Calorimetric studies on phase transitions arising from orientational order-disorder of the molecular axes of ferrocene and its derivatives

Abstract

Heat capacities of the channel inclusion compound, Fe(C 5D 5) 2 · 3(NH 2) 2CS, and two ferrocenium salts, [Fe(C 5H 5)(C 6H 6)] + (PF 6) − and [Fe(C 5H 5) 2] + (PF 6) −, have been measured with adiabatic calorimeters between 13 and 393 K. Five phase transitions were found for Fe(C 5D 5) 2 · 3(NH 2) 2CS corresponding to those for Fe(C 5H 5) 2 · 3(NH 2) 2CS. The dominant phase transitions at 145.8 and 160.6 K are responsible for the onset of reorientational order-disorder of the molecular axis of Fe(C 5D 5) 2 in the clathrate cavity. The mass-effect of the guest ferrocene molecule on the phase transitions was not remarkable. The ferrocenium salt, [Fe(C 5H 5)(C 6H 6] +(PF 6) −, exhibited four phase transitions and two glass transition phenomena at low temperatures while its analog, [Fe(C 5H 5) 2] +(PF 6) −, brought about only three phase transitions without showing the glass transition. The higher-temperature phase transitions in these two salts have been assigned to the reorientational order-disorder mechanism of the molecular axes of the cations in the pseudo-cavities formed by eight PF − 6 anions. For the origin of the lower-temperature phase transitions in these two salts, three possibilities have been discussed. Among them, plausible origin is likely to be an order-disorder change of PF − 6 anion in the lattice. An important unsettled problem common to these three compounds is a question whether or not the Fe(C 5D 5) 2 and the cations, [Fe(C 5H 5)(C 6H 6)] + and [Fe(C 5H 5) 2] +, are still reorienting around their molecular axes even at the lowest-temperature phase.