Do mono- or diphenol substitutions in phenanthroline-based ligands serve in effective separation of Am3+/Eu3+ ions?- Insights from DFT calculations

Abstract

ABSTRACT Partitioning of trivalent actinides (An(III)) from lanthanides (Ln(III)) during Spent Nuclear Fuel (SNF) reprocessing presents a major challenge due to the exhibition of very similar chemical properties. Although an array of phenanthroline-based ligands have surfaced to solve this problem, the recently reported soft – hard-donor-combined N, N’-diethyl-N.N’-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen) ligand has gained much attention. In this work, Et-Tol-DAPhen has been electronically modified with di and mono substitutions of phenol at the 4th and 5th positions. The structural properties and extraction abilities of these modified ligands have been studied using DFT calculations. The results reveal that the ligand (ETDP3) with diphenol substitutions at the 4th position has an excellent extraction capability with high selectivity toward Am(III) over Eu(III) ion. Electronic structure and bonding analyses provide insights into the nature of metal-ligand bonds in the ten coordinated ML(NO3)3 complexes [M = Am and Eu] and convey that in all the complexes the M-O bonds are stronger than the M-N bonds. The quantum theory of atoms in molecules (QTAIM) reveals the weak C-H – O interactions in these ML(NO3)3 complexes. The charge analysis explains the exceptional complexation behavior by determining the donor–acceptor interactions in the Am complexes. ΔΔG values obtained from thermodynamic analysis indicate the preferential selectivity of these ligands toward trivalent Am over Eu ions. Around 22% increased covalency is observed between M(NO3)3 and L in the Am complexes compared to that of the Eu complexes. Overall, this computational study offers an understanding of how beneficial it is to have mono- and diphenol substitutions at the fourth and fifth position of the phenanthroline-based ligands for effective actinide/lanthanide separation.