Unprecedented transformation of [I−·I3−] to [I42−] polyiodides in the solid state: structures, phase transitions and characterization of dipyrazolium iodide triiodide

Abstract

Dipyrazolium iodide triiodide, [C₃N₂H₅⁺]₂[I⁻·I₃⁻], has been synthesized and studied by means of X-ray diffraction, differential scanning calorimetry, dielectric measurements, and UV-Vis spectroscopy. Two reversible, solid–solid phase transitions (Imma (I) ↔ (II) ↔ Pbam (III)) at 254 K and 182/188 K respectively have been revealed. The anionic network experiences spectacular changes associated with a huge rebuilding of the inorganic network from [I⁻·I₃⁻] to [I₄²⁻]. The low frequency dielectric relaxation process occurs in phase II with the activation energy of ca. 34 kJ mol⁻¹. The molecular motion of the pyrazolium cations in [C₃N₂H₅⁺]₂[I⁻·I₃⁻] has been studied by means of proton magnetic resonance studies (¹H NMR). The ferroelastic properties of all phases have been confirmed by polarizing microscopy observations. The molecular mechanism of the phase transitions in the compound is proposed.