Ferryl protonation in oxoiron(IV) porphyrins and its role in oxygen transfer

Nicholas C. Boaz, Seth R. Bell, John Taylor Groves

Research output: Contribution to journalArticle

42 Scopus citations

Abstract

Ferryl porphyrins, P-FeIV=O, are central reactive intermediates in the catalytic cycles of numerous heme proteins and a variety of model systems. There has been considerable interest in elucidating factors, such as terminal oxo basicity, that may control ferryl reactivity. Here, the sulfonated, water-soluble ferryl porphyrin complexes tetramesitylporphyrin, oxoFeIVTMPS (FeTMPS-II), its 2,6-dichlorophenyl analogue, oxoFeIVTDClPS (FeTDClPS-II), and two other analogues are shown to be protonated under turnover conditions to produce the corresponding bis-aqua-iron(III) porphyrin cation radicals. The results reveal a novel internal electromeric equilibrium, P-FeIV=O ⇆ P+-FeIII(OH2)2. Reversible pKa values in the range of 4-6.3 have been measured for this process by pH-jump, UV-vis spectroscopy. Ferryl protonation has important ramifications for C-H bond cleavage reactions mediated by oxoiron(IV) porphyrin cation radicals in protic media. Both solvent O-H and substrate C-H deuterium kinetic isotope effects are observed for these reactions, indicating that hydrocarbon oxidation by these oxoiron(IV) porphyrin cation radicals occurs via a solvent proton-coupled hydrogen atom transfer from the substrate that has not been previously described. The effective FeO-H bond dissociation energies for FeTMPS-II and FeTDClPS-II were estimated from similar kinetic reactivities of the corresponding oxoFeIVTMPS+ and oxoFeIVTDClPS+ species to be ∼92-94 kcal/mol. Similar values were calculated from the two-proton P+-FeIII(OH2)2 pKaobs and the porphyrin oxidation potentials, despite a 230 mV range for the iron porphyrins examined. Thus, the iron porphyrin with the lower ring oxidation potential has a compensating higher basicity of the ferryl oxygen. The solvent-derived proton adds significantly to the driving force for C-H bond scission.

Original languageEnglish (US)
Pages (from-to)2875-2885
Number of pages11
JournalJournal of the American Chemical Society
Volume137
Issue number8
DOIs
StatePublished - Mar 4 2015

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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