ullerenes: synthesis and structure

    In 1985, Kroto, Smalley and coworkers discovered that, by subjecting graphite to laser radiation at >10 000 K, new allotropes of carbon were formed. The fullerenes are named after architect Buckminster Fuller, known for designing geodesic domes. Each fullerene is molecular and the family includes C₆₀, C₇₀, C₇₆, C₇₈, C₈₀ and C₈₄. Several synthetic routes to fullerenes have been developed; C₆₀ and C₇₀ are the major components of the mixture formed when graphitic soot is produced as graphite rods are evaporated (by applying an electrical arc between them) in a helium atmosphere at ≈130 bar and the vapour condensed. Extraction of the soot into benzene yields a red solution from which C₆₀ and C₇₀ can be separated by chromatography. Hexane or benzene solutions of C₆₀ are magenta, while those of C₇₀are red. Both C₆₀ and C₇₀ are now available commercially, and this has encouraged rapid exploration of their chemical properties. Figure a shows the structure of C₆₀. Although a number of X-ray diffraction studies of C₆₀ have been carried out, the near-spherical shape of the molecule has led to frustrating orientational disorder (see Section 18.3) problems. The C₆₀ molecule belongs to the Ih point group and consists of an approximately spherical network of atoms which are connected in 5- and 6-membered rings; all the C atoms are equivalent, as indicated by the fact that the ¹³C NMR spectrum of C₆₀ exhibits one signal (δ + ‏143). The rings are arranged such that no 5-membered rings are adjacent to each other. Thus, C₆₀ (the smallest fullerene that can be isolated as a stable species) satisfies the Isolated Pentagon Rule (IPR).† The separation of the 5-membered rings by 6-membered rings is easily seen in the schematic representation of C₆₀ shown in Figure1.1b which also gives a bonding scheme.

Fig.1.1 (a) The structure of the fullerene C60; the approximately spherical molecule is composed of fused 5- and 6-membered rings of carbon atoms. [X-ray diffraction at 173K of the benzene solvate C₆₀   4C6H6, M.F. Meidine et al. (1992) J. Chem. Soc., Chem. Commun., p. 1534.] (b) A representation of C60, in the same orientation as is shown in (a), but showing only the upper surface and illustrating the localized single and double carbon–carbon bonds.

Each C atom is covalently bonded to three others in an approximately trigonal planar arrangement; the relatively large surface of the ‘sphere’ means that there is only slight deviation from planarity at each C centre. There are two types of C^_C bond: those at the junctions of two hexagonal rings (6,6- edges) are of length 139 pm, while those between a hexagonal and a pentagonal ring (5,6-edges) are longer, 145.5 pm. These differences indicate the presence of localized double and single bonds, and similar bonding descriptions are appropriate for other fullerene cages. We consider chemical evidence for the presence of C=C double bonds below. After C₆₀, the next smallest fullerene to satisfy the IPR is C₇₀. The C₇₀ molecule has D₅h symmetry and is approximately ellipsoidal (Figure1.2); it comprises 6- and 5-membered rings organized so that, as in C₆₀, 5-membered rings are never adjacent. The ¹³C NMR spectrum of C₇₀ confirms that there are five C environments in solution, consistent with the solid state structure (Figure1.2a).

Fig.1.2 The structure of C₇₀ determined from an X-ray diffraction study of C₇₀ 6S₈ [H.B. Burgi et al. (1993) Helv. Chim. Acta, vol. 76, p. 2155]: (a) a ball-and-stick representation showing the five carbon atom types, and (b) a space-filling diagram illustrating the ellipsoidal shape of the molecule.