Effects of proton acceptors on formation of a non-heme iron(IV)-oxo complex via proton-coupled electron transfer

Rates of formation of a non-heme iron(IV)-oxo complex, [Fe ^IV(O)(N4Py)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2- pyridyl)methylamine), via electron-transfer oxidation of [Fe^III(OH) (N4Py)]²⁺ in acetonitrile (MeCN) containing H₂O (0.56 M) were accelerated as much as 390-fold by addition of proton acceptors such as CF₃COO^-, TsO^- (p-MeC₆H ₄SO₃^-), NsO^- (o-NO₂C ₆H₄SO₃^-), DNsO^- (2,4-(NO₂)₂C₆H₃SO₃ ^-), and TfO^- (CF₃SO₃^-). The acceleration effect of proton acceptors increases with increasing basicity of the proton acceptors. The one-electron oxidation potential of [Fe ^III(OH)(N4Py)]²⁺ was shifted from 1.24 to 0.96 V vs SCE in the presence of TsO^- (10 mM). The electron-transfer oxidation of Fe^III-OH complex was coupled with the deprotonation process by proton acceptors in which deuterium kinetic isotope effects were observed when H ₂O was replaced by D₂O.