Chemistry Reference
In-Depth Information
Fig. 34 Supramolecular ZnP/C
60
molecular wire
7 The Future: Non-IPR Fullerenes
As stated above, C
60
is by far the most abundant and common of all fullerenes. The
question is why, among the many possible cages that can be formed with carbon
atoms, the one containing 60 atoms is favored? Furthermore, since all fullerenes Cn
are constituted by hexagons (
n
22) and pentagons
(12 for all fullerene cages, which are responsible for the curved geometry), why
among the 1,812 possible isomers for 60 carbon atoms was only the icosahedral
symmetry
I
h
-C
60
molecule (soccer-ball shape) formed?
These intriguing questions were answered by Kroto, who proposed that the local
strain increases with the number of bonds shared by two pentagons (pentalene
units), thus affording less-stable molecules. This rule was coined as the “isolated
pentagon rule” (IPR), stating that all pentagons must be surrounded by hexagons,
thus forming the corannulene moiety [
16
]. The resonance destabilization that
results from the adjacent pentagons (8
20 with the exception of
n
¼
ˀ
electrons which do not satisfy the H¨ckel
rule) and reduction of the
-orbital overlapping because of cage curvature explains
the lower stability of non-IPR fullerenes [
257
]. A head-to-tail exclusion rule
has also been proposed to explain the higher stability of fullerenes obeying the
IPR rule [
258
].
For a precise number of carbon atoms forming a cage, the number of non-IPR
fullerene isomers is very much larger than the IPR ones. Furthermore, in addition to
doubly fused pentagons found in non-IPR fullerenes, triple directly fused pentagons
and more recently triple sequentially fused pentagons have been reported [
259
].
Therefore there is great interest in the search for the huge number of expected non-
IPR fullerenes whose chemical reactivity and properties should be different from
those known for IPR fullerenes [
260
].
In order to achieve non-IPR fullerenes, two different strategies have been
developed to increase their stability, namely through endohedral and exohedral
derivatization [
261
]. In both approaches the key issue to stabilize non-IPR
fullerenes focuses on how to release or decrease the strains of fused pentagons.
The first endohedral strategy involves encaging a metal cluster inside the
fullerene cage. The bending strain on fused pentagons is significantly decreased
ˀ
