Hexane-1,6-diammonium dinitrate

The hexane-1,6-diammonium cation of the title compound, C6H18N2 2+·2NO3 −, lies across a crystallographic inversion centre and shows significant deviation from planarity in the hydrocarbon chain. This is evident from the torsion angle of −64.0°(2) along the N—C—C—C bond and thse torsion angle of −67.1°(2) along the C—C—C—C bonds. An intricate three-dimensional hydrogen-bonding network exists in the crystal structure, with each H atom on the ammonium group exhibiting bifurcated interactions to the nitrate anion. Complex hydrogen-bonded ring and chain motifs are also evident, in particular a 26-membered ring with graph-set notation R 4 4(26) is observed.

The hexane-1,6-diammonium cation of the title compound, C 6 H 18 N 2 2+ Á2NO 3 À , lies across a crystallographic inversion centre and shows significant deviation from planarity in the hydrocarbon chain. This is evident from the torsion angle of À64.0 (2) along the N-C-C-C bond and thse torsion angle of À67.1 (2) along the C-C-C-C bonds. An intricate three-dimensional hydrogen-bonding network exists in the crystal structure, with each H atom on the ammonium group exhibiting bifurcated interactions to the nitrate anion. Complex hydrogen-bonded ring and chain motifs are also evident, in particular a 26-membered ring with graph-set notation R 4 4 (26) is observed.

Comment
The crystal structure of the title compound (I) adds to our current ongoing studies of long-chained diammonium salts. Colourless needle-like rectangular crystals of hexane-1,6-diammonium dinitrate were synthesized and formed as part of our structural chemistry study of the inorganic mineral acid salts of hexane-1,6-diamine. A search of the Cambridge Structural Database (Version 5.30, February 2009 release; Allen, 2002) revealed that this compound had not previously been determined.
The diammonium hexane chain lies across a crystallographic inversion centre and hence the asymmetric unit contains one nitrate anion and one-half of the hexane diammonium cation. The hydrocarbon chain is also not extended as is common in long chained hydrocarbons but shows significant folding and deviation from planarity. This is clearly evident from the torsion angle along the N1-C1-C2-C3 bond (-64.0°(2)) and along the C1-C2-C3-C3 i bond (-67.1° (2)). Selected torsion angles can be found in Table 1. The molecular structure of (I) is shown in Figure 1.  Table 2. Since the hydrogen bonding network is complex, we focus on one particularly interesting hydrogen-bonding ring motif in the structure. Figure 4 shows a view of two diammonium cations and two nitrate anions (viewed down the c axis) that are hydrogen bonded together to form a large, 26-membered ring motif with graph set notation R 4 4 (26). Another smaller ring motif is evident as a result of the bifurcated hydrogen-bond interaction with the nitrate anion and this ring has the graph-set notation R 2 1 (4) but is not depicted graphically. Chain motifs also exist and were identified with Mercury (Macrae et al.), but again, these are not shown graphically.

Experimental
Compound (I) was prepared by adding 1,6-diamino-hexane (0.50 g, 4.30 mmol) to 55% nitric acid (2 ml, 42.5 mmol) in a sample vial. The mixture was then refluxed at 363 K for 2 h. The solution was cooled at 2 K h -1 to room temperature.
Colourless rectangular needles of hexane-1,6-diammonium dinitrate were collected and a suitable single-crystal was selected for the X-ray diffraction study.