Ruthenium catalysts for water oxidation involving tetradentate polypyridine-type ligands

Abstract

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.