meso-Substitution Activates Oxoiron(IV) Porphyrin π-Cation Radical Complex More Than Pyrrole-ß-Substitution for Atom Transfer Reaction.

There have been two known categories of porphyrins: a meso-substituted porphyrin like meso-tetramesitylporphyrin (TMP) and a pyrrole-ß-substituted porphyrin like native porphyrins and 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin (TMTMP). To reveal the chemical and biological function of native hemes, we compare the reactivity of the oxoiron(IV) porphyrin π-cation radical complex (Compound I) of TMP (TMP-I) with that of TMTMP (TMTMP-I) for epoxidation, hydrogen abstraction, hydroxylation, sulfoxidation, and demethylation reactions. Kinetic analysis of these reactions indicated that TMP-I is much more reactive than TMTMP-I when the substrate is not sterically bulky. However, as the substrate is sterically bulkier, the difference of the reactivity between TMP-I and TMTMP-I becomes smaller, and the reactivity of TMP-I is comparable to that of TMTMP-I for a sterically hindered substrate. Since the redox potential of TMP-I is almost the same as that of TMTMP-I, we conclude that TMP-I is intrinsically more reactive than TMTMP-I for these atom transfer reactions, but the steric effect of TMP-I is stronger than that of TMTMP-I. This is contrary to the previous result for the single electron transfer reaction: TMTMP-I is faster than TMP-I. DFT calculations indicate that the orbital energies of the Fe═O moiety for TMTMP-I are higher than those for TMP-I. The difference in steric effect between TMP-I and TMTMP-I is explained by the distance from the mesityl group to the oxo ligand of Compound I. Significance of the pyrrole-ß-substituted structure of the hemes in native enzymes is also discussed on the basis of this study.

Small Reorganization Energy for Ligand-Centered Electron-Transfer Reduction of Compound I to Compound II in a Heme Model Study.

The electron-transfer (ET) processes from electron-donor substrates to oxoiron(IV) porphyrin π-cation-radical species (Cpd I) are key steps in their oxygenation reactions. Here, we have evaluated the rate constants of the outer-sphere ET reduction of Cpd I model complexes of meso-tetramesitylporphyrin (1) and 2,7,12,17-tetramesityl-3,8,13,18-tetramethylporphyrin (2) in light of the Marcus theory of ET to determine the ET reorganization energies (λ). The λ values of the ligand-centered ET reduction of Cpd I model complexes are much smaller than those of the metal-centered ET reduction of various oxoiron(IV) complexes. Moreover, the λ value of 1 is larger than that of 2, resulting from the difference in the nature of the a1u/a2u porphyrin π-cation-radical orbitals.