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 to generate two equivalents of H2O is more downhill than the analogous 2e–/2H+ process to generate H2O2 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 are not capable of promoting 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 a single sp3 hybridized carbon on the tetrapyrrole periphery. This unconventional structural motif significantly attenuates the electronic properties of these architectures, and is manifest in multielectron redox properties that are not observed for 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 these new platforms can selectively promote the ORR via the 4e–/4H+ pathway to generate water. Detailed voltametric 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 mono-cobalt macrocycles. The potential application of these new architectures for incorporation into fuell cell cathodes will also be discussed.

Nonaromatic tetrapyrrole metal complexes supporting a multielectron redox chemistry for efficent dioxygen activation

253rd National Meeting (2017)

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