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Science Concentrates

November 1, 2004 | A version of this story appeared in Volume 82, Issue 44

K+ key to Celtic clusters

Using a "shrink-wrapping" strategy, chemists have made an isopolyoxotungstate cluster anion that captures trace potassium ions [J. Am. Chem. Soc., 126, 13880 (2004)]. Leroy Cronin of the University of Glasgow, in Scotland, and colleagues' shrink-wrapping technique employs bulky organic cations that prevent the cluster anions from aggregating, thereby allowing the researchers to control the cluster anion's size and, possibly, its architecture and properties. The isopolyoxotungstate anion discussed in the report, K[H12W36O120]11­ (shown, red = O, blue = W, blue polyhedra = WO6, purple sphere and polyhedron = K+ and its coordination environment), has a Celtic ring shape that surrounds a potassium cation in the cluster's central cavity. The presence of this potassium ion, along with the crucial role it plays in the structure's assembly, surprised the researchers because they used no potassium in their starting materials. With some chemical detective work, Cronin reports, his group traced the cation's presence to a base bath used to clean the glassware. In the future, he hopes to capitalize on the host-guest chemistry of this serendipitous finding.

Using Rayleigh to differentiate nanotubes

Determining whether an individual single-walled carbon nanotube (SWNT) is semiconducting or metallic just got easier, thanks to researchers at Columbia University. A group led by Tony F. Heinz and Louis E. Brus has demonstrated that Rayleigh scattering spectroscopy can be used to identify the electronic transitions of both metallic and semiconducting nanotubes [Science, published online Oct. 28, http://dx.doi. org/10.1126/science.1103294]. The researchers report that this method is noninvasive and, unlike other SWNT-differentiating techniques, is general for nanoscale objects. Scientists had thought that Rayleigh scattering--a technique that uses elastic light scattering from small, polarizable objects to get spectroscopic information--would give only extremely weak signals when used to characterize nanoscale objects. However, Heinz and Brus's team found they were able to obtain Rayleigh scattering spectra with a high signal-to-noise ratio from individual carbon nanotubes when they used a white light source of laser brightness. The researchers hope to use the technique to probe other nanostructures.

Peptide causes iron exporter's demise

Researchers have pinpointed how the peptide hormone hepcidin regulates iron uptake and distribution in the body. A team led by Jerry Kaplan of the University of Utah's School of Medicine and Tomas Ganz of the David Geffen School of Medicine at UCLA has found that the hairpin-shaped peptide controls iron levels by binding to ferroportin, an iron-exporting protein found on the surface of specific iron-containing cells [Science, published online Oct. 28, 1126/science.1104742]. Using radiolabeled hepcidin and chemical cross-linking, the team showed that binding of hepcidin to ferroportin causes the iron exporter to be internalized and degraded by the cell. Without ferroportin, iron remains trapped in the cell. The team suggests that blocking the hepcidin-ferroportin interaction might be a useful strategy for treating anemia of inflammation, a blood iron deficiency associated with too much hepcidin. They also propose that a form of hepcidin might be used to treat hemochromatosis, a common genetic disorder that causes excess iron to accumulate in body tissues.

Selective tetramerization yields 1-octene

Petrochemical companies use the linear -olefins 1-hexene and 1-octene as comonomers to make polyethylene, and several firms have developed highly selective chromium catalysts to make 1-hexene by an ethylene trimerization process. But the analogous tetramerization to produce 1-octene has been elusive. The proposed mechanism for trimerization involves a seven-membered metallacycle intermediate consisting of a chromium atom and three conjoined ethylene molecules. Expansion of the ring to the nine members needed to form 1-octene was thought to be unlikely to occur, until now. An international team led by Annette Bollmann of South Africa-based Sasol Technology explored a variety of Cr(III) catalysts with diphosphine ligands to make 1-octene, finding several combinations that provide nearly 70% selectivity for the olefin [J. Am. Chem. Soc., published online Oct. 20, http://]. Optimization led to production rates of more than 500 kg of product per kg of catalyst per hour. Sasol is increasing production of 1-octene to meet global demand, which is expected to grow at 6 to 8% per year.

Mild route to C, CN materials

A single-step method to thermally decompose a nitrogen-rich tetrazine (shown) under mild conditions and reproducibly control the composition and morphology of the carbon or carbon nitride products has been developed by My Hang V. Huynh and Michael A. Hiskey of Los Alamos National Laboratory and coworkers [Angew. Chem. Int. Ed., 43, 5658 (2004)]. The researchers discovered that by carefully heating the sensitive tetrazine and controlling the reaction times, they could prepare an assortment of materials. To make nearly pure carbon nanospheres, they heated the tetrazine in air to 150 °C over two hours; carbon nanopolygons were made by heating for one hour. Carbon nitride (C3N4) materials with leaflike or with mixed rope- and ball-like morphologies were similarly prepared by prolonged heating. When an N2 atmosphere was used, C3N5 materials were produced. The method is attractive because the only by-product is N2 and the decomposition is controllable without applied pressure, they note. The materials have several potential applications that depend on their size and shape, including battery electrodes and in catalysis.



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