Activation and Functionalization of the Uranyl Ion by Electrochemical Reduction

Abstract

Interconversion of the oxidation states of uranium enables separations and reactivity schemes involving this element and contributes to technologies for recycling of spent nuclear fuels. The redox behaviors of uranium species impact these processes, but use of electrochemical methods to drive reactions of molecular uranium complexes and to obtain molecular insights into the outcomes of electrode-driven reactions has received far less attention than it deserves. Here, we show that electro-reduction of the uranyl ion (UO₂²⁺) can be used to promote stepwise functionalization of the typically unreactive oxo groups with exogenous triphenylborane (BPh₃) serving as a moderate electrophile, avoiding the conventional requirement for a chemical reductant. Parallel electroanalytical, spectrochemical, and chemical reactivity studies, supported by spectroscopic findings and structural data from X-ray diffraction analysis on key reduced and borylated products, demonstrate that our electrochemical approach largely avoids undesired cross reactions and disproportionation pathways; these usually impact the multicomponent systems needed for uranyl functionalization chemistry. Joint computational studies have been used to map the changes associated with U–O activation and to quantify the free energy differences related to key reactions. Taken together, the results suggest that electrochemical methods can be used for selective interconversion of molecular actinide species, reminiscent of methods commonly employed in transition metal redox catalysis.