Investigation of enhanced Am selectivity for Eu in solvent extraction using a BTPhen ligand substituted with halogen

The effect of bromine (Br) on the separation of Am/Eu using BTPhen was investigated using DFT calculations. The simulated results agreed well with the reported experimental result of Br substitution. It is concluded that the contribution of the d orbital becomes less important by increasing the number of Br atoms, that is, the relative contribution of the f orbital becomes important. The Am f orbital contributed to both bonding and antibonding interactions with the ligand, whereas the Eu f orbital contributed to the antibonding interaction with the ligand. To study the halogen effect systematically, we introduced a series of new halogen atoms (chlorine, fluorine, iodine) into BTPhen. When the electronegativity of the halogen atom increases, the ΔG for complex formation shifts to the positive direction, and the ΔΔG which shows the difference in ΔG between Am and Eu becomes a large negative value, suggesting that the Am selectivity is larger. This is due to the increased Δρ _BCP (= ρ _BCP(Am) - ρ _BCP(Eu)) between the metal and the ligating nitrogen atom with an increase of electronegativity of the halogen atom.