SCIENCE & TECHNOLOGY
DIGGING FOR NOVEL FLUOROFULLERENES Collaborative team makes molecules that look like a deflated soccer ball and a ringed planet The infrared spectrum of C 6 0 F 2 0 points fluorine atoms can make! to a highly symmetrical structure. And the 19 One might think that the F nuclear magnetic resonance spec partially fluorinated fultrum—obtained by collaborator Joan M. lerenesC 6 0 F 1 8 andC 6 0 F 2 0 Street of the University of Southampton, would be very similar molecules. But recent England—shows a single, sharp line, indiresearch results indicate that they are dramatically different: C 6Q F 18 has been described as a tortoise shell or a par tially deflated soccer ball, while C 6 0 F 2 0 is reminiscent of the ringed planet Saturn. The structure of C 60 F 18 was reported last year by a Russian-British collabo rative team led by chemists Olga V. Boltalina of Moscow State University and Roger Taylor of Sussex University [Angew. Chem. Int. Ed., 39,3273 (2000); C & E N , Sept. 18, 2 0 0 0 , page 47}. Boltalina's group prepared both fluo- B0LTALINA (left) and TAYLOR rofullerenes by reacting C 6 0 with K 2 PtF 6 at 460 °C under reduced pressure eating that all 20 fluorine atoms are equiv in a glass reactor. Taylor's group fished out alent. Only one possible structure is con the major product (C60F18) and character sistent with all the spectroscopic data, ized it. Collaborators at the Nesmeyanov according to the researchers: a structure Institute of Organoelement Compounds in which a jagged (CF) 20 chain runs along in Moscow obtained the compound's Xthe "equator" of the cage. ray crystal structure. Furthermore, calculations indicate that The findings show that one-half of the C 6 0 F 2 0 is highly strained, causing the cage fullerene cage—the half studded with the to be distorted: The poles are flattened and 18 fluorine atoms —is flattened, like a the fluorine-ringed equator is expanded, beaten-in soccer ball. In the middle of this suggestive of a spinning planet. T h e flattened region is a hexagonal ring that researchers have dubbed the molecule "satappears to be fully aromatic because its six urnene" [Angew. Chem. Int. Ed., 4 0 , 787 C - C bonds are all the same length. The 18 (2001)}. fluorine atoms are arranged like a crown They also have proposed a mechanis around this special hexasubstituted ben tic scenario that explains how C 60 F 18 and zene ring. C 60 F 18 is produced in the reactor along with many other fullerene derivatives, and one of the minor by-products was identi fied from its mass spectrum as C 6 0 F 2 0 . To characterize it further, the researchers needed larger amounts of this fluorofullerene, so Boltalina's group scaled up the preparation using a fluorinating mixture of manganese(III) fluoride and potassium fluoride. The larger scale synthesis enabled Taylor and coworkers to isolate several mil SATURN EN Ε The C60F20 molecule ligrams of C 6 0 F 2 0 by high-performance liq is depicted in an equatorial view. uid chromatography—enough to establish The fluorine atoms are shown in its extraordinary structure by spectro greenish-yellow. scopic means. HAT A DIFFERENCE TWO
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C 6 0 F 2 0 can form under the same reac tion conditions via two different but related pathways. Each pathway involves steps in which fluorines are added to a pair of contiguous carbons. But this addi tion can proceed according to two dif ferent patterns, one leading to C 6 0 F 1 8 and the other to C 6 0 F 2 0 . The formation of C 6 0 F 1 8 is favored because it contains an aromatic ring and C 6 0 F 2 0 does not. C o n s i s t e n t w i t h this scenario, t h e amount of C 6 0 F 1 8 produced is 20 times thatofC60F20. THE GEOMETRY of the saturnene cage and the electron-withdrawing effect of its fluo rines suggest that this molecule could be of interest for future photonic or photovoltaic applications. With this in mind, Boltalina and coworkers are try ing to boost the yield of the compound so that there will be sufficient quantities of it for electrochemical and other rel evant experiments. In addition, Taylor is trying to grow saturnene crystals that are large enough for X-ray analysis. C 60 F 18 and C 6 0 F 2 0 are not the only unusual fluorofullerenes that the Russ ian and British researchers have uncov ered. They also have isolated both iso mers of C 6 0 F 3 6 , which look "very unfullerenelike," in Taylor's words. One isomer has three flat aromatic faces, and the other has four. Another amazing structure in this fam ily is C 6 0 F 4 8 , which Boltalina calls "an indented fullerene" because of "the dras tic distortions" in the carbon cage around the only six double bonds that remain unfluorinated. The Moscow-Sussex team, though, was not first with this one: The synthesis and structure determination of C 6 0 F 4 8 was first reported by an American team in 1994. Beyond these molecules, the BoltalinaTaylor team has isolated and characterized many other fluorofullerenes, including C
60 F 2> C60F16> C 60 F 17 CF 3> V a r i ° U S
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oxides and dioxides of fluorofullerenes, and fluorinated derivatives of C 70 , C 76 , C 78 , C 82 , and C 84 . Most of these compounds have been reported within the past few years, and Boltalina presented highlights of the research last month at the 15th Win ter Fluorine Conference in St. Petersburg Beach, Fla. At that conference, Boltalina suggested that this work is like excavating at an archaeological site and never knowing what you'll turn up. In the case of some of these oddly shaped fluorofullerenes, she remarked, "we dug out something very valuable."—RON DAGANI HTTP://PUBS.ACS.ORG/CEN
