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/Ru IV =O couple clearly support such a mechanism. However, in the cases of [Ru(tpy)(bpy)(OH2)]2+ and [Ru(tpy)(bpm)(OH 2)]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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry