Experimental and theoretical studies of the scandium carbide endohedral metallofullerene Sc2C2@C82 and its carbene derivative
Experimental and theoretical studies of the scandium carbide endohedral metallofullerene Sc2C2@C82 and its carbene derivative
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DOI：
10.1002/anie.200701049
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Nagase, Shigeru
中科院分区：
文献类型：
--
作者：
Iiduka, Yuko;Wakahara, Takatsugu;Nagase, Shigeru
内嵌金属富勒烯作为具有空富勒烯所不具备的独特性质的新型球形分子引起了人们的特别关注。[1-3]许多工作已经进行了金属富勒烯与Sc，Y和La原子内C82和C84笼封装。其中，碳化钪内嵌金属富勒烯，如Sc2C2@C84 [4，5]和Sc3C2@C80 [6，7]是最令人感兴趣的，因为C2单元与几个金属原子一起封装，这对碳化钪内嵌金属富勒烯的化学非常重要。对于Sc2 C84金属富勒烯，已经分离出三种异构体（I，II和III）。[8，9]最丰富的异构体Sc2 C84（III）根据其X射线光电子、[10] 13 C NMR、[9] 45 Sc NMR、[11] IR、[12]和拉曼[13]光谱测量、粉末X射线分析[14]和理论计算[15]进行了表征和讨论，前提是两个Scatom被封装在C84的D2 d异构体中。然而，我们最近观察到Sc2 C84（III）的改进的13 C NMR光谱，显示总共17条线（11个全强度信号，5个半强度信号和16强度信号），[16]与以前的13 C NMR研究不同。[9]新观察到的13 C NMR图谱不能通过将两个Sc原子置于满足孤立五边形规则的C84的任何异构体中来解释。我们已经提出，13 C NMR谱图是由两个C原子以及两个Sc原子被封装在C82的C3 v异构体内的事实来解释的。最近，通过同步加速器X射线粉末衍射数据的MEM（最大熵方法）/Rietveld分析发现，Sc 2C2@C82结构是正确的，尽管Sc 2@C84结构曾经通过MEM/Rietveld分析确定。[17]为了验证Sc2 C84（III）是碳化钪金属富勒烯（Sc2C2@C82（III）），进行了X射线单晶分析和密度泛函计算。通过密度泛函计算优化的Sc2C2@C82（III）的结构如图1所示。[18]电子结构描述为（Sc_2C_2）_4 + C_82_4 π，这是由Sc_2C_2到C_82的四电子转移的结果。当包封的Sc2 C2部分具有弯曲结构并且两个Sc原子不相等时，结构最稳定。这一结果似乎与13 CNMR谱（16个信号）和45 Sc NMR谱（只有一个信号）相矛盾，13 CNMR谱显示Sc2C2@C82（III）具有C3 v对称性，45 Sc NMR谱显示两个Sc原子是等效的。这种情况可以解释为Sc 2C 2中的Sc和C原子可以在NMR时间尺度上快速旋转和移动。通过循环伏安法（CV）测量的Sc2C2@C82（III）的氧化还原电位和
Endohedral metallofullerenes have attracted special attention as new spherical molecules with unique properties that are unexpected for empty fullerenes.[1–3] Much work has been carried out on metallofullerenes with Sc, Y, and La atoms encapsulated inside C82 and C84 cages. Among these, scandium carbide endohedral metallofullerenes, such as Sc2C2@ C84 [4, 5] and Sc3C2@ C80,[6, 7] are the most interesting because of the encapsulation of the C2 unit together with several metal atoms, which is very important to the chemistry of scandium carbide endohedral metallofullerenes. For the Sc2C84 metallofullerene, three isomers (I, II, and III) have been isolated.[8, 9] The most abundant isomer, Sc2C84 (III), was characterized and discussed in terms of its X-ray photoelectron,[10] 13C NMR,[9] 45Sc NMR,[11] IR,[12] and Raman [13] spectroscopic measurements, powder X-ray analysis,[14] and theoretical calculations [15] on the premise that two Scatoms were encapsulated inside the D2d isomer of C84. However, we have very recently observed an improved 13C NMR spectrum of Sc2C84 (III) that shows a total of 17 lines (11 full-intensity signals, fivehalf-intensity signals, and one 1/6-intensity signal),[16] unlike the previous 13C NMR study.[9] The newly observed 13C NMR pattern is not explained by placing two Sc atoms inside any of the isomers of C84 that satisfy the isolated-pentagon rule. We have suggested that the 13C NMR pattern is explained by the fact that two C atoms as well as two Sc atoms are encapsulated inside the C3v isomer of C82. Very recently, it has been found that the Sc2C2@ C82 structure is correct by MEM (maximum-entropy method)/Rietveld analysis of synchrotron X-ray powder diffraction data, though the Sc2@ C84 structure was once determined by MEM/Rietveld analysis.[17]To verify that Sc2C84 (III) is a scandium carbide metallofullerene (Sc2C2@ C82 (III)), X-ray single-crystal analysis and density functional calculations were carried out. The structure of Sc2C2@ C82 (III), optimized by density functional calculations, is shown in Figure1.[18] The electronic structure is described as (Sc2C2) 4+ C82 4À as a result of four-electron transfer from Sc2C2 to C82. The structure is most stable when the encapsulated Sc2C2 moiety has a bent structure and two Sc atoms are not equivalent. This result seems contradictory to the 13CNMR spectrum (16signals), which shows that Sc2C2@ C82 (III) has C3v symmetry, and the 45Sc NMR spectrum (only one signal), which shows that the two Sc atoms are equivalent. This situation is explained by the fact that the Sc and C atoms in Sc2C2 are allowed to rotate and move rapidly on the NMR time scale. The redox potentials of Sc2C2@ C82 (III), measured by cyclic voltammetry (CV) and