Ferrocenium electrochemical

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... 

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... 

As far as the AEP values are concerned, it cannot be ruled out that the molecular rearrangements that occur on passing from ferrocene to ferrocenium ion might also play a role, even if minimal, in their departure from the value of 59 mV, or in lowering the degree of electrochemical reversibility of the process. 

In support of the electrochemical evidence, the molecular structure of the biferrocenium ion in [( -CsF Fe -CsfLOf -CsfLOFe -CsHs)] [I3] shows that both pairs of cyclopentadienyl rings have an eclipsed conformation, Figure 16. Furthermore, the mean Fe-Cp(Centroid) distance is equivalent in both the ferrocenyl units and equal to 1.68 A. Speculatively, this value is intermediate between the values previously observed for ferrocene and ferrocenium ions, thus supporting charge delocalization between the two centres.27... 

Attempts have been performed to analyse (in terms of rate constants) the steric-to-electrochemical factors which can influence the interaction of ferrocenium derivatives with reduced GOD, or the ability of ferrocenes to act as mediators in the enzymatic oxidation of glucose as a function of cyclopentadienyl substituents.63... 

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. 

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). 

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. 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

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. 

Beer and co-workers have investigated this concept using self-assembled monolayers of the l,l -bis(alkyl-iV-amido)ferrocene 106 on gold electrodes [72]. The pendant disulphide groups serve to covalently anchor the receptor to the gold surface. In electrochemical experiments on 106 in acetonitrile/dichloro-methane solution, anion-induced cathodic shifts of the ferrocene/ferrocenium... 

Electrochemical studies confirmed the presence of redox-active nanoparticles. Differential pulse and cyclic voltammetry studies were conducted. Cyclic voltammetry showed that the complex displays an electrochemically reversible ferrocene/ ferrocenium couple (Figure 9.6). The oxidation potential for the hybrid CPMV-Fc conjugate and free ferrocenecarboxylic acid in solution was determined E1/2 of CPMV-Fc was 0.23 V, and Elj2 of free ferrocenecarboxylic acid was 0.32 V versus the Ag/AgCl electrode, respectively. This shift is expected for the conversion of the carboxyl group of ferrocenecarboxylic acid to an amide on coupling to the virus capsid, since the amide is less electron-withdrawing. 

An alternative to light-related detection is an electrochemical response. If the sensor and analyte are in solution then cyclic voltammetry can be used to detect changes in redox potential between the free sensor and its complex with the analyte. Supramolecular applications of this approach were pioneered by Beer who linked crown ethers to electrochemically responsive ferrocenium [1] and cobalticinium [14] groups. In the former case a response was detected when cations complementary to the crown ether cavity were added to acetonitrile solutions of the sensors in the latter, anions were detected by an acyclic receptor. 

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