Synthesis, structure and valence-trapping vs. detrapping for new trinuclear iron pentafluoro benzoate complexes: possible recognition of organic molecules by 57Fe Mössbauer spectroscopy.

New mixed-valence trinuclear iron pentafluorobenzoate complexes were synthesized. Their valence-detrapping and/or valence-trapping phenomena were studied by (57)Fe Mössbauer spectroscopy and X-ray crystallography. For [Fe3O(C6F5CO2)6(py)3]·CH2Cl2 (1), a valence-trapped state was observed at low temperatures, while the valence-detrapped state was observed at room temperature. Removal of CH2Cl2 from 1 gives the de-solvated [Fe3O(C6F5CO2)6(py)3] (2) where the valence was trapped at room temperature. The CH2Cl2-free 2 can reversibly absorb and desorb CH3CN; the process was followed by (57)Fe Mössbauer spectroscopy by monitoring valence-trapping and valence-detrapping phenomena. Organic molecules such as benzene, toluene, ethylbenzene, cumene, and xylene are also trapped by 2 and affect the iron valence states. However, small molecules such as H2O and CO2 do not affect the valence-trapped state of 2. Three xylene isomers trapped within the nano-void of 2 were distinguished by (57)Fe Mössbauer spectroscopy at room temperature.

Spin crossover phenomenon accompanying order-disorder phase transition in the ligand of [FeII(DAPP)(abpt)](ClO4)2 compound (DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) and its successive self-grinding effect.

The spin crossover phenomenon of the recently described spin crossover complex [FeII(DAPP)(abpt)](ClO4)2 [DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] accompanying an order-disorder phase transition of the ligand was investigated by adiabatic heat capacity calorimetry, far-IR, IR, and Raman spectroscopies, and normal vibrational mode calculation. A large heat capacity peak due to the spin crossover transition was observed at T(trs) = 185.61 K. The transition enthalpy and entropy amounted to Delta(trs)H = 15.44 kJ mol-1 and Delta(trs)S = 83.74 J K-1 mol-1, respectively. The transition entropy is larger than the expected value 60.66 J K-1 mol-1, which is contributed from the spin multiplicity (R ln 5; R: the gas constant), disordering of the carbon atom of the six-membered metallocycle in the DAPP ligand, and one of the two perchlorate anions (2R ln 2), and change of the normal vibrational modes between the high-spin (HS) and low-spin (LS) states (35.75 J K-1 mol-1). The remaining entropy would be ascribed to changes of the lattice vibrations and molecular librations between the HS and LS states. Furthermore, [Fe(DAPP)(abpt)](ClO4)2 crystals disintegrated and became smaller crystallites whenever they experienced the phase transition. This may be regarded as a successive self-grinding effect, evidenced by adiabatic calorimetry, DSC, magnetic susceptibility, and microscope observation. The relationship between the crystal size and the physical quantities is discussed.

Degree of disorder in cubic mesophases in thermotropics: thermodynamic study of a liquid crystal showing two cubic mesophases.

Heat capacity of a thermotropic mesogen ANBC(22) (4(')-alkoxy-3(')-nitrobiphenyl-4-carboxylic acid with 22 carbon atoms in alkyl chain) showing two cubic mesophases was measured by adiabatic calorimetry between 13 and 480 K. Excess enthalpies and entropies due to phase transitions were determined. A small thermal anomaly due to the cubic Im3m-->cubic Ia3d phase transition was successfully detected. Through an analysis of chain-length dependence of the entropy of transition, the sequence of two cubic mesophases (with space groups Ia3d and Im3m) is deduced for thermotropic mesogens ANBC(n). It is shown that the disorder of the core arrangement decreases in the order of Sm-C-->cubic (Im3m)-->cubic (Ia3d) while that of the chain in the reverse order cubic (Ia3d)-->cubic (Im3m)-->Sm C.

The Valence-Detrapping Phase Transition in a Crystal of the Mixed-Valence Trinuclear Iron Cyanoacetate Complex [Fe(3)O(O(2)CCH(2)CN)(6)(H(2)O)(3)].

A mixed-valence trinuclear iron cyanoacetate complex, [Fe(3)O(O(2)CCH(2)CN)(6)(H(2)O)(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, [CoFe(2)O(O(2)CCH(2)CN)(6)(H(2)O)(3)] (2). The mixed-valence states at various temperatures were also determined by (57)Fe Mössbauer spectroscopy. The Mössbauer 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&thremacr; with Z = 6 and a = 20.026(1) Å, c = 12.292(2) Å; at 135 K, a = 19.965(3) Å, c = 12.145(4) Å; and at 100 K, the crystal system changes into triclinic system, space group P&onemacr;, with Z = 2 and a = 12.094(2) Å, b = 12.182(3) Å, c = 12.208(3) Å, 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 Fe(3)O 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&thremacr; 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 CoFe(2)O 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 Mössbauer 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.