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 activation (e.g., by Fe porphyrin in Cytochrome P450) and O production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M(OH) complexes remain a rarity. Herein we describe the synthesis of Mn(OH)(ttppc) (3) (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 3. The complexes Mn(O)(ttppc) (4) and Mn(HO)(ttppc) (1·HO) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di- tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k = 2.73(12) × 10 M s, which is ∼3 orders of magnitude larger than 4 ( k = 17.4(1) M s). Reaction of 3 with a series of para-substituted 2,6-di- tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k = 510(10)-36(1.4) M s 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(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn(O) complex, or even some Fe(O) complexes.