Base-enhanced catalytic water oxidation by a carboxylate-bipyridine Ru(II) complex.

In aqueous solution above pH 2.4 with 4% (vol/vol) CH3CN, the complex [Ru(II)(bda)(isoq)2] (bda is 2,2'-bipyridine-6,6'-dicarboxylate; isoq is isoquinoline) exists as the open-arm chelate, [Ru(II)(CO2-bpy-CO2(-))(isoq)2(NCCH3)], as shown by (1)H and (13)C-NMR, X-ray crystallography, and pH titrations. Rates of water oxidation with the open-arm chelate are remarkably enhanced by added proton acceptor bases, as measured by cyclic voltammetry (CV). In 1.0 M PO4(3-), the calculated half-time for water oxidation is ∼7 µs. The key to the rate accelerations with added bases is direct involvement of the buffer base in either atom-proton transfer (APT) or concerted electron-proton transfer (EPT) pathways.

Electro-assembly of a chromophore-catalyst bilayer for water oxidation and photocatalytic water splitting.

The use of electropolymerization to prepare electrocatalytically and photocatalytically active electrodes for water oxidation is described. Electropolymerization of the catalyst Ru(II)(bda)(4-vinylpyridine)2 (bda=2,2'-bipyridine-6,6'-dicarboxylate) on planar electrodes results in films containing semirigid polymer networks. In these films there is a change in the water oxidation mechanism compared to the solution analogue from bimolecular to single-site. Electro-assembly construction of a chromophore-catalyst structure on mesoporous, nanoparticle TiO2 films provides the basis for a dye-sensitized photoelectrosynthesis cell (DSPEC) for sustained water splitting in a pH 7 phosphate buffer solution. Photogenerated oxygen was measured in real-time by use of a two-electrode cell design.

One-electron activation of water oxidation catalysis.

Rapid water oxidation catalysis is observed following electrochemical oxidation of [Ru(II)(tpy)(bpz)(OH)](+) to [Ru(V)(tpy)(bpz)(O)](3+) in basic solutions with added buffers. Under these conditions, water oxidation is dominated by base-assisted Atom Proton Transfer (APT) and direct reaction with OH(-). More importantly, we report here that the Ru(IV)═O(2+) form of the catalyst, produced by 1e(-) oxidation of [Ru(II)(tpy)(bpz)(OH2)](2+) to Ru(III) followed by disproportionation to [Ru(IV)(tpy)(bpz)(O)](2+) and [Ru(II)(tpy)(bpz)(OH2)](2+), is also a competent water oxidation catalyst. The rate of water oxidation by [Ru(IV)(tpy)(bpz)(O)](2+) is greatly accelerated with added PO4(3-) with a turnover frequency of 5.4 s(-1) reached at pH 11.6 with 1 M PO4(3-) at an overpotential of only 180 mV.

Multiple pathways in the oxidation of a NADH analogue.

Oxidation of the NADH analogue, N-benzyl-1,4-dihydronicotinamide (BNAH), by the 1e(-) acceptor, [Os(dmb)3](3+), and 2e(-)/2H(+) acceptor, benzoquinone (Q), has been investigated in aqueous solutions over extended pH and buffer concentration ranges by application of a double-mixing stopped-flow technique in order to explore the redox pathways available to this important redox cofactor. Our results indicate that oxidation by quinone is dominated by hydride transfer, and a pathway appears with added acids involving concerted hydride-proton transfer (HPT) in which synchronous transfer of hydride to one O-atom at Q and proton transfer to the second occurs driven by the formation of the stable H2Q product. Oxidation by [Os(dmb)3](3+) occurs by outer-sphere electron transfer including a pathway involving ion-pair preassociation of HPO4(2-) with the complex that may also involve a concerted proton transfer.

Solar water splitting in a molecular photoelectrochemical cell.

Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. We describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell. It uses a derivatized, core-shell nanostructured photoanode with the core a high surface area conductive metal oxide film--indium tin oxide or antimony tin oxide--coated with a thin outer shell of TiO2 formed by atomic layer deposition. A "chromophore-catalyst assembly" 1, [(PO3H2)2bpy)2Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH2)](4+), which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO2 shell. Visible photolysis of the resulting core-shell assembly structure with a Pt cathode resulted in water splitting into hydrogen and oxygen with an absorbed photon conversion efficiency of 4.4% at peak photocurrent.

Application of the rotating ring-disc-electrode technique to water oxidation by surface-bound molecular catalysts.

We report here the application of a simple hydrodynamic technique, linear sweep voltammetry with a modified rotating-ring-disc electrode, for the study of water oxidation catalysis. With this technique, we have been able to reliably obtain turnover frequencies, overpotentials, Faradaic conversion efficiencies, and mechanistic information from single samples of surface-bound metal complex catalysts.

Accumulation of multiple oxidative equivalents at a single site by cross-surface electron transfer on TiO2.

The photodriven accumulation of two oxidative equivalents at a single site was investigated on TiO2 coloaded with a ruthenium polypyridyl chromophore [Ru(bpy)2((4,4'-(OH)2PO)2bpy)](2+) (Ru(II)P(2+), bpy = 2,2'-bipyridine, ((OH)2PO)2-bpy = 2,2'-bipyridine-4,4'-diyldiphosphonic acid) and a water oxidation catalyst [Ru(Mebimpy) ((4,4'-(OH)2PO-CH2)2bpy)(OH2)](2+) (Ru(II)OH2(2+), Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, (4,4'-(OH)2PO-CH2)2bpy) = 4,4'-bis-methlylenephosphonato-2,2'-bipyridine). Electron injection from the metal-to-ligand charge transfer (MLCT) excited state of -Ru(II)P(2+) (-Ru(II)P(2+)*) to give -Ru(III)P(3+) and TiO2(e(-)) was followed by rapid (<20 ns) nearest-neighbor -Ru(II)OH2(2+) to -Ru(III)P(3+) electron transfer. On surfaces containing both -Ru(II)P(2+) and -Ru(III)OH2(3+) (or -Ru(III)OH(2+)), -Ru(II)OH2(2+) was formed by random migration of the injected electron inside the TiO2 nanoparticle and recombination with the preoxidized catalyst, followed by relatively slow (µs-ms) non-nearest neighbor cross-surface electron transfer from -Ru(II)OH2(2+) to -Ru(III)P(3+). Steady state illumination of coloaded TiO2 photoanodes in a dye sensitized photoelectrosynthesis cell (DSPEC) configuration resulted in the buildup of -Ru(III)P(3+), -Ru(III)OH(2+), and -Ru(IV)═O(2+), with -Ru(IV)═O(2+) formation favored at high chromophore to catalyst ratios.

Coordination chemistry of single-site catalyst precursors in reductively electropolymerized vinylbipyridine films.

Reductive electropolymerization of [Ru(II)(PhTpy)(5,5'-dvbpy)(Cl)](PF6) and [Ru(II)(PhTpy)(5,5'-dvbpy)(MeCN)](PF6)2 (PhTpy is 4'-phenyl-2,2':6',2″-terpyridine; 5,5'-dvbpy is 5,5'-divinyl-2,2'-bipyridine) on glassy carbon electrodes gives well-defined films of poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(Cl)](PF6)} (poly-1) or poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(MeCN)](PF6)2} (poly-2). Oxidative cycling of poly-2 with added NO3(-) results in the replacement of coordinated MeCN by NO3(-) to give poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(NO3)](+)}, and with 0.1 M HClO4, replacement by H2O occurs to give poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(OH2)](2+)} (poly-OH2). Although analogous aqua complexes (e.g., [Ru(tpy)(bpy)(OH2)](2+)) undergo rapid loss of H2O to MeCN in solution, poly-OH2 and poly-OH2(+) are substitutionally inert in MeCN. The substitution chemistry is reversible, with reductive scans of poly-1 or poly-OH2 in MeCN resulting in poly-2, although with some loss of Faradaic response.

Redox mediator effect on water oxidation in a ruthenium-based chromophore-catalyst assembly.

The synthesis, characterization, and redox properties are described for a new ruthenium-based chromophore-catalyst assembly, [(bpy)(2)Ru(4-Mebpy-4'-bimpy)Ru(tpy)(OH(2))](4+) (1, [Ru(a)(II)-Ru(b)(II)-OH(2)](4+); bpy = 2,2'-bipyridine; 4-Mebpy-4'-bimpy = 4-(methylbipyridin-4'-yl)-N-benzimid-N'-pyridine; tpy = 2,2':6',2"-terpyridine), as its chloride salt. The assembly incorporates both a visible light absorber and a catalyst for water oxidation. With added ceric ammonium nitrate (Ce(IV), or CAN), both 1 and 2, [Ru(tpy)(Mebim-py)(OH(2))](2+) (Mebim-py = 2-pyridyl-N-methylbenzimidazole), catalyze water oxidation. Time-dependent UV/vis spectral monitoring following addition of 30 equiv of Ce(IV) reveals that the rate of Ce(IV) consumption is first order both in Ce(IV) and in an oxidized form of the assembly. The rate-limiting step appears to arise from slow oxidation of this intermediate followed by rapid release of O(2). This is similar to isolated catalyst 2, with redox potentials comparable to the [-Ru(b)-OH(2)](2+) site in 1, but 1 is more reactive than 2 by a factor of 8 due to a redox mediator effect.

Accelerating slow excited state proton transfer.

Visible light excitation of the ligand-bridged assembly [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH(2))(4+)] (bpy is 2,2'-bipyridine; L is the bridging ligand, 4-phen-tpy) results in emission from the lowest energy, bridge-based metal-to-ligand charge transfer excited state (L(-•))Ru(b)(III)-OH(2) with an excited-state lifetime of 13 ± 1 ns. Near-diffusion-controlled quenching of the emission occurs with added HPO(4)(2-) and partial quenching by added acetate anion (OAc(-)) in buffered solutions with pH control. A Stern-Volmer analysis of quenching by OAc(-) gave a quenching rate constant of k(q) = 4.1 × 10(8) M(-1) • s(-1) and an estimated pK(a)* value of ~5 ± 1 for the [(bpy)(2)Ru(a)(II)(L(•-))Ru(b)(III)(bpy)(OH(2))(4+)]* excited state. Following proton loss and rapid excited-state decay to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH)(3+)] in a H(2)PO(4)(-)/HPO(4)(2-) buffer, back proton transfer occurs from H(2)PO(4)(-) to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(bpy)(OH(2))(4+)] with k(PT,2) = 4.4 × 10(8) M(-1) • s(-1). From the intercept of a plot of k(obs) vs. [H(2)PO(4)(-)], k = 2.1 × 10(6) s(-1) for reprotonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK(a) values intermediate between pK(a)(H(3)O(+)) = -1.74 and pK(a)(H(2)O) = 15.7.

Role of proton-coupled electron transfer in the redox interconversion between benzoquinone and hydroquinone.

Benzoquinone/hydroquinone redox interconversion by the reversible Os(dmb)(3)(3+/2+) couple over an extended pH range with added acids and bases has revealed the existence of seven discrete pathways. Application of spectrophotometric monitoring with stopped-flow mixing has been used to explore the role of PCET. The results have revealed a role for phosphoric acid and acetate as proton donor and acceptor in the concerted electron-proton transfer reduction of benzoquinone and oxidation of hydroquinone, respectively.

Sensitized photodecomposition of organic bisphosphonates by singlet oxygen.

During efforts to stabilize metal oxide bound chromophores for photoelectrochemical applications, a novel photochemical reaction has been discovered. In the reaction, the bisphosphonate functional groups -C(PO(3)H(2))(2)(OH) in the metal complex [Ru(bpy)(2)(4,4'-(C(OH)(PO(3)H(2))(2)bpy)](2+) are converted into -COOH and H(3)PO(4). The reaction occurs by sensitized formation of (1)O(2) by the lowest metal-to-ligand charge transfer excited state(s) of [Ru(bpy)(2)(4,4'-(C(PO(3)H(2))(2)(OH))(2)(bpy))](2+)* followed by (1)O(2) oxidation of the bisphosphonate substituent. A related reaction occurs for the bisphosphonate-based drug, risedronic acid, in the presence of O(2), light, and a singlet oxygen sensitizer ([Ru(bpy)(3)](2+) or Rose Bengal).

An amide-linked chromophore-catalyst assembly for water oxidation.

The synthesis and analysis of a new amide-linked, dinuclear [Ru(bpy)(2)(bpy-ph-NH-CO-trpy)Ru(bpy)(OH(2))](4+) (bpy = 2,2'-bipyridine; bpy-ph-NH-CO-trpy = 4-(2,2':6',2"-terpyridin-4'-yl)-N-[(4'-methyl-2,2'-bipyridin-4-yl)methyl]benzamide) assembly that incorporates both a light-harvesting chromophore and a water oxidation catalyst are described. With the saturated methylene linker present, the individual properties of both the chromophore and catalyst are retained including water oxidation catalysis and relatively slow energy transfer from the chromophore excited state to the catalyst.

Water oxidation intermediates applied to catalysis: benzyl alcohol oxidation.

Four distinct intermediates, Ru(IV)═O(2+), Ru(IV)(OH)(3+), Ru(V)═O(3+), and Ru(V)(OO)(3+), formed by oxidation of the catalyst [Ru(Mebimpy)(4,4'-((HO)(2)OPCH(2))(2)bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) and 4,4'-((HO)(2)OPCH(2))(2)bpy = 4,4'-bismethylenephosphonato-2,2'-bipyridine] on nanoITO (1-PO(3)H(2)) have been identified and utilized for electrocatalytic benzyl alcohol oxidation. Significant catalytic rate enhancements are observed for Ru(V)(OO)(3+) (~3000) and Ru(IV)(OH)(3+) (~2000) compared to Ru(IV)═O(2+). The appearance of an intermediate for Ru(IV)═O(2+) as the oxidant supports an O-atom insertion mechanism, and H/D kinetic isotope effects support net hydride-transfer oxidations for Ru(IV)(OH)(3+) and Ru(V)(OO)(3+). These results illustrate the importance of multiple reactive intermediates under catalytic water oxidation conditions and possible control of electrocatalytic reactivity on modified electrode surfaces.

Concerted electron-proton transfer (EPT) in the oxidation of tryptophan with hydroxide as a base.

Tryptophan is unique among the redox-active amino acids owing to its weakly acidic indolic proton (pK(a) ≈ 16) compared to the -O-H proton of tyrosine (pK(a) = 10.1) or the -S-H proton of cysteine (pK(a) = 8.2). Stopped-flow and electrochemical measurements have been used to explore the roles of proton-coupled electron transfer and concerted electron-proton transfer (EPT) in tryptophan oxidation. The results of these studies have revealed a role for OH(-) as a proton acceptor base in EPT oxidation of N-acetyl-tryptophan but not for other common bases. The reorganizational barrier for (N-acetyl-tryptophan)(+/•) self-exchange is also estimated.