CHEMICAL & ENGINEERING
NEWS OF THE WEEK 4UPMST5I, 2|0Q2 I- gD|TgP| BY 3AN\ÇÉ LONG|& |AALOK MERITA
ORGANOMETALLIC
CHEMISTRY
BUCKY FERROCENES Hybrid molecules combine key features of ferrocenes and fullerenes
T
AKE TWO BEAUTIFUL, FAS-
cinating molecules, fuse them into one, and the re sult is likely to capture people's imaginations. That, in essence, is what Eiichi Nakamura has done. The Uni versity of Tokyo chemistry pro fessor and his coworkers have prepared molecular hybrids of ferrocenes and fullerenes, two families of molecules whose rich scientific legacies have intersect ed before, but never like this. In a preliminary communica tion, the Tokyo chemists describe the synthesis, on a multigram scale, of the first two "bucky fer-
CHIME RAS Molecular structures of the f erroceneC60 (above) and -C70 (below) hybrids were deter mined by X-ray diffraction.
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rocenes"[J.Am. Chem. Soc, 124, 9354 (2002)}. In both, aniron(II) atom is sandwiched between a discrete cyclopentadienyl (Cp) ring and a cyclopentadienide ring that is part of a C 6 0 or C 70 cage. In the C 6 0 hybrid, which is ex tremely stable, the cyclopentadi enide ring coordinated to the iron is isolated from the rest of the cage byfivesp3 carbons, each car rying a methyl group. These methyls provide the proper elec tronic environment to make the pentagon they surround a suit able 6iT-electron ligand for the metal ion, Nakamura explains. The crown of five methyl groups also makes the pentagon flatter, "so that the pentagon's 2p orbitals and the iron orbitals overlap bet ter with each other." To make the somewhat less stable C 7 0 -ferrocene hybrid, Nakamura's team was able to get away with placing only three methyl groups around the cage's cyclopentadienide ring. The Japanese team prepares the C 6 0 -ferrocene hybrid in a two-step process: First, C 6 0 is methylated in 95% yield on a 10g scale to give C 60 (CH 3 ) 5 H. This compound is then heated with [FeCp(CO) 2 } 2 in benzonitrile at 180 °C, producing the bucky fer rocene in 52% isolated yield. A similar synthesis produces the C70-ferrocene hybrid in 31% iso lated yield from C 7 0 (CH 3 ) 3 H. Nakamura expects to improve the scale of these syntheses. "The hybrid compound can
not only be oxidized reversibly like ferrocene but can also be re duced reversibly like fullerene," the researchers point out in their paper. Since ferrocene itself can only be oxidized, not reduced, the hybrid offers a unique combina tion of redox properties. Nakamura is hopeful that they will be able to synthesize the par ent C 6 0 ferrocene—one without any methyl substituents—although calculations by other researchers suggest that the instability of the unsubstituted hybrid will make its synthesis very difficult. Nakamura says his group can make other bucky ferrocenes as well as other metallocenes, so "the chemistry can be significantly ex panded in the future." He also be lieves the method could be used to construct metallocene structures not only on higher fullerenes but also on the hemispherical end caps of carbon nanotubes. Nakamura and colleagues demonstrated last year that an ionic fullerene derivatives can be made to assemble into vesicles (C&EN, March 12, 2001, page 12). He expects that once the bucky ferrocenes are derivatized with water-soluble groups, they will be amenable to forming redox-tunable vesicles that might be useful for controlled release of drugs or other substances. The hybrid molecules could al so be used as building blocks to construct other types of nanostructures having useful photonic or electronic properties. Chem istry professor Ian Manners of the University ofToronto believes the synthesis of these hybrids "opens up many exciting new research opportunities" and possible ap plications in materials science. Not only that, Manners tells C&EN, but "these are beautiful molecules—an inspiring sight for any synthetic chemist!"—R0 Ν DAGANI
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