Tuning the structure of disulfonated phenanthroline based ligands for effective separation of Am(III)/Eu(III) ions : A DFT investigation

Abstract

The disulfonated N,N′-diphenyl-2,9-diamide-1,10-phenanthroline (DS-Ph-DAPhen) ligand, is recently added to the list of efficient water-soluble heterocyclic ligands for the separation of actinides from lanthanides in nuclear waste management. This ligand contains phenyl sulfonate group which improves the solubility in aqueous medium. To understand the role of sulfonate group in selective binding of Am3+ over Eu3+ ions, a systematic investigation is executed on the designed disulfonated N,N′-diethyl-N,N′-diphenyl-2,9-diamide-1,10-phenanthroline (DS-Et-Ph-DAPhen) ligands. Here, we provide insights into the nature of metal–ligand bond in ML(NO3)3 complexes [M = Am and Eu] to trace the origin of selectivity in the separation process using scalar relativistic ZORA/DFT calculations. The quantum theory of atoms in molecules (QTAIM) and energy decomposition analysis (EDA) are carried out to understand the stabilizing interactions and binding energies of these ML(NO3)3 complexes respectively. Results reveal that DS-Et-Ph-DAPhen ligands show preferential binding to Am3+ where MO bond length is shorter and stronger than that of MN bonds. Computed separation factors indicate that meta-substitution is most favourable for extraction. Overall this study brings out the importance of sulfonate group in the stripping of Am3+ from Eu3+ and will shed light on the structural role played by sulfonate moiety for the nuclear waste treatment.