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According to the green development requirements of carbon neutrality and carbon peaking, the effective separation of lanthanides and actinides is one of the key factors for nuclear energy to become a sustainable energy source. In recent years, o-phenanthroline-based ligands have been proven to be effective in the separation of lanthanides and actinides. In this work, based on 5,9b-dihydro-4aH-cyclopenta[1,2-b:5,4-b']dipyridines, we designed six N-heterocyclic ring ligands and theoretically studied their extraction capacity and separation selectivity for the Am(III) and Eu(III) ions. Various theoretical methods were used to analyze the properties of the ligands and study the bonding nature of the ligands with the metal ions. Thermodynamic parameters were calculated to measure the extraction ability of the ligands to the metal ions and to explore the separation capacity of the ligand for the Am(III) and Eu(III) ions. The calculated results show that the five- and six-membered N-heterocyclic ring side chains of the ligands and the distribution of the N atoms on the side chain rings have obvious effects on the bonding of the ligands to metal ions and on the extraction and separation properties of the ligands for the metal ions. It was found that the extractants with six-membered ring side chains possess an extraction ability slightly better than that of the ligands with five-membered ring side chains and that the ligands with a pair of adjacent N atoms on the side chains have a stronger separation selectivity for the Am(III)/Eu(III) ions. The theoretical research in this work will help to understand the details of binding and extraction properties of similar ligands and provide insights for the future design of five- and six-membered heterocyclic ligands.

Designing ligands that can effectively separate actinide An(III)/lanthanide Ln(III) in the solvent extraction process remains one of the key issues in the treatment of accumulated spent nuclear fuel. Nitrogen donor ligands are considered as promising extractants for the separation of An(III) and Ln(III) due to their environmental friendliness. Four new macrocyclic N-donor hexadentate extractants were designed and their coordination with Am(III) and Eu(III), as well as their extraction selectivity and separation performance for Am(III) and Eu(III), were investigated by scalar relativistic density functional theory. A variety of theoretical methods have been used to evaluate the properties of the four ligands and the coordination structures, bonding properties, and thermodynamic properties of the complexes formed by the four ligands with Am(III) and Eu(III). The results of various wavefunction analysis methods including NBO analysis, quantum theory of atoms in molecules (QTAIM) analysis, and so on show that Am(III) has a stronger coordination ability with the ligands than Eu(III) due to the Am 5f orbitals more involved in bonding with the ligands than the Eu 4f orbitals, and the bonding environment of the N-donor in the ligand has a significant effect on its coordination ability of the metal ions. Thermodynamic analysis of the solvent extraction process shows that all of the four N-containing macrocyclic ligands have good extraction selectivity and separation performance for Am(III) and Eu(III). This study provides theoretical support for designing potential nitrogen-containing macrocyclic extractants with excellent separation performance.