Pyridine-2,6-bis(thioether) (SNS) Complexes of Ruthenium as Catalysts for Transfer Hydrogenation

Coordination of the pincer ligand 2,6-bis(tert-butylthiomethyl)pyridine (SNStBu) to Ru(PPh3)3Cl2 and Ru(MeCN)4Cl2 led to the isolation of the complexes Ru(SNStBu)Cl2(PPh3) (1) and Ru(SNStBu)Cl2(MeCN) (4), respectively. Substitution of one chloride ligand from 1 and 4 with acetonitrile proved facile via salt metathesis with NaBPh4 to yield [Ru(SNStBu)Cl(MeCN)(PPh3)]BPh4 (2) and [Ru(SNStBu)Cl(MeCN)2]BPh4 (5). Reaction of 1 with an excess of AgBF4 in acetonitrile afforded the dicationic complex [Ru(SNStBu)(MeCN)2(PPh3)](BF4)2 (3); however reaction of 4 under analogous conditions afforded the unusual silver-bridged dimer [Ru(SNStBu)(MeCN)2Cl]2[μ-Ag(MeCN)2]2(BF4)4 (6), which retained the second chloride ligand on ruthenium. The less sterically encumbered compounds 5 and 6 were shown to have fluxional conformational arrangements associated with an inversion of the tetrahedrally hybridized thioether centers. Compounds 1−4 were also investigated as catalysts for the transfer hydrogenation of acetophenone. Steric congestion within these complexes was found to negatively impact on catalyst activity, with the less sterically encumbered complexes yielding the highest activity. In the case of 4 an extremely high turnover frequency of ca. 87 000 h−1 was achieved for this reaction.