Water oxidation with ruthenium catalysts

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.Chemistry, Department o

Component Analysis of Dyads Designed for Light-Driven Water Oxidation

A series of seven dyad molecules have been prepared utilizing a [Ru­(tpy)­(NN)­I]⁺ type oxidation catalyst (NN = 2,5-di­(pyrid-2′-yl) pyrazine (<b>1</b>), 2,5-di-(1′,8′-dinaphthyrid-2′-yl) pyrazine (<b>2</b>), or 4,6-di-(1′,8′-dinaphthyrid-2′-yl) pyrimidine (<b>3</b>). The other bidentate site of the bridging ligand was coordinated with 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), or a substituted derivative. These dinuclear complexes were characterized by their ¹H NMR spectra paying special attention to protons held in the vicinity of the electronegative iodide. In one case, <b>10a</b>, the complex was also analyzed by single crystal X-ray analysis. The electronic absorption spectra of all the complexes were measured and reported as well as emission properties for the sensitizers. Oxidation and reduction potentials were measured and excited state redox properties were calculated from this data. Turnover numbers, initial rates, and induction periods for oxygen production in the presence of a blue LED light and sodium persulfate as a sacrificial oxidant were measured. Similar experiments were run without irradiation. Dyad performance correlated well with the difference between the excited state reduction potential of the photosensitizer and the ground state oxidation potential of the water oxidation dyad. The most active system was one having 5,6-dibromophen as the auxiliary ligand, and the least active system was the one having 4,4′-dimethylbpy as the auxiliary ligand

A Molecular Light-Driven Water Oxidation Catalyst

Two mononuclear Ru­(II) complexes, [Ru­(ttbt)­(pynap)­(I)]­I and [Ru­(tpy)­(Mepy)₂(I)]I (tpy = 2,2′;6,2″-terpyridine; ttbt = 4,4′,4″-tri-<i>tert</i>-butyltpy; pynap = 2-(pyrid-2′-yl)-1,8-naphthyridine; and Mepy = 4-methylpyridine), are effective catalysts for the oxidation of water. This oxidation can be driven by a blue (λ_max = 472 nm) 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 intramolecular system with blue light produces oxygen with a higher turnover number than the analogous intermolecular component system under the same conditions

A Ru(II) Bis-terpyridine-like Complex that Catalyzes Water Oxidation: The Influence of Steric Strain

The complexation of 2,9-dicarboxy-1,10-phenanthroline (DPA) with [Ru­(tpy)­Cl₃] (tpy = 2,2′;6,2″-terpyridine) provides a six-coordinate species in which one carboxyl group of DPA is not bound to the Ru­(II) center. A more soluble tri-<i>t-</i>butyl tpy analogue is also prepared. Upon oxidation, neither species shows evidence for intramolecular trapping of a seven-coordinate intermediate. The role of the tpy ligand is revealed by the preparation of [Ru­(tpy)­(phenq)]²⁺ (phenq = 2-(quinol-8′-yl)-1,10-phenanthroline) that behaves as an active water oxidation catalyst (TON = 334). This activity is explained by the expanded coordination geometry of the phenq ligand that can form a six-membered chelate ring that better accommodates the linear arrangement of axial ligands required for optimal pentagonal bipyramid geometry. When a 1,8-naphthyidine ring is substituted for each of the two peripheral pyridine rings on tpy, increased crowding in the vicinity of the metal center impedes acquisition of the prerequisite reaction geometry