Synthesis and characterization of cyclo-pentazolate salts of iron(iii) and aluminum(iii)

Abstract

The assembly of cyclo-pentazolate (cyclo-N₅⁻) anions with counterions has thrived in recent years owing to their potential as high energy density materials. However, high-valence metal pentazolate salts are still rare, thus hindering the recognition of the structures and properties of new metal pentazolate salts. Herein, two trivalent metal pentazolate hydrates, namely, Fe(H₂O)₆(N₅)₃·9H₂O (1) and Al(H₂O)₆(N₅)₃·9H₂O (2), were synthesized via a metathesis reaction. The compounds were characterized by infrared spectroscopy, single-crystal X-ray diffraction, electrospray ionization mass spectrometry, elemental analysis, and differential scanning calorimetry. The cyclo-N₅⁻ anion decomposition temperature for compound 2 is 141.4 °C, which means it has the highest thermal stability among all cyclo-N₅⁻-based materials reported so far. Single-crystal X-ray analysis revealed that there are no direct σ-bonds between the metal centers and cyclo-N₅⁻ anions in the crystal lattices of 1 and 2. Hydrogen bonding and π–π stacking interactions in the crystal lattices of 1 and 2 were evaluated. The proportion of hydrogen bonds is more than 75%. Four kinds of hydrogen bond ring motifs are formed via O–H⋯N and O–H⋯O interactions: tetrameric R⁴₄(10), pentameric R⁵₅(12) and R⁵₅(17), and nonameric R⁹₉(21) motifs. This study deepens our understanding of the structure of cyclo-N₅⁻-based complexes and is expected to contribute to the further development of pentazole chemistry.