Electrochemistry of ferrocenes

Crooks, R. M. and Bard, A. J. (1988a) Electrochemistry in near-critical and supercritical fluids. Part VI. The electrochemistry of ferrocene and phenazine in acetonitrile between 25 and 300°C. J. Electroanal. Chem. 243, 117-131. 

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. 

Several problems are encountered when potentials in different solvents are sought compared to the potential scale in water. A variety of approaches [186-194] have been followed to attack this problem usually the approach has been to introduce some kind of nonthermodynamic assumption, such as the supposition that certain large, monovalent ions (Rb, CS ) [191] or redox systems [186-188] of the charge type n/n + 1 (preferably 0/ + 1 [186,187]) have a nearly equal free energy of solvation in the two solvents so that the free energy of a transfer of the reference ion is small. The redox couples [194] ferro-cenium/ferrocene and bis(biphenyl)chromium(I)/bis(biphenyl)chromium(0) (BCr" "/BCr) have been recommended as reference redox systems for measurements in nonaqueous solvents however, an investigation concluded [195] that the electrochemistry of ferrocene in MeCN at microelectrodes was far from ideal, as some film formation may occur. 

An extensive study of the electrochemistry of ferrocene, methyl viologen, and ferricyanide was conducted in SDS-dodecane-l-pentanol-0.1 M NaCl microemulsion [69]. The ferricyanide was confined to the aqueous phase, while the methyl viologen was partitioned. The shifts in the half-wave potentials were analyzed in terms of the model of Ohsawa et al. [70]. Mackay and coworkers [71-75] investigated microemulsions as media for the degradation of the nerve agent simulant / -nitrophenyldiphenyl phosphate (PNDP) using iodosobenzoate as catalyst. Luminescence and electrochemical studies were conducted in SDS, CTAB, and cetyltrimethylammonoum chloride (CTAC) microemulsions used... 

Figure 1. (a) Scheme for the reaction sequence of the electrochemically coupled enzymatic oxidation of glucose, (b) The effect of the enzymatic reaction on the electrochemistry of ferrocene using a gold disc of 25lim diameter at 5mV s... 

Alcohols are often used as cosurfactants in microemulsions, and insight has been obtained from the electrochemistry of ferrocene alcohols (4). Oxidations of FcOHCio, FcOHCu, and FcOHCig [34] were nearly reversible and controlled by diffusion in microemulsions of DDAB, CTAC, or SDS. In micellar solutions, electrode reactions were more complex and reflected strong adsorption of the ferrocene alcohols onto the electrode. 

One of the first CPs to be studied as a modified electrode was P(Py), at which the electrochemistry of ferrocene (FeCp2) was studied [55]. This FeCp2 system (E° +0.42 V vs. SCE) showed near ideal reversible electrochemical behavior, with proportional to indicating diffusion limited processes, cathodic/anodic peak current ratios of near unity, and cathodic/anodic peak separations close to 70 mV. More recently, poly (bithiophene) modified electrodes have been used to monitor the electrochemistry of FeCp2 as well as / -benzoquinone, and to probe the semiconducting behavior of this CP [56]. Ferrocene appears popular as a near-reversible probe material for CP-modified electrodes, having been studied with poly(thiophene) and other CP modified electrodes as well [57]. 

Weaver JEF, Breadner D, Deng F, Ramjee B, Ragogna PJ, Murray RW (2011) Electrochemistry of ferrocene-functionalized phosphonium ionic liquids. J Phys Chem C 115 19379-19385... 

The electrochemistry of some single-atom bridged ferrocene dimers gives a separation of two le waves. The carbon atoms in CROH and C = 0 can communicate between ferrocene units in 32 and 33 (101). Diferrocenyl sulfide, 34, shows significant AE° values, 0.29-0.33 V, indicating the presence of strong internuclear interactions (102,103). 

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... 

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... 

The second-generation 02" biosensors are mainly based on the electron transfer of SOD shuttled by surface-confined or solution-phase mediators, as shown in Scheme 2(b). In 1995, Ohsaka et al. found that methyl viologen could efficiently shuttle the electron transfer between SOD and the glassy carbon electrode and proposed that such a protocol could be useful for developing 02 biosensors [125], Recently, Endo et al. reported an 02, biosensor based on mediated electrochemistry of SOD [148], In that case, ferrocene-carboxaldehyde was used as the mediator for the redox process of SOD. The as-developed 02 biosensor showed a high sensitivity, reproducibility, and durability. A good linearity was obtained in the range of 0 100 pM. In the flow cell system, tissue-derived 02 was measured. 

One final issue remains to be resolved Of the portion of the AEpi that is due to resistance, what part is caused by solution resistance and what part is caused by film resistance To explore this issue we examined the electrochemistry of a reversible redox couple (ferrocene/ferricinium) at a polished glassy carbon electrode in the electrolyte used for the TiS 2 electrochemistry. At a peak current density essentially identical to the peak current density for the thin film electrode in Fig. 27 (0.5 mV see ), this reversible redox couple showed a AEpi of 0.32 V (without application of positive feedback). Since this is a reversible couple (no contribution to the peak separation due to slow kinetics) and since there is no film on the electrode (no contribution to the peak separation due to film resistance), the largest portion of this 0.32 V is due to solution resistance. However, the reversible peak separation for a diffusional one-electron redox process is —0.06 V. This analysis indicates that we can anticipate a contribution of 0.32 V -0.06 V = 0.26 V from solution resistance in the 0.5 mV sec control TiS2 voltammogram in Fig. 27. 

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. 

Assume that a disk-shaped electrode (gold, platinum, carbon, etc.) has been coated with a Film of poly (vinyl ferrocene) (Table 13.2). This can be accomplished by dissolving the polymer in chloroform, applying a drop of the solution to the electrode surface, and allowing the solvent to evaporate. The electrochemistry of the resulting polymer film-coated electrode can be investigated using the same electrochemical cell and equipment as described in the previous example. 

In the previous section, we demonstrated the micrometer droplet size dependence of the ET rate across a microdroplet/water interface. Beside ET reactions, interfacial mass transfer (MT) processes are also expected to depend on the droplet size. MT of ions across a polarized liquid/liquid interface have been studied by various electrochemical techniques [9-15,87], However, the techniques are disadvantageous to obtain an inside look at MT across a microspherical liquid/liquid interface, since the shape of the spherical interface varies by the change in an interfacial tension during electrochemical measurements. Direct measurements of single droplets possessing a nonpolarized liquid/liquid interface are necessary to elucidate the interfacial MT processes. On the basis of the laser trapping-electrochemistry technique, we discuss MT processes of ferrocene derivatives (FeCp-X) across a micro-oil-droplet/water interface in detail and demonstrate a droplet size dependence of the MT rate. 

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