Endohedral metallofullerenes

Abstract

Endohedral metallofullerenes (fullerenes with metal atom(s)encapsulated) are novel forms of fullerene-based materials which haveattracted wide interest during the past eight years, not only inphysics and chemistry but also in such interdisciplinary areas asmaterials and biological sciences. In this paper, advances in theproduction, separation (isolation) and various spectroscopiccharacterizations of endohedral metallofullerenes are presented in anattempt to clarify their structural, electronic and solid stateproperties. Endohedral metallofullerenes are normally produced by DCelectric arc discharge of metal/graphite composite rods used aspositive electrodes. The metallofullerenes can also be produced by theso-called laser furnace method which incorporates laser vaporizationof the composite rods under high temperature (ca 1000 °C). Theendohedral metallofullerenes so far produced are centred on group 2and 3 metallofullerenes such as Sc, Y, La, Ca, Sr and Ba as well aslanthanide metallofullerenes (Ce-Lu). These metal atoms have beenencapsulated in higher fullerenes, especially in C82. Thesemetallofullerenes have easily been extracted by solvents from primarysoot. By using an elaborate high-performance liquid chromatographytechnique, the metallofullerenes are completely purified and isolatedlike C60 and C70. Synchrotron x-raydiffraction, 13C NMR andultra-high vacuum scanning tunnelling microscopy (UHV-STM) studies haverevealed that metal atoms are indeed encapsulated by the carbon cageand that the metal atoms are not in the centre of the fullerene cagebut very close to the carbon cage, indicating the presence of a strongmetal-cage interaction. It has been revealed by electron spinresonance and also by theoretical calculations that substantialelectron transfers take place from the encaged metal atom to thecarbon cage: intrafullerene electron transfers. Good examples arelanthanum and yttrium metallofullerenes which have the charge statesof La3+@C823- and Y3+@C823-. One of the mostdistinct features of such a metallofullerene is superatom character,by which the metallofullerene can be viewed as a positively chargedcore metal surrounded by a negatively charged carbon cage. Such amolecule has a great similarity to the superatom concept firstproposed by Watanabe and Inoshita in a semiconductor heterostructurecomprising a spherically symmetric positively charged core.Structural and electronic analyses based on x-ray diffraction andUHV-STM measurements indeed provide evidence of such character. Otherelectronic and magnetic properties of the metallofullerenes are alsointriguing. Endohedral metallofullerenes will be metal, small-gapsemiconductors or insulators depending upon the fullerene size and thekind and the number of metal atom encapsulated. Finally, someprospective applications of metallofullerenes are presented.