Convenient synthesis of fluoride-alkoxides of Nb(v) and Ta(v): a spectroscopic, crystallographic and computational study

Abstract

The synthesis and the spectroscopic characterization of fluoride-alkoxides of niobium and tantalum in the highest oxidation state are reported. Suspensions of MF₅ (M = Nb, Ta) in a chlorinated solvent reacted with up to three equivalents of ROSiMe₃ (R = Me, Et, Ph) to afford polynuclear derivatives and variable amounts of FSiMe₃. Thus MF₄(OR) (R = Et, Ph) and MF₃(OR)₂ were obtained by selective 1 : 1 and 1 : 2 reactions almost exclusively as single isomeric products; otherwise mixtures of MF₄(OMe) species were afforded from the equimolar reactions of MF₅ with MeOSiMe₃. The 1 : 3 reaction of TaF₅ with MeOSiMe₃ led to different forms of TaF₂(OMe)₃. The synthesis of TaF(OPh)₄ was forced by high temperature conditions or the use of a large excess of PhOSiMe₃. DFT studies were carried out in order to predict, in the distinct cases, the most stable structures of the metal products. The molecular structures of [NbF₂(OPh)₂(μ-F)]₃ and [TaF(OPh)₃(μ-OPh)]₂ were ascertained by X-ray diffraction.