Electrocatalytic Proton Reduction By A Cobalt Complex Containing A Proton-Responsive Bis(Alkylimdazole)Methane Ligand: Involvement Of A C-H Bond In H-2 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 k(obs) of approximate to 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 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.