The Valence-Detrapping Phase Transition in a Crystal of the Mixed-Valence Trinuclear Iron Cyanoacetate Complex [Fe3O(O2CCH2CN)6(H2O)3]
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- 作者：Tadahiro Nakamoto ; Minoru Hanaya ; Motomi Katada ; Kazutoyo Endo ; Susumu Kitagawa ; and Hirotoshi Sano
- 刊名：Inorganic Chemistry
- 出版年：1997
- 出版时间：September 24, 1997
- 年：1997
- 卷：36
- 期：20
- 页码：4347 - 4359
- 全文大小：587K
- 年卷期：v.36,no.20(September 24, 1997)
- ISSN：1520-510X
文摘
A mixed-valence trinuclear iron cyanoacetate complex,[Fe3O(O2CCH2CN)6(H2O)3](1), was prepared, and thenature of the electron-detrapping phase transition was studied by amultitemperature single-crystal X-ray structuredetermination (296, 135, and 100 K) and calorimetry by comparison withan isostructural mixed-metal complex,[CoFe2O(O2CCH2CN)6(H2O)3](2). The mixed-valence states at various temperatureswere also determined by57Fe Mössbauer spectroscopy. TheMössbauer spectrum of 1 showed a valence-detrappedstate at room temperature.With decreasing temperature the spectrum was abruptly transformedinto a valence-trapped state around 129 K,well corresponding to the heat-capacity anomaly due to the phasetransition (Ttrs = 128.2 K) observed inthecalorimetry. The single-crystal X-ray structure determinationrevealed that 1 has an equilateral structure at296and 135 K, and that the structure changes into an isosceles one at 100K due to the electron trapping. The crystalsystem of 1 at 296 K is rhombohedral, space groupR
with Z = 6 and a =20.026(1) Å, c = 12.292(2) Å; at135K, a = 19.965(3) Å, c = 12.145(4)Å; and at 100 K, the crystal system changes into triclinic system,space groupP
, with Z = 2 and a =12.094(2) Å, b = 12.182(3) Å, c =12.208(3) Å, 
= 110.04(2)
, 
=108.71(2), 
=109.59(2). The X-ray structure determination at 100 Ksuggests that the electronically trapped phase of 1 atlowtemperature is an antiferroelectrically ordered phase, because thedistorted Fe3O molecules, which are expectedto possess a nonzero electronic dipole moment, oriented alternativelyin the opposite direction with respect to thecenter of symmetry. On the other hand, no heat-capacity anomalywas observed in 2 between 7 and 300 K, andX-ray structure determination indicated that 2 shows nostructure change when the temperature is decreased from296 K down to 102 K. The crystal system of 2 at 296 Kis rhombohedral, space group R with Z =6 and a =19.999(1) Å, c = 12.259(1) Å; at 102 K,a = 19.915(2) Å, c = 12.061(1) Å.Even at 102 K the CoFe2O complexstill has a C3 axis, and the three metal ionsites are crystallographically equivalent because of a staticpositionaldisorder of two FeIII ions and one CoII ion.The activation energy of intramolecular electron transfer of1 in thehigh-temperature disordered phase was estimated to be 3.99 kJmol-1 from the temperature dependence oftheMössbauer spectra with the aid of the spectral simulationincluding the relaxation effect of intramolecular electrontransfer. Finally the phase-transition mechanism of 1was discussed in connection with the intermoleculardielectricinteraction.
with Z = 6 and a =20.026(1) Å, c = 12.292(2) Å; at135K, a = 19.965(3) Å, c = 12.145(4)Å; and at 100 K, the crystal system changes into triclinic system,space groupP, with Z = 2 and a =12.094(2) Å, b = 12.182(3) Å, c =12.208(3) Å, = 110.04(2), =108.71(2), =109.59(2). The X-ray structure determination at 100 Ksuggests that the electronically trapped phase of 1 atlowtemperature is an antiferroelectrically ordered phase, because thedistorted Fe3O molecules, which are expectedto possess a nonzero electronic dipole moment, oriented alternativelyin the opposite direction with respect to thecenter of symmetry. On the other hand, no heat-capacity anomalywas observed in 2 between 7 and 300 K, andX-ray structure determination indicated that 2 shows nostructure change when the temperature is decreased from296 K down to 102 K. The crystal system of 2 at 296 Kis rhombohedral, space group R with Z =6 and a =19.999(1) Å, c = 12.259(1) Å; at 102 K,a = 19.915(2) Å, c = 12.061(1) Å.Even at 102 K the CoFe2O complexstill has a C3 axis, and the three metal ionsites are crystallographically equivalent because of a staticpositionaldisorder of two FeIII ions and one CoII ion.The activation energy of intramolecular electron transfer of1 in thehigh-temperature disordered phase was estimated to be 3.99 kJmol-1 from the temperature dependence oftheMössbauer spectra with the aid of the spectral simulationincluding the relaxation effect of intramolecular electrontransfer. Finally the phase-transition mechanism of 1was discussed in connection with the intermoleculardielectricinteraction.