Crystallographic Characterization of the Structure of the Endohedral Fullerene {Er2@C82 Isomer I} with Cs Cage Symmetry and Multiple Sites for Erbium along a Band of Ten Contiguous Hexagons

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Abstract
The structure of one of the three previously separated isomers of {Er2@C82} has been determined through a single-crystal X-ray structure determination of the noncovalent adduct, {Er2@C82 Isomer I}·{CoII(OEP)}·1.4(C6H6)·0.3(CHCl3). The C82 cage is identified specificlly as the Cs(82:6) isomer (one of nine possible isolated pentagon isomers) from the crystallographic data. The carbon atoms of the C82 cage were individually identified and refined with only a constraint that required the two halves of the cage to possess similar bond lengths. Although the carbon cage is well ordered at 113 K, the erbium atoms are disordered. The electron density within the cage of {Er2@C82 Isomer I} has been modeled with two major sites with occupancies of 0.35 and 21 other individual erbium sites with occupancies ranging from 0.138 to 0.011. These erbium sites all reside near the walls of the fullerence and cluster near a band of ten contiguous hexagons that encircles the carbon cage. Since two other isomers of C82 (C3v(82:8) and C2v(82:9)) have a similar band of ten contiguous hexagons, it is tempting to speculate that the other two known isomers of {Er2@C82} have these cage structures.