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.

A mononuclear nonheme manganese(IV)–oxo complex binding the Ce4+ ion, [(dpaq)MnIV(O)]+–Ce4+ (1‐Ce4+), was synthesized by reacting [(dpaq)MnIII(OH)]+ (2) with cerium ammonium nitrate (CAN). 1‐Ce4+ was characterized using various spectroscopic techniques, such as UV/Vis, EPR, CSI‐MS, resonance Raman, XANES, and EXAFS, showing an Mn−O bond distance of 1.69 Å with a resonance Raman band at 675 cm−1. Electron‐transfer and oxygen atom transfer reactivities of 1‐Ce4+ were found to be greater than those of MnIV(O) intermediates binding redox‐inactive metal ions (1‐Mn+). This study reports the first example of a redox‐active Ce4+ ion‐bound MnIV‐oxo complex and its spectroscopic characterization and chemical properties.

Binding of Lewis acidic metal ions and Brønsted acid at the metal–oxo group of high-valent metal–oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal–oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese­(IV)–oxo complex binding triflic acid, {[(dpaq)­MnIV(O)]–HOTf}+ (1–HOTf). First, 1–HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn–O stretching frequencies of 1–HOTf (e.g., νMn–O at ∼793 cm–1) and 1–M n+ (M n+ = Ca2+, Zn2+, Lu3+, Al3+, or Sc3+) and the Lewis acidities of H+ and M n+ ions, suggesting that H+ and M n+ bind at the metal–oxo moiety of [(dpaq)­MnIV(O)]+. Interestingly, a single-crystal structure of 1–HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn­(IV)–oxo complex but a Mn­(IV)–hydroxide complex, [(dpaq)­MnIV(OH)]­(OTf)2 (4), with a Mn–O bond distance of 1.8043(19) Å and a Mn–O stretch at 660 cm–1. More interestingly, 4 reverted to 1–HOTf upon dissolution, demonstrating that 1–HOTf and 4 are interconvertible depending on the physical states, such as 1–HOTf in solution and 4 in isolated solid. The reactivity of 1–HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1–M n+ complexes; an interesting correlation between the Mn–O stretching frequencies of 1–HOTf and 1–M n+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

A mononuclear manganese(V)–oxo complex with tetraamido macrocyclic ligand (TAML), [MnV(O)(TAML)]− (1), is a sluggish oxidant in oxidation reactions. Herein, a mononuclear manganese(V)–oxo TAML cation radical complex, [MnV(O)(TAML+.)] (2), is reported. It was synthesized by reacting [MnIII(TAML)]− with 3.0 equivalents of [RuIII(bpy)3]3+ or upon addition of one‐electron oxidant to 1 and then characterized thoroughly with various spectroscopic techniques along with DFT calculations. Although 1 is a sluggish oxidant, 2 is a strong oxidant capable of activating C−H bonds of hydrocarbons (i.e., hydrogen atom transfer reaction) and transferring its oxygen atom to thioanisoles and olefins (i.e., oxygen atom transfer reaction).