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

The complexation of perrhenate (ReO4-) anions by the uranyl (UO22+) cation has been investigated by joint 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. According to MD simulations in aqueous solution, the UO2(ReO4)(2) complex quickly dissociates to form UO2(H2O)(5)(2+), while in acetonitrile, a stable UO2(ReO4)(5)(3-) species forms from dissociated ions. In the ILs, the UO2(ReO4)(n)(2-n) complexes (n = 1 to 5) remained stable along the dynamics, and to assess their relative stabilities, we computed the free energy profiles for stepwise ReO4- complexation to uranyl. In the two studied ILs, complexation is favored, leading to the UO2(ReO4)(5)(3-) species in [Bmi][Tf2N] and to UO2(ReO4)(4)(2-) 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)](2)(4-) 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.