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

• Published in: Acta crystallographica. Section C, Crystal structure communications Crystal structure communications, 2017-09, Vol.73 (9), p.660-666
• Main Authors: Martin, Kenneth L., Breton, Gary W.
• Format: Article
• Language: English
• Subjects: Aromatic compounds; catenated N atoms; Computers; Crystal structure; Crystallography; Dimers; Equilibrium; Fish; Free radicals; Molecular structure; NMR; Nuclear magnetic resonance; PCB; Polychlorinated biphenyls; proton NMR; radical dimer; Radicals; Shielding; shielding chemical shift; tetrazane; urazole radicals; X-rays; π‐stacking interactions
• ISSN: 2053-2296; 0108-2701; 2053-2296; 1600-5759
• DOI: 10.1107/S2053229617010786

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, C24H28N6O4, 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 1H NMR spectrum. Armed with this information, we were able to decipher the more complicated 1H NMR spectrum obtained from a dimer that was monosubstituted at the ortho position with a methyl group. ortho‐Substituted 1‐arylurazole radicals favor the formation of tetrazane dimers rather than existing predominantly as independent free radicals like many other urazole radicals. The X‐ray crystal structure of one of these dimers agreed well with a computational model and readily explained unusual shielding effects observed for some signals in the 1H NMR spectrum.