N -(3-Nitrophenyl)maleimide

The title compound, C10H6N2O4, belongs to a series of N-aryl­maleimides, which can be used as photoinitiators for free-radical polymerization. The dihedral angles between the planes of the benzene and imide rings are 56.2 (1) and 52.9 (1)° in the two independent mol­ecules in the asymmetric unit.

There is considerable activity related to the use of N-substituted maleimides as photoionizers for free-radical polymerization, where the maleimide can produce the initiating radical species (Pyriadi & Nabeel, 1988;Andersson et al., 1996;Hoyle et al., 1999). In continuing the structural studies on N-substituted maleimide systems, to study the behaviour of C aryl -N distance and imide/benzene interplanar angle, the crystal structure determination of m-nitrophenylmaleimide, C 10 H 6 N 2 O 4 , (I), was undertaken. The reactivity of N-aromatic maleimides in photopolymerization processes as a function of the angle between the maleimide and benzene rings has been analysed (Miller et al., 2000). The p-nitrophenylmaleimide (p-NPM) system has been reported by our research group (Moreno-Fuquen et al., 2003). This structure has a close analogy to the title compound and it has been used as a model for comparison.
A perspective view of the two independent molecules in the asymmetric unit of the title compound, showing the atomic numbering scheme, is given in Fig. 1. Focusing on the N-C aryl bond length, in the title compound the N2-C5 and N4-C15 distances are 1.424 (4) and 1.421 (4) Å , respectively. These values are close to the N-C aryl bond length for p-nitrophenylmaleimide (Moreno-Fuquen et al., 2003) and are slightly smaller than the average value reported for nine Narylmaleimide derivatives (Miller et al., 2000). The benzene ring mean plane is rotated 56.2 (1) and 52.9 (1) with respect to the imide ring mean plane. These values are dictated probably by the weak hydrogen bond between an O atom of the maleimide group and a C atom of the benzene ring. The rotation is smaller in the case of p-NPM, which has an angle of 42.98 (5) . This is consistent with the literature values, where other maleimides with bulky ortho substituents show angles of rotation greater than 80 . Other bond lengths and internal geometrical parameters of the title compound (Table 1) are similar to those in p-NPM. There are no significant intermolecular hydrogen bonds in the structure.

Experimental
Reagents and solvents for the synthesis were obtained from Aldrich Chemical Co., and were used without additional purification. The title compound was prepared by taking equimolar quantities of m-nitroaniline and maleic anhydride in nitrobenzene and refluxing at 513 K for 3 h. The reaction product was filtered and washed with hexane and then it was dissolved in a mixture of ethyl acetate-hexane (15% hexane) in order to purify it by column chromatography. The solid was crystallized from chloroform, giving pale-yellow prisms with a melting point of 395 (1) K.  Table 1 Selected geometric parameters (Å , ).
In the absence of significant anomalous scattering, Friedel pairs were merged. H atoms were located in electron-density difference maps and subsequently treated as riding atoms, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C).
We are grateful to the Instituto de Química Física Rocasolano, CSIC, Spain, for the use of the Cambridge Structural Database System. The authors also acknowledge the Universidad del Valle, Colombia for partial financial support.

Figure 1
The asymmetric unit of the title compound with the atomic labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as spheres of arbitrary radii.