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 (H 2 O) 3 ] 2+ ( 1, Cp* = η 5 -C 5 Me 5 ) as a catalyst precursor in water. Complex 1 has been characterized by X-ray structure analysis, 1 H 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 1 H 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 1 H NMR and atmospheric pressure chemical ionization mass spectrometry (APCI-MS). A possible mechanism for this transfer hydrogenation is proposed.