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

Abstract

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.

Mechanistic Insights into the Formation of 1-Alkylidene/Arylidene-1,2,4-triazolinium Salts: A Combined NMR/Density Functional Theory Approach

“…Upon the addition of tetrabutylammonium iodide, however, the respective product iodide salts ( 5a–e and 6a–e ) could be obtained by applying a recently reported selective precipitation protocol from acetonitrile and diethyl ether. 41 As predicted by the revised reaction mechanism, a regio- and stereoselective synthesis of all 1-benzylidene-1,2,4-triazolinium salts was achieved with isolated yields ranging from 40 to 60% ( 5a–e and 6a–e , Scheme 3 B). In contrast to the 1-alkylidene salts ( 4a–c ), a complete conversion with the aromatic aldehydes was achieved in shorter reaction times (16–72 vs 5 h) and a lower molar excess (neat vs 2 equiv), which is likely ascribable to a preferred formation of products 5a–e and 6a–e through the mesomeric stabilization of the phenyl moieties.…”

“…Additionally, liquid/liquid extraction procedures with aqueous solutions and chromatographic purification steps were found to be unfit since, during these steps, water-induced product degradation was observed. Upon the addition of tetrabutylammonium iodide, however, the respective product iodide salts ( 5a–e and 6a–e ) could be obtained by applying a recently reported selective precipitation protocol from acetonitrile and diethyl ether . As predicted by the revised reaction mechanism, a regio- and stereoselective synthesis of all 1-benzylidene-1,2,4-triazolinium salts was achieved with isolated yields ranging from 40 to 60% ( 5a–e and 6a–e , Scheme B).…”

“…After addition of Et 2 O, the product iodides could be isolated. 41 Further purification was achieved by recrystallization from MeOH/Et 2 O mixtures.…”

“…The formed colored solution was separated and mixed with a solution of tetrabutylammonium iodide dissolved in CH 3 CN. After addition of Et 2 O, the product iodides could be isolated . Further purification was achieved by recrystallization from MeOH/Et 2 O mixtures.…”

2-Arylazoimidazoles Revamped by Quarternization or Dimerization; Another Gain in Functionality of an Industrial Dyestuff Family by Task-Specific Side-Chain Substituents

“…The instrumentation of hot stage nicroscopy (HSM) and Thermogravimetric Analysis (TGA) is described in detail in a preceding communication. 22 Diffraction intensity data for the single crystal structure determinations of 1, 3, 4S and 7 • 0.5 H2O were recorded with a Oxford Diffraction Xcalibur Ruby Gemini diffractometer and those for 2 • 0.5 4-VBnCl, 5 and 7 were recorded with a Bruker D8 Quest Photon 100 diffractometer, using MoKα radiation (λ = 0.7107 Å) or in the case of 4S and 7 CuKα radiation (λ = 1.54184 Å) . The crystal structures were solved by Direct Methods with SHELXT 27 or SIR 2002 28 and refined by full-matrix least-squares techniques using SHELXL 29 .…”