As reaction intermediate proposed in the catalytic cycle of cytochrome d, peroxo-iron(III) chlorin complexes are formed and characterized by using synthetic models. The peroxo complexes have been successfully formed by the reaction of chloro-iron(III) complexes of tetramesitylchlorin (H(2)TMC) and octaethylchlorin (H(2)OEC) with 2 equiv of the superoxide anion in acetonitrile under an argon atmosphere. The peroxo complexes of iron(III) chlorins afford characteristic electronic absorption spectra, in which the Soret band and the characteristic band for chlorin complexes are largely red-shifted, as compared with those of the parent ferric chlorin complexes. ESR spectra of the peroxo chlorin complexes exhibit resonances at g approximate to 9 and 4.2, indicative of rhombic high-spin ferric states. Further, the oxygen-oxygen stretching mode of the peroxo complexes is observed at 806 cm(-1), implying that the peroxide ligands are bound to the iron in a bidentate fashion. On the basis of these spectral features, the peroxo-iron(III) chlorin complexes are suggested to have electronic and molecular structures similar to those of the corresponding porphyrin complexes. On the contrary, the paramagnetic deuterium NMR signals of the saturated pyrrole (pyrroline) rings of the peroxo chlorin complexes exhibited an unusually large splitting into downfield and upfield regions. The unusual splitting of the hyperfine-shifted pyrroline deuterium resonances could be explained by the large deformation of the pyrroline rings in the peroxo chlorin complexes.