Chemistry Reference
In-Depth Information
et al.
1997
) and in general for any charged fullerene. (Aihara
2001
) Very recently,
we have shown in a systematic study that the selection of the most suitable cage for
the formation of all of the most common
C
2n
(2n
66-104) EMFs reported to date
is governed by the maximum aromaticity criterion (MARC), which determines that
the most aromatic negatively charged fullerene cage is the most stable one (Garcia-
Borràs et al.
2013b
). This rule is based on one of the unique features of these EMFs
species, which is the formal electronic transfer that occurs from the metal cluster to
the fullerene cage. It has been shown that this negatively charge transferred to the car-
bon structure is mainly located on five membered rings (5-MRs), (Rodríguez-Fortea
et al.
2010
) which makes them more aromatic (Garcia-Borràs et al.
2013b
). As a
whole, there are two main factors that determine the stability of the fullerenes: strain
energy and aromaticity. The aromaticity of the hollow neutral fullerenes is very low,
and then reducing strain energy is the main factor that governs their stability. That
is why all empty fullerenes follow the IPR rule. On the contrary, in highly charged
fullerenes or EMFs, the aromaticity plays a major role. Thus, maximizing the total
aromaticity is the main stabilizing factor, which could lead to both an IPR or non-IPR
isomer as the most stable fullerenic cage.
The structure, stability, and reactivity of the fullerenic cage among other factors
depend on: (i) the size of the metal cluster encapsulated inside, and (ii) the formal
electronic transfer from the metal cluster to the fullerene cage. For those species
involving small rare-earth atoms, i.e. Sc, Y, and lanthanides from Gd to Lu, the
I
h
-
C
80
cage is usually the most abundant (Chaur et al.
2007
; Stevenson et al.
2004
).
The
I
h
-C
80
fullerene is also considered to be the best cage for trapping TNT or
M
2
clusters, although is the most unstable among the seven IPR structures of C
80
fullerene (Stevenson et al.
1999
;Luetal.
2012
). Similar situations are found for
other fullerene cages such as the
D
3h
-C
78
,
C
2
-C
78
, and
C
2v
-C
82
. The preference for
the
I
h
-C
80
cage among all possible C
80
IPR and non-IPR isomers is mainly attributed
to the higher aromaticity of the negatively charged
I
h
-C
80
cage in the EMFs (Garcia-
Borràs et al.
2013b
). In addition to that, the inner cavity of
I
h
-C
80
is relatively large;
the shape of the cage maximizes the separation between the negatively charged 12
pentagons (the so-called maximum pentagon separation rule); (Rodríguez-Fortea
et al.
2010
) and
I
h
-C
80
presents a large LUMO
=
4 gap (Campanera et al.
2005
; Rodríguez-Fortea et al.
2010
; Valencia et al.
2007
). Apart from the highly
stable TNT EMF, Sc
3
N@
I
h
-C
80
, and the related X
3
N@
I
h
-C
80
(X
+
3-LUMO
+
Sc, Y, Gd, Tb,
Dy, Ho, Er, Tm, Lu) and mixed metal nitride clusterfullerenes M
x
Sc
3
−
x
N@
I
h
-C
80
,
(X
=
Gd, Dy, Lu, Y, Ce, Nd, Tb, Er, Lu, Ti) and M
x
L
3
−
x
N@
I
h
-C
80
(Lu
x
Y
3
−
x
, CeLu
2
,
Y
2
Ti), many other
I
h
-C
80
cluster-based EMFs have been reported (Yang et al.
2011
;
Lu et al.
2012
). For instance, the metal clusters La
2
,Ce
2
, and Y
3
, (Guldi et al.
2010
; Popov et al.
2010
; Akasaka et al.
1997
; Suzuki et al.
1995
) the metal carbides
Sc
3
C
2
and Sc
4
C
2
, (Tan and Lu
2006
; Tan et al.
2006
) the metal hydrocarbides Sc
3
CH,
(Krause et al.
2007
) metal carbonitride Sc
3
CN, (Wang et al.
2010b
) and metal oxides
Sc
4
O
2
and Sc
4
O
3
have been encapsulated inside the
I
h
-C
80
cage. (Stevenson et al.
2008
; Mercado et al.
2010
) Interestingly, all these X@
I
h
-C
80
species have a formal
charge transfer from the metal cluster to the cage of six electrons, i.e. X
6
+
@
I
h
-C
6
80
.
=
