Coordination Chemistry of Glycolic Acid and Uranyl

Glycolic acid (α-hydroxyacetic acid, HA) is a simple, water-soluble carboxylic acid ligand commonly found in the environment. It can coordinate with metal ions through both its carboxyl and hydroxyl oxygen atoms. The complexation of uranyl ions with HA has been investigated using potentiometry, absorption spectroscopy and Raman spectroscopy. Five complex species were identified: (UO₂A)⁺, UO₂A₂, (UO₂A₃)^–, (UO₂A₃H_–1)^2– and (UO₂A₃H_–2)^3–. Their stability constants were determined through potentiometric and absorption spectroscopic methods, and the coordination modes of HA were elucidated using Raman spectroscopy in aqueous solutions with pH＜5. For the first three species (UO₂A)⁺, UO₂A₂ and (UO₂A₃)^–, the stability constants measured by potentiometry are 2.36±0.09, 4.02±0.01 and 5.33±0.27 respectively, which are consistent with the results that calculated from absorption spectroscopy and previous reports. The latter two species, (UO₂A₃H_–1)^2– and (UO₂A₃H_–2)^3–, are reported here for the first time, with stability constants determined by potentiometry as 1.66±0.05 and –3.11±0.06, respectively. The molar absorption spectra of the five complexes were obtained using absorption spectroscopy. The spectra of the first three species are quite similar, while the characteristic peaks of uranyl in the latter two species are nearly absent. This indicates that the dissociation of hydrogen atoms from the hydroxyl groups of glycolic acid significantly affects the uranyl absorption spectra. Raman spectroscopy was used to infer the coordination modes of glycolic acid in the complexes. The Raman shift peaks for ${\mathrm{UO}}_2^{2+}$, (UO₂A)⁺, UO₂A₂ and (UO₂A₃)^– were found at 869, 858.7, 848.4, and 843.3 cm^–1, respectively. According to the empirical relationship between the Raman shift of the uranyl group and ligand ability (i.e. a larger Raman shift corresponds to a stronger ligand ability), in the (UO₂A)⁺ and UO₂A₂, glycolate forms a five-membered ring chelate with uranyl through both a carboxyl and a hydroxyl oxygen atom. Due to steric hindrance, the uranyl equatorial plane cannot accommodate three five-membered cyclic ligands simultaneously. Consequently, in the complex (UO₂A₃)^–, two ligands form a five-membered ring chelate, while the third ligand is coordinated in a four-membered ring mode through two carboxyl oxygen atoms. The simpler carboxyl-terminal coordination results in smaller Raman shifts compared to the more stable chelating coordination mode. In (UO₂A₃)^–, the hydrogen atoms from the glycolate hydroxyl groups in the five-membered ring can sequentially dissociate, forming (UO₂A₃H_–1)^2– and (UO₂A₃H_–2)^3–, and resulting the Raman peak of uranyl to a lower wavenumber.