Outer-sphere Electron-transfer Reactions Research Articles

Kinetics and Mechanism of the Outer-Sphere Oxidation of Horse-Heart Cytochromec by an Anionic Chromium(v) Complex – Kinetic Evidence for Precursor Formation and a Late Electron-Transfer Transition State

The irreversible outer-sphere electron-transfer reaction between trans-bis(2-ethyl-2-hydroxybutanoato(2–))oxochromate(v) and cytochrome cII was investigated as a function of pH, concentration, temperature and pressure. The plot of the observed pseudo-first order rate constant as a function of the CrV concentration shows a clear trend towards saturation at higher CrV concentrations, from which the precursor formation constant and the electron-transfer rate constant could be separated (K = 37 ± 5 M−1 and kET = 1510 ± 180 s−1 at pH 4.8 and 279 K). In the low CrV concentration range the second-order electron-transfer rate contants were measured as a function of temperature (ΔH# = 20.9 ± 0.6 kJ mol−1; ΔS# = –79.9 ± 2.1 J K−1 mol−1; ΔG# (279 K) = 43.2 kJ mol−1). High-pressure experiments were performed at two different pH values. The kinetic (stopped-flow) and thermodynamic (electrochemical) measurements as a function of pressure enabled the construction of a volume profile for the system at 279 K. The activation volumes for the redox process are –9.2 ± 0.2 (pH 5.0) and –11.1 ± 0.8 cm3 mol−1 (pH 4.7), and the overall reaction volumes were estimated to be –7 ± 2 (pH 5.0) and –10 ± 2 cm3 mol−1 (pH 4.7) . The transition state of the redox reaction lies to a large extent on the product side and can be described as “late”. The results are discussed in comparison to earlier measurements using cobalt and ruthenium complexes as reaction partners for cytochrome c.

Relationship among Redox Potentials, Proton Dissociation Constants of Pyrrolic Nitrogens, and in Vivo and in Vitro Superoxide Dismutating Activities of Manganese(III) and Iron(III) Water-Soluble Porphyrins

The log kcat values for the dismutation of O2•- by a series of monohydroxoiron(III) and aquamanganese(III) porphyrins, including ortho, meta, and para isomers of 5,10,15,20-tetrakis(N-alkylpyridiniumyl)porphyrins, were found to vary linearly with the metal-centered redox potentials (E1/2) for the M(III)/M(II) couple. Each 120 mV increase in E1/2 imparted a 10-fold increase in kcat. The observed behavior is in accord with the Marcus equation for outer-sphere electron-transfer reactions, suggesting that the same mechanism is operative for iron and manganese porphyrins. The Marcus plot enabled us to estimate the self-exchange rate constants of monohydroxoiron porphyrins to be ∼1 order of magnitude higher than those of aquamanganese porphyrins. Furthermore, E1/2 values for all of the metalloporhyrins investigated were linearly related to the acid dissociation constants (pKa3) of the pyrrolic nitrogen of the metal-free porphyrins, indicating that either E1/2, or the more readily measured pKa3, may be useful in predicting SOD activity in vitro. The most potent compounds investigated, with respect to SOD activity, are those of the ortho N-alkylpyridiniumyl series. Ortho N-alkylpyridiniumyl groups are more electron withdrawing than are the meta or para groups, thus imparting a more positive redox potential and a correspondingly higher SOD activity. Sufficiently positive potentials, or sufficiently low pKa3 values, are necessary for useful SOD activity, but so is the absence of toxicity. Despite their favorable redox potentials and SOD activities, all Fe(III) porphyrins investigated were toxic to Escherichia coli under both aerobic and anaerobic conditions and to both SOD-deficient and SOD-proficient strains. Only the ortho and meta manganese isomers of the N-alkylpyridiniumyl series (MnIIITE-2-PyP5+, MnIIITM-2-PyP5+ and MnIIITM-3-PyP5+) significantly protected SOD-deficient E. coli and allowed growth in an aerobic minimal medium. In previous work, we established that the lower toxicity of these compounds is due to diminished ability to bind to nucleic acids. The Mn(III) complexes are preferable to the Fe(III) complexes for SOD mimics possibly due to a lower tendency for axial ligation. We propose E1/2 ≥ +0.05 V vs NHE and/or pKa3 ≤ 2.0 as necessary requirements for Mn porphyrins to be considered useful SOD mimics.

Effects of electronic structure on chiral induction in outer-sphere electron transfer reactions of complexes with the C3 symmetric ligand, 1,4,7-triazacyclononane-1,4,7-tris[2′( R)-2′-propionate](-3)

The ligand 1,4,7-triazacyclononane-1,4,7-tris[2′( R)-2′-propionate](-3)(( R)-tacntp 3−), binds stereospecifically to transition metal ions. The structures of the complexes [Cr(( R)-tacntp)]·NaBr and [Fe(( R)-tacntp)]·H 2O have been determined by X-ray crystallography. Both complexes have the Λ-configuration but the conformation of the chelate rings in Λ-[Cr(( R)-tacntp)] is (λ,λ,λ) with a geometry close to octahedral while in Λ-[Fe(( R)-tacntp)] it is (δ,δ,δ) and the geometry is closer to that of a trigonal prism. Chiral induction in the electron transfer reactions of Λ-[Co(( R)-tacntp)], Λ-[Fe(( R)-tacntp)] and Λ-[Mn(( R)-tacntp)] with [Co(( RR,SS)-chxn) 3] 2+ has been investigated. All three reactions are outer-sphere and four isomeric [Co(( RR,SS)-chxn) 3] 3+ products are identified in each case. The oxidants Λ-[Fe(( R)-tacntp)] and Λ-[Mn(( R)-tacntp)] show very similar selectivities, quite different from those of Λ-[Co(( R)-tacntp)]. Reasons for this behavior are discussed.

Enantioselectivities in Electron-Transfer and Excited State Quenching Reactions of a Chiral Ruthenium Complex Possessing a Helical Structure

The outer-sphere electron-transfer reactions between diastereomers of Ru(menbpy)3•+ (menbpy = 4,4‘-di{(1R,2S,5R)-(−)-menthoxycarbonyl}-2,2‘-bipyridine) and enantiomers of Co(acac)3 and Co(edta)- have been studied by pulse radiolysis. Δ-Ru(menbpy)3•+ rapidly reduces Co(acac)3 in 85% EtOH/H2O (1 mM NaH2PO4) with second-order rate constants of (2.1 ± 0.1) × 107 and (7.8 ± 0.2) × 106 M-1 s-1 for the Δ- and Λ-Co(acac)3 enantiomers, respectively, and an enantioselectivity factor (ketΔ/ketΛ) of 2.7. Λ-Ru(menbpy)3•+ preferentially reduces Λ-Co(acac)3 with an enantioselectivity factor (ketΔ/ketΛ) of 0.8. Activation volume data (ΔV‡) suggest that the association between the Δ−Δ isomers in the encounter complex allows closer interaction of the metal centers than between the other isomer combinations. The value of ketΔ/ketΛ for the reaction of Δ- and Λ-Co(edta)- with Δ-Ru(menbpy)3•+ is 1.2. Electron-transfer reactions of seven racemic Ru(L)3•+ (L = substituted phenanthroline) complexes with Co(acac)3 were also studie...

Deprotonation of the Transition Metal Hydride (η5-C5Me5)(PMe3)IrH2. Synthesis and Chemistry of the Strongly Basic Lithium Iridate (η5-C5Me5)(PMe3)Ir(H)(Li)

Treatment of (η5-C5Me5)(PMe3)IrH2 (1) with tert-butyllithium gives (η5-C5Me5)(PMe3)Ir(H)(Li) (2) as a bright yellow solid. NMR evidence indicates that the lithium iridate 2 is aggregated in benzene, is converted to a single symmetrical species in THF, and is present as a dimer in DME. Treatment of 2 with 3,3-dimethylbutane trifluoromethanesulfonate-1,2-syn-d2 (3-syn-d2) gave the alkylated hydridoiridium complex 4a-anti-d2, which was converted to the corresponding chloride Cp*(PMe3)Ir(CHDCHDCMe3)(Cl) (4c-anti-d2) by treatment with CCl4. Analysis of this material by NMR spectroscopy showed that it was contaminated with ≤15% syn isomer. The alkylation therefore proceeds with predominant inversion of configuration at carbon, indicating that the major pathway is an SN2 displacement and not an outer-sphere electron-transfer reaction. Protonation studies carried out on iridate 2 with organic acids of varying pKa allowed us to estimate that the pKa of the dihydride 1 falls in the range 38−41, making it less acidic than DMSO and more acidic than toluene. This represents the least acidic transition metal hydride whose pKa has been quantitatively estimated. Treatment of 2 with main group electrophiles allowed the preparation of several other hydridoiridium derivatives, including Cp*(PMe3)Ir(SnPh3)(H) (5a), Cp*(PMe3)Ir(SnMe3)(H) (5b), and Cp*(PMe3)Ir(BR2)(H) (6a, R = F; 6b, R = Ph). Reaction of 2 with acid chlorides and anhydrides leads to acyl hydrides Cp*(PMe3)Ir(COR)(H), and fluorocarbons also react, giving products such as Cp*(PMe3)Ir(C6F5)(H) in the case of hexafluorobenzene as the electrophile.

A theoretical study of outersphere electron transfer reactions in electrolyte solutions

A microscopic theory of outersphere electron transfer reactions in electrolyte solutions is presented. Both static and dynamic effects of solvent and ion atmosphere on rates of electron transfer are calculated by employing molecular models. The donor–acceptor system is composed of two spheres and the electrolyte solution is composed of dipolar solvent molecules and ions which are treated at the same molecular level. A microscopic expression for the free energy of activation is derived by using density functional theory. The dynamic effects are calculated by using a molecular hydrodynamic theory which properly includes finite wave vector modes of relaxation of solvent and ion atmosphere. Explicit numerical results are presented for the activation free energy and the rate constant of electron transfer in solutions of varying ion concentration. It is found that ion atmosphere can make an important contribution to the activation free energy at finite ion concentration although the net increase in the activation energy is not very significant for the solutions studied in this work. This happens because, with increase of ion concentration, the ion atmosphere contribution to the total activation free energy increases, whereas the solvent contribution shows a decreasing trend. The solvent behaves as an effective less polar medium due to screening by ions and, therefore, its contribution to the activation free energy decreases as ion concentration is increased.

Self-assembled monolayers on electrode surfaces: a probe for redox kinetics

The formation and characterization of self-assembled monolayers of organosulfur compounds like alkanethiols and dialkyl (di)sulfides on metal surfaces such as gold are areas of current research interest. The presence of an aromatic ring in a thiol molecule can enhance the binding between Au and the thiol, resulting in the formation of compact and impervious self-assembled monolayers. The formation of a monolayer of 2-mercaptobenzothiazole (MBT), containing an aromatic group with a fused thiazole ring but no long alkyl chain, is achieved on a gold electrode surface. Voltammetric investigations of ferro/ferricyanide and ferrous/ferric redox systems carried out on this Au|MBT electrode are reported. Further, the possibility of using such an Au|MBT electrode to distinguish between inner and outer sphere electron transfer reactions is indicated.

Solvent effects on outersphere electron transfer reactions in mixed dipolar liquids

A microscopic theory of outersphere electron transfer reactions occurring in mixed dipolar liquids is presented. Both static and dynamic effects of solvent mixture on rates of electron transfer processes are calculated by employing molecular models. The donor–acceptor system is composed of two spheres between which the electron is transferred. The sizes of the donor and acceptor can be different. The solvent mixture is composed of different kinds of spherical molecules with embedded dipoles. The molecules of the different components are distinguished by their sizes and dipole moments. A microscopic expression for the free energy of activation is derived by using density functional theory and constrained variational approach. The dynamic effects are calculated by using a molecular hydrodynamic theory which properly includes the finite wavevector modes of relaxation of dipolar mixture. In the numerical calculations, the solvent molecules of different components are taken to be of equal size but different polarity and mole fraction. Explicit numerical results are obtained for the activation free energy and the rate constant of electron transfer between a donor and an acceptor of varying sizes and the importance of preferential solvation is elucidated. The effects of changing the donor–acceptor separation on the activation free energy and the rate constant are also discussed.

Inner sphere and outer sphere electron transfer reactions in atom transfer radical polymerization

AbstractThe participation of electron transfer (ET) processes in atom transfer radical polymerization (ATRP) is discussed. Inner sphere electron transfer process (atom transfer) is the inherent part of ATRP. It includes the homolytic cleavage of C‐halogen bond in dormant macromolecular alkyl halides species by the Cu(I) activator with the rate constant kact≈︁100±1 M−1s−1. This process is reversible and the deactivation of growing radicals by X‐Cu(II) species is very fast with the rate constant of deactivation kact≈︁107±1 M−1s−1. However, in addition to the atom transfer process (inner sphere ET), outer sphere ET may also occur. Growing radicals can either be oxidized by Cu(II) species to carbocations or reduced by Cu(I) species to carbanions. These reactions were observed in the polymerization of p‐methoxystyrene and initiation with 2‐bromomalonates, respectively. In order to avoid these side reactions the catalyst structure should be fine tuned for the particular monomer and the range of targeted molecular weights. In addition, some side reactions can be avoided by slow addition of initiator (e.g. for benzhydryl halides), catalyst (for malonates and CCl4 as initiators) and by reducing polymerization temperature.

Magnetokinetic effects in outer-sphere electron-transfer reactions: on the role of spin-orbit and hyperfine couplings

Magnetokinetic effects in the outer-sphere electron transfer between Co(sep)2+ and Cl2 were investigated between 0 and 7 T. The dependence of this reaction rate constant on the magnetic induction bears similarities to those observed in other outer-sphere electron-transfer reactions of Co coordination compounds. Sources of such effects are discussed on the basis of the quantum-mechanical electron-transfer theory.

Magnetokinetic effects in outer-sphere electron-transfer reactions: on the role of spin-orbit and hyperfine couplings

Magnetokinetic effects in the outer-sphere electron transfer between Co(sep)2+ and Cl2 were investigated between 0 and 7 T. The dependence of this reaction rate constant on the magnetic induction bears similarities to those observed in other outer-sphere electron-transfer reactions of Co coordination compounds. Sources of such effects are discussed on the basis of the quantum-mechanical electron-transfer theory.

Electrostriction effects on electron transfer reactions in solution. I. Adiabatic regime

The outer-sphere unimolecular electron transfer reactions, DA⇌D+A−, in polar solvents are studied theoretically. By employing a recent theory of fluctuating cavities [H. J. Kim, J. Chem. Phys. 105, 6818, 6833 (1996)], we construct a nonequilibrium free energy formulation that accounts for both the solvation and electrostriction aspects of the changing solute dipole moment during the reaction. With a two-state diabatic description for the solute electronic structure, we obtain a two-dimensional free energy surface in terms of the cavity radius a and a solvent coordinate that gauges the nonequilibrium solvent orientational polarization Por. The equilibrium a values for the reactant and product states are found to differ significantly, indicating that the cavity reorganization plays an important role in the reaction free energetics and dynamics. The reaction mechanism and rate constant are analyzed via the solution-phase reaction path, which is found to vary markedly with the relative time scales of the Por and a motions. To be specific, the charge transfer proceeds in a concerted fashion with active participation of both Por and a when their characteristic times are comparable. By contrast, a stepwise mechanism results when there is a large mismatch between the Por and a time scales. In the slow cavity limit where a is much slower than Por, the reorganization through the cavity size adjustment precedes the barrier crossing that occurs exclusively along the Por coordinate. In the opposite fast cavity regime where Por is mainly responsible for activation and deactivation, it is found that the saddle point on the free energy surface is no longer rate-determining due to the nonlinear residual entropic effects. A brief comparison with other reaction systems involving charge shift in solution is also made.

Theoretical study of electronic transmission coefficient of outer-sphere electron self-exchange reactions in solution: a hydration function scheme

On the basis of the thermodynamic characteristics of the hydration process of metal ions, a new theoretical scheme is presented. It is applied to determining the theoretical values of the electronic transmission coefficient of electron self-exchange reactions in solution in terms of the original Landau-Zener formalism. This scheme gets around the shortcomings of the summation method of the inner-sphere and outer-sphere contributions and directly adopts accurate hydration potential functions to determine relevant quantities. The hydration potential functions are analytically determined from the experimental spectroscopic data and the hydration energy data. Two important quantities are the slopes of the potential energy surfaces and the coupling matrix elements. The slopes are obtained from the hydration potential functions and the activation parameters are obtained in terms of a new proposed self-exchange activation model. The coupling matrix elements are determined from adiabatic and diabatic activation energies which are obtained from the new activation model and experiment, and are compared with the ab initio values in recent literature. Theoretical results of the electronic transmission coefficients of electron transfer reactions obtained using the values of slopes from the hydration potential functions are found to be in close agreement with the corresponding experimental values obtained from the experimental rate data. These theoretical values of the electronic transmission coefficient and the experimental values are found to be less than unity. This indicates that outer-sphere electron transfer reactions in solution involving hydrated transition metal ions studied in this work are nonadiabatic in nature. The applicability of this scheme is also discussed.

Nonlocal dielectric response of water and reorganization energies for outer sphere electron transfer reactions

New results on the nonlocal dielectric function of liquid water (Ph. A. Bopp, A. A. Kornyshev, G. Sutmann, Phys. Rev. Let. 76, 1280 (1996)) are discussed and used to calculate reorganization free energies for electron exchange reactions between solute ions. The energies are shown to be several times larger than those given by the classical Marcus formula for the born sphere model of an ion. However, they are reduced to Marcus or even lower values for modified charge density models, which take into account the delocalization of the ion excess charge distribution.

Quantum energy gap law of outer-sphere electron transfer reactions: A molecular dynamics study on aqueous solution

The quantum energy gap law for electron transfer (ET) reactions in water is examined. Molecular dynamics (MD) simulation analysis is carried out to obtain the solvent reorganization energies, time correlation functions (TCF), spectral density functions, and quantum rate constants. Their dependence on the reaction free energy and on the donor–acceptor distance is explored along with the solvent isotope effects. Properties of the imaginary-time saddle-point for the TCF expression of the ET rate formula are also examined. The high-frequency intramolecular vibrational modes of the solvent water are found to present marked quantum effects on the ET rate, while their contribution to the static reorganization energy is small (less than 6%). The energy gap dependence of the quantum activation free energy is shown to become nearly independent of the donor–acceptor distance when renormalized by the reorganization energy. Approximations to compute quantum rate constants from MD simulation data are briefly discussed in light of the present results.

Electron transfer reactions in electrolyte solutions: effects of ion atmosphere and solvent relaxation

A molecular theory of the effects of solution dynamics on the rates of adiabatic outer-sphere electron transfer reactions in electrolyte solutions is presented. The theory correctly includes the dynamics of both ions and solvent molecules through frequency dependent ion and solvent susceptibilities. The rate of electron transfer is calculated by employing the well-known Grote-Hynes theory. The barrier-crossing frequency of electron transfer is found to depend rather strongly on the salt concentration dependent solvent longitudinal relaxation time and on the ionic conductivity. Numerical results are presented for some common aqueous and non-aqueous electrolyte solutions.

Outer electron transfer in methylene chloride: concentration, salt, and temperature dependences of the oxidation of trans-ReX2(cis-1,2 bis(diphenylphosphino)ethylene)2 (X = Cl, Br) by the clathrochelate [Co(1,2-cyclohexanedionedioximate)3(BButyl)2]BF4

The oxidation of ReC1 2 ( cis -1,2 bis(diphenylphosphino)ethylene) 2 and ReBr 2 ( cis -1,2 bis(diphenylphosphino)ethylene) 2 by the clathrochelate [Co(1,2-cyclohexanedione dioximate) 3 (BButyl) 2 ]BF 4 have been studied by the stopped-flow method as a function of temperature (−85 to −50°C), added Bu 4 NBF 4 (0–0.1 M), and reactant concentration in methylene chloride. The rate constants were fit to a mechanism involving separate paths for the free ion and the ion paired Co(III) oxidant. The result was the enthalpy and entropy of ion pairing for both Bu 4 NBF 4 and the oxidant, and the enthalpy and entropy of activation for both the free ion path and the ion paired path for each reaction. This study makes this one of the best characterized outer-sphere electron transfer reactions occurring in the low dielectric constant solvent methylene chloride. The results will allow comparison of the reactivity of these mononuclear complexes with that of metal-metal bonded dirhenium complexes. Using the temperature dependences of the reduction potentials of the Re and Co complexes as well as decamethylferrocene, and the kinetics of the reduction of the Co(III) complex by decamethylferrocene in methylene chloride, also as a function of added salt, estimates of the high electron self-exchange rate constants of the two Re complexes have been made. They are both very similar and are ca 10 8 M −1 S −1 at 25°C.

Theoretical Study of Landau−Zener Electronic Transmission Factor for Outer-Sphere Electron Transfer Reactions in Solution

On the basis of the original Landau−Zener formalism, this paper presents an accurate scheme for determining the theoretical values of the electronic transmission factor in terms of the experimental vibrational spectroscopic and thermodynamic data of the complex ions in solution. The quantity of central importance in this regard is the slope of the potential energy surface, and it is obtained from the accurate Morse function and the modified harmonic oscillator potential function determined from the experimental vibrational spectroscopic data and thermodynamic data. Also, the slopes are obtained using the classical improved average dipole orientation method. The various activation parameters are obtained in terms of a new improved self-exchange activation model. The coupling matrix elements are determined from adiabatic and nonadiabatic activation energies which are obtained from the new activation model and experiments, and are compared with the ab initio values in recent literatures. Theoretical results of electronic transmission factors of electron transfer reactions obtained using the values of slopes from the accurate potential functions (the Morse function and the modified harmonic oscillator potential) as well as the harmonic oscillator potential and the classical ion−dipole capture force field potential are found to be in close agreement with the experimental values of electronic transmission factor obtained from experimental data of the rate constants. These theoretical values of electronic transmission factors, as well as those from experimental data of rate constants, are found to be less than unity. These results indicate that outer-sphere electron transfer reactions in solution involving aquo complexes of the transition metal ions studied in this work are nonadiabatic in nature. The further extension of this theoretical scheme is also discussed.

Calculations of Intramolecular Reorganization Energies for Electron-Transfer Reactions Involving Organic Systems

The determination of intramolecular reorganization energies for self-exchange outer-sphere electron-transfer (ET) reactions is presented using a recently published method. From ab initio calculations, the electronic structure and equilibrium geometry, q, for the neutral, qeq(0), and monocharged, qeq(+/−), organic molecules are obtained along with the force constant matrix F. A projected force constant f is determined as f = (ΔqTFΔq)/|Δq|2, where Δq = qeq(0) − qeq(+/−). The intramolecular reorganization energy is obtained as λi = ((f(0)f(+/-))/(f(0) + f(+/-)))|Δq|2. It is shown that, due to the inclusion of the coupling between the various vibrational coordinates, this approach is an improvement compared to conventional methods for calculating the intramolecular reorganization energy. The method is tested on the reductive ET of aromatic hydrocarbons, the oxidative ET of substituted benzenes, and oxidative/reductive ET of benzyl radicals.

Electrolyte dynamics effect on adiabatic outersphere electron transfer reactions

A theoretical study of the effects of electrolyte solution dynamics on adiabatic outersphere electron transfer reactions is presented. The theory is self-contained and it properly includes the diffusive and non-diffusive modes of relaxation of ions and solvent molecules. Numerical results are obtained for the rate of electron transfer in a model electrolyte solution consisting of spherical ions in Stockmayer solvent and are compared with the rate of electron transfer in the pure Stockmayer solvent. It is found that the deviation from the prediction of transition state theory is higher for electron transfer in the electrolyte solution and this is because of the extra frictional contribution from slow ion atmosphere relaxation.