Reversible structural phase transition and dielectric behaviour associated with the ordering-disordering of molecular rotation in an organic supramolecular solid

A pure organic single crystal, [H(2)dabco][Ma](2) (1) ([H(2)dabco](2+) = diprotonated 1,4-diazabicyclo[2,2,2] octane, [Ma] = malic acid), was prepared and characterised. Differential scanning calorimetry, potential energy calculations, and dielectric property measurements indicated the occurrence of a reversible structural phase transition at similar to 250 K, while single-crystal X-ray diffraction revealed that 1 possesses a unique three-dimensional network structure with the [H(2)dabco](2+) cation bridged by two [Ma](-) anions through N-H center dot center dot center dot O hydrogen bonds. At 296 K, the [H(2)dabco](2+) cation exists in a rotationally disordered state and is fixed into the two-dimensional layered structure of [Ma](-) anions through N-H center dot center dot center dot O hydrogen bonds. Upon cooling to 100 K, the above cation is frozen into an ordered state and shifts towards the top anion and the bottom anion at one end along the hydrogen bond. The origin of the phase transition and the dielectric anomaly is ascribed to the order-disorder transformation of [H(2)dabco](2+) cations and the resulting relative molecular rotation from the equilibrium position. (C) 2021 Published by Elsevier B.V.