In this manuscript, we further investigate
the use of Lindqvist polyoxovanadate alkoxide (POV–alkoxide)
clusters as homogeneous molecular models of reducible metal oxides
(RMO), focusing on the structural and electronic consequences of forming
one or two oxygen-deficient sites. We demonstrate the reactivity of
a neutral POV–alkoxide cluster, [V6O7(OCH3)12]0, with a
reductant, revealing routes for controlling metal-to-oxygen ratios
in self-assembled polynuclear ensembles through post-synthetic modification.
The outlook of this science is bolstered by the fact that, in both
cases, O-atom removal reveals reduced V ions at the surface of the
cluster. Extending our entry into small-molecule activation mediated
by surface defect sites, we report the reactivity of mono- and divacant
clusters with a model substrate, tert-butyl isocyanide,
demonstrating the electronic consequences of small-molecule coordination
to reduced ions in RMO materials.
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.
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.
Lindqvist polyoxovanadate‐alkoxide (POV‐alkoxide) clusters are excellent candidates for applications in energy storage and conversion due to their rich electrochemical profiles. One approach to tune the redox properties of these cluster complexes is through substitutional cationic doping within the hexavanadate core. Here, we report the synthesis of a series of tungsten‐substituted POV‐alkoxide clusters with one and two tungsten atoms. Soft landing of mass‐selected ions was used to purify heterometal POV‐alkoxides that cannot be readily separated using conventional approaches. The soft landed POV‐alkoxides are characterized using infrared reflection‐absorption spectroscopy and electrospray ionization mass spectrometry. The redox properties of the isolated ions are examined using an in situ electrochemical cell which enables traditional in vacuo electrochemical measurements inside of an ion soft landing instrument. Although the overall cluster core retains redox activity after tungsten doping, vanadium‐based redox couples (VV/VIV) are shifted substantially, indicating a pronounced effect of a heteroatom on the electronic structure of the core.
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