HOMOGENEOUS HYDROGENATION OF ALKYNES AND OF 1,4-CYCLOHEXADIENE IN THEPRESENCE OF THE CLUSTERS RU-3(CO)(7)(MU-PPH2)(2)(C6H4), RU-4(CO)(11)(MU(4)-PPH)(C6H4), RU-3(CO)(7)(MU-PPH2)(2)(HC2PH) AND RU-4(CO)(11)(MU(4)-PPH)(C2PH2)

The title complexes, containing phosphido or phosphinidene bridges, ca talyze the hydrogenation of alkynes and of 1,4-cyclohexadiene (1,4-CHD ). The benzyne-substituted clusters Ru-3(CO)(7)(PPh2)(2)(C6H4) (1) and Ru-4(CO)(11)(PPh)(C6H4) (2) show the highest hydrogenation activity y et observed for substituted metal carbonyl clusters towards alkynes; t he activity is related to the nature of the alkyne substrate, C2Et2 < EtC2Ph < C2Ph2. The alkyne complexes Ru-3(CO)(7)(PPh2)(2)(HC2Ph) (3) a nd Ru-4(CO)(11)(PPh)(C2Ph2) (4), structurally closely related to 1 and 2, have also been examined in comparable reactions; complex 3 shows v ery high activity, especially towards 1,4-CHD. Organometallic intermed iates could not be isolated but direct and indirect evidence supportin g a reaction pathway based on cluster catalysis was obtained; this wil l require the formation of an active site, dihydrogen activation and i nsertion of the substrate into M-H bonds. Possible alternative mechani sms are also discussed. (C) 1998 Elsevier Science S.A. All rights rese rved.