Complexation of trivalent lanthanides and actinides with diethylenetriaminepentaacetic acid: Theoretical unraveling of bond covalency

Modeling separation of lanthanides via heterogeneous ligand binding

Individual lanthanide elements have physical/electronic/magnetic properties that make each useful for specific applications. Several of the lanthanides cations (Ln3+) naturally occur together in the same ores. They are notoriously difficult to separate from each other due to their chemical similarity. Predicting the Ln3+ differential binding energies (ΔΔE) or free energies (ΔΔG) at different binding sites, which are key figures of merit for separation applications, will help design of materials with lanthanide selectivity. We apply ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) to calculate ΔΔG for Ln3+ coordinated to ligands in water and embedded in metal-organic frameworks (MOFs), and ΔΔE for Ln3+ bonded to functionalized silica surfaces, thus circumventing the need for the computational costly absolute binding (free) energies ΔG and ΔE. Perturbative AIMD simulations of water-inundated simulation cells are applied to examine the selectivity of ligands towards adjacent Ln3+ in the periodic table. Static DFT calculations with a full Ln3+ first coordination shell, while less rigorous, show that all ligands examined with net negative charges are more selective towards the heavier lanthanides than a charge-neutral coordination shell made up of water molecules. Amine groups are predicted to be poor ligands for lanthanide-binding. We also address cooperative ion binding, i.e., using different ligands in concert to enhance lanthanide selectivity.

Vibrational anisotropy decay resolves rare earth binding induced conformational change in DTPA

Elucidating the relationship between metal-ligand interactions and the associated conformational change of the ligand is critical for understanding the separation of lanthanides via ion binding. Here we examine DTPA, a multidentate ligand that binds lanthanides, in its free and metal bound conformations using ultrafast polarization dependent vibrational spectroscopy. The polarization dependent pump-probe spectra were analyzed to extract the isotropic and anisotropic response of DTPA's carbonyl groups in the 1550-1650 cm-1 spectral region. The isotropic response reports on the population relaxation of the carbonyl stretching modes. We find that the isotropic response is influenced by the identity of the metal ion. The anisotropy decay of the carbonyl stretching modes reveals a faster decay in the lanthanide-DTPA complexes than in the free DTPA ligand. We attribute the anisotropy decay to energy transfer among the different carbonyl sites - where the conformational change results in an increased coupling between the carbonyl sites of metal-bound DTPA complexes. DFT calculations and theoretical simulations of energy transfer suggest that the carbonyl sites are more strongly coupled in the metal-bound conformations compared to the free DTPA. The stronger coupling in the metal bound DTPA conformation leads to efficient energy transfer among the different carbonyl sites. Comparing the rate of anisotropy decay across the series of metal bound DTPA complexes we find that the anisotropy is sensitive to the charge density of the central metal ion, and thus can serve as a molecular scale reporter for lanthanide ion binding.

Preorganization of N, O-hybrid phosphine oxide chelators for effective extraction of trivalent Am/Eu ions - A computational study

N, O-hybrid phosphine oxide ligands with N-heterocyclic cores are the advanced extractants for extracting actinides over lanthanides. Yet, the challenging task in designing an efficient hybrid ligand is tracing the...

Topology of the Electron Density and of Its Laplacian from Periodic LCAO Calculations on f-Electron Materials: The Case of Cesium Uranyl Chloride

The chemistry of f-electrons in lanthanide and actinide materials is yet to be fully rationalized. Quantum-mechanical simulations can provide useful complementary insight to that obtained from experiments. The quantum theory of atoms in molecules and crystals (QTAIMAC), through thorough topological analysis of the electron density (often complemented by that of its Laplacian) constitutes a general and robust theoretical framework to analyze chemical bonding features from a computed wave function. Here, we present the extension of the Topond module (previously limited to work in terms of s-, p- and d-type basis functions only) of the Crystal program to f- and g-type basis functions within the linear combination of atomic orbitals (LCAO) approach. This allows for an effective QTAIMAC analysis of chemical bonding of lanthanide and actinide materials. The new implemented algorithms are applied to the analysis of the spatial distribution of the electron density and its Laplacian of the cesium uranyl chloride, Cs2UO2Cl4, crystal. Discrepancies between the present theoretical description of chemical bonding and that obtained from a previously reconstructed electron density by experimental X-ray diffraction are illustrated and discussed.

Charge Density Analysis of Actinide Compounds from the Quantum Theory of Atoms in Molecules and Crystals

The nature of chemical bonding in actinide compounds (molecular complexes and materials) remains elusive in many respects. A thorough analysis of their electron charge distribution can prove decisive in elucidating bonding trends and oxidation states along the series. However, the accurate determination and robust analysis of the charge density of actinide compounds pose several challenges from both experimental and theoretical perspectives. Significant advances have recently been made on the experimental reconstruction and topological analysis of the charge density of actinide materials [Gianopoulos et al. IUCrJ, 2019, 6, 895]. Here, we discuss complementary advances on the theoretical side, which allow for the accurate determination of the charge density of actinide materials from quantum-mechanical simulations in the bulk. In particular, the extension of the Topond software implementing Bader's quantum theory of atoms in molecules and crystals (QTAIMAC) to f- and g-type basis functions is introduced, which allows for an effective study of lanthanides and actinides in the bulk and in vacuo, on the same grounds. Chemical bonding of the tetraphenyl phosphate uranium hexafluoride cocrystal [PPh4+][UF6-] is investigated, whose experimental charge density is available for comparison. Crystal packing effects on the charge density and chemical bonding are quantified and discussed. The methodology presented here allows reproducing all subtle features of the topology of the Laplacian of the experimental charge density. Such a remarkable qualitative and quantitative agreement represents a strong mutual validation of both approaches-experimental and computational-for charge density analysis of actinide compounds.

Tailoring the selectivity of phenanthroline derivatives for the partitioning of trivalent Am/Eu ions – a relativistic DFT study

Preferential binding of actinides over lanthanides using tailored phenanthroline derivative ligands through relativistic DFT calculations.

Theoretical Prediction of the Potential Applications of Phenanthroline Derivatives in Separation of Transplutonium Elements

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 (N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide, L_a) and its derivatives (5-bromo-(N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide, L_b), and 5-(4-(λ¹-oxidaneyl)phenyl)-(N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9- dicarboxamide, L_c) 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 L_c is a better electron donor to actinides than L_a and L_b. The possible extracted complexes AnL(NO₃)₃ and [AnL₂(NO₃)]²⁺ (L = L_a, L_b, L_c; 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 L_c 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 L_c. 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.