Chemistry Reference
In-Depth Information
Fig. 26 Molecular structures of La@
C
2
v
-C
82
,La
2
@
I
h
-C
80
, and Gd
3
N@
I
h
-C
80
the presence of an energetically low-lying fourfold degenerate LUMO, which
enables this fullerene to accept up to six electrons, and the electronic distribution
may be represented by the ionic model: (Sc
3
N)
6+
@(C
80
)
6
[
195
].
Considering the TNT method, endohedrals of the type M
3
N@
I
h
-C
80
have been
prepared for very different metals ( M
Sc, Y, La, Ce, Nd, Gd, Tb, Dy, Ho, Er,
etc
.
) and, in general, the metal cluster rotates freely inside the fullerene, with the
M
3
N unit adopting a planar geometry, except for the case of Gd
3
N@
I
h
-C
80
(Fig.
26
), where the nitride ion is out of the plane of the three gadolinium ions [
196
].
Upon increasing the size of the encapsulated metal, the yield of cluster fullerenes
usually decreases, and a distribution of molecules is obtained with larger metal ions
favoring larger cages. For example, with gadolinium six cages are formed:
Gd
3
N@C
78
,Gd
3
N@C
80
,Gd
3
N@C
82
,Gd
3
N@C
84
,Gd
3
N@C
86
, and Gd
3
N@C
88
[
197
], whereas for the larger lanthanum only the formation of three very large
cages, La
3
N@C
88
,La
3
N@C
92
, and La
3
N@C
96
, is observed [
198
]. In addition,
Echegoyen and co-workers found a remarkable influence of the cage structure on
the electrochemistry of the Gd
3
N@C
2
n
family [
197
]. The cage size does not seem to
affect significantly the reduction potential of these compounds, which displayed
very similar first reduction potentials, but the oxidation potentials shift from
+0.58 V vs Fc
+
/Fc in Gd
3
N@C
80
to +0.06 V vs Fc
+
/Fc in Gd
3
N@C
88
, which
suggests that the HOMO of the TNT-endofullerenes is probably cage-centered.
ΒΌ
5.2 Chemical Reactivity of Endohedral Fullerenes
The chemical functionalization of endohedral metallofullerenes is essential to
generate materials easy to process for multiple potential applications. Initial
experiments on the functionalization of endohedral fullerenes demonstrated a
high reactivity and the formation of multiple adducts or regioisomeric mixtures
[
199
]. However, a remarkable regioselectivity has been observed in a few cases
depending on the encapsulated cluster, metal species, carbon cage size or
symmetry.
More specifically, the
I
h
isomer of the C
80
carbon cage presents high symmetry,
with only two possible [1,2] addition sites: bonds between two hexagonal rings
