Electrocatalytic hydrogen evolution from water at low overpotentials with cobalt complexes supported by redox-active bipyridyl-NHC donors

Three cobalt complexes bearing tunable, redox-active bipyridyl N-heterocyclic carbene (NHC)-based ligands have been studied for electrocatalytic hydrogen evolution from aqueous solu-tions. The effect of structural modifications to the ligand framework is investigated across the cata-lyst series, which includes a non-macrocyclic derivative (1-Co) and 16-(2-Co) and 15-(3-Co) mem-bered macrocycles. A structure-activity relationship is demonstrated, in which the macrocyclic complexes have greater activity compared to their non-macrocyclic counterpart with the most rigid catalyst, supported by the 15-membered macrocycle, performing best overall. Indeed, 3-Co catalyz-es H2 evolution from aqueous pH 4 acetate buffer with a Faradaic efficiency of 97％ at a low over-potential of 330 mV. Mechanistic studies are consistent with formation of a cobalt-hydride species that is subsequently protonated to evolve H2 via a heterolytic pathway.