pH-Dependent Transfer Hydrogenation, Reductive Amination, and Dehalogenation of Water-Soluble Carbonyl Compounds and Alkyl Halides Promoted by Cp*Ir Complexes

This paper reports pH-dependent transfer hydrogenation, reductive amination, and dehalogenation of water-soluble substrates with the organometallic aqua complexes [Cp*Ir III (H 2 O) 3 ] 2+ ( 1, Cp* = η 5 -pentamethylcyclopentadienyl), [(Cp∧py)Ir III (H 2 O) 2 ] 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 p K 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.