Highly Efficient N-Pivot Tripodal Diglycolamide Ligands for Trivalent f-Cations: Synthesis, Extraction, Spectroscopy, and Density Functional Theory Studies

A series of four N-pivot tripodal diglycolamide (DGA) ligands, where three DGA moieties are attached to the central N atom via spacers of different lengths and with varying alkyl substituents on the amidic nitrogen of DGA (L_I-L_IV), were studied for their extraction and complexation ability toward trivalent lanthanide/actinide ions, including solvent extraction, complexation using spectrophotometric titrations, and luminescence spectroscopic studies. Introduction of a methyl group on the amidic nitrogen atom gives rise to a 400 fold increase of the Eu distribution ( D) value [L_III (NMe) vs L_II (NH)] at 1 M HNO₃. Enlargement of the spacer length between the pivotal N atom and the DGA moieties with one carbon atom results in a 14 times higher D_Eu value [L_I (C3) vs L_II (C2)]. Slope analyses showed that Eu³⁺ was extracted as a bis-solvated species with all four ligands. The compositions of the Eu³⁺/L complexes were further confirmed by spectroscopic measurements, its formation constants following the order: L_III > L_IV > L_I > L_II. Luminescence spectroscopy and electrospray ionization mass spectrometry revealed that all four ligands form [Eu(L)₂(NO₃)₃] complexes. Density functional theory and thermodynamic parameters corroborated the existence of [Eu(L)₂(NO₃)₃] complexes.

Crystal structures of two bis-carbamoyl-methyl-phosphine oxide (CMPO) compounds

Two bis-carbamoyl-methyl-phosphine oxide compounds, namely {[(3-{[2-(di-phen-yl-phosphino-yl)ethanamido]-meth-yl}benz-yl)carbamo-yl]meth-yl}di-phenyl-phos-phine oxide, C36H34N2O4P2, (I), and diethyl [({2-[2-(di-eth-oxy-phosphino-yl)ethanamido]-eth-yl}carbamo-yl)meth-yl]phospho-nate, C14H30N2O8P2, (II), were synthesized via nucleophilic acyl substitution reactions between an ester and a primary amine. Hydrogen-bonding inter-actions are present in both crystals, but these inter-actions are intra-molecular in the case of compound (I) and inter-molecular in compound (II). Intra-molecular π-π stacking inter-actions are also present in the crystal of compound (I) with a centroid-centroid distance of 3.9479 (12) Å and a dihedral angle of 9.56 (12)°. Inter-molecular C-H⋯π inter-actions [C⋯centroid distance of 3.622 (2) Å, C-H⋯centroid angle of 146°] give rise to supra-molecular sheets that lie in the ab plane. Key geometric features for compound (I) involve a nearly planar, trans-amide group with a C-N-C-C torsion angle of 169.12 (17)°, and a torsion angle of -108.39 (15)° between the phosphine oxide phospho-rus atom and the amide nitro-gen atom. For compound (II), the electron density corresponding to the phosphoryl group was disordered, and was modeled as two parts with a 0.7387 (19):0.2613 (19) occupancy ratio. Compound (II) also boasts a trans-amide group that approaches planarity with a C-N-C-C torsion angle of -176.50 (16)°. The hydrogen bonds in this structure are inter-molecular, with a D⋯A distance of 2.883 (2) Å and a D-H⋯A angle of 175.0 (18)° between the amide hydrogen atom and the P=O oxygen atom. These non-covalent inter-actions create ribbons that run along the b-axis direction.

Novel diamide ligands with a central carbonyl group and their comparative evaluation with the diglycolamide ligand: synthesis, extraction, DFT and chromatographic studies

The extraction behaviour of U(VI), Th(IV) and Nd(III) was investigated as a function of nitric acid concentration for diamide based extractants, namely, N,N,N′,N′-tetraoctyl-3-carbonylpentanediamide (TOCPDA) and 4-carbonyl-heptanedioic acid bis-dioctylamide (CHADA). In addition, the distribution ratio was also measured for Pu(IV) and Sr(II) with 1.1 M CHADA in n-dodecane. These extractants were synthesized by adopting simple acid, amine coupling reaction with DCC (dicyclohexylcarbodiimide) and DMAP (N,N′-dimethylaminopyridine) as the coupling agent. The newly synthesized extractants were characterized by FT-IR, NMR, Mass, CHNS and HPLC. The extraction results indicated that CHADA shown has better extraction behavior for U(VI) compared to TOCPDA. In addition, CHADA coated HPLC column was examined for the retention behaviour of U(VI), Th(IV), and Nd(III). Computation studies based on density functional theory (DFT) were carried out to understand the complexing behaviour of U(VI), Pu(IV) and Sr(II) with CHADMA and TMCPDA.

Unprecedented Inversion of Selectivity and Extraordinary Difference in the Complexation of Trivalent f Elements by Diastereomers of a Methylated Diglycolamide

When considering f elements, solvent extraction is primarily used for the removal of lanthanides from ore and their recycling, as well as for the separation of actinides from used nuclear fuel. Understanding the complexation mechanism of metal ions with organic extractants, particularly the influence of their molecular structure on complex formation is of fundamental importance. Herein, we report an extraordinary (up to two orders of magnitude) change in the extraction efficiency of f elements with two diastereomers of dimethyl tetraoctyl diglycolamide (Me₂ -TODGA), which only differ in the orientation of a single methyl group. Solvent extraction techniques, extended X-ray absorption fine structure (EXAFS) measurements, and density functional theory (DFT) based ab initio calculations were used to understand their complex structures and to explain their complexation mechanism. We show that the huge differences observed in extraction selectivity results from a small change in the complexation of nitrate counter-ions caused by the different orientation of one methyl group in the backbone of the extractant. The obtained results give a significant new insight into metal-ligand complexation mechanisms, which will promote the development of more efficient separation techniques.

Electrochemical oxidation of trivalent americium using a dipyrazinylpyridine modified ITO electrode

We present here the electrochemical oxidation of Am(iii) to AmVO2+ and AmVIO22+ in pH 1 nitric acid using a mesoporous tin-doped indium oxide electrode modified with a covalently attached dipyrazinylpyridine ligand. The applied potential affects the distribution of Am oxidation products. At potential 1.8 V, only Am(v) is observed, while increasing the potential to as much as 2.0 V, results in oxidation of Am(iii) to Am(v) and subsequent oxidation of Am(v) to Am(vi). At applied potentials >2.0 V, Am(iii) is oxidized to Am(v), while Am(vi) is reduced to Am(v). The latter reduction reaction is likely due to the increased rate of hydrogen peroxide formation from the 2-electron oxidation of water at the electrode at these high potentials. The development of future ligand modified electrodes for actinide oxidations must consider how they facilitate Am oxidations while disfavoring unwanted or competing reactions.

Separation of trivalent actinides and lanthanides using various ‘N’, ‘S’ and mixed ‘N,O’ donor ligands: a review

Separation of trivalent actinide (An) and lanthanide (Ln) elements is one of the burning topics in the back end of the nuclear fuel cycle due to the similarity in their chemical behaviour. A significant amount of research is being carried out worldwide to develop suitable ligands for the separation of the trivalent actinides and lanthanides. Some of the research groups are engaged in continuous improvement of the di-ethylene-triamine-penta acetic acid (DTPA) based Ln/An separation method, whereas extensive research is going on for the development of the lipophilic and hydrophilic ‘N’ donor heteropolycyclic ligands as the actinide selective ligand. A number of ‘S’ donor ligands are also explored for the Ln/An separation. In the present review, we made an attempt to highlight various separation processes based on soft donor ligands developed for Ln/An separations. Beside the conventional solvent extraction processes, separation possibilities membrane based and solid phase extraction techniques are evaluated for the Ln/An separation and are compiled in the present review.

Binding affinity of pyridines with AmIII/CmIII elucidated by density functional theory calculations

In recent decades, N-heterocyclic ligands have been extensively used in the separation of lanthanides/actinides, whereas the selective extraction of amercium or curium has been very challenging. Using density functional theory calculations, this study is devoted to the investigation of the binding affinity of a series of modelling pyridine ligands with Am^III and Cm^III. The structure-property correlations between the amercium and curium systems and the binding affinity were obtained, and promising strategies for efficient separation of Am^III/Cm^III were proposed.

Unfolding the complexation and extraction of Am3+ and Eu3+ using N-heterocyclic aromatic diphosphonic acids: a combined experimental and DFT study

Extraction behaviour of Am³⁺ with three ‘N,O’-donor N-heterocyclic aromatic diphosphonates is explained using spectroscopic and DFT studies.

A first phosphine oxide-based extractant with high Am/Cm selectivity

A new phosphine oxide ligand demonstrates high selectivity for the Am–Cm pair with SF = 2.9–3.5 and the Am–Eu pair with SF = 7.3–8.5 in the range of 0.1–3 M nitric acid.

Pillararenes as macrocyclic hosts: a rising star in metal ion separation

Pillararenes are macrocyclic oligomers of alkoxybenzene akin to calixarenes but tethered at the 2,5-positions via methylene bridges. Benefiting from their unique pillar-shaped architecture favorable for diverse functionalization and versatile host-guest properties, pillararenes decorated with chelating groups worked excellently as supporting platforms to construct extractants or adsorbents for metal ion separation. This feature article provides a detailed summary of pillararenes in Ln/An separation by liquid-liquid extraction and heavy metal separation by solid-liquid extraction. The preorganization effect of the rigid pillararene framework has a profound impact on the extraction of metal ions, and a unique extraction mechanism is observed when employing ionic liquids as solvents. The rich host-guest chemistry of pillararenes enables construction of a wide variety of supramolecular materials as metal ion adsorbents. We also discuss the differences between pillararenes and several well-known macrocycles, with a focus on the metal-ligand coordination and its influencing factors. We hope this review will provide useful information and unleash new opportunities in this field.

Coordination of Nd(iii) and Eu(iii) with monodentate organophosphorus ligands in ionic liquids: spectroscopy and thermodynamics

The thermodynamics and coordination nature of the complexes of Ln(iii) with three monodentate organophosphorus ligands in ionic liquids have been elucidated and illustrated by spectroscopic and calorimetric techniques.

Evaluation of two aza-crown ether-based multiple diglycolamide-containing ligands for complexation with the tetravalent actinide ions Np4+ and Pu4+: extraction and DFT studies

Two multiple diglycolamide (DGA)-containing extractants where the DGA arms are tethered to the nitrogen atoms of two aza-crown ether scaffolds, a 9-membered aza-crown ether containing three ‘N’ atoms (L_I) and a 12-membered aza-crown ether containing four ‘N’ atoms (L_II), were evaluated for the extraction of the tetravalent actinide ions Np⁴⁺ and Pu⁴⁺. The tripodal ligand with three DGA arms (L_I) was relatively inferior in its metal ion extraction properties as compared to the tetrapodal ligand with four DGA arms (L_II) and Pu⁴⁺ ion was better extracted than Np⁴⁺ ion with both the ligands. A solvation extraction mechanism, where species of the type ML(NO₃)₄ are extracted, was found to be operative for both the ligands involving both the tetravalent actinide ions. While the extraction of the metal ions increased with the feed nitric acid concentration up to 4 M, a sharp decline in the extraction was seen after that. Quantitative extraction (>99%) of the actinide ions was observed with L_II from 4 M HNO₃, suggesting the possible application of the ligands for actinide partitioning of high-level waste. The structure and the composition of the complexes were optimized by DFT computations.

Two multiple diglycolamide (DGA)-containing aza-crown ether ether-based extractants were evaluated for the extraction of the tetravalent actinide ions Np⁴⁺ and Pu⁴⁺.

The structural evolution and tunable photoluminescence of f-element bearing coordination polymers of the 2,4,6-tri-α-pyridyl-1,3,5-triazine ligand

An attempt at expanding the family of f-element bearing TPTZ coordination polymers has resulted in fifteen new complexes with topologies that evolved along the periodic table and tunable photoluminescence properties.

Supramolecular ligands for the extraction of lanthanide and actinide ions

A selection of supramolecular ligands designed to extract f-elements.

Highly Efficient and Selective Dissolution Separation of Fission Products by an Ionic Liquid [Hbet][Tf2N]: A New Approach to Spent Nuclear Fuel Recycling

Here, we propose the use of carboxyl-functionalized ionic liquid, [Hbet][Tf₂N], to separate the fission products from spent nuclear fuels. This innovative method allows the selective dissolution of neutron poisons, lanthanides oxide, as well as some fission products with high yield, leaving most of the UO₂ matrix and minor actinides behind in the spent nuclear fuel and accomplishing the actinides recovery as a group. Water-saturated [Hbet][Tf₂N] can dissolve lanthanides oxide from simulated spent nuclear fuel with a dissolution ratio of 100% at 40 °C. However, the dissolution of uranium is almost negligible (<1%) under the same conditions. This big difference in dissolution provides a novel separation approach to spent nuclear fuel recycling and may open new perspectives for spent nuclear fuel reprocessing. The recovery of Nd and U from metal-loaded ionic liquids and the recyclability of the ionic liquid [Hbet][Tf₂N] have also been investigated. Furthermore, a U/ x value related to the lattice energy U of metal compound M _xO _y is used to elaborate the solubility. This work represents the first case for efficient fission products removal by selective dissolution, avoiding the complete dissolution of spent nuclear fuel, the producing of the large high-level radioactive waste, and reducing environmental hazards.

Effect of an alkyl substituent and spacer length in benzene-centered tripodal diglycolamides on the sequestration of minor actinides

Three benzene-centered tripodal diglycolamide (Bz-T-DGA) ligands, where diglycolamide (DGA) moieties are tethered to the central benzene ring through a methylene spacer and having either a hydrogen atom (LI) or an isopentyl group (LII) attached to the N-atom, and DGA moieties attached via an ethylene spacer and having an isopentyl group attached to the N-atom (LIII), were studied for their complexation and extraction abilities towards trivalent actinides and lanthanides. The distribution ratio of Am(iii) and Eu(iii) with 1 mmol L-1 ligand in 5% iso-decanol/n-dodecane followed the order: LII > LIII > LI. The substitution of the H atom with the isopentyl group on the N-atom of the DGA moieties resulted in two orders of magnitude enhancement in the extraction ability of the ligand. On the other hand, increase in the spacer length between the benzene ring and the DGA moieties resulted in several fold reduction in the extraction ability of the ligand. Spectroscopic studies with Eu3+ ions in acetonitrile also confirmed the metal/ligand complex formation constant in the order: LII > LIII > LI. Luminescence decay lifetimes of Eu3+/ligand complexes confirmed the absence of water molecules and that all the primary coordination sites of the metal ion are occupied by the ligands.

Recent Advances in the Separation of Rare Earth Elements Using Mesoporous Hybrid Materials

Over the past decades, the need for rare earth elements (REEs) has increased substantially, mostly because these elements are used as valuable additives in advanced technologies. However, the difference in ionic radius between neighboring REEs is small, which renders an efficient sized-based separation extremely challenging. Among different types of extraction methods, solid-phase extraction (SPE) is a promising candidate, featuring high enrichment factor, rapid adsorption kinetics, reduced solvent consumption and minimized waste generation. The great challenge remains yet to develop highly efficient and selective adsorbents for this process. In this regard, ordered mesoporous materials (OMMs) possess high specific surface area, tunable pore size, large pore volume, as well as stable and interconnected frameworks with active pore surfaces for functionalization. Such features meet the requirements for enhanced adsorbents, not only providing huge reactional interface and large surface capable of accommodating guest species, but also enabling the possibility of ion-specific binding for enrichment and separation purposes. This short personal account summarizes some of the recent advances in the use of porous hybrid materials as selective sorbents for REE separation and purification, with particular attention devoted to ordered mesoporous silica and carbon-based sorbents.

Quantum Chemistry Study on the Extraction of Trivalent Lanthanide Series by Cyanex301: Insights from Formation of Inner- and Outer-Sphere Complexes

The extraction of lanthanide series by Cyanex301, i.e., bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HC301), has been modeled by density functional theory calculation, taking into account the formation of both inner- and outer-sphere complexes. The inner-sphere complex Ln(C301)₃ and the outer-sphere complex Ln(H₂O)₉(C301)₃ are optimized, followed by the analysis of interaction energy, bond length, Laplacian bond orders, and Mulliken populations. The covalency degree increases in Ln-S and Ln-O bonds in the inner- and outer-sphere complexes, respectively, as the lanthanide series is traversed. Mulliken population analysis indicates the important role of the 5d-orbital participation in bonding in the formation of inner- and outer-sphere complexes. Two thermodynamic cycles regarding the formation of inner- and outer-sphere complexes are established to calculate the extraction Gibbs free energies (ΔG _extr), and relaxed potential energy surface scan is utilized to model the kinetic complexation of C301 anion with hydrated metal ions. Light lanthanides can form both inner- and outer-sphere complexes, whereas heavy lanthanides only form outer-sphere complexes in biphasic extraction. After adopting the data of forming inner-sphere complex for light Ln(III) and that of forming outer-sphere complexes for heavy Ln(III), the trend of the calculated -ΔG _extr agrees very well with that of the experimental distribution ratios on crossing the Ln(III) series. Results from this work help to theoretically understand the extraction behavior of Cyanex301 with respect to different Ln(III).

A diglycolamide-functionalized TREN-based dendrimer with a ‘crab-like’ grip for the complexation of actinides and lanthanides

TREN-G1-DGA, a multiple DGA ligand, was synthesized and evaluated for the extraction of actinides and fission product ions.

Synthesis and radiometric evaluation of diglycolamide functionalized mesoporous silica for the chromatographic separation of actinides Th, Pa and U

Grafting of DGA on mesoporous silica affords a sorbent with the potential to separate U from Pa and Th.

Understanding Am3+/Cm3+ separation with H4TPAEN and its hydrophilic derivatives: a quantum chemical study

Quantum chemical calculations have been used to help understand the back-extraction and separation of Am³⁺/Cm³⁺ with H₄TPAEN and its two hydrophilic derivatives.