Macrocyclic control of electron transfer to high valent uranium in heterobimetallic complexes

The redox properties of the uranyl ion, UO₂²⁺, influence the chemistry required for nuclear fuel reprocessing, but little spectroscopic insight is available to support design strategies for influencing the redox properties of uranium complexes. Here, structural studies with X-ray diffraction analysis, electrochemical methods, and Raman spectroscopy have been used to examine one strategy for influencing uranyl redox chemistry, namely co-encapsulation of UO₂²⁺ and secondary metal cations (Cs⁺, Rb⁺, K⁺, Na⁺, Li⁺, and Ca²⁺) in macrocyclic ligands. Two ligands are compared in this work that differ in the denticities of their secondary cation binding sites (pentadentate vs. hexadentate), enabling direct quantification of influences on the redox and vibrational properties of the uranyl moiety. The U^VI/U^V thermodynamic reduction potential is correlated with the effective Lewis acidity of the secondary metal cations; solid-state and solution-phase Raman spectra show that this effect can be attributed to electrostatics that effectively drive diminished electron donation to uranium in adducts of more strongly Lewis acidic cations. The heterogeneous electron transfer (ET) rates for U^VI/U^V redox processes, however, depend on both the strength of cation binding in the macrocycles and the Lewis acidity of the cations, suggesting opportunities for molecular design in development of reagents for nuclear fuel reprocessing/separations.