Electrocatalytic Proton Reduction by a Cobalt Complex Containing a Proton‐Responsive Bis(alkylimdazole)methane Ligand: Involvement of a C−H Bond in H2 Formation

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. The HBMIMPh2 ligand in the homoleptic cobalt(II) complex [Co(HBMIMPh2)2]2+ ([1]2+) was found to act as both a proton‐ and a redox‐responsive ligand. Using [1]2+ as an electrocatalyst in the hydrogen evolution reaction (HER) revealed an unprecedented interplay between a cobalt(II)‐hydride moiety and a ligand C−H bond in the formation of the H−H bond (see scheme).