A Theoretical Study\non the Influence of Five- and\nSix-Membered N‑Heterocyclic Ring Side Chains of the N‑Donor\nExtractants on Am(III)/Eu(III) Extraction and Separation

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