The secondary coordination sphere and axial ligand effects on oxygen reduction reaction by iron porphyrins: a DFT computational study
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Oxygen reduction reaction (ORR) catalyzed by a bio-inspired iron porphyrin bearing a hanging carboxylic acid group over the porphyrin ring, and a tethered axial imidazole ligand was studied by DFT calculations. BP86 free energy calculations of the redox potentials and pK a’s of reaction components involved in the proton coupled electron transfer (PCET) reactions of the ferric-hydroxo and -superoxo complexes were performed based on Born–Haber thermodynamic cycle in conjunction with a continuum solvation model. The comparison was made with iron porphyrins that lack either in the hanging acid group or axial ligand, suggesting that H-bond interaction between the carboxylic acid and iron-bound hydroxo, aquo, superoxo, and peroxo ligands (de)stabilizes the Fe–O bonding, resulting in the increase in the reduction potential of the ferric complexes. The axial ligand interaction with the imidazole raises the affinity of the iron-bound superoxo and peroxo ligands for proton. In addition, a low-spin end-on ferric-hydroperoxo intermediate, a key precursor for O–O cleavage, can be stabilized in the presence of axial ligation. Thus, selective and efficient ORR of iron porphyrin can be achieved with the aid of the secondary coordination sphere and axial ligand interactions.
KeywordsComputational chemistry Density functional theory Dioxygen Heme Model compound Porphyrin Electrochemistry
This work was financially supported by Scientific Research (C) (No. 24550080) and Scientific Research on Innovative Area (No. 25109535) to T.O. from JSPS and by the Elemental Science and Technology Project to Y.N. from the MEXT of Japan. NP thanks the Mitsubishi UFJ Trust Scholarship. JGL is grateful to the NSF of China (No. 21271072, 21571062), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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