Thorium(IV) and Uranium(IV) Thioether and Selenoether Complexes: Synthesis and An-ER2 (E = S, Se) Bonding Comparison

Reactions of the rigid thioether- and selenoether-containing ligand salts [{Li(AE2Ph2)}2] (E = S or Se; AE2Ph2 = 4,5-bis(phenylchalcogenido)-2,7,9,9-tetramethylacridanide) with ThCl4(dme)2 or UCl4 (for E = Se) afforded the actinide chalcogenoether complexes [(AE2Ph2)2ThCl2] (E = S (1), Se (2)), and [(ASe2Ph2)2UCl2] (3). X-ray crystal structures of 1-3 revealed tetravalent actinide cations complexed to two κ3-coordinated AE2Ph2 ligands, with Th-ER2 and U-ER2 distances below the sum of the covalent radii. Complexes 1-3 provide extremely rare examples of thorium-thioether, thorium-selenoether, and uranium-selenoether bonds, and 1 and 2 contain the shortest known Th-SR2 and Th-SeR2 distances. DFT and QTAIM calculations confirm the presence of significant An(IV)-ER2 interactions in 1-3 and provide insight into the extent of covalency in the An-ER2 bonds.

Theoretical Study of Am(III) and Eu(III) Separation by a Bipyridyl Phosphate Ligand

Actinide An(III) and lanthanide Ln(III) are known to exhibit similar chemical properties; thus, it is difficult to distinguish them in the separation of highly radioactive waste liquids. One potential method to efficiently separate actinides and lanthanides involves the design and development of phosphorus-oxygen-bonded ligands with solvent extraction separation. Here, a bipyridine phosphate ligand with two isopropyl and phosphate groups is introduced to selectively extract actinides. The electronic structure, bonding properties, thermodynamic behavior, and quantum theory of atoms in molecules (QTAIM) of Am(III) and Eu(III) complexes with the bipyridine phosphate ligands were analyzed by using density functional theory (DFT) calculations. The analysis demonstrates that the Am-N bond exhibits stronger covalent characteristics than the Eu-N bond, indicating that the bipyridine phosphate ligand had better selectivity for Am(III) than for Eu(III) in terms of binding affinity. The thermodynamic analysis established the complex [ML(NO3)2(H2O)2]+ as the most stable species during the complexation process. The results indicate great potential for utilizing the bipyridine phosphate ligand for the effective separation of An(III)/Ln(III) in spent fuel reprocessing experiments.

Recent Advances in the Study of Trivalent Lanthanides and Actinides by Phosphinic and Thiophosphinic Ligands in Condensed Phases

The separation of trivalent actinides and lanthanides is a key step in the sustainable development of nuclear energy, and it is currently mainly realized via liquid-liquid extraction techniques. The underlying mechanism is complicated and remains ambiguous, which hinders the further development of extraction. Herein, to better understand the mechanism of the extraction, the contributing factors for the extraction are discussed (specifically, the sulfur-donating ligand, Cyanex301) by combing molecular dynamics simulations and experiments. This work is expected to contribute to improve our systematic understanding on a molecular scale of the extraction of lanthanides and actinides, and to assist in the extensive studies on the design and optimization of novel ligands with improved performance.

Synthesis and Properties of Sulfur-Containing Organophosphorus Extractants Based on Red Phosphorus, Alkyl Bromides, and Elemental Sulfur

In order to obtain sulfur-containing organophosphorus compounds that are promising as extractants of heavy metals, the interaction of elemental phosphorus and sulfur with alkyl bromides catalyzed using strong bases was studied. According to the task, the reaction of non-toxic and non-flammable red phosphorus with alkyl bromides under conditions of phase transfer catalysts (PTC), followed by the introduction of elemental sulfur into the reaction medium, were studied. It is shown that alkyl bromides interact with red phosphorus when heated (95-105 °C, 5-8 h) under conditions of phase transfer catalysts (PTC) in a two-phase system: a 60% aqueous solution of KOH-toluene-benzyltriethylammonium chloride (BTEAC) forming a mixture of organophosphorus compounds along with alkylphosphines (57-60%), are the main reaction products; alkylphosphine oxides are also formed (40-43%). The introduction of elemental sulfur (solution in toluene) at the final stage of the process into the reaction mass cooled to 40-60 °C leads to the expected alkylphosphine sulfides, which are the result of the interaction of alkylphosphines with sulfur. The formation of complex mixtures of products prevents the release of target alkylphosphine sulfides in individual form. However, the synthesized mixture of alkylphosphine sulfides and alkylphosphine oxides without separation into individual components is promising for studying its extraction properties in relation to heavy metals. Testing of the extraction properties of synthesized mixtures of alkylphosphine sulfides and alkylphosphine oxides in relation to heavy metals (Ni, Co, Zn, Pb) and noble metals (Ag) showed that the resulting mixtures of tertiary phosphine oxides and phosphine sulfides are highly effective extractants. The degree of extraction in relation to Ni, Co, Zn, and Pb varies from 99.90 to 99.99%, and for Ag from 99.56 to 99.59%.

A multi-faceted approach to probe organic phase composition in TODGA systems with 1-alcohol phase modifiers

The effect of varying 1-alcohol alkyl chain length on extraction of lanthanides (Lns), H₂O, and H⁺ was studied with tetraoctyl diglycolamide (TODGA) via solvent extraction coupled with FT-IR investigations. This multi-faceted approach provided understanding regarding the relationship between extracted Lns, H₂O and H⁺, 1-alcohol volume fraction, and 1-alcohol alkyl chain length. Under acidic conditions there is competition with 1-alcohols and their ability to solubilize aggregates and incidentally induce third phase formation by increasing the extraction of H₂O. At low 1-alcohol concentrations (5 vol%), the trend for 1-alcohol alkyl lengths in solubilizing the aggregates is 1-hexanol > 1-octanol > 1-decanol. Shorter alkyl chains suppress aggregation, ultimately resulting in lower H₂O concentrations and less available TODGA to hydrogen bond with H⁺. Increasing the 1-alcohol concentration to 30 vol% results in the opposite trend, with longer alkyl chains suppressing aggregation. These results suggest this approach is effective at probing trends in the organic phase micro-structure, and indicates trends across the Ln period with various 1-alcohol alkyl chain lengths are a function of outer-sphere coordination.

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.

Uranium(IV) Thio- and Selenoether Complexes: Syntheses, Structures, and Computational Investigation of U-ER2 Interactions

Rigid thioether- and selenoether-containing pincer proligands H[AS₂ Ph 2 ] (1) and H[ASe₂ Ph 2 ] (2) were synthesized, and deprotonation provided the potassium salts [K(AS₂ Ph 2 )(dme)] (3) and [K(ASe₂ Ph 2 )(dme)₂ ] (4). Reaction of two equivalents of 3 or 4 with [UI₄ (dioxane)₂ ] afforded the uranium thioether complex [(AS₂ Ph 2 )₂ UI₂ ] (5) and the first example of a uranium-selenoether complex, [(ASe₂ Ph 2 )₂ UI₂ ] (6). X-ray structures revealed distorted square antiprismatic geometries in which the AE₂ Ph 2 ligands are κ³ -coordinated. The nature of the U-ER₂ bonding in 5 and 6, as well as methyl-free analogues of 5 and 6 and a hypothetical ether analogue, was investigated computationally (including NBO, AIM, and ELF calculations) illustrating increasing covalency from O to S to Se.