Stabilization of uranyl(V) by dipicolinic acid in aqueous medium

Preparation of a stable U(V) complex in an aqueous medium is a challenging task owing to its disproportionation nature (conversion into more stable U(VI) and U(IV) species) and sensitivity to atmospheric oxygen. The stable uranyl (UO22+)/dipicolinic acid (DPA) complex ([(UO2)-O-(VI)(DPA)(OH)(H2O)](-)) was formed at pH 10.5-12.0, which was confirmed by potentiometric and spectrophotometric titrations, and NMR, ESI-MS and EXAFS spectroscopy. The complex [(UO2)-O-(VI)(DPA)(OH)(H2O)](-) can be electrochemically reduced on the Pt electrode at -0.9 eV (vs. Ag/AgCl) to [(UO2)-O-(V)(DPA)(OH)(H2O)](2-) in aqueous medium under an anaerobic environment. According to cyclic voltammetric analysis, a pair of oxidation and reduction waves at E'(0) = -0.592 V corresponds to the [(UO2)-O-(VI)(DPA)(OH)(H2O)](-)/[(UO2)-O-(V)(DPA)(OH)(H2O)](2-) redox couple and the formation of [(UO2)-O-(V)(DPA)(OH)(H2O)](2-) was confirmed by the electron stoichiometry (n = 0.97 +/- 0.05) of the reduction reaction of [(UO2)-O-(VI)(DPA)(OH)(H2O)](-). The pentavalent uranyl complex [(UO2)-O-(V)(DPA)(OH)(H2O)](2-) was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [(UO2)-O-(V)(DPA)(OH)(H2O)](2-) complex is stable at pH 10.5-12.0 in anaerobic water for a few days. DFT calculation shows the strong complexing ability of DPA stabilizing the unstable oxidation state U(V) in aqueous medium.

Automated summary

The stable uranyl/dipicolinic acid complex was successfully prepared in an aqueous medium, and could be electrochemically reduced to form a stable pentavalent uranyl complex under anaerobic conditions. The complexation ability of DPA in stabilizing the unstable U(V) oxidation state in aqueous medium was demonstrated through various spectroscopic and analytical techniques.