Evidence for Uranium(VI/V) Redox Supported by 2,2'-Bipyridyl-6,6'-dicarboxylate

The 2,2'-bipyridyl-6,6'-dicarboxylate ligand (bdc) has been shown in prior work to effectively capture the uranyl(VI) ion, UO₂²⁺, from aqueous solutions. However, the redox properties of the uranyl complex of this ligand have not been addressed despite the relevance of uranium-centered reduction to the nuclear fuel cycle and the presence of a bipyridyl core in bdc, a motif long recognized for its ability to support redox chemistry. Here, the bdc complex of UO₂²⁺ (1-UO) has been synthetically prepared and isolated under nonaqueous conditions for the study of its reductive chemical and electrochemical behavior. Spectrochemical titration data collected using decamethylcobaltocene (Cp*₂Co) as the reductant demonstrate that 1e^- reduction of 1-UO is accessible, and companion near-infrared and infrared spectroscopic data, along with theoretical findings from density functional theory, provide evidence that supports the accessibility of the U(V) oxidation state. Data obtained for control ruthenium complexes of bdc and related polypyridyl dicarboxylate ligands provide a counterpoint to these findings; ligand-centered reduction of bdc in these control compounds occurs at potentials more negative than those measured for reduction of 1-UO, further supporting the generation of uranium(V) in 1-UO. Taken together, these results underscore the usefulness of bdc as a ligand for actinyl ions and suggest that it could be useful for further studies of the reductive activation of these unique species.