Saturation Kinetics in Phenolic O–H Bond Oxidation by a Mononuclear Mn(III)–OH Complex Derived from Dioxygen

The
mononuclear hydroxomanganese­(III) complex, [MnIII(OH)­(dpaq)]+, which is supported by the amide-containing
N5 ligand dpaq (dpaq = 2-[bis­(pyridin-2-ylmethyl)]­amino-N-quinolin-8-yl-acetamidate) was generated by treatment
of the manganese­(II) species, [MnII(dpaq)]­(OTf), with dioxygen
in acetonitrile solution at 25 °C. This oxygenation reaction
proceeds with essentially quantitative yield (greater than 98% isolated
yield) and represents a rare example of an O2-mediated
oxidation of a manganese­(II) complex to generate a single product.
The X-ray diffraction structure of [MnIII(OH)­(dpaq)]+ reveals a short Mn–OH distance of 1.806(13) Å,
with the hydroxo moiety trans to the amide function
of the dpaq ligand. No shielding of the hydroxo group is observed
in the solid-state structure. Nonetheless, [MnIII(OH)­(dpaq)]+ is remarkably stable, decreasing in concentration by only
10% when stored in MeCN at 25 °C for 1 week. The [MnIII(OH)­(dpaq)]+ complex participates in proton-coupled electron
transfer reactions with substrates with relatively weak O–H
and C–H bonds. For example, [MnIII(OH)­(dpaq)]+ oxidizes TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol),
which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol,
in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect
of 1.8 observed for this reaction implies a concerted proton–electron
transfer pathway. The [MnIII(OH)­(dpaq)]+ complex
also oxidizes xanthene (C–H BDFE of 73.3 kcal/mol in dimethylsulfoxide)
and phenols, such as 2,4,6-tri-t-butylphenol, with
BDFEs of less than 79 kcal/mol. Saturation kinetics were observed
for phenol oxidation, implying an initial equilibrium prior to the
rate-determining step. On the basis of a collective body of evidence,
the equilibrium step is attributed to the formation of a hydrogen-bonding
complex between [MnIII(OH)­(dpaq)]+ and the phenol
substrates.