Use of Soft Heterocyclic N-Donor Ligands To Separate Actinides and Lanthanides

A phenanthroline-derived ligand and its complexation with Pd(ii): from ligand design, synthesis and Pd(ii) complexes structures to its application

This paper reports the first example of a phenanthroline-derived bis-opened-triazine ligand showing high selectivity and fast extraction rate toward Pd(ii).

Exploring electronic effects on the partitioning of actinides(iii) from lanthanides(iii) using functionalised bis-triazinyl phenanthroline ligands

Revealing how soft N-type donor ligands achieve minor actinide selectivity is fundamental in the design of new and improved extractants for advanced future fuel cycles.

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.

On the stoichiometry and stability of americium(III) complexes with a hydrophilic SO3-Ph-BTP ligand, studied by liquid-liquid extraction

1:1 and 1:2 complexes of americium(III) with a hydrophilic anionic SO₃–Ph–BTP⁴⁻ ligand were detected in acidic aqueous nitrate solutions by a solvent extraction method. The determined conditional stability constants of these complexes, logβ₁ = 4.35 ± 0.07 and logβ₂ = 7.67 ± 0.06, related to 1 M aqueous solutions, are much lower than the literature values for the analogous curium species, determined by TRLFS in very dilute aqueous solutions. There is also no evidence for the existence of the 1:3 Am³⁺ complex similar to the reported curium(III) complex. A hypothesis has been formulated to explain these discrepancies. It suggests the necessity to carefully check the equilibria in each phase of solvent extraction systems containing two competing ligands—lipophilic and hydrophilic.

Dithio- and Diselenophosphinate Thorium(IV) and Uranium(IV) Complexes: Molecular and Electronic Structures, Spectroscopy, and Transmetalation Reactivity

We report a comparison of the molecular and electronic structures of dithio- and diselenophosphinate, (E2PR2)(1-) (E = S, Se; R = (i)Pr, (t)Bu), with thorium(IV) and uranium(IV) complexes. For the thorium dithiophosphinate complexes, reaction of ThCl4(DME)2 with 4 equiv of KS2PR2 (R = (i)Pr, (t)Bu) produced the homoleptic complexes, Th(S2P(i)Pr2)4 (1S-Th-(i)Pr) and Th(S2P(t)Bu2)4 (2S-Th-(t)Bu). The diselenophosphinate complexes were synthesized in a similar manner using KSe2PR2 to produce Th(Se2P(i)Pr2)4 (1Se-Th-(i)Pr) and Th(Se2P(t)Bu2)4 (2Se-Th-(t)Bu). U(S2P(i)Pr2)4, 1S-U-(i)Pr, could be made directly from UCl4 and 4 equiv of KS2P(i)Pr2. With (Se2P(i)Pr2)(1-), using UCl4 and 3 or 4 equiv of KSe2P(i)Pr2 yielded the monochloride product U(Se2P(i)Pr2)3Cl (3Se-U(iPr)-Cl), but using UI4(1,4-dioxane)2 produced the homoleptic U(Se2P(i)Pr2)4 (1Se-U-(i)Pr). Similarly, the reaction of UCl4 with 4 equiv of KS2P(t)Bu2 yielded U(S2P(t)Bu2)4 (2S-U-(t)Bu), whereas the reaction with KSe2P(t)Bu2 resulted in the formation of U(Se2P(t)Bu2)3Cl (4Se-U(tBu)-Cl). Using UI4(1,4-dioxane)2 and 4 equiv of KSe2P(t)Bu2 with UCl4 in acetonitrile yielded U(Se2P(t)Bu2)4 (2Se-U-(t)Bu). Transmetalation reactions were investigated with complex 2Se-U-(t)Bu and various CuX (X = Br, I) salts to yield U(Se2P(t)Bu2)3X (6Se-U(tBu)-Br and 7Se-U(tBu)-I) and 0.25 equiv of [Cu(Se2P(t)Bu2)]4 (8Se-Cu-(t)Bu). Additionally, 2Se-U-(t)Bu underwent transmetalation reactions with Hg2F2 and ZnCl2 to yield U(Se2P(t)Bu2)3F (6) and U(Se2P(t)Bu2)3Cl (4Se-U(tBu)-Cl), respectively. The molecular structures were analyzed using (1)H, (13)C, (31)P, and (77)Se NMR and IR spectroscopy and structurally characterized using X-ray crystallography. Using the QTAIM approach, the electronic structure of all homoleptic complexes was probed, showing slightly more covalent bonding character in actinide-selenium bonds over actinide-sulfur bonds.

Synthesis of new 4′-(N-alkylpyrrol-2-yl)-2,2′: 6′,2″-terpyridines via N-alkylation of a pyrrole moiety

New 4′-(N-alkylpyrrol-2-yl)-2,2′:6′,2″-terpyridines were synthesized by N-alkylation of a pyrrole substituent with alkyl halides in dimethyl sulfoxide.

Theoretical prediction of Am(iii)/Eu(iii) selectivity to aid the design of actinide-lanthanide separation agents

Selective extraction of minor actinides from lanthanides is a critical step in the reduction of radiotoxicity of spent nuclear fuels. However, the design of suitable ligands for separating chemically similar 4f- and 5f-block trivalent metal ions poses a significant challenge. First-principles calculations should play an important role in the design of new separation agents, but their ability to predict metal ion selectivity has not been systematically evaluated. In this work, we examine the ability of several density functional theory methods to predict selectivity of Am(iii) and Eu(iii) with oxygen, mixed oxygen-nitrogen, and sulfur donor ligands. The results establish a computational method capable of predicting the correct order of selectivities obtained from liquid-liquid extraction and aqueous phase complexation studies. To allow reasonably accurate predictions, it was critical to employ sufficiently flexible basis sets and provide proper account of solvation effects. The approach is utilized to estimate the selectivity of novel amide-functionalized diazine and 1,2,3-triazole ligands.

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.

Efficient masking of corrosion and fission products such as Ni(II) and Pd(II) in the presence of the minor actinide Am(III) using hydrophilic anionic or cationic bis-triazines

Water soluble anionic and cationic bis-triazine ligands are able to suppress (mask) the extraction of corrosion and fission products such as Ni(II) and Pd(II) that are found in PUREX raffinates. Thus it is possible to separate these elements from the minor actinide Am(III). Although some masking agents have previously been developed that retard the extraction of Pd(II), this is the first time a masking agent has been developed for Ni(II).

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.

The structures of CyMe4-BTBP complexes of americium(iii) and europium(iii) in solvents used in solvent extraction, explaining their separation properties

The structure around the absorbing atom in the dicomplexes of CyMe₄-BTBP of americium(iii) and europium(III) have been studied in solvent used in solvent extraction.

Effective Am(iii)/Eu(iii) separations using 2,6-bis(1,2,4-triazin-3-yl)pyridine (BTP) functionalised titania particles and hierarchically porous beads

Titania particles and beads functionalised with a modified BTP ligand have been used to selectively extract Am(iii) over Eu(iii) from 0.01 M nitric acid solutions.

Synthesis of functionalised BTPhen derivatives – effects on solubility and americium extraction

Separation of the minor actinides (Am/Cm) from spent nuclear fuel post-PUREX process is expected to play a key part in new reprocessing methodologies.

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 the actinides Am(iii) and Cm(iii) by electronic modulation of bis-(1,2,4-triazin-3-yl)phenanthrolines

It has been shown that modification of the phenanthroline backbone of CyMe₄-BTPhen leads to subtle electronic modulation, permitting differential ligation of Am(iii) and Cm(iii) resulting in separation factors up to 7.

Probing the difference in covalence by enthalpy measurements: a new heterocyclic N-donor ligand for actinide/lanthanide separation

BQPhen, a new tetra-dentate nitrogen-donor ligand, forms stronger complexes with Am³⁺ than Eu³⁺ or Nd³⁺, due to a more exothermic enthalpy of complexation for the Am³⁺ complexes, implying a higher degree of covalence in the BQPhen complexes with Am³⁺ than Eu³⁺ or Nd³⁺.

Ditopic CMPO-pillar[5]arenes as unique receptors for efficient separation of americium(iii) and europium(iii)

A unique host–guest recognition process involving a new class of homoditopic CMPO-pillar[5]arenes and lanthanides was revealed to proceed in a stepwise manner, and correlated to the efficient and selective separation of americium(iii) from europium(iii) under acidic feed conditions.

The selectivity of diglycolamide (TODGA) and bis-triazine-bipyridine (BTBP) ligands in actinide/lanthanide complexation and solvent extraction separation – a theoretical approach

Charge transfer from the ligand to valence hybrid (mainly d) metal orbitals nearly equally stabilizes cationic Eu^III and Am^III TODGA complexes.

Trivalent uranium phenylchalcogenide complexes: exploring the bonding and reactivity with CS2 in the Tp*2UEPh series (E = O, S, Se, Te)

The trivalent uranium phenylchalcogenide series, Tp*2UEPh (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, E = O (1), S (2), Se (3), Te (4)), has been synthesized to investigate the nature of the U-E bond. All compounds have been characterized by (1)H NMR, infrared and electronic absorption spectroscopies, and in the case of 4, X-ray crystallography. Compound 4 was also studied by SQUID magnetometry. Computational studies establish Mulliken spin densities for the uranium centers ranging from 3.005 to 3.027 (B3LYP), consistent for uranium-chalcogenide bonds that are primarily ionic in nature, with a small covalent contribution. The reactivity of 2-4 toward carbon disulfide was also investigated and showed reversible CS2 insertion into the U(III)-E bond, forming Tp*2U(κ(2)-S2CEPh) (E = S (5), Se (6), Te (7)). Compound 5 was characterized crystallographically.

CMPO-calix[4]arenes with spacer containing intramolecular hydrogen bonding: effect of local rigidification on solvent extraction toward f-block elements

To understand intramolecular hydrogen bonding in effecting liquid-liquid extraction behavior of CMPO-calixarenes, three CMPO-modified calix[4]arenes (CMPO-CA) 5a-5c with hydrogen-bonded spacer were designed and synthesized. The impact of spacer rotation that is hindered by introduction of intramolecular hydrogen bonding upon extraction of La(3+), Eu(3+), Yb(3+), Th(4+), and UO2(2+) has been examined. The results show that 5b and 5c containing only one hydrogen bond with a less hindered rotation spacer extract La(3+) more efficiently than 5a containing two hydrogen bonds with a more hindered rotation spacer, demonstrating the importance of local rigidification of spacer in the design of extractants in influencing the coordination environment. The large difference in extractability between La(3+) and Yb(3+) (or Eu(3+)) by 5b (or 5c), and the small difference by 5a, suggests intramolecular hydrogen bonding do exert pronounced influence upon selective extraction of light and heavy lanthanides. Log-log plot analysis indicates a 1:1, 2:1 and 1:1 stoichiometry (ligand/metal) for the extracted complex formed between 5b and La(3+), Th(4+), UO2(2+), respectively. Additionally, their corresponding acyclic analogs 7a-7c exhibit negligible extraction toward these metal ions. These results reveal the possibility of selective extraction via tuning local chelating surroundings of CMPO-CA by aid of intramolecular hydrogen bonding.

Pillar[5]arene-based diglycolamides for highly efficient separation of americium(iii) and europium(iii)

Pillar[5]arenes were functionalized with ten diglycolamide (DGA) arms on both sides of the pillar framework and were found to exhibit excellent separation and extraction efficiency towards Am(iii) and Eu(iii) at high acidity.

Synthesis and f-element ligation properties of NCMPO-decorated pyridine N-oxide platforms

Syntheses of hybrid N-attached CMPO decorated pyridine N-oxide ligands, computational, spectroscopic and structural characterization of their coordination interactions with lanthanide ions and survey solvent extraction analyses are described.

Selective extraction of scandium from yttrium and lanthanides with amic acid-type extractant containing alkylamide and glycine moieties

The liquid–liquid extraction of rare earth metal ions (scandium (Sc³⁺), yttrium (Y³⁺) and the lanthanides (La³⁺, Nd³⁺, Eu³⁺ and Dy³⁺)) was investigated using N-[N,N-di(2-ethylhexyl)aminocarbonylmethyl]glycine (D2EHAG).