Hydrogen Evolution Catalyzed by Heteroleptic Mononuclear Co(II) Complexes Through Integrated Electrochemical and Computational Studies

ABSTRACT

The development of efficient molecular cobalt electrocatalysts for the hydrogen evolution reaction (HER) is of considerable interest for sustainable hydrogen production. Herein, three heteroleptic Co(II) complexes, [Co( L ^(OMe) )bpyCl]ClO ₄ ( 1 ), [Co( L ^(OMe) )dmbpyCl]ClO ₄ ( 2 ), and [Co( L ^(OMe) )phenCl]ClO ₄ ( 3 ) ( L = 2‐chloro‐4,6‐bis(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)‐1,3,5‐triazine), were synthesized and comprehensively characterized, including single‐crystal X‐ray diffraction (SCXRD), which confirmed distorted octahedral geometries around the Co(II) center for all. Their electrocatalytic HER activity was investigated in anhydrous acetonitrile using acetic acid as the proton source. All complexes exhibited pronounced catalytic current enhancement in the ligand‐centered reduction region, affording HER overpotentials of 0.53–0.61 V. Controlled‐potential electrolysis confirmed hydrogen evolution with faradaic efficiencies of 92.5%, 98.4%, and 74.4% for complexes 1 , 2 and 3 , respectively. Post‐catalytic rinse tests, UV–vis spectroscopy, electrochemical stability studies, and field‐emission scanning electron microscopy/energy‐dispersive x‐ray spectroscopy (FE‐SEM/EDS) analyses confirmed the homogeneous nature of the catalysts. Combined electrochemical and theoretical investigations indicate ligand‐centered reduction prior to the cobalt hydride formation. Complex 2 exhibited the highest TOF and Faradaic efficiency under the AcOH electrolysis conditions, highlighting the role of ancillary‐ligand electronic modulation in tuning the redox properties and HER activity, establishing a clear structure–activity relationship for the rational design of efficient cobalt‐based molecular HER electrocatalysts.