This paper reports pH-dependent transfer hydrogenation, reductive amination, and dehalogenation of water-soluble substrates with the organometallic aqua complexes 2+ (2, Cp∧py ) η 5 -(tetramethylcyclopentadienyl)methylpyridine), and [Cp*Ir III (bpy)(H 2 O)] 2+ (3, bpy ) 2,2′bipyridine) as catalyst precursors and the formate ions HCOONa and HCOONH 4 as hydrogen donors. Because of the difference in the electron-donating ability of the Cp*, Cp∧py, and bpy ligands, the Lewis acidity of the iridium ions of 1-3 are ordered in strength as follows: 1 > 2 > 3. Complexes 1-3 are reversibly deprotonated to form the catalytically inactive hydroxo complexes [(Cp*Ir III ) 2 (µ-OH) 3 ] + (5), [{(Cp∧py)Ir III } 2 (µ-OH) 2 ] 2+ (6), and [Cp*Ir III (bpy)-(OH)] + (7) around pH 2.8, 4.5, and 6.6, respectively. The deprotonation behavior of 1-3 indicates that the more Lewis acidic iridium ions would lower the pK a values of the coordinated H 2 O ligands. As a function of pH, the catalyst precursors 1 and 3 react with the formate ions to form the hydride complexes [(Cp*Ir III ) 2 (µ-H)(µ-OH)(µ-HCOO)] + (8) and [Cp*Ir III (bpy)(H)] + (9), respectively, which act as active catalysts in these catalytic reductions. A similar hydride complex would be formed from the reaction of 2 with the formate ions, though we have no definite structural information on the hydride complex. The structures of 3(OTf) 2 ‚H 2 O (OTf ) CF 3 SO 3 -), [(Cp∧py)Ir III Cl 2 ] (4), 6(OTf) 2 , 7(OTf)‚2H 2 O, and 8(PF 6 ) were unequivocally determined by X-ray analysis.

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