Separation
of trivalent actinides An(III) from lanthanides Ln(III) is a worldwide
challenge owing to their very similar chemical behaviors. It is highly
desirable to understand the nature of selectivity for the An(III)/Ln(III)
separation with various ligands through theoretical calculations because
of their radiotoxicity and experimental difficulties. In this work,
we have investigated three dithioamide-based ligands and their extraction
behaviors with Am(III) and Eu(III) ions using the scalar-relativistic
density functional theory. The results show that the dithioamide-based
ligands have stronger electron donating ability than do the corresponding
diamide-based ones. All analyses including geometry, Mulliken population,
QTAIM (quantum theory of atoms in molecules), and NBO (natural bond
orbital) suggest that the Am–S/N bonds possess more covalency
compared to the Eu–S/N bonds, and the M–S bonds have
more covalent character than the M–N bonds. Thermodynamic results
reveal that N
2,N
9-diethyl-N
2,N
9-di-p-tolyl-1,10-phenanthroline-2,9-bis(carbothioamide)
(L
1
) has a stronger complexing
ability with metal ions owing to its rigid structure and that N
6,N
6′-diethyl-N
6,N
6′-di-p-tolyl-[2,2′-bipyridine]-6,6′-bis(carbothioamide)
(L
2
) shows a higher selectivity
for the Am(III)/Eu(III) separation. In addition, these dithioamide-based
ligands possess Am(III)/Eu(III) selectivity higher than those of the
corresponding diamide-based ones, although the former have weaker
complexing ability with metal ions, probably due to the greater covalency
of the M–S bonds. This theoretical evaluation provides valuable
insights into the nature of the selectivity for the Am(III)/Eu(III)
separation and information on designing of efficient An(III)/Ln(III)
separation with dithioamide-based ligands.
Pyridylpyrazole ligands have shown
excellent competence for partitioning
actinides from lanthanides. As far as we know, the preorganization
structure of the ligand has a great impact on the extraction separation
ability. However, the mechanism that works well for some ligands but
fails for others needs to be clearly elucidated. In this work, we
designed three various pyridylpyrazole ligands, BPP, BPBP, and BPPhen,
and further preorganized one or both side pyrazole rings of these
ligands. The properties of these ligands and the coordination structures,
bonding nature and thermodynamic behaviors of the related Am(III)
and Eu(III) complexes have been systematically studied in a theoretical
fashion. All analyses of geometries, charge transfer, QTAIM (quantum
theory of atoms in molecules) and NBO (natural bond orbital) suggest
that the Am–N bonds possess more covalence compared to that
of Eu–N bonds. According to the thermodynamic results, increasing
the rigidity of the bridging skeleton rather than the side chain can
enhance the extraction ability and Am(III)/Eu(III) selectivity of
the ligand. This work may identify the reasonability of a useful approach
on achieving highly efficient Am(III)/Eu(III) separation through tuning
the preorganization level of the ligand and further provide meaningful
theoretical basis on the input of preorganization toward ligand design
and screening.
Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) is one of the most important steps in spent nuclear fuel reprocessing. However, it is very difficult and challenging to separate them due to their similar chemical properties. Recently the pyridylpyrazole ligand (PypzH) has been identified to show good separation ability toward Am(III) over Eu(III). In this work, to explore the Am(III)/Eu(III) separation mechanism of PypzH at the molecular level, the geometrical structures, bonding nature, and thermodynamic behaviors of the Am(III) and Eu(III) complexes with PypzH ligands modified by alkyl chains (Cn-PypzH, n = 2, 4, 8) have been systematically investigated using scalar relativistic density functional theory (DFT). According to the NBO (natural bonding orbital) and QTAIM (quantum theory of atoms in molecules) analyses, the M-N bonds exhibit a certain degree of covalent character, and more covalency appears in Am-N bonds compared to Eu-N bonds. Thermodynamic analyses suggest that the 1:1 extraction reaction, [M(NO)(HO)] + PypzH + 2NO → M(PypzH)(NO)(HO) + 5HO, is the most suitable for Am(III)/Eu(III) separation. Furthermore, the extraction ability and the Am(III)/Eu(III) selectivity of the ligand PypzH is indeed enhanced by adding alkyl-substituted chains in agreement with experimental observations. Besides this, the nitrogen atom of pyrazole ring plays a more significant role in the extraction reactions related to Am(III)/Eu(III) separation compared to that of pyridine ring. This work could identify the mechanism of the Am(III)/Eu(III) selectivity of the ligand PypzH and provide valuable theoretical information for achieving an efficient Am(III)/Eu(III) separation process for spent nuclear fuel reprocessing.
The nature of the Am(iii)/Eu(iii) separation for two types of ester and amide based ligands with a phenanthroline skeleton was systematically explored from a theoretical perspective.
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