The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)-oxo porphyrin π-cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)+. FeIV(O)(p-Y-PyO)]+ (1-Y) (tmp = meso-tetramesitylporphyrin, p-Y-PyO = para-substituted pyridine N-oxides, and Y. = OCH3, CH3, H, Cl). In all of the oxidation reactions, the reactivities of 1-Y are found to follow the order 1OCH3 > 1-CH3 > 1-H > 1-Cl; negative Hammett p values of -1.4 to -2.7 were obtained by plotting the reaction rates against the σP values of the substituents of p-Y-PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)-oxo porphyrin π-cation radicals bearing electron-donating axial ligands are more reactive in oxo-transfer and hydrogen-atom abstraction reactions. These results are counterintuitive since iron(IV)-oxo porphyrin π-cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO-H bond and thereby enhances its H-abstraction activity. In addition, it weakens the Fe=O bond and encourages oxo-transfer reactivity. Both are Bell-Evans-Polanyi effects, however, in a series of neutral ligands like p-Y-PyO, there is a relatively weak trend that appears to originate in two-state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)-oxo porphyrin π-cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.
- Axial ligand effect
- Density functional calculations
- Enzyme models
- Two-state reactivity