fullerene, endohedral (rus. фуллерен, эндоэдральный otherwise эндофуллерен; эндометаллофуллерен abbr., ЭМФ; ЭФ; ТМН ЭФ) — fullerene molecule that contains one or more atoms or simplest molecules inside the carbon cage.

Description

Endohedral fullerenes can be divided into two main groups. The first includes those endohedral fullerenes that contain atoms of non-metals or simple molecules (e.g., nitrogen, phosphorus, helium, xenon, CO, etc.). The second consists of endohedral fullerenes which encapsulate metal atoms or carbide, nitride and sulphide metal-containing clusters, i.e. endohedral metallofullerenes (EMF). Among the latter, a subfamily of particularly stable M3N-containing endohedral fullerenes (where M = Sc, Y or a lanthanide metal), known as trimetallic nitride fullerenes, is to be mentioned.

The distinctive feature of endohedral metallofullerenes that makes them special among other derivatives is their strong donor-acceptor interaction between the metal atoms and the carbon case, which results in considerable electron transfer to the latter, the electronic state of metal atoms becoming similar to that observable in common inorganic salts.

Endohedral fullerenes are described with the notation Mm@Cn, where M is the encapsulated atom or cluster, and the subscripts m and n indicate the number of endohedral atoms (or molecules) and the number of carbon atoms in a fullerene molecule, respectively. The IUPAC-recommended name for La@C82 is {82} fullerene-incar-lanthanum and should be written as iLaC82, but this is rarely used in literature.

Methods to produce endohedral fullerenes depend on the nature of the endohedral particles inside. For example, fullerenes with inert gas atoms entrapped are produced by applying high inert gas pressure (several thousand atmospheres) to the fullerene sample at elevated temperatures (600-1000°C). Another approach to such compounds is ion implantation: bombardment of the fullerene target with accelerated ions of the desired element. Ion implantation is also used to obtain endohedral fullerenes with nitrogen, phosphorus and alkali metal atoms.

The yield of endohedral products in the above approaches is typically low, much isolation efforts being thus required. Most of the endohedral metallofullerenes are synthesised using a different way, by means of arc-discharge and laser evaporation of graphite doped with the respective metal oxides or salts in the helium atmosphere. The arc discharge method, developed originally to produce pristine fullerenes, is currently the main method of EMF synthesis in macroscopic quantities (with the yield of up to several percent). To synthesise trimetallic nitride species, a small amount of nitrogen or ammonia (about 1% vol.) is added to the helium atmosphere.

A large body of empiric evidence suggests that formation of EMF molecules upon arc-discharge proceeds via the assembly of a carbon cage around endohedral atoms rather than random trapping of these atoms into the almost ready cages. Consequently, the isomeric distribution of resulting molecules is influenced by donor-acceptor interactions between the forming carbon cage and the endohedral atoms. Thus, the structures of synthesisable endohedral compounds frequently differ from the available pristine fullerenes. The abundance of higher fullerene cages, such as C80, C82, C84, etc., is much increased and in many cases the products do not obey the isolated pentagon rule (see fullerene). A number of endohedral compounds with metals of the scandium subgroup, some other transition metals, lanthanides, and alkaline earth metals have been obtained in such a way.

The endohedral molecules are isolated from the soot via extraction with organic solvents (toluene, o-xylene, carbon disulphide, o-dichlorobenzene, N, N-dimethylformamide, etc.). The extract is then separated by means of multi-stage high performance liquid chromatography (HPLC).

Many endohedral metallofullerenes are highly polymerisable because of the open-shell electronic structure of their carbon cages, which hampers their isolation. At the same time, despite the negative charge on the carbon cage, some of these compounds show significantly more pronounced electron-withdrawing properties than even pristine fullerenes, and can thus exist as stable anions, such as . Functionalisation of those compounds may result in stable molecules with odd numbers of functional groups.

Endohedral metallofullerene chemistry remains relatively unexplored though a number of their derivatives have been produced with different chemical groups.

The applications most actively discussed for endohedral metallofullerenes are related to the biomedical field. The carbon cage provides perfect protection against contact of endohedral particles with the environment and thus against potential adverse effects of such exposure on the body. In this regard, endohedral metallofullerenes can be considered as radiotherapy agents (if radioactive endohedral atoms are introduced), contrast agents for magnetic resonance tomography (in the case of paramagnetic endohedral atoms) or labels of other kinds. The problem of delivery of such agents to the desired organs can be addressed through exohedral functionalisisation with appropriate groups (see biofunctionalisised nanomaterials).

Illustrations

Structural features of endohedral metallofullerenes.
Structural features of endohedral metallofullerenes.

Authors

  • Gusev Alexander I.
  • Kareev Ivan E.
  • Ioffe Ilya N.

Sources

  1. Koltover, V.K. Endohedral fullerenes: from chemical physics to nanotechnology and nanomedicine. In: “Progress in Fullerene Research” (Ed. M. Lang), New York: Nova Science Publishers, 2007, pp. 199-233.
  2. Heath J. R., O’Brien S.C., Zhang Q. et al. Lanthanum Complexes of Spheroidal Carbon Shells // J. Am. Chem. Soc. 1985. V. 107. P. 7779–7782.
  3. Shinohara H. Endohedral metallofullerenes // Reports on Progress in Physics. 2000. V. 63. P. 843– 892. A.V Yeletsky Endohedral structures (in Russian) // UFN. 2000. V. 170. P. 113–142.
  4. L.N Sidorov et al. Fullerenes. (in Russian) — Moscow: Ehkzamen, 2005. — 687 P.

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