Way to Enforce Selectivity via Steric Hindrance: Improvement of Am(III)/Eu(III) Solvent Extraction by Loaded Diphosphonic Acid Esters

Completely preorganized bis-lactam-1,10-phenanthroline ligands with high stability for efficient separation of Am(III) over Eu(III)

N,O-Heterocyclic ligands such as 2,9-diamide-1,10-phenanthroline dicarboxamide (DAPhen) and bis-lactam-1,10-phenanthroline (BLPhen) exhibit excellent separation performance for Am(III) and Eu(III) in high-level liquid waste. However, DAPhen-based ligands show poor extraction capacity, and BLPhen ligands suffer from decomposition in acidic solutions, which hinders their application in practical separation processes. To develop ligands with superior performance, two new completely preorganized and highly stabilized bis-lactam-1,10-phenanthroline (BLPhen) ligands with varying alkyl chain lengths were synthesized, demonstrating exceptional extraction and separation of Am(III) from Eu(III) with maximum separation factors of 68 and 53, respectively. DFT calculations and various analytical techniques confirmed that the ligands form 1 : 1 complexes with metal ions, with stronger bonding interactions for Am(III), highlighting their potential for spent nuclear fuel processing.

Role of H+ in Solvent Extraction of Am(III) and Eu(III) Using a Polydentate Bipyridine N-,O-Donor Ligand: Chemical Equilibrium States and Kinetic Aspects

In this work, the influence of protonation on the kinetics and thermodynamics of extraction of the Am/Eu pair using N-heterocyclic dicarboxylic acid diamide N,N'-diethyl-N,N'-bis(4-ethylphenyl)-[2,2'-bipyridine]-6,6'-dicarboxamide (L) was investigated. The extraction efficiency of the ligand did not decrease, even at a nitric acid concentration 4 times higher than that of the ligand in the organic phase. X-ray diffraction analysis established that protonation leads to the preorganization of the ligand due to the reversal of bipyridyl rings into the binding conformation when both nitrogen atoms are turned to one side. Also, the effect of protonation on the chemical shifts of the functional binding groups (bipyridyl and amide) in solution was demonstrated by NMR spectroscopy. However, the presaturation of the organic phase with nitric acid did not lead to significant changes in the surface activity of the ligand. For extraction kinetics, a mechanism was proposed in which protonation and preorganization of the ligand was the limiting stage.

Coordination of Fluorine-Substituted 1,10-Phenanthroline Diphosphonates with Americium(III) and Lanthanides(III): Solvent Extraction, Complexation, XRD, and Theoretical Study

Hybrid N,O-donor ligands based on 1,10-phenanthroline are a promising class of compounds for processing high-level waste. Here, we synthesized novel phenanthroline-based diphosphonates containing electron-withdrawing fluorine atoms in alkyl substituents. We studied their extraction properties for Am(III) and, for the first time, for the entire series of lanthanides(III). The extraction of nitric acid for these diphosphonates was also studied. It has been shown that replacing one hydrogen atom with a fluorine atom in the ethyl substituent of a diphosphonate leads to a decrease in the affinity of the ligands for metal cations. Replacing two hydrogen atoms leads to a decrease in the affinity of the ligands for both metal cations and protons. But such modifications led to a change in the nature of the extraction of Am(III) and Eu(III) from nitric acid solutions, and as a result, the diphosphonate with two substituted hydrogen atoms retains its extraction properties when extracted from 5 mol/L HNO3. The complexation in solution was studied by using ultraviolet-visible (UV-vis) titration for Nd(NO3)3 and Eu(NO3)3. The study of complexation in solid form using SC-XRD revealed the formation of complexes of the composition LnL(NO3)3, as well as the possibility of the formation of hydrolyzed binuclear complexes [Eu(μ2,κ4-(RO)2P(O)Phen(O)2(OR))(NO3)2]2. Density functional theory (DFT) calculations were performed to gain more insight into coordination properties and describe experimental data. It was shown that introducing fluorine atoms decreases the charge modules for both Nphen and OP═O and decreases the protonation energy.

Visible Light Photoredox Catalysis in the Synthesis of Phosphonate-Substituted 1,10-Phenanthrolines

Photoredox-catalyzed phosphonylation of bromo-substituted 1,10-phenanthrolines under visible light irradiation was studied. The reaction was shown to proceed under mild conditions with Eosin Y as a photocatalyst in DMSO under blue light irradiation. It is transition-metal-free and affords the target phosphonate-substituted 1,10-phenanthrolines in moderate yields (26-51%) in 22 to 40 h. The rate and selectivity of the reaction depend largely on the position of the bromine atom, as well as on the nature and position of other substituents in the 1,10-phenanthroline core.

Bipyridyldicarboxamides and f-metals: the influence of electron effects on the structure, stability, separation, and photophysical properties of their complexes

In this work, three isomeric fluorinated bipyridyldicarboxamides were studied to evaluate the impact of the fluorine atom position on the structure, stability, Am(III)/Ln(III) separation, and photophysical properties of their complexes. The complexes of the fluorinated amides have a metal-to-ligand composition of 1 : 1, which is independent of the fluorine atom position or lanthanide metal. The bipyridyl fragments in the fluorinated complexes are flattened compared with those in unsubstituted ones. Ln-to-heteroatom distances are more affected by steric hindrance in the ligand and further by lanthanide ion radius contraction. This leads to significant effectivity of heavy lanthanide extraction compared with the light ones, particularly for 4F diamide. Fluorination leads to a slight variation in the excited triplet state of the complexes, and hence, the effectiveness of luminescence increases for Eu, Sm, and Tb complexes. Moreover, fluorination significantly affects the CIE chromaticity coordinates for the complexes.

Progress in phenanthroline-derived extractants for trivalent actinides and lanthanides separation: where to next?

Spent nuclear fuel (SNF) released from reactors possesses significant radioactivity, heat release properties, and high-value radioactive nuclides. Therefore, using chemical methods for reprocessing can enhance economic efficiency and reduce the potential environmental risks of nuclear energy. Due to the presence of relatively diffuse f-electrons, the chemical properties of trivalent lanthanides (Ln(III)) and actinides (An(III)) in SNF solutions are quite similar. Separation methods have several limitations, including poor separation efficiency and the need for multiple stripping agents. The use of novel multi-dental phenanthroline-derived extractants with nitrogen donor atoms to effectively separate An(III) over Ln(III) has been widely accepted. This review first introduces the development history of phenanthroline-derived extractants for extraction and complexation with An(III) over Ln(III). Then, based on structural differences, these extractants are classified into four categories: nitrogen-coordinated, N,O-hybrid coordinated, highly preorganized structure, and unsymmetric structure. Each category's design principles, extraction, and separation performance as well as their advantages and disadvantages are discussed. Finally, we have summarized and compared the unique characteristics of the existing extractants and provided an outlook. This work may offer a reliable reference for the precise identification and selective separation between An(III) and Ln(III), and point the way for future development and exploration.

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.

Sensing and extraction of hazardous metals by di-phosphonates of heterocycles: a combined experimental and theoretical study

In this study, pyridine and phenanthroline diphosphonate ligands were investigated for the first time from the context of solvent extraction and potentiometric sensing of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II) cations. The extraction efficiency under the same conditions for phenanthroline-diphosphonates is considerably higher than that for pyridine ligands. At the same time, the pyridine-diphosphonates show pronounced selectivity towards lead in this metal series. The extraction systems with phenanthroline diphosphonates provided the most efficient extraction of Cd(II) and Pb(II) cations (D > 90). The newly developed pyridine and phenanthroline diphosphonate ligands have proven to be highly effective components in plasticized polymeric membranes. These ligands can be utilized to construct potentiometric ion sensors that exhibit a notable response specifically towards Pb(II) cations. Among the previously reported tetradentate ligands, the phenanthroline diphosphonate ligand, when incorporated into plasticized polymeric membranes, demonstrated the highest sensitivity towards d-metals and Pb(II). The structure of the single crystal complex of Pb(II) and Cd(II) with pyridine-diphosphonates was studied by X-ray diffraction analysis (XRD). The geometry of Cu(II), Zn(II), Cd(II) and Pb(II) complexes and the energy effect of the complex formation, including pseudo-oligomerization reactions, were determined by DFT calculations. The high sensing and extraction efficiency of diphosphonates with respect to Pb(II) is consistent with the minimum values of complex formation energies. The variation in sensory and extraction properties observed among the studied diphosphonate ligands is influenced by the ability to form polynuclear complexes with Pb(II) cations, whereas such properties are absent in the case of Cd(II) cations.

Selective Complexation and Separation of Uranium(VI) from Thorium(IV) with New Tetradentate N,O-Hybrid Diamide Ligands: Synthesis, Extraction, Spectroscopy, and Crystallographic Studies

An unmet challenge in the thorium-uranium fuel cycle is the efficient separation of uranium from thorium. Herein, two new tetradentate N,O-hybrid ligands, N,N'-diethyl-N,N'-di-p-tolyl-2,2'-bipyridine-6,6'-dicarboxamide (Et-Tol-BPDA) and N,N'-diethyl-N,N'-di-p-tolyl-2,2'-bipyrimidine-4,4'-dicarboxamide (Et-Tol-BPymDA), comprising a bipyridine or bipyrimidine core and amide moieties were designed and synthesized for selectively complexing and separating U(VI) from Th(IV). The high U(VI)/Th(IV) extraction selectivity was achieved by Et-Tol-BPDA (SF_U/Th = 33 at 3 M HNO₃) and Et-Tol-BPymDA (SF_U/Th = 73 at 3 M HNO₃) in nitric acid solutions. The extraction process for U(VI) or Th(IV) with these two ligands primarily proceeded through the solvation mechanism, as evidenced by slope analyses. Thermodynamic studies for the extraction of U(VI) and Th(IV) revealed a spontaneous process. Results from UV-vis spectroscopic titration and slope analyses demonstrated that U(VI) and Th(IV) each form a 1:1 complex with the two ligands both in the monophasic organic solution and the biphasic extraction system. The stability constants of the 1:1 complexes of Et-Tol-BPDA or Et-Tol-BPymDA with U(VI) were found to be larger than those with Th(IV), which coincide well with the high U(VI)/Th(IV) extraction selectivity. The solid-state structures of Et-Tol-BPDA, Et-Tol-BPymDA, and 1:1 complexes of the two ligands with U(VI) or Th(IV) were analyzed by X-ray diffraction technique. The results from this work implicate the potential of bipyridine- and bipyrimidine-derived diamide ligands for uranium/thorium separation.

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

Rare-earth elements are widely used in high-end technologies, the production of permanent magnets (PMs) being one of the sectors with the greatest current demand and likely greater future demand. The combination of Nd and Dy in NdFeB PMs enhances their magnetic properties but makes their recycling more challenging. Due to the similar chemical properties of Nd and Dy, their separation is expensive and currently limited to the small scale. It is therefore crucially important to devise efficient and selective methods that can recover and then reuse those critical metals. To address these issues, a series of heptadentate Trensal-based ligands were used for the complexation of Dy3+ and Nd3+ ions, with the goal of indicating the role of coordination and solubility equilibria in the selective precipitation of Ln3+-metal complexes from multimetal non-water solutions. Specifically, for a 1:1 Nd/Dy mixture, a selective and fast precipitation of the Dy complex occurred in acetone with the Trensalp-OMe ligand at room temperature, with a concomitant enrichment of Nd in the solution phase. In acetone, complexes of Nd and Dy with Trensalp-OMe were characterized by very similar formation constants of 7.0(2) and 7.3(2), respectively. From the structural analysis of an array of Dy and Nd complexes with TrensalR ligands, we showed that Dy invariably provided complexes with coordination number (cn) of 7, whereas the larger Nd experienced an expansion of the coordination sphere by recruiting additional solvent molecules and giving a cn of >7. The significant structural differences have been identified as the main premises upon which a suitable separation strategy can be devised with these kind of ligands, as well as other preorganized polydentate ligands that can exploit the small differences in Ln3+ coordination requirements.

Pyridine-di-phosphonates as chelators for trivalent f-elements: kinetics, thermodynamic and interfacial study of Am(III)/Eu(III) solvent extraction

The fractionation of high-level radioactive waste from nuclear power plants simplifies the handling of its components, and facilitates the reduction of radiotoxic effects on the environment. The search and study of new ligands for solvent extraction, as one of the methods in fractionation, remains a complex and important research task. In this work, four pyridine diphosphonate ligands were synthesized. These ligands are part of the class of the N,O-donor extractants, which are selective towards Am(III). The separation factor SF(Am/Eu) for the best extractant reached values up to 10. The influence of the substituents on the efficiency of extraction and complexation of trivalent f-elements, the kinetics of extraction, and the behavior of the ligand at the interface were described. The effect of nitric acid concentration on the extraction was shown. The stoichiometry of the complexes was determined by slope analysis in solvent extraction experiment and verified by spectrophotometric titration in acetonitrile. Liquid tension experiments with a pendant drop method revealed the interfacial properties of the ligands in "F-3 solvent/H₂O" and "F-3 solvent/HNO₃" systems. The relationship between the surface activity and the ligand structure was shown. Studies of the extraction kinetics were performed in a modified Lewis cell. The effect of the ligand structure on the extraction rate was shown. The DFT calculation with the B3LYP density functional was used to explain the extraction properties of the ligands, including selectivity. The calculation of the pre-organization energy of the ligands explained the kinetics and extraction patterns for the studied series.

Theoretical unraveling of the separation of trivalent Am and Eu ions by phosphine oxide ligands with different central heterocyclic moieties

The treatment of nuclear spent fuels, especially the separation of minor actinides, is an imperative task for the healthy development of the nuclear industry. Up to now, it still remains a worldwide challenge to separate trivalent An³⁺ from Ln³⁺ because of their similar chemical properties. Therefore, investigating the mechanism behind the selective extraction of An³⁺ by theoretical methods is necessary. In this work, three phosphine oxide ligands with the same side structures but different bridging frameworks, Ph₂PyPO, Ph₂BipyPO and Ph₂PhenPO, were investigated theoretically, and compared with each other using relativistic density functional theory. The results of QTAIM and MBO suggest that the Am-N bonds in the studied complexes have more covalent character than those in the Eu-N bonds, whereas the PDOS analysis indicates that more overlap exists between Am-5f and the Ph₂PyPO's N-2p orbital than between Am-5f and Ph₂BipyPO's N-2p, and Am-5f and Ph₂PhenPO's N-2p orbital. However, the studied ligands all possess stronger affinities towards Am³⁺ than Eu³⁺, which partly results in the Am³⁺ selectivity towards Eu³⁺ in these three ligands. The calculated reaction free energy can reproduce the Am/Eu separation ability difference of three ligands well. This work offers some useful information for An/Ln separation of phosphine oxide ligands, and may help to design more efficient An³⁺/Ln³⁺ separation ligands.

New tetradentate N,O-hybrid phenanthroline-derived organophosphorus extractants for the separation and complexation of trivalent actinides and lanthanides

Comparison of the extraction and separation properties between two novel phenanthroline-derived organophosphorus ligands, Et-Ph-BPPhen and Et-Ph-PIPhen.