Organometallic ionic liquids from alkyloctamethylferrocenium cations: thermal properties, crystal structures, and magnetic properties

Abstract

Alkyloctamethylferrocenium salts with the Tf₂N anion ([Fe(C₅Me₄C_nH_2n+1)(C₅Me₄H)][Tf₂N]; Tf₂N = bis(trifluoromethanesulfonyl)amide) were prepared, and their ionic liquid properties, thermal properties, crystal structures, and magnetic properties were investigated. The melting points of the Tf₂N salts were near room temperature, and decreased with increasing alkyl chain length up to n = 8 and then increased. The salts with PF₆ and NO₃ anions were also prepared. The melting points of the PF₆ salts were higher than 100 °C. Most of these salts exhibited phase transitions in the solid state. The sum of the entropies of the melting and solid phase transitions was nearly independent of the alkyl chain length for salts with short alkyl chains, whereas those for salts with longer alkyl chains (n ≥ 10 for Tf₂N salts, n ≥ 6 for PF₆ salts) increased with increasing alkyl chain length. Crystal structure determinations revealed that the short chain salts form simple alternately packed structures of cations and anions in the solid state, and that the long chain salts form lamellar structures, in which the alkyl chains are aligned parallel between the layers. The effects of magnetic fields on the crystallization of the paramagnetic ionic liquids were investigated, and revealed that the Tf₂N salts with n = 4 exhibited magnetic orientation when solidified under magnetic fields. The magnetic orientation was shown to be a bulk phenomenon, and the importance of the magnetic anisotropy of the crystal structure was suggested in comparison with the response of other Tf₂N salts.