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. 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 principal g tensor axes are performed for resolution of the 'H, 4115N and 59 Co 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 it orbitals. The analysis reproduces the experimental g 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 B, 2 , 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.

Use of EPR Spectroscopy to Unravel Reaction Mechanisms in (Catalytic) Bond Activation Reactions: Some Selected Examples

“…The total line-width, the anisotropy of the gtensor, as well as hyperfine coupling to 59 Co as well as 14 N changes from one spectrum to the other. The 14 N hyperfine from the porphyrin N-atoms is observed only for the (OEP)Co(II) form [48].…”

“…Hence, cobalt has often been used as a substitute for the iron center in hemoglobin and myoglobin to investigate the mechanism of O 2 activation. Figure 3 (bottom) shows representative spectra of the cases discussed above for cobalt before and after O 2 binding [48]. The total line-width, the anisotropy of the gtensor, as well as hyperfine coupling to 59 Co as well as 14 N changes from one spectrum to the other.…”

“…The activation of O 2 by transition metal complexes is probably one of the oldest examples of the use of EPR spectroscopy to study and understand the activation of small molecules [47,48]. The ''easiest'' and the most studied metal for this activation has been cobalt [47,48].…”

“…In contrast to the Co(III)-superoxo cases, these are many spin systems and spin-spin coupling needs to be considered here. For the Fe(III)-superoxo case (with [48] porphyrin ligands), usually the unpaired electron of the d 5 -LS iron center antiferromagnetically couples with the unpaired electron of the superoxo radical to deliver a diamagnetic ground state. For the Cu(II)-superoxo case, a similar case can be imagined [56][57][58].…”

“…The ''easiest'' and the most studied metal for this activation has been cobalt [47,48]. This has been done for practical purposes even though cobalt is not the metal of choice for activating and fixing O 2 in bio-systems [3].…”