Electrocatalytic proton reduction by mononuclear cobalt complexes of bis(benzimidazolyl)pyridine ligands: experimental and theoretical study

This work presents an electro-catalytic and photochemical study of the Co(II) complexes [Co(L¹)₂](ClO₄)₂ (1), [Co(L²)₂]Cl₂ (2), and [Co(L³)₂](ClO₄)₂ (3), where L¹ = 2,6-bis(1H-benzo[d]imidazole-2-yl)pyridine, L² = 2,6-bis(4,5-dimethyl-1H-benzo[d]imidazole-2-yl)pyridine, and L³ = 2,6-bis(4-fluoro-1H-benzo[d]imidazole-2-yl)pyridine. These complexes were investigated as catalysts for hydrogen production from acids. In electro-catalytic tests, the complexes were evaluated in a non-aqueous solvent with moderately strong trifluoroacetic acid (TFA) as the substrate, showing catalytic wave onset at −0.93 V, −1.13 V, and −1.06 V vs. Fc⁺/Fc for complexes 1, 2, and 3, respectively. The complexes demonstrated turnover frequencies of 26.42 s⁻¹, 1.56 s⁻¹, and 1.26 s⁻¹ at a 14 : 1 acid-to-catalyst ratio, with hydrogen production detected by gas chromatography. Zinc complexes (4–6) with the same ligands were also screened for electrocatalytic hydrogen evolution to identify the active site for catalysis. The cobalt complexes were also tested for photochemical proton reduction in water at pH 7, using [Ru(bpy)₃]²⁺ as the photosensitizer and ascorbic acid as the sacrificial electron donor. The cobalt complexes thus demonstrate effectiveness as catalysts for both electro-catalytic and photo-catalytic proton reduction in non-aqueous and aqueous solutions.