Disordered structure of propane-1,2-diaminium dichloride

In the title compound, C3H12N2 2+·2Cl−, the cations are disordered over two well resolved positions in a 0.525 (13):0.475 (13) ratio. The disorder involves two C atoms which assume positions that make an almost mirror-symmetrical system. Similar disorder is observed both at room temperature and at 120 (1) K. The conformation of the NCCN chain in both components is close to trans (the torsion angles ca ±170°), while that of CCCN chain is close to gauche (±50°). In the crystal, a network of relatively strong N—H⋯Cl hydrogen bonds connects the cations and anions into one-cation-deep layers parallel to (001); there are R 2 4(8) and R 2 4(11) ring motifs within the plane. The planes are only loosely connected by van der Waals contacts and electrostatic interactions between cations and anions.

In the title compound, C 3 H 12 N 2 2+ Á2Cl À , the cations are disordered over two well resolved positions in a 0.525 (13):0.475 (13) ratio. The disorder involves two C atoms which assume positions that make an almost mirror-symmetrical system. Similar disorder is observed both at room temperature and at 120 (1) K. The conformation of the NCCN chain in both components is close to trans (the torsion angles ca AE170 ), while that of CCCN chain is close to gauche (AE50 ). In the crystal, a network of relatively strong N-HÁ Á ÁCl hydrogen bonds connects the cations and anions into onecation-deep layers parallel to (001); there are R 2 4 (8) and R 2 4 (11) ring motifs within the plane. The planes are only loosely connected by van der Waals contacts and electrostatic interactions between cations and anions.

Comment
The study of polycation structure, counterions and the nature of the interaction between low-molecular-weight salts with some electrolites in solutions are very important to understanding behaviour biogenic polyamines under normal physiological conditions. Aliphatic biogenic polyamines in biological systems exist as polycations which interact with nucleic acid polyanions. The crystal structure of the salts of these amines are therefore essential in the modeling of nucleic acids. Complexes formed from DNA and polycations containing primary amine grups are relevant because of their potential use in gene therapy, modifying the conformation and the state of aggregation of DNA (Hosseinkhani et al., 2004;Ziebarth et al., 2009;Itaka et al., 2010). During our study on metal promoted synthesis ofSchiff base complexes derived from various polyamines and salicylaldehyde we isolated the crystals of some salts of polyamines (Pospieszna-Markiewicz et al., 2006, 2007. Here we report another salt, propane-1,2-diaminium dichloride (I, Scheme 1).
It turned out that in the crystal structure the cation is heavily disordered -disorder involves two carbon atoms -between the two positions with the site occupation factors of 0.525 (13) and 0.475 (13). Both alternative positions refined quite well (anisotropically) without any kind of restraints. Fig. 1 shows one of the alternatives (most occupied) and Both alternative cations have opposite signs of torsion angles. The conformation of N1-C2-C3-N4 chain is extended (torsion angles are 172.0 (4)° and -168.5 (4)° for more and less occupied part, respectively) while the C21-C2-C3-N4 torsion angles are 50.0 (10)° and -47.0 (11)°. Such conformation -we will call it tg -is the most popular among the simple propane-1,2-diaminium salts (for instance it was found in the structures of hydrogenarsenate (Todd & Harrison, 2005), bis(6-carboxypyridine-2-carboxylate) (Aghabozorg et al., 2008), or tetrafluoro-beryllium (Gerrard & Weller, 2002). The other possibilities are also observed, for instance in some simple hydrates (e.g. arsenate monohydrate, Lee & Harrison, 2003) the g + gcombination is also reported.
In the crystal structure the strong N-H···Cl hydrogen bond connects molecules into two-dimensional, one-molecule deep layers parallel to (001) plane (Fig. 3). The motifs formed can be described, using graph set notations, as rings R 2 4 (8) and R 2 4 (11). It might be noted that for both alternatives the hydrogen atoms involved in these interactions are practically in the same positions. Each chloride anion accepts three hydrogen bonds, in flattened trigonal pyramid coordination. The layers in turn are loosely connected probably by electrostatic interactions between the charged species (Fig. 4).

Experimental
To a methanol solution (10 ml) of salicylaldehyde (0.043 ml, 0.4 mmol) a methanol solution (10 ml) of 1,2-diaminepropane (0.017 ml, 0.2 mmol) was added dropwise with stirring. After 5 minutes a methanol solution (20 ml) of ErCl 3 .6H 2 O (0.0764 g, 0.2 mmol) was added. The reaction was carried out at room temperature for 75 minutes. The solution volume was than supplementary materials sup-2 reduced to 10 ml by roto-evaporation and after 7-14 days of slow diffusion of THF into the solution at 6 °C white crystals suitable for X-ray were formed.

Refinement
Hydrogen atoms were located geometrically (C(methyl)-H 0.98 Å, C2-H 1.00 Å, C3-H 0.99 Å, N-H 0.91 Å) and refined in a riding model approximation; the U iso values of H atoms were set at 1.2 (1.5 for CH 3 and NH 3 groups) times U eq of their carrier atom. Fig. 1. Anisotropic ellipsoid representation of I together with atom labelling scheme. The ellipsoids are drawn at 50% probability level, hydrogen atoms are depicted as spheres with arbitrary radii; hydrogen bonds are shown as dashed lines. Only major part of the disordered cation is shown.