An Easily Prepared Monomeric Cobalt(II) Tetrapyrrole Complex that Efficiently Promotes the 4e–/4H+ Peractivation of O2 to Water

The selective 4e–/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. Monomeric aromatic tetrapyrroles (such as porphyrins, phthalocyanines, and corroles) coordinated to Co(II) have been considered as oxygen reduction catalysts due to their low cost and relative ease of synthesis. How- ever, these systems have been repeatedly shown to be selective for O2 reduction by the less desired 2e –/2H+ pathway to yield hydrogen peroxide. Herein, we report the initial synthesis and study of a Co(II) tetrapyrrole complex based upon a non-aromatic isocorrole scaffold that is competent for 4e–/4H+ ORR. This Co(II) 10,10-dimethyl isocorrole (Co[10- DMIC]) is obtained in a just four simple steps and excellent yield from a known dipyrromethane synthon. Evaluation of the steady state spectroscopic and redox properties of Co[10-DMIC] against those of Co(II) porphyrin ([Co(TPFPP)]) and corrole ([Co(TPFPC)(PPh3)]) homologs demonstrated that the light harvesting and electrochemical properties of the isocorrole are distinct from those displayed by more traditional aromatic tetrapyrroles. Further, investigation of the ORR activity of Co[10-DMIC] using a combination of electrochemical and chemical reduction studies revealed that this simple, unadorned monomeric Co(II) tetrapyrrole is ~85% selective for the 4e–/4H+ reduction of O2 to H2O over the more kinetically facile 2e–/2H+ process that delivers H2O2. By contrast, the same ORR evaluations conducted for the Co(II) porphyrin and corrole homologs demonstrated that these traditional aromatic systems catalyze the 2e–/2H+ conversion of O2 to H2O2 with near complete selectivity. Despite being a simple, easily prepared, monomeric tetrapyrrole platform, Co[10-DMIC] supports an ORR catalysis that has historically only been achieved using elaborate porphyrinoid-based architectures that incorporate pendant proton-transfer groups, ditopic molecular clefts, or which impose cofacially ori- ented O2 binding sites. Accordingly, Co[10-DMIC] represents the first simple, unadorned, monomeric metalloisocorrole complex that can be easily prepared and which shows a privileged performance for the 4e–/4H+ peractivation of O2 to water as compared to other simple Co(II) tetrapyrroles.