Hydrogenation of Imines Catalyzed by 2-(Aminomethyl)pyridine-Based Ruthenium and Osmium Complexes

CNN pincer ruthenium(II) and osmium(II) complexes of general formula RuCl(CNN)(dppb) (1 a-c) and OsCl(CNN)(dppb) (2 a,b) (dppb=Ph2P(CH2)4PPh2, HCNN=[6-(p-tolyl)-2-pyridyl]methylamine (4), N-methyl-[6-(p-tolyl)-2-pyridyl]methylamine (7) and N,N-dimethyl-[6-(p-tolyl)-2-pyridyl]methylamine (8)) were prepared and assessed as catalysts in the hydrogenation of a variety of imines. Among the examined complexes, the Ru-complex 1 a, RuCl(4)(dppb), showed the best activity, reducing N-aryl and N-alkyl imines to the related amines in high yields (up to 100 %) with a catalyst loading of 0.1-1 mol% and under mild conditions (5 atm H2, 40-70 °C). The Os-complexes exhibited a lower activity than their ruthenium analogues. Despite this, the Os-complex 2 a, OsCl(4)(dppb), was successful in the reduction of N-benzylideneaniline at 70 °C, affording a rare example of application of osmium complexes in the catalytic hydrogenation of imines.