Understanding the Exohedral Functionalization of Endohedral Metallofullerenes - Exotic Properties of Carbon Nanomatter - page 92

Fig. 4.12 Energy profiles at B3LYP-D/TZP//B3LYP-D/DZP level of theory of the Diels-Alder
cycloaddition between La@ C 2v -C 82 and Cp and Cp* for the attack on the most reactive bond
(bond o ), when dispersion effects were included ( black ) and not included ( red ). The stationary
points represented are: (1) reactants, (2) reactant intermediate structure, (3) transition state, and
(4) product. Relative energy values are given in kcal mol − 1 . (Reprinted with permission from
(Garcia-Borràs et al. 2013a ). Copyright 2013 Wiley)

dispersion corrections, the Cp monoadduct would be more stable than the Cp* one.
Nevertheless, when dispersion interactions are considered, this situation is inverted,
with the Cp* adduct being the one largely more stabilized. The reason is the higher
stability of the Cp* reactant complex due to dispersion interactions. This result
confirms again that dispersion corrections are essential for analyzing the reactivity
of fullerenes and related compounds.

4.4 Conclusions

In this chapter, we have presented several comprehensive computational inves-
tigations of the exohedral functionalization of endohedral metallofullerenes. The
Diels-Alder (DA) cycloaddition is one of the most important and widely used reac-
tions for the functionalization of fullerene compounds. First, we have focused our
attention on the effect of different metallic clusters on the Diels-Alder cycloaddi-
tion. We have compared the Diels-Alder reactivity of the empty D 3h -C 78 fullerene
and Sc 3 N@ D 3h -C 78 ,Y 3 N@ D 3h -C 78 , and Ti 2 C 2 @ D 3h -C 78 EMFs. Our results show
that the encapsulation of the metal cluster reduces the DA reactivity of the fullerenic
cage, both from a thermodynamic and kinetic point of view. However, the three EMFs
present very different regioselectivities. For instance, the most favorable adducts un-
der kinetic control are for type A [6, 6] 1 bond for hollow D 3h -C 78 , type B [6, 6] 6
bond for Sc 3 N@ D 3h -C 78 , type D [5, 6] d bond for Y 3 N@ D 3h -C 78 , and type D [5, 6]
c bond for Ti 2 C 2 @ D 3h -C 78 EMFs. One key factor to explain the differences in the
DA regioselectivity is the strain energy of the cage caused by the metallic cluster.
Whereas for the large clusters (i.e. Y 3 N and Ti 2 C 2 ) the DA attack occurs at the bonds