ESR Studies of A_1uand A_2uOxoiron(IV) Porphyrin π-Cation Radical Complexes. Spin Coupling between Ferryl Iron and A_1u/A_2uOrbitals

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.

EPR study of a novel [Fe–porphyrin] catalyst

“…Of immediate relevance to our results is the work of Fujii et al [42] for substituted Fe-porphyrins in organic solvent [42].…”

“…(S΄=1/2). The coupling is assumed to be a weak exchange interaction J [18,24,42]. Within the formalism of the weakly coupled (S=1 and than g ‫||‬ results from ferromagnetic coupling [24,42].…”

“…Since phonon-induced transitions within a doublet are forbidden in first order, spin relaxation is dominated by Orbach processes via excited states. In such systems, the temperature dependence of the spin-lattice relaxation rate (1/T) can provide an estimate of the energy separation between the ground from the excited states (∆E) [24,42]. The spin relaxation is dominated by Orbach processes via excited doublet states at all but the lowest temperatures.…”

“…The value of eff g ⊥ is determined by the exchange interaction with the ferryl moiety (S = 1). To a first approximation [24,42] [43] and J/D is the ratio of exchange coupling to the zero-field splitting …”

Spectroscopic and Kinetic Characterization of Peroxidase-Like π-Cation Radical Pinch-Porphyrin-Iron(III) Reaction Intermediate Models of Peroxidase Enzymes

“…Although, the activation energies to produce compound 0, quantified in other work [40] ESR spectra of FePPic-I show a radical signal different to the reported for HRP-I [35][36][37][38][39][40]; although, it is possible that these signals contain the one at g ~ 2 for axial spectra of qms species and the axial spectra with s = 3/2. It is clear that each ESR spectrum is a snapshot of some of the changes in the oxidation state of the iron ion.…”