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

Vanadium is accommodated in both the framework (V) and central positions (V) in the Keggin-type polyoxometalates (POMs) [VVMO] (M = Mo, W; VVM) and in the central position in [VMO] (VM). The structures of the VVM class have been determined by X-ray crystallography and compared to those of VM 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 VVM and VM 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 VMo occurs at the molybdenum framework site to give VMoMo. In contrast, EPR spectra of one-electron reduced VW at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give VW. In the case of VVM, the initial reduction occurs at the framework V site to give VVM. The second electron is delocalized over the Mo framework in two-electron reduced VVMoMo, whereas it is accommodated on the central V site in VVW. The distance between V and V in VVW estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in VVW from crystallographic data. Simulations of the cyclic voltammograms as a function of CFSOH acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [-BuN][PF]) and allowed acid association constants to be estimated and compared with the literature values available for [XVMO] (X = S ( = 3), P and As (= 4); M = Mo, W). The interpretation of the voltammetric data is supported by V NMR measurements on the oxidized V forms of the POMs.