Fluorine-19 Nuclear Magnetic Resonance Spectroscopy
Many more FFs have been studied by NMR spectroscopy than by X-ray crystallography, both because many compounds have not yet yielded suitable crystals and because NMR spectra of mixtures can be, in many cases, readily interpreted. Although NMR analysis cannot, in general, lead to unambiguous structure elucidation, it can rule out many structure possibilities on the basis of symmetry (number and intensities of peaks) and coupling-constant patterns.
Nuclear magnetic resonance studies that simply support the results of X-ray structure determinations discussed above will not be included here because of space limitations.
c60Fx
Small amounts of the FFs C60F2, C60F4, C60F6, C60F8, C60F16, and C60F20 were isolated by HPLC from the crude reaction products of C60 fluorinations with K2PtF6.[15-19,21] Their 19F NMR spectra are consistent with sequential, contiguous addition patterns that result from a series of 1,2 additions of F atoms. Diagrams showing these additions for C60F2, C60F4, and C60F6 are shown in Fig. 3. Note that NMR spectroscopy is a particularly suitable technique for distinguishing between the three patterns shown for C60F6. Structure S would result in an NMR spectrum with three equal intensity peaks (this was what was observed), structure O would result in a spectrum with one peak, and structure T would result in a spectrum with four peaks with relative intensities 1:1:2:2. Even in cases where the number of peaks and their relative intensities cannot distinguish between two different patterns, a distinction based on coupling constant patterns may still be possible.
Fig. 3 Diagrams showing likely F-atom addition patterns for C60F2 and C60F4 and three possible F-atom addition patterns C60F6. The solid circles represent C-F bonds. The 19F NMR spectrum of C60F6 is consistent with the diagram labeled S.
The 19F NMR spectrum of C60F20 is interesting because of its simplicity. It consists of a single peak, requiring that all 20 F atoms are symmetrically related.[19] The only possible contiguous addition structure has D5d symmetry with the 20 C-F bonds arranged on the ”equator” of a C60 cage. The structure, shown in Fig. 4, is reminiscent of the planet Saturn, and C60F20 has been tentatively named saturnene.
c60 FxOy
The isolation and 19F NMR analysis of small quantities of the oxafluorofullerenes C60F2O, C60F4O, C60F6O, C60F8O, C60F16O, and several isomers of C60F18O led to the conclusion that they are all intramolecular ethers like the isomer of C60F18O that had been structurally characterized.1-18-1 Schlegel diagrams for the likely structures of C60F2O, C60F4O, C60F6O, C60F8O, based on the distinctive chemical shifts of F atoms on the two ether C atoms, are shown in Fig. 5. The contiguous pattern of F atom addition is clearly seen, and the O atom is always inserted into the weakest C(sp3)-C(sp3) bond.
Fig. 4 Schlegel diagram and an artist’s rendition of the proposed structure of C60F20 (saturnene). The structure has D5d symmetry; therefore all 20 F atoms are equivalent. The solid circles in the Schlegel diagrams represent C-F bonds.
Fig. 5 Schlegel diagrams showing likely F- and O-atom addition patterns for C60F2O, C60F4O, C60F6O, and C60F8O based on 19F NMR spectra. The solid circles represent C-F bonds.
PHYSICAL PROPERTIES OF FLUOROFULLERENES
Solubilities in Organic Solvents
The statement that FFs are more soluble than their parent fullerene can be found in a growing number of publications. Not only is this statement incorrect in many instances, the quantitative data on which it would be properly based have not yet been published in the open literature. Quantitative solubilities could only be measured after selectively fluorinated fullerenes became available in macroscopic amounts. Some of the recently measured solubilities of C60F36 and C60F48 in hydrocarbon solvents are listed in Table 2.[59]
It was found that the solubilities of C60F48 in aromatic hydrocarbon solvents were lower, not higher, than the corresponding solubility of C60. The solubilities for C60F18 were even lower than those for C60F48. Nevertheless, the solubilites are high enough so that toluene can be used as the eluent for HPLC purifications. On the other hand, the solubilites of C60F48 in aliphatic hydrocarbon solvents were orders of magnitude higher than for C60, and those for C60F36 were ca. 10-50 times higher than for C60. In contrast to the solubility of FFs in aliphatic and aromatic hydrocarbon solvents, bromofullerenes have such low solubilities that little synthetic work with them can be anticipated in common organic solvents.[9]
Table 2 Solubilities of fluorofullerenes in hydrocarbon solvents at 25°Ca
 |
 |
 |
 |
 |
|
 |
 |
 |
 |
 |
|
|
n-Pentane |
0.26(4) |
11.9(1) |
46 |
46 |
2.2 x 103 |
|
n-Hexane |
0.36(4) |
7.1(1) |
20 |
6.7 |
1.3 x 102 |
|
n-Heptane |
0.43(3) |
6.6(1) |
15 |
6.7 |
1.0 x 102 |
|
n-Decane |
0.43(1) |
5.5(1) |
13 |
4.5 |
57 |
|
Benzene |
0.91(1) |
0.44 |
|||
|
Toluene |
0.86(1) |
0.22 |
|||
|
Mesitylene |
0.19(1) |
0.09 |
|||
Whenever a FF is crystallized from an aromatic hydrocarbon or halogenated aromatic hydrocarbon solvent, one or more solvent molecules per FF are strongly bound in the lattice (see the ”X-Ray Crystallography” section). For example, crystals containing one toluene molecule per C60F18 are stable up to 190°C, at which temperature the solvent evaporates, leaving behind solvent-free C60F18.[47- The structure of C60F48 discussed above was determined with a crystalline modification containing two mesitylene molecules per FF. A drawing of the formula unit is shown in Fig. 6. The two symmetry-related mesitylene molecules lie directly above and below the molecule along the C3 symmetry axis. Further solid state and solution studies will be needed to more fully understand the relative solubilities of fullerenes and FFs.
As far as oxygenated solvents are concerned, the early literature must, once again, be approached with caution. For example, acetone and tetrahydrofuran (THF) were used in the early 1990s to extract FFs from reaction vessels.[28,29- It was later found that these solvents, as well as methanol, completely degrade FFs with 36 or more F atoms, even to the point of rupturing the C60 cage.[60] The decomposition rate in THF increased as the residual water content of the solvent increased. On the other hand, C60F18 has recently been found to be stable for weeks in carefully dried THF or 1,2-dimethoxyethane.[61]
Fig. 6 Ball-and-stick plot of the C60F48-2 C6H3Me3 formula unit determined by X-ray crystallography (C6H3Me3 = 1,3,5-C6H3Me3 = mesitylene). Hydrogen atoms on the mesitylene molecules have been omitted for clarity. The open and hatched circles represent C and F atoms, respectively. The distances between the three molecules are to scale.
Table 3 Thermodynamic properties (kJ mol 1) of fluorofullerenes and C60
 |
 |
 |
 |
 |
|
C60F48 |
- 7563 ±166 |
130±7 |
- 7454 ±166 |
287±4 |
|
C60F36 |
-5362±201 |
139±8 |
- 5223 ±201 |
295 ±6 |
|
C60F18 |
202± 10 |
- 1450d |
302 |
|
|
C60 |
- 2355±15 |
174±6 |
- 2586 ±14 |
Thermodynamic Properties
The compositional purity, thermal stability, and availability of gram quantities of C60F18, C60F36, and C60F48 have allowed some of their thermodynamic properties to be investigated. Selected data are listed in Table 3.[13,62-65]
The vaporization properties of all three compounds have been studied[66,67] and parallel the sublimation enthalpies listed in Table 3. The vapor pressure of C60F48 is marginally higher than that of C60F36 (e.g., 0.45 vs. 0.17 Pa, respectively, at 560 K).[68] The major isomers of these compounds are either nonpolar or have negligible polarity; they are spheroidal, essentially non-polarizable, fluorocarbon nanoparticles with weak inter-molecular interactions. In contrast, the vapor pressures of C60 and C60F18 are orders of magnitude lower, but for different reasons. The extended p-electron system in C60 renders its polarizable, which leads to a relatively large sublimation enthalpy. On the other hand, the large dipole moment of C60F18 leads to strong intermolecular interactions and a large sublimation enthalpy.
Combustion calorimetry was used to measure standard molar enthalpies of formation, Afffm both in the solid state and in the gas phase (the two values differ only by the standard molar enthalpies of sublimation).[13,62,63] The importance of these measurements is that it allowed the average C-F bond dissociation energies, ca. 290 kJ mol-1, to be determined. Temperature-dependent heat capacity measurements for C60F36 revealed an order-disorder phase transition at 329.6 K (AtrsHo = 7 kJ mol-1).[69,70] An ordered body-centered tetragonal to plastic face-centered cubic phase transition was also observed in a variable temperature X-ray diffraction study.c
Properties of Fluorofullerene Ions
Ionic properties of FFs have been of particular interest to both gas-phase and solution chemists as the expectation was that addition of many electronegative F atoms should significantly enhance the electron accepting ability of the fullerene. Indeed, the very first measurements of EA (C60F2)[71] and EA(C60F46,48)[72] showed them to be 2.72 and 4.06 eV, respectively, higher than EA(C60), which is 2.67 eV.[73] Even more remarkable was the discovery made by gas-phase scientists that FFs formed doubly charged anions in the gas phase either under high-energy collisions of C60Fx- with inert gases[74] or as the result of a thermal desorption electron-capture process.[69,70] In the latter case, C60F482 – was found to be exceptionally stable toward ejection of the second electron under collision conditions, or in the process of metastable decay, this was explained by the existence of the Coulomb barrier to the removal of the excess electron. The ability to form stable negative ions is a manifestation of the exceptional electron-withdrawing character of FFs. Another manifestation was observed in electrochemical studies of C60F48,[75] C60F36,[76] and C60F18,[77] with reported Ered values of 1.38, 0.58 and 0.04 V, respectively, vs. SCE [cf. Ered(C60) = - 0.43 V vs. SCE].[77] The FF C60F48 is believed to have the most positive Ered among known organic electron acceptors. A comparison of EAs[77] and Ered values shows that, as expected, the electron-accepting ability of FFs increases as the number of F atoms increases.
CHEMICAL PROPERTIES OF FLUOROFULLERENES
Formation of Oxafluorofullerenes
In a series of earlier papers,considerable attention was paid to the presence of oxygenated fluorofullerene species C60FxOy in the products of some fluorination reactions (with F2, interhalogens, or XeF2). The origin of these by-products was believed to be either exposure of fluorofullerenes in air or formation during the synthesis because of the presence of adventitious O2 or H2O. However, the first hypothesis was not confirmed as solid FFs can be kept in open air for years without any change in chemical composition (O.V. Boltalina, unpublished data). The latter reason seems more feasible; moreover, this is confirmed by direct observations by the MSU group of the increased content of C60F34O (or C60F18O) in the crude product of reaction between C60 and MnF3 (or K2PtF6), used for large-scale preparation of C60F36 (or C60F18) under reduced pressure of 10"2 Torr, in comparison with the product obtained in situ in a mass spectrometer (pressure 10" 6 Torr).[79] A similar effect, i.e., the enhanced formation of oxygen-containing species, also occurred in the presence of hydrolysis products of some fluorinating reagents, such as high-valent metal fluorides.
Addition Reactions
The chemistry of C60 includes the attachment of pairs of atoms or groups of atoms across double bonds, resulting in C60Xx derivatives, and various types of cycloadditions yielding monoadducts, bisadducts, or polyadducts.[9] If a fullerene derivative with some of its reaction sites occupied is used in an addition reaction, one might expect a more restricted set of products with fewer added functional groups. The simplest examples that can be found in the literature are further fluorinations of FF substrates with specific compositions: C60F18,[80] C60F36,[81] and C60F48.[82] The latter substrate was used in a reaction with F2 in an attempt to achieve a higher degree of fluorination. However, the reaction led to the rupture of the fluoroful-lerene cage. In case of fluoridations of C60F18 with MnF3 and C60F36 with F2 or XeF2, selective formation of C60F36 and C60F48, respectively, with the expected isomer compositions, were reported. This is another piece of evidence confirming the stability of specific isomers of [60]full-erenes with 36 and 48 F atoms.
At present, C60F18, with its bare hemisphere, and other FFs with relatively few F atoms, appear to be the most suitable substrates for addition chemistry. The availability of these compounds allows one to explore reactions similar to those carried out with pristine C60. At present, only a few examples have been reported: reactions of anthracene with C60F18 and C60F20[83,84] and the reaction of C60F18 with tetrathiafulvalene (TTF).[85] In the reaction with anthracene, only 1:1 cycloaddition products were formed with both FFs because of steric hindrance caused by the presence of F atoms on the cage, while C60 itself formed monoadducts and several bisadducts in similar reactions. In the case of addition of TTF to C60F18, six-electron cycloaddition occurred through the terminal C-C bond in the fulvalene (accompanied by elimination of two F atoms) and yielded as a main product an asymmetric C60F16- TTF monoadduct. Another recent example concerns the CF3-radical addition to C60F18, with silver trifluoroacetate used as source of trifluoro-methyl radicals. Gas-phase species with up to six attached trifluoromethyl groups to C60F18 were observed by mass spectrometry.
Substitution Reactions
Earlier reports on the chemical reactivity of FFs involved impure mixtures, so isolation and characterization of the products was not achieved. These studies concerned introduction of the phenyl and methyl groups[86] or the formation of various hydroxides and oxides from hydrolysis reaction of relatively compositionally pure C60F36, although characterization of those products was not complete.[87] Two types of substitution reactions were more systematically studied using C60F18 as a substrate: electrophilic substitutions into aromatics in the presence of Lewis acid catalyst and nucleophilic substitutions. The first study of the phenylation of C60F18, in which FeCl3 was used as the catalyst, resulted in the formation of a triphenylated derivative.[51] This was followed by a more extensive study of a variety of aromatic addends.[88] Monosubstituted, disubstituted, and trisubstituted products C60F18 _nArn were isolated (n = 1-3) for Ar=4-tolyl, 4-methoxyphenyl, 4-phenoxyphenyl, and 4-chlorophenyl. In all cases, substitution occurred at the three least sterically hindered F atoms (the F atoms outside of the 15-F-atom belt in C60F18). For the bulky substituents Ar=2-fluorenyl, 2-biphenylyl, and 1- and 2-naphthyl, mono-adducts C60F17Ar were the dominant products. The Lewis acids SbF5 and TiCl4 were also found to be effective catalysts for these reactions.
The reaction of a fluorofullerene with a bromomalo-nate diester (Bingel reaction) was first studied by MALDI mass spectrometry and resulted in the substitution of up to three F atoms with bromomalonate groups (A.L. Mir-akyan, personal communication to O.V. Botalina, 2000). Later isolation and spectroscopic and crystallographic characterization of the monosubstituted, disubstituted, and trisubstituted products revealed that instead of the expected (by analogy with similar reaction of C60) methanofullerene formation, nucleophilic substitution of up to three bromomalonate groups for F atoms in C60F18 occurred, with the three F atoms leaving from the least sterically hindered positions. The trisadduct had a prominent emerald-green color because of the formation of 18p annulene belt of electrons with high conjugation.1-54-1 This new type of fluorofullerene derivative with various electron-donor substituents was extensively studied with the goal of designing of donor-acceptor systems to be probed in photovoltaic devices.[89] Mono- and bis-addition products of the Prato reaction with C60F18 ([3 +2] dipolar cycloaddition with aldehydes and amino acids) were obtained, with a much greater variety of isomeric products than in the case of Prato reactions with C C60.
CONCLUSION
The first decade of the fluorine chemistry of fullerenes has passed. Joint efforts of research chemists worldwide laid the foundation for further developments in this fascinating branch of fullerene chemistry. At present, fluorine-containing fullerenes represent the largest group of derivatives with the same single substituent that have been synthesized in macroscopic amounts, isolated into specific compositionally and/or isomerically pure compounds, and fully or partially structurally characterized. Fundamental physical quantities such as formation enthalpies, vapor pressures, heat capacities, reduction potentials, ionization energies, and electron affinities were determined for few specific fluorofullerenes, which constitutes the basis for more advanced theoretical and experimental physical-chemical and chemical studies with these compounds. Developments of new nonconventional approaches to the preparation of fluorine-containing derivatives provides hope that compounds with new compositions and structures will be obtained in the future.
Basic physical properties such as solubility, stability, thermal resistance, and resistance toward light and moisture were studied, all showing that fluorofullerenes represent a promising class of new chemical materials, sufficiently robust to be probed/examined/used for various practical applications. Although the high price remains an obstacle for advancing these materials, as it does for other fullerene-related applications, recent activities of large international companies as well as small nanotech businesses should bring down the prices of fullerenes once planned ton-scale production facilities come on line.
As for the possible applications of fluorinated full-erenes that were discussed in the past decade, which include their use as cathode materials in Li-batteries, as lubricants, and as oxidizing and fluorinating reagents, more research with pure materials under controlled conditions is needed (early reports on the performance of FFs in batteries or as lubricants were based on studies with complex mixtures of FFs contaminated with various oxafluorofullerenes). Current interest in the use of FFs in photovoltaic devices is based on their ability to function as good electron acceptors. In this regard, the report that the charge-transfer lifetime of a complex of C60F18 and tetrathiofulvalene is 780 nsec is very promising.[91] The possibility that FFs might be useful in lithography technologies has also been suggested recently.
