Perrhenate Complexation by Uranyl in Traditional Solvents and in Ionic Liquids: A Joined Molecular Dynamics/Spectroscopic Study

The complexation of perrhenate (ReO4-) anions by the uranyl (UO22+) cation is investigated by joined molecular dynamics simulations and spectroscopic (UV-Vis, TRLFS and EXAFS) studies in aqueous solution, acetonitrile and three ionic liquids (ILs), namely [Bmi][Tf2N], [Me3BuN][Tf2N] and [Bu3MeN][Tf2N] that are based on the same Tf2N- anion (bis(trifluoromethylsulfonyl)imide) and either Bmi+ (1-butyl,3-methylimidazolium), Me3BuN+ or Bu3MeN+ cations. They show that ReO4- behaves as a weak ligand in aqueous solution and as a strong ligand in acetonitrile and in the ILs. In aqueous solution, the simulated UO2(ReO4)2 complex quickly dissociates to form the UO2(H2O)52+ species, while in acetonitrile the simulated UO2(ReO4)53- complex forms from dissociated ions and remains stable along 100 ns of dynamics. When the UO2(ReO4)n2-n complexes (n = 1 to 5) are simulated in the ILs, the uranyl cation remains coordinated to the ReO4- ligands, and to additional OTf2N oxygens when n < 5. To assess the relative stabilities of these complexes, we computed the free energy profiles for stepwise ReO4- complexation by uranyl (PMF calculations). In the two studied ILs, perrhenate complexation is favoured, leading to the UO2(ReO4)53- species in [Bmi][Tf2N] and to UO2(ReO4)42- in [Bu3MeN][Tf2N]. Furthermore, in both acetonitrile and [Bmi][Tf2N] solutions, MD and PMF simulations support the formation of dimeric uranyl complexes [UO2(ReO4)4]24- with two bridging ReO4- ligands. The simulation results are qualitatively consistent with spectroscopic observations in the different solvents, without firmly concluding, however, on the precise composition and structure of the complexes in the solutions.