Filling a Fullerene

NANOSCIENCE

Using organic synthesis as a scalpel and stitches, Japanese researchers have performed "molecular surgery" on a buckyball. A group at Kyoto University creates an opening in the molecule, inserts H₂ into the cavity, and then, in just four steps, closes up the C₆₀ framework to construct the endohedral fullerene H₂@C₆₀ [Science, 307, 238 (2005)].

The key point "is that we showed, for the first time, the great potential of organic synthesis for the production of endohedral fullerenes, which thus far has relied on hardly controllable physical methods under extreme conditions," says chemistry professor Koichi Komatsu, who spearheaded the effort.

Previously, Komatsu's group had prepared a C₆₀ derivative containing a 13-membered-ring orifice that they quantitatively filled with H₂ (C&EN, June 16, 2003, page 5). In the latest work, Komatsu and coworkers Michihisa Murata and Yasujiro Murata synthetically "sew" this hole to make the nanocontainer.

"Our success depends on the presence of a readily removable sulfur atom on the rim of the orifice," Komatsu says. Once the hydrogen is trapped within the fullerene, the researchers oxidize this sulfide group to a sulfoxide. The sulfoxide is then excised photochemically, shrinking the hole by one atom. Titanium-mediated coupling of carbonyl groups tightens the opening to an eight-membered ring.

The opening is then small enough that the team can heat the compound without losing the hydrogen cargo. The fullerene derivative rearranges--presumably via thermally allowed electrocyclization--restoring the C₆₀ cage and eliminating 2-cyanopyridine and diphenylacetylene in the process.