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(H2O)(3)](2+) (1, CP* = eta (5)-pentamethylcyclopentadienyl), [(Cp boolean AND py)Ir-III(H2O)(2)](2+) (2, Cp boolean AND py = eta (5-)(tetramethylcyclopentadienyl)methylpyridine), and [Cp*Ir-III(bpy)(H2O)(2+) (3, bpy = 2,2'-bipyridine) as catalyst precursors and the formate ions HCOONa and HCOONH4 as hydrogen donors. Because of the difference in the electron-donating ability of the Cp*, Cp boolean AND 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)(mu -OH)(3)(+) (5), [{(Cp boolean AND py)Ir-III}(2)(mu -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 H2O ligands. As a function of pH, the catalyst precursors I and 3 react with the formate ions to form the hydride complexes [(Cp*Ir-III)(2)(mu -H)(mu -OH)(mu -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).H2O (OTf = CF3SO3-), [(Cp boolean AND py)(IrCl2)-Cl-III] (4), 6(OTf)(2), 7(OTf). 2H(2)O, and S(PF6) were unequivocally determined by X-ray analysis.