Theoretical insights into selective separation of trivalent actinide and lanthanide by ester and amide ligands based on phenanthroline skeleton

Abstract

Phenanthroline based ligands have shown potential performance for partitioning trivalent actinides from lanthanides. In this work, we have explored four ester and amide ligands based on the phenanthroline skeleton and elucidated the separation mechanism between Am(III) and Eu(III) ions. The molecular geometries and extraction reactions of the metal–ligand complexes were modeled by using scalar-relativistic density functional theory. The results show that the amide based ligands have stronger coordination ability with the metal ions than the corresponding ester based ligands. According to the thermodynamic results, ligands N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (L²) and N,N′-(1,10-phenanthroline-2,9-diyl)bis(N-ethyl-P-methyl-N-(p-tolyl)phosphinic amide) (L⁴) appear to have the strongest complexing ability, which is supported by the result of electrostatic potential (ESP) and the M–O_L bond orders. Moreover, ligand L² has excellent selectivity for Am(III)/Eu(III) among the four ligands. Additionally, the bonding properties between the metal ions and the ligands reveal that the Am(III)/Eu(III) selectivity stems from the Am–N bonds with more covalent character, which is supported by the analysis of the hardness of the ligands and the bond orders. This work provides useful information for understanding the Am(III)/Eu(III) selectivity of phenanthroline derived ligands bearing ester and amide groups.