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)­Cl2(L) [R = 1-adamantyl (Ad, 1), C6H5 (2), and 2,6-Me2C6H3 (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 Me2AlCl; 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., Mn = 1.71–2.66 × 106, Mw/Mn = 2.27–2.53].
On the basis of V nuclear magnetic resonance (51V 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 Me2AlCl 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 Me2AlCl, whereas
no significant changes in the spectra were observed in presence of
MAO.