Hydrogen‐Bonded Polyrotaxane Cation Structure in Nickel Dithiolate Anion Radical Salts: Ferromagnetic and Semiconducting Behavior Associated with Structural Phase Transition. Issue 28 (17th April 2019) - Shirakawa, Yuki
Abstract: 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 ] − (dmit 2− = 1, 3‐dithiole‐2‐thione‐4, 5‐dithiolate). (H‐bpy + )(DB‐24‐crown‐8)[Ni(dmit)2 ] − crystallized as two polymorphs, crystals1 and2 . Crystal1 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 crystal1, 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. Crystal2 has a similar pseudo‐polyrotaxane structure but showed no phase transition. This suggests that the looser crystal packing in crystal1 induces the structural change of the pseudo‐polyrotaxane, perturbing the electron system of [Ni(dmit)2 ] − . Abstract : Time to switch ! A pseudo‐polyrotaxane structure has been introduced into the [Ni(dmit)2 ] − single crystal (dmit 2− =1, 3‐dithiole‐2‐thione‐4, 5‐dithiolate). The molecular motion of the flexible pseudo‐polyrotaxane caused a phase transition perturbing the arrangement of [Ni(dmit)2 ] − (see figure), which resulted in the semiconductor/insulator transition associated with ferromagnetic behavior at low temperatures.