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Preparation and characterization of oxoiron(IV) chlorin complexes as the first models for a reaction intermediate in the catalytic cycle of cytochrome d

As models for a reaction intermediate in the catalytic cycle of cytochrome d, two types of oxoferryl chlorin complexes, (TPC)FeIVO(N-MeIm) and (TPC)FeIVO (TPC = tetraphenylchlorin, N-MeIm = N-methylimidazole), have been prepared for the first time by an autoxidation reaction of (TPC)Fe 11 with O2. Oxyiron(II)(TPC) and μ-peroxo-bridged iron(III) dimer, (TPC)FeIIIOOFeIII(TPC), are also detected in the course of the reaction. The six-coordinated (TPC)FeIVO(N-MeIm) complex is produced by adding N-methylimidazole to the μ-peroxo-bridged iron(III) chlorin dimer at -80 °C. The μ-peroxo-bridged iron(III) dimer, the formation of which has been confirmed by paramagnetic NMR, undergoes homolytic cleavage of the O-O bond upon laser illumination to yield the five-coordinated (TPC)FeIVO complex. The absorption spectrum of (TPC)FeIVO(N-MeIm) shows a characteristic redshifted band at 630 nm as observed in the case of the oxoferryl intermediate of cytochrome d. Proton NMR spectra of (TPC) FeIVO(N-MeIm) exhibit a small hyperfine shift of the pyrrole protons, consistent with the oxoferryl formulation. However, the paramagnetic NMR resonances of the saturated pyrrole (pyrroline) ring protons show unusual splitting into upfield and downfield region, suggesting deformation of the pyrroline ring of the chlorin complex. While the unusually high frequency of v(FeIV=O) (815 cm-1) and large isotopic shift (Δv = 46 cm-1) observed for the oxoferryl intermediate of cytochrome d have been attributed to the chlorin macrocycle (heme d), the five- and six-coordinated oxoferryl chlorin complexes reported here exhibit v(FeIV=O) frequencies and isotopic (16O/18O) frequency shifts nearly identical to those of the corresponding porphyrin complexes, respectively.