Visible and Resonance Raman Spectra of Low Valent Iron Porphyrins
Format of Original
6 p.; 29 cm
Inorganica Chimica Acta
The resonance Raman spectrum of Fe(TPP)2− was obtained after the three-electron electrochemical reduction of Fe(TPP)(Cl). The coulometric reduction was carried out in the presence of bis(triphenylphosphoanylidine)ammonium chloride in DMF in order to avoid the formation of iron–σ-alkyl complexes. The resonance Raman spectrum of the intermediate oxidation states (Fe(TPP) and Fe(TPP)−) were consistent with previous work. The spectrum of the three-electron product, Fe(TPP)2−, obtained at 442 nm, was qualitatively similar to the two-electron reduced product, Fe(TPP)−, with an intense ν2 band at 1537 cm−1. The high frequency bands generally decreased in energy, contrary to the expectations based on the X-ray crystallographic core size. In particular, the ν2 and ν10 bands decreased by 18 and 22 cm−1. A small increase was observed for the ν4 band (+4 cm−1). While these changes were not consistent with the measured core size, they were in agreement with other porphyrin π-anion radicals such as Zn(TPP)− and VO(OEP)−. Based on the resonance Raman spectra, Fe(TPP)2− can be formulated as an iron(I) π-radical anion. Significant backbonding between the dπ-orbitals of the iron to the eg* orbital of the porphyrin, though, is probably occurring. As a result, the complex is probably not a pure π-anion radicals.