Resonance Raman investigation of nitric oxide bonding in iron porphyrins: detection of the Fe-NO stretching vibration

With Soret-excited resonance Raman spectroscopy (RRS), we have detected the ν(FeIINO) and ν(FeIIINO) stretching vibrations for several iron porphyrins, including octaethylporphyrin (OEP), tetraphenylporphyrin (TPP), heme-5, SP-13, SP-14, and SP-15. This result enables us to make the first comparison of the e(FeIINO) stretching frequencies in iron porphyrins (∼527 cm⁻¹) and hemoproteins (∼554 cm⁻¹), which is of interest if the protein's effect on the Fe-NO bond is to be assessed. Solvent effects on the Fe¹¹-NO bond strength were significant; for FeII(OEP)(pyridine)(NO), the ν(Fe¹¹-NO) stretching frequency was observed 5 cm⁻¹ higher in CH₂Cl₂ (527 cm⁻¹) than in benzene (522 cm⁻¹). The electronic trans effect was investigated by preparing complexes with N-methylimidazole, pyridine, and tetrahydrofuran as proximal ligands, and steric effects were assessed by comparing hindered (1,2-dimethylimidazole) and unhindered (N-methylimidazole) axial bases. Interestingly, the FeIINO bond strength proved insensitive to alterations in the trans base. These results are contrasted with previous FeIICO, FeII-O₂, and FeIIIICN studies, in which the iron-ligand bond strength was found to be dramatically affected by properties of the proximal ligand.