Novel pure Pnma–P212121 ferroelastic phase transition of 1,4-diisopropyl-1,4-diazonia-bicyclo[2.2.2]octane tetra-chlorobromo-M(ii) (M = Mn and Co)

Abstract

Two novel phase transition compounds Dip-DABCO tetra-chlorobromo-M(II) (M = Mn and Co), (Dip-DABCO = 1,4-diisopropyl-1,4-diazonia-bicyclo[2.2.2]octane) C₁₂H₂₆N₂·MnBrCl₃ (1) and (C₁₂H₂₆N₂)₄·(CoBr_1.25Cl_2.75)₄ (2) were synthesized, and their structures have been determined by means of single-crystal X-ray diffraction. The two compounds are isomorphous. Differential scanning calorimetry (DSC) measurements indicated that compounds 1 and 2 underwent a reversible phase transition at ca. 245.2 K with a hysteresis width of 4.4 K and at ca. 222.3 K with a hysteresis width of 5.5 K, respectively. The variable-temperature single-crystal X-ray diffraction data suggests that the phase transition was from high crystallographic symmetry with a space group of Pnma to a low-symmetry state with a space group of P2₁2₁2₁. That is, Landau symmetry breaking occurred with a pure GPT (mmm–222). The ordering of twisting motions of the 1,4-diazoniabicylo[2,2,2]octane ring may have driven the phase transition.