Role of H⁺ in Solvent Extraction of Am(III) and Eu(III) Using a Polydentate Bipyridine N-,O-Donor Ligand: Chemical Equilibrium States and Kinetic Aspects

In this work, the influence of protonation on the kinetics and thermodynamics of extraction of the Am/Eu pair using N-heterocyclic dicarboxylic acid diamide <i>N</i>,<i>N</i>'-diethyl-<i>N</i>,<i>N</i>'-bis(4-ethylphenyl)-[2,2'-bipyridine]-6,6'-dicarboxamide (<b>L</b>) was investigated. The extraction efficiency of the ligand did not decrease, even at a nitric acid concentration 4 times higher than that of the ligand in the organic phase. X-ray diffraction analysis established that protonation leads to the preorganization of the ligand due to the reversal of bipyridyl rings into the binding conformation when both nitrogen atoms are turned to one side. Also, the effect of protonation on the chemical shifts of the functional binding groups (bipyridyl and amide) in solution was demonstrated by NMR spectroscopy. However, the presaturation of the organic phase with nitric acid did not lead to significant changes in the surface activity of the ligand. For extraction kinetics, a mechanism was proposed in which protonation and preorganization of the ligand was the limiting stage.