N,N′-Bis(2-aminobenzyl)ethane-1,2-diaminium dinitrate

In the title salt, C16H24N4 2+·2NO3 −, both the cation and anion are placed in general positions, although the cation displays non-crystallographic inversion symmetry, with the aliphatic chain extended in an all-trans conformation. The benzene rings are almost parallel, with a dihedral angle between their mean planes of 3.3 (6)°. The nitrate ions are placed in the vicinity of the protonated amine groups, forming efficient N—H⋯O inter-ion hydrogen bonds. Each nitrate ion in the asymmetric unit bridges two symmetry-related cations, forming an R 4 4(18) ring, a common motif in organic ammonium nitrate salts. This results in the formation of chains along [010] with alternating cations and anions. The neutral amine groups are involved in slightly weaker N—H⋯O hydrogen bonds with the nitrate O atoms, and there are also a number of C—H⋯O hydrogen bonds present. The resulting supramolecular structure is based on a two-dimensional network extending in the ab plane.

In the title salt, C 16 H 24 N 4 2+ Á2NO 3 À , both the cation and anion are placed in general positions, although the cation displays non-crystallographic inversion symmetry, with the aliphatic chain extended in an all-trans conformation. The benzene rings are almost parallel, with a dihedral angle between their mean planes of 3.3 (6) . The nitrate ions are placed in the vicinity of the protonated amine groups, forming efficient N-HÁ Á ÁO inter-ion hydrogen bonds. Each nitrate ion in the asymmetric unit bridges two symmetry-related cations, forming an R 4 4 (18) ring, a common motif in organic ammonium nitrate salts. This results in the formation of chains along [010] with alternating cations and anions. The neutral amine groups are involved in slightly weaker N-HÁ Á ÁO hydrogen bonds with the nitrate O atoms, and there are also a number of C-HÁ Á ÁO hydrogen bonds present. The resulting supramolecular structure is based on a two-dimensional network extending in the ab plane.

Comment
The title salt crystallized unexpectedly, when attempting the crystallization of a macrocyclic molecule, resulting from the Schiff condensation between 2,6-diacetylpyridine and N,N′-bis(2-aminobenzyl)ethane-1,2-diamine (Garza Rodríguez, 2010). The synthesis was carried out via a template reaction, using Mn 2+ as metal center, and analytical data showed that the Mn 2+ complex was formed, with nitrate as counter ions. However, this compound is almost insoluble in organic solvents, like MeOH, EtOH, acetone and ethyl acetate, impeding the preparation of single crystals. Only slight solubility was obtained in hot acetonitrile. Slow evaporation of MeCN over 3 weeks afforded a mixture of amorphous brown solids and colourless needle-shaped crystals. We assume that the brown solids should be a mixture of manganese oxides, resulting from the hydrolysis of the complex induced by trace amounts of water and dissolved O 2 . Minutes amounts of H 3 O + are then released, which promote the formation, and finally the crystallization of the nitrate salt of the protonated amine.
Title compound ( Fig. 1) crystallizes in a non-centrosymmetric space group, with all atoms placed in general positions.
However, the dication (C 16 H 24 N 4 ) 2+ presents a non-crystallographic inversion center, with the central aliphatic chain extended in the all-trans conformation. This conformation was previously obtained for the same cation crystallized as ptoluenesulfonate salt, although in that case, the dication was placed on a crystallographic inversion center (Garza Rodríguez et al., 2011). The free amine, for which the X-ray structure is also known (Rodríguez de Barbarín et al., 2007) has a different solid state conformation, although preserving the centrosymmetric character. For the nitrate salt reported here, departure from centrosymmetry is small, as reflected, for example, by the dihedral angle between benzene rings, limited to 3.3 (6)°.
Nitrate ions positions are determined by the formation of hydrogen bonds with ammonium NH 2 + groups in the cation.
Each independent anion, N21 and N25, bridges two cations related by cell translation in the [0 1 0] direction, forming a R 4 4 (18) ring motif (Etter, 1990;Fig. 2). This arrangement seems actually to be common in crystal structures involving ammonium and nitrate species, and R 4 4 (18) motifs are also formed in salts closely related to the title compound, for example with N,N′-dibenzylethane-1,2-diammonium (Liu et al., 2007) or N,N′-bis(4-chlorobenzyl)ethane-1,2-diammonium . For such salts, the crystal structure is invariably based on edge-fused R 4 4 (18) rings, which afford a one-dimensional linear supramolecular structure. In the case of the title compound, chains run along the short b axis, and no significant interchain interactions are observed (Fig. 2

Experimental
An amount of 2,6-diacetylpyridine (735 mg, 4.50 mmol) in ethanol (180 ml) was mixed with the templating reagent Mn(NO 3 ) 2 ·xH 2 O (1.130 g) and refluxed for 30 min. Then, N,N′-bis(2-aminobenzyl)ethane-1,2-diamine (1.302 g, 4.80 mmol, dissolved in 25 ml of ethanol) was slowly added, and the mixture further refluxed for 1 h. The resulting solid was filtered from hot ethanol, washed with cold ethanol, and dried under reduced pressure. A solution of the solid in hot CH 3 CN was left to crystallize for 3 weeks. After this time, only one product was obtained as single crystals, in low yield, which was identified by X-ray diffraction as the title nitrate salt.

Refinement
H atoms for aromatic CH and methylene CH 2 groups were placed in idealized positions, and refined with C-H bond lengths fixed to 0.93Å and 0.97Å, respectively, and U iso (H) = 1.2U eq (C). Amine and ammonium H atoms were found in a difference map, and refined freely, although the geometry for NH 2 group was restrained to sensible target values: bond lengths N-H were restrained to 0.90 (2)Å and H···H separations were restrained to 1.54 (3)Å. Isotropic displacement parameters were computed in this case as U iso (H) = 1.5U eq (N).