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A flattened fullerene

Superoxides on Mars destroy organics Scientists believe they have demonstrat­ed why martian soil is so reactive and consequently why no organic com­pounds have been found on the planet's surface. In the 1970s, the Viking lander's examination of martian soil turned up the first evidence for this chemical reactivity. But a complete explanation of this finding has remained elusive. Scientists have hy­pothesized that extremely reactive super­oxide ions (02"), which easily destroy or­ganic compounds, are formed in the mar­tian environment. Albert S. Yen, a planetary scientist at the Jet Propulsion Laboratory, Pasadena, Calif., and col­leagues there and at California Institute

of Technology and Chemiotics Consult­ing in Newton, Mass., found lab evidence that the superoxide ion is indeed the cul­prit [Science, 289, 1909 (2000)]. The group exposed grains of minerals typical of martian soil to martian atmospheric conditions and temperatures. They also bombarded the samples with UV light similar to that impinging on Mars's sur­face. Superoxide ions formed readily, in­dicating that they are the "most straight­forward explanation for the unusual reac­tivity of the martian soil and for the apparent absence of organic molecules," the authors write.^

First metallaquinone: Carbene and zwitterion The first example of a compound in which one of the oxygen atoms of a quinone has been replaced by a metal turns out to have a dual personality \J.Am. Chem. Soc, 122, 8797 (2000)]. David Milstein, head of the department of organ­ic chemistry at Weizmann Institute of Science, Re-hovot, Israel, and co­workers have prepared a metallaquinone in which a ruthenium atom replac­es one of the two oxygens that are nor­mally found in a p-quinone. Quantum chemical calculations by assistant pro­fessor Jan M. L. Martin and coworkers in the same department confirm that the gas-phase structure is quinoidal. IR and NMR spectroscopic studies indicate that the metallaquinone's character is solvent dependent: The molecule inter-converts between a quinoid-Ru(O) car­bene form in nonpolar solvents (top structure) and a Ru(II)-quinolate zwit-terionic form in polar solvents (bot­tom). The researchers are investigat­ing the compound's optical and catalyt­ic properties.^

MoS2 'onions' help reduce wear and tear Solid lubricants like molybdenum disul­fide and related compounds are useful in vacuum, space, and other applica­tions where liquids are impractical to

use. But these materials often decom­pose in the presence of oxygen or water. Now, researchers in the electrical engi­neering division of the University of Cambridge have developed a procedure for preparing MoS2 films that results in a highly wear-resistant product that stands up to high levels of humidity [Nature, 407,164 (2000)]. By ablating a MoS2 target with an arc discharge in the presence of high-pressure nitrogen, research associate Manish Chhowalla and professor Gehan A. J. Amaratunga produce clusters of hollow fullerene-like MoS2 "onions" in the form of thin films. Whereas sputtering and other methods for preparing MoS2 nanoparticles result in amorphous particles or particles with just short-range order, the new method leads to highly ordered structures, the researchers point out. Tribological tests, in which a coefficient of friction is measured while surfaces slide past one another, show in an atmosphere of ni­trogen and 45% humidity, that the new lubricant reduces friction by a factor of 10 compared with sputtered MoS2. The group attributes the good performance to the presence of curved S-Mo-S planes that prevent oxidation and preserve the material's layered structure.^

Science Roundup • A linear rotaxane dimer that can be chemically induced to stretch or con­tract in a manner that resembles natural muscle has been synthesized by Jean-Pierre Sauvage's group at Louis Pasteur University, Strasbourg, France [Angew. Chem. Int. Ed, 39, 3284 (2000)]. • Using molecular beam methods and a transmission grating with nanometer-scale slits, J. Peter Toennies of Max Planck Institute for Fluid Dynamics Re­search in Gottingen, Germany, and co­workers have determined that the bond length in the extremely weakly bound helium dimer is roughly 52 A [Phys. Rev. Lett, 85, 2284 (2000)]. Conventional spectroscopy and scattering techniques are too energetic to probe this fragile species. • This year's Antarctic ozone hole is the largest ever observed, covering a record 28.3 million sq km, NASA re­ports. Scientists think the hole's surpris­ingly large extent may be due to the ex­treme intensity of this season's Antarc­tic vortex—the upper stratospheric air current that isolates air over the South Polar region.^

In 1996, a team of Russian and British re­searchers reported the synthesis of an unusual fluorinated fullerene, C60F18. The 19F NMR spectrum of this com­pound indicated that all the fluorine at­oms are bound to one hemisphere of the fullerene cage. Now, having determined

the single-crystal X-ray structure • of C6oF18—the first for a fluo-V ^ rinated fullerene—the

#H*/ v ^S> team has discov-iJy^/N^yl N^ ered that one half

m » \ j ^ \ | 1 of the fullerene \ ^jj jl\\ ca&e is flattened, mi00 \ \ M [# like a partially de-J «^ / / // flated soccer ball fa

(shown) [Angew. 4 ^ \ Chem. Int. Ed, 39, 3273

* (2000)]. Moreover, in the center of this flattened re­

gion is a hexagonal ring whose C-C bonds are all the same length, indicating that it is fully aromatic. "It is the first truly benzenoid ring identified on a fullerene surface," according to the re­search team, which is led by chemists Olga V. Boltalina of Moscow State Uni­versity and Roger Taylor of Sussex University in England. The 18 fluorine atoms are arranged like a crown around this special hexasubstituted benzene ring. The unusual electronic properties of this compound make it "a particularly interesting candidate for future applications in photonics and photovoltaics," Taylor says.^

SEPTEMBER 18,2000 C&EN 4 7

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