Heat Capacity and Phase Transition of the Mixed-Valence Compound [Fe₃O(O₂CCH₃)₆(py)₃CHCL₃)

The heat capacity of the mixed-valence complex [Fe₃O(O₂CCH₃)₆(py)₃](CHCl₃), where py is pyridine, has been measured with an adiabatic calorimeter between 14 and 300 K. A phase transition with two peaks closely centered at 207.14 and 208.19 K has been found. The enthalpy and entropy of the phase transition are ΔH = 5107 ± 44 J mol^-1and ΔS = 28.10 ± 0.44 J K^-1mol^-1. By comparing the present calorimetric results with available⁵⁷Fe Mossbauer, X-ray structural, and solid-state²H NMR data for the complex with a CDC1₃solvate molecule, we concluded that the phase transition is associated with the onset of intramolecular electron transfer in the mixed-valence Fe₃0 complexes and the onset of orientational disordering of the chloroform solvate molecules. The former contribution to ΔS is R In 4 while the latter is R In 8, where R is the gas constant. The total entropy gain, R In 32 (=28.82 J K^-1mol^-1), agrees well with the observed ΔS. In the high-temperature phase, each Fe₃0 complex is dynamically interconverting between four vibronic states, in one of which the complex has C₃symmetry and is electronically delocalized. In each of the other three states, the “extra” electron is trapped on one of the iron ions. The contribution for the solvate molecule is in keeping with a model derived from the²H NMR studies. In this model the C-H vector of the CHC1₃solvate molecule is moving between four equally probable positions, one with the C-H vector along the C₃-symmetry axis, which is also the axis along which the Fe₃0 complexes are stacked. The other three positions have the C-H vector directed off the C₃axis on a cone that makes an angle of 24.7° with the C₃axis. Since the CHC1₃solvate molecule sits on a 32 symmetry site, the number of the total positions of the C-H vector is 8 (4 up and 4 down), and hence the entropy gain for the CHC1₃going from static in one position to moving dynamically between these eight positions is R In 8. The present complex confirms that dynamics of a solvate molecule in a solid-state lattice dramatically affect the rate of intramolecular electron transfer in mixed-valence complexes. The relationship between the rate of intramolecular electron transfer and disordering of the solvate molecules is discussed by comparing the present results with the phase transitions found in the analogous complexes [Fe₃O(O₂CCH3)₆(4-Me-py)3](CHCl₃) and [Fe₃O-(O₂CCH3)₆(4-Me-py)₃](CH₃CCl₃), where 4-Me-py is 4-methylpyridine.