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% HClO₄). The observed second-order rate constant (k_obs) of sulfoxidation of thioaniosoles by [(N4Py)­Fe^IV(O)]²⁺ increases linearly with increasing concentration of HClO₄ (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(Me₂bpy)₃]²⁺ (Me₂bpy = 4,4-dimehtyl-2,2′-bipyridine) to [(N4Py)­Fe^IV(O)]²⁺ increases with increasing concentration of HClO₄, exhibiting second-order dependence on HClO₄ concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron transfer from [Fe^II(Me₂bpy)₃]²⁺ to [(N4Py)­Fe^IV(O)]²⁺ to yield [Fe^III(Me₂bpy)₃]³⁺ and [(N4Py)­Fe^III(OH₂)]³⁺. The one-electron reduction potential (E_red) of [(N4Py)­Fe^IV(O)]²⁺ in the presence of 10 mM HClO₄ (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of E_red vs log­[HClO₄] 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.