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Contribution of Coulomb Interactions to a Two-Step Crystal Structure Phase Transformation Coupled with a Significant Change in Spin Crossover Behavior for a Series of Charged FeII Complexes from 2,6-Bis(2-methylthiazol-4-yl)pyridine

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journal contribution
posted on 08.01.2018, 15:54 by Kazuyuki Takahashi, Mitsunobu Okai, Tomoyuki Mochida, Takahiro Sakurai, Hitoshi Ohta, Takashi Yamamoto, Yasuaki Einaga, Yoshihito Shiota, Kazunari Yoshizawa, Hisashi Konaka, Akito Sasaki
A series of [FeII(L)2]­(BF4)2 compounds were structurally and physically characterized (L = 2,6-bis­(2-methylthiazol-4-yl)­pyridine). A crystal structure phase transformation from dihydrate compound 1 to anhydrous compound 3 through partially hydrated compounds 2 and 2′ upon dehydration was found. Compounds 1 and 3 exhibited a gradual spin crossover (SCO) conversion, whereas compounds 2 and 2′ demonstrated two-step and one-step abrupt SCO transitions, respectively. An X-ray single-crystal structural analysis revealed that one-dimensional and two-dimensional Fe cation networks linked by π stacking and sulfur–sulfur interactions were formed in 1 and 3, respectively. A thermodynamic analysis of the magnetic susceptibility for 1, 2′, and 3 suggests that the enthalpy differences may govern SCO transition behaviors in the polymorphic compounds 2′ and 3. A structural comparison between 1 and 3 indicates that the SCO behavior variations and crystal structure transformation in the present [FeII(L)2]­(BF4)2 compounds can be interpreted by the relationship between the lattice enthalpies mainly arising from Coulomb interactions between the Fe cations and BF4 anions as in typical ionic crystals.