Formation of mu-oxo dimer by the reaction of (meso-tetraphenylporphyrinato)iron(III) with various imidazo les

Reactions between (meso-tetraphenylporphyrinato)iron(III) perchlorate [Fe(t pp)]ClO4 and various imidazoles have been examined in CD2Cl2 solutions. H-1 NMR analysis revealed the formation of three kinds of complex; mu -oxo dim er, mono-imidazole adduct, and bis-imidazole adduct. The product ratios cha nged 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 mu -oxo dimer and mono-imidazole adduct. However, by the addition of excess amount of imidazole. either the mu -oxo dimer or bis-imidazole adduct was formed exclusively depending on the bulk iness of the imidazole used. In the case of bulky imidazole such as 2-methy lbenzimidazole or 2-isopropyl-1-methylimidazole, the mu -oxo dimer was form ed quantitatively. In the case of less bulky imidazole such as parent imida zole or 1-methylimidazole, bis-imidazole adduct became the sole product. Th e results have been explained in terms of the difference in steric interact ions between the axial ligands and porphyrin core; the severe steric repuls ion 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 p roton from coordinated water, and lead to the formation of mu -oxo dimer. T hus, the role of bulky imidazoles in these reactions has some relevance to that of distal histidine in hemoglobin and peroxidase. (C) 2001 Published b y Elsevier Science B.V.