Reduction of high-valent uranium represents both a strategy for sequestration of this radioactive element from the environment as well as a key step in handling/recycling of nuclear fuels, but electron transfer to uranium via electrochemical methods has received less attention than it deserves. Here, the influences of secondary metal ions and ligand structure on uranyl redox cycling have been mapped for two modular families of heterobimetallic [U(μ-OAr)2Mn+] complexes that differ in the denticity of the binding site for secondary metal ions (pentadentate vs. hexadentate). Both families of complexes display reversible UVI/UV chemistry, enabling determination of thermodynamic reduction potentials and heterogeneous electron transfer rate constants. Optimized, rapid reduction occurs only when tight supramolecular binding of the secondary metals is balanced with their inherent Lewis acidities, leading to characteristic “volcano” plots that result from the interplay of these factors. Quantified trends in these data, based on findings from both solid-state structural and solution-phase studies, offer a mechanistic framework for optimization of uranium redox cycling and design rules for supramolecular structures to promote efficient electrode-driven actinide chemistry.

Optimizing the redox chemistry of the uranyl ion with heterobimetallic chemistry

ACS Spring 2022

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