Effective
separation and recovery of chemically similar transplutonium
elements from adjacent actinides is extremely challenging in spent
fuel reprocessing. Deep comprehension of the complexation of transplutonium
elements and ligands is significant for the design and development
of ligands for the in-group separation of transplutonium elements.
Because of experimental difficulties of transplutonium elements, theoretical
calculation has become an effective means of exploring transplutonium
complexes. In this work, we systematically investigated the coordination
mechanism between transplutonium elements (An = Am, Cm, Bk, Cf) and
two crown ether macrocyclic ligands [N,N′- bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6
(H2bp18c6) and N,N′-bis[(6-methylphosphinic-2-pyridyl)methyl]-1,10-diaza-18-crown-6
(H2bpp18c6)] through quasi-relativistic density functional
theory. The extraction complexes of [Anbp18c6]+ and [Anbpp18c6]+ possess similar geometrical structures with actinide atoms
located in the cavity of the ligands. Bonding nature analysis indicates
that the coordination ability of the coordinating atoms in pendent
arms is stronger than that in the crown ether macrocycle because of
the limitation of the macrocycle. Most of the coordination atoms of
the H2bp18c6 ligand have a stronger ability to coordinate
with metal ions than those of the H2bpp18c6 ligand. In
addition, the bonding strength between the metal ions and ligands
gradually weakens from Am to Cf, which is mainly attributed to the
size selectivity of the ligands. Thermodynamic analysis shows that
the H2bp18c6 ligand has a stronger extraction capacity
than the H2bpp18c6 ligand, while the H2bpp18c6
ligand is superior in terms of the in-group separation ability. The
extraction capacity of the two ligands for metal ions gradually decreases
across the actinide series, indicating that these crown ether macrocycle
ligands have size selectivity for these actinide cations as a result
of steric constraint of the crown ether ring. We hope that these results
offer theoretical clues for the development of macrocycle ligands
for in-group transplutonium separation.