THE ROLE OF THE AXIAL LIGAND IN MESO-TETRAARYLMETALLOPORPHYRIN MODELS OF THE P-450 CYTOCHROMES

With iodosobenzene as an oxygen source, MnTPP(Cl), [MnTPP(H2O)2]ClO4, MnTPPNO3, FeTPP(Cl), [FeTPP(H2O)2]ClO4 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(H2O)2]ClO4 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 D4h metal e(g)(d(xz), d(yz)) and porphyrin e(g)(pi*) orbital mixing in six-coordinate manganese porphyrins prevents electron loss from the porphyrin a2u 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)(pi*) orbitals, thus preventing a protective orbital interaction. The effects of sigma and pi-charge donation from the trans axial ligand of the model P-450 catalysts are differentiated. It is proposed that a donation can accelerate oxene transfer from the catalyst to the substrate, while pi-charge donation can additionally alter the energy of the acceptor orbital on the active catalyst that is responsible for initiating substrate oxidation.