A new polymorph of 2,6-dimethoxybenzoic acid

A new crystalline form of 2,6-dimethoxybenzoic acid, C9H10O4, crystallizing in a tetragonal unit cell has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent molecule with a synplanar conformation of the carboxy group. The sterically bulky o-methoxy substituents force the carboxy group to be twisted away from the plane of the benzene ring by 65.72 (15)°. The carboxy group is disordered over two sites about the C—C bond [as indicated by the almost equal C—O distances of 1.254 (3) and 1.250 (3) Å], the occupancies of the disordered carboxym H atoms being 0.53 (5) and 0.47 (5). In the known orthorhombic form reported by Swaminathan et al. [Acta Cryst. (1976), B32, 1897–1900], due to the antiplanar conformation adopted by the OH group, the molecular components are associated in the crystal in chains stabilized by linear O—H⋯O hydrogen bonds. However, in the new tetragonal polymorph, molecules form dimeric units via pairs of O—H⋯O hydrogen bonds between the carboxy groups.

A new crystalline form of 2,6-dimethoxybenzoic acid, C 9 H 10 O 4 , crystallizing in a tetragonal unit cell has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent molecule with a synplanar conformation of the carboxy group. The sterically bulky o-methoxy substituents force the carboxy group to be twisted away from the plane of the benzene ring by 65.72 (15) . The carboxy group is disordered over two sites about the C-C bond [as indicated by the almost equal C-O distances of 1.254 (3) and 1.250 (3) Å ], the occupancies of the disordered carboxym H atoms being 0.53 (5) and 0.47 (5). In the known orthorhombic form reported by Swaminathan et al. [Acta Cryst. (1976), B32, 1897-1900, due to the antiplanar conformation adopted by the OH group, the molecular components are associated in the crystal in chains stabilized by linear O-HÁ Á ÁO hydrogen bonds. However, in the new tetragonal polymorph, molecules form dimeric units via pairs of O-HÁ Á ÁO hydrogen bonds between the carboxy groups.

Comment
In this paper it is reported the crystal structure of a new polymorph, (I), of 2,6-dimethoxybenzoic acid, produced unexpectedly during an attempt to synthesize cocrystals of boronic acid with of 2,6-dimethoxybenzoic acid. The known form, (II) ( Fig. 3), of of 2,6-dimethoxybenzoic acid is orthorhombic in the space group P2 1 2 1 2 1 and crystallizes with one molecule in the asymmetric unit (Swaminathan et al., 1976;Bryan & White, 1982;Portalone, 2009). In (II), due to the antiplanar conformation adopted by the OH group, the molecular components are associated in the crystal in chains stabilized by linear O-H···O hydrogen bonds.
The title new polymorph (I) is tetragonal in the space group P4 1 2 1 2. The asymmetric unit of (I) comprises a non-planar independent molecule, as the o-methoxy substituents force the carboxy group to be twisted away from the plane of the phenyl ring by 65.72 (15)° (Fig. 1). The carboxy group, which adopts a synplanar conformation, is almost completely disordered, as The pattern of bond lengths and bond angles of the phenyl ring is consistent with that reported in the structure determination of (II), and a comparison of the present results with those obtained for similar benzene derivatives in the gas phase (Schultz et al., 1993;Portalone et al., 1998) shows no appreciable effects of the crystal environment on the ring deformation induced by substituents. Analysis of the crystal packing of (I), (Fig. 2), shows that the molecular components form the conventional dimeric units observed in benzoic acids (Leiserowitz, 1976;Kanters et al., 1991;Moorthy et al., 2002). Indeed, the structure is stabilized by usual intermolecular C 2 2 (8) O-H···O interactions (Etter et al., 1990;Bernstein et al., 1995;Motherwell et al., 1999) (Table 1) which link the molecules into dimers through the disordered carboxy moieties [symmetry code: (i) -y + 1, -x + 1, -z + 3/2].

Experimental
Polymorph (I) was formed during cocrystallization in a 1:1 molar ratio of 2,6-dimethoxybenzoic acid (1 mmol, Sigma Aldrich at 99% purity) and phenylboronic acid (1 mmol, Sigma Aldrich at 97% purity). The two components were dissolved in water (10 ml) and gently heated under reflux for 3 h. After cooling the solution to an ambient temperature, only one crystal suitable for single-crystal X-ray diffraction was grown by slow evaporation of the solvent after two weeks. Unfortunately, any attempts to produce more crystals of polymorph (I) by repeating the crystallization conditions were unsuccessful. Crystallization of 2,6-dimethoxybenzoic acid carried out under a wide range of different sets of conditions (different solvents, different molar ratio, different cosolute molecules) led systematically to the orthorhombic polymorph.

Refinement
All H atoms were identified in difference Fourier maps, but for refinement all C-bound H atoms were placed in calculated positions, with C-H = 0.97 Å (phenyl) and 0.97-0.98 Å (methyl), and refined as riding on their carrier atoms. The U iso supplementary materials sup-2 values were kept equal to 1.2U eq (C, phenyl). and to 1.5U eq (C, methyl). The remaining two half H atoms of the carboxy group were freely refined and their occupancy factors constrained to sum to unity. In the absence of significant anomalous scattering in this light-atom study, Friedel pairs were merged. Fig. 1. The molecular structure of (I), showing the atom-labelling scheme. Displacements ellipsoids are at the 50% probability level.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )