858342-97-9Relevant academic research and scientific papers
Strong Inhibition of O-Atom Transfer Reactivity for MnIV(O)(π-Radical-Cation)(Lewis Acid) versus MnV(O) Porphyrinoid Complexes
Zaragoza, Jan Paulo T.,Baglia, Regina A.,Siegler, Maxime A.,Goldberg, David P.
, p. 6531 - 6540 (2015)
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 Bronsted 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. (Figure Presented).
