Synthesis and Structural Analysis of (Arylimido)vanadium(V) Complexes Containing Phenoxyimine Ligands: New, Efficient Catalyst Precursors for Ethylene Polymerization

A series of (arylimido)vanadium(V) complexes containing phenoxyimine ligands of the type V(NAr)Cl₂[O-2-R-6-{(2,6-ⁱPr₂C₆H₃)NCH}C₆H₃] [Ar = 2,6-Me₂C₆H₃; R = H (1a), Me (1b), ^tBu (1c)] have been prepared in 64−78% yields from V(NAr)Cl₃ with LiO-2-R-6-[(2,6-ⁱPr₂C₆H₃)NCH]C₆H₃. The structures for 1a−c determined by X-ray crystallography indicate that these complexes have distorted square-pyramidal structures around vanadium, consisting of the arylimido ligands in the apical site of a distorted square-pyramid and the N,O-chelate ligand, which forms a plane including both the aryloxo ring and the imino nitrogen. The Cl−V−Cl bond angle for 1c [90.49(2)°] is apparently smaller than those in 1a,b [91.97(7)°, 91.25(2)°, respectively], and the V−N(imine) bond distance in 1c [2.203(2) Å] is shorter than those in 1b,c [2.216(4), 2.2165(17) Å, respectively]. These complexes (especially 1c) exhibited notable catalytic activities for ethylene polymerization in the presence of MAO, and the ortho substituent in the aryloxo ligand strongly affected the catalytic activity; the activity of the ^tBu analogue (1c) was higher than that of the reported V(NAr)Cl₂(O-2,6-Me₂C₆H₃) under the same conditions. The reaction of 1.1 equiv of 2-Me-6-{(2,6-ⁱPr₂C₆H₃)NCH}C₆H₃OH with V(NAr)Me(NC^tBu₂)₂ afforded another methyl complex without reaction with the methyl group occurring; unique reactivity of the methyl complex with phenol could thus be observed.