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 RuVO species. In contrast, Ru­(II) complexes containing 4-t-butyl-2,6-di-1′,8′-(naphthyrid-2′-yl)-pyridine (and its bisbenzo-derivative), an equatorial water, and two axial 4-picolines follow the thermodynamically more favorable “direct pathway” via [RuIVO]2+, which avoids the higher oxidation state [RuV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 [RuIII–OOH]2+ and [RuIV–OO]2+) at an applied potential below the RuVO/RuIV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 RuV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.