Endohedral metallofullerenes fullerenes

Thrash TP, Cagle DW, Alford JM, Wright K, Ehrhardt GJ, Mirzadeh S, Wilson LJ (1999) Toward fullerene-based radiopharmaceuticals high-yield neutron activation of endohedral 165Ho metallofullerenes. Chem. Phys. Lett. 308 329-336. 

A first striking observation of this study was the dramatic difference in electron-accepting abilities of isomeric endohedral fullerenes. This difference reaches 0.22 V for the isomers of Yb C76 and even 0.3 V between two isomers of Yb C82. The cage size also influences the reduction potentials. The [CJ and [C2v] isomers of Yb C82 are the best electron acceptors of the family and the second isomer of Yb C76 the weakest electron acceptor. In general, however, these metallofullerenes are better electron acceptors than the available empty cages. The metallofullerenes with bigger cages, Yb C82 and Yb C84, also accept more easily a fifth electron than the others. 

Figure 6.29. Images of the calculated structures involved in the hole-repairing mechanism for endohedral fullerene growth. Shown from left to right are the Cyg open cage, top and side views of the CgY fragment, and the final Y Cg2 metallofullerene. Reproduced with permission from Gan, L.-H. Wang, C.-R. J. Phys. Chem. A 2005,109, 3980. Copyright 2005 American Chemical Society.

The fullerenes were first discovered by laser ablation cluster ion formation in a mass spectrometer.89 Soon after the identification of C60 as a magic number cluster, [LaC60]+ was also identified.90 Later, macroscopic quantities of fullerenes were produced by the carbon arc method91 soon followed by the macroscopic production of the lanthanum endohedral fullerenes.92 Fullerene chemistry has developed rapidly since the preparation of macroscopic quantities of these compounds. The development of the metallofullerenes has been hampered by the low yield... 

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 metallofullerenes. The accepted symbol to indicate encapsulation is e.g. U C28, La Cg2. 

La Cg2 was reported in macroscopic quantities (Chai et al. 1991). It was in the same study, where the symbol was introduced to denote that the former atom or molecule is encaged inside the fullerene molecule. These species containing atoms or molecules inside the fullerene cages are called endohedral fullerenes (EFs). The most studied compounds are the so-called endohedral metallofullerenes (EMFs) due to their high abundancy (Akasaka and Nagase 2002). In 1991, the first structural characterization by X-ray diffraction of an EMF was reported (Chai et al. 1991). 

While many other metallic and nonmetallic elements have been encapsulated in Mlerenes, rare earth element endohedral metallofullerenes were the first endohedral fiillerenes synthesized. All the rare earth elements, with the exception of Pm, have been encapsulated inside fullerenes. Indeed, the rare earth-containing endohedral fullerenes dominate the literature on this subject, most likely because the compounds are relatively easy to synthesize, in contrast to potential transition-metal-containing endohedral fullerenes. 

It was shown how both endohedral and exohedral fullerenes can be inserted in nanotubes. In the peapods containing endohedral fullerenes (for instance Ce Cg2), HRTEM images showed interesting rotation and translation motion of the trapped spheroids. Exohedral metallofullerenes, CsC io have been synthesized and successfully encapsulated into SWCNTs via a new chemical reduction of Cgo molecules into anions. The addition of iodine to already prepared peapods allowed the coalescence of Cgo directly inside the nanotubes. Indeed, after heating at 550 C, iodine-doped peapods, inside the Cgo molecules molecules have been transformed in a tubular structure. Khlobystov et al. were able to perform reactions on the inner surface of carbon nanotubes in the presence of catalyti-cally active atoms of rhenium and monitor the whole process via HRTEM. ... 

The results of Dunk et al [48] provide experimental insight into how endohedral fullerenes are formed in carbon vapor and allow for a complete description of ftillerene formation to be developed. Larger U C2 endohedral metallofullerenes U C36 and U C44 are shown to form with a small probability based on U C28 as their precursor. 

Cagle D W, Kennel S J, Mirzadeh S, Alford J M and Wilson L J 1999 In vivo studies of fullerene-based materials using endohedral metallofullerene radiotraoers Proc. Natl Acad. Sc/. USA 96 5182-7... 

Nagase S, Kobayashi K, Akasaka T, Wakahara T (2000) Endohedral metallofullerenes theory, electrochemistry, and chemical reactions. In Kadish KM, Ruoff RS (eds.) Fullerenes chemistry, physics, and technology. Wiley, New York, pp. 395 136. 

Zhang S, Sun D, Li X, Pei F, Liu S (1997) Synthesis and solvent enhanced relaxation property of water-soluble endohedral metallofullerenes. Fullerene Sci. Tech. 5 1635-1643. 

These examples demonstrate the well-known process of polymerization initiated by anion-radicals. Our next consideration is devoted to an unusual case of initiation. Intercalation of fullerenes by metals results in the formation of fullerene-metal derivatives. Paramagnetic metallofullerenes (anion-radicals) are the fullerenes doped with endohedral metal. According to calculations and structural studies, LaCs2, for example, contains La in the center of one hexagonal ring of the fuller-ene cage (Akasaka et al. 2000, Nishibori et al. 2000, Nomura et al. 1995). Intrafullerene electron transfer in metallofullerenes is possible (Okazaki et al. 2001). 

The carbon arc is a versatile method to generate a wide variety of nanostruct-ured carbon materials such as fullerenes (7,11), endohedral metallofullerenes (12-14), nanotubes (6,8-10,15), nanocapsules filled with metals (or carbides) (16-18), and so forth. Arc discharge is characterized by the high electric current... 

This review describes the preparation, characterization, and properties of all nonpolymeric complexes that contain a metal removed from the fullerene also are included. The article does not cover the essentially ionic fullerides MmC (4) or the endohedral metallofullerenes MmC (8), which have been reviewed previously. The extended fullerenes, or so-called carbon nanotubes, which have hollow centers and can be filled with metal salts, also are not discussed. The majority of complexes involve 7r-bonds and, apart from alkyl lithium fullerides, the potentially useful synthetic area of o- complexes has not been explored. Table I shows the occurrence of metal-bound adducts across the periodic table. 

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