Endohedral fullerenes atoms

One particularly striking aspect of endohedral fullerene complexes is the surprising ability of the carbon cage to contain highly reactive species such as atomic nitrogen.1801 N C6o (54) has been prepared... 

In contrast to carbon, which forms structures derived from both sp2 and sp3 bonds, silicon is unable to form sp2 related structures. Since one out of four sp3 bonds of a given atom is pointing out of the cage, the most stable fullerene-like structure in this case is a network of connected cages. This kind of network is realized in alkali metal doped silicon clathrate (19), which were identified to have a connected fullerene-like structure (20). In these compounds, Si polyhe-dra of 12 five-fold rings and 2 or 4 more six-fold rings share faces, and form a network of hollow cage structures, which can accommodate endohedral metal atoms. Recently, the clathrate compound (Na,Ba), has been synthesized and demonstrated a transition into a superconductor at 4 K (21). The electronic structure of these compounds is drastically different from that of sp3 Si solid (22). 

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

Yamada M, Akasaka T, Nagase S (2010) Endohedral metal atoms in pristine and functionalized fullerene cages Acc Chem Res 43 92-102... 

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 15.30 X-ray crystal structure of a derivative of the endohedral fullerene complex Sc3N C80 (Sc and N atoms shown as large spheres).51...

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. 

NMR chemical shifts of the appropriate endohedral fullerenes (fullerenes that contain helium atoms)... 

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-... 

The monometalhc endohedral fullerenes are typified by M C82 (M = Sc, Y, La), and ESR studies of these molecules indicate that they are best described as the charge-separated species M + C82 For all three species, two distinct signals are seen, indicating the presence of two isomers of M C82, although it is not known whether these correspond to any of the observed isomers of the parent ffillerene C82 EXAFS studies of Y Cs2 indicate that each yttrium atom has seven nearest neighbor carbon atoms at 2.35 A as well as another yttrium atom at 4.05 A. These data are not consistent with a simple endohedral structure and further studies are required to clarify this apparent contradiction. 

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. ... 

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