Packing polymorphism in the crystal structure of 4,5-dimethoxy-2-nitrobenzyl acetate

The title compound shows two packing polymorphs, in which the molecular structures are planar and essentially similar. One crystal shows intermolecular C—H⋯O and π–π interactions, while the other crystal exhibits several modes of intermolecular C—H⋯O interactions.


Chemical context
Polymorphism is of interest in crystallization, phase transition, material synthesis and the pharmaceutical industry because differences in the crystal packing and/or conformation of compounds with the same formula can change the chemical and physical properties, including solubility, bioavailability and so forth (Moulton & Zaworotko, 2001;Matsuo & Matsuoka, 2007;Yu, 2010). We have been investigating silane coupling agents and thiols with distal functional groups protected by photolabile 2-nitrobenzyl groups (Edagawa et al., 2012). During the course of photoremoval studies of these materials, we found that the simple ester, 4,5-dimethoxy-2nitrobenzyl acetate, which releases acetic acid on photo-irradiation, forms two different types of crystals, orange rods and yellow needles. Here, we report the crystal structures of these two polymorphs of the title compound.

Supramolecular features
Although the two crystals crystallize in the same space group (P2 1 /c) with Z 0 = 1, their packing modes are different. In the orange crystal, the molecules are connected by an intermolecular C-HÁ Á ÁO interaction [C11-H11BÁ Á ÁO4 i ; symmetry code: (i) 1 À x, À 1 2 + y, 3 2 À z; Table 1] between the methoxy group and the carbonyl group, forming a helical chain along the b axis as shown in Fig. 2, left. In addition, a interaction between the benzene rings with a centroidcentroid distance of 3.6087 (11) Å links the chains to be stacked along the c axis. In the yellow crystal, the molecules located in the plane perpendicular to the ac plane are connected by C-HÁ Á ÁO interactions (Table 2) between methoxy groups [C10-H10BÁ Á ÁO6 ii ; symmetry code: (ii) 1 À x, 1 À y, 2 À z] and between acetyl groups [C9-H9BÁ Á ÁO4 iii ; symmetry code: (iii) Àx, À 1 2 + y, 1 2 À z], forming a sheet structure parallel to (302) (Fig. 2, right).
In the orange crystal, the molecules are stacked in columnar structures viainteractions along the c axis ( Fig. 3, left). In contrast, nointeractions are observed in the yellow crystal. The molecules are therefore terraced along the diagonal line of the a and c axes as shown in Fig. 3, right. As a result of these packing differences, the volume of the unit cell of the orange crystal is larger than that of the yellow one, i.e., the orange crystal contains slightly more void space than the yellow one. This would account for the predominant growth of the yellow crystals.

Synthesis and crystallization
4,5-Dimethoxy-2-nitrobenzyl alcohol (0.714 g, 3.35 mmol), acetic anhydride (0.63 ml, 6.66 mmol), Et 3 N (1 ml) and CH 2 Cl 2 (20 ml) were placed in a 100 mL flask, and the mixture was stirred at ambient temperature overnight. The mixture was extracted with CH 2 Cl 2 (20 ml Â 3), washed with brine, dried over MgSO 4 , and evaporated to give a yellow solid (0.773 g, 90% yield). The solid was crystallized by slow evaporation from a mixed solution of ethyl acetate and hexane  The molecular structures of the title compound polymorphs, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.
(1:1). Orange crystals were occasionally obtained in small amounts, but the yellow crystals grew predominantly.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were located geometrically and refined using a riding model, with C-H = 0.99 Å and U iso (H) = 1.2U eq (C) for methylene H atoms, C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for aromatic H atoms, and C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for methyl H atoms.