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

Abstract

We describe herein the hydrogen-atom transfer (HAT)/proton-coupled electron-transfer (PCET) reactivity for Fe(IV)-oxo and Fe(III)-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, [Fe(IV)N4Py(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.