Endohedral fullerenes: Encyclopedia II - Endohedral fullerenes - Endohedral metallofullerenes

Endohedral fullerenes - Endohedral metallofullerenes

Doping fullerenes with electro positive metals takes place in an arc reactor or via laser evaporation. The metals can be transition metals like scandium, yttrium as well as lanthanides like lanthanum and cerium. Also possible are endohedral complexes with elements of the alkaline earth metals like barium and strontium and alkali metals like potassium and tetravalent metals like uranium, zirconium and hafnium. The synthesis in the arc reactor is however unspecific. Besides unfilled Fullerenes, endohedral metallofullerenes develop with different cage sizes like La@C₆₀ or La@C₈₂ and as different isomer cages. Aside from the dominant presence of mono-metal cages, numerous di-metal endohedral complexes and the tri-metal Fullerenes like Sc₃@C₈₂ were also isolated.

In 1998 a discovery drew large attention. With the synthesis of the Sc₃N@C₈₀ for the first time, the inclusion of a molecule fragment had succeeded into a fullerene cage. This compound can be prepared by arc-vaporization at temperatures up to 1100 °C of graphite rods packed with Scandium(III) oxide iron nitride and graphite powder in a K-H generator in a nitrogen atmosphere at 300 Torr ^[1].

Endohedral metallofullerenes are characterised by the fact that electrons will transfer from the metal atom to the fullerene cage and that the metal atom takes a position off-center in the cage. The size of the charge transfer is not always simple to determine. In most cases it is between 2 and 3 charge units, in the case of the La₂@C₈₀ however it can be even about 6 electrons such as in Sc₃N@C₈₀ which is better described as [Sc₃N]⁺⁶@[C₈₀]^-6. These anionic fullerene cages are very stable molecules and do not have the reactivity associated with ordinary empty fullerenes. They are stable in air up to very high temperatures (600 to 850°C) and the Prato reaction yields only a monoadduct and not multi-adducts as with empty fullerenes.

The lack of reactivity in Diels-Alder reactions is utilised in a method to purify [C₈₀]^-6 compounds from a complex mixture of empty and partly filled fullerenes of different cage size ^[2]. In this method Merrifield resin is modified as a cyclopentadienyl resin and used as a solid phase against a mobile phase containing the complex mixture in a column chromatography operation. Only very stable fullerenes such as [Sc₃N]⁺⁶@[C₈₀]^-6 pass through the column unreacted.

In Ce₂@C₈₀ the metal atoms are found to be untouchable and display a three-dimensional random motion ^[3]. This is evidenced by the presence of only two signals in the ¹³C-NMR spectrum. It is possible to force the metal atoms to a standstill at the equator as shown by x-ray crystallography when the fullerene is exahedrally functionalized by an electron donation silyl group in a reaction of Ce₂@C₈₀ with 1,1,2,2-tetrakis(2,4,6-trimethylphenyl)-1,2-disilirane.

Adapted from the Wikipedia article "Endohedral metallofullerenes", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki