5 research outputs found

    Linear Free Energy Relationships in the Hydrogen Evolution Reaction: Kinetic Analysis of a Cobaloxime Catalyst

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    Kinetic analysis of hydrogen production catalyzed by Co­(dmgBF<sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub> (dmgBF<sub>2</sub> = difluoroboryl-dimethylglyoxime) was performed in acetonitrile with a series of <i>para</i>-substituted anilinium acids. It was determined that the mechanism of hydrogen evolution is governed by three elementary steps; two are acid concentration and p<i>K</i><sub>a</sub> dependent, whereas the third was shown to be intrinsic to the catalyst, likely reflecting either H–H bond formation or H<sub>2</sub> release. The kinetics of the first proton transfer step, the protonation of the singly reduced catalyst, were evaluated using foot-of-the-wave analysis, as well as current–potential analysis for voltammograms displaying total catalysis behavior. Analysis of the total catalysis peak shift required the empirical determination of a new equation for the ECEC′ catalytic mechanism using digital simulations. The kinetics of the second proton transfer stepassigned to protonation of the doubly reduced, singly protonated speciesand the acid-independent step were determined by analyzing the plateau current of the catalytic wave over a range of acid concentrations. Both proton transfer steps follow linear free energy relationships of log­(<i>k</i>) vs acid p<i>K</i><sub>a</sub>. These linear relationships give slopes of −0.94 and −0.77 for the first and second proton transfers, respectively, indicating that both steps become faster with increasing acid strength

    Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>–</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

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    Facile and selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast as 10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup> under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O<sub>2</sub> reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O<sub>2</sub> to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup> while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst

    Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>–</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

    No full text
    Facile and selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast as 10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup> under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O<sub>2</sub> reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O<sub>2</sub> to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup> while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst

    Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>–</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

    No full text
    Facile and selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast as 10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup> under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O<sub>2</sub> reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O<sub>2</sub> to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup> while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst

    Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>–</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

    No full text
    Facile and selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e<sup>–</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast as 10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup> under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O<sub>2</sub> reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O<sub>2</sub> to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup> while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst
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