Spin crossover behavior of 1,3,4-oxadiazole based dinuclear iron(II) complexes und functionalized phosphonates as bridging ligands in inorganic-organic hybrid-materials

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Abstract

The change between the electronic states high-spin and low-spin is called spin crossover or spin transition. The transition can be induced by a change in temperature, pressure, or the irradiation with light. The requirement for such properties is a 3d transition metal ion with the electronic configuration 3d4 to 3d7. Crucial for the switchability is a coordination environment that can stabilize both, the High-Spin and Low-Spin state. Therefore, the ligand field has to have the appropriate strength to split the d-orbitals up to the point, where the preferred state is determined by the external stimuli. One of the most studied metal ions in this matter is probably iron(II) in an octahedral coordination geometry. Especially dinuclear compounds are of great interest because both, intermolecular and intramolecular interaction can be observed. Furthermore, a third mixed [HS-LS] state can occur. Herein, several new compounds based on a symmetric 2,5-disubstituted 1,3,4-oxadiazole ligand are reported, that were synthesized using different iron(II) metal salts. With single crystal X-Ray diffraction, Mössbauer-spectroscopy and magnetic measurements, the spin crossover properties of the novel complexes were investigated. The measurements revealed a strong dependence on classical and non-classical hydrogen bonds between the complex cation and the counter ions: a change of the hydrogen bond acceptor at the counter ions results in a different spin crossover behavior. Additionally, the number of contacts between a cation and its next neighbors via hydrogen bonds mediated by the counterions has a strong influence on the nature of the spin transition. The steric hindrance and flexibility of the ligand has to be considered as well. Compared to analog complexes based on thiadiazole and triazole ligands, they differ not only on a structural level, but also significantly in their spin crossover characteristics. The origin for these changes is found in the substitution of the hetero atom in the five membered ring. This new family of oxadiazole based dinuclear iron(II) compounds is an ideal model system to illustrate the diversity and complexity of spin crossover compounds and pronounce the important part of intermolecular contacts.

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