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-UO2) 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-UO2 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-UO2, further
supporting the generation of uranium(V) in 1-UO2. 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.