ChemInform Abstract: Heat Capacity and Phase Transition of the Mixed‐Valence Compound Biferrocenium Triiodide

Abstract

The results of the measurement of heat capacity at constant pressure, C,,, of the mixed-valence compound biferrocenium triiodide with an adiabatic calorimeter between 14 and 360 K are reported. This compound is known to show a trapped-valence state at low temperature on the 57Fe Mossbauer time scale and a detrapped average valence state at high temperatures. Heat capacity data are presented to show that biferrocenium triiodide exhibits a phase transition in the same temperature region where the valence detrapping occurs on the S7Fe Mossbauer time scale. The present calorimetric measurements indicate a main C, peak centered at 328 K and two subsidiary small peaks at 312 and 346 K. These three peaks may be attributed to the phase transition arising from different sample histories (mainly different grain sizes of the specimen). The phase transitions are characterized by weak cooperativity; the excess heat capacity due to the phase transition is observed over a wide temperature region from -220 to -360 K. The transition entropy was found to be ASm = (1.77 & 0.06) J K-' mol-'. This value is unexpectedly small even if one takes into account only the contribution from the onset of intramolecular electron transfer in the binuclear cations, which amounts to R In 2 (=5.76 J K-' mol-'), where R is the gas constant. Based on the temperature acquisition of AS,,,, the average of the potential energy difference between two electron-localized states, ( FeAFeB') and ( FeArrrFeB), was estimated. The result indicates incompleteness of the phase transitions. A qualitative microscopic model for the phase transition is presented. Intermolecular interactions in the stacks of mixed-valence cations are suggested to be important. In order to have a phase transition it is also important to have interactions between a given cation and its neighboring 1,- anions. In order to examine the role of the 13- counteranion, far-infrared and Raman spectra have been recorded in the range below 400 cm-' as a function of temperature. The symmetric stretching mode (ul) of the I,- anion was observed at 110 cm-I in the Raman spectrum while the deformation (u2) and asymmetric stretching (u,) modes were observed at 55 and 136 cm-' in the IR spectrum. The observation of such a mutual exclusion of vibrational modes between the IR and Raman experiments suggests that the I,- anion in biferrocenium triiodide has nearly a linear symmetrical form, and thus a charge oscillation between two distorted forms I--I-I and I-I--I- does not occur in the range 101-340 K. At the very least the distortion of the 1,- anion in biferrocenium triiodide is not as appreciable as it is in other statically distorted cases. IR and Raman data are also presented for the analogous mixed-valence compound 1',6'-dichlorobiferrocenium triicdide hemiicdine. In keeping with the X-ray structural results, which show an asymmetric It- anion in this compound, all three I,- vibrational modes are seen both in the IR and Raman spectra.