Homogeneous hydrogenation of alkynes and of 1,4-cyclohexadiene in the presence of the clusters Ru3(CO)7(μ-PPh2)2(C6H4), Ru4(CO)11(μ4-PPh)(C6H4), Ru3(CO)7(μ-PPh2)2(HC2Ph) and Ru4(CO)11(μ4-PPh)(C2Ph2)

The title complexes, containing phosphido or phosphinidene bridges, catalyze 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 yet observed for substituted metal carbonyl clusters towards alkynes; the activity is related to the nature of the alkyne substrate, C2Et2 < EtC2Ph < C2Ph2. The alkyne complexes Ru-3(CO)(7)(PPh2)(2)(HC2Ph) (3) and Ru-4(CO)(11)(PPh)(C2Ph2) (4), structurally closely related to 1 and 2, have also been examined in comparable reactions; complex 3 shows very high activity, especially towards 1,4-CHD. Organometallic intermediates could not be isolated but direct and indirect evidence supporting a reaction pathway based on cluster catalysis was obtained; this will require the formation of an active site, dihydrogen activation and insertion of the substrate into M-H bonds. Possible alternative mechanisms are also discussed. (C) 1998 Elsevier Science S.A. All rights reserved.