Abstract
Effects of ligand and ligand substituents in a series of (imido)vanadium(V) complexes containing monodentate anionic ancillary donor ligands of the type, V(NAr)Cl2(L) [Ar = 2,6-Me2C6H3; L = OAr, O-2,6-Ph2C6H3, N = CtBu2, 1,3-tBu2(CHN)2C = N, WCA-NHC (anionic N-heterocyclic carbenes with a weakly coordinating borate moiety)] and V(NAd)Cl2(OAr) (Ad = 1-adamantyl), were explored by the solution V K-edge XANES (pre-edge and edge region) spectra. No significant differences in the pre-edge (and the edge) peaks were observed between these complexes which fold a distorted 4 coordinate tetrahedral geometry around vanadium; the shoulder-edge peaks ascribed to the presence of V_Cl bonds were also observed in all cases. No significant spectral changes were observed when the phenoxy analogues (exhibiting catalytic activities for ethylene/norbornene copolymerization) were treated with MAO, and even addition of norbornene, suggesting preservation of the oxidation state. Remarkable changes in the XANES spectrum (decrease in the intensity of the pre-edge peak and low-energy shift of the edge region) were observed after addition of AliBu3 toward V(NAr)Cl2(WCA-NHC), and no notable changes were observed upon further addition of AliBu3 and norbornene; these results suggest that the formed species, presumably vanadium(III), act directly as the active species in this catalysis.
