The heat capacity under constant pressure, C,, of oxo-centered mixed-valence [Fe30(02CCH3)6(3-Me-py)3].3h-aMse b-epeny measured with an adiabatic calorimeter between 12 and 350 K, where 3-Me-py is 3-methylpyridine, Four phase transitions were found to occur at 181, 263.5,271.5, and 282.5 K. The cumulative enthalpy and entropy changes due to these four phase transitions were calculated to be &H = 3410 f 130 J mol-' and 42 = 13.71 f 0.65 J K-' mol-I, respectively. By comparison of the present results with the S7Fe Mossbauer spectroscopy and the X-ray structural work already done for this complex, it is concluded that these phase transitions involve at various temperatures intramolecular electron transfer in the mixed-valence Fe30 complexes and the order-disorder phenomenon of the pyridine solvate molecules jumping between two energetically equivalent positions in the solid state. The contributions from these two phenomena to the transition entropy are R In 3 and R In 2, respectively, and the sum is 14.90 J K-I mol-I. This accounts well for the observed value of 13.71 f 0.65 J K-' mol-'. DTA thermograms for the solid solutions of [Fe11'2Fe",_,Co1*,0(02CCH3)6(3-Me-py)3]w.h3e-reM xe -=p 0y,,0 .25,0.5,0.75, and 1. O, have been recorded between 80 and 350 K. The highest temperature phase transition found for the x = 0.0 compound is linearly shifted to a lower temperature with increasing x. The concentration dependence of the transition temperature, Tc(x), is described by the linear equation Tc(x) (K) = 282.54 - 13.08~. The lowest temperature phase transition at 181 K is easily undercooled. By comparison of the molar entropies of the undercooled and nonundercooled states at a common temperature (196.8395 K in the present case), it has been concluded that the undercooled phase has no residual entropy, that is, the molecules in this phase are in an ordered state at 0 K. A model for the observed phase transitions has been devised that accounts for all of the X-ray structural, spectroscopic, and heat capacity results for [Fe30(02CCH3)6(3-Me-py)3].3-BMeel-owpy 1.8 1 K all three iron ions are inequivalent and only one vibronic state of the Fe30 complex is populated. At 181 K the thermal energy becomes comparable to intermolecular interactions and there is a phase transition such that the potential-energy diagram of the Fe30 complex changes. At temperatures above 181 K two vibronic states (i.e., two minima in the potential-energy diagram) are populated. Finally, in the high-temperature phase tran$ition which culminates at 282.5 K and has two anomalies at 263.5 and 271.5 K there is an onset of motion involving the 3-Me-py solvate molecule jumping between its two lattice sites. As a result the Fe,O complex tends to become more equilateral than it is at low temperatures and all three vibronic states of the Fe,O complex are populated to some degree.
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Sorai et al. (1987) studied this question.