Fullerene exohedral

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 ... 

It can be also very important for determining the number of isomers of higher fullerenes of regioisomers of exohedral fullerene adducts and for carrying out mechanistic investigations on the escape of the endohedral guest and reversible addition reactions [125]. The Xe NMR spectrum ofXe C5Q has also been reported [126]. 

Most exohedral fullerene derivatives of preparative importance are formed by one or several successive 1,2-additions to [6,6]-bonds. For a systematic analysis of the regiochemistry of such reactions it is very useful to introduce a simple site labeling system [3]. The relative positional relationships (such as ortho, meta and para in... 

So far, all efforts to generate, isolate and characterize heterofuUerenes via Kratsch-mer-Huffman vaporization of graphite in the presence of hetero-element-containing compounds such as boron nitride (BN) or cyanogen (CN)2 have failed. An alternative route for the direct formation of heterofuUerenes is cluster rearrangement within exohedral fullerene derivatives such as iminofullerenes and azafuUeroids. The first hints of success by this approach were obtained from mass spectrometry investigations of the cis-l-diazabishomo[60]fullerene 3 [12], the n-butylamine adduct 4 [12] the 1,2-epiminofullerene 5 [11] and the cluster opened ketolactam 6 [2]. 

In general, addihon products of Cjq tend to revert to the parent fullerene by eliminahng the addends. This aspect wiU be discussed in more detail in Sechon 14.4, since the retention of the shuctural type has been frequently used as aromaticity criterion. However, in many cases exohedral fullerene adducts exhibit thermal and chemical stability that is high enough to allow their use as building blocks in synthetic chemistry and technological applications. 

An interesting offshoot in the context of carbon-based nanomaterials is the role of external perturbations in modulating their physical and chemical characteristics. These perturbations can include cations or neutral atoms. In this context, we examined the magnetic properties of exohedral fullerenes of alkali metal fullerides (Aj,-C6o. A = Na, K, Rb, Cs) [148-150] and the spin properties of endohedral fullerenes (A C6o, A = N, P, As, O, S) [128,151] (Fig. 34.11). The most interesting aspect of the experimental... 

Fig. 34.11. Ground triplet (d D2 ) and excited singlet (s C2a) states of (Cgo) with two views (for the alkali cation doped exohedral fullerenes), and spin-containing endohedral fullerenes [128,148].

Most of the exohedral fullerene derivatives of preparative importance are formed by one or several formal 1,2-additions to [6,6]- or [5,6]-bonds. For a suitable discussion of the regiochemistry of such fullerene derivatives it is very valuable to introduce a simple and clear site labeling system, which allows facile description of the constitution of a given fullerene derivative. We first introduced a very descriptive nomenclature [177,188] for the assignment of the relative positional relationships (like ortho, meta and para in benzene chemistry) of addend carrying bonds in Cgg derivatives with labeling the corresponding bonds as cis-n (n = 1 -3), e, e", trans-n (n= 1-4) (Fig. 15). 

It was shown how both endohedral and exohedral fullerenes can be inserted in nanotubes. In the peapods containing endohedral fullerenes (for instance Ce Cg2), HRTEM images showed interesting rotation and translation motion of the trapped spheroids. Exohedral metallofullerenes, CsC io have been synthesized and successfully encapsulated into SWCNTs via a new chemical reduction of Cgo molecules into anions. The addition of iodine to already prepared peapods allowed the coalescence of Cgo directly inside the nanotubes. Indeed, after heating at 550 C, iodine-doped peapods, inside the Cgo molecules molecules have been transformed in a tubular structure. Khlobystov et al. were able to perform reactions on the inner surface of carbon nanotubes in the presence of catalyti-cally active atoms of rhenium and monitor the whole process via HRTEM. ... 

As far as fullerenes decorated by a shell of metal atoms are concerned, which we refer to as exohedral fullerenes, we can identify several possibilities which maintain the full symmetry of the bare structure. For C6o, there are = 12 sites above the centers of the pentagons, 2 = 20 sites above the centers of the hexagons, 3 = 30 sites above the centers of the electron-rich bonds between neighboring pentagons, 4 = 60 sites above the C atoms, and ns = 60 sites above the electron-poor bonds of the pentagons. We can place metal atoms at any of these sets of sites to form a shell of... 

A number of fullerene compounds have been developed that have unusual chemical, physical, and biological characteristics, and are being used or have the potential to be used in a host of new applications. Some of these compounds involve atoms or groups of atoms that are encapsulated within the cage of carbon atoms (and are termed endo-hedral fullerenes). For other compounds, atoms, ions, or clusters of atoms are attached to the outside of the fullerene shell (exohedral fullerenes). 

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