Fullerenes electrochemical reduction

Reduction of fullerenes to fullerides — Reversible electrochemical reduction of Ceo in anhydrous dimethylformamide/toluene mixtures at low temperatures leads to the air-sensitive coloured anions Qo" , ( = 1-6). The successive mid-point reduction potentials, 1/2, at -60°C are -0.82, -1.26, -1.82, -2.33, —2.89 and —3.34 V, respectively. Liquid NH3 solutions can also be used. " Ceo is thus a very strong oxidizing agent, its first reduction potential being at least 1 V more positive than those of polycyclic aromatic hydrocarbons. C70 can also be reversibly reduced and various ions up to... 

The electroreduction of iV-(haloethyl)amides 8 at a mercury cathode in DMF or acetonitrile gives rise to the corresponding iV-(2,2-dichlorovinyl)amides 9 in good to excellent yields (equation 5)25. The electrochemical reduction of fluorinated fullerene has been reported26. 

The two-electron reduction of Ceo to produce Ceo can be achieved electrochemically in the presence of methyl iodide to yield the dimethyl adduct, Mc2Ceo [139]. In this context, catalytic currents are observed for the electrochemical reduction of Ceo or C70 to produce Ceo or C70 , respectively, in the presence of vicinal dibromides [140] or a,to-dihaloalkanes [141, 142]. Electrosynthesis of methano-fullerenes has been achieved by the reaction of Ceo with j o-brominated and... 

Fullerene has also been incorporated in redox-active dendrimers. In 62 the Ceo unit has been modified by connecting a single polyaryl-ether dendritic branch [132]. Electrochemical reduction in CH2CI2 of 62 features three reversible waves, which occur at a potential more negative than that of the free Cso molecule. This shift was ascribed to an insulating effect on the connected dendritic structure. 

Fullerenes have high electron affinity so that electrochemical reduction processes are easily observed. The solution-phase electrochemical response of Cgg and C70 fullerenes in MeCN/toluene mixtures at low temperature consisted of six reversible one-electron reductions at potentials between -0.97 to -3.26 V vs. Fc7Fc+ (Xie et al., 1992). This can be represented as successive one-electron transfer processes ... 

Electrochemical reductions of fuUerene films in the presence of Brnsted acids yield hydrogenated fullerenes H Cgg, where n depends on the acid, its concentration, and on the electrode potential. Hydrogenated fullerene films behave as semiconductors with increased photoefficiency [67]. 

The fulleride anions generated by electrochemical reduction may be reacted with electrophiles, yielding functionalized fullerenes like, for instance, alkylated derivatives. 

In this contribution we present a review on recent developments in the field of advanced clustered and polymeric carbon materials. The carbon clusters to be discussed refer to the recently discovered cage type molecules of the fullerene family rather than to a conventional aggregation of a small number of atoms. The polymeric carbon materials are carbyne like pure carbon chains grown from teflon by an internal electrochemical reduction process. ... 

The sequences of cycloaddition-retro-cycloaddition could indeed be used as a smart strategy to carry out protection-deprotection protocols that could selectively add or remove addends from fullerenes while leaving others unperturbed. Therefore, retro-cycloaddition protocols have been reported by the use of thermal treatment [37], microwave irradiation [38], chemical reduction [39], electrochemical reduction [40], or electrochemical oxidation [41],... 

The CeoPd polymers can be also prepared by electrochemical reduction of Geo in the presence of PdCl2(NCPh)2, in the form of redox-active, black films that coat the electrode. Addition of PPh3 to the film dissolves it to produce C6oPd(PPh3)2. Different kinds of films are produced by varying the Pd compound and the relative concentration of the precursors. The process can be applied successfully to a modified fullerene 2 -ferrocenylpyrrolidinio-[3, 4 l,2][C6o]fulletene. Electrodes modified with these films have been applied in electrochemical studies. ... 

In general, the methanofullerenes formed by the Bingel reaction are stable and have been used widely for the synthesis of new materials. However, as a drawback, they show liability under reductive conditions. Electrochemical reduction is sufficient to induce a reverse Bingel reaction in di(alkoxycarbonyl)methanofullerenes, generating the parent fullerene and starting malonate as products. ... 

In this chapter we concentrate on reduction processes of carbon-rich systems. The formation of anions and radical anions of -conjugated monocyclic systems, cyclophanes, bowls and fullerenes is described. Carbon-rich compounds can be reduced directly by contact with alkali metals Li, Na, K, Rb and Cs, which have a low reduction potential. Proton, carbon and lithium NMR and EPR spectroscopies are the main methods used to gain a better understanding of the mono- and polycyclic systems in solution. Special attention will be given to modes of electron delocalization, aromaticity, anti-aromaticity, as well as aggregation, bond formation and bond cleavage processes of diamagnetic electron transfer products. Electrochemical reductions will be briefly discussed. 

Recently, the electrochemistry of fullerenes and their derivatives has gained much attention [33]. Cgo, C70 and higher fullerenes were reduced electrochemi-cally, and six reduction waves were observed for both Cgo and C70 [34], as well as for most of the higher fullerenes [35]. The energy levels that were obtained from these experiments were mostly in line with MO calculations. The electrochemistry of numerous fullerene derivatives was studied to compare their electron affinities and energy levels with their parent fullerenes. Electrochemically induced isomer-izations can be observed in CV, as is the case in the rearrangement of fulleroids to methanofullerenes [36]. 

No X-ray diffraction studies on structure were performed however, the NMR studies provided evidence that the Co species should have a structure closely related to that of analogous Ru or Rh derivatives whose X-ray structures are known, that is, the Cso unit being 77 -bonded to the metal center via one of its G=C units. Electrochemical reduction has been performed to afford the dianion species. The fullerene unit was shown to be weakly coordinated to Co as it could be easily displaced by smooth electrophiles such as I2, maleic anhydride, or PPhs, regenerating the free C6o- At approximately the same time, Baird et al. studied the electron transfer between Ceo and NaCo(CO)4. This reaction led to a transient compound NaCo(CO)3[C6o], which was unstable and led in refluxing THE solution to NaCo[C6ol 3THF Unfortunately, this carbonyl-free compound could not be analyzed by X-ray diffraction... 

Since electrochemical reduction of the fullerene films involves incorporation of cations to form a new phase, the nature of the background electrolyte cation is the most important factor determining the electrochemical behavior of the film. Properties of the new phase, such as solubility, conductivity and kinetics of the phase reorganization affect the electrochemistry of the film. The nature of the solvent, on the other hand, mainly affects the solubility of the reduced film. Since the reduced fullerenes dissolve easily in many non-aqueous solvents (see Section 1.1), the electrochemical studies of the thin fullerene films have been limited mainly to acetonitrile with a few studies in propylene carbonate [136,144] or y-butyrolactone [151]. Unless otherwise stated, all the results described below have been obtained with acetonitrile as solvent. 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

The electrochemical features of the next higher fullerene, namely, [70]fullerene, resemble the prediction of a doubly degenerate LUMO and a LUMO + 1 which are separated by a small energy gap. Specifically, six reversible one-electron reduction steps are noticed with, however, a larger splitting between the fourth and fifth reduction waves. It is important to note that the first reduction potential is less negative than that of [60]fullerene [31]. 

Nakanishi, T, Ohwaki, H., Tanaka, H., Murakami, H., Sagara, T. and Nakashima, N. (2004) Electrochemical and chemical reduction of fullerenes C o and C70 embedded in cast films of artificial lipids in aqueous media, f Phys. Chem. B, 108, 7754-7762. 

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

The first chemical transformations carried out with Cjq were reductions. After the pronounced electrophilicity of the fullerenes was recognized, electron transfer reactions with electropositive metals, organometallic compounds, strong organic donor molecules as well as electrochemical and photochemical reductions have been used to prepare fulleride salts respectively fulleride anions. Functionalized fulleride anions and salts have been mostly prepared by reactions with carbanions or by removing the proton from hydrofullerenes. Some of these systems, either functionalized or derived from pristine Cjq, exhibit extraordinary solid-state properties such as superconductivity and molecular ferromagnetism. Fullerides are promising candidates for nonlinear optical materials and may be used for enhanced photoluminescence material. 

The UV/Vis spectra (Figure 3.2) of the chestnut brown solutions of the monoadducts CjqHR, particularly the intensive bands at = 213, 257 and 326 nm, are close to those of Cjq, demonstrating their electronic similarity [4]. The biggest changes in the spectra compared with Cjq appear in the visible region. The typical features of Cjq between X = 400 and 700 nm are lost, and a new and very characteristic band at X = 435 nm appears, which is independent of the nature of R. Also, the electrochemical properties of CjqHR are comparable with those of Cjq [5, 19]. The first three reversible reduction waves shift about 100 mV to more negative potentials. Therefore, the fullerene core in these monoadducts still exhibits remarkable electron-acceptor properties, which is one reason for almost the identical chemical reactivity compared with CgQ. 

Electric fleld gradient, 22 214-218 Electroabsorption spectroscopy, 41 279 class II mixed-valence complexes, 41 289, 291, 294-297 [j(jl-pyz)]=+, 41 294, 296 Electrocatalytic reduction, nickel(n) macro-cyclic complexes, 44 119-121 Electrochemical interconversions, heteronuclear gold cluster compounds, 39 338-339 Electrochemical oxidation, of iron triazenide complexes, 30 21 Electrochemical properties fullerene adducts, 44 19-21, 33-34 nickeljll) macrocyclic complexes, 44 112-113... 

The electrochemical properties of TNT-EMFs, M3N C2n n > 39) differ from those of the empty cage fullerenes (see Fig. 6) due to the interaction of the metal cluster with the carbon cage and because the structure of these carbon cages are generally different. As a consequence, the reductive processes are electrochemically irreversible but chemically reversible. The oxidative processes occur at lower potentials because the HOMO orbital is mainly localized on the trimetallic nitride clusters and the HOMO-LUMO gaps in solution are smaller [25,58]. The endohedral metallo-fullerenes M C2n show similar behavior but even smaller HOMO-LUMO gaps [59]. 

At this point, it is important to indicate that a very large number of C-bridged cyclopropanated fullerene derivatives undergo irreversible reduction processes leading to the removal of the addend and recovery of the pristine parent fullerene. The process has been advantageously used in electrosynthetic procedures, and thus a separate section covering the electrochemically induced retro-cyclopropanation reaction is presented later in this chapter (see Sect. 6.1.5.2). A number of other C-bridged cyclopropanated derivatives will be discussed there. 

A large number of cyclopropanated derivatives of Cgo in which the bridging atom is an electron rich transition metal (see Fig. 16) such as Pt, Pd, Ni, Ir, W, Mo, and Rh has been reported. Their electrochemical properties have been reviewed [83, 141, 142] and, in general, reductions are Cgo centered and negatively shifted with respect to those of pure Cgo, while oxidations are metal centered. In most cases, however, the first reduction is accompanied by breakage of the carbon-metal bonds and recovery of the pristine [60] fullerene. In multiadduct derivatives, the breakage occurs in a stepwise manner. 

Fullerene is an ideal candidate as a component of molecular batteries because it shows six chemically and electrochemically reversible, one-electron reduction70 and one oxidation process.71 In particular, the first reduction process occurs at easy accessible potentials (—0.98 V versus Fc +/Fc in MeCN/toluene solution at 263 K)70 and it is thus the most suitable process to exploit in charge storing devices. To covalently append fullerene to the dendritic structure, chemical functionalization of the bucky-ball is necessary. Fortunately, most of its derivatives keep the reversible electrochemical properties of Ceo, at least for the first reduction process, which usually occurs at more negative potentials than that of fullerene. 

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