5 research outputs found
Linear Free Energy Relationships in the Hydrogen Evolution Reaction: Kinetic Analysis of a Cobaloxime Catalyst
Kinetic
analysis of hydrogen production catalyzed by CoÂ(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 stepî—¸assigned
to protonation of the doubly reduced, singly protonated speciesî—¸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 logÂ(<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
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 Oî—¸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 Oî—¸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
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 Oî—¸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 Oî—¸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
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 Oî—¸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 Oî—¸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
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 Oî—¸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 Oî—¸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