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INORGANIC CHEMISTRY
Buckminsterfullerene (c60) has been dubbed "the most beautiful molecule" largely because it possesses the highest possible of all symmetries--icosahedral. But there are other icosahedral molecules out there, and two striking examples that have fullerene-like structures but aren't based on carbon were unveiled last week in Science [300, 778 and 781 (2003)].
In the As-Ni cluster, the vertices of one polyhedron abut the faces of the other and vice versa. Thus, each atom of the As20 cage caps a Ni3 face, and conversely, there is a nickel atom centered in each As5 ring. The interpenetrating nature of the As20 and Ni12 polyhedra "gives rise to a 'dimpled' golf-ball-like structure defined by 32 atoms equally distributed about the surface of a sphere," the researchers explain. "These 32 atoms form 60 equivalent triangular faces that give rise to the geodesic polyhedron with [icosahedral] symmetry, but the Ni atoms are displaced inward from the surface of the sphere." The other example of an inorganic fullerene-like molecule was reported by Manfred Scheer of the University of Karlsruhe, in Germany, and his colleagues Junfeng Bai in China and Alexander V. Virovets in Russia. They reacted copper(I) chloride with a ferrocene-like complex consisting of an iron atom sandwiched between two ligands, one of them being cyclo-P5. In the spherical cluster that results, the P5 rings have been stitched together by CuCl moieties to form a P60Cu25Cl5 cage. In this cage, the P5 rings "are surrounded by six-membered P4Cu2 rings that result from the coordination of each of the phosphorus lone pairs to CuCl metal centers, which are further coordinated by P atoms of other cyclo-P5 rings," the researchers point out. Thus, five- and six-membered rings alternate as in the fullerenes. Although the shell contains no carbon atoms, organic groups are coordinated to the P5 rings and to some of the copper atoms. With an outer diameter of 2.13 nm, this cage molecule is about three times as large as C60. But it is certainly not the largest symmetric inorganic cluster to be prepared. Even more complex clusters based on molybdenum oxide have been reported in recent years by Achim Müller's group at the University of Bielefeld, in Germany. Some of these inorganic "superfullerenes," such as one containing 132 molybdenum atoms, share structural similarities with the new As-Ni and P-Cu-Cl clusters. In an accompanying commentary in Science, Müller notes that spherical clusters "will be preferred for numerous applications in nanotechnology because of their inertness and stability. Additional desirable properties include solubility and structural features such as nanometer-scale cavities and pores, as in the case of the Mo132 cluster, where both the size of the pores and the overall size can be adjusted." Müller also suggests that one day the beauty of these spherical clusters may not just lie in their symmetry but in their utility. |
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TOTIPOTENT
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A team of U.S. and French scientists are reporting in Science that they have coaxed both male and female mouse embryonic stem cells to develop into eggs, the first time that stem cells have been induced to develop into germ line cells. Image shows structures morphologically similar to ovarian follicles induced in culture.
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