Synthesis in soot: the new molecular cages.

A year ago, chemists at Pennsylvania State University discovered a new class of hollow, cage-like compounds, called metallo-carbohedrenes, or met-cars (SN: 4/18/92, p.250). Because of the compounds' molecular architecture - symmetrical balls built from 12 carbon atoms and eight metal atoms - scientists speculated that met-cars could make good catalysts, semiconductors, and possibly superconductors. But an annoying obstacle blocked the study of met-car materials: Only small numbers of the molecules could be produced, and then only in the gas phase.

Met-cars will no longer be shrouded in mystery. The Penn State team has found a way to synthesize bulk quantities of the molecules in the solid state, announced principal researcher A. Welford Castleman Jr. at last week's meeting of the American Chemical Society in Denver.

"This will open the door to exploring the chemistry of met-cars in a meaningful way," says Castleman. He and his colleagues describe their breakthrough in detail in the April 9 issue of SCIENCE.

The group made the first met-cars while creating and manipulating clusters of hydrocarbons and titanium in a plasma reactor with a technique called laser vaporization. "We could only make [limited batches of] molecules that way," notes Castleman. Still working with gaseous forms, the researchers discovered they could replace the hydrocarbons with a titanium-graphite rod. Since fullerenes-similarly cage-like-can be produced in the solid state by passing an electric current between two graphite rods in a chamber with helium, the team decided to try the same technique using titanium-graphite rods. They prepared their rods with titanium and later vanadium, pressing the metal and carbon powders together and baking them.

Indeed, the black soot that remained after the rods were vaporized contained met-cars (and often fullerenes). Once they analyzed the sooty substance - which happily proved to be stable in air - they tinkered with the composition of the rods. So far, the team can produce soot containing 1 percent met-cars, but they are working to improve that yield. They also hope to develop better methods for extracting the product, says Castleman.

Shiv Khanna, a theoretical physicist at Virginia Commonwealth University in Richmond, considers this work "extremely important." He notes, "From the beginning, we've wondered what structures met-cars could form and what physical properties they might have. Now real experiments will be able to tell us."