Low-temperature syntheses of vanadium(III) and molybdenum(IV) bromides by halide exchange

Abstract

Halide-exchange reactions of VCl₄ or MoCl₅ with dry HBr at temperatures between ca.–50 and +20 °C afforded the corresponding bromides, VBr₃ or MoBr_4′, in good yields, presumably via the formation of unstable higher-valent bromide intermediates which undergo spontaneous loss of bromine. In corresponding reactions with Hl the exchange was incomplete and mixed halides were obtained. At room temperature the VCl₄–Hl system gave VCl₃ in an almost quantitative yield. By reaction of MoBr₄ with tetrahydrofuran (thf)[MoBr₃(thf)₃] was isolated and its crystal structure determined: orthorhombic, space group Pbcn(no. 60), a= 8.812(2), b= 13.882(5), c= 14.279(3)Å, Z= 4, R= 0.063. The molecule has the usual meridional geometry, typical of other [MoX₃(thf)₃] derivatives (X = Cl or l).