Energetics and structure of uranium(VI)-acetate complexes in dimethyl sulfoxide.

The thermodynamics of the complexation between uranium(VI) and acetate in dimethyl sulfoxide (DMSO) was studied at 298 K in an ionic medium of 0.1 mol dm(-3) tetrabutyl ammonium perchlorate. The results show that the uranyl ion forms three strong successive mononuclear complexes with acetate. The complexes, both enthalpically and entropically stabilized, are significantly more stable in DMSO than in water. This feature can be ascribed to the weak solvation of acetate in DMSO. The thermodynamic parameters for the formation of the uranium(VI) complexes with acetate in DMSO are compared with those with ethylenediamine in the same solvent. The difference between the two ligand systems reveals that, for the complexation reactions involving charge neutralization, the reorganization of the solvent gives a very important contribution to the overall complexation energetics. The coordination mode of acetate in the uranyl complexes and the changes of the solvation sphere of UO(2)(2+) upon complexation were investigated by FT-IR spectroscopy in DMSO and in acetonitrile/DMSO mixtures. In addition, DFT calculations were performed to provide an accurate description of the complexation at the molecular level. The experimental and calculated results suggest that acetate is solely bidentate to UO(2)(2+) in the 1:1 and 1:3 complexes but mono- and bidentate in the 1:2 complexes. The DFT calculations also indicate that the medium effects must always be taken into account in order to gain accurate information on the complex formation in solution. In fact, the relative stability of the reaction products changes markedly when the DFT calculations are carried out in vacuum or in DMSO solution.