A. Ikezaki et M. Nakamura, Formation of mu-oxo dimer by the reaction of (meso-tetraphenylporphyrinato)iron(III) with various imidazo les, J INORG BIO, 84(1-2), 2001, pp. 137-144
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.