Understanding solvent polarity effects on the separation of uranyl porphyrin-derivative complexes

Solvation is a crucial task in developing efficient and selective extractants for actinide elements, but an understanding and perspective of solvent effects on the extraction of uranyl are still lacking. Herein, we present investigations into solvent effects on the geometry, stability and bonding properties of five uranyl porphyrin derivative complexes (UO₂(L)) in four solvents via relativistic quantum chemical calculations, and reveal some trends in the influence of solvent polarity on uranyl compounds. All five [L]^2- ligands equatorially coordinate [UO₂]²⁺ in a hexa-dentate (κ⁶) fashion. Thus, the uranium center is bound to six N atoms by U-N bonds, and the properties of the U-N bond are affected by the ligands rather than by the solvent. The relative stability of these UO₂(L) complexes is obviously affected by the solvent polarity; lower polar solvents stabilize more UO₂(L) complexes from higher polar solvents, resulting in more extensive interaction of U-N. By computing the reaction energy, we track the conversion of pure [UO₂]²⁺ to UO₂(L) complexes in different solvents; the uranyl dipentafluorobenziamethyrin species exhibit the greatest selectivity to higher polarity solvents, with significant orbital interactions of U-N accounting for this stability and selectivity. This study provides a general procedure for theoretical screening of the binding ability and solvent selectivity of macrocyclic ligands towards uranyl, and searching for suitable ligands and solvents that will later be applied in nuclear science disciplines.