Sila-Explosives Offer A Better Bang

By swapping a silicon atom for a carbon atom in a pair of common high explosives, a German research team has devised a new class of compounds with the potential to outperform traditional organic nitrogen explosives and whose production and use could have a lower environmental impact (J. Am. Chem. Soc., DOI: 10.1021/ja071299p).

The work by explosives experts Thomas M. Klapötke and Burkhard Krumm of Ludwig Maximilians University, in Munich, and silicon chemist Reinhold Tacke and coworkers of the University of Würzburg appears to be the first systematic study of silicon-based high explosives, the researchers say.

The team synthesized Si(CH₂ONO₂)₄ and Si(CH₂N₃)₄, which are analogs of pentaerythritol tetranitrate (PETN) and pentaerythrityl tetraazide, respectively. The "unexpected extreme sensitivity" of the silicon compounds limited the ability of the researchers to fully characterize them, Klapötke tells C&EN, and they "are far too shock-sensitive for any practical application." But work to prepare more stable versions of the sila-explosives is under way.

Explosives such as PETN and trinitrotoluene (TNT) that contain organic nitrate and nitro groups (ONO₂, NO₂) are toxic and form toxic by-products during their production and use, the researchers note. Klapötke's group and others have been working in conjunction with the U.S. Army Research Laboratory to develop alternatives that form benign by-products, such as N₂ from azides or possibly silicon-nitrogen compounds.

Tacke's group recently took key steps toward the sila-explosives by synthesizing Si(CH₂Cl)₄ and Si(CH₂OH)₄, the silicon analogs of the starting materials used to make the carbon-based explosives. The Würzburg researchers previously have used the silicon-carbon switch strategy to synthesize drugs and odorants. The initial work on Si(CH₂N₃)₄ by graduate student Dennis Troegel led to a terrible explosion, Tacke says, which prompted him to call on Klapötke's group to collaborate and carry out further experiments.

This research "is another excellent example of recent trends inchemistry and physics to transfer successful carbon compounds andchemistry to parallel silicon-based ones," comments Bernhard Hidding of Heinrich Heine University, in Düsseldorf, Germany. Hidding is part of a team that has simulated the performance of liquid silanes and cyclosilanes as alternatives to hydrocarbon rocket and jet fuels, such as kerosene. "It seems that the analogy between carbon and silicon compounds can indeed be driven much further than ever anticipated, yielding astonishing results like these highly sensitive, high-energy sila-explosives," Hidding says.