Complexation and Extraction of Trivalent Actinides and Lanthanides by Triazinylpyridine N-Donor Ligands

Carbon NMR Titration Could Be More Informative for In-Situ Lanthanide Coordination Chemistry Investigation

In situ characterization of coordination species and their evolution are crucial for effective lanthanide separation and recycling. Current technical approaches often operate under constrained conditions, requiring complex equipment, experimental setups, and sometimes intricate data interpretation. Herein, we demonstrate that carbon NMR titrations offer a valuable approach for in situ coordination analysis of lanthanides, particularly for highly fused preorganized ligands that offer more NMR-active carbons than traditional proton NMR. Two representative ligands of both lipophilic and hydrophilic in nature were investigated, and the resulting carbon NMR titration data were analyzed. Comparisons with IR and single-crystal data showed that NMR provided insights not only into the evolution of coordination species but also into changes in the electron distribution during complex formation. Additionally, we discussed inconsistencies between atomic charge populations obtained from carbon NMR and those from Mulliken and Hirshfeld calculations. With advancements in NMR technology and the availability of higher-field NMR instruments, we believe NMR titrations will play an increasingly significant role in unravelling the complex solution coordination chemistry of f-block elements.

Selective Crystallization Separation Driven by Structural Divergence in Lanthanide Mixed-Organic Systems

Lanthanide separation remains challenging due to their shielded 4f orbitals, which prevent bonding with ligand orbitals. The slight decrease in the Ln3+ ionic radii from La3+ to Lu3+ leads to highly similar coordination environments, making their differentiation even more difficult. In this study, a unique complex organic system based on 1,10-phenanthroline-2,9-dicarboxylic acid (H2PDA), N,N'-dimethylformamide (DMF), and its decomposition products was developed to achieve selective crystallization separation of lanthanide elements. Through the study of the crystallization periodicity of lanthanides in the solvothermal systems, we enabled the efficient crystallization separation of lanthanides such as La/Ce, La/Sm and La/Lu, with binary separation factors of 2.0 ± 0.1, 8.9 ± 0.1, and 26.9 ± 3.1, respectively.

Ab Initio Speciation of Tc-Gluconate Complexes in Aqueous Systems

Tc-gluconate complexes in aqueous systems were recently reported and characterized by Tc L3-edge X-ray absorption near-edge structure (XANES) measurements [Dardenne, K.; Inorg. Chem. 2021, 60, 12285-12298]. The puzzling result was reported that the Tc L3-edge XANES of the sample containing Tc(IV)-gluconate species differs substantially from that of the Tc(IV)O2(am,hyd) hydrous oxide reference sample, whereas the Tc K-edge XANES spectra did not differ significantly. We studied this observation theoretically and tracked the unknown Tc(IV)-gluconate species in a three-step procedure: (1) developing chemical models, (2) optimizing the equilibrium structures of the models, and (3) simulating the corresponding Tc L3-edge XANES spectra. We identified the [Tc(IV)(Glu-2H)2(H2O)2]2- structure as the most likely Tc(IV)-gluconate species present in our samples and explain the substantial difference between the two Tc L3-edge XANES spectra. Additionally, we revisited the Tc(V)-gluconate species and identified the [Tc(V)O(Glu-H)2]- structure as the most likely Tc(V)-gluconate species in our sample.

Room temperature crystal field splitting of curium resolved by circularly polarized luminescence spectroscopy

Coordination of Cm(iii) with a chiral decadentate ligand N,N,N',N'-tetrakis[(6-carboxypyridin-2-yl)methyl]-1,2-diaminocyclohexane (tpadac) generated complexes with strong luminescence allowing for the unprecedented measurement of well-resolved Cm(iii) circularly polarized luminescence spectra. Quantitative resolution of the electronic structure of the [Cm(tpadac)][K] complexes was achieved at room temperature, highlighting the strength of the combination of luminescence and circularly polarized luminescence spectroscopies to unravel the fundamental electronic structure of Cm(iii). These results are a clear demonstration that these spectroscopies are powerful yet simple tools for the fundamental understanding of electronic structure, which opens the door to future investigations of other Cm(iii) complexes in geometries relevant to nuclear applications, and even other 5f-elements.

New synthetic pathway towards BTrzPhen-tetraol: a hydrophilic 2,9-bis-triazolyl-1,10-phenanthroline ligand for selective americium stripping

Selectively separating Am(III) from nuclear waste streams is an extremely challenging task due to the presence of the trivalent lanthanides and Cm(III). 1,10-Phenanthroline ligands decorated with 1,2,4-triazines or 1,2,3-triazoles have emerged as promising extractants for achieving such separation. In this article, a new robust synthetic pathway towards the hydrophilic, CHON compliant bistriazoylphenanthroline ligand BTrzPhen-tetraol is reported. BTrzPhen-tetraol was synthesised both as the hydrochloride and as a free base with overall yields of 66% and 48%, respectively. The ligand demonstrated excellent solubility and stability in dilute nitric acid solutions, with no observable decomposition after three days in 0.5 mol L-1 HNO3 at 50 °C. Additionally, it exhibited rapid stripping kinetics for Am(III) and Eu(III). Liquid-liquid extraction experiments conducted with BTrzPhen-tetraol, TODGA, and radiotracers of Am(III), Cm(III), and Eu(III) yielded maximum Eu(III)/Am(III) and Cm(III)/Am(III) separation factors of 84 and 2.4, respectively, at 0.26 mol L-1 HNO3. Notably, the separation factors achieved with BTrzPhen-tetraol are comparable to those of existing systems. While only a minor influence of the BTrzPhen concentration on the distribution ratios of Am and Cm was observed under the given conditions, these results highlight the effectiveness of hydrophilic BTrzPhen ligands for selective americium stripping and encourage further optimisation to enhance performance.

Structural Regularities, Thermal Stability, and Nature of Chemical Bonding in the Series of Actinide Double Sulfates Cs[An(SO4)2(H2O)3]·H2O (An = U, Np, Pu, or Am)

Investigation of the properties of the trivalent light actinide compounds is hindered by their low stability under normal conditions. In this study, the An3+ double sulfates Cs[An(SO4)2(H2O)3]·H2O (An = U, Np, Pu, or Am) were synthesized and characterized by complementary methods. Their structure was solved using single-crystal X-ray diffraction (XRD), and peculiarities of the sulfate anion environment were addressed with vibrational spectroscopy. The oxidation states of the actinides were confirmed by using X-ray absorption near edge spectroscopy (XANES) and solid-state absorption spectroscopy. Changes in the local environment of Am ions caused by self-irradiation are observed after several months of storage. Decomposition of the compounds in air and in the inert atmosphere at temperatures up to 1000 °C and the final products were studied using thermal analysis and powder diffraction. Computational investigation employing approaches such as QTAIM, Löwdin bond order analysis, and atomic charge calculations was used to investigate trends in the nature of chemical bonds in these compounds. It is shown that the covalent interaction decreases from U to Am with a corresponding increase in ion charge.

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.

Computation-Aided Development of Next-Generation Extractants for Trivalent Actinide and Lanthanide Separation

The chemical similarities between trivalent actinides [An(III)] and lanthanides [Ln(III)] present a significant challenge in differentiating and separating them, which is a key step toward closing the nuclear fuel cycle. However, the existing separation approaches commonly suffer from demerits such as inadequate separation factors, limited stripping efficiency, and undesired coextraction. In this study, a novel unsymmetrical phenanthroline-derived amide-triazine (Et-Tol-CyMe4-ATPhen) extractant was first designed and then screened with theoretical computation. Meanwhile, they were successfully synthesized by using a de novo construction method. As expected, Et-Tol-CyMe4-ATPhen exhibited a favorable extraction ability for Am(III) and minimal extraction for Ln(III), thereby achieving an extremely selective An(III)/Ln(III) separation with a separation factor of over 280. Furthermore, Am(III) could be easily and effectively stripped from the loaded phases using dilute nitric acid. The underlying coordination mechanisms were thoroughly elucidated by using 1H NMR, ESI-MS, UV-vis absorption spectrometry, photoluminescence spectrometry, and single-crystal X-ray diffraction. This work holds promise for addressing the current challenges in An(III)/Ln(III) separation and represents a pioneering endeavor in developing next-generation extractants from first-principles calculation.

Evaluation of Multidentate Ligands Derived from Ethyl 1,2,4-triazine-3-carboxylate Building Blocks as Potential An(III)-Selective Extractants for Nuclear Reprocessing

Bis-1,2,4-triazine ligands are amongst the most promising soft N-donor ligands for the partitioning of trivalent actinides from trivalent lanthanides; a key separation proposed in the future reprocessing of spent nuclear fuels. In an effort to improve the extraction properties of these benchmark ligands, we propose herein a general ligand design approach that is inspired by the field of drug discovery, and we apply it to a new class of ligands in which the bidentate 3-(2-pyridyl)-1,2,4-triazine unit of the benchmark ligands is replaced by a bidentate 1,2,4-triazine-3-carboxamide unit. A series of nine novel ligands were synthesized by reactions of readily available ethyl 1,2,4-triazine-3-carboxylate building blocks with different polyamine cores and evaluated for their ability to extract and separate Am(III) and Cm(III) from Eu(III). One of the reported ligands can co-extract Am(III) and Eu(III) from nitric acid into cyclohexanone, albeit with no selectivity between the metal ions. NMR titration experiments suggested that ligand 23 b formed a chiral 1 : 1 complex species with La(III) but not Lu(III) or Y(III), suggesting the coordination cavity of the ligand is sensitive to the size of the metal ion. The structures and thermodynamic parameters for the proposed complexes were further supported by DFT calculations.

Plant extract mediated in-situ synthesis of iron/manganese alginate hydrosphere and its excellent recovery of rare earth elements in mine wastewater

The recovery of rare earth elements (REEs) is a major issue based on environmental governance and sustainable resource utilization. In this study, we developed a novel hydrogel material (Fe/Mn@ALG) by anchoring Fe/Mn NPs on alginate spheres, where Fe/Mn NPs were in-situ synthesized using Euphorbia cochinchensi leaf extract as reduced and protection agents. The Fe/Mn@ALG was applied directly to real mine wastewater, generating efficient and selective recovery of REEs with the coexistence of numerous competing metal ions. As results have shown, Fe/Mn@ALG was a useful adsorbent for REEs with an adsorption efficiency 78.62 % achieved, which was also confirmed by distribution coefficients (Kd), up to 2451.66 mL·g-1. Furthermore, Fe/Mn@ALG exhibited preferential response to REEs over other metal ions with the separation factor (SF) being up to 240. This great adsorption performance and selectivity toward REEs were attributed to its specific surface area, oxygen-rich functional groups and negatively charged surface in acid wastewater. Furthermore, REEs could be greatly desorbed from Fe/Mn@ALG with output concentration being three times higher than the initial concentration. Additionally, Fe/Mn@ALG maintained its good adsorption performance with efficiency reaching 72.24 % after five reuses. Overall, Fe/Mn@ALG can be considered as a promising candidate for wastewater remediation and sustainable management of resources.

Effect of the stability of 1,3-bis(diphenylphosphoryl)-2-oxapropane complexes on the separation of lanthanide ions and their detection

Phosphoryl podands of neutral type with a flexible ethylene glycol chain and diphenylphosphorylmethyl end groups are known for their complexation properties towards various cations. In this work, the complexation process between 1,3-bis(diphenylphosphoryl)-2-oxapropane (L) and lanthanide ions was studied. Namely, the stability constants of lanthanide complexes with L in acetonitrile were estimated by the method of spectrophotometric titration. It was found that the stability constants of Ln3+ complexes with L increase in the lanthanide series, which is consistent with the extraction and ion-selective properties of L. The extraction ability of L and this ligand in the presence of ionic liquids (ILs) such as methyltrioctylammonium nitrate (MTOAN) and bis[(trifluoromethyl)sulfonyl]imide 1-butyl-3-methylimidazolium (C4mimTf2N) was studied. It was found that in the presence of ILs, L extracts the elements of the yttrium subgroup of lanthanides much better than those of the cerium subgroup: SFLu/Ce(L-MTOAN) = 2.29; SFLu/Ce(L-C4mimTf2N) = 110.38. The use of the L-C4mimTf2N mixture in the processes of lanthanide separation into subgroups is much more efficient than the use of L without the addition of ILs or the use of the L-MTOAN mixture. The ion-selective properties of L towards Ln3+ ions were studied. Podand L exhibits potentiometric selectivity to Lu3+ ions.

Introducing Phosphate Ester into DAPhen by Propyl Enhanced the Selectivity for UO22+ over Th4

A new type of phenanthroline carboxamide(DAPhen)-phosphate ester ligand (L1/L2) was synthesized for the selective separation of U(VI) over Th(IV). Liquid-liquid extraction experiments showed that the introduction of phosphate ester could increase the extraction ability of ligands for U(VI), especially L2, which showed high selectivity for the separation of U(VI) over Th(IV). The slope analysis indicated that L1 could form 1:1 and 1:2 complexes with U(VI) and 1:1 complexes with Th(IV). NMR titration revealed that the DAPhen unit of ligands combined with one U(VI) to form 1:1 complexes, and then the phosphate ester unit of the 1:1 complexes further combined with another U(VI) to form 1:2 complexes. Ligands provide only the DAPhen unit to Th(IV) to form 1:1 complexes. The crystal structures found 1:2 complexes of L1 and U(VI), 1:1 complexes of L2 and U(VI), and 1:1 complexes of L1 and Th(IV). The larger stability constant (log β) of the 1:1 complexes of L2 with U(VI) than that of the 1:1 complexes of L1 with U(VI) showed that the binding ability of U(VI) with the DAPhen unit of L2 is stronger than that of U(VI) with the DAPhen unit of L1. This study provides new ideas for designing extractants with excellent properties.

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.

Amino Acid Decorated Phenanthroline Diimide as Sustainable Hydrophilic Am(III) Masking Agent with High Acid Resistance

Hydrophilic actinide masking agents are believed to be efficient alternatives to circumvent the extensive hazardous organic solvents/diluents typically employed in the liquid-liquid extraction for nuclear waste management. However, the practical application of hydrophilic ligands faces significant challenges in both synthetic/purification procedures and, more importantly, the acid resistance of the ligands themselves. Herein, we have demonstrated the combination of phenanthroline diimide framework with a biomotif of histidine flanking parts could achieve efficient separation of trivalent lanthanides/actinides (also actinides/actinides) under high acidity of over 1 M HNO3. This approach leverages the soft-hard coordination properties of N, O-hybrid ligands, as well as the energetically favored imides for metal coordination and the multiple protonation of histidine. These factors collectively contribute to the synthesis of an easily accessible, highly water-soluble, superior selective, and acid-resistant Am(III) masking agent. Thus, we have shown in this paper, by proper combination of synthetic N, O-hybrid ligand with amino acid, trivalent lanthanide and actinide separation could be efficiently fulfilled in a more sustainable manner.

Tetradentate Ligand's Chameleon-Like Behavior Offers Recognition of Specific Lanthanides

The surging demand for high-purity individual lanthanides necessitates the development of novel and exceptionally selective separation strategies. At the heart of these separation systems is an organic compound that, based on its structural features, selectively recognizes the lighter or heavier lanthanides in the trivalent lanthanide (Ln) series. This work emphasizes the significant implications resulting from modifying the donor group configuration within an N,O-based tetradentate ligand and the changes in the solvation environment of Ln ions in the process of separating Lns, with the unique ability to achieve peak selectivity in the light, medium, and heavy Ln regions. The structural rigidity of the bis-lactam-1,10-phenanthroline ligand enforces size-based selectivity, displaying an exceptional affinity for Lns having larger ionic radii such as La. Modifying the ligand by eliminating one preorganization element (phenanthroline → bipyridine) results in the fast formation of complexes with light Lns, but, in the span of hours, the peak selectivity shifts toward middle Ln (Sm), resulting in time-resolved separation. As expected, at low nitric acid concentrations, the neutral tetradentate ligand complexes with Ln3+ ions. However, the change in extraction mechanism is observed at high nitric acid concentrations, leading to the formation and preferential extraction of anionic heavy Ln species, [Ln(NO3)x+3]x-, that self-assemble with two ligands that have undergone protonation, forming intricate supramolecular architectures. The tetradentate ligand that is structurally balanced with restrictive and unrestrictive motifs demonstrates unique, controllable selectivity for light, middle, and heavy Lns, underscoring the pivotal role of solvation and ion interactions within the first and second coordination spheres.

Extraction and complexation studies with 2,6-bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine in the presence of 2-bromohexanoic acid

To improve the understanding of the extraction chemistry of An(iii) and Ln(iii) with N-donor ligands 2,6-bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine (C4-BPP) in the presence of 2-bromohexanoic acid was investigated. Extraction studies showed an excellent separation factor of SFAm(III)/Eu(III) ≈ 200 and SFAm(III)/Nd(III) ≈ 60 in comparison with the structurally similar ligand 2,6-bis(5-neopentyl-1H-pyrazol-3-yl)pyridine C5-BPP (SFAm(III)/Eu(III) ≈ 100), even though C5-BPP showed significantly higher stability constants. Time-resolved laser fluorescence spectroscopy (TRLFS) studies revealed the formation of the ternary 1 : 1 and 1 : 2 complexes [Eu(C4-BPP) n (2-bromohexanoate) m ](3-m)+ (n = 1-2) ( and ). [Eu(C4-BPP)2(2-bromohexanoate) m ](3-m)+ was the relevant complex species in solvent extraction. In contrast, Cm(iii) form stable 1 : 3 complexes. The ability of 2-bromohexanoic acid to replace C4-BPP from the inner coordination sphere of Eu(iii) but not from Cm(iii) is due to a more favorable complexation of Cm(iii) over Eu(iii) with C4-BPP. This resulted in a notably more efficient separation of An(iii) and Ln(iii) in comparison with C5-BPP.

6-(6-Methyl-1,2,4,5-Tetrazine-3-yl)-2,2'-Bipyridine: A N-Donor Ligand for the Separation of Lanthanides(III) and Actinides(III)

Here, we report the synthesis of the 6-(6-methyl-1,2,4,5-tetrazine-3-yl)-2,2'-bipyridine (MTB) ligand that has been developed for lanthanide/actinide separation. A multimethod study of the complexation of MTB with trivalent actinide and lanthanide ions is presented. Single-crystal X-ray diffraction measurements reveal the formation of [Ce(MTB)2(NO3)3], [Pr(MTB)(NO3)3H2O], and [Ln(MTB)(NO3)3MeCN] (Ln = Nd, Sm, Eu, Gd). In addition, the complexation of Cm(III) with MTB in solution was studied by time-resolved laser fluorescence spectroscopy. The results show the formation of [Cm(MTB)1-3]3+ complexes, which occur in two different isomers. Quantum chemical calculations reveal an energy difference between these isomers of 12 kJ mol-1, clarifying the initial observations made by time-resolved laser fluorescence spectroscopy (TRLFS). Furthermore, quantum theory of atoms in molecules (QTAIM) analysis of the Cm(III) and Ln(III) complexes was performed, indicating a stronger covalent contribution in the Cm-N interaction compared to the respective Ln-N interaction. These findings align well with extraction data showing a preferred extraction of Am and Cm over lanthanides (e.g., max. SFAm/Eu = 8.3) at nitric acid concentrations <0.1 mol L-1 HNO3.

2,6-Bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine: Effects of the Peripheral Aliphatic Side Chain on the Coordination of Actinides(III) and Lanthanides(III)

To improve our understanding of the interaction mechanism in trivalent lanthanide and actinide complexes, studies with structurally different hard and soft donor ligands are of great interest. For that reason, the coordination chemistry of An(III) and Ln(III) with 2,6-bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine (C4-BPP) has been explored. Time-resolved laser fluorescence spectroscopy (TRLFS) studies have revealed the formation of [Cm(C4-BPP)n]3+ (n = 1-3) (log β1' = 7.2 ± 0.4, log β2' = 10.1 ± 0.5, and log β3' = 11.8 ± 0.6) and [Eu(C4-BPP)m]3+ (m = 1-2) (log β1' = 4.9 ± 0.2 and log β2' = 8.0 ± 0.4). The absence of the [Eu(C4-BPP)3]3+ complex shows a more favorable complexation of Cm(III) over that of Eu(III). Additionally, complementary NMR measurements have been conducted to examine the M(III)-N bond in Ln(III) and Am(III) C4-BPP complexes. 15N NMR data have revealed notable differences in the chemical shifts of the coordinating nitrogen atoms between the Am(III) and Ln(III) complexes. In the Am(III) complex, the coordinating nitrogen atoms have shown a shift by 260 ppm, indicating a higher fraction of covalent bonding in the Am(III)-N bond compared with the Ln(III)-N bond. This observation aligns excellently with the differences in the stability constants obtained from TRLFS studies.

Remarkable Improvement in Am3+ and Cm3+ Separation Using a Cooperative Counter Selectivity Strategy by a Combination of Branched Diglycolamides and Hydrophilic Polyaza-heterocycles

Separation of Am3+ and Cm3+ is one of the most challenging problems in the back-end of the nuclear fuel cycle. In the present work, we exploited the cooperative effect of the opposite selectivity of hydrophobic branched DGA derivatives and hydrophobic N-donor heterocyclic ligands taken in two different phases to achieve improved separation behavior. A systematic study was performed using a series of DGA derivatives to understand the effect and the position of branching in the alkyl chains on the separation behavior of Am3+ and Cm3+. A separation factor (S.F.) value as high as 10 for Cm3+ over Am3+ was obtained in the case of TiBDGA (N,N,N',N'-tetra-iso-butyl diglycolamide) using SO3PhBTPhen ((phenanthroline-2,9-diyl)-1,2,4-triazine-5,5,6,6-tetrayltetrabenzenesulfonic acid) as the aqueous complexant, which is the highest reported value so far for the ligand-based separation of Am3+ and Cm3+ without involving any oxidation or reduction step. The high selectivity favoring Cm3+ ion extraction in the case of this DGA derivative is also explained with the help of computational studies.

Understanding the Complexation of Alkyl-Substituted Nitrilotriacetamides with Uranium: A Study by Absorption Spectroscopy and Microcalorimetry

Complexation thermodynamics of UO22+ ions with a series of alkyl-substituted nitrilotriacetamides (NTA) was investigated by absorption spectroscopy and microcalorimetry. The hexamethyl derivative of NTA (HMNTA) forms the weakest two successive complexes with UO22+ ions with stability constants of log β11 = 3.5 ± 0.1 and log β12 = 6.1 ± 0.1. The formation constant values increased linearly with increasing alkyl chain length of the substituents from hexamethyl NTA to hexabutyl NTA (HBNTA) and to hexahexyl NTA (HHNTA). The complexation with each ligand was both enthalpy and entropy driven with exothermic enthalpy changes of ΔH11 = -14.7 ± 1.0 kJ/mol, ΔH12 = -10.2 ± 0.8 kJ/mol for HMNTA, ΔH11 = -19.2 ± 1.2 kJ/mol, ΔH12 = -16.4 ± 1.1 kJ/mol for HBNTA, and ΔH11 = -21.3 ± 1.4 kJ/mol, ΔH12 = -19.4 ± 2.3 kJ/mol for HHNTA. Similarly, the positive entropy changes with each ligand were ΔS11 = 18.1 ± 2.7 J/mol/K, ΔS12 = 82.9 ± 3.8 J/mol/K for HMNTA, ΔS11 = 14.4 ± 1.2 J/mol/K, ΔS12 = 87.2 ± 4.2 J/mol/K for HBNTA, and ΔS11 = 16.1 ± 2.4 J/mol/K, ΔS12 = 92.6 ± 3.1 J/mol/K for HHNTA. Structural features of the complex suggest the participation of two ligands coordinating in a bidentate mode via the carbonyl oxygens. The [UO2L2]2+ complexes appear to be noncentrosymmetric with two ligands and one water molecule occupying the equatorial plane of the dioxo uranyl cation. The structure of the complex was confirmed by 1H NMR titration, EXAFS measurements, and DFT calculations.

Amine-Terminated Phenanthroline Diimides as Aqueous Masking Agents for Am(III)/Eu(III) Separation: An Alternative Ligand Design Strategy for Water-Soluble Lanthanide/Actinide Chelating Ligands

Selective actinide coordination (from lanthanides) is critical for both nuclear waste management and sustainable development of nuclear power. Hydrophilic ligands used as masking agents to withhold actinides in the aqueous phase are currently highly pursued, while synthetic accessibility, water solubility, acid resistance, and extraction capability are the remaining problems. Most reported hydrophilic ligands are only effective at low acidity. We recently proved that the phenanthroline diimide skeleton was an efficient building block for the construction of highly efficient acid-resistant hydrophilic lanthanide/actinide separation agents, while the limited water solubility hindered the loading capability of the ligand. Herein, amine was introduced as the terminal solubilizing group onto the phenanthroline diimide backbone, which after protonation in acid showed high water solubility. The positively charged terminal amines enhanced the ligand water solubility to a large extent, which, on the other side, was believed to be detrimental for the coordination and complexation of the metal cations. We showed that by delicately adjusting the alkyl chain spacing, this intuitive disadvantage could be relieved and superior extraction performances could be achieved. This work holds significance for both hydrophilic lanthanide/actinide separation ligand design and, concurrently, offers insights into the development of water-soluble lanthanide/actinide complexes for biomedical and bioimaging applications.

A hydrolytically stable complexant for minor An separation from Ln in process relevant diluents

The evaluation of bis-1,2,4-triazine complexants containing eight-carbon, alkoxy-functionalized phenyl substituents at the 3,3'-positions for selective minor actinide extraction in simulated high level waste is reported. The complexant retained chemoselective extraction efficiency of actinides over lanthanides while breaking through the nonpolar diluent solubility and hydrolytic stability barriers that limit many BTP complexants. Thus, we report a BTP complexant that is readily soluble in both kerosene and isooctanol without forming precipitates, emulsions, or third phase after contact with nitric acid and extracts 241Am3+ from 154Eu3+. Separations and spectroscopic data are reported herein.

Synthesis of Fused [1,2,3]-Triazoloheteroarenes via Intramolecular Azo Annulation of N-Tosylhydrazones Catalyzed by 1,8-Diaza-bicyclo[5.4.0]undec-7-ene

The structural diversity of triazoloheteroarenes render this moiety an attractive synthon for drug discovery, C-H functionalization, and complexant design for minor actinide separations. While contemporary work has demonstrated the capacity to leverage downstream functional group interconversion of the triazolopyridine, a broadly applicable method tolerant of diverse heteroaryl constructs and pendant functionality to obtain triazoloheteroarenes remains under reported. In this work, the serendipitous discovery of a metal, azide, and oxidant free transformation of various heteroaryl N-tosylhydrazones of carbaldehydes and ketones to the corresponding [1,2,3]-triazoloheteroarene via intramolecular azo annulation using a substoichiometric amount of 1,8-diaza-bicyclo[5.4.0]undec-7-ene is described. These results substantively improve upon previous approaches offering efficient access to the described heterocycles. Discovery of reaction conditions, method optimization, complexant, pyridine, and heteroarene substrate scope, as well as relevant scale-up reactions are reported herein.

Preorganization Effects on Eu(III) Ion Coordination by Dipyridyl-Phenanthroline Ligands: A Combined Experimental and Theoretical Analysis

Although 1,10-phenanthroline has been proven to hold a strong complexing capacity for f-block elements and their derivatives have been applied in many fields, research on more highly or completely rigid phenanthroline ligands is still rare due to the challenging syntheses. Here, we reported three tetradentate ligands 2,9-di(pyridin-2-yl)-1,10-phenanthroline (L1), 12-(pyridin-2-yl)-5,6-dihydroquinolino[8,7b][1,10]phenanthroline (L2), and 5,6,11,12-tetrahydrobenzo[2,1-b:3,4-b']bis([1,10]phenanthroline) (L3) with increasing preorganization on the side chain; among which, L3 is fully preorganized. Their complexation reactions with Eu(III) were systematically investigated by electrospray ionization mass spectrometry (ESI-MS), UV-vis titrations, and single-crystal structures. It is found that all three ligands form only 1:1 M/L complexes with Eu(III). The single-crystal structures revealed that the three ligands hold similar coordination modes, while their stability constants determined by UV-vis titrations were L3 (4.80 ± 0.01) > L2 (4.38 ± 0.01) > L1 (3.88 ± 0.01). This trend is supported not only by the thermodynamic stability of rigid ligands compared to free ligands but also by the conclusion that rigid ligands exhibit faster reaction rates (lower energy barrier) than free ligands kinetically. This work is helpful in providing theoretical guidance for the subsequent development of highly preorganized chelating ligands with strong coordination ability and high selectivity for f-block elements.

Determination of ultra-trace level 241Am in marine sediment and seawater by combining TK200-TK221 tandem-column extraction chromatography and SF ICP-MS

Sound strategies for marine chemical monitoring call for measurement systems capable of producing comparable analytical results with demonstrated quality. This work presents the development and validation of a new analytical procedure for the determination of the 241Am mass fraction in marine sediment and seawater samples at low levels. The procedure includes a tandem-column extraction chromatography for separation of 241Am and sector field-inductively coupled plasma mass spectrometry (SF ICP-MS) for its determination. The separation is based on the application of two new extraction resins, TK200 and TK221. The acid leaching method was employed for the pre-treatment of marine sediments, while Fe(OH)3 co-precipitation was used for Am pre-concentration in seawater samples. The extraction behaviors of Am on TK221 resins in the different acidic mediums were investigated. The removal capabilities of the tandem TK200-TK221 columns for the 241Am in the presence of interfering elements including Pu, Pb, Hg, Bi, Tl, Pt, Hf, U, and Th were carefully investigated and the corresponding decontamination factors (DFs) estimated to be in the range from 104 to 106. The main interfering element Pu was efficiently removed with a DF of about 6 × 105. Matrix rare earth elements (REEs) in marine sediments were further removed by the application of TEVA resins. 241Am mass fraction was quantified by the application of external calibration and SF ICP-MS. Following the recommendations of the ISO/IEC 17025 guidelines, the validation of the analytical procedure was accomplished by executing it on the certified reference material (CRM) IAEA-385 (marine sediment) and the seawater IAEA-443 reference materials (RM). The obtained results showed that 241Am mass fractions were accurately determined in both reference samples, with excellent reproducibility (2.1 % and 7.6 %) and low LODs (0.4 fg g-1 and 0.2 fg g-1). The relative expanded uncertainties (k = 2) obtained were 17.1 % and 29.0 %, respectively. The overall analytical times for the application of the proposed procedure on the marine sediment and seawater samples were evaluated to be only about 9 h and 6.5 h, respectively. It shows great advantages for its potential applications for emergency monitoring of 241Am contamination in the marine environment.

Theoretical insights into selective extraction of Am(III) from Cm(III) and Eu(III) with asymmetric N-heterocyclic ligands

Separation of lanthanide (Ln) and minor actinide (MA) elements and mutual separation between minor actinide elements (e.g. Am(III) and Cm(III)) represent a crucial undertaking. However, separating these elements poses a significant challenge owing to their highly similar physicochemical properties. Asymmetric N-heterocyclic ligands such as N-ethyl-6-(1H-pyrazol-3-yl)-N-(p-tolyl)picolinamide (Et-p-Tol-A-PzPy) and N-ethyl-N-(p-tolyl)-1,10-phenanthroline-2-carboxamide (ETPhenAm) have recently received considerable attention in the separation of MAs over Ln from acid solutions. By changing the central skeleton structures of these ligands and introducing substituents with different properties on the side chains, their complexation behavior with Am(III), Cm(III), and Eu(III) may be affected. In this work, we explore four different asymmetric N-containing heterocyclic ligands, namely Et-p-Tol-A-PzPy (L1), N-ethyl-6'-(1H-pyrazol-3-yl)-N-(p-tolyl)-[2,2'-bipyridine]-6-carboxamide (L2), N-ethyl-9-(1H-pyrazol-3-yl)-N-(p-tolyl)-1,10-phenanthroline-2-carboxamide (L3), and ETPhenAm (L4) using density functional theory (DFT). The calculated results demonstrate the potential of ligands L1-L4 for the extraction and separation of Am(III), Cm(III), and Eu(III). Ligand analysis shows that ligand L3 binds more easily to the central metal atom, in line with the stronger extraction capacity of L3. In spite of the higher covalence between the side chain and the central metal atom for complexes with L1-L3, the main chain seems to control the stability of the extraction complexes. The preorganized 1,10-phenanthroline backbone also further enhances the extraction performance of L3 and L4. The difference in coordination ability between the side chain donors of these ligands and metal ions may affect their separation efficiency. This work presents theoretical insights into synthesizing novel ligands for separating trivalent actinides by adjusting N-heterocyclic ligands.

N-Heterocyclic Carbene to Actinide d-Based π-bonding Correlates with Observed Metal-Carbene Bond Length Shortening Versus Lanthanide Congeners

Comparison of bonding and electronic structural features between trivalent lanthanide (Ln) and actinide (An) complexes across homologous series' of molecules can provide insights into subtle and overt periodic trends. Of keen interest and debate is the extent to which the valence f- and d-orbitals of trivalent Ln/An ions engage in covalent interactions with different ligand donor functionalities and, crucially, how bonding differences change as both the Ln and An series are traversed. Synthesis and characterization (SC-XRD, NMR, UV-vis-NIR, and computational modeling) of the homologous lanthanide and actinide N-heterocyclic carbene (NHC) complexes [M(C5Me5)2(X)(IMe4)] {X = I, M = La, Ce, Pr, Nd, U, Np, Pu; X = Cl, M = Nd; X = I/Cl, M = Nd, Am; and IMe4 = [C(NMeCMe)2]} reveals consistently shorter An-C vs Ln-C distances that do not substantially converge upon reaching Am3+/Nd3+ comparison. Specifically, the difference of 0.064(6) Å observed in the La/U pair is comparable to the 0.062(4) Å difference observed in the Nd/Am pair. Computational analyses suggest that the cause of this unusual observation is rooted in the presence of π-bonding with the valence d-orbital manifold in actinide complexes that is not present in the lanthanide congeners. This is in contrast to other documented cases of shorter An-ligand vs Ln-ligand distances, which are often attributed to increased 5f vs 4f radial diffusivity leading to differences in 4f and 5f orbital bonding involvement. Moreover, in these traditional observations, as the 5f series is traversed, the 5f manifold contracts such that by americium structural studies often find no statistically significant Am3+vs Nd3+ metal-ligand bond length differences.

Next-Generation 3,3'-AlkoxyBTPs as Complexants for Minor Actinide Separation from Lanthanides: A Comprehensive Separations, Spectroscopic, and DFT Study

Progress toward the closure of the nuclear fuel cycle can be achieved if satisfactory separation strategies for the chemoselective speciation of the trivalent actinides from the lanthanides are realized in a nonproliferative manner. Since Kolarik's initial report on the utility of bis-1,2,4-triazinyl-2,6-pyridines (BTPs) in 1999, a perfect complexant-based, liquid-liquid separation system has yet to be realized. In this report, a comprehensive performance assessment for the separation of 241Am3+ from 154Eu3+ as a model system for spent nuclear fuel using hydrocarbon-actuated alkoxy-BTP complexants is described. These newly discovered complexants realize gains that contemporary aryl-substituted BTPs have yet to achieve, specifically: long-term stability in highly concentrated nitric acid solutions relevant to the low pH of unprocessed spent nuclear fuel, high DAm over DEu in the economical, nonpolar diluent Exxal-8, and the demonstrated capacity to complete the separation cycle with high efficiency by depositing the chelated An3+ to the aqueous layer via decomplexation of the metal-ligand complex. These soft-N-donor BTPs are hypothesized to function as bipolar complexants, effectively traversing the organic/aqueous interface for effective chelation and bound metal/ligand complex solubility. Complexant design, separation assays, spectroscopic analysis, single-crystal X-ray crystallographic data, and DFT calculations are reported.

Lanthanide transport in angstrom-scale MoS2-based two-dimensional channels

Rare earth elements (REEs), critical to modern industry, are difficult to separate and purify, given their similar physicochemical properties originating from the lanthanide contraction. Here, we systematically study the transport of lanthanide ions (Ln3+) in artificially confined angstrom-scale two-dimensional channels using MoS2-based building blocks in an aqueous environment. The results show that the uptake and permeability of Ln3+ assume a well-defined volcano shape peaked at Sm3+. This transport behavior is rooted from the tradeoff between the barrier for dehydration and the strength of interactions of lanthanide ions in the confinement channels, reminiscent of the Sabatier principle. Molecular dynamics simulations reveal that Sm3+, with moderate hydration free energy and intermediate affinity for channel interaction, exhibit the smallest dehydration degree, consequently resulting in the highest permeability. Our work not only highlights the distinct mass transport properties under extreme confinement but also demonstrates the potential of dialing confinement dimension and chemistry for greener REEs separation.

Renormalized-Residue-Based Multireference Configuration Interaction Method for Strongly Correlated Systems

The implementation of multireference configuration interaction (MRCI) methods in quantum systems with large active spaces is hindered by the expansion of configuration bases or the intricate handling of reduced density matrices (RDMs). In this work, we present a spin-adapted renormalized-residue-based MRCI (RR-MRCI) approach that leverages renormalized residues to effectively capture the entanglement between active and inactive orbitals. This approach is reinforced by a novel efficient algorithm, which also facilitates an efficient deployment of spin-adapted matrix product state MRCI (MPS-MRCI). The RR-MRCI framework possesses several advantages: (1) It considers the orbital entanglement and utilizes highly compressed MPS structure, improving computational accuracy and efficiency compared with internally contracted (ic) MRCI. (2) Utilizing small-sized buffer environments of a few external orbitals as probes based on quantum information theory, it enhances computational efficiency over MPS-MRCI and offers potential application to large molecular systems. (3) The RR framework can be implemented in conjunction with ic-MRCI, eliminating the need for high-rank RDMs, by using distinct renormalized residues. We evaluated this method across nine diverse molecular systems, including Cu2O22+ with an active space of (24e,24o) and two complexes of lanthanide and actinide with active space (38e,36o), demonstrating the method's versatility and efficacy.

Rationally designed nanotrap structures for efficient separation of rare earth elements over a single step

Extracting rare earth elements (REEs) from wastewater is essential for the growth and an eco-friendly sustainable economy. However, it is a daunting challenge to separate individual rare earth elements by their subtle differences. To overcome this difficulty, we report a unique REE nanotrap that features dense uncoordinated carboxyl groups and triazole N atoms in a two-fold interpenetrated metal-organic framework (named NCU-1). Notably, the synergistic effect of suitable pore sizes and REE nanotraps in NCU-1 is highly responsive to the size variation of rare-earth ions and shows high selectivity toward light REE. As a proof of concept, Pr/Lu and Nd/Er are used as binary models, which give a high separation factor of SFPr/Lu = 796 and SFNd/Er = 273, demonstrating highly efficient separation over a single step. This ability achieves efficient and selective extraction and separation of REEs from mine tailings, establishing this platform as an important advance for sustainable obtaining high-purity REEs.

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.

Spin-Adapted Externally Contracted Multireference Configuration Interaction Method Based on Selected Reference Configurations

As one kind of approximation of the full configuration interaction solution, the selected configuration interaction (sCI) methods have been shown to be valuable for large active spaces. However, the inclusion of dynamic correlation beyond large active spaces is necessary for more quantitative results. Since the sCI wave function can provide a compact reference for multireference methods, previously, we proposed an externally contracted multireference configuration interaction method using the sCI reference reconstructed from the density matrix renormalization group wave function [J. Chem. Theory Comput. 2018, 14, 4747-4755]. The DMRG2sCI-EC-MRCI method is promising for dealing with more than 30 active orbitals and large basis sets. However, it suffers from two drawbacks: spin contamination and low efficiency when using Slater determinant bases. To solve these problems, in this work, we adopt configuration state function bases and introduce a new algorithm based on the hybrid of tree structure for convenient configuration space management and the graphical unitary group approach for efficient matrix element calculation. The test calculation of naphthalene shows that the spin-adapted version could achieve a speed-up of 6.0 compared with the previous version based on the Slater determinant. Examples of dinuclear copper(II) compound as well as Ln(III) and An(III) complexes show that the sCI-EC-MRCI can give quantitatively accurate results by including dynamic correlation over sCI for systems with large active spaces and basis sets.

Redox stabilization of Am(v) in a biphasic extraction system boosts americium/lanthanides separation efficiency

Americium (Am) is a key radioactive element in consideration in nuclear waste treatment. Separation of Am from the fission products, lanthanides, is a prerequisite to minimize the hazardous impact of Am and make utilization of rare Am isotopes, but it represents a great challenge due to the chemical similarity between the two groups of elements. Herein, we realize the separation by first oxidizing Am(iii) to high valent Am(vi) and then converting it to Am(v) in situ in a biphasic extraction system with Bi(v) oxidant incorporated in an organic phase. Am(v) is highly stabilized during the separation process and this leads to record high Ln/Am separation factors (>105) in a single contact over a wide range of acidities.

Coordination and fragmentation chemistry of CyMe4-BTPhen complexes with lanthanides and actinides: A combined investigation by ESI-MS and DFT calculations

To further understand the complexation and fragmentation during the extraction process, the formation of 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-12,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BTPhen) complexes with lanthanides (Ln = La, Ce, Nd, Sm, Eu, Yb) and actinides (UO22+, Th4+) was observed by electrospray ionization mass spectrometry (ESI-MS) technique and density functional theory (DFT) calculations. Mass spectrometry titrations showed the variation relationship of different complexes in acetonitrile. For lanthanides, the major complexes were 1:2 species ([Ln(L)2]3+ and [Ln(L)2(NO3)]2+) with a ratio of 1:2, which were observed at the initial addition of Ln3+, whereas the species ([Ln(L)(NO3)2]+) with a ratio of 1:1 was detected when the [Ln]/[L] concentration ratio reached 1.0. For UO22+ and Th4+ complexes, 1:1 or 1:2 species ([UO2L(NO3)]+, Th(L)2(NO3)3+ and Th(L)2(NO3)22+) were formed. The fragmentation chemistry of both the ligand and the complex cations was characterized in detail by collision-induced dissociation. The fragmentation process of CyMe4-BTPhen was unfolded sequentially on both sides of the ligand by cleavage of C-C and C-N bonds. DFT calculations provided a detailed analysis of the structures and thermodynamics of those complexes, which indicated that the stable complexes formed in acetonitrile solution were consistent with the ESI-MS results.

Hydroxyl-group functionalized phenanthroline diimides as efficient masking agents for Am(III)/Eu(III) separation under harsh conditions

The separation of Lns(III) from radioactive Ans(III) in high-level liquid waste remains a formidable hydrometallurgical challenge. Water-soluble ligands are believed to be new frontiers in the search of efficient Lns/Ans separation ligands to close the nuclear fuel cycles and dealing with current existing nuclear waste. Currently, the development of hydrophilic ligands far lags behind their lipophilic counterparts due to their complicated synthetic procedures, inferior extraction performances, and acid tolerances. In this paper, we have showed a series of hydroxyl-group functionalized phenanthroline diimides were efficient masking agents for Am(III)/Eu(III) separation under high acidity (˃ 1 M HNO3). Record high SFEu(III)/Am(III) of 162 and 264 were observed for Phen-2DIC2OH and Phen-2DIC4OH in 1.25 M HNO3 which represents the best Eu(III)/Am(III) separation performance at this acidity. UV-vis absorption, NMR and TRLFS titrations were conducted to elucidate the predominant of 1:1 ligand/metal species under extraction conditions. X-ray data of both the ligand and Eu(III) complex together with DFT calculations revealed the superior extraction performances and selectivities. The current reported hydrophilic ligands were easy to prepare and readily to scale-up, acid tolerant and highly efficient, together with their CHON-compatible nature make them promising candidates in the development of advanced separation processes.

Selective Separation of Americium(III), Curium(III), and Lanthanide(III) by Aqueous and Organic Competitive Extraction

Adding hydrophilic ligands into aqueous solutions for the selective binding of actinides(III) is acknowledged as an advanced strategy in Ln(III)/An(III) separation. In view of the recycling and radioactive waste disposal of the minor actinide, there remains an urgent need to design and develop the appropriate ligand for selective separation of An(III) from Ln(III). Herein, four novel hydrophilic ligands with hard-soft hybrid donors, derived from the pyridine and phenanthroline skeletons, were designed and synthesized as masking agents for selective complexation of An(III) in the aqueous phase. The known N,N,N',N'-tetraoctyl diglycolamide (TODGA) was used as lipophilic extractant in the organic phase for extraction of Ln(III), and a new strategy for the competitive extraction of An(III) and Ln(III) was developed based on TODGA and the above hydrophilic ligands. The optimal hydrophilic ligand of N,N'-bis(2-hydroxyethyl)-2,9-dicarboxamide-1,10-phenanthroline (2OH-DAPhen) displayed exceptional selectivity toward Am(III) over Ln(III), with the concentrations of HNO3 ranging from 0.05 to 3.0 M. The maximum separation factors were up to 1365 for Eu/Am, 417.66 for Eu/Cm, and 42.38 for La/Am. The coordination mode and bonding property of 2OH-DAPhen with Ln(III) were investigated by 1H NMR titration, UV-vis spectrophotometric titration, luminescence titration, FT-IR, ESI-HRMS analysis, and DFT calculations. The results revealed that the predominant species formed in the aqueous phase was a 1:1 ligand/metal complex. DFT calculations also confirmed that the affinity of 2OH-DAPhen for Am(III) was better than that for Eu(III). The present work using a competitive extraction strategy developed a feasible alternative method for the selective separation of trivalent actinides from lanthanides.

Theoretical investigation on the ligands constructed from phenanthroline and five-membered N-heterocyclic rings for bonding and separation properties of Am(III) and Eu(III)

The ligands, derived from the combination of phenanthroline and various five-membered N-heterocyclic rings, were subject to a comprehensive investigation for their potential in the extraction and separation of actinides and lanthanides. This study employed DFT methods to thoroughly explore the properties of both phenanthroline (Ph) and the diverse five-membered N-heterocyclic rings (R1-R8). Additionally, tridentate ligands RlPh (l = 1-8) and tetradentate ligands RlPhRr (l, r = 1-8) were analyzed in detail, encompassing their electrostatic potential (ESP), protonation energy, coordination bonding with the metals Am(III) and Eu(III), and the thermodynamics of extraction separation for Am(III) and Eu(III). The findings highlight that the electrostatic potential (ESP) and binding capabilities of the five-membered N-heterocyclic ring units serve as effective predictors for the properties of intricate tridentate and tetradentate ligands, as well as their coordination bonding affinity with metals. The ligands' binding energy is closely associated with their ESP, and notably, the binding energy of tridentate and tetradentate ligands correlates well with the binding energies of their constituent structural units. The computational results reveal that the R2 unit, along with its corresponding tridentate ligand R2Ph and tetradentate ligands R2PhRr, exhibits the highest ESP, superior binding energies, and the strongest coordination bonding affinity with the metals. The theoretical calculations further identify several promising extractants for the effective separation of Am(III) and Eu(III). The tridentate ligands R1Ph, R7Ph, and R4Ph, and the tetradentate ligands R4PhR4, R6PhR6, R2PhR2, R1PhR5 and R3PhR6 were identified as having excellent separation performance for Am(III) and Eu(III). This study would provide insights for the design of extractants for the separation of Am(III) and Eu(III) by use of five-membered N-heterocyclic rings as structural units.

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+.

Complexation of Am(III) and Eu(III) with DRAPA (N,N-dialkyl-6-amide-pyridine-2-carboxylic acid)

The similarity of the coordination chemistry of Am(III) and Eu(III) and two homologous tridentate ligands, N,N-di-2-ethylhexyl-6-amide-pyridine-2-carboxylic acid (DEHAPA, HL') in solvent extraction and N,N-dimethyl-6-amide-pyridine-2-carboxylic acid (DMAPA, HL) in aqueous solution and in the solid state, is revealed structurally and spectroscopically with complexes ML'3 (org), ML3 (aq) and ML3 (s), respectively.

Tuning aminopolycarboxylate chelators for efficient complexation of trivalent actinides

The complexation of trivalent lanthanides and minor actinides (Am3+, Cm3+, and Cf3+) by the acyclic aminopolycarboxylate chelators 6,6'-((ethane-1,2-diylbis-((carboxymethyl)azanediyl))bis-(methylene))dipicolinic acid (H4octapa) and 6,6'-((((4-(1-(2-(2-(2-hydroxyethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)pyridine-2,6-diyl)bis-(methylene))bis-((carboxymethyl)azanediyl))bis-(methylene)) dipicolinic acid (H4pypa-peg) were studied using potentiometry, spectroscopy, competitive complexation liquid-liquid extraction, and ab initio molecular dynamics simulations. Two studied reagents are strong multidentate chelators, well-suited for applications seeking radiometal coordination for in-vivo delivery and f-element isolation. The previously reported H4octapa forms a compact coordination packet, while H4pypa-peg is less sterically constrained due to the presence of central pyridine ring. The solubility of H4octapa is limited in a non-complexing high ionic strength perchlorate media. However, the introduction of a polyethylene glycol group in H4pypa-peg increased the solubility without influencing its ability to complex the lanthanides and minor actinides in solution.

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.

A Simple yet Efficient Hydrophilic Phenanthroline-Based Ligand for Selective Am(III) Separation under High Acidity

Highly selective hydrophilic ligands were believed to be an efficient way to overcome the massive amount of hazardous organic solvent used in the liquid-liquid extraction process and stood as a new frontier in the Lns(III)/Ans(III) partition. Current reported hydrophilic ligands suffer from harsh preparation conditions, inferior extraction performances, limited available chemical structures, and inability to carry out extraction under high acidity. In this article, we report a simple yet efficient carboxylic group modified phenanthroline-diimide ligand which displayed unexpected Lns(III)/Ans(III) and Ans(III)/Ans(III) separation capabilities in 1.5 M HNO3. Unique dimeric architectures for Eu(III) complexes were observed, which could be the origin of the outperforming selectivity and acid resistance. We believe this crystal engineering approach could inspire a renaissance in searching for new functional groups and coordination modes for efficient, high-acid-tolerance Lns(III)/Ans(III) separation ligands.

Tuning the Alkyl Chain of Nitrilotriacetamide for Selectively Extracting Trivalent Am over Eu Ions

The successful management and safe disposal of high-level nuclear waste necessitate the efficient separation of actinides (An) from lanthanides (Ln), which has emerged as a crucial prerequisite. Mixed donor ligands incorporating both soft and hard donor atoms have garnered interest in the field of An/Ln separation and purification. One such example is nitrilotriacetamide (NTAamide) derivatives, which have demonstrated selectivity in extracting minor actinide Am(III) ions over Eu(III) ions. Nevertheless, the Am/Eu complexation behavior and selectivity remain underexplored. In the work, a comprehensive and systematic investigation has been conducted for [M(^RL)(NO₃)₃] complexes (M = Am and Eu) utilizing relativistic density functional theory. The NTAamide ligand (^RL) is substituted with various alkyl groups, namely, methyl, ethyl, propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, and n-octyl. Thermodynamic calculations show that the alkyl chain length in NTAamide is capable of tuning the separation selectivity of Am and Eu. Moreover, the differences in calculated free energies between Am and Eu complexes are more negative for R = Bu-Oct than Me-Pr. This indicates that elongation of the alkyl chain can increase the efficiency of selective separation of Am(III) from Eu(III). Based on the quantum theory of atoms in molecules and charge decomposition analyses, it has been observed that the strength of Am-^RL bonds is higher than that of Eu-^RL bonds. This disparity is attributed to a greater degree of covalency in Am-^RL bonds and a higher level of charge transfer from ligands to Am within complexes containing these bonds. Energies of occupied orbitals with the central N character are recognized overall lower for [Am(^OctL)(NO₃)₃] than for [Eu(^OctL)(NO₃)₃], indicative of stronger complexation stability of the former. These results offer valuable insights into the separation mechanism of NTAamide ligands, which can help guide the development of more powerful agents for An/Ln separation in future applications.

Theoretical Insights on the Complexation of Americium(III) and Europium(III) with Diglycolamide- and Dimethylacetamide-Functionalized Calix[4]arenes

Separation of minor actinides from lanthanides is one of the biggest challenges in spent fuel reprocessing due to the similar physicochemical properties of trivalent lanthanides (Ln(III)) and actinides (An(III)). Therefore, developing ligands with excellent extraction and separation performance is essential at present. As an excellent pre-organization platform, calixarene has received more attention on Ln(III)/An(III) separation. In this work, we systematically explored the complexation behaviors of the diglycolamide (DGA)/dimethylacetamide (DMA)-functionalized calix[4]arene extractants for Eu(III) and Am(III) using relativistic density functional theory (DFT). These calix[4]arene-derived ligands were obtained by functionalization with two or four binding units at the narrow edge of the calix[4]arene platform. All bonding nature analyses suggested that the Eu-L complexes possess stronger interaction compared to Am-L analogues, resulting in the higher extraction capacity of the these calix[4]arene ligands toward Eu(III). Thermodynamic analysis demonstrates that these pre-organized ligands on the calix[4]arene platform with four binding units yield better extraction abilities than the single ligands. Although DMA-functionalized ligands show stronger complexation stability for metal ions, in acidic solutions, the calix[4]arene ligands with DGA binding units have better extraction performance for Eu(III) and Am(III) due to the basicity of the DMA ligand. This work enabled us to gain a deeper understanding of the bonding properties between supramolecular ligands and lanthanides/actinides and afford useful insights into designing efficient supramolecular ligands for separating Ln(III)/An(III).

Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides

Partitioning of americium from lanthanides (Ln) present in used nuclear fuel plays a key role in the sustainable development of nuclear energy1-3. This task is extremely challenging because thermodynamically stable Am(III) and Ln(III) ions have nearly identical ionic radii and coordination chemistry. Oxidization of Am(III) to Am(VI) produces AmO22+ ions distinct with Ln(III) ions, which has the potential to facilitate separations in principle. However, the rapid reduction of Am(VI) back to Am(III) by radiolysis products and organic reagents required for the traditional separation protocols including solvent and solid extractions hampers practical redox-based separations. Herein, we report a nanoscale polyoxometalate (POM) cluster with a vacancy site compatible with the selective coordination of hexavalent actinides (238U, 237Np, 242Pu and 243Am) over trivalent lanthanides in nitric acid media. To our knowledge, this cluster is the most stable Am(VI) species in aqueous media observed so far. Ultrafiltration-based separation of nanoscale Am(VI)-POM clusters from hydrated lanthanide ions by commercially available, fine-pored membranes enables the development of a once-through americium/lanthanide separation strategy that is highly efficient and rapid, does not involve any organic components and requires minimal energy input.

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.

Complexation of a Nitrilotriacetate-Derived Triamide Ligand with Trivalent Lanthanides: A Thermodynamic and Crystallographic Study

Non-heterocyclic N-donor nitrilotriacetate-derived triamide ligands are one of the most promising extractants for the selective extraction separation of trivalent actinides over lanthanides, but the thermodynamics and mechanism of the complexation of this kind of ligand with actinides and lanthanides are still not clear. In this work, the complexation behaviors of N,N,N',N',N″,N″-hexaethylnitrilotriacetamide (NTAamide(Et)) with four representative trivalent lanthanides (La³⁺, Nd³⁺, Eu³⁺, and Lu³⁺) were systematically investigated by using ¹H nuclear magnetic resonance (¹H NMR), ultraviolet-visible (UV-vis) and fluorescence spectrophotometry, microcalorimetry, and single-crystal X-ray diffractometry. ¹H NMR spectroscopic titration of La³⁺ and Lu³⁺ indicates that two species of 1:2 and 1:1 metal-ligand complexes were formed in NO₃^- and ClO₄^- media. The stability constants of NTAamide(Et) with Nd³⁺ and Eu³⁺ obtained by UV-vis and fluorescence titration show that the complexing strength of NTAamide(Et) with Nd³⁺ is lower than that with Eu³⁺ in the same anionic medium, while that of the same lanthanide complex is higher in ClO₄^- medium than in NO₃^- medium. Meanwhile, the formation reactions for all metal-ligand complexes are driven by both enthalpy and entropy. The structures of lanthanide complexes in the single ClO₄^- and NO₃^- medium and the mixed one were determined to be [LnL₂(MeOH)](ClO₄)₃ (Ln = La, Nd, Eu, and Lu), [LaL₂(EtOH)₂][La(NO₃)₆], and [LaL₂(NO₃)](ClO₄)₂, separately. The average bond lengths of lanthanide complexes decrease gradually with the decrease in ionic radii of Ln³⁺, indicating that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, which coincides with the trend of the complexing strength obtained by spectroscopic titration. This work not only reveals the thermodynamics and mechanism of the complexation between NTAamide ligands and lanthanides but also obtains the periodic tendency of complexation between them, which may facilitate the separation of trivalent lanthanides from actinides.