New nickel-containing homogeneous hydrogenation catalysts -: Structures of [Ni(o-MeO-dpppol)Cl2] and [Ni(dcpe)Cl2]

The didentate phosphane ligands 1,3-bis(di(ortho-methoxyphenyl)phosphanyl)-2-propanol (o-MeO-dpppol) and 1,2-bis(dicyclohexylphosphanyl)ethane (dcpe) have been used in a study towards catalytic hydrogenation using homogeneous nickel catalysts. The nickel halide complexes [Ni(o-MeO-dpppol)X-2] and [Ni(dcpe)X-2] (X = Cl, Br or I) have been synthesized and were characterized using electronic absorption and IR spectroscopy. The structures of the complexes [Ni(o-MeO-dpppol)Cl-2] and [Ni(dcpe)Cl-2] have been determined by X-ray crystallography. The nickel ions in these complexes are in (distorted) square-planar geometries with NiP2Cl2 chromophores. The synthesized halide complexes and in situ mixtures of nickel acetate and the ligands were tested on catalytic activity in homogeneous hydrogenation. The ligand dcpe yields very active catalysts; turnover numbers up to 3000 in I h have been obtained. Generally, it appeared that the catalytic activities are higher in methanol than in methanol/dichloromethane mixtures. It was found that the catalytic activities observed for complexes containing the ligand o-MeO-dpppol are less reproducible than those of catalysts with the ligand dcpe. This lower reproducibility is probably related to the fact that the former ligand is more readily oxidized. (C) 2003 Elsevier Science B.V. All rights reserved.