NEWS OF THE NEWS OF WEEK THE WEEK SUPRAMOLECULAR OPEN AND SHUT Urea-ion stoppers (green, black) can be reversibly removed, allowing Ca2+ ions (pink) to enter the capsule (Mo is blue; 0, red).
ARTIFICIAL CELLS ALLOW ION ENTRY Porous inorganic capsules serve as models for biological ion-transport processes
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HEMISTS IN GERMANY HAVE
constructed artificial cells with stoppered pores that open for the uptake of calcium ions and then close. The process resembles the functioning ofgated calcium channels in biological cell membranes. The cells—spherical nanoscale polyoxomolybdate clusters—were constructed by Achim Muller and coworkers at the University of Bielefeld (Angew. Chem. Int. Ed., published online Nov. 8, dx.doi. org/10.1002/anie.200502202). Each cell has 20 Mo 9 0 9 pores
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CHEMISTRY
that are noncovalently bonded to protonated urea ions that act as stoppers. The Bielefeld team added Ca2+ ions to an aqueous solution of the molybdate capsules and examined the precipitated product by Xray crystallography. The analysis revealed not only that Ca 2+ had entered the capsules but also that the urea stoppers had closed the pores after ion entry. "The presence of a large number of added cations, like Ca2+, in the vicinity or at the surface of the highly charged capsules decreases
CATALYSIS
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Co2-TO-CO ROUTE Copper boryl complex catalyzes C0 2 reduction in well-characterized system
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long known how to convert carbon dioxide into more I useful useful compounds, but humans are ai still striving to meet this fundamental challenge in practical ways. A new step in CL /0 1 direction has now been this taken with the development Cu ta of a catalytic system that reduces C 0 2 to C O in solution, Boryl complex 0 = c = 0 with high \/ turnover n u m b e r s and frequencies {J.
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Diboroxane
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Bis(pinacolato)diboron
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Borate complex R = 2,6-diisopropylphenyl or cyclohexyl 10
Am. Chem. Soc. 2005,127,17196). The system, reported byjoseph P. Sadighi, assistant professor of chemistry at Massachusetts Institute ofTechnology, and coworkers David S. Laitar and Peter Muller, is based on a copper® boryl complex with an N-heterocyclic carbene ligand. This complex quickly abstracts an oxygen atom from C0 2 , yielding CO and a borate complex. The latter can then be reacted with bis (pinacolato) diboron to regenerate the boryl complex. The purloined oxygen is retained in a diboroxane by-product. The chemists HIGH TURNOVER Boryl complex achieved the highabstracts an oxygen atom from C0 2 to est turnover numyield CO and a borate complex, which b e r s - 1 , 0 0 0 per reacts with bis(pinacolato)diboron to copper—at elevatregenerate the boryl complex. ed temperatures.
ACTERIA AND PLANTS HAVE
C & E N / NOVEMBER 28, 2005
the electrochemical gradient across the artificial cell membrane," Muller explains. "As a result, the affinity for the positively charged urea guests decreases and the guests are released. The uptake of ions also results in a cell response in the sense that the structure of the encapsulated water/electrolyte assembly changes," he adds. "We now intend to study systematically the interaction of our artificial cells with their environments and investigate how the properties of the cells can be tuned by changing their charges and 'internal clothes,' as well as the strength of the interactions of the stoppers in the pores," Mûller says. "This type of investigation could yield information about the functioning of cell membrane calcium channels in physiological processes and also provide insights into counterion transport of pathological relevance."—MICHAEL FREEMANTLE
Using a less bulky carbene ligand led to a less stable boryl complex but one that worked faster before it decomposed. This faster complex achieved 100 turnovers within one hour at ambient temperature and below. Bis(pinacolato)diboron's acceptance of the oxygen atom derived from C 0 2 is essentially irreversible. That's "a deal-breaker in terms of practicality," Sadighi says. If one had "an oxygen acceptor that could later liberate the oxygen photochemically, we would have achieved both halves of an energy conversion cycle that consumes C0 2 ," he tells C&EN. "This facile catalytic reduction represents, potentially, the first half." "This chemistry won't solve the world's C 0 2 problem," Sadighi says. But because the oxygen abstraction and catalyst turnover involve well-defined reactants and products, unlike some other C0 2 -to-CO reduction systems, he thinks further studies could point the way to more practical systems.—RON DAGANI WWW.CEN-0NLINE.ORG
