ESR Studies of A1uand A2uOxoiron(IV) Porphyrin π-Cation Radical Complexes. Spin Coupling between Ferryl Iron and A1u/A2uOrbitals

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Abstract
This study shows the ESR spectra of oxoiron(IV) porphyrin π-cation radicals of 1−8 in dichloromethane−methanol (5:1) mixture. We reported in a previous paper that oxoiron(IV) porphyrin π-cation radicals of 1−4 are in an a1u radical state while those of 5−8 are in an a2u radical. The ESR spectra (g⊥eff ∼ 3.1 and g∥eff ∼ 2.0) for the a1u radical complexes, 1−4, appear quite different from those reported previously for the oxoiron(IV) porphyrin π-cation radical of 5 (gy = 4.5, gx = 3.6, and gz = 1.99). The unique ESR spectra of the a1u radical complexes rather resemble those of compound I from Micrococcus lysodeikticus catalase (CAT) and ascorbate peroxidase (ASP). This is the first examples to mimic the ESR spectra of compound I in the enzymes. From spectral analysis based on a spin Hamiltonian containing an exchange interaction, the ESR spectra of 1−4 can be explained as a moderate ferromagnetic state (J/D ∼ 0.3) between ferryl S = 1 and the porphyrin π-cation radical S‘ = 1/2. The magnitudes of zero-field splitting (D) for ferryl iron and isotropic J value, estimated from the temperature-dependence of the half-saturation power of the ESR signals, are ∼28 and ∼+8 cm-1, respectively. A change in the electronegativity of the β-pyrrole substituent hardly changes the ESR spectral feature while that of the meso-substituent slightly does owing to the change in the E/D value. On the basis of the present ESR results, we propose the a1u radical state for compound I of CAT and ASP.

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