Fullerene Chemistry and Electrochemistry

Fullerene chemistry has become a very active research field, largely because of the uniqueness of the C60 molecule [10.1] and the variety of fullerene derivatives that appear to be possible. The synthesis of crystalline M3C60 (M = K, Rb, Cs) compounds by the chemical reduction of C60 with alkali metals led to the discovery in 1991 of moderately high temperature (Tc ~ 20 K) superconductivity in these compounds [10.2]. Since that date, chemists have learned how to generate a diverse group of fullerene derivatives, where molecular fragments are bonded to the C60 cage, leaving the cage essentially intact, although the C-C bond lengths in the vicinity of the attachment are perturbed [10.3-7]. Many of these chemical reactions have taken advantage of the electrophilic properties of fullerenes, that is, their tendency to attract electrons.

Because all the carbon bonds in the fullerene molecule are satisfied within the molecular shell, substitution reactions involving the exchange of one chemical group for another, as is common in organic chemistry [10.8,9], are not possible, although substitution of boron for carbon in the shell has been observed in molecular beam experiments [10.1]. Furthermore, catastrophic decomposition of the carbon shell occurs in oxygen above 400°C [10.10,11],

Chemical groups have been attached to the fullerene molecule through bonds between carbon atoms in the fullerene shell and transition metal, nitrogen, oxygen, or carbon atoms in the adduct. The reactive site in the fullerene is, in most cases, the C=C (double) bond bridging pentagons [or in other words, the double bond which is located at the fusion of two

Fig. 10.1. Schematic view of a Cjq molecule emphasizing the octahedral arrangement of pyracylene units. A pyracylene unit contains two pentagons and two hexagons and a central (reactive) double bond. The pyracylene unit centered on the (100) axis is highlighted, and all rings associated with these units are shaded in the figure. Double bonds are located at the fusion of hexagonal rings.

nit (shaded). The chemical behavior of C60 has been reported to be similar to that of electron-deficient polyalkenes, -(RC=CR)„- where R represents an alkyl, hydrogen, or aromatic group [10.4,7].

The limited availability of gram quantities of fullerenes, other than C60 and C70, has largely confined fullerene chemistry to the study of chemical reactions based on C60 and C70, with most of the chemical studies reported for C60 and some smaller effort given to chemical reactions based on C70. Almost no chemical synthesis studies involving higher-mass fullerenes have thus far been published.

In this chapter, we first review some of the general characteristics of chemical reactions involving fullerenes, classify these reactions into major categories (reduction, bridging, addition, polymerization, host-guest com-plexing, etc.), then give a few examples of chemical reactions in several of these categories. Intercalation reactions in the solid state are discussed in Chapter 8 and in §9.3, while the synthesis, extraction, and purification of fullerenes are discussed in §5.1, §5.2, and §5.3, including the use of chromatography for separations. Also related to fullerene chemistry is fullerene surface science, discussed in §17.9.

10.1. Practical Considerations in Fullerene Derivative Chemistry

Since many of the fullerene reactions seem to be reversible and the amount of the fullerene material available to most chemists is relatively small, fullerene chemistry is one of the most challenging research areas at the cut

ting edge of the chemical sciences. To add to the challenge, chemical analysis of fullerene derivatives using a variety of spectroscopic techniques can be difficult, because of the poor solubility of these species in water or common organic solvents. Regarding solid-state chemistry, the crystallization of many of the fullerene-derived products has been found to be difficult, so that definitive structural determinations are complicated by the unavailability of single-crystal specimens of adequate quality and size. Fullerenes have also been found to form solvates with many organic solvents, so that attempts to grow crystals from solution instead yield crystals of the solvated species.

The chemical reactivity of C60 can be strongly photosensitive and oxygen sensitive. For example, C60 can be polymerized by ultraviolet (UV)-visible light (see §7.5) and the chemical reactivity of polymerized C60 differs from that of the individual C60 monomers. Furthermore, the characteristics of the polymerization process are highly sensitive to the presence of oxygen [10.12,13]. Thus C60 must be stored and handled with appropriate care to avoid unwanted polymerization and photoinduced intercalation of 02 prior to use, or while carrying out chemical reactions, or while characterizing the reaction products. In some cases, oxygen in the presence of light can be used as a cage-opening process [10.14-16] to promote specific chemical reactions.

10.2. General Characteristics of Fullerene Reactions

Since C60 is a unique molecule in a number of ways, it can be expected that the chemistry governing C60 will also have some unique characteristics. It is further believed that chemical reactions involving any fullerene have some common characteristics. In this section we discuss some of these characteristics.

The bonding in C60 can be described approximately by an sp2 configuration. However, curvature of the fullerene cage leads to a small admixture of sp3 character. All fullerene molecules share the closed cage structure (see the discussion of Euler's theorem in §3.1). As the number of carbon atoms in the cage increases, the curvature is reduced, and the chemical behavior should approach that of graphite without the dangling bond edge sites.

Another feature of importance to chemical reactions with C60 arises from its unique structure. The regular truncated icosahedron is the only one out of over 12,000 possible structures that can be formed from 60 carbon atoms [10.17] which has pentagons separated from each other (the isolated pentagon rule). It is believed that after chemical reactions have occurred, there will be little change to the C60 shell, and the isolated pentagon rule will still be valid [10.4], thus preventing double bonds and the attendant strain from occurring in the pentagonal rings [10.18]. Referring to Fig. 10.2, where the three possible placements of two pentagons relative to a hexagon in a fullerene shell are presented, it is seen that only the configuration shown in Fig. 10.2(a) satisfies the requirement that all five bonds of the pentagon are single bonds, and Fig. 10.2(a) is indeed the configuration found in the C60 molecule and most of its derivatives. Another way to reduce strain is to introduce aromaticity in the pentagonal ring to reduce charge accumulation, as discussed below.

For purposes of understanding the chemical reactivity of C60, researchers view this molecule as a nonoverlapping, octahedral arrangement of six pyra-cylene units with one reactive double bond located in the center of each unit. This arrangement may be visualized from inspection of Fig. 10.1, where the pyracylene units are shaded for clarity. Four pyracylene units are arranged around the belt of the C60 molecule, and the remaining two units are centered on the top and bottom of the molecule. The reactive double bonds are also indicated symbolically in Fig. 10.1. Finally, four of the eight hexagons (unshaded) that are isolated from one another by the pyracylene units are evident in the figure.

Because C60 also contains double bonds, chemists often classify the C60 molecule (and other fullerenes as well) as an alkene. Most chemists do not generally classify fullerenes as aromatic molecules, which are ring molecules with alternating single and double bonds that resonate with one another [10.19]. The presence of the pentagons in C60, with their single bonds, localizes the double bonds at specific sites, unlike the situation typical of aromatic molecules such as benzene, in which the v electrons in the double bond are delocalized around the hexagonal ring. Double bonds are thought to be absent in the pentagonal rings because of the attendant strain energy [10.4], Among the class of alkenes, C60 has an unusually large number of equivalent reaction sites (30, corresponding to the number of double bonds per molecule), leading to the possibility of a large number of different re

Fig. 10.2. The three possible dispositions of two pentagonal rings adjacent to a hexagonal ring: configurations (b) and (c) are inconsistent with having only single bonds on the pentagonal edges, while configuration (a) has only single bonds on the pentagonal edges and is the favored disposition for CM and its derivatives [10.4], a b c a c

Fig. 10.2. The three possible dispositions of two pentagonal rings adjacent to a hexagonal ring: configurations (b) and (c) are inconsistent with having only single bonds on the pentagonal edges, while configuration (a) has only single bonds on the pentagonal edges and is the favored disposition for CM and its derivatives [10.4], action products resulting from reactions with a single reagent. Thus, most chemical reactions with fullerenes are not selective.

Many of the addition reactions of C60 and C70 involve the scission of the central double bond in the pyracylene unit. Several examples of fullerene derivatives obtained in this way are given below. As various chemical groups bond to the two carbons at the double bond location, the carbon atoms are pulled out of their equilibrium positions, the local structure of the fullerene is modified, and the high degree of symmetry of C60 is reduced, thereby activating many new (primarily C atom motion) Raman and infrared vibrational modes (see §11.3.1).

Whereas the CM molecule rotates rapidly [1010 s'1 as determined from nuclear magnetic resonance (NMR) measurements [10.20,21] see §16.1.4] in the lattice at room temperature, the formation of adducts in a chemical reaction serves to restrict the C60 rotations. Thus chemical reactions affect many physical properties of fullerenes, such as those relating to phase transitions associated with this rotational motion (see §7.1.3).

In general, the fullerenes are electron attracting, which chemists call "electrophilic" [10.4,7]. Thus chemical reactions with electron-donating species (called nucleophiles by chemists) are preferred. It follows that C60 and C70 are easily reduced (i.e., they accept electrons from the nucleophile). Consequently, these fullerenes are considered to be (mild) oxidizing agents. Chemical reactions can also be used to "functionalize" fullerenes, which refers to the attachment of new chemical groups for the purpose of altering specific chemical or physical properties. The new surface group may also allow a subsequent reaction to occur, producing a new fullerene derivative. For example, the addition of ethylenediamine [QH^NF^] to Qo can be used to produce a "hairy ball" that is water soluble [10.22]. Reactions involving the scission of the it bond in the central C=C double bond of the pyracylene unit are further discussed in §10.6 and §10.7.

In Fig. 10.3 we show some general categories of chemical reactions known to occur for C60 (and to some extent also for C70) [10.4], In the following sections we comment on a number of these categories for chemical reactions and give a few examples for each case. Note that in Fig. 10.3, which is adapted from Ref. [10.4], we have rearranged the figure, grouping the various fullerene derivatives according to their structural similarity. Three main groups have been identified. Approximately half the products in the figure are derived from hydrogenation, alkylation, and halogenation, resulting in a radial covalent bond between the Qo and the adduct. In some cases, the species or groups of the same type that are attached to a single ball can be quite numerous [e.g., Br(24), F(60), H(60), CH3(24), and the amine group RNH(12), where the number in parentheses is the reported maximum value of that species], and the maximum number of a

Hatogenation

Bridging and Cyehaddltloo

Hatogenation

Bridging and Cyehaddltloo

Fulleroids

Fig. 10.3. Some general categories of reactions known to occur with CMI (and to a lesser extent with C70), based on a figure of Ref. [10.4]. Here R denotes a functional group.

Fulleroids

Fig. 10.3. Some general categories of reactions known to occur with CMI (and to a lesser extent with C70), based on a figure of Ref. [10.4]. Here R denotes a functional group.

given adduct may be determined by steric hindrance. Another group in the figure corresponds to derivatives where the adduct is attached by a bridge whose ends are bonded across the reactive double bond in a pyracylene unit. Multiple additions of identical larger adducts, attached in this way, are also possible. Thus, similar to the case of the first group, one reagent can generate a variety of fullerene derivatives which differ simply by the number of additions. This multiplicity of products leads to an interesting diversity of derivatives, with different chemical and physical properties. The third (and last) group in Fig. 10.3 contains derivatives which one normally associates with the solid state, i.e., polymers and host-guest solids or clathrates and intercalation compounds (e.g., K^C60).

10.3. Reduction and Oxidation of C60 and C70

The propensity of C60 and C70 to take on extra electrons is an important driving force in many fullerene-based chemical reactions and is the focal point of this section. In §10.3.1, we make general remarks about the relationship between the electrophilicity of the pyracylene units within the C60 and C70 molecules and the reducibility of C60. In §10.3.2, we discuss the electrochemical reduction of C60 and C70 in solution, as well as the more difficult electrochemical oxidation of C60. Battery applications of electrochemical redox reactions are presented in §20.4. In §10.3.3, we present limited evidence for the chemical oxidation of C60.

10.3.1. Fullerene Reduction—General Remarks

The high electron affinity of C60 and C70 strongly favors their reduction relative to their oxidation. In a chemical reduction reaction, electronic charge is transferred to the fullerene, leading to anion formation Q0~, or more generally to a higher density of electron charge on the fullerene. Cyclic voltammetry studies of C60 have shown (see §10.3.2) that reversible reduction can be carried out to yield fullerene anions C£0~ [10.23-25] and C70" [10.26-28] for 1 < n < 6 [10.23]. The addition of 6 electrons to C60 is sufficient to completely fill the lowest unoccupied molecular orbital (LUMO) hu (or fiu) level (see §12.1); both the tiu and /lu notations appear in the literature.

In Fig. 10.4, the effect of addition of one electron to a pyracylene (shaded) unit of C6(l is shown. It is proposed [10.4,7] that the addition of a sixth it electron to the pentagonal ring (introducing a net charge -e on the ring) is favored because it forms the aromatic cyclopentadienyl radical within the pyracylene unit. The five dots, located where the double bonds once existed (see Fig. 10.4), represent the unpaired electrons left after the dissociation of these bonds.

When considering reduction reactions from a chemical and structural point of view, the addition of six electrons to C60 presents an especially symmetric configuration. Since one pyracylene unit is involved with each

ir electron, forming an aromatic 6ir cyclopentadienyl radical. The five dots on the figure represent unpaired electrons created by forming the cyclopentadienyl radical.

electron addition of the type shown in Fig. 10.4, the addition of six electrons would symmetrically involve all six pyracylene constituents of the C60 molecule (see Fig. 10.1). Furthermore, the addition of six nucleophilic (electron-donating) groups, one per pyracylene unit, is commonly found in fullerene chemistry [10.4].

In discussing sites for chemical reactions, chemists often make use of projection diagrams of fullerenes, such as the Schlegel projection of C60 and C70 shown in Fig. 10.5 [10.4], These diagrams show bonding relations for all bonding sites for these fullerenes on a planar projection. In each of these diagrams, three of the six pyracylene units are easily identified, since the two pentagons of these pyracylene units are outlined in boldface in the figure.

Many of the reduction reactions of C60 and C70 of importance to physical measurements in the solid state have been made with alkali metal and alkaline earth species. In many cases, the reaction is deliberately controlled so that exactly three electrons are added to each C60 molecule, as, for example, for superconductivity studies in M3C60 compounds (M = K, Rb and appropriate alkali metal alloy compounds such as M,_^M^C60) (see §15.1). Another interesting example of a reduction reaction is the addition of the charge transfer complex TDAE (tetrakis-dimethylamino-ethylene) to form an unusual magnetic compound (see §8.7.3 and §18.5.2). The reducibility of C60 and C70 allows these molecules to be hydrogenated [10.29] or alkylated [10.30] as, for example, by CH3 addition. In these reactions, the C£0~ anion has been proposed as an intermediate species in the synthesis route [10.30].

Fig. 10.5. Schlegel diagrams for (a) C(l0 and (b) C70 molecules. Three of the six pyracylene units for each molecule are highlighted. The three other pyracylene units in each molecule are located symmetrically with respect to those highlighted by bold lines [10.4],

10.3.2. Electrochemical Fullerene Reduction and Oxidation

Electrochemistry of the free molecule is important for identifying the various ionization states that can be achieved. Historically, cyclic voltammetry studies showed at an early time that Qq and C70 could easily be reduced [10.23,25,29], In these experiments, a solution containing C60 or C70 is placed in an electrochemical cell containing two chemically inert (redox) electrodes and a third reference or "standard" electrode. A potential is established between the inert electrodes. The choice of the solvent in the cell is critical in keeping the electrochemically generated Q0~ anions in solution. The solvent also affects, to some degree, the values of the reduction potentials at which specific anions are first formed. An electrolyte is also added to raise the cell conductance and should be chosen so that charge is exchanged between the fullerenes and the inert electrodes, and not with the electrolyte. Furthermore, the charge state of the electrolyte should remain unchanged during the electrochemical reaction. The cell potential is measured between one of the inert electrodes and the reference electrode.

For the case of C60, a mixture of acetonitrile and toluene usually provides a suitable solvent for study of the charging of the free C60 molecules. In the electrochemical reduction process, C60 molecules or Q0~ anions near the negative electrode are reduced, picking up one electron. Figures 10.6(a)-(d) show the cyclic voltammograms for the reversible reduction of (a, b) C60 and (c, d) C70 in a toluene/acetonitrile solution at -10°C [10.27]. The electrolyte, in this case, is a phosphorus hexafluoride salt. In both Figs. 10.6(a) and (c), the upper curves are the voltammograms and the lower curves (b, d) are the differential pulsed polarograms (DPP), which provide a more accurate means of determining the cell potential required to form a specific fullerene anion. At the various voltage peaks shown in Fig. 10.6(a) and (c), the C"0 and C70~ molecular ions pick up one additional electron. The experimental results [10.27] show the possibility of adding six electrons to both Cfio and C70. These six electrons completely fill the t,u LUMO level of the icosahedral C^ molecular ion. The stability of the C^ and C"0 anions suggests that they should engage in reversible electron transfer with many electron donors (nucleophiles), including metals and metal complexes. Electrochemistry can thus provide an important route for the synthesis of C60-metal compounds and charge transfer salts.

Thus far, no report has been given for the reversible electrolytic oxidation of C60, i.e., removing electrons from the highest occupied molecular orbital (HOMO) level in an electrolytic cell. All electrochemical attempts to oxidize C60 have led to the oxidation of the electrolyte instead [10.31,32], However, the irreversible formation of C^ has been reported [10.25],

Cyclic voltammetry studies, such as shown in Fig. 10.6, have been frequently used by chemists for the characterization of fullerene deriva-

Potential (Volts vs Fc/Fc*)

Potential (Volts vs Fc/Fc*)

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