Probing the surrounding of a cobalt(II) porphyrin and its superoxo complex by EPR techniques

Abstract

Octaethylporphyrinato-cobalt(II), Co(II)OEP, was studied by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) in frozen solutions of methanol, tetrahydrofuran and pyridine diluted into chloroform. Oxygenation led to the respective paramagnetic superoxo complexes Co(III)OEP · py · O 2 −⋅ The EPR spectra demonstrate strong differences in the axial ligation states ((base-on)/(base-off)) and ease of the oxygenation process. Additional ENDOR studies with partial orientation selection along the principalg tensor axes are performed for resolution of the1H,14/15N and59Co hyperfine coupling constants. This allows a comparison of the electron spin density distribution of the superoxo complex and its precursor. The data are interpreted in the framework of a more rigorous and detailed theoretical configuration interaction model than previously presented in the literature. The theoretical treatment shows that the structure of the superoxo complex is best described in a three-orbital model with contributions from the cobalt 3d, 4s, and oxygen π orbitals. The analysis reproduces the experimentalg and Co hyperfine data yielding the relative energies of the MO’s and the MO coefficients for the description of the spin density distribution in the Co(II) complex and its superoxo complex. To demonstrate the generality of the approach and possible applications, a comparison is made with the vitamin B12r system. Furthermore, it provides detailed insight into the electronic and geometric changes resulting from axial ligation and oxygenation of the Co(II)OEP complex; this information can be used for predictions of the catalytic activity.