Fullerene redox reactions

A water-soluble Cj-symmetrical trisadduct of Cjq showed excellent radical scavenging properties in vitro and in vivo and exhibits remarkable neuro-pro tective properties [7,8]. It is a drug candidate for the prevention of ALS and Parldnsoris disease. Concerning the reaction mechanism, nucleophilic additions and radical additions are closely related and in some cases it is difficult to decide which mechanism actually operates [92]. For example, the first step in the reaction of f-eo with amines is a single electron transfer (SET) from the amine to the fullerene. The resulting amines are finally formed via a complex sequence of radical recombinations, deprotonations and redox reactions [36]. 

Strong evidence exists for electron hopping in photoinduced transmembrane redox reactions mediated by Ceo and C70 fullerenes across planar bilayer membranes... 

It occurs catalytically on the surface of Fe nanoparticles grown from Fe(CO)5. Also, the conventional synthesis of nanotubes by catalytic CVD from acetylene or methane can be formally considered as redox reaction. Nevertheless, the electrochemical model of carbonization (Sections 4.1.1 and 4.1.2) is hardly applicable for CVD and HiPco, since the nanotubes grow on the catalyst particle by apposition from the gas phase, and not from the barrier film (Figure 4.1). The yield and quality of electrochemically made nanotubes are usually not competitive to those of catalytic processes in carbon arc, laser ablation, CVD and HiPco. However, this methodology demonstrated that nanotubes (and also fullerenes and onions (Section 4.3)) can be prepared by soft chemistry" at room or sub-room temperatures [4,5,101]. Secondly, some electrochemical syntheses of nanotubes do not require a catalyst [4,5,95-98,100,101]. This might be attractive if high-purity, metal-free tubes are required. 

In this light, a simple addition, rather than a redox reaction, was expected to dominate the reaction of [60]fullerene with the above mentioned radiolytically generated carbon- and heteroatomic-centered radicals (12-18,21,22). Spectrophotometric evidence for the postulated adduct formation stems from the broad 900 nm absorption band (Figure 1), substantially different from those of the singly reduced or oxidized fullerene, i.e. the 7i-radical anion (1080 nm) and 7i-radical cation (960 / 980 nm), respectively (12,13). 

Addition and redox reactions lead to covalent exohedral adducts and salts, respectively. Subsequent transformations of specifically activated adducts pave the way to other classes of fullerene derivatives (Fig. 1). These are heterofuller-enes, defined degradation products or partial structures, open cage species and endohedral fullerenes. 

Addition reactions, electron transfer reactions, and reactions involving the opening of the fullerene cage (chemical surgery) have been thoroughly studied on fullerenes. Other reactions such as nucleophilic additions, cycloaddition reactions, free-radical additions, halogenations, hydroxylation, redox reactions, and metal transition complexations have been reported for Cgo as well. Furthermore, fullerenes are easily reduced by electron-rich chemical reagents as well as electrochemically. Their oxidation, however, is considerably more difficult to achieve [17]. Thus, electrochemical measurements showed the formation from the monoanion to the hexaanion [18]. 

These evaluations suggest that Ceo and related fullerenes should be more reactive than benzene. However, it is difficult to directly compare the reactivities of these typical representatives of two- and three-dimensional aromatics. In contrast to benzene and other planar aromatics, the jt systems of the fullerenes have no boundaries. In other words, the fullerenes contain no hydrogens that can be replaced via substitution reactions. As a consequence, a chemical transformation of fullerenes is always accompanied with a change of the structure, whereas retention of the structural type is characteristic for reactions of benzene-like aromatics. Two main types of primaiy chemical transformations are possible addition reactions and redox reactions. 

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... 

Preparation of mono-adducts of fullerene - for studies on electrostatic interactions - was undertaken by cyclopropanation of fullerene with appropriately functionalised malonic esters 1 (Bingel reaction) to form 2. Coupling with the tert-butyl protected oligoamide-amino-dendron 3 and subsequent hydrolysis lead to the water-soluble fullerene dendron 5, which can carry up to nine negative charges after depro to nation. After association with the zinc complex of cytochrome C, photoinduced electron transfer (PET) from the redox protein to the fullerene can be accomplished, which was studied by fluorescence spectroscopy. 

An important field where the cyclopropanation reaction finds growing application is the construction of dendrimers possessing fullerenes either as functional cores [26-33] or branches [34-38]. Dendrimers can serve as building blocks for the construction of organized materials with nanosize precision due to the well-defined three-dimensional structure they possess. An issue of great importance is to incorporate photoactive and/or redox-active units at the center of the dendrimer in order to establish these types of materials as molecular devices. An example of an organofullerene material that has the potential to serve as a core building block for the construction of dendrimeric compounds... 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

Pulse radiolysis of 2-propanol has also been used to investigate elementary redox and radical reactions of fullerenes in solution [18]. For example, reduction of Ceo to C6o by Cs (k > 10 dm mol" s ) and (CH3)2 COH [k = (5 2) x 10 dm mor s ] was observed, and Ceo " was found to be stable for hours in the absence of oxygen. Ceo is insoluble in water, but its reduction in this solvent by (CH3)2 C0H was achieved by imbedding the fullerene in y-cyclodextrin to form a soluble guest-host complex in a 9 1 (vol %) water 2-propanol mixture. In this case, the reaction rate was two times slower than that in neat 2-propanol. Information was also obtained [18] on the addition of CH3 to Ceo in 2-propanol by the follow-... 

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