Ligand Substituent Effects on Light-Driven Hydrogen Evolution by Cobalt(II) Tripodal Iminopyridine Catalysts under Precious-Metal-Free Conditions

Cobalt tripodal iminopyridine complexes [Co(L)]²⁺ (1–4), where L=cis,cis,–1,3,5-tris(pyridine-2-carboxaldimino)-cyclohexane (1) and its derivatives, bearing substituents of varying electronic properties at the pyridine moieties, act as water reduction catalysts (WRCs) under visible light (λ > 420 nm) in aqueous acetonitrile (2.5 %, v/v), using tri(n-butyl)triazatriagulenium (TATA⁺) as the organic photosensitizer and triethylamine (TEA) as the sacrificial donor. In the presence of added acetic acid, enhanced turnover numbers (TONs) of 1430–2810 were recorded with the substituted catalysts (2–4), which are attributed to the varied catalytic potential (E_cat) upon the introduction of the substituents.