title: Crystal Structures and Phase Sequences of Metallocenium Salts with Fluorinated Anions: Effects of Molecular Size and Symmetry on Phase Transitions to Ionic Plastic Crystals.
abstract: Sandwich compounds often exhibit various phase transitions, including those to plastic phases. To elucidate the general features of the phase transitions in metallocenium salts, the thermal properties and crystal structures of [Fe(C5Me5)2]X ([1]X), [Co(C5Me5)2]X ([2]X), and [Fe(C5Me4H)2]X ([3]X) have been investigated, where the counter anions (X) are Tf2N (=(CF3SO2)2N-), OTf (=CF3SO3-), PF6, and BF4. The Tf2N salts commonly undergo phase transitions from an ordered phase at low temperatures to an anion-disordered phase, followed by a plastic phase and finally melt at high temperatures. All these salts exhibit a phase transition to a plastic phase, and the transition temperature generally decreases with decreasing cation size and increasing anion size. The crystal structures of these salts comprise an alternating arrangement of cations and anions. About half of these salts exhibit phase transitions at low temperatures, which are mostly correlated with the order-disorder of the anion.
authors.affiliation: Department of Chemistry, Faculty of Science, Kobe University, Rokkodai, Nada, Hyogo, 657-8501, Japan. tmochida@platinum.kobe-u.ac.jp.
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Mochida
authors.affiliation: Department of Applied Chemistry, Faculty of Engineering, Tokyo University of Science, Yamaguchi, Sanyo-Onoda, Yamaguchi, 756-0884, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Funasako
authors.affiliation: Department of Chemistry, Faculty of Science, Kobe University, Rokkodai, Nada, Hyogo, 657-8501, Japan.
authors.forename: Mai
authors.initials: M
authors.lastname: Ishida
authors.affiliation: Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi, Chiba, 274-8510, Japan.
authors.forename: Shingo
authors.initials: S
authors.lastname: Saruta
authors.affiliation: Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi, Chiba, 274-8510, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Kosone
authors.affiliation: Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi, Chiba, 274-8510, Japan.|Research Center for Materials with Integrated Properties, Faculty of Science, Toho University, Miyama, Funabashi, Chiba, 274-8510, Japan.
authors.forename: Takafumi
authors.initials: T
authors.lastname: Kitazawa
keywords: crystal structures; metallocenes; phase transitions; plastic crystals; structure elucidation