Resonance Raman Characterization of Highly Reduced Iron Octaethylporphyrin: n- (n = 0, 1, and 2)

Article abstract of DOI:10.1021/ja00235a028

Resonance Raman, EPR, and visible absorption spectra were observed for highly reduced iron-octaethylporphyrin complexes, ^n- which were obtained in tetrahydrofuran (THF) by the sodium mirror contact reduction technique.Raman and EPR spectra were measured with the same preparation as characterized by the visible absorption spectrum which showed clear isosbestic points in each stage of the oxidoreduction: Fe^III(OEP)<*>Fe^II(OEP)(THF)<*>^-<*>^2-.Raman lines of ^- and ^2- were qualitatively assigned on the basis of the frequency shifts observed for (15)N-substitution and meso-deuteriation. ^- which yielded EPR signals at g_<*> = 2.26 and g_<*> = 1.93 at 77 K but no signals at room temperature gave the ν₄ line at the same frequency as the Fe^II complex but the ν₁₀ line at much lower frequency at room temperature.The overall spectral pattern of ^- in THF was distinctly different from that of Fe^II(OEP) in THF and showed clear change upon freezing of the solution, in agreement with the reported spin transition upon freezing.Accordingly, ^- in THF solution was assigned to the high-spin Fe^I porphyrin. ^2-, which showed a hyper type absorption spectrum and was EPR silent, gave the ν₄ line at a considerably lower frequency but the C_aC_m stretching modes at frequencies similar to those of the low-spin Fe^I(OEP) in frozen THF.The ν₄ mode of ^2- exhibited an appreciable frequency shift upon meso-deuteriation as well as upon (15)N-substitution in contrast with other cases.This implied more mixing of the C_aN and C_aC_m stretching modes and thus increased ?-conjugation of the two bonds.Frequencies of the modes involving the C_bC_b stretching character like, ν₂ and ν₁₁ were distinctly lower for ^2-, suggesting the formation of a porphyrin ?-anion radical.