Fullerene Endohedral Compounds

Tsuchiya T, Akasaka T, Nagase S (2010) New vistas in fullerene endohedrals functionalization with compounds from main group elements. Pure Appl Chem 82 505-521... 

Most of studies have been devoted to the CHA analysis by placing the nucleus at the centre of the spherical box. However, the confined atom may conceivably be located anywhere inside the spherical box. For atoms trapped in fullerene cages, in the theory of endohedral compounds, there exist experimental and theoretical studies that show that the confined atom is not at the centre of M C60 [72-78]. A similar problem has been studied in... 

Fig. 4.8 Representation of all non-equivalent bonds of the Ng2 C6o compound. The activation energies (in kcal mop ) corresponding to the Diels-Alder cycloaddition reaction between 1,3-butadiene and all non-equivalent bonds for all considered noble gas endohedral compounds. Ng2 Cjo has been represented on the right. A grey scale has been used to represent the different noble gases endohedral compounds black color is used to represent the helium-based fullerene, light grey for neon, medium grey for argon, dark grey for krypton, and white for xenon...

Fullerenes can also form compounds with atoms encapsulated within the cage structure, termed endohedral compounds and designated M C , where the symbol signifies that the M atom is encapsulated within the C cage, e.g., U C2g, Y Cgo, Y2 Cg2, La2 Cgo, La Cg2- Fullerene aggregates are bonded by van der Waals forces (c.f. graphite) and will permit the entrapment of alkali metal ions such as Li, Na, K, Rb and Cs [87]. 

Fullerene Endohedral Complexes. The possibility of trapping atoms or molecules inside carbon frameworks is a very peculiar property of fullerenes. Today there is indeed strong evidence that metal atoms can actually be trapped inside fullerene cages. A nomenclature for this peculiar kind of compounds has been even suggested (M Cn) for a metal atom M located inside a C fullerene. 

Endohedral metal fullerenes can be detected in relatively small amounts in the mass spectra in the laser vaporization cluster beams (vide supra). However, macroscopic quantities of these compounds may be produced rather readily either by vaporization in a laser furnace apparatus or by arc-burning of a composite rod of graphite and the corresponding metal oxide. In Fig. 4.50, a mass spectrum which illustrates the formation of a series of fullerene endohedral yttrium complexes obtained by laser vaporization of a composite graphite/ Y2O3 rod at 1200 ""C is reproduced. Among these species there is also one, Y2 Cs2, which corresponds to the inclusion of a metal cluster in the fullerene ball. 

The organic inclusion compound 2,4,6-tris-(4-bromophenoxy)-l,3,5-triazine was used to create an ordered arrangement of endohedral fullerenes in a crystalline host matrix <06CPL327>. 

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). 

The study of Sc2C2 Cg2 [C3v] and Sc3C2 C80 began with a misunderstanding. Their redox properties were first interpreted as if these compounds were Sc2 Cg4 and Sc3 C82 before Nagase et al. showed that they were in reality metal carbide endohedral fullerenes.62,63... 

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]. 

To close this section on trimetal nitride endohedral fullerenes, we present and discuss data from potassium intercalation experiments on Sc3N C80. As was the case for Kx(Tm C82), x-ray photoemission measurements of the K 2p and C Is signals can be used to give a good indication of the overall potassium concentration in such intercalation compounds. Figure 16 displays the K 2p photoemission lines for several intercalation levels in Kx(Sc3N C80). 

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. 

New carbon compounds, namely, endohedral metallofullerenes (EMF.v) are promising building blocks to be used in the design of nanosized materials of a new generation. They exhibit unique electrical, magnetic and chemical properties. Now fullerene chemistry is most developed due to that fullerenes are accessible to a wide range of researchers. However, chemical properties of EMFs are very weakly studied because of problems of the synthesis and isolation of pure EMF.v in preparative quantities. 

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