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

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