Implications of Charge and Heteroatom Dopants on the Thermodynamics and Kinetics of Redox Reactions in Keggin-Type Polyoxometalates.

Abstract

The utilization of polyoxometalate-based materials is largely dictated by their redox properties. Detailed understanding of the thermodynamic and kinetic efficiency of charge transfer is therefore essential to the development of polyoxometalate-based systems for target applications. Toward this end, we report electrochemical studies of a series of heteroatom-doped Keggin-type polyoxotungstate clusters [PW₁₂O₄₀]^3- (PW ₁₂ ), [VW₁₂O₄₀]^3- (V _in W ₁₂ ), [P(VW₁₁)O₄₀]^4- (PV _out W ₁₁ ), and [V(VW₁₁)O₄₀]^4- (V _in V _out W ₁₁ ) to elucidate the role of the identity and spatial location of heteroatoms and overall cluster charge on the rate constants of electron transfer and redox reaction entropies. Electrochemical analyses of the polyoxotungstates reveal that the kinetics of electron transfer for W-based redox processes change as a function of the redox activity of the heteroatom, whereas the spatial location of the heteroatom dopant does not significantly impact the electrokinetics. Variable temperature cyclic voltammetry measurements in organic solutions containing noncoordinating electrolyte ions establish that redox reaction entropies are primarily dictated by the overall charge of the clusters. Specifically, the redox entropy exhibits a good linear relationship with the dielectric continuum function Z _ox ² - Z _red ² (Z _ox = charge of oxidized species, Z _red = charge of reduced species). Finally, our experimental data do not show a prominent correlation between the kinetics of electron transfer and redox entropy, implying that the charge-transfer kinetics are not solely governed by structural reorganization. Taken together, these results highlight how structural and electronic parameters can influence the kinetics and thermodynamics of charge transfer in polyoxotungstates and provide insights into the design of polyoxometalate compounds with target redox properties.