Theoretically investigating the ability of phenanthroline derivatives to separate transuranic elements and their bonding properties

Abstract

With the development of green chemistry, it is necessary to extract minor actinides from toxic high-level liquid waste. Hence, we need to explore the nature of bonding between ligands and actinide cations in order to design excellent ligands. In the present work, we systematically investigate the potential separation ability of typical tetradentate ligands (1,10-phenanthroline-2,9-diyl)bis((1H-1,2,4-triazol-1-yl)methanone) (L_triazol), (1,10-phenanthroline-2,9-diyl)bis((1H-pyrazol-1-yl)methanone) (L_pyrazol) and (1,10-phenanthroline-2,9-diyl)bis((1H-pyrrol-1-yl)methanone) (L_pyrrol) for trivalent lanthanides and actinides through quasi-relativistic density functional theory (DFT). The coordination number of the possible extracted complexes of 1 : 1 type AnL(NO₃)₃ and 1 : 2 type [AnL₂(NO₃)]²⁺ (An = Np, Pu, Am, Cm; L = L_triazol, L_pyrazol, L_pyrrol) is 10 in a nitric acid environment. Both the electrostatic potential and natural atomic charge analyses of the ligands show that L_pyrazol has stronger ability to attract actinide cations than L_triazol and L_pyrrol, which may be caused by different amide substituents. The QTAIM (quantum theory of atoms in molecules) and IRI (interaction region indicator) analysis validate that there is a weak covalent interaction between the ligand and the actinide metal ion. The analyses including geometry, WBI (Wiberg bond index), NBO (natural bond orbital), and thermodynamics suggest that the interaction between the ligand and actinide cations gradually decreased from Np to Cm. Our theoretical study may contribute to a better understanding of the nature and law of bonding between actinide ions and ligands, and pave the way for designing robust ligands for in-group separation of transuranic complexes in the future.