A mononuclear non-heme manganese(V)-oxo complex, [Mn(V)(O)(TAML)](-) (1), was synthesized by activating dioxygen in the presence of olefins with weak allylic C-H bonds and characterized structurally and spectroscopically. In mechanistic studies, the formation rate of 1 was found to depend on the allylic C-H bond dissociation energies (BDEs) of olefins, and a kinetic isotope effect (KIE) value of 16 was obtained in the reactions of cyclohexene and cyclohexene-d10. These results suggest that a hydrogen atom abstraction from the allylic C-H bonds of olefins by a putative Mn(IV)-superoxo species, which is formed by binding O2 by a high-spin (S = 2) [Mn(III)(TAML)](-) complex, is the rate-determining step. A Mn(V)-oxo complex binding Sc(3+) ion, [Mn(V)(O)(TAML)](-)-(Sc(3+)) (2), was also synthesized in the reaction of 1 with Sc(3+) ion and then characterized using various spectroscopic techniques. The binding site of the Sc(3+) ion was proposed to be the TAML ligand, not the Mn-O moiety, probably due to the low basicity of the oxo group compared to the basicity of the amide carbonyl group in the TAML ligand. Reactivity studies of the Mn(V)-oxo intermediates, 1 and 2, in oxygen atom transfer and electron-transfer reactions revealed that the binding of Sc(3+) ion at the TAML ligand of Mn(V)-oxo enhanced its oxidizing power with a positively shifted one-electron reduction potential (ΔEred = 0.70 V). This study reports the first example of tuning the second coordination sphere of high-valent metal-oxo species by binding a redox-inactive metal ion at the supporting ligand site, thereby modulating their electron-transfer properties as well as their reactivities in oxidation reactions.

A mononuclear non-heme Mn­(III)–aqua complex, [(dpaq)­MnIII(OH2)]2+ (1, dpaq = 2-[bis­(pyridin-2-yl­methyl)]­amino-N-quinolin-8-yl-acet­amidate), is capable of conducting hydrogen atom transfer (HAT) reactions much more efficiently than the corresponding Mn­(III)–hydroxo complex, [(dpaq)­MnIII­(OH)]+ (2); the high reactivity of 1 results from the positive one-electron reduction potential of 1 (E red vs SCE = 1.03 V), compared to that of 2 (E red vs SCE = −0.1 V). The HAT mechanism of 1 varies between electron transfer followed by proton transfer and one-step concerted proton-coupled electron transfer, depending on the one-electron oxidation potentials of substrates. To the best of our knowledge, this is the first example showing that metal­(III)–aqua complex can be an effective H-atom abstraction reagent.

Mononuclear nonheme manganese­(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)­MnIV(O)]+-M n+ (1-Mn+, M n+ = Ca2+, Mg2+, Zn2+, Lu3+, Y3+, Al3+, and Sc3+) (dpaq = 2-[bis­(pyridin-2-ylmethyl)]­amino-N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese­(III) complex, [(dpaq)­MnIII(OH)]+, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M n+). The Mn­(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M n+ on the reactivity of 1-M n+ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M n+, such as 1-Sc3+ ≈ 1-Al3+ > 1-Y3+ > 1-Lu3+ > 1-Zn2+ > 1-Mg2+ > 1-Ca2+, follows the Lewis acidity of M n+ bound to the Mn–O moiety; that is, the stronger the Lewis acidity of M n+, the higher the reactivity of 1-M n+ becomes. In sharp contrast, the reactivity of 1-M n+ in the HAT reaction was reversed, giving the reactivity order 1-Ca2+ > 1-Mg2+ > 1-Zn2+ > 1-Lu3+> 1-Y3+> 1-Al3+ ≈ 1-Sc3+; that is, the higher is Lewis acidity of M n+, the lower the reactivity of 1-M n+ in the HAT reaction. The latter result implies that the Lewis acidity of M n+ bound to the Mn–O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M n+; cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C–H bond activation reactions.

We report the synthesis of manganese(iii)-peroxo (Mn(O)) and manganese(iii)-hydroperoxo (Mn(OH)) complexes by activating dioxygen (O) and the amphoteric reactivity of the Mn(iii)-hydroperoxo complex in electrophilic and nucleophilic reactions.