Fulleride Anions

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. 

These phenomena are related to internal properties of the molecule. Among organic molecules, the behavior of and its chemically modified forms is unique. 

Reductive transformations of fullerenes have not only been carried out to prepare fulleride salts. The fulleride ions themselves are reactive species and easily undergo subsequent reactions, for example, with electrophiles. Therefore, the anions provide a valuable synthetic potential for fullerene chemistry. 

Theoretical calculations of the molecular orbital levels of show that the lowest 

Therefore, Cjq was predicted to be a fairly electronegative molecule, being reducible up to the hexaanion [8]. Indeed, this was supported by very early investigations carried out with in solution, namely cyclic voltammetry studies, which showed its facile and stepwise reduction. In these first investigations, Cgg was reduced by two [9] and three [10] electrons. 

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A broadening and red-shifting was observed for the 330 nm absorption and also for the 255 nm absorption [11], which can also be accounted by a diminution in symmetry [33], In the visible region, spectral changes produce the colors of the different fulleride anions, which are dark red-purple for Cgg , red-orange for and dark red-brown for [17]. 

Fulleride anions are often more soluble, especially in more polar solvents, than the parent fullerenes. For example, in bulk electrolysis experiments with tetra-n-butylammonium perchlorate (TBACIO4) as supporting electrolyte, carried out in acetonitrile where Cjq is completely insoluble, fairly concentrated, dark red-brown solutions of 50 can be obtained [81]. Upon reoxidation, a quantitative deposition of a neutral Cjq film on the surface of a gold/quartz crystal working electrode takes place. This Cjq film can be stepwise reductively doped with TBA, leading to (Cjo )... 

Whereas Cjq is insoluble and inert in liquid ammonia without any cosolvent, the fulleride anions Cjq"" n = 1 ), generated electrochemically with KI as supporting electrolyte, dissolve completely in this polar medium [15]. Further reductions lead to the ammonia-insoluble potassium salts of the penta- and hexaanions. 

Another semiconducting fulleride salt, [Ru(bpy)3](C5o)2 with bpy = 2,2 -bipyridine, crystallizes on the Pt electrode surface out of dichloromethane solutions saturated with [Ru(bpy)3]PF5 within a few minutes [79]. The NIR spectra of benzonitrile solutions of this salt demonstrate that the only fulleride anion present is 55 . The temperature dependence of the conductivity is typical for a semiconductor, with the room temperature conductivity being 0.01 S cm and the activation energy 0.1 kj mol (0.15 eV). It was postulated that there is an electronic overlap between the two ions of this salt leading to a donation of electron density from the 55 to the ligand orbitals in the [Ru(bpy)3] " AI 0.7) [79]. 

The electrochemical generation of fulleride anions can also be used to synthesize covalent organofullerene derivatives by quenching the anions with electrophiles. This was exemplified in the synthesis of dimethyldihydro[60]fuUerene, the simplest dialkyl derivative of [87]. For this purpose benzonitrile solutions of Cjq and tetra-tert-butylammonium perchlorate (TBACIO4) where exhaustively electroreduced in a dry-box to yield a dark red solution of Treatment of this solution with an... 

Only the fulleride anions C jo ( = 1-6) show broad (diagnostic) peaks in the near-IR (NIR) spectrum (82, 83). 

R = Bul e = H from Saccharin Scheme 7. Some electrophilic substitution reactions of alkyl fulleride anions. 

In general, electrochemical studies of Cg2 have been devoted to the study of its endohe-dral metallo complexes, in which metal atoms are trapped inside the fullerene cage. Part of the fascination with these molecules stems from two aspects of their electronic structures (1) Unlike noble gas endofullerenes, metallofullerenes are formed from a stable ion-pair association between a fulleride anion of charge n and a cation of charge n, which cannot escape from the anionic cage nor react with other substances outside the... 

In solution, all six possible fulleride anions can be prepared and have been studied by various methods. The results are summarized in the review by Reed and Bolskar [18]. The most common reduction methods are the reaction with alkali metals or electrochemistry. In these cases, marked solvent dependence is observed indicating that the effect of the environment is not negligible even in dilute solutions. 

This latter result proves beyond doubt that fulleride anions can be regarded as preserving their molecular identity. To show lower symmetry without a constraining crystal field cannot be explained by any other mechanism. 

E24.28 In Na2C o, all of the tetrahedral holes are filled with sodium cations within the close-packed array of fulleride anions—in other words, it has an antifluorite structure (see Section 3.9(a)). In NajC >, all of the tetrahedral holes and all of the octahedral holes are filled with sodium cations within the fee lattice of fulleride anions. 

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

In the solid state, the [Cgo] cages are arranged in layers with hexagonal packing, although the cages are well separated [K(crypt-222)j cations reside between the layers of fulleride anions. 

Cyclic voltammetry may be used to study species not accessible by other means, e.g. the observation of a transient Tl(II) species, [Tl2] (see Section 13.9), and the reversible generation of fulleride anions and of [Ceof and [Cgo] (see Section 14.4). 

In general intercalation compounds often show interesting properties as electronic conductors. That is also the case of the products of the intercalation of alkali metals in buckminsterfullerene. As discussed above, this process results in an electron transfer from guest to host thus giving rise to fulleride anions. Such a charge transfer is fundamentally due to the large electron affinity and low reduction potential that characterize the fullerenes. 

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