Design of Crystalline Spaces for Molecular Rotations in Crystals

4-Methylanilinium derivatives were used to introduce spaces for molecular rotation in crystals. The [Ni­(dmit)2]− (dmit2– = 2-thioxo-1,3-dithiole-4,5-dithiolate) salts with supramolecular cations of dibenzo[18]­crown-6 (DB[18]­crown-6) and 4-methylanilinium derivatives, (4-methylanilinium+)­(DB­[18]­crown-6)­[Ni­(dmit)2]− (1), (2-fluoro-4-methylanilinium+)­(DB­[18]­crown-6)­[Ni­(dmit)2]− (2), and (3-fluoro-4-methylanilinium+)­(DB­[18]­crown-6)­[Ni­(dmit)2]− (3) were synthesized. The potential energy curves for the molecular rotations of the cations in the crystals had double minimum shapes with maxima of 100, 210, and 230 kJ mol–1 for crystals 1, 2, and 3, respectively. Introduction of a methyl substituent at the p-position was effective in reducing the potential energy maxima. For crystals 2 and 3, large dielectric responses originating from the flip-flop motions of the cationic molecules were observed upon applying an AC voltage. The temperature-dependent magnetic susceptibilities of complexes 1, 2, and 3 followed the Curie–Weiss law, showing weak antiferromagnetic interactions.