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[Co(HBMIMPh2)2](BF4)2 (1) [HBMIMPh2=bis(1‐methyl‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)methane] was investigated for its electrocatalytic hydrogen evolution performance in DMF using voltammetry and during controlled potential/current electrolysis (CPE/CCE) in a novel in‐line product detection setup. Performances were benchmarked against three reported molecular cobalt hydrogen evolution reaction (HER) electrocatalysts, [Co(dmgBF2)2(solv)2] (2) (dmgBF2=difluoroboryldimethylglyoximato), [Co(TPP)] (3) (TPP=5,10,15,20‐tetraphenylporphyrinato), and [Co(bapbpy)Cl](Cl) (4) [bapbpy=6,6′‐bis‐(2‐aminopyridyl)‐2,2′‐bipyridine], showing distinct performances differences with 1 being the runner up in H2 evolution during CPE and the best catalyst in terms of overpotential and Faradaic efficiency during CCE. After bulk electrolysis, for all of the complexes, a deposit on the glassy carbon electrode was observed, and post‐electrolysis X‐ray photoelectron spectroscopy (XPS) analysis of the deposit formed from 1 demonstrated only a minor cobalt contribution (0.23 %), mainly consisting of Co2+. Rinse tests on the deposits derived from 1 and 2 showed that the initially observed distinct activity was (partly) preserved for the deposits. These observations indicate that the molecular design of the complexes dictates the features of the formed deposit and therewith the observed activity. [ABSTRACT FROM AUTHOR]

Homogeneous electrocatalytic proton reduction is reported using cobalt complex [1](BF4)2. This complex comprises two bis(1‐methyl‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)methane (HBMIMPh2) ligands that contain an acidic methylene moiety in their backbone. Upon reduction of [1](BF4)2 by either electrochemical or chemical means, one of its HBMIMPh2 ligands undergoes deprotonation under the formation of dihydrogen. Addition of a mild proton source (acetic acid) to deprotonated complex [2](BF4) regenerates protonated complex [1](BF4)2. In presence of acetic acid in acetonitrile solvent [1](BF4)2 shows electrocatalytic proton reduction with a kobs of ≈200 s−1 at an overpotential of 590 mV. Mechanistic investigations supported by DFT (BP86) suggest that dihydrogen formation takes place in an intramolecular fashion through the participation of a methylene C−H bond of the HBMIMPh2 ligand and a CoII−H bond through formal heterolytic splitting of the latter. These findings are of interest to the development of responsive ligands for molecular (base)metal (electro)catalysis. [ABSTRACT FROM AUTHOR]