Ferrocenium/ferrocene redox potential

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

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. 

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. 

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). 

The C90 cage has 46 possible constitutional isomers, out of which only five can be isolated. The electrochemistry of C90 shows two oxidations and six reductions. The redox potentials for C90 are given in Table 8.1. The first reduction potential appears at 0.49 V versus ferrocene/ferrocenium, thus making C90 the easiest to reduce among the empty cage fullerenes. 

Redox potentials of cobaltocene and substituted cobaltocenes (Section 7.3) have been determined for the reduction of cobaltocenium to cobaltocene, a potential of —0.95 V (vs. SCE) in acetonitrile or —0.86 V (vs. SCE) in CH2CI2 was measured (—1.35 V vs. the ferrocene/ferrocenium couple in aprotic solvents). The large potential difference between oxidation of ferrocene and cobaltocene is intriguing, since it is related in a simple fashion to the difference in the HOMCULUMO gap... 

In a series of papers on the total syntheses of alkaloids, Baran and coworkers have recently reported that enolates of carbonyl compounds undergo oxidative coupling with indoles and pyrroles in the presence of oxidants such as copper(II) and iron(III) salts . A detailed study of the oxidative cyclization reported in equation 15 has shown that 26 is converted into 27 with the highest yields when Fe(acac)3 is the oxidant, presumably due to its high redox potential (+1.1 V vs. the ferrocenium/ferrocene couple in THF solution ), which is the most positive among all the oxidizing agents tested for the transformation. 

The electrochemistry of ferrocene-type ligands and their complexes is reviewed in detail by Zanello in Chapter 7. Hence the present description discusses only briefly some unique features of dppf complexes. These complexes are generally expected to exhibit a ferrocene-centered oxidation process. The general interest lies in the modification of the redox potential of the ferrocene/ferrocenium couple on phos-phination of the Cp rings, complexation of the resultant dppf ligand, and variations among the various known coordination modes of the ligand. 

Such bis(diphenylphosphino)ferrocene-palladium complexes commonly undergo a reversible one-electron oxidation, centered on the ferrocene moiety, and an irreversible one-electron reduction, centered on the palladium fragment. The relevant redox potentials are reported in Table 7-29, together with those of related complexes. It must be noted that the ferrocene-based one-electron oxidation leads to ferro-cenium-palladium complexes that are more stable than the free diphenylphosphino-ferrocenium ion. 

The oxetane-derivatized hole conductors span a broad range of redox potentials between 0.0 and 0.5 V vs. the ferrocene/ferrocenium redox couple, which is a standard reference in organic electrochemistry. Thus, this class of materials is ideally suited to bridge the gap to low-lying HOMO levels of an emitter polymer. This becomes particularly important for blue-emitting polymers such as polyfluorenes. 

Aldridge et al. have demonstrated that a similar boryl-ferrocene 81 can be used as a selective colorimetric sensor for F . When F was added to a GH2CI2 solution of 81, under aerobic conditions, a color change from orange to pale green was observed. This did not occur with any other anion tested. Spectroscopic and electrochemical measurements suggest that complexation of F causes the spontaneous formation of a ferrocenium species, that is, the 150 mV anodic shift in the Fc/Fc redox potential caused by F complexation lowers the redox potential enough for the 81 2F complex to be aerobically oxidized. 

Another class of mixed-metal anion receptors has been investigated which possess redox reporter groups based on two different metal complexes. This enables the quahtative comparison of their comparative anion-sensing abih-ties. Macrocycles 35 and 36 combine the Ru (bpy)3 moiety with a bridging ferrocene or cobaltocenium imit [29]. Electrochemical experiments in acetonitrile solution revealed that the Ru VRu redox potential was insensitive to anion binding, whereas the ferrocene/ferrocenium (in 35) and cobal-tocene/cobaltocenium (in 36) redox couples were shifted cathodically (by 60 mV and 110 mV respectively with chloride). However, the first reduction of Ru°(bpy)3, a Hgand-centred process based on the amide substituted bipyridyl, was also found to imdergo an anion induced cathodic shift (40 mV and 90 mV with chloride for 35 and 36, respectively). 

The ferrocene/ferrocenium couple is still the lUPAC recommended reference for reporting redox potentials in nonaque-ous solvents [37]. Recently, Connelly and Geiger strongly urged workers to report... 

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