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

Abstract

1,4-Dimethyl-1,4-diazabicyclo[2.2.2]octane bis(perchlorate), C₈H₁₈N₂²⁺·2ClO₄⁻, was synthesized and separated as colorless rodlike crystals. Differential scanning calorimetry (DSC) measurements revealed that this compound underwent a reversible phase transition at ca. 201.7 K with a hysteresis of 5.1 K width, which was also confirmed by dielectric measurements. Single crystal X-ray diffraction data suggested that there was a transition from a room temperature phase with the space group of Pnma (a = 15.5417(14) Å, b = 13.3677(12) Å, c = 20.7728(19) Å, V = 4315.7(7) ų and Z = 12) to a low temperature one with a space group of P2₁/n (a = 13.331(3) Å, b = 15.185(4) Å, c = 20.477(5) Å, β = 90.895(3)°, V = 4144.8(17) ų and Z = 12), 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.