How Do the Axial and Equatorial Ligands Modulate the Reactivity of a Metal-Bound Terminal Oxidant? An Answer from the Hypochlorite Adduct of Iron(III) Porphyrin
How Do the Axial and Equatorial Ligands Modulate the Reactivity of a Metal-Bound Terminal Oxidant? An Answer from the Hypochlorite Adduct of Iron(III) Porphyrin
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DOI：
10.1021/acscatal.2c01840
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发表时间：
2022-08
期刊：
影响因子：
12.9
通讯作者：
Sawako Yokota;Yuna Suzuki;S. Yanagisawa;T. Ogura;S. Nozawa;M. Hada;Hiroshi Fujii
中科院分区：
文献类型：
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作者：
Sawako Yokota;Yuna Suzuki;S. Yanagisawa;T. Ogura;S. Nozawa;M. Hada;Hiroshi Fujii
过渡金属络合物的末端氧化剂加合物不仅是高价氧物种的前体，而且是催化各种氧化反应的活性化合物。虽然关于过氧物、过氧酸和碘代芳烃的末端氧化剂加合物已经有了各种报道，但关于次氯酸盐与金属络合物的加合物的报道很有限。我们成功地制备了具有不同吸电子取代基的铁(III)-卟啉络合物的双次氯酸盐加合物。光谱(紫外-可见吸收、核磁共振、电子顺磁共振和扩展的X射线吸收精细结构)研究和密度泛函理论计算表明，随着卟啉和轴向配体的给电子体效应增强，次氯酸盐的Fe-OCL和O-Cl键变弱，次氯酸盐络合物变得更不稳定。此外，对次氯酸盐络合物的环氧化、氯化、亚硫氧化和吸氢反应的动力学分析表明，随着Fe-OCL和O-Cl键的减弱，次氯酸盐与铁结合的活性变得更强。这些结果使我们能够回答轴向和赤道配体如何控制铁结合的次氯酸盐的反应性的问题。关键是电子从充满的OCLπ*-轨道向空位的Fe3dz2-和4p-轨道的迁移，这是形成Fe-OCL配位键的关键。由于Fe3dz2-和4p-轨道的能量增加，配体的电子给体效应的增强降低了电子从OCLπ*-轨道上的迁移。这种变化使得OCLπ*-轨道上的电子密度较高，Fe-O和O-Cl键较弱，次氯酸盐络合物发生反应。这将是控制过渡金属络合物的其他末端氧化剂加合物的反应活性的共同机制。
Terminal oxidant adducts of transition-metal complexes have been known as not only the precursors of high-valent oxo species but also the reactive compounds that can catalyze various oxygenation reactions. While there have been various reports on the terminal oxidant adducts of peroxides, peracids, and iodosylarenes, the report of the hypochlorite adduct of metal complexes has been limited. We succeed in the preparation and characterization of bis-hypochlorite adducts of iron(III) porphyrin complexes having various electron-withdrawing substituents. The spectroscopic (ultraviolet–visible absorption, nuclear magnetic resonance, electron paramagnetic resonance, and extended X-ray absorption fine structure) studies and density functional theory calculations indicate that as the electron-donor effect of the porphyrin and axial ligands becomes stronger the Fe–OCl and O–Cl bonds of the iron-bound hypochlorite become weaker and thus the hypochlorite complex becomes more unstable. Furthermore, the kinetic analyses of the hypochlorite complexes for the epoxidation, chlorination, sulfoxidation, and hydrogen abstraction reactions indicate that as the Fe–OCl and O–Cl bonds become weaker the iron-bound hypochlorite becomes more reactive. These results allow us to answer the question how the axial and equatorial ligands control the reactivity of the iron-bound hypochlorite. The key is the electron migration from the fully filled OCl π*-orbitals to the vacant Fe 3dz2- and 4p-orbitals, which is essential for making the Fe–OCl coordination bond. An increase in the electron-donor effect of the ligand decreases the electron migration from the OCl π*-orbitals because of an increase in the energies of the Fe 3dz2- and 4p-orbitals. This change makes the electron density in the OCl π*-orbitals high, the Fe–O and O–Cl bonds weak, and the hypochlorite complex reactive. This would be a common mechanism for controlling the reactivity of other terminal oxidant adducts of transition-metal complexes.