Synthesis and Structural Analysis of Four Coordinate (Arylimido)niobium(V) Dimethyl Complexes Containing Phenoxide Ligand: MAO-Free Ethylene Polymerization by the Cationic Nb(V)−Methyl Complex
K. Koide, J. Yi, M. Kuboki, S. Yamazoe, N. Nakatani, K. Nomura
Organometallics in press.

Four coordinate (arylimido)niobium(V) dimethyl complexes containing 2,6-di-tert-butylphenoxy ligand, Nb(NAr)- Me2(O-2,6-t Bu2C6H3) [Ar = 2,6-i Pr2C6H3 (5a), 2,6-Me2C6H3 (5b), 2-MeC6H4 (5c)], were prepared from the bis(dimethylamide) analogues, Nb(NAr)(NMe2)2(O-2,6-t Bu2C6H3) (2a−c), by treatment with AlMe3. Structures with a distorted tetrahedral geometry of the bis(dimethylamide) complexes (2a−c) and the dimethyl complexes (5b,c) were determined by X-ray crystallography. The solution Nb K-edge XANES spectra (X-ray absorption near edge structure, in toluene at 25 °C) showed that pre-edge intensities in the dimethyl complexes (5a,b) were higher than those in the bis(dimethylamide) complexes (2a,b) and their pre-edge absorptions (including their shoulder edge absorptions) were highly affected by the ligand sets employed; these peak assignments are well explained by TD-DFT (time dependent density functional theory) simulation as a results of s−d transition and degree in subsequent d−p hybridization. The dimethyl complexes (5a−c) showed good catalysis capabilities for ethylene polymerization in n-octane in the presence of methylaluminoxane (MAO) especially at 80 °C (ca. 808−1140 kg-PE/mol-Nb·h). Treating 5a with [Ph3C][B(C6F5)4] in THF gave the cationic [Nb(N2,6-i Pr2C6H3)Me(O-2,6-t Bu2C6H3)(THF)]+ [B(C6F5)4] − (6a), which polymerized ethylene at 80 °C upon addition of Al(nC8H17)3 to afford ultrahigh molecular weight polymers with unimodal molecular weight distributions (Mn = 1.04−1.21 × 106 , Mw/ Mn = 1.60−1.86); the results strongly suggest that the polymerization proceeded via the cationic alkyl species.