off with time and became very slow after about 1 6 , 0 0 0 sec. At this stage the uptake corres-ponded to reduction of Rh*"'"* to Rh*. Rh*** + H — i W Rh* + 2H+ ( 3 . 1 ) No hydrogenation of maleic acid was observed. In the absence of maleic acid, Rh*** is rapidly reduced by H^ to the metal''. Spectrophotometric measure-ments showed that the Rh*** chloro complexes did not complex with maleic acid. The Rh* state must be stabilized by complexing with the maleic acid in a rapid step which prevents further reduction or disproportionation to the metal. Rh + Maleic acid > Complex (3.2) A rate law of the form -d[H2]/dt = -d[Rh m]/dt = k[Rh m][H 2] (3.3) indicates that at constant [H2] the reaction becomes one of pseudo first. order. A good linear plot of log vs. time was obtained from the rate data, assuming the decrease in is proportional to the H2 uptake, (Figure 2 ) . From the slope of the rate plot the.pseudo first order rate _ L constant can be evaluated and, taking the solubility of H2 as 6 . 1 X 10 —1 80 8 l mole atm. in this medium * , the bimolecular rate constant k was cal-culated. The values, obtained for two different i n i t i a l [Rh 1 1 1], 0.51 and 0 . 5 9 M * sec. *, confirm that the reaction is first order in rhodium and agree with the values obtained for the catalytic reduction of Fe*** under similar k 5 conditions ' . This indicated that the rate determining step is the same for both systems. The original red Rh*** solution, which showed absorption maxima at kOk m/i, £•= 67 and 511 m/t, £ = 7 1 + , gradually became light yellow during the course of the reaction; the spectrum of the final solution exhibited absorp-tion maxima at 3 5 3 m/t, £= 1*00 and 1*25 n^ ., 6 = 1 8 0 , (Figure 3 ) . The same yellow species in solution was obtained by reducing the Rh***/maleic acid/ 3 M HC1 solution with Ti***, (in the absence of maleic acid, Ti*** rapidly reduces Rh*** to the metal). The solution certainly contained a complex of maleic acid with rhodium in a reduced valency state and the stoichiometry of the reaction with H2 indicated a Rh* complex. A number of experiments were carried out using various chloro 82 rhodium complexes, prepared according to Wolsey and coworkers , in aqueous acid solutions. With the cationic species JRh(H20)g]3+ and [Rh(H 0)^01 ] + in solution a rapid uptake of H2 was observed and reduction to Rh° occurred with a resulting heterogeneous catalytic reduction of maleic acid. With the solution containing predominantly the 5:1 or 6:1 chloro species reduction to the Rh* - maleic acid complex again occurred, but there was also slight reduc-tion to the metal; the resulting heterogeneous hydrogenation complicated the uptake plots. Because of the irreproducibility of the aqueous Rh***/Hg/ maleic acid systems, which probably resulted from slight reduction of Rh*** to the metal, further kinetic study was not undertaken. Chloro rhodium complexes in acid aqueous solution also could not be shown to catalyze homogeneously the hydrogenation of other olefinic substances such as fumaric acid, acrylic acid, methacrylic acid and trans-cinnamic acid. In a l l of these systems metallic rhodium was produced and this resulted in heterogeneous hydrogenation of the substrate. 3:02 Non-aqueous systems In attempts to prevent the slight production of metallic rhodium, similar experiments in some non-aqueous solvents were carried out. In both tetrahydrofuran and dioxan solution, metal production resulted in heterogene-ous hydrogenation. The polar solvent N,N-dimethylacetamide (DMA) , however, was found to be a suitable medium for study of the RhCl^'SH^O catalyzed homo-geneous hydrogenation of maleic acid, other substituted ethylenes and ethylene i t s e l f . 3:03 The rhodium chloride - maleic acid system in dimethylacetamide The experiments consisted of adding excess maleic acid to a neutral solution of RhCl^'SHgO in DMA, followed by hydrogenation. In contrast to the aqueous system complete homogeneous hydrogenation of maleic acid occurred. Figures h and 5 show typical gas uptake plots at 80° for a variety of rhodium concentrations and H pressures; in the absence of RhCl *3H 0 there was no 31 Time, sec Figure h. Rate plots for the rhodium chloride catalyzed hydrogena-tion of maleic acid in DMA at 8 0 ° , ( 7 2 5 mm H , 0 . 0 5 M maleic acid). [Rh] : () 0 . 0 0 2 5 M, (O) 0 . 0 0 5 0 M, ( • ) 0 . 0 0 7 5 M. 32 0 2 0 0 0 1*000 6 0 0 0 Time, sec Figure 5 . Rate plots for the rhodium chloride catalyzed hydrogena-tion of maleic acid in D M A at 8 0 ° , ( 0 . 0 0 5 M RhCl , 0 . 0 5 M maleic acid), [ H J : ( V ) 0 . 5 1 X 1 0 " 3 M , ( Q ) I - 1 * ? X 1 0 ~ 3 M , ( A ) 1 - 9 3 X 1 0 ~ 3 M , ( { • > 2 . 5 3 X 10 J M . measureable uptake over 1 0 , 0 0 0 sec. The i n i t i a l rate falls off somewhat for about the first 1 5 0 0 sec and then becomes essentially linear almost up to the end-point wl.en rhodium metal deposited. The total uptake agreed well with the stoichiometry,for complete reduction of the maleic acid present and reduction of Rh*** to Rh°. The succinic acid was readily isolated and iden-tified. The visible absorption spectrum of a freshly prepared solution of RhCl^ 'SH^ O in DMA changed slowly with time, presumably due to equilibration between species containing coordinated chloride and DMA. This change could be accomplished rapidly by heating to 8 0 ° and the steady spectrum of the red solution showed absorption maxima at kh2 mju, £= 335 and a shoulder at 530 mfi, £'-•150, (Figure 6). No reactions such as carbonyl abstraction^ were observed between RhCl^'SH^O and DMA under the conditions cf the kinetic experiments. There was no evidence for formation of a Rh*** - maleic acid complex. During the catalyzed hydrogenation reactions, the red Rh*** solu-I 8h tions soon became yellow indicating possible reduction to the Rh state A spectrum of the yellow solution towards the end of the reaction showed absorption maxima at h2k mju., £= 2kOt (Figure 7 ) . On standing in air the yellow solution reverted slowly back to the original red solution. This re-verse reaction could be accomplished quite quickly using oxygen at higher temperatures and the stoichiometry of the oxygen uptake corresponded closely to oxidation, via a two equivalent process, of Rh* to Rh*** as seen from Figure 8 , (this incidentally analyzes for a good first order dependence on Rh* with a pseudo first order rate constant = 7 . 8 2 X 10 ^sec. * ) . Evidence for the Rh* state also is obtained from the fact that a n.m.r. proton spectrum of a sufficiently concentrated solution gave no metal-hydrogen bond signal in the range of 1 2 - U 0 f , indicating the absence of any Rh***H species, (e.g. Rh***H , XXX XXX Rh H(maleic acid) or Rh (maleic acid)H 2 species). This observation does 3h 1.2 Time, sec Figure 8. Rate p lot fo r oxidat ion of the Rh - maleic ac id complex i n DMA at 63°, (0.005 M Rh 1 - maleic ac id , 7>5 mm 0 ). Inset: p lo t of log [Rh 1 - maleic acid] agaiust t ime. ON not however rule out small concentrations of a Rh***H species since a signal for metal-H bonds is only observed i f the solution is sufficiently concen-trated8''. As in the aqueous systems, in the absence of maleic acid Rh*** „'s rapidly reduced by H2 to the metal. No traces of metallic rhodium (a powerful heterogeneous catalyst) became visible t i l l near the end point of the hydro-genation reactions, and light scattering experiments using a spectrofluori-meter indicated the absence of any colloidal matter. These points, together with the general reproducibility of the experiments and the reactions of the yellow solutions, indicated that a purely homogeneous system involving Rh* was being studied. 'When an experiment was carried out using a slightly greater than 1:1 ratio of maleic acid to Rh***, no rhodium metal deposited until the uptake was equal to that required for reduction of Rh*** to Rh*; this indicates that the olefin complex is a 1:1 species. 3:0k Kinetics of the RhCl3'3H20/maleic acid/Hg system in DMA The H2 uptake plots did not analyze for simple first or second order reactions; the i n i t i a l slope and the slope of the final linear region were thus measured. The rates of reaction derived from these measured slopes are summarized in Tables I - IX for experiments in which the reaction tempera-ture and concentrations of Rh***, H 2 > maleic acid, chloride, perchlorate and acid were varied. 3:05 The in i t i a l region The i n i t i a l rate is seen to be first order in rhodium and hydrogen (Table I, Figures 9 & 10) and independent of maleic acid concentration while Table I Rhodium trichloride catalyzed hydrogenation of maleic acid Kinetic data at 8 0 ° in DMA The effect o° Rh* 1 1], |~H j and ["Maleic acid] on the i n i t i a l reaction rate. (Rh I i r)x H 2 (HJx [Maleic acid] x Initial rate k l 1 0 2 , M mm 3 * 10 , M 1 0 2 , M of H2 uptake x 1 0 5 , M sec - 1 M sec 0.25 725 2 . 3 2 5 . 0 1 . 1 8 2.03 0 . 5 0 7 2 5 2.32 5 . 0 1 . 8 0 1 . 5 5 0 . 5 0 7 2 5 2 . 3 2 5.0 2 . 0 8 1 . 7 9 0 . T 5 7 2 5 2 . 3 2 5 . 0 2 . 6 9 1 . 5 5 1 . 0 0 7 2 5 2 . 3 2 5 . 0 3.lh 1 . 5 9 0 . 5 0 6 3 0 . 2 0 5 . 0 0 . 2 1 2 . 1 0 0 . 5 0 160 0 . 5 1 5 . 0 O.kl 1 . 6 2 0 . 5 0 275 0 . 8 7 5 . 0 0 . 7 7 1 . 7 8 • 0 . 5 0 k6k 1 . H 7 5 . 0 1 . 1 2 1 . 5 2 0.50 6 l i 1 . 9 3 5 . 0 1 . 5 9 1 . 6 5 0 . 5 0 797 2 . 5 3 5 . 0 2 . 0 3 1 . 6 0 0 . 2 5 725 2.32 1 5 . 0 1 . 2 0 2 . 0 7 0 . 5 0 7 2 5 2.32 1 0 . 0 2 . 0 5 1 . 7 7 0 . 5 0 7 2 5 2 . 3 2 1 5 . 0 2 . 0 0 1 . 7 2 0 . 5 0 7 2 5 2 . 3 2 1 5 . 0 2 . 1 0 1 . 8 1 0 . 5 0 7 2 5 2 . 3 2 5 . 0 2.0k . 1 . 7 6 a 0 . 5 0 7 2 5 2 . 3 2 1 0 . 0 2.03 i . 7 5 a See Appendix II DQ in place of H, 39 1+0 ( H J X 1 0 3 , M Figure 1 0 . Dependence of the rate of Rh* - maleic acid complex formation on [HJ , (DMA, 8 0 ° , 0 . 0 0 5 M RhCl 3, 0 . 0 5 M maleic acid). the l a t t e r was v a r i e d from 0 . 0 5 to C . 1 5 M (Table I ) . Thus the rate law i s i n i t i a l l y -d[H 2 ]/dt = . k 1 [ i ' h I I I ] [ H 2V (3.U) Values of k^ at 80° are given i n Table I; the mean value i s 1 .71 M * sec. *; k^ was e s s e n t i a l l y independent of addition of up to 0 .05 M p-toluene s u l f o n i c a c i d (a strong acid) and up to 0 . 1 0 M s u c c i n i c a c i d (Table I I ) . The values of —1 —1 k^ c a l c u l a t e d from the slopes of Figures 9 & 10 were 1 . 6 2 M sec. and 1 . 6 o M -* sec. * r e s p e c t i v e l y . There was no measureable k i n e t i c isotope e f f e c t within experimental e r r o r (k^ /k^ — 1 . 0 ) . The k i n e t i c evidence and the evidence i n section (3:03) suggest the i n i t i a l rate involves again production of a Rh* - maleic a c i d complex as i n the aqueous system, (section 3:02). k R h 1 1 1 + H 2 —-*-> Rh 1 + 2H+ (3 . 1 ) Rh 1 + Maleic a c i d f a S t > Complex ( 3 . 2 ) K i n e t i c measurements over the temperature range of 50 - 8 5 ° y i e l d e d a good Arrhenius p l o t (Table I I I , Figure 1 1) and the a c t i v a t i o n parameters AH1'**= 17.3 ± 1 .5 k c a l and A S = - 9 . 2 ± U.l e.u. The rate of disappearance of the Rh*** red c o l o r , on formation of the Rh* - maleic a c i d complex at 80° was followed continuously (section 2:0k) by the decrease i n o p t i c a l density at a 530 m/o absorption shoulder, (Figure 6 ) . Allowance f o r absorption due to the Rh* - maleic a c i d complex at 530 ra.fx was made when analyzing the r e s u l t s . A p l o t of ( - L o s[ Rh*** I.^ AJ - l°§l^ htime t ^ vs. t (Figure 12) gave a good s t r a i g h t l i n e confirming a f i r s t order dependence i n Rh***. I f a f i r s t order dependence i n H 2 was assumed, a k^, value of 1.2U M~* s e c . - * was estimated, which i s considered to' be i n f a i r agreement with the Table II Rhodium trichloride catalyzed hydrogenation of maleic acid Kinetic data at 80° in DMA The effect of additives on the i n i t i a l reaction rate. [ R h m ] x (H21X [Maleic acid]x I n i t i a l rate k l 102, M mm 3 * 10 , M 102, M of Hg uptake x 10^ , M sec * M. sec 0.50 725 2.32 10.0 2.12 1.83a 0.50 725 2.32 10.0 1.35 l . l 6 b 0.50 725 2.32 10.0 0.91 0.78c 0.50 725 2.32 10.0 0.82 0.71d 0.50 725 2.32 10.0 0.75 0.65e 0.50 725 2.32 5.0 2.00 f 1.72 0.50 725 2.32 5-0 2.50 2.15s 0.50 725 ,2.32 10.0 2.k6 2.12h 0.50 725 2.32 10.0 2.50 2.151 * See Appendix II a 0.05 M LiClO^. b 0.0k M LiClO^, 0.01 M L i C l . C 0.02 M LiClO^, 0.03 M L i C l . d 0.05 M L i C l . 6 1.20 M L i C l . f 0.05 M Succinic acid. 0.10 M Succinic acid. 0.05 M p-toluene sulfonic acid. 1 0.03 M p-toluene sulfonic acid, 0.02 M LiC10.. Table III Rhodium trichloride catalyzed hydrogenation of maleic acid Temperature dependence of k^ in DMA [Rh 1 T I] = 0.50 X 10~ 2 M; [Maleic acid] = 5.0 X I O - 2 M Temp. [Hjx I n i t i a l rate k, °C 1 0 3, M- * of H 2 uptake M - 1 s e c - 1 x 1 0 ^ , M sec * 5 0 . 0 7 0 . 0 7 5 . 0 8 0 . 0 8 5 . 0 2.kl 2.36 2.3k 2.32 2.30 0 . 2 2 0 . 9 1 1 . 5 7 1.9k 2 . 8 1 0 . 1 8 0 . 7 7 1 . 3 U 1 . 6 7 ' 2.kk * See Appendix II kk Figure 11. Arrhenius plot for the Rh - maleic acid complex forma-tion (DM, 0 . 0 0 5 M RhCl , 0 . 0 5 M maleic acid). ^5 k^ . values l i s t e d in Table I considering the quite different techniques used. 3 : 0 6 Effect of chloride on the i n i t i a l region An inverse dependence on added' chloride concentration was noted over the range 0 - 0 . 0 5 M when the total salt strength was kept constant by addition of perchlorate; further increase in (pi ] had apparently l i t t l e effect, (Table II).. A number of experiments were carried out at 1 . 2 M chloride, where the lower limiting i n i t i a l rate had been attained. The spectrum of the i n -i t i a l Rh*** solution exhibited absorption maxima at kkk mil, £ = 3 ^ 3 and 5 ^ 8 mfi, £ = 1 1 5 , (Figure 6 ) . On reaction of this solution with hydrogen in the presence of maleic acid, a yellow solution (X 1+35 m/x., £= 3^+2) again IT19.X resulted, (Figure 7 ) . The gas uptake plot now consisted of a short i n i t i a l linear region which tended quite sharply to a second linear region of much greater slope; this decreased quite sharply at the completion of the reaction, (Figure 13). The total hydrogen uptake corresponded closely to that required for reduction of maleic acid to succinic acid and reduction of Rh*** to Rh°. The uptake plots were analyzed in the same manner as described for experiments with no added chloride. The i n i t i a l rate is again f i r s t order in III Rh and Hg, (Table IV, Figures lh & 1 5 ) , and independent of maleic acid, (Table IV'). Values of k^ at 8 0 ° (Table IV), calculated according to equation (3.U), are about half the values obtained at similar conditions with no added chloride. The values of k^ calculated from the slopes of Figures 1** & 1 5 were O.7I+ M * sec. * and O . 7 6 M * sec. * respectively. A very small kinetic iso-H 2 D 2 tope effect (k^ /k^ &1.06) was observed. Kinetic measurements over the temperature range of 70 - 8 5 ° yielded an Arrhenius plot, (Table V, Figure l 6 ) , and the activation parameters &H + = 1 7 . 3 ± 1 . 0 kcal andAS.+ = - 1 0 . 8 ± 2 . 8 e.u. 6.01 — - 1 Rh :( A ) 0.0025 M, (Q) 0.0050 M, ( • ) 0.00T5 M. Table IV Rhodium trichloride catalyzed hydrogenation of maleic acid Kinetic data at 80° in 1.2 H LiCl/DMA The effect of [Rh***], [H ], and [Maleic acid] on the i n i t i a l reaction rate. [ R h m ] x H2 [Hjx [Maleic acidjx Initial rate k l 102, M mm 3 * 10 , M 102, M of Hg uptake x 10^, M sec * M sec 0.25 725 2.32 5.0 '.. 0.U2 0.73 0.50 725 2.32 5.0 0.85 0.1k 0.50 725 2.32 5.0 0.85 0.1k 0.T5 725 2.32 5.0 1.25 0.72 1.00 . 725 2.32 5.0 1.72 0.1k 0.50 182 0.58 5.0 0.21 O.Ik 0.50 365 1.16 5.0 0.55 0.9k 0.50 5Vr 1.73 5.0 0.70 0.81 0.50 725 2.32 10.0 0.75 O.65 0.50 725 2.32 5.0 0.81 0.70a * See Appendix II 9. Dc in place of H, 50 Table V Rhcrlium trichloride catalyzed hydrogenation of maleic acid Temperature dependence of in 1.2 M LiCl/DMA [Rh 1 1 1] = 0.50 X IO - 2 M; [Maleic acid] = 5.0 X 10~ 2 M Temp. i H2J X I n i t i a l rate k 1 °C 10 3, M of H 2 uptake M - 1 sec x 10 5, M s e c - 1 70.0 2.36 0.U2 0.36 75.2 2.3U v 0.59 ' 0.51 80.0 2.32 0.85 0.71* 86.3 2.29 1.1+8 1.31 # See Appendix II 52 bo o h3 -0.5 2.90 2.86 (1/T) X 103, K - 1 Figure 16. Arrhenius plot for the Rh 1 - maleic acid complex forma-tion (1.2 M LiCl/DMA, 0.005 M RhCl^ 0.05 M maleic acid). 3:07 The hydrogenation of maleic acid The slope of the extended linear region of the uptake graphs, (Figures k & 5 ) , was presumably associated with the hydrogenation of the maleic acid; these rates are given in Table VI. There seems to be a good second order dependence on rhodium, (Figure 17). The dependence on Hg also appears to be second order up to con--3 centrations of 2 X 10 M, but at higher concentrations i t falls to more nearly first order, (Figure 1 8 ) . Varying the maleic acid concentration resulted in a complex inverse rate dependence (Table VI). These hydrogenation experiments were repeated at a later date and the same general dependences seemed to result; however, the second linear rates were much greater (Table VII), although the i n i t i a l rates were essen-t i a l l y the same as in the earlier work. The only difference in the two sets of experiments was in the use of different batches of RhCl^*3Hg0. Addition of chloride over the range 0 - 0 . 0 5 M when the total salt strength was constant had l i t t l e effect on the second rate, (Table VII). At 1.2 M chloride, the kinetics of the extended linear region, (Figure 13), were much simpler and the reactions were always reproducible, (Table VIII). There was a reasonable first order dependence on rhodium, (Table VIII, Figure 19) and a good first order dependence on Hg upto 2.32 X -3 10 M, (Table VIII, Figure 20); the rate was also now independent of the maleic acid concentration, (Table VIII). When deuterium was used in place of hydrogen only a small decrease in the rate of hydrogenation was observed, H2 °2 (k ? /kg = 1 . 0 8 ) . Thus, this experimental data (Table VIII) for the second reaction f i t a rate law of the form: -d[Hg]/dt = kg[Rh] [H2] (3.5) Table VI Rhodium trichloride catalyzed hydrogenation of maleic acid Kinetic data at -30° in DMA The effect of [Rh], [Kg] and [Maleic acio) on the second (linear) reaction rate. [Rh]x [Maleic acid] x Second rate 1 0 2 , M mm 3 * 10 , M 1 0 2 , M of Hg uptake x 1 0 5 , M s e c - 1 0 .25 7 2 5 2 .32 5 . 0 . 0 . 1 7 0.50 7 2 5 2 . 3 2 5 . 0 0 . 6 9 0 . 7 5 7 2 5 2 . 3 2 5 . 0 1 . 5 3 1 . 0 0 7 2 5 2 . 3 2 • 5 . 0 2 . 5 3 0 . 5 0 107 0.31* 5 . 0 o.oi* 0 . 5 0 1 6 0 0 . 5 1 5 . 0 0 . 0 9 0 . 5 0 . 275 0 . 8 7 5 . 0 0 . 1 U 0 . 5 0 3 5 5 1 . 1 3 5 . 0 0 . 2 2 0 . 5 0 l*6U 1.1*7 5 . 0 0 . 3 5 0 . 5 0 6 l l 1 . 9 3 5 . 0 0 . 5 9 0 . 5 0 7 9 7 2 . 5 3 5 . 0 0 . 8 2 0 . 5 0 7 2 5 2.32 1 0 . 0 0 . 2 6 0 . 5 0 7 2 5 2 . 3 2 1 5 . 0 o.oi* See Appendix II 5 5 [Rh] 2 X 1 0 6 , M2 Figure IT. Dependence of the rate of hydrogenation of maleic acid on (Rh] , (DMA, 8 0 ° , 7 2 5 mm H g , 0 . 0 5 M maleic acid). 56 Table VII Rhodium trichloride catalyzed hydrogenation of maleic acid Kinetic data at 8 0 ° in DMA The effect of [Rh] , [Maleic acidj and additives on the second (linear) , reaction rate.T [Rh] x H 2 [H2]x [Maleic acid]x Second Rate 1 0 2 , M mm 3 * 1 0 ° , M 1 0 2 , M of ^ uptake x 1 0 ^ , M sec * 0 . 2 5 7 2 5 2 . 3 2 1 5 . 0 0 . U 0 0 . 5 0 7 2 5 2 . 3 2 5 . 0 1 . 7 0 0 . 5 0 7 2 5 2 . 3 2 1 5 - 0 1 . 3 2 0 . 5 0 7 2 5 2.32 1 5 . 0 1 . 2 7 0 . 5 0 725 2.32 2 5 . 0 1 . 1 5 0 . 5 0 7 2 5 2.32 1 0 . 0 l.U8& . 0 . 5 0 7 2 5 2.32 1 0 . 0 1 . 3 5 b 0 . 5 0 7 2 5 2 . 3 2 1 0 . 0 1 . 3 7 ° 0 . 5 0 7 2 5 2 . 3 2 1 0 . 0 1 . 2 9 d 0 . 5 0 7 2 5 2 . 3 2 1 0 . 0 1 . 9 7 6 0 . 5 0 7 2 5 2 . 3 2 5 . 0 l . U 3 f 0 . 5 0 7 2 5 2 . 3 2 . 5 . 0 1 . 6 7 g 0 . 5 0 7 2 5 2 . 3 2 1 0 . 0 ' 1 . 7 2 h 0 . 5 0 7 2 5 2 . 3 2 1 0 . 0 1.6k1 +Batch of RhCl 3-3H 2 0 different from that used for experiments in Table VI. See Appendix II a 0 . 0 5 M LiClO^. b 0 . 0 l * M LiClO^, 0 . 0 1 M L i C l . ° 0 . 0 2 M LiClO^, 0 . 0 3 M L i C l . d 0 . 0 5 M L i C l . 6 1 . 2 0 M L i C l . f 0 . 0 5 M Succinic acid 2 h 0 . 1 0 M Succinic acid. 0 . 0 5 M p-toluene sulfonic acid. 1 0 . 0 3 M p-toluene sulfonic acid. Table VIII Rhodium trichloride catalyzed hydrogenation of maleic acid Kinetic data at 8 0 ° i r 1 . 2 M LiCl/DMA The effect of [Rh] , ( H2] , and (Maleic acid] on the second (linear) reaction rate. K [Hg]x (Maleic acid] x Second rate k 2 1 0 2 , M mm 1 0 3 , M * 2 10 , M of Hg uptake x 1 0 ^ , M sec * M sec 0.25 7 2 5 2 . 3 2 5 . 0 1 . 0 6 1 . 8 3 0 . 5 0 7 2 5 2 . 3 2 5 . 0 1 . 8 6 1 . 6 0 0 . 5 0 7 2 5 2 . 3 2 5 . 0 1 . 8 1 1 . 5 6 0 . 7 5 7 2 5 2 . 3 2 5 . 0 2 . 5 7 1.1+8 1 . 0 0 7 2 5 2.32 5 . 0 3 . 0 2 1 . 3 0 0 . 5 0 1 8 2 0 . 5 8 5 . 0 0.1+8 I . 6 5 0 . 5 0 365 1 . 1 6 5 . 0 0 . 9 8 1 . 6 8 0 . 5 0 5 V f 1 . 7 3 5 . 0 1 . 3 9 1 . 6 1 0 . 5 0 7 2 5 2 . 3 2 1 0 . 0 1 . 9 7 1 . 7 0 0 . 5 0 7 2 5 2.32 5 . 0 1 . 6 0 l.l+ 6 a * See Appendix II Dg in place of Hg. 0 5 . 0 1 0 . 0 [Rh] X 1 0 3 , M Figure 1 9 . Dependence of the rate of hydrogenation of maleic acid on [Rh] , ( 1 . 2 M LiCl/DMA, 8 0 ° , 7 2 5 mm H , 0 . 0 5 M maleic acid). [H2] X 1 0 3 , M Figure 2 0 . Dependence of the rate of hydrogenation of maleic acid on [H2] , ( 1 . . 2 M LiCl/DMA, 8 0 ° , 0 . 0 0 5 M Rh, 0 . 0 5 M maleic acid). At constant [II J this rate law gives.-, the required pseudo zero order kinetics i f the [Rh) terra also remains constant.;. this is thought to represent a constant concentration of a Rh* - maleic acid conplex (see section 3". 1 6 ) . Values for the rate constant k^ are summarized in Table VIII. The values of k^ calculated from the slopes of Figures 19 and 20 were 1.1+7 M-* sec. -* and 1 . 6 l M-* sec. -* respectively. A good Arrhenius plot was obtained from kinetic measurements over the temperature range of 70 - 8 5 ° , (Table IX, Figure 2 1), and yielded the activa-tion parameters AH 2+ = 18.U ± 1 . 0 kcal and ASg* = - 6 . 1 ± 2 . 8 e.u. for the maleic acid reduction in 1 . 2 M chloride media. 3 : 0 8 Hydrogenation of other substituted ethylenes in dimethylacetamide solution A number of reactions using the substituted ethylenes: fumaric acid, maleic anhydride, diethyl maleate, maleic acid monoamide, tetracyanoethylene and hexene-1 as substrates were also investigated. It was hoped to learn more about the factors governing the st a b i l i t y of the Rh* - olefin complex formed and also i t s l a b i l i t y for subsequent hydrogenation. Though detailed kinetic studies were not carried out, representative reactions for each system were studied. The uptake plots were similar to those described for the maleic acid systems and were analyzed in the same way. The kinetic data obtained are shown in Table X. ' Increasing the concentration of maleic anhydride and diethyl maleate caused a marked decrease in the rate of their hydrogenation as observed for the maleic acid system for similar conditions (Table VI & VII). The data for the maleic anhydride reactions indicate that both the complex formation with Rh* and the subsequent hydrogenation reaction are probably f i r s t order in rhodium. Table IX Rhodium trichloride catalyzed hydrogenation of maleic acid Temperature dependence of kg in 1 . 2 M LiCl/DMA [Rh] = 0 . 5 0 X 1 0 ~ 2 M; [Maleic acid] = 5 . 0 X 1 0 ~ 2 M Temp. Second rate °C 3 * 10 \ M of Hg uptake M sec x 1 0 5 , M s e c - 1 7 0 . 0 2 . 3 6 0 . 8 1 0 . 7 0 7 5 . 2 2 . 3 H 1 . 3 1 1.13 8 0 . 0 2.32 1 . 8 U 1 . 5 8 8 6 . 3 2 . 2 9 2 . 7 1 2.1+0 See Appendix II 6 1 - 0 . 21 I 1 L_ : I I ! _ 2 . 7 8 2 . 8 2 2 . 8 6 2 . 9 0 (1/T) X 10 , K Figure 21. Arrhenius plot for the rhodium chloride catalyzed hydrogenation of maleic acid (1.2 M LiCl/DMA, 0.005 M Rh, 0.05 M maleic acid). 6 2 Table X Summary of kinetic data at 80 for various substituted ethylenes in DMA [H ] = 2 . 3 2 X 1 0 ~ 3 M [Rh]x Organic substrate [substrate]x I n i t i a l rate Second rate 1 0 2 , M 1 0 2 , M . of Hg uptake of Hg uptake x 1 0 ^ M sec * x 10^ M sec * 0.50 Maleic acid 0 . 5 0 Maleic acid. 0 . 5 0 Maleic acid 0 . 5 0 Fumaric acid 0 . 5 0 Fumaric acid 0 . 5 0 Fumaric acid 5 . 0 1 0 . 0 1 5 - 0 5 . 0 1 5 . 0 2 5 . 0 1 . 8 0 2 . 0 5 2 . 0 0 <[ 1 . 5 8 £ 1 . 1 6 £ 0 . 6 9 - 0 . 2 6 0 . 8 8 * slight Rh°^ 0 . 2 5 Maleic anhydride 0 . 5 0 Maleic anhydride 0 . 5 0 Maleic anhydride 1 0 . 0 1 0 . 0 2 0 . 0 2 . 6 1 U . 6 6 5 . 0 9 1 . 8 1 3 . 6 3 - 0 . 3 5 0 . 5 0 Diethyl Maleate 0 . 5 0 Diethyl Maleate 0 . 5 0 Diethyl Maleate 5 . 0 1 0 . 0 1 5 . 0 6 . U 0 6 . 6 0 7 . 8 0 2 . 5 0 < 0 . 0 7 < 0 . . 0 7 0 . 5 0 Maleic Acid Monoamide 1 0 . 0 0 . 5 0 Tetracyanoethylene 1 0 . 0 0 . 5 0 Hexene-1 5 . 0 0 . 3 7 N i l ; yellow ppte Rh°j; . 0 . 1 1 Values not reliable - metal may be present The RhCl^'3HgO/maleic acid monoamide system showed a very slow in i t i a l reaction with hydrogen in comparison to the other systems. When RhCl^'SHgO in DMA was mixed with tetracyanoethylene a yellow solid began to precipitate; on applying hydrogen to this mixture at 86° m'•• gas uptake was observed. With fumaric acid as substrate, metallic rhodium was sometimes observed and hence the rates obtained may not be valid. Metallic rhodium was always produced using hexene-1 as substrate. 3:09 The rhodium chloride catalyzed hydrogenation of ethylene in dimethylacetamide RhCl^'SKgO in DMA acted as an efficient catalyst for the homogeneous hydrogenation of ethylene. Solutions of RhCl_«3Ho0 in DMA absorbed a H /C0H, mixture at 1*0° to id 2 ^ 4 give an i n i t i a l rapid uptake followed by a second region of somewhat lower rate. After a small amount of i n i t i a l uptake the solution became yellow and remained a bright yellow until the uptake f e l l off^ due to substantial decrease in partial pressure of and OgH^ , at which time rhodium metal began to de-posit. The yellow solution gradually became orange red when exposed to air. Exposure of this solution to air for a very long time resulted in the forma-tion of the original reddish RhCl^ *SH^ O/DMA solution. Thus, i t seems reason-able that the yellow solution may contain a Rh* ethylene complex, produced by ethylene rapidly complexing with Rh* produced by reduction of Rh***. Analysis of the gas mixture by gas chromatography after a consider-able amount of gas uptake, but prior to any deposition of rhodium metal, indicated the production of ethane. When RhCl^'SHgO in DMA was subjected to a purely C^ H^ atmosphere at temperatures of TO - 85 ° , a similar yellow solution resulted (see Chapter IV). If this yellow solution was subjected to a Hg/CgH^ mixture at ho , hydrogena-tion of CgH^ to ethane again occurred. Detailed kinetic studies on these CgH^ /Hg systems were not carried out since the system using the solid substrate maleic acid was much more -amenable for study. Discussion 3:10 The reaction in aqueous solution The experiments involving the maleic acid system in aqueous solution showed that chloro rhodium complexes (with rhodium in the t r i - or univalent state) are not effective catalysts for the homogeneous hydrogenation of the olefinic bond in maleic acid. An olefin complex is formed with univalent rhodium but this shows no reaction with Hg. This contrasts somewhat with ruthenium systems: Ru*** chloro complexes have been shown to be ineffective as catalysts for the homogeneous hydrogenation of olefinic bonds but the. Ru** 9 II complexes were active . A 1:1 complex of Ru with maleic acid is formed, but this reacted with Hg to yield succinic acid and free Ru** again. The rate determining step for the formation of the Rh* - maleic acid complex is thought to involve a heterolytic splitting of Hg by the catalytic species with the formation of a metal hydride: k Rh*** + H_ •. * " Rh***H~ + H+ (3.6) * -1 -This rate determining step is suggested since the determined rate constant is the same as that for the Hg catalyzed reduction of Fe*"L* Production of Rh* must proceed by the rapid step Rh***H~ > Rh* + H+ (3.T) In the absence of maleic acid, rhodium metal could then be produced by further reduction of Rh* by Hg or by a disproportionation reaction. There is s t a b i l i -zation of t i e Rh* state by complexing with maleic acid only when anionic chlororhodate (III), complexes are i n i t i a l l y present; with cationic complexes present metal i.s precipitated. The d acceptor orbitals of the chloride ion may be important for stabilizing the low valency state of the metal, the stabilization increasing with the number of chloride ligands present. The more ready production of metal in systems using fumaric, acrylic, methacrylic and trans-cinnamic acids indicates that Rh* complexes with these ligands are less stable than the maleic acid complex. This could be readily 86 rationalized in terms of steric and electronic effects 3:11 The reaction in non-aqueous solution RhCl^'SHgO in DMA solutions is an effective homogeneous hydrogenation catalyst for the reduction of certain substituted ethylenes and ethylene i t -III I self . Hg i n i t i a l l y reduces Rh to Rh which rapidly complexes with the olefin present, the Rh* - olefin complex being the effective catalyst for the homogeneous hydrogenation of the olefinic bond. 3:12 I n i t i a l reaction When maleic acid was used as the olefinic substrate in DMA solution, the kinetics of the i n i t i a l Rh* - maleic acid complex formation gave the rate law -d [H2] /dt = -d[Rh***]/dt = k^Rh***] [H2] (3.8) The production of Rh* from Rh*** must proceed through some inter-mediate which involves activation of molecular hydrogen by Rh***. As mentioned • 66 in section 1:03 - 1 : 0 5 , this process involves fission of molecular hydrogen to form one or two metal-hydrogen bonds. Formation of one metal hydrogen bond can result by either a heterolytic or homolytic s p l i t t i n g mechanism and forma-tion of two s.uch bonds by dihydride formation. The energies required for the heterolytic and homolytic spl i t t i n g of fi7 in water can he estimated by a straight forward calculation -. Splitting into free atoms (homolytic splitting): H2(aq) 5> 2H-(aq) ( 3 - 9 ) requires about 100 kcal, whereas the ionic s p l i t t i n g (heterolytic s p l i t t i n g ) : H2(aq) 5> H+(aq) + H~(aq) (3 .10) requires only about 33 kcal. These figures are based on the heats of hydration for the proton of - 2 6 0 kcal, for hydride ion of - 1 0 8 kcal, and for H 2 of - 2 kcal. Consequently, unless two hydrogen atoms can be accepted simultaneously by some species in solution, the ionic splitting should be favored in solvents 83 of high polarity. DMA can be regarded as a moderately high polar solvent and i t seems l i k e l y that the i n i t i a l step w i l l involve a heterolytic splitting of hydrogen with the formation of a Rh***-H intermediate as in aqueous solu-tion. Heterolytic s p l i t t i n g i s observed for the activation of molecular hydrogen by the octahedral species RMpy^Cl^ in ethanol which results in elimination of hydrogen chloride and the formation of Rhtpy^ClgH which has a Rh-H bond detectable by n.m.r.88. Other Rh*1"'" hydrides such as [Rh (NH^) ^ jp + , [Rh(CN)1|(H20)H]2~, [Rh(trien)ClH] + have been reported 1 5. A mechanism involving formation of a dihydride species in the i n i t i a l Rh*** reduction is very unlikely. Dihydride formation has been reported for square planar Rh* or Ir* complexes. Reaction with H 2 forms octahedral Rh*** or Ir*** dihydride complexes in which the hydrogen is present as anionic ligands**' 3^' 8^'^ 0. Thus, the production of Rh* from Rh*** must result through a monohydride intermediate. Dihydride formation could possibly take place by reaction of the newly formed Rh* with resulting in a Rh*** di-hydride species. This dihydride species would then have to react rapidly with, maleic acid to form a reasonably stable rhodium dihydride maleic acid complex. Rh* + H2 > Rh***H2 (3.11) Rh***H2 + MA > Rh***H2(MA) (3.12) (The reaction with maleic acid must be rapid since in the absence of maleic acid rhodium metal forms rapidly). The absence of a signal for a Rh-H bond, the comparison with the aqueous system, and the preference for the overall simpler process with its more likely chemistry suggest that dihydride forma-tion is very unlikely prior to a step involving coordination to the Rh*. The following mechanism for the i n i t i a l reaction which results in the formation of a 1:1 Rh*(maleic acid) complex is suggested: Rh*** + H_ = k Rh***H" + H+ (3.6) d -1 Rh***H" > Rh* + H+ (3.7) Rh* + MA Rh*(MA) (3.13)' (coordinated ligands omitted; MA = maleic acid) Reaction (3.7) is likely to be fast since rhodium metal is produced very rapid-ly in the absence of maleic acid. The observed invariance of the rate with added strong acid is consistent with this since the reverse of reaction (3.6) becomes negligible. Thus the rate determining step for hydrogen uptake would appear to be the heterolytic splitting of H2. The absence of any measureable isotope effect suggests that the breaking of H-H bonds and making of Rh-H bonds occurs in a concerted process. It seems that the reduction of Rh*** to Rh° w i l l proceed through a Rh* state followed by further reaction of Ph* with or by a disproportiona-tion reaction: 2Rh* + H 2 > 2Rh° + 2H+ (3.1U) 2Rh* > Rh** + Rh° (3.15) 3:13 Effect of chloride on the i n i t i a l reaction The inverse dependence of the reaction rate with added chloride reached a limiting rate when the added chloride was about ten times the rhodium concentration in solution. The limiting rate was about one third that of a reaction in which no chloride was added. Reaction (3.6) may involve re-placement of chloride ion, k Rh***Cl + K_ r - 3 ^ ~HRh***Cl . + C l " + H + (3.16) n 2 , n-l k - l and increasing the [ci ] could increase the reverse of reaction (3.l6) and give rise to the inverse dependence. However since in the absence of maleic acid rhodium metal is deposited readily, even at high (ci ] , reaction of the Rh***H~ species to Rh° via Rh* is l i k e l y to be faster than the reverse of reaction (3.l6); in any case an acid inhibition would also be expected. The reaction more l i k e l y involves an equilibrium of the type T T T T T T ' Rh Cl Rh Cl . + Cl (3.IT) n n-l (coordinated ligands other than Cl omitted) with the Rh***Cl _ species being more reactive than the Rh***Cl species for n-l * . n r . 6 9 the heterolytic splitting of H?. The spectrum of the i n i t i a l solution at higher [ c i ] indicates that some higher chloro species are present. The limiting lower rate at about 0 . 1 M.chloride (Table II) suggests that the rhodium is then essentially a l l present a^ Rh***Cl and the rate constant k^ evaluated at 1 . 2 M chloride in the results (Table IV) refers to the activity of this species. Assuming that the [RhCl ]:[RhCln ]^ is at least 1 0 : 1 at the 0 . 1 M chloride puts an upper limit of about 10 2 M for the equilibrium constant K; this indicates that for the experiments with no added chloride using — 3 III 5 X 10 M Rh a maximum of 80% dissociation could occur according to equa-tion (3.IT). The rate data (Table II) then show that the activity of the Rh***Cl . species is at least 3 times that of the Rh***Cl species. In n-1 1 n aqueous solution the corresponding activity was found to increase with increas-ing number of coordinated chlorides but this was for anionic species only 5; in the DMA solvent the species at no added chloride must be cationic or neutral and the higher positive charge of the Rh***Cln ^ species could be an important factor. The rate constant k^ evaluated at no added chloride (Table I) is best regarded as a composite constant reflecting contribution from both chloro species. 3:1^ Solvent effects It is of interest to consider briefly the effect of solvent on the i n i t i a l rate of reaction and on the activation parameters (Table XI). Comparison of data in 3 M HC1 and DMA media indicates that the sol-vent changes the rate without changing the basic mechanism for the Rh* -maleic acid complex formation. Comparison of the rate constants with the dielectric constants of the two solvents shows that the less polar solvent aids the reaction. Rates of reactions producing ions in solution generally increase with Table XI Rate constants and activation parameters for Rh* - maleic acid complex formation at 8 0 ° Initial Solvent Dielectric k l AS + Media -1 -1 Complex Constant M sec kcal e.u. RhCl3*3H20 3 M HC1 8 0 0 . 5 5 * — _ - 0 . 5 6 b 2k.6h 9.0 RhCl3-3H20a DMA 37.8 1 . 6 7 17.3 -9.2 RhCl3'3H20a 1.2 M LiCl/DMA Q.lh 17.3 -10.8 Rh(Et 2S) 3Cl 3 C DMA 3 7 . 8 1.82 12.9 -19.2 present work reference h reference 100 7 1 the polarity of the solvent and the increase is governed largely by the change in the entropy of activation during the formation of a polar transition state, (i.e. 'freezing' of solvent molecules around i o n s ) ^ * 5 ^ 2 . If charge separation occurs and other effects are not too important, (i.e. acceptor-donor properties of solute-solvent), the rate constant would be expected to be a marked func-tion of the dielectric constant. Rates of hydrogen addition to complexes su^h as IrCltCOKPPh^g a n < i RhCl^Ph^)^ are increased by the use of more polar solvents and these findings have been interpreted in terms of a polar transi-, . 1 1 , 8 9 tion state ' . In the Rh* - maleic acid systems then some other factor such as the coordinating a b i l i t y , (donor strength), of the solvents is l i k e l y to be im-portant . 9 3 Lantzke and Watts have studied the replacement by solvent of coordinated molecules of a number of ethylenediamine cobalt (III) complexes and suggest that the strength of the solvents as ligands to be H^ O — D M A « gk DMF<DMSO. According to Gutman , however, the donor strength of DMA is greater than for water. In any case DMA may be expected to complex to some degree with the rhodium. A number of transition metal salts have been prepared which con-tain DMA and i t is generally concluded that DMA is coordinated to the metal 9 5 . 9 8 * through the oxygen . The increased i n i t i a l rate observed in DMA could result then from a solvent assisted dissociation mechanism which has been suggested for many octa-9 9 hedral complexes . This solvent assisted dissociation mechanism may occur in the production of the Rh***Cl n species suggested in reaction ( 3 . 1 7 ) . Solutions of RhCl *3H 0 in DMA have yielded an incompletely characterized red solid which on the basis of infra red and partial analysis may be Rh(DMA)(H 20) 2Cl 3. The activation parameters are given in Table X I along with those for a similar system using RMEt^S^Cl^ in D M A * * " ^ ' * ^ * . The increase l h rate of the i n i t i a l reaction in D M A compared to HC1 is reflected in the lowe.-AH^+. As expected the entropy values alone would favor reaction in the more polar solvent. 3:15 Effect of different substrates on the i n i t i a l reaction It i s not too clear why the i n i t i a l rates of uptake should vary for different substrates, since the i n i t i a l rate law in the maleic acid system shows no dependence on the maleic acid concentration. The i n i t i a l rates for the maleic anhydride and diethyl maleate systems are essentially i n -dependent of substrate concentration (Table X ) although the slight increase with increasing concentration could be significant; this could indicate be-tween zero and f i r s t order in olefin concentration, which can be explained by slow interaction of the olefin with some slowly produced intermediate Rh*** species. No reaction was detected spectrophotometrically between Rh*** and maleic acid and this is consistent with zero order dependence on the lat t e r . Tetracyanoethylene however reacted with Rh***,'in the absence of H^, to give a yellow precipitate; C^H^ i t s e l f also reacts with Rh*** to give a Rh*('CgHj+) complex (Chapter IV). Further experiments to investigate possible interaction of Rh*** with the other substrates would be necessary to help explain the i n i t i a l rate variation. 3:l6 The catalytic hydrogenation The Rh* - maleic acid complex in D M A functioned as a catalyst for the hydrogenation of maleic acid. The hydrogenation was homogeneous and rhodium metal, a powerful heterogeneous catalyst, did not result u n t i l the amount o.1* maleic acid present was insufficient to stabilize the Rh* in the form of a Rh* - maleic acid complex. 3:^ 7 Hydrogenation in the absence of added chloride The kinetics obtained in the absence of added chloride indicated a second order dependence on Rh, between f i r s t and second order in H^, and a complex inverse dependence on maleic acid. Thus a simple hydrogenation mechan ism is not involved or else is present associated with other competitive reactions. No simple mechanism can be formulated to give these kinetic depend ences although the maleic acid dependence and the second order dependence on Rh.suggest that Rh - maleic acid dimers are involved. The formation of a Rh*(maleic a c i d ) 2 species is l i k e l y , particularly at higher maleic concentra-tions. The Rh*(maleic a c i d ) 2 species might then dimerize, and dimer complexes of rhodium are generally inactive as catalysts for hydrogenation and similar 11,1+6 reactions ' Some dimeric complexes of Rh* which are known and have received con-siderable study are [ ( C ^ ) 2RhCl] [ R h C l f C ^ 2 * 0 2 , [RhCl(CO ),J ^ , [(diene)RhCl] 2 1 0 3, [(.CglL^ JgRhCl] 2 * ° U , [ R h C K C ^ ) 2] 2*° 5 , and [ R h C l ^ P ) ^ * * which a l l feature a chloride bridging structure and seem to require two coor-dinated molecules of the unsaturated ligand. In the case of the Rh* - maleic acid system in DMA a similar chloride bridged dimer may be formed to some extent. The complex [(C 2H^) 2RhCl] 2 was inactive for the homogeneous hydro-genation of ethylene in DMA; a Rh*(C2H^) complex prepared in DMA and probably monomeric was however quite active (see Chapter IV). f j ^ H ^ ) 2 R h C l ] 2 in ben-zene in the presence of various phosphines does function as a homogeneous catalyst for the hydrogenation of styrene**^; the dimer complex is almost certainly cleaved by the phosphine ligand to an active monomeric species. 7h Inverse dependences on the olefin concentration for the rhodium chloride catalyzed hydrogenation of diethyl maleate and maleic anhydride were also observed (Table X). The rate of hydrogenation almost approached zero at the higher concentrations used in the diethyl maleate system. A dimethyl maleate rhodium (i) co-nplex, having the formula 107 (CgHgOj, ) RhCl, has recently been described although whether i t is monomeric or dimeric is not reported. In the infra-red spectrum of (C^HgO^J^RhCl the band of the free C = C bonds is absent, which is at about l6H5 cm * in liquid dimethyl maleate, while the band of the C = 0 ester groups is present, having approximately the same position as in liquid dimethyl maleate. Thus the it electrons of the C = C bond must be involved in bonding to the Rh*. The irreproducibility of the hydrogenation rates at low chloride with a different batch of RhCl^'SH^O (although the kinetic dependences seemed the same) is perhaps not too surprising in view of the complex chemistry known to be associated with aqueous solutions of this salt, due to the presence of hydroxy or oxy bridged polymeric species*^8. It has been reported that [RhC^CO^lg and similar bridged complexes cry r D are cleaved by donor ligands or in some cases by chloride ion ' . . When the Rh* - maleic acid system was investigated in a high chloride DMA media, (1.2 M LiCl), no dependence at a l l on maleic acid was observed and the kinetics simplified to first order in both Rh and H^ . Thus in this system, the chloride ion may cleave any Rh* maleic acid dimer which may form or more likely may simply prevent formation of the dimer. 3:18 Hydrogenation at high chloride concentration Mechanisms of catalytic hydrogenation have been discussed in detail 1-3 11 by a number of workers ' . The catalytic hydrogenation mechanism can be thought of as involving three basic steps: (l) substrate activation, (2) hydro-gen activation, (3) hydrogen transfer. The kinetic data and observations on the 7 5 Rh* - maleic acid system at high [ci ] suggest that these three basic steps are l i k e l y to be present for the Rh* - maleic acid catalyzed hydrogenation of maleic acid. The substrate activation occurs through coordination of maleic_acid to Rh* to form a Rh* - maleic acid complex. This is a rapid step and the rate determining step is thought to involve subsequent reaction of this with H^i this yields succinic acid and Rh* again which complexes with further maleic acid. k Rh*(MA) + H 2 ——> Rh* + SA ( 3 - l 8 ) Rh* + MA f a s t > Rh*(MA) ( 3 . 1 9 ) (coordinated ligands other than MA. omitted; MA = maleic acid, SA = succinic acid) Such a mechanism yields the hydrogenation rate law -d[H 2]/dt = k2[Rh*(MA)] [H2] ( 3 . 2 0 ) which was obtained experimentally (see section 3 : 0 7 ) . The hydrogen activation may be considered to occur by a homolytic or heterolytic s p l i t t i n g mechanism (see section 1 : 0 5 ) . Heterolytic s p l i t t i n g would involve the following scheme which can be written in analogy with the Ru** Catalyzed hydrogenation of maleic and fumaric acids in HC1 solutions^ (see section 1 : 0 9 ) . I C H, - Rh -I (fast) A Rh - || - Rh - | I C 1 ~~C / S / \ +H + ( 3 . 2 1 ) +Nc — / v 1 \ / H R h - t H - C - C - H - 5 -I / \ I N — / Rh - C - C - H I / \ This mechanism features a rate determining heterolytic splitting of by the Rh* olefin resulting in the formation of a hydridorhodium (I) olefin complex. The following step involves rearrangement, of the hydrido 7 f - o l e f i n complex to a (T-alkyl complex by an 'insertion' of the olefin into the metal hydride, bond. 1 0 9 Cossee has pointed out that i f this insertion forms part of the catalytic reaction a few requirements must be f u l f i l l e d . There must be some complex formation between the hydride ion and the Rh-olefin complex which means that either a vacant coordination site should be present or at least only a l i t t l e energy should be involved in replacement by the hydride of an-other ligand or solvent molecule. A high energy of complex formation between the metal olefin and H w i l l inhibit further reaction, since the energy of the tr-alkyl complex must be lower than that of the hydrido tf-olefin complex so as to provide a driving force for the reaction. Also the energy barrier for the migration of the hydride should be low. This energy barrier is caused by the energy to break, at least p a r t i a l l y the Rh hydride bond and, in addi-tion, by rearrangements in structure such as changing the configuration around the carbon atoms of the olefin bond from trigonal to tetrahedral. In the Rh maleic acid system the observations already considered seem to suggest that the requirements of hydride formation with the Rh olefin complex and i t s l a b i l i t y are met. The hydrogenation reaction is completed In the above sequence by electrophilic attack on the metal bonded carbon atom by a proton. The origin of the proton in the reduction of maleic acid by Ru** has been shown to come from the solvent^. However in the Rh* maleic acid system in DMA (a non pro-tonic solvent) this step involving electrophilic attack on the metal carbon atom seems less l i k e l y , and in such a mechanism may be rate determining. Addition of a strong acid (p-toluene sulfonic acid) had no effect on the rate. An alternative f i n a l step formulation could involve a rate determin-ing H ? s p l i t t i n g by the <r-alkyl complex, I N _ / - R h - C - C - H + H. | / \ 2 slow. H I Rh + H - NC - Cy- H (3.22) and such reactions of rhodium alkyls with hydrogen to yield metal hydrides have been postulated**^. The rhodium hydride would have to complex rapidly with more olefin, this then rearranging to give the v""-alkyl intermediate again; the same kinetics would result. However, the Rh* - maleic acid ^-complex is considered to be a much more l i k e l y stable species in solution than the cr-alkyl intermediate and thus a mechanism involving activation of the hydrogen through dihydride formation is preferred: V Rh H„ H V L 1—H L CH I (Rh*** L j — H L (3.23) Rh HC - CH I I (where L = CI, H^ O, solvent; S = solvent) Hydride formation is accompanied by formal oxidation of the metal; the second step involves hydrometallation .of the olefinic bond via a cis ligand rearrange-ment as proposed for the intermediate step in the previous heterolytic scheme. This mechanism is similar to that proposed for the IrCl(CO) (PPh^g catalyzed hydrogenation of maleic acid in DMA***, and the RhC^PPh^).^ catalyzed hydro-genation of olefins in benzene solution**. In the latter however, the 78 dihydride is thought to form prior to reaction with olefin. The small deuterium isotope effect observed is similar to that observed for the oxidative addition of H0(D ) to the square planar - 6 . 1 e.u. are in the normal range for a bimolecular reaction in solution 112 between an ion and a neutral molecule . The entropy of activation i n d i -cates that there is l i t t l e charge redistribution during formation of the activated complex. Such reactions are expected to be favored in a medium of lower dielectric constant due to greater attraction between the ion and d i -1 1 2 pole and induced dipole of the neutral molecule . This could be a factor promoting the homogeneous hydrogenation in DMA compared to aqueous media. More important factors are l i k e l y to be: l ) the role of the DMA as a donor ligand, 2 ) the increased solubility (X 1 0 ) of H ? under the experimental con-ditions in the DMA media compared to aqueous systems. An intermediate or activated complex in which both the olefin and are coordinated to the metal atom seems necessary for hydrogenation to occur. Species which can activate molecular hydrogen quite rapidly and rever-sibly, eg. IrCl(CO)(PPh^)^ in benzene, and RhClCPPh^g i f i pyridine or aceto-n i t r i l e , are unable to catalytically hydrogenate olefins at 2 5 ° and one atmosphere or do only very slowly 1 1' 1 1"''. A dihydride species which forms is coordinately saturated and thus there is no vacant site for olefin coordina-tion. Similarly Rh1 dimers such as [RhCl(PPh^g] 2 react with Hg but are inactive catalysts due to no vacant coordination s i t e s 1 1 . H H Ph^P. CI PPh H H Complexes such as IrClCOCPPh^)^ and RhCJ^PPh^^ do a c t a s hydrogenation catalysts however after a solvent assisted dissociation of a triphenyl-phosphine l i g a n d * 1 ' 1 1 1 and the production of a coordination s i t e . The process of hydrogen transfer Is uncertain; Wilkinson** has^ suggested i t may involve the simultaneous transfer of both hydrides, each by a three centre transition state, i f the olefin complex occupies a position . cis to both Rh-H bonds. ( 3 . 2 M CHAPTER IV REACTIONS BETWEEN RHODIUM ( i l l ) CHLORIDE AND ETHYLENE k:01 Introduction During the investigation of the catalyzed hydrogenation of olefinic compounds (Chapter III), some interesting reactions were observed between ethylene itself and solutions of RhCl^'SH^O, (see section 3 : 0 9 ) . Reaction between metal ions and olefins in solution is of particular interest because the complexes formed are possible intermediates for a variety of catalyzed reactions of the olefins. Kinetic studies of formation of olefin complexes are usually complicated by subsequent rearrangement or decomposition of the complex and therefore relatively few studies of this type of reaction have been made. U :02 The reaction in aqueous media When ethylene was bubbled through a dilute solution of RhCl^'SH^O in water at room temnerature, the i n i t i a l red solution (\ 508 mu, €= 9 6 .7 ' max n with increasing absorption below U50 m/m) rapidly became yellow ( N m a x ^57 m^ , €= 8 8 . 8 ; 3 6 0 mju., C= 8 5 ) , although there was no measureable ethylene uptake. The spectrum of the final yellow solution is quite similar to that reported for the (Rh(H O^Clg] cation ' and thus i t appears that ethylene may catalyze, at room temperature, the replacement of chloride ligand by water in the pro-duction of the (Rh^HgO^Cl^]* cation from solutions of rhodium trichloride. Molecular hydrogen has been reported to catalyze the same substitution process 8 1 and a mechanism involving a Rh**''" hydride intermediate was postulated*^. A 113 I very recent report however indicates that such reactions go via RL inter-mediates; this certainly seems more l i k e l y for the system involving ethylene (a reducing agent) and this reaction is presently under further investigation in this laboratory**^. When this reaction was attempted at 7 5 ° , metallic rhodium formed quite quickly. Treatment of RhCl *3H20 in 3 M HC1 with ethylene at U0° showed no indication of gas uptake within three hours and the visible absorption spectra remained unchanged. When the reaction was run in 3 M HC1 at 7 5 ° » ethylene was consumed slowly and Rh° was produced. U:03 The reaction in dimethylacetamide solution Solutions of RhCl^'SHgO in DMA absorbed ethylene at measureable rates from 70 - 85° without metal production and gave uptake plots of the kind shown in Figure 22; no uptake occurred in the absence of RhCl^SHgO. Ethylene consumption was practically linear for the f i r s t 1000 sec and then slowly de-creased with time; for the reaction shown using 0.02 M rhodium the gas consump-tion after eight hours corresponded to greate'r than a 5:1 mole ratio of C^H^Rti and the reaction was s t i l l proceeding slowly. Analysis for reaction products in solution gave acetaldehyde (about 1 mole for each mole of Rh***) and a small amount of acetone. Gas phase analysis by mass spectrometry showed the presence of acetaldehyde, butene-1, butene-2 and unconsumed ethylene. The original red solution (\ Uh2 mii, £= 335; 530 m/u, €-"150) became yellow max fa"max hlO mjj., £~>6h0) indicating l i k e l y reduction to the Rh state , and on standing in air this reverted slowly back to the original red solution. h:0k Kinetics of the reaction in DMA The ethylene consumption plots (Figure 22) did not analyze for simple f i r s t or second order reactions but the i n i t i a l slopes could be measured with reasonable accuracy and these are summarized in Table XII for experiments in which concentrations of Rh'***, ethylene, and added chloride, perchlorate, acid, and water were varied. The results show a f i r s t order dependence on Rh*** at least upto 0 . 0 2 M (Figure 2 3 ) and values of the pseudo f i r s t order rate cons-tant k' are tabulated. The dependence on ethylene is less than one, the order decreasing with increasing ethylene concentration and the rate tending to approach a limiting value (Figure.2k). A marked inverse dependence on added chloride concentration was noted over the range 0 - 0 . 0 5 M when the total salt strength was kept constant by addition of perchlorate (Table XII). Addition of 0 . 0 5 M toluene sulfonic acid (a strong acid) had essentially no effect on the i n i t i a l rate. The addition of small amounts of water (0 - 0 . 9 M) caused an increase in the i n i t i a l rate of ethylene consumption. The rate of disappearance of the original Rh*** red color at 8 0 ° in the presence of ethylene was followed continuously by the decrease in optical density at 5 5 0 myu, € ^ I I I = l k k t (Figure 2 5 ) . Allowance for absorption due to the resulting Rh complex at 550 mjx was made when analyzing the results. A plot of (log [ R n ^ n i t i a l ] ~ * ° g [ R htime t]^ a S a * n s t time (Figure 2 6 ) gave a good straight line confirming a f i r s t order dependence on Rh, and giving a k 1 value /- -k - 1 of 5 . 6 X 10 sec in good agreement with the value from a gas uptake experi-ment at corresponding conditions. U : 0 5 Discussion of the reaction in DMA The results clearly indicate that the ethylene i n i t i a l l y reduces Rh*** to the univalent state, acetaldehyde being the other product of the redox process. The olefin must then complex rapidly with Rh* to prevent dispro-portionation of this to metal. The production of butenes indicates that a subsequent catalytic dimerization of ethylene occurs. 8'+ Table XII Ethylene reduction 01' rhodium chloride Summary of kinetic date, at 80° in DMA [ R h m ] x [c 2Hjx I n i t i a l rate h 1 k' x io" 2 2 * - 1 1.0 , M mm 10 , M of uptake sec , S - 1 x 10 , M. sec 0.25 730 i+.o 0.17 6.9 0.75 730 1+.0 0.1*6 6 . 2 1 . 0 0 730 k.o 0.69 6.9 1.50 730 h.O 1 . 1 2 l.k 2 . 0 0 730 h.o 1.58 1.9 1 . 0 0 83 0.1+5 0.15 1.5 1 . 0 0 1 1 2 0.6l 0 . 2 2 2 . 2 1 . 0 0 1 2 8 0.70 0 . 2 2 2 . 2 1 . 0 0 l l * 2 0.78 0.2U 2.1+ 1 . 0 0 1 6 0 0.88 0.33 3.3 1 . 0 0 2 1 6 1 . 2 0.1+0 1+.0 1 . 0 0 369 2 . 0 0.1*9 k.9 1 . 0 0 1*73 2.6 0.51* 5.k 1.00 6 0 2 3.3 0.63 6.3 1 . 0 0 796 U.3 0.61* 6.1* 1 . 0 0 730 i+.o 0 . 1 6 ^ b 1 . 0 0 730 h.O 0.27 2 ' T c 1 . 0 0 730 k.o 0.1*2 k'2l • 1 . 0 0 730 h.O 0.73 7' 3e 1 . 0 0 730 h.O . 1.02 10.2 J 1 . 0 0 730 h.O 1 . 0 6 1 0 .6 1 1 . 0 0 730 h.O 0.86 8 - 6 h 1 . 0 0 730 U ,0 1.73 17.3h 1 . 0 0 730 I+.O 1.77 17.7 * See Appendix II + k* = ( I n i t i a l rate of uptake)/ R h 1 1 1 a 0 . 0 5 M L i C l . b 0.03 M L i C l , 0.02 M LiClO^. C 0.02 M L i C l , 0.03 M LiCIO,. d 0.01 M L i C l , O.Ql* M LiC10>. 6 0.05 M LiCIO,. f 0.05 M toluene sulfonic acid. S 0.1* M HgO. h 0.7 M HgO. 1 0.9 M HO. 85 0 0 . 5 0 1 . 0 0 1 . 5 0 2 . 0 0 (Rh 1 1 1 ] X 1 0 2 , M Figure 2 3 . Dependence of i n i t i a l rate on [ R h 1 1 1 ] , 8 0 ° , 730 mm C H. , DMA solution. 2 O i - l DMA solution. a5 o •rH - P P ( O 1 1 0 0 0 _L 2000 3000 Time, sec U000 5 0 0 0 Figure 2 5 . Optical density plot at 5 5 0 nyi for loss of original chloro rhodium ( i l l ) species, 8 0 ° , 730 mm C_H, , 0 . 0 1 M Rh 1 1* . € D,III = ^ * ' 2 4' i n i t i a l ' Rh lkkt 1 cm c e l l , DMA. solution. M B M - H M -P Xi o >-3 cd •H •P r H - H M C M *H .G bO O .-3 0.75L 0 . 5 0 0 . 2 5 h 1 0 0 0 2 0 0 0 Time, sec. I l l Figure 2 6 . F i r s t order plot for reaction of Rh with C^E^, 80 III 730 mm C^H,, 0.01 M Rh. ... DMA solution. 2 4' i n i t i a l ' The k i n e t i c s of the i n i t i a l region are s i m i l a r to those observed previously for the formation of a ruthenium ( i l ) - o l e f i n complex i n aqueous 115 HC1 solut io i . and suggest that formation of the rhodium ( i ) -e thy lene com-plex proceeds through a two-step d i ssoc ia t ion mechanism of the type, k • ~ R h m c i r - 1 - - R h I I 3 : c i n + C l " (U . l ) n n - l - v k - l R h 1 1 1 ^ ^ + C 2 H U —2-> [ R h ^ l ^ C ^ ) ] { k ~ n ) . (It.2) I These steps could then be followed by steps such as H 2 ° I I — R h C l . + CH.CHO + 2H (it.3) n - l 3 T fa c f f Rh C l + X C2H^ > complex • (it.lt.) Assuming the steady state approximation for the Rh^^^Cl^ species t h i s mechanism y i e l d s the rate - law d ( c 2 H j k ^ K ^ C l J ^ H j (4.5) at ^ J c i " ) .+ k 2 [ c 2 H j At constant (CgHjJ and [ci ] (the l a t t e r determined by the so lu t ion e q u i l i b r i a of the RhCl 3 «3H 2 0 i n D M A ) , t h i s becomes = R ' [ R h m c i ] '(4.6). L n J dt where k' = k ^ [ C g l l J '/k (ci~] + k g [c H J . Values of k' are ca lcu lated from the i n i t i a l slope using equation ( i t .6) and these are l i s t e d i n Table X I I . 88 The expression for k' can be rewritten as 1 1 . k _ i [ C 1 " - = > + — r , ( 4 . 7 ) k * - k l k l k 2 [ C 2 H 4 J Considering the uncertainty in measuring i n i t i a l slopes the plot of 1/k' versus 1/[Cgll^] shown in Figure 27 is reasonable and yields the values k ^ 1 0 ~ 3 s e c - 1 and k ^ C l " ] / ^ ^ 0 . 0 2 5 M. A plot of 1/k' versus added [.Cl~] - 3 - 1 is shown in Figure 28 and the straight line drawn gives k^ 1 . 3 5 x 10 sec and k ^/k2 c£ 5 . 4 . Combining with the k ^[C 1 ] / ^ 2 v a^- u e above indicates that the free ( c i .] at the start of these experiments using 10 2 M RhCl^'SHgO is — 3 about 5 x 10 M. Thus n in equations ( 4 . 1 ) to ( 4 . 6 ) is l i k e l y to be 3 or 2 . The observed rate constant k' and the derived rate constant k^ must be consi-dered as composite constants probably reflecting contributions from more than one rhodium ( i l l ) complex. In the absence of added water the oxygen of the acetaldehyde must presumably come from the trihydrate and the H^ O molecule required for reaction ( 4 . 3 ) may be coordinated in the Rh*** intermediate (I) along with coordinated DMA solvent molecules to make a coordination number of 6 for octahedral con-figuration (anhydrous RhCl^ was found to be insoluble in DMA). I could dissociate to give a hydroxo species this then decomposing via a tT-bonded 116 complex t o products as i n the e thy lene r e d u c t i o n o f o the r meta l ions " I + H20' > L R h C l n _ 1 ( 0 H ) ( C 2 H 1 + ) ] ( 3 _ n ) + '+ K+ (4.8) II II > \ci . - Rh - CH_ -• CH0 - Oh] (4 .9) i. n - 1 d d > III III > CI ,Rh* + CHoCH0 + H + (4.10) n-1 3 89 o CD CO ro t o H x: H I 1 0 0 2 0 0 Figure 2 7 . Dependence of i n i t i a l rate on the ethylene concentra-tion plotted in accord with equation (U.T) ( 8 0 , 0 . 0 1 M Rh, DMA solution). 0 0.02 0.04 Added [ci~] , M Figure 2 8 . Dependence of i n i t i a l rate on the chloride concentra-tion plotted in accord with equation (4.7), ( 8 0 ° , 0 . 0 1 M Rh, 730 mm C H^ , [ci~] + [ciO^ -] = 0.05 M, DMA solution). J 9 1 The agreement between the k' values from the spectrophotometry and gas uptake studies is significant since i t indicates that reaction (4 . 3 ) between the intermediate I and H^ O i s not so rapid that a contribution to the i n i t i a l gas uptake results from reaction ( 4 . 4 ) ; the latter has to be very-rapid following reaction ( 4 . 3 ) . Addition of H^ O gives an increase in the i n -i t i a l rate; this could result from an increase in the rate of reaction ( 4 . 3 } and the subsequent absorbtion of X moles of C^H^ in reaction ( 4 . 4 ) . On i n -creasing the water concentration the rate seems to approach a limiting value about 3 times that of the rate in the absence of added water, suggesting that X is l i k e l y to be 2 in reaction ( 4 . 4 ) and that the resulting Rh* complex could be [cigRhfCgH^g]" It i s of interest that in the corresponding palladium (II) system in aqueous solution**^* reaction ( 4 . 9 ) is postulated as rate-determining, the 7/-complex formation being a rapid equilibrium; reaction ( 4 . 8 ) was also written as an equilibrium, this giving rise to an inverse acid dependence in the rate-law. A mechanism such as T T T ^ T T T — Rh CI + C0H, „ Rh CI .(C0H, ) + CI (4.11) n 2 4 n-1 2 4 v • •' T T T "\ T + Rh CI -.(C.H,) ~ T ~ > Rh CI _ + CH "CHO + 2H (4.12) n-1 2 4 HgO n-1 3 would give a rate-law d ( c 2 H U ] k 3K[Rh I I 3 :total] [ C 2 H J dt [c i~] + K [ C 2 H J which is the same form as the rate-law of equation ( 4 . 5 ) . However, no (4.13) spectral or gas uptake evidence to indicate any equilibrium such as ( 4 . 1 l ) was obtained and for this reason as well as the parallel with the Ru** - Cg^4 system, the dissociation mechanism is strongly favoured. The extent of solvent participation in reaction (U.l) is of course 117 uncertaii-. Recent reports indicate that aquation reactions of rhodium ( i l l ) complexes ;-how considerably more S^2 character than the corresponding well-studied cobalt ( i l l ) reactions. Studies of substitution reactions of rhodium ( i l l ) complexes in non-aqueous solvents have not been reported previously. Measuirments on the temperature dependence of k' over the range of 70. - 8 5 ° (Table XIII) at an essentially constant ethylene concentration sufficiently high that k 1 ^ k^ yielded an Arrhenius plot (Figure 2 9 ) and the activation parameters AH =*= = 9 . 5 - 1.5 kcal and AS1*5 =-1+5.5 - h.3 e.u. Large negative entropies of activation such as this are found for ionization reac-tions in solvents of moderate polarity and are explained by the freezing of 9 2 solvent molecules around the incipient ions on going to the activated state . Many fruitless attempts were made to isolate the Rh* ethylene com-plex. Cramer has reported a reaction between rhodium trichloride hydrate and ethylene in aqueous methanol solution at 2 5 ° when the dimer complex ( C g H ^ \ R n 2 C 1 2 e v e r v t u a-Hy precipitates ; the properties of this compound in DMA are quite different from those of the Rh* ethylene complex obtained in the present study. The u.v. and visible spectra are quite different and a solution of the dimer complex was rapidly converted to metal at 8 0 ° even under an ethylene atmosphere. The dimer is clearly not the catalytically active species either for dimerization or hydrogenation (see section 3 :17). 1+6 Cramer has reported an important paper on the RhCl^ catalyzed dimerization of ethylene to linear butenes in ethanolic HC1 solutions and suggests that the anion (cigRhtCgHj^) ]~ is the most l i k e l y active catalyst for dimeriza-tion and the present work indicates that this species is present. The dimerization mechanism postulated by Cramer involves the following features: (i) the reversible oxidation of Rh* by a proton, acid to give a Rh*** hydride, ( i i ) a rate-determining insertion of coordinated olefin between an alkyl Table XIII Ethylene reduction of rhodium chloride in DMA Temperature dependence of k 1 _ o -1 Temp. C k' X 10 , sec 70 It.5 75 6.5 80 6.9 85 8.5 Determined for 10 M Rh , U.O x 10 M C Ii group and the metal ion to which i t is attached, ( i i i ) the l a b i l i t y of hydrogen in Rh"'"'1"* alkyl and olefin complexes (see section 1 : 0 9 ) . Both proton and halide essential for dimerination w i l l also be present in the present system. CHAPTER V DIRECT CARBONYLATION OF RHODIUM ( i l l ) CHLORIDE COMPLEXES IN AQUEOUS SOLUTION 5:01 Introduction -This chapter describes studies directed at the preparation of some rhodium carbonyl complexes; the search for possible effects of the CO group on catalytic activity prompted this work. Since ruthenium (III) carbonyl complexes have been obtained from 31 the direct reaction of ruthenium (III) chloride with CO under mild conditions i t seemed reasonable to try this method for the preparation of rhodium car-bonyl complexes from rhodium (III) chloride complexes. The kinetics of this direct reaction were investigated in aqueous media and are discussed in this chapter. Some catalytic properties of the prepared rhodium carbonyl com-plexes are discussed in Chapter VII. 5:02 The reaction of'RhCl3'3H20 in aqueous solution with CO Solutions of RhCl^'SHgO in H^ O absorbed CO very readily at room temperature; the i n i t i a l red solution becoming yellow. Gas uptake and the presence of a yellow solution suggested the possible formation of a rhodium (i) carbonyl complex. The reaction was too fast in order to provide good kinetic data. RhCl3*3H20 in 3 M HC1 absorbed CO at measureable rates from 65 - 85° and gave uptake plots of the type shown in Figure 30. The reaction was espec-97 8 0 ° , U50 ram CO, R h 1 1 1 : ('& ) 0 . 0 1 2 5 M, (O) 0 . 0 2 5 0 M, 3 M HC1. ially interesting since it exhibited autocatalysis. The total measureable gas uptake corresponded to a 2:1 mole ratio of CO:Rh and, experiments in the presence of a soda lime tube resulted in a measureable gas uptake correspond-ing to a 3:1 mole ratio of CO:Rh. Thus one mole of CO^ is probably produced for every three moles of CO consumed. These observations suggest an overall stoichiometry for the reaction given by the following equation: Rh 1 1 1 + 3C0 > RhI(C0)2 + C02 (5.1) (chloride and water ligands are omitted) A mass spectral analysis of the gas mixture at the end of the reaction also indicated the production of CO^ . The original red solution, (^max hOk m/i, £= 67; 511 m/i, £= lk) became yellow (X 325 m/i, fe~>5700) indicating likely reduction to the Rh* state. This yellow solution when exposed to air soon returned to the original red solution. The rhodium carbonyl reaction product, formed as the yellow solution containing probably the anionic species (Rh(C0)2Cl2]~, could be precipitated by the addition of a methanol solution of tetraphenylarsonium chloride to a concentrated volume of the yellow solution. The properties of the isolated solid corresponded to those reported by Vallarino 5 T for (Ph). As + (Rh(CO) Cl ]" (m.pt. l8l°, I.R.(KBr disc):v = 2050 cm * ) . J Treatment of the final yellow solution with triphenylphosphine evolved CO and gave an excellent yield of trans RhCl(CO) (PPh )g2''*^, (Analysis Fotind: Rh} lU.6; Cl, 5.1; C, 6k.6; P, 8.8; H, k.3%. Calc. for RhCl(C0)(PPh3)2: Rh, lU.9; Cl, 5.1; C, 6k.k; 0, 2.3; P, 9-0; H, k.3%. I.R.(KBr disc):v>CQ = 1982 cm"1. M.pt. 189.- 191 (decomp. ) )..- • 99 5:03 Kinetics of the reaction in HC1 media ' The nature of the uptake plots (Figure 30) suggested that the reac-tion exhibited autocatalysis. The plots were consistently found to yield maximum slopes at a point close to 30% of the total CO uptake, regardless of the i n i t i a l [Rh***] used. When these uptake plots were differentiated graphically, the results always gave curves of the type shown in Figure 31, which were nearly symmetrical around the 50% reaction point. It is readily shown that a rate proportional to the product [Rh***] [Rh* ] becomes a maximum when the [Rh***] = [Rh*] = 1/2 [Rh] , and therefore, i t is reasonable to assume that the dominant term in the rate expression has a rhodium dependence expressed by the equation -d[co]/dt = k2;[Rh***][Rh*] (5.2) where k^1 is a pseudo second order rate constant that s t i l l contains a l l chem-ical and physical variables except the dependence on rhodium. Peters and ll8 Byerley have found previously that the reduction of cupric salts in aqueous solutions by CO at high pressure also exhibited this type of behavior. It is important to note that the curves in Figure 31 show a non-zero rate at the high [Rh***] end of the curve or, in other words, there is a significant i n i t i a l rate of uptake in the plots shown in Figure 30. This shows a finite reaction rate at zero [Ph*] values and this must be taken into account by adding an additional term of the form k^'(Rh***]n to equation (5.2). The rate law now becomes: -d[co]/dt = ^'[Rh***] 0 + k2'[Rh***][Rh*] (5.3) The occurrence of a zero intercept for the high [Rh*]part of the curve in Figure 31 provides evidence that no term of significance to the CO uptake 100 5 .0L . [Rh 1 1 1], M Figure 31. Typical plots of -d [co]/dt vs. [Rh 1 1 1] , 8 0 ° , 1+50 mm CO, [^h111] i n i t i a l : ( D ) 0 ' 0 2 5 M»'(0) 0 . 0 5 0 M, 3 K HC1. measurements, which is a function of only Rh*, exists. . 5 : 0 4 The i n i t i a l reaction Since no Rh* is present at the >'tart of the reaction and the amount of Rh* produced in the i n i t i a l stage of the reaction i s very small relative to the Rh*** present, the i n i t i a l slow uptake of CO (Figure 30) probably proceeds mainly by a path which gives rise to the f i r s t term of rate equation (5.3). The kinetics of the f i r s t part of the overall process were determined from i n i t i a l slopes which could be measured with reasonable accuracy.- The i n -i t i a l rates obtained-by this method are summarized in Table XIV for experi-ments in which the [Rh***] and [co] were varied. Duplicate runs of some of the experiments at later dates showed that the i n i t i a l rates were quite re-producible. The results show a good f i r s t order dependence on Rh*** at least - 4 upto 0 . 0 5 M and a f i r s t order dependence on CO at least upto 4.30 X 10 M (Table XIV, Figures 32 & 33). Thus the rate law for the i n i t i a l reaction can be formulated as: -d[co]/dt = k j R h * * * ] ^ ] (5.4) Values of k^ at 8 0 ° were calculated and are given in Table XIV. The values of k^ calculated from the slopes of Figures 32 & 33 were 0.53 M~* s~* and 0.54 M * s * respectively. Kinetic measurements over the temperature range of 65 - 85° yielded a good Arrhenius plot (Table XV, Figure 34) and the activation parameters AlLjf = 2 5 . 9 ± 1.0 kcal andAS^ = 13.2 ±2.5 e.u. 5 : 0 5 The reaction involving the autocatalytic species The observations and results of sections 5:02 - 5:04 suggest that a 102 TabJ-.e XIV Carbonylation of rnodium chloride Summary of kinetic, data i.i 3 M KC1 at 8 0 ° [ R h i n ] x CO [ c o]x I n i t i a l rate k l 1 0 2 , M mm 1+ * 10 , M of CO uptake xlO^, M sec. * M sec. xlO*, M *sec. * 1 . 2 5 U50 3.80 2 . 5 0 0 . 5 3 0 . 7 9 1 . 2 5 1+50 3 . 8 0 2 . 5 0 0 . 5 3 0 . 7 9 2 . 5 0 1+50 3 . 8 0 . 5 . 1 2 0 . 5 ^ 0 . 6 5 2 . 5 0 1+50 3 . 8 0 5 . 1 2 0 . 5 1 * 0 . 6 7 3 . T 5 1+50 3 . 8 0 7 . 3 6 0 . 5 ^ 0 . 6 1 5 . 0 0 1+50 3 . 8 0 9 . 8 5 0 . 5 2 0 . 6 3 5 . 0 0 1+50 3 . 8 0 1 0 . 1 1 0 . 5 3 0 . 6 3 2 . 5 0 85 0 . 7 2 1 . 2 6 0 . 7 0 0 . 6 2 2 . 5 0 175 1.1+8 . 2 . 0 1 0 . 5 ^ 0 . 6 2 2 . 5 0 22l+ 1 . 8 9 2 . 9 6 0 . 6 3 -2 . 5 0 2 2 6 1 . 9 1 2 . 7 7 0 . 5 8 0 . 6 3 2 . 5 0 300 2 . 5 3 3.1+3 0 . 5 1 * 0 . 6 8 2 . 5 0 300 2 . 5 3 3.1+3 0 . 5 ^ . 0 . 6 8 2 . 5 0 305 2 . 5 8 3 . 5 0 0.5!+ 0 . 6 8 2 . 5 0 336 2 . 8 6 - - O . 6 5 2 . 5 0 377 3 . 1 8 1+.00 0 . 5 0 O . 6 5 2 . 5 0 5 0 5 1+.27 5 . 7 7 0.5k 0 . 6 6 * r -1 SI [COJ computed from solubility data of Seidell assuming the solubility to be the same as in pure water. "t"k calculated from f u l l integrated equation (5.10) using appropriate [RhJ , CO and corresponding k value. 103 101+ Table XV Carbonylation of rhodium chloride Temperature dependence of k^ ([Rh 1 1 1] = 0.025 W; 3 M HCl) Temp. CO [co] x V °C - ' mm 4 * 10 , M M sec. 65 6 0 0 5 . 1 5 0 . 1 0 70 5 6 0 4 . 7 3 0 . 1 5 75 510 4 . 3 0 0.32 8 0 4 5 0 3 . 8 0 0 . 5 4 84 ' 3 9 4 3 . 3 8 0 . 7 6 * H computed from solubility data of Seidell 8 1 assuming the solubility to be the same as in pure water. 106 species [RhtCOjgClg] is probably the autocatalytic agent involved in the overall.reaction. The kinetic data for the i n i t i a l reaction (section 5:0H) now suggests the formulation of rate law (5 .3 ) as: -d[co] /dt = k^Rh 1 1 1] [CO] + ^ ' [ R h 1 1 1 ] ^ 1 ] (5 .5 ) The following kinetic data provide further evidence for this rate law and enable the calculation of the rate constant kg'* Maximum rate measurements were made over.a series of experiments at different i n i t i a l rhodium concentrations (Table XVI). When these maximum rates were plotted against the i n i t i a l rhodium concentrations at a constant [co] a linear relation was not observed (Figure 35 ) . A plot of these maximum rates against the product [Rh 1 1 1] X [Rh 1] at the point of maximum rate, did however result in a reasonable linear dependence (Figure 35 ) . The linear dependence supports the dominance of the second term in the rate equation (5.5) for this part of the reaction. The fact that the plot of maximum rate against [Rh 1 1 1] X [Rh*] does not quite pass through the origin can be interpreted as indicat-ing that the f i r s t term of the rate equation (5 .5) is not completely insigni-ficant at this stage of the reaction. A value of kg' = 0.067 M * s * was calculated directly from the slope of the lihe'ar plot in Figure 35. In order to analyze for the CO dependence of the reaction involving the Rh* carbonyl species, maximum rate measurements were made over a series of experiments in which the partial pressure of CO, (p' ), was varied while the i n i t i a l [Rh***] was always the same (Table XVI). When these maximum rates were plotted against the P'^Q used, no simple order dependence in CO was ob-served (Figure 36 ) . It is evident from these experiments that at higher pressures of CO the reaction rate increases only slightly with pressure. Examination of rate law (5 .5) suggests that this slight increase in reaction 1 0 8 Table XVI Carbonylation of rhodium chloride Maximum rates in 3 M HC1 at 8 0 ° [ R h m ] x CO [co]x Maximum rate x 1 0 2 , M mm 4 10 , M 1 0 5 , M s" 1 . 1 . 2 5 U50 3 . 8 0 0 . 4 7 1 . 2 5 U50 3 . 8 0 0 . 4 9 2 . 5 0 4 5 0 3 . 8 0 1 . 4 4 2 . 5 0 450 3 . 8 0 1 . 4 5 3 . 7 5 4 5 0 3 . 8 0 2 . 7 8 5 . 0 0 450 3 . 8 0 4 . 5 7 ' 5 . 0 0 4 5 0 3 . 8 0 4 . 5 6 . 2 . 5 0 85 0 . 7 2 0 . 8 1 2 . 5 0 1 7 5 1.48 1 . 0 9 2 . 5 0 1 7 5 1.48 1.13 2 . 5 0 2 2 6 1 . 9 1 1.24 2 . 5 0 2 2 6 1 . 9 1 1.23 2 . 5 0 300 2 . 5 3 1 . 2 9 2 . 5 0 305 2 . 5 8 1 . 3 3 2 . 5 0 336 2 . 8 6 1 . 3 4 2 . 5 0 350 2 . 9 0 1 . 3 8 2 . 5 0 377 3 . 1 8 1 . 3 5 2 . 5 0 505 4 . 1 7 1 . 5 2 [co] computed from solubility 8 l data of Seidell assuming the solubility to be the same as in pure water. 110 r a t e mty r e s u l t from the CO dependence o f the f i r s t term i n the r a t e law. I f one assumes that t h i s i s so, the slope o f maximum r a t e against p' p r. of Figure 36 rhould provide a value f o r k^. The value of k^ obtained by t h i s method i s 0.31 M * s * which i s i n good agreement w i t h the values o f k^.ob-t a i n e d from thi* measurements o f i n i t i a l slopes (Table XIV). A l s o , r a t e law (5.5) suggests t h a t the second term, k g ' [ R h 1 1 1 ] [ R h 1 ] , should be i d e n t i f i e d , w i t h the i n t e r c e p t found i n Figure 36. The value of k^' obtained from t h i s i n t e r c e p t i s 0.063 M * s * which i s i n good agreement w i t h the value of k g 1 obtained from the p l o t of maximum r a t e against [ R h m ] x [ R h 1 ] , (Figure 35). Thus i t would appear t h a t the r e a c t i o n i n v o l v i n g the a u t o c a t a l y t i c species i s v i r t u a l l y independent of [ co]at higher CO pressures. Thus kg' becomes a t r u e second order r a t e constant kg and the r a t e law f i n a l l y then becomes -d[co]/dt = k j R h 1 1 1 ] ^ ] + k ^ R h 1 1 1 ] ^ 1 ] (5.6) Figure 36 suggests t h a t the pressure e f f e c t would be f e l t on the second term o f the r a t e law only at CO pressures so low t h a t the formation of the [ R h I ( C 0 ) 2 C l 2 ] " species from Rh and CO becomes r a t e determining, (see s e c t i o n 5 : 0 7 ) . ' I t i s of i n t e r e s t t o con s i d e r the r a t i o o f maximum r e a c t i o n r a t e t o i n i t i a l r e a c t i o n r a t e i n order t o c a l c u l a t e values o f kg. This r a t i o can be expressed as: r K H / ' H i a x ' " i K " ] ! ^ * fcgh111] W W ( 5 7 ) " ( - d [ 0 0 ] / * ) I n l t " ( k j R h 1 1 1 ] [CO] ) I n i t -Since the at maximum r e a c t i o n r a t e w i l l be h a l f the [ R h " 1 ] i n i t i a l l y , equation (5.7) can be formulated as: _ 21 JMax _ . R . R _ 2k1[co] + 2 ( 5 * 8 ) where [^h^]y a x 1 S ^he concentration of the autocatalytic species at the point of maxinum rate. Values of kg obtained by using equation (5.8) and the data of Tables XIV and XVI are summarized in Table XVII. The values agree reasonably with those obtained from the plots of Figures 35 and 36. Since, -d[co]/dt = -2d[Rh m]/dt = 2d[RhI]/dt (5-.9) the following expression (5.10) can be obtained by integration of rate law (5.6): log(k + kgfRh 1]) - l o g [ R h m ] = (k • + k ? [ R h ] T o t a l ) t k.b + log k x« - l o g [ R h ] T o t a l . (5.10.) where k^' = k^[co] and t is the reaction time. It is possible to calculate values of kg from this expression for a particular experiment by using the appropriate k^' values determined from i n i t i a l slopes (Table XIV). Values of kg were calculated from the f u l l integrated expression, at the maximum rate when [Rh 1 1 1] = [Rh1] for experiments in which the [Rh] and [co] were indepen-dently varied, and are summarized in Table XIV. The values of kg calculated by this method agreed well with values estimated by the other methods. A further method of calculating kg is to consider the f i n a l part of the overall reaction and reconsider the integrated expression (5.10). Once the [Rh*] has built up to an appreciable concentration and[Rh***]has corres-pondingly decreased the second term of rate equation (5.6) w i l l account for the major part of the rate of CO consumption. Thus, in equation (5.10) the 113 Table X V I I Carbonylation of rhodium chloride Calculation of k^ by use of equation (5.8) (80 , 3 M HC1) [ R h 1 1 1 ^ [co] x Max Rate k l k2 2 1* * -1 -1 -1 -1 10 , M 10 , M I n i t . Rate M sec M sec 1.25 3.80 1.88 0.53 0.091 2.50 3.80 2.82 0.5^ 0.076 3.75 3.80 3.78 0.5*t 0.072 5.00 3.80 It. 52 0.53 O.O65 5.00 3.80 It.63 0.52 0.065 2.50 0.72 8.3U 0.5^ 0.055 2.50 1.U8 5.63 0.5U 0.066 2.50 1.91 U.U8 0.58 0.071 2.50 2.53 3.77 0.51* 0.071 2.50 2.58 3.80 0.5^ O.07I+ 2.50 3.18 3.38 0.50 0.073 2.50 It.27 2.6U 0.51* 0.079 * r 1 81 IC0J computed from s o l u b i l i t y data of S e i d e l l assuming the s o l u b i l i t y to be. the same as i n pure water. Ilk log term containing k^', can be simplified since k^' w i l l be considerably less than both kg[Rh*] and k 2 T o t a l * Equation (5.10) reduces to logfah 1 ] - l o g [ R h m ] = k2[Rh] t + log k-k.b " M H Total " * ° g k 2 ( 5 ' n ) A similar expression can also be obtained by integration of rate law (5.2) which takes no account at a l l of the f i r s t reaction. Expression (5.11) suggests that a plot of log [Rh1] - l o g [ R h m ] against time should give a straight line of slope ^ 2 Total''1**^ a n t i ^ n t e r c e P ' t l o S ~ L O G O^JTotal ~ log kg. The slope and the intercept should therefore provide values of kg. This type of plot was carried out for a number of experiments, (Figure 37), and the straight lines drawn give the kg values summarized in Table XVIII. Again quite consistent kg values are obtained. As expected the plots f a l l off badly from linearity in the f i r s t part of the reaction. A rapid uptake of CO occurred at 80° when RhCl^'SHgO was added to a solution which contained Rh* at a concentration at least equal to the RhCl3'3HgO added (Figure 38). The uptake plot resembled the latter half of the uptake plots shown in Figure 30. The i n i t i a l rapid uptake provides support for the autocatalytic nature of the Rh* carbonyl species; measurement of the i n i t i a l slope and application of rate law (5 .6) enables calculation of a value for kg. The values of kg obtained (Table XIX) were in good agreement with values from other methods of analysis. Kinetic measurements over the temperature range of 65 - 85° yielded a good Arrhenius plot (Table XX, Figure 39) and the activation parameters AHgt = 12.5 ± 1.0 kcal andASg=t = - 29.2 ±2.5 e.u. 115 0 2 0 0 0 Uooo 6 0 0 0 Time, sec Figure 3 7 . Plots of log^Rh 1 (CO) ClJ'] - log ^ h 1 1 1 ] vs. time for various CO pressures ( 8 0 ° , 0 . 0 2 5 M Rh, P'CO: (A) 85 ram, (Q) 175 mm, ( D) 2 2 6 mm, ( V ) 450 mm, 3 M HCl). Table XVIII Carbonylation of rhodium chloride Calculation of kg by use of equation (5.11) ( 8 0 ° , 3 M HCl) [Rh I I ] C]x [co]x k 2 k 2 1 0 2 , M 10 , M M sec M sec 2.50 0 .72 0 .060 0 .059 2 .50 1.U8 0 .063 O.O65 2.50 1 . 9 1 0 .071 0 .072 2 . 5 0 3 .18 0 .078 0 .073 2 .50 3.80 0 . 0 8 1 0 .073 3 .75 3 .80 0.071+ 0 .067 5 .00 3 .80 O.O65 0.071 * r 1 81 I COj computed from solubility data of Seidell assuming the solubility to be the same as in pure water. a r I kg calculated from slope of log([Rh (C0)g Clg]") - l o g f R h1 1 1 ] vs. time plot b kg calculated from intercept of log ((Rh* (C0)gClg]~) - log[Rh I I I1 J vs. time plot. Table XIX Carbonylation of rhodium chloride Calculation of kg from experiment of R h 1 1 1 addition to JRhI(C0)gClg]~ ( 8 0 ° , 3 M HC1) [ R h m ] x [Rh I(C0)gClg]~ x [C0]x Total rate x k 2 1 0 2 , M 2 1* 10 , M 10 , M * 5 - 1 1 0 ; , M s M ^ s - 1 1.15 1 .15 3 .80 1 . 3 2 . ' 0 . 0 8 1 2.1U 2.11* 3 .80 3.28 0.062. * r i 81 IC0J computed from solubility data of Seidell assuming the solubility to be the same as in pure water. 117 s CVJ o H o 8 0 0 1 6 0 0 2H00 Time, sec Figure 3 8 . Rate plot for R h 1 1 1 reduction when [Rh*(CO^Cl^]~ present i n i t i a l l y , ( 8 0 ° , 1*50 mm CO, 0 .0211* M Rh 1 1 1, 0 . 0 2 1 * * M [hh^CO) Cl ]~, 3 M HCl). 118 Table XX Carbonylation of rhodium chloride Temperature dependence of k^ ( [mi] = . 0 2 5 M, 3 M HC1) Temp. CO [co"j: k 2 o h * - 1 - 1 C mm 1 0 , M M sec 6 5 6 0 0 ' 5 . 1 5 0 . 0 2 8 70 5 6 0 It.7 3 0 . 0 3 6 7 5 510 3.hh 0 . 0 5 0 8 0 U50 3 . 8 0 0 . 0 6 6 Qh 3 9 1 * 3 . 3 8 0 . 0 8 0 [co] computed from solubility data of S e i d e l l 8 * assuming the solubility to be the same as in pure water. 119 120 5 : 0 6 The effect of chloride concentration on reaction rate Experiments were carried out in which the [c i "] was varied while a constant acidity was maintained. This was accomplished by decreasing the [HCl] and adding sufficient HCIO^ to provide a 3 M acid media. The kinetic data obtained for these experiments are summarized in Table X X I . The results indicate a slight inverse dependence on chloride for the i n i t i a l rate. Over the range of chloride concentration 0 . 5 0 M to 3 . 0 M, the essential species present is the 5 : 1 chloro complex'', and the variation in reaction rates under these conditions is probably not due to the presence of different chloro species. This slight inverse dependence may well arise from a variation in CO solubility on altering the reaction media; the calcula-ted CO s o l u b i l i t i e s consistent with the i n i t i a l rate are shown in Table X X I . Hg shows increased solubility with increasing [ H C I O ^ ] in solutions with a constant t o t a l concentration of HCl and HCIO^. A much greater inverse dependence on chloride was observed however for the maximum rate (Table X X I , Figure Uo), which furthermore i s essentially independent of the [co] (see section 5 : 0 5 ) . When the inverse of the maximum rate was plotted against the ( c i ] a straight line resulted (Figure Uo). When the reaction was carried out with Na^RhClg'^HgO in 3 M HCl, both the i n i t i a l and maximum rates are increased. The 6 : 1 chloro complex is the main species present^ and values of k^ = 0 . 7 8 M * sec * and kg = 0 . 1 2 M * sec * were estimated. The corresponding values for reactions involving the 5 : 1 chloro species in the same media were k^ = 0 . 5 * * M * sec * and kg = 0 . 0 6 5 M"1 s e c - 1 . 5 : 0 7 Discussion Carbon monoxide reacts then with aqueous solutions of rhodium 121 Table XXI Carbonylation of rhodium chloride Effect of [c i "] on the rates of reaction ( 8 0 ° , 3 M HC1) [Rhjx [co]x [ci~] [ H 3 0 + ] I n i t i a l rate x Max. rate x 1 0 2 , M 1 0 2 , M * M M 1 0 5 , M sec." 1 1 0 5 , M sec." 2 . 5 0 3 . 8 3 . 0 3 . 0 0 . 5 1 l.M» 2 . 5 0 ( 4 . 8 ) 2 . 0 3 . 0 O . 6 5 1 . 8 9 2 . 5 0 ( 4 . 9 ) 1 . 0 3 . 0 0 . 6 6 2 . 5 6 2 . 5 0 ( 5 - 5 ) 0 . 5 3 . 0 0 . 7 4 3 . 5 1 Values in brackets are calculated using the i n i t i a l reaction rates and the known, solubility of 3 . 8 x 10 M in 3 M HC1. 0.75 chloride complexes under relatively mild conditions to form the [Rh(C0) 2Cl 2] ~ species. Treatment with triphenylphosphine to give trans RhCl(CO)(PPh^)^ provides a general rapid and efficient way of preparing such complexes, giving a better rout i v than the more familiar decarbonylation reactions**' 2 3 2^'^ 3 t'7 Chatt and Shaw have also reported a similar direct procedure which involves treating rhodium trichloride with CO in boiling ethanol for 3 hr., to give a pale yellow solution; addition of tertiary phosphine or arsine (2 mol. per rhodium atom) evolves CO and affords excellent yields of complexes of the type trans - RhCl(CO)lv, . Their pale yellow solution must almost certainly contain the [Rh(CO) 2Cl 2] species. Similar procedures involving direct use of carbon monoxide for the preparation of ruthenium carbonyl complexes con-31 119 120 taining tertiary phosphines and arsines have also been reported ' ' The Rh*** reduction and carbonylation reaction were of special i n -terest since the reaction involved a true autocatalytic step. The experimental kinetic data and observations were found to f i t the rate law: -d[co]/dt = 2d[Rh*(C0)2]/dt = [Rh***] (k-Jco] + k2(_Rh*]) (5.12) which is consistent with the following mechanism: k Rh***-0H2 + CO —*-> [Rh***-C02H] + H + (5.13) [Rh***-C02H] f a s t > Rh* + C0 2 + H + ' (5.1*0 Rh* + 2C0 f a s t > Rh*(C0) 2 (5.15) T T T T ? T + Rh"\(C0)2 + mi (H20) 2Rh 1(C0) 2 + C0 2 + 2H (5.16) (chloride ligands omitted) The f i r s t step (reaction 5-13) involves a carbon monoxide "insertion" reaction and has beer: frequently postulated as the rate determining step for 1 2 1 - 1 2 U the reduction of metal ions by carbon monoxide . It has been pointed out that before the "insertion" reaction can occur the CO must coordinate to 12S the metal . Furthermore there is evidence for a number of cases that.it is the group i n i t i a l l y bonded to the metal (in this case a water ligand) which migrates onto th^ CO after c o o r d i n a t i o n ^ ^ ^ Reaction (5.13) probably involves a Cl displacement by CO to form a Rh***(CO) species, Y [ c i 5 R h m - O H 2 ] 2 " + CO — > [ c i ^ R h m - C o ] ~ + C l " ( 5 . 1 7 ) and this is followed by the migration of the HgO group: H2° 0 r l " T T T ^ 1 - r T T T ^ "i P- + [Cl^Rh -CO] > [Cl^Rh -C-OHj + H ( 5 - l 8 ) After transfer of two electrons to the metal, the intermediate I undergoes a rapid decomposition to form Rh*, COg and H + (equation 5.1*0 followed by the rapid reaction of Rh* with CO (equation 5.15). The reaction of Rh* with CO must be rapid since "uncoordinated" Rh* in HCl solution rapidly gives Rh 5 r • metal . Thus the f i r s t three steps (equations 5.13 - 5.15) give rise to the production of Rh*(C0) 2 in the i n i t i a l overall reaction: Rh***(H20) + 3C0 > Rh*(C0) 2 + C0 2 + 2H+ (5.19) It is of interest to compare the data for the reactions of a 3 M HCl solution of Rh*** with CO and H 2 > 125 k^, M sec. AH+, k'cal . AS+, e.u. CO system 25.9 13.2 Hg system 0.56 2U.6 9.0 (Chapter 3 ref. k). The data agree closely suggesting a common mechanism for the two reactions, for example an i n i t i a l rate-determining S^l dissociation of a chlororhodate species. However the reactions are both s t r i c t l y f i r s t order in gas reactant and show no significant inverse dependence on Cl . It seems most l i k e l y that the reactions involve a direct S^ 2 mechanism with replacement of Cl by CO or Hg (which f i n a l l y coordinates as H ) and that the transition states give rise to similar activation parameters. It is of interest to note that very similar parameters have been estimated for the reaction of both CO and Hg with [ c i 5 R u m ( H 2 0 ) ] 2 ~ under the same c o n d i t i o n s 3 1 , 6 , &H* Q = 2U kcal,AS* Q = 7 e.u. AH* = 23.1 kcal,ASff = 6 e.u.). rtg Equation 5«l6 represents the autocatalytic stage of the reaction. The absence of a CO dependence at higher CO pressures suggests that this rate determining step involves a bimolecular reaction between the (W^COgClg]" species and the [ c i ^ h ^ ^ H g O ) ] 2 - species. The detailed path of this reaction is thought to involve a mixed valence bridged transition state^Rh 1...X...Rh 1 1 1] (X = C l , or less l i k e l y CO), analogous to that described for the P t 1 1 catalyz-IV 127 ed substitution reactions of Pt complexes ; this involves a two equiva-lent electron exchange in which a single halide bridge seems to be clearly indicated. The reaction of [kh(HgO)C1^]2_ with pyridine to form trans -(Rhpy^Clg]+ is catalyzed by reducing agents. Rund, Basolo and P e a r s o n 1 1 3 ' 1 2 8 have similarly suggested a bridged mechanism involving a planar Rh 1 complex. • 1 2 6 A planar Rh* complex forms a bridged intermediate (through its axial position) with an inert Rh*** ion and also coordinates a sixth ligand giving, an inter-mediate such as I (equation 5.22) [Rh(H 20)Cl 5] 2- J&£££* Rh* (5.20) Rh* + k py -£^-> [ R h ( p y ) J + ( 5.2l) [Rh(py)H]+ + [Rh(H20)Cl^]2~ + H20 slow^ [H 20-Rh*(py) u-Cl-Rh***Cl^-H 2o]~ (5.22) Electron transfer then occurs with subsequent breaking of the labile Rh* ligand bonds to give the Rh*** pyridine product and regenerate the Rh* catalyst. I > trans-(Rh(py)lt(H20)Cl]2+ + Rh* (5.23) trans-[Rh(py) l t(H 20)Cl] 2 + + Cl" f a S t> trans-[Rh(py)1+C12]+ + H20 (5.24) A similar chloride bridging mechanism can be written for the present CO system: [Rh*(C0)2Cl2]~ + (Rh***(H20)Cl5]2~ + H20 [(H 20)Cl 2Rh*(CO) 2-Cl-Rh***Cl l i-H 2o] 3 ( 5 . 2 5 ) [(C0)2Rh***(H20)Cl3] + Rh* ( 5 . 2 6 ) 127 followed by, Rh 1 + 2C0 : > Rh I(C0) 2 ( 5 . 2 7 ) C l 3 ( H 2 C ) R h m ( C O ) 2 " i n s e r t i o n ^ Cl3(CO)Rh***-CO-OH + H + (5.28) The R h 1 1 1 produci after electron transfer would be Rh***(C0) 2(H 20)Cl 3; this species with both coordinated CO and HgO could undergo a rapid insertion reaction to yield the product Rh*(CO)Cl 3 (cf. reaction (5.14)); this species i t s e l f can then react as such or with a further coordinated CO with Rh*** to give a bridged intermediate again. The autocatalytic stage could alternatively take place through a CO bridged intermediate (Rh*...CO...Rh***): (CO) (CO) Rh*(CO) + Rh***(H20) > Rh*...(CO)...Rh***(0H2) (5.29) CO 8^ I > Rh* + Rh***^0H2 ( 5 . 3 0 ) followed by the rapid "insertion" again. The carbonyl bridge is thought to be much less l i k e l y than the chloride bridge particularly since the former requires substitution into a relatively substitution inert Rh*** complex; substitution of an uncoordinated CO molecule has been seen to be relatively slow (the i n i t i a l reaction (5.13)). The observed CO dependence on the second rate at low [ c o ], (see section 5 : 0 5 ) , could arise from a slower rate of formation of [Rh*(C0) 2Cl 2]". Since "uncoordinated" Rh* in HC1 rapidly gives the metal, at least one of the CO groups must complex rapidly. The rate of addition of the second CO could possibly become much slower at lower [ c o ] and thus [Rh*(CO) 2C1 2]~ formation 1 2 8 could become rate determining. The inverse chloride dependence observed for the autocatalytic rate may result from the effect of chloride ion on the mixed valence bridged inter-mediate. Rh* chloride bridged dimer complexes can be cleaved by Cl ion as well as other donors ' . e.g. [Rh(C0) 2Cl] 2 + 2C1" = 2[Rh(C0)2Cl2]" (5.31) Since the postulated bridged intermediate probably involves only one bridging ligand, cleavage by chloride would presumably occur more readily than in the dimer cases for which i t has been found. The efficiency of the reduction of the newly formed Rh*** species (equation 5 . 2 8 ) requires the presence of a co-ordinated H20 which originates as a sixth ligand coordinating axially on the [Rh*(CO)2Cl2]" species; 'replacement' of water by Cl at some stage at higher chloride concentrations would clearly inhibit such a mechanism. The activation parameters A H ^ =12.5 kcal and A S ^ = - 29.2 e.u. indicate that the more favorable reduction path through the bridged inter-mediate is due to the much lower activation energy; the entropy of activation is , as expected, much less favorable than for the 'direct' reduction. A few activation parameters for redox reactions involving bridged intermediates have been reported for some Co , Cr , and Pt systems ; the entropies of activation are generally quite large and negative and the activa-tion energies range upto about 15 kcal. The 6:1 chloro rhodate (III) species was more reactive than the 5:1 species for the reaction with CO in 3 M HCl. A similar result has been found for corresponding reactions with H , and is explained by the expected greater lability of the chloride in the hexachloro species''. It is interesting to note, that as was found for the 5:1 species, the rate constant for reduction of 129 - 1 - 1 fRh I I ICl^~P~ bv both CO and H 0 are aL7;ain very similar, ( k n n = 0.7k M sec. ku = 0 . 7 1 M"1 s e c . - 1 ) . H2 CHAPTER VI THE FORMATION OF SOME RHODIUM CARBONYL COMPLEXES IN DIMETHYLACETAMIDE SOLUTION . . Reactions of dimethylacetamide solutions of RhCl^'SHgO with CO have shown some interesting differences with the corresponding reaction-in aqueous solutions. 6 : 0 1 Stoichiometry Solutions of RhCl^'S'HgO in DMA were found to absorb CO at measure-able rates in the temperature range 30 - 8 5 ° ; the total measured uptake of CO corresponded closely to 2 moles of CO per mole of Rh**"''. In the absence of RhCl^'SHgO no CO consumption was apparent. At U0° CO was taken up in two distinct stages, the f i r s t mole of CO being taken up rapidly relative to the second mole of CO uptake, (Figure h i ) . When the reaction was carried out at higher temperature, a very rapid uptake occurred i n i t i a l l y correspond-ing to about one mole of CO per mole of Rh*** used. This was followed' by a much slower uptake involving a further mole of CO. When the reactions were carried out with Na^RhClg'^H^O, the uptake rate was much faster for both stages, than when similar concentrations of RhCl^'SHgO were used. The original red color of RhCl «3H o 0 in DMA, (X hhQ mil, £ = 3 6 0 ; ° 3 2 max ~ 530 m/t (shoulder), £'J150; (Figure 6 ) ) did not change greatly on absorbtion of the f i r s t mole of CO (>• 395 m/i, £ = 540; '/-» 520 m/t (shoulder), £ - 7 8 ) , thus suggesting the possibility of a Rh***(C0) complex in solution. After uptake of the second mole of CO, the solution became yellow {\ "° 3 6 0 , 1 ' . . . • * ' max £ - J 6 T 0 ) indicating l i k e l y formation o" the Rh* state. A yellow solid was obtained when the f i n a l second stage reaction solution was treated with a slightly acid (HCl) methanol solution containing tetraphenylarsonium chloride. The infra-red of this solid was identical to the solid obtained from the rhodium chloride - CO reactions in toueous solution (section 5 : 0 2 ) which was formulated as Ph^As[Rh(C0) 2C1 2]. A mass spectrometric analysis performed on the reaction gas atmosphere after consumption of about one mole of CO per mole of Rh*** indi-cated only slight traces of C0 2 > again suggesting l i t t l e Rh* formation in the i n i t i a l stage of the overall reaction. After uptake of the second mole of CO, mass spectrometric analysis and a simple chemical test (BaCO^ forma-tion) indicated the presence of considerably more C0 2 than at the end of the f i r s t stage ('Jfour to five times as much). Treatment of the reaction solution (0.03 M in Rh***) at the end of the f i r s t stage with 0 2 at H 0 ° for several hours did not result in any appar-ent gas uptake or evolution. A similar treatment with Hg at k0° resulted in a gas uptake (0 . 0 2 M) which is reasonably consistent with reduction of Rh*** to Rh°. Rh***(C0) + 3 H_ > Rh° + CO + 3H+ (6.1) 2 When excess maleic acid was added to the reaction solution (0 . 0 0 9 M Rh***) of the f i r s t stage and then treated with H 2 at 8 0 ° C, the uptake of Hg(0 .008 M) corresponded closely to that required for reduction of Rh*** to Rh*. The solution during the course of this reaction became a bright yellow suggesting the presence of a Rh* complex; the carbonyl group is thought to be s t i l l present 133 e.g. Rh I I ] C(CO) + H_ > RhI(CO)(MA) + 2H+ (6.'2) (ligands other than CO & MA omitted; MA = maleic acid) (see section 7:03) 6:02 Kinetics of the f i r s t stage Since the Rh**"1" does not appear to he reduced the stoichiometry of the reaction for the f i r s t stage can be represented by: Rh*** + CO > Rh***(C0) (6.3) (ligands other than CO omitted) The formation of Rh***(C0) was found to obey the second-order rate law -d[Rh***]/dt = d[Rh***(C0)l/dt = k^Rh 1 1 1] [CO] (6.4) At constant CO concentration the reaction thus exhibited pseudo-first order kinetic behavior. Values of k.^ were determined using equation (6.4) from the slopes of f i r s t order log plots such as that depicted in Figure 1+2; the stoichiometric relation K " j • V « C c o ) ] - W11}^, (6.5) was used to compute [Rh***] from the observed CO uptake. The values of k^ thus obtained are summarized in Table XXII. Addition of chloride upto 0.03 M. had essentially no effect on the value of k^. Measurements of k^ over the temperature range 30 - 5 0 ° (Table XXIII) yielded a f a i r Arrhenius plot (Figure 43) from which the following _ 0 . 8 0 D £> U O 03 •3 CM O U 1 0 0 0 3000 - 0.1+0 III, 2 0 0 0 Time, sec. Figure 1+2. Kinetic plot for the formation of Rh^^CO) in DMA ( 0 . 0 3 0 M Rh 1 1 1, 755 mm CO, 1 + 0 ° ) : ( O ) c o absorbed; ( Q ) log [Rh 1 1 1] The broken line corresponds to the uptake of 1 mole of CO per mole of Rh 1 1 1. 135 Table XXII Formation of the Rh**"'' carbonyl species in DJ4A Summary of kinetic data at 1*0° [ R h m ] x CO [co] x k l . ' 102, M mm 3 * 1 0 J , M M sec 1 . 0 0 7 5 5 5.31* 0 . 1 6 • 1 . 5 0 7 5 5 5.31* 0 . 1 7 2 . 0 0 7 5 5 5.31* 0.17 3 . 0 0 7 5 5 5.31* 0 . 1 7 3 . 0 0 7 5 5 5.31* 0 . 1 6 . 3 . 0 0 9 6 0 . 6 8 0.23 3 . 0 0 1 8 5 1.31 0 . 1 9 3 . 0 0 2 9 1 2 . 0 6 0 . 1 8 3 . 0 0 380 2 . 7 0 0 . 1 8 3 . 0 0 1*95 3 . 5 0 0 . 1 6 3 . 0 0 5 9 5 U . 2 0 0 . 1 6 3 . 0 0 6 5 6 1*.61» 0 . 1 7 3 . 0 0 6 9 5 1+.91 0 . 1 7 3 . 0 0 7 9 5 5 . 6 3 0 . 1 6 2 . 0 0 7 5 5 5.31* 0 . 1 9 a 2 . 0 0 7 5 5 5.31* 0 . l 8 b 1 . 2 0 7 5 5 5 . 3 V 0 . 1 9 ° * See Appendix II a 0 . 0 0 8 3 M" L i C l , 0 . 0 0 8 3 M LiCIO, ; b 0 . 0 1 6 6 M L i C l , C 0 . 0 3 M L i C l . Table XXIII Formation of the Rh*** carbonyl species in DMA Temperature dependence of k^ ( [Rh] = 0 . 0 3 M; [co] = 5 . 3 1 * x 1 0 ~ 3 M *) Temp., °C k^ -, M s 30 0 . 0 6 3 35 0 . 0 8 9 1*0 0 . l 6 0 Uo 0 . 1 7 0 U 5 0 . 2 6 7 50 0.31*5 50 0 . 3 5 0 * See Appendix II 138 activation parameters were determined: AH f = 1 7 . 0 ± 1 . 5 k c a l . A S ^ = - 8 . 3 ± 4 . 5 e.u. 6 : 0 3 Kinetict of the second stage The measured uptake of CO for the second stage corresponded closely to one mole of CO per mole of Rh***(CO) present; the production of a Rh* carbonyl complex is indicated. The reaction can be formulated stoichiometrically as: H20-Rh***(C0) + 2C0 > Rh*(C0) 2 + COg + 2H+ ( 6 . 6 ) The reaction is extremely slow at the conditions used and only a few runs were carried out. At constant CO pressure the reaction exhibited pseudo-first order kinetic behavior, the formation of the Rh*(C0)2 species obeying the rate-law: -d[Rh***]/dt = k2'[Rh***] ( 6 . 7 ) The reaction was essentially independent of CO pressure and thus kg' = k 2« Values of kg determined from the slopes of f i r s t order log plots (Figure 4 4 ) are given in Table XXIV. Measurements of k 2 over the temperature range 40 - 8 5 ° (Table XXV) yielded an Arrhenius plot (Figure 4 5 ) and the activation parameters AH 2* = 1 3 . 6 ± 1 . 5 kcal andAS2=r= - 1 9 . 2 ± 4 . 5 e.u. 6:04 Discussion The kinetic results and observations indicate that RhCl^'SHgO in 139 Table XXIV Formation of the Rh""" carbonyl species in DMA Summary of kinetic data Temp [R h m ] x CO [co]x It k 2 x 10 " °C 1 0 2 , M mm 1 0 3 , M - 1 sec Uo 1 . 0 0 755 5.3U 0 . 2 1 Up. 2 . 0 0 755 5 .31* 0 . 2 1 80 2 . 0 0 730 U.57 2.58 80 3 . 0 0 730 U.57 2 . 6 7 80 3 . 0 0 390 2 .32 2.12 80 3 . 0 0 730 U.57 5 . 8 7 a * See Appendix II a Na RhCl^«12H20 used in place of RhCl^rLO Table XXV Formation of the Rh* carbonyl species.in DMA Temperature dependence of k 2 ([Rh] = 0.03 M; [co] = 5.3 x 1 0 ~ 3 M *) O 4 Temp., C k 0 x 10 , sec ItO 0 . 2 1 a 6 5 1 . 2 3 80 2 . 6 7 ' 8 5 . .3 .1*3 * o Solubility of CO at 1*0 used in computing k 2 (see Appendix II). a [Rh] = 0 . 0 2 M in this case. DMA undergoes an "insertion-reduction" reaction with CO to yield [Rh^CO^Clg] as was found in aqueous solution (Chapter V); however, in DMA the kinetics are much different. In DMA at Uo° the overall reaction is s p l i t into two.distinct stages: (l) addition reaction of CO to a chloro rhodate (III) species, followed by ( 2 ) a slow "insertion" reaction to pro-vide Rh*. This result clearly indicates the necessity for coordination of the unsaturated CO molecule before "insertion" can take place; i f , as seems 1 2 6 l i k e l y , such insertion reactions are in fact ligand migration reactions then this necessity becomes obvious. The addition reaction probably is similar to the S^2 displacement postulated for the reaction in aqueous solution although the possibility of reaction through an i n i t i a l S_,l dissociation involving a coordinated H_0 ligand cannot be ruled out. The increase in rate for the reaction in DMA compared to 3 M HC1 is seen to be due to the lower AR^1; which might well result from easier displacement of the leaving ligand - l i k e l y to be a co-ordinated solvent molecule in the DMA system. Comparison of the entropy values alone suggest that the reaction would be favored in the more polar solvent. It is interesting to note that very similar activation parameters result for both CO and Hg with RhCl^RgO in DMA (see Chapter III, Table XI). Compared to the reaction in aqueous solution, no autocatalysis results in DMA since in this solvent the [_Rh*(COJ^ClJ forms very slowly compared to the Rh***(CO) species, thus the mechanism for the production of [Rh*(CO) CI ] " in DMA can be formulated as: (HgORh 1 1 1 + CO — > (H 0)Rh-Ii:t(CO) ( 6 . 8 ) k (H20)RhIi:tCO —-> Rh m(COOH) + H + ( 6 . 9 ) Rh***(COOH) f a S t> Rh1 + C02 + H+ (6.10) Rh1 + 2C0 f a s t> RhI(C0)2 (6.11) (coordinated ligands other than H^ O and CO omitted) vhere kg <§: k^ Jbo] . In the aqueous system reaction (6.9) is very fast and the subsequent autocatalytic step involving the bridged [Rh***...Cl...Rh*] intermediate occurs (section 5:07). The water molecule necessary for. the insertion reaction in DMA must come from the HgO originally coordinated to the Rh***. The obvious experiment to use anhydrous rhodium chloride in DMA, in an attempt to stop at the Rh***(C0) stage, was prevented due to the complete insolubility of the salt. The activation parameters for the decomposition of the Rh***(C0) complex are reasonable; a large negative entropy of activation is common for ionization reactions in solvents of medium polarity. The decomposition of the Rh***(C0) complex by water is strictly analogous to the decomposition of the Rh***(CgH^) complex in DMA solution postulated in the studies described,in Chapter IV (section U:05) although no direct rate data for this latter system could be estimated. CHAPTEP VII CATALYTIC PROPERTIES OF SOME Ri'ODIUM CARBONYL COMPLEXES 7 : 0 1 Introduction The carbonyl complexes resulting from the direct reaction of CO with solutions containing rhodium chloro complexes, (Chapters V and VI), were examined with respect to their activity as catalysts for the hydro-genation of maleic acid and for the reduction of ferric chloride. These studies were carried out in both aqueous and DMA media and some preliminary kinetic experiments were performed in instances where catalytic activity was apparent. 7 : 0 2 Activity of chloro carbonyl complexes in aqueous solution Maleic acid substrate When a pale yellow solution of the [RI^CCO ) C l ] ~ , 0 . 0 2 5 M in 3 M HCl, was treated with at 8 0 ° for 3 hours some Rh metal resulted and CO evolution was apparent; the solution was s t i l l slightly yellow. When this reaction was attempted in the presence of maleic acid ( 0 . 1 2 5 M) no Rh metal formed, although gas evolution ( 0 . 0 1 2 5 M) occurred over 5 hours of reaction. The final solution was a more intense yellow, (continuum below 350 m/t) which was quite different from that due to the original [Rh^CO^Clg] ~ solution or the previously found Rh* - maleic acid complex (Figure 3 ) . A similar in-tense yellow solution resulted when a solution of [Rh*(C0) Cl_l~ and maleic 1»*5 acid were shaken under an atmosphere of for several hours at 80°; in the absence of maleic acid some traces of Rh metal occurred. The [Rh I(C0) 2Cl 2]~ species thus decomposes slowly under N to give Rh metal, presumably through disproportio/iation of Rh*. Maleic acid prevents this and the results indicate that maleic acid replaces one of the CO groups to give a species [Rh*(CO)(MA)C1 ] ; as in the corresponding system without the carbonyl group no catalytic hydrogenation of the olefin seems to occur. The somewhat easier reduction of [Rh*(CO) Clg] to Rh metal in the presence of H 2 suggests that a slow H2 activation reaction such as [Rh*(C0) 2Cl 2]" + H 2 [Rh*(C0)2ClH]" + H+ + Cl" (7.1) (or [Rh*(C0) 2Cl 2) _ + H 2 =^=^ [Rh*(C0)ClgH]2" + H+ + CO ) (7.2) (cf. equation 3.6) might be occurring with subsequent decomposition of the hydride to the metal. eg. [Rh*(CO) C1H]~ => Rh° + Cl~ + 1 H + 2C0 2 Ferric chloride substrate A number of reactions were carried out using solutions of [Rh*(CO)gClg]~ and FeCl^^HgO in 3 M HC1 media. A solution containing equal concentrations (0.05 M) of [Rh*(CO)gClg]~ and FeCl3'6HgO was placed under a Ng atmosphere at 80° and agitated for 2 hours. Over this period of time con-tinuous gas evolution was observed as the i n i t i a l orange solution became red and some Rh metal formed. When this reaction was carried out on a 3 M HC1 solution which contained about 8 times as much FeCl3*6HgO as [Rh(CO.)gClg]~, 0.0094 M, gas was evolved over 3 hours of reaction. The i n i t i a l orange-yellow solution gradually became orange and no Rh metal formed. A t i t r a t i o n of the f i n a l reaction solution with KMnO^ suggested the presence of Fe**, • O .O56 M, in the f i n a l reaction solution. The visible absorption spectra of the f i n a l solution was measured and an absorption shoulder at <° 510 mfi vas observed. Rhodium chloride solutions in 3 M HCl have absorption maxima at 511 m/*> (. £ = . The concentration of R h 1 1 1 was calculated to be 0 . 0 0 9 0 M, almost equal to the concentration of the Rh* used in the reaction. These results indicate that the Rh* of the [Rh*(CO^Cl^) species is involved in electron transfer reactions with Fe*** to produce Fe** and a Rh*** species. The noncomplementary electron transfer reaction between the Rh* and Fe*** may take place by a mechanism involving a transient Rh** species, Fe*** + Rh* ^ = Fe** + Rh** (7.3) although the kinetics of this system were not studied. The alternative to invoking the unstable Rh** species would be a one step termolecular reaction: 2Fe*** + Rh* 5> 2Fe** + Rh*** , ( 7 . 5 ) but such a mechanism has rarely been postulated for reactions involving only 129 131 metal ions ' . The electron transfer could take place through a mixed valence chloride bridged intermediate as postulated for the reduction of - m - 2-2 by [Rh*(CO)gCl2] in section 5:07 or presumably by an outer sphere mechanism. The amount of Fe** produced indicates that the CO ligands also act as reductants; a 0.0091* M solution of [Rh*(C0) 2Cl 2]~ could reduce a 0.057 M Fe solution: 2H 0 Rh*(CO) + bFe111 ~> Rh 1 1 1 + 2C02 + 6Fe** + *4H + (7.6) (ligands other than CO omitted) The CO ligand.-- could effectively act as reductants by decomposition of inter-mediate Rh*** carbonyl species to Rh* by the insertion-reduction reaction described in section 5:07. These reactions of [Rh*(C0) 2Cl 2")~ with FeCl^^HgO were also inves-tigated in the presence of a: CO atmosphere. In cases where the concentration of FeCl3*6H20 was greater than the concentration of [Rh*(C0)2Cl2"] used, 0 . 0 3 8 M, the reaction proceeded initially with continuous gas evolution followed by CO uptake which eventually subsided. When FeCl3*6H20 or [Rh*(C0)2Cl2] solutions were treated separately with CO at 8 0 ° no evolution or uptake occurred. The absorption spectra of the final Rh/Fe reaction solu-tion indicated the absence of ferric chloride species and the presence of • '[Rh*(C0) C l ] ~ at its i n i t i a l concentration ( 0 . 0 3 5 M). The [Rh*(C0)2Clo]~ could be extracted by the addition of sufficient triphenylphosphine in ben-zene solution to the final solution. Rh*Cl(C0)(PPh^Jg was precipitated by the addition of ethanol to the benzene extract. The colorless aqueous phase of solution remaining on separation of the benzene extract was titrated with KMnO^ ; the result indicated the presence of Fe**.in the final reaction solu-III tion at a concentration nearly equal to the original amount of Fe used in the reaction. A few reactions were carried out using RhCl^'SHgO and FeCl^^HgO in 3 M HCl with CO at 8 0 ° . A slow uptake of CO was observed for the first half (time scale) of the reaction followed by a more rapid reaction for the second . half of the reaction (Figure k6). The total uptake in reactions using differ-ent [Rh*** ] always corresponded to 2 moles of CO for every mole of Rh*** used. 11*8 20.0 0 4000 8 0 0 0 12000 Time, sec Figure 1*6. CO uptake plot for the reduction of R h 1 1 1 in the presence of F e 1 1 1 . ( 0 . 1 0 M Rh 1 1 1, 0 . 1 0 M F e 1 1 1 , 1*50 nun CO, 8 0 ° , 3 M'HCl. Again complete reduction of Fe*** occurred.and the rhodium species [Rh*(CO) Cl"]"* finally resulted. The uptake plot (Figure 46) is initiall y non autocatalytic and suggests that the [Rh*(C0)2Cl2]~ species reacts more rapidly with Fe*** than with Rh***. In-these systems under CO atmospheres, [Rh*(C0)2Cl2]~ reduces Fe to Fe and the resulting Rh species is again converted to [Rh*(C0)2Cl2] ~ by CO. 7:03 Activity of chloro carbonyl.complexes in DMA Maleic acid substrate As already mentioned in section 6:01 the Rh***(C0) species in the presence of maleic acid in DMA absorbed H2 at a measureable rate at 80°, the total uptake of H2 corresponding to slightly less than one mole of H2 per III mole of Rh present. Since in the absence of maleic acid, the Rh (CO) was reduced to the metal by Hg, i t seems likely that a Rh* complex is formed containing at least some maleic acid. The original orange-red solution of the Rh***(C0) species (\ 395 nut, € = 540; <-> 520 mix. (shoulder), £^78), became increasingly more yellow over the course of the reaction; the final reaction solution was a very bright yellow (X' 335 n^ u, 6 ^ 1900; 405 myu, (shoulder), 6— 500). Thus the spectral evidence also seems to suggest the formation of a Rh* complex, possibly containing both CO and maleic acid since the spectrum is quite different from that observed for the Rh* - maleic acid complex described in section 3:03, and there was no evidence for CO evolution. The Rh* complex resulting from the Rh***(C0) species does not appear to be active for hydrogenation whereas the Rh* - maleic acid complexes described in section 3:,07 are quite active. l2 For the H reduction of the Rh***(C0) species in the presence of 150 maleic acid a good straight line was obtained for a plot of log fa111 (CO)] against time for a gas uptake experiment at constant Hg pressure. O.n the basis of a first order dependence on hydrogen, the rate constant k was cal-culated to be 0.122 M~* sec - 1 at 8 0 ° . eg. Rh m(C0) + H2 k > RhI(C0) + 2H+ (T.T) T f a ^ t T Rh (CO) + Maleic acid > Rh (CO)(Maleic acid) ( T . 8 ) The rate of reduction of RhCl3*3HgO in DMA by Hg in the presence of maleic acid is more than ten times faster than for the Rh***(C0) species under corresponding conditions. The coordinated CO has an inhibiting effect on the Hg reduction of Rh**"'" as well as on catalytic hydrogenation by the resulting Rh*(C0)(maleic acid) species; no hydrogenation of maleic acid seemed to occur. The Rh*(C0)g species which was prepared in DMA (section 6:0l) was mixed with maleic acid in DMA and subjected to treatment with Hg at 8 0 ° . During the first few minutes of the reaction gas was evolved and then over the next 3 hours only a slight uptake of gas was apparent. The yellow color of the i n i t i a l solution increased in intensity over this period of reaction time (increased.absorption below 350 raft,). These observations suggest at least partial replacement of CO by maleic acid during the reaction. No hydrogenation of maleic acid was observed. Decrease in activity with introduction of carbonyl groups into the 31 metal complex has been observed for some chlororuthenate (ll) complexes in their ability to catalyze hydration of acetylenes and for Rl^CltPPh^).^ i n its catalytic hydrogenation properties**. The latter has been rationalized in the following way: a strong Tj'acid such as CO will withdraw electron density from the metal and will increase the energy required for the species to activate molecular Hg in the oxidative addition step**. The following 15 step: H CI MA CI MA Rh1 / H2 ^ /Rh 1 1 1 / f7.9) C O C I CO (assumed trans) CI (cf. equation 3:23) (where MA = maleic acid) would thus be unfavorable in catalytic hydrogenation by the carbonyl species. The reason for the inhibition of the reduction of the i n i t i a l Rh*''"'1' octa-hedral complex by after substitution of a CO ligand is not too clear. Ferric chloride substrate When FeCl3«6H"20 and the Rh (CO)g snecies in DMA were treated with CO at 8 0 ° , a slight evolution was observed in i t i a l l y . No further evolution or uptake was apparent in the following hour. The orange-yellow color of the i n i t i a l reaction mixture became yellow at the end of this time. As in the aqueous system, i t seems likely that the Rh*(C0)2 species may be active in reducing Fe*** to Fe** and since CO is present, any Rh*** formed could be converted back to Rh*(C0)2. 7:04 Activity of RhCl(C0)(PPh ) g in DMA solution Since RhCl(CO)(PPh3)2 is insoluble in aqueous solution, some 30 studies on its activity were made in DMA solution. Vaska and Rhodes have studied the activity of RhCl(CO)(PPh3)2 in toluene and benzene solution for the hydrogenation of ethylene and acetylene and report that the conversion of ethylene to ethane at 6 0 ° was quite low after long periods. V/hen 0 . 0 1 M RhCl(CO) (PPh 3) 2 in DMA was treated with Hg-a*; 80 for 5 hours no appreciable uptake occurred. A very slow reaction occurred when the same solution containing excess maleic acid was treated with Hg at temp-eratures up to 8 5 ° . The corresponding iridium complex has been shown tc be 111 active for hydrogenation under similar conditions When a DMA solution of 0 . 0 2 M RhCl(CO) (FPh )2 and 0.0375 M FeCl '6^ 0 was shaken under a Ng atmosphere for 17 hours at 60° there was essentially no reaction, showing that no Fe**"'" reduction occurs via electron transfer. However treatment of a similar solution with Hg at 60° resulted in a rapid uptake of hydrogen. The number of moles of Hg consumed was equal to half the number of moles of FeCl 3 * 6 H g 0 used; thus indicating complete re-duction of Fe*** to Fe**. . The kinetics of this reaction appear to be some-what involved and are to be studied further. It seems l i k e l y that Hg activation through the following reaction: RhCl(C0)(PPh 3)g + Hg ^ = ± RhCl(C0)(PPh3)gHg (7.10) w i l l be involved. This reaction is well substantiated for the corresponding 132 iridium complex . The equilibrium must l i e well to the l e f t for the rhodium system, since solutions of the complex give no measureable Hg uptake. CHAPTER VIII GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE'WORK Solutions of RhCl^'BHgO have been found to undergo a number of interesting and significant reactions which involve activation of relatively inert gas molecules such as H^ and CO. Since the present work was commenced in 1965 there has been a considerable amount of data published on studies involving catalytic reactions with rhodium complexes generally, but kinetic studies involving rhodium trichloride have been restricted to systems involv-ing polymerization and isomerization of organic unsaturated compounds. The present studies show that a solution of RhCl^'SH^O in dimethyl-acetamide i s an effective catalyst for the homogeneous hydrogenation of a variety of substituted ethylenes as well as ethylene i t s e l f (Chapter III). A mechanism postulated involves an i n i t i a l H^ reduction of Rh**''" to Rh* which is stabilized in solution by rapid complexing with the olefin; the resulting Rh* olefin complexes subsequently react with to produce the saturated organic compound and Rh* again. Similar studies in aqueous solution show that Rh* olefin complexes are again formed but there appears to be no subsequent hydrogenation. Ethylene i t s e l f has been found to react with RhCl^'SHgO in a var-iety of ways (Chapter IV). In neutral aqueous solution at ambient tempera-tures a catalyzed ligand substitution (H^ O displacing Cl~) has been observed; at temperatures above about U 0 ° metallic rhodium precipitates. In dimethyl-acetamide at 8 0 ° ethylene reduces Rh*** to a Rh* ethylene complex, this sub-sequently acting as a dimerization catalyst for the production of butenes. The mechanism postulated for the production of the Rh* ethylene complex in -volves i n i t i a l formation of a Rh*** ethylene complex followed by decomposi-tion of this by water. Introduction of carbonyl ligands into rhodium chloride complexes has- been found to inhibit catalytic activity at least for the hydrogenation of olefinic substrates (Chapter VII). There is evidence for the production of Rh* olefin complexes containing carbonyl groups but there is no evidence for subsequent reaction with H^ . Carbonyl groups were found to be conveniently introduced into rhodium complexes by direct carbonylation of solutions of RhCl^SHgO. The kinetics of these carbonylation reactions proved most interesting. In aqueous solution (Chapter V) the anionic species [Rh*(CO^Clg] - is auto-catalytically produced; the mechanism postulated involves i n i t i a l production of some [Rh^CO^Clg] through a well established CO "insertion" reaction involving a chlororhodate ( i l l ) complex. The Rh* dicarbonyl is then involved in a two electron transfer process via a chloride bridged [Rh*...Cl...Rh*** ] intermediate to produce further Rh* more ef f i c i e n t l y than the direct insertion-reduction process. The carbon monoxide is thus being catalytically activated through coordination to rhodium for subsequent reduction of Rh*** resulting from the electron transfer process. An inorganic substrate, Fe*** has been catalytically reduced in this manner. The activation of the carbon monoxide as a Rh* carbonyl in aqueous solution is thought to be of particular interest; the studies so far have involved only reduction of Rh*** and Fe***. An easy preparation of the square planar complex trans - Rh*Cl(CO) (PPh^g resulted incidentally from these carbonylation studies; as in the case of corresponding studies with ruthenium chloride solutions i t is thought that a whole range of rhodium (I) carbonyl complexes could be prepared by addition of ligands to solutions containing the [RhfCOj^Clg] anion. Studies of direct carbonylation of RhCl^'SHgO in DMA solution have shown the importance of the presence of a water molecule for the production of the [Rh^COjgClg]~ species from an i n i t i a l l y formed Rh***(CO) complex (Chapter VI). The kinetic studies generally have given information on the sub-stitution reactions of Rh*** and Rh* complexes. Other workers have recently suggested that substitution reactions of Rh*** complexes in aqueous solution show considerably more S^ 2 character than their corresponding well studied Co*** analogues. The present studies in aqueous solution are in agreement with this. Studies of substitution reactions of Rh*** species in non-aqueous media have not previously been reported. The reaction with C^ H^ and CO show quite idfferent kinetics and there is insufficient data to draw any general conclusions. The work in this thesis has obviously demonstrated the importance of solvent effects although this was not the prime purpose of the study. Both the donor power of the solvent as a ligand, and its dielectric pro-perties are important and i t would be of considerable interest to carry out similar studies in a wide range of solvents in order to investigate their effect. 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Al:01 Rhodium salts Rhodium (III) trichloride was obtained a.s anhydrous RhCl^ and as RhCl^'SHgO from Platinum Chemicals. Na^RhClg'^HgO was obtained from K and K Laboratories. The rhodium salts were stored at a l l times over silica gel in desiccators. /i-dichlorotetraethylenedirhodiura (I), (C^H^ ^ RhgCl^, was prepared 32 according to the method of Cramer from ethylene and RhCl^ 'SH^ O in aqueous methanol solution and stored under refrigeration. A l : 0 2 Gases Purified hydrogen and nitrogen were obtained from Canadian Liquid Air Co.; the hydrogen was passed through a Deoxo catalytic purifier to remove traces of oxygen before use. Carbon monoxide, ethylene, acetylene and deuter-ium were obtained as CP. grade from Matheson Co. Al : 0 3 Substrates Inorganic Ferric salts were obtained as FeCl^*611^0 (AR grade) from Fisher Scientific, FetCiO^^'oHgO from Alfa Inorganics and anhydrous FeCl^ from K and K Laboratories. Ferrous chloride was obtained as FeCl^ 'l+HgO from British Drug House. 1 6 5 Organic A l l organic reagents were puchased as C.P. grade. Maleic, fumaric, and succinic acids (Eastman Kodak) were recrystallized from water, water-ethanol, and water respectively before use. Their purity was checked by melting point.determinations. Diethyl maleate, maleic anhydride, maleic acid • mono-amide and phenylpropiolic acid ( a l l products from K and K Laboratories) and hexene (Eastman Kodak) were used without further purification. Tetra-cyanoethylene was obtained from Eastman Kodak and was freshly sublimed before use. A1:0U Solvents N,N-dimethylacetamide (b.p. 1 6 5 - 1 6 6 ° ) was obtained from Eastman 8 2 Organic Chemicals (Kodak). Purification involved s t i r r i n g over calcium hydride under nitrogen atmosphere for ho hours, followed by d i s t i l l a t i o n under reduced pressure. The constant boiling fraction, ( 5 8 ° C —11.0 mm), was collected directly onto Linde kk molecular seive and stored under nitro-gen atmosphere at a l l times. Fisher Scientific concentrated HC1 was diluted to give stock HC1 solut ions. Stock HCIO^ solutions were obtained by dilution of reagent grade 60% perchloric acid solution obtained from Nichols Chemical Company. A l : 0 5 Other materials Lithium chloride and lithium perchlorate were AR grade obtained from A l l i e d Chemical Co. p-toluene sulfonic acid and triphenylphosphine were of C.P. grade from Eastman Kodak; the latter was freshly recrystallized from benzene/ethanol before use. Tetraphenylarsonium chloride from K and K Laboratories was C.P. grade. Potassium permanganate used was AR grade from Fisher Scientific. Titanium ( i l l ) chloride was obtained as 98.6% solid from Alfa Inorganics. A l l other chemical substances used were of reagent grade. D i s t i l l e d water was always used when water dilutions or aqueous solution preparations were necessary. APPENDIX II VAPOR PRESSURE AND SOLUBILITY DATA FOR N, N-DIMETHYLACETAMIDE A2:01 Vapor pressure of DMA The vapor pressure of DMA over the temperature range 0 - 100° C i s shown i n Figure kf. This data was obtained from the Dupont product 8 3 information b u l l e t i n on DMA A 2 : 0 2 S o l u b i l i t y of gases i n DMA The s o l u b i l i t i e s of hydrogen, ethylene, and carbon monoxide i n DMA at s p e c i f i c temperatures and pressures were estimated by the method described i n Chapter I I , section 2:03. A 2 : 0 3 Hydrogen s o l u b i l i t y i n DMA Figure kQ shows the s o l u b i l i t y of Hg from 0 - 8 0 0 mm at 8 0 ° C. The s o l u b i l i t y d i d not change appreciably over the temperature range of kO - 8 0 ° C or on the addition of 1 . 2 M L i C l (Table XXVI). The s o l u b i l i t y values used i n Table I - X of Chapter I I I are taken d i r e c t l y from Figure kQ. A2:0k Ethylene s o l u b i l i t y i n DMA Figure k9 shows the s o l u b i l i t y of ethylene i n DMA from 0 - 8 0 0 mm at 8 0 ° C; the s o l u b i l i t y was unaffected by addition of 0 . 0 5 M L i C l or LiClO^ and va r i e d l i t t l e over the temperature range 70 - 8 5 ° C (Table XXVI). ON 170 The solubility values used in Tables XII and XIII of Chapter IV ar.e taken directly from Figure 49. A2 : 0 5 Carbon monoxide solubility in DMA Figure 50 shows the solubility of carbon monoxide in DMA from 0 - 8 0 0 mm at 40° C; the solubility changed l i t t l e over the temperature range 40 - 8 0 ° C (Table XXVI). The solubility values used in Tables XXII -XXV are taken directly from Figure 5 0 . Table XXVI Effect of temperature and additives on solubility of gases in DMA Gas p' Temp., Additive Estimated Solubility gas mm °C x 1 0 3 , M H2 725 80 - 2.32 H2 7 5 0 40 - 1.85 H2 7 2 5 8 0 1.2 M Li C l 2.00 C2 H4 730 80 - 40.0 C2 H4 730 80 0.05 M LiCl 39.0 C2 H4 730 80 0.05 M LiClO^ 39.0 C2 H4 730 70 42.0 CO 704 40 - 4.96 CO 704 8 0 _ 4.57 TIT
Thesis/Dissertation
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Chemistry
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Kinetic studies on catalytic properties of rhodium complexes in solution
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