Crystal structures of saturn-like C50Cl10 and pineapple-shaped C64Cl4: geometric implications of double- and triple-pentagon-fused chlorofullerenes.

Fullerenes are spherical cage carbon molecules constructed from hexagons and exactly 12 pentagons. Theoretically, according to the so-called Isolated Pentagon Rule (IPR), fullerenes with isolated pentagons are relatively stable and synthesizable. This same rule predicts that fullerenes with fused pentagons are too reactive to be isolated. IPR-violating fullerenes, however, have been reported to be stabilized by either encapsulation with metal atoms (or their carbide/ nitride clusters) or external derivatization. Fullerenes featuring fused pentagons have been characterized by computationally corroborated C NMR spectroscopy. Detailed information regarding their geometries and intermolecular interactions, however, requires single-crystal X-ray characterization, as in the case of Sc3N@C68. [9] The crystal structures of two double-fused-pentagon endohedral metallofullerenes, namely La@C72(C6H3Cl2) [10] and Tb3N@C84, [11] have also been determined. Highly accurate structural parameters established by X-ray crystallography are rare for exohedral derivatives of IPR-violating fullerenes. Prinzbach et al., for example, have recently reported the crystal structures of polybrominated dodecahedrane, which is the smallest IPR-violating fullerene derivative known to date. Herein we report two crystal structures of IPR-violating fullerenes (C50 (no. 271) and C64 (no. 1911)) [2,13] stabilized by exohedral chlorination. The former is the smallest fullerene for which a cage can be constructed without triplets of directly or sequentially fused pentagons, while the latter is a triple-fused-pentagon fullerene. The crystallographic data reported herein provide unequivocal structural information for these IPR-violating fullerene derivatives (e.g., detailed geometric parameters, molecular curvature, pyramidalization angles, aromaticity, intermolecular interactions) and the property implications thereof. The X-ray structures of C50Cl10 (1) and C64Cl4 (2) are shown in Figure 1. The symmetries of these IPR-violating chlorofullerenes are identical to those of their carbon cage parents, in other wordsD5h-C50 (1a) for 1 and C3v-C64 (2a) for 2. The former is a Saturn-like chlorofullerene molecule constructed symmetrically from five pairs of two fused pentagons whose fused edges are covalently bound by ten chlorines in a manner that resembles the rings around Saturn (another Saturn-like halogenated molecule is C60F20, which was synthesized by Taylor). In contrast, 2 has a pineappleshaped structure with four chlorine atoms covalently attached