Carbide Cluster Metallofullerenes: Structure, Properties, and Possible Origin

Endohedral metallofullerenes (EMFs) are hybrid molecules with different metallic species encapsulated inside the fullerene cages. In addition to conventional EMFs that contain only metal ions, researchers have constructed novel compounds that encapsulate metallic dusters of nitride, carbide, oxide, cyanide, and sulfide. Among these structures, carbide duster metallofullerenes (CCMFs) are unique because their synthesis requires only graphite and the metal source. As a result the molecular structures of CCMFs are particularly difficult to characterize. Two carbon atoms are encapsulated inside the cage, but they do not participate in constructing the cage framework. Recent X-ray crystallographic studies of EMFs have allowed researchers to unambiguously identify CCMFs (MxC2@C-2n). Previously most of these structures had been described as conventional EMFs M-x@C2n+2. Most of these species are scandium-containing compounds such as Sc3C2@I-n(7)-C-80 [not Sc-3@C-3v(7)-C-82], Sc2C2@C-2v(5)-C-80 [not Sc-2@C-82], Sc2C2@C-s-(6)-C-82 [not Sc-2@C-s(10)-C-84], Sc2C2@C-2v(9)-C-82 [not Sc-2@C-2v (17)-C-84], Sc2C2@C-3v(8)-C-82 [not Sc-2@D-2d(23)-C-84], and Sc2C2@D-2d(23)-C-84 [not Sc-2@C-86]. Additional examples of CCMFs include Gd2C2@D-3(85)-C-92, Sc2C2@C-2v(6073)-C-68, Ti2C2@D-3h(5)-C-78, M2C2@C-3v(8)-C-82, M2C2@C-s(6)-C-82 (M = Y, Er, etc), Y2C2@C-84, Y2C2@D-3(85)-C-92, Y2C2@D-5(450)-C-100, and Lu3C2@D-2(35)-C-88. The existence of so many CCMF species reminds us that the symbol '@' (which denotes the encapsulation status of EMFs) should be used with caution with species whose molecular structures have not been determined unambiguously. This Account presents a detailed summary of all aspects of CCMFs, including historically erroneous assignments and corrected structural characterizations, along with their intrinsic properties such as electrochemical and chemical properties. We emphasize structural issues, features that are fundamental for understanding their intrinsic properties. Finally, we discuss the formation mechanism and possible origin of duster EMFs, not just CCMFs.