Maximum Aromaticity As a Guiding Principle for the Most Suitable Hosting Cages in Endohedral Metallofullerenes

Encapsulation of metal clusters in endohedral metallofullerenes takes place in cages that in most cases are far from being the most stable isomer in the corresponding hollow fullerenes. In large metal clusters and small fullerene cages, the election is mainly driven by the need to reduce strain. For the not so large metal clusters, it is usually said that the cage is chosen to minimize the electrostatic repulsion between charged five-membered rings.¹ We have proved that maximizing the aromaticity is the main driving force that determines the most suitable hosting carbon cage for a given metallic cluster.² This simple rule, the so-called Maximum ARomaticity Criterion (MARC), allow us to directly compare IPR (isolated pentagon rule) and non-IPR anionic isomers regardless the number of adjacent pentagon pairs (APP) they have. We have tested the MARC for the majority of endohedral metallofullerenes from C₆₆ to C₁₀₄ reported to date and the results show that MARC successfully predicts the most suitable C_2n cage for hosting a given metal cluster among the manifold existing C_2n isomers. In addition, using the Maximum ARomaticity Criterion, one can justify why the IPR rule is not always fulfilled when EMFs are considered.

[Figure 1 here]

Figure 1. Classification in terms of the ALA index of the anionic IPR and non-IPR fullerene isomers with 1-3 Adjacent Pentagon Pairs (APPs) for the most common C_2n (2n = 66–104) EMFs reported to date. Isomers experimentally observed are marked using black dots, and their corresponding isomer numbers are indicated. The ALA prediction ordering for the experimentally observed isomers are given in parentheses. 

1. A. Rodríguez-Fortea, N. Alegret, A.L. Balch, J.M. Poblet, Nat. Chem. 2010, 2, 955.
2. M. Garcia-Borràs, S. Osuna, M. Swart, J.M. Luis, and M. Solà, Angew. Chem. Int. Ed. 2013, 52, 9275.