Vanadium-Containing Keggin-Type Polyoxometalates, [VM 12 O 40 ] 3- and [VVM 11 O 40 ] 4- (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 out V ) and central positions (V in V ) in the Keggin-type polyoxometalates (POMs) [V in V V out V M 11 O 40 ] 4- (M = Mo, W; V in V V out V M 11 ) and in the central position in [V in V M 12 O 40 ] 3- (V in V M 12 ). The structures of the V in V V out V M 11 class have been determined by X-ray crystallography and compared to those of V in V M 12 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 V V out V M 11 and V in V M 12 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 V Mo 12 occurs at the molybdenum framework site to give V in V Mo V Mo 11 . In contrast, EPR spectra of one-electron reduced V in V W 12 at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give V in IV W 12 . In the case of V in V V out V M 11 , the initial reduction occurs at the framework V out V site to give V in V V out IV M 11 . The second electron is delocalized over the Mo framework in two-electron reduced V in V V out IV Mo V Mo 10 , whereas it is accommodated on the central V site in V in IV V out IV W 11 . The distance between V in IV and V out IV in V in IV V out IV W 11 estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in V in V V out V W 11 from crystallographic data. Simulations of the cyclic voltammograms as a function of CF 3 SO 3 H acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [ n -Bu 4 N][PF 6 ]) and allowed acid association constants to be estimated and compared with the literature values available for [XV out V M 11 O 40 ] n - (X = S ( n = 3), P and As ( n = 4); M = Mo, W). The interpretation of the voltammetric data is supported by 51 V NMR measurements on the oxidized V V forms of the POMs.