Controlling mixed-valence states of pyridyldiimino-bis(o-phenolato) ligand radical in uranyl(VI) complexes

Combination of a uranyl(VI) ion (U^VIO₂²⁺) with a redox-active ligand results in characteristic electronic structures that cannot be achieved by either component alone. In this study, three U^VIO₂²⁺ complexes that bear symmetric or asymmetric 2,6-diiminopyridine-based ligands were synthesized and found to exhibit a first redox couple between -1.17 V and -1.31 V (vs. Fc^0/+) to afford singly reduced complexes. The unique electronic transitions of the singly reduced U^VIO₂²⁺ complexes observed in the NIR region allowed us to combine spectroelectrochemistry and time-dependent density functional theory (TD-DFT) calculations to determine the redox-active site in these U^VIO₂²⁺ complexes, i.e., to clarify the distribution of the additional unpaired electron. By exploiting the push-pull effect of electron-donating and -withdrawing substituents, the ligand-based π-radical of the singly reduced U^VIO₂²⁺ complexes, which tends to delocalize over the ligand, can be localized to specific sites.