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Effect of Imidazole and Phenolate Axial Ligands on the Electronic Structure and Reactivity of Oxoiron(IV) Porphyrin π-Cation Radical Complexes: Drastic Increase in Oxo-Transfer and Hydrogen Abstraction Reactivities

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journal contribution
posted on 16.03.2009, 00:00 by Akihiro Takahashi, Takuya Kurahashi, Hiroshi Fujii
To study the effect of axial ligands on the electronic structure and reactivity of compound I of peroxidases and catalases, oxoiron(IV) porphyrin π-cation radical complexes with imidazole, 2-methylimidazole, 4(5)-methylimidazole, and 3-fluoro-4-nitrophenolate as the axial ligands were prepared by ozone oxidation of iron(III) complexes of 5,10,15,20-tetramesitylporphyrin (TMP) and 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin (TMTMP). These complexes were fully characterized by absorption, 1H, 2H, and 19F NMR, electron paramagnetic resonance (EPR), and electrospray ionization mass spectrometry (ESI-MS) spectroscopy. The characteristic absorption peak of compound I at approximately 650 nm was found to be a good marker for estimation of the electron donor effect from the axial ligand. The axial ligand effect did not change the porphyrin π-cation radical state, the a2u state of the TMP complexes, or the a1u radical state of both the TMTMP complexes and compound I. The ferryl iron and porphyrin π-cation radical spins were effectively transferred into the axial ligands for the a2u complexes but not for the a1u complexes. Most importantly, the reactivity of the oxoiron(IV) porphyrin π-cation radical complex was drastically increased by the imidazole and phenolate axial ligands. The reaction rate for cyclooctene epoxidation was increased 100- to 400-fold with axial coordination of imidazoles and phenolate. A similar increase was also observed for the oxidation of 1,4-cyclohexadiene,N,N-dimethyl-p-nitroaniline and hydrogen peroxide. These results suggest extreme enhancement of the reactivity of compound I by the axial ligand in heme enzymes. The functional role of axial ligands on the compound I in heme enzymes is discussed.

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