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) was incorporated into anion radical salts, [Ni(dmit)2]- (dmit2- = 1,3-dithio-2-thione-4,5-dithiolate). (H-bpy+)(DB[24]crown-8)[Ni(dmit)2]- crystallized into two polymorphs: crystals 1 and 2. Crystal 1 had a lower density and looser packing structure. In crystal 1, H-bpy+ formed a one-dimensional hydrogen bonding chain, which passed though the crown ether ring of DB[24]crown-8. DB[24]crown-8 took a U-shape conformation, in which two phenylene rings sandwiched one of the pyridyl rings of H-bpy+ to stabilize the structure. The [Ni(dmit)2]- anions arranged in the layer parallel to the (101(-)) plane with uniform side-by-side interaction. Structural phase transition was observed at 235 K, accompanied by ordering of the polyrotaxane structure. In crystal 1, at 173 K, H-bpy+ was twisted along the central C-C bond, which perturbed the [Ni(dmit)2]- arrangement through short C-H•••S contacts. As a result, semiconducting behavior, with activation energy of 0.21 eV, became insulating below 235 K. The crystal exhibited ferromagnetic interaction because of the weak side-by-side interaction between [Ni(dmit)2]-. Crystal 2 had a similar pseudo-polyrotaxane structure but showed no phase transition. This suggested that a looser crystal packing induced the structural change of pseudo-polyrotaxane, perturbing the electronic system of [Ni(dmit)2]-.