SCIENCE/TECHNOLOGY
Systematic Chemistry of C 60 Beginning To Emerge Buckminsterfullerene, QQ, in contrast to its physical stability, is quite reactive chemically—a feature chemists are exploiting Rudy M. Baum, C&EN San Francisco
Chemists are tinkering with buckminsterfullerene, the C 60 molecule, and their efforts are beginning to pay off. Research from a number of labs suggests that C 60 , as stable as it is physically, is a ready participant in a number of classes of chemical reactions. The recent discoveries about C 60 , s reactivity are the initial bricks in the construction of a comprehensive fullerene chemistry that ultimately will turn these carbon cage molecules into useful reagents for synthetic chemistry.
"C 6 0 was initially thought to be thoroughly inert," says Paul J. Krusic, a Du Pont chemist who is at the forefront of probing C60's reactivity. "Scientists have called it uniquely stable, and they have been impressed by the resilience of its icosahedral framework. But C 60 actually is very reactive chemically, particularly so with free radicals." Krusic and others point out that investigation of C 60 by chemists has lagged somewhat behind characterization of the molecule by physicists and materials scientists. That was natural for a number of reasons. Two physicists—Donald R. Huffman, of the University of Arizona, and Wolfgang Kratschmer, of the Max Planck Institute for Nuclear Physics, Heidelberg, Germany—first discovered how to make bulk quantities of C 60 . The two chemists who first suggested the existence of C 60 —Richard E. Smalley, of Rice University, and Harry W. Kroto, of
the University of Sussex, England— are physical chemists whose expertise is spectroscopy. And one of the most startling early discoveries about C 60 is that it is a relatively high-temperature superconductor when it is doped with a number of alkali metals, one of a number of C 60 properties that drew materials scientists to the study of fullerenes. Now, a systematic fullerene chemistry is beginning to emerge. For example, Krusic, Edel Wasserman, also of Du Pont, and Keith F. Preston, John R. Morton, and Petra N. Keizer, of the National Research Council of Canada, recently described the reaction of C 60 with photochemically generated benzyl radicals (C&EN, Nov. 25, page 17). The chemists characterize C 60 as a "radical sponge" because the molecule has, in effect, 30 carbon-carbon double bonds to which free radicals can add. Although the chemists had previously shown that radicals readily add to
Addition of three benzyl groups to C60 produces remarkably stable allylic radical (left); analogous trimethyl C60 radical forms (right), but does not exhibit unusual stability December 16, 1991 C&EN
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Science/Technology
Diphenyldiazomethane adds to Ceo, "inflating11 its cage
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Νi^N—C
C 60 , no structural information was initially available about the reaction products. Krusic, Preston, and their collabo rators have now used electron spin resonance (ESR) spectroscopy to characterize the products of the reaction b e t w e e n b e n z y l radicals (C 6 H 5 CH 2 ·) and C 60 . They have shown that from one to at least 15 benzyl radicals add to C60. And they 18
December 16, 1991 C&EN
have discovered that the addition of three or five benzyl groups to C 60 produces radical products that are re markably stable. The ESR data strongly suggest that one of these stable radicals is an allylic radical that arises from the ad dition of three benzyl groups to adja cent double bonds radiating from a five-membered ring on C60. (Buckminsterfullerene has the geometry of a truncated icosahedron, a polyhe dron with 12 pentagonal faces and 20 hexagonal faces.) The unpaired elec tron is substantially localized on the three allylic carbons, even though there are adjacent carbon-carbon double bonds over which it might be expected to delocalize. The other stable radical is a cyclopentadienyl radical that results from the addition of five benzyl groups to the double bonds radiating from one five-membered ring on C60. In this species, the unpaired electron is shared equally by the five carbon at oms of the five-membered ring. The Du Pont and Canadian chem ists attribute the extraordinary sta bility of these two radicals to steric protection afforded by the bulky benzyl groups that form a protective "canopy" over the localized, un paired electron, as well as to reso nance stabilization of the radical by those benzyl groups. By contrast, they have shown that up to 34 meth yl radicals add to C60, but that neither the trimethyl nor pentamethyl C 60 radicals appear to be unusually sta ble. Preliminary data indicate a num ber of other alkyl radicals react dif ferently with C60 than either methyl or benzyl radicals, Preston says. "This appears to be an excellent method for functionalizing C 60 ," Krusic says. He also notes that free radicals play an important role in polymerizations, and suggests these sorts of reactions "may have applica tions in polymer chemistry involv ing C60. One can imagine producing C 60 with long chains radiating away from it, polymers of C 60 arranged like beads in a necklace, or polymers containing C 60 units strung together with spacers between them." Fred Wudl, a chemistry professor at the University of California, San ta Barbara, is one of the pioneers of efforts to produce chemical deriva tives of C60. Wudl and UCSB collab-
C60 encapsulates a lanthanum atom orators T. Suzuki, Q. Li, O. Almarsson, and K. C. Khemani recently re ported a reaction that results in the "systematic inflation of C 60 " to pro duce fullerene derivatives Wudl terms "fulleroids" (C&EN, Nov. 25, page 17). Research in Wudl's lab has al ready demonstrated that nucleophilic addition reactions to C 60 are facile and produce myriad products. However, the electronic character of these compounds is drastically al tered from the parent C 60 molecule, Wudl points out, to the point where only minor conjugation among the π orbitals of the carbon cage re mains. The inflation reaction devised by Wudl modifies the basic fullerene skeleton while retaining the same number of π orbitals and the atten dant unusual electronic character of C60. Instead of a nucleophile, the UCSB chemists react the fullerene with a dipolar molecule, diphenyl diazomethane. They reasoned that diphenyldiazomethane would add to a carbon-carbon double bond in C 60 to form a five-membered heterocycle. This would be followed by elimination of N 2 to form a transient cyclopropyl group made up of two carbon atoms of the fullerene and the methyl carbon of the diphenylmethyl group. The bond between the two fullerene carbons of this cy clopropyl group then would open to accomplish the fullerene expansion. The reaction produced a product that was isolated and purified by chromatography. The spectroscopic and physicochemical properties of the compound strongly suggest the
methyl carbon of the diphenylmethyl group is incorporated into the fullerene cage to produce a species t h a t is b e s t c h a r a c t e r i z e d as (C 6 H 5 ) 2 C 61 . At the recent Materials Research Society (MRS) meeting in Boston, Wudl presented the x-ray crystal structure of the 4,4'-dibromodiphenyl fulleroid [(C 6 BrH 4 ) 2 C 61 ], which was determined by collaborators at the University of California, Los Angeles. This structure determination indicates the fulleroid derivative is an inflated fullerene. Wudl and coworkers have already used this chemistry to produce a variety of fulleroids. For example, the UCSB researchers have produced and have begun to characterize difulleroids. In these compounds, two C 61 fulleroid cages are linked to each other via a phenyl group. Both the para- and meta-difulleroids have been synthesized. Wudl believes these fullerene derivatives open up a wide range of possible fullerene synthetic chemistry. Another approach to functionalizing fullerenes was presented at the MRS meeting by Glen P. Miller, of the corporate research laboratories of Exxon Research & Engineering. Working in collaboration with Exxon scientists Chang S. Hsu, Long Y. Chiang, Hans Thomann, and Marcelino Bernardo, Miller has shown oxidized C 60 reacts with a number of nucleophiles to produce symmetrically substituted alkoxylated and arylated fullerene derivatives.
Miller points out that, although C 60 is difficult to oxidize electrochemically, the molecule is readily oxidized in protic superacid media. (A superacid is any Br0nsted acid stronger than 100% sulfuric acid.) Miller and collaborators oxidize pure C 60 to form the C 60 radical cation in the superacids known as magic acids. These are mixtures of fluorosulfuric acid (FS0 3 H) and antimony pentafluoride (SbF5). They have shown that mixtures of oxidized C 60 in magic acids are stable indefinitely if handled properly. The oxidized C 60 can be "trapped by suitable nucleophiles to form the corresponding nucleophilic addition products," Miller says. 13C nuclear magnetic resonance spectroscopy and mass spectrometry suggest that two, four, and six nucleophilic groups add symmetrically to the C 60 radical cation. The Exxon researchers have shown methanol, 1-butanol, and benzene all add to the C 60 radical cation. They have also reacted C 6 0 + with 1,6-hexanediol to produce the 1,6difulleroxyhexane. This is a species consisting of two C 60 groups linked through ether linkages to either end of a six-carbon chain. Miller points out that 1,6-difulleroxyhexane is not an electronically closed shell structure unless the two C 60 cages at the ends of the hexanediol bridge couple, which would result in what he calls an "earmuff ether" structure. "We expect this synthetic proce-
Apparatus generates C6o by vaporizing graphite
dure to work with a wide variety of nucleophiles, leading to new materials with wide-ranging properties," Miller says. Miller's work dovetails nicely with recently reported research by George A. Olah and coworkers at the University of Southern California on production of polyarenefullerenes obtained by acid-catalyzed fullerenation of aromatics [/. Am. Chem. Soc, 113, 9387 (1991)]. Olah and USC collaborators Imre Bucsi, Christian Lambert, Robert Aniszfeld, Nirupam J. Trivedi, Dilip K. Sensharma, and G. K. Surya Prakash have shown that aluminum trichloride and strong acids catalyze the addition of aromatics such as benzene and toluene to C60. In this reaction, hydrogen and the aromatic group add across the fullerene double bonds. At least 12 aromatic molecules can add to C 60 , the chemists report. And other aspects of C 60 's reactivity are being discovered. At the MRS m e e t i n g , for e x a m p l e , Exxon's Chiang discussed three different types of electrophilic substitution reactions that involve C 60 and lead to the production of fullerols, C 60 molecules with up to 14 hydroxy 1 groups attached to them, research done in collaboration with Exxon scientists Ravi Opasani and John W. Swirczewski. Also at the MRS meeting, Douglas A. Loy, of Sandia National Laboratories, described the synthesis and characterization of a C60-p-xylylene copolymer. Loy and Sandia coworker Roger A. Assink react pure C 60 with xylylene produced by the thermolysis of paracyclophane. All of these reactions point toward a rich fullerene chemistry bei n g d e v e l o p e d in t h e c o m i n g months and years. Of course, a variety of other research on C 60 and the fullerenes continues unabated. For example, at the MRS meeting, Smalley presented an update on research in his lab on fullerenes with metal atoms trapped inside the carbon cage. These so-called endohedral fullerene complexes are likely to possess unique properties that have possible practical importance. Smalley and Rice coworkers Yan Chai, Ting Guo, Changming Jin, Robert E. Haufler, L. P. Felipe Chibante, Jan Fure, Lihong Wang, and J. December 16, 1991 C&EN
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Science/Technology Michael Alford produce lanthanumcontaining fullerenes by laser vaporization of graphite impregnated with lanthanum oxide. Smalley has introduced a symbolism for designating fullerenes that is particularly useful for describing these metal-containing species (C&EN, Sept. 2, page 6). In it the "@" symbol designates an entity that is encapsulated by the fullerene cage. Thus, C 60 containing a single lanthanum atom is represented by La@C60. At the MRS meeting, Smalley described efforts to further characterize the endohedral metallofullerenes. Samples of metallofullerenes prepared at Rice were studied by x-ray photoemission spectroscopy by John H. Weaver and coworkers Gary H. Kroll and Tim R. Ohno at the University of Minnesota. These studies, which will be reported shortly in Chemical Physics Letters, together with the air stability of the samples, indicate both lanthanum and yttrium are encapsulated by fullerenes. La@C60 is the most abundant lanthanumcontaining fullerene. La@C82 seems to be the second most abundant species. Previous research in Smalley's laboratory had established that La@C82 was the only lanthanumcontaining fullerene that was soluble in toluene. According to Smalley, "yttrium appears to be incorporated into fullerene cages with even greater facility than lanthanum." Y@C 60 , again, is the most abundant species. In addition to the fullerenes containing a single yttrium atom, the researchers produce complexes containing two yttrium atoms, Y2@Cn, with Y 2 @C 82 especially prominent. Like La@C82, Y@C82 can be readily extracted from soot with boiling toluene. Y 2 @C 82 is also soluble in this solvent, Smalley says. Researchers at IBM's Almaden Research Center, San Jose, Calif., have used a modification of the carbonarc fullerene production technique developed by Huffman and Kratschmer to produce the first bulk (milligram) quantities of pure metallofullerenes, IBM chemist Constantino S. Yannoni reported in Boston. In research that will be reported shortly in Nature, Yannoni and IBM collaborators Robert D. Johnson, Mattanjan S. de Vries, Jesse R. Salem, and Don20
December 16, 1991 C&EN
ald S. Bethune used ESR spectroscopy to characterize solid-state La@C82 and La@C 82 in toluene solution. Their results indicate that the lanthanum atom in La@Cn endohedral fullerene complexes is inside the fullerene cage and in the 3+ oxidation state. These findings lead to some fascinating possibilities. The unique chemical properties of La@C82 versus the other lanthanum-containing fullerenes led Smalley and others to speculate that the lanthanum donated two electrons to the fullerene cage, giving it the electronic properties of C 84 , a particularly stable empty fullerene. Johnson suggests the finding that lanthanum in La@C82 is an La3+ argues against this explanation. Smalley isn't so sure. "The only thing I have been able to figure out that could explain why La@C 82 , uniquely, of all the lanthanum-containing fullerenes, dissolves in toluene is that it behaves, to the outside world, like it's a fullerene, because toluene dissolves fullerenes," he says. "So something has to be unique about 82. If you assume that two electrons go into the cage, then that would make it an 84-electron shell, which is the first stable electron count after 70." Smalley's group has reproduced the IBM ESR study, and he agrees with the interpretation that it is La3+ that is inside La@C82. Smalley paints a picture of this species that has the small La3+ ion "levitated" inside the C822" cage, which possesses a closed electronic shell analogous to that of C84. This leaves a lot of empty space around the lanthanum, and Smalley believes the third, unpaired electron from the lanthanum probably resides predominantly in "Rydberg-like" orbitals between the lanthanum and the inside of the fullerene cage. "That makes physical sense to me. It seems to be consistent with the ESR results and it is consistent with 82 being special," Smalley says. According to Johnson, the IBM group has already produced bulk samples of a number of endohedral fullerene complexes containing lanthanum and yttrium, and the researchers have begun to characterize these compounds. They find, for ex-
ample, that unlike La@C82, the other lanthanum-containing fullerenes are soluble in pyridine. This suggests the complexes are either polar or, at least, polarizable. One other recent research discovery that may or may not relate directly to fullerenes is the discovery of helical microtubules of graphitic carbon by Sumio Iijima, of Japan's NEC Corp.'s Fundamental Research Laboratories (C&EN, Nov. 11, page 25). These remarkable structures grow at the negative end of the electrode used for evaporating graphite in a procedure much like that devised by Huffman and Kratschmer for producing fullerenes. Transmission electron microscopy (TEM) reveals each needle comprises coaxial tubes of graphitic sheets—that is, sheets made up of hexagonally bonded carbon atoms—ranging in number from two to about 50. On each tube, the hexagons are arranged in a helix around the needle's axis. Iijima believes the helical structure may aid in the growth process that forms the tubes. In Iijima's TEM images, all or nearly all of the tubes are capped. This leads Smalley and others to speculate that the tubes are, in fact, large fullerene structures. In many of their early publications, Smalley and Kroto and their coworkers speculated about "giant fullerenes" that contained from 100 to several hundred carbon atoms. They assumed these would be large versions of C60—large, hollow, generally spherical structures. Now, Smalley thinks that may not be the case. Research at UCLA by Robert L. Whetten, François Diederich, and coworkers, Smalley notes, indicates C 76 and C 84 have hexagons arranged in a helix. And there are, in fact, reasons to suspect that distributing the 12 pentagons of a giant fullerene as far apart as possible in a spherical structure would lead not to the most stable fullerene, but rather, to one that is susceptible to reactions that would break it down. "In fact, there never was any reason to think the giant fullerenes would be large versions of C 60 ," Smalley says. "It may turn out that the canonical giant fullerene is not a large, spherical object, but rather a buckytube." Π
