Unravelling the mechanism of f-element extraction by phenanthroline-diamides: A case of 4,7-substituted 1,10-phenanthroline-2,9-diamides

Management of high-level waste is essential for the further sustainable development of nuclear energy. Solvent extraction is one of the technologically acceptable methods for carrying out such processing. The search for new selective extractants is an urgent task that requires systematic research into the structure-properties relationship. 1,10-phenanthroline-2,9-diamides (DAPhen) is a promising ligand class for processing such solutions. Identification of the binding and separation mechanism is the most important fundamental question for any separation system. In this work, we systematically studied of the mechanism of extraction and complex formation by this class of compounds on the example of 4,7-substituted aliphatic 1,10-phenanthroline-2,9-diamides. For systematic comparison, we conducted liquid-liquid extraction studies of f-elements and nitric acid, determination of binding constants (UV–vis and luminescence titration), and structural studies of complexes. Using these methods, it was shown how electron-withdrawing substituents (-Cl) and electron-donating (-OBu) significantly affect the Brønsted and Lewis basicity, the stoichiometry of the complexes formed and the trends in the extraction of f-elements. Moreover, XRD study of an array of complex compounds allowed us to establish the structural features that determine the efficiency and selectivity of liquid-liquid extraction.