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), C(5)H(12)N(2)O·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, C(5)H(14)N(2)O(2+)·2Br(−), measured at 142 K, crystallized in the ortho­rhom­bic space group P2(1)2(1)2(1). 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.