Reactivity and Regioselectivity of Noble Gas Endohedral Fullerenes Ng@C-60 and Ng(2)@C-60 (Ng = He-Xe)

Recently, it was shown that genuine Ng-Ng chemical bonds are present in the endohedral fullerenes Ng(2)@C-60 in the case of Ng=Xe, while it is more debatable whether a chemical bond exist for Ng = Ar and Kr. The lighter homologues with helium and neon are weakly bonded van der Waals complexes. The presence of a noble gas dimer inside the cage is expected to modify the exohedral reactivity of the C-60 cage with respect to that of free C-60. To investigate the impact of encapsulated diatomic noble gas molecules on the chemical reactivity of C-60, we analyzed the thermodynamics and the kinetics of [4+2] Diels-Alder cycloaddition of 1,3-cis-butadiene at all non-equivalent bonds in free C-60, Ng@C-60, and Ng(2)@C-60 (Ng=He, Ne, Ar, Kr, and Xe). Our BP86/TZP calculations reveal that introduction of single noble gas atoms in Ng@C-60 and noble gas dimers He-2 and Ne-2 in Ng(2)@C-60 has almost no effect on the exohedral reactivity compared to free C-60, in agreement with experimental results. In all these cases cycloaddition is clearly favored at the [6,6] bonds in the fullerene cage. For the endohedral compounds He-2@C-60 and Ne-2@C-60 a slight preference (by less than 2 kcal mol(-1)) for bonds closer to the C-5 symmetry axis is found. This picture changes dramatically for the endohedral compounds with heavier noble gas dimers. Encapsulation of these noble gas dimers clearly enhances the reaction, both under thermodynamic and kinetic control. Moreover, in the case of Xe-2@C-60, addition to [6,6] and [5,6] bonds becomes equally viable. These reactivity changes in endohedral fullerenes are attributed to stabilization of the LUMO, increased fullerene strain energy, and greater compression of the encapsulated Ng(2) unit along the He to Xe series.