Exohedral Reactivity

First, the Diels-Alder reaction between 1,3-cis-butadiene and Cgo has been studied as reference. The reaction is favored over the pyracylene [6,6] bond that presents a reaction energy of —20.7 kcal mol and an activation barrier of 12.7 kcal-moP The [5,6] bonds are substantially less reactive as the reaction and activation energies obtained are 15.4 and 8.3 kcal-mol less favorable. The noble gas encapsulation hardly affects the exohedral reactivity of the cage, i.e. differences of less than 0.4 kcal-mol were observed for both the reaction energies and barriers. 

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]. 

Compared to small two-dimensional molecules, for example the planar benzene, the structures of these three-dimensional systems are aesthetically appealing. The beauty and the unprecedented spherical architecture of these molecular cages immediately attracted the attention of many scientists. Indeed, Buckminsterfullerene CgQ rapidly became one of the most intensively investigated molecules. For synthetic chemists the challenge arose to synthesize exohedrally modified derivatives, in which the properties of fullerenes can be combined with those of other classes of materials. The following initial questions concerned the derivatiza-tion of fullerenes What kind of reactivity do the fullerenes have Do they behave like a three-dimensional superbenzene What are the structures of exohedral fullerene derivatives and how stable are they ... 

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]. 

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. 

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. 

MetaUofuUerenes that could actually be isolated have been examined for their reactivity as well. Exohedral functionaHzation was achieved, for example, by photochemical reactions, Diels-Alder addition, etc. In the reaction with disihranes, the thermal variant was found to be viable besides the photochemical addition... 

The reactivity of [IrH(CO)(PPh3)2] 908 with both C60 and C70 fullerenes has been studied by Usatov et 7 he resulting exohedral monohydrido complexes were formed with complete stereo- and... 

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