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 (HBMIM (Formula presented.)) 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 HBMIM (Formula presented.) 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 k_obs 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 HBMIM (Formula presented.) ligand and a Co^II−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.