FIELD OF THE INVENTION

The present invention relates to novel salts of fullerenes, also known as fulleride salts, their preparation and use.

BACKGROUND OF THE INVENTION

Diamond and graphite are two well known allotropic forms of carbon. Another form, the fullerenes, have been prepared by graphite volatilization (See W. Kratschmer et al , Nature, 347, p. 354 (1990)). Potassium and other metal complexes of fullerenes have been observed in the gas phase by mass spectrometry (See D. M. Cox et al , J. Chem. Phvs. 88(3), 1588 (1988)).

Fullerenes are hollow molecules composed only of carbon atoms and constitute a new allotropic form of carbon. Typically, fullerenes each have carbon atoms arranged as 12 pentagons, but differing numbers of hexagons having the formula C2n where n is equal to or greater than 16 (hereinafter "fullerenes"). The pentagons are required in order to allow the curvature and eventual closure of the closed surface upon itself. The most abundant species of fullerenes identified to date is the C60 molecule or Buck insterfullerene (here¬ inafter "C6θ"). C60 consists of 12 pentagons and 20 hexagons. However, other species, including C70 have also been identified.

SUMMARY OF THE INVENTION

The portion of this invention that relates to Cβo fulleride salts was first disclosed by applicants at a seminar in Boston, Massachusetts on November 29, 1990, and subsequently published in Mat. Res. Soc. Svmp. Proc. Vol. 206, p. 659 (1991). This invention provides new compositions of matter having the formula A _n C _x , wherein C _x is a fullerene anion wherein x is preferably selected from the group consisting of C60 and C70 and wherein A is a monovalent cation.

Preferred monovalent cations, in accordance with the present inven¬ tion, include ammonium cations, alkyl ammonium cations such as quater¬ nary ammonium cations, alkali metal cations, phosphonium and arsoniu cations, especially organophosphonium and organoarsonium ions, partic¬ ularly phenylphosphonium and phenylarsonium cations. The preferred fullerene used in the practice of the present invention is C60-

In one embodiment of the present invention, the fulleride salt compounds of the present invention are prepared by passing an electric current through (electrolyzing) a non-aqueous solution of fullerenes in the presence of a soluble salt containing a cation, A, of the A _n Cχ compound to be formed. The electric potential is applied for a time sufficient to generate fullerene anions in the solution. In another embodiment of the present invention, the fulleride salt compounds are prepared in the solid state. The compounds of the present invention exhibit reversible electrochemical reduction; and, consequently, are particularly useful as electrode components in electrochemical cells such as secondary batteries.

The fullerene anions, Ceo'-^ and C70 ^~ y, wherein y is a charge of from 1 to 3, specifically 1, 2, and 3 in the compound of the present invention contain unpaired electrons and thus have paramag¬ netic properties. In the case of, for example, C6o~l and Cδo----, these properties are confirmed by electron spin resonance spectroscopy ("ESR") for the tetralkyl ammonium fulleride salt containing the C60~* monoanion. The magnitude of the magnetic susceptibility of the compound of the present invention varies with the temperature accord¬ ing to the Curie-Weiss law, which is known to one having ordinary skill in the art. Since a one-to-one magnetic susceptibility tempera¬ ture correspondence exists, these compounds may be used as magnetic thermometers.

Similarly, fulleride salts containing C60" ¹ monoanions may be used as semiconductors (See P. M. Allemand et al., J. Am. Chem. Soc., 113,2780 (1991)) while those containing C60" ³ and alkali-metals may be used as superconductors (See A. F. Hebard et al., Nature 350,600 (1991)).

Alkali salts of C60 also can serve as starting materials for the preparation of other materials. The reaction of the lithium salt of Ceo, for example, with alkyl halides yield alkyl derivatives of fullerenes (See J. W. Bausch et al., J. Am. Che . Soc 113,3205 (1991)) that may be useful as polymer blends, composites and building blocks.

Further, the A _n C _x fulleride salts are stable in the absence of air and other reactive molecules. Their free radical character may make them suitable as spin labels. Spin labels are usually organic molecules that contain an unpaired electron (for example, a nitroxyl radical) and are used to render diamagnetic molecules to which they are attached susceptible to analysis by magnetic spin resonance techniques. The A _n C _x spin labels mixed, for example with polymers, may allow valuable information concerning polymer dynamics and struc¬ tures to be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention encompasses novel compositions of matter having the formula A _n C _x , wherein C _x is a fullerene anion, wherein C _x is preferably selected from the group consisting of Cδo and C70, wherein A is a monovalent cation and wherein n is an integer from 1 to 3 inclusive, specifically 1, 2, and 3. The cation, A, may be selected from a wide range of cations. The fullerene anion is further selected from the group consisting of monovalent, divalent and tri- valent anions. Its valence, however, depends on the formula of the monovalent cation, A _n . Thus, for example where the monovalent cation, A _n , has the formula (A ⁺¹ )ι, C _x will be a monovalent anion; where A _n has the formula (A+l)2, C _x will be a divalent anion; for (A+l)3, C _x will be a trivalent anion. Particularly preferred cations for A include ammonium and alkyl ammonium cations, organophosphoniu or organoarsonium cations, especially tetraorganophosphoniu and tetra- organoarsonium ions, such as tetraphenylphosphonium and tetraphenyl- arsonium cations, alkali metal cations, and the like. Among the alkali metal cations, Li, Na and K are especially preferred. The

preferred fullerene used in the practice of the present invention is C60-

The starting materials for the practice of the present invention can be obtained from commercial sources. In addition, the fullerenes may be prepared by graphite volatilization (see W. Kratschmer, et al, Nature. 347, p. 354 (1990)).

When the compounds of the present invention are made by electrolyzing a non-aqueous solution containing fullerenes and a soluble salt of a cation, A, such as one of the aforementioned cat¬ ions, the electrochemical reduction of fullerenes is conducted prefer¬ ably in a low or high polarity solvent as required to dissolve the reactants, but which will be inert to the reaction. "Non-aqueous" as used herein means solvent systems wherein water, if present, is electrochemically and chemically inert. Thus, by way of example, toluene, dichloromethane in the case where the cation, A, is organic and a high polarity solvent such as dimethyl sulfoxide when the cation A is inorganic. Single, binary or multicomponent mixtures of tetrahy- drofuran, ethylene chloride, toluene, xylenes, dimethyl sulfoxide, dichloromethane, benzene are particularly suitable for dissolving the fullerenes. Additionally, the salt containing the cation, A, of desired compound, A _n C _x , must have some solubility in the organic solvent system used. Typically, therefore, salts containing organo groups such as, but not restricted to, R4NCI, R4ASU, R4PCI, R4NPF6, and R4NBF4 are particularly suitable. In the foregoing salts, R is selected from the group consisting of hydrogen and an organic moiety. The organic moiety should be chosen in order to render the salt soluble in the solvent system. It is within the skill of one of ordinary skill in the art to make a selection of the appropriate solvent system. R may, for example, be selected from alkyl groups having from 1 to 16 carbon atoms and phenyl groups. Other useful salts include alkali metal salts having anions containing sufficient organic moieties to render the salt soluble in the solvent system. Representative examples include NaBPh4, KBPh4 where ("Ph" as used herein means phenyl or substituted phenyl group). When inorganic salts containing the cation A are used, the solvent system is more

polar. Inorganic salts such as NaBF4, KC1 and KBr should be used with mixtures of tetrahydrofuran, dimethyl sulfoxide, dimethyl formamide, toluene and the like. The relative ratio of components of the solvent mixtures should be such as to insure some degree of solubility of both starting materials; i.e., fullerenes (which are soluble in relatively non-polar solvents) and inorganic salts (more soluble in polar sol¬ vents like dimethyl sulfoxide). It is within the skill of a person with average skill in the art to select the optimum mixture of sol¬ vents suitable for a given inorganic salt containing a desired A cation. The resultant A _n C _x fulleride salts need not be soluble in the solvent mixtures used and it is preferred that they are not if isola¬ tion of the solid salt is desirable.

A commercially available PAR (Princeton Applied Research) System equipped with Pt wire auxiliary electrode, standard calomel reference electrode (SCE) and a Pt gauze working electrode may be used to reduce, for example, up to gram quantities of the particular fullerene. Other commercial units may be used to prepare larger quantities of fulleride salts. A potentiostat regulates the potential necessary to produce the desired reduction state (salt of anion) of the fullerene in the particular solvent system. The molar ratio of salt to fullerene used in the solvent system will be generally greater than 3:1 and, preferably, will be in the range of about 10 to 20. The potential may be applied using any known source of direct current after immersing the working and reference electrodes into the solvent system. The auxiliary electrode may be isolated from the working electrode compartment during application of the electric potential, the mixture may be stirred, but such is not necessary. Typically, the reaction takes place at room temperature and pressure under a blanket of inert gas. Alternatively, an inert gas (for example, N2, Ar) may be bubbled through the solvent mixture. For example, for the produc¬ tion of CβO" ¹ monoanion from Cδo a potential of -0.45 V versus SCE in the DMSO solvent system using KBr as supporting electrolyte resulted in production of only the fullerene salts containing C60 monoanion. Since the potential necessary to produce the anions of desired valence is solvent dependent its value can be determined via cyclic volta - metry (CV) or differential pulse polarography (DPP). Care must be

6 -

taken in choosing solvent system, electrolyte, temperature, electrode type, etc. to obtain reversible well separated waves in CV at slow scan speeds. These conditions are necessary for selective production of the particular fullerene salt and to ensure that once produced they are not decomposed during the bulk electrolysis. For typical examples of well separated waves in CV and DPP data on Ceo and C70 fullerenes, see S. M. Gorun et. al, Mat. Res. Soc. Svmp. Proc. 206, 659 (1991).

The process of the present invention may be used to selec¬ tively generate the salt of the fullerene anions in solutions of the fullerenes and other hydrocarbons by applying a current to the solu¬ tion, wherein the current is of sufficiently low voltage to selective¬ ly reduce the particular fullerene to the correspondent salt of the fullerene anion without reducing the other hydrocarbons. The fuller¬ ene anions have the formula C _χ -y, wherein C _x is a fullerene, prefera¬ bly a fullerene selected from the group consisting of Cδo and C70 and wherein y is an integer of from 1 to 3, specifically 1, 2 and 3. The range of electric potentials will vary with the solvent system, but can readily be selected by one having ordinary skill in the art. The electric potential is from about zero to about -0.7 V, when the fullerene anion is selected from the group consisting of C6o~l (when the fullerene is Ceo) and C70" ¹ when the fullerene is C70; it is from about -0.80 V to about -1.1 V when the fullerene anion is selected from the group consisting of Cδθ"2 (when the fullerene is Cδo) and C7o ^* 2 (when the fullerene is C60) and C7o~ ² (when the fullerene is C70); and it is from about -1.3 V to about -1.7 V when the fullerene anion is selected from the group consisting of CβO" ³ (when the fuller¬ ene is Cβo) and Zη ~ (when the fullerene is C70). For example, 60~l _> which is prepared from Cδo at a potential of -0.70 V in 1:2 dichloromethane/toluene mixtures, may be selectively prepared as the salt of the C60~l anion from mixtures of C60 with other hydrocarbons having higher potentials (that is, having a chemical inertness within the range of potentials at which the Cδo" ¹ anion is produced) by applying the foregoing chemical potentials to the solution. The salt of the fullerene anion may be present in the solution or precipitated therefrom. The actual voltage may vary, depending on the solvent in which the reduction is conducted. However, the potential should be

chosen within the range of the electrochemical potential necessary to generate the particular salt of the fullerene anion and within the range of chemical inertness for the other hydrocarbons. (For techni¬ cal details see, for example, P. T. Kissinger and Wm. H. Heine an, Editors, Laboratory Techniques in Electrochemistry-, Marcel Decker, Inc., N.Y. (1984); and A. Bard and L. Faulkner, Electrochemical Methods. Wiley and Sons, N.Y. (1980)).

During the electrolysis, the progress of the reaction is monitored by measuring the amount of current that passes through the solution. Alternatively, since the fullerene anions are paramagnetic while the starting fullerenes are diamagnetic, quantitative ESR spectroscopy can be used for the same purpose. Similar results are expected for C70"- ¹ monoanion. (See, for example M. A. Greaney et al . , J. Phvs. Chem. 95,7142 (1991) for the ESR signatures of the C60' ¹ and C70" ¹ and D. Dubois et al., J. Am. Chem. Soc. 113,4364 (1991) for C60 ^" -*-)- For Ceo, applicants found that the most convenient method to detect Ceo"* is the monitoring of the near infrared spectrum of the solution. A strong absorption band centered at approximately 1065 nm is present in Ceo"*-*, but missing in CδO- This band, which is attrib¬ uted to the H0M0-LUM0 (highest occupied molecular orbital-lowest unoccupied molecular orbital) electronic transition of C50 ^"1 disap¬ pears upon oxidation of CδO" ¹ to δO and can be regenerated upon further reduction. Its position and intensity is practically indepen¬ dent of the A cation, being observed for both organic (e.g., BU4N) and inorganic (e.g., K) salts of C60" » in a variety of solvents. As used herein, "Bu" means "butyl".

The A _n C product may be isolated by precipitation; for example, using a non-solvent or precipitating agent, such as toluene or ether or by reducing the volume of the solvent used via vacuum distillation or low temperature freezing. Using this technique, only the fullerene salt precipitates. For example, if KC1 is used as a source of potassium cation, only KCβn, not KC1 is precipitated. Only traces of chloride ions are detectable in the KC60 solid precipitate via scanning electron microscopy energy dispersive spectra technique.

The following examples are intended to demonstrate the invention and not l imit it in any way.

EXAMPLES

In all of the following examples, the solvents were dried and degassed according to standard methods. A blanket of inert gas prevented the contact of the reaction mixture with the atmosphere. All reactions are carried out at ambient temperature and pressure unless stated otherwise. Mixtures of C60/C70 were obtained by solvent extracting the soot produced via the carbon arc synthesis method, as stated in D. M. Cox et al., J. Am. Chem. Soc. 113, 2940 (1991). Pure C60 was produced by chromatography from mixtures of Cδo and C70 fullerenes, as described in the literature. See, for example, D. M. Cox et al., J. Am. Chem. Soc. 113, 2940 (1991).

Example 1

A methylene chloride/toluene solution (2/1 v/v) containing 0.025 g Cδo and 0.5 g (Bu4N)+l(PF6) ^_1 was electrolyzed at. -0.70 V vs. SCE for a time sufficient to reduce CδO- Electronic spectroscopy and ESR spectra indicated the formation of the alkyl ammonium Cδo fuller¬ ide salt, (Bu4N)+lC60 ^_1 - Cyclic voltommetry and DPP confirmed the presence of Ceo- .

Example 2

The procedure specified in Example 1 was employed using C70 and produced the alkylammonium C70 ^*1 fulleride salt (Bu4N)+lC70" ¹ , as shown by electrochemical and ESR analysis.

Example 3

A tetrahydrofuran solution containing 0.002 g Cδo and 0.30 g NaBF4 was electrolyzed at -0.7 V vs. SCE for a time sufficient to reduce the Ceo. The properties of the resulting inorganic fulleride salt Na+lCβO" ¹ are similar to those of its BU4N+C60 ^" -*- counterpart in

Example 1 (as determined by electronic and magnetic resonance spec¬ troscopy).

Example 4

A polar solvent system consisting of a dimethyl sulfoxide DMSO/toluene solution (6/1 v/v) containing a suspension of 0.1 g Cδo and 0.25 KC1 was electrolyzed at -0.45 V vs. SCE for a time sufficient to produce C60~*- The solution turned red at the end of the reduc¬ tion. Addition of toluene (about 5:1 toluene/DMSO) and refrigeration overnight at -20°C allowed for the removal of most of DMSO and some toluene as a frozen solid. Addition of freshly distilled diethyl ether to the remaining solution resulted in the precipitation of KC60 as a black powder, which was isolated via filtration or decanting the liquid. Electronic and ESR spectra of DMSO solutions of the solid confirmed the presence of C60" in the inorganic K+Cδo"-- fulleride salt. Only traces of chloride ions were detected via scanning elec¬ tron microscopy energy dispersive spectroscopy, which also confirmed the presence of the KCδo salt.

Example 5

The procedure of Example 4 was repeated using KBr instead of KC1 to produce K+1C60 ^"1 fulleride salt.

Example 6

This example illustrates the use of CβO as a solid state electrode component. A toluene solution of CδO w s deposited on a glassy carbon electrode. Upon solvent evaporation, the electrode was coated with solid Cδø. The coated electrode was immersed in an acetonitrile solution containing 0.4 g BU4NPF6 as the supporting electrolyte. Ceo and its anions are not soluble in acetonitrile. Cyclic voltametric scans revealed the reversible formation of Ceo ^"1 - C60"-**, Cβθ" anions in the solid film. The additions of electrons to C60 was reversible; the CV scans are similar to those obtained previ¬ ously by using a solution of Cδo- A control CV analysis of a

suspension of C60 in acetonitrile solution containing the same sup¬ porting electrolyte revealed practically no fullerene-anion formation.