Synthesis and stability of 2-tetrazenium salts

Abstract

The geometry and electronic structure of (E)-N₄Me₄ (1) and the (E)-N₄Me₄H⁺ and the (E)-N₄Me₅⁺ cations was examined by a DFT approach. By using the B3LYP/6-31+G(d,p) model we showed that the terminal nitrogen atoms in 1 are strongly basic, as evidenced by their highly negative NBO charges in comparison to the azo nitrogen atoms. Interestingly, protonation of 1 to form the (E)-N₄Me₄H⁺ cation does not result in significant changes in the NBO charges of the protonated nitrogen atom, which is in contrast with classical views that describe tetracoordinated nitrogen atoms as being positively charged. Insight into the thermal stability of salts of the (E)-N₄Me₄H⁺ and the (E)-N₄Me₅⁺ cations was gained experimentally by DSC measurements of two salts of the (E)-N₄Me₄H⁺ cation, namely with chloride (2) and picrate (3) anions and the iodide salt of the (E)-N₄Me₅⁺ cation (4), which were synthesized by protonation of 1 with hydrochloric (2) and picric (3) acids and by methylation of 1 with methyl iodide (4), respectively. Compounds 2–4 were characterized by analytical (elemental analysis and mass spectrometry) and spectroscopic (¹H/¹³C NMR, IR/Raman and UV spectroscopies) methods. Protonation and methylation of 1 to form the (E)-N₄Me₄H⁺ (compounds 2 and 3) and (E)-N₄Me₅⁺ (compound 4) cations, respectively, appears to occur at the terminal nitrogen atoms, in keeping with the results of the NBO analysis and the higher stabilization energy of the conformations with a protonated/methylated terminal nitrogen atom. The geometry optimization by the B3LYP/6-31+G(d,p) method points at very weak N3–N4 bonds (N4 = protonated/methylated nitrogen atom), which explains the formation of dimethylammonium picrate in the thermal decomposition of picrate salt 3 and suggests that dialkylaminium radicals (R₂N⁺˙) are involved in the decomposition pathway.