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

Abstract

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 ≈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.