A Complete Guide on the Influence of Metal Clusters in the Diels-Alder Regioselectivity of Ih-C80 Endohedral Metallofullerenes

The chemical functionalization of endohedral metallofullerenes (EMFs) arouses considerable interest for the possibility to synthesize new species with potential applications in materials science and medicine. Experimental and theoretical studies on the reactivity of endohedral metallofullerenes are scarce. To improve our understanding of the endohedral metallofullerene reactivity, we have systematically studied with density functional methods the Diels-Alder cycloaddition between s-cis-1,3-butadiene and practically all X@I_h-C₈₀ EMFs synthesized up to date: X = Sc₃N, Lu₃N, Y₃N, La₂, Y₃, Sc₃C₂, Sc₄C₂, Sc₃CH, Sc₃NC, Sc₄O₂, and Sc₄O₃.^1,2 We have studied both the thermodynamic and the kinetic regioselectivity, taking into account the free rotation of the metallic cluster inside the fullerene. This systematic study has been made possible through the use of the Frozen Cage Model, a computationally cheap approach to accurately predict the exohedral regioselectivity of cycloaddition reactions in EMFs. Our results show that the reactivity of all metal clusters studied is lower than that of the hollow I_h-C₈₀ cage. Except for the Y₃ cluster, the addition takes place predominantly over the [5,6] bond. In many cases, however, a mixture of the two possible regioisomers is predicted. In general, the [6,6] addition is favored in EMFs that have a larger charge transfer from the metal cluster to the cage or have a voluminous metal cluster inside. The present investigation represents the first complete and exhaustive guide on the reactivity of I_h‑C₈₀ based EMFs.

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1. M. Garcia-Borràs, S. Osuna, J.M. Luis, M. Swart, and M. Solà, Chem. Eur. J. 2013, DOI: 10.1002/chem.201302202.

2. S. Osuna, R. Valencia, A. Rodríguez-Fortea, M. Swart, M. Solà, J.M. Poblet, Chem. Eur. J. 2012, 18, 8944.