Ionic liquids containing the urea moiety for recognition of halides and complex anions

A series of novel imidazolium based ionic liquids containing the urea moiety were designed and synthesized for anion recognition. 1-Ethylurea-3-methylimidazolium acetate ([Eumim]OAc) was used as the receptor for the halides and complex anions (BF ₄ ^-, PF ₆ ^-, BPh ₄ ^-). ¹H NMR spectra showed that the urea protons and imidazolium C(2) proton of the receptor ([Eumim]OAc) moved upfield on addition of various anions. A Job plot showed that the [Eumim]OAc receptor formed a 1:1 complex with BPh ₄ ^-. X-ray diffraction analysis and the molecular modeling study revealed that the conformations of [Eumim]OAc and [Eumim]BPh ⁴ were different. The conformational change of the cation was caused by anion exchange, and may provide an alternative to current methods for recognition of anions.