Syntheses and structural investigations of new half sandwich Ir(III) and Rh(III) amine compounds and their catalytic transfer hydrogenation of aromatic ketones and aldehydes in water

Abstract

Six new compounds, [IrCp*(p-R-N-(pyridin-2-ylmethyl)aniline)Cl]PF6 (R=H (1); F (2); OCH3 (3)), and [RhCp*(p-R-N-(pyridin-2-ylmethyl)aniline)Cl]PF6 (R=H (4); F (5); OCH3 (6)), (where Cp*=pentamethylcyclopentadiene) have been synthesized. Furthermore, the new [IrCp*(N-methyl-N-(pyridin-2-ylmethyl)aniline)Cl]PF6 (7) and [RhCp*(N-methyl-N-(pyridin-2-ylmethyl)aniline)Cl]PF6 (8) compounds were synthesized to allow an investigation of the role of the NH group and NCH3 in the proposed reaction mechanism of transfer hydrogenation. The synthesized compounds were characterized by 1H NMR, 2D NMR, 13C{1H} NMR, HR-Mass spectroscopy, IR spectroscopy and elemental analysis. 1H NMR studies revealed that the N,N’ amine and NCH3 protected N,N’ bidentate ligands coordinated to the Ir(III) and Rh(III) metal ions and the compounds 1-8 exhibit diastereoisomers in DMSO-d6 solution at room temperature. Time dependent 1H NMR studies show that the Ir(III) amine compounds 1-3 are oxidized to imine complexes by molecular oxygen. The crystal structures of the complexes [Cp*IrCl(N-(pyridin-2-ylmethyl)aniline)]PF6 (1) and [Cp*RhCl(p-OCH3-N-(pyridin-2-ylmethyl)aniline)]PF6 (6) have been resolved by single crystal X-ray diffraction. The Ir(III) and Rh(III) amine compounds were compared in the catalytic transfer hydrogenation reaction. The Rh(III) amine and imine compounds were found to be more versatile than the Ir(III) amine and imine compounds. Thus, the Rh(III) compounds were particularly effective for the catalytic transfer hydrogenation of aromatic carbonyl groups in water with sodium formate and formic acid as the hydrogen source under pH dependent acidic conditions. The higher activation of compounds 1–3 vs 7 and 4–6 vs 8 supports a dominant mechanism in which the NH moiety contributes to the activation of the carbonyl compounds. Furthermore, the Rh(III) amine catalyst (4) can be easily recovered from the reaction medium and reused for at least three catalytic cycles without decrease in conversions.