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 ](BF 4 ) 2 . This complex comprises two bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane (HBMIM Ph2 ) 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 Ph2 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 Density Functional Theory (BP86) reveal that dihydrogen formation takes place in an intramolecular fashion through the participation of a methylene C-H bond of the HBMIM Ph2 ligand and a Co II -H bond through formal heterolytic splitting of the latter. These results represent the joint involvement of a base metal-hydride and a C-H bond in electrocatalytic proton reduction and are of interest to the development of responsive ligands for molecular (base)metal (electro)catalysis.