Chemistry Reference
In-Depth Information
5.1 Metallofullerenes and TNT Endofullerenes
Smalley and co-workers demonstrated in 1991 that a family of lanthanum
containing fullerenes were produced under a modified Kr¨tschmer-Huffman reac-
tor and that extraction with toluene yielded mostly La@C
82
(Fig.
26
), which was
the first endohedral fullerene to be isolated [
186
]. La@C
82
has an electronic state
best described as [La]
3+
[C
82
]
3
with an open-shell electronic structure that is a
consequence of a three-electron transfer from lanthanum to C
82
[
187
]. The resulting
electron spin of La@C
82
imposes a unique chemical reactivity comparable to
radical species, inducing magnetism on the molecular scale, or an enhanced elec-
tron conductivity [
188
,
189
].
Since the isolation and characterization of the first mono-endohedral
metallofullerene, La@C
82
, many other classical M@C
2
n
(M
¼
Sc, Y, La, Ce, Gd,
etc
.
) have been obtained, with a C
2
v
-C
82
most abundant cage [
176
]. In all these
fullerene cages filled with a single metal, the metal is not in the center of the cage
but tends to coordinate with the cage carbons, being situated under a hexagonal ring
along the
C
2
axis. As a result, the distribution of charge density is highly anisotropic
over the surface, with electrophiles and nucleophiles selectively attacking the two
different regions [
190
].
In the case of the endohedral fullerenes containing two metal, the M
2
@C
80
(M
La, Ce, etc
.
) cage is typically obtained, with the two isomers
I
h
and
D
5
h
as the
most abundant [
176
]. In the case of these endohedral metallofullerenes, not only is
the metal-cage interaction important but also the metal-metal interaction is crucial
for the positioning and moving of the metal atoms. In the case of M
2
@
I
h
-C
80
structures, it has been demonstrated how the metal atoms circulate three-
dimensionally [
191
], in contrast to M
2
@
D
5
h
-C
80
species, where the metallic
atoms circulate two-dimensionally along a band of ten contiguous hexagons inside
the
D
5
h
-C
80
cage [
192
].
However, for a long period of time the development of the chemistry of
endohedral metallofullerenes was impeded by the relatively low yields in which
they were produced. An important breakthrough in this chemistry occurred in 1999,
when Dorn and co-workers reported the production of trimetallic nitride clusters
with high yields [
193
]. In the trimetallic nitride template (TNT) method, packed
graphite rods (metal oxide/carbon/catalyst) are burned in the presence of a dynamic
flow of He/N
2
and afforded macroscopic quantities of materials such as Sc
3
N@C
80
,
with yields that exceed those of the third most abundant (next to C
60
and C
70
) empty
cage C
84
, produced under normal conditions.
The isolation of Sc
3
N@C
80
in macroscopic quantities has facilitated the study of
its physical structure and chemical reactivity [
180
]. From the seven possible
constitutional isomers for C
80
satisfying the isolated pentagon rule (IPR), interest-
ingly only the two least stable empty isomers with
I
h
and
D
5
h
symmetries are the
ones that predominate when they are filled with metallic nitride clusters, the
I
h
isomer certainly being the most abundant [
194
]. When considering the electronic
structure of
I
h
-C
80
it is possible to rationalize this observation. It is characterized by
¼
