Complexation of uranyl with chelidamic acid: Crystal structures, binding strength, and electrochemical redoxes

Chelidamic acid (denoted as H₃L in this paper) is a naturally occurring derivative of dipicolinic acid that functions as a versatile ligand. The complex formation of chelidamic acid with UO₂²⁺ has been investigated by potentiometry in a 0.1 M NaClO₄ solution. The potentiometry identified 1:1 uranyl–chelidamate complexes (UO₂HL, UO₂L₂⁻), 1:2 uranyl–chelidamate complex (UO₂L₂⁴⁻), and a hydrolytic complex UO₂(OH)₂L²⁻. The possible UO₂H₂L₂²⁻ structure was determine by crystal analysis. Chelidamate molecules coordinate to uranyl mainly through the pyridine–carboxylate chelation pocket (O, N, O), and 4-pyridinol–OH group is protonated. The presence of hydrolysate UO₂(OH)₂L^2– was further confirmed by nuclear magnetic resonance spectroscopy (NMR). In an alkaline aqueous medium under anaerobic conditions, the complex U^VIO₂(OH)₂L²⁻can be electrochemically reduced to pentavalent uranyl.