Cobaltocenium reduction

Porphyrin oxidation Porphyrin reduction Cobaltocenium reduction... 

Another variety is the ester type of receptors, containing carboxycobaltocenium units esterified with various diphenols, 72 and 73, which introduce aromatic spacers [122]. Such compounds accept Br guests and shift the cobaltocenium reduction wave towards more negative potentials. In this instance anion recognition and fixation seem to be based simply on electrostatic interactions. 

As an extension of this work,Kaifer et al. prepared a series of PPI dendrimers (G-l-G-4) functionalized with cobaltocenium at the periphery [48] and studied their electrochemical behavior and binding interactions with P-CD. While the positively charged cobaltocenium-terminated dendrimer 27G-1 is not com-plexed by P-CD in aqueous media, electrochemical reduction of the dendrimer in the presence of excess P-CD triggers the formation of a multisite inclusion complex with this host to provide pseudorotaxane-terminated dendrimer 28G-1 (Fig. 10). Similar electrochemical and binding behavior toward P-CD was observed for the G-2 and G-3 dendrimers. However, the resulting multisite inclu-... 

The electrochemical properties were investigated by CV and SWV. The effects of adding anions to solutions of the porphyrin are summarized in Table 20. Interestingly, the porphyrin reduction waves are not significantly perturbed by any anionic guest, but the cobaltocenium moieties do show cathodic shifts of up to 225 mV with the dihydrogenphosphate anions. 

As seen the complex displays either a single ferrocenyl-centred oxidation (which however looks like it is partially chemically reversible) or a single cobaltocenium-centred reduction (which is affected by electrode adsorption), thus testifying that no interaction exists among the different metallocene units. 

In a similar manner to manganocene, cobaltocene is easily oxidized to the corresponding cobaltocenium ion [Co(f/5-C5H5)2]+, which, as illustrated in Figure 48, exhibits two distinct chemically reversible reductions corresponding to Coni/Con and Con/CoI.88... 

The first redox-active receptor for anions was the bis-cobaltocenium macrocylic ester 17 reported by Beer and Keefe in 1989.25 FAB-MS and FT-IR evidence indicates that 17 can bind anions in the oxidized state. Reduction of the cobaltoceniums to the zero state would be expected to weaken this interaction, and indeed a modest 45 mV j/2 shift is observed in acetonitrile upon addition of 4 equivalents of Br. ... 

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. 

Dendrimers Terminated with Cobaltocenium and Ferrocene-Cobaltocenium Units Like ferrocene, cobaltocenium is an excellent organometallic moiety to incorporate in or functionalize dendritic systems. As already discussed, it is indeed isoelectronic with ferrocene, highly stable, positively charged complex, which undergoes a reversible monoelectronic reduction to yield the neutral cobaltocene. 

They are used as voltammetric solvent alone or by mixing with C02. Usually they have wide potential windows. Olsen and Tallman [26] measured in supercritical chlorodifluoromethane the oxidation wave of ferrocene (Fc° -> Fc+) and the reduction wave of cobaltocenium ion (Cc+->Cc°). The difference between their halfwave potentials was 1.28 V, in good agreement with 1.31 V obtained in various non-aqueous solutions (Section 8.2.2). Recently, Abbott and Eardley [27] studied the reduction of C02 in a mixed SCF (C02/HFC-134a, %HFc-i34a = 0.3) using Pt and Pb electrodes. At 60 °C and 260 bar, the faradaic efficiencies (%) of (COOH)2,... 

Figure 12.5 Cyclic voltammograms for the reduction of 1 mM cobaltocenium (Cp2Co+) hexafluorophosphate and oxidation of 1 mM ferrocene (Cp2Fe) in acetonitrile recorded at a band electrode (width = 4.6 / m) at a scan rate of 10 mV s 1. The supporting electrolyte is tetrabutylammonium hexafluorophosphate at (A) 0.02 M, (B) 0.2 mM, (C) 2.0 mM, and (D) 20 mM. [From Ref. 68, reprinted with permission of the copyright holder.]...

Figures 11.10 (a) and (b) show that the voltammetry of these couples in a range of RTILs is nearly electrochemically reversible. Note however that, unlike the ferrocene- and cobaltocenium-based couples, the reduction potentials are likely to vary significantly from one RTIL to another. In experimental practice it is also important to verify that the calibration molecules do not interfere chemically with the voltammetric process under study. For example, we have investigated the oxidation of molecular hydrogen in the presence of TMPD and observed a reaction of the two species, as noted by the disappearance of the reverse-peak of the first redox couple (see Figure 11.11). This implies that the peak potentials ofTMPD +/TMPD are no longer obvious, and that this redox couple cannot be used as an internal reference in this type of experiment.

Redox potentials of cobaltocene and substituted cobaltocenes (Section 7.3) have been determined for the reduction of cobaltocenium to cobaltocene, a potential of —0.95 V (vs. SCE) in acetonitrile or —0.86 V (vs. SCE) in CH2CI2 was measured (—1.35 V vs. the ferrocene/ferrocenium couple in aprotic solvents). The large potential difference between oxidation of ferrocene and cobaltocene is intriguing, since it is related in a simple fashion to the difference in the HOMCULUMO gap... 

Cobaltocene is stable as a monomer under all conditions, showing no tendency to dimerize, as does the rhodium congener rhodocene. Cobaltocenium salts are easily prepared by several routes, including mild oxidation of cobaltocene, and are stable as cations. However, neutral electron-rich monomers are no longer stable if the aromaticity of one of the Cp rings is perturbed by interposed CH2 groups (e.g. CpCo(cyclohexadienyl)). Dimerization of a C-C bond adjacent to the jr-system frequently occurs (Scheme 28). The process can be followed electrochemically on reduction of the CpCo()] -E)+ cation to the neutral sandwich, which then dimerizes. Logically, the same dimerization is observed with so-called half-open cobaltocene, that is, a bis(pentadienyl)cobalt, which has one pentadienyl see... 

Cobaltocene is the most frequently used single-electron reductant although its standard oxidation potential is not very negative E° = —1.31 V relative to FeCp2 in DME). Thus, it can only reduce organometallic compounds which are very easy to reduce. It is readily prepared under an inert atmosphere, but another drawback is that it is air-sensitive in both solution and the solid state and must be sublimed just before use [120]. Its properties as a reductant have been reviewed [21, 121, 122]. Noteworthy applications of cobaltocene are single-electron reductions of neutral complexes to their monoanions giving ion pairs with cobaltocenium as the counter-cation ... 

A particular emphasis is to be made on these systems the recognition of ferrocene-based substrates can be switched off by monoelectronic oxidation of the substrate itself, whereas in the case of positively charged guests the binding interaction can be electrochemically activated by either a mono- (for cobaltocenium) or bi-electronic (for viologen) reduction process [86]. 

Wang Y, Mendoza S, Kaifer AE (1998) Electrochemical reduction of cobaltocenium in the presence of P-cyclodextrin. Inorg Chem 37 317-320... 

We reported the first transition metal centered anion receptor to operate solely through electrostatic attraction in 1989 [9, 186]. Receptor 74 contains two positively charged, 18-electron, air stable, redox active cobaltocenium moieties. The reversible reduction potential of these redox active centers was observed to shift cathodically (up to 45 mV) on the addition of excess bromide ions. This... 

Figure 2.20 Reduction current vs. potential 4>sci across the space-charge region at -GaAs in acetonitrile with cobaltocenium (CoCp2 ) R = charge-transfer resistance, Ug = electrode potential (Meier etal., 1999).

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