Crystal structure of dimethyl 4,4′-dimethoxybiphenyl-3,3′-dicarboxylate

The title compound is an intermediate in the synthesis of linkers for coordination polymers. Centrosymmetric molecules are packed along the a axis to form corrugated layers parallel to the ac plane.


Chemical context
The title compound is an intermediate in the synthesis of 4,4 0dimethoxybiphenyl-3,3 0 -biphenyldicarboxylic acid, an organic linker for use in the synthesis of coordination polymers (Lundvall et al., 2016). The title compound, C 18 H 18 O 6 , has previously been reported (Wang et al., 2009;Kar et al., 2009), however, its crystal structure was undetermined up until now.

Structural commentary
The asymmetric unit of the title compound comprises one half of the molecule, with an inversion centre located at the midpoint of the benzene-benzene bond (Fig. 1). The benzene rings are coplanar due to symmetry. This is somewhat unexpected since a slight torsion between the two rings is a common feature in biphenyl compounds. The methyl carboxylate substituents are oriented trans relative to the benzene-benzene bond, and the plane of the substituent makes a dihedral angle of 18.52 (8) relative to the parent benzene ring. The methoxy substituent is nearly coplanar with the parent benzene ring, and a torsion angle ISSN 2056-9890 C5-C4-O1-C9 of only À5.22 (15) is observed. The methyl groups of the methyl carboxylate and methoxy substituents are oriented away from each other to accommodate the steric demands of these groups.

Supramolecular features
The molecules are packed in the unit cell with the axis of the biphenyl scaffolds parallel to each other. The axis of the biphenyl moiety is oriented approximately 20 off the a axis of the unit cell (Fig. 2), and the molecules form corrugated layers extending parallel to the ac plane (Fig. 3). The packing is not directed by strong intermolecular bonding since the shortest OÁ Á ÁH contact is about 2.5 Å (Table 1). However, weak C-HÁ Á ÁO interactions between neighbouring molecules seem to have an influence on the crystal packing (Fig. 4). The molecular structure of the title compound with atom labels and 50% probability displacement ellipsoids. Non-labelled atoms are generated by the symmetry code (Àx + 1 2 , Ày + 1 2 , Àz). H atoms have been omitted for clarity.

Figure 4
Graphical representation of the shortest intermolecular OÁ Á ÁH contacts, illustrated as dashed blue lines.

Figure 3
Packing diagram of the title compound viewed along the a axis. H atoms have been omitted for clarity.

Synthesis and crystallization
The title compound was synthesized by a slightly modified procedure of the method described by Wang et al. (2009). Synthetic details are given in the Supporting Information of our recent contribution (Lundvall et al., 2016). Single crystals suitable for structure determination were obtained by recrystallizing the title compound from chloroform solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were positioned geometrically at distances of 0.95 (CH) and 0.98 Å (CH 3 ) and were refined using a riding model with U iso (H) = 1.2U eq (CH) and U iso (H)=1.5U eq (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.