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Title: Ferryl Protonation in Oxoiron(IV) Porphyrins and Its Role in Oxygen Transfer

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 in this study, 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 $$\leftrightarrows$$ 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.
Authors:
 [1] ;  [1] ;  [1]
  1. Princeton Univ., NJ (United States). Dept. of Chemistry
Publication Date:
OSTI Identifier:
1210362
Grant/Contract Number:
SC0001298
Type:
Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 137; Journal Issue: 8; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Research Org:
Energy Frontier Research Centers (EFRC). Center for Catalytic Hydrocarbon Functionalization (CCHF)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Contributing Orgs:
CCHF partners with University of Virginia (lead); Brigham Young University; California Institute of Technology; Colorado School of Mines; University of Maryland; University of North Carolina, Chapel Hill; University of North Texas; Princeton University; The Scripps Research Institute; Yale University
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; catalysis (homogeneous); catalysis (heterogeneous); bio-inspired; hydrogen and fuel cells; materials and chemistry by design; synthesis (novel materials)