Strong Inhibition of O-Atom Transfer Reactivity for MnIV(O)(π-radical-cation)(Lewis acid) Versus MnV(O) Porphyrinoid Complexes

摘要

The oxygen atom transfer (OAT) reactivity of two valence tautomers of a Mn^V(O) porphyrinoid complex was compared. The OAT kinetics of Mn^V(O)(TBP8Cz) (TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato³⁻) reacting with a series of triarylphosphine (PAr3) substrates were monitored by stopped-flow UV-vis spectroscopy, and revealed second-order rate constants ranging from 16(1) to 1.43(6) × 10 M⁻¹ s⁻¹. Characterization of the OAT transition state analogs Mn^III(OPPh3)(TBP8Cz) and Mn^III(OP(o-tolyl)3)(TBP8Cz) was carried out by single-crystal X-ray diffraction (XRD). A valence tautomer of the closed-shell Mn^V(O)(TBP8Cz) can be stabilized by the addition of Lewis and Brønsted acids, resulting in the open-shell Mn^IV(O)(TBP8Cz^•+):LA (LA = Zn^II, B(C6F5)3, H⁺) complexes. These Mn^IV(O)(π-radical-cation) derivatives exhibit dramatically inhibited rates of OAT with the PAr3 substrates (k = 8.5(2) × 10⁻³ − 8.7 M⁻¹ s⁻¹), contrasting the previously observed rate increase of H-atom transfer (HAT) for Mn^IV(O)(TBP8Cz^•+):LA with phenols. A Hammett analysis showed that the OAT reactivity for Mn^IV(O)(TBP8Cz^•+):LA is influenced by the Lewis acid strength. Spectral redox titration of Mn^IV(O)(TBP8Cz^•+):Zn^II gives E_red = 0.69 V vs SCE, which is nearly +700 mV above its valence tautomer Mn^V(O)(TBP₈Cz) (E_red = −0.05 V). These data suggest that the two-electron electrophilicity of the Mn(O) valence tautomers dominate OAT reactivity and do not follow the trend in one-electron redox potentials, which appear to dominate HAT reactivity. This study provides new fundamental insights regarding the relative OAT and HAT reactivity of valence tautomers such as M^V(O)(porph) versus M^IV(O)(porph^•+) (M = Mn or Fe) found in heme enzymes.