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

It is well known that the trivacant anions α-B-[XW 9 O 33 ] 9- react with vanadyl ions to give the sandwich-type polyoxometalates [(V IV O) 3 (XW 9 O 33 ) 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 IV O) 3 (XW 9 O 33 ) 2 ] 12- allowing full characterization both in solution using UV-vis and multinuclear ( 17 O, 51 V, and 183 W) NMR spectroscopies and in the solid state by single-crystal X-ray diffraction. Structural analysis of the oxidized [(V V O) 3 (XW 9 O 33 ) 2 ] 9- polyanions is consistent with the idealized D 3 h symmetry, while solution studies reveal a fair hydrolytic stability in a wide pH range from 0 to 6. Besides, the D 3 h polyanions either as reduced or oxidized forms [(VO) 3 (AsW 9 O 33 ) 2 ] 9/12- have been identified as the thermodynamic product that results from the conversion of the C 2 v polyanion [(H 2 O)(VO) 3 (AsW 9 O 33 ) 2 ] 9/12- through moderate heating. Conversely, the Sb III -containing derivative gives exclusively the D 3 h polyanion, probably either due to the extended lone pair of the trigonal Sb III heterogroup that prevents the formation of the C 2 v 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.