Selective separation of Am(III) from Eu(III) by 2,9-Bis(dialkyl-1,2,4-triazin-3-yl)-1,10-phenanthrolines: A relativistic quantum chemistry study

The complexation properties of Am(III) and Eu(III) with a serial of novel N-heterocyclic tetradentate ligands, 2,9-bis(dialkyl-1,2,4-triazin-3-yl)-1,10-phenanth-rolines (BTPhens), have been calculated by density functional theory (DFT) coupled with relativistic small-core pseudopotential. It is found that the nitrogen atoms in triazine rings favor metal cations more than those in phenanthroline skeleton. The Am-N bond lengths are comparable to or even shorter than those of Eu-N bonds based on the larger radii of Am(III) than Eu(III). The effect of alkyl substituents on the complex structures is limited but they can adjust the complexation reaction and extraction kinetics. Additionally, the solvent effect can decrease the probability of the complexation reactions greatly and the solvation energies of Am(III) and Eu(III) might be the primary driving force in the complexation selectivity. The complexation reactions for forming ML(NO₃)₃ and [ML₂(NO₃)]²⁺ complexes are believed to dominate the selective extraction separation of Am(III) from Eu(III) in nitric acid media, which agrees well with the experimental results. The comparisons of BTPhens with BTBPs in dipoles and electronic structures confirm that the BTPhens exhibit more favourable extraction kinetics and selectivity for minor actinides.