Computational, ¹H NMR, and X-ray structural studies on 1-arylurazole tetrazane dimers

Abstract

Nitrogen-centered urazole radicals exist in equilibrium with tetrazane dimers in solution. The equilibrium established typically favors the free-radical form. However, 1-arylurazole radicals bearing substituents at the ortho position favor the dimeric form. We were able to determine the structure of one of the dimers (substituted at both ortho positions with methyl groups), namely 1,2-(2,4-dimethylphenyl)-2-[2-(2,4-dimethylphenyl)-4-methyl-3,5-dioxo-1,2,4-triazolidin-1-yl]-4-methyl-1,2,4-triazolidine-3,5-dione, CHNO, via X-ray crystallography. The experimentally determined structure agreed well with the computationally obtained geometry at the B3LYP/6-311G(d,p) level of theory. The preferred syn conformation of these 1-arylurazole dimers results in the two aromatic rings being proximate and nearly parallel, which leads to some interesting shielding effects of certain signals in the H NMR spectrum. Armed with this information, we were able to decipher the more complicatedH NMR spectrum obtained from a dimer that was monosubstituted at the ortho position with a methyl group.

Factors affecting the dimerization of persistent nitrogen‐centered 1‐phenyl urazole radicals to tetrazanes

“…As noted above and reported previously, urazole radicals 2(a‐d) • exist in an equilibrium with the corresponding dimers 2‐2 that greatly favor the tetrazane dimers (Scheme ). This is most readily confirmed by the observation that 1 H and 13 C NMR spectra may be collected on solutions of these compounds (although the spectra for 2a • and 2b • are very broad at room temperature, vide infra ), whereas for solutions of 1a • and 1b •, in which the urazole radicals are much more prevalent, collection of such data is not possible because of the obscuring paramagnetic nature of the radical form.…”

“…This is most readily confirmed by the observation that 1 H and 13 C NMR spectra may be collected on solutions of these compounds (although the spectra for 2a • and 2b • are very broad at room temperature, vide infra ), whereas for solutions of 1a • and 1b •, in which the urazole radicals are much more prevalent, collection of such data is not possible because of the obscuring paramagnetic nature of the radical form. As reported earlier, we were able to isolate and purify tetrazane 2c ‐ 2c and then analyze its structure via X‐ray crystallography . The N―N bond joining the 2 urazole rings was found to be not unusual in length (1.385 Å) suggesting it was a rather typical bond as might be expected given the predominance of the dimer species in solution.…”

“…In our previous work, we suggested that the tetrazane form 2 ‐ 2 was favored by these particular urazole radicals because, unlike 1a• and 1b• , radicals 2 • could not benefit from the stabilizing effect of delocalization of radical spin into the aromatic ring because of steric hindrance from the ortho substituents that prevent the required coplanarity of the urazole and aromatic ring systems. Thus, we hypothesized that in the absence of such radical stabilization, the dimer form is preferred.…”

“…Oxidation of 1‐arylurazoles 1a,b is known to lead to formation of the corresponding air‐stable N ‐centered urazole radicals ( 1a •, 1b •), which are persistent in solution (Scheme ) . Both of these urazole radicals are known to be in equilibrium with the corresponding N―N tetrazane dimer, 1 ‐ 1 , with the equilibrium thought to only weakly favor the dimer species .…”

“…We recently reported on the oxidation of some 1‐arylurazoles ( 2a‐d ), bearing at least one substituent at the ortho positions of the aromatic ring, that give rise to urazole radicals 2 • that instead more strongly favor the dimer form in solution (Scheme ) . Unlike radicals 1 •, concentration of solutions of all 4 of these radicals afforded the corresponding dimers, 2 ‐ 2 , as stable solids that could be purified by chromatography.…”