Water oxidation with ruthenium catalysts



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CeIV-activated water oxidation: A family of 29 mononuclear RuII complexes have been prepared and characterized by 1H NMR, electronic absorption, and cyclic voltammetry. These complexes are studied as catalysts for water oxidation. The complexes may be divided into three basic types. The type-1 complexes involve a [RuII(NNN)(NN)(X)]n (NNN = tridentate ligand, NN = a bidentate ligand, and X = halogen and n = 1+; X = H2O and n = 2+). The type-2 complexes contain a NNN, two molecules of 4-methylpyridine (pic), and halogen or H2O to form a [RuII(NNN)(pic)2(X)]n complex. The type-3 complexes contain no water molecule and thus are constructed from a tetradentate ligand (NNNN) and two molecules of pic to provide a [RuII(NNNN)(pic)2]2+ complex. In general the type-2 catalysts are more reactive than the type-1. The type-2 iodo-catalyst shows first-order behavior and, unlike the bromo- and chloro-catalysts, does not require water-halogen exchange to show good activity. The importance of steric strain and hindrance around the metal center is examined. The introduction of three t-butyl groups at the 4, 4′, and 4″ positions of tpy (ttbt) sometimes improves catalyst activity, but the effect does not appear to be additive. Photo-activated water oxidation: Two mononuclear RuII complexes, [Ru(ttbt)(pynap)(I)]I and [Ru(tpy)(pic)2(I)]I (tpy = 2,2';6,2''-terpyridine; pynap = 2-(pyrid-2'-yl)-1,8-naphthyridine), are effective catalysts for the oxidation of water. This oxidation can be driven by a blue LED light source using [Ru(bpy)3]Cl2 (bpy = 2,2'-bipyridine) as the photosensitizer. Sodium persulfate acts as a sacrificial electron acceptor to oxidize the photosensitizer that in turn drives the catalysis. The presence of all four components: light, photosensitizer, sodium persulfate, and catalyst are required for water oxidation. A dyad assembly has been prepared using a pyrazine-based linker to join a photosensitizer and catalyst moiety. Irradiation of this intra-molecular system with blue light produces oxygen with a higher turnover number than the analogous intermolecular component system under the same conditions.



Water oxidation, Ru catalyst, Water splitting