Revisiting Pentazole: An Investigation into the Intriguing Molecule Exhibiting Dual Organic and Inorganic Characteristics

Pentazole (cyclo-HN₅) is a unique heterocycle categorized as both an organic and inorganic compound. However, attempts to synthesize and characterize cyclo-HN₅ have been unsuccessful thus far. In this study, we synthesized a cyclo-HN₅ solution and investigated the spectra, structure, aromaticity, acidity, and stability of cyclo-HN₅. The lone pair of electrons on the protonated N atom of cyclo-HN₅ participates in π-electron delocalization, forming two N═N bonds. Further investigations suggest that cyclo-HN₅ exhibits significantly decreased π aromaticity and slightly lower σ aromaticity than cyclo-N₅^-. Experimental results suggest that pure cyclo-HN₅ is unstable at ambient temperatures and pressures, but it can be isolated at high pressures or stabilized in solution by abundant hydrogen bonds. The pK_a of cyclo-HN₅ was determined as 1.63 (H₂O, 25 °C) via potentiometric titration, indicating that cyclo-HN₅ is a medium-strong acid. This study reveals the fundamental structure and properties of cyclo-HN₅, thereby providing important data for advancing cyclo-HN₅ chemistry.