Theoretical Insights into Preorganized Pyridylpyrazole-Based Ligands toward the Separation of Am(III)/Eu(III)

Pyridylpyrazole ligands have shown excellent competence for partitioning actinides from lanthanides. As far as we know, the preorganization structure of the ligand has a great impact on the extraction separation ability. However, the mechanism that works well for some ligands but fails for others needs to be clearly elucidated. In this work, we designed three various pyridylpyrazole ligands, BPP, BPBP, and BPPhen, and further preorganized one or both side pyrazole rings of these ligands. The properties of these ligands and the coordination structures, bonding nature and thermodynamic behaviors of the related Am(III) and Eu(III) complexes have been systematically studied in a theoretical fashion. All analyses of geometries, charge transfer, QTAIM (quantum theory of atoms in molecules) and NBO (natural bond orbital) suggest that the Am-N bonds possess more covalence compared to that of Eu-N bonds. According to the thermodynamic results, increasing the rigidity of the bridging skeleton rather than the side chain can enhance the extraction ability and Am(III)/Eu(III) selectivity of the ligand. This work may identify the reasonability of a useful approach on achieving highly efficient Am(III)/Eu(III) separation through tuning the preorganization level of the ligand and further provide meaningful theoretical basis on the input of preorganization toward ligand design and screening.