Fluorination fullerenes

Another, more recent, fluorine contribution to the development of new branches of chemical science is connected with fullerenes, the first members of this family of close-caged molecules being obtained only about a decade ago. Fullerenes, having the composition C , C70, C76, C84.. . , represent the new... 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

Allowing fluorine gas at low pressure to react with the solid fullerene (Scheme 9.1)... 

Scheme 9.4 Schematic drawing of the experimental set-up for simultaneous in situ Knudsen-cell synthesis (fluorination) and mass spectral characterization of fullerenes [36].

Because the forces of attraction prevail when molecules are brought into sufficiently dose proximity under normal conditions, release is best effected if both the strength of the interaction and the degree of contact are minimized. Aliphatic hydrocarbons and fluorocarbons achieve the former effect, finely divided solids the latter. Materials such as microcrystalline wax [64742 42-3] and hydrophobic silica [7631-86-9] combine both effects. Some authors refer to this combined effect as the ball bearing mechanism. A perfluoroalkylated fullerene nanosphere would perhaps be the ultimate example of this combined effect (17). These very general mechanistic remarks can be supplemented by publications on the mechanism of specific classes of release agents such as metallic stearates (18), fatty acids and fluorinated compounds (19), and silicone-coated rdease papers (20,21). The mechanism of release of certain problem adherents, eg, polyurethanes, has also been addressed (22,23). 

Fullerenes can be derivatized by various means. For example, reaction with fluorine gas proceeds stepwise to the formation of colorless CeoFeo, which, according to the 19F nuclear magnetic resonance (NMR) spectrum, contains just one type of F site and so evidently retains a high degree of symmetry.9 In view of the low adhesion typical of fluorocarbons, this spherical molecule is expected to have extraordinary lubricant properties. Curiously, bromination of Ceo is reversible on heating otherwise, the reactions of fullerenes resemble those of alkenes or arenes (aromatic hydrocarbons). 

Fullerenes C60 and C70 are successfully fluorinated54 with manganese(III) fluoride. In a Knudsen cell at 560-650 JC, the major product from C60 is C60F36, whereas in a sealed tube at 330°C, the oxide C60F36O is the major product, with a little C60F34. Fluorination of C70 in a sealed tube at 390°C gives mainly C70F38, with some of the F,6 and F40 compounds. The use of cobalt(III) fluoride and iron(III) fluoride is mentioned, but without details. 

Tungsten(VI) fluoride does not intercalate into graphite.29 but it does seem to react in some way46 with fullerenes C60/C70, but not as a fluorinating agent. 

The 19F NMR spectrum of the cycloadduct from the reaction between 2//-thiopyran-2-thionc, derived from tetrathiofulvalene by loss of CS2, and the fluorinated fullerene C60F18 shows only 16 fluorine atoms indicating that the [2+2]-cycloaddition involved the loss of two F atoms. The most upfield signal is associated with the F atoms adjacent to the donor sulfur addend <2003GEJ2008>. 

C o solubility increases in the similar sequence, from iodobenzene to chlorobenzene. The exception is fluorobenzene in which C6o solubility is lower than in iodobenzene. Apparently, in the case of interaction between fluorobenzene and C6o fullerene the factor of high fluorine electronegativity prevails. Moreover, as Table 3 indicates the fluorobenzene nitration gives rise to mainly para-isomer and very little ort/zo-isomers. Consequently, the entire negative charge is localized in the /jara-position in a fluorobenzene molecule. Therefore, as with Cgo solubility in alkyl derivatives of benzene (Table 2), one can anticipate that for the C6o molecule that is an electrophilic reagent, the ortho-position will be the more preferential location for electrophilic attack than the /w/ra-position. 

Thus, for the first time it is shown that carbonic nanomaterials (fullerene, single-and multiwall nanotubes, nanofibers) demonstrate high activity at cryogenic conditions (77K) in reactions of chain halogenation (F2, Cl2) with kinetic chain length up to 104 -105. The ESR spectra of active free- radical intermediates were recorded. The presence of vibration bands of C-Cl bonds in products has been indicated by IR method. For the first time chain fluorination of carbonic nanofibers, mono- and multiwall nanotubes has been performed at low temperatures. 

Glazkov, V.P., Zhukov, V.P., Somenkov, V.A., Shil shtein, S.Sh. (1997) Neutron diffraction investigation of C60 fullerene and C60F4S Fluorine-fullerene at high pressure, Poverhnost (Surface) 7, 45-49 (in Russian). 

Mass spectrometric techniques have been largely employed for the characterization of fullerenes, as documented in some recent reviews180-182. Several articles deal with MS of fluorinated products of C60 and C70 fullerenes183"186. Mass spectra of crude samples from... 

Recently, a direct fluorination on the C60 fullerene was attempted. Selig and coworkers examined the reaction mixture by MS and concluded that up to 52 hydrogens were replaced by fluorine although the most abundant product was found to be C60H20F40284. Taylor, Holloay, Kroto and coworkers performed a similar reaction and found that several fluorinated compounds had been formed with an average of C60H10F50. The C—F bonds in these derivatives are very sensitive, and any contact with a nucleophile such as water or bicarbonate results in an immediate loss of all the fluorine atoms285. 

The electroreduction of iV-(haloethyl)amides 8 at a mercury cathode in DMF or acetonitrile gives rise to the corresponding iV-(2,2-dichlorovinyl)amides 9 in good to excellent yields (equation 5)25. The electrochemical reduction of fluorinated fullerene has been reported26. 

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