Spectroscopic Study on Aqueous Uranyl(VI) Complexes with Methoxy- and Methylbenzoates: Electronic and Steric Effects of the Substituents.

Aqueous complexation of uranyl(VI) ions with methoxy- and methylbenzoates in 0.1 M NaClO solutions was studied by means of UV-vis absorption and Raman spectroscopy. The predominance of 1:1 complexation (uranyl to ligand) was verified for all uranyl carboxylates under acidic conditions (-log [H] < 3.2), and absorption spectra, stability constants, and symmetric stretching frequencies of the uranyl group of the complexes were determined for the first time. For meta- and para-substituted benzoates, a linear free energy relationship (LFER) was observed between the equilibrium constants for the protonation (log β) and uranyl complexation (log β) reactions, and the electronic effects of the substituents were successfully described by the Hammett equation. In the case of ortho-substituted benzoates, the stability constant of uranyl 2-methoxybenzoate is slightly lower than the LFER trend, which is generally explained by the destabilization of cross-conjugation in the uranyl complex due to the steric hindrance between the reaction center and adjacent methoxy group. On the contrary, the stability constant of uranyl 2-methylbenzoate is comparable to the LFER trend, implying that the steric effect is relatively insignificant for the smaller methyl group. The utility of such thermodynamic correlations between the uranyl-substituted benzoates is useful for the molecular understanding and predictive modeling of chemical interactions between actinyl(VI) ions and various organic carboxyl groups.