HBr or not HBr? That is the question: crystal structure of 6-hydroxy-1,4-diazepane-1,4-diium dibromide redetermined.

Liu et al. [Chin. J. Struct. Chem. (1996). 15, 371-373] reported the structure of 6-hydroxy-1,4-diazepane di(hydrogen bromide), C5H12N2O·2HBr, which was interpreted in terms of neutral diazepane and HBr molecules. We found, however, ample evidence that the formation of an organic salt, consisting of a diammonium cation and two bromide anions, is more plausible. This interpretation is also in agreement with thermogravimetric analysis and with the observed solution behaviour. The crystal structure of 6-hydroxy-1,4-diazepane-1,4-diium dibromide, C5H14N2O2+·2Br-, measured at 142 K, crystallized in the orthorhombic space group P212121. The structure displays O-H...Br and N-H...Br hydrogen bonding. Contact distances are given. A search in the Cambridge Structural Database for the singly-bonded H-Br moiety revealed a total of 69 structures. The question, whether these structures really include HBr as neutral molecules or rather Br- anions and a protonated substrate such as an amine, is addressed.