Switchable dielectric phase transition in tris(1-(chloromethyl)-1,4-diazabicyclo[2.2.2]octane) tetra(tetrafluoroborate) dichloride

•Temperature-induced structural phase transition•Symmetry breaking occurred with mmmF2/m.•Phase transition undergoes from Pnma to P21/c.

Tris(1-(chloromethyl)-1,4-diazabicyclo[2.2.2]octane) tetra(tetrafluoroborate) dichloride, [C7H15ClN22 +]3·[BF4−]4·2Cl−, was synthesized and separated as colorless rodlike crystals. Differential scanning calorimetry detected that this compound underwent a reversible phase transition at ca. 242.9 K with a hysteresis of 3.7 K width. Single crystal X-ray diffraction data suggested that a room-temperature phase with a space group of Pnma (a = 20.0644(12) Å, b = 7.3121(4) Å, c = 25.6369(15) Å, V = 3761.3(4) Å3 and Z = 4) was transited to a low temperature one with a space group of P21/c (a = 7.184(3) Å, b = 25.579(10) Å, c = 20.074(8) Å, β = 91.348(6)°, V = 3688(3) Å3 and Z = 4), and symmetry breaking occurred with an Aizu notation of mmmF2/m. The order–disorder transition of BF4− anions and the ordering of twisting motions of dabco rings may drive the phase transition.

Graphical abstractA novel mononuclear metal–organic compound, tris(1-(chloromethyl)-1,4-diazabicyclo[2.2.2]octane) tetra(tetrafluoroborate) dichloride, [C7H15ClN22 +]3·[BF4−]4·2Cl−, was synthesized. X-ray diffraction data suggested that the phase transition undergoes from a room temperature phase with a space group of Pnma to a low temperature one with a space group of P21/c, and symmetry breaking occurred with an Aizu notation of mmmF2/m.Figure optionsDownload full-size imageDownload as PowerPoint slide