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

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