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* = η⁵-pentamethylcyclopentadienyl), [(CpΛpy)Ir^III(H₂O)₂]²⁺ (2, CpΛpy = η⁵-(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*, CpΛpy, 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), [{(CpΛpy)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 H₂O 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·H₂O (OTf = CF₃SO₃^-), [(CpΛpy)Ir^IIICl₂] (4), 6(OTf)₂, 7(OTf)·2H₂O, and 8(PF₆) were unequivocally determined by X-ray analysis.