Revisiting the Three Vanadium Sandwich-Type Polyoxometalates: Structures, Solution Behavior, and Redox Properties

It is well known that the trivacant anions α-B-[XW₉O₃₃]^9- react with vanadyl ions to give the sandwich-type polyoxometalates [(V^IVO)₃(XW₉O₃₃)₂]^12- with X = As^III or Sb^III. Nevertheless, the oxidized derivatives have been obtained selectively by electrochemical oxidation from the fully reduced derivatives [(V^IVO)₃(XW₉O₃₃)₂]^12- allowing full characterization both in solution using UV-vis and multinuclear (¹⁷O, ⁵¹V, and ¹⁸³W) NMR spectroscopies and in the solid state by single-crystal X-ray diffraction. Structural analysis of the oxidized [(V^VO)₃(XW₉O₃₃)₂]^9- polyanions is consistent with the idealized D₃ symmetry, while solution studies reveal a fair hydrolytic stability in a wide pH range from 0 to 6. Besides, the D₃ polyanions either as reduced or oxidized forms [(VO)₃(AsW₉O₃₃)₂]^9/12- have been identified as the thermodynamic product that results from the conversion of the C₂ polyanion [(H₂O)(VO)₃(AsW₉O₃₃)₂]^9/12- through moderate heating. Conversely, the Sb^III-containing derivative gives exclusively the D₃ polyanion, probably either due to the extended lone pair of the trigonal Sb^III heterogroup that prevents the formation of the C₂ arrangement or the lability of the oxo-metalate bonds that favor chemical exchange. The electrochemical studies of sandwich-type polyoxometalates revealed that each {V═O} group gives rise to a one-electron transfer process. At last, the redox properties appear strongly altered in the 0.3-5 pH range, consistent with proton-coupled electron transfers.