Molecular Box Holds Promising Future For Host-Guest Chemistry

May 04, 1998

CHAMPAIGN, Ill. -- In recent years, chemists have become increasingly interested in making tiny geometric shapes that can hold small molecules or individual atoms. To the chemist's inventory of such shapes -- compact spheres of C60 and test-tube-like forms of carbon nanotubes -- researchers at the University of Illinois have added an important new element -- a molecular box.

"We have built a molecular box that is approximately 5 angstroms on a side with an internal volume of about 132 angstroms," said Thomas Rauchfuss, professor of chemistry and a researcher at the university's Beckman Institute for Advanced Science and Technology. "In a molecular sense, that's pretty spacious, and opens an exciting new dimension to future applications of host-guest chemistry."

In host-guest chemistry, the presence of a guest species is detectable through changes that occur in the chemical properties of the host, Rauchfuss said. "Such chemistry will likely form the molecular basis of future sensing technology."

The new molecular boxes are highly symmetrical, with alternating corners of cobalt and rhodium atoms, bridged by 12 carbon-nitrogen ligands. The octahedral binding nature of the metal atoms provides the necessary three-dimensional geometry to assemble the boxes.

"The box-like cores of the resulting molecular cages resemble subunits of the Prussian blue lattice, which is a very stable dye," Rauchfuss said. "But, unlike Prussian blue and similar zeolites and molecular sieves -- which are insoluble -- the new molecular boxes are easily dissolved."

Although a fundamental shape, boxes have not been prepared previously because their construction requires so many individual components -- eight corners and 12 edges -- all arranged in a particular manner. To construct the molecular boxes, Rauchfuss, visiting postdoctoral research associate Kevin Klausmeyer and research chemist Scott Wilson begin with special, pre-organized building blocks.

"Our basic building blocks are four cobalt 'corners' that each come with three built-in edges," Rauchfuss said. "To these pieces we add four additional corners -- the rhodium atoms -- to form the box."

To facilitate assembly of the boxes, the researchers replace some of the carbon-nitrogen ligands with cyclopentadienyl (Cp) groups that block the faces of the metal octahedron and prevent further growth. The Cp groups not only prevent the individual boxes from fitting together into chains, they also force the polymerization to grow toward the middle of the box.

"The Cp groups are crucial, because they allow us to assemble a molecule instead of a polymer," Rauchfuss said. "But, they are also interesting because there is so much organic chemistry we can do with them. Down the road, we want to make boxes that are negatively or positively charged in order to modify the host-guest chemistry."

The researchers described the new molecular boxes at the American Chemical Society national meeting in Dallas, March 29-April 2.

University of Illinois at Urbana-Champaign

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