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

Development of rapid, robust water oxidation catalysts remains an essential element in solar water splitting by artificial photosynthesis. We report here dramatic rate enhancements with added buffer bases for a robust Ru(II) polypyridyl catalyst with a calculated half-time for water oxidation of ∼7 μs in 1.0 M phosphate. The results of detailed kinetic studies provide insight into the water oxidation mechanism and an important role for added buffer bases in accelerating water oxidation by concerted atom–proton transfer

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

Multiple Pathways in the Oxidation of a NADH Analogue

Oxidation of the NADH analogue, <i>N</i>-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

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

In aqueous solution above pH 2.4 with 4% (vol/vol) CH(3)CN, 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)(CO(2)-bpy-CO(2)(−))(isoq)(2)(NCCH(3))], 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 PO(4)(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

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O²⁺ 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

Concerted Electron–Proton Transfer (EPT) in the Oxidation of Cysteine

Cysteine is the most acidic of the three common redox active amino acids with p<i>K</i>_a = 8.2 for the thiol compared to p<i>K</i>_a = 10.1 for the phenol in tyrosine and p<i>K</i>_a ≈ 16 for the indole proton in tryptophan. Stopped-flow and electrochemical measurements have been used to explore the role of proton-coupled electron transfer (PCET) and concerted electron–proton transfer (EPT) in the oxidations of <i>L</i>-cysteine and <i>N</i>-acetyl-cysteine by the polypyridyl oxidants M­(bpy)₃³⁺ (M = Fe, Ru, Os) and Ru­(dmb)₃³⁺ (bpy is 2,2′-bipyridine, and dmb is 4,4′-dimethyl-2,2′-bipyridine). Oxidation is rate limited by initial 1e^– electron transfer to M­(bpy)₃³⁺, with added proton acceptor bases, by multiple pathways whose relative importance depends on reaction conditions. The results of these studies document important roles for acetate (AcO^–) and phosphate (HPO₄^2–) as proton acceptor bases in concerted electron–proton transfer (EPT) pathways in the oxidation of <i>L</i>-cysteine and <i>N</i>-acetyl-cysteine with good agreement between rate constant data obtained by electrochemical and stopped-flow methods

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(PO3H2)2(OH) in the metal complex [Ru(bpy)2(4,4'-(C(OH)(PO3H2)2bpy)]2+ are converted into −COOH and H3PO4. The reaction occurs by sensitized formation of 1O2 by the lowest metal-to-ligand charge transfer excited state(s) of [Ru(bpy)2(4,4'-(C(PO3H2)2(OH))2(bpy))]2+* followed by 1O2 oxidation of the bisphosphonate substituent. A related reaction occurs for the bisphosphonate-based drug, risedronic acid, in the presence of O2, light, and a singlet oxygen sensitizer ([Ru(bpy)3]2+ or Rose Bengal)