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
(N
2,N
9-diethyl-N
2,N
9-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide, La) and its derivatives (5-bromo-(N
2,N
9-diethyl-N
2,N
9-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide,
Lb), and 5-(4-(λ1-oxidaneyl)phenyl)-(N
2,N
9-diethyl-N
2,N
9-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.