Valence tautomerism of new pyrazolo[4,3-e]tetrazole[4,5-b][1,2,4]triazines

New pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazines 5 and 6 were prepared as new biological active compounds with potential anticancer activity referring to our previous study. The 1H NMR spectra reveal tautomeric equilibrium azido and tricyclic forms of these compounds in solution. The molecular and crystal structures of 5 and 6 were determined by the X-ray analysis. The X-ray investigations show that in the crystalline state the compounds 5 and 6 exist as the linear tricyclic pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine tautomeric form. The analysis of the intra- and intermolecular interactions indicates the weak C–H⋯N hydrogen bonds and π⋯π interactions in crystal of 6 and van der Waals forces only in crystal of 5 as the non-covalent interactions influencing the molecular packing. The theoretical calculations at the ab initio DFT/B3LYP/6-311++G(d,p) level showed the azido form (3 and 4) as one existing in the gaseous phase and the coexistence of the tricyclic linear (5 and 6) and azido tautomeric forms in solution, wherein the contribution of the linear form compared to the azido form changes with polarity of the solvent.