Evaluation of Multidentate Ligands Derived from Ethyl 1,2,4-triazine-3-carboxylate Building Blocks as Potential An(III)-Selective Extractants for Nuclear Reprocessing

Abstract

Bis-1,2,4-triazine ligands are amongst the most promising soft N-donor ligands for the partitioning of trivalent actinides from trivalent lanthanides; a key separation proposed in the future reprocessing of spent nuclear fuels. In an effort to improve the extraction properties of these benchmark ligands, we propose herein a general ligand design approach that is inspired by the field of drug discovery, and we apply it to a new class of ligands in which the bidentate 3-(2-pyridyl)-1,2,4-triazine unit of the benchmark ligands is replaced by a bidentate 1,2,4-triazine-3-carboxamide unit. A series of nine novel ligands were synthesized by reactions of readily available ethyl 1,2,4-triazine-3-carboxylate building blocks with different polyamine cores, and evaluated for their ability to extract and separate Am(III) and Cm(III) from Eu(III). One of the reported ligands is able to co-extract Am(III) and Eu(III) from nitric acid into cyclohexanone, albeit with no selectivity between the metal ions. NMR titration experiments suggested that ligand 23b formed a chiral 1:1 complex species with La(III) but not Lu(III) or Y(III), suggesting the coordination cavity of the ligand is sensitive to the size of the metal ion. The structures and thermodynamic parameters for the proposed complexes were further supported by DFT calculations. The approach could be employed as a screening tool to facilitate future design of improved actinide-selective ligands for closing the nuclear fuel cycle.