Strong Periodic Tendency of Trivalent Lanthanides Coordinated with a Phenanthroline-Based Ligand: Cascade Countercurrent Extraction, Spectroscopy, and Crystallography

Complexation and extraction of trivalent actinides over lanthanides using highly soluble phenanthroline diamide ligands with different side chains

Although phenanthroline diamide ligands have been widely reported, their limited solubility in organic solvents and poor performance in the separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) at high acidity are still clear demerits. In this study, we designed and synthesized three highly soluble phenanthroline diamide ligands with different side chains. By introducing alkyl chains and ester groups, the ligands solubility in 3-nitrotrifluorotoluene is increased to over 600 mmol/L, significantly higher than the previous reported phenanthroline diamide ligands. Based on anomalous aryl strengthening, benzene ring was incorporated to enhance ligand selectivity toward Am(III). Extraction experiments demonstrated favorable selectivity of all the three ligands towards Am(III). The optimal separation factor (SFAm/Eu) reaches 53 at 4 mol/L HNO3, representing one of the most effective separation of An(III) over Ln(III) under high acidity. Slope analysis, single crystal structure analysis, as well as titration of ultraviolet visible spectroscopy, mass spectrometry, and nuclear magnetic resonanc confirmed the formation of 1:1 and 1:2 complex species between the metal ions and the ligands depending on the molar ratio of metal ions in the reaction mixture. The findings of this study offer valuable insights for developing phenanthroline diamide ligands for An(III)/Ln(III) separation.

Enhancing the Selectivity of Trivalent Actinide over Lanthanide Using Asymmetrical Phenanthroline Diamide Ligands

Phenanthroline diamide ligands have been widely used in the separation of trivalent actinides and lanthanides, but little research has focused on extractants with asymmetrical substitutes. Two novel asymmetrical phenanthroline-based ligands N2,N2,N9-triethyl-N9-tolyl-1,10-phenanthroline-2,9-dicarboxamide (DE-ET-DAPhen) and N2-ethyl-N9,N9-dioctyl-N2-tolyl-1,10-phenanthroline-2,9-dicarboxamide (DO-ET-DAPhen) were first synthesized in this work, whose extraction ability and complexation mechanism to trivalent actinides [An(III)] and lanthanides [Ln(III)] were systematically investigated. The ligands dissolved in n-octanol exhibit good extraction ability and high selectivity toward Am(III) in acidic solutions. The complexation mechanism of the ligands with Ln(III) in solution and solid state was analyzed using slope analysis, 1H NMR spectrometric titration, ESI-MS, and calorimetric titration. It is revealed that the ligands complex with Am(III)/Eu(III) with 1:1 stoichiometry. The stability constant (log β) of the complexation reaction of Eu(III) with DE-ET-DAPhen determined by UV-vis spectrophotometric and calorimetric titration is higher than that of DO-ET-DAPhen, indicating the stronger complexation ability of DE-ET-DAPhen. Meanwhile, the calorimetric titration results show that the complexation process is exothermic with a decreased entropy. The structures of 1:1 complexes of Eu(III) and Nd(III) with DE-ET-DAPhen were analyzed through single-crystal X-ray diffraction. This work proves that ligands containing asymmetrical functional groups are promising for An(III)/Ln(III) separation, which shows great significance in efficient extractants designed for the spent nuclear fuel reprocessing process.

Kinetic features of solvent extraction by N,O-donor ligands of f-elements: a comparative study of diamides based on 1,10-phenanthroline and 2,2'-bipyridine

A variant of microfluidic setup design for the study of extraction kinetics has been proposed. Mass transfer constants for Am(III) and Eu(III) and observed rate constants were obtained for N-,O-donor ligands featuring phenanthroline and bipyridyl cores. The possibility of determining rate constants for cations independently of each other makes it possible to observe the kinetic effect of separation. The extraction rate was found to be lower for the bipyridyl ligand, compared to phenanthroline. The values of the rotation barriers for the ligands were calculated using the DFT method. The values correlate with the obtained low extraction rate for the bipyridyl ligand. Also, crystallographic data showing anti-conformation for the bipyridyl ligand align with the kinetic data. Surface tension was also determined for the systems with the studied ligands. It is shown that at equal ligand concentrations, the value of surface tension agrees with the extraction rate. Furthermore, it is shown that for the bipyridyl ligand, prior contact of the organic phase with nitric acid significantly affects the surface tension.

New asymmetric tetradentate phenanthroline chelators with pyrazole and amide groups for complexation and solvent extraction of Ln(III)/Am(III)

To tune the complexation and solvent extraction performance of the ligands with a 1,10-phenanthroline core for trivalent actinides (An3+) and lanthanides (Ln3+), we synthesized two new asymmetric tetradentate ligands with pyrazole and amide groups, i.e., L1 (N,N-diethyl-9-(5-ethyl-1H-pyrazol-3-yl)-1,10-phenanthroline-2-carboxamide) and its analogue L2 with longer alkyl chains (N,N-dihexyl). The complexation of the ligands with Ln3+ was confirmed by 1H NMR titration and X-ray crystallography, and stability constants were measured in methanol by spectrophotometric titration. The asymmetric ligands exhibited an improved performance in terms of selective solvent extraction of Am3+ over Eu3+ in strongly acidic solutions compared to their symmetric analogues. The improved selectivity of the asymmetric ligands was interpreted theoretically by density functional theory simulations. This study implies that combining different functional groups to construct asymmetric ligands may be an efficient way to tune ligand performance with regard to An3+ separation from Ln3+.

4-Oxo-7-fluoro-1,10-phenanthroline-2,9-diamides: Synthesis, Structural Features, Lanthanide Complexes, and Am(III)/Ln(III) Solvent Extraction

A highly efficient synthetic approach was developed for the synthesis of unsymmetrical 1,10-phenanthroline-2,9-diamides with two different substituents in the fourth and seventh positions of the phenanthroline core. The structures of these ligands were confirmed using various spectral methods including 2D-NMR and X-ray analysis. Quantum chemical calculations supported the presence of tautomeric forms of these ligands. Furthermore, it was discovered that these compounds exhibit polydentate ligand behavior toward lanthanide nitrates. The structural characteristics of the complexes formed between these ligands and lanthanide nitrates were investigated both in the solid state and in solution. To further understand the binding properties of these novel unsymmetrical ligands, the binding constants for potential complexes were quantitatively measured by using UV-vis spectrophotometric titration. This allowed for a comprehensive analysis of the binding affinity and stability of these complexes. Extraction experiments of f-elements were performed for symmetrical and unsymmetrical diamides. Overall, this study presents significant advancement in the synthesis and characterization of unsymmetrical 1,10-phenanthroline-2,9-diamides and provides valuable insights into their potential applications as polydentate ligands for lanthanide nitrates.

Size Selective Ligand Tug of War Strategy to Separate Rare Earth Elements

Separating rare earth elements is a daunting task due to their similar properties. We report a "tug of war" strategy that employs a lipophilic and hydrophilic ligand with contrasting selectivity, resulting in a magnified separation of target rare earth elements. Specifically, a novel water-soluble bis-lactam-1,10-phenanthroline with an affinity for light lanthanides is coupled with oil-soluble diglycolamide that selectively binds heavy lanthanides. This two-ligand strategy yields a quantitative separation of the lightest (e.g., La-Nd) and heaviest (e.g., Ho-Lu) lanthanides, enabling efficient separation of neighboring lanthanides in-between (e.g., Sm-Dy).

Noncoordinating Secondary Sphere Ion Modulates Supramolecular Clustering of Lanthanides

The role of counterions in molecular recognition of lanthanides is underexplored, especially when they exhibit weak interactions with the metal cations. Here, we report a complementary and comprehensive investigation integrating theoretical calculations with X-ray absorption fine structure spectroscopy, dynamic light scattering, and small-angle X-ray scattering to reveal atomic-scale structural features beyond the immediate coordination sphere of a system used for rare-earth element separations. Our results indicate the formation of an unusual T-shaped outer-sphere lanthanide complex, containing two ligands and two nitrate ions in the first coordination sphere, whereas the third nitrate is weakly coordinated and resides in the second shell. This unique structural arrangement causes inhomogeneous charge distribution, leading to self-assembly of the complexes into larger nanoclusters through sterically directed electrostatic interactions in the nonpolar medium. Our findings point to the importance of "noncoordinating" anions in defining the degree of supramolecular aggregation and ion cluster assembly.

Atomistic Insights into Structure and Dynamics of Neodymium(III) Complexation with a Bis-lactam Phenanthroline Ligand in the Organic Phase

Rare-earth elements (REEs) such as neodymium are critical materials needed in many important technologies, and rigid neutral bis-lactam-1,10-phenanthroline (BLPhen) ligands show one of the highest extraction performance for complexing Nd(III) in REE uptake and separation processes. However, the local structure of the complexes formed between BLPhen and Nd(III) in a typical organic solvent such as dichloroethane (DCE) is unclear. Here, we perform first-principles molecular dynamics (FPMD) simulations to unveil the structure of complexes formed by BLPhen with Nd(NO₃)₃ in the DCE solvent. BLPhen can bind to Nd(III) in either 1:1 or 2:1 fashion. In the 1:1 complex, three nitrates bind to Nd(III) via the bidentate mode in the first solvation shell, leading to the formation of a neutral complex, [Nd(BLPhen)(NO₃)₃]⁰, in the organic phase. In contrast, there are two nitrates in the first solvation shell in the 2:1 complex, creating a charged complex, [Nd(BLPhen)₂(NO₃)₂]⁺. The third nitrate was found to be far away from the metal center, migrating to the outer solvation shell. Our simulations show that the binding pocket formed by the two rigid BLPhen ligands allows ample space for two nitrates to bind to the Nd(III) center from opposite sides. Our findings of two nitrates in the first solvation shell of the 2:1 complex and the corresponding bond distances agree well with the available crystal structure. This study represents the first accurate FPMD modeling of the BLPhen-Nd(III) complexes in an explicit organic solvent and opens the door to more atomistic understanding of REE separations from first principles.

MOFs with 12-Coordinate 5f-Block Metal Centers

We have constructed an unprecedented MOF platform that accommodates a range of 5f-block metal ions (Th⁴⁺, U⁴⁺, Np⁴⁺, Pu⁴⁺) as the primary building block. The isoreticular actinide metal-organic frameworks (An-MOFs) exhibit periodic trends in the 12-coordinate metal environment, ligand configuration, and resulting ultramicroporosity. It holds potential in distinguishing neighboring tetravalent actinides. The metal ionic radius, carboxylate bite angle, anthracene plane twisting, interligand interactions, and countercation templating collectively determine an interplay between solvation, modulation, and complexation, resulting in a coordination saturation of the central actinide, while lanthanide counterparts are stabilized by the formation of a dimer-based motif. Quantum chemical calculations indicate that this large coordination number is only feasible in the high-symmetry environment provided by the An-MOFs. This category of MOFs not only demonstrates autoluminescence (4.16 × 10⁴ counts per second per gram) but also portends a wide-bandgap (2.84 eV) semiconducting property with implications for a multitude of applications such as hard radiation detection.

Luminescent Complexes of Europium (III) with 2-(Phenylethynyl)-1,10-phenanthroline: The Role of the Counterions

New antenna ligand, 2-(phenylethynyl)-1,10-phenanthroline (PEP), and its luminescent Eu (III) complexes, Eu(PEP)₂Cl₃ and Eu(PEP)₂(NO₃)₃, are synthesized and characterized. The synthetic procedure applied is based on reacting of europium salts with ligand in hot acetonitrile solutions in molar ratio 1 to 2. The structure of the complexes is refined by X-ray diffraction based on the single crystals obtained. The compounds [Eu(PEP)₂Cl₃]·2CH₃CN and [Eu(PEP)₂(NO₃)₃]∙2CH₃CN crystalize in monoclinic space group P2_1/n and P2_1/c, respectively, with two acetonitrile solvent molecules. Intra- and inter-ligand π-π stacking interactions are present in solid stat and are realized between the phenanthroline moieties, as well as between the substituents and the phenanthroline units. The optical properties of the complexes are investigated in solid state, acetonitrile and dichloromethane solution. Both compounds exhibit bright red luminescence caused by the organic ligand acting as antenna for sensitization of Eu (III) emission. The newly designed complexes differ in counter ions in the inner coordination sphere, which allows exploring their influence on the stability, molecular and supramolecular structure, fluorescent properties and symmetry of the Eu (III) ion. In addition, molecular simulations are performed in order to explain the observed experimental behavior of the complexes. The discovered structure-properties relationships give insight on the role of the counter ions in the molecular design of new Eu (III) based luminescent materials.