[CuCln]2-n Ion-Pair Species in 1-Ethyl-3-methylimidazolium Chloride Ionic Liquid-Water Mixtures: Ultraviolet-Visible, X-ray Absorption Fine Structure, and Density Functional Theory Characterization

We studied the coordination environment about Cu(II) in a pure ionic liquid, 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl), and in binary mixtures of this compound with water across the entire range of compositions, using a combination of X-ray absorption line structure (XAFS), ultraviolet-visible (UV-vis) spectroscopy, and electronic structure calculations. Our results show a series of stages in the ion pairing of the divalent cation, Cu(II), including the contact ion pairing of Cu2+ with multiple Cl- ligands to form various CuCln(2-n) polyanions, as well as the subsequent solvation and ion pairing of the polychlorometallate anion with the EMIM+ cation. ion-pair formation is strongly promoted in [EMIM]Cl by the low dielectric constant and by the extensive breakdown of the water hydrogen-bond network in [EMIM]Cl-water mixtures. The CuCl42- species dominates in the [EMIM]Cl solvent, and calculations along with spectroscopy show that its geometry distorts to C-2 symmetry compared to Did in the gas phase. These results are important in understanding catalysis and separation processes involving transition metals in ionic liquid systems.