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

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 (v(3)(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 v(3,as)(COO) and symmetric v(3,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 (v(3)(UO2), v(3,as)(COO), v(3,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-O-water distance of similar to 2.36 angstrom, which is shorter than the U-O-water distance of similar to 2.40 angstrom of the hydrated uranyl ion. For all complexes the U-O-carboxyl distance of the bidentately coordinated carboxylic group is similar to 2.47 angstrom, while the monodentately coordinated carboxylic group of the 1:3 U(VI)-suc complex has a U-O-carboxyl distance of similar to 2.36 angstrom, that is, similar to the short U-O-water distance in the 1:1 and 1:2 complexes.