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

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)(mu-F)](3) and [TaF(OPh)(3)(mu-OPh)](2) were ascertained by X-ray diffraction.