Chemistry Reference
In-Depth Information
because of the strong interactions between the fused pentagons and the metal
cluster.
Endohedral fullerenes have been known from the earliest times of fullerenes (see
above). Although theoretical studies predicted in the early 1990s that elusive non-
IPR fullerenes could be stabilized by the presence of clusters encapsulated in the
fullerene cage, the first non-IPR fullerenes, namely Sc
2
@C
66
[
262
] and Sc
3
N@C
68
[
263
], were obtained in 2000. It is important to note that carbon cages in endohedral
fullerenes are different from those observed in empty fullerenes. Therefore, favor-
able electronic interactions between the encapsulated species with the carbon cage
are required for the stabilization of the resulting endohedral fullerene.
This method brings about the productions of other different non-IPR
metallofullerenes, such as trimetallic nitride fullerenes (i.e., Tb
3
N@C
84
[
264
],
Sc
3
N@C
70
[
265
], and Y
3
N@C
78
[
266
] etc.), metal carbide endofullerene
Sc
2
C
2
@C
68
[
267
], and metal cyanide endofullerene Sc
3
NC@C
78
[
268
].
A simple and qualitative rule to predict the stability of a given endohedral is
based on the calculated HOMO-LUMO gap for the resulting “ionic” endofullerene.
This energy gap can be roughly calculated from the (LUMO-3)-(LUMO-4) gap
determined for the neutral cage, thus predicting the most stable IPR and non-IPR
endofullerenes [
269
].
Remarkably, non-IPR endohedral metallofullerenes (fullerenes containing one
or more metal atoms in the inner cavity) show a strong coordination of the metal
atoms to the fused pentagons, similar to that observed for a variety of organometal-
lic species in which the concave face of the pentalene unit is coordinated to the
metal atom [
270
]. In contrast, IPR endofullerenes show a motion for the
encapsulated metals or clusters in the inner cavity. In some cases involving
endofullerenes endowed with only one metal atom, the metal generally coordinates
with the cage and motion is difficult.
On the other hand, the second strategy based on exohedral derivatization has
afforded a variety of non-IPR derivatives based on the remarkable reactivity of the
fused pentagons, thus changing the carbon bond hybridization and releasing the
bending strains. The first small fullerene
#271
C
50
(the Fowler-Manolopoulos nomen-
clature to differentiate isomers is specified by symmetry and/or by spiral algorithm)
was trapped and stabilized by chlorine atoms as
#271
C
50
Cl
10
in 2004 [
271
].
Since then the fullerene cage has been exohedrically functionalized by introducing
hydrogen or chloride atoms on the cage surface, to produce a variety of non-IPR
fullerene derivatives, such as C
64
H
4
[
272
], C
71
H
2
[
273
], C
78
Cl
8
[
274
], etc.
This stabilization of the resulting non-IPR fullerene derivatives has been
accounted for by the “strain-relief principle” resulting from the rehybridization
from sp
2
to sp
3
carbon atoms, as well as by the “local aromaticity principle,” which
involves maintaining the local aromaticity of the un-derivatized sp
2
carbon skeleton
that remains after the derivatization process. Based on both principles, it has been
possible to predict the stability of a variety of exohedrically functionalized non-IPR
fullerenes.
