Site-selective halogenation of mixed-valent vanadium oxide clusters

Abstract

Here, we expand on the synthesis and characterization of chloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) clusters, to include the halogenation of mixed-valent vanadium oxide assemblies. These findings build on our previously disclosed results describing the preparation of a mono-anionic chloride-functionalized cluster, [V₆O₆Cl(OC₂H₅)₁₂]¹⁻, by chlorination of [V₆O₇(OC₂H₅)₁₂]²⁻ with AlCl₃, aimed at understanding the electronic consequences of the introduction of halide-defects in bulk metal oxides (e.g. VO₂). While chlorination of the mixed-valent POV-ethoxide clusters was not possible using AlCl₃, we have found that the chloride-substituted oxidized derivatives of the Lindqvist vanadium-oxide clusters can be formed using TiCl₃(thf)₃ with [V₆O₇(OC₂H₅)₁₂]ⁿ (n = 1−, 0) or WCl₆ with [V₆O₇(OC₂H₅)₁₂]⁰. Characterization of the chloride-containing products, [V₆O₆Cl(OC₂H₅)₁₂]ⁿ (n = 0, 1+), was accomplished via¹H NMR spectroscopy, X-ray crystallography, and elemental analysis. Electronic analysis of the redox series of Cl-doped POV-alkoxide clusters via infrared and electronic absorption spectroscopies revealed all redox events are localized to the vanadyl portion of the cluster, with the site differentiated V^III–Cl moiety retaining its reduced oxidation state across a 1.9 V window. These results present new synthetic routes for accessing chloride-doped POV-alkoxide clusters from mixed-valent vanadium oxide precursors.