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Heat capacity and phase transitions of the mixed-valence compound hexakis(acetato)tris(3-methylpyridine)oxotriiron.3-methylpyridine

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Authors
Sorai, Michio; Shiomi, Yutaka; Hendrickson, David N.; Oh, Seung M.; Kambara, Takeshi
Issue Date
1987-01
Publisher
American Chemical Society
Citation
Inorg. Chem. 1987, 26, 223
Abstract
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.
ISSN
0020-1669 (print)1520-510X (online)
Language
English
URI
http://hdl.handle.net/10371/5634
DOI
https://doi.org/10.1021/ic00249a005
https://doi.org/10.1021/ic00249a005
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Chemical and Biological Engineering (화학생물공학부)Journal Papers (저널논문_화학생물공학부)
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