Oxidative addition of H-SiR₃ to Di- and triruthenium carbonyl complexes bearing a bridging azulene ligand: Isolation of new silylruthenium complexes and catalytic hydrosilylation of ketones

Oxidative addition of PhMe₂SiH to di- and triruthenium carbonyl clusters bearing 4,6,8-trimethylazulene as the bridging ligand was studied in relation to mechanisms of hydrosilylation of ketones catalyzed by these complexes. Reaction of PhMe₂SiH with (μ₃,η⁵:η⁵-4,6,8-trimethylazulene)Ru ₃(CO)₇ (3) resulted in liberation of a CO ligand, oxidative addition of the Si-H bond, and hydrogenation of one carbon-carbon double bond in the azulene ligand to form a novel 46-electron cluster, (μ₂,η³:η⁵-4,5-dihydro-4,6,8-trimethy lazulene)Ru₃(H)(SiMe₂Ph)(CO)₆ (6). In contrast, (μ₂,η³:η⁵-4,6,8-trimethylazulene)Ru ₂(CO)₅ (4) reacted with HMe₂SiPh to give (μ₂,η³:η⁵-4,5-dihydro-4,6,8-trimethy lazulene)Ru₂(CO)₅(SiMe₂Ph)₂ (7), which has a unique Ru→Ru dative bond, by way of oxidative addition of two molecules of PhMe₂SiH to the starting diruthenium complex followed by hydrogenation of a carbon-carbon double bond in the azulene ligand. In contrast to the fact that the diruthenium complexes 4 and 7 are not catalytically active, the triruthenium clusters 3 and 6 are catalysts for the hydrosilylation of acetophenone with moderate catalytic activity. NMR observation of intermediates in the catalytic hydrosilylation of acetophenone using 6 as catalyst suggests the existence of a reaction pathway without a cluster fragmentation, in which the triruthenium cluster is involved in the catalytic cycle.