Novel pyrazolylphosphite– and pyrazolylphosphinite–ruthenium(II) complexes as catalysts for hydrogenation of acetophenone
The new compounds and potential ligands 2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)ethyldiphenlyphosphinite (L1), 2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)ethyldiethylphosphite (L2), 2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)ethyl-diethylphosphite (L3) and 2-(3,5-diphenyl-1H-pyrazol-1-yl)ethyldiethylphosphite (L4) were prepared from the reaction of (3,5-(disubstituted)pyrazol-1H-yl)ethanol and the appropriate phosphine chloride. The phosphinite (L1) and phosphites (L2–L4) and 2-(3,5-diphenyl-1H-pyrazol-1-yl)ethyldiphenylphosphinite (L5) were reacted with [Ru(p-cymene)Cl2]2 to afford the ruthenium(II) complexes [Ru(p-cymene)Cl2(L1)] (1), [Ru(p-cymene)Cl2(L2)] (2), [Ru(p-cymene)Cl2(L3)] (3), [Ru(p-cymene)Cl2(L4)] (4), and [Ru(p-cymene)Cl2(L5)] (5). All ruthenium complexes were characterized by a combination of NMR spectroscopy, elemental analysis and, in selected cases, by single crystal X-ray crystallography. Complexes 1–5 and [Ru(p-cymene)Cl2(L6)] (6) (prepared from 2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyldiphenylphosphinite (L6)) were investigated as catalysts for both transfer and molecular hydrogenation of acetophenone to 1-phenylethanol. At 80 °C the percent conversion of acetophenone in transfer hydrogenation was moderate to high over 10 h (42–87%); for molecular hydrogenation acetophenone, conversions were as high as 98% in 6 h.