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
