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

The bis­(trifluoromethanesulfonyl)­amide
anion (Tf2N),
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 Tf2N 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.