Synthesis of alpha-diimine V(III) complexes and their role as ethylene polymerisation catalysts

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