Dual-aromaticity in nitrogen-rich compounds: from fundamental concepts to the application of high-energy-density materials

Abstract

Nitrogen-rich heterocycles have long been regarded as promising candidates for high-energy-density materials (HEDMs), yet the mechanism of their inherent stability remains elusive. This review highlights the emerging concept of dual-aromaticity, the coexisting π- and σ-electron delocalization, as a fundamental electronic configuration that enhances both structural stability and energy density in such systems. We show that σ-aromaticity composed of electron lone-pairs, originating from sp²-hybridized nitrogen atoms, complements π-aromaticity in stabilizing five-membered rings such as cyclo-N₅ˉ and its CH-, P-, and As-substituted analogs. Acidic coordination (e.g., H₃O⁺, NH₄⁺) is shown to play a synergistic role by reinforcing hydrogen-bonding networks that preserve delocalization and suppress protonation-induced destabilization, particularly above a critical acidity (c ≥ 4). Leveraging quantum chemical descriptors, we further demonstrate how the dual-aromaticity paradigm guides crystal engineering, from acidic and metal-coordinated frameworks. Finally, we show that in-plane bridging or hinge-like linkers enhances σ-aromaticity at the expense of the π component, tuning molecular flexibility and crystal phases to construct 3D networks of delocalized L-shell valence electrons, offering a viable strategy toward ambient-stable all‑nitrogen crystals (e.g., N_8, N₁₀). By integrating electronic structure theory, synthetic reactivity, and materials design, this review offers a unified, theory-guided and practice-oriented roadmap for leveraging dual-aromaticity in molecular and crystal engineering, advancing next-generation HEDMs for aerospace, defense, and energy technologies.