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

Abstract

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. © 2012 American Chemical Society.