Hindered rotation in a novel 1,2,4-triazinyl phenanthroline (t-phen) ligand leading to improved separation of Am3+ and Eu3+ vis-a-vis 1,2,4-triazinyl bipyridine (t-bipy): a computational validation of the experimental results

A novel nitrogen donor ligand having a more pre-organized structure, 1,2,4-triazinyl phenanthroline (t-phen), is synthesized and evaluated for lanthanide-actinide separation in the present work. The extraction and selectivity for Am3+ over Eu3+ was found to be improved with t-phen as compared to the analogous 1,2,4-triazinyl bipyridine (t-bipy), which was explained by analyzing their conformational energies and energy differences between the frontier orbitals (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) with the help of density functional theoretical calculations. Higher covalence in Am3+ complexes as compared to the Eu3+ complexes was indicated by more shared electrons between Am3+ and bonded 'N' atoms, shorter 'Am-N' bonds and higher overlap between the orbitals of Am3+ and ligands in the frontier orbitals of the complexes in the case of both the ligands. Natural bond order analysis also supports these observations.