Crystal structure of a new polymorph of 3-acetyl-8-methoxy-2H-chromen-2-one

A new polymorphic form of 3-acetyl-8-methoxy-2H-chromen-2-one is described and compared with the previously reported polymorph. In the crystal, hydrogen bonds, π–π interactions and antiparallel C=O⋯C=O interactions give rise to a helical supramolecular architecture


Chemical context
Derivatives of 2H-chromen-2-one are some of the most important heterocycles in natural and synthetic organic chemistry. These substances are bioactive compounds and have a wide range of applications in the medical field (Gaudino et al., 2016) showing, for example, anti-HIV, antimutagenic, anticancer and antitumor activities among others (Vekariya & Patel, 2014). They are synthesized using classical methodologies such as the Pechmann or Knoevenagel reactions, as well as recent methodologies such as the metathesis cyclization (Salem et al., 2018) or alkynoates cyclization (Liu et al., 2018).
The disposition of the crystalline lattices of coumarin derivatives is driven by a great variety of intermolecular interactions (Santos-Contreras et al., 2009). This working group has reported the participation ofstacking interactions, hydrogen-bonding and dipole-dipole interactions involving the carbonyl group (Gó mez-Castro et al., 2014) in the determination of the 1D, 2D and 3D supramolecular assemblies of crystalline structures for different compounds (Gonzá lez-Padilla et al., 2014). This report describes the structure of a second polymorph of the title compound and the importance of C-HÁ Á ÁO, C OÁ Á ÁC O andstacking intermolecular interactions in crystal packing.

Supramolecular features
The crystal network of the title compound (polymorph II) is assembled by zigzag shaped molecular layers that extend approximately in the (012) and (012) planes, forming an angle of 116.2 . In the flat section of the zigzag layer R 3 3 (18) motifs are formed by C6-H6Á Á ÁO2 ii and C12-H12BÁ Á ÁO11 i hydrogen bonds (Table 1). These intermolecular interactions impart stability to the 2D sheet, while weak C14-H14AÁ Á ÁO2 iii interactions generate an R 2 3 (16) motif at the intersection of the planes (Fig. 2). Adjacent layers, separated by a distance of 3.4083 (5) Å , are connected bystacking interactions with a centroid-to-centroid distance of 3.600 (9) Å and a slippage of 1.160 Å . In addition to the stacking, layers are stabilized by antiparallel C OÁ Á ÁC O interactions (Allen et al., 1998) involving the acetyl group separated by a distance of 3.1986 (17) Å .
The supramolecular array of polymorph II exhibits a helical conformation, like polymorph I (Li et al., 2012). However, in polymorph II the C OÁ Á ÁC O interactions form the central axis of the helix whilstinteractions between the aromatic and lactone rings, aligned in a head-to-tail conformation, control the rotation of the structure. In polymorph I, the helical axis is built by hydrogen bonds and face-to-facestacking interactions of the benzofused rings (Fig. 3). For polymorph I, a complete rotation of the helix is performed in 11.5 Å , while in polymorph II the displacement of the helix in a whole rotation is 12.4 Å .

Hirshfeld surface and 2D fingerprint plots
In order to better understand the crystal packing of both polymorphs, Hirshfeld surface analyses and 2D fingerprint plots were carried out using Crystal Explorer 17.5 (Turner et al., 2017). From the analysis of the Hirshfeld surfaces (Fig. 4), it is evident that there are differences between the chemical environments of these two identical molecules. In Fig. 4, the Hirshfeld surfaces for polymorph I show a series of strong short contacts (big red dots) corresponding to hydrogen bonds stabilizing the 2D sheets. The planar areas above and below the rings are whereinteractions (small red dots) take place, giving rise to the 3D network. On the other hand, polymorph II is stabilized by a short directional hydrogen bond and the sum of weak interactions with longer contact distances than in polymorph I. This suggests that polymorph II may be the less stable between these two phases of the title compound. To quantitatively compare polymorphs I and II in terms of their crystal packing, 2D fingerprint plots were developed and analysed. The character of the fingerprints plots for both polymorphs is similar, with small differences in the relative contributions of each type of interaction to the Hirshfeld surface. The weak interactions include CÁ Á ÁH (C-HÁ Á Á ), CÁ Á ÁO (C OÁ Á ÁC O, C OÁ Á Á) and CÁ Á ÁC (-), as well as short directional interactions such as HÁ Á ÁO (Fig. 5).
Although polymorphs I and II exhibit the same type of intermolecular interactions, the way these common interactions contribute to the packing in each polymorph differs in each case. The minor differences in which weak intermolecular interactions contribute to the formation of the crystal (Fig. 6), give rise to distinct polymorphs as suggested by Hasija & Chopra (2019). As can be seen in Fig ORTEP plot of polymorph II of the title compound with the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Packing of molecules in polymorph II by C-HÁ Á ÁO hydrogen bonding and the packing of parallel sheets connected via C OÁ Á ÁC O and weakinteractions. Dotted lines depict the intermolecular interactions.
forces in the crystal formation of both polymorphs are HÁ Á ÁH and OÁ Á ÁH interactions, but CÁ Á ÁO and CÁ Á ÁH short contacts in polymorph I make slightly bigger contributions to build the lattice, while in polymorph II, hydrogen bonding andstacking contribute in greater proportions. . This structure, which we call polymorph I, is assembled by parallel flat sheets that extend along the b axis. It is worth mentioning that the acetyl coumarin without any substituent also forms at least two polymorphic forms (A and B; Munshi et al., 2004) with subtle differences in intermolecular interactions, which include weak C-HÁ Á ÁO and C-HÁ Á Á inter-actions. Form A crystallizes with head-to-head stacking being favored during nucleation, while form B prefers a head-to tailstacking. This is similar to the two polymorphs of the title compound.

Synthesis and crystallization
The Hirshfeld surfaces for polymorphs I and II showing both sides of the molecules. Red areas represent contacts shorter than the sum of the van der Waals radii, blue areas represent zones where the shortest distance between atoms is larger than the sum of van der Waals radii and white areas are zones close to the sum of van der Waals radii.

Figure 5
Comparison of several intermolecular interactions (blue areas) involved in the crystal packing of polymorphs I and II by decomposition of twodimensional fingerprint plots. Green areas represent a greater abundance of close contacts and the full fingerprint appears beneath each decomposed plot as a grey shadow.

Figure 6
Relative contributions to the Hirshfeld surface for the major intermolecular contacts in polymorphs I and II.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2

3-Acetyl-8-methoxy-2H-chromen-2-one
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.