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Endohedral fullerene


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Rendering of a molecule of C_60 containing a noble-gas atom.

*Endohedral fullerenes* are fullerenes that have additional atoms, ions,
or clusters enclosed within their inner spheres. The first lanthanum C_60
complex was synthesized in 1985 called La@C_60 . The @ sign in the name
reflects the notion of a small molecule trapped inside a shell. Two types
of endohedral complexes exist: *endohedral metallofullerenes* and
*non-metal doped fullerenes* ^[1] .


[edit ] Endohedral metallofullerenes

Doping fullerenes with electropositive 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 , 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@C_60 or La@C_82 and as different isomer cages. Aside from the dominant
presence of mono-metal cages, numerous di-metal endohedral complexes and
the tri-metal carbide fullerenes like Sc_3 C_2 @C_80 were also isolated.

In 1998 a discovery drew large attention. With the synthesis of the Sc_3
N@C_80 , the inclusion of a molecule fragment in a fullerene cage had
succeeded for the first time. 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 .^[2]

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 La_2 @C_80 however it can be even
about 6 electrons such as in Sc_3 N@C_80 which is better described as
[Sc_3 N]^+6 @[C_80 ]^-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 [C_80 ]^-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 [Sc_3 N]^+6
@[C_80 ]^-6 pass through the column unreacted.

In Ce_2 @C80 the two metal atoms exhibit a non-bonded interaction.^[3]
Since all the six-membered rings in C80-Ih are equal^[3] the two
encapsulated Ce atoms exhibit a three-dimensional random motion ^[4] .
This is evidenced by the presence of only two signals in the ^13 C-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 Ce_2 @C_80 with
1,1,2,2-tetrakis(2,4,6-trimethylphenyl)-1,2-disilirane.


[edit ] Non-metal doped fullerenes

Saunders in 1993 showed the formation of endohedral complexes He@C_60 and
Ne@C_60 when C_60 is exposed to a pressure of around 3 bar of the noble
gases^[5] . Under these conditions about one out of every 650,000 C_60
cages was doped with a helium atom. The formation of endohedral complexes
with helium , neon , argon , krypton and xenon as well as numerous adducts
of the He@C_60 compound was also demonstrated^[6] with pressures of 3
kbars and incorporation of up to 0.1% of the noble gases.

While noble gases are chemically very inert and commonly exist as
individual atoms, this is not the case for nitrogen and phosphorus and so
the formation of the endohedral complexes N@C_60 , N@C_70 and P@C_60 is
more surprising. The nitrogen atom is in its electronic initial state (^4
S_3/2 ) and is therefore to be highly reactive. Nevertheless N@C_60 is
sufficiently stable that exohedral derivatization from the mono- to the
hexa adduct of the malonic acid ethyl ester is possible. In these
compounds no charge transfer of the nitrogen atom in the center to the
carbon atoms of the cage takes place. Therefore ^13 C-couplings , which
are observed very easily with the endohedral metallofullerenes, could only
be observed in the case of the N@C_60 in a high resolution spectrum as
shoulders of the central line.

The central atom in these endohedral complexes is located in the center of
the cage. While other atomic traps require complex equipment, e.g. laser
cooling or magnetic traps , endohedral fullerenes represent an atomic trap
that is stable at room temperature and for an arbitrarily long time.
Atomic or ion traps are of great interest since particles are present free
from (significant) interaction with their environment, allowing unique
quantum mechanical phenomena to be explored. For example, the compression
of the atomic wave function as a consequence of the packing in the cage
could be observed with ENDOR spectroscopy . The nitrogen atom can be used
as a probe, in order to detect the smallest changes of the electronic
structure of its environment.

Contrary to the metallo endohedral compounds, these complexes cannot be
produced in an arc. Atoms are implanted in the fullerene starting material
using gas discharge (nitrogen and phosphorus complexes) or by direct ion
implantation . Alternatively, endohedral hydrogen fullerenes can be
produced by opening and closing a fullerene by organic chemistry methods.


[edit ] References

1. *^ * Periodic table of endohedral fullerene atoms

2. ^ ^/*a*/  ^/*b*/  Ge,
Z; Duchamp, Jc; Cai, T; Gibson, Hw; Dorn, Hc (2005). "Purification
of endohedral trimetallic nitride fullerenes in a single, facile
step". /Journal of the American Chemical Society/ *127* (46):
16292–8. doi :10.1021/ja055089t
. PMID
16287323
. 
3. ^ ^/*a*/  ^/*b*/ 
K.Muthukumar; J.A.Larsson (2008). "Explanation of the different
preferential binding sites for Ce and La in M2@C80 (M = Ce, La".
/Journal of Materials Chemistry/ *18*: 3347–51. doi
:10.1039/b804168g
. 
4. *^ * Yamada, M; Nakahodo, T; Wakahara, T; Tsuchiya,
T; Maeda, Y; Akasaka, T; Kako, M; Yoza, K; Horn, E; Mizorogi, N;
Kobayashi, K; Nagase, S (2005). "Positional control of
encapsulated atoms inside a fullerene cage by exohedral addition".
/Journal of the American Chemical Society/ *127* (42): 14570. doi
:10.1021/ja054346r
. PMID
16231899
. 
5. *^ * M. Saunders, H. A. Jiménez-Vázquez, R. J. Cross,
and R. J. Poreda (1993). "Stable compounds of helium and neon.
He@C60 and Ne@C60". /Science / *259*
(5100): 1428–1430. doi
:10.1126/science.259.5100.1428
. PMID
17801275
. 
6. *^ * Martin Saunders, Hugo A. Jimenez-Vazquez, R.
James Cross, Stanley Mroczkowski, Michael L. Gross, Daryl E.
Giblin, and Robert J. Poreda (1994). "Incorporation of helium,
neon, argon, krypton, and xenon into fullerenes using high
pressure". /J. Am. Chem. Soc. / *116*
(5): 2193–2194. doi
:10.1021/ja00084a089
. 

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Categories : Fullerenes
| Supramolecular chemistry