pH-dependent transfer hydrogenation of water-soluble carbonyl compounds with [Cp*IrIII(H2O)3]2+ (Cp* = η5-C5Me5) 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* = eta(5)-C5Me5) as a catalyst precursor in water. Complex 1 has been characterized by X-ray structure analysis, H-1 NMR, and potentiometric titration experiments. The active catalyst, a dinuclear mu-hydride complex [(Cp*Ir-III)(2)(mu-H)(mu-OH)(mu-HCOO)](+) (2), has been isolated and characterized by H-1 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 H-1 NMR, and atmospheric pressure chemical ionization mass spectrometry (APCI-MS). A possible mechanism for this transfer hydrogenation is proposed.