Hydrogen-Bonded Polyrotaxane Cation Structure in Nickel Dithiolate Anion Radical Salts: Ferromagnetic and Semiconducting Behavior Associated with Structural Phase Transition.

The pseudo-polyrotaxane structure of [(H-bpy+ )- (DB-24-crown-8)]∞ (H-bpy+ = monoprotonated 4,4-bipyridinium; DB-24-crown-8 = dibenzo-24-crown-8) has been incorporated into the anion radical salt [Ni(dmit)2 ]- (dmit2- = 1,3-dithiole-2-thione-4,5-dithiolate). (H-bpy+ )(DB-24-crown-8)[Ni(dmit)2 ]- crystallized as two polymorphs, crystals 1 and 2. Crystal 1 was found to have a lower density and looser packing structure in which H-bpy+ forms a one-dimensional hydrogen-bonding chain that passes though the crown ether ring of DB-24-crown-8. DB-24-crown-8 adopts a U-shaped conformation in which two phenylene rings sandwich one of the pyridyl rings of H-bpy+ to stabilize the structure. The [Ni(dmit)2 ]- anions are arranged in a layer parallel to the (10) plane with uniform side-by-side interactions. A structural phase transition was observed at 235 K, accompanied by ordering of the polyrotaxane structure. In crystal 1, at 173 K, H-bpy+ is twisted around the central C-C bond, which perturbs the arrangement of [Ni(dmit)2 ]- through short C-H⋅⋅⋅S contacts. As a result, the semiconducting behavior, with an activation energy of 0.21 eV, becomes insulating below 235 K. The crystal exhibits ferromagnetic interactions because of the weak side-by-side interactions between [Ni(dmit)2 ]- anions. Crystal 2 has a similar pseudo-polyrotaxane structure but showed no phase transition. This suggests that the looser crystal packing in crystal 1 induces the structural change of the pseudo-polyrotaxane, perturbing the electron system of [Ni(dmit)2 ]- .