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O zjawisku spin-crossover. Polimeryczne kompleksy Fe(II) z pochodnymi 1,2,4-triazolu

Połomka P.

Wiadomości Chemiczne

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2004

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[Z] 58, 5-6

477--496

EN

This paper is a short review of a spin-crossover phenomenon in complexes of iron(II). This phenomenon is related to the thermally induced transition between a diamagnetic low spin ground state (LS or S=0) and the excited paramagnetic high spin state (HS or S=2). Numerous compounds switch from a stable electronic state to another state in a reversible and detectable fashion in response to an appropriate and controllable external perturbation. This transition is accompanied by a strong thermochromism between rose and white colors. In certain cases, a hysteresis effect is accompanying this transition, which confers a memory effect to the system. The potential applications of spin transition (ST) molecular materials in the field of display and data processing has led to intensive studies of this phenomenon in the last ten years. For this purpose the compounds need to show a large thermal hysteresis, with the middle of the hysteresis loop falling as close as possible to room temperature. To achieve this goal iron(II) ST molecular materials are designed and synthesized as polymers in order to increase intermolecular interactions. In this paper some polymers based on 1,2,4-triazole derivatives are described because of their bridging nature leading to a strong cooperativity. The influence of various parameters, namely type of counter ion, R substituent in 4- position in the triazole ring, number of noncoordinated lattice water molecules, on the structure and ST regime of Fe(II)-1,2,4-triazole polymers is depicted. Some of the compounds presented might be of potential use as the electronic displays or switches.

2

Spin-entropy driven charge transfer phase transition in iron mixed-valence system

Kojima N., Itoi M., Ono Y., Okubo M., Enmoto M.

Materials Science

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2003

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Vol. 21, No. 2

181-189

EN

We have synthesized iron mixed-valence complexes, (n-CnH2n+1)4N[FeIIFeIIIX3] (X = mto(C2O3S), dto(C2O2S2), tto(C2OS3)) and have investigated their physical properties by means of 57Fe Mössbauer spectroscopy, magnetic susceptibility and electrical resistivity measurements. From the analysis of 57Fe Mössbauer spectra, magnetic susceptibility and electrical resistivity, we have discovered a new type of first order phase transition around 120°K for (n-CnH2n+1)4N[FeIIFeIII(dto)3](n = 3, 4), where the charge transfer transition between FeII and FeIII occurs reversibly. In the higher temperature phase, the FeIII (S = 1/2) and FeII (S = 2) sites are coordinated by six S atoms and six O atoms, respectively. In the lower temperature phase, on the other hand, the FeIII (S = 5/2) and FeII (S = 0) sites are coordinated by six O atoms and six S atoms, respectively. Moreover, we have found the ferromagnetic phase transition in this system. The ferromagnetic order is induced by the charge transfer interaction between the FeIII and FeII sites. Moreover, we propose various multifunctional properties for (n-CnH2n+1)4N[FeIIFeIII(mto)3] and (n-CnH2n+1)4N[FeIIFeIII(tto)3].

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Spin crossover in a supramolecular feii-feiii system

Ohta H., Sunatsuki Y., Ikuta Y., Matsumoto N., Lijima S., Akashi H., Kambe T., Kojima M.

Materials Science

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2003

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Vol. 21, No. 2

191-198

EN

The structure of [FeII(H3L)](ClO4)2ˇ3H2O, where H3L is a tripodal hexadentate ligand derived from the 1:3 condensation of tris(2-aminoethyl)amine and 4-formylimidazole, has been determined by X-ray crystallography at 113 and 293°K. A spin transition was indicated from the Fe-N bond distances. The temperature dependence of the magnetic susceptibility revealed that the complex undergoes a gradual spin transition in the temperature range 150-270°K. A mixed-valence complex, [FeII(H3L)][FeIII(L)](BF4)2 1.5H2O, was prepared by the controlled deprotonation of the protonated species, [FeII(H3L)](BF4)2ˇ1.5H2O, under aerobic conditions, and the X-ray structure was determined at 293°K. Two species, [FeII(H3L)]2+ and [FeIII(L)], are linked by imidazole-imidazolate (NH-N) hydrogen bonds to form a puckered sheet structure. Magnetic susceptibility measurements and Mössbauer spectra provided evidence of spin-crossover at both the FeII and FeIII sites. There are three accessible electronic states: (LS FeII-LS FeIII), (HS FeII-LS FeIII), and (HS FeII-HS FeIII) that occur in passing from lower to higher temperatures.