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

Extraction and Complexation Investigation of Palladium(II) by a Nitrilotriacetate-Derived Triamide Ligand

Effective and selective separation and recovery of the fission product palladium from high-level liquid waste are conducive not only to reducing its hazards to the public health and environment but also to alleviate the pressure on the increasing demand for natural palladium. Herein, the Pd²⁺ extraction in an HNO₃ solution with a nitrilotriacetate-derived triamide ligand NTAamide(n-Oct) and the complexation between them were investigated. Using n-octanol as a diluent, NTAamide(n-Oct) demonstrated an excellent selectivity, strong extractability, and high loading capacity for Pd²⁺ extraction. Combined with the results of single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, electrospray ionization-mass spectroscopy, microcalorimetric titration, and slope analysis, the extracted complexes were determined as [PdL₂](NO₃)₂ and [PdL₂][Pd(NO₃)₄] (where L denotes the NTAamide ligand) in 0.10 and 3.0 mol/L HNO₃ solutions, respectively. The extraction model closely depended on the solvation state of Pd²⁺ in the HNO₃ solution. An ion-pair extraction model was proposed and discussed.

Advancing the Am Extractant Design through the Interplay among Planarity, Preorganization, and Substitution Effects

Advancing the field of chemical separations is important for nearly every area of science and technology. Some of the most challenging separations are associated with the americium ion Am(III) for its extraction in the nuclear fuel cycle, ²⁴¹Am production for industrial usage, and environmental cleanup efforts. Herein, we study a series of extractants, using first-principle calculations, to identify the electronic properties that preferentially influence Am(III) binding in separations. As the most used extractant family and because it affords a high degree of functionalization, the polypyridyl family of extractants is chosen to study the effects of the planarity of the structure, preorganization of coordinating atoms, and substitution of various functional groups. The actinyl ions are used as a structurally simplified surrogate model to quickly screen the most promising candidates that can separate these metal ions. The down-selected extractants are then tested for the Am(III)/Eu(III) system. Our results show that π interactions, especially those between the central terpyridine ring and Am(III), play a crucial role in separation. Adding an electron-donating group onto the terpyridine backbone increases the binding energies to Am(III) and stabilizes Am-terpyridine coordination. Increasing the planarity of the extractant increases the binding strength as well, although this effect is found to be rather weak. Preorganizing the coordinating atoms of an extractant to their binding configuration as in the bound metal complex speeds up the binding process and significantly improves the kinetics of the separation process. This conclusion is validated by the synthesized 1,2-dihydrodipyrido[4,3-b;5,6-b]acridine (13) extractant, a preorganized derivative of the terpyridine extractant, which we experimentally showed was four times more effective than terpyridine at separating Am³⁺ from Eu³⁺ (SF_Am/Eu ∼ 23 ± 1).

Determination of 241Am in Environmental Samples: A Review

The determination of 241Am in the environment is of importance in monitoring its release and assessing its environmental impact and radiological risk. This paper aims to give an overview about the recent developments and the state-of-art analytical methods for 241Am determination in environmental samples. Thorough discussions are given in this paper covering a wide range of aspects, including sample pre-treatment and pre-concentration methods, chemical separation techniques, source preparation, radiometric and mass spectrometric measurement techniques, speciation analyses, and tracer applications. The paper focuses on some hyphenated separation methods based on different chromatographic resins, which have been developed to achieve high analytical efficiency and sample throughput for the determination of 241Am. The performances of different radiometric and mass spectrometric measurement techniques for 241Am are evaluated and compared. Tracer applications of 241Am in the environment, including speciation analyses of 241Am, and applications in nuclear forensics are also discussed.

Marking actinides for separation: Resonance-enhanced multiphoton charge transfer in actinide complexes

Precise separation and purification of f-block elements are important and challenging especially for the reduction of nuclear waste and the recycling of rare metals but are practically difficult mainly because of their chemical similarity. A promising way to overcome this difficulty is controlling their oxidation state by nonchemical processes. Here, we show resonance-enhanced multiphoton charge transfer in actinide complexes, which leads to element-specific control of their oxidation states owing to the distinct electronic spectra arising from resonant transitions between f orbitals. We observed oxidation of trivalent americium in nitric acid. In addition, we found that the coordination of nitrates is essential for promoting the oxidation reaction, which is the first finding ever relevant to the primary process of photoexcitation via resonant transitions of f-block elements. The resonance-enhanced photochemical process could be used in the nuclear waste management, as it would facilitate the mutual separation of actinides, such as americium and curium.

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.

Probing the Difference in the Complexation of Trivalent Actinides and Lanthanides with a Tridentate N,O-Hybrid Ligand: Spectroscopy, Thermodynamics, and Coordination Modes

Comparatively revealing the complexation behavior of trivalent actinides and lanthanides with functional ligands in aqueous solution is of great importance to enrich our knowledge on the fundamental coordination chemistry of trivalent f-block elements and to control the fate of minor actinides in nuclear fuel cycles. In this work, the complexation of Am(III) and Nd(III), representatives for trivalent actinides and lanthanides, respectively, with a N,O-hybrid ligand 6-(dimethylcarbamoyl)picolinic acid (DMAPA, denoted as HL) was investigated by absorption spectroscopy, calorimetry, X-ray crystallography, and density functional theory (DFT) calculations. Successive formation of 1:1, 1:2, and 1:3 (metal/ligand) complexes of Am(III) and Nd(III) with DMAPA was identified, and the corresponding thermodynamic parameters were determined. The binding strength of Am(III) with DMAPA is slightly stronger than that of Nd(III), and the complexation of Nd(III) with DMAPA is mainly entropy-driven. The crystal structure of the 1:2 Nd(III)/DMAPA complex and the DFT calculation shed additional light on the coordination and structural characteristics of the complexes. In contrast to the Nd-N bond in the Nd(III)/DMAPA complex, the Am-N bond in the Am(III)/DMAPA complex exhibits more covalency, which contributes to the slightly stronger complexation of Am(III) with DMAPA.

Theoretical Insights into the Selective Separation of Am(III)/Eu(III) Using Hydrophilic Triazolyl-Based Ligands

Designing ligands with efficient actinide (An(III))/lanthanide (Ln(III)) separation performance is still one of the key issues for the disposal of accumulated radioactive waste and the recovery of minor actinides. Recently, the hydrophilic ligands as promising extractants in the innovative Selective ActiNide Extraction (i-SANEX) process show excellent selectivity for Am(III) over Eu(III), such as hydroxylated-based ligands. In this work, we investigated the selective back-extraction toward Am(III) over Eu(III) with three hydrophilic hydroxylated triazolyl-based ligands (the skeleton of pyridine L^a, bipyridine L^b, and phenanthroline L^c) using scalar-relativistic density functional theory. The properties of three hydrophilic hydroxylated ligands and the coordination structures, bonding nature, and thermodynamic properties of the Am(III) and Eu(III) complexes with three ligands have been evaluated using multiple theoretical methods. The results of molecular orbitals (MOs), quantum theory of atoms in molecules (QTAIMs), and natural bond orbital (NBO) reveal that Am-N bonds possess more covalent character compared to Eu-N bonds. The thermodynamic results indicate that the complexing ability of L^b and L^c with metal ions is almost the same, which is stronger than that of L^a. However, L^a has the best Am(III)/Eu(III) selectivity among three ligands, which is attributed to the largest difference in covalency between Am-N_trzl and Eu-N_trzl bonds in ML^a(NO₃)₃. This work provides an in-depth understanding of the preferential selectivity of the hydrophilic hydroxylated ligands with An(III) over Ln(III) and also provides theoretical support for designing potential hydrophilic ligands with excellent separation performance of Am(III)/Eu(III).

Liquid-Liquid Extraction of the Eu(III) Cation by BTP Ligands into Ionic Liquids: Interfacial Features and Extraction Mechanisms Investigated by MD Simulations

What happens at the ionic-liquid (IL)/water interface when the Eu³⁺ cation is complexed and extracted by bis(dimethyltriazinyl) pyridine "BTP" ligands has been investigated by molecular dynamics and potential of mean force simulations on the interface crossing by key species: neutral BTP, its protonated BTPH⁺ form, Eu³⁺, and the Eu(BTP)₃³⁺ complex. At both the [BMI][Tf₂N]/water and [OMI][Tf₂N]/water interfaces, neither BTP nor Eu(BTP)₃³⁺ are found to adsorb. The distribution of Eu(BTP)₂³⁺ and Eu(BTP)³⁺ precursors of Eu(BTP)₃³⁺, and of their nitrate adducts, implies the occurrence of a stepwise complexation process in the interfacial domain, however. The analysis of the ionic content of the bulk phases and of their interface before and after extraction highlights the role of charge buffering by interfacial IL cations and anions, by different amounts depending on the IL. Comparison of ILs with octanol as the oil phase reveals striking differences regarding the extraction efficiency, the affinity of Eu(BTP)₃³⁺ for the interface, the effects of added nitric acid and of counterions (NO₃^- vs Tf₂N^-), charge neutralization mechanisms, and the extent of "oil" heterogeneity. Extraction into octanol is suggested to proceed via adsorption at the surface of water pools, nanoemulsions, or droplets, with marked counterion effects.

Promising Lipophilic PyTri Extractant for Selective Trivalent Actinide Separation from High Active Raffinate

Within a spent nuclear fuel recycling strategy, in the past few years, the pyridine-bis-triazole unit was found to be rather effective and selective in minor actinide (MA) separation from synthetic high active raffinate (HAR). In this research work, the main features of the recently studied PTEH ligand were investigated in order to evaluate its potentialities in SANEX-like processes. Its applicability in advanced separation processes was demonstrated, even at process temperatures. It manifested satisfactory extraction properties for a successful selective An separation from Ln, easy cation release, and adequate extraction kinetics as well as outstanding hydrolytical and radiolytical stability. All the results collected in this work allowed the scientists on the committee of the H2020 GENIORS project to promote PTEH as a concrete alternative to the reference CyMe₄-BTBP ligand.

Ultra-Efficient Americium/Lanthanide Separation through Oxidation State Control

Lanthanide/actinide separation is a worldwide challenge for atomic energy and nuclear waste treatment. Separation of americium (Am), a critical actinide element in the nuclear fuel cycle, from lanthanides (Ln) is highly desirable for minimizing the long-term radiotoxicity of nuclear waste, yet it is extremely challenging given the chemical similarity between trivalent Am(III) and Ln(III). Selective oxidation of Am(III) to a higher oxidation state (OS) could facilitate this separation, but so far, it is far from satisfactory for practical application as a result of the unstable nature of Am in a high OS. Herein, we find a novel strategy to generate stable pentavalent Am (Am(V)) through coordination of Am(III) with a diglycolamide ligand and oxidation with Bi(V) species in the presence of an organic solvent. This strategy leads to efficient stabilization of Am(V) and an extraordinarily high separation factor (>10⁴) of Am from Ln through one single contact in solvent extraction, thereby opening a new avenue to study the high-OS chemistry of Am and fulfill the crucial task of Ln/Am separation in the nuclear fuel cycle. The synergistic coordination and oxidation process is found to occur in the organic solvent, and the mechanism has been well elucidated by quantum-theoretical modeling.

NanoTafla Nanocomposite as a Novel Low-Cost and Eco-Friendly Sorbent for Strontium and Europium Ions

Now the wide use of nanooxides is attributed to their remarkable collection of properties. Nanocomposites have an impressive variety of important applications. A thermal decomposition approach provides a more optimistic method for nanocrystal synthesis due to the low cost, high efficiency, and expectations for large-scale production. Therefore, in this study a new eco-friendly nanooxide composite with sorption characteristics for europium (Eu(III)) and strontium (Sr(II)) was synthesized by a one-step thermal treatment process using earth-abundant tafla clay as a starting material to prepare a modified tafla (M-Taf) nanocomposite. The synthesized nancomposite was characterized by different techniques before and after sorption processes. Different factors that affected the sorption behavior of Eu(III) and Sr(II) in aqueous media by the M-Taf nanocomposite were studied. The results obtained illustrated that the kinetics of sorption of Eu(III) and Sr(II) by the M-Taf nanocomposite are obeyed according to the pseudo-second order and controlled by a Langmuir isotherm model with maximum sorption capacities (Q _max) of 25.5 and 23.36 mg/g for Eu(III) and Sr(II), respectively. Also, this novel low-cost and eco-friendly sorbent has promising properties and can be used to separate and retain some radionuclides in different applications.

Influence of Electronic Modulation of Phenanthroline-Derived Ligands on Separation of Lanthanides and Actinides

The solvent extraction, complexing ability, and basicity of tetradentate N-donor 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BT- Phen) and its derivatives functionalized by Br, hydroxyphenyl, nitryl were discussed and compared. It was demonstrated that four BTPhen ligands are able to selectively extract Am(lll) over Eu(lll). It was notable that the distribution ratio of 5-nitryl-CyMe4-BTPhen for Eu(lll) was suppressed under 0.02, which was much lower compared to DEu(lll) = 1 by CyMe4-BTPhen. The analysis of the effect of the substituent on the affinity to lanthanides was conducted by UV/vis and fluorescence spectroscopic titration. The stability constants of various ligands with Eu(lll) were obtained by fitting titration curve. Additionally, the basicity of various ligands was determined to be 3.1 ± 0.1, 2.3 ± 0.2, 0.9 ± 0.2, 0.5 ± 0.1 by NMR in the media of CD3OD with the addition of DClO4. The basicity of ligands follows the order of L1 > L2 > L3 > L4, indicating the tendency of protonation decreases with the electron-withdrawing ability increase.

Complexation Behaviors of a Tridentate Phenanthroline Carboxamide Ligand with Trivalent f-Block Elements in Different Anion Systems: A Thermodynamic and Crystallographic Perspective

The counteranion has a strong influence on the complexation behavior of tridentate phenanthroline carboxamide ligands with actinides and lanthanides, but the thermodynamic and underlying interaction mechanism at the molecular level is still not clear. In this work, a tridentate ligand, N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA), was synthesized, and the effects of different anions (Cl^-, NO₃^-, and ClO₄^-) on the complexation behavior of Et-Tol-PTA with typical lanthanides were thoroughly studied by using ¹H nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-vis) spectrophotometry, and single-crystal X-ray diffraction. The NMR spectroscopic titration of Lu(III) showed that there were three species (1:1, 2:1, and 3:1 ligand-metal complexes) formed in Cl^- solution systems while two species (2:1 and 1:1) were formed in NO₃^- and ClO₄^- solution systems. When Et-Tol-PTA was titrated with La(III), two species (2:1 and 1:1) were formed in NO₃^- systems and only one species (1:1) was formed in Cl^- and ClO₄^- systems. In addition, the stability constant was determined via UV-vis spectroscopic titration, which showed that the complexation strength between Et-Tol-PTA and Eu(III) decreased in the following order: ClO₄^- > NO₃^- > Cl^-. This indicated that Et-Tol-PTA had the strongest complexation ability with Eu(III) in the ClO₄^- system. The structures of Et-Tol-PTA complexed with EuCl₃, Eu(NO₃)₃, and Eu(ClO₄)₃ were further elucidated by single-crystal X-ray diffraction and agreed well with the results of UV-vis titration experiments. The results of this work revealed that the mechanisms of complexation of lanthanides with the asymmetric ligand Et-Tol-PTA were strongly affected by different anionic environments in solution and in the solid state. These findings may lead to the improvement of the separation of trivalent actinides and lanthanides in nuclear waste.

Theoretical insights into the separation of Am(iii)/Eu(iii): designing ligands based on a preorganization strategy

The extraction behaviors of Am(iii) and Eu(iii) were investigated using phenanthroline and bipyridine ligands based on a preorganization strategy.

New Route to Amide-Functionalized N-Donor Ligands Enables Improved Selective Solvent Extraction of Trivalent Actinides

A new general synthetic route to selective actinide extracting ligands for spent nuclear fuel reprocessing has been established. The amide-functionalized ligands separate Am(III) and Cm(III) from the lanthanides with high...

Opportunities and challenges of applying advanced X-ray spectroscopy to actinide and lanthanide N-donor ligand systems

N-donor ligands such as n-Pr-BTP [2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine] preferentially bind trivalent actinides (An3+) over trivalent lanthanides (Ln3+) in liquid-liquid separation. However, the chemical and physical processes responsible for this selectivity are not yet well understood. Here, an explorative comparative X-ray spectroscopy and computational (L3-edge) study for the An/Ln L3-edge and the N K-edge of [An/Ln(n-Pr-BTP)3](NO3)3, [Ln(n-Pr-BTP)3](CF3SO3)3 and [Ln(n-Pr-BTP)3](ClO4)3 complexes is presented. High-resolution X-ray absorption near-edge structure (HR-XANES) L3-edge data reveal additional features in the pre- and post-edge range of the spectra that are investigated using the quantum chemical codes FEFF and FDMNES. X-ray Raman spectroscopy studies demonstrate the applicability of this novel technique for investigations of liquid samples of partitioning systems at the N K-edge.

Relativistic Multiconfigurational Ab Initio Calculation of Uranyl 3d4f Resonant Inelastic X-ray Scattering

We applied relativistic multiconfigurational all-electron ab initio calculations including the spin-orbit interaction to calculate the 3d4f resonant inelastic X-ray scattering (RIXS) map (3d3/2 → 5f5/2 U M4 absorption edge and 4f5/2 → 3d3/2 U Mβ emission) of uranyl (UO22+). The calculated data are in excellent agreement with experimental results and allow a detailed understanding of the observed features and an unambiguous assignment of all involved intermediate and final states. The energies corresponding to the maxima of the resonant emission and the non-resonant (normal) emission were determined with high accuracy, and the corresponding X-ray absorption near edge structure spectra extracted at these two positions were simulated and agree well with the measured data. With the high quality of our theoretical data, we show that the cause of the splitting of the three main peaks in emission is due to the fine structure splitting of the 4f orbitals induced through the trans di-oxo bonds in uranyl and that we are able to obtain direct information about the energy differences between the 5f and 4f orbitals: Δ5f δ/ϕ - 4f δ/ϕ, Δ5f π* - 4f π, and Δ5f σ* - 4f σ from the 3d4f RIXS map. RIXS maps contain a wealth of information, and ab initio calculations facilitate an understanding of their complex structure in a clear and transparent way. With these calculations, we show that the multiconfigurational protocol, which is nowadays applied as a standard tool to study the X-ray spectra of transition metal complexes, can be extended to the calculation of RIXS maps of systems containing actinides.

Molecular Forces in Liquid-Liquid Extraction

The phase transfer of ions is driven by gradients of chemical potentials rather than concentrations alone (i.e., by both the molecular forces and entropy). Extraction is a combination of high-energy interactions that correspond to short-range forces in the first solvation shell such as ion pairing or complexation forces, with supramolecular and nanoscale organization. While the latter are similar to the long-range solvent-averaged interactions in the colloidal world, in solvent extraction they are associated with lower characteristic lengths of the nanometric domain. Modeling of such complex systems is especially complicated because the two domains are coupled, whereas the resulting free energy of extraction is around k_BT to guarantee the reversibility of the practical process. Nevertheless, quantification is possible by considering a partitioning of space among the polar cores, interfacial film, and solvent. The resulting free energy of transfer can be rationalized by utilizing a combination of terms which represent strong complexation energies, counterbalanced by various entropic effects and the confinement of polar solutes in nanodomains dispersed in the diluent, together with interfacial extractant terms. We describe here this ienaics approach in the context of solvent extraction systems; it can also be applied to further complex ionic systems, such as membranes and biological interfaces.

Prediction of An(III)/Ln(III) Separation by 1,2,4-Triazinylpyridine Derivatives

The effect of frustrated Lewis donors on metal selectivity between actinides and lanthanides was studied using a series of novel organic ligands. Structures and thermodynamic energies were predicted in the gas phase, in water, and in butanol using 9-coordinate, explicitly solvated (H₂O) Eu, Gd, Am, and Cm in the +III oxidation state as reactants in the formation of complexes with 2-(6-[1,2,4]-triazin-3-yl-pyridin-2-yl)-1H-indole (Core 1), 3-[6-(2H-pyrazol-3-yl)pyridin-2-yl]-1,2,4-triazine (Core 2), and several derivatives. These complexations were studied using density functional theory (DFT) incorporating scalar relativistic effects on the actinides and lanthanides using a small core pseudopotential and corresponding basis set. A self-consistent reaction field approach was used to model the effect of water and butanol as solvents. Coordination preferences and metal selectivity are predicted for each ligand. Several ligands are predicted to have a high degree of selectivity, particularly when a low ionization potential in the ligand permits charge transfer to Eu(III), reducing it to Eu(II) and creating a half-filled f⁷ shell. Reasonable separation is predicted between Cm(III) and Gd(III) with Core 1 ligands, possibly due to ligand donor frustration. This separation is largely absent from Core 2 ligands, which are predicted to lose their frustration due to proton transfer from the 2N to the 3N position of the pyrazole component of the ligands via tautomerization.

Double-Walled Metal-Organic Framework with Regulable Pore Environments for Efficient Removal of Radioactive Cesium Cations

An anion double-walled metal-organic framework [Co₂Li₄(BTC)₃(DMF)(H₂O)·(CH₃)₂N]_n (1) based on heterobimetallic Li⁺ and Co²⁺ ions was successfully constructed. Utilizing selective destruction and formation of Co-O/Co-N bonds in the metal chains, [Co₂Li₄(BTC)₃(py)(H₂O)·(CH₃)₂N]_n (2) and [Co₂Li₄(BTC)₃(pi)(H₂O)·(CH₃)₂N]_n (3) with the same skeleton but distinct pore structures can be surprisingly obtained. Additionally, compounds 2 and 3 can be transformed into [Co₂Li₄(BTC)₃(H₂O)₂·(CH₃)₂N]_n (4) by soaking them in an ethanol solution. This kind of single-crystal-to-single-crystal transformation successfully regulates the pore structure of MOFs and enriches the diversity of the pore wall on the premise of retaining the original framework. Most impressively, compound 1 shows high adsorption capacity for Cs⁺ cations and is a good candidate to selectively accommodate Cs⁺ among the common alkali metal ions, which is future identified by single-crystal X-ray diffraction and inductively coupled plasma mass spectrometry (ICP-MS) test. Meanwhile, compound 1 can selectively adsorb methylene blue (MB) and crystal violet (CV) molecules over Rhodamine B (RMB).

Comparative Investigation into the Complexation and Extraction Properties of Tridentate and Tetradentate Phosphine Oxide-Functionalized 1,10-Phenanthroline Ligands toward Lanthanides and Actinides

Two new phosphine oxide-functionalized 1,10-phenanthroline ligands, tetradentate 2,9-bis(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-BPPhen, L¹ ) and tridentate 2-(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-MPPhen, L² ), were synthesized and studied comparatively for their coordination with trivalent actinides and lanthanides. The complexation mechanisms of these two ligands toward trivalent f-block elements were thoroughly elucidated by NMR spectroscopy, UV/vis spectrophotometry, fluorescence spectrometry, single-crystal X-ray diffraction, solvent extraction, and theoretical calculation methods. NMR titration results demonstrated that 1 : 1 and 1 : 2 (metal to ligand) lanthanides complexes formed for L¹ , whereas 1 : 1, 1 : 2 and 1 : 3 lanthanide complexes formed for L² in methanol. The formation of these species was validated by fluorescence spectrometry, and the corresponding stability constants for the complexes of Nd^III with L¹ and L² were determined by using UV/vis spectrophotometry. Structures of the 10-coordinated 1 : 1-type complexes of EuL¹ (NO₃ )₃ and [EuL² (NO₃ )₃ (H₂ O)] Et₂ O in the solid state were characterized by X-ray crystallography. In solvent-extraction experiments, L¹ exhibited extremely strong extraction ability for both Am^III and Eu^III , whereas L² showed nearly no extraction toward Am^III or Eu^III due to its high hydrophilicity. Finally, the structures and bonding natures of the complex species formed between Am^III /Eu^III and L¹ /L² were analyzed in DFT calculations.

A review of the alpha radiolysis of extractants for actinide lanthanide separation in spent nuclear fuel reprocessing

Radiation stability is one of the key properties to enable the efficient use of extractants in spent nuclear fuel with high radioactivity. The last several decades have witnessed a rapid progress in the radiation chemistry of extractants. A variety of studies and reviews pertinent to the radiation stability of extractants have been published. However, a thorough summary for the alpha radiolysis results of extractants is not available. In this review, we survey the development of alpha radiolysis of extractants for actinide lanthanide separation and compare their radiolysis behaviors induced by alpha particles and gamma rays. The discussion of alpha radiolysis of extractants is divided into three parts according to the functional groups of extractants (i.e., phosphine oxide, amide and bis-triazinyl bipyridines). Given the importance of radiation source to carry out alpha irradiation experiment, we first give a brief introduction to three practicable alpha radiation sources including alpha emitting isotopes, helium ion beam and reactor. We hope this review will provide useful information and unleash a broad palette of opportunities for researchers interested in radiation chemistry.

Theoretical Study of Effects of Solvents, Ligands, and Anions on Separation of Trivalent Lanthanides and Actinides

Due to its associated low CO₂ emissions, nuclear energy production is rapidly growing. In this context, the treatment of high-level liquid waste (HLLW) of nuclear plants is of high concern to both scientific and industrial communities. Specifically, the separation of An(III) and Ln(III) cations when processing nuclear fuel is a vitally important, yet challenging, step within HLLW because An(III) and Ln(III) have similar chemical properties in solution. To guide the choice of relevant ligands, anions, and solvents for this separation step, in this work, we calculate and compare the free energy of formation of different Am(III) and Eu(III) complexes (which are typical and important An(III) and Ln(III) cation examples), involving two different ligands and three different counter ions in four different solvents. Based on our calculations, we predict that the chosen solvent is a key factor in the extraction of Am(III) and Eu(III) in treatment of HLLW. This study supports a systematic, computation-assisted screening of solvents and extractive ligands with counter anions as a proficient method to optimize the separation of Ln(III) and An(III).

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

Phenanthroline-diamide ligands have been reported in the selective separation of actinides over Eu(III); on the contrary, relevant basic coordination chemistry studies are still limited, and extraction under actual application conditions is rarely involved. In this work, N,N'-diethyl-N,N'-ditolyl-2,9-diamide-1,10-phenanthroline [Et-Tol-DAPhen (L)] was applied to explore the coordination performance of lanthanides in simulative high-level liquid waste. For the first time, cascade countercurrent extraction was conducted with Et-Tol-DAPhen as the extractant, which reveals the periodic tendency of the extraction efficiency of lanthanides to decrease gradually as the atomic number increases. Comparison of elements with similar radii verifies the hypothesis that the increase in the atomic number leads to a decrease in the ionic radius, thus reducing the coordination and extraction capacity of ligands. Slope analysis, electrospray ionization mass spectrometry, and ultraviolet-visible titration results show that the ligand forms 1:1 and 1:2 complexes with lanthanides and the coordination ability follows the tendency of extraction efficiency, and the first crystal structures of Lns(III) with a phenanthroline-diamide ligand, i.e., [LaL(NO₃)₃(H₂O)] and [LaL₂(NO₃)₂][(NO₃)], were obtained, which confirms the conclusions described above. This work promises to enhance our comprehension of the chemical properties of Lns(III) and offer new clues for the design and synthesis of novel separation ligands.

Design of phosphoryl containing podands with Li+/Na+ selectivity using machine learning

In this work we demonstrated, that machine learning opens a way for real design of ligands with required metal ion selectivity. We performed the ensemble QSPR modelling of the Li⁺/Na⁺ complexation selectivity and the stability constants for the Li⁺L and Na⁺L complexes of phosphoryl podands in nonaqueous solvent THF/СНCl₃ (4:1 v/v). The models were built and cross-validated using MLR with the ISIDA QSPR program and SVM with the libSVM package. The program SVMsmf was implemented to fulfil an ensemble modelling using libSVM and the Substructural Molecular Fragments (SMF) descriptors. SMF were used as descriptors for the ensemble modelling, properties predictions by consensus models and design of combinatorial library of new ligands. SMF such as the P=O group, the ether and P=O groups bound through the aromatic ring contribute significantly to the Li⁺/Na⁺ selectivity. The developed models were applied for the prediction of the studied properties for a focused virtual library of 3057 phosphoryl podands generated using SMF contributions promising for selective binding of lithium. Consensus models selected hits for a synthesis by combinatorial library screening. Among the constructed selective ligands - hits, three new podands were synthesized, for which the experimentally estimated selectivity is in satisfactory agreement with that predicted.

Bis-(1,2,4-triazin-3-yl) ligand structure driven selectivity reversal between Am3+ and Cm3+: solvent extraction and DFT studies

Selectivity between Am3+ and Cm3+ was investigated after their aqueous complexation with three structurally tailored hydrophilic bis-(1,2,4-triazin-3-yl) ligands followed by their extraction with N,N,N'N'-tetraoctyl diglycolamide (TODGA) dissolved in an ionic liquid (C4mim·Tf2N). The three hydrophilic ligands used were SO3PhBTP, SO3PhBTBP, and SO3PhBTPhen. It was evident from the solvent extraction studies that SO3PhBTP formed a stronger complex with Cm3+ than with Am3+, but SO3PhBTPhen showed better complexation ability for Am3+ than for Cm3+, and SO3PhBTBP showed no selectivity for the two actinide ions. DFT calculations indicated that the coordinating 'N' atoms in BTP were more co-planar in the complex and this co-planarity was higher in the Cm3+ complex as compared to that in Am3+. In the case of BTBP and BTPhen ligands, on the other hand, the co-planarity was more pronounced in the Am3+ complexes. Mayer's bond order calculations of M-N bonds in the complexes also indicated a reversal of the complexation ability of the BTP and BTPhen ligands for Am3+ and Cm3+. Calculations of the complexation energies further supported the higher selectivity of the BTP ligand for Am3+ by -52.0 kJ mol-1, and better selectivity of the BTPhen ligand for Cm3+ by -24.7 kJ mol-1.

Influence of a N-Heterocyclic Core on the Binding Capability of N,O-Hybrid Diamide Ligands toward Trivalent Lanthanides and Actinides

N,O-hybrid diamide ligands with N-heterocyclic skeletons are one of the promising extractants for the selective separation of actinides over lanthanides in a highly acidic HNO₃ solution. In this work, three hard-soft donor mixed diamide ligands, pyridine-2,6-diylbis(pyrrolidin-1-ylmethanone) (Pyr-PyDA), 2,2'-bipyridine-6,6'-diylbis(pyr-rolidine-1-ylmethanone) (Pyr-BPyDA), and (1,10-phenanthroline-2,9-diyl)bis(pyrrolidin-1-ylmethanone) (Pyr-DAPhen), were synthesized and used to probe the influence of N-heterocyclic cores on the complexation and extraction behaviors with trivalent lanthanides and actinides. ¹H NMR titration experiments demonstrated that 1:1 metal-to-ligand complexes were mainly formed between the three ligands and lanthanides, but 1:2 type complexes were also formed between tridentate Pyr-PyDA and Lu(III). The stability constants (log β) of these three ligands with two typical lanthanides, Nd(III) and Eu(III), were determined through spectrophotometric titration. It is found that Pyr-DAPhen formed the most stable complexes, while Pyr-PyDA formed the most unstable complexes with lanthanides, which coincided well with the following solvent extraction results. The solid-state structures of 1:1 type complexes of these three ligands with La(III), Nd(III), and Er(III) in nitrate media were identified by a single-crystal X-ray diffraction technique. Nd(III) and Er(III) were 10-coordinated with Pyr-PyDA, Pyr-BPyDA, and Pyr-DAPhen via one ligand molecule and three nitrate ions. La(III), because of its larger ionic radius, was 11-coordinated with Pyr-DAPhen through one ligand molecule, three nitrate ions, and one methanol molecule. Solvent extraction experiments showed that the preorganized phenanthroline-derived Pyr-DAPhen had the best extraction performance for trivalent actinide among the three ligands tested. This work provides some experimental insights into the design of more efficient ligands for trivalent actinide separation by adjusting the N-heterocyclic cores.

Substituent Effect on the Selective Separation and Complexation of Trivalent Americium and Lanthanides by N,O-Hybrid 2,9-Diamide-1,10-phenanthroline Ligands in Ionic Liquid

The extraction and complexation of trivalent americium (Am) and lanthanides (Ln) by four 2,9-diamide-1,10-phenanthroline (DAPhen) ligands with different alkyl substituent groups on the diamide moiety in an ionic liquid (IL), C₄mimNTf₂, were studied through a combination of batch extraction, spectroscopic, and calorimetric approaches. All four DAPhen ligands can achieve selective separation of Am(III) from Eu(III), but the detailed extractability and the extraction kinetics are affected significantly by the length of the alkyl substituent groups. UV-vis absorption spectrophotometric titrations indicate that Ln(III) coordinates with all four ligands in a 1:2 mode in the ionic liquid and the binding strength decreases with the increase of the alkyl chain length. The complexation of the DAPhen ligands with Ln(III) in the ionic liquid is driven by highly positive entropies and opposed by endothermic enthalpies. A luminescence spectroscopy study suggests that each DAPhen ligand coordinates in a tetradentate form with Eu(III). This work further unravels the unique extraction and coordination behavior in an ionic liquid system and offers additional guidelines to design more efficient DAPhen ligands for Ln(III)/An(III) separation.

Coordination of trivalent lanthanum and cerium, and tetravalent cerium and actinides (An = Th(IV), U(IV), Np(IV)) by a 4-phosphoryl 1H-pyrazol-5-olate ligand in solution and the solid state

Structural investigations of three actinide(iv) 4-phosphoryl 1H-pyrazol-5-olate complexes (An = Th(iv), U(iv), Np(iv)) and their cerium(iv) analogue display the same metal coordination in the solid state. The mononuclear complexes show the metal centre in a square antiprismatic coordination geometry composed by the two O-donor atoms of four deprotonated ligands. Detailed solid state analysis of the U(iv) complex shows that dependent on the solvent used altered arrangements are observable, resulting in a change in the coordination polyhedron of the U(iv) metal centre to bi-capped trigonal prismatic. Further, single crystal analyses of the La(iii) and Ce(iii) complexes show that the ligand can also act as a neutral ligand by protonation of the pyrazolyl moiety. All complexes were comprehensively characterized by NMR, IR and Raman spectroscopy. A single resonance in each of the 31P NMR spectra for the La(iii), Ce(iii), Ce(iv), Th(iv) and Np(iv) complex indicates the formation of highly symmetric complex species in solution. Extended X-ray absorption fine structure (EXAFS) investigations provide evidence for the same local structure of the U(iv) and Np(iv) complex in toluene solution, confirming the observations made in the solid state.