Theoretical characterization of existing and new fullerene receptors

We have studied the ability of 26 receptors to catch the fullerenes C-60 and C-70. The prediction of which host displays the largest affinity with fullerenes is complicated by the fact that some hosts are extremely flexible. For example, the cyclotriveratrylene (CTV) based host, with three 2-[9-(1,3-dithiol-2-ylidene) anthracen-10(9H)-ylidene]-1,3-dithiole (exTTF) pincers, has an interaction energy that is quite modest and even lower than that determined for the famous C60H28 buckycatcher. Notwithstanding this energetic difference, experimental results indicated that the exTTF-CTV host has an association constant comparable to those reported for metalloporphyrins. In line with the recent experimental results we found that when three corannulene pincers are attached to cyclotriveratrylene, the ability of the host to interact with fullerene is not improved with respect to the C60H28 buckycatcher, which has two corannulene pincers. Our theoretical calculations showed that the reason for such an outcome is that the corannulene pincers are stacked and thus a large amount of energy is required to break the intramolecular dispersion interactions that keep the structure stacked. A similar scenario was found when we attached one, two and three exTTF pincers to pentakis(1,4-benzodithiino) corannulene. Bearing these results in mind, and considering that the C60H28 buckycatcher is somewhat rigid, we alkylated the rim of the corannulene pincers. The alkylated buckycatcher is able to interact with C-60 with an interaction energy that is larger than that corresponding to the unsubstituted host. Therefore, functionalization of the C60H28 buckycatcher seems to be the most promising road to the synthesis of new fullerene receptors that are not based on metalloporphyrins. The design of new hosts must be pursued with the aim of finding receptors whose most stable conformations are similar to that expected in the supramolecular complex.