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Successful synthesis of neutral N₆ opens door for future energy storage

Nitrogen finally joins the elite tier of elements like carbon that can form neutral allotropes—different structural forms of a single chemical element. Researchers from Justus Liebig University, Giessen, Germany, have synthesized neutral hexanitrogen (N₆)—the first neutral allotrope of nitrogen since the discovery of naturally occurring dinitrogen (N₂) in the 18th century that is cryogenically stable and can be prepared at room temperature.

This new study, in Nature, synthesized hexanitrogen (N₆) via gas-phase reaction, with the main ingredients being chlorine (Cl₂) or bromine (Br₂) and an extremely reactive and explosive solid silver azide (AgN₃), under reduced pressure.

The researchers spread AgN₃ on the inner surface, and a gaseous halogen (Cl₂ or Br₂) was passed through the solid under reduced pressure at room temperature. The reaction triggered by the process produced N₆ alongside byproducts chloronitrene (ClN) and hydrazoic acid (HN₃).

These molecules were then trapped in argon matrices—an inert matrix of solid argon—at cryogenic conditions (10 Kelvin) to stabilize and isolate the highly reactive N₆.

Molecular forms of nitrogen are highly promising as carbon-neutral and high-energy-density materials. Upon decomposition, they release a large amount of energy as they break down into their stable N₂ form, a non-toxic, inert gas, unlike conventional fuels that produce greenhouse gases such as CO₂.

Unfortunately, N₂ is the only naturally occurring allotrope (molecular form) of nitrogen, which, due to its inert nature arising from exceptionally strong triple bonds, is unsuitable for use as a fuel.

For decades, scientists have tried synthesizing larger neutral nitrogen molecules as energy materials but failed due to the extremely unstable nature of polynitrogen molecules.

Previous studies have detected the azide radical (-N₃) and the second N₄ via spectroscopy, but their structure remained a mystery. On the theoretical front, the structures of N₄ to N₁₂ have been predicted, yet none have been experimentally isolated, as they are considered too unstable.

This study broke the trend by not only successfully synthesizing the neutral N₆ molecule but also identifying its linear, acyclic structure with C_2h symmetry. The molecule consists of a chain of six nitrogen atoms where two azide (N₃) units and three nitrogen atoms are held together by double bonds joined by a single N–N bond in the center.

The mechanism likely involved a two-step gas-phase reaction. At first, the gaseous Cl₂ or Br₂ reacted with silver azide to produce silver halide (AgX, where X = Cl or Br) and halogen azide (XN₃). The halogen azide formed in the first step reacted with another molecule of silver azide to produce silver halide and hexanitrogen (N₆).

The N₆ produced at room temperature remained stable at cryogenic temperatures, allowing the researchers to isolate it as a pure film at 77 K—the temperature at which nitrogen turns liquid. Computational calculations revealed that the molecule had a half-life of 35.7 milliseconds at room temperature and over 132 years at cryogenic conditions.

The researchers also discovered that N₆, upon decomposition, releases an exceptional amount of energy—2.2 times more per unit mass than the known explosive TNT and twice that of RDX.

They emphasize that the preparation of a metastable molecular nitrogen allotrope beyond N₂ not only advances fundamental scientific understanding but also holds potential for future energy storage applications.