Redox potentials of electron transfer

Fig. 8-7. Dependence of the yield in chloro-de-diazoniations on the redox potential of electron transfer reagents (from Galli, 1988 Fc = ferrocene).

Moore GR, Pettigrew GW, Rogers NK (1986) Factors influencing redox potentials of electron transfer proteins. Proc Natl Acad Sci USA 83 4998-4999... 

Figure 3. The three-state hypothesis for the redox potentials of electron transfer in the 4Fe-4S active centers of ferredoxins and HiPIP (7). The redox potential differs over a 1-V range (see also Ref. 2 for discussion).

Redox potentials of the halide ions explain that direct electron release to the benzenediazonium ion takes place only with iodide (and astatide, At ). This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light or other special procedures. For bromo- and chloro-de-diazoniations, catalysis by cuprous salts (Sandmeyer reaction) is necessary. For fluorination, the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen, i.e. a heterolytic dediazoniation without electron transfer. GaUi demonstrated that in chloro-de-diazoniations the yield is strongly dependent on the redox potential of electron transfer catalysts (highest yields with Cu" and Sn +), but that the rate of electron transfer influences the yield also. Electron transfer is likely to be the rate-limiting step of aryl radical formation in dediazoniations catalyzed by transition metal salts. 

In such timescales, it is possible to study very fast heterogeneous electron transfer rate constants [48]. Diffusion layers as thin as a few nanometers are characteristic for such fast scan rates [50]. Coupled homogeneous chemical reaction steps become less important, and highly reactive intermediates can be detected [48]. The chemical reversibility of electrode reactions increases and thus redox potentials of electron transfer reactions involving extremely unstable species become available [48]. 

Complex formation with Fe(II) and Fe(III) both on solid and solute phases has a dramatic effect on the redox potentials thus, electron transfer by the Fe(II), Fe(III) system can occur at pH = 7 over the entire range of the stability of water Eh (-0.5 V to 1.1 V). 

The position of a system within one of these series is established by its redox potential E (see p. 18). The redox potential has a sign it can be more negative or more positive than a reference potential arbitrarily set at zero (the normal potential of the system [2 H /H2]). In addition, E depends on the concentrations of the reactants and on the reaction conditions (see p.l8). In redox series (4), the systems are arranged according to their increasing redox potentials. Spontaneous electron transfers are only possible if the redox potential of the donor is more negative than that of the acceptor (see p.l8). 

It has been postulated that complexes of electron-transfer proteins in a membrane are of graduated redox-potential. The electron transfer occurs through channels provided by a complex sequence of ligands and bonds are conjugated molecules like carotenoids. These proteins are able to accept electrons from excited Chi on one membrane side (anode) and donate them to an acceptor of more positive redox potential on the other side (cathode). The membrane provides a resistance for an ion current from anode to cathode which closes the electrochemical circuit, and converts excitation energy into chemical free energy AG (Figure 9.4 b). 

Considerable potential exists to design surface modified electrodes which can mimic the behaviour of electronic components. For example, a rectifying interface can be produced by using two-layer polymer films on electrodes. The electroactive species in the layers have different redox potentials. Thus electron transfer between the electrode (e.g. platinum) and the outer electroactive layer is forced to occur catalytically by electron transfer mediation through the inner electroactive layer. 

Both the thermodynamics and kinetics of electron transfer reactions (redox potentials and electron transfer rates) have steric contributions, and molecular mechanics calculations have been used to identity them. A large amount of data have been assembled on Co3+/Co2+ couples, and the majority of the molecular mechanics calculations reported so far have dealt with hexaaminecobalt (III/II) complexes. 

Figure 8. The azide anion, when enclosed in the dizinc(II) cryptate [Zn2(44)]causes the quenching of the facing anthracene fragment an electron is transferred from the electron rich anion to the photoexcited spacer. Inclusion of cyanate anion does not alter fluorescence emission, due to lack of the suitable redox potential allowing electron transfer.

Macrobicyclic cobalt compounds satisfy all these requirements. Their additional advantage is that the redox potentials and electron-transfer rates may be varied on introduction of different apical substituents, by changing the charge of the complexes via protonation or deprotonation, or by altering steric factors. This allows one to select the most suitable complexes as ETAs. 

Figure 8.26. Representative Fe(II)/Fe(III) redox couples at pH = 7. (phen = phen-anthroline sal = salicylate porph = porphyrin = valid for [HCO = 10 M.) Complex formation with Fe(II) and Fe(III) both on solid and solute phases has a dramatic effect on the redox potentials thus electron transfer by the Fe(II),Fe(III) system can occur at pH = 7 over the entire range of the stability of water (-0.5 to 1.1 V). (= Fe 0)2 Fe refers to Fe adsorbed inner-spherically to a surface of a hydrous ferric oxide. The range of redox potentials for heme derivatives given on the right illustrates the possibilities involved in bioinorganic systems.

Gou, P, Hanke, G.T., Kimata-Ariga, Y., Standley, D.M., Kubo, A., Taniguchi, I., Nakamura, H., and Hase, T. (2006). Higher order structure contributes to specific differences in redox potential and electron transfer efficiency of root and leaf ferredoxins. Biochemistry 45, 14389-14396. 

Following photoexcitation using a laser pulse at 355 nm, emission is observed from the monolayers with an excited state lifetime (6.2 ps) that exceeds that of the complex in solution (1.4 ps). It appears that weak electronic coupling between the adsorbates and the electrode means that the excited states are not completely deactivated by radiationless energy transfer to the metal. As illustrated in Fig. 13, in the first report of its land, we used voltammetry at megavolt per second scan rates to directly probe the redox potentials and electron transfer characteristics of electronically excited species. 

Bock CR, Connor JA, Gutierrez AR, Meyer TJ, Whitten DG, Sullivan BP, Nagle JK (1979) Estimation of excited-state redox potentials by electron-transfer quenching. Application of electron-transfer theory to excited-state redox processes. J Am Chem Soc 101(17) 4815-4824. doi 10.1021/ja00511a007... 

Figure 630 Band energy of different semicondncting oxides and standard potentials of electron-transfer redox reactions [31].

Determining Equilibrium Constants for Coupled Chemical Reactions Another important application of voltammetry is the determination of equilibrium constants for solution reactions that are coupled to a redox reaction occurring at the electrode. The presence of the solution reaction affects the ease of electron transfer, shifting the potential to more negative or more positive potentials. Consider, for example, the reduction of O to R... 

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