Synthesis, crystal structure and phase transitions of a series of imidazolium iodides

Abstract

The reaction of imidazole with hydroiodic acid leads to three products crystallizing as ionic salts; [C₃N₂H₅⁺][I⁻], [C₃N₂H₅⁺]₂[I₄²⁻] and [C₃N₂H₃I₂⁺][I⁻]. All the analogs were characterized by single-crystal X-ray diffraction, while the first two were additionally studied by calorimetric, dilatometric, dielectric and proton magnetic resonance methods. At room temperature (RT), [C₃N₂H₅⁺][I⁻] adopts the centrosymmetric, trigonal space group (R). The crystal structure consists of disordered imidazolium cations and discrete I⁻ ions. [C₃N₂H₅⁺][I⁻] undergoes two discontinuous phase transitions (PTs) at 180/185 K and 113/123 K (cooling–heating), both of them governed by the imidazolium cation dynamics. [C₃N₂H₅⁺]₂[I₄²⁻] consists of disordered imidazolium cations and quite rare and exotic [I₄]²⁻ tetraiodide counterion. It undergoes continuous PT at 204 K of the ferroelastic type with a symmetry change from orthorhombic Fddd to monoclinic C2/c. The mechanism of PT is complex and consists of ‘order–disorder’ and ‘displacive’ contributions that are assigned to the dynamics of cations and to the distortion of the [I₄²⁻] rods, respectively. [C₃N₂H₃I₂⁺][I⁻] is built up of discrete 4,5-diiodoimidazolium cations and isolated I⁻ ions. A characteristic feature of this compound is the presence of a layered structure in which moieties are held together by strong I⋯I halogen interactions and N–H⋯I hydrogen bonds.