Fullerenes redox properties

Chen N, Klod S, Rapta P et al (2010) Direct Arc-discharge assisted synthesis of CeoH2(C3H5N) a cis-l-pyrrolino Ceo fullerene hydride with unusual redox properties. Chem Mater 22 2608-2615... 

This chapter presents an up-to-date account of the redox properties of the pristine fullerenes and a large number of their derivatives as revealed by electrochemical studies in solution. The description here is as exhaustive as possible, although not completely comprehensive due to the large number of reports on the subject that have appeared over the years. A section on electrosynthesis of fullerene derivatives is included, with special emphasis on the retro-cyclopropanation reaction, a reaction that has led to the formation of novel derivatives as well as... 

The covalent chemistry of fullerenes has developed very rapidly in the past decade in an effort to modify fuUerene properties for a number of applications such as photovoltaic cells, infrared detectors, optical limiting devices, chemical gas sensors, three-dimensional electroactive polymers, and molecular wires [8, 25, 26, 80-82]. Systematic studies of the redox properties of Cgo derivatives have played a crucial role in the characterization of their unique electronic properties, which lie at the center of these potential applications. Furthermore, electrochemical techniques have been used to synthesize and separate new fullerene derivatives and their isomers as well as to prepare fullerene containing thin films and polymers. In this section, to facilitate discussion of their redox properties, Cgo derivatives have been classified in three groups on the basis of the type of attachment of the addend to the fullerene. In group one, the addends are attached via single bonds to the Cgo surface as shown in Fig. 6(a) and are referred to as singly bonded functionalized derivatives. The group includes... 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

The study of Sc2C2 Cg2 [C3v] and Sc3C2 C80 began with a misunderstanding. Their redox properties were first interpreted as if these compounds were Sc2 Cg4 and Sc3 C82 before Nagase et al. showed that they were in reality metal carbide endohedral fullerenes.62,63... 

Very similar results were obtained from the CV studies of ( )-38 and ( )-39, but the observed anodic shifts of the first redox couples upon complexation with K+ were smaller (50 mV for ( )-38 and 40 mV for ( )-39). The reduction of the anodic shift from 90 mV (in ( )-37) to 40 mV (in ( )-38) can be explained by an increasing average distance between the cation bound to the crown ether and the fullerene surface, as the addition pattern changes from trans-1, to trans-2, and to trans-3 [55], Additionally, the effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of ( )-37 were investigated. As expected, all electrochemical data clearly demonstrate a much larger interaction between crown-ether-bound cations with the negatively charged than with the neutral fullerene core [55],... 

Ferrocene is composed of a pair of 6-7r-electron carbon arrays and a 6-d-electron iron(II) atom. Ferrocene-fullerene donor-acceptor dyads carry all the requisites for electron-transfer phenomena. However, data for the formation of ferrocene-fullerene hybrids are not abundant. Some such dyads have already been synthesized following the methodology of 1,3-dipolar cycloaddition of the appropriate azome-thine ylides to C60, with either variable-spacing building blocks or a rigid-bridge all-cj-bonded framework, in order to tune the redox properties of the system [40,234, 248-251]. Another novel dyad that contained two covalently bound ferrocene units was recently synthesized via cyclopropanation of the fullerene core [252]. 

The CV measurements indicate that the oxidation state of the yttrium atom in Y Cs2 (Suzuki et al., 1996) was close to that of La Cs2, probably 3-i. The electrochemistry of Y Cs2 is almost identical to that of La Cs2 (Anderson et al., 1997a Suzuki et al., 1996). Other mono-metallofullerenes such as Y Cs2, Ce Cs2 and Gd Cs2 have a similar tendency in their redox properties (Suzuki et al., 1996). Anderson et al. (1997b) reported CV data on Sc3C2 Cso- CV measurements were also taken on other lanthanide fullerenes such as Pr Cs2, Nd Cs2, Tb Cs2, E)y C82, Ho C82, Er C82 and Lu C82 by Wang et al. (1997). The first reduction potentials of all seven M C82 were found to locate within a close vicinity... 

A variation of the above strategy has been used to produce the related [2]-rotax-ane 16 in 15% yield. In this case, two potentially redox-active fullerenes act as the stoppers. CgQ was chosen for inclusion in this product because of its interesting electrochemical and electronic properties and, in particular, because it is a strong electron acceptor. Although the substituted fullerene stoppers appear to have a significant influence on the redox properties of the metal centre (an anodic shift occurs), the converse is not true and the metal does not significantly affect the redox properties of the fullerene groups. 

In this chapter we concentrate exclusively on the most important aspects of the electrochemistry of 50 and the other pristine higher fullerenes, namely, C70, 75, 73, Cg2, and Cg4. A vast number of derivatives of 50 and a few of C70, 75, and C7g have been prepared, and their redox properties have been studied using electrochemical techniques. However, due to space limitations, it is impossible to cover this subject here and to give it the justice that it deserves. Since several comprehensive reviews on fullerenes and their derivatives have appeared recently, the reader is referred to these for details [6]. Nevertheless, it has been recently discovered that controlled potential electrolysis can be used as a synthetic tool for the preparation of new fullerene derivatives, as well as for the separation of otherwise inseparable fullerene isomers. Thus, a short section covering electrosynthesis of fullerene derivatives is included at the end of this chapter. 

A 5 1 ratio mixture of the two most abundant constitutional isomers of C78, C2U-C78, and D3-C78, have been separated from the other fullerenes. Selegue and coworkers reported the first study on the redox properties of the C2 isomer in a toluene/acetoni-trile mixture at — 15°C. Five reversible and one irreversible reduction waves were... 

Bourgeois, J.-P. Echegoyen. L. Fibbioli, M. Pretsch, E. Diederich. F. Regioselective synthesis of trans-l fuller-ene i.v-adducts directed by a crown ether tether Alkali metal cation modulated redox properties of fullerene-crown ether conjugates. Angew. Chem. Int. Ed. 1998. 37. 2118-2121. 

The redox properties of a macromolecule comprised of a fuUerene core functionalized with a porphyrin and 10 dendritic branches of various lengths have been reported. The dendritic branches were found to affect not only the redox properties of the fullerene core but also the electrochemical behavior of the porphyrin both the fullerene- and the porphyrin-based reductions shifted to more negative potentials as the size of the dendritic branches increased [69]. 

Considering that the conductivity and superconductivity of doped fullerenes is widely revised in a different chapter of this book, we will centre our attention on the use of Cso as an acceptor in the preparation of charge-transfer complexes and the redox properties of organofullerenes. 

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