Chemistry Reference
In-Depth Information
Fig. 25 Drawing for the
cationic metallofullerene
Li
+
@C
60
and 3,000 atm of noble gas), although the occupation level of the guest is as low as
0.1-1% [
181
]. In a second approach, molecules as water have been introduced in
C
60
and C
70
cages by using organic reactions in the so-called “molecular surgery”
[
182
]. This strategy consists in a series of steps which involve making an incision in
the fullerene cage to form an opening on the surface, inserting the desired molecule
through the opening, and finally closing the hole to reproduce the fullerene cage
while retaining the guest species.
The case of H
2
O@C
60
is quite remarkable, since single-crystal X-ray analysis of
the complex [H
2
O@C
60
octaethylporphyrin) reveals that, in
contrast to many metallofullerenes where the metal often adopts an off-center
location and does not move freely, the O atom is located at the center of C
60
,
with the O-H bonds pointing towards the Ni atoms. In addition, H
2
O@C
60
and the
empty fullerene can be separated quite easily by HPLC on a pyrenylated stationary
phase, in stark contrast to the case of noble gas atoms or H
2
endohedrals. This easy
access to a non-hydrogen-bonded H
2
O molecule inside the apolar fullerene cage
allows the investigation of the properties of the isolated H
2
O molecule as well as the
modification of the exohedral chemical reactivity of a unique
wet
C
60
.
From the two categories of endohedral fullerenes mentioned above, the elec-
tronic properties of metallofullerenes are particularly promising considering that
they are featured by a charge transfer from the encapsulated metal atoms to the
carbon cage, forming a non-dissociating salt that consists of metal cation(s)
encapsulated in a fulleride anion [
183
]. This electron-transfer was regarded to
stabilize not only the encapsulated species but also the fullerene cage that can
sometimes be otherwise unstable in the empty form. A striking example of this
ionic model was the isolation of the cationic endohedral metallofullerene Li
+
@C
60
(Fig.
25
)[
184
], which can only be stabilized significantly in ambient conditions
when it co-exists with an appropriate counteranion. For example, the crystal
structure of the salt [Li
+
@C
60
](PF
6
)
exhibits a strong interaction between Li
+
,
residing inside the C
60
cage, and PF
6
on the outside, the interaction being shown to
occur through the six-membered rings [
185
].
In the following sections we are going to concentrate on mono- and divalent
metallofullerenes and metallic nitrides, since these metallofullerenes have been
extensively investigated and their chemical reactivity reasonably explored.
(NiOEP)
2
] (OEP
¼
