Abstract
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.
Highlights
6-Hydroxy-1,4-diazepane was first synthesized by Saari et al (1971) and has been used as a tridentate facially coordinating metal-complexing agent (Liu et al, 1997a)
The free ligand has been isolated as a dihydrogen bromide, C5H12N2OÁ2HBr, and its crystal structure has been reported [Liu et al, 1996; Cambridge Structural Database (CSD; Groom et al, 2016) refcode TOKTIW]
3.72% and wR2 = 6.31%) were marginally better than those of model B (R1 = 3.84% and wR2 = 6.63%). These results clearly show that the crystal structure analysis alone does not allow discrimination with certainty between the two models
Summary
6-Hydroxy-1,4-diazepane (dazol) was first synthesized by Saari et al (1971) and has been used as a tridentate facially coordinating metal-complexing agent (Liu et al, 1997a). The free ligand has been isolated as a dihydrogen bromide, C5H12N2OÁ2HBr, and its crystal structure has been reported [Liu et al, 1996; Cambridge Structural Database (CSD; Groom et al, 2016) refcode TOKTIW]. The authors postulated crystallization of the neutral diazepane as a free base together with two HBr molecules. From a chemical point of view, the formation of discrete HBr molecules beside a basic entity is surprising, even taking into account that the situation in the solid state does not necessarily reflect the well-known acid–base properties in aqueous solution. NMR spectroscopy, and (ii) prepared a crystalline sample of the title compound (see Scheme), repeated the structure determination reported by Liu et al (1996) and performed additional thermogravimetric measurements to elucidate the solid-state properties
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