Structure and properties of novel endohedral metallofullerenes

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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.