Chemistry Reference
In-Depth Information
The experimental isolation of some of these EMFs species led to the wrong as-
signment of the fullerene cage and cluster encapsulated inside due to experimental
difficulties in the characterization of these EMFs. It should be emphasized that EMFs
are obtained generally in low yield and are often difficult to isolate. For instance,
the first example of a carbide cluster EMF, the Sc
2
C
2
@
D
2d
(23)-C
84
was initially as-
sumed to be Sc
2
@C
86
(Wang et al.
1999
,
2001
). Similarly, a theoretical study on the
smallest metal carbide EMFs synthesized up to date Sc
2
C
2
@
C
2v
-C
68
, argued that the
synthesized EMF corresponds to Sc
2
@
C
2v
(7854)-C
70
(Shi et al.
2006
; Zheng et al.
2012
). On the other hand, neither Y
2
C
82
nor Y
3
C
80
were found to be metal carbide
EMFs and they were assigned to be Y
2
@C
82
and Y
3
@C
80
classical EFs (Nishibori
et al.
2006
; Popov et al.
2010
). Interestingly, the initially assigned Ti
2
@C
80
later was
found to be Ti
2
C
2
@
D
3h
(78:5)-C
78
by
13
C NMR and density functional theory (DFT)
calculations (Cao et al.
2001
; Jaffiol et al.
2003
; Iwasaki et al.
2004
; Tan and Lu
2005
;
Yumura et al.
2005
). The
D
3h
(78:5)-C
78
cage in Ti
2
C
2
@
D
3h
(78:5)-C
78
represents
the smallest fullerene cage obeying the isolated pentagon rule (IPR) that has a metal
carbide encapsulated. For non-IPR cages, the smallest metal carbide disclosed so far
is Sc
2
C
2
@C
68
(Shi et al.
2006
). From an electronic point of view, metal carbides can
formally transfer four electrons to the fullerene cage as in (Sc
2
C
2
)
4
+
@(C
82
)
4
−
or six
electrons like in (Ti
2
C
2
)
6
+
@(C
78
)
6
−
or (Sc
3
C
2
)
6
+
@(C
80
)
6
−
(Valencia et al.
2008
).
These observations demonstrate the paramount importance of both experiment and
theory to correctly characterize these fascinating compounds.
The above-mentioned electron-transfer confers EMFs a different reactivity than
the hollow counterparts. The exohedral functionalization of EMFs is essential for the
final goal of obtaining new materials of interest for future applications in the fields of
biomedicine and materials science. These species have interesting physicochemical
properties with many potential interesting applications in the fields of magnetism,
superconductivity, nonlinear optical (NLO) properties, radioimmunotherapy, and
magnetic resonance imaging (MRI) contrast agents, among others. The most re-
markable example is the gadolinium based EMFs Gd
3
N@
I
h
-C
80
. This fullerene can
be effectively used in MRI as contrast agent (Fatouros et al.
2006
).
4.1.2
Exohedral Functionalization of Endohedral
Metallofullerenes
In fullerene structures at least six possible different C-C bonds might be present:
types A to F (represented in Fig.
4.1
). Adjacent pentagon pairs (APPs) are present in
non-IPR carbon cages where types E and F can be found.
Because X@
I
h
-C
80
EMFs are those most abundant, their exohedral function-
alization have been intensively explored in the quest for new materials of interest
for future applications in biomedicine and materials science (Rivera-Nazario et al.
2013
). The first exohedral functionalization of an EMF was a Diels-Alder (DA) cy-
cloaddition of 6,7-methoxyisochrom-3-one on Sc
3
N@
I
h
-C
80
(Iezzi et al.
2002
). The
icosahedral
I
h
-C
80
cage is highly symmetric (see Fig.
4.2
), and taking into account
