Catalytic water oxidation by ruthenium(II) quaterpyridine (qpy) complexes: Evidence for ruthenium(III) qpy-N,N''-dioxide as the real catalysts

Polypyridyl and related ligands have been widely used for the development of water oxidation catalysts. Supposedly these ligands are oxidation-resistant and can stabilize high-oxidation-state intermediates. In this work a series of ruthenium(II) complexes [Ru(qpy)(L)2] (qpy= 2,2' :6',2":6'',2'''-quaterpyridine; L = substituted pyridine) have been synthesized and found to catalyze Ce-driven water oxidation, with turnover numbers of up to 2100. However, these ruthenium complexes are found to function only as precatalysts; first, they have to be oxidized to the qpy-N,N'''- dioxide (ONNO) complexes [Ru(ONNO)(L)2] which are the real catalysts for water oxidation.