PH-Dependent Transfer Hydrogenation of Ketones with HCOONa as a Hydrogen Donor Promoted by (η6-C6Me6)Ru Complexes

Abstract

The paper reports on the development of a new class of water-soluble organometallic catalysts for pH-dependent transfer hydrogenation. An organometallic aqua complex [(η6-C6Me6)RuII(bpy)(H2O)]2+ (1, bpy = 2,2‘-bipyridine) acts as a catalyst precursor for pH-dependent transfer hydrogenation of water-soluble and -insoluble ketones with HCOONa as a hydrogen donor in water and in biphasic media. Irrespective of the solubility of the ketones toward water, the rate of the transfer hydrogenation shows a sharp maximum around pH 4.0 (in the case of biphasic media, the pH value of the aqueous phase is adopted). In the absence of the reducible ketones, as a function of pH, complex 1 reacts with HCOONa to provide a formato complex [(η6-C6Me6)RuII(bpy)(HCOO)]+ (2) as an intermediate of β-hydrogen elimination and a hydrido complex [(η6-C6Me6)RuII(bpy)H]+ (3) as the catalyst for the transfer hydrogenation. The structures of 1(PF6)2, 2(HCOO)·HCOOH, and [(η6-C6Me6)RuII(H2O)3]SO4·3H2O {4(SO4)·3H2O}, the starting material for the synthesis of 1, were unequivocally determined by X-ray analysis.