Uranyl(VI) Complexes in and from Imidazolium Acetate Ionic Liquids: Carbenes versus Acetates?

Abstract

Formation of uranyl(VI)-N-heterocyclic carbene complexes from 1-alkyl-3-methylimidazolium acetate ionic liquid carbene sources was investigated by theoretical methods, combining classical molecular dynamics ab initio molecular dynamics and static DFT calculations. The interaction between the UO2(2+) cation and the acetate anion was found to be very strong, in accordance with the hard and soft acids and bases principle. The calculations, however, indicate that the preference of the acetate anions to coordinate monodentately, together with the Coulombic repulsion between these anionic ligands, provides accessible sites for the carbene in the corresponding uranyl-acetato complexes. According to the calculations the carbene is bound to the uranium atom of the [UO2(OAc)3](-) complex with significant strength; ΔE value of -79.4 kJ mol(-1) was obtained. Comparison with the analogous, but experimentally confirmed, reaction between the present ionic liquid and CO2 indicates that this binding strength should be enough for the reaction to occur. The analogous aqua complexes exhibit lower stability, showing that traces of moisture should not prohibit the reaction via competitive coordination. The results obtained suggest a mild and very convenient method for preparing carbene complexes of metals in general, including those with the uranyl cation, which have been so far limited to a few exotic examples.