Ferrocenes ferrocenium system

We have investigated the ferrocene/ferrocenium ion exchange to determine the effects of different solvents on electron-transfer rates. There is probably only a very small work term and very little internal rearrangement in this system. Thus the rates should reflect mostly the solvent reorganization about the reactants, the outer-sphere effect. We measured the exchange rates in a number of different solvents and did not find the dependence on the macroscopic dielectric constants predicted by the simple model [Yang, E. S. Chan, M.-S. Wahl, A. C. J. Phys. Chem. 1980, 84, 3094]. Very little difference was found for different solvents, indicating either that the formalism is incorrect or that the microscopic values of the dielectric constants are not the same as the macroscopic ones. 

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 ferrocene/ferrocenium reference redox system at platinum fulfills these requirements fairly well [4-6]. Another system which has been recommended is bis(biphenyl)chromium (0)/bis(biphenyl)chromium (+1) (BCr+ /BCr) [5, 7]. Several other systems have been suggested, and used sporadically, such as cobaltocene/cobaltocenium, tris(2,2 -bipyridine) iron (I)/tris(2,2 -bipyridine) iron (0), Rb+/Rb(Hg), and so on. 

For several tautomeric systems ketones/enols, imines/enamin and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative a-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide a-carbonyl radical and a-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V. 

C-P-Im triad 49 was prepared in order to further investigate this phenomenon [184]. An electron acceptor moiety based on the 4,5-dinitro-l,8-naphthalenedicar-boximide system was used in this triad, as this imide is more easily reduced that pyromellitimide (—0.88 V versus ferrocene/ferrocenium as an internal reference, compared to —1.24 V for the imide moiety of 48), and can therefore serve as an acceptor for a wider variety of donors. Excitation of the porphyrin moiety of 49... 

In aprotic (as well as in protic and mixed) media, the two reference systems of choice are ferrocene/ferrocenium and bis (biphenil) chromium (I/O). The pen-tamethylcyclopentadienyl analog of the former was recently shown to yield higher performance [49, 50]. Among other typical electrodes, Ag/AgN03 should be mentioned. We can also mention special reference systems suitable for certain solvents, such as amalgam electrodes based... 

The proximity of the anion-binding core of the calixpyrrole to the electrochemically active ferrocene moiety led to the reasonable expectation (based on electrostatic considerations) that in both compounds 14 (Figure 17) and 15 (Figure 18) a cathodic shift in the ferrocene/ferrocenium redox wave would be observed as an anion was bound. However, in contrast to these expectations, anodic shifts were observed upon the addition of certain anions to 14 and 15 (Table 4). In the case of the -ferrocenylamidocalix[4)pynole 14, cathodic. shifts of 14 and 207 mV were observed for the Fc /Fc couple upon addition of F and Cl anions, respectively. On the other hand, the addition of H2POJ anions to this system was found to lead to an anodic shift of 9 mV (Table 4). An anodic... 

Cyclic voltammograms were obtained on a Bioanalytical Systems CV-27 instrument samples were dissolved in dry THE containing 0.1 M [Et4N][PF0] as supporting electrolyte. The voltammograms were obtained at a scan rate of 100 mV/sec, and Ei/2 values were determined relative to ferrocene/ferrocenium as an internal standard. The electrode array consisted of a saturated calomel reference electrode and platinum disk (working) and wire (auxilliary) electrodes. Potentials were uncorrected for junction effects. 

A well-defined redox couple can be used to calibrate an RE or as an internal standard in electrochemical experiments. The reference redox couple must be stable for the duration of the measurement, and must exhibit a repeatable potential in the system used. A good reference redox couple (63) for nonaqueous, and some carefully controlled aqueous systems, is the ferrocene/ferrocenium (FcIFc ) couple at 0.5-10.0 mM concentration. Standard reduction potentials, E°, for various solvents (64) are listed in Table 4.10. Other couples can be found in References (64-66). 

As mentioned above, ferrocene is amenable to electrophilic substitution reactions and acts like a typical activated electron-rich aromatic system such as anisole, with the limitation that the electrophile must not be a strong oxidizing agent, which would lead to the formation of ferrocenium cations instead. Formation of the CT-complex intermediate 2 usually occurs by exo-attack of the electrophile (from the direction remote to the Fe center. Fig. 3) [14], but in certain cases can also proceed by precoordination of the electrophile to the Fe center (endo attack) [15]. 

The structure of [Ir(cod)(dppf)]PF6 shows approximately square-planar geometry at Ir, and the cp rings of the dppf ligand are close to parallel and staggered.592 The systems [Ir(cod)(LL)]C104, where LL = dppf, l-diphenylphosphino-2-(7V,7V-dimethylamino)methyl ferrocene and 1,6-diferrocene-2,5-diazahexane, catalytically trimerize PC=CH to 1,3,5-triphenylbenzene.593 The electrochemistry of [Ir(dppf)2]BPh4 shows two one-electron reductions at —1.560 V and -1.755 V vs. ferrocenium/ ferrocene.753... 

Because of their reversible electrochemical properties, ferrocene [biscyclopentadie-nyl-iron(II), FeCp2 and cobaltocenium [biscyclopentadienyl-cobalt(III), CoC p2 1 I are the most common electroactive units used to functionalize dendrimers. Both metallocene residues are stable, 18-electron systems, which differ on the charge of their most accessible oxidation states zero for ferrocene and + 1 for cobaltocenium. Ferrocene undergoes electrochemically reversible one-electron oxidation to the positively charged ferrocenium form, whereas cobaltocenium exhibits electrochemically reversible one-electron reduction to produce the neutral cobaltocene. Both electrochemical processes take place at accessible potentials in ferrocene- and cobaltocenium-containing compounds. 

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