In 1999, chemistry professor Harry C. Dorn introduced the scientific world to an exotic new type of metal-containing fullerene: a C₈₀ cage encapsulating a trimetallic nitride cluster such as Sc₃N (C&EN, Sept. 20, 1999, page 54). Now, Dorn's group at Virginia Polytechnic Institute & State University, Blacksburg, has synthesized and characterized the first organic derivative of this cage complex [J. Am. Chem. Soc., 124, 524 (2002)].

	REACTION DU JOUR [4+2] cycloaddition yields metallofullerene derivative.

The achievement, he says, is "a major stepping stone" toward the preparation of functionalized endohedral metallofullerenes that could find use in the diagnosis and treatment of medical conditions.

A fullerene containing gadolinium, such as Gd₂ScN@C₈₀, could provide an improved contrast-enhancing agent for magnetic resonance imaging (MRI) of the body. And fullerenes encapsulating a radioactive atom such as holmium could be useful as tracers or anticancer agents.

The problem with such metallofullerenes, though, is that they don't dissolve in aqueous fluids, so getting them into tissues is difficult. Functionalizing the cage complex with an organic group that can be made water soluble, however, would eliminate that difficulty. That's what Dorn and his coworkers have done.

The team prepares the metallofullerenes in an electric arc furnace by vaporizing, in the presence of nitrogen, a graphite rod packed with a powdered metal oxide such as Sc₂O₃. The desired product--Sc₃N@C₈₀--is separated from the C₆₀- and C₇₀-containing soot and is purified.

To derivatize the compound, graduate student Erick B. Iezzi treated a few milligrams of it with an isochromanone, a reagent that adds to a fullerene double bond via a [4 + 2] cycloaddition.

Dorn's collaborators at the University of California, Davis--Alan L. Balch and Marilyn M. Olmstead--have confirmed the structure of the derivative using single-crystal X-ray diffraction. That work will be published separately.

Although the derivative itself is not water soluble, it could easily be made water soluble by replacing its methoxy groups with carboxyl groups, for instance.

Chemistry professor Stephen R. Wilson of New York University, a pioneer in exploring the medical applications of fullerenes, calls Dorn's work "extremely exciting" because, by making metallofullerenes water soluble, Dorn has opened the door to using them in the body. Radioactive metals can be delivered to tissues in the form of metal chelates, Wilson points out, but the metal can get loose and wander where it shouldn't. But when the radionuclide is encased in a fullerene, it can't leak out.

Dorn's metallofullerene synthesis, in which the trimetallic nitride serves as a template for growing a fullerene shell around the nitride cluster, is "a breakthrough," in Wilson's view, because it allows metal-containing fullerenes to be produced in much higher yields than are otherwise possible. And now that the Virginia Tech group has shown that these metallofullerenes can be functionalized in much the same way as empty fullerenes, Wilson says, it "opens up a new field of chemistry."

The limited availability of metallofullerenes has hampered efforts to study and apply them, Dorn says. His group is now gearing up to produce hundreds of milligrams of the trimetallic nitride-containing fullerenes. The technology has been licensed to Luna Innovations, a company in Blacksburg that is developing these compounds into MRI contrast agents and other commercial products.--

[Previous Story] [Next Story]

Chemical & Engineering News
Copyright © 2002 American Chemical Society