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

The development of molecular electrocatalysts for fuel-forming reactions, such as the hydrogen-evolving reaction (HER) or the reduction of carbon dioxide, is generally hindered by their susceptibility to dioxygen in practical applications, which results from the concomitant formation of reactive oxygen species. The concept of a secondary coordination sphere (SCS) has been widely adopted in designing molecular electrocatalysts to promote the aforementioned energy-conversion reactions. The impact of this supernumerary interaction through the SCS on the oxygen-tolerant properties of molecular electrocatalysts is less explored. A HER electrocatalyst, 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. In this study, mechanistic studies of how this complex undergoes HER under aerobic conditions were conducted. The results reveal that the oxygen reduction reaction (ORR) predominates in the presence of molecular oxygen. Further studies uncover the 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 how the interplay of proton transfer routes between the secondary coordination sphere and exogenous pathway can impact reactivity and product selectivity.