Enhanced Am/Eu separation ability of disulfonated diamide N-heterocyclic ligands by adjusting N-, O-donor affinity: A theoretical comparative study

Separation of minor actinides (MAs) from lanthanides (Ln) is a vital and challenging stage in advanced nuclear waste treatment. Development of water-soluble back-extraction ligand is regarded as a feasible alternative method for high efficiency An(III)/Ln(III) separation. Therefore, theoretical investigating on the deep mechanism behind the selective back-extraction of An(III) is very important. In this work, Am- and Eu-complexes with three water-soluble disulfonated N-heterocyclic ligands, disulfonated N,N’-diphenyl-2,9-diamide-1,10-phenanthroline (DS-Ph-DaPhen, L1), disulfonated N,N’-diphenyl-2,9-diamide-2,2′-Dipyridine (DS-Ph-DADipy, L2), and disulfonated N,N’-diphenyl-2,9-diamide 5H-cyclopenta[2,1-b:3,4-b’]-Dipyridine (DS-Ph-Da-5H-Cyclo-PentaDipy, L3) were systematically investigated using quasi-relativistic density functional theory (DFT) method. Electrostaic potential (ESP) and restrained electroStatic potential (RESP) atomic charge on donor atoms indicate that N atoms’ affinity toward metal ions in L2 and L3 was increased by replaceing the phenanthroline ring with dipyridine ring and 5H-Cyclo-PentaDipyridine ring, while the affinity of O atoms seem to decrease from L1 to L2 and L3. Through Wiberg bond indices and quantum theory of atoms in molecule (QTAIM) analyses, three ligands’ Am(III) preference may be attributed to the weak but different extend partial covalency in Am-N and Eu-N bonds. Based on the NBO population and PDOS analyses, slightly more overlap of Am-5f and N/O-2p orbitals than those between Eu-4f and N/O-2p orbitals. Based on the theoretical calculation, the covalency difference between Am-N and Eu-N bonds seems to have inverse correlation with the intensity of M−N bonds. However, all the theoretical results suggest that, by increasing N’s affinity and weakening O’ affinity at the same time, ligand’s Am(III)/Eu(III) separation ability can be improved. This work offers some useful information for changing ligands’ An(III)/Ln(III) back-extraction and separation abilities by adjusting the affinity of donor N and O atoms in its central N-heterocyclic skeleton, and may shed light for future designing of more efficient An(III)/Ln(III) back-extraction ligands.