Activation of dioxygen (O2) in enzymatic and biomimetic reactions has been intensively investigated over the past several decades. More recently, O–O bond formation, which is the reverse of the O2-activation reaction, has been the focus of current research. Herein, we report the O2-activation and O–O bond formation reactions by manganese corrole complexes. In the O2-activation reaction, Mn(V)-oxo and Mn(IV)-peroxo intermediates were formed when Mn(III) corroles were exposed to O2 in the presence of base (e.g., OH−) and hydrogen atom (H atom) donor (e.g., THF or cyclic olefins); the O2-activation reaction did not occur in the absence of base and H atom donor. Moreover, formation of the Mn(V)-oxo and Mn(IV)-peroxo species was dependent on the amounts of base present in the reaction solution. The role of the base was proposed to lower the oxidation potential of the Mn(III) corroles, thereby facilitating the binding of O2 and forming a Mn(IV)-superoxo species. The putative Mn(IV)-superoxo species was then converted to the corresponding Mn(IV)-hydroperoxo species by abstracting a H atom from H atom donor, followed by the O–O bond cleavage of the putative Mn(IV)-hydroperoxo species to form a Mn(V)-oxo species. We have also shown that addition of hydroxide ion to the Mn(V)-oxo species afforded the Mn(IV)-peroxo species via O–O bond formation and the resulting Mn(IV)-peroxo species reverted to the Mn(V)-oxo species upon addition of proton, indicating that the O–O bond formation and cleavage reactions between the Mn(V)-oxo and Mn(IV)-peroxo complexes are reversible. The present study reports the first example of using the same manganese complex in both O2-activation and O–O bond formation reactions.

The rate constants of the C═C epoxidation and the C-H hydroxylation (i.e., allylic C-H bond activation) in the oxidation of cyclohexene by a high-valent iron(IV)-oxo porphyrin π-cation radical complex, [(TMP)Fe(O)(Cl)] (1, TMP = meso-tetramesitylporphyrin dianion), were determined at various temperatures by analyzing the overall rate constants and the products obtained in the cyclohexene oxidation by 1, leading us to conclude that reaction pathway changes from the C═C epoxidation to C-H hydroxylation by decreasing reaction temperature. When cyclohexene was replaced by deuterated cyclohexene (cyclohexene-d), the epoxidation pathway dominated irrespective of the reaction temperature. The temperature dependence of the rate constant of the C-H hydroxylation pathway in the reactions of cyclohexene and cyclohexene-d by 1 suggests that there is a significant tunneling effect on the hydrogen atom abstraction of allylic C-H bonds of cyclohexene by 1, leading us to propose that the tunneling effect is a determining factor for the switchover of the reaction pathway from the C═C epoxidation pathway to the C-H hydroxylation pathway by decreasing reaction temperature. By performing density functional theory (DFT) calculations, the reaction energy barriers of the C═C epoxidation and C-H bond activation reactions by 1 were found to be similar, supporting the notion that small environmental changes, such as the reaction temperature, can flip the preference for one reaction to another.

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Heme compound II models bearing electron-deficient and -rich porphyrins, [FeIV(O)(TPFPP)(Cl)]– (1a) and [FeIV(O)(TMP)(Cl)]– (2a), respectively, are synthesized, spectroscopically characterized, and investigated in the chemoselectivity and disproportionation reactions using cyclohexene as...