pH-dependent transfer hydrogenation of water-soluble carbonyl compounds with [Cp*Ir^III(H₂O)₃]²⁺ (Cp* = η⁵C₅Me₅) as a catalyst precursor and HCOONa as a hydrogen donor in water

This paper reports a pH-dependent hydrogenation of water-soluble carbonyl compounds by hydrogen transfer from HCOONa as a hydrogen source (transfer hydrogenation) promoted by [Cp*Ir^III(H₂O)₃]²⁺ (1, Cp* = η⁵-C₅Me₅) as a catalyst precursor in water. Complex 1 has been characterized by X-ray structure analysis, ¹H NMR, and potentiometric titration experiments. The active catalyst, a dinuclear μ-hydride complex [(Cp+Ir^III)₂(μ-H)(μ-OH)(μ-HCOO)]⁺ (2), has been isolated and characterized by ¹H NMR, IR, and electrospray ionization mass spectrometry (ESI-MS). The rate of this transfer hydrogenation shows a sharp maximum at pH 3.2 because the active catalyst 2 is generated from the reaction of 1 with HCOONa at pH 3.2 in the highest yield. The series of the carbonyl compounds consists of a straight chain aldehyde (n-butyraldehyde), a cyclic aldehyde (cyclopropanecarboxaldehyde), a ketone (2-butanone), an aldehyde-acid (glyoxylic acid), and a keto-acid (pyruvic acid). Products were determined by ¹H NMR and atmospheric pressure chemical ionization mass spectrometry (APCI-MS). A possible mechanism for this transfer hydrogenation is proposed.