Discrete spherical hexadecavanadates incorporating a bromide with oxidative bromination activity

Abstract

Two discrete hexadecavanadates, (n-Bu₄N)₄[V₁₆O₃₈(X)] (X = Cl⁻ (1) and X = Br⁻ (2)), were synthesized by a reaction of [V₁₀O₂₆]⁴⁻ with a template anion resulting in the incorporation of chloride or bromide in the {V16} spherical cluster framework. The reaction of [V₁₀O₂₆]⁴⁻ with p-toluenesulfonic acid proceeded under an aerobic environment to give 2 in the presence of an excess amount of bromide anion, which acted as both a template anion and a reducing reagent for the formation of the mixed-valence framework. For the synthesis of cluster 1, additional reductive conditions were required due to the weak reducing ability of the chloride anion. The crystal structures of 1 and 2 were determined using single-crystal X-ray diffraction analysis. Both were found to consist of a discrete [V^V₉V^IV₇O₃₈(X)]⁴⁻ framework by the linkage of VO₅ pyramidal units. Cyclic voltammetric studies of 1 and 2 in acetonitrile showed a series of stepwise reversible redox processes, which were due to the redox of the spherical polyoxovanadate frameworks. The oxidative bromination reactions of aromatic substrates were also investigated using cluster 2 as a catalyst under aerobic conditions.