Reactivity and Regioselectivity of Noble Gas Endohedral Fullerenes Ng@C60 and Ng2@C60 (Ng=He–Xe)

AbstractRecently, it was shown that genuine NgNg chemical bonds are present in the endohedral fullerenes Ng2@C60 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 C60 cage with respect to that of free C60. To investigate the impact of encapsulated diatomic noble gas molecules on the chemical reactivity of C60, we analyzed the thermodynamics and the kinetics of [4+2] Diels–Alder cycloaddition of 1,3‐cis‐butadiene at all nonequivalent bonds in free C60, Ng@C60, and Ng2@C60 (Ng=He, Ne, Ar, Kr, and Xe). Our BP86/TZP calculations reveal that introduction of single noble gas atoms in Ng@C60 and noble gas dimers He2 and Ne2 in Ng2@C60 has almost no effect on the exohedral reactivity compared to free C60, 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 He2@C60 and Ne2@C60 a slight preference (by less than 2 kcal mol−1) for bonds closer to the C5 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 Xe2@C60, 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 Ng2 unit along the He to Xe series.