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.