La 2 @C 72 : Metal-Mediated Stabilization of a Carbon Cage S. Stevenson, P. Burbank, K. Harich, Z. Sun, and H. C. Dorn* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061 P. H. M. van Loosdrecht, M. S. deVries, J. R. Salem, C.-H. Kiang, R. D. Johnson, and D. S. Bethune IBM Research DiVision, Almaden Center, 650 Harry Rd., San Jose, California 95120 ReceiVed: December 8, 1997; In Final Form: February 23, 1998 In this study, we report production, isolation, and characterization for the relatively small endohedral metallofullerene, La 2 @C 72 . As described, La 2 @C 72 is readily isolated from conventional electric-arc-generated carbon/metal soot. This new species was purified by HPLC chromatography and characterized by laser desorption mass spectrometry and UV-vis spectroscopy. The mass spectrum also demonstrates the presence of the monometal species, La@C 72 , but the absence of empty-cage C 72 . Since empty-cage C 72 has not been successfully isolated to date, the results of the present study are in agreement with the argument for metal- mediated stabilization of the C 72 carbon cage by lanthanum ions. The chromatographic retention data suggest that the electronic structure of La 2 @C 72 is consistent with a (La 3+ ) 2 @C 72 6- species and the prediction of a relatively small dipole moment. Introduction The recent discovery and isolation of metal-encapsulated fullerenes (A m @C 2n , m ) 1-3, n ) 30-70) represents a unique class of new compounds. These endohedral metallofullerenes 1-28 are of special interest because of their unique structural arrangement and potential applications. 2,3,29 For example, predicted applications already include possible uses as nonlinear optical devices, 3 catalysts, 3 superconductors, 2,29 lasers, 2 and ferroelectric materials. 2,3 Although most endohedral metallo- fullerenes isolated to date have cages of 80 carbons or greater (Sc 2 @C 74 , 17,18 Sc 2 @C 84 , 3,17,18,24,26 Sc 3 @C 82 , 22,23,25-27 Y@C 82 , 12,13,26 Y 2 @C 90 , 21 La@C 82 , 14-16,19-21 La 2 @C 80 , 19,21 Gd@C 82 , 16,26,28 Gd 2 @C 90 , 21 Er 2 @C 82 , 27 and Pr@C 82 26 ), a notable exception is the smaller cages (e.g., C 70 -C 74 ) which have only a limited number of isolated pentagon rule (IPR) satisfying cage isomers. Most of the endohedrals isolated to date are mono- or dimet- allofullerenes (MMF and DMF). For the C 72 empty cage, only one IPR allowed isomer is predicted (D 6d ) which leads to a limied number of anticipated encapsulated endohedral metal- lofullerene isomers, A m @C 72 (Figure 1). In similar fashion, the empty-cage C 74 fullerene has only one IPR-allowed structure (D 3h ). 17,18 In experimental agreement, Sc 2 @C 74 has also been purified, 32 but without concomitant isolation of the correspond- ing empty-cage C 74 fullerene. Thus, there is a paucity of examples for those endohedrals with the smaller C 72 -C 74 type cage structures apparently stabilized by the presence of encap- sulated metal atoms. In this paper, we report the production, separation, and characterization of a unique dimetal metallofullerene (DMF) species, La 2 @C 72 . As previously noted, there is only one possible C 72 empty-cage structure 33 (D 6d symmetry) that satisfies the isolated pentagon rule. 32-34 In this regard, it should be noted that the computer-generated structure for La 2 @C 72 (Figure 1) with the metal atoms along the D 6 symmetry axis is one of only a few possible isomers and shown only for illustrative purposes. Experimental Section The production method used here to obtain metallofullerenes is very similar to the original Kra ¨tschmer-Huffman 35 arc- burning method. Cored rods packed with a mixture of graphite and either pure lanthanum, La 2 O 3 , or lanthanum carbide were arc-burned in a fullerene generator under a dynamic He atmosphere. 27 During this process metallofullerene and fullerene- rich soot formed which deposits on the cooled chamber walls. In addition, a carbide-like deposit formed on the counter electrode, and optimum results were obtained by also repeatedly back-burning the deposit on the anode. The rod preparation parameters utilized graphite rods (diameter of 6 mm, cored to 4 mm) filled with a La 2 O 3 /graphite mixture with a metal concentration doping of 0.5-5 atomic %. Typical electric-arc * To whom correspondence should be addressed. Figure 1. Computer-generated structure for La2@C72 (C72 D6d cage symmetry). The metal atoms are placed along the D6d symmetry axis. 2833 J. Phys. Chem. A 1998, 102, 2833-2837 S1089-5639(98)00452-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/07/1998