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Proton-coupled electron transfer at modified electrodes by multiple pathways.

ABSTRACT: In single site water or hydrocarbon oxidation catalysis with polypyridyl Ru complexes such as [Ru(II)(Mebimpy)(bpy)(H(2)O)](2+) [where bpy is 2,2'-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog [Ru(II)(Mebimpy)(4,4'-bis-methlylenephosphonato-2,2'-bipyridine)(OH(2))](2+) 2-PO(3)H(2), accessing the reactive states, Ru(V) = O(3+)/Ru(IV) = O(2+), 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(2+) or forms of 2-PO(3)H(2) to Ru(IV) = O(2+) 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.

SUBMITTER: Chen Z

PROVIDER: S-EPMC3248485 | biostudies-literature | 2011 Dec

REPOSITORIES: biostudies-literature

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Proton-coupled electron transfer at modified electrodes by multiple pathways.

Chen Zuofeng Z Vannucci Aaron K AK Concepcion Javier J JJ Jurss Jonah W JW Meyer Thomas J TJ

Proceedings of the National Academy of Sciences of the United States of America 20111212 52

In single site water or hydrocarbon oxidation catalysis with polypyridyl Ru complexes such as [Ru(II)(Mebimpy)(bpy)(H(2)O)](2+) [where bpy is 2,2'-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog [Ru(II)(Mebimpy)(4,4'-bis-methlylenephosphonato-2,2'-bipyridine)(OH(2))](2+) 2-PO(3)H(2), accessing the reactive states, Ru(V) = O(3+)/Ru(IV) = O(2+), at the electrode interface is typically rate limiting. The higher oxidation states are accessible by ...[more]

PMID: 22160681

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Project description:Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BI2P) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The set of TPAs spans more than 6 pK a units. These compounds have been designed to explore the role of the bridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular proton translocation across either two (the BIP series) or three (the BI2P series) acid/base sites. These molecular constructs feature an electrochemically active phenol connected to the TPA group through a benzimidazole-based bridge, which together with the phenol and TPA group form a covalent framework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistry demonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogen-bonded network to a TPA group. The experimental data show the benzimidazole bridges are non-innocent participants in the PCET process in that the addition of each benzimidazole unit lowers the redox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPA group. Using a series of hypothetical thermodynamic steps, density functional theory calculations correctly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on the nature of the final protonated species and provided insight into the thermodynamic role of dibenzimidazole units in the PCET process. This information is crucial for developing molecular "dry proton wires" with these moieties, which can transfer protons via a Grotthuss-type mechanism over long distances without the intervention of water molecules.

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Proton-coupled electron transfer at modified electrodes by multiple pathways.

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Proton-coupled electron transfer at modified electrodes by multiple pathways.

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