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

• Published in: Inorganic chemistry 2015-08, Vol.54 (16), p.7873-7884
• Main Authors: Tong, Lianpeng, Kopecky, Andrew, Zong, Ruifa, Gagnon, Kevin J, Ahlquist, Mårten S. G, Thummel, Randolph P
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b00915

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.