Origin of the Enhanced Reactivity of μ-Nitrido-Bridged Diiron(IV)-Oxo Porphyrinoid Complexes over Cytochrome P450 Compound I

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• 作者：Matthew G. Quesne ; Dhurairajan Senthilnathan ; Devendra Singh ; Devesh Kumar ; Pascale Maldivi ; Alexander B. Sorokin ; Sam P. de Visser
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：April 1, 2016
• 年：2016
• 卷：6
• 期：4
• 页码：2230-2243
• 全文大小：799K
• ISSN：2155-5435

文摘

μ-Nitrido-bridged diiron porphyrins and phthalocyanines are known to be efficient oxidants that are able to oxidize some of the strongest C–H bonds in nature, such as the one in methane. The origin of their catalytic efficiency is poorly understood, and in order to gain insight into the structural and electronic features of this chemical system, we performed a detailed and systematic study into their chemical properties and reactivities using density functional theory. Our work shows that μ-nitrido-bridged diiron porphyrins and phthalocyanines are highly active catalytic oxidants, which react with methane with very low reaction barriers and a rate-determining hydrogen-atom-abstraction step. Furthermore, the μ-nitrido-bridged diiron porphyrin and phthalocyanine complexes react with a free energy of activation that is more than 10 kcal mol^–1 lower in energy than that found for cytochrome P450 Compound I, which is known to be one of the most efficient C–H hydroxylating agents in Nature. We have analyzed the electronic configuration of reactants and transition states in detail and have identified the key properties of the oxidants that lead to this rate enhancement. In particular, the potency of the oxidant is related to the orbital mixing patterns along the Fe–O axis, whereby the axial iron(IV)-nitrido group donates sufficient electron density to affect the pK_a of the oxo group as well as the strength of the O–H bond formed in the iron(IV)-hydroxo complex. The studies confirm that μ-nitrido diiron-oxo complexes should react via oxygen atom transfer readily even with strong C–H bonds as in methane. The results are analyzed with orbital diagrams, valence bond, and thermochemical cycles and explain the intricate details of the mechanism and the properties of the oxidant.