The effects of cationic odd–even alkyl chains on the crystal structure and energetic properties of cyclo-N5− salts

Abstract

The change in chemical or physical properties of materials composed of repeated methylene units with odd or even numbers of linkages is a well-documented phenomenon in organic chemistry and materials science. So far, such behavior has not been reported for the important class of cyclo-pentazolate anion (cyclo-N₅⁻) salts. In this study, we successfully synthesized and characterized four novel cyclo-N₅⁻ salts, where the alkyl chains in the tetrazole cations contain varying odd and even carbon numbers. X-ray single-crystal diffraction analysis reveals distinct structural arrangements dependent on the carbon atom parity in the tetrazolium cation's alkyl chain. When the alkyl chain contains an odd number of carbon atoms, the tetrazolium cation, cyclo-N₅⁻, and hydrogen bonds adopt a helical configuration, resulting in a highly ordered crystalline structure. Conversely, alkyl chains with an even number of carbon atoms lead to planar arrangements of these components, manifesting in relatively disordered crystal structures. Our findings disclose the important effect of minor changes in the molecular conformation of organic cations on the crystal structure, sensitivity and energetic properties of cyclo-N₅⁻ salts, demonstrating the control of key properties of cyclo-N₅⁻ salts by alkyl chain engineering of the cations.