N,N-Dimethoxyimidazolium Derivatives as Ion Pair Constituents of Energetic Redox Couples : Model Studies by Thermal Analysis and Crystallography

By utilizing an expedient anion exchange protocol and starting with an improved synthesis of 2-azido-1,3-dimethoxyimidazolium hexafluorophosphate, a series of six metathetical salts, including the energetic nitrate and perchlorate as well as the halides, was prepared. We focused on the crystallographic and thermal characterization of these compounds, representing intrinsically reactive hybrid organic / inorganic salts, and conducted a small comparative study with the related 2-methyl-1,3-dimethoxyimidazolium salts. The latter compounds were expected to thermally disintegrate less sluggishly since the covalent azido-group as intrinsic blasting initiator is missing. Both 2-azido- and 2-methylimidazolium nitrate exhibit a very sharp thermal onset of complete disintegration. In contrast, the respective perchlorates display a continuous thermogravimetric weight loss over a broad temperature range. Additionally, X-ray single crystal structure determinations were performed for eight of the new compounds as well as for two 2-azido-1,3-dimethoxyimidazolium salts with a mixed anion composition. Phase purity was also checked for five of the congeners using powder X-ray diffraction (Pawley fitting). Notably, in the 2-azido-imidazolium cation, the intra-ring bonds of the N¹−C² type were found to be significantly shorter than N¹−C⁴ type bonds, and a characteristic asymmetry between the two N¹−C²−N(azido) bond angles was observed. The experimental cation geometry was reproduced by DFT calculations on selected compounds.