Structure of a Surprising Non-isolated Pentagon Rule Endohedral Metal Carbide Made Possible by Synchrotron Radiation

Endohedral metal carbides suggest an intriguing relationship to the growth mechanism of endohedral fullerenes; however, due to crystal quality requirements, only a half dozen of these have been characterized by single crystal X-ray diffraction. Likewise, few examples of nonisolated pentagon rule (non-IPR) endohedral fullerenes have been crystallographically characterized, beginning with Sc3N@C68 in 2003 [1]. Since that time, the use of synchrotron radiation has increased our ability to determine the structures of these relatively rare fullerenes. The metal clusters encaged in non-IPR fullerenes include Tb3N, Gd3N, Sc2S and involve a range of cage sizes from C66 to C84. To date, the carbides include only C2 fragments, cage sizes in the range C68 to C92 and the metals Sc, Ti, Y, and Gd. We recently determined the first crystal structure of a non-IPR endohedral that contains a metal carbide. That endohedral fullerene, paramagnetic Gd2C2@C1(51383)-C84, belongs to one of 51592 possible isomers of the C84 cage, of which 24 follow the isolated pentagon rule (IPR). Our study reveals a single violation of the IPR; that is, the fullerene cage possesses one pentalene unit where two pentagons are touching. Theoretical calculations as well as C NMR studies anticipated this result for the diamagnetic Y2C2@C84 congener. Geometric features, including positioning of the Gd2C2 unit within the cage in comparison to related carbide endohedrals will be presented.