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](BF ₄ ) ₂ . This complex comprises two bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane (HBMIM Ph 2 ) ligands that contain an acidic methylene moiety in their backbone. Upon reduction of [1](BF ₄ ) ₂ by either electrochemical or chemical means, one of its HBMIM Ph 2 ligands undergoes deprotonation under the formation of dihydrogen. Addition of a mild proton source (acetic acid) to deprotonated complex [2](BF ₄ ) regenerates protonated complex [1](BF ₄ ) ₂ . In presence of acetic acid in acetonitrile solvent [1](BF ₄ ) 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 Ph 2 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.
(© 2020 The Authors. Published by Wiley-VCH GmbH.)