Speciation of Copper(II) Complexes in an Ionic Liquid Based on Choline Chloride and in Choline Chloride/Water Mixtures

A deep-eutectic solvent with the properties of an ionic
liquid
is formed when choline chloride is mixed with copper­(II) chloride
dihydrate in a 1:2 molar ratio. EXAFS and UV–vis–near-IR
optical absorption spectroscopy have been used to compare the coordination
sphere of the cupric ion in this ionic liquid with that of the cupric
ion in solutions of 0.1 M of CuCl2·2H2O
in solvents with varying molar ratios of choline chloride and water.
The EXAFS data show that species with three chloride ions and one
water molecule coordinated to the cupric ion as well as species with
two chloride molecules and two water molecules coordinated to the
cupric ion are present in the ionic liquid. On the other hand, a fully
hydrated copper­(II) ion is formed in an aqueous solution free of choline
chloride, and the tetrachlorocuprate­(II) complex forms in aqueous
choline chloride solutions with more than 50 wt % of choline chloride.
In solutions with between 0 and 50 wt % of choline chloride, mixed
chloro–aquo complexes occur. Upon standing at room temperature,
crystals of CuCl2·2H2O and of Cu­(choline)­Cl3 formed in the ionic liquid. Cu­(choline)­Cl3 is
the first example of a choline cation coordinating to a transition-metal
ion. Crystals of [choline]3[CuCl4]­[Cl] and of
[choline]4[Cu4Cl10O] were also synthesized
from molecular or ionic liquid solvents, and their crystal structures
were determined.