Multi-scale simulations of complexation behavior of lanthanides and actinides with soft-hard donor-based ligands

Americium redox battery is emerging as a continuous power source by leveraging the long half-life of Am-241. There is a need to separate Am from lanthanides and actinides. Therefore, the present study discusses the structures, bonding and thermodynamics of complexes of Eu3+, Am3+, and Cm3+ with soft donor bis(2,4,4-trimethylpentyl) dithiophosphinic acid, mixed soft-hard donors bis(2,4,4-trimethylpentyl) thiophosphinic acid and hard donor ligand di-2, 4, 4-trimethylpentylphosphinic acid and their methyl truncated analogue using density functional theory (DFT), conductor-like screening model for real systems (COSMO-RS) and ab-initio molecular dynamics (AIMD) simulation. The high selectivity of Am3+ ion over Eu3+ and Cm3+ ions is demostrated through higher covalency in the Am-donor atom, the most negative complexation free energy and a smaller partial atomic charge on the Am3+ ion for soft donor (S atom) ligand, cyanex-301. The Wiberg bond indices are also seen to be higher for Am3+ ion over Eu3+ and Cm3+ ions, which indicates stronger complexation of Am3+ ion with cyanex-301. The calculated partial density of states (PDOS) of f orbitals also indicates higher covalency of Am3+ ion towards soft “S” donor. The present ab-initio findings might be useful for the design and development of new ligand molecules for mutual selectivity of lanthanides and actinides.