A DECEPTIVELY SIMPLE CASE OF SELECTIVE HYDROGENATION OF PHENYLACETYLENE TO STYRENE CATALYZED BY A CIS-HYDRIDO(MU-2-DIHYDROGEN)RUTHENIUM(II) COMPLEX

Depending on the substrate to metal complex ratio (2, 3, 4), the reaction of the cis-hydrido(eta2-dihydrogen)ruthenium(II) complex [(PP3)Ru(H)(H2)]BPh4 (2) with HC=CPh in tetrahydrofuran gives the sigma-alkenyl [(PP3)Ru{C(H)=C(H)Ph}]+ (detected spectroscopically), the sigma-alkynyl [(PP3)Ru(C=CPh)]Bph4.THF (6), or the eta3-butenynyl E-[(PP3)Ru(eta3-PhC3CHPh)}BPh4 (3a,b) {PP3 = P(CH2CH2PPh2)3]. The latter product is isolated as a 3:1 mixture of geometric isomers displaying a different anchoring mode of the butenynyl ligand to the metal center. The cis-hydride(eta3-dihydrogen)ruthenium(II) complex 2 is an active catalyst precursor for the selective hydrogenation of phenylacetate to styrene. A kinetic study carried out in 1,2-dichloroethane solution shows that the rate of the catalytic hydrogenation reaction is proportional to the initial concentration of the catalyst precursor, second order with respect to dihydrogen pressure, and independent of substrate concentration. At very low concentrations of HC=CPh (<0.12 M), the order of the catalytic rate with respect to substrate concentration tends to 1. In light of the kinetic study and of the reactivity of 2 towards HC=CPh, a reaction mechanism is proposed which essentially involves the usual cycle adopted by mono(hydrido)metal catalysts. In fact, unlike the related iron derivative [(PP3)Fe(H)(H2)]BPh4 previously investigated, dihydrogen in 2 behaves as a weakly bound ligand which is readily displaced by phenylacetylene. Other ruthenium species, namely the butenynyl complexes 3a,b and the alkynyl 6, play a significant role in the catalysis cycle as side intermediates.