CHEMICAL & ENGINEERING
NfiWfllIflffllÎMWËiM AUGUST 19, 2002
ELUSIVE
EDITED BY JAN IC E LO NG & LOUISA WRAY DALTO N
SPECIES
AN AROMATIC RING OF FIVE NITROGENS Long-sought cyclo-N$~~ anion has been experimentally detected in the gas phase Christe's group synthesized the first salt of the V-shaped N 5 + cation a few years ago. T h a t cation was the first new all-nitrogen species to be prepared in Christe isolable quantities in more than a century With N 5 + in hand, Christe, who is also a research professor at the University of Southern California, sought to combine it with azide (N 3 ~). But after several attempts that yielded "nothing but explosions," Christe says, they concluded that N 5 + N 3 ~ "cannot be made." Their focus then shifted to the predicted N 5 ~, which calculations suggested would be more likely to mate with N 5 + to give a neutral, stable N 5 + N 5 ~ allotrope. Now, Christe and colleagues provide experimental evidence that N 5 " is no longer hypothetical—it can actually be generated inside a mass spectrometer usC5H4Ning the electrospray ionization method. Their starting point isj&-hydroxyphenylpentazole (HPP), one of several known derivatives of the unknown parent pentazole, HN 5 . Pentazolate anion The researchers infused a solution of H P P into the spectrometer's ion CRUCIAL CLEAVAGE Afterp-hydroxyphenylpentazole loses source, where it was iona hydrogen atom during electrospray ionization tandem mass ized to the corresponspectrometry, it can follow either of two fragmentation pathding phenolate anion. ways, depending on the collision voltage. The high-voltage pathway leads to the pentazolate anion. They found that, when
W
HILE ELEMENTS SUCH AS
carbon, phosphorus, and sulfur can exist in several different forms known as allotropes, nitrogen exists in only one stable form—N 2 . But scientists are getting closer to preparing an allotrope of nitrogen. T h e latest advance in this quest is the first experimental detection, in the gas phase, of the pentazolate anion, cyc/o-Ns~, the all-nitrogen counterpart to the cyclopentadienide anion (C 5 H 5 "). T h e 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)}.
8
C&EN
/ AUGUST
19,
20G2
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 N 5 ~ anion subsequently loses N 2 to yield N 3 ~, which further supports its identification. The scientists obtained additional evidence for the formation of the N 5 ~ anion using 15 N-labeled H P P The experimental results, together with theoretical calculations, "establish beyond doubt that the observed N 5 ~ 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 N 5 " 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 N 5 ~ in solution from H P P 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 N 5 ~ salts, perhaps by reacting a precursor such as H P P with a strong nucleophile. Experiments along these lines are in progress. Because N 5 ~ is aromatic and isoelectronic with C 5 H 5 ~, Christe says, its use as a pentahapto ligand for transition metals could open an entirely new realm: inorganic nitrogen analogs of metallocenes.—RON DAGANI HTTP://PUBS.ACS.ORG/CEN
