Thermomorphic Behavior of the Ionic Liquids [C4mim][FeCl4] and [C12mim][FeCl4]

The iron-containing ionic liquids 1-butyl-3-methylimidazolium tetrachloroferrate(III) [C(4)mim][FeCl(4)] and 1-dodecyl-3-methylimidazolium tetrachloroferrate(III) [C(12)mim][FeCl(4)] exhibit a thermally induced demixing with water (thermomorphism). The phase separation temperature varies with IL weight fraction in water and can be tuned between 100 °C and room temperature. The reversible lower critical solution temperature (LCST) is only observed at IL weight fractions below ca. 35 % in water. UV/Vis, IR, and Raman spectroscopy along with elemental analysis prove that the yellow-brown liquid phase recovered after phase separation is the starting IL [C(4)mim][FeCl(4)] and [C(12)mim][FeCl(4)], respectively. Photometry and ICP-OES show that about 40 % of iron remains in the water phase upon phase separation. Although the process is thus not very efficient at the moment, the current approach is the first example of an LCST behavior of a metal-containing IL and therefore, although still inefficient, a prototype for catalyst removal or metal extraction.