Heat capacity and phase transitions of the mixed-valence compound [Fe3O(O2CCH3)6(py)3](py)

Abstract

The heat capacity under constant pressure, C,, of oxo-centered mixed-valence [ Fe30(02CCH3)6(py)3(p] y) has been measured with an adiabatic calorimeter between 12 and 300 K. Four phase transitions were found to occur at 11 1.4, 112.0, 185.8, and 191.5 K. These transitions can be classified basically into two groups: one is a lower-temperature phase transition with two C, peaks in the 11 1-1 12 K region and the other is a higher-temperature phase transition evolving from - 11 5 K to culminate in two C, peaks in the 185-191 K region. The total enthalpy and entropy of these phase transitions were determined by estimating plausible normal heat capacities to give 4940 J mol-' and (30.58 f 0.83) J K-' mol-', respectively. By comparing the present results with the 57Fe Mossbauer spectroscopy and the X-ray structural work reported for this complex, it is concluded that these phase transitions are associated with the intramolecular electron transfer in the mixed-valence Fe30 complexes and the orientational disordering of the pyridine solvate molecules about the crystallographic C3 axis. DTA data for the solid solutions, [Fe"',Fe'11,Co*',o(02ccH3)6(py)3](py), indicate that the phase transition having a C, peak at 185.8 K is due to the orientational order-disorder phenomenon associated with the pyridine solvate molecules. The observed transition entropy is much larger than the value (RI n 3) expected to result from electronic delocalization in the Fe1112Fe1t1r iads and the orientational disorder (R In 3) about the C3 axis of the pyridine solvate molecules. Two possibilities to account for the remaining entropy are discussed: one is associated with changes in the orbital degeneracy of the Fe(I1) ion and the other is the rotation of the pyridine solvate molecules about their pseudo-C6 axes.