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Nanotube Magic
[C&EN, April 16, 2001]

[C&EN, April 23, 2001]

C60 Superconductivity
[C&EN, Dec. 4, 2000]

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Martin Saunders

R. James Cross

Yves Rubin

Fred Wudl

Kendall N. Houk

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April 23, 2001
Volume 79, Number 17
CENEAR 79 17 pp. 11
ISSN 0009-2347
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Operation provides the best route yet for putting gas atoms inside the cage


Molecular surgeons have chemically opened a hole in the C60 cage and inserted a helium atom or a hydrogen molecule into the cavity. In so doing, they have demonstrated the most efficient route developed thus far to place guest atoms inside a fullerene cage [Angew. Chem. Int. Ed., 40, 1543 (2001)].

A fullerene bislactam with a "mouth" on top can be stuffed with a helium atom, as shown above. A top view of the empty bislactam as a space-filling model (below) shows the mouthlike opening below the two oxygen atoms (red).
In the past decade, such endohedral complexes have been prepared via a number of "brute force" methods. For example, metal atoms have been encapsulated inside fullerene cages by vaporizing metal-graphite composites using an electric arc or a laser. And at Yale University, chemistry professors Martin Saunders and R. James Cross and their coworkers have shown that noble-gas atoms can be squeezed into an intact fullerene by subjecting it to high temperature and high gas pressure.

These methods, though, typically produce only small amounts of the endohedral complex. In the Yale high-pressure experiments, for instance, only one in 1,000 fullerene molecules end up with a noble-gas atom inside.

In 1995, chemistry professor Fred Wudl of the University of California, Los Angeles, and colleagues succeeded in chemically severing two adjacent bonds in C60 to open up a small hole or crack in the cage. The opening wasn't big enough to allow a guest molecule to enter the cage.

But the work inspired UCLA associate chemistry professor Yves Rubin to try a similar approach to make the interior of the fullerene accessible. In 1999, Rubin and coworkers Georg Schick and Thibaut Jarrosson reported an unusual reaction sequence that efficiently opens the largest aperture that has ever been formed in a fullerene shell. Their one-pot reaction sequence begins with the addition of a bisazide to three carbon-carbon bonds of C60 and leads to the formation of a bislactam, a fullerene derivative with an orifice that is shaped like an open mouth.

To find out what temperatures would be required to force specific gases through the opening, Rubin's group collaborated with UCLA chemistry professor Kendall N. Houk and postdoc Michael D. Bartberger, who performed the necessary high-level calculations. The results predicted that helium would enter the fullerene cage at a significantly lower temperature than would molecular hydrogen, which has an elongated shape and therefore a larger surface area.

That is indeed what the researchers have found in their experiments, which were performed in collaboration with Saunders, Cross, and postdoc Guan-Wu Wang at Yale. Under forcing conditions (about 300 ºC and 475 atm), about 1.5% of the bislactam molecules ended up playing host to a guest helium atom--a 15-fold greater yield than in the earlier Yale experiments that squeezed pressurized helium into intact C60. The researchers also found that they were able to produce H2-containing bislactam in up to 5% yield at 400 ºC and 100 atm. This is by far the best result yet obtained for the direct introduction of a gas into a fullerene, Rubin points out, and it bodes well for future experiments aimed at achieving higher levels of encapsulation.

Wudl agrees, saying the work is "a good proof of principle" for the surgical approach.

Compared to the hydrogen incorporation results, lower levels of helium are found inside the fullerene because the helium atoms can escape from the cage more easily, even at 100 ºC, Rubin tells C&EN. So an important goal of his research is to find a way to "suture" the broken bonds and restore the cage to its original form.

Open fullerenes, though, afford an opportunity to study the dynamics of how small molecules or ions pass through constricted channels in chemically well-defined systems. Such studies, Rubin notes, are relevant to the regulation of ion flow by membrane protein channels.

Ultimately, Rubin hopes that this surgical approach will allow the production of sizable quantities of fullerenes containing metal atoms. These complexes are expected to have interesting and useful physical properties that could lead to new types of magnetic materials, superconductors, and contrast agents for magnetic resonance imaging.

But first, Rubin says, fullerenes with even larger openings will have to be engineered. And, Wudl suggests, using larger fullerenes would allow larger species to be encapsulated.

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