RESONANCE RAMAN CHARACTERIZATION OF FERRIC AND FERRYL PORPHYRIN PI-CATION RADICALS AND THE FE[IV]=O STRETCHING FREQUENCY

Resonance Raman (RR) spectra of porphyrin pi-cation radicals were investigated for the Fe(III) and Fe(IV) states by using (tetramesitylporphyrinato)iron complexes [(TMP)Fe]. Vibrational modes were assigned on the basis of the isotopic frequency shifts upon N-15 and meso-C-13 substitution and the reported normal coordinate analysis. For both the Fe(III) and Fe(IV) porphyrin pi-cation radicals, the nu-4 band was significantly shifted to lower frequency and thus the previous assignment by Kincaid et al. (J. Am. Chem. Soc. 1989, 111, 735) should be modified. The RR spectra exhibited no temperature dependence between -100 and +10-degrees-C and between -100 and -80-degrees-C for the Fe(III) and Fe(IV) porphyrin pi-cation radicals, respectively, indicating the presence of one type of radicals, presumably the a2u radical judging from the low-frequency shifts of the nu-2 mode. The controversy about the Fe(IV) = O stretching mode (nu-Fe = O) of the Fe(IV) = O porphyrin nu-cation radical was clarified by demonstrating that the two nu-Fe = O RR bands appear at 829 and 801-802 cm-1 in the presence of EtOH and n-PrOH while a single band appears at 831 or 801 cm-1 in the presence of MeOH or t-BuOH, respectively. The difference between the nu-Fe = O frequencies in the presence and absence of MeOH were interpreted reasonably in terms of the effect of the fifth ligand.