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Proton-Promoted Oxygen Atom Transfer vs Proton-Coupled Electron Transfer of a Non-Heme Iron(IV)-Oxo Complex

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journal contribution
posted on 29.02.2012, 00:00 by Jiyun Park, Yuma Morimoto, Yong-Min Lee, Wonwoo Nam, Shunichi Fukuzumi
Sulfoxidation of thioanisoles by a non-heme iron­(IV)–oxo complex, [(N4Py)­FeIV(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 (kobs) of sulfoxidation of thioaniosoles by [(N4Py)­FeIV(O)]2+ increases linearly with increasing concentration of HClO4 (70%) in acetonitrile (MeCN) at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)­FeIV(O)]2+, the observed second-order rate constant (ket) of electron transfer from one-electron reductants such as [FeII(Me2bpy)3]2+ (Me2bpy = 4,4-dimehtyl-2,2′-bipyridine) to [(N4Py)­FeIV(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 [FeII(Me2bpy)3]2+ to [(N4Py)­FeIV(O)]2+ to yield [FeIII(Me2bpy)3]3+ and [(N4Py)­FeIII(OH2)]3+. The one-electron reduction potential (Ered) of [(N4Py)­FeIV(O)]2+ in the presence of 10 mM HClO4 (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of Ered vs log­[HClO4] also indicates involvement of two protons in the PCET reduction of [(N4Py)­FeIV(O)]2+. The PCET driving force dependence of log ket 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 kobs values of acid-promoted sulfoxidation of thioanisoles by [(N4Py)­FeIV(O)]2+ with the ket values of PCET from one-electron reductants to [(N4Py)­FeIV(O)]2+ at the same PCET driving force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)­FeIV(O)]2+ to thioanisoles rather than outer-sphere PCET.