Synthesis and Structural Analysis of (Imido)vanadium Dichloride Complexes Containing 2-(2′-Benz-imidazolyl)pyridine Ligands: Effect of Al Cocatalyst for Efficient Ethylene (Co)polymerization

(Imido)vanadium(V) dichloride complexes containing 2-(2'-benzimidazolyl)-6-methylpyridine ligand (L) of type V(NR)Cl 2 (L) [R = 1-adamantyl (Ad, 1 ), C 6 H 5 ( 2 ), and 2,6-Me 2 C 6 H 3 ( 3 )] have been prepared, and their structures were determined by X-ray crystallography as distorted trigonal bipyramidal structures around vanadium. Reactions with ethylene using 1-3 in the presence of methylaluminoxane (MAO) afforded a mixture of oligomer and polymers, and the compositions were affected by the imido ligand employed. By contrast, 1-3 exhibited remarkable catalytic activities for ethylene polymerization in the presence of Me 2 AlCl; the phenylimido complex ( 2 ) exhibited the highest activity [80 100 kg-PE/mol-V·h turn over frequency (TOF, 2 850 000 h -1 , 792 s -1 )]. The ethylene copolymerizations with norbornene afforded ultrahigh-molecular-weight copolymers with uniform molecular weight distributions and compositions [e.g., M n = 1.71-2.66 × 10 6 , M w / M n = 2.27-2.53]. On the basis of V nuclear magnetic resonance ( 51 V NMR), electron spin resonance, and V K-edge X-ray absorption near-edge structure (XANES) spectra of the catalyst solution, the observed difference in the catalyst performance in the presence of (between) MAO and Me 2 AlCl cocatalyst should be due to the formation of different catalytically active species with different oxidation states. Apparent changes in the oxidation state were observed in the (especially in the NMR and XANES) spectra upon addition of Me 2 AlCl, whereas no significant changes in the spectra were observed in presence of MAO.