Visible Light-Driven Hydrogen Evolution from Water Catalyzed by A Molecular Cobalt Complex

An approximately planar tetradentate polypyridine ligand, 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)­quinoline (ppq), has been prepared by two sequential Friedländer condensations. The ligand readily accommodates Co­(II) bearing two axial chlorides, and the resulting complex is reasonably soluble in water. In DMF the complex shows three well-behaved redox waves in the window of 0 to −1.4 V (vs SHE). However in pH 7 buffer the third wave is obscured by a catalytic current at −0.95 V, indicating hydrogen production that appears to involve a proton-coupled electron-transfer event. The complex [Co­(ppq)­Cl2] (6) in pH 4 aqueous solution, together with [Ru­(bpy)3]­Cl2 and ascorbic acid as a sacrificial electron donor, in the presence of blue light (λmax = 469 nm) produces hydrogen with an initial TOF = 586 h–1.