Simple cationic sandwich complexes that contained alkyl- or halogen substituents provided ionic liquids (ILs) with the bis(perfluoroalkanesulfonyl)imide anion. Ferrocenium- and cobaltocenium ILs [M(C5H4R1)(C5H4R2)][Tf2N] (M=Fe, Co) and arene–ferrocenium ILs [Fe(C5H4R1)(C6H5R2)][Tf2N] were prepared and their physical properties were investigated. A detailed comparison of their thermal properties revealed the effects of molecular symmetry and substituents on their melting points. Their viscosity increased on increasing the length of the substituent on the cation and the perfluoroalkyl chain length on the anion. Upon cooling, ILs with low viscosities exhibited crystallization, whereas those with higher viscosities tended to exhibit glass transitions. Most of these salts showed phase transitions in the solid state. A magnetic-switching phenomenon was observed for the paramagnetic ferrocenium IL, which was associated with a liquid/solid transformation, based on the magnetic anisotropy of the ferrocenium cation. 57Fe Mössbauer spectroscopy was applied to [Fe(C5H4nBu)2][Tf2N] to investigate the vibrational behavior of the iron atom in the crystal and glassy states of the ferrocenium IL.
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