Heptane-1,7-diaminium dinitrate

In the title molecular salt, C7H20N2 2+·2NO3 −, the crystal structure exhibits an unusual back-to-back paired double-stacked packing arrangement culminating in an overall double zigzag pattern of the dications. The nitrate anions form a ring around one pair of double-stacked dications. An intricate three-dimensional N—H⋯O and N—H⋯(O,O) hydrogen-bonding network exists in the crystal structure.

In the title molecular salt, C 7 H 20 N 2 2+ Á2NO 3 À , the crystal structure exhibits an unusual back-to-back paired doublestacked packing arrangement culminating in an overall double zigzag pattern of the dications. The nitrate anions form a ring around one pair of double-stacked dications. An intricate three-dimensional N-HÁ Á ÁO and N-HÁ Á Á(O,O) hydrogenbonding network exists in the crystal structure.

Comment
The crystal structure of the title compound (I) adds to our current ongoing investigations of long-chained diammonium inorganic mineral acid salts (van Blerk & Kruger, 2009). Colourless crystals of heptane-1,7-diammonium dinitrate were obtained and analyzed by single-crystal X-ray diffraction techniques. This material forms part of our structural chemistry study of the inorganic mineral acid salts of the n-alkyldiamines. A search of the Cambridge Structural Database (Version 5.32, Allen, 2002) revealed that this compound had not previously been determined.
The asymmetric unit of compound (I) contains one diammonium dication and two nitrate anions with all atoms occupying general positions. The hydrocarbon chain is also fully extended with very slight deviations from planarity chain as is evident from the torsion angles along the hydrocarbon chain (tabulated in Table 1). The molecular structure of (I) is shown in Fig. 1. anions. An extensive three-dimensional hydrogen-bonding network is also formed of N-H···O hydrogen bonds.
A close-up view of selected hydrogen bonding interactions can be viewed in Fig. 3. The three-dimensional hydrogen bonding network is built and linked through hydrogen bonding interactions between the ammonium groups of the dication and the nitrate anions. Clear evidence of bifurcated hydrogen bonding interactions can also be seen in this illustration. The hydrogen bond distances and angles for the title compound (I) can be found in Table 2.

Experimental
Compound (I) was prepared by adding heptane-1,7-diamine (0.50 g, 3.84 mmol) to 55% nitric acid (2 ml, 42.5 mmol, Merck) 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 crystals of heptane-1,7-diammonium dinitrate were collected and a suitable single-crystal was selected for the X-ray diffraction study.

Refinement
H atoms were geometrically positioned and refined in the riding-model approximation, with C-H = 0.97 Å, N-H = 0.89 Å, and U iso (H) = 1.2U eq (C) or 1.5U eq (N). For (I), the highest peak in the final difference map is 1.05 Å from O3 and the deepest hole is 0.87 Å from O3.