Crystal structures of methyl 3,5-dimethylbenzoate, 3,5-bis(bromomethyl)phenyl acetate and 5-hydroxybenzene-1,3-dicarbaldehyde

The crystals of methyl 3,5-dimethylbenzoate are composed of strands of C—H⋯O=C bonded molecules, which are further arranged into layers. As a result of the presence of two bromomethyl substituents in 3,5-bis(bromomethyl)phenyl acetate, molecular dimers formed by crystallographically non-equivalent molecules are connected to structurally different two-dimensional aggregates in which the bromine atoms participate in Br⋯Br bonds of type I and type II. In the case of 5-hydroxybenzene-1,3-dicarbaldehyde,which possesses three donor/acceptor substituents, the molecular association in the crystal creates a close three-dimensional network comprising Caryl—H⋯Ohydroxy, Cformyl—H⋯Oformyl and O—H⋯Oformyl bonds.


Chemical context
Studies on molecular recognition of carbohydrates by artificial receptors revealed that macrocyclic compounds bearing two flexible side-arms represent effective and selective receptors for complexation of glucopyranosides. The binding properties of these compounds depend on the nature of their building blocks, among others, the type of bridging units that connect two aromatic platforms (Lippe & Mazik, 2013, 2015Amrhein et al., 2016. The design of such receptor architectures was inspired by the results of our crystallographic studies on receptor-carbohydrate complexes (Mazik et al., 2005; for recent examples, see Kö hler et al., 2020Kö hler et al., , 2021. For the syntheses of macrocycles consisting of benzenebased bridges, various 2-or 5-substituted benzene-1,3-dicarbaldehydes have proven to be useful starting materials. Benzene derivatives with methyl or bromomethyl groups in positions 1 and 3 are used to prepare the latter compounds. The crystal structures of three 1,3,5-substituted benzenes, serving as precursors for the syntheses of the macrocyclic compounds mentioned above, are described in this work.

Structural commentary
The title compounds 1 and 3 crystallize in the monoclinic system (space group P2 1 /c, Z = 4), whereas compound 2 crystallizes in the triclinic space group P1 with two independent but conformationally similar molecules (A and B) in the asymmetric unit of the cell. In compound 1 (Fig. 1), the plane through the methyloxycarbonyl unit is tilted at an angle of 8.70 (8) with respect to the benzene ring. In the independent molecules of 2 (Fig. 2), the planes passing through the ester units are inclined at angles of 62.9 (1) and 81.3 (1) , respectively, to the plane of their arene ring. The two bromine atoms of each molecule are located on opposite sides of the benzene ring. In the crystal of the 5-hydroxybenzene-1,3-dicarbaldehyde (3) (Fig. 3), the molecule deviates slightly from planarity, with the formyl groups rotated out of the benzene ring at angles of 4.43 (16) and 4.04 (16) .

Supramolecular features
In the crystal structure of 1, the molecules are arranged into layers extending parallel to the crystallographic [101] plane (see Fig. 4). Within a given layer, the molecules are linked in strands via C-HÁ Á ÁO C bonds [d(HÁ Á ÁO) 2.57 Å ; Table 1], with a methyl H atom acting as the donor. No directional interactions are present between the molecular strands of a layer. With the participation of a H atom of the methyl ester unit, the linkage between the molecules of adjacent layers occurs by C-HÁ Á Á contacts (Nishio et al., 2009) with a HÁ Á ÁCg distance of 2.77 Å . Fig. 5 shows a packing excerpt of the crystal structure viewed in the direction of the layer normal.
The excerpt of the crystal structure of 2 shown in Fig  Perspective view of the molecular structure of 2. Anisotropic displacement ellipsoids are drawn at the 50% probability level.

Figure 3
Perspective view of the molecular structure of 3. Anisotropic displacement ellipsoids are drawn at the 50% probability level.

Figure 1
Perspective view of the molecular structure of 1. Anisotropic displacement ellipsoids are drawn at the 50% probability level.

Figure 4
Packing diagram of 1 viewed down the crystallographic b-axis.
an eight-membered ring motif. In the direction of the crystallographic a-axis, the connection of the dimers occurs through -Á (face-to-face) interactions (Tiekink & Zukerman-Schpector, 2012) with a centroid-centroid distance of 3.653 (1) Å and an offset of 1.592 Å between the interacting arene rings. Viewing the crystal structure of compound 3 in the direction of the a-axis reveals a stacking arrangement of molecules (Fig. 8). Along the stacking axis the centroid-centroid distance of 3.735 (1) Å between consecutive molecules indicates the presence of offsetinteractions. As is obvious from Fig. 9, showing the mode of non-covalent bonding in the crystal, the   Mode of intermolecular non-covalent interactions in the crystal structure of 3. The cyclic supramolecular synthons are marked by colour highlighting.  Table 3], thus creating a supramolecular synthon with the graph set R 4 4 (17) (Etter, 1990;Etter et al., 1990;Bernstein et al., 1995) in which four molecules take part. The OH group is also involved in formation of an inversionsymmetric ring motif of the structure R 2 2 (8). Another supramolecular motif corresponding to the R 2 2 (14) graph set is formed by the formyl groups of inversion-related molecules.
Suitable crystals of compounds 2 and 3 for X-ray analysis were obtained by slow evaporation from a hexane solution, while crystals of 1 were grown from a subcooled melt.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Hydrogen atom H1 in 3 was located in a difference-Fourier map and freely refined. Other H atoms were positioned geometrically and refined isotropically using a riding model with C-H = 0.93-0.98 Å and U iso (H) = 1.2-1.5U eq (C).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.27708 (14) 0.33954 (14) 0.48553 (8)   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. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.33 e Å −3 Δρ min = −0.28 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.