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Journal Articles Journal of the American Chemical Society Year : 2018

Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica-Supported Catalyst

Abstract

Here we report on the use of a silanol-decorated polyoxotungstate, [SbW9O33(tBuSiOH)3]3\textendash (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)3·thf] (Mes = 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW9O33(tBuSiO)3V(thf)]3\textendash (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d2-VIII high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal·mol\textendash 1 more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated-vanadium atom dispersed on silica surfaces [(\equivSi\textemdash O)3VIII(OH2)], an intermediate that is often proposed in a Mars\textendash van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW9O33(tBuSiO)3VO]3\textendash (3). In compound 2, the d2-electrons are localized in degenerated d(V) orbitals, whereas in the electronically analogous bireduced-[SbW9O33(tBuSiO)3VO]5\textendash, 3·(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bireduced heteropolyanion derivative with completely decoupled d1-V(IV) and d1-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e., thf (σ-donor) vs oxo (π-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O3VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3·(2e).

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Dates and versions

hal-02887488 , version 1 (02-07-2020)

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Teng Zhang, Albert Solé-Daura, Sarah Hostachy, Sébastien Blanchard, Céline Paris, et al.. Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica-Supported Catalyst. Journal of the American Chemical Society, 2018, 140 (44), pp.14903--14914. ⟨10.1021/jacs.8b09048⟩. ⟨hal-02887488⟩
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