Ferrocenes ferrocenium

In 90% aqueous DMSO. Measured vs. Ag/Ag" " in acetonitrile rev reversible, irrev irreversible. "Kinoshita et al. (1994). In DMSO-water (molar ratio 85 15). In DMSO. The potential determined vs. ferrocene/ferrocenium was converted to the value vs. Ag/Ag by adding -(-0.083 V (Komatsu etal., 1982a). 

The 1,4,7-trithiacyclononane ligand, [9]aneS3, zinc complex was synthesized to compare with the electrochemistry of related complexes and showed an irreversible oxidation and an irreversible reduction at +1.30 V and —1.77 V vs. ferrocene/ferrocenium, and the X-ray crystal structure of the bis macrocycle zinc complex was reported.5 0,720... 

In the attached ferrocene nonadiabatic case (Figure 1.24), large variations of a with potential are detected, thanks to the conjunction of two factors (1) the ferrocene-ferrocenium couple entails a rather small reorganization energy (about 0.85 eV), consisting essentially of a solvent reorganization contribution, hence the large variation of a with potential and (2) at the... 

FIGURE 1.24. Potential-dependent forward and backward rate constants of the ferrocene-ferrocenium couple attached to a gold electrode hy a long-chain alkane thiol assembled together with unsubstituted alkane thiols of similar length. Solid line use of Equations (1.37) to (1.39) with X, = 0.85 eV, ks — 1.25 s 1. Adapted from Figure 4A in reference 65, with permission from the American Association for the Advancement of Science. 

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... 

The addition of stoichiometric amounts of Ni2+, Cu2+ and Zn2+ to solutions of [28]—[32] in acetonitrile led to large anodic shifts of the respective ferrocene/ferrocenium redox couple of up to 190 mV in the case of [29] and Cu2+ (Table 8). Analogous experiments in water at pH values 10.5-12 revealed that [28]—[32] electrochemically recognize these transition metal cations in the aqueous environment (Table 9). 

When an equimolar mixture of Ni2+, Cu2+ and Zn2+ was added to aqueous electrochemical solutions of [29] and [30] the ferrocene-ferrocenium redox couple shifted anodically by an amount approximately the same as that induced by the Cu2+ cation alone. This result suggests that [29] and [30] are... 

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. 

Oxidation potentials of the conjugated anions measured in DMSO solution and referenced to the ferrocene/ferrocenium couple. 

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. 

The well known ferrocene/ferrocenium oxidation possesses the features typical of electrochemical reversibility. This foreshadows the substantial maintenance of the original molecular geometry on passing from decamethylferrocene to decamethylferrocenium. As a matter... 

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. 

It can be also deduced that the potential of the ferrocene/ferrocenium couple, for the same solvent, also varies with the supporting electrolyte. 

Such behaviour, in particular the slight elongation of the Fe-C distance following the one-electron removal, is a common feature which has been found in different, structurally resolved, ferrocene/ferrocenium couples. Table 5 shows a few examples. 

Most of the countless ferrocene derivatives which have been synthesized display a one-electron ferrocene/ferrocenium oxidation process at potentials roughly predictable on the basis of the inductive effects of the substituents. It should be noted, however, that at times the introduction of substituents renders the corresponding ferrocenium species unstable. 

One of these studies, which requires sophisticated apparatus, involved the ferrocene/ferrocenium oxidation process.4,5 Figure 18 compares the response obtained with platinum and superconducting electrodes in the temperature range of the Tc. 

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). 

Shifts of the anodic current peak potential of the ferrocene/ferrocenium couple produced by the presence of anions (2 equiv.) added as their tetrabutylammonium salts (Af = (fp,(free) - p (anion)l). 

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. 

The outer-sphere one-electron reduction of CO2 leads to the formation of the 02 radical anion. In dry dimethylformamide, the C02/ C02 couple has been experimentally determined to be —2.21 V vs. standard calomel electrode (SCE) or approximately —2.6 V vs. the ferrocene/ferrocenium couple [21,22]. From pulse radiolysis experiments, the reduction potential of CO2 is —1.90 V vs. the SHE in water (—2.14 V vs. SCE) [23]. Theoretical calculations have been used to calculate the contributions of various factors to the reduction potential of CO2. These include the electron affinity of CO2,... 

Both in acetonitrile and in other non-aqueous solvents, a major problem arises in terms of the manner in which the potential values are reported by various investigators. Koepp, Wendt, and Strehlow [6] noted that hydrogen ion is the poorest reference material on which to base nonaqueous potentials because of the extreme differences in its solvation in various solvents. On the basis of an investigation of the solvent dependence of 18 redox couples, these investigators concluded that ferrocene/ferrocenium ion (i.e. bis(cyclopentadienyl)iron(III/II), abbreviated as Fc+ /Fc°) and/or cobal-tocene/cobalticenium ion represented optimal potential reference materials for nonaqueous studies. On the basis of their minimal charge (+1, 0) and their symmetry (treated as though they were roughly spherical), the potentials of these two redox couples are presumed to be relatively independent of solvent properties. 

The first members of the fullerene family to be discovered were C60 and C7o-The electrochemical properties of these compounds have been well characterized. Both fullerenes show six reversible reductions and one oxidation by cyclic voltammetry.4 5 The reductions are almost equally spaced, with the first reduction occurring at — 1.0 V versus ferrocene/ferrocenium couple. Successive reductions occur approximately 400mV apart (Fig. 8.1). 

Figure 8.1 Cyclic voltammetry (a) and Osteryoung square wave voltammetry (b) of Cgo (acetonitrile/toluene + 0.1 M ( -Bu)4NPF6), using a glassy carbon electrode (GCE) working and ferrocene/ferrocenium (Fc/Fc +) couple as an internal reference. Reprinted with Permission from Ref. 4. Copyright 1992 American Chemical Society.

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. 

A first group of compounds Ce Cg2, Gd Cg2, Y Cg2, and the major [C2v] and minor [Cs] isomers of La Cg2 and Pr Cg2 showed two oxidation steps, the first reversible and the second irreversible, even at scan rates up to 1 V/s. The low potential of the first oxidation step, close to that of the ferrocene/ferrocenium couple (see Table 8.3 and Fig. 8.4), made these compounds rather good electron donors. These compounds could also be reduced in four to six distinct steps, most of them reversible, and their reducing ability was found even higher than that of and similar to that of the major isomer of Cg2 (C2). Noticeably, all these compounds had a very low electrochemical HOMO-LUMO gap (A ,gap<0.50 V). In addition, similar UV/Visible spectra were obtained for all of them,28 suggesting also similar electronic structures. ESR showed that Y Cg229 and both isomers of La CX249 52 are radical species and consequently that the formal oxidation state of the metal in these structures is probably + 3. Therefore, their low HOMO-LUMO gap is probably a consequence of their open-shell electronic structure. 

The efficient on/off switching of fluorescence from substituted zinc porphyrin-ferrocene dyads 16a and 16b is achieved through redox control of the excited-state electron transfer quenching.26 This redox fluorescence switch is based on the switching of the excited-state electron transfer from the ferrocene to the zinc porphyrin through the use of the ferrocene/ferrocenium (Fc/Fc +) redox couple. 

Figure 4 Dark currents in DSSCs with the standard I /I2 redox couple (solid line) and with a kinetically much faster redox couple, ferrocene/ferrocenium, FeCp2 + /0. A = tetrabutylammonium. The charge-transfer resistance, Rct (see Fig. 1), of the 1 2 couple is 106 times greater than that of the FeCp2+/0 couple, leading to what is sometimes mistaken as diode behavior in the dark for the cell containing the 1 2 couple. (Data from Ref. 49.)...

Ohmic effects render Epc more negative, Epa more positive, AEp and 8Ep larger, and X smaller than the true values. Since experimental approaches to elimination of iRu errors are not foolproof (see Chap. 7), the presence of ohmic distortions should be tested by measurements on a Nernstian couple such as ferrocene/ferrocenium under conditions identical to those used to probe the test compound. In principle, errors in the measured CV parameters for a test compound can be eliminated by referencing its responses to those of the Nernstian standard. Note that this approach is accurate only if the current level of the standard, rather than its concentration, is equal to that of the test compound, since the diffusion coefficients of the two species may appreciably differ. 

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