Catalytic Hydrogenation of Benzonitrile by Triruthenium Clusters: Consecutive Transformations of Benzonitrile on the Face of a Ru₃ Plane

Reactions of the triruthenium clusters {Cp*Ru­(μ-H)}₃(μ₃-H)₂ (<b>1</b>) and (Cp*Ru)₃(μ₃-H)­(μ-H)₂(μ-CO) (<b>12</b>) (Cp* = η⁵-C₅Me₅) with benzonitrile were investigated in relation to the selective hydrogenation of benzonitrile to benzylamine. Benzonitrile undergoes consecutive transformations into μ₃-η²:η²(⊥)-nitrile, μ₃-η²:η²(⊥)-imidoyl, μ₃-η²(∥)-imidoyl, μ₃-η²-alkylideneamido, μ₃-imido, and μ-amido ligands on the Ru₃ plane accompanied by the uptake of dihydrogen. The reactions are analogues of nitrile hydrogenation on a metal surface. The complexes are structural models of chemisorbed species and catalyze the hydrogenation of benzonitrile. Complex <b>1</b> catalyzes benzonitrile hydrogenation without additives but exhibits only moderate selectivity toward benzylamine. Although the μ₃-benzylimido complex {Cp*Ru­(μ-H)}₃(μ₃-NCH₂Ph) (<b>4</b>) was obtained by reaction of <b>1</b> with benzonitrile, it was readily transformed into to the μ₃-imidoyl complexes (Cp*Ru)₃(μ-H)₂{μ₃-η²:η²(⊥)-PhCNH} (<b>3</b>) and (Cp*Ru)₃(H)₄{μ₃-η²(∥∥)-PhCNH} (<b>4</b>), which are key intermediates in secondary imine formation. Two benzonitrile molecules were incorporated on the Ru₃ plane under the reaction conditions, which decreases the selectivity of primary amine formation. In contrast, μ-carbonyl complex <b>12</b> suppresses the incorporation of additional benzonitrile ligands and the formation of μ₃-imidoyl species due to the presence of CO. These features of <b>12</b> bring about significant improvement in the selectivity of benzonitrile hydrogenation and produce benzylamine in 92% yield