Water Oxidation with Mononuclear Ruthenium(II) Polypyridine Complexes Involving a Direct RuIV=O Pathway in Neutral and Alkaline Media

The catalytic water oxidation mechanism proposed for many single-site ruthenium complexes proceeds via the nucleophilic attack of a water molecule on the Ru-V=O species. In contrast, Ru(II) complexes containing 4-t-buty1-2,6-di- 1',8'-(naphthyrid-2'-y1)-pyridine (and its bisbenzo-derivafive), an equatorial water, and two axial 4-picolines follow the thermodynamically more favorable "direct pathway via [Ru-IV=O](2+), which avoids the higher oxidation state [Ru-V=O](3+) in neutral and basic media. Our experimental and theoretical results that focus on the pH dependent onset catalytic potentials indicative of a PCET driven low energy pathway for the formation of products with an O-O bond (such as [Ru-III-OOH](2+) and [Ru-IV-OO](2+) at an applied potential below the Ru-V=O/Ru-IV=O couple clearly support such a mechanism. However, in the cases of [Ru(tpy)(bpy)(OH2))(2+) and [Ru(tpy)(bpm)(OH2))(2+), the formation of the Ru-V=O species appears to be required before O-O bond formation. The complexes under discussion provide a unique functional model for water oxidation that proceeds by four consecutive PCET steps in neutral and alkaline media.