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The decarbonylation of aldehydes using ruthenium porphyrins Tarpey, Bláithín 1982

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THE DECARBONYLATI ON OF ALDEHYDES USING RUTHENIUM PORPHYRINS by BLAITHIN TARPEY B.Sc, University College, Galway, 1977 ( A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Chemi stry) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA © B l a i t h i n Tarpey, 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or pu b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. requirements for an advanced degree at the University Department of The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 i i ABSTRACT The t o p i c of t h i s t h e s i s i s the c a t a l y t i c and stoichiometric decarbonylation of aldehydes. A carbonylation r e a c t i o n , the reaction of RuTPP(n-Bu 3P) 2 with CO gas was also studied. A c a t a l y t i c decarbonylation system, using RuTPPCPPh.^ as c a t a l y s t plus added excess tri-n-butylphosphine i n acetonitrile-dichloromethane, was employed to decarbonylate several organic aldehydes. This system proved to be an e f f e c t i v e decarbonylation agent, which also exhibited some s e l e c t i v i t y . Some mechanistic studies were c a r r i e d out in an attempt to elucidate the d e t a i l s of the reaction mechanism. From these and other findings i n t h i s laboratory, a tentative mechanism was pro-posed f o r the decarbonylation reaction. A r a d i c a l mechanism, involving Ru(III) intermediates was favoured on the basis of e.s.r. data, spectral evidence and c y c l i c voltammetry studies. However, no intermediates were i s o l a t e d . The f u l l d e t a i l s of the mechanism, such as the precise nature of the postulated intermediates, have not been f u l l y determined, although there are in d i c a t i o n s that solvated species, l i k e RuTPP(CO)(CH^CN), may be important. The stoichiometric reaction between RuTPPCn-Bu-^P^ and phenylacetaldehyde was also studied. The k i n e t i c analysis indicated that the rea c t i o n , equation i , RuTPP(n-Bu 3P) 2 + C 6H 5CH 2CH0 ^ RuTPP(CO)(n-Bu 3P) + n-Bu3P + C gH 5CH 3 equation i followed a pseudo-first-order rate law in the absence of excess phosphine, however, the actual k obsd values were found to be irr e p r o d u c i b l e . Some features of t h i s reaction, such as the f a i l u r e of the reaction to go to i i i completion, and also the e f f e c t of trace oxygen on the reaction were i investigated. The carbonylation of RuTPP(n-Bu 3P) 2 by CO gas was of i n t e r e s t , with respect to the c a t a l y t i c r e a ction, which seems to involve pre-treatment with CO (or aldehyde) to form a phosphine monocarbonyl. The reaction was found to follow a simple d i s s o c i a t i v e mechanism, which i s s i m i l a r to that observed f o r analogous M(porp)L 2, where M = Ru or Fe. A K value of 0.054 at 26°C was obtained and in d i v i d u a l rate constants were calculated also. The temperature dependence of K was used to f i n d the thermodynamic parameters AS° and AH° f o r the reaction. Some electrochemical studies were performed on RuTPP(n-Bu 3P) 2 and the corresponding monocarbonyl, RuTPP(CO)(n-Bu 3P). Electrochemical oxidation of RuTPP(CO)(n-Bu 3P) yi e l d e d a i r - c a t i o n r a d i c a l , whereas bromine oxidation appeared to e f f e c t oxidation of the metal. i v TABLE OF CONTENTS ABSTRACT i 1 TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i ABBREVIATIONS x ACKNOWLEDGEMENTS x i i CHAPTER 1. INTRODUCTION 1 1.1 Decarbonylation of Aldehydes 1 1.2 Ruthenium MetalloDorohyrins 6 CHAPTER 2. EXPERIMENTAL 12 2.1 General Methods 12 2.1.1 Elemental Analysis 12 2.1.2 Gas-Liquid Chromatography . . 12 2.1.3 V i s i b l e Spectrometry 12 2.1.4 Mass Spectrometry ." 12 2.1.5 Infra-Red Spectroscopy 12 2.1.6 C y c l i c Voltammetry 12 2.1.7 Photolysis 13 2.1.8 Gases 13 2.1.9 Solvents 13 2.1.10 Materials 13 2.1.11 Nuclear Magnetic Resonance Spectroscopy 13 2.2 Decarbonylation Procedure 13 2.3 Spectrophotometry Kinetic Measurements 14 2.4 Complexes 16 V CHAPTER 3. THE REACTION OF RuTPP(n-Bu 3P) 2 AND CO 19 3.1 Spectral C h a r a c t e r i s t i c s 1 9 3.2 Treatment of Data 2 2 3.3 Discussion 30 CHAPTER 4. THE CATALYTIC DECARBONYLATION OF ALDEHYDES .36 4.1 The Use of RuOEP(CH 3CN) 2 to Decarbonylate Aldehydes . . . 36 4.2 Discussion 40 4.3 The Use of RuTPP(PPh 3) 2 to Decarbonylate Aldehydes . . . . 41 4.3.1 Other Aldehydes 47 4.3.2 Further Studies 48 4.4 Discussion 49 4.4.1 The Role of the Phosphine Liqands 50 4.4.2 The Role of A c e t o n i t r i l e 51 CHAPTER 5. THE STOICHIOMETRIC REACTION OF RuTPP(n-Bu 3P) 2 WITH PIIENYLACETALDEHYDE 53 5.1 Kinetics and Spectral C h a r a c t e r i s t i c s 53 5.2 The Bromine Oxidation of RuTPP(n-Bu 3P) 2 58 5.3 The Bromine Oxidation of RuTPP(CO)(n-Bu3P) in CH 2C1 2 62 5.4 E f f e c t of Oxygen on the Stoichiometric Decarbonylation Reaction 66 5.5 Discussion 68 CHAPTER 6. CONCLUSIONS 77 Suggestions f o r Further Studies 78 v i LIST OF TABLES Table Page 1.1 v(CO) Values i n Cm"1 f o r Various Ru(II) Porphyrins . . . . 11 111.1 Equilibrium Constants f o r the Reaction of RuTPP(n-Bu 3P) 2 with CO at D i f f e r e n t Temperatures . . . 24 1 1 1 . 2 Rate Constants f o r the Reaction of RuTPP(n-Bu 3p) 2 . . . . 28 1 1 1 . 3 Reaction of RuTPP(n-Bu 3P) 2 with CO in Toluene at 26°C 30 1 1 1 . 4 Kinetic and Equilibrium Data f o r the Reactions of Fe and Ru.Porphyrin Complexes with CO in Toluene 31 1 1 1 . 5 Log K^ Values f o r Ligand Binding to a Series of Co Porphyrins 32 1 1 1 . 6 Comparison of Axial Ligand L a b i l i t i e s of Iron and Ruthenium Phthalocyanine Adducts 34 IV. 1 The Decarbonylation of Aldehydes Using a RuTPP(PPh 3) 2/n-Bu 3P System 45 V. l k obsd Values Obtained f o r Various Ru(II) Concen-t r a t i o n s at 3.42xlO"2M Aldehyde .56 vi i LIST OF FIGURES Figure Page II. 1 Evacuable C e l l f o r Optical Density Measurements 15 111 -1 Spectral Changes f o r the Reaction of RuTPP(n-Bu 3P) 2 in Toluene at 26°C with No Added Phosphine 20 I I I . 2 The Reaction of RuTPP(n-Bu 3P) 2 with 1 atm of CO in Toluene at 1.67xl0" 4M Phosphine 21 111.3 F i r s t Order Plot f o r the Reaction of RuTPP(n-Bu 3P) 2 with CO in Toluene at 31 °C 23 1 1 1 . 4 Log A -A /A -A Versus [n-Bu-P] at Constant CO o e e ° ° 3 Pressure 25 1 1 1 . 5 Van't Hoff Plot f o r the Reaction of.RuTPP(n-Bu 3P) 2 i n Toluene 26 1 1 1 . 6 A Plot of k obsd" 1 Versus [C0]/[L] f o r the Reaction of RuTPP(C0)(n-Bu 3P) and n-Bu3P at 26°C 29 1 1 1 . 7 Possible Structure f o r the Five-coordinate Inter-mediate 33 IV.1 The Reaction of Ru0EP(CH 3CN) 2 with Phenylacetaldehyde at Room Temperature in Benzene. (1) Ru0EP(CH 3CN) 2, (2) RuOEP(CO)(CH3CN) 37 IV.2 The Reaction of RuOEP(CH 3CN) 2 with Phenylacetaldehyde at 26°C in Benzene 39 IV.3 RuTPP(PPh 3) 2 in CH 2C1 2 Solution 42 IV.4 The GLC Trace of the Products of the Reaction of RuTPP(PPh,) 9 with Phenyl acetaldehyde 45 v i i i IV. 5 Spectral Changes f o r the C a t a l y t i c Decarbonylation of Phenylacetaldehyde Using RuTPP(PPh 3) 2/n-Bu 3P as Catalyst. (1) Reaction Mixture A f t e r 15 Min. (2) Final Spectrum 46 V. l The Reaction of RuTPP(n-Bu 3P) 2 with Phenylacetaldehyde i n CH 2C1 2 at 26°C, (A) and i n Toluene, (B) 54 V.2 F i r s t Order Plot f o r Reaction of RuTPP(n-Bu 3P) 2 with 3.42xlO"2M Phenylacetaldehyde at 26°C 55 V.3 Reaction of RuTPP(n-Bu 3P) 2 with Phenylacetaldehyde Showing Secondary Reaction (2). (3) i s the Spectrum Obtained A f t e r Addition of n-Bu3P to (2) 59 V.4 A Plot of k obsd Versus yL of Phenylacetaldehyde . . . . 60 V.5 Spectral Changes f or the Bromine Oxidation of RuTPP(n-Bu 3P) 2 in CH 2C1 2 (1). The Final Spectrum (2) 61 V.6 Spectral Changes f o r the Bromine Oxidation of RuTPP(C0)(n-Bu 3P) i n CH 2C1 2 63 V.7 C y c l i c Voltammetry of RuTPP(CO)(n-Bu3P) in Toluene-A c e t o n i t r i l e with 0.1M E l e c t r o l y t e 64 V.8 Product of Electrochemical Oxidation of RuTPP(C0)(n-Bu 3P) (1). Aft e r Reduction with TBAB (2) 65 V.9 The Reaction of RuTPP(n-Bu 3P) 2 with 3.42xlO"2M Phenylacetaldehyde. (1) I n i t i a l Spectrum. (2) 10% Reaction a f t e r 30 Min, No Oxygen Admitted. (3) Final Spectrum, 60 Min a f t e r Addition.of Oxygen 67 V.10 Spectra of Two Ru(III) Porphyrin Species 69 ix V.II Spectra of Two ir-Cation Radicals Produced by (1) Bromine and (2) Electrochemical Oxidation of Ru0EP(C0)(py) 70 V.12 Spectra of ir-Cation Radicals 72 V.13 The Reaction of RuTPP(n-Bu 3P) 2 with Phenylace-taldehyde to Give a Ru(III) Species 73 V.14 A P l o t of p0 2 Versus k obsd f o r the Reaction of RuTPP(n-Bu,P) 9 with Phenylacetaldehyde 75 X Abbreviations Atm atmosphere A t absorption at any time, t A g absorption at the equilibrium position A Q absorption of the reactant A^ absorption of the product CH^CN a c e t o n i t r i l e °C degrees centigrade cm centimetres CO carbon monoxide DMF N,N-dimethylformamide AH enthalpy change f o r the reaction °K degrees Kelvin K equilibrium constant k-| k i n e t i c rate constant k_i k i n e t i c rate constant k 2 k i n e t i c rate constant k_2 k i n e t i c rate constant k^ o v e r a l l rate constant f o r the forward reaction k o v e r a l l rate constant f o r the reverse reaction r k obsd pseudo-first-order rate constant M molarity M(porp)L five-coordinate metal porphyrin complex M(porp)L 2 six-coordinate metal porphyrin complex M(porp)(C0)L carbonylated six-coordinate metal porphyrin complex M(porp)LL' mixed axi a l ligand six-coordinate metal porphyrin complex Mp mesoporphyrin IX dianion minute mi 1 1 i l i t r e mass spectrometry m i c r o l i t r e nitrogen tri-n-butylphosphine oxygen octaethylporphyrin dianion octamethyltetrabenzoporphyrin dianion phthalocyanine dianion protoporphyrin IX dianion pyridine tri-ruthenium dodecacarbonyl ruthenium(II) reconstituted myoglobin ruthenium(III) reconstituted myoglobin entropy change f o r the reaction second temperature time tetrabutyl ammonium boranate tetrahydrofuran tetraphenylporphyrin dianion molar e x t i n c t i o n c o e f f i c i e n t frequency Acknowledgements I would l i k e to thank Drs. D. Dolphin and B.R. James f o r t h e i r supervision during the past few years, and also f o r t h e i r assistance in the production of t h i s t h e s i s . I also wish to thank M. Barley f o r his help i n carrying out some experiments and f o r useful discussion. Thanks also to a l l my friends f o r assistance i n proofreading and typing. 1 CHAPTER 1 INTRODUCTION 1.1 Decarboxylation of Aldehydes The purpose of t h i s chapter i s to review the relevant l i t e r a t u r e on t r a n s i t i o n metal catalyzed decarbonylation reactions and some ruthe-nium porphyrin chemistry, e s p e c i a l l y that of ruthenium porphyrin phosphine systems. The decarbonylations of aldehydes, acyl halides, aroyl halides 1 -3 and ketones are useful and important reactions in organic chemistry. T r a n s i t i o n metal complexes have been investigated f o r use as s t o i c h i o -metric and c a t a l y t i c decarbonylation agents, some of which, notably / \ 4-9 RhCl(PPh^)3> have been used f o r homogeneous decarbonylation. The RhCl(PPh^)3 complex acts as a stoichiometric homogeneous decarbonylation agent under mild conditions; the reactions with aldehydes, a c y l , and aroyl halides are summarized by equation I . l . ^ " ^ ' 1 ^ RCOX + RhCl(PPh 3) 3 >- RX + trans-RhCl (CO) ( P P h 3 ) 2 + PPh 3 (X = H,C1) equation 1.1 In the case of long chain a l i p h a t i c aldehydes some o l e f i n i s also produced, see equation 1.2 RhCl(PPh 3) 3 + RCH2CH2CH0 »- RhCl (CO) ( P P h 3 ) 2 + RCH 2CH 3 or H 2 + RCH. = CH 2 equation 1.2 If a beta hydrogen i s present s i m i l a r reactions take place with acyl 5 halides, i n which case the only product of decarbonylation i s o l e f i n . The mechanism for decarbonylation of acid chlorides,jproposed by 4 5 7 11-14 Baird, Nyman and Wilkinson, is shown in scheme 1.1. ' ' This mechanism has been supported by some kinetic data, and isolation of two intermediates, 2 and 3, in the case of R = aryl. The active intermediate is thought to be 1, which is formed as RhCHPPhg)^ dissociates in solu-tion.^ This species is also thought to be important in the catalytic 15 16 hydrogenation of olefins. ' CO PKjP Rh PPh 4. RCOCI CI RhCl(PPh 3) RCO CO CI Rh' CI P P h , PPh„ (5) - C O Ph3P — R h — C l -|- RCOCI PPh, CO (1) (4) Ph 3P—Rh—PPh, - f RCI CI RCO Rh (2)c CO Rh Cl (3) PPh, PPh, PPh, PPn* Cl Scheme 1.1. The Decarbonylation of Acyl Halides by RhCl (PPh-,) 3 The f i r s t step i n the reaction i s thought to be an oxidative addi-tion to 1, to y i e l d the five-coordinate acyl complex 2, which undergoes f a c i l e rearrangement to 3. The l a s t step i s a reductive elimination to y i e l d 4, and alk y l halide. This reaction, however, cannot be made c a t a l y t i c at useful temper-atures since the carbonyl complex formed i s stable and does not lose CO even at high temperatures. An unsuccessful attempt was made to reverse the decarbonylation of RhCl(CO)(PPh^)^ (c.f. equation. 1),-in molten PPh 3-•j o Another approach was to regenerate the active complex by photolysis, since many metal carbonyls d i s s o c i a t e CO in t h i s way. However, photol-y s i s of the carbonyl complex r e s u l t s i n oxidation of the ligands, CO and PPh 3 to C0 2 and P(Ph) 30, r e s p e c t i v e l y , but the photoproduct obtained may 18 be converted to RhCl(PPh 3) 3. The photoproduct was not i s o l a t e d but was thought to be an oxygen adduct, formed as a r e s u l t of trace oxygen present during photolysis. The decarbonylation of acid chlorides and aldehydes can be driven c a t a l y t i c a l l y at very high temperatures. Ohno and T s u j i , among others, have used RhCl(CO)(PPh 3) 2 to c a t a l y t i c a l l y decarbonylate several aroyl 1 4 5 19 halides. ' ' ' The f i r s t step i s not the i r r e v e r s i b l e oxidative addi-t i o n to form 2, but the formation of complex 5 (scheme 1.1) which loses CO on heating to give the five-coordinate acyl complex. Although the c a t a l y t i c decarbonylation using RhCl(CO)(PPh 3) 2 works smoothly f o r aryl 5 compounds, giving up to 76% y i e l d f o r cinnamaldehyde, i t was found that undesirable side reactions occurred when a l i p h a t i c aldehydes were used. Most of the mechanistic work has been performed on acid halides, since no intermediates had been i s o l a t e d f o r the corresponding aldehyde 20 reactions. Recently, Suggs has used a cyclometallation reaction, to i s o l a t e the generally unstable intermediate complexes thought to be 4 present i n the decarbonylation of aldehydes. The acyl hydride, 6 , obtained i s the f i r s t such intermediate found and provides support f o r the mechanism discussed f o r decarbonylation of acyl halides. Rauchfuss has studied acyl complexes of metals, in view of t h e i r 21 possible intermediacy in decarbonylation and other such reactions. An unusual ruthenium complex, 7, which undergoes f a c i l e intramolecular decarbonylation, led him to postulate a mechanism involving a n (n-bonded) carbonyl, instead of the usual oxidative addition pathway. The r a t i o n a l e f o r t h i s suggestion was based on the observation that ruthenium(IV) i s 22 not usually a r e a d i l y attainable oxidation state. Other metal compounds have been employed as decarbonylation c a t a l y s t s ; 1 P d C l ? has been used to decarbonylate substituted O PPhj 6 Ru OC \ C l 7 P 5 23 cinnamaldehydes i n high y i e l d . Another palladium-based system using 24 palladium metal c a t a l y t i c - a l l y converts myrtenal to apopinene. The ruthenium complex, { ^ C l ^ ^ H ^ C C g H ^ P l g l C l has been studied as + CO a stoichiometric decarbonylation agent. 1 However the major product in these cases i s often o l e f i n , and the hydrogen released reduces aldehyde to alcohol. Recently a series of rhodium complexes have been shown to be use-f u l decarbonylating c a t a l y s t s , under r e l a t i v e l y mild conditions, at pr pc 150°C. ' The c a t a l y s t s e x h i b i t long term s t a b i l i t y , high turnover numbers, and some s e l e c t i v i t y . Mechanistic studies to date indicate that the oxidative addition step i s the rate determining step. How-ever, there i s no d i r e c t evidence f o r Rh(III) intermediates. The other steps are the usual a c y l - a l k y l migration step, which obviously requires some rearrangement and opening of the chelate ligand and reductive elimination, see diagram on p. 6, although the f u l l mechanistic d e t a i l s have not been elucidated as yet. The temperature at which the c a t a l y s t operates i s very important in decarbonylations, since at higher temperatures there are many side reactions which may occur; many aldehydes are known to undergo extensive thermal decomposition. A c a t a l y s t should also show some long term s t a b i l i t y , measured by the t o t a l number of turnover numbers achieved. 6 + 1.2 Ruthenium Metalloporphyrins Until recently, i n t e r e s t i n metalloporphyrin chemistry was based mainly on the b i o l o g i c a l s i g n i f i c a n c e of metalloporphyrins as the active s i t e s in n a t u r a l l y occurring oxygen and electron transport systems, p a r t i c u l a r l y those containing i r o n . However, recent studies have shown that, in addition to being good models f o r molecules such as haemoglobin 27 2! and myoglobin,metal!oporphyrins can also e x h i b i t c a t a l y t i c a c t i v i t y . ' 27 Several metal!oporphyrins catalyze oxidations using oxygen, while some cobalt porphyrins have recently been shown to e l e c t r o c a t a l y t i c a l l y reduce oxygen to water. ' A Ru(II) porphyrin system has also been 30 found to c a t a l y t i c a l l y decarbonylate aldehydes. Furthermore, although simple Fe(II) porphyrin systems are not stable to autoxidation, i t was anticipated that the Ru(II) analogues, being more s u b s t i t u t i o n - i n e r t , would not oxidize so r e a d i l y and might thereby f a c i l i t a t e the study of oxygen a c t i v a t i o n . In the early seventies the f i r s t reports appeared on the synthesis 31 and c h a r a c t e r i z a t i o n of some Ru(II) porphyrins. The general synthetic 7 route, using Ru 3(C0)-| 2, and the free base of the porphyrin, leads to 32 the formation of a monocarbonyl. R u 3 ( C 0 ) 1 2 + H2P EtOH RuP(CO)(EtOH) (P = OEP, TPP, MpIX) equation 1.3 The enhanced l a b i l i t y of axi a l ligands in metalloporphyrin com-plexes, compared to re l a t e d octahedral non-porphyrin systems, i s demon-strated by the extensive chemistry of these b i s - l i g a n d species, scheme 33 1.2. Unlike the corresponding Fe(II) systems the Ru(porp)l_ 2 species are generally a i r - s t a b l e i n the s o l i d state, although oxidation to Ru(III) can occur in solu t i o n . Following the reported r e a c t i v i t y of RuP(N 2)L RUP(0 2)L ^ Ruinp R u P L -e RUP(CO)L Scheme 1.2. The Chemistry of RuPL 2 Species Fe(porp)l_ 2 with oxygen at low temperatures, to give the oxygen adduct, 3 4 studies were i n i t i a t e d in t h i s laboratory on the r e a c t i v i t y of Ru(porp)l_2 with at both sub-zero and ambient temperatures. It was discovered that RuOEP(CH 3CN) 2 in DMF or pyrrole s o l u t i o n , absorbed 1 mole of 0 2 per 35 ruthenium atom, at ambient temperatures. In addition to binding 0 2 r e v e r s i b l y the complex also bound CO to form the monocarbonyl. Some preliminary k i n e t i c data f o r the reaction with 0 2 was analyzed d i s s o c i a t i v e mec shown in equation 1.4 for a echanism, l i k e a corresponding Fe(II) s y s t e m 3 ^ 5 - 3 8 as -L +X M(porp)L 2 ^ + [ _ " M(porp)L ^ _ x ^  M(porp) (X) (L) (X = o 2 , c o ) equation 1.4 The oxygen adduct, Ru(porp)(0 2)(L), was not i s o l a t e d , as oxida-35 ti o n to Ru(III) occurred during the work-up procedure. Thus the nature of the Ru0 ? bond remains uncertain. I n i t i a l l y a Ru(III)-0 2~ superoxide formulation was favoured, as polar aprotic solvents, which would tend to s t a b i l i z e such a charge-separated species, enhanced 39 oxygenation. The absence of an e.s.r. signal also indicated Ru(III) 39 formation. Other evidence suggested a peroxoruthenium(IV) system, although Ru(IV) i s a rare oxidation state. Following the success in reacting 0 2 with Ru0EP(CH 3CN) 2 and RuMpIX(DMF)2, i t was anticipated that reconstituted myoglobin, RuMb, might bind 0 2. However, treatment of RuMb with 0 2 in phosphate buffer at 0°C gave only i r r e v e r s i b l e oxidation to RuMb"1". The mechanism involved i s thought to be an outer sphere oxidation of a six-coordinate moiety, within the protein. 9 An important feature of some Ru(porp)L 2 complexes, i n i t i a l l y 31a noted by Chow and Cohen, was t h e i r a b i l i t y to abstract CO from organic substrates. These workers found that RuTPP(aniline) 2 in re f l u x i n g a c e t i c anhydride formed RuTPP(C0)(aniline). Subsequently, during the preparation of RuMpIX(DMF)2, which involved prolonged photolysis of RuMpIX(CO)(EtOH) in DMF, c a t a l y t i c decarbonylation of the solvent o c c u r r e d . ^ Ru(porp)(DMF) 2 >- Ru(porp)(CO)DMF + HNMe2 hv,DMF Other Ru(porp)L 2 species exhibited strong a f f i n i t i e s f o r CO. Kinetic studies on Ru0EP(CH 3CN) 2 in toluene at 30°C, yi e l d e d an equilibrium con-stant K, of ca. 10 4, f o r the reaction with CO to give RuOEP(CO)(CH 3CN). 4 1 These observations led us to attempt development of a photocatalytic system using Ru(porp)l_ 2 for the decarbonylation of aldehydes. I n i t i a l l y , the stoichiometric reaction of Ru0EP(CH 3CN) 2 with phenylacetaldehyde was investigated. The CO abstraction was rapid, but attempts to make the reaction photocatalytic were unsuccessful, as small excesses of a c e t o n i t r i l e , which seemed to be essential f o r the photochemical loss of CO from the ruthenium carbonyl, i n h i b i t e d the reaction. Another approach to a c a t a l y t i c decarbonylation, i s to somehow l a b i l i z e the CO of the metal carbonyl adduct, as the main problem in the c a t a l y t i c cycle i s the removal of CO from the adduct formed, e.g. as in the RhCl(PPh^)^ system (see above). Until recently the carbonyl in Ru(porp)(CO)(L) has been considered r e l a t i v e l y non-labile, although photolysis i n the presence of excess ligand, L, was known to regenerate the Ru(porp)l_2 species. However, some phosphine ligands have been 10 shown to thermally displace CO from Ru(porp)(CO)(EtOH), leading to 43 Ru(porp)(phosphine^ complexes. ' The ligands tri-n-butylphosphine and PPhgCHgPPr^j f o r example, r e a d i l y displace CO from both the OEP and TPP complexes, whereas triphenylphosphine w i l l displace CO from TPP complexes only, and that requires prolonged heating with a large excess 44 of phosphine. " Tri-n-butylphosphine, in contrast, r a p i d l y forms the bis phosphine complex using a small excess, at ambient temperatures. The porphyrin phosphine systems were reactive towards several small gas molecules, such as O2, CO, SO2, N^. and were also i n v e s t i -30 gated as potential decarbonylation c a t a l y s t s . R u T P P ^ P h ^ * which diss o c i a t e s a phosphine ligand r a p i d l y in s o l u t i o n , c a t a l y t i c a l l y decarbonylated phenylacetaldehyde in CH2C12 at ambient temperatures, but the system was i n e f f i c i e n t , and yielded low turnover numbers. The objective of the work was to design a complex which would ( i ) abstract CO from aldehydes and ( i i ) form a carbonyl adduct con-taini n g a l a b i l e CO. Tri-n-butylphosphine not only displaces CO r e a d i l y from Ru(porp)(CO)(EtOH) but a l s o , being a good iT-acceptor, l a b i l i z e s a trans CO, Chapter 3. This increased l a b i l i t y i s possibly r e f l e c t e d in the vCO values f o r the monocarbonyl complexes, see Table 1.1. RuTPP(CO)(n-BugP) has a CO frequency s i g n i f i c a n t l y higher than the triphenylphosphine analogue, implying less back-bonding in the butyl system. Based on such f a c t o r s , a small excess of tri-n-butylphosphine was added to a solution of RuTPP^Ph^^ in dichloromethane and aceto-n i t r i l e i n an attempt to f i n d an e f f e c t i v e c a t a l y t i c decarbonylation system. It has been reported previously that n i t r i l e solvents have a s t a b i l i z i n g e f f e c t on intermediate species in some rhodium catalyzed decarbonylations. 1 This system, RuTPP(PPh 3) 2/tri-n-butylphosphine, in CH9C19/CH^CN, was found to c a t a l y t i c a l l y decarbonylate several n Table 1.1. v(CO) Values in cm-1 f o r Various Ru(II) Porphyrins'^ COMPOUND v(C0) SOLID 0 v(C0) SOLUTION Ru0EP(C0)(Et0H) 1928 1928 a Ru0EP(C0)(py) 1939 Ru0EP(C0)(PPh 3) 1954 1953 9 Ru0EP(C0) 2 1990 RuTPP(C0)(py) 1943 RuTPP(CO) [(p-MeOPh) 3P] 1950 RuTPP(C0)(PPh3),, 1956 RuTPP(C0) 2 2005 RuTPP(C0)(n-Bu 3P) 1980 b Ru0EP(C0)(Cy 3P) 1935 a a I n CHC1 I l n CC1 4? So l i d deposited on NaCl plates. aldehydes at ambient temperatures. Details of t h i s c a t a l y t i c system are discussed in Chapter 4. The reactions of RuTPP(n-Bu 3P) 2 with CO. gas (Chapter 3) and phenylacetaldehyde (Chapter 5) have been studied with a view to elucid a -tion of the mechanism of decarbonylation. Kinetic parameters have been obtained f o r the reaction with CO. 12 CHAPTER 2  EXPERIMENTAL 2.1 General Methods 2.1.1 Elemental Analysis Microanalyses were c a r r i e d out by Mr. P. Borda of t h i s department. 2.1.2 Gas-Liquid Chromatography Gas l i q u i d chromatography was performed on a Hewlett-Packard 5830A or a Carle 113 instrument, using 0V101 and 0V17 columns. 2.1.3 V i s i b l e Spectrometry V i s i b l e spectra were recorded on a Cary (Model 17) spectrometer, f i t t e d with a thermostated c e l l compartment f o r k i n e t i c studies. Quartz c e l l s of path length 1 cm were used. 2.1.4 Mass Spectrometry Mass spectra were recorded on a Varian MAT CH or a KRATOS/AEI MS-902. GLC mass spectrometry was c a r r i e d out on a VG Micromass 12 instrument. 2.1.5 Infra-Red Spectroscopy A l l i n f r a - r e d spectra were recorded on a Perkin-Elmer 457 grating spectrophotometer c a l i b r a t e d with polystyrene. 2.1.6 C y c l i c Voltammetry C y c l i c voltammetry was performed in a H c e l l and potentials were measured at a Pt electrode against a Ag/AgCl reference electrode. 13 2.1.7 Photolysis Photolyses were performed using a 600 D Bl-PIN tungsten halogen lamp. 2.1.8 Gases P u r i f i e d argon was supplied by Canadian Liquid A i r Ltd. and used without further p u r i f i c a t i o n . C P . grade carbon monoxide was obtained from Union Carbide. 2.1.9 Solvents A l l solvents were d i s t i l l e d under an i n e r t atmosphere, and handled 45 under argon using Schlenk techniques. Dichloromethane (spectral grade) was d i s t i l l e d from Cah^, as was a c e t o n i t r i l e (Burdick and Johnson, d i s -t i l l e d i n gl a s s ) . Toluene (Fisher S c i e n t i f i c Co., spectral grade) was d i s t i l l e d from sodium-potassium amalgam. Benzene (Eastman Kodak, spectral grade) was d i s t i l l e d from sodium benzophenone k e t y l . 2.1.10 Materials A l l aldehydes were repeatedly d i s t i l l e d , usually under vacuum. Tri-n-butylphosphine was also d i s t i l l e d several times under vacuum. Both aldehydes and phosphine were stored under argon. 2.1.11 Nuclear Magnetic Resonance Spectroscopy Proton n.m.r. spectra were recorded at 270 MHz with a U.B.C,N.M.R. centre modified Nicolet-Qxford 11-270 spectrometer. Tetramethylsilane was used as internal standard.-2.2 Decarbonylation Procedure A few milligrams of R u T P P ^ P h ^ were dissolved i n CH^C^, under argon, and to th i s were added 10-25 mL of CH^CN, making up a solution of 14 -4 ca. 10 molarity. A small amount of CO was blown over the solution forming RuTPP(CO)(PPh 3), characterized spectrophotometrically (addition of excess substrate at t h i s stage also generates the carbonyl complex). The solution was s t i r r e d continuously under argon, and samples were with-drawn using syringes and analyzed by GLC. Variations of t h i s experimental procedure were also t r i e d in an attempt to study the reaction more sys-tematically, since r e a c t i v i t y tended to be ir r e p r o d u c i b l e . Some solutions were degassed p r i o r to addition of aldehyde, and CO was not admitted to the system; in these cases the aldehyde served as a source of CO, to produce RuTPP(CO)(PPh 3). As usual 5-10 yL of tri-n-butylphosphine were added and argon was blown through the s o l u t i o n . Products were c o l l e c t e d in a cold trap, using a hexane-1iquid nitrogen slush. Unfortunately t h i s type of procedure was not convenient with v o l a t i l e solvents, such as CH3CN, and in some cases b e n z o n i t r i l e was substituted. 2.3 Spectrophotometry Kinetic Measurements Due to the s e n s i t i v i t y of c e r t a i n materials (solutions of the porphyrin complexes, aldehyde substrates etc.) used, and also the nature of the experiment, a l l o p t i c a l density measurements were c a r r i e d out in an evacuable c e l l , F i g. II.1. The c e l l was maintained at constant temper-ature by placing i t in a thermostated c e l l compartment, which was attached to a Tanson c i r c u l a t i n g bath. In a t y p i c a l run using CO gas, a solution of the ruthenium complex was placed in the bulb (B) and degassed by three freeze-pump-thaw cycles. The c e l l was placed in the thermostated compartment f o r approximately 30 min to e q u i l i b r a t e in vacuo. Af t e r the i n i t i a l spectrum was recorded, a known pressure of gas was admitted to the c e l l , which was shaken vigor-ously to ensure complete mixing of gas in s o l u t i o n . The corresponding \ Quartz cell Figure II.1. Evacuable C e l l f o r Optical Density Measurements 16 carbonylation reaction was then monitored to completion. Since the concentrations of the complexes used were low (of the order of 2-8xlO~^M) -3 -1 and the s o l u b i l i t y of carbon monoxide i s approximately 6x10 M atm , the gas concentrations i n s o l u t i o n , and the p a r t i a l pressure of the gas above the solution were assumed to remain constant throughout any one experi-ment. The pressure of the gas was measured using a mercury manometer. 2.4 Complexes Carbonyl(ethanol)octaethylporphyri natorutheni um(11) Ru0EP(C0)(Et0H) was prepared by a method s i m i l a r to that of Tsutsui et a l . j ^ ^ a s described in a previous thesis from t h i s l a b o r a t o r y . 4 1  Analysis: calculated f o r C 3 GH 5 C )N 40 2Ru: C, 66.19%; H, 7.07%; N, 7.92%. Found: C, 66.13%; H, 7.18%; N, 7.86%. V i s i b l e Spectrum: \ m a x in nm: 549, 518, 395, 375 (sh). log e : 4.35, 4.12, 5.25. Bi s(acetoni tri 1e)octaethylporphyri natorutheni um(11) RuOEP(CH 3CN) 2 was prepared by a general method described by Whitten et a l . 4 3 Analysis: calculated f o r C^H^NgRu: C, 67.13%; H, 6.99%; N, 11.75%. Found: C, 67.50%; H, 6.94%; N, 11.63%. V i s i b l e Spectrum: \ m x - n m . ^ m ^ m ^ 3 U ( s h ) > 3 8 5 ( s h ) > ] o g £ . 4.23, 4.02, 5.34. Bis(tri-n-butylphosphine)tetraphenylporphyrinatoruthenium(lI) RuTPP(n-Bu 3P) 2 was prepared by adding an excess of t r i - n - b u t y l -phosphine ( 3 x l 0 ~ 2 moles) to a 10"4M solution of RuTPP(PPh 3) 2 in CH2Cl2-Me0H (4:1, v/v) at room temperature. The dichloromethane was removed slowly using N 2 and the complex c r y s t a l l i z e d out of MeOH. The c r y s t a l s were 17 washed with cold hexane and dried in vacuo. Analysis: calculated f o r C g g H g ^ P o R u : C, 73.02%; H, 7.39%; N, 5.03%. Found: C, 73.00%; H , 7.25%; N , 4.84%. V i s i b l e Spectrum: X m a x in nm: 560, 527, 437. log e: 4.01, 4.17, 5.30. Bi s ( t r i phenylphosphi ne)tetraphenylporphyri natoruthenium(11) / 44 45 RuTPP(PPh 3) 2 was prepared by a method described previously. ' Analysis: calculated f or CggH^N^Ru: C, 77.59%; H, 4.72%; N, 4.52%. Found: C, 77.22%; H, 4.78%; N , 4.36%. V i s i b l e Spectrum: X m a x in nm: 550, 515.7, 435, 413.3. log e : 3.38, 3.74, 5.02, 4.41. Carbonyl(tri-n-butylphosphine)tetraphenylporphyrinatoruthenium( II) RuTPP(CO)(n-Bu^P) was prepared by bubbling CO through a solution of RuTPP(n-Bu 3P) 2 (10"4M) in ca. 25 mL CH 2Cl 2-Me0H (5:1, v/v). The product was c r y s t a l l i z e d out of cooled methanolic so l u t i o n a f t e r removal of CHgCl2 by vaporization. The product was washed with cold methanol and dried in vacuo. Analysis: calculated f or C 5 y H 5 5N 40PRu: C, 72.65%; H , 5.87%; N , 5.96%. Found: C, 68.46%; H, 5.60%; N , 5.39%. V i s i b l e Spectrum: X in nm: 581, 542, 420. log e: 3.96, 4.17, 5.50. Mass Spectrometry: m/e values: 916 (M-C0) +, 714 (RuTPP) +. Both the mass spectrum and v i s i b l e spectrum obtained indicate that the compound i s indeed as formulated. However, the analyses indicate . that some impurity may be present in the c r y s t a l s . At f i r s t i t was suspected that some solvent, probably methanol, had c r y s t a l l i z e d with the s o l i d complex. The i . r . and H^ n.m.r. spectra showed no peaks due to free methanol or dichloromethane. 18 The pattern of peaks in the n.m.r. spectrum suggests that two phosphine complexes are present i n so l u t i o n . The main set of peaks at +0.30 <5 (P-(CH 2) 2CH 2CH 3), -1.45 6 (P-CHgCHgCHgCHg) and -2.70 S (P-CH 2(CH 2) 2CH 3) are assigned to the RuTPP(CO)(n-Bu 3P) complex by 44 comparison with the spectrum of RuOEP(CO)(n-Bu 3P). Another set of peaks occur at +0.36 <5, -1.14 6 and -2.28 5. These are assigned to the bis phosphine complex, RuTPP(n-Bu 3P) 2. This was confirmed by comparison with the n.m.r. of an authentic sample of RuTPP(n-Bu 3P) 2. The s t a r t i n g material, RuTPP(n-Bu 3P) 2 i s not l i k e l y to be the impurity giving r i s e to the poor a n a l y s i s , since the percentage carbon of the two compounds i s . so close, see above. The impurity i s probably some excess n-Bu3P which, in concentrated solutions of the complexes, would e a s i l y displace CO from RuTPP(CO)(n-Bu3P) forming RuTPP(n-Bu 3p) 2. 19 CHAPTER 3 THE REACTION OF RuTPP(n-Bu 3P) 2 AND CO 3.1 Spectral C h a r a c t e r i s t i c s Toluene was used as a solvent to study t h i s r e action, f o r the sake of comparison with previous studies. On addition of about one atmosphere of CO to a solution of RuTPP(n-Bu 3P) 2, a series of spectral changes was observed as a function of time. There were several clean i s o s b e s t i c points and the reaction appeared to go to completion, forming RuTPP(CO)(n-Bu 3P), judging by the spectral changes (see Figure I I I . l ) . To e s t a b l i s h pseudo-first-order conditions with respect to phosphine concentrations, small amounts of tri-n-butylphosphine were added to the solutions p r i o r to reaction with CO. In order to follow the rate of reaction s u c c e s s f u l l y the excess phosphine used was less than twenty-fold since an equilibrium i s set up according to the following equation. RuTPP(n-Bu 3P) 2 . + C 0 > RuTPP(C0)(n-Bu 3P) + n-Bu3P equation I I I . l The p o s i t i o n of the equilibrium obtained i s dependent on the CO pressure used and the amount of excess phosphine added. At pressures of CO less than one atmosphere, with no excess phosphine, the reaction again did not go to completion. This i s i n contrast to the analogous 41 OEP system, where the reaction went to completion under a l l pressures of CO, down to about 0.21 atmospheres. Figure III.2 shows t y p i c a l spectral changes c h a r a c t e r i s t i c of the reaction (the Soret bands are not shown, as they are o f f the scale at the concentrations used). As with the Ru0EP(n-Bu,P) 9 system, excess Figure 111.1. Spectral Changes f o r the Reaction of CO with RuTPP(n-Bu qP) 9 in Toluene at 26°C with No Added Phosphine 22 phosphine i s not necessary to generate the correct spectrum f o r s i x -coordinate species. In the i n i t i a l spectrum of RuTPP(n-Bu 3P) 2, there are bands at 560nm (log e = 4.01), 527nm (log e = 4.17), and 437nm (log e = 5.30). The spectrum of RuTPP(CO)(n-Bu 3P) has bands at 581nm (log e = 3.96), 542nm (log e = 4.17) and 420nm (log e = 5.50). The l a t t e r are determined from spectral data using one atmosphere of CO, when i t i s assumed that the reaction has gone to completion. At any equilibrium p o s i t i o n , bands of both s t a r t i n g complex and carbonyla-t i o n product were present. Reaction times varied from 25 min to 90 min , depending on temperature and concentration of ligands. 3.2 Treatment of Data The k i n e t i c data were obtained by following the decrease in spec-t r a l i n t e n s i t y of the 527nm band of the bis-phosphine complex. Plots A -A j v e r s u s time gave good s t r a i g h t l i n e s , Figure III.3. A g i s the absorbance at equilibrium and A^ i s the absorbance at any time, t, during the reaction and A i s the absorbance at t = 0. This demon-• o strates that the reaction follows pseudo-first-order k i n e t i c s under the conditions of the experiment. The CO concentration i s much larger (about 10 times) than that of the porphyrin. The overall v a r i a t i o n i n the spectral changes of the reaction with CO pressure and phosphine concen-t r a t i o n i s consistent with the simple one-to-one equilibrium shown in equation I I I . l , f o r which [RuTPP(C0)(n-Bu 3P)] [n-Bu 3P] K [RuTPP(n-Bu 3P) 2] [CO] This can be written as log K = log [ A Q - A e / A e - A j - log[C0] + log[Bu 3P] • r- 1 r-time, sec TOO 150 200 Figure III.3. F i r s t Order Plot f o r the Reaction RuTPP(n-Bu 3P) 2 with CO in Toluene at 31°C 24 where, A Q = absorbance of RuTPP(n-Bu 3P) 2, and Aro = absorbance of product, estimated by ext i n c t i o n c o e f f i c i e n t s and A g = absorbance at equilibrium. The CO pressure i s converted to molarity using the known s o l u b i l i t y of 4fi CO i n toluene. The equilibrium constants were calculated from plots of log A ( )-A e/A e-A o o versus log [n-Bu 3P], at a fixed CO pressure. In a l l cases s t r a i g h t l i n e plots of slope = -1.0 ± 0.15 were obtained (Figure III.4). The K values thus obtained are l i s t e d i n Table I I I . l . A plo t of In K versus 1/T (Figure III.5) gave a good s t r a i g h t l i n e from which the f o l -lowing thermodynamic parameters were obtained; AS° = 78.9J/deg ± 20J/deg AH° = 30.5kJ ± 4kJ. Table I I I . l . Equilibrium Constants f o r the Reaction of RuTPP(n-BUqP) 9 with CO at Di f f e r e n t Temperatures Temp. °C 21 26 31 36 ; 41 K 0.049 : 0.054 : 0.068 0.083 0.107 Some individual rate constants were also c a l c u l a t e d , assuming that the mechanism corresponds to that of the Ru0EPL 2 (where L = n-Bu^P, or CH^CN) systems, i . e . a d i s s o c i a t i v e mechanism, equation III.2. k k RuTPP(n-Bu 3P) 2 1 ^ RuTPP(n-Bu3P) 2 ^ RuTPP(CO)(n-Bu 3P). k - l k-2 equation 111.2 Since an equilibrium e x i s t s , k_ 2 cannot be assumed to be n e g l i g i b l e , and the rate law becomes Figure III.4. Log A ^ / A ^ Versus [n-BUgP] at Constant CO Pressure ;ln K -3.1 -3.2 -3.3 -3.4 1/T x 10 3 ( K - 1 ) Figure III.5. Van't Hoff Plot f o r the Reaction of RuTPP(n-Bu~P) 9 with CO in Toluene 27 k 1k 2[C0] [RuTPP(n-Bu 3P) 2] k_ ]k_ 2[n-Bu 3P] [RuTPP(CO)(n-Bu3P)] k_1 [n-Bu3P] + k 2[C0] k_1 [n-Bu 3P]+ k 2[C0] which can be rewritten, rate = k f[RuTPP(n-Bu 3P) 2] - k r[RuTPP(C0)(n-Bu 3P)] where k^ i s the overall pseudo-first-order rate constant f o r the forward reaction and k i s the overall pseudo-first-order rate constant for the r reverse reaction. The CO and phosphine concentrations remain constant 47 during one experiment. This expression can now be integrated to give (k- + k j t = In A -A /A.-A , f r o e t e A g = absorbance at equilibrium A^ = absorbance at any time, t A = absorbance of i n i t i a l RuTPP(n-Bu 0P) 0. o 3 2 PI ots of In A 0-A e/A t-A e versus time, gave good s t r a i g h t l i n e s of k n k 9 [CO] + k ,k 9 [n-BuqP] slope = , 1 % n n 1 " ' , " % „ n - J = k obsd. K k_1 [n-Bu 3P] + k 2 [CO] Due to the complicated form of t h i s rate expression, no simple graphical method y i e l d s the k i n e t i c rate constants. Therefore, a l t e r n a t i v e pro-cedures were used. One method of obtaining data i s to look separately at the reverse reaction, RuTPP(C0)(n-Bu 3P) + n-Bu3P »- RuTPP(n-Bu 3P) 2 + CO. A f t e r adding one atmosphere of CO to a degassed solution of RuTPP(n-Bu 3P) 2 and allowing the reaction to go to completion, a large excess of phosphine 28 was added under the CO atmosphere, pushing the reaction in equation 1 completely to the l e f t . Under these conditions, k-j i s n e g l i g i b l e , and rate i s given by k ,k 7 [n-BuoP][RuTPP(CO)(n-Bu.P)] rate = ~ _ , J k 2[C0] + k_i [n-Bu 3P] which may be expressed rate = k obsd [RuTPP(C0)(n-Bu 3P)]. This can be arranged to give k obsd" 1 = k _ 2 _ 1 + k 2[C0]/k_-,k_ 2[n-Bu 3P] A p l o t of k obsd - 1 versus [C0]/[L] should give a s t r a i g h t l i n e of slope = k2/k_-|k_2 and intercept k _ 2 _ 1 , Figure III.6. Because the forward reaction does not go to completion at less than one atmosphere of CO, k-j cannot be determined d i r e c t l y . However, knowing K, k_ 2, and k2/k_-|, k-j can be calculated (Table III.2). The k obsd values calcu-lated from these constants agree with k obsd found experimentally, see Table III.3. Table III.2. Rate Constants f o r the Reaction of RuTPP(n-Bu3P.)2 and CO at 26°C k-jsec - 1 k2/k_-, k _ 2 s e c _ 1 1.316xl0" 3 3.366 8.94xl0~ 2 30 Table III.3. Reaction of RuTPP(n-Bu~P) ? with CO in Toluene at 26°C J [n^Bu 3P]x!0 4 moles/1itre CO pressure atm 3 a k obsdx lO 1 3 sec" 1 3 b k obsd x 10° s e c - 1 1.67 0.966 1.67 2.07 1.98 0.966 1.97 2.21 2.38 0.966 2.08 2.39 3.38 0.966 2.19 2.83 3.90 0.966 2.55 3.06 ?Found experimentally. Calculated. 3.3 Discussion From the data in Table 111.2, i t i s apparent that the si x - c o o r d i -nate bis-phosphine complex of RuTPP(n-Bu 3P) 2 loses a phosphine ligand (k-j) ca. 70 times slower than the mixed carbonyl-phosphine species gives up to a CO molecule, (k_2), and t h i s i s the major f a c t o r govern-ing the equilibrium constant, (K* = k-jk 2/k_ 1 k _ 2 ) . The k_-j/K2 value shows that the five-coordinate intermediate has a s l i g h t k i n e t i c preference f o r CO binding over n-Bu3P. The approximately f i f t e e n - f o l d d i f f e r e n c e in the equilibrium constants f o r the analogous phosphine complexes, RuOEP(n-Bu 3P) 2 and RuTPP(n-Bu 3P) 2, Table III.4, i s r e f l e c t e d in the higher value of k_-|/k2 found i n the TPP system. A possible r a t i o n a l e f o r t h i s may be increased s t e r i c hindrance i n the TPP system, the phenyl group (at r i g h t angles to the porphyrin plane) a s s i s t i n g the removal of the bulky tri-n-butylphosphine group. S t e r i c influences on the binding of CO would be small within the two porphyrin systems. 31 Table 111.4. Kinetic and Equilibrium Data f o r the Reactions of Fe and Ru Porphyrin Complexes with CO in Toluene COMPLEX k l k _ l / k 2 k-2 K TEMP.°C Ru0EP(CH 3CN) 2 a 2.41xl0~ 3 0.176 3.4xl0" 7 4.0xl0 4 30 Ru0EP(n-Bu 3P) 2 a 2.11xl0 - 3 0.061 5.35xl0~ 2 0.677 31 RuTPP(n-Bu 3P) 2 b 1.32xl0" 3 0.273 8.93xl0" 2 0.054 26 Fe0MBP(pip) 2 c 1020 1.7 0.25 2340 23 Fe0MBP(py) 2 c 490 26 0.09 209 23 FePpIX(pip) 2 c 20 0.002 0.06 23,000 23 FeTPP(pip) 2 c 11 0.002 0.52 150,000 23 F e P c ( p i p ) 2 c 0.5 3.33 0.13 0.85 23 •S. Walker, M.Sc. Thesis, University of B r i t i s h Columbia. This work. B.R. James, K.J. Reimer and C T . Wong, J. Am. Chem. Soc. 99, 4815 (1977). E l e c t r o n i c factors could also play a r o l e as there i s a difference i n the two porphyrin b a s i c i t i e s , see pK 3 in Table III.5. pK 3 r e f e r s to the addition of a proton to the neutral porphyrin + + [ P H 2 ] p H + H pH- (pK 3 = pH - log - — ^ - ) c [ p H 3 ]-• and i s used as a measure of porphyrin b a s i c i t y . This decreased b a s i c i t y of TPP leads to a greater a f f i n i t y f o r donor ligands and r e s u l t s i n 48 stronger metal-porphyrin bonding. This trend has been observed in the study of amine ligand binding to cobalt porphyrin complexes where a decrease in porphyrin b a s i c i t y correlated with :an increase i n log K^, 49 Table I IT.5. The equilibrium .studied was the formation of the 1:1 adducts and K. refe r s to the equilibrium constant f o r that reaction when L = py. 32 L + CoP ^ ^ LCoP \ ' equation 111.5 The decreased b a s i c i t y of TPP may also be correlated with the equilibrium constant, K f o r the carbonylation (equation III.l.) and thus to the k_-|/k^ value; t h i s r e s u l t s in a larger k_ 1/k 2 value, where n-Bu3P, a better-a donor i s favoured over CO,- thus increasing k - j . 4 9 In"the analogous OEP .complex the increasing b a s i c i t y r e s u l t s in a smaller value of k_ -j /k 2 • Table III.5. Log K|_ Values f o r Ligand Binding to a Series of Co Porphyrins PORPHYRIN pK 3 log K L OEP 6.0 2.85 MpTXDME 5.8 3.25 PpIXDME 4.8 3.78 TPP 3.0 2.92 There i s very l i t t l e d i f f e r e n c e in the CO ' o f f rates f o r the two bis-phosphine systems, which implies that the dominant fa c t o r in k_ 2 i s the trans e f f e c t of the tri-n-butylphosphine ligand. Comparison of k_ 2 values f o r the phosphine and a c e t o n i t r i l e systems, shows a difference 5 of the order of 10 . The high trans e f f e c t of tri-n-butylphosphine, being a good ir-acid, l a b i l i z e s CO much more than a c e t o n i t r i l e , which i s 50 a poorer ir-acceptor. A hypothesis has been advanced attempting to r e l a t e k i n e t i c data, in p a r t i c u l a r k_-|/k2> to the structure of the five-coordinate interme-51 diate in the reaction mechanism outlined in equation III.2. Data on 33 Fe(porp)(airline^ and FePcCamine^ systems are given i n Table III.4, together with those of Ru(porp)l_2 systems. The five-coordinate i n t e r -mediate of the PpIX and TPP systems was considered to have a structure s i m i l a r to that of A, with the Fe atom out of the porphyrin plane and 51 high spin. Larger values of k ^ 2 * such as those f o r OMBP and Pc, are thought to be consistent with an intermediate which i s low or in t e r -mediate spin and has a structure comparable to B. The value of k_^/k2 obtained f o r RuTPP(n-Bu 3P) 2 i s more comparable to those of the iron OMBP and Pc systems, rather than the TPP system, implying that the interme-diate state resembles B more than A, Figure 111.7. Comparing the data f o r the Fe and RuTPP systems, there i s a dramatic d i f f e r e n c e i n the values of k_^/k 2, which suggests that the structure of the postulated intermediate i s affected by the change i n metal. The numbers suggest that the Fe(porp)(n-Bu-^P^ geometry would be cl o s e r to A, while that of • 1 i / , \ / „ , \ J / N , \ [ / ^ \ \ - — N / K ' — N N N B Figure III.7. Possible Structures f o r the Five-coordinate Intermediate the ruthenium analogue i s cl o s e r to B. This e f f e c t may be due to the superior iT-backbonding a b i l i t y of Ru(II) compared to Fe(I I ) , tending 52 to keep the metal i n the plane of the porphyrin. Of the Fe sub-group, six-coordinate metalloporphyrins, only Fe(II) complexes show high spin states, which i s explained by the f a c t that the energy s p l i t t i n g between 52 the d , d , d , and d 2 2, d 2 increases i n the order Fe<<Ru<0s. xy xz yz x ~y z 34 A recent study on the axial ligand s u b s t i t u t i o n reactions of Ru(II)PcL 2 indicates that these reactions again occur via a simple 53 d i s s o c i a t i v e mechanism. (T)RuPc(L) (T)RuPc + X -L +L +X -X (T)RuPc + L (T)RuPc(X) T = trans group, X = nucleophile, L = leaving group. 54 The same mechanism holds for FePcL 2 systems, and the k _ - j / r a t i o s are s i m i l a r f o r both Fe and Ru systems. The large difference in k-j values, Table III.6, shows that FePcL 2 systems are more l a b i l e than RuPcL^ systems, thought to r e s u l t from greater ( a x i a l ) Tr-backbonding a b i l i t y of Ru(II). The f a c t that the k -|/k2 r a t i o s are s i m i l a r f o r ruthenium and iron phthalocyanine systems, whereas f a i r l y large d i f f e r -ences e x i s t within analogous porphyrin systems,may be explained by the greater r i g i d i t y of the phthalocyanine ligand which tends to keep the Fe more in the plane, such that B represents more the geometry of both Table III.6. Comparison of Axial Ligand L a b i l i t i e s of Iron and Ruthenium Phthalocyanine Adducts^3 Trans Group Leaving Group k^FeJ/k^Ru) P(0Bu) 3 P(0Bu) 3 20,000 P(0Bu) 3 Melm 890 P(0Bu) 3 py 260 P(Bu) 3 P(0Bu) 3 100,000 k, (Fe) at 21°C in acetone, k-j (Ru) at 25°C in acetone. 35 55 the Fe and Ru five-coordinate intermediates. Thus a l l three ruthenium porphyrins and possibly ruthenium phthalocyanine systems have f i v e -coordinate intermediates whose structures are s i m i l a r and are thought to resemble that of B in Figure 111.7, with the Ru in the porphyrin plane. It was hoped that the study of the RuTPPCn-Bu^P^ reaction with CO might give some i n s i g h t into the mechanism of the c a t a l y t i c aldehyde decarbonylation reaction, or at l e a s t the i n i t i a t i o n step, which appears to involve pre-treatment with CO. The r e l a t i v e l y high value of k_ 2 indicates that the CO of the monocarbonyl i s quite l a b i l e as i s also suggested by v(C0) f o r t h i s complex, see Table 1.1. The r e l a t i v e l y low K value indicates that the equilibrium (equation I I I . l ) l i e s to the l e f t _5 when small amounts of phosphine are present, even down to 10 M. The dramatic e f f e c t of tri-n-butylphosphine on the position of the equi-lib r i u m implies that the r e l a t i v e concentrations of tri-n-butylphosphine and aldehyde are l i k e l y to be c r i t i c a l in the c a t a l y t i c decarbonylation sequence. 36 CHAPTER 4 THE CATALYTIC DECARBONYLATION OF ALDEHYDES 4.1 The Use of RuQEP(CH 3CN) 2 to Decarbonylate Aldehydes The t i t l e complex was o r i g i n a l l y selected as a decarbonylation agent because of i t s rapid and clean reaction with CO in toluene solu-t i o n , 4 1 and also because the monocarbonyl thus formed, Ru0EP(C0)(CH3CN) can be conveniently decarbonylated by photolysis i n the presence of 42 excess a c e t o n i t r i l e . I t was anticipated that the reaction with aldehyde could be made c a t a l y t i c using v i s i b l e l i g h t , as in equation IV. 1. Ru0EP(CH 3CN) 2 — >- Ru0EP(C0)(CH3CN)+RH+CH3CN ' hv, CH3CN • equation IV.1 Benzene was chosen as solvent because of the ready s o l u b i l i t y of the OEP complexes in i t . Furthermore, the benzene was stable under the conditions of the experiment. A simple non-catalytic reaction at room temperature was c a r r i e d out i n i t i a l l y using a lxlO~ 4M solution of Ru0EP(CH 3CN) 2 in degassed _ p benzene containing 10 M phenylacetaldehyde. A rapid (2-5 min ) colour change from the c h a r a c t e r i s t i c purple of Ru0EP(CH 3CN) 2 to the orange coloured carbonyl complex, RuOEP(CO)(CH3N) indicated that decarbonyla-ti o n had occurred. The spectral changes f o r the reaction are shown in in Figure IV.1, showing Ru0EP(CH 3CN) 2 with bands at 527nm (log e = 4.23) and 496nm (log e = 4.02), s h i f t e d to 549nm (log e = 4.35) and 518nm (log e = 4.12), c h a r a c t e r i s t i c of RuOEP(CO)(CH,CN)• The intense Soret 37 Figure IV.1. The Reaction of RuOEP(CH3CN)2 with Phenylacetaldehyde at Room Temperature i n Benzene. (1) RuOEP(CH.CN)?, (2) Ru0EP(C0)(CH3CN) 38 band at 405nm (log e = 5.34) correspondingly s h i f t s to 395nm (log e = 5.25). An impurity band at ca. 610nm measured in the f i n a l spectrum r e s u l t s during addition of aldehyde and i s l i k e l y due to some oxidation of RuOEP(CH 3CN) 2 by impurities i n the aldehyde, by comparison with spectral data obtained f o r a i r oxidation of t h i s complex. As excess a c e t o n i t r i l e i s thought to be necessary f o r regenerating the b i s - a c e t o n i t r i l e species, the same reaction was c a r r i e d out in the presence of a three-fold excess of a c e t o n i t r i l e . However, under these conditions no decarbonylation of the aldehyde took place, according to reaction IV.1, over a period of several days. During the photo-induced decarbonylation of RuOEP(CO)(CH^CN), a five-coordinate intermediate i s probably formed, which then r a p i d l y adds a CH3CN ligand. Solutions of RuOEP(CO)(CH3CN) in benzene, with excess phenylacetaldehyde and a ten-fold excess of a c e t o n t r i l e , were i r r a d i a t e d using v i s i b l e l i g h t in the hope that any such five-coordinate intermediate would be capable of abstracting CO, even i n the presence of excess a c e t o n i t r i l e . However, no toluene was formed, and no spectral changes were observed. The length of time of photolysis, and concentra-tions of both aldehyde and a c e t o n i t r i l e (including no added a c e t o n i t r i l e ) were varied, but with no success. The i n i t i a l rate of the thermal CO abstraction by RuOEP(CH 3CN) 2 was quite rapid, and attempts were made to obtain rate constants f o r the reaction by spectrophotometry. However, the d i l u t e solutions - 5 - ? (10~ M in Ru, 10" M in aldehyde) required f o r t h i s experiment were e a s i l y 'oxidized' by trace impurities in the aldehyde, and no data relevant to the decarbonylation were obtained; the noted i s o s b e s t i c systems are thought to r e f e r to the oxidation process (Figure IV.2). 39 Figure IV.2. The Reaction of RuOEP(CH3CN)2 with Phenylacetaldehyde at 26°C in Benzene 40 Other b i s spec ies such as RuOEP(py) 2 , RuOEP(THF)2 were generated i n s i t u , by pho to l ys i ng the carbony l i n the app rop r i a te s o l v e n t , and reac ted w i th pheny laceta ldehyde. However, no deca rbony la t i on occur red i n the presence of excess a x i a l l i g a n d . 4 .2 D i s c u s s i o n The proposed r e a c t i o n scheme, equat ion IV .1 , r evea l s two main problems. F i r s t l y , the r e a c t i o n RuOEP(CH 3CN) 2 + RCHO »- RuOEP(CO) (CH3CN) + CHgCN + RH does not proceed i n the presence of excess a c e t o n i t r i l e and second l y , success fu l regenera t ion o f the c a t a l y s t from p h o t o l y s i s o f RuOEP(CO)(CH^CN) p lus 1 e q u i v a l e n t of a c e t o n i t r i l e does not occu r . S ince the carbonyl i s p h o t o - l a b i l e , i t s d isp lacement by l i g h t would not be expected to depend on the amount o f excess a c e t o n i t r i l e p resen t . Th i s i s shown to be the case i n a separa te experiment which demonstrated tha t the r e a c t i o n , CHoCN RuOEP(CO)(CH3CN) »- Ru0EP(CH3CN)2+C0 proceeded w i th on ly th ree equ i va len t s of a c e t o n i t r i l e p resen t . I f CO were e jec ted g i v i n g a f i v e - c o o r d i n a t e i n t e r m e d i a t e , such a s p e c i e s , having a vacant c o o r d i n a t i o n s i t e , would be a b e t t e r cand ida te f o r o x i d a t i v e a d d i t i o n by aldehyde which i s the f i r s t s tep i n many decar -4 - 1 2 bony la t i on r e a c t i o n s . However, s i nce the on ly product o f p h o t o l y s i s i s RuOEP(CO)(CH 3CN), a t any added CH^CN c o n c e n t r a t i o n , i t was concluded tha t l i g h t would not remove CO i n the presence of excess a ldehyde. A r e a c t i o n i n which RuOEP(CO)(CH3CN) was t r ea ted w i th an excess of a c e t o n i t r i l e over aldehyde d id r e s u l t i n the fo rmat ion of RuOEP(CH 3CN) 2. There i s no p o s s i b i l i t y o f genera t ion o f RuOEP(CH qCN) ? and subsequent 41 conversion to RuOEP(CO)(CH^CN) by abstraction from the aldehyde, i . e . c a t a l y s i s , as no build-up of toluene i s observed. 41 From the k i n e t i c studies on the carbonylation reaction RuOEP(CH 3CN) 2 + CO *- RuOEP(CO) (CH^CN) + CH3CN the five-coordinate intermediate prefers to bind CO over CH^CN k i n e t i c -a l l y by a f a c t o r of 5, and also the mixed ligand species gives up a CO ca. 10 4 times more slowly than the bis species loses CH^CN. I f the reaction, equation IV.1, i s presumed to follow the same d i s s o c i a t i v e mechanism, see Chapter 3, the k i n e t i c data could account f o r the f a i l u r e of Ru0EP(C0)(CH3CN) to lose CO in the presence of a source of carbonyl, although these data r e f e r to a thermal system. No mechanistic studies have been performed on the p h o t o l y t i c decarbonylation. The other problem, f o r c i n g Ru0EP(CH3CN)2 to abstract CO in the presence of excess a c e t o n i t r i l e , could be possibly overcome by increas-ing the concentration of aldehyde r e l a t i v e to a c e t o n i t r i l e . In the analogous reaction with CO an excess of CH^CN does not prevent the carbonylation reaction from going to completion. 4 1 4.3 The Use of RuTPP(PPh 3) 2 to Decarbonylate Aldehydes Several ruthenium porphyrin complexes incorporating various 44 phosphine ligands were synthesized in t h i s laboratory, and these were investigated f o r use as decarbonylation c a t a l y s t s . The prepara-ti o n of RuTPP(PPh 3) 2 (8), i s described in Chapter 2. During the r e f l u x i n g procedure the p o s i t i o n of the v(C0) changed from 1950 cm - 1 to 1915 cm - 1, before complete loss of bands in t h i s region. Complex (8) also i s known to d i s s o c i a t e a phosphine ligand r a p i d l y in s o l u t i o n , giving an unsual spectrum, see Figure IV.3. A solution of RuTPP(PPh 3) 2, 42 43 (8) i n Ch^Cl,, shows absorptions at 412nm, 435nm, and 510nm. Addition of CO gas gives a d i s t i n c t colour change and a spectrum (9) with bands at 412, 530, and 565nm, re s p e c t i v e l y , s i m i l a r to that of RuTPP(CO)(EtOH) species. On addition of triphenylphosphine to t h i s solution a further species (10) i s generated, which i s neither (8) nor (9), and which has absorptions at 420, 540 and 575 nm. This spectrum i s s i m i l a r to that obtained f o r RuTPP(CO)(n-Bu^P) in the presence or absence of excess phosphine and i s att r i b u t e d to RuTPP(CO)(PPh 3). The spectrum of (9) i s t y p i c a l of a RuTPP(CO)(EtOH) species, or other RuTPP(C0)(S) species 31 where S i s a weakly coordinating solvent, such as CH^CN, or THF. Although i t i s known from X-ray c r y s t a l data that S occupies a coordi-56 nation s i t e i n the s o l i d state, i t i s not cl e a r whether t h i s i s true when the complex i s in solu t i o n . The s i m i l a r i t y in the spectra of a l l of these RuTPP(C0)(S) and the RuTPP(CO)(PPh 3) species suggests that perhaps the si x t h a x i a l ligand d i s s o c i a t e s i n s o l u t i o n , giving a f i v e -coordinate Ru(porp)(C0) species. This implies that i n d i l u t e solution and in the absence of excess triphenylphosphine, RuTPP(CO)(PPh 3) disso-cia t e s a phosphine according to the following scheme. Addition of excess phosphine to (8) gives the t y p i c a l bis-phosphine spectrum, with bands at 434 and 515nm. RuTPP(PPh 3) 2 K RuTPP(PPh 3) + PPh 3 CO RuTPP(PPh 3)(C0) K ^ RuTPP(CO) + PPh 3 10 'PPh, RuTPP(PPh 3). 44 Refluxing or warm solutions of RuTPPCPPh^ in C ^ C ^ were employed to decarbonylate phenylacetaldehyde c a t a l y t i c a l l y , but turn-over numbers were not very high. Figure IV.4 shows a chromatograph of one such reaction mixture. The turnover numbers were estimated using the internal standard method (by adding a known concentration of a standard compound to a known quantity of the reaction mixture). The response f a c t o r of the product being estimated i s compared to the response f a c t o r of the standard. This r a t i o of response factors should be constant over the concentration range studied. Using the r a t i o n a l e discussed in the introduction, the reaction was modified by adding excess tri-n-butylphosphine, which i s known to 44 displace CO more r e a d i l y than triphenylphosphine, and the reaction was c a r r i e d out in a c e t o n i t r i l e solutions. Typical spectral changes f o r the c a t a l y t i c reaction are shown in Figure IV.5. Solutions were -5 -2 i n i t i a l l y ca. 5x10 M in Ru, 10 M in aldehyde, and small amounts (5-10 yL) of tri-n-butylphosphine were added. Addition of larger amounts of phosphine gave formation of RuTPP(n-Bu 3P) 2 and no decarbonylation. A f t e r addition of phosphine, spectral changes indicated the presence of two species with Soret bands at 435nm (a bis-phosphine) and 416nm, which i s l i k e l y to be a monocarbonyl complex. The f i n a l spectrum obtained,when decarbonylation had ceased, i s also shown in Figure IV.5; the main feature i s a broad Soret at 410nm. The colour of these i n a c t i v e solutions was greenish-brown. The range of turnover numbers obtained f o r decarbonylation of phenylacetaldehyde under a range of conditions varied considerably, Table IV.1. Several attempts were made to reproduce turnover numbers and reaction rates (Chapter 2). However, the reaction was found to be generally i r r e p r o d u c i b l e , sometimes not showing any c a t a l y t i c a c t i v i t y Table IV.1. The Decarbonylation of Aldehydes Using a RuTPP(PPh 3) 2/n-Bu 3P System30(b) Substrate Major Product (Z) C 6H 5CH0 Benzene (100) C^CH-CHCHO (trans) Styrene (100) Conversion (time i n hour) C^R^CH^CHO p-CN-C^B^CHO n-C 6H 1 3CHO 2-Ethylbutanal aCUO ^-v. ([J) (70);(J) ( 3 0 ) 10(1), 20(12) 10(5) 20(10) Toluene (95) 30(1), 90(4) B e n r o n i t r i l e (100) 15(12) 10(1) 30(1) Turn-over ( f i r s t hour) 10 20 10 3- 50 20 n - C 6 H u (65) n-C 5H 1 2 (85) O ( 6 0 )O D" C6 H1A ( 5 ) (35) 30(1), 50(18) 90(50) 10' 10" 2 x 10' 10' ,TOlUtMt (15 turnovers) Figure IV.4. The GLC Trace of the Products of the Reaction of RuTPP(PPru) ? with Phenylacetaldehyde Figure iV.5. Spectral Changes f o r the C a t a l y t i c Decarbonylation of Phenylacetaldehyde Using RuTPPfPPh^Wn-BuaP- as Catalyst. (1) Reaction Mixture A f t e r 15 Min. (2) Final Spectrum 47 while other times y i e l d i n g very high turnover numbers. Modifications of the experimental procedure s t i l l gave variable turnover numbers in a non-reproducible manner. The general lack of r e p r o d u c i b i l i t y and the composition of some of the decarbonylation products (see Table IV.1) indicated that a rad i c a l process might be involved. Experiments with rad i c a l i n h i b i t o r s confirmed t h i s ; two d i f f e r e n t r a d i c a l i n h i b i t o r s , hydroquinone and 2,6 d i - t e r t - b u t y l - p - c r e s o l , were found to i n h i b i t production of toluene from phenylacetaldehyde in the c a t a l y t i c system. Solutions containing the r a d i c a l i n h i b i t o r s (at 10 M) retained t h e i r i n i t i a l orange-red colour f o r weeks, no spectral changes and no toluene being detected. 4.3.1 Other Aldehydes A range of aldehydes was investigated, with a view to probing the mechanism of decarbonylation. No general trend was apparent. Heptanal was decarbonylated but the percentage conversion was quite low; in the case of a longer chain a l i p h a t i c aldehyde, dodecanal, no decarbonylation took place over several days. Of the aromatic aldehydes, benzaldehyde proved d i f f i c u l t to decarbonylate. A f t e r several unsuccessful attempts, some decarbonylation products were eventually detected by another worker 57 in t h i s laboratory. Two of the substituted benzaldehydes (p-nitro and p-cyano) gave somewhat higher, turnover numbers than benzalde-hyde, but there was no i n d i c a t i o n of a r e l a t i o n s h i p between c a t a l y t i c a c t i v i t y and the su b s t i t u t i o n pattern of these aldehydes. The decar-bonylations seem to be reasonably s e l e c t i v e with aromatic aldehydes, y i e l d i n g the expected product. However, small amounts of other products were obtained when an a l i p h a t i c system was decarbonylated, Table IV.1. The decarbonylation of cyclohexanecarboxaldehyde, which was c a r r i e d out 48 in b e n z o n i t r i l e , yielded two products which were c o l l e c t e d in a cold trap and i d e n t i f i e d by GC-MS as cyclohexane and methylcyclopentane. A l l attempts to i s o l a t e the ruthenium end product or any interme-diates using conventional chromatographic techniques were unsuccessful. 4.3.2 Further Studies In addition to the studies described above, various other experi-ments were c a r r i e d out in an attempt to elucidate d e t a i l s of the reaction mechanism. 9 These are described here because of t h e i r relevance to the discussion of the c a t a l y t i c reaction. C y c l i c voltammetry was performed on both RuTPPI^ systems (L = t r i -n-butyl phosphine, triphenylphosphine) and also the RuTPP(PPh 3) 2/n-Bu 3P c a t a l y t i c system, and some evidence f o r Ru(III) intermediates comes from these voltammograms. During decarbonylation of the indane aldehyde, some i r r e v e r s i b l e waves were seen at -0.1V, and in the corresponding phenylacetaldehyde system, waves at -0.08V and +0.08V were thought to r e s u l t from Ru(III) species. Infra-red measurements during decarbonylation of the indane alde-hyde revealed a small peak at 2015 cm - 1, which could be due to a Ru(III) h y d r i d e . ^ " Some e.s.r. studies were also performed, using the very slow c a t a l y t i c decarbonylation of pyridine-2-aldehyde, and cyclohexen-4-al. Organic free r a d i c a l s , g = 2.00, were detected in both of these systems. During decarbonylation of phenylacetaldehyde, a low temperature broad signal at g - 2.20 was assigned to a low spin d^ Ru(III) species, although Ru(III) porphyrin systems do not normally give detectable aWork performed by G. Domazetis, P.D.F. 49 CO e.s.r. s i g n a l s . Kinetic studies were hampered by the lack of repro-d u c i b i l i t y . A f i r s t - o r d e r dependence on aldehyde was observed, but the rate constants were not reproducible. 4.4 Discussion The generally accepted mechanism of decarbonylation, involving oxidative addition of aldehyde, CO migration, and reductive elimination, would appear to be u n l i k e l y f o r t h i s system. This mechanism would require coordination numbers greater than s i x , and also Ru(IV) i n t e r -22 mediates, which are unusual, though p l a u s i b l e . The data so f a r suggest a r a d i c a l mechanism, and Ru(III) i n t e r -mediates. A t e n t a t i v e mechanism i s outlined in scheme IV.1. Decar-bonylation of the acyl ra d i c a l i s thought to be metal a s s i s t e d , giving Scheme IV.1. The Mechanism of Decarbonylation Using the RuTPP(PPh 3) 2/n-Bu 3P System a Ru(II) carbonyl, which i s subsequently decarbonylated by n u c l e o p h i l i c attack by tri-n-butylphosphine. This phosphine can displace coordinated carbonyl, as exemplified by the reaction RuTPP(CO)(n-Bu 3P) + n-Bu3P t K > RuTPP(n-Bu 3P) 2 + CO This reaction occurs thermally in toluene at 26°C with a K value of 18.5, see Chapter 3. 50 It was hoped that i d e n t i f i c a t i o n of the species present during c a t a l y s i s would be accomplished v i a spectrometry. The spectral changes fo r the c a t a l y t i c reaction are shown in Figure IV.5. After addition of aldehyde (or CO) and tri-n-butylphosphine, two bands appear in the Soret region; one at 435-437nm due to a bis-phosphine species, the other usually found between 416 and 418nm, i s probably due to a carbonyl adduct. In the v i s i b l e region a band at 510nm s h i f t s to 530nm with a band at 560nm. The spectral c h a r a c t e r i s t i c s of t h i s region are very s i m i l a r to those of RuTPP(PPh 3) 2 a f t e r addition of CO (see Figure IV.3), although the bis(tri-n-butylphosphine) complex also has absorptions close to these wavelengths, at 527nm and 560nm. There are no absorp-tions due to the RuTPP(CO)(n-Bu^P) species, which has a Soret band at 420nm. From the spectral data i t may be concluded that in the active c a t a l y s t solutions there are two main species present. One i s a b i s -phosphine, ei t h e r a mixed phosphine system, or a bi s ( t r i - n - b u t y l p h o s -phine) system, and the other may be RuTPP(CO)(S), a solvated species. The f i n a l spectrum, obtained when c a t a l y s i s i s complete, usually a f t e r 4-24 hours, has a Soret band which appears from 405-415nm with a band at 530nm and a shoulder at 560nm. This spectrum i s common to a l l the decarbonylations, i r r e s p e c t i v e of the aldehyde, and seems to be that of a RuTPP(CO) species. 4.4.1 The Role of the Phosphine Ligands From some experiments with RuTPP(n-Bu 3P) 2> which does not appear to c a t a l y t i c a l l y decarbonylate phenylacetaldehyde, i t seems that the triphenylphosphine ligand i s necessary f o r the c a t a l y t i c r e a ction, e i t h e r in an i n i t i a t i o n step or a propagation step. This r o l e i s probably related to the manner in which RuTPP(PPhJ ? dissociates a 51 phosphine in sol u t i o n . The d i s s o c i a t i o n of phosphine from RuTPP(PPh 3) 2 i s unusual, as the analogous RuTPP(n-Bu 3P)2 complex shows no such behaviour. This d i s s o c i a t i o n may be germane to the c a t a l y t i c decar-bonylation reaction. 4.4.2 The Role of A c e t o n i t r i l e As mentioned previously, n i t r i l e solvents have been found to s t a b i l i z e three coordinate intermediates in decarbonylation r e a c t i o n s , 1 thus preventing dimerization of the active species. The n i t r i l e solvents may have the same type of e f f e c t i n the present system, i . e . preventing dimerization or aggregation of unsaturated intermediates. However, i t i s more l i k e l y that the formation of solvated species such as RuTPP(phosphine)(CH 3GN), for. which there i s some spectral, evidence, i s the primary r o l e of a c e t o n i t r i l e . B e n z o n i t r i l e appears to have the same e f f e c t as a c e t o n i t r i l e and was used in many reactions where v o l a t i l e products were being c o l l e c t e d . The c a t a l y t i c reaction does occur in absence of n i t r i l e solvents, but y i e l d s were not as good in CH^Cl2 alone. There i s evidence, then, f o r the occurrence of Ru(III) interme-diates from e.s.r. data and also c y c l i c voltammetry. If a r e l a t i v e l y high concentration of Ru(III) b u i l t up, i t should be observable in the v i s i b l e spectrum of the c a t a l y s t s o l u t i o n , although small amounts would not be detectable. There i s also strong evidence f o r a rad i c a l type mechanism. The f a c t that the c a t a l y t i c reaction was t o t a l l y i n h i b i t e d by ra d i c a l i n h i b i t o r s , f o r several t r i a l s , indicates that free r a d i c a l s are d e f i n i t e l y involved. The other evidence f o r these comes from e.s.r. where signals due to free r a d i c a l s were detected in frozen c a t a l y s t s olutions. Formation of R species i s also favoured in view of the MS of the decarbonylated cyclohexanealdehyde which revealed the presence 52 of methylcyclopentane as a product. The rearrangement shown below i s p l a u s i b l e , as the 5-hexene-l-yl r a d i c a l i s known to rearrange as shown. 59 This R species could be s t a b i l i z e d by a metal complex, in a cage reac-t i o n . This type of mechanism was considered by Walborsky and Al l e n f o r the stoichiometric decarbonylation of aldehydes by RhCl(PPh 3) 3. The i r e p r o d u c i b i l i t y of the reaction and poor k i n e t i c data also support V ci .Pph3 Rh H PPh, Cl h' •Ph, H PPh, Scheme IV.2. The Decarbonylation of Aldehydes Using RhCl(PPh 3) 3 t h i s theory. It i s possible that the eventual destruction of the porphyrin ri n g i s also a r e s u l t of a radi c a l attack on the porphyrin. 53 CHAPTER 5 THE STOICHIOMETRIC REACTION OF RuTPP(n-Bu 3P) 2 WITH PHENYLACETALDEHYDE 5.1 Kinetics and Spectral C h a r a c t e r i s t i c s This reaction was studied with a view to elu c i d a t i n g the mech-anism of the c a t a l y t i c reaction discussed i n Chapter 4, in p a r t i c u l a r the nature of the i n i t i a t i o n step of the reaction, in which the mono-carbonyl, RuTPP(CO)(n-Bu-jP) i s thought to be formed. Dichloromethane was chosen as solvent i n i t i a l l y because of the ready s o l u b i l i t y of the porphyrin complexes in i t , and also to f a c i l i -tate any detection of toluene. To ensure that only a stoichiometric decarbonylation was taking place, the reaction mixture was tested f o r toluene by GLC, at the end of the reaction. Toluene was not detected at low concentrations of ruthenium c a t a l y s t , and a stoichiometric amount _3 was detected at 10 M ruthenium. RuTPP(n-Bu 3P) 2 + RCHO *• RuTPP(CO)(n-Bu 3P) + RH + n-Bu3P equation V.l The reaction was followed spectrophotometrically, and several clean i s o s b e s t i c points were observed, Figure V . l . The d i r e c t i o n of these spectral changes was the same as that observed f o r the analogous reac-tion with CO, Chapter 2. At high aldehyde to ruthenium r a t i o s , and in the absence of added excess phosphine, the reaction exhibited pseudo-/A -A \ f i r s t - o r d e r dependence as shown by plots (Figure III.2) of 1og( f l _fl°°j versus time, where i Figure V.I. The Reaction of RuTPP(n-Bu3P) with Phenylacetaldehyde [3.42xT0"2;M] in 0\?C~\ 26°C, (A) and in Toluene, (B) 55 56 = absorption at time, t A = absorbance of f i n a l species A Q = absorbance of i n i t i a l species In the presence of any added phosphine, even one mole/Ru, the reaction did not proceed and no carbonyl was formed. Table V.l shows the various k obsd values obtained f o r varying concentrations of ruthenium at the same i n i t i a l concentration of Table V . l . k obsd Values Obtained f o r Various Ru(II) Concentrations at 3.42xlO"2M Aldehyde [ R u ( I I ) ] x l 0 5 moles/1itre k obsdx 10 3 s e c _ l 3.28 1.52 a 2.69 1.57 a 3.66 1.21 a 1.90 1.53 a 3.33 1.01 a 3.66 1.21 a 4.11 0.96 b 1.90 1.27 b 2.66 1.73 b 2.69 1.57 b 2.58 0.78° 1.69 1.16 c 5.08 1.50 c F i r s t batch. ^Second batch. 'Third batch, addition of 0 2 a f t e r degassing. 57 aldehyde. The value of k obsd was not independent of the ruthenium concentration (the normal behaviour for first-order dependence on ruthenium) and there was no regular trend in k obsd as a function of the ruthenium concentration. Thus, although each individual run showed good first-order behaviour, the evaluated k obsd values were scattered and non-reproducible. An attempt was made to study the dependence on aldehyde concentration, however, this was complicated by the interfer-ence of a secondary reaction, which occurred at the higher aldehyde concentrations, see below. Using the extinction coefficients of the carbonyl complex, more careful analysis revealed that the reaction was not going to completion as originally assumed, but appeared to be going to an equilibrium position. This raised the possibility of the toluene product being involved in an equilibrium such as: RuTPP(n-Bu3P)2 + RCHO ^ ^ RuTPP(CO)(n-Bu3P) + Bu3P + RH equation V.2 which could imply a potentially exciting carbonylation of toluene! Indeed, the reaction when carried out in toluene, Figure V.l, did not proceed to any appreciable extent. However, addition of small amounts of toluene, e.g. 100-fold excess over ruthenium had l i t t l e effect on the measured k obsd values or the apparent equilibrium position, which argues against an equilibrium involving toluene. An attempt was made to reverse the reaction, equation V .3 . RuTPP(C0)(n-Bu3P) + BU3P T o 1 u e n e > RuTPP(n-Bu3P)2 +C6H5CH0 equation V.3 However, no visible absorption bands due to RuTPP(n-Bu?P)? were observed. 58 The use of higher concentrations of aldehydes usually resulted in a loss of i s o s b e s t i c s and general broadening of the spectrum in the 530nm region, before the 'A^' of the decarbonylation reaction could be recorded, Figure V.3. This problem was circumvented by using a Guggen-heim analysis of the data f o r which 'A 1 does not have to be known. J oo Figure V.4 shows some data obtained for one set of runs but l a t e r sets were found to be i r r e p r o d u c i b l e . The secondary reaction appeared to be re l a t e d to the p u r i t y and amount of the aldehyde used. I t was i n i t i a l l y suspected that t h i s reaction may have been a degradation reaction involving attack on the porphyrin r i n g . However, addition of excess tri-n-butylphosphine to solutions in which the 'secondary' reaction has taken place regenerated the i n i t i a l spectrum completely, Figure V.3, i n d i c a t i n g that the 'secondary' reaction i s not porphyrin breakdown and i s probably a r e v e r s i b l e oxidation reaction. However, i f the reaction solutions were l e f t to stand f o r more than 1-2 days, only p a r t i a l regen-eration of the bis species was achieved. Some impurity in the aldehyde was thought to be responsible f o r the 'secondary' reaction, but i t was not c l e a r from the data whether the RuTPP(CO)(n-Bu^P) product, or the RuTPP(n-Bu 3P) 2 s t a r t i n g material was being oxidized. Some redox studies were c a r r i e d out to help resolve t h i s oxidation problem. 5.2 The Bromine Oxidation of RuTPP(n-Bu,P) 2 Bromine oxidation of RuTPP(n-Bu 3P) 2 was considered l i k e l y to generate Ru(III)TPP(n-Bu 3P) 2. Small aliquots of a lxlO~ 2M bromine solution in CH 2Cl2 were therefore added to a 5x10" M Ru(II) solution u n t i l oxidation was complete, as judged by spectral changes, Figure V.5. There was a loss of i n t e n s i t y at 527nm and 560nm, giving r i s e to a broad absorption in t h i s region, with a shoulder at 580nm. Absorption at nm 600 Figure V.3. Reaction of RuTPP(n-BuoP) 2 with Phenylacetaldehyde Showing Secondary Reaction ( 2 ) . (3) i s the Spectrum Obtained a f t e r Addition of n-Bu^P to (2) 60 61 62 437nm decreased, and the Soret s h i f t e d to 424nm. The increased absorp-ti o n at 360nm i s t y p i c a l of Ru(III) metalloporphyrin complexes, as are the other spectral changes. The i s o s b e s t i c points and the general spec-t r a l changes f o r t h i s oxidation reaction are s i m i l a r to those obtained f o r the reaction of RuTPPtn-Bu^P^ with phenylacetaldehyde. The colour of the solution a f t e r bromine oxidation i s o l i v e green and addition of cp TBAB or tri-n-butylphosphine regenerates the i n i t i a l spectrum of RuTPP(n-Bu 3P) 2. 5.3 The Bromine Oxidation of RuTPP(CO)(n-Bu 3P) in C H Q C I Q Spectral changes f o r t h i s reaction are shown in Figure V.6. There i s a general decrease and broadening of a l l the absorptions, giving a less intense Soret at 422nm and a featureless v i s i b l e region. A broad absorption at ca. 360nm i s again i n d i c a t i v e of Ru(III) forma-t i o n . Reduction of t h i s bromine-oxidized solution with TBAB did gen-erate a spectrum s i m i l a r to that of the carbonyl, but some loss in resol u t i o n had occurred. The colour of the oxidized solution again i s dark green. The general features of th i s oxidation imply that i t i s Ru(II) to Ru(III) oxidation, and not u-cation r a d i c a l formation by oxidation of the porphyrin r i n g , see below. It has been shown that oxidation of Ru(porp)(CO)(L) complexes, where L = amines, always occurs at the porphyrin r i n g rather than at C O the metal, although a recent study on the influence of axial ligands on the redox potentials of RuTPPL^ systems claims that oxidation occurs 44 at the metal atom, even when CO i s a ligand. It was of i n t e r e s t , then, to do a co n t r o l l e d electrochemical oxidation of RuTPP(CO)(n-Bu^P) to see i f the product of t h i s type of oxidation might d i f f e r from the product of a bromine oxidation. 63 Figure V.6. Spectral Changes for the Bromine Oxidation of RuTPP(C0)(n-Bu 3P) in CH ?C1 2 64 • An electrochemical oxidation of the carbonyl complex, from 0.0 to +2.0V, was c a r r i e d out i n a s o l u t i o n of t o l u e n e - a c e t o n i t r i l e (1:1) using 0.1M n-Bu 4N +C10 4" as the supporting e l e c t r o l y t e . A c y c l i c voltammogram was obtained, Figure V.7, with waves at 0.42V, 0.77V, and 1.0V. The Figure V.7. C y c l i c Voltammetry of RuTPP(CO)(n-Bu 3P) in Toluene-Acetonitrile with 0.1M E l e c t r o l y t e main peak, 1.0V i s thought to be due to a u-cation r a d i c a l by comparison 64 with other findings in t h i s laboratory. The other minor peaks are thought to be due to some impurities. The solution was then e l e c t r o l y s e d at 1.0V, y i e l d i n g an olive-green s o l u t i o n , whose spectral c h a r a c t e r i s t i c s are shown in Figure V.8. The peak at 640nm i s t y p i c a l of cation r a d i c a l 64 formation. Addition of TBAB to t h i s s o l u t i o n y i e l d s an orange-red so l u t i o n which appeared, from i t s spectral c h a r a c t e r i s t i c s , to be a mixture of RuTPP(CO)(CH^CN) and RuTPP(n-Bu 3P) 2. The broad band at 530nm with a shoulder at 560nm and a Soret at 412nm i s t y p i c a l of RuTPP(C0)S. 3 1 A second Soret at 437nm indicates the presence of a small amount of RuTPP(n-Bu 3P)2» the remainder of the spectrum being masked by that of the carbonyl complex. These-results imply that during electrochemical 65 Figure V.8. Product of Electrochemical Oxidation of RuTPP(C0)(n-Bu 3P) (1). A f t e r Reduction with TBAB (2) oxidation RuTPP(CO)(n-Bu 3P) i s converted to RuTPP +°(CO)(n-Bu^P) which possibly loses a phosphine ligand. The presence of t h i s excess phos-phine in solution may be the reason f o r the formation of a small amount of the bis species a f t e r reduction. A solution of the cation r a d i c a l , sealed under argon,was found to revert to the reduced RuTPP(C0)S a f t e r a few hours. This behaviour i s t y p i c a l of i r - c a t i o n r a d i c a l s in s o l u t i o n . 5.4 E f f e c t of Oxygen on the Stoichiometric Decarbonylation Reaction The rate of decarbonylation varied i r r e p r o d u c i b l y from one exper-iment to another and t h i s v a r i a t i o n was not related to the concentration of the ruthenium s t a r t i n g material, Table V . l . In an attempt to make the reaction more reproducible the solutions were degassed by three freeze-thaw cycles p r i o r to addition of aldehyde. In some cases the aldehyde was added to the solvent and degassed p r i o r to addition to the s o l i d complex. This was in contrast to the usual procedure, where degassing of solvent was accomplished by an argon purge. The degassed solutions of RuTPPtn-Bu^P),, did not now e f f e c t any decarbonylation of phenylace-taldehyde over several hours. In a further series of experiments, vary-ing pressures of oxygen were admitted to the reaction mixtures a f t e r degassing had taken place, Figure V.9. In these cases the reaction proceeded only a f t e r 0 2 had been admitted; the reactions y i e l d e d pseudo-f i r s t - o r d e r rate constants as usual (Table V . l ) . It was concluded that trace amounts of 0 2 were necessary f o r stoichiometric decarbonylation. The CH^Cl2 solvent used in these experiments had not been rou t i n e l y degassed since i t had been d i s t i l l e d under argon and was considered to be argon-saturated. The aldehyde, d i s t i l l e d in vacuo and stored under argon, was also presumed to be 0 2 free. The RuTPPtn-Bu^P),, complex i t s e l f does not react with 09 in solution over a period of several days 1 Figure V.9.- The Reaction of RuTPR(n-Bu3P)p_:with ,3.42x10 Phenylacetaldehyde. (1) I n i t i a l Spectrum. (2) 10% Reaction a f t e r 30 Min, No Oxygen Admitted. (3) Final Spectrum, 60 Min a f t e r Addition of Oxygen 68 at ambient temperatures. V i s i b l e l i g h t does not appear to be a factor since a reaction which was c a r r i e d out in the dark, except f o r 30 second i n t e r v a l s to record absorbances, proceeded in the usual manner, i . e . a pseudo-first-order decay of s t a r t i n g material. An attempted decarbonylation reaction, in which a radi c a l inhib-i t o r (2,6 d i t e r t - b u t y l - p - c r e s o l ) was added to the solvent in ca. 10" M concentration before d i s s o l u t i o n of the s o l i d RuTPP(n-Bu 3P) 2, f a i l e d to proceed; no change in colour or spectral c h a r a c t e r i s t i c s was observed a f t e r 3 hours, the normal time f o r these reactions to go to completion. 5.5 Discussion The oxidation of RuTPP(n-Bu 3P) 2 y i e l d e d the expected product, Ru(III)TPP(n-Bu 3P) 2 +, i d e n t i f i e d by i t s v i s i b l e spectrum. Figure V.10 shows the spectrum obtained f o r the analogous Ru(III)0EP(Bu 3P) 2 +and Ru(III)TPP(PPh 3)p +complexes, the former being produced by electrochemical oxidation and the l a t t e r by B r 2 o x i d a t i o n . ^ 4 The oxidation i s r e v e r s i b l e since addition of TBAB to solutions of Ru(III)TPP(n-Bu 3P) 2 + regenerates the s t a r t i n g material. An excess of tri-n-butylphosphine also seems to act as a reducing agent. Oxidation of Ru(porp)(C0)L species i s generally thought to occur at the porphyrin r i n g , rather than at the metal centre, y i e l d i n g i r-cation r a d i c a l species. Ru(II) n - c a t i o n ra d i c a l species have been well charac-t e r i z e d by t h e i r u . v . - v i s i b l e spectra, and also by e.s.r. data. Recent studies in t h i s laboratory have shown that oxidation of Ru0EP(C0)L (L = py) cn gives r i s e to two d i f f e r e n t products both n - c a t i o n r a d i c a l s , see Figure V . l l . The existence of two possible e l e c t r o n i c states f o r i r-cation r a d i -cals i s well-known, and the formation of one or other of these states seems to depend on the axial ligand, the counter ion present, and the nature of 65 the porphyrin. nm 500 6oo Figure V.10. Spectra of Two Ru(III) Porphyrin Species N 0 IidTosTvT o . o Figure V.II. Spectra of. Two i r - C a t i o n Radicals Produced by (1) Bromine and (2) Electrochemical Oxidation of RuOEP(CO)(py) Bromine oxidation of RuTPP(CO)(n-Bu^P), from the spectral data, does not r e s u l t in the formation of a -rr-cation r a d i c a l but a Ru(III) complex. Electrochemical oxidation does r e s u l t in the formation of RuTPP +'(C0)L, as judged by spectral data and chemical r e a c t i v i t y . The spectrum of th i s carbonyl oxidation product may be compared with those of various other -rr-cation r a d i c a l s , shown in Figure V.12. The general s i m i l a r i t i e s , decreased and broadened Soret bands at ca. 400nm, broad peaks from 500-600nm, and absorption above 600nm, are e a s i l y noted. Obviously the change in metal w i l l modify the spectral c h a r a c t e r i s t i c s as w i l l the solvent. From the spectrum of the reduced species, Figure V.8, i t appears that some ligand exchange has occurred, producing mainly RuTPP(C0)S and some RuTPP(n-Bu3P),>, which suggests that phosphine may have been l o s t by the. -rr-cation radical allowing a c e t o n i t r i l e , which i s in large excess, to coordinate instead. The free phosphine released would then react with the carbonyl complex to give the bis-phosphine complex. The spontaneous regeneration of RuTPP(C0)(S) from solutions of the -rr-cation r a d i c a l standing under argon, probably due to trace amounts of reducing agent in the solvent, i s also t y p i c a l of these ruthenium(II) cation r a d i c a l species, although some cation r a d i c a l s were found to be 65c quite stable in so l u t i o n . As yet no e.s.r. studies have been c a r r i e d out on t h i s product to confirm the nature of the cation r a d i c a l . It was hoped that a study of the redox properties of these two complexes, RuTPP(CO)(n-Bu3P) and RuTPP(n-Bu 3P) 2, would be of aid in explaining the non-catalytic decarbonylation of phenylacetaldehyde, equation V . l . It i s probable that some impurity in the aldehyde i s responsible f o r oxidation of eit h e r s t a r t i n g material or product thus . r e s u l t i n g in the loss of is o s b e s t i c s a f t e r approximately 70% reaction, and generation of a second set of isos b e s t i c s seems to support t h i s idea. A (NillTPPy-PFg-400 ' 500 ' COO ' TOO W o * length, nm 900 400 500 600 700 Wavelength,nm Optical absorption tpectra (in CH]C12) of Co(II)OEP ( ); [CoOIDOEPrCKV ( ); and [Co(IH)OEP] J* 2C104~(----). Figure V.l2. Spectra of -rr-Cation Radicals 73 Examination of the spectral data indicates that i t i s oxidation of the metal, giving a ruthenium(III) species, which i s occurring, see Figure V.l3. The broad absorbance ca. 360nm and general broadening i n the Figure V.l3. The Reaction of RuTPP(n-Bu3P)2 with Phenylacetaldehyde to Give a Ru(III) Species region 500-600nm i s t y p i c a l of Ru(lII) and the spectral changes are more t y p i c a l of Ru(III)TPP(n-Bu 3P) 2 + than Ru(III)TPP(CO)(n-Bu 3P) +, which has weaker absorbance below 400nm than the bis-phosphine species. This agrees with the observation that RuTPP^-Bu^P^ has a lower oxidation potential (0.5V) than the corresponding carbonyl, since CO tends to s t a b i l i z e the lower valence s t a t e . 4 4 It would be expected that i n a reaction mixture containing both the bis-phosphine and carbonyl complex, the bis-phosphine complex would be p r e f e r e n t i a l l y oxidized. Thus i t was concluded that the f a i l u r e of reaction V.l to go to completion was due, not to the s e t t i n g 74 up of an equilibrium, but to the interference of a secondary reaction involving oxidation of the s t a r t i n g material, RuTPP(n-Bu 3P) 2. Another important feature of t h i s reaction i s the influence of 0 2 on the progress of the decarbonylation. Since e f f i c i e n t l y degassing the solvent e f f e c t i v e l y i n h i b i t s the decarbonylation of phenylacetaldehyde i t appears that trace 0 2 i s necessary f o r i n i t i a t i o n and/or propagation of the reaction. It i s not c l e a r from the graph, Figure V.14, i f there i s any l i n e a r dependence of the reaction rate on pOp. The large v a r i a -tions i n k obsd values, Table V . l , may also be explained by t h i s 'dependence 1, e s p e c i a l l y as there i s a r e l a t i v e l y large v a r i a t i o n from batch to batch compared with the v a r i a t i o n s found within each batch. The d i f f e r e n t batches are sets of data obtained at d i f f e r e n t times, using d i f f e r e n t batches of solvent and aldehyde. The complete i n h i b i t i o n of the reaction by a r a d i c a l i n h i b i t o r suggests that 0 2 may be i n i t i a t i n g the reaction via a free ra d i c a l process. Organic oxidations frequently require 0 2 as a reagent and many of these reactions are free radical processes. One such reaction, the oxidation of methacrolein, i s c a r r i e d 66 out in the presence of a metal c a t a l y s t , usually a Co complex. A k i n e t i c study of t h i s reaction indicated a dependence on the f i r s t power of the 0 2 pressure. This e f f e c t of 0 2 was ascribed to i t s role in the abstraction of aldehyde hydrogen by the metal c a t a l y s t in the i n i t i a t i o n step of the reaction M n + + RCHO *• M^"1)"1" + RCO + H + since the f i r s t step of the reaction was thought to involve coordination of the aldehyde, part of the abstraction of H may have been accelerated by the presence of 0 o. 75 76 / Co < 0 = C However, t h i s explanation did not f u l l y account f o r the oxygen depend-ence of the r e a c t i o n . ^ This e f f e c t of oxygen, aiding in the abstraction of aldehyde hydrogen, i s a p o s s i b i l i t y f o r the stoichiometric decarbonylation reaction where the f i r s t step may be formation of a Ru(111)H and a r a d i c a l . However, more data on the e f f e c t of oxygen are needed before the d e t a i l s of t h i s reaction are elucidated. The question of whether trace oxygen i s a fa c t o r i n the c a t a l y t i c decarbonylation reaction, which i s thought to proceed via a free r a d i c a l mechanism, remains although some reactions have been c a r r i e d out i n degassed solvents (freeze-thaw) in attempts to improve r e p r o d u c i b i l i t y , no s i g n i f i c a n t e f f e c t was observed. However, t h i s does not rule out the p o s s i b i l i t y of oxygen playing a r o l e since trace oxygen may not have been t o t a l l y excluded. CHAPTER- 6 77 CONCLUSIONS The RuTPP(n-Bu 3P)2 complex reacted with CO i n toluene to give an equilibrium mixture with RuTPP(CO)(n-Bu^P). The CO i s e a s i l y displaced by an excess of tri-n-butylphosphine. Large excesses of t r i - n - b u t y l -phosphine i n h i b i t e d the reaction e n t i r e l y and added excess phosphine i s not necessary f o r pseudo-first-order k i n e t i c s . K i n etic and thermo-dynamic parameters were obtained f o r t h i s system and the values compared with those found f o r other Ru(porp )L2 systems and also some porphyrin and Pc systems containing Fe. A structure f o r the postulated f i v e -coordinate intermediate i s proposed, based on a comparison of the k ^l^^ values. It was concluded that a l l three Ru(porp )L2 species investigated, and possibly R u P ^ species, have five-coordinate intermediates whose structures are similar^with the metal atom in the plane of the porphyrin and of low or intermediate spin. A comparison of the k _ - [ / f ° r two MTPPL2 systems suggests that the nature of the metal influences the structure of the five-coordinate intermediate, the FeTPPL2 intermediate tending to be high spin with the metal more out of the porphyrin plane. Inspection of the K values f o r the analogous RuTPP(CO)(n-Bu^P) and RuOEP(CO)(n-Bu^P) complexes implies that the porphyrin has a strong influence on the equilibrium constant. Two possible reasons f o r t h i s e f f e c t were c i t e d : s t e r i c hindrance in the TPP sytem causing an increase in the value of k ^ and thus increasing K, and the difference in porphyrin b a s i c i t i e s , TPP being less basic than OEP. The RuTPP(PPh 3)2 complex, with added tri-n-butylphosphine in acetonitrile-dichloromethane, was found to be an e f f i c i e n t decarbonyla-t i o n c a t a l y s t f o r some organic aldehydes. Susbstituted benzaldehydes were decarbonylated more r e a d i l y than benzaldehyde. The decarbonylation 78 was i n h i b i t e d by r a d i c a l i n h i b i t o r s . Decarbonylation of cyclohexane-carboxaldehyde resulted in the production of some methylcyclopentane ascribed to the fragmentation of a r a d i c a l species, RCO. Other studies, e.g. e.s.r., c y c l i c voltammetry and i . r . suggested that Ru(III) i n t e r -mediates were involved as well as free r a d i c a l s . The existence of a solvated species in the c a t a l y s t solutions was indicated by spectral evidence. The d i s s o c i a t i o n of RuTPP(PPh 3) 2 and RuTPP(CO)(PPh 3) in d i l u t e s o l u t i o n indicated that some solvated species, such as RuTPP(CO)(S), is l i k e l y to play an important r o l e in the c a t a l y t i c decarbonylation. The stoichiometric reaction between RuTPP(n-Bu 3P) 2 and phenyl-acetaldehyde did not go to completion. However, a possible equilibrium involving toluene was ruled out and a secondary reaction, probably o x i -dation of RuTPP(n-Bu 3P) 2, i s considered to be the reason f o r the observed 'equilibrium.' Pseudo-first-order k i n e t i c s were observed f o r t h i s reac-t i o n in the absence of added phosphine. In the presence of stoichiometric amounts of tri-n-butylphosphine the reaction f a i l e d to proceed. The pseudo-first-order rate constants calculated were ir r e p r o d u c i b l e and scattered, implying that r a d i c a l formation might be implicated in a rate determining step. Trace oxygen was necessary f o r the i n i t i a t i o n and/or propagation of the stoichiometric decarbonylation. Suggestions f o r Further Study As regards the study of the carbonylation reaction i t would be i n t e r e s t i n g to obtain more data on RuTPP and other Ru porphyrin systems in order to generate a more complete picture. The main problem in obtaining more concrete evidence of i n t e r -mediates, etc., in the c a t a l y t i c reaction i s the i n s o l u b i l i t y of the TPP complexes in a c e t o n i t r i l e s o l u t i o n . The p o s s i b i l i t y of detecting and 79 i s o l a t i n g intermediates would increase i f the c a t a l y s t solutions were more concentrated; c o l l e c t i o n and i d e n t i f i c a t i o n of products would also be f a c i l i t a t e d . One way to increase the s o l u b i l i t y of the porphyrin i s to attach hydrocarbon ' t a i l s ' , which could be accomplished without serious d i f f i c u l t y as TPP i s the easiest and cheapest synthetic porphyrin to make. There i s , however, the p o s s i b i l i t y that any such modification to the porphyrin might adversely a f f e c t i t s c a t a l y t i c a c t i v i t y . As free r a d i c a l s seem to be implicated in the c a t a l y s i s , some deuteration studies might prove useful . A possible experiment i s the decarbonylation of a mixed aldehyde and deutero-aldehyde system. This type of experiment would indicate whether or not a bimolecular pathway was involved. Deuteration at the aldehyde hydrogen of cyclohexanecar-boxaldehyde, f o r instance, may provide some information about the nature of the proposed r a d i c a l intermediate. Kinetic isotope studies would give some indications about the rate determining step although rate con-stants were found to be nonreproducible. A l l of these experiments would also p r o f i t from increased c a t a l y s t concentration y i e l d i n g larger quan-t i t i e s of product to investigate. One of the major flaws in t h i s c a t a l y t i c system i s the lack of long term s t a b i l i t y , which combined with poor s o l u b i l i t y , prevents the accumulation of decarbonylated products. It i s not c l e a r how the c a t a l y s t i s destroyed. C e r t a i n l y the spectra indicate that some metal oxidation occurs, probably again due to trace impurities in the alde-hydes; free r a d i c a l s are also l i k e l y candidates f o r attack on the por-phyrin r i n g . 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