Searching for long intra-annular nitrogen chains: Synthesis, characterization, and decomposition mechanism of tetrazolo[1,5-b][1,2,4]triazines

Highlights: • Tetrazolo[1,5-b][1,2,4]triazines were synthesized and fully characterized by analytical and spectroscopic methods. • The thermal decomposition mechanism of tetrazolo[1,5-b][1,2,4]triazines were firstly studied and described. • The decomposition pathways of tetrazolo[1,5-b][1,2,4]triazines are opening in tetrazole rings, followed by N₂ elimination. The synthesis and analysis of tetrazolo[1,5-b][1,2,4]triazine (1), 5,6,7,8-tetrahydrotetrazolo[1,5-b][1,2,4]triazine (2), 5,8-dinitro-5,6,7,8-tetrahydrotetrazolo[1,5-b][1,2,4]triazine (3), tetrazolo[1,5-b][1,2,4]triazin-7(8H)-one (4), 6,7-dimethyltetrazolo[1,5-b][1,2,4]triazine (5), and 6,7-dimethyl-5,6,7,8-tetrahydrotetrazolo[1,5-b][1,2,4]triazine (6) (intra-annular N5 chains) and the thermal decomposition mechanism of these compounds are described. These tetrazolo[1,5-b][1,2,4]triazines were characterized by ¹H and ¹³C NMR, mass spectrometry, elemental analysis, and differential scanning calorimetry (DSC). Compounds 2, 4, 5, and 6 were further confirmed by single-crystal X-ray diffraction. Electrostatic potentials were estimated by density functional theory to establish the molecular electron distribution and their stabilities. Careful characterization indicated that these compounds containing N5 chains exhibited relatively high thermal stability. The study of the thermal decomposition mechanism indicates that the tetrazole ring opening of the fused aza-cyclic compounds, followed by N₂ elimination is predicted to be the primary decomposition channel, whether or not they are conjugated systems, and whether or not they have substituent groups. The results could lay the groundwork for the development of new long catenated nitrogen atom chains in fused heterocycles.