David M. Stanbury

Reduction Potentials Involving Inorganic Free Radicals in Aqueous Solution

Publisher Summary This chapter discusses the reduction potentials involving inorganic free radicals in aqueous solution. Because free radicals are usually transients, knowing their thermodynamic properties is primarily useful in mechanistic studies. Thus, the useful redox couples associated with a given free radical correspond to plausible elementary steps in reaction mechanisms. All potentials are expressed against the normal hydrogen electrode (NHE). Apart from the NHE, the standard state for all solutes is the unit molar solution at 25 °C. This violates the usual convention for species, such as O 2 that occur as gases, but because the rates of bimolecular reactions in solution are significant, the unit molar standard state is most convenient. The emphasis is on electron transfer reactions in which no bonds are formed or broken, electron transfer reactions in which concerted electron transfer and bond cleavage could occur, and certain atom transfer reactions. The chapter also presents a tabulation of ∆ f G ° values for all the radicals. A common approach in estimating the thermochemistry of aqueous free radicals is to use gas-phase data with approximations of solvation energies.

Autoxidation kinetics of aqueous nitric oxide

Reports on the kinetics of the autoxidation of aqueous nitric oxide are discussed. It is concluded that the correct rate law is ‐d[NO]/dt = 4k aq[NO]2 [O2] with k aq = 2 × 106 M−2 · s−1 at 25°C and that a recent report of a rate law zero order in NO is incorrect.

Standard electrode potentials involving radicals in aqueous solution : inorganic radicals (IUPAC Technical Report)

Abstract Recommendations are made for standard potentials involving select inorganic radicals in aqueous solution at 25 °C. These recommendations are based on a critical and thorough literature review and also by performing derivations from various literature reports. The recommended data are summarized in tables of standard potentials, Gibbs energies of formation, radical pKa’s, and hemicolligation equilibrium constants. In all cases, current best estimates of the uncertainties are provided. An extensive set of Data Sheets is appended that provide original literature references, summarize the experimental results, and describe the decisions and procedures leading to each of the recommendations.

Proton-coupled electron-transfer oxidation of phenols by hexachloroiridate(IV).

One-electron oxidation of phenol, 2,4,6-trimethylphenol, and 2,6-dimethylphenol by [IrCl(6)](2-) in aqueous solution has a simple pH dependence, indicating slow bimolecular oxidation of ArOH and faster oxidation of ArO(-). H/D kinetic isotope effects as large as 3.5 for oxidation of ArOH support concerted proton-coupled electron transfer with water as the proton acceptor.

Direct Oxidation of l-Cysteine by [FeIII(bpy)2(CN)2]+ and [FeIII(bpy)(CN)4]-

The oxidation of L-cysteine by the outer-sphere oxidants [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution is highly susceptible to catalysis by trace amounts of copper ions. This copper catalysis is effectively inhibited with the addition of 1.0 mM dipicolinic acid for the reduction of [Fe(bpy)2(CN)2]+ and is completely suppressed with the addition of 5.0 mM EDTA (pH<9.00), 10.0 mM EDTA (9.010.0) for the reduction of [Fe(bpy)(CN)4]-. 1H NMR and UV-vis spectra show that the products of the direct (uncatalyzed) reactions are the corresponding Fe(II) complexes and, when no radical scavengers are present, L-cystine, both being formed quantitatively. The two reactions display mild kinetic inhibition by Fe(II), and the inhibition can be suppressed by the free radical scavenger PBN (N-tert-butyl-alpha-phenylnitrone). At 25 degrees C and micro=0.1 M and under conditions where inhibition by Fe(II) is insignificant, the general rate law is -d[Fe(III)]/dt=k[cysteine]tot[Fe(III)], with k={k2Ka1[H+]2+k3Ka1Ka2[H+]+k4Ka1Ka2Ka3{/}[H+]3+Ka1[H+]2+Ka1Ka2[H+]+Ka1Ka2Ka3}, where Ka1, Ka2, and Ka3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H. For [Fe(bpy)2(CN)2]+, the kinetics over the pH range of 3-7.9 yields k2=3.4+/-0.6 M(-1) s(-1) and k3=(1.18+/-0.02)x10(6) M(-1) s(-1) (k4 is insignificant in the fitting). For [Fe(bpy)(CN)4]- over the pH range of 6.1-11.9, the rate constants are k3=(2.13+/-0.08)x10(3) M(-1) s(-1) and k4=(1.01+/-0.06)x10(4) M(-1) s(-1) (k2 is insignificant in the fitting). All three terms in the rate law are assigned to rate-limiting electron-transfer reactions in which various thiolate forms of cysteine are reactive. Applying Marcus theory, the self-exchange rate constant of the *SCH2CH(NH2)CO2-/-SCH2CH(NH2)CO2- redox couple was obtained from the oxidation of L-cysteine by [Fe(bpy)(CN)4]-, with k11=4x10(5) M(-1) s(-1). The self-exchange rate constant of the *SCH2CH(NH3+)CO2-/-SCH2CH(NH3+)CO2- redox couple was similarly obtained from the rates with both Fe(III) oxidants, a value of 6x10(6) M(-1) s(-1) for k11 being derived. Both self-exchange rate constants are quite large as is to be expected from the minimal rearrangement that follows conversion of a thiolate to a thiyl radical, and the somewhat lower self-exchange rate constant for the dianionic form of cysteine is ascribed to electrostatic repulsion.

Oxidation of glutathione by hexachloroiridate(IV), dicyanobis(bipyridine)iron(III), and tetracyano(bipyridine)iron(III).

The aqueous oxidations of glutathione (GSH) by [IrCl(6)](2-), [Fe(bpy)(2)(CN)(2)](+), and [Fe(bpy)(CN)(4)](-) are described. All three reactions are highly susceptible to catalysis by traces of copper ions, but this catalysis can be fully suppressed with suitable chelating agents. The direct oxidation by [IrCl(6)](2-) yields [IrCl(6)](3-) and GSO(3)(-); some GSSG is also obtained in the presence of O(2). The two Fe(III) oxidants are reduced to their corresponding Fe(II) complexes with nearly quantitative formation of GSSG. The kinetics of these reactions have been studied at 25 °C and μ = 0.1 M between pH 1 and 11. All three reactions have rate laws that are first order in [M(ox)] and [GSH](t) and show a general increase in rate with increasing pH. Detailed studies of the pH dependence enable the rate law to be elaborated with terms for reaction of the individual protonation states of GSH. These pH-resolved rate constants are interpreted with a mechanism having rate-limiting outer-sphere electron-transfer from the various thiolate forms of GSH.

VANISHINGLY SLOW KINETICS OF THE CLO2/CL- REACTION : ITS QUESTIONABLE SIGNIFICANCE IN NONLINEAR CHLORITE REACTIONS

Abstract There is considerable disagreement regarding the rate of the aqueous reaction 2ClO 2 + Cl − + H 2 O → HClO 2 + ClO 2 − + HOCl. Brays early (1906) measurements indicate a third-order rate constant of 1 × 10 −3 M −2 s −1 at 60 °C, while recent kinetic modelling studies of chlorite oscillators use rate constants ranging from 5 × 10 1 to 8 × 10 7 M −2 s −1 at 25 °C. We report attempts to make direct measurements on the reaction at 25 °C. The reaction is very slow and photosensitive: in 0.1 M HCl with 0.8 mM ClO 2 less than 5% of the ClO 2 is consumed in 3 h, and in 0.01 M HClO 4 /1 mM NaCl with 0.3 mM ClO 2 less than 10% is consumed in 2 weeks. Free energy calculations indicate that the reaction has an equilibrium constant too small for the reaction to occur by itself. In principle the reaction can be driven by the addition of product scavengers. Moreover, if appropriate scavengers are present to remove the purported intermediates ClClO 2 and ClO 2 − then the reaction could have the sought third-order rate law. Support for a finite but low rate constant ( k > 3.8 × 10 −5 M −2 s −1 ) for this process comes from a consideration of a prior report on the kinetics of the reverse reaction in view of the principle of microscopic reversibility. Finally, ab initio calculations on ClClO 2 , an intermediate in the reaction, lead to an upper limit of 1.2 × 10 −5 M −2 s −1 ; within the uncertainty in the hydration energy of ClClO 2 , this result is in agreement with the lower limit given above. Thus, while an accurate value for the rate constant at 25 °C is not yet in hand, it is clearly orders of magnitude less than has been used in several recent kinetic simulations.

Oxidation of Phenol by Tris(1,10-phenanthroline)osmium(III)

Outer-sphere oxidation of phenols is under intense scrutiny because of questions related to the dynamics of proton-coupled electron transfer (PCET). Oxidation by cationic transition-metal complexes in aqueous solution presents special challenges because of the potential participation of the solvent as a proton acceptor and of the buffers as general base catalysts. Here we report that oxidation of phenol by a deficiency of [Os(phen)(3)](3+), as determined by stopped-flow spectrophotometry, yields a unique rate law that is second order in [osmium(III)] and [phenol] and inverse second order in [osmium(II)] and [H(+)]. A mechanism is inferred in which the phenoxyl radical is produced through a rapid PCET preequilibrium, followed by rate-limiting phenoxyl radical coupling. Marcus theory predicts that the rate of electron transfer from phenoxide to osmium(III) is fast enough to account for the rapid PCET preequilibrium, but it does not rule out the intervention of other pathways such as concerted proton-electron transfer or general base catalysis.

RECENT ADVANCES IN ELECTRON-TRANSFER REACTIONS

Publisher Summary The field of electron-transfer chemistry is presently in a stage of rapid growth and diversification. This chapter aims to sketch aspects of present areas of research activity, using the presentations at the three conferences (2002 Dalton Discussion meeting in Kloster Banz and the 2001 and 1999 Gordon Research Conferences on Inorganic Reaction Mechanisms in Ventura) as a guide to significant recent advances. The chapter discusses experimental studies of single-electron transfer at dinuclear transition-metal complexes, across electrode interfaces, between and within metalloproteins, to main-group radicals, and between organic and inorganic molecules. In parallel with these experimental studies, theoretical studies are preceding apace, with notable advances in the understanding of outer sphere electron-transfer and two-electron transfer. Another notable trend has been the increasing technical sophistication in the field of molecules and mechanisms. Techniques with increasingly improved time resolution, reaction conditions that are increasingly demanding of control, and calculations that require greater computational power are hallmarks of these studies.

Oxidations at Sulfur Centers by Aqueous Hypochlorous Acid and Hypochlorite: Cl+ Versus O Atom Transfer

Sulfur-containing compounds are known to be susceptible to oxidation by aqueous HOCl, but the factors affecting the rates of these reactions are not well-established. Here we report on the kinetics of oxidation of thiosulfate, thiourea, thioglycolate, (methylthio)acetate, tetrathionate, dithiodiglycolate, and dithiodipropionate at 25 °C and 0.4 M ionic strength. These reactions obey the general rate law -d[OCl-]/dt = (kOCl-[OCl-] + kHOCl[HOCl])[substrate] with some exceptions: tetrathionate and the two disulfides undergo rate-limiting hydrolysis at high pH, and dithiodiglycolate has an additional term in the rate law that is second order in [substrate]. The reactions of HOCl are believed to have a Cl+ transfer mechanism, and in the case of thiosulfate the rate of hydrolysis of the ClS2O3- intermediate was determined. In the case of thiourea evidence was obtained for thiourea monoxide as a long-lived product. It is shown that sulfite and species with terminal sulfur atoms have kHOCl values in the vicinity of 1 × 109 M-1 s-1, while SCN- and thioethers react somewhat more slowly; tetrathionate, trithionate, and disulfides react much more slowly. Comparison of the rate constants with those for oxidation of these sulfur substrates by H2O2 and [Pt(CN)4Cl2]2- shows that HOCl reacts a few orders of magnitude more rapidly than [Pt(CN)4Cl2]2- and ∼9 orders of magnitude more rapidly than H2O2. Many of the kHOCl values are leveled by the high electrophilicity of HOCl. It is proposed that the kOCl- values correspond to oxygen-atom transfer mechanisms, as supported by LFERS (linear free energy relationships) relating these rate constants to those for reactions of H2O2 and [Pt(CN)4Cl2]2-.