Theoretical Insights into the Separation of Nd(III)/Dy(III) by Amine-Terminated Phenanthroline Diimide Ligands

Rare earth elements (REEs) such as Nd(III) and Dy(III) are critical components in advanced technologies, but their similar properties pose significant challenges for efficient separation. Amine-terminated phenanthroline diimide ligands (Phen-2DIC2NH2, Phen-2DIC4NH2, and Phen-2DIC6NH2) have emerged as promising candidates for f-element separation due to their tunable structures and strong coordination with lanthanides. Herein, we systematically investigate the separation mechanism of Nd(III)/Dy(III) using these ligands via a combination of density functional theory (DFT) calculations, molecular dynamics (MD) simulations, and advanced bonding analyses. The results reveal that Phen-2DIC4NH2 exhibits the optimal Nd(III)/Dy(III) selectivity, attributed to its balanced electron-donating ability, moderate structural flexibility, and enhanced coordination with metal ions. Bonding analyses (QTAIM, EDA-NOCV, AdNDP) demonstrate that metal-ligand bonds possess both ionic and covalent characters, with Dy(III) forming stronger interactions than Nd(III) due to its smaller ionic radius and higher charge density. Thermodynamic calculations confirm spontaneous complexation reactions, with Phen-2DIC4NH2 showing the most favorable Gibbs free energy difference (ΔΔ<i>G</i>) for Nd(III)/Dy(III) separation. MD simulations further validate the dynamic stability of the complexes and reveal ligand exchange mechanisms governed by nitrate ion coordination modes. This work provides profound theoretical insights into the structure-selectivity relationship of phenanthroline-based ligands and offers guidelines for the rational design of efficient extractants for REE separation.