Imidazole,imidazolate, and hydroxide complexes of (protoporphyrin IX)iron(III) and its dimethyl ester as model systems for ferric hemoproteins: Electron paramagnetic resonance and electronic spectral study

The EPR and electronic spectral changes upon titration of systems consisting of (protoporphyrin IX)iron(III) chloride (Fe(PPIX)Cl) or its dimethyl ester (Fe-(PPIXDME)Cl) and imidazole derivatives with tetrabutylammonium hydroxide solution have been measured at 77 and 298 °K in various solvents. The EPR and electronic spectra of the melt of Fe(PPIXDME)Cl in imidazole derivatives have been also measured. The imidazole derivatives studied here were imidazole and 4-methyl-, 4-phenyl-, 2-methyl-, 2,4-dimethyl-, 1-methyl-, and 1-acetylimidazole. The spectral changes upon addition of hydroxide were markedly different between the systems containing NH imidazoles (BH), with a dissociable proton, and those containing NR imidazoles (BR), without it. In the former systems, five spectral species were successively formed at 77 °K and were assigned to following complexes: [Fe(P)(BH)₂]⁺, Fe(P)(BH)(B), [Fe(P)(B)₂]^?, Fe(P)(BH)(OH), and [Fe(P)(B)(OH)]^?, where P is PPIX or PPIXDME. In the latter systems, initial complex, [Fe(P)(BR)₂]⁺, was found to be changed to final complex, Fe(P)(BR)(OH), through an intermediate at 77 °K. At 298 °K, both systems were found to react with hydroxide to finally form Fe(P)(OH). The crystal field parameters were evaluated using the EPR g values in low-spin complexes studied here and in hemoproteins. The five regions corresponding to five low-spin complexes could be distinguished in crystal field diagrams.