Dimerization of terminal alkynes catalyzed by chloro(η5-pentadienyl) bis(triphenylphosphine)ruthenium(II) and kinetics of phosphine substitution

Exchange of PMe2Ph for PPh3 in (η5-pentadienyl)ruthenium{bis(triphenylphosphine)}chloride, (η5-C5H7)Ru(PPh3)2Cl (1) under first order conditions proceeds rapidly in THF at room temperature. A pseudo-first order rate constant of 17 ± 2 × 10−4 s−1 is obtained for the reaction at 21 °C. The rate constant is essentially independent of the phosphine concentration. The activation parameters, ΔH† = 16.1 ± 0.4 kcal mol−1 and ΔS† = −16 ± 1 cal K−1 mol−1 differ from those reported for phosphine exchange in CpRu(PPh3)2Cl (2) and (η5-indenyl)Ru(PPh3)2Cl (3). The reaction of 1 with PMe2Ph is about 70 times faster than the reaction of 2 at 30 °C and some 40 times faster than the reaction of 3 at 20 °C. (η5-C5H7)Ru(PPh3)2Cl(1) is more active than the ruthenium(II) complexes 2, 3, and TpRu(PPh3)2Cl (4) in the catalytic dimerization of terminal alkynes with nearly quantitative conversion of PhCCH and FcCCH at ambient temperature in 24 h. The enhanced substitution rate is accompanied by >50% conversion of phenylacetylene to oligomeric products. Reaction of 1 with NaPF6 in acetonitrile yields the cationic ruthenium(II) complex [(η5-C5H7)Ru(PPh3)2(CH3CN)][PF6] (7). The latter complex is much less active in reactions with phenylacetylene than 1 but avoids the formation of oligomeric products.