N-(3,4-Dimethylphenyl)maleamic acid

The title compound, C12H13NO3, crystallizes with four independent molecules in the asymmetric unit. The N—H bond and the C=O bond in the amide segment are anti to each other. The C=C double bond is cis configured and an intramolecular O—H⋯O hydrogen bond is formed in each molecule. The mean planes through the aromatic ring and the amide group –NHCO– are inclined at angles of 17.4 (3), 20.8 (2), 16.2 (2) and 11.2 (3)° in the four molecules. In the crystal, intermolecular N—H⋯O hydrogen bonds link the molecules into ribbons along the b axis.

The title compound, C 12 H 13 NO 3 , crystallizes with four independent molecules in the asymmetric unit. The N-H bond and the C O bond in the amide segment are anti to each other. The C C double bond is cis configured and an intramolecular O-HÁ Á ÁO hydrogen bond is formed in each molecule. The mean planes through the aromatic ring and the amide group -NHCO-are inclined at angles of 17.4 (3), 20.8 (2), 16.2 (2) and 11.2 (3) in the four molecules. In the crystal, intermolecular N-HÁ Á ÁO hydrogen bonds link the molecules into ribbons along the b axis.
MT and JK thank the Grant Agency of the Slovak Republic (VEGA 1/0817/08) and Structural Funds, Interreg IIIA, for financial support in purchasing the diffractometer.

Comment
As a part of studying the effect of ring and side chain substitutions on the crystal structures of biologically important amides (Gowda, Foro, Saraswathi & Fuess, 2009;Gowda, Foro, Saraswathi, Terao & Fuess, 2009;Gowda, Tokarčík et al., 2009;Prasad et al., 2002), the crystal structure of N-(3,4-dimethylphenyl)-maleamic acid (I) has been determined. The asymmetric unit of the cell contains four independent molecules (Fig. 1). The conformations of the N-H and C=O bonds in the amide segment of the structure are anti to each other and those of the amide O atom and the carbonyl O atom of the acid segment are also anti to each other. But the amide O atom is anti to the H atom attached to the adjacent C atom, while the carboxyl O atom is syn to the H atom attached to its adjacent C atom (Fig.1). In the structure of (I), the rare anti conformation of the C=O and O-H bonds of the acid group has been observed, similar to that obsrved in N-(2,6-dimethylphenyl)maleamic acid (Gowda, Tokarčík et al., 2009) andN-phenylmaleamic acid (Lo &Ng, 2009), but contrary to the more general syn conformation observed for C=O and O-H bonds of the acid group in N-(2,6-dimethylphenyl)succinamic acid (Gowda et al., 2009b).
The various modes of interlinking carboxylic acids by hydrogen bonds is described elsewhere (Leiserowitz, 1976).

Experimental
To a solution of maleic anhydride (0.025 mol) in toluene (25 ml) was added dropwise a solution of 3,4-dimethylaniline (0.025 mol) also in toluene (20 ml) with constant stirring. The resulting mixture was warmed with stirring for over 30 min and set aside for additional 30 min at room temperature for the completion of reaction. The mixture was then treated with dilute hydrochloric acid to remove the unreacted 3,4-dimethylaniline. The resultant solid N-(3,4-dimethylphenyl)maleamic acid was filtered under suction and washed thoroughly with water to remove the unreacted maleic anhydride and maleic acid. It was recrystallized to constant melting point from ethanol. The purity of the compound was checked by elemental analysis and characterized by its infrared spectra. The single crystals used in X-ray diffraction studies were grown in an ethanol solution by slow evaporation at room temperature.

Refinement
All H atoms bonded to C and N atoms were positioned with idealized geometry (C-H = 0.93 or 0.96 Å, N-H = 0.86 Å) and refined using a riding model. H atoms of the carboxyl groups were located in a difference map and finally refined with O-H distance fixed at 0.88 Å. The U iso (H) values were set at 1.2U eq (C aromatic , N, O) and 1.5U eq (C methyl ). Two methyl groups (C12 and C72) exhibit orientational disorder in hydrogen atoms positions. In both groups two sets of H atoms were refined with equal occupancies of 0.50. The U values of the fragment C62, C63, C64, C65, C66 and C72 and of the supplementary materials sup-2 atom pairs C42-C43 and N2-C21 were subject to a restraint (DELU instruction), i.e. the components of the displacement parameters in the direction of the bond were restrained to be equal within an effective standard deviation 0.005. Because of low anomalous scattering power the absolute structure cannot be determined reliably and therefore 5143 Friedel pairs were merged.    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.