Interactions Between Hydrated Cerium(III) Cations and Carboxylates in an Aqueous Solution: Anomalously Strong Complex...

Interactions between hydrated Ce³⁺ and various carboxylates are of fundamental interest. Anomalously strong interactions with Ce³⁺ occur when diglycolic acid (DGA) is added into a Ce³⁺ aqueous solution, unlike various other carboxylic acids. Herein, 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, addition of 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 of 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 by varying the pH and ionic strength, we confirm that this equivalence is an inherent property of the Ce³⁺-DGA system. The Lewis acid-base theory cannot explain why DGA binds to 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 to Ce³⁺, facilitated by the central oxygen. Therefore, DGA enables efficient quenching through the chelate effect when it binds to Ce³⁺.