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

Abstract

A water stable pentavalent uranyl complex, [U ^(V)O ₂(DPA)(OH)(H ₂O)] ²⁻ was synthesized electrochemically and characterized by coulometry, absorption and XAFS spectroscopy. The complex is stable in respect of disproportionation in anaerobic water.

Abstract

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 (UO ₂ ²⁺)/dipicolinic acid (DPA) complex ([U ^(VI)O ₂(DPA)(OH)(H ₂O)] ⁻) 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 [U ^(VI)O ₂(DPA)(OH)(H ₂O)] ⁻ can be electrochemically reduced on the Pt electrode at −0.9 eV ( vs. Ag/AgCl) to [U ^(V)O ₂(DPA)(OH)(H ₂O)] 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 [U ^(VI)O ₂(DPA)(OH)(H ₂O)] ⁻/[U ^(V)O ₂(DPA)(OH)(H ₂O)] ²⁻ redox couple and the formation of [U ^(V)O ₂(DPA)(OH)(H ₂O)] ²⁻ was confirmed by the electron stoichiometry ( n = 0.97 ± 0.05) of the reduction reaction of [U ^(VI)O ₂(DPA)(OH)(H ₂O)] ⁻. The pentavalent uranyl complex [U ^(V)O ₂(DPA)(OH)(H ₂O)] ²⁻ was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [U ^(V)O ₂(DPA)(OH)(H ₂O)] 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.