Effects of Supporting Electrolyte and Solvent on the Thermodynamics and Kinetics of Reduction Reactions in a Keggin-Type Polyoxotungstate

We report the thermodynamic and kinetic behavior of reduction reactions of the Keggin-type polyoxometalate (POM) [PW12O40]3– ((PW12)3–) in the presence of TBAPF6, KPF6, and LiPF6 supporting electrolytes. Room-temperature electrochemical analyses of (TBA)3(PW12) (TBA+ = tetrabutylammonium) reveal anodic shifts of the second and third reduction potentials in the presence of KPF6 and LiPF6 in acetonitrile (MeCN) and N,N-dimethylformamide (DMF), arising from ion-pairing interactions between the reduced charge states of the POM and alkali cations. Variable-temperature cyclic voltammetry analyses reveal ion-pairing behavior between alkali cations and the most reduced charge states of the POM, and associated disruption of POM solvation shell, to shift redox entropies (ΔSredox) positively as compared to the cation-uncoupled (PW12)3–/(PW12)4– reduction. Collectively, these results highlight that tuning the identity of supporting electrolyte cation and solvent can influence both the kinetics and thermodynamics of electron transfer in POMs, providing experimental handles for controlling electrochemical properties for targeted applications in electrochemistry.