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Keywords:

  • iron;
  • ligand effects;
  • enzyme models;
  • oxidation;
  • porphyrinoids;
  • reaction mechanisms

Abstract

The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [FeIII(tpfpp)X]/H2O2/HOO system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl or CF3SO3) in acetonitrile under basic conditions at −15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [FeIII(tpfpp)(OOH)] and low-spin [FeIII(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O[BOND]O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic O[BOND]O bond cleavage is observed for high- [FeIII(tpfpp)(OOH)] and low-spin [FeIII(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [FeIII(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [FeIII(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.