Recent progress in the studies of endohedral metallofullerenes

This overview presents the recent progress in the area of endohedral metall ofullerenes in the past several years. The important results have been summ arized as follows: (1) Many metals including Group 3 metals, most of the la nthanide series elements, and Group 2 metals have been encapsulated into a fullerene cage to form mono-, di-, and trimetallofullerenes by using the ar c-evaporation technique. (2) Some endohedral metallofullerenes such as Grou p 3 metals, most of the lanthanide series elements, Group 2 metals, and som e of their isomers have been successfully isolated and purified by a two-st ep or several-step HPLC technique. By using high-temperature and high-press ure extraction with pyridine, Ln@C-80, Ln@C-82, and Ln2@C-80 for most rare- earth metals have been selectively extracted in high yield (about 1% of the saw soot) from fullerenes and other size metallofullerenes. (3) The endohe dral nature of metallofullerenes such as Y@C-82, Sc2@C-84, and Sc@C-82 has been finally confirmed by synchrotron X-ray powder diffraction. The symmetr ies and the structures of metallofullerenes such as Ca@C-82(III), La-2@C-80 (I-h), Sc-2@C-84(D-2d), and Sc-2@C-84(C-s) have been confirmed by NMR measu rements. (Lb) The information on the electronic structures and properties o f endohedral metallofullerenes has been obtained by various spectrometric m eans Such as EPR, W-vis-MR, XPS, CV. It is generally accepted that three-el ectron transfer is favorable when M = Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er , Lu but Sc, Eu, Sm, Yb, Tm, Ca, Sr, Ba prefer to donate two electrons to t he fullerene cages. (5) Several chemical reactions of endohedral metalloful lerenes have been reported in which reagents are disilacyclopropane, digerm acyclopropane, diphenyldiazomethane, and trifluoroacetic acid. (6) Mass spe ctrometry provided the crucial evidence that led to the discovery of metall ofullerenes in 1985 and has always played a key role in their identificatio n and characterization, Ion-mobility measurements of gas-phase ions have ob tained the information of structures and the formation mechanism of endohed ral metallofullerenes. till Theoretical calculations on the endohedral meta llofullerenes have made an important contribution to the studies on the sym metry of the cage, the position of metal atom(s) inside the cage, the numbe r of electronic transfer between metal atom(s) and fullerene cage, etc. (C) 2000 Elsevier Science S.A. All rights reserved.