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Science & Technology

May 7, 2007: Volume 85, Number 19; p. 54

Science & Technology Concentrates

Unique C-H Activation
δ Aromaticity Spied In Metal Clusters
Molecules' Open Arms Influence Nanoshapes
Protein Motion Guides Photosynthesis
Zipper Structures Play Key Role In Amyloid Aggregates

Unique C-H Activation

A new allyl nitrosyl complex of tungsten selectively activates the terminal C-H bond of linear alkanes under ambient conditions in a new way that could prove useful in chemical syntheses (J. Am. Chem. Soc. 2007, 129, 5372). The complex (structure on left) is thermally unstable, losing neopentane [C(CH₃)₄] at 20 °C by abstracting a hydrogen atom from the allyl ligand. The intermediate diene complex (center) then reacts with the alkane solvent—in this case, n-pentane—to add an n-pentyl ligand, essentially hiding the displaced terminal hydrogen within the diene ligand to restore the allyl ligand. The n-pentyl complex (right) can react with I₂ to yield 1-iodopentane, the starting point for further derivatizations. "The most intriguing aspect of this chemistry," notes Peter Legzdins of the University of British Columbia, in Vancouver, who led the research, "is that the C-H bond activation yields a thermally stable—and isolable&mdah;allyl complex instead of a thermally unstable hydrido alkyl complex."

δ Aromaticity Spied In Metal Clusters

Courtesy of Alexander Boldyrev (both)

Computational studies and gas-phase photoelectron spectroscopy experiments have confirmed the existence of delocalized bonding involving the d atomic orbitals of transition metals. Research on the planar Ta₃O₃^- cluster (shown at left) by Alexander I. Boldyrev of Utah State University, Lai-Sheng Wang of Washington State University, Richland, and coworkers provides the first experimental evidence of a δ-aromatic molecule and suggests that δ aromaticity exists in other planar transition-metal complexes (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200700442). The concept of aromaticity was introduced into organic chemistry to describe delocalized bonding in planar, cyclic, conjugated molecules such as benzene. Aromaticity has since been used to describe bonding in main-group and organometallic compounds, in particular in all-metal clusters. These compounds may exhibit multiple σ and π aromaticity or antiaromaticity involving s, p, or d atomic orbitals. In the case of Ta3O3–, the researchers show that the d orbitals participate in π and δ delocalized bonding (δ molecular orbital at right). The next challenge, the researchers say, is to look for Φ aromaticity, which might occur in metals with f orbitals.

Molecules' Open Arms Influence Nanoshapes

A nanoparticle's shape and size can influence its properties just as much as its chemistry, so the ability to predict the former from the latter would be a boon to nanoscientists. Now, a research team at the Chinese Academy of Sciences, in Beijing, has taken a step closer to doing just that for nanoparticles made from substituted benzenes (Chem. Commun. 2007, 1623). Hongbing Fu, Jiannian Yao, and coworkers prepared ortho, meta, and para isomers of bis(iminopyrrole)benzene and then observed the formation of nanospheres, nanowires, and nanocubes, respectively. Fu and Yao explain that intermolecular hydrogen bonding between the iminopyrrole arms hanging from the benzene ring of the isomers plays a critical role in determining which nanoparticle shape eventually forms. The ortho isomer, with its arms held at a 60° angle from one another, likely dimerizes before aggregating into spheres, they note. The extended arms of the meta and para isomers, they reason, are more likely to join up to form longer chains. The 120° m-bis(iminopyrrole)benzene chains take on a zigzag shape and end up as nanowires, whereas the 180° p-bis(iminopyrrole)benzene chains don't mesh together as well and form truncated cubes.

Protein Motion Guides Photosynthesis

A study of the initial stages of electron transfer in bacterial photosynthesis reinforces previously obtained evidence that photosynthetic proteins do more than simply provide the scaffolding on which to hang chromophores (Science 2007, 316, 747). Neal W. Woodbury and coworkers at Arizona State University studied the rate of the initial electron-transfer reaction in a collection of native and mutant photosynthetic reaction center proteins from a light-harvesting bacterium. By combining their data with modeling, Woodbury and coworkers conclude that slow protein motions—not simply the precise positions of chromophores—control the rate of initial electron transfer. They suggest that such dependence on protein dynamics might help explain photosynthesis' enviable efficiency.

Zipper Structures Play Key Role In Amyloid Aggregates

Two years ago, David Eisenberg of UCLA and a collaborative international group obtained the first atomic structure of an amyloid fibril formed from a yeast prion protein and found that an interdigitated β-sheet structure called a steric zipper was essential to fibril formation (C&EN, June 13, 2005, page 9). Now, the same group has confirmed that steric zippers are a common structural feature in such aggregates by determining the atomic structures of microcrystals formed by 30 peptides associated with Alzheimer's disease, Parkinson's disease, type 2 diabetes, Creutzfeldt-Jakob disease, and other amyloid-related conditions (Nature, DOI: 10.1038/nature05695). The researchers used specialized techniques, including synchrotron radiation microcrystallography, to obtain structures of the amyloid microcrystals, which otherwise would be too small to permit structural analysis. The work suggests that "amyloid diseases are similar not only on the fibril level but also on the molecular level," the researchers write.