Preparation of a neutral nitrogen allotrope hexanitrogen C2h-N6 - PubMed (original) (raw)

Preparation of a neutral nitrogen allotrope hexanitrogen C2h-N6

Weiyu Qian et al. Nature. 2025 Jun.

Abstract

Compounds consisting only of the element nitrogen (polynitrogens or nitrogen allotropes) are considered promising clean energy-storage materials owing to their immense energy content that is much higher than hydrogen, ammonia or hydrazine, which are in common use, and because they release only harmless nitrogen on decomposition1. However, their extreme instability poses a substantial synthetic challenge and no neutral molecular nitrogen allotrope beyond N2 has been isolated2,3. Here we present the room-temperature preparation of molecular N6 (hexanitrogen) through the gas-phase reaction of chlorine or bromine with silver azide, followed by trapping in argon matrices at 10 K. We also prepared neat N6 as a film at liquid nitrogen temperature (77 K), further indicating its stability. Infrared and ultraviolet-visible (UV-Vis) spectroscopy, 15N-isotope labelling and ab initio computations firmly support our findings. The preparation of a metastable molecular nitrogen allotrope beyond N2 contributes to our fundamental scientific knowledge and possibly opens new opportunities for future energy-storage concepts.

© 2025. The Author(s).

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Conflict of interest statement

Competing interests: W.Q., A.M., and P.R.S. are inventors on European patent application EP24194869 (16 August 2024), submitted by the Justus Liebig University Giessen, which covers a method for producing molecular polynitrogens.

Figures

Fig. 1

Fig. 1. All known neutral molecular nitrogen allotropes and preparation of N6.

a, Discovery timeline (year given), composition and structure (the structure of N4 has not been determined). b, Reaction sequence used in this study. r.t., room temperature.

Fig. 2

Fig. 2. Infrared spectra of N6 isotopomers and side products.

a, Lower trace: computed anharmonic infrared spectrum of N6 at B3LYP/def2-TZVP, including the ν8 + ν9 combination. Middle trace: difference spectrum showing the changes after 8 min of 436 nm irradiation of the products of the reaction of Cl2 with AgN3. Upper trace: difference spectrum showing the changes after 6 min of 436 nm irradiation of the reaction products of Br2 with AgN3. b, Difference spectrum of a neat N6 film at 77 K showing the changes after 8 min of 436 nm irradiation. c, Bottom to top traces: computed anharmonic infrared spectrum of N6, 15NNNNN15N (1a), 15NNN15NNN (1b) and NN15N15NNN (1c) at B3LYP/def2-TZVP, including the ν8 + ν9 combination; difference spectrum showing the changes after 8 min of 436 nm irradiation of the reaction products of Br2 with AgN3; difference spectrum showing changes after 8 min of 436 nm irradiation of the reaction products of Br2 with Ag15N14N14N. Matrix sites from natural abundance and isotope-labelled HN3 (#) and H2O (*) are marked.

Fig. 3

Fig. 3. Measured and computed UV-Vis spectrum of N6 and molecular orbitals involved in the electronic transitions.

Experimental difference UV-Vis spectrum reflecting changes following 4 min of 436 nm irradiation of the reaction products of Br2 with AgN3 in argon at 10 K. Inset, computed [TD-B3LYP/def2-TZVP] electronic transitions for N6 and molecular orbitals involved.

Fig. 4

Fig. 4. Computational analyses for N6.

a, Potential energy profile (Δ_G_298K, kcal mol−1) for N6 at CCSD(T)/cc-pVTZ. The optimized parameters of N6 are given in Ångstrom (normal font), degrees (italics), natural charges in bold and natural bond orders in bold italics. Insets, computed NN bond lengths for N2, _trans_-HNNH, hydrazine and HN3 at CCSD(T)/cc-pVTZ. b, Contour line map of the Laplacian of the electron density of N6; solid and dashed lines represent positive and negative regions, respectively. c, ELF map.

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