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

Abstract

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 cocatalyst but higher selectivities for 1-butene. A turnover frequency of 22 800 mol C2H4/mol Ni·h was observed for 11 in the presence of 800 equiv of MAO. The selectivities were in the range 70−85% for the C4 olefins and in the range 33−38% for 1-butene within the C4 fraction. For these P,N ligands, increasing the degree of alkyl substitution at the carbon α to P leads to higher activities for ethylene oligomerization and more selective formation of α-olefins. The nature of the N-heterocycle influences the activity, as shown by the turnover frequencies of 58 100 mol C2H4/mol Ni·h for 11 in the presence of 6 equiv of AlEtCl2 and 45 900 mol C2H4/mol Ni·h for the related phosphinooxazoline complex 16. Under these conditions, the selectivity to 1-butene within the C4 fraction was 11% for 11 and 20% for 16. When MAO was used as a cocatalyst (800 equiv), a similar trend was observed for the activity, but the selectivity to 1-butene within the C4 fraction increased to 38% for 11 and 37% for 16.