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

Pentazole ( cyclo -HN 5 ) is a unique heterocycle categorized as both an organic and inorganic compound. However, attempts to synthesize and characterize cyclo -HN 5 have been unsuccessful thus far. In this study, we synthesized a cyclo -HN 5 solution and investigated the spectra, structure, aromaticity, acidity, and stability of cyclo -HN 5 . The lone pair of electrons on the protonated N atom of cyclo -HN 5 participates in π-electron delocalization, forming two N═N bonds. Further investigations suggest that cyclo -HN 5 exhibits significantly decreased π aromaticity and slightly lower σ aromaticity than cyclo -N 5 - . Experimental results suggest that pure cyclo -HN 5 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 K a of cyclo -HN 5 was determined as 1.63 (H 2 O, 25 °C) via potentiometric titration, indicating that cyclo -HN 5 is a medium-strong acid. This study reveals the fundamental structure and properties of cyclo -HN 5 , thereby providing important data for advancing cyclo -HN 5 chemistry.