10.5. Halogenation Reactions

Similar to the hydrogénation reactions of C60 discussed in §10.4, halogenation reactions of C60 forming C^X^ (X = Br, Cl, F) have been reported for a wide variety of n values. In these reactions the halogen is attached to a carbon atom on the fullerene shell through a single radial covalent bond, formed by using one electron provided by the fullerene and the other by the halogen. The greatest reactivity is with fluorine and the species C60F36 has been reported by several groups [10.29,37], Complete fluorina-tion to C60F60 has also been reported [10.38], based on a single 19F line in the NMR spectrum. Polychlorofullerenes are produced by passing Cl2 gas through C60 at elevated temperatures (300-400°C), and up to 24 CI atoms have been added to a C60 molecule [10.39]. Bromination of C60 has also been reported, for chemical reactions carried out in the 20-55°C range. X-ray crystallographic evidence has been provided for C60Br6, C60Br8, and C60Br24 [10.40,41], These x-ray studies of C60Br^ (x = 6,8,24) showed that the attachment of Br is accomplished by placing Br at symmetrically bonding vertices of the C60 molecule (see Fig. 10.8). Other possibilities might be found for the placement of Br atoms around the C60 shell to obtain other C60Br„ compounds.

Ultraviolet irradiation of fullerene/chlorine mixtures in chlorinated solvents permits fast chlorination under very mild conditions. A very high chlorine content has been found in a photochlorinated fullerene sample, which by elemental analysis was found to fit the average formula C60C140 [10.42], although it was clear that a distribution of chlorinated products CgoCln had been obtained.

Halogenated fullerenes such as C60C1„ can be used in substitution reactions to attach aromatic groups sequentially to the fullerene shell. In this case, the aromatic molecule reacts with the halogenated C60 in the presence of a Lewis acid catalyst. For example, starting with C60C1„ in the presence of benzene and an A1C13 catalyst, phenylation of C60 with up to 22 phenyl (C6H5) groups to form C60(C6H5)„ (with n < 22) has been observed with mass spectroscopy [10.39].

10.6. Bridging Reactions

Addition of functional groups to C60 and C70 can be accomplished through the formation of a bridge across the reactive C=C double bonds. Nitrogen, carbon, oxygen, and transition metal atoms such as Ni, Pt, or Pd [10.3-7] have been found to bridge these C=C bonds.

The simplest example of a bridging fullerene derivative is the covalently bonded epoxide QqO [see Fig. 10.10(a)], which is an important building

pentagonal ring bond of C^. The structure in (a) is favored for C^ in forming CmO and the structure (b) is expected to occur for C70 in forming C70O [10.4].

block for more complex synthesis paths, whereby other species attach to the oxygen, or the oxygen participates in a substitution reaction (see §10.3.3). With dimethyl dioxirane [(CH3)2C02], Qo either removes the oxygen to form the epoxide or forms [C60C(CH3)2O2], a dioxirane derivative [10.43]. Use of 13C NMR [10.44] and x-ray diffraction techniques [10.45] established the structure of C^O to be a bridge attachment with elongated bonds of a single oxygen at the double-bond position of a pyracylene unit, as shown in Fig. 10.10(a) [10.46-48]. It is interesting to note that the crystal structure for C60O is the same (fee) as for C60 itself and with a similar lattice constant [10.45], except that the presence of the oxygen weakly inhibits the rotation of the fullerene in the lattice, thereby increasing the temperature T01 for the structural phase transition (see §8.7.2). Whereas C60O follows the bonding scheme shown in Fig. 10.10(a), the bonding of oxygen to C70 is expected to form an oxido-annulene type bridge structure, shown in Fig. 10.10(b), where the annulene structure is defined as [-CH=CH-]„. Furthermore, in the oxido-annulene form, the oxygen was proposed to attach itself at the waist of the C70 molecule, where there is a high density of hexagons [10.49]. On the other hand, the waist hexagons are more graphitic and, therefore, less reactive, suggesting that the oxygen might instead attach at the polar caps, in analogy with C60. The bridge arrangement shown in Fig. 10.10(b) has also been proposed for the addition of methylene (CH2) to C60, where the carbon atom is at the bridge site.

We now give a few examples of carbon bridge reactions. In one example, X in Fig. 10.10(a) isXs CAr2, where Ar denotes an aromatic group, and a carbon atom forms the bridge between the fullerene and each of the Ar groups. A specific example of a C(Ar)2 attachment is the fullerene derivative C61(C6H5)2, which is illustrated in Fig. 10.11. Here the attachment of

two aromatic (C6H5) phenyl groups forms a diphenylfulleroid C61(C6H5)2, where the term fulleroid refers to C61, denoting a C60 cage to which a carbon atom is added at a double-bond site in a pyracylene unit [10.3], consistent with Fig. 10.10(a). In this process the double bond becomes saturated and the adjoined carbon atom has elongated bridge bonds to the C60 cage (see Fig. 10.11). The number of tt electrons associated with the fulleroid (C61) is two less than for the neutral C61, with two dangling bonds available for bonding as shown in Fig. 10.11 [10.7]. As a result of the phenylation reaction, the authors place a double bond in a pentagonal ring, in contrast with the view [10.4] that the pentagons on the fullerene shell remain unstrained. However, it is possible that the stress created by the double bond in the pentagon is relieved by an increase in the bond length between the bridging carbon and the carbon from the C60 molecule.

Another example of a carbon bridge reaction of the type shown in Fig. 10.10(a) is the reaction yielding a fulleroid (p-BrC6H4)2C61 (see Fig. 10.12 where X = Br), which forms hexagonal crystals, and the molecular unit is closely related to Fig. 10.11. In the crystalline form, the structure of this 4-4' dibromodiphenyl fulleroid has been studied by x-ray diffrac-

Fig. 10.12. A water-soluble fulleroid of the form CmiC(C6H4X)2. One simple example of this molecular derivative is X = Br and another more complex example may have the potential to inhibit the key viral enzyme HIV-1 protease, where X = H0C(0)(CH2)2 C(0)NH(CH2)2—. Connecting the species X to the fulleroid is a C6H4 group shown in the diagram [10.50-52].

tion [10.7], It is of interest to note that, in the CAr2 addition shown in Fig. 10.12, all the pentagons retain their single bonds, in contrast to the CAr2 addition in Fig. 10.11. From a physicist's standpoint, the addition of a CAr2 group in both Figs. 10.11 and 10.12 causes a charge redistribution on the rings surrounding the carbon bridge, which is not easily described by the simple schematic diagrams shown in either of these figures.

Fullerene derivatives of the CAr2 carbon bridge type have been discussed for practical applications to control the binding of drugs. The fulleroid shown in Fig. 10.12 also serves as a schematic version of a water-soluble C60 derivative which has the potential for inhibiting growth of the human immunodeficiency virus (HIV) by blocking the active site of the viral enzyme HIV-1 protease. In this case the Ar group is a substituted phenyl group C6H4X, where X = H0C(0)(CH2)2C(0)NH(CH2)2- [10.50-52], The X group might also be a drug which is weakly bound to the phenyl group, so that selective transfer of the drug group X can take place in a controlled way.

In the preceding sections, we have referred to several fullerene derivatives that have involved the attachment of functional groups by bridging to the fullerene shell, through one of the oxygen and carbon bridges of the type shown in Figs. 10.10(a) and (b). We now discuss the formation of fullerene derivatives based on bridging via metal atoms [10.6,53]. An early example of this type of fullerene derivative is shown in Fig. 10.13, where a direct bridge to C70 is made through the Ir atom to two triphenylphos-phine groups, in terms of the bridge type shown in Fig. 10.10(a). As can be seen in Fig. 10.13, each of the phosphine groups donates two electrons. Two of these donated electrons bond to each of the CO and CI and the remaining two electrons bond the Ir to the C70. This example also points to the multiplicity of compounds that can be formed by attachments to lower-

Fig. 10.13. The addition of Ir(CO)Cl(PPh3)2 to C70 as an example of a metal bridge reaction [10.54], As shown, the Ir metal atom bonds to the two (PPh3) groups, to the CO and CI, and to the C70 shell.

symmetry fullerenes of higher-mass. For example, three crystallographically different pyracylene bridge site attachments can be made to C70, leading to 15 different isomers of Ir(CO)Cl(PPh3)2C70.

The metal Pt has been used to attach triethylphosphine groups by reacting C60 with [(C2H5)3P]4Pt in benzene [10.5]. In this case, two triethylphos-phide groups are bonded as ligands to each Pt atom through the donation of two electrons from each phosphorus atom to the Pt, forming a coordination bond which is covalent in character. In turn, the platinum atom shares two of its valence electrons with two adjacent carbons of the C60 molecule, thus forming two bridging bonds. The same authors have also used Pt and Pd metal bridges to synthesize symmetric structures (see Fig. 10.14), such as [(Et3P)2Pt]6C60 and [(Et3P)2Pd]6C60, where ethyl is denoted by Et = C2H5, and the metal atom is attached at the double-bond site [see Fig. 10.10(a)], one metal atom per pyracylene unit [10.5]. Structural studies using x-ray diffraction have been applied to these Pt-bridged fullerene derivatives [10.5]. In preparing many of the metal bridge compounds, specific functional groups are added for special chemical reasons, while other

Fig. 10.14. Structure of [(Et3P)2Pt]6C60 looking down a threefold symmetry axis under idealized Th point group symmetry. The Pt forms a direct metal bridge bond to the fullerene shell and also bonds to two triethylphosphine (Et3P) groups. The Et=C2H5 terminations are denoted by a single white ball [10.5].

Fig. 10.14. Structure of [(Et3P)2Pt]6C60 looking down a threefold symmetry axis under idealized Th point group symmetry. The Pt forms a direct metal bridge bond to the fullerene shell and also bonds to two triethylphosphine (Et3P) groups. The Et=C2H5 terminations are denoted by a single white ball [10.5].

Fig. 10.15. The osmylation of Cm by 0s04 in i-butylpyridine to form a large metal bridge derivative [10.53], In the figure the small black ball denotes the Os and the four balls marked with an X denote the oxygen. The oval with a slash denotes the nitrogen of the pyridine and the four carbons of the ¿-butyl groups are shown.

chemical species might be selected to enhance the chemical stability of the compound.

Figure 10.15 shows a metal bridge reaction involving cycloaddition, where the metal complex is the osmylated C60 compound [10.53]. In this reaction a metal bridge is made by the Os atom through two oxygens to the fullerene forming a pentagonal ring, and for this reason this metal bridge reaction is also classified as a "cycloaddition" reaction, which is further discussed in §10.7. The 0s04 group in Fig. 10.15 attaches as a bridge of the type shown in Fig. 10.10(a). Four oxygens are attached to the osmium, accounting for six electrons, since two oxygen bonds attach the entire complex back to the C60 cage (see Fig. 10.15). Two i-butylpyridine groups are coordinated to the Os atom through single covalent bonds, formed from two electrons provided by the nitrogen atom in the i-butylpyridine group.

10.7. Cycloaddition Reactions

Cycloaddition reactions are also possible with fullerenes. As a result of a cycloaddition reaction, a four-, five-, or six-membered ring is fused to the outside of the fullerene shell in such a way that one side of the ring is also part of the cage. An example of cycloaddition is shown in Fig. 10.16 in which a six-membered ring is fused to a C60 shell with the evolution of CO. Here the attachment to the C60 molecule is across the double bond connecting two pentagons of a pyracylene unit as shown in Fig. 10.10(a). It is readily checked that the bonding requirements are satisfied for all the atoms in the aromatic ring structures shown in the figure. Because of the bulkiness of many of the ring additions, the number of rings that can be attached to a single fullerene is limited by steric considerations.

C60 can undergo "4+2" and "2+2" cycloaddition reactions with other organic molecules. In these reactions, C=C double bonds are broken on both

Fig. 10.16. An example of a Diels-Alder reaction yielding the cycloaddition product of a 5,6-dimethylene-l,4-dimethyl-2,3-diphenyl group to C«, to form a fullerene derivative that is unable to undergo the reverse reaction because of the release of the CO by-product. The chemical notation of the reaction product focuses on the benzene ring on the right, where two phenyl (C6H5) groups attach at sites 2 and 3, while methyl (CH3) groups attach at sites 1 and 4, and finally at the two remaining sites, 5 and 6, methylene groups (CH2) attach [10.55,56],

Fig. 10.16. An example of a Diels-Alder reaction yielding the cycloaddition product of a 5,6-dimethylene-l,4-dimethyl-2,3-diphenyl group to C«, to form a fullerene derivative that is unable to undergo the reverse reaction because of the release of the CO by-product. The chemical notation of the reaction product focuses on the benzene ring on the right, where two phenyl (C6H5) groups attach at sites 2 and 3, while methyl (CH3) groups attach at sites 1 and 4, and finally at the two remaining sites, 5 and 6, methylene groups (CH2) attach [10.55,56], molecules and new bonds form, depending on the number of tt electrons involved in these bonds. A "4+2" addition implies the rearrangement of four 7r electrons from a conjugated diene unit (C=C-C=C) on the organic reactant molecule and two v electrons from a double bond in the fullerene. Similarly, a "2+2" cycloaddition to C60 implies a bond rearrangement in which two pairs of v electrons, one pair from C60 and the other pair from the reacting organic molecule, are redistributed to form new single bonds joining the molecules. The latter has been proposed for the formation of the C60 dimer in the solid state with photoassistance, in which two C60 shells are joined by a four-membered ring (see Fig. 7.20) [10.12],

Aji example of a "4+2" cycloaddition reaction in solution is the multiple addition of cyclopentadiene (C5H6, with two double bonds in the ring) to C60. Up to six cyclopentadiene addends have been attached to the fullerene, each across the reactive double bond of the pyracylene unit [see Fig. 10.10(a)]. One such addition is shown in Fig. 10.17 [10.57], The addition of each cyclopentadiene occurs by breaking the double bond in the pyracylene unit and forming two new single bonds to the cyclopentadiene, as shown in the figure. Since this addition reaction is reversible, analysis of the products has been facilitated by stabilization of the "cycloadded" C60 through saturation of the remaining double bond in the cyclopentene ring with hydrogen [10.57].

Gas-phase cycloaddition of cyclopentadiene (C5H6) and cyclohexadiene (C6H8) to Qo and C70 has also been accomplished. In this case, the ring molecule was added to a C60 and C70 radical cation (C*(|, C7(J), formed by 50 eV electron impact [10.58]. The addition has been proposed to occur at the site of the positive charge, which is localized on the C-C bond between the pentagonal rings of the pyracylene unit. The resulting adduct is structurally similar to that of Fig. 10.17, with the difference that a positive charge is delocalized on the C60 surface.

An example of a "2+2" cycloaddition reaction in solution is given by the attachment of up to six benzyne (C6H4) groups to the C60 shell [10.59]. Analysis of these reaction products by JH NMR and 13C NMR analysis indicates that the benzyne is connected to the fullerene shell by two single carbon bonds, thus forming a four-membered ring.

Cycloaddition reactions can be used not only to form derivatives of C60 but also to introduce functional groups to a Qo adduct. For example, photo-catalytic addition of a substituted cyclohexenone (C5H7RCO, R=H, CH3) to C60, produces a "2+2" cycloaddition product in which the ketone [an organic molecule that contains the group -(C=0)] is linked to the shell by single C-C bonds forming a four-membered ring [10.60]. For other examples of "2+2" cycloaddition relevant to polymer chain formation, see §10.10.2 and §7.5.1.

10.8. Substitution Reactions

Since C60 contains only strongly bonded carbon atoms (and no hydrogen atoms), conventional substitution reactions do not occur for C60 or other higher-mass fullerenes. However, substitution reactions do occur on derivatives of C60 containing additional groups. For example, KgQo reacts with CH3I to yield (CH3)6C60 and KI, where methyl groups are substituted for potassium atoms. Numerous C60 derivatives can be prepared from nucle-ophilic (electron-donating) substitution of substituted C60, especially in the case of halogenated compounds. Thus, fluorinated C60 reacts vigorously with strong nucleophiles like amines as well as with weak nucleophiles like acetic acid to produce C60 adducts containing oxygen and nitrogen groups [10.61]. Alcohol derivatives of C60 [e.g., C60(OH)„], called "fullerols," can also be prepared by nucleophilic substitution of a nitrosated [C60(NO)+] derivative [10.62].

Another example in which C60 participates in a substitution reaction is in the formation of alkoxylated and arylated derivatives obtained by elec-trophilic substitution of halogenated compounds in the presence of an acid catalyst, as explained in §10.5.

A free radical, denoted by a superscript dot, is a chemical group with a dangling bond. Figure 10.3 shows schematically the reaction of a radical group with C60 to yield a fullerene derivative [i.e., C60 +nR* (C60R„)*], where the attachment of the functional group is at one site and the unpaired electron is at another site. The electrophilicity of C60 makes it highly reactive toward the addition of free radicals. Reactions involving free radicals lead to the formation of addition products, as in the case of hydrogenated and alkylated C60 (see §10.4), or to the formation of highly stable radicals, as is the case for the addition of benzyl radicals for C60 [10.61],

Some of the radicals which react readily with C60 include Me', Ph*, PhS*, PhCH2*, CBr3', CC13', CF3', and Me3CO', where Me and Ph denote methyl and phenyl groups, respectively [10.4], Models for the attachment of free radical groups to C60 are shown in Fig. 10.18 and lead to a free radical derivative of CM. In Fig. 10.18(a), three R* groups [where R* could, for example, be a benzyl group (C6H5-CH*)] are attached at the sites indicated by R and the unpaired electron is localized within the pentagon ring, forming an "allylic" radical. However, when five groups are attached to all five available sites in a cyclopentadienyl configuration, then the unpaired electron becomes delocalized within the pentagon ring, forming a "cyclopentadienyl" radical, as shown in Fig. 10.18(b).

Thus, the addition of methyl radicals (Me*) to obtain (CH3)„C60 for 1 < n < 34, or benzyl radicals C6H5CH2' to obtain (C6H5CH2)„C60 for 1 < n < 15, has been demonstrated [10.61]. In the case of the addition of

10.9. Reactions with Free Radicals

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