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Mechanistic understanding of low-energy pathways for catalytic water oxidation by Ru mononuclear polypyridyl complexes

In contrast to the proposed catalytic water oxidation mechanism by most of single-site ruthenium complexes that proceed via the nucleophilic attack of a water molecule on the RuV=O species, ruthenium(II) complexes containing 4-t-butyl-2,6-di-1',8'-(naphthyrid-2'-yl)-pyridine (or its bisbenzo-derivative), water and two 4-picoline ligands show the reaction mechanism proceeding by 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 results 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 is required before O–O bond formation. Our complexes provide a unique functional model for water oxidation that proceeds by four consecutive PCET steps in neutral and alkaline media.

Inorg. Chem. 2013, 52, 8845–8850, DOI: 10.1021/ic401023w