Mechanistic Investigations of a Hydrogen-Evolving Cobalt Diimine-Dioxime Complex in an Oxygen Environment: Roles of Secondary Coordination Sphere, Bro̷nsted Acid, and Axial Ligand

The concept of a secondary coordination sphere (SCS) has been widely adopted in designing molecular electrocatalysts to promote energy-conversion reactions, such as the hydrogen-evolution reaction (HER) or the reduction of carbon dioxide. The role of SCS in the oxygen-tolerant properties of molecular electrocatalysts is less explored. An HER electrocatalyst, the cobalt diimine-dioxime complex, is one of the metal complexes designed by the concept of SCS to facilitate HER and retain its reactivity in an oxygen environment. Nevertheless, the mechanism underlying its oxygen tolerance remains unclear. This study revealed that in the presence of molecular oxygen, the oxygen reduction reaction (ORR) predominates over the hydrogen evolution reaction (HER) for this cobalt complex. Further investigations suggest that intramolecular proton transfer through SCS and intermolecular proton transfer from exogenous proton sources mutually dictate the product selectivity of ORR between H₂O₂ and H₂O, thereby determining the stability of the complex under HER. In addition, the choice of labile ligands has emerged as a useful factor in enhancing oxygen tolerance. These findings provide valuable design principles for developing oxygen-tolerant molecular catalysts and shed light on the reactivity and product selectivity controlled by the interplay of proton transfer routes.

The mechanism of the hydrogen evolution reaction (HER) catalyzed by cobalt diimine dioxime in the presence of molecular oxygen was examined. It was observed that the oxygen reduction reaction (ORR) is favored over the HER in the presence of molecular oxygen. The product distribution between H₂O and H₂O₂ varies with the pK_a values of the Bro̷nsted acids, highlighting that the proton relay site is crucial in determining product selectivity.