Synthesis of nickel complexes with bidentate N,O-type ligands and application in the catalytic oligomerization of ethylene

The dinuclear complexes [Ni(micro-Cl){(4,5-dihydro-4,4-dimethyloxazol-2-yl)methanol}](2)Cl(2) and [Ni(micro-Cl){(pyridin-2-yl)methanol}](2)Cl(2) 16 have been synthesized in high yields by reaction of NiCl(2) with 2 mol. equiv. of the ligands 4,5-dihydro-4,4-dimethyloxazol-2-yl)methanol 13 or (pyridin-2-yl)methanol 15, respectively. The reaction of NiCl(2) with 3 mol. equiv. of 15 afforded in high yield the mononuclear, octahedral mer-[Ni{(pyridin-2-yl)methanol}(3)Cl(2)] complex 18. The reaction of 16 with NaH led to the deprotonation of one of the pyridine alcohol ligands to form [Ni{(pyridin-2-yl)methanol}{(pyridin-2-yl)methanolate}Cl] 21 in which the metal is coordinated by one pyridine alcohol and one pyridine alcoholate ligand. The crystal structures of the dinuclear, chloride-bridged octahedral complexes in 14.C(6)H(12) and in 16.3CH(2)Cl(2) and of the mononuclear, octahedral complex 18 in 18.CH(2)Cl(2) have been determined by X-ray diffraction. In the latter case, intermolecular OH...Cl bonding interactions generate a centrosymmetric pseudo-dimer. Complexes 14, 16, and 21 have been tested in ethylene oligomerization with AlEtCl(2) (Al/Ni ratios of 2, 4 or 6) or MAO (50, 100 or 200 equiv.) as co-catalysts under 10 bar of ethylene and yielded mostly dimers and trimers. Complex 16 in the presence of 6 equiv. of AlEtCl(2) proved to be the most active system with a turnover frequency (TOF) up to 187 500 C(2)H(4) (mol Ni h)(-1). Complex 16 with 200 equiv. of MAO was also the most active, with TOF up to 104 300 C(2)H(4) (mol Ni h)(-1) under 30 bar of ethylene.