Bis(dimethylammonium) 2,2′-(1,3,6,8-tetraoxo-2,7-diazapyrene-2,7-diyl)diacetate

The asymmetric unit of title compound, 2C2H8N+·C18H8N2O8 2−, comprises one crystallographically independent dimethylammonium cation and half of a 2,2′-(1,3,6,8-tetraoxo-2,7-diazapyrene-2,7-diyl)diacetate dianion. The anion lies on an inversion centre and the two carboxylate groups are in trans positions based on the naphthaleneteracarboxylic diimide group. The crystal packing is stabilized by N—H⋯O hydrogen bonds between cations and anions, as well as by π–π interactions between the naphthaleneteracarboxylic diimide groups [centroid–centroid distance = 4.812 (3) Å].


Comment
The assembly of functionalised organic molecules in the solid state has attracted much attention in crystal engineering and materials science (Pantos et al., 2007). The prediction of organic crystal structures is a central aim of the development of successful synthetic strategies. In general, precise control over solid state assembly processes will facilitate the synthesis of complex functional materials imbued with desirable optical, electronic, magnetic properties starting from carefully chosen yet relatively simple molecular precursors (Pigge, 2011). We are interested in utilizing acid-functionalized naphthalaleneteracarboxylic diimide derivative as starting materials in crystal engineering approaches to a range of functional organic materials. Herein we report an unexpected organic salt compound 2(C 2 H 8 N). (C 18 Table 1. The overall hydrogen bonding interaction makes a 12-atom ring and a 4-atom ring, as shown in Figure 2. On the other hand, because of the large π-conjugated skeleton in the naphthalaleneteracarboxylic diimide moiety, the strong intermolecular π···π interactions are formed with the perpendicular distance between planes of 3.32 Å. Therefore, the crystal packing of I is stabilized both by N-H···O hydrogen bonds and π···π interactions.

Experimental
A mixture of 1,4,5,8-naphthalaleneteracarboxylic diimide-N,N'-diacetic acid (38.32 mg), 4,4'-bipyridyl (31.24 mg) in DMF (3 ml) was sealed in a 25 ml Teflon-lined stainless steel reactor and heated at 373 K for 72 h. Single crystals of the title compound were obtained after cooling the solution to room temperature, and washed with DMF. The yield is calculated 60%.

Refinement
All H atoms were located in difference maps and refined independently with isotropic displacement parameters.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.