Fig. 10.18. Structure of free radical derivatives of cm, where the unpaired spin is (a) localized forming an "allylic" free radical and (b) delocalized in the symmetric "cyclopentadienyl" configuration [10.4]. Note that only the region

benzyl radicals, as was described in the previous paragraph, the C6q radical resulting from the addition of three or five benzyl radicals is specially stable above 50° C. The stability of these radicals has been attributed to the steric protection of the surface radical sites by the surrounding benzyl substituents. Finally, as described in §10.10.2, radical formation by fullerene molecules plays an important role in polymerization reactions.

10.10. Host-Guest Complexes and Polymerization

10.10.1. Host-Guest Complexes

The term "host-guest" complex refers to a cluster of molecules or atoms (the host structure), which defines a cavity, containing a "guest" molecule, atom, or ion. Implicit in the definition is the condition that the guest is bound to the host structure through either van der Waals or ionic interactions and not by any covalent bonds. In many host-guest complexes, the geometry of the host structure is such that the host cannot form first, followed by the insertion of the guest species. Rather the formation of the host-guest complex requires the simultaneous presence of the guest and the constituent units of the host structure. If the complex is stabilized by the van der Waals interaction, either a "clathrate" or an "inclusion" complex is formed, depending on whether the cavity is a tube or channel (inclusion complexes) or the cavity completely surrounds the guest species (clathrate complex). Complexes can bond together to form larger complexes leading to a solid-state structure. For example, we have discussed C^ clathrate compounds in previous chapters, in which CM is the guest species located inside cages defined by organic solvent molecules. If the complex is stabilized by charge transfer, the complex is referred to as a "charge transfer" complex. Examples of clathrate and charge transfer complexes are given below.

Before discussing these complexes, however, we should remind the reader that C60 can also participate in a host-guest compound as the host. Two well-known examples discussed previously are the MXC60 (M = alkali metal) compounds and C60(O2)i. The former is a charge transfer compound, since the lattice is stabilized by the electron transfer from the M atoms to form C60 anions, while the latter does not involve charge transfer between the C60 lattice and the 02 physisorbed in the octahedral interstices of the fee lattice. We have also used the term "intercalation" compounds for the M^Qq and C60(O2)Jl. compounds, because the intercalant is observed to diffuse or "intercalate" into the host structure. In the case of the M^C^q compounds, a structural rearrangement of the host occurs for x = 1,4,6. Strictly speaking, the term intercalation applies only to two-dimensional host materials and refers to the introduction of guest species into layers between the host lattice planes (see §2.14).

Several examples of C60 host-guest complexes have been reported where C60 is the guest molecule, e.g., C60-hydroquinone [10.63], C60 y-cyclodextrin [10.64], and C60 calixarenes [10.65]. For example, a 3:1 clathrate complex is formed from C50 and hydroquinone. Hydroquinone (C6H602) is a ben-zenoid ring with OH groups on two opposing vertices of the ring, and these OH groups participate in intermolecular hydrogen bonding (i.e., an intermolecular H-O-H link). In this complex, each C60 molecule is in contact with a total of 18 hydroquinone molecules. According to a simple model, twelve of the 18 hydroquinone molecules are located in the region of the north and south poles (six on each pole) of the C60 molecules and their planar rings are parallel to the fullerene surface. The remaining six hydroquinone molecules are centered on a ring around the equator of the C60 and contact the fullerene "edge on." It has been suggested that the short distance between the C60 and the hydroquinone molecules near the north and south poles of the enclosed Qq molecules is caused by a charge transfer interaction between the hydroquinone and the C60, resulting in a very compact structure. Classic past examples of hydroquinone host-guest complexes [10.63] involve 3:1 complexes with methanol (but not ethanol), S02, C02 and argon (but not neon) as the guest species, where the 3:1 refers to the relative concentrations of hydroquinone to guest species.

Two further examples of C60 clathrate complexes involve interesting "cup-shaped" molecules. In the case of cyclodextrin (CD) hosts, a 2:1 y-cyclodextrin:C60 complex is formed from an aqueous solution of C60 and y-cyclodextrin [10.64], The C60 becomes trapped between two molecular "cups" of y-cyclodextrin. The shape and size of the cups, or truncated cones, for a-, (3-, and y-cyclodextrin, which contain six, seven, and eight glucose units, respectively, are shown in Fig. 10.19, while in Fig. 10.20 we show, respectively, a schematic top view of /3-cyclodextrin and a perspective view of a-cyclodextrin. As can be seen in Fig. 10.19, the cavity in the a-, ¡3-, and y-cyclodextrin (CD) is too small to accommodate a C60 molecule deep within the cup. Two y-cyclodextrins, however, have been shown to complex around a C60 guest, and a side view and top view of the 2:1 complex are shown in Fig. 10.21 [10.64]. Cyclodextrins are nontoxic, and as a result they find applications in drug and food encapsulation [10.9],

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