MOLECULAR CLUSTERS IN HOMOGENEOUS CATALYSIS - KINETIC AND CHEMICAL EVIDENCE FOR THE PARTICIPATION OF TRIRUTHENIUM CLUSTER COMPLEXES IN A CLUSTER-PROMOTED CATALYTIC-HYDROGENATION OF DIPHENYLACETYLENE

The cluster complex [Ru3(mu-H)(mu3-ampy)(CO)8(PPh3)] (1) (Hampy = 2-am ino-6-methylpyridine) promotes the selective homogeneous hydrogenation of diphenylacetylene to stilbene (353 K, P(H2) < 1 atm). A kinetic st udy of the catalytic reaction, which is first order in cluster concent ration, as well as the isolation and characterization of the compounds [Ru3(mu3-ampy)(mu,eta1:eta2-Ph2C2H)(CO)7(PPh3)] (2) and (mu-H)2(mu3-a mpy)(mu,eta1:eta2-Ph2C2H)(CO)6(PPh3)] (3), which are the products of t he sequential reactions of 1 with diphenylacetylene and hydrogen, indi cate that the catalytic species are trinuclear and that the rate deter mining step in the catalytic cycle is an isomerization reaction in com plex 3 which favors the reductive coupling of the hydride and alkenyl ligands to give cis-stilbene. Complex 1 does not react with hydrogen u nder the conditions used in the catalytic runs.