Polymorphs and Structural Phase Transition of [Ni(dmit)2]− Crystals Induced by Flexible (trans-Cyclohexane-1,4-diammonium)(Benzo[18]crown-6)2 Supramolecule

Structurally flexible trans-cyclohexane-1,4-diammonium (CHDA2+) forms a hydrogen-bonded sandwich-type supramolecule with benzo[18]crown-6 in (CHDA2+)(benzo[18]crown-6)2[Ni(dmit)2]2 (1) (dmit2–: 2-thioxo-1,3-dithiole-4,5-dithiolate). During the crystallization, the three crystal polymorphs 1a, 1b, and 1c were obtained in the space groups P21/c, P1̅, and P1̅, in which the orientation of the benzene rings of benzo[18]crown-6 in the sandwich-type supramolecular cations and the [Ni(dmit)2]− arrangements were different. Crystal 1c had a structural phase transition at ∼200 K, at which the unit cell volume of the low-temperature phase (1d) became twice that of the high-temperature phase (1c). The [Ni(dmit)2] π-dimer was observed in crystals 1a, 1b, 1c, and 1d. A double-minimum-type potential energy profile was observed for the rigid rotation of CHDA2+ around the N–N axis. Among four new crystals, the relatively low potential energy barrier of ∼120 kJ mol–1 for crystal 1b was consistent with the temperature- and frequency-dependent dielectric constants. The crystal polymorph and structural phase transition were coupled in (CHDA2+)(benzo[18]crown-6)2[Ni(dmit)2]−2.