Structures and spin states of mono- and dinuclear iron(II) complexes of imidazole-4-carbaldehyde azine and its derivatives |
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| Authors: | Yukinari Sunatsuki Ryohei Kawamoto Kunihiro Fujita Hisashi Maruyama Takayoshi Suzuki Hiroyuki Ishida Masaaki Kojima Seiichiro Iijima Naohide Matsumoto |
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| Institution: | 1. Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan;2. National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8566, Japan;3. Department of Chemistry, Faculty of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan |
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| Abstract: | Mononuclear Fe(H2LR)2]X2 (R = H, 2-Me, 5-Me, 2-Et-5-Me; X = ClO4, BF4) and dinuclear Fe2(H2LR)3]X4 complexes containing imidazole-4-carbaldehyde azine (H2LH) and its derivatives prepared by condensation of 4-formylimidazole, 2-methyl- or 5-methyl-4-formylimidazole, or 2-ethyl-4-methyl-5-formylimidazole, with hydrazine in a 2:1 mole ratio in methanol, were prepared and their magnetostructural relationships were studied. In the mononuclear complexes, H2LR acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H2LR acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate structure. At room temperature, Fe2(H2LH)3](ClO4)4 and Fe2(H2L2-Me)3](ClO4)4 were in the high-spin (HS) and low-spin (LS) states, respectively. The results are in accordance with the ligand field strength of H2L2-Me with electron-donating methyl groups being stronger than H2LH, with the order of the ligand field strengths being H2L2-Me > H2LH. However, in the mononuclear Fe(H2LH)2](ClO4)2 and Fe(H2L2-Me)2](ClO4)2 complexes, a different order of ligand field strengths, H2LH > H2L2-Me, was observed because Fe(H2LH)2](ClO4)2 was in the LS state while Fe(H2L2-Me)2](ClO4)2 was in the HS state at room temperature. X-ray structural studies revealed that the interligand steric repulsion between a methyl group of an H2L2-Me ligand and the other ligand in Fe(H2L2-Me)2](ClO4)2 is responsible for the observed change in the spin state. The same is true for Fe(H2L2-Et-5-Me)2](ClO4)2, while Fe(H2L5-Me)2](ClO4)2 does not involve such a steric congestion and stays in the LS state over the temperature range 5–300 K. Two kinds of crystals (polymorphs) were isolated for Fe2(H2LH)3](BF4)4 and Fe2(H2L2-Et-5-Me)3](ClO4)4, and they exhibited different magnetic behaviors. |
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