Fig. 10.7. (a) Voltammogram of the first reduction/oxidation cycle of pure Qq (3.16 mg). Dots are the current is averaged over the 1 h duration of > each 10 mV step. The open circuit voltage relaxed to 1.2 V after reducing to 0.2 V. (b) Voltage vs composition obtained by integration of voltammogram (a). The error in x is ±5%. The structure associated with reduction occurs at 2.3, 1.9, 1.5, and 1.0 V [10.33],
is shown as the positive current trace [10.33]. By integration of the current that flows from the start of the cycle to each point on the reduction voltam-mogram, the amount of Li delivered to the working electrode is monitored. Assuming that one Li atom is inserted per electron passing through the external circuit, a plot is then made of the working electrode voltage vs the stoichiometry of the working electrode, as shown in Fig. 10.7(b). The well-defined features A, B, and C in Fig. 10.7(a) correspond to well-defined steps in Fig. 10.7(b), where A, B, and C correspond to x = 0.5, 2.0, and 3.0 in LijC60. The weak feature at D corresponds to x = 4.
It is interesting to note that the roughly constant 0.5 V separation between A, B, C, and D in Fig. 10.7(a) corresponds approximately to that observed for electron reduction of C£0~ up to n = 4 in solution. One should conclude, therefore, as do the authors of Ref. [10.33], that (Li+)4CgQ has formed at D in Fig. 10.7(b). The passage of further charge leads to the insertion of neutral Li, and for x > 4 we presume that Lix,.4(Li+)4C^0 is formed up to x ~ 11. It remains an open question where the four Li+ ions reside and how they are distributed between tetrahedral and possible multiply occupied octahedral sites.
For the oxidation cycle, the separation between each peak is ~0.5 V as seen in Fig. 10.7(a). The constant value of this peak-to-peak separation would be interpreted as implying small changes in energy between molecular ion states separated by a single electronic charge [10.33], The magnitude and repetition rate of the potential steps in Fig. 10.7(b) are chosen to be small enough to give good resolution and large enough to monitor the time response of the system [10.33]. By taking large voltage steps, a general overview of the electrochemistry of the system is obtained over a wide voltage range, while study of the response for small steps provides detailed information about the equilibrium conditions pertaining to each feature in the voltammogram. A detailed analysis of the reduction peak "A" shows this process to be an unusual first-order transition, corresponding to a difficult initiation of the Li intercalation at 2.34 V Peak "B" corresponds to a more usual first-order process, while peak "C" behaves like a solid solution process with a voltage overlap between the reduction and oxidation cycles [10.33]. Some examples for which electrochemical intercalation into C60 has been successfully demonstrated include Li [10.33] and Na [10.32].
By the term chemical oxidation of C60 we refer to a chemical reaction which converts neutral C60 to a cation Cg0. This is to be distinguished from the formation of the covalently bonded C60O epoxide compound (see §10.6), or the physisorption of molecular 02 within the crystalline C60 structure to form a clathrate (see §8.7.2 and §9.3.2), or the photochemical intercalation of dioxygen into C60 in the solid state (see §10.10).
As stated above, the reduction of C60 is much easier to carry out than its oxidation, although oxidation reactions have been observed. For example, a radical cation form of C60 has been reported based on the oxidation of C60 by XeF2 [10.34] or the reaction with a strong superacid. In the latter case, C60 was oxidized by a mixture of fluorosulfuric acid and antimony pentafluoride FS03H:SbF5 [10.35], which was shown to yield largely stable Cg0/superacid mixtures. The cation reacts readily with different types of nucleophiles (electron donors) to produce a variety of addition products, probably through a chain reaction mechanism in which carbocations (a group of carbons that possess a positive charge localized on one of the carbon atoms) act as intermediates, similar to the nucleophilic addition to carbon-carbon double bonds in alkenes [10.9]. In these reactions, multiple attachment of the adduct is carried out through the continuous transfer of the positive charge from one carbon atom of one double bond to another carbon atom in a different double bond, or to another species in the reaction medium. For example, can produce symmetrically substituted alkoxylated [such as C60(OCH3)„, n = 2,4 or 6] and arylated [such as C60(C6H5)22] fullerene derivatives by reacting with alcohols such as methanol and butanol [10.35].
10.4. Hydrogénation, Alkylation, and Amination
The fullerene derivatives obtained by hydrogénation and alkylation attach functional groups to the fullerene shell by radical single bonds. Hydro-genated derivatives of C60 and C70 have been synthesized by chemical, electrochemical, and catalytic methods. Thus, C60H18 and C60H36 have been prepared by a Birch reduction [10.29], which refers to a chemical process in which Li metal in the presence of f-butanol is used to reduce the C60 (or C70) to the monoanion Q, (or C^), which then leads to hydrogen attachment. High levels of hydrogen attachment (C^H^ and C70H46) have also been obtained by gas-phase catalytic reactions [10.36]. Even further hydrogénation has been accomplished by high-pressure (~175 psi of H2) reactions of C60 in an electrochemical cell, leading to QoHeo [10.36]. The working electrode in this case was a composite of C^ with ~17% Ag, and these cells contained a 30% KOH solution. These results were interpreted as encouraging evidence for the use of C^H, in battery applications [10.36] (see §20.4.2). Although hydrogénation of C60 is relatively easy to accomplish, the hydrogenated reaction products are unstable, change their solubility with time, and show signs of oxygen uptake [10.4].
Neutral Qq molecules do not readily react with an alkyl (C„H2„+]) group in an alkylation reaction. More generally, C60 does not readily react with electrophilic (electron-attracting) species, as already mentioned in §10.3.3. To promote reactions with electrophilic species such as alkyls, C60 is first reduced to an anion, as, for example, by reaction with an alkali metal species, similar to the Birch-type reaction mentioned above, and this is followed by reaction with an alkyl halide, such as CH3I, to yield methylated C60 and Kl as a by-product. Compounds such as C60(CH3)„ with stoichiometries rt = 6,8, and 24 have been prepared by such a substitution reaction [10.30]. The stoichiometry determination of these compounds is usually based on proton and 13C NMR characterization and field ionization mass spectrometry. In C60(CH3)„, the methyl (CH3) groups are connected to the fullerene shell by a single C-C bond (see Fig. 10.8), as is the case also for a hydrogen or a halogen attachment (see §10.5). The maximum number of alkyl groups that can be added to C60 without any groups being connected to adjacent C atoms on the shell is 24.
Alkylation of Qq and C70 may also be accomplished by nucleophilic (electron-donating) addition and substitution reactions of C60 and its derivatives. For example, addition products have been obtained from the reaction of an aromatic moiety with protonated Cg0 (see §10.3.3). The formation of mixed derivatives [QoiCH^joPh,,), C60(CH3)i-C4H9] and monoalkylated derivatives C60H(C2H5) have been produced by reactions with charged nucleophiles following standard synthetic routes for alkylation reactions [10.22]. In the above chemical formula, Ph represents a phenyl group (C6H5), and "t-" refers to a carbon linked to three groups. The tertiary carbon in this i-C4H9 group attaches to one of the carbons participating in a double bond prior to the reaction; the other CH3 group in this fullerene derivative may attach to the other carbon in the same double bond or to a carbon at another double bond site.
Addition of an amino group to C60 is also possible. It has been reported that Qo, via nucleophilic addition reactions, may undergo multiple attachments of primary amines (RNH2), where R is an alkyl or aryl (aromatic) group terminated by an amine group (-NH2). The first step in the addition is the formation of a single C-N bond linking the amine group to one of the carbons at a reactive C=C bond site of the fullerene. The adjacent C atom at this double bond site attracts a hydrogen atom from the terminating amine group, forming a C-H bond, as shown in Fig. 10.9. For example, C60 has been functionalized by the nucleophilic (electron-donating) attachment of ethylenediamine (NH2CH2CH2NH2). Multiple attachments of this type to a single fullerene molecule have also been reported [10.22].
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