Chemists probe new crystalline vistas.

Chemists probe new crystalline vistas

Unload a box of hollow rods and connectors shaped like an X twisted at the waist. Start building. Before long, you'll have structure resembling diamond's tetrahedral lattice or the hexagonal structural motif of lonsdaleite, another all-carbon mineral. Using molecules as the rods and copper atoms as the linkers, two australian Chemists have begun to develp what they hope will become a set of cemical Tinkertoys for making novel materials with "unprecedented and possibly useful properties."

Compared with the tight crystal lattice of diamond or sodium chloride, regular molecular frameworks made with rodlike units host spacious interconnected cavities. By controlling the size of the rods, and thereby the size of the cavities within the framework, the scientists hope to tailor-make materials that can isolate specific components of chemical mixtures. The high proportion of empty space in these new structures also suggests possibilities for inventing extremely light solid materials, the researchers propose. Mostly, though, they say they don't know what to expect.

"I see the most important possibilities in the area of catalysis," remarks chemist Richard Robson of the University of Melbourne in Parkville, Victoria. He and crystallographer Bernard F. Hoskins hope to design frameworks into which only specific reactant molecules can enter and then leave after being transformed into specific products at internal catalytic sites.

In the July 19 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, the scientists describe "the first example of a deliberately designed and constructed infinite framework consisting of tetrahedral centers linked together by rod-like units." In an experiment Robson calls "simple in the extreme," they built the molecular network by replacing each of four roughly spherical acetonitrile groups (a carbon atom bonded to three hydrogen atoms and one carbon-nitrogen couple), loosely hooked to a common copper atom, with a more complex chemical made of four rodlike molecular segments, each capable of bonding to a copper atom.

The liquid-phase reaction produces colorless, diamond-like crystals. "If you keep the crystals under the liquid they grow out of, they retain their sharp edges, nice shiny faces and internal clarity," Robson says. "If you take the crystals out and dry them, they become milky and opaque because they lose some of their [structural] regularities."

Using X-ray crystallography to probe the crystal's architecture, hoskins found the framework resembles an extra-ariy diamond-like lattice. The framework molecules constitute roughly one-third of the crystal's volume. Solvent and ions fill the rest of the space, apparently as a liquid that seems able to flow between the solid framework's polyhedral cavities.

The researchers assess their work as a proof-of-concept effort showing the feasibility of designing and building new "infinite" crystalline frameworks by linking rod-like segments in three dimensions. Researchers elsewhere have focused on different tactics to achieve somewhat similar results. At the University of Minnesota at Minneapolis, for instance, chemist Margaret C. Ette uses weak intermolecular bonds called hydrogen bonds to coax two kinds of chemicals into novel solid materials by forcing them to crystallized together.