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

摘要

The complexation of perrhenate (ReO4~-) anions by the uranyl (UO2~(2+)) 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 TfjN~- anion (bis(trifluoromethylsulfony!)-imide) and either Bmi~+ (l-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 dim eric 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.