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.

This paper reports a pH-dependent hydrogenation of water-soluble carbonyl compounds by hydrogen transfer from HCOONa as a hydrogen source (transfer hydrogenation) promoted by [Cp*IrIII(H2O)3]2+ (1, Cp* = η 5-C5Me5) as a catalyst precursor in water. Complex 1 has been characterized by X-ray structure analysis, 1H NMR, and potentiometric titration experiments. The active catalyst, a dinuclear μ-hydride complex [(Cp*IrIII)2(μ-H)(μ-OH)(μ-HCOO)]+ (2), has been isolated and characterized by 1H NMR, IR, and electrospray ionization mass spectrometry (ESI-MS). The rate of this transfer hydrogenation shows a sharp maximum at pH 3.2 because the active catalyst 2 is generated from the reaction of 1 with HCOONa at pH 3.2 in the highest yield. The series of the carbonyl compounds consists of a straight chain aldehyde (n-butyraldehyde), a cyclic aldehyde (cyclopropanecarboxaldehyde), a ketone (2-butanone), an aldehyde-acid (glyoxylic acid), and a keto-acid (pyruvic acid). Products were determined by 1H NMR and atmospheric pressure chemical ionization mass spectrometry (APCI-MS). A possible mechanism for this transfer hydrogenation is proposed.

The reaction of (Phebox)SnMe3 (4; Phebox = 2,6-bis(oxazolinyl)phenyl) and [(cyclooctene)2RhCl]2 in the presence of CCl4 provided the air-stable and water-tolerant (Phebox)RhCl2(H2O) complexes 5. These neutral (noncationic) aqua complexes 5 acted as asymmetric catalysts for enantioselective allylation of aldehydes with allyltin reagents in the presence of 4 Å molecular sieves (MS 4A). Furthermore, these aqua complexes could be recovered quantitatively from the reaction media. Detailed mechanistic studies of this catalytic system using X-ray and NMR spectroscopy revealed that the (Phebox)RhCl2 fragment, generated by releasing H2O from aqua complex 5, is an active catalyst and the reaction proceeds by a Lewis acid catalyzed mechanism. The relative stereochemistry of the major adduct of the reaction of benzaldehyde with crotyltin reagents was anti (threo). The observed anti diastereoselectivity and si-face attack of allyltins on the carbonyl carbon of aldehydes were explained by the inverse antiperiplanar transition-state model.