Theoretical Prediction of the Potential Applications of Phenanthroline Derivatives in Separation of Transplutonium Elements.

Recovery of transplutonium elements from adjacent actinides is extremely complicated in spent fuel reprocessing. Uncovering the electronic structures of transplutonium compounds is essential for designing robust ligands for in-group separation of transplutonium actinides. Here, we demonstrate the in-group transplutonium actinides separation ability of the recent developed phenanthroline ligand Et-Tol-DAPhen (N2,N9-diethyl-N2,N9-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide, La) and its derivatives (5-bromo-(N2,N9-diethyl-N2,N9-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide, Lb), and 5-(4-(λ1-oxidaneyl)phenyl)-(N2,N9-diethyl-N2,N9-di-p-tolyl-1,10-phenanthroline-2,9- dicarboxamide, Lc) through quasi-relativistic density functional theory (DFT). Both electrostatic potential and molecular orbital analyses of the ligands indicate that the electron-donating group substituted ligand Lc is a better electron donor to actinides than La and Lb. The possible extracted complexes AnL(NO3)3 and [AnL2(NO3)]2+ (L = La, Lb, Lc; An = Am, Cm, Bk, Cf) possess similar structures. Bonding nature analysis validates that the covalent interactions of the metal-ligand bonds are enhanced across actinide series from Am to Cf, which stem from the energy degeneracy of the 5f orbitals of actinides and the 2p orbitals of the ligand coordinating atoms. The Lc ligand displays slightly stronger covalent bonding compared to the other two ligands. Simultaneously, thermodynamic analysis confirms the stronger metal-ligand bonding of the Cf3+ complexes and the higher stability of the extraction species with Lc. Consequently, the covalency between the DAPhen derivatives and transplutonium actinides seems to be positively correlated with the extraction ability of these ligands. Nevertheless, these ligands exhibit diverse separation abilities to in-group actinide recovery. Therefore, the enhancement of covalency does not necessarily lead to the improvement of separation ability due to different extraction capabilities. We hope that these results will provide some inspiration for designing novel ligands for in-group transplutonium separation.