Dimethyl 3,3′-dimethoxybiphenyl-4,4′-dicarboxylate

In the title compound, C18H18O6, the biphenyl moiety is twisted with a dihedral angle of 29.11 (10)°. The carbomethoxy groups form C—C—C—O torsion angles of −18.3 (3) and −27.7 (3)° with the attached rings, as a result of steric hindrances from the nearby methoxy groups. In the absence of stacking interactions and with no H⋯O contacts shorter than 2.7 Å, the packing is dominated by weaker van der Waals interactions.

In the title compound, C 18 H 18 O 6 , the biphenyl moiety is twisted with a dihedral angle of 29.11 (10) . The carbomethoxy groups form C-C-C-O torsion angles of À18.3 (3) and À27.7 (3) with the attached rings, as a result of steric hindrances from the nearby methoxy groups. In the absence of stacking interactions and with no HÁ Á ÁO contacts shorter than 2.7 Å , the packing is dominated by weaker van der Waals interactions.

Comment
The title compound is an intermediate in the synthesis of 3,3′-dimethoxy-4,4′-biphenyldicarboxylic acid, an organic linker for use in the synthesis of MOFs (Metal-Organic Frameworks). The title compound has previously been reported (Zhou et al., 2007), but its crystal structure was unknown until this publication.
There is a twist between benzene rings, which is a common feature in biphenyl compounds. The methoxy substituents are nearly coplanar with their parent benzene rings. On the opposite, the methyl carboxylate substituents are not co-planar with the adjacent benzene rings, and the corresponding dihedral angles differ between the two halves of the molecule.
The methyl groups of the methoxy and methyl carboxylate substituents are oriented away from each other to accommodate the sterical demands of these groups. The long axis of the molecules is oriented in the [101] direction and two-dimensional corrugated layers parallel to the ac plane can be imagined. The packing does not appear to be directed by any strong intermolecular bonding, although some long range interaction might influence the ordering of the molecules. Indeed, the carbonyl O atoms O5 and O2 are oriented towards H12 and H2 of neighbouring molecules in a near linear fashion. However, since the O-H distances are very long (>2.7 Å), they are unlikely to be a major factor in the crystal packing.

Experimental
The title compound was synthesized by a slightly modified version of the method used by Zhou et al. (2007). In the Ullmann-coupling of 2 equivalents of methyl 4-iodo-2-methoxybenzoate to form the title compound, the reaction temperature was increased to 225 °C and the reaction time was set to 8 h. The title compound was extracted from the reaction mixture by repeated washing with warm ethyl acetate and subsequent filtering to remove solid particles. The resulting 1 H NMR spectrum is in good agreement with what was reported by Zhou et al. (2007).
Single crystals suitable for XRD analysis were obtained by recrystallizing the title compound from ethyl acetate.

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 0.93 (CH) and 0.96 Å (CH 3 ) and refined using a riding/rotating model with U iso (H)=1.2 U eq (CH) and U iso (H)=1.5 U eq (CH 3 ).  The molecule of the title compound with atom labels and 50% probability displacement ellipsoids. Hydrogen atoms are omitted for clarity. 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.