Platinum Metals Rev., 1991, 35, (2), 70
Carbonylation Reactions in Aqueous or Mixed Solvent Systems
Catalysis by Ruthenium Carbonyl Complexes
In recent years much interest has been focused on carbonylation reactions in aqueous or biphasic systems catalysed by platinum group metals complexes. This is due to the reactivity of platinum group metals ions as catalysts designed to activate small molecules. When used in C, chemistry a variety of reactions can be catalysed by these complexes, in particular by ruthenium complexes. Ruthenium has several oxidation states and co-ordination numbers, and is relatively cheap. Additionally, ruthenium catalysts are stable and their reaction conditions are usually mild, making them excellent for homogeneous catalysis. Various reactions of ruthenium carbonyl complexes and ruthenium saloph dichloride complexes with carbon monoxide have been investigated in this laboratory, and are considered here.
The importance of C1 chemistry and its sources of natural gas or synthesis gas have increased considerably in recent years (1). This is due mainly to the reactivity of platinum group metals ions as catalysts which are designed to activate small molecules, such as carbon monoxide, hydrogen, olefins, oxygen and nitric oxide under mild conditions (2). The importance of homogeneous catalysis in the next decade will be mostly focused on low energy synthetic routes (3), with minimum or zero byproducts. This approach needs careful planning of synthetic routes, with fewer steps to achieve product selectivity and specificity under milder conditions. The platinum group metals ions of the 4d series, namely ruthenium, rhodium, and palladium occupy a unique place in the Periodic Table, and offer a balance between the thermodynamic stability and kinetic lability of the complexes. Although rhodium (4) and palladium (5) complexes have been widely used in industry, the application of rhodium as a catalyst has been comparatively less well-known. Ruthenium offers the advantage of being available in several oxidation states varying from 0 to 8 and co-ordination numbers from 4 to 7, and these factors together with its lower cost make it an excellent candidate for homogeneous catalysis.
The ruthenium complex ethylenediamine-tetraacetatoaquo-ruthenium(III) [Ru(EDTA-H) (H2O)] (1) reacts with carbon monoxide to give [RuII(EDTA-H)(CO)] (2) which is ultimately reduced to [RuII(EDTA-H)(CO)]– (3). In our laboratory we have been investigating the chemistry of [RuIII(EDTA-H)Cl]– (6–8) and its reactivity for the activation of oxygen (9, 10), nitrogen (11, 12), hydrogen (13, 14) nitric oxide (15) and carbon monoxide (16–21). The [RuIII(saloph)Cl2]–(4) (where saloph = bis(salicylaldehyde)-o-phenylenediimine) (22) is an excellent catalyst for oxygenation (23) and oxidative and reductive carbonylation (24, 25). In the present paper the catalysis of complex [RuIII(EDTA-H) (CO)]– (3) in the water gas shift reaction (16), the hydroformylation of olefins (17), the carbonylation of cyclohexene (18), amines (19), ammonia (20), benzyl chloride (21) and the catalysis of [RuII(saloph)Cl2]2– in oxidative and reductive carbonylations (24, 25) will be discussed. The reactions proceed under mild conditions of temperature and pressure, with a high degree of specificity in most cases.
Reactions of [RuIII(EDTA-H)Cl]–
Interaction of Carbon Monoxide with [RuIII(EDTA-H)(H2O)]
The interaction of carbon monoxide with [RuIII(EDTA-H) (H2O)] (1) gives an aquo-carbonyl species 2 which loses a proton to form the hydroxo species 2a (Scheme I). The equilibrium between 2 and 2a is pH dependent. Complex 2a forms the µ -carbonyl-µ -hydroxo dimer 2b at pH 3.5. Heating complexes 2a or 2b to 60°C in an atmosphere of carbon monoxide at 10 to 15 atmospheres gives the stable ruthenium(II) carbonyl 3, which is used as a catalyst in a number of reactions. The structure of the ruthenium(II) carbonyl 3, see Figure 1, indicates that CO retains its nucleophilic character in the complex, which is thus an excellent catalyst for CO insertion in M-O, M-C and M-N bonds. The Ru-C bond length of coordinated CO is larger than, and the C-0 distance shorter than, diat of most other ruthenium(II) carbonyls (26), indicating a weaker bonding of Ru-CO in this complex.
Catalysis of the Water Gas Shift Reaction
The water gas shift reaction is essentially a reaction between carbon monoxide and water to form hydrogen and carbon dioxide:
The reaction is exothermic and acts as a source of hydrogen in ammonia plants (27, 28). The source of the carbon monoxide is the gasification of coal or natural gas. The conventional catalyst (27) chromium activated iron oxide (Fe3O4/Cr2O3) or cobalt-molybdenum catalyst (CoO/MoO3) are used for the reaction at a temperature of 400-460°C and at a pressure of 200 atmospheres of CO, or CO + H2.
In view of the extreme importance of this reaction to industry as a source of hydrogen we have studied the catalysis of the water gas shift reaction by [RuII(EDTA-H)(CO)]– (3) at 45-80°C and 15-35 atm of CO (16). Since the reaction is exothermic, the optimum temperature of the reaction catalysed by 3 is about 50°C, at 15 atmospheres of CO. From the kinetic data the mechanism shown in Scheme II is proposed for the water gas shift reaction catalysed by 3.
It has been suggested that the rate determining step of the reaction is the oxidative addition of H20 to RuII(EDTA-H)(CO) (3) to form the hydrido carbonyl species 3a and 3b. The formation of 3a and 3b as intermediates is supported by NMR of the hydrides in solution, which gives peaks at – 20.1 and 8.1 ppm, respectively, for hydrides cis and trans to CO, respectively. Insertion of CO into the RuIV-OH bond in a fast step gives the η1 -formato intermediate, 3c, which dissociates in a fast step to form CO2, H2 and LRuII(H2O), and this gives 3 on reaction with CO, thus completing the catalytic cycle. The values of the equilibrium constant, K1, and the rate constant at 50°C and 15 atm of CO are 11/M and 80.0/min, respectively, with a turnover of 350 moles of CO2/H2 per mole of the catalyst per hour; the highest reported so far in the liquid phase. The activation energy of the reaction is 1.47 kcal/mole which amounts to a decrease of about 45 kcal/mole for the proposed dissociation of H2O into H and OH in the rate determining step. The entropy and enthalpy of activation are 2.45 e.u. (entropy units) and 0.83 kcal/mole.
Hydroformylation of l-Hexene
Recently much attention has been diverted to water soluble catalyst in order to achieve a high conversion of end olefins to the linear aldehydes in the oxo reaction (29, 30). The use of the water soluble phosphine ligands, especially the sulphonated phosphines (29, 30), offers a simple biphasic system wherein the Rh(I) catalyst is maintained in the aqueous phase, whereas the organic phase contains the reactants and products with different ligands. The use of a soluble Rh(I) catalyst and 1-hexene as substrate gives a linear to branched chain aldehyde ratio ranging from 9:1 to 18:1 at 80°C and at a CO pressure of 8 atmospheres (30). The complex [RuII(EDTA-H)CO]– (3) catalyses the hydroformylation of 1-hexene to 98 per cent 1-heptaldehyde in a homogeneous system at 1.30°C and 80 atmospheres of syngas in a contact time of 12 hours (17). The mechanism of the reaction is depicted in Scheme III. The values of the equilibrium constants K1, K2, and K3 and the rate constant, k, at 130°C (80 atm of CO) are 4.9/M, 190/M, 5.3/M and 0.32/min, respectively.
Allylic and Vinylic Carbonylation of Cyclohexene
The complex [RuII(EDTA-H)(CO)]– (3) catalyses the carbonylation of cyclohexene in an alcohol : water mixture of ratio 80 : 20 to give the vinylic aldehyde cyclohexene-1-carboxylaldehyde (20%), allylic aldehyde, cyclohexene-3-carboxylaldehyde (20%) and cyclohexane carboxylaldehyde (60%) (18). The optimum reaction conditions are 120°C and 20 atmospheres of CO partial pressure. The carbonylations of 1-, 3- and 4-methyl cyclohexenes conducted at 160°C and 20 atmospheres of CO partial pressure gave the corresponding alcohols, l-(2-methylcyclo-hexene)methanol, l-(4-methylcyclohexene)-methanol and l-(5-methylcyclohexene)-methanol, respectively. The rate of carbonylation has first order dependence with respect to catalyst, CO pressure and substrate concentrations, respectively. The mechanism of the reaction is given in Scheme IV.
The substrate cyclohexene reacts with complex 3 in a pre-equilibrium step K1 to form the mixed ligand η 2 -olefin complex 3c. The rate determining step of the reaction is considered to be the insertion of the ruthenium catalyst in the allylic C-H bond to form an allylic intermediate 3d. The next step, CO insertion (or alkyl migration) which is a well-known step in homogeneous catalysis, affords an acyl species 3e which, on reductive elimination and simultaneous double bond migration gives cyclohexene-1-carboxyaldehyde and cyclohexene-3-carboxyaldehyde, regenerating species 3 by reaction with CO. Cyclohexane carboxyaldehyde is formed by hydrogenation of cyclohexene-1-carboxyaldehyde and of cyclohexene-3-carboxyaldehyde by hydrogen gas released by the water gas shift reaction. The occurrence of the water gas shift reaction under the reaction conditions has been established in our earlier studies (16). The values of the constants K1 and k at 120°C and 20 atmospheres of CO are 1.9/M and 1.43/min, respectively. The activation energy of the reaction was 16 kcal/mole. The endothermicity of the reaction reflects on the energy needed to insert CO into the allylic C-H bond of cyclohexene.
Carbonylation of Primary and Secondary Amines
The complex [RuII(EDTA-H)(CO)]– (3) catalyses the carbonylation of diethylamine and triethylamine (19) at a CO pressure of 5-26 atm and temperatures in the range 80-100°C in aqueous media. The products of the carbonylation of diethylamine are N,N-diethylformamide (80%) and also N,N-tetraethylurea (20%), whereas triethylamine gives 100 per cent N,N-diethylpropionamide. The observed rates for the carbonylation of diethylamine and triethylamine are 5.9 ×10’Vminand 12.5 ×10–3/min, respectively, at 100°C and 25 atm of CO. The rates are in accord with the higher basicity of triethylamine as compared to diethylamine. The mechanisms of the reaction are depicted in Scheme V.
Carbonylation of Ammonia
The extension of the carbonylation of tertiary and secondary amines is the carbonylation of ammonia (20) which was conducted over a range of CO pressures 50-80 atm and in the temperature range 50-60°C. The main product of the reaction is urea, with small quantities of formaldehyde. The reaction gives a new route for urea synthesis from carbon monoxide and ammonia. The synthesis of urea through the established ammonium carbamate route requires temperatures of the order of 400-500°C and a high pressure of 200-300 atm. The catalysis of carbonylation of ammonia by 3 drastically reduces the input of energy required for the synthesis of urea, see Scheme VI.
The rate determining step of the reaction is the homolytic cleavage of NH3 into H and NH2 followed by the oxidative addition to [RuII(EDTA-H)CO]– (3) to form a Ru(IV) hydrido imide complex 3h. Insertion of CO into the Ru-N bond gives the η 1 -formamide intermediate 3i which reacts with another molecule of NH3 to give urea, H2 and 3, thus completing the catalytic cycle. The proposed rate determining step of the reaction is the oxidative addition of NH3 to LRuII-CO to give 3h (Scheme VI).
Carbonylation of Aryl Halides
The carbonylation of benzyl chloride (21) is catalysed by the complex [RuII(EDTA-H)(CO)]– (3) at 80°C and 20 atmospheres of CO in an 8:2 ethanol:water mixture. The products of the carbonylation of benzyl chloride are phenylacetic acid and ethylphenylacetate in a 4:6 molar ratio. The reaction proceeds with 100 per cent conversion of the substrates to products, with an overall turnover frequency of 44 moles of the products per hour.
The mechanism of the reaction as depicted in Scheme VII shows the equilibrium formation of the carbonyl complex in step 1. The rate determining step of the reaction is the oxidative addition of RC1 to [RuII(EDTA-H)(CO)]– (3) to form the Ru(IV)-alkyl complex 3J. Fast insertion of CO into the M-C bond of 3J forms the aroyl intermediate 3K. Nucleophilic attack of OH– or RO– on the carbonyl carbon of 3K gives the products RCOOH or ester RCOOC2H5 and the catalyst (Scheme VII).
Reactions of [Ru(saloph)Cl2]–
Reductive Carbonylation of Nitrobenzene
The reductive carbonylation of nitrobenzene to phenylurethane is a very important reaction from the view point of the production of polyurethane foam and fibres (31). Various transition metal ion catalyst have been used for this reaction (32, 33). The ruthenium(III) complex [Ru(saloph)Cl2]– (4) (saloph = bis-(salicylaldehyde)-o-phenylenediimine) catalyses the reductive carbonylation of nitrobenzene in ethanol at 15 atmospheres of CO and at 160°C to phenylurethane, with a turnover rate of 80 moles of product per mole of the catalyst per hour (24). The mechanism of the reaction is depicted in Scheme VIII.
In the proposed mechanism (Scheme VIII) complex 4 reacts with a molecule of CO to give [LRuIII(CO)Cl] species 4a in a pre-equilibrium step. The solution spectrum of the carbonylation experiment, conducted in the absence of substrate nitrobenzene, showed a peak at 370 nm which is attributed to the LMCT band of the carbonyl complex 4a. In a second pre-equilibrium step, nitrobenzene forms a mixed ligand complex 4b which reacts with another CO molecule in a rate determining step to give Ru(III) nitroso complex 4c by the deoxygenation of the –NO2 group of nitrobenzene and the release of CO2. The solution spectrum of the reaction mixture withdrawn while the reaction was in progress showed the characteristic peak of a Ru-NO bond at 316 am. The nitroso complex 4c quickly takes up one more molecule of CO in a fast step, to give a nitrido intermediate species 4d via the deoxygenation of the nitroso –NO group. Migration of CO to the electrophilic nitrogen atom of the M≡N+-R intermediate gives the co-ordinated phenylisocyanate complex 4e which reacts with ethanol in the presence of Cl– in a fast step to give phenylurethane; simultaneously regenerating the active catalytic species 4a.
The activation energy, Ea, calculated from the temperature dependence of the rate of carbonylation of nitrobenzene in the range 140-160°C is 38 kcal/mole. The ∆H≠ and ∆S≠ values calculated at 160°C are 37.0 kcal/mole and +86.0 e.u., respectively. The ∆H≠ value of 37.0 kcal/mole observed in this study indicates that the reductive carbonylation of nitrobenzene is highly endothermic in nature. This is expected because of the bond breaking reaction in the transition state. The high positive value of ∆S≠ ( + 86.0 e.u.) reflects on the various dissociative steps involved in the formation of the active intermediate. The catalytic activity of a Schiff base complex depends on the presence of labile axial chloride groups to facilitate such dissociation and the formation of a nitroso complex species 4c. The driving force for the reaction is thus the high positive entropy of the reaction.
Oxidative Carbonylation of Cyclohexylamine
The oxidative carbonylation of different primary and secondary amines catalysed by Co(salen) (where salen = bis(salicylaldehyde)-ethylenediimine) have been reported (34, 35). The turnover number based on the amount of amine converted was found to be 0.50-2.0. Palladium catalysts have been reported to catalyse the oxidative carbonylation of amines to urethanes (36).
The complex K[RuIII(saloph)Cl2] 4 selectively catalyses the oxidative carbonylation of cyclohexylamine in ethanol medium to cyclohexylurethane at 160°C and at CO + O2 (1:0.5) pressure of 21 aim (25). A turnover number of 30 mole per mole catalyst per hour was observed in this reaction. The activation energy for the reaction was 21.0 kcal/mole.
The oxidative carbonylation of amines has certain advantages over the reductive carbonylation of nitro complexes (37). Compared to the reductive carbonylation of nitrobenzene by complex 4 (24), the oxidative carbonylation of cyclohexylamine proceeds at an activation energy which is 16 kcal/mole lower than that of the former. The mechanism of the oxidative carbonylation of cyclohexylamine is depicted in Scheme IX. It may be seen that the mechanisms of the reductive and oxidative car-bonylations differ in the routes leading to the formation of the isocyanate species 4d. In the mechanism of oxidative carbonylation proposed in Scheme IX, species 4d is suggested to be formed by intramolecular oxygenation of the primary amine by the Ru(V)-oxo species 4g, with the simultaneous elimination of a water molecule. In addition to the above differences, the reactions also differ in the stoichiometry of CO required for the reaction. In the case of oxidative carbonylation only one mole of CO is required for the entire reaction (Scheme IX), whereas for the reductive carbonylation three moles of CO are required for the reaction.
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