Covalent azides have been investigated for more than hundred years. Hydrazoic acid (HN3) for instance was synthesized for the first time by Curtius in 1890. Aside from HN3, the halogen azides XN3 (X = F, Cl, Br, I) are the simplest azides. Only IN3 has been structurally characterized by single-crystal X-ray diffraction to date. The growth of suitable single crystals of halogen azides in general is difficult owing to their extreme sensitivity towards small pressure variations.
Stephan Schulz, University of Duisburg-Essen, Essen, Germany, and colleagues present the single-crystal X-ray structure of bromine azide, BrN3. In contrast to IN3, which forms chains, BrN3 forms a helical structure in the solid state. Such a structural feature has not been previously observed in covalent p-block azide chemistry.