2,2′-[2,3,5,6-Tetramethyl-p-phenylenebis(methyleneoxy)]dibenzoic acid

The asymmetric unit of the title compound, C26H26O6, contains only a half-molecule, the other half being generated by an inversion center. The two carboxyphenoxymethyl units occupy the 1,4-positions of the central aromatic ring. The central ring and the six linked C atoms are almost planar, with a maximum deviation of 0.0286 (17) Å, and the plane makes a dihedral angle of 75.50 (6)° with the benzene ring. In the crystal, strong O—H⋯O hydrogen bonds between the carboxyl groups of adjacent molecules and C—H⋯π interactions link the molecules into zigzag chains along (220) and (10); the two types of chain are arranged alternately, forming a three-dimensional framework.

The asymmetric unit of the title compound, C 26 H 26 O 6 , contains only a half-molecule, the other half being generated by an inversion center. The two carboxyphenoxymethyl units occupy the 1,4-positions of the central aromatic ring. The central ring and the six linked C atoms are almost planar, with a maximum deviation of 0.0286 (17) Å , and the plane makes a dihedral angle of 75.50 (6) with the benzene ring. In the crystal, strong O-HÁ Á ÁO hydrogen bonds between the carboxyl groups of adjacent molecules and C-HÁ Á Á interactions link the molecules into zigzag chains along (220) and (110); the two types of chain are arranged alternately, forming a three-dimensional framework.

Comment
The title compound (I), was designed as a ligand for preparing MOF materials. This paper reports its single crystal structure in the solid state. The asymmetric unit contains only half of the molecule, with the other half generated by an inversion center at (0,1/2,0). (Fig 1.). The two branches of benzenic-carboxylate acid groups of the title compound occupy the 1, 4 positions of the central aromatic ring to form a line. The central ring with its linked six C atoms, similar to the one observed in Britton (2003), is almost planar, with a maxima deviation of -0.0286 (17)° for C13 and makes a dihedral angle of 75.50 (6)°w ith the benzene ring. Strong C-H···π bond was observed in the structure, with a perpendicular distance of 2.700 (3)° to the C9 C10-C12-C9 i C10 i C12 i ring plane (i:-x, -y, -z). Meanwhile, the structure of the two carboxylate groups linked by the strong H-bond (Table 1) is comparable to that described in Bailey & Brown (1967) and Glidewell et al. (2004). Strong hydrogen bondings between the carboxylate groups of the adjacent molecules link the molecules of title compound into zig-zag chains along (220) and (-110) directions, respectively (Fig. 2), and these two chains were arranged alternatively to form a 3D framework (Fig. 3).
The precipitate was filtrated and was put into 30 ml methanol. NaOH aqueous solution (20 ml, 2 mol/L) was added and the solution was stirred for 8 hours under refluxing. After cooling to room temperature, the pH value of the clear solution was adjusted to 2 by dilute hydrochloric acid. The clear solution was allowed to evaporate slowly under inert atmosphere.
Prismatic crystals of the title compound were obtained after 2 days. The crystals were filtered, washed by cold EtOH and dried in air.

Refinement
All H atoms were positioned geometrically and refined using a riding model with C-H = 0.930 Å and U iso (H) = 1.2 U eq (C) for aromatic hydrogens, and with C-H = 0.960 Å and U iso (H) = 1.5 U eq (C) for H atoms of the methyl groups, and with C-H = 0.970 Å and U iso (H) = 1.2 U eq (C) for H atoms of the methylene group, and with O-H = 0.820 Å and U iso (H)

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.