Theoretical design of polyazole based ligands for the separation of Am(III)/Eu(III)

The extraction of radioactive minor actinides (An(III)) from lanthanides (Ln(III)) is an extremely important step in nuclear waste reprocessing. Designing ligands with high-performance actinide-selectivity remains an essential task. Recent works have reported that some polyazole based ligands exhibit good An(III)/Ln(III) separation performance. Herein, we first evaluated the effects of different polyazole side chains on the Am(III)/Eu(III) selectivity by exploring three pyridine-derived polyazole ligands L1, L2 and L3 with 1,2,4-triazole, 1,2,3-triazole, and pyrazole side chains, respectively, using scalar relativistic theoretical methods. The coordination structures, bonding properties and thermodynamic behaviors of AmL(NO3)3 and EuL(NO3)3 complexes were investigated, which clarifies that the side chains do affect the electronic structure of ligand and its selectivity for Am(III)/Eu(III) ions. Moreover, L1 with 1,2,4-triazole side chains exhibited the highest selectivity for Am(III) over Eu(III) while the lowest complexation ability for metal ions among the three pyridine-derived polyazole ligands. Subsequently, we designed a new ligand L4 containing 1,2,4-triazole side chains and a preorganized phenanthroline backbone. Theoretically, such a new ligand was verified to show stronger complexation ability and higher selectivity for Am(III)/Eu(III) ions than L1. This work clarifies the complexation nature of polyazole based ligands with Am(III)/Eu(III) ions and provides design strategies for highly efficient polyazole based ligands for An(III)/Ln(III) separation.