Redox potentials ferrocenes

Values are for aqueous solution vs. NHE. The /si/2 values are close to the values in acetonitrile vs. SCE because the ferricenium/ferrocene redox potential shifts by 0.25 V (vs. SCE) on going from acetonitrile to water while the SCE potential in water is 0.24 V [6, 7). 

The electrochemistry associated with these redox-active cryptands is quite intriguing. As pointed out earlier, anodic shifts of the ferrocene redox potential may be used via the Nernst equation to estimate the decrease in binding capacity (Xj/Ki) on coordination with a cation. Beyond this, however, if Kj is determined independently as is the case for 38 (m, n = 2) [63] then Xj may be calculated and correlated with the ratio of cationic radius/charge (Fig. 6-7) — data that reveal that increasing charge density on the cation destabilizes the complex between the oxidised cryptand and the cation, presumably by charge repulsion [68]. Alkali metal cations gave only small (< 20 mV) anodic shifts with this cryptand. 

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

Table IV lists the redox potentials of conjugated ferrocene oligomers (mainly dimers with a single bridge). Potential values are denoted against different reference electrodes as given in the references. The values can be primarily compared using the relationship mentioned in the footnote of the table, although care should be taken with some errors derived from junction potentials which depend on experimental conditions. There have been several reports on the quantitative estimation of structural factors affecting internuclear electron delocalization.

Azo-bridged ferrocene oligomers also show a marked dependence on the redox potentials and IT-band characteristics of the solvent, as is usual for class II mixed valence complexes 21,22). As for the conjugated ferrocene dimers, 2 and 241 the effects of solvents on the electron-exchange rates were analyzed on the basis of the Marcus-Hush theory, in which the t/max of the IT band depends on (l/Dop — 1 /Ds), where Dop and Ds are the solvent s optical and static dielectric constants, respectively (155-157). However, a detailed analysis of the solvent effect on z/max of the IT band of the azo-bridged ferrocene oligomers, 252,64+, and 642+, indicates that the i/max shift is dependent not only on the parameters in the Marcus-Hush theory but also on the nature of the solvent as donor or acceptor (92). 

A suitable extrathermodynamic approach is based on structural considerations. The oldest assumption of this type was based on the properties of the rubidium(I) ion, which has a large radius but low deformability. V. A. Pleskov assumed that its solvation energy is the same in all solvents, so that the Galvani potential difference for the rubidium electrode (cf. Eq. 3.1.21) is a constant independent of the solvent. A further assumption was the independence of the standard Galvani potential of the ferricinium-ferrocene redox system (H. Strehlow) or the bis-diphenyl chromium(II)-bis-diphenyl chromium(I) redox system (A. Rusina and G. Gritzner) of the medium. 

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

The selected electrodes (five n-type and four p-type) were used to obtain kinetic current vs. potential data in solutions containing poised ferrocene redox couples (50% oxidized, 50% reduced) (37.391. The electrode potential was varied over a range of at least 0.5 V to over 1.0 V. Three couples were examined ferrocene (FER) itself, decamethylferrocene (DFER) and acetylferrocene (AFER). The reduction potentials of DFER and AFER with respect to FER (which is assigned a value of 0.0) are -0.50 and +0.25 V, respectively. The reduction potentials for all three couples are located between the CBE and VBE of the WSe2-CH3CN interface. 

Redox potentials are referenced versus the ferrocenium/ferrocene (Fc+/Fc) couple (irr) peak potentials are given for irreversible processes. 

The bis-benzo-15-crown-5 ferrocene compound [7] containing two vinylic linkages was formed in a mixture of three isomeric components, the cis-cis, cis-trans and trans-trans isomers, which proved inseparable. However, the precedent of insignificant differences found between the magnitudes of the metal cation-induced anodic shifts in the ferrocenyl redox potentials of the respective separated cis and trans isomers [2a] and [2b] led us to use the same isomeric mixture of [7] throughout the subsequent FABMS and electrochemical group 1 and 2 metal cation complexation experiments,... 

Interestingly, the sulfur-linked bis-crown ligand [8] shows an unprecedented cathodic potential shift upon addition of K+ cations to the electrochemical solution (Table 3). It is believed to be a conformational process that causes the anomalous shift of the ferrocene/ferrocenium redox couple and not a through-space or through-bond interaction, as these effects would produce the expected anodic potential shift of the ferrocene redox couple. The origin of the effect may be a redirection of the lone pairs of the sulfur donor atoms towards the iron centre upon complexation. This would increase the electron density... 

Refer to footnote a Table 23. h pa and pc represent the anodic and cathodic current peak potentials of the ferrocene/ferrocenium redox couple of the free ligand. Cathodic shifts in the ferrocene redox couples produced by the presence of anion (5 equiv) added as the tetrabutyl-ammonium salts. As the concentration of the anion increased, the ferrocene/ferrocenium redox couple began to exhibit the features of an EC mechanism. 

This contrasts with the assumption made until a few years ago that the redox potential of the ferrocene/ferrocenium couple was independent of solvent and fixed at a constant value of + 0.400 V vj. NHE. It is, in fact, this controversial assumption that is at the originla of the IUPAC recommendation113 (not yet always followed) of expressing the potential of any redox couple in non-aqueous solvents with respect to the potential of the [Fefa5-C5H5)2]/[Feft5-C5H5)2]+ couple. 

The comparison with the redox potential of ferrocene highlights the electron-withdrawing effect of each trisulfur chain (which renders more difficult the removal of electrons). 

As seen, the original ferrocene molecule oxidizes at E° = -0.05 V (vs. SCE). In the presence of Cu(I) ions the redox potential shifts to E0/ = + 0.40 V. This means that before the complexation, the application of a potential of about 0.1 V generates a current flow, whereas in the presence of Cu(I) ions, in order to generate the same current, we must apply a potential value of about + 0.6 V. It is likely that in the presence of different cations, the potential shift would be different, thus allowing the specific recognition of cations. 

As an example, the variation of the redox potential of the ferrocene/ ferrocenium oxidation with solvent is considered. This has already been cited in Chapter 4, Table 2, and is now more extensively reported in Table 8. 

In a systematical study, Golovin et al. investigated a series of metallocene derivatives in terms of their redox potentials, mass transport properties, and chemical and electrochemical stabilities in both electrochemical test cells and commercial-size AA rechargeable cells.Figure 43 shows the complete voltammetric scan of the ferrocene-containing elec-... 

The effect of these ferrocene-based additives on overcharge protection is shown in Figure 44, where AA cells based on lithium, LhMn02, and electrolytes with or without additives were overcharged. In the absence of these redox shuttles (A), the cell voltage continues to rise, indicating the occurrence of major irreversible decompositions within the cell whereas the presence of shuttle agents (B—E) locks the cell potential in the vicinity of their redox potentials... 

As mentioned earlier in Chapter 5, there are ion-radicals capable of forming hydrogen-bond complexes with neutral molecules. Such complexation significantly changes the redox potential comparatively to that of an initial depolarizer. Of most importance is that the formation of ion-radicals is a reversible process. In other words, the redox-switched effect operates in this host-gnest systems. Scheme 8.5 illnstrates the effect realized in the systems of ferrocene/ferrocenium (Westwood et al. 2004) and of nitrobenzene/the nitrobenzene anion-radical (Bn et al. 2005). 

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