Aqueous Uranium(VI) Complexes with Acetic and Succinic Acid: Speciation and Structure Revisited

Abstract

We employed density functional theory (DFT) calculations, and ultraviolet-visible (UV-vis), extended X-ray absorption fine-structure (EXAFS), and attenuated total reflection Fourier-transform infrared (IR) spectroscopy analyzed with iterative transformation factor analysis (ITFA) to determine the structures and the pH-speciation of aqueous acetate (ac) and succinate (suc) U(VI) complexes. In the acetate system, all spectroscopies confirm the thermodynamically predicted pH-speciation by Ahrland (1951), with the hydrated uranyl ion and a 1:1, a 1:2 and a 1:3 U(VI)-ac complex. In the succinate system, we identified a new 1:3 U(VI)-suc complex, in addition to the previously known 1:1 and 1:2 U(VI)-suc complexes, and determined the pH-speciation for all complexes. The IR spectra show absorption bands of the antisymmetric stretching mode of the uranyl mojety (υ3(UO2)) at 949, 939, 924 cm(-1) and at 950, 938, 925 cm(-1) for the 1:1, 1:2 and 1:3 U(VI)-ac and U(VI)-suc complexes, respectively. IR absorption bands at 1535 and 1534 cm(-1) and at 1465 and 1462 cm(-1) are assigned to the antisymmetric υ3,as(COO) and symmetric υ3,s(COO) stretching mode of bidentately coordinated carboxylic groups in the U(VI)-ac and U(VI)-suc complexes. The assignment of the three IR bands (υ3(UO2), υ3,as(COO), υ3,s(COO)) and the stoichiometry of the complexes is supported by DFT calculations. The UV-vis spectra of the equivalent U(VI)-ac and U(VI)-suc complexes are similar suggesting common structural features. Consistent with IR spectroscopy and DFT calculations, EXAFS showed a bidentate coordination of the carboxylic groups to the equatorial plane of the uranyl moiety for all uranyl ligand complexes except for the newly detected 1:3 U(VI)-suc complex, where two carboxylic groups coordinate bidentately and one carboxylic group coordinates monodentately. All 1:1 and 1:2 complexes have a U-Owater distance of ∼2.36 Å, which is shorter than the U-Owater distance of ∼2.40 Å of the hydrated uranyl ion. For all complexes the U-Ocarboxyl distance of the bidentately coordinated carboxylic group is ∼2.47 Å, while the monodentately coordinated carboxylic group of the 1:3 U(VI)-suc complex has a U-Ocarboxyl distance of ∼2.36 Å, that is, similar to the short U-Owater distance in the 1:1 and 1:2 complexes.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

“…Carboxylate ligands including acetate [152,185], citrate [186], glycolate [187], lactate [187], malate [187], oxalate [152,188], succinate [185], tartrate [187] and tricarboxylate [187] ions, readily form complexes with the uranyl cation and are important species in natural and biological systems. Hexavalent uranium is a hard Lewis acid that prefers hard O-donors; thus, carboxylate complexes are stable in water and have formation constants (log K) that range between 2 and 16 [189–192].…”

“…This assignment discrepancy is likely attributed to overlapping vibrational bands from various uranyl species and (the lack of) band deconvolution used during spectral analysis. This also occurred for succinic acid, which also chelates to the uranyl cation through mono- or bidentation coordination modes and displays multiple vibrational frequencies in ATRIR spectra (950, 938, and 925 cm −1 ) [185]. …”

Exploring Actinide Materials Through Synchrotron Radiation Techniques

“…Rencently, Lucks et al investigated the structures of U(VI) complexes with acetate and succinate ligands in aqueous solution at different pH using ultra‐violet‐visible absorption (UV‐vis), EXAFS and IR absorption spectroscopy coupled with iterative transformation factor analysis (ITFA) and DFT calculations. In the acetate (ac) system, the 1:1, 1:2 and 1:3 U‐ac complexes occurred with increasing pH and these complexes exhibited exclusively bidentate coordination.…”

The pH dependence of Am(III) complexation with acetate: an EXAFS study

“…In the literature a number of EXAFS studies on the interaction of acetate with different actinides can be found [Th(IV) (Rao et al, 2004), U(VI) (Bailey et al, 2004;Jiang et al, 2002;Lucks et al, 2012), Np(IV) (Takao et al, 2012), Np(V,VI) (Takao et al, 2009)]. However, no EXAFS data on the complexation of Am(III) or other trivalent actinides with acetate are available.…”