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

It is well known that the trivacant anions alpha-B-[XW9O33]9-react withvanadyl ions to give the sandwich-type polyoxometalates [(VIVO)3(XW9O33)2]12-withX=AsIIIor SbIII. Nevertheless, the oxidized derivatives have been obtained selectivelyby electrochemical oxidation from the fully reduced derivatives [(VIVO)3(XW9O33)2]12-allowing full characterization both in solution using UV-vis and multinuclear (17O,51V,and183W) NMR spectroscopies and in the solid state by single-crystal X-ray diffraction.Structural analysis of the oxidized [(VVO)3(XW9O33)2]9-polyanions is consistent withthe idealizedD3hsymmetry, while solution studies reveal a fair hydrolytic stability in awide pH range from 0 to 6. Besides, theD3hpolyanions either as reduced or oxidizedforms [(VO)3(AsW9O33)2]9/12-have been identified as the thermodynamic productthat results from the conversion of theC2vpolyanion [(H2O)(VO)3(AsW9O33)2]9/12-through moderate heating. Conversely, theSbIII-containing derivative gives exclusively theD3hpolyanion, probably either due to the extended lone pair of the trigonal SbIIIheterogroup that prevents the formation of theC2varrangement or the lability of the oxo-metalate bonds that favor chemicalexchange. 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-coupledelectron transfers.

Automated summary

This study identifies a new type of polyoxometalate and characterizes its properties through chemical analysis and physical experiments. The results show that this polymer exhibits different properties in different pH ranges, and proton-coupled electron transfers are involved in the redox process.