Site-Selective Halogenation of Polyoxovanadate Clusters: Atomically Precise Models for Electronic Effects of Anion Doping in VO₂

Abstract

We report the synthesis and characterization of a monochloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) cluster, which can serve as a molecular model for halogen-doped vanadium oxide (VO₂) materials that have recently attracted great interest as advanced materials for energy-saving smart window applications. Chloride-substituted variants of the Lindqvist vanadium-oxide cluster were obtained via two distinct chemical pathways: (1) direct halogenation of the isovalent parent POV-alkoxide architecture, [V₆O₇(OC₂H₅)₁₂]^-2 with AlCl₃ and (2) coordination of a chloride ion to a coordinatively unsaturated vanadium center within a cluster that bears a single oxygen-atom vacancy, [V₆O₆(OC₂H₅)₁₂]⁰. Notably, our direct halogenation constitutes the first example of selective, single-site halide doping of homometallic metal oxide clusters. The chloride-containing compound, [V₆O₆Cl(OC₂H₅)₁₂]^-1, was characterized by ¹H NMR spectroscopy and X-ray crystallography. The electronic structure of the chloride-functionalized POV-alkoxide cluster was established by infrared, electronic absorption, and X-ray photoelectron spectroscopy and revealed formation of a site-differentiated V^III ion upon halogenation. Cyclic voltammetry was employed to assess the electrochemical response of halide doping. A comparison of the Cl-VO₂ model to the fully oxygenated cluster, [V₆O₇(OC₂H₅)₁₂]^-2, provides molecular-level insights into a new proposed mechanism by which halogenation increases the carrier density in solid VO₂, namely, through prompting charge separation within the material.