The Exohedral Diels-Alder Reactivity of the Titanium Carbide Endohedral Metallofullerene Ti2C2@D3h-C78: Comparison with D3h-C78 and M3N@D3h-C78 (M=Sc and Y) Reactivity

The chemical functionalization of endohedral (metallo)fullerenes has become a main focus of research in the last few years. It has been found that the reactivity of endohedral (metallo)fullerenes may be quite different from that of the empty fullerenes. Encapsulated species have an enormous influence on the thermodynamics, kinetics, and regiochemistry of the exohedral addition reactions undergone by these species. A detailed understanding of the changes in chemical reactivity due to incarceration of atoms or clusters of atoms is essential to assist the synthesis of new functionalized endohedral fullerenes with specific properties. Herein, we report the study of the DielsAlder cycloaddition between 1,3-butadiene and all nonequivalent bonds of the Ti2C2@D3h-C78 metallic carbide endohedral metallofullerene (EMF) at the BP86/TZP//BP86/DZP level of theory. The results obtained are compared with those found by some of us at the same level of theory for the D3h-C78 free cage and the M3N@D3h-C78 (M=Sc and Y) metallic nitride EMFs. It is found that the free cage is more reactive than the Ti2C2@D3h-C78 EMF and this, in turn, has a higher reactivity than M3N@D3h-C78. The results indicate that, for Ti2C2@D3h-C78, the corannulene-type [5,?6] bonds c and f, and the type B [6,?6] bond 3 are those thermodynamically and kinetically preferred. In contrast, the D3h-C78 free cage has a preference for addition to the [6,?6] 1 and 6 bonds and the [5,?6] b bond, whereas M3N@D3h-C78 favors additions to the [6,?6] 6 (M=Sc) and [5,?6] d (M=Y) bonds. The reasons for the regioselectivity found in Ti2C2@D3h-C78 are discussed.