Competitive Coordination of Uranium(VI) and Thorium(IV) by Tetradentate Phenanthroline Ligands: Structural and Spectroscopic Investigation of U(VI)-Th(IV) Mixed Ion Pair Formation

Abstract

Tetradentate phenanthroline-based ligands have demonstrated ability to efficiently extract uranium(VI) from nitric acid solutions. While their coordination with U(VI) has been extensively studied, their behavior toward tetravalent actinides under conditions relevant to spent nuclear fuel reprocessing remains poorly understood. Here, we investigated the extraction and coordination chemistry of Th(IV) in the presence of excess U(VI).Using a combination of solvent extraction, UV-Vis, Raman, and luminescence spectroscopy, we demonstrated that when the organic phase is pre-loaded with the uranyl ion pair [UO₂L(NO₃)]⁺[UO₂(NO₃)₃]^-, Th(IV) undergoes a selective anion-exchange reaction with the trinitratouranyl anion, forming the mixed ion pair [UO₂L(NO₃ )]⁺[Th(NO₃)₅]^-. Single-crystal X-ray diffraction further confirms the formation of heterometallic U-Th complexes, including ([UO₂L(NO₃)]⁺)₂[Th(NO₃)₆]^2-, providing direct structural evidence for mixed-metal ion pairing. These results revealed a previously unrecognized mechanism of Th(IV) uptake driven by anion exchange rather than direct ligand coordination.