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

摘要

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O_(2) activation (e.g., by Fe porphyrin in Cytochrome P450) and O_(2) production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M~(IV)(OH) complexes remain a rarity. Herein we describe the synthesis of Mn~(IV)(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~(V)(O)(ttppc) (4 ) and Mn~(III)(H_(2)O)(ttppc) (1 ·H_(2)O) 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) = 2.73(12) × 10~(4) M~(–1) s~(–1), which is ∼3 orders of magnitude larger than 4 (k _(2) = 17.4(1) M~(–1) s~(–1)). Reaction of 3 with a series of para -substituted 2,6-di-tert -butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, t Bu, H) gives rate constants in the range k _(2) = 510(10)–36(1.4) M~(–1) s~(–1) 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~(IV)(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn~(V)(O) complex, or even some Fe~(IV)(O) complexes.