Light-Driven Proton Reduction in Aqueous Medium Catalyzed by a Family of Cobalt Complexes with Tetradentate Polypyridine-Type Ligands

A series of tetradentate 2,2′:6′,2″:6″,2‴-quaterpyridine-type
ligands related to ppq (ppq = 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)­quinoline)
have been synthesized. One ligand replaces the 1,10-phenanthroline
(phen) moiety of ppq with 2,2′-bipyridine and the other two
ligands have a 3,3′-polymethylene subunit bridging the quinoline
and pyridine. The structural result is that both the planarity and
flexibility of the ligand are modified. Co­(II) complexes are prepared
and characterized by ultraviolet–visible light (UV-vis) and
mass spectroscopy, cyclic voltammetry, and X-ray analysis. The light-driven
H2-evolving activity of these Co complexes was evaluated
under homogeneous aqueous conditions using [Ru­(bpy)3]2+ as the photosensitizer, ascorbic acid as a sacrificial electron
donor, and a blue light-emitting diode (LED) as the light source.
At pH 4.5, all three complexes plus [Co­(ppq)­Cl2] showed
the fastest rate, with the dimethylene-bridged system giving the highest
turnover frequency (2125 h–1). Cyclic voltammograms
showed a significant catalytic current for H2 production
in both aqueous buffer and H2O/DMF medium. Combined experimental
and theoretical study suggest a formal Co­(II)-hydride species as a
key intermediate that triggers H2 generation. Spin density
analysis shows involvement of the tetradentate ligand in the redox
sequence from the initial Co­(II) state to the Co­(II)-hydride intermediate.
How the ligand scaffold influences the catalytic activity and stability
of catalysts is discussed, in terms of the rigidity and differences
in conjugation for this series of ligands.