Aromatic hexazine rings — [N₆]⁴⁻ — an all-nitrogen analogue of benzene, have finally been synthesized by researchers using high-pressure, laser-heated synthesis. The hexazine rings are present within a complex K₉N₅₆ structure containing [N₆]⁴⁻ and [N₅]⁻ rings as well as neutral nitrogen dimers.
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Previously, aromatic hexazine, [N₆]⁴⁻, was a hypothetical chemical structure which would follows Hückel rule for aromaticity: a planar ring molecule will have aromatic properties if it has 4n + 2 π electrons, where n is a non-negative integer.
Compounds with a high proportion of nitrogen tend to exhibit explosive properties, given the relative stability of molecular nitrogen (N₂ gas) (see: The Explosive Chemistry of Nitrogen: A Fascinating Journey From 9th Century to the Present), making their synthesis both difficult and risky, hence why a synthesis for [N₆]⁴⁻ has been so long in coming and why it would need to be performed at high pressure.
Prior to this, all-nitrogen aromaticity, had restricted to the [N₅]⁻ pentazolate anion. A similar synthesis reported in 2022 produced the anti-aromatic [N₆]²⁻, starting from potassium azide (KN₃).
The results are published in Nature Chemistry:
Dominique Laniel et al.: Aromatic hexazine [N₆]⁴⁻ anion featured in complex structure of the high-pressure potassium nitrogen compound K₉N₅₆. Nature Chemistry (2023). DOI: https://doi.org/10.1038/s41557-023-01148-7