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

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Abstract

This paper reports pH-dependent transfer hydrogenation, reductive amination, and dehalogenation of water-soluble substrates with the organometallic aqua complexes [Cp*IrIII(H2O)3]2+ (1, Cp* = η5-pentamethylcyclopentadienyl), [(Cp∧py)IrIII(H2O)2]2+ (2, Cp∧py = η5-(tetramethylcyclopentadienyl)methylpyridine), and [Cp*IrIII(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∧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*IrIII)2(μ-OH)3]+ (5), [{(Cp∧py)IrIII}2(μ-OH)2]2+ (6), and [Cp*IrIII(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 pKa values of the coordinated H2O ligands. As a function of pH, the catalyst precursors 1 and 3 react with the formate ions to form the hydride complexes [(Cp*IrIII)2(μ-H)(μ-OH)(μ-HCOO)]+ (8) and [Cp*IrIII(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∧py)IrIIICl2] (4), 6(OTf)2, 7(OTf)·2H2O, and 8(PF6) were unequivocally determined by X-ray analysis.

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