Exohedral

Akasaka T, Nagase S, Kobayashi K, Suzuki T, Kato K, Yamamoto K, Funasaka H and Takahashi T 1995 Exohedral derivatization of an endohedral metallofullerene Gd Cg2 J. Chem. See., Chem. Commun. 1343-4... 

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

Modifications of the conduction properties of semiconducting carbon nanotubes by B (p-type) and N ( -type) substitutional doping has also been dis-cussed[3l] and, in addition, electronic modifications by filling the capillaries of the tubes have also been proposed[32]. Exohedral doping of the space between nanotubes in a tubule bundle could provide yet an-... 

Exohedral functionalization of [60]-fullerene by [4 -i- 2] cycloadditions. Diels-Alder reactions of [60]-fullerene with electron-rich 2,3-dioxysubstituted-1,3-butadienes [146]... 

Averdung J., Torres-Garcia G., Luftmann H., Schlachter L, Mattay J. Progress in Fullerene Chemistry From Exohedral Functionalization to HeterofuUerenes. 

Keywords Diels-Alder reaction of exohedral functionalization of fullerenes and preparation of heterocyclic fullerenes... 

Another method to prepare mixed Li/Al derivatives has been developed by ligand-exchange reactions, starting from the A14N4 cube 39 and LiPHR (R = cyclohexyl) (68). Thus, the outcome of the latter is the rhombododecahedral Li4Al4P6 cluster 40, which bears exohedral THF donor ligands at the lithium centers (Eq. 23) (68). 

Huang and Freiser (132, 133) were able to prepare exohedral metal C60 ions [MC60]+ by direct reaction of the bare metal ions Fe+, Ni+, Co+, Cu+, Rh+, and La+ with Cgo vapor produced from a heated probe. The [MC60]+ ions when subjected to low-energy collision-induced dissociation with argon all produced the Cg0 ion. These results show that the metal ions attach to the outer surface of C60. The exohedral metallofullerene ions differ from the endohedral metallofullerenes produced by laser ablation of metal oxide-graphite mixtures and support the observations of Smalley and co-workers (148) who found that endohedral metallofullerene ions dissociate by loss of C2 units. 

Witte P, Beuerle F, Hartnagel U, Lebovitz R, Savouchkina A, Sali S, Guldi D, Chionakis N, Hirsch A (2007) Water-solubility, antioxidant activity and cytochrome C binding of four families of exohedral adducts of C60 and C7Q. Org. Biomol. Chem. 5 3599-3613. 

The two most commonly used derivatization methods for exohedral functionalization are the nucleophilic cyclopropanation with malonates (Bingel, 1993) and the formation of fulleropyrrolidines (Maggini et al., 1993). Both of these protocols have been used extensively to produce water-soluble fullerenes for biomedical applications. Other stable water-soluble fullerene adducts have also been reported (Hirsch and Brettreich, 2005). Sections 3.2.2-3.2.5 will give a short overview on the state-of-the-art of water-soluble fullerene derivatives and outline some general trends for designing such molecular structures. 

Another important method for preparation for exohedrally functionalized fullerenes is the 1,3-dipolar cycloaddition of in s/Yw-generated azomethine ylides to C60 yielding fulleropyrrolidines (Maggini et al., 1993). Further functionalization is facilitated either by the use of adequate aldehydes for the azomethine ylide formation or quatemization of the pyrrolidine nitrogen atom. Both bisaddition (Kordatos et al.,... 

In conclusion, by varying the number and kind of the attached substructures for exohedrally functionalized fullerenes the solubility in water can be fine-tuned. From a pharmacological point of view, a well-balanced arrangement of both hydrophilic and lipophilic behaviour is required in order to achieve favourable biodistribution. Amphiphilic monoadducts containing long lipophilic alkyl chains like 8 are promising candidates for potential medical applications. 

The first exohedral metallafullerene to be isolated and structurally characterized was the adduct [( 2-C6o) (K20,0)-Os(04)(4- er -butylpyr-idine) ], Figure 19 24... 

Let us turn our attention now to the electrochemical aspects of a few exohedral metallafullerenes. 

Fig. 1.3 Functionalization pathways for SWNTs (a) defect-group functionalization, (b) covalent side-wall functionalization, (c) noncovalent exohedral functionalization with surfactants, (d) noncovalent exohedral functionalization with polymers, and (e) endohedral functionalization with, for example, C60. For methods (b)-(e), the tubes are drawn in idealized fashion, but defects are found in real situations. From [103] with kind permission of Wiley.

Carbon nanostructures can be doped by the introduction/interaction of foreign atoms. Different doping categories include (a) exohedral doping or intercalation, (b) endohe-dral doping or encapsulation or (c) in-plane or substitutional doping. 

The chemical modification of CNTs can be endohedral (inside the cavity of the tube) or exohedral [42]. There are some examples in the literature that have demonstrated the filling of CNTs with fullerenes, biomolecules (proteins, DNA), metals and oxides that have been driven inside by capillary pressure [39, 42, 72-78]. However, in this section we will focus on exohedral functionalization, taking place just at the external walls of the tubes. Both covalent (chemical-bond formation) and noncovalent (physiadsorption) functionlizations can be carried out. In the following... 

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 ... 

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]. 

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