Benzoic acid–2,2′-biimidazole (2/1)

In the title compound, C6H6N4·2C7H6O2, the asymmetric unit contains a half-molecule of biimidazole and one benzoic acid molecule. The unit cell contains two biimidazole molecules and four benzoic acid molecules, giving the reported 2:1 ratio of benzoic acid to biimidazole. The biimidazole molecule is located on an inversion center (passing through the central C—C bond). Strong N—H⋯O and O—H⋯N hydrogen bonds link the benzoic acid molecules with the neutral biimidazole molecules, which lie in planar sheets. In the crystal packing, the parallel sheets are related by a twofold rotation axis and an inversion centre, respectively, forming an interwoven three-dimensional network via weak C=O⋯π intermolecular interactions between neighboring molecules.

In the title compound, C 6 H 6 N 4 Á2C 7 H 6 O 2 , the asymmetric unit contains a half-molecule of biimidazole and one benzoic acid molecule. The unit cell contains two biimidazole molecules and four benzoic acid molecules, giving the reported 2:1 ratio of benzoic acid to biimidazole. The biimidazole molecule is located on an inversion center (passing through the central C-C bond). Strong N-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds link the benzoic acid molecules with the neutral biimidazole molecules, which lie in planar sheets. In the crystal packing, the parallel sheets are related by a twofold rotation axis and an inversion centre, respectively, forming an interwoven three-dimensional network via weak C OÁ Á Á intermolecular interactions between neighboring molecules.

Comment
Compounds containing the 2,2'-biimidazole moiety have been the focus of several investigations not only due to their biological activity, but also due to their contribution to the field of crystal engineering (Matthews, et al. 1990;Tadokoro & Nakasuji, 2000). In these compunds weak interactions, such as C-H···O and C=O···π, play crucial roles in building the overall three-dimensional structure (Mori & Miyoshi, 2004;Li & Yang, 2006;Gao et al., 2009).
The asymmetric unit of compound (I) contains one benzoic acid and 1/2 neutral biimidazole molecule, in which the imidazole rings are coplanar ( Fig. 1). Each biimidazole molecule is linked to two benzoic acids via strong N-H···O and O-H···N hydrogen bonds (Table 1) twithin planar sheets ( Figure 2). These sheets further assemble to layers via weak C=O···π (see Table 1, Cg1 for centre of N1/C1/N2/C3/C2) interactions between neighboring molecules and arrange alternatively and across along b and c axis in two-dimensional structure, and the dihedral angle of the planes are 92.7°. In contrast, two groups of these parallel layers on a twofold rotation axis and inversion centre forming a zigzag conformation along c axis in whole three-dimensional network as shown in Fig. 3.

Experimental
Benzoic acid (0.25 g, 2 mmol) and biimidazole (1 mmol) were dissolved in water(10 ml) by adding 1.4 ml of 2 M HCl while stirring. The solutions were stirred for 1 h, then filtered. Filtrate was left to stand at room temperature. Crystals suitable for data collection appeared after a few weeks by slow evaporation of the aqueous solvent.

Refinement
H atoms attached to C atoms were placed in geometrically idealized positions, with Csp 2 = 0.93 Å, and constrained to ride on their carrier atoms, with U iso (H) = 1.2 Ueq (C). H atoms attached to N1 and O1 atoms were located in difference Fourier maps and refined with U iso (H for N) = 0.06 Å 2 and U iso (H) = 1.5 Ueq (O); N-H distance is 0.88 (5) Å and the O-H distance is 0.856 Å. Fig. 1. A view of the structure of compound (I) with displacement ellipsoids drawn at the 50% probability level, the biimidazole sits on a center of symmetry passing through the C1-C1 bond. Symmetry code: (i) 1 -x, 1 -y, 1 -z.  Benzoic acid-2,2'-biimidazole (2/1) 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 > σ(F 2 ) is used only for calculating Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq N1 0.3655 (