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 (1) and (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (12) (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 1 catalyzes benzonitrile hydrogenation without additives but exhibits only moderate selectivity toward benzylamine. Although the μ3-benzylimido complex {Cp*Ru(μ-H)}3(μ3-NCH2Ph) (4) was obtained by reaction of 1 with benzonitrile, it was readily transformed into to the μ3-imidoyl complexes (Cp*Ru)3(μ-H)2{μ3-η²:η²(⊥)-PhCNH} (3) and (Cp*Ru)3(H)4{μ3-η²(∥∥)-PhCNH} (4), 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 12 suppresses the incorporation of additional benzonitrile ligands and the formation of μ3-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. [ABSTRACT FROM AUTHOR]