Electrode kinetics for the reduction of central hetero VinV and framework addenda VoutV and WVI atoms in the [VinVoutW11O40]4− polyoxometalate: Comparisons with [SVoutW11O40]3− and [VinW12O40]3

•Reduction of [SVoutW11O40]3−, [VinVoutW11O40]4− and [VinW12O40]3− has been investigated by FT-AC voltammetry.•Thermodynamics and electrode kinetics have been estimated from simulation of FT-AC voltammograms.•Sites and reduction levels of polyoxometalates drastically affect the electrode kinetics. The electrode kinetics associated with the polyoxometalate (POM) [VinVoutW11O40]4-/5-/6−, [VinW12O40]3-/4-/5− and [SVoutW11O40]3-/4-/5− (Vin – central hetero vanadium atom, Vout – framework addenda vanadium atom) reduction processes in CH3CN (0.10 M [Bu4N][PF6]) have been determined by Fourier Transformed large amplitude alternating current voltammetry at a glassy carbon electrode. [VinVoutW11O40]4− provides an interesting situation where both the VinV and VoutV vanadium atoms can be reduced to the VIV state. The heterogeneous charge transfer rate constant kVout0′ value for the framework [VinVVoutVW11O40]4−/[VinVVoutIVW11O40]5− process of 0.15 cm s−1 (reversible formal potential E0ʹ = −223 mV vs Fc0/+) is almost ten times that of 0.020 for the kVin0′ (E0ʹ = −1027 mV vs Fc0/+) one associated with the central [VinVVoutIVW11O40]5−/[VinIVVoutIVW11O40]6− reaction. The significant decrease in the electrode kinetics for the process involving reduction of the central vanadium could in principle be attributed to differences in the Vin and Vout coordination geometry or a longer distance required for electron transfer between the electrode surface and Vout. However, neither of these factors is considered to be dominant as kVin0′ associated with the [VinVW12O40]3−/[VinIVW11O40]4- reaction is 0.19 cm s−1 (E0ʹ is −114 mV vs Fc0/+). With this POM, the more negative second process (E0ʹ = −918 mV vs Fc0/+) is associated with a WVI/WV framework reduction with a k0ʹ value of 0.080 cm s−1. With the second control POM, [SVoutW11O40]3−, the initial very fast framework VoutV/VoutIV reduction step (E0ʹ = 145 mV vs Fc0/+) has a k0ʹ value of 0.19 cm s−1 and is followed at considerably more negative potentials (E0ʹ = −1343 mV vs Fc0/+) by the slightly slower framework WVI/WV process with k0ʹ = 0.13 cm s−1. Thus, the k0ʹ values are most strongly influenced by the negative charge (n value) associated with the POMn-(/n+1)- reaction, being close to reversible when n = 3 as in the [VinW12O40]3−4− and [SVoutW12O40]3−/4− reactions, but much slower when n = 5 as in the case of the [VinVoutW11O40]5-/6- reaction. Accordingly, the initial reduction step is always faster than the second one that occur