Ruthenium catalysts for water oxidation involving tetradentate polypyridine-type ligands

A series of Ru(II) complexes that behave as water oxidation catalysts were prepared involving a tetradentate equatorial ligand and two 4-substituted pyridines as the axial ligands. Two of these complexes were derived from 2,9-di-(pyrid-2'-yl)-1,10-phenanthroline (dpp) and examine the effect of incorporating electron-donating amino and bulky t-butyl groups on catalytic activity. A third complex replaced the two distal pyridines with N-methylimidazoles that are more electron-donating than the pyridines of dpp and potentially stabilize higher oxidation states of the metal. The tetradentate ligand 2-(pyrid-2'-yl)-6-(1'',10''-phenanthrol-2''-yl)pyridine (bpy-phen), possessing a bonding cavity similar to dpp, was also prepared. The Ru(II) complex of this ligand does not have two rotatable pyridines in the equatorial plane and thus shows different flexibility from the [Ru(dpp)] complexes. All the complexes showed activity towards water oxidation. Investigation of their catalytic behavior and electrochemical properties suggests that they may follow the same catalytic pathway as the prototype [Ru(dpp)pic2](2+) involving a seven-coordinated [Ru(IV)(O)] intermediate. The influence of coordination geometry on catalytic performance is analyzed and discussed.