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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.
Ogo et al. (Wed,) studied this question.