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 UO22+ 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 UO22+ 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.