Naphthalene-2,6-dicarboxylic acid–1-methylpyrrolidin-2-one (1/2)

The asymmetric unit of the title compound, C12H8O4·2C5H9NO, contains one half-molecule of naphthalene-2,6-dicarboxylic acid (NDA) and one molecule of 1-methylpyrrolidin-2-one (NMP): the NDA molecules lie on the crystallographic twofold rotation axes. In the crystal, the components are linked by strong O—H⋯O hydrogen bonds and C—H⋯O interactions.

The asymmetric unit of the title compound, C 12 H 8 O 4 Á-2C 5 H 9 NO, contains one half-molecule of naphthalene-2,6dicarboxylic acid (NDA) and one molecule of 1-methylpyrrolidin-2-one (NMP): the NDA molecules lie on the crystallographic twofold rotation axes. In the crystal, the components are linked by strong O-HÁ Á ÁO hydrogen bonds and C-HÁ Á ÁO interactions.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SU2234).
Naphthalene-2,6-dicarboxylic acid-1-methylpyrrolidin-2-one (1/2) B. Wu, G. Peng, Y. Cheng, X. Li and J. Liu Comment Naphthalene-2,6-dicarboxylic acid (NDA) is an important monomer for producing polyester and polyurethane materials and liquid crystal polymers (LCP). During the manufacturing process, the impurities in NDA, such as 6-formyl-2-naphthoic acid (FNA), debase the quality of the products dramatically. So the purification of NDA is very important however, this process is difficult (Nagase et al., 2004). Although many methods have been proposed in this field, they are either too complex or not cost effective. Recently, we have obtained crystals of the title compound, a mixture of NDA and N-Methyl Pyrrolidone (NMP). We call this phenomenon adductive crystallization and intend to apply this crystallization technique to the purification of NDA.
The title compound crystallized in the orthorhombic space group Fdd2, and the molecular structure is shown in Fig. 1.
The asymmetric unit contains one half-molecule of NDA and one molecule of NMP. The pyrrolidone group has an envelope conformation with atom C9 at the flap. The dihedral angle between the mean planes of the naphthalene ring of the NDA molecule and the pyrrolidone ring of the NMP molecule is 22.39 (15)°.
In the crystal the NDA and NMP molecules are linked by strong O-H···O hydrogen bonds and C-H···O interactions ( Fig. 2 and Table 1).

Experimental
The title compound was obtained by putting 0.1 g of Naphthalene-2,6-dicarboxylic acid (NDA) into 1 ml of N-Methyl Pyrrolidone (NMP) at room temperature and then leaving the mixture in the freezer, which was maintained at 255 K, for 72 h. During the process, we observed the gradual disappearance of the NDA powder and the appearance of colourless needle-like crystals of the title compound.

Refinement
In the final cycles of refinement, in the absence of significant anomalous scattering effects, Friedel pairs were merged and   Table 1 for details].