2-Aminoterephthalic acid dimethyl ester

Single crystals of the title compound, C10H11NO4, an intermediate in the industrial synthesis of yellow azo pigments, were obtained from the industrial production. The molecules crystallize as centrosymmetic dimers connected by two symmetry-related N—H⋯O=C hydrogen bonds. Each molecule also contains an intramolecular N—H⋯O=C hydrogen bond. The dimers form stacks along the a-axis direction. Neighbouring stacks are arranged into a herringbone structure.

Single crystals of the title compound, C 10 H 11 NO 4 , an intermediate in the industrial synthesis of yellow azo pigments, were obtained from the industrial production. The molecules crystallize as centrosymmetic dimers connected by two symmetry-related N-HÁ Á ÁO C hydrogen bonds. Each molecule also contains an intramolecular N-HÁ Á ÁO C hydrogen bond. The dimers form stacks along the a-axis direction. Neighbouring stacks are arranged into a herringbone structure.
In order to determine the crystal structure of compound I and to search for different crystallographic phases, hydrates or solvates, a polymorph screening was performed. Different crystallization methods were used including (1) recrystallization from various solvents and solvent mixtures by heating and subsequent slow cooling, (2) overlaying a solution of the compound by an anti-solvent, (3) diffusion of an anti-solvent into a solution of the compound via the gas phase. The solvents included the most common organic solvents, e.g. dimethylsulfoxide, N,N-dimethylacetamide, N-methylpyrrolidone, ethers, esters, alcohols as well as acids, bases and water. According to X-ray powder diffraction no additional phases were formed.
Single crystals were obtained from technical synthesis. The structure of the molecule is shown in Fig. 2. The central six-membered ring is planar [mean deviation from best plane: 0.002 (2) Å]. The angle between the plane of the six-membered ring and the planes of the carboxy groups attached to C3 and C6 is 3.3 (2) and 5.4 (2)°, respectively. The molecules form centrosymmetric dimers connected by two symmetry-related N-H···O═C hydrogen bonds (Table 1). The second H atom of the NH 2 group is involved in an intramolecular N-H···O═C hydrogen bond. The crystal packing is shown in Figures   3 and 4. The molecules stack along the crystallographic a-direction. The interplanar distance in the stack is 3.396 (2) Å.
Neighbouring stacks show a herringbone arrangement.

Experimental
The crystals of compound I were obtained from Clariant GmbH, Frankfurt am Main.

Refinement
The H atoms attached to C atoms were geometrically positioned and were constrained using C planar -H = 0.95 Å, C methyl -H = 0.98 Å, U iso (H) = 1.2U eq (C planar ) and U iso (H) = 1.5U eq (C methyl ). The H atoms at the NH 2 group were taken from a difference Fourier synthesis and were refined. The torsion angles about the O-C methyl groups were refined. Fig. 1. Preparation of Pigment Yellow 213 in which compound (I) is used as preproduct. Fig. 2. Molecular structure of compound I, with atom labels and anisotropic displacement ellipsoids (drawn at 50% probability level) for non-H atoms.   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 > σ(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.