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 (μ 3 -H) 2 ( 1 ) and (Cp*Ru) 3 (μ 3 -H)(μ-H) 2 (μ-CO) ( 12 ) (Cp* = η 5 -C 5 Me 5 ) with benzonitrile were investigated in relation to the selective hydrogenation of benzonitrile to benzylamine. Benzonitrile undergoes consecutive transformations into μ 3 -η 2:η 2 (⊥)-nitrile, μ 3 -η 2:η 2 (⊥)-imidoyl, μ 3 -η 2 (∥)-imidoyl, μ 3 -η 2 -alkylideneamido, μ 3 -imido, and μ-amido ligands on the Ru 3 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 1 catalyzes benzonitrile hydrogenation without additives but exhibits only moderate selectivity toward benzylamine. Although the μ 3 -benzylimido complex {Cp*Ru(μ-H)} 3 (μ 3 -NCH 2 Ph) ( 4 ) was obtained by reaction of 1 with benzonitrile, it was readily transformed into to the μ 3 -imidoyl complexes (Cp*Ru) 3 (μ-H) 2 {μ 3 -η 2:η 2 (⊥)-PhCNH} ( 3 ) and (Cp*Ru) 3 (H) 4 {μ 3 -η 2 (∥∥)-PhCNH} ( 4 ), which are key intermediates in secondary imine formation. Two benzonitrile molecules were incorporated on the Ru 3 plane under the reaction conditions, which decreases the selectivity of primary amine formation. In contrast, μ-carbonyl complex 12 suppresses the incorporation of additional benzonitrile ligands and the formation of μ 3 -imidoyl species due to the presence of CO. These features of 12 bring about significant improvement in the selectivity of benzonitrile hydrogenation and produce benzylamine in 92% yield.