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experimental/theoretical study the regioselectivity of the 1,3-dipolar cycloaddition
reaction was compared for the mixed species Sc
x
Gd
3
−
x
N@
I
h
-C
80
(x
0-3) (Chen
et al.
2007
). Experiments showed that the regioselectivity of the reaction was dras-
tically changed as the [5, 6] regioisomer was obtained in the case of Sc
3
N@
I
h
-C
80
,
whereas the [6, 6] one for Gd
3
N@
I
h
-C
80
. Our calculations indicate that the differ-
ence in reactivity observed experimentally is basically due to the
lower activation
barrier
found for the Gd
3
N@
I
h
-C
80
[6, 6] addition (15.7 kcal mol
−
1
for the [6, 6]
addition as compared to 16.5 kcal mol
−
1
for the [5, 6] one). The kinetics of the
Diels-Alder reaction at Gd
3
N@
I
h
-C
80
is therefore crucial to fully understand the
experimental findings.
=
4.2.3
The Influence of Metal Clusters on the Diels-Alder
Regioselectivity of I
h
-C
80
Endohedral Metallofullerenes
In order to improve our understanding of the endohedral metallofullerene reactivity
and the regioselectivity changes due to the different nature of the metal clus-
ter encapsulated, we have systematically studied with density functional methods
(ZORA-BLYP-dDsC/TZP//ZORA-BP86-D
2
/DZP) the Diels-Alder cycloaddition
between
s-cis
-1,3-butadiene and practically all X@
I
h
-C
80
synthesized up to date:
X
Sc
3
N, Lu
3
N, Y
3
N, La
2
,Y
3
,Sc
3
C
2
,Sc
4
C
2
,Sc
3
CH, Sc
3
NC, Sc
4
O
2
, and Sc
4
O
3
(see Fig.
4.8
). Using the Frozen Cage Model (FCM), which is a computationally
cheap approach to accurately predict the exohedral regioselectivity of cycloaddition
reactions in EMFs, (Garcia-Borràs et al.
2012b
) we have studied both the thermo-
dynamic and the kinetic regioselectivity of the process taking into account the free
rotation of the metallic cluster inside the fullerene.
In Fig.
4.2
the two non-equivalent bonds for
I
h
-C
80
fullerene are represented. Even
though
I
h
-C
80
fullerene has only two different non-equivalent bonds, the complexity
of the theoretical study of X@
I
h
-C
80
reactivity and regioselectivity arises from the
free rotation that the inner metal clusters present (see for instance the previous 2.2
section). Indeed, the study presented here has only been made possible through the
use of the FCM method, which in the first phase scans and finds the most stable
products of a given reaction in EMFs at a low computational cost, which is followed
in the second phase by a full (non-frozen) exploration of the reactivity for a small
number of selected most reactive bonds.
In that sense, we have reported in Fig.
4.9
Gibbs reaction energies and Gibbs
reaction barriers for all the studied systems considering the most favorable orientation
of the inner metallic cluster. As said before, 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
while
−
18.1 kcal mol
−
1
. The reaction
barriers are found to be 3.4 and 2.4 kcal mol
−
1
for [5, 6] and [6, 6] additions at this
level of theory. Thus, results indicate that the
I
h
-C
80
empty cage is very reactive and
thermodynamic and kinetic products are not the same.
−
the corresponding value for the [6, 6] addition is
