3,3′-(p-Phenylenedimethylene)di-1H-imidazol-1-ium bis(4-nitrobenzoate)–4-nitrobenzoic acid (1/2)

The asymmetric unit of the title compound, C14H16N4 2+·2C7H4NO4 −·2C7H5NO4, comprises one-half of the 3,3′-(p-phenylenedimethylene)di-1H-imidazol-1-ium dication, which lies on an inversion centre, one 4-nitrobenzoate anion and one 4-nitrobenzoic acid molecule. In the crystal, the components are linked into a two-dimensional network parallel to (110) by O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds.


Comment
Over the past few years, efforts have been focused on the investigation of coordination polymers with flexible ligands. Diimidazole flexbile ligands such as 1,4-bis(1H-imidazol-1-yl)methylbenzene (bix) find numerous applications in constructing metal-organic coordination polymers as they can rotate freely about methylene carbon atoms to adjust to the coordination environment. We report here the crystal structure of the title compound.
The asymmetric unit comprises one-half of a bix 2+ dication lying on an inversion centre, one 4-nitrobenzoate anion and one neutral 4-nitrobenzoic acid molecule (Fig. 1). Bond distances and angles are normal (Chen et al., 2010).
In the crystal structure, the dications, anions and neutral 4-nitrobenzoic acid molecule are interlinked by O-H···O, N-H···O and C-H···O hydrogen bonds (Table 1), forming a two-dimensional hydrogen-bonded network parallel to the (110).
The mixture was stirred and refluxed for 1 h and then filtered. The resulting colourless solution was allowed to stand in air for two weeks. Colourless crystals of the title compound suitable for X-ray diffraction analysis were obtained by slow evaporation of the solution.

Refinement
H atoms were positioned geometrically [O-H = 0.85 Å, N-H = 0.86 Å and C-H = 0.93 or 0.97 Å] and refined using a riding model, with U iso (H) = 1.5U eq (O) and 1.2U eq (C,N). Fig. 1. The asymmetric unit of the title compound, showing the atomic numbering and 30% probability displacement ellipsoids. Atoms labelled with the suffix A are generated by the symmetry operation (-x, 1-y, -z).

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 supplementary materials sup-3 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.