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

The redetermined crystal structures of hexane-1,6-diammonium dichloride, C(6)H(18)N(2)(2+) x 2 Cl(-), (I), hexane-1,6-diammonium dibromide, C(6)H(18)N(2)(2+) x 2 Br(-), (II), and hexane-1,6-diammonium diiodide, C(6)H(18)N(2)(2+) x 2 I(-), (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) degrees, 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) degrees. All three salts exhibit thermotropic polymorphism, as is evident from differential scanning calorimetry analysis and variable-temperature powder X-ray diffraction studies.