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

Abstract

The dinuclear complexes [Ni(µ-Cl){(4,5-dihydro-4,4-dimethyloxazol-2-yl)methanol}]₂Cl₂14 and [Ni(µ-Cl){(pyridin-2-yl)methanol}]₂Cl₂16 have been synthesized in high yields by reaction of NiCl₂ 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 NiCl₂ with 3 mol. equiv. of 15 afforded in high yield the mononuclear, octahedral mer-[Ni{(pyridin-2-yl)methanol}₃Cl₂] 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·C₆H₁₂ and in 16·3CH₂Cl₂ and of the mononuclear, octahedral complex 18 in 18·CH₂Cl₂ 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 AlEtCl₂ (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 AlEtCl₂ proved to be the most active system with a turnover frequency (TOF) up to 187 500 C₂H₄ (mol Ni h)⁻¹. Complex 16 with 200 equiv. of MAO was also the most active, with TOF up to 104 300 C₂H₄ (mol Ni h)⁻¹ under 30 bar of ethylene.