4,4′-Dimethoxybiphenyl-3,3′-dicarboxylic acid

The title compound, C16H14O6, was recrystallized under solvothermal conditions. The molecules are located on inversion centres, with one complete molecule generated from the asymmetric unit by inversion. There are intramolecular O—H⋯O hydrogen bonds involving the carboxylic acid group and the O atom of the adjacent methoxy group. In the crystal, molecules are linked via O—H⋯O hydrogen bonds, forming chains propagating along [100]. The chains are linked via C—H⋯O hydrogen bonds, forming sheets parallel to (001).

The title compound, C 16 H 14 O 6 , was recrystallized under solvothermal conditions. The molecules are located on inversion centres, with one complete molecule generated from the asymmetric unit by inversion. There are intramolecular O-HÁ Á ÁO hydrogen bonds involving the carboxylic acid group and the O atom of the adjacent methoxy group. In the crystal, molecules are linked via O-HÁ Á ÁO hydrogen bonds, forming chains propagating along [100]. The chains are linked via C-HÁ Á ÁO hydrogen bonds, forming sheets parallel to (001).
We acknowledge the support from the Norwegian Research Council (project 190980), inGAP and the Department of Chemistry, UiO.
Supporting information for this paper is available from the IUCr electronic archives (Reference: QM2105). The title compound has previously been reported (Wang et al., 2009) as an intermediate in the synthesis of an arylamide. The crystal structure was however not reported in this publication.
The structure of the title compound C 16 H 14 O 6 , has an orthorhombic Ibam symmetry. The asymmetric unit of the compound contains one half of the molecule, with the complete molecule being generated by an inversion centre. The two benzene rings appear as planar relative to each other and the carboxylic acid groups are located in a trans fashion with regards to the bond between the benzene rings. Biphenyl compounds commonly feature a torsion angle between the benzene rings, and the relatively large thermal parameters of the atoms furthest away from the molecular axis could indicate that a small torsion angle is present. Thus, the apparent planar configuration of the benzene rings might be considered a crystallographical artifact. Intramolecular hydrogen bonding between H1 and O3 directs the orientation of the hydroxyl group. Intermolecular hydrogen bonds between the O1 oxygen atoms of neighbouring molecules arrange the molecules in one-dimensional zigzag chains. These chains are further packed to form two-dimensional layers stabilized by hydrogen bonds between the carbonyl oxygen (O2) and one aromatic hydrogen (H5). It is worth noting that the carboxylic acid dimer motif thus is absent in this structure. The molecules are ordered along the c axis in a staggered motif with an intermolecular distance equal to one half of the c axis. This distance might indicate some weak π-π stacking interaction between the two-dimensional layers.

Experimental
The title compound was synthesized by a slightly modified version of the method used by Wang et al. (2009).
In the synthesis of methyl 5-iodo-2-methoxybenzoate, the reaction time was increased from 30 to 60 minutes.
In the Ullmann-coupling of 2 equivalents of methyl 5-iodo-2-methoxybenzoate to form dimethyl 4,4′-dimethoxy-3,3′dicarboxylate, the reaction temperature was increased to 225 °C and the reaction time was set to 8 h.
In the synthesis of the title compound, dimethyl 4,4′-dimethoxy-3,3′-dicarboxylate and potassium hydroxide was stirred in a mixture of water and THF under reflux for 18 h. The mixture was concentrated under reduced pressure, washed with diethyl ether and acidified with nitric acid. The product was separated from the mixture by filtration and washed with water. The 1 H NMR spectrum of the title compound is in good agreement with what was reported by Wang et al. (2009).
The title compound (151 mg, 0.5 mmol) was subjected to solvothermal conditions (H 2 O, 100 °C for 2 days) in the precence of Ca(NO 3 ) 2˙4 H 2 O (118 mg, 0.5 mmol) and NaOH (40 mg, 1.0 mmol). The procedure did not yield the desired MOF, the title compound was however recrystallized into single crystals suitable for X-ray diffraction.

Refinement
The structure was refined by full-matrix least squares using SHELXL97 (Sheldrick, 2008) as implemented in the WinGX suite (Farrugia, 2012). H-atoms were positioned geometrically at distances of 0.82 (OH), 0.93 (CH) and 0.96 Å (CH 3 ) and refined using a riding model with U iso (H)=1.2 U eq (CH) and U iso (H)=1.5 U eq (OH and CH 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.