Phase transitions and thermal properties of decamethylferrocenium salts with perfluoroalkyl-sulfonate and -carboxylate anions exhibiting disorder

Decamethylferrocenium salts with perfluoroalkylsulfonate anions (−CnF2n+13SOCnF2n+1SO3−; n = 1, 4, and 8) and perfluoroalkylcarboxylate anions (−CnF2n+12COCnF2n+1CO2−; n = 1–4) were prepared to investigate the effects of the anion on the thermal behaviors of the salts. Differential scanning calorimetry (DSC) measurements revealed that all the salts exhibited phase transitions in the solid state. Salts with an octamethylferrocenium cation or decamethylcobaltocenium cation exhibited different phase sequences from those of the corresponding decamethylferrocenium salts. Structural changes associated with the phase transitions were investigated crystallographically for two salts. The phase transition in [Fe(C5Me5)2](CF3SO3) at −121.6 °C was accompanied by ordering of the cation conformations into eclipsed and staggered conformations in different ratios. The unit cell volume became six times larger and the space group changed from Pmn21 to P1 in the low-temperature phase. The phase transition in [Fe(C5Me5)2](C3F7CO2) at −115 °C was not accompanied by any change in the unit cell or the space group (P21/c). The libration or displacements of the anions that existed in the high-temperature phase was suppressed in the low-temperature phase, associated with a slight rotational displacement of the molecules. Anions with longer perfluoroalkyl chains exhibited disorder in the solid state even at low temperatures.