Complex, endohedral

Crystal structures of a few fullerenes containing endohedral atoms have been recently solved (apart from the noble gas trapping Kr C60,22a the metal atoms containing Sc3 N C78,22b Sc3 N C80 and Sc2 ErN C80>22c Y C82,22d Sc C82,22e Er2 C82,22f Sc3 Cs2,228 Sc2 C84,22h Sc2 C2 C84,221), but unfortunately their electrochemical behaviour is not known. Electrochemical information is also limited for those endohedral complexes which lack definitive structural characterization.23... 

The endohedral complex formation can change considerably the properties of the host and guest. The most striking cases, exemplifying the tremendous... 

Interaction-induced spectra of endohedral complexes of buckminster-fullerene. p. 261, 1995. In Ref. [9]. 

AC and AEC complexation is also effected efficiently by other macrocyclic ligands such as the spherands 13, cryptospherands 14 [2.9, 2.10], calixarenes [2.38, A.6, A.23], torands [2.39], etc., some of them, for instance the spherands displaying particularly high stabilities. A special case is represented by the endohedral complexes of fullerenes in which the cation (Sr2, Ba2+, lanthanides) is locked inside the closed carbon framework [2.40],... 

TABLE 12. Encaged ion population (IVion), HOMO and LUMO energies (eV) and stabilization energies (AE, kcalmol-1) of endohedral complexes of Ygo (Y = C, Si and Ge) with various ionsaA.c... 

The Global HSAB Principle Stabilization of Endohedral Complexes of Cgo> Si60 and Ge60 with Monoatomic Cations and Anions... 

For the first time, a systematic uniform ab initio study of endohedral complexes of C60, Si60 and Ge60 with monoatomic cations (Li+, Na+, K+ and Rb+) and anions (F-, Cl% Br and P) was performed. The 3-21 G level (3-21+G for the anions) was used. This could be achieved by application of the multiplicative integral approximation [86], as implemented in the program BRABO [87]. The sequence studied for the cages bears a close resemblance with isomorphic substitution in zeolites modifying its catalytic activity via bridging hydroxyls. 

As can be seen, the populations are very close to the isolated ion populations. Almost no charge transfer occurs between the ions and the surrounding cage, justifying the term "endohedral complexes". Moreover, it turned out that the typical shell structure of the ions is completely conserved. In a second step, the complexation energies AE were computed as ... 

Table 5 BSSE corrected stabilization energies of the endohedral complexes.

For the first time the small fullerene molecule C28 was discovered in structure of endohedral complex U C28 [11]. The entire first attempts to extract the isolated molecule C28 in purely kind failed. Theoretical calculations show that isolated molecule C2g is radical with 4 non-paired electrons (S = 2), localized on atoms, disposed in vertexes which are common for each three neighboring pentagons. Exactly this fact explains the high activity of isolated molecule C28 and unsuccessful attempts to get it in purely kind. 

Collision-induced fragmentation of Ceo" (n = 1 3) with the noble gases helium and neon at high energy (typically 6keV) results in the formation of encapsulated products, e g. [He C6o]"" The retention of the gas atoms in the fragmentation products [He C2m]" (2w = 58, 56, 54, etc.) is consistent with endohedral complex formulation. Furthermore, [He C6o] has been neutralized in the gas phase to produce He C6o (lifetime >90 p,s), which was then successfully... 

The endohedral fullerenes U C28 and U C6o have been reported to be sufficiently stable to allow chemical isolation. Endohedral complexes of the smaller fullerenes (C2 , 2n < 60) are conunonly observed in molecular beam experiments (Section 4.1), but this stabilization of C28 by uranium atom encapsulation is remarkable. A theoretical study of [7i-C28]" and Td-C2iiU (25) indicate that they are stable molecules, and that the laboratory preparation of their derivatives may be viable. ... 

Table 6 Half-Wave Reduction and Oxidation Potentials (in V vs. Fc/Fc ) of Q2 and Some of Its Endohedral Complexes Obtained by CV...

In this chapter on chemically modified fullerenes, I will discuss only a few representative molecules which incorporate fullerenes, and focus attention on the best-characterized compounds. A comprehensive review of chemical reactions with fullerenes has just been published by Taylor and Walton.[Ta93] It is not appropriate here to attempt a synthesis of the emerging field of the chemical properties of the fullerenes. Covered elsewhere in this book are ioni-cally bonded fullerides, such as the superconductor KsCeo (Chapter IV) and endohedral complexes, in which various atoms are captured inside the hollow carbon shell (Chapter VI). 

Returning to La Cn Alvarez et al. showed that the Kratschmer-Huffman contact arc method, with rare-earth oxides mixed with graphite, also produced significant amounts of endohedral complexes. A191] They focused their attention on La2 Cn, and found a phenomenon similar to what was seen in La C while the raw soot contains en-dohedrals with a broad spectrum of n, the toluene extract contains only La2 C8o and La C82- That result was also confirmed by Ross et al., who... 

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