Discovering the colorful new world of tin.

Discovering the colorful New World of tin

Like Old World explorers setting off for uncharted lands brimming with material riches, organic chemists have been probing carbon's look-alike atoms -- largely silicon, but most recently tin. As a starring element in such things as protein, diamond, Velcro, DNA, pencils, plastic wrap, stickum-paper, nylon and ice cream, carbon has earned celebrity status among atoms. But silicon has gained in prestige, and new research is raising the status of tin.

All three elements share the same column in the periodic table and exhibit many similar chemical behaviors. Though scientists are still uncovering the fundamentals of silicon and tin chemistry, they already have learned how to string silicon atoms into polysilanes -- silicon-based polymers good for making fibers, hard coatings and other materials. "It's as though we've discovered a new continent," says silicon-chemistry pioneer Robert West at the University of Wisconsin-Madison. "Because of its analogy to carbon, and yet difference, silicon compounds are quite exciting to work with."

The same goes for tin compounds, says chemist Lawrence R. Sita. At Carnegie Mellon University in Pittsburgh, he and Richard D. Bickerstaff have made a tin version of a pinwheel-shaped molecule called propellane, a chemical structure known only in theory until chemists assembled a real one out of carbon and hydrogen in 1982. Sita views the tin version of propellane and the lessons learned in its making as a Rosetta stone that may help open the field of organo-tin chemistry.

"We would like to uncover the [type of] general rules for structure and bonding for tin compounds that we already have in organic [carbon-based] chemistry," Sita told SCIENCE NEWS. The Pittsburgh chemists will report their findings in a forth-coming JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. "It's new and quite exciting," West says.

Five tin atoms form the hub of the new propellane. Two of them, called bridgehead atoms, bind to other tin atoms in the hub. The other three hub atoms bind both to the bridgehead atoms and to two bulky hydrocarbon constituents that form the blades of the pinwheel and prevent the structure from buckling and snapping.

"The most surprising feature of the molecule is its color," Sita says. The intense blue-violet hue can signify unusual optical and electronic properties, suggesting that the compound might serve as a basis for useful new materials. Now the researchers are trying to use transition metals such as cobalt for linking tin propellane molecules. "We'd like to use the tin propellane as the structural unit to form a rigid [molecular] rod," Sita says. In a solid phase, he reasons, such molecules would tend to order themselves along one axis and exhibit magnetic behavior due to a sufficient number of unpaired electrons, yielding a moldable polymer with magnetic properties.

In the May 10 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, he and Bickerstaff report making the first organo-tin compounds having two fused squares of tin atoms. Solutions of the compounds are "thermochromic," starting out colorless at frigid liquid-nitrogen temperatures and then turning yellow, orange and finally deep orange-red when they reach room temperature. "In tin chemistry the rule of thumb is 'if it's colored it's important,'" Sita says. The next step, he adds, is to develop sturdy tin-based polymers by linking the fused squares into ladder-like structures. "We anticipate a new vista of novel materials."