Vanadium-Containing Keggin-Type Polyoxometalates, [VM12O40]3- and [VVM11O40]4- (M = Mo, W): Structural Characterization and Voltammetric, NMR, and EPR Studies Related to Electrochemical Reduction at Framework and Central Vanadium Sites

Sousuke Yokoyama; Shinya Azuma; Yohei Eguchi; Keisuke Kodani; Takuya Hasegawa; Shuhei Ogo; Hiromi Ota; Si-Xuan Guo; John F Boas; Jie Zhang; Alan M Bond; Tadaharu Ueda

doi:10.1021/acs.inorgchem.3c01921

Vanadium-Containing Keggin-Type Polyoxometalates, [VM₁₂O₄₀]^3- and [VVM₁₁O₄₀]^4- (M = Mo, W): Structural Characterization and Voltammetric, NMR, and EPR Studies Related to Electrochemical Reduction at Framework and Central Vanadium Sites

Inorg Chem. 2024 Jan 8;63(1):117-128. doi: 10.1021/acs.inorgchem.3c01921. Epub 2023 Dec 23.

Authors

Sousuke Yokoyama¹, Shinya Azuma¹, Yohei Eguchi¹, Keisuke Kodani¹, Takuya Hasegawa², Shuhei Ogo^{3

4}, Hiromi Ota⁵, Si-Xuan Guo⁶, John F Boas⁷, Jie Zhang⁶, Alan M Bond⁶, Tadaharu Ueda^{3

4

8}

Affiliations

¹ Department of Applied Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan.
² Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan.
³ Department of Marine Resource Science, Faculty of Agriculture and Marine Science, Kochi University, Nankoku 780-8502, Japan.
⁴ Marine Core Research Institute, Kochi University, Nankoku 783-8502, Japan.
⁵ Division of Instrumental Analysis, Department of Instrumental Analysis and Cryogenics, Advanced Science Research Center, Okayama University, Okayama, 700-8530, Japan.
⁶ School of Chemistry, Monash University, Clayton, Victoria 3800, Australia.
⁷ School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia.
⁸ MEDi Center, Kochi University, Kochi 780-0842, Japan.

PMID: 38141018
DOI: 10.1021/acs.inorgchem.3c01921

Abstract

Vanadium is accommodated in both the framework (V_out^V) and central positions (V_in^V) in the Keggin-type polyoxometalates (POMs) [V_in^VV_out^VM₁₁O₄₀]^4- (M = Mo, W; V_in^VV_out^VM₁₁) and in the central position in [V_in^VM₁₂O₄₀]^3- (V_in^VM₁₂). The structures of the V_in^VV_out^VM₁₁ class have been determined by X-ray crystallography and compared to those of V_in^VM₁₂ reported previously. A major feature of interest with POMs is their capacity for very extensive reduction, particularly when protonation accompanies the electron transfer step. With V_in^VV_out^VM₁₁ and V_in^VM₁₂ POMs, knowledge as to whether reduction occurs at V or M sites and the concomitant dependence on acidity has been obtained. Frozen solution EPR spectra obtained following bulk electrolysis showed that the one-electron reduction of V_in^VMo₁₂ occurs at the molybdenum framework site to give V_in^VMo^VMo₁₁. In contrast, EPR spectra of one-electron reduced V_in^VW₁₂ at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give V_in^IVW₁₂. In the case of V_in^VV_out^VM₁₁, the initial reduction occurs at the framework V_out^V site to give V_in^VV_out^IVM₁₁. The second electron is delocalized over the Mo framework in two-electron reduced V_in^VV_out^IVMo^VMo₁₀, whereas it is accommodated on the central V site in V_in^IVV_out^IVW₁₁. The distance between V_in^IV and V_out^IV in V_in^IVV_out^IVW₁₁ estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in V_in^VV_out^VW₁₁ from crystallographic data. Simulations of the cyclic voltammograms as a function of CF₃SO₃H acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [n-Bu₄N][PF₆]) and allowed acid association constants to be estimated and compared with the literature values available for [XV_out^VM₁₁O₄₀]^n- (X = S (n = 3), P and As (n= 4); M = Mo, W). The interpretation of the voltammetric data is supported by ⁵¹V NMR measurements on the oxidized V^V forms of the POMs.