Planar versus non-planar: The important role of weak C—H⋯O hydrogen bonds in the crystal structure of 5-methylsalicylaldehyde

Wavy layers of molecules are formed in the crystal structure of 5-methylsalicylaldehyde due to weak C—H⋯O interactions between methyl groups and the aromatic ring system. Molecules form columns in which the methyl groups are oriented in opposite directions layer-by-layer along cell axis a. In the molecule, the hydroxyl substituent is bound intramolecularly to the aldehyde group at the ortho position.

The crystal structure of 5-methylsalicylaldehyde (5-MSA; systematic name 2-hydroxy-5-methylbenzaldehyde), C 8 H 8 O 2 , was discovered to be a textbook example of the drastic structural changes caused by just a few weak C-HÁ Á ÁO interactions due to the additional methylation of the aromatic ring compared to salicylaldehyde SA. This weak intermolecular hydrogen bonding is observed between aromatic or methyl carbon donor atoms and hydroxyl or aldehyde acceptor oxygen atoms with d(DÁ Á ÁA) = 3.4801 (18) and 3.499 (11) Å . The molecule shows a distorted geometry of the aromatic ring with elongated bonds in the vicinity of substituted aldehyde and hydroxyl carbon atoms. The methyl hydrogen atoms are disordered over two sets of sites with occupancies of 0.69 (2) and 0.31 (2).

Chemical context
Salicylaldehydes form an important and widely used group of compounds in the pharmaceutical and agrochemical industry (Kirchner et al., 2011). They have a functional role as metabolites in eukaryotic plants and as nematicides (Caboni et al., 2013;Kim et al., 2008). As part of a series of co-crystallization experiments in which the title compound was used as a coformer, single-crystals of 5-methylated salicylaldehyde (5-MSA) were obtained and characterized by single-crystal X-ray diffraction. Its crystal structure is reported herein and compared to the unsubstituted form of salicylaldehyde (SA) [Kirchner et al. (2011); refcode YADJOE in the Cambridge Structural Database (Groom et al., 2016)]. Even though 5-MSA carries just one additional methyl group compared to the latter, a very large difference in melting point is observed. Whereas SA is a liquid at room temperature, 5-MSA is a crystalline solid with a melting point of 328-330 K.

Supramolecular features
The large difference in melting point between SA and 5-MSA is unequivocally related to the different way the two molecules pack in the crystal lattice. Layers of SA molecules are arranged in almost perfect sheets, resulting in a layered structure roughly along the a axis. The distance between these layers of molecules can be analysed by the distance between the centroids (Cg) of the phenyl rings with d(CgÁ Á ÁCg) = 3.7838 (11) Å (Figs. 2 and 3). No intermolecular hydrogenbonding interactions can be detected in the range d(DÁ Á ÁA) = 2.5-3.5 Å .

Figure 2
The crystal packing (DIAMOND; Brandenburg, 1999) of SA viewed along the a axis. -stacking interactions are indicated by blue dashed lines.

Figure 3
The crystal packing (DIAMOND; Brandenburg, 1999)  [d(DÁ Á ÁA) = 2.5-3.5 Å ] apart from van der Waals interactions. Three C-HÁ Á ÁO interactions are present between either aromatic or methyl C atoms and aldehyde or alcohol oxygen atoms: two close to 3.5 Å with C10Á Á ÁO8 = 3.499 (2) Å and C5Á Á ÁO8 = 3.4801 (18) Å and corresponding C-HÁ Á ÁO angles of 152 and 149.3 (13) , respectively. The third and shortest interaction, has a C6Á Á ÁO9 distance of 3.4053 (18) Å and an angle of 138.7 (12) ( Table 1). The latter results in a R 2 2 (8) ring, a graph set very often observed in the centrosymmetric structures of aromatic acids and aldehydes due to the occurrence of inversion centres between molecules (Fig. 4). In this manner, pairs of molecules are connected to each other by weak intermolecular interactions.
The most significant consequence of the additional interactions compared to SA, however, can be seen in the distances between the phenyl rings and the geometry of how they are arranged towards each other. There are two distances between the centroids of the phenyl rings, one within significance range, the other one slightly above, with d(CgÁ Á ÁCg) = 3.7539 (11) and 4.7456 (13)  The crystal packing (DIAMOND; Brandenburg, 1999) of 5-MSA viewed along the a axis. Hydrogen-bonding interactions are shown as blue dashed lines.

Figure 5
The crystal packing (DIAMOND; Brandenburg, 1999) of 5-MSA viewed along the c axis. -stacking interactions are indicated by blue dashed lines drawn between the centroids of the aromatic rings.

Figure 6
The crystal packing (DIAMOND; Brandenburg, 1999) of 5-MSA viewed along the a axis. -stacking interactions are indicated by blue dashed lines drawn between centroids of the aromatic ring. are formed instead, whereby the 5-MSA molecules form columns in which the methyl groups are oriented in opposite directions layer-by-layer along the a axis (Figs. 5 and 6). The stronger -stacking of the aromatic rings combined with the additional weak intermolecular interactions provides a logical explanation for the difference in melting points between SA and 5-MSA and is a perfect textbook example of the drastic structural changes caused by just a few weak C-HÁ Á ÁO interactions due to an additional methylation of the aromatic ring.

Synthesis and crystallization
The title compound, together with a catalytic volume of ethanol solvent, was ground in a mortar and pestle into a dried powder, which was then dissolved in 1.5 mL of the solvent and allowed to crystallize. Single crystals of suitable quality were selected directly from the dried crystalline precipitate.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The structure solution was not straightforward. A first attempt to solve the structure in space group P2 1 /c was unsuccessful. The structure solution was carried out in P1 and then transformation using PLATON (Spek, 2009) to the correct space group P2 1 /c took place. The hydrogen atoms of the methyl substituent show disorder with an occupancy of 0.69 (2) at positions H10A, H10B, H10C and 0.31 (2) at positions H10D, H10E, H10F. They were included at idealized positions riding on the parent carbon atom, with isotropic displacement parameters U iso (H) = 1.5U eq (CH 3 ).
Refinement of the corresponding site-occupation factors of the methyl-group hydrogen atoms was carried out using a free variable so that their sum is unity. All other hydrogen atoms were located individually in a difference-Fourier map and refined isotropically. SHELXL (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

2-Hydroxy-5-methylbenzaldehyde
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. Chicken Wire Problem: first structure solution in P1 then transformation using Platon to P2(1)/c (Brandenburg, 1999 Symmetry codes: (i) x+1, −y+1/2, z+1/2; (ii) x+1, y, z; (iii) −x+1, −y+1, −z−1.