Theoretical Probing of Size-Selective Crown Ether Macrocycle Ligands for Transplutonium Element Separation

Abstract

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.

Effects of Electron‐Withdrawing and ‐Donating Substituents in N‐Donor Scorpionate Ligands and the Metal 5 f/4 f Orbitals on Am(III)/Eu(III) Complexation and Separation

“…The delocalization index (DI) of the M−N bond can be used as a measure of the electron sharing between the N and M basins. A higher value of DI indicates more electron sharing between the two basins and generally a stronger bond between the two bonded atoms [24] . According to Table 1, the donor N atom of the ligands shares more electrons with Am than with Eu, so forms a stronger bond with Am than with Eu.…”

“…A higher value of DI indicates more electron sharing between the two basins and generally a stronger bond between the two bonded atoms. [24] According to Table 1, the donor N atom of the ligands shares more electrons with Am than with Eu, so forms a stronger bond with Am than with Eu. For the same metal, the N donor atom of the ligands L b and L c with the electron-withdrawing group shares less electrons with the metal, while the N atom of the ligands L d and L e with the electron-donating group shares more electrons with the metal, consistent with the above analysis.…”

Understanding the Coordination Chemistry of AmIII/CmIII in the DOTA Cavity: Insights from Energetics and Electronic Structure Theory

“…Thus, theoretical computational simulation is an alternative. 14,15 Furthermore, a full understanding of the fundamental physicochemical properties of actinide complexes at the atomic scale also plays a key role in describing the behavior of actinides in solution.…”

“…This method has fewer limiting factors compared to alternative approaches and is therefore widely used. − However, due to the high radioactivity and chemical toxicity of actinides, experimental study of the chemical behavior of actinides in an aqueous solution is not only difficult to conduct but also costly. Thus, theoretical computational simulation is an alternative. , Furthermore, a full understanding of the fundamental physicochemical properties of actinide complexes at the atomic scale also plays a key role in describing the behavior of actinides in solution.…”

Semirigid Ligands Enhance Different Coordination Behavior of Nd and Dy Relevant to Their Separation and Recovery in a Non-aqueous Environment

“…A valuable approach for the recovery of REEs is the use of multifunctional ligands, which can selectively separate lanthanide − or actinide cations − or a mixture of both. − These ligands were specifically engineered to express different metal-to-ligand interaction geometries and strengths for a specific metal–ligand entity. In a slightly different approach, Bogart et al showed that dysprosium and neodymium preferentially formed monomeric and dimeric entities, respectively, when using a polydentate N′N3O3 donor ligand.…”