Formation of μ-oxo dimer by the reaction of (meso-tetraphenylporphyrinato)iron(III) with various imidazoles

Reactions between (meso-tetraphenylporphyrinato)iron(III) perchlorate [Fe(tpp)]ClO₄ and various imidazoles have been examined in CD₂Cl₂ solutions. ¹H NMR analysis revealed the formation of three kinds of complex; μ-oxo dimer, mono-imidazole adduct, and bis-imidazole adduct. The product ratios changed to a great extent depending on the amount and nature of imidazoles. In general, addition of less than 1.0 equiv of imidazole relative to [Fe(tpp)]ClO₄ led to the formation of both μ-oxo dimer and mono-imidazole adduct. However, by the addition of excess amount of imidazole, either the μ-oxo dimer or bis-imidazole adduct was formed exclusively depending on the bulkiness of the imidazole used. In the case of bulky imidazole such as 2-methylbenzimidazole or 2-isopropyl-1-methylimidazole, the μ-oxo dimer was formed quantitatively. In the case of less bulky imidazole such as parent imidazole or 1-methylimidazole, bis-imidazole adduct became the sole product. The results have been explained in terms of the difference in steric interactions between the axial ligands and porphyrin core; the severe steric repulsion prohibits the formation of bis-adduct in the case of bulky imidazoles. As a result, bulky imidazoles prefer to behave as a base; they abstract a proton from coordinated water, and lead to the formation of μ-oxo dimer. Thus, the role of bulky imidazoles in these reactions has some relevance to that of distal histidine in hemoglobin and peroxidase.