Effects of Molecular Structure on Phase Transitions of Ionic Plastic Crystals Containing Cationic Sandwich Complexes

Salts of sandwich complexes often exhibit a plastic phase at high temperatures. To determine a molecular design that can achieve a plastic phase at lower temperatures, we synthesized [CoCp2][X] and [Ru(Cp)(C6H6)][X] (Cp = C5H5) with various anions (X = monocarba-closo-dodecaborate (CB11H12-:), B(CN)(4)(-), CF3BF3-, OTf (-), BF4-, C(CN)(3)(-), and tris(pentafluoroethyl)trifluorophosphate (FAP(-))) and investigated their phase behaviors. All of the salts except the C(CN)(3) and FAP salts exhibited a plastic phase. The phase transition temperature to the plastic phase tended to decrease with decreasing anion size. This tendency contrasts that observed in octamethyl- and decamethylferrocenium salts. Although the phase transition temperatures of most salts were high, those of [CoCp2] [CF3BF3], [Ru(Cp)(C6H6)][CF3BF3], and [Ru(Cp)(C6H6)][BF4] were below 300 K. The plastic phases of the salts had a CsCl-type structure. Crystal structure determinations at low temperatures revealed that the cations and anions were arranged alternately in most of the salts. However, the C(CN)(3) salts exhibited a stacking arrangement of the cations, which is responsible for the absence of a plastic phase.