Theoretical Insights Into The Selective Extraction Of Americium(?) Over Europium(?) With Amide-based Ligands

Separation of trivalent actinides An(?)from lanthanides Ln(?)is a worldwide challenge owing to their very similar chemical behaviors.It is highly desirable to understand the nature of selectivity for the An(?)/Ln(?)separation with various ligands through theoretical calculations because of their radiotoxicity and experimental difficulties.In this work,we have investigated three dithioamide-based ligands and four ester and amide ligands based on phenanthroline skeleton and their extraction behaviors with Am(?)and Eu(?)ions using the scalar-relativistic density functional theory.The main results are summarized as follows:(1)The results show that the dithioamide-based ligands have stronger electron donating ability.The M-S bonds have distinct?character,and Am6d orbitals and Eu 5d orbitals have predominant contributions to the?bonding based on the bonding nature.And there is a more covalent contribution to Am-L bonding than to Eu-L bonding in the same type of complexes.Moreover,the covalency of the M-S bond is larger than that of the M-N bond.Thermodynamic results reveal that L~1 has a stronger complexing ability with metal ions owing to its rigid structure and that L~2shows a higher selectivity for the Am(?)/Eu(?)separation.In addition,these dithioamide-based ligands possess Am(?)/Eu(?)selectivity higher than those of the corresponding diamide-based ones,although the former have weaker complexing ability with metal ions,probably due to the greater covalency of the M-S bonds than the M-O bonds.(2)Phenanthroline based ligands have shown highter acid resistance and potential performance for partitioning trivalent actinides from lanthanides than pyridine ligands,and their related researches have attracted wide attention from scientists.In addition,ligands containing phosphate and amide groups also have good Am(?)/Eu(?)extraction and separation effects.We have explored four ester and amide ligands based on the phenanthroline skeleton and elucidated the separation mechanism between Am(?)and Eu(?)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 Am–N_L bonds in all complexes are shorter than the corresponding Eu–N_L bonds.The NBO analyses and thermodynamic results show that there is a stronger complexing interaction between the ligands and the Am(?)ion compared to the Eu(?)ion.Our results clearly verify that the relatively harder oxygen atoms offer these ligands higher coordination affinities toward both of the An(?)and Ln(?)ions compared to the relatively softer nitrogen atoms.Moreover,the amide based ligands have a stronger complexing ability compared to the corresponding ester based ligands,and ligand L~2 shows the best selectivity for Am(?)/Eu(?)among the four ligands.