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 VIII 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.

Atomically precise vanadium-oxide clusters

“…24). 237 Introduction of a surface defect, in the formation of an oxygenatom vacancy, resulted in a anodic shift of the redox events by ~0.37 V in comparison to that of the parent cluster, [V 6 O 7 (OC 2 H 5 ) 12 ] n . Similar anodic shifts are observed upon chlorination, where, in this case, each redox-event is shifted anodically by ~ 0.2 V, relative to that of the parent, fullyoxygenated cluster.…”

Trace Hydrogen Sulfide Sensing Inspired by Polyoxometalate-Mediated Aerobic Oxidation

“…Concurrently, distinct shifts in V 2p binding energies were observed from 517.0 to 515.9 eV (V 2p 3/2 ) and from 523.8 to 523.0 eV (V 2p 1/2 ) upon H 2 S exposure, consistent with the reduction of V V to V IV in the POM ( Figure 4 b). 41 , 42…”

The crystal structure of hexakis(2-(pyridin-2-ylamino)pyridin-1-ium) decavanadate(V) dihydrate, C60H64N18O30V10

“…Polyoxovanadates, as an outstanding large important branch of anionic metal-oxo clusters, have led to a rapid growth due to their structural features and promising applications in diverse areas, such as catalysis, electricity, medicine and functional materials [3][4][5][6]. The design and synthesis of new polyoxovanadates architectures with well-defined structures expand the range of polyoxovanadates-inspired hybrid materials [7,8].…”