Theoretical insights into Am(III)/Eu(III) separation with electronically regulated hydrophilic phenanthroline-based ligands

• Substituent effects on Am(III)/Eu(III) separation were systematically elucidated through theoretical calculations. • The Nₚₕₑₙ atoms were identified as the decisive coordination sites controlling selectivity. • A clear theoretical mechanism underlying Am(III)/Eu(III) selective separation was established. As the demand for nuclear energy and the increasing radioactive waste, safe disposal of nuclear waste is a global challenge. A key hurdle in treating nuclear waste is separating minor actinides from lanthanides. Ligands with high lanthanide/actinide separation efficiency are key to the treatment of high-level liquid waste via liquid-liquid extraction. Therefore, it is crucial to deeply comprehend the electronic structures including substituent effect of ligands and the corresponding Am(III)/Eu(III) complexes. Herein, we selected phenanthroline-based hydrophilic ligand ( L 1 ) and its electron-donating (-OMe, -phenol) substitutes L 3 , L 4 as well as electron-withdrawing (-Br) substitute L 2 to systematically elucidate the structural properties, bonding characteristics, and thermodynamic properties using scalar relativistic density functional theory. The electrostatic potential (ESP) and molecular orbital analyses reveal that L 3 and L 4 feature more negative ESP values and enhanced metal-binding capabilities. The analyses of bonding nature indicate that the difference of the covalency between ligand and Am(III)/Eu(III) ions mainly originates from M-N phen bond, which is probably attributed to the Am(III)/Eu(III) selectivity. Thermodynamic analyses reflect that L 3 and L 4 ligands enhance the binding energy with metal ions owing to more electron density in the cavity and easier electron transfer to metal ions, while L 2 enhances the Am(III)/Eu(III) selectivity owing to the pronounced covalency difference between Am-N phen and Eu-N phen . These results demonstrate that electronic regulation on the phenanthroline backbone greatly influences the back-extraction performance of Am(III)/Eu(III) ions. This work provides the theoretical foundation for developing electronically regulated hydrophilic phenanthroline-based ligands with highly efficiency Am(III)/Eu(III) selectivity. The effects of substituents on phenanthroline-based hydrophilic ligand for the Am(III)/Eu(III) selectivity were systematically evaluated using scalar relativistic DFT. The results indicated that the electron-donating functional ligands modified possess the stronger binding ability to Am(III)/Eu(III) ions and the ligands modified by the electron-withdrawing group enhances the Am(III)/Eu(III) selectivity.