CARBONYL CLUSTERS AS HOMOGENEOUS CATALYSTS - KINETIC AND MOLECULAR ASPECTS OF THE HYDROGENATION OF DIPHENYLACETYLENE PROMOTED BY AN ALKENYL-BRIDGED TRIRUTHENIUM CLUSTER COMPLEX

The cluster complex [Ru-3(mu(3)-ampy)(mu-eta(1):eta(2)-PhC=CHPh)(CO)(8)] (1; Hampy = 2-amino-6-methylpyridine), which contains a bridging alkenyl ligand, promotes the selective homogeneous hydrogenation of diphenylacetylene to cis- and trans-stilbene under very mild conditions (333 K, P(H-2) < 1 atm). Compound 1 is the only metal complex observed by IR spectroscopy in the catalytic solution. This complex reacts with hydrogen at room temperature, in the absence of diphenylacetylene, to give a mixture of cis- and trans-stilbene and the known cluster compounds [Ru-3(mu-H)(mu(3)-ampy)(CO)(9)] (2) and [Ru-6(mu-H)(6)(mu(3)-ampy)(2)(CO)(14)] (3); however, these products are not observed in the presence of diphenylacetylene. The reaction of 1 with carbon monoxide gives the nonacarbonyl derivative [Ru-3(mu(3)-ampy)(eta(1)-PhC=CHPh)(CO)(9)] (4), which contains a terminal alkenyl ligand. Reaction of complex 1 with HBF4.OEt(2) gives the cationic compound [Ru-3(mu-H)(mu(3)-ampy)(mu-eta(1):eta(2)-PhC=CHPh)(CO)(8)][BF4] (5), which has been crystallographically characterized. Crystal data for 5.CH2Cl2: triclinic, space group P $($) over bar$$ 1, a = 10.357(9) Angstrom, b = 11.279(5) Angstrom, c = 16.751(9) Angstrom, alpha = 105.42(6)degrees, beta = 97.68(6)degrees, gamma = 100.61(4)degrees, Z = 2. Complex 5 reacts with [PPN][BH4] to give cis- and trans-stilbene as well as a coordinatively unsaturated derivative which on exposure to diphenylacetylene regenerates complex 1. These results, coupled with a kinetic study of the catalytic hydrogenation reaction, which is first order in cluster and hydrogen concentrations and negative or zero order (depending on the [substrate]:[catalyst] ratio) in diphenylacetylene concentration, support a hydrogenation mechanism. This mechanism corresponds to the general rate law -d[Ph(2)C(2)]/dt = {k(a) + k(b)[Ph(2)C(2)](-1)} [1] (P(H-2)), in which the catalytic species are trinuclear, and consists of two coupled catalytic cycles, one working preferably at [Ph(2)C(2)]:[1] < 50 and the other at [Ph(2)C(2)]:[1] > 50.