30 research outputs found

    Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol Complex: Stepwise Oxidation and Concerted Reduction

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    Proton-coupled electron transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII)-inspired [Ru­(bpy)<sub>2</sub>(phen-imidazole-Ph­(OH)­(<sup><i>t</i></sup>Bu)<sub>2</sub>)]<sup>2+</sup>, in which Ru­(III) generated by a flash–quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen-bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the Ru<sup>III/II</sup> couple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru­(III) is energetically favorable by both stepwise and concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with <i>k</i><sub>ET</sub> = 2.7 × 10<sup>7</sup> s<sup>–1</sup> and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λ<sub>DA</sub> = 1.16 eV and 280 < <i>H</i><sub>DA</sub> < 540 cm<sup>–1</sup> consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen-evolving complex. In our PSII-inspired complex, the recombination reaction activation energy is <2 kcal mol<sup>–1</sup>. The reaction is nonadiabatic (<i>V</i><sub>PCET</sub> ≈ 22 cm<sup>–1</sup> (H) and 49 cm<sup>–1</sup> (D)) and concerted, and it exhibits an unexpected inverse KIE = 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels

    O–O Radical Coupling: From Detailed Mechanistic Understanding to Enhanced Water Oxidation Catalysis

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    A deeper mechanistic understanding of the key O–O bond formation step of water oxidation by the [Ru­(bda)­(L)<sub>2</sub>] (bdaH<sub>2</sub> = 2,2′-bipyridine-6,6′-dicarboxylic acid; L is a pyridine or isoquinoline derivative) family of catalysts is reached through harmonious experimental and computational studies of two series of modified catalysts with systematic variations in the axial ligands. The introduction of halogen and electron-donating substituents in [Ru­(bda)­(4-X-py)<sub>2</sub>] and [Ru­(bda)­(6-X-isq)<sub>2</sub>] (X is H, Cl, Br, and I for the pyridine series and H, F, Cl, Br, and OMe for the isoquinoline series) enhances the noncovalent interactions between the axial ligands in the transition state for the bimolecular O–O coupling, resulting in a lower activation barrier and faster catalysis. From detailed transition state calculations in combination with experimental kinetic studies, we find that the main contributor to the free energy of activation is entropy due to the highly organized transition states, which is contrary to other reports. Previous work has considered only the electronic influence of the substituents, suggesting electron-withdrawing groups accelerate catalysis, but we show that a balance between polarizability and favorable π–π interactions is the key, leading to rationally devised improvements. Our calculations predict the catalysts with the lowest Δ<i>G</i><sup>⧧</sup> for the O–O coupling step to be [Ru­(bda)­(4-I-py)<sub>2</sub>] and [Ru­(bda)­(6,7-(OMe)<sub>2</sub>-isq)<sub>2</sub>] for the pyridine and isoquinoline families, respectively. Our experimental results corroborate these predictions: the turnover frequency for [Ru­(bda)­(4-I-py)<sub>2</sub>] (330 s<sup>–1</sup>) is a 10-fold enhancement with respect to that of [Ru­(bda)­(py)<sub>2</sub>], and the turnover frequency for [Ru­(bda)­(6-OMe-isq)<sub>2</sub>] reaches 1270 s<sup>–1</sup>, two times faster than [Ru­(bda)­(isq)<sub>2</sub>]

    Inverse Kinetic Isotope Effect in the Excited-State Relaxation of a Ru(II)–Aquo Complex: Revealing the Impact of Hydrogen-Bond Dynamics on Nonradiative Decay

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    Photophysics of the MLCT excited-state of [Ru­(bpy)­(tpy)­(OH<sub>2</sub>)]<sup>2+</sup> (<b>1</b>) and [Ru­(bpy)­(tpy)­(OD<sub>2</sub>)]<sup>2+</sup> (<b>2</b>) (bpy = 2,2′-bipyridine and tpy = 2,2′:6′,2″-terpyridine) have been investigated in room-temperature H<sub>2</sub>O and D<sub>2</sub>O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the <sup>3</sup>MLCT state that initiates H-bond dynamics with the solvent and the <sup>3</sup>MC state that serves as the principal pathway for nonradiative decay

    Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine–Dicarboxylate Complexes

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    In order to gain a deeper mechanistic understanding of water oxidation by [(bda)­Ru­(L)<sub>2</sub>] catalysts (bdaH<sub>2</sub> = [2,2′-bipyridine]-6,6′-dicarboxylic acid; L = pyridine-type ligand), a series of modified catalysts with one and two trifluoromethyl groups in the 4 position of the bda<sup>2–</sup> ligand was synthesized and studied using stopped-flow kinetics. The additional −CF<sub>3</sub> groups increased the oxidation potentials for the catalysts and enhanced the rate of electrocatalytic water oxidation at low pH. Stopped-flow measurements of cerium­(IV)-driven water oxidation at pH 1 revealed two distinct kinetic regimes depending on catalyst concentration. At relatively high catalyst concentration (ca. ≥10<sup>–4</sup> M), the rate-determining step (RDS) was a proton-coupled oxidation of the catalyst by cerium­(IV) with direct kinetic isotope effects (KIE > 1). At low catalyst concentration (ca. ≤10<sup>–6</sup> M), the RDS was a bimolecular step with <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> ≈ 0.8. The results support a catalytic mechanism involving coupling of two catalyst molecules. The rate constants for both RDSs were determined for all six catalysts studied. The presence of −CF<sub>3</sub> groups had inverse effects on the two steps, with the oxidation step being fastest for the unsubstituted complexes and the bimolecular step being faster for the most electron-deficient complexes. Though the axial ligands studied here did not significantly affect the oxidation potentials of the catalysts, the nature of the ligand was found to be important not only in the bimolecular step but also in facilitating electron transfer from the metal center to the sacrificial oxidant

    Electrocatalysis on Oxide-Stabilized, High-Surface Area Carbon Electrodes

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    A procedure is described for preparing and derivatizing novel, high surface area electrodes consisting of thin layers of nanostructured ITO (Sn­(IV)-doped indium tin oxide, <i>nano</i>ITO) on reticulated vitreous carbon (RVC) to give RVC|<i>nano</i>ITO. The resulting hybrid electrodes are highly stabilized oxidatively. They were surface-derivatized by phosphonate binding of the electrocatalyst, [Ru­(Mebimpy)­(4,4′-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpy)­(OH<sub>2</sub>)]<sup>2+</sup> (Mebimpy = 2,6-bis­(1-methylbenzimidazol-2-yl)­pyridine; bpy = 2,2′-bipyridine) (<b>1-PO</b><sub><b>3</b></sub><b>H</b><sub><b>2</b></sub>) to give RVC|<i>nano</i>ITO-Ru<sup>II</sup>-OH<sub>2</sub><sup>2+</sup>. The redox properties of the catalyst are retained on the electrode surface. Electrocatalytic oxidation of benzyl alcohol to benzaldehyde occurs with a 75% Faradaic efficiency compared to 57% on <i>nano</i>ITO. Electrocatalytic water oxidation at 1.4 V vs SCE on derivatized RVC|<i>nano</i>ITO electrode with an internal surface area of 19.5 cm<sup>2</sup> produced 7.3 μmoles of O<sub>2</sub> in 70% Faradaic yield in 50 min

    Nonaqueous Electrocatalytic Oxidation of the Alkylaromatic Ethylbenzene by a Surface Bound Ru<sup>V</sup>(O) Catalyst

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    The catalyst [Ru­(Mebimpy)­(4,4′-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpy­(OH<sub>2</sub>)]<sup>2+</sup>, where Mebimpy is 2,6-bis­(1-methylbenzimidazol-2-yl)­pyridine and 4,4′-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine, attached to nanocrystalline Sn­(IV)-doped In<sub>2</sub>O<sub>3</sub> (nanoITO) electrodes (nanoITO|Ru<sup>II</sup>–OH<sub>2</sub><sup>2+</sup>) has been utilized for the electrocatalytic oxidation of the alkylaromatics ethylbenzene, toluene, and cumene in propylene carbonate/water mixtures. Oxidative activation of the surface site to nanoITO|Ru<sup>V</sup>(O)<sup>3+</sup> is followed by hydrocarbon oxidation at the surface with a rate constant of 2.5 ± 0.2 M<sup>–1</sup> s<sup>–1</sup> (<i>I</i> = 0.1 M LiClO<sub>4</sub>, <i>T</i> = 23 ± 2 °C) for the oxidation of ethylbenzene. Electrocatalytic oxidation of ethylbenzene to acetophenone occurs with a faradic efficiency of 95%. H/D kinetic isotope effects determined for oxidation of ethylbenzene point to a mechanism involving oxygen atom insertion into a C–H bond of ethylbenzene followed by further 2e<sup>–</sup>/2H<sup>+</sup> oxidation to acetophenone

    Lability and Basicity of Bipyridine-Carboxylate-Phosphonate Ligand Accelerate Single-Site Water Oxidation by Ruthenium-Based Molecular Catalysts

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    A critical step in creating an artificial photosynthesis system for energy storage is designing catalysts that can thrive in an assembled device. Single-site catalysts have an advantage over bimolecular catalysts because they remain effective when immobilized. Hybrid water oxidation catalysts described here, combining the features of single-site bis-phosphonate catalysts and fast bimolecular bis-carboxylate catalysts, have reached turnover frequencies over 100 s<sup>–1</sup>, faster than both related catalysts under identical conditions. The new [(bpHc)­Ru­(L)<sub>2</sub>] (bpH<sub>2</sub>cH = 2,2′-bipyridine-6-phosphonic acid-6′-carboxylic acid, L = 4-picoline or isoquinoline) catalysts proceed through a single-site water nucleophilic attack pathway. The pendant phosphonate base mediates O–O bond formation via intramolecular atom-proton transfer with a calculated barrier of only 9.1 kcal/mol. Additionally, the labile carboxylate group allows water to bind early in the catalytic cycle, allowing intramolecular proton-coupled electron transfer to lower the potentials for oxidation steps and catalysis. That a single-site catalyst can be this fast lends credence to the possibility that the oxygen evolving complex adopts a similar mechanism

    Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine–Dicarboxylate Complexes

    No full text
    In order to gain a deeper mechanistic understanding of water oxidation by [(bda)­Ru­(L)<sub>2</sub>] catalysts (bdaH<sub>2</sub> = [2,2′-bipyridine]-6,6′-dicarboxylic acid; L = pyridine-type ligand), a series of modified catalysts with one and two trifluoromethyl groups in the 4 position of the bda<sup>2–</sup> ligand was synthesized and studied using stopped-flow kinetics. The additional −CF<sub>3</sub> groups increased the oxidation potentials for the catalysts and enhanced the rate of electrocatalytic water oxidation at low pH. Stopped-flow measurements of cerium­(IV)-driven water oxidation at pH 1 revealed two distinct kinetic regimes depending on catalyst concentration. At relatively high catalyst concentration (ca. ≥10<sup>–4</sup> M), the rate-determining step (RDS) was a proton-coupled oxidation of the catalyst by cerium­(IV) with direct kinetic isotope effects (KIE > 1). At low catalyst concentration (ca. ≤10<sup>–6</sup> M), the RDS was a bimolecular step with <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> ≈ 0.8. The results support a catalytic mechanism involving coupling of two catalyst molecules. The rate constants for both RDSs were determined for all six catalysts studied. The presence of −CF<sub>3</sub> groups had inverse effects on the two steps, with the oxidation step being fastest for the unsubstituted complexes and the bimolecular step being faster for the most electron-deficient complexes. Though the axial ligands studied here did not significantly affect the oxidation potentials of the catalysts, the nature of the ligand was found to be important not only in the bimolecular step but also in facilitating electron transfer from the metal center to the sacrificial oxidant

    Selective Electrocatalytic Oxidation of a Re–Methyl Complex to Methanol by a Surface-Bound Ru<sup>II</sup> Polypyridyl Catalyst

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    The complex [Ru­(Mebimpy)­(4,4′-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpy)­(OH<sub>2</sub>)]<sup>2+</sup> surface bound to tin-doped indium oxide mesoporous nanoparticle film electrodes (nanoITO-Ru<sup>II</sup>(OH<sub>2</sub>)<sup>2+</sup>) is an electrocatalyst for the selective oxidation of methylrhenium trioxide (MTO) to methanol in acidic aqueous solution. Oxidative activation of the catalyst to nanoITO-Ru<sup>IV</sup>(OH)<sup>3+</sup> induces oxidation of MTO. The reaction is first order in MTO with rate saturation observed at [MTO] > 12 mM with a limiting rate constant of <i>k</i> = 25 s<sup>–1.</sup> Methanol is formed selectively in 87% Faradaic yield in controlled potential electrolyses at 1.3 V vs NHE. At higher potentials, oxidation of MTO by nanoITO-Ru<sup>V</sup>(O)<sup>3+</sup> leads to multiple electrolysis products. The results of an electrochemical kinetics study point to a mechanism in which surface oxidation to nanoITO-Ru<sup>IV</sup>(OH)<sup>3+</sup> is followed by direct insertion into the rhenium–methyl bond of MTO with a detectable intermediate

    Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis

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    Catalytic O<sub>2</sub> evolution with <i>cis</i>,<i>cis</i>-[(bpy)<sub>2</sub>(H<sub>2</sub>O)­Ru<sup>III</sup>ORu<sup>III</sup>(OH<sub>2</sub>)­(bpy)<sub>2</sub>]<sup>4+</sup> (bpy is 2,2-bipyridine), the so-called blue dimer, the first designed water oxidation catalyst, was monitored by UV–vis, EPR, and X-ray absorption spectroscopy (XAS) with ms time resolution. Two processes were identified, one of which occurs on a time scale of 100 ms to a few seconds and results in oxidation of the catalyst with the formation of an intermediate, here termed [3,4]′. A slower process occurring on the time scale of minutes results in the decay of this intermediate and O<sub>2</sub> evolution. Spectroscopic data suggest that within the fast process there is a short-lived transient intermediate, which is a precursor of [3,4]′. When excess oxidant was used, a highly oxidized form of the blue dimer [4,5] was spectroscopically resolved within the time frame of the fast process. Its structure and electronic state were confirmed by EPR and XAS. As reported earlier, the [3,4]′ intermediate likely results from reaction of [4,5] with water. While it is generated under strongly oxidizing conditions, it does not display oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS analysis demonstrates a considerably modified ligand environment in [3,4]′. Raman measurements confirmed the presence of the O–O fragment by detecting a new vibration band in [3,4]′ that undergoes a 46 cm<sup>–1</sup> shift to lower energy upon <sup>16</sup>O/<sup>18</sup>O exchange. Under the conditions of the experiment at pH 1, the [3,4]′ intermediate is the catalytic steady state form of the blue dimer catalyst, suggesting that its oxidation is the rate-limiting step
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