Dichotomous Hydrogen Atom Transfer vs Proton-Coupled Electron Transfer During Activation of X鈥揌 Bonds (X = C, N, O) by Nonheme Iron鈥揙xo Complexes of Variable Basicity

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• 作者：Dandamudi Usharani ; David C. Lacy ; A. S. Borovik ; Sason Shaik
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：November 13, 2013
• 年：2013
• 卷：135
• 期：45
• 页码：17090-17104
• 全文大小：869K
• 年卷期：v.135,no.45(November 13, 2013)
• ISSN：1520-5126

文摘

We describe herein the hydrogen-atom transfer (HAT)/proton-coupled electron-transfer (PCET) reactivity for Fe^IV鈥搊xo and Fe^III鈥搊xo complexes (1鈥?b>4) that activate C鈥揌, N鈥揌, and O鈥揌 bonds in 9,10-dihydroanthracene (S1), dimethylformamide (S2), 1,2-diphenylhydrazine (S3), p-methoxyphenol (S4), and 1,4-cyclohexadiene (S5). In 1鈥?, the iron is pentacoordinated by tris[N鈥?tert-butylureaylato)-N-ethylene]aminato ([H₃buea]^3-) or its derivatives. These complexes are basic, in the order 3 1 > 2. Oxidant 4, [Fe^IVN4Py(O)]²⁺ (N4Py: N,N-bis(2-pyridylmethyl)bis(2-pyridyl)methylamine), is the least basic oxidant. The DFT results match experimental trends and exhibit a mechanistic spectrum ranging from concerted HAT and PCET reactions to concerted-asynchronous proton transfer (PT)/electron transfer (ET) mechanisms, all the way to PT. The singly occupied orbital along the O路路路H路路路X (X = C, N, O) moiety in the TS shows clearly that in the PCET cases, the electron is transferred separately from the proton. The Bell鈥揈vans鈥揚olanyi principle does not account for the observed reactivity pattern, as evidenced by the scatter in the plot of calculated barrier vs reactions driving forces. However, a plot of the deformation energy in the TS vs the respective barrier provides a clear signature of the HAT/PCET dichotomy. Thus, in all C鈥揌 bond activations, the barrier derives from the deformation energy required to create the TS, whereas in N鈥揌/O鈥揌 bond activations, the deformation energy is much larger than the corresponding barrier, indicating the presence of a stabilizing interaction between the TS fragments. A valence bond model is used to link the observed results with the basicity/acidity of the reactants.