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 Friedlander 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)Cl-2] (6) in pH 4 aqueous solution, together with [Ru(bpy)(3)]Cl-2 and ascorbic acid as a sacrificial electron donor, in the presence of blue light (lambda(max) = 469 nm) produces hydrogen with an initial TOP = 586 h(-1).