Redox behavior of cerium in heteropolyoxotungstate complexes

Abstract

In situ XAFS spectroelectrochemistry is used to characterize the oxidation state and coordination environment of cerium in two different heteropolytungstates, the Wells–Dawson [Ce(α-2-P₂W₁₇O₆₁)₂]^17– anion, and the Preyssler [CeP₅W₃₀O₁₁₀]^12– anion. Ce(III), coordinated to the Wells–Dawson framework, is oxidized at an applied potential of +0.37 V vs. Ag/AgCl. This is 1.15 V less than the Ce(IV)/Ce(III) standard reduction potential of +1.52 V vs. Ag/AgCl, demonstrating that Ce(IV) is stabilized in this heteropolyanion. In contrast, Ce(III) when encapsulated in the Preyssler anion is not oxidized at potentials higher than would be required for its oxidation in a non-complexing medium, indicating a stabilization of Ce(III). This is despite the 12– charge on the anion. The different redox behavior of Ce is understood in terms of the different coordination environments afforded by the two heteropolyanions. The relative importance of saturative coordination versus electrostatic stabilization in the redox behavior of these two complexes is discussed.