Second-Sphere Hydrogen-Bond Donors and Acceptors Affect the Rate and Selectivity of Electrochemical Oxygen Reduction by Iron Porphyrins Differently

The factors that control the rate and selectivity of 4e-/4H+ O2 reduction are important for efficient energy transformation as well as for understanding the terminal step of respiration in aerobic organisms. Inspired by the design of naturally occurring enzymes which are efficient catalysts for O2 and H2O2 reduction, several artificial systems have been generated where different second-sphere residues have been installed to enhance the rate and efficiency of the 4e-/4H+ O2 reduction. These include hydrogen-bonding residues like amines, carboxylates, ethers, amides, phenols, etc. In some cases, improvements in the catalysis were recorded, whereas in some cases improvements were marginal or nonexistent. In this work, we use an iron porphyrin complex with pendant 1,10-phenanthroline residues which show a pHdependent variation of the rate of the electrochemical O2 reduction reaction (ORR) over 2 orders of magnitude. In-situ surface-enhanced resonance Raman spectroscopy reveals the presence of different intermediates at different pH's reflecting different rate-determining steps at different pH's. These data in conjunction with density functional theory calculations reveal that when the distal 1,10-phenanthroline is neutral it acts as a hydrogen-bond acceptor which stabilizes H2O (product) binding to the active FeII state and retards the reaction. However, when the 1,10-phenanthroline is protonated, it acts as a hydrogen-bond donor which enhances O2 reduction by stabilizing FeIII-O2.- and FeIII-OOH intermediates and activating the O-O bond for cleavage. On the basis of these data, general guidelines for controlling the different possible ratedetermining steps in the complex multistep 4e-/4H+ ORR are developed and a bioinspired principle-based design of an efficient electrochemical ORR is presented.

Automated summary

Controlling the factors that affect the rate and selectivity of 4e-/4H+ O2 reduction is crucial for efficient energy transformation and understanding the final step of respiration in aerobic organisms. This study explores the use of artificial systems with different second-sphere residues to enhance the rate and efficiency of O2 reduction. By investigating the behavior of an iron porphyrin complex with pendant 1,10-phenanthroline residues, the researchers discovered that the pH-dependent variations in the reaction rate are due to different intermediates formed at different pH levels. These findings, along with density functional theory calculations, provide insights into designing an efficient electrochemical O2 reduction reaction based on bioinspirations.