Hydrogen-bonding motifs and thermotropic polymorphism in redetermined halide salts of hexamethylenediamine.

The redetermined crystal structures of hexane-1,6-diammonium dichloride, C6H18N22+·2Cl−, (I), hexane-1,6-diammonium dibromide, C6H18N22+·2Br−, (II), and hexane-1,6-diammonium diiodide, C6H18N22+·2I−, (III), are described, focusing on their hydrogen-bonding motifs. The chloride and bromide salts are isomorphous, with both demonstrating a small deviation from planarity [173.89 (10) and 173.0 (2)°, respectively] in the central C—C—C—C torsion angle of the hydro­carbon backbone. The chloride and bromide salts also show marked similarities in their hydrogen-bonding inter­actions, with subtle differences evident in the hydrogen-bond lengths reported. Bifurcated inter­actions are exhibited between the N-donor atoms and the halide acceptors in the chloride and bromide salts. The iodide salt is very different in mol­ecular structure, packing and inter­molecular inter­actions. The hydro­carbon chain of the iodide straddles an inversion centre and the ammonium groups on the diammonium cation of the iodide salt are offset from the planar hydro­carbon backbone by a torsion angle of 69.6 (4)°. All three salts exhibit thermotropic polymorphism, as is evident from differential scanning calorimetry analysis and variable-temperature powder X-ray diffraction studies.