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

The paper reports on the development of a new class of water-soluble organometallic catalysts for pH-dependent transfer hydrogenation. An organometallic aqua complex [( η 6 -C 6 Me 6 )Ru II (bpy)(H 2 O)] 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 [( η 6 -C 6 Me 6 )Ru II (bpy)(HCOO)] + ( 2 ) as an intermediate of β -hydrogen elimination and a hydrido complex [( η 6 -C 6 Me 6 )Ru II (bpy)H] + ( 3 ) as the catalyst for the transfer hydrogenation. The structures of 1 (PF 6 ) 2, 2 (HCOO)·HCOOH, and [( η 6 -C 6 Me 6 )Ru II (H 2 O) 3 ]SO 4 ·3H 2 O { 4 (SO 4 )·3H 2 O}, the starting material for the synthesis of 1, were unequivocally determined by X-ray analysis.