Endohedral Reactivity

The corresponding calculations on the endohedral and exohedral complexes of CgQ with H, F or the methyl radical as guest predict the same behavior [98]. In all cases the formation of endohedral covalent bonds is energetically unfavorable due to the analogous strain arguments, even if such reactive species as F atoms are exposed to the inner surface of Cjq. 

These investigations focused for the first time on a new aspect of topicity that takes into account the influence of the shape of a bent structure on its reactivity. The remarkable inertness of the inner surface contrasts with the pronounced reactivity of the outer concave surface of Cjq. Almost unperturbed, atomic species or reactive molecules can be studied at ambient conditions once they are encapsulated by CgQ. Moreover, the wavefunction of the guest atom can be influenced by a permanent distortion of the Cjq cage. This was demonstrated by exohedral derivatization of N Cgo, leading to a lowering of the f symmetry, which influences the ESR spectra of the paramagnetic guest [99-103]. 

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Such a mode of steric protection that furnishes both stability and reactivity can be referred to as peripheral steric protection in contrast with the conventional proximal steric protection shown in Figure 11.2. For this purpose, we sought to develop nanoscale bowl-shaped molecular cavities large enough to serve as a reaction environment for the functionality of and secure enough to suppress the bimolecular decomposition processes of the endohedral reactive species. 

Stabilization of highly reactive species in molecular bowls and capsules with an endohedral functionality 97LA2393. 

One particularly striking aspect of endohedral fullerene complexes is the surprising ability of the carbon cage to contain highly reactive species such as atomic nitrogen.1801 N C6o (54) has been prepared... 

On the other hand, and as discussed before, the chemical reactivity of CNT sidewalls increases vhth the tube curvature (i.e. decrease of the tube diameter), due to the increase of the pyramidalization angle and greater strain energy per atom [37,38]. Such pyramidalization of the CNT atoms causes the exohedral lobes of the orbitals to be larger than their endohedral counterparts. The reactivity of the surface is thus enhanced by the pronounced exposure of the hybrid orbitals from the exterior, which favors the orbital overlap vhth incoming reactants [38]. 

Another striking development with endohedral fullerenes was the synthesis of N C5q, the first example of encapsulation of a reactive nonmetal atom [128-132]. 

Since then, several studies on the chemical reactivity of endohedral metallofullerenes have been performed in order to understand their basic chemical properties. The endohedral metallofullerenes Gd C82 [137], La2 C80, and Sc2 C84 [138] showed similar behavior towards disilirane, forming the corresponding adducts via C-Si bonds. Also, similar functionalization of La C82 was achieved with digermirane [ 139,140]. However, when diphenyldiazomethane was added to La C82 an exohedral adduct was formed via C-C bonds [ 140,141 ]. This organic functionalization of endohedral metallofullerenes opened the way to chemically tune the electronic properties of the so-produced materials, the degree of charge transfer, and ultimately their electronic properties. 

Cardona, C.M., Kitaygorodskiy, A., and Echegoyen, L. (2005) Trimetallic nitride endohedral metallo-fullerenes reactivity dictated by the encapsulated metal cluster. Journal of the American Chemical Society, 127, 10448-10453. 

The thermal addition of digermirane to La Cg2 has been reported to occur even at 20 °C [270]. This is in sharp contrast with the fact that empty fullerenes react only photochemically. Such an enhanced reactivity of endohedral metallofullerenes may be ascribed to a significantly larger electron affinity (3.22 eV) of La Cg2 as compared to Ceo (2.57 eV) and C70 (2.69 eV) [271]. 

An important clue for the disappearance of many endohedral species in the process of solvent-extraction is given by Bandow et al.[Ba92] Handling the soot and performing the solvent extraction under anaerobic conditions, they observed LaCre and LaCs4 in the mass spectra of the solvent extract. Evidently, these species are susceptible to oxidation from the atmosphere and/or dissolved air, which either destroys or immobilizes them. This result calls for a careful reevaluation of their reactivity in an ion trap or similar experiment. 

For hollow objects like the fullerenes, a general distinction has to be made between outside and inside reactivity. Modifications to the outside are termed exohedral functionalization, and those to the inside are endohedral. Both variants are observed for the fullerenes. Classical fullerene chemistry deals with exohedral functionalization by one or more groups attached to the carbon atoms. Endohedral chemistry, on the other hand, studies compounds consisting of atoms or small molecules included in the cavity within the fullerene cage. The exohedral processes may further be divided into covalent and noncovalent interactions with the reaction partner. 

The driving force of fullerene reactivity most of aU originates from their curved surface. Any addition from the outside to one or more of the already prepyramidal-ized carbon atoms leads to sp -hybridization and thus reduces the strain in the fullerene molecule. An addition from the inside, on the contrary, would increase the strain and consequently is impossible. In addition the overlap of the participating orbitals would be insufficient (Figure 2.33). Endohedrally included atoms or molecules accordingly are not bound to the carbon skeleton, but they exist inside of it as more or less free entity (refer to Section 2.5.4). 

Tml2, and that of other isolated uncommon divalent complexes there are also two endohedral metallofullerenes Sc Cg4 (Takahashi et al., 1995) and Tm C82 (Kirbach and Dunsch, 1996 Pichler et al., 1997) for which spectroscopic evidence points to the divalent nature of Sc and Tm, respectively they are worth mentioning here but they will not be discussed further. We will also consider the important issue of chemical systems in which uncommon divalent species may be present or have been postulated as intermediates, or displaying divalent-like reactivity. We will also report on the few low-valent scandium species that have been reported and finally on the zero-valent compounds. 

Several reports on the chemistry of endohedral lanthanide fullerene complexes indicate that they are quite similar in reactivity to the empty fullerenes. However, their redox chemistry is very rich since they can also get easily oxidized in contrast to Cgo and C70 [13b,24]. Reactions with a disiUrane [cyclo-... 

Heterofullerenes with Heteroatoms Other Than N, B. After unsuccessful attempts by the Smalley group, Clemmer et al. reported the formation of CnNb+ (n = 28-50) clusters upon pulsed laser vaporization of a mixed NbC/graphite composite rod [144]. From the difference between even and odd n CnNb+ ions in their chemical reactivity towards O2 and N2, evidence was found for the odd-C numbered cages to be (quasi)heterofullerenes instead of endohedral complexes. No clusters with a C/Nb ratio of 60 1 were detected, however. 

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