The oxygen reduction reaction (ORR) is critical for fuel cells, but typically requires expensive metals, such as platinum. Molecular systems offer a unique opportunity to study the mechanisms for the ORR in a controlled experimental setting. For example, iron porphyrin systems are known in nature for their ability to bind and activate O2, and have been widely studied as catalysts for molecular ORR. These molecular catalysts often require high overpotentials (h) to reach high turnover frequencies (TOF); however, they offer an easily functionalizable platform and a model opportunity to study ORR at the atomic-level. Computational studies can augment these experimental studies, often giving atomic-level insight. In this presentation, we discuss a series of Fe porphyrin and other macrocycles to show the role that ligand design can have on the mechanism, both through electronic structure modification at the metal and by affecting the secondary coordination sphere. One promising system is the Fe o-TMA porphyrin, which features cationic trimethylaminium groups decorating phenyl groups on the porphyrin. In the presence of acetic acid, these complexes exhibit higher TOFs for ORR at a given h relative to other Fe porphyrins. We show that the FeIII/II reduction potential is sensitive to coordinating anions in solution, as changing from neutral to anionic ligands in the axial position features an 800 mV negative shift in reduction potential. The ability to coordinate such anionic groups is favorable in the cationic Fe o-TMA porphyrin. The O2 binding energies for this catalyst are similar to those for other porphyrin systems lacking the cationic moieties, despite the change in charge of the complex and the potential for through-space interactions with the ligand. We also investigate an Fe-TMP porphyrin to show how trace amounts of water can alter the selectivity determining steps. Water can preferentially solvate an HOO- anion, encouraging release of the anion and yielding H2O2 as product. Finally, we investigate porphyrin-like bathophen macrocycles to show that protonatable groups on the ligand can alter the overall charge of the complex. We investigate the role this change in charge alters mechanism for the ORR. Together, these studies shed light into how secondary sphere interactions and electronic effects work together to affect small molecule binding and activation.

Interplay of ligand and surroundings in Fe-based macrocycles for the oxygen reduction reaction

ACS Spring 2022

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