4,4′-[(5-Carboxy-1,3-phenylene)bis(oxy)]dibenzoic acid

In the title compound, C21H14O8, the central benzene ring makes dihedral angles of 77.8 (6) and 75.9 (5)° with the outer benzene rings. In the crystal, molecules are linked by O—H⋯O hydrogen bonds involving carboxyl groups, forming one-dimensional ladders. Two-dimensional layers are formed by interpenetration of these one-dimensional ladders.

In the crystal structure, two benzene rings, β(composed of C 8 -C 13 ) and γ (composed of C 15 -C 20 ) are connected to the center ring (α, composed of C 1 -C 6 ) by ether bond. The dihedral angle between α and β is 77.8 (6)°, and between α and γ is 75.9 (5)° (Fig. 1). Strong intermolecular O-H···O hydrogen bonds are formed between the carboxylic acid groups of neighboring molecules (Table 1), which link the molecules to one-dimensional supra-molecular ladder (Fig. 2). The interpenetration among the one-dimensional molecular ladders which are parallel produce two-dimensional layer (Fig. 3).

Experimental
The title compound was synthesized by a modified literature method (Neogi et al.2009). Methyl 3,5-dihydroxylbenzoate (1.68 g,10 mmol) was dissolved in DMF (50 ml). To this solution was added K 2 CO 3 (7 g,51 mmol) and 4-fluorobenzonitrile (2.4 g,20 mmol). The mixture was heated under reflux for 2 days. The resulting solution was poured in 250 ml icecold water and kept over-night. The yellow compound was filtered and washed several times with water. The yellow compound (3.73 g, 10 mmol) was allowed to reflux with 6 N NaOH solution (50 ml) for 12 h, cooled to room temperature and acidified with HCl (6 N). Colorless crystalline product was obtained and isolated by filtration, washed with water and dried in vacuum. Zn(NO 3 ) 2 (0.075 g,0.25 mmol), 4,4′-(5-carboxy-1,3-phenylene)bis(oxy)dibenzoic acid (0.098 g,0.25 mmol), were mixed in water (5 ml). The mixture were placed in a 25 ml Teflon-lined stainless steel autoclave and heated autogenously under pressure for 2 d at 393 K. After cooling to room temperature, the block-shaped colourless crystals were obtained.

Refinement
All hydrogen atoms bonded to O and C were fixed in ideal positions, with C-H = 0.93 (aromatic) and O-H = 0.82 Å, and treated as riding on their parent atoms with U iso (H)=0.08 Å 2 .  The molecular structure of the title compound with the atom-numbering scheme, with 50% probability displacement ellipsoids.

Figure 2
The packing of title compound, showing one ladder of molecules connected by O-H···O hydrogen bonds (dashed lines).
H atoms not involved in hydrogen bonding have been omitted for clarity.

Figure 3
The interpenetration among the one-dimensional ladders, showing two-dimensional layer. H atoms not involved in hydrogen bonding have been omitted for clarity. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.28 e Å −3 Δρ min = −0.22 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.