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, C₆H₁₈N ₂²⁺·2Br^-, (II), and hexane-1,6- diammonium diiodide, C₆H₁₈N₂ ²⁺·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 hydro-carbon backbone. The chloride and bromide salts also show marked similarities in their hydrogen-bonding inter-actions, with subtle differences evident in the hydrogen-bond lengths reported. Bifurcated inter-actions are exhibited between the N-donor atoms and the halide acceptors in the chloride and bromide salts. The iodide salt is very different in mol-ecular structure, packing and inter-molecular inter-actions. The hydro-carbon 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 hydro-carbon 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.