Synthesis and Characterization of a Dinuclear Iron(II) Spin Crossover Complex with Wide Hysteresis

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• 作者：Birgit Weber ; Eike S. Kaps ; Jaroslava Obel ; Klaus Achterhold ; Fritz G. Parak
• 刊名：Inorganic Chemistry
• 出版年：2008
• 出版时间：November 17, 2008
• 年：2008
• 卷：47
• 期：22
• 页码：10779-10787
• 全文大小：290K
• 年卷期：v.47,no.22(November 17, 2008)
• ISSN：1520-510X

文摘

The magnetic properties and results from X-ray structure analysis for a new pair of iron(II) spin-crossover complexes [FeL1(meim)₂](meim) (1(meim)) and [Fe₂L2(meim)₄](meim)₄ (2(meim)₄), with L1 being a tetradentate N₂O₂²⁻ coordinating Schiff-base-like ligand [([3,3′]-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentane-dionato)(2-)N,N′,O²,O²′], L2 being an octadentate, dinucleating N₂O₂²⁻ coordinating Schiff-base-like ligand [3,3′,3′′,3′′′]-[1,2,4,5-phenylenetetra(iminomethylidyne)]tetra(2,4-pentanedionato)(2-)N,N′,N′′,N′′′,O²,O²′,O²′′,O²′′′], and meim being N-methylimidazole, are discussed in this work. Crystalline samples of both complexes show a cooperative spin transition with an approximately 2-K-wide thermal hysteresis loop in the case of 1(meim) (T_1/2↑ = 179 K and T_1/2↓ = 177 K) and an approximately 21-K-wide thermal hysteresis loop in the case of dinuclear complex 2(meim)₄ (T_1/2↑ = 199 K and T_1/2↓ = 178 K). For a separately prepared powder sample of 2, a gradual spin transition with T_1/2 = 229 K is observed that was additionally followed by Mössbauer spectroscopy. The results from X-ray structure analysis give a deeper insight into the molecule packing in the crystal and, by this, help to explain the increase of cooperative interactions during the spin transition when going from the mononuclear to the dinuclear complex. Both compounds crystallize in the triclinic space group P1̅, and the X-ray structure was analyzed before and after the spin transition. The change of the spin state at the iron center is accompanied by a change of the O−Fe−O angle, the so-called bite of the equatorial ligand, from about 109° in the high-spin state to 89° in the low-spin state. The cooperative interactions responsible for the thermal hysteresis loop are due to elastic interactions between the complex molecules in both cases. However, due to the higher symmetry of the dinucleating ligand in 2(meim)₄, a 3D network of short contacts is formed, while for mononuclear complex 1(meim), a 2D layer of linked molecules is observed. The spin transition was additionally followed in solution using ¹H NMR spectroscopy for both complexes. In both cases, a gradual spin transition is observed, and the increase of cooperative interactions when going from the mononuclear to the dinuclear system is solely attributed to the extended network of intermolecular contacts.