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 O2 activation (e.g.,
by Fe porphyrin in Cytochrome P450) and O2 production (e.g.,
by the Mn cluster in Photosystem II). However, well-characterized
mononuclear MIV(OH) complexes remain a rarity. Herein we
describe the synthesis of MnIV(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 MnV(O)­(ttppc) (4) and MnIII(H2O)­(ttppc) (1·H2O) 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 k2 = 2.73(12) × 104 M–1 s–1, which is ∼3
orders of magnitude larger than 4 (k2 = 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, tBu, H) gives rate constants
in the range k2 = 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 MnIV(OH) complex
is a much more powerful H atom abstractor than the higher-valent MnV(O) complex, or even some FeIV(O) complexes.