Theoretical Study on the Coordination and Separation Capacity of Macrocyclic N-Donor Extractants for Am(III)/Eu(III)

Designing ligands that can effectively separate actinide An(III)/lanthanide Ln(III) in the solvent extraction process remains one of the key issues in the treatment of accumulated spent nuclear fuel. Nitrogen donor ligands are considered as promising extractants for the separation of An(III) and Ln(III) due to their environmental friendliness. Four new macrocyclic N-donor hexadentate extractants were designed and their coordination with Am(III) and Eu(III), as well as their extraction selectivity and separation performance for Am(III) and Eu(III), were investigated by scalar relativistic density functional theory. A variety of theoretical methods have been used to evaluate the properties of the four ligands and the coordination structures, bonding properties, and thermodynamic properties of the complexes formed by the four ligands with Am(III) and Eu(III). The results of various wavefunction analysis methods including NBO analysis, quantum theory of atoms in molecules (QTAIM) analysis, and so on show that Am(III) has a stronger coordination ability with the ligands than Eu(III) due to the Am 5f orbitals more involved in bonding with the ligands than the Eu 4f orbitals, and the bonding environment of the N-donor in the ligand has a significant effect on its coordination ability of the metal ions. Thermodynamic analysis of the solvent extraction process shows that all of the four N-containing macrocyclic ligands have good extraction selectivity and separation performance for Am(III) and Eu(III). This study provides theoretical support for designing potential nitrogen-containing macrocyclic extractants with excellent separation performance.

Lanthanide SMMs Based on Belt Macrocycles: Recent Advances and General Trends

Enhancement of axial magnetic anisotropy is the central objective to push forward the performance of Single-Molecule Magnet (SMM) complexes. In the case of mononuclear lanthanide complexes, the chemical environment around the paramagnetic ion must be tuned to place strongly interacting ligands along either the axial positions or the equatorial plane, depending on the oblate or prolate preference of the selected lanthanide. One classical strategy to achieve a precise chemical environment for a metal centre is using highly structured, chelating ligands. A natural approach for axial-equatorial control is the employment of macrocycles acting in a belt conformation, providing the equatorial coordination environment, and leaving room for axial ligands. In this review, we present a survey of SMMs based on the macrocycle belt motif. Literature systems are divided in three families (crown ether, Schiff-base and metallacrown) and their general properties in terms of structural stability and SMM performance are briefly discussed.

Control of magnetic anisotropy by macrocyclic ligand distortion in a family of DyIII and ErIII single molecule magnets

A family of hexaazamacrocyclic lanthanide complexes, [Ln(Ln)(NCS)3] (LnIII = Dy, Er; n = 1-3) has been synthesized and characterized by single-crystal X-ray diffraction, magnetic measurements and ab initio calculations. Macrocyclic ligands (Ln) differ in the lateral spacers, which are aliphatic chains with two and three carbons (for Ln, n = 1 and 2, respectively), and an aromatic ring for Ln = 3. Modification of the macrocycle spacer tunes planarity and rigidity of the equatorial coordination for both oblate (Dy) and prolate (Er) lanthanide ions. Ac-susceptibility studies showed that four of the six complexes are field induced single molecule magnets (SMMs). Trends in magnetic relaxation properties are rationalized with the aid of ab initio multireference calculations, highlighting the combined influence of macrocycle planarity, lanthanide electronic density distribution and intermolecular interactions for the achievement of slow demagnetization.

Antenna triplet DFT calculations to drive the design of luminescent Ln3+ complexes

Density functional theory-based methods have been exploited to look into the structural, vibrational and electronic properties of antenna ligands, all of them being crucial factors for the reliable design of customized luminescent lanthanide (Ln3+) complexes. The X-ray structures, UV-Vis absorption spectra and triplet (T1) energies of three novel β-diketone ligands with a thienyl group and naphthyl (L1), phenanthryl (L2), and pyrenyl (L3) polycyclic aromatic hydrocarbons as substituents have been modelled. Vibronic progressions provide a strong contribution to the L1 and L2 absorption spectra, while the L3 absorption spectrum needs the assumption of different conformational isomers in solution. T1 energies have been estimated either through the vertical- or the adiabatic-transition approach. The comparison with the phosphorescence spectra of Gd3+ complexes allowed us to infer that the latter approach is the most suitable one, in particular when sizable ligands are involved. Results obtained for the isolated antennas can be directly compared with those of the corresponding Ln3+ complexes, due to the unanimously accepted assumption that the excitation is ligand-centred.

Insights into bonding interactions and excitation energies of 3d-4f mixed lanthanide transition metal macrocyclic complexes

In this contribution, a computational study of equatorial bound tetranuclear macrocycle (butylene linked) [LnZn(HOM^Bu)]³⁺ (Ln = La³⁺, Ce³⁺) complexes was carried out. Here, the electronic structure, bonding interaction and excitation energies were studied within the relativistic density functional theory framework. From the electronic structure analysis, the frontier molecular orbitals (FMOs) were strongly localized in the d-orbitals of the Zn centers and the f-orbitals of the lanthanide ions. Besides, the inner MOs were found to exhibit a π-character from the organic part of the macrocyclic chain. EDA-NOCV was used as a tool for evaluating the bonding interaction, taking the trinuclear metallomacrocycle (ZnHOM^Bu) and the lanthanide center as fragments. This analysis showed that the interaction between these fragments was slightly covalent; with this covalency being the result of a charge transfer from the metallomacrocyclic ring to the lanthanide. This phenomenon was observed in the deformation density channels obtained from the EDA-NOCV study; in which π- and σ-charge transfer was observed. Finally, the TD-DFT study of the excitation energies evidenced three sets of bands: the first set with the highest intensity represented the ligand to metal charge transfer bands; the second set could be attributed to the 3d-4f electronic transitions between the metal centers; and the third set represented the f-f bands found for the open-shell cerium complex. This class of complexes accomplishes the "antenna effect" principle, which states that highly absorptive transition-metal (TM) complexes can be used to enhance the luminescence of poorly emissive systems, and are introduced in this study as self-sensitizer bimetallic d-f systems with potential applications in near infra-red (NIR) technologies.