Ionic liquid, glass or crystalline solid? Structures and thermal behaviour of (C4mim)2CuCl3

Abstract

The ionic liquid (C₄mim)₂CuCl₃ was synthesised from a mixture of copper(I) chloride and 1-butyl-3-methylimidazolium chloride (C₄mimCl) and investigated using crystallographic and thermoanalytical methods. In the crystalline state, the compound consists of C₄mim⁺ cations and triangular [CuCl₃]²⁻ anions and forms three different modifications, which are connected through phase transitions at 227 and 203 K. The high and intermediate temperature phases crystallise in the space group C2, whereas the low temperature phase exhibits the space group P2₁. The three crystal structures are related through an isomorphic and a klassengleiche symmetry transition, respectively. The solid undergoes congruent melting at 320 K. The enthalpy of fusion was determined to be 25.7 kJ mol⁻¹. The melting process is irreversible and the ionic liquid can be supercooled to its glass transition at 221 K.