Saturation kinetics in phenolic O-H bond oxidation by a mononuclear Mn(III)-OH complex derived from dioxygen

Academic Article

Abstract

  • 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, [Mn III(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 [Mn III(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. © 2014 American Chemical Society.
  • Authors

    Digital Object Identifier (doi)

    Author List

  • Wijeratne GB; Corzine B; Day VW; Jackson TA
  • Start Page

  • 7622
  • End Page

  • 7634
  • Volume

  • 53
  • Issue

  • 14