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

The removal of the most long-lived radiotoxic elements from used nuclear fuel, minor actinides, is foreseen as an essential step toward increasing the public acceptance of nuclear energy as a key component of a low-carbon energy future. Once removed from the remaining used fuel, these elements can be used as fuel in their own right in fast reactors or converted into shorter-lived or stable elements by transmutation prior to geological disposal. The SANEX process is proposed to carry out this selective separation by solvent extraction. Recent efforts to develop reagents capable of separating the radioactive minor actinides from lanthanides as part of a future strategy for the management and reprocessing of used nuclear fuel are reviewed. The current strategies for the reprocessing of PUREX raffinate are summarized, and some guiding principles for the design of actinide-selective reagents are defined. The development and testing of different classes of solvent extraction reagent are then summarized, covering some of the earliest ligand designs right through to the current reagents of choice, bis­(1,2,4-triazine) ligands. Finally, we summarize research aimed at developing a fundamental understanding of the underlying reasons for the excellent extraction capabilities and high actinide/lanthanide selectivities shown by this class of ligands and our recent efforts to immobilize these reagents onto solid phases

Synthesis and Screening of Modified 6,6′-Bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenzo[<i>e</i>][1,2,4]triazin-3-yl)-2,2′-bipyridine Ligands for Actinide and Lanthanide Separation in Nuclear Waste Treatment

Effects of chloro and bromo substitution at the 4-position of the pyridine ring of 6,6′-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenzo­[<i>e</i>]­[1,2,4]­triazin-3-yl)-2,2′-bipyridine (CyMe₄-BTBP) have been studied with regard to the extraction of Am­(III) from Eu­(III) and Cm­(III) from 0.1–3 M HNO₃. Similarly to CyMe₄-BTBP, a highly efficient (<i>D</i>_Am > 10 at 3 M HNO₃) and selective (SF_Am/Eu > 100 at 3 M HNO₃) extraction was observed for Cl-CyMe₄-BTBP and Br-CyMe₄-BTBP in 1-octanol but in the absence of a phase-transfer agent

Lanthanide Speciation in Potential SANEX and GANEX Actinide/Lanthanide Separations Using Tetra-N-Donor Extractants

Lanthanide­(III) complexes with N-donor extractants, which exhibit the potential for the separation of minor actinides from lanthanides in the management of spent nuclear fuel, have been directly synthesized and characterized in both solution and solid states. Crystal structures of the Pr³⁺, Eu³⁺, Tb³⁺, and Yb³⁺ complexes of 2,9-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe₄-BTPhen) and the Pr³⁺, Eu³⁺, and Tb³⁺ complexes of 6,6′-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2′-bypyridine (CyMe₄-BTBP) were obtained. The majority of these structures displayed coordination of two of the tetra-N-donor ligands to each Ln³⁺ ion, even when in some cases the complexations were performed with equimolar amounts of lanthanide and N-donor ligand. The structures showed that generally the lighter lanthanides had their coordination spheres completed by a bidentate nitrate ion, giving a 2+ charged complex cation, whereas the structures of the heavier lanthanides displayed tricationic complex species with a single water molecule completing their coordination environments. Electronic absorption spectroscopic titrations showed formation of the 1:2 Ln³⁺/L_N₄‑donor species (Ln = Pr³⁺, Eu³⁺, Tb³⁺) in methanol when the N-donor ligand was in excess. When the Ln³⁺ ion was in excess, evidence for formation of a 1:1 Ln³⁺/L_N₄‑donor complex species was observed. Luminescent lifetime studies of mixtures of Eu³⁺ with excess CyMe₄-BTBP and CyMe₄-BTPhen in methanol indicated that the nitrate-coordinated species is dominant in solution. X-ray absorption spectra of Eu³⁺ and Tb³⁺ species, formed by extraction from an acidic aqueous phase into an organic solution consisting of excess N-donor extractant in pure cyclohexanone or 30% tri-<i>n</i>-butyl phosphate (TBP) in cyclohexanone, were obtained. The presence of TBP in the organic phase did not alter lanthanide speciation. Extended X-ray absorption fine structure data from these spectra were fitted using chemical models established by crystallography and solution spectroscopy and showed the dominant lanthanide species in the bulk organic phase was a 1:2 Ln³⁺/L_N‑donor species

BTBPs versus BTPhens: Some Reasons for Their Differences in Properties Concerning the Partitioning of Minor Actinides and the Advantages of BTPhens

Two members of the tetradentate <i>N</i>-donor ligand families 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) currently being developed for separating actinides from lanthanides have been studied. It has been confirmed that CyMe₄-BTPhen <b>2</b> has faster complexation kinetics than CyMe₄-BTBP <b>1</b>. The values for the HOMO–LUMO gap of <b>2</b> are comparable with those of CyMe₄-BTBP <b>1</b> for which the HOMO–LUMO gap was previously calculated to be 2.13 eV. The displacement of BTBP from its bis-lanthanum­(III) complex by BTPhen was observed by NMR, and constitutes the only direct evidence for the greater thermodynamic stability of the complexes of BTPhen. NMR competition experiments suggest the following order of bis-complex stability: 1:2 bis-BTPhen complex ≥ heteroleptic BTBP/BTPhen 1:2 bis-complex > 1:2 bis-BTBP complex. Kinetics studies on some bis-triazine <i>N</i>-donor ligands using the stopped-flow technique showed a clear relationship between the rates of metal ion complexation and the degree to which the ligand is preorganized for metal binding. The BTBPs must overcome a significant (ca. 12 kcal mol^–1) energy barrier to rotation about the central biaryl C–C axis in order to achieve the <i>cis</i>–<i>cis</i> conformation that is required to form a complex, whereas the <i>cis</i>–<i>cis</i> conformation is fixed in the BTPhens. Complexation thermodynamics and kinetics studies in acetonitrile show subtle differences between the thermodynamic stabilities of the complexes formed, with similar stability constants being found for both ligands. The first crystal structure of a 1:1 complex of CyMe₄-BTPhen <b>2</b> with Y­(NO₃)₃ is also reported. The metal ion is 10-coordinate being bonded to the tetradentate ligand <b>2</b> and three bidentate nitrate ions. The tetradentate ligand is nearly planar with angles between consecutive rings of 16.4(2)°, 6.4(2)°, 9.7(2)°, respectively

BTBPs versus BTPhens: Some Reasons for Their Differences in Properties Concerning the Partitioning of Minor Actinides and the Advantages of BTPhens

Two members of the tetradentate <i>N</i>-donor ligand families 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) currently being developed for separating actinides from lanthanides have been studied. It has been confirmed that CyMe₄-BTPhen <b>2</b> has faster complexation kinetics than CyMe₄-BTBP <b>1</b>. The values for the HOMO–LUMO gap of <b>2</b> are comparable with those of CyMe₄-BTBP <b>1</b> for which the HOMO–LUMO gap was previously calculated to be 2.13 eV. The displacement of BTBP from its bis-lanthanum­(III) complex by BTPhen was observed by NMR, and constitutes the only direct evidence for the greater thermodynamic stability of the complexes of BTPhen. NMR competition experiments suggest the following order of bis-complex stability: 1:2 bis-BTPhen complex ≥ heteroleptic BTBP/BTPhen 1:2 bis-complex > 1:2 bis-BTBP complex. Kinetics studies on some bis-triazine <i>N</i>-donor ligands using the stopped-flow technique showed a clear relationship between the rates of metal ion complexation and the degree to which the ligand is preorganized for metal binding. The BTBPs must overcome a significant (ca. 12 kcal mol^–1) energy barrier to rotation about the central biaryl C–C axis in order to achieve the <i>cis</i>–<i>cis</i> conformation that is required to form a complex, whereas the <i>cis</i>–<i>cis</i> conformation is fixed in the BTPhens. Complexation thermodynamics and kinetics studies in acetonitrile show subtle differences between the thermodynamic stabilities of the complexes formed, with similar stability constants being found for both ligands. The first crystal structure of a 1:1 complex of CyMe₄-BTPhen <b>2</b> with Y­(NO₃)₃ is also reported. The metal ion is 10-coordinate being bonded to the tetradentate ligand <b>2</b> and three bidentate nitrate ions. The tetradentate ligand is nearly planar with angles between consecutive rings of 16.4(2)°, 6.4(2)°, 9.7(2)°, respectively

New furoquinoline alkaloid and flavanone glycoside derivatives from the leaves of <i>Oricia suaveolens and Oricia renieri</i> (Rutaceae)

<div><p>Fractionation of the methanol extract of the leaves of <i>Oricia renieri</i> and <i>Oricia suaveolens</i> (Rutaceae) led to the isolation of 13 compounds including the hitherto unknown furoquinoline alkaloid named 6,7-methylenedioxy-5-hydroxy-8-methoxy-dictamnine <b>(1)</b> and a flavanone glycoside named 5-hydroxy-4′-methoxy-7-<i>O</i>-[α-l-rhamnopyranosyl(1‴→5″)-β-d-apiofuranosyl]-flavanoside <b>(2)</b>, together with 11 known compounds <b>(3</b>–<b>13)</b>. The structures of the compounds were determined by comprehensive analyses of their 1D and 2D NMR, mass spectral data and comparison. All compounds isolated were examined for their activity against human carcinoma cell lines. The alkaloids <b>1</b>, <b>5</b>, <b>12</b>, <b>13</b> and the phenolic <b>2</b>, <b>8</b>, <b>11</b> tested compounds exhibited non-selective moderate cytotoxic activity with IC₅₀ 8.7–15.9 μM whereas compounds <b>3</b>, <b>4</b>, <b>6</b>, <b>7</b>, <b>9</b> and <b>10</b> showed low activity.</p></div