Anion-Ordering Phase Transitions in Biferrocenium and Ferrocenium Salts with the Bis(trifluoromethanesulfonyl)amide Anion

[Image: see text] The bis(trifluoromethanesulfonyl)amide anion (Tf(2)N), which is a common component of ionic liquids, often exhibits disorder in the solid state. In this study, the phase transitions and crystal structures of the Tf(2)N salts of 1,1‴-dineopentyl-1′,1″-biferrocene (=npBifc), 1′,1″-biferrocene (=Bifc), ferrocene, and cobaltocene (1–4, respectively) were compared. All the salts exhibited phase transitions at low temperatures, which are accompanied by anion ordering, though the ordering was not complete in 2 and 3. X-ray crystallographic investigation revealed that the cations and anions in 1 and 2 adopted alternating arrangements and segregated columnar arrangements, respectively. The cation in 1 exhibited a symmetrical, average-valence structure in the room-temperature phase owing to rapid valence tautomerization, whereas the cation exhibited an unsymmetrical structure in the low-temperature phase. The cation in 2 exhibited an unsymmetrical, trapped-valence structure in both phases. The cation valence states in these salts were accounted for by the electrostatic interactions between the cations and anions. The crystal structures and phase behavior of the ferrocenium salt 3 were very different from those of 4.