Mechanisms of Surface Electron Transfer. Proton-Coupled Electron Transfer

Trammell, Scott A.; Wimbish, John C.; Odobel, Fabrice; Gallagher, Laurie A.; Narula, Poonam M.; Meyer, Thomas J.

doi:10.1021/ja9821854

Cited by 74 publications

(112 citation statements)

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“…Electrocatalysis by deliberate introduction of PCET pathways at modified electrode surfaces has been reported (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). This study is important in extending solution reactivity, for example, toward water and hydrocarbon oxidation catalysis, to electrode interfaces in device configurations.…”

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confidence: 95%

“…The role of PCET in electrochemical reactivity has been discussed in a series of papers (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). In a cyclic voltammetry (CV) study on the Os-based couples cis-½Os IV ðbpyÞ 2 ðpyÞðOÞ 2þ ∕ cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ (where bpy is 2,2′-bipyridine) and cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ ∕cis-½Os II ðbpyÞ 2 ðpyÞðOH 2 Þ 2þ , Savéant and coworkers (7) concluded that oxidation of Os II Rate accelerations were observed with added proton acceptor bases, the Britton-Robinson buffer (phosphate, citrate, borate, and acetate), accompanied by the appearance of a solvent KIE of 2-2.5.…”

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“…For the surface-bound analog of ½Ru II ðtpyÞðbpyÞðOH 2 Þ 2þ (tpy is 2,2′:6′,2″-terpyridine) 1, ½Ru II ðtpyÞð4;4 0 -ðPO 3 H 2 CH 2 Þ 2 bpyÞ ðOH 2 Þ 2þ ð4;4 0 -ðPO 3 H 2 CH 2 Þ 2 bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine) 1-PO 3 H 2 , on planar Sn(IV)-doped In 2 O 3 (ITO) electrodes, a reversible Ru IV ¼O 2þ ∕Ru III -OH 2þ wave appears but only at high surface coverages (11). This observation was attributed to the surface coverage-dependent, cross-surface disproportionation mechanism in Scheme 1.…”

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Proton-coupled electron transfer at modified electrodes by multiple pathways

Chen

Vannucci

Concepcion

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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In single site water or hydrocarbon oxidation catalysis with polypyridyl Ru complexes such as ½Ru II ðMebimpyÞðbpyÞðH 2 OÞ 2þ [where bpy is 2,2′-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog ½Ru II ðMebimpyÞ ð4,4 0 -bis-methlylenephosphonato-2,2 0 -bipyridineÞðOH 2 Þ 2þ 2-PO 3 H 2 , accessing the reactive states, Ru V ¼O 3þ ∕Ru IV ¼O 2þ , at the electrode interface is typically rate limiting. The higher oxidation states are accessible by proton-coupled electron transfer oxidation of aqua precursors, but access at inert electrodes is kinetically inhibited. The inhibition arises from stepwise mechanisms which impose high energy barriers for 1e − intermediates. Oxidation of the Ru III -OH 2þ or Ru III -OH 2 3þ forms of 2-PO 3 H 2 to Ru IV ¼O 2þ on planar fluoride-doped SnO 2 electrode and in nanostructured films of Sn(IV)-doped In 2 O 3 and TiO 2 has been investigated with a focus on identifying microscopic phenomena. The results provide direct evidence for important roles for the nature of the electrode, temperature, surface coverage, added buffer base, pH, solvent, and solvent H 2 O∕D 2 O isotope effects. In the nonaqueous solvent, propylene carbonate, there is evidence for a role for surface-bound phosphonate groups as proton acceptors.concerted electron-proton transfer | metal oxide electrode | surface modification | electrocatalysis | spectroelectrochemistry E lectron transfer reactions involving pH-dependent protoncoupled electron transfer (PCET) couples with a change in proton content between oxidation states, such as quinone/hydroquinone (Q∕H 2 Q) or M¼O∕M-OH∕M-OH 2 oxo/hydroxo/aqua transition metal couples, are often slow at inert electrodes (1-5). For these couples, the change in protonation state and the requirement to add or lose protons adds to the normal kinetic barrier to electron transfer. The PCET effect arises from the fact that oxidation or reduction at the electrode occurs by electron transfer without a change in proton content. This effect restricts interfacial mechanisms to electron transfer followed by proton transfer (ET-PT) or proton transfer followed by electron transfer (PT-ET). Both involve high-energy intermediates in nonequilibrium protonation states.As an example, oxidation of H 2 Q, H 2 Q − e − → H 2 Q þ• , occurs at E o0 ¼ 1.10 V vs. normal hydrogen electrode (NHE) independent of pH (E o0 is the formal potential) (2). The thermodynamic potential for H 2 Q oxidation at pH 7,. An inert electrode at pH ¼ 7 creates an overpotential of 0.47 V for the initial electron transfer in the ET-PT sequence. Following electron transfer, thermodynamic equilibrium is reached by proton loss from H 2 Q þ• to the surrounding medium at the prevailing pH with ΔE o0 ¼ 1.10 V − 0.059fpH − ðpK a ðH 2 Q þ• Þg with pK a ðH 2 Q þ• Þ ¼ −0.95. Oxidation can also occur by PT-ET with initial loss of a proton from H 2 Q to give HQ − followed by oxidation to HQ• at E o0 ðHQ • ∕HQ − Þ ¼ 0.46 V. For this mechanism, an inhibition to rate arises from the pH depende...

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Proton-coupled electron transfer at modified electrodes by multiple pathways

Chen

Vannucci

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et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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“…Grafting sensitizers on the surface of TiO 2 with the phosphonic acid group has been reported earlier by us [4][5][6] and other groups. [7][8][9][10][11][12][13] Recently, we have demonstrated the significance of using an amphiphilic polypyridyl ruthenium sensitizer to achieve unprecedented stability in DSCs at elevated temperatures. 14 Also, self-assembling of ω-alkylphosphonic acids on a wide range of metal oxide surfaces has been of great scientific and practical interest due to the ability to form robust and compact monolayers.…”

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Amphiphilic Ruthenium Sensitizer with 4,4‘-Diphosphonic Acid-2,2‘-bipyridine as Anchoring Ligand for Nanocrystalline Dye Sensitized Solar Cells

et al. 2004

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A new amphiphilic polypyridyl ruthenium complex, cis-Ru(dpbpy)(dnbpy)(NCS) 2 (dpbpy ) 4,4′-diphosphonic acid-2,2′-bipyridine; dnbpy ) 4,4′-dinonyl-2,2′-bipyridine), was synthesized and demonstrated as an efficient sensitizer. The sensitizer was characterized by NMR, voltammetry, ATR-FTIR, resonance Raman, as well as electronic absorption and emission spectra. Nanocrystalline dye-sensitized solar cells (DSCs) with this new sensitizer generated g8.0% power conversion efficiencies under illumination with simulated air mass 1.5 sunlight. This is the first time such a high solar-to-electricity power conversion efficiency for DSCs has been reached by a sensitizer bearing non-carboxylic acid anchoring groups. Additionally, the devices exhibited excellent stability under light soaking at 55 to 60°C. Time-resolved nanosecond laser experiments revealed that the dye regeneration rate of this new sensitizer seems to be quite similar but the charge recombination is significantly slower in comparison to the analogues carboxylated sensitizer.

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“…Large k H ͞k D KIEs have been observed in other reactions, including H-abstraction by methyl radicals in low-temperature glasses (11), the photoenolization of ortho-methyl aryl ketones in rigid media (12), and in certain enzymatic reactions (13)(14)(15). For PCET reactions, values of up to 30 have been observed in solution (16) and Ͼ70 on an oxide surface (17).…”

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Colossal kinetic isotope effects in proton-coupled electron transfer

Huynh

Meyer

2004

Proc. Natl. Acad. Sci. U.S.A.

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P roton-coupled electron transfer (PCET) reactions are at the heart of important biological processes, including photosynthesis and respiration (1-4). These reactions avoid high-energy intermediates through the concerted transfer of an electron and a proton. PCET is distinguished from H-atom transfer (HAT) in that the transferring electron and proton come from different sites in the reducing agent rather than from a single chemical bond (5-10). Nuclear quantum effects often play an important role, as evidenced experimentally by the kinetic isotope effect (KIE), the ratio of rate constants for the hydrogen and deuterium forms of the reducing agent. Large k H ͞k D KIEs have been observed in other reactions, including H-abstraction by methyl radicals in low-temperature glasses (11), the photoenolization of ortho-methyl aryl ketones in rigid media (12), and in certain enzymatic reactions (13-15). For PCET reactions, values of up to 30 have been observed in solution (16) and Ͼ70 on an oxide surface (17).In earlier work, we described a PCET reaction with a k 1 (H 2 O)͞k 1 (D 2 O) KIE of 175 at room temperature based on the reduction of benzoquinone (Q) by an osmium complex containing a phosphorus-hydrogen bond (18). We report here values of 198 and 455 for PCET reactions from sulfur and nitrogenhydrogen bonds based on an extended analysis of reactions reported earlier in preliminary accounts. 3.0 ϫ 10 Ϫ4 M to 8.0 ϫ 10 Ϫ2 M. The temperature of solutions during the kinetic studies was maintained to within Ϯ0.1°C with use of Lauda RM6 and Thermo Neslab RTE 7 circulating water baths and monitored with an Omega HH-51 thermocouple probe. All rate constants cited in this work are reported as the averages of at least three or more independent experiments.Cyclic voltammetric experiments were measured with the use of Princeton Applied Research (PAR) models 263A and 273 potentiostats, and bulk electrolyses were performed with a PAR model 173 potentiostat͞galvanostat. All measurements were conducted in a three-compartment cell in 1:1 (vol͞vol) CH 3 CN͞ H 2 O with 1.0 M NH 4 PF 6 as the supporting electrolyte. A glassy carbon working electrode was used for aqueous measurements. All potentials are referenced to the saturated sodium chloride͞ calomel electrode (SSCE, 0.236 V vs. the normal hydrogen electrode) at room temperature and are uncorrected for junction potentials. In all cases, the auxiliary electrode was a platinum wire. The solution in the working compartment was deoxygenated by N 2 or argon bubbling. All redox couple potentials are the average of four independent measurements and are Ϯ2 mV. Results and DiscussionThe kinetics of the reactions shown in Eqs. 1-3 were studied in 1:1 (vol͞vol) CH 3 CN͞H 2 O mixtures at 25.0 Ϯ 0.1°C in 1.0 M NH 4 PF 6 ͞HPF 6 mixtures. The details of the reaction in Eq. 2 with phosphorus as the proton-donor atom were reported in ref. 18, and preliminary accounts with S and N were reported in refs. 19 and 20. The structures of the Os(V) nitrogen-based electronproton acceptor and Os(IV) sulfur-ba...

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Mechanisms of Surface Electron Transfer. Proton-Coupled Electron Transfer

Cited by 74 publications

References 10 publications

Proton-coupled electron transfer at modified electrodes by multiple pathways

Proton-coupled electron transfer at modified electrodes by multiple pathways

Amphiphilic Ruthenium Sensitizer with 4,4‘-Diphosphonic Acid-2,2‘-bipyridine as Anchoring Ligand for Nanocrystalline Dye Sensitized Solar Cells

Colossal kinetic isotope effects in proton-coupled electron transfer

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