Spin Crossover Behavior in a Series of Iron(III) Alkoxide Complexes

The synthesis, crystal structures, magnetic behavior, and electron paramagnetic resonance studies of five new FeIII spin crossover (SCO) complexes are reported. The [FeIIIN5O] coordination core is constituted of the pentadentate ligand bztpen (N5) and a series of alkoxide anions (ethoxide, propoxide, n-butoxide, isobutoxide, and ethylene glycoxide). The methoxide derivative previously reported by us is also reinvestigated. The six complexes crystallize in the orthorhombic Pbca space group and show similar molecular structures and crystal packing. The coordination octahedron is strongly distorted in both the high- and low-temperature structures. The structural changes upon spin conversion are consistent with those previously observed for [FeIIIN4O2] SCO complexes of the Schiff base type, except for the Fe–O­(alkoxide) bond distance, which shortens significantly in the high-spin state. Application of the Slichter–Drickamer thermodynamic model to the experimental SCO curves afforded reasonably good simulations with typical enthalpy and entropy variations ranging in the intervals ΔH = 6–13 kJ mol–1 and ΔS = 40–50 J mol–1 K–1, respectively. The estimated values of the cooperativity parameter Γ, found in the interval 0–2.2 kJ mol–1, were consistent with the nature of the SCO. Electron paramagnetic resonance spectroscopy confirmed the transformation between the high-spin and low-spin states, characterized by signals at g ≈ 4.47 and 2.10, respectively. Electrochemical analysis demonstrated the instability of the Fe­(II) alkoxide derivatives in solution.本文报道了五种新型三价铁（FeIII）自旋交叉（spin crossover, SCO）配合物的合成、晶体结构、磁学行为及电子顺磁共振研究。该类配合物的[FeIIIN5O]配位核由五齿配体bztpen（提供N₅配位点）与一系列烷氧基阴离子（乙氧基、丙氧基、正丁氧基、异丁氧基及乙二醇氧基）构成。本课题组此前报道的甲氧基衍生物也被重新研究。六种配合物均结晶于正交晶系Pbca空间群，展现出相似的分子结构与晶体堆积方式。无论高温还是低温结构，其配位八面体均发生显著畸变。自旋转换过程中的结构变化与此前报道的希夫碱类[FeIIIN4O2]型自旋交叉配合物基本一致，仅Fe-O（烷氧基）键长在高自旋态（high-spin state）下发生显著缩短。将斯利赫特-德里克默（Slichter–Drickamer）热力学模型应用于实验自旋交叉曲线后，获得了拟合效果良好的模拟结果，其典型焓变与熵变区间分别为ΔH=6~13 kJ·mol⁻¹与ΔS=40~50 J·mol⁻¹·K⁻¹。协同性参数Γ的估算值区间为0~2.2 kJ·mol⁻¹，该结果与自旋交叉行为的本质相符。电子顺磁共振（electron paramagnetic resonance, EPR）波谱法证实了高自旋态（high-spin state）与低自旋态（low-spin state）之间的相互转变，其特征信号的g值分别约为4.47与2.10。电化学分析结果表明，溶液中的Fe(II)烷氧基衍生物存在不稳定性。