State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan, Hubai 430074, China.
Acc Chem Res. 2013 Jul 16;46(7):1627-35. doi: 10.1021/ar4000086. Epub 2013 May 1.
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 clusters of nitride, carbide, oxide, cyanide, and sulfide. Among these structures, carbide cluster 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@C2n). Previously most of these structures had been described as conventional EMFs Mx@C2n+2. Most of these species are scandium-containing compounds such as Sc3C₂@Ih(7)-C₈₀ [not Sc₃@C3v(7)-C₈₂], Sc₂C₂@C2v(5)-C₈₀ [not Sc₂@C₈₂], Sc₂C₂@Cs(6)-C₈₂ [not Sc₂@Cs(10)-C₈₄], Sc₂C₂@C2v(9)-C₈₂ [not Sc₂@C2v(17)-C₈₄], Sc₂C₂@C3v(8)-C₈₂ [not Sc₂@D2d(23)-C₈₄], and Sc₂C₂@D2d(23)-C₈₄ [not Sc₂@C₈₆]. Additional examples of CCMFs include Gd₂C₂@D₃(85)-C₉₂, Sc₂C₂@C2v(6073)-C₆₈, Ti₂C₂@D3h(5)-C₇₈, M₂C₂@C3v(8)-C₈₂, M₂C₂@Cs(6)-C₈₂ (M = Y, Er, etc.), Y₂C₂@C₈₄, Y₂C₂@D₃(85)-C₉₂, Y₂C₂@D₅(450)-C₁₀₀, and Lu₃C₂@D₂(35)-C₈₈. 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 cluster EMFs, not just CCMFs.
笼内金属富勒烯（EMFs）是一种具有不同金属物种的杂化分子，被包裹在富勒烯笼内。除了仅包含金属离子的传统 EMFs 外，研究人员还构建了新型化合物，将氮化物、碳化物、氧化物、氰化物和硫化物的金属簇包裹在内。在这些结构中，碳化物簇金属富勒烯（CCMFs）是独特的，因为它们的合成只需要石墨和金属源。因此，CCMFs 的分子结构特别难以表征。两个碳原子被包裹在笼内，但它们不参与构建笼状结构。最近对 EMFs 的 X 射线晶体学研究使研究人员能够明确识别 CCMFs（MxC₂@C2n）。以前，这些结构中的大多数都被描述为传统的 EMFs Mx@C2n+2。这些物种大多数是含有钪的化合物，如 Sc3C₂@Ih(7)-C₈₀[不是 Sc₃@C3v(7)-C₈₂]、Sc₂C₂@C2v(5)-C₈₀[不是 Sc₂@C₈₂]、Sc₂C₂@Cs(6)-C₈₂[不是 Sc₂@Cs(10)-C₈₄]、Sc₂C₂@C2v(9)-C₈₂[不是 Sc₂@C2v(17)-C₈₄]、Sc₂C₂@C3v(8)-C₈₂[不是 Sc₂@D2d(23)-C₈₄]和 Sc₂C₂@D2d(23)-C₈₄[不是 Sc₂@C₈₆]。CCMFs 的其他例子包括 Gd₂C₂@D₃(85)-C₉₂、Sc₂C₂@C2v(6073)-C₆₈、Ti₂C₂@D3h(5)-C₇₈、M₂C₂@C3v(8)-C₈₂、M₂C₂@Cs(6)-C₈₂（M=Y、Er 等）、Y₂C₂@C₈₄、Y₂C₂@D₃(85)-C₉₂、Y₂C₂@D₅(450)-C₁₀₀ 和 Lu₃C₂@D₂(35)-C₈₈。如此多的 CCMF 物种的存在提醒我们，对于其分子结构尚未明确确定的物种，应谨慎使用符号“@”（表示 EMFs 的封装状态）。本账户详细总结了 CCMFs 的各个方面，包括历史上的错误分配和修正的结构特征，以及它们的电化学和化学性质等固有性质。我们强调结构问题，这些问题是理解其固有性质的基础。最后，我们讨论了簇 EMFs 的形成机制和可能的起源，而不仅仅是 CCMFs。
