Site-Selective Halogenation of Polyoxovanadate Clusters: Atomically Precise Models for Electronic Effects of Anion Doping in VO_2

摘要

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 (VO_2) 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, [V_6O_7(OC_2H_5)_(12)]~(-2) with AlCl_3 and (2) coordination of a chloride ion to a coordinatively unsaturated vanadium center within a cluster that bears a single oxygen-atom vacancy, [V_6O_6(OC_2H_5)_(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, [V_6O_6Cl(OC_2H_5)_(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-VO_2 model to the fully oxygenated cluster, [V_6O_7(OC_2H_5)_(12)]~(-2), provides molecular-level insights into a new proposed mechanism by which halogenation increases the carrier density in solid V02, namely, through prompting charge separation within the material.