Ferrocene/ferrocenium redox couple

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. 

The accessible ferrocene/ferrocenium redox couple of ferrocene has led to its frequent use in electrochemical anion sensors. The chemical and structural similarity between ferrocene and cobaltocenium has meant that receptors based on these complexes often share the same design. The most relevant difference is that the ferrocene derivatives are neutral (until oxidised to ferrocenium),have no inherent electrostatic interaction with anions and therefore their complexes with anions exhibit lower stability constants. 

A very unusual mixed-metal receptor has been prepared by Jurkschat and co-workers [68]. Macrocycle 102 comprises two ferrocene reporter units linked together by two Lewis acidic organotin spacers. Electrochemical measurements in dichloromethane solution show anion-induced cathodic shifts of the ferrocene/ferrocenium redox couple of 130 mV for chloride, 210 mV for fluoride and 480 mV for dihydrogenphosphate. 

Another example of ET in an inhomogeneous medium is the three-zone interfacial assembly depicted in Figure 3.27. To model As for ET between a film-modified metal electrode and a ferrocene/ferrocenium redox couple in contact with aqueous solvent [49], the Poisson equation was solved with the following parameters s = 1.8 and 0I = 78.0 (water) = s0ll = 2.25 (alkane film) = eom = oo (metal), a = d = 3.8 A (effective radius of redox group), and Aq = e [23]. 

Figure 18.6 Comparison of the electrochemical windows of RMIm(HF)2 3F. W.E. GC disk C.E. Pt plate scanning rate 10mVs The potential is referred to the potential of ferrocene/ ferrocenium redox couple in each salt, (a) DMIm(HF)2.3F (b) EM m(HF)2 j= (c) PrM m(HF)2 (d) BMIm(HF)2.3F (e) PeMlm(HF)2 3F (f) HMIm(HF)2.3F. Vertical dotted lines denote cathode and anode limits.

We briefly mention here the use of the ferrocene/ferrocenium redox couple to mediate electron transfer on the oxidation (anodic) side, especially in derivatized electrode. This broad area has been reviewed [349]. For instance, polymers and dendrimers containing ferrocene units have been used to derivatize electrodes and mediate electron transfer between a substrate and the anode. Recently, ferrocene dendrimers up to a theoretical number of 243 ferrocene units were synthesized, reversibly oxidized, and shown to make stable derivatized electrodes. Thus, these polyferrocene dendrimers behave as molecular batteries (Scheme 42). These modified electrodes are characterized by the identical potential for the anodic and cathodic peak in cyclic voltammetry and by a linear relationship between the sweep rate and the intensity [134, 135]. Electrodes modified with ferrocene dendrimers were shown to be efficient mediators [357-359]. For the sake of convenience, the redox process of a smaller ferrocene dendrimer is represented below. 

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. 

Molina and co-workers have investigated the urea-crown ether functionalised ferrocene 27 [25]. This receptor-produced anion induced cathodic shifts in the ferrocene/ferrocenium redox couple of 52 mV with fluoride and 190 mV with dihydrogenphosphate. In acetonitrile solution on addition of 2 equivalents of K ions a dramatic attenuation in the anion-induced cathodic shift was observed, with dihydrogenphosphate giving rise to a shift of only 50 mV. 

Cyclic voltammetry (CV) is often used to determine the electrochemical properties of D-A copolymers. Usually, the CV setup consists of a Ag/AgNOs reference electrode, a platinum wire counterelectrode and a glassy carbon electrode with a drop-cast polymer film. First, the CV curve of ferrocene/ ferrocenium redox couple (Fc/Fc ) should be measured to calibrate the reference electrode. The polymer s HOMO energy level in electron volts is calculated from the onset of the oxidation peak ( ox) according to eqn (15.4). If the reduction peak of polymer is not clearly visible, the LUMO level can be approximated from the HOMO measured by CV and the bandgap measured by UV-vis. 

Cyclic voltamograms of 4 and 8 show the reversible oxidation event to be shifted 0.30 V and 0.32 V, respectively, more positive than the ferrocene/ferrocenium redox couple. These data would suggest that the iron centers in 4 and 8 are much less capable of stabilizing a positive charge. It is logical to argue they would also form a much less stable a-ferrocenyl carbocation intermediate hence, we tend to rule out such a species in the homopolymerization of complex 4. 

Discuss critically the statement that the ferrocene/ferrocenium redox couple is fully electrochemically reversible in acetonitrile solution at a platinum electrode. 

The difference of 10 mV is significant since the ferrocene/ferrocenium redox couple is often used as a redox marker in non-aqueous electrochemistry (acting as an internal reference electrode), including that using room temperature ionic hquids as solvents. The correction for the difference in diffusion coefficients is necessary to give a valid reference scale. 

Because of its insolubility, redox potentials of SmCp2 are difficult to obtain. However, electrochemical reduction of (C5H5)3Sm in THF at mercury, gold and platinum electrodes have been used to estimate the redox potential (Bond et al., 1986). The E1/2 value was found to be —2.66 V vs. ferrocene/ferrocenium redox couple. The redox potential is considerably more negative than for the reduction of Sm(III) in water (Morss, 1976) indicating the impact of the Cp ligand on Sm(II) redox chemistry. 

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