The valence-detrapping phase transition in a crystal of the mixed-valence trinuclear iron cyanoacetate complex [Fe3O(O2CCH2CN)(6)(H2O)(3)]

A mixed-valence trinuclear iron cyanoacetate complex, [Fe3O(O2CCH2CN)(6)(H2O)(3)] (1), was prepared, and the nature of the electron-detrapping phase transition was studied by a multitemperature single-crystal X-ray structure determination (296, 135, and 100 K) and calorimetry by comparison with an isostructural mixed-metal complex, [CoFe2O(O2CCH2CN)(6)(H2O)(3)] (2) The mixed-valence states at various temperatures were also determined by Fe-57 Mossbauer spectroscopy. The Mossbauer spectrum of 1 showed a valence-detrapped state at room temperature. With decreasing temperature the spectrum was abruptly transformed into a valence-trapped state around 129 K, well corresponding to the heat-capacity anomaly due to the phase transition (T-trs = 128.2 K) observed in the calorimetry. The single-crystal X-ray structure determination revealed that 1 has an equilateral structure at 296 and 135 K, and that the structure changes into an isosceles one at 100 K due to the electron trapping. The crystal system of 1 at 296 K is rhombohedral, space group R (3) over bar with Z = 6 and a = 20.026(1) Angstrom., c = 12.292(2) Angstrom; at 135 K, a = 19.965(3) Angstrom, c = 12.145(4) Angstrom; and at 100 K, the crystal system changes into triclinic system, space group P (1) over bar, with Z = 2 and a = 12.094(2) Angstrom, b = 12.182(3) Angstrom, c = 12.208(3) Angstrom, alpha = 110.04(2)degrees, beta = 108.71(2)degrees, gamma = 109.59(2)degrees. The X-ray structure determination at 100 K suggests that the electronically trapped phase of 1 at low temperature is an antiferroelectrically ordered phase, because the distorted Fe3O molecules, which are expected to possess a nonzero electronic dipole moment, oriented alternatively in the opposite direction with respect to the center of symmetry. On the other hand, no heat-capacity anomaly was observed in 2 between 7 and 300 K, and X-ray structure determination indicated that 2 shows no structure change when the temperature is decreased from 296 K down to 102 K. The crystal system of 2 at 296 K is rhombohedral, space group R (3) over bar with Z = 6 and a = 19.999(1) Angstrom, c = 12.259(1) Angstrom; at 102 K, a = 19.915(2) Angstrom, c = 12.061(1) Angstrom. Even at 102 K the CoFe2O complex still has a C-3 axis, and the three metal ion sites are crystallographically equivalent because of a static positional disorder of two Fe-III ions and one Co-II ion. The activation energy of intramolecular electron transfer of 1 in the high-temperature disordered phase was estimated to be 3,99 kJ mol(-1) from the temperature dependence of the Mossbauer spectra with the aid of the spectral simulation including the relaxation effect of intramolecular electron transfer. Finally the phase-transition mechanism of 1 was discussed in connection with the intermolecular dielectric interaction.