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 porphyrins, (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 mu-oxo complexes investigated and the synthetic analogue (TPPFe)2O (where TPP = tetraphenylporphyrin) was made with the help of O-18 substitution. The dependence of the frequencies nu(s) and nu(a) 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-degrees and 170-degrees, respectively. The excitation profiles (351.1-647.1 nm) of the porphyrin core marker bands for all the mu-oxo complexes studied are in good agreement with the expected general trends of the resonance enhancement with moderate vibronic coupling. However, for the mu-oxo physiological-type porphyrin complexes, the nu-4 marker band exhibits an unexpected strong enhancement maximum near the maximum found in the excitation profile of the nu(a) and nu(s) (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.