Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction

Chivukula V. Sastri, Jimin Lee, Kyungeun Oh, Jin Lee Yoon, Junghyun Lee, Timothy A. Jackson, Kallol Ray, Hajime Hirao, Woonsup Shin, Jason A. Halfen, Jinheung Kim, Lawrence Que, Sason Shaik, Wonwoo Nam

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Abstract

The reactivities of mononuclear nonheme iron(IV)-oxo complexes bearing different axial ligands, [FeIV(O)(TMC)(X)]n+ [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH 3 (1-NCCH3), CF3COO- (1-OOCCF3), or N3- (1-N3)], and [FeIV(O)(TMCS)] + (1′-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4, 8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh3 and hydrogen atom abstraction from phenol O - H and alkylaromatic C - H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh3, the reactivity order of 1-NCCH3 > 1-OOCCF3 > 1-N3 > 1′-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)-oxo unit upon replacement of CH3CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C - H and phenol O - H bonds, i.e., 1′-SR > 1-N3 > 1-OOCCF3 > 1-NCCH3. Furthermore, a good correlation was observed between the reactivities of iron(IV)-oxo species in H atom abstraction reactions and their reduction potentials, Ep,c, with the most reactive 1′-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)-oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.

Original languageEnglish (US)
Pages (from-to)19181-19186
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume104
Issue number49
DOIs
StatePublished - Dec 4 2007

Keywords

  • Biomimetics
  • High-valent iron-oxo intermediate
  • Nonheme iron enzymes
  • Oxygen activation

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    Sastri, C. V., Lee, J., Oh, K., Yoon, J. L., Lee, J., Jackson, T. A., Ray, K., Hirao, H., Shin, W., Halfen, J. A., Kim, J., Que, L., Shaik, S., & Nam, W. (2007). Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction. Proceedings of the National Academy of Sciences of the United States of America, 104(49), 19181-19186. https://doi.org/10.1073/pnas.0709471104