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(C Me ) ]X ([1]X), [Co(C Me ) ]X ([2]X), and [Fe(C Me H) ]X ([3]X) have been investigated, where the counter anions (X) are Tf N (=(CF SO ) N ), OTf (=CF SO ), PF , and BF . The Tf N 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.

Phase Transitions, Crystal Structures, and Magnetic Properties of Ferrocenium Ionic Plastic Crystals with CF3BF3 and Other Anions

“…The phase sequences of these salts are shown in Figure . Several salts exhibited additional phase transitions, as has been reported for other ferrocene‐containing salts, which may be related to the order/disorder of the anion.…”

“…With respect to the cation orientation, each unit cell of these salts contains two types of cations that are oriented with their C 5 axes almost perpendicular to each other. The salts of 1 and 2 often exhibit unidirectional alignment of the cations in the crystal;,, therefore, the present salts are exceptional. In [ 1 ][CF 3 BF 3 ] (space group P nnm , Z =2), the carbon and boron atoms in the anion are disordered (0.5:0.5).…”

“…As discussed previously, a plausible explanation for the overall tendency is that the contacts between the cations get larger with decreasing radius ratio ( ρ = r small ion / r large ion ), suppressing the molecular rotation due to steric hindrance of the methyl groups, which probably led to the increase in the transition temperature . In contrast to the case of [MCp 2 ][X] (M=Fe, Co) discussed above, the transition temperatures of the CF 3 BF 3 and OTf salts were comparable in [ 1 ][X] ([ 1 ][CF 3 BF 3 ]: T C =416.9 K, [ 1 ][OTf]: 419.7 K) and [ 2 ][X] ([ 2 ][CF 3 BF 3 ]: T C =∼270 K, [ 2 ][OTf]: T C =288.6 K). This is ascribed to [ 1 ] + and [ 2 ] + having considerably larger cation volumes (351.3 and 315.7 Å 3 , respectively) than [MCp 2 ] + , which reduces the influence of the local charge distribution in the anion on the cation–anion electrostatic interactions.…”

“…The phase transition temperatures of [ 1 ][X] and [ 2 ][X] are shown in Figure b along with the data for the BF 4 , PF 6 , OTf, and Tf 2 N salts . The phase transition temperatures of [ 2 ][X] are ∼100 K lower than those of [ 1 ][X] with the same anion.…”

“…Many salts containing sandwich complexes exhibit a plastic phase because of their nearly spherical molecular shape; however, the temperature of the phase transition ( T C ) to the plastic phase is generally high, and only a few salts are known to exhibit a plastic phase at room temperature . In the pursuit of a molecular design to lower the transition temperature, we developed various ionic plastic crystals containing cationic sandwich complexes and investigated the correlation between molecular size and transition temperature . By investigating diamagnetic salts containing Co or Ru, we found that CF 3 BF 3 − is a useful anion that lowers the transition temperature .…”