Interactions between hydrated cerium(III) cations and carboxylates in aqueous solution: Anomalously strong complex formation with diglycolate suggesting a chelate effect

Interactions between hydrated Ce³⁺ and various carboxylates are of fundamental interest. Anomalously strong interactions with Ce³⁺ occur when adding diglycolic acid (DGA) into a Ce³⁺ aqueous solution, unlike various other carboxylic acids. The complex formation constants of Ce³⁺ with these acids are evaluated via absorption and emission spectra. Hydrated Ce³⁺ emits fluorescence with unity quantum yield; however, adding various carboxylates statically quenches the fluorescence when Ce³⁺‒carboxylate complexes form, because the fluorescence lifetime is constant irrespective of the carboxylate concentration. In the observed static quenching, the complex formation constants obtained from the absorption and emission spectra (K_abs and K_em) agree well. The binding Ce³⁺ by the conjugate Lewis bases, i.e., carboxylates is approximately inversely proportional to the pH. Adding DGA into the system also statically quenches the fluorescence, but far more efficiently, even in a much weaker solution. We rigorously deduce K_abs and K_em of Ce³⁺ with DGA without any approximation using comparable concentrations. Careful fittings provide equivalent K_em and K_abs values, and varying the pH and ionic strength confirms this equivalence is an inherent property of the Ce³⁺-DGA system. Lewis acid–base theory cannot explain DGA binds Ce³⁺ ~1000 times more strongly than the other carboxylates. This anomalously strong binding may be due to a chelate effect caused by the DGA’s central oxygen atom, which forms a five-membered ring with the conjugate Lewis bases of DGA; double chelate rings can also form, while bis-deprotonated DGA binds Ce³⁺, facilitated by the central oxygen. Therefore, DGA enables efficient quenching through the chelate effect when it binds Ce³⁺.