Isomerization of olefins by phosphine-substituted ruthenium complexes and influence of an 'additional gas' on the reaction rate

Phosphine-substituted ruthenium carbonyls have often been used as catalytic precursors in reactions such as the hydrogenation or the hydroformylation of olefins. To collect evidence on the coordination of the olefin as a prel iminary step of these reactions we have investigated the isomerization of h ex-l-ene, in hydrocarbon solvent, in the presence of the phosphine-substitu ted ruthenium carbonyls Ru(CO)(3)(PR3)(2), Ru-3(CO)(9)(PR3)(3) and Ru(CO)(2 )(OAc)(2)(PR3)(2) [R = Bu, Ph]. When using Ru(CO)(3)(PPh3)(2) the rate of t he reaction shows a partial first order with respect to the concentration o f the catalyst and of the substrate. The activation parameters were also ev aluated and the activation entropy is negative. A reaction scheme involving the displacement of a carbonyl ligand with formation of a pi -olefin-ruthe nium complex is suggested. The rate of the reaction significantly changes i f an alcohol is used as solvent. This behaviour is attributed to a modifica tion of the catalytic precursor with formation of a ruthenium hydride. This hypothesis is confirmed by the identification of an alkoxy ruthenium hydri de. The isomerization of olefins by phosphine-substituted ruthenium carbony ls is retarded by the presence of an 'additional gas' such as dinitrogen. T his influence is more evident than the analogous one reported in the hydrof ormylation reaction: the same pressure of the 'additional gas' present in t he reaction vessel reduces the rate of the isomerization to a larger extent , i.e. the presence of 1000 bar of nitrogen decreases in otherwise identica l experiments the isomerization conversion of hex-l-ene from 95.6% to 25.8% . An analogous effect is also caused by the presence of argon and xenon. He lium, on the other hand, does not display any influence. These data are an indication of an interaction between the 'additional gas' and a catalytical ly active transition metal complex. (C) 2001 Elsevier Science B.V. All righ ts reserved.