Structure and properties of novel endohedral metallofullerenes

自首次成功提纯/分离金属富勒烯以来，已有许多纯金属富勒烯的结构和光谱研究报道。然而，提纯仍然是金属富勒烯研究中最重要和最关键的部分之一。这仅仅是因为电弧放电产生的金属富勒烯的正常产量在数量上非常有限，通常是(60)的3-5%。高效的净化方案是绝对必要的。两级高效液相分离方案已被证明是一种有用的方法。一种分离各种类型金属富勒烯的方法，尽管目前用于分离富勒烯的高效液相色谱柱相当昂贵。通过使用这些高纯度的样品，进行了结构研究。Y@C82(I)和5c2@C84(III)的内面体结构最终通过Rietveld/MEM(最大熵法)同步加速器粉末X射线衍射法确定，得到了最近的金属-碳和Sc-Sc距离等信息。然而，这些金属富勒烯的异构体结构尚未确定。另一方面，高分辨~(13)C-核磁共振实验为我们提供了关于富勒烯异构体结构的详细信息。作为例子，我们给出了SC2@C84(异构体III)和Ca@C82(异构体III)的异构体结构的~(13)C-核磁共振结果。事实上，这些~(13)C-核磁共振实验首次证实了面内金属富勒烯(如空位高富勒烯)结构异构体的存在。本文的最后部分将讨论一些有用的判据来解释单金属富勒烯的UV-VIS-近红外吸收光谱。

Since the first successful purification / isolation of endohedral .metallo-fullerenes many structural and spectroscopic studies on pure metallo-fullerenes have been reported. And yet, purification still remains as one of the most important and crucial parts of metallofullerene studies. This is simply because that the normal yield of metallofullerenes produced by arc discharge is very limited in amount, typically 3 - 5 percent of that of (60. Efficient purification schemes are definitely needed. The two-stage HPLC separation protocol has been proven to be a useful. method for isolating various types of metallofullerenes although the current HPLC columns for fullerene separation are fairly expensive.By using these highly purified samples, structural studies have been performed. Endohedral structures of Y@C82 (I) and 5c2@C84 (III) have been finally confirmed by synchrotron powder X-ray diffraction studies with the Rietveld / MEM (maximum entropy method) analysis, where such information as the nearest metal-carbon and Sc - Sc distances were obtained. However, the isomer structures of these metallofullerenes have not yet determined.High resolution 13C-NMR experiments, on the other hand, have so far provided us detailed information on isomer structures of fullerenes. Here, we present 13C-NMR results for the isomer structures of Sc2@C84 (isomer III) and Ca@C82 (isomer III) as examples. In fact, the presence of structural isomers of endohedral metallofullerenes (like empty higher fullerenes) has been confirmed, for the first time, by these 13C-NMR experiments.Some useful criteria for interpreting UV-Vis-near IR absorption spectra of mono-metallofullerenes will be discussed in the final part of the present article.