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Kinetics and mechanism of hydrogenolysis of a ruthenium(II) acyl complex Joshi, Ajey Madhav 1986

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KINETICS AND MECHANISM OF HYDROGENOLYSIS OF A RUTHENIUM!II) ACYL COMPLEX by AJEY MADHAV JOSH I B . S c , University of Poona, Poona, India, 1980 M.Sc , Indian Ins t i tu te of Technology, Bombay, India, 1983 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CHEMISTRY We accept th is thesis as conforming to the required standard The University of B r i t i s h Columbia September, 1986 © Ajey Madhav Josh i , 1986 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 D a t e Ocf- \1 DE-6(3/81) i i ABSTRACT This thesis describes the results of kinetic investigations into the hydrogenolysis of the ruthenium-acyl complex dicarbonylchloro-(norbornenoyDbis(triphenylphosphine)ruthenium(II ),RuCl(C0'C7H9) (C0) 2-(PPh3)2» 2. N,N-Dimethylacetamide (DMA) and toluene solutions of 2 react with one mole equivalent of H 2 to give HRuCl(C0) 2(PPh 3) 2, 3, and the unsaturated aldehyde product C^ HgCHO, 4. The subsequent, relatively slow hydrogenation of 4 by a further mole of H 2 to give the saturated aldehyde CyH-jiCHO, 5, was found to be catalyzed by 3. A detailed kinetic study on the hydrogenolysis of 2 in DMA solvent revealed a first-order rate dependence on LRuJjotaV a n inverse dependence on added [PPh^], and a f i r s t - to zero-order dependence on H 2 pressure. The following mechanism is proposed to account for these observations: k l RuCl(C0«C 7H 9)(C0) 2(PPh 3) 2 < > [RuCl(C0'C 7H g)(C0) 2(PPh 3)] + PPh3 k - l k, [RuCl(C0«C7H9)(C0)2(PPh3)] + H 2 • [HRuCl(C0) 2(PPh 3)] + C7H9CH0 K' [HRuCl(C0) 2(PPh 3)] + PPh3 ^ • HRuC1(C0) :fPPh 3l : Values of k-j and k_-j/k2 have been evaluated at 65°C (k-j = 4.6±0.5xl0 s~\ k_-|/k2 = 1.7±0.2) and the act ivat ion enthalpy and entropy for the k-j step determined (AH* = 69±7 kJmole" 1 , As* = -126+13 JK" 1 mole' 1** New, and as yet incompletely character ized, ruthenium complexes of stoichiometry "RuC l (C 6 H 9 ) (PPh 3 ) 2 " , 6, H RuCl(C 6 H 8 )(C 6 H g )(PPh 3 ) 2 -benzene solvate", 7, and RuCl^CgHg) (PPh 3 ) 2 , 8, have been synthesized by the reactions of 1,4-cyclohexadiene (CgHQ) with HRuCl(PPh 3) 3 'C 6Hg (complexes 6 and 7), and RuC l 2 (PPh 3 ) 3 (complex 8) , respect ive ly . Reactions of 6, 7, and 8 in DMA with H 2 and with CO were invest igated, and the organic and inorganic products i den t i f i ed : [H 2 RuC l (PPh 3 ) 2 ] 2 • C 6 H 1 2 [HRuCl(C0) 2(PPh 3) 2 + CgHg No react ion RuCl 2 (CO) 2 (PPh 3 ) 2 + C 6 H 8 CO TABLE OF CONTENTS Paoe ABSTRACT i i TABLE OF CONTENTS iv LIST OF TABLES v i i i LIST OF FIGURES ix LIST OF ABBREVIATIONS x i i ACKNOWLEDGEMENTS xiv Chapter I . INTRODUCTION. . 1 1.1 General Introduction 1 1.2 Homogeneous Hydrogenation 3 1.2.1 Hydrogen Act ivat ion 4 1.2.2 Hydrogenolysis 7 1.2.3 Substrate Act ivat ion 8 1.3 Hydroformylation 11 1.4 Aim of Work 18 Chapter I I . EXPERIMENTAL 19 2.1 Mater ia ls 19 2.1.1 Solvents 19 2.1.2 Gases 19 2.1.3 Other Mater ia ls 19 2.1.4 Ruthenium Compounds . . 20 V Page 2.1.4.1 Dichlorotr is( tr iphenylphosphine)- . . ruthenium(II) 20 2.1.4.2 Chlorohydridotr is(tr iphenylphosphine)-mthenium(II) - benzene solvate . . . 21 2.1.4.3 Bromohydri do t r i s ( t r t phenyl phosphine)-ruthenium(II) - DMA solvate 21 2.1.4.4 Chlorohydrido(norbornadiene)bis-(triphenylphosphine)ruthenium(II) . . 22 2.1.4.5 Bromohydrido(norbornadi ene)bis-(triphenylphosphine)ruthenium(II). . 23 2.1.4.6 Dicarbonylchloro(norbornenoyl)bis-( t r i phenyl phosphine)ruthem'um( II) 23 2.1.4.7 Dicarbonylchlorohydridobis-(triphenylphosphine)ruthenium(II) 24 2.1.4.8 Carbonylchloro(propiony1)bis-(triphenylphosphine)ruthenium(II) 24 2.2.1 Reaction of HRuCl(PPh 3)«C 6H 6 with 1,4-cyclohexadiene 25 2.2.2 Reaction of RuC l 2 (PPh 3 ) 3 with 1,4-cyclohexadiene 28 VI Page 2.3 Instrumentation 28 2.4 Gas-uptake Apparatus 31 2.4.1 The Apparatus 31 2.4.2 Procedure for a Typical Experimental Run . . 34 2.4.3 Measurement of Gas So l ub i l i t i e s 35 Chapter III . RESULTS AND DISCUSSION 37 3.1 The Ruthenium-acyl Complex 37 3.2 Hydrogenolysis of RuCl(C0«C 7 H 9 )(C0) 2 (PPh 3 ) 2 , 2 . . . 38 3.2.1 Stoichiometry of the Reaction, and Product Ident i f i ca t ion 40 3.2.2 Rate Measurements 45 3.3 Analysis of the Kinet ic Data 58 3.3.1 Dependence of the Rate on Ruthenium Concentration, [Ru]j 59 3.3.2 Dependence of the Rate on Added PPh 3 Concentration 61 3.3.3 Dependence of the Rate on Hydrogen Concentration 61 3.4 Dependence of the Rate-constant on Temperature . . 63 3.5 Discussion 67 v i i Page Chapter IV. REACTIONS OF RUTHENIUM(II)-CYCLOHEXADIENE COMPLEXES 72 4.1 React ions of " R u C l ( C 6 H g ) ( P P h 3 ) 2 " , 6 72 4 .1 . 1 React ion of " R u C l ( C 6 H g ) ( P P h 3 ) 2 " , 6, w i t h H 2 72 4 . 1 . 2 React ion of " R u C l ( C 6 H g ) ( P P h 3 ) 2 " , 6, w i t h CO 75 4.2 React ions of R u C l 2 ( C 6 H g ) ( P P h 3 ) 2 , 8 75 4.3 D i s cu s s i on 77 Chapter V GENERAL CONCLUSIONS AND SOME RECOMMENDATIONS FOR FUTURE WORK 82 5.1 General Conc lus ions 82 5.2 Suggest ions f o r Future Work 83 REFERENCES 84 viii LIST OF TABLES Table TItie Page III-l H2 solubility data in DMA at 65°C 49 II1-2 Kinetic data for hydrogenolysis of RuCl(C0*C7Hg)-(C0)2(PPh3)2 in DMA at 65°C 51 III-3 Temperature dependence of k.u 65 LIST OF FIGURES F i gu r e T i t l e Page 1.1a - 1.1c Schematic r ep re sen ta t i on of the types of metal -hydrogen i n t e r a c t i o n s 5 1.2 Schematic r ep re sen ta t i on of i n t e r a c t i o n between a metal and an a lkene 9 2.1 XH n.m.r . spectrum of R u C l 2 ( C 6 H g ) ( P P h 3 ) 2 i n CDC1 3 29 2.2 Anaerobic spec t r a l c e l l used f o r U V - v i s i b l e spectroscopy 30 2.3 The cons tan t -p res su re gas uptake apparatus . . . . 32 3.1 A t y p i c a l r a t e p l o t f o r H 2 uptake by £ i n DMA at 65°C 39 3.2 3 1 P { 1 H } n.m.r. spec t ra i n DMA of 2, (a) be fore and (b ,c ) a f t e r hydrogeno lys i s by H 2 , and that of (d) an au thent i c sample of HRuC l ( C0 ) 2 ( PPh 3 ) 2 41 3.3 High f i e l d lH n.m.r. spectrum of £ i n CDC1 3 . . . . 43 3.4 Gas chromatograms i n to luene of (a) the r e a c t i on m ix tu re a f t e r ~ 1 mole equ iva lent H 2 uptake by 2, (b) a standard s o l u t i o n of C 7H 9CH0 44 3.5 A t y p i c a l r a t e p l o t f o r the consumption of the second mole of H 0 by DMA so l u t i o n s of 2 at 65°C 46 3.6 Rate p l o t f o r the HRuC l (CO) 2 ( PPh 3 ) 2 - c a t a l y zed hydrogenat ion of C 7H QCH0 47 X LIST OF FIGURES Figure Ti t i e Page 3.7 H 2 s o l ub i l i t y in DMA at 65°C 50 3.8 Rate plots for the hydrogenolysis of 2 in DMA, at 65°C, at various [Ruly 53 3.9 A rate plot analyzed for 1st order Ru-dependence, [Ru] y = 7.58xlO" 3M 54 3.10 Dependence of the hydrogenolysis rate on [Ru]j at 65°C 55 3.11 Dependence of the rate on added [PPh.,], at constant -3 [Ru] T (4.56x10 M), under 1 atm H 2 pressure . . . . 56 3.12 Dependence of the rate on the H 2 pressure at [Ru]j= 4.56 x 10"3M (a) without and (b) with added PPh 3 (9.66xlO" 4M) 57 3.13 Plot of l o g ( i n i t i a l rate) against log([Ru]y) . . 60 3.14 Dependence of 1/rate on added PPh 3 concentration according to equation 3.10 . . . . . 62 3.15 Dependence of the rate on [H23 according to equation 3.10 64 3.16 Dependence of the rate-constant for hydrogenolysis of 2 on temperature 66 x i LIST OF FIGURES • F i gu re T i t l e Page 4.1 Rate p l o t f o r H 2 uptake i n DMA by "RuC l (CgH g ) -( P P h 3 ) 2 " at 25°C 73 4.2 Rate p l o t f o r CO uptake i n DMA by "RuC l (CgH g ) -( P P h 3 ) 2 \ at 30°C ? 6 4.3 Rate p l o t f o r CO uptake i n DMA by R u C l 2 ( C 6 H g ) ( P P h 3 ) 2 , at 50°C 78 x i i LIST OF ABBREVIATIONS The fo l lowing l i s t of abbreviat ions, most of which are commonly adopted in chemical l i t e ra tu re , w i l l be employed in th i s thes is : 1 3 C( 1 H} proton broad-band decoupled carbon n.m.r. Cp cyclopentadienyl, C^ H^  CYHD cyclohexadiene, C 6 H 8 DMA N,N-dimethylacetamide, CH 3C0N(CH 3) 2 dppe l,2-bis(diphenylphosphino)ethane (C 6 H 5 ) 2 PCH 2 CH 2 P(C 6 H 5 ) 2 Et e thy l , C 2 H 5 i . r . inf ra-red J coupling constant, Hz L,L' ligands In natural logarithm M molar concentration or metal atom Me methyl, CH3 n.m.r. nuclear magnetic resonance P phosphine ligands or pressure 3 1 P{ 1 H} proton broad-band decoupled phosphorus n.m.r. PPh 3 triphenylphosphine, (CgH 5) 3P P t o l 3 t r i (p- to ly l )phosph ine, (p-CH 3C gH 4) 3P [Ru] T t o ta l i n i t i a l concentration of RuCl(C0»C 7H 9)(C0) 2(PPh 3) UV u l t r a - v i o l e t X hal ide l igands, CI, Br or I x i i i 6 chemical s h i f t , ppm e molar ext inct ion coef f i c ient v frequency, cm~^  Some of the compounds used during the course of th i s work have been referred to by numeral symbols in th i s thesis for the sake of brevi ty and s imp l i c i t y . Following is the l i s t of complexes corresponding to each such symbol: Symbol Compound 1, HRuCl(NBD)(PPh 3) 2 2 RuCl(C0 'C 7 H 9 ) (C0) 2 (PPh 3 ) 2 3 HRuCl(CO) 2(PPh 3) 2 4 C7HgCH0 5 C7H1-,CH0 "RuCl (C 6 H 9 ) (PPh 3 ) 2 " "RuCl(C 6H g)(C 6H 8)(PPh 3) 2-benzene solvate" RuCl 2 (C 6 Hg)(PPh 3 ) 2 0 II C0»R denotes acyl substituent-(-C-R) X I V ACKNOWLEDGEMENTS I am extremely grateful to Professor B.R. James for his expert guidance and encouragement throughout the course of th i s work. I thank him for his time and utmost patience. I am indebted to Dr. Ian Thorburn for his assistance and helpful discussions throughout. I also thank members of the James gang for the i r support and fr iendship, and for making useful suggestions regarding th i s work in general. I wish to express my grat i tude to Mr. David Thackray for proof-reading some early dra f ts . The assistance and cooperation of various departmental services are gratefu l ly acknowledged. 1 CHAPTER I INTRODUCTION 1.1 General Introduction Catalysis remains one of the most active fields of chemical research and has been highlighted in the Pimentel report* as one of key significance. The last three decades have witnessed an ever-increasing interest in the study of homogeneous catalysis resulting in tremendous advances and development in the area. A large number of coordination and organometallic compounds, particularly those of transition metals, have been tested for catalytic activity toward a wide range of chemical reactions; these include hydrogenation of unsaturated organic substrates, olefin isomerization, dehydration of tertiary alcohols, hydroformylation, and water-gas shift reaction, to name but a few. The relative merits and demerits of homogeneous and heterogeneous catalysts, especially from an industrial point of view, have been widely 3-6 discussed. " Homogeneous catalysis offers the following advantages over the industrially more prevalent heterogeneous catalysis: i) Higher activity of homogeneous catalysts allows use of milder reaction conditions. i i ) Higher selectivity, specificity, and reproducibility of homogeneous catalysts are strongly desirable for industrial processes. i i i ) Catalytic properties of homogeneous catalysts may be varied systematically by changing the ligands. 2 iv) Ease of kinetic and mechanistic studies of homogeneous systems by conventional spectroscopic and kinetic techniques allows for more detailed mechanistic investigations. Oxygen- and moisture-sensitivity can be a problem in homogeneous catalysis, but the principal drawback is the diff i c u l t y in separation of the reaction product(s) from the soluble catalyst. However, in recent years attempts have been made to overcome this problem by anchoring homogeneous catalysts on either insoluble inorganic supports such as s i l i c a , alumina and zeolites, or on organic polymer supports such as polystyrene cross-linked with divinylbenzene (DVB).^ A drawback of such cross-linked supports is the nonequivalence of active sites because of differences in accessibility and microenvironment with resulting 8 differences in activity and stab i l i t y . However, this problem may be solved by using noncross-1inked polymer supports such as polyamides which also exhibit superior mechanical and thermal st a b i l i t i e s compared to the Q cross-linked styrene supports. A major thrust in research on metal cluster compounds is 9-11 supposedly aimed at mimicking catalytic sites at a surface and, to a lesser extent, development of "heterogenized" homogeneous c a t a l y s t s . 1 0 - 1 2 In a related vein, a better knowledge of homogeneous catalysis particularly at a cluster site may lead to a better understanding of 9-14 heterogeneous systems. The number of industrial processes currently using homogeneous catalysts is s t i l l relatively small. These processes include the Wacker process, the Oxo process, methanol carbonylation, oligomerization of 3 d ienes , and some Z i e g l e r - N a t t a systems. Use of homogeneous c a t a l y s t s con ta i n i ng c h i r a l l i gands f o r syntheses of o p t i c a l l y a c t i v e amino ac ids i s one of the most e x c i t i n g , recent developments. A number of comprehensive reviews have been p u b l i s h e d . 1 ^ " 1 9 C u r r e n t l y , the Monsanto Chemical Company u t i l i z e s a W i l k i n son - t ype rhodium(I) c a t a l y s t w i t h c h i r a l d iphosphine l i gands f o r l a r g e s c a l e asymmetric syn thes i s of the amino a c i d drug L-Dopa which i s used f o r treatment of Pa r k i n son ' s d i s ease . Homogeneous c h i r a l c a t a l y s t systems are be ing deployed a l so f o r i n d u s t r i a l s c a l e asymmetric syn thes i s of L - a s pa r t i c a c i d and L -pheny la l an ine , which are used in the syn thes i s of a popular sugar s u b s t i t u t e food a d d i t i v e , aspartame -a methyl es te r of the 20 d i pep t i d e L - a spa r t y l - L - pheny l a l a n i n e - be t t e r known by i t s tradename "Nutrasweet" . 1.2 Homogeneous Hydrogenat ion A l though the f i r s t repor t on homogeneous hydrogenat ion dates back to the yea r 1938 when Ca l v i n repor ted c a t a l y t i c r educ t i on of quinone to 21 hydroquinone us ing a cuprous ace ta te - q u i n o l i n e system, i t was only i n the ea r l y s i x t i e s that i n t e r e s t i n t h i s area rece i ved a boost . Some of the p i onee r i ng work was done by Ha lpe rn ' s group i n t h i s Un i v e r s i t y between 1 9 5 3 - 6 1 . 2 2 P a r t l y because of h i s t o r i c a l reasons and p a r t l y because of obvious p o t e n t i a l commercial a p p l i c a t i o n s , a major po r t i on of the vast l i t e r a t u r e on homogeneous c a t a l y s i s accumulated to date dea ls w i th homogeneous hydrogenat ion of unsaturated organ ic subs t r a t e s . A number of systems 4 have been studied in considerable deta i l and the i r mechanisms deduced. L i te rature on homogeneous hydrogenation, both ca ta l y t i c as well as sto ich iometr ic , i s very well-documented. A number of excel lent, comprehensive reviews * " and spec ia l ized texts , books and book 27-31 sections are also ava i lab le . The four basic processes occurring during homogeneous hydrogenation of an unsaturated substrate by a t rans i t i on metal complex are: i ) ac t ivat ion of molecular hydrogen i i ) substrate act ivat ion i i i ) hydrogen transfer to the substrate iv) release of the substrate and, in the case of a ca ta l y t i c react ion, regeneration of the ca ta lys t . 1.2.1 Hydrogen Act ivat ion Cata ly t i c properties of a metal complex are determined to a great extent by the coordination number and electron conf igurat ion of the 32 metal. Ac t i v i t y and spec i f i c i t y of a hydrogenation cata lyst are further influenced by the e lectronic propert ies, including it-effects, a-effects, 33 and also the s te r i c properties of the surrounding l igands. These factors supposedly provide the basis for cata lyst design. For a metal complex to be able to act ivate hydrogen or the substrate, coordinat ive unsaturation at the metal centre i s usually 5 essential. Coordinatively saturated complexes, which generally are unreactive toward hydrogen for lack of a vacant active site, may become active in solution through dissociation of a labile ligand. Not surprisingly, complexes with d 8 electron configuration which are often coordinatively unsaturated, or the d 6 complexes which can easily become so by loss of a ligand, constitute the largest group of hydrogenation catalysts. However, complexes of T i , Hf, Zr and Nb which have d 2 or d 3 electron configurations have also been investigated for possible catalytic 34 activity with positive results. The nature of interaction between molecular hydrogen and a metal complex is also dictated by the electronic and steric properties of the 3 33 surrounding ligands; ' the principal types of such an interaction are i) end-on (Fig. 1.1a, 1.1b) and i i ) sideways interaction (Fig. 1.1c). 24,33a M M H 2 M H, acts as an electron donor H2 acts as an electron acceptor 3-centre,2-electron transition state F i g . 1.1a F i g . 1.1b F i g . 1.1c 6 35 Kubas and co-workers have reported i so la t i on of molybdenum and tungsten phosphine complexes with side-on bonded H 2 l igand, analogous to the t rans i t i on state as in F i g . 1.1c. More recent ly, Morris et a l . have successful ly i so lated s imi lar iron and ruthenium r) 2-dihydrogen complexes of the type trans- [M( n 2 -H 2 ) (H) (dppe) 2 ] B F V The crysta l structure of the iron complex c l ea r l y shows the symmetrically coordinated -n 2-H 2 l igand. The energetics of hydrogen act ivat ion have important consequences on the ca ta l y t i c a c t i v i t y of the metal complex; whi le too strong a metal-hydrogen (M-H) bond may retard or even prevent the transfer of hydride to the coordinated substrate, too weak an interact ion between the metal and hydrogen may resul t in a concentration of the M-H species too small to achieve an appreciable reaction rate. 37 As noted by Halpern, act ivat ion of hydrogen may involve a) homolytic cleavage, b) di hydride formation, or c) overal l hetero lyt ic s p l i t t i n g ; these can be represented by the general equations 1.1-1.3. Hetero lyt ic s p l i t t i n g Homolytic s p l i t t i n g Dihydride formation (1.1b) (1.2) (1.3) (1.1a) 7 Fu r t he r , hydrogen a c t i v a t i o n may lead to hydrogeno lys i s of a 38 me t a l -me t a l l o i d bond such as M-Ge, M-S i , but most commonly a meta l -carbon (M-C) bond (eq . 1 .4) . R-MX n + H 2 H-MX h + R-H (1.4) These d i f f e r e n t processes of hydrogen a c t i v a t i o n have been d i scussed i n 39 40 d e t a i l In a number of review a r t i c l e s on hydrogenat ion. ' Of p a r t i c u l a r re levance to the t op i c of t h i s t h e s i s i s the hydrogeno lys i s of meta l -carbon bonds, which w i l l be cons idered b r i e f l y i n the f o l l o w i n g s e c t i o n . 1.2.2 Hydrogeno lys i s Hydrogeno lys i s , which fo rma l l y resembles a net h e t e r o l y t i c 38 s p l i t t i n g , i s exemp l i f i ed by the r eac t i on s shown below (eqs. 1 .5 -1 .7 ) . (1.5) (1.6) (1.7) 8 The importance of s tud ies on hydrogeno lys i s can hard ly be overemphasized, cons i de r i ng that a vast ma jo r i t y of s t o i c h i ome t r i c as we l l as c a t a l y t i c hydrogenat ion and hydro formy la t ion r eac t i on s i n v o l v e hydrogeno lys i s of meta l -carbon bond as the f i n a l s t e p . 3 8 , 4 1 In a d d i t i o n , i n a number of ins tances hydrogeno lys i s of metal a l k y l or a r y l d e r i v a t i v e s prov ides a way of generat ing the c a t a l y t i c a l l y a c t i v e hydr ide spec i e s . Such i s presumably the case w i th Z i e g l e r hydrogenat ion c a t a l y s t systems (eqs. 1.8a - 1 .8c ) . A lkene hydrogenat ion ca ta l y zed by Z i e g l e r - t y p e c a t a l y s t s probably i nvo l ves hydrogeno lys is of the metal a l k y l formed a f t e r i n i t i a l a l k y l a t i o n of the metal complex by an a l k y l a t i n g 42 agent such as A1R ? ; t h i s leads to the c a t a l y t i c a l l y a c t i v e monohydride. MX + R0A1 n o RMX - + H 9 n- i c R 0A1X + R-MX . c n-1 HMX . + R-H n-1 HMX . + ^C = C / n- i / \ • HMX . . n - 1 > k R'-MX n-1 (1.8a) (1.8b) (1 .8c) 1.2.3 Substrate Act ivat ion For hydrogenat ion to proceed, coo rd i na t i on of the unsaturated subs t r a t e to the metal i s u sua l l y necessary at some stage dur ing the c a t a l y t i c c y c l e . Coo rd ina t i on of an unsaturated organ ic s ub s t r a t e , such as an a lkene, to metal i s v i s u a l i z e d as over lap of the f i l l e d n-donor 9 o r b i t a l s of the a lkene w i th the o-acceptor o r b i t a l s of the metal ( F i g . 1 .2) . The s y n e r g i s t i c i n t e r a c t i o n - back donat ion of e l e c t r on dens i t y from f i l l e d metal non-bonding o r b i t a l s to the empty ant ibond ing o r b i t a l s of the a lkene - f u r t h e r helps s t a b i l i z e the meta l -a l kene bonding. F i g . 1.2 Reduct ion of e l e c t r on dens i t y on the coord ina ted a lkene, as evidenced by a decrease i n the double bond cha ra c t e r , a c t i v a t e s the a l kene . The subs t r a t e must coo rd ina te to the metal i n a p o s i t i o n c i s to the hydr ide f o r a subsequent hydrogen t r a n s f e r to occur . E l e c t r on wi thdrawing subs t i t uen t s on an o l e f i n and lower o x i d a t i o n s t a t e of the 43 metal tend to favour coo rd i na t i on of the o l e f i n . Depending upon whether hydrogen a c t i v a t i o n precedes or f o l l ows subs t r a t e a c t i v a t i o n , r e a c t i on pathways of hydrogenat ion have been 10 c l a s s i f i e d as o c cu r r i ng v i a 1) a hydr ide or s a tu ra t e rou te and i i ) an unsa tu ra te rou te , r e s p e c t i v e l y ; the former i s encountered more f r equen t l y . A lkene hydrogenat lon ca t a l y zed by HRuC l ( PPh 3 ) 3 p rov ides an e x ce l l e n t example of the hydr ide rou te (eqs. 1.9a, 1 . 9 b ) . ^ -P HRuClP 3 + a lkene +P HRuCl ( a ! k e n e ) P 2 ^ I — • RuCl (a l ky l ) ? 2 (1.9a) +P RuC l ( a l k y l ) P , -P R u C l ( a l k y l ) P 3 +H, HRuClP 3 + a lkane (1.9b) Hydrogenat ion of a lkenes ca t a l y zed by R h C l ( P P h 3 ) 3 i s thought to proceed by e i t h e r a sa tu ra t e or an unsa tu ra te rou te depending on the 45-47 subs t r a t e , but sometimes v i a both the routes s imu l taneous l y . A s i m p l i f i e d unsa tu ra te route i s shown i n scheme 1.1. Rh C l ( P P h 3 ) 3 + a lkene j—• RhCl ( a l k e n e ) ( P P h 3 ) 3 a lkane +PPh \ -PPh. RhC l ( a l k e ne ) ( PPh 3 ) 2 +PPh H R h C l ( a l k y l ) ( P P h 3 ) 3 HRhCKa l ky l ) ( P P h 3 ) 2 -PPh. Scheme 1.1 11 1.3 Hydroformylation 48 Discovered by Roelen in 1938, hydroformylation, also known as the "Oxo" react ion, refers to the addit ion of carbon monoxide and hydrogen to an alkene to produce an aldehyde as represented by the general equation 1.10. Hydroformylation i s i ndus t r i a l l y one of the most RCH=CH, + CO + H 9 • RCH~CH,CH0 + RCHCHO (1.10) important and widely u t i l i z e d homogeneous ca ta l y t i c processes, with the annual worldwide production of aldehydes from unsaturated hydrocarbons 49 50 running into several mi l l i ons of tons. ' The aldehydes themselves have l i t t l e d i rect app l i ca t ion . However, they serve as excel lent intermediates for the production of commercially more important der ivat ives such as alcohols (eqs. 1.11a,1.11b), acids (eq. 1.12), and amines (eq. 1.13), formed by hydrogenation, ox idat ion, and reductive amination, respect ive ly .^ 0 H 2 RCHoCH0 • RCH0CH0OH (1.11a) r n 2 OH base RCH2CHCHCH0 R (1.11b) H 2 RCH2CH2CHCH20H < RCH2CH=CCH0 R R ) 12 [0] RCH2CH0 • RCH2C00H (1.12) RCH2CH0 + H 2 + NH3 • RCHgCHgNHg + HgO (1.13) R = a l k y l , aryl A large number of t rans i t i on metal complexes catalyze the 41 50 51 hydroformylation react ion, ' ' but cobalt and rhodium carbonyls as well as phosphine - containing der ivat ives are by far the most e f fec t ive and e f f i c i en t ca ta lys t s . Ruthenium cata lys ts , although not as e f f i c i en t , are promising especia l ly because of the i r re l a t i ve l y low cost, high s t a b i l i t y , and high capacity for homologation react ions. Of l a te , ro platinum complexes have also received increasing attent ion. Hydroformylation has been the subject of numerous exhaustive 54 55 reviews, ' and while the majority of advanced inorganic chemistry 56 texts have sections dealing with th i s top ic , a substantial amount of patent l i t e r a t u r e on hydroformylation continues to be published every year. A number of review a r t i c l e s describe the k ine t i cs and mechanisms of hydroformylation react ions. Many cobalt and rhodium systems have been invest igated in considerable d e t a i l . The studies on the cobalt systems involve re l a t i ve l y high temperatures (80-200°C) and high C0/H 2 gas pressures (20-350 atm). The now generally accepted mechanism for hydroformylation of alkenes catalyzed by Co 2(C0)g, the hydride HCo(C0)4 being the act ive species, was f i r s t proposed by Heck and Breslow in 1961, 57 and i s out l ined in equations 1.14-1.19. 13 Co 2 <C0) 8 + H 2 • 2HCo(C0) 4 (1.14) HCo(C0) 4 • HCo(C0) 3 + CO (1.15) HCo(CO) 3 + RCH=CH2 • HCo(CO) 3(RCH=CH 2) • RCH £ CH 2 Co(C0) 3 (1.16) (1.18) c o CO H ? 1 HCo(C0) 4 HCo(C0) 3 < V RCH2CH2CH0 \ — RCH 2CH 2C0Co(C0) 3 (1.17) CO C o 2 ( C 0 ) 8 C o 2 ( C 0 ) ? (1.19) HCo(CO) 4 In the l a t e s i x t i e s , W i l k i n son and co-workers suggested d i s s o c i a t i v e (D) as we l l as a s s o c i a t i v e (A) mechanisms f o r a lkene hydro fo rmy la t ion c a t a l y z ed by rhod ium-t r ipheny lphosph ine systems under CO mi l d c ond i t i o n s (25°C, 1 atm C0 /H 2 p r e s su r e ) ; the mechanisms are summarized 1n scheme 1.2. The complex HRh (C0 ) 2 ( PPh 3 ) 2 was proposed as the key i n te rmed ia te that reacted w i t h the a l kene . L a t e r , a s i m i l a r mechanism was proposed by W i l k i n son et a l . f o r ruthenium-phosphine 59 systems. The ru then iu key a c t i v e In te rmed ia te . systems. The ruthenium(O) complex R u ( C 0 ) 2 ( P P h 3 ) 3 was proposed as the 14 15 Based on these and other mechanistic studies, hydroformylation of alkenes catalyzed by a t rans i t i on metal complex may be seen as consist ing of: i ) formation of a metal hydride complex, unless the precursor i t s e l f is a hydride, i i ) formation of a metal-alkyl intermediate presumably via a i t -o l e f i n i c complex, i i i ) CO insert ion to give the corresponding acyl intermediate, and f i n a l l y , iv) hydrogenolysis of the acyl complex by a) to afford the product aldehyde with regeneration of the metal hydride complex (eq. 1.18), or by b) the metal hydride to give the aldehyde and dinuclear metal precursor (eq. 1.19). The various intermediates involved have been isolated and/or characterized spectroscopica l ly for a number of systems including those of cobalt , rhodium, i r id ium, and ruthenium. As described ea r l i e r (Section 1.2.1), the k inet i cs and mechanisms of the steps ( i ) and ( i i ) have been studied in much deta i l in re la t ion to hydrogenation of alkenes. S im i l a r l y , voluminous work has been done on e luc idat ion of the k ine t i c and mechanistic aspects of carbonyl insert ion reactions using manganese a lky l carbonyls in p a r t i c u l a r . 6 0 ' 6 ^ The review A1 AO A a r t i c l e s by Wojc ick i , Calderazzo, and Kuhlmann et a l . among others provide exce l lent , deta i led accounts of the progress in the understanding 64 of carbonyl insert ion react ions. Recent work from Halpern's group on 16 nucleophi l ie ca ta lys i s of the CO migratory insert ion reaction shows evidence for a d i ssoc ia t i ve trapping mechanism (Scheme 1.3). LnM(C0)R + S L nM(C0.R)(S) L nM(C0.R)(S) + L' • L nL'M(C0.R) + S s LnM(C0)R + L* -—*• LnL*M(C0.R) (S = solvent) Scheme 1.3 In contrast, very l i t t l e i s known about the important product-forming step involv ing hydrogenolysis of an acyl complex. A major problem concerns the general i n s t ab i l i t y of acyl complexes. Meta l-acy ls , especia l ly those of group VIII metals, are k i ne t i c a l l y unstable and may undergo very f a c i l e carbonyl de- insert ion as well as reductive el iminat ion react ions, which i s why of course, complexes of cobalt , rhodium, ruthenium, and palladium usual ly make good hydroformylation and decarbonylat ion^ ca ta l ys t s . As noted above (see eqs. 1.14-1.19), hydrogenolysis of the acyl complex can be accomplished using either H 2 or a metal hydride. The l a t t e r allows for stoichiometr ic conversion of o le f ins to aldehydes 6 6 (eqs. 1.20,1.21). R-CH=CH2 + 2HCo(C0)4 • RCHgCHgCHO + Co 2 (C0) ? (1.20) or a l te rna t i ve l y , R-CH=CH2 + 2HCo(C0)4 + CO • RCHgCHgCHO + Co 2(C0)g (1.21) 17 Under hydroformylation conditions, however, hydrogenolysis by H 2 seems to prevai l . The non-detection of HCo(C0)4 by i . r . has been considered consistent with th i s , 6 ' 7 while some recent work of Markd and coworkers6^~7 (Veveals more substantial evidence. This recent work involves one of the very few kinetic investigations of hydrogenolysis of an acyl complex by H .^ Rate measurements on the reactions of (n-C 3H 7*C0)Co(C0) 4 and (i-C 3H 7«C0)Co(C0) 4 with H 2 and with HCo(C0)4 showed that H 2 is less reactive than the carbonyl hydride at lower temperatures but i ts relative reactivity increases with temperature so that under the high temperature and pressure conditions of hydroformylation hydrogenolysis by H 2 dominates. The complex acetyldicarbonylbis(trimethylphosphite)cobalt(I) reacts with H 2 stoichiometrically to form a hydridocobalt species and acetaldehyde (eq. 1 .22) . 7 1 CH 3«C0Co(C0) 2[P(0Me) 3] 2 + H 2 • HCo(C0) 2[P(0Me) 33 2 + CH3CH0 (1.22) Thomas, 7 1 in a study of kinetics of this hydrogenolysis reaction, reported at a Chemical Society meeting: i) a f irst-order rate dependence on the acylcobalt complex concentration and, i i ) a zero-order rate-dependence on the H 2 pressure. It was suggested that the rate controll ing step involved loss of a coordinated phosphite ligand, this step being followed by a relatively fast reaction with H 0 . 18 1.4 AIM OF WORK As pointed out above there are very few reports of k ine t i c studies of hydrogenolysis by H 2 of t rans i t i on metal acyl complexes. While the recent work of Markd and c o w o r k e r s 6 8 - 7 0 and the report in 1971 by Thomas71 concern mainly acyl complexes of cobalt , and are probably the only such studies made so fa r , no k ine t i c studies on hydrogenolysis of ruthenium acyl complexes have been reported. The ruthenium(II) acyl complex, d icarbonylch lorobis( t r iphenyl-phosphine)norbornenoylruthenium(II), RuCl(C0«C 7H g)(C0) 2(PPh 3) 2 , was f i r s t 17 synthesized and characterized in th is laboratory. This a i r - s tab le , 6-coordinate, 18-electron ruthenium(II) complex was found in preliminary studies during the present work to react slowly with H 2 , and the system appeared to be an excel lent candidate for conducting a detai led invest igat ion into a hydrogenolysis react ion (products, stoichiometry and mechanistic aspects). The resu l ts of th i s invest igat ion are described and discussed in Chapter I I I . Results of some experiments on related complexes, branching out from the main body of th is work, are included in Chapter IV. 19 C H A P T E R I I E X P E R I M E N T A L 2.1 Mater ia ls 2.1.1 Solvents Spectral or ana ly t i ca l grade solvents were obtained from MCB, BDH, Mal l inckrodt, F isher, Eastman or A ldr ich Chemical Co. Benzene, toluene and hexanes were ref luxed with, and d i s t i l l e d from, sodium metal/benzophenone under a nitrogen atmosphere. N,N'-dimethylacetamide (DMA) was s t i r r ed with CaH2 for at least 24 h, vacuum d i s t i l l e d at 35-40°C, and stored under argon in the dark. Methanol, ethanol, and dichloromethane were d i s t i l l e d after ref lux ing with the appropriate drying agents (Mg/I 2 for MeOH and EtOH, P 2 0 5 for CH 2C1 2). Diethyl ether and . n-octane were used as supplied without further pu r i f i c a t i on . A l l solvents were deoxygenated pr ior to use. 2.1.2 Gases Pur i f i ed argon (H.P.), nitrogen (U.S.P.) , oxygen (U.S.P.) , carbon monoxide ( C P . ) , hydrogen (U.S.P.) and ethylene ( CP . ) were obtained from Union Carbide Canada L td . ; a l l except hydrogen were used without further pu r i f i c a t i on . Hydrogen was passed through an Engelhard Deoxo ca ta l y t i c hydrogen pu r i f i e r to remove traces of oxygen. 2.1.3 Other Mater ia ls Reagent grade triphenylphosphine (Strem Chemicals, Inc.) and 20 t r i e t h y l am i n e (MCB) were used as supp l i ed , w i thout f u r t h e r p u r i f i c a t i o n , f o r s yn the t i c purposes. Norbornadiene (Eastman) and 1 ,4-cyc lohexad iene ( A l d r i c h ) were p u r i f i e d by pass ing through a column of a c t i v a t ed alumina ( F i s h e r , A-950 neut ra l chromatographic grade 80-200 mesh) p r i o r to use. 5-Norbornene-2-carboxaldehyde was obta ined from A l d r i c h and p u r i f i e d by vacuum d i s t i l l a t i o n at 58-60°C, and s tored under argon. Norbornane-2-carboxaldehyde was prepared by hydrogenat ion of a methanol (20 ml) s o l u t i o n of the norbornene precursor (5 ml) under a H 2 atmosphere (4 atm) us ing Pd/C as a c a t a l y s t ; methanol was pumped o f f a f t e r 5 h and the product was p u r i f i e d by d i s t i l l a t i o n . 2 .1 .4 Ruthenium Compounds The ruthenium, supp l i ed on loan by Johnson Matthey L t d . , was obta ined as RuC l 3 *xH 2 0 . Depending upon the batch , the ruthenium content va r i ed from 35-40%. A l l s yn the t i c r e a c t i o n s , un less s p e c i f i e d o therw i se , were c a r r i e d out under an atmosphere of argon, employing Schlenk techn iques , as a l l the ruthen ium(II ) complexes prepared i n the course of t h i s work were s u s c ep t i b l e to o x i d a t i o n by a i r , at l e a s t i n s o l u t i o n . 2 .1 .4 .1 D i ch1o ro t r1s ( t r i pheny1phosph ine ) ru then ium( I I ) , R u C l 2 ( P P h 3 ) 3 . 7 3 Hydrated ruthenium t r i c h l o r i d e , RuC l 3 *xH 2 0 (2.0 g, 7 mmol) was r e f l u x ed i n methanol (400 ml) f o r 15 minutes under a N 2 atmosphere. The s o l u t i o n was then coo l ed , PPh 3 (11.5 g, 43.4 mmol) added, and the s o l u t i o n r e f l u x ed fo r 3 h. The dark brown product which p r e c i p i t a t e d on coo l i ng 21 was then fi l t e r e d , washed with methanol and vacuum dried. Yield - 6.65 g (99%); calcd. for C ^ H ^ C l ^ R u C: 67.61%, H: 4.69%, CI: 7.41%; found C: 67.60%, H: 4.70%, CI: 7.20%. The 3 1P{ !H} n.m.r. data agree with 73r those reported. 2.1.4.2 Chlorohydridotr1s(triphenylphosphine)ruthen1um(II) - benzene  solvate, HRuCl(PPh 3) 3-C gH 6. 7 4 , 7 5 A benzene (400 ml) suspension of RuCl 2(PPh 3) 3(4.5 g, 4.7 mmol) was stirred with triethylamine, Et 3N (0.07 ml, 5.0 mmol) under a H 2 atmosphere for 18 h at room temperature. The violet product was fil t e r e d , washed f i r s t with ethanol (3 x 10 ml) to remove Et3N«HCl, then with diethyl ether, and vacuum dried. The complex is air-sensitive and blackens after a few hours exposure to air in the solid state. Its solutions are extremely air-sensitive and turn green within minutes of exposure to air. Yield - 4.1 g (87%); calcd. for C^H^C^Ru C: 71.86%, H: 5.19%, CI: 3.54%; found C: 71.87%, H: 5.21%, CI: 3.40%. v R u _ H : 2028 cm"1. 20 °C 6CDC1 : ~ 1 7 * 7 5 PP m (RLI-H, q, 26 Hz). The spectroscopic data agree with 3 those reported in the l i t e r a t u r e . 7 4 2.1.4.3 Bromohydridotris(tripheny1phosphine)ruthem'um(II) DMA solvate, HRuBr(PPh 3) 3»DMA. 7 6 A procedure similar to that used for preparation of the preceding compound (Section 2.1.4.2) was followed, but using RuBr 2(PPh 3) 3 as 22 precursor (1.0 g, 0.955 mmol) and DMA (10 ml) as the solvent. Addit ion of a base such as Et 3N was not necessary as the DMA solvent is basic enough to react with the HBr formed. The v io le t product was washed with DMA (2x2 ml) and vacuum dr ied. S imi lar to the preceding compound, th i s complex is a i r - sens i t i ve both in so l i d state as well as in so lut ion. Y ie ld - 0.52 g (51.6%); ca lcd. for CggH^NBrOP-jRu, C: 65.95%, H: 5.25%, N: 1.33%; found C: 65.68%, H: 5.24%, N: 1.41%. v R u _ H : 2035 cm" 1 ( l i t e r a tu re - 2030) . 7 6 The i . r . carbonyl stretch of DMA was observed at 1652 cm" 1 ( l i t e r a tu re - 1650) , 7 6 very c lose to the frequency for the uncoordinated solvent (1660 cm"1) ind icat ing that the DMA merely occupies a l a t t i c e s i t e in the c r y s t a l , s im i la r to the benzene molecule in HRuC l (PPh 3 ) 3 -C 6 H 6 . 7 7 6 2 0 ° C : -16 .46 ppm(Ru-H, q, 26 Hz). L 6 U 6 The precursor RuBr£(PPh 3) 3 complex was kindly made ava i lab le by Dr. I. Thorburn of th i s laboratory. 2.1.4.4 Ch io rohyd r l do tno rbo rnad i ene )b i s ( t r i pheny 1phosphine)  r u then i um( I I ) , H R u C l ( N B D ) ( P P h 3 ) 2 . 7 5 This complex was prepared by adding norbornadiene (2 ml) to a benzene (50 ml) suspension of HRuCl(PPh 3) 3*C gH 6 (1.9 g, 1.9 mmol), and then s t i r r i n g for 18h at room temperature. The resu l t ing red-brown solut ion was f i l t e r e d to remove any insoluble mater ia ls, and the f i l t r a t e reduced to « 25 ml and added to hexane (100 ml). The pale brown so l i d which prec ip i tated on s t i r r i n g for 6-8 h was f i l t e r e d , washed with hexane 23 (2x10 ml) and vacuum dried. Yield - 1.0 g (70%); C^H-^ClPgRu requires C: 68.47%, H: 5.21%, CI: 4.71%; found C: 68.41%, H: 5.20%, - i ?n °r CI: 4.59%. v R u _ H : 2082 cm . 6^ ^ :-8.90 ppm(Ru-H, t, 24 Hz). 31 1 P{ H) n.m.r. spectrum in CD2C12 showed a single peak at 40.4 ppm. The 72 spectroscopic data agree with those reported in the literature. 2.1.4.5 Synthesis of bromohydrido(norbornadiene)b1s(tr1pheny1phosphine)  ruthenlum(II), HRuBr(NBD)(PPh3)2. This compound was prepared by a procedure similar to that used for preparing the chlorohydrido counterpart (Section 2.1.4.4), but using HRuBr(PPh3)3»DMA (0.45 g, 0.43 mmol) as precursor. The precipitated light brown solid was washed with diethyl ether (2x5 ml) in addition to hexane (2x5 ml) on f i l t r a t i o n . Yield - 0.265 g (78%); C 4 3H 3 gBrP 2Ru requires C: 64.66%, H: 4.93%, Br: 10.01%; found C: 64.61%, H: 5.04%, Br: 9.85%. v R y _ H : 2108 cm"1. o|°°C:-8.43 ppm (Ru-H, t, 24.6 Hz). The 31 1 P{ H} n.m.r. signal in CgDg appeared at 39.4 ppm. 2.1.4.6 Dlcarbonylchloro(norbornenoy1)bis(triphenylphosphine)  ruthenium(II), RuCl(C0-C 7H 9)(C0) 2(PPh 3) 2. 7 2 A benzene (15 ml) solution of HRuCl(NBD)(PPh3)2 (0.3 g, 3.6 mmol) was stirred under CO atmosphere for 18 h. The resulting yellow solution was filtered to remove any insoluble materials, and the f i l t r a t e reduced in volume to « 5 ml. The yellow product, which precipitated after addition of hexanes (20 ml) to the f i l t r a t e and stirring for 3 h, was then fi l t e r e d , washed with hexane (2x5 ml) and dried in vacuo. Yield - 0.24 g 24 (73%). Calcd. for C 4 6 H 3 9 0 3 C l P 2 Ru , C: 65.91%, H: 4.66%, and CI: 4.24%; found C: 65.75%, H: 4.61%, and CI: 4.11%, v C Q : 2024 s, 1940 vs, 31 1 1910 sh; v c = Q : 1610 w. The P{ H} n.m.r. s inglet was observed at 30.4 ppm in DMA. The spectroscopic data are in agreement with those 72 reported in the l i t e r a t u r e . 2.1.4.7 D icarbony l c h l o r o hyd r i dob i s ( t r i pheny l phosph ine ) ru then ium( I I ) , H R u C l ( C 0 ) 2 ( P P h 3 ) 2 . 7 8 A DMA (20 ml) suspension of HRuCl(PPh 3) 3»CgH 6 (1.0 g, 1 mmol) was s t i r r ed under CO for 18 h. The purple suspension changed to a yel low so lut ion. The solut ion was concentrated to ~10 ml by removing the DMA under reduced pressure at ~40°C. The white so l id that prec ip i tated out was f i l t e r e d , washed f i r s t with ethanol (2x5 ml) to remove any traces of DMA, and then with acetone (5 ml), and dried under vacuum. Y ie ld - 0.43 g (60%); ca l cd . for C 3 gH 3 1 C10 2 P 2 Ru, C: 63.55%, H: 4.32%, CI: 4.95%; found C: 63.15%, H: 4.35%. 6 C D C 1 3 : " 4 • 5 0 p p m { R u " H ' *» 1 9 A H z ) * T h e 3 1 p ^ H > n.m.r. spectrum in DMA showed a s inglet at 39.5 ppm. The spectroscopic 78 data agree with the l i t e r a tu re data. This complex was also prepared using benzene as solvent instead of DMA fol lowing a s imi la r procedure. The white so l i d obtained was washed with hexane and dried under vacuum. 2.1.4.8 Ca rbony l ch l o ro (p rop iony ! )b i s ( t r i pheny1phosph ine ) ru then ium( I I ) , RuC l ( C0»C 2 H 5 ) ( C0 ) {PPh 3 ) 2 . 7 9 A benzene (10 ml) suspension of HRuCl (CO) 9(PPh.,) 9 (0.3 g, 25 0.42 mmol) was heated at 80°C under an ethylene atmosphere (1.5 atm) for 5 h. Ethanol (10 ml) was added to the cooled so lu t ion, af ter venting the excess gas pressure. The so lut ion was reduced to approximately half the volume, and the prec ip i tated bright yellow so l i d f i l t e r e d and washed with ethanol (2x5 ml), and dried under vacuum. Recrys ta l l i za t ion from dichloromethane/ethanol produced bright yellow crysta ls of RuCl(C0*C 2H 5)(C0)(PPh 3) 2*CH 2Cl 2 , which after prolonged drying (~3 days) under vacuum, afforded the propionyl complex RuCl(C0»C 2H 5)(C0)(PPh 3) 2-Y ie ld - 0.285 g (91%); ca lcd. for C 4 0 H 3 5 C10 2 P 2 Ru, C: 64.38%, H: 4.73%; C l : 4.76%; found C: 64.66%, H: 4.58%, C l : 5.02%. v C Q : 1950 vs, 1921 s, 1892 s cm"1 ( l i t e ra tu re - 1957 vs, 1926 s, 1900 s ) , 7 9 -1 79 v c = Q : 1637 vs cm ( l i t e ra tu re - 1643 vs) . In dichloromethane solut ion v C Q a n d v c = Q were observed at 1946, 1919 and 1640 cm"1 ( l i t e ra tu re - 1948, 1920, 1640) , 7 9 respect ive ly . The 3 1 P{ ] H} n.m.r. in CH 2C1 2 showed a s ingle peak at 31.5 ppm. 2.2.1 Reaction of HRuCl(PPh3)3»CgHg with 1,4-cyclohexadiene In an attempt to prepare a ruthenium(II) hydridodiene complex s im i la r to HRuCl(NBD)(PPh 3) 2, and possibly the corresponding acyl complex by subsequent react ion with CO, a benzene (20 ml) suspension of HRuCl(PPh 3) 3»C 6H 6 (0.7 g, 0.7 mmol) was s t i r r ed with 1.3 ml (13.8 mmol) of 1,4-cyclohexadiene (CgHg) at ~ 55°C. The i n i t i a l l y purple suspension, which changed into a dark red solut ion over ~ 48 h, was l e f t s t i r r i n g for another 12 h and then cooled to room temperature, reduced in volume to ~ 8 ml and f i l t e r e d to remove the unreacted HRuCl(PPh 3) 3 . Hexane (20 ml) was added to the f i l t r a t e and the solut ion s t i r r ed for 6 h. The brown 26 so l i d that prec ip i tated was f i l t e r e d of f , washed with hexane (2x5 ml) and vacuum dr ied. The yellow f i l t r a t e was retained for further work-up. Y ie ld - 0.23 g (44%); calcd. for "RuCl (C 6 H g ) (PPh 3 ) 2 " , C 4 2 H 3 g C l P 2 Ru , C: 67.97%, H: 5.26%, C l : 4.79%; found C: 68.28%, H: 5.21%, C l : 4.70%. The i . r . spectrum showed no Ru-H absorption, and also the n.m.r. spectrum, which has yet to be f u l l y interpreted, showed no 31 1 h igh- f i e ld hydride resonance(s). The P{ H} n.m.r. spectrum showed just a s ing let at 40.0 ppm, which is quite close to the norbornadiene analog (40.4 ppm). This brown complex was extremely a i r - sens i t i ve in so lut ion, and so l i d samples exposed to a i r slowly turned green within a few days. The nature of the brown so l i d is discussed further in Chapter IV. The yellow f i l t r a t e (benzene:hexane) obtained along with the brown product was concentrated to give ~ 2 ml of a dark red o i l . Addit ion of hexane (10 ml) prec ip i tated a yellow s o l i d , but much of i t redissolved during washing with hexane. The r e l a t i v e l y high s o l ub i l i t y in a number of organic solvents (CH 2C1 2, MeOH, acetone, Et 2 0) posed d i f f i c u l t i e s in i so-lat ion of th is yel low complex. However, addit ion of about 5 ml n-octane to the red o i l fol lowed by re f r igera t ion at 0°C for 2 days, prec ip i tated the yellow complex that was quick ly f i l t e r e d and dried as such under vacuum for 2 days. Y ie ld - 0.16 g. The i . r . spectrum showed no absorption in the hydride region. The H^ n.m.r. spectrum in CgDg at 20°C was again complicated, and has not yet been interpreted; there was, however, a complex pattern of weak high f i e l d s ignals in the -6 to -7 ppm 31 1 region ind icat ive of metal hydride(s) impur i t ies . The P{ H} spectrum showed a broad s ing let at 31.9 ppm, and also a few very low intens i ty s ignals which could not be assigned. 27 When the reaction between HRuCl(PPh 3) 3*CgH 6 and 1,4-cyclohexadiene was carr ied out for a longer period of time (~ 100 h instead of 60 h) , only the yellow product was obtained, no brown complex could be i so lated. Of in teres t , benzene solut ions of the brown s o l i d , henceforth represented as , , RuC l (C 6 H 9 ) (PPh 3 ) 2 " , when heated with excess 1,4-CYHD at ~ 55°C for 36 h, also y ie lded solut ions of the yellow complex. Based on these resul ts the chemistry in benzene can be summarized as: O -PPh, -V "RuC l (C 6 H 9 ) (PPh 3 ) 2 " (2.1) 55 °C brown product 55 °C O t yel low product (2.2) This yellow complex is tentat ive ly formulated as "RuCl(C 6Hg)(CgHg)(PPh 3) 2" (see Chapter IV) and is probably a benzene solvate (calcd. for C 5 4 H 5 3 C l P 2 Ru , C: 72.35%, H: 5.89%; found C: 71.87%, H: 6.21%). Reactions of both the brown and the yellow products with H 2 and with CO were invest igated (Chapter IV). These invest igat ions are incomplete but they do shed some l ight on the nature of the brown and the yellow species; further studies are necessary to establ ish the exact ident i ty of the two products. 28 2.2.2 Reaction of RuC l 2 (PPh 3 ) 3 with 1,4-cyclohexadiene A benzene (30 ml) suspension of RuC l 2 (PPh 3 ) 3 (0.5 g, 0.52 mmol) was s t i r r ed with 1,4-CYHD (0.75 ml, 9.4 mmol) at 60°C for 60 h, during which time the dark brown suspension changed to a reddish yellow solut ion containing a reddish orange prec ip i ta te . The mixture was reduced in volume to ~ 10 ml and the c r y s ta l l i ne so l i d was f i l t e r e d off , washed with hexane (2x5 ml), and dried in vacuo. Y ie ld - 0.20 g (50% based on "RuC l 2 (C 6 H 8 ) (PPh 3 ) 2 " formulat ion); calcd. for C 4 2 H 3 gC l 2 P 2 Ru , C: 64.91%, H: 4.87%, C l : 9.14%; found C: 64.95%, H: 4.64%, C l : 9.61%. The UV-vis ib le spectrum in DMA showed absorption maxima at 370 nm (e = 1940 M _ 1 cm _ 1 ) and ~ 475 nm (broad, z = 480 M ' W 1 ) . 31 1 The P{ H} n.m.r. spectrum in CD 2C1 2 showed a s ingle peak at 28.9 ppm, and a small amount of free PPh 3 (~ 3%) was also observed. The n.m.r. spectrum in CDC 1^  is shown in F ig . 2.1 The set of peaks in the 6 = 7-8 ppm region is assigned to the protons of the two PPh 3 l igands (30 H). The remaining signal at 6 5.40 ppm integrates to 8 H, which is assigned to the coordinated diene moiety. Reactions of th is complex with H 2 and with CO are discussed in Chapter IV. 2.3 Instrumentation Infrared spectra were recorded on a Nicolet 5 DX FT-IR spectrophotometer as Nujol mulls between Csl p lates, unless spec i f ied otherwise. UV-vis ib le spectra were recorded on a Perkin Elmer 552A spectrophotometer with thermostatted c e l l compartments, using anaerobic spectral ce l l s of path length 1.0 or 0.1 cm (F ig. 2.2). ro F l'9- 2 A : * H n - m - r - spectrum of RuCl2(CgH8)(PPh0)o in CDC10. 0.0 ppm '3'2 1 9* 2 - 2 : Anaerobic spectral ce l l used for UV-v is ib le spectroscopy. 31 H and P{ H}-nuclear magnetic resonance spectra were recorded on a Bruker WP80 (80 MHz for 1 H , 32.44 MHz for 3 1 P{ 1 H} ) , a Varian XL300 (300 MHz for 1 H , 121.4 MHz for 3 1 P{ 1 H} or a Bruker WH400 (400 MHz for lH) spectrometer, a l l operating in Fourier-transform mode, using tetramethyl-s i lane (TMS) and PPh 3 (~ -6 ppm w. r . t . 85% H 3P0 4) as standards. The spectrometers were equipped with var iab le temperature attachments which 31 were used when necessary. A l l P n.m.r sh i f t s are reported r e l a t i ve to 85% H 3 P0 4 , with downfield sh i f t s taken as pos i t i ve . Gas chromatographic analyses were performed on a Carle AGC311 (constant temperature) gas chromatograph equipped with a thermal conductivity detector (TCD) as well as a flame ion izat ion detector (FID), or a temperature programmable Hewlett Packard 5830A instrument equipped with a TCD. 0V101 or 5% Carbowax cap i l l a ry columns were used with helium as the ca r r i e r gas. Gas uptakes for stoichiometr ic or k ine t i c studies were measured on the constant pressure apparatus as described in the fol lowing sect ion. Elemental analyses were performed by Mr. P. Borda of th i s department. 2.4 Gas-uptake Apparatus 2.4.1 The Apparatus The constant pressure gas-uptake apparatus, used for s to i ch io -metric determinations and k ine t i c studies of reactions with small gas molecules H ? , CO and 0 ? , i s represented schematically in F ig . 2.3. F i g . 2.3; The constant pressure gas uptake apparatus. 33 The apparatus consisted p r inc ipa l l y of three parts, v i z . the reaction vessel , the measurement un i t , and the gas-handling part. The reaction vessel was a smal l , pyrex two-neck f lask (A) of approximately 25 ml capacity, with a dimpled bottom, and equipped with a dropping side-arm bucket (B). The f lask was connected to a cap i l l a ry oil-manometer (F) containing n-butyl phthalate - a l i qu i d of neg l ig ib le vapour pressure, at tap E through a f l e x i b l e glass sp i ra l tube (C). When cl ipped to a piston-and-wheel mechanical shaker (G) driven by a Welch var iab le speed e l e c t r i c motor, the f lask could be held immersed in a thermostatted o i l -ba th (H) - a four l i t r e glass beaker f i l l e d with s i l i c one o i l in an insulated wooden box. The cap i l l a ry manometer (F) was connected through tap I to a gas measuring burette, consist ing of a prec is ion bored tube (J) of known diameter, and a mercury reservoir (K). This measuring unit was thermostatted at 25°C in a water bath (M). The gas burette was connected v ia a Fischer and Porter te f lon needle valve (0) to the gas-handling part of the apparatus, which included a gas i n l e t (P), a mercury manometer (Q) and a Welch Duo-Seal rotary vacuum pump (R). Sta in less steel jacketed, e l e c t r i c a l l y i so lated heating co i l s were used to heat the two baths. Thermostatting was achieved by using Juno thermo-regulators and Merc-to-Merc relay control c i r c u i t s , along with mechanical s t i r r i n g . Temperature of the two baths could be maintained with in ± 0.05°C of the set temperatures. The gas uptake was measured with a ve r t i c a l l y mounted cathetometer, and times were recorded from a Lab-Chron 1400 timer. 34 I t was extremely important that the apparatus be mainta ined l e a k - f r e e dur ing a run . High vacuum Apiezon N grease was used f o r g reas ing a l l ground g lass j o i n t s and taps i n the apparatus. 2.4.2 Procedure f o r a Typ i c a l Exper imenta l Run In a t y p i c a l experiment, 10 ml of DMA (or the app rop r i a te so l ven t ) were p i pe t t ed i n t o the r ea c t i on f l a s k . The complex was weighed on an a n a l y t i c a l ba lance i n the small sample bucket (B) and suspended i n the f l a s k on the s ide-arm hook. The f l a s k was then at tached to the gas-hand l ing pa r t of the apparatus at po in t S through the g l a s s s p i r a l tube. A f t e r a c a r e f u l degassing of the s o l u t i o n by a f r eeze and thaw s t a t i c vacuum procedure, repeated at l e a s t t h r i c e , hydrogen was in t roduced i n t o the f l a s k t i l l the gas p ressure was s l i g h t l y l e s s than that de s i r ed . Taps D and T were c l o s ed , the s p i r a l arm and the f l a s k removed from the gas-hand l ing par t and at tached to the c a p i l l a r y manometer (F) at j o i n t E w i th the tap D s t i l l c l o s e d . The f l a s k was p laced in the o i l bath preheated to the des i r ed temperature, and c l i p ped to the mechanical shaker which was then s t a r t e d . The r e s t of the apparatus upto tap D was evacuated keeping the taps E, I , L, N and 0 open. About 10 minutes (~25-30 minutes f o r gas pressures of 0.5 atm or l e s s ) were a l lowed f o r thermal e q u i l i b r a t i o n between the r ea c t i on vesse l and the o i l - b a t h , and the shaker was then stopped. Meanwhile, hydrogen (or the approp r i a te gas) was admitted i n to the r e s t of the apparatus u n t i l the gas pressure was somewhat lower than that d e s i r e d . Tap D was opened and the p ressure ad jus ted to the des i r ed va l ue . 35 The needle-valve (0) and taps I and L were closed, and the i n i t i a l reading of the mercury level in the gas measurement burette (J) taken. The gas pressure in the gas-handing part was raised over that in the reaction f l ask . The apparatus was now ready for a run. The bucket containing the complex was dropped into the solut ion by turning the side-arm, and the shaker and the timer were started. For slow gas uptakes tap E was kept closed most of the time, but was opened ~ 2 minutes before taking a reading to allow for equ i l i b ra t ion , and closed again. The gas uptake indicated by the dif ference in the o i l levels of the cap i l l a ry manometer (F) was measured by carefu l ly al lowing the gas into the mercury reservoir v ia the needle valve (0) to balance the o i l leve ls and noting the corresponding r i s e in the mercury level of the burette at appropriate time in te rva l s . From knowledge of the diameter of the gas burette, the volume of gas consumed could be calculated from the r i s e in the height of the mercury level and expressed as moles of gas uptake per l i t r e of so lut ion. _5 The apparatus was precal ibrated and a value of 1.98 x 10 moles of gas uptake per centimeter r i s e in the mercury level was ca lcu lated. 2.4.3 Measurement of Gas S o l u b i l i t i e s The gas-uptake apparatus described above was used for measure-ment of s o l ub i l i t y of hydrogen in DMA at spec i f i c temperatures and pressures. Typ ica l l y , DMA (10 ml) was degassed using the freeze-thaw 36 procedure, the taps D and T were then closed, and the f lask along with the sp i ra l arm transferred to the cap i l l a ry manometer. After the system upto the tap D was evacuated, hydrogen was introduced, and i t s pressure adjusted approximately to that required. Tap D was opened and the exact pressure noted, the taps J , and L, and the needle valve (0) were c losed, and the timer and the shaker started. The immediate gas-uptake was then measured. 37 CHAPTER I I I RESULTS AND DISCUSSION 3.1 The Ruthenium-acyl Complex The ru then ium!I I ) norbornadiene (NBD) complex HRuC l (NBD) (PPh 3 ) 2 , l 75 f i r s t repor ted by W i l k i n son et a l . i n 1968, e x i s t s i n s o l u t i o n as a s i n g l e isomer, w i t h the meta l , the hydr ide , and an a lkene n-bond i n a 72 cop lanar c i s arrangement which i s f avourab le f o r o l e f i n i n s e r t i o n . React ion of t h i s hydr idod iene complex w i t h carbon monoxide, dep i c ted i n the equat ion 3 . 1 , i s i r r e v e r s i b l e and leads to the format ion of the acy l complex d i c a rbony l ch l o r o {no rbo rnenoy l ) b i s ( t r i pheny l phosph i ne ) -ru then ium( I I ) , R u C l ( C 0 ' C 7 H 9 ) ( C 0 ) 2 ( P P h 3 ) 2 , 2, v i a consecu t i ve o l e f i n and carbony l i n s e r t i o n s i n to the meta l -hyd r i de and the me t a l - a l k y l bonds, + . , 72 r e s p e c t i v e l y . 3 CO H R u C l ( C 7 H g ) ( P P h 3 ) 2 • R u C l ( C 0 . C 7 H 9 ) ( C 0 ) 2 ( P P h 3 ) 2 (3.1) H 1 2 38 Detai led H, J ± P{ A H} , and CrH} n.m.r. studies, including se lec t ive homonuclear decoupling experiments, by Dekleva from th is laboratory showed that the acyl substituent occupies the thermodynamically less favourable 72 endo pos i t ion of the norbornene moiety, as represented by 2. During preliminary studies in the present work, DMA and toluene solut ions of 2 were found to absorb H 2 very slowly at 1 atm H 2 pressure and 40-60°C over a period of several hours. It was decided to invest igate th i s reaction in more deta i l with regard to the products, the stoichiometry, and the k ine t i c s of the react ion. The resul ts and discussion of these studies are described in the fol lowing sect ions. The bromo- complex HRuBr(NBD)(PPh3)2 (Section 2.1.4.5) reacted with ~ 3 mole equivalents of CO in DMA solvent at 30°C to give presumably an acyl complex analogous to 2. 3.2 Hydrogenolysis of RuCl(C0«C 7 H g )(C0) 2 (PPh 3 ) 2 , 2. The DMA and the toluene solut ions of 2 absorbed ~ one mole equivalent of hydrogen over about 20 hours at 65°C under 1 atm pressure of H 2 . The i n i t i a l yel low colour of the solut ions became more intense during the gas uptake. A typica l plot of the H 2-uptake against time in DMA is shown in F i g . 3.1. An extremely slow absorption of H 2 was then noted beyond the f i r s t mole equivalent uptake, unt i l f i n a l l y a tota l of ~ 1.9 mole equivalents of H 2 were consumed in ~ 130 hours. The 'ex t ra ' H 2 consumed ar ises from hydrogenation of the i n i t i a l l y formed aldehyde product (see equation 3.3). DMA solut ions of the propionyl complex RuCl(C0*C 2H 5)(C0)(PPh 3) 2 f a i l ed to react with 1 atm H„ at 65°C. 2.5 n 2 H 1.5 H CO VO 0.5 H 0 + 1000 2000 3000 4000 5000 Time , m i n 6000 7000 8000 9000 F i g . 3 . 1 : H ? uptake by 2 in DMA at 65°C under 760 tor r H 9 pressure. 40 3 .2 .1 S to i ch lomet ry of the Reac t i on , and Product I d e n t i f i c a t i o n As seen from the F i g . 3 . 1 , the DMA so l u t i on s of 2 f i n a l l y absorb approx imate ly two moles of h*2 per mole of the ruthenium complex. Th i s obse rva t i on i s exp la ined by ( i ) hydrogeno lys is of 2 by the f i r s t mole of H 2 l e ad i ng to a hydr idoruthenium product (3) and the corresponding aldehyde (4 ) : R u C l ( C 0 . C 7 H 9 ) ( C 0 ) 2 ( P P h 3 ) 2 + H 2 - HRuC l ( C0 ) 2 ( P Ph 3 ) 2 + C 7H gCH0 (3.2) CO C l ^ | P CO p. p H I R u .CO 'CO Cl CHO and ( i i ) a r e l a t i v e l y slow hydrogenat ion of 4 to the sa tu ra ted aldehyde (5) by a second mole of H 0 : a 30 »4 ppm 39.5 30»4 ppm c 39.5 ppm J 39.5 ppm 31n/l. F i g . 3 .2 : P{ H} n.m.r. spect ra i n DMA at 20°C of (a) 2 (c) 2 + H 2 , a f t e r heat ing f o r 20 h at 65°C and (b) 2 + H 2 , a f t e r heat ing f o r 2 h at 65°C ~ (d) the au then t i c sample of HRuC l ( C0 ) 2 ( P Ph 3 ) 2 . 42 31 1 The hydrogenolysis of 2, in DMA, was monitored by P{ H} n.m.r. spectroscopy. The spectra are presented in F ig . 3.2 along with that of an authentic sample of HRuCl(CO) 2(PPh 3) 2 , which was prepared by carbonylation of a DMA solut ion of HRuCl(PPh 3) 3«C gH 6 (see Section 2.1.4.7) . A s ing let appeared at 39.5 ppm and grew in intens i ty as the H 2 reaction proceeded; after ~ 1 mole equivalent H 2-uptake the n.m.r. signal due to 2 (30*4 ppm) had disappeared. Removal of DMA from the f i na l solut ion y ie lded a pale yel low so l id which showed strong i . r . absorption bands at 2042, 2036, 1994 and 1982 cm" 1 , corresponding to those of the carbonyl absorption bands of HRuCl(C0) 2(PPh 3) 2 (2043, 2035, 1993 and 1981 c m " 1 ) , 7 8 which however is a white s o l i d . A weak absorption was also observed at ~ 1886 cm" 1 , which i s charac te r i s t i c of an intense yel low photoisomerization product of HRuCl(C0) 2 (PPh 3 ) 2 . The yel low colour of the product iso lated from the uptake solut ions is presumably due to th i s photoisomerization product, 81 which i s of unknown geometry. F i na l l y , a high f i e l d hydride resonance 20 °C 1 at 6Q D C 1 = -4.49 ppm ( t , 19.4 Hz) in the H n.m.r. spectrum of 3 3 78 (F ig . 3.3) confirms the ident i ty of the inorganic product. The complex, HRuCl(C0) 2(PPh 3) 2 , was f i r s t prepared in th i s 78 laboratory. Although a number of geometrical isomers are possible, only one has been iso lated in pure form (the white so l id) and assigned the c i s - c i s - t r an s - structure shown for 3 in equation 3.2. 43 • i i 1 1 — i 1—t i - 2 - 3 - 4 - 5 - 6 -7 ppm F i g - 3 .3 : High f i e l d H n.m.r. spectrum of 3 i n CDCU . The unsaturated aldehyde product 4 formed dur ing the hydrogeno lys i s was detected and i d e n t i f i e d by gas chromatography by comparison w i t h a commerc ia l ly a v a i l a b l e sample of 5-norbornene-2-carboxa ldhyde. Us ing a 3 m 0V101 column at 100°C on a Ca r l e AGC311 instrument equipped w i t h a f lame i o n i z a t i o n de tec to r (FID) , a r e t e n t i o n t ime of 14.8 min was measured f o r both 4 and the au thent i c sample of C^HgCHO i n to luene ( F i g . 3 . 4 ) . Dur ing hydrogeno lys i s of the CH^C^ so l u t i o n s of 2, the d isappearance of the acy l i . r . absorp t ion band at 1610 cm" 1 was accompanied by the appearance of a new i . r . absorp t ion band at 1717 cm" 1 , which i s i d e n t i c a l to the carbonyl s t r e t c h of the aldehyde C,H oCH0 i n CH 0 C1 0 . 44 Fig. 3.4: Gas chromatograms of (a) the reaction mixture after ~ 1 mole equivalent H2 uptake by 2 in toluene, (b) a standard solution of C7H9CHO in toluene. 45 The product of the hydrogenation reaction 3.3 was s im i l a r l y detected and i den t i f i ed using gas chromatographic techniques, by comparison with CyH^CHO (retention time 13.0 min under the condit ions noted above), which was prepared by hydrogenation of the commercially ava i lab le precursor (4). The saturated aldehyde (5) could be detected by GC after approximately 0.9 mole equivalent of H 2 uptake; only ~ 5-10% of 5 was detected after 1 mole H 2 uptake (F ig . 3.4). An i n i t i a l rate of about 8 1 3.5 x 10 Ms was calculated for consumption of the second mole of H 2 at 65°C ([Ru] T = 4.56 x 10" 3 M, P„^ = 760 tor r ) in the range 0.95-1.30 mole equivalent h"2-uptake (F ig . 3.5). A very slow absorption of H 2 was observed by DMA solut ion of CyHgCHO in the presence of a stoichiometr ic amount of 3 at 65°C, with -8 i n i t i a l rates of the same order of magnitude (~ 10" ) as observed for the second mole of H^-uptake by solut ions of 2. The hydrogenation of 4 was also found to be catalyzed by 3 at 65°C in DMA; with [Ru] = 4.00 x 10" 3 M and [4] = 0.17 M, an i n i t i a l l inear rate of 1.35 x 10~6 Ms" 1 was observed (F ig . 3.6). The data for th is s ingle experiment suggest the ca ta l y t i c react ion is c lose to 1st order in unsaturated aldehyde concentration. 3.2.2 Rate Measurements A constant pressure gas-uptake apparatus described ear l i e r (Sections 2.4.1, 2.4.2) was used to monitor the reaction between the 46 6.5 F l ' 9 - 3 - 5 - A typ ica l rate plot for the consumption of the second mole of H 2 by DMA solut ions of 2 at 65°C, [Ru]y = 4.56xlO" 3M 47 6000 7000 Time , s F 1 9- 3 - 6 : Ra*e P^t for the HRuCl(C0 2)(PPh 3) 2-catalyzed hydrogenation of C7HgCH0 fn DMA at 65°C 48 ruthenium acyl complex and Hr,. The H 2 gas-uptakes were measured at to ta l -3 ruthenium complex concentrations, [Ru]y, ranging from 2.5x10 to _3 8.0x10 M, at d i f ferent H^-pressures (190-760 t o r r ) , and at various temperatures (50-70°C), changing only one parameter at a time. Most measurements were made at 65°C, the rates becoming inconveniently slower at lower temperatures. Rates were measured for the i n i t i a l uptakes of upto ~ 0.3-0.4 mole equivalents of h^. DMA was chosen as the solvent over toluene because of the convenient lower vapour pressure of the former (10-35 t o r r ) 8 2 compared to that of toluene (100-300 t o r r ) 8 3 over the temperature range employed (50-70°C). Of interest , l i t t l e difference was observed in the rates of the reaction using either DMA or toluene as solvent (see TABLE I I I - 2 ) . The rates of H 2 uptake by 2 remained close to l inear over a longer range (upto ~ 0.4 mole equivalents uptake) than expected. This l i nea r i t y of rates, even beyond ~ 0.2 mole equivalents (20%) i n i t i a l uptake, is presumably due to addit ional contr ibut ion from the concurrent hydrogenation of the product aldehyde 4 (more discussion on p. 68) according to react ion 3.3. Rates calculated from the analyses of rate data for 1st order Ru-dependence within ind iv idual runs in the i n i t i a l ~ 20% region were found to be almost ident ica l to the i n i t i a l rates measured d i r e c t l y from the respective H 2 uptake plots (see for example, F i g . 3.9 and TABLE I I I - 2 ) . Therefore, i n i t i a l rates, which could be measured very conveniently, have been employed throughout th is study. The s o l u b i l i t y of hydrogen in DMA at 65°C at various h"2-pressures was determined using the procedure described in Section 2.4.3, and was found to obey Henry's law at least upto one atmosphere pressure of hydrogen (TABLE I I I - l , F i g . 3.7). The K value defined as the ra t io of 49 the molar s o l ub i l i t y of hydrogen [H^] to the hydrogen pressure P(H 2) in to r r , was calculated from the slope to be 2 .86x l0 - 6 Mtor r " 1 . Further h^-solubi l i ty data at d i f ferent temperatures were taken from ea r l i e r 84 _ 6 , measurements from th i s laboratory (K = 2.32 , 2 . 7 0 and 2.82x10 Mtorr at 30, 50, and 60°C respect ive ly) . TABLE 111-1 So l ub i l i t y of H 2 in DMA at 65°C. PH , to r r [H 2] x 10 3 , M 760 570 380 190 2.19 1.62 1.05 0.56 K = [H 2 ] /P H = 2.86x l0" 6 Mtorr" 50 3 2.5 H 9 0 0 H 2 pressure , t o r r F i g . 3.7: H 2 s o l u b i l i t y i n DMA at 65°C. 51 TABLE II1-2 •Kinetic data for hydrogenolysis of RuCl (C0«C 7H g)(C0) 2(PPh 3) in DMA at 65°C. [Ru] T X [H 2] I n i t i a l rate x lO 3 , M tor r x lO 3 M x lO 7 , Ms" 1 2.50 760 2.17 1.11 3.13 760 2.17 1.42 3.88 760 2.17 1.75 4.56 760 2.17 1.95 4.81 760 2.17 2.09 6.03 760 2.17 2.69 7.58 760 2.17 3.40 4.56 660 a 1.89 1.96 4.56 660 1.89 1.93 4.56 570 1.63 1.99 4.56 380 1.09 1.91 4.56 190 0.54 2.01. 4.56 760 2.17 1.22° 4.56 570 1.63 1.09b 4.56 300 0.86 0.73 b 4.56 760 2.17 0.73 c 4.56 760 2.17 0.53 d 4.56 760 2.17 0.46 e 4.56 760 2.17 0 .00 f 4.80 760 2.05 0.679 4.80 760 0.62 n (a) + P C Q i 100 t 0 r r > P Tota l : 7 6 0 t o r r . (b) - ( f ) At added [PPhJ x W = (b) 0.966 M, (c) 2.42 M, (d) 3.87 M, (e) 4.83 M, and (f) 45.6 M (g) Rate in DMA at 50°C (h) Rate in toluene at 50°C. Error in the rate values is estimated at ~5-105» from least squares analyses of the i n i t i a l - r a t e p lo ts . 52 The dependence of the i n i t i a l rates on the to ta l ruthenium concentration [Rujy, hydrogen pressure, and added triphenylphosphine were studied at 65°C. The reaction rates were also measured using h /^CO mixtures in place of hydrogen to study any possible ef fect of external CO on the hydrogenolysis. The k ine t i c data are summarized in TABLE I I I -2 . F i na l l y , the temperature dependence of the i n i t i a l rates was investigated at a s ingle ruthenium concentration in the absence of added PPh^ (TABLE I I I -3 ) . The rate plots for the hydrogenolysis at 65°C under one atmosphere hydrogen pressure for d i f ferent to ta l ruthenium concentrations ranging _3 between 2.5-8.0 xlO M can be seen in F ig . 3.8. The ind iv idua l rate plots analyzed well for a f i r s t - o rde r dependence on [Ru] (for example F ig . 3.9). The plot of the i n i t i a l rate against the to ta l ruthenium concentration is shown in F ig . 3.10 and is f i r s t order in [Ruj-j-. The ef fect of added PPh 3 was investigated at a to ta l ruthenium concentration of 4.56x10 M, under 1 atm H 2 pressure. The rate of hydrogenolysis was found to decrease with added phosphine as seen in the F i g . 3.11, and at ~ 10-fold excess of PPh 3 no H 2 uptake was observed over 24 hours. The effect of varying the H 2 pressure was studied at [Ru] T = 4 .56x l0" 3 R without and with added PPh 3 (F ig. 3.12). Without PPh 3 > a zero-order dependence on H 2 pressure was observed from 760 to 190 to r r , whi ls t at the added PPh 3 concentration of 9.66xl0" 4 M, a decrease in the rate of hydrogenolysis was observed on decreasing the H 2 pressure from 760 to 300 t o r r . For a reaction carr ied out under a mixture of H 9 and CO 53 4 . 5 -4 -20000 25000 Time , s F*9» 3«8- Rate plots for the hydrogenolysis of 2 fn DMA, at 65°C, at various [Ru]j. [Ru] T x l0 3 ,M = (a) 7.58, (b) 6.03, (c) 4.81 and (d) 3.88. 54 -1.8 - 2 Rate obtained from the slope (initial 20% region) 3.5xl0"7 Ms"1 -2 .2--2.4 o -2 .6-- 2 . 8 -- 3 --3 .2 -r 5 i 10 15 r 3 20 i 25 Time t, x 10 , s 30 Fig. 3 .9 : A r a t e p l o t analyzed f o r 1 -o rde r Ru-dependence i n DMA at 65°C [ R u ] T = 7.58 x 1 0 " 3 M 55 F l'9- 3 - 1 0 : Dependence of the hydrogenolysis rate on [Ru] T at 65°C. 56 57 3 2.5-0 . 5 -- T I 1 1 1 I 1 I 1 100 200 300 400 500 600 700 600 900 pressure , t o r r Fig. 3.12: Dependence of the rate on the H 2 pressure at constant [Ru3T (a) added [PPh 3] = 0 (b) added [PPh 3] = 9.66 x 10"4 M. [Ru] T = 4.56xlO'3M 58 ( P „ =660 tor r and P^Q=100 t o r r ) , the I n i t i a l rate was essent ia l ly the same as that measured under just 660 to r r H 2 (TABLE II1-2) . 3.3 Analysis of the K inet i c Data To explain the f i r s t - o rde r dependence on [Ru] T , the inverse dependence on added [PPh 33, and the f i r s t - to zero-order dependence on H 2 pressure, the fol lowing mechanism i s proposed: k l RuCl(C0.C 7 H 9 ) (C0) 2 (PPh 3 ) 2 [RuCl(C0.C 7H 9)(C0) 2(PPh 3)] + PPh 3 (3.4) k - l k [RuCl(C0'C 7 H 9 )(C0) 2 (PPh 3 )] + H 2 — ^ [HRuCl(C0) 2(PPh 3)] + C7HgCH0 (3.5) K' [HRuCl(C0) 2(PPh 3)] + PPh 3 — • HRuCl(C0) 2(PPh 3) 2 (3.6) where k p k_^ , and k 2 are the rate constants of the indiv idual steps, and K' represents the equi l ibr ium constant for reaction 3.6. Applying the steady-state treatment to the intermediate, [RuCl(C0»C 7H 9)(C0) 2(PPh 3)], the fol lowing rate law is obtained: d[Ru] T d[H ?] k , k ? [Ru] T [H ? ] Rate = - L = i_ = Li L i (3.7) dt dt k_j[PPh 3] + k 2 [H 2 ] 59 3.3.1 Dependence of the Rate on Ruthenium Concen t r a t i on , [ R u ] T At constant [H 2] and constant [PPh 3] , the rate equation 3.7 reduces to: Rate = k o b s - [ R u ] T (3.8) where k , k 9 [ H 9 ] k b = 1 2 2 (3.9) 0 b S k_ 1 [PPh 3 ]+k 2 [H 2 ] The Ru-dependence was measured in the absence of added PPh3» The f i r s t - o rde r Ru dependence, evident from the s t r a i gh t - l i ne plots shown in F igs . 3.9 and 3.10, requires that the [PPh 3] term remains e f fec t ive ly constant and/or neg l ig ib le . The independence of the reaction rate on H 2 under the condit ions at which the Ru dependence was measured, requires that k 2 [H 2 ] » k_ 1 [PPh 3 ] , and th i s coupled with what must be a large value for K' (since the product is HRuCl(C0) 2(PPh 3) 2 and [PPh 3] cannot increase markedly during any s ing le run), leads to the simple f i r s t -o rde r in metal. The slope of the plot of l o g ( i n i t i a l rate) vs. log([Ru] T ) also gives n=l (F ig . 3.13), where n i s the order of the reaction in Ru. A k Q b s value of 4 . 4x l 0 " 5 s " 1 i s obtained from the slope of the plot of i n i t i a l rate against [Ru] T (F ig . 3.9). 60 F ig . 3.13: P lot of l o g ( i n i t i a l rate) against log([Ru] T) 61 3.3 .2 Dependence of the Rate on Added PPh 3 Concent ra t i on The rate equation 3.7 can be rearranged to give: k - l 1 1 • [PPh 3] + (3.10) Rate k 1 k 2 [Ru ] T [H 2 ] J k 1 [Ru ] T At constant [Ru]j and [H 2 ] , therefore, the plot of 1/rate vs. added [PPh 3] should be a stra ight l i n e . The rate constant k p and the re l a t i ve magnitudes of k_j and k 2 can then be calculated from the intercept, and the slope, respect ive ly . Such a plot (F ig . 3.14) gives a stra ight l i n e with slope = 3.59xl0 9 sM~2, and intercept = 4.86x l0 6 sM" 1 , from which a kj value of 4.5xl0~ 5s~ 1 , and a k_^/k2 value of 1.6 are obtained for condit ions when [Ru] T = 4.56xlO" 3M and [H 2] = 2.17xlO" 3M, at 65°C. 3 .3 .3 Dependence of the Rate on Hydrogen Concent ra t i on Under the condit ions that k_ 1[PPh 3] « k 2 [H 2 ] in the absence of added PPh 3 , the rate equation 3.7 reduces to: Rate = kjCRuDj (3.11) which is independent of [H 2 ] , as found experimentally. For [Ru]j = 4.56x10 M, in the absence of added PPh 3 , the rates remained reasonably constant between the H 2 pressure range 190-760 torr (Table II1-2, F i g . 3.12). However, the rate dependence on [H 2 ] should eventually go from zero- to f i r s t - o rde r as the hydrogen concentration i s lowered, when 62 25 OH 1 1 1 1 O 1 2 3 4 5 Added [PPh 3 ] x l O 3 , M g. 3.14: Dependence of 1/rate on added PPh 3 concentration according to equation 3.10 [Ru] T = 4 .56x l0" 3 M, P H = 760 to r r 63 k_-|CPPh-^ J becomes comparable to I^Lr^]. Further, the rate should become s t r i c t l y f i r s t - o rde r in O^] i f and when k_-|[PPh3] » k^CH^]. A l terna-t i v e l y , the k_i[PPh^] » l^tHg] condit ion can be achieved by addit ion of PPhg, although such an addit ion would also resu l t in an overa l l lowering of the rates due to the inverse dependence on [PPh^]. The effect of [H^] on the rate, at an added [PPhg] of 9.66xl0~ 4M, is shown in F ig . 3.12. The added [PPh^] of 9.66xl0~4M is apparently not large enough to obtain a s t r i c t l y f i r s t - o rde r dependence; nevertheless, the decrease in the rate with the lowering of pressure reveals the required zero- to f i r s t - o rde r t r ans i t i on . According to equation 3.10, the plot of 1/rate against l/Lh^L at constant [Ru]j and added [PPh^], is expected to y i e l d a stra ight l ine; k-| and k_-|/k2 can be obtained as before from the intercept and the slope, respect ive ly . For [Ru] T = 4.56xl0" 3M, at an added [PPh 3] = 9.66xl0" 4M, such a plot (F ig. 3.15) gives a stra ight l ine , and values of k-j= 5.0x10"^ s _ 1 and k_1/k2 =1 .9 at 65°C are calculated from the intercept (4.43xl0 6 sM" 1) and the slope (7.94xl0 3 s) of th is p lo t . 3.4 Dependence of the Rate-constant on Temperature The dependence of k^ on temperature was studied between the range 50-70°C; the data are summarized in Table 111-3. The plot of l n ( k Q b s / T ) vs. 1/T y ie lds a reasonably good stra ight l ine (F ig. 3.16). Values of AH* = 69±7 kJmole" 1 and AS* = -126±13 JK^mole" 1 are calculated from the slope and the intercept, respect ive ly. 64 F l ' 9 - 3 - 1 5 : Dependence of the rate on [H 2] according to equation 3.10, [Ru] T = 4 .56x l0" 3 M, added [PPh 3] = 9 .66x l0 ' 4 M 65 TABLE I I1 -3 Dependence of k Q b s on temperature Temperature, K k o b s x l ° 5 ' s _ 1 323 1.40 328 2.31 333 2.97 338 4.45 343 7.05 66 3.16: Dependence of the rate-constant for hydrogenolysis of 2 on temperature 67 3.5 Discussion The approximate 2 mole equivalent H 2 uptake by solut ions of RuCl(C0«C 7 H 9 )(C0) 2 (PPh 3 ) 2 , 2, is explained by the hydrogenolysis of the compound 2 (reaction 3.2), and the subsequent hydrogenation of the unsaturated aldehyde product 5-norbornene-2-carboxaldehyde, 4, to the saturated norbornane-2-carboxaldehyde, 5 (reaction 3.3) . The inorganic product of the hydrogenolysis is i dent i f i ed as HRuCl(C0) 2(PPh 3) 2 , 3. RuC l (C0 .C 7 H 9 MC0) 2 (PPh 3 ) 2 + H 2 • HRuCl (C0) 2 (PPh 3 ) 2 + CyHgCHO (3.2) 2 3 4 ~ ~ ~ C?H9CH0 + H 2 — • C ? H n CH0 ( 3 > 3 ) Independent measurements of H 2 uptake by solut ions of 4 in the presence of 3 show that the hydrogenation to 5 is ca ta l y t i c in The i n i t i a l rate of hydrogenation measured in th i s ca ta l y t i c react ion at 65°C, for a system with [aldehyde]: [Ru] » 42 was 1.35xlO~ 6Ms _ 1 (Ru] = _3 4.00 x 10 M). The i n i t i a l rate of hydrogenation measured for a system containing mole equivalents of Ru and aldehyde (at a metal concentration -8 1 the same as in the ca ta l y t i c system) was - 3 . 8 x 10 Ms . If the ca ta l y t i c react ion were f i r s t order in aldehyde substrate, then the -8 expected i n i t i a l ca ta l y t i c rate would be (~ 3.8x10 x42) i . e . 1.6xlO" 6Ms"Vhis is close to the experimentally noted 1.35xlO~6Ms"^ value, and strongly indicates that the ca ta l y t i c rate is Ist-order in unsaturated aldehyde. These data are also consistent with the rate of consumption of 68 the second mole equivalent of H £ 3.5xlO~°Ms ; [Ru] T = 4.56xlO~°M) measured during the H 2 uptake by 2 (F ig. 3.5). The hydrogenation of the unsaturated aldehyde, which i s ~ 7 times slower than the hydrogenolysis react ion, causes l i t t l e interference in measurement of the i n i t i a l rates of the hydrogenolysis react ion. However, the rate data analyses for a l s t -order ruthenium dependence within a s ing le run (F ig . 3.9) c lear l y show a deviation from the s t r a i gh t - l i ne log vs. time p lo t , towards a higher rate than expected, beyond about a 0.7 mole H 2-uptake. The addit ional contr ibut ion to the rate of H 2-uptake i s at t r ibuted to the concurrent hydrogenation of 4. The re l a t i ve l y slow rate of hydrogenation of the unsaturated aldehyde i s consistent with previous f ind ings. In ea r l i e r studies from 7fth th i s laboratory, 3 was found to be qui te i ne f f i c i en t as an o l e f i n hydrogenation cata lys t , under temperature and H 2-pressure condit ions QC s imi la r to those used during the present work. Fahey in 1973 reported on the RuCl 2(C0) 2(PPh2) 2 - catalyzed se lect ive hydrogenation of diene and t r i ene substrates to monoenes, and these reactions were considered to involve 3 as the ca ta l y t i ca l l ay act ive species. Much more severe condit ions were used (130-160°C, 15-20 atm P g ) , and the rates were n 2 slowest for the hydrogenation of internal alkenes. Cata ly t i c a c t i v i t y of Ru complexes is commonly found to decrease with the introduct ion of n-acceptor carbonyl g roups . 8 6 As mentioned in the Introduction (Section 1.2.1), the presence of a vacant coordination s i t e on the metal i s usually essential for H 2 act ivat ion to take place. The 6-coordinate ruthenium-acyl complex 69 RuCl(CO-CyHg)(C0) 2(PPh 3) 2 may become coordinat ively unsaturated through loss of a coordinated l igand. The pos s i b i l i t y of CO d issoc iat ion from 2 i s ruled out because (a) the rate of hydrogenolysis remains unaffected by the presence of external CO and (b) the ruthenium product i s s t i l l a d icarbonyl . Indeed, the k ine t i c data argue unambiguously for loss of coordinated phosphine in a slow (kj) step (eq. 3.4). -5 -1 The k^  value of 4.5x10 s obtained from the dependence of the hydrogenolysis rate on added [PPh^] i s , with in experimental error, the -5 -1 same as the k Q b s value of 4.4x10 s obtained from the [Ru]j-dependence, measured in the absence of added phosphine. Under the l a t t e r condit ions, c lear l y k_ 1[PPh 3] « k 2 [H 2 ] (eq. 3.7), with the rate being independent of H 2 from 190-760 to r r (0.54-2.17xlO" 3M). The steady state concentration of PPhg must remain extremely low, and indeed the f a i l u r e to detect any free 31 1 PPh 3 by P{ H} n.m.r. spectroscopy in solut ions of 2 means that the equi l ibr ium constant K for the d issoc iat ion of PPh 3 i s immeasurably small . In any case, the hydrogenolysis of 2 does not result in a build-up of f ree PPh-j as the reaction proceeds; the equi l ibr ium constant K1 for reaction O C 3.6 must presumably be very large, and the o r i g i na l l y d issoc iated PPh 3 quickly coordinates to the [HRuCl(C0) 2(PPh 3)] species to form 3. The k_^/k2 ra t io of 1.6, obtained from the inverse dependence of hydrogenolysis rate on added [PPh 3] (F ig . 3.14), means that the rates of the phosphine associat ion to, and hydrogenolysis of, RuCl(CO'C^Hg)-(C0) 2 (PPh 3 ) , (eqs. 3.4, 3.5) become comparable at re l a t i ve l y low added [PPh 3 L Consequently, in the presence of added phosphine, the Hp-dependence i s expected to go from zero- to f i r s t - o rde r . Such a sh i f t 70 in h^-dependence i s evident from the data of F i g . 3.12b (Table I I I -2 ) , even at the re l a t i ve l y small added [PPh 3] of 9.66xlO" 4M ([PPh 3]/[Ru] = 0.21) and, since the [h^] i s of the same order as the [PPh 3 ] , th i s immediately implies qua l i t a t i ve l y that the k_^/k2 ra t io is c lose to unity. -5 -1 Also, the k^  value of 5.0x10 s and the k^/k,, ra t io of 1.9 obtained from the H^-dependence data (F ig . 3.15) are in acceptable agreement with those determined experimentally from the inverse PPh3-dependence. The measured AH* value of 69 ± 7 kJmole" 1 for the hydrogenolysis reaction refers to the k^  step and i s thus the enthalpy of act ivat ion for phosphine d issoc iat ion from 2 in DMA solvent. If the act ivated complex resembles c lose ly the f ive-coordinate intermediate, then the 69 kJmole - 1 could approximate to the bond d issoc iat ion energy of the Ru-P bond. However, the entropy of act ivat ion (see below) strongly suggests that solvat ion of the reaction intermediate i s important, and solvat ion energies are thus l i k e l y contr ibut ing to the overal l act ivat ion enthalpy value. The entropy of act ivat ion has a large negative value (-126 ± 13 J K _ 1 m o l e - 1 ) , which i n i t i a l l y might appear contrary to the expectation based on a d i ssoc ia t i ve mechanism. The only p laus ib le explanation for a negative AS must l i e in differences in solvat ion of the ground state reactant and act ivated state; the degree of solvat ion in the l a t t e r must be greater. For a t rans i t i on state close to complete d issoc iat ion of the phosphine, solvat ion of the 5-coordinate intermediate and free phosphine could lead to an overal l negative entropy of ac t i va t ion . A negative AS* value for a rate determining l igand d issoc iat ion step i s unusual; in solvents such as toluene the entropy of act ivat ion is invar iably pos i t i ve 71 87 for such a process. The nature of the solvent ( in th i s system, of coordinating and polar character) c l ea r l y plays a key ro le . Hydrogenolysis reactions (cf. eq. 3.2) are known to proceed often v ia oxidat ive addit ion of H 2 , followed by fast reductive el iminat ion of 58 59 the products, ' and a 7-coordinate Ru(IV)-H 2 t rans i t i on state or even a T T ? ^ f i Ru -(T) -H 2 ) are both p laus ib le (k 2 - s tep) . Hetero ly t ic act ivat ion of H 2 seems un l i ke ly since the hydrogenolysis of 2 proceeds at about the same rates in both DMA and toluene solvents (Table I I I - 2 ) . I t would be of considerable interest to measure the corresponding k ine t i cs and act ivat ion parameters for the hydrogenolysis react ion in toluene. The i n ab i l i t y of the propiony! complex RuCl(C0 'C 2 H 5 )(C0)(PPh 3 ) 2 , to react with H 2 i s surpr is ing in view of the a v a i l a b i l i t y of a vacant coordination s i t e on th i s 5-coordinate, supposedly square-pyramidal 7 9 complex. Although blocking of the coordination s i t e by a coordinating DMA solvent molecule is poss ib le, the complex is known to react revers ib ly 79 with CO very rap id ly and th i s was demonstrated for the complex in DMA so lut ion: RuCl(C0*C 2H 5)(C0)(PPh 3) 2 + CO — • RuCl(C0«C 2 H c )(C0) 2 (PPh 3 ) 2 (3.12) RuCl(C0-C 2 H 5 ) (C0) 2 (PPh 3 ) 2 + CO [Ru(C0-C 2H 5)(CO) 3(PPh 3) 2]C1 (3.13) 72 CHAPTER IV REACTIONS OF RUTHENIUM(II)-CYCLOHEXADIENE COMPLEXES R e a c t i o n s o f new c y c l o h e x a d i e n e d e r i v a t i v e s of r u t h e n i u m ( S e c t i o n s 2.2.1 and 2.2.2) w i t h H 2 and w i t h CO were i n v e s t i g a t e d ; p r e l i m i n a r y r e s u l t s o f t h e s e s t u d i e s a r e d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n s . 4.1 R e a c t i o n s of " R u C l ( C 6 H 9 ) ( P P h 3 ) 2 " , 6 4.1.1 R e a c t i o n of " R u C l ( C 6 H 9 ) ( P P h 3 ) 2 " , 6, w i t h H 2 The brown p r o d u c t , 6, o b t a i n e d from the r e a c t i o n o f H R u C l ( P P h 3 ) 3 ' C 6 H 6 w i t h 1,4-CYHD.CgHg, ( S e c t i o n 2.2.1), r e a c t e d w i t h ~ 2.45 mole e q u i v a l e n t s o f H 2 p e r Ru i n DMA, a t 1 atm H 2 p r e s s u r e . The H 2 uptake p l o t i s shown i n F i g . 4.1 and i n d i c a t e s a r e l a t i v e l y r a p i d r e a c t i o n w i t h an i n i t i a l 2 mole e q u i v a l e n t s o f H 2,...followed by a much s l o w e r uptake o f the f i n a l ~ 0.5 mole e q u i v a l e n t o f H 2 . The i n i t i a l l y r e d d i s h brown s o l u t i o n o f 6 t u r n e d deep r e d i n c o l o u r as the uptake p r o c e e d e d . The r e a c t i o n was a l s o f o l l o w e d by v i s i b l e s p e c t r o p h o t o m e t r y , and a s i n g l e a b s o r p t i o n maximum was o b s e r v e d a f t e r c o m p l e t i o n o f the r e a c t i o n ( ^ m a x = 500 nm, e = 1425 M'^cm" 1). The r e a c t i o n a l s o p r o c e e d e d i n C H 2 C 1 2 and CgHg 31 1 s o l v e n t s . The room t e m p e r a t u r e P{ H} n.m.r. spectrum o f t h e p r o d u c t ( s ) i n CgDg showed peaks a t 70.5 and 45.9 ppm. These r e s u l t s are s i m i l a r t o t h o s e o b t a i n e d by D e k l e v a i n t h i s l a b o r a t o r y d u r i n g an i n v e s t i g a t i o n of the r e a c t i o n o f H R u C l ( N B D ) ( P P h 3 ) 2 > 1, w i t h H 2 . 7 2 a » 8 8 D e k l e v a f o u n d t h a t 1 r e a c t e d w i t h ~ 2.5 moles of H 2 per mole o f Ru; t h e p r o d u c t s were shown t o be a d i m e r i c r u t h e n i u m complex, [ H 9 R u C l ( P P h . ) 9 ] 9 (X „ a t 500 nm, e = 1410 M ^ c r r f 1 ; 3 1 P { 1 H } n.m.r. . c o c c max 3 2.5-\ 1.5-| EC 1-T 0.5 i o-T-o i i i 10 20 30 i 40 I I I 50 60 70 Time x 10" 3 , s ao —r-90 100 110 Fig. 4.1: Rate plot for H2 uptake in DMA by "RuCl(CgHg)(PPh3)2" at 25°C [Ru] = 2.72x10"° M. Inset - Initial portion of the rate plot expanded. 74 72a 88 at 70.8 and 46.1 ppm i n C g D g ) , and f r e e norbornane {eq. 4 . 1 ) . ' 1 HRuCl (NBD)(PPh 3 ) 2 + 2.5 H 2 • - [HgRuC l ( P Ph 3 ) 2 3 2 + C y H 1 2 (4.1) 1 2 The i no rgan i c product of the r ea c t i on of 6 w i t h H 2 must c l e a r l y be [ H 2 R u C l ( P P h 3 ) 2 ] 2 by comparison of the spec t roscop i c da ta , wh i l e c y c l o -hexane was detected q u a l i t a t i v e l y by gas chromatography. There fore s i m i l a r to r e a c t i o n 4 . 1 , the r e a c t i on between 6 and H 0 can be w r i t t e n as: 1 " R u C l ( C 6 H 9 ) ( P P h 3 ) 2 " + 2.5 H 2 • _ [ H 2 R u C l ( P P h 3 ) 2 D 2 + C g H 1 2 (4.2) I n t e r e s t i n g l y , the y e l l ow complex, 7, the second product obta ined from the r e a c t i on of HRuC l ( PPh 3 ) 3 »C g H g w i t h 1,4-CYHD (Sec t ion 2 . 2 . 1 , eq. 2 . 2 ) , a l s o reac ted w i t h H 2 (~ 4 mole equ iva len t s of H 2 based on the " R u C l ( C g H g ) ( C g H 8 ) ( P P h 3 ) 2 - b e n z e n e so l v a t e " f o rmu l a t i on ; 4.5 mole equ i va l en t s would be expected f o r a r eac t i on such as 4.2a) to g i ve products which were found to be i d e n t i c a l to those formed i n r e a c t i o n 4 . 2 . 1 " R u C l ( C g H 9 ) ( C g H 8 ) ( P P h 3 ) 2 " + 4.5 H ? • - D ^ R u C l ( P P h 3 ) 2 ] 2 + 2 C g H 1 2 (4.2a) 75 4 .1 . 2 Reac t ion of " R u C l ( C 6 H 9 ) ( P P h 3 ) 2 " w i t h CO DMA solut ions of 6 rapidly reacted with CO (1 atm) at 30°C. The brown so l i d turned ye l lowish, instantaneously, in the so l i d state on introduct ion of CO to the react ion- f lask . This was then followed by CO uptakes of ~ 1.7 - 1.85 mole equivalents per mole of Ru (F ig . 4.2) by the so lut ion which gradually turned from reddish brown to yel low in colour during the uptake. The inorganic product of th is reaction was i den t i f i ed as HRuCl(C0) 2(PPh 3) 2 , 3, by 3 1 P{ 1 H} n.m.r. (39.5 ppm, s inglet) and also by i . r . spectroscopy ( v C Q : 2042, 2033, 1992 and 1982 cm" 1 ) . The <2 mole equivalent uptakes are attr ibuted to what appears to be a so l i d state reaction of 6 with CO. Thus, unl ike the reaction between HRuCl(NBD)-(PPh 3 ) 2 and CO which leads to the formation of the acyl complex (Section 3.1), the reaction of 6 with CO presumably resul ts in diene displacement with resu l t ing formation of the dicarbonyl species, 3 (eq. 4 .3) : "RuCl(C 6 H 9 ) (PPh 3 ) 2 " + 2 CO • HRuCl(C0) 2(PPh 3) 2 + CgHg (4.3) Reaction of the yellow "RuCl(C gH g)(C gHg)(PPh 3) 2-benzene solvate", 7, with CO in benzene also resulted in the formation of 3. 4 . 2 React ions of RuC l 2 ( C 6 H g ) ( P Ph 3 ) 2 , 8 DMA solut ions of 8 f a i l e d to react with 1 atm H 2 at 65°C. Previous work from th is laboratory has shown that a norbornadiene analogue, RuC l 2 (NBD)(Pto l 3 ) 2 , i s also unreactive towards H 2 under comparable 89 condit ions. 2.5 n 1.6-3 QZ 0.5 H °1 1 1 - r 1 i 1 - i 1 1 r 0 1000 2000 3000 4000 5000 6000 7000 6000 9000 10000 11000 T i m e , s F l ' 9 - 4 - 2 - Rate plot for CO uptake in DMA by "RuCl (CgHgHPPh^", at 30°C [Ru] = 2.32xl0" 3 M 77 However, 8 was found to react with CO, and at 50°C under 1 atm CO DMA solutions of 8 absorbed ~ 1.9 mole equivalents of CO (F ig . 4 .3) . The i n i t i a l l y orange solut ion changed to pale yellow during the uptake, while 31 1 a new peak appeared at 16.9 ppm in the P{ H} n.m.r. spectrum, accompanied by the simultaneous disappearance of the 28.9 ppm signal due to 8. The inorganic product was ident i f i ed as c i s - c i s - t rans RuCl 2 (CO) 2 (PPh 3 ) 2 by comparison of the P{ H} n.m.r. data 90 with those reported in l i t e ra tu re . Thus, 8 almost cer ta in ly undergoes a diene displacement by CO as shown below: RuC l 2 (C 6 H 8 ) (PPh 3 ) 2 + 2 CO • RuC l 2 (C0) 2 (PPh 3 ) 2 + CgHg (4.4) 4.3 Discussion The brown complex 6 has the stoichiometry RuCl(CgHg)(PPh 3) 2, and consistent with th is when reacted with H 2 , 6 takes up ~ 2.5 mole equiva-lents of H 2 and forms [H 2 RuC l (PPh 3 ) 2 ] 2 and free cyclohexane (eq. 4 .2) . The expected formulation "HRuCl(CgH Q)(PPh 3) 2" has problems in that a Ru-H could not be detected for 6 by i . r . or H^ n.m.r. spectroscopy; th is seems to support the existence of a cyclohexenyl (or n - a l l y l ) product rather than a 'hydridodiene' species (eq. 4 .5) . A l te rnat i ve ly , such non-detection of a hydride may resu l t from a fast equi l ibr ium conversion between the hydridodiene species and the species resu l t ing from hydride transfer (eq. 4 .5) . 2.5 • • • I I l I 1000 2000 3000 4000 5000 6000 7000 8000 Time , s F ig . 4.3: Rate plot for CO uptake 1n DMA by RuCl 2 (C 6 Hg)(PPh 3 ) 2 at 50°C [Ru] = 4.00 x l O ' 3 M 79 HRuCl(CgH8)(PPh3) ; RuCl(C 6 H 9 )(PPh 3 ) 2 (4.5) Ru H 6a Ru 6b Ru 6c etc. Some support for such an equilibrium is provided by the reactivity of 6. The reaction of 6 with CO results in the formation of HRuCl(C0) 2(PPh 3) 2 by a net displacement of the diene, while 6 reacts with a further mole of 1,4-cyclohexadiene leading to the formation of the yellow complex, 7, which appears to contain both the C g H g and the diene (CgHg) moieties: "RuCl(CgH9)PPh3)J 6 C 6 H 8 , "RuCl(C 6H 9)(CgHg)(PPh 3) 2" 7 (4.6) That 7 takes up ~ 4 mole equivalents of H 2 , and yields the same products as those obtained during reaction 4.2, is consistent with the formulation shown. The "RuCl(CgH 9)(PPh 3) 2" species is extremely reactive. For example, the species turns yellow in the solid state in the presence of CO. These reactivity observations can be rationalized i f 6 existed at least part ia l ly as the coordinatively and/or electronically unsaturated 80 "hydride transferred" (6b, 6c) species, which must then react with CO 91 almost instantaneously. Werner and Feser have reported on a rapid equi l ibr ium conversion between a Rh(III) hydridoethylene complex and the corresponding ethyl-comp1 ex: [CpRhH(C 2H 4)PMe 3]+ — • [CpRh(C 2H 5)PMe 3] + (4.7) The hydridoethyl ene complex was found to undergo ethylene replacement by CO and not give an acyl insert ion product, and yet y ie lded an ethy l -ethyl ene- comp 1 ex on reaction with C 2 H 4 : [CpRhH(C 2H 4)PMe 3]+ + CO • [CpRhH(C0)PMe3]+ + C 2 H 4 (4.8) [CpRhH(C 2H 4)PMe 3]+ + ^ 2^T~^ [CpRh(C 2H 5)(C 2H 4)PMe 3] + (4.9) S im i l a r l y , the chemistry of 6 would be consistent with a rapid equi l ibr ium 1 13 shown in eq. 4 .5 . Low temperature H and C n.m.r. studies are required to elucidate further the exact nature (geometry, conf igurat ion in solut ion) of 6 (and 7). The invest igat ion of the reaction between 1,4-cyclohexadiene and RuC l 2 (PPh 3 ) 3 (Section 2.2.2) was prompted by the interest in the nature of 6 and 7. The orange product, 8, obtained during th is reaction has the stoichiometry RuCl 2 (CgH g ) (PPh 3 ) 2 , consistent with displacement of a phosphine by diene. Interest ing ly , low temperature *H n.m.r. studi< in C D C 1 3 show that a l l 8 protons of the diene are equivalent ( 6 C D C 1 81 5.40 ppm), at least down to -60°C. The *H n.m.r. chemical sh i f t for the 92 diene protons is c lose to that reported by Bennett and Smith for the coordinated arene protons of RuCl 2(CgHg)(PPh 3) (6CDC1 = 5 , 3 8 PPm^* 3 One possible explanation for the equivalence of the 8 diene protons is that the 1,4-cyclohexadiene may isomerise to 1,3-cyclo-hexadiene; rapid movement of the conjugated double-bonds in the l a t t e r , at least on the n.m.r. t ime-scale, could then result in the observed equivalence of the 8 protons. Such isomerization of 1,4- to 1,3-cyclo-hexadiene (before a subsequent dehydrogenation to an arene der ivat ive) has 92 been suggested previously; further studies are needed to explore th i s po s s i b i l i t y . 82 CHAPTER V GENERAL CONCLUSIONS AND SOME RECOMMENDATIONS FOR FUTURE WORK 5.1 General Conc lus ions The analyses of the various kinetic data for the hydrogenolysis of the acyl bond in RuCl(C0'C 7Hg)(C0) 2(PPh 3) 2 are internally consistent and a mechanism has been proposed to account for the data (eqs. 3.4-3.6). Of interest, the first-order metal-dependence, and the zero-order H2-dependence in the absence of added phosphine, when the rate determining step becomes PPh3 dissociation, are very similar to the results reported by Thomas71 on the cobalt(I)-phosphite system (Chapter I). While the similarity in the mechanism of hydrogenolysis of the Co(I) and the Ru(II) systems may be coincidental, the prospect of a general mechanism involving ligand dissociation as the primary slow step seems like l y , at least for the hydrogenolysis of coordinatively and electronically saturated metal-acyl complexes. Obviously, more extensive kinetic studies on related systems, even for complexes of the same metal, are necessary before any conclusion regarding the generality of any mechanism can be 35 reached. The kinetic instability of metal-acyl complexes will be a major hurdle in attempting systematic kinetic studies on hydrogenolysis of such complexes. A new ruthenium(II) complex of the stoichiometry "RuCl(C 6H 9)(PPh 3) 2", 6, a CYHD derivative formally analogous to HRuCl(NBD)(PPh3)2, was prepared and its reactions with H? and CO investigated. While 6 reacted with H2 in a manner identical to the NBD analogue, an attempt to prepare the corresponding acyl complex by 83 carbonylation was unsuccessful. Instead, the reaction of 6 with CO resulted in the formation of HRuCl ( CO^(PPh^ , presumably through a diene-displacement reaction (eq. 4 .4) . The product 7 from the reaction between 6 and 1,4-CYHD seems to have the stoichiometry "RuCKCgHgMCgHg)-(PPh 3 ) 2 " . Another new complex RuCl^CgHgMPPh.^, 8, was synthesized by react ion of RuCl^tPPh^)^ and 1,4-CYHD; the diene moiety exhib i ts f lux iona l behaviour, that makes a l l 8 protons equivalent even at -60°C. The character izat ion and study of these new Ru(II)-CYHD complexes is s t i l l in the prel iminary stages. 5.2 Suggestions fo r Future Work As wel l as further studies (pa r t i cu la r l y n.m.r.) to elucidate f u l l y the nature of complexes 6, 7 and 8, other routes to Ru-CO.R species should be pursued. Ruthenium(O) complexes of the type RufCO),,^)^, (P = 93 phosphine) are known to undergo f a c i l e oxidat ive addit ion react ions, and such reactions with acy l/aroy l hal ides may provide a convenient route to the syntheses of the required complexes, which could then be used for systematic k ine t i c studies on hydrogenolysis of the Ru-C bond. 84 REFERENCES 1. G.C. Pimentel i n the NRC repor t "Oppor tun i t i e s i n Chemis t ry " , Na t iona l Academy P ress , Washington D . C , 1985; through execut i ve summary from CAEN, Oct . 14, 13 (1985). 2. (a) J . T s u j i , "Organic Syn thes i s by means of T r a n s i t i o n Metal Complexes", S p r i n ge r - Ve r l a g , New York, 1975. (b) G.W. P a r s h a l l , "Homogeneous C a t a l y s i s " , Wi ley I n t e r s c i e n c e , New York, 1980. (c) J . P . Col lman and L.S. 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