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Science & Technology

August 4, 2008
Volume 86, Number 31
pp. 36-37

Science & Technology Concentrates

Uranium-Aluminum Bond A First

S. Minasian & J. Krinsky
New actinide-group 13 complex contains an unsupported bond between uranium (blue) and aluminum (green).

A novel complex that contains a uranium-aluminum bond adds to the growing field of metal-metal bond chemistry (J. Am. Chem. Soc., DOI: 10.1021/ja8042382). The molecule, created by John Arnold, Jamin L. Krinsky, and Stefan G. Minasian of the University of California, Berkeley, is the first to contain a bond between an actinide and a group 13 metal. That the bond doesn't require any bridging groups to hold it together "means the uranium-aluminum interaction is a reasonably strong one, all on its own," Arnold says. The uranium-aluminum interaction is stabilized by a pentamethylcyclopentadiene (Cp*) ligand and three trimethylsilyl-substituted cyclopentadiene ligands. Arnold's group drew its initial inspiration from several lines of work in other labs, including the discovery of bonds between lanthanides and the Cp*Al ligand and the knowledge that Cp*Al is loosely analogous to carbon monoxide, which is known to bind to lanthanides and actinides. "We were curious to extend that work to actinides," Arnold says. The group is now exploring the reactivity of the complex and working to synthesize related compounds.

Magnesium Nitride Serves Up Ammonia

Magnesium nitride (Mg3N2) is a convenient source of ammonia for a host of reactions that produce nitrogen-containing compounds, a University of Cambridge research team reports (Org. Lett., DOI: 10.1021/ol801398z). The stable low-cost commercially available reagent, which produces NH3 when treated with water or certain alcohols, is an alternative to premade NH3 solutions or gas cylinders that can be cumbersome to use on a small scale. Gemma E. Veitch, Katy L. Bridgwood, and Steven V. Ley demonstrated the utility of Mg3N2 in methanol as a reagent for the direct conversion of a broad range of esters to primary amides (carboxamides). In a separate study, the researchers used Mg3N2 to prepare dihydropyridines via the Hantzsch condensation of ethyl acetoacetate with various aldehydes in ethanol (Org. Lett., DOI: 10.1021/ol801399w). Magnesium nitride was previously known to release ammonia in water, the researchers note, but it had not been used to generate ammonia in organic syntheses until now. The team is still exploring the scope of the method, Bridgwood says. A paper on the preparation of pyrroles will be published soon, she adds, and experiments involving pyrimidine synthesis from nitriles, nucleophilic aromatic substitution, and continuous-flow reactions are ongoing.

More Clues To Biotoxin Assembly

A newly discovered natural product is providing additional hints about how algae construct deadly toxins (Org. Lett., DOI: 10.1021/ol801243n). Ladder polyethers, named for their runglike structures, are toxins found in ecologically devastating algal blooms called red tides. In 1985, Columbia University's Koji Nakanishi suggested that the polyethers could arise from a cascade of epoxide-opening reactions, but it's still not clear how the ladders' fused ether rings are assembled. Now, a team led by Jeffrey L. C. Wright of the Center for Marine Science at the University of North Carolina, Wilmington, has detected trace amounts of brevisamide, a compound featuring just a single ether ring, in algae extracts. Brevisamide's structure, Wright says, supports the notion that an epoxide-opening pathway generates the ether ring. The structure also indicates the direction in which biosynthetic proteins assemble the carbon skeleton. Compared with other types of natural products, researchers know little about how these toxins are made, so "each new clue strongly influences our evolving hypothesis," says Timothy F. Jamison of MIT, who also explores ladder polyether biosynthesis.

New Growth In Inorganic Nanopeapods

Angew. Chem. Int. Ed.
View Enlarged Image
Platinum "peas" rest inside CoAl2O4 "pods."

Nanoscientists have discovered a new way to grow inorganic nanopeapods???nanoscale shells that enclose a row of nanoparticles. The latest process, developed by Lifeng Liu of the Max Planck Institute of Microstructure Physics, in Halle, Germany; Woo Lee of the Korea Research Institute of Standards & Science, in Daejon, South Korea; and coworkers allows facile control of both the size and separation of platinum nanoparticles within a CoAl2O4 nanoshell (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200801931). To create the nanopeapods, the researchers first electrodeposit nanowires composed of alternating layers of cobalt and platinum within a nanoporous anodic aluminum oxide membrane. Annealing the membrane at 700 °C prompts the cobalt to react with the alumina, forming continuous CoAl2O4 "pods." The heating also causes the platinum to agglomerate into spherical "peas" within these shells. The lengths of the cobalt and platinum segments determine the diameter and distance between each of the platinum peas. Long cobalt segments lead to large spaces between the particles, whereas short cobalt segments form more tightly packed peas.

Surface Patterning With Nanoprecision

Courtesy of Manfred Buck
Micrograph shows a supramolecular-SAM hybrid. A superimposed molecular model (center) depicts how the hydrogen-bonded network corrals the SAM.

By corralling self-assembled monolayers (SAMs) within the pores of a supramolecular network, chemists in Europe have developed a method for creating patterns over large surface areas with nanoscale precision (Nature 2008, 454, 618). Manfred Buck and coworkers at Scotland's University of St. Andrews invented the new fabrication platform, which they say could be used to build biosensors or molecular electronics. The researchers form their supramolecular networks on a gold surface by capitalizing on the hydrogen-bonding interaction (shown) between 1,3,5-triazine-2,4,6-triamine and perylene-3,4,9,10-tetracarboxylic diimide. The triamine's threefold symmetry gives rise to a honeycombed network full of hexagonal pores, which are subsequently filled with organothiol-based SAMs. "The combination of the network with SAMs offers considerable design flexibility, with the network providing an exact definition of structures in the substrate plane and the SAM permitting separate surface modification," the researchers note, adding that "the hybrid system can provide control on a length scale and at a precision not readily achievable otherwise."

Gene Activators Mimic Effects Of Exercise

Couch potatoes, rejoice! A research team led by Ronald M. Evans of the Salk Institute for Biological Studies, in La Jolla, Calif., has identified two compounds that increase the ability of muscle cells to burn fat and that significantly improve endurance for physical activity in mice (Cell, DOI: 10.1016/j.cell.2008.06.051). The compounds could aid obese people and patients who suffer from muscle-wasting diseases such as muscular dystrophy. A group led by Evans had previously shown that genetic engineering of mice to enhance the activity of the PPARδ gene modified their muscle composition and enabled the animals to run twice as far as normal mice. Now, Evans and coworkers have determined that the investigational drug GW1516 reproduces this effect in normal mice but only if the mice exercise. The researchers found that another compound, 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), boosts endurance even in mice that are not put on an exercise regimen. AICAR activates muscle genes that are normally turned on by exercise. Recognizing that athletes might abuse these drugs, the team is developing a mass spectrometry technique to detect the compounds and their metabolic by-products in blood and urine.

Structures Of Neutral Gold Clusters Revealed

Vibrational spectroscopy reveals the structures of neutral gold clusters, such as Au7 and Au20, for the first time.

Gold nanoparticles are touted for their ability to oxidize carbon monoxide in applications such as cleaning up automobile exhaust. In particular, atoms at the edges of gold clusters are thought to be catalytically reactive due to their low coordination to other atoms. Spectroscopic techniques used to study the electronic and geometric structures of these nanoparticles have been limited to charged clusters, however. A team led by Andr?? Fielicke of the Fritz Haber Institute of the Max Planck Society, in Berlin, has now used a far-infrared vibrational spectroscopy technique to determine the gas-phase structures of three neutral gold clusters: Au20, Au19, and Au7 (Science 2008, 321, 674). Neutral Au20 has the same symmetrical pyramid structure as its analogous anion, Au20-, the researchers observed. But neutral Au7 has a structure distinct from either its cationic or anionic forms. The change in Au7's structure as its electron density increases from the cationic, to neutral, to anionic forms corresponds to a decrease in average atom coordination, the researchers found. This shift facilitates binding of molecules such as CO that would not ordinarily bind to gold atoms with higher coordination numbers, Fielicke says.

Biomimetically Built Cancer Inhibitors

An international research team has reported the total syntheses of exiguamine A and B, which are potent inhibitors of a promising enzyme target implicated in several types of cancer (Nat. Chem. Biol., DOI: 10.1038/nchembio.107). Raymond J. Andersen of the University of British Columbia, Vancouver; Dirk Trauner of the University of California, Berkeley; and coworkers employed a biomimetic strategy to synthesize the compounds. They built a quinone intermediate using simple amino acid-derived building blocks and then proceeded to exiguamine A and B by means of an oxidative cascade of reactions. Exiguamine A and B are natural products derived from a marine sponge found in the waters of Papua New Guinea. The compounds inhibit the enzyme indoleamine-2,3-dioxygenase (IDO) with nanomolar affinity. IDO is involved in the degradation of tryptophan, and it protects a growing fetus from its mother's immune system. But the enzyme is also overexpressed in many tumor cell lines, correlates with a poor prognosis for certain cancers, and seems to play a role in the "immune escape" mechanisms that solid tumors use to avoid recognition by the immune system, the authors note. "Our general synthetic strategy should give rise to many exiguamine analogs, which could serve as useful candidates for drug development," they write.

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