Large metal ions in a relatively small fullerene cage: the structure of Gd3N@C2(22010)-C78 departs from the isolated pentagon rule.

An isomerically pure sample of Gd(3)N@C(78) has been extracted from the carbon soot formed in the electric-arc generation of fullerenes using hollow graphite rods packed with Gd(2)O(3) and graphite powder under an atmosphere of helium and dinitrogen. Purification has been achieved by chromatographic methods and the product has been characterized by mass spectrometry, UV/vis absorption spectroscopy, and cyclic voltammetry. Although a number of endohedral fullerenes have been found to utilize the D(3h)(5)-C(78) cage, comparison of the spectroscopic and electrochemical properties of the previously characterized Sc(3)N@D(3h)(5)-C(78) with those of Gd(3)N@C(78) reveals significant differences that indicate that these two endohedrals do not possess the same cage structure. A single crystal X-ray diffraction study indicates that the fullerene cage does not follow the isolated pentagon rule (IPR) but has two equivalent sites where two pentagons abut. The endohedral has been identified as Gd(3)N@C(2)(22010)-C(78). Two of the gadolinium atoms of the planar Gd(3)N unit are located within the pentalene folds formed by the adjacent pentagons. The third gadolinium atom resides at the center of a hexagonal face of the fullerene.