Proton-promoted oxygen atom transfer vs proton-coupled electron transfer of a non-heme iron(IV)-oxo complex

Sulfoxidation of thioanisoles by a non-heme iron(IV)-oxo complex, [(N4Py)Fe ^IV(O)] ²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N- bis(2-pyridyl)methylamine), was remarkably enhanced by perchloric acid (70% HClO ₄). The observed second-order rate constant (k _obs) of sulfoxidation of thioaniosoles by [(N4Py)Fe ^IV(O)] ²⁺ increases linearly with increasing concentration of HClO ₄ (70%) in acetonitrile (MeCN)at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)Fe ^IV(O)] ²⁺, the observed second-order rate constant (k _et) of electron transfer from one-electron reductants such as [Fe ^II(Me ₂bpy) ₃] ²⁺ (Me ₂bpy = 4,4-dimehtyl-2,2′-bipyridine) to [(N4Py)Fe ^IV(O)] ²⁺ increases with increasing concentration of HClO ₄, exhibiting second-order dependence on HClO ₄ concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron transfer from [Fe ^II(Me ₂bpy) ₃] ²⁺ to [(N4Py)Fe ^IV(O)] ²⁺ to yield [Fe ^III(Me ₂bpy) ₃] ³⁺ and [(N4Py)Fe ^III(OH ₂)] ³⁺. The one-electron reduction potential (E _red) of [(N4Py)Fe ^IV(O)] ²⁺ in the presence of 10 mM HClO ₄ (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of E _red vs log[HClO ₄] also indicates involvement of two protons in the PCET reduction of [(N4Py)Fe ^IV(O)] ²⁺. The PCET driving force dependence of log k _et is fitted in light of the Marcus theory of outer-sphere electron transfer to afford the reorganization of PCET (λ = 2.74 eV). The comparison of the k _obs values of acid-promoted sulfoxidation of thioanisoles by [(N4Py)Fe ^IV(O)] ²⁺ with the k _et values of PCET from one-electron reductants to [(N4Py)Fe ^IV(O)] ²⁺ at the same PCET driving force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)Fe ^IV(O)] ²⁺ to thioanisoles rather than outer-sphere PCET.