Axial ligand effect on the catalytic activity of biomimetic Fe-porphyrin catalyst: An experimental and DFT study

- The anionic state of the axial imidazole affects drastically the catalytic activity.
- Deprotonation of the axial imidazole results to a 10-fold enhanced catalytic activity.
- Imidazolate leads to increased electron donation ability to the central metal.
- Increased electron donation favors the formation of the FeIVOPor+ state.

Electrostatic interactions occurring in the immediate environment of the metal center in native enzymes influence the charge distribution and electron donation ability, regulating activity. Herein, using an iron-porphyrin biomimetic complex, we have investigated the effect of the protonation state of the axial imidazole on the catalytic oxidation of pentachlorophenol. We show that the catalytic efficiency is drastically affected by the anionic state of the axial ligand. The chemical events occurring during the catalytic cycle were monitored by a detailed analytical study. EPR, UV-Vis and low temperature UV-Vis together with theoretical DFT and TD-DFT calculations provide evidence that deprotonation of the axial imidazole leads to increased electron donation ability to the central metal, enhancing the formation of ferryl species. Imidazole favored the formation of a pure porphyrin-radical model at the high-valent FeIVOPor+ state while imidazolate presented a significant axial ligand-radical character. The theoretical calculations suggest that the enhanced catalytic activity of the deprotonated conformation is attributed to the increased electron donation ability, favoring the formation of the ferryl species and suggesting that the “push” effect prevails over the “pull” effect.

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