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Insights into proton-coupled electron transfer mechanisms of electrocatalytic H2 oxidation and production.

ABSTRACT: The design of molecular electrocatalysts for H(2) oxidation and production is important for the development of alternative renewable energy sources that are abundant, inexpensive, and environmentally benign. Recently, nickel-based molecular electrocatalysts with pendant amines that act as proton relays for the nickel center were shown to effectively catalyze H(2) oxidation and production. We developed a quantum mechanical approach for studying proton-coupled electron transfer processes in these types of molecular electrocatalysts. This theoretical approach is applied to a nickel-based catalyst in which phosphorous atoms are directly bonded to the nickel center, and nitrogen atoms of the ligand rings act as proton relays. The catalytic step of interest involves electron transfer between the nickel complex and the electrode as well as intramolecular proton transfer between the nickel and nitrogen atoms. This process can occur sequentially, with either the electron or proton transferring first, or concertedly, with the electron and proton transferring simultaneously without a stable intermediate. The electrochemical rate constants are calculated as functions of overpotential for the concerted electron-proton transfer reaction and the two electron transfer reactions in the sequential mechanisms. Our calculations illustrate that the concerted electron-proton transfer standard rate constant will increase as the equilibrium distance between the nickel and nitrogen atoms decreases and as the pendant amines become more flexible to facilitate the contraction of this distance with a lower energy penalty. This approach identifies the favored mechanisms under various experimental conditions and provides insight into the impact of substituents on the nitrogen and phosphorous atoms.

SUBMITTER: Horvath S

PROVIDER: S-EPMC3465444 | biostudies-literature | 2012 Sep

REPOSITORIES: biostudies-literature

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Insights into proton-coupled electron transfer mechanisms of electrocatalytic H2 oxidation and production.

Horvath Samantha S Fernandez Laura E LE Soudackov Alexander V AV Hammes-Schiffer Sharon S

Proceedings of the National Academy of Sciences of the United States of America 20120423 39

The design of molecular electrocatalysts for H(2) oxidation and production is important for the development of alternative renewable energy sources that are abundant, inexpensive, and environmentally benign. Recently, nickel-based molecular electrocatalysts with pendant amines that act as proton relays for the nickel center were shown to effectively catalyze H(2) oxidation and production. We developed a quantum mechanical approach for studying proton-coupled electron transfer processes in these ...[more]

PMID: 22529352

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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.

Project description:Redox events in which an electron and proton are exchanged in a concerted elementary step are commonly referred to as proton-coupled electron transfers (PCETs). PCETs are known to operate in numerous important biological redox processes, as well as recent inorganic technologies for small molecule activation. These studies suggest that PCET catalysis might also function as a general mode of substrate activation in organic synthesis. Over the past three years, our group has worked to advance this hypothesis and to demonstrate the synthetic utility of PCET through the development of novel catalytic radical chemistries. The central aim of these efforts has been to demonstrate the ability of PCET to homolytically activate a wide variety of common organic functional groups that are energetically inaccessible using known molecular H atom transfer catalysts. To do so, we made use of a simple formalism first introduced by Mayer and co-workers that allowed us to predict the thermodynamic capacity of any oxidant/base or reductant/acid pair to formally add or remove H· from a given substrate. With this insight, we were able to rationally select catalyst combinations thermodynamically competent to homolyze the extraordinarily strong E-H σ-bonds found in many common protic functional groups (BDFEs > 100 kcal/mol) or to form unusually weak bonds to hydrogen via the reductive action of common organic π-systems (BDFEs < 35 kcal/mol). These ideas were reduced to practice through the development of new catalyst systems for reductive PCET activations of ketones and oxidative PCET activation of amide N-H bonds to directly furnish reactive ketyl and amidyl radicals, respectively. In both systems, the reaction outcomes were found to be successfully predicted using the effective bond strength formalism, suggesting that these simple thermochemical considerations can provide useful and actionable insights into PCET reaction design. The ability of PCET catalysis to control enantioselectivity in free radical processes has also been established. Specifically, multisite PCET requires the formation of a pre-equilibrium hydrogen bond between the substrate and a proton donor/acceptor prior to charge transfer. We recognized that these H-bond interfaces persist following the PCET event, resulting in the formation of noncovalent complexes of the nascent radical intermediates. When chiral proton donors/acceptors are employed, this association can provide a basis for asymmetric induction in subsequent bond-forming steps. We discuss our efforts to capitalize on this understanding via the development of a catalytic protocol for enantioselective aza-pinacol cyclizations. Lastly, we highlight an alternative PCET mechanism that exploits the ability of redox-active metals to homolytically weaken the bonds in coordinated ligands, enabling nominally strong bonds (BDFEs ∼ 100 kcal) to be abstracted by weak H atom acceptors with concomitant oxidation of the metal center. This "soft homolysis" mechanism enables the generation of metalated intermediates from protic substrates under completely neutral conditions. The first example of this form of catalysis is presented in the context of a catalytic C-N bond forming reaction jointly mediated by bulky titanocene complexes and the stable nitroxyl radical TEMPO.

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Insights into proton-coupled electron transfer mechanisms of electrocatalytic H2 oxidation and production.

Publications

Insights into proton-coupled electron transfer mechanisms of electrocatalytic H2 oxidation and production.

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