Chemistry Reference
In-Depth Information
be attacked. Finally, we focus our attention on the essential role of the dispersion
interactions to reproduce the experimental results of the exohedral cycloaddition on
EMFs.
4.1
Introduction
4.1.1
Endohedral Metallofullerenes
The encapsulation of metals and small molecules inside a fullerene cavity, i.e. the
formation of endohedral fullerenes, was considered right after the C
60
discovery
(Heath et al.
1985
; Kroto et al.
1985
). Endohedral metallofullerenes (EMFs) have
attracted increasing attention in the last few years for several reasons: (i) the pro-
duction yield of EMFs is generally higher than for other endohedral fullerenes, (ii)
the potential application of these molecules in medicine and materials science, (iii)
the metal cluster strongly interacts with the fullerene cavity leading to quite unique
materials (Akasaka and Nagase
2002
; Chaur et al.
2009
;Luetal.
2012
; Osuna et al.
2011a
; Rodríguez-Fortea et al.
2011
; Yang et al.
2011
).
The first example of stable endohedral metallofullerene, La@C
82
, was reported
in. (Chai et al.
1991
) In 1999, the synthesis, isolation, and characterization of the
first member of the metallic tri-nitride template (TNT) family, i.e. Sc
3
N@C
80
was
reported. (Stevenson et al.
1999
) In fact, Sc
3
N@C
80
is the third most abundant
fullerene only after C
60
and C
70
. Since then, many other EMFs have been synthesized
that range from single metal atoms to metallic carbides, nitrides, oxides, and sulfides.
(Chaur et al.
2009
) A variety of EMFs with fullerene carbon cages that range from
C
68
to C
104
and metal atoms that are generally from Group 3 or lanthanides have
been reported (Chaur et al.
2009
; Yang et al.
2011
).
Nowadays it is well-known that fullerenes can encapsulate atoms, ions, metallic
clusters, and small molecules such as H
2
, CO, H
2
O, NH
3
or CH
4
(for reviews see (Liu
and Sun
2000
; Guha and Nakamoto
2005
; Murata et al.
2008
; Yamada et al.
2010
;
Rodríguez-Fortea et al.
2011
;Luetal.
2011
; Chaur et al.
2009
). These endohedral
fullerenes (EFs) can be classified in different classes: (Chaur et al.
2009
) (i) Clas-
sical EFs of the type X@
C
2n
,X
2
@
C
2n
, and X
3
@
C
2n
, with X
=
metal, noble gas,
small molecule and 60
≤
2n
≤
88); (ii) metallic nitride EMFs such as M
3
N@
C
2n
,
with M
96; (iii) metallic carbide EMFs like M
2
C
2
@
C
2n
,
M
3
C
2
@
C
2n
,M
4
C
2
@
C
2n
, hydrogenated metallic carbide M
3
CH@
C
2n
, and metal-
lic nitrogen carbide M
3
CN@
C
2n
with 68
=
metal and 68
≤
2n
≤
92; (iv) metallic oxide EMFs of the
type M
4
O
2
@
C
2n
and M
4
O
3
@
C
2n
; and v) metallic sulfide M
2
S@
C
2n
.
The isolated-pentagon rule (IPR) formulated by Kroto (
1987
) states that all pen-
tagons must be surrounded by hexagons to alleviate the strain produced by two
fused pentagons (a pentalene unit). While the isolated pentagon rule (IPR) is strictly
obeyed by all pure-carbon fullerenes isolated to the date, (Lu et al.
2008
) an increas-
ing number of EMFs have been synthesized that present non-IPR cages. Therefore,
the IPR rule appears to be more a suggestion than a rule for these species (Kobayashi
≤
2n
≤
