Chemistry Reference
In-Depth Information
Fig. 3 Empty fullerenes C
60
and C
70
(
left
) and endohedral fullerenes La@C
82
and Sc
3
N@C
80
(
right
)
1.2 General Remarks on the Chemical Reactivity of Fullerenes
Buckyballs
constitute a family of closed cage carbon allotropes that contains
2 (10 +
H
) carbon atoms, where
H
is the number of hexagons, while the number
of pentagons is always fixed at 12. In principle, an unlimited number of fullerene
structures could result. However, the simplest and most abundant is C
60
, which is
formed by 60 carbon atoms -12 pentagons and 20 hexagons-, followed by C
70
.C
60
has icosahedral symmetry and a diameter of 7.8
˚
. An important structural motif of
fullerenes is the so-called “isolated pentagon rule,” which means that all pentagons
in the molecule must be isolated from other pentagons, since structures with fused
pentagons are highly destabilized due to the increase in strain energy and resonance
destabilization [
16
] (Fig.
3
).
In contrast to diamond and graphite, which are sparingly soluble in organic
solvents, fullerenes are soluble in some organic solvents. They undergo a variety of
chemical reactions in solution to afford a huge number of fullerene derivatives
which, in general, preserve the outstanding chemical, physical, and electrochemical
properties of pristine fullerenes. The study of the chemical reactivity of fullerenes
has experienced an unprecedented development during the last two decades and is
expected to continue on this steep slope.
The singular 3D geometry of fullerenes containing 30 or more highly reactive
double bonds constitutes a new scenario where a variety of different chemical
reactions can be tested. The convex surface of fullerenes offers new possibilities
for the study of new reactions and mechanisms under severe geometrical
constraints on carbon atoms showing a singular sp
2.3
hybridization [
17
].
The contributions stemming from well-established and versatile protocols to
functionalize fullerenes chemically have yielded a broad spectrum of intriguing,
tailor-made fullerene derivatives. The remarkable properties of the latter are con-
tinuously under investigation and form the basis in the search for practical
applications of fullerenes.
As mentioned above, the C
60
molecule is formed from 12 pentagons and 20
hexagons linked by single and double carbon-carbon bonds. The calculated bond
distances reveal subtle differences between the [5,6]- and [6,6]-bonds with values of
1.45 and 1.38
˚
, respectively. Due to the mixed character of 1,3,5-cyclohexatrienes
