New Nickel Ethylene Oligomerization Catalysts Bearing Bidentate P,N-Phosphinopyridine Ligands with Different Substituents α to Phosphorus

The new phosphinopyridine ligands rac-2-[(diphenylphosphanyl)benzyl]pyridine (6), rac-2-[1‘-(diphenylphosphanyl)ethyl]pyridine (7), and 2-[1‘-(diphenylphosphanyl)-1‘-methyl]ethylpyridine (8) with variable substitution at the carbon α to P were used for the synthesis of the paramagnetic Ni(II) complexes [NiCl2(P,N)] 9−11, respectively. The complex 11·0.5CH2Cl2 has been shown by X-ray diffraction to have an almost planar coordination geometry about the metal, although the coordination sphere of all complexes in solution was determined by the Evans method to be distorted tetrahedral. The Ni complexes provided activities for the catalytic oligomerization of ethylene up to 58 100 mol C2H4/mol Ni·h (11) in the presence of only 6 equiv of AlEtCl2. The selectivity for C4 olefins reached 81% for 9 in the presence of 2 equiv of AlEtCl2, but the selectivity toward 1-butene was only 11−14%. In the presence of 400 or 800 equiv of methylalumoxane (MAO), complexes 9−11 yielded lower activities than with AlEtCl2 as cocat...