Temperature-induced reversible structural phase transition of 1-(chloromethyl)-1,4-diazoniabicyclo[2.2.2]octane bis(perchlorate)

Abstract

1-(Chloromethyl)-1,4-diazoniabicyclo[2.2.2]octane bis(perchlorate), C₇H₁₅ClN₂²⁺·2ClO₄⁻, was synthesized and separated as colorless block crystals. Differential scanning calorimetry (DSC) measurements detected that this compound underwent a reversible phase transition at ca. 143.9 K with a hysteresis width of 12.7 K, which was also confirmed by dielectric measurements. Single crystal X-ray diffraction data suggested that the compound may undergo phase transition (170 K) from a room temperature phase with a space group of Pbca (a = 13.585(4) Å, b = 12.926(4) Å, c = 16.012(5) Å, V = 2811.8(16) ų and Z = 8) to a low temperature one with a space group of P2₁/c (a = 12.659(4) Å, b = 16.121(5) Å, c = 13.197(4) Å, β = 91.231(4)°, V = 2692.5(14) ų and Z = 8.), and symmetry breaking occurred with an Aizu notation of mmmF2/m. The order–disorder transition of ClO₄⁻ anions and the ordering of twisting motions of the dabco ring may drive the phase transition.