Science Concentrates

C-H bonds oxidized selectively

Researchers have helped address a long-standing problem in organic chemistry by finding a way to oxidize unactivated alkane C-H bonds under mild conditions in a highly regioselective and chemoselective manner. Oxidizing C-H bonds selectively is challenging because their bond strength is high and the harsh conditions typically required to break them tend to adversely affect other parts of organic molecules. Now, assistant professor of chemistry Melanie S. Sanford and coworkers Lopa V. Desai and Kami L. Hull at the University of Michigan, Ann Arbor, report that iodobenzene diacetate, C₆H₅I(OCOCH₃)₂, can be used as a stoichiometric oxidant to carry out selective Pd(II)-catalyzed oxidations of unactivated C-H bonds in pyridines and oxime ethers (shown, Ac = acetyl) [J. Am. Chem. Soc., 126, 9542 (2004)]. "The method we report is extremely practical and allows access to many valuable, and in many cases difficult-to-access, organic products," Sanford says. She and her coworkers are currently investigating the mechanism of the reaction and the range of substrates and oxidants that can be used in it.

Nitrogenase caught reducing

Nitrogenase is the much-studied enzyme that catalyzes the reduction of N₂ to NH₃ and the reduction of other triply bonded substrates such as alkynes. The X-ray structure of the enzyme's active site has been known for a decade, yet chemists still don't know whether reduction occurs at one or more iron atoms in the enzyme's FeMo-cofactor or at a molybdenum atom. Brian M. Hoffman of Northwestern University and colleagues now report the first detailed description of a trapped nitrogenase reduction intermediate (shown), providing new evidence supporting iron as the reduction site [J. Am. Chem. Soc., 126, 9563 (2004)]. The intermediate was generated by freezing a reaction of isotopically labeled propargyl alcohol with a modified nitrogenase and was characterized in a series of electron-nuclear double resonance spectroscopy experiments. Pinpointing the origin of the hydrogen atoms--either the alcohol or the solvent--allowed the researchers to propose that a cyclic intermediate forms when the triple bond of the alcohol coordinates to an iron atom. This is circumstantial evidence that N₂ binds to the same site, they say, but direct evidence is still needed.

New analyzer for miniature mass spec

R. Graham Cooks of Purdue University and his colleagues have designed a new type of mass analyzer that offers advantages for portable, miniaturized mass spectrometers [Anal. Chem., 76, 4595 (2004)]. A hybrid of two other types of mass analyzer, the team's rectilinear ion trap (RIT) is more sensitive than the cylindrical ion trap (CIT), another mass analyzer used in miniature instruments. The improved sensitivity is the result of being able to lengthen the RIT in the z dimension, compensating for any losses in ion-trapping capacity as the x and y dimensions are shrunk to decrease the operating voltage. In contrast, all three CIT dimensions must be shrunk proportionally. The higher ion-trapping capacity of the RIT results in a 40-fold improvement in the signal-to-noise ratio relative to a similarly sized CIT. Cooks and his colleagues continue to develop the RIT. "We are investigating new fabrication methods to make extremely cheap 'throwaway' mass analyzers," as well as ultraminiaturized instruments, he says.

Clue to Ti's role in H₂ storage

In 1997, scientists reported that doping sodium aluminum hydride (NaAlH₄) with certain titanium compounds produces a material that can reversibly release and take up hydrogen under moderate conditions in the solid state. These results made NaAlH₄ appear to be a more promising hydrogen-storage material than it had been. But how titanium catalyzes the release and reabsorption of hydrogen has remained a mystery. Now, Jason Graetz at Brookhaven National Laboratory, Trevor A. Tyson at New Jersey Institute of Technology, and their coworkers have uncovered a clue. Using X-ray absorption near-edge spectroscopy, the team has found evidence that the titanium resides on the surface of NaAlH₄ in the form of amorphous titanium aluminide (TiAl₃) "rather than entering the bulk material and replacing other atoms or occupying empty spots within the lattice," Graetz says [Appl. Phys. Lett., 85, 500 (2004)]. "Our finding is the first step toward an even more interesting discovery: determining exactly how TiAl₃ helps the hydride release and reabsorb hydrogen." Understanding that mechanism could lead to better dopants and hydrogen-storage materials.

Nanotubes on a bender

When grown by direct-current plasma-enhanced chemical vapor deposition, carbon nanotubes align themselves along the direction of the applied electric field in the plasma environment. Generally, this field runs perpendicular to the substrate upon which the tubes are grown, but at the edges of the sample holder--where the electric field lines tend to bend sharply--the nanotubes are inclined to incline. Recognizing the potential of this biased growth, researchers at the University of California, San Diego, developed a method for growing carbon nanotubes that zigzag (shown) [Nano Lett., published online July 27, http://dx.doi.org/10.1021/nl049121d]. Sungho Jin and colleagues used field-concentrating conductor plates to dramatically manipulate electric field lines, thereby inducing the zigzag growth. Although Jin and his group had to halt tube growth each time they changed the direction of the field, they are developing motorized rotational controls so that carbon nanotubes with complex morphologies can be grown continuously.