The role of the axial ligand in meso-tetraarylmetalloporphyrin models of the P-450 cytochromes

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With iodosobenzene as an oxygen source, MnTPP(Cl), [MnTPP(H 2O) 2]ClO 4, MnTPPNO 3, FeTPP(Cl), [FeTPP(H 2O) 2]ClO 4 and (FeTPP) 2O were used as model P-450 catalysts for the oxidation of cyclohexene in methylene chloride, benzene, benzonitrile and acetonitrile. The effect of added methanol and 4-methylpyridine was also investigated. A surprisingly high (80%) yield of cyclohexene oxide returned by [MnTPP(H 2O) 2]ClO 4 in benzonitrile required a reconsideration of the accepted role of the trans axial ligand in model P-450 reactions. Performance differences between the MnTPP + and FeTPP + derivatives are attributed to differences in metal—porphyrin orbital mixing. It is proposed that D 4 h metal e g (d xz , d yz ) and porphyrin e g (π*) orbital mixing in six-coordinate manganese porphyrins prevents electron loss from the porphyrin a 2 u orbital. Porphyrin ligand oxidation in pentacoordinate manganese complexes is attributed to an unsymmetrical metal—porphyrin interaction. The metal e g (d xz , d yz ) orbitals of six-coordinate iron porphyrin complexes are thought to lie below the porphyrin e g (π*) orbitals, thus preventing a protective orbital interaction. The effects of σ and π charge donation from the trans axial ligand of the model P-450 catalysts are differentiated. It is proposed that σ donation can accelerate oxene transfer from the catalyst to the substrate, while π charge donation can additionally alter the energy of the acceptor orbital on the active catalyst that is responsible for initiating substrate oxidation.