Synthesis of α-diimine V(iii) complexes and their role as ethylene polymerisation catalysts

Abstract

Two novel α-diimine vanadium complexes, [PhNC(Me)–C(Me)NPh]V(THF)Cl₃ (1) and {[2,6-(i-Pr)₂Ph]NCH–CHN[2,6-(i-Pr)₂Ph]}VCl₃ (2) were synthesized by ligand exchange reaction of VCl₃(THF)₃ with the appropriate α-diimine ligand. The chemical structure of 1 was determined by X-ray crystallography and found to consist of a vanadium atom in a distorted octahedral geometry with the oxygen atom of the coordinated THF, two nitrogen atoms of the diimine ligand and one chlorine atom in the same plane. Complexes 1 and 2 were characterized by FT-IR, ¹H NMR, elemental analysis and 2 was identified as the THF free V(III) diimine complex. The behavior of 1 and 2 as ethylene polymerisation catalysts was preliminarily explored after activation with MAO or AlEt₂Cl and compared with that of {2,6-bis[2,6-(i-Pr)₂PhNC(Me)]₂(C₅H₃N)}VCl₃ (3) under similar conditions. Upon activation of 1–3 with AlEt₂Cl, moderate polymerisation activities were observed at −40 °C whereas the corresponding MAO activated catalysts exhibit lower activity. The molar concentration of the active sites at −40 °C is rather low (found in the range 5–12% of total vanadium) and results from slow alkylation of the complexes followed by reduction of V(III) catalysts to inactive V(II) species. At 50 °C an increase of the ethylene polymerisation activities of the 1–3-AlEt₂Cl catalysts was observed and the trend of the productivity values follows the steric hindrance at the metal center. Alkylation of the diimine ligands of the 1–3 catalysts with AlEt₂Cl producing deactivation of the active sites is only partially active under polymerisation conditions whereas the reaction of 1 and 2 with MAO produces fast degradation of the active species.