Strong Inhibition of O‑Atom Transfer Reactivity for MnIV(O)(π-Radical-Cation)(Lewis Acid) versus MnV(O) Porphyrinoid Complexes

The oxygen atom transfer (OAT) reactivity of two valence tautomers of a MnV(O) porphyrinoid complex was compared. The OAT kinetics of MnV(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) × 104 M–1 s–1. Characterization of the OAT transition state analogues MnIII(OPPh3)­(TBP8Cz) and MnIII(OP­(o-tolyl)3)­(TBP8Cz) was carried out by single-crystal X-ray diffraction (XRD). A valence tautomer of the closed-shell MnV(O)­(TBP8Cz) can be stabilized by the addition of Lewis and Brønsted acids, resulting in the open-shell MnIV(O)­(TBP8Cz•+):LA (LA = ZnII, B­(C6F5)3, H+) complexes. These MnIV(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 MnIV(O)­(TBP8Cz•+):LA with phenols. A Hammett analysis showed that the OAT reactivity for MnIV(O)­(TBP8Cz•+):LA is influenced by the Lewis acid strength. Spectral redox titration of MnIV(O)­(TBP8Cz•+):ZnII gives Ered = 0.69 V vs SCE, which is nearly +700 mV above its valence tautomer MnV(O)­(TBP8Cz) (Ered = −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 MV(O)­(porph) versus MIV(O)­(porph•+) (M = Mn or Fe) found in heme enzymes.