Low-frequency vibrations of ferroprotoporphyrin-substituted imidazole complexes. A resonance Raman study

This article reports the low-frequency regions of resonance Raman spectra of five- and six-coordinated ferroprotoporphyrin complexes in aqueous solution with or without detergent. For high-spin complexes having their iron atom monocoordinated to variously substituted imidazoles or to dimethylformamide, the frequency of a band observed between 194 and 237 cm⁻¹ (labelled band II) primarily depends on the $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ of the axial ligand. In the absence of steric effects from the axial ligand, the lower is the $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ of ligand, the higher the frequency of band II. We previously assigned band II to a mode essentially involving the Fe-N(pyrrole) bonds. The above $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ dependence is readily explained, in the frame of this assignment, in terms of a decrease in the Fe-N(pyrrole) bond strength (and of an increase in bond length) when the basicity of the axial ligand increases. On the other hand, the alternative assignment of band II to a stretching mode of Fe-N(axial ligand) is inconsistent with the observed $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ dependence. As far as hexacoordinated complexes are concerned, specific bands are observed at 203, 194 and 176 cm⁻¹ for imidazole, 1-methylimidazole and pyridine, respectively. These bands are assigned, on the basis of isotopic substitutions, to a summetric stretching mode of the axial ligands [ν(N-Fe-N)]. Band II is observed at 265 cm⁻¹ for these low-spin complexes, a frequency expected from the short Fe-N(pyrrole) bond lengths of nearly planar ferroporphyrins.