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

Two novel α-diimine vanadium complexes, [PhN=C(Me)-C(Me)=NPh]V(THF)Cl3 (1) and {[2,6-(i-Pr)2Ph]N=CH-CH=N[2,6-(i-Pr)2Ph]}VCl 3 (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)2PhN=C(Me)]2(C5H 3N)}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.