Complexation properties and structural character of lanthanides complexes of O,N-hetero donor ligand BIZA

Complexation Behaviors of a Tridentate Phenanthroline Carboxamide Ligand with Trivalent f-Block Elements in Different Anion Systems: A Thermodynamic and Crystallographic Perspective

The counteranion has a strong influence on the complexation behavior of tridentate phenanthroline carboxamide ligands with actinides and lanthanides, but the thermodynamic and underlying interaction mechanism at the molecular level is still not clear. In this work, a tridentate ligand, N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA), was synthesized, and the effects of different anions (Cl^-, NO₃^-, and ClO₄^-) on the complexation behavior of Et-Tol-PTA with typical lanthanides were thoroughly studied by using ¹H nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-vis) spectrophotometry, and single-crystal X-ray diffraction. The NMR spectroscopic titration of Lu(III) showed that there were three species (1:1, 2:1, and 3:1 ligand-metal complexes) formed in Cl^- solution systems while two species (2:1 and 1:1) were formed in NO₃^- and ClO₄^- solution systems. When Et-Tol-PTA was titrated with La(III), two species (2:1 and 1:1) were formed in NO₃^- systems and only one species (1:1) was formed in Cl^- and ClO₄^- systems. In addition, the stability constant was determined via UV-vis spectroscopic titration, which showed that the complexation strength between Et-Tol-PTA and Eu(III) decreased in the following order: ClO₄^- > NO₃^- > Cl^-. This indicated that Et-Tol-PTA had the strongest complexation ability with Eu(III) in the ClO₄^- system. The structures of Et-Tol-PTA complexed with EuCl₃, Eu(NO₃)₃, and Eu(ClO₄)₃ were further elucidated by single-crystal X-ray diffraction and agreed well with the results of UV-vis titration experiments. The results of this work revealed that the mechanisms of complexation of lanthanides with the asymmetric ligand Et-Tol-PTA were strongly affected by different anionic environments in solution and in the solid state. These findings may lead to the improvement of the separation of trivalent actinides and lanthanides in nuclear waste.