Hydrogen Evolution Catalysis by Cobalt Complexes of an Aza‐bridged Bis‐1,10‐phenanthroline Ligand Bearing Pendant Basic Sites

A novel series of molecular Co(II) complexes with aza‐bridged bis‐1,10‐phenanthroline (bpa) ligands have been synthesized and reported for the catalytic hydrogen evolution reaction (HER). Various nitrogen donor substituents are introduced at the aza‐bridge of the bis‐phenanthroline moieties render intramolecular basic sites in the complexes’ second‐coordination sphere. Modifying the pendant nitrogen donor significantly influences the catalytic HER performance. Among these, the cobalt complex with the (2‐pyridyl)methyl substituted bis‐phenanthroline ligand (L3) exhibits the most photocatalytic HER efficiency in both organic and aqueous media. Under optimized conditions with [Ru(bpy)3]2+ as a sensitizer and ascorbic acid as an electron donor, [CoII(L3)(TfO−)2] achieves an initial turnover frequency (TOF) of 1882 ± 65 h−1 per catalyst and a turnover number (TON) of 4842 ± 122 in a 3‐hour reaction period under the irradiation of visible light. Combined experimental and theoretical evidences illustrate that phenanthroline moieties of the bpa ligands act as electron mediators during the HER catalysis, while the pendant nitrogen donor substituents mediate proton transfer. This work provides a unique example for understanding the activity‐structure relationship of homogeneous HER catalyst concerning the redox‐active properties and second coordination sphere basic sites of organic ligand platforms.