Crystal structures of dimethyl 5-iodoisophthalate and dimethyl 5-ethynylisophthalate

The 5-iodo- and 5-ethynyl-substituted dimethyl isophthalates show molecular frameworks with methyl carboxylate moieties being tilted or perfectly planar with respect to the benzene ring, respectively. Crystal structures feature a three- or two-dimensional supramolecular aggregation in the iodo and ethynyl derivatives, respectively, supported by C—H⋯I and C—H⋯O hydrogen bonding as well as I⋯O and π–π interactions.


Chemical context
In recent years, the design of solid porous framework materials (MacGillivray, 2010;Furukawa et al., 2013;Eddaoudi et al., 2015) has become a very important topic in the field of supramolecular crystal engineering (Desiraju et al., 2011). Associated with it, so-called linker molecules featuring a geometrically rigid structure frequently being of linear, trigonal or tetrahedral shape and having carboxylic acid functions as terminal groups play a key role in building such systems (Lin et al., 2006;Hausdorf et al., 2009;Zheng et al., 2010). In the course of the synthesis of the respective linkers, the title compounds (I) and (II), both being 5-substituted dimethyl isophthalates, are much used intermediates. However, these compounds are not only synthetically significant but also show interesting structures in the crystalline state, as demonstrated herein.

Structural commentary
The molecular structures of the title compounds, (I) and (II), are illustrated in Fig. 1a and 1b, respectively. Taking into account experimental error, the bond distances within the isophthalate framework agree well with those found in the crystal structure of dimethyl isophthalate (Gallagher, 2012). Compound (I) crystallizes in the orthorhombic space group Pna2 1 with one molecule in the asymmetric unit. The molecule adopts a twisted conformation with the mean planes defined ISSN 2056-9890 by the methyl carboxylate moieties inclined at angles of 12.6 (2) and 6.0 (2) with respect to the plane of the benzene ring. Compound (II) crystallizes in the orthorhombic space group Pnma with the molecule located on a symmetry plane, i.e. the molecule is perfectly planar. However, the molecule adopts approximate C 2v symmetry with the atoms C2, C5, C11 and C12 lying on a non-crystallographic bisecting symmetry plane.
Furthermore,arene interactions with a centroid-centroid distance of 3.566 (1) Å and a slippage of 1.325 Å between the interacting aromatic rings stabilize the crystal structure along the stacking axis of the molecular sheets.

Database survey
The search in the Cambridge Structural Database (CSD, Version 5.38, update May 2017; Groom et al., 2016) for metasubstituted derivatives of dimethyl isophthalate excluding their metal complexes, solvates and salts gave 18 hits. None of these compounds represents a 5-halogen-and 5-alkynylsubstituted dimethyl isophalate. The parent compound, dimethyl isophthalate (CSD refcode GOHRUS; Gallagher & Mocilac, 2012) crystallizes in space group Pna2 1 with two conformationally similar molecules in the asymmetric unit. The independent molecules participate in different ways in non-covalent bonding. One of them is involved in the formation of linear strands with the molecules connected by C-H aryl Á Á ÁO C bonds. Interstrand association is accomplished byarene stacking. Molecules related by the twofold screw axis are also linked via C-H aryl Á Á ÁO C Symmetry code: (i) Àx þ 1; Ày þ 1; z À 1 2 .

Figure 2
Packing diagram of compound (I) viewed down the a axis. Dashed lines represent hydrogen-bonding interactions. Table 2 Hydrogen-bond geometry (Å , ) for (II).

Figure 3
Packing excerpt of compound (II) viewed down the b axis. Dashed lines represent hydrogen-bonding interactions.

Figure 1
Perspective view of the molecular structures of the title compounds, (a) (I) and (b) (II), with atom labelling. Anisotropic displacement ellipsoids are drawn at the 40% probability level.
bonding to form helical strands. In addition, these strands are stabilized bystacking forces.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were positioned geometrically and refined using a riding model with C-H distances of 0.94-0.98 Å and U iso (H) = 1.5U eq (C-methyl) or U iso (H) = 1.2U eq (C) for other H atoms.

1,3-Dimethyl 1-iodocyclohexa-3,5-diene-1,3-dicarboxylate (I)
Crystal data 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. Refined as a 2-component twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq I1 0.81504 (2) 0.66115 (2)    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.17 e Å −3 Δρ min = −0.18 e Å −3 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.