A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O<sub>2</sub> activation (e.g., by Fe porphyrin in Cytochrome P450) and O<sub>2</sub> production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M<sup>IV</sup>(OH) complexes remain a rarity. Herein we describe the synthesis of Mn<sup>IV</sup>(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn<sup>V</sup>(O)­(ttppc) (<b>4</b>) and Mn<sup>III</sup>(H<sub>2</sub>O)­(ttppc) (<b>1</b>·H<sub>2</sub>O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i><sub>2</sub> = 2.73(12) × 10<sup>4</sup> M<sup>–1</sup> s<sup>–1</sup>, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i><sub>2</sub> = 17.4(1) M<sup>–1</sup> s<sup>–1</sup>). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i><sub>2</sub> = 510(10)–36(1.4) M<sup>–1</sup> s<sup>–1</sup> and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn<sup>IV</sup>(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn<sup>V</sup>(O) complex, or even some Fe<sup>IV</sup>(O) complexes.