Nickel and iron complexes with oxazoline- or pyridine-phosphonite ligands; synthesis, structure and application for the catalytic oligomerisation of ethylene

Abstract

The bis(oxazolinyl)phenylphosphonite ligand 5 (bis(4,4-dimethyl-2-(1-hydroxy-1-methylethyl)-4,5-dihydrooxazole)phenylphosphonite, NOPON^Me2)) and the new pyridine-phosphonite ligand 6 (2-ethyl(1′-methyl-1′-hydroxy)pyridine-6H-dibenz[c,e][1,2]oxaphosphorin) have been used for the preparation of the mononuclear complexes [NiCl₂(NOPON^Me2)] 18 and [NiCl₂(6)₂] 19, respectively, which catalyze the oligomerisation of ethylene with activities up to 57300 mol C₂H₄ mol Ni⁻¹ h⁻¹ (19 in the presence of only 6 equivalents of AlEtCl₂). The selectivities for C₄ dimers were as high as 90% (18 in the presence of only 2 equivalents of AlEtCl₂) with selectivities for 1-butene of 21–22% of the C₄ fraction. In the presence of 400 or 800 equivalents of MAO as cocatalyst, complex 19 yielded turnover frequencies of 7400 mol C₂H₄ mol Ni⁻¹ h⁻¹ and 13200 mol C₂H₄ mol Ni⁻¹ h⁻¹, respectively. The selectivities for 1-butene and ethylene dimers were similar to those obtained with AlEtCl₂. The fact that 19 with a cyclic phosphonite moiety leads to higher activities and selectivities than 18 which contains an acyclic phosphonite group underlines the importance of the ligand on the catalytic properties of its metal complex. An unprecedented dinuclear iron complex [FeCl₂(4,4-dimethyl-2-{(1-hydroxy-1-methyl)ethyl}-4,5-dihydrooxazolate)]₂20 was also obtained which contains two pentacoordinated metal centers coordinated by a bridging-chelating oxazoline-alcoholate. Complexes 18–20 are paramagnetic in solution, as determined by the Evans method.