Theoretical Insights into the Substitution Effect of Phenanthroline Derivatives on Am(III)/Eu(III) Separation

Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) poses a huge challenge in the reprocessing of spent nuclear fuel due to their similar chemical properties. N,N'-Diethyl-N,N'-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen) is a potential ligand for the extraction of An(III) from Ln(III), while there are still few reports on the effect of its substituent including electron-withdrawing and electron-donating groups on An(III)/Ln(III) separation. Herein, the interaction of Et-Tol-DAPhen ligands modified by the electron-withdrawing groups (CF₃, Br) and electron-donating groups (OH) with Am(III)/Eu(III) ions was investigated using scalar relativistic density functional theory (DFT). The analyses of bond order, quantum theory of atoms in molecules (QTAIM), and molecular orbital (MO) indicate that the substitution groups have a slight effect on the electronic structures of the [M(L-X)(NO₃)₃] (X = CF₃, Br, OH) complexes. However, the thermodynamic results suggest that a ligand with the electron-donating group (L-OH) improves the extraction ability of metal ions, and the ligand modified by the electron-withdrawing group (L-Br) has the best Am(III)/Eu(III) selectivity. This work could render new insights into understanding the effect of electron-withdrawing and electron-donating groups in tuning the selectivity of Et-Tol-DAPhen derivatives and pave the way for designing new ligands modified by substituted groups with better extraction ability and An(III)/Ln(III) selectivity.