RESONANCE RAMAN PURSUIT OF THE CHANGE FROM FE-II-O2 TO FE-III-OH VIA FE-IV=O IN THE AUTOXIDATION OF FERROUS IRON PORPHYRIN

Resonance Raman (RR) and visible absorption spectra were observed for autoxidation intermediates of ferrous tetramesitylporphyrin [(TMP)FeII] to the ferric hydroxy derivative [(TMP)FeIIIOH] via (TMP)FeIIO2, (TMP)FeIIIOOFeIII(TMP), and (TMP)FeIV=O. The O-O stretching [v(O2)] and FeII-O2 stretching [v(FeII-O2)] Raman bands were simultaneously observed at 1171 and 522 cm-1, respectively, for the (TMP)FeIIO2 in toluene solution at -100 °C for the first time. The present data together with the reported IR data for the solution samples indicate a linear inverse correlation between v(O2) and v(FeII-O2) frequencies similar to that between v(CO) and v(FeII-CO), but the data from heme proteins fall off the line. Upon raising the sample temperature to -70 °C, formation of (TMP)FeIIIOOFeIII(TMP) was confirmed by 1H NMR and its visible absorption spectrum was determined. However the peroxo-bridged dimer was so photolabile that it was decomposed into (TMP)FeIV=O by laser illumination even at -70 °C, and therefore, no oxygen isotope sensitive RR band assignable to (TMP)FeIIIOOFeIII(TMP) was identified. (TMP)FeIV=O was also photolabile and yielded the photoproduct, the same as the case of thermal decomposition, but (TMP)FeIV=O gave the FeIV=O stretching [v(FeIV=O)] Raman band at 843 cm-1, which is in agreement with the value reported for the five-coordinate oxoferryl complex. The reduction rate of (TMP)FeIV=O to (TMP)FeIIIOH was different between the toluene-h8 and -d8 solutions, suggesting that it proceeds via hydrogen abstraction from toluene. Presumably, the FeIV=O bond has a partial radical character, which increases upon electronic excitation, and this is the reason why decomposition of (TMP)FeIV=O is accelerated by laser illumination. © 1990, American Chemical Society. All rights reserved.