New catalysts for the chemoselective reduction of α,β-unsaturated ketones: Synthesis, spectral, structural and DFT characterizations of mixed ruthenium(II) complexes containing 2-ethene-1,3-bis(diphenylphosphino)propane and diamine ligands

The novel trans-[RuCl2(dpme)(L)] complexes (1L1–1L4) {dpme is H2CC(CH2PPh2)2 and L is 1,2-ethanediamine (L1), 1,3-propanediamine (L2), 2,2-dimethyl-1,3-propanediamine (L3) and 1,4-butanediamine (L4)} were obtained by reacting trans-[RuCl2(dpme)2] with an excess amount of the corresponding diamine in CH2Cl2 as the solvent. One of the diphosphine ligands was quantitatively replaced by the corresponding diamine, even when excess diamine was added. In solution the trans-[RuCl2(dpme)(diamine)] isomer configuration was confirmed by NMR spectroscopy and the X-ray crystal structure of 1L2 was also determined. These diphosphine complexes (1L1–1L4) have exhibited significant activity and selectivity as hydrogenation catalysts for α,β-unsaturated ketones. Increasing the chelating ring of the diamine ligands from five and six to seven decreases the turnover frequencies (TOFs). The catalytic activities were compared to the analogous trans-[RuCl2(dppp)(L)] (2L1–2L4) {dppp is H2C(CH2PPh2)2}. Complexes 1L1–1L4 catalyzed the hydrogenation of α,β-unsaturated ketones faster than the previously reported complexes 2L1–2L4. Density functional theory (DFT) calculations have been carried out to study the shift in the Ru(III)/Ru(II) couple and the catalytic activity for two representative complexes, 1L2 and 2L2.

Graphical abstractThe novel trans-[RuCl2(dpme)(L)] complexes {dpme is H2CC(CH2PPh2)2 and L is 1,2-ethanediamine, 1,3-propanediamine, 2,2-dimethyl-1,3-propanediamine and 1,4-butanediamine} were obtained by reacting trans-[RuCl2(dpme)2] with an excess amount of the corresponding diamine in CH2Cl2 as the solvent. In solution the trans-[RuCl2(dpme)(diamine)] isomer configuration was confirmed by NMR spectroscopy and the X-ray crystal structure. These diphosphine complexes have exhibited significant activity and selectivity as hydrogenation catalysts for α,β-unsaturated ketones. Increasing the chelating ring of the diamine ligands from five and six to seven decreases the turnover frequencies (TOFs). The catalytic activities were compared to the analogous trans-[RuCl2(dppp)(L)] {dppp is H2C(CH2PPh2)2}. Density functional theory (DFT) calculations have been carried out to study the shift in the Ru(III)/Ru(II) couple and the catalytic activity for two representative complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide