Effect of imidazole and phenolate axial ligands on the electronic structure and reactivity of oxoiron(IV) porphyrin pi-cation radical complexes: drastic increase in oxo-transfer and hydrogen abstraction reactivities.
To study the effect of axial ligands on the electronic structure and reactivity of compound I of peroxidases and catalases, oxoiron(IV) porphyrin pi-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, (1)H, (2)H, and (19)F 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 pi-cation radical state, the a(2u) state of the TMP complexes, or the a(1u) radical state of both the TMTMP complexes and compound I. The ferryl iron and porphyrin pi-cation radical spins were effectively transferred into the axial ligands for the a(2u) complexes but not for the a(1u) complexes. Most importantly, the reactivity of the oxoiron(IV) porphyrin pi-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.
Takahashi
Akihiro
A
Institute for Molecular Science and Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, and Department of Functional Molecular Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8787, Japan.
Kurahashi
Takuya
T
Fujii
Hiroshi
H
D004583:Electrons; D006859:Hydrogen / Q000737:chemistry; D062385:Hydroxybenzoates / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*
D062385:Hydroxybenzoates; D007093:Imidazoles; D008024:Ligands; D011166:Porphyrins; C028565:porphyrin pi cation radical; C066317:ferryl iron; C029899:imidazole; D006859:Hydrogen; D007501:Iron; C017616:phenolic acid; D010100:Oxygen