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 (CHDA 2+ ) forms a hydrogen-bonded sandwich-type supramolecule with benzo[18]crown-6 in (CHDA 2+ )(benzo[18]crown-6) 2 [Ni(dmit) 2 ] 2 ( 1 ) (dmit 2–: 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 P 2 1 /c, P 1̅, and P 1̅, 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 CHDA 2+ 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 (CHDA 2+ )(benzo[18]crown-6) 2 [Ni(dmit) 2 ] − 2 .