Synthesis and Evaluation of Lipophilic BTBP Ligands for An/Ln Separation in Nuclear Waste Treatment: The Effect of Alkyl Substitution on Extraction Properties and Implications for Ligand Design

Theoretical investigations into the bonding and separation properties of non-rigid, partially rigid, and rigid ligands derived from Et-Tol-PTA with trivalent lanthanides and actinides

The separation of trivalent actinide elements from lanthanide elements represents one of the most formidable challenges within the context of nuclear waste partitioning and transmutation (P&T) processes. Consequently, we embarked on a systematic investigation aimed at elucidating the bonding properties and thermodynamic behavior of a N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA) ligand in conjunction with trivalent actinide and lanthanide elements. This investigation involved the utilization of various density functional theory (DFT) methods and a comparative analysis between small-core pseudopotential basis sets and all-electron basis sets. It was found that well-performing results were achieved using the PBE0 functional in both bond length and thermodynamic energy calculations, with minimal impact being exerted by the basis set on the results. Furthermore, an exploration was carried out into the bonding and thermodynamic properties of trivalent actinides and lanthanides with ligands derived from Et-Tol-PTA, encompassing non-rigid (La), partially rigid (Lb, Lc), and rigid (Ld) ligands. Thermodynamically, advantages in the separation of Am(III)/Eu(III) were exhibited by Lb and Lc ligands, while excellent performance in the separation of Am(III)/Cm(III) was demonstrated by the La ligand. Analyses conducted using quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), and natural bond orbital (NBO) methodologies revealed the presence of partial covalent character in the bonds between oxygen (O) and metal (M), as well as between nitrogen (N) and metal (M), with a higher degree of covalent character being observed in O-Am and N-Am bonds compared to O-Cm/Eu and N-Cm/Eu interactions.

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.

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.

Deep Eutectic Solvents: Promising Co-solvents to Improve the Extraction Kinetics of CyMe4-BTBP

In this communication, we report on the use of deep eutectic solvents (DESs) for processing nuclear waste, with a view to selectively recovering minor actinides (MA) from highly active raffinate solutions. DESs are an interesting new class of green and eco-sustainable solvents. Herein, a representative family of DES was tested as a co-solvent for MA/lanthanides partitioning based on Selective ActiNide EXtraction (SANEX)-like hydrometallurgical processes. The reference system exploits the CyMe₄-BTBP lipophilic extractant for selective MA recovery, but the slow kinetics is the main limitation toward the industrial implementation. A selection of hydrophilic DESs has been proposed as a phase transfer catalyst and tested to improve the process performances. In this work, the radiochemical stability and the extraction behavior of these DESs have been ascertained. Moreover, a preliminary optimization of system composition has been achieved. This study underlines a catalytic effect of DES that can be proficiently exploited to enhance CyMe₄-BTBP extraction and selectivity.

Impact of Solvent Polarity on the Ligand Configuration in Tetravalent Thorium N-Donor Complexes

A combined NMR spectroscopic and theoretical study on the complexation of diamagnetic Th(IV) with 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP) was performed. Different ligand configurations were observed for [Th(nPr-BTP)₃]⁴⁺ complexes depending on the solvent's ability to actively form hydrogen bonds. In polar aprotic solvents, a complex is observed, which is isostructural with [M(nPr-BTP)₃]³⁺ (M = Am, Ln) complexes studied earlier. In contrast, ¹H, ¹³C, and ¹⁵N NMR spectra recorded in polar protic solvents showed twice as many signals, indicating a breakdown of symmetry. Supported by density functional theory (DFT) calculations, this difference is explained by the solvent effect on the steric arrangement of the propyl moieties located on the triazine rings. Important information on bonding properties was obtained by ¹⁵N NMR. In contrast to the respective Am(III) complex showing a significant covalent contribution, the Th(IV)-BTP interaction is mainly electrostatic.

Stronger Hydration of Eu(III) Impedes Its Competition against Am(III) in Binding with N-donor Extractants

The significance of understanding the interaction between actinide(III)/lanthanide(III) (An(III)/Ln(III)) and N-donor extractants lies in the importance of efficient An³⁺/Ln³⁺ separation in advanced nuclear fuel cycles and the high expectation of the application of N-donor extractants. This work reports a density functional theory study aiming at a plausible explanation of the origin of the selectivity of the ligands in An³⁺/Ln³⁺ separation and an evaluation of the influence of the bridging groups of typical N-donor extractants. Five bis(triazine) N-donor ligands were considered, differing in their denticity dictated by their bridging groups and in the flexibility of these bridging groups. The results showed much stronger hydration of Eu(III) in comparison to Am(III) in the ligand exchange of aqua ligands by N-donor ligands, while there was a moderate difference in their interaction strengths with the N-donor ligands. This implicated that the distinct difficulty in desolvating Eu(III) and Am(III) may govern their selectivity in liquid-liquid extraction. The analysis of the role of the bridging groups of the ligands confirmed the importance of a ligand to be equipped with preorganized binding sites to minimize the perturbation of entropy. We tentatively propose that this conclusion may hold in the explanation of the low selectivity of oxygenated extractants and the high selectivity of extractants with soft donors in An³⁺/Ln³⁺ separation.

Effect of Heterocyclic Ring on LnIII Coordination, Luminescence and Extraction of Diamides of 2,2'-Bipyridyl-6,6'-Dicarboxylic Acid

We have synthesized and examined several complexes of lanthanides with diamides of 2,2′-bipyridyl-6,6′-dicarboxylic acid bearing various hetaryl-based side chains for the elucidation of the effect of the heterocycle on the structure and properties of the ligands. The multigram scale methods for the preparation of various N-alkyl-hetaryls and their diamides were developed. The solid state structure of 6-methyl-2-pyridylamide of 2,2′-bipyridyl-6,6′-dicarboxylic acid possesses a flat structure where the conformation is completely different from that previously observed for N-alkylated 2,2′-bipyridyl-6,6′-dicarboxamides and 2,6-pyridinedicarboxamides. The complexes of new ligands were synthesized and NMR and X-Ray studied their structure in solution and solid state. The results demonstrate that complexes possess the same structures both in solid state and in solution. Stability constants of the complexes were less when comparing with dimethyl-substituted diamides, but higher than for unsubstituted dianilide. Contrarily, the extraction ability of 2-pyridyl-diamide is significantly lower than for corresponding anilide. Specific interaction of extractant with solvent molecules, which is not available for electron-sink pyridine amides, can explain this. The luminescence of new Eu complexes was significantly higher than for all previously 2,2′-bipyridyl-6,6′-dicarboxamides and QY reaches 18%. Asymmetry ratios of Eu complexes were 25% higher when compared other complexes with 2,2′-bipyridyl-6,6′-dicarboxamides, which indicates large deviation from the inversion center.

Exploring the Subtle Effect of Aliphatic Ring Size on Minor Actinide-Extraction Properties and Metal Ion Speciation in Bis-1,2,4-Triazine Ligands

The synthesis and evaluation of three novel bis-1,2,4-triazine ligands containing five-membered aliphatic rings are reported. Compared to the more hydrophobic ligands 1-3 containing six-membered aliphatic rings, the distribution ratios for relevant f-block metal ions were approximately one order of magnitude lower in each case. Ligand 10 showed an efficient, selective and rapid separation of Am^III and Cm^III from nitric acid. The speciation of the ligands with trivalent f-block metal ions was probed using NMR titrations and competition experiments, time-resolved laser fluorescence spectroscopy and X-ray crystallography. While the tetradentate ligands 8 and 10 formed Ln^III complexes of the same stoichiometry as their more hydrophobic analogues 2 and 3, significant differences in speciation were observed between the two classes of ligand, with a lower percentage of the extracted 1:2 complexes being formed for ligands 8 and 10. The structures of the solid state 1:1 and 1:2 complexes formed by 8 and 10 with Y^III , Lu^III and Pr^III are very similar to those formed by 2 and 3 with Ln^III . Ligand 10 forms Cm^III and Eu^III 1:2 complexes that are thermodynamically less stable than those formed by ligand 3, suggesting that less hydrophobic ligands form less stable An^III complexes. Thus, it has been shown for the first time how tuning the cyclic aliphatic part of these ligands leads to subtle changes in their metal ion speciation, complex stability and metal extraction affinity.

Time-resolved and steady-state irradiation of hydrophilic sulfonated bis-triazinyl-(bi)pyridines - modelling radiolytic degradation

Efficient separation of the actinides from the lanthanides is a critical challenge in the development of a more sophisticated spent nuclear fuel recycling process. Based upon the slight differences in f-orbital distribution, a new class of soft nitrogen-donor ligands, the sulfonated bis-triazinyl-(bi)pyridines, has been identified and shown to be successful for this separation under anticipated, large-scale treatment conditions. The radiation robustness of these ligands is key to their implementation; however, current stability studies have yielded conflicting results. Here we report on the radiolytic degradation of the sulfonated 2,6-bis(1,2,4-triazin-3-yl)pyridine (BTP(S)) and 6,6'-bis(1,2,4-triazin-3-yl)-2,2'-bipyridine (BTBP(S)) in aerated, aqueous solutions using a combination of time-resolved pulsed electron techniques to ascertain their reaction kinetics with key aqueous radiolysis products (eaq-, H˙, ˙OH, and ˙NO3), and steady state gamma radiolysis in conjunction with liquid chromatography for identification and quantification of both ligands as a function of absorbed dose. These data were used to construct a predictive deterministic model to provide critical insight into the fundamental radiolysis mechanisms responsible for the ligands' radiolytic stability. The first-order decays of BTP(S) and BTBP(S) are predominantly driven by oxidative processes (˙OH and, to a lesser extent, H2O2), for which calculations demonstrate that the rate of degradation is inhibited by the formation of ligand degradation products that undergo secondary reactions with the primary products of water radiolysis. Overall, BTP(S) is ∼20% more radiolytically stable than BTBP(S), but over 90% of either ligand is consumed within 1 kGy.

An overview of eight- and nine-coordinate N-donor solvated lanthanoid(III) and actinoid(III) ions

The use of replacement lanthanoid ions in actinoid chemistry is commonplace, which requires a full understanding of the similarities and differences between the two series. This overview lists, compares and discusses the available crystallographic data for N-donors for the lanthanoids and the actinoids using their trivalent state as a natural starting point for comparison.

Design, synthesis, and evaluation of nickel dipyridylmethane complexes for Coordination-Induced Spin State Switching (CISSS)

The coordination of pyridine to a nickel(ii) dipyridylmethane complex changes the spin state.

Preparation and characterization of a novel macroporous silica-bipyridine asymmetric multidentate functional adsorbent and its application for heavy metal palladium removal

The effective removal of heavy metal ¹⁰⁷Pd(II) from highly active liquid waste (HLW) is very valuable for reducing its hazardous and risk to public health and environment. For this purpose, a novel silica-bipyridine multidentate functional adsorbent was synthesized by vacuum infusing a new asymmetric N-donor ligand CA-MTBP (bipyridine derivative) into the macroporous SiO₂-P support. SEM, N₂ adsorption-desorption isotherms, TGA, XRD, FT-IR, ²⁹Si solid-state NMR and XPS spectroscopy were utilized to systematically characterize the physicochemical properties of the adsorbent. The characterization results indicated that CA-MTBP was successfully immobilized onto the pores of SiO₂-P by intermolecular interaction. Strong hydrogen-bonding interactions identified by single crystal structure of the ligand and ²⁹Si NMR may play a key role in achieving this immobilization. TGA and TOC studies showed that CA-MTBP/SiO₂-P had excellent thermal stability and highly HNO₃ resistance. EDS and XPS investigations provided directly evidences for Pd(II) being selectively adsorbed onto the adsorbent. The adsorbent had excellent adsorption capability, fast adsorption kinetics and high selectivity for Pd(II) over other typical tested metals in HNO₃ media.

Extraction of minor actinides, lanthanides and other fission products by silica-immobilized BTBP/BTPhen ligands

Novel BTBP [bis-(1,2,4-triazin-3-yl)-2,2′-bipyridine]/BTPhen [bis-(1,2,4-triazin-3-yl)-1,10-phenanthroline] functionalized silica gels have been developed to extract minor actinides, lanthanides and other fission products.

Selective separation of trivalent f-ions using 1,10-phenanthroline-2,9-dicarboxamide ligands in ionic liquids

1,10-Phenanthroline-2,9-dicarboxamide complexants decorated with alkyl chains and imidazolium cations have been studied for extraction of trivalent f-ions into imidazolium ionic liquids.

Separation and complexation of palladium(ii) with a new soft N-donor ligand 6,6′-bis(5,6-dinonyl-1,2,4-triazin-3-yl)-2,2′-bipyridine (C9-BTBP) in nitric acid medium

Both 1 : 1 and 2 : 1 Pd–C9-BTBP complexes were found in solution by ¹H NMR titration.

Pd-Catalyzed Diamination of 1,2,4-Triazinyl Complexant Scaffolds

As part of ongoing efforts in this laboratory to design and synthesize multidentate soft-N-donors as effective complexants for chemoselective minor actinide extraction from used nuclear fuel, a series of aminated mono-1,2,4-triazinylpyridines were required. This study focuses on streamlining convergent access to a diverse array of functionalized N-donors using Pd-catalysis from a common synthon affording access to pyridinyl triazines as the 4,4'-amino derivatives which are commercially limited and unsuccessful in traditional condensation chemistry. A general Pd-catalyzed method for the double amination of functionalized pyridinyl-1,2,4-triazines with low catalyst/ligand loadings enabling the formation of 16 novel complexants is presented.

Hydrophilic sulfonated bis-1,2,4-triazine ligands are highly effective reagents for separating actinides(iii) from lanthanides(iii) via selective formation of aqueous actinide complexes

Tetrasulfonated bis-1,2,4-triazine ligands can selectively complex and separate actinides from lanthanides in aqueous nitric acid with very high selectivities.

We report the first examples of hydrophilic 6,6′-bis(1,2,4-triazin-3-yl)-2,2′-bipyridine (BTBP) and 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen) ligands, and their applications as actinide(iii) selective aqueous complexing agents. The combination of a hydrophobic diamide ligand in the organic phase and a hydrophilic tetrasulfonated bis-triazine ligand in the aqueous phase is able to separate Am(iii) from Eu(iii) by selective Am(iii) complex formation across a range of nitric acid concentrations with very high selectivities, and without the use of buffers. In contrast, disulfonated bis-triazine ligands are unable to separate Am(iii) from Eu(iii) in this system. The greater ability of the tetrasulfonated ligands to retain Am(iii) selectively in the aqueous phase than the corresponding disulfonated ligands appears to be due to the higher aqueous solubilities of the complexes of the tetrasulfonated ligands with Am(iii). The selectivities for Am(iii) complexation observed with hydrophilic tetrasulfonated bis-triazine ligands are in many cases far higher than those found with the polyaminocarboxylate ligands previously used as actinide-selective complexing agents, and are comparable to those found with the parent hydrophobic bis-triazine ligands. Thus we demonstrate a feasible alternative method to separate actinides from lanthanides than the widely studied approach of selective actinide extraction with hydrophobic bis-1,2,4-triazine ligands such as CyMe₄-BTBP and CyMe₄-BTPhen.

Europium, uranyl, and thorium-phenanthroline amide complexes in acetonitrile solution: an ESI-MS and DFT combined investigation

ESI-MS and density functional theory (DFT) methods were combined to elucidate the complexation mechanisms of tetradentate phenanthroline amide ligand with Eu(iii), U(vi), and Th(iv) in an acetonitrile solution.

Effective separation of Am(iii) and Eu(iii) from HNO3 solutions using CyMe4-BTPhen-functionalized silica-coated magnetic nanoparticles

It has been shown that CyMe₄-BTPhen-functionalized silica-coated maghemite (γ-Fe₂O₃) magnetic nanoparticles (MNPs) are capable of quantitative separation of Am(iii) from Eu(iii) from HNO₃ solutions.

Extraction mechanism and γ-radiation effect on the removal of Eu3+ by a novel BTPhen/[Cnmim][NTf2] system in the presence of nitric acid

The extraction mechanism of Eu³⁺ by using BTPhen/[C_nmim][NTf₂] (n = 2, 4, 8) was investigated in the presence of nitric acid, and the new extraction system shows high radiation stability.

Complexation of lanthanides, actinides and transition metal cations with a 6-(1,2,4-triazin-3-yl)-2,2':6',2''-terpyridine ligand: implications for actinide(III)/lanthanide(III) partitioning

The quadridentate N-heterocyclic ligand 6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2' : 6',2''-terpyridine (CyMe(4)-hemi-BTBP) has been synthesized and its interactions with Am(III), U(VI), Ln(III) and some transition metal cations have been evaluated by X-ray crystallographic analysis, Am(III)/Eu(III) solvent extraction experiments, UV absorption spectrophotometry, NMR studies and ESI-MS. Structures of 1:1 complexes with Eu(III), Ce(III) and the linear uranyl (UO(2)(2+)) ion were obtained by X-ray crystallographic analysis, and they showed similar coordination behavior to related BTBP complexes. In methanol, the stability constants of the Ln(III) complexes are slightly lower than those of the analogous quadridentate bis-triazine BTBP ligands, while the stability constant for the Yb(III) complex is higher. (1)H NMR titrations and ESI-MS with lanthanide nitrates showed that the ligand forms only 1:1 complexes with Eu(III), Ce(III) and Yb(III), while both 1:1 and 1:2 complexes were formed with La(III) and Y(III) in acetonitrile. A mixture of isomeric chiral 2:2 helical complexes was formed with Cu(I), with a slight preference (1.4:1) for a single directional isomer. In contrast, a 1:1 complex was observed with the larger Ag(I) ion. The ligand was unable to extract Am(III) or Eu(III) from nitric acid solutions into 1-octanol, except in the presence of a synergist at low acidity. The results show that the presence of two outer 1,2,4-triazine rings is required for the efficient extraction and separation of An(III) from Ln(III) by quadridentate N-donor ligands.