Solvation of uranyl(II), europium(III) and europium(II) cations in basic room-temperature ionic liquids: A theoretical study

We report a molecular dynamics study of the solvation of UO22+, Eu3+ and Eu2+ ions in two "basic" (Lewis acidity) room-temperature ionic liquids (IL) composed of the 1-ethyl-3-methylimidazolium cation (EMI+) and a mixture of AlCl4- and Cl- anions, in which the Cl-/AlCl4- ratio is about 1 and 3, respectively. The study reveals the importance of the [UO2Cl4](2-) species, which spontaneously form during most simulations, and that the first solvation shell of europium is filled with Cl- and AlCl4- ions embedded in a cationic EMI+ shell. The stability of the [UO2Cl4](2-) and [(EuCl6)-Cl-III](3-) complexes is supported by quantum mechanical calculations, according to which the uranyl and europium cations intrinsically prefer Cl- to the AlCl4- ion. In the gas phase, however, [(EuCl6)-Cl-III](3-) and [(EuCl6)-Cl-II](4-) complexes are predicted to be metastable and to lose two to three Cl- ions. This contrasts with the results of simulations of complexes in ILs, in which the "solvation" of the europium complexes increases with the number of coordinated chlorides, leading to an equilibrium between different chloro species. The behavior of the hydrated [Eu(OH2)(8)](3+) complex is considered in the basic liquids; the complex exchanges H2O molecules with Cl- ions to form mixed [EuCl3(OH2)(4)] and [EuCl4(OH2)(3)](-) complexes. The results of the simulations allow us to better understand the microscopic nature and solvation of lanthanide and actinide complexes in "basic" ionic liquids.