Simple Outer-sphere Electron Transfer Research Articles

Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis.

Homogeneous electrochemical catalysis of N2O reduction to N2 is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N2O. Taking into account the previously estimated reorganization energy required to form the N2O radical anions leads to an estimation of the N2O/N2O˙− standard potential in acetonitrile electrolyte. The direct reduction of N2O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N2O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N2O and emphasizes the differences between chemical and redox catalytic processes.

Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium\u2013oxygen cell

Lithium–oxygen cells, in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide stored in the cathode. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for the operation of the lithium–oxygen cell. It is essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain fast charging. Here, we investigate the kinetics of lithium peroxide oxidation by several different classes of redox mediators. We show that the reaction is not a simple outer-sphere electron transfer and that the steric structure of the mediator molecule plays an important role. The fastest mediator studied could sustain a charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here.

Hemoglobin and Myoglobin as Reducing Agents in Biological Systems. Redox Reactions of Globins with Copper and Iron Salts and Complexes.

In addition to reversible O2 binding, respiratory proteins of the globin family, hemoglobin (Hb) and myoglobin (Mb), participate in redox reactions with various metal complexes, including biologically significant ones, such as those of copper and iron. HbO2 and MbO2 are present in cells in large amounts and, as redox agents, can contribute to maintaining cell redox state and resisting oxidative stress. Divalent copper complexes with high redox potentials (E0, 200-600mV) and high stability constants, such as [Cu(phen)2]2+, [Cu(dmphen)2]2+, and CuDTA oxidize ferrous heme proteins by the simple outer-sphere electron transfer mechanism through overlapping π-orbitals of the heme and the copper complex. Weaker oxidants, such as Cu2+, CuEDTA, CuNTA, CuCit, CuATP, and CuHis (E0 ≤ 100-150mV) react with HbO2 and MbO2 through preliminary binding to the protein with substitution of the metal ligands with protein groups and subsequent intramolecular electron transfer in the complex (the site-specific outer-sphere electron transfer mechanism). Oxidation of HbO2 and MbO2 by potassium ferricyanide and Fe(3) complexes with NTA, EDTA, CDTA, ATP, 2,3-DPG, citrate, and pyrophosphate PPi proceeds mainly through the simple outer-sphere electron transfer mechanism via the exposed heme edge. According to Marcus theory, the rate of this reaction correlates with the difference in redox potentials of the reagents and their self-exchange rates. For charged reagents, the reaction may be preceded by their nonspecific binding to the protein due to electrostatic interactions. The reactions of LbO2 with carboxylate Fe complexes, unlike its reactions with ferricyanide, occur via the site-specific outer-sphere electron transfer mechanism, even though the same reagents oxidize structurally similar MbO2 and cytochrome b5 via the simple outer-sphere electron transfer mechanism. Of particular biological interest is HbO2 and MbO2 transformation into met-forms in the presence of small amounts of metal ions or complexes (catalysis), which, until recently, had been demonstrated only for copper compounds with intermediate redox potentials. The main contribution to the reaction rate comes from copper binding to the "inner" histidines, His97 (0.66nm from the heme) that forms a hydrogen bond with the heme propionate COO- group, and the distal His64. The affinity of both histidines for copper is much lower than that of the surface histidines residues, and they are inaccessible for modification with chemical reagents. However, it was found recently that the high-potential Fe(3) complex, potassium ferricyanide (400mV), at a 5 to 20% of molar protein concentration can be an efficient catalyst of MbO2 oxidation into metMb. The catalytic process includes binding of ferrocyanide anion in the region of the His119 residue due to the presence there of a large positive local electrostatic potential and existence of a "pocket" formed by Lys16, Ala19, Asp20, and Arg118 that is sufficient to accommodate [Fe(CN)6]4-. Fast, proton-assisted reoxidation of the bound ferrocyanide by oxygen (which is required for completion of the catalytic cycle), unlike slow [Fe(CN)6]4- oxidation in solution, is provided by the optimal location of neighboring protonated His113 and His116, as it occurs in the enzyme active site.

A kinetic study of the oxidations of 2-mercaptoethanol and 2-mercaptoethylamine by heteropoly 11-tungsto-1- vanadophosphate in aqueous acidic medium

The kinetics of oxidation of 2-mercaptoethanol and 2-mercaptoethylamine by the heteropoly 11-tungsto-1- vanadophosphate anion, [PVVW11O40]4−, have been studied spectrophotometrically in aqueous perchloric acid at 25 °C. EPR and optical studies show that [PVVW11O40]4− is reduced to the one-electron reduced heteropoly blue, [PVIVW11O40]5−, whilst the thiols are oxidized to the corresponding disulphides, RSSR. Spectrophotometric titrations show that the stoichiometry of both reactions is 1:1. At constant pH, the reactions show simple second-order kinetics with first-order dependence of rate on both [oxidant] and [thiol]. At constant [thiol], the rate of the reaction increases with increasing pH. Plots of kobs/[thiol]t versus 1/[H+] are linear with finite intercepts, showing that both the undissociated thiol (RSH) and thiolate ion (RS−) are reactive species. Generation of RS· from RSH proceeds via a separated-concerted proton–electron transfer mechanism. The reaction of thiolate ion is a simple outer-sphere electron transfer reaction. By applying the Marcus theory, the self-exchange rate constants for the couples HOCH2CH2S·/HOCH2CH2S− and H3N+CH2CH2S·/H3N+CH2CH2S− were evaluated as 3 × 109 and 2.2 × 108 M−1 s−1, respectively, at 25 °C.

In Situ Kinetics Studies in All-Vanadium Redox Flow Batteries

We report results of polarization measurements resolved for the negative and positive electrodes of vanadium redox batteries (VRBs) using a dynamic hydrogen electrode in an operating battery cell. Electrochemical experiments with symmetric electrolyte feeds were also performed. Greater kinetic polarization is observed at the negative (V3/2+) electrode compared to the positive electrode (V5/4+), in contrast with previously reported ex situ measurements. For the positive electrode, the polarization in the low-current regime was modest and was not kinetically controlled. The relative rates of reaction are a surprise since it might be expected that the V3/2+ redox reaction is a simple outer-sphere electron transfer.

Concerted Proton–Electron Transfers. Consistency between Electrochemical Kinetics and their Homogeneous Counterparts.

The concerted proton-electron transfer (CPET) oxidation of phenol with water (in water) and hydrogen phosphate as proton acceptors provides a good example for testing the consistency of the electrochemical and homogeneous approaches to a reaction, the comprehension of which raises more mechanistic and kinetic challenges than that of a simple outer-sphere electron transfer. Comparison of the intrinsic kinetic characteristics (obtained at zero driving force of the CPET reaction) shows that consistency is indeed observed after a careful identification and quantitation of side factors (electrical work terms, image force effects). Water (in water) appears as a better intrinsic proton acceptor than hydrogen phosphate in both cases in terms of reorganization energy and pre-exponential factor, corroborating the mechanism by which electron transfer is concerted with Grotthus-type proton translocation in water. Detailed compared analysis of the approaches also revealed that modest but significant electric field effects may be at work in the electrochemical case. Comparison with phenoxide ion oxidation, taken as a reference outer-sphere electron transfer, points to a CPET precursor complex that possesses a precise spatial structure allowing the formation of one or several H-bonds as required by the occurrence of the CPET reaction, thus decreasing considerably the number of efficient collisions compared with those undergone by structureless spherical reactants.

Influence of Organic Solvents on the Kinetics of Electron Transfer and the Adsorption at Highly Oriented Pyrolytic Graphite

Acetonitrile and several other common organic solvents have been found to dramatically influence the kinetics of electron transfer and the adsorption at freshly cleaved highly oriented pyrolytic graphite (HOPG). Ferricyanide and anthraquinone-2,6-disulfonate were used to investigate the effect of several pretreatment procedures including cleaving, electrochemical pretreatment, and thermal etching on the activity of basal plane HOPG. The results of this work suggest that although active sites produced by cleaving are relatively stable in air and in aqueous solution, exposure to organic solvents quickly and irreversibly passivates them toward simple outer-sphere electron transfer and adsorption. In contrast, electrochemically pretreated and thermally etched HOPG is much less sensitive to organic solvents, suggesting that the natures of the active sites produced by these methods are significantly different. As a result, it is important to take into account this behavior when using freshly cleaved HOPG in surface and electrochemical studies.

“Thermal” SRN1 Reactions: How Do They Work? Novel Evidence that the Driving Force Controls the Transition between Stepwise and Concerted Mechanisms in Dissociative Electron Transfers

In SRN1 reactions, unlike in conventional nucleophilic substitutions, the nucleophile does not react directly with the electrophile but with a radical resulting from its reductive cleavage. Many SRN1 substitutions require an external stimulation involving the injection of a catalytic amount of electrons. In “thermal” SRN1 reactions, there is no other source of initiating electrons than the nucleophile which is usually a poor electron donor. Such reactions are unlikely to be initiated by a simple outersphere electron transfer from the nucleophile followed by the cleavage of the substrate anion radical. Rather, initiation follows a mechanism in which electron transfer and bond cleavage are concerted. These conclusions are based on a full analysis of a model system involving 4-nitrocumyl chloride as the substrate and the 2-nitropropanate ion as the nucleophile where all the pertinent thermodynamic and kinetic parameters were determined by direct or indirect electrochemical methods. They extend to other examp...

THE KINETICS OF ELECTRON TRANSFER FOR A SERIES OF IRON(II) CYCLIDENE COMPLEXES

Abstract The kinetics of oxidation of a series of iron(II) cyclidene complexes by tris(1,10-phenanthroline)cobalt(III) in methanol have been measured by stopped flow spectrophotometry. The reactions obey a first order rate law when the cobalt(III) complex is present in large excess. The corresponding second order rate constants fall in the range 5.0–130 × 105 M−1s−1 (25°C). A linear correlation between the logarithm of the rate constant and the iron(III)/(II) redox potential indicates that the reactions behave as simple outer-sphere electron transfer processes. The self-exchange rate constants for the iron cyclidene complexes have been estimated from the Marcus equation and found to vary between 0.7 × 107 M1s1 and 9 × 107 M1s1. The dependence of the self-exchange rate constant on ligand structure is discussed.

Homogeneous Electron Transfer Catalysis of the Electrochemical Reduction of Carbon Dioxide. Do Aromatic Anion Radicals React in an Outer-Sphere Manner?

Electrochemically generated anion radicals of aromatic nitriles and esters possess the remarkable property to reduce carbon dioxide to oxalate with negligible formation of carboxylated products. They may thus serve as selective homogeneous catalysts for the reduction of CO2 in an aprotic medium. The catalytic enhancement of the cyclic voltammetric peaks of these catalysts is used to determine the rate constant of the electron transfer from these aromatic anion radicals to CO2 as a function of the catalyst standard potential. Substituted benzoic esters allowed a particularly detailed investigation of the resulting activation-driving force relationship. Using 14 different catalysts in this series made it possible to finely scan a range of reaction standard free energies of 0.4 eV. Detailed analysis of the resulting data leads to the conclusion that the reaction is not a simple outer-sphere electron transfer. It rather consists in a nucleophilic addition of the anion radical on CO2, forming an oxygen (or nit...

Theoretical Models of Hot Carrier Effects at Metal‐Semiconductor Electrodes

The physical and electrochemical properties of hot carriers at metallized semiconductor electrodes in contact with a redox electrolyte are described. Theoretical ballistic mean‐free paths, , for hot carriers in Ag, Cu, Au, Al, Ru, Pd, and Pt, were calculated as a function of excess energy above the Fermi energy. At 0.5 eV hot, was 580 Å for Ag, 420 Å for Cu and Au, 90 to 130 Å for the remaining metals, with Pt having the lowest value. Using the Marcus‐Gerischer model, currents at the equilibrium potential were calculated as a function of built‐in barrier, film thickness, and reorganization energy for Au and Pt. The current for a simple outer sphere electron transfer reaction is predicted to be enhanced when the film thickness becomes . For Au/n‐Si diodes, the enhancement for reorganization energy, , 100 Å Au and built‐in barriers of 0.4 to 0.6 eV are predicted to be 60 to 10 times. Larger enhancements are predicted for larger reorganization energies, the maximum effect being predicted for . The stage most likely to limit observation of hot carriers effects is slow vibrational or vibrational‐rotational relaxation of the redox acceptor or donor. By a combination of theoretical calculations and empirical data, we conclude that it is possible to observe hot carrier effects under real conditions. Experimental data that support the theoretical development and the hot carriers concept are presented in the following paper.

Heterogeneous and homogeneous redox catalysed reduction of some sulfilimines

The homogeneous and electrode two-electron reduction of some sulfonylsfulfilimines was investigated in order to obtain more quantitative kinetic and thermodynamic information.The two processes were analysed on the basis of homogeneous redox catalysis and voltammetric results, respectively. Both reactions were shown, in most cases, to be controlled by the first endergonic electron uptake. The kinetic data for the process were interpreted on the basis of simple theories for electron-transfer reactions, allowing intrinsic thermodynamic and activation parameters of the charge-transfer steps to be estimated. The results were found to be more consistent with the hypothesis of a simple outer-sphere electron transfer followed by a fast first-order reaction rather than with a concerted bond breaking-charge transfer step. No relevant differences were observed owing to the replacement of ethyl with phenyl groups in the sulfilimine moiety, the substitution effect being apparent only from a thermodynamic point of view. On the contrary, the introduction of a CN group in the sulfonyl moiety increased sensibly the stability of the intermediate radical anion. This caused the electrode process kinetics to be partially dependent on the rdaical anion decay, allowing the quantitative evaluation of its lifetime.

Ionic strength effect on the ascorbate reduction of sperm whale and horse heart metmyoglobins

Kinetics of the reduction of sperm whale skeletal muscle and horse heart metmyoglobins (metMb) by ascorbic acid (H 2A) were studied in a nitrogen atmosphere at 25 °C, at different ionic strengths (NaCl) and between pH 7.29 and 8.29. The second order rate constants were determined for the reductions of metMb(H 2O) by ascorbate (HA − and A 2−). It was found that the ascorbate reduction is faster for horse heart metMb than for sperm whale metMb at a higher ionic strength, and that at a lower ionic strength similar reactivities are observed. The logarithm of the rate constants correlates to the square root of the ionic strength. This is explained by a simple outer-sphere electron transfer mechanism. It was found that the effective charge of sperm whale metMb is higher than that of horse heart metMb and that there is no specific interaction of ascorbate with metMb.

Kinetics of electrocatalysis of dibromoalkyl reductions using electrodes with covalently immobilized metallotetraphenylporphyrins

Abstract : The reduction of PhCHBrCH2Br, PhCHBrCHBrPh, and CH2BrCHBrCH3 at the surfaces of electrodes to which cobalt (II) or copper (II) tetra(p-aminophenyl)porphyrin has been covalently attached is strongly catalyzed by reduction of the metalloporphyrin. The rate of electrocatalytic reduction was measured using rotated disk electrode voltammetry, and was independent of the amount of metalloporphyrin on the electrode above an estimated monomolecular coverage level. The results are consistent with theory which assumes that the rate of diffusion of electrochemical charge through the porphyrin layer is faster than the rate of diffusion of catalytic substrate through the layer. Comparison of the electrocatalytic rates for the different substrates indicates the electron transfer mediation involves specific interactions between substrate and metalloporphyrin rather than being a simple outer sphere electron transfer event. Potential step chronoamperometry is introduced as an alternative method for electrocatalytic measurements at modified electrodes. (Author)

Oxidation of heme proteins by copper(II) complexes. Rates and mechanism of the copper catalysed autoxidation of cytochrome c, myoglobin and hemoglobin

The mechanism of the autoxidation of ferrocytochrome c catalysed by some copper(II) complexes has been estblished. In most cases the rate determining step was found to be the one electron oxidation of the cytochrome by the copper(II) species, which was followed by the rapid reoxidation of the resulting Cu(I) complexes by molecular oxygen. The rate constants for the electron transfer reactions of several copper(II) complexes with ferrocytochrome c, myoglobin and hemoglobin have been measured at 25 °C. It was shown that these rates for the copper complexes, as well as those for several other oxidants, are consistent with simple outer sphere electron transfer through the heme edge for all the heme proteins. the redox potential of the reacting species was found to be the dominant factor in determining the relative rate of the electron transfer reactions. This enables the catalytic activity of different copper complexes to be predicted.

Contribution of ionic effects to electron transfer reactions

This paper reports an initial effort to combine the theory of ionic conductivity, due to Friedman, with the theory of simple outer sphere electron transfer reactions. In this work the ions, which do not participate in the electron transfer transitions, are considered individually. The solvent is considered, as elsewhere, as a dielectric continuum. The calculations are carried out for extreme ionic dilution, a Debye–Huckel limit. The purpose of this work is to attempt to provide a more detailed account of the effect of ions present in the solution on the reaction rate. The effect which arises manifests itself through the reaction rate constant expression. Specifically, we find that as a result of the interaction between the electron transfer system and the ions in solution, transport coefficients associated with ionic mobilities in solution appear in the rate constant expression. The nature of the ionic effect is similar to the effect found when one considers either damping of the electron transfer transition state or damping of the energy states of the polarization modes of the dielectric continuum.