Chemistry Reference
In-Depth Information
Fig. 4.6
Representations of
a
the relative position of the metal nitride with respect to the attacked
C
−
C bond of the fullerene cage;
b
non-equivalent bonds present in the
I
h
and
D
5h
cages
the exohedral reactivity of the most important and abundant EMFs, i.e. the Sc
3
N@
I
h
-
C
80
and its Sc
3
N@
D
5h
-C
80
isomer, and the (bio)chemically relevant lutetium and
gadolinium-based EMFs for both
I
h
-C
80
and
D
5h
-C
80
cages. As performed in previ-
ous studies, we analyze the thermodynamics and the kinetics of the DA cycloaddition
of
s-cis
-1,3-butadiene on all different bonds of the
I
h
-C
80
and
D
5h
-C
80
cages.
The study of the reactivity on the M
3
N@C
80
EMFs is not straightforward since
the metal nitride can rotate freely in the interior of the carbon cage cavity. This has
been observed in NMR experiments (Duchamp et al.
2003
; Stevenson et al.
1999
;
Yang et al.
2008
) and has been confirmed by computational studies (Campanera
et al.
2002
; Popov and Dunsch
2008
; Rodríguez-Fortea et al.
2006
). In this study, we
considered eight orientations of the M
3
N unit (see Fig.
4.6
a) inside the
I
h
-C
80
cage,
and five orientations (1, 2, 4, 5 and 8) for the
D
5h
-C
80
isomer. The lower number of
orientations studied in the case of the
D
5h
cage is mainly due to the higher number
of non-equivalent bonds present in the fullerene structure. In the case of
I
h
-C
80
only
two different addition sites are possible: a type-D [5, 6] (corannulene) ring junction
and a type-B [6, 6] junction. In contrast, the less symmetric
D
5h
-C
80
cage presents
nine different C
C bonds: four [5, 6] bonds (type-D) and five [6, 6] bonds (one
type-A, three type-B and one type-C, see Fig.
4.6
b).
As observed in the previously studied Sc
3
N@
D
3h
-C
78
, all EMFs are less reactive
than their homologous hollow cages. Thus, for the pristine hollow
I
h
-C
80
fullerene,
the Gibbs reaction energy for the addition on [5, 6] bond is
−
35.7 kcal mol
−
1
−
18.1 kcal mol
−
1
. When
the Sc
3
N is encapsulated inside
I
h
-C
80
, the lowest energy adduct corresponds to the
[5, 6] addition (see Fig.
4.7
) with a Gibbs reaction energy of
while the corresponding value for the [6, 6] addition is
−
17.2 kcal mol
−
1
.
The addition is more stable than the [6, 6] one by more than 12 kcal mol
−
1
. The
energy difference between the corresponding TSs is, however, notably smaller (
ca.
4 kcal mol
−
1
). Experimentally, the 1,3-dipolar cycloaddition of N-ethylazomethine
−
