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August 19, 2002
Volume 80, Number 33
CENEAR 80 33 p. 8
ISSN 0009-2347


ELUSIVE SPECIES

AN AROMATIC RING OF FIVE NITROGENS
Long-sought cyclo-N5- anion has been experimentally detected in the gas phase

RON DAGANI

While elements such as carbon, phosphorus, and sulfur can exist in several different forms known as allotropes, nitrogen exists in only one stable form--N2. But scientists are getting closer to preparing an allotrope of nitrogen.

8033NOTW
Christe
PHOTO BY RON DAGANI
The latest advance in this quest is the first experimental detection, in the gas phase, of the pentazolate anion, cyclo-N
5-, the all-nitrogen counterpart to the cyclopentadienide anion (C5H5-). The discovery was made by chemists Ashwani Vij and Karl O. Christe of the Air Force Research Laboratory at Edwards Air Force Base in California and coworkers [Angew. Chem. Int. Ed. 41, 3051 (2002)].

Christe's group synthesized the first salt of the V-shaped N5+ cation a few years ago. That cation was the first new all-nitrogen species to be prepared in isolable quantities in more than a century.

With N5+ in hand, Christe, who is also a research professor at the University of Southern California, sought to combine it with azide (N3-). But after several attempts that yielded "nothing but explosions," Christe says, they concluded that N5+N3- "cannot be made."

Their focus then shifted to the predicted N5-, which calculations suggested would be more likely to mate with N5+ to give a neutral, stable N5+N5- allotrope.

Now, Christe and colleagues provide experimental evidence that N5- is no longer hypothetical--it can actually be generated inside a mass spectrometer using the electrospray ionization method.

Their starting point is p-hydroxyphenylpentazole (HPP), one of several known derivatives of the unknown parent pentazole, HN5. The researchers infused a solution of HPP into the spectrometer's ion source, where it was ionized to the corresponding phenolate anion. They found that, when accelerated in an electric field and allowed to collide with inert gas molecules, the anion follows different fragmentation pathways depending on the collision voltage. At low voltages, the pentazole ring molecule breaks apart. But at high voltages, the C–N bond is cleaved, releasing the intact pentazolate anion. The N5- anion subsequently loses N2 to yield N3-, which further supports its identification.

The scientists obtained additional evidence for the formation of the N5- anion using 15N-labeled HPP. The experimental results, together with theoretical calculations, "establish beyond doubt that the observed N5- species must be the long-sought pentazolate anion," they write.

"This is a very exciting development," comments Piotr Kaszynski, an associate professor of chemistry at Vanderbilt University in Nashville. This work opens up the possibility of studying the properties--and perhaps even the chemistry--of the N5- anion in the gas phase and comparing the results directly with calculations, he says.

Kaszynski and colleagues reported earlier this year that they had tried without success to generate N5- in solution from HPP using controlled ozonolysis [J. Org. Chem., 67, 1354 (2002)].

Because substituted phenylpentazoles are easily accessible, Christe believes it should be possible to synthesize and isolate N5- salts, perhaps by reacting a precursor such as HPP with a strong nucleophile. Experiments along these lines are in progress.

Because N5- is aromatic and isoelectronic with C5H5-, Christe says, its use as a pentahapto ligand for transition metals could open an entirely new realm: inorganic nitrogen analogs of metallocenes.

CRUCIAL CLEAVAGE After p-hydroxyphenylpentazole loses a hydrogen atom during electrospray ionization tandem mass spectrometry, it can follow either of two fragmentation pathways, depending on the collision voltage. The high-voltage pathway leads to the pentazolate anion.



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