Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo "cubane" complexes containing the Mn4O46+ and Mn4O47+ core types

Marcelino Maneiro, Wolfgang F. Ruettinger, Emilie Bourles, George L. McLendon, G Charles Dismukes

Research output: Contribution to journalArticle

46 Citations (Scopus)

Abstract

The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn4O4(O2PPh2)6, 1, and [Mn4O4(O2PPh2) 6]+, 1+, with phenothiazine (pzH). Both pcet reactions form 1H, by H transfer to 1 and by hydride transfer to 1+. Surprisingly, the rate constants differ by only 25% despite large differences in the formal charges and driving force. The driving force is proportional to the difference in the bond-dissociation energies (BDE > 94 kcal/mol for homolytic, 1H → H + 1, vs. ≊127 kcal/mol for heterolytic, 1H → H- + 1+, dissociation of the O - H bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (ΔAH = -1.2 kcal/mol and ΔS = -32 cal/mol·K; 25-6.7°C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no H/D kinetic isotope effect (kH/kD = 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H + pzH → 1H2 + pz.). These lines of evidence indicate that formation of a reactive precursor complex before atom transfer is rate-limiting (conformational gating), and that little or no N - H bond cleavage occurs in the transition state. H-atom transfer from pzH to alkyl, alkoxyl, and peroxyl radicals reveals that BDEs are not a good predictor of the rates of this reaction. Hydride transfer to 1+ provides a concrete example of two-electron pcet that is hypothesized for the O - H bond cleavage step during catalysis of photosynthetic water oxidation.

Original languageEnglish
Pages (from-to)3707-3712
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume100
Issue number7
DOIs
Publication statusPublished - Apr 1 2003

    Fingerprint

Keywords

  • Bond-dissociation energy
  • Hydrogen atom transfer
  • Kinetic isotope effect
  • Manganese
  • Proton transfer

ASJC Scopus subject areas

  • Genetics
  • General

Cite this