7-Aminoheptylazanium iodide

The absolute structure of the title compound, [H3N-(CH2)7-NH2]I, has been determined from the diffraction experiment, the Flack parameter refining to −0.02 (2). In the crystal, adjacent symmetry-related cations are connected by head-to-tail R′H2N+—H⋯NH2 R hydrogen bonds, forming chains along [010]. The remaining four H atoms attached to the amino and the azanium group form weak hydrogen bonds to neighbouring iodide anions, producing a three-dimensional hydrogen-bonded network. The amino group and the aliphatic chain of the 7-aminoheptylazanium cation show an exact all-trans conformation, within experimental uncertainties. The azanium group, to fulfill the needs of hydrogen bonding, is twisted out of the plane defined by the C atoms of the aliphatic chain, the C—C—C—N torsion angle being −65.4 (4)°.

The absolute structure of the title compound, [H 3 N-(CH 2 ) 7 -NH 2 ]I, has been determined from the diffraction experiment, the Flack parameter refining to À0.02 (2). In the crystal, adjacent symmetry-related cations are connected by head-totail R 0 H 2 N + -HÁ Á ÁNH 2 R hydrogen bonds, forming chains along [010]. The remaining four H atoms attached to the amino and the azanium group form weak hydrogen bonds to neighbouring iodide anions, producing a three-dimensional hydrogen-bonded network. The amino group and the aliphatic chain of the 7-aminoheptylazanium cation show an exact alltrans conformation, within experimental uncertainties. The azanium group, to fulfill the needs of hydrogen bonding, is twisted out of the plane defined by the C atoms of the aliphatic chain, the C-C-C-N torsion angle being À65.4 (4) .
Our longstanding interest in the structural chemistry of α,ω-diazaniumylalkanes is focused on their versatility as templates for the synthesis of new polyiodides (Reiss & Engel, 2002;Reiss & Engel, 2004;Reiss, 2010). However, only a limited number of high-quality crystal structure determinations on α-azaniumyl-ω-aminoalkane salts have been described (Luciawati et al., 2011;Pienack et al., 2007). Furthermore, the positions of the hydrogen atoms of the hydrogen bond donating groups are not well resolved in all cases (Natarajan et al., 1996, Qian et al., 2007. This contribution presents a rare example of a crystal structure of an α-azaniumyl-ω-aminoalkane without any disorder. The asymmetric unit of the title compound consists of one 7-aminoheptylazanium cation and one iodide anion. The bond lengths and angles within the organic cation are, with C-C bond lengths between 1.497 (5) Å to 1.517 (4) Å and slightly shorter C-N distances, 1.462 (4) Å and 1.481 (4) Å, as expected. The azanium group, to fulfill the needs of hydrogen bonding, is twisted out of the plane defined by the carbon atoms of the all-trans conformation aliphatic chain, the C5-C6-C7-N2 torsion angle being -65.4 (4)° ( Fig.1 and Fig. 3) Cations are connected to symmetry-related units by head-to-tail R'H 2 N + -H···NH 2 R hydrogen bonds. As a result of this primary connection, one-dimensional zigzag chains along [010] are formed (Fig. 1). According to a generally accepted classification (Steiner, 2002), these N + -H···N hydrogen bonds can be described as medium strong. Both hydrogen atoms of the amino group and two of the three hydrogen atoms of the azaniumyl group form hydrogen bonds with neighbouring iodide anions. These weak N-H···I hydrogen bonds (Table 1) connect the above-mentioned chains into a three-dimensional framework ( Fig. 2 and 3). This framework can be classified by graph sets (Etter et al. 1990) as built of two smaller ring motifs [R 2 4 (8) and R 4 6 (12); (Fig. 2)] in the hydrophilic region of the structure and a ring motif R 2 4 (24) that includes the alkyl chains (Fig. 3).
Depending on the reaction conditions, the title compound is sometimes contaminated with a small amount of the darkcoloured α,ω-diazaniumylheptane tetraiodide, (H 3 N-(CH 2 ) 7 -NH 3 )I 4 (Reiss, 2010). To verify the purity of the synthesized material, powder diffraction data of a representive part of the bulk phase were collected on a Huber G600 diffractometer (transmission, Cu Kα1, step width: 0.03°, 20 sec./step). A profile fit (Kraus & Nolze, 2000) on the powder diffraction data based on the structure model obtained from the single-crystal experiment proved the identity of the bulk phase with the investigated single-crystal (Fig. 4). This finding is supported by the Raman spectrum collected which does not show the I 4 2--specific absorption band at 175 cm -1 .

Refinement
All hydrogen atoms were located from a difference Fourier synthesis. The positional parameters of hydrogen atoms of the NH 2 and the NH 3 group were refined with soft N-H distance restraints; the final range of N-H distances is 0.87 (2) -0.90 (2) Å. All hydrogen atoms of the CH 2 groups were refined using a riding model; C-H = 0.97 Å and U iso (H) = 1.2U eq (C).

Special details
Experimental. The Raman spectrum was measured using a Bruker MULTIRAM spectrometer (