The selective 4e–/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. The partial reduction (2e–/2H+) of dioxygen generates hydrogen peroxide via a process that is significantly less exothermic. Despite the fact that the 4e–/4H+ reduction of O2 is more exothermic than the analogous 2e–/2H+ process, most transition metal complexes show a preference for the incomplete reduction reaction. Tetrapyrrole complexes of cobalt and iron have been heavily studied as platforms for the oxygen reduction reaction (ORR). In general, monotopic cobalt and iron macrocycles, including porphyrins and corroles do not promote the 4e–/4H+ ORR to generate H2O with high selectivities. It is against this backdrop that we have developed efficient synthetic strategies for preparation of a variety of novel nonaromatic tetrapyrrole macrocycles that contain one or two sp3 hybridized carbons on the tetrapyrrole periphery. These unconventional structural motifs attenuate the structures and electronic properties of the tetrapyrrole architectures, and significantly improve the ability of these ligands to donate multiple electrons, as compared to more traditional porphyrinic complexes. In addition to reporting the synthesis, photophysics and redox properties of a variety of new nonaromatic tetrapyrrole metal complexes, we will demonstrate that the cobalt complexes of some of these platforms can selectively promote ORR via the 4e–/4H+ pathway to generate water. Detailed voltammetric and rotating ring disk electrode experiments will be described to demonstrate how these new systems are superior O2 reduction catalysts when compared to most previously studied monomeric cobalt macrocycles. The potential application of these new architectures for incorporation into fuel cell cathodes will also be discussed.

Nontraditional porphyrinoid scaffolds as efficient electrocatalysts for the oxygen reduction reaction

254th National Meeting (2017)

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