Reversible ferroelastic phase transition of N-chloromethyl-1,4-diazabicyclo[2.2.2]octonium trichlorobromoaquo copper(II)

A novel mononuclear metal–organic compound, N-chloromethyl-1,4-diazabicyclo-[2.2.2]octonium trichlorobromoaquo copper(II), Cu(LCH2Cl) (H2O) Br2.75Cl0.25 (1) (L + CH2Cl = N-chloromethyl-1,4-diazabicyclo[2.2.2]octonium ion) was synthesized. Differential scanning calorimetry (DSC) results showed that this compound underwent a reversible phase transition at ca. 190 K with a hysteresis of 11 K width and single crystal X-ray diffraction data suggested that there was a transition from a room-temperature phase with the space group of Pnma (a = 17.790(2) Å, b = 9.0000(12) Å, c = 8.2263(11)Å, V = 1317.1(3) Å3 and Z = 1) to a low-temperature one with the space group of P21/n (a = 8.184(4) Å, b = 17.681(8) Å, c = 8.941(4) Å, β = 91.218(6)°, V = 1293.5(10) Å3 and Z = 1). Symmetry breaking occurred with the Aizu notation of mmmF2/m. The ordering of twisting motions of 1,4-diazoniabicylo[2,2,2]octane ring may drive the phase transition.