Endohedral fullerenes encapsulation

Many elements have been encapsulated in fullerenes, including most of the Group 1 and Group 2 elements. All noble gases with the exception of Rn have been encapsulated. Endohedral fullerenes are also known for all the rare earth elements (except Pm) where the bonding is largely ionic, similar to that foimd for the Groups 1 and 2 elements. Only a few transition metals and nonmetals such as Ti, Zr, Hf, N, C, and P have been encapsulated, which can be attributed, in part, to synthetic difficulties. 

A. L. Balch, FuUerenes as encapsulating hosts preparation, detection, and structures of endohedral fullerenes, in Encyclopedia of Supramolecular Chemistry (eds J. L. Atwood and J. W. Steed), Marcel Dekker, New York, 2004, p. 579. 

Another striking development with endohedral fullerenes was the synthesis of N C5q, the first example of encapsulation of a reactive nonmetal atom [128-132]. 

Pure carbon spheres of Cf,(l the fullerenes—react avidly with free radicals with a higher antioxidant ability than the naturally occurring vitamin E (21). Endohedral fullerenes have been shown to be capable of encapsulating a variety of atoms such as radiotracers or noble gases, thereby making them efficient excipients in the delivery of radioisotopes to cancer cells or in magnetic resonance imaging (MRI) (22). 

One of the most remarkable properties of fullerenes is their ability to encapsulate atoms, ions and small molecules, to form the so-called endohedral fullerene complexes, denoted... 

Figure 1 The structures of endohedral fullerenes with encapsulated tetrahedral MX molecules and...

Figure 7 The structure of endohedral fullerene clusters with benzene and borazole molecules encapsulated into Cs4 ( >6a).

Up to 1999, only metal atoms [1-5], metal clusters [6,7], metal nitrides [55-57], and noble gas atoms [58-60] were observed to be encaged inside C60, C70, or various sizes of higher fullerenes. The experimental evidence for carbon atoms or metal-carbon compounds (carbides) being encapsulated inside fullerenes had not yet been observed. In 2000, Shinohara et al. succeeded in the first production, isolation, and spectroscopic characterization of a scandium carbide endohedral fullerene (Sc2C2) C84. Following this, the first experimental evidence based on synchrotron X-ray diffraction was presented and revealed that the Sc carbide is encapsulated in the form of a lozenge-shaped Sc2C2 cluster inside the D2d-C84 fullerene [8]. 

Fullerenes can encapsulate various atoms within the cages, and these compounds have been referred to as endohedral fullerenes. For example, the symbolic representations La C6o and La2 Cso indicate that the fullerene cage encapsulates one and two lanthanum atom(s), respectively. The IUPAC description refers to these fullerenes species as incar-fullerenes, and the formulas are written as t LaCeo and tl Cso, (i is derived from incarcerane). Some metal endohedral fullerenes are listed in Table 14.2.1. The endohedral fullerenes are expected to have interesting and potentially very useful bulk properties as well as a fascinating chemistry. Some non-metallic elements, such as N, P, and noble gases, can be incarcerated into fullerenes to form N 0,0, P C6o, N C o, Sc3N C80, Ar Oo, etc. 

Endohedral fullerenes are molecules in which an atom or group of atoms is encapsulated within the cage. In the case of metal encapsulation the compounds are commonly referred to as endohedral metallofiillerenes. The accepted symbol to indicate encapsulation is , e g. U C28, La C82-... 

To date, all the confirmed isolable endohedral fullerenes contain metals either scandium, yttrium, or one of the f-block elements may be encapsulated, and compounds containing up to three metals have been reported. The proportions of the endohedral fiillerenes observed depends upon the manner by which they are extracted from the soot. For lanthanum, predominantly La Cg2 and l. i2 C%o are isolated by solvent extraction techniques, whereas direct sublimation from the soot yields La C6o, La C7o, La C74, and La C82- An explanation for this behavior is that most of the endohedral fullerenes are present as insoluble salts, e.g. [La C6o]" [Ceo], and only La Cg2 and La2 C8o are sufficiently redox stable to be extracted as the neutral molecules. 

The endohedral fullerenes U C28 and U C6o have been reported to be sufficiently stable to allow chemical isolation. Endohedral complexes of the smaller fullerenes (C2 , 2n < 60) are conunonly observed in molecular beam experiments (Section 4.1), but this stabilization of C28 by uranium atom encapsulation is remarkable. A theoretical study of [7i-C28]" and Td-C2iiU (25) indicate that they are stable molecules, and that the laboratory preparation of their derivatives may be viable. ... 

Yang, H., Lu, C., Liu, Z. et al (2008) Detection of a family of gadolinium-containing endohedral fullerenes and the isolation and crystallographic characterization of one member as a metal-carbide encapsulated inside a large fullerene cage. Journal of the American Chemical Society, 130, 17296-17300. 

Zuo, T.M., Olmstead, M.M., Beavers, C.M. etal. (2008) Preparation and structural characterization of the 7h and the Dsh isomers of the endohedral fullerenes TmsNOCgo icosahedral Cgo cage encapsulation of a trimetallic nitride magnetic cluster with three uncoupled Tm + ions. Inorganic Chemistry, 47, 5234-5244. 

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