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

• Published in: Journal of the American Chemical Society 2013-11, Vol.135 (45), p.17090-17104
• Main Authors: Usharani, Dandamudi, Lacy, David C, Borovik, A. S, Shaik, Sason
• Format: Article
• Language: English
• Subjects: Carbon - chemistry; Electrons; Ferric Compounds - chemistry; Hydrogen - chemistry; Iron Compounds - chemistry; Models, Molecular; Nitrogen - chemistry; Oxidants - chemistry; Oxygen - chemistry; Protons
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja408073m

We describe herein the hydrogen-atom transfer (HAT)/proton-coupled electron-transfer (PCET) reactivity for FeIV–oxo and FeIII–oxo complexes (1–4) that activate C–H, N–H, and O–H bonds in 9,10-dihydroanthracene (S1), dimethylformamide (S2), 1,2-diphenylhydrazine (S3), p-methoxyphenol (S4), and 1,4-cyclohexadiene (S5). In 1–3, the iron is pentacoordinated by tris[N′-tert-butylureaylato)-N-ethylene]aminato ([H3buea]3‑) or its derivatives. These complexes are basic, in the order 3 ≫ 1 > 2. Oxidant 4, [FeIVN4Py(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···H···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–Evans–Polanyi 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–H bond activations, the barrier derives from the deformation energy required to create the TS, whereas in N–H/O–H 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.