Ligand‐Directed Pathway Divergence in Electrocatalytic Hydrogen Evolution by Rationally Designed Cobalt(II) Complexes

ABSTRACT In the quest for efficient molecular catalysts for the hydrogen evolution reaction (HER), we report two cobalt(II) complexes bearing redox‐innocent and non‐innocent ligands. Two cobalt(II) complexes, [Co( L1 )Cl](ClO 4 ) ( 1 ) and [Co( L2 )(bpy)Cl](ClO 4 ) ( 2 ) (where L1 = 1‐(1‐methyl‐1H‐benzo[d]imidazol‐2‐yl)‐N,N‐bis(pyridin‐2‐ylmethyl)methanamine and L2 = 4 / ‐(anthracen‐9‐yl)‐2,2 / :6 / ,2 // ‐terpyridine) bearing redox inactive ( L1 ) as well as redox active ligand ( L2 ) systems were synthesized and thoroughly characterized by different spectroscopic and analytical tools. The molecular structures of complexes were confirmed using single‐crystal x‐ray diffraction. Their electrocatalytic hydrogen evolution studies were performed using acetic acid as an external proton source, and the generated hydrogen was quantified by gas chromatography with faradaic efficiency above 90%. Theoretical calculations have also been performed to support the experimental results and develop the mechanism at an atomistic level. It was observed that ligand modification can shift the Co(II)/(I) redox potential significantly, but this will not assure the generation of a cobalt‐hydride active intermediate at less negative potentials, as the formed low valent cobalt species could be less nucleophilic to directly attack H + from acetic acid. Post‐catalytic analyses were also performed to check the integrity of metal complexes as molecular catalysts.