Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (13/14), 2004 - 2010
Nothing but Nitrogen
Chemists are excited about a chain of five
nitrogen atoms / The first new nitrogen
variant in over one hundred years
Compounds that consist only of atoms of a single element have always fascinated chemists - take for example the fullerenes, which are made of pure carbon. Californian chemists, Karl O. Christe and William W. Wilson, are now making sure that they get similar hype with a molecular cation consisting only of five nitrogen atoms. Until now, there were only two compounds that contained only nitrogen: nitrogen itself - the major component of air - and the azide ion, a charged molecule made of three nitrogen atoms, which was discovered over 109 years ago.
What made this new compound so difficult to make is also the reason for the high level of interest in it: it is highly explosive. This stems from the fact that - as we know from computer calculations - all molecules that contain at least two neighboring nitrogen atoms tend to easily decompose to form ultra-stable dinitrogen, and generally a lot of energy is released during this process. Christe and his co-workers thus had to search for a synthetic route that avoids this decomposition. Recently, they were successful in synthesizing their compound from selected starting materials at low temperature in hydrogen fluoride - a substance that even eats through glass.
The new, V-shaped compound carries a positive charge that must be balanced by an oppositely charged ion - it is therefore "merely" part of a salt. That this compound could be produced at all, unlike the many other "nitrogen-only" molecules that exist only on the theoretician's computer screen, is due to the fact that the bonds between the nitrogen atoms are in an intermediate state between single and double bonds. This intermediate state stabilizes the ‘undecided’ molecule. However, the substance is still very difficult to handle; in initial analyses, a few milligrams destroyed a measuring device.
This substance will thus remain more of a laboratory curiosity. The production of larger quantities is a risky undertaking that only a few laboratories in the world would dare attempt. Nevertheless, the "chemical community" is now eagerly waiting for Christe to combine his N5+ salt with a compound of the azide ion, N3-. This could result in the formation of N8 - the first crystalline form of pure nitrogen.
Both substances consist of several intricately connected rings, made of carbon and oxygen, that must be linked in exactly the manner prescribed by nature. To find the way out of the "synthesis labyrinth" (Nicolaou) and to obtain these compounds, the Californian chemists had to develop a whole series of new strategies and reactions. Finally, after several dozen separate steps, they had a few milligrams of nature-identical substance in hand - starting from compounds that can be bought from any chemical supplier.