Experimental Discovery and Electronic Structure of Free State Aminopentazole N₅NH₂ and It's Cation

All-nitrogen compounds (ANC) that are environment-friendly and promising high-energy-density materials (HEDMs). N₅NH₂, an all-nitrogen building block with functional groups, can be converted into bridged-pentazole N₁₀, azopentazole N₁₂, etc. But N₅NH₂ synthesized by previous methods was bound in supramolecular frameworks and not isolated for further research. In this article, we synthesized free-state N₅NH₂ through radical amination of cyclo-N₅ ^‒ and its cation N₅NH₃ ⁺ by the directional hydrolysis of imido-pentazoles. The new structures were verified by MS, IR, NMR and powder diffraction, and their aromaticity, HOMO-LUMO, ESP, etc. were calculated by DFT. Experimental results show these compounds are only stable at low temperature. For instance, N₅NH₃ ⁺Cl^‒ is stable below ‒20 °C for 12 hours. Harmonic oscillator model of aromaticity (HOMA), electron localization function (ELF), NICS_ZZ(1)_π, localized orbital locator (LOL), and anisotropy of the induced current density (ACID) show N₅NH₂ and N₅NH₃ ⁺ have aromaticity, and the order of aromaticity and stability is N₅NH₃ ⁺ < N₅NH₂ < cyclo-N₅ ^‒, consistent with experiments. The HOMO-LUMO energy gaps of N₅NH₂ (ΔE = 10.05 eV) and N₅NH₃ ⁺ (ΔE = 10.14 eV) is lower than that of cyclo-N₅ ^‒ (ΔE = 11.46 eV), indicating higher reaction activity. The synthesis of free-state N₅NH₂ is a crucial step in all-nitrogen construction and promotes the energetic materials field toward all-nitrogen.