Coordination Structures of Uranium(VI) and Plutonium(IV) in Organic Solutions with Amide Derivatives

Carbamide and monoamide derivatives are very promising molecules to achieve U(VI) and Pu(IV) extraction and separation from spent nuclear fuels through solvent extraction. Herein, coordination structures of U(VI) and Pu(IV) complexes with carbamide derivatives were characterized using X-ray crystallography as well as infrared, UV-visible, and EXAFS spectroscopies. Coordination structures are compared to those obtained for monoamide derivatives in order to better understand the role of coordination chemistry in extraction properties. Single crystals were first synthesized with a short alkyl chain carbamide analog. Carbamide complexation in the solid state is found analogous to that in the monoamide. In organic solution, upon solvent extraction from nitric acid aqueous solution, it is shown that both amide derivatives can bind in the inner and outer coordination spheres of uranium(VI) and plutonium(IV). The amount of outer sphere coordination complexes increases with the amount of nitric acid. With uranium(VI), at a nitric acid concentration up to 5 mol·L^-1, amide derivatives operate predominantly in the inner coordination sphere. In contrast, Pu(IV) coordination geometry is much more sensitive toward acid concentration or ligand structure than U(VI). Pu(IV) changes from inner sphere complexation at 0.5 mol·L^-1 HNO₃ to mostly outer sphere complexation at 4 mol·L^-1. The proportion of outer-sphere complexes is strongly influenced by the ligand structure. Higher Pu(IV) extraction is found to be correlated with the amount of Pu(IV) outer sphere species. Secondary interactions in the outer sphere coordination shell appear to be of primary importance for plutonium extraction.