Synthesis and reactivity of niobium complexes having a tripodal triaryloxide ligand in bidentate, tridentate, and tetradentate coordination modes

Abstract

The synthesis and reactivity of niobium complexes incorporating a tripodal triphenol (tris(3,5-tert-butyl-2-hydroxylphenyl)methane = H₃[O₃]) have been investigated. Addition of one equivalent of NbCl₅ in CH₃CN to H₃[O₃] in toluene led to partial HCl elimination, giving [H(O₃)]NbCl₃(CH₃CN) (1) with a bidendtate bis(aryloxide) ligand and a pendant phenol arm. Treatment of 1 with THF afforded [H(O₃)]NbCl₃(THF) (2). Deprotonation of 1 with NEt₃ in toluene promoted coordination of the pendant phenol group to generate (Et₃NH)[(syn-O₃)NbCl₃] (3-syn). Prolonged heating of 3-syn resulted in clean conversion to the anti isomer (3-anti). Attempted deprotonation of 2 with PhCH₂MgCl provided [H(O₃)]Nb(CH₂Ph)₃ (4), in which alkylation took place at the metal center but the pendant phenol arm remained intact. When 3-syn was treated with PhCH₂MgCl, [O₃C]Nb(CH₂Ph) (5) was produced via C–H activation of the methine C–H bond. The analogous reaction with 3-anti provided a benzylidene complex [anti-O₃]Nb(CHPh)(THF) (6). During the course of the reaction, the anti ligand conformation is retained. Upon heating, 4 underwent methine C–H and phenol O–H activation, yielding the metalatrane 5. Complexes 1, 3-syn, 3-anti, 4, and 5 were characterized by X-ray diffraction.