Endohedral Complexes
As endohedral complexes are called fullerenes , an atom or a cluster has been introduced into the cavity. There are two classes of compounds.
Doping with electropositive metals (metal fulleride)
The doping with electropositive metals takes place in the arc reactor or by laser vaporization. The elements of the III. Subgroup Sc, Y and La, as well as from the group of lanthanoids Ce, Pr, Sm, Gd, Dy, Ho, Er, Tm, Lu. Furthermore, endohedral complexes with elements of the second main group (Ca, Sr, Ba), alkali metals (Li, Na, K, Cs) and tetravalent metals (U, Zr, Hf) are known.
The synthesis in the arc reactor is, however, unspecific. In addition to unfilled fullerenes , endohedral metal fullerides are produced in various cage sizes (La @ C 60 , La @ C {82} ) and in various isomeric cages (Sc @ C 82 ). In addition to the dominant monometal cages, numerous bimetal-endohedral compounds and the trimetal fullerene Sc 3 @C 82 have also been isolated.
In 1998, a discovery caused a sensation. With the synthesis of Sc 3 N @ C 80 , the inclusion of a molecular fragment in a fullerene cage was successful for the first time. The trimetallic nitride fullerides Ho 3 N @ C 80 and Er 3 N @ C 80 were then synthesized and analyzed spectroscopically. In contrast to Sc 3 N @ C 80 , the trimetal nitride cluster of Er 3 N @ C 80 has a planar structure in the cage instead of a tetragonal structure as in Sc 3 N @ C 80 . Furthermore, the endohedral trimetal nitride fullerides ErSc 2 N @ C 80 and Er 2 ScN @ C 80 could be produced and separated by means of preparative HPLC .
Endohedral metal fullerides are characterized by the fact that electrons are transferred from the metal atom to the cage and the metal atom occupies a non-central position in the cage. The size of the charge transfer is not always easy to determine. In most cases it is between two and three charge units, but in the case of the La 2 @C 80 it is even 6 electrons.
Doping with non-metal elements
Saunders was able to prove the existence of the endohedral complexes He @ C 60 and Ne @ C 60 in 1993 . These complexes are formed when C 60 is heated to 600 ° C for five hours at a pressure of approx. 2500 bar. Under these conditions, only one of the approx. 650,000 C 60 cages was doped with an He atom. In the meantime, endohedral complexes of helium, neon, argon, krypton and xenon as well as numerous adducts of He @ C 60 have been detected.
While noble gases are chemically inert and therefore always appear atomically, the discovery of nitrogen or phosphorus endohedral complexes is very unusual. Of these complexes, N @ C 60 , N @ C 70 and P @ C 60 have so far been detected and isolated. The nitrogen atom is in its electronic ground state ( 4 S 3/2 ) and is therefore to be regarded as highly reactive. Nevertheless, N @ C 60 is so stable that exohedral derivatization is possible and mono-, bis-, and hexadducts of ethyl malonate can be synthesized. With these compounds there is no charge transfer from the nitrogen atom in the center to the carbon atoms of the cage. Therefore 13 C-couplings, which can be easily observed in the endohedral metallofullerenes, could only be detected with high resolution as shoulders of the middle line in the case of the N @ C 60 . The central atom of the noble gas and nitrogen or phosphorus endohedral compounds is exactly in the middle of the cage.
While other atomic traps normally only require a great deal of equipment, such as. B. by laser cooling of atoms or ions in magnetic traps, endohedral fullerenes represent an atomic trap that is stable at room temperature for almost any length of time. Atomic or ion traps are of great interest because here particles are excluded from any interaction with them Environment. In this way the intrinsic properties of these particles can be observed. This includes B. the compression of the atomic wave function as a result of the encapsulation in the cage, which could be observed by electron-nuclear double resonance . The nitrogen atom can thus be used as a probe to detect the smallest changes in the electronic structure of its environment.
synthesis
In contrast to the metal endohedral compounds, these complexes cannot be generated in an electric arc. In the production of these compounds, unfilled fullerene cages are used as the starting material and the atoms are implanted. The synthesis of nitrogen or phosphorendohedral fullerenes is achieved by gas discharge, high frequency discharge, or direct ion implantation.