Resonance Raman enhancement of the oxo-bridged dinuclear iron center: Vibrational modes in iron(III) physiological-type porphyrin complexes

Oxo‐bridged dinuclear iron(III) complexes of two physiological‐type prophyrins, (DPDEFe)2O and (PPDEFe)2O (where DPDE = deuteroporphyrin dimethyl ester and PPDE = protoporphyrin dimethyl ester), were investigated using resonance Raman and infrared spectroscopy. The assignment of the porphyrin vibrational modes was performed using polarization measurements and previous studies concerning the corresponding mononuclear iron(III) complexes, (DPDEFe)Cl, (PPDEFe)Cl, and related compounds. The assignment of the FeOFe axial bridge vibrational modes of the two μ‐oxo complexes investigated and the synthetic analogue (TPPFe)2O (where TPP = tetraphenylporphyrin) was made with the help of 18O substitution. The dependence of the frequencies vs and va on the FeOFe angle for (TPPFe)2O is in good agreement with the angle value previously determined by x‐ray diffraction data. The values of the FeOFe angle in (DPDEFe)2O and (PPDEFe)2O were estimated to be close to 160° and 170°, respectively. The excitation profiles (351.1–647.1 nm) of the porphyrin core marker bands for all the μ‐oxo complexes studied are in good agreement with the expected general trends of the resonance enhancement with moderate vibronic coupling. However, for the μ‐oxo physiological‐type porphyrin complexes, the v4 marker band exhibits and unexpected strong enhancement maximum near the maximum found in the excitation profile of the va and vs (FeOFe) modes. These enhancement maxima tend to correspond to a minor oxo → Fe(III) charge‐transfer band in the electronic absorption spectra. A mixing of several electronic levels under low symmetry may be responsible for this enhancement phenomenon.