ja211641s_si_001.pdf (261.01 kB)
Proton-Promoted Oxygen Atom Transfer vs Proton-Coupled Electron Transfer of a Non-Heme Iron(IV)-Oxo Complex
journal contribution
posted on 2012-02-29, 00:00 authored by Jiyun Park, Yuma Morimoto, Yong-Min Lee, Wonwoo Nam, Shunichi FukuzumiSulfoxidation 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.